Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems 0323904041, 9780323904049

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ADVANCED NANOMATERIALS AND NANOCOMPOSITES FOR BIOELECTROCHEMICAL SYSTEMS

Micro and Nano Technologies Series

ADVANCED NANOMATERIALS AND NANOCOMPOSITES FOR BIOELECTROCHEMICAL SYSTEMS Edited by

NABISAB MUJAWAR MUBARAK Petroleum and Chemical Engineering, Faculty of Engineering, Universiti Teknologi Brunei, Bandar Seri Begawan, Brunei Darussalam

ABDUL SATTAR JATOI Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan

SHAUKAT ALI MAZARI Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan

SABZOI NIZAMUDDIN Australian Rivers Institute and School of Environment and Science, Griffith University, Nathan Campus, Queensland, Australia

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-90404-9 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisitions Editor: Ana Claudia A. Garcia Editorial Project Manager: Rafael Guilherme Trombaco Production Project Manager: Kamesh R Cover Designer: Greg Harris Typeset by Aptara, New Delhi, India

Contributors

Tallam Aarti CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India Omid Akhavan Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran; Department of Physics, Sharif University of Technology, Tehran, Iran Alberto Alvarez-Gallegos Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Mexico Abdul Hakeem Anwer Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Sciences, Aligarh Muslim University, Aligarh, India Rezoana Bente Arif Department of Electrical and Electronic Engineering, IUBAT-International University of Business Agriculture and Technology, Uttara, Dhaka, Bangladesh Muhammad Asad Department of Chemistry, Kohat University of Science and Technology, Kohat, Pakistan Behzad Ataie-Ashtiani Department of Civil Engineering, Sharif University of Technology, Tehran, Iran Aarti Atkar CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India; AcSIR – Academy of Scientific & Innovative Research, CSIR-IICT, Hyderabad, India Sandra Edith Benito-Santiago Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico Bushra Department of Chemistry, Kohat University of Science and Technology, Kohat, Pakistan Felipe Caballero-Briones Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico M. Castillo-Juárez Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico Bibiana Cercado Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C. (CIDETEQ), Querétaro, México xiii

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Contributors

Vicente Compañ Escuela Técnica Superior de Ingenieros Industriales (ETSII), Departamento de Termodinámica Aplicada, Universitat Politécnica de Valencia, Valencia, Spain Jorge Escorihuela Universitat de Valencia, Departamento de Química Orgánica, Valencia, Spain Saeed Fatima CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India; AcSIR - Academy of Scientific & Innovative Research, CSIR-IICT, Hyderabad, India Natarajan Gnanaseelan Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico Jesús Guerrero-Contreras Departamento de Ingeniería Eléctrica-Electrónica, Tecnológico Nacional de México, Instituto Tecnológico de Saltillo, Saltillo, México Chongshen Guo School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China Ekhlas Kadum Hamza Control & System Engineering Department, University of Technology, Iraq Shahad Nafea Jaafar Control & System Engineering Department, University of Technology, Iraq Ajith James Jose Department of Chemistry, St. Berchmans College, Changanassery, Kottayam, Kerala, India Marziyeh Jannesari Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran; Department of Physics, Sharif University of Technology, Tehran, Iran Keshava Joshi Department of Chemical Engineering, SDM College of Engineering and Technology (V.T.U., Belagavi), Dharwad, Karnataka, India Sathish-Kumar Kamaraj Instituto Politécnico Nacional (IPN)-Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Altamira, Mexico Nishat Khan Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Sciences, Aligarh Muslim University, Aligarh, India Mohammad Zain Khan Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Sciences, Aligarh Muslim University, Aligarh, India

Contributors

Annamalai Senthil Kumar Nano Bioelectrochemistry Research Lab, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology University, Vellore, Tamil Nadu, India; Carbon dioxide and Green Technology Research Centre, Vellore Institute of Technology University, Vellore, Tamil Nadu, India Shaukat Ali Mazari Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan Nabisab Mujawar Mubarak Petroleum and Chemical Engineering, Faculty of Engineering, Universiti Teknologi Brunei, Bandar Seri Begawan, Brunei Darussalam Nawshad Muhammad Department of Dental Materials, Institute of Basic Medical Sciences, Khyber Medical University Peshawar, Peshawar, Pakistan Lakshmipathy Muthukrishnan Department of Conservative Dentistry and Endodontics, Saveetha Dental College & Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India Pedro Nava-Diguero Universidad Tecnológica de Altamira, Altamira, Mexico Lokeshwari Navalgund Department of Chemical Engineering, SDM College of Engineering and Technology (V.T.U., Belagavi), Dharwad, Karnataka, India Umar Nishan Department of Chemistry, Kohat University of Science and Technology, Kohat, Pakistan Sabzoi Nizamuddin Australian Rivers Institute and School of Environment and Science, Griffith University, Nathan Campus, Queensland, Australia Manideep Pabba CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India Abdur Rahim Department of Chemistry, COMSAT University Islamabad, Islamabad Campus, Pakistan Antonia Sandoval-González CONACYT-Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C. (CIDETEQ) Quéretaro, México Vinayaka B. Shet Nitte (Deemed to be University), NMAM Institute of Technology (NMAMIT), Department of Biotechnology Engineering, Nitte, India Sundergopal Sridhar CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India; AcSIR - Academy of Scientific & Innovative Research, CSIR-IICT, Hyderabad, India

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Contributors

Mohammadhossein Taghipour Department of Materials Engineering, University of Tabriz, Tabriz, Iran Shabnam Taghipour Department of Civil Engineering, Sharif University of Technology, Tehran, Iran Manju Venkatesan Nano Bioelectrochemistry Research Lab, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology University, Vellore, Tamil Nadu, India Chiranjeevi Srinivasa Rao Vusa Chemical Sensors Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Kanpur, India Mei Yan School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China Muhammad Zahoor School of Engineering, RMIT University, Australia Jixiang Zou School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China

Contents

Contributors

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About the editors Foreword

xvii xxi

Preface Acknowledgments

xxiii xxv

1. Introduction to the microbial electrochemical system

1

Bibiana Cercado 1.1 Electrochemical cells and bioelectrochemical systems (BESs) 1.2 Biological fundamentals of BESs 1.3 Electroactive biofilm 1.4 Applications of BESs 1.5 Electrodes and bioelectrodes 1.6 Membranes 1.7 Electrochemical cell design 1.8 Characterization of BESs 1.9 Conclusions and perspectives References

2. Electricity generation with the use of microbial electrochemical systems

1 3 5 6 7 8 9 10 12 13

19

M. Castillo-Juárez, Pedro Nava-Diguero and Felipe Caballero-Briones 2.1 2.2 2.3 2.4

Introduction to microbial electrochemical systems Electrogenic organisms Typical applications for microbial electrogenesis Principles of microbial electrochemical systems: fuel cells (MFCs) and electrolysis cells (MECs) 2.5 MFC performance: operation parameters 2.6 MFC optimization 2.7 Challenges to improve MFC performance at real-life scale 2.8 Perspectives, the future of MFCs 2.9 Concluding remarks Acknowledgments References

19 24 27 32 36 38 47 49 50 51 51

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3. Overview of wastewater treatment approaches related to the microbial electrochemical system

57

Saeed Fatima, Tallam Aarti and Sundergopal Sridhar 3.1 Introduction 3.2 Current research on wastewater treatment techniques 3.3 Comparison between conventional systems and microbial electrochemical systems for wastewater treatment 3.4 Classification of microbial electrochemical systems 3.5 Working principle and mechanism microbial electrochemical systems for wastewater treatment 3.6 Bottlenecks and troubleshooting involved in MESs 3.7 Conclusions and future prospects References

4. Synthesis and application of nanocomposite material for microbial fuel cells

57 60 61 62 64 72 74 75

81

Antonia Sandoval-González and Bibiana Cercado 4.1 Introduction 4.2 Synthesis of nanocomposite materials used in microbial fuel cells 4.3 Characterization of nanocomposites materials used as electrodes in microbial fuel cells 4.4 Nanoparticles-based electrodes 4.5 Performance of nanomaterials in anodes and cathodes 4.6 Conclusions References

5. Classification of nanomaterials and nanocomposites for anode material

81 83 91 96 99 102 102

109

Mei Yan, Jixiang Zou and Chongshen Guo 5.1 5.2 5.3 5.4 5.5

Introduction Carbon-based nanomaterials and nanocomposites Transition metal and/or transition metal oxide decorated carbonaceous anode Conductive polymers improved carbonaceous nanocomposites Other nanocomposites (transition metal/transition metal oxide/polymer/carbon/ transition metal carbide, etc.) 5.6 Other nanomaterials or nanostructure for improving anode performances 5.7 Future challenge of nanomaterial/nanocomposite material 5.8 Conclusions References

109 110 115 118 120 121 123 123 124

Contents

6. Properties of nanomaterials for microbial fuel cell application

133

Manju Venkatesan, Vicente Compañ, Annamalai Senthil Kumar, Jorge Escorihuela, Chiranjeevi Srinivasa Rao Vusa and Sathish-Kumar Kamaraj 6.1 Bioelectrochemical energy generation systems principle and types 6.2 Components of MFC 6.3 Properties of vital components and their intrinsic factors to enhance electricity output 6.4 Different types of nanomaterials in MFC 6.5 Outlook and future perspective References

7. Advanced nanocomposite material for wastewater treatment in microbial fuel cells

133 136 138 150 160 161

169

Aarti Atkar, Manideep Pabba and Sundergopal Sridhar 7.1 Introduction 7.2 Microbial fuel cell (MFC) as an emerging source of energy 7.3 Role of nanocomposite materials in MFCs 7.4 Conclusions and future prospects Acknowledgment References

8. Nanostructured electrode materials in bioelectrocommunication systems

169 172 174 181 181 182

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Ekhlas Kadum Hamza and Shahad Nafea Jaafar 8.1 Introduction 8.2 Theory background 8.3 Bioelectrochemical system 8.4 Bioelectrochemical fuel cell 8.5 Conclusion and future perspectives References

9. Nanomaterials supporting biotic processes in bioelectrochemical systems

187 188 190 196 201 202

205

Rezoana Bente Arif and Nabisab Mujawar Mubarak 9.1 Introduction 9.2 Nanomaterials used in biocell 9.3 Toxicity of NPs and toxicity reduction by NPs in MFC 9.4 Conclusions References

205 205 214 216 216

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10. Nanomaterials supporting direct electron transport

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Abdul Hakeem Anwer, Nishat Khan and Mohammad Zain Khan 10.1 Introduction 10.2 Mechanism of electron transfer—electron release 10.3 The current state of knowledge about electrode–bacteria interactions 10.4 Conclusion and future perspectives References

11. Nanomaterials supporting oxygen reduction in bio-electrochemical systems

221 223 228 235 236

241

Muhammad Zahoor, Sabzoi Nizamuddin and Shaukat Ali Mazari 11.1 Introduction 11.2 Material synthesis and characterization 11.3 Role of nanomaterials in oxygen reduction in bio-electrochemical systems 11.4 Chemical kinetics reaction mechanisms 11.5 Outlook and challenges References

12. Nanomaterials for ion-exchange membranes

241 244 248 255 257 257

261

Ajith James Jose 12.1 Introduction 12.2 Ion exchange membranes (IEMs) 12.3 Nanomaterials for IEMs 12.4 Methods available for nanomaterials incorporation in IEMs 12.5 Nanomaterials used in IEMs 12.6 Factors affecting the performance of nanomaterial incorporated IEMs 12.7 Applications of nanomaterial incorporated IEMs 12.8 Advantages and disadvantages of nanomaterial incorporated IEMs 12.9 Conclusion and future scopes References

13. Nanomaterials supporting indirect electron transport

261 261 265 266 267 272 273 275 276 277

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Umar Nishan, Bushra, Muhammad Asad, Nawshad Muhammad and Abdur Rahim 13.1 Introduction 13.2 Nanomaterials supporting indirect electron transport in bioelectrochemical system 13.3 Nanomaterials role in indirect electron transport in azo dyes reduction 13.4 Conclusions References

279 281 285 291 291

Contents

14. Techno-economic analysis of microbial fuel cells using different nanomaterials

295

Lakshmipathy Muthukrishnan, M. Castillo-Juárez, Pedro Nava-Diguero, Felipe Caballero-Briones, Alberto Alvarez-Gallegos and Sathish-Kumar Kamaraj 14.1 Introduction 14.2 Microbial fuel cells and energy 14.3 Circular bioeconomy of MFCs 14.4 Techno-economic assessment of MFCs 14.5 Performance of MFCs 14.6 Use of nanomaterials in MFCs 14.7 Market survey of nanomaterials 14.8 Life cycle assessment (LCA) of MFCs 14.9 Nanomaterials reusability 14.10 Conclusions References

15. Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

295 301 301 305 306 309 312 314 317 319 320

327

Shabnam Taghipour, Marziyeh Jannesari, Mohammadhossein Taghipour, Behzad Ataie-Ashtiani and Omid Akhavan 15.1 Introduction 15.2 Carbon-based nanomaterials and synthesis methods 15.3 Application of carbon-based nanomaterials in bioelectrochemical systems 15.4 Graphene-based nanomaterials as the anode electrode 15.5 Microbial electrolysis cells 15.6 Conclusions and future perspectives References

16. Synthesis and application of graphene-based nanomaterials for microbial fuel cells

327 329 333 336 341 344 344

357

Sandra Edith Benito-Santiago, Natarajan Gnanaseelan, Jesús Guerrero-Contreras, Sathish-Kumar Kamaraj and Felipe Caballero-Briones 16.1 Introduction 16.2 Materials for anode 16.3 Materials for cathode 16.4 Synthesis and application of graphene-based nanomaterials for microbial fuel cells 16.5 Conclusion and future outlook References

357 358 358 364 371 371

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17. Future development, prospects, and challenges in application of nanomaterials and nanocomposites 377 Vinayaka B. Shet, Keshava Joshi and Lokeshwari Navalgund 17.1 Introduction 17.2 Future developments 17.3 Perspectives 17.4 Outlook and challenges References Index

377 377 380 388 389 393

Dedication

In the name of Allah, the Most Gracious and the Most Merciful. First of all, I would like to raise my heartfelt gratitude and appreciation to Allah S.W.T for the permission, guidance, wisdom, and blessing for all these years till now, when I have reached this important destination of my journey in life to accomplish my goal. Finally, I would like to present my most heartfelt and warmest appreciation to the great parents and parents-in-law (may ALLAH SWT bless and reward them), brothers and sisters who always encouraged and supported me during the completion of the book. Special and heartiest gratitude to my dearest wife,Muna Tasnim Mukhtaruddin and kids,Muhammad Fayyad, Muhammad Fawwaz, and Mulaika Faleeha, for their invariable encouragement endless sacrifices, patience, understanding, ideas, and inspiration from time to time in finishing the book smoothly and timely. Dr. Nabisab Mujawar Mubarak In the name of Allah, the Most Gracious and the Most Merciful. I am thankful to almighty Allah for the successful completion of this task. Thanks to my parents, sisters, brothers, wife, and children. Dr. Abdul Sattar Jatoi This book would not have been possible without sacrifices from the family. I dedicate this book to my loving wife and my lovely daughter Aysha. Dr. Shaukat Ali Mazari I, Dr. Sabzoi Nizamuddin, would like to praise the almighty Allah and his prophet (PBUH) for the blessings on us. I would like to thank my mentors, supervisors, colleagues, and parents for their support throughout my research and academic career. At last, I would heartily say thanks to the editorial team, including Dr. N.M Mubarak, Dr. A.S Jatoi, and Dr. S.A Mazari for their consistent support in finalizing this project. Dr. Sabzoi Nizamuddin

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Preface

In the past two decades, extensive research has been conducted to improve the performance of microbial electrochemical systems (MES) in product generation and treatment of wastewater and their industrial application scale. In expanding these techniques, several economic issues regarding the feasibility of the process have been studied. This economic feasibility must be evaluated concerning environmental sustainability and efficiency; that is why in the first place, these techniques are considered. Recent advancements in nanomaterial and nanocomposites for the bioelectrochemical system open new research areas for the development of fuel cells. The performance of cathode and anode material used in the fuel cells is responsible for converting chemical energy into electrical energy through a redox reaction. In recent years, various studies aimed at improving the efficiency and production of fuel cells have recognized the importance of electrode materials. The progress made for microbial fuel cell (MFC) electrode materials have been discussed in this book. The main aim of this application is to study the improvement of energy production through the alteration of anode materials to follow obscurity with microorganisms. The arrangement of the book’s contents gives researchers working/affiliated with environmental, wastewater management, biological, and composite nanomaterials applications with a one-stop solution. It also discusses the functionality of nanomaterial-based electrodes for microbial electrochemical systems. This book offers recent advancements and progress in applying nanomaterials and nanocomposites-based materials for bioelectrochemical systems. Also provide various properties, pollutant removal efficiency, application as a biosensor, and many more features related to the Bioelectrochemical system. We are thrilled to have received contributions from respected authors worldwide, and we are very grateful for their support. We thank them all for their interest and quick submission.The enthusiasm and efforts of the authors in the subject of bioelectrochemical systems with the usage of the bioelectrochemical system, which we think will be required reading for the growing number of researchers in this field. Each chapter has been thoroughly examined and through several phases, such as quality and plagiarism checks. Those chapters that passed the initial evaluation were peer-reviewed for professional comments by two independent peer reviewers. Only chapters that got good feedback were forwarded to the writers for editing. We got 24 abstract submissions. However, only 17 were selected for full paper submission. We appreciate the writers’ honesty in refining their chapters and constructively responding to comments/suggestions, and we thank them for their patience and understanding. We would like to thank all competent peer reviewers for taking the time to xxiii

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Preface

examine the work and provide helpful recommendations and feedback. Their support allowed the book to meet international requirements. Finally, we editors would like to express our heartfelt appreciation to Elsevier’s editorial staff (Rafael G. Trombaco), who guided us through every stage of the book’s development, beginning with the proposal, creating ELSA, updating as needed, producing proofs, creating a professional cover page, and finally publishing. Without their help, this book would never have gotten to this point. A/P. Dr. Nabisab Mujawar Mubarak Petroleum and Chemical Engineering, Faculty of Engineering, Universiti Teknologi Brunei, Bandar Seri Begawan, Brunei Darussalam Dr. Abdul Sattar Jatoi Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan Dr. Shaukat Ali Mazari Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan Dr. Sabzoi Nizamuddin Australian Rivers Institute and School of Environment and Science, Griffith University, Nathan Campus, Queensland, Australia

Foreword

Bioelectrochemical systems (BES) utilize the principles of extracellular electron transfer by selective anaerobic microorganisms called exoelectrogens and represent a multipurpose technology for energy production, product synthesis, product recovery, and wastewater treatment. Carbon and metal-based nanoparticles and conductive polymers could contribute to the growth of thick anodic and cathodic microbial biofilms, leading to enhanced electron transfer between the electrodes and the biofilm. Extending active surface area, increasing conductivity, and biocompatibility are significant attributes of promising nanomaterials used in MFC modifications. The recent development of nanomaterials displays significant features for bioelectrochemical systems that have brought immense interest in the scientific community, where researchers employ them for various applications. Editors have well-structured the contents of this book. They covered the basic approaches/concepts/case studies, issues in these techniques, synthesis of different nanocomposites, and applications in various fields, including environmental, agricultural, and membrane technology, energy, and sensor applications.In my opinion,it is an excellent reference book for both academia and industry. Prof Dr. Mohammad Khalid Graphene & Advanced 2D Materials Research Group (GAMRG) School of Engineering and Technology, Sunway University No.5 Jalan Universiti, Bandar Sunway, Petaling Jaya Selangor, Malaysia

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About the editors

Dr. Nabisab Mujawar Mubarak is an Associate Professor in the Faculty of Engineering, Universiti Teknologi Brunei, Brunei Darussalam. He serves as a scientific reviewer in numerous Chemical Engineering and Nano Technology journals. In research, Dr. Mubarak has published more than 270 journal papers, 30 conference proceedings and authored 30 book chapters, and the H-index is 54. His interest areas are carbon nanomaterials synthesis, magnetic biochar production using microwave, and wastewater treatment using advanced materials. He is a recipient of the Curtin Malaysia Most Productive Research award, outstanding faculty of Chemical Engineering award, Best Scientific Research Award London, and an exceptional scientist in publication and citation by i- Proclaim, Malaysia. He also has the distinction of being listed in the top two percent of the world’s most influential scientists in chemicals and energy. The List of the Top 2% Scientists in the World compiled and published by Stanford University is based on their international scientific publications, a number of scientific citations for research, and participation in the review and editing of scientific research. Dr. Mubarak is a Fellow Member of the Institution of Engineers Australia, a Chartered Professional Engineer (CPEng) of The Institution of Engineers Australia, and a Chartered Chemical Engineer of the Institute of Chemical Engineering (IChemE), UK. He has published 4 books and is co-editor for ongoing Elsevier-edited books: (1) Nanomaterials for Carbon Capture and Conversion Technique,(2) Advanced nanomaterials and nanocomposites for Bioelectrochemical Systems, (3) Water Treatment Using Engineered Carbon Nanotubes, and (4) Emerging Water Pollutants: Concerns and Remediation Technologies. Address: Petroleum and Chemical Engineering, Faculty of Engineering, Universiti Teknologi Brunei, Bandar Seri Begawan, Brunei Darussalam. Research Interests: Advanced carbon nanomaterials synthesis via microwave technology, MXene synthesis and its application in wastewater treatment and energy storage, graphene/CNT buckypaper for strain sensor application, biofuels, magnetic buckypaper, immobilization of enzymes, protein purification, magnetic biochar production using a microwave, and wastewater treatment using advanced materials.

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About the editors

Dr. Abdul Sattar Jatoi is an Assitant Professor in the Chemical Engineering department at Dawood University of Engineering and Technology, Karachi, Pakistan. He has over 11 years of experience in Academics and Research. Dr. Jatoi has expertise in Microbial electrochemical systems for environmental and energy applications. He has published over 93 papers, including 12 book chapters. He has two books in the process of editing. He is a peerreviewed member of more than 50 scientific journals. 70 works published in various international conference proceedings. Address: Department of Chemical Engineering, Dawood University of engineering and technology, Karachi, Pakistan. Research Interests: Microbial electrochemical system, Emerging pollutant treatment via different approaches, Sustainable nanomaterials development; Wastewater treatment, Green-water treatment technology; Separation & Purification Techniques; EnviroAnalytical Techniques. Dr. Shaukat Ali Mazari is an Assistant Professor in the Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan. He also held the positions of Director Quality Enhancement Cell and Director Postgraduate Studies for two years, respectively. Dr. Mazari holds a Ph.D. degree from Malaya, Kuala Lumpur, Malaysia. He has coauthored more than 65 SCI articles and has an H-Index of 21. He is co-author of 10 book chapters and co-editor of 3 ongoing books. His research focuses on chemical and environmental engineering. He is a reviewer for several high-quality chemical and environmental engineering journals and serves as a referee for several funding agencies. Dr. Mazari specializes in carbon capture, conversion and storage, nanomaterials, thermodynamic modeling, and artificial intelligence in chemical engineering. Address: Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan. Research Interests: Carbon dioxide capture, conversion and storage, material synthesis and characterization, the environmental impact of nanomaterials, process modeling and simulation, application of machine learning and deep learning models.

About the editors

Dr. Sabzoi Nizamuddin is currently working as a Postdoctoral researcher at Australian Rivers Institute and school of Environment and Science, Griffith University, Nathan Campus, Queensland, Australia. He worked as a Research Fellow at the School of Engineering, RMIT University, from July 2019 to August 2021. He has received his Ph.D. in Chemical Engineering from RMIT University Australia, Master of Chemical Engineering from University Malaya Malaysia, and Bachelor of Chemical Engineering from Dawood College of Engineering and Technology Pakistan. During his Ph.D. studies, he was awarded the Research Excellence award from the school of Engineering, RMIT University, based on publications’ high quantity and high quality. He has been the author of more than 100 articles in peer-reviewed Scopus/SCI/ESCIindexed journals and 5 book chapters for Elsevier and Springer publishers. He has presented his findings at domestic and international conferences.

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Acknowledgments

I thank Prof. Zohrah, Vice-Chancellor, and higher management of Universiti Teknologi Brunei, and colleagues in the Department of Petroleum and Chemical Engineering for the continuous support and encouragement. My special thanks go to all my co-editors and authors for their valuable contributions. Dr. Nabisab Mujawar Mubarak I would like to take this opportunity to express my sincere gratitude to the higher management of Dawood University of Engineering and Technology and colleagues in the Chemical Engineering Department for the continuous support and encouragement at all times. My special thanks go to all my coeditors and authors for their valuable contributions. Dr. Abdul Sattar Jatoi I would like to take the opportunity to express my sincere gratitude to the management of the Dawood University of Engineering and Technology, Karachi, for their support. I would like to express special thanks to my student Mazhar Ali, who helped me resolve several online formatting issues. I am also thankful to Dr. Mubarak, Dr. Abdul and Dr. Nizamuddin for their support. Nonetheless, this book would not have been possible without contributions from the book chapter authors. A big thanks go to them. Dr. Shaukat Ali Mazari I also thank my colleagues, Dr. Mubarak & Dr. Abdual, and Dr. Shaukat for their constant motivation. This book would not have been possible without their support and cooperation. Finally, thank all the authors who contributed high research value chapters. Dr. Sabzoi Nizamuddin

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CHAPTER 1

Introduction to the microbial electrochemical system Bibiana Cercado Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C. (CIDETEQ), Querétaro, Mexico

1.1 Electrochemical cells and bioelectrochemical systems (BESs) BESs are based on electrochemical cells in the fuel cell or electrolysis cell mode. A fuel cell is an electrochemical device that converts the chemical energy of a fuel into electrical energy flowing between two electrodes, from anode to cathode, and as an ionic current in an electrolyte. The electrodes are immersed in the electrolyte in which the current flows under migration and diffusion phenomena. The fuel cell operates continuously as long as fuel is fed to the cell (Larminie & Dicks, 2003). The electric current produced in fuel cells results from an oxidation reaction at the anode. The anode acts as a current collector and transfers the charge to the cathode through a circuit external to the cell. The electrons reaching the cathode are used in a reduction reaction, and charge balance is achieved by the migration of ions in the electrolyte between the electrodes. Oxidation and reduction reactions in fuel cells are thermodynamically favorable and spontaneous, thus requiring the installation of an external resistor between the anode and cathode to control the current flowing in the cell. In contrast, the electric current flowing in an electrolysis cell is the sum of the current produced by the oxidation reaction at the anode and the amount of current that is supplied externally. The current flow from the anode to the cathode can also be forced by changing the electrode potential and the cell voltage bock (Bockris et al., 2000). The electrical current needs to be forced when the potential of the anodic reaction is more positive than or very close to the value of the potential of the cathodic reaction, so there is no (or insufficient) electromotive force for the flow of electrons from the anode to the cathode to occur. However, the electrode potential, and consequently the oxidation and reduction reactions, can be modified by an external device, such as a power supply or a potentiostat, to perform the desired reactions. The feature that differentiates electrochemical cells from BESs is the composition of the electrode materials. BES electrodes are composed of a film of electroactive microorganisms, thus forming microbial bioelectrodes that differ from enzymatic

Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00014-0

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Advanced nanomaterials and nanocomposites for bioelectrochemical systems

bioelectrodes (Freguia et al., 2012). Microbial bioelectrodes have a different potential (E) than the bare electrode, so the cell potential is also modified (Ecell = Epositive − ENegative ). BESs are generally installed with bioanodes, while cathodes may or may not be colonized by biofilms. The predominance of bioanodes is partly because oxidation reactions are more widespread than reduction reactions in microorganisms. The biofilm developed on the electrode material or substrate has the ability to act as a type of catalyst. Microorganisms, being discrete units containing an array of enzymes acting in a synchronized manner,lead to oxidation/reduction of molecules more efficiently than enzyme arrays on electrodes. In addition, the biofilm has been observed to reduce the resistance to charge transfer at the electrode-electrolyte interface (Martin et al., 2013). The biofilm-electrode system is the fundamental element that defines a BES, both for energy production in microbial fuel cells (MFCs) and for chemical compound formation in microbial electrolysis cells (MECs) using organic and inorganic pollutants as fuel or substrate for the microorganisms. 1.1.1 Historical development of BESs The earliest investigations of microbial electrochemical cells date back to 1911, with work reported by M. C. Potter in Proceeding of the Royal Society of London. Series B Biological Sciences (84:160-276). Potter presented the use of yeast as a biological intermediate for the production of electric current in a fuel cell. The cell was fed different concentrations of glucose, and the output voltage was directly related to the glucose concentration. These changes in cell voltage were attributed to yeast activity. Potter’s bioelectrochemical device received no further attention for a long period. It was not until the 1990s that research on BESs increased. Research has focused on the charge transfer from microbial species to electrodes, as well as on the identification of the nutrients most favorable for the production of electric current by electroactive species (Santoro et al., 2017). Electrochemical cells were installed with bioanodes with the addition of synthetic redox mediators to improve charge transfer. The electrode–electrolyte systems in the cathodic chamber were mainly abiotic in the initial phase of BES development. Redox mediators such as ferricyanide were also frequently added to the cathodic chamber during that period. In the first half of the 2000s, the installation of MFCs in marine environments expanded rapidly. Marine sediments were used as a source of electroactive inoculum, and the high conductivity of seawater was exploited as a natural electrolyte for the operation of ocean sensors. These devices were powered by energy from MFCs (Abbas et al., 2017). The installation of BESs in natural environments led to the exclusion of synthetic redox mediators and sparked interest in the use of more complex substrates such as

Introduction to the microbial electrochemical system

chitin, cellulose, and starch (Pant et al., 2010). In addition, the use of BESs in natural environments highlighted the erosion suffered by the electrodes, thus initiating proposals for modification of the materials (Kumar et al., 2013) In approximately 2003, the use of wastewater as fuel for BESs was proposed. The application of BESs as a novel technology for pollutant removal encouraged new cell designs for higher volume operation, as well as the search for lower-cost installation materials (Rabaey et al., 2005). Studies on anode modifications proliferated in approximately 2007. Electrode modification strategies were focused mainly on the application of conductive polymer films, heteroatom doping, and control of the micro- and macro-structure of carbonaceous materials (Pham et al., 2009). Platinum was the model catalyst used for cathodic reactions in early BES research; later, platinum-doped carbon electrodes were widely used (Cheng et al., 2006). Research on MECs for the production of commercially valuable chemicals began in approximately 2005. Bioelectrosynthesis of hydrogen (Liu et al., 2005) was followed by the production of methane (Cheng et al., 2009) and organic acids (Rabaey & Rozendal, 2010) in the cathode compartment of the BES. Due to the market interest in hydrogen, other scalable cathode materials, such as nickel and stainless steel in the form of meshes and foams, have been proposed (Selembo et al., 2009). Since 2011, the function of electrolysis cells has expanded to pollutant removal; as a result, research on the design and modification of electrode materials has grown rapidly. Since then, a wide diversity of materials, structures, and methods of both composite preparation and material modification have been proposed for use as electrodes and membranes (Palanisamy et al., 2019).

1.2 Biological fundamentals of BESs The operation of a BES is based on the metabolic activity of electrochemically active microorganisms capable of giving or receiving electrons to or from an electrode. Not all microorganisms present in a bioelectrochemical cell are able to transfer charge; in addition, a fraction of microorganisms are suspended in the electrolyte, and another fraction is attached to the solid electrode material. The charge or electron flow in the BES originates from metabolic reactions (catabolism) and is used in other metabolic reactions (anabolism) inside the microbial cells or is transferred to the electrode; therefore, the entire electrode is expected to be colonized by microorganisms. However, abiotic chemical reactions may occur in the electrode material if the biofilm colonization is heterogeneous,since areas of the electrode are uncovered and exposed to chemicals in the electrolyte (Jatoi et al., 2021, 2022). The recognized mechanisms of charge transfer at the electrode-biofilm interface correspond to three types. Transfer by direct contact between the microorganism and

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Figure 1.1 Schema of mediated electron transfer and examples of redox molecules.

the electrode, transfer through cellular structures such as pili or flagella, and transfer mediated by redox molecules (Kracke et al., 2015; Shi et al., 2016). In addition to the above mechanisms, transfer between species possibly also influences charge transfer in the BES (Barua & Dhar, 2017). Direct transfer occurs through the contact of the microbial cell and the electrode. Microbial cells have an energy metabolism that involves enzyme complexes in a process called the electron transport chain. The enzyme complexes are embedded in the periplasmic membrane, the intermembrane space, and the outer membrane of the cell. A diversity of cytochromes has been identified that vary among microbial species; however, direct electron transfer is mostly attributed to c-type cytochromes (Costa et al., 2018). Mediated transfer occurs when the microbial cell separates from the electrode. Charge transport is accomplished by redox molecules that are capable of cyclic oxidation and reduction without changing their elemental composition (Martinez & Alvarez, 2018). The charge is transferred from the microbial cell to the redox molecule in its oxidized form; once the redox molecule receives the charge, it is reduced and directed toward the electrode by a combination of diffusion and migration phenomena. The redox molecule in the vicinity of the electrode gives up the charge to the electrode and when oxidized again restarts the charge transport cycle (Fig. 1.1). Redox mediators are either added to the electrochemical cell or produced by the microorganisms. Some synthetic redox mediators are toxic to microorganisms; in

Introduction to the microbial electrochemical system

addition, charge transfer with natural mediators is often less efficient due to their concentration being limited by their production rate and diffusional limitations (Torres et al., 2010). These conditions result in a less frequent operation of BESs with mediators. Charge transfer across cell membrane extension structures occurs on the same basic principle as direct contact transfer. Cytochrome-type enzyme complexes have been identified in flagella and pili. The advantage of flagella is the greater separation distance between the microbial cell and the electrode. The advantage of pili is that they allow several contact points per microbial cell (Tremblay et al., 2012). The three mechanisms of charge transfer to or from the electrode are enhanced by the biofilm arrangement of the microorganisms. The organization of microorganisms into communities is common because it allows communication for maximum utilization of nutrients and maintenance in the environment in which they are found.

1.3 Electroactive biofilm The discovery of charge transfer between microorganisms and solid materials was observed during the reduction of iron compounds by species of the genus Geobacter in marine environments. This charge transfer process has also been observed in other species belonging mostly to the class of deltaproteobacteria (Lovley & Holmes, 2022). In addition to their electron transfer characteristics, the microbial species suitable for the preparation of bioelectrodes must be prone to grow in biofilm-like communities. The development of a biofilm starts with the adhesion of bacteria on a solid surface, the bacteria spread and during the process expel polymeric substances composed mainly of carbohydrates and proteins. The exopolymeric substances form a gel in which the bacteria are embedded and protect themselves from adverse environmental conditions (Borole et al., 2011; Saratale et al., 2017). The bacterial colonies adhering to the support extend first horizontally and then vertically until they reach a thickness that prevents the arrival of nutrients to the cells forming the first layer. In some cases, the lack of nutrients causes the death of the oldest layer and the detachment of the entire biofilm Fig. 1.2. In other cases, erosion of biofilms occurs due to the flow of fluids in contact with the outermost layer (Cercado et al., 2012) (Garrett et al., 2008). All of the above processes make the biofilms heterogeneous and dynamic elements, thus affecting the bioelectrode preparation. Biofilms can be developed in the laboratory from pure cultures consisting of a single species, from mixed cultures of two or three known species, and from consortia obtained from wastewater or natural environments (Chabert et al., 2015). The selection of the type of electroactive biofilm for bioelectrode preparation depends on the BES application.

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(A)

(B)

Figure 1.2 Schema of biofilm formation and micrograph of biofilm from wastewater on carbon fibers.

1.4 Applications of BESs BES applications have diversified over time; energy production in the form of electric current is the most widespread application in MFCs. In addition, the utilization of waste matter as fuel enhanced the application of MFCs as a new technology for wastewater treatment (Kumar et al., 2018). The removal of organic matter is a more frequent application than the removal of inorganic matter via anodic oxidation (Cercado-Quezada et al., 2010); however, the chemolithotrophic metabolism of certain microorganisms has allowed the removal of recalcitrant inorganic compounds (Das et al., 2019) and metals (Bagchi & Behera, 2020). BESs have been used in MEC mode for hydrogen production in the cathodic chamber (Kadier et al., 2020), and they have also been used for methane enrichment in biogas in anaerobic digester-type reactors modified with electrodes (Yu et al., 2018). Gaseous fuels can be obtained simultaneously by using the anode chamber for biogas collection and the cathode chamber for hydrogen collection (Segundo-Aguilar et al., 2021). Methane production has also been achieved by reducing carbon dioxide supplemented externally to the cathodic chamber in a bioelectrosynthesis process (Zhen et al., 2015). The synthesis of value-added chemicals from carbon dioxide is a recent application of BESs. Products that have been obtained from the cathodic reduction of carbon dioxide include methane, short carbon chain organic acids such as acetate and butyrate, as well as solvents such as ethanol. This process uses an external flow of carbon dioxide that may come from another process (Zhen et al., 2017). Since the formation of valuable products occurs at the cathode, the material and catalysts must be carefully selected for specific reactions. Water desalination is performed by placing a third chamber between the anode and cathode chambers, creating a microbial desalination cell (MDC). The saline water in the central chamber is deionized by migrating anions to the positive electrode and cations

Introduction to the microbial electrochemical system

to the negative electrode. MDCs require the placement of anion and cation exchange membranes in the same device (Sayed et al., 2020). For this reason, research on improved and antibiofouling membranes is of special interest for MDC. Nutrient removal and recovery using BESs are two potential applications for the agri-food industry. Nitrogen in wastewater can be recovered by various processes in which electrodes, anodes, or cathodes, are introduced to modify the flow of electrons in the natural nitrogen cycle. Hybrid bioelectrochemical processes include nitrification, denitrification, and anaerobic ammonia oxidation (Sun et al., 2020). Metal recovery in BESs is an emerging application that is associated with mining activity. The process is electrolytic and depends on the reduction potential of the metal ions, chemical species changes depending on pH, and the current flowing through the BES. Metals can be recovered as precipitates at the bottom of the cell or as deposits on the electrodes. Metal ions that have been successfully removed or recovered are Co(II/II), Cr(VI), Cu(II), Cu(II), Ag(I), Hg(II), Se(VI) and Cd(II) (Dominguez-Benetton et al., 2018). Among the most recent applications of BESs is in energy storage in the form of a biocapacitor. In addition to the intrinsic capacitive properties of carbon, the biofilm’s ability to store charge makes energy storage more efficient (Caizán-Juanarena et al., 2020). The fundamentals of capacitance in biofilms are still under investigation; however, the preparation of carbonaceous materials with high capacitance and promoting the development of a homogeneous biofilm is of interest for this application.

1.5 Electrodes and bioelectrodes The primary function of anodes in BESs is the oxidation of matter by microbial activity. Therefore, greater importance has been given to the biocompatibility that exists between carbon-based materials and microbial cells. Among the research on electrodes modified with heteroatoms, nitrogen-doped carbon-based electrodes are the most frequently reported (Guo et al., 2020). Anodes are modified to favor colonization by bacteria, maintain a homogeneous biofilm and increase charge transfer between microorganisms and electrodes. Several modification strategies have been identified; first, rough texture appears to be more favorable than smooth texture in the support material, materials with macroporosity favor nutrient diffusion in the biofilm, and highly graphitized carbons favor charge conduction (Cercado et al., 2013; Cercado et al., 2016). Other modifications of carbon anodes have been the inclusion of metal particles, in some cases as pure metal nanoparticles and in others as metal oxides. Various conductive polymers have also been tested to modify the biocompatibility, texture, and conductivity of carbonaceous materials or metals used as supports (Guo et al., 2015).

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The main function of cathodes in MFCs is the oxygen reduction reaction. Because this reaction has slow kinetics, cathode modification has focused primarily on improving the catalytic properties of the material compared to platinum as a model catalyst. The functions of cathodes in MECs are hydrogen gas production, methane, and organic acid synthesis, removal of pollutants, and metal recovery or removal. Because of the multiple functions of cathodes, hybrid materials incorporating two or more modification strategies are necessary. Of particular interest are the use of zeolite imidazole frameworks as precursors (Shao et al., 2022), as well as other metal organic frameworks (J. Li et al., 2022). Residue-based carbons (S. Li et al., 2021) and biogenic metal nanoparticles (Sallam et al., 2021) are also interesting strategies due to their sustainability. The most recent electrode preparation strategies consist of doping metallic and nonmetallic particles into nanostructures to form porous macrostructures (Shao et al., 2022). Given the complexity of the preparation procedure, the electrodes thus obtained must have very specific applications, such as pharmaceutical removal. The synthesis of some valuable products by abiotic cathodic reactions requires the maintenance of environments uncontaminated by microorganisms, so antimicrobial materials have been prepared for this purpose (Jiang et al., 2021). Another BES component that requires antimicrobial properties is the membrane separating the electrodes. Thus, the preparation of materials for use as electrodes in BESs can be grouped into materials that promote or inhibit the development of biofilms, materials for pollutant removal, and materials for the synthesis of value-added compounds. These various requirements open up a large field of research in electrode materials (Fig. 1.3).

1.6 Membranes The membranes in the BES prevent contact between the electrodes, and their function is to separate the reactions occurring at each electrode, the electrolytes (anolyte and catholyte) and the gas flows between the anode and cathode chamber. The membrane regulates the transport of ions from one electrolyte to another depending on the type of membrane and the ions present in the electrolytic solutions. The most common types of membranes used in BESs are anionic and cationic polymeric membranes; however, other types of membranes include bipolar, mosaic, and amphoteric membranes. Cationic membranes contain negatively charged groups such as sulfonates and carboxylates that control the passage of cations between electrolytes. Other membrane classifications include perfluorinated membranes such as Nafion and nonperfluorinated membranes such as sulfonated poly (ether ether ketone) (SPEEK) (Shabani et al., 2020). Membrane modifications are aimed at improving the most relevant factors, such as ion exchange capacity, internal electrical resistance, gas diffusion, water uptake, mechanical resistance, and antifouling. Membrane (bio)fouling is of particular interest, and this

Introduction to the microbial electrochemical system

Figure 1.3 Approaches on materials for installation of bioelectrochemical systems.

phenomenon is caused by the deposition and strong adhesion of biological and nonbiological particles. (Bio)fouling causes a decrease in mass and charge transfer processes across the membrane, which negatively affects the overall performance of the BES. Strategies to prevent (bio)fouling in membranes include the use of enzymes for quorum sensing disruption,the use of quaternary ammonium compounds,and the preparation of composites based on metal oxides and toxic metal nanoparticles (Nasruddin & Bakar, 2021). The challenges for membrane preparation are the use of sustainable materials as well as preparation methods with minimal generation of environmentally harmful wastes.

1.7 Electrochemical cell design The first microbial bioelectrochemical cells were constructed with a conventional architecture consisting of two chambers separated by an ion exchange membrane. Cell designs evolved along with their applications and the findings encountered during their operation.

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Research on BES for wastewater treatment opted for a single-chamber membranefree design. Further improvements included air cathodes forming membrane-cathode assemblies (Kardi et al., 2019). Scale-up of BES was performed from the point of view of industrial effluent depollution; however, the larger volume BES showed poor performance (Escapa et al., 2015). Successful cases of increased operating volume have been reported for modular units in series and parallel electrical and hydraulic connections instead of large BES volumes (Walter et al., 2020). Another effective design is the installation of BESs forming baffles inside channels (Kadier et al., 2016b). Therefore, the preparation of installation materials for BES should not be of large size but in large numbers for scale-up. Applications of BESs as sensors and for the investigation of charge transfer phenomena at the biofilm–electrode interface have led to the miniaturization of designs (Ye et al., 2018). Flat plate designs are the most common; however, there are other designs that allow the testing of several materials simultaneously on a multiple electrode array (Tahernia et al., 2019). The basic criteria for the design of an electrochemical reactor are shared with the BES. The criteria cover vessel geometry and volume, electrode arrangement and number of electrodes, membrane placement, and location of liquid and gas ports. Analysis of the bioelectrochemical reactor design from an electrochemical engineering approach has identified the need for improvements in volumetric bioelectrodes (Hernández-García et al., 2020). Even for lab-scale BESs, the electrochemical engineering approach should be incorporated into the investigations. Unfortunately, there is no standardized bioelectrochemical cell design that would allow comparisons between investigations, both in terms of operating conditions and installation materials. In this regard, a well-characterized conventional design for abiotic processes, such as parallel plate electrochemical cells, can be useful for attributing results to the parameter under investigation rather than to BES design.

1.8 Characterization of BESs BESs are characterized by at least three types of parameters: physicochemical, biological, and electrochemical (Table 1.1). Evaluation with respect to pollutant removal in both biotechnological processes and electrochemical processes results in a larger number of variables to evaluate (and control) compared to other processes (Sharma et al., 2014). Physicochemical parameters include pH, conductivity, temperature, oxidizable matter, and total organic carbon, and if applicable, any specific nutrient. All these measurements are performed in both the anolyte and the catholyte (Kadier et al., 2016a). The biological parameters measured in most BESs are the microbial biomass in the inoculum and changes in the suspended biomass throughout the operation of the device. The biofilm is investigated during the colonization and maturation phases; characteristics such as the degree of support coverage,thickness,and amount of exopolymeric substances

Introduction to the microbial electrochemical system

Table 1.1 Parameters for characterization of bioelectrochemical systems. Start phase Final phase

Biological

Physical chemical

Electrochemical

Microbial biomass in inoculum: protein, volatile solid suspended Substrate concentration:organic, inorganic molecules, chemical oxygen demand, total organic carbon pH Conductivity Dissolved oxygen Temperature Open circuit potential and voltage Electrochemical impedance spectroscopy Cyclic voltammetry Linear voltammetry Chronoamperometry

Biofilm coverage Substrate consumption Substrate consumption rate Degree of stability

Electrode corrosion Coulombic efficiency Cathodic efficiency Energy efficiency Product yield

are variables of interest. The ratio of live cells to total biofilm, redox mediator production and enzyme production are indicators of activity (Babauta et al., 2012). Electrochemical parameters are measured with techniques adapted to the biofilmelectrode interface using a potentiostat/galvanostat. The techniques on the bioelectrodes are performed starting with the one that disturbs the biological phase the least. Thus, the first technique to characterize the BES is typically the open circuit potential (OCP) (Turick et al., 2019). The OCP shows the changes due to the colonization of the electrode and indicates a characteristic value useful during the comparison of bioelectrodes or entire BESs. It is worth noting that the obtained OCP value is pseudostationary due to natural changes in the biofilm. Another technique applied to characterize bioelectrodes is electrochemical impedance spectroscopy (EIS). EIS determines the distribution of resistances at the electrode and electrochemical cell levels (Dominguez-Benetton et al., 2012). The ohmic resistances correspond to the macrocharacteristics of the BES (distance between electrodes, electrolyte conductivity, contact between carbonaceous and metallic material, presence of membrane). The charge transfer type resistances correspond to the phenomena of the biofilm-electrode interface that include contact type, degree of colonization, activity of microorganisms, and presence of exopolymeric substances (Kretzschmar & Harnisch, 2021). Linear and cyclic voltammetry (LSV, CV) are applied to identify oxidation/reduction signals associated with nutrient use and/or biofilm activity itself. For example, the presence of cytochromes or the production of redox mediators has been identified for Geobacter and Shewanella species (Babauta et al., 2012; Ruiz et al., 2020). The LSV technique is mainly used in the characterization of modified cathodes (Santoro et al., 2017).

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Chronoamperometry (CA) is used to simulate the conditions of one of the half-cells (anodic or cathodic) without intervening factors related to the presence of the membrane (Cercado-Quezada et al., 2010). CA makes it possible to keep the potential of an electrode fixed over time and is therefore used for the controlled bioelectrosynthesis of compounds (Rabaey & Rozendal, 2010). Physicochemical, biological, and electrochemical parameters are used in combination to calculate the efficiencies of the BES. The coulombic efficiency relates the energy obtained as electric current to the energy in the matter consumed by the microorganisms (Logan et al., 2006). The current production is normalized to the electrode area for both the MFC and MEC to obtain the current density; in some cases, the current is normalized to the membrane area (An & Lee, 2013), which seems pertinent for studies of modified membranes. The energy produced in the MFC is reported as the power density, while the product obtained in the MEC is reported as the cathode efficiency. Power is the product of the cell potential times the current density produced, while cathodic efficiency is the percentage ratio of the product obtained (or removed) at the cathode to the electric current flowing in the cell (Logan et al., 2006) (Logan & Rabaey, 2012). Other efficiency parameters used in biotechnology have been adapted to evaluate the performance of BESs. For example, the product/substrate yield (YP/S ), where an attempt is made to correlate the product obtained in the cathodic chamber with the substrate consumed in the anodic chamber (Lu et al., 2012). The difficulty of using parameters between two chambers is that many other factors are involved in each chamber that can bias the yield result without being identified. Analysis of possible interactions between parameters must be done for each BES design and set of operating conditions, as unreliable calculations could be made by simply following reported equations. For example, the calculation of the Coulombic efficiency is a function of the matter consumed in the anode chamber, but organic matter is oxidized by electroactive and nonelectroactive microorganisms, resulting in an overestimation of the organic matter consumption.

1.9 Conclusions and perspectives BESs are devices based on electrochemical cells into which a microbial biological element is introduced. The rise of BES development is relatively recent; however, as a hybrid device coming from the union of electrochemical technology, environmental sciences, biotechnology, and materials sciences, it has given rise to a wide variety of research lines, some of which are listed below. r The interaction of microorganisms with electrode materials is the central feature in BESs, so the preparation, modification, and improvement of both biofilm and electrode is of great interest to obtain the expected benefits according to the function of the BES.

Introduction to the microbial electrochemical system

r

r

r

r

r

The costs of raw materials and resources required for the modification of materials should be minimized. It is recommended that research on new materials include economic information and demonstrate the feasibility for real applications. High-volume BESs involve the use of sustainable and recyclable materials, as well as simple methods of electrode modification. In this scenario, more research is needed on the availability of sustainable raw materials, achieving standardized characteristics of the prepared materials and determining their durability. Handling and disposal of spent and unusable materials is an issue that has not yet arisen in the history of BES development, but strategies to address health risks, especially from some nanomaterials, could be envisaged. Smaller-scale devices with portable features allow the installation of carefully selected and more expensive materials. However, reuse of the devices and their components is also desirable due to the cost of noble metals and the complexity required for their preparation. BESs installed with new materials must demonstrate their competitiveness compared to conventional materials with market applications so that their eventual industrial manufacture can be promoted.

References Abbas, S. Z., Rafatullah, M., Ismail, N., & Syakir, M. I. (2017). A review on sediment microbial fuel cells as a new source of sustainable energy and heavy metal remediation: Mechanisms and future prospective. International Journal of Energy Research, 41(9), 1242–1264. https://doi.org/10.1002/er.3706. An, J., & Lee, H. S. (2013). Implication of endogenous decay current and quantification of soluble microbial products (SMP) in microbial electrolysis cells. RSC Advances, 3(33), 14021–14028. https://doi. org/10.1039/c3ra41116h. Babauta, J., Renslow, R., Lewandowski, Z., & Beyenal, H. (2012). Electrochemically active biofilms: Facts and fiction. A review. Biofouling, 28(8), 789–812. https://doi.org/10.1080/08927014.2012.710324. Bagchi, S., & Behera, M. (2020). Assessment of Heavy Metal Removal in Different Bioelectrochemical Systems: A Review. Journal of Hazardous, Toxic, and Radioactive Waste, 24(3), 04020010. https://doi.org/10.1061/(ASCE)HZ.2153-5515.0000500. Barua, S., & Dhar, B. R. (2017). Advances towards understanding and engineering direct interspecies electron transfer in anaerobic digestion. Bioresource Technology, 244, 698–707. https://doi.org/10.1016/ j.biortech.2017.08.023. Bockris, J. O., Reddy, A. K. N., & Gamboa-Aldeco, M. (2000). Modern Electrochemistry 2A. Fundamentals of Electrodics. New York: Kluwer Academic. Borole, A. P., Reguera, G., Ringeisen, B., Wang, Z. W., Feng, Y., & Kim, B. H. (2011). Electroactive biofilms: Current status and future research needs. Energy and Environmental Science, 4(12), 4813–4834. https://doi. org/10.1039/c1ee02511b. Caizán-Juanarena, L., Borsje, C., Sleutels, T., Yntema, D., Santoro, C., Ieropoulos, I., Soavi, F., & ter Heijne, A. (2020). Combination of bioelectrochemical systems and electrochemical capacitors: Principles, analysis and opportunities. Biotechnology Advances, 39, 107456. doi:10.1016/j.biotechadv.2019.107456. Cercado, B., Auria, R., Cardenas, B., & Revah, S. (2012). Characterization of artificially dried biofilms for air biofiltration studies. Journal of Environmental Science and Health - Part A Toxic/Hazardous Substances and Environmental Engineering, 47(7), 940–948. https://doi.org/10.1080/10934529.2012.667292.

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Palanisamy,G.,Jung,H.Y.,Sadhasivam,T.,Kurkuri,M.D.,Kim,S.C.,& Roh,S.H.(2019).A comprehensive review on microbial fuel cell technologies: Processes, utilization, and advanced developments in electrodes and membranes.Journal of Cleaner Production,221,598–621.https://doi.org/10.1016/j.jclepro.2019.02.172. Pant, D., Van Bogaert, G., Diels, L., & Vanbroekhoven, K. (2010). A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresource Technology, 101(6), 1533–1543. https://doi.org/10.1016/j.biortech.2009.10.017. Pham, T. H., Aelterman, P., & Verstraete, W. (2009). Bioanode performance in bioelectrochemical systems: Recent improvements and prospects. Trends in Biotechnology, 27(3), 168–178. https://doi.org/ 10.1016/j.tibtech.2008.11.005. Rabaey, K., Clauwaert, P., Aelterman, P., & Verstraete, W. (2005). Tubular microbial fuel cells for efficient electricity generation. 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A critical review on recent proton exchange membranes applied in microbial fuel cells for renewable energy recovery. Journal of Cleaner Production, 264, 121446. doi:10.1016/j.jclepro.2020.121446. Shao, C., Wu, L., Wang, Y., Qu, K., Chu, H., Sun, L., Ye, J., Li, B., & Wang, X. (2022). An open superstructure of hydrangea-like carbon with highly accessible Fe-N4 active sites for enhanced oxygen reduction reaction. Chemical Engineering Journal, 429, 132307. doi:10.1016/j.cej.2021.132307. Sharma, M., Bajracharya, S., Gildemyn, S., Patil, S. A., Alvarez-Gallego, Y., Pant, D., Rabaey, K., & DominguezBenetton, X. (2014). A critical revisit of the key parameters used to describe microbial electrochemical systems. Electrochimica Acta, 140, 191–208. https://doi.org/10.1016/j.electacta.2014.02.111. Shi, L., Dong, H., Reguera, G., Beyenal, H., Lu, A., Liu, J., Yu, H. Q., & Fredrickson, J. K. (2016). Extracellular electron transfer mechanisms between microorganisms and minerals. 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Tahernia, M., Mohammadifar, M., Hassett, D. J., & Choi, S. (2019). A fully disposable 64-well papertronic sensing array for screening electroactive microorganisms. Nano Energy, 65, 104026. doi:10.1016/j.nanoen. 2019.104026. Torres, C. I., Marcus, A. K., Lee, H. S., Parameswaran, P., Krajmalnik-Brown, R., & Rittmann, B. E. (2010). A kinetic perspective on extracellular electron transfer by anode-respiring bacteria. FEMS Microbiology Reviews, 34(1), 3–17. https://doi.org/10.1111/j.1574-6976.2009.00191.x. Tremblay, P. L., Aklujkar, M., Leang, C., Nevin, K. P., & Lovley, D. (2012). A genetic system for Geobacter metallireducens:Role of the flagellin and pilin in the reduction of Fe(III) oxide.Environmental Microbiology Reports, 4(1), 82–88. https://doi.org/10.1111/j.1758-2229.2011.00305.x. Turick, C. E., Shimpalee, S., Satjaritanun, P., Weidner, J., & Greenway, S. (2019). Convenient non-invasive electrochemical techniques to monitor microbial processes: Current state and perspectives. Applied Microbiology and Biotechnology, 103(20), 8327–8338. https://doi.org/10.1007/s00253-019-10091-y. Walter, X. A., Santoro, C., Greenman, J., & Ieropoulos, I. A. (2020). Scalability and stacking of selfstratifying microbial fuel cells treating urine. Bioelectrochemistry, 133, 107491. https://doi.org/10.1016/ j.bioelechem.2020.107491. Ye, D., Zhang, P., Zhu, X., Yang, Y., Li, J., Fu, Q., Chen, R., Liao, Q., & Zhang, B. (2018). Electricity generation of a laminar-flow microbial fuel cell without any additional power supply. RSC Advances, 8(59), 33637– 33641. https://doi.org/10.1039/C8RA07340F. Yu, Z., Leng, X., Zhao, S., Ji, J., Zhou, T., Khan, A., Kakde, A., Liu, P., & Li, X. (2018). A review on the applications of microbial electrolysis cells in anaerobic digestion. Bioresource Technology, 255, 340–348. https://doi.org/10.1016/j.biortech.2018.02.003. Zhen, G., Kobayashi, T., Lu, X., & Xu, K. (2015). Understanding methane bioelectrosynthesis from carbon dioxide in a two-chamber microbial electrolysis cells (MECs) containing a carbon biocathode. Bioresource Technology, 186, 141–148. https://doi.org/10.1016/j.biortech.2015.03.064. Zhen, G., Lu, X., Kumar, G., Bakonyi, P., Xu, K., & Zhao, Y. (2017). Microbial electrolysis cell platform for simultaneous waste biorefinery and clean electrofuels generation: Current situation, challenges and future perspectives. Progress in Energy and Combustion Science, 63, 119–145. https://doi.org/10.1016/ j.pecs.2017.07.003.

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CHAPTER 2

Electricity generation with the use of microbial electrochemical systems M. Castillo-Juárez a, Pedro Nava-Diguero b and Felipe Caballero-Briones a

a Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico b Universidad Tecnológica de Altamira, Altamira, Mexico

2.1 Introduction to microbial electrochemical systems Microbial electrochemical systems (MES) also known as bio electrochemical systems and microbial electrochemical technologies are based on the ability of some microorganisms to catalyze different electrochemical reactions, specifically, reactions that involve electron transfer, allowing to recover energy and some added-value byproducts, and can serve in wastewater treatment reducing organic matter, heavy metals, and other pollutants, depending on the architectures (H. Wang & Ren, 2013). MES use a group of microorganisms capable to transport electrons to, or from insoluble electron acceptors, or donors as part of their metabolism (Otero et al., 2020), these electrons are transfer out of cell membranes (extracellular electron transport) to the electrode either directly through membrane-bound protein structures such as pili, c-type cytochrome, and filaments, or by using mobile electron shuttles, that act as mediators for indirect electron transfer. These microorganisms, either called electrochemically active bacteria, anode respiring bacteria, exoelectrogen, or electricigen microbes, convert the chemical energy stored in organic or inorganic substrates to electrical energy during their anaerobic respiration (H. Wang & Ren, 2013; Jatoi et al., 2021, 2022). MES consists of two elements: anode and cathode where the oxidation and reduction reactions occur respectively, where at least one of these is microbially catalyzed, giving origin to bioanode and biocathode terms. In bioanodes, exoelectrogenic bacteria oxidize organic or inorganic matter anaerobically to discharge electrons which are transferred through the electron transport chain to the electrode directly or indirectly.In biocathodes, bacteria called electrotrophs receive the electrons from the cathode directly or via some redox mediators to reduce compounds (Sharma et al., 2014). Microbial physiology, electrode material, and surface properties, redox potentials, electrolyte chemistry, nature of substrate, and imposed operational conditions define the final performance in the MES (Sharma et al., 2014). Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00021-8

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Figure 2.1 Figure shows a representative of microbial electrochemical systems.

MES are classified into four large groups depicted in Fig. 2.1, also named MXC where X means different applications (Aguilar-González et al., 2016): microbial fuel cells (MFC) for generating electricity and wastewater treatment; microbial electrolysis cells (MEC) to produce mainly inorganic compounds, such as hydrogen; microbial electrosynthesis cells (MES) to synthesize organic chemical compounds, and microbial desalinization cells (MDCs) to desalinize water in combination with other functions. MFC are devices that use microorganisms to oxidize organic and inorganic matter to generate electric energy and consist of two electrodes, an anode and a cathode joined by an external wire to close an electric circuit, as presented in Fig. 2.2. The containers of electrodes are separated by a proton exchange membrane (PEM). Before the operation startup, the microorganisms are incubated in the anolyte to create a biofilm onto the anode surface. The electrons generated by the microbial substrate oxidation in the anodic chamber (with anaerobic conditions) are pumped to the anode either directly through membrane-bound protein structures such as pili, c-type cytochrome or and filaments, or by using mobile electron shuttles as described before, and brought to the cathode through the external circuit that connects the electrodes and feeds a charge. Also, protons are generated by the oxidation from substrate in the anodic chamber: they diffuse in anaerobic conditions, and transfer through PEM to cathodic chamber. Finally, electrons and protons combine to reduce oxygen molecules from air or another electron acceptor from a catholyte,producing water and an electric current that can feed a charge. Power outputs of MFC have been reported in terms of the cell volume or the electrode areas: for example, some authors report as much as 280 mWm−2 per electrode area (Zhang et al., 2014) and 102.93 mWm−3 per cell volume (Guo et al., 2014). Other

Electricity generation with the use of microbial electrochemical systems

Figure 2.2 Typical construction of a microbial fuel cell (MFC).

metric that has been used to determine the energy yield of MFCs is the normalized energy recovery (NER) to represent energy data, and it is expressed in two units: NERV in kWhm−3 , based on the volume of the treated wastewater in an MFC, or the produced power divided by wastewater flow rate, as shown in Eq. (2.1), and NERCOD in kWh.kg.COD−1 , based on the amount of organic substrates, measured as chemical oxygen demand (COD) removed in an MFC, or the produced power divided by removed COD and wastewater flow rate, as represented by (Eq. 2.2) (Xiao et al., 2014). In addition, the NER of the continuously operated MFCs based on kWh/kg COD, is represented by Eq. (2.3) (Shabani et al., 2020). NERV = NERCOD = NER =

Power × Time (t ) Anode chamber volume

Power × Time (t ) COD removed within time (t)

Power Wastewater flow rate × COD

(2.1)

(2.2)

(2.3)

NER values as high as 0.1 kWhm−3 and 0.3 kWhkg−1 COD have been reported (Bhowmick et al., 2019). It is important to mention, that despite the aforementioned values of energy output, there is a lot of dispersion in the reported data, in dependence of the substrate, the bacteria, cell architecture, electrode materials, between other, so there is plenty of room for MFC optimization.

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Figure 2.3 Typical microbial electrolysis cell.

MEC is a system to which electric energy is provided to achieve a given process or formation of chemical products, mainly inorganic. These systems use electrons that arrive at the cathode, supplied by an external source, to combine them with the protons from the metabolic activity of the microbes, to produce H2 as presented in Fig. 2.3. The system must be under anaerobic conditions to avoid that the electrons and protons combining with oxygen. This process is known as electrohydrogenesis or microbial electrolysis. The calculations to determine the necessary voltage are based on Gibbs free energy for redox reaction. The biodegradable organic matter to be used in these systems can be of variable composition, from simple molecules to complex mixtures. The reported MEC efficiencies or hydrogen yield are also wide, some values are 974 ± 116 mL H2 .gacetate −1 (González-Pabón et al., 2021), and 511.02 ml 2 g−1 VS (J. Huang et al., 2020). Microbial electrosynthesis cells convert CO2 into multicarbon products using bacteria and electrical energy using microbial electrosynthesis,as shown in Fig.2.4.The key merits of this technology are self-regeneration, self-adjustment of catalyst quantity according to the conversion rate requirement, and versatility in fuel generations and its pathways (Rabaey & Rozendal, 2010). Several laboratory-scale studies have demonstrated the use of acetogens that have the ability to convert various syngas components as CO, CO2 , and H2 to carbon compounds, such as acetate, butyrate, butanol, lactate, and ethanol (Venkata Mohan et al., 2014). MDC is a modification from MFC reactors (Luo et al., 2012) integrated with an electrodialysis to simultaneously treat wastewater,desalinate brackish or seawater,and produce electric energy (Ramírez-Moreno et al., 2021). A pair of ion exchange membranes

Electricity generation with the use of microbial electrochemical systems

Figure 2.4 Microbial electrosynthesis cell.

Figure 2.5 Microbial desalination cell.

known as desalinization chamber is located between anode and cathode chambers, as depicted in Fig. 2.5. MDC uses electroactive internal sludge. When the biofilm is formed on the anode surface, the bio catalysis process begins by oxidation reaction in watery sludge (Carmalin Sophi et al., 2016). This results in a potential difference between both electrodes and if there is an external load connected between the anode and cathode the flow of electric current appears. Thus, anions and cations migrate through the respective membranes, increasing the salt concentration in the anodic and cathodic compartments, while decreasing the salt concentration in the saline compartment (Ramírez-Moreno et al., 2021).

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Table 2.1 Three kinds of microbes in MES. Group 1 Group 2

Group 3

E. coli Bacillus sp.

Brevibacillus sp. PTH1 Pseudomonas sp. enabled Brevibacillus sp.

Shewanella putrefaciens Geobacter sulfurreducens

It is worth to mention the inherent advantages of the MES compared with other systems such as photovoltaics, hydrogen photocatalysts, electrolyzers and desalination technologies as membranes, for example. The microbial systems can be seen as product/energy harvesting that stripe added-value products such as energy, chemicals, or hydrogen from waste substrates. The investment and technology intensity of MES is reduced in comparison with other technologies and can be considered as more sustainable in terms of environmental impact and carbon footprint as will describe in subsequent sections.They can be reproduced and operated even in rural communities without special facilities or training for operation and maintenance.

2.2 Electrogenic organisms In the core of MES there is a unique group of microbes called electrochemically active bacteria, that have the capability to oxidize organic or inorganic electron donors, and transfer electrons to the anode electrode (H. Wang & Ren, 2013). To the better understanding and control of an MES, it is important to describe these organisms, specifically how do they transfer these electrons. Although only a few electroactive microorganisms have been studied in detail about their electron transfer mechanism, and even there are many species assigned as electroactive, there is not a common definition about what electroactivity is (Koch & Harnisch, 2016). There are three kinds of microbes in MES. The first group are those organisms that cannot directly donate an electron to an anode and need mediators to get it. The second group known as electroactive bacteria or “electricigens” or “exoelectrogens” or “electrogens” or “anodophiles” or “anode-respiring bacteria,” are able to form a conductive biofilm on the anode. And the last group are those which are cocultured with other microorganisms to perform the electron release and transfer activities (Roy & Pandit, 2018). Table 2.1 shows some examples of microorganisms of each kind. About the electron transfer from the bacteria to the anode, the most now recognized transfer modes are the indirect, which requires a catalyst supplemented with external redox mediators such as riboflavin or humic acid (Sevda et al., 2018); and direct, where bacteria form a conductive biofilm and transfer electrons directly to the anode by endogenous mediator secretion or by cellular appendages such as pili,nanowires or c-type cytochromes. Fig. 2.6 schematizes the electron transfer modes.

Electricity generation with the use of microbial electrochemical systems

Figure 2.6 Electron transfer mode (adapted from (Roy & Soumya, 2019)). Modified from Roy, S. & Pandit, S. (2018). Microbial electrochemical system: Principles and application. In Biomass, Biofuels, Biochemicals: Microbial Electrochemical Technology: Sustainable Platform for Fuels, Chemicals and Remediation (pp. 19–48). Elsevier. https://doi.org/10.1016/B978-0-444-64052-9.00002-9.

To increase electricity generation required for practical applications of MES is necessary to understand the electrochemically active bacteria and microbial community performance in these systems. To improve this, the analysis and comprehension of techniques for studying the characteristics of exoelectrogenic microbes and microbial communities such as the morphology, genetic potential, and metabolic capacity, are important to understand electricity generation in MES. The first way to characterize bacteria is its isolation from the substrate to know its physiological, genetic, and functional properties without interferences. There are strategies to grow up pure cultures as examples: dilution (the streak plate, the spread plate, and the pour plate) and special U-tube. Fig. 2.7 shows a scheme for bacteria isolation. After isolation,to identify and compare bacterial pure cultures and their strains,RNAgene sequencing, operational taxonomic units, and polymerase chain reaction test (PCR) are the regular used tools. For example, 11 different bacterial phyla such as Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, Firmicutes, Synergistetes, Spirochaetes, TM7, SR1, Tenericutes, and Proteobacteria were identified. using 16S rDNA libraries and massive DNA semiconductor sequencing (Kamaraj et al., 2019). To study the cellular structure, transmission electron microscopy, atomic force microscope, scanning electron microscope and confocal/fluorescence microscopy can be used. Microbial morphology of the surface carbon felt electrodes have been observed

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Figure 2.7 Diagram for bacteria isolation.

Electricity generation with the use of microbial electrochemical systems

by scanning electron microscopy with an energy dispersive X-ray detector (Song et al., 2020). Finally, to characterize the bioelectrochemical activity, electrochemical methods such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) are commonly used (Zhi et al., 2014). A scanning EIS technique was proposed to confirm the variation of internal resistance, charge transfer resistance, and ohmic resistance of microbial fuel cells (MFCs) (Kim et al., 2021). The CV analysis was used for knowing electron transfer pathways in bioanodes in MFCs (You et al., 2021). It is important to notice that in MFCs there are no isolated organisms, rather they form complex relationships in the environment. These interactions can have advantages and disadvantages for them. These interactions are mutualism, predator-prey, amensalism, competition, neutralism and, commensalism (nitrification and methanogenesis). The study of these interactions helps to know the community behavior (Zuñiga et al., 2017). Fig. 2.8 presents microbial activity using different computational strategies for their study. About techniques for studying microbial community, the population dynamic is an indicator of electrochemical activity and the methods used are most probable number (MPN), MPN- polymerase chain reaction (PCR) and, quantitative PCR (Zhi et al., 2014). To increase the MFC output from the bacteria point of view, MFCs with mixed-culture biofilm communities have a remarkable power performance, also nonelectrogenic microbes in mixed cultures are important for the microbial ecology of MFCs because they provide a local anaerobic environment for anaerobic electrogens to achieve higher power production. Microbial interactions studies will let to improve the understanding of the performance of an MFC (M. Li et al., 2018).

2.3 Typical applications for microbial electrogenesis Microbial electrogenesis (ME) is a source of green energy in which certain types of bacteria using electrochemically active proteins on their outer membranes, can directly transfer these electrons to electrically conductive materials and generating an electrical current. The typical applications for microbial electrogenesis are shown in Fig. 2.9 and will be discussed below. 2.3.1 Wastewater treatment and energy generation Activated sludge process is the most used system to treat wastewater in the world but requires a high capital investment for operation and maintenance. Sewage sludge is an organic waste generated in wastewater treatment plants (WWTP). The treatment of sludge constitutes 20–60% of the total operating expenditures of WWTP. Electrogenesis in microbial fuel cells is a promising approach to extract useful energy from sewage sludge.

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Figure 2.8 Different experimental approaches and computational strategies are applied for the study of microbial communities. From Zuñiga, C., Zaramela, L. & Zengler, K. (2017). Elucidation of complexity and prediction of interactions in microbial communities. Microbial Biotechnology, 10(6), 1500–1522. https://doi.org/10.1111/1751-7915.12855.

Electricity generation with the use of microbial electrochemical systems

Figure 2.9 Applications for ME.

However, its impact on ultimate sludge disposal has not been well evaluated. The forms of energy in wastewater are organic matter, nutrients (N, P), and thermal energy. Getting and taking advantage from this kind of energies could be a sustainable solution (Gude, 2016). A MFC can be utilized into a conventional sludge treatment wetland to enhance its performance. Wang et al. have shown that this combination of systems presents a satisfactory efficiency for sludge dewatering and mineralization despite a high sludge loading rate (SLR) (7.07 % of dewatering rate, 40.43 % of volatile solid (VS) removal and 80.95 % of total COD removal) and recycled the bioenergy simultaneously. The maximum voltages (0.7 V) are presented by the increasing of SLR.This result about heavy metal and fecal bacteria agrees with Uggetti and Caicedo and the final residual sludge from the wetlands may be used for agricultural reuses as fertilizer or soil amendment. And it is a promising technology for simultaneous sludge treatment and energy recovery (Wang et al., 2021) even at extremely low COD concentrations (Hiegemann et al., 2016). In pharmaceutical industrial wastewater, the reports demonstrated that MFC significantly reduced COD and total dissolved solids from the pharmaceutical wastewater as much as 80% and 35%, respectively. The effluent also contains antiseptics that would reduce the microbial activity; therefore, further work is needed on degrade these antiseptics in a parallel process to enhance the efficiency of MFC (Rashid et al., 2021). Table 2.2 presents some reported MFC efficiencies in terms of treatment of wastewater of different origin, evaluated through the reduction of COD, and of electricity generation,

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Table 2.2 Performance in wastewater treatment and electrical generation of some reported MFC. Wastewater source COD removal Power generation References

Domestic Brewery Industrial acid mine drainage and municipal wastewater Domestic Swine farm Urine wastewater Food Dairy

79 ± 7% 88% 15%

116 mW 0.0965 kWh/m3 14 mW/m3

(Feng et al., 2014) (Dong et al., 2015) (Vélez-Pérez et al., 2020)

26% 83.80% 52.70% 67.92% 92.20%

12.8 W/m3 175.7 W/m2 63 mW m−3 0.236 W/m2 3.2 ± 0.3 Wm

(Koffi & Okabe, 2020) (Zhuang et al., 2012) (Yang et al., 2020) (Wang et al., 2020) (Sanjay & Udayashankara, 2021)

reported in different ways, such as total power, volume, and area power. The Table depicts the dispersion of achieved efficiencies in COD removal and power generation, and also points out to the need of an unified metric for reporting MFC performance. 2.3.2 Hydrogen generation Hydrogen gas has a high potential as an environmentally acceptable energy for transport. MEC could be produce green hydrogen at greater efficiencies than fermentation and in theory at around one-tenth of the electrical energy input of water electrolysis, because of the cathodic reaction of the protons comes from the oxidation of the organic matter contained in wastewater, which could additionally reduce the costs for wastewater treatment (Cotterill et al., 2016). Reports show that, with an applied voltage of 0.9 V, current generation and H2 production reached maxima of 7.4 A m−3 and 0.19 ± 0.04 L/L/day, respectively (Kadier et al., 2016). The current challenges for practical MEC implementation are the hydrogen production rate and the lowering of energy input. To prevent methane evolution and allow a stable hydrogen production,the use of hydrophilic porous materials for PEMs has been reported (Zhao et al., 2021). Other options are to do an alkaline thermal pretreatment to the sludge to promote his bioaccessibility and biodegradability (Zhao et al., 2021). Table 2.3 shows a compendium of different reported Table 2.3 Efficiency of some reported MECs. Type H2 generation Power generation Input voltage References

Two-chamber MEC Cubic single-chamber MEC Single chamber MEC

0.14 m3 m−3 d−1 2.46 mA 3.72±0.13 mol- 193.95 ± 15.42 H2 /mol-acetate A/m3 0.71 L/L/D 5.6–13.7 A/m2 .

1V 0.8 V

(Jain & He, 2020) (Cui et al., 2021)

1.36 V

Single-chamber MEC

2.03 ± 0.01 2.03 ± 0.04 mol-H2 /molA/m2 glucose

0.7 V

(L. Wang et al., 2021) (Z. Li et al., 2021)

Electricity generation with the use of microbial electrochemical systems

Figure 2.10 Schematics of a biosensor.

MECs and its respective efficiencies in terms of architectures, H2 generation rates, and power generation, including the voltages that are used to produce it. Voltages as low as 0.7 V and H2 as high as 3.7 mol H2 per mol of fuel have been reported, indicating the great room for innovation and optimization of this technology. 2.3.3 Biosensors Biosensors are analytical devices that convert a biological response into a quantifiable and processable signal. Fig. 2.10 shows the elements of a biosensor and its function. Biosensors must meet a series of key requirements: the bioreceptor has to be suitable for the specific application and has to show low variation between experiments; its response must be accurate, precise, reproducible, and linear in the analytical range of interest; its reaction has to be independent from other physical parameters. Besides, for mass application, it is desirable to be cheap, small, portable, and simple (Di Lorenzo, 2016). The biosensors based on microbial electrogenesis are promising for-monitoring water samples containing various organic matters and environmental contaminants (Chung & Dhar, 2021). For example, the biological oxygen demand (BOD), is an index of organic content and gives an indication of how much oxygen would be required for microbial degradation. In an MFC, the BOD is directly proportional to the concentration of the oxidized substrate; thus MFCs can be used as a BOD sensor (Roy & Pandit, 2018). On the other hand, self-powered and reusable microbial fuel cell biosensor for toxicity detection in heavy metal polluted water have been reported. With such a biosensor was possible to determine the toxicity order of several cations: Ni2+ < Zn2+ < Cu2+ < Cr6+ (Naik & Jujjavarapu, 2021). Different toxins could also be detected using MFCbased biosensors (Naik & Jujjavarapu, 2021). A double chamber MFC was reported for on-line monitoring ammonium (NH4+ -N) in municipal wastewater; the reported results were closely compared with traditional methods (Do et al., 2021). Limitations for MFC biosensors include substrate concentration, high internal resistance of the system, diffusion of oxygen into the anode chamber, permeability of the

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PEM, oxygen supply and consumption in the cathode chamber as well as the interference of environmental factors (Velasquez-Orta et al., 2020).

2.4 Principles of microbial electrochemical systems: fuel cells (MFCs) and electrolysis cells (MECs) The equations from 4 to 8 describe the anode and cathode reactions that occur in an MES, making the following considerations: (1) NADH acts as an electron provider and oxygen acts as a terminal electron acceptor: (2) the reaction on the surface anode is thermodynamically propitious; (3) the Nernst equation, where the theoretical voltage generation is 1.1 V (Roy & Pandit, 2018). Anode reactions NAD+ + H+ + 2 e− → NADH 0 = E0 − EAnode

[NADH] RT ln = −0.32 V nF [NAD+ ][H+ ]2

(2.4) (2.5)

Cathode reactions O2 + 4 H+ + 4 e− → 2 H2 O 0 = E0 − ECathode

[NADH] RT ln = +0.84 V nF [O2 ]2 [H+ ]2

0 0 0 ETotal = ECathode − EAnode = +0.84 V − (−0.32 V) = 1.16 V

(2.6) (2.7) (2.8)

where: E0 standard cell potential 0 0 , ECathode standard half-cell anode (oxidation) and cathode (reduction) potenEAnode tials J R = 8.314472 Kmol (Universal gas constant) T temperature in Kelvin C F = 96, 485.3383 mol (Faraday constant) 2.4.1 Microbial fuel cell The metabolic activity of several microorganism has been exploited on the electrode surfaces in the microbial fuel cells. Lower power density is the main limitation of MFC. However, the merits are potential technology in economical wastewater treatment and biological path for the recovery of energy along with wastewater treatment in an environmentally sustainable manner. In a microbial fuel cell, both anode and cathode are kept in the separate chambers in aqueous solutions which are separated by membranes. Electrons and protons are

Electricity generation with the use of microbial electrochemical systems

Figure 2.11 Selection of electrochemically active biocatalyst by electrochemical (Bioelectrolysis) and chemical (ferric citrate) as final terminal electron acceptors. Modified from Allyn & Bacon. (2007). Iopsychology and the Foundations of Neuroscience. https://slideplayer.com/slide/6248769/.

generated by a microorganism that oxidizes the fuel (electron donor) in the anode chamber. The electrons are transported to the cathode chamber through an external circuit, where the reduction reaction takes place. As a result of oxidation at the anodic chamber, CO2 is produced. Water and electricity are generated in the cathode chamber after reducing oxygen to water, consuming protons, and electrons (Scott, 2016). A microorganism forms an electroactive biofilm in the anode. Chemical stability, corrosion resistance, mechanical strength, and toughness are important factors to be considered for choosing the anodic current collector. Performance of fuel cells is improved by having anode material with a large surface area. Graphite rod, activated carbon, carbon cloth, reticulated vitreous carbon (VRC) are majorly used as anode materials. There is a separation membrane, either call PEM between the anode and cathode chambers, and protons are transported through the membrane. Oxygen (or another electron acceptor) reduction reaction takes place in the cathode chamber. Its performance determines the current density (Das, 2018). The factors affecting the performance of MFC are the rate of microbial oxidation and electron transfer, proton transfer through the membrane and electron acceptor supply, and reduction process mechanism at the cathode (Gil et al., 2003). To improve the anode efficiency, Junxian Hou et al. coated a carbon cloth with electrochemical reduced graphene oxide and polyaniline. With this methodology, the authors improved the power density, efficient charge transfer, and bacterial biofilm loading. Further, Sathish-Kumar et al. proposed a chemical and electrochemical selection of electrochemically active microorganisms. The hybrid inoculum produced an increased power density and the electrochemical stressed harvested inoculum accelerates the oxidation of organic removal efficiency (Fig. 2.11).

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Figure 2.12 Pencil MFC prototypes (100 mL and 1 L) connected in series. From SathishKumar, K., Dr. (2017). Prototipos desarrollados en nuestro grupo. https://sites.google.com/site/ drkamarajsathishkumar/home/m-prototipos-desarrollados-en-nuestro-grupo.

In another report, Xiao et al. used graphene as electrode material with an improved electricity generation with respect to activated carbon. Large surface area and open structure of graphene were the reasons for the improved performance. Both reduced graphene oxide sheets and particle showed increased power density. The microbial fuel cell operates under various conditions such as differences in temperature, pH, electron acceptor, electrode surface areas, reactor size, and operating time. Many types of new bacteria are constantly being discovered to transfer an electron to the anode (anodophillic) or capability to transfer the electrons between bacteria (Logan & Regan, 2006a) On the other hand, novel architectures are being constantly tested, for example, a novel design called pencil MFC, shown in Fig. 2.12 was registered in the Mexican patent office by S.K. Kamaraj et al., with the potential to scale up the process of MFC in the wastewater sector. 2.4.2 Microbial electrolysis cell Hydrogen could play a major role in the global energy scenario. It has a high specific energy on mass basis around 9.5 kg of H, equivalent to 25 kg of gasoline. Hydrogen has a broad demand as raw materials in petrochemical, food, microelectronics, ferrous and non-ferrous, metal, chemical and polymer synthesis and as an energy carrier in clean sustainable energy (Midilli et al., 2005). Hydrogen gas produced using conventional

Electricity generation with the use of microbial electrochemical systems

Figure 2.13 Microbial electrolysis cell.

methods from fossil fuels, i.e. gray hydrogen, leads to the release of uncontrolled carbon dioxide which further adds up global warming emissions.Blue hydrogen accounts for that produced from methane steam reforming and CO2 storage. Renewable water electrolysis, i.e. green hydrogen, may be an available option but it suffers from demerits such as higher energy requirement (5.6 kWh m−3 H2 ) and low efficiency (56–73%) (Logan et al., 2008a). However, with the help of microorganisms in a bioelectrochemical cell, green hydrogen could be also generated at an affordable cost. In a bioelectrolysis system, oxidation and reduction are catalyzed by microorganisms. Anode and cathode are electrically connected by the external circuit which paves the way to harvest or supply electrical energy to the bioelectrochemical cell according to the intended process, as depicted in Fig. 2.13. Like the microbial fuel cell, exoelectrogens are electrochemical active microorganisms that are capable to transfer the electrons from in and out of the cell to the catalyze chemical reaction in the microbial electrolysis cell. The electrons are transported from anode to cathode through an external circuit in a microbial fuel cell. Protons are transferred to the anaerobic cathode through membranes under acidic conditions. The protons (H+ ) are used for the generation of hydrogen and other useful products such as hydrogen peroxide. Still, this reaction cannot occur spontaneously, so extra electrical energy is supplied to fulfill the required energy to drive the reactions. Cathodic reactions under acidic conditions 2H+ +2e− → H2

(2.9)

2H2 O + 2e− → H2 +2OH−

(2.10)

Under alkaline conditions

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In microbial electrolysis cell, both anode and cathode are anaerobic to promote the growth of microorganisms such as Geobacter spp and other exogenic or methogenic organisms (Logan & Regan, 2006a).

2.5 MFC performance: operation parameters The monitoring parameters with which conventional electrochemical systems are generally described have evolved to adjust to the performance of the MES, such as the power density (PD), which was initially defined as the ratio of power per area. surface of the electrode, and subsequently, as the power in relation to the volume of the chambers, either anodic or cathodic (Rabaey et al., 2005); or in other reports, the coulombic efficiency (CE), which describes the conversion efficiency from electron donors (organic) to electric current (Ng et al., 2009) where the amount of electron donors is based on the organic compounds removed (Z. Wang & He, 2020). Wang & He proposed an improved evaluation parameter based on the CE, which was called coulombic recovery (CR), shown in Eq. 2.11. t0 Electrical output load 0 It(A∗s = Coulombs)   CR = =   mol e− Electrical input load (mol )∗4 ∗ COD 96485 Coulombs O initial 2 − mol e mol O2 (2.11) The CE parameter is often used to demonstrate how good a system is in its performance, although its definition is based on the consumption of COD, which can lead to misunderstandings, especially when COD removal reaches low values. It has been observed that the performance of a MFC obviously declines in terms of the efficiency of COD removal as time passes, and therefore, in the generation of current. However, in parallel it has also been observed that the CE tends to increase, probably due more to the effect of the decrease in the efficiency of COD removal rather than due to the generation of current. For this reason, it has been suggested to consider the total input (or initial) COD in the definition of CR, instead of the COD consumption efficiency used in the definition of the CE. In this way, the CR is not affected by the efficiency of the organic removal and visualizes the initial organic input as the total of electrons available for conversion. Of course, with this change, the CE will always be greater than CR in value, but the latter is more stable against variations in COD consumption between experiments. The power density (PD) is the parameter regularly used in the report of the experiments, generally as a function of the processing volume in the anode chamber of the reactors. From the analysis of its mathematical definition, it is noticeable that this parameter tends to decrease as the size of the reactor increases even though the cells provide the same level of energy; which implies that the larger the volume of the system, the lower the electricity generation, even when it contains similar operational

Electricity generation with the use of microbial electrochemical systems

characteristics in terms of substrate, bacterial community, pH, and other conditions (Ge et al., 2013; Z. Wang & He, 2020). Therefore, the convenience of using two additional energy parameters, which are not dependent on the size of the reactors, or the type of substrate, or the type of separator between the anodic and cathode chambers, used in continuous flow experiments, has recently been analyzed (Ge et al., 2013). These parameters are generically called NER, from where the volume-based NER (NERv ) and the NER based on the amount of COD consumed by microorganisms (NERCOD ) are described by Eqs. (2.1)–(2.3). In the reports where they have been used, NER values fluctuate within a narrow range of 10−1 to 10−2 kWhm−3 or kWhkg−1 COD, even when the systems increase the anode volume several orders of magnitude, which means that no obvious correlation between volume and NER’s is appreciable (Zheng et al., 2013). So the NER can show the energy performance of a system regardless of the size of the reactor, so they can serve as a benchmark either the operating characteristics of different MFC could be different (Z. Wang & He, 2020). For example, the substrate rich in organic compounds contained in the anode chamber, is recognized as a factor that affects the potential of the anode, the development of the bacterial consortium and the quality of the treated effluent, in addition to energy recovery (Rabaey et al., 2011). The NER analysis from different substrates has shown that both parameters, the volumetric one and the one based on COD consumption, have a similar range of values in single molecule substrates, appreciating significant differences when the complexity increases, but in all cases, within the range from 10−1 to 10−2 kWhm−3 or kWhkg−1 COD (Ge et al., 2013). Like any galvanic cell in which reactions take place at the anode and the cathode, an appropriate separation between the two is required, to prevent interferences due to the diffusion of oxygen, organic compounds, and other compounds. These separations can be ionic type membranes (anodic,AEM;cathodic,CEM;or protonic,PEM),nonionic (inert) or even operated without a separator (by oxygen gradient). The NER analysis compared to the installation or not of separators indicates that the MFC’s without separator and inert separator provide a high level of NER with a similar range of values in both parameters than those that use ionic separators, which showed variations between them, independently of the substrate (Ge et al., 2013). As it contains information on energy in relation to the flow and characteristics of wastewater (COD), this analysis suggested the convenience of using the NERv and NERCOD parameters as comparative performance indicators, although the indicators commonly used today in the literature, such as power density (PD), coulombic efficiency (CE) and energy efficiency (EE), have their own importance, given the complexity of the operation and performance of MFC’s Moreover, it is advisable to add other indicators in addition to these, such as the hydraulic retention time (HRT), the organic load rate, the removal efficiency, and even economic factors, among other (Z. Wang & He, 2020).

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In addition to the comparative purpose of the NERs between different experiments, NER analysis allows to carry out an energy balance and determine how far a process is from the energy neutral or energy positive treatment, that is, if the process will provide enough energy to cover the demand of the equipment involved (such as feed and recirculation pumps). With the NER analysis has been estimated that the energy content of organic compounds is approximately 3.86 kWhkg−1 COD and that MFCs can recover less than 2 kWhkg−1 COD at most, although most of the reported experiments recover less than 1 kWhkg−1 COD. In general, it is agreed that, for MFCs, the delivery of volumetric NERv lie below 0.1 kWhm−3 or for NERCOD below 0.3 kWhkg−1 COD removed (Ge et al., 2013; Z. Wang & He, 2020).

2.6 MFC optimization The foregoing discussion indicates MFCs technology shall be improved through the optimization of architectures, operation parameters, microbiology and construction materials,in addition to seeking secondary energy support alternatives,equally sustainable such as anaerobic or algae digestion bioreactors, or even photovoltaic panels or wind towers, to name a few. From the mathematical definition of the monitoring parameters such as CE, CR or NERV, and NERCOD , it is appreciated that any factor that encourages energy production is critical. The reported literature has identified among these factors the configuration of the cell, the conditions of the substrate, the type of bacterial community, and the type of operation (Choudhury et al., 2017). These considerations can apply to batch or continuous flow systems. In the case of the cell configuration, the relationship between the size and the spacing of the electrodes, as well as the anodic volume of treatment must be proportionally conserved so that the power is maintained (geometric similarity). In addition, the materials used in the anode, cathode, and separation membrane (if used) should have been pre-tested and high performance assured. With reference to the anodic volume, it must be kept at a low unit capacity in such a way that the increase in treatment capacity is due to the increase in processing units (modularization) and not due to the size of the reactor (Logan et al., 2008a; Li et al., 2008). Regarding the substrate characteristics such as COD content, conductivity, pH, and temperature, they must be considered in the design of the MFC (Liang et al., 2018; Sengodon & Hays, 2012). In the decision related to the bacterial community, it must be considered whether it will be inoculated or whether the consortium associated with the stream to be treated will be maintained. This decision is related to the characterization results (Choudhury et al., 2017; Sengodon & Hays, 2012). Operationally, the type of cell to be used should be considered, if it will be used with a single chamber, double chamber, upflow or stacked cell, and if an ionic separator

Electricity generation with the use of microbial electrochemical systems

Table 2.4 Some examples of hydraulic capacity of MFC’s related to type of WW to treat. Pilot Scale Wastewater Hydraulic % of flow effective Scale HRT (h) capacity (m3 d−1 ) type rate1 volume1

Domestic2 Residential2 Dairy3

∼300,000 residents ∼8000 residents ∼100 residents Produce ∼140 m3 of milk per day

6 6 12 12

228,000 6080 40 1400

0.1 0.1 1 5

∼57000 ∼1520 ∼200 ∼35000

Adapted from Wang and He (2020) with own data. Modified from Wang, Z. & He, Z. (2020). Frontier review on metal removal in bioelectrochemical systems: mechanisms, performance, and perspectives. Journal of Hazardous Materials Letters, 1, 100002. https://doi.org/10.1016/j.hazl.2020.100002

will be used, inert or without separator (Choudhury et al., 2017; Logan et al., 2008a). If there will be independent flow currents for the anode and cathode chambers, or if the chambers will be connected in series for the same stream, i.e. the effluent from the anode chamber is passed through an aeration station and recirculated to the cathode chamber, before discharging it. Also, if the cell will operate with a uniform hydraulic residence time (HRT) for the entire process in both chambers, or if they will be different for each of the chambers, taking into consideration that, in the latter circumstance, a long HRT at the cathode leads to an insufficient supply of dissolved oxygen,increases the internal resistance and decreases the power density, so this would have to be determined in advance for the system under test. In addition, maintenance on the electrodes to reduce biofilms is key to keep power output (Liang et al., 2018; Logan et al., 2008b; Sengodon & Hays, 2012). 2.6.1 Scaling criteria The efforts to find the best operation conditions, materials, and configurations for maximum power output, aimed to scaling up, raise the question: What criteria define that an experiment should not be considered laboratory, but rather pilot plant level? It should be borne in mind that the purpose of a pilot test is to provide technical specifications to design a full-scale system, and therefore, a pilot system must have the characteristic of scaling linearly to full size, without significant variations in the key configuration and parameters of operation. For wastewater treatment systems, the range of 0.1 to 5% of the hydraulic capacity of the full-size facility is usually considered (Metcalf & Eddy, 2014). The trend is that the larger the full-size system, the lower the percentage of hydraulic capacity the pilot system should have. In addition, the type of WWs, the operational environment,and the stability of performance (via long-term operation) must be taken into account (Wang & He, 2020). From Table 2.4, the hydraulic capacity of the real treatment plants in m3 d−1 is presented. The values are obtained by multiplying the hydraulic retention time expressed in days with the percentage of mass flow to be considered and by 1000 to convert it to volume, in liters. For example, a domestic WWTP treatment plant with moderate organic

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load (WWTP) with input mass flow of 228,000 m3 d−1 would require the pilot plant to have a processing capacity of 57 m3 (0.1% of installed capacity) or some smaller WWTP with a capacity of 6080 m3 d−1 , the required pilot effective processing volume would be 1520 L. In addition to the global dimensional similarity, it is required to preserve the geometric similarities of the components and performance of the studied system considering the type of WW, the operational environment, and its stability in long-term operation. HRT and MFC meaning hydraulic retention time and:microbial fuel cell,respectively Data were taken from: 1. Metcalf and Eddy (2014). 2. Data based in 100 l a person per day from Howard and Bartram (2003). 3. Lara et al. (2009). Wang & He reported the existence of so-called “pilot MES studies” but only 17.2% could be of the size to handle an expected pilot scale hydraulic capacity based on the characteristics of the WW they treated, and only studies with 2 m3 of scaling were compliant with the capacity required for a small WWTP. They also reported that MES studies for the treatment of high resistance WW from industry or agriculture or for residential WW, only 37.5% could meet the criterion of 1–5% of hydraulic capacity; regarding the operational environment criterion: 20.7% used synthetic water, 10.3% had a short operational time (96%

46.11 mW∗ h

313 h (MF1) 79.9 ± 4.3% 0.44 ± 0.03 W∗ m−3

(S. Huang et al., 2021) IPE123 0.13 ± 0.00 kWh (Lu et al., 2017) m-3 0.037 ± 0.008 kWh kg-COD-1

(continued on next page)

Electricity generation with the use of microbial electrochemical systems

Single chamber

Proton exchange membrane Nafion 117

Sustrate Produced Water containing pollutants Synthetic wastewater Produced Water containing pollutants

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Shape Tubular upflow

Upflow

Sustrate seafood processing industry wastewater chocolaterie wastewater Municipal wastewater

96 tubular MFC modules, which were placed as 12 × 8 arrays with 12 MFCs in each row and 8 rows in total 50 stacked MFC Municipal modules with a wastewater total volume of 1000 L

Anode Activated carbon fibre felt

Cathode Activated carbon fibre felt

Proton exchange membrane fluorinated membrane – Nafion

HRT 4–30 h

COD remotion 72–83%

Power Density NER 2.21 Wm−3

Reference (Jayashree et al., 2016)

Carbon fiber veil

Carbon fiber veil

Nafion 117

15 h

70%

98.8 mW/m2

(Subha et al., 2019)

carbon brush

carbon cloth cation exchange 18 h coated with membrane, nitrogen doped Ultrex CMI7000 activated carbon powder

76.80%

nm

(Ge & He, 2016)

Activated carbon

Activated carbon

70–80%

7– 60W∗ m−3

cation exchange membranes (CEM, Tianwei Company)

2h

0.033 ± 0.005 kWh per m3

(Liang et al., 2018)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 2.5 MFC with different configurations and performances—cont’d

Table 2.6 Detailed design of some MFC from the literature.

Shape

Sustrate

Anode

Cathode

Proton exchange membrane

HRT

COD remotion

2.01W∗ m−3

Reference

(Rashid et al., 2021)

7.0–8.6 W∗ m3 (Hang et al., 2018)

Electricity generation with the use of microbial electrochemical systems

Dual truncated Municipal solid The outer The inner Membrane- 4 80.55% membrane-less wastewater to paraboloid paraboloid cup less days MFC composed form the cup made made with of two biofilm in the with graphite. A paraboloidanode, after that graphite phosphate shaped cups, one Pharmaceutical buffer solution inside the other. Industries with neutral wastewater was pH was used as used in anode catholyte. Three chamber (a Dewatered sludge graphite graphite fiber Two of 42 42% cylindrical fiber brush brush Nafion 117 days anodic chamber and two cubic cathodic chambers)

Power Density

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Table 2.7 Pilot scale MFC plants and their performance parameters. Removed Strategy Volume COD Power Application

Reference

Modularized MFC 200 L 75% system consisting of 96 tubular MFC modules for treating municipal wastewater A bespoke stack of 19.2 L 48% 12 MFC modules

200 mW

to drive a 60 W DC (Ge & He, 2016) pump for catholyte recirculation

150 mW continuous

6 modules

95–120 mW

LED lighting system (Xavier Alexis (not directly, were Walter et al., used a booster 2018) circuit and battery) charging of (Xavier Alexis mobile smartphone Walter et al., (42 h) 2017)

10.5 L nm

In wastewater treatment the objectives remain to maximize power and reduce COD (Premier et al., 2016). Considering the challenges in scaling up MFCs, applying an MFC system for small scale wastewater treatment may be more feasible. In general, there are two strategies for scaling up MFCs, increase the size of reactor and stacking multiple reactors into one system (modularization). Optimizing the coordination between MFC modules for the best performance will be a key challenge for modularized systems (Ge & He, 2016). To treat urine is known that to achieve the scaling up can be done by enlarging the size of electrodes (Jadhav et al., 2021) even these can has negative effects when the electrode height is more than 5-6 cm (Walter et al., 2020) For commercial applications, the costs of the main elements of the cell, particularly the cathode catalyst are still high (Khalili et al., 2017), therefore an intensive research effort for efficient and cheap catalysts is on the run. Control and instrumentation of the scaled-up plants as well as the combined use of green energies such as photovoltaics to run the pumps and control instruments is devised. The modeling of scaled-up plants is still on early stages; as discussed above, not only the single-cell features but the relation with influent flux and recharge, the hydraulic retention time, the air circulation, the environmental temperature that affects the bioactivity of microorganisms, the pH, the meteorological parameters such as relative humidity, air speed, barometric pressure, that influence the CO2 release, the cathode wetting, and the biological activity, are parameters that must be studied in a pilot scale to provide an operation model of the plant to be scaled.Also,the catalyst efficiencies,the reusability and maintenance of the plant including “resetting” the anodes once a resistive biofilm is reached, the variability of the substrate, the homogenization to an optimal particle size in the influent, are points to optimize in large scale plants. Nevertheless, some attempts to build pilot-scale plants have been tested in several groups, and some of them are shown in Table 2.7.

Electricity generation with the use of microbial electrochemical systems

2.7 Challenges to improve MFC performance at real-life scale Successful scaling up of MFC depends on its construction economically. The higher operating cost and low power output hinder commercial scaling up of the MFC. The usage of expensive electrode materials, current collector, catalyst, and separator materials cause higher cost of the MFC. The capital cost required for the MFC to treat waste water is 30 times more than traditional domestic water treatment (Do et al., 2018). The structure should possess high surface area to support biofilm and withstand the weight of the water and biofilm. The application of cheap electrode materials such as carbon paper and graphite rod are impended due to their inherent lack of durability, structural strength, and cost. The long-term stability of electrode is another issue in wastewater treatment technology. The electrode materials used MFC should sustain higher catalytic properties in the wake of problems such as various degradations, corrosion, fouling of active surfaces, and loss enzyme activities (Logan & Regan, 2006b; Kim et al., 2007; Osman et al., 2010). The electrode materials impact the performance of MFC through adhesion of microbes, electron mobility and electrochemical efficiency. In wastewater treatment technology, the performance of membrane is declined by the biofouling, which hinders the proton exchange. Position and configuration of electrodes in the reactor also crucial for enhanced performance of the MFC. MFC cannot function at the low temperatures as the microbial reactions are slow at these temperatures. The wires which connect the electrodes in MFC effects its performance directly in electron transfer and energy generation (Abbassi & Yadav, 2020). PEM in MFC transfers the protons to the cathode chamber and maintain charge neutrality. Nafion is customarily used as PEM in MFC. However, Nafion consumes the 40% of total cost of an entire MFC system. Newly investigated PEM should have higher porosity, proton conductivity, brittleness and low cost in order realize the future scaling up process of MFC. Yet another issue with the PEM is that limitations in proton (H+ ) transfer, which hampers the overall process efficiency of the MFC. The usage of supplemented substances (For example Mg2+ , NH4 + , Na+ , and K+ ) at anode compartment and employment of aqueous electrolyte at anode/cathode compartment reduce H+ transfer (Chandrasekhar, Kadier, Kumar, Nastro, & Jeevit, 2017). Fouling at the membrane limits the large-scale application of MFC for wastewater treatment.Biofouling breaks the proton migration and completion for substrate utilization (Do et al., 2018). The higher internal resistance poses major challenge in the MFC scaling up for different applications, which directly affects the power generation. Anodic resistance, cathodic resistance and ohmic resistance majorly contribute to the internal resistance of the MFC. Ohmic resistance happens between electrolyte and PEM during the charge transfer. Diffusion resistance occurs at the interface between electrodes and surroundings of the electrolyte. The formation of electrical double layer at electrode-electrolyte interface store electrical charge, which act as an electrical capacitor (Liang et al., 2007).

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Another limiting factor for large scale implementation of MFC is overpotentials, which inhibits direct flow of electrons from bacteria. Overpotential reduce the overall energy efficiency of MFC and classified as activation overpotential, concentration polarization and ohmic losses. Electrodes in the MFC require certain activation energy to carry redox reactions cathode and anode, which termed as activation overpotential. Augmenting roughness and specific surface area of electrodes, addition of catalyst to the electrode and increasing operation temperature are the ways to reduce the activation overpotential. Concentration polarization arises due to faster oxidation at the anode and subsequent transfer to the surface. Large oxidative force of anode causes this problem and affects the diffusion. Ohmic losses are related to the ohmic resistance which are discussed above (ElMekawy et al., 2013). The power density from wastewater treatment lower than 0.5 W m−2 at the optimum conditions.The higher internal resistance,poor biofilm kinetics and substrate degradation are responsible for low power density. Other factors which could affect power density during the scaling up of the MFC are external mass transfer resistance due to incomplete mixing and inhomogeneous biofilm structure on the electrode. Staking the MFC in parallel would result in improved power density and columbic efficiency (Do et al., 2018; L. He et al., 2017). However, staking would result in unforeseeable situation as some of the unit cell would act as power supplying batteries and rest as power consuming loads (Gupta et al., 2021). 2.7.1 Manufacturing, cost, carbon footprint, and comparison with clean electricity technologies As discussed in the previous section, capital cost of MFC is exceptionally high due to costly electrode materials and membrane. Ultrex or Nafion films are generally used as membrane for large-scale MFC, which costs ca. $110 m−2 . The costs of MFC installation using recent innovations for the treatment capacity of 25 Kg COD m3 d−1 with a lifetime of 10 years are $100 m−2 for anode, $1500 m−2 for cathode, $1m−2 for separator, and $5000 m−3 for reactor. By comparison, the cost of MFC is 30 times higher than conventional activated sludge system. A-J Wang et al estimated that full-scale capital costs range from $735 m−3 to $ 36000 m−3 in accordance with different design and configuration. MFCs excel in the cheap operational cost and easy maintenance. MFCs operate with much lesser energy of about $0.12 kWh−1 -COD than conventional systems, which operate at 0.6 KWh−1− COD (Axelsson et al., 2012; A. J. Wang et al., 2020). Efforts are undertaken to reduce the capital cost and enhance the performance of MFCs. Ge et al reported a 200-L modularized MFC system under non laboratory conditions for treatment of municipal wastewater. The capital cost estimated for this system was $ 6064, which is comparable to the small wastewater treatment plants standardized under treatment capacity (gpd- gallon per day). The capital cost of 200 L MFC was $58 per gpd of treatment capacity, where small wastewater treatment plants costed $70 per gpd

Electricity generation with the use of microbial electrochemical systems

per capacity for capacity of 1000 gpd (Ge & He, 2016). The large-scale implementation of MFC at long term turns exceptionally profitable. Abourached et al conducted a case study on economic returns of large scale MFC technology to treat wastewater prior irrigation. The capital and operating cost for 20 years of the conventional wastewater treatment plants at Massachusetts is $68.2 million, whereas MFC costs $ 6.4 million. The net profit by utilizing MFC for treatment of fruit wastewater before irrigation for the period of 20 years stands at $ 7.1 million in San Joaquin Valley of California (here water values $440 per ac-ft and electricity cost is 15.5 cents per Kwh) (Abourached et al., 2016). MFCs are used to treat the wastewater and simultaneously produce the electricity and useful biomaterials sustainably, which reduce the carbon footprint. Constructed wetlands (CW) are incorporated with MFC to treat the wastewater and generate the electricity simultaneously. Wang et al studied greenhouse gas emission of integrated CWMFC system for the application of wastewater treatment. It cut the CWG by 5.9%– 32.4% of CO2 , 17.9%–36.9% of CH4, and 7.2%–38.7% of N2 O. The reference base values of GWG emission (CO2 -8.43–12.06 g m−2 .d, CH4 -3.39–4.90 mg m−2 .d, and 3.39–4.90 mg m−2 .d for N2 O) are from published papers of CW from the period of 1994 to 2013 (Wang, Tian, Liu, & Zhao, 2018). Chen et al simultaneously reduced the CO2 using the MFC and produced electricity and biomaterial (poly hydroxy alkanoates) during the waste treatment of water. The addition of electron shuttling mediators (e.g., aminophenols) remarkably enhanced the electron transfer efficiency of electrochemically active microorganisms with CO2 reduction rate of 40–60 Faraday efficiency (Chen et al., 2013).

2.8 Perspectives, the future of MFCs The future of microbial fuel cell applications is very wide, as detected since the first decade of the 21st century. The electricity generation with the use of MES, coupled with added-value processes such as wastewater treatment or water desalination as well as added-value products such as hydrogen, fuels and chemicals pose a large interest in make these technologies commercially viable. MES emerge as an alternative for the generation of renewable energy and water treatment both in rural environment as well as in smart cities, closely related to the sustainability of water, energy and pollution alignment with the objectives and goals 6 and 7 of Sustainable Development of the United Nations, i.e. clean water and sanitation, and affordable and non-polluting energy respectively. The challenge of this technology is mostly associated with the low energy yield that is still achieved from the experiments carried out in developed and implemented plants. More research is needed focused on microorganisms, improvement of materials, optimizing the design of the architecture to positively contribute to its scalability in processing and obtaining electricity.

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A change of focus is necessary in terms of expectation and consider this achieved energy that could be obtained, as a backup energy, rather than one used in production and distribution line, as it is obtained by multipurpose as mentioned above considering that the sanitation service is obtained. With this challenge clarified and delimited, wastewater treatment and added-value products and processes will be able to provide environmental and economic benefits. If it is possible to make use of this technology in an optimal way to complete it as operational and commercially viable, this same resulting energy can be used to maintain the installation. Among the advantages of using microbial fuel cells for wastewater treatment and electricity production are the following points: 1. They present a high theoretical energy conversion of over 70%, due to the fact that the chemical energy of the wastewater is converted directly into electrical energy, but also can produce added-value products such as hydrogen, chemicals and fuels and recover metals or in combination with other green technologies such as photocatalysis or photovoltaics, can degrade persistent pollutants from water. 2. MFCs can convert biomass at temperatures between 15 and 40°C and with low concentrations of organic matter in the water, both problematic conditions for conventional digesters. 3. They produce less CO2 than any other current technology using fossil fuels, so the few emissions they produce do not require any treatment. 4. They can be implemented in remote locations where there is a demand for basic electricity consumption and wastewater treatment. 5. Low operating cost, it provides a cheap way to produce electric energy and also to revalue liquid wastes. But despite the advantages of low marginal cost and high energy recovery, microbial fuel cells still have a way to go before they become economically viable, mainly due to their high capital costs. Compared to conventional water treatment systems (0.01–0.1 €/kg dissolved organic carbon) MFCs have a capital cost of 8 € per kg COD removed from wastewater (Logan & Regan, 2006a). Of the total initial cost, approximately half corresponds to the anode and cathode, since this is where the largest investment is made to solve the second major problem of CBMs: low power density. The power density of a standard MFC is around 0.05-1 W per m2 of projected electrode surface area.

2.9 Concluding remarks The UN 2030 agenda is set out in its objectives 6 and 7, clean water and energy for everyone.

Electricity generation with the use of microbial electrochemical systems

This chapter gave an overview of the knowledge available today on electricity generation with the use of MES. Such systems allow satisfying the purposes of treating water and generating energy, as well as producing added-value products and processes, such as water desalination,among others.Several types of MES and their applications have been developed. The center of these technologies is the microorganisms in the substrate, being the electrogenic ones that occupy the fundamental role of these systems. The fundamentals and applications of MES are reviewed in the present Chapter to understand their importance as a proposal for the mitigation of water and energy problems. The challenge of this technology is associated with the low energy yield that is still achieved from the experiments carried out in developed and implemented plants but despite the advantages of low marginal cost and high energy recovery, microbial fuel cells still have a way to go before they become economically viable, mainly due to their high capital costs. Understanding the operation and interactions of each of the parts involved in its design is vital to be able to make improvements and satisfy the energy output required in practical applications.

Acknowledgments This work was financed by the SIP-IPN Innovation Grant 2020-0890.

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Venkata Mohan, S., Velvizhi, G., Vamshi Krishna, K., & Lenin Babu, M. (2014). Microbial catalyzed electrochemical systems: A bio-factory with multi-facet applications. Bioresource Technology, 165(C), 355– 364. https://doi.org/10.1016/j.biortech.2014.03.048. Walter, X. A., Santoro, C., Greenman, J., & Ieropoulos, I. A. (2020). Scalability and stacking of selfstratifying microbial fuel cells treating urine. Bioelectrochemistry, 133, 1–8. https://doi.org/10.1016/ j.bioelechem.2020.107491. Walter, Xavier Alexis, Merino-Jiménez, I., Greenman, J., & Ieropoulos, I. (2018). Journal of Power Sources, 392, 150–158. doi:10.1016/j.jpowsour.2018.02.047. Walter, Xavier Alexis, Stinchcombe, A., Greenman, J., & Ieropoulos, I. (2017). From the lab to the field: Self-stratifying microbial fuel cells stacks directly powering lights. Applied Energy, 192, 575–581. doi:10.1016/j.apenergy.2020.115514. Wang, A. J., Wang, H. C., Cheng, H. Y., Liang, B., Liu, W. Z., Han, J. L., Zhang, B., & Wang, S. S. (2020). Electrochemistry-stimulated environmental bioremediation: Development of applicable modular electrode and system scale-up. Environmental Science and Ecotechnology, 3, 1–9. doi:10.1016/j.ese.2020.100050. Wang, H., & Ren, Z. J. (2013). A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnology Advances, 31(8), 1796–1807. https://doi.org/10.1016/j.biotechadv.2013.10.001. Wang, L., Long, F., Liang, D., Xiao, X., & Liu, H. (2021). Hydrogen production from lignocellulosic hydrolysate in an up-scaled microbial electrolysis cell with stacked bio-electrodes. Bioresource Technology, 320, 1–10. doi:10.1016/j.biortech.2020.124314. Wang, M., Wang, Z., Hu, F., Fan, L., & Zhang, X. (2020). Polyelectrolytes/α-Fe2O3 modification of carbon cloth anode for dealing with food wastewater in microbial fuel cell. Carbon Resources Conversion, 3, 76–81. https://doi.org/10.1016/j.crcon.2020.02.004. Wang, Z., & He, Z. (2020). Frontier review on metal removal in bioelectrochemical systems: Mechanisms, performance, and perspectives. Journal of Hazardous Materials Letters, 1, 1–6, 100002. https://doi.org/10.1016/j.hazl.2020.100002. Xiao, L., Ge, Z., Kelly, P., et al. (2014). Evaluation of normalized energy recovery (NER) in microbial fuel cells affected by reactor dimensions and substrates. Bioresource Technology, 77–83. Yang, N., Liu, H., Jin, X., Li, D., & Zhan, G. (2020). One-pot degradation of urine wastewater by combining simultaneous halophilic nitrification and aerobic denitrification in air-exposed biocathode microbial fuel cells (AEB-MFCs). Science of the Total Environment, 748, 1–11. doi:10.1016/j.scitotenv.2020.141379. Yaqoob, A. A., Ibrahim, M. N. M., & Rodríguez-Couto, S. (2020). Development and modification of materials to build cost-effective anodes for microbial fuel cells (MFCs):An overview.Biochemical Engineering Journal, 164, 1–14. https://doi.org/10.1016/j.bej.2020.107779. You, J., Chen, H., Xu, L., Zhao, J., Ye, J., Zhang, S., Chen, J., & Cheng, Z. (2021). Anodic-potentialtuned bioanode for efficient gaseous toluene removal in an MFC. Electrochimica Acta, 375, 1–8. doi:10.1016/j.electacta.2021.137992. Zhang, F., Yongtae, A., Logan, B. E., et al. (2014). Treating refinery wastewaters in microbial fuel cells using separator electrode assembly or spaced electrode configurations. Bioresource Technology, 46–52. Zhao, N., Liang, D., Li, X., Meng, S., & Liu, H. (2021). Hydrophilic porous materials provide efficient gasliquid separation to advance hydrogen production in microbial electrolysis cells. Bioresource Technology, 337, 1–7. doi:10.1016/j.biortech.2021.125352. Zhi, W., Ge, Z., He, Z., & Zhang, H. (2014). Methods for understanding microbial community structures and functions in microbial fuel cells: A review. Bioresource Technology, 171, 461–468. https://doi.org/ 10.1016/j.biortech.2014.08.096. Zhuang, L., Zheng, Y., Zhou, S., Yuan, Y., Yuan, H., & Chen, Y. (2012). Scalable microbial fuel cell (MFC) stack for continuous real wastewater treatment. Bioresource Technology, 106, 82–88. https://doi.org/ 10.1016/j.biortech.2011.11.019. Zuñiga, C., Zaramela, L., & Zengler, K. (2017). Elucidation of complexity and prediction of interactions in microbial communities. Microbial Biotechnology, 10(6), 1500–1522. https://doi.org/10.1111/1751-7915.12855.

CHAPTER 3

Overview of wastewater treatment approaches related to the microbial electrochemical system Saeed Fatima a,b, Tallam Aarti a and Sundergopal Sridhar a,b

a CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India b AcSIR - Academy of Scientific & Innovative Research, CSIR-IICT, Hyderabad, India

3.1 Introduction In India, the demand for freshwater, as well as the deposition of wastewater, is increasing rapidly due to urbanization and industrialization (Trivedi, 2010). Consequent to the speedy rise in population and growing water demand, the burden on water sources in India is rapidly increasing as well as water accessibility per capita is dropping day by day (Seow et al., 2016). The gigantic supply of domestic water to meet these needs generates about 70%–80% of wastewater as stated by the Ministry of Urban Development, Govt. of India (Sahasranaman & Ganguly, 2018). Another current significant issue is energy scarcity due to overpopulation and the expansion of cities. Energy usage in different sectors is increasing sharply leading to heavy consumption of energy resources (Shukla, 1997). To tackle both of these problems, an efficient process has to be developed that minimizes these effects. Furthermore, the better way to overcome this issue is to capture the energy from biomass present in the wastewater. However, the organic matter present in the wastewater contains energy-rich particles that could be a new source of energy generation (Johnson et al., 1965). In the early 1980s, industrial and domestic wastewater was most commonly treated using aerobic and lagoon process techniques. Based on the degree of solid content present in the wastewater,the treatment procedure has been varied (Speece, 1983). However, the most popular treatment technologies like aerobic activated sludge, anaerobic digestion, are not efficient in perfectly capturing the energy potential of wastewater due to their high capital cost, sludge generation, high energy input, etc. Also, electrochemical advanced oxidation processes can decompose organic compounds into less refractory products, and even mineralize them to carbon dioxide, water, and other inorganic species. The efficiency of this conventional technique is often limited by the weak mass transfer of the pollutant molecules, and the energy consumption is still at a relatively high level for commercial uses (Pan et al., 2019). Microbial electrochemical systems (MESs) can be considered as an alternative to these techniques owing to their Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00010-3

c 2023 Elsevier Inc. Copyright  All rights reserved.

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cost-effectiveness and sustainability. The research and development of MESs exposed many challenges and investigations in the field of material science and engineering, electrochemistry, microbiology, and other related areas (Wang & Ren, 2013; Jatoi et al., 2021, 2022). Moreover, MESs offer a scalable forum for all those various engineering functions to be created, as well as a special environment to consider the largely untapped microbial electrochemistry. Although several emerging environmental systems only have single or multiple features, the MES Platform’s flexibility has led to the discovery of scores of new functions. About all MESs work on the same concept where the biodegradable substances, like waste products, are oxidized by microbes to produce electricity (Shehab et al., 2013). The MESs have been explored and established thoroughly over the last decade, which creates a whole new multi-sectoral field for technology development that links up microbiology, electrocatalysis, chemical and materials sciences, engineering, and many other fields. MESs has proved to be a sustainable technology in wastewater treatment due to their intrinsic feature of producing bioenergy along with recovery of better quality products. Fig. 3.1 gives an overview of the major MESs. In 1911, Potter was the first to announce the development of actual microbial biosynthesis a century earlier, but scientific attention for this idea has only recently exploded, contributing to an unprecedented spike in the number of journal publications (Pugazhenthiran et al., 2016). There are some outstanding articles on the origin and evolution of MESs (Borole et al., 2011; Feng et al., 2013), as well as the substances, structures, and microbiome cultures in various systems. The majority of the papers address the biochemical conversion that takes place in the presence of microbes or biocatalysts in MESs, where energy is generated in the form of fuel or electricity. Thereafter, the interaction between microbes and electrodes has been explored for a deeper understanding of the first principles, scientific developments along with microbial and technological improvements (Feng et al., 2013; Pugazhenthiran et al., 2016). The interaction between the microbial cells and electrode can either be through capacitive and faraday interactions. During capacitive interactions, the lipid layer of a microorganism comes in contact with the electrode and displaces water molecules and ions from the electrode, leading to a change in the electrochemical capacity of electrodes due to which current flows in the circuit. During faraday interactions, redox reactions take place mediated by microbial and molecular species involved in extracellular electron transfer (EET) (Nelson & Guarino, 1969). In all MESs the microorganisms have to perform extracellular electron transfer (EET), that is, transport of electrons across the cell envelope and set up an electrical contact with the electron acceptor such as an electrode maintained at ample oxidizing potential (Yang et al., 2012). The study of EET is based upon two most commonly used microorganisms, that is, Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA, both Gram-negative mesophilic bacteria that can transfer electrons to extracellular substrates during respiration (Otero et al., 2020). Geobacter is capable of making biofilms that have long-range electron conductions from cells located at a distance of ˃50 μm from the electrode (Nelson & Guarino, 1969).

Overview of wastewater treatment approaches related to the microbial electrochemical system

Figure 3.1 Overview of microbial electrochemical systems.

For MESs, the interface of biology and electrochemistry is used and a biological moiety (e.g., a cell, enzyme, or organelle) plays a key role (Schröder et al., 2015). Examples of such categories are biofuel cells, biosensors, and bioelectrocatalysis. They are widely used for water desalination and wastewater treatment (Wang & Ren, 2013). Recently developed MESs like microbial fuel cells (MFCs) is a promising technique to generate electric power using microorganisms to catalyze different electrochemical reactions, to capture the energy in wastewater for diverse purposes (Franks & Nevin, 2010). Energy production by electrochemical processes or conventional combustion requires fuel to

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provide electrons and an electron acceptor (oxidizer). In MESs, organic matter is the fuel, and oxygen is the primary oxidizer for aerobic respiration by bacteria. However, many other soluble chemical species can serve as oxidizers for anaerobic bacteria, including nitrate, sulfate, and carbon dioxide. Bacteria known as exoelectrogens have the ability to transfer electrons outside the cell to insoluble electron acceptors, such as iron and other metal oxides, or to electrodes in bioelectrochemical systems (Patil et al., 2012). The research on MESs for treating wastewater has been intensified in the last 5 years leading it to an advanced level of development. Microbial electrochemistry is the backbone for the working of all MESs. A wide range of potential applications of MESs has arisen as a result of the ability to extract and use the current and potential generated by bacteria. Scaling up MESs is important for real field applications with enhanced bioelectricity generation and concurrent wastewater treatment (Butti et al., 2016). Therefore, pilot-scale studies are needed to fully predict the functionality and long-term survival of the material as well as to examine the effectiveness of such devices over some time with changes in temperature, energy, and maintenance (Krantz & Kifferstein, 1998). The present chapter aims to provide an overview of MESs. Conventional and advanced MESs for wastewater treatment are well presented throughout the manuscript. It covers different types of MESs like microbial electrolysis cells (MECs),MFCs,microbial remediation cells, biosensors, etc., including their merits, challenges as well as future prospects which can help in further developments in the research area of MESs for wastewater treatment.

3.2 Current research on wastewater treatment techniques Over several years, the primary aim of urban wastewater treatment was to minimize the amount of sediment, oxygen-demanding products, dissolved oxides, and infectious bacteria present in hazardous wastewater. However, in current history, much emphasis is being put on developing methods for the treatment of wastewater from industrial and urban sources (Kaur et al., 2012). Quite a lot of traditional methods have been interpreted for the treatment of wastewater associated with chemical material rich in organic contaminants. In the 1960s, the use of adsorbents has piqued interest by most researchers (Hernandez & Osma, 2020; Jatoi et al., 2021, 2022; Mubarak et al., 2014; Mehmood et al., 2020; Yee et al., 2018; Nizamuddin et al., 2019; Lim et al., 2018; Mazari et al., 2021; Khan et al., 2020), for the eradication of organic substances from watercontaining waste biomass, which can be one of the preliminary treatment methods. Activated carbon is the most commonly used adsorbent for the treatment of bio-waste water rich in organic matter. Nevertheless, owing to its increased price as well as a 10%–15% deduction during the regeneration process chose alternative adsorbents such as sawdust, lignite, peat, fly ash for the removal of copper trace metal. Whereas Johansson

Overview of wastewater treatment approaches related to the microbial electrochemical system

et al., suggested the use of assertive filtration using an alkaline medium to remove phosphorus from domestic sewage water (Johansson & Hylander, 1998). Another primary technique to oxidize waste biomass present in water is by injecting ozone, which is an excellent oxidizing agent, as well as a disinfectant that can destroy a wide range of toxins, likely aromatic and phenolic hydrocarbons. According to Martínez et al., ozone-treated water is ideal to use in the cultivation of freshly harvested crops (Martínez et al.,2011).The secondary procedure for the treatment of wastewater includes biological processes for the elimination of saturated and unsaturated contaminants in the presence of a microbial medium (Logan, 2010; Tchobanoglous et al., 1991; Hernandez & Osma, 2020). In biological treatment, the wastewater can be subjected either to an aerobic or anaerobic process in a reactor containing bacteria as the microbial media. The microbes usually convert the organic matter of the waste biomass into water, ammonia, and carbon dioxide and detoxify the remaining inorganic content (Joss et al., 2006). Furthermore, tertiary treatment methods play a key role in the production of safe drinking water. Vaporization, evaporation, crystallization, electrodialysis, electrocoagulation, and membrane separation processes (reverse osmosis, nanofiltration), are some of the methods used for the posttreatment. The nature of wastewater and the process economy provide the backbone for selecting the suitable treatment systems for wastewater. Highly contaminated water has a tint and contains solid residues which are primarily treated with preliminary and secondary systems followed by tertiary treatment technologies. A secondary treatment process is indeed not necessary unless the biological oxygen demand is high. Whereas, tertiary treatment methods can only be employed when the wastewater is colorless and free of particles and is tainted by synthetic, chemical, and microbial contaminants. In specific, potentially toxic compounds pollute underground water, and that only requires tertiary technologies to handle it. On the other hand, surface water contaminated by natural, or bacterial impurities needs secondary and tertiary treatment. The forms of toxins found in the sample determine the postsecondary treatment strategy, and the best decision can be made by considering the above-mentioned discussions. However, the wastewater is heavily contaminated and colored by industrial waste key elements, agricultural, and biochemical contaminants, necessitating a well-balanced combination of primary, secondary, and tertiary treatment technologies (Gupta et al., 2012).

3.3 Comparison between conventional systems and microbial electrochemical systems for wastewater treatment Rapid industrialization is resulting in an enormous amount of wastewater generation. Various technologies are emerging for the proper utilization of it as a potential source of energy and byproducts recovery. The energy potential of wastewater could be five times more than the energy consumed to treat it. A conventional process like activated sludge for wastewater treatment needs about twice power for aeration and pumping as compared

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to other processes (Logan, 2010). Membrane-based bioreactors are also in huge demand but they need more energy for operation. Anaerobic digestion also has limitations like they need concentrated stream and warmer temperature. The potential of MESs to treat wastewater while subsequently reducing the need to handle sludge generation and low solids production is exponentially rising. Table 3.1 gives the data on the comparison of conventional systems with MESs for wastewater treatment.

3.4 Classification of microbial electrochemical systems MESs present a rapidly growing research field taking inputs from electrochemistry,microbiology, engineering, and polymer science (Wang & Ren, 2013). The area is vast and the graph for the development in these technologies is stretching day by day. These systems can be of various kinds depending on the purpose, but all systems share one common principle in the anode, at which biodegradable substrates, such as waste materials, are oxidized by microorganisms and generate electrical current (Ramírez-Vargas et al., 2018). The applied electrical voltage can be used as an indicator to differentiate between primary MESs and secondary MESs. In primary systems, the used voltage thermodynamically lies within the physiological range of the used microorganisms. While secondary systems are often indicated by strongly negative or strongly positive electrode potentials. Based on the electron transfer they can be classified as: Primary electrochemical systems: In these systems, EET is either directly from cell to acceptor or mediated by electron shuttles following faradays processes (Meng et al., 2017). Secondary electrochemical systems: These systems utilize indirect interactions and the bio conditions, electrochemical processes are controlled by the adjustment of microbial environment, that is, pH, oxygen pressure, metabolite concentrations, etc. (Meng et al., 2017). Based on the type of application they can be classified as: 1. Microbial electrolytic cells (MECs). 2. MFCs. 3. MDCs. 4. Microbial capacitive deionization cells. 5. Microbial remediation cells. 6. Biosensors. With the novel reactor configurations, researchers aim at developing technically and economically competitive MESs to curtail the gap between old and new technologies for wastewater treatment. Also, wastewater is a complex mixture of many components and requires robust and versatile technologies for its treatment. Biochemicals, fuels, electricity, etc. can be produced by treating wastewater through MESs making it useful in both ways (Butti et al., 2016).

Table 3.1 Comparison of MESs with conventional systems for wastewater treatment. Microbial Conventional electrochemical Objective Advantages of MES over conventional process process process

Anaerobic digestion

Microbial fuel cell

Reduction of Total Suspended Solids (TSS)

Aeration reactor

Microbial fuel cell

Heavy Metal Removal (Cu(II), Cr(VI), Co(III), Hg(II), Se(IV)

Chemical precipitation

Microbial remediation cells

Reduction of toxic chemicals

Membrane biofilm reactor (MBfR) Capacitive Deionization (CDI)

Microbial fuel cell (MFC)

Desalination

Microbial capacitive desalination Cell(MCDC)

MFC can convert 80% of potential energy into bioenergy whereas with anaerobic digestion the energy conversion can go up to 35-40%. Diluted wastewater streams can also be employed for treatment in MFCs. The minimal amount of solid residue production can be the main benefit in MFC as it is 10 times less than aeration. The operational cost of MFC is 20-30% less when compared to aeration reactors. Chemical precipitation involves a large number of chemicals for treatment that pose a serious concern for discharge. Chemical precipitation can’t be used to remove heavy metal ions which are less than 100 mg/l in concentration. Higher removal efficiencies up to 99% can be achieved in MRCs for Cu, Cd, Cr removal. MFC configuration is capable of deducing harmful chemicals like perchlorate from the wastewater sludge up to 24mg/L when compared to biological process MBfR. The desalination efficiency of MCDC for wastewater is 7-25 times greater than conventional CDI. Total Dissolved Solids deposition occurs in CDI and MCDCs pose no solids depositions as there can be a larger area due to their three-chamber configuration.

Smita S Kumar et al. (2019)

Huggins et al. (2013)

Smita S Kumar et al. (2019)

Butler et al. (2010)

Forrest et al. (2012)

Overview of wastewater treatment approaches related to the microbial electrochemical system

Biogas, bioethanol, and bio hydrogen production

References

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Figure 3.2 Overview of microbial electrochemical systems.

3.5 Working principle and mechanism microbial electrochemical systems for wastewater treatment 1. MECs: The principle involved in MECs is a cathodic reduction of protons through an external potential to produce hydrogen gas and other useful compounds. They rely on nonspontaneous reactions catalyzed by Electroactive anode respiring bacteria (EARB) (Escapa et al., 2016). These bacteria possess the ability to conserve energy from electron transfer to an electron acceptor (e.g., a solid-state electrode), and play a lead role in current density generation. Fig. 3.2 illustrates the mechanism involved in MECs. Here, The EARB gets accumulated at the anode and performs the oxidation of organic substrate present in wastewater by a series of redox reactions to carbon dioxide (CO2 ) and release protons (H+ ) along with electrons (e− ) into the solution. The H+ ions diffuse to the cathode through a protonexchange membrane which is fixed between the anode and cathode compartment

Overview of wastewater treatment approaches related to the microbial electrochemical system

(Kadier et al., 2018). Electrons, which are one of the by-products of bacteria metabolism flows through the outer circuit to the cathode. Due to the nonspontaneous reaction, the Gibbs free energy of the reaction is positive, and theoretical cell voltage is negative, therefore, a power source is required to initiate the reaction (Rousseau et al., 2020). The electrical energy to the MECs can be supplied in two modes either potentiostatic or galvanostatic. The galvanostatic mode is based on input current flow to the electrodes while the potentiostatic mode is based on the fixed potential between the electrodes. Apart from H2 , MECs are capable of producing methanol, ethanol, hydrogen peroxide, and other chemicals. MECs require power input of 0.1–0.6 V to support the reactions (Li et al., 2017). The self-sustaining microbial biocatalysts eliminate the need for valuable metals on the anode. The hydrogen or other products can be fully recovered with high conversion efficiency. The current density is affected by the reactor configuration of MEC. There are several MECs like single-chamber MEC, two-chamber MEC, coupled MEC system, etc. The effect of different operational variables needs to be studied to optimize the performance of MECs in terms of cost and production. 2. MFCs: The basic principle of MFCs include oxidation of organic and inorganic matter present in wastewater using bacteria as catalysts. MFCs work on the principle of converting chemical energy to electrical energy through enzyme catalytic reactions Fig. 3.3. In an anode chamber, the microorganisms oxidize the substrate (Pandit et al., 2017). The protons released during substrate catabolism pass into the cathodic chamber through an ion exchange membrane, meanwhile the electrons go to the cathode via an external wire. The electrons and protons combine with oxygen at the cathode to produce water. Here the electrical energy is recoverable from the external circuit as the overall reaction is thermodynamically favorable (Kumar et al., 2017). MFCs operate in galvanic mode i.e. the redox reactions at the electrodes produce a voltage which in turn drives electrons through electric devices. The functioning of MFCs depends on the type of microbes present in them. The choice of microorganisms depends on their catalytic activity and electron generation capacity from the organic substrate present in wastewater. Higher output can be achieved by changing reactor configuration, optimizing Ph, conductivity, and surface area of cathode and anode (Koroglu et al., 2018). Different type of wastewaters is used as a substrate for MFCs. The food processing industry, beverage industry, dairy industry, and distilleries proved to be powerful industries to provide substrate for MFCs. Based on the electrolytes used in fuel cells they can be of several types such as alkaline fuel cells, phosphoric acid fuel cells, solid oxide fuel cells, molten carbonate fuel cells, polymer electrolyte membrane fuel cells, etc. (Choudhury et al., 2021). Various MFCs models including the bulk liquid model, biofilm model, and electrochemical model are developed for wastewater treatment in MFCs.

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Figure 3.3 Microbial fuel cell and its principle.

3. Microbial Desalination Cells (MDCs): MDCs are one of the neoteric techniques that incorporates MFCs and electrodialysis for wastewater treatment, subsequently producing electricity. It can be used either independently or can be combined with other techniques. The difference in ion concentration in the anode and desalination chamber is the driving force for the motion of ions in the MDC. They can be of several types including Batch MDCs: Osmotic MDC, Tubular air cathode MDC, photosynthetic MDC, Continuous MDCs: up-flow MDC, continuous flow MDC (Sayed et al., 2020). MDCs can be three-compartment cells consisting of an anode chamber, desalination chamber, and cathode chamber. MDCs work on a similar principle as MFCs (Fig. 3.4). In the anodic compartment, the anode respiring bacteria forms a biologically active film around the anode. These bacteria degrade the organic matter present in wastewater releasing electrons and protons.The protons pass through the cation exchange membrane into the cathode compartment. Water forms after the cathodic reaction along with bioelectricity production. Salt removal takes place in the desalination chamber due to the potential difference between anode and cathode. The anions (Cl− , SO4 2− ) and cations (Na+ , Mg2+ ) from saltwater move to

Overview of wastewater treatment approaches related to the microbial electrochemical system

Figure 3.4 Schematic of microbial desalination system.

anode and cathode compartments through the anion exchange membrane and cation exchange membrane respectively. In total, 99% of salts are removed in this manner through MDCs producing more energy than conventional systems (Shehab et al., 2013). The desalination rate gives an insight into salt removal in a typical MDC system (Fig. 3.4). However, the desalination efficiency decreases due to the electroosmotic force which causes water molecules to move from the desalination chamber along with ions through ion exchange membrane Stacked cell arrangement can recover more energy as compared to any other configuration. MDCs pose conspicuous benefits as they do not require any external source of energy for performing ion separation in wastewater (Forrestal et al., 2012). By incorporating MDCs with other aerobic treatment processes, organic matter can be effectively removed from wastewater along with desalination treatment. MDCs are an energy-intensive process rather than conventional desalination technologies that require 2–15 kW h m−3 of energy for producing fresh water from seawater (Zhang & He, 2015). MDCs can also be combined with MECs for metal removal applications (Saeed et al., 2015). 4. Microbial Capacitive Deionization Cells (MCDCs): MCDCs is an extension of MDCs. They consist of three compartments, namely, anodic compartment, capacitive deionization compartment (CDC), and cathodic compartment Fig. 3.5. MCDCs works on the principle of the electro sorption process (Shrestha et al., 2018). The CDC consists of highly porous carbon-based electrodes at an electrode potential of about 0.8–1.4V. Salt ions from the feed travel into electrical double layers that are present as the capacitive charge storing systems. The electrons released by electron

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Figure 3.5 Membrane assisted microbial capacitive deionization cell.

respiring bacteria in the anodic compartment migrate to electrodes through anion and cation exchange membranes in the cathodic compartment to separate ions through capacitive storage, that is, non-Faradic charge storage (Wen et al., 2014). MCDCs work efficiently for the efficient treatment of low salinity water. These are similar to MDCs except for the fact that they need a much lower DC (direct current) voltage of about 1.4V for electric double layer formation (Feng et al., 2013). They have a high potential to overcome PH fluctuation and salt migration problems of MDCs. The carbon electrodes can be coated with the ion exchange polymer layer to increase the electrosorption efficiency and compactness of the MCDCs. Electrical discharge of electrodes needs to be done once the electrodes reach their maximum salt adsorption capacity (Forrestal et al., 2012). 5. Microbial Remediation Cells (MRCs): MRCs are a type of MFCs that are used to remove heavy metal contaminants and perchlorates from wastewater. The toxic metals like U(VI), Cr(VI), Cu(II), Cd(II), Hg(II), Se(IV), Co(III/II), Pb(II), etc., can be converted to less toxic or insoluble form by MRCs (Yaqoob et al., 2020). If the biocathodes are used biosorption also takes place along with chemical reduction. The basic concept relies on the fact that the electroactive biofilm formation takes place at the anode and the biological degradation of water occurs which releases electrons and protons (Meng et al., 2017). The protons travel to the cathode through the ion exchange membrane and the electrons carry out the reduction of the toxic metals in the cathodic chamber. The reduced metal ions either precipitate in the solution or get deposited at the cathode. If the reduction potential of the cathode is less than the anodic potential, then external energy needs to be supplied to carry out

Overview of wastewater treatment approaches related to the microbial electrochemical system

the metal reduction. These can also be used for groundwater remediation (Dummi et al., 2017). The reactor type configuration can be used for removing U(VI), Cr(VI), etc. from groundwater. Single and dual-chamber types of cells can be used but the latter produces high power as compared to the former one. Various factors including electrode material, membrane type, substrate, temperature, and solution Ph, etc., will affect the metal removal efficiency of the MRCs. 5.1 Constructed Wetlands-MFC: CWs are engineered systems designed to treat wastewater in controlled conditions with the help of plants and microorganisms along with wetland vegetation and soil media. CWs can be combined with MFCs to explore the possibility of wastewater treatment in a natural environment. The biofilm development takes place with the help of wetlands plants and leaves. The roots that are buried in the wetland allow for the growth of microorganisms (Wu et al., 2015). The in-depth profile of CWs presents a system with anaerobic zones in the bottom and aerobic zones on top. Anode and cathode are fixed in the anaerobic and aerobic zones respectively. The EARB present in the anodic zone, that is, bottom zone decomposes the organic matter, releasing electrons to the anode. These electrons flow from bottom to top, that is, to the aerobic cathode zone through the external circuit. The protons traveled in the aerobic zone take part in the cathodic reduction of oxygen to form water (Gupta et al., 2020). Cws can have several advantages of being operated on low solar energy input and maintenance, low production of sewage sludge, and low cost of operation. 6. Biosensors: They detect physiological and biochemical changes in a biological environment. A biosensor consists of three elements i.e. a bioreceptor, transducer, and a signal processing system. The biological element or bioreceptor interacts with the analyte and its response is converted into an electrical signal by the transducer. This electrical signal is amplified and measured with the help of a signal processing system. These sensitive devices are able to detect the composition of species without human intervention. Biosensors find their important applications in food, water, marine, and medicine fields (Ivars-Barceló et al., 2018). The pharmaceutical industry needs selective sensors to improve the quality of products and reduce byproducts. Rapid detection of microbial contamination in food makes them essential for the food industry to improve food quality. They are also used for real-time monitoring of biohazardous material in the environment. MFC-based biosensors are gaining special attention for the detection of BOD and toxicity in water samples due to lower operational costs and environmental risks. There are several biosensors available such as thermal, calorimetric, piezoelectric, optical, and electrochemical. 6.1 Electrochemical biosensors: These are the most compact and controllable which can operate even in turbid media with excellent instrumental sensitivity. An electrode is used as the transduction element in this biosensor. These can

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be conductometric, potentiometric, and amperometric based on the detection method. In the electrochemical biosensing process, the chemical potential of a particular species in solution is measured by comparing it with the reference electrode. The biochemical reactions produce or consume ions that cause changes in the electrical properties of the solution. The electrochemical detector detects it by a biological interactive substance during a biochemical active process. The failure of these sensors can occur due to surface contamination. The membrane can be used to avoid this contamination by foreign materials from a reaction (Mehrvar & Abdi, 2004). Electrochemical biosensors can be classified as: a) Amperometric sensors: These sensors measure the changes in current on the working electrode due to direct oxidation of the products in the biochemical reaction. They are rapid, inexpensive, and easily disposable. This biosensor is quite common among the three electrochemical biosensors due to its high sensitivity. Due to their lower limit of detection and simplicity amperometric detectors are commonly used with biocatalytic and affinity sensors (Ronkainen et al., 2010). In addition, hydrodynamic amperometric techniques can provide significantly enhanced mass transport to the electrode surface. An example of an amperometric biosensor is the aforementioned glucose biosensor, which is based on the amperometric detection of hydrogen peroxide. b) Potentiometric biosensors: They detect the potentials at the working electrode along with charge accumulation due to selective binding at the electrode surface. They measure the accumulation of a charge potential at the working electrode compared to the reference electrode in an electrochemical cell when zero or no significant current flows between them (Bakker & Pretsch, 2005). For potentiometric biosensors, the measurements are done by the relationship between the concentration and potential which is governed by the Nernst equation. It also gives information about the ion activity in an electrochemical reaction. These sensors prove suitable for measuring low concentrations like nickel, manganese, mercury, and arsenate ions in tiny sample volumes. c) Conductometric biosensors: This type of sensor detects the change in conductance between a pair of metal electrodes in a bulk solution. Many advanced fabrication techniques are available for the development of commercially viable biosensor electrodes for various industrial applications. They are widely used for handling small volumes of liquids with high precision. These are strongly associated with enzymes in most cases. Due to enzymatic reaction the changes in ionic strength, and thus the conductivity of a solution between two electrodes takes place (Grieshaber et al., 2008). There are

Overview of wastewater treatment approaches related to the microbial electrochemical system

examples of successful development of these devices for practical application, such as drug detection in human urine and pollutant detection in environmental testing. A biosensor provides rapid and reliable information continuously, reversibly, and accurately.The influence of external parameters like changes in temperature and Ph affect the current/power generation which in turn disturbs the evaluations (Karunakaran et al., 2015). Glucose biosensor is widely used nowadays for testing the blood glucose level in diabetic patients. Also for impedance measurements using a biosensor, it is prepared on an electrode such that the electron transfer rate is selectively modulated by the analyte. Capacitive biosensors were made by Berggren, Bjarnason, and Johansson (2001) to detect antigens, antibodies, proteins, DNA, and heavy metals with detection limits as low as 10−15 . These types of biosensors detect the changes in electrochemical and/or electrical properties that are caused by the blocking ability of self-assembled monolayers. Electron impedance spectroscopy (EIS): In MESs, the EIS technique is used to optimize the power output, coulombic efficiency, and COD removal efficiency. It is used to investigate the electrochemical performance of anode and cathode subjected to change in surrounding conditions. The response of EARB to changes in physiological conditions like redox potentials, electrolyte chemistry, mass transport parameters, material properties of electrodes, etc. can also be recorded with EIS. EIS captures steady-state characteristics of the MES systems over a wide range of frequencies (106 –10−4 Hz) (Borole et al., 2010). The energy/charge storage behavior of the system can be easily understood with EIS. The electrical equivalents circuits comprising resistors, capacitors, and inductors are used to develop arbitrary models based on EIS data. The measurements can be done in either way, that is, using two electrodes (anode, cathode) cell configuration or three-electrode (anode, cathode, and reference electrode) cell configuration (Fig. 3.6). Mostly in all systems cathode will be acting as a working electrode and anode as reference or counter electrode. The charge and mass transfer reactions, biokinetics mechanisms, hydrodynamics, and surface electrochemistry, etc. can be obtained using Equivalent Electrical Circuit (EEC) models (Sindhuja et al., 2016). These can be arranged in series/parallel configurations. The transfer functions derived from the basic laws of the processes involved like electrokinetics, diffusion, partition, etc. can be used to describe the system. A model describing the impedance behavior of the system can be used as the starting point for analysis. EIS also gives an insight into the charge and mass transfer process associated with oxygen reduction reactions at the cathode. Here a 5–20 Mv of an alternating signal is applied to disturb the working electrode, and the electrochemical response of the circuit is observed and recorded. The data obtained can be validated using Kramers-Kroning transforms (Kim et al., 2021). However, systems linearity, stability, and causality must be ensured to apply this method. So the system should be stable and reversible. To overcome this drawback fast Fourier transform method can be used which uses noise signal to obtain the current signal, which is achieved by mixing ac voltages of

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Figure 3.6 Impedance measurement using three-electrode configuration.

Figure 3.7 Equivalent Electrical Circuit of a Microbial electrochemical system: Ra, anode resistance; Rs, ohmic resistance; Rc, cathode resistance; CPE, constant phase element.

various frequencies to an electrochemical cell.EIS is also used by impedimetric biosensors for direct analytic detection, immune detection, nucleic acid detection, etc. Modern impedance analyzers also contain data evaluation software and fitting programmers which allow the calculation of transfer functions according to an equivalent circuit (Lisdat & Schäfer, 2008). The internal resistance Rin consisting of charge transfer resistance, ohmic resistance, and diffusion resistance can be calculated with EIS to assess the performance of MFCs (Fig. 3.7). The charge transfer resistance of the equivalent circuit can be obtained by ‘Nyquist Plot’ graphs. The charge transfer resistance gives an idea about the lifetime of the membrane, electrochemical kinetics of reactions, and microbial activities in the cell. Both Nyquist plots and bode plots can be used to present the data of impedance measurement (Tiquia-Arashiro & Pant, 2020).

3.6 Bottlenecks and troubleshooting involved in MESs The challenges associated with the development of MESs include the high operating cost of the system constituents along with membrane, durability, and strength of electrodes,

Overview of wastewater treatment approaches related to the microbial electrochemical system

intermittent power generation, corrosion of the electrodes, voltage reversal, etc., ultracapacitor can be used to store the diffused energy (Hassan et al., 2021). MESs proved to be versatile technology for wastewater treatment in various fields. However, bioelectronics and biosensors need to be explored to increase the energy production rate through MESs. To overcome the drawbacks associated with the high cost of the electrodes, the waste materials can be used to produce functional materials for electrodes. The replacement of expensive components can be achieved by the use of several biodegradable secondary materials, lingo cellulosic biomass, as porous separators, and charcoal derived from pyrolysis of lingo cellulosic biomass (biochar), as conductive and electro-active material for electrodes. in a circular economy approach (Xie et al., 2015). Broader studies can be carried out using wastewater for MESs to reduce the cost of the process, increasing the feasibility and sustainability. Due to the combination of two systems, that is, bacterial and electrochemical the overall system becomes complex and it becomes difficult to find its constraints. Close monitoring of bacterial growth and its metabolism is essential for the efficient performance of MESs. Various surrounding factors like pH, temperature, salinity, proton flux, and oxidant need to be optimized for the smooth functioning of biocathodes in the system. Redox mediators are required for simulating the interaction of microbes with electrodes. Metabolic engineering and synthetic biology approaches should be applied to microorganisms to suppress higher resistance and product inhibition. The electro-simulation of microbial metabolism paves the way toward an integrated system to optimize product synthesis, redox-limited reactions, and the performance of biocatalysts (Shantonu et al., 2016). Biocatalysts like photosynthetic bacteria, cyanobacteria, mixed microbiome, acidophiles, thermophiles, alkaliphiles, halophiles, psychrophiles, hyperthermophiles, etc. play an important role in EET transport at the anode in most MESs. But the different electron uptake mechanisms by these catalysts involved in the process are still not known properly. To increase the productivity and electrical output of the system a deep understanding of these mechanisms and their interactions with electrodes is crucial. The multifunctional enzymes (moonlighting proteins) involved in the respiratory electron transport chain can be focused on further research to improve the activity of biocatalysts (Krishna et al., 2019). The ultramodern MESs based biosensors can’t be used efficiently to detect different toxic pollutants due to their long response time and inconsistent operation (BadihiMossberg et al., 2007). The continuous supply of energy can help in increasing their sensitivity and accuracy. Various adaptive approaches can be applied to solve the problems related to reactor configuration sludge degradation and for the continuous removal of salts from wastewater. Further study needs to be made on the interactions between the microbes for a proper understanding of the sludge stabilization mechanism in MCDCs (Xing et al., 2020). To reduce the energy consumption in a few MESs, the recirculation rates can be reduced along with optimizing the operating conditions. The

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potential energy losses associated with bacterial metabolism, mass transfer, and the current generation are irreversible which limits the reactants flux, proton diffusion, reaction activation, and charge transfer (Sharma et al., 2014). At higher current densities mass transport of chemical species by diffusion to the electrodes becomes insufficient leading to mass transfer losses. Maintaining high bulk concentrations and proper distributions of oxidants around the cathode can minimize mass transport losses. Substrate crossover occurs in MESs due to diffusion and electro-osmosis of other species along with ions to the cathode through the membrane. Hence, the Columbia efficiency and potential drop reduction occur at the cathode. Advancements in membrane materials and cathode catalysts can be deployed to curtail its effect on the system’s performance (Daud et al., 2015). The cost of electrodes can be overcome by indirect electrostimulation and electric syntrophic techniques as they don’t need electron exchange with the electrodes. Biocompatible, biogenic, and low-cost materials can be state of art materials to scale up MESs. Different approaches are scrutinized to improve the cathode performance by decreasing the activation energy barrier, increasing the area of the reaction interface, optimizing the temperature and oxidant concentration (Zhang & Angelidaki, 2016). There is still no clear vision on how to run these technologies for real-time use with robustness, flexibility, and stability. Long-term stability of the output is mandatory to use these systems for prolonged periods.

3.7 Conclusions and future prospects There are several physicochemical and biological methods available for water and wastewater treatment. But over ten years of scientific research and technological development, the usefulness of MESs has greatly increased, and their efficiency has reached heights. These systems successfully eliminated the problem of solid waste generation and more space requirements with their simple/compact design, easy operation, and higher efficiency. The present MESs advancements offer sufficient mechanistic proof of certain microorganisms’ ability to energetically interphase their system through a variety of methods. MESs face numerous challenges like the cost of membranes, cathode catalyst cost, membrane fouling, and pH imbalance for their commercialization. These problems can be overcome by using nonprecious cathode catalysts and cost-effective membranes. The ion exchange membranes can also be combined with graphite sheets to avoid problems of high ionic concentration around them. Interaction relations are so important in a hybrid environment of MESs. Existent characterization techniques provide basic tools for describing those relationships, usually ranging between microscopic levels to surface structure levels with high resolution through the system characterization. As a consequence, the MESs has evolved as effective methods for energy generation from wastewater or certain valuable products. It can be regarded as a green technology as it generates less sludge and by-products.

Overview of wastewater treatment approaches related to the microbial electrochemical system

The study of the mechanisms throughout the progression of various scientific areas of interest showed that microbial electrochemical devices are seen as a forerunner of modern MES science, the dynamics that underpin the electron transfer pathways that control interactions among microbes, and electrodes in MES are of great interest to researchers. However, when designing scientific bio-hybrid structures, the scope of reengineering must be carefully considered. Advancements in the design and implementation of emerging technology initiatives, like Nano-manipulation and manufacturing, as well as selective multidisciplinary collaboration, may open up new avenues for picking microbes to fuel MES advancement. Hybrid MES systems can be employed for future use instead of using them as standalone systems to use for multiple applications at one time. They can also be combined with advanced oxidation methods for treating wastewater from metal industries. Future advances in MES necessitate progress in the construction of a common standard language. This method would allow for the reduction of the negative consequences of the use of ambiguous terms that will encourage researchers to concentrate on basic design and experimentation activities in the long run. MESs can be further improvised with well-characterized constituents along with advanced properties, which will help in the assessment of system functioning,writing technologies for wastewater production, energy generation, and Biocomputing. Finally, by combining advanced materials like nanoparticles with microelectronic designs, technological bioengineering instruments, and microorganisms, there could be scope for great advancement in the area of MESs for wastewater treatment.

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Synthesis and application of nanocomposite material for microbial fuel cells Antonia Sandoval-González a and Bibiana Cercado b a

b

CONACYT-Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C. (CIDETEQ) Quéretaro, México Centro de Investigación y Desarrollo Tecnológico en Electroquímica, S. C. (CIDETEQ), Querétaro, México

4.1 Introduction Throughout history, the accelerated growth of humanity, industrial activities, and natural processes have caused environmental pollution, which has gone from being light and reversible to having an environment in which it is no longer possible to live without consequences. However, there are two main problems that, if not solved, could cause a worldwide environmental and economic crisis: the generation of electric energy and the scarcity of water due to its high consumption and contamination by industrial, domestic, and agricultural activities (Webster et al., 2018). An alternative to minimize the water problem is to look for wastewater treatment alternatives to give them a second use and thus avoid consuming more drinking water in activities where it is not required. On the other hand, conventional energy generation has low conversion efficiency, thus resulting in large atmospheric emissions that have deteriorated the environment. This suggests that it is necessary to look at sustainable green technologies and mitigate this problem with the best efficiencies. One of the systems that are attractive to solve this problem has to do with the development of microbial fuel cells (MFCs), which transform the chemical energy of organic compounds (degradation) into electrical energy. Microbial fuel cells have different designs, they can be single-chamber, two-chamber, tubular, and air-cathode (Di Palma et al., 2018; Masoudi et al., 2021; Slate et al., 2019; Jatoi et al., 2021, 2022). Twochamber or H-type MFCs are characterized because they have two compartments, the anode zone, and the cathode zone, which have a separator (usually the proton exchange membrane, PEM) that prevents contact between the two zones, but allows the transfer of protons from the anode to the cathode. The anode chamber contains the wastewater, the microorganisms, and the anode electrode, where the microorganisms form a biofilm on the electrode surface for the oxidation of organic matter and the transfer of electrons (pili, cytochrome c,shuttles).These electrons are transferred to the cathode through an external resistor, at the cathode the oxygen reduction reaction (ORR) is promoted to form water, Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00019-X

c 2023 Elsevier Inc. Copyright  All rights reserved.

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by supplying oxygen, electrons, and protons that came from the anode zone. However, although MFCs are promising devices to provide sustainable solutions for future energy needs, they are not yet commercial due to several factors affecting their performance, such as low electron transfer, low output power density, high cost of materials (membranes and catalysts), low catalytic activity, and limited conductivity (Saranya et al., 2019; Yousefi et al., 2018). These characteristics are related to the low electron transfer between the anode and cathode, due to the properties presented by the materials used as electrodes, it is necessary to improve their performance and efficiency (Kaur et al., 2020). Nowadays, there are several traditional materials studied to give solutions to this problem, the most common are carbon-based (Ghasemi et al., 2013), and others such as stainless steel, which is widely studied due to its low cost and high mechanical strength, but presents problems of corrosion and low biocompatibility with microorganisms (Dessie et al., 2020), in some cases to achieve high power density it is necessary to use several anodes and cathodes in the same system (Masoudi et al., 2021). However, because the cathode electrode presents lower efficiencies, many of the materials are focused on improving their reaction kinetics, such as the case of Cu2 O/rGO (Xin et al., 2020). In recent years, the use of nanotechnology for the efficient design of materials has had very advantageous results compared to traditional materials, because it allows changing their properties according to the manipulation of the synthesis method used. Among the physical properties of nanomaterials are the type of surface (smooth or rough), size, shape, width to height ratio, specific surface area, distribution, ability to dissolve, structure, adhesion to each other, etc., and chemical properties: molecular structure, composition, state (solid, liquid, gas), surface chemistry, the attraction of water molecules and oils or fats. Nanomaterials are usually between 1 and 100 nm in size, with different shapes: nanotubes, nanospheres, spinels, stars, fibers, flakes, etc. The surface of the nanomaterial will influence the degradation of contaminants and energy production since it is a determining factor in the growth of the biofilm. If we have materials with a very large thickness or very smooth surface, it will hinder the diffusion of the substrate to the interior of the material, preventing the formation of the biofilm in that area (Cai et al., 2020). A suitable alternative to limit these problems is the use of nanocomposite materials. Nanocomposite materials are classified into (1) polymer matrix nanocomposites, (2) ceramic matrix nanocomposites, and (3) metal matrix nanocomposites, and the nanocomposites used in MFCs are sought to have mechanical and chemical stability, greater electroactive area, strong interfacial adhesion between the biofilm and the nanocomposite, improved biocompatibility, and increased electrical conductivity, favoring the rate of electron transfer (Jafary et al., 2018; Wilberforce et al., 2022). Fig. 4.1 shows the number of published works on the development of nanocomposite materials with applications in MFC. Fig. 4.1 shows that there is a large number of research in the development of separating membranes of the anodic and cathodic zone, this is because, among all the elements that

Synthesis and application of nanocomposite material for microbial fuel cells

Figure 4.1 Nanocomposite materials are used in MFCs to generate electrical energy. Search source https://www.sciencedirect.com/ (accessed February 2022).

make up the fuel cells, the membranes are the most expensive and limit scaling up to a commercial level and even to a pilot plant. In this book chapter, we will address important issues in the development of catalytic materials such as their synthesis, physicochemical and electrochemical characterization, cell testing, and their performance in terms of their power density.

4.2 Synthesis of nanocomposite materials used in microbial fuel cells In this section, we present the synthesis methods most commonly used in the development of nanocomposites with applications in microbial fuel cells. To choose a suitable synthesis method, we must consider the cost of the process, stability, durability, limited contamination, ease to handle, excellent product yield, whether it is feasible to consider large-scale production. There are physical and chemical synthesis methods for obtaining nanocomposites, which must have a close relationship between the synthesis conditions (quantity, distribution, structure, morphology, mass, surface, composition, etc.) and the properties required for the application of the designed nanocomposite. Among the physical and chemical methods are solid, liquid, and gas-phase synthesis methods. Nanocomposites obtained by solid-phase present high yields and can be scaled up to an industrial level, but it is not easy to control particle size, purity, and morphology. Liquid-phase synthesis methods are characterized by being simple, easy to handle, and particle size can be controlled, but they consume more energy, increasing the cost of

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Figure 4.2 Different methods of nanocomposite synthesis and their application in an MFC.

synthesis. On the other hand, to generate well-dispersed small particles with high surface activity and purity, gas-phase methods must be used (Yang & Park, 2019). Nanomaterials that are dispersed in the matrix (polymeric, ceramic, or metallic) can be spherical, fibers, tubes, wafers, or sheets (Goyal, 2017). Fig. 4.2 shows some methods of nanocomposites synthesis. 4.2.1 Hydrothermal synthesis of nanocomposites The hydrothermal synthesis method helps to have a better dissolution of poorly soluble species under normal conditions. It consists of heating water above its boiling point (100°C), causing the pressure inside the container to be higher than 1 atmosphere, which will only be reached if the container is closed. Water at these conditions (100°C and 1 atm) acts as a reducing agent (releasing oxygen), causing changes in the oxidation state of the elements involved in the synthesis. Depending on the synthesis temperature and pressure conditions, different morphologies will be obtained, as well as compact or separated structures. In some cases, organic cations are used to nucleate the product environment, generating cavities with desired diameters. The equipment used for the

Synthesis and application of nanocomposite material for microbial fuel cells

synthesis are autoclaves or microwaves (Meng et al., 2016), and the characteristics of the solvent (dielectric constant, boiling point, temperature, polarity, viscosity, etc.) and the reaction time are of great influence on the properties of the nanocomposite material. Finally, it is important to highlight that hydrothermal synthesis is highly employed due to its versatility, it has little loss of reagents, and the composition of the material to be synthesized is controlled through a phase or multiphase chemical reactions. In this type of synthesis, other solvents can also be used, receiving the name “solvothermal synthesis” (Adschiri et al., 2000; Gan et al., 2020; Meng et al., 2016). 4.2.2 Sol–gel Sol–gel is an interesting chemical synthesis and deposition method; in the last stages of the synthesis process, it allows to control of the characteristics of the nanocomposite material at the molecular level. In addition, it is inexpensive, safe, and it is possible to have nanoparticles, thin films, ultrafine powders, and fibers of the desired size, porosity, and crystallinity. This technique also makes it possible to visualize gel formation by first dispersing the colloidal solid particles (usually metal-organic compounds) in a liquid phase and through hydrolysis and polymerization reactions a colloidal suspension or sol is formed. Sols are classified as lyophobic (the solvent-particle interaction is weak) and lyophilic (strong solvent-particle interaction). When the forces between two particles are repulsive, they prevent the agglomeration of the particles allowing the sols to be stable. Finally, with the help of additives, the system collapses and the wet gel is formed, which depending on the desired application can be heat treated to have amorphous solids, crystalline solids, or macromolecules (Pomogailo, 2005; Ullattil & Periyat, 2017). 4.2.3 Chemical reduction Chemical reduction is one of the most highly employed methods of synthesis of nanocomposite materials because it does not require sophisticated equipment to be carried out, it can be performed in an aqueous medium or with other solvents. Also, structural and morphological parameters of the obtained nanocomposite can be controlled, which can be modified to meet the specific requirements that are sought. The chemical reduction of metal precursor salts can be carried out with reducing agents, through the use of microwaves or by electrochemical reduction. The metal precursor dissolved in the medium is reduced and transformed into metal atoms for different mechanisms, these atoms are dispersed to meet with metal ions, other atoms, or even clusters, this leads to the formation (atom-atom collisions) and growth (atom-nucleus and nucleus-nucleus collisions) of stable nuclei. The size of the nucleus will depend on the metal-metal interaction and the redox potential difference between the metal salt and the reducing agent. When nucleation times are short, monodisperse particles are obtained, but if the nucleation and growth stages are carried out at the same time, a

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material with dispersed particle size will be obtained, and finally, if the Oswald maturation process takes place, large particles will predominate in the material. There are several reducing agents that can be used, but the most commonly used are sodium citrate (Na3 C6 H5 O7 ) and sodium borohydride (NaBH4 ). If a nanocomposite material with small particles is sought, a strong reducing agent is used, although there is a risk that the small particles will form clusters when precipitating. Using weak reducing agents in this technique, nanocomposites with large nanoparticles can be obtained (Ferrando, 2016; Nair & Laurencin, 2007; Saif et al., 2022). 4.2.4 Microwaves The method of synthesis of nanocomposite materials through the application of microwaves consists of homogeneously dispersing the precursors in the solvent, which can be water or organic solvents. The aim is that the solvent has a high dielectric constant; the higher dielectric constant, the greater the solvent’s affinity to interact with the microwaves. The precursors dispersed in the solvent are irradiated by the microwaves (2450 MHz with a length of 12.24 cm) by dipolar polarization or ionic conduction, the dipolar rotation generates shocks and friction with neighboring molecules, while in the ionic conduction the electrical resistance of the medium to the flow of ions will give off heat, these two mechanisms increase the kinetic energy and the temperature of the medium. Parameters such as temperature (ramp), heating rate, precursor concentration, nature of the solvent, and reaction time, are involved and control the nucleation, growth, and properties of nanocomposites (Horikoshi & Serpone, 2016). 4.2.5 Sonochemistry The method of nanocomposite synthesis by sonochemistry uses high-frequency ultrasound to generate volume oscillations and bubble formation in a liquid medium containing dispersed precursors. At high powers, compression and rarefaction waves are generated, rarefaction causes cavitation, which allows (1) nucleation, (2) formation, (3) growth, and (4) implosive collapse of bubbles, producing intense heating (5000 K), high pressure (1000 atm) and high heating and cooling velocities, causing transient and very localized zones with extremely high temperature and pressure gradients. Temperature and pressure contribute to the formation of nuclei from the precursors, which are finally transformed into nanocomposites with characteristics and properties according to the reaction time, the applied power, the frequency emitted by the equipment, the concentration of the precursors, the pH of the medium, etc. (Fuentes et al., 2021). 4.2.6 Synthesis for polymers Dispersing nanoparticles into polymeric matrices has brought great advantages to polymer applications due to the improvement of their properties, such as higher thermal stability, higher mechanical strength, increased barrier properties, as well as

Synthesis and application of nanocomposite material for microbial fuel cells

decreased aggregation. However, to achieve these properties it is important to choose the right nanoparticles and the polymer in which they will be embedded because not all nanoparticles and polymers have the same affinity. Another challenge is to ensure that the nanoparticles are homogeneously dispersed in the polymer matrix, which is achieved with good interfacial compatibility between the nanoparticles and the polymer. There are different synthesis methods for the fabrication of polymer nanocomposites: (a) melt-mixing, in this category, (1) extrusion, (2) water-assisted, (3) melt compounding, (4) atomic layer deposition, and (5) injection molding; other synthesis methods are (b) mixing techniques such as (1) solution blending, (2) shear mixing, and (3) casting. Finally, it can be found the methods: (c) In-situ, (d) electrospinning, and (e) laser sintering. These synthesis methods aim to produce polymer matrix nanocomposites that have uniform dispersion and no aggregations, although not all of the above synthesis methods are applicable to obtain nanocomposites for use in microbial fuel cells. In-situ polymerization allows the fabrication of nanocomposites that are thermodynamically stable and consists of dispersing nanoparticles in a monomer (melt with high shear forces),and then initiating polymerization on the surface of the nanoparticles, through heat or radiation and/or by a catalyst. This process allows covalent bonding between the nanoparticles and the polymer matrix through various condensation reactions. Epoxy nanocomposites constitute the majority of research and applications using in-situ polarization. Melt blending is most commonly used to prepare nanocomposites of clays with thermoplastic and elastomeric polymer matrices. The polymer is melted and combined with the desired amount of nanoparticles using an extruder or injection molding, the melting is carried out in the presence of inert gas. The melt blending is not harmful to the environment due to the absence of organic solvents. Melt blending is industrially interesting because it is cost-effective to produce nanocomposite materials in large quantities (Verma & Goh, 2019). Solvent casting consists of pouring a mixture containing the solvent with the nanoparticles and the polymer into a mold. The thermodynamic and kinetic properties of this nanocomposite will be a function of the solvent removal/evaporation rate, mixture composition, and distribution of the nanoparticles in the polymer. Electrospinning is an effective method suitable for producing porous structures and laser sintering avoids aggregation (Colijn & Schroën, 2021). Table 4.1 shows some nanocomposite materials that were used as electrodes and membranes in the microbial fuel cells. The nanocomposite materials in Table 4.1, showed better results compared to the commercial materials currently used in MFCs, and this is due to the transformation of traditional materials to composite nanomaterials which improve their properties that include: (1) optical, (2) electrical and magnetic, (3) catalytic activity, (4) sensorial, (5) biocompatibility, (6) thermal stability, (7) roughness, (8) mechanical, (9) hardness, (10) strength, (11) toughness, (12) lightweight, (13) barrier, (14) rheological, (15) chemical resistance, (16) corrosion resistance, (17) flame retardancy, (18) smart response, (19) surface area, (20) size, (21) surface energy, (22) number of surface atoms, (23) surface defect,

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Fe3 O4 /Au – NCs-3DGF Modified graphite felt – SnO2 /PPy/ SnO2 -sandwiched Modified graphite felt – SnO2 /Ppy Modified graphite felt – SnO2 NiO/graphene – GO – GO/ZnO – GO/TiO2 – rGO – NiWO4 – NWG – rGO-CC – NiWO4 -CC – NWG (0.5 – mmol)-CC NWG (1.0 mmol)-CC – NWG (3.0 – mmol)-CC Au@PANI/CC –

–

Hydrothermal

2980 ± 54

(Song et al., 2020)

–

Sol–gel

130.00

(Imran et al., 2021)

–

73.09

–

37.21

– – – – – – – – – –

Hydrothermal Solvothermal

Solvothermal

Solvothermal

(Wu et al., 2018a) (Yaqoob et al., 2021)

(Geetanjali et al., 2021) (Geetanjali and Kumar, 2019)

1.128 ± 42 743 ± 33

– – –

3.632 W m−2 292 × 10−3 912 × 10−3 608 × 10−3 253.00 372.00 1458.00 210 ± 10 256 ±12 462 ± 21

Core-shell composite. Modification of electrode by spray method

∼332 ± 52

(Kirubaharan et al., 2019)

(continued on next page)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 4.1 Nanocomposite materials prepared with different synthesis methods and used in microbial fuel cells. Electrodes Power density Anode Cathode Membrane Synthesis method References mW m−2

Fe-t-MOF/PANI – – – – –

– – – –

– – –

PPy@TiO2 PAni@TiO2 PAni-Co-PPy)@TiO2 SGO-TiO2 -PAni GO-TiO2 -PAni TiO2 -PAni Ni-Co/SPAni Modified graphite felt PANI/β-MnO2 Modified graphite felt β-MnO2 Modified graphite felt PANI and β-MnO2 Ni-Co/MGO Ni-Co/GO

– – – – – – – –

– – –

–

Hydrothermal Chemical reduction Electrochemical polymerization Conventional polymerization route Microwave-assisted In situ oxidative Polymerization Muti step process

Chemical reduction Hydrothermal and in-situ chemical oxidative polymerization

– – –

680.00 143.00 1502.78

(Kaur et al., 2021) (Ghasemi et al., 2021) (Li and Zhou, 2018)

763 ± 38 665 ± 33 1692 ± 15

(Pattanayak et al., 2020) (Mahalingam et al., 2021) ∼637.79 ± 32 (Pattanayak et al., ∼828.32 ± 41 2021) ∼987.36 ± 49 904.18 (Papiya et al., 2020) 734.12 561.50 ∼659.79 (Papiya et al., 2018) 248.00 (Zhou et al., 2018) 183.00 204.00

Chemical reduction

1003.18 889.60

(Papiya et al., 2019) (continued on next page)

Synthesis and application of nanocomposite material for microbial fuel cells

– – – – – – – –

– CNT/Pt Graphite felt decorated α-Fe2 O3 /polyaniline K-(PPy-Co-PANI)-rGO PPy-Co-PANI)-rGO rGO-CuS-ZnS

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90

Electrodes Anode

Cathode

Membrane

Synthesis method

Power density References mW m−2

–

–

SPSEBS+ 6%STNT

Solution casting

138.00

–

–

–

–

–

–

– – – – – – –

– – – – – – –

PHA-PHB Solvent blending (15%p/p) APSf /SMWCNT Solution casting and solvent evaporation Layer-by-layer Alternately dipping into assembled positive and negative chitosan/ suspensions. montmorillonite PES5_S Melt compounding and extrusión. PES10_S PES15_S PES20_S PES0 Melt-blending PES5 PES20

601.00 304 .20

(Sugumar and Dharmalingam, 2022) (Sirajudeen et al., 2021) (Nazia et al., 2020)

119.58 ± 19.16 (Yousefi et al., 2018)

18.70 ± 0.48 (Bavasso et al., 2021) 65.24 ± 5.79 47.82 ± 4.32 32.52 ± 3.34 0.08 ± 0.01 (Di Palma et al., 2018) 1.66 ± 0.21 9.59 ± 1.18

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 4.1 Nanocomposite materials prepared with different synthesis methods and used in microbial fuel cells—cont’d

Synthesis and application of nanocomposite material for microbial fuel cells

(24) surface functionalization, (25) surface composition, (26) porosity, (aa) shape, etc. The numerous properties of the new materials require also numerous specific characterization techniques.

4.3 Characterization of nanocomposites materials used as electrodes in microbial fuel cells 4.3.1 Structural characterization Structural characterization is the internal organization of the nanocomposite, which depends on the synthesis, sample preparation, and interpretation of data. Below we will outline some techniques that are of importance for the characterization of nanocomposites developed for application as electrodes in microbial fuel cells. 4.3.1.1 X-ray diffraction (XRD) X-ray diffraction (XRD) is a structural technique widely used in materials design. Diffraction patterns are obtained by measuring the angles of the primary X-ray beam diffracted by the crystalline phases present in the nanocomposites, according to Bragg’s Law, or can also be obtained through a database (e.g., JADE) that contains information to identify what is contained in the nanocomposites under study. The result of this measurement can give well-defined diffractograms or amorphous diffractograms, the latter are characterized by having very broad peaks in which the diffraction patterns cannot be identified, making the technique inappropriate for obtaining useful information in the design of the material. On the other hand, when you have diffractograms with welldefined crystallographic peaks you can: (1) identify the phase or phases present in the nanocomposite, (2) the crystal structure, (3) the lattice parameter, (4) the distribution of crystallographic planes, and (5) the size of the crystals, which are obtained with the intensity of the diffraction peaks. This technique is nondestructive and measurements are performed on solid-state samples (powder, film, pellet), and no special sample preparation is required (Ameh, 2019). Nanocomposites such as S,N-GR/Fe3 C/CC (Xia et al., 2021), NiO/graphene (Wu et al., 2018a), GO/ZnO, and GO/TiO2 (Yaqoob et al., 2021), are some examples of materials characterized with XRD on and then used in MFC. The above examples showed defined cristal structures with cristal size less than 50 nm. However, characteristic peaks were not detected in some nanocomposites; for instance in PAni-Co-PPy@TiO2 (Pattanayak et al., 2021). 4.3.1.2 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) is a characterization technique widely used when micrographs are required to show the interaction of nanoparticles with their support or matrix, as well as for nanocomposites. The different electron detectors play an

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important role in SEM measurements; other important technical parameters are the contrast mechanisms in the microscope, the material, the topography, the voltage, the magnetism, and the crystallographic orientation based on the electron emission, as well as the lateral resolution, which depends on the special distribution of the emitted secondary electrons (Dunlap & Adaskaveg, 1997; Oatley, 1982; Seiler, 1983). Then, SEM helps to investigate the morphology of the surface (smoothness, roughness), the homogeneity of the dispersion of the nanomaterials on the matrix that contains them; moreover, in the case of nanocomposites, the matrix can be ceramic, polymeric, and/or metallic. SEM is an expensive technique; although the sample does not require complex preparation, attention must be paid so as not to affect the results; moreover, the micrographs obtained are not difficult to interpret. Another characteristic of the material that can be analyzed in SEM is the chemical composition of the samples that can be identified and quantified through energy-dispersive X-ray spectroscopy (EDS) in microscopes having an auxiliary device. In EDS, an image is selected to be focused in different areas (surface or crosssection) to know the relative composition of the nanocomposite. The information obtained is the atomic percentage and weight percentage of the elements present in the nanocomposite. It is recommended to make several measurements so that the average value is a homogeneous result of the scanning of the sample.Finally,there is a field electron microscope (FESEM) in which higher magnification can be achieved and this leads to higher quality images, compared to SEM (Karak, 2018). NiO/Graphene nanocomposite showed nanoflakes assembled microspheres that were wrapped by graphene nanosheets to form a porous structure (Wu et al., 2018a). ZnO and TiO2 showed a uniform distribution on GO, with a particle size within 80–100 nm (Yaqoob et al., 2021). NWG showed agglomeration nanoparticles but these particles were anchored on the rGO layer (Geetanjali et al., 2021). BioAu/MWCNT (Wu et al., 2018b) and tadpole-shaped were observed in S,N-GR/Fe3 C/CC (Xia et al., 2021). 4.3.1.3 Transmission electron microscopy (TEM) Transmission electron microscopy (TEM) is an expensive, nondestructive technique, and for its measurement to be successful, the sample preparation (dispersion method in solution, chemical fixation, washing, etc.) plays an important role. Moreover, the sample must be thin, that is, transparent to electrons. The principle of the technique consists in irradiating the nanocomposite with a beam of electrons, some of these electrons are transmitted, others are scattered, and some others present interactions that can produce light emissions, secondary electrons, X-rays, etc. From this, the transmission/scattering of the electrons form images, the crystal structure is obtained from the diffraction of the electrons and the X-ray emission gives information on the elemental composition of the composite. Other measurements such as internal/external diameter ratio in the case of nanotubes, higher resolution of the nanomaterials at their interfaces, fringes, interplanar distance,dislocations,and defects,are made using high-resolution transmission

Synthesis and application of nanocomposite material for microbial fuel cells

electron microscopy (HRTEM). The histogram of the nanoparticle size distribution is obtained from the images either by manually measuring a large number of particles or with the specific software of the equipment. Misinterpretation of the results in TEM or HRTEM micrographs will spoil all the benefits of the technique (Zuo & Spence, 2017). NiO/Graphene nanocomposite showed NiO nanoparticles with the (200) cristal plane and interplanar distance of 0.21 nm (Wu et al., 2018a). In S,N-GR/Fe3 C/CC composite was detected the lattice spacing of graphite carbon and Fe, ∼0.34 and ∼0.24 nm, respectively (Xia et al., 2021). GO/ZnO showed the irregular distribution of ZnO on the GO surface. Finally, in PAni-Co-PPy)@TiO2 nanocomposite, the TiO2 NPs were spheric and cubic shaped with agglomeration (Pattanayak et al., 2021). 4.3.1.4 Thermogravimetric analysis (TGA) Thermogravimetric analysis determines the thermal degradation temperature of the nanocomposites, the weight of the residue at different heating temperatures, and the carbon residue or the amount of undegraded product present in the nanocomposites. The sample must be crushed into very small pieces since the measurement will determine the loss or gain of mass as a function of temperature. To obtain homogeneous results, it is important to take care of the speed at which the heating is performed, otherwise, the materials may undergo undesired transformations. The process consists of heating the sample in a suitable atmosphere and applying a controlled temperature ramp. The results are obtained in form of a graph in which the variation of the mass (ordinate axis) with respect to the temperature or time of measurement (abscissa axis). The graph allows for determining the stability of the nanocomposite, the composition of the intermediates, and the final residue. This technique is of great importance for nanocomposites that have a polymeric matrix (Corcione & Frigione, 2012). GO/ZnO and GO/TiO2 nanocomposites showed stability of 75% and 60% at 800°C, respectively (Yaqoob et al., 2021). 4.3.1.5 X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) is a technique that requires high vacuum conditions to provide information on the elemental identification and chemical states of the element, the relative composition of the surface constituents, and the valence band structure of the nanocomposites. The XPS spectrum is obtained by measuring the kinetic energy provided by the binding energy and the number of electrons escaping from the nanocomposite surface. In this technique the depth of information is in the range of 3 nm to 12 nm, here 95% of all photoelectrons are scattered when reaching the surface (3λ). This phenomenon depends on the nanocomposite under study due to the kinetic energy of its electrons (Watts & Wolstenholme, 2019). For example, in PAni-CoPPy@TiO2 was confirmed the presence of TiO2 NPs incorporated into the copolymer (Pattanayak et al., 2021).

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4.3.2 Electrochemical characterization of nanomaterials 4.3.2.1 Cyclic voltammetry (CV) Cyclic voltammetry is a versatile potentiodynamic electrochemical technique that is widely used. This technique is characterized by the application of a potential difference between the working and reference electrode to generate a current–potential (I–E) output curve, showing oxidation/reduction processes. The shape of the I–E curve will depend on the exchange of electrons between the electrolyte (depending on its nature) and the working electrode. Other factors affecting the I–E curve are the pH, the presence or absence of a chemical reaction, the phase transformation, adsorption phenomena, corrosion of the electrodes, connections, etc. In bioelectrochemical cells, I–E curves can be obtained but they are sometimes difficult to interpret because apart from the nanocomposite as the electrode, microorganisms modify the characteristics of the system electrode–electrolyte (Kissinger & Heineman, 1983). 4.3.2.2 Chronoamperometry (CA) Chronoamperometry is a basic electrochemical technique involving pulses of potential on the working electrode from a value where no faradaic process occurs to a potential where the surface concentration of the electroactive species is zero. In this technique, a given potential is applied and the current due to diffusion is monitored with respect to time, obtaining a t-I curve. This curve allows the study of diverse parameters of the system such as the electroactive area and the diffusion coefficient of electroactive species in solution. The curve also shows when the current decreases as the reactant are depleted (Bard & Faulkner, 2001). In the case of microbial fuel cells, it allows determining the catalytic activity of the microorganisms in presence of a specific substrate. This catalytic activity is also measured as electrical energy production and elimination of organic matter in the electrolyte. 4.3.2.3 Electrochemical impedance spectroscopy (EIS) Electrochemical impedance spectroscopy (EIS) is based on the application of an AC voltage or sinusoidal current with low amplitude in a certain frequency range, the current or voltage responds to the system with the same frequency and intensity, but with a certain phase shift angle (φ). Impedance (z) is the ratio of an applied voltage to current, which represents the opposition to the flow of electrons or current in a circuit due to the presence of resistors, capacitors, and inductors. Impedance is expressed in the form of a complex number, where the resistance is the real component and the capacitive reactance is the imaginary component. The analysis of the system response gives information about the interface, the interface structure, and the reactions taking place on it. EIS is a very sensitive technique, so it must be used with care, and it is not always well understood, especially in microbial cells due to all the variants that are present in the biological system (Lasia, 2014).

Synthesis and application of nanocomposite material for microbial fuel cells

Figure 4.3 Structural, electrochemical characterization techniques, and cell tests representation of nanocomposites materials used in microbial fuel cells.

4.3.3 Evaluation of nanomaterials in microbial fuel cells The potential–current (E–I) and current–power density (I–P) polarization curves are used to determine the maximum power production of an MFC and the current-related losses: (1) the activation overpotential causes a voltage drop due to the slow reaction taking place at the electrode surface, these losses are associated with the activation resistance, the type of catalyst and the electroactive area. (2) The ohmic overpotential is caused by the electronic, ionic, and contact resistances between the components inside the cell. Finally, (3) a concentration overpotential is also observed in the polarization curves, which is associated with mass transport losses and is characterized by the change of the reactant concentration at the electrode surface. Changes in reactant concentration are associated with microbial consumption. These overpotentials are always present, decreasing the open circuit potential and the power density delivered by the MFC. It is recognized that the power density efficiencies are low due to the electrode material used, electrolyte, substrate, and chamber separator (Fan et al., 2008; Logan et al., 2006). Fig. 4.3 shows a scheme of representative techniques applied to characterize nanostructured materials used in MFCs.

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4.4 Nanoparticles-based electrodes 4.4.1 Anodes The anodes of the microbial fuel cells serve the function of collecting electrons from the biofilm-forming bacteria and transferring the charge to the cathode through external connections, which are usually highly conductive metal mesh or wires. The anodes also act as solid support for the development of the electroactive biofilm. Therefore, materials for the preparation of bioanodes must-have characteristics that fulfill both functions. Charge collection is related to an intrinsic property of the material called capacitance. Capacitance involves not only energy harvesting but also energy storage. This property has been explored in bioanodes with a capacitor function rather than energy production such as in microbial fuel cells. Carbon fabric was modified with manganese ferrite (MnFe2 O4 ) showing a capacitive nature that favored the half-cell potential of the anode in a microbial fuel cell (Khilari et al., 2015). Charge transfer is an interfacial phenomenon between the surface of the electrode and the biofilm which is formed by bacteria and exopolymeric substances (mainly proteins and sugar chains). The biofilm has a more variable nature than the electrode material as it depends on the microbial species in the inoculum, as well as their physiological state. Therefore, it is necessary to explore the charge transfer between the biofilm and the modified materials. As mentioned before, the EIS is an electroanalytical technique to estimate the resistance to charge transfer in the interphase electrode-biofilm. The application of EIS has been reported for an anode prepared by the hydrothermal synthesis method to obtain alpha-FeOOH nanowires on carbon paper. The nanowires reduced the charge transfer resistance (Xian et al., 2021). Impedance analysis of carbon paper electrodes coated with iron-doped zinc oxide nanoparticles (Fe-ZnO) also showed a decrease in charge transfer resistance compared to a control anode (Muthukumar et al., 2019). Similar behavior was reported for anodes prepared with carbon felt, carbon cloth, and graphite support for iron/iron oxide (Fe/Fe2 O3 ) nanoparticles (Mohamed et al., 2018). Carbonaceous materials are widely used in the preparation of anodes due to their high biocompatibility with microorganisms and because of the content of oxygenated surface groups that enable further functionalization. The carbonaceous support materials are in the form of paper, cloth, activated carbon fiber, and felt (Mohamed et al., 2018); graphene material derived from organic waste has also been used (Yaqoob et al., 2021). Metallic materials such as stainless steel, titanium mesh, and copper foil have alternatively been used as supports but with a subsequent carbon particle deposition (Pu et al., 2022). The modification of electrodes with nanoparticles is very flexible since it is possible to use nanoparticles of metals, nonmetals, and composites. The use of pure metals is less frequent and is applied in the evaluation of electroactive microbial species and for the biosynthesis process. For example, electricity production by Shewanella spp. was

Synthesis and application of nanocomposite material for microbial fuel cells

evaluated on a carbon felt electrode impregnated with multi-walled carbon nanotubes containing Au nanoparticles (Calderon et al., 2020). Palladium nanoparticles deposited on dry biomass of Shewanella oneidensis were used as an anode catalyst (Ogi et al., 2011). A large number of researches have focused on the preparation of metal oxide nanoparticles in order to favor charge transfer using low-cost materials. Among the most recent research is the preparation of anodes via green synthesis of ZnO and TiO2 nanoparticles that were introduced into lignocellulose-derived graphene oxide (Yaqoob et al., 2021). In another research, the synthesis of alpha-FeOOH nanowires on carbon paper appeared to show advantages over nanoparticles as the nanowires conducted electrons over longer distances (Xian et al.,2021).Additionally,electrodeposition synthesis of iron/iron oxide (Fe@Fe2 O3 ) nanoparticles with nitrogen-doped carbon quantum dots were proposed to improve biofilm development on the electrode (Habibi et al., 2021). The preparation of nanocomposites that include metal nanoparticles is also a common strategy for anode preparation. An anode was prepared by impregnating activated carbon fiber with cerium oxide, and then carbon nanofibers were grown on the substrate by catalytic chemical vapor deposition. The graphitic carbon nanofibers and the microporous substrate improved the application of the material as a photoanode (Pophali et al., 2021). Polymers such as polyaniline, polyethyleneimine, and polythiophene have been used as components of nanocomposites.A carbon fabric modified with I2 -doped nanoparticles of polythiophene was used to increase the anode conductivity (Rajendran et al., 2021). Eco-friendly and low-cost synthesis of titanium dioxide nanoclusters was achieved by growing the nanoparticles in situ by S. cerevisiae on a polyethyleneimine-functionalized carbon felt (Duarte & Kwon, 2020). The problem of low biocompatibility of metal nanoparticles has been addressed by encapsulation of Au nanoparticles with polyaniline as a biocompatible and conductive polymer (Kirubaharan et al., 2019). In addition, the toxicity of some metals and metal oxides toward microorganisms has been evaluated for Fe-ZnO (Muthukumar et al., 2019). 4.4.2 Cathodes The cathode chamber in microbial fuel cells typically hosts the ORR. The ideal catalyst for the ORR is platinum; however, it is widely recognized that it is high cost, low availability, and highly susceptible to poisoning; all of which preclude its use in scaling up microbial fuel cells. ORR on noble metals involves complex mechanisms, adsorbed species, and soluble intermediates to create a network of reactions that are not fully understood. Because ORR is present in many electrochemical systems, its optimization continues to be investigated. One aspect to be solved about ORR is the low reaction rate and therefore the need for better catalysts. The design of new catalysts for ORR aims not only to replace the use of noble metals and accelerate the reaction but also to increase the stability and durability of the material.

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Table 4.2 Diverse requirements for the design of new catalytic nanomaterials. Environmental and economic aspects

Sustainable material

Available all over the world Low cost compared to Pt Abundant storage

Catalytic properties

Textural features

Long-term desirable features

Efficient for ORR via the four-electron pathway Abundant active sites

High surface area

Stability

Improved mass transfer

Durability

High exposure to catalytic sites Low resistance to charge transfer

Homogenously dispersed Anti(bio)fouling nanoparticles Hierarchical texture, Anticorrosion micro- and meso-porous structure High graphitization degree of carbon materials Good adsorption capacity

Efficient conductive network in nanocomposites

The requirements for the design of new catalysts are varied and from a practical point of view, it is difficult to meet all of them in a single catalytic material. The catalyst should preferably be synthesized with sustainable materials, have enough homogeneously dispersed active sites, have a high surface area, and be inexpensive (Table 4.2). In addition, the improved catalyst has to operate in a favorable environment for biological processes such as in the case of microbial fuel cells. Due to all the above considerations, there is a great diversity of catalytic materials involving both metallic and nonmetallic nanoparticles, carbonaceous materials, and polymers. Polymers are employed with several functions: conductive phase between the other elements that compose the catalyst, protective film of metallic nanoparticles, and precursors of carbonaceous material. Nonmetallic nanoparticles are less frequently used compared to metallic nanoparticles for the preparation of catalysts due to the difference in electrical conductivity between them. Nonmetallic nanoparticles are synthesized using mainly the elements: N, F, B, and S. While for the synthesis of metallic nanoparticles, the use of Fe, Co, Ni predominates. In some other cases, nanoparticles of Cd, Ag, Pd, and Pt have also been used. Metals are exploited not only in their pure state but also as metal oxides, alloys, or forming metal-organic-framework (MOF) structures as precursors of the catalyst. Recently the use of perovskites has also been proposed as an innovative catalyst (Nourbakhsh et al., 2020), and sustainable synthesis of the catalyst has been achieved by the biosynthesis of metal nanoparticles through bacterial processes (Wu et al., 2020). During catalyst design, nanoparticles can be placed on the support material either as dopants, homogeneously deposited on flat or spherical surfaces, dispersed between

Synthesis and application of nanocomposite material for microbial fuel cells

fibers, encapsulated, or embedded in the bulk material. The way the nanoparticles are interconnected with the support depends on the structure of the latter, for that reason many carbonaceous structures have been explored. The catalyst support forms part of the cathode which is generally made of carbonaceous material. The forms of carbon used in catalyst design are activated carbon and graphitic carbon in the form of graphene oxide and reduced graphene oxide; the higher the graphitization degree the higher the electrical conductivity. As for metallic particles, carbonaceous material can also have a sustainable source when obtained from waste biomass after pyrolytic processing and activation (Ma et al., 2016). Carbon materials can be used with the structure of sheets,nanosheets,nanotubes,multiwalled carbon nanotubes, fibers, nanofibers, flakes, shells, and spheres. The combination of structures of carbon materials is intended to create a specific texture and hierarchical porosity, as well as to modify the surface area of the material to promote nanoparticles binding. The design of catalysts for ORR in MECs has evolved from the use of nonmetallic nanoparticles decorating a single carbon form and structure (S. Zhong et al., 2014), through the use of bimetallic nanoparticles (Gupta et al., 2017), mixed types of nanoparticles to obtain a synergistic effect (Liu et al., 2020), the use of different forms (activated, graphitic) and structures (fibers, cloth, etc.) of carbon as support (Du et al., 2019), to form increasingly complex structures such as leaf structures (M. Zhong et al., 2021) and flower superstructures (Shao et al., 2022). Table 4.3 shows a brief description of the design of catalysts in which nanoparticles have been used to improve the ORR. Although ORR is the most frequent reaction in the cathodic chamber of microbial fuel cells, there are other reported functions for the cathode such as the reduction of Cr(VI) (Pophali et al.,2021),or an antibacterial property (Li et al.,2021).Thus,the removal of contaminants is among the most recent focus for the design of catalysts. New designs for cathode catalysts bring challenges that still need to be addressed. The synthesis methods involve several steps with the possibility of adding handling errors for each step; in addition, detachment of nanoparticles from the supports must be avoided. Notably, the possibility of massive or large-scale fabrication of the catalysts needs to be addressed in current microbial fuel cell research.

4.5 Performance of nanomaterials in anodes and cathodes The performance parameters of nanomaterials used for anode oxidation reactions vary from those reported for cathodes in microbial fuel cells. In the anode chamber, oxidation of organic matter by microorganisms is expected to occur; therefore, the percentage removal of chemical oxygen demand is a relevant parameter (Muthukumar et al., 2019). The current density produced from a microbial activity is the basic electrochemical parameter that indicates the electroactivity of the microorganisms (Pophali et al., 2021).

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Table 4.3 Overview of nanoparticles used as cathode catalyst in microbial fuel cells. Nanomaterial References

NCMO/rGO/CNT Nickel-cobalt manganite supported on reduced graphene oxide/carbon nanotube prepared by a hydrothermal method to form spinel nanoparticles. Co/Ni@GC/NCNTs/CNFs Graphene carbon-coated with CoNi alloy nanoparticles, Metallic nanoparticles were grown on the tips of N-doped carbon nanotubes that were vertically aligned on carbon nanofibers. Mesoporous silica nanoparticles were introduced in Fe-doping zeolitic imidazolate framework-8 (ZIF-8) precursor. The materials were assembled into a flower-like superstructure interlaced by porous nanosheets. Co3 O4 -NC/CF Co3 O4 nanoparticles were dispersed on nitrogen-doped carbon nanoflakes that were vertically assembled on carbon fibers. Co-N-PC@CNTs A double ZIF precursor is used to synthesize a leaf-like cobalt/nitrogen co-doped porous carbon embedded with carbon nanotubes. CNF/CuO2 NP/ACF A photocathode was prepared by impregnating activated carbon fiber with cuprous oxide nanoparticles, followed by growing carbon nanofibers on the activated carbon fiber using catalytic chemical vapor deposition. CoNC@AC Cobalt-nitrogen-carbon nanotube were implanted on activated carbon by growing Co/Zn-based metal organic frameworks (MOF) on activated carbon followed by thermal pyrolysis. CC-PANI-Pt Polymerization of aniline onto carbon cloth for electrodeposition of platinum nanoparticles though chronoamperometry and cyclic voltammetry. NC@CoNC/rGO Core/shell carbon materials doped with Co and N were prepared from bimetallic MOF through pyrolysis method after being interconnected by reduced graphene oxide. BioAu/MWCNT Carbon cloth modified with nanocomposites of multi-walled carbon nanotubes blended with BioAu. Biogenic gold nanoparticles were reduced from aqueous Au(III) and in situ Cu(II) co-reduction. LaCoO3 , LaMnO3 , and LaCo0.5Mn0.5O3 perovskite-type oxide nanoparticles were used as cathode catalysts. N-CNT/rGO nanosheet Co-glycolate nanoparticles uniformly coated on reduced graphene oxide nanosheets were reduced to Co nanoparticles acting as catalysts for the growth of carbon nanotubes which were doped with nitrogen. N/S-Fe-CNF/CDC Nitrogen, sulfur and carbon as dopants on iron nanoparticles-dispersed on carbon nanofibers. Carbon nanofibers were grown over carbide-derived carbon. Thiourea was used as source of N, S and C.

(Qavami and Ghasemi, 2022) (Li et al., 2022)

(Shao et al., 2022)

(Li et al., 2021)

(Zhong et al., 2021)

(Pophali et al., 2021)

(Liu et al., 2020)

(Zerrouki et al., 2022) (Zaho et al., 2020)

(Wu et al., 2020)

(Nourbakhsh et al., 2020) (Du et al., 2019)

(Pophali et al., 2019)

(continued on next page)

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Table 4.3 Overview of nanoparticles used as cathode catalyst in microbial fuel cells—cont’d Nanomaterial

References

Co-NCNT NP Cobalt nanoparticles with nanopolyhedra form synthesized via pyrolysis of bimetallic MOF were embedded in nitrogen-doped carbon nanotubes. AA/Ni NP/CNF Alumina nanoparticles to increase the electrical conductivity of the electrode; nickel nanoparticles catalyzed the grow of the carbon nanofibers on an activated carbon microfiber substrate by chemical vapor deposition. N-B/C Nitrogen and boron dopants on carbon nanoparticles.

(Zhang et al., 2019)

(Gupta et al., 2017)

(Zhong et al., 2014)

Additionally, the relationship between the above two parameters is reported as coulombic efficiency (Calderon et al., 2020). Given the central role of electroactive microorganisms attached to the anode, the analysis of microbial communities, identification of species, and quantification of their abundance in the biofilm are also occasionally investigated (Xu et al., 2018). The performance of catalyst nanomaterials for cathodes, in addition to those already described for anodes, also includes kinetic parameters such as the exchange current, the half-wave potential, and the onset potential. Stability is reported in some investigations as the percentage retention from the maximum catalytic capacity, also is reported the number of hours of operation or the number of cycles (Pophali et al., 2019). Due to the difference in performance parameters between anode and cathode, power density is the most commonly used comparative parameter for microbial fuel cells. However, it should be considered that power density is the result of fuel cell components and conditions that are not under study; for instance, the inoculum and nutrients. Regarding the inoculum, variables such as microbial species, population density, and physiological state should be standardized. With respect to nutrients, the variability in chemical composition, concentration of each compound, bioavailability, toxicity, and reaction by-products also need to be controlled. Therefore, comparisons of the effectiveness of nanomaterials in microbial fuel cells need to be made from a global point of view. The power density in nanoparticle-modified anodes is reported in two major ranges with an extreme value outside these ranges (reports in the period 2022–2017). The lowperformance range is from 0.9 mW m−2 to 310 mW m−2 , while the high-performance range spans from 616 mW m−2 to 5800 mW m−2 . A maximum power density value outside the high range has been reported as 25900 mW m−2 . This power density was obtained with an anode prepared from titanium oxide nanoclusters grown in situ with S. cerevisiae as a template on polyethyleneimine functionalized carbon felt (Duarte & Kwon, 2020). The power density in modified cathodes can be grouped into a low range from 309 mW m−2 to 704 mW m−2 ,intermediate range from 889 mW m−2 to 1630 mW m−2 ,

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and high range from 2045 mW m−2 to 3200 mW m−2 for the same period mentioned above. The maximum power density value was achieved in a cathode modified with flower-shaped porous carbon nanospheres. These structures were obtained by molding Fe-doping precursors and using mesoporous silica nanoparticles as templates. The spheres were bonded with nanosheets from the inside to the outside which optimized the spacing, increased the surface area, and improved the transport to the active sites (Shao et al., 2022). Overall, the diversity of materials and methods of preparing nanoparticles to modify anodes and cathodes is vast; however, improvements in power density seem to increase more slowly for cathodes than for anodes. The catalytic capability of microorganisms could be a route to explore and exploit in nanomaterials preparation for cathodes.

4.6 Conclusions In this chapter, we have shown diverse methods of synthesis of nanocomposite materials with applications in the anode and cathode of microbial fuel cells. In recent years, research on improving nanocomposite materials has increased dramatically. However, there are still major challenges to advancing the properties of the nanomaterials, such as preparation of materials that are nontoxic to microorganisms, increased affinity of microorganisms to the material, an increase of the electroactive area, better understanding of the physical structure—electrochemical properties relationship, and standardization of the MFC system to obtain comparable results attributed to the improvements in the nanomaterial.

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CHAPTER 5

Classification of nanomaterials and nanocomposites for anode material Mei Yan, Jixiang Zou and Chongshen Guo School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, China

5.1 Introduction Bioelectrochemical systems (BESs) can convert chemical energy of organic substrates into electricity, generating useful products (e.g. hydrogen, methane, formate, acetate, etc.) or desalinating water. Microbial fuel cell (MFC) is a typical and the most studied BESs, representing over 75% of publications in 2016 (Santoro et al., 2017). Microbial fuel cells are devices that employ microbes as the catalyst to oxidize organic or inorganic matter and generate electricity. Electrons produced by the microorganism from the substrates are transferred to the anode and then flow to the cathode by an external circuit containing a resistor or a load. By conversion, positive current flows from the positive terminal (cathode) to the negative terminal (anode). In this device, the substrates that are oxidized at the anode by the microbes can be replenished continuously or intermittently (Logan et al.,2006).The main feature of BESs is utilization of biological catalytic redox activity of microbes. Anode as the habitat of microbes plays a key role for the overall performances. The electrochemically active bacteria (EAB) on the anode can oxidize organic matters and then transfer electrons to the anode via multiple extracellular electron transfer (EET) pathways such as direct transfer via c-type outer membrane cytochromes, longdistance through conductive bacterial pili or microbial nanowires and mediated transfer by electron shuttles (Wu et al., 2018). Thereafter, the electron acceptors (usually refers to oxygen) in cathode accept the electrons came from the anode, thus generating current. An excellent anode should enrich abundant EAB and facilitate electrons transfer to the external circuit. Traditional carbon materials, such as carbon cloth, carbon fibers, carbon paper, graphite rod, etc., have been widely used as anodes in the microbial fuel cells because of their preferable biocompatibility, electrical conductivity, chemical stability, and low cost. However, low specific surface area and the hydrophobic surfaces inhibit the contact of the microorganisms with the traditional carbon materials. Meanwhile, low accessible active sites of them show that the electrocatalytic activity is not ideal. In order to improve the performances of MFCs, various nanomaterials and nanocomposites have been developed as MFC anodes. Among these anodes, carbonaceous anodes are still the major research areas. In this chapter, carbon nanotubes, graphene and graphene Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00020-6

c 2023 Elsevier Inc. Copyright  All rights reserved.

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oxide, transition metal and transition metal oxide, conductive polymers, etc. modified carbonaceous nanomaterials and their nanocomposites anodes are summarized.

5.2 Carbon-based nanomaterials and nanocomposites 5.2.1 Carbon nanotubes 3D-MWCNT (three-dimensional multiwall carbon nanotube) was coated on carbon cloth as both anode and cathode previously. On the merits of the high specific surface area and electrical conductivity of CNT, the maximum power density of CNT-coated anode was as high as 65 mW·m–2 , which was 250% over the non-CNT-coated anode (26 mW·m–2 ) and 148% over the control one (both anode and cathode without CNT coating, 44 mW·m–2 ). The CNT decoration facilitated biofilm formation on the anode surface, and meanwhile enhanced reduction activity on the cathode surface (Tsai et al., 2009). Sun et al. developed a layer-by-layer assembled CNT-modified carbon paper anode, which significantly decreased the interfacial charge transfer resistance from 1163  to 258  (Sun et al., 2010). Wang et al. certificated that carbon nanotube could transform the irreversible electrochemistry behavior of S. oneidensis’s cell surface cytochromes (L. Peng et al., 2010). In addition, CNT not only enhanced electron transfer (ET) between the biofilm and electrode, but also provided a suitable microbial environment for the sulfate-reducing bacteria immobilization (Bai et al., 2013). Adding CNT powders and Geobacter sulfurreducens together into the anode chamber formed a composite biofilm on the anode, which reduced the startup time and anodic resistance (P. Liang et al., 2011). Moreover, bamboo-like nitrogen doped CNT (bamboo-NCNTs), CNT deposited on porous graphite felts or graphite electrode, MWCNT and nanopowder impregnated on graphite anode, welding assembly of 3D CNTs on a carbon-fiber paper (CNT-CF), have been reported to improve the MFCs performances of power generation (Ci et al., 2012; Mohanakrishna et al., 2012; Q. Yang et al., 2019; X. Zhang et al., 2013; Y. Zhao et al., 2011). Furthermore, CNTs coated graphite felt anode not only generated high electric power but also improved the phenanthrene removal rate in sediment microbial fuel cells (Y. Liang et al., 2020). The morphology of CNT is an important influence factor for the high performance of MFCs. Vertically aligned MWCNT on the silicon chip was applied as the anode in a 75 μL microsized air cathode MFC (Mink & Hussain, 2013). The maximum current density and power density of the MWCNT anode are 880 mA·m–2 and 19.36 mW·M–2 , respectively, being 800% and 600% more than that of the expensive gold anode (156 mA·m–2 and 2.96 mW·M–2 ), 2200% and 1900% much more than that of the inexpensive nickel anode (39 mA·m–2 and 1.12 mW·M–2 ). Ren et al. compared three different CNT-based anode materials (Fig. 5.1) of VACNT (vertically aligned CNT), RACNT (randomly aligned CNT), and SSLbL (spin-spray layer-by-layer) CNT (Ren et al.,2015).A bare gold electrode was used as the control.The sheet resistance of VACNT

Classification of nanomaterials and nanocomposites for anode material

Figure 5.1 Schematic illustration of three types of CNT-based anodes (VACNT, RACNT and SSLbL) and the bare gold anode: the thickness of the biofilm formation by Geobacter sp. on these anodes.

and RACNT are three orders of magnitude higher than that of SSLbL CNT and bare gold electrodes.The biofilm thicknesses of VACNT,RACNT,SSLbL CNT,and bare gold anodes are 3.3 ± 0.2 μm, 6.5 ± 0.5 μm, 9.0 ± 1.0 μm, and 1.8 ± 0.2 μm, respectively. The bare gold anode has the lowest sheet resistance but thinner biofilm than others, indicating that CNTs attract more exoelectrogens. Of the three CNT-based anodes, the low sheet resistance results in high biofilm thickness. In addition, the vertical structure of VACNT inhibits exoelectrogens inside the VACNT forest, while RACNT films with porous features can accommodate exoelectrogens between CNTs. The SSLbL CNT film is composed of horizontally oriented CNTs conjugated with polymers, which gave the thickest biofilm and the highest power density of 3320 ± 40 W·m–3 . VACNT grown on low-cost stainless-steel mesh can be used as both anode and cathode in MFCs (Amade et al., 2016). The structure of CNT-coated substrates can also affect the performances of MFCs. Cui et al. reported CNT-sponge (CNT coated on macroporous sponge) and CNTtextile (CNT coated on porous intertwined textile) electrodes (Xie et al., 2011; Xie et al., 2012). Both of them comprise an open macroporous structure for the efficient mass transport and a microporous CNT layer for the strong anode-biofilm interaction.

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However, compared with CNT-textile, the 3D continuous CNT coating and tunable pore diameters of sponge prevented interruption of contact fibers, resulting in lower internal resistance and higher maximum current density. Directly growing carbon nanotube networks (NanoWeb) on a conductive reticulated vitreous carbon (RVC) scaffold formed a new biocompatible electrode with a hierarchical porous structure (Flexer et al., 2013). The new NanoWeb-RVC electrode not only enhanced the rate of EET, but also ensured an efficient mass transfer. Comparing with sponge and RVC as the support substrates, a facile and scalable one-step chemical vapor deposition synthesis method of 3D CNT sponges without base materials was developed, which can be applied in broad microbial electrochemical systems (Erbay et al., 2015; Zhou et al., 2017). Functionalized carbon nanotubes can further improve their performances in BESs.An amine-terminated ionic liquid functionalized carbon nanotubes (CNT-IL) increase surface positive charges and nitrogen functional groups of the CNTs-based anode. CNT-IL composite improves the microbial adhesion, and resultantly promotes the flavin-mediated ET and the direct ET between exoelectrogen and the anode (Wei et al., 2016). Meanwhile, some works found that other functional groups such as carboxyl, hydroxyl, and nitrophenyl, improved anodes that could also enhance MFCs performances by increasing the bioelectrochemical catalytic performance of electroactive biofilm (M. Fan et al., 2017; Iftimie & Dumitru, 2019; Jiang et al., 2018). 5.2.2 Graphene and graphene oxide Graphene is a single-atom-thick sheet consisting of sp2 hybridized carbon atoms (Hou et al., 2013; Lim et al., 2018). Due to its unique nanostructure, excellent conductivity, large surface area, mechanical strength, and high good electrocatalytic activities, graphene has been widely used in the BES. 3D porous graphene aerogel (GA) with a high capacitance as an anode increased the output voltage and strengthened the stability for a long-time electricity generation (Yu et al., 2018). The MFC equipped with GMS (graphene mixed with polytetrafluoroethylene coated on stainless steel mesh) anode produces a maximum power density of 2668 mW·m–2 , which is 18 times higher than that obtained with SSM (stainless steel mesh) anode. Such a excellent performance can be attributed to the high surface area of graphene, which promoted bacteria adhesion on the anode surface (Y. Zhang et al., 2011). Graphene/carbon cloth improved the power density and energy conversion efficiency, being attributed to the high biocompatibility of graphene (J. Liu et al., 2012). Nevertheless, many researchers have reported that graphene possesses antibacterial activity. Chen et al. studied the antibacterial activity of graphene toward exoelectrogens biofilms on the surfaces of graphene-modified anodes (GMAs) (J. Chen et al., 2015). The results suggested that the antibacterial activity of graphene had no negative effect on the electricity generation of MFCs with GMAs, although it reduced the Shewanella oneidensis MR-1 biofilms viability and electrochemical activity

Classification of nanomaterials and nanocomposites for anode material

during the initial biofilm growth stage. The internal bacteria were affected by the antibacterial of graphene (membrane and oxidative stress) via direct physical contact between graphene and bacteria, however, the outer bacteria were less interfered as the mature biofilm contains extracellular polymeric substances. Meanwhile, the improved charge conductivity of the anode in the presence of graphene remarkably enhanced the electrons transfer, thus generating higher power density and long-term stability. Xiao et al. investigated the effects of graphene morphology on the performance of MFCs via comparing two types of graphene. Crumpled graphene particles with a 3D open porous structure modified carbon cloth anode has a higher surface area than regular graphene sheet modified one, and resultantly enhanced bioreaction/electrochemical kinetics and mass transfer (Xiao et al., 2012). A graphene oxide nanoribbons (GONRs) network was fabricated on carbon paper by an electrophoretic deposition method in the absence of any binder (Y. X. Huang et al., 2011). The GONRs network was prepared with MWCNTs as precursor, retaining the feature of a high aspect ratio. The GONRs can act as nanowires like conductive cellular pili produced by some electrochemically active microbes like Shewanella, and the nanoporous structure in the GONRs network provides a large electrochemical active surface area, thus significantly enhancing the EET process. In situ formation of bacteria/graphene network via bacterial respiration also facilitated the EET kinetics (Y. Yuan et al., 2012). Tang et al. reported a novel method for in situ formation of graphene layers on the surface of a graphite electrode via the electrochemical exfoliation of a graphite plate. The obtained graphene-layer-based graphite plate electrode (GL/GP) produced 1.72-, 1.56- and 1.26-times the maximum power density of the graphite plate (GP), EG/GP (exfoliated gaphene modified GP) and rGO/GP (chemically reduced graphene modified GP) anodes, respectively (Tang et al., 2015). A high-performance macroporous 3D graphene sponge anode can be prepared by freeze-drying under different cooling rate. It provided 3D open porous space for the microbial colonization (W. Chen et al., 2014; L. Huang et al., 2016). Xie et al. reported a similar electrode structure with a lower cost: graphene coatings on sponge with a stainless steel current collector (G-S-SS) (Xie et al., 2012). The electrode with graphene coating can simutaneously enhance EET and catalyze the cathodic oxygen reduction reaction (ORR), upon which graphene-based electrodes enabled MFCs powering a commercial clock (Call et al., 2017). Compared to commonly used graphene nanosheets, positively-charged ionic liquid (1-(3-aminopropyl)-3-methylimidazolium bromide) functionalized graphene nanosheets (IL-GNS) gave better MFC performance by increasing the number of negatively-charged bacterial cells attached on the surface of anode, and accelerating the inoculation of bacterial cells on the anode due to the electrostatic interactions (Fig. 5.2) (C. Zhao et al., 2013). Nitrogen-doped graphene sheets also lead to similar results. The nitrogen doping in GNS generated three binding configurations within the carbon lattice: graphitic N, pyridinic N and pyrrolic N. The higher positive charges are expected to exist in pyrrolic N

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Figure 5.2 Schematic illustration of the interactions between the CP/IL-GNS anode and S. oneidensis cells.

than pyridinic N. The delocalization of positive charges also exhibited higher bacterial affinity through electrostatic interaction. In addition, the increased sp2 configurations in graphitic and pyridinic conformation enhanced the electron transfer via the π conjugation (Kirubaharan et al., 2015). Inducing hydrophilic functional groups or doping hydrophilic graphene oxide into graphene-modified anodes can also enhance bacteria attachment and promote electron transfer from bacteria to the anode surface (Chang et al., 2017; Najafabadi et al., 2016; N. Yang et al., 2016). The maximum power density of the MFC with graphene- cetyltrimethylammonium bromide (G-CTAB) anode with both positively charged and hydrophilic properties was 3.7 times higher than that of the MFC with the graphene anode (Xue et al., 2017). Moreover, MFC with graphene oxide modified anode exhibited good energy generation as well as extraordinary wastewater treatment (Khalid et al., 2018). 5.2.3 Other carbonaceous nanomaterials and nanocomposites Although one-dimensional carbon nanotube and two-dimensional graphene display good conductivity, excellent electrochemical stability and biocompatibility, they still have their shortcomings. For example, CNT is hard to form 3D architecture with enough interspace for bacterial growth, and the reduced graphene oxide (rGO) nanosheets are easy to aggregate due to their strong Van der Waals interactions. MWCNTs@rGO hybrid tailored by inserting MWCNTs as bridges into the rGO skeleton, overcame above shortcomings, in which it formed a 3D hierarchically porous structure with large surface area and excellent biocompatibility for rich bacterial biofilm and high electron transfer rate (L. Zou et al., 2016). Introducing of amino group into the carbon nanotube/reduced graphene oxide composite anode increased the accumulation of exoelectrogen and further facilitated electricity generation (J. Li et al., 2020; Yi et al., 2020). Except carbon nanotube and graphene-modified anode materials, other carbon nanomaterials are also reported. Activated carbon nanofiber nonwoven (ACNFN) has

Classification of nanomaterials and nanocomposites for anode material

an ultra-thin, macroporous interconnected structure that enlarges the interior accessible surface area and decreases mass transport limitations. The maximum current density of a single chamber MFC with ACNFN anode was 2715 A·m–3 , which is even higher than that with CNT-sponge composite anode in the literature (2500 A m–3 ) (Manickam et al., 2013; Xie et al., 2012). Single-wall carbon nanohorns modified a stainless-steel anode are also better than a carbon nanotube modified electrode (J. Yang et al., 2017). One-step carbonization of dandelion seeds with a high nitrogen content afforded selfdoped-nitrogen porous carbon nanosheets, which greatly increased the surface area and reduced the internal resistance (Xing et al.,2020).This is likewise a potential modification material for the high efficient MFC anodes with simple preparation and low cost.

5.3 Transition metal and/or transition metal oxide decorated carbonaceous anode 5.3.1 Transition metal modified carbonaceous anodes or transition metal/carbon nanocomposites In order to improve the anodic performance, various metal coatings have been developed for the carbon-based materials. Gold does not contain functional groups, such as carboxylic acids, alcohols, and quinines, making the gold surface to be not suitable for the bacterial adhesion. However, the high conductivity of gold could enhance the electron transfers from bacteria to the electrode, which is favorable for exoelectrogens to attach to the electrode, and thus accelerate the biofilm growth (Guo et al., 2012; Sun et al., 2010). Compared with Pd nanoparticles, the current density of Au nanoparticles decorated graphite anode is 9-fold higher than that of the Pd anode, being attributed to the better conductivity of Au (4.88 × 107 S·m–1 ) than Pd (9.48 × 106 S·m–1 )(Y. Fan et al., 2011). Alatraktchi et al. certificated that power generation increased with Au nanoparticles density on the electrodes (Alatraktchi et al., 2014). Gold nanoparticles fabricated by microbial methods (microorganisms reducing Au(III) ions) have better biocompatibility and higher catalytic activity as compared with chemical methods (Wu et al., 2018). MFCs with graphene/Au composite modified carbon paper anodes significantly improved the current generation and power density. The synergistic effects of graphene sheets and Au nanoparticles support large surface area and increase the conductivity and biocompatibility as well (Cheng et al., 2018; Zhao et al., 2015). GA has a continuous 3D macroporous structure, which is suitable for bacterial immobilization and efficient mass transport. The decoration of GA with Pt nanoparticles is a free-standing anode with high charge transfer capability and it does not need external substrate (Zhao et al., 2015). Besides these, the inexpensive Fe nanoparticles are a good candidate for modifying electrodes due to the low cost and high conductivity in contrast to the noble metal materials (S. Xu et al., 2012). Modification of carbon-based nanomaterials, for instance reduced graphene oxide and carbon nanotube, on some metal substrates, such as Ni foam,

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Ni mesh and stainless steel mesh, is another good choice for obtaining high efficient anodes (H. Wang et al., 2013; Y. Yang et al., 2018; Y. Zhang et al., 2011). 5.3.2 Transition metal oxide decorated carbonaceous anodes or transition metal oxide/carbon nanocomposites Titanium dioxide stands as one of the most attractive metal oxides and has been widely studied in many fields owing to its advantages of rich abundance, low cost, good biocompatibility, physical and chemical stability. However, the low electrical conductivity of TiO2 significantly restricts its application in MFCs because its low electron transfer efficiency significantly reduces the power output. To overcome this drawback, the combination of TiO2 with many carbonaceous materials, such as carbon nanotube, graphene sheets, and carbon nanofibers, has been carried out and proven to be effective in improving the electron transfer efficiency as well as offering high surface area (C. E. Zhao et al., 2014; Garcia-Gomez et al., 2015; Wen et al., 2013). For example, TiO2 nanowires on the surface of carbon paper acted as the role of pili, promoting long-distance EET in the bacterial network (Jia et al., 2016). TiO2 has been intensively investigated in supercapacitor due to its semiconductive properties.Integrating TiO2 with egg white protein-derived carbon formed a nanostructured capacitive layer, which could provide extra charge localizing sites to facilitate electron transfer, realizing simultaneously convert and store renewable energy in MFCs (Tang et al., 2015). In addition, N-doping could generate new energy levels near the bottom of conduction band of TiO2 due to oxygen vacancy, resulting in more positive flat-band potential and larger transient charge storage capacity (Yin et al., 2017). Besides, the addition of nano-Fe3 O4 to the anode also boosts the transient charge storage capacity in virtue of its unique electric property for transferring electrons between Fe(II) and Fe(III) and higher conductivity than most of other metal oxides (X. Peng et al., 2013). Addition of carbon nanotube or reduced graphene oxide can enhance the electron transportation efficiency of Fe3 O4 (Jia et al., 2016; Ma et al., 2020). Meanwhile Fe3 O4 helps to attach the CNT on anode with the magnetic attraction between Fe3 O4 and the magnet. FeIII oxide has been proven to have a high affinity for c-type outer membranes cytochromes (OmcA and Mtrc) of Shewanella species (dissimilatory iron-reducing bacteria). FeIII oxide can also encourage Shewanella to release flavins, which can be regarded as electron shuttles to mediate indirect electron transfer. Consequently, graphene/Fe3 O4 composites reduced the startup time and improved bacterial affinity (Shewanella oneidensis MR-1), EET efficiency and long-term stability (Song et al., 2017). Carbon cloth modified with MnO2 , Pd and Fe3 O4 nanoparticles were used in MFCs to remove pharmaceutically active compounds (PhACs)(H. Xu et al., 2018). Diclofenac (DCF), ibuprofen (IBF) and carbamazepine (CBZ) were effectively removed in the MFCs with the MnO2 , Fe3 O4 or Pd modified anode at a removal rate of 81.5%–84.0% for CBZ, 48.7%–52.6% for DCF and 18.8%– 20.1% for IBF. High-throughput sequencing results showed that the MnO2 and Fe3 O4

Classification of nanomaterials and nanocomposites for anode material

modified anodes enriched 72.1% of the exoelectrogen Geobacter, while the Pd decorated anode enriched 38.5% of Geobacter and 16.1% of Sphaerochaeta, possibly because Geobacter can directly use MnO2 and Fe3 O4 as electron acceptors. Heteroatom doping and coating with an electron mediator on the anode surface could improve its conductivity, corrosion resistance,and chemical stability.Self-supported N-doped C/Fe3 O4 -nanotube composite arrays grown on carbon cloth (CC@N-C/Fe3 O4 ) could, serve as a bioelectrode in BES with a hierarchical 3D open nanostructure. This nanostructure with many channels makes it easy for microbial adhesion and diffusion of substrates internally within the bioelectrode. The BES equipped with this bioelectrode displayed a current density of 4.11 mA·cm–2 and coulombic efficiency of 89% after 3 months of operation,which were 2 and 3 times higher than that with carbon cloth electrode, respectively (Y. Wang et al., 2021). The MFC with RuO2 nanoparticles-coated carbon felt anode reached the maximum power density of 3.8 W·m–2 , which is 17 times higher than that obtained with the bare anode (Lv et al., 2012). However, no bacteria were attached on the RuO2 surfaces, which may be due to the toxic property of RuO2 . The electricity generation should be attributed to the planktonic cells. The other transition metal oxide like MnO2 has been applied in MFCs because of its excellent properties, such as non-toxicity, low cost, high specific pseudo-capacitance and superior biocompatibility (C. Zhang et al., 2016). Graphene, multiwall carbon nanotubes and halloysite nanotubes can enhance the conductivity, while the dispersion of MnO2 increased the wettability of the composite electrodes (Y. Chen et al., 2016; Fu et al., 2014). FeO nanoparticles modified carbon paper and graphene oxide-zeolite composite also showed the hydrophilic behavior which is helpful to microorganism attachment (Harshiny et al., 2017; Paul et al., 2018). Besides, multiwalled carbon nanotube/SnO2 and reduced graphene oxide/SnO2 nanocomposites have been developed as desirable anode materials for the MFCs (Mehdinia et al., 2014a, 2014b). It has been reported that binary metal oxides hold multiple oxidation states and show better super-capacitive compared to single-component metal oxides. Bimetal oxide (NiWO4 ) and reduced graphene oxide nanocomposites modified anodes were studied in single-chambered microbial fuel cell. In terms of microbial diversity, NiWO4 /rGO has shown the highest abundance of exoelectrogenic bacteria γ -proteobacteria (42.37%), followed by NiWO4 and rGO, while carbon cloth contained the lowest amount of γ -proteobacteria (11.23%). NiWO4 /rGO-based single-chambered microbial fuel cell has shown 6.9-fold higher power output (1458 mW·m–2 ) than plain carbon cloth anode (212 mW·m–2 ) (Geetanjali et al., 2021). 5.3.3 Transition metal and transition metal oxide comodified carbonaceous anodes Incorporation of both transition metal and transition metal oxides can make the best of their respective advantages and make up some shortcomings. Mohamed et al. reported cobalt/cobalt oxide nanoflakes deposited carbonaceous anodes in microbial fuel cells.

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The cobalt nanoflakes were sheathed by a thin layer of cobalt oxide. Cobalt nanoflakes increased the surface area, decreased the electron transfer resistance, and improved the electrochemical properties of the anode surface, while the thin layer of cobalt oxide improved the surface hydrophilicity, overcame the toxicity risk of the pure cobalt, and increased the adhesion of negatively-charged bacteria to surfaces primarily due to its positive charge. Accordingly, in air-cathode microbial fuel cells working by real wastewater, the treated anodes revealed good performance as compared with the untreated electrodes (Mohamed et al., 2017; Jatoi et al., 2021, 2022). Similarly, iron nanoparticles were enveloped with a thin layer of iron oxides. The Fe/Fe2 O3 nanoparticles were deposited on three different carbonaceous anodes: carbon felt, carbon cloth, and graphite to drive MFCs based on the real industrial wastewater. This structure enhanced the wettability of the anode for bacterial adhesion and the degradation rate of organic wastes, meanwhile it decreased the electron transfer resistance. Consequently, the power density was increased by 385%, 170%, and 130% for the carbon felt, carbon cloth, and graphite anodes, respectively. Moreover, the MFC based on the novel electrodes achieved more than 80% removal of organic compounds from wastewaters (Mohamed et al., 2018). Within contrast to the pure culture, the MFC with the metal and metal oxides comodified electrodes powered by mixed biocatalyst cultures enriched various exoelectrogenic bacteria, generating more than two times of power density (Mohamed et al., 2018). Song et al. reported a high-performance anode of Fe3 O4 /Au NCs-3DGF that is a core/satellite structured Fe3 O4 /Au nanocomposite-incorporated 3D macroporous graphene foam. Fe3 O4 nanoparticles (Fe3 O4 NPs) were used as high affiliative anchors to improve bacterial attachment, while gold nanoparticles coated on the Fe3 O4 NPs surfaces served as conductive bridges to enhance EET, and the 3D macroporous graphene foam rendered the inner surface for bacterial adhesion. The synergistic effect of the Fe3 O4 /Au NCs-3DGF anode made MFC displayed 71-fold maximum volumetric power density higher than that of a commercial graphite rod anode (2980 ± 54 vs 41 ± 4 mW·m−2 ) (Song et al., 2020). Wu et al. developed a carbon paper anode modified with CNT/Au/TiO2 nanocomposites. The MFC equipped with this anode delivered a maximum power density of 2.4 mW·m−2 ,which was three times larger than that obtained from the MFC with bare carbon paper (Y. Wu et al., 2013).

5.4 Conductive polymers improved carbonaceous nanocomposites Conductive polymers and their composites have been used to modify anodes to improve the performances of MFCs. Among them, polyaniline (PANI) and polypyrrole (PPy) are the most commonly used materials. Qiao et al. reported a carbon nanotube/polyaniline composite (CNT/PANI) modified anode with improved electrochemical activity and ameliorative biocompatibility (Qiao et al., 2007). The hydrophilicity and positively charged nature of PANI in neutral solutions is ready for the negatively charged bacterial adhesion, while the introduction of high conductive CNT improved the surface area

Classification of nanomaterials and nanocomposites for anode material

and conductivity of PANI in neutral solution (Cui et al., 2015). Wu et al. reported a PANI/CNT composite modified anode via layer-by-layer self-assembly technique and graft polymerization, which realized long-term stability (Wu et al., 2018). Yellappa et al. prepared PANI/CNT composite by in situ oxidative chemical polymerization method. The MFCs with PANI/CNT coated stainless steel mess (SSM-PANI/CNT) anode generated the maximum power density of 48 mW·m−2 and chemical oxygen demand (COD) removal efficiency of 80%, being better than SSM-PANI-anode (38 mW·m–2 , 65%) and SSM-anode (28 mW·m–2 , 58%)(Yellappa et al., 2019). In contrast to the traditional flat anode, 3D microporous graphene can support more internal space for bacteria loading. Yong et al. developed a 3D macroporous and monolithic graphene/PANI anode which facilitated electron transfer and provided multiplexed and highly conductive pathways (Yong et al., 2012). Electrochemically reduced graphene oxide and polyaniline nanofibers were coated on carbon cloth (PANI-ERGNO/CC). MFC with PANIERGNO/CC anode yielded a maximum power density of 1390 mW·m–2 , being three times larger than that with the carbon cloth anode (Hou et al., 2013). PANI networks grown on graphene nanoribbons-coated carbon paper showed much higher performances than those of each individual component as anode (Zhao et al., 2013). In situ polymerization of aniline on graphene oxide and carbonization at 1600 °C, formed a carbon composite with good catalytic activity toward glucose oxidation (Kang et al., 2017). PANI/rGO decorated mesophase pitch-based carbon fiber brush anode promoted power production in MFCs. The PANI improved the surface roughness and surface potential of carbon fibers, thus enhancing the microorganism adhesion and electrogenic performances of MFCs (N. Zhao et al., 2018). PANI/rGO modified carbon cloth anode enhanced the gathering of exoelectrogen Geobacter with the abundance as high as 81.4% and high expression of electrogenesis-related outer-surface octaheme c-type cytochrome OmcZ (Lin et al., 2019). Moreover, brush-like PANI and vertically aligned PANI greatly improved the performances of MFCs because of special morphology (Zhai et al., 2019; W. Zhang et al., 2017). Compared with PANI, PPy has good conductivity even in the neutral pH solution. A PPy-CNTs composite modified anode showed better electrochemical activity than that of plain carbon cloth. A mediatorless MFC with 5 mg·cm–2 PPY-CNTs modified anode exhibited the maximum power density of 228 mW·m–2 (Y. Zou et al., 2008). Yang et al. reported a rGO@PPy hybrid with a macroporous sandwich-like structure. The reduced graphene oxide layer was shielded by the PPy layer,which protected exoelectrogenic bacteria from the antibacterial effect of graphene. The microporous structure with about 100 μm pore size provided more sites for bacteria affinity and growth, as well as facilitated nutrient diffusion (L. Yang et al., 2019). Chitosan (CHIT) polymer can also form 3D macroporous scaffolds with a large pore size for bacterial colonization of internal pores. Moreover, the distribution of CNTs throughout the chitosan scaffold reduced the resistance and cross-linking agent glyoxal strengthened the CHIT-CNT matrix and structural stability. MFC with this anode showed an open circuit voltage of 600 mV and a maximum power

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density of 4.75 W·m–3 at a current density of 16 A·m–3 (Higgins et al., 2011). Poly(3,4ethylenedioxythiophene) (PEDOT), an important derivative of organic conducting polythiophene, possesses better stability than that of PPy and shows high conductivity even in neutral solution.Besides,the PEDOT backbone is positively charged,which make the surface of PEDOT/graphene (G/PEDOT) hybrid form a compact biofilm easily.The G/PEDOT anode generated 15 times maximum power density higher that of carbon paper anode in a H-shaped MFC (873 vs 55 mW·m–2 ) (Y. Wang et al., 2013). (Poly NIsopropylacryl- amide) (PNIPAM) hydrogels filled with highly conductive graphene and carbon nanotubes exhibited the power density and current density up to 434 mW·m–2 and 3606 mA·m–2 , respectively (Kumar et al., 2014). Polydopamine (PDA) as a wellknown biomacromolecule with rich functional groups (amino, catechol, oxime, and carboxylic acid groups) can enhance hydrophilicity of the MFC anode. The synergistic effect of PDA, rGO, and carbon cloth improved the startup speed, power generation, and degradation efficiency of Congo red in microbial fuel cells (H. Liu et al., 2021).

5.5 Other nanocomposites (transition metal/transition metal oxide/polymer/carbon/transition metal carbide, etc.) Qiao et al. presented a nanostructured polyaniline/mesoporous TiO2 composite anode in Escherichia coli microbial fuel cells. The composite with 30 wt% PANI gave the best bio- and electrocatalytic performance, which was possible because the addition of PANI formed a nanostructured network, enhancing the electron transfer rate (Qiao et al., 2008). Modification of carbon paper with vertically oriented TiO2 nanosheets and 20 cycles of PANI (TiO2 -20PANI/CP) exhibited the lowest charge transfer resistance and the largest transient charge storage capacity (Yin et al., 2019). Titanium substoichiometric oxides, known as Magneli phases, have been already used to prepare electrically conducting supports. In situ modified titanium suboxides (Ti4 O7 , TS) with polyaniline and graphene (TSGP) provided a new alternative for the efficient anode. The MFC with TSGP anode produced the maximum power density of 2073 mW·m–2 , which is 12.7 times that with the carbon cloth control (Z. L. Li et al., 2019). A nanocomposite of MWCNTMnO2 /PPy was electrochemically deposited on carbon cloth electrode. This electrode showed good electrical conductivity of 0.1185 S·m–1 and exhibited a maximum power density of 1125.4 mW·m–2 in a mediatorless MFC (Mishra & Jain, 2016). MFC with a multi-walled MnO2 /polypyrrole/MnO2 nanotubes (NT-MPMs) modified carbon cloth anode generated the maximum power density up to 32.7 ± 3 W·cm–3 , which is 1.3 times higher than that of carbon cloth (H. Yuan et al., 2016). α-Fe2 O3 nanorod and chitosan were modified on indium tin oxide by a layer-by-layer assembly technique.MFC with the (Fe2 O3 /CS)4 /ITO anode produced current higher than other modified anodes (Ji et al., 2011). Magnetite (Fe3 O4 ) magnetic nanoparticles (MNP) were modified on Toray carbon paper (TCP), which can be easily separated under an external magnetic

Classification of nanomaterials and nanocomposites for anode material

field. Then the MNP was encapsulated by polypyrrole and polyaniline copolymer, acquiring new properties of increased thermal stability and electrochemical capacitance. The MFC with the fabricated anode achieved high degradation (89%) and decolorization (> 68.5%) of the toxic dye (Sarma et al., 2018). Transition metal molybdenum carbide has been reported to exhibit platinum-like electrocatalytic activity toward the bacteria metabolites. Wang et al. prepared a nano-molybdenum carbide/carbon nanotube (Mo2 C/CNTs) composite modified carbon felt anode. The carbon felt anode with 16.7 wt% Mo Mo2 C/CNTs composite displayed a comparable electrocatalytic activity base on E. coli to that with 20 wt% Pt as anode electrocatalyst. The superior performance of the developed electrode can be ascribed to the bifunctional electrocatalysis of Mo2 C/CNTs. The composite facilitates the formation of biofilm as well as exhibits the electrocatalytic activity toward the oxidation of hydrogen, which is the common metabolite of E. coli (Y. Wang et al., 2014). The hybrid of Mo2 C nanoparticle with graphene also greatly promoted the adhesion of Shewanella putrefaciens cells and delivered a maximum power density of 1697 mW·m–2 , more than 13-fold over the plain carbon cloth anode (L. Zou et al., 2019). Zeng et al. reported a cost-effective synthetic method for molybdenum carbide nanoparticles modified carbonized cotton textile (Mo2 C/CCT) electrode. The electrode offers macroscale porous structure with a high specific surface area (832.17 m2 ·g–1 ) for bacterial adhesion (Zeng et al., 2018). Iron carbide nanoparticles dispersed in porous graphitized carbon (Nano-Fe3 C@PGC) also exhibited an outstanding electrocatalytic activity and high electronic conductivity. MFCs with Nano-Fe3 C@PGC anode yielded a power density of 1856 mW·m−2 (M. Hu et al., 2019). Many dissimilatory metal-reducing bacteria, such as Geobacter species and Shewanella oneidensis, use Fe(III) as the electron acceptors,while elemental sulfur mediates electron-shuttling during bacterial iron reduction. Wang et al. fabricated FeS2 nanoparticles decorated graphene electrode (FeS2 /rGO). The anode not only enhanced enrichment of Geobacter species but also promoted EET, thus generating high power density of 3220 mW·m−2 in the acetate-feeding MFC and a power density of 310 mW·m−2 in effluent from a beer factory wastewater. The MFCs with FeS2 /rGO anode in acetate medium successfully ran an electromagnetical pendulum (R. Wang et al., 2018). Porous Bacterial cellulose (BC) is a promising scaffold for electrode because of its intrinsic 3D network structure, super-hydrophilic nature and ultrafine mechanical properties. In-situ polymerization of aniline on BC fibers provided suitable conductivity. Such BC/PANI anode achieved maximum power density of 117.76 mW·m−2 in a current density of 617 mA·m−2 (Mashkour et al., 2016).

5.6 Other nanomaterials or nanostructure for improving anode performances Electron transfer via mediators as an indirect EET pathway plays an important role in highly efficient electricity output. Sharma et al. reported an MFC with novel electron

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(A)

(B)

(C)

Figure 5.3 (A) and (B) SEM images of bare carbon cloth and TiN nanoarrays in-situ grown on carbon cloth, (C) microbial community structure of the biofilms on the both anodes.

mediators. The novel mediators are nanofluids that were formed by dispersing nanocrystalline platinum anchored CNTs in water. The MFC with nanofluids and CNT-based electrodes showed 2470 mW·m–2 of power density, 6-fold increase compared to graphite electrodes (386 mW·m−2 ) (Sharma et al., 2008). Use of carbon nanoparticles for bacteria immobilization in mediator type MFC is also a good method for improving power density. Yuan et al. reported a carbon cloth anode coated with Proteus vulgaris-containing carbon paste, obtaining 2.5 times improvement of power density comparing with P. vulgaris suspended in bulk solution (Y. Yuan et al., 2009). The improved performance was attributed to the decrement in ohmic, charge transfer and diffusion resistances. Su et al. further reported that the hybrid biofilms formed by TiO2 @TiN nanocomposite with Shewanella loihica PV-4 increased both flavin secretion and cytochrome c expression, thus improving EET efficiency (Su et al., 2020). A programmable biohybrid system composed of DNA polymerization of CNTs and silica nanoparticles containing the exoelectrogenic bacterium Shewanella oneidensis could be applied even beyond microbial biosensors, bioreactors, and fuel cell systems (Y. Hu et al., 2020). Champigneux et al. studied the effect of surface nano/micro-structuring on the early formation of biofilm. They found that the nano-rough surface is beneficial for the early stage of electroactive biofilm formation. The micro-pillars could increase the surface available site for biofilm growth and the optimal micro-roughness should balance the largest surface area and the lowest possible mass transport limitation (Champigneux et al., 2018). Liu et al. further confirmed this result (Liu et al., 2021). They reported that TiN nanoarrays insitu grown on carbon cloth were easily for fast attachment of biofilm. Moreover, the nanoarrays and micrometer sized carbon fibers enriched 97.2% of the well-known model exoelectrogens Geobacter on the anode surface (Fig. 5.3), which is the highest ratio among all of the reports. Experiment results and density functional theory calculation certificate that metallic TiN nanoarrays can obtain electrons easily from c-type cytochromes of Geobacter and timely transfer the electrons to external circuit. This makes Geobacter soli

Classification of nanomaterials and nanocomposites for anode material

express flagella and pili, and chemotactic toward TiN nanoarrays, leading to the high enrichment of Geobacter soli on the anode. More importantly, the structure of biofilm covered nanoarrays appears like bridges that set up by piers, which are not restricted by mass transport with the flowing of anolyte.

5.7 Future challenge of nanomaterial/nanocomposite material The reports of nanomaterias/nanocomposite materials discussed above display that the use of nanostructured materials can significantly improve the current density and power density of MFCs, which is attributed to the increasing active surface area and fast electron transfer rate. Good biocompatibility and excellent conductivity are two key factors that affect the anode performances. Except this, reasonable structural design can enhance bacterial attachment and affect microbial community distribution. Hydrophilic, positively charged and rough surfaces facilitate negatively charged bacterial enrichment. As compared with two-dimensional materials, the three-dimensional porous structure can provide more attachment sites for bacterial attachment, and is conducive to the longdistance electrons transmission. Nevertheless, micrometer-scale macroporous structures are necessary to avoid biological clogging. Thus nanomaterials/nanocomposites that satisfy all the above conditions are good candidate for anode. However, there are still no nanomaterials/nanocomposites available for anodes in large-scale practical application. In view of this, low cost, high stability, and easy preparation are important parameters.

5.8 Conclusions Carbon nanotubes and graphene with high specific surface area and electrical conductivity, possess high electrocatalytic activity. Their antibacterial activity only affects the initial biofilm attachment but does not decrease the electricity generation of MFCs. One-dimensional CNT is beneficial for electrons transfer along its long axis direction, but lack of multidirectional connections because it cannot form a three-dimensional frame structure. Two-dimensional graphene layered structure is easy to aggregate, and the three-dimensional GA can overcome this shortcoming, but pore size should be carefully optimized to avoid the biological blockage of the pore. In addition, large-scale and low-cost graphene and CNTs are still a challenge in the near future. Except noble metals, transition metals and their oxides are abundant and low-cost. Transition metal oxides with supercapacitor property boost the transient charge storage capacity, but their semiconductive property results in low electron transfer rate. The electrical conductivity of precious metals is undoubtedly ideal. However, the high price precludes their use as substitutes. Other transition metals with low cost seem like a better choice, but their biocompatibility and corrosion resistance need to be improved. Conductive polymers

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can change carbonaceous anodic surface from hydrophobic to hydrophilic and positively charged nature increases negatively charged bacterial attachment. In short, the above diverse nanomaterials have their own advantages and shortcomings, those nanocomposites can combine the advantages and overcome the corresponding shortcomings, eventually generating unexpected results but meanwhile increasing the complexity. More efficient anode materials should be developed. The nanostructure of anode is also important and reasonable structure design can accelerate the initial formation of biofilm and shorten the battery start-up time. The interaction between bacteria and anode material need to be further studied, so as to point out the direction for targeted screening of electroactive bacteria. More electroactive bacteria that can decompose complicated organic compositions such as heterocyclic aromatic in wastewater should be screened to purify sewage and generate electricity or produce other useful products.

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Classification of nanomaterials and nanocomposites for anode material

Zhang, W., Xie, B., Yang, L., Liang, D., Zhu, Y., & Liu, H. (2017). Brush-like polyaniline nanoarray modified anode for improvement of power output in microbial fuel cell. Bioresource Technology, 233, 291–295. https://doi.org/10.1016/j.biortech.2017.02.124. Zhang, X., Epifanio, M., & Marsili, E. (2013). Electrochemical characteristics of Shewanella loihica on carbon nanotubes-modified graphite surfaces. Electrochimica Acta, 102, 252–258. https://doi.org/ 10.1016/j.electacta.2013.04.039. Zhang, Y., Mo, G., Li, X., Zhang, W., Zhang, J., Ye, J., Huang, X., & Yu, C. (2011). A graphene modified anode to improve the performance of microbial fuel cells. Journal of Power Sources, 196(13), 5402–5407. https://doi.org/10.1016/j.jpowsour.2011.02.067. Zhao, C., Gai, P., Liu, C., Wang, X., Xu, H., Zhang, J., & Zhu, J. J. (2013). Polyaniline networks grown on graphene nanoribbons-coated carbon paper with a synergistic effect for high-performance microbial fuel cells. Journal of Materials Chemistry A, 1(40), 12587–12594. https://doi.org/10.1039/c3ta12947k. Zhao, C., Wang, Y., Shi, F., Zhang, J., & Zhu, J. J. (2013). High biocurrent generation in shewanellainoculated microbial fuel cells using ionic liquid functionalized graphene nanosheets as an anode. Chemical Communications, 49(59), 6668–6670. https://doi.org/10.1039/c3cc42068j. Zhao, C. E., Gai, P., Song, R., Zhang, J., & Zhu, J. J. (2015). Graphene/Au composites as an anode modifier for improving electricity generation in Shewanella-inoculated microbial fuel cells. Analytical Methods, 7(11), 4640–4644. https://doi.org/10.1039/c5ay00976f. Zhao, C. E., Wang, W. J., Sun, D., Wang, X., Zhang, J. R., & Zhu, J. J. (2014). Nanostructured graphene/TiO2 hybrids as high-performance anodes for microbial fuel cells. Chemistry - A European Journal, 20(23), 7091–7097. https://doi.org/10.1002/chem.201400272. Zhao, N., Ma, Z., Song, H., Wang, D., & Xie, Y. (2018). Polyaniline/reduced graphene oxide-modified carbon fiber brush anode for high-performance microbial fuel cells. International Journal of Hydrogen Energy, 43(37), 17867–17872. https://doi.org/10.1016/j.ijhydene.2018.08.007. Zhao, S. L., Li, Y. C., Yin, H. J., Liu, Z. Z., Luan, E. X., Zhao, F., Tang, Z. Y., & Liu, S. Q. (2015). Threedimensional graphene/Pt nanoparticle composites as freestanding anode for enhancing performance of microbial fuel cells. Science Advances, 1, e1500372. https://doi.org/10.1126/sciadv.1500372. Zhao, Y., Watanabe, K., & Hashimoto, K. (2011). Hierarchical micro/nano structures of carbon composites as anodes for microbial fuel cells. Physical Chemistry Chemical Physics, 13(33), 15016–15021. https://doi.org/10.1039/c1cp21813a. Zhou, Y., Hou, J., Chen, W., & Liu, Z. (2017). Carbon nanotube sponge 3D anodes for urine-powered microbial fuel cell. Energy Sources Part A: Recovery Utilization and Environmental Effects, 39(14), 1543–1547. https://doi.org/10.1080/15567036.2017.1339220. Zou, L., Huang, Y., Wu, X., & Long, Z. e. (2019). Synergistically promoting microbial biofilm growth and interfacial bioelectrocatalysis by molybdenum carbide nanoparticles functionalized graphene anode for bioelectricity production. Journal of Power Sources, 413, 174–181. https://doi.org/ 10.1016/j.jpowsour.2018.12.041. Zou, L., Qiao, Y., Wu, X. S., & Li, C. M. (2016). Tailoring hierarchically porous graphene architecture by carbon nanotube to accelerate extracellular electron transfer of anodic biofilm in microbial fuel cells. Journal of Power Sources, 328, 143–150. https://doi.org/10.1016/j.jpowsour.2016.08.009. Zou, Y., Xiang, C., Yang, L., Sun, L. X., Xu, F., & Cao, Z. (2008). A mediatorless microbial fuel cell using polypyrrole coated carbon nanotubes composite as anode material.International Journal of Hydrogen Energy, 33(18), 4856–4862. https://doi.org/10.1016/j.ijhydene.2008.06.061.

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Properties of nanomaterials for microbial fuel cell application Manju Venkatesan a, Vicente Compañ b, Annamalai Senthil Kumar a,c, Jorge Escorihuela d, Chiranjeevi Srinivasa Rao Vusa e and Sathish-Kumar Kamaraj f

a Nano Bioelectrochemistry Research Lab, Department of Chemistry, School of Advanced Sciences, Vellore Institute of Technology University, Vellore, Tamil Nadu, India b Escuela Técnica Superior de Ingenieros Industriales (ETSII), Departamento de Termodinámica Aplicada, Universitat Politécnica de Valencia, Valencia, Spain c Carbon dioxide and Green Technology Research Centre, Vellore Institute of Technology University, Vellore, Tamil Nadu, India d Universitat de Valencia, Departamento de Química Orgánica, Valencia, Spain e Chemical Sensors Laboratory, Department of Chemical Engineering, Indian Institute of Technology, Kanpur, India f Instituto Politécnico Nacional (IPN)-Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Altamira, Mexico

6.1 Bioelectrochemical energy generation systems principle and types Bioelectrochemical systems (BESs) use living organisms and/or enzymes for the conversion of chemical energy to electrical energy vice versa in energy conversion systems (Barelli et al., 2021; Berk & Canfield, 1964; Gunaseelan et al., 2020; Jatoi et al., 2021, 2022). The bioelectrochemical energy conversion systems link biochemical pathways with electrochemical reactions. There are broadly two types of bioelectrochemical systems based on biocatalyst used in the system. They are (1) microbial fuel cells (MFCs) and (2) enzymatic fuel cells (EFCs) consisting of microorganisms and enzymes as biocatalysts. The operating principle of MFCs and EFCs are shown in Figs. 6.1A and B, respectively. The microorganisms such as bacteria or yeast present on the electrode surface transfer the metabolically generated electrons to the anode surface in MFC. The transferred electrons are then transported to the cathode surface via an external electrical connection generating electricity. The electrons that reach the cathode surface react with protons and an electron acceptor (O2 ) to produce water, as shown in Fig. 6.1A. The EFCs use specific isolated enzymes to convert chemical energy to electrical energy based on substratespecific reactions. For example, glucose oxidase enzyme-modified electrode as bioanode and laccase enzyme-modified electrode as biocathode were used to produce electricity by converting glucose to gluconolactone, as shown in Fig. 6.1B. The wonder of the MFCs is using microorganisms that are able to degrade a wide range of organic compounds to produce electricity. Also, microorganisms have the ability to degrade organic matter into CO2 and water. Hence, MFCs are used for various applications such as wastewater treatment (Boas et al., 2022; Guo et al., 2020; He, 2017; Leu et al., 2016; Boas et al., 2022; Guo et al., 2020; He, 2017; Leu et al., 2016), bioenergy production (Kumar et al., 2015), Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00007-3

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Figure 6.1 The operating principle of (A) MFCs and (B) EFCs.

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Figure 6.2 The operating principle of MDCs.

biosensors (Cho et al., 2020; Cui & Lai, 2019), battery (Xie et al., 2013), powering of mobile phone (I. A. Ieropoulos et al., 2013) and self-powered biosensors. The MFC was first reported by Potter MC in 1991 (Potter, 1911), he used certain species of bacteria to disintegrate the organic matter. He found the disintegration of organic compounds by microorganisms with the liberation of electrical energy. MFC systems allow reaching power outputs in the order of 4 W m−2 and its working principle boosted the interest of several researchers to produce electrical energy directly from the biological degradation of wastes. Currently, most of the progress achieved to use MFC for this purpose was at the lab scale (Greenman et al., 2021; Hiegemann et al., 2019). Scaling up this technology has recently gotten important and comes under intensive investigation. The first attempt to scale up is to increase the volume (Mehravanfar et al., 2019). Presently, the stacked systems were developed to increase the power output in which the single MFC units are connected either series or parallel or a combination of both (I. A. Ieropoulos et al., 2013; I. Ieropoulos et al., 2008; Li et al., 2008; Y. Gajda et al., 2020; Yuan et al., 2013). Different studies in stacked systems revealed that the combination of parallel–series configuration delivers high power outputs with more extended time than other configurations. However, a recent study shows that the series configuration is cost-effective to achieve the highest power output and chemical oxygen demand removal under the lowest operating costs (Mehravanfar et al., 2019). So far, the largest tacked MFC (Liang et al., 2018) reported is modularized MFC with 50 stacked MFC modules, each with a volume of 20 L, while the smallest MFC reported is microfluidic MFC (1.5 μL anode chamber and 4 μL cathode chamber) (Qian et al., 2011). The scale and size of the bioelectrode and its chamber are likely to have the most significant effect on the formation and behavior of the biofilms that colonize on electrode surfaces (Greenman et al., 2021) and MFC power output (Fig. 6.2).

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There are other bioelectrochemical systems with a small change in the principle of MFCs in literature, as shown in Figs. 6.2 and 6.3. They are (1) microbial electrolysis cells (MECs), (2) microbial electrosynthesis (MESs), and (3) microbial desalination cells (MDCs). The MDCs produce electricity (G0).The MDCs are self-powered desalination systems and the energy required to perform desalination is produced by the metabolism of microorganisms when wastewater is degraded (Ramírez-Moreno et al., 2019). An additional chamber must be integrated between the anodic and cathodic chambers to perform desalination using MFC. The chambers separated using an anion and a cation exchange membrane, as shown in Fig. 6.2. The MECs are also bioelectrochemical hydrogen production systems (Yang et al., 2021) that utilize bioanode and cathodic process as shown in Fig. 6.3A. The principle of MESs is similar to MECs but the MESs uses biocathode to reduce CO2 to value-added chemicals by consuming the H2 (Rabaey & Rozendal, 2010), as shown in Fig. 6.3B. The electrical energy is supplied to both MECs and MESs to produce fuel by microbial electrolysis. Several MFC based systems were developed for electricity production, wastewater treatment, fuel synthesis, desalinization, bioremediation, metal recovery, self-powered biosensor, etc. due to their versatility. These MFC-based BES systems are further categorized into five types based on the feed stoke and components (1) conventional MFCs, (2) plant MFCs, (3) sediment MFCs, (4) osmotic MFCs, and (5) Photobioelectrochemical fuel cell (PBFC) (Apollon et al., 2022; Berchmans, 2018; Gunaseelan et al., 2020). The performance of the MFCs mainly depends on their components such as microorganisms, biofilm, electrodes, and separators (membrane). In this book chapter, we have discussed the main components of MFC and their properties, different nanodimentional materials used for MFC, and their intrinsic properties to use as anode, cathode, and membrane. Finally, we discussed the mechanisms and interactions of nanomaterials with biofilm followed by the advancement expected to the real field in the future.

6.2 Components of MFC Various designs and configurations of MFCs were reported in the literature and they can be gathered into different main categories (1) single-chamber MFC (SCMFC), (2) dual-chamber MFC, (DCMFC), (3) stacked MFC, (4) Up-flow MFC, and (5) microfluidic MFC (Boas et al., 2022; Krishnaraj et al., 2015; Prasad et al., 2006). In addition, to design various operating conditions and sizes are reported in the literature to improve the performance of the MFC. The main components of different MFCs designs and configurations are microorganisms, biofilm, electrodes (anode and cathode), membranes,electrolytes,anode and cathode chamber and their volume.Among them,the microorganisms, biofilm, electrodes, and membranes are vital components and playing a pivotal role in the power output.

Figure 6.3 The operating principle of (A) MECs and (B) ME.

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6.3 Properties of vital components and their intrinsic factors to enhance electricity output 6.3.1 Microorganisms The trillions of microorganisms are found on the Earth according to a recent report (Locey & Lennon, 2016). The potential of microorganisms to use in various MFC technology depends on it is external electron machinery which helps in transferring the electrons to an electrode surface and vice versa. The molecular machinery means the transfer/acceptance of electrons between the microorganisms and electrode via the proteins, biomolecules, and/or genes which are exist in the inner and outer membrane of the microorganisms. These microorganisms can be categorized into three domains bacteria, Archaea and Eukarya based on their cellular life form. In addition to that it can be further classified into two main categories in MFC (1) exoelectrogens and (2) electrotrophs. The exoelectrogens are the microorganism that transfer the electron to an anode from the substrate while the electrotrophs needs electrons as feed to grow, hence accepting the electrons (Logan, 2019). The properties of microorganism to use for MFC is given in Table 6.1. The most studied microorganisms are bacteria which exist in a wide range of habitats. But, all the bacteria are not electrochemically active and the electrochemically active bacteria (exoelectrogens) at the anode are reviewed and given in the literature (Z. Jiang et al., 2018; Kumar et al., 2015; Greenman et al., 2021). Exoelectrogens are Ochrobactrum anthropic, Rhodopseudomonas palustris, Rhodobacter sphaeroides, Acidiphilium cryptum, Rhodoferax ferrireducens, Shewanella putrefactions, Shewanella oneidensis, Geobacter metallireducens, Geobacter sulfurreducens, Clostridium butyricum, Clostridium beijerinckii, and Arcobacter butzleri. The most studied bacteria in MFC are Geobacter sulfurreducens and Shewanella oneidensis. The Geobacter sulfurreducens is a rod-shaped gram-negative bacteria known as electricigens that can oxidize many carbon molecules. These bacteria were colonized on the surface of the graphite electrode and used as bioanode to convert acetate to CO2 and produce electricity. The electricity produced on this Geobacter sulfurreducensbased MFC is 65 mA m−2 on the electrode surface and 163 to 1,143 mA m−2 in poised-potential mode (Bond & Lovley, 2003). The Shewanella oneidensis is another bacteria that can reduce metal ions and carbon molecules. It is a facultative bacterium, capable of surviving and proliferating in both aerobic and anaerobic conditions. Hence, the S. oneidensis is used to reduce and adsorb heavy metals in wastewater. It also has the ability to produce electricity by consuming acetate and lactate (Bond & Lovley, 2003). The beauty of bacteria is the formation of biofilm on the electrode surface which is very important in MFC based technology. These biofilms are either transfer or accept the electrons and plays a pivotal role in the performance of the MFC technology.

Properties of nanomaterials for microbial fuel cell application

Table 6.1 The general and specific properties of components microorganism, electrodes and membrane. Components General properties Specific property to use in MFC

Microorganism (i) Easy to grow (preferable-prolific (i) It should have electron machinery, growth), (ii) ability to form a (ii) it should be either biofilm, exoelectrogens or electrotrophs, (iii) (iii) thickness can be controlled it should have the ability to form by changing the flow and growth stable conductive biofilms on the rate electrode surface, (iv) the formed biofilm should transfer or accepts the electrons through the electrode surface, (v) high population of active cells in the biofilm, (vi) biofilm with mother layer for long term use Bioelectrode (i) High electrical conductivity, (i) Biocompatible, (ii) it should have (Bioanode) (ii) high surface to volume, an affinity towards microbes to form (iii) biocompatible, (iv) low a biofilm, (iii) it should accelerate (production) cost, (v) easily the biocatalysis of microorganism available, (vi) chemically inert, (iv) porous and perfusable, (vii) durable, (viii) firm or flexible (v) bioanode contains solid, (ix) antifouling exoelectrogens which transfer the metabolically generated electrons to the electrode surface, (vi) no leakage through the electrodes because of capillary action, (vii) act as only end terminal electron acceptor, (viii) it should have strong interaction with biofilm, (ix) it should be stable in the mixture of microbial inoculum (Bio)cathode It should have general properties An electrode with properties same as bioanode including, (i) reduction of electron acceptor (oxygen), (ii) gas (O2 ) adsorption property, and (iii) antibacterial activity can be used as cathode in MFC. The biocathode can also be used whose specific properties are same as bioanode but uses electrotrophs instead of exoelectrogens Membrane (i) Mechanical and chemical (i) High cation exchange particularly stability, (ii) no electrical proton, (ii) inhibit the biofilm conductivity, (iii) high cation formation to avoid clogging and/or anion exchange capacity, (antifouling and antibacterial (iv) water-uptake capacity, activity), (iii) impermeable to O2 , (v) high ionic conductivity, (iv) reduce the substrate leakage (vi) impermeable to gases from one chamber to another, that is, it should have low cross-over

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6.3.2 Biofilm The biofilm is structured microbial communities formed naturally by microorganisms on the living or non-living surfaces (Costerton et al., 1978). The biofilm is known over 300 years ago, and “early work” shows that the microbes can adhere and grow on wet surfaces when exposed to nutrients (Henrici, 1933; Zobell & Allen, 1935). A highly diverse range of bacteria has been found to form biofilm on the living and non-living surfaces (Greenman et al., 2021). Different forces govern the attachment of cells to the solid surface (1) Lifshitz–van der Waals (LW) or electrodynamic interactions, (2) Lewis acid-base (AB), and (3) electrical double layer (EL) which are clearly explained by the extended Derjaguin, Landau, Verwey, Overbeek theory (XDLVO) (van Oss, 2008). The variables which are thought to be important in cell attachment and biofilm formation are (Greenman et al., 2021; Donlan, 2002; Hodgson et al., 1995): r Cell surface properties- flagella, pili, fimbriae, capsules, and/or slime formation. r Inoculum- single or mixed inoculum. r Cell culture medium- type, concentration, pH, redox, transfer of metal ion, O , 2 inhibitory compounds, and temperature. r Physicochemical properties of bulk fluid stoke. r Types of the substrate (electrode in MFC)- its texture, roughness, and hydrophobicity. r Conditioning layer- Conditioning the surface with acids, proteins, polymers to increase the adsorption of cells. The performance of MFC depends on the thickness of the biofilm, the population on electrode, substrate, and electron transferability of the biofilm to deliver high power densities. The biofilm thickness can be on a micrometer scale of 30–50 μM. If it is higher than 50 μM, there is a loss of energy due to bacteria’s internal transfer. Geobacter sulfurreducens biofilms with a thickness of ∼20 μm and 45 μm were formed and found that the biofilm with a thickness of ∼20 μm was more electrochemically active than thick 45 μm biofilmsun (Sun et al., 2016). This might be due to the diffusion limiting of thick biofilm. Also, the population of cells (accumulation of cells) on the electrode affects the performance of the MFC. The thick mature biofilm (>50 μm) of Shewanella oneidensis strain MR-1 with low populations was formed by applying the external load of 1M while the thinnest biofilm < 5 μm was formed with the heaviest load of 100 (McLean et al., 2010). The thinner biofilms showed higher power output than the thick biofilms (Pasternak et al., 2018). The above studies prove that the active cells are more important than the thickness of the biofilm. In addition, the selection of inoculum is also important. Both mixed and pure cultures (inoculum) have been used to produce bioelectricity in MFC technology. There are debatable reports in the literature regarding which inoculum is preferred to produce high energy. Few studies show that mixed cultures produce high power density than pure while other few studies show that pure cultures can also produce high power density

Properties of nanomaterials for microbial fuel cell application

(Greenman et al., 2021; Kumar et al., 2015; Rengasamy & Berchmans, 2012). For example, the continuous-flow miniStack MFC was developed using carbon cloth as electrodes, acetate as fed, and G. sulfurreducens as exoelectrogens to produce electricity (Nevin et al., 2008).They used pure and mixed cultures (sewage sludge inoculum) to develop bioanode. They found that the pure culture delivered a maximum power density of 1900 mW m−2 , approximately 21% higher than the mixed cultures. Another study with a selective inoculum of mixed culture of known bacteria, e.g. Pseudomonas aeruginosa, Azospira oryzae, Acetobacter peroxydans and Solimonas variicoloris have been shown to produce 100% higher power density than the unknown inoculum (anaerobic sludge) (Inoue et al., 2011). Hence, the selecting suitable bacterial inoculum (pure or mixed culture) with a suitable substrate is preferred to extract more energy for the current generation. For example, G. sulfurreducens can reduce acetate with ∼100% electron recovery to generate electricity (Richter et al., 2008). In addition to the above properties for biofilm formation, the detachment behavior of the biofilm is another important crucial key process in continuous MFC performance and biofilm stability. There are different mechanisms in the biofilm detachment process, such as (1) erosion and shearing (continuous removal of small clumpy portions of the biofilm), (2) sloughing (rapid and massive removal), (3) abrasion, and (4) natural process of shedding. The MFC with perfusion electrodes is suited for continuous operation because the biofilms form on highly porous material, do not limit the growth, and can be controlled by the flow rate of the medium (Ledezma et al., 2012). Also, there is a monolayer of microorganisms on the electrode called the mother layer, a core layer that produces new daughter cells (Helmstetter & Cummings, 1963; You et al., 2015). The new daughter cells are formed, shed, and washed away by the laminar flow of the bulk medium at high liquid shear rates. Again, new daughter cells can be formed, and biofilm remains as a constant number of cells with time, and with constant flow, the biofilm quickly reaches a dynamic steady state. The biofilm with the mother layer can be stable for a long time (years). So, MFC with such biofilms can run for a long time. 6.3.3 Electrode An ideal electrode in electrochemical applications should have high electrical conductivity, high active sites, low chemical reactivity, and physical stability. In addition, an electrode with high surface area, high abundance and low cost gets priority in electrochemical applications. The carbon (graphite, rod, plate, foam, brush, carbon paper, carbon cloth, etc.) and metals (Au, Ag, Cu, Ti, etc.) are used as electrodes in electrochemical applications. But, the electrodes must have a high affinity toward microorganism to attach on the surface to form stable biofilm for MFC applications. The microbial attachment on carbon materials is higher than the most metals except for a few, such as stainless steel (SS) and titanium (Ti). Most of those metals are prone to corrosion. The

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carbon-based electrode materials are proven to be economically viable, have an affinity toward microorganisms (biocompatible), are chemically inert, and expedite the electron transfer in microorganisms. The electrode materials in MFC can be divided into (bio)anode and (bio)cathode. The properties of electrodes used in MFC are given in Table 6.1. Many conventional carbon-based electrodes graphite, carbon paper, carbon felt, carbon cloth, etc. have been used in MFC. However, each electrode has their own advantages and disadvantages. For example, graphite electrodes have a low surface area. Carbon paper is very thin, fragile, and lack microscopic and nanoscopic structures which play a vital role in bacterial attachment and extracellular electron transfer (EET). The disadvantage of carbon cloth and felt is the leakage of liquid through the carbon cloth and felt due to capillary action when exposed to long time. In addition, conventional flat electrodes with small pore sizes are in accessible for bacteria to the interior of these flat electrodes. Different forms of carbon materials are prepared (shown in Fig. 6.4A) to increase the surface area and microbial attachment. Each form has its own advantages and disadvantages (Yaqoob et al., 2020). The brushes are composed of graphite bristles/fibers and linked to a conductive metal (SS and Ti) provides higher surface area for microbe attachment than other carbon materials (Logan et al., 2006). However, one should explore the optimum diameter of the brush to reduce the distance between anode and cathode to produce higher power output and to avoid ohmic losses. It’s easy to reduce the distance between flat electrodes than 3D electrodes to avoid ohmic losses but results in lower power densities (Yaqoob et al., 2020). Carbon veil electrodes are proven to be the best electrodes (Gajda et al., 2020), particularly on a large scale due to (1) sufficient microchannels, (which allow permeation of bacteria and nutrients through advective transport) and (2) low resistance due to thread continuity. Cathodes used in MFC are of two types (1) electrode with microorganisms, and (2) electrode without microorganisms (Cai et al., 2020; Mashkour et al., 2021a). A biocathode should have electrotrophs instead of exoelectrogens. However, both type of cathodes should reduce electron acceptors (widely O2 is used). Hence, both should have gas adsorption properties in addition to oxygen reduction reaction activity. Further, the electrodes are modified with various materials such as bulk carbon, carbon nanomaterials, metals, metal oxides, metal nanoparticles, metal carbides, conducting polymers, composites (shown in Fig. 6.4B) (1) to enhance the biofilm growth, (2) to accelerate the biocatalysis of the biofilm, (3) to improve the stability, and (4) power output in MFC applications (Cai et al., 2020; Mashkour et al., 2021a; Narayanasamy & Jayaprakash, 2020; Yaqoob et al., 2020; Yaqoob et al., 2020). The materials in nanodimention are getting importance due to its novel properties such as high surface-to-volume ratio, significant number of activation sites, providing high surface for biofilm leads to enhancement in activity. Moreover, the ORR rate and gas adsorption can be improved by the nanoscale

Properties of nanomaterials for microbial fuel cell application

Figure 6.4 (A) Different forms of electrodes and (B) different types of materials are used to enhance the performance of an electrode.

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materials. The antibacterial activity of the metal and metal-oxides NPs is a key factor for improving durability of the air-cathode. The different types of nanomaterials used for MFC applications are discussed in Section 6.4.

6.3.4 Electron transport mechanisms between microorganisms and an electrode After developing a suitable biofilm on the suitable electrode (bioelectrode), the microorganism on the bioelectrode should transfer the metabolically generated electrons from their outer cell membrane to the electrode to produce electricity. There are different types of electron transfer mechanisms (1) direct electron transfer (DET) and (2) mediated electron transfer (MET). The direct electron transfer is further categorized into (1) DET via outer-surface c-type cytochromes (OM c-Cyts), (2) DET by long-range electron transfer via microbial nanowires, and (3) DET facilitated by coated conducting polymer, as shown in Fig. 6.5. In MET, the redox shuttles are used for electron transfer, as shown in Fig. 6.5D. The redox shuttles can be of two types (1) natural endogenous mediator and (2) synthetic exogenous mediator. The natural endogenous mediator is in-situ formed the flavins (FMN, FAD, and riboflavin) secreted by the microorganisms and/or the complex between OM c-Cyts and secreted flavin (Y. Yang et al., 2015; Kumar et al., 2016). For example, G. sulfurreducens secretes flavin molecules such as riboflavin (RF) in the single-layer biofilms. The secreted RF combines with the OM c-Cyts and forms a complex that further transfers the electron to the electrode surface (Kumar et al., 2016). In S. oneidensis, the complex of cytochromes–flavins mediates an electron transfer mechanism. For example, FMN acts as a cofactor for cytochrome MtrC and RF for cytochrome OmcA (Y. Yang et al., 2015). The synthetic exogenous mediators are known for use in MFCs such as methylene blue, thionin, neutral red, 2, 6-dichlorophenol, indophenol, safranine-O, phenothiazine, and benzyl viologen (Greenman et al., 2021). Till now, only a few genera and species have been shown to follow direct conductive mechanisms including Shewanella, Geobacter, Rhodoferax ferrireducens, Pelotomaculum thermopropionicum,Geothrix,and Geoalkalibacter (Greenman et al.,2021;Kumar et al.,2018).These bacteria are attached to the electrode surface through cytochrome C or nanowires/pili and transfer the electron directly. The G. sulfurreducens follows the DET mechanism for extracellular electron transport in multilayered biofilms (Kumar et al., 2016). The direct electron transport mechanism from cells to the anode in Geobacter has been provided by lovely (Lovley, 2008). The NAD+ is reduced to NADH within the cell when the electron-rich substrate (acetate or lactate) is oxidized. The electron received by cell is transferred to the electrode via dehydrogenase and cytochrome system consisting of quinone/menaquinone pool, periplasmic proteins MacA, PpcA, and outer membrane proteins, OmcE and OmcS. A series of redox reactions spanning the inner cytoplasmic

(A)

(B)

(C)

(D)

Properties of nanomaterials for microbial fuel cell application

Figure 6.5 From Frattini, D., Karunakaran, G., Cho, E.-B. & Kwon, Y. (2021). Sustainable Syntheses and Sources of Nanomaterials for Microbial Fuel/Electrolysis Cell Applicationss: An Overview of Recent Progress. Processes, 9 (7). Schematic representation of different electron transfer mechanisms on anode (A) DET by outer membrane c-Cytochrome (OM c-Cyts), (B) DET by conductive (type IV) pili and/or nanowires (NW), (C) DET facilitated by coated polymer, and (D) MET by externally added synthetic mediator and/or self-secreted natural mediator (flavins). Adopted from the literature.

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membrane across the periplasmic space occurs until the electron is conducted across the outer membrane to the anode electrode via the outer membrane cytochromes OmcE and OmcS. The transport of electron within multilayers of cells may also occur via bacterial nanowires i.e. the dense part of conductive microorganisms. 6.3.5 Membranes The performance of MFC depends mainly on each electron that is produced at the anode and an equivalent proton which must be transported to the cathode through the electrolyte to sustain the current. Proton exchange membrane (PEM) is one of the most critical components in the configuration and operation of the MFC as it physically divides the anode from the cathode and allows a flow of protons to pass through it to the cathode. Also, the PEM restricts oxygen flux to the anode chamber to prevent a decrease in coulombic efficiency (X.-Z. Rismani-Yazdi et al., 2011; Yuan et al., 2011). The primary purposes of a membrane in an MFC are as follows (Apollon et al., 2022; Peighambardoust et al., 2010): r Separate the anodic chamber from the cathodic one. r Minimize back-diffusion of oxygen to the anodic chamber and reduce the substrate flux from the anode to the cathode. r Increase the coulombic efficiency (CE), reducing the flux of the oxygen from the cathode chamber to the solution in the anode chamber. r Ensure an efficient and sustainable operation over time. It is imperative to ensure that the membrane’s resistance is small enough to reduce ohmic losses in proton transport through it. Ideally, the materials used in an MFC as separators should inhibit the transfer of materials such as the substrate and be impermeable to gases such as H2 , N2, or electron acceptors to the anode (especially O2 ), while facilitating the passage of protons to the cathode with high efficiency. The typical commercial membranes, including Ultrex and Nafion, are generally used in MFC (Kamaraj et al., 2015; Mashkour et al., 2021a). These membranes have disadvantages, including the high cost of the membrane (1500 USD m−2 for Nafion 115),oxygen leakage from cathode to anode, water permeability, substrate loss, transport crossover of cations other than protons like Na+ , K+ , NH4 + , Ca2+ , and Mg2+ contained in the growth medium, where they are typically present in concentrations about 100 times as high as that of protons (Rismani-Yazdi et al., 2011; Beauger et al., 2013; Rozendal et al., 2006). Subsequently, these cations combine with the sulfonated groups of Nafion© and inhibit the migration of protons produced during substrate degradation, causing a decrease in MFCs performance. The decrease in the MFC performance is due to the significant ohmic losses in the membrane, accompanied by a simultaneous pH reduction in the anode chamber and a corresponding pH increase in the cathode side that in turn affects electrode reactions (Rozendal et al.,2006).High water uptake capacity,high ion exchange

Properties of nanomaterials for microbial fuel cell application

capacity, and low oxygen permeability are the essential properties of a better membrane, among the other properties (shown in Table 6.1) in MFC. Water uptake capacity: The water uptake capacity of a membrane can also be defined as the average number of water molecules per the conducting functional group. water uptake (WU) =

WC IEC × 100 × Mw

(6.1)

Where IEC represents the ion-exchange capacity (i.e., OH ion content per gram of polymer, meq g−1 ), Mw, 18 g mol−1 , is the molecular weight of water, and WC is the water content. The water content (WC) of a membrane is often reported in percent and defined on the dry polymer basis and calculated by comparing the dry weight (Wdry ) and the wet weight (Wwet ) of a membrane according to the following expression: water content (WC) =

(Wwet − Wdry ) Wdry

(6.2)

The amount of absorbed water in a membrane is an important parameter because it is related to proton conductivity. 6.3.6 Ion exchange capacity (IEC) The ion exchange capacity of a membrane is another crucial parameter in MFC because the ionic transport properties of the membrane depend on the amount and type of the ion exchange groups. IEC (given in meq. H+ /g) of the membrane can be calculated using the following equation: ion exchange capacity (IEC) =

VNaOH × CNaoh ) Wdry

(6.3)

Where VNaOH and CNaOH represent the volume and concentration (in mol L−1 ) of the NaOH solution used in the titration, and Wdry is the dry weight of the membrane in g. 6.3.7 Oxygen permeability Oxygen permeability is one of the most critical parameters in MFC. Using the same concept as in direct methanol fuel cells is interesting to evaluate the β parameter of the membrane, defined as the ratio of the proton conductivity (σ ) of the membrane and its oxygen permeability (P). Both parameters σ and P are intrinsic, and then β parameter will be a number specific of the membrane. Therefore, the evaluation of this parameter is important, because it is independent from the membrane thickness (Nasef et al., 2006). Therefore, the parameter β helps to predict the MFC performance for a specific membrane as function of a characteristic factor, which is a theoretical consideration for the suitable estimation of MFC performance under practical operating

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conditions. It is a specific value for a membrane because it is based on two membranes intrinsic properties, σ and P. Consequently, a simultaneous and equivalent increment of proton conductivity and oxygen permeability would not cause any change in the MFC performance. Oxygen permeability can be calculated using electrochemical techniques (Compañ et al., 2002; Brennan et al., 1986; Wilke & Chang, 1955). Briefly, when the global cathode is maintained at 0.75 V with respect to the silver anode, all the oxygen passing through the sample is reduced at the cathode, and the following electrochemical reaction occurs. O2 + 2H2 O + 4e− → 4OH−

(6.4)

In the steady-state conditions (t), the current intensity (i) can be written as i=

nFADκp Lav

(6.5)

And the apparent permeability (P) can be obtained as Dk =

I · Lav = B · I · Lav nAF p

(6.6)

where I is the steady-state current intensity (in amperes), n is the number of electrons exchanged in the electrodes for each molecule of oxygen (n = 4); and F is the Faraday constant (= 96,487 C mol−1 vol O2 (STP) = 96487 A s/22400 cm3 O2 (STP). A is the surface area of the gold-plated cap in contact with the membrane, and p is the O2 partial pressure difference across the membrane = 15.5 cm Hg. B is a constant of the cell (B = (nFA p)–1 ); in our case, it is equal to B = 0.02629 cm3 O2 (STP) cm–2 A–1 s–1 cm Hg–1 under the given conditions, and Lav harmonic average thickness of the sample calculated from five measurements in five regions of the central zone of the membrane with 2.5 mm radius. The oxygen permeability measurement, according to Eq. (6.3), does not take into account the resistance to the transmission of oxygen by the liquid boundary layers between the membrane and the cathode and over the membrane, if we take into account these resistances, a new equation should be developed, it is       Dk Dk Dk = + (6.7) Lav App Lav Mem Lav BL Where (L = Dk) BL is the boundary layers resistance. Eq. (6.4) is used to determine the oxygen permeability coefficient of the membrane. For this, we have considered that the boundary layer permeability is equivalent to the water permeability (P = (Dk)water = 93 Barrers). This value is obtained considering that the oxygen diffusion coefficient and solubility are D = 30 × 10–6 cm2 s–1 , k = 3.1 × 10–5 cm3 of O2 cm–3 mmHg–1 , in water

Properties of nanomaterials for microbial fuel cell application

Table 6.2 Conductivity (σ ) and characteristic number (β) determined for different polymeric membranes. Membrane Thickness (m) P (Barrers) (mS/cm) =/ P (S. s. cmHg cm–3 )

Nafion-117 Nafion-PVA-15 Nafion-PVA-23 SPEEK-35PVA (Water) SPEEK-35PVA (DMAc) SPEEK-30PVB-35PVA Agar 2% Agar 6 wt% Agar 6 wt%-KCl 10 wt% Agar 8 wt%-KCl 10 wt% Agar 2 wt%-KCl 8 wt%

1835 15±1 23±2 92±3 89±3 70±5 274±8 815±24 737±22 813±24 313±9

11.5±0.8 0.80± 0.11 1.01±0.16 1.33±0.16 0.77±0.10 1.34±0.20 21.3±1.0 10.6±0.3 21.7± 0.4 12.4±1 19.1±1

31±2 5.9±0.2 5.6±0.2 5.4±0.2 3.4±0.1 10.3±1.5 1.81±0.05 2.32±0.07 7.10±0.20 5.80±0.17 1.91±0.06

(2.7±0.2)x107 (7.4±1.3)x107 (5.5±1.1)x107 (4.1±0.6)x107 (4.4±0.2)x107 (7.69±2.3)x107 (8.6±0.4)x105 (2.1±0.1)x106 (3.2±0.2)x106 (4.8±0.2)x106 (1.0±0.1) x106

β = σ /P is defined by the ratio of the proton conductivity of the membrane to its oxygen permeability. The work temperature for these data was 30 °C.

solution at 25 °C, respectively (Wilke & Chang, 1955). Values of the oxygen permeability measured in our laboratory for different PEMs used in MFCs are gathered in Table 6.2. 6.3.8 Membrane conductivity In a single chamber microbial fuel cell (SCMFC), the membrane is one of the most important parts of the device, and it is generally considered as the heart of MFCs. The protons arrive at the cathode which together with the electrons coming from the anode subsequently, are combined at the cathode side with molecular oxygen, to produce water, aided by a catalyst that favors electrochemical reactions and consequently generating electrical power. The two most well-known and widely used methods to measure the in-plane conductivity of polymeric membranes and conductors are: (1) 2-point probe and (2) 4-point probe. On one hand, the 2-point probe involves two electrodes being adhered to opposite sides of sample, usually a rod or rectangular prism is put in contact with the sample, and after applying a known voltage the current can be obtain with an amperemeter. Then, applying Ohm’s law the resistance of the sample can be calculated. Reciprocally,applying a current across the material,the voltage can be determined using a voltmeter. From both current and voltage, the resistance of the sample can be determined. On the other hand, the 4-point probe technique is similar in concept, but it uses four terminals instead of two. The four probes can be arranged in a variety of geometries, but they are often placed in a straight line with some known spacing between the probes. However, measurements of the conductivities in-plane are very different compared to measurements trough plane, being two third orders of magnitude higher than those obtained through plane. Considering that protons must pass through the membrane to

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reach the cathode, the characterization of the conductivity of a PEM by through plane methods gives a more realistic value of the conductivity of the membrane. The most widely used technique for the analysis of ionic conductivity along a range of frequency and temperatures is the electrochemical impedance spectroscopy. This method allows to obtain the conductivity and diffusivity of protonic charge carriers. In a general experiment, measurements are carried out under humid conditions, and samples are completely hydrated to reproduce conditions like those on the fuel cell. From the dielectric measurements, the dc-conductivity (σ dc ) can be obtained using three different procedures; however, all methods are indirect, because they need the criteria to give the value of the dc-conductivity. Therefore, the dc-conductivity (σ dc ) can be obtained (1) from the relationship between the complex dielectric permittivity (ɛ∗) and complex conductivity, (2) another method to determine the σ dc is by means of the Nyquist diagrams, where the imaginary part is plotted versus the real part of impedance (−Z vs Z ), and (3) the third method to obtain the σ dc is from the Bode diagram. In Table 6.2, we have gathered some conductivities values obtained for different membranes used as PEMs in MFCs.

6.4 Different types of nanomaterials in MFC The materials in nano-dimension are getting importance in electrochemical applications because of their novel properties such as a high surface area to volume ratio, low internal resistance, a significant number of active sites, high catalytic activity, etc. In addition, many nanomaterials are shown to accelerate biocatalysis by providing high surface area and strong interaction for bacterial attachment. On the other hand, few nanomaterials enhance the gas adsorption property, ORR activity, and antibacterial activity of the electrode, which can be used as air-cathode. Membranes modified with NMs (TiO2 , SiO2, and graphene oxide (GO)) can reduce the substrate leakage,lower the oxygen crossover, and increase proton conductivity and water uptake capacity. Hence, nanomaterials are used to modify anode, cathode, and membrane in MFC applications to improve the power output and stability. Different types of NMs such as carbon nanomaterials, metal nanoparticles, metal oxide nanoparticles, metal sulfide nanoparticles, metal carbides, nano-sized polymers, and composites of the above are used in MFC applications as either anode or cathode, or membranebased on their unique properties. Unique properties of NMs to use as an anode, cathode, and membrane. Nanomaterials are getting significant importance in MFC applications to modify bioanode and cathode.The below-mentioned properties of NMs are the foremost reasons for anode modification r High surface to volume ratio. r Facilitates charge extraction efficiency by acting as bridges facilitate efficient EET from microorganisms to the electrode.

Properties of nanomaterials for microbial fuel cell application

r

Few NMs are highly biocompatible, particularly bioderived NMs. Nanostructured porous NMs facilitate the growth of microorganism. Functionalities on the surface of NMs increase bacteria-NMs interaction. r NMs can interconnect the functionalities on the cell membrane. r The direct contact of the NMs, electrode, and redox centers (cell) enhances the EET. r Metals centers act as redox centers to transfer electrons. In the case of cathode modification: r NMs increased the rate of ORR. r It provides more number of active sites for the reduction reaction. r NMs have high gas adsorption property. r NMs have antibacterial activity, that is, it inhibits the biofilm formation on its surface. While the NMs incorporated membrane exhibits an increase in the water uptake capacity, proton conductivity, and ion exchange capacity. The O2 cross-over is reduced, and membrane biofouling was also reduced. A few examples of NMs used for anode, cathode, and membrane modifications are discussed below. r r

6.4.1 Nanomaterials used for anode modification and their intrinsic properties The metabolic electrons produced by the catabolic process in the bacterial cytoplasm are transferred to the electrode surface through a series of DET or MET. These electron-transfer processes generally involve sluggish electron hopping through redox centers or multiple redox cycles, severely limiting the charge-transfer efficiency. It is essential to modify anodic electrodes to efficiently extract the electrons to the electrode. Different nanomaterials such as carbon-based NMs and composites, metal and metal oxides, metal carbides, and nanopolymers have been used to modify the surface of anode to extract the metabolic electrons to the electrode for MFC applications. Few NMs used for anode modifications are shown in Table 6.3 and discussed in the below sections. 6.4.2 Carbon materials Carbon nanomaterials are widely used NMs for bioelectrode modification owing to their high surface area for microbial attachment and stability in the mixture of microbial inoculum. For example, carbon nanomaterials, including CNTs, MWNTs, and graphene (Gr) have been used for anode modification to improve the MFC performance. The carbon sheet was modified with MWCNTs to enhance the growth of Enterobacter cloacae as bioanode. To investigate the effect of MWCNTs modified anode in MFC performance, the DCMFC was constructed using the above Enterobacter cloacae/MWNTs modified carbon sheet as anode and Pt coated carbon sheet as a cathode. This work found a 256% increase in the power output on MWCNTs modified bioanode than the

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Power density (mW/m2 )∗ Control Modified References

Microorganism

Anode (control)

NMs

Cathode

Enterobacter cloacae

Carbon paper

MWCNTs

Carbon sheet

256% improvement

Mixed culture

Carbon cloth

CNT-Textile

Pt/Carbon cloth

Geobacter sulfurreducens Mixed culture Shewanella putrefaciens Escherichia coli

Carbon sheet Carbon paper Carbon felt SS

CNTs Graphene Bio-reduced GO PEDOT/Gr/NiNPs

– Cabon paper Pt sheet Graphite

48% improvement in 3D sponge than CNT-Textile 200% improvement 368 7 0.015 240 1800 3200

Saccharomyces cerevisiae

Carbon felt

AuNPs

–

381

2271

Shewanella

rGO-Ag scaffold

Carbon paper

6600

500

Mixed culture

Graphite

Shewanella AgNPs hybrid Iron carbide nanoparticles Iron oxide/carbon nanocomposite Graphene with PANI

Shewanella loihica PV-4 Carbon felt Mixed culture

Carbon cloth

Pt/carbon cloth 997

1856

Pt/carbon paper 220

797

carbon fiber brush –

306

155

Graphite plate

1

117 136

Shewanella loihica PV-4 TiO2 nano sheet/ carbon cloth Mixed culture Graphite plate

PANI/TiO2 nano sheet/ carbon cloth Wet BC-PANI

Mixed culture

Graphite plate

Wet BC-PPy

Graphite plate

1

Mixed culture/ SCMFC Shewanella xiamenensis

BC-CNT

PANI modified BC-CNT TiO2 modified BCPANI

Activated carbon-SS Graphite

20% improvement

BC-PANI

63% improvement

137

179

(Nambiar & Limson, 2009) (Xie et al., 2011)

(Jiang et al., 2018) (Guo et al., 2014) (Zhu et al., 2019) (Hernández et al., 2019) (Duarte et al., 2019) (Bocheng et al., 2021) (Jiang et al., 2014) (Q. Yang et al., 2021) (Lin et al., 2019) (Q. Yang et al., 2021) (Mashkour et al., 2016) (Mashkour et al., 2017) (Mashkour et al., 2020) (Truong et al., 2021)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 6.3 Different nanomaterials used for anode modifications.

Properties of nanomaterials for microbial fuel cell application

bare carbon sheet (Nambiar & Limson, 2009). Also, CNTs promote scaffold porosity for biofilm formation and improve electrocatalysis. Recently, nano hybrids of MWCNTs blended with bacteria-derived AuNPs (MWCNTs/BioAu) were used in MFC and it had the shortest start up time (6.74 days) and high power density (aprroximately 178 mW m−2 ) with that of unmodified control (Wu et al., 2018). The CNTs have hydrophobic and hydrophilic contact points for the microbe to attach firmly to the anode surface (Z. Jiang et al., 2018). For example, CNT-coated textile and CNT-coated sponge was developed. The CNT-coated sponge achieved a maximum current density of about 48% higher than CNT-textile because the three-dimensional (3D) CNT-coated sponge promoted microbial colonization (Xie et al., 2011). However, the carbon NMs are toxic to bacteria if the concentration of the NMs exceeds its toxic threshold. Different concentrations of CNTs suspensions were added in the biofilm growth medium G. sulfurreducens to investigate CNTs toxicity effect on microbial growth (Jiang et al., 2018). The formed biofilm achieves a 37% decrease in the current density compared to the G. sulfurreducens biofilm formed using a growth medium without CNTs. This work revealed that the maximum MWCNTs concentration required to improve G. sulfurreducens biofilm performance is 0.1 mg mL−1 . However, the effect of NMs might differ for different microorganisms, so one should consider the concentration of NMs and their toxic threshold. Another interesting two-dimensional (2D) carbon nanomaterial used in MFC is graphene. The exciting properties of Gr are electrical and thermal conductivity, specific surface area, high electron mobility, and mechanical strength. Carbon paper was modified with Gr (Gr/CP), and anaerobicsludge was used as anodic inoculum of MFCs (W. Guo et al., 2014). The MFC with Gr/CP bioanode achieved a stable maximum power density of 368 mW m–2 under 1,000 external resistance, which was 51% higher than the blank anode. The scanning electron microscopy study revealed an increase in the surface roughness of GR/CP, which is favorable for high populations of bacteria attachment. Moreover, the orientation of Gr (plate or crumpled) is also essential (Xiao et al., 2012) because the crumpled graphene can double electricity generation. The bioreduced GO modified anode with shewanella putrefaciens delivers 240 times higher power density than its control which indicates that effect of method of preparation of GO (Zhu et al., 2019). The thin layer of reduced GO was formed on carbon brush (CB) by an electrophoretic deposition to use as anode in MFC. The RGO-modified CB enhanced the performance of the MFC, where the power density increased more than 10 times (from 33 mWm–2 to 381 mWm–2 ), as shown in Fig. 6.6A (Sayed et al., 2021). 6.4.3 Metal nanoparticles Several metal and metal oxide nanoparticles are used for MFC applications. It was found that biogenic nanoparticles can serve as a bridge to facilitate electron transfer and

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(B) (C)

(D)

(E)

(F)

Figure 6.6 From Sayed, E. T., Alawadhi, H., Olabi, A. G., Jamal, A., Almahdi, M. S., Khalid, J. & Abdelkareem, M. A. (2021). Electrophoretic deposition of graphene oxide on carbon brush as bioanode for microbial fuel cell operated with real wastewater. Developments of Hydrogen Fuel Cell Technologies, 46 (8), 5975–5983. https://doi.org/10.1016/j.ijhydene.2020.10.043. (A) Polarization curves for MFCs using plain carbon brush (PCB) and electrophorotic reduced GO modified carbon brush (RGO-CB) adopted from (Sayed et al., 2021), (B) abundance of extracellular electron transfer related functional genes omcB and omcZ in anode biofilm (PRC, PC, CC refers to MFC equipped with PANI/RGO/CC, PANI/CC and carbon cloth, respectively) adopted from (Lin et al., 2019), (C) electrochemical impedance analysis of CP, rGO, rGO-PDDA anode adopted from (Ma et al., 2020), (D) Voltage output profile of the MFC with Ag3Pt-based cathode operated for 40 days in the first trial and 40 days in the second trial after regeneration of cathodes and membranes adopted from (Noori et al., 2018), (E) the polarization plots of MFC with nanofiber Nafion reinforced membrane (Kamaraj et al., 2015), and (F) the MFC with MEA cathode (CNT-BC-Nano Zycosil) adopted from (Mashkour et al., 2021a).

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

(A)

Properties of nanomaterials for microbial fuel cell application

interconnect cell networks. Various metal nanoparticles (Au, Ag, Ni, etc.) were used for bioanode modification. For instance, AuNPs were used to modify carbon felt electrode polyethyleneimine-functionalized carbon felt for yeast-based MFC. The above-modified electrode achieved a power density of 2271 mW m–2 . In-depth analysis of high-resolution scanning electron microscopy revealed a strong interaction between the yeasts‘ external cellular membranes and the AuNPs. In contrast, a weak interaction was observed with the carbon fibers. All these confirm the excellent biocompatibility of the AuNPs (Duarte et al., 2019). Recently, Shewanella –AgNPs hybrids have been developed for MFC applications. The charged silver ions released from the rGO-Ag scaffold facilitate the Shewanella attachment to the rGO/Ag scaffold and form a dense biofilm. The resulting Shewanella -Ag MFCs deliver a maximum current density of 6600 mW cm–2 and a single-cell turnover frequency of 8.6 × 105 s–1 . These are the higher values reported to date in MFCs (Bocheng et al., 2021). The scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) elemental mapping studies of the bacteria on the RGO/Ag anode after completing MFC cycles revealed enrichment of Ag around the Shewanella (near, inside, and across the membrane area). This might be due to the intermembrane and intramembrane reduction of metals to nanoparticles. The so-formed nanoparticles act as metallic wire and boost charge extraction of metabolic electrons of the slow electron-transfer process of the cell (Bocheng et al., 2021).

6.4.4 Transition metal-based nanoparticles (metal sulfide, metal oxide, metal carbide) Metal oxide and sulfide NPs also used for anode modification to enhance the current output in MFCs. The Shewanella biofilm doped with iron sulfide NPs found crystalline iron sulfide nanoparticles intimate contact with the cell membrane at electrode/NPs interface. These biogenic NPs act as bridges to facilitate efficient EET from Shewanella cells to electrode surfaces (X. Jiang et al., 2014). The iron oxide can promote electron transfer inside biofilm as an electrical conduit or interface by accumulation on the cell surface (Nakamura et al., 2009). The nano-colloid form of Fe2 O3 self-assemble the Shewanella in the form of an interconnected network resulting in 50-fold more power generation compared to the control (Kato et al., 2013; Nakamura et al., 2009). Few iron oxides are electrically nonconductive, but the oxygen functional groups on the surface enhance the biofilm growth. Hence, carbon materials are mixed with iron oxide to improve conductivity. The bacteria-derived iron oxide-carbon nanocomposite (BioFeOx/C) was recently developed for MFC. The Shewanella precursor adhered on the carbon felt electrode was directly carbonized to form Bio-FeOx/C. The MFC with the Bio-FeOx/C electrode exhibits the maximum power density of 797.0 mW m–2 , much higher than obtained with the conventional carbon felt anode (226.1 mW m–2 ) (Q. Yang et al., 2021).

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Metal carbide NPs are inexpensive nanomaterials that can be used as an alternative for high-cost NMs. Recently, Hu et al. used iron carbide NPs (FeCNPs) modified carbon felt as an electrode for biofilm formation. The power density of FeCNPs modified bioanode increased by about 200% due to the decrease in charge transfer resistance after FeCNPs modification. The FeCNPs could enrich the exoelectrogenic biofilm of mixed cultures and speed up the redox reaction. Also, the FeCNPs have Fe similar to microbial “heme Fe” resulting in higher biocompatibility to biofilms (X. Jiang et al., 2014). Usually, the metal oxides are coated on electrode surface to use in MFC due to its biocompatibility, environmental stability, and low price. Also, it could facilitate direct interspecies electron transfer. Recently, the different concentration of Fe3 O4 was added in the anodic chamber with 2.5 mL fresh medium. The single chamber MFC with 4.5 g L–1 Fe3 O4 delivers power density of 399±9 mW m–2 while the single chamber MFC without Fe3 O4 delivers power density of 255±25 mW m–2 . All these indicate that higher amount of Fe3 O4 may inhibit the performance, so one should consider the concentration of metal oxides too (Zheng et al., 2022). 6.4.5 Polymers Few biocompatible polymer-based materials can provide a favorable environment for biofilm formation. 3D polymer composites and aerogels were prepared for MFC by mixing with NMs and found an enhancement in power densities. Reduced graphene oxide (rGO) incorporated with PANI/Geobacter bioanode showed a 190% increase in the power density. This work revealed that rGO/PANI composite enhanced the growth of microorganisms with an abundance as high as 81.4% (Lin et al., 2019), shown in Fig. 6.6B. The TiO2 nanosheets/CC were modified with PANI, which offers a 63% increase in the power density (Q. Yang et al., 2021). Another interesting polymer is nanocelluose, a low-cost polymer with excellent biocompatibility. Porous hydrogel bacterial cellulose (BC) -based anodes coated by conductive polymers (PANI (Mashkour et al., 2016) and polypyrrole (Mashkour et al., 2017)) exhibit higher power density (>100%) than the control. The BC nanofibers have permanent access of bacteria to nutrients and prevent bacteria spoilage and clogging of pores due to capillary action. The BC-PANI was further modified using CNT (Mashkour et al., 2020) or TiO2 (Truong et al., 2021) to improve the power density. The composite of NiNPs, PEDOT, and Gr modified SS with E. coli achieved a high power density of 3200 mW cm–2 than SS. GrNiNPs-PEDOT film shows enhanced biocompatibility between microorganisms and modified electrode (Hernández et al., 2019). 6.4.6 Polyelectrolyte modified NMs Another study revealed that the polyelectrolyte could reduce ohmic resistance and charge transfer resistance of the Gr, leading to high power density. For example, Poly diallyl dimethylammonium chloride (PDDA)-rGO modified bioanode achieved a higher

Properties of nanomaterials for microbial fuel cell application

peak power density of 5029 mW m–2 compared to CP (921.3 mW m–2 ) and rGO (2006 mW m–2 ) (Ma et al., 2020). The electrochemical impedance analysis revealed the reduction in ohmic and charge-transfer résistance of CP and rGo after PDDA modification, as shown in Fig. 6.6C. Mechanisms on bioanode in MFC The NMs in bioelectrode can enhance electron transfer by different pathways, r Inside membrane mechanism—NMs can transfer the electrons through the membrane and can facilitate the electron transfer by acting as a bridge between the inner membrane and outer membrane of the cell. r Interspecies mechanism—NMs can diffuse inside the biofilm and enhance the electron transfer by interconnecting the cells. r Inside biofilm mechanisms—NMs can promote electron transfer inside biofilm as electrical conduit or interface by accumulation on the cell surface. r Interface mechanism—NMs are placed between electrode and microorganism, and the interaction leads to achieving a high population on the electrode. r The few NMs (metal-based NMs, that is, iron oxide) act as a mediator to enhance electron transfer. 6.4.7 Nanomaterials used for cathode modification in MFC and their intrinsic properties The oxygen reduction reaction is desired to occur at the highest rates on a cathode. Precious metal (Pt) is a known high-performance electrocatalyst for ORR. The cost and bacteria’s poisonous effect on Pt-based catalysts is its limitations. Various electrocatalysts such as transition metal and metal oxides (Fe, Ni, Co, Mn, etc.), carbon nanomaterials, metal-organic frame works (MOF), polymer, and their composites are developed to enhance the ORR catalysis. Few catalysts used for MFC are shown in Table 6.4. As shown in the table, the N-doped CNTs with Co NPs (Zhang et al., 2019), metal (Co, Ni) and nitrogen-doped carbon NMs (Tang et al., 2016; Zhong et al., 2019), metal oxides (Ni/NiO, SnO2 , Co3 O4 ) (Choi et al., 2020; Tiwari et al., 2020; L. Jiang et al., 2020) and modified Gr (Valipour et al., 2016; W. Kodali et al., 2018; Yang et al., 2019) are used as cathode for MFC applications. Nitrogen-doped CNT was used to modify carbon cloth cathode. The catalytic activity of the N-CNT/CC electrode in ORR was higher than the Pt-coated CC. The cathode generated a maximum power density of 542 mW m–3 , higher than that of the Pt-coated one (approx. 500 mW m–3 ). In addition to nitrogen, P and Co also doped in carbon materials to enhance the power output. Also, the cathode modified with CoMOF shows power density 2350 mW m–2 vs. 2002 mW m–2 measured for Pt (Zhao et al., 2020). But bacteria’s poisonous effect, that is, formation of biofilm on air-cathode, is an important challenge in MFC technology. The formed biofilm decreases the active sites of ORR on the cathode surface.

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Microorganisms

Anode

Cathode (control) Nanomaterial in cathode

Power density (mW/m2 ) Control Cathode

References

Escherichia coli

CC

Pt-coated CP

2520

4336

(Tang et al., 2016)

2021 744 489

1683 773 1630

(Valipour et al., 2016) (Yang et al., 2019) (Choi et al., 2020)

1020

2030

(Kodali et al., 2018)

714

2479

(Zhong et al., 2019)

2252

888

(Zhang et al., 2019)

200% improvementa 6.25 467

(Tiwari et al., 2020)

Mixed culture Mixed culture Mixed culture Mixed culture

Mixed culture Mixed culture Mixed culture Mixed culture a CP, carbon

Co/Ni and N codoped carbonous NPs on CP Carbon Brush Pt/CC RGO/CC CC Pt/CC GO-Zn/CoO on CC Carbon Felt Pt-coated carbon Ni-sheathed NiOx felt nanoparticle Carbon Brush Activated Iron aminoantipyrinecarbon-carbon graphene black nanosheet Carbon Felt Activated carbon Co/Nitrogen-doped CNTs modified AC Carbon felt Activated carbon Pyrolised ZnCo zeolitic imidazolate framework – Carbon felt SnO2 -polyaniline modified carbon felt Graphite felt Stainless steel NiFe layered double (SS) ydroxide- Co3 O4 / SS

paper; CC, carbon cloth.

(Jiang et al., 2020)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 6.4 NMs modified cathode for MFC applications.

Properties of nanomaterials for microbial fuel cell application

The NMs, silver nanoparticles, and iron oxide incorporated carbon materials inhibit the growth of bacteria. The incorporation of silver in the catalyst layer of the composite helped recover 95% of the initial power density after 90 cycles (E. Yang et al., 2016). The electrode with Ag3 Pt catalyst could perform consistently for 40 days of the operational period was not found poised with microorganisms (Noori et al., 2018), shown in Fig. 6.6D. The ZnONPs and CoNPs with antimicrobial properties have also inhibited the growth of biofilm on the cathode surface (W. Yang et al., 2019) and achieved stable performance for a long time. All of these confirm that the inhibition of biofilm on cathode surface is another crucial factor in higher ORR activity and gas adsorption property.

6.4.8 Nanomaterials used for membrane modification and their intrinsic properties Different polymers, agar, and composites are used to make membranes for MFC applications to improve the water uptake capacity and proton conductivity (X.-Z. Beauger et al., 2013; Hernández-Flores et al., 2019; Kamaraj et al., 2015; Peighambardoust et al., 2010; Yuan et al., 2011). Recently, polymer nanofibers reinforced membranes are also prepared to increase the performance of the membrane in fuel cell applications (Kamaraj et al., 2015; Mollá & Compañ, 2011). For example, two first reinforced membranes were made up of Nafion polymer deposited between polyvinyl alcohol (PVA) nanofibers. The membranes showed outstanding stability, high proton conductivity, and enhanced mechanical and barrier properties. The membranes were characterized in a single chamber microbial fuel cells (SCMFCs) using electrochemically enriched high sodic saline hybrid H-inocula (Geobacter metallireducen,Desulfurivibrio alkaliphilus,and Marinobacter adhaerens) as biocatalyst. A maximum power density of 1053 mW m−3 at a cell voltage of 340 mV was achieved with the Nafion-PVA-15 membrane, shown in Fig. 6.6E. The Nafion-PVA15 membrane displayed the lowest total internal resistance (RCT ≈ 522 ) (Kamaraj et al., 2015). On the other hand, the nanomaterials are incorporated into the polymer to make membranes for MFC. A conventional Nafion modified with TiO2, and the formation of Ti-OH groups in the TiO2 -based nanocomposite membrane increased the exchange sites over the membrane, leading to higher ion conductivity than Nafion (bajestani, 2016). It also provides a higher water uptake comparatively. The sulfonated TiO2 modified sulfonated polystyrene ethylene butylene polystyrene (SPSEBS) composites (Ayyaru & Dharmalingam,2015) showed higher proton conductivity,much lower oxygen cross-over, and much lower cost (200 USD m−2 ) than commercial Nafion 117. The SiO2 modified Sulfonated poly (ether ether ketone) (SPEEK) based membrane exhibits higher water uptake and proton conductivity compared to the bare SPEEK (Sivasankaran & Sangeetha, 2015).

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The carbon nanomaterial (GO) modified quaternized polysulphone (QPSU) anion exchange membrane also exhibits better properties than bare QPSU (Elangovan & Dharmalingam, 2016). The MFC with GO-QPSU membrane achieved a power density of 1050 mW m−2 . The MFC with the Fe3 O4 NPs modified polyethersulfone (PES) membrane exhibits a nine-time increment in the PES membrane’s power density (Di Palma et al., 2018). The MFC with SPEEK-Ag nanoparticles achieved a power density of 160 mW m−2 than SPEEK membrane (Kugarajah & Dharmalingam, 2021). The nano-zycosil treated bacterial cellulose-carbon nanotubes based membrane was used for membrane electrode assembly (MEA) based MFC (shown in Fig. 6.6F), which achieved a power density of 1790 mW m−2 with 250% of columbic efficiency improvement by using the MEA compared to gas diffusion electrode (Mashkour et al., 2021). However, increasing the NMs content in the polymer/NMs composite can decrease the water uptake capacity, increase oxygen cross-over owing to the lack of functional groups, and improve void matrix, that is, pores. Therefore, the composition of polymer/nanocomposite is an essential parameter in membrane development. In addition to membrane composition, the membrane electrode assembly has been of great importance recently because it delivers high power density than SCMFCs.

6.5 Outlook and future perspective Nanomaterials improve the performance of MFC by providing a high surface-to-volume ratio, facilitating bacterial growth and attachment, enhancing biocatalysis and charge excretion efficiency when we are using as bioelectrode. It also enhances the rate of ORR and gas adsorption properties in the cathode. The incorporation of NMs in membrane increased water uptake property, proton conductivity, resistivity, etc. However, there are certain limitations and challenges to use NMs for MFCs, given below: r The effect of nanomaterials might differ for each species, i.e. the bacterials growth can be enhanced using NM for a few bacterias. At the same time, the same NMs can inhibit the growth of other few bacteria. For example, the AgNPs are toxic to E. coli growth while enhancing Shewanella’s growth (Qian et al., 2011). r The specific concentration of NMs is also essential. If the concentration of NMs exceeds the toxic-threshold concentration, it will inhibit the growth and activity of biofilm. r The properties of NMs depends on size and morphology. Hence, an intensive investigation is needed to understand the structure–activity relationship. r Eventhough the metal oxides promote biofilm growth, the electrical conductivity of metal-oxides is limiting their performance. r The carbon NMs prepared using differnt approaches have different properties. It should be taken into consideration during experimental design.

Properties of nanomaterials for microbial fuel cell application

r

The NMs enhances the ORR rate and gas adsorption property. But they should have an antimicrobial activity to withstand microbial environments for longer time. r The NMs in the membrane are also expected to have an antimicrobial activity to avoid clogging of pores by biofilm formation. The substrate cross-over is increased after modification of membrane with NMs due to the occupation of the functional groups with NMs and formation of the voids matrix. Hence, one should consider the composition and structure of the membrane. r The membrane electrode assembly of anode and cathode using NMs and desired biocompatible polymers (bacterial cellulose) enhances the performance of MFC with MEA. r It’s changing to work with unknown inoculum from sludge because pathogenic bacteria can exist in it. Handling those is a real risk to living beings. Hence, one should consider such strange environmental features. Even though the NMs have been widely used in MFC, the cost of the materials limits the scale-up of MFC. There are other challenges during MFC operation (1) aggregation of NMs, (2) inactivation due to adsorption of components from fed, and (3) leaching of NMs. Hence, intensive investigation is needed to understand the sustainability of this whole technology.

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Advanced nanocomposite material for wastewater treatment in microbial fuel cells Aarti Atkar a,b, Manideep Pabba a and Sundergopal Sridhar a,b

a CSIR-Indian Institute of Chemical Technology, Membrane Separations Laboratory, Process Engineering & Technology Transfer Division, Hyderabad, India b AcSIR – Academy of Scientific & Innovative Research, CSIR-IICT, Hyderabad, India

7.1 Introduction In the upcoming future, the world will face an energy crisis due to a lack of energy sources hence, to resolve this kind of issues researchers are developing an alternative sources of energy like renewable sources such as solar, wind, biomass, tidal, etc. Currently, people are using nonrenewable sources for the production of energy such as oil, natural gas, coal, and nuclear energy-giving tremendous energy but ultimately reducing the level of nonrenewable sources present on the earth. It gives a substantial amount of energy but doesn’t fulfill the demand of the people due to the increase in the population. Oil, natural gas, and coal are also called fossil fuels that formed within the Earth from dead plants and animals over a million years. Fig. 7.1 shows an increase in the growth of the population which requires a lot of energy. If societies are dependent only on fossil fuels, then soon the fossil fuel will finish in the upcoming future. The fuel cell is one of the most promising technologies, due to its eco-friendly and efficient perception in producing electrical energy. A microbial fuel cell (MFC) is a bioelectrochemical device that converts chemical energy into electrical energy using microbiological activity. MFC is renowned for its twin function ability which is proficient in producing power and at the same time treating wastewater (Ahn & Logan, 2010; Ghasemi et al., 2013; Jatoi et al., 2021, 2022). Currently, the scale-up of MFC is still not possible due to the low power density, high cost, and short durability (Babauta et al., 2018; K. Ben Du et al., 2007; Leong et al., 2013; Liew et al., 2014). MFC was divided into three parts—membrane,electrode,and proton exchange membrane (PEM). The membrane separates the anode and cathode compartment. Electrodes present in the anode and cathode chamber will act as catalysts. The membrane is an essential part of MFC because it ensures that an equal amount of protons produced by microorganisms are transferred from the anode to the cathode compartment. Therefore, the selection of material is important which can carry protons without swelling of the Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00013-9

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Figure 7.1 MFC as an alternative source of electricity production.

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membrane for a long period. Decomposition of organic and inorganic material can be done by microorganisms that generate electrons, and they can be transferred onto the surface of an anode using pili or mediators. There are two ways to transfer the electrons onto the surface of an anode: (1) direct transfer and (2) indirect transfer. In a direct transfer, some species of bacteria utilize their pili/nanowires to transfer the electrons directly on the anode surface whereas, a mediator can be used to transfer the electrons onto an anode surface in an indirect transfer. Mediators such as neutral red (Park & Zeikus, 2000), methylene blue (Rahimnejad et al., 2011), thionine (Rahimnejad et al., 2012), methyl orange, bromocresol green, methyl red, neutral red (Babanova et al., 2011), and ferricyanide (Parkash and Aziz, 2015) are different redox-active mediators, which aids as transporter/shuttle for electrons. By improving the performance of the membrane, catalyst, and electrode MFC efficiency can be enhanced. It can be done by selecting and optimizing nanosized materials, and different designs of the electrode. Various types of materials were used for membrane synthesis by different researchers such as graphene oxide (GO) (Cao et al.,2011;Choi et al.,2012),modified GO (Heo et al., 2013), silver GO (Ag-GO) (Bao et al., 2011), Nafion/TiO2 (Bazrgar Bajestani & Mousavi, 2016), Ag GO-GO-SPEEK (Kien Ben Liew et al., 2020) to improve the performance of MFC. Different types of novel materials and designs are being developed to make electrodes. Anode electrodes made by nanocomposite material such as bifunctional manganese ferrite/poly-aniline hybrid (Khilari et al., 2015), Bamboo-NCNTs (Ci et al., 2012), CNT Sponge (Erbay et al., 2015), CNT-based anode (Thepsuparungsikul et al., 2014) to improve the MFC performance. In MFC, bacteria can be used as a biocatalyst to oxidize organic and inorganic matter to produce electrical energy. Conventionally, Pt was used as a catalyst but the high cost of the Pt material does not make it feasible for scale-up. Hence, the researcher studied various type of nanomaterials that act as a catalyst and gives a good performance with a low cost when compared to Pt catalyst. Vanadium is also used as a catalyst in many chemical reactions to reduce environmental problems and is also available on earth abundantly (Ghoreishi et al., 2014). Hence, the overall performance of MFC depended on two components- electrodes, and PEM. The efficiency of MFC can be enhanced by using advanced nanosized materials. The incorporation of nanoparticles in a polymeric solution is used to develop a novel PEM that aids to boost power generation. Electroplating is an alternative way to deposit a metal coating on another piece of metal. The deposited metal in electroplating becomes a part of that electrode. In this book chapter, we have concentrated on several types of nonmaterial utilized to produce enhanced anode and cathode electrodes. All of these nonmaterial have also been employed to synthesis the PEMs as well as improved their performance as compared to the Nafion membrane. The basic principle of MFC and its application in wastewater treatment is also mentioned. In the foreseeable future, MFC will be the most promising technology for treating massive amounts of wastewater while also producing power.

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This will be a notable step in the area of science in obtaining an alternate energy source.

7.2 Microbial fuel cell (MFC) as an emerging source of energy Renewable energy is attracting a lot of attention around the globe. MFCs recently arisen as a renewable electrochemical development in this sense (Modi et al., 2017). The global quest for fresh and safe freshwater resources is continuing to be motivated by growing demands for a clean water supply. To prevent water scarcity and environmental destruction, wastewater purification and recycling are crucial. Reliable technology and the reduced energy consumption is essential to fulfilling these objectives. An MFC is a system that degrades organic material and produces green energy as a sort of electricity by using exoelectrogenic biofilm on the anode (Malaeb et al., 2013). MFCs have gained attention as an environmentally sustainable electrochemical innovation and are considered promising for wastewater disposal, including the elimination of hazardous elements from wastewater and the production of bioelectricity at the same time. In the last decade, comprehensive research into fuel cell technology has resulted in the introduction of different types and groups of fuel cells. MFC promises to be a potential bioelectricity supply and wastewater treatment technique. The anode of an MFC generates energy by oxidizing organic material as well as the cathode’s oxygen reduction. MFCs have superiority over PEMFCs, DMFCs, and DBFCs. This type of fuel cell doesn’t need expensive fuel sources such as pure hydrogen, methanol, or sodium borohydride, as well as a particular catalyst for fuel oxidation. Although MFC is a rather environmentally friendly and renewable form of energy, it also has low voltage and power intensity. The other major limitations of MFC are its high Internal resistance is, and low oxygen transport. Because of these limitations, the commercialization of MFCs has been challenging. The schematic diagram of MFC was shown in Fig. 7.2. To boost the cathode catalytic performance, several studies have been conducted (Liew et al., 2015). The anode and cathode electrodes of a MFC are comparable to those of a battery. Electric power is generated by the voltage differential between the anode and cathode, as well as electron flow in the outside circuit. A two-chamber MFC is a common configuration. There are sections for aerobic (cathode) and anaerobic (anode) activity which are separated by a PEM or a salt bridge. While in the anode compartment, electrochemically active microorganisms break down biodegradable organic materials, creating electrons (e) and protons (H+ ). Protons permeate into the cathode compartment via the PEM, whereupon they mix with electrons and oxygen to produce water. Biological substrates for the anode can range from simple carbohydrates like glucose and acetate to more substantial molecules like starch, complicated wastes, sediments, and a variety of other organic and inorganic components. In the MFC, microorganisms play a crucial function. An MFC is a combination of electron transport and electrochemical

Advanced nanocomposite material for wastewater treatment in microbial fuel cells

Figure 7.2 Schematic diagram of MFC.

process. According to the latest research, MFCs could give the potential for long-term energy generation from biodegradable chemicals. MFCs are being extensively studied for a variety of contaminants, including carbohydrate oxidation, COD elimination from landfill leachate, and ferrous iron (Fe2+) oxidation in acid mine drainage. Single chamber MFC has been created in recent times. Notwithstanding its usefulness and prospective advantages in the treatment of wastewater, the MFC technique as a whole is still in its early stages (Yadav et al., 2012). Bioelectrochemical scheme techniques have recently been explored as a substitute due to their potential involvement in removing pollutants and generating power from wastewater via microbial metabolisms. MFCs can clean wastewater while also generating power, making them a great solution for water and energy concerns. Furthermore, organics contained in surplus sludge as a result of various treatment procedures are utilized to generate energy in MFCs. The MFC is a remarkable innovation with distinct energy, environmental, and economic operating characteristics. It’s a platform technology with a wide range of applications, including power generation, wastewater treatment, biohydrogen production, and biosensors (Palanisamy et al., 2019). MFCs create new possibilities for wastewater treatment and power production at the same time. Because of its potential to process wastewater and produce energy at the same time, the MFC technique looks promising as a feasible solution to current biological wastewater treatment plants (WWTPs) (H. Liu et al., 2005). As a result, one of the top priorities for MFC is product innovation and implementation to convert wastewater treatment from an energy-intensive operation to one that is a renewable, energyefficient, or energy-generating system. For realistic implementations of MFCs for the treatment of wastewater and bioenergy generation, scale-up is crucial. Nevertheless, at

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this moment, full-scale deployment of this wastewater treatment technology is not easy due to some economic and technical challenges that must be tackled. The key obstacles for commercializing reliable MFCs in the wastewater treatment sector are due to their high-cost components, etc. To overcome these limitations researchers are working to develop budget-friendly components, finding structures that can be used at a wider level, and designing equipment that is cost-efficient and reliable in energy production (F. Zhang et al., 2010). Standard MFCs have low power intensities and poor power conversion effectiveness.Their realistic uses are limited due to slow electron transport among bacteria and electrodes. Multiple advances have been taken to increase electron transfer rates at the bacteria/electrode junction, including the use of tiny diffusive electron intermediaries like neutral red, quinone, and thionine to boost electron transport from ions, donors to the surface of the anode. External intermediaries, on the other hand, are costly and harmful, and they inflict operating losses. Utilizing nanomaterials as anode components to dramatically increase MFC processing power output is one approach that could resolve this problem (Mehdinia et al., 2014). Exo-electrogens found in the anolyte of the anode chamber act as a biocatalyst in a standard MFC.Under anaerobic environments,organic material in wastewater is oxidized, emitting electrons and protons together with carbon dioxide. Exocellular, the produced electrons are transmitted to the anode’s conductive substrate, where they are transmitted to the cathode through an external circuit. The PEM allows protons to migrate to the cathode. The oxygen reduction reaction (ORR) happens at the cathode, resulting in bioelectricity (Modi et al., 2017). Nevertheless, as opposed to other more proven fuel cell technologies, the MFC has poor operational productivity as well as the massive cost of its parts in comparison to the minimal price of the water it processed which are the two most important impediments to the commercial viability of MFC. MFC’s efficiency has increased in current times as a result of using the affordable nanocomposite substances in the electrode which are highly efficient and physically stable like nanostructured carbon in comparison to older Pt on carbon, it has a greater interface size and greater electrochemical catalytic performance (Ghasemi et al., 2013). As a result, there is a surge of attention in seeking the most cost-effective renewable and efficient energy resources with quite minimal to zero emissions (H. Liu et al., 2006).

7.3 Role of nanocomposite materials in MFCs Nanotechnology, a branch of advanced science, has been popular in recent years. This technology made a breakthrough in the scientific and technological research of operational compounds with improved characteristics due to the nanoscale size of the substances. Nanotechnology has sparked a lot of attention over the years, and because of its interdisciplinary existence, it can be used in a variety of disciplines (Youtie et al., 2009). Several scientists concur that nanoparticles are least expensive than traditional substances

Advanced nanocomposite material for wastewater treatment in microbial fuel cells

and also nanotechnology causes lower pollution and contamination. Since nanomaterials need fewer resources and energy to manufacture, they produce fewer wastes. In recent decades, the exponential development of modern nanotechnology-based innovations has transformed life and spawned a slew of new businesses. Nanotechnology involves the study of tiny or ultra-fine structures with a diameter of fewer than 100 nanometers. Particles in this sized class have unique characteristics that set them apart from traditional materials.As a result,nanomaterials can be used to improve the performance of nanocomposite materials. As a result, nanomaterials can be used to improve the performance of nanocomposite materials. Higher surface area, stronger penetrability, and behavior, along with improved chemical specificity, are some of the most innovative characteristics of nanoparticles. The advantage of a greater surface area is that it has better surface activation over bulk materials, resulting in higher chemical reaction speeds and better catalytic performance, and thus improves the system’s efficiency. Nanotechnology has indeed been widely employed in the manufacturing of innovative systems for renewable energy generation, such as solar and wind turbines, fuel cells as well as a new wave of battery and energy storage devices like capacitors and supercapacitors. Many produced nanoparticles are usually made of carbon, silicon, or metal oxides (Ghasemi et al., 2013). Several nanomaterials are being used in recent times to boost MFC efficiency and power density output. It’s because of its ability to develop an interaction between the bacterial biofilm and the electrode’s surface, allowing electron transport. Nanosized metal oxide semiconductor materials have a huge surface region, are nontoxic, biocompatible, and chemically stable. Nanocomposite substances can be organic or inorganic, and their use in science and manufacturing is progressively increasing. Polymeric nanocomposites are by far the most common kind of nanocomposites which in recent times attracted a lot of interest due to their remarkable properties in contrast to typical microcomposites (Ghasemi et al., 2013). Membranes made with nanomaterials, particularly nanofibers, have been the subject of current research aimed at designing convenient and cost-effective membranes for MFC systems. The unusual mechanical characteristics of nanofibers, as well as their enormous surface area, have drawn the greatest scientific interest in this study. Because of Van der Waals interactions, CNTs and CNFs have a high clustering capability. The peculiar mechanical characteristics of nanofibers, as well as their wider coverage size, have drawn the most focus from the research community to this study (Lim et al., 2012). Bioelectrochemical systems (BES) are systems where a microbe or purified enzyme catalyzes at a minimum one electrode process. The oxidation of organic or inorganic electron donors by self-sustaining electro-active microorganisms at the anode is a common concept in nearly all BES systems (Baca et al., 2016). Abundant nanosized structures like graphene and polyaniline have been developed and used to change the MFC electrode and construct high-performance MFC anodes, to boost the performance of the MFC’s power output. Carbon nanotube (CNTs)- sponge

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MFC electrodes were created by covering a sponge with CNTs according to Xie et al. Platinum (Pt) and gold (Au) have traditionally been considered precious metals and are used as electrodes. They are the most effective ORR catalysts in terms of oxygen reduction through a four-electron reduction route. The exorbitant expense of platinum (Pt) catalysts and other precious metals employed in the oxygen reduction process (ORR) makes them even less attractive and unsuitable for application. Different forms of catalytic substances like metal-metal complexes and electro-conductive polymers have grabbed the interest of several researchers due to their strong catalytic properties in the oxygen reduction process under neutral circumstances. The catalytic performance of nitrogen-doped carbon compounds against ORR catalysts was also good, although the manufacturing method was complicated, preventing them from being used on a big basis. Manganese oxide is a metal oxide that has been widely researched as an ORR catalyst due to its unique characteristics like low cost, and lack of environmental impact. Manganese oxide/carbon nanotube (MnO2 /CNT) hybrids have been suggested to improve the underlying poor electronic conductivity of manganese oxide. CNTs are carbon nanostructures with high electrical conductivity and specific surface area, which are important features for catalysts. MnO2 was also mixed with different carbon materials, such as graphene, and interestingly, the MnO2 graphene nanocomposite outperformed the Pt/C catalyst in terms of catalytic activity and resistance (Liew et al., 2015). 7.3.1 Proton exchange membranes based on nanocomposites PEMs important role and plays one of the most crucial components in fuel cells. They are proficient in moving protons efficiently from anode to cathode compartment. Furthermore, PEMs must be capable to prevent the movement of other materials from anode and cathode chambers, such as substrate or oxygen. Ultrex, Nafion, Bipolar membranes, Dialyzed membranes, Polystyrene and divinylbenzene with a sulfuric acid group, glass wool, nanoporous filter, and microfiltration membranes have all been used as PEM in MFC (Rozendal et al., 2008; Sun et al., 2009; Zuo et al., 2008). Nafion is one of the most popular PEMs used as a reference PEM than aforesaid membranes (Jana et al., 2010). Despite knowing that Nafion is a widely used PEM, it has several disadvantages likely significant expenses, oxygen leakage from the cathode to anode, substrate loss, cation transport and accumulation rather than protons, and biofouling (Chae et al., 2008). Due to these drawbacks, researchers across the globe are focusing to develop a new type of PEM that resolved these downfalls and also gives adequate performance than Nafion membrane. Recently, membrane science and technology have received a lot of attention, owing to the vast and promising applications of polymer/inorganic nanoparticle membranes in energy, environment, and biomedical materials. Here are some different types of PEM which have been developed by researcher across the globe and are listed in Table 7.1.

Advanced nanocomposite material for wastewater treatment in microbial fuel cells

Table 7.1 Comparison of proton conductivity, power density, and COD removal efficiency of the different advanced proton exchange membranes. Proton S.I. Membrane Power density COD removal References conductivity

1

2

3

4

5 6

7 8

9

Supported Ionic 103.9 mW m–3 Liquids Membranes (Silms) Polyvinylidene 4.9 mW m–2 Fluoride (PVDF)/ Nafion Nanofiber Activated Carbon 6.1 × 10–2 S cm–1 3.7 W m–3 Derived From Coconut Shell (ACCS/Clay) Activated Carbon 57.64 mW m–2 Nanofiber (ACNF)/Nafion Nanocomposite GO-SPEEK 1.48 × 10–3 S cm–1 902 mW m–2

89.1%

(HernándezFernández et al., 2015)

>70%

(Shahgaldi et al., 2014)

Sulfonated poly 2.56 × 10–5 (ether ether ketone)/poly(ether sulfone) sulfonated biochar 0.077 S cm–1 (SBC-600) Phosphoric 0.073 S cm–1 acid-doped poly(ether sulfone benzotriazole) Fe3 O4 /PES

170 mW m–2

68 ± 6%

1.14 w m–3

81 ± 6.6%

427 mW cm–2

20 mW m–2

81.05 ± 0.08% (Neethu et al., 2019)

82.3%

(Ghasemi et al., 2012)

83.01%

(Leong et al., 2015) (Lim et al., 2012)

(Chakraborty et al., 2020) (K. Wang et al., 2018)

(Rahimnejad et al., 2012)

Sulfonated TiO2 and sulfonated polystyrene-ethylene-butylene-polystyrene (SPSEBS) nanocomposite membrane are prepared by Ayyaru et al. (Ayyaru & Dharmalingam, 2015). According to him, SPSEBS gives the peak power density which was 4 times greater than Nafion 117. The sulfonated composite membrane produced 124% higher power output which improved the proton conductivity and also decreased the oxygen permeability and internal resistance of the sulfonated composite membrane. Nafion/TiO2 nanocomposite membrane was used by Bajestani et al. (Bazrgar Bajestani & Mousavi, 2016), for MFC application. As stated by the author, DMF solvent was better among all solvents and gives the best morphology, the highest porosity, and maximum proton conductivity. Silver GO and GO nanoparticles in sulfonated polyether

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ketone membrane were used for power generation using MFC by Liew et al., (Kien Ben Liew et al., 2020). Proton conductivity was observed 54.2% higher and reduced the oxygen diffusion coefficient about 76.7% than Nafion 117 membrane reported in this article. Nanocomposite membrane composed of nanoalumina within sulfonated PVDF-co-HFP/Nafion blend was synthesized by Vikash Kumar (Kumar et al., 2016). 7.3.2 Nanocomposite materials for electrode fabrication The size of nanomaterials is around 1–100 nm. The properties of NMs are size-dependent and it’s different from bulk parts.The combination of these unique properties of NMs and other materials formed nanocomposites used to improve electrode performance (Kaur et al., 2020). The selection of material for electrodes plays a key role in a MFC. They should be inexpensive, nontoxic, easy to handle, improved power density, environment-friendly, and noncorrosive. To get better efficiency of MFCs, a wide range of electrode materials have been used,ranging from single metal electrodes to composite nanomaterials.The bacterial biofilm formed at the anode acts as a catalyst. The use of chemical energy available in the organic compounds is converted into electrical energy at the same time it reduces oxygen and forms water in the cathode chamber. The performance of MFC in terms of bacterial adhesion, electron transfer, and electrochemical efficiency depends on the selection of good electrode material. The researchers are attempting to scale up the production of electricity using various carbon-based nanomaterials such as carbon paper, carbon felt, carbon fiber, and carbon nanotube-based composites. To carry out MFC technology into practice, material costs must be reduced and power densities have to be increased (Mustakeem, 2015). Furthermore, the cathode materials should have catalytic properties for reducing oxygen, whereas, the anode electrode plays a vital role in the generation of electricity. In recent times, carbon-based materials used such as MWCNT, SWCNT, CNT with different functional groups attached showed promising physical, chemical, and structural properties (Thepsuparungsikul et al., 2014). The CNT and other metal-based anodes with corresponding outputs were summarized in Table 7.2. Stainless steel (SS) mesh coated with MnO2 /carbon nanotube and polymethylphenyl siloxane material is used to fabricate the cathode for power generation in the MFC which was reported by Chen et al. (2012). MnO2 /CNT and PMPS showed higher cathode performance of single-chamber MFC than Pt/C and PDMS cathode material with a low cost of fabrication. Therefore, it will be great material in the preparation of cathode electrodes for commercialization in wastewater treatment. However, the performance of MFC is dependent on the selection of electrode material. Porous material has a high surface area which leads to a high amount of electricity generation with improved performance of the MFC. Thus, to improve the electricity, and mechanical strength of MFCs, carbonaceous materials such as carbon rods, carbon

Advanced nanocomposite material for wastewater treatment in microbial fuel cells

Table 7.2 Comparison of CNT and metal-based anode electrode and power density. Electrode S.I Anode Cathode Power density References

01

09

Activated Carbon (AC)/Polyurethane Cube (PU) PU/Graph/ PPy Polypyrrole Polyurethane based activated carbon sponge CNT coated sponge (0.2) CNT sponge (0.3) Reduced graphene oxide nickel foam Graphene sponge Graphene oxide-CNT TiO2

10 11

NiO/PANI MnO2 Carbon Felt

02 03

04 05 06 07 08

Graphite felt

22 mW m–3

(Sudirjo et al., 2020)

Fecl3

225 mW m–3

Pt/C air cathode

926 mW m–2

(Pérez-Rodríguez et al., 2016) (Liu et al., 2015)

CNT-sponge Pt

990 W m–3

(Xie et al., 2012)

Pt-CC CC

2150 W m–3 661

(Erbay et al., 2015) (Wang et al., 2013)

Carbon paper Carbon cloth (CC) stainless steel mesh (SSM) carbon felt (CF) Ti sheet

427 434

(Xie et al., 2012) (Hassan et al., 2012)

2870

(Ying et al., 2019)

1079 3580

(Zhong et al., 2018) (Zhang et al., 2015)

paper, carbon fiber brush, reduced GO and, graphite felt and metal has been considered as the components of MFC. These compounds can be included in other materials like polymer and metals to achieve good mechanical strength, electricity, thermal stability. SS is considered an effective electrode because of its good mechanical strength, constancy, and resistance to corrosion. Several researchers have been developed modified anode materials by incorporating them into conducting materials (e.g., polyaniline (PANI)) and/or materials with high surface area and porosity, like carbon nanotube (CNTs), to improve fuel cell performance. 7.3.3 Application of MFCs in domestic and industrial wastewater treatment The MFC received lots of courtesy due to its wide applications in wastewater treatment and bioenergy production. Furthermore, using electrochemical reactions, this technology allowed the conversion of embedded chemical energy within organic/inorganic waste into electrical energy using electrochemical reactions As a result, this technology could be useful in wastewater treatment, heavy metal/toxic compound bioremediation, and other applications. Wastewater treatment is a highly energy-intensive process that requires 950 to 2850 KJ/m3 of wastewater requiring treatment. On other hand, some notable studies show that wastewater contains 9.3 times more energy than that used to treat an identical

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Figure 7.3 Schematic diagram of application of MFC in wastewater treatment and electricity production.

volume, highlighting the desire to capture this energy through the use of MFC. The schematic diagram of the application of MFC in wastewater treatment and electricity production in Fig. 7.3. Wastewater treatment remains the most basic sanitation requirement in order to protect the ecosystem, aquatic life, and further that could be used as drinking water reserves. Throughout the world, the activated sludge process is currently established for the treatment of wastewater. This process produces superior results within a reasonable processing time, but this process is energy-intensive which requires high capital, and space, operating, and maintenance costs. The activated sludge process requires aeration, which can count up to 75% of WWTP energy costs, while the treatment and disposal of sludge may count up to 60% of the total operation cost. The United States spends $25 billion annually on domestic wastewater treatment, and another $300 billion is needed for improving publicly owned treatment works. Anaerobic treatment has been practiced for high-strength wastewater and various industrial wastewater and excess sludge streams from the WWTPs. This technology has been developed for over a century to treat wastewater successfully while recovering valuable bioenergy. Direct disposal

Advanced nanocomposite material for wastewater treatment in microbial fuel cells

of wastewater produced by domestic, agricultural, and industrial facilities has a variety of environmental consequences, including eutrophication of surface waters, hypoxia, and algal blooms that contaminate the potential of drinking water supplies. Current wastewater treatment processes consume a lot of resources and chemicals, and they need a lot of money upfront (Gude, 2016). According to Young Ahn et al., energy needs for wastewater aeration was 50% of operating cost and also produced many residual solids which are difficult to treat further and to dispose of. The MFC was efficient to treat domestic wastewater at ambient and mesophilic temperatures. Temperature is also an important factor that affects the performance of MFC. Surajbhan Sevda et al., studied the treatment of complex wastewater using a MFC. Complex wastewater contains total dissolved solids, particulate matter, microscopic nutrients, etc., which can be used as substrate in MFCs and are also effective to cure the high COD removal efficiencies. According to Choudhury et al., dairy wastewater also contains potential substrates that can be treated by MFC to produce stable electricity. Thus, MFC is one of the easiest ways to generate steady power and at the same time, it can treat domestic, dairy, and industrial wastewater. MFC has a high potential to recover heavy metals, hydrogen, and other substrates which can be further used as fertilizer. Using this technology, we can reduce the operating cost of the plant, increase the power and current density. It is easy to operate and does not require skilled labor. Nowadays, researchers are trying to scale up this technology to generate an alternative resource for energy production. In the upcoming future, MFC might be used in the industry to treat wastewater in a continuous process.

7.4 Conclusions and future prospects In this book chapter, we have written about advanced nanocomposite material used in an MFC for wastewater treatment application. MFC is an alternative way to reduce the cost of commercializing WWTPs and simultaneously produce the electricity used for further processes. Some new advanced material used to synthesis of PEM has been discussed in this chapter and also mentioned its application in treating the wastewater. Furthermore, researchers are trying to innovate new material used for the synthesis of the electrode, and PEM with novel nonmaterial to improve the performance of MFC. Based on the current research trends, it is to be noted that, there are a lot of future scopes, where MFC can be a replacement for activated sludge method for wastewater treatment as MFC has the capability to produce electricity and treat wastewater simultaneously.

Acknowledgment The authors acknowledge Membrane Separation Laboratory, CSIR- Indian Institute of Chemical Technology, for providing research facilities, platform, and financial assistance. NetSCOFAN project GAP0837 is acknowledged for supporting this research work. The manuscript communication number is IICT/Pubs./2023/008.

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Common abbreviations MFC NM SS GO TiO2 Ag-GO SPEEK CNT NCNT Pt DBFCs PEMFCs DMFCs ORR SPSEBS AC PU Ppy DMF PVDF-co-HFP/Nafion CSMM PEO/GO PEM Al2 O3 COD PVDF ACCS SBS Fe3 O4 /PES MWCNT SWCNT MnO2 NiO/PANI WWTP CC Ti SSM

Microbial fuel cell Nanomaterial Stainless steel Graphene oxide Titanium oxide Silver graphene oxide Sulfonated Poly(Ether Ether Ketone) Carbon nanotubes Norma; carbon nanotubes Platinum Direct borohydride fuel cell Proton Exchange Membrane Fuel Cells Direct Methanol Fuel Cell Oxidation-reduction Reaction Sulfonated Polystyrene-Ethylene-Butylene-Polystyrene Activated Carbon Polyurethane Cube Polypyrrole Dimethylformamide Polyvinylidene fluoride co-polymer hexafluoropropylene Nafion Charged Surface Modifying Macromolecule Poly (ethylene oxide)/graphene oxide Proton exchange membrane Aluminum oxide Chemical oxygen demand Polyvinylidene Fluoride Activated Carbon Derived From Coconut Shell Sulfonated biochar Iron oxide/polyethersulphone Multi-walled carbon nanotube Single-walled carbon nanotube Manganese dioxide Nickel Oxide/Polyaniline Wastewater treatment plant Carbon Titanium Stainless steel mesh

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Liu, M., Zhou, M., Yang, H., Zhao, Y., & Hu, Y. (2015). A cost-effective polyurethane based activated carbon sponge anode for high-performance microbial fuel cells. RSC Advances, 5(102), 84269–84275. https://doi.org/10.1039/c5ra14644e. Malaeb, L., Katuri, K. P., Logan, B. E., Maab, H., Nunes, S. P., & Saikaly, P. E. (2013). A hybrid microbial fuel cell membrane bioreactor with a conductive ultrafiltration membrane biocathode for wastewater treatment. Environmental Science and Technology, 47(20), 11821–11828. https://doi.org/10.1021/es4030113. Mayahi, A., Ilbeygi, H., Ismail, A. F., Jaafar, J., Daud, W. R. W., Emadzadeh, D., Shamsaei, E., Martin, D., RahbariSisakht, M., Ghasemi, M., & Zaidi, J. (2015). SPEEK/cSMM membrane for simultaneous electricity generation and wastewater treatment in microbial fuel cell.Journal of Chemical Technology and Biotechnology, 90(4), 641–647. https://doi.org/10.1002/jctb.4622. Mehdinia, A., Ziaei, E., & Jabbari, A. (2014). Facile microwave-assisted synthesized reduced graphene oxide/tin oxide nanocomposite and using as anode material of microbial fuel cell to improve power generation. International Journal of Hydrogen Energy, 39(20), 10724–10730. https://doi.org/10.1016/ j.ijhydene.2014.05.008. Modi, A., Singh, S., & Verma, N. (2017). Improved performance of a single chamber microbial fuel cell using nitrogen-doped polymer-metal-carbon nanocomposite-based air-cathode. International Journal of Hydrogen Energy, 42(5), 3271–3280. https://doi.org/10.1016/j.ijhydene.2016.10.041. Mustakeem (2015). Electrode materials for microbial fuel cells: Nanomaterial approach. Materials for Renewable and Sustainable Energy, 4, 22. https://doi.org/10.1007/s40243-015-0063-8. Neethu, B., Bhowmick, G. D., & Ghangrekar, M. M. (2019). A novel proton exchange membrane developed from clay and activated carbon derived from coconut shell for application in microbial fuel cell. Biochemical Engineering Journal, 148, 170–177. https://doi.org/10.1016/j.bej.2019.05.011. Palanisamy,G.,Jung,H.Y.,Sadhasivam,T.,Kurkuri,M.D.,Kim,S.C.,& Roh,S.H.(2019).A comprehensive review on microbial fuel cell technologies: Processes, utilization, and advanced developments in electrodes and membranes.Journal of Cleaner Production,221,598–621.https://doi.org/10.1016/j.jclepro.2019.02.172. Park, D. H., & Zeikus, J. G. (2000). Electricity generation in microbial fuel cells using neutral red as an electronophore. Applied and Environmental Microbiology, 66(4), 1292–1297. https://doi.org/10.1128/ AEM.66.4.1292-1297.2000. Parkash, A., & Aziz, S. (2015). Utilization of sewage sludge for production of electricity using mediated salt bridge based dual chamber microbial fuel cell. Journal of Bioprocessing and Biotechniques, 5(8), 1. http://dx.doi.org/10.4172/2155-9821.1000251. Pérez-Rodríguez, P., Ovando-Medina, V. M., Martínez-Amador, S. Y., & Rodríguez-de la Garza, J. A. (2016). Bioanode of polyurethane/graphite/polypyrrole composite in microbial fuel cells. Biotechnology and Bioprocess Engineering, 21(2), 305–313. https://doi.org/10.1007/s12257-015-0628-5. Rahimnejad, M., Ghasemi, M., Najafpour, G. D., Ismail, M., Mohammad, A. W., Ghoreyshi, A. A., & Hassan, S. H. A. (2012). Synthesis, characterization and application studies of self-made Fe 3O4/PES nanocomposite membranes in microbial fuel cell. Electrochimica Acta, 85, 700–706. https://doi. org/10.1016/j.electacta.2011.08.036. Rahimnejad, M., Najafpour, G. D., Ghoreyshi, A. A., Shakeri, M., & Zare, H. (2011). Methylene blue as electron promoters in microbial fuel cell. International Journal of Hydrogen Energy, 36(20), 13335–13341. https://doi.org/10.1016/j.ijhydene.2011.07.059. Rahimnejad, M., Najafpour, G. D., Ghoreyshi, A. A., Talebnia, F., Premier, G. C., Bakeri, G., Kim, J. R., & Oh, S. E. (2012). Thionine increases electricity generation from microbial fuel cell using Saccharomyces cerevisiae and exoelectrogenic mixed culture. Journal of Microbiology, 50(4), 575–580. https://doi.org/ 10.1007/s12275-012-2135-0. Rozendal, R. A., Sleutels, T. H. J. A., Hamelers, H. V. M., & Buisman, C. J. N. (2008). Effect of the type of ion exchange membrane on performance, ion transport, and pH in biocatalyzed electrolysis of wastewater. Water Science and Technology, 57(11), 1757–1762. https://doi.org/10.2166/wst.2008.043. Shahgaldi, S., Ghasemi, M., Wan Daud, W. R., Yaakob, Z., Sedighi, M., Alam, J., & Ismail, A. F. (2014). Performance enhancement of microbial fuel cell by PVDF/Nafion nanofibre composite proton exchange membrane. Fuel Processing Technology, 124, 290–295. https://doi.org/10.1016/j.fuproc.2014.03.015. Sudirjo, E., Constantino Diaz, P. Y., Cociancich, M., Lisman, R., Snik, C., Buisman, C. J. N., & Strik, D. P. B. T. B. (2020). A thin layer of activated carbon deposited on polyurethane cube leads to new conductive bioanode for (plant) microbial fuel cell. Energies, 13(3), 574. doi:10.3390/en13030574.

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CHAPTER 8

Nanostructured electrode materials in bioelectrocommunication systems Ekhlas Kadum Hamza and Shahad Nafea Jaafar Control & System Engineering Department, University of Technology, Iraq

8.1 Introduction Nanostructures have paved the way to previously untenable oral drug delivery paths, from high surface area advantages regarding the drug nanocrystals to an increase in the penetration and aspect ratio provided via CNTs. Nanostructures have many distinctive properties which might be linked with other materials, forming a synergistic multifunctional theranostic “all-in-one” system (Deng et al., 2019). Exoelectrogens are a distinctive microorganisms class with the capability of shuttling (directly) the electrons exogenously to the external surfaces with no involvement regarding the artificial mediators (Kumar et al., 2016; Tang et al., 2010). One of the main limitations to further applications of bioelectrochemical systems was the fairly low electron transfer between the anode and the bacteria because of high internal resistance; thus, decreased power generation. For improving the performance related to bioelectrochemical systems, one must understand the metabolic impediments, the dynamics regarding electrochemically active bacterial communities in the biofilms, and the way that bacterial species have the ability to be involved in the extracellular electron transfer (Ng et al., 2017). As stated via a recent study, MFC is one of the effective technologies with regard to wastewater treatment. Yet, their electricity output was extremely low; that is why such technology’s commercialization remains a distant dream. There are many literature articles on bioelectrochemical systems. The Jatoi et al. (2021, 2022) attempts to analyze and show new nanostructured materials, acquired via electro-spinning approach, for designing 3D arrangements of electrodes and enhancing the energy efficiency related to energy production devices. Actually, designing novel 3D nanostructured electrodes enhances such devices’ energy efficiency, optimizing the production of energy, acquired via novel technologies of renewable energy. The presented chapter focuses on such devices for generating power output via electrochemical conversion of various fuels, like the wastes, and environmental compounds, including CO2 .

Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00004-8

c 2023 Elsevier Inc. Copyright  All rights reserved.

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A study conducted by Bakshi (Bakshi et al., 2018) reports the proof-of-concept demonstration and rapid fabrication related to hyperbranched gold nanostructure-based electro-chemical immunosensor to detect the protein biomarkers at pg/mL levels and optimizing a simple direct electro-chemical process for quickly generating uniformly distributed high surface area within 30 min. A study conducted by Choi and Yoon (2015) thoroughly reviews many nanomaterials utilized to fabricate the electrochemical capacitor electrodes and provides a summarization of electric double layer capacitors, hybrid capacitors, and pseudo capacitors. As in this paper (Selim et al., 2017), the involvement of microbiology, electrochemistry, and materials sciences in the efficient development and design of BESs are discussed intensively. A study conducted by Srikanth, Kumar, and Puri (2018) discussed the simplicity of utilizing BES as a possible selection to harness energy from various wastes which are generated from petroleum refineries. A study conducted by Selim et al. (2017) showed that the electrode modifications with conductive nanostructured elements were extremely sensitive and enabled direct assay for biofilm formation with no treatments. So, morphological changes in the bacterial cell wall, after switching from a planktonic state to a biofilm matrix, have been imaged by means of SEM; also, the modifications in the cell wall chemical composition have been monitored via EDX. Therefore, the developed microbial electrochemical system has been utilized effectively for monitoring the changes in biofilm matrix within various stresses by direct measurement of the electron exchanges. This chapter’s objective is to understand the basics of Nanostructure electrodes and understanding the major electrochemistry principles. Understanding the applications, instrumentation, and working principles of electrochemical methods. Understanding how the electrochemical methods are utilized for analyzing extra-cellular electron transfer at the interfaces of biofilm-electrodes. Explaining the simplicity of capturing indirect electron transfer (IDET) and direct electron transfer (DET) from electro-active cofactors, which are buried in the native protein or electro-chemically active microorganism to the surface of the electrode via electro-chemical methods. Understanding the way that the strategies of immobilization allow observing the DETs from microorganisms and enzymes on the electrode surface. The basic functions and background of the microbial fuel cell (MFC) as well as understanding the many components and configurations of MFCs, also their contributions to performance and knowing the different MFC-based techniques,and understanding their working principle.Cognition,the latest development in the electrochemical sensor systems for healthcare monitoring.

8.2 Theory background This part will explain in detail what the nanostructure means and how to relate it to a bioelectrochemical system and from the applications that we used in this chapter

Nanostructured electrode materials in bioelectrocommunication systems

are MFCs and MECs, they are two examples of rapidly-developing biotechnology, and another application is Healthcare Applications with bioelectrochemical. 8.2.1 Nanostructure Nanotechnology is providing a direct and efficient way of creating new phenomena and characteristics via a decrease in the sizes of materials with no changes in the chemical composition of the material. Also, nanotechnology can be defined as the capability for manipulating or controlling devices, materials, and systems at the nanometer scale for creating structures with new functions and properties created at the length level (Mirkin, 2000). The size of nanostructures is normally not more than 100 nanometers. Besides, a nanometer is one-billionth of a meter, which is approximately 100,000 times small compared to the human hair width and 100 times small compared to geometrics generally utilized in commercial semiconductor manufacturing nowadays. Nanostructures are of high importance in the development of engineering and scientific technologies at the nanoscale. Recently, nanostructures were the major focus of many studies due to their unique properties influencing the electrical, physical, biological, chemical, and optoelectrical properties. The nanostructures’ composition, including a bimetallic, monometallic, metal oxide, magnetic, hybrid, semiconductor, composite, and so on., was frequently utilized as a base for their classification, while the nanostructured materials utilized in nanosensors manufacturing were: CNTs (extremely high electron conductivity and high surface area), nanoscale wires (the ability of high detection sensitivity), biomaterials, thin films, polymers, also metal and metal oxides NPs (Abdel-Karim et al., 2020; Mubarak et al., 2014). A dramatic improvement in the chemical properties of nanostructures is also achievable because of the high surface-to-volume ratio of nanometer-sized materials. The ability to prepare nanostructures is essential to modern science and technology. This technology was developed to prepare a large number of inverse opals using self-assembled nanostructures-synthetic opals as templates (Cherukuri et al., 2004; Gheith et al., 2005; Mattson et al.,2000).This method provides a simple way to synthesize 3D nanostructures. To date, 2D nanostructures (or quantum wells) have been prepared using techniques such as molecular beam epitaxy by the semiconductor community (Stone et al., 2004). 0D nanostructures (or quantum dots) have also been obtained using various chemical methods (Birkenhauer et al., 2004). This project will investigate some routes used to fabricate new nanostructured materials. These include the use of Inverse Opals and Aligned carbon nanotubes (CNT) as templates and the synthesis of novel nanofibers by electrospinning techniques. Novel properties and functions of the resulting nanomaterials due to the reduction in size are also demonstrated in this research work. Fuel cells were optimum primary energy conversion devices with regard to remote site location and have applications, even though an assured electrical supply was needed

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Figure 8.1 From Rajendran, S., Naushad, M. & Balakumar, S. (2019). The work chart for nanostructuring materials’ development.

for power distribution and power generation (Rajendran et al., 2019). Fig. 8.1 shows the work chart for nanostructuring materials’ development.

8.3 Bioelectrochemical system BES application, an up-to-date multifaceted invention in science working at interface of electrochemistry and biotechnology, to handle many petroleum waste types and valorizing them to value-added or energy products will cause an increase in the refineries sustainability. BES might be providing a complementary or alternative process with lesssevere process conditions as well as high selectivity to certain reactions for disposing of the waste streams in the industry of petroleum (Srikanth et al., 2018). In addition, the bio electro analytical approaches were a class of methods in analytical chemistry that were significant for grasping the mechanism of electron transfer between the electrochemically active agents (exoelectronic bacteria) along with their cognate receivers (i.e., the anodes), through evaluating potential (i.e., volts) and/or current (amperes) in the electro-chemical cells. There are 3 major categories in each one of the electrochemical analytical approaches presented as follows: (1) potentiometric systems—evaluating the differences in potential between electrodes; (2) coulometric systems—evaluating cell current throughout time, (3) voltametric systems—evaluating the current with variation of the potential. 1. The main BES benefits are (1) net positive energy gain or low-energy input required because of biological interventions, (2) reactions’ selectivity, (3) microbes’

Nanostructured electrode materials in bioelectrocommunication systems

Figure 8.2 From Abdel-Karim, R., Reda, Y. & Abdel-Fattah, A. (2020). Journal of the Electrochemical Society, 167(3). https://doi.org/10.1149/1945-7111/ab67aa. Diagram of common BES, which includes its main benefits and multifaceted functions.

adaptability to produce various energy/products, (4) inexpensive operation and design, (5) reactions at ambient conditions, (6) self-regeneration related to biocatalyst, (7) possibilities of high-value uplifts in market. Also, BES might be operated with the use of enzymes or microbes depending on its application as well as the various BES advantages, as can be seen in Fig. 8.2. The potential to integrate physical, biological, and chemical components throughout the operation of BES providing a possibility for initiating many reactions, including electrochemical, biochemical, physic-chemical, bioelectrochemical, and so on, which have been cohesively referred to as the bioelectrochemical reactions (Srikanth et al., 2018). Fig. 8.2 shows the diagram of common BES,which includes its main benefits and multifaceted functions. 8.3.1 Bioelectrochemical systems: how they work MFC and MEC were two examples of rapidly developing biotechnologies, commonly referred to as BES, combining the electrochemical and biological processes for generating hydrogen, electricity, or other significant chemicals. MECs and MFCs were electrochemical cells’ types (batteries were considered as another example). Electrochemical cells include 2 electrodes, cathode, and anode, which were joined via an external wire

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for completing the electrical circuit. MFCs were developed for producing electricity, while the electricity is used by MECs for driving the chemical reactions at cathode for producing hydrogen and/or other chemicals. MECs and MFCs have the ability to achieve this through the use of wastewater, and, in the process, they might be removing the organic matter from wastewater. With regard to MECs and MFCs, distinctive microorganisms’ types, generally bacteria, breaking down the organic material, as indicated in the wastewater, at the anode within anaerobic (with no oxygen) conditions. In the case when the organic material is breaking down, electrons are released via protons (i.e. the positively-charged hydrogen ions), bacteria (i.e., the negatively charged particles), as well as CO2 into solution. Besides, the electrons are collected via the anode, which is after that traveling to cathode through an external circuit (electric current might flow). Protons travel through solution in the cell to the cathode. The CO2 might be captured and reused. With regard to MFCs, the electricity was generated through extracting it from electron carrying external circuit. Also, electrons that arrive at the cathode within aerobic conditions, for instance, with the existence of oxygen, are combined with oxygen and protons, generally from air to form water. MEC is specified as modified MFCs. In the case, when excluding the oxygen from the cathode, the electrons will be released via the bacteria in the case of breaking down organic matter arriving at the cathode combined with protons for producing hydrogen. The reaction doesn’t happen in a spontaneous way, and some amount of external energy (along with that created via bacteria) must be added to the system for driving such a process. MECs might be set up in a way that other, preferably high-value, products like caustic soda might be formed. Fig. 8.3 shows BES utilizing wastewater treatment as instance. Diagram of a common configuration of the two major bioelectrochemical wastewater treatment systems: (1) MFC, (2) MEC for hydrogen production. 8.3.2 Extracellular electron transfer (EET) The way that microorganisms are driving BESs is one of the major issues facing researchers; before commercially using BES, the performance of electrodes and bacteria must be enhanced; thus, the “electron transfer” is also improved. This might result in more production of hydrogen, electricity, or other chemicals. Specific microorganisms’ species (bacteria) with the ability of releasing the electrons from inside of their cells to an electrode were the base for BESs. The approach in which such electrically-active bacteria are doing this is referred to as EET (Harnisch et al., 2011). There are two major electron transfer approaches direct transfer of the electrons from the bacteria that is physically attached to the electrode and indirect electron transfer from bacteria that isn’t physically attached to the electrode. 1. Direct transfer of the electrons between bacteria and electrode may happen in two ways.

Nanostructured electrode materials in bioelectrocommunication systems

Figure 8.3 BES utilizing waste-water treatment as instance. Diagram of a common configuration of the 2 major bioelectrochemical wastewater treatment systems: (A) MFC, (B) MEC for hydrogen production.

2. In the case of the existence of physical contacts between the outer structures of the membrane of microbial cell and electrode surface. Those outer structures will be linked as well to the microorganism’s inner structures, which allows the transport of the electrons from within microbial cell, via the wall of the membrane and in a direct manner on to electrode. 3. In the case of the transfer of electrons between microorganism and electrode via tiny projections (which are referred to as the nanowires or the pili) extending from the microorganism’s outer membrane and attach themselves to electrode. Due to the fact that the nanowires have the ability of reaching across tens of the microns, the microorganisms that are more distant from electrode may still be maintaining direct contact with electrode. In that regard, DET requiring the physical contact between electrode and microbe has been accomplished through producing a conductive layer in outer-membranes, which is observed in Fig. 8.4A (Harnisch et al., 2011). However, the bacterial nanowires have been created by Shewanella Oneidensis MR1 in its outer membrane for extra-cellular transport of the electron (El-Naggar et al., 2010; Pirbadian et al., 2014; Reguera et al., 2005).

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Figure 8.3, cont’d.

Indirect electron transfer from bacteria to an electrode happens through long-range electron shuttle compounds that could naturally exist (for instance, in the wastewaters), or they could be produced from the actual bacteria. The electrons are transported initially to the surface of the bacterial cell and they are gathered and then transported by the shuttle compounds to the electrode. Through the use of one or several of those methods for transferring the electrons to electrode, microorganisms have the ability of growing around the electrode, which results in the buildup of multilayered films, which have been

Nanostructured electrode materials in bioelectrocommunication systems

Figure 8.4 From Selim, H. M. M., Kamal, A. M., Ali, D. M. M. & Hassan, R. Y. A. (2017). Electroanalysis, 29(6), 1498–1505. https://doi.org/10.1002/elan.201700110. DET through the natural conductive cell-wall formation; B: DET through natural conductive nanowires (pili) formation.

referred to as the biofilms. The larger the electrode’s surface area, the higher it is the possibility for the development of the bacterial films. This implies in turn that there is a higher likelihood for generation of the electron, in turn meaning a greater amount of the electricity, hydrogen, or other chemical compounds may be generated (Rozendal et al., 2008).

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Figure 8.5 From Li, S., Cheng, C. & Thomas, A. (2017). Advanced Materials, 29(8). https://doi.org/ 10.1002/adma.201602547. Schematic showing the setup, benefits, applications, and output of a microbial fuel cell (MFC).

8.4 Bioelectrochemical fuel cell In the fuel cell context, the electro-chemical cell can be defined as a device producing electric current a redox reaction result. It includes two half-cells. Every one of those half cells includes one electrolyte and one electrode. The cathode is an electrode where the reduction (i.e., electron gain) takes place, whereas, the anode is an electrode in which the oxidation (i.e., electron loss) happens. Microbial electrochemical technology (MET) is quite good for converting the waste into the energy as well as other important products. The MET was helpful to develop a wide range of the implementation such as the MFC, microbial electrolysis cells (MEC), bioelectrochemical treatment systems (BET), metal recovery, microbial electrosynthesis (MES), and microbial desalination cells (MDC). Optimally, the MFC includes 2 compartments, a cathode chamber, and anode chamber, separated through a proton exchange membrane (PEM), as can be seen from Fig. 8.5. Previous discussions have firmly supported the significance of the electrode materials in the future MFC development as one of the reliable technologies. The optimal material of the anode must be identified for the high specific surface area, good electrical conductivity, porosity, biocompatibility, high mechanical strength, lower costs, and good chemical stability. Those characteristics play a role in an enhancement of the microorganisms’ attachment to an anode, more preferable transfer of the substrate

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mass, and enhancement of degradation rate of the substrate and the efficiency of the power generation. A variety of the materials that range from the stainless steel to the versatile carbon-based materials in a form of the graphite foam, brushes, felt, CNT, and graphene were studied as anodes The catalytic materials, such as the polyaniline–platinum composites, polypyrrole, etc., resulted in the enhancement of the efficiency of the MFC by several times, due to the fact that they have facilitated the direct microbial metabolites’ oxidation (in this section, various anode materials that have been lately explored will be discussed, such as the carbon- based materials, metal oxide electrodes, metals, and alloys electrodes, composite and functionalized electrodes, nanostructured materials, as well as the conducting polymers. For the purpose of the improvement of the properties and efficiency of the fundamental MFC was the main emphasis of the research, up until now, the devices utilizing bioelectrochemistry principles are gaining interest in parallel. Table 8.1 shows some of the MFC based Technologies. 8.4.1 Electron transfer for MFC Carriers are specific molecules that have the ability to carry chemo signals or molecular structures that are having the data. Utilizing molecules as data carriers in molecular communication has been noticed in biological networks. The carriers that are utilized can be molecular motors or calcium ions. Molecular motors such as kinesin, dynein and myosin are proteins, which give rise to movements and utilize chemical power, that carries an information molecule packet from transmitter to receiver (Harnisch et al., 2011). requirement for establishing a transfer of electrons from microbial cell to an electrode is that the electrons will be transferred from within the membrane of the microbial cell to its outside, either through physical transfer of the reduced compounds or through the electron that hop across membrane with the use of membrane-bound redox enzymes. For a sufficient transfer of the electrons, linking species have to be fulfilling the requirements below: 1. It has to have low over-potential of oxidation at certain surfaces of the electrode to sufficient electroactive species. 2. It has to have the ability of physically contacting the surface of the electrode. 3. The linking species’ standard potential has to be as near as possible to primary substrate’s redox potential, or have to at least be considerably negative to that of oxidants, (typically the oxygen). 8.4.2 Healthcare applications with bioelectrochemical systems In areas of biological analysis, healthcare, environmental monitoring, food safety, and chemical analyses, biosensors became one of the most significant practical tools. In comparison with the traditional analytical approaches, biosensors are considerably smaller

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1-Sediment MFC (SMFC)or benthicMFCs (BMFC)

The BMFC includes an anode that is placed at anoxic area and cathode that is placed at oxic area. Electrodes are connected via a load, for instance, a sensor or resistor

It had operated toward the utilization of the gradients of reduction–oxidation, which can be found over the interfaces of sediment–water. The microbial aerobic respiration takes place at the interface of sediment–sea-water, in which the oxygen is predominant. At conditions of the deeper anoxic sediments, the microbes utilize the oxidants for performing the sulfate fermentation, reduction and methano-genesis. 2-Body Fluid Batteries Applications of the fuel cells in the IMDs (i.e. implantable medical devices) has been considered as one of the very promising fields, due to the fact that it will result in the considerable reduction of the pain and cost of replacing the lithium-ion batteries in the implants.

3-Sensors of Toxicity

Biotoxicity assays are presently highly significant for the assessment of the general toxicity of a certain sample. Which is why, the MFC has the ability of providing non-energy-consuming, inexpensive systems of toxicity detection, where the generated signal amplitude may give aninsight about the sample’s toxicity degree.

MFC that is implanted in colon, the anode (composed of a biocompatible material) has been attached to the intestine’s inner walls in which the anaerobic microorganisms are predominant, and cathode has been placed in middle part of intestine, in which there’s oxygen. None-the-less, the power output has been low, and that may can be a result of the low O2 concentration (Han etal. 2010). Determining the toxicity of the waste-water that contains pesticides, antibiotics, and heavy metals, and inhibition of detectable levels have been recorded.

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Table 8.1 Word shows some of the MFC-based technologies. MFC Technologies Description

4-Quantification & Identification

6- MFCs as BOD Sensors

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5-Analyte Sensors

This fuel cell application is nearly 50 years old, where The electrode system included a silver peroxide the scientists have been capable of developing an cathode and a platinum anode. It operated on a electrode system to quantify the microorganism concept of microorganisms’ oxidation and silver populations peroxide reduction, and it might estimate the populations of the microbial cells in a range between 108 cells/mL and 109 cells/mL. A membrane filter has been utilized for trapping microbes, and the redox dyes have been utilized as mediators for the transfer of the electrons (Matsunaga et al., 1979). Genes that are responsible for degrading the target None-the-less, such sensitivity and specificity can’t be substrate are inserted up-stream of any of the accomplished in the MFC (Abrevaya et al., 2015). reporter genes, synthesizing the enzyme that is responsible for degradation and generating detectable signals. This approach provides an added benefit of having The MFC-based biosensors produce a signal that is the ability for avoiding interference that results equal to the sample’s carbonaceous biological from the oxidation of the inorganic substances, oxygen demands. MFC-based BOD sensors have like the iron or a sulfur and nitrogen compound been designed based on a predicted BOD value. For the samples in which the BOD values < 10mg/l, the oligotrophic bacteria have been utilized for the colonization of the anode (Rabaey et al. 2009).

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devices, providing real-time information and are optimal for fast analysis and measurement (Li et al., 2017). There are applications in the areas of the healthcare, classifying electro-chemical sensors to 3 main types, which are (1) wearable electro-chemical sensing systems and (2) POC sensing systems. 8.4.3 POC sensing systems POC testing has been characterized as conducting a medical prognostic or diagnostic study close to the patient for providing fast results, which is why, the actual total analysis is going to be fast and simple without using complicated or expensive instrumentations. The latest improvements in the sensor fabrication technologies, nanotechnology, microfluidics, microelectronics, and polymers resulted in producing sufficient POC diagnostic devices for the monitoring of the healthcare. The combination of the microfluidics (manipulation of sample fluids’ only nano or picoliters) with the microelectronics provides the ability for the accurate control of the fluids in the assay, and resulted in making them preferred candidates for the replacement of the conventional experimental methods (Logan et al., 2006) according to technological advancements, the electrochemical POC sensors were categorized to the first, second, and future generation POC devices. Fig. 8.6 shows the using LMP-91000 evaluation board has been re-configured for the purpose of performing three electrode cyclic measurements of voltammetry for the purpose of achieving the electro-chemical biosensing of the cortisol (Rozendal et al., 2006). The micro-electrodes for the estimation of the cortisol have been fabricated through the immobilization of the monoclonal ant cortisol antibody onto self-assembled monolayer (SAM) modified gold micro-electrodes. 8.4.4 Wearable electrochemical sensing systems The wearable sensing devices are capable of monitoring the health data and tracking the physiological activities in the real time. The sensors in the wearable format are preferable for the long-term monitoring of the biological signals (i.e., minimally invasive or noninvasive), as wearable gadgets are in a continuous contact with the body of the wearer. The rigid solid materials of the electrode or the traditional electrodes aren’t appropriate for the body wearable applications and are incompatible with the current flexible electronic devices. The impact of the variety of the material geometries and electrode materials was researched for the purpose of improving the wearable biosensor device functionality. The researchers have lately made considerable attempts toward the development of the wearable electro-chemical sensors, which have the ability of monitoring the analytes in the biofluids like the saliva, tears, wound fluids, and sweat (Pareek et al., 2019; Sackmann et al., 2014; Shetti et al., 2019). The characteristics of those materials may be tuned toward being biocompatible, light-weight, and soft for the comfortable wear.

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Figure 8.6 From Rozendal, R. A., Hamelers, H. V. M. & Buisman, C. J. N. (2006). Environmental Science and Technology, 40(17), 5206–5211. https://doi.org/10.1021/es060387r. Electrodes integrated in portable potentiostat for the detections of the cortisol.

8.5 Conclusion and future perspectives Since 1911, bioelectrochemical systems are in continues progress. Many types of nanosensors have been reviewed, categorized and discussed according to energy source, structure, and materials. In general, optical nanosensors are very useful for chemicals monitoring inside cell. Many nanostructured materials applied for nanosensors were presented such as CNT, polymers, and biomaterials. Despite the relatively short history of nanosensors, the progress established in this area has been remarkable. With the continuing progress in nanotechnology tools and increasing research on the nanoscale phenomena, one may expect further achievements in the field of nanosensors. This can be reached through the enhanced performance of existing nanosensors and newer nanosensors based on novel mechanisms. This chapter reviewed the recent advances made in electrochemical sensing systems for healthcare monitoring. Electrochemical sensors were classified into three categories based on their functions and applications. POC sensors are suitable for frequent monitoring or single-use applications. Wearable sensors are much more suitable for activity tracking or closely monitoring therapeutic progression. Recent advances in material

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engineering brought electro-chemical sensing technologies to the next levels. Fabrics and flexible substrate-integrated sensors, being attached to human skin, monitor elevations in biomarkers levels continuously and alert the wearer for possible threats. Implantable sensors, on the other hand, find applications in monitoring neurochemicals in vivo in real-time. Although direct monitoring of blood biomarker levels (POC sensor) assures accurate health information, the invasive needle-pinching step to collect blood often results in patients (particularly young patients) being reluctant to adopt the process. Sensors in wearable or implantable form may avoid such burden of repeated blood collections, but it is quite invasive (implanted sensor) and requires periodic replacement of the sensor owing to biofouling and its short lifetime.

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Logan, B. E., Hamelers, B., Rozendal, R., Schröder, U., Keller, J., Freguia, S., Aelterman, P., Verstraete, W., & Rabaey, K. (2006). Microbial fuel cells: methodology and technology. Environmental Science and Technology, 40(17), 5181–5192. doi:10.1021/es0605016. Mattson, M. P., Haddon, R. C., & Rao, A. M. (2000). Molecular functionalization of carbon nanotubes and use as substrates for neuronal growth. Journal of Molecular Neuroscience, 14(3), 175–182. doi:10.1385/JMN:14:3:175. Mirkin, C. A. (2000). Programming the assembly of two- and three-dimensional architectures with DNA and nanoscale inorganic building blocks. Inorganic Chemistry, 39(11), 2258–2272. doi:10.1021/ic991123r. Mubarak, N., Abdullah, E., Jayakumar, N., Sahu, J. (2014). An overview on methods for the production of carbon nanotubes. Journal of Industrial and Engineering Chemistry, 20(4), 1186–1197. Ng, I. S., Hsueh, C. C., & Chen, B. Y. (2017). Electron transport phenomena of electroactive bacteria in microbial fuel cells: a review of Proteus hauseri. Bioresources and Bioprocessing, 4(1), 1–17. doi:10.1186/s40643017-0183-3. Pareek, A., Shanthi Sravan, J., & Venkata Mohan, S. (2019). Graphene modified electrodes for bioelectricity generation in mediator-less microbial fuel cell. Journal of Materials Science, 54(17), 11604–11617. doi:10.1007/s10853-019-03718-y. Pirbadian, S., Barchinger, S. E., Leung, K. M., Byun, H. S., Jangir, Y., Bouhenni, R. A., Reed, S. B., Romine, M. F., Saffarini, D. A., Shi, L., Gorby, Y. A., Golbeck, J. H., & El-Naggar, M. Y. (2014). Shewanella oneidensis MR-1 nanowires are outer membrane and periplasmic extensions of the extracellular electron transport components. Proceedings of the National Academy of Sciences of the United States of America, 111(35), 12883– 12888. doi:10.1073/pnas.1410551111. Rajendran, S., Naushad, M., & Balakumar, S. (2019). Nanostructured materials for energy related applications. Reguera, G., McCarthy, K. D., Mehta, T., Nicoll, J. S., Tuominen, M. T., & Lovley, D. R. (2005). Nature, 435(7045), 1098–1101. doi:10.1038/nature03661. Rozendal, R. A., Hamelers, H. V. M., & Buisman, C. J. N. (2006). Environmental Science and Technology, 40(17), 5206–5211. doi:10.1021/es060387r. Rozendal, R. A., Hamelers, H. V. M., Rabaey, K., Keller, J., & Buisman, C. J. N. (2008). Trends in Biotechnology, 26(8), 450–459. doi:10.1016/j.tibtech.2008.04.008. Sackmann, E. K., Fulton, A. L., & Beebe, D. J. (2014). Nature, 507(7491), 181–189. doi:10.1038/nature13118. Selim, H. M. M., Kamal, A. M., Ali, D. M. M., & Hassan, R. Y. A. (2017). Electroanalysis, 29(6), 1498–1505. doi:10.1002/elan.201700110. Shetti, N. P., Bukkitgar, S. D., Reddy, K. R., Reddy, C. V., & Aminabhavi, T. M. (2019). Colloids and Surfaces B: Biointerfaces, 178, 385–394. doi:10.1016/j.colsurfb.2019.03.013. Srikanth, S., Kumar, M., & Puri, S. K. (2018). Bioresource Technology, 265, 506–518. doi:10.1016/j.biortech. 2018.02.059. Stone, G. W., Ellis, S. G., Cox, D. A., Hermiller, J., O’Shaughnessy, C., Mann, J. T., Turco, M., Caputo, R., Bergin, P., Greenberg, J., Popma, J. J., & Russell, M. E. (2004). New England Journal of Medicine, 350(3), 221–231. doi:10.1056/NEJMoa032441. Tang, X., Du, Z., & Li, H. (2010). Electrochemistry Communications, 12(8), 1140–1143. doi:10.1016/j.elecom. 2010.06.005.

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CHAPTER 9

Nanomaterials supporting biotic processes in bioelectrochemical systems Rezoana Bente Arif a and Nabisab Mujawar Mubarak b

a Department of Electrical and Electronic Engineering, IUBAT-International University of Business Agriculture and Technology, Uttara, Dhaka, Bangladesh b Petroleum and Chemical Engineering, Faculty of Engineering, Universiti Teknologi Brunei, Bandar Seri Begawan, Brunei Darussalam

9.1 Introduction The demand for electricity is in a growing phase as time goes on. Most of the commercial methods available for electricity generation, such as natural gas, coal, nuclear energy, and so on release an extreme amount of carbon dioxide into the environment. To suppress this emission of CO2 gas as well as to minimize the environmental pollution besides the usage of renewable energy, microbial fuel cell (MFC) is a favorable method for different purposes; for example, wastewater treatment, electro-hydrogenic process, and biosensing applications (Cao et al., 2009; Cheng & Logan, 2007; Yifeng Zhang & Angelidaki, 2011; Jatoi et al., 2021, 2022). The addition of nanomaterials in MFCs in various aspects showed a noticeable uplift in cell performance. Nanomaterials have been utilized in different ways to improvise cell performance. In some cases, they are used for modification of the electrodes (Alatraktchi et al., 2014; Yang et al., 2012) for better electricity generation, wherein in other cases, they can be utilized as electrode catalysts (Singh & Verma, 2015). Since MFC is a process that deals with organic materials, the biocompatibility of the system is a huge concern. Environmental remediation can be achieved by using nanomaterials in MFCs (Bikshapathi et al., 2012). Due to having a huge surface area for better chemical reaction, nanomaterials are preferred to be used in MFC. Considering multiple advantages of the different nanoparticles, they are considered to be quite compatible to be used in MFCs. In this chapter, the application of different nanomaterials in MFC for improving its performance is described.

9.2 Nanomaterials used in biocell During the last few decades, platinum (Pt) has been ruling as a catalyst in MFC. Pt has been exhibiting great performance in MFC regarding the power as well as the current density. However, its high cost is a vital concern for the availability of MFC Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00006-1

c 2023 Elsevier Inc. Copyright  All rights reserved.

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in various fields. On the other hand, due to having a different texture, size, and behavior of nanoparticles, they are gradually becoming a better choice for researchers. The larger surface area of the nanoparticles boosts up the reaction, and thus the oxygen reduction reaction (ORR) gets accelerated. Furthermore, using NPs in MFC, the bacterial growth on the electrodes can be minimized to improve the performance of the cell dramatically. 9.2.1 Carbon nanotubes In recent times, the activated carbon, as well as carbon nanotubes (CNT), has been having increasing demand in improvisation of the performance of MFC to replace the existing catalyst, which is comparatively more expensive for commercialization. In order to fasten the ORR, a versatile form of Carbon is being utilized in the form of a catalyst or modified electrode in MFC. Different forms of CNT obtained from bio plants were demonstrated in the past (J. Tang et al., 2019; L. Zhou et al., 2016; Mazari et al., 2021), which are more cost-effective, eco-friendly, higher ORR rate, and showed almost similar or a little better performance in terms of power density than metallic catalyst such as Pt or modified with Pt. in MFC. Recently Huang et al. (2021) demonstrated that catalyst with CNT showed the best performance in terms of voltage as well as electrical conductivity and energy. Platinum is the most popular catalyst in MFC. However, due to its high cost, it has been under research to get replaced by other materials. Due to having larger surface in nanoparticle than a bulk particle, nanomaterial, especially CNT with composite materials, are preferably the best option to replace Pt for higher efficacy in MFC. Pt/CNT composite illustrated better ORR performance as well as efficiency, electricity power generation with reduced cost than only Pt catalyst in MFC (Halakoo et al., 2015). MnCO2 O4 /C-60 nanocomposite catalyst provided a better ORR rate than Pt/C catalyst and came up with better efficiency than m- MnCO2 O4 /C-60 in MFC (Hu et al., 2015). Because of exhibiting high performance in ORR as a result of high electron transformation, highest power and efficiency compared to CNT and functionalized CNT, MnO2 /f-CNT composite was considered to be a better catalyst in MFC. And also, due to its harmless nature, it can be replaced by a costly Pt-C catalyst (Liew et al., 2015). Huang et al. (2015) utilized Ni/CNT composite to reduce the cost and improve the ORR of MFC analyzing the characteristics of Ni/CNT composite using transmission electron microscopy (TEM), XRD, and Raman spectroscopy and found that 77% of Ni/CNT combination provided the highest electron transformation (Fig. 9.1). Moreover, some CNT composites are capable of reducing the toxicity of the material into MFC; e.g., Zhou et al. (S. Zhou et al., 2020) utilized n-doped CNT activated carbon composite catalyst obtained from paper clay in order to reduce harmful chromate (VI) to less detrimental chromate (III) as well as to improve the performance of MFC. Recently, including CNT into other combinations of materials such as cobalt/nitrogen provides

Nanomaterials supporting biotic processes in bioelectrochemical systems

Figure 9.1 Adapted with permission from Huang, J., Zhu, N., Yang, T., Zhang, T., Wu, P. & Dang, Z. (2015). Nickel oxide and carbon nanotube composite (NiO/CNT) as a novel cathode non-precious metal catalyst in microbial fuel cells. Biosensors and Bioelectronics, 72, 332–339. https://doi.org/ 10.1016/j.bios.2015.05.035. Electron Pathway and NiO/CNT Catalytic Process (after Huang et al., 2015).

an increased rate of ORR notably, which is equivalent to the rate in Pt/C (Zhong et al., 2021). On the other side, although Pt/C provides the highest power density, in terms of cost, it is yet not appropriate to get commercialized. Rather Xu et al. (2019) found that MnS2 /CNT composite provides almost 22 times less per watt cost than Pt/C, although and on top of that, the production cost was significantly less in CNT than Pt/C. Instead of experiencing better ORR acceleration, low cost, and improvement in the power density, for long-lasting usage, further improvement in CNT catalyst in MFC is required. Zhang et al. (2018) demonstrated that the MFC power density increased until three months after production in the case of CNTs-modified graphite felt (CNTs-GF), but after 13 months, it started decreasing. An MFC without the membrane along with CNT-based catalyst was proposed by Christwardana and Kwon (2017), where yeast/CNT exhibited significantly high-power density than only the CNT catalyst. All in all, it can be said that CNT-based catalysts are providing a promising future of MFC in terms of cost, longevity, faster ORR, and in some cases, better power density. Fig. 9.1 provides Electron Pathway and NiO/CNT Catalytic Process (Huang et al., 2015). 9.2.2 Gold nanoparticles Gold Nanoparticle (AuNP) is one of the highly used nanoparticles for different purposes, and likewise, it is also quite popularly used in MFCs in order to improve the cell performance in terms of power and electricity generation, self-cleaning of the cell, efficacy development, acceleration of the reaction rate and so on. Using the different amounts of AuNP in MFC, the power can be improved as well as the microbial

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Figure 9.2 Adapted with permission from Han, T. H., Khan, M. M., Kalathil, S., Lee, J. & Cho, M. H. (2013). Simultaneous enhancement of methylene blue degradation and power generation in a microbial fuel cell by gold nanoparticles. Industrial & Engineering Chemistry Research, 52(24), 8174–8181. https://doi.org/10.1021/ie4006244. A Schematic Diagram for Methylene Blue (MB) Degradation in MFC Using AuNP (after Han et al., 2013).

community in the cell can be controlled. AuNP can be utilized with its diverse density on the Carbon made electrode, and with a higher density of AuNP thickness on the carbon electrode, the power, and current density can be increased. Also, the fewer amount of AuNP provides less period of deployment of COD (Han et al., 2013) which is illustrated in Fig. 9.2. Another example of Au NP on carbon fiber in MFC was demonstrated by Duarte et al., 2019), where the usage of Au nanoflower on the surface-functionalized carbon fiber anode showed the best power density amid other combinations used in that particular study. AuNP has also been beneficial in wastewater treatment using MFC. A harmful chemical, methylene blue, which is a commonly used fabric dye, contaminates the water and is also harmful to living objects. Using MFC with positive changed AuNP, the reduction of this chemical is perfectly viable. Again, by controlling the layer of AuNP peptide, the charge transfer rate of the MFC cell can be controlled, and the power density of the cell can be improved (Jahnke et al., 2019). 9.2.3 Silver nanoparticles Another famous nanoparticle used in MFC is silver nanoparticle or silver of nanoparticle with other materials. In wastewater treatment, the different fungus starts inhabiting the cathode and affects the performance of the MFC where silver nanoparticle (Ag-NP) is a promising material to eradicate these fungi and improve the efficiency or the performance of the cell. One research illustrated that addition of Ag-NP into MFC at 5%, and 10% upgrades the power density and the coulombic efficiency significantly. The power density increased to 7.9 ± 0.5 W/m3 and 9.8 ± 0.3 W/m3 for Ag-NP of 5% and 10% respectively,

Nanomaterials supporting biotic processes in bioelectrochemical systems

Figure 9.3 Adapted with permission from Saravanakumar, K., MubarakAli, D., Kathiresan, K., Thajuddin, N., Alharbi, N. S. & Chen, J. (2016). Biogenic metallic nanoparticles as catalyst for bioelectricity production: a novel approach in microbial fuel cells. Materials Science and Engineering: B, 203, 27–34. https://doi.org/10.1016/j.mseb.2015.10.006. Spherical Shaped Silver Nanoparticles used in Microbial Fuel Cell (after Saravanakumar et al., 2016).

from 1.7 ± 0.3 W/m3 for MFC without Ag-NP. On top of that, the increased coulombic efficiency was 10%–26% and 12%–29% for 5% and 10% Ag-NP as a catalyst in MFC, which was 8%–14.5% without Ag-NP (Noori et al., 2016). Spherical Ag-NP is one of the metallic NP catalysts in MFC (Fig. 9.3), which is beneficial for the oxygen-free condition to improve the bioelectricity in a cost-effective and environment-friendly manner (Saravanakumar et al., 2016). Composite of Ag-NP with other materials is helpful to uplift the ORR, coulombic efficiency as well power density to some extent. Accumulation of biofilm on the cathode in MFC is a concerning phenomenon as it reduces the efficacy of the system. Composite with Ag-NP and Ag-NP with iron oxide (Ma et al., 2015) Ag/FeS/PGC composite catalyst (Sun et al., 2017) in most of the cases showed better effectiveness than only conventional Pt/C catalyst in the antibacterial as well ORR process which assisted to improvise the power density and the efficiency of the MFC. Furthermore, in order to remove the salt and other biomaterials in Microbial desalination cell with Ag-SnO2 composite catalyst provided 1.67 times larger power density and noteworthy higher coulombic efficiency than desalination cell without any catalyst (Anusha et al., 2018).

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On top of that, due to large surface area of Ag-NP/AC composite provides effective nature in bacterial removal and prevention of the formation of biofilm in wastewater treatment with a stable cell voltage and adequate power density (Sallam et al., 2021). Recently, new approaches in order to gain better proton conduction, higher blockage on oxygen passing, and high bacterial removal phenomena Ag-NP-based membranes are showing decent natures. Liew et al. (2020)) attained that as a cell membrane, AgGOGO-SPEEK provided better performance than Nafion 117 in terms of internal resistance, thickness as well as the inhibition of biofilm on the electrode, power density, coulombic efficiency, and proton conductivity. Although AgGO-GO-SPEEK in some cases showed a little inferior performance than GO-SPEEK, it provided highly stable behavior with time. In low-energy wastewater treatment, silver nanoparticle coated on commercially available membrane lessened the aggregation of bacteria on the membrane significantly (Hirsch et al., 2019).

9.2.4 Zinc-modified nanoparticles in MFC activities The usage of Zinc Nanoparticles (Zn-NP) has been uplifting in the past few years and providing satisfactory outcomes. Due to their larger surface area for being metal nanoparticles and because of the shape, comparatively higher bandgap, they are capable of suppressing bacterial growth, which can be utilized for wastewater treatment, food treatment, hygiene maintainer in industrial production and other bioprocessing systems (Hossain et al., 2014; Yamamoto et al., 1998). Escherichia coli is a common bacterial pathogen in biosamples which is a big concern to be eliminated. ZnO nanoparticles can defend both Gram-positive and Gram-negative E. coli bacteria (L. Zhang et al., 2008) which is beneficial to keep the electrode protected in MFC from bacteria habitation. ZnO NP, along with di-ethylene glycol, were capable of replacing the bacterial layer from the membrane, leaving the increased permeability of the membrane (Brayner et al., 2006). Furthermore, Carbone et al. (Carbone et al., 2017) proposed a faster E. coli removal process using ZnO NP by optical characterizations of time and temperature. As ZnO NPs are toxic to the bacteria, they can also remove the bacteria in biogas production from sludge (Nguyen et al., 2015). Zn NPs are well-suited metal NPs for electricity generation utilizing the biowastes in MFC. ZnO-CuO photocathodes used in photocatalytic fuel cells showed better absorption of visible light due to having a larger surface area. On top of that, this property of ZnO NP also assisted in reducing the toxic biowaste, which all together helped improve the performance of the cell in terms of electricity generation and waste removal as shown in Fig. 9.4 (Bai et al., 2016). Wastewater contaminated with Zn2+ showed a significant positive result in bacterial reduction and assisted in improving the cell voltage (Wang et al., 2020). This phenomenon is illustrated in Fig. 9.5. Zn

Nanomaterials supporting biotic processes in bioelectrochemical systems

Figure 9.4 Adapted with permission from Bai, J., Wang, R., Li, Y., Tang, Y., Zeng, Q., Xia, L., Li, X., Li, J., Li, C. & Zhou, B. (2016). A solar light driven dual photoelectrode photocatalytic fuel cell (PFC) for simultaneous wastewater treatment and electricity generation. Journal of Hazardous Materials, 311, 51– 62. https://doi.org/10.1016/j.jhazmat.2016.02.052. Photocatalytic Fuel Cell using Zn/CuO Nanoparticle Photocathodes (after Bai et al., 2016).

Figure 9.5 Adapted with permission from Wang, Q., Lv, R., Rene, E. R., Qi, X., Hao, Q., Du, Y., Zhao, C., Xu, F. & Kong, Q. (2020). Characterization of microbial community and resistance gene (CzcA) shifts in up-flow constructed wetlands-microbial fuel cell treating Zn (II) contaminated wastewater. Bioresource Technology, 302, 122867. https://doi.org/10.1016/j.biortech.2020.122867. Schematic of MFC for the Treatment of Zinc Contaminated Wastewater (after Wang et al., 2020).

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NPs also is used as a catalyst to enhance the performance of MFC by increasing the ORR and the electron transportation rate between two electrodes. Cobalt and ZnO with graphene oxide were demonstrated by Yang et al. (2019), where Zn and Co were taken at a 1:1 ratio as an ORR catalyst. It has been demonstrated in that study that the biofilm growth on cathode was discontinued upgrading the maximum cell power up to 773 mWm–2 which is more significant than conventional Pt/C catalyst-based MFC. Zn NPs are also beneficial in designing photo-catalytic fuel cells. A photo-catalytic fuel cell with ZnFe2 O4 /Ag/Ag3 VO4 was used as a photoelectrode in one study by Tang et al. (2020), which improves the cell performance and can be utilized in solar hydrogen manifestation as well as the improvement in solar power with the removal of biowaste. Nano Co0.5 Zn0.5 Fe2 O4 catalyst prepared by sol–gel combustion showed the outstanding performance to accelerate the ORR with minimal charge transfer and coulombic efficacy of 43.3%. Moreover, it was significantly less costly than Pt metal catalyst (Das et al. 2018). 9.2.5 Others Nickel and titanium are other two metals found to be used in different MFCs either as a catalyst or composite material, or electrodes. The increase in pollutants in the environment and natural energy shortage is becoming a great concern day by day. In this circumstance, different metal-dependent batteries are pretty popular to produce energy, among which Zn-air batteries are primarily in use (Liu et al., 2016; Zhu et al., 2011). Besides, Zn-air batteries having extraordinary energy capacity, high storage capacity, and nondetonating behavior, its discharging performance (Li et al., 2013), better electrode catalysts are challenges to improve ORR as well as cell performance where Pt is considered the most used catalyst (Bu et al., 2016). However, due to having a very high cost, its replacement with different nanomaterials or other biomaterials are under research. Nickel composite-based nanomaterials are pretty efficient to improve the stability of the cell by improvising the ORR as well as the electron transformation rate. For example, nickel-cobalt (Ni1 Co3 ) CNT composite catalyst demonstrated by Deng et al. (2018) increased the ORR of the cell and also increased the electron transfer rate. By means of pyrolysis thermal decomposition method of Ni/Co salt, CNT, polyaniline, and dicyandiamide. Nico/CNT was utilized as a catalyst in carbon paper cathode. It was illustrated that Ni/CO-CNT composite improved the electron transfer up to 3.78 with an excellent ORR and convenient current density and cell voltage. Furthermore, Nickel NPs have been noticed to be used in bio MFC where E. coli was used as the catalyst. Singh and Verma (2015) demonstrated that Nickel NP-based CNT electrode in MFC uplifted the open-circuit voltage power density of the bio MFC nine times higher than MFC with only activated carbon fiber. A schematic of MFC with Ni/NPs

Nanomaterials supporting biotic processes in bioelectrochemical systems

Figure 9.6 Adapted with permission from Singh, S. & Verma, N. (2015). Fabrication of Ni nanoparticles-dispersed carbon micro-nanofibers as the electrodes of a microbial fuel cell for bioenergy production. International Journal of Hydrogen Energy, 40(2), 1145–1153. https://doi.org/ 10.1016/j.ijhydene.2014.11.073. Schematic of Nickel Nanoparticle Modified Electrodes in MFC (after Singh & Verma, 2015).

modified electrode is exhibited in Fig. 9.6. Ni/NPs were also beneficial to improve the electricity generation from E. coli biocatalyst and increase the ORR of the MFC. The chemical vapor deposition method was adopted to synthesis Ni/NP on the electrode. In the literature, it was assayed the presence of Ni/NP in microfluidic MFC provided approximately 76% of the power in lithium rechargeable batteries which was demonstrated by (Mousavi et al. (2019). It was found that the large surface area of Ni/NP in the MFC cell improved the performance of the cell and also reduced the electrode gaps, which reduced the cell resistance. It also increased the amount of the attached bacteria in the cell. The magnetic field process was utilized to synthesis Ni/NPs in the mentioned research. Moreover, in another research, Nickel foam along with graphene NP was used in algae cathode for the removal of Cadmium ions. Due to combining graphene NP with Nickel, the power density improved significantly than the power density in the case of only using Nickel (Yu Zhang et al., 2018). Besides different nanomaterials, titanium NPs are also used in some cases. The use of TiO2 NP in MFC provides better conductivity, faster electron transfers, and more bacterial agglomeration, which as a result helps improve the cell performance in terms

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(A)

(B)

Figure 9.7 Adapted from Guo, T., Wang, C., Xu, P., Feng, C., Si, S., Zhang, Y., Wang, Q., Shi, M., Yang, F., Wang, J. & Zhang, Y. (2019). Effects of the structure of TiO2 nanotube arrays on its catalytic activity for microbial fuel cell. Global Challenges, 3(5), 1800084. https://doi.org/10.1002/gch2.201800084 A Schematic of the Structure of Microorganism on (A) TiO2 nanotubes arrays-HF; (B) TiO2 nanotubes arrays-NH4 F Electrodes (after Guo et al., 2019).

of power and current density (Deng et al., 2019). Again, modified TiO2 nanotubes arrays shown in Fig. 9.7 can be utilized as electrodes to vary the cell performance (Guo et al., 2019). On top of that, bioanode within low cost and high stability can be developed by utilizing TiO2 nanotubes arrays (Feng et al., 2016). Cobalt compound nanoparticles are also used to improve the cell ORR as well as performance replacing the conventional metallic MFC (Das et al., 2018).

9.3 Toxicity of NPs and toxicity reduction by NPs in MFC With the increased popularity of nanoparticles in various fields, the usage of NPs in MFC has been upgrading over time. Although the application of NPs in MFC was not that significant before 10 years, the number of assays regarding the involvement of NPS in MFC has been rising for the last ten years. The keywords used to find out the research number that deals with NPs in MFC, were “Nanoparticle” AND “MFC.” As demonstrated in Fig. 9.8A, it was found that in the last ten years, the implication of NPs

Nanomaterials supporting biotic processes in bioelectrochemical systems

(A)

(B)

Figure 9.8 Comparison of the number of research (A) from 2011 to 2020, and (B) from 2001 to 2010 Regarding NPs in MFC.

in MFC has risen dramatically and with the highest range of 612 in a year. On the other hand, this number was way too small between 2001 and 2010, as illustrated in Fig. 9.8B. It indicates the onward popularity of NPs in MFCs in recent years. The NPs can be beneficial in the reduction of toxicity of MFC, as well as they can be harmful to the system. In spite of increasing the amount of application of nanoparticles in MFC for improving cell performance, there is some inconvenience of using NPs in MFC since there are bioproducts which in some cases may get affected by the NPs and result in a degradation of the cell performance. Since the bacterial microorganisms are the source of electron transformation and production of electricity in MFC, any hamper in their presence causes the reduction of electricity production from the cell. Due to their small size and high surface energy NPs specifically metal oxide NPs impacts microorganisms negatively in MFC. Baek and An (2011) demonstrated the toxicity level of different metal oxide NPs on various bacteria such as E. coli, B. subtilis, and S. aureusin in one research. The toxicity level of these metal oxide NPs was characterized by dispersing them into the culture medium and, after a specific period counting the reduced number of the colony-forming unit. It was examined in that research that Copper Oxide was the most toxic metal oxide NP for MFC where ZnO, NiO, and Sb2 O3 followed accordingly. Most common nanomaterial used in MFC as electrode catalysts is CNTs. However, extreme usage of CNT (De Volder et al., 2013) may cause hamper in ecological issues in MFC. In some recent assays, it was found that single-walled CNT (SWCNT), as well as multi-walled (MWCNT), can affect the number of microorganisms in biosamples. For example, SWCNT interrupts the bioaccumulation of S. benedicti (Ferguson et al., 2008) and E. foetida (Petersen et al., 2009), where MWCNT reduces the accumulation of C. plumosus. On the contrary, CNT can be helpful to Polycyclic Aromatic Hydrocarbons

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(PAHs) toxic material reduction (Shrestha et al., 2015). SWCNT can affect the microbial community of the soil negatively as the excessive presence of it affects the amount of phospholipid fatty acid significantly in soil (Jin et al., 2014). Therefore, usage of soil with SWCNT in MFC may affect cell performance. AgNP is another nanomaterial that is quite popularly used in MFC. However, in some phenomena, it can be toxic for MFC. Recently, Zakaria and Dhar (Zakaria & Dhar, 2020) illustrated that disclosure of AgNP in MFC reduced the current density from 29 ± 2.0 A/m3 to 20 ±2.2 A/m3 . Moreover, due to its extremely small dimension, it was found with the help of TEM to be penetrated into the cell, which deteriorated the cell performance, and also, AgNP was noticed to be quite harmful toward some bacteria in the MFC. Moreover, excessive usage of AgNP may create cell toxicity (Akter et al., 2018) because of its small dimension, large surface area, and higher surface energy. Nanomaterials are also used in microbial biosensors for pathogen detection. The addition of MWCNT in the electrode in the microbial system can be utilized to analyze the cytotoxicity (Hassan et al., 2014). Although the NPs may affect the cell performance in some phenomena, their significant contribution to the improvements of the MFC efficacy development cannot be ignored.

9.4 Conclusions Different nanomaterials have the potential to improve the MFC performance to generate electricity. Nanomaterials have been applied to MFC as the catalyst to enhance the current density, power density, efficacy of the cell, and the acceleration of ORR. Moreover, nanomaterials are used in MFC to lessen the bacterial growth on the electrodes, which puts down the electron transfer rate among the electrodes, and thus the cell performance degrades. On the other hand, nanomaterials have a larger surface area than bulk materials, which assist in increasing the reaction rate on the electrodes and the overall cell. Nanomaterials have successfully been used to replace nonmetallic electrodes. However, in some cases, the power generation of nanomaterial-based MFC equivalent to the metal-based MFC is yet to be achieved. Instead, modification of MFC features with different nanomaterials is quite effective in low current flow requirements and laboratory usage. In the future, further improvement in the configuration of nanostructures in MFC may open a new era in the significant improvement of the MFC cell performance.

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Liu, Q., Wang, Y., Dai, L., & Yao, J. (2016). Scalable Fabrication of Nanoporous Carbon Fiber Films as Bifunctional Catalytic Electrodes for Flexible Zn-Air Batteries. Advanced Materials, 28(15), 3000–3006. https://doi.org/10.1002/adma.201506112. Ma, M., You, S., Gong, X., Dai, Y., Zou, J., & Fu, H. (2015). Silver/iron oxide/graphitic carbon composites as bacteriostatic catalysts for enhancing oxygen reduction in microbial fuel cells. Journal of Power Sources, 283, 74–83. https://doi.org/10.1016/j.jpowsour.2015.02.100. Mazari, S. A., Ali, E., Abro, R., Khan, F. S. A., Ahmed, I., Ahmed, M., Nizamuddin, S., Siddiqui, T. H., Hossain, N., Mubarak, N. M., Shah, A. (2021). Nanomaterials: Applications, waste-handling, environmental toxicities, and future challenges – A review. Journal of Environmental Chemical Engineering, 9(2), 105028. Mousavi, M. R., Ghasemi, S., Sanaee, Z., Nejad, Z. G., Mardanpour, M. M., Yaghmaei, S., & Ghorbanzadeh, M. (2019). Improvement of the microfluidic microbial fuel cell using a nickel nanostructured electrode and microchannel modifications. Journal of Power Sources, 437, 1–9. https://doi.org/10.1016/ j.jpowsour.2019.226891. Nguyen, D., Visvanathan, C., Jacob, P., & Jegatheesan, V. (2015). Effects of nano cerium (IV) oxide and zinc oxide particles on biogas production. International Biodeterioration & Biodegradation, 102, 165–171. https://doi.org/10.1016/j.ibiod.2015.02.014. Noori, Md. T., Jain, S. C., Ghangrekar, M. M., & Mukherjee, C. K. (2016). Biofouling inhibition and enhancing performance of microbial fuel cell using silver nano-particles as fungicide and cathode catalyst. Bioresource Technology, 220, 183–189. https://doi.org/10.1016/j.biortech.2016.08.061. Petersen, E. J., Pinto, R. A., Landrum, P. F., & Weber, Jr. W. J. (2009). Influence of Carbon Nanotubes on. Pyrene Bioaccumulation from Contaminated Soils by Earthworms. Environmental Science & Technology, 43(11), 4181–4187. https://doi.org/10.1021/es803023a. Sallam, E. R., Khairy, H. M., Elnouby, M. S., & Fetouh, H. A. (2021). Sustainable electricity production from seawater using Spirulina platensis microbial fuel cell catalyzed by silver nanoparticles-activated carbon composite prepared by a new modified photolysis method. Biomass and Bioenergy, 148, 1–8. https://doi. org/10.1016/j.biombioe.2021.106038. Saravanakumar, K., MubarakAli, D., Kathiresan, K., Thajuddin, N., Alharbi, N. S., & Chen, J. (2016). Biogenic metallic nanoparticles as catalyst for bioelectricity production: A novel approach in microbial fuel cells. Materials Science and Engineering: B, 203, 27–34. https://doi.org/10.1016/j.mseb.2015.10.006. Shrestha, B., Anderson, T. A., Acosta-Martinez, V., Payton, P., & Cañas-Carrell, J. E. (2015). The influence of multiwalled carbon nanotubes on polycyclic aromatic hydrocarbon (PAH) bioavailability and toxicity to soil microbial communities in alfalfa rhizosphere. Ecotoxicology and Environmental Safety, 116, 143–149. https://doi.org/10.1016/j.ecoenv.2015.03.005. Singh, S., & Verma, N. (2015). Fabrication of Ni nanoparticles-dispersed carbon micro-nanofibers as the electrodes of a microbial fuel cell for bio-energy production. International Journal of Hydrogen Energy, 40(2), 1145–1153. https://doi.org/10.1016/j.ijhydene.2014.11.073. Sun, Y., Dai, Y., Duan, Y., Xu, X., Lv, Y., Yang, L., & Zou, J. (2017). Biofouling inhibition on nano-silver/ferrous sulfide/partly-graphitized carbon cathode with enhanced catalytic activity and durability for microbial fuel cells. Carbon, 119, 394–402. https://doi.org/10.1016/j.carbon.2017.04.064. Tang, J., Wang, Y., Zhao, W., Ye, W., & Zhou, S. (2019). Porous hollow carbon tube derived from kapok fibres as efficient metal-free oxygen reduction catalysts. Science China Technological Sciences, 62(10), 1710–1718. https://doi.org/10.1007/s11431-018-9453-0. Tang, L., Liu, L., Chen, Q., Yang, F., & Quan, X. (2020). The construction and performance of photocatalytic-fuel-cell with Fe-MoS2/reduced graphene oxide@carbon fiber cloth and ZnFe2 O4 /Ag/Ag3 VO4 @carbon felt as photo electrodes. Electrochimica Acta, 362, 1–10. https://doi.org/ 10.1016/j.electacta.2020.137037. Wang, Qian, Lv, Ruiyuan, Rene, Eldon R., Qi, Xiaoyu, Hao, Qiang, Du, Yuanda, Zhao, Congcong, Xu, Fei, & Kong, Qiang (2020). Characterization of microbial community and resistance gene (CzcA) shiftsin up-flow constructed wetlands-microbial fuel cell treating Zn (II) contaminated wastewater. Bioresource Technology, 302 (April 2020), 1–9. https://doi.org/10.1016/j.biortech.2020.122867. Xu, Y., Zhou, S., & Li, M. (2019). Enhanced bioelectricity generation and cathodic oxygen reduction of air breathing microbial fuel cells based on MoS2 decorated carbon nanotube. International Journal of Hydrogen Energy, 44(26), 13875–13884. https://doi.org/10.1016/j.ijhydene.2019.04.040.

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CHAPTER 10

Nanomaterials supporting direct electron transport Abdul Hakeem Anwer, Nishat Khan and Mohammad Zain Khan Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Sciences, Aligarh Muslim University, Aligarh, India

10.1 Introduction Almost all living things perform oxidation–reduction reactions to perform energy transformation. The majority of species utilize soluble oxidizing and reducing agents to accept or donate electrons, respectively (Karthikeyan et al., 2012, 2015). However, many microorganisms may obtain energy by transfers of electrons from or to extracellular substrates, which operate as electron acceptors or donors, respectively. Extracellular electron transfer (EET) is the term given to this kind of microbial metabolism. EET is a common natural process that is essential for the biogeochemical cycling of a variety of elements and finds use in several applications. In bioelectrochemical systems such as microbial fuel cells (MFC), microbial electrosynthesis (MES) and microbial electrolysis cells (MEC),EET plays a critical role.MFCs are primarily bioanodic systems that generate electric energy by connecting bioanodic and cathodic reactions (such as ferricyanide or O2 ) (Karthikeyan et al., 2009). It requires microbes-electrode EET and microbes performing the function are referred to as exoelectrogens. MFC finds a variety of applications, including bioremediation, power generation, and biosynthesis (Danish Khan et al., 2015). Similar to MFCs, MECs also utilize bioanodes and low-power input to produce H2 at the cathode by setting a differential voltage (Aiken et al., 2019). MES, on the other hand, is a biocathode-driven method that utilizes microbial community to generate bioelectrocommodities or biofuels from carbon dioxide (CO2 ) at applied potential by conducting EET between microbes and electrode/cathode. This form of EET is also called microbial electron uptake (EU). Microbes capable of EU are called “electrotrophs” and can consume electrical energy from electrodes or electrons from electroconductive surfaces to produce value-added products (Rowe et al., 2018). MES may occur by either indirect or direct EU, according to several reports. In direct EU, microbes attached to the electroconductive materials (such as cathodes or iron minerals) receive electrons directly from them (Ha et al., 2017; Zhang et al., 2013). Indirect EU, on the other hand, involves microbes using indirect electrochemically generated or supplied diffusible chemicals to obtain electrons from conductive materials (Fig. 10.1). The main Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00016-4

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Figure 10.1 Schematic of extracellular electron uptake (EU) pathways adopted in microbial electrosynthesis (MES) at different poised cathode potentials (PCP) versus standard hydrogen electrode (SHE). (Medox mediator oxidation, Medred mediator reduction, PHB polyhydroxybutyrate).

Nanomaterials supporting direct electron transport

objective of this chapter is to address the current developments and difficulties associated with direct electron uptake from the solid electrodes in an MES, as this mode is less explored than indirect EU. This chapter details the involvement of direct electron uptake in MES (Table 10.1). In addition, the aim of this chapter is to emphasize that when analyzing microbe-charged surface interactions, applications and mechanistic studies go hand in hand.

10.2 Mechanism of electron transfer—electron release In microbial EET, a chain of protein networks is involved in transferring the electrons metabolically generated to external electron acceptors. The mechanism involved in EET is still under research. The three main mechanisms collectively contributing to the EET include, (1) direct electron transfer involving outer membrane cytochromes (OM c-Cyts) (Estevez-Canales et al., 2015), (2) mediated transport via cell-secreted flavins (Marsili et al., 2008), and (3) nanowires conducted electron transport. Okamoto et al. in contrast to the mediated electron transfer via shuttling of electrons by cell-secreted flavins such as flavin mononucleotide (FMN) and riboflavin (RF) have recently suggested the formation of bound-flavin semiquinones formed by RF/FMN binding to OM c-Cyts, OmcA and MtrC proteins. As compared to the free flavin, the bacterial EET of bound-flavin semiquinones is enhanced by 103 to 105 -times indicating a critical role of OM c-Cytsflavins in the process of microbial EET (Okamoto et al., 2013). Bacteria generate nanowires, a sort of intricated electrical wiring, to transfer electrons to electron acceptors. So far, various literature has reported nanowires as bacterial pili extensions however, recently it has been proposed that these nanowires extend from outer membrane and periplasm instead of being pili-based structures. The excellent electrical conductivity of these nanowires is attributed to the presence of multiheme cytochromes on them. The role of microbial nanowires in EET interests researchers and is still under investigation and its detailed understanding will have a significant effect on bioelectricity generation, nanobiotechnological applications, bioremediation, and corrosion (Hauser & Zhang,2010;Malvankar & Lovley,2012,2014).Further,Malvankar and Lovley (2014) have suggested the metal-like conductivity of nanowires produced by Geobacter Sulfurreducens as nanowires conductivity considerably increased on cooling, a characteristic of metallic conduction. Furthermore, with falling pH, a two-fold increase in conductivity was observed in the nanowires. The observation suggests proton doping of the pili formation of p-type carriers as a result of protonation as has also been observed in the case of organic metals (e.g., polyaniline) (Terje & Skotheim, 2007). On the other hand, Shewanella oneidensis MR-1 has been reported to produce p-type nanowires with tuneable electronic performance. For S. oneidensis MR-1, the primary charge carriers are found to be holes formed on bacterial nanowires (Leung et al., 2013). However, research is still undergoing

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A possible mechanism for the EU/biological phenomena PCP versus SHE References has been identified

Group of microorganisms

Application

Electrotrophs

Acetogens

MES

Graphite electrode serving as −0.4 an electron acceptor for the conversion of carbon dioxide to acetate by the bacterium S. ovata Co-culture of Sporomusa The electrons produced by NA ovata/Desulfobulbus sulfide oxidation by D. propionicus propionicus were subsequently utilized by S. ovata to convert CO2 to acetate on a graphite cathode. Moorella Carbon dioxide (CO2 ) −0.4 thermoautotrophica, reduction to acetate through Moorella thermoacetica EU at 60°C is temperature (thermophile) dependent. Sporomusa sphaeroides, Carbon dioxide (CO2 ) is −0.4 Clostridium ljungdahlii, reduced to acetic acid by Sporomusa silvacetica, using a graphite electrode in Clostridium aceticum a fixed condition.

MES

MES

MES

Sporomusa ovata

(Nevin et al., 2010)

(Gong et al., 2013)

(continued on next page)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 10.1 Direct electron uptake by various electrotrophs for MES.

Methanogens

Electromethanogenesis

Electromethanogenesis

MES

Electromethanogenesis

Direct electron uptake from the cathode to transformation CO2 to CH4. Mixed methanogenic In the MES system, culture electrically reduced neutral red (NR) was the only source of reducing power for the metabolism and development of a mixed methanogenic culture. Methanococcus maripaludis Efficient EU is achieved via (electromethanogenic the activity of archaeon) surface-associated redox enzymes and formate dehydrogenases. Mixed methanogenic The applied cathodic voltage culture as well as the surrounding environmental variables are important factors in “electrometabolism.” Methanothermobacter The voltage used as a thermautotrophicus reducing power to convert source CO2 to CH4 is evaluated.

−1.0

−1.5

−0.6

−0.456

NA

(continued on next page)

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Electromethanogenesis

Methabacterium palustre

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Application

Electrotrophs

Electromethanogenesis

Methanosarcina and Methanosaeta (acetoclastic methanogens

Photoautotrophic MES microbes

Rhodopseudomonas palustris TIE-1

No application explored Rhodopseudomonas palustris TIE-1

No application explored Rhodopseudomonas palustris TIE-1

A possible mechanism for the EU/biological phenomena PCP versus SHE References has been identified

Inhibition of hydrogen −0.7 (H2 )-utilizing methanogens by antibiotic pretreatment was shown to be effective, whereas enhancement of acetoclastic methanogen growth was found to be considerable. PHB development inside cells +0.1 using light as an energy source, poised electrodes as an electron supply (through direct EU), and CO2 as a carbon supplier. The pioABC operon affects +0.1 direct electron absorption from the cathode, which is required for photoautotrophic growth through ferrous iron (Fe(II)) oxidation. The direct EU was increased +0.1 by 3.8 times by electrochemically depositing Prussian blue (PB) complex.

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 10.1 Direct electron uptake by various electrotrophs for MES—cont’d

Nanomaterials supporting direct electron transport

to establish a consensus on the mechanism involved in the process of microbial (Bonanni et al., 2013; Bond et al., 2012; Jatoi et al., 2021, 2022). 10.2.1 Mechanism of electron transfer—electron uptake Some bacteria power their metabolism by detecting negatively charged surfaces through the uptake of electrons and utilizing these surfaces as electron donors. Recent research has revealed that bacteria may absorb electrons from cathodes to manufacture high-value fuels (acetate, hydrogen and methane) through a technique called microbial electrosynthesis abbreviated as MES Fig. 10.1 (Lovley, 2011; Nevin et al., 2010). This technology has shown promising results in converting CO2 to value-added chemicals and fuels (Patil et al., 2015). There have been few studies that describe the bacterial electron absorption method at the cathode, and a deeper knowledge of the mechanism is required for the technique’s realistic implementation (Rosenbaum et al., 2011). In the case of Geobacter species, the electrochemical analysis of c-Cyts has revealed that the probable reversibility of its electron transferability (Rosenbaum et al., 2011). Additionally, the shared role of hydrogenase enzymes and c-Cyts in the uptake of electrons by microbes has been suggested by a few studies (Barton et al., 2007; Perdra et al., 1996). The respiratory pathway has also been reported to reverse for S. oneidensis MR-1 reductive metabolism. In S. oneidensis, fumarate reduction is severely reduced by deleting periplasmic cytochrome MtrA and OM c-Cyt anchoring protein-MtrB. More research is needed to determine the specific process involved in the process of electron uptake by bacteria from the cathode. 10.2.2 Role of the electrode in extracellular electron transfer MFC and MES technologies were developed in response to the discovery of bacterial bidirectional EET. However, due to a number of challenges associated, such as weak electrical connection between microbes and electrodes, low biofilm conductivity, and a lack of knowledge of the bidirectional EET process, these developments are still a long way from becoming commercially viable. The conductivity of biofilms is thought to be a key component in the EET process’ efficiency (Malvankar et al., 2012). Unfortunately, biofilm conductivity alone is insufficient to allow electrons to flow freely between the electrode and the biofilm. To improve the bidirectional EET process, material scientists have recently begun to use sophisticated nanostructured materials such as noble metal nanoparticles, graphene, conducting polymers carbon nanotubes, semiconductors, and inorganic nanomaterials, as bioanodes and biocathodes. The bidirectional EET process has improved dramatically as a result of the efforts utilizing nanostructured materials. This study deals with the latest advancements in nanostructured materials in the bacterial bidirectional EET process. Previous knowledge of the basic principles of bacterial adhesion to solid surfaces is important since the development of biofilms

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is one of the key processes of both techniques (MFC and MES) (Anwer et al., 2021). Several investigations, including theoretical and practical research, focused on the study of the bacterial connection to different nanostructured surfaces. The benefits of utilizing nanomaterials like this are addressed in order to improve electron transport in BESs along with present difficulties and future prospects.

10.3 The current state of knowledge about electrode–bacteria interactions The present state of knowledge on the interaction of the microbial community with electrode surfaces is addressed in this section. The information gained from the interactions between bacteria and electrodes may be used directly to the creation of methods for the production of effective materials for anode and biocathode. The majority of microorganisms prefer to form biofilms on electrode surfaces to provide protection from a wide range of environmental challenges and toxins (Donlan, 2002). Several studies have been conducted on the development of bacterial biofilms, and microbes are known to produce extracellular polymeric substances (EPS) in order to adhere to electrode surfaces. The genetic analysis of flagella and pili has shown that they are also involved with the development of biofilms (Tran et al.,2011).Various parameters influence the development of a biofilm including wettability (of biofilm and electrode), surface topography, surface charge, cell–cell signaling and the number of EPS produced (Eboigbodin et al., 2005; Li & Logan, 2004). Among these, it is well established that surface topography plays a crucial role in the early phases of biofilm development (Chang et al., 2015). As a possible tool for studying bacterial life in nanoscale spaces, nanotechnology has emerged as a promising avenue of investigation (Hochbaum & Aizenberg, 2010). Nanomaterials provide a range of options for tuning the electrode surface to understand the relation between the bacterial cells and refashioned surfaces. For example, by using various porous nanomaterials as electrodes, it is possible to study the impact of roughness and porosity of surfaces on the development of biofilms. To design effective electrode materials, it is essential to first get a knowledge of the bacterial affinity for electrode surfaces, which may be accomplished by conducting experiments. Recently, silicon (Si) nanowires were used to investigate the bacterium–nanomaterial interaction as their physical interactions can be measured with precision. Sakimoto, Liu, Lim, and Yang (2014) immersed Si nanowires in Sporomusa ovata cells present in growth medium while continuously applying voltage. At high ionic concentrations, S. ovata cells aligned parallel to the nanowires, whereas when the ionic concentration is low, microbes adhered to the nanowires in arbitrary directions. The results indicate that the bacterial cells on the nanowires were orientated and selfassembled as a consequence of the salt treatment. In a similar manner, a study recently demonstrated the ability of S. oneidensis cells to detect and attach to Si nanowires while swimming. The findings of the research showed the preference of S. oneidensis to remain

Nanomaterials supporting direct electron transport

on the Si nanowires as compared to on the silicon substrate and revealed the critical role of nano-sized surfaces in biofilm formation. Nanostructured titanium dioxide biofilm with approximately 20 nm surface roughness, according to Singh et al. (2011)), promotes biofilm development, while a further increase in surface roughness hinders the formation of bacterium adhesions. MitikDineva et al. (2008)) reported the promotion in microbial adhesion by three times on a surface with nano-scale roughness as compared to micro- or macro-sized surfaces. The effect of nano-roughness on bacterial adhesion was examined by growing a biofilm of Pseudomonas isachenkonni (a proteobacterium) on a glass surface that had been changed to have a different nano-scale roughness than the original. When comparing the modified and unmodified glass surfaces, a greater number of bacterial cells have adhered to the modified surface. On the nano-sized rough surface, a morphological change of the cells is evident by a rise in cell size and production of EPS. These findings clearly demonstrated that the nano-surface stimulated the cellular metabolism of the microbial community. The role of surface wettability on the formation of biofilm and the EET processes involved has been recently demonstrated by Ding, Lv, Zhu, Jiang, and Liu (2015)). The influence of wettability on the process to EET was investigated, using a tin-doped by In2 O3 (ITO) with varied wettability to grow a biofilm of Shewanella loihica PV-4 at a stable applied potential. An EET activity, 5-times higher than that observed by the biofilm developed on the hydrophobic substrate was reported using biofilm developed on ITO glass. Electrochemical studies indicated that surface wettability improved the electrical connection between microorganisms and electrodes as it changes the microbeelectrode interface local polarity. This local polarity change affects the OM c-Cyts redox state, enabling them to remain in a mainly reduced state. The EET process was greatly aided by the redox state change in the OM c-Cyts with the hydrophilic surface. In brief, porosity or high surface roughness permits microbes to adhere securely to the electrode surface, allowing them to quickly transfer electrons to the electrode. Furthermore, biofilm formation and the EET process are promoted by the hydrophilic surfaces. 10.3.1 Materials utilized in the cathode of the MES Carbon-based materials are the most often utilized cathodes in MES systems for CO2 bioconversion. Fig. 10.2 shows the use of a variety of available commercially and surfacemodified catalysts for the cathode in CO2 fed MES systems throughout time,as well as the performance gains that have occurred (in terms of cathode surface-based output rates). In MFC, the microbe electrode (anode) is used, while in MES, the microbe electrode (cathode) is used, with the same or different mode. 10.3.2 Carbon-based cathode materials Carbon materials, in general, have grown more popular, with graphite, in particular, being the most often utilized commercially. It is mostly utilized in the shape of blocks or

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Figure 10.2 Showing the timeline evolution of cathode materials for reduction of CO2 in MES systems. SS—stainless steel; Au—gold; CNT—carbon nanotube; Pd—palladium; Si-TiO2 ; RVC—reticulated vitreous carbon; Ni—nickel; TEPA—tetraethylene pentamine; 3D—three-dimensional; MWCNT—multiwall carbon nanotube; rGO—reduced graphene oxide; PEDOT: PSS—poly(3,4-ethylenedioxythiophene): polystyrene.

plates, rods or sticks, and granules. Carbon materials are usually plane-sheet structured with inherent benefits of low residual current, wide electrochemical window, ease of modification, reusability, relative inertness, recyclability, extremely high biocompatibility, and sufficient electrical conductivity in aqueous solution (Anwer et al., 2019; Zhou et al., 2011). However, the single graphite rod or block restricts the performance in terms of increasing productivity due to its limited porosity and accessible surface area for microbe adhesion. To obtain a high volumetric acetate rate of production from CO2 , the Marshall group suggested utilizing granular graphite as a cathode andusingd an enhanced electrosynthesizing mix of microbial culture species (Marshall et al., 2012, 2013). The presence of enhanced bacterial cell adhesion and growth on such cathodes overtime was confirmed by electron micrographs. Because of their large volumetric surface area and porosity, granular graphite materials are well suited for promoting electrode–microorganism interactions and sustaining the electrogenic biofilm activity. In comparison to graphite electrodes, other 2D (two-dimensional) thin morphology carbon materials such as carbon cloth and carbon plate are extensively utilized because they have chemical stability, superior electrical conductivity, flexibility, higher porosity and lightweight. Carbon cloth was employed in MES systems in early research by Zhang et al.because of its simplicity,ease of additional surface modification and flexibility (Zhang et al., 2013). However, such planar 2-D electrodes have many drawbacks including high internal resistance, low reactive specific surface area, high activation overpotential, poor electrocatalytic activity and rapid formation of a passivation film on the surface of an

Nanomaterials supporting direct electron transport

electrode, thus restricting their application in MES systems. Three-dimensional (3D) materials such as carbon felt and carbon fiber rod electrodes have been investigated in MES systems (Table 10.2). The electrodes’ 3D structure provides a more surface area for biofilm development and reduces mass transfer difficulties. Additionally, electrodes having a three-dimensional structure are more adaptable to spatial alteration, and their catalytic reactive areas facilitate electron exchange via either direct or mediated mechanisms (Li et al., 2017). Bajracharya et al. improved the rate of acetate synthesis by using carbon felt cathode with an additional stainless steel current collector and reducing the voltage applied to –690 mV against SHE for hydrogen synthesis to facilitate electron transfer in recent research (Bajracharya et al., 2015). MES systems operating in galvanostatic mode have also been successfully used to show their application in the bioconversion of CO2 . H2 can be created continuously at cathodes under galvanostatic control by supplying a steady current. This allows for nonlimiting H2 -based bioproduction when using galvanostatic control. Recently, it has been revealed that unaltered RVC foam may be used directly as cathodes (Bajracharya et al., 2015). Acetate was produced at a high rate (196.8 g m–2 d–1 ) from CO2 in this research due to the galvanostatic control and the electrode’s large surface area, as well as the use of biocathodes in continuous flow operating mode (Bajracharya et al., 2015; LaBelle & May, 2017). In MES systems, stainless steel (SS) is more widely utilized as a current collector in association with carbon-based cathodes as its direct usage is restricted mostly owing to its low biocompatibility.However, it has been shown that it has the potential to be used as cathodes, particularly in H2 -based bioproduction (Blanchet et al., 2015). 10.3.3 Nanomodified carbon-based cathode materials Biocathode optimization for CO2 MES has become more popular, with the usage of surface-modified and purpose-built materials being suggested (Table 10.3). An electrochemical pretreatment procedure is carried out on electrodes prior to surface modification, which may result in greater exposure of the active surfaces for subsequent modification and immobilization. For the purpose of removing contaminants from the electrodes’ surfaces, they are exposed to chemical or thermal treatment. Carbon cloth and carbon felt, are among the most frequently used base materials because of their large surface area, ease of commercial accessibility, high porosity, flexibility for microbial adhesion, large surface area, and simplicity of spatial adjustment (Aryal et al., 2016). For instance, Zhang et al. changed carbon cloth electrodes by immobilizing chitosan, 3-aminopropyltriethoxysilane, melamine, polyaniline, cyanuric chloride, gold, ammonia, carbon nanotube (CNT) cotton, palladium, and carbon nanotube polyester for CO2 reduction along with S. ovata (Zhang et al., 2013). As compared to the traditional carbon cloth cathode, chitosan, apolysaccharide-rich in amino and hydroxyl groups, boosted the rate of acetate synthesis by 7.6-fold, making it the most effective of the changed

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Graphite stick Batch Carbon rod Batch

−400 −201

Graphite granular Carbon fiber rod

Batch

SS felt Graphite rod Graphite granular Carbon felt with -SS

Current Rate density (g/m2 /d) (A/m2 )

Maximum titer products (g/L)

CE in acetate (%)

−0.207 −4.1

0.063 0.2

2-oxobutyrate 86 ± 21 CH4 Ng

−590

Sporomusa ovata 1.38 Anaerobic digestion 0.0431 slurry Brewery wastewater Ng

Ng

10.5

H2 , CH4

69

Batch

−400

Bog sediment

0.063

−0.03

0.02

35.2 ± 4.4

Batch Batch Continuous

−690 −600 −600

12.8 38 2.7

−1.5 −3.23 −12.3

0.127 0.34 Ng

Batch

−1140

20.4

−5

13.5

–

60.8

Carbon felt Batch with -SS Carbon cloth Batch

−895

Acetobacterium woodii Brewery wastewater Anaerobic digester sludge Enriched mix culture (from UASB effluents) Wastewater sludge

Ethanol, butyrate, H2 , butanol and propionate – H2 , formate CH4

40

−10

0.63

−800

Enriched mixed 92.54 culture from syngas

−20

2.8

Graphite block Batch

−690

S. ovata

Ng

0.1

H2 , ethanol, 45 ± 7 and CH4 Ethanol, – butanol and butyrate Ethanol 87.6 ± 6.5

33.28

81 40 ± 4 28.9

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 10.2 MES systems cathode materials (Aryal et al., 2017). Applied Carbon Operation potential (mV Microbial culture or material mode inoculum source versus SHE)

Nanomaterials supporting direct electron transport

Table 10.3 For MES modified cathode material.

Electrode materials

Carbon FELT Three dimensional-graphene carbon felt electrode Graphite stick Graphite stick–Ni Nano wire Carbon cloth Carbon cloth–reduced graphene oxide tetraethylene pentaamine Carbon cloth 3-aminopropyltriethoxysilane Chitosan Cyanuric chloride

Applied potential (mV versus SHE)

CE (%)

400 ± 10 2450 ± 160

–690

86 ± 3

123 282 88 ± 8 321 ± 53

100 890 191 ± 10 686 ± 146

–400

82 ± 14

–690

83 ± 3

30 ± 7 95 ± 20 229 ± 56 205 ± 50

71 ± 11 206 ± 11 475 ± 18 451 ± 79

–400

76 ± 12 86 ± 12 83 ± 10 81 ± 16

Production rate (mMd–1 m–2 )

Current density (mAm–2 )

136.5 ± 43.5 925.5 ± 29.4

cathodes. Improved biocompatibility was said to have allowed bacterial cells to generate stronger electrostatic contacts with the chitosan-modified electrode, thereby leading to this product’s increased electrostatic ability. Biofilm development on the cathode is further assisted by the catalytic surface with appropriate pore-size characteristics. In addition, alterations in carbon cloths with gold (Au), palladium (Pd), and nickel (Ni) metal nanoparticles led in relation with untreated electrodes to an increase of 6, 4.7, and 4.5 times in the acetate production rates correspondingly. The low charge transfer resistances and high nanoparticular conductivity have been proposed to be the source of increased MES acetate production (Zhang et al., 2013). This has also been observed when cotton and polyester-fabric-based cathodes were modified with CNTs.The treated cathodes increased the production of acetate by 3.4 and 3.2 times as compared with the control carbon cloth cathode. For bioelectrochemical devices, CNTs are among the best modifiers for constructing electrodes because of their high conductivity and biocompatibility however, the high cost, restricts their usage (Wang, 2005). It has also been reported that Ni nanowires covered with a graphite substrate were up to 50 times rougher after being exposed to microwave treatment than untreated graphite (Nie et al., 2013). The porous nature of Ni-nanowire graphite greatly strengthened the connections between cathode and microorganisms, resulting in 2.3 times increase in the generation rate of acetate over untreated electrodes (Nie et al., 2013). Jourdin et al. used a chemical vapor deposition (CVD) technique to create a NanoWeb-RVC cathode, which resulted in a 33.3-fold increase in the rate of acetate synthesis as compared to the unmodified carbon plate electrode in their study. Because of its large surface area and macroporous size, modified NanoWeb-RVC is feasible for mass transfer and biofilm development in a wide range of applications (Jourdin et al., 2014). Additionally, these researchers used an

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electrodeposition technique to create CNT-modified RVC electrodes (dubbed EPD3D) and revealed a high synthesis rate of acetate from CO2 utilizing an enhanced, sustained acetogenic population (Jourdin et al., 2015). The use of multi-walled carbon nanotubes (MWCNTs) modified RVC cathodes in MES systems led to the highest recorded acetate production rate of up to 1330 g m–2 d–1 . The soaring bioproductive rate was met with the use of highly open microporous RVCs with pore dimensions of 0.6 mm which produced a favorable equilibrium between high active surface area and effective mass transfer between the mass electrolyte and biocathode. Bioproduction, with the remarkable rate of bioproduction, was obtained by using a 0.6 mm pore sized microporous RVC that promoted effective mass transfer while also enabling enough surface area for biofilm development (Jourdin et al., 2016). Liu and colleagues created the photocathode with silicon-titanium oxide (Si-TiO2 ) for direct sunlight harvesting in a very innovative manner. With S. ovata, this photocathode could produce up to 6 g L–1 acetate using sunlight as the only source of electrons (Liu et al., 2015). A recent study by Cui et al. utilized 3D carbon felt cathodes modified with iron oxide and found that the rate of acetate production increased by 4.8 times when compared to the rate of acetate production while using an unmodified 3D carbon felt cathode (Cui et al., 2017). It has been proposed that the partially conductive characteristic of iron oxide might have helped in increasing the efficacy of EET. Graphene, which is gradually being utilized in assembling and modifying the nontraditional carbonaceous substrate, has also found application as cathodes for MES systems. Additionally, Chen et al. prepared tetraethylenepentamine (TEP) from solutions on a carbon cloth electrode after reducing graphene on the electrode. The electric charge interacts with the biofilm to increase acetate synthesis by 11.8-fold as compared to the untouched electrode (Chen et al., 2016). A 3D carbon felt/graphene electrode was designed by Aryal et al. for the formation of a well-developed biofilms, and they found 6.8 times increase in acetate production when compare to unmodified carbon felt. A similar research group continued working on graphene paper-based flexible and freestanding cathodes and observed an 8-fold increase in acetate production than carbon paper cathodes. In MES, the use of graphene-based electrodes would be advantageous due to the cheap cost of manufacturing (Segal, 2009). The usage of additional composite electrodes such as gas diffusion electrodes (GDEs) and activated carbon VITO-CoRE® electrodes has also been studied (Mohanakrishna et al., 2015). The porous composite activated carbon gas diffusion electrode offers a perfect three-phase interface (gas–liquid–solid). CO2 was diffused using a hydrophobic gas diffusion layer,followed by a current collector and a catalyst layer,which allows bacteria to synthesize chemicals in a controlled environment. The use of a GDE assured an enhanced and regulated transport of CO2 to the biocatalyst. The mass transfer coefficient (kLa) was observed to be twice as high for GDE as compared to the sparged systems. On the other hand, the formation of a thin diffusion layer on the surface of the electrode was frequently found to be a restriction for CO2 solubility (Alvarez-Gallego et al., 2012). The

Nanomaterials supporting direct electron transport

VITO-CoRETM electrode, which has a plastic inert support as the cathode, was created by the same research group. In single-chamber reactors, the use of a cathode with a large surface area resulted in increased biofilm development as well as increased production of MES from CO2 (Mohanakrishna et al., 2015). 10.3.4 Photo-active semiconductors modified cathode A photo biocathode works on the less-energy-consuming route because it may supply the reducing equivalent (electron) onsite to electroactive bacteria (EAB) for the targeted synthesis of biochemicals from CO2 . However, since this is a relatively new idea, there are certain difficulties that must be overcome. Examples of such characteristics are sufficient biocompatibility, high CO2 adsorption capability, and low or nil generation of reactive oxygen species (ROS). The one-dimensional semiconductor nanostructures, such as nanorods, nanowires, and nanotubes, for example, demonstrated much lower CO2 mass transfer losses, as well as better biocompatibility and stability (Imani et al., 2016; Liao et al., 2011). The EAB-based photosynthetic microbial electrosynthesis (PMES) was demonstrated with the new technique’s initial application, in which the novel silicon TiO2 nanowire composite cathode was interfaced with S. Ovata to produce acetate from CO2 (Liu et al., 2015). It will also be feasible to utilize this cathode as part of a singlechamber PMES system, which will be a more practical design for field size research due to the lower manufacturing costs and simplicity of maintenance (Gavilanes et al., 2019). Later developments included covering the TiO2 -passivated n+/p-Si with a thin layer of Ni-Mo alloy through sputtering and utilizing it as the photo biocathode in a PMES paired with a Pt-coated abiotic anode (Nichols et al., 2015). Methanosarcina barkeri hybridized with Ni-Mo–TiO2 -n+/p-Si was able to collect visible light at 740 nm and showed minimal overpotential reduction of CO2 to CH4 when compared to previous designs. It is noteworthy that the metabolism of M. barkeri is not adversely affected by this hybrid cathode. A photo biohybrid cathode, consisting of a solar collector made of p-type cobalt (II, III) oxide (Co3 O4 )/mesoporous nickel (Ni) foam and an EAB biocatalyst, was recently developed for the effective conversion of CO2 to formic acid (HCOOH) in PMES by Wu et al. (Wu et al., 2020). As previously stated, hybrid photoelectrodes based on semiconductor-metal are superior to single semiconductors in a number of aspects (Cai et al., 2020).

10.4 Conclusion and future perspectives The chapter summarizes the role of nanomaterials in improving the direct electron transfer between microorganisms and electrodes to achieve enhanced biochemical synthesis from CO2 using MES. Cathode material with high surface area, biocompatibility, conductivity, and porosity are the key requirement for the efficient performance of

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MES. The process of Microbial electrosynthesis and EET can be significantly enhanced by using advanced nanostructures. This improvement in the nanostructure-modified electrodes can be attributed to low internal resistance, large surface area, and high conductivity. A close contact between bacterial nanowires and OM c-Cyts owing to the nanomaterial modification paves the way for an effective EET process. Recently, the 3D configuration of electrodes has garnered attention as a promising approach for improved MES performance. More elaborate research is needed to understand the nanomaterials’ involvement in cellular metabolism to be able to more efficiently enhance the EET and overall electrosynthesis. Similar to EET, biofilm engineering is also important for bioelectrocatalysis to assure evenly distributed biofilm cell activity. Electrode cost being one of the major bottlenecks calls for the development of cheap and efficient materials for the successful upgradation of technique to a practical scale. Water electrolysis is the most common anodic reaction so far and further research should be focused on identifying other anodic substrates for achieving overall system productivity in MES. Finally, other than acetate research should be focused on carbon chain elongation to produce highcarbon compounds in MES.

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Jourdin, L., Freguia, S., Donose, B. C., Chen, J., Wallace, G. G., Keller, J., & Flexer, V. (2014). A novel carbon nanotube modified scaffold as an efficient biocathode material for improved microbial electrosynthesis. Journal of Materials Chemistry A, 2(32), 13093–13102. doi:10.1039/C4TA03101F. Jourdin, L., Grieger, T., Monetti, J., Flexer, V., Freguia, S., Lu, Y., Chen, J., Romano, M., Wallace, G. G., & Keller, J. (2015). High acetic acid production rate obtained by microbial electrosynthesis from carbon dioxide. Environmental Science and Technology, 49(22), 13566–13574. doi:10.1021/acs.est.5b03821. Jourdin, L., Lu, Y., Flexer, V., Keller, J., & Freguia, S. (2016). Biologically induced hydrogen production drives high rate/high efficiency microbial electrosynthesis of acetate from carbon dioxide. ChemElectroChem, 3(4), 581–591. doi:10.1002/celc.201500530. Karthikeyan, R., Ganesh, V., & Berchmans, S. (2012). Bio-electrocatalysis of Acetobacter aceti through direct electron transfer using a template deposited nickel anode. Catalysis Science and Technology, 2(6), 1234–1241. doi:10.1039/c2cy20022h. Karthikeyan, R., Sathish kumar, K., Murugesan, M., Berchmans, S., & Yegnaraman, V. (2009). Bioelectrocatalysis of Acetobacter aceti and Gluconobacter roseus for current generation. Environmental Science and Technology, 43(22), 8684–8689. doi:10.1021/es901993y. Karthikeyan, R., Wang, B., Xuan, J., Wong, J. W. C., Lee, P. K. H., & Leung, M. K. H. (2015). Interfacial electron transfer and bioelectrocatalysis of carbonized plant material as effective anode of microbial fuel cell. Electrochimica Acta, 157, 314–323. doi:10.1016/j.electacta.2015.01.029. LaBel, le, E. V., & May, H. D. (2017). Energy efficiency and productivity enhancement of microbial electrosynthesis of acetate. Frontiers in Microbiology, 8(May), 756. doi:10.3389/fmicb.2017.00756. Leung, K. M., Wanger, G., El-Naggar, M. Y., Gorby, Y., Southam, G., Lau, W. M., & Yang, J. (2013). Shewanella oneidensis MR-1 bacterial nanowires exhibit p-type, tunable electronic behavior. Nano Letters, 13(6), 2407–2411. doi:10.1021/nl400237p. Li, B., & Logan, B. E. (2004). Bacterial adhesion to glass and metal-oxide surfaces. Colloids and Surfaces B: Biointerfaces, 36(2), 81–90. doi:10.1016/j.colsurfb.2004.05.006. Li, S., Chong, C., & Arne, T. (2017). Carbon-based microbial-fuel-cell electrodes: from conductive supports to active catalysts. Advanced Materials, 29(8) 1602547. Liao, J. Y., Lei, B. X., Kuang, D. B., & Su, C. Y. (2011). Tri-functional hierarchical TiO2 spheres consisting of anatase nanorods and nanoparticles for high efficiency dye-sensitized solar cells. Energy and Environmental Science, 4(10), 4079–4085. doi:10.1039/c1ee01574e. Liu, C., Gallagher, J. J., Sakimoto, K. K., Nichols, E. M., Chang, C. J., Chang, M. C. Y., & Yang, P. (2015). Nanowire-bacteria hybrids for unassisted solar carbon dioxide fixation to value-added chemicals. Nano Letters, 15(5), 3634–3639. doi:10.1021/acs.nanolett.5b01254. Lovley, D. R. (2011). Powering microbes with electricity: Direct electron transfer from electrodes to microbes. Environmental Microbiology Reports, 3(1), 27–35. doi:10.1111/j.1758-2229.2010.00211.x. Malvankar, N. S., & Lovley, D. R. (2012). Microbial nanowires: A new paradigm for biological electron transfer and bioelectronics. Chemsuschem, 5(6), 1039–1046. doi:10.1002/cssc.201100733. Malvankar, N. S., & Lovley, D. R. (2014). Microbial nanowires for bioenergy applications. Current Opinion in Biotechnology, 27, 88–95. doi:10.1016/j.copbio.2013.12.003. Malvankar, N. S., Tuominen, M. T., & Lovley, D. R. (2012). Biofilm conductivity is a decisive variable for high-current-density Geobacter sulfurreducens microbial fuel cells. Energy and Environmental Science, 5(2), 5790–5797. doi:10.1039/c2ee03388g. Marshall, C. W., Ross, D. E., Fichot, E. B., Norman, R. S., & May, H. D. (2012). Electrosynthesis of commodity chemicals by an autotrophic microbial community. Applied and Environmental Microbiology, 78(23), 8412– 8420. doi:10.1128/AEM.02401-12. Marshall, C. W., Ross, D. E., Fichot, E. B., Norman, R. S., & May, H. D. (2013). Long-term operation of microbial electrosynthesis systems improves acetate production by autotrophic microbiomes. Environmental Science and Technology, 47(11), 6023–6029. doi:10.1021/es400341b. Marsili, E., Baron, D. B., Shikhare, I. D., Coursolle, D., Gralnick, J. A., & Bond, D. R. (2008). Shewanella secretes flavins that mediate extracellular electron transfer. Proceedings of the National Academy of Sciences of the United States of America, 105(10), 3968–3973. doi:10.1073/pnas.0710525105. Mitik-Dineva, N., Wang, J., Mocanasu, R. C., Stoddart, P. R., Crawford, R. J., & Ivanova, E. P. (2008). Impact of nano-topography on bacterial attachment. Biotechnology Journal, 3(4), 536–544. doi:10.1002/ biot.200700244.

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Mohanakrishna, G., Seelam, J. S., Vanbroekhoven, K., & Pant, D. (2015). An enriched electroactive homoacetogenic biocathode for the microbial electrosynthesis of acetate through carbon dioxide reduction. Faraday Discussions, 183, 445–462. doi:10.1039/c5fd00041f. Nevin, K. P., Woodard, T. L., Franks, A. E., Summers, Z. M., & Lovley, D. R. (2010). Microbial electrosynthesis: Feeding microbes electricity to convert carbon dioxide and water to multicarbon extracellular organic compounds. mBio, 1(2), e00103–10. doi:10.1128/mBio.00103-10. Nie, H., Zhang, T., Cui, M., Lu, H., Lovley, D. R., & Russell, T. P. (2013). Improved cathode for high efficient microbial-catalyzed reduction in microbial electrosynthesis cells. Physical Chemistry Chemical Physics, 15(34), 14290–14294. doi:10.1039/c3cp52697f. Nichols, Eva M., Joseph, J. Gallagher, Chong, L., Yude, S., Joaquin, R., Yi, Y., Yujie, S., Peidong, Y., Michelle, C. Y. C., & Christopher, J. C., (2015). “Hybrid bioinorganic approach to solar-to-chemical conversion.” Proceedings of the National Academy of Sciences 112(37) 11461–11466. Okamoto, A., Shafeer, K., Xiao, D., Kazuhito, H., Ryuhei, N., & Kenneth, H. Nealson. (2014). Cell-secreted flavins bound to membrane cytochromes dictate electron transfer reactions to surfaces with diverse charge and pH. Scientific Reports, 4(1), 1–8. Patil, S. A., Gildemyn, S., Pant, D., Zengler, K., Logan, B. E., & Rabaey, K. (2015). A logical data representation framework for electricity-driven bioproduction processes. Biotechnology Advances, 33(6), 736–744. doi:10.1016/j.biotechadv.2015.03.002. Perdra,A.S.,Franco,R.,Feio,M.J.,Pinto,C.,Lampreia,J.,Reis,M.A.,Calvete,J.,Moura,I.,Beech,I.,Lino,A.R., & Moura, J. J. G. (1996). Characterization of representative enzymes from a sulfate reducing bacterium implicated in the corrosion of steel. Biochemical and Biophysical Research Communications, 221(2), 414–421. doi:10.1006/bbrc.1996.0610. Rosenbaum, M., Aulenta, F., Villano, M., & Angenent, L. T. (2011). Cathodes as electron donors for microbial metabolism: Which extracellular electron transfer mechanisms are involved? Bioresource Technology, 102(1), 324–333. doi:10.1016/j.biortech.2010.07.008. Rowe, A. R., Rajeev, P., Jain, A., Pirbadian, S., Okamoto, A., Gralnick, J. A., El-Naggar, M. Y., Nealson, K. H., Ribbe, M. W., Ajo-Franklin, C., & Barstow, B. (2018). Tracking electron uptake from a cathode into Shewanella cells: Implications for energy acquisition from solid-substrate electron donors. mBio, 9(1), e02203–17. doi:10.1128/mbio.02203-17. Sakimoto, K. K., Liu, C., Lim, J., & Yang, P. (2014). Salt-induced self-assembly of bacteria on nanowire arrays. Nano Letters, 14(9), 5471–5476. doi:10.1021/nl502946j. Segal, M. (2009). Selling graphene by the ton. Nature Nanotechnology, 4(10), 612–614. doi:10.1038/nnano. 2009.279. Singh, A. V., Vyas, V., Patil, R., Sharma, V., Scopelliti, P. E., Bongiorno, G., Podestà, A., Lenardi, C., Gade, W. N., & Milani, P. (2011). Quantitative characterization of the influence of the nanoscale morphology of nanostructured surfaces on bacterial adhesion and biofilm formation. PLoS One, 6(9). doi:10.1371/journal.pone.0025029. Terje, A., & Skotheim, J. (2007). Handbook of Conducting Polymers (3rd ed., p. 1680). Boca Raton: CRC press. Tran, V. B., Fleiszig, S. M. J., Evans, D. J., & Radke, C. J. (2011). Dynamics of flagellum-and pilus-mediated association of Pseudomonas aeruginosa with contact lens surfaces. Applied and Environmental Microbiology, 77(11), 3644–3652. doi:10.1128/AEM.02656-10. Wang, J. (2005). Carbon-nanotube based electrochemical biosensors: A review. Electroanalysis, 17(1), 7–14. doi:10.1002/elan.200403113. Wu, J., Han, X., Li, D., Logan, B. E., Liu, J., Zhang, Z., & Feng, Y. (2020). Efficient CO2 conversion to formic acid in a novel microbial photoelectrochemical cell using a visible-light responsive CO3 O4 nanorodarrayed photocathode. Applied Catalysis B: Environmental, 276, 119102. doi:10.1016/j.apcatb.2020.119102. Zhang,T.,Nie,H.,Bain,T.S.,Lu,H.,Cui,M.,Snoeyenbos-West,O.L.,Franks,A.E.,Nevin,K.P.,Russell,T.P.,& Lovley, D. R. (2013). Improved cathode materials for microbial electrosynthesis. Energy and Environmental Science, 6(1), 217–224. doi:10.1039/c2ee23350a. Zhou, M., Chi, M., Luo, J., He, H., & Jin, T. (2011). An overview of electrode materials in microbial fuel cells. Journal of Power Sources, 196(10), 4427–4435. doi:10.1016/j.jpowsour.2011.01.012.

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CHAPTER 11

Nanomaterials supporting oxygen reduction in bio-electrochemical systems Muhammad Zahoor a, Sabzoi Nizamuddin b and Shaukat Ali Mazari c a

School of Engineering, RMIT University, Australia Australian Rivers Institute and School of Environment and Science, Griffith University, Nathan Campus, Queensland, Australia c Department of Chemical Engineering, Dawood University of Engineering and Technology, Karachi, Pakistan b

11.1 Introduction The world population is growing at an exponential rate and so does the demand for energy and water, especially, in the developing countries (Baloch et al., 2018). At current pace, the world energy consumption is expected to reach 19,177 million tons of oil equivalent (Mtoe) by 2040 (2019). It could lead to the increase in amount CO2 emissions in the atmosphere from 33.2. gigatons (Gt) in 2018 to 41.3 Gt in 2040. Recently, the world has witnessed its first climate change induced famine in Madagascar, Africa due to drought worst in last four decades. Consequently, the research has been shifted towards the cheaper, eco-friendly alternate sources to address the current challenges (Hossain et al., 2020; Hwang et al., 2008; Mazari et al., 2021). In recent times, bioenergy has gained much popularity due to its eco-friendly nature and multiple benefits (Ryckebosch et al., 2011). Researchers have proposed various methods incorporating waste biomass to produce energy reducing the overall burden on the environment (Hussaro, 2014; Qureshi et al., 2019; Zahoor et al., 2021). Within this domain, the bio-electrochemical systems have gained popularity and is referred as green technology due to benefits of wastewater treatment, electricity generation, and chemicals production (Chandrasekhar, 2018). Bio-electrochemical systems (BESs) consists of bacterial systems used for the generation of electricity by treating wastewater along with organic matter, also generating the valuable chemicals including biogas, biopolymers, and alcohols, etc. (Banu et al., 2019). Microorganisms are important component of the BES and play a pivotal role in oxidizing the organic matter by the metabolic activities. In BES, microbe’s carryout oxidation and reduction reactions at the same time. Generally, in all types of BES, organic matter (substrate) oxidation due to the microbial activities takes place at the anode producing the electricity, however, at the cathode, various outcomes can be obtained such as hydrogen production and other valuable chemicals based on the type of BES used. Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00017-6

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Figure 11.1 Classification of bio-electrochemical system (BES). From Banu et al. (2019).

Various schemes have been incorporated in the BES, such as, (1) anaerobic oxidation– reduction reactions (ORR) taking place at the anode, (2) anaerobic oxidation–reduction reactions (ORR) taking place at the cathode,(3) absorption/desorption on the electrodes, and (4) combination of the above-mentioned mechanisms depending on the type of BES (Banu et al., 2019; Chandrasekhar, 2018). In recent times, the BES are classified based on the operation mechanism; (1) microbial fuel cell, (2) microbial electrosynthesis (MES), microbial electrolysis cell (MEC), microbial desalination cell (MDC), and microbial solar cell (MSC). Fig. 11.1 illustrates the classification of BES (Banu et al., 2019; Harnisch & Schröder, 2010; Liu et al., 2014; Oliveira et al., 2017).

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

In the past,majority of the research carried out related to the BES dealt with microbial fuel cells (MFC). Michael Potter (1911) has first studied the bio-electrochemical system in the form of MFC, generating the electric current by utilizing several microorganisms, degradation of substrate converting the chemical energy into electrical energy (Zheng et al., 2020; Jatoi et al., 2021, 2022). The MFC is consists of an anode and a cathode separated by an ion exchange membrane and connecting external circuit (Rinaldi et al.,2008). Electrons and protons/cations are released from the anode due to the consumption of substrate by the electro-active microorganism attached to the anode electrode immersed in the organic matter (fed) solution. The electrons are transferred through the external circuit to the cathode, and the protons transfer from the ion exchange membrane from the anode compartment towards the cathode compartment closing the circuit (Rinaldi et al., 2008; Zerrouki et al., 2022). The bacteria at the anode are anaerobic and the bacteria at the cathode are aerobic. There is also a possibility for the elimination of cathode chamber by directly exposing the cathode to the atmosphere, which is referred as single chamber MFC, it reduces the cost and simplifies the design. Reduction reaction takes place at the cathode in the presence of oxygen, for example, the migrated proton and the electron combines with the free oxygen, forming water the cathode (Rinaldi et al., 2008). Oxygen is considered as the ideal acceptor due to its abundant availability, low-cost and high oxidation potential, though, the oxidation reduction reaction (ORR) efficiency at the cathode majorly affects the overall performance of the MFC (Feng et al., 2011). The ORR mechanism is the major hurdle behind the low efficacy as it is very sluggish and slow resulting in low power outputs, moreover, a major setback behind the upscaling of the process (Li et al., 2020; Liu et al., 2014). It has been reported that cathode ORR efficiency can be enhanced by the application of a catalyst at the cathode electrode which increases the electric flux from the fast movement of electrons (Hernandez & Osma, 2020; Kang et al., 2018; Liu et al., 2014; Narayanaswamy Venkatesan & Dharmalingam, 2016). Various catalysts have been developed to enhance the efficiency of the MFC by increasing the ORR rate, such as (1) abiotic cathodes, (2) bio-cathode (biocatalyst), and (3) enzymes. For the bio-cathodes, low cost is the advantage, though, it gives low power outputs. Conversely, the enzymes give high power output, but they have limitation of high cost. Abiotic cathode, platinum (Pt), a precious metal has been regarded as a suitable catalyst for the ORR at the cathode, however, there are some limitations such as high cost, rare metal, and deactivation in the acidic environment by the pollutants (Feng et al., 2011; Shahbazi Farahani et al., 2019). Therefore, there is a need to explore the effective catalyst material for the cathode to enhance electron kinetics. Recently, researchers have turned their attention towards the catalyst with less expensive material and high catalytic activity potential of ORR at the cathode. Chemically modified carbon-based nanomaterials with tunable structure and morphology including carbon nanotubes (CNTs), graphene (2D nanomaterials), carbon nanofibers, and graphite form can be utilized as a catalyst for the ORR (Mustakeem, 2015). Also, other nano materials can be used as a catalyst to enhance the functionality of

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the MFCs, such materials are (1) metal nanoparticles (i.e., platinum, silver, gold, palladium, and copper), Oxides of metals (i.e., manganese oxide, titanium oxide, silicon dioxide, and aluminum oxide), quantum dots (i.e., ZnS, Cds), and nanocomposites (Khoo et al., 2020). The nanostructures are reserves high level of stability, large surface area, and have high conductive nature (Chandrasekhar, 2018). Therefore, these materials have high potential to be exercised as a catalyst at the cathode and can be utilized as a backing material. Fig. 11.2 adopted from Chandrasekhar (2018) shows the types of catalyst used for the ORR at the cathode. This chapter focuses on classification of bioelectrochemical systems and materials as catalysts for supporting oxygen reduction in bio-electrochemical systems. Nanomaterial synthesis and characterization for supporting oxygen reduction in bio-electrochemical systems. Role of nanomaterials in oxygen reduction in bio-electrochemical systems is discussed from the application and performance point of view.Chemical kinetics,reaction mechanisms, opportunities and challenges are also highlighted.

11.2 Material synthesis and characterization 11.2.1 Material synthesis Nanomaterial’s synthesis requires strict level of controls over the shape, size, and crystallinity of the material, as it is vital for the application of the material in various fields. There are numerous methods exists for synthesis of nanomaterials with their possible advantages and disadvantages which are categorized as (1) top-down and (2) bottomup techniques (Kafle, 2020; Lim et al., 2018). Fig. 11.3 illustrates the difference between above-mentioned techniques. The top-down technique carryout the synthesis of the nanomaterials by conserving the original properties of the material but reducing/cutting the bulk materials to nano particles (refer to Fig. 11.3). The main advantage of this approach is that it provides significant control over the production process, though, helps in mass production of nanomaterials. The major drawback of this approach is that surface structure of the nanomaterials is not perfect as desired, and its processed patterns are crystallographic ally damaged. The bottom-up technique involves the construction of materials from the bottom. The complex nanostructures are created by utilizing the nanoparticles, atoms, and molecules as building block of the nanomaterials. This gives freedom to the scientist to engineer the properties for the desired application. Furthermore, it offers uniformity of the final product, desired surface structure, reduced cost, and can be upscaled. In this section, commonly used synthesis methods are discussed using both top-down and bottom-up approaches (Khan et al., 2017). 11.2.1.1 Chemical vapor deposition (CVD) The CVD technique is a bottom-up methodology and is used for the synthesis of CNT in various forms and graphene (Gr) nanosheets (Yazdi et al., 2016). Conventionally, a

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

Figure 11.2 Oxidation-reduction reactions (ORR) catalyst incorporated in bio-electrochemical system-microbial fuel cells (BES-MFC). From Chandrasekhar (2018).

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Figure 11.3 Difference between top-down and bottom-up nanomaterials synthesis techniques. From Kafle (2020).

CVD setup includes the following: (1) gas carrying system, (2) tubular furnace, and (3) gas removal system. The commonly used substrate for graphene sheets is copper (Cu). The CVD technique has two types (1) thermal CVD and (2) plasma-enhanced CVD. In thermal CVD, chemical reaction is carried out at an elevated temperature, gaseous hydrocarbon feedstock is decomposed and introduced to the surface of the substrate to get deposited on it. In plasma-enhanced CVD, the chemical reaction of the feedstock occurs under vacuum where the decomposed material is deposited as a thin film on the surface of the substrate (Pareek & Mohan,2018).CNTs can be developed in various forms in bulk amount including powdered form, thick or thin film form, coiled or straight, entangled or aligned, and preferred structure on the desired location of the substrate. The CNTs are synthesized by a chemical reaction between transition metal catalyst particles with feedstock of gaseous hydrocarbons (Kuppurangam et al., 2019; Yazdi et al., 2016). CVD technique is also used for the synthesis of Gr from a carbon source on a metal foil substrate in a wide-area, single layered sheets (Pareek & Mohan, 2018). The CVD technique has high rate of growth, can be used for the hard materials, and has good accuracy. However, the process is complex, the layer formed on the substrate must not be pure, and toxic, corrosive gases are produced (Kafle, 2020). 11.2.1.2 Hydrothermal process Hydrothermal process is one of the oldest processes and is a bottom-up methodology, it is also referred as green technique due to its closed system operation for the synthesis unaltered huge nanocrystals with specific dimensional control (Anwer et al., 2019). It involves high temperature and high-pressure water activity crying out disintegration of the precursor salts (metal, metal oxide, etc.). It is extensively used for the synthesizing of metal oxides, quarts, and single crystals (e.g., N-doped graphene) in an apparatus made of steel pressure vessel (autoclave) (Kuppurangam et al., 2019). The temperature gradient

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

is maintained between to end of the growth chamber. The precursor salt is supplied along water, which at the hot end dissolves the salt and deposits at the colder end resulting in growing the crystals (Kafle, 2020). The improved variant of this procedure is referred as the solvo-thermal process giving freedom for the choosing from wide variety of solvents. Furthermore, this technique offers greater adaptability for controlling the size of the molecules and crystalline structure of the material (Anwer et al., 2019). 11.2.1.3 Sol–gel method Sol–gel technique is a type of chemical methods used to synthesize the multi component metallic oxide-based nanoparticles through simple and cost-effective method. It is commonly preferred due to its operation at the low temperature in a liquid phase providing pure oxide precursors (Kafle, 2020). It is a methodology in which Solution of appropriate precursor is formed and are changed into the gel through the utilization of precipitating mediator, transformed substance dimensions vary between nano to picometers (Anwer et al., 2019). 11.2.1.4 Mechanical exfoliation As clear from the name, mechanical exfoliation is top-down technique which mechanically peels the material layer by layer using lateral and normal forces. It is also referred as “Scotch tape method” as it can extract a layer of nanomaterial from the bulk material with the use of scotch tape (Khan et al., 2017). After peeling of the material, the resultant material yields a large amount of nanomaterial which can be transferred to the required surface of the support material via wet or dry routes (Khan et al., 2017). This technique is widely used for the metal oxides and graphene-based nanomaterials such as graphene oxide (Zhang et al., 2017). The advantage of this process is that it provides high quality product as no chemical reaction is involved, however, it is very tiring, time consuming, and challenging for large scale production (Olabi et al., 2020). 11.2.1.5 Chemical exfoliation, oxidation, and reduction This technique is a top-down technique in which layers are separated by the intercalating and decomposition of bulk materials crystals. It is an ideal technique used for the synthesis of graphene oxide (GO) on a large scale in a much cheaper way. This technique does not distort the overall structure of the material. However, it can increase the overall number of layers due to the presence of chemical reagent (Olabi et al., 2020). The graphene oxide can be further reduced by removing the unnecessary groups to give reduced graphene oxide (rGO). The nanomaterial produced through this technique is highly conductive and favors heterogenous electron transfer rate (Anwer et al., 2019). 11.2.2 Material characterization Nanomaterials now-a-days are synthesized alone or using two or more materials conjugates, making a nanocomposite with superior properties. Nanomaterials are commonly

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synthesized to obtain unique properties. However, the internal properties and how these nanocomposites will behave for the ORR in bio-electrochemical system-microbial fuel cells (BES-MFC) can be determined through the characterization of the material. The material characterization is divided into two types, (1) physical properties and (2) chemical properties. The physical properties aim to determine the properties such as size, shape, crystal structure, dispersion, crystallinity, surface properties and optical characterization to fully describe them. On the other hand, chemical characterization aims to measure various properties such as concentration, zeta potential, bonding methodology, other conjugated material, and composition of the nanomaterials (Kafle, 2020). Commonly used techniques for the characterization of the nanomaterials are as follows: UV-visible spectroscopy uses absorption/reflectance mechanism and showers light on the material within visible and adjacent ranges. This absorption/reflectance of light by various molecules in nanomaterials confirms their presence. Particle size, shape and dispersion is determined through various equipment’s such as scanning electron microscope (SEM), atomic force microscope (AFM), and transmission electron microscope. X-ray diffraction (XRD) is used to determine the extent of crystallization. Thermogravimetric analysis (TGA) is used to determine the weight loss due to thermal decomposition when the material is subject to the high heat. Fourier transform infrared spectroscopy (FTIR) is used to determine the presence of certain chemical group by determining the vibration stretching frequency. Brunauer-Emmett-Teller (BET) is used to determine the surface area of the nanomaterials. Static light scattering (SLS) technique is used to determine particle size distribution in the nanostructure. High-resolution transmission electron microscopy (HRTEM) is used to determine the crystallographic structure of the material through imaging at an atomic scale. Electron energy loss spectroscopy (EELS) is used to determine the loss of energy and ionization potential of atom. Energy dispersion X-ray spectroscopy (EDS/EDX) is an analytical tool for the determination of chemical characterization due to the unique electron structures. The material is investigated through scattering of light on the material and analyzed through X-rays (Anwer et al., 2019; Kafle, 2020; Srivastava, 2012).

11.3 Role of nanomaterials in oxygen reduction in bio-electrochemical systems In BES-MFC, exoelectrogenic bacteria carryout the biological oxidation reaction at the anode chamber resulting in freeing protons/cations and electrons. The produced electrons transport through an external channel to the cathode chamber and the protons/cations through a permeable membrane. The oxygen reduction reaction undergoes through a catalyst on the cathode, as the oxygen is abundantly available in the

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

atmosphere and has high electric potential (Chandrasekhar, 2018; Kuppurangam et al., 2019). However, the ORR in the BES-MFC has three major limiting factors which hampers the overall efficiency such as activation energy, ohmic losses, and mass transfer losses. The activation energy losses occur due to the lower ORR kinetics at the electrode owing to high energy requirement (498 kJ/mol) for the breaking the double bond of oxygen. Ohmic losses occurs due to the resistance to the flow of protons/cations and electrons, and mass transfer losses occur due to the lack of transfer of the reactants to the active sites of the catalyst. To overcome the above-mentioned limiting factors and enhance the overall efficiency of the ORR, the cathode must be modified with catalyst which offer high electrical conductivity with electron accepting capabilities to reduce the ohmic losses, high catalytic activity to reduce the required activation energy for the double bond of the oxygen, and the catalyst should facilitate the swift transfer of the electrons to reduce the mass transfer losses (Kannan & Gnana Kumar, 2016). 11.3.1 Carbon-based nanomaterial catalyst Recently, carbon-based nanomaterial has gained success enhancing the ORR as a catalyst in the cathode chamber of the BES-MFC.Carbon-based nanomaterials offer high surface area and high conductivity. Besides, they can be incorporated as a supporting material due to their inexpensiveness compared with other materials. The carbon-based nanomaterial catalyst used in the BES-MFC are carbon nanotubes, carbon nanofibers, and graphene etc. Still, their mechanism for the catalytic reaction has not been understood (Wang et al., 2014). 11.3.1.1 Carbon nanotubes (CNT) catalyst Carbon nanotubes are considered as a suitable material for the ORR catalyst as it has high catalytic conductivity, corrosion resistive nature, chemical and mechanical stability, lower cost, greater surface area, and high functionality with other groups (Chandrasekhar, 2018). The high surface area provides homogenous distribution of the catalytic unit to the supporting electrode material reducing the mass transfer losses, and it offers higher catalytic activity reducing activation energy losses (Mikhaylova et al., 2011). Oxidation capability of CNTs is induced by the oxidative treatment which enhances the solubility, better dispersion of the catalyst, and improves the active sites for the ORR. Sulfuric acid, nitric acid, or mixture of both acids is used for the oxidative treatment of CNTs to make them functionalized (Yazdi et al., 2016). Lu et al. (2013) pretreated the CNTs with nitric acid, the treatment enhanced the electroconductivity by 60%. Similarly, Ghasemi et al. (2013) also reported the treatment of CNT with mixture of acid which resulted in the enhanced ORR catalysis when compared against the carbon paper. Nitrogen doped CNTs (NCNTs) has shown great potential towards the enhancement of the ORR kinetics. The nitrogen addition imposes the uniform charge distribution at the

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Figure 11.4 Single cell bio-electrochemical system-microbial fuel cells (BES-MFC) configuration incorporating the carbon nanotubes (CNTs) for the oxidation-reduction reactions (ORR). From Chandrasekhar (2018).

surface of CNT easing down the oxygen interaction and adsorption (Yazdi et al., 2016). Feng et al. (2011) investigated the NCNTs in the BES-MFC. The NCNT electrode based MFC showed superior result in terms of power density 12.8% higher than the conventional Pt/C electrode based MFC. The results showed higher catalytic activity and durability against the conventional electrode. He et al. (2016) investigated NCNTs grew vertically aligned on the carbon cloth.The results demonstrated the favoring the ORR by enhancing the power density by 8.6% against the conventional Pt based cathodes. The above studies demonstrate the potential benefits for the incorporation of the carbonbased materials for enhancing the ORR in BES-MFC. Fig. 11.4 shows the configuration of BES-MFC incorporating the CNTs for the ORR. 11.3.1.2 Graphene (Gr)/graphite catalyst Graphene is a carbon-based two-dimensional nanostructure catalyst in the form of a honeycomb design. The Gr offers excellent characteristics which involves large surface area, high chemical and mechanical strength, lower-cost, along good thermal and electrical conductivity (Olabi et al., 2020). It has a reputation of enhancing ORR as

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

a catalyst at cathode in the BES-MFC due to significant effect of the electron shuttling rate caused by the presence of the oxygen containing compounds when compared against the other carbon-based catalyst. It should be kept in mind that ORR mechanism and catalyst action varies with various types of carbon-based materials (Chandrasekhar, 2018; Olabi et al., 2020). Santoro et al. (2017) studied the 3-D graphene sheets against the activated carbon as a cathode catalyst, the results demonstrated higher power density of 2.05 W/m2 for graphene nanosheets against the activated carbon with only 1.01 W/m2 . Similarly, Xiao et al. (2012) reported high power density of 3.3 W/m2 using crumpled graphene and 2.5 W/m2 using flat graphene as a catalyst at the cathode showing 10 times higher power density than bare carbon cloth. Graphene oxide (GO) and reduced graphene oxide (rGO) are extensively recognizable derivatives of the graphene. They have gained popularity due to high electrical conductivity, greater mass transfer with linear dispersion of electron, and stable support to enhance the reaction rate for the catalyst (Chandrasekhar, 2018). However, they are mostly investigated in the form of composite and have not been utilized independently (Shaari & Kamarudin, 2017). Similar to CNTs, nitrogen doping on graphene is also researched, the nitrogen doping promotes the ORR at the cathode with 4-electron pathway which enhances the performance of the BES-MFC (Kannan & Gnana Kumar, 2016). Feng et al. (2011) reported the use of nitrogen doped graphene nanosheets (NGNs) raised the ORR activity. The power density produced by the use of NGNs is equivalent to the power density of the Pt/C catalyst. Ci et al. (2012) investigated the use of NGNs as an ORR catalyst at the cathode of MFC generating 10% higher power output compared against the Pt/C. The peak current density reported for NGNs was 8.75 mA/cm2 which is greater than the 5.12 mA/cm2 of the Pt/C commercial catalyst (Ci et al., 2012). 11.3.2 Metal–carbon-based nanomaterial catalyst This section provides insight on the metal–carbon-based nanomaterials used as a catalyst for the ORR in the BES-MFC. 11.3.2.1 Metal–CNT catalyst Conventionally, the metal catalyst, for example, Pt is deposited on the support surface such as carbon cloth etc. Since CNTs have the ability to enhance the overall ORR and have the potential to be used as a supporting material. This methodology gives insight to the new possibilities to access the suitability of the catalyst for enhancing the power density and raising the overall efficiency of the BES-MFC by combination of metal catalyst with CNTs (Chandrasekhar, 2018; Yazdi et al., 2016). Zhang et al. (2011) reported that the MnO2 /CNT to be an effective catalyst than the individual MnO2 and CNTs. The power density produced by the MnO2 /CNT as a catalyst during

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ORR was 210 mW/m−2 which comparable against than the conventional commercial Pt/C cathode (229 mW/m−2 ) with much lower cost.Similarly,manganese oxide (MnO2 ) has also been investigated on the vertical array CNTs on the grown over stainless steel mesh. The outcome revealed higher SAV ratio for the adsorption of oxygen during ORR (Amade et al., 2015). Halakoo et al. (2015) prepared the Pt/CNT composite and investigated different cathodes by varying the amount of Pt conjugate with the CNTs. The results demonstrated that the Pt/CNT nanomaterial catalyst delivered promising performance while keeping less dependence on the precious metals. CNT coupled with nickel oxide (NiO) catalytic activity has been investigated. The results demonstrated enhanced catalytic performance for the conjugate NiO/CNT when compared against the individual catalysts (Y. Huang et al., 2015). The CNT catalyst on platinum support has been tested as a high-power density and low-cost cathode. The power efficiency of the CNT/Pt electrode has shown up to 32% raise in the power yield in the MFC (Ghasemi et al., 2013). 11.3.2.2 Metal–graphene catalyst Similarly, to the CNTs, Graphene has the reputation of acting as an active support for the metal catalyst (Chandrasekhar, 2018; Lim et al., 2018; Oliveira et al., 2017). Its major application has been seen along with the transition metals to minimize the shortcoming of lower catalytic activity and mass transfer (Pareek & Mohan, 2018). Y. Zhang et al. (2012) utilized iron tetrasulfophthalocyanine (FeTsPc) conjugated with the graphene. The evaluation of the ORR power density proved it a good alternative to Pt cathode, however, remained behind the MnO2 . Mashkour et al. (2017) determined the use of separate materials titanium oxide (TiO2 ) and hybrid graphene (HG) conjugate on the graphene (GP) electrode as a modified catalyst for the ORR. Significant performance enhancement has been recorded for GP-HG catalyst with overall power density of 220 mW/m2 , 30 mW/m2 for the TiO2 -GP against the 30 mW/m2 of simple GP electrode. 11.3.2.3 Metal-activated carbon catalyst Metals have superior catalytic ability but activated carbon has been used as a supporting material for ORR to compensate for the shortcomings and increasing the overall yield. Transition metal oxide, Cobalt Oxide supported with nitrogen doped activated carbon has been put to test. Investigation results showed the conjugate significantly increases the power density BES-MFC by 122.5% which is higher than the commercially used Pt/C (1201.4 mW/m2 ), showing potential for commercial use (Q. Huang et al., 2017). Similarly, Ge et al. (2016) hydrothermally fabricated the nano-urchin-like nickel cobaltite and mixed with activated carbon, utilized it as an electrode for the ORR in BES-MFC. The results showed 2.28-time high power density against the bare activated carbon electrode. Nitrogen doped carbon powder (NDCP) has been put into test for improvement of

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

catalytic activity of ORR. The outcome depicted that the high loading of the NDCP gives compatible performance as of the Pt/C catalyst. The lower overpotential, low cost and high chemical and mechanical stability gives NDCP an edge over the conventional catalyst for the ORR (Shi et al., 2012). 11.3.2.4 Metal–polymer catalyst Similar to metal–polymer anode, it has been proposed to incorporate the conducting polymers as support material for metal-based catalyst at the cathode of the BES-MFC (Antolini, 2015). PANI-MnO2 nanocomposite was investigated for the electrocatalytic activity for the ORR. The reported outcomes showed that the BES-MFC has enhanced power density due to synergistic effect created by both materials. The PANI-MnO2 catalyst activity improved due to the reduction of distance between the electrolyte and the electrode (Ansari et al., 2016). PANI and vanadium oxide (V2 O5 ) nanocomposite was synthesized and applied as a cathode catalyst to investigate the change brought by the catalyst to ORR (Ghoreishi et al., 2014). The outcome illustrated that PANI/V2 O5 generates a power density of 79.26 mW/m2 , which comparatively than V2 O5 and PANI by 65.31 mW/m2 and 42.4 mW/m2 , respectively. Furthermore, the columbic efficiency of the PANI/V2 O5 gives 16% which is 10% higher than the Pt catalyst (Ghoreishi et al., 2014). The above literature demonstrates the potential of using metal–polymer composites for enhancing the efficiency of the ORR at cathode. 11.3.3 Polymer-based nanomaterial catalyst This section demonstrates the use of polymer-based nanomaterials as a catalyst for the ORR in the BES-MFC. The conducting polymers including polyaniline (PANI), polythiophene, polypyrrole (PPy), and poly(3-methyl)thiophene strengthen due to the presence of π -conjugate show greater ionization potential, reduced energy transitions, high electron affinity and high electrical conductivity. The catalytic activity of the polymer-based catalyst can be altered by changing the polymer structure, carbon supporting material, and nitrogen bonding locations (Kannan & Gnana Kumar, 2016; Rinaldi et al., 2008). 11.3.3.1 Polymer–CNT catalyst The polymer based CNTs catalyst is synthesized through the following methods (1) Adsorption of polymers on the surface of the CNTs, it involves the wrapping of CNTs with the polymer chain. (2) Covalent bonding between CNTs and the polymers by in situ polymerization or through chemically attaching the polymers on the surface of the CNTs (Chandrasekhar, 2018). Jiang et al. (2014) tested the polyaniline (PANI) and CNT conjugate for the ORR at the cathode of BES-MFC for wastewater treatment and power generation. The results illustrated power density for PANI/CNT cathode greater

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than the pure CNT cathode. However, the power density is lower than the Pt/C catalyst (Jiang et al., 2014). Ghasemi et al. (2016) demonstrated the use of PPy/CNT as a novel cathode catalyst for the ORR at the cathode. The results showed the power density for PPy/CNT catalyst around 113 mW/m2 compared to the Pt/C catalyst 122.7 mW/m2 and the coulomb efficiency attained was 21% and 24.6% respectively. Thus, making the PPy/CNT catalyst as a prominent candidate to be used as a cheap and effective alternative compared to the precious metal-based catalyst. 11.3.3.2 Polymer–graphene (Gr) catalyst Similarly, the conducting polymers efficacy can be enhanced by doping them with graphene. Ren et al. (2013) prepared PANI/graphene nanosheets modified catalyst for reducing the oxygen at the cathode. The outcomes showed improved electrical conductivity and reduced activation energy with power density of 99 mW/m2 , making it a cheaper and effective alternate catalyst for the ORR. Graphene doped polyaniline (PANI) displayed an enhanced conductivity of 10 S.cm−1 compared with 2 S.cm−1 of pure PANI (Wang et al., 2014). Ansari et al. (2014) prepared PANI/Gr conjugate by in situ oxidative polymerization of aniline and graphene along surfactants and CTAB. It made formal distribution of Gr on the PANI structure which enhanced the electrical conductivity due to the π –π bond between PANI and Gr. The investigation showed high cathodic activity at an elevated temperature of 150 °C giving improved cell voltage and high-power density 0.01795 W/m2 , leading to overall enhanced performance against the plain carbon cathode (0.00921 W m2 ) (Ansari et al., 2014). 11.3.4 Metal/polymer/carbon-based nanomaterial composite catalyst It is clear that the metal oxides possess great potential towards the ORR, but poor electrical conductivity is barrier behind their lone application for which they are combined with conductive polymers as discussed in Section 11.3.2.4. Still, the application of Metal/polymer on the bare electrode may constrain the conductivity and reduces the electron and mass transfer. Keeping all this in mind, the composites of metal/polymer/carbon-based nanomaterials are potential contender for enhancing the performance of the BES-MFC with the effective catalyst for the ORR, reducing the activation energy for oxygen, high electrical conductivity, increased surface area and improved mass transfer rate (Yazdi et al., 2016). Nguyen et al. (2016). Similarly, Pattanayak et al., (2019) fabricated Ni–NiO/PPy–rGO nanocomposite catalyst by in situ polymerization pyyrole (Py) on reduced graphene oxide, using it as support material for depositing the nickel–nickel oxide Ni–NiO and used it as an ORR catalyst in the cathode chamber of BES-MFC. The synergistic effect of the composite material enhances the electrocatalytic activity for the ORR. The results demonstrated superior stability of the Ni–NiO/PPy–rGO nanocomposite with reduction potential of 0.535 V against the Pt/C catalyst with 0.521 V. The current density and power density of the Ni–NiO/PPy–rGO

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

nanocomposite determined were 2134.56 mA/m2 and 678.79 ± 34 mW/m2 , respectively. While for the commercial Pt/C catalyst the values measured were 1788.2 mA/m2 and 481.02 ± 24 mW/m2 respectively. As an effective catalyst, nanocomposite can be further optimized to replace conventional catalysts in BES-MFCs (Pattanayak et al., (2019). Yuan et al. (2011) synthesized PANI/carbon black (C) composite supported with FePc and investigated the electrocatalytic activity of the composite for the ORR at the cathode chamber of BES-MFC. The results suggested that the absorption of FePc on PANI/C enhances the catalytic activity by shifting the towards the positive potential and this results in increment of the peak current. The power density reported for the PANI/C/FePc was 630.5 mW/m2 compared to 575.6 mW/m2 for the Pt cathode. The economical analysis suggested that the power to cost ration for PANI/C/FePc is 7.5 times greater than the conventional cathode (Yuan et al., 2011). Lu et al. (2013) fabricated manganese (Mn)/polypyrrole(Ppy)/CNT-based novel oxygen reduction catalyst for the BES-MFC. The nanocomposite gives stable performance delivering power density of 213 mW/m2 with loading of 2 mg/cm2 , equivalent to the performance delivered by the Pt/C based commercial catalyst (Lu et al., 2013). This shows that the composite materials have massive potential, once optimized, practical application with upscaling of BES-MFCs will be feasible.

11.4 Chemical kinetics reaction mechanisms There are various options available for the electron acceptors in the aqueous medium of the cathode chamber of the MFC, such as dichromate, ferricyanide, persulfate, and permanganate etc. (Ben Kuppurangam et al., 2019; Liew et al., 2014). However, oxygen is regarded as the most suitable candidate as an electron acceptor in the cathode chamber of the MFC, due to its abundance, low-cost high oxidation potential (Feng et al., 2011). The oxygen molecules reduce at the cathode by accepting the free electrons coming from the anode chamber, it is referred as the ORR. There are series of steps in the ORR at the cathode based on the type of catalyst being used. Commonly, these processes are classified as namely 4-electrons pathway and 2-electron pathway. The 4-electrons pathway involves water (H2 O) production, while the 2-electron pathway generates hydrogen peroxide which ultimately leads to the reduction in electron potential (EP). Fig. 11.5 shows the mechanism of oxygen reduction at the cathode catalyst, (a) complete reduction, and (b) partial reduction (Stacy et al., 2017; Wang et al., 2014; Zhang et al., 2005). The following ORR takes place cathode chamber of the MFC.   O2 + 4H+ + 4e− → 2H2 O EPO2 /H2 O = 0.816 V (11.1)   O2 + 2H+ + 2e− → 2H2 O2 EPO2 /H2 O2 = 0.257 V

(11.2a)

  2H2 O2 + 4H+ + 2e− → 2H2 O EPH2 O2 /H2 O = 1.375 V

(11.2b)

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Figure 11.5 Mechanism of oxygen reduction at the cathode catalyst, (A) complete reduction and (B) partial reduction. From Stacy et al. (2017).

Ideally, the ORR at the cathode ought to progress with the 4-electron pathway (reaction 1), or either by 2-electron pathway, producing hydrogen peroxide and then instantly reacting with oxygen to form water. The electron potential in both pathways is same, that is, 0.816 V in reaction 1 and ((1.375 + 0.257)/2 = 0.816 V) in reaction 11.2a and 11.2b (Harnisch & Schröder, 2010). In the full reduction mechanism, dissociative adsorption of oxygen occurs when the oxygen molecule adsorbed at the surface of the catalyst. The ORR rate is determined by the adsorption of oxygen initially at the electrode catalyst surface, before it undergoes reduction to water or hydrogen peroxide. After the O2 adsorption, the transfer of electrons takes place with the addition of hydrogen, producing OOH ions. Further addition of hydrogen results in second electron transfer. The location of second hydrogen bonding decides the overall reaction fate, for the 4-electron pathway, the second hydrogen combines with the absorbed

Nanomaterials supporting oxygen reduction in bio-electrochemical systems

oxygen atom at the catalyst, it will produce two OH atoms. Furthermore, hydrogen addition with electron transfer results in production of two water molecules. For the 2-electron pathway, the second hydrogen combines with the oxygen already bonded with first hydrogen atom resulting in the production of hydrogen peroxide. As per density functional theory (DFT) determines that the 4-electron pathway is thermodynamically stable and it could enhance the overall efficiency of the ORR (Stacy et al., 2017). The generation of hydrogen peroxide reduces the overall electrode potential and current density, it also acts as an aggressive oxidizing agent that can reduce the overall stability of the cathode by attacking the catalyst and electrode support material (Harnisch & Schröder, 2010). The ORR pathway taking place at the cathode with various catalyst can be determined by rotating disc electrode (RDE) analysis or rotating ring disc electrode (RRDE) analysis is used to determine the number of electrons involved (Ben Liew et al., 2014). Therefore, to push the ORR in the direction of the 4-electron pathway, the catalyst development requires extensive amount of care to develop a catalyst which can reduce the activation energy required to break the double bond between the oxygen molecules. Till now, various nanomaterials are incorporated as an ORR catalyst improving the kinetics of the ORR and enhancing the generation of power (Wang et al., 2014).

11.5 Outlook and challenges The primary goal behind the massive research of the BES-MFC technology is to enhance the overall efficiency and reduce cost. Cathode catalyst is the major hurdle behind these goals, the ideal catalyst must be cheaper, simple to prepare, stable, high ionic conductivity and high mechanical strength. The process parameters such as concentration, nature of the substrate, acidic/basic environment, temperature, and pH value have a massive role behind the high overpotential and low faradic efficiency of MFC (Mustakeem, 2015). ORR kinetics are significantly affected by the operating conditions; thus, cheaper, stable and highly active catalyst is essential for the ORR kinetics. Moreover, process losses due to various overpotentials (e.g., concentration, activation, etc.) must be reduced by the optimization of the process. Also, process optimization can enhance power density at the anode by raising the microbial population with optimum conditions. Lastly, electron transfer mechanism can be fast-tracked by incorporating the suitable catalyst coating on the electrode surface (Chandrasekhar, 2018).

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Pareek, A., & Mohan, S. V. (2018). Biomass, biofuels, biochemicals: Microbial electrochemical technology: Sustainable platform for fuels,chemicals and remediation (pp.75–97).Elsevier.doi:10.1016/B978-0-444-64052-9.00004-2. Pattanayak, P., Papiya, F., Kumar, V., Pramanik, N., & Kundu, P. P. (2019). Sustainable Energy & Fuels, 3(7), 1808–1826. doi:10.1039/c9se00055k. Qureshi, S. S., Nizamuddin, S., Baloch, H. A., Siddiqui, M. T. H., Mubarak, N. M., & Griffin, G. J. (2019). Biomass Conversion and Biorefinery, 9(4), 827–841. doi:10.1007/s13399-019-00381-w. Ren, S., Lei, H., Wang, L., Bu, Q., Chen, S., & Wu, J. (2013). Biosystems Engineering, 116(4), 420–426. Rinaldi,A.,Mecheri,B.,Garavaglia,V.,Licoccia,S.,Di Nardo,P.,& Traversa,E.(2008).Energy and Environmental Science, 1(4), 417–429. doi:10.1039/b806498a. Ryckebosch, E., Drouillon, M., & Vervaeren, H. (2011). Biomass and Bioenergy, 35(5), 1633–1645. doi:10.1016/ j.biombioe.2011.02.033. Santoro, C., Kodali, M., Kabir, S., Soavi, F., Serov, A., & Atanassov, P. (2017). Journal of Power Sources, 356, 371–380. doi:10.1016/j.jpowsour.2017.03.135. Shaari, N., & Kamarudin, S. K. (2017). Renewable and Sustainable Energy Reviews, 69, 862–870. doi:10.1016/ j.rser.2016.07.044. Shahbazi Farahani, F., Mecheri, B., Majidi, M. R., Placidi, E., & D’Epifanio, A. (2019). Carbon-supported Fe/Mn-based perovskite-type oxides boost oxygen reduction in bioelectrochemical systems. Carbon, 145, 716–724. doi:10.1016/j.carbon.2019.01.083. Shi, X., Feng, Y., Wang, X., Lee, H., Liu, J., Qu, Y., He, W., Kumar, S. M. S., & Ren, N. (2012). Bioresource Technology, 108, 89–93. doi:10.1016/j.biortech.2011.12.078. Srivastava, R. (2012). International Journal of Green Nanotechnology: Biomedicine, 4(1), 17–27. doi:10.1080/ 19430892.2012.654738. Stacy, J., Regmi, Y. N., Leonard, B., & Fan, M. (2017). Renewable and Sustainable Energy Reviews, 69, 401–414. doi:10.1016/j.rser.2016.09.135. Wang, L., Lu, X., Lei, S., & Song, Y. (2014). Journal of Materials Chemistry A, 2(13), 4491–4509. doi:10.1039/ c3ta13462h. Wang, Z., Cao, C., Zheng, Y., Chen, S., & Zhao, F. (2014). ChemElectroChem, 1(11), 1813–1821. doi:10.1002/ celc.201402093. Xiao, L., Damien, J., Luo, J., Jang, H. D., Huang, J., & He, Z. (2012). Journal of Power Sources, 208, 187–192. doi:10.1016/j.jpowsour.2012.02.036. Yazdi, A. A., D’Angelo, L., Omer, N., Windiasti, G., Lu, X., & Xu, J. (2016). Biosensors and Bioelectronics, 85, 536–552. doi:10.1016/j.bios.2016.05.033. Yuan, Y., Ahmed, J., & Kim, S. (2011). Journal of Power Sources, 196(3), 1103–1106. doi:10.1016/j. jpowsour.2010.08.112. Zahoor, M., Nizamuddin, S., Madapusi, S., & Giustozzi, F. (2021). Process Safety and Environmental Protection, 147, 1135–1159. doi:10.1016/j.psep.2021.01.032. Zerrouki, A., Kameche, M., Ait Amer, A., Tayeb, A., Moussaoui, D., & Innocent, C. (2022). Environmental Technology (United Kingdom), 43(9), 1359–1369. doi:10.1080/09593330.2020.1829088. Zhang, J., Vukmirovic, M. B., Xu, Y., Mavrikakis, M., & Adzic, R. R. (2005). Angewandte Chemie, 117(14), 2170–2173. doi:10.1002/ange.200462335. Zhang, Y., Hu, Y., Li, S., Sun, J., & Hou, B. (2011). Journal of Power Sources, 196(22), 9284–9289. doi:10.1016/j.jpowsour.2011.07.069. Zhang, Y., Liu, L., Van Der Bruggen, B., & Yang, F. (2017). Journal of Materials Chemistry A, 5(25), 12673–12698. doi:10.1039/c7ta01511a. Zhang, Y., Mo, G., Li, X., & Ye, J. (2012). Journal of Power Sources, 197, 93–96. doi:10.1016/j.jpowsour. 2011.06.105. Zheng, T., Li, J., Ji, Y., Zhang, W., Fang, Y., Xin, F., Dong, W., Wei, P., Ma, J., & Jiang, M. (2020). Frontiers in Bioengineering and Biotechnology, 8(2019), 31 IEA. doi:10.3389/fbioe.2020.00010.

CHAPTER 12

Nanomaterials for ion-exchange membranes Ajith James Jose Department of Chemistry, St. Berchmans College, Changanassery, Kottayam, Kerala, India

12.1 Introduction Membrane science is an emerging area in chemistry, especially in applied chemistry and chemical engineering which have vast industrial and public health applications. It also plays a vital role in the field of alternative energy and separation applications. On considering the separation applications of membrane science, numerous membranes have been studied and utilized industrially in different processes like nanofiltration, ultrafiltration, microfiltration, electrodialysis, reverse osmosis, and pervaporation. Membranes and its properties play a major role in the feasibility of all these processes. Among the separation membranes, ion exchange membranes are one of the advanced separation membranes. Whilst having so many applications it has a few drawbacks that limit its efficiency. So, to face the challenges associated with ion exchange membranes, lot many researchers have been working world widely and they have investigated various approaches to enhance their properties. Among those the incorporation of nanomaterials into ion exchange membranes is one of the popular approaches employed. Therefore, this chapter provides you a snapshot of different types of ion exchange membranes along with the current status of nanomaterials incorporation in ion exchange membranes. This chapter also covers the methods adopted to incorporate nanomaterials into the ion exchange membranes, their properties, factors affecting their properties along with advantages and disadvantage.

12.2 Ion exchange membranes (IEMs) Ion exchange membranes play a significant role in addressing energy and environment related problems, thereby improving our standard of living, mainly in water treatment or purification, energy production, and energy storage. Due to their increased academic and industrial values, research on the development and applications of nanomaterial-based IEMs got more attention from various fields. As the name implies, IEMs are membranes that help to exchange ions between solutions. Usually, an IEM consists of charged groups or ion exchange groups attached to the three-dimensional polymer backbone. As the ions present are charged, they selectively absorb counter-ions and prevents the same Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00009-7

c 2023 Elsevier Inc. Copyright  All rights reserved.

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charge from penetration. Also because of water absorption IEMs displays gelling states in electrolyte solutions. 12.2.1 Types of IEMs IEMs are broadly classified into cation exchange membranes (CEMs) and anion exchange membranes (AEMs) depending on the type of ionic groups. Anion exchange membranes are (AEMs) membranes with fixed positively charged groups which prevent the passage of cations but permit the passage of anions. Whereas CEMs are membranes with fixed negatively charged groups which prevent the passage of anions but allow the passage of cations (Alabi et al., 2018). IEMs can also be classified by their structure and basic material, (1) amphoteric ion-exchange membranes in which both negatively and positively fixed ionic groups are randomly distributed, (2) bipolar membranes which consist of a cationand an anion-exchange membrane laminated together, and (3) mosaic ion-exchange membranes which are composed of macroscopic domains of polymers with negatively fixed ions and those with positively fixed ions randomly distributed in a neutral polymer matrix. 12.2.2 Fundamental properties of IEMs The charged groups (anions or cations) attached to the membranes make the IEMs differ from other polymer films and they give IEMs the properties that they possess. The amount (ion exchange capacity) and species of the charged groups and their distribution in the membrane along with the amount of water molecules adsorbed on the membrane due to these groups decide the properties of IEMs. 12.2.2.1 Ionic transport across the membrane Fundamental process behind a cation and an anion exchange membrane is that it allows the selective permeation of cations and anions respectively. Usually, Donnan equilibrium theory is used to understand the functioning of IEMs.When the ion exchange membrane is immersed in an electrolyte solution, counter-ions are ion exchanged and adsorbed on the membrane, and the co-ion is also adsorbed on the membrane and finally the ratio of ions in the membrane phase attains equilibrium. 12.2.2.2 Membrane potential The algebraic sum of Donnan and diffusion potentials obtained by the partition of ions into the pores as well as the mobility of ions within the membrane phase compared with the external phase is usually termed as the membrane potential (Khodabakhshi & Asgari, 2018). The membrane potential is generally measured using sodium chloride or potassium chloride solutions, which have nearly equal cation and anion mobilities. When both surfaces of an IEM come in contact with solutions containing different

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counter-ions a membrane potential is generated. As an example, when one surface of the cation exchange membrane is in contact with KCl solution and the other side with NaCl solution, potassium ions will selectively get ion exchanged with the membrane and easily permeate through it compared with sodium ions. This is due to the higher affinity of potassium ions for cation exchange membranes and smaller ionic diameter of potassium ions. Because of this the membrane surface of the sodium chloride side becomes positive. Membrane potential is generally high when the concentration of the mobile counter-ion is high and the membrane is highly cross-linked. The potential also is high if the anion exchange membrane has lower water content. 12.2.2.3 Diffusion When electrolyte solutions of different concentration are placed on both sides of an IEM, the electrolytes diffuse from a high-concentration side to a low concentration side. The diffused amount directly affects the current efficiency and the purity of products, so the diffusion coefficient of electrolytes through IEMs is important in practical applications. In some cases, the permeability coefficient is determined instead of the diffusion coefficient. The diffusion coefficient is expressed in cm2 s−l and the permeability coefficient in cm s−l . 12.2.2.4 Permselectivity When different kinds of ions charged with the same sign approaches the membrane the permeations of such ions across the membrane are not equivalent each other. This phenomenon is termed as perm-selectivity between ions charged with the same sign. The permselectivity between ions is calculated using the following equation: PAB =

tA/ tB CA/ CB

(12.1)

where tA : transport numbers of A ions in the membrane, tB : transport numbers of B ions in the membrane, CA : average concentrations of A ions during electrodialysis, CB : average concentrations of B ions during electrodialysis. The perm selectivity between ions through an ion exchange membrane in electrodialysis is measured using a two-or four-compartment cell. An IEM should be highly permeable for counter ions but should be impermeable to co-ions. 12.2.2.5 Transport number It is related to the ratio of the concentration of ion exchange groups in the membrane to the concentration of the outer solution. It is a measure of the permselectivity of

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counter-ions through the IEM. It is usually measured from membrane potential and by electrodialysis. 12.2.2.6 Electrical resistance It is an important property of IEM which is expressed as electrical resistance per unit area (cm2 ). Usually, it is measured in sodium chloride or potassium chloride solution but it changes remarkably with the species of counter-ions and also with the concentration of the electrolyte solution. This is due to the increase in Donnan adsorbed salts and shrinking of the membranes with increasing external electrolyte solution. Commercially available IEMs have electrical resistance of about 2–10 cm2 . Usually, low resistance IEMs are preferred in electrochemical processes. 12.2.2.7 Osmosis and electro-osmosis When a membrane is in contact with electrolyte solutions differing in concentration counter-ions (cations) and co-ions (anion) diffuse from a high concentration side to a low-concentration side. With IEM accelerated (anomalous positive) or depressed (negative) osmotic behaviors are observed. The anomalous positive osmosis is observed when the mobility of counter-ions in the cation exchange membrane is larger than that of anions. The negative osmosis occurs when the mobility of counter-ions in the cation exchange membrane is less than that of anions. In case of electro-osmosis when an electric current is passed across an ion exchange membrane, counter-ions are transferred through the membrane with water molecules (electro-osmotic water). This is because with applied current the liquid in the membrane will get the same charge as of the counter-ions and hence the liquid moves with the counter-ions toward the same direction. 12.2.2.8 Ion exchange capacity (IEC) and water content Ion exchange capacity (IEC) is an important tool that determines the efficiency of an IEM. As various kind of ion exchange groups are utilized in IEMs, with the amount and species of ion exchange groups the IEC of IEM changes. IEC is expressed as milligram equivalent per gram of membrane (meq./g) specified with dry or wet state of the membrane. In general, the ion exchange capacity of commercially available hydrocarbon type membranes is 0.5–3.5 mg equivalent per gram dry membrane. IEC is usually determined by measuring the amount of specified counter-ions in the membrane, using titration, after the process of ion exchange with other ions or solution, using a suitable indicator. Water content of IEM changes with the species of counter-ions and with the concentration of electrolyte solution with which the membranes are allowed to equilibrate. It is expressed as the amount of water per gram of sodium or potassium ion-form cation

Nanomaterials for ion-exchange membranes

exchange membrane or of chloride ion-form anion exchange membrane (g-H2 O/g ionic form wet or dry membrane). 12.2.2.9 Mechanical strength The IEM should be mechanically strong and should have a low degree of swelling or shrinking in transition from dilute to concentrated ionic solutions. 12.2.2.10 Chemical stability The IEM should be stable over the entire pH-range and in the presence of oxidizing agents.

12.3 Nanomaterials for IEMs The pursuit for developing IEMs with excellent properties made research to synthesize and develop robust IEMs through different approaches. Some of these include variation of functional groups (Jeong et al., 2015), inclusion of additives, surface modifications, adjustment of cross-linking polymers, etc. (Safronova et al., 2016). Incorporation of nanomaterials (NMs) into IEMs which comes under the category of inclusion of additives has now been investigating widely as a means of improving their properties. Thus developed IEMs are mainly used for fuel cell rather than electrodialysis (ED) because of potential toxicity of nanomaterial but still research is ongoing in this field to overcome such difficulties. 12.3.1 Use of nanomaterials in IEMs Many research works have shown that with nanomaterial incorporation the IEMs properties got improved especially its ion exchange capacity (Hosseini et al., 2012). Though understanding on the mechanisms by which the nanomaterials improve the properties of IEMs is limited. Mainly two different mechanisms are available to explain the way by which the nanomaterials improve the properties of IEMs. Among these the notable one is that nanomaterials provide additional ionic groups for ion exchange if the nanomaterials are functionalized (Mazari et al., 2021). That is if the nanomaterials are highly functionalized and if its quantity is high in the IEM, it will significantly increase the ion exchange capacity of the membrane (Bai et al., 2015). But addition of nonfunctionalized nanomaterials also resulted in improvements in the IEC of IEMs. And this means that there must be some other mechanism(s) responsible for the improved properties of IEMs (Zuo et al., 2009). Another is the ionic cluster dispersion mechanism (ICDM),in which the nanomaterial incorporation enables the creation of interconnected ion conducting pathways within the membrane matrix of IEMs. This results in formation of more ion conducting channels which provide more pathways for ion transport thereby

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Figure 12.1 Schematic representation of solution blending process.

increasing the IEC. It is the mechanism in the improvement of properties of IEMs with addition of nonfunctionalized nanomaterials (Li et al., 2009).

12.4 Methods available for nanomaterials incorporation in IEMs Several methods are available for incorporating nanomaterials in IEMs. They usually include: 12.4.1 Solution blending It is the commonly used technique for the synthesis of nanomaterial incorporated IEMs as it is usually used for polymeric nanocomposites fabrication. In this method nanoparticles are blended into the polymer matrix (Fig. 12.1) easily using a solvent and this solution is casted into a membrane. After this the solvent is separated through different techniques like evaporation, solution immersion, etc. 12.4.2 In situ polymerization It is almost similar to the solution bending process except that it uses monomers instead of polymers. That is, it requires an extra polymerization step to complete the formation of ion exchange membrane.The polymerization process is carried out after the incorporation of nanomaterials into the monomer solution. It is more time consuming and costly than solution bending process.

Nanomaterials for ion-exchange membranes

12.4.3 Melt mixing This process does not require any solvent instead here the nanomaterials are dispersed in a molten polymer matrix. Usually, thermoplastic polymers are employed because it is unsuitable for thermosetting polymers. At higher temperatures the thermoplastic polymer and the nanomaterials are mixed using techniques like injection molding, extrusion, etc. As it is taking place at higher temperatures there is chance of decomposition of the polymer and nanomaterials, degradation of surface modifiers and insufficient dispersion of nanomaterials, etc. 12.4.4 In situ sol–gel Compared to solution blending process the main difference is that it is completed by the hydrolysis and polycondensation of the precursor after the dispersion of nanomaterial precursor in the polymer solution. Hence it is more time consuming and costly than solution blending. Its applicability is limited as it can be used only to incorporate nanomaterials having precursors.

12.5 Nanomaterials used in IEMs A large number of studies on IEMs with different nanomaterial incorporations have been reported word widely. A direct comparison on all these studies will be difficult as diverse test conditions used in all these research works. The properties of IEMs with nanomaterials like the transport number and ionic exchange capacity, etc. vary significantly with the test conditions and also depending on the measurement method used. So here we are only trying to provide a brief outlook on the studies conducted on IEMs with different nanomaterial incorporations especially CNT, graphene, oxides, and metal nanoparticles (Nizamuddin et al., 2018). 12.5.1 Carbon-based nanomaterials in IEMs 12.5.1.1 Carbon nanotube (CNT) and its varieties in IEMs CNTs are one-dimensional hollow cylindrical tubes, made up of atom thick layers of carbon atoms, having a diameter measuring on the nanometer scales which are usually classified as single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Single-walled CNTs are cylinders formed from a single graphene sheet having diameters ranging from 0.4 nm to 3 nm whereas multi-walled CNTs consist of concentric rolls of graphene sheets. Due to their fascinating electrical and mechanical properties they have been used in numerous applications like sensors, energy storage and energy conversion devices, displays and semiconductor devices, etc. (Muhammad et al., 2021).

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As CNT’s have high aspects ratio and surface area they can be used as reinforcement materials in polymer nanocomposites. Because of their large aspect ratio, they have been intensively explored in constructing long-range ion conducting paths within a membrane matrix. By selecting appropriate ion-conducting groups attached to the surface of CNTs one can well optimize the microenvironment of the ionic channels. Also, CNTs one dimensional nature contributes to the formation of one-dimensional long-range channel along the interfaces between functional CNTs and polymer matrix. Studies by Heidary et al. revealed that a series of membranes prepared with the polymeric matrix containing aminated multi-walled carbon nanotubes shows improved transport properties, hydrophilicity, and energy consumption (Heidary et al., 2021). M. Qui et al. proposed can effective strategy to improve both the conductivity and alkaline stability of anion exchange membranes (AEMs) by the incorporation of imidazolium ionic liquids (ImILs) modified 1D carbon nanotubes (IL@CNT) into imidazolium-based poly (ether ketone) (ImPEEK). In their work they attached two types of ionic liquids (IL-M and ILB) with different alkaline to CNTs. They found that the introduction of ionic liquids to the CNT provided the hybrid membranes with additional ion hopping positions and 1D long-range ion-conducting channels (Qiu et al., 2019) S. M. Hosseini and co-workers found that comparison to their pristine counterpart, polycarbonate/styrene–butadiene rubber (PC/SBR) CEMs embedded with MWCNTs, displayed enhanced CEMs properties. They observed improvements in membrane potential, surface charge density, permselectivity, and transport number. Furthermore, superior ionic permeability, ion flux, ionic conductivity, current efficiency, energy consumption and thermal stability were also observed (Hosseini et al., 2010). H. Fan and co-authors developed nanocomposite cation exchange membranes by incorporating sulfonic acid-functionalized carbon nanotubes, sCNTs, in sulfonated poly(2,6-dimethyl-1,4-phenyleneoxide) polymer matrix. Thus, fabricated IEMs maintained their permselectivity along with improved conductivity behavior (Fan et al., 2020). Even though CNT inclusion gives excellent results it is difficult to obtain a homogeneous dispersion of CNT in solvents and they may aggregate into form clusters which can reduce IEM performance. 12.5.2 Graphene and its varieties in IEMs Graphene, the 2D honeycomb arrangement of carbon atoms, found so much interest among researchers due to its high mechanical strength, high-electrical conductivity, highthermal conductivity, and large specific surface area. Because of these unique properties, graphene and its varieties like graphene oxide (GO) and reduced graphene oxide (rGO) are widely used as additives in nanocomposites membranes. The problem of low hydroxide ion conductivity compared to protons and the reduce thermal and chemical stability of anion exchange membranes (AEM) can be overcome by incorporating graphene as it has good hydrogen ions (H+ , OH− ) selectivity and conductivity. Likewise GO can bind

Nanomaterials for ion-exchange membranes

with the functional groups of the polymer backbone as it contains oxygenated groups consisting of epoxide, hydroxyl, carbonyl, and carboxyl functional groups, etc. (GarcíaCruz et al., 2016). Also GO is used as additives for nanocomposite IEMs because of its easy dispersion in many of the solvents used in membrane casting of IEMs. L. García-Cruz and co-workers introduced graphene oxide as dopant to a mixed matrix membrane based on chitosan (CS) and poly(vinyl) alcohol (PVA). They found that the incorporation of graphene oxide considerably increased the thermal stability of the membrane. But it exhibited a low conductivity because the introduced graphene oxide did not change the crystallinity of the membrane. This membrane acted as a good physical barrier for alcohol permeability because of the reinforced structure created by the hydrogen bonds between the graphene oxide filler and the mixed polymer matrix (García-Cruz et al., 2016). H. Bai et al. in their work prepared a nanohybrid matrix by incorporating phosphorylated graphene oxide (PGO) nanosheets into chitosan (CS) matrix. The highly conductive channels formed along the PGO surface was found to significantly enabled proton conduction under both hydrated and anhydrous conditions (Bai et al., 2015). A heterogeneous cation exchange membranes based on polyvinylchloride mixed matrix embedded with graphene oxide nanoplates were prepared by S. M. Hosseini et al. using solution casting. An increase in the concentration of graphene oxide nanoplates up to a level in the matrix improved surface hydrophilicity and IEC of the membranes after that both these properties decreased. But membrane potential, transport number, selectivity and conductivity of the membrane increased with graphene oxide increase. Even though an initial increase in graphene oxide concentration decreased the ion permeability and flux initially after it showed a sharp increasing trend at higher concentrations of graphene oxide. But the water content lowered by increasing the GON content (Hosseini et al., 2020). W. Liu and co-authors prepared graphene oxide—P84 co-polyimide composite membranes through vacuum filtrating of graphene oxide on the porous P84 anion exchange supports followed by crosslinking with ethylenediamine which helped the stacked GO nanosheets to achieve necessary stability to overcome their inherent dispersibility in water environment. These membranes are applied to the diffusion dialysis process to separate H2 SO4 from the H2 SO4 /FeSO4 mixture and they showed superior separation factor than commercially available membranes. They also used to separate HCl from the organic acidic liquor containing HCl/glyphosate and this indicated that have good separation ability for the acidic solutions (W.Liu et al.,2020).It is found that functionalized nanomaterials have improved dispersion in solvents and casting solutions and they could also provide additional functional groups for ion exchange and hence their properties. K. Gerani and co-workers reports that for a sulfonated polyether sulfone-based cation-exchange membranes its transport properties like ionexchange capacity, transport number, and conductivity as well as thermal stabilities got enhanced by the incorporation of sulfonated graphene oxide rather than graphene oxide

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(Gerani et al., 2017). A. Alabi and co-authors incorporated negatively charged graphenebased nanomaterials into a noncharged poly(vinylidene fluoride) (PVDF) matrix to fabricate a nanocomposite cation exchange membranes (CEMs) by an in-house developed mold-casting technique. They incorporated modified graphene oxide or reduced graphene oxide nanosheets into ion exchange group carriers using sulfonic groupbearing agents and introduction of these modified nanosheets provided the ion exchange capabilities to the PVDF matrix. Due to the action of graphene-based nanomaterials as pore fillers these nanocomposite CEM’s stiffness increased and hence they displayed lower linear swelling ratios which are good for membrane stability. They showed good IEC and permselectivity and they showed good electrodialysis with considerable salt removal rates although the energy consumption results of the novel nanocomposite CEMs were higher than the conventional polymeric CEM (Alabi et al., 2020). D. Zhang et al. introduced 5-mercaptotetrazole modified graphene oxides into poly(2,6-dimethyl-1,4-phenylene oxide)-based anion exchange membranes which showed improved alkaline stability, enhanced Young’s modulus and low methanol permeability because the quaternized graphene oxide changed the microphase structure of the membranes with more ion clusters which helped for ion conduction (Zhang et al., 2020). Even though there are so many favorable results there are some problems that should be concerned like the homogeneity or the stability of the graphene-based layers on the surface of IEMs under different conditions and more studies are required on this topic. 12.5.3 Oxide-based nanomaterials in IEMs Usually, we can find IEMs with single metal oxide, multi–metal oxide and mixed metal oxide as additives in polymer matrix. Due to their chemical inertness, control over the particle size and size distribution, their simple fabrication techniques and comparatively inexpensive precursors make Silica nanoparticles the commonly used oxide in IEMs especially in fuel cell applications. L. Liu et al. prepared a series of novel composite anion exchange membranes for alkaline fuel cell by incorporating quaternized mesoporous silica nanoparticles into the chloromethylated polysulfone matrix. They found that with incorporation of silica nanoparticles ion exchange capacity (IEC) values, the bicarbonate conductivity, the water uptake, and the swelling-resistant properties of the membrane were greatly improved (L. Liu et al., 2015). M. Moghadasi and co-authors fabricated anion exchange membranes based on polyethersulfone which is modified using pristine and functionalized silica nanoparticles. The content of nanoparticle and the type of functional group associated with it influenced the water content of the membrane. Also, highest ion exchange capacity and conductivity were obtained for polyethersulfone modified with functionalized silica nanoparticles (Moghadasi & Mortaheb, 2017). S. M. Hosseini et al. prepared polyvinylchloride/styrene–butadiene-rubber blend heterogeneous cation exchange membranes and employed Iron–nickel oxide (Fe2 NiO4 )

Nanomaterials for ion-exchange membranes

nanoparticle as additive in membrane fabrication. Membrane ion exchange capacity, permselectivity,charge density,membrane potential and transport number were improved initially by increase of additive loading and then they began to decrease by more additive concentration. Membranes exhibited lower potential, selectivity and transport number for bivalent ions compared to monovalent ones. Membrane water content also found to decease with increase in Fe2 NiO4 concentration. They also developed a new type of cation exchange membrane using polyvinylchloride which was modified by BaFe12 O19 /CuO composite nanoparticles. They observed an enhancement in membrane water uptake whereas a decrease in membrane areal electrical resistance with BaFe12 O19 /CuO composite nanoparticles inclusion in the matrix. Up to 2 wt% addition of the nanoparticles ion exchange capacity, transport number, membrane potential, ion permeability/flux, permselectivity, surface charge density, etc. of the membrane improved sharply but with further increase of additive concentration all the parameters showed a decreasing trend (Hosseini et al., 2020). D. V. Golubenko, Shaydullin, and Yaroslavtsev (2019) introduced nanoparticles of Zr, Ti, and Si oxides in ion-exchange membranebased on sulfonated polystyrene nanocomposites and investigated the influence of acid– base properties of these particles on the physicochemical and transport properties of the membrane. With silica Doping water uptake and conductivity of the membrane increased whereas with introduction of titanium or zirconium oxide these properties of the membranes reduced. They report that the reason for this increase or decrease is a peculiar cross-linking of the membrane via salt bridge formation. Even though metal oxides like titanium oxide (TiO2 ), aluminum oxide (Al2 O3 ), zinc oxide (ZnO), etc. are less frequently used in combination with IEMs they also found to be improving the properties of IEMs. P. F. Msomi et al. prepared a series of anion exchange membranes based on poly (2,6-dimethyl-1,4-phenylene) (PPO) and polysulfone (PSF) blended with TiO2 and found that the water uptake, swelling ratio, ion exchange capacity, ion conductivity (IC) and thermal stability of the AEMs increased with increasing content of TiO2 within the composite membrane (Msomi et al., 2020). S. M. Hosseini and co-authors prepared mixed matrix heterogeneous cation exchange membranes using polyvinylchloride with aluminum oxide (Al2 O3 ) nanoparticles as additive. They found that membrane water content was enhanced by increase of additive concentration in prepared membrane but there was a decrease in ion exchange capacity. The membrane permselectivity, membrane areal electrical resistance and transport number were improved initially with increase of aluminum oxide nanoparticles concentration and then showed a decreasing tendency by more increase in additive content whereas an opposite effect is observed in the case of membrane ionic permeability and flux (Hosseini et al., 2014). Synthesized a novel electrolyte polymeric membrane by introduction of zinc oxide (ZnO) nanoparticles in a cross-linked PVA matrix for direct methanol fuel cell (DMFC) applications in which zinc oxide nanoparticles served as inorganic ion exchangers. The fabricated membranes exhibited a good thermal and chemical stability,

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high percentage of water uptake, a low percentage of methanol uptake along with transport number and ion exchange capacity comparable to commercially available membranes. 12.5.4 Metal nanoparticle-based IEMs There was not that much literature on metal nanoparticle incorporated IEMs other than Ag nanoparticle.J.Zhu et al.synthesized silver (Ag) nanoparticles loaded cation-exchange membranes and they found that proper particle size and dispersibility of the silver nanoparticles improved the mechanical properties of the membranes. The distribution of Ag nanoparticles near ion-exchange groups increased the aggregation of water molecules around them which helped in the formation of array broad ion-transport channels and this improved the efficiency of ion transport in the membranes. This membrane exhibited an enhanced NaCl removal ratio of with a high current efficiency and a low energy consumption (Zhu et al., 2019).

12.6 Factors affecting the performance of nanomaterial incorporated IEMs From above discussed contents we can definitely say that numerous factors influence the properties as well as performance of the ion exchange membranes. The two main factors include the quantity of the nanoparticle incorporated, whether it is functionalized or not. We can see that in some works the IEMs performance shows positive correlations with the content of nanomaterial whereas some report varying correlations of IEMs properties with the nanomaterials content. However, majority of the works report improvements in properties of IEMs at some specific quantity of nanomaterial. There were also results that showed insufficient improvement in properties like ion exchange capacity of IEM when there is masking or isolation of functional groups due to excessive quantity of nanomaterials. We can see that when functionalized nanomaterials are incorporated in the polymer matrix it showed enhanced properties compared to its pristine variety incorporated membrane. Another factor that influences the properties of ion exchange membranes is the properties of the additives or nanomaterial incorporated. If the nanomaterials possess properties such as mechanical strength, electrical conductivity, hydrophilicity, adsorption capacity, etc. it can be helpful in to producing membrane with better characteristics. That is if the nanoparticle has good electrical conductivity it will result in the IEMs with improved conductivity. The hydrophilic nature of the nanomaterial may help in entrapping water there by enhancing proton conductivity. In case of CNTs as its diameter is large enough to accommodate water molecule it can provide additional water channel which in turn increase the membrane water uptake and ionic permeability.

Nanomaterials for ion-exchange membranes

The nanoparticles may modify the pores and channels present in the system and can improve its mechanical strength, permselectivity, etc. The functional groups provided by the nanomaterials can improve the ionic transport and counter-ion permselectiviy whereas charged groups provided by them may improve ion-exchange capacity. If the interfacial bonding between the polymer matrix and additive nanomaterials is strong enough then it will reflect as improvement in the mechanical property of the membrane. Particle size, aspect ratio, etc. can also play a significant role in the overall performance of the membranes. A well-designed comparative study under standard conditions can only give information of effect of nanomaterial on the properties of IEMs.

12.7 Applications of nanomaterial incorporated IEMs Ion exchange membranes found attention and much interest from the academic and industrial fields, due to their potential applications in various fields especially like energy production and storage. Although ion exchange membranes can be used in many fields, they are mainly used in electrochemical or electromembrane processes such as electrodialysis, diffusion dialysis, separation of electrolytes and solid polymer electrolytes for fuel cells and reverse flow batteries. In these desalination by electrodialysis is the main process used in water treatment or purification, fuel cells are used for energy production and reverse flow batteries for energy storage (Siddiqui et al., 2019). Desalination is a process that employed to remove the salts and minerals from water medium. Different methods are used for water desalination and these include reverse osmosis, electrodialysis, nanofiltration, etc. The most important large-scale application of electrodialysis is water desalination. Industrial application of ion exchange membranes first started in the field of electrodialysis. It uses a constant electric field and selective ion-exchange membranes to move salt ions from water. Fig. 12.2 shows the schematic structure of a typical vertical sheet-flow type electrodialyzer in which anion exchange membranes, cation exchange membranes and gaskets (namely desalting cells and concentrating cells) are arranged alternately. Through the feeding frames a salt solution to be desalinated is supplied which flows through entrance ducts, entrance slots, currentpassing portions, and exit slots, and collected from the exit ducts to the outside of the stack. Another desalination method is membrane capacitive deionization in which ions are removed by applying an electrical field through an aqueous solution that flows among porous electrodes which are oppositely placed, and in front of which IEMs are positioned (Biesheuvel & van der Wal, 2010). Another application of ion exchange membrane comes in fuel cells which generate electricity by a chemical reaction. There are different kinds of fuel cells which includes solid oxide fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell (MCFC), direct methanol fuel cell (DMFC), polymer electrolyte fuel cell (PEFC), etc. and Ion exchange membranes are usually incorporated into PEFC and DMFC. The general design of the

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Figure 12.2 Schematic representation of a vertical flow type electrodialyzer.

Figure 12.3 Schematic representation of a fuel cell.

mentioned fuel cells is similar, except for the electrolyte, which determines the operating temperature. Fig. 12.3 shows schematic representation of a fuel cell. A typical polymer electrode fuel cell is shown in Fig. 12.3. It consists of a fuel electrode, an air electrode, and an ion exchange membrane. A direct methanol fuel cell whose key part is a proton exchange membrane can generate electricity directly from methanol and it have advantages like low weight, simple system design, high energy density at low operating temperature, low emission, etc. The development of the proton exchange membranes with a lower cost and higher performance is necessary and hence researchers focused on making new PEMs with high proton conductivity, durability, low fuel crossover, and low cost. The utilization of novel polymers and the incorporation of

Nanomaterials for ion-exchange membranes

Figure 12.4 Schematic representation of a reverse flow battery.

inorganic nanomaterials as additives into the polymer matrix is a good approach for the development of new membrane material. Recently anion exchange membrane fuel cells are used widely used in replacement for proton exchange membranes due to its high expense as it uses platinum-based catalyst (Biesheuvel & van der Wal, 2010). Fig. 12.4 shows schematic representation of a reverse flow battery. Redox flow batteries (RFBs) are regarded as promising large-scale energy storage and conversion devices. In this, oxidation–reduction reaction occurs between two chemical species on the inactive electrode surfaces of a battery. The term redox flow battery is used because in this battery the chemical species are stored outside the battery and supplied to it by pumps. It consists of consisting of solution tanks dissolving active electrolytes, pumps for circulating the electrolysis solution among the tanks and the cells, and pipelines. A cation exchange membrane is used to separate the electrolysis cell from the anode cell and cathode cell. AC transmitted from a power station is converted to DC through an A/D inverter, and the resulting charge is transmitted into the battery and a typical RFB is shown in Fig. 12.4. So, electrode and ion exchange membrane materials have vital role in the cell performance of RFBs in terms of efficiency, output power density, rate performance and cyclability. Application of nanomaterials to obtain advanced electrode and membrane materials can prompt improvements in RFBs performance (Long et al., 2021).

12.8 Advantages and disadvantages of nanomaterial incorporated IEMs Above-discussed studies shows that incorporation of nanomaterials in the polymer matrix is best approach for improving the properties of IEMs. Due to nanomaterial introduction IEMs displayed improved characteristics compared to an individual polymeric membrane and even comparable to commercially available systems and can find applications in

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fuel cell, water treatment, electrodialysis, etc. But when the nanomaterial loading level exceeds a certain concentration the properties of the membranes such as conductivity and permselectivity are decreased. This excess nanomaterial levels may also result in inaccessibility of functional groups and hence decrease in both conductivity and permselectivity. If the nanoparticles are deposited on the surface of the membrane it will result in the inaccessibility of functional groups, resistance for mass transfer or barrier for ionic migration and penetration.If there exists good compatibility between nanomaterial, the polymer matrix and distribution of the nanomaterial in the matrix, it will yield membranes with good stability and mechanical strength as well as good ion exchange parameters. This compatibility and fine distribution could be achieved by selecting the appropriate materials and improving the particles dispersion. The inorganic particle is functionalized with functional groups that can interact chemically with the polymer matrix thus improving the compatibility of the additive particle. If the nanomaterial introduced make the membrane brittle it will adversely affect its performance.

12.9 Conclusion and future scopes Nanomaterials incorporated IEMs can be considered as beneficial candidates for different applications like water purification, energy production and storage, etc. They are utilized in systems like electrodialysis, fuel cells, reverse flow battery, etc. Usually, nanomaterial incorporated into a polymer matrix to achieve composite membranes which can exhibit improvement in ion exchange capacity, permselectivity, mechanical strength, thermal stability, and water uptake. The commonly used nanomaterials include single walled and multi walled carbon nanotubes, graphene and its varieties and single and mixed oxides. Metal nanoparticles other than silver are rarely used. The nanomaterial properties, the quantity of nanomaterial added and distribution of the nanomaterial in the polymer matrix are highly influenced the properties and performance of ion exchange membranes. From the above discussions we can find that IEMs which have been modified with functionalized nanoparticle demonstrated superior properties compared to the IEMs modified with its virgin part. Nanomaterials incorporated IEMs should be studied under standard conditions to understand the real effect of these nanomaterials in their properties. Research in the field of nanomaterial-based IEMs is still widely open if we consider the varieties of polymers, nanomaterials, and preparation routes available. As selection of suitable polymer type is not only the prime factor influencing IEM properties with varying nanomaterial content and achieving good distribution of the nanomaterial in the matrix a large variety of composite IEMs can be fabricated. Since there are numerous routes are available for nanomaterial functionalization, composite IEMs of different varieties can still be synthesized and investigated for suitable properties. The final outcome of nanomaterial incorporation in IEMs should be to achieve improvement in the overall performance of the IEMs and it should not confine to isolated benefits.

Nanomaterials for ion-exchange membranes

To achieve significant results computer based numeric modeling and simulation would also be used in addition to laboratory experimental research. Along with standardized experimental conditions real time analysis of the membranes beyond the lab frame should be conducted to accurately assess performance of the different nanomaterial incorporated nanocomposite IEMs and to understand their working mechanisms.

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CHAPTER 13

Nanomaterials supporting indirect electron transport Umar Nishan a, Bushra a, Muhammad Asad a, Nawshad Muhammad b and Abdur Rahim c a

b c

Department of Chemistry, Kohat University of Science and Technology, Kohat, Pakistan Department of Dental Materials, Institute of Basic Medical Sciences, Khyber Medical University Peshawar, Peshawar, Pakistan Department of Chemistry, COMSAT University Islamabad, Islamabad Campus, Pakistan

13.1 Introduction Bioelectrochemical system can transform chemical energy to electrical energy using the chemistry between microorganisms and anode (Yuan et al., 2019; Jatoi et al., 2022). It is considered to be one of the most promising techniques in various fields such as in environmental monitoring, biosensing, waste water treatment, sustainable system for energy production (Borole, 2012; Jatoi et al., 2022). The microbial catalyst, which can transmit metabolically produced electrons to an external electrode, is the most important component of bioelectrochemical system technology. In bioelectrochemical system flow of electrons produced in external circuit measured as electric current as microorganisms or enzymes are involved in the oxidation or reduction reactions. Bioelectroactive microorganisms include bacteria, fungi, and archaea are known to be capable of electro activity. They have the capability to interact with cathode or anode (Bajracharya et al., 2016). On these substrates, electrons produced by microorganisms like as bacteria are transferred to the anode. They either flow to the cathode through a conductive substance that includes a resistor, or the gadget is operated with a load on it (i.e., producing electricity that runs a device) (Kracke et al., 2015). Indirect electron transfer (IET) or mediated electron transfer (MET) and direct electron transfer (DET) are two types of electron transfer between microorganisms and metal ions or electrodes. DET occurs when a microbial cell comes into direct contact with a solid-state electron acceptor/donor. For DET of bacteria in a bioelectrochemical system, outer membrane redox proteins such as cytochrome c and other electron transfer molecules associated with the microbial cell outer surface must establish a biofilm on the surface of the solid state electrode (Philips et al., 2016). Pili, electronically conductive protein filaments that are often referred to as “nanowires,’’ can also transport electrons directly between the electrode surface and microorganisms. Both Shewanella and Geobacter, model organisms, have been discovered Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00003-6

c 2023 Elsevier Inc. Copyright  All rights reserved.

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to generate pili while growing on the surface of electrode, allowing transfer of electron from the electrode surface throughout the biofilm (Pirbadian et al., 2014). The second electron transfer process relies on a soluble electron shuttle, also known as redox mediator, which transports electrons between the bacterial cells and electrode. Chemical compounds known as electron shuttles or redox mediators can enter bacterial cells and reduce (by accepting electrons) before being reoxidized (by transferring these electrons) in a microbial fuel cell’s electron acceptor or anode electrode. Because most microorganisms do not directly exchange electrons, with electrodes, redox mediators are frequently utilized as (endogenous or self-produced by microorganism) or added as (artificial or exogenous) to enhance transfer of electron (Evelyn et al., 2014). Primary metabolites, such as sulfides or ferrous ions or secondary metabolites such as phenazines are some examples of microbially produced electron shuttles or redox mediators (Pham et al., 2009). Pseudomonas aeruginosa excreted phenazines and flavins produced by Shewanella are considered the best redox mediators or electron shuttles produced by microorganisms. Microorganisms can also use artificial or exogenous electron shuttles or redox mediators to mediate the extracellular electron transfer of nonelectrogenic bacteria or to boost the capacity of electrodes. Some examples of exogenous or artificial redox mediators or electron shuttles used in bioelectrochemical system are humic acids, 2,6dichlorophenol indophenol, 1,4-naphthoquinone, anthraquinone-2-sulfonate (AQS), potassium ferricyanide, pyocyanin, thionine, resazurin, anthraquinone-2,6-disulfonate (AQDS), neutral red, and methylene blue (Van der Zee & Cervantes, 2009). Bioelectrochemical systems rely upon bioelectrocatalysts of microbial as well as enzymatic nature (Freguia et al., 2012). Another type of bioelectrochemical system is that using enzymes as catalyst (Zheng et al., 2020). Enzymatic biofuel cells are a form of bioelectrochemical device that uses oxidoreductase enzymes as electrocatalysts to convert energy to electricity by oxidizing an organic substrate and/or reducing oxygen or peroxide. The cathode and anode electrodes of the fuel cell can be assembled thanks to enzyme selectivity for their substrates. The use of various types of polymers and conductive nanomaterials as electrodes allows for a higher specific surface area, a higher number of wired enzymes in a unit area, and easier electron transmission between the active site of the enzyme and the electrode (Barelli et al., 2019). In enzymatic fuel cells, mediated electron transfer and direct electron transfer are two different electron transport paths due to which electrons transfer from the redox center of the biocatalyst at the anode (i.e., bioanodic enzyme) to the electrode. DET occurs when an electron produced by oxidation catalyzed by a bioanodic enzyme’s redox center transfer directly to surface of electrode and is captured as current. When another component is utilized as a mediator to transfer electrons between the enzyme catalyst and the electrode surface, MET develops (Yu & Myung, 2021). Even when DET-capable enzymes are used, mediators are frequently used to boost current density. A mediator acts as an electron shuttle, accepting one or more electrons from the electrode’s surface and delivering them to the enzyme’s active site to boost kinetics

Nanomaterials supporting indirect electron transport

(Johnson et al., 2003). Unfortunately, direct electron transport necessitates a direct link between the electrode and the enzyme, which is not always possible due to the active site’s position and orientation within the protein.Direct electron transport is only possible when the distances between the active site in the enzyme and electron relay, as well as the distance between electron relay and the surface of the electrode are less than the tunneling distance (roughly 1.5 nm), allowing the electrode and enzyme to make electronic contact. On the other hand, the distance between the catalytic core and the electrode material may be too great if the active center is deeply entrenched in the enzyme. In this scenario, mediators, which are small molecules with sufficient redox potential and activity, could be utilized as shuttles to increase electron transportation. In comparison to direct electron transport, mediated or indirect electron transfer results in higher catalytic currents. Some criteria, such as the redox potential, stability, and solubility under working conditions, as well as the features of the enzyme and the mediator, must be considered when choosing a suitable mediator. This is because the mediator’s hydrophobic/hydrophilic characteristics, as well as the mediator’s size and shape, affect the mediator’s penetration near to the enzyme’s redox site (Pereira et al., 2018). Laccases, oxidoreductases, and peroxidases are among these enzymes that have the potential to target a wide range of organic contaminants. These enzymes can transform a variety of substrates into less hazardous insoluble molecules that can be easily removed from waste. Due to the resistant nature of organic contaminants, these enzymes are sometimes unable to act on them. In the presence of redox mediators which are specific low molecular weight molecules these resistant substrates are transformed into less hazardous forms. A redox mediator boosts the pace of an enzyme-catalyzed reaction and expands the substrate selection range (Husain & Husain, 2008). The ability of an amperometric biosensor to measure substrate-dependent currents depends on electron-transfer activities between the active sites of enzymes (immobilized) and the electrode surface. Furthermore, taking into consideration the heterogeneous nature of the sensor compounds, the suggested redoxmediator (carbon-paste based) test method is an acceptable instrument for a preliminary assessment of numerous redox mediators in combination with a number of distinct enzymes (Ivanova et al., 2003). Nanomaterials support indirect electron transport by two methods (Fig. 13.1). This chapter discusses the development applications of nanomaterials for supporting indirect electron transport.

13.2 Nanomaterials supporting indirect electron transport in bioelectrochemical system 13.2.1 Nanomaterials as electron shuttles or redox mediators to facilitate indirect electron transport Direct electron transmission via conductive cytochromes and nanowires, as well as indirect electron transfer via soluble electron shuttles, can be facilitated by nanomaterials, resulting in a significant improvement in the performance of bioelectrochemical systems.

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Figure 13.1 Schematic for nanomaterials involved in indirect electron support in bioelectrochemical systems.

As a result, new nanomaterials have been studied for their ability to increase extracellular transfer of electron via the mediator pathway by adsorbing microbial electron shuttles or acting as solid mediators. Some nanomaterials can act as redox mediators in addition to boosting extracellular electron transport through adsorption and maintaining a high concentration of mediators near the electrode. To promote indirect electron transport, nanomaterials can act as redox mediators or electron shuttles (Fig. 13.2). Modification of anode with nanomaterials reduces the barriers of indirect electron transport. These mediators can collect electrons and then transport them to target compounds, assisting in the decomposition of pollutants at the same time (Wang et al., 2021). By acting as electron sinks and shortening the distance between bacteria and the anode, nanoparticles can help with electron transmission. Metal nanoparticles have been shown, in the literature, to have a role in electron transfer reactions by functioning as electron mediators for electron transfer from solutions to electrodes. Nanoparticles can carry electrons on their surface for electron transfer because of their large surface/volume ratio. The charges on the surface of the nanoparticles and the electrode surface generate a potential difference, allowing electron transport (Suravaram, 2016). Nanotubes of carbon such as SWCN (single walled carbon nanotubes) and graphene (graphene oxide, reduced graphene oxide, or pure graphene) are nanocarbon materials with excellent thermal, mechanical and electrical properties. As a result, they have

Nanomaterials supporting indirect electron transport

Figure 13.2 Schematics of nanomaterials supporting indirect electron transport methods.

been commonly used as redox mediators in electron transport that is either indirect or mediated. Carbon nanotubes (CNTs) operate as a redox mediator to promote indirect electron transport in order to establish electronic connection with redox enzymes serving as intermediates for the electron transfer because of their small diameter and quinone structure (Kalathil & Pant, 2016; Mubarak et al., 2014). Kalathil et al. (Kalathil et al., 2013) described a CNT/MnO2 nanocomposite as the anode for MFC. The Mn4+ in the nanocomposite may improve electron transport between the anode material and microorganisms, hence increasing electron conduction. MnO2 can also be utilized to store electrons because of its super capacitance, which is comparable to that of the cytochromes found in electrochemically active microorganisms’ outer cell membranes. MnO2 acts as a redox mediator in the composite, storing and releasing electrons to the electrode to aid indirect electron transport. Schematic of nanomaterials as redox mediators is presented in Fig. 13.3. 13.2.2 Anode modification with nanomaterials to support indirect electron transport Because the efficacy of mediated electron transport is greatly reduced when a biofilm forms, a biofilm-free electrode would be useful for efficient and stable operation of bioelectrochemical systems, particularly those that use indirect electron transport. As a result, it is important to develop electrode materials that can both restrict biofilm formation and respond sensitively to redox mediators. According to Zhang et al. (2015), this substance, tungsten trioxide (WO3 nanorods), has the ability to efficiently suppress

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Figure 13.3 Schematic of nanomaterials as redox mediators.

the formation of biofilm. As a result, it is an excellent electrode material for enhancing mediated or indirect extracellular electron transfer. Graphene composites have a long history of usage in bioelectrochemical systems (BES). Extracellular electron transport efficiency was greatly increased with the interaction between polyaniline nanostructure and electro active bacteria surface when the morphology and structure of polyaniline (PANI) were properly adjusted. In a bioelectrochemical system, polyaniline would be an excellent choice for a graphene composite electrode. The model electroactive bacteria S. oneidensis MR-1 was used to see how graphene and/or PANI affected extracellular electron transport between the bacteria and a specific electrode. After polyaniline nanostructures were modified with 46 and 55 A cm−2 for GP/PANI (graphite paper/polyaniline) and GO/PANI (graphene/PANI composite electrode), respectively, mediated or indirect (flavins mediated) electron transport was considerably found. It was discovered that graphene and polyaniline (PANI) nanostructures have a synergistic impact in boosting flavin-mediated or indirect transfer of electron in S. oneidensis MR-1 (Sun et al., 2017). MFCs seeded with Shewanella oneidensis MR-1, a well-studied exoelectrogen, had their carbon fabric anodes modified with nitrogen-doped carbon nanoparticles. The introduction of nitrogen-doped carbon nanotubes enhanced absorption of flavins at anode (a soluble electron mediator secreted by S. oneidensis MR-1), supporting shuttlemediated indirect extracellular transfer of electron. Zhang et al. (2015) reported a simple and effective strategy to significantly improve extracellular electron transfer process of S. loihica PV-4 as a model electrogenic microbe by adding antimony-doped tin oxide (ATO) nanoparticles with 5–10 nm size acted as active microelectrodes, with a concentration of 2 mM in the electrochemical reactor. In the presence of antimony-doped nanoparticles (ATO), the outer addition of riboflavin resulted in an evident enhanced of roughly 5 A

Nanomaterials supporting indirect electron transport

for the microbial current, suggesting that the extracellular electron transport indirectly mediated by flavins is accelerated by ATO nanoparticles. An amine-terminated ionic liquid (IL-NH2 ) is utilized to functionalize carbon nanotubes in MFCs to increase interfacial electron transport of Shewanella putrefaciens (S. putrefaciens) anode. Not only does the CNT-IL composite improve S. putrefaciens cell adhesion, but it also promotes flavin-mediated electron transport between the cells and the anode. It is worth noting that carbon nanotubes-ionic liquid (CNT-IL) is better for mediated or indirect electron transfer than DET. The CNT-IL/carbon cloth anode has a threefold better power density than the CNT anode in batch-mode S. putrefaciens MFCs and has outstanding long-term stability. This CNT-IL could be a useful anode material for microbial fuel cells (Wei et al., 2016).

13.3 Nanomaterials role in indirect electron transport in azo dyes reduction Carbon quantum dots (tiny carbon nanoparticles), commonly known as C-dots, were discovered to have hydroxyl and carboxyl functional groups on their surface after being produced from coconut husk.The presence of these functional groups on a carbon matrix endowed the C-dots with the ability to transport and conduct electrons. Dye reductionbased transfer of electron activity monitoring of a mixed microbial culture with C-dots indicated a 172% increase in transfer of electron activity, supporting the significance of C-dots in boosting microbial culture redox activity. This improvement was achieved by using C-dots as electron shuttles between the microbes and the dye to facilitate indirect electron transfer. In the experiment, the pace with which electrons were moved to methylene blue dye by microorganisms with electron transfer ability of carbon quantum dots was precisely proportionate to the rate at which it was decolored to its reduced leuco-form (Vishwanathan et al., 2016). The production mediators of electron in soluble extracellular metabolites by (Pseudoalteromonas sp. CF10-13) performed major function in dye complex Nephtol Green B (NGB) degradation and decolorization via extracellular electron transfer (indirect electron transport mechanism). With suggested mechanisms of Nephtol Green B (NGB) degradation, H2 S is a critical component. The use of Fe– S nanoparticles in the breakdown of Nephtol Green B dye avoids the risks of H2 S and ferric iron, allowing dye resources to be recovered. As a result, Pseudoalteromonas sp. CF1013 possibly transferred sulfonic acid groups to H2 S, that is in some ways consistent to the suggested NGB biodegradation processes (Cheng et al., 2019). Shewanella oneidensis MR-1, an electrochemically active bacterium, was combined with photocatalysts. For the destruction of contaminants, silver phosphate (Ag3 PO4 ) nanoparticles form a BPRDS (biophotoelectric reductive degradation system). Oneidensis MR-1 secretes flavins that act as electron shuttles, allowing for indirect EET. Shewanella, on the other hand, cannot directly degrade Rhodamine B. In the biophotoelectric

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reductive degradation system, reductive riboflavin could not drive the deethylation of Rhodamine B and operate as an electron shuttle to speed up RhB degradation. In this process, visible light activates the photocatalyst, silver phosphate nanostructures, which produce holes and photogenerated electrons. In VB, these photogenerated holes act as an electron shuttle, taking electrons from Shewanella cells via flavin. Through the Ndeethylation mechanism, photoexcited electrons destroy Rhodamine B. To improve reduction of azo dyes in the cathode of microbial fuel cells and bioelectricity production, a novel modified form of cathode approach was developed on (CP) using immobilization of redox mediators (RMs) with self-assembled peptide nanotubes (PNTs) as the carrier. Both the oxidative and reductive peak current densities were significantly greater for the riboflavin/PNTs and anthraquinone-2,6-disulfonate/PNTs modified electrodes than for the redox mediators or peptide nanotubes (PNTs) alone. The coupling effects of redox mediators and peptide nanotubes (PNTs) on the modified electrode have been demonstrated using modified electrodes and raw carbon paper electrodes. The self-assembled peptide nanotubes (PNTs) may increase electronic conductivity and specific surface area of cathodes, as well as provide a suitable carrier for the immobilization of redox mediators, which could lead to improved Orange II decolorization and power production. Peptide nanotubes, as the encapsulating matrix for RMs, had a higher rate of decolorization of orange II than Redox mediators immobilized by Nafion, which could be attributed to PNTs’ unique nanostructure and electronic conductivity (Xu et al., 2017).

13.3.1 Nanomaterials role in indirect electron transport in bioelectrochemical biosensor In amperometric biosensors, metallic nanoparticles have been widely researched as electron transfer mediators. While oxide and nonmetallic nanoparticles such as semiconductor nanoparticles can also boost the electron transfer rate between electrodes and proteins (Hayat et al., 2014). The electrochemistry of glucose oxidase (GOx) immobilized on a graphite rod electrode augmented with Au-NPs was examined. The aim of the study was the determination of the efficacy and application of Au-NPs in improving glucose oxidase-based amperometric glucose biosensors. Amperometric biosensors work by measuring the current associated with the oxidation and reduction of species that is electrically active and is participated in the recognition process. The use of Au-NPs increased the rate of facilitated electron transfer, according to this study. When using amperometric biosensors, the sensitivity of the electrochemically established method is dependent on the diameter of Au-NPs adsorbed on the surface of the graphite rod electrode. The electrocatalytic activity of GOx/Au-NPs/graphite electrodes based on smaller Au-NPs (3.5 nm and 6 nm) was found to be higher than that of the diameter of Au-NPs (13 nm). Owing to the larger surface area of Au-NPs they have the capability of

Nanomaterials supporting indirect electron transport

holding more enzyme molecules. By enabling electron transmission between electrode surface and redox enzyme, the presence of Au-NPs allows mediated electron transfer across the conducting circuit. Furthermore, a spectrophotometric investigation found that nanoparticles of gold can operate as mediator of transferring electron in dissolved glucose oxidase (GOx) and Au-NPs-based systems (German et al., 2010; Mehmood et al., 2020). A possible technique to fabricate a glucose biosensor was to covalently bind glucose oxidase (GOx) to a gold nanoparticle monolayer modified Au electrode. When ferocementhol was utilized as a synthetic redox mediator, the glucose oxidase mounted on gold nanoparticles showed outstanding electrocatalytic activity for glucose oxidation. The AuNPs can help with electron transport between the electrode surface and analyte, as well as enzyme loading and function (Zhang et al., 2005). Nanomaterials as electron mediators in electrochemical biosensors are becoming increasingly important. Metallic nanostructures are an interesting choice because of their strong conductivity and higher surface/volume ratio.When silver nanowires were utilized to build a new catechol electrochemical biosensor, the advantages of enhancing the aspect ratio of the electron mediator were studied.Studies using atomic force microscopy (AFM) revealed that the enzyme was distributed uniformly throughout the AgNWs, maximizing the surface of contact. Because of enhanced area of contact, transfer of electron between electrode surface and the enzyme is enhanced. This results in a LOD of 2.7 × 10−6 for tyrosinase immobilized onto AgNWs (AgNWs-Tyr), which is a factor of one lower than the LOD of 3.2 × 10−5 M for tyrosinase immobilized AgNPsTyr (Salvo-Comino et al., 2021). Covalent ferrocene carboxaldehyde adsorption on multiwalled carbon nanotubes (MWNTs) produced a nanocomposite with good water dispersion to allow the electrode and glucose oxidase to communicate electrically. It was possible to cast multiwalled carbon nanotube nanocomposites treated with ferrocene (MWNTs-Fc) on electrode surfaces. Glucose oxidase was subsequently mounted on the nanostructure film with the help of chitosan to create a reagentless amperometric glucose sensor.The nanocomposites were characterized using FTIR spectra and cyclic voltammetry.The addition of ferrocene as an electron transfer mediator and MWNTs as a conductor increased the enzymatic response to glucose oxidation significantly. With a linear range of 3.8 × 10−3 mol L−1 and LOD of 3.0 × 10−6 mol L−1 the new biosensor showed a quick reaction to glucose (Qiu et al., 2009). To achieve electrical transduction of the biochemical process, enzyme-modified NTs were immobilized onto electrode surfaces in the presence of either natural redox mediators (e.g., H2 O2 ) or synthetic redox mediators (e.g., ferrocene derivatives) to shuttle electrons between the redox protein and the conducting support. In the

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absence of glucose oxidase (GOx), no reaction was achieved using pure ferrocenylderivatized single wall nanotube (Fc-SWNT)/amphiphilic pyrrole derivative film, indicating that the SWNT/polymer system cannot stimulate glucose oxidation. In fact, after adding an appropriate diffusional redox mediator, ferrocenemethanol, to the buffer solution, methanol/GOx/poly-2 films with a comparable composition to the (FcSWNT/GOx/poly-2 the films mentioned above were discovered to be glucose catalytically sensitive.All of this suggests that FcSWNT may provide preferential electrical wiring routes for the enzyme, most likely via aligned ferrocenyl groups (although NT intrinsic conductivity may also play a role), underlining the potentialities of redox-active NTs for the development of mediated biosensors (Callegari et al., 2004). The amperometric approach was utilized to produce an enzyme-free nanobiosensor for acetylcholine (Ach) detection using nano-sized copper particles. Copper can oxidize organic and biological molecules via a chemical reaction between those substrates and Cu (III) active species near the anodic edge of voltammograms in alkaline solutions, via a redox mediation electron transfer mechanism near the anodic edge of voltammograms in alkaline solutions. According to this idea, the electrooxidation of ACh on copper-based electrodes happened via the active Cu (III) species. In comparison to the microstructured copper, the appearance of the two anodic peaks at lower potentials with higher corresponding currents was linked to the two steps of electrooxidation of the Ach in such a way that the copper nanoparticles can resolve the two fine steps of the electrooxidation process. The use of nanoparticles in an amperometric method for acetylcholine sensing with enhanced sensitivity proved successful (Jabbari et al., 2008). The electrochemical and spectroscopic properties of bis-pyrene-ABTS (2,2 -Azinobis (3-ethylbenzothiazoline-6sulfonic acid) encapsulated nanoparticles were examined. An intraelectron transfer chain between nearby redox units of clustered nanoparticles and the mono- and trinuclear Cu sites of bilirubin oxidases to intimate mediated or indirect electron transfer demonstrates their usage as electron shuttles or redox mediators. The redox nanoparticle approach yields higher current densities for mediated O2 reduction as compared to similar bioelectrode cells with dissolved mediator. The redox nanoparticles showed improved catalytic stability over 2 days, indicating a stabilizing influence that the polymeric framework displays. As bioelectrocatalysis mediators, bioinspired nanoparticles have the potential to be beneficial in future biosensors and biofuel cells (Gross et al.,2017).

13.3.2 Nanomaterials facilitate indirect electron transport for power or bioelectricity generation The metal or metal oxide modified anodes, such as MnO2 , Fe3 O4 , and Pd nanoparticles, produced better power densities in bioelectrochemical systems than the carbon black anode. Nanoparticles of MnO2 , Fe3 O4 , and Pd might improve electron transport from

Nanomaterials supporting indirect electron transport

anode bacteria to anode electrode. This improvement could be attributable to improved electron transmission, internal resistance was lowered, and microbial populations were carefully enriched. Furthermore, the loaded Fe3+ and Mn4+ on the Fe3 O4 and MnO2 anodes could operate as mediators of electron, tuning long-distance extracellular transport of electron between the bacteria and anode via a facilitated electron transfer process. Decoration of anode with Pd, Fe3 O4 , and MnO2 resulted in different performance in eliminating pharmaceutically active compounds from microbial fuel cells, according to the findings. When compared to the modified form of carbon black (CB) control anode, carbamazepine (CBZ), ibuprofen (IBF), and diclofenac (DCF) were further successfully removed in microbial fuel cells with the Pd, Fe3 O4 , and MnO2 anode. MFCs’ enhanced power production and PhACs removal performance may be due to improved electron transport, a greatly enhanced microbial community, and a unique catalytic activity of the metal or metal oxides on the anode electrode (Xu et al., 2018). (MnO2 )/(MWCNTs) composites for use as innovative anodes in BMFCs are generated in this study through a direct redox reaction between permanganate ions (MnO4 ) and (MWCNTs). The MnO2 /MWCNTs anode has improved power density, kinetic activity and wettability in comparison to the plain graphite (PG) anode, according to the findings. The deposition of MnO2 on multiwall carbon nanotubes (MWCNTs) can aid in the enrichment of Mn-related bacteria and considerably contribute to the provision of additional electrons. The metal Mn (IV) acts as an electron transfer mediator. The Mn (VI)/Mn (II) pair serves as a shuttle, carrying electrons from Mn-related bacteria to anode and then to the cathode via the outside circuit. In summary, a Mn4+ /Mn2+ electron shuttle is produced as a result of the anodic contact between the modified form of layer and the biofilm microbial action of bacteria, which improves the kinetic activity (Fu et al., 2014). It has been discussed how to build a new microbial fuel cell (MFC) employing carbon nanotube (CNT)-based electrodes and novel electron mediators. Nanofluids were created by distributing nanocrystalline platinum attached carbon nanotubes (CNTs) in water to create the new mediators. Despite its inability of regeneration with oxygen and toxicity, in the cathode compartment, hexacyanoferrate was used as the ultimate electron acceptor because it has been shown to improve cathode performance. This study proved the capacity of noble metals dispersed on CNTs to create high energy from even basic bacteria like E. coli. The concept of electron shuttles and using new nanomaterials for electrodes to improve MFC performance has been proposed (Sharma et al., 2008). To get better electrocatalytic performance, the MFC anode is made up of a nanoporous molybdenum carbide functionalized carbon felt electrode with a high threedimensional hierarchical porous architecture. The nanoporous molybdenum carbide (Mo2 C) functionalized anode significantly improves microbial electrocatalysis in microbial fuel cells MFCs, resulting in a fivefold increase in long-term electricity generation stability and power density. The significant improvement is due to the incorporation

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of a rough molybdenum carbide (Mo2 C) nanomaterials interface into carbon pore of comparatively large size architecture, which promotes microbial enlargement with high inner electron shuttle (flavin) excretion and a large number of available nanopores, which increases electrochemically active surface area. It is undeniable that nanoporous molybdenum carbide (Mo2 C) functionalization enhances the catalytic current of natural electron shuttles by enhancing their redox potential. This shows that flavins have a greater thermodynamic energetic force to transport electrons to a nanoporous molybdenum carbide-functionalized electrode. Furthermore, endogenous flavin quantity in microbial fuel cells with all Mo2 C@CF anodes are somewhat diverse but significantly more than in functionalized carbon felt microbial fuel cells, implying that the nanopore molybdenum carbide attachment may stimulate S. putrefaciens CN32 cells to release additional electron shuttles via biotic–abiotic interaction (Zou et al., 2017). The anode for S. putrefaciens CN32 MFC was made from a honeycomb-like hierarchical porous carbon–silica composite made from distiller’s grains for the first time. To begin with, the PC/Si-macro II’s porous structure allows free space for bacteria to migrate in and create a biofilm. The electron shuttles (flavins) can be trapped in the mesopores at the same time, allowing the two electroactive sites to contact the electrode surface simultaneously for a direct two-electron transfer. Furthermore, the contact-based DET process via outer membrane c-type cytochromes may benefit from a siliceous crust surface because it provides a pleasant surface for increased bacterium attachment. As a result, the PC/Si-II anode enhances both the DET and MET processes, resulting in the highest bioelectrocatalysis activity (Wu et al., 2018). Humic acid, as an electron shuttle, aids in the speeding up transfer of electron during natural biogeochemical transformations. Investigation of humic acid’s possible applications could lead to new bioelectrochemical system development concepts. Using MFC system that generates energy, the electron transfer availability of humic acid was examined. Humic acid was loaded into supercapacitor ferroferric oxide (Fe3 O4 ) to generate a stable wrappage (HA@Fe3 O4 ) as the doped materials. Humic acid was shown to minimize microorganism metabolic over potential and hence increase the efficiency of electron transmission between the anode and the bacterium by a significant amount. The microbial fuel cell with HA@Fe3 O4 -doped anode, which has greater chemical oxygen demand removal efficiency and electron consumption rate than the undoped anode, achieved a maximum power density of 1487.06 mW m−2 . Furthermore, the presence of Fe3 O4 speeds up the electron transfer process between humic acid and the anode, lowering the energy required for electron transport. Furthermore, the HA@Fe3 O4 doped anode has good stability and could provide a wider substrate utilization range for microorganism growth, resulting in a high-performance output to microbial fuel cells. Because of its ability to shuttle electrons, this research lays the groundwork for using humic acid in the creation of bioelectrochemical systems (Huang et al., 2019).

Nanomaterials supporting indirect electron transport

To aid the shuttling of electrons from inside the cell to the electrodes outside the cell, a number of mediators, such as neutral red, anthraquinone-1,6-disulfonic acid (AQDS), and 1,4-naphthoquinone, were used in many previous investigations. Several investigations have demonstrated that mediators immobilized on graphite anodes considerably improve power. All nanoparticle decorated electrodes, including 11 Au decorated anodes and five Pd decorated anodes, showed increased current output. The current generation’s large change was primarily due to improved bacterium–electrode interactions, rather than nonbiological processes. Cell-bound outer membrane cytochromes, conductive bacterium generated appendages (nanowires), and self-produced mediators (flavins) have all been shown to have a part in S. oneidensis electron transmission. The conductive nanoparticles may improve electron transfer efficiency by increasing direct contact between the electrodes and the bacterial outermembrane enzyme. They may also provide a vast surface area for mediators to transport electrons from bacterial internal enzymes (Fan et al., 2011).

13.4 Conclusions Nanomaterials acting as redox mediators or electron shuttles and modification with anode play an important role to facilitate or support indirect electron transport mechanism in different bioelectrochemical systems. Mostly carbon nanomaterials promote indirect electron transport acting as electron sink and reduce the distance between microbes and anode in bioelectrochemical systems. Nanomaterials store electrons due to its high capacitance and then release to anode serve as solid state redox mediators or electron shuttles. Modified anode with nanomaterials such as nitrogen-doped carbon nanoparticles, carbon nanotubes, carbon nanocomposites, and metal oxides nanoparticles promote indirect electron transport enhance absorption and secretions of endogenous redox mediators near anode to support or facilitate indirect electron transport. Due to high surface to volume ratio and high conductivity of nanomaterials as electron shuttles or redox mediators in electrochemical biosensing is taking an important role also support indirect or mediated electron transport in enzymatic-based bioelectrochemical systems.

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Kracke, F., Vassilev, I., & Krömer, J. O. (2015). Microbial electron transport and energy conservation – The foundation for optimizing bioelectrochemical systems. Frontiers in Microbiology, 6, 575. https://doi.org/10.3389/fmicb.2015.00575. Mehmood,A.,Mubarak,N.,Khalid,M.,Walvekar,R.,Abdullah,E.,Siddiqui,M.,Baloch,H.A.,Nizamuddin,S., & Mazari, S. (2020). Graphene based nanomaterials for strain sensor application–a review. Journal of Environmental Chemical Engineering, 8(3), 103743. Mubarak, N., Abdullah, E., Jayakumar, N., & Sahu, J. (2014). An overview on methods for the production of carbon nanotubes. Journal of Industrial and Engineering Chemistry, 20(4), 1186–1197. Pereira, A. R., Sedenho, G. C., de Souza, J. C. P., & Crespilho, F. N. (2018). Advances in enzyme bioelectrochemistry. Anais Da Academia Brasileira de Ciencias, 90(1), 825–857. https://doi.org/10.1590/ 0001-3765201820170514. Pham, T. H., Aelterman, P., & Verstraete, W. (2009). Bioanode performance in bioelectrochemical systems: Recent improvements and prospects. Trends in Biotechnology, 27(3), 168–178. https://doi.org/ 10.1016/j.tibtech.2008.11.005. Philips, J., Verbeeck, K., Rabaey, K., & Arends, J. B. A. (2016). Electron transfer mechanisms in biofilms. In: Cotter, P.D., (Ed.) Microbial electrochemical and fuel cells: Fundamentals and applications (pp. 67–113). Aarhus, Denmark: Elsevier Inc. https://doi.org/10.1016/B978-1-78242-375-1.00003-4. Pirbadian, S., Barchinger, S. E., Leung, K. M., Byun, H. S., Jangir, Y., Bouhenni, R. A., Reed, S. B., Romine, M. F., Saffarini, D. A., Shi, L., Gorby, Y. A., Golbeck, J. H., & El-Naggar, M. Y. (2014). Shewanella oneidensis MR-1 nanowires are outer membrane and periplasmic extensions of the extracellular electron transport components. Proceedings of the National Academy of Sciences, 111(35), 12883–12888. https://doi.org/ 10.1073/pnas.1410551111. Qiu, J. D., Zhou, W. M., Guo, J., Wang, R., & Liang, R. P. (2009). Amperometric sensor based on ferrocenemodified multiwalled carbon nanotube nanocomposites as electron mediator for the determination of glucose. Analytical Biochemistry, 385(2), 264–269. https://doi.org/10.1016/j.ab.2008.12.002. Salvo-Comino, C., Martin-Pedrosa, F., Garcia-Cabezon, C., & Rodriguez-Mendez, M. L. (2021). Silver nanowires as electron transfer mediators in electrochemical catechol biosensors. Sensors, 21(3), 899. https://doi.org/10.3390/s21030899. Sharma, T., Mohanareddy, A., Chandra, T., & Ramaprabhu, S. (2008). Development of carbon nanotubes and nanofluids based microbial fuel cell. International Journal of Hydrogen Energy, 33(22), 6749–6754. https://doi.org/10.1016/j.ijhydene.2008.05.112. Sun, D. Z., Yu, Y. Y., Xie, R. R., Zhang, C. L., Yang, Y., Zhai, D. D., Yang, G., Liu, L., & Yong, Y. C. (2017). In-situ growth of graphene/polyaniline for synergistic improvement of extracellular electron transfer in bioelectrochemical systems. Biosensors and Bioelectronics, 87, 195–202. https://doi.org/10.1016/j.bios.2016.08.037. Suravaram, S. (2016). Assembling nanostructured connections in bio-electrochemical systems. Van der Zee, F. P., & Cervantes, F. J. (2009). Impact and application of electron shuttles on the redox (bio)transformation of contaminants: A review. Biotechnology Advances, 27(3), 256–277. https://doi.org/ 10.1016/j.biotechadv.2009.01.004. Vishwanathan,A.S.,Aiyer,K.S.,Chunduri,L.A.A.,Venkataramaniah,K.,Siva Sankara Sai,S.,& Rao,G.(2016). Carbon quantum dots shuttle electrons to the anode of a microbial fuel cell. 3 Biotech, 6(2), 228. https://doi.org/10.1007/s13205-016-0552-1. Wang, R., Li, H., Sun, J., Zhang, L., Jiao, J., Wang, Q., & Liu, S. (2021). Nanomaterials facilitating microbial extracellular electron transfer at interfaces. Advanced Materials, 33(6), 2004051. https://doi.org/ 10.1002/adma.202004051. Wei, H., Wu, X. S., Zou, L., Wen, G. Y., Liu, D. Y., & Qiao, Y. (2016). Amine-terminated ionic liquid functionalized carbon nanotubes for enhanced interfacial electron transfer of Shewanella putrefaciens anode in microbial fuel cells. Journal of Power Sources, 315, 192–198. https://doi.org/10.1016/j.jpowsour.2016.03.033. Wu, X., Qiao, Y., Shi, Z., & Li, C. M. (2018). Enhancement of interfacial bioelectrocatalysis in Shewanella microbial fuel cells by a hierarchical porous carbon–silica composite derived from distiller’s grains. Sustainable Energy & Fuels, 2(3), 655–662. https://doi.org/10.1039/C7SE00560A. Xu, H., Quan, X., Xiao, Z., & Chen, L. (2017). Cathode modification with peptide nanotubes (PNTs) incorporating redox mediators for azo dyes decolorization enhancement in microbial fuel cells. International Journal of Hydrogen Energy, 42(12), 8207–8215. https://doi.org/10.1016/j.ijhydene.2017.01.025.

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Xu, H., Quan, X., Xiao, Z., & Chen, L. (2018). Effect of anodes decoration with metal and metal oxides nanoparticles on pharmaceutically active compounds removal and power generation in microbial fuel cells. Chemical Engineering Journal, 335, 539–547. https://doi.org/10.1016/j.cej.2017.10.159. Yu, S., & Myung, N. V. (2021). Recent advances in the direct electron transfer-enabled enzymatic fuel cells. Frontiers in Chemistry, 8, 620153. https://doi.org/10.3389/fchem.2020.620153. Yuan, H. R., Deng, L. F., Qian, X., Wang, L. F., Li, D. N., Chen, Y., & Yuan, Y. (2019). Significant enhancement of electron transfer from Shewanella oneidensis using a porous N-doped carbon cloth in a bioelectrochemical system. Science of the Total Environment, 665, 882–889. https://doi.org/10.1016/j.scitotenv.2019.02.082. Zhang, F., Yuan, S. J., Li, W. W., Chen, J. J., Ko, C. C., & Yu, H. Q. (2015). WO3 nanorods-modified carbon electrode for sustained electron uptake from Shewanella oneidensis MR-1 with suppressed biofilm formation. Electrochimica Acta, 152, 1–5. https://doi.org/10.1016/j.electacta.2014.11.103. Zhang, S., Wang, N., Yu, H., Niu, Y., & Sun, C. (2005). Covalent attachment of glucose oxidase to an Au electrode modified with gold nanoparticles for use as glucose biosensor. Bioelectrochemistry, 67(1), 15–22. https://doi.org/10.1016/j.bioelechem.2004.12.002. Zhang,X.,Liu,H.,Wang,J.,Ren,G.,Xie,B.,Liu,H.,Zhu,Y.,& Jiang,L.(2015).Facilitated extracellular electron transfer of Shewanella loihica PV-4 by antimony-doped tin oxide nanoparticles as active microelectrodes. Nanoscale, 7(44), 18763–18769. https://doi.org/10.1039/c5nr04765j. Zheng, T., Li, J., Ji, Y., Zhang, W., Fang, Y., Xin, F., Dong, W., Wei, P., Ma, J., & Jiang, M. (2020). Progress and prospects of bioelectrochemical systems: Electron transfer and its applications in the microbial metabolism. Frontiers in Bioengineering and Biotechnology, 8, 10. https://doi.org/10.3389/fbioe.2020.00010. Zou, L., Lu, Z., Huang, Y., Long, Z. e., & Qiao, Y. (2017). Nanoporous Mo2 C functionalized 3D carbon architecture anode for boosting flavins mediated interfacial bioelectrocatalysis in microbial fuel cells. Journal of Power Sources, 359, 549–555. https://doi.org/10.1016/j.jpowsour.2017.05.101.

CHAPTER 14

Techno-economic analysis of microbial fuel cells using different nanomaterials Lakshmipathy Muthukrishnan a, M. Castillo-Juárez b, Pedro Nava-Diguero c, Felipe Caballero-Briones b, Alberto Alvarez-Gallegos d and Sathish-Kumar Kamaraj e

a Department of Conservative Dentistry and Endodontics, Saveetha Dental College & Hospitals, Saveetha Institute of Medical and Technical Sciences, Chennai, Tamil Nadu, India b Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico c Universidad Tecnológica de Altamira, Altamira, Mexico d Centro de Investigación en Ingeniería y Ciencias Aplicadas, Universidad Autónoma del Estado de Morelos, Cuernavaca, Mexico e Instituto Politécnico Nacional (IPN)-Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Altamira, Mexico

14.1 Introduction Energy security remains one of the shared issues across the globe for which continued efforts have been taken to frame policies for economic sustainability. With the world getting populated and long-term shifts in temperatures and weather patterns, there is a high-level threat toward securing access to water and energy resources. As a result, there are lots of research studies going on in for evaluating and implementing alternative energy resources to satisfy the demand-and-supply chain of the growing population. According to the International Energy Outlook 2016 (World Energy Demand and Economic Outlook, 2016) reference a substantial growth in total world energy consumption has surged from 549 quadrillion British thermal units (Btu) in 2012 to 629 quadrillion Btu in 2020 and is expected to shoot up to 815 quadrillion Btu by 2040, accounting for 48% increase since 2012. In order to counterbalance the energy crisis, hydropower, biomass, wind, geothermal, and solar radiation have been sought after as alternative renewable resources for generating sustainable energy. In accordance, microbial fuel cell (MFC) remains one such technology, which has recently emerged to meet the demands. For instance, MFC-based wastewater treatment technology has been successful in providing clean water and green energy (Pant et al., 2012). Furthermore, the diverse capacity of the MFC relies upon the conversion of any kind of organic material into electricity through bioelectrochemical reactions utilizing microorganisms or enzymatic catalysis, toward sustainability of agricultural and industrial processes (Deval et al., 2017). More interestingly, the performance characteristics of Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00018-8

c 2023 Elsevier Inc. Copyright  All rights reserved.

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MFCs have outnumbered conventional technologies such as anaerobic digester and aerated lagoon. Improved conversion ability of MFCs (organic wastes are converted into electricity), circumvention of additional gas treatment and external energy input have made MFCs more efficient toward widespread applications. Before moving on to the efficiency of MFCs, it is quintessential to explore the principle behind the working of MFCs. The earliest description of the biocatalystlity to transport electrons directly to an electrode with the aid of an external mediator was discovered by Potter in 1911 (Potter, 1911). Since then, there have been several attempts to bring this technology to a more viable scale with more economical feasibility. Thus, MFC has emerged as an alternative energy system that converts the organic materials into electricity with the aid of a biocatalyst at the ambient temperature. This can employ a wide range of soluble or dissolve complex organic materials/wastewater and renewable biomass influent/effluent as a substrate that further offers the dual benefits of renewable direct electrical energy generation with simultaneous wastewater/remediation, which makes the whole process sustainable. Biocatalysts in the anode chamber are capable of oxidizing the diverse organic substrate, liberating electrons and protons. The electrons in turn travel to the cathode site via an external circuit and protons diffuse through the proton exchange membrane (PEM). These protons and electrons subsequently combine at the cathode side with molecular oxygen to produce water as illustrated in Fig. 14.1. Alongside, the reactions occurring at both the chambers could be given as: Anode: CH3 COO− + 2 H2 O → 2 CO2 + 7 H− + 8 e− Cathode: O2 + 4 e− + 4 H+ → 2 H2 O The by-products derived from these reactions include free electrons which flow through the circuit to generate energy in the form of electrical energy (Ucar et al., 2017). 14.1.1 MFCs into electricity generation MFCs harbor microorganisms for generating electricity and some of which have been involved in the syntrophic degradation of organic compounds (electron donors). As a result of degradation, the electrons are transferred directly to the outer surface of the cell thereby reducing the participation of an extracellular terminal electron acceptor. In addition, there may be some soluble metal species, which are reduced outside the cell because they are too large to enter the cell, as is probably the case for humic substances (Lovley et al., 1996, 1998). Moreover, there are certain soluble metal species, such as Fe(III) and U(VI) which are either chelated with citrate or complexed with carbonate to facilitate extracellular reduction (Coppi et al., 2007; Gralnick & Newman, 2007; Marshall et al., 2006; Shelobolina et al., 2007; Jatoi et al., 2021). Oxidation of organic matter coupled with the reduction of Fe(III) and Mn(IV) oxides plays an important role in regulating the carbon, iron and manganese cycles

Techno-economic analysis of microbial fuel cells using different nanomaterials

Figure 14.1 Schematic representation of the working of single-chamber microbial fuel cell. From Sathish-Kumar, K., Vignesh, V. & Caballero-Briones, F. (2017). Sustainable power production from plantmediated microbial fuel cells. In Sustainable agriculture towards food security (pp. 85–107). Springer Singapore. https://doi.org/10.1007/978-981-10-6647-4_6.

in sedimentary environments in turn influencing the fate of diverse trace metals and phosphates (Thamdrup, 2000). On the other hand, anaerobic oxidation of organic contaminants by the reduction of Fe(III) plays a pivotal role in the bioremediation of groundwater (Lovley, 1997) thereby stimulating dissimilatory metal reduction. Such approaches have shown promising results and have been implied for immobilizing toxic metals on the subsurface (Anderson et al., 2003; Lloyd et al., 2003). Furthermore, MFCs tend to utilize electrochemically active microbes in order to generate electricity by biodegrading the substrates. The power generation too varied with respect to the type of substrate used as shown in Table 14.1. However, extracellular transfer of electrons to minerals such as Fe(III) and Mn(IV) oxides (Gralnick & Newman, 2007; Lovley et al., 2004; Shi et al., 2007) and electrodes (Afkar et al., 2005; Kim et al., 2006) is probably of the greatest interest at present because of their environmental significance and practical applications in MFC. There are various reports touted on microbe–metal/electrode interactions (Gralnick & Newman, 2007; Lovley et al., 2004) that typically followed two major mechanisms viz. Direct and Indirect electron transfer.

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Table 14.1 Variation in power and current density with different substrates used. Type of substrate Power density Current density References

Glucose

Lactate Acetate

Wastewater

2.4 W m−2 290 mW m−2 1641 mW m−2 2066 mW m−2 2686 mW m−2 1303 mW m−3 4.75 mW m−3 1487 mW m−2 27.4 mW m−2 900 mW m−2 4.22 W m−3 1098 mW m−2

5042 mA m−2 568 mA m−2 7 A−2 8 A−2 0.30 mA cm−2 2.74 A−2 16 A−3 500 μA cm−2 98.6 mA m−2 1500 mA m−2 1600 mA m−2 4.62 mA cm−2 7.2 A−2

(Sharma et al., 2008) (Sun et al., 2010) (Zhu et al., 2011) (Zhu et al., 2011) (Zhang et al., 2011) (Feng et al., 2010) (Higgins et al., 2011) (Liu et al., 2008) (Li et al., 2011) (Wang et al., 2011) (Zhang et al., 2011) (Khilari et al., 2014) (Iftimie and Dumitru, 2019)

No permission required.

14.1.2 Direct electron transfer mechanism The main element in direct electron transfer is the ability of microbes to switch their metabolism from a soluble electron donor (hydrogen, glucose, acetate, etc.) or acceptor (oxygen, nitrate, fumarate, etc.) to a solid electron donor or acceptor at the surface of a conductive electrode as illustrated in Fig. 14.2 (Liu et al., 2011; Rabaey et al., 2005). Earlier reports state that some bacterial species such as Desulfuromonas acetoxidans,Geobacter sulfurreducens, Geobacter metallireducens, Rhodoferax ferrireducens, Desulfobulbus propionicus, and Enterococcus gallinarum (Holmes et al., 2004; Kim et al., 2005; Liu et al., 2011) were able to transfer electrons directly to an electrode surface. However, the direct transfer of electrons from bacteria to the conductive material was found controversial for S. putrefaciens, C. butyricum, Aeromonas hydrophila, and S. oneidensis (Erable et al., 2010). The flow of electrons encountered with these strains was a combination of a direct transfer (through components of the cell membrane) and an indirect transfer via mediators secreted by the bacteria (abiotic oxidation products derived from biological fermentation or through electrochemical mediators). 14.1.2.1 Oxidation of bacterial metabolism product The first studies on electrochemically active bacteria (EABs)/conductive material interfaces showed bioelectricity generation by oxidation of products derived from bacterial fermentation on the anode surface. These products such as H2 , alcohols, ammonia or H2 S or HS are oxidized at the anode (Fig. 14.3A). Recently, Clostridium (Clostridium beijerinckii, Clostridium butyricum) and other fermentative bacterial (Escherichia coli K12) strains were used to produce hydrogen electrochemically on the anode (Niessen et al., 2006; Tong et al., 2008). These studies indicated high current densities up to 30 Amps per square

Techno-economic analysis of microbial fuel cells using different nanomaterials

(A)

(B)

Figure 14.2 Direct electron transfer. (A) Membrane-bound electron transfer (B) nanowire-mediated electron transfer. From Sathish-Kumar, K., Vignesh, V. & Caballero-Briones, F. (2017). Sustainable power production from plant-mediated microbial fuel cells. In Sustainable agriculture towards food security (pp. 85–107). Springer Singapore. https://doi.org/10.1007/978-981-10-6647-4_6.

meter (A m−2 ) of an electrode (at + 0.20 V/Ag–AgCl) with heat-treated soil (compostbased fertilizers and manure) and tungsten carbide electrodes. Compounds other than hydrogen were also exploited in biological fuel cells. For example, the sulfate-reducing bacteria present in marine sediments were able to reduce sulfate compound into HS− or S2− (depending on pH), which oxidized into S0 directly on the anode surface (Lovley, 2006; Reimers et al., 2006; Ryckelynck et al., 2005). 14.1.2.2 Electron transfer by artificial electrochemical mediators Some nonfermentative bacteria act as an electrode as an electron acceptor to achieve the desired conversion but require the use of mediators for the electron transfer.

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(A)

(B)

(C)

Figure 14.3 Indirect electron transfer: (A) Oxidation of bacterial metabolism product; (B) Electron transfer by artificial electrochemical mediators; (C) Bacteria that produce their mediators. From SathishKumar, K., Vignesh, V. & Caballero-Briones, F. (2017). Sustainable power production from plant-mediated microbial fuel cells. In Sustainable agriculture towards food security (pp. 85–107). Springer Singapore. https://doi.org/10.1007/978-981-10-6647-4_6.

Electrochemical mediators are molecules that can be oxidized and reduced and then recycled successively. In their oxidized form, they are able to cross the cell membrane, accept electrons from at least one electron donor within the cell and then transfer throughout the cell under in the reduced form to finally oxidize and transfer electrons on the anode (Qiao et al.,2010) (Fig.14.3B).Electrochemical mediators were often used with bacterial species such as E. coli, Pseudomonas sp., Proteus sp., and Bacillus sp., which were unable to transfer electrons from their internal metabolism outside the cell.The most used mediators were thionine (Fig. 14.3B), neutral red, 2-hydroxy-1,4-naphthoquinone, and

Techno-economic analysis of microbial fuel cells using different nanomaterials

different kinds of phenazine. Since it has not been demonstrated that the microorganisms are able to maintain their growth in the presence of electrochemical mediators and the biological fuel cells have been operated continuously, requiring a constant presence of mediators that increased the cost of their applications. These disadvantages discouraged the use of mediators. Finally, electrochemical mediators were often toxic and could not be released into the environment without prior treatment. 14.1.2.3 Bacteria that produce their own mediators Bacteria such as Pseudomonas spp., Shewanella putrefaciens, or Geothrix fermentans (Karthikeyan et al., 2015) are able to produce electrochemical mediators to increase the extracellular electron transfer (Zhang et al., 2015; Jatoi et al., 2022). The bacterium, Pseudomonas aeruginosa has been described as producing phenazine molecules in turn increasing electron transfer as measured on the electrode (power obtained 116 mW m−2 ). In contrast, the mutant strains of P. aeruginosa, which could not synthesize electrochemical mediators, have achieved only 6 mW m−2 representing only 5% of the power observed with the nondeficient strain (Fig. 14.3C) (Bosire & Rosenbaum, 2017). With new properties and opportunities for technological and commercial development,applications of nanoparticles have been shown or proposed in areas as diverse as microelectronics, coatings and paints, and biotechnology (Fogel & Limson, 2016).

14.2 Microbial fuel cells and energy MFCs have been harnessed successfully for energy production such as electric power and biofuels. MFCs harbor MFCs utility to generate energy, to fulfill the increasing demand without the release of harmful byproducts or gases into the environment. For instance, glucose was fermented by Clostridium butyricum in order to produce hydrogen (Cabrol et al., 2017). Some of the products produced using MFCs have been enlisted in Table 14.2.

14.3 Circular bioeconomy of MFCs Conventionally, linear economic models have been employed to produce energy and chemicals and valuable products at the cost of finite resources thereby discarding the remains as wastes. In order to reduce the “take-make-dispose” concept in the linear model, sustainable, and renewable economic models to validate, reduce, and recycle the wastes have been proposed (Venkata Mohan et al., 2016). In this view, circular bioeconomic models have also been suggested as alternatives to linear economic models in terms of enhancing the recycling and reducing the biomass-derived wastes for the production of value-added products with the least negative impact on the environment (Venkata Mohan et al., 2016). For instance, in the wastewater treatment process, the main

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Table 14.2 MECs and their value-added product production. MEC system Production

References

Low-energy-input Dual-chamber MEC

(Zhang et al., 2019a)

16L MEC designed for H2 and NH3 recovery

MFCs-ammonia electrolysis cell coupled system (AEC)

MFCs with anaerobic digestion in a two-step process BES in alkaline MEC system harboring Enterococcaceae and Geo-alkalibacter

Electricity-assisting MFC

MFCs with graphite plates as electrodes

Hydrogen production (Maximum yield 28.7 L kg−1 ; max. H2 production rate of 2.5 m mol L−1 D−1 . Energy recovery efficiency 215.3% with total energy conversion of 11.3%. Production using lignocellulosic materials subjected to SSF technique. NH4+ transport rates up to 15.3 g N d−1 m−2 and H2 production rates up to 0.2 L H2 L−1 reactor d−1 achieved by bioelectrochemical system (BES). Production using pig slurry and some bacterial groups to clear off vertical stratification. Four air-cathode MFCs connected in series and substrate shifting showed H2 production rate of 59 μL g Mo2 C/N-rGO−1 h−1 and bioelectricity generation. NH4 + –N removal efficiency at 71% with H2 production rate at 42 μL Mo2 C/N-rGO−1 h−1 . This was accomplished by using AEC coupled system with Mo2 C/N-doped graphene nanocomposite. Production of BioH2 increased by 276%. Microbial metabolism mediated wastewater treatment and energy production. Higher current intensities corresponding to 2 mA, 71.4 A m−1 and 55% of CE in the anode of MEC involving consortia of bacteria (fermentative bacteria, alkaline exoelectrogens, and homoacetogens). Produced by the microbial degradation of alkaline glycerol. Circuit current of 9–78 mA m−2 along with H2 synthesis in several MFCs. This obviously increased the power supply for H2 production. Acid-rich effluents were used as feed in order to increase the efficiency of MFC. Members of Lactobacillaceae, Bacillaceae, Clostridia, and Pseudomonas aeruginosa were used to colonize cathode. Microbial systems and the material attributed for municipal waste treatment.

(Corbari et al., 2019)

(Zhang et al., 2019b)

(Florio et al., 2019) (Badia-Fabregat et al., 2019)

(Sun et al., 2009)

(Oh, 2005)

(continued on next page)

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Table 14.2 MECs and their value-added product production—cont’d MEC system

Production

References

Microbial electrolysis system (MES)

Ethanol, butanol, or propanediol produced by electricity-aided anaerobic microbial metabolic reactions. Glucose, glycerol, and other organic compounds were utilized by the microbes from the nutrient waste. Ethanol production by Saccharomyces cerevisiae by fermentation of glucose and bioelectrochemical oxidation.

(Mostafazadeh et al., 2017)

Ethanol production by the synergistic metabolic activities of Geobacter sulfurreducens and Clostridium cellobioparum and by fermentation of glycerol (ethanol yield up to 10 g L−1 )

(Speers et al., 2014)

Production of ethanol was at its maximum (11.52 g L−1 ) by the anaerobic digestion of Saccharomyces cerevisiae and glucose degradation in bio-electro-Fenton (BEF) system.

(Birjandi et al., 2016)

Microbe–enzyme cooperative bioelectrochemical system MEC (microbial electrolysis cell) driven by customized microbial consortium Glucose degradation in a bio-electro-Fenton system-driven MFCs

(Pagnoncelli et al., 2018)

objective pertains to clean the effluent prior to their discharge into water bodies thereby preventing pollution. In the conventional wastewater treatment plants, there generally involves pretreatment, primary, secondary, and tertiary treatments. While going through the process, primary treatment involves a stationary reservoir to settle down heavy solids and make the rest of the solids and other materials float. This is subjected to secondary treatment involving the microbial intervention of removal of organic and other inorganic materials. During the tertiary treatment, physical and chemical treatment is employed to purify the water and discharged it into the surrounding environment (Yin, 2010). Among treatment processes, secondary treatment involves biological degradation of dissolved organic matters in a controlled biological reactor. During biological degradation, there is a large consumption of electricity as the aerobic digestion of wastes demands large quantity of uninterrupted electricity. It is the aeration of activated sludge that has a great influence on wastewater treatment where the aerobic microbes are supplied with oxygen to expedite the conversion of wastes into CO2 , H2 O, and other organic constituents. Although this approach is the most preferred one due to the fast degradation of organic matters,low capital,and maintenance costs,it suffers from operating costs while circulating the air through aerobic tanks. Further, the process generates chemicals with long-carbon chains making the solid wastes difficult to dispose (Chan et al., 2009). Alternatively, anaerobic digestion is adopted in order to convert any organic wastes into biogas and CO2 in the absence of O2 . This process involves hydrolysis, acidogenesis,

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Figure 14.4 The significance of MFCs in environmental clean-up and production of energy (electricity/H2 ). CO2 released into the environment as a byproduct (during the microbial degradation of organic wastes) is reused by the biomass for their growth. The same CO2 is then distributed through the ecosystem as a sustainable carbon-neutral technology and is used to generate electricity by anaerobic microbial reactions. No permission required.

and methanogenesis for efficient conversion of wastes into methane (50–75%) and CO2 (20–45%). Even though the operational cost and solid waste remains are lower than those of the aerobic system, the time taken for anaerobic digestion exceeds weeks to months for completion (Fardin et al., 2018). On the other hand, the recruitment of BESs helped reduce the heavy amount of electricity being consumed for the aerobic digestion process. Alongside, they are capable enough to produce and utilize electricity for further electrochemical reactions. Moreover, the use of BESs confers innumerous applications such as rapid degradation of wastes through anodic reactions and electricity generation through anaerobic microbial reactions (Santoro et al., 2017). This is one such instance of circular bioeconomy where the use of MFCs generates electricity (energy) by decomposition of organic wastes through anaerobic microbes. In this line, the circular bioeconomy model remains the most promising approach over linear model aerobic digestion. Furthermore, based on the consumption of fossilized fuels and the emission of greenhouse gases, the aforesaid model has innumerous environmental benefits as illustrated in Fig. 14.4.

Techno-economic analysis of microbial fuel cells using different nanomaterials

This approach laid the foundation and emerging attention for CO2 sequestration and microbial synthesis in which microbes were harnessed to utilize CO2 and wastewater as organic feed. Typically, these bioelectrocatalysts are capable enough to convert CO2 into carbon-based industrially valuable products by directly recruiting electrons obtained from microbial reactions. For instance, CO2 released as a byproduct is reused in a CO2 -looping system as a reactant thereby reducing the emission into the environment (Sadhukhan et al., 2016). Moreover, carboxylic acids and alcohols remain the value-added products of MESs as reported (Christodoulou et al., 2017). There have been other products such as C1–6 which were produced as a result of carbon chain elongation and additional synthesis units. This CO2 /wastewater recycling system serves as a classical example of a circular bioeconomy model and economically viable approach (Bajracharya et al., 2017). Such renewable and sustainable system pertains to development and improvement in economic viability and performance.

14.4 Techno-economic assessment of MFCs MFCs constitute one of the economically viable waste-to-energy conversion approaches destined to remove dissolved organic matters over the conventional linear model. Although MFCs do not rely on aerobic conditions facilitating bacterial reactions, the economic feasibility of MFCs in treating wastewater over aerobic digestion need clarity. Initially, the wastewater treatment plants operate on huge capital costs, standard operating conditions, plant size and desired final products. For instance, there is number of variables accounting for operational costs of the waste water treatment plants. The operating costs of ∼360 wastewater treatment plants of 10,00,000–80,00,000 m3 year−1 accounted to nearly $0.12–$0.75 m−3 whereas those with 1,50,000–10,00,00,000 m3 year−1 was found to be in the range of $0.08–$0.99 m−3 (Hernandez-Sancho et al., 2011; Niu et al., 2016). Despite the operational and capital costs, specific operating conditions and the types of plants need to be compared for economic feasibility. Earlier reports on the wastewater treatment economics using MFCs have been well documented. The industrial-scale reactor could perform treating 100 m3 d−1 of wastewater with 2000 mg BOD L−1 at 20% coulombic efficiency and 85 W m−3 of power density thereby removing 85% of BOD. For better results, the anolyte volume used accounted for 33.3 m3 made to run for 8 h of hydraulic retention time (HRT). In accordance, the net value worked out to $3,80,528/33.3 m3 of MFC for a time span of 10 years, but devoid of maintenance and operating costs. On the other hand, the realistic setup demands a power density of 5W m−3 for 6 L a volume anode chamber which was found to be lesser than 5 W m−3 in an MFC-driven system for processing larger volumes of wastes. Thus, aerobic wastewater was more economical and beneficial than MFC but which lack experimental data (Fornero et al., 2010).

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On the other hand, the use of an MFC of 20 L volume could efficiently generate a 13-fold power density of 144 W m−3 and a volumetric resistivity of 1.2 m m−3 . It was also inferred that in controlled solutions, the MFCs were able to generate a power density of >1 kW m−3 . To make MFCs much more efficient and economically feasible, high power density, coulombic, and COD removal efficiency need to be improved by treating a large volume of wastewater (Dekker et al., 2009). Upon comparing the techno-economic analysis of MFC-driven wastewater treatment and aerobic wastewater treatment, innumerous differences in the operating parameters were identified. The compartments of a fuel cell were assembled with carbon cloth anode and cathode and their performances compared with conventional activated sludge process. Studies conducted by setting up the effluent flow rate with COD of 15,000 mg L−1 at 54 m3 per day, with a wide range of removal of 40–90% and coulombic efficiency (CE) of 2–30% attributed mostly toward wastewater treatment and electricity generation. In this scenario, MFCs could utilize 10% of the CE to remove 60% of COD and thereby generating 7.0 and 3.5 V, respectively (Trapero et al., 2017).

14.5 Performance of MFCs The performance of MFC is influenced by various factors such as electrode materials, the membranes, the mediators, electrode configuration and the microbial strain used. As the rate of flow of electrons between microbes and electrodes (anode/cathode) directly limits the performance of a MFC, the material with which electrodes are made is a major factor. In the anode chamber, the organic substances are oxidized by the metabolic activities of exoelectrogens (the microbes) and the electrons generated are then transferred to the external circuits via multiple extracellular electron-transfer pathways. Moreover, the electrons upon reaching the cathode get consumed by the reactions occurring on the cathode, enabling a closed electrical circuit thereby enabling electrical energy harvesting. The performance characteristics of anode remain the mainstay in the application of MFCs. In order to surpass the limitations, there need to be an improvement in the electron transfer efficiency and anode performance. This demands optimization of operation conditions, using electro-active microbial committees in the anode chamber, genetically engineered exoelectrogens and modified anode material for increasing the efficiency of extracellular electron transfer. There were several approaches made out of which the modification of anode material showed required up-gradation in the performance of MFCs, for the scale-up and commercialization. The characteristics of an ideal anode material are excellent conductivity, high surface area for better attachment of bacteria and its growth, enhanced biofilm formation through surface modification along with good extracellular electron transfer efficiency. Due to the possibility of using wastewater as a substrate for MFCs, it not only functions as a source of energy

Techno-economic analysis of microbial fuel cells using different nanomaterials

Figure 14.5 Functionalization of the electrode using different types of nanomaterials for improving the efficiency of MFCs. No permission required.

but a potential method of water treatment. Thus, MFCs are not only typical for industries but also in research (Elmekawy et al., 2014; Singh et al., 2016; Yu et al., 2015). In order to improve the efficiency of electron transfer, there are several nanomaterials that have been used in the anode to accelerate the extracellular reactions. This approach showed promising results in the improvement in power generation by MFCs. The nanomaterials usually comprise of polymers, oxides, metals, and their composites. Recently, carbon nanotubes (CNTs), graphite granules, and other metallic nanomaterials have found profound implications as anode nanomaterial. The functionalization of nanomaterials on the electrode has been illustrated in Fig. 14.5. Conversely, the use of nanomaterials still poses some challenges that interfere with MFCs unstable system performance, cost-effectiveness, and electron-recovery rates. To understand the need for functionalization, the technical glitch in the working of traditional carbon-based anodes for MFCs has been briefed. MFCs of any kind showcase the use of carbon as anodic material for their low resistance to charge transfer and cost-effectiveness. Further, the large surface area facilitating microbial attachment and growth, anticorrosive properties and stability remain the hallmarks. Then crude carbon was replaced by carbon cloth and carbon paper which had some limitations pertaining to durability and, relatively high cost. Although such materials can be used as anodic material, the large resistance offered limited their use. Graphite rods were preferred over

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(A)

(B)

Figure 14.6 Illustration of the effect of modified anode with nanomaterials: (B) over unmodified anode; (A) in terms of electron transfer. EAB, electrochemically active bacteria; EPS, exopolysaccharides. No permission required.

carbon cloth and paper in MFCs owing to good electrical conductivity, chemical stability, and low cost. But increased surface area could not be achieved to facilitate microbial attachment. Later, graphite fiber brush (with conductive corrosion-resistant titanium wire) ensheathing the anode was used in MFCs. This not only enhanced the specific surface area for microbial attachment but also significantly enhanced power generation (Zhou et al., 2011). MFCs are capable of generating low operating voltage (Vop ) on a par with the electromotive force (Ethermo ) termed as thermodynamically predicted potentials. As a result, there is energy loss in the form of activation, bacterial metabolism, mass transfer and ohmic loss. One of the prime reasons for this loss accounted to biofouling of anode caused due to excess of biofilm formation and organic compounds release which in turn reduces the electron transfer. There were various strategies adopted to reduce this activation that include increasing electrode catalysis, the introduction of mediators to facilitate electron transport, increasing the surface area, enriching electrogenic biofilm on the anode and optimizing operational parameters both inside compartments of anode and cathode. Among various strategies, the recruitment of nanomaterials has been preferred to modify anode which in turn promote the formation of electroactive bacteria and electron transfer (Pandit et al., 2017) as illustrated in Fig. 14.6.

Techno-economic analysis of microbial fuel cells using different nanomaterials

14.6 Use of nanomaterials in MFCs Nanomaterials of different types have been employed for the functionalization of anode surfaces. Metal oxide nanoparticles such as manganese oxide, iron oxide, titanium oxide, etc. have been widely used to enhance the adhering capacity of microbial strains in turn increasing the electrochemical activity and electron transport thereby increasing the energy output. The nanomaterials with their catalysts and their application in MFCs have been enlisted in Table 14.3. From the table, it is evident that different nanomaterials have a specific role in enhancing the electron transfer and power density of MFC. For instance, iron oxide could promote extracellular electron transport (EET) through two different mechanisms: by the formation of an electrical conduit inside biofilm and as an interface by accumulating on the surface of the cell. When inside the biofilm, these particles could promote the expression of c-type cytochromes which in turn generates electricity. Iron oxide nanoparticles on the anode surface facilitate attachment/self-assembly of Shewanella sp. in the form of an interconnected network resulting in 50-fold increase in power generation. On the other hand, as an interface, these iron oxide nanoparticles serve as a redox couple between Fe2+ and Fe3+ at the anodic interface thereby accelerating the EET process (Kato et al., 2012, Kato et al., 2013). CNTs and their hybrid material have been recognized as bioelectrode modifiers to promote scaffold porosity for biofilm formation and to enhance electrocatalysis. The use of CNTs to increase the surface area of anode in turn increases the biofilmforming potential and improved electrocatalysis. There are several methods such as physio-adsorption process, noncovalent interactions, direct electron transfer and chemical oxidation adopted to functionalize CNTs. Furthermore, improve the electrolytic activity of CNTs, they are modified with metal nanoparticles and metal colloids. In MFCs, metals like stainless steel, titanium and platinum are used to fabricate anode, but owing to the surface properties carbon-based materials have taken advantage. For instance, a CNT-coated sponge was prepared and investigated for its performance by offering lower resistance along with greater stability. Moreover, it had a three-dimensional scaffold with a uniform microporous structure promoting microbial colonization. This helped in achieving the maximum current density about 48% higher in comparison to the CNT-textile, when operated under the same condition. The extracellular electron transfer through microbes gets boosted up under the application of three-dimensional nanostructured electrodes. In addition, nitrogen-doped CNTs also find an application in MFC for power output enhancement. Nitrogen-doped CNT was prepared similar to bamboo with the catalytic pyrolysis of ethylenediamine which enhances the MFC performance through several mechanisms like enhanced electrochemical performance, biocompatibility as well as increased active sites for electrochemical reactions. CNTs can also enhance biofilm formation due to the presence of micropores (∼5–10 μm) and

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References

V2 O5 nanorods (nonsustainable) Palladium NPs on carbon cloth MnO2 NPs on carbon cloth (nonsustainable) Fe3 O4 on carbon cloth (nonsustainable) Fe nanorods (nonsustainable)

Single chamber

Cathode catalyst

1073 ± 18

2067 ± 25

(Ayyaru et al., 2019)

Dual chamber

Anode

824 ± 36

680 ± 28

(Xu et al., 2018)

Dual chamber

Anode

782 ± 37

680 ± 28

(Xu et al., 2018)

Dual chamber

Anode

728 ± 33

680 ± 28

(Xu et al., 2018)

Dual chamber

Cathode catalyst

66.4 mW m−3

10.6 mW m−3

WO3 NPs on carbon felt (nonsustainable) Palladium NPs on carbon cloth AuNPs on carbon paper (nonsustainable) MnOx nanorods (nonsustainable) Polyaniline (PANI)/multiwalled CNTs

Single chamber

Anode

1280

490

(Liu and Vipulanandan, 2017) (Varanasi et al., 2016)

Dual chamber

Anode

605

534

(Quan et al., 2015)

Dual chamber

Anode

346

174

(Guo et al., 2012)

Single chamber

Cathode catalyst

772.8 mW m−3

236.7 mW m−3

(Liu et al., 2010)

Single chamber

Cathode catalyst

476

367

(Jiang et al., 2014)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 14.3 Metal and metal oxide nanoparticles, their use in MFCs and relative power density. Power density Control power Nanomaterial MFC type Component (mW m−2 ) density (mW m−2 )

Techno-economic analysis of microbial fuel cells using different nanomaterials

mesopores (∼100 μm) on structures on the anode (Higgins et al., 2011; Xie et al., 2011; Yu et al., 2015). CNTs along with graphene were shown to be effective materials for use in MFC applications. A common property exhibited by both the materials attributed toward increased surface area facilitates proficient electron transport and enhanced effectiveness in MFCs (Ngaw et al.,2017).Moreover,the combinatorial effect of the use of CNTs along with microbial strains such as E.coli in MFCs had also been reported.With the insertion of CNTs, there was a significant increase in the power density in the order of 2470 mW m−2 on a par with 386 mW m−2 generated by control (Vishwanathan et al., 2016). In addition, quantum dots (QDs) have also been found to enhance the efficiency of MFCs with an improved performance 22.5% increase in power density. Equally competent on the catalytic activity, these carbon-based materials are too expensive and cannot be sourced from sustainable resources and processes (Christwardana & Kwon, 2017). Metals and metal oxide nanomaterials have gained attention due to their greater interaction toward the biological components which in turn could easily support electron transport toward electrodes. As electron transport constitutes one of the key components, the stacking of carbon anode support with nanoparticles had enhanced the growth of biofilm on their surface thereby improving the efficiency of MFCs (Duarte & Kwon, 2020; Duarte et al., 2019). Moreover, the excellent catalytic activities of transition metals and transition metal oxides have been harnessed and used as oxygen reduction reaction (ORR) catalysts in MFCs (Oliveira et al., 2020). Wu et al. (2018) have shown that carbon cloth anodes modified using Au NPs and Shewanella oneidensis MR-1 bacteria were effective in potentially generating a power density of about 178.34 ± 4.79 mW m−2 .The performance achieved attributed to 56.11% more than the performance of the control cells. Yet another study by Saravanakumar et al. (2016) showed the effectiveness of Au and Ag NPs synthesized using Trichoderma sp. in improving the catalytic activity of MFCs under anaerobic conditions. The highest current produced by this biosynthesized material accounted to 342.80 mA. On the other hand, phytochemical (Amaranthus dubius) mediated synthesis of Fe3 O4 NPs was found to be very effective in the MFC thereby generating a power density of 145.5 mW m−2 on a par with the control cells (Harshiny et al., 2017). Similarly, eucalyptus leaves mediated synthesis of zerovalent iron nanoparticles were used in MFCs (Xiao et al., 2018) with great improvement in performance. Recently, Citrobacter sp. mediated synthesis of Pd NPs and their implications toward MFCs has shown greater efficiency in terms of a higher power density of 539.3 mW m−3 (Matsena et al., 2020). Tahernia, Mohammadifar, Feng, and Choi (2020) have fabricated biogenic Pd nanoparticles using S. oneidensis MR-1 and found to be effective with an enhanced performance up to 75%. In parallel, Karim et al. (2020) have reported on the simultaneous synthesis of Au NPs and their decoration on MFC anode electrodes for special interactions of biofilms. As a result, Au NPs decorated anode generated 62.5% power density quite higher than

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the plain carbon anode. The set-up facilitated good adhesion of bacteria, amplified the conductivity, reduced internal resistance and improved the overall performance of MFC. Besides innumerous nanoparticles and their applications in MFCs, there remains a challenge with the type of nanoparticles. For instance, not all the metal nanoparticles shall bring in the same effect as an electrical bridge in MFC. Mateo, Cañizares, Rodrigo, and Fernandez-Morales (2018) have reported that the anode electrode of MFC decorated by Ni nanoparticles has shown the highest power density over the anode decorated with either Au or Pt. This decides the economic feasibility where the noble metalbased nanomaterials could be substituted by mere cheaper metal/metal oxides for MFC applications. It was also inferred from the table that the introduction of metals and metal oxide nanoparticles in MFCs could effectively increase the performance characteristics of MFCs. Moreover, the absolute power generation depends on factors such as specific substrates, cell design and operating conditions provided the decoration of nanomaterials on anode or cathode. The anodic decoration solely depends on the size and shape of the nanomaterials used whereas the cathodic catalysts require NPs loading. Besides, conjugated or composite materials have been equal competitors to improve the performance of microbial electrochemical devices. Composites of metals/oxides/carbon-based materials and conjugated forms such as polymers, layer-by-layer structures have been preferred for studying the interactions among carbon, polymers and microbial strain used. For instance, the fabrication of anode with mesoporous TiO2 /polyaniline composites in E. coli-based MFCs have demonstrated a two-fold increase in the power density of 1495 mW m−2 (Qiao et al., 2008). Similarly, waste corn stalks and pomelo skins mediated preparation of Fe3 O4 and partly graphitized carbon nanocomposites were used as cathode catalysts in MFCs and found capable of generating a power density of 1502 ± 30 mW m−2 over 1192 ± 33 mW m−2 generated by control. The high oxygen reduction reactivity, low charge transfer resistances and long-term stability of nanocomposite decorated electrodes of MFCs serve as low cost and high-efficiency catalysts for enhanced power generation (Ma et al., 2014). Moreover, nanocomposites of polyethersulfone with Fe3 O4 nanoparticles have been used as proton exchange membranes in MFCs by Di Palma et al. (2018). A maximum power density of 9.59 ± 1.18 mW m−2 and a current density of 38.38 ± 4.73 mA m−2 have been generated using 20 wt. % of nanoparticles.

14.7 Market survey of nanomaterials Nanomaterials have gained popularity and started penetrating the technological world to meet the demand-and-supply chain. Despite, their market penetration and diffusion into an industrial level, they face some challenges such as high capital cost, scalability, multiplication/miniaturization approach, COD removal rate, and value-added products manufacturing. A microelectrochemical device is represented by electrodes (anode and

Techno-economic analysis of microbial fuel cells using different nanomaterials

cathode), proton exchange membrane (PEM), reactor case, current collectors, and other accessories.Moreover,the performance characteristics of MFCs are dependent on various factors such as the pH, temperature, cell design, surface structure, microbial strain, solution conductivity, nutrient concentration, and charge transfer efficiency. There have been several approaches to overcome the shortcomings and to improve the efficiency of MFCs such as shifting toward a green chemistry approach, use of cheaper metal/metal oxide nanomaterials, carbon-based composites, etc. (Christwardana et al., 2018; Gajda et al., 2018; Iannaci et al., 2020). The criticality relies on reducing the costs of electrode material used in microbial electrochemical technologies to gain market attractiveness. The cost abatement needed to become feasible for use can only be achieved by excluding the use of noble metals and scavenging raw carbon materials from secondary or natural sources and then transforming them into performant nanostructured materials using green synthesis routes that are cheap and effective. Due to these driving reasons and additionally to overcome the problems arising from toxicity and low performance, sustainable approaches have been developed to resolve these shortcomings. Two steps are quintessential for these steps to be successful. First, the choice of materials and controlled synthesis protocol capable of converting a renewable, natural, and bulk material into a nanomaterial. Second, to prioritize the use of green synthesized materials as components in MFCs and to demonstrate the feasibility and performance characteristics in a reliable and cost-effective fashion (Sciarria et al., 2020). The main objective of various MFC research studies is to facilitate and improve the compatibility of microorganisms toward anode materials for diverse applications such as wastewater treatment and energy production. In order to achieve, cost-effective materials remain one of the main concerns for sustainable and environmentally friendly approaches. One such material is stainless steel which was found very effective in improving the power density and coulombic efficiency of MFCs. Therefore, significant progress has been made by incorporating stainless steel into various anode materials. The insertion of steel woolPANI-polypyrrole nanocomposites into MFCs has shown a higher power density of 2880 mW m−2 so (Sonawane et al., 2018). In this line, graphene, exhibiting a large surface area and excellent electrical conductivity has been widely used as an anode material in MFCs. The power density of graphene-modified stainless steel mesh anode in MFC accounted for 2668 mW m−2 (Yuan & He, 2015). Similarly, N-doped TiO2 nanosheets in MFCs have been shown to improve the electronic properties of carbon paper anodes. It has been noted that the bacteria-anode interaction had led to an increase in energy production with minimal electron loss. In addition, TiO2 nanofilms doped with carbon paper and calcined in NH3 atmosphere at different temperatures (400°C, 500°C, 600°C, and 700°C) showed that the one calcined at 600°C had the best performance of the electrode reaching 196% and with maximum power density compared to control (Yin et al., 2017).

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A modified anode based on multiwalled carbon nanotubes (MWCNTs) was also prepared to improve the performance of the MFC. Compared to bare carbon fabric anodes, the efficient growth of E. coli on the MWCNT, MWCNT-COOH, and MWCNT-NH2 -doped anodes was reported. Therefore, the maximum power density recorded with an anode modified by MWCNT-COOH attributed to 560.4 mW m−2 (Fan et al., 2017). Thence, these types of anode modifications help improve the power generation and stability of the MFC. Nanomaterial electrode materials in MFC provide a promising tool for high hydrogen production because MFC has the potential to generate electricity by treating wastewater. Although the yield and purity of hydrogen is still a difficult problem, there are still many opportunities in the development of electrode materials for the production of hydrogen (Zhao & Ci, 2018). FCs embrace the promising potential to catalyze aggressive players in various markets because of their wide scope of utilizations. Also, because of their high measured quality, wide power range and variety of properties among various kinds, FCs have applications going from bikes to expansive coage control plants as fuel cells can hypothetically be utilized for any energy requesting application. Endeavors toward the commercialization of fuel cells in the compact hardware, stationary power age and transportation areas are in progress for sure. For example, Elon Musk (of Tesla) has claimed hydrogen fuel cells are more of a marketing ploy for auto-makers, and not a long-term strategy; in contrast, Japan has announced its ambitious plan to become the first hydrogen society by 2022.

14.8 Life cycle assessment (LCA) of MFCs Life cycle assessment (LCA) remains one of the well-established tools to comprehensively determine the potential environmental and human health impacts of a product throughout its life cycle. It all starts with the extraction of raw materials, manufacturing, transport, and end-use with the disposal of residues. It involves the scope and goal, life cycle inventory, assessment and interpretation (Muthukrishnan et al., n.d.). It works on the need to analyze the level of environmental damage caused by the developed products and processes. In simple terms, LCA forms a base for an environmental evaluation framework. It is an internationally standardized methodology for qualitative and quantitative evaluation of environmental impacts of the developed products through all stages of their life cycles and the framework has been presented in ISO 14040:2006 (Finkbeiner & Klöpffer, 2014). The LCA protocol is a mandate to provide a thorough description of the processes and the technical challenges faced during the product development. LCA analysis is divided into different steps that include Goal and scope definition (mainly concerned with the goals, purpose of the assessment and decision making on the product). Once the goals are defined, identifying the intended applications and the degree of analytical

Techno-economic analysis of microbial fuel cells using different nanomaterials

Figure 14.7 Mandatory stages involved in LCA. No permission required.

depth and rigor of the study is critical. Second, the scope definition, that includes description and characteristics of the product of concern, functional unit and reference flow, system boundary, assessed impact categories and related impact assessment methods, assumptions/limitation (Fig. 14.8). When the goals and scope have been defined, the life cycle inventory (LCI) is prepared. The LCI is the compilation and quantification of inputs and outputs for a given product system throughout its life cycle. The LCI in turn forms the basis for calculating the potential environmental impacts of the product or process developed. It comprises of detailed tracking of all the inputs and outputs of the product system, including raw resources or materials, energy by type, water and emissions to air, water and land by specific substance. Moreover, all resources must use and emissions associated with the life cycle stages in the defined system boundary are captured in the LCI. The following life cycle stages may be included in the LCI viz. raw material acquisition and preprocessing, capital goods, production, product distribution and storage, end-user stage, logistics, and end-of-life (Fig. 14.9). The choice of system boundary will have a large effect on study results depending on the direct impacts linked to the biofuel pathway (attributional LCA modeling) and/or indirect effects beyond the pathway (consequential LCA modeling). Indirect effects are most often associated with indirect land-use change (iLUC) emissions of CO2 due to the market-driven demand for more land to compensate for food production lost to biofuels. Several studies concluded that the choice of method to allocate inventory data among biofuel pathway products and coproducts has an overwhelming effect on LCA results. Depending on the goal and scope of the study, one or more impact categories may be included in the life cycle impact assessment (LCIA).

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Figure 14.8 Life cycle assessment and system boundary. No permission required.

Figure 14.9 Life cycle impact assessment framework. No permission required.

Techno-economic analysis of microbial fuel cells using different nanomaterials

Table 14.4 LCA studies and their respective green-house gas emissions. System adopted Green-house gas emissions References

Activated sludge – Primary sedimentation; Agricultural sludge (secondary clarification) Activated sludge treatment + Biological treatment + Agricultural sludge treatment Constructed wetlands integrated with MFC Pretreatment + MEC + Posttreatment Pretreatment + MFC + Posttreatment Pretreatment + MDC + Posttreatment

0.055 kg m−3

(Stokes and Horvath, 2010)

0.195–0.213 kg m−3

(Niero et al., 2014)

8.49 g m−3

(Corbella et al., 2017)

−8.5 kg m−3 , −2.28 to 6.58 kg m−3 , −0.53 to 5.72 kg m−3

(Zhang et al., 2019)

Based on the goal and scope, there are more impact categories included in the LCA. Besides, global warming and energy consumption, a complete environmental evaluation of negative impacts to soil, water, air, human health, and ecosystems have been illustrated in Fig. 14.9. There are mid-point indicators viz. human toxicity, ozone layer depletion, global warming, and eutrophication and the end-point indicators focus on environmental damage. For better aspects, the choice of LCA methodology plays a pivotal role in framing system boundary, source of inventory data, unit process inputs and decisions on coproduct allocation. There have been various studies on the choice of LCA studies and their respective greenhouse gas emissions as enlisted in Table 14.4.

14.9 Nanomaterials reusability The purity, quality, and size of the nanomaterials are often good enough for the target application. Depending on the particular synthesis environment, different sizes, and morphologies can be obtained, but the exact control of these parameters needs to be investigated further to unveil the precise mechanisms occurring in these complex systems. On the other hand, these types of nanomaterials can be efficiently used in MFCs and MECs with benefits from the point of view of economic feasibility. For instance, MFCs containing nanocarbon materials or metal NPs from secondary sources have a potential power density boost ranging from a few hundred to a few thousand mW cm−2 . This implies that cheap and reliable production and the availability of such sustainable nanomaterials is crucial to propelling both fields further toward industrialization. Even for these devices, solid demonstrations of scaled-up systems that are capable of producing

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and supplying a sufficient amount of energy (MFC) or the production of convenient value-added products (MEC) are necessary. Breakthroughs in nanotechnology could provide technologies that would contribute to worldwide energy security and supply. A report published by Rice University (Texas) identified numerous areas in which nanotechnology could contribute to more efficient, inexpensive, and environmentally sound technologies than are readily available. Although the most significant contributions may be to unglamorous applications such as better materials for exploration equipment used in the oil and gas industry or improved catalysis, nanotechnology is being proposed in numerous energy domains, including solar power, wind, clean coal, fusion reactors, new generation fission reactors, fuel cells, batteries, hydrogen production, storage, and transportation and a new electrical grid that ties all the power sources together. The main challenges where nanotechnology could contribute are: Lower the costs of photovoltaic solar energy tenfold, to achieve the commercial photocatalytic reduction of CO2 to methanol, to create a commercial process for direct photoconversion of light and water to produce hydrogen, in lowering the costs of fuel cells between tenfold and a hundredfold and create new and sturdier materials, to improve the efficiency and storage capacity of batteries between tenfold and a hundredfold for automotive and distributed generation applications,to develop thermochemical processes with catalysts to generate hydrogen from water at temperatures lower than 900°C at commercial costs and to create CO2 mineralization methods that can work on a vast scale without waste streams. Solving these challenges will take many years, but commercial and public research institutes are already exploiting nanotechnology for energy applications. Bell Labs, for instance, has started exploring the possibility of producing a microbattery that would still work 20 years after purchase by postponing the chemical reactions that degrades traditional batteries. Other research studies are underway in the mission of dispensing batteries with nanotubes-based “ultra” capacitors powerful enough to propel hybridelectric cars. The development of various advanced nanomaterials-based anodes like CNTs, graphene, porous carbon, and the metallic nanomaterials have been used for designing and construction of evolved MFCs.The electrocatalysis occurring at the anode in the case of MFCs is quite complicated as it involves bio catalytic process carried out by microbes. Hence, the electrocatalysis enhancement in MFCs can strategically be carried out by modifying electrode material and microbial cells. There is a systematic relation between bioelectrochemical cell configuration and the anode potential. This anode potential affects the microbial attachment, growth, diversity and response from biofilm. The principle of the MFC electrode can be understood as bacteria turned into supercapacitor electrodes. The nanomaterials have high conductivity, excellent biocompatibility with microbial inoculum mixture, and good stability find application in MFC designing and fabrication. Intimate bacterial adhesion for growth and efficient electron transfer make

Techno-economic analysis of microbial fuel cells using different nanomaterials

their ways in research approach and realization. These nano-engineered anode materials improve the power output for a cleaner and more sustainable energy production through MFCs. Different carbon-based electrode materials ranging from classic graphene to add functional polymers can be fabricated by the process of electro spinning. But the power generation and the electrode costs are not quite suitable currently to be commercialized. There is a need of further research on easy, economic, and high-efficiency electrode preparation. The electron-transfer mechanisms responsible for biocatalytic processes occurring must be guided for the preparation of novel MFC’s electrode materials.

14.10 Conclusions Nanotechnology has spread its wings in offering extensive avenues for improving the efficiency of MFCs. In this chapter, the applications of nanomaterials in different MFC compartments and their improved efficiency have been discussed in detail. For a better life cycle, high-surface area/biocompatible nanomaterials are the boon in developing thick biofilms on electrodes’ surfaces. Second, the importance of metal, metal-oxide and conductive NPs used as bridges in abstaining electron transfer resistance has been elucidated.Besides discussing numerous advantages,investigations on the effect of nanoparticle morphology and electron transfer are yet to be deciphered. Another important factor is the toxicity of NPs and CNTs which need immediate addressing as it may harm both microbial biofilms and the environment when released. The level of toxicity solely depends on the size and concentration of NPs. Alongside, the adverse effects associated with electrode modifications using NPs and CNTs could be moderated by adding biocompatible nanomaterials such as PANI. On the other hand, the use of nanoporous materials would relatively increase cathodic gas adsorption in turn increasing the microbial electrosynthesis of chemicals. The beneficial effects of metal oxide- and MOF-based nanocomposites in MFCs confer greater stability in microbial environments in turn increasing the efficiency in catalyzing the ORR. So such materials are the most preferred ones and inexpensive alternatives to Pt. Similarly, doping of nitrogen and other metals can extend ORR active sites of carbonous nanomaterials providing more space for microbial adhesion. Nanomaterials have vastly improved the efficiency of MFC applications by zeroing of external current supply but more proficient in treating wastewater and sensing. However, for a stable and reproducible response, the components of MFCs should still be improved. Environmental impacts of nanomaterials used in MFC modification and their release into soil and water demand an extra vigil. Surpassing the challenge would determine the suitability and overall sustainability of nano-based MFC technology. Guidelines pertaining to environmental safety, hunting in for cost-effective nanomaterial, standard operating protocols, and proper disposal would eventually lead to building a safer and healthier society paving way for a disruptive technological boom in MFCs.

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Techno-economic analysis of microbial fuel cells using different nanomaterials

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CHAPTER 15

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems Shabnam Taghipour a,∗, Marziyeh Jannesari b,c,∗, Mohammadhossein Taghipour d, Behzad Ataie-Ashtiani a and Omid Akhavan b,c a

Department of Civil Engineering, Sharif University of Technology, Tehran, Iran Institute for Nanoscience and Nanotechnology, Sharif University of Technology, Tehran, Iran Department of Physics, Sharif University of Technology, Tehran, Iran d Department of Materials Engineering, University of Tabriz, Tabriz, Iran b c

15.1 Introduction The rapid population growth and consequently increase in industries has caused major energy consumption and environmental contamination. Water pollution as one of the most important environmental issues has gained huge worldwide attention (Madima et al., 2020). The industrial regions constantly produce a considerable volume of wastewater at high rates and finally discharge it without adequate purification which is a result of lax enforcement of the rules, illegal, and improper application of components (Elmobarak et al.,2021).Pollutants can also enter aquatic environments indirectly through penetration of agriculture-related substances such as pesticides, herbicides, and fertilizers (Taghipour et al., 2022a; Taghipour et al., 2022b; Taghipour et al., 2021; Taghipour et al., 2022c). Previous studies were centralized on the detection of acute health outcomes induced by individual contaminants and their short-term negative consequences on the environment. Over time as scientific investigations progressed, and new emerging pollutants were detected, studies disclosed the severe effects of these pollutants in the long term (Oller et al., 2011). So far various chemicals (Donkadokula et al., 2020; Hendi et al., 2021; Khorsandi et al., 2019; Saleh et al., 2020; Shi et al., 2021), physical and biological (Taghipour & Ayati, 2015, 2017, 2019; Taghipour et al., 2017) treatment processes have been utilized for water and wastewater purification. Besides numerous advantages of physical and biological treatment such as simplicity, low cost, and applicability for a large volume of wastewater, these methods suffer from disadvantages such as long start-up period, posttreatment requirement of the effluent, larger area requirement, creating odor ∗

These authors contributed equally.

Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00008-5

c 2023 Elsevier Inc. Copyright  All rights reserved.

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problems, and ineffectiveness in the removal of hard-degradable or toxic compounds (Musa & Idrus, 2021). Thus, the development of more efficacious technologies for mineralization or conversion of hard degradable compounds to harmless compounds is necessary. In the past decades, fossil fuel was the main power resource that has negatively affected the environment. Moreover, the detrimental gas emission from these fuels into the atmosphere prompted scientists to investigate superseded energy resources. In addition to environmental effects, depletion of fossil fuel resources and insecure fossil product prices were among other driving forces for finding a new source of power such as renewable energy in industries (Olabi et al., 2020). Thus, the daily increase in global consciousness about the energy-environment nexus is evoking interest in the development of new technologies that can minimize environmental footprints while energy production and energy utilization during environmental purification (Wang et al., 2015). Among various processes, bioelectrochemical systems (BECs) are considered a revolutionary novel approach with sustainable, green, and more eco-friendly operations. This technology utilized the acquired energy from biomass and wastewater for the production of hydrogen, electrical energy, and other high-value biochemical products via redox reactions in the presence of a biocatalyst (e.g., bacteria; Akhavan & Ghaderi, 2011; Kumar et al., 2017). EC employs microbial–electrochemical mechanisms using electrodes as electron acceptors/donors for the production of sustainable energy for on-situ usage. Oxidation and reduction reactions are conducted in anode and cathode, respectively (Zheng et al., 2020). The performance of BEC is strongly dependent on various physical and operational factors such as the composition of the wastewater, temperature, hydraulic retention time (HRT), reactor configuration, and electrode materials (Noori et al., 2020). HRT refers to the duration in which wastewater remains in the reactor. The higher flow rate (or lower HRT) will lead to a higher amount of organic loading rate as the food inside the reactor. The studies have proved that higher OLRs have ameliorated the functionality of BES and volumetric treatment rates (Leicester et al., 2020). Apart from the straight effect of temperature on microorganisms in the BES, it can affect the ion membranes and the electrodes. Lower temperature can reduce the locomotion of H2 O molecules and soluble ions and consequently increase the system resistance. Studies have affirmed that pretreatment of anodes by heating can enhance the power generation due to the reduction in the C–O composition on the electrode surface as well as the increment of the nitrogen to carbon ratio (Zhang et al., 2019). With this background, this brief review focuses on the synthesis of carbon-based nanomaterials (e.g., graphene-based nanostructures), their application in bioelectrochemical systems (e.g., microbial fuel cells, microbial electrolysis cells), and valuableadded chemical production (hydrogen and hydrogen peroxide generation) are well described.

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

Figure 15.1 Two main categories for the synthesis of nanomaterials.

15.2 Carbon-based nanomaterials and synthesis methods The term “nanoscale” describes NMs with the size range of 1–100 nanometers, but generally, materials with at least one dimension in the range of 1 up to several hundred nanometers are considered as NMs (Taghipour et al.,2019).NMs can be fabricated by two main approaches including bottom-up and top-down approaches which are illustrated in Fig. 15.1. The bottom-up strategy refers to the growth of graphene sheets by combining the basic structural units to fabricate graphene directly via molecular organic precursors (Dhand et al., 2013; Hosseini et al., 2011; Hosseini et al., 2018; Mossa Hosseini et al., 2011). In other words,the intended substance is fabricated in an atom-by-atom or molecule-by-molecule approach to form a uniform size, controlled morphology, and well-distributed material (Akhavan et al., 2012; Khan et al., 2019). In the top-down approach, the aim is to break the bulk materials and weaken the van der Waals forces to detach the layers from each other and turn them into smaller particles by physical methods (e.g., grinding, milling, and crushing; Khan, 2020). The technologies in the synthesis of NMs and their dependency on each of the bottom-up and top-down categories are represented in Fig. 15.2. NMs benefit from privileges such as a large surface/volume ratio, unrivaled optical, mechanical, and thermal characteristics, and facile derivatization methods (Zhang et al., 2013). Owing to these unique features, NMs play a crucial role in electricity generation and enhancing the conversion of carbon oxide into beneficial products and/or valueadded chemicals (Khadir et al., 2021). During the past years, many efforts have been devoted to modifying or developing electrocatalysts (in both cathodic and anodic chambers) by using various NMs including carbon-based NMs to improve the active surface area, electron transfer, bacteria cohesion, durability, biocompatibility, and electrochemical inertness and yield. Among numerous carbon-based NMs, graphene-based nanostructures have gained significant attention owing to their exceptional properties which will be discussed in the next sections (Mazari et al., 2021).

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Figure 15.2 The major synthesis approaches of nanomaterials.

15.2.1 Graphene-based NMs Graphene is a 2D material made up of a single graphite layer of sp2 -bonded carbon atoms with a hexagonal lattice structure which was first discovered by Andre Geim and Konstantin Novoselov in 2004 (Siqueira et al., 2017). So far, graphene-based NMs such as graphene oxide (GO) and reduced graphene oxide (rGO) have been extensively researched in the field of electrochemistry compared to other carbonaceous nanostructures due to their particular physical, chemical, and electrochemical characteristics such as high theoretical surface area of a monolayer (2630 m2 g−1 ; Matte et al., 2011), high electrical conductivity (1738 siemens m−1 ; Rahighi et al., 2021; Weiss et al., 2012) excellent thermal durability and conductivity (up to 601 °C and 3080–5150 Wm K−1 , respectively; Ghosh et al., 2008; Wu et al., 2009), remarkable mechanical strength with Young’s modulus (0.8– 1.0 TPa; Papageorgiou et al., 2017), distinguished optical transmittance (97.7%; Xiong et al., 2019), desirable biocompatibility (Akhavan, 2016), superior electrochemical activity (Chen et al., 2014), wide potential windows (Bora et al., 2018), and low charge transfer resistance (RCT; Gao & Duan, 2015). Furthermore, in graphene, the charge can transport a lengthier interatomic distance connoting a higher flow rate in comparison to other electrode materials (Olabi et al., 2020). Considering the above-mentioned properties and their cheapness to acquire in high quality, graphene-based NMs appear to be a wise choice in bioelectrochemistry to overcome the existing energy and healthcare issues. Besides, rich functionalities can be obtained in the hierarchical graphene materials for enhanced efficiency as well. The major synthesis methods of graphene based on bottom-up and top-down approaches are demonstrated in Fig. 15.3. The main reason for developing various

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

Figure 15.3 Two main approaches in the synthesis of graphene.

technology in graphene synthesis is to enhance production yield and superior control of the material characteristics. In order to control the growth of 2D graphene sheets, the utilized precursors usually contain benzene ring molecules with highly reactive functional groups (Dhand et al., 2013). Epitaxial growth refers to the thermal decomposition of hydrocarbons on a substrate followed by a cooling procedure to the formation of carbonaceous nanostructures from carbon. Mostly, hexagonal structural silicon carbide (4H-SiC and 6H-SiC) decomposes at high temperatures (1200–1600 °C) and vacuum (˂10−10 Torr) to sublime Si atoms and eventuate to the growth of graphene sheets by aggregation of excessive carbon atoms (Ahmadpur, 2022; Ahmadpur & Gokasar, 2021; Pang et al., 2021; Yazdi et al., 2016). The obtained material from this method benefits from having a large surface area but suffering from nonhomogeneity. To prevail over this deficiency, the application of alternative substrates such as polytetrafluoroethylene (Xia et al., 2018) and CO2 laser (as the heating step; Aïssa et al., 2015) has been reported by researchers. Chemical vapor deposition (CVD) is another commonly used method in the synthesis of thin nanofilm of graphene on metallic catalysts at elevated temperatures (650–1000 °C). The transition metal catalyst (e.g., Cu (Huet et al., 2020), Ni (Huang & Ruoff, 2020), and Fe (An et al., 2011)), disintegrate hydrocarbonic gases into C and H atoms. This method consisted of the following steps: mass reactant transport, precursor reaction, gas molecules diffusion, precursor adsorption, diffusion of the precursor into the substrate, surface reaction, desorption of products, and finally the elimination of the probable byproducts (González et al., 2020; Shi et al., 2020). The quality of the obtained graphene

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(A)

(B)

(C)

Figure 15.4 Schematic depiction of mechanical force in possible directions during mechanical exfoliation: (A) lateral force, (B) normal force, and (C) nondirectional force.

is considerably related to the quantity of the precursors and the reaction rate and temperature (Khan, 2020). This technology offers large-scale production of uniform graphene with a low number of defects, and an adjustable number of graphene layers. Pyrolysis or solvothermal method involves the application of a solvent interacting with a carbon precursor (e.g., ethanol (Speyer et al., 2018), alcohol, n-propanol (Khan, 2020), carbon tetrachloride (Quan et al., 2014)) in the equimolar ratio of ethanol/sodium to synthesis graphene layer (Olabi et al., 2021). This procedure is carried out in a stainless steel autoclave vessel at high pressures with subsequent sonication for the separation of graphene sheets from sodium-ethoxide (Azizi-Lalabadi & Jafari, 2021). Among widely used top-down concepts, mechanical exfoliation has caught much attention from researchers. The aim of this method is to peel graphene from bulk graphite on the layer by layer base by prevailing the van der Waals bonds among adjoining graphene flakes (Yi & Shen, 2015). In this way, external force in various directions including normal, lateral, and/or in no direction will be applied as demonstrated in Fig. 15.4. The normal force is applied in micromechanical cleavage by Scotch tape. The longitudinal or transverse force can be applied among two graphite layers by the selflubricating method (Yi & Shen, 2015). Solvent-based exfoliation is one of the most commonly utilized approaches among liquid-phase exfoliation which exfoliates unmodified graphite via ultrasonication in a

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

solvent (Lavin-Lopez et al., 2016). The selected solvent should possess adequate solubility, surface tension (preferably similar to that of graphene i.e. 40–50 mJ m−2 ; Singh et al., 2015), and surface tension (preferably similar to that of graphene, i.e., 70–80 mJ m−2 ; Hernandez et al., 2008) in order to defeat van der Waals bond. Considering the solubility of graphene in several solvents and the aforementioned parameters, the application of N-methyl-2-pyrrolidone (NMP) have eventuated the highest percentage of single-layer graphene. One of the considerable benefits of solvent-based exfoliation of nonmodified graphite is the low consumption of solvent (Mishra and Badekai, 2020). To enhance production yield, various parameters, for example, sonication time (Khan et al., 2010), the addition of assistant (Geng et al., 2010), and further thermal treatment (Oh et al., 2012) should be optimized. Electrochemical exfoliation of graphite has emerged as a facile method in the fabrication of graphene-based structures with two approaches: the anodic (exertion of positive bias, roughly +10–20 V) and cathodic exfoliations (exertion of negative bias; Ambrosi & Pumera, 2016) which rely on the power supply applied on the graphite electrodes (Liu et al., 2019; Prola et al., 2013). By using a positive electric current, electrons migrate from the graphitic anode surface and subsequently make a positive charge which causes the intercalation of bulky negative ions (e.g., NO3 − , Cl− , and SO4 2− ; Ambrosi & Pumera, 2016). Thus, the enlargement of the interlayer distance among graphene sheets causes their effective exfoliation. Besides structural damage, using high voltage simplifies the functionalization of the exfoliated graphene by the generation of oxygen groups. Therefore, in the existence of negative potential in the anodic exfoliation and positive intercalation ions (e.g., tetraalkylammonium [H(CH2 )n ]4 N+ (Gorduk, 2021), Li+ (Shi et al., 2017)), researchers hampered the generation of oxygen functionalities and successfully achieved higher quality graphene. Generally, exfoliation yield via anodic exfoliation compared to cathodic has shown much better performance. Electrodes in electrolytes can be made up of graphite in various shapes including plate/flake/rod/foil or powder to produce a bulk amount of graphene (Yang et al., 2020). The quality of the products vigorously depends on the exerted potential and the type of the electrolyte.

15.3 Application of carbon-based nanomaterials in bioelectrochemical systems 15.3.1 Main principles of the bioelectrochemical systems Bioelectrochemical systems (BECs) are the systems employing the electrochemically active microorganisms to catalyze the production of bioelectricity and also value-added chemicals (such as H2 , hydrogen peroxide, and other valuable products) from waste organic matters. In these systems, electrons are produced in the former as biochemical

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fuel cells and consumed in the latter as biochemical electrolysis cells, respectively. To create a complete electric circuit in a classic form of BSE, an external conductive wire was utilized to connect the anode and the cathode electrons (Dhanjai et al., 2018). On the anode side, organic waste materials (paper recycling (Cheng et al., 2011), textile industry (Fang et al., 2015), domestic (He et al., 2017), pharmaceutical (Zhang et al., 2015), and brewery (ElMekawy et al., 2015) wastewaters) as the energy sources are consumed and oxidized to generate CO2 through bioactive agents (i.e., microorganisms or enzymes). Simultaneously, electrons from anaerobic respiration are either directly or indirectly captured by the anode electrode to transfer toward the cathode cell through the conductive wire (Reguera et al., 2005; Zhao et al., 2017). In contrast to the similar spontaneous reactions that occurred in the anode resulting in producing electrical energy, the different reactions requiring external energy injection as a driving force in the cathode cells, lead to producing various value-added materials such as water, hydrogen, hydrogen peroxide, acetate, etc. (Dhanjai et al., 2018; Reguera et al., 2005). 15.3.2 Microbial fuel cells 15.3.2.1 Electron transfer mechanisms in microbial fuel cells Classic microbial fuel cells (MFCs) consist of one anode and one cathode electrode separated from each other through a proton exchange membrane (PEM; Santoro et al., 2017). On the anode side, bacteria grow as bioactive sites in the form of biofilms through the oxidation of waste organic matter as the fuel for the bacteria to produce electrons (and protons; Franks & Nevin, 2010). The produced electron sank through the anode electron will be transferred to the cathode by means of an external circuit and the protons pass through the PEM (Cheng et al., 2017). Extracellular electron transfer (EET) in MFCs, the process in which electrons transfer from the cellular membranes of the electrogenic bacterial strains to the electrodes via metabolism, has been considered the eminent factor for harvesting energy (Call & Logan, 2008; Kadier et al., 2018). Different strategies have been proposed for the EET including direct electron transfer by means of microbially conductive pili joining the C-type cytochromes with the electrode surfaces, and the mediated electron transfer (MET) occurring via either exogenous or endogenous electron mediators (Saratale et al., 2017). Direct electron transfer commonly occurs through physical contact between the proteins in the outer membrane of the bacteria and the electrode surfaces (Katuri et al., 2012; Malvankar et al., 2015; Zhao et al., 2017). Pilli in the special bacteria with high conductivity can serve as a biological nanowire to shuttle the electrons from the biofilm to the electrode surfaces (Steidl et al., 2016). Therefore, for effective electron transfer to yield a high-performance MFC, both electron trapping and bacterial attachment are critical for anode electrodes from functional and also structural perspectives (Sonawane et al., 2017). In fact, the appropriate material for the anode is the one that

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

can provide a desirable bacterial attachment and facilitate electron flow (Sonawane et al., 2017). 15.3.3 Electrode material selection In spite of the environmentally friendly, it has been shown that one of the major bottlenecks for the practical applications of the BFCs is their low efficiency. The performance of the MFCs is tremendously influenced by the properties of the system components such as electrodes, catalysts, membrane, and the strength of attachment of the formed biofilm on the electrodes. Both electron trapping and bacterial attachment are critical for anode electrodes from functional and also structural perspectives. In fact, the appropriate material for the anode is the one that can provide a desirable bacterial attachment and facilitate electron flow (Davis & Higson, 2007). The composition of electrode materials highly determines the biocatalytic activities of the microorganisms and the electron flow (ElMekawy et al., 2017; Justino et al., 2017). From the material perspective, various physico-chemical properties of the electrodes including bioadhesion, biocompatibility, electric conductivity, porosity, surface area, catalytic activity, and chemical stability are highly influenced by the electrode composition (Li et al., 2017). Beyond these vital properties, the price of materials should be reasonable to afford a cost–benefit balance. Several kinds of materials including metals, stainless steel, and carbon-based materials have been utilized for bioenergy harvesting via microorganisms (Kondaveeti et al., 2018). However, low conductivity, small surface area, and material cost are the main impediments prohibiting the commercialization application of the MEC systems. Possessing good conductivity and anticorrosion properties, carbon-based materials have been considered as a proper material in the fabrication of electrodes. Carbonbased materials such as graphite (in the form of sheet, rod, and brush; Picot et al., 2011), carbon felt (Hidalgo et al., 2016), carbon paper (Xian et al., 2021), and carbon cloth (Liu et al., 2020) have been employed to fabricate commercially anode electrodes. By means of these materials, however, an inappropriate biofilm attachment limits the electron transfer and ultimately results in low MFC performance. To address these restrictions, in recent years, efforts have been made to produce nanotechnology-based electrodes to provide more enlarged surface areas facilitating bacterial growth and their attachment to the electrode surfaces and enhancing the electron transfer efficiency as well as oxygen reduction reaction (ORR) kinetics (Farrukh et al., 2021). Among other nanomaterials, carbon-based nanomaterials introduced in different shapes and sizes as zero-, one-, two-, and three-dimensional materials (such as fullerenes (0D), (single or multiwall) carbon nanotubes (SWCNTs, MWCNTs ) (1D), nanosheets of graphene (2D) and networks of 3D graphene nanomaterials, respectively; Siqueira et al., 2017) have drawn much attention in MECs (Akhavan & Ghaderi, 2010; Mohan et al., 2019).

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15.4 Graphene-based nanomaterials as the anode electrode The physio-chemical properties of the material composition play a tremendous role in energy harvesting (Jannesari et al., 2023). In recent 10 years, graphene and its derivatives, as the two-dimensional (2D) materials composed of a single layer of atomic carbon, have tremendously drawn much attention in a wide spectrum of scientific, technological, and industrial applications due to their extraordinary physio-chemical properties (Akhavan, 2010; Akhavan & Ghaderi, 2010; Akhavan et al., 2016; Mehmood et al., 2020). In fact, its high conductivity (reported to be 64 mS cm−1 ; Wang et al., 2008), high surface area to volume ratio (theoretically proposed as 2630 m2 g−1 which is much higher compared to that of graphite 10 m2 g−1 ), thermal stability, and high mechanical strength as well as its reasonable price (Jannesari et al., 2018) and feasible industrial production (Akhavan et al., 2016; Jannesari et al., 2020) introduce a multifaceted material. Hence, this multifunctional material has been considered a great promising candidate in a wide range of applications from biomedical fields (Akhavan et al., 2016; Jannesari et al., 2020) to nanobubble formation (Akhavan et al., 2016; Jannesari et al., 2020), wastewater treatment, and recently in (bio)electrochemical processes (ElMekawy et al., 2017; Olabi et al., 2020). Astonishing electron mobility, high rates of charge transport, and its electrocatalytic activity encourage graphene materials as appropriate electrodes in electrochemical applications (Brownson & Banks, 2010; Chen et al., 2010; Pumera, 2009). In addition, the oxygen functional group at the surface and edges of the graphene family can provide a desirable control over its functionality (Jannesari et al., 2018), significantly influencing the electrochemical efficiency through the heterogenous shuttling rate of electrons as compared to other carbon-based electrode materials like CNT and graphite (Novoselov et al., 2004). The efficiency of MFCs is substantially influenced by the EET rate, ohmic, activation, and mass transport losses as the auxiliary voltage needed for the currently lost restitution, occurring during the processes of charge transfer, electrochemical reaction, and mass transport (Rismani-Yazdi et al., 2008). Graphene-based electrodes can address these challenges by providing more active sites for the microbes for improving the ET rate (Ahn & Logan, 2013; Hutchinson et al., 2011). On the other hand, due to the high surface area to volume ratio and suitable biocompatibility, resulting in more bacterial attachment and growth on the surface of the electrodes, graphene (Gr)-based nanomaterials have demonstrated excellent power density and energy harvesting efficiency (Dhanjai et al., 2018). Despite several privileges, there exist some controversial aspects to using graphene in BEC systems. One important feature refers to the biocompatibility of these kinds of materials with respect to bacterial growth. Some researchers have confirmed antibacterial aspects of graphene-based nanomaterials due to the oxidative stress-induced on the bacterial cells and their destructive effect on the integrity of the cell membrane

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

(Akhavan & Ghaderi, 2012; Akhavan et al., 2011; Jannesari et al., 2020; Zhao et al., 2013) resulting in cytoplasmic content. While some others have considered Gr-based nanomaterials as propitious candidates potentially catalyzing microbial growth through expressing endogenous electron mediators or signaling molecules leading to the EET accelerating (ElMekawy et al., 2017). The interactions between bacterial cells and the Gr-based nanomaterials electrodes might be influenced by the size and their physiochemical properties such as the shape and their functional groups. Another argumentative concern about these materials for use as the anode electrode is their surface charges. In general, both the cellular membrane of the bacteria and the surface of Gr-based nanomaterials are negatively charged (Jannesari et al., 2018, 2020; Zhao et al., 2013). Therefore, the electrostatic repulsive force of the same charges may attenuate the bioadhesion of the bacteria on the electrode surfaces, leading to reducing the rate of ET. Hence, to obtain better efficiency, physical (shape and structure changing) or chemical modifications have been suggested to completely exploit the different capacities of these materials. 15.4.1 Physical amendment of graphene-based electrodes To increase more surface area to volume ratio and conquer high internal resistance as the main restricting factors, for gaining high power density and energy conversion, Gr-based nanomaterials have been modified by some physical techniques (ElMekawy et al., 2013; Pasupuleti et al., 2016). Crumpling graphene sheets through compression capillary further enhanced the surface area to volume ratio as well as aggregation resistance for creating a suitable anode for MFC to harvest a power density as high as 3.6 W m−3 when compared to the system possessing a flat rGO anode with a power density of 2.7 W m−3 (Xiao et al., 2012). Employing GO nanoribbons with a high ratio of length to diameter accelerating ET rate and serving as a nanowire is another effective enhancement of the physical modification of the anode (Huang et al., 2011). Another strategy applied to significantly promote the MFC performance is to exploit porous structures of Gr-based nanomaterials or nanomeshed graphene. This approach supplies an extra surface area (Akhavan, 2010; Akhavan & Ghaderi, 2013; Wang et al., 2008) for effective interfacial shuttling and lessens the diffusion resistance by providing more active sites with a more uniform distribution (ElMekawy et al., 2017; Yong et al., 2014). An example of this approach is to use plasma-enhanced chemical vapor deposition to fabricate N2 -doped graphene electrodes possessing high pore density (Kirubaharan et al., 2015). As a result, the surface area availability as well as edge plane exposure (due to the more defective appearance) remarkably amplified the catalytic activities and ultimately the more efficient performance of the system. More importantly, these days, 3D graphene with interconnected networks creating either macro-, micro-, or mesoporous structures (having the pore sizes of larger than

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50 nm, less than 2 nm, and 2–50 nm, respectively) have attracted a great deal of attention in MFC systems (ElMekawy et al., 2017; Wang et al., 2013; Yu et al., 2017). It has been shown that to boost microbial performance, the size range of the porosity network is pivotal (Chong et al., 2019). In fact, a few micrometers of pore sizes facilitate penetration of the microbial cells, however, no effective improvement can be achieved (Huang et al., 2016). In contrast, clogging can occur in the presence of a few tens of micrometer pores (Lepage et al., 2012). Instead, biofilms are successfully able to penetrate inside 3D structures with a porosity size in the range of a few hundred micrometers (Huang et al., 2016; Yong et al., 2012). This effect can be attributed to the bacterial sizes of 250–2000 nm. Hence, the graphene-based macroporous anodes have attained better productivity (Chong et al., 2019). In fact, these structures provide increased surface area to facilitate the interfacial electron transfer (between the biofilms and the anode surfaces) along with the internal bacterial colonization and biocatalyst immobilization simultaneously (Chen et al., 2012; Mashkour et al., 2016). To achieve better efficiency, other kinds of material can be combined with the 3D graphene structures in the form of composite materials. For example, stainless steel was incorporated into the 3D graphene sponge material to improve electrical conductivity (Xie et al., 2012). Another important example is the fabrication of nickel foam deposited on the rGO 3D network as the anode electrode in an MFC system (Song et al., 2018; Yu et al., 2018). Moreover, the uniform porosity in the scaffold affords an oversized surface area for the bacterial colonization and also facilitates the efficient culture media mass diffusion resulting in a power density of 661 W m−3 (Wang et al., 2013). In a recent study, an aerogel nonagglomerated 3D graphene-based anode electrode with a lamellar structure and hydrophilic surface was produced. This electrode demonstrated a value of 583.8 W m−3 for maximal power density which was five times larger than that of the least hydrophilic electrodes (Li et al., 2020). Poly diallyldimethylammonium chloride (PDDA) was also employed for crosslinking graphene nanosheets (2D) and carbon fiber paper to fabricate a 3D biocompatible graphene with output power and current density enhancements of 41% and 23% as compared to those of the bioanode in the absence of 3D graphene (Shi et al., 2021). 15.4.2 Graphene modified anode with utilizing the conductive polymers Modern anodes are also prepared by using graphene-based materials in combination with some conductive polymers such as polyaniline (PANI; Hou et al., 2013), polypyrrole (PPy; Sun et al., 2019), polytetrafluoroethylene (PTFE; Zhang et al., 2011), ionic liquid (1-(3aminopropyl)-3-methylimidazolium bromide; Zhao et al., 2013), Nafion (Zhang et al., 2014), chitosan (Cui et al., 2018), and polyurethane (PU; Dhanjai et al., 2018; Tremblay et al., 2020). Coating the graphene anodes with the polymers promote hydrophobicity as well as biocompatibility of the anode surface resulting in vigorous interactions between the anode surface and the microbial cells (Dhanjai et al., 2018). One other influencing

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

aspect in the combination of the anode electrode with polymers is modifying the electrostatic surface charges of the anode enabling the negatively charged microorganism to absorb on the modified positively charged anodes (Ren et al., 2013). Among others, PANI has been widely applied as a conductive polymer to modify graphene-based anode in MFCs due to its suitable stability, reasonable price, and straightforward fabrication methods. In one study it has been shown that a power density as high as 1.390 W m−2 (three times greater than that of the original anode electrode) can be achieved by an anode electrode composed of graphene and PANI which amend CC anode. In addition, the intense electrostatic attraction force between the positively charges PANI functional groups and the negatively charged bacterial cells has significantly encouraged biofilm formation (Ren et al., 2013). Another example is functional groups of imine (NH) and pyridine (N=C) in a hybrid of rGO-Apy/CC which has tremendously enhanced bacterial colonization along with the rate of EET. These effects led to obtaining a power density as high as 1.253 W m−2 approximately two times higher than that of the unmodified CC electrode (Gangadharan et al., 2016). In addition, functional groups of imine (NH) and pyridine (N=C) as active sites can reinforce π –π electronic transition between the bacterial cell and the electrode reducing the thickness of the electrode double layer originating from two opposite charges due to the activation energy decline at the interface of the electrode and the biofilm (Gangadharan et al., 2016). 15.4.3 Graphene-modified anode composite with metal oxide Good biocompatibility, high surface area to volume ratio, and chemical stability of metal oxide nanostructures as semiconductors have provided suitable modifier materials to enhance the performance of MFC systems (Mehdinia et al., 2014). The metal oxides including SnO2 (Amalraj et al.,2020;Mehdinia et al.,2014),TiO2 (Park et al.,2020),AgO, CuO, and Al2 O3 (Chaturvedi et al., 2021) have been widely investigated in doping or blending (as composite) with natural derived precious carbon due to their reinforcement role in the bacteria-anode adhesion to form a stronger biofilm. This effect accelerates the electron movements from the bacterial cell membrane to the anode surface. The focal role of metal oxide in improving the anode electrode is attributed to its high electrical conductivity. Moreover, metal oxide modification of graphene provides a tremendous impact on decreasing the cost and preventing the corrosion of the electrode (Yaqoob et al., 2020). Among others, SnO2 and TiO2 have attracted the most attention to modifying graphene-based anode electrodes. A graphene/TiO2 nanostructure fabricated through a straightforward method of microwave-assisted solvothermal demonstrated a significant performance enhancement as a result of increasing the specific surface area, pore-volume, active groups, and most importantly electrical conductivity of the hybrid nanostructure (Mehdinia et al., 2014; Zhao et al., 2014). Recent research investigated a graphene-based anode electrode modified with loading ZnO nanoparticles to obtain a power density of

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1350 × 10−3 mW m−2 and efficiency of approximately 91% for Pb2+ removal (Yaqoob et al., 2021). 15.4.4 The principles behind the cathodic electrode side In spite of the fact that organic substrate as the main source of energy generation is degraded biologically at the anode side of MFCs, the efficiency of the cathode electrode at which the electron acceptor can be reduced is also detrimental to the biofuel cell’s performance. As the privileged electron acceptor in the cathode electrode of MFCs, its availability as well as its reduction kinetics, oxygen plays a substantial role in the performance of electricity current harvesting (Dhanjai et al., 2018). Different fourelectron or two-electron pathways have been considered for ORR processes. Less energy dissipation along with the direct water production through the oxygen direct conversion instead of any intermediate generation make the four-electron pathway favored for ORR. The reactions in different pathways are demonstrated in the following equations: Two-electron pathway (Eqs. (15.1)–(15.3)): − O2 + H2 O + 2e− → HO− 2 + OH , E0 = −0.065 V vs. NHE

(15.1)

HO− + H2 O + 2e− → 3OH− , E0 = 0.867 V vs. NHE

(15.2)

− HO− 2 → 3OH + O2

(15.3)

Four-electron pathway (Eq. (15.4)): O2 + 4H+ + 4e− → 4H2 O, E = 1.229 V vs. NHE

(15.4)

To accelerate the slow ORR processes, exquisite metals of gold (Xiao et al., 2016), palladium (Wang et al., 2019), and platinum (Kondaveeti & Min, 2013) have been exploited as catalysts. However, their overprice along with their instability in chemical and biological environments restricts their scale-up applications. To overcome these constraints, meaning reducing the electrode’s cost and simultaneously improve the ORR kinetics, various kinds of materials such as non-noble metals, carbon-based materials (Qiao et al., 2010), metal oxides, and metal alloys (Hindatu et al., 2017; Mohamed et al., 2017) were utilized and examined. 15.4.5 Graphene-based cathode electrode for MFCs Among others, graphene as an allotrope of carbon has attracted a great deal of attention due to its low-cost and feasible large-scale synthesis (Jannesari et al., 2020) as well as extraordinary properties of excellent electrical conductivity, high mechanical strength, corrosion resistance (ElMekawy et al., 2017; Jannesari et al., 2018; Olabi et al., 2020). In fact, this family of materials has been considered the best candidate for cathode catalysts in MFCs to enhance oxygen reduction reactions.

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

Diverse nonprecious metals and metal oxides such as manganese dioxide (MnO2 ), tin oxide (SnO2 ), nickel foam, and titanium dioxide (TiO2 ) have been applied in catalyzing ORR processes in MFCs due to their low cost, chemical stability, suitable biocompatibility, and appropriate catalytic activity. A wide spectrum of metal-based/graphene composites such as SnO2 /rGr, MnO2 /Gr, Ni/rGO, Fe/Gr, and TiO2 /Gr has been prepared and introduced as cathode electrodes for MFCs (Zhang et al., 2014).

15.5 Microbial electrolysis cells A strong focus was drawn on recovering value-added products from wastewater over the recent decades. A huge amount of organic matter in agricultural, industrials, and municipal wastewaters can be potential precursors for simultaneous power and valuable chemical production. Recently, microbial electrochemical systems have been considered as promising technologies to promote the efficiency of the traditional systems in generating desirable chemicals with enhanced selectivity as a result of tuning the redox balance which controls the microbial metabolism (Jatoi et al., 2021). 15.5.1 The basic mechanism of microbial electrolysis cells In the microbial electrolysis cells (MEC), the produced electrons and protons at the anode electrode are consumed at the cathode side for electrochemical oxidation–reduction reactions (ORR) of biodegradable carbon sources to produce valuable products of methane, hydrogen, hydrogen peroxide (H2 O2 ), etc. To this end, microorganisms as selfrenewable, low-priced, and corrosion-resistant biocatalysts are employed (Kadier et al., 2020a, 2020b). Different reactions in MEC systems have been carried out in various reactors of single, dual, integrated, and continuous-flow chambered reactors (Zhou et al., 2013). In dual-chambered reactors, the reactions have been proceeded based on supplying external energy to overcome the unfavorable coupled redox reaction in which proton reduces to hydrogen in contrast to the oxygen reduction into water (which occurs in the dual standard chambers in a microbial fuel cell;Tremblay et al.,2020).However,it is worth noting that a portion of the required energy for the cathode reactions can be achieved through the process of organic carbon oxidation. Therefore, the required external voltage stands lower as compared to that for water electrolysis. Therefore, the main advantage of the biological oxidation reaction is attributed to the less activation required electricity energy (Dhanjai et al., 2018). 15.5.2 Graphene-based cathodic electrodes in value-added product 15.5.2.1 Hydrogen generation Microbial electrolysis cell systems have developed the production of valuable products such as hydrogen. Electrogenic microorganisms have been exploited in the MECs for producing hydrogen from waste organic materials by spending a small extra energy (Chakraborty et al., 2020; Tremblay et al., 2017; Yuan & He, 2015).

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To produce hydrogen through this method, the selection of material for the cathode electrode is crucial. To achieve a sustainable and swift hydrogen evolution, it is of great importance to select a desirable material for cathode generation, possessing the ability to deal with the challenges of energy barrier and their commercialization (meaning reducing price). Recently, a wide range of nonprecious materials from metal (e.g., nickel, and nickel alloys and stainless steel mesh) and metal oxide (like cobalt oxide; Call et al., 2009; Chaurasia et al., 2021; Jayabalan et al., 2019; Jayabalan et al., 2020) to carbonaceous materials (including carbon brush,cloth,and paper as well as graphite) have been attracted much attention to be applied in MEC systems (Merino-Jimenez et al., 2016; Pasupuleti et al., 2015; Walcarius et al., 2013). However, employing these materials has not provided a sufficient rate and suitable performance for hydrogen evolution. Graphene with outstanding properties has exhibited dealing with these restrictions and offers suitable MEC performance. A nickel foam/graphene hybrid introduced as the cathode observably enhanced HER (with the average rate of 1.31 mL H2 at an external bias of 0.8 V) in comparison with bare nickel. The obtained HER was also comparably superior to that of Pt/C cathode foam (Cai et al., 2016). In another study, two metal oxide–graphene nanocomposites of NiO/rGO and Co3 O4 /rGO have been introduced as hydrogen production catalysts in MEC utilizing wastewater from the sugar industry as the source of energy. The results showed that NiO/rGO-coated cathode obtained better performance and simultaneously less overpotential at 600 mV as well as the least resistance in Nyquist plots as compared to Co3 O4 /rGO and also bare Ni-foam. In this study, the maximum HER of 4.38 mmol L−1 D−1 and the recovery of cathodic hydrogen of 20.8% was achieved at an applied voltage of 10.0 V. Ultimately, the composite of NiO/rGO recorded the performance of MEC 1.19 and 2.68 times greater than that of Co3 O4 /rGO and bare Ni-foam as respect to the hydrogen production, respectively (Jayabalan et al., 2020). 15.5.2.2 Hydrogen peroxide generation Hydrogen peroxide (H2 O), as a high oxidant material produced as the intermediate in the two-electron pathway, remains a strong destructive effect on the electrodes. H2 O2 as a strong oxidant employed in scientific, technological, and industrial fields has been produced through a common method of anthraquinone oxidation. As a result of using precious catalysts of palladium and generation of hazardous organic byproducts during the period of H2 O2 storage, this technique is considered really expensive and nongreen (Campos-Martin et al., 2006). Hence, a green and more cost-effective method is still of great interest for producing H2 O2 . To address these restrictions, many efforts in the last century proposed abiotic electrochemical techniques to produce H2 O2 more efficiently in a low-priced manner. More recently, considering reducing energy consumption simultaneously using up the organic waste matter as the precursor chemicals have resulted in the development of bioelectrochemical systems as a

Synthesis and application of carbon-based nanomaterials for bioelectrochemical systems

cost-effective and environmentally-friendly alternative technique for H2 O2 generation (Feng et al., 2021). In both abiotic and biotic electrochemical systems, the two-electron pathway of oxidation–reduction reaction is responsible for H2 O2 production (An et al., 2019; Chen et al., 2016; Das et al., 2020; Zhao et al., 2019). In addition to the extraordinary physicochemical properties of graphene and its derivatives (such as their exceptional electrical conductivity, high specific surface area, Zzs, and the ability to tune their functional groups), economical aspects have favored these materials in MECs for producing value-added chemicals such as H2 O2 . As described previously, oxygen molecules are reduced to H2 O2 or H2 O through − 2e pathway or 4e− pathway at the cathode surface, respectively (Dhanjai et al., 2018). As the oxygen functional groups in carbon-based materials (i.e., rGO, CNT, carbon black, etc.) are loosely bonded to their backbone structures, these materials mostly experience the 2e− pathway producing H2 O2 . In fact, doped O and N heteroatoms play a pivotal role to regulate and conduct the graphene derivatives into the 2e− pathway (Yang et al., 2018). During the synthesis processes of GO and rGO, oxygen is spontaneously doped as the most common heteroatom and creates a wide range of oxygen functional groups on the surface of graphene-based materials. Simulation studies have demonstrated that the carbon atoms adjoining the oxygen functional groups of carboxyl (COOH) and epoxy (COC) can effectively drive the ORR process in the 2e− pathway (Lu et al., 2018). Interestingly, a report has revealed that H2 O2 was produced at a higher rate of 131% by using oxidized graphene as an air-cathode electrode when compared to the plain graphene cathode (Dong et al., 2018). On the other hand, it has been shown that overindulgent oxidation negatively influences the electrical conductivity of the graphene structures, making them inappropriate as electrochemical catalysts (Kim et al., 2018). Therefore, for gaining efficient performance, it is necessary to appropriately regulate the degree of oxidation in graphene-based materials. In contrast to oxygen, N-doped graphene favored the 4e− pathway, leading to H2 O2 production. In fact, the electron properties and surface charges of the graphene materials can be feasibility adjusted through N doping resulting in positively charged graphene (Li et al., 2009). Recent studies have shown that the number and the position of N doped in the structure can regulate the ORR pathway type.It means that there can exist kinds of N-doped graphene with the H2 O2 ORR pathway preferred (Guo et al., 2016; Su et al., 2019) in contrast to the previous researches emphasized on the N-doped graphene favoring 4e− pathway (Kannan, 2016; Liu et al., 2016; Qu et al., 2010). In addition, the other main role of the N doped in graphene structure lies in oxygen adsorption enhancement, encouraging the 2e− the pathway to produce H2 O2 . This effect mainly is attributed to the more electronegativity of N atoms (3.04) as compared to the C atoms (2.55), inducing a positively charged to the C atoms adjacent to the N atoms, which facilitates electron trapping to produce H2 O2 as a result of accelerating oxygen absorption (Guo et al., 2016; Qu et al., 2010).

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In addition to the chemical functional groups doped in the graphene-based derivatives, the amount and the configuration of physical defects induced in these structures during the synthesis processes tremendously affect the preferred ORR pathway with respect to their electronic structures.In fact,it has been established that defects have the ability to conduct the ORR process in a 2e− pathway (Chen et al.,2018;Terrones et al.,2012). Beyond the functional groups and physical defects, the composite form of graphenebased materials with nanoparticles such as metal or metal oxide nanoparticles have displayed a positive effect on the H2 O2 production pathway (Mi et al., 2019; Moraes et al., 2014). Graphene-supported Ni electrocatalysts demonstrated promoted H2 O2 selectivity generation through the 2e− pathway under alkaline conditions. This effect was attributed to the isolated Ni atoms as well as oxygen functional groups presented in the graphene structure (Song et al., 2020).

15.6 Conclusions and future perspectives So far most of the BEC-related studies were conducted on the laboratory scale with limited kinds of contaminants and concentrations, therefore a massive scale-up is suggested. Additionally, as is mentioned in several reports, BECs can be efficient in the degradation of both reduced and oxidized pollutants. In theory, reduced pollutants are being oxidized at the anode side and oxidized pollutants are being reduced at the cathode side which means that additional energy maybe will be required. In the case of utilized electrode material, graphene has surely existed in the searchlight of materials compared to others for many years. The availability of commercial and high-quality graphene at cheap prices is considered as a promising material in MFCs and MECs technologies. Biocompatibility, conductivity, and corrosion resistance are other important properties of an efficient electrode in the presence of high pollutant concentration and long realtime contact. Finally, overcoming the challenges of harvesting the produced energy during treatment is also among the high priorities in the commercialization of BECs technology.

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Synthesis and application of graphene-based nanomaterials for microbial fuel cells Sandra Edith Benito-Santiago a, Natarajan Gnanaseelan a, Jesús Guerrero-Contreras b, Sathish-Kumar Kamaraj c and Felipe Caballero-Briones a

a Instituto Politecnico Nacional, Materials and Technologies for Energy, Health and Environment, CICATA Altamira, Altamira, Mexico b Departamento de Ingeniería Eléctrica-Electrónica, Tecnológico Nacional de México, Instituto Tecnológico de Saltillo, Saltillo, México c Instituto Politécnico Nacional (IPN)-Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada, Altamira, Mexico

16.1 Introduction Every year 3928 km3 of water withdrawn globally from earth and 56% of it was discharged into atmosphere again in the form of drainage water from industries, municipality, and agricultural fields. It is estimated that 80% of water is released to atmosphere without adequate treatment (Progress on Sanitation et al., 2015). Wastewater effluents potentially affect the quality of receiving water bodies according to its quality of chemical, microbiological concentration, and compositions (Okereke et al., 2016; Yaradoddi et al., 2020). Water is having direct influence on the economy and its primary areas such as agricultural outcomes, industrial productions, and environment. Treatment of water becomes essential even to mitigate the global warming. The conventional water treatment methods like water disinfection, decontamination, and desalination may solve the problem. But its adaptations are inhibited by its intense chemical processes, higher energy, and capital requirements (Shannon et al., 2008). Microorganism diverges metabolic activity has been harnessed on the surfaces of the electrode in the microbial fuel cells (MFCs). MFCs are constructed with anode and cathode chambers, which are separated by membrane. Microorganism in the anode chamber oxidizes the fuel and thereby electrons and protons are generated. The electrons travel to cathode chamber via external circuit and results in the reduction. Water and electricity are generated in cathode chamber after reducing oxygen to water consuming protons and electrons. Lower power density of MFC limits the practical scaling up. However, the advantages are potential technology in economical wastewater treatment and biological path for the recovery of energy along with wastewater treatment in environmentally sustainable way. The limitations with MFC could be nullified with Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00005-X

c 2023 Elsevier Inc. Copyright  All rights reserved.

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graphene-based materials which have excellent characteristics such as large accessible surface, excellent mechanical strength, and remarkable flexibility. These properties make the graphene-based materials suitable for electrode material for MFCs. For example, the use of graphene-based material in anode chamber of MFC results in enhancement of catalytic activity, electron mobility, and reducing the polarization phenomenon. High active sites, improved interconnection between catalyst and electrolyte and enhanced ORR rate are realized in case of cathode chamber. This book chapter discusses the application of graphene-based nanomaterials for the applications of electrode materials for MFCs with elaboration of different synthesis processes (Ci et al., 2015; Yu et al., 2016; Jatoi et al., 2022).

16.2 Materials for anode In the anode, one of the most important aspects is the selection of the material with which it will be built and thus ensure its working efficiency. It has been seen that a low transfer of electrons translates into a deficiency in electrical performance in the MFC. There are a large variety of materials that have been widely used for the construction of electrodes, in which are found activated carbon cloth, carbon felt, cloth, rod, mesh, fiber, paper, brushes, 3D graphite, glassy carbon, granular graphite, graphite block, graphite felt, and graphite oxide, also using different materials to improve (Bhargavi et al., 2018; Wang et al., 2011). To increase the electron transportation, it is necessary to promote the rate of organic substrate oxidation at the surface of anode electrode, this could be possible developing anode materials. A high oxidation rate led to the high generation of electrons which can travel toward the cathode. In the literature, information has been found about graphite, graphene, or graphene oxide as anode due this modification can bring high electron transfer which led a high performance in the output voltage of the total system (Kim et al., 2021; Koffi & Okabe, 2020; Logan et al., 2007; Yang et al., 2018). The biocompatibility of anode is also very important in MFCs operation for better outcomes in terms of energy production. The produced microbes are in direct contact with the surface of anode. If the anodic material is not biocompatible with the microbial growth,then the generation of electrons will be decreased.Due to the above,the materials must not be toxic (Bian et al., 2018; Griškonis et al., 2020; Jatoi et al., 2021).

16.3 Materials for cathode The use of graphene oxide as a carbon-based support can improve the electrical conductivity and catalytic activity of the composites due to presence of reactive functional groups that make chemical functionalization easier and, therefore, the dispersion of the catalyst (Aliyev et al., 2019). On the other hand, spinel ferrites have been studied as electrocatalysts for their magnetic behavior (Lahiri & Sengupta, 1991). Studies related to

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

these materials show that when applying an external magnetic field leads to increase in oxygen transport, translating into an improvement in ORR (Zhang et al., 2022). It is reported that this enhancement is caused by the Kelvin force which promotes the transfer of paramagnetic molecular oxygen due to its unpaired spin (Sun et al., 2020). The oxygen reduction reaction (ORR) is one of the key reactions at the cathode in an MFC. Oxygen is an ideal electron acceptor due to its high oxidation potential, low cost and the formation of water as the final product (Logan et al., 2006). There are two mechanisms of electron transfer in aqueous solutions:(I) reduction via four electrons from O2 to H2 O, and (II) reduction via two electrons from O2 to hydrogen peroxide (H2 O2 ). Where the former being the preferred mechanism in MFC applications (Dange et al., 2022; Erable et al., 2012). However, the ORR mechanism depends on several parameters such as the nature of the electrode, the pH and current density. The kinetics of ORR on the surface of an electrode is strongly related to the adsorption of intermediates on the catalyst surface (O∗ , OH∗ , and OOH∗ ). O2 is strongly adsorbed on the electrode surface at high potentials, therefore the proton and electron cannot be transferred to the highly stable molecule. The reaction takes place in acidic and alkaline media involving the formation of oxygen intermediates such as OH, O2 − , O, H2 O2 , and HO2 − . According to the above, the catalyst must have an optimal binding strength for the intermediate products. The characteristic of a catalyst depends on its electronic structure. Metals with d-states present high energy relative to the Fermi energy and offer a stronger interaction of the electrode surface with the intermediates (Nørskov et al., 2009; Wu & Yang, 2013). One approach to reduce expensive Pt-based cathodes has been carbon-based electrodes with or without catalysts such as transition metals, metal oxides, in which electrocatalytic activity toward ORR equal or even higher than Pt has been reported (Ando et al., 2010; Wang et al., 2018). Among these oxides, spinel-type materials are a class of material with the formula AB2 O4 where A and B are either the same or different transition metals. The different valences in spinels play an important role in enhancing ORR activity by offering more active sites. To improve the electronic conductivity of the cathode, electrically conductive supports such as graphene, activated carbon, or graphitized carbon could be used. Table 16.1 summarizes the materials used to make electrodes (anode and cathode) for MFC, including carbon materials and metal oxides and its power density. The cubic spinel structure (A)[B2 ]O4 characterized by the Fd3m space group, facecentered cubic, consists of a close packing of oxygens occupying positions 32e in which one eighth of the interstices or tetrahedral sites (position 8a) and half of the octahedrons (position 16d),are occupied by cations.There are then twice as many cations in octahedral coordination (B) than in tetrahedral coordination (A) with the oxygens. The nonideal structure is derived from the ideal one by translation of the oxygens from their ideal position in the [111] direction from the nearest cation A, as shown in Fig. 16.1.

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Materials

Carbon-based material

Metal and metal oxides

Electrodes

Anode CNT/Carbon cloth Graphene oxide MWCNT/CNP/epoxy resin Carbon cloth Carbon felt Graphene/CN/carbon felt Carbon felt Carbon felt Carbon felt

Cathode Carbon cloth Carbon paper Carbon cloth Carbon cloth Carbon fiber felt Carbon electrode plates Carbon felt/GO@ Fe3 O4 Carbon felt/GO@ NiFe2 O4 NiO-CuO/Graphene

Surface area of anode (cm2 )

Power density (mW m−2 )

— — —

65 102 245.34

4 2.5 —

679.7 784 199.24

1

109.22

1

508.16

16.63

21.25

References

(Tsai et al., 2009) (Chen et al., 2015) (Mohanakrishna et al., 2012) (Qiao et al., 2015) (Yang et al., 2017) (Huang et al., 2021) (Caballero-Briones et al., 2018) (Caballero-Briones et al., 2018) (Khater et al., 2022)

Advanced nanomaterials and nanocomposites for bioelectrochemical systems

Table 16.1 Electrode materials for microbial fuel cell.

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

(A)

(B)

Figure 16.1 (A) The inverse spinel structure of Fe3 O4 , consisting of an FCC oxygen lattice, with tetrahedral (A) and octahedral (B) site. (B) Scheme of the exchange interaction in magnetite. From Fodjo, E. K., Gabriel, K. M., Serge, B. Y., Li, D., Kong, C. & Trokourey, A. (2017). Selective synthesis of Fe3 O4 Au x Ag y nanomaterials and their potential applications in catalysis and nanomedicine. Chemistry Central Journal, 11(1). https://doi.org/10.1186/s13065-017-0288-y.

λ is a parameter that describes the degree of investment and takes values of 0 and 0.5. For a normal spinel, λ ≈ 0 and the formula is (A)[B2 ]O4 (Chaumont & Burgard, 1979). For an inverse spinel, λ = 0.5 and the formula is (B)[AB]O4 . This cationic arrangement could provide the electron transfer to carry out the oxygen reduction reaction. For example, magnetite nanoparticles have magnetic properties, biocompatibility, and possibility of functionalization (Daoush, 2017; Wallyn et al., 2019). There are two approaches to obtain nanoparticles with inverse spinel or not structure (Burns & Glazer, 2013). The first one in called top-down with physical methods, whereas the second one is called bottom-up with chemical and biological methods. In the scheme, Fig. 16.2 is represented each method depending on its category. Physical methods to produce metallic oxide nanoparticles are feasible for large-scale production. These methods are based on solid, gas, vapor, or plasma state reaction. However, achieving a preferential particle size results in long production times, making it an expensive and limited method (Bell, 2005). Among these processes are ball milling, laser

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Figure 16.2 Classification of synthesis methods for magnetite nanoparticles. From Niculescu, A. G., Chircov, C. & Grumezescu, A. M. (2022). Magnetite nanoparticles: Synthesis methods – A comparative review. Methods, 199, 16–27. https://doi.org/10.1016/j.ymeth.2021.04.018.

ablation, electron beam, deposition, sputtering, aerosol spray, and pyrolysis (Satyanarayana, 2018). Ball milling is used as a size reduction method. It is used as a size reduction method in which a cylindrical container is used to introduce the precursor inside it together with micrometric-sized spheres whose function is grinding (Marcelo et al., 2020). On the other hand, laser ablation consists of irradiating the sample with an Nd:YAG laser beam, in this way the size, shape, or phase composition of the particles with relatively uniform sizes of around 15 nm can be controlled (Luo et al., 2018). Another physical process is through the electron beam, the beam is directed at the sample, obtaining nanoparticles through the evaporation of the initial precursor (Fu et al., 2018). Another deposit method is through sputtering. It consists of ejecting atoms from the surface of the material using energetic particles, for example an ion beam of inert gas (Miyata et al., 2006). Aerosol spray pyrolysis synthesis technique it is used to obtain small-sized particles and its main advantage is the production of materials with a single composition starting from a mixture of materials in a solution. Due to the above, it is a promising scalable synthesis method in which nanoparticles with spherical morphology can be produced (Hadjipanayis et al., 1992). In the other hand, chemical methods offer the manufacture of nanoparticles starting from molecular entities. Various chemical methods can be found in the literature to obtain metal oxide particles with magnetic properties with nanometric sizes (Ansari et al., 2019; Genuzio et al., 2020; Mandziak et al., 2018). The coprecipitation method is widely used in which the ions contained in a salt solution are precipitated by adjusting the pH on the surface of the substrate (Cabrera et al., 2008). Thermal decomposition is used

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

to obtain dispersed and uniform crystalline nanoparticles with magnetic susceptibility to an external magnetic field. This technique bases its principle on the decomposition of organometallic ions at high temperatures (Unni et al., 2017). Another method of synthesizing of magnetic nanoparticles (MNPs) is by sol–gel process. This chemical route starts from a molecular precursor that is dissolved in water or alcohol and is converted to a gel through heating and constant stirring by hydrolysis/alcoholysis. Subsequently, the process is wet, so it is necessary to dry and finally the application of the gel (polymeric three-dimensional network) (Bokov et al., 2021), after the gel is dried to obtain the nanomaterial. Also, microemulsion is another method, this technique carried out with a polar phase and stable thermodynamically solution (Yarbrough et al., 2020). The advantages of this process are large interfacial area, ultralow interfacial tension and monodispersed nanoparticles, controlling nanoparticle size, morphology, and homogeneity (Chin et al., 2014). Other chemical process for magnetic nanoparticles fabrication is by sonochemistry that involves dissociation and formation of chemical bonds. This is possible because the materials are under ultrasonic irradiation (above 20 kHz) and form acoustic cavitation (formation, growth, and implosive collapse bubbles) (Machado et al., 2021). Hydrothermal method, this synthesis process consists of reacting a solution in a wide temperature range, from room temperature to high temperatures, which concludes in the formation of nanomaterials. Furthermore, it is possible to control the temperature through the pressure conditions during the synthesis (Gan et al., 2020). Another method of synthesis for the preparation of magnetic nanoparticles is assisted by microwaves, being able to obtain ultra-small sizes. With microwave radiation the heating is in the whole system. Ion conduction and dipole polarization are two types of heating mechanisms; molecules in the mixing solution react by microwave energy, some particles must be polar (Brollo et al.,2017).In the other hand,chemical reduction is one of the most commonly techniques used to synthesis of nanoparticles. This process involves heating a solution with metallic salts and it is required a reducing agent (Pulit et al., 2013). The reduction of metallic salts has three stages, the first is by reducing agents, following by stabilization of the ionic complex and controlling the size with capping agent (sodium citrate, surfactants, etc.). After adding the reduced agent, the metallic atoms precipitates and nucleates (Khan et al., 2016; Krishna et al., 2021). The electrochemical synthesis has some advantages as potentially low cost, is used to produce a wide variety of metallic nanoparticles, operates a low temperature, it can obtain high purity of materials (Li et al., 2013). The system operates by using two electrodes (anode and cathode) both under a solution mixture. The setup is polarized by external potential/current source. There are parameters that involve the quality of nanoparticles, as current density, solvent polarity, distance between electrodes, temperature, stir, etc. (Bensebaa, 2013). Solvothermal method has been used to fabricate nanoparticles. The synthesis method can use aqueous or nonaqueous solvents at high temperatures above boiling point and its possible control size distribution, shape and crystalline phases varying

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A

B

A

Figure 16.3 Structure of graphite, composed of graphene sheets linked by electrostatic force called Van der Waals bond. From Katsnelson, M. I. (2007). Graphene: Carbon in two dimensions. Materials Today, 10(1–2), 20–27. https://doi.org/10.1016/S1369-7021(06)71788-6.

pressures above 1 bar (Nunes et al., 2019; Shaikh et al., 2020; Yáñez-Vilar et al., 2009). Polyol synthesis method use glycol solvent to suspending metal precursor following elevating temperature and stir to fabricate metallic nanoparticles. With this process it is possible controlling the particle growth, nucleation, and agglomeration (Fiévet et al., 2018; Lee et al., 2014). The biological method has been development to be eco-friendly, nontoxic, and with biomedical applications. This kind of method uses microorganisms to prepare inorganic nanoparticles. The nanoparticles are produced by biogenic enzymatic process, and it can be classified as intracellular and extracellular synthesis. It consists of the organism grab target ions from the solution and turn the metal ions through enzymes (Li et al., 2011).

16.4 Synthesis and application of graphene-based nanomaterials for microbial fuel cells Graphene is a two-dimensional nanomaterial; it consists of a single layer of carbon atoms which are in a hexagonal structure that is built on the hexagonal Bravais lattice whose base consists of two identical atoms associated with each lattice point (Muzyka et al., 2017) as shown in Fig. 16.3.

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

Figure 16.4 Carbon atoms with π and σ bonds.

The π bonds that generate an electronic cloud on the plane of the sheet are responsible for the electronic properties of graphene, while the σ bonds are responsible for the high hardness (Li et al., 2011). In Fig. 16.4, the bonds of the graphene sheet are schematized. The electronic properties of graphene depend on the number of graphene layers. Single layer graphene (SLG) and bi-layer graphene (BLG) are zero gap semiconductors, with only electrons and holes, respectively. When the graphene layers are greater than 3 but less than 10, the conduction and valence bands begin to overlap and free charge carriers appear (Morozov et al., 2005). 16.4.1 Introduction to graphene oxide The synthesis of graphene oxide (GO) can essentially be divided into two categories: bottom-up methods where simple carbon molecules are used to construct pristine graphene, and top-down methods where layers of graphene derivates are extracted from a carbon source, typically graphite. The focus on top-down methods, which first generate GO and/or rGO, is more popular for realizing graphene derivates, particularly for use in nanocomposite materials. GO is also known as graphite oxide or graphitic acid. It was first synthesized by chemist Benjamin Collins Brodie in 1859 (Feicht et al., 2019), using a multilevel oxidation of graphite. Brodie used the combination of potassium chlorate (KClO3 ) with nitric acid (HNO3 ) together with graphite, finding carbon, oxygen, and hydrogen (Vaka et al., 2020; Brodie, 1859; Agudosi et al., 2020). Some of the current synthesis processes for obtaining graphene oxide are the Brodie, Staudenmaier, or Hummers methods or modifications of these. Graphene oxide (GO) and reduced graphene oxide (rGO) have become the mainly studied materials as an

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Figure 16.5 Lerf–Klinowski model indicating the presence of carboxylic acids on the periphery of the graphene oxide basal plane. From He, H., Klinowski, J., Forster, M. & Lerf, A. (1998). A new structural model for graphite oxide. Chemical Physics Letters, 287(1–2), 53–56. https://doi.org/10.1016/S0009-2614(98)00144-4.

alternative material to graphene. GO sheets are several hundred nanometers to a few microns in diameter. Its structure is two-dimensional (2D) semiplanar with a thickness of approximately 1 nm (Zhang et al., 2009). It usually has linear wrinkles and a greater surface roughness compared to pristine graphene, due to the presence of carbons with sp3 hybridization and to the fact that GO contains functional groups. Active oxygenates such as hydroxyl and epoxy present on both sides of the GO layers. However, there are discussions about the proposed models of the structure of graphene oxide, with Anton Lerf and Jacek Klinowski being the model widely used to describe graphite oxide as a structure with hydroxyl and epoxy groups outside the basal plane of the sheet. Whereas carboxylic and carbonyl groups are present at the edges of the sheet. These functional groups found between the layers of the GO, give it the ability to be hydrophilic. In Fig. 16.5, the graphene oxide sheet with the functional groups is observed; carbonyl (C=O), carboxyl (COOH), epoxy (C–O–C), and hydroxyl (–OH), at the edges and on the plane of the sheet, as shown in Fig. 16.5. The advantage of GO is that it can be chemically modified to tailor its properties for multiple applications. The main chemical modifications of GO to have different functionalities or properties can be classified as (i) reduction, or removal of oxygenated groups from graphene oxide; (ii) functionalization, which consists of creating covalent bonds between graphene oxide and organic molecules; (iii) doping, when heteroatoms are introduced into the carbon rings in the sheet, and (iv) decoration, which uses the oxygenated groups of GO as reactive centers to nucleate nanoparticles of inorganic materials. A case of interest in the application of GO decoration is the production of electrodes (anode or cathode) for fuel cells.

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

Figure 16.6 Reaction mechanism between sodium nitrate and sulfuric acid. From Guerrero-Contreras, J. & Caballero-Briones, F. (2015). Síntesis de óxido de grafeno como plataforma nanoscópica para materiales funcionales.

16.4.2 Synthesis method of graphene oxide The Hummers method consists of the oxidation of graphite to form graphite oxide and after exfoliation of it by intercalation of ions such as NO3 − between the laminar space, forming graphene oxide (GO). The process begins by adding the oxidizing and exfoliating reagents to the graphite together with a strong acid (H2 SO4 and/or H3 PO4 ) while the temperature is maintained at 5°C to prevent the graphene sheets from breaking, subsequently the temperature is increased to give step to the exfoliation process and after this stage water is added to raise the temperature up to 98°C. In this stage, oxygen is supplied to the system to form the carboxyl, epoxy, carbonyl, and hydroxyl groups. The reaction is terminated by oxidizing the permanganate ion to Mn4+ with hydrogen peroxide to form insoluble MnO2 . To recover the material, the remaining SO4 2− and Mn4 + ions are removed with HCl and finally the graphene oxide is dried at a temperature of 50°C to remove the trapped water and in turn ensure that the functional groups do not decompose at this temperature. The recovered GO is dispersible in water, although it depends on the level of oxidation, that is, the number of functional groups found on the edges and in the basal plane of the sheet (Lim et al., 2018; Guerrero-Contreras & Caballero-Briones, 2015). In Fig. 16.6, the reaction between NaNO3 and H2 SO4 is illustrated. The resulting nitrate ion intercalates between the sheets of graphite bound by Van der Waals interactions, allowing access to oxidants. Potassium permanganate, the most widely used oxidant, activates the oxidation process.Dimanganese heptaoxide (Mn2 O7 ) is more reactive than KMnO4 ,but it explodes at temperatures above 55°C or when it comes into contact with organic compounds, so the first part of the oxidation process occurs at low temperatures, as already mentioned. Mn2 O7 reacts with double bonds (Trömel & Russ, 1987), in the presence of acids with double bonds it generates epoxide groups, as shown in Fig. 16.7. Furthermore, with the addition of sulfuric acid, the carbon double bonds found at the edges of the graphite sheets form compounds called alkyl acid sulfates, generated by the addition of hydrogen to a carbon double bond and a bisulfate ion to the other (Morrison & Boyd, 1987). Fig. 16.8 describes the process described above. The resulting compound is esters of sulfonic acids. The result of adding water to the reaction in the presence of the acid generates alcohols, due to the hydration of alkenes, the

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Figure 16.7 Evolution stages of permanganate to Mn2 O7 to produce an epoxide. From GuerreroContreras, J. & Caballero-Briones, F. (2015). Síntesis de óxido de grafeno como plataforma nanoscópica para materiales funcionales.

Figure 16.8 Formation process of acid sulfates in carbons with double bond. From Guerrero-Contreras, J. & Caballero-Briones, F. (2015). Síntesis de óxido de grafeno como plataforma nanoscópica para materiales funcionales.

Figure 16.9 Hydroxyl group formation process in the graphene structure. From Guerrero-Contreras, J. & Caballero-Briones, F. (2015). Síntesis de óxido de grafeno como plataforma nanoscópica para materiales funcionales.

main source of alcohol formation in graphite (Wiberg & Saegebarth, 1957). In Fig. 16.9, the training process is outlined. After the alcohols are formed, the epoxide groups are more reactive, one mechanism for the production of epoxide groups is through the preoxidation of carbon–carbon double bonds, this process takes place in the presence of water and reactive ions at high temperatures. Fig. 16.10 illustrates the process described. In the process of formation of the hydroxyl and epoxide functional groups by the reaction of the permanganate ion with two carbon atoms joined by a double bond, a substitutional displacement of the manganese ion is generated with hydrogen atoms

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

Figure 16.10 Preoxidation process for the generation of epoxides. From Guerrero-Contreras, J. & Caballero-Briones, F. (2015). Síntesis de óxido de grafeno como plataforma nanoscópica para materiales funcionales.

Figure 16.11 Reaction process of permanganate ions for the formation of hydroxyl and epoxides.

of the water molecule that has been included in the synthesis process. The resulting hydroxyl groups that do not react do so with the hydrogen atom of the partner functional group, releasing a molecule of water leading to the formation of the epoxide. The above formation mechanism is shown in Fig. 16.11. For the formation of acid sulfates, alcohols and epoxides to occur, temperatures close to room temperature are necessary, while for oxidation with H2 O2 to occur, temperatures close to 100°C are necessary (Wiberg & Saegebarth, 1957). With the previously described the process of intercalation of the hydroxyl and epoxide oxygen groups between the graphene sheets causes the interplanar distances to increase up to three times, starting from 3.32 Å in the pristine graphite up to 8.32 Å in the GO. Table 16.2 outlines the graphene oxide synthesis methods and its variations. Table 16.2 Summary of graphene oxide methods. Carbon Reaction Methods Oxidants source time

Temperature (°C)

Brodie, 1859 Graphite KClO3 , HNO3 3–4 days Staudenmaier, Graphite KClO3 , HNO3 , 96 h 1898 H2 SO4 Hummers, Graphite KMnO4 , NaNO3 , 2 h 1958 H2 SO4

60 Room temperature 20–35–98

Marcano, 2010 Dimiev, 2016

50

Ranjan, 2018

Graphite H2 SO4 , H3 PO4 , 1.5 h KMnO4 Graphite (NH4 )2 S2 O8 , 98% 3–4 h H2 SO4 , fuming H2 SO4 Graphite H2 SO4 , H3 PO4 , 24 h KMnO4

Room temperature

References

(Brodie, 1859) (Staudenmaier, 1898) (Hummers & Offeman, 1958) (Marcano et al., 2010) (Dimiev & Eigler, 2016)

Room (Ranjan et al., temperature– 2018) 35–95

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Figure 16.12 Summary of primary synthesis methods for MGO nanoparticles. From He, Y., Yi, C., Zhang, X., Zhao, W. & Yu, D. (2021). Magnetic graphene oxide: Synthesis approaches, physicochemical characteristics, and biomedical applications. TrAC Trends in Analytical Chemistry, 136, 116191. https://doi.org/10.1016/j.trac.2021.116191.

16.4.3 Synthesis of metal oxides with graphene oxide There are physical and chemical methods applied to modify or add metal oxides to graphene oxide structure. Fig. 16.12 shows two types of synthesis approaches, the first one is by chemical methods and the last one is by physical methods. As previously described, the methods for obtaining graphene oxide and due to its characteristics of oxide groups can be chemically or physically modified by incorporating nanoparticles of metal oxides, which can confer magnetic characteristics depending on the concentration of metal ions (Barakat et al., 2018). The synthesis methods for fabrication GO/metal oxide nanocomposites are summarized in Fig. 16.12. In the recent literature, it can be found the enhance of ORR by accelerating charge transfer with composites GO/metal oxide due act as electrocatalysts due to their electronic configuration, these materials possess a magnetic state that can be harnessed and induce an external magnetic field on them using permanent magnets to enhance the efficiency of the ORR. The improvement in its performance is caused by two forces involved: the Lorenzian force that depends on the magnetic properties of the electrolytic species, and the Kelvin force that depends on the motion of paramagnetic species of the system, specifically paramagnetic O2 (Zakrzewska et al., 2022).

Synthesis and application of graphene-based nanomaterials for microbial fuel cells

16.5 Conclusion and future outlook The MFCs still facing the challenges in requirement of low power output,high capital and operational cost, intensive labor, difficult in attaining the repeated identical performances, contamination risks, and life-span issues (Chandrasekhar et al., 2017). Other major challenges in the future include stability, long-term performance, and scaling up process form lab scale to full scale (Breheny et al., 2019). High internal resistance, slow microbial reaction at low temperature potential possibility of biofouling, and expensive cation exchange membrane hinder the practical applications of MFC (Surya Ramadan et al., 2017). The recent progress with the application of graphene oxide material in MFC could lead to efficient harvesting of electrical energy and wastewater treatment in near future. Graphene-based material offers large specific surface area and accommodates numerous microorganisms on the macropores, which largely enhance the generation of electricity in the MFCs. Cathode material with large surface area in-house high number of active sites. Biocompatibility of the electrode material is possible with application of hierarchical and porous structure. High specific surface area, excellent catalytic performance, and improved electrical properties make graphene-based material ideal candidate for the MFCs (Yu et al., 2016). As described above, the modification of graphene oxide with metal oxide materials with magnetic properties gives it good performance compared to using only carbon-based materials. Due to the nanometric size of the metal oxide particles (MO), the contact surface area on the carbon-based material, anode and/or cathode, increases. Additionally, the power output of MFCs can be improved by using an external magnetic field in the magnetic graphene oxide electrodes (MGO) to enhance the ORR, as described in the literature (Zeng et al., 2018).

References Agudosi, E. S., Abdullah, E. C., Numan, A., Mubarak, N. M., Khalid, M., & Omar, N. (2020). A review of the graphene synthesis routes and its applications in electrochemical energy storage. Critical Reviews in Solid State and Materials Sciences 45(5), 339–377. https://doi.org/10.1080/10408436.2019.1632793. Aliyev, E., Filiz, V., Khan, M. M., Lee, Y. J., Abetz, C., & Abetz, V. (2019). Structural characterization of graphene oxide: Surface functional groups and fractionated oxidative debris. Nanomaterials, 9(8), 1180– 1194. https://doi.org/10.3390/nano9081180. Ando, T., Izhar, S., Tominaga, H., & Nagai, M. (2010). Ammonia-treated carbon-supported cobalt tungsten as fuel cell cathode catalyst. Electrochimica Acta, 55(8), 2614–2621. https://doi.org/10.1016/ j.electacta.2009.12.039. Ansari, S., Ficiarà, E., Ruffinatti, F., Stura, I., Argenziano, M., Abollino, O., Cavalli, R., Guiot, C., & D’Agata, F. (2019). Magnetic iron oxide nanoparticles: synthesis, characterization and functionalization for biomedical applications in the central nervous system. Materials, 12(3), 465. https://doi.org/10.3390/ ma12030465. Barakat, N. A. M., El-Deen, A. G., Ghouri, Z. K., & Al-Meer, S. (2018). Stable N-doped & FeNi-decorated graphene non-precious electrocatalyst for oxygen reduction reaction in acid medium. Scientific Reports, 8(1). https://doi.org/10.1038/s41598-018-22114-1.

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CHAPTER 17

Future development, prospects, and challenges in application of nanomaterials and nanocomposites Vinayaka B. Shet a, Keshava Joshi b and Lokeshwari Navalgund b

a Nitte (Deemed to be University), NMAM Institute of Technology (NMAMIT), Department of Biotechnology Engineering, Nitte, India b Department of Chemical Engineering, SDM College of Engineering and Technology (V.T.U., Belagavi), Dharwad, Karnataka, India

17.1 Introduction The microbial fuel cell (MFC) is becoming promising in the current era since it converts chemical energy into electricity due to the metabolic activity of the microorganisms. Even though advancement has been made to enhance efficiency of the MFC, there is a further challenge to improve the efficiency by adopting current technology.Development of affordable and effective MFC with economical production certainly plays a promising role in expanding the widespread application in near future. The components of the MFC consist of electrodes, membrane, and the circuit. The improvements essential for MFC were standstill during recent years due to the constraints in the technology, however amalgamation of nanomaterials with MFC have given the breakthrough in improving electrode, membrane, and circuit. Nanomaterials are having potential to enhance the efficiency due to increased surface area, conductivity, catalytic activity, chemical stability, corrosion resistance (Kaur et al., 2020; Jatoi et al., 2021). The chapter is focused on the future developments, perspectives, scale up criteria, and challenges involved in the application of nanomaterials in the MFC.

17.2 Future developments Fig. 17.1 presents future development, challenges, scale up criteria, and advantages of MFC. 17.2.1 Electrode The efficient functioning of fuel cell electrodes plays a key role. While developing electrodes the following property of the material used for construction must be considered. Advanced Nanomaterials and Nanocomposites for Bioelectrochemical Systems DOI: https://doi.org/10.1016/B978-0-323-90404-9.00001-2

c 2023 Elsevier Inc. Copyright  All rights reserved.

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Figure 17.1 Future development, challenges, scale up criteria, and advantages of MFC.

Increased surface area: Leads to the enhanced reaction site and escalates the electron transfer. Conductivity and circuit: All the electrons generated at the interface of anode have to reach the cathode through the circuit. The circuit should offer the least resistance in order to achieve maximum throughput of the fuel cell. Minimum resistance: The MFC offers resistance, contributed by electrode and membrane. Stability and durability: The electrode undergoes corrosion due to the oxidation and reduction reaction taking place at the electrodes. Electrodes should be stable under both acidic and alkali conditions. Economic and scale up: The electrodes should be cost effective and facilitate scale up of the fuel cell. The performance of the MFC and stability depends on the conducive environment provided by anode in biofilm formation. Critical step involved in the power generation consists of transfer of electrons from microorganisms to anode. The nanomaterials are promising due to the properties exhibited at nanoscale. The electrode can be developed using nanomaterials or the electrodes can be decorated with nanomaterials to attain the desired property. The electrode having greater surface area, better conductivity, anticorrosion, adhesion of microorganisms are under development. Carbon, metal, and composite nanomaterials are explored to achieve the maximum throughput of the fuel cell. 17.2.2 Carbon-based nanomaterial The conventional carbon-based materials, such as graphite plates, carbon paper, carbon felt, carbon brush, exhibit a shortfall in terms of electron acceptor and microbial

Future development, prospects, and challenges in application of nanomaterials and nanocomposites

carrier. Carbon-based nanomaterials such as carbon nanotube, carbon nanosphere offers greater surface area for the adhesion of microorganism and transfer of extracellular electrons. Carbon nanotube (CNT) has the potential to be used for the development of electrodes due to greater surface area, conductivity, mechanical strength, thermal stability. To enhance the surface area, 3D electrodes are developed. However, 3D electrodes encountered the problem such as small pore size restricting the bacterial penetration and poor conductivity. CNT-modified anode offers enhanced power density. CNT has the promising impact on biofilm formation and electron transfer, hence suitable for the anode electrode. Surface modification is essential to enhance the interaction of anode with microorganism. Nitrogen functionalities improve the adhesion of microorganisms. CNT inhibits the proliferation of cells and causes cell death. The cytotoxicity properties pose a challenge in the development of electrodes. To overcome the challenge, dosage of CNT used for the modification has to be assessed and regulated (Liu et al., 2020; Mazari et al., 2021). 17.2.3 Metal-based nanomaterial Metal-based nanomaterials offer better catalysis, biocompatibility, and conductivity (Kardi et al., 2017). Decorated metal nanoparticles on anode act as bridge between bacteria and electrode to transfer electrons. Drawback of metal or metal oxide nanoparticle cannot build 3D structure on the interface of carbon-based material to resist the reduction in mass transfer induced by thick film of cell. Lack of nutrient supply to the inner core leads to cell death and leads to nonconductive debris on the electrode surface. Further cytotoxicity of the metal nanoparticles has to be assessed. 17.2.4 Nanocomposite material The nanocomposite materials are developed from carbon, metal, metal oxide, and conducting polymer delivers the expected properties of electrode. The power density can be escalated by using nanocomposite material. Coating of iron and copper oxide nanoparticles on the carbon-based electrode has better efficiency. The base material of the electrode is coated with nanomaterial to develop nanocomposites. Graphene, polyaniline, carbon nanotube based nanocomposites are reported to be developed to incorporate desired properties on the electrode. Besides, the coated polymer protects the electrode from corrosion. Graphene-based composite materials have reported exhibiting enhanced power density and better energy conversion between bacteria and electrode in MFC. The composite materials also enhance the adhesion of microorganism on the surface to facilitate the cell-to-cell electron transfer. Hence, graphene-based composite materials are potential for development of efficient MFC (Ci et al., 2015; Khan et al., 2020). However, developing simplified economic synthesis process of composite material for

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large-scale production and exploring the mechanism of oxygen reduction reaction to develop modeling technique are essential. The conducting polymer polyaniline exhibits promising electrochemical activity and catalytic behavior. Hence, polyaniline is explored in the MFC development. It has several advantages such as easy processability, good biocompatibility, controllable electrical conductivity, and environmental stability. The nanocomposites of polyaniline are used as coating material for the electrodes of the MFC to enhance the generation of power. The resistance offered for the electronic transmission between microorganism and electrode can be reduced by coating polyaniline on electrodes due to its inherent electrical conductivity. The polyaniline nanocomposite also contributes in increasing the surface area of the electrodes and enhanced power generation in the MFC. The anodic modification prevailed the adhesion of microorganism on the electrode in turn improved performance of the MFC. Conductivity property of the polyaniline can be further enhanced by introducing dopants such as copper doped zinc oxide and zinc oxide. Amount of dopants essential for achieving the maximum conductivity can be optimized. 17.2.5 Membrane The membrane used in the MFC should possess properties such as water uptake, ionic conductivity, and overcome biofouling. Nanomaterials explored in the development of nanomembranes are in a nascent stage of investigation. Conducting polymer polypyrrole is reported in the development of proton ion exchange nanomembranes. 17.2.6 Metal organic frameworks The research on developing new materials has resulted in metal organic framework (MOF). MOFs are highly microporous crystalline material composed of metal ions and organic ligands. MOF offers increased surface area, adjustable pore size, structure diversity, improved conductivity, and biocompatibility. MOFs are potential materials to be explored for electrodes in the MFC. Adhesion of microorganism and biofilm formation can be enhanced by incorporating biological molecules such as amino groups to anodic materials. Standardization of biofunctional groups in the development of electrodes to achieve optimum adhesion and biofilm formation has to be investigated. Various biomolecules suitable to be incorporated into the anodic materials should be screened. Potential nanomaterials used in the MFC are depicted in Fig. 17.2.

17.3 Perspectives Current global energy crisis is resultant because of the shortage of natural resources and also the safety, efficiency, and environmental components of fossil fuels foot it back as a favored energy source. To overcome the problems of the energy crisis, researchers and scientists have to work to meet the future need of energy which is more consistent, supportable, and clean energy. The global energy demand is going to rise by 18 billion

Future development, prospects, and challenges in application of nanomaterials and nanocomposites

Figure 17.2 Potential nanomaterials used in the MFC and its properties.

tones by 2035 to the present need of 12 billion tones as per the International Energy Agency (IEA). The ecological imbalance is occurring as all the natural resources are being depleted to meet the current energy demand. As energy need is increasing at global level, which is due to increase in population, there is need for alternative energy sources that are much cleaner, cheaper, and environmental friendly. Today researchers are looking for alternative sources for energy generation and one among them is MFCs technology, which produces energy from microorganisms (Venkata Mohan et al., 2008). The MFC is found to be the alternative technology for power generation which is considered as a cleaner, safer, and efficient process and also does not produce any toxic by-products. Hence, in current necessity to meet energy demand, MFCs have shown to be an effective technology for energy production and also recovery and conversion of chemical energy into electricity. Many researchers are working with MFC technology and found that the performance of MFC directly depends on the kinetics of the electrode reactions within the fuel cell and in turn electrode reaction is influenced by the choice of the material (Choudhury et al., 2017; Huang et al., 2021; Palanisamy et al., 2019). An extensive range of materials has been studied and tested to improve the performance of MFCs. Nanocomposite materials ensure the potential to develop materials optimal for electrode manufacture to

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improve the power generation. Nanomaterials are becoming promising technology for fuel cell applications and to improve the properties such as excellent conductivity, thermal stability, mechanical strength, and resistance to corrosion. Some nanomaterials are being incorporated into other materials like polymer and metal oxides. Nanocomposites thus designed have been effectively used as electrode materials in MFCs.

17.3.1 Research In today’s modern world to meet the energy demand, researchers and government are looking for eco-friendly supportive source of energy. MFCs are substituting the source of other energy production systems due to clean and renewable energy resources. MFCs also deal with dual-purpose solutions that are for both electricity generation and wastewater treatment. The power generation in MFCs is restricted due to many investigations in the performance of MFC needed. Researchers are working with different scientific and engineering fields of construction and analysis and principles of understanding materials and techniques for better performance of MFC. There are two principal types of biofuel cell, MFC and enzymatic fuel cell (EFC) and these devices convert chemical energy to electrical energy via electrochemical reactions concerning microbes and enzymes. The perception of MFC was not well appreciated until 1999, from the day it was discovered by Potter in 1911, due to its dual purpose of electricity production and wastewater treatment which was studied. In the past 10 decades, the indication of tremendous interest from the scientific community has led to a promising source to afford power in a cost effective method. Many factors influence the performance of MFC, but more research is focused on electrode material as it is straight away influencing the potential and performance in commercialization of MFC. Numerous materials have been studied to improve the performance of MFC right from single metal to nanomaterial and hence receiving worldwide attention due to the production of clean and renewable energy. An understanding and identification of all the components is essential in the design of MFC to improve the power outputs. The knowledge of multidisciplinary approach with environmental engineering,material science engineering,electrochemistry,microbiology, molecular biology is very essential to understand and perform well with MFC. The challenge of the researcher is in understanding the complexity in techniques of studying the MFCs. In the present scenario, MFCs produce lower power output due to many factors related to anodic and cathodic cells, the chemical species in the electrolyte, microbial species, membranes, cell configuration, and operating conditions. Many of the MFCs at the lab and pilot plant are designed for simultaneous power production and wastewater treatment. Currently, research is extended to desalination process, carbon capture, biosensors, hydrogen production, and bioremediation. The basic principles for any application remain the same,but kinetic studies and electron transfer mechanisms vary

Future development, prospects, and challenges in application of nanomaterials and nanocomposites

from one product to the other and hence depth study is needed for understanding the different applications. Researchers have worked for the development and optimization of various types of cells like single, dual, stacked fuel cells. Investigators have used different types of bacteria like gram-negative bacteria (Escherichia coli, Bacillus violaceus, Pseudomonas fluorescens, Klebsiella pneumonae, Pseudomonas methanica, Proteus vulgaris, Desulfuromonas acetoxidans, Geobacter sulfurreducens, Methylovorus dichloromethanicum, Stenotrophomonas acidaminiphila, Methylovorus mays) and algae. Today’s core emphasis of researchers in MFC development is to improve power generation through the interaction of microorganisms with anode material from wastewater and determine the best compatibility. A main concern of the researcher is to develop cost effective, sustainable, and eco-friendly material. Stainless steel has shown high efficiency for improving the power density and coulomb of MFC. Composite stainless steel and graphene have been widely used as an anode material which have shown large surface area and excellent conductivity.Nanomaterial such as gold particle and iron oxide particle has shown excellent performance of MFC for simultaneous electricity generation and wastewater treatment (Kumar et al., 2018). MFC plays an important role in reduction of heavy metals, organic compounds, toxic elements, generation of hydrogen energy from various organic matters. Many researchers demonstrated that nanostructured materials, including carbon nanotubes (CNTs), graphene, activated carbon fiber, metal, metal oxides, and conductive polymers, have shown many appreciable properties such as good conductivity, large specific surface area, and excellent catalytic activity. The unique electrochemical properties of nanomaterial provide the strong interactions between bacteria and anode. These anodic reactions have both good and efficient metabolic rate of microbes and electrode interfacial charges, where the need of anode here is to increase the formation of biofilm and improve the transfer rate (Angelaalincy et al., 2018). Many researchers are exploring macroporous material to improve the density of bacteria on anode. However, still the power density can be enhanced two to three times by embedding the anode material with good conductive nanowire or nanoporous structure. Nanomaterials are being developed rapidly due to their advancement in a wide range of applications and these materials have a high efficiency process in transferring electrons from microbes to anode. Nanomaterials are rich both in micropores and nanopores, where the microporous structure is conductive to the growth of biofilm, and the nanoporous structure can enhance the direct electrochemistry of electronic medium for further achieving efficient biocatalysis and electrocatalysis. Carbon nanotubes are widely used as anode due to its very excellent conductivity, stability, anticorrosion. Metal or metal nanoparticles embedded with carbon materials have revealed the excellent charge transfer rate and the best biocompatibility. The excellent physical, chemical properties along with biocompatibility of nanomaterial are bringing much enhanced performance of MFCs.

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17.3.2 Performance of MFCs In today’s life, energy plays an important role in sustaining processes and due to increased energy demands, researchers have focused on the research and development of low-cost and long-lasting energy sources. Among them MFCs potentially represent the best source of low cost and eco-friendly referred as green energy production, where they operate by converting organic waste into electricity.The applications of nanomaterials for modifying the main components of MFC systems and their effect on cell performance are reviewed. Nanomaterials play an important role in MFCs; however, to enhance the properties of few nanomaterials (e.g., reduced graphene oxide (rGO) and carbon nanotubes (CNTs)), they have been incorporated into other materials (like polymers and metals) to add beneficial properties such as excellent conductivity, thermal stability, mechanical strength, and resistance to corrosion. Nanocomposites thus formed have been successfully utilized as electrode materials in MFCs. Innovations in the design of anode materials led to the formation of various materials with improved power density and efficiency. Carbonbased materials have been used widely in the manufacture of anode due to their high porosity, large surface area, and good electrical conductivity. To improve the performance of electrodes, various nanocomposites with enhanced properties (e.g., high mechanical strength, electrical conductivity, thermal stability) have been proposed and developed in the last few years. The main focus of MFC research has been put to enhance the interaction of microorganisms from wastewater treatment with the anode material and to determine their compatibility. Nonetheless, more efforts are still needed to increase power generation from wastewater. Cost effective materials are one of the major concerns of researchers as MFCs should be sustainable and eco-friendly. As discussed, stainless steel is highly efficient for enhancing the power density and coulomb efficiency of MFCs. Noticeable improvements were hence attained by incorporating stainless steel into various anode materials. Carbon- and metal-based nanoparticles and conductive polymers could contribute to the growth of thick anodic and cathodic microbial biofilms, leading to enhanced electron transfer between the electrodes and the biofilm. Extending active surface area, increasing conductivity, and biocompatibility are among the significant attributes of promising nanomaterials used in MFC modifications. Among the various nanocatalysts used on the cathode side, metal-based nanocatalysts such as metal oxides and metal–organic frameworks (MOFs) are regarded as inexpensive and high performance alternatives to the conventionally used high-cost Pt. In addition, polymeric membranes modified with hydrophilic and antibacterial nanoparticles could lead to higher proton conductivity and mitigated biofouling compared to the conventionally used MFCs. Overview of performance of MFC is given in Table 17.1.

Future development, prospects, and challenges in application of nanomaterials and nanocomposites

Table 17.1 Performance overview of MFC. Sl. No. 1

Material type FeO nanoparticle

Advantages Ease of synthesis and separation Excellent biodegradability Ability to bind multiple targeted compounds Large surface to volume ratio

Disadvantages High cost of synthesis Limits scale up process Poor dispersion abilities

2

TiO2 nanoparticle

Good biocompatibility Good corrosion resistance Large specific area High porosity

Poor electrical conductivity

3

Carbon nanotubes (CNT)

Low cost High surface area Relatively high conductivity and durability

Poor porosity Biocompatibility

4

Graphene

Excellent electrical and thermal stability High surface area and excellent electrical conductivity

Performance The biosynthesized FeO nanomaterial used as anode in MFC is an efficient material which is nontoxic, ecofriendly with better physic-chemical properties. The maximum power output produced was 145 mW m−2 and power density increased to 31%. Also these enhance the biofilm formation by promoting cell adhesion The power density of TiO2 as cathode in MFC was up to 70.39 mW m−2 . The maximum power density delivered by a TiO2 /polyaniline composite as an anode material was reported to be 1459 mW m−2 CNTs promote scaffold porosity for biofilm formation, along with enhancing electrocatalysis. The maximum power density achieved by the CNT/rGO anode was 1137 mW m−2 , which is 8.9 times higher than that of carbon cloth anode MFC The maximum power density of carbon felt Mo2 C/CNT composite was recorded as 170 mW Though graphene has high energy output as it provides higher surface area, but the orientation of its fabricated electrode matters the most. Researches show the utilization of crumpled graphene can double the generation of electricity. Utilizing RGO/SnO2 (reduced graphene oxide/tin oxide) in MFC produces 1624 mW m−2 of power density. Reduced graphene coated with Polyaniline (PANI) in carbon cloth prepared in suitable solvent of phosphate buffer [86] produced power density of 1390 mW m−2 .

References (Harshiny et al., 2017)

(Qiao et al., 2008)

(Wu et al., 2018)

(Zhou et al., 2019)

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Table 17.1 Performance overview of MFC—cont’d Sl. No.

Material type

Advantages

5

Cobalt oxide nanoparticles

Excellent electrochemical activity Good antimicrobial

6

7

8

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Disadvantages

Performance

A microbial fuel cell using the as-prepared Co/N-CNT as the cathode catalyst achieved a maximum power density of 1260 mW m−2 , which was 16.6% higher than that based on state-of-art Pt/C catalyst (1080 mW m−2 ). Silver nanoparticles High electrical and The Shewanella-silver MFCs thermal deliver a maximum current conductivity. density of 3.85 milliamperes per High chemical square centimeter, power density stability. of 0.66 milliwatts per square High antimicrobial centimeter, and single-cell activity turnover frequency of 8.6 × 105 per s, which are all considerably Nontoxic higher than those of the best MFCs reported to date. The hybrid MFCs features an excellent fuel-utilization efficiency, with a coulombic efficiency of 81%. The maximum power density (3006 mW m−3 ) and current density (34,100 mA m−3 29) of MFC-Ag were 30 found to be significantly higher than the MFC-FC and MFC-blank. Molybdenum carbon Large specific surface Low energy density The characterization results show nanoparticles area that the carbonized cotton textile High conductivity modified with Mo2 C nanoparticles offers a great Low cost specific surface area (832.17 m2 g−1 ) for bacterial adhesion. The MFC using Mo2 C/CCT anode delivers the maximum power density of 1.12 W m−2 , which is 51% and 116% higher than that of CCT and unmodified carbon felt anodes under the same conditions. Graphite rod High conductivity Low porosity and limited The maximum power density and chemical surface area for achieved with a graphite rod as stability bacterial adhesion the electrode material is 26 mW m−2 Graphite fiber High surface area The power density achieved using Low electrode graphite fiber brush was resistance 422 mW m−2 . In another study, graphite brush was used as an anode, while cobalt tetra methyl phenylporphyrin (CoTMPP) carbon cloth was used as the cathode. This system demonstrated a power density of up to 2400 mW m−2 Graphene-modified Good porosity High cost The power density is stainless steel mesh Good electrical 2668 mW m−2 conductivity

References (Yang et al., 2019)

(Cao et al., 2021; Ali et al., 2019)

(Zeng et al., 2018)

(Zejie and Lim, 2020)

(Siddarth et al., 2019)

(Yuan and He, 2015)

Future development, prospects, and challenges in application of nanomaterials and nanocomposites

17.3.3 Scale up The scale up technology in any system is to improve the performance and meet the demand. Investigators are working toward the development of MFC technology from laboratory scale to pilot plant investigations in the range of 1–1000 L. There is a large scope of substantial progress toward scale up and practical application of MFCs in the last 10 years. There is a great demand for alternative renewable energy today to overcome the ecological imbalance and protect the environment. MCFs are being scaled up to meet the energy demand along with large-scale treatment of wastewater plants. The most important requisite is to increase the size of the reactor and treatment capability with max power generation. Today MFCs are used for effective waste water treatment, energy generation, as biosensors, small power generation systems, and sediment batteries. MFC technology commercialization has a variety of challenges to overcome with the scale up ratios. The self-sustained energy operating plant of wastewater treatment at the field level has not been conducted yet and needs to overcome the problems of energy output per unit volume, voltage losses, enlarged sizes. The concept still has many critical issues for feasibility like reactor configuration,electrode material,catalyst,high fabrication cost, material cost, operational stability, internal resistance, lower efficiency of mixed culture biofilm on electrode and slow degradation kinetics, electrochemical capabilities of various microbes (Clauwaert et al., 2008). But still the valued application of MFCs is established in wastewater treatment and operational electronic appliances. More research emphasis and scale up are with tubular reactor configuration than on flat plate design. The greater focus with flat plate design will lead to a significant improvement of MFC form lab scale to pilot scale.In today’s waste to energy conversion technologies,the overall power densities of scale up MFC are low and competitive. Researchers are handling issues with electrode configuration like spacing, size, orientation, shape, surface area, bacterial adhesion, electrochemical efficiency to determine the maximum power attainable in scale up design. Still many scale up designs have used the expensive membrane materials even with the availability of less and better performing materials. Even there is much concentration needed for longer run of the technology with low cost to make this MFC technology competitive with other waste to energy technologies. Studies are concentrated to scale up the power production using different materials like carbon-based materials, carbon fibers, carbon nanotubes composites, metals embedded in nanocomposites, noble metals. More focus of the researchers is to reduce the cost of material and maximize power densities in MFC technology. Though many criteria are available for selecting the anode and cathode electrodes, both should have good surface area, porosity, conductivity, stability, durability, cost, and its access ability. To increase the surface area, the resistance and porosity are to be decreased, high electrical conduction of material makes electron flow with less resistance. Further high surface area enhances the electrode kinetics and have better biofilm formation. The rough surface of

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material increases the durability but has chances of fouling and decreases the long-term performance of MFC (Xu et al., 2016). Hence choice of material is very important in MFC technology with durability and stability in both acidic and basic environments. The material used for MFC technology for commercialization should be of low cost, ease of availability, and sustainable. Carbon nanotubes have shown promising material because of their unique properties and it is also reported that it has strong cell adhesion, cell attachment and growth properties (Munir et al., 2019). The overall functionality of a unit depends on factors like microbial inoculum and concentration, substrate composition and concentration, loading pH, feeding rate and configuration, temperature, electrode material, ion exchange membrane/separator, and reactor configuration (Seelam et al., 2016). The operational performance is bounded by start-up, physio-chemical parameters, substrate composition, feed loading rates, microbial culture, and its stability.

17.4 Outlook and challenges The global energy challenge is to meet the demand of energy without much consumption of nonrenewable fossil fuel and also to protect the environment and climate change. In this regard MFC mechanism has been an attractive renewable source of energy which converts waste to usable form of energy. MFC is a green technology as it converts chemical energy to electrical energy without any combustion with their ability of using different bacteria. In the last 15 years significant advancement has been made in the design of electron transfer mechanism, kinetics, membrane properties, cell design, and few pilot plants have been set up (Santoro et al.,2017;Jatoi et al.,2022).Energy produced by MFC is not put into large-scale applications,even though the energy produced is clean,effectively abundant of microbes at free cost and recyclable without any toxic by-products (Choi, 2015). Though all such advancement is observed, still there are challenges to improve efficiency and scale up of MFC. The present challenge is the electrode material and its properties for scale up and to enhance efficiency, hence researchers have focused on different combinations of materials and composites mainly nanomaterials. The novel electrode material should have better charge transfer, high surface area, high porosity, high conductivity, and biocompatibility for better growth of biofilm on the surface. The global nanomaterial is having a great future with high annual growth rate but the challenge is the environmental sustainability of these materials. These materials are one-, two-, or three-dimensional with special electrical, optical, surface area, biocompatibility to be used as an efficient electrode. The 2D and 3D materials have played as excellent and vital roles as electrode modifiers in MFCs (Kaur et al., 2020). Metal organic frames are accepted as exceptional ones as they provide more surface area and undergo excellent catalytic properties with high chemical stability, thermal stability, porosity and by doping with nanomaterial it is enhanced as an effective catalyst. Noble metals play an important role and produce highest power

Future development, prospects, and challenges in application of nanomaterials and nanocomposites

production, but they are often limited to huge costs and treatment. Though macroporous electrodes are promising ones in enhanced power generation they are prone to clog and hinder the long-time usage. Materials like brass, aluminum, and copper display the toxic toward microbial growth and few metal-based anodes are susceptible to low mechanical strength and corrosion (Sawant et al., 2017). Development and effective affordable MFC with low cost of production will be a promising role in future. Still the challenge exists in investigators in developing robust, durable and composite material for anode fabrication to improve the performance and scale up. The other challenging issues are the reactor volume, start-up time, and large-scale units. The cost and possibility of biofouling and clogging influence the design of the reactor. The space between the anode and cathode and the surface area are one of the challenges again to improve the performance of MFC in larger setups. The units are connected in parallel or series or any combination to target the current requirement. Researchers suggest that MFCs connected parallel yield higher electrical energy compared to that in series (Liu et al., 2018). Start-up time is one of the limiting factors in huge wastewater treatment plants and it is one of the limiting factors for MFC operation. Though MFC provides better treatment technologies compared to others in terms of lower biomass production, no aeration, no temperature adaptation to achieve high efficiency, but in contrast the long-term commercial adoption is a far challenging issue. MFC technology is a promising source for alternative renewable energy globally because of its capability of converting all wastes into energy directly through catalysis of bacteria. Though nanomaterials are showing promising results for certain limitations, challenges still remain for the real-time scale up (Kuppurangam et al.,2019).Modern nanotechnology is one of the encouraging solutions with more effectiveness and efficiency to meet the future demand of energy as there is substantial growth in the global human population. Researchers have to still exploit the nanotechnology in MFC to attain greater energy, high efficiency, low cost, scale up criteria, real-time application. The outlook for the future development and scaling of sustainable MCF with nanotechnology is the need of the hour. Efforts and more investments are needed in the area of MFC to make it commercially available one day.

References Ali, J., Wang, L., Waseem, H., Sharif, H. M. A., Djellabi, R., Zhang, C., & Pan, G. (2019). Bioelectrochemical recovery of silver from wastewater with sustainable power generation and its reuse for biofouling mitigation. Journal of Cleaner Production, 1425–1437. https://doi.org/10.1016/J.JCLEPRO.2019.07.065. Angelaalincy, M. J., Navanietha Krishnaraj, R., Shakambari, G., Ashokkumar, B., Kathiresan, S., & Varalakshmi,P.(2018).Biofilm engineering approaches for improving the performance of microbial fuel cells and bioelectrochemical systems. Frontiers in Energy Research, 6, 63. https://doi.org/10.3389/fenrg.2018.00063. Cao, B., Zhao, Z., Peng, L., Shiu, H.-Y., Ding, M., Song, F., Guan, X., et al. (2021). Silver nanoparticles boost charge-extraction efficiency in Shewanella microbial fuel cells. Science, 373(6561), 1336–1340. https://doi.org/10.1126/SCIENCE.ABF3427.

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391

Index

Page numbers followed by “f” and “t” indicate, figures and tables respectively.

A Active oxygenates, 366 Aerobic techniques, 57

B Bacteria isolation, 26f Bimetal oxide, 91 Bioanodes, 2 Biocathode, 142 Bioelectrochemical fuel cell, 196 electron transfer, 197 Bioelectrochemical system, 133, 175, 190 anode modification, 283 azo dyes reduction, 285 benefits, 190 electron shuttles, 281 extracellular electron transfer, 192 future perspectives, 201 healthcare applications, 197 indirect electron transport, 281 nanomaterials role, 286, 288 principle and types, 133 wearable electrochemical sensing systems, 200 work, 191 Bioelectrochemical systems, 1, 101, 328, 335 abiotic chemical reactions in, 3 anodes modification, 7 primary function, 7 applications, 6 energy storage, 7 in hydrogen production, 6 in methane production, 6 nutrient removal and recovery, 7 as sensors, 10 synthesis of value-added chemicals from carbon dioxide, 6 water desalination, 6 bioanodes, 2 cathodes, 8 charge transfer, 3, 4f, 5

direct, 4 direct contact transfer, 5 mediated, 4 using redox mediators, 4 composition of electrode materials, 1 earliest investigations of, 2 electron flow in, 3 high-volume, 13 membranes in, 8, 9f common types of, 8 modifications, 8 strategies to prevent (bio)fouling, 9 metal recovery in, 7 in natural environments, 2 operation of, 3 parameters for characterization of, 11t, 12 biofilm-electrode interface, 11 biological, 10 electrochemical, 11 open circuit potential, 11 oxidation/reduction signals, 11 physicochemical, 10 possible interactions between parameters, 12 product/substrate yield, 12 for pollutant removal, 3 use of wastewater as fuel, 3 Bioelectrocommunication systems nanostructure, 189 theory background, 188 Biofilms, 140 in bioelectrode preparation, 5 electrode system, 2 erosion of, 5 heterogeneous, 5 from wastewater, 6f Biological oxygen demand (BOD), 31 Biosensor amperometric, 70 capacitive, 31 conductometric, 70 electrochemical, 50 393

394

Index

glucose, 31 potentiometric, 70 Bottom-up strategy, 329

Deltaproteobacteria, 5 Direct electron transfer, 144, 188, 299f mechanism, 298 Dual-chamber MFC (DCMFC), 136

Electrochemically active bacteria, 24, 101 Electrochemical mediators, 299, 301 Electrode, 377 Electrode–bacteria interactions, 228 carbon-based cathode materials, 229 cathode of MES, 229 nanomodified carbon-based cathode materials, 231 photo-active semiconductorsmodified cathode, 235 Electrode–electrolyte systems, 2 Electrode fabrication, 178, see also Microbial fuel cell (MFC) Electrode modification strategies, 3 Electrode potential, 1 Electrogenic microorganisms, 341 Electrohydrogenesis of microbial electrolysis, 22 Electrolysis cell, 1 electric current flowing in, 1 Electromotive force, 1 Electron impedance spectroscopy (EIS), 31 Electron transfer extracellular electron transfer, 227 mechanism, 223, 227 Electrotrophs, 138 Energy security, 295 Enzymatic fuel cells, 133, 382 Epitaxial growth, 331 Epoxy nanocomposites, 86 Equivalent electrical circuit (EEC) models, 26f, 31 energy/charge storage behavior, 31 measurements, 25f, 31 Exoelectrogens, 138 Extracellular electron transfer (EET), 58, 142, 192, 221

E

F

Electrical double layer, 140 Electroactive anode respiring bacteria (EARB), 64 Electrochemical biosensor, 50 Electrochemical cell design, 9 criteria, 10 electrochemical engineering approach, 10 flat plate designs, 10 formation of baffles, 10 single-chamber membrane-free design, 10 Electrochemical cells, 2 Electrochemical exfoliation of graphite, 333 Electrochemical impedance spectroscopy, 11, 27

Fuel cell, 1 anode, 1 cathode, 1 electric current produced, 1 oxidation and reduction reactions, 1

C Capacitive biosensors, 31 Carbonaceous nanomaterials, 86, 96 Carbon-based nanomaterials, 378 synthesis methods, 329 Carbon nanotubes, 82, 92, 206 anodes, 83f bamboo-like nitrogen doped CNT, 82 carbon-fiber paper, 82 CNT-coated substrates, 83 functionalized, 84 Catalytic nanomaterials, 98t Cationic membranes, 8 Cellulose, 2 Chemical kinetics reaction mechanisms, 255 Chemical reduction of metal precursor salts, 85 Chemical vapor deposition (CVD), 331 Chitin, 2 Chitosan, 92 Chronoamperometry, 12 Conductive polymers, 92 Conductometric biosensor, 70 Constructed Wetlands-MFC, 69 Coulombic efficiency, 146 Cytochrome-type enzyme complexes, 5

D

G Gibbs free energy for redox reaction, 22 Glucose biosensor, 31 Gold nanoparticles (AuNP), 207 Graphene, 85, 331f, 364 aerogel, 85

Index

based composite materials, 379 based nanomaterials, 336 cetyltrimethylammonium bromide, 86 effects, 85 modified anodes, 85, 339 nanosheets, 86 oxide nanoribbons, 85 synthesis of metal oxides, 370 Graphene oxide, 85 Graphite rods, 307

H Hummers method, 367 Hydrogen bioelectrosynthesis of, 3 peroxide generation, 342 as sustainable energy, 34 Hydrothermal method, 363

I Indirect electron transfer, 188, 300f Industrial-scale reactor, 305 Ion exchange capacity (IEC), 147 Ion exchange membranes (IEM), 261 chemical stability, 265 diffusion, 263 electrical resistance, 264 fundamental properties, 262 ion exchange capacity, 264 ionic transport across the membrane, 262 mechanical strength, 265 melt mixing, 267 membrane potential, 262 nanomaterials, 265 nanomaterials incorporation, 266 osmosis and electro-osmosis, 264 permselectivity, 263 in situ polymerization, 266 in situ sol–gel, 267 solution blending, 266 transport number, 263 types, 262

L Lagoon process techniques, 57 Lifshitz–van der Waals, 140 Linear and cyclic voltammetry, 11 Liquid-phase synthesis methods, 83

M Mediated electron transfer (MET), 144 Metal based nanomaterials, 379 Metal organic frames, 380, 388 Microbial bioelectrodes, 1 Microbial capacitive deionization cells, 67, 68f Microbial communities, 28f Microbial desalination cells (MDC), 6, 22, 23f, 67f, 136, 196 desalinization chamber of, 22 principle, 135f produce electricity, 136 types, 66 use of electroactive internal sludge, 22 Microbial electrochemical systems (MES), 19, 20f, 57, 58, 59f, 64f anode and cathode, 19 bottlenecks and troubleshooting involved, 72, 73 classification, 62 based on application, 62 based on electron transfer, 62 primary electrochemical systems, 62 secondary electrochemical systems, 62 interface of biology and electrochemistry, 59 microbial electrolysis cells, 20 microbial electrosynthesis cells, 20 microbial fuel cells, 20 microorganisms used in, 19 principles, 32 scalable forum, 57 scaling up, 60 Microbial electrochemical technology (MET), 196 Microbial electrogenesis, 27 applications, 29f as biosensor, 31f, 31 COD removal, 29 hydrogen generation, 30 sludge treatment, 29 toxicity detection in heavy metal polluted water, 31 wastewater treatment and energy generation, 27, 30t Microbial electrolysis cells (MECs), 2, 3, 12, 20, 22f, 35f, 36, 136, 196, 221, 301, 341 cathodic reactions under acidic conditions, 35 under alkaline conditions, 35 efficiencies, 22, 30t exoelectrogens, 35

395

396

Index

functions of cathodes in, 8 use of hydrogen in, 34 Microbial electrosynthesis, 20, 23f, 196, 221 advantages of, 24 applications of, 25 bacterial pure cultures and their strains, 25 electron transfer mode, 24, 25f investment and technology intensity of, 24 microbes used in, 24, 24t redox mediators, 24 Microbial fuel cell (MFC), 169, 172 anode modification, 151 bacteria in, 138 biofilm, 140 carbon materials, 151 categories, 138 cathode modification, 157 cathodes in, 142 components, 136 conventional, 136 electrode, 141 electrode fabrication, 178 electrode materials, 141 electron transport mechanisms, 144 enhance electricity output, 138 future perspective, 160 future prospects, 181 ion exchange capacity, 147 membranes conductivity, 149 membranes modification, 159 metal nanoparticles, 153 microfluidic, 136 microorganisms, 138 miniStack, 140 nanocomposite materials, 174 nanomaterials, 150 osmotic, 136 oxygen permeability, 147 performance, 140 plant, 136 polyelectrolyte modified NM, 156 polymers, 156 proton exchange membranes, 176 sediment, 136 transition metal-based nanoparticles, 155 up-flow, 136 zinc-modified nanoparticles, 210 Microbial fuel cells (MFCs), 2, 12, 59, 81, 221, 295, 334, 357, 377

advantages of, 50 based wastewater treatment treatment, 295 basic principle of, 65, 66f bioeconomy, 301 challenges, 47 characteristics and performance, 42, 43 commercial applications, 46 components, 377 design, 45t electrode materials for, 360t functions of cathodes in, 8 hydraulic capacity, 39, 39t, 40 interactions, 27 manufacturing cost, 48 in marine environments, 2 modeling of scaled-up plants, 46 normalized energy recovery, 20, 21 operation parameters, 36, 37 optimization of, 38, 40 output, 27 oxygen reduction reaction (ORR), 81 performance, 306 power outputs of, 20 principles, 32 proton exchange membrane (PEM), 20 prototypes, 34f significance, 304f techno-economic assessment of, 305 technology, 381, 387, 389 two-chamber, 81 Microbial remediation cells (MRCs), 68 Most probable number (MPN), 27

N Nafion, 8, 47 Nanocomposite materials, 82, 83f, 83, 88, 379 challenges, 94 chemical reduction, 85 electrochemical characterization chronoamperometry, 94 cyclic voltammetry, 94 electrochemical impedance spectroscopy, 94 hydrothermal synthesis method, 84 for improving anode performances, 93 for bacteria immobilization, 93 formation of biofilm, 93 of polymer nanocomposites, 86 sol–gel, 85 sonochemistry, 86

Index

thermogravimetric analysis, 93 transformation of properties, 87 using microwaves, 86 Nanomaterials anode modification, 151 in biocell, 205 carbon-based nanomaterials, 267 carbon materials, 151 carbon nanotubes, 206 cathode modification, 157 gold nanoparticles, 207 graphene, 268 in ion exchange membranes, 267 membrane modification, 159 metal nanoparticle, 272 metal nanoparticles, 153 in microbial fuel cell, 150 nanomaterial incorporated, 273 oxide-based nanomaterials, 270 performance, 272 polyelectrolyte modified NM, 156 polymers, 156 silver nanoparticles, 208 synthesis approaches, 330f transition metal-based nanoparticles, 155 zinc-modified nanoparticles, 210 Nanomaterials supporting oxygen reduction bio-electrochemical system, 248 carbon-based nanomaterial catalyst, 249 chemical exfoliation, 247 chemical vapor deposition, 244 graphite catalyst, 250 hydrothermal process, 246 material characterization, 247 material synthesis, 244 mechanical exfoliation, 247 sol–gel method, 247 Nano-molybdenum carbide, 93 Nanoparticles-based electrodes anodes, 96, 97 cathodes, 97, 98, 99, 100 design of catalysts, 99 nonmetallic nanoparticles and metallic nanoparticles, 98 performance parameters, 99, 101 power density, 101 stability, 101 structures of carbon materials, 99 Natural endogenous mediator, 144

Nitrogen-doped graphene sheets, 86 Nonmetallic nanoparticles, 98

O Organic matter oxidation, 296 Oswald maturation process, 85 Oxidation of bacterial metabolism product, 298 Oxidation reaction, 2 in fuel cells, 1 Oxygen permeability, 147 Oxygen reduction reaction (ORR), 174, 205, 358

P Photo-bioelectrochemical fuel cell (PBFC), 136 Platinum-doped carbon electrodes, 3 Pollutants, 327 Polymer-based nanomaterial catalyst, 253 carbon-based nanomaterial composite catalyst, 254 polymer–CNT catalyst, 253 polymer–graphene catalyst, 254 Proton exchange membrane (PEM), 146, 169, 176, 196, 296, see also Microbial fuel cell (MFC) Pt-based cathodes, 359 Pyrolysis, 332

R Redox shuttles, 144 natural endogenous mediator, 144 synthetic exogenous mediator, 144 Reduction reaction in fuel cells, 1

S Scale up technology, 387 Scanning electron microscopy (SEM), 91 Silver nanoparticles, 208 composite, 207 Single chamber microbial fuel cell (SCMFC), 136, 149 Solvent-based exfoliation, 332 Solvothermal method, 332, 363 Sonochemistry, 86 Starch, 2 Sulfonated polystyrene-ethylene-butylene-polystyrene (SPSEBS) nanocomposite membrane, 177 Synthetic exogenous mediator, 144

397

398

Index

T Transition metal, 87, 91 modified carbonaceous anodes, 87 oxide, 91 Transmission electron microscopy (TEM), 92 Two-electron pathway, 340

Wastewater treatment techniques, 27, 30t biological treatment, 61 conventional systems and microbial electrochemical systems, comparison, 61, 63t reasearch on, 60 for safe drinking water, 61 Water desalination, 6

W Wastewater effluents, 357 Wastewater treatment plants, 172

X X-ray diffraction (XRD), 91