Materials for Hydrogen Production, Conversion, and Storage 1119829348, 9781119829348

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Materials for Hydrogen Production, Conversion, and Storage

Scrivener Publishing 100 Cummings Center, Suite 541J Beverly, MA 01915-6106 Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Materials for Hydrogen Production, Conversion, and Storage

Edited by

Inamuddin Tariq Altalhi Sayed Mohammed Adnan and

Mohammed A. Amin

This edition first published 2023 by John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA and Scrivener Publishing LLC, 100 Cummings Center, Suite 541J, Beverly, MA 01915, USA © 2023 Scrivener Publishing LLC For more information about Scrivener publications please visit www.scrivenerpublishing.com. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. Wiley Global Headquarters 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Limit of Liability/Disclaimer of Warranty While the publisher and authors have used their best efforts in preparing this work, they make no rep­ resentations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchant-­ ability or fitness for a particular purpose. No warranty may be created or extended by sales representa­ tives, written sales materials, or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further informa­ tion does not mean that the publisher and authors endorse the information or services the organiza­ tion, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Library of Congress Cataloging-in-Publication Data ISBN 9781119829348 Front cover images supplied by Wikimedia Commons Cover design by Russell Richardson Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines Printed in the USA 10 9 8 7 6 5 4 3 2 1

Contents Preface xxi 1 Transition Metal Oxides in Solar-to-Hydrogen Conversion 1 Zuzanna Bielan and Katarzyna Siuzdak 1.1 Introduction 2 1.2 Solar-to-Hydrogen Conversion Processes Utilizing Transition Metal Oxides 3 1.2.1 Photocatalysis 3 1.2.2 Photoelectrocatalysis 5 1.2.3 Thermochemical Water Splitting 6 1.3 Transition Metal Oxides in Solar-to-Hydrogen Conversion Processes 7 1.3.1 Photocatalysis and Photoelectrocatalysis 7 8 1.3.1.1 TiO2 1.3.1.2 α-Fe2O3 16 1.3.1.3 CuO/Cu2O 20 1.3.2 Thermochemical Water Splitting 23 24 1.3.2.1 Fe3O4/FeO Redox Pair 1.3.2.2 CeO2/Ce2O3 and CeO/CeO2-δ Redox Pairs 25 1.3.2.3 ZnO/Zn Redox Pair 27 1.4 Conclusions and Future Perspectives 28 References 29 2 Catalytic Conversion Involving Hydrogen from Lignin Satabdi Misra and Atul Kumar Varma List of Abbreviations 2.1 Introduction 2.1.1 Background of Bio-Refinery and Lignin 2.1.2 Lignin as an Alternate Source of Energy 2.1.3 Lignin Isolation Process 2.2 Catalytic Conversion of Lignin

41 41 42 42 44 45 45 v

vi  Contents 2.2.1 Lignin Reductive Depolymerization into Aromatic Monomers 47 2.2.2 Catalytic Hydrodeoxydation (HDO) of Lignin 48 2.2.3 Hydrodeoxydation (HDO) of Lignin-Derived-Bio-Oil 51 Summary and Outlook 52 References 53 3 Solar–Hydrogen Coupling Hybrid Systems for Green Energy Bilge Coşkuner Filiz, Esra Balkanli Unlu, Hülya Civelek Yörüklü, Meltem Karaismailoglu Elibol, Yağmur Akar, Ali Turgay San, Halit Eren Figen and Aysel Kantürk Figen 3.1 Concept of Green Sources and Green Storage 3.2 Coupling of Green to Green 3.3 Solar Energy–Hydrogen System 3.3.1 Photoelectrochemical Hydrogen Production 3.3.1.1 PEC Materials 3.3.1.2 Photoelectrochemical Systems 3.3.2 Electrochemical Hydrogen Production 3.3.2.1 Polymer Electrolyte Membrane Electrolysis Cell (PEMEC) 3.3.2.2 Alkaline Electrolysis Cell (AEC) 3.3.2.3 Solid Oxide Electrolysis Cell (SOEC) 3.3.3 Fuel Cell 3.3.4 Photovoltaic 3.4 Thermochemical Systems 3.5 Photobiological Hydrogen Production 3.6 Conclusion References

65

66 67 67 68 70 73 74 75 76 77 78 79 80 82 84 85

4 Green Sources to Green Storage on Solar–Hydrogen Coupling 97 A. Mohan Kumar, R. Rajasekar, P. Sathish Kumar, S. Santhosh and B. Premkumar 4.1 Introduction 98 4.1.1 Hybrid System 99 101 4.2 Concentrated Solar Thermal H2 Production 4.3 Thermochemical Aqua Splitting Technology 103 for Solar–H2 Generation 4.4 Solar to Hydrogen Through Decarbonization of Fossil Fuels 105 4.4.1 Solar Cracking 106 4.5 Solar Thermal-Based Hydrogen Generation Through Electrolysis 107 4.6 Photovoltaics-Based Hydrogen Production 107

Contents  vii 4.7 Conclusion References 5 Electrocatalysts for Hydrogen Evolution Reaction R. Shilpa, K. S. Sibi, S. R. Sarath Kumar, R. K. Pai and R.B. Rakhi 5.1 Introduction 5.2 Parameters to Evaluate Efficient HER Catalysts 5.2.1 Overpotential (o.p) 5.2.2 Tafel Plot 5.2.3 Stability 5.2.4 Faradaic Efficiency and Turnover Frequency 5.2.5 Hydrogen Bonding Energy (HBE) 5.3 Categories of HER Catalysts 5.3.1 Noble Metal-Based Catalysts 5.3.2 Non-Noble Metal-Based Catalysts 5.3.3 Metal-Free 2D Nanomaterials 5.3.4 Transition Metal Dichalcogenides 5.3.5 Transition Metal Oxides and Hydroxides 5.3.6 Transition Metal Phosphides 5.3.7 MXenes (Transition Metal Carbides and Nitrides) Conclusion References

109 110 115 116 117 117 118 119 119 120 121 121 125 126 129 130 132 132 134 134

6 Recent Progress on Metal Catalysts for Electrochemical Hydrogen Evolution 147 Tejaswi Jella and Ravi Arukula 6.1 Introduction 148 6.1.1 Type of Water Electrolysis Technologies 148 6.1.1.1 Alkaline Electrolysis (AE) 149 6.1.1.2 Proton Exchange Membrane Electrolysis (PEME) 149 6.1.1.3 Solid Oxide Electrolysis (SOE) 149 6.2 Mechanism of Hydrogen Evolution Reaction (HER) 149 6.2.1 Performance Evaluation of Catalyst 151 6.3 Various Electrocatalysts for Hydrogen Evolution Reaction (HER) 153 6.3.1 Noble Metal Catalysts for HER 153 6.3.1.1 Platinum-Based Catalysts 153 6.3.1.2 Palladium Based Catalysts 155 6.3.1.3 Ruthenium Based Catalysts 157 6.3.2 Non-Noble Metal Catalysts 158

viii  Contents 6.3.2.1 Transition Metal Phosphides (TMP) 6.3.2.2 Transition Metal Chalcogenides 6.3.2.3 Transition Metal Carbides (TMC) 6.4 Conclusion and Future Aspects References

158 162 163 164 165

7 Dark Fermentation and Principal Routes to Produce Hydrogen 181 Luana C. Grangeiro, Bruna S. de Mello, Brenda C. G. Rodrigues, Caroline Varella Rodrigues, Danieli Fernanda Canaver Marin, Romario Pereira de Carvalho Junior, Lorena Oliveira Pires, Sandra Imaculada Maintinguer, Arnaldo Sarti and Kelly J. Dussán 7.1 Introduction 182 7.2 Biohydrogen Production from Organic Waste 183 7.2.1 Crude Glycerol 186 7.2.1.1 Dark Fermentation of Crude Glycerol to Biohydrogen and Bio Products 187 7.2.2 Dairy Waste 189 7.2.2.1 Dark Fermentation of Dairy Waste to Biohydrogen and Bioproducts 190 7.2.3 Fruit Waste 193 7.2.3.1 Dark Fermentation of Fruit Waste to Hydrogen and Bioproducts 194 7.3 Anaerobic Systems 198 7.3.1 Continuous Multiple Tube Reactor 206 7.4 Conclusion and Future Perspectives 209 Acknowledgements 210 References 210 8 Catalysts for Electrochemical Water Splitting for Hydrogen Production Zaib Ullah Khan, Mabkhoot Alsaiari, Muhammad Ashfaq Ahmed, Nawshad Muhammad, Muhammad Tariq, Abdur Rahim and Abdul Niaz 8.1 Introduction 8.2 Water Splitting and Their Products 8.3 Different Methods Used for Water Splitting 8.3.1 Setup for Water Splitting Systems at a Basic Level 8.3.2 Photocatalysis 8.3.3 Electrolysis

225

226 229 229 229 230 232

Contents  ix 8.4 Principles of PEC and Photocatalytic H2 Generation 8.5 Electrochemical Process for Water Splitting Application 8.5.1 Water Splitting Through Electrochemistry 8.6 Different Materials Used in Water Splitting 8.6.1 Water Oxidation (OER) Materials 8.6.2 Developing Materials for Hydrogen Synthesis 8.6.3 Material Stability for Water Splitting 8.7 Mechanism of Electrochemical Catalysis in Water Splitting and Hydrogen Production 8.7.1 Electrochemical Water Splitting with Cheap Metal-Based Catalysts 8.7.2 Catalysts with Only One Atom 8.7.3 Electrochemical Water Splitting Using Low-Cost Metal-Free Catalysts 8.8 Water Splitting and Hydrogen Production Materials Used in Electrochemical Catalysis 8.8.1 Metal and Alloys 8.8.2 Metal Oxides/Hydroxides and Chalogenides 8.8.3 Metal Carbides, Borides, Nitrides, and Phosphides 8.9 Uses of Hydrogen Produced from Water Splitting 8.9.1 Water Splitting Generates Hydrogen Energy 8.9.2 Photoelectrochemical (PEC) Water Splitting 8.9.3 Thermochemical Water Splitting 8.9.4 Biological Water Splitting 8.9.5 Fermentation 8.9.6 Biomass and Waste Conversions 8.9.7 Solar Thermal Water Splitting 8.9.8 Renewable Electrolysis 8.9.9 Hydrogen Dispenser Hose Reliability 8.10 Conclusion References

232 233 233 233 233 235 235 235 236 236 237 238 238 239 239 240 240 241 241 241 241 242 242 242 242 243 243

9 Challenges and Mitigation Strategies Related to Biohydrogen Production 249 Mohd Nur Ikhmal Salehmin, Ibdal Satar and Mohamad Azuwa Mohamed 9.1 Introduction 249 9.2 Limitation and Mitigation Approaches of Biohydrogen Production 252 9.2.1 Physical Issues and Their Mitigation Approaches 252

x  Contents 9.2.1.1 Operating Temperature Issue and Its Control 252 9.2.1.2 Hydraulic Retention Time (HRT) and Optimization 252 9.2.1.3 High Hydrogen Partial Pressure – Implication and Overcoming the Issue 253 9.2.1.4 Membrane Fouling Issues and Solutions 254 9.2.2 Biological Issues and Their Mitigation Approaches 256 9.2.2.1 Start-Up Issue and Improvement Through Bioaugmentation 256 9.2.2.2 Biomass Washout Issue and Solution Through Cell Immobilization 256 9.2.3 Chemical Issues and Their Mitigation Approaches 257 9.2.3.1 pH Variation and Its Regulation 257 9.2.3.2 Limiting Nutrient Loading and Optimization 257 9.2.3.3 Inhibitor Secretion and Its Control 258 9.2.3.4 Byproduct Formation and Its Exploitation 260 9.2.4 Economic Issues and Ways to Optimize Cost 260 9.3 Conclusion and Future Direction 265 Acknowledgements 266 References 266 10 Continuous Production of Clean Hydrogen from Wastewater by Microbial Usage 277 P. Satishkumar, Arun M. Isloor and Ramin Farnood 10.1 Introduction 278 10.2 Wastewater for Biohydrogen Production 279 10.3 Photofermentation 281 10.3.1 Continuous Photofermentation 283 10.3.2 Factors Affecting Photofermentation Hydrogen Production 286 10.3.2.1 Inoculum Condition and Substrate Concentration 286 10.3.2.2 Carbon and Nitrogen Source 287 10.3.2.3 Temperature 288 10.3.2.4 pH 288 10.3.2.5 Light Intensity 288 10.3.2.6 Immobilization 290 10.4 Dark Fermentation 291 10.4.1 Continuous Dark Fermentation 292

Contents  xi 10.4.2 Factors Affecting Hydrogen Production in Continuous Dark Fermentation 296 10.4.2.1 Start-Up Time 296 10.4.2.2 Organic Loading Rate 296 10.4.2.3 Hydraulic Retention Time 297 10.4.2.4 Temperature 301 10.4.2.5 pH 302 10.4.2.6 Immobilization 302 10.5 Microbial Electrolysis Cell 304 10.5.1 Mechanism of Microbial Electrolysis Cell 304 10.5.2 Wastewater Treatment and Hydrogen Production 305 10.5.3 Factors Affecting Microbial Electrolysis Cell Performance 308 10.5.3.1 Inoculum 308 10.5.3.2 pH 308 10.5.3.3 Temperature 308 10.5.3.4 Hydraulic Retention Time 308 10.5.3.5 Applied Voltage 310 10.6 Conclusions 310 References 311 11 Conversion Techniques for Hydrogen Production and Recovery Using Membrane Separation 319 Nor Azureen Mohamad Nor, Nur Shamimie Nadzwin Hasnan, Nurul Atikah Nordin, Nornastasha Azida Anuar, Muhamad Firdaus Abdul Sukur and Mohamad Azuwa Mohamed 11.1 Introduction 320 11.2 Conversion Technique for Hydrogen Production 321 11.2.1 Photocatalytic Hydrogen Generation via Particulate System 321 11.2.2 Photoelectrochemical Cell (PEC) 324 11.2.3 Photovoltaic-Photoelectrochemical Cell (PV-PEC) 325 11.2.4 Electrolysis 327 11.3 Hydrogen Recovery Using Membrane Separation 329 (H2/O2 Membrane Separation) 11.3.1 Polymeric Membranes 330 11.3.2 Porous Membranes 331 11.3.3 Dense Metal Membranes 332 11.3.4 Ion-Conductive Membranes 333

xii  Contents 11.4 Conclusion Acknowledgements References

335 336 336

12 Geothermal Energy-Driven Hydrogen Production Systems 343 Santanu Ghosh and Atul Kumar Varma Abbreviations 344 12.1 Introduction 345 12.2 Hydrogen – A Green Fuel and an Energy Carrier 347 12.3 Production of Hydrogen 348 12.3.1 Fossil Fuel-Based 348 12.3.2 Non-Fossil Fuel-Based 349 12.4 Geothermal Energy 353 12.4.1 Introductory View 353 12.4.2 Types and Occurrences 354 12.5 Hydrogen Production From Geothermal Energy 355 12.5.1 Hydrogen Production Systems 355 12.5.2 Working Fluids 369 12.5.3 Assimilation of Solar and Geothermal Energy 370 12.5.4 Chlor-Alkali Cell and Abatement of Mercury and Hydrogen Sulfide (AMIS) Unit 372 12.5.5 Hydrogen Liquefaction 374 12.5.6 Hydrogen Storage 375 12.6 Economics of Hydrogen Production 377 12.6.1 A General Overview 377 12.6.2 Economy of Hydrogen Yield Using Geothermal Energy 379 12.7 Environmental Impressions of Geothermal Energy-Driven Hydrogen Yield 381 12.8 Conclusions 382 References 384 13 Heterogeneous Photocatalysis by Graphitic Carbon Nitride for Effective Hydrogen Production 397 Kiran Kumar B., B. Venkateswar Rao, Sashivinay Kumar Gaddam, Ravi Arukula and Vishnu Shanker 13.1 Introduction 398 13.1.1 Typical Heterogeneous Photocatalysis Mechanism 399 13.1.2 Necessity of the Photocatalytic Water Splitting 400 401 13.2 g-C3N4-Based Photocatalytic Water Splitting 13.2.1 Influence of the g-C3N4 Morphology on Photocatalytic Water Splitting 402

Contents  xiii 13.2.1a g-C3N4 Thin Nanosheets-Based Photocatalytic Water Splitting 13.2.1b Porous g-C3N4-Based Photocatalytic Water Splitting 13.2.1c Crystalline g-C3N4-Based Photocatalytic Water Splitting 13.2.2 Metal Doped Photocatalytic Water Splitting 13.2.3 Semiconductor/g-C3N4 Heterojunction for Photocatalytic Water Splitting 13.3 Future Remarks and Conclusion References

402 404 405 406 407 408 409

14 Graphitic Carbon Nitride (g-CN) for Sustainable Hydrogen Production 417 Zaib Ullah Khan, Mabkhoot Alsaiari, Saleh Alsayari, Nawshad Muhmmad and Abdur Rahim 14.1 Introduction 418 14.2 Various Methods for Hydrogen Production 421 14.3 Production of Hydrogen from Fossil Fuels 422 14.3.1 Steam Reforming 422 14.3.2 Gasification 422 14.4 Hydrogen Production from Nuclear Energy 422 14.4.1 Water Splitting by Thermochemistry 422 14.5 Hydrogen Production from Renewable Energies 423 14.5.1 Electrolysis 423 14.5.2 Photovoltaic Solar 423 14.5.3 Wind Method for Producing Hydrogen 423 14.5.4 Biomass Gasification Use for Hydrogen Production 424 14.5.5 Agricultural or Food-Processing Waste that Contains Starch and Cellulose 424 14.6 Preparation of g-C3N4 Materials 425 14.6.1 Sol-Gel Method for Making Graphitic Carbon Nitride 426 14.6.2 Hard and Soft-Template Method 426 14.6.3 Template-Free Method for Making Graphitic Carbon Nitride 428 14.7 Properties of g-C3N4 Materials 429 14.7.1 Stability 429 14.7.1.1 Thermal Stability 429 14.7.1.2 Chemical Stability 430 14.7.1.3 Electrochemical Properties 430

xiv  Contents 14.8 The Advantages of Sustainable Hydrogen Production and Their Applications 14.8.1 Hydrogen Applications 14.9 Hydro Processing in Petroleum Refineries and Their Usage 14.9.1 Hydrocracking 14.9.2 Hydrofining 14.9.3 Ammonia Synthesis 14.9.4 Synthesis of Methanol 14.9.5 Electricity Generation from Hydrogen 14.9.6 Applications for Green Hydrogen 14.9.7 Replacing Existing Hydrogen 14.9.8 Heating 14.9.9 Energy Storage 14.9.10 Alternative Fuels 14.9.11 Fuel-Cell Vehicles 14.10 Conclusion References

430 430 431 431 431 432 433 433 434 434 435 435 435 436 436 436

15 Hydrogen Production from Anaerobic Digestion 441 Muhammad Farhan Hil Me, Mohd Nur Ikhmal Salehmin, Hau Seung Jeremy Wong and Mohamad Azuwa Mohamed 15.1 Introduction 441 15.2 Basic Overview of Anaerobic Digestion 443 15.3 How to Obtain Hydrogen from Anaerobic Digestion 445 15.3.1 Single-Stage Reactor 445 15.3.2 Two-Stage Reactor 445 15.3.3 Feedstock and Resulting Hydrogen 446 15.4 Challenges and Mitigation Strategies in Biohydrogen Production 447 15.4.1 Combating Microbial Competition 447 15.4.2 Enhancing Biohydrogen Production Yield by Technical and Operational Adjustments 448 15.4.3 Minimizing Inhibition by Byproducts from Pretreatments 450 15.4.4 Minimizing Inhibition by Metal Ions 451 15.4.5 Minimizing In-Process Inhibition 452 15.4.5.1 Volatile Fatty Acids and Alcohols 452 15.4.5.2 Ammonia 453 15.4.5.3 Hydrogen 453 15.5 Practicality of Technologies at Industrial Scale 453

Contents  xv 15.6 Conclusion Acknowledgements References 16 Impact of Treatment Strategies on Biohydrogen Production from Waste-Activated Sludge Fermentation Rajeswari M. Kulkarni, Dhanyashree J.K., Esha Varma, Sirivibha S.P. and Shantha M.P. 16.1 Introduction 16.2 Methods of Production of Hydrogen Using WAS 16.2.1 Dark Fermentation 16.2.2 Photofermentation 16.2.3 Microbial Electrolysis Cell 16.3 Physical Treatment Methods 16.4 Chemical Treatment Methods 16.5 Conclusions References

456 456 456 465 466 467 468 469 470 471 486 504 505

17 Microbial Production of Biohydrogen (BioH2) from Waste-Activated Sludge: Processes, Challenges, and Future Approaches 511 Abhispa Bora, T. Angelin Swetha, K. Mohanrasu, G. Sivaprakash, P. Balaji and A. Arun 17.1 Introduction 512 17.2 Hydrogen and Waste-Activated Sludge 513 17.2.1 Hydrogen 513 17.2.2 Waste-Activated Sludge 514 17.3 Mechanisms of Hydrogen Production 514 515 17.3.1 H2 Production by Dark Fermentation Process 516 17.3.2 H2 Production by Photofermentation Process 17.3.3 Using Microbial Electrolysis Cell 518 520 17.4 H2 Production by Microalgae Using Waste 520 17.4.1 Bottlenecks of H2 Production 521 17.4.2 Key Factors Influencing H2 Production 17.5 Recent Endeavors to Enhance H2 Production 522 17.5.1 Recent Advancements in Dark Fermentation 522 17.5.2 Recent Advances in Photofermentation 526 17.5.3 Recent Advances in Microbial Electrolysis Cell 527 17.6 Future Approaches 528 17.7 Conclusion 528 References 529

xvi  Contents 18 Perovskite Materials for Hydrogen Production 539 Surawut Chuangchote and Kamonchanok Roongraung 18.1 Current Problems of Technology for Hydrogen Production 540 18.2 Principle of Perovskite Materials 540 18.2.1 Oxide Perovskite 542 18.2.1.1 Titanate-Based Oxide Perovskite (ATiO3)542 18.2.1.2 Tantalate-Based Oxide Perovskite (ATaO3)544 18.2.1.3 Niobate-Based Oxide Perovskite 545 18.2.2 Halide Perovskite 547 18.2.2.1 Conventional Halide Perovskite 547 18.2.2.2 Lead-Free Halide Perovskites 548 18.3 Synthesis Process for Perovskite Materials 549 18.3.1 Microwaves 550 18.3.2 Sol-Gel 550 18.3.3 Hydrothermal/Solvothermal 551 18.3.4 Precipitation 553 18.3.5 Hot-Injection 553 18.4 Hydrogen Production from Solar Water Splitting 554 18.4.1 Photocatalytic System 555 18.4.2 Photoelectrochemical System 556 18.4.3 Photovoltaic–Electrocatalytic System 559 18.5 Conclusion and Future Perspectives 562 References 563 19 Progress on Ni-Based as Co-Catalysts for Water Splitting 575 Arti Maurya, Kartick Chandra Majhi and Mahendra Yadav 19.1 Introduction 576 19.1.1 Thermodynamic Aspects of Hydrogen Production 577 19.1.2 Different Processes for the Photocatalytic Hydrogen Evolution by Water Splitting 578 19.1.3 Photocatalyst 578 19.1.3.1 Homogeneous Photocatalysis 578 19.1.3.2 Heterogeneous Photocatalysis 579 19.2 Photocatalytic Hydrogen Generation System 581 19.2.1 Electron Donor and Electrolyte/Sacrificial Reagent 581 19.2.2 Loading of Co-Catalyst 581 19.2.3 Photocatalytic Activity Efficiency 583

Contents  xvii 19.3 Semiconductor Materials 584 19.3.1 Oxide-Based Semiconductor and Their Composites 584 19.3.2 Non-Oxide-Based Semiconductor and Their Composites 586 19.3.3 Polymer/Carbon Dots/Graphene-Based and Carbon Nitride-Based Photocatalyst and Their Composites 588 19.4 State of Art for the Nickel Used as Photocatalyst 591 19.5 Progress of Ni-Based Photocatalyst for Hydrogen Evolution 592 19.5.1 Metallic Form of Ni Used as Co-Catalyst 592 19.5.2 Ni-Based Oxide and Hydroxide Used as Co-Catalyst for Hydrogen Production 594 19.5.3 Ni-Based Sulfides Used as Co-Catalyst and Photocatalyst 596 19.5.4 Ni-Based Phosphides Used as Co-Catalyst Towards Hydrogen Production 598 19.5.5 Ni-Based Complex Act as Co-Catalyst for Hydrogen Production 600 19.5.6 Other Ni-Based Co-Catalyst for Hydrogen Production 602 19.6 Conclusion and Future Perspective 608 Author Declaration 609 Acknowledgment 609 References 609 20 Use of Waste-Activated Sludge for the Production of Hydrogen Hülya Civelek Yörüklü, Bilge Coşkuner Filiz and Aysel Kantürk Figen 20.1 Introduction 20.2 WAS to Hydrogen Production 20.2.1 Biohydrogen Production 20.2.1.1 Dark Fermentation 20.2.1.2 Photofermentation 20.2.1.3 Microbial Electrolysis Cell 20.2.2 Thermochemical Hydrogen Production 20.2.2.1 Pyrolysis 20.2.2.2 Gasification 20.2.2.3 Super Critical Water Gasification

625 626 629 629 629 632 634 635 636 639 643

xviii  Contents 20.3 Conclusion Remarks References 21 Current Trends in the Potential Use of the Metal-Organic Framework for Hydrogen Storage Maryam Yousaf, Muhammad Ahmad, Zhi-Ping Zhao, Tehmeena Ishaq and Nasir Mahmood 21.1 Introduction 21.2 Structure of MOFs 21.3 Mechanism of H2 Storage by MOFs 21.4 Strategies to Modify the Structure of MOFs for Enhanced H2 Storage 21.4.1 Tuning the Surface Area, Pore Size, and Volume of MOFs 21.4.2 Enhancement in Unsaturated Open Metal Sites 21.4.3 MOFs with Interpenetration 21.4.4 Linker Functionalization of MOFs 21.4.5 Hybrid and Doping of MOFs 21.5 Conclusions and Future Recommendations Acknowledgement References 22 High-Density Solids as Hydrogen Storage Materials Zeeshan Abid, Huma Naeem, Faiza Wahad, Sughra Gulzar, Tabassum Shahzad, Munazza Shahid, Muhammad Altaf and Raja Shahid Ashraf 22.1 Introduction 22.2 Metal Borohydrides 22.2.1 Lithium Borohydride 22.2.2 Sodium Borohydride 22.2.3 Potassium Borohydride 22.3 Metal Alanates 22.3.1 Lithium Alanate 22.3.2 Sodium Alanate 22.4 Ammonia Boranes 22.5 Metal Amides 22.5.1 Lithium Amide 22.5.2 Sodium Amide 22.6 Amine Metal Borohydrides 22.6.1 Amine Lithium Borohydrides 22.6.2 Amine Magnesium Borohydrides

645 646 655 656 657 659 661 661 663 665 667 668 674 675 675 681

682 683 683 685 687 688 688 690 691 693 693 694 696 696 697

Contents  xix 22.6.3 Amine Calcium Borohydrides 22.6.4 Amine Aluminium Borohydrides 22.7 Conclusion References

698 699 699 699

Index 707

Preface

The extensive awareness and environmental concern are driving the global civilization towards cleaner and green energy production. This ultimately leaves no option other than using hydrogen as a fuel that has almost no adverse environmental impact. But hydrogen poses several hazards in terms of human safety as its mixture of air is prone to potential detonations and invisible fires. The permeability of cryogenic storage can induce frostbite as it leaks through metal pipes. In short, there are a lot of challenges at every step to strive for emission-free fuel. As the density of hydrogen is very low, efficient methods are being developed and engineered to store it in a small volume. Hydrogen can leak at a rate as low as 4 μg/sec to catch fire hazards and thus its detection poses a serious challenge both in terms of safety and expense. Both renewal and non-renewal sources are targeted as feedstocks for the production of hydrogen. The non-renewal feedstocks mainly of petroleum are the major contributor to date but there is a future perspective in renewal source comprising mainly of water splitting via electrolysis, radiolysis, thermolysis, photocatalytic water splitting, and biohydrogen routes which are being extensively worked out. When American physicist Richard Feynman said, “There is plenty of room at the bottom”, material science filled plenty of scope for improved properties that can be exploited to overcome the enormous challenge of harnessing energy from hydrogen.This book edition mainly targets the current and future material for the production, conversion, and storage of the cleaner fuel – hydrogen. The scope and limitations both in terms of engineering and cost have been discussed. Materials for Hydrogen Production, Conversion, and Storage describes mainly the production of hydrogen from various sources along with the protagonist materials involved. Further, the extensive and novel material involved in conversion technologies is discussed. The book also covered the details of storage materials of hydrogen for both physical and chemical systems. This book should be useful for engineers, environmentalists, xxi

xxii  Preface governmental policy planners, non-governmental organizations, faculty, researchers, students from academics, and laboratories that are linked to various functional materials related to hydrogen production, conversion, and storage capacity. Based on the book’s objective, this issue edition is divided into 22 chapters: Chapter 1 summarizes the possibility of hydrogen production from water in the solar-driven processes in the presence of transition metal oxides. Photo(electro)catalytic and thermochemical paths are described, with detailed characteristics, challenges, and problems. Lastly, future possibilities of the most popular metal oxide-based semiconductors are covered. Chapter 2 discusses the role of lignin as a renewable and sustainable energy source and its valorization through feasible methods. This chapter mainly focuses on the catalytic conversion of lignin into value-added fuels which has the potential to meet the energy gap between the demand and supply of conventional fossil fuels. Chapter 3 details various solar-hydrogen coupling hybrid systems for green energy applications. Photo-, electro-, thermo-, and bio-chemical solar systems to hydrogen production are also discussed. The classification of these systems, their fundamentals, and their components is presented as well, in addition to the future perspective for green energy applications. Chapter 4 includes various methods of conversion of solar energy into hydrogen. This includes concentrated solar thermal H2 production; thermo-chemical aqua splitting technology for solar-H2 production; solar-H2 through de-carbonization of fossil fuels; solar cracking; and solar thermal-based hydrogen generation through electrolysis and photovoltaic based hydrogen production. Chapter 5 encompasses the role of electrocatalysts in electrocatalytic water splitting hydrogen evolution reaction. The basic mechanism of hydrogen evolution reaction and the significant parameters that qualify an efficient electrocatalyst are discussed. Various state-of-art catalysts for electrocatalytic generation of hydrogen through water splitting are also discussed. Chapter 6 mainly focuses on the modern advancements in the composition and formulating of nanostructured catalysts of noble/non-noble metal-based materials for hydrogen evolution reactions (HER). The key challenges, perspectives, and opportunities for developing new catalysts for efficient electrochemical water splitting are also discussed. Chapter 7 presents the biohydrogen production associated with the generation of secondary metabolites through dark fermentation. Details of principal metabolic pathways from specific organic wastes and principal microbiota involved are discussed. Additionally, it shows bioreactor

Preface  xxiii projects’ main advances in biomass and operational optimization in wastewater-fed bioH2-producing systems. Chapter 8 describes the process of electrocatalytic water splitting for hydrogen production. The electrocatalyst foundations for water splitting, as well as the characteristics of a good electrocatalyst for hydrogen, are also discussed. Chapter 9 highlights the prevailing issues associated with bioreactor operation and the recent advancement in alleviating the challenges of biohydrogen production. Four challenges are identified and discussed, namely physical, biological, chemical, and economical. Chapter 10 addresses various microbes used in continuous hydrogen production from a large array of wastewaters. Photo-fermentation, dark fermentation, and microbial electrolytic cells are discussed in detail. Continuous hydrogen production is emphasized. Factors that affect hydrogen yield and hydrogen production rate are also discussed. Chapter 11 reviews several conversion techniques for hydrogen evolution by water splitting using photocatalysis, photoelectrocatalysis, and photovoltaic-photoelectrochemical systems. On top of that, several types of membrane separation for hydrogen recovery are also discussed. Chapter 12 emphasizes the applications of geothermal energy for hydrogen production that can be used as the principal energy carrier in the upcoming hydrogen era. The methods of hydrogen synthesis, thermodynamic efficiencies, economy, and environmental impacts are elaborated. Hence, this chapter brushes a portrait of a hydrogen-based greener sustainable future. Chapter 13 provides the current advancements in design and morphology changes of g-C3N4 including porous, crystalline, thin-nanosheets, ­metal-doping/g-C3N4, and semiconductor/g-C3N4 heterogeneous photocatalysts for improving the H2 production by photocatalytic water splitting. Moreover, the fundamental challenges and future outlooks herein photocatalytic water splitting for the evolution of H2 energy are highlighted. Chapter 14 elaborates the sustainable production of hydrogen by using graphitic carbon nitride (g-C3N4), as the utilization of g-CN in H2 with high specific surface area transformations, power modules, sun-­oriented cells, supercapacitors, and lithium batteries offers new freedoms. This record gives an examination of the effect of ecological testing on hydrogen-producing innovation from sustainable and non-renewable ­ sources, with an accentuation on its utilization. Chapter 15 recapitulates the fundamentals behind anaerobic digestion to produce hydrogen and highlighted the challenges and mitigation strategies

xxiv  Preface in biohydrogen production. Finally, the practicality of anaerobic digestion technologies at an industrial scale is discussed. Chapter 16 presents information about the synthesis of hydrogen as an alternative to fossil fuel from abundantly available waste-activated sludge. Dark fermentation, photo fermentation, and microbial electrolysis cell methods used for hydrogen production are also discussed. Moreover, this chapter also explains various physical, chemical, and physicochemical treatments adopted to produce hydrogen along with the process conditions maintained. Chapter 17 briefly describes the disadvantages of using fossil fuels. Recently, BioH2 is considered as an alternative for fossil fuels as it can be generated from renewable sources like biomass and wastes. This chapter concentrates on the prospective use of waste-activated sludge as raw material for H2 generation. Chapter 18 enumerates the basic principle of perovskite materials, including the structure of oxide and halide perovskites with the synthesis processes. Various modifications of the perovskite materials are discussed. The recent developments in solar water splitting for hydrogen production, including photocatalysis, photoelectrochemical, and photovoltaic-electrocatalysis are reviewed in this chapter. Chapter 19 briefly discusses the mechanism involved in hydrogen production with the help of a photocatalyst. Additionally, the role of co-catalyst and sacrificial reagent are discussed. Also, previously reported different nickel/ nickel-based photocatalysts for hydrogen production are discussed in detail. Chapter 20 explains the concept of waste-activated sludge used for the production of hydrogen-based on thermochemical and biological processes. The potential strategies and prospects of thermochemical and biological processes for hydrogen energy systems are well compared and presented based on their advantages, drawbacks, and future feasibility. Chapter 21 showcases hydrogen storage potential and the general mechanism involved in hydrogen storage by metal-organic frameworks (MOFs). Furthermore, the effect of structural modifications of MOFs to enhance their H2 storage capacities is discussed. Future recommendations are also outlined to overcome existing drawbacks in MOFs structure to make them acceptable for commercial H2 storage. Chapter 22 presents an overview of the most prominent high-density solids that are potential hydrogen storage materials and are anticipated as key enablers for the hydrogen economy. The aspects of hydrogen storage capacity, kinetics, and thermodynamics are briefly discussed for each class of materials in addition to their limitations and performance enhancement techniques.

Preface  xxv Highlights: • Provides a broad overview of present and upcoming materials for the hydrogen generation, conversion, and storage • Introduces the readers and professionals with a solid foundation in the broad and expanding field of hydrogen generation, conversion, and storage • Explores current procedures used in the production of hydrogen • Details of hydrogen as an alternate source of energy from fossil fuels, water resources, and biomass Inamuddin Tariq Altalhi Sayed Mohammed Adnan Mohammed A. Amin

1 Transition Metal Oxides in Solarto-Hydrogen Conversion Zuzanna Bielan and Katarzyna Siuzdak* Centre of Plasma and Laser Engineering, The Szewalski Institute of Fluid-Flow Machinery, Polish Academy of Science, Gdańsk, Poland

Abstract

Taking into account rapid technological development and growing global Earth population, the needs for clean energy sources are still unmet. Realizing the limited access to fossil fuels as well as the care for natural environment, the focus onto the clean hydrogen generation is fully justified. Due to its high energy capacity and ecofriendly water as a combustion product, hydrogen can be regarded as an ideal fuel. Moreover, the hydrogen production can be realized via water hydrolysis supported by the solar radiation regarded as clean, inexpensive, and renewable power source available everywhere. Among others, transition metal oxides can be used as materials that capture light and then carry out the solar-to-hydrogen conversion via photo(electro)catalytic or thermochemical path. Recently we can observe enormous numbers of works dedicated to the morphology control at the nanoscale, adding novel functionalities or elaboration of synthesis method that within several minutes provides us nanomaterials capable of decomposing water. In this regard, potential challenges, problems, and recent advances associated with the most popular metal oxide-based semiconductors are discuses in order to further development of efficient large-scale hydrogen-based technologies. Keywords:  Transition metal oxide, solar-to-hydrogen conversion, photocatalysis, photoelectrocatalysis, water splitting, TiO2

*Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (1–40) © 2023 Scrivener Publishing LLC

1

2  Materials for Hydrogen Production, Conversion, and Storage

1.1 Introduction Due to increasing demand on electricity and fuel, the world economy is paying more and more attention to renewable energy sources, such us geothermal, hydro, wind, nuclear, solar, and biofuels [1]. Hydrogen as a green energy carrier dates back to the Industrial Revolution in XVIII century [2]. After the second World War, this element attracts enormous attention due to undoubtedly benefits, including almost zero waste production during using (very low NOx and CO2 emission), ease in storage and transport (liquid and gas form) as well as the highest specific energy contents (higher heating value (HHV) and lower heating value (LHV) equal to 141.8 MJ/kg and 119.96 MJ/kg, respectively) [3–6]. Nevertheless, even if H2 is presented as environmentally friendly fuel, its production is mainly based on fossil fuels. Taking into account the fabrication route, one may distinguish hydrogen of different colors. About 95% of hydrogen is produced via coal gasification (brown) or methane reforming (grey). Blue hydrogen production process is similar to the previously mentioned grey, but the by-product CO2 is captured which eliminates the carbon wastes. However, the most desirable and clean is green hydrogen, the production of which does not emit any greenhouse gasses to the atmosphere. Most of the green hydrogen production processes base on the natural photosynthesis process, which use the solar energy and converts it into chemical forms. In this regard, artificial photosynthesis refers to any solardriven mechanisms, both oxidation and reduction, allowing for producing solar fuel. In the case of hydrogen production, two main groups could be specified: the use of solar generated electricity in water electrolysis process, as well as the direct solar water splitting. The first process, known as “indirect electrolysis” is characterized by 70%–85% conversion efficiency, depending on the type and operation conditions [7, 8] and about 99% of share in total global green hydrogen production [3]. The presented group of processes is mostly represented by photovoltaics (PV) – electrolysis hybrid system [9, 10]. Nonetheless, in such system, the overall price of hydrogen need to be high enough to counterbalance the all costs of production [7]. Aside the costs, the other doubt regarding described electricity-to-­ hydrogen conversion method concerns transformation of one effective energy carrier to another and, further, back into electricity again, which may be questionable, as electricity is a valuable energy carrier itself. In this regard, water splitting by direct use of solar irradiation (often presented as separate group - yellow hydrogen) is seen as a future of hydrogen

Metal Oxides in Solar-to-Hydrogen Conversion  3 production technologies. Furthermore, the efficiency losses due to heat– electricity transformation in this solution do not take place, which works to its advantage [11]. However, current commercially available solar-based hydrogen production processes are characterized by rather low efficiency and scale (with an upward trend from ~5% in 2012 [12] up to 16%–17% in 2020 [13]). Therefore, research efforts are mainly focused on increasing the effectivity of hydrogen generation and usage of renewable energy sources, while maintaining low operating costs. At this point, metal oxides, most favorable from d-block, are an attractive class of materials for water splitting due to their abundance in Earth crust, low production cost, relatively good stability, and rather facile preparation procedure, allowing for their large-scale demand [14]. Therefore, the presented work is focused on the most recent approaches in the field of transition metal oxides applied in solar-to-hydrogen conversion, their impact onto the efficiency of water splitting reactions, as well as modifications made to increase the hydrogen percentage share in total global energy demand.

1.2 Solar-to-Hydrogen Conversion Processes Utilizing Transition Metal Oxides Regarding direct solar-to-hydrogen conversion one can distinguish following processes: photobiological [15], thermochemical [16], as well as photo(electro)catalytic [17, 18]. In photobiological water splitting, hydrogen is generated through the biological changes, in the presence of hydrogen-­ catalyzing enzymes produced by green algae as well as photosynthetic bacteria [19] and no additional transition metal oxides are required in the system. In this regard, photobiological processes were excluded from the further discussion.

1.2.1 Photocatalysis To initiate the photocatalytic reaction, an appropriate wavelength must be absorbed by semiconductor particle. The wavelength is determined by the width of the bandgap (Eg) and the energy difference between the conduction band (CB) and valence band (VB). For effective photoexcitation and promotion of electrons from VB to CB, the photons’ energy should be equal or even higher than photocatalyst’s bandgap energy. Nevertheless, excitation of electron is only a first step. For efficient hydrogen generation

4  Materials for Hydrogen Production, Conversion, and Storage via photocatalytic water splitting, the CB of semiconductor must be more negative than the hydrogen evolution level (HER; H+/H2 E = 0 V/NHE) [20], as it is presented in Figure 1.1. In this regard, confirmed high photocatalytic activity towards water/air purification does not determine water splitting reaction, while for effective ●OH radical production VB of semiconductor must be more positively charged than water oxidation level (OER; O2/H2O E = 1.23 V/NHE). Nonetheless, bandgap position is only one among many other factors determining effective photocatalytic water splitting. Takanabe in his work

Conduction band

-1

H2 light

Band gap

Vacuum

V/NHE

H+

electron

+1

-4.5

H+/H2

0

1.23 eV O2/H2O

+2

H2O

Valence band

hole

+3

O2

Photocatalyst

Figure 1.1  Fundamental mechanism of semiconductor-based photocatalytic water splitting for hydrogen generation. (Reproduced from Chen et al. [21]). Time scale

fs

ps

ns

μs

Exciton separation: -Excition binding energy -Dielectric contsant

Photon absorption: -Band gap -Band positions -Direct/indirect bandgap -Absorption coefficient -Optical penetration depth -Refractive index -Scattering/reflection

s

Carrier transport: H2 + 1/2O2 -Conductivity/resistivity -Space charge layer/Depletion width Overall efficiency -Flatband potential -IPCE/QE -Surface state/potential determining ions -STH

6

4

2 1

ms

3

Carrier diffusion: -Effective mass of carriers -Carrier lifetime -Carrier mobility -Diffusion length

5

H2O Mass transfer -iR drop -pH gradient -Diffusion -Viscosity -Effective ion size -Activity coeff icient

Catalytic efficiency -Electrocatalytic activity (Exchange current density) -Transfer coefficient -Tafel slope -Activation energy -Charge transfer resistance

Figure 1.2  Parameters associated with efficient photocatalytic water splitting. (Reproduced from Takanabe [22]).

Metal Oxides in Solar-to-Hydrogen Conversion  5 [22] listed six crucial parameters, including photon absorption, exciton separation, carrier diffusion, carrier transport, catalytic efficiency, and mass transfer (see Figure 1.2). Presented factors overlap with each other like gears; lower yield on one step results in the decrease of overall effectivity.

1.2.2 Photoelectrocatalysis Similar to the photocatalysis reaction, in photoelectrocatalysis (PEC), first, the semiconductor’s electron needs to be excited from VB to CB by photon adsorption. Excited electrons and created holes migrate separately to the surface of electrodes where they, at the electrode-electrolyte interface, participate in the water splitting reactions with the absorbed reactants [23]. Nonetheless, in the opposite to typical photocatalysis reaction, the photogenerated e− and h+ are separated by gradient potential. By using the external circuit, the electrons are transferred to the counter electrode; thus, hydrogen and oxygen generation reactions are carried out on the opposite electrodes [24], as presented in Figure 1.3. Similar to the photocatalysis process, semiconductors used as photoelectrodes must meet almost identical requirements, i.e. capability of light absorption, good durability, and low cost as well as appropriate band potentials [25]. However, the material has to be deposited onto the conductive substrate acting as current collector. Moreover, extremely important is the stability of photoelectrode’s material in strongly acidic and alkalic

e- e- e- eεinterface e-

eEF Ohmic contact

hv

3

∆Eº

εgen

h+ Photoanode 2

4H+ + 4e¯ 2H2 eOPHER 1.23 V OPOER Pt

2H2O 4H+ + 4e¯ +O2

Counter electrode

2 εtransport

Electrolyte

Figure 1.3  Fundamental mechanism of photoelectrochemical water splitting with transition metal oxide as photoanode and Pt as counter electrode, where: ΔE0 – difference in OER/HER potentials, e− – electron, h+ – hole, EF – Fermi level, hv – light, ε – energy of the electron quantum state. (Reproduced from Lee et al. [26]).

6  Materials for Hydrogen Production, Conversion, and Storage electrolytes, since the higher the OH−/H+ ions concentration, the faster the water splitting reactions occur. Equally important, strong electrolytes reduces the initial potential [23]. Due to photo(electro)corrosion, the semiconductor photoelectrode’s surface could be degraded, which significantly reduce their water splitting potential [27].

1.2.3 Thermochemical Water Splitting The thermochemical water splitting using the solar energy was first described in the 70s, as a modification of nuclear power plants’ heat utilization-­ based water splitting processes [28–30]. Heat, needed for reaction’s initiation, is provided directly from the Sun with use of solar concentrators. Moreover, in opposite to photo(electro)catalysis, the thermochemical water splitting allows for use entire solar spectrum, regardless of the wavelength [31]. However, even if the direct high temperature water splitting is possible above 2500 K, generated oxygen and hydrogen could form an extremely explosive mixture. Taking this issue into account, it is necessary to separate them to eliminate the possibility of explosion. The two-step thermochemical water splitting with metal-based materials as catalysts ensures that hydrogen and oxygen are produced separately in different redox reactions, as it is presented in Figure 1.4. Moreover, it allows for significant decrease of reactions’ temperature. In the first step, metal oxide (MOox) is thermally O thermal reduction MOOX

temperature

TTR

MO

MOred + 1 2 O2

thermochemical cycle

TWS

MOred+ H2O

MO O

MOOX + H2

water splitting H

O

H

H H

Figure 1.4  The general mechanism of the thermochemical water splitting. MO means metal-based material, in oxidized (MOox) and reduced (MOred) forms. (Reproduced from Roeb et al. [32]).

Metal Oxides in Solar-to-Hydrogen Conversion  7 reduced (MOred) with the O2 release. In second, exothermic step, reduced metal oxide returns to its original state with simultaneous water hydrolysis and H2 evolution [33]. What is also important, the whole process is closed in cycle, with only water and heat as an input stream and O2 and H2 as products [34].

1.3 Transition Metal Oxides in Solar-to-Hydrogen Conversion Processes As it was presented briefly in previous section, the solar-to-hydrogen conversion processes could not take place without proper materials which catalyze the generation of H2, mostly transition metal oxides. Due to almost identical semiconductors used in photocatalysis and photoelectrolysis processes, their applications will be discussed together.

1.3.1 Photocatalysis and Photoelectrocatalysis For both photocatalysis and photoelectrocatalysis, the first factor determining the possibility of effective hydrogen conversion is the width of bandgap width and its edges’ position relative to the water oxidation/

Vacuum

0

E

NiO

NHE

-2.5 -2.0 -3.0

-1.5

-3.5

-1.0

-4.0 -4.5 -5.0 -5.5 -6.0 -6.5 -7.0 -7.5

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-8.0

Cu2O CuO 1.7 eV

Sb2O5 ZnO 2.2 eV

Fe2O3

Co3O4 2.07 eV

3.0 eV

3.2 eV

2.1 eV

WO3

TiO2

Nb2O5

H2/H2O

SnO2 3.2 eV

2.6 eV

3.3 eV

3.4 eV

H2O/O2

3.8 eV

Figure 1.5  Bandgap values as well as VB and CB positions for various transition metal oxides used for water splitting. (Based on Ke et al. [35] and Lee et al. [26]).

8  Materials for Hydrogen Production, Conversion, and Storage reduction reaction. Figure 1.5 shows bandgap values together with its VB and CB positions for most popular transition metal oxides. As it is shown, the average bandgap of presented metal oxides is about 2.8 eV. Almost all valence bands are more positive than OER, while for only some semiconductors conduction bands are more negatively located than HER. What is also important, for optimal use of solar energy, bandgap ca. 2 eV is required [36]. Nonetheless, as presented by Thimsen et al. [37] several materials which meet these criteria are characterized by relatively low solar-to-hydrogen conversion efficiencies. This requires an individual approach to each of presented material and discussing them one by one in the following chapters.

1.3.1.1 TiO2 1.3.1.1.1 Photocatalysis

Among all transition metal oxides used in photocatalytic water splitting, titanium(IV) oxide is the most popular and widespread. It was the first described by Fujishima and Honda in 1972 [38], presenting a as followed scheme of water decomposition:

TiO2 + 2hv → 2e− + 2h+ 1 2h + + H 2O ® O2 + H + 2



2e− + H+ → H2

(1.1) (1.2) (1.3)

The overall reaction:



1 H 2O + 2hv ® O2 + H 2 2

(1.4)

Nevertheless, wide bandgap (~3.2 eV) of TiO2 allows for using almost only UV light for photoexcitation, thus strictly limiting the solar-to-­ hydrogen conversion efficiency (about 5%) [39]. In order to overcome this difficulty, several methods are applied, including (i) surface modification with noble and semi-noble metals [40, 41], (ii) doping with nonmetals [42–44], (iii) introduction of intrinsic defects into crystal structure [45, 46], (iv) heterojunction with other semiconductors [47–49], and

Metal Oxides in Solar-to-Hydrogen Conversion  9 (v) sensitization with dyes [50]. Another approach is to select the synthesis parameters in such way, to obtain TiO2 with different shapes. Through such change, different crystal surfaces are exposed causing significant change of photocatalytic activity. In Figures 1.6 to 1.8 various structures of titania for hydrogen production, including 2D nanosheets, decahedral anatase particles (DAP), and octahedral anatase particles (OAP), are presented. Wei et al. [51] in their work compared the hydrogen generation efficiency between the plasmonic metal-modified OAP and DAP TiO2 (see Figure 1.9). They found that in UV/Vis light range DAP is more photoactive than OAP and commercial spherical titania due to better charge separation (~274 µmol·h−1 and 190 µmol·h−1, respectively). Shortly, excited electrons migrate to {101} while holes to {001} facet. In OAP, which consists of only one facet, efficient charge separation is tough (see Figure 1.10). (a) But30-8h

(b) But30-13h

1 µm

(c) But30-18h

1 µm

(d) But30-24h

1 µm

1 µm

Figure 1.6  2D TiO2 nanosheets with {001} exposed facet (Reproduced fromDudziak et al. [52]). (b)

(a)

(d)

(c) A

M

100 nm

100 nm

100 nm

100 nm

Figure 1.7  Decahedral anatase particles (DAP) with exposed {001} and {101} facets. Reproduced from Janczarek et al. [53]). (a) N2H4

(c) NH3

(b)

0)

(100)

(10

(e)

Truncation A

Size (nm)

1µm

Truncation A

Counts

43.8º

0-50 50-100 100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 500-550 550-600

Truncation A

100 90 80 70 60 50 40 30 20 10 0

(100 )

(e) 45.4º

0-50 50-100 100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 500-550

Counts

46.4º 45 40 35 30 25 20 15 10 5 0

(d) (d)

b)

1µm

Size (nm)

Figure 1.8  Decahedral anatase particles (DAP) (a, b) and rectangular prisms-like anatase particles (c, d) with exposed {101}, {100} and {010}. (Reproduced form Kowalkińska et al. [54]).

10  Materials for Hydrogen Production, Conversion, and Storage (b) 300 Au/OAP Ag/OAP Cu/OAP

200

H2 generation / μmol

H2 generation / μmol

(a) 300

100

0

0

15

30 45 irradiation time / min

200

100

0

60

Au/DAP Ag/DAP Cu/DAP

0

60

15 30 45 irradiation time / min

Figure 1.9  Comparison between metal-modified OAP (a) and DAP (b) in hydrogen evolution reaction. (Reproduced from Wei et al. [51]). (a)

UV

(b)

h+ (001)

e¯

O2 (101)

e¯ e¯ h+

e¯ e¯

h+ h+ A

UV

(101) e¯ e¯

e¯

O2 –• O2

h+

A

–•

O2 e¯

h+

A+•

A+•

Figure 1.10  Decahedron (a) and octahedron (b) mechanisms of photocatalytic reactions under UV light. (Reproduced from Wei et al. [51]).

Going further, a brief outline of the other TiO2-modification’s representatives used for solar-to-hydrogen conversion is presented in Table 1.1. Titania surface modification with noble and semi-noble metals is the most popular method of increasing the H2 generation effectivity. Numerous publications are released every year, presenting novel approaches using platinum [55], gold [56], and silver [57], but also other transition metals [58, 59]. The Fermi level of presented metals is located lower comparing to titanium(IV) oxide which allows for effective separation of photogenerated electrons and holes, thereby preventing their undesired recombination and return to its basic state [60]. Furthermore, referring to the research by Zielińska-Jurek et al. [61], smaller metal particles (5–6 nm) are more desirable for significant increase of TiO2 photoactivity. Also, the dispersion and concentration of metal nanoparticles strictly affects the final solar-to-­ hydrogen conversion efficiency [62]. Too small concentration would not affect the photocatalytic process efficiency while too high could even block

Metal Oxides in Solar-to-Hydrogen Conversion  11 Table 1.1  Summary of photocatalytic H2 evolution with using TiO2-based photocatalysts. H2 evolution [µmol·h-1]

References

2% wt. Ag/Cu; MeOH/ H2O (50:50 vol%); UV/ Vis Hg lamp (22 mW/ cm2); 60 min

0.04 - 0.53

[63]

CuOx-C/TiO2

50 mg photocatalyst; 5% wt. glycerol; 300W Xe lamp; 3 h

7.7 - 433.3

[64]

Pt-TiO2

0.1% Pt; 100 mg photocatalyst; 100 ml H2O; 300 W Xe lamp (λ > 300 nm)

1.8 – 4.4 (A) 0.5 – 3.3 (R)

[65]

Pt/F-TiO2 nanosheets

20 mg photocatalyst; EtOH/H2O = 4:3; 350 W Xe lamp (20 mW/cm2)

3.0 – 333.5

[66]

TiO2/SrTiO3

18 cm3 photocatalyst; UV lamp (λmax = 254 nm; 5400 µW·cm-2); H2O/ MeOH = 8:1; 300 min

386.6

[67]

TiO2-ZnO

1-10 wt% ZnO; 0.1 g photocatalyst; EtOH/ H2O = 1:1; Hg lamp (λmax = 254 nm; 2.2 mW·cm-2); 10 h

915 - 1297

[68]

Black TiO2

0.02 g photocatalyst; MeOH:H2O = 1:1; Full spectrum solar simulator (1 Sun power); 22 days (5 h/day)

200

[69]

AnataseBrookite TiO2 nanosheets

20 mg photocatalyst; MeOH:H2O = 1:4; 350 W Xe lamp (15 A); 5 h

690 - 1312

[70]

Photocatalyst

Experimental conditions

DAP/Ag-Cu

A – anatase; R – rutile, MeOH – methanol, EtOH – ethanol.

12  Materials for Hydrogen Production, Conversion, and Storage the active sites on the semiconductor’s surface and prevents the photons absorption [71]. Second, a popular method for increasing the H2 evolution process is application of the heterojunction of TiO2 with other semiconductors as a photocatalyst. In most cases, the other counterpart is characterized by lower bandgap energies and/or its CB and VB are located in more suitable positions, in relation to HER/OER than TiO2. Depending on the semiconductors’ mutual positions, three types of heterojunction systems could be distinguished (see Figure 1.11). The II type of heterojunction, also appearing in the literature as direct Z-scheme, is the most desirable, due to efficient charge carrier separation [72, 73]. As a consequence of favorable bands’ energies electrons and holes could be directly transferred between the semiconductors, increasing its lifetime. A relatively new (comparing to the rest) method of increasing the TiO2 photoactivity is the design and awareness introduction to its structure the intrinsic defects. Blue (Ti3+/oxygen vacancies) or black (incorporated H-doping) titania as well as unintentionally arising Ti/O defects and surface disorders could cause the extension of light absorption up to visible region due to bandgap narrowing or the formation of additional electronic states [74, 75]. Comparing to other modification methods, defects formation inside the crystal structure is more effortless, with using UV light [45] or hydrothermal reaction assisted with oxidizing environment [46]. This was shown by Wu et al. [45] indicating that yellow, ultra-small Ti-defected TiO2 achieved higher H2 production from formaldehyde comparing with normal, un-modified titanium(IV) dioxide assisted with co-catalysts. The range of H2 evolution presented in the literature for TiO2-based photocatalysts is very wide and ranges from several micromoles even up to dozen of millimoles per hour [76], depending on the light source, modifiers, time of irradiation etc. Therefore, it is impossible to clearly indicate “the best” result. Certainly in a few years, today’s satisfactory results will not be treated as such, due to significant photocatalysis development. What is also worth to mention, in almost all cases described in Table 1.1, the H2 generation is conducted in the presence of sacrificial electron donors (SED), such as methanol, ethanol, formic acid, EDTA etc. [77–79]. Type I

CB

CB VB

A

B

Type II VB

CB VB A

CB

CB

B

VB

B

Type III CB VB

VB

A

Figure 1.11  Three types of semiconductors’ heterojunction systems. (Reproduced form Janczarek and Kowalska [73]).

Metal Oxides in Solar-to-Hydrogen Conversion  13 (a)

2H+

(b)

H2

H2

e¯ Au

e¯ Au

e¯ TiO2

e¯ TiO2

hv h+

h+

e¯ CH2O + H+

hv

H2O O2

2H+

h+

h+

•CH2OH + H+ CH3OH

Figure 1.12  Water splitting (a) and hydrogen generation in the presence of SED (b), on the example of Au@TiO2. (Reproduced from Hainer et al. [77]).

Some researches postulate that reaction with SED could not be defined as “water splitting” due to only half of the water splitting reactions occur [80], which is presented as Equations (1.5) to (1.7). As it is presented in Figure 1.12b, use of sacrificial scavengers cause production of hole trapping radicals which are at the same time electron donors, taking part in H2 generation reaction. Thus, the number of steps from excitation to final product is greater, though they lead to increased H2 generation, while the photocatalytic “true water splitting” process, as it was named by Hainer et al. [77] could be very inefficient [39].

TiO2 + hv → e− + h+

(1.5)

CH3OH + h+ → ∙CH2OH + H+

(1.6)

2e− + H+ → H2

(1.7)

1.3.1.1.2 Photoelectrocatalysis

While TiO2 in photocatalysis process could be used in various forms (suspension, thin layer) and its content was given in units of mass per volume, in the case of photoelectrocatalysis using bulk semiconductors is impossible because current collector is required. In this regard, titanium(IV) oxide photoanode is frequently from the start obtained as a film on conducting material (fluorine-doped tin oxide (FTO) glass, iodine-doped tin oxide (ITO) glass) [81, 82] or directly produced by etching of titanium sheet [83]. Wide range of titania structures used in photoelectrocatalytic water splitting includes e.g. nanotubes (NTs), nanorods (NRs) and nanowires (NWs) (see Figure 1.13). Comparing to the planar films, mentioned systems allow

14  Materials for Hydrogen Production, Conversion, and Storage (a)

(b)

1 µm

(c)

(d)

500 nm

Figure 1.13  Titania nanorods (a), nanotubes (b) with their cross-section image (c) and nanowires (d). (Reproduced from Qu and Lai [84] (a), Haryński et al. [85] (b-c) and Asiah et al. [86] (d)).

for more efficient charge carriers transport and are characterized by higher surface area available for redox reactions [87]. Similar to the previously described photocatalysis process, TiO2 used in photoelectrocatalysis water splitting definitely must be modified, in order to reach increased activity towards HER (see Table 1.2). Atabaev et al. [88] through modification of titania nanorods surface with Pt nanoparticles obtained about 86% higher HER photocurrent comparing to the un-modified TiO2 NRs. In the work of Dong et al. [89] nickel-doped TiO2 nanotubes were presented as excellent material for hydrogen generation. Moreover, Ni acted as electron trap, extending the electron-hole recombination time. A popular approach is also creating heterojunctions with other semiconductors such as iron oxides, zinc oxide, or graphitic carbon nitride. The last of the mentioned compounds has been used in work of Xiao et al. [90]. 1D TiO2 NTs photoanode covered with g-C3N4 layer reached almost 78 times higher saturation photocurrent than pure TiO2 NTs (0.090 mA·cm−2 and 0.007 mA·cm−2, respectively). Additional introduction of oxygen vacancies into titanium(IV) oxide crystal structure, acting as charge carriers separators, caused even higher photocurrent value of 0.723 mA·cm−2, measured for 1.23 V vs. RHE. Another carbon-based material used as heterojunction with TiO2 is γ-graphyne. Those two-dimensional, newly emerging carbon materials with large surface area and high stability are characterized with higher mobility of carriers than graphene and could achieve photocurrent equal to 3.95 mA·cm−2 [91]. Further, Kupracz et al. [92] presented an innovate method of FeOx-modified TiO2 NTs synthesis with use of pulse laser treatment. Obtained samples were tested as working electrodes and reveled significantly increased transient photocurrents comparing to reference bare TiO2, which is shown in Figure 1.14. Similar research was also conducted by Fu et al. [93]. By coating the α-Fe2O3 nanoarrays with metallic Au nanoparticles and external TiO2 layer, the maximum photocurrent during photoelectrocatalytic water splitting reached 1.05 mA·cm−2 with an Incident Photon to Converted Electron (IPCE) of 26% at 340 nm.

Metal Oxides in Solar-to-Hydrogen Conversion  15 Table 1.2  Summary of photoelectrocatalytic H2 evolution using TiO2-based materials. All photocurrents are given for 1.23 V vs. RHE. Experimental conditions

Photocurrent [mA·cm-2]

Pt-TiO2 NRs on FTO substrate

AM 1.5 solar sun simulator (100 mW·cm-2), 0.1 M NaOH electrolyte (pH = 13.6)

0.36 (bare TiO2 NRs) 0.67 (Pt-TiO2 NRs)

[88]

Ni-doped TiO2 NTs

150 W Xe lamp (100 mW·cm-2), 1.0 M KOH electrolyte

0.40 (bare TiO2 NTs) 0.60 – 0.85 (Ni-TiO2 NTs)

[89]

0D/1D g-C3N4/ TiO2 NTs with oxygen vacancies

300 W Xe lamp with 420 nm cut off filter (λ > 420 nm), 0.1 M Na2SO4 electrolyte

0.007 (bare TiO2 NTs) 0.723 (0D/1D g-C3N4/OV-TiO2 NTs

[90]

FeOx-TiO2 NTs

150 W Xe lamp with AM 1.5 filter, 0.5 M NaOH electrolyte

0.166 (UV/Vis light) 0.0044 (Vis light)

[92]

α-Fe2O3/Au/ TiO2

AM 1.5 solar sun simulator (100 mW·cm-2), 1 M NaOH electrolyte

1.05

[93]

P@TiO2 NTs

AM 1.5 solar sun simulator (100 mW·cm-2), 0.1 M KOH electrolyte

0.2 (black P@TiO2) 0.6 (fibrous red P@ TiO2)

[94]

TiO2/CdS/ CdSe

300 W Xe AM 1.5 solar sun simulator (100 mW·cm-2), 0.01 M Na2SO3/Na2S electrolyte

2.01

[95]

γ-graphyne/ TiO2

500 W Xe lamp, 0.01 M Na2SO4 electrolyte (pH = 7)

3.95

[91]

Material

References

16  Materials for Hydrogen Production, Conversion, and Storage 200

(a)

j (μA/cm2)

160

TiO2NTs FT-us30

TiO2-us60

TiO2-us30 FT-us60

TiO2NTs FT-us30

TiO2-us30 FT-us60

TiO2-us60

5

120

4

80

3 2

40 0 100

7 (b) 6

1 120

140 t (s)

160

180

0 100

120

140 t (s)

160

180

Figure 1.14  Transient photocurrents for bare and FeOx-modified TiO2 NTs registered at +0.5 V vs. Ag/AgCl/0.1 M KCl under chopped dark/UV/Vis light (a) and dark/Vis light (b). (Reproduced from Kupracz et al. [92]).

Obtained  via vapor deposition of phosphorus allotropes, P@TiO2 NTs were, in turn, characterized by about 1.30% of IPCE at 450 nm, while their photocurrent value was in the range of 0.2–0.6 mA·cm−2 (for black and fibrous red phosphorus, respectively) [94].

1.3.1.2 α-Fe2O3 Comparing to the TiO2, α-Fe2O3 (iron(II) oxide, hematite) is characterized by lower bandgap value, ca. 2.1 eV, thus being capable of absorbing the visible light (maximum ~600 nm). It is also a highly stable, nontoxic, and low-cost material, with a promising perspective as a photo(electro)catalyst [96]. However, its valence band is more positive than H+/H2 potential, which practically precludes the water splitting reaction [97]. Moreover, due to the high recombination rate of electrons and holes present, α-Fe2O3 efficient modification methods in order to overcome presented difficulties are required.

1.3.1.2.1 Photocatalysis

α-Fe2O3 as a powder used in photocatalytic water splitting can exhibit different shapes: 2D hexagonal plates [98], rod-like nanoparticles [99], flower-like nanoparticles [100], nanospheres [101], etc. Each morphology is characterized by different physicochemical properties that directly related with their photocatalytic activity. Nevertheless, for profitable water splitting, hematite indisputably has to be modified on surface and/ or crystal structure level or heterojunction with suitable semiconductor.

Metal Oxides in Solar-to-Hydrogen Conversion  17 The  summary of most widespread methods is provided together with H2 evolution values in Table 1.3. She et al. [98] combined small, flat hexagonal plates of α-Fe2O3 with 2D graphitic carbon nitride nanosheets in order to obtain efficient Z-scheme heterojunction. After 5 hours of irradiation the value of generated hydrogen was about 3·104 µmol·h−1 and the external quantum efficiency (QE) at λ = 420 nm reached almost 45%. Moreover, photocatalytic stability of 2D α-Fe2O3/g-C3N4 tested in five subsequent water splitting cycles revealed its good stability. The research conducted by Wang et al. [102] on Fe2O3@ MnO2/C3N4 core-shell nanocubes shows that, without carbon nitride support, Fe2O3@MnO2 are almost inactive in hydrogen generation reaction. The best result regarding water splitting was meanwhile achieved with a 20% share of C3N4 support in the total photocatalyst mass. In turn, Krishnan et al. [103] obtained CeO2-Fe2O3 mixed oxide heterojunction by using the thermal decomposition method. Cerium oxide is a semiconductor with bandgap energy of ~3 eV with VB and CB energies equal to 2.56 eV and −0.44 eV, respectively [104]. For this reason, CeO2 Table 1.3  Summary of photocatalytic H2 evolution using α-Fe2O3-based photocatalysts. Experimental conditions

H2 evolution [µmol·h-1]

References

2D α-Fe2O3/gC3N4

0.01 g photocatalyst, 10% triethanolamine solution in H2O, 300 W Xe lamp (λ > 400 nm), Pt co-catalyst, 5 h

3·10

[98]

Core-shell Fe2O3@ MnO2 nanocubes on C3N4 support

30 mg photocatalyst, 300 W Xe lamp (λ > 190 nm)

124 (Fe2O3@ MnO2/C3N420% sample)

[102]

CeO2-Fe2O3

1 g photocatalyst, solar light simulator (1000 mW·cm-2), 1 h

360 – 580

[103]

Photocatalyst

4

18  Materials for Hydrogen Production, Conversion, and Storage

could be used in photocatalytic H2 generation processes; however, its use is limited to UV light only. Through heterojunction with visible light active α-Fe2O3, efficient solar-to-hydrogen conversion could be reached.

1.3.1.2.2 Photoelectrocatalysis

In almost all cases described in the literature and related to photoelectrocatalytic hydrogen generation with use of α-Fe2O3-based materials, iron(II) oxide is deposited on the conducting material, e.g. FTO or ITO, see Table 1.4. Kleiman-Shwarsctein et al. [105] used in their work electrodeposition method for obtaining thin film of hematite additionally doped with Mo and/or Cr. α-Fe2O3-based photoanodes with 5% Cr and 15% Mo were characterized by IPCE values equal to 6% and 12%, respectively (0.4 V vs. Ag/AgCl, 400 nm), which was from 2.2 to 4 times higher than for undoped hematite film. Further, Kyesmen et al. [106] described the possibility of effective use of α-Fe2O3/CuO heterojunction for photoelectrochemical water splitting. Thin films of iron(II) oxide and copper oxide on FTO were obtained by using the dip coating and 70-day aging processes. Comparing to α-Fe2O3 reference, photocurrent density for the Table 1.4  Summary of photoelectrocatalytic H2 evolution using α-Fe2O3-based materials. All photocurrents are given for 1.23 V vs. RHE. Experimental conditions

Photocurrent [mA·cm-2]

References

Cr/Mo-α-Fe2O3

1000 W Xe lamp with monochromator and cut-off filters; 1 M NaOH electrolyte

N/D

[105]

α-Fe2O3/CuO

1 Sun solar simulator, Na2SO4 electrolyte (pH = 5.8)

0.53

[106]

α-Fe2O3/Au/ZnO

AAA solar simulator (AM 1.5 G), 0.5 M NaOH electrolyte

0.25

[107]

Bi-layered α-Fe2O3

Solar simulator AM 1.5 (100 mW·cm-2), 1 M KOH electrolyte

0.143

[108]

Material

N/D – no data.

Metal Oxides in Solar-to-Hydrogen Conversion  19

α-Fe2O3/CuO heterojunction photocurrent density was 19 times higher, which confirms its positive impact on photoelectrocatalytic water splitting. Unfortunately, the additional tests revealed poor stability of prepared photoanode in acidic electrolytes, which significantly inhibits its largescale applications. In turn, Kant et al. [107] used chemical spray pyrolysis technique for obtaining α-Fe2O3/Au/ZnO heterojunction on ITO. Due to more negative CB of ZnO in comparison to α-Fe2O3, photoexcited electrons transfer from ZnO to α-Fe2O3 and further to the Pt counter electrode where they took part in hydrogen generation process, as it is presented in Figure 1.15. Moreover, Au nanoparticles significantly improve the charge separation, leading directly to enhanced PEC water splitting. A completely different approach to α-Fe2O3-based photoanodes was presented in work by Lucas-Granados et al. [108]. Using anodization performed in F− rich environment they successfully obtained bi-layered α-Fe2O3 foil, with top nanosphere layer and bottom formed by nanotube layer (see Figure 1.16). Presented material, additionally annealed in argon at 500°C revealed the photocurrent density of 0.143 mA·cm−2 during the water splitting photoelectrochemical reaction, which no doubt proves the suitability of iron(II) oxide as semiconductor for solar-to-hydrogen conversion. e¯

e¯

(-2.92eV) e¯e¯ e¯ e¯

e¯

e¯

SPR excitation e¯e¯ e¯e¯e¯

e¯ e¯ e¯ e¯e¯ e¯ Au (-5.3eV) 2.2eV

h+h+h+ α-Fe2O3

e¯

e¯

e¯

e¯

H2

Pt

e¯ Defects (-5.35eV)

2H+

3.2eV H2O

Vo”

O2

ZnO

Figure 1.15  Proposed schematic charges transfer paths in α-Fe2O3/Au/ZnO heterojunction system during photoelectrocatalytic water splitting. (Reproduced from Kant et al. [107]).

20  Materials for Hydrogen Production, Conversion, and Storage (a)

150

i (mA/cm2)

125

I Formation of initial oxide layer

I

II Formation of porous by the flouride ions

II

100

III

75

III Formation of nanotubes

50 25 0

0

100

200

300

400

500

600

700

800

900

t (s)

(c)

(b)

200 nm

200 nm

Figure 1.16  Current density vs. time graph during anodization with schematic steps of bi-layered α-Fe2O3 formation (a) together with final material SEM images: top view (b) and cross section (c). (Reproduced from Lucas-Granados et al. [108]).

1.3.1.3 CuO/Cu2O Both copper oxides are p-type semiconductors with narrow bandgaps equal to 1.7 eV and 2.2 eV for CuO and Cu2O, respectively [109]. Additionally, their CB is more negative than HER, making those compounds suitable for photo(electro)catalytic water splitting. Nonetheless, mentioned oxides are relatively weakly stable, therefore susceptible to oxidation and photocorrosion. Hence, different methods of stabilizing them are being sought, while maintaining their photoactive properties.

1.3.1.3.1 Photocatalysis

As it was briefly summarized in Table 1.5, Wang et al. [110] described a CuxO@TiO2 core-shell structure, where the core was copper oxide and the shell was commercial titania. Such structure was proposed in order to prevent the photocorrosion of Cu2O by separating it from the oxidative environment. Obtained heterojunction was characterized not only by high hydrogen evolution efficiency, but also a good stability during long, 8-hour irradiation process. Further, Dubale et al. [111] presented an efficient photocatalytic hydrogen production using chrysanthemum-like Cu2O/CuO composite additionally supported on trimesic acid-based carbon. High H2 evolution rate (2670 µmol·h−1 for the best sample calcined at 350°C) as

Metal Oxides in Solar-to-Hydrogen Conversion  21 Table 1.5  Summary of photocatalytic H2 evolution using CuO/Cu2O-based photocatalysts. H2 evolution [µmol·h-1]

References

10 mg photocatalyst, MeOH/H2O (50:50 vol%), Hg lamp (λ > 290 nm), 60 min

50 – 210

[110]

C@Cu2O/CuO

10 mg photocatalyst, MeOH/H2O 1:4, 350 W Xe lamp with UV-cutoff filters (40 mW·cm-2), 6 h

1310 – 2670

[111]

g-C3N4/CuO

0.1 g photocatalyst, 300 W Xe lamp with cutoff filter (λ > 420 nm), 10% triethanolamine solution in H2O, 8 h

432 – 937

[112]

50 – 160

[113]

Photocatalyst

Experimental conditions

Core-shell CuxO@TiO2

RuO2/Cu2O

0.5 g photocatalyst, 300 W Xe lamp with cutoff filter (λ > 450 nm), pH = 7, 400 h

well as external QE equal to 52.4% in visible light could suggest that nonnoble metal photocatalysts are promising materials for large-scale processes. P-n semiconductor heterojunctions are also a widespread treatment for increasing the efficiency of hydrogen generation. In the works by Li et al. [112] as well as Banerjee and Mukherjee [113] copper oxides were combined with g-C3N4 and RuO2, respectively. Both mentioned n-type oxides had positive impact on photocatalytic water splitting occurring at CuO/ Cu2O composite.

1.3.1.3.2 Photoelectrocatalysis

In contrast to previously described TiO2 and α-Fe2O3-based photoelectrocatalytic H2 generation processes, where studied material was photoanode, copper oxides are usually used for photocathode formation (see Table 1.6). It is related to unfavorable photocorrosion and photooxidation reactions as well as its p-type behavior [114]. In the work of Srevarit et al. [81] an FTO/CuO/TiO2 photocathode was successfully obtained and tested towards photoelectrocatalytic

22  Materials for Hydrogen Production, Conversion, and Storage Table 1.6  Summary of photoelectrocatalytic H2 evolution using CuO/Cu2Obased materials. All photocurrents are given for 0 V vs. RHE. Material

Experimental conditions

Photocurrent [mA·cm-2]

FTO/CuO/TiO2

LED diode, 150 min

n.d. 5800 µL of generated H2

[113]

FTO/Al/Cu2O/ NiS

300 W Xe lamp (100 mW·cm-2, AG 1.5 G), 0.1 M Na2SO4 electrolyte

- 5.16

[115]

p-Cu2O/ZnO on Ti substrate

300 W Xe lamp (500 mW·cm-2, AG 1.5 G), 0.5 M Na2SO4 electrolyte (pH 5.7)

- 3.03/- 7.23

[116]

ZnO/Cu2O/rGO

300 W Xe lamp (100 mW·cm-2, AG 1.5 G), 0.5 M Na2SO4 electrolyte

- 10.11

[117]

FTO/FeOOH/ Cu2O

Xe lamp, 0.1 M, Na2SO4 electrolyte

- 1.5 (FeOOH/ Cu2O) - 0.66 (Cu2O)

[118]

References

water splitting. As photoanode FTO/WO3/BiVO4 heterojunction was used. This allowed for the efficient generation of 5800 µL of hydrogen during a 150 min irradiation process. Subsequently, hydroxyl radical, generated via H2O oxidation by photogenerated hole (h+) at the anode surface, allowed for almost 100% degradation of E. coli bacteria cells. Use of aluminum nanoparticles as a plasmonic photosensitizer as well as NiS co-catalyst on Cu2O photocathode was in turn described by Chen et al. [115]. Measured photocurrent density equal to 5.16 mA·cm−2 and was about eight times greater than for bare Cu2O electrode. Heterojunction of copper and zinc oxides grown on a titanium foil was used for highly efficient PEC water splitting by Tawfik et al. [116]. Obtained material was about 25 times more effective comparing to bare Cu2O/FTO photocathode (7.23 mA·cm−2 and 0.29 mA·cm−2, respectively). Even higher photocurrent in similar reaction conditions was obtained for ZnO/Cu2O/rGO heterojunction, described by Hou et al. [117]. Electrodeposition of reduced graphene oxide (rGO) sheets

Metal Oxides in Solar-to-Hydrogen Conversion  23 on ZnO/Cu2O allowed for photocurrent increase up to ~10 mA·cm−2. In contrast, in the work of Zhou et al. [118] the FeOOH layer was loaded as a hole transfer intermediate layer between Cu2O and FTO. This contributed to significantly improve of the copper(I) oxide photocathode stability.

1.3.2 Thermochemical Water Splitting In thermochemical water splitting process, two temperature ranges are distinguished: below and above 1000°C. Iodine–sulphur (I-S) cycles as well as alkali metal redox cycles (Na-Redox) belong to the group of low temperature processes, with operating temperatures about 850°C and 600°C, respectively [119]. Nonetheless, usually they are related with incomplete reactions or electrochemical steps, together with necessity of additional H2 separation steps, as well as hazardous and corrosive reactants/products [34]. The high temperature two-step cycles of thermochemical water splitting, presented in Figure 1.17, are mainly based on ferrites (Fe3O4/FeO), cobalt oxides (Co3O4/CoO), cerium oxides (CeO2/CeO2-δ or Ce2O3/CeO2), manganese oxides (Mn3O4/MnO), or perovskites (mostly based on lanthanum, strontium and manganese [120, 121]). The reduction and oxidation steps’ temperature ranges as well as their enthalpy’s (ΔH) are presented in Table 1.7 for selected metal oxides’ redox pairs. In the following subchapters selected transition metal oxides redox pairs used in thermochemical water splitting will be discussed in detail.

Na-Redox

Water splitting cycles (redox steps)

I-S Cycles Fe3O4/FeO SnO2/SnO ZnO/Zn Co3O4/CoO Mn3O4/MnO CeO2/Ce2O3

CeO2/CeO2-σ

500

1000 1500 Temperature (ºC)

2000

Figure 1.17  Temperature ranges of most common thermochemical water splitting cycles using transition metal oxides. (Based on Miyaoka [119]).

24  Materials for Hydrogen Production, Conversion, and Storage Table 1.7  Reduction and hydrolysis steps’ enthalpy and temperature ranges for different metal oxides’ redox pairs used in thermochemical water splitting. Reduction step

Hydrolysis step

Redox pair

ΔH [kJ·mol-1]

T [°C]

ΔH [kJ·mol-1]

T [°C]

References

Fe3O4/ FeO

609

1400-2000

-267

600

[11, 122, 123]

SnO2/ SnO

557

1600

-286

450-600

[34]

ZnO/Zn

312-376

1000-2150

-104

600

[124–126]

Co3O4/ CoO

196

1175

N/D

800-1000

[127, 128]

Mn3O4/ MnO

189

1500-1850

-40

370-900

[129]

CeO2/ Ce2O3

445.4

2000

-139

400-600

[130, 131]

CeO2/ CeO2-δ

279

< 2000

no data

< 1000

[132–134]

N/D – no data.

1.3.2.1 Fe3O4/FeO Redox Pair The reduction and hydrolysis of Fe3O4/FeO redox pair is provided below.

1 Fe3O4 ® 3FeO + O2 2

(1.8)

H2 O + 3FeO → Fe3 O4

(1.9)



The first reaction, thermal reduction of Fe3O4, is highly endothermic and requires heating to minimum 1400°C [11], or even up to 2000°C [122]. In contrast, the hydrolysis of FeO is slightly exothermic and belongs to the group of low-thermal reactions (about 600°C) [123]. In all scientific literature concerning the use of thermochemical water splitting, researches are mainly concentrated at lowering the process temperatures while maintaining the hydrogen generation yield. Karatza et al. [135] were studying the effect of pre-treatment on effective low temperature (250°C–310°C) water splitting over commercially available Fe3O4 pellets.

Metal Oxides in Solar-to-Hydrogen Conversion  25 They discovered that magnetization pre-treatment as well as electric power brought to reactive media caused about 0.12 mg/h H2 production. At the same time, the temperature of the process (not less than 300°C) showed a secondary effect. Also, from almost the beginning, various modifications of Fe3O4 are being considered. Roeb et al. [136] in their research studied the theoretical changes in thermochemical H2 production by using zincand nickel-modified iron (II,III) oxides. They discovered that replacement of 40% of Fe for Ni, forming NiFe 2y Fe32 y O4 y /2 structure, leads to higher H2 yield (in conversion to 1 mol of metal oxide per 1 cycle) than analogous amount of Zn2+ ions. Similar research was conducted by Amar et al. [137], considering that NiFe2O3 stabilizes on yttria oxide (Y2O3) as the best material for thermochemical generation among single and doped spinel or their mixtures. They also proved that with 13.07 ml/g of generated hydrogen, NiFe2O3/Y2O3 is suitable for large-scale thermochemical water splitting. Yet another approach to the hydrogen generation process was presented by Safari and Dincer [138]. They proposed four-step iron-chlorine cycle, with steps as follows:

3FeCl2 + 4H2O → Fe3O4 + 6HCl + H2    ΔH = +156 kJ (1.10) Fe3O4 + 8HCl → FeCl2 + 2FeCl3 + 4H2O   ΔH = −244 kJ (1.11) 2FeCl3 → 2FeCl2 + Cl2          ΔH = −160 kJ(1.12) Cl2 + H2O → 2HCl + 0.5O2       ΔH = +59 kJ (1.13) At 525°C the hydrogen molar flow rate was equal to 0.86 kmol·h−1, and decreasing to 0.56 kmol·h−1 with the temperature increase to 925°C.

1.3.2.2 CeO2/Ce2O3 and CeO/CeO2-δ Redox Pairs In contrast to other transition metal oxides cycles, cerium (IV) oxide-based process could be carried out in two ways: by thermal reduction to Ce2O3 (Equations 1.14–1.15) or to CeO2-δ (Equations 1.16, 1.17). In the case of CeO2 to Ce2O3 reduction, the higher temperature is needed due to almost two times higher reaction enthalpy in comparison to CeO2−CeO2-δ cycle (450 kJ·mol−1 and 279 kJ·mol−1, respectively).



1 2CeO2 ® Ce 2O3 + O2 2

(1.14)

26  Materials for Hydrogen Production, Conversion, and Storage

Ce2O3 + H2O → 2CeO2 + H2

(1.15)



δ CeO2 ® CeO2−δ + O2 2

(1.16)



CeO2 − δ + δH2O → CeO2 + H2

(1.17)

In research works from the last 5 years, both of the cerium(IV) oxidebased redox pairs are described. In the work of Arifin and Weimer [139] commercial ceria was used for kinetics and mechanistic study of thermochemical water splitting. First, the CeO2 powder was reduced by laser treatment (500 W continuous wave near-IR laser). In the next step, obtained material was oxidized back during the hydrogen evolution reaction. Based on the theoretical kinetic model, observed water splitting reaction best fits to the first-order kinetic model, with the optimum hydrolysis temperature range 750–950°C. Further, Lee et al. [140] described efficient use of a CeO2 supported on porous ceramic (mullite nanofibers) for water splitting. Porous matrix prevents from aggregation and deactivation of catalyst at high temperatures, while maintaining high H2 generation (5.2 ml·g−1, 1500°C, 1200 min). Similarly, Meng et al. [141] used in their research metal-­ modified reduced CeO2-based ceramics, with the general Ce0.9M0.1O2-δ formula, where M is Mg, Ca, Sr, Sc, Y, Dy, Zr, or Hf. It was revealed that the amount of generated H2 in 9 subsequent cycles was significantly increased, kmol 0.0010

CeO2

CeO(g)

0.0009 0.0008 0.0007 0.0006 0.0005 CeO1.83

0.0004

CeO1.72

0.0003

Ce2O3

0.0002 0.0001 0.0000

0

500

1000

1500 2000 Temperature (ºC)

2500

3000

Figure 1.18  Required temperature ranges for cerium oxides equilibrium (p = 200 mbar, N2 atmosphere). (Reproduced from Abanades and Flamant [142]).

Metal Oxides in Solar-to-Hydrogen Conversion  27 comparing to the CeO2 reference, due to doping with higher valence cations. In turn, Abanades and Flamant [142] in their article from 2006, for the first time described the CeO2/Ce2O3 redox pair, while confirming the necessity of higher temperature during the metal oxide reduction step (see Figure 1.18). Their researches were further extended by Felinks et al. [143], which were focused on heat recovery during high ( Eg light [17]. Therefore, a semi-conductor with the appropriate band position and bandgap and energy efficiency of at least 10% is required for practical use of the processes mentioned above for water splitting [20]. Table 3.2  The materials of photoelectrodes. Type of photoelectrodes

Material

References

Photoanodes

TiO2

[21]

ZnO

[22]

CdS

[29]

Fe2O3

[34]

WO3

[34]

GaN

[50]

Si

[28]

Cu

[32]

Pt

[54]

Photocathodes

70  Materials for Hydrogen Production, Conversion, and Storage Photoelectrochemical water splitting is one of the hydrogen production methods that work on the idea of converting solar energy using photovoltaic materials. Using a semiconducting material for photoelectrochemical via the presence of light, water splitting is a high-efficiency and low-cost photocatalyst that makes the oxidation and reduction steps. The semi-conductor photoelectrodes can easily capture light and convert it into energy used in the reactions. Also, the external electricity or chemical bias is required for water electrolysis and enhances the reaction kinetics for providing enough voltage for the photoelectrochemical cell to perform reaction at the desired conditions. Electrons are excited, and electron-hole pairs are produced by a redox reaction in the photoelectrode with photon absorption. The molecule can be oxidized by the holes created, and the electron can decrease H to H2 [19, 21–25]. The studies on improving efficient and long-shelf live systems for splitting of water to produce H2 and O2 in the presence of solar spectrum based on reactions are given in Equations 3.1–3.3:

O2 + 4 H + +  4e − → 2H 2O         Eox = 1.23 − 0.059 V  ( pH = 0 ) or   E 0 = +0.82 V  ( pH = 4e − → 2H 2O         Eox = 1.23 − 0.059 V  ( pH = 0 ) or   E 0 = +0.82 V  ( pH = 7 ) (3.1) 

4 H + +  4e − → 2 H 2                     Ered = 0 − 0.059 V  ( pH = 0 ) or   E 0 = −0.41 V  ( pH = 7 e − → 2 H 2                     Ered = 0 − 0.059 V  ( pH = 0 ) or   E 0 = −0.41 V  ( pH = 7 )  (3.2) 

2 H 2O →   2 H 2   +  O2    Eoverall   reaction =   −1.23 V      

(3.3)

where 2.46 V is the total voltage held in hydrogen (H2) as a result of the reduction of two electrons and protons [20]. Water (H2O) is transparent and does not absorb visible light, even though visible light has enough energy to divide it into hydrogen (H2) and oxygen (O2) (Equations 3.5–3.7). Intermediates (light-harvesting system: semiconducting catalyst) are required to accomplish water photocleavage through a cyclic route (Equation 3.7).

3.3.1.1 PEC Materials The semi-conductors used in photoelectrochemical cells are n- and p-type. Because effective water-splitting needs a suitable semi-conductor to convert photon energy to hydrogen, photoelectrode material selection is the

Solar–Hydrogen Coupling Hybrid Systems  71 most critical design component of PEC systems [17]. Photo-anodes are made from n-type semiconductors, which have an abundance of electrons (e). P-type semiconductors, on the other hand, have an excess of electron holes (p) and operate as a photocathode. Thus, different configuration between photoanode, photocathode, metal anode, and metal cathode might be used in the photoelectrochemical cell. When the photoanode is exposed to light with a higher energy than its energy bandgap, electrons are excited and rise from the valence band (EV) to the conduction band (EC). They then flow to the metal cathode or photocathode through the external circuit. As a result, a photocurrent is created, which causes water to split in the production of hydrogen and oxygen. Energy bandgap, the characteristic value of individual semi-­ conductors, is the difference in energy between the two levels (Ec – Ev = EG). Photons with a higher energy than the semiconductor’s energy bandgap can be absorbed, resulting in electron-hole pairs. Then, on the counter electrode’s external circuit, they’re transmitted to a semiconductor-liquid junction or an ohmic rear contact [26]. The semi-conductor photoelectrode has some properties for meeting system targets such as a bandgap range between 1.8 and 2.2 eV for effective visible-light conversion, efficient e−-h+ pair separation to avoid recombination, and isolated charges that transport quickly; valance and conduction band edge locations that are appropriate; non-corrosiveness and high chemical stability in the electronic environment; and low price [27]. For photoelectrochemical systems, several materials have been reported, such as TiO2, WO3, ZnO, Cu2O, graphene, etc. Almost none of them are meeting targets, some of them have large bandgaps, and some are not chemically stable [28]. For that reason, using a modified complex structure during the preparation of electrodes is another alternative for improving the system properties to reach desired performance. For this aim, electrodes were modified by chemical vapor deposition [29, 30], electrochemical deposition [31], electrodeposition [32], sol-gel [33], spin-coating [34], spray pyrolysis [35], and other related methods [17]. These techniques affect the crystallinity, size, bandgap, pH dependency, and light sensibility of the material, which is effective for photochemical performance in terms of photocurrent density of the system [25]. Compared to amorphous materials, highly organized crystalline materials perform significantly better, mainly affecting synthesis temperature and pressure based on the method [36]. The size of the particle has an inverse proportion with the photoelectrochemical efficiency; the smaller the size, the higher the electrokinetics between the electron and hole. The bigger particles have better band bending characteristics, and charge extraction occurs at the electrode-electrolyte

72  Materials for Hydrogen Production, Conversion, and Storage interface [37]. The dimensionality of the nanomaterials such as 0, 1, 2, and 3 affects the materials’ surface area and thickness and affects the UV light-­ harvesting efficiency. It allows for simple charge carrier transfer onto surfaces and improves hydrogen production efficiency. The zero dimensions (0D) nanostructure are quantum dots (QDs) such as carbon [38], CdS, CdSe [39], graphene [40], Co3O4 [41], and graphitic carbon nitride [42] that absorbs visible light efficiently and has excellent photocatalytic activity. The one-dimension (1D) nanostructures such as nanorods, nanowires, and nanotubes made of TiO2 [43], SiC [44], WO3 [45], and ZnO [46] show good properties in the water-splitting process. The two-dimension (2D) structures, such as nanosheets and nanofilms are generally used for water oxidation [47, 48]. The three-dimensional (3D) nanostructures are also notable for their excellent photoactivity capability. It improves light absorption by shortening the distance between photogenerated holes and the electrolyte/electrode contact, allowing for more efficient light absorption [49]; the narrow bandgap is another critical factor for good efficiency. Nanomaterials have a narrow bandgap that enables a wide range of light absorption. For effective photoelectrochemical water splitting, the bandgap should be between 1.6 and 2.2 eV. The band edge location is precise, and photo charge carrier mobility is greater in this bandgap range [25]. The total charge of the surface of the material is strongly affected by the pH value of the electrolyte solutions that could be corrosive for the electrodes. The pH value of the system has to enhance the reactivity, and together with this, materials have to show good stability [50]. Titanium dioxide (TiO2), zinc oxide (ZnO), quantum dots (QDs), hematite (α-Fe2O3), tungsten trioxide (WO3), bismuth vanadate (BiVO4), graphene, and graphene-based materials are the favorable materials that are used in several nanoforms as a semi-conductor. TiO2 is the most often utilized material for water splitting and hydrogen production experiments [51–55]. Because bulk TiO2 has a bandgap of 3.03–3.18 eV, it has low efficiency in solar light absorption, despite its stability and inexpensive cost, making it potentially useful in water splitting. Among TiO2 nanomaterials, the TiO2 nanotube is the best option [51]. ZnO has similar optoelectronic characteristics compared to TiO2. Water splitting with ZnO nanoparticles has also been widely studied. It has a comparable energy band structure to TiO2 (bandgap 3.3 eV). Because of its poor photocatalytic quantum efficiency and limited visible light absorption, ZnO has low photocatalytic efficiency [21, 22, 56–61]. QDs have an effective band structure that can maximize the solar light absorption efficiency by changing size and dimensions. QDs have four main qualities: (i) adjustable absorption capabilities, (ii) excellent stability, (iii) solar spectrum suitability, and (iv) various

Solar–Hydrogen Coupling Hybrid Systems  73 bandgap sensitizations. Cd, CdSe, Ag2S, PBS, and CuInS2 QDs have been reported to generate hydrogen via water-splitting reactions [56, 57, 62]. Hematite has a small bandgap of 2.1 eV, allowing it to absorb around 40% of solar energy, and its stability in water medium provides compatibility [63]. WO3 has a bandgap around ~2.5–2.7 eV. High crystallinity, porosity, the capacity to capture 12% of solar light, moderate hole diffusion length, superior chemical stability, and simple and inexpensive production methods are advantages of WO3 nanomaterials [64–66]. Due to its excellent absorption capacity and bandgap energy of 2.4–2.6 eV, the BiVO4 nanoparticle has attracted a lot of attention in the field of water splitting [67, 68]. Graphene shows unique reactivity and adsorption properties against to the water splitting reaction. Especially, graphene sheets increased photocatalytic hydrogen production activity by providing more significant active adsorption cite which affects the interfacial charge process. However, graphene and graphene oxide show hydrophobic and hydrophilic characteristics, respectively. In addition to this graphene oxide is more appropriate for water splitting because of its photocatalytic activity. By appropriately substituting edge site oxygen atoms, graphene oxide may be produced either p-doped or n-doped [25, 69].

3.3.1.2 Photoelectrochemical Systems A photoelectrochemical system is an effective way to split water and turn light energy into electric energy. Solar photon conversion devices, or solar cells, are divided into three categories such as photovoltaic electrolyser, single PEC, and photocatalytic based slurry systems. In the first, the electrolysis process is driven by a photovoltaic-­electrolyzer setup that contains two separated primary components. In the second system, a photocatalytic setup with photoactive semi-­conductor particles floats freely in a fluid as a slurry. The PEC cell’s third system is using semi-conductors which are separated via membrane in an aqueous electrolyte-filled cell [70]. Several different configurations have been improved to avoid dangerous reactions, separate H2, and O2, increase the efficiency, optimize the space between anode and cathode, and form uniform current distributions [71]. Single photoelectrode, dual photoelectrode, photoelectrochemical/photovoltaic systems (PEC/PV), and their hybrid systems are integrated with various type solar cells that researchers put attention on to improve effective energy production from solar energy. In single photoelectrode systems, two electrodes are placed into electrolytes where photoanode (n-type semi-conductor) or photocathode (p-type semi-conductor) is placed in the direction of sunlight irradiation. Detrimental band edge

74  Materials for Hydrogen Production, Conversion, and Storage locations are a significant bottleneck in single photoelectrodes, leading to unfeasible consideration of the single electrode systems. The external electrical, chemical, or photovoltaic cell or multiple semi-­ conductor layers are required to improve system efficiency. Dual p ­ hotoelectrode systems solved the problems of single electrode systems mainly. For splitting water or absorbing additional light, sufficient photovoltage can be provided by using of high and low bandgap n-type (photoanode) and p-type (photocathode) semi-conductors as a pair [60, 61]. These types of cells generally design in several ways: P mode, T mode, wired T mode, and wireless T mode [72]. PEC/PV systems (conventional solar cells) are another alternative way for hydrogen production. Photoelectrodes couple with the photovoltaic unit. PV cells provide high enough photovoltage for electrodes to be driven by sunlight [73]. These conventional solar cell systems generally combined silicon-based anode combination of single or double injection. Multi-junction systems are also designed to improve the energy efficiency of the systems to absorb light with 350–500 nm wavelength range. The photoanodes are generally made of Fe2O3, WO3, TiO2, CdS, ZnO, and BiVO4, etc., based materials, together with this GaP, Si, Cu, Pt, and Ru preferred photocathode content. Electrolyte solutions are generally acidic, basic, or neutral solutions such as KNO3, NaOH, Na2SO4, etc. The instability of the PV units is the major problem of these systems, and hybrid PEC-chloralkaline systems have been improved. The amount of solar radiation that is directed towards PV/T is used to create necessary power for electrolysis to generate Cl2, H2, and NaOH. Then, photocathode immediately produces H2 gas and NaOH from the part fed to the PEC reactor [74]. Another option for enhancing visible light to generate energy is to combine photoelectrodes with dye-sensitized solar cells. Wide-band-gap semi-conductor and molecular photosensitizers such as TiO2 are used for the covering of the surface of parts. Dye-sensitized n-type, p-type, and p-n type designs are preferred to get high enough electrical bias. Another alternative system is a perovskite-based solar cell (PSC), which has acknowledged optoelectronic qualities such as a suitable bandgap, high charge carrier mobility, and considerable light absorption coefficient. The stability of these devices takes attention, but there have been limited studies on these due to their low efficiency reported as about 2.4% to 10% [75, 76].

3.3.2 Electrochemical Hydrogen Production As a potentially carbon-free fuel, hydrogen is expected to play a critical role in profound decarbonization across all economic sectors [77]. Solar

Solar–Hydrogen Coupling Hybrid Systems  75 energy can sustainably make hydrogen by electrolyzing water at low temperatures [78]. Electrolysis is one of the methods of producing hydrogen in non-fossil ways [79]. Water electrolysis is described as splitting a water molecule into hydrogen and oxygen via an electrochemical reaction powered by electrical or thermal energy. Electrolytic water splitting is a promising option for producing hydrogen and oxygen using electricity. However, electrolysis produces only 0.25 percent of hydrogen worldwide. Pure water is highly resistive to electricity, and electrolysis can only take place if the water is acidic or alkaline. As a result, ions in the water must be present for the water electrolysis process to conduct electricity [80]. The electrolyzer is a device that uses electricity to electrolyze water. An electrolyzer is made up of two electrodes separated by an electrolyte. At the anode, water oxidizes to produce oxygen, four positively charged hydrogen ions (protons), and a similar number of electrons. As illustrated in Equations 3.4 and 3.5, hydrogen ions interact with electrons from the external circuit to produce hydrogen gas [81].



Anode reaction: 2 H 2O → O2 + 4 H + + 4e −

(3.4)



Cathode reaction: 4 H + + 4e − → 2H q

(3.5)

Alkaline electrolysis (AEL), proton exchange membrane (PEM), and to a lesser extent, solid oxide electrolysis (SOEL) are the methods for water electrolysis that are now deemed practical [82].

3.3.2.1 Polymer Electrolyte Membrane Electrolysis Cell (PEMEC) In the early 1960s, NASA developed the PEM fuel cell (PEMFC), which is similar in concept to the PEM type electrolyzer. However, the high cost of manufacturing has hindered the commercialization of fuel cells during the next 30 years. PEMFCs are high-performance, simple-structured systems that can operate at low operating temperatures and are of interest as sustainable environmental and energy systems. Anode, membrane electrolyte, and cathode components make up a PEMFC system that can transform into an electrolysis cell by connecting the anode and cathode in reverse mode. PEM electrolysis cells allow hydrogen production from hydrogen carrier fuels such as methanol and water. A typical PEM electrolysis cell features current density characteristics (1.0–2.0 A.cm-2) and voltage efficiency (67%–87%) at 50°C –80°C temperature with pressure less than

76  Materials for Hydrogen Production, Conversion, and Storage 30 bar, and voltage 1.8–2.2. Moreover, the start-up time would be less than 15 minutes [78, 83, 84]. PEM electrolyzers may be combined with renewable energy to produce sustainable hydrogen fuel, maintaining high current density and high voltage efficiency while lowering operating costs. The solid phase of the PEM electrolyte allows the cell to be compact while decreasing ohmic losses caused by the nature of the operations. However, due to the acidic and corrosive working environment of the PEM electrolyzer, the cost of cell components, current collectors, and separator plates increase in the system. Therefore, Pt, Ru, and Ir-based electrocatalysts and titanium-based current collectors and plates are required for long-term performance in PEMEC systems [85]. Furthermore, due to their proton conductive characteristics, gas-solid polysulfone membranes (Nafion®, fumapem®) are frequently used as electrolyte components to generate high purity hydrogen gas and oxygen as by-products [78]. PV and EC system integration is based on a fundamental logic that predicts transmitting the electrical energy generated by solar radiation on PV directly to an EC system. Following the introduction of Fujishima and Honda’s [19] electrolysis using PV, the hybrid system developed and expanded with a variety of photovoltaic technology and electrolyzers. Concentrated solar radiation is transformed to a photo absorber by line-focusing or point-focusing optical devices to supply adequate energy required by PEMEC [86]. Fujii et al. [87] proposed concentrated PV solar panels to create electricity directed to PEM systems. Concentrated PV cells (commercial InGaP/GaAs/Ge triple-junction PV cells) with PEMEC have been continued to work by maintaining their stability at a yield of 12% and higher. Nakamura et al. [88] developed a CPV mono module integrated to a PEMEC with a Pt electrode and achieved a 31% CPV module energy with a 24.4% hydrogen conversion efficiency.

3.3.2.2 Alkaline Electrolysis Cell (AEC) The alkaline electrolysis method is a cutting-edge technology for producing hydrogen fuel. Even though alkaline electrolysis cells (AEC) are manufactured on a large scale, installation and maintenance costs are still expensive. Moreover, AECs have a short operating life and use a lot of energy during the electrolysis process [89]. As a system, AEC consists of a cathode, dissociator, electrolyte, and anode. There is a liquid electrolyte between the anode and cathode components in its traditional design,

Solar–Hydrogen Coupling Hybrid Systems  77 which decreases cell efficiency and voltage. Hence, the “zero-gap” concept is utilized to eliminate the design problem caused by the traditional approach [89, 90]. The conductivity of the liquid electrolyte determines the operating conditions of AEC. The most typical electrolyte is a solution of 20%–30% potassium hydroxide (KOH) by weight, while the optimum conductivity temperature range of the KOH solution is 50°C–80°C. A low-cost sodium hydroxide solution is used instead of a KOH solution. The NaOH solution, on the other hand, has a lower conductivity than the KOH solution [90]. In comparison to acidic environments, the operation of AECs in the primary environment provides higher reaction chelation in the electrodes. The primary environment of the AEC allows the catalyst to be used with high electrical efficiency. As a result, it provides high yields for low-cost metals such as nickel, cobalt, and iron [84, 91]. Renewable energy sources such as solar and wind can be coupled with AECs. On the other hand, system combinations generate an ecologically dynamic structure since energy generation from renewable sources is dependent on external variables. As a result, research towards AE cells that can better adapt to variable environments is proceeding [89, 90]. For example, as an integrated hydrogen production via solar energy technology, Đukić (2015) has designed PV-AEC systems and achieved 77% system efficiency [92]. On the other hand, it was also studied PV-AEC technology using NaOH electrolyte [93]. They calculated a stable system capable of producing 2-L hydrogen in 4 hours at an average irradiance of 800 W.m−2.

3.3.2.3 Solid Oxide Electrolysis Cell (SOEC) Since its launching in 1980, solid oxide cells (SOC), all cell components made of solid materials, have attracted much attention [78]. Solid oxide electrolysis cells (SOEC) that provide comprehensive options for fuels and water can create a concept of sustainable energy. Furthermore, SOECs are more beneficial than PEMEC and AECs due to their better chemical efficiency with a minor electrical input [94]. Water molecules supplied to the cathode are split into H2 and oxygen ions under the electrical current provided by the energy sources. The reduced O2 ions move through the electrolyte and oxidize into oxygen gas at the anode, while hydrogen gas is emitted at the cathode. Maintaining intracellular stability of SOEC components is essential due to the high-temperature operating conditions. Like solid oxide fuel cells,

78  Materials for Hydrogen Production, Conversion, and Storage the SOEC system often uses yttrium doped zirconia (YSZ) with dense ionic conducting material as the electrolyte. YSZ electrolytes are known for their physical and chemical stability in high-temperature conditions. Other than YSZ, Scandinavia stabilized zirconia, ceria-based, and lanthanum-­based electrolytes are also investigated for their conductivity as a SOEC electrolyte [95]. Even though the same materials are used for both SOFC and SOEC, other operating conditions vary, such as partial pressure and electrical conditions [96]. Hybrid solar systems are being investigated for supplying the energy and heat requirements of electrolysis. According to thermodynamic principles, the Gibbs free energy decreases as the temperature rises in the water separation reaction, lowering the current required for the electrolysis process. To supply heat and energy for SOEC, Wang et al. (2020) proposed combining PV cells and photon-enhanced thermionic emission (PETE) [97], while Mohammadi and Mehrpooya (2019) used a parabolic trough collector to meet the SOEC’s energy and heat requirements [98]. Furthermore, Akikur et al. (2014) proposed the system with parabolic trough solar collectors and PV panels to provide heat energy and electrical requirements for SOEC [99].

3.3.3 Fuel Cell Hydrogen fuel cells are a device that transforms hydrogen into electricity and contributes to renewable energy [100]. The hydrogen combustion reaction in a fuel cell is split into two electrochemical half-reactions:



H 2  2 H + + 2e −

(3.7)



1 O2 + 2 H + + 2e −  H 2O 2

(3.8)

Considering these reactions, electrons transferred from the fuel are forced to pass through the external circuit. It creates an electric current. Separation is carried out with the help of an electrolyte. An electrolyte is a material that allows ions to flow but, on the contrary, does not allow electrons to flow. More specifically, a fuel cell has two electrodes separated by an electrolyte, where two electrochemical reactions occur [101]. There have been some studies that can serve the purpose of G2G. For example, Calise et al. studied a green energy production plant and a fuel

Solar–Hydrogen Coupling Hybrid Systems  79 cell-based power generation system to address electrical and thermal demand [102]. Dispenza et al. offered an in-depth investigation of the first Italian hydrogen refueling station for hydrogen buses, which was used to power a hybrid electric fuel cell minibus, an electric minivan, and two fuel cell electric bicycles as part of a Smart City idea [103]. According to Gambini et al. metal hydrides can be used as hydrogen storage devices for fuel cells [104], and lanthanum alloys can be used in small-scale fuel cell applications [105]. According to Brooks et al., fuel cell technology can be used in various aircraft applications, and hydrogen can help the aviation industry lessen its environmental impact [106].

3.3.4 Photovoltaic PV is still the most widely used solar system, owing to many features, including its low cost compared to other solar systems, its ability to work at various scales, and its ease of installation. PV-hydrogen coupling is, in reality, the most developed solar-hydrogen coupled system [107, 108]. A solar cell, also known as a photovoltaic cell, is an electrical device that uses the photovoltaic effect, physical and chemical phenomena, to convert light energy directly into electricity. Photovoltaic modules, also known as solar panels, are made up of solar cells, the building components [109]. Since PV hydrogen coupling is highly preferred, many studies have been carried out in this area. When hydrogen production is combined with renewable energy, the cost of hydrogen reduces, and its income is increased [110]. Schnuelle et al. looked at the interaction of solar systems with proton exchange membrane (PEM) and alkaline electrolyzers, giving dynamic modeling of the individual systems and their integration [111]. Ready et al. propose a method for producing hydrogen that combines photovoltaic cells with solar thermal energy [112]. Boundaries et al. investigates the impact of geography on the performance of PV-hydrogen coupling [113]. Also, Bhattacharyya et al. investigate the performance of a PV-hydrogen system under various environmental conditions [114]. Solar radiation can be converted into chemical energy such as storable and even portable energy carriers like hydrogen, drawing attention to long-term energy supply. Many substances can be considered for the potential hydrogen evolution process as green energy. Water splitting hydrogen production is the most studied and developed method belong the solar-based processes, and the fundamental principle of the electrolysis method is the splitting of water under a particular current to hydrogen and oxygen [45]. Electrolysis and several other water separation technologies

80  Materials for Hydrogen Production, Conversion, and Storage such as thermochemical are becoming prominent due to their promising operating conditions. PV panels provide the operating current and operating temperature required for hydrogen production in solar to hydrogen systems with electrolysis cells (EC). Therefore, two variables are considered limiting factors on hydrogen development in this system: PV modules and EC systems. Nakamura et al. (2015) investigated the PV limitation factor by evaluating solar to hydrogen (STH) efficiency using concentrated PV modules, which increased CPV efficiency to 31% and PEM EC-STH efficiency to 24.4%. In this system, the energy transmitted from PV to the electrolysis cell must be constant [88]. Therefore, for some PV types, a single module will not meet a specific condition that would allow the EC system to start up; however, the system may be supported by additional renewable energy sources like wind and hydro. Another approach besides the other renewable sources is developing concentrated photovoltaic (CPV) technology to provide energy and heat to EC. Based on the radiation intensity gathered by receivers, CPV can be classified as low, medium, or high concentration [115]. Another limiting factor in PV-EC hybrid systems is the use of electrolyzers to create hydrogen through electrochemical conversion. Different electrolysis cells are developed to overcome the limitations as well as obtain optimum efficiency. Component development and compatibility, current requirements, operation conditions, and other aspects are investigated to enhance EC efficiency. Electrolyzers are classified into different categories based on the membrane that uses: polymer electrolyte, alkaline electrolyte, and solid oxide electrolyte cells [116].

3.4 Thermochemical Systems In solar thermochemical systems, solar radiation heats the reflector/­ collector and is concentrated through the systems such as solar furnaces or solar power towers. The concentrated solar radiation is captured in a solar receiver/reactor, and the solar energy is converted into chemical energy. The initial phase in the solar heat conversion process is to use mirrors called collectors to reflect incoming rays and concentrate them into receivers/­reactors, which are solar reactors for thermochemical reactions [117]. As a critical component of thermal power generation systems, solar concentrators are classified into three categories based on concentration factors (low 10x, medium 10–100x, and high >100x) and two categories based on optical features (linear or point focus) [115]. Point focus optic

Solar–Hydrogen Coupling Hybrid Systems  81 systems (suitable for high concentration up to 11575×) contain square Fresnel lenses, parabolic dish, or central receivers with heliostat fields. Linear focus optic systems, on the other hand, contain linear Fresnel lenses, parabolic troughs, and cylindrical parabolic concentrators [118]. Many parallel rows of collectors are used in linear focusing optical PV systems to convert solar radiation into heat and energy. Fresnel lenses work by refracting the sun’s rays to a single point using prisms contained inside the lenses. Each reflector/mirror of Fresnel lenses in a linear Fresnel system is spaced in a series of extended, thin, low profile, and horizontally to focus direct solar radiation on a stationary receiver set on a linear tower. Fresnel lenses are advantageous because of their low cost and lightweight [115, 117, 118]. Solar radiation is concentrated into the receiver via parabolic dish concentrators, which have rotational parabolic symmetry. Parabolic dish systems with a high concentration factor can achieve great temperature and electrical efficiency. On the other hand, parabolic trough lenses (PTL) achieve high concentration ratios due to their mass scale installations. PTL reflects solar energy in a single beam onto a receiver in the parabola’s center, allowing temperatures to reach 400°C. As a point focus system, solar towers follow the sun and direct its rays to the heat exchanger/ receiver at the top of the center tower through mirrors installed in a large area. The obtained temperature with a high concentration ratio can reach 800°C–1000°C and more at solar tower systems [115, 119]. The solar processes can be classified into three groups such as electrochemical, photochemical, and thermochemical. The thermochemical process possesses advantages over the other methods because of its high overall efficiency and lower operation cost [120]. The thermochemical process focused on solar radiation is a convenient method for the production of ‘solar fuels.’ In this manner, the solar energy is captured in a solar reactor and converted into chemical energy [121]. Five routes such as solar thermolysis, solar thermochemical cycles, solar cracking, solar reforming, and solar gasification are maintained to produce fossil fuels. All these production routes consist of endothermic reactions, water (for solar thermolysis, thermochemical cycles, and reforming process), and fossil fuels (for solar cracking, reforming, and gasification) are used as raw materials [122]. Solar thermolysis of water enables the production of hydrogen gas as solar fuel at higher temperatures (above 2500 K). In this process, concentrated solar energy is utilized to split water into hydrogen and oxygen gases. However, this dissociation reaction is reversible, and the separation of hydrogen from the production gas (H2 and O2 mixture) is essential to prevent an explosion. In this regard, various low and high-temperature

82  Materials for Hydrogen Production, Conversion, and Storage separation methods can be applied [123]. Due to the lower efficiency of solar water thermolysis, water splitting thermochemical cycles are recommended for the high efficient hydrogen production. Thermochemical cycles contain the multi-step thermal splitting of water. In this process, metal oxides generate hydrogen from water at T1300 K [124]. In the three routes, reforming, cracking, and gasification, fossil fuels are used as raw materials for hydrogen and syngas production. Here, the greenhouse gas emission considered the main disadvantage can be neglected due to the use of concentrated solar energy to operate these processes. Solar cracking is utilized for the generation of lighter hydrocarbons and hydrogen from fossil fuels. Methane is used as raw material for solar cracking processes, and hydrogen and solid carbon are formed as reaction products [125]. It is also an endothermic reaction, and the reaction is carried out in a solar reactor. The reactor design plays a significant role in the efficiency of the reaction. There are two types of solar reactors such as direct and indirect irradiated reactors [121]. The direct irradiated reactors are used for direct heating of the reactants through solar radiation. In the indirect irradiated reactors, the solar radiation is absorbed by carbon black (absorber) materials, and the heat is transported to the reactant (generally methane). Its disadvantage is the particle deposition on the window in the reactor system. In the solar reforming process, hydrocarbons react with steam or carbon dioxide, and catalysts reduce reaction temperatures. The syngas is obtained in the solar steam reforming process. This reaction is conducted at temperatures between 800°C–1000°C, and nickel or nickel-based materials are used as catalysts. In direct heating of reactant gas, the concentrated solar energy passes through the window to the catalyst bed in the reactor system, and the reforming reaction takes place [117]. Coal is used in the steam gasification reaction, and this process consists of two essential steps. In the first step, the coal is pyrolyzed at 1200 K, and char, carbon monoxide, carbon dioxide, hydrogen, and methane occur as reaction products. In the second step, char is gasified with steam or carbon dioxide to produce syngas [126]. In solar coal gasification, the concentrated solar radiation is absorbed by coal, and steam or carbon dioxide is used as gasifying agents.

3.5 Photobiological Hydrogen Production Biological hydrogen production is a promising environmentally friendly method and produces hydrogen at ambient temperatures and pressures

Solar–Hydrogen Coupling Hybrid Systems  83 while using less energy [127]. Moreover, the method can be carried out using different microorganism species and carbon sources (Figure 3.2). Biological hydrogen production is carried out using two different phenomena based on light dependency: photo-fermentative or dark fermentative. Photobiological hydrogen production is the conversion of organic matter to hydrogen through the metabolism of photosynthetic bacteria under anaerobic light conditions [128]. Three methods are preferred for photobiological hydrogen production, which vary the types of microorganism and their metabolic pathways followed for hydrogen production. Direct biophotolysis is the first pathway, a combination of chemical and biological processes decomposing water molecules through photosynthesis to transfer hydrogen. Scenedesmus spp., Chlorococcum spp., and Chlorella spp., are the most commonly preferred species producing hydrogen with a high yield using this pathway. The second process is indirect photolysis. Cyanobacteria or microalgae convert organic substances to hydrogen in two steps by using light to synthesize carbohydrates from the substrate first and then producing hydrogen via synthesized carbohydrates under dark conditions. Indirect photolysis is generally not preferred for biohydrogen production since this method has low yield and complicated decomposition of crop plants [129]. Photofermentation is the third method used to convert organic substances to hydrogen by photosynthetic heterotrophic microorganisms. Photofermentation is a cost-­effective and safe method among all photobiological methods [127]. One of the most critical components of this process, different microorganism species can be used in biohydrogen production. Because hydrogen production yield is being correlated with microbial activities, researchers are focused on improving biochemical efficiency. Even though the use of genetic engineering techniques to manipulate the genetics of microorganisms is a well-studied area in biohydrogen production [130], green algae species such as Chlorella spp., Scenedesmus obliquus, Chlamydomonas reinhardtii, Dunaliella salina, and Spirulina platensis spp.; cyanobacteria such

Photobiological Hydrogen Production

Enzymatic process

Metabolic process

Anaerobic digestion

Figure 3.2  Hydrogen production based on photochemical systems.

Fermentation

84  Materials for Hydrogen Production, Conversion, and Storage as Anabaena; and photosynthetic bacteria such as Rhodospirillum rubrum and purple nonsulfur bacteria are generally preferred species due to their high H2 bio-convertibility yields, already [127, 131]. Usage of biomass or living organisms such as waste-activated sludge or algal waste is a potential replacement for fossil fuels used for hydrogen production due to being renewable sources, unlike fossil fuels and producing hydrogen alongside CO2 uptake. Because of its availability, high organic content, and low cost, many organic wastes have gotten attention. Even though various biological and chemical processes are needed for photo­ biological hydrogen production from plants, algae, or cyanobacteria [132], algal biomass is a promising source with both medium-light transmittance and abundant. Nevertheless, photobiological hydrogen production is still in the early stages of research compared to other biofuels like biodiesel and bioethanol [127, 133]. Photobiological hydrogen production has been reported to be primarily affected by environmental factors. Among all factors, light intensity and duration significantly impact algal hydrogen generation due to a light-dependent system [134]. Moderately low light intensity is optimal for microalgal growth for biohydrogen production because of the self-­ inhibition occurrence with the increment of light intensity. Temperature and pH are also crucial factors that might cause a change for cyanobacteria and microalgae in hydrogen generation yield. The optimum temperature and pH for photobiological hydrogen production vary according to species [135]. In addition, different carbon sources and their concentrations can cause a change in the hydrogen production yield of cyanobacteria due to inducing electron transportation which affects nitrogenase activity [136]. A comprehensive metabolic remodeling, which comprises novel pathways and enzyme enhancement via metabolic engineering, might lead to significant improvements in photobiological hydrogen production. In addition, genetically engineered microorganisms, several of which are now already available for green algae and cyanobacteria, can help to enhance hydrogen production yield [137, 138].

3.6 Conclusion Coupling between solar energy systems and hydrogen generations benefits adaptation and realization of G2G system from small to large scale applications. Significantly, hybrid solar systems such as photo (photovoltaic,

Solar–Hydrogen Coupling Hybrid Systems  85 photochemical, photoelectrochemical, photobiological) coupled with fuel cells and electrolyser increase the energy system’s total efficiency and sustainability requirements in the domain of the G2G concept. The replacement of the traditional fuel and battery storage system with hybrid solar energy systems using hydrogen will significantly contribute to the transition to a carbon-free economy and green agreement, especially since green production and storage are made. Although there is a lot of knowledge and investigation of the G2G system based on solar hydrogen, the main barriers against commercializing the aforementioned hybrid systems are the initial cost of such installation, high cost of investment, socio-political obstacles, and lack of motivation to adapt of the private and also public sector. However, many recommendations and regulations are considered to overcome existing barriers and encourage the sector, such as developing tools or software to design the most efficient hybrid solar-hydrogen combination, fund research on the subject, support entrepreneurs, and financial aid companies. This study focused on hybrid solar systems coupled with hydrogen and “photo” technologies, highlighting green hydrogen production and storage coupling with solar energy. Therefore, in an increasing relation to developing smart cities integrated with green and sustainability concepts, the G2G system will play an essential role in limiting fossil fuel consumption, decreasing CO2 emission, using renewable energy sources, and reducing waste. Further, the hybrid systems can generate and store hydrogen during the day, then convert it into an energy provider at night using the electrolyser system as a fuel cell. As a result, smart grid technologies are proposed to be integrated with the solar-hydrogen system to build smart cities to develop integration networks and create solutions for urban environments to optimize resources without the carbon footprint.

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94  Materials for Hydrogen Production, Conversion, and Storage 122. Steinfeld, A., Solar thermochemical production of hydrogen—A review. Sol. Energy, 78, 603, 2005. 123. Baykara, S., Experimental solar water thermolysis. Int. J. Hydrog. Energy, 29, 1459, 2004. 124. Charvin, P., Stéphane, A., Florent, L., Gilles, F., Analysis of solar chemical processes for hydrogen production from water splitting thermochemical cycles. Energy Convers. Manage., 49, 1547, 2008. 125. Rodat, S., Abanades, S., Flamant, G., Co-production of hydrogen and carbon black from solar thermal methane splitting in a tubular reactor prototype. Sol. Energy, 85, 645, 2011. 126. Gregg, D.W., Aiman, W.R., Otsuki, H.H., Thorsness, C.B., Solar coal gasification. Sol. Energy, 24, 313, 1980. 127. Sivaramakrishnan, R., Shanmugam, S., Sekar, M., Mathimani, T., Incharoensakdi, A., Kim, S.H., Parthiban, A., Geo, V.E., Brindhadevi, K., Pugazhendhi, A., Insights on biological hydrogen production routes and potential microorganisms for high hydrogen yield. Fuel, 291, 120136, 2021. 128. Argun, H. and Kargi, F., Photo-fermentative hydrogen gas production from dark fermentation effluent of ground wheat solution: Effects of light source and light intensity. Int. J. Hydrog. Energy, 35, 4, 1595, 2010. 129. Das, D. and Veziroglu, T.N., Advances in biological hydrogen production processes. Int. J. Hydrog. Energy, 33, 21, 6046, 2008. 130. Poudyal, R.S., Tiwari, I., Koirala, A.R., Masukawa, H., Inoue, K., Tomo, T., Najafpour, M.M., Allakhverdiev, S., II, Veziroğlu, T.N., Hydrogen production using photobiological methods, in: Compendium of Hydrogen Energy, V. Subramani, A. Basile, N. Veziroğlu (Eds.), pp. 289–317, Woodhead Publishing, Amsterdam, 2015. 131. Hu, J., Comparisons of biohydrogen production technologies and processes, in: Waste to Renewable Biohydrogen, Q. Zhang, C. He, J. Ren, M. Goodsite (Eds.), pp. 71–107, Academic Press, 2021. 132. Yin, Y. and Wang, J., Enhanced biohydrogen production from macroalgae by zero-valent iron nanoparticles: Insights into microbial and metabolites distribution. Bioresour. Technol., 282, 110, 2019. 133. Yadav, G., Shanmugam, S., Sivaramakrishnan, R., Kumar, D., Mathimani, T., Brindhadevi, K., Pugazhendhi, A., Rajendran, K., Mechanism and challenges behind algae as a wastewater treatment choice for bioenergy production and beyond. Fuel, 285, 119093, 2021. 134. McKinlay, J.B. and Harwood, C.S., Photobiological production of hydrogen gas as a biofuel. Curr. Opin. Biotechnol., 21, 3, 244, 2010. 135. Kaushik, A. and Anjana, K., Biohydrogen production by Lyngbya perelegans: Influence of physicochemical environment. Biomass Bioenerg., 35, 3, 1041, 2011.

Solar–Hydrogen Coupling Hybrid Systems  95 136. Kaushik, A. and Sharma, M., Exploiting biohydrogen pathways of cyanobacteria and green algae: An industrial production approach, in: Biohydrogen Production: Sustainability of Current Technology and Future Perspective, A. Singh and D. Rathore (Eds.), pp. 97–113, Springer, New Delhi, 2017. 137. Nyberg, M., Heidorn, T., Lindblad, P., Hydrogen production by the engineered cyanobacterial strain Nostoc PCC 7120 ΔhupW examined in a flatpanel photobioreactor system. J. Biotechnol., 215, 35, 2015. 138. Khetkorn, W., Rastogi, R.P., Incharoensakdi, A., Lindblad, P., Mademwar, D., Pandey, A., Larroche, C., Microalgal hydrogen production–A review. Bioresour. Technol., 243, 1194, 2017.

4 Green Sources to Green Storage on Solar–Hydrogen Coupling A. Mohan Kumar1, R. Rajasekar1*, P. Sathish Kumar2, S. Santhosh1 and B. Premkumar3 Department of Mechanical Engineering, Kongu Engineering College, Erode, Tamilnadu, India 2 Department of Mining Engineering, Indian Institute of Technology Kharagpur, West Bengal, India 3 Department of Food Technology, Kongu Engineering College, Erode, Tamilnadu, India 1

Abstract

Solar energy will be an important resource in the forthcoming power development of the universe. Hydrogen is a renewable energy transporter and an efficient alternative resource for combustible fuels and carbon-dioxide release to save earth from climate change. The integrated solar-H2 system can be applied into various standalone operations. This chapter comprises various methods of conversion of solar energy into hydrogen, which includes concentrated solar thermal hydrogen production, ­thermo-chemical aqua splitting technology for solar-H2 production, solar-H2 through de-carbonization of fossil fuels, solar cracking, solar thermal-based hydrogen generation through electrolysis, and photovoltaics-based hydrogen production. Keywords:  Solar-hydrogen, solar cracking, aqua splitting technology, photovoltaics, de-carbonization, electrolysis

*Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (97–114) © 2023 Scrivener Publishing LLC

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98  Materials for Hydrogen Production, Conversion, and Storage

4.1 Introduction Global warming is a major problem faced by many countries due to the release of greenhouse gases, namely, carbon dioxide, methane, nitrous oxide, etc. The main source of harmful gases is red-hot of combustible gases for various applications, namely, vehicle transportation, electricity production, and heat generation [1]. To eliminate global warming, it is essential to replace fossil fuels to natural resources such as cosmic and air energy. Since solar energy is clean and has plentiful availability, it has many benefits compared to other energy resources. However, energy production using solar is not continuous and needs an energy storage system [2]. Hydrogen is an excellent option to store surplus electrical energy produced from renewable source. Among the different methods of hydrogen production process, electrolysis method of hydrogen production is commonly used. The hydrogen-solar coupling is a very attractive method of solar energy storage system compared to the other method because of different advantages of hydrogen such as clean, with plenty of availability, and low operating temperature [3, 4]. Especially, hydrogen produced by using solar energy in water electrolysis process is a very reliable process [4]. In particular, hydrogen produced based on water electrolysis coupled with photovoltaic method is a favorable way of natural energy. However, the performance of the photovoltaic system reduces at elevated temperature [5, 6]. Enhancement of the electrical energy without compromising performance of the photovoltaics is the best approach [7]. The energy on the surface of the photovoltaics is dissipated as waste heat, which cannot be utilized as electrical energy [8]. To improve the effectiveness of the photovoltaic cell, make H2O circulation on the surface of the cell. Photovoltaic cell is a very favorable energy conversion method, which converts electrical and heat energy at the same time [9]. It also provides heat to the domestic applications such as water heater, which saves energy required to heat water. The financial payback period of photovoltaic thermal system is advanced due to the energy extract from the source [10]. The financial and atmospheric parameters are also required to compute photovoltaic thermal system efficiency; the energy payback period for the photovoltaic thermal system is 10 months to 172 months [11]. Duel energy system gives the idea about the integration of the dual energy system and improving energy quality with dual energy storage system. The combined system of fuel cell and the photovoltaic cell provides consistency in energy storage for domestic applications [12, 13]. The photovoltaic and fuel cell integrated system consists of the photovoltaic

Green Sources to Green Storage  99 module and fuel cell stack, which contain electrolyzer to generate hydrogen [14–16]. Fuel cell is a very good energy conversion system, which converts maximum hydrogen into electricity with negligible loss. The predominant benefits of the proton exchange membrane fuel cell (PEMFC) is quick start, lesser emissions, small operating temperature, and higher energy density [17, 18]. The integration unit of low temperature proton exchange membrane and high purity hydrogen production is most suitable for portable applications. It is also used as static power generation unit. In recent years, people are living in distributed places in remote areas, hence the alternative power source required to overcome the issue. The installation cost of the cosmic power method is advanced and the variation of the solar radiation produced irregular energy generation. The important factor to enhance presentation of the system is appropriate selection of the components.

4.1.1 Hybrid System The diagrammatic representation of the hybrid solar hydrogen energy method for the rural application is shown in the Figure 4.1. The hybrid system comprises the hydrogen storage container, battery, fuel cell, electric

PVT

1

H2O

11 14 AC/DC Converter AC 20 Electric Power load

13

Load controller

15

21

2 23

18

17 DC

12 Pump

DC

16 DC 22

Battery

DC

19

PEMFC

Hydrogen Tank

H 2O Tank 3

10

9

DHW

DC

7 8 Compresser

6

4 5 PEM Electrolyzer

H2O & O2

O2 Sep O2

Figure 4.1  Diagrammatic representation of the integrated solar system for standalone operation in residential applications.

100  Materials for Hydrogen Production, Conversion, and Storage energy load. The photovoltaic thermal is performing as generator of electricity and heat energy. The photovoltaic thermal of the solar panel is joined as series and gives thermal energy to water container through the pump positioned at the foot of the container. The photovoltaic thermal method is chosen based on the power requirement, and the solar irradiation is produced on the installation site. Table 4.1 shows the hydrogen’s critical role of energy conversion to climate change. The electrical energy output from the photovoltaic thermal system is achieved with maximum power point tracking (MPPT). The electrical energy output is produced by the MPPT joined with the AC/DC converter via the DC power line. The excess electrical power generated by the cosmic irradiation compared to the required electrical load is stored in the proton exchange membrane, which is stored as hydrogen and oxygen in separate storage tank. The proton exchange membrane electrolyzers are having many benefits compare to the electrolyzers such as maximum current density, maximum safety, small in size, and user friendliness. The electrical power generated by the cosmic irradiation is lesser compare to what is required, and then it is compensated by the proton-exchange membrane fuel cell (PEMFC). In integrated S-H energy system, oxygen is not stored. However, the medical oxygen can be compressed and deposited. The reusable water is separated from the proton exchange membrane fuel cell. The PEMFC gives electric power with high stability and also a range of operational and atmospheric conditions. In integrated S-H energy system, energy stored in batteries is produced by the photovoltaic thermal models and fuel cells. The batteries are very important for the continuous electrical energy required by the integrated cosmic hydrogen energy system. Table 4.1  Hydrogen’s critical role during the energy conversion to climate change. S. no.

Sustainable energy integration

Decarburization of energy system

6

Flexible systems

Cleaner transport

7

Surges accessibility

Cleaner industrial processes

8

Increased resilience

Cleaner outputs to meet the needs

9

Multigenerational option

Renewable feedstock to all sectors

10

Enhanced efficiency

Declined environmental impact and increased sustainability

Green Sources to Green Storage  101 The  battery is very helpful when the photovoltaic and photo-exchange membrane fuel cell produces lesser electric power compare to the solar power. Similarly, the solar power load surpasses the load capability of the fuel cell then the power is deposited in the battery. To increase the life of the battery, the gaseous cell is twisted when the electric power reduces the preset value.

4.2 Concentrated Solar Thermal H2 Production Owing to the high cost of the electrolyzers, the electricity produced by the alternative method, that is, conversion of the water into H2 and O2, can be used in the fuel cell [19]. The oxygen and hydrogen produced as reaction molecules in fuel cell during the energy production can be converted into water in a closed system [20]. For the upcoming years, replacement of the combustible fuel power plants and the power supplied to the national grids is mandatory [21]. The conversion performance of the Solar to H2 is the major indication of the performance of the plant. The past investigation reveals that the solar to H2 conversion performance may be enhanced up to 12% in the photovoltaics-based system [22]. The hydrogen production efficiency in concentrated photovoltaics varies from 18% to 24.8% under different condition. When the maximum power in the photovoltaics and maximum power in electrolyzers meets at the particular value of straight normal irradiation, the maximum solar to hydrogen effectiveness will be achieved. In real practice of solar hydrogen power plant, the direct normal irradiation and photovoltaic maximum power continuously change throughout the day. These changes may reflect on the efficiency of the power plant [23]. To reduce expenses for the hydrogen storage and transportation, homogeneous power compression is applied on the site of electrical power generation [24]. As shown in Figure 4.2, the solar energy produces hydrogen fuel via different paths namely, thermochemical (solar to thermal energy); in electrolysis process, solar is transformed into electrical power by using photovoltaics. The cosmic energy is transformed into the photonic energy through photocatalysis [25]. Hydrogen production by photonic method is still under investigation. The homogeneous, heterogeneous, and different photo electrochemical methods are available to produce hydrogen products [26]. Yi et al. investigated the bismuth tungstate photocatalysts with nanostructures having ability to split water molecules to the hydrogen [27]. Zamfirescu et al. [28] reported the water electrolysis, in which the dual photo electrochemical method has been used and thermodynamically analyzed. The results reported that the

102  Materials for Hydrogen Production, Conversion, and Storage

Hydrosol production process

Form of energy

Solar energy

Thermal energy

Electrical energy

Biochemical energy

Thermo chemical water splitting

High temperature electrolysis

Enzymatic process

Thermolysis

Hybrid thermo chemical

Metabolic process

Thermo chemical gasification/ pyrolysis

Hydrogen sulfide cracking

Conventional electrolysis

Photonic energy

Photocatalysis (photoelectrochemical, photochemical)

Anaerobic digestion Fermentation

Hydrogen

Figure 4.2  Production of the solar hydrogen via different routes.

cell has received 20% of the solar light and the hydrogen conversion efficiency is 4%. The flat plate cosmic collector installed after the cell captured approximately 60% of the solar radiation. The hydrogen production uses bacteria or algae from solar energy in photobiological process [29]. The concentrated parabolic trough is used as solar collector; it is greatly capable of conversion of solar to hydrogen in commercial market. Due to its being economical, the conversion of solar to hydrogen is challenging in the market. However, compared to the other energy conversion process, solar to hydrogen conversion is less efficient; it is one barrier facing solar to hydrogen conversion [30]. For electrolysis, thermolysis, and thermochemical process, water is used as chemical source; dual source of fossil gaseous and water is used for the cosmic steam reforming process and cosmic gasification process. The thermal energy of the concentrated photovoltaics is used to maintain the maximum temperature during hydrogen production [31]. A significant irradiation from the reactor can reduce due to the elevated temperature process. Additionally, a powerful technique is necessary to separate the generated hydrogen and oxygen to prevent explosion. There are two types of the separation techniques, namely, electrolytic and effusion separation.

Green Sources to Green Storage  103 Tube for quenching H2O feed

Concentrated solar energy

Window

Cavity

Figure 4.3  Diagrammatic representation of hydrogen production using solar reactor for the thermolysis process.

Two decades ago, investigation was carried out to generate hydrogen by the thermolysis process. This method of production did not reveal much attention on the practical solar hydrogen production due to lacking of high temperature tolerance. Baykara [32] reported the generation of hydrogen by using solar reactor with the capacity of 1 kilowatt in separation of water, as shown in Figure 4.3. Water absorbs heat from the wall and is evaporated before filling in the cavity. Quenching steam is passed through the four jets to reduce temperature which are in normal direction to the reactor. The effectiveness of this method was reported as 1.1% and the hydrogen mole fraction was found to be 0.00012 and 0.03 at the temperature of 1500 K and 2500 K, respectively.

4.3 Thermochemical Aqua Splitting Technology for Solar–H2 Generation When thermochemical method of conversion of solar to water is performed at the mid temperature level, it’s about 1200 K and the separation of the oxygen and hydrogen is not a big problem. The thermochemical process of metal oxide and redox reaction consists of two steps, namely, solar and non-solar. Equations are shown below. First step (solar)



y M x O y → xM + O2 2

(4.1)

104  Materials for Hydrogen Production, Conversion, and Storage Second step (non-solar)

xM + yH2 O → Mx Oy + yH2

(4.2)

where M represents metal and MxOy denotes a metal oxide. At the beginning of the solar-based procedure, endothermic cosmic thermal separation of metal oxide to the metal occurs; later, the exothermic non-solar founded process happened, and the metal hydrolysis is carried out to produce H2 [33]. The initial compound in Equation (4.1) is metal oxide, which is produced and applied in Equation (4.2); therefore the thermochemical process is cyclic process. The H2 and O2 are produced in dual different processes; no gas separation process is needed [34]. Additionally, a pure form of hydrogen has been obtained from this process; this can be used directly in the PEMFC. Diver et al. introduces a thermo-chemical reactor, which consists of reactor, receiver, fixed blades, and recuperator; by using ferrite reactor, the materials move in opposite direction as shown in Figure 4.4. At the front of the rotating rings, the cosmic radiation has directed in the top of the section. The mid rings were used to get the workable heat and hydrolysis reaction is carried out with cosmic power of 15 kW. The efficiency of hydrogen production was reported as 32.3 %. The thermochemical cycles are classified as Concentrated solar radiation

O2 outlet

Thermal reduction zone

Recuperation zone

Recuperation zone

Water oxidation zone

H2O Inlet H2O, H2 Outlet

Figure 4.4  Diagrammatic representation of the hydrogen production using thermochemical solar reactor.

Green Sources to Green Storage  105 volatile and non-volatile cycles based on the metal oxide phase during the process. In the nonvolatile cycle phase, quenching is not required because the oxide remains as solid phase. However, in volatile phase, transition of solid to gas stage happened and diminished as metal oxide. Hence quenching is required for this phase change [35]. Chambon et al. investigated that the production of zinc and Tin (II) oxide from separation of compressed Tin(IV)oxide and zinc oxide by using 1 kilowatt solar system; 1%–3% of thermochemical cycle efficiency was found. Recently, thermochemical process has been investigated with the materials, namely, magnesium chloride, copper chlorine, hybrid sulfur, and hydride iodine [36]. The convenience to handle the reactions and moderate temperatures are required for magnesium chlorine; it is the best method in thermochemical water splitting technology compared to other technologies to produce hydrogen [37]. Dincer proposed a combined method of magnesium–chlorine cycle and the traditional electrolysis process for producing suitable method and declining required electrical work. Copper–chlorine is most possible method of the cosmic to H2 conversion with highest temperature condition, which was universally developed [37]. Researchers from different countries are achieved a case study to determine solar H2 generation using copper and chlorine cycle [38]. In total, 785 MW energy is received by the solar plant. However, 309 MW solar energy was only absorbed by the molten salt; therefore the total plant efficiency is 20% and the production rate is 0.89 kg hydrogen per second. Hydrogen generation using hybrid sulfur follows thermochemical and electrochemical steps. The endothermic thermochemical steps need a catalyst and high temperature (approximately 800°C). This process takes 1/3 part of electricity for electrolysis [39]. In a series, all the responses are carried out in liquid or gas phase to decline electrical power needs for transport. Decomposition of the sulfuric acid takes place at the first step over a catalytic bed at elevated temperature to produce liquid sulfur dioxide and oxygen. At the second step, the sulfur dioxide responds with iodine to produce hydriodic acids with different gravimetric characteristics and sulfur. Hydriodic acid disintegrated at the temperature of 300°C–350°C to produce the decomposition reactor [40].

4.4 Solar to Hydrogen Through Decarbonization of Fossil Fuels Production of hydrogen from fossil fuels is at various temperatures ranging from 200°C to 3000°C. The solar thermochemical process is classified as three types, namely, solar gasification, solar cracking, and solar reforming.

106  Materials for Hydrogen Production, Conversion, and Storage

4.4.1 Solar Cracking In solar cracking method, the larger molecules are broken into tiny molecules, namely, oil and natural gas, which is represented by the following Equation (4.3).

Cx H y → xC( gr ) +



y H2 2

(4.3)

In this chemical reaction, a carbon and H2-rich liquid stage is produced; many researchers investigated to produce hydrogen in solar cracking of methane method [41]. Due to the absorption of the methane by infrared radiation, the process of heating happened within the spectral range. In indirect method, the cosmic radiation power is captured by the carbon black and moved to methane [42]. Irradiation prototypes investigated by the Abanades and Flamant at the lab scale are shown in Figure 4.5. In direct irradiation prototype, the Argon and methane are mixed using a mixture, which is passing through graphite nozzle. The testing of the irradiation is carried out with different nozzle geometries and concluded that it influence more on the chemical conversion and thermochemical efficiency; the range of thermochemical efficiencies was obtained between 0.5% and 1.8 % [43].

Inlet Ar+ CH4 Concentrated solar radiation

Inlet Ar+ CH4

H 2O

Graphite Nozzle

H2O H2O

Product outlet

Figure 4.5  Solar-to-hydrogen through solar cracking of methane.

Green Sources to Green Storage  107

H2O

Heat engine

H2 O2

Electrolyzer Electrical generator

Concentrating collector

Figure 4.6  Diagrammatic representation of solar to hydrogen using solar thermal process.

4.5 Solar Thermal-Based Hydrogen Generation Through Electrolysis The solar thermal founded H2 generation method consists of four key parts, namely, heat engine, concentrating collector, electrical generator, and electrolyzer as shown in Figure 4.6. The hotness storage unit has added to this method for uninterrupted stream of thermal power to the heat engine. Cosmic radiation focused on the absorber, which is committed to the heat engine; portion of the power is transformed to the useful mechanical effort and remaining energy is unused. The mechanical work is converted into the electrical power by using electrical generator. This electrical energy is utilized for the water electrolysis. The generator and the heat engine have moving parts, hence more maintenance is required [44].

4.6 Photovoltaics-Based Hydrogen Production The combined system of photovoltaics and low temperature electrolysis is a best system to generate H2 on both solar section and electrolysis. In the beginning of the 70s, photovoltaic system combined with water electrolysis was used for hydrogen production [45]. Due to the environmental issues, most suitable method of converting solar power to the electricity is direct method of photovoltaic source to electrical energy. Lodhi established perception of the cosmic to H2 using photovoltaic cells as alternative of liquid

108  Materials for Hydrogen Production, Conversion, and Storage and gaseous form of fossil fuels [46]. The cosmic to H2 generation effectiveness is 16%; similarly, the efficiency of electrolysis and photovoltaic systems is 80% and 20%, respectively [47]. According to Joshi et al. [48], calculation of the power efficiency of photovoltaics founded cosmic-H2 conversion is 3.48% to 4.84 %. This is due to the efficiency of the photovoltaic system [49]. Tributsch reported that if the efficiency electrolysis system is about 70% to 75% then the of conventional silicon-based photovoltaics is about 8% to 14% [50]. The commercial distribution of the solar-hydrogen systems is reduced due to the low effectiveness of the cosmic-H2 generation [51]. The advantages of the photovoltaic systems are: it is an ecofriendly technology, it does not emit harmful gases, and there is noise contamination during operation. Also, the maintenance is not complex due to lesser moving parts. However, the photovoltaic system is costlier. Kothari et al. reported that the solar–hydrogen production process efficiency is increased up to 25% to 30% by sequencing of photovoltaic cells and electrolysis [52]. Ulleberg et al. [53] observed the effectiveness of the solar-H2 systems at low temperatures and low solar radiations, and reported that the size of the solar-H2 system influences more on the performance of the system. Efficiency of the photovoltaic system diminishes at huge temperature; heat is transferred to electrolyzer, where the performance of the system improves. The photovoltaics is made up of semiconductors, under the exposures of the solar radiations the electron hole couples are detached by the electrical field about the connection. According to the semiconductor materials the photovoltaics is categorized into three generations [54]. The silicon founded photovoltaics is a first generation photovoltaics, which is available up to 85% in the commercial market. This type of the photovoltaics offered single and multi crystalline with efficiency of 25.6% and 20.4%, respectively. The third-generation photovoltaics is multi-junction solar cells, which are huge cost and the different type of the concentrator is used to concentrate solar irradiation to the smaller area to avoid the higher cost of flat plate solar cells [55]. The intensity of the solar radiation has been increased up to 200 suns to 1000 suns by using curved mirrors [56]. However, the concentrated photovoltaics plays an important character in the market, and the consideration has been given to increase the hat elimination rate and decrease the fabrication cost [57]. Different types of concentrating technology have been used, namely, central receiving system, 2d focusing reflector, linear concentrating reflector, 2d concentrator, and linear concentrating lens [58]. A diagrammatic representation of concentrated

Green Sources to Green Storage  109 Receiver PV array PV Cooling system

Parabolic disc

Load

MPPT DC/DC Electrolyzer

DC/AC DC Main line

DC/DC

DC/DC

Fuel cell

Water storage Oxygen storage Hydrogen storage

DC/AC Hydrogen Compressor

Figure 4.7  A diagrammatic representation of CPV‐based hydrogen production.

photovoltaic-based hydrogen production is shown in Figure 4.7 [59]. To improve the performance of the concentrated photovoltaics, the device called extreme power point tracking is used at the production. The produced electrical energy is given to the key DC (direct current) power line after passing through the extreme power point tracking device and the DC/DC converter. Solar tracker is used to enhance the effectiveness of the system. The concentrated photovoltaics is considered for solar beam direction; a solar tracker is needed to track solar beam direction in all the times. If solar beam is not absorbing solar radiation properly, the electrical energy generation reduced to zero.

4.7 Conclusion The solar–hydrogen system gives many advantages as a clean energy source. Interruption of solar energy is an unavoidable characteristic of this uncontaminated basis that might be rectified by solar–H2 coupling in its place of outdated power source. This chapter discusses in detail about integrated solar–hydrogen system for household application. Also discussing about different methods used to convert solar energy into hydrogen, it includes photovoltaics and solar cracking techniques.

110  Materials for Hydrogen Production, Conversion, and Storage

References 1. Koponen, J., Kosonen, A., Ruuskanen, V., Huoman, K., Niemelä, M., Ahola, J., Control and energy efficiency of PEM water electrolyzers in renewable energy systems. Int. J. Hydrogen Energy, 42, 29648, 2017. 2. Babu, C. and Ponnambalam, P., The role of thermoelectric generators in the hybrid PV/T systems: A review. Energy Convers. Manage., 151, 368, 2017. 3. Ferrero, D. and Santarelli, M., Investigation of a novel concept for hydrogen production by PEM water electrolysis integrated with multi-junction solar cells. Energy Convers. Manage., 148, 16, 2017. 4. Acar, C. and Dincer, I., Review and evaluation of hydrogen production options for better environment. J. Clean. Prod., 218, 835, 2019. 5. Yang, Z., Zhang, G., Lin, B., Performance evaluation and optimum analysis of a photovoltaic-driven electrolyzer system for hydrogen production. Int. J. Hydrogen Energy, 40, 3170, 2015. 6. Zafar, S. and Dincer, I., Thermodynamic analysis of a combined PV/T–fuel cell system for power, heat, fresh water and hydrogen production. Int. J. Hydrogen Energy, 39, 9962, 2014. 7. Skoplaki, E. and Palyvos, J.A., On the temperature dependence of photovoltaic module electrical performance: A review of efficiency/power correlations. Sol. Energy, 83, 614, 2009. 8. Kumar, A., Baredar, P., Qureshi, U., Historical and recent development of photovoltaic thermal (PVT) technologies. Renew. Sust. Energ. Rev., 42, 1428, 2015. 9. Tyagi, V., Kaushik, S., Tyagi, S., Advancement in solar photovoltaic/thermal (PV/T) hybrid collector technology. Renew. Sust. Energ. Rev., 16, 1383, 2012. 10. Herrando, M., Markides, C.N., Hellgardt, K.A., UK-based assessment of hybrid PV and solar-thermal systems for domestic heating and power: System performance. Appl. Energy, 122, 288, 2014. 11. Good, C., Environmental impact assessments of hybrid photovoltaic–­ thermal (PV/T) systems–A review. Renew. Sust. Energ. Rev., 55, 234, 2016. 12. Kusiak, A., Zhang, Z., Verma, A., Prediction, operations, and condition monitoring in wind energy. Energy, 60, 1, 2013. 13. Ahmadi, P., Dincer, I., Rosen, M.A., Multi-objective optimization of a novel solar-based multigeneration energy system. Sol. Energy, 108, 576, 2014. 14. Ahmadi, P., Dincer, I., Rosen, M.A., Energy and exergy analyses of hydrogen production via solar-boosted ocean thermal energy conversion and PEM electrolysis. Int. J. Hydrogen Energy, 38, 1795, 2013. 15. Akrami, E., Nemati, A., Nami, H., Ranjbar, F., Exergy and exergoeconomic assessment of hydrogen and cooling production from concentrated PVT equipped with PEM electrolyzer and LiBr-H2O absorption chiller. Int. J. Hydrogen Energy, 43, 622, 2018.

Green Sources to Green Storage  111 16. Hacatoglu, K., Dincer, I., Rosen, M.A., Exergy analysis of a hybrid solar hydrogen system with activated carbon storage. Int. J. Hydrogen Energy, 36, 3273, 2011. 17. Wu, C., Yin, M., Zhang, R., Li, Z., Zou, Z., Li, Z., Further studies of photodegradation and photocatalytic hydrogen production over Nafion-coated Pt/ P25 sensitized by rhodamine B. Int. J. Hydrogen Energy, 45, 22700, 2020. 18. Othman, R., Dicks, A.L., Zhu, Z., Non precious metal catalysts for the PEM fuel cell cathode. Int. J. Hydrogen Energy, 37, 357, 2012. 19. Véjar, S., Campos, J., Sebastian, P., Characterization of the electrical energy consumption of a building for the dimensioning of a solar–hydrogen energy system. Int. J. Energ. Res., 34, 962, 2010. 20. Manoharan, Y., Hosseini, S.E., Butler, B., Alzhahrani, H., Senior, B.T.F., Ashuri, T., Krohn, J., Hydrogen fuel cell vehicles; current status and future prospect. Adv. Sci., 9, 2296, 2019. 21. Jia, J., Seitz, L.C., Benck, J.D., Huo, Y., Chen, Y., Ng, J.W.D., Bilir, T., Harris, J.S., Jaramillo, T.F., Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30%. Nat. Commun., 7, 13237, 2016. 22. Fujii, K., Nakamura, S., Sugiyama, M., Watanabe, K., Bagheri, B., Nakano, Y., Characteristics of hydrogen generation from water splitting by polymer electrolyte electrochemical cell directly connected with concentrated photovoltaic cell. Int. J. Hydrogen Energy, 38, 14424, 2013. 23. Khaselev, O., Bansal, A., Turner, J., High-efficiency integrated multijunction photovoltaic/electrolysis systems for hydrogen production. Int. J. Hydrogen Energy, 26, 127, 2001. 24. Islam, S., Dincer, I., Yilbas, B.S., System development for solar energy-based hydrogen production and on-site combustion in HCCI engine for power generation. Sol. Energy, 136, 65, 2016. 25. Yang, Y., Zhang, C., Huang, D., Zeng, G., Huang, J., Lai, C., Zhou, C., Wang, W., Guo, H., Xue, W., Boron nitride quantum dots decorated ultrathin porous g-C3N4: Intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation. Appl. Catal. B Environ., 245, 87, 2019. 26. Yi, H., Huang, D., Qin, L., Zeng, G., Lai, C., Cheng, M., Ye, S., Song, B., Ren, X., Guo, X., Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production. Appl. Catal. B Environ., 239, 408, 2018. 27. Yi, H., Qin, L., Huang, D., Zeng, G., Lai, C., Liu, X., Li, B., Wang, H., Zhou, C., Huang, F., Nano-structured bismuth tungstate with controlled morphology: Fabrication, modification, environmental application and mechanism insight. Chem. Eng. J., 358, 480, 2019. 28. Zamfirescu, C., Naterer, G., Dincer, I., Water splitting with a dual photo-­ electrochemical cell and hybrid catalysis for enhanced solar energy utilization. Int. J. Energ. Res., 37, 1175, 2013.

112  Materials for Hydrogen Production, Conversion, and Storage 29. Hosseini, S.E., Abdul Wahid, M., Jamil, M., Azli, A.A., Misbah, M.F., A review on biomass-based hydrogen production for renewable energy supply. Int. J. Energ. Res., 39, 1597, 2015. 30. El-Emam, R.S. and Özcan, H., Comprehensive review on the techno-­ economics of sustainable large-scale clean hydrogen production. J. Clean. Prod., 220, 593, 2019. 31. Pregger, T., Graf, D., Krewitt, W., Sattler, C., Roeb, M., Möller, S., Prospects of solar thermal hydrogen production processes. Int. J. Hydrogen Energy, 34, 4256, 2009. 32. Baykara, S., Experimental solar water thermolysis. Int. J. Hydrogen Energy, 29, 1459, 2004. 33. Fuqiang, W., Lanxin, M., Ziming, C., Jianyu, T., Xing, H., Linhua, L., Radiative heat transfer in solar thermochemical particle reactor: A comprehensive review. Renew. Sust. Energ. Rev., 73, 935, 2017. 34. Roeb, M., Neises, M., Säck, J.-P., Rietbrock, P., Monnerie, N., Dersch, J., Schmitz, M., Sattler, C., Operational strategy of a two-step thermochemical process for solar hydrogen production. Int. J. Hydrogen Energy, 34, 4537, 2009. 35. Agrafiotis, C., Roeb, M., Sattler, C., A review on solar thermal syngas production via redox pair-based water/carbon dioxide splitting thermochemical cycles. Renew. Sust. Energ. Rev., 42, 254, 2015. 36. Ghandehariun, S., Naterer, G., Dincer, I., Rosen, M., Solar thermochemical plant analysis for hydrogen production with the copper–chlorine cycle. Int. J. Hydrogen Energy, 35, 8511, 2010. 37. Ozcan, H. and Dincer, I., Comparative performance assessment of three configurations of magnesium–chlorine cycle. Int. J. Hydrogen Energy, 41, 845, 2016. 38. Naterer, G., Suppiah, S., Rosen, M., Gabriel, K., Dincer, I., Jianu, O., Wang, Z., Easton, E., Ikeda, B., Rizvi, G., Advances in unit operations and materials for the CuCl cycle of hydrogen production. Int. J. Hydrogen Energy, 42, 15708, 2017. 39. Gorensek, M.B., Hybrid sulfur cycle flowsheets for hydrogen production using high-temperature gas-cooled reactors. Int. J. Hydrogen Energy, 36, 12725, 2011. 40. Yadav, D. and Banerjee, R., A review of solar thermochemical processes. Renew. Sust. Energ. Rev., 54, 497, 2016. 41. Abanades, S., Kimura, H., Otsuka, H., A drop-tube particle-entrained flow solar reactor applied to thermal methane splitting for hydrogen production. Fuel, 153, 56, 2015. 42. Abanades, S., Kimura, H., Otsuka, H., Hydrogen production from ­thermo-catalytic decomposition of methane using carbon black catalysts in an indirectly-irradiated tubular packed-bed solar reactor. Int. J. Hydrogen Energy, 39, 18770, 2014.

Green Sources to Green Storage  113 43. Abanades, S. and Flamant, G., Production of hydrogen by thermal methane splitting in a nozzle-type laboratory-scale solar reactor. Int. J. Hydrogen Energy, 30, 843, 2005. 44. Siddiqui, O. and Dincer, I., Examination of a new solar-based integrated system for desalination, electricity generation and hydrogen production. Sol. Energy, 163, 224, 2018. 45. Bilgen, E., Solar hydrogen from photovoltaic-electrolyzer systems. Energy Convers. Manage., 42, 1047, 2001. 46. Lodhi, M., A hybrid system of solar photovoltaic, thermal and hydrogen: A future trend. Int. J. Hydrogen Energy, 20, 471, 1995. 47. Rzayeva, M., Salamov, O., Kerimov, M., Modeling to get hydrogen and oxygen by solar water electrolysis. Int. J. Hydrogen Energy, 26, 195, 2001. 48. Joshi, A.S., Dincer, I., Reddy, B.V., Exergetic assessment of solar hydrogen production methods. Int. J. Hydrogen Energy, 35, 4901, 2010. 49. Joshi, A.S., Dincer, I., Reddy, B.V., Performance analysis of photovoltaic systems: A review. Renew. Sust. Energ. Rev., 13, 1884, 2009. 50. Tributsch, H., Photovoltaic hydrogen generation. Int. J. Hydrogen Energy, 33, 5911, 2008. 51. Kelly, N.A., Gibson, T.L., Cai, M., Spearot, J.A., Ouwerkerk, D.B., Development of a renewable hydrogen economy: Optimization of existing technologies. Int. J. Hydrogen Energy, 35, 892, 2010. 52. Kothari, R., Buddhi, D., Sawhney, R., Comparison of environmental and economic aspects of various hydrogen production methods. Renew. Sust. Energ. Rev., 12, 553, 2008. 53. Miland, H. and Ulleberg, Ø., Testing of a small-scale stand-alone power system based on solar energy and hydrogen. Sol. Energy, 86, 666, 2012. 54. Lewis, N.S., Toward cost-effective solar energy use. Science, 315, 798, 2007. 55. Price, J.S., Grede, A.J., Wang, B., Lipski, M.V., Fisher, B., Lee, K.-T., He, J., Brulo, G.S., Ma, X., Burroughs, S., High-concentration planar microtracking photovoltaic system exceeding 30% efficiency. Nat. Energy, 2, 17113, 2017. 56. Apostoleris, H., Stefancich, M., Chiesa, M., Tracking-integrated systems for concentrating photovoltaics. Nat. Energy, 1, 16018, 2016. 57. Chong, K., Siaw, F., Wong, C., Wong, G., Design and construction of non-­ imaging planar concentrator for concentrator photovoltaic system. Renew. Energy, 34, 1364, 2009. 58. Chong, K.-K., Lau, S.-L., Yew, T.-K., Tan, P.C.-L., Design and development in optics of concentrator photovoltaic system. Renew. Sust. Energ. Rev., 19, 598, 2013. 59. Burhan, M., Shahzad, M.W., Ng, K.C., Concentrated photovoltaic (CPV): Hydrogen design methodology and optimization, in: Advances in Hydrogen Generation Technologies, IntechOpen, London, United kingdom, 2018.

5 Electrocatalysts for Hydrogen Evolution Reaction R. Shilpa1, K. S. Sibi1, S. R. Sarath Kumar2*, R. K. Pai 3† and R.B. Rakhi4‡ Department of Physics, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala, India 2 Department of Nanoscience and Nanotechnology, University of Kerala, Kariavattom Campus, Thiruvananthapuram, Kerala, India 3 Technology Mission Division, Department of Science and Technology (DST), Ministry of Science and Technology, Government of India, New Delhi, India 4 Material Sciences and Technology Division, CSIR – National Institute of Interdisciplinary Science and Technology (CSIR-NIIST), Thiruvananthapuram, Kerala, India 1

Abstract

The world has been in the midst of a crucial energy crisis over the past few decades. The detrimental effects on the environment caused by non-renewable energy sources, such as fossil fuels, coal, and petroleum, demand clean as well as sustainable alternatives. There has been increasing emphasis on the development of renewable and clean energy devices such as bioenergy devices, hydrogen fuel cells, and photovoltaic devices. Hydrogen stands as solace as it is a compromising candidate to replace fossil fuels. As hydrogen is not available in free form on earth; it is customary to implement methods to extract hydrogen from its compounds like hydrocarbons and water. One of the simplest and easiest methods to accomplish this is via electrocatalytic water splitting. The present chapter gives a brief introduction to hydrogen generation through the mechanism of water electrolysis. This chapter also discusses various parameters that quantify the electrocatalytic behavior of a catalyst chosen and the various electrocatalysts used so far. Keywords:  Electrocatalysts, hydrogen evolution, overpotential, Tafel slop *Corresponding author: [email protected]; [email protected] † Corresponding author: [email protected] ‡ Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (115–146) © 2023 Scrivener Publishing LLC

115

116  Materials for Hydrogen Production, Conversion, and Storage

5.1 Introduction Hydrogen generation through electrocatalytic water splitting has gained huge attention to meet the constantly increasing energy demands of the global population since not only has it got a high gravimetric energy density but also the byproduct of the reaction is water. For enhancing the efficiency of hydrogen evolution reaction (HER), suitable electrocatalysts accelerating the rates of the HER should be chosen [1–3]. An electrocatalyst affects the activation energy of an electrochemical reaction, which is a function of the voltage at which the reaction occurs. Thus, electrocatalysts change the oxidation–reduction potential at the electrode–electrolyte interface. Generally, electrocatalysts are chosen based on three parameters: activity, stability, and selectivity [4, 5]. The current density generated in a reaction defines an active electrocatalyst. A quantitative description of the activity of a material is given by the Tafel equation, which relates the rate of an electrochemical reaction to the overpotential of the reaction [6, 7]. The stability of an electrocatalyst refers to its ability to withstand the potentials at which the electrochemical transformations occur. A durable electrocatalyst thus preserves its morphology as well as electrochemical characteristics throughout the reaction [8]. A selective catalyst interacts with particular substrates producing a single product, a concept explained by the Sabatier principle specifying the optimum bond strength between the catalysts and the reactants [9, 10]. Electrocatalysts are categorized into two: heterogeneous and homogeneous. Heterogeneous electrocatalysts may not be in the same phase as the reactants whereas homogeneous ones are soluble and in the same phase as that of the reactants [11]. Most of the catalysts routinely used are heterogeneous, acting on the surface of the electrode or the electrode itself. Efficient HER using metal alloys, enzymes, metal oxides, metal dichalcogenides, and bioinspired molecular electrocatalysts has been reported [12–14]. Figure 5.1 shows the general classification of electrocatalysts. Electrochemical splitting of water to liberate clean, efficient, sustainable hydrogen fuel to replace the traditional fossil fuels requires an optimal catalyst that lowers the energy to be supplied to overcome the barrier potential to initiate the HER. Metallic platinum and platinum-based compounds are widely accepted as the best HER catalysts by virtue of their low values of hydrogen binding energy and Gibbs free energy and also it requires low energy for activating the desorption of hydrogen from the catalyst surface. Studies show that catalyst candidates belonging to platinum group metals (PGMs) satisfy the demands of lower value of overpotential and higher

Electrocatalysts for Hydrogen Evolution Reaction  117 Electrocatalysts

Heterogeneous

Bulk Materials

Nanoparticles

Homogeneous

Nanomaterials

Enzymes

Carbon-Based Materials

Metal-Organic Frameworks

Inorganic Coordination Complexes

Figure 5.1  Schematic diagram of the general classification of electrocatalysts.

value of current density. However, scarcity of resources and the high cost of these materials necessitate developing novel, earth-abundant, cheap, and stable catalysts to extend this technology industrially [15].

5.2 Parameters to Evaluate Efficient HER Catalysts 5.2.1 Overpotential (o.p) For a real thermodynamic reaction to proceed at a favorable rate, an amount of energy is required called the thermodynamic potential. Under standard conditions, the Nernst potential for HER (EN.H) given by the Nernst equation is zero. But practically it is needed to supply an additional amount of energy. Overpotential (ƞ) is defined as the excess amount of energy required to drive a chemical reaction [16, 17]. This extra potential compensates for the potential drop due to the inherent resistance of the catalyst used, the resistance of the solvent, and the resistance of the system arising due to contact between other interfaces. If iR is the ohmic potential drop in the system [18], the potential applied to initiate the HER is written as



E = EN.H + iR + ƞ

(5.1)

The o.p at a current density (c.d) of 1 mA cm−2 is known as the onset o.p Generally, o.p at 10 mA cm−2 (ƞ10) is used as the main component to evaluate the selectivity of a material. An efficient, robust, and active catalyst is characterized by a smaller value of ƞ10 [19]. Figure 5.2 shows the schematic diagram of HER energetics.

118  Materials for Hydrogen Production, Conversion, and Storage (a)

(b)

Eact

ηIO, I ηIO, II ηIO, III

Ei

η -10 mA/cm2

E H2 EHER

2H+

E0

I’

I

II

III

j

iR

Figure 5.2  Schematic representation of (a) HER energetics and (b) polarization curves of various electrocatalysts.

5.2.2 Tafel Plot Tafel plot is another important parameter that determines an efficient electrocatalyst. The Tafel equation (5.2) relates the rate of an electrochemical reaction to the o.p required:



ƞ = b log (j / jo),

(5.2)

where j is the c.d, jo is the exchange current density (current density at zero o.p), and b is the Tafel slope (T.s). The T.s can be calculated from the rectilinear portion of the Tafel plot. Extrapolating the Tafel plot to zero o.p gives the value of jo. It is given by the expression,



b=

1 2.303RT αnF

(5.3)

Here, R is the ideal gas constant; T, the temperature, α, the charge transfer coefficient; F, the Faraday constant; and n, the number of electrons transferred (for HER, n = 2) [20, 21]. Smaller values for the T.s b and high jo are the properties of an efficient catalyst. Thus from Equation (5.3), it is clear that a catalyst with a small b value possesses a high charge transfer ability [22]. Another method to find b by using the parameters ƞ and the charge transfer resistance was proposed by Vrubel et al. [23] (Figure 5.3).

Electrocatalysts for Hydrogen Evolution Reaction  119 Slope I

η I

Slope II

II log jo.II log jo.I

log j

Figure 5.3  Schematic representation of T.s.

5.2.3 Stability As discussed earlier, a stable electrocatalyst should withstand the changes in potential or current throughout the electrochemical reaction. The stability of an electrocatalyst can be determined by repeated voltammetric analyses, wherein the catalyst is subjected to repetitive cycling at some fixed scan rate (say, 100 mV s−1). The stability of the material is ensured by a negligible change in o.p even after several cycles [24]. Galvanostatic or potentiostatic electrolysis also confirms a stable catalyst. By monitoring the changes in c.d in the potentiostatic mode or the changes in potential in the galvanostatic mode for at least 10 hours, the stability of a catalyst can be determined. Thus the stability and endurance of a catalyst are indicated by longer duration without changes in current or potential [25].

5.2.4 Faradaic Efficiency and Turnover Frequency Faradaic efficiency is a quantity that explains how well the charge carriers, i.e., electrons, take part in an electrochemical reaction. It qualifies a selective catalyst. In water electrolysis for hydrogen evolution, Faradaic efficiency is the ratio of the actual quantity of experimentally generated hydrogen to the theoretically calculated value of hydrogen. The theoretical value of hydrogen generation can be obtained through either constant current or constant potential electrolysis. Comparing it with the actual amount of hydrogen generated defines the inherent catalytic efficiency of the candidate chosen. The kinetic theory of the hydrogen evolution mechanism is explained by the parameter, turnover frequency (TOF). Evaluating the rate of catalysis in terms of TOF was first proposed by Boudart et al. in 1968. TOF is the number of molecules of reacting species in an active site per second. In other words, it is also defined as the frequency of turnover of molecules

120  Materials for Hydrogen Production, Conversion, and Storage of the reacting species into the desired product per unit time. TOF of an efficient HER electrocatalyst is calculated as



TOF =

jA 4nF

(5.4)

where j is the c.d, A is the electrode’s surface area, n is the mole number and F the Faraday’s constant.

5.2.5 Hydrogen Bonding Energy (HBE) The basic mechanism of HER involves the diffusion of protons onto the surface of the electrocatalyst to form adsorbed hydrogen (Had), as a result of the reaction with an electron (Volmer reaction). The decomposition of this adsorbed hydrogen to molecular hydrogen occurs through two steps: either by the diffusion of another proton followed by a reaction with an electron (Heyrovsky reaction) or two of the hydrogen atoms adsorbed onto the surface of the electrocatalyst may combine to liberate molecular hydrogen (Tafel reaction) [26, 27]. Electrochemical hydrogen adsorption – Volmer reaction:

H2O + e− ↔ Had + OH− (alkaline media)

(5.5)

H3O+ ↔ e− Had + H2O (acidic media)

(5.6)

Electrochemical desorption – Heyrovsky reaction:

H2O + e− + Had ↔ H2 (g) + OH− (alkaline media)

(5.7)

H+ e− + Had ↔ H2 (g)  (acidic media)

(5.8)

Recombination of adsorbed hydrogen:



2 Had ↔ H2 (g)   (both alkaline and acidic media)

(5.9)

Hydrogen bonding energy explains how efficiently the proton diffusion and decomposition (to form molecular hydrogen) occurs on the catalyst surface [28]. If HBE is too low, the number density of protons adsorbed onto the surface of the catalyst will be less, leading to a lower production of molecular hydrogen. Alternatively, if HBE is too high, protons will adhere

Electrocatalysts for Hydrogen Evolution Reaction  121 to the catalyst surface too strongly, making it difficult to detach. This leads to poisoning and corrosion of the electrocatalyst used. Thus, as explained by the Sabatier principle, the interaction between the reactants should be neither too strong nor too weak. Electrocatalysts falling in the crown region of the volcano curve are viable HER catalysts. Also, since the normal potential of the hydrogen electrode is referenced as zero, the Gibbs free energy, ΔGH, of the candidate chosen should be close to zero.

5.3 Categories of HER Catalysts 5.3.1 Noble Metal-Based Catalysts

Exchange Current for H2 Evolution, -log i, .A cm-2

Among the various electrocatalysts used so far, PGMs including platinum, palladium, ruthenium, iridium, and rhodium are recognized as highly efficient HER candidates compared with other metal-based materials. Owing to its optimum value of o.p and suitable Gibbs free energy which is almost equal to zero, platinum (Pt) stands as the most suited HER catalyst. Figure 5.4 shows the position of Pt in the volcano curve of various electrocatalysts. Placed next to the summit of the volcano curve, Pt proves to be the best candidate for electrocatalytic HER due to its small T.s and o.p [29, 30].

Pt

3

Ro Rh Ir Au

5

Ni Cu

Co Fe W

7 Sn Bi Zn Ga

9

Ti

Ag

Pb Cd

Mo Ti Nb

Ta

In

30 50 70 M-H Bond Strength/kcal mol-1

90

Figure 5.4  Volcano plot showing various HER catalysts. [Adapted and reprinted with permission (30) from copyright 1972, Elsevier].

122  Materials for Hydrogen Production, Conversion, and Storage Various attempts have been made using platinum nanoparticles (NPs) for enhanced HER electrocatalysis. The activities of Pt NPs are mainly determined by their size, shape, and facets. Contrary to the expectation of an increase in activity as the size of NPs decreases, it is found that the activities of Pt NPs decrease in the 1–3 nm range of atomic size [31]. It was reported by Tan and co-workers that the lowest value of activation energy was exhibited by the nearest neighbor (100)-facet bridge site pairs and it contributes to approximately 75% of the activity of the nanoparticles [32]. Ma et al. demonstrated that the size effects of Pt NPs supported on carbon nanotubes (CNTs) and found that NPs of size about 1.5 nm shows much higher HER performance than other size ranges in all pH solutions, and has mass activity 23–36 times that of commercially available Pt/C [33]. Chen et al. investigated the HER catalytic performance of flower-like platinum-cobalt-ruthenium alloy in 1 molar KOH and observed a very low o.p of 22 mV, small T.s of 46 mV dec−1, and high exchange current density (3.30 mA cm−2) [34]. Increasing metal utilization efficiency by dispersing single metal atoms on suitable supports was observed by Liu et al. Pt NPs dispersed onto onion-like nanospheres of carbon (Pt/OLC; 0.27 wt% Pt) exhibited 38 mV as o.p and high turnover frequency for HER in the acidic electrolyte [35]. The electrocatalytic HER activity of Pt particles dispersed on the walls of carbon nanotubes was reported by Tavakkoli et al. [36]. Zhang et al. directed his work to boost the electrocatalytic activity of MXene. Single Pt atoms immobilized onto the Mo vacancies of double transition metal MXene Mo2TiC3Tx exhibited high catalytic ability with ƞ10 of 30 and ƞ100 of 77 mV respectively [37]. Another work with Pt particles on MXenes was investigated by Zhang and co-workers. Pt NPs deposited 2D Ti3C2Tx MXenes showed optimal HER activity with ƞ10 of 67.8 mV, close to that of Pt/C catalyst available commercially (64.2 mV) [38]. The earth-abundant noble metal palladium (Pd), which is cheaper than Pt, shows favorable HER catalytic activity attributed due to the similarity in atomic size to Pt and good hydrogen adsorbancy. An alloy of palladium and silver, free from platinum, as HER electrocatalyst was studied extensively by Yao et al. Nanoporous Pd-Ag-Al electrodes showed low ƞ values of 63 and 108 mV at 200 mA cm−2 and smaller T.s of about 26 and 56 mV dec−1 in half molar H2SO4 and 1 molar KOH respectively [39] (Figure 5.5). A composite of porous palladium nanoparticles and carbon nitride was synthesized and its electrocatalytic properties were determined by Bhowmik and co-workers. The Pd-CNx composites showed a low onset potential of −12 mV, a T.s of 35 mV dec−1, and a c.d of 10 mA cm−2 at a lower o.p of −55 mV (Pd loading: 0.043 mg cm−2) [40]. Chandrasekaran

Electrocatalysts for Hydrogen Evolution Reaction  123

57 mV dec-1

-100 -150

Overpotential (V)

Current density (mA cm-2)

-200 0.0 1 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 Potential (V versus RHE) (d) (e) 0.6 0 -50

Pd53AI17 Pd65Ag17AI17 Pd22Ag63AI15 Ag90AI10 Pt/C

116 mV dec-1

0.5

0.12 73 mV dec-1

Ag90AI10

Pd53AI17

45 mV dec-1 Pd65Ag17AI17 26 mV dec-1 Pt/C 23 mV dec-1

CoMoP/C CoP/BCN

MoC NiCo2P5 Ni0.85Co0.11Se2

MoC5 CoN5

Pt/C Ru@C2N

0.05

Pd65Ag17Al17 Pd22Ag63Al16 Pd23AI17

Overpotential at 10 mA cm-2 (V)

0.10

56 mV dec-1

Pd22Ag63AI15

0.09 0.06

Pd65Ag17AI17

10 100 Current dendity (mA cm-2)

0.03 -200 10 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 (g) Potential (V versus RHE) 0.25 0.5 m H2SO4 1 M KOH 0.20 0.15

Pt/C

Pd22Ag63AI15

100 Current dendity (mA cm-2)

-50

-100

Pd66Ag17AI17 Initial 10,000th

14 mV

1 mV s-1 -150 -0.3 -0.2

(f)

-0.1

0.0

Potential (V versus RHE) 0

-50

Pd66Ag17AI17 Initial 30,000th

-100 1 mV s-1 -150 -0.3 -0.2

-0.1

0.0

Potential (V versus RHE) MoS27@NPG

144 mV dec-1

0.1

Pd53AI17

N-WC

153 mV dec-1

0.2

(c) 0

Current dendity (mA cm-2)

-150

Pd53AI17 Pd65Ag17AI17 Pd22Ag63AI15 Ag90AI10 Pt/C

236 mV

Ag90AI10

Current dendity (mA cm-2)

-100

0.3

dec-1

RuP2/NPC MoNi4 NiO/Ni-CNT NiFeO2/CFP Pt2Ni3 NiO/Ni Cr2O3/NiO/Ni NiCoP Pt/NGNs Pt-MoS2 Pt/PCM CoS-P/CNT MoS2/CoSe2 PICoFe/CN Ni-C-N Ru/GLC Ni/C Mo2C MoP CoSe2 FeP Fe-CoP/Ti (Fe0.49Co0.52)S2

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Figure 5.5  (a) Polarization curves and (b) corresponding Tafel plots of various palladium-based catalysts in 1M KOH (c) polarization curves of nanoporous Pd66Ag17Al17 before and after 10,000 cycles from −0.2 to 0.1 V in 1M KOH (d) Polarization curves and (e) corresponding Tafel plots of various palladium-based catalysts in 0.5M H2SO4 (f) polarization curves of nanoporous Pd66Ag17Al17 before and after 10,000 cycles from −0.2 to 0.1 V in 0.5M H2SO4 (g) comparison of o.p’s with previously mentioned catalysts. [Adapted and reprinted with permission (39) from copyright 2019, American Chemical Society].

and co-workers synthesized carbon dots doped with nitrogen supported by palladium nanoparticles composite (n-Pd@NDCDs) through hydrothermal carbonization and thermolytic reduction. The onset potential of n-Pd@NDCDs was about 0.195 VRHE and T.s of the composite observed through voltammetric and EIS methods were 135 and 141.8 mV dec−1, respectively. Also, the value of ƞ10 of n-Pd@NDCDs was determined as 291 mV [41]. Kelly et al. introduced the concept of monolayer amounts of noble metals deposited on carbides as cheaper and highly active electrocatalysts for hydrogen generation. Pd supported on carbides of tungsten and molybdenum synthesized using physical vapor deposition of Pd on WC

124  Materials for Hydrogen Production, Conversion, and Storage and Mo2C, showed excellent HER activity and electrochemical stability. Thus, monolayer metals deposited either on WC or Mo2C proved to be better candidates for HER water electrolysis [42]. Chao and co-workers used Pd nanorings alloyed with Cu-Pt dual sites as HER electrocatalysts with excellent activity in an acidic medium with a much low o.p of 22.8 mV [43]. Another work using palladium nanoparticles as HER electrocatalyst was reported by Nagamahesh and coworkers [44]. Ramakrishna et al. explained the synthesis strategy of carbon nanotubes doped with nitrogen, supported with palladium (Pd/NCNTs). The as-prepared Pd/NCNTs used as hydrogen electrodes in polymer exchange membrane (PEM) water electrolyzer proved to be an excellent alternative for Pt-based electrocatalysts for HER [45]. Among all the 4d metals, ruthenium (Ru) has the highest surface energy and has an inherent Ru-H bond strength equivalent to 65 kcal mol−1, which is much favorable for releasing the adsorbed hydrogen from its surface [46, 47]. The morphological properties of Ru NPs and their correlation to the electrocatalytic properties were studied by Nielsen et al. and founded that Ru NPs favor settling in the sizes of diameter 1.75, 2.5, and 3.0 nm, resulting in high surface area specimens which in turn enhances the electronic properties [48]. Subsequently, more research has been focused on Ru NPs as viable electrocatalysts. For the first time, Zheng et al. demonstrated the effect of the crystalline structure of Ru nanocatalyst on the electrocatalytic behavior. Face-centered cubic crystallographic structure of Ru NPs exhibited a 2.5-fold higher hydrogen generation rate than the Pt catalysts [49]. Mahmood et al. reported a Ru-based catalyst that performs HER in both acidic and basic mediums. Ru nanoparticles were dispersed in a nitrogenated holey two-dimensional carbon structure (Ru@C2N) and it was shown to exhibit much favorable turnover frequencies and smaller o.p values (at 10 mA cm–2) of 13.5 mV in acidic solution and 17.0 mV in basic solution as well as outstanding stability in both media [50]. Single-atom Ru-based catalyst was tried as HER electrocatalyst under all pH conditions by Wang and co-workers. Ru atoms attached to the surface of MoS2 nanosheets that were supported by a carbon cloth exhibited high catalytic activity, and stability of Ru-MoS2/CC was much higher as compared to bare MoS2/CC. The DFT calculations reveal that the synergistic effects of Ru and MoS2 account for the optimal activity of the catalyst material [51]. Though the low o.p’s and high stability of PGMs make them the best choice for HER water splitting; the high-cost due to scarcity of noble metals necessitates the search for novel, earth-abundant, and cheaper candidates.

Electrocatalysts for Hydrogen Evolution Reaction  125

5.3.2 Non-Noble Metal-Based Catalysts Among the non-noble metals, Nickel (Ni) has proved to be the most active for catalyzing HER as it offers a minimum value of Gibbs free energy of hydrogen adsorption and maximum value of exchange current density [52, 53]. Voltammetric studies carried out by Miles and Thomason revealed that the performance of this category of metals follows the decreasing order with Ni and Cu having the highest and lowest activity and Mo, Co, W, and Fe in between [19, 54]. To get the best outcome, numerous engineering protocols such as heteroatom doping, defect engineering, and interface engineering were adapted to design and synthesize various Ni-based electrocatalysts [55, 56]. Gong et al. disclosed that heterostructures of nickel oxide and nickel on carbon nanotube sidewalls are efficient HER electrocatalysts. The interaction between the nickel ion and the carbon nanotube enables the material to exhibit similar activity as that of platinum [57]. Porous graphene doped with atomic nickel was studied as an efficient HER catalyst by Qiu et al. The composite showed superior activity with an o.p value of approximately 50 mV and a small T.s of 45 mV dec−1, in 0.5 M H2SO4 solution. Voltammetric results proved that the material exhibits excellent cyclic stability [58]. Ahn et al. investigated the electrochemical response of electrodeposited nickel dendrites in alkaline water electrolysis [59]. Chu et al. concluded that carbon-coated nickel-nickel oxide composite behaves as a suitable electrocatalyst in the acidic medium. The composite showed a lower o.p of 108 mV (vs. RHE) at 10 mA cm−2, a smaller T.s of 44 mV dec−1, and good stability in a 0.5 M H2SO4 solution [60] (Figure 5.6). A novel nickel-cobalt-sulfide catalyst was prepared and investigated as HER electrocatalyst by Irshad and co-workers. The catalyst showed an onset o.p of 150 mV and 280 mV o.p at 10 mA cm−2 and exhibited two different T.s of 93 and 70 mV dec−1 corresponding to two dissimilar mechanisms [61]. Like nickel, metallic cobalt and cobalt-based materials have been studied extensively as HER catalysts. It was Zou et al. who first reported the synthesis of cobalt-embedded nitrogen-rich-carbon nanotubes. The as-prepared composite proved to be an excellent candidate for water splitting HER. Compared to multiwalled carbon nanotubes, this catalyst showed a small T.s of 69 mV dec−1 [62]. Another work with cobalt-doped FeS2 was done by Wang et al. in 2015 [63]. In 2018, Zhang et al. discussed the performance of cobalt NPs compressed into porous carbon nanofibers doped with nitrogen. The catalyst optimized at 800°C gave an o.p of only 159 mV at 10 mA cm−2 in standard acidic solution [64]. Cobalt/cobalt oxide NPs incorporated onto carbon-graphene composite doped with nitrogen were studied

126  Materials for Hydrogen Production, Conversion, and Storage

Ni-MOF-GO(9%)/GCE Ni/NiO@C/GR-700-8/GCE Ni/NiO@C/GR-800-8/GCE Ni/NiO@C/GR-900-8/GCE bare GCE

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Figure 5.6  (a) LSV curve (b) corresponding Tafel plots (c) the double layer capacitance and (d) Nyquist plots of various Nickel-based HER electrocatalysts [Adapted and reprinted with permission (62) from copyright 2018, Elsevier Ltd.].

as HER electrocatalyst by Wen et al. The as-prepared catalyst showed stability in both acidic and alkaline media with o.p’s of 140 and 136 mA cm−2, respectively [65]. Studies thus reveal that cobalt and cobalt-based materials are also promising candidates for electrocatalytic water splitting hydrogen evolution. Other non-noble metal-based catalysts are also being widely studied for HER application [66].

5.3.3 Metal-Free 2D Nanomaterials The discovery of graphene by Geim and Novoselov in 2004 led to tremendous insights into various research fields including electrocatalysis. The estimated theoretical value of high surface area, higher values of modulus of elasticity, and high thermal and electrical conductivities favor graphene nanosheets (GNS) as a suitable candidate for HER catalysis [67]. Though non-metal organic catalysts do not show much fair HER activity, Jiao et al. in 2016 demonstrated the catalytic functioning of heteroatom-doped graphene material, by carrying out electrochemical measurements and

Electrocatalysts for Hydrogen Evolution Reaction  127 density functional theory calculations. Results obtained experimentally was satisfactory with the theoretical calculations and the catalyst exhibited a lower T.s of 120 mV dec−1 [68]. Low stability and activity are the major problems while using metal-free catalysts instead of precious m ­ etal-based catalysts. However, Deng et al. solved this by incorporating CoNi nanoalloy into ultrathin graphene shells. The study of the electrocatalytic performance of the catalyst shows values of ƞ1 almost equal to zero and ƞ10 of 142 mV [69]. To increase the efficiency of the catalysts, single-atom catalysis through atomic layer deposition is a widely accepted strategy [70]. Sun et al. reported improved catalytic activity of platinum doped graphene nanosheets [71, 72]. Even though the basal planes of pristine graphene are less active, replacing the sp2 carbon atoms by heteroatom doping tremendously increases its catalytic performance. One such approach was done by Ito et al., by co-doping nitrogen, and sulfur onto nanoporous graphene [73]. Recently in 2020, Joyner et al. reported graphene-supported MoS2 nanostructures as efficient HER electrocatalysts [74] (Figure 5.7). One among the other allotropes of carbon, graphyne, a planar sheet of one-atom-thick sp and sp2-bonded carbon atoms, stands as the best choice for HER electrocatalysis. Xue and co-workers carried out selfcatalyzed growth of graphydine (graphyne-2) nanowires array on Cu foam.

V

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Figure 5.7  (a) Comparison of HER activity of various rGO-MoS2 composites (b) activity of 1rGO-2MoS2 as a function of sulfurization time (c) stability tests of 1rGO-1MoS2 (d) T.s of various rGO-MoS2 composites (e) T.s of 1rGO-2MoS2 (f) T.s’s of stability tests. [Adapted and reprinted with permission (76) from copyright 2020, American Chemical Society].

128  Materials for Hydrogen Production, Conversion, and Storage The material exhibited an onset o.p of 52 mV, a low T.s of 69 mV dec−1, and o.p’s of 79 and 162 mV at 10 and 100 mA cm−2 respectively, thereby proving to be the best candidate for hydrogen evolution reaction [75]. The quest for precious metal-free catalysts led to the exploration of one of the first artificial polymers, carbon nitride, as effective hydrogen evolution catalysts. Graphitic carbon nitride (g-C3N4) consists of a 2D crystal of hexagonal carbon network in which N is substituted with carbon by the sp2 hybridization [76]. Zhao et al. reported excellent HER activity, for g-C3N4 nanoribbons/GNS composite with a T.s of 54 mV dec−1, a low value of ƞ1 of 84 mV, and an o.p of 207 mV at 10 mA cm−2 [77]. The electrocatalytic behavior of g-C3N4 coupled with nanoporous GNS co-doped with sulfur and selenium was studied by Shinde and co-­ workers in 2015. The material exhibited stability for a longer period compared to those of commercially available Pt/C catalysts [78]. Recently Paul and co-workers studied the HER activity of metal encapsulated graphitic layers. The as-synthesized cuprous oxide (Cu2O) encapsulated with g-C3N4 shows o.p of 148.7 mV, with an exchange c.d of 12.8 mA cm−2, in alkaline medium [79]. As a metal-free candidate, black phosphorus (BP) has also been studied for HER electrocatalysis. The low stability and poor electrical conductivity of BP could be overcome by preparing composites with suitable materials. He et al. deposited molybdenum disulfide (MoS2) onto black phosphorus nanosheets. The composite exhibited ƞ10 value of 85 mV and an exchange c.d value of 0.66 mA cm−2. The voltammetric and potentiostatic tests showed the enhanced stability of BP through MoS2 deposition [80]. MoS2/ BP heterostructure for pH-universal hydrogen evolution was explained by Liang et al. The MoS2/electro-exfoliated black phosphorus composite exhibited low o.ps of 126 mV in acidic media, 237 mV in basic media, and 258 mV in neutral media [81]. The electrocatalytic HER activity of molybdenum diselenide (MoSe2) – BP heterostructures was extensively studied by Li and co-workers. The material showed the properties of lower value of potential needed to initiate the reaction (200 mV), small T.s (97 mV·dec−1), and excellent stability proving its potential in electrocatalytic applications [82]. Another heterostructure of BP was synthesized by Suragtkhuu et al. by integrating ultrathin few-layer BP nanosheets and graphene doped with boron [83]. The low o.p of 385.9 mV at 10 mA cm−2 and a low charge transfer resistance (Rct) of only 5.5 Ω in H2SO4 solution show that the material is an active HER electrocatalyst. The electrocatalytic HER performance of BP decorated with metal clusters was also investigated by Zhang et al. They showed the high performance of BP decorated with cobalt metal clusters over others [84].

Electrocatalysts for Hydrogen Evolution Reaction  129

5.3.4 Transition Metal Dichalcogenides The benign structural and electronic properties of transition metal dichalcogenides (TMDs) make them applicable in various electrocatalytic applications, most importantly electrochemical water splitting hydrogen evolution [85]. Extensive studies have been going on using various TMDs like molybdenum sulfide (MoS2), molybdenum selenide (MoSe2), tungsten sulfide (WS2), tungsten selenide (WSe2), cobalt sulfide (CoS2), cobalt selenide (CoSe2), and so on. Among these, MoS2 is widely explored due to its multi-crystal structure (e.g. 2H (hexagonal), 1T (trigonal), and 3R(rhombohedral) crystal structures), bandgap, abundant active surface sites, and high specific surface area [19]. Chen et al. synthesized vertically aligned MoS2 films by RF-magnetron sputtering under different argon gas pressures. The best HER performance was seen in the sample prepared at 0.8 Pa with low onset o.p of 149 mV [86]. Yang et al. discussed the electrochemical HER activity of etched oxygen-incorporated molybdenum disulfide nanosheets [87]. The alkaline HER capability of MoS2 enhanced by tuning orbital orientation was reported by Zang et al. The MoS2 doped with carbon exhibited an ƞ10 value of 49 mV, which is a promising value than that of bare MoS2 [88]. For MoS2 nanodots, Benson et al. reported a T.s of 61 mV dec−1 with an onset potential of −0.09 V vs. RHE [89]. Lai et al. recorded the HER activity of nitrogen-doped carbon nanofiber/ molybdenum disulfide nanocomposite. The material proved excellent activity with a small o.p of 108 mV, a high c.d of 8.7 mA cm−2 at ƞ 10 = 200 mV, a low T.s of 61 mV dec−1, and excellent cyclic stability [90]. Another composite of molybdenum disulfide was synthesized, and the HER catalytic performance was explained by Tang and co-workers. The nanocomposite of MoS2 and reduced graphene oxide doped with nitrogen exhibited low onset o.p of −5mV vs. RHE, and a smaller value of T.s of 41.3 mV dec−1 in the acidic medium [91]. Facile synthesis of MoS2/GNS for electrocatalysis was reported by Ye et al. The material showed a low onset o.p of 80 mV and a small T.s of 48.0 mV dec−1 in 0.5 M H2SO4 [92]. Another molybdenum-based dichalcogenide, molybdenum diselenide (MoSe2), has also been explored as HER electrocatalyst because of its favorable electrical conductivity and excellent catalytic ability. Li et al. introduced a strategy to construct MoSe2 nanosheets/BP nanosheet heterostructures to improve the conductivity of bare MoSe2, thereby increasing its catalytic behavior. Electrocatalytic analysis showed that the composite material exhibited a small T.s value of 97 mV·dec−1 [82]. In another work by Park et al., MoSe2 embedded CNT-reduced graphene oxide composite exhibited a small o.p and T.s of 240 mV at a c.d of 10 mA cm‒2 and 53 mV

130  Materials for Hydrogen Production, Conversion, and Storage dec−1, respectively [93]. Engineering MoSe2 and its reduced graphene oxide composites for better HER studies were carried out by Sarker et al. [94]. Binder-free MoSe2 nanoflakes were synthesized by Barough and co-workers and the material was found suitable for HER activity as it offered 107 mV o.p and 75 mV dec−1 T.s [95]. Vikraman et al. adopted a facile synthesis strategy to develop MoSe2 nanolayers. A low o.p of 88 mV (at 10 mA·cm−2) and a high exchange c.d of 0.845 mA·cm−2 were exhibited by the vertically aligned layered MoSe2 structure [96]. Yang et al. studied the improvements in the hydrogen generation rate of MoSe2 by doping. Nickel-doped MoSe2 gave a low ƞ10 value of 181 mV with 4.5 at.% Ni-doped in 1 M KOH which was much lower than bare MoSe2 (335 mV) [97]. Other groups of TMDs used as HER electrocatalysts include WS2, WSe2, CoS2, NiS2, and so on. Tran et al. investigated the HER catalytic activity of cobalt-tungsten and nickel-tungsten ternary sulfides [98]. Yang and co-workers, demonstrated enhancing the HER activity by incorporating nickel and cobalt onto amorphous tungsten sulfide. The material annealed at 210°C offered a T.s of 55 mV dec−1 [99]. The effect of a conductive oxide core in WS2 to enhance the hydrogen evolution performance was studied by Seo et al. [100]. Tan et al. adopted a bottom-up approach to synthesize WS2/WS3 film-like material. The WS2.64 reported a lowest T.s of 43.7 mV dec−1 [101]. Li et al. fabricated molybdenum-tungsten sulfide supported on carbon cloth via electrodeposition for hydrogen evolution [102]. Thin films of WSe2 deposited onto a tungsten foil offered 350 mV o.p to drive the hydrogen-evolution reaction at (10 mA cm−2) in aqueous 0.5 M H2SO4 [103]. Wang and co-workers reported a small T.s of 57.6 mV dec−1 for WSe2/GNS composite [104]. Cobalt-doped WSe2/MWCNTs heterostructures exhibited exceptional electrocatalytic activity in both acidic and alkaline mediums [105]. As TMDs represent a large class of compounds, it offers various candidate materials for HER applications. Extensive studies have been carried out with molybdenum telluride (MoTe2) [106], iron sulfide (FeS2) [107], cobalt sulfide (CoS2) [108], nickel sulfide (Ni3S2) [109] (Figure 5.8), and so on.

5.3.5 Transition Metal Oxides and Hydroxides The poor electrocatalytic performance obtained for transition metal oxides (TMO) or hydroxides (TMH) due to their low conductivity hinders the use of pristine TMOs and TMHs widely. Hence, many works emphasize methods like doping, reducing thickness, and making hybrid composites and heterostructures to enhance their catalytic behavior. The electrocatalytic ability of mesoporous molybdenum oxide was reported by Luo et al.

Electrocatalysts for Hydrogen Evolution Reaction  131 (a)

(b)

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Figure 5.8  FE-SEM images of (a) Ni3S2@NF (b) and (c) CoSx/Ni3S2@NF (d) cuboid shaped CoSx [Adapted and reprinted with permission (111) from copyright 2018, American Chemical Society].

The freshly prepared MoO3–x only requires a lower value of ƞ10 of about 104 mV in 0.1 M KOH [110]. Imran et al. reported excellent hydrogen evolution activity for tungsten-doped MoO3/GNS nanocomposite with onset potential 50 mV, a T.s of 46 mV dec−1, and good durability up to 2000 cycles in the acidic medium [111]. Zhao and co-workers developed a bifunctional electrode with manganese dioxide nanosheet arrays on nickel foam for overall water electrolysis [112]. Yan and co-workers prepared three-dimensional self-supported cobalt molybdenum oxide through the hydrothermal calcination method. The composite material promised an o.p of 68 mV at 10 mA cm−2 and 178 mV at 100 mA cm−2 in 1 M KOH and a T.s of 82 mV dec−1 [113]. Wu et al. reported the catalytic behavior of metallic tungsten oxide-carbon mesoporous nanowires. The material gave onset o.p of 35 mV, ƞ10 of 58 mV, low T.s of 46 mV dec−1, and excellent stability for 10 h [114]. Tungsten oxide nanoparticles were also reported to be promising HER candidates in acidic and near-neutral media by Nayak et al. [115]. Conductive tungsten oxide nanosheets, exhibiting a smaller value of ƞ10 of 38 mV and a low T.s of 38 mV dec−1, were proved to be efficient HER candidates by Zheng et al. [116]. Wondimu and co-workers reported the hydrogen generation rate of nanocomposites comprising of tungsten

132  Materials for Hydrogen Production, Conversion, and Storage oxide nanoplates doped with iron and reduced graphene oxide. The material offered a low o.p of 54.60 mV and a T.s of 41.99 mV dec−1. LSV studies confirm the stability of the material for 2000 cycles [117]. Wang et al. [118] reported the electrocatalytic behavior of platinum-nickel hydroxides, while Xing and co-workers [119] reported nanoporous nickel hydroxide synthesized by electrodeposition on nickel film as HER electrocatalyst.

5.3.6 Transition Metal Phosphides Another class of compounds of transition metals, transition metal phosphides (TMPs) has been found applicable in various research fields, particularly in electrocatalytic water splitting due to its good electrical conductivity, replacing the conventional metal-based catalysts. Ma et al. reported the HER activity of nickel cobalt phosphide with three-dimensional architecture. The sample exhibited small ƞ10 values of 80 mV for 0.5 M H2SO4 and 105 mV for 1 M KOH, small T.s’s of 37 mV dec−1 for 0.5 M H2SO4, and 79 mV dec−1 for 1 M KOH [120]. Huang et al. reported that cobalt phosphide (CoP) nanorods exhibited an o.p of 167 mV and 171 mV (at a c.d of 20 mA cm−2) in acidic and basic solution, respectively [121]. Another work with cobalt phosphide nanoparticles in a strong acidic medium was reported by Popczun and co-workers. The CoP electrode gave a lower value of ƞ10 of −85 mV and was stable over 24 h [122]. Molybdenum phosphide (MoP) and their composite structures are also good candidates for electrocatalytic HER. Xing et al. reported closely interconnected MoP nanoparticles as suitable hydrogen evolution electrocatalysts [123]. Xiao et al. also reported the HER activity of MoP nanoparticles [124]. Zhang et al. noted that the hybrid of MoP with carbon nanotube exhibited ƞ10 values of 83 mV in acidic, 102 mV in neutral, and 86 mV in basic medium [125]. Ge et al. studied the HER performance of hierarchical MoP coupled with carbon under pH-universal conditions [126]. These reports proved that transition metal phosphides are a good choice as better HER electrocatalysts and it paved the way to extend the study onto other TMPs like tungsten phosphide [127–129], iron phosphide [130–132], nickel phosphide [133–136], and so on.

5.3.7 MXenes (Transition Metal Carbides and Nitrides) Recently, transition metal carbides (TMCs) and transition metal nitrides (TMNs) have emerged as effective candidates for HER electrocatalysis, replacing noble metals. It was in 2011 that this class of materials called MXenes was first discovered and synthesized by selective etching of

Electrocatalysts for Hydrogen Evolution Reaction  133 aluminium from the MAX phase to produce Ti3C2Tx [137, 138]. Among all the TMCs, molybdenum carbides are the most explored as a HER candidate. Molybdenum oxide and molybdenum carbide coated carbon black were tested as HER electrocatalyst in acidic media by Zhang et al. The hybrid exhibited an onset o.p of −121 mV and a much fair T.s of −69 mV dec−1 in 0.5 M H2SO4 solution. Also, the material showed excellent stability than the commercially available Pt/C catalyst [139]. Liao et al. reported the HER activity of nanoporous Mo2C nanowires [140]. Ma and co-workers suggested that ultra-fine molybdenum carbide nanoparticles mixed with carbon showed promising catalytic activity towards hydrogen evolution [141]. Liu et al. suggested that tungsten carbide also fits best for HER catalysis [142]. Recently, Li and coworkers reported a new method for synthesizing Ti3C2 MXene. They adopted a facile method to synthesize modified Ti3C2 MXene through a one-step gamma radiation strategy and coupled it with amorphous MoS2, forming a hybrid 2D/2D structure (Figure 5.9). The hybrid material exhibited enhanced activity with a T.s of 41 mV dec−1, a low excess potential of 196 mV at a c.d of 50 mA cm−2, and durable stability [143]. Wu et al. turned their work towards transition metal nitrides as bifunctional catalysts. The synthesized porous cobalt/tungsten nitride polyhedron gave sufficient performance in alkaline medium with low ƞ10 value of 27 mV and 232 mV for hydrogen and oxygen evolution reaction, respectively [144]. Miao et al. studied the use of hexagonal molybdenum nitride supported by boron and carbon nanotubes doped with nitrogen for hydrogen evolution from seawater. The composite material exhibited a small ƞ10 of 78 mV at 10 mA cm−2, promising to be a good substitute to noble metal-based catalysts [145]. The HER performance of carbides and nitrides of molybdenum prepared through a urea glass route method was reported by Ma and co-workers [146]. Xie and coworkers also reported the activity of atomically thin molybdenum nitride nanoparticles. Surface studies revealed that surface Mo atoms provide for excellent proton

LiF-HCL etching

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Ti3AIC2

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Figure 5.9  Schematic diagram of synthesis of modified-Ti3C2/MoS2 [Adapted and reprinted with permission (145) from copyright 2020, American Chemical Society].

134  Materials for Hydrogen Production, Conversion, and Storage adsorption and charge transfer [147, 148]. Yan et al. reported tungsten nitride modified with phosphorus on reduced graphene oxide for electrocatalysis. The improvement in the activity of tungsten nitride is by virtue of the interaction of phosphorus with reduced graphene oxide and tungsten nitride. The hybrid material showed a ƞ10 value of only 85 mV in acidic media [149]. From these works, it is clear that two-dimensional inorganic materials, which are hydrophilic and inherits metallic conductivity, ascertain the requirements concomitant with HER electrocatalysts [150–152].

Conclusion Electrocatalytic water splitting to produce hydrogen gas is an advanced energy conversion method [153]. The efficacy of platinum-based metals as the best choice of electrocatalysts is ascribed to its higher affinity towards hydrogen adsorption. As a result of the scarcity of resources and high cost of PGMs, research has led to the exploration and synthesis of new HER candidates [154]. Insights into the basic mechanisms and principles of HER are crucial for selecting and designing new electrocatalytic materials. To expand this technology of hydrogen production industrially, novel, stable, and robust electrocatalysts should be designed and synthesized.

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Electrocatalysts for Hydrogen Evolution Reaction  143 durable performance for electrocatalytic hydrogen evolution reaction. Int. J. Hydrogen Energy, 42, 8130, 2017. 112. Zhao, Y., Chang, C., Teng, F., Zhao, Y., Chen, G., Shi, R., Waterhouse, G.I.N., Huang, W., Zhang, T., Defect-engineered ultrathin Δ-MnO2 nanosheet arrays as bifunctional electrodes for efficient overall water splitting. Adv. Energy Mater., 7, 1, 2017. 113. Yan, Q., Yang, X., Wei, T., Wu, W., Yan, P., Zeng, L., Zhu, R., Cheng, K., Ye, K., Zhu, K., Yan, J., Cao, D., Wang, G., Self-supported cobalt–molybdenum oxide nanosheet clusters as efficient electrocatalysts for hydrogen evolution reaction. Int. J. Hydrogen Energy, 44, 21220, 2019. 114. Wu, R., Zhang, J., Shi, Y., Liu, D., Zhang, B., Metallic WO2-carbon mesoporous nanowires as highly efficient electrocatalysts for hydrogen evolution reaction. J. Am. Chem. Soc., 137, 6983, 2015. 115. Nayak, A.K., Verma, M., Sohn, Y., Deshpande, P.A., Pradhan, D., Highly active tungsten oxide nanoplate electrocatalysts for the hydrogen evolution reaction in acidic and near neutral electrolytes. ACS Omega, 2, 7039, 2017. 116. Zheng, T., Sang, W., He, Z., Wei, Q., Chen, B., Li, H., Cao, C., Huang, R., Yan, X., Pan, B., Zhou, S., Zeng, J., Conductive tungsten oxide nanosheets for highly efficient hydrogen evolution. Nano Lett., 17, 7968, 2017. 117. Wondimu, T.H., Chen, G.C., Kabtamu, D.M., Chen, H.Y., Bayeh, A.W., Huang, H.C., Wang, C.H., Highly efficient and durable phosphine reduced iron-doped tungsten oxide/reduced graphene oxide nanocomposites for the hydrogen evolution reaction. Hydrogen Energy, 43, 6481, 2018. 118. Wang, L., Zhu, Y., Zeng, Z., Lin, C., Giroux, M., Jiang, L., Han, Y., Greeley, J., Wang, C., Jin, J., Platinum-nickel hydroxide nanocomposites for electrocatalytic reduction of water. Nano Energy, 31, 456, 2017. 119. Xing, Z., Gan, L., Wang, J., Yang, X., Experimental and theoretical insights into sustained water splitting with an electrodeposited nanoporous nickel hydroxide@nickel film as an electrocatalyst. J. Mater. Chem., 5, 7744, 2017. 120. Mu, J., Li, J., Yang, E.C., Zhao, X.J., Three-dimensional hierarchical nickel cobalt phosphide nanoflowers as an efficient electrocatalyst for the hydrogen evolution reaction under both acidic and alkaline conditions. ACS Appl. Energy Mater., 1, 3742, 2018. 121. Huang, Z., Chen, Z., Chen, Z., Lv, C., Humphrey, M.G., Zhang, C., Cobalt phosphide nanorods as an efficient electrocatalyst for the hydrogen evolution reaction. Nano Energy, 9, 373, 2014. 122. Popczun, E.J., Read, C.G., Roske, C.W., Lewis, N.S., Schaak, R.E., Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide nanoparticles. Angew. Chemie - Int. Ed., 53, 5427, 2014. 123. Xing, Z., Liu, Q., Asiri, A.M., Sun, X., Closely interconnected network of molybdenum phosphide nanoparticles: A highly efficient electrocatalyst for generating hydrogen from water. Adv. Mater., 26, 5702, 2014.

144  Materials for Hydrogen Production, Conversion, and Storage 124. Xiao, P., Sk, M.A., Thia, L., Ge, X., Lim, R.J., Wang, J.Y., Lim, K.H., Wang, X., Molybdenum phosphide as an efficient electrocatalyst for the hydrogen evolution reaction. Energy Environ. Sci., 7, 2624, 2014. 125. Zhang, X., Yu, X., Zhang, L., Zhou, F., Liang, Y., Wang, R., Molybdenum phosphide/carbon nanotube hybrids as pH-universal electrocatalysts for hydrogen evolution reaction. Adv. Funct. Mater., 28, 1, 2018. 126. Ge, R., Huo, J., Liao, T., Liu, Y., Zhu, M., Li, Y., Zhang, J., Li, W., Hierarchical molybdenum phosphide coupled with carbon as a whole pH-range electrocatalyst for hydrogen evolution reaction. Appl. Catal. B Environ., 260, 118196, 2020. 127. McEnaney, J.M., Chance Crompton, J., Callejas, J.F., Popczun, E.J., Read, C.G., Lewis, N.S., Schaak, R.E., Electrocatalytic hydrogen evolution using amorphous tungsten phosphide nanoparticles. Chem. Commun., 50, 11026, 2014. 128. Pu, Z., Ya, X., Amiinu, I.S., Tu, Z., Liu, X., Li, W., Mu, S., Ultrasmall tungsten phosphide nanoparticles embedded in nitrogen-doped carbon as a highly active and stable hydrogen-evolution electrocatalyst. J. Mater. Chem. A, 4, 15327, 2016. 129. Wang, X.D., Xu, Y.F., Rao, H.S., Xu, W.J., Chen, H.Y., Zhang, W.X., Kuang, D., Bin,Su, C.Y., Novel porous molybdenum tungsten phosphide hybrid nanosheets on carbon cloth for efficient hydrogen evolution. Energy Environ. Sci., 9, 1468, 2016. 130. Callejas, J.F., McEnaney, J.M., Read, C.G., Crompton, J.C., Biacchi, A.J., Popczun, E.J., Gordon, T.R., Lewis, N.S., Schaak, R.E., Electrocatalytic and photocatalytic hydrogen production from acidic and neutral-pH aqueous solutions using iron phosphide nanoparticles. ACS Nano, 8, 11101, 2014. 131. Du, H., Gu, S., Liu, R., Li, C.M., Highly active and inexpensive iron phosphide nanorods electrocatalyst towards hydrogen evolution reaction. Int. J. Hydrogen Energy, 40, 14272, 2015. 132. Lv, C., Peng, Z., Zhao, Y., Huang, Z., Zhang, C., The hierarchical nanowires array of iron phosphide integrated on a carbon fiber paper as an effective electrocatalyst for hydrogen generation. J. Mater. Chem. A, 4, 1454, 2016. 133. Hu, C., Lv, C., Liu, S., Shi, Y., Song, J., Zhang, Z., Cai, J., Watanabe, A., Nickel phosphide electrocatalysts for hydrogen evolution reaction. Catalysts, 10, 188, 2020. 134. Popczun, E.J., McKone, J.R., Read, C.G., Biacchi, A.J., Wiltrout, A.M., Lewis, N.S., Schaak, R.E., Nanostructured nickel phosphide as an electrocatalyst for the hydrogen evolution reaction. J. Am. Chem. Soc., 135, 9267, 2013. 135. Pan, Y., Liu, Y., Liu, C., Nanostructured nickel phosphide supported on carbon nanospheres: Synthesis and application as an efficient electrocatalyst for hydrogen evolution. J. Power Sources, 285, 169, 2015. 136. Yu, J., Li, Q., Chen, N., Xu, C.Y., Zhen, L., Wu, J., Dravid, V.P., Carbon-Coated Nickel Phosphide Nanosheets as Efficient Dual-Electrocatalyst for Overall Water Splitting. ACS Appl. Mater. Interfaces, 8, 27850, 2016.

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6 Recent Progress on Metal Catalysts for Electrochemical Hydrogen Evolution Tejaswi Jella* and Ravi Arukula† Department of Chemistry, Anurag University, Hyderabad, Telangana, India

Abstract

At present, electrochemical water splitting technology emerged as a zero emission and sustainable source for hydrogen evolution; however, electrocatalyst is inevitable to attain a high yield of hydrogen. Owing to its high catalytic stability and activity, platinum (Pt) compounds are employed as the most efficient catalysts in this field. Nevertheless, the scarcity and over price limits its practical utilization. It is necessary to evolve innovative catalyst with improved properties and reduced cost to acquire the required hydrogen production. Herein, we concentrated on the modern developments in the composition and composing of nanostructured catalysts of noble metal and non-noble metal-based materials for hydrogen evolution reactions. Contemporary approaches and attentions have been scrutinized for the synergistic structure, composition, construction, electronic arrangement, and active centers of the catalyst for intensifying the activity and durability. In addition to this, the challenges, perspectives, and opportunities for developing new catalysts for efficient electrochemical water splitting are also outlined. Keywords: Water splitting, hydrogen evolution reaction, electrocatalyst, transition metals, and noble metals

*Corresponding author: [email protected] † Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (147–180) © 2023 Scrivener Publishing LLC

147

148  Materials for Hydrogen Production, Conversion, and Storage

6.1 Introduction Hydrogen (H2) energy is one of the most favorable candidates for next-­ generation energy sources, as it is with a high gravimetric energy density of 120–142 MJ kg−1, trivial carbon emission, and eco-friendly nature [1–3]. Hydrogen-powered vehicles deliver the similar performance compared to conventional vehicle fuel but with zero carbon emission and other pollutants. Currently, the major portion of H2 is produced by the processing of fossil fuel, which emits CO2 gas leading to global warming. In this context, water electrolysis has attracted significant attention as water and electricity are the only precursors. Besides, obtained H2 with high quality can be directly used as feedstock or fuel gas [4–10]. As a result, the advancement of water splitting and hydrogen generation methods has countless insistence and significance [11–13]. In a standard electrochemical cell, H2 and O2 are produced through reduction at the cathode and oxidation at the anode, driven by external voltage supplied across the cell. One of the major obstacles to commercialize this technology is the sluggish reaction kinetics of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) leading to low efficiency. Hence, it is important to use electrocatalysts at the electrodes to control the energy barrier of the reaction (237 kJ/mol) [14–20]. Owing to the high stability and activity, platinum and Pt-based materials are widely used as catalysts for electrochemical HER. But scarcity and high cost limit its commercialization [21–23]. Hence, to reduce the usage of Pt-based catalysts, great attention has been devoted to exploiting new inexpensive catalysts with improved activity and stability [24–32]. Yet, numbers of catalysts are probed for the HER comprising noble-metal-based materials, earth abundant transition metals oxide (hydroxides), chalcogenides, phosphides, nitrides and carbides, functional metal-free carbon, and various composites, but their practical applications are limited by pH requirement. In this chapter, we systematically discussed the recent progresses of design, fabrication, and advancement of numerous catalysts for electrochemical water splitting for hydrogen evolution including nanostructured noble and non-noble metal-based catalysts.

6.1.1 Type of Water Electrolysis Technologies Based on the electrolyte used, electrochemical water splitting is categorized into three types; they are (i) alkaline electrolysis (AE), (ii) proton exchange membrane electrolysis (PEME), and (iii) solid oxide electrolysis (SOE).

Electrocatalysts for Hydrogen Evolution Reaction  149

6.1.1.1 Alkaline Electrolysis (AE) For the first time, Troostwijik and Diemann established the electrolysis phenomenon in 1789, and currently, the most recognized technique for H2 production is alkaline electrolysis on an industrial scale in the world [33–37]. In this strategy, two electrodes are dipped in 20%–30% of KOH caustic electrolyte solution, and O2 is generated at the anode by oxidizing the water, at cathode H2 produced by reduction of water molecule, a membrane that avoids the mixing of hydrogen with oxygen [38, 39]. However, the resulting air bubbles and deposited metal cations reduce the electrodes effective surface area and increase the resistance of the electrolyte which can lead to low current densities, reduced catalytic activity, and finally low efficiencies [40].

6.1.1.2 Proton Exchange Membrane Electrolysis (PEME) In 1960, the concept of polymer membrane electrolyte was proposed to conquer the challenges of alkaline electrolysis [41]. This is further improved by using solid membrane electrolyte of sulphonated polystyrene. The advantages of polymer electrolyte membrane include low membrane thickness, high proton conductivity, and low air permeability, the trampled design of the apparatus, and operations at high-pressure [42–45].

6.1.1.3 Solid Oxide Electrolysis (SOE) Donitz and Erdle first reported the solid oxide electrolysis in 1980s using the ceramic compound as an electrolyte which operates relatively at high temperature with high H2 production efficiencies. Now, this technology is still unfledged and further research is required to develop a new, inexpensive, and stable material [46, 47]. Alkaline electrolysis is established and widespread hydrogen evolution technology among all the electrolysis [48].

6.2 Mechanism of Hydrogen Evolution Reaction (HER) Hydrogen evolution is one of the half-cell reactions at the cathode to produce H2 gas, which typically occurs on the surface of the catalyst in one of the two different steps (as depicted in Figure 6.1). Formation of active intermediates (H*) is the first step of the reaction following electrochemical adsorption of H atom on the surface of the catalyst by gaining an electron from the electrode, which is called as Volmer reaction. Based on the pH

150  Materials for Hydrogen Production, Conversion, and Storage +

H

(a) H

+

H

H

O

H

H

e

e

e

e

H e

e

H e

Heyrovsky reaction

e

e

H

H H

H H

e

H

O

H

+

O

+

H

H

O

(b)

Volmer reaction H e

H e

(c)

H H H e

H e

Tafel reaction

Step-1: Formation of H*

Step-2: Evolution of H2

  Figure 6.1  Hydrogen evolution reaction by two mechanisms on electrode surface (Self-drawn).

of the electrolyte, either H+ from acidic electrolyte (Equation 6.1) or H2O undergoes reduction (Equation 6.2).

H3O+ + e− H2O + e−

H* + H2O  (Volmer - Acidic medium)

(6.1)

H* + OH−  (Volmer - Alkaline/neutral medium)

(6.2)

The second step is the formation of hydrogen molecule by electrochemical desorption of H* along with an electron and bound to proton, which is the Heyrovsky reaction (Equations 6.3 and 6.4) or by the chemical combination of two adjacent H* atoms without electron transfer (Tafel step, Equation 6.5).

H* + H3O+ + e−

H2 + H2O (Heyrovsky - acidic medium)

(6.3)

Electrocatalysts for Hydrogen Evolution Reaction  151 H2 + OH− (Heyrovsky - alkaline/neutral medium)(6.4)

H* + H2O + e−

H* + H*

H2 (Tafel reaction under acidic or alkaline)

2H+ + 2e−

(6.5)

H2 (The overall reaction)

Evolution of hydrogen molecule can occur through either Heyrovsky or Tafel step but both the reactions are triggered by the Volmer reaction. The rate of HER strongly relies on the binding strength of reactive intermediates (∆GH*) on the catalyst [49]; either too low or too high ∆GH* shows a detrimental effect on HER, therefore it is important to consider the ∆GH in the development of catalyst. Based on theoretical calculations and observations, in 2004, Nørskov’s group summarized a general volcano plot (Figure 6.2) between exchange current density and Gibbs free energy [50], which provides an intuitive description of the activities of different metals. The metal which is placed (Pt) on the top of the volcano curve appears to be optimal with the best HER catalytic activity.

6.2.1 Performance Evaluation of Catalyst The evaluation process is required to correlate the performance of different electrocatalysts, which is discussed by overpotential, Tafel slope, and exchange current density, which can be calculated from polarization curves. Turnover frequency and Faradic efficiency were identified from

10-1 10-2

Pt Re

j0 (A cm–2)

10-3

Pd Rh

10-4

Ni2P (001) MoS2 edge

Co

W

10-6

10-8 –0.8

Ir

Ni

10-5

10-7

Pt (111)

Au

Cu

Nb

Au (111) Mo

–0.6

–0.4

Ag

–0.2

0.0 0.2 ΔGH* (eV)

0.4

0.6

0.8

Figure 6.2  Activity of the various electrocatalysts for HER, a volcano plot. [Replicated with permission from Ref. [50] copyright 2005 ECS].

152  Materials for Hydrogen Production, Conversion, and Storage experimental outcomes and the catalyst selectivity can be determined by theoretical calculations. I. Overpotential The Potential difference between the experimentally observed values and the thermodynamically calculated potential of an electrochemical reaction is called as overpotential [51]. This is measured by using linear sweep voltammetry (LSV) and potential to drive 10 mA.cm−2 of current density can be considered as benchmark for assessing the performance of catalyst [52]. The lower the overpotential, the higher will be the catalytic activity. II.  Tafel Slope and Exchange Current Density Tafel equation imparts the relation between overpotential and the current density, which is expressed as:



η = a + b log j



η = overpotential, b = slope, j = current density Tafel plot slope (η vs. log j) provides the information about reaction kinetics and mechanism on the surface of electrocatalyst [53]. The lower Tafel slope indicated, the lesser overpotential is required to improve the current density, which implies the tendency of rapid charge transfer. The current density at equilibrium potential (η = 0) called exchange current density jo, describes the inherent charge transfer between electrode and electrolyte. Higher exchange current density indicates higher charge transfer and lower energy barrier. Hence, for an ideal catalyst the Tafel slope and exchange current density are with minimum and maximum essential values, respectively. III.  Faradic Efficiency The ratio of the amount of hydrogen evolution practically to the theoretically calculated hydrogen production is the Faradic efficiency. Experimental values are determined by gas chromatography method. IV.  Turnover Frequency Turnover frequency gives the information about the amount of products formed per active site per unit time,

Electrocatalysts for Hydrogen Evolution Reaction  153 indicating the intrinsic activity of catalyst; it is calculated using the following equation:



TOF =

jA α Fn

where: J = Current density corresponding to the fixed overpotential A = Working electrode surface area α = Number of electrons involved from the catalyst F = Faradays constant (96485 C mol−1) n = number of moles of material loading on the electrode Besides, stability is another parameter to estimate the catalyst performance, which is measured by observing the dissimilarity fashion in potential or current by time at constant current density or voltage.

6.3 Various Electrocatalysts for Hydrogen Evolution Reaction (HER) 6.3.1 Noble Metal Catalysts for HER Noble metal (Pt, Pd, Ru, Ir, etc.) compounds are still preferred as the most competent catalysts for HER due to their great intrinsic catalytic activity and ability to achieve industrial hydrogen production demands [54]. Though, the real-time utilizations of aforementioned materials are hindered by its price, agglomeration, and less stability during HER reaction. To overcome these challenges, systematic developments of catalysts are required, downscaling the particle size [55–58]; blending with another low-cost, earth-abundant metal will decrease the cost [59–62] and molecular engineering effectively improves the stability of catalyst [63–65].

6.3.1.1 Platinum-Based Catalysts Despite the tremendous progress that occurred in the development of non-precious catalysts towards HER, still, Pt-based noble metal materials is deemed to be state-of-the-art; extended research has focused to further improve the catalytic properties [66–68]. Amplified efficiency can be reached by one of the adaptable ways of downscaling the catalyst nanoparticles to single atoms. Self-limiting surface reactions like atomic

154  Materials for Hydrogen Production, Conversion, and Storage layer deposition have the capability to switch the size and scattering of particles on a substrate [69, 70]. Recently, atomic layer deposition (ALD) technique is used to synthesize single platinum atoms and clusters borne on nitrogen-doped graphene nanosheets by Cheng and his co-authors [71]. ALD50Pt/NGNs and ALD100pt/NGNs were prepared by 50 and 100 ALD cycles, respectively, which controlled the size and dispersion of Pt catalyst on NGNs surface [69, 72, 73]. The scanning transmission electron microscopy (ADF-STEM) studies revealed the uniform dispersion of single Pt atoms along with small Pt clusters on NGNs for ALD50Pt/NGNs and for ALD100Pt/NGNs; along with single Pt atoms, clusters have grown to form nanoparticles. These are further confirmed by DFT calculations, as prepared ALDPt/NGNs exhibit excellent HER catalytic activity with lower Tafel slope 29 mV.dec−1, high current density 16 mA.cm−2 at 0.05 mV overpotential, and 37.4 times higher mass activity than commercial Pt/C in 0.5M H2SO4 at room temperature. The superior catalytic effect of ALDPt/ NGNs due to the more number of unoccupied d-orbitals on a single Pt atom was attributed to stronger interaction and effective electron transfer between Pt-H atom [74, 75]. The increased ALD cycles improves the number of clusters and nanoparticles in the ALDPt/NGNs and leads to the decreased catalytic activity. These results revealed that the utilization of all the atoms and decreasing the cost of the catalyst can be attained by single Pt atoms and clusters [76]. Moreover, the downscaling, functionalization of supporting material will improve the catalyst performance intensely as it increases the electronic conductivity and the surface area. For instance, Luo et al. reported an inexpensive Pt-based electrocatalyst anchored over functionalized Vulcan Carbon (VC) by explaining the role of supporting material [77]. The Vulcan carbon was first functionalized with Ruthenium and ethylene glycol using hydrothermal treatment and then used to immobilize the Pt nanoparticles through the ALD method. The functionalized VC influences the metal loading, particle size, and distribution of Pt which can be characterized by HAADF-STEM measurements. As synthesized, Pt/Ru/VC composite catalyst demonstrates excellent activity under 0.5 M H2SO4 by 23 mV of reasonable over potential at 10mA/cm2 current density. The large ECSA, high mass activity, lower charge transfer resistance, and strongest hydrogen adsorption on the catalyst are attributed to the exceptional catalytic performance. Besides, the functionalized VC stabilizes the catalyst by resisting the agglomeration of Pt without activity loss even after 3000 cycles, where commercial 20 wt% Pt/C catalysts with almost 26 mV of a negative shift in the polarization curve at 10 mA/cm2 after the 3000 cycles indicate the lowest stability. The above result demonstrates that the electronic and structural properties of Pt were strongly affected by

Electrocatalysts for Hydrogen Evolution Reaction  155 functionalized VC. Alloying with low cost, earth-abundant metal is another strategy to improve the stability and utilization efficiency of the catalyst [78–81]. By using alloying process, Xie et al. [82] successfully fabricated monocrystalline Pt-Ni branched nanocages through one pot synthesis with Ni-rich alloy@Pt-rich alloy core-shell structure followed by etching the core in acidic solution. The well-epitaxial Pt-Ni nanocages structure was characterized by HAADF-STEM analysis and perceived the dimensions of branches as 200 ± 20 nm of length, 50 ± 5 nm of the width, and the wall thickness to be 2.8 nm. Benefiting from the monocrystalline branched structure, the catalyst possesses an extensive active surface area, and all the directions are being approachable to the reagent. In addition, Ni easily reduces the OH− ions as it is with higher standard oxidation potential and forms Pt-Ni(OH)2 interface between electrode and electrolyte, which could lower the kinetic barrier and promote the Volmer step (dissociation of a water molecule to form Had intermediate) [66]. As a result, the prepared catalyst exhibits superior HER catalytic activity with a small Tafel slope of 73mV•dec−1 and larger exchange current density than commercial Pt/C. Binary compounds [82–87], ternary compounds [88–90], and the addition of water-dissociating materials [91] are alternative ways to improve the catalyst properties. Despite superior HER catalytic activity of Pt-based material, poor catalytic activity under alkaline, neutral medium, and high cost limit their commercial application. For an alternate electrocatalyst, one should consider the hydrogen-binding energy, cost, and stability under a wide range of pH. In this regard, Palladium (Pd) is favored, due to great tendency and selectivity for hydrogen adsorption, reversible hydride formation, and moreover the cost almost five times lesser than Pt entrust the Pd and Pd based materials as excellent alternate to Pt catalyst [92, 93].

6.3.1.2 Palladium Based Catalysts Pd nanoparticles with different morphologies including nanocages, nanoframes, branched structures, multi-components, and core-shell structures are favorable due to their excellent catalytic performance [94]. As mentioned earlier, integrating the metal with supporting material enhances the catalytic property by synergic effect and reduces the cost by low metal loading. Among the carbon materials, graphene is considered as the most promising component as a supporting material. Recently, Bhowmik et al. [95] synthesized a porous Pd nanoparticles-carbon nitride composite (Pd-CNx) from ultrasound reduction of PdCl4 with NaBH4 in the presence of graphitic carbon nitride nanosheets. TEM and FE-SEM measurements revealed that the Pd NPs confined CNx interconnected to form a porous

156  Materials for Hydrogen Production, Conversion, and Storage morphology, which is beneficial for the superior activity of the synthesized catalyst. At a small loading (0.043 mg/cm2) of Pd, catalyst shows an excellent HER activity with low overpotential, Tafel slope, and higher exchange current density of −55 mV, 35 mV/dec, and 0.40 mAcm−2, respectively, and also displayed very high stability in strongly acidic media, which is much higher than that of commercial Pt/C. The porous structure of the catalyst and strong metal-carbon (Pd-C) bond improves the charge transport and synergetic effects between CNx support and Pd-NPs which leads to the exceptional HER activity and durability of the catalyst. As supporting material plays a vital role in catalyst performance, the employment of different compounds as supporting layers is another way to enhance the catalyst properties. With the advantage of strong interactions and tunable electron coupling between metal and metal oxide, blending of metal nanoparticles with metal oxide has attracted much attention to developing new electro catalysts with improved activity (e.g. Pd/W18O49, Pt/Ti 0.7Mo0.3O2, Pt/ TaB2) [96–98]. Accordingly, Chen et al. [99] prepared sub-nano-sized palladium clusters encapsulated in porous CeO2 nanorods (Pd NCs@CeO2) through a simple wet chemical method. As expected, with the small size, clean surface, and the strong electronic interactions between Pd NCs and ceria nanorods, the Pd NCs@CeO2 composite shows enhanced catalytic activity and stability towards HER with an onset potential of −0.036V vs. RHE and great current density even after the 5000-cycle test. The enhanced catalytic activity of Pd NCs@CeO2 is mainly due to the effective electron transfer between Pd and oxygen atoms. Jiang et al. reported a new class of nanoporous Pd-Ag surface alloy supported on nanoporous Pd-Ag-Al alloy electrode as notable electrocatalyst for HER in both basic and acidic medium [100]. Compared to simple Pd surface atoms, the Pd-Ag alloy surface has a low hydrogen binding strength, which favors effective hydrogen production in both basic and acidic media by harmonizing the water dissociation and hydrogen evolution. The prepared composite NP Pd66Ag17Al17 has an ultralow over the potential of −11 and −16.8mV at a current density of 10 mA.cm−2 much lower than that of Pt/C (−35 and −34.8 mV) in both 0.5 M H2SO4 and 1M KOH electrolytes, respectively. The superior HER activity in the basic medium was validated by 56 mV/ dec of ultralow Tafel slope, which is the lowest value among the tested catalysts including Pt/C (∼57 mV.dec−1). Furthermore the catalyst stability analyzed by relating the LSV curves recorded sooner and later continuous potential cycling between −0.2 and 0.1 V at a scan rate of 100 mV/s in 1 M KOH electrolyte; there is only −14 mV negative shifts after 10000 cycles at the current density of 100 mA cm−2 while Pt/C shows −46 mV negative shifts. The exceptional activity accompanied by the unique architecture of

Electrocatalysts for Hydrogen Evolution Reaction  157 continuous Pd-Ag-Al skeleton improves the electron transfer and electrolyte accessibility and also promote H2 evolution. As an economical and viable alternative to Pt, ruthenium (Ru) has gained considerable attention due to their Pt-like hydrogen bond strength (~ 65 kcal mol−1) [101–104], significant anti-corrosion ability [105], and lower price (4% to that of Pt) [106]. Moreover, the remarkable water dissociation ability of Ru makes it a promising candidate for HER [107].

6.3.1.3 Ruthenium Based Catalysts Ruthenium-based materials like oxides, phosphides, sulphides, and bimetallic were extensively examined for electrochemical hydrogen evolution reaction [108–112]. In spite of this, agglomeration of Ru nanoparticles during the synthesis and poor stability at high current density are the major challenges for the widespread utilization of Ru catalyst. In this context, a suitable supporting material is crucial to optimize the geometric structure, to enhance the catalytic activity, and endorse the enduring stability [113– 115]. Recently, Li et al. designed and constructed an impressive ultrafine Ru nanoparticle anchored over the graphene hollow nanospheres (GHSs) through a template-assisted approach [116]. The three-dimensional hallow nanospheres provide larger surface area for the well distribution of Ru NP, enhance the active sites, lessen the aggregation, and promote the catalytic performance [117–119]. By combining the ultrafine structured Ru NP and hallow spherical shape of graphene, the resultant Ru/GHSs catalyst evinced phenomenal catalytic property with low overpotential of 24.4 mV at 10 mA.cm−2 of current density, a small Tafel slope of 34.8 mV dec−1, along with high stability in 1.0 M KOH solution compared to commercial 20% Pt/C. Embedding the metal into the supporting material is another way to decrease the aggregation and way to improve the metal catalytic activity. Li et al. reported an outstanding electrocatalyst for HER by embedding the ruthenium-cobalt alloy in hallow carbon spheres (RuCo@HCSs) which outperforms with low overpotential and Tafel slope in wide range of pH [120]. This is attributed to the synergic influence of the hollow carbon layer and Ru-Co alloy, which could govern the bond strength of carbon– hydrogen throughout the HER process [121, 123]. DFT calculations confer the remarkable activity of hallow carbon, due to the electron transfer from metal nanoparticles and lower hydrogen dissociation energy. Transition metal compounds have been tested as substitutes for carbon-based supporting materials as these are having good electrical conductivity and stability in acidic medium. Combination transition metal with noble metal will enhances the HER activity as well. Based on this view, Manna et al.

158  Materials for Hydrogen Production, Conversion, and Storage combined Ru nanoparticles with CoSe nanocrystals to achieve an active nanocomposite material, which is synthesized by using a low-cost and simple single-step colloidal method [124]. Evaluation of prepared catalyst [Ru-CoSe] for HER admitted that, until 40CV cycles the activity was steadily increased and then stabilized. Investigations suggest that the catalyst underwent a transformation by losing the Co cations during HER; Ru-CoSe NCs were converted to Ru-CoOx/Co(OH)x-CoSe2 which was the actual catalyst for HER. The Ru-CoSe NCs exhibits overpotential of 152 mV, which was lower than CoSe NCs (228mV) to achieve 10 mA/cm2 of current density. The Tafel slopes were calculated to find the HER kinetics and recognized that Ru-CoSeNCs (37 mV/dec) has near value with Pt/C reference electrode (35.5 mV/dec), while CoSe with a higher slope of 46 mV/dec indicates that decorated Ru nanoparticles enhance the HER kinetics. For instance, various electrochemical properties of several electrocatalysts for hydrogen production using water splitting strategy are presented in Table 6.1.

6.3.2 Non-Noble Metal Catalysts Though the noble metal-based catalysts exhibit superior activity, low abundance and overprice restrain their real time application. So it is imperative to develop economical, stable, and high-performance catalysts to reach the desired hydrogen production for water splitting for real time applications. In this context, transition metal compounds have attracted great attention in the view of ampleness and low price; moreover, the unpaired electrons in d-orbital favor the electrocatalytic process [125] and make them as promising candidates. However, high agglomeration and low electrical conductivity hampered reaching optimal activity and stability [126]. It is necessary to engineer their composition and morphologies to obtain the required properties. In this view, transition metal sulphide, carbides, and oxides are considered as most encouraging catalysts.

6.3.2.1 Transition Metal Phosphides (TMP) Owing to their outstanding catalytic activity, stability in both acidic and basic pH, low cost, and more abundance, transition metal phosphides (TMP) are considered to be significant electrocatalysts for HER [127, 128]. Besides, phosphides have tunable electronic structures, metal-like electrical conductance, and resistance towards corrosion [129–132]. In 2005, Liu and Rodriguez first discovered the great potential of TMPs as HER electrocatalysts. However, there is still great room to further boost

Electrocatalysts for Hydrogen Evolution Reaction  159

Table 6.1  Electrochemical properties of various electrocatalysts for hydrogen production. Catalyst

Electrolyte

Overpotential (mV)

Current density (mA.cm−2)

Tafel Slope (mV.dec−1)

Ref

ALDPt/NGNs

0.5M H2SO4

0.05

16

29

[71]

Pt/Ru/VC

0.5M H2SO4

23

10

30.6

[77]

Pt-Ni nanocages

0.1M KOH

104

10

73

[82]

PdCNx

0.5M H2SO4

-55

40

35

[95]

Pd NCs@CeO2

0.5M H2SO4

--

--

235

[99]

Pd-Ag

0.5M H2SO4 1M KOH

63 108

200 200

26 56

[100]

Ru/GHS

1.0M KOH

24.4

10

34.8

[116]

RuCo@HCSs

1.0M KOH 1.0M neutral

21 49

10 10

32 59

[120]

Ru@CoSe

0.5M H2SO4

152

10

37

[124]

Ox-MoP/CNT MoP/CNT

0.5M H2SO4

114 160

10 10

51.6 54.5

[133] (Continued)

160  Materials for Hydrogen Production, Conversion, and Storage

Table 6.1  Electrochemical properties of various electrocatalysts for hydrogen production. (Continued) Catalyst

Electrolyte

Overpotential (mV)

Current density (mA.cm−2)

Tafel Slope (mV.dec−1)

Au/CoP@NC-3

0.5M H2SO4 1M KOH

118.1 140.9

10 10

57.75 72.48

[142]

FeP/CC

0.5M H2SO4

58

10

45

[145]

MoS2/TiO2

0.5M H2SO4 1M KOH

350 700

10 10

48 60

[171]

Ni3S2/NF

1M KOH

48.1

10

88.2

[172]

CoNiSe2/NF

1M KOH

106

10

MoxC/Cu

1M KOH 0.5M H2SO4

169 194

200 200

98 74

[182]

Co-MoC

0.5M H2SO4

200

100

38

[190]

Ref

[174]

Electrocatalysts for Hydrogen Evolution Reaction  161 their HER activity and stability for large-scale application. Wang et al. first reported MoP as an outstanding HER electrocatalyst for both the electrolytes [133] and profess that phosphorization modifies the metal properties and the degree of phosphorization determines the stability and activity. For instance, Metal-rich form (Mo3P) requires high overpotential (∼500 mV) as compared to MoP (125 mV) to reach 10 mA.cm−2 of current density. Though MoP shows better activity even in bulk-form, synthesis of crystalline MoP is still a challenge that needs to be resolved by developing new preparation methods. Besides, the conductivity and stability of a catalyst are further enhanced by introducing a supporting material [134–136]. In this view, Qamar et al. demonstrated an oxalate guided nonhydrolytic method to obtain small MoP nanoparticles well dispersed on carbon nanotube surface [137]. The small particle size with homogenous distribution, high specific and active surface area, conductivity, interfacial charge transfer kinetics, and turnover frequency induce the superior activity for MoP/ CNT electrode. As prepared MoP/CNT catalyst showed an excellent durability at 10 mA cm−2 of current density under acidic medium. The performance of catalyst can be improved by various methods in which hetero atom doping in electrode is one of the promising strategies, for instance, Mo doped Ni2P [138], Mn doped Ni2P [139], Ni doped CoP [140, 141]. Li et al. [142] synthesized a new gold incorporated CoP nanoparticles dispersed on N-doped carbon (Au/CoP@NC-3) by pyrolysis of Au@ZIF-67-3 at 900°C under Argon atmosphere and subsequent phosphorization at low temperature, which shows better HER activity with overpotential of 118.1 and 140.9 mV at 10 mA.cm−2 current density and the Tafel slopes 57.75 and 72.48 mV/dec in acid and alkaline solutions, respectively, much lower than CoP@NC. The modified electronic structure of active CoP enhances the effective surface area and optimizes H adsorption consequently improving the HER activity via electron transfer from Au. Utilization of multiple components in electrode preparation is one of the detrimental processes, which we can overcome by minimizing the components. For example, in general, electrocatalysts coated on current collectors using polymer binders as film-forming agents, by avoiding the polymer binder more active sites can expose, facilitate electron diffusion and reduce the series resistance [143] leading to improved catalytic activity [144] which can be achieved by growing the active phases on conductive substrates as the current collectors. Luo and co-workers developed self-assembled FeP nanorods decorated on carbon cloth (FeP NAs/CC) through low-temperature phosphidation of Fe2O3 NAs/CC [145]. As a binder-free three-dimensional electrode, it has shown excellent catalytic activity with a low Tafel slope of 45 mV/dec, large

162  Materials for Hydrogen Production, Conversion, and Storage exchange current density of 0.50 mA/cm2 in acidic medium and to reach the 10 mA/cm2 of current density, it requires 58 mV of overpotential.

6.3.2.2 Transition Metal Chalcogenides Due to its easy availability, inexpensive, high redox nature, good catalytic activity, and stability, transition metal chalcogenides developed as a popular group of HER catalyst [146, 147]. Moreover, it is notified that all the cofactors in hydrogenases ubiquitously involve metal sulphur interactions [148–153]. By considering the hydrogenases active sites, increasing significant attention have been focused on transition metal chalcogenides, such as CoSx [118–120, 154–156], CoSe2 [157–159], FeS [160–162], NiSx [163], NiSex [164–166], and MoSex [167, 168] as potential HER catalysts. By considering the exclusive structural and electronic characteristics, exposed active sites and molybdenum disulphides (MoS2) were widely tested as electrocatalysts for HER. In the year of 2005, based on the density functional theory calculations, Himmemann et al. predicted that the edges of MoS2 act as active sites for HER [169, 170]. This was experimentally confirmed and the catalytic properties further improved by modifying the morphology of MoS2, which leads to rise in the extent of active sites, the catalytic activity, and the electrical conductivity. Recently, Alberto and group synthesized a core-shell catalyst with manifold active edges and specific surface by decorating single crystalline MoS2 nanosheets on TiO2 nanorods arrays which couple the superior charge transport properties [171]. The as-prepared nanocomposite shows superior hydrogen evolution activity in basic and acidic media with 48 and 60 mV dec−1 respectively. Structural engineering of nanomaterials is another strategy to improve the catalytic activity. For instance, Fu et al. synthesized high-efficient special nanostructured nickel sulphides directly grow on current collectors to develop a 3D network (Ni3S2 nanorods@Ni-nanosheets) [172]. The 3D Ni3S2/NF was synthesized by the simple solvothermal method in which hydrazine hydrate, cetyltrimethyl ammonium bromide (CTAB), and sulphur powder treated with Ni foam at 160°C for 12 h. Ni3S2 nanorods can be obtained by using CTAB in the synthetic process and nanosheets formed using hydrazine hydrate. From Figure 6.3, low magnified SEM image displays complete coverage of NF with Ni3S2 nanorods and an enlarged SEM image (Figure 6.3b and Figure 6.3c) demonstrates the diameter of nanorods. In addition to this, from Figure 6.3d, the transition electron microscopy (TEM) images further confirm the microstructure of Ni3S2 nanorods coated with nanosheets. The HRTEM images of Figures 6.3e and f show both nanosheets and rods

Electrocatalysts for Hydrogen Evolution Reaction  163 (a)

(b)

(c)

20µm (e)

500nm (f)

41

0.

(d)

1µm

Ni

3S 2

S2 Ni 3

01)

2

i3 S m(

1)

8n

(02

m (1

5nm

200nm

5nm

0.2

m

8n

1n

0

2

0.28 nm (021) Ni3S2

(1

1) N

i 3S )N

01

E

0.2

0.4

m

n .41

S2 Ni 3

02

F

(1

nanorod

nm

nanosheet

) 01

5nm

Figure 6.3  (a–c) are the SEM images, (d) TEM is the image of Ni3S2/NF, and (e–f) HRTEM images of Ni3S2/NF from (d) [Replicated with permission from Ref. [172] copyright 2018 ACS].

exhibit clear lattice periphery with interplane distance of 0.28 and 0.41 nm. It is demonstrated the formation of Ni3S2 nanorods@nanosheets homojunction, which dramatically improves the catalytic effect by enhancing the electron transport kinetics and ion diffusion compared with the single nanorods and nanosheets. Compared to single metal dichalcogenides, mixed metal dichalcogenides are generally shows better performance because their synergetic effect of various metals nurture the defects in structure and motivate a part of idle sites to obtain enhanced catalytic activities [173]. Li et al. prepared a novel cobalt-nickel selenides nanorods on porous nickel foam (Co0.75 Ni0.25 Se/NF) and tested for HER, which has shown overpotential of 106 mV at 10 mA/cm2 current density [174]. The synergy between the Co and Ni determined by theoretical calculations explained that it is advantageous to improve the internal structural properties and electrical conductivity of the catalyst, and depress the HER binding energy as well as water adsorption energy.

6.3.2.3 Transition Metal Carbides (TMC) It has been confessed that the addition of carbon into the transition metal lattices will provide greater electronic density of states of d-band at the

164  Materials for Hydrogen Production, Conversion, and Storage Fermi level and, hence high catalytic activities [175, 176]. By considering the distinctive electronic structure and profusion, recently, TMCs have gained massive attention in hydrogen evolution technology [177–179]. Molybdenum carbides have been considered as a viable compound due to their Pt-like d-band electronic structure, HER activity, and low cost [180, 181]. Chen et al. developed the porous MoxC nano ribbons and nanoflowers over the copper foam (MoxC/Cu), copper foil, and nickel foam (MoxC/Ni) by in situ two-step thermal procedures [182]. The synthesized catalysts consisting of MoC/Mo2C heterostructure exhibited enhanced HER charge transfer kinetics with contrary actions of adsorption and desorption of H+ and reduced H by Mo2C and MoC, respectively [183]. When comparing the catalytic properties of MoxC on different supporting materials, MoxC on Cu has excellent activity with 136 mV of overpotential to attain current density of 100 mA/cm2 and 98 mV.dec−1 of Tafel slope in 1M KOH, while MoxC/Ni has greater overpotential of 338mv and Tafel slope of 257 mV.dec−1. From EIS studies, it has been observed that MoxC/Cu exhibits about 11 times lower interfacial charge transfer resistance compared to MoxC/Ni, due to the stronger adsorption and better electrical contact with copper and extreme charge transfer kinetics [184]. The 7 mV rise in the overpotential at 100 mA/cm2 during the i-t test suggests the outstanding durability towards HER in 1M KOH. The superior activity can be ascribed to the porous copper foam which accelerates electron transfer and mass transport. Incorporation of a distinct transition metal ions into TMC [185–187] can effectively improve the formation of heterogeneous phases which regulate the catalytic property and increase the degree of graphitization for carbon support that enhance the conductivity and stability of the carbon-contained TMC composite catalyst [188, 189]. Lin et al. synthesized Co-doped MoC by pyrolysis of MoOx-amine precursors [190]. The prepared Co-Mo2C nanowires exhibited excellent HER kinetics by 200 mV of overpotential and 39 mV.dec−1 Tafel slope to attain a current density of 100 mA.cm−2 under 0.5 M H2SO4 solutions. Multi-heteroatom doping N, P-doped Mo2C [191], N, P or N, and S dual-doped Mo2C catalysts [192] are developed to analyze the HER catalytic activity.

6.4 Conclusion and Future Aspects Hydrogen evolution from electrochemical water splitting gained great attention due to the availability of water and zero emissions. Nevertheless, the energy efficacy of water electrolysis is obstructed by the slow HER

Electrocatalysts for Hydrogen Evolution Reaction  165 and OER reaction kinetics and high overpotential. So it is indispensable to utilize catalysts to enhance the reaction kinetics and energy production. Catalyst is a key component, which reduces the overpotential and increases the reaction kinetics for efficient H2 evolution. Taken together, impressive advancement have been done in the design of electrocatalyst for water splitting for hydrogen generation by different strategies. Herein, we discussed the development of electrocatalyst and their activity including noble metals and non-noble metal catalysts. Platinum-group catalysts are preferred as a superior catalyst for HER, but high cost and low abundance limit their real-time application. In this context, various strategies like low metal loading, downscaling, the material size, and alloying with other lowcost materials are used to overcome these issues. The adoption of transition metal-based compounds as electrocatalysts is another approach to achieve high energy efficiency. Transition metal phosphides, sulphides, selenides, carbides, oxides, etc., have been evaluated for HER. Molecular and structure engineering, coating on various supporting materials, and blending with other metals are the adapted ways to improve the catalyst performance. In addition, there is a wide-ranging window for further development of energy efficiency and decline in cost of water electrolysis by developing the facile and inexpensive methods for the synthesis of electrocatalysts on a commercial scale. By understanding the mechanism, as it differs in different pH and temperature conditions, further investigation required. Overall, the present concerns in the development of universally employable approaches for low-cost and efficient catalysts have been addressed by aforementioned forthcoming concepts.

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Electrocatalysts for Hydrogen Evolution Reaction  179 176. Gao, Q., Zhang, W., Shi, Z., Yang, L., Tang, Y., Structural design and electronic modulation of transition-metal-carbide electrocatalysts toward efficient hydrogen evolution. Adv. Mater., 31, 1802880, 2019. 177. Meng, T. and Cao, M., Transition metal carbide complex architectures for energy-related applications. Chem. Eur. J., 24, 16716, 2018. 178. Chen, W.F., Muckerman, J.T., Fujita, E., Recent developments in transition metal carbides and nitrides as hydrogen evolution electrocatalysts. Chem. Commun., 49, 8896, 2013. 179. Houston, J.E., Laramore, G.E., Park, R.L., Surface electronic properties of tungsten, tungsten carbide, and platinum. Science, 185, 258, 1974. 180. Zheng, W., Cotter, T.P., Kaghazchi, P., Jacob, T., Frank, B., Schlichte, K., Zhang, W., Su, D.S., Schuth, F., Schlogl, R., Experimental and theoretical investigation of molybdenum carbide and nitride as catalysts for ammonia decomposition. J. Am. Chem. Soc., 135, 3458, 2013. 181. Wan, C., Regmi, Y.N., Leonard, B.M., Multiple phases of molybdenum carbide as electrocatalysts for the hydrogen evolution reaction. Angew. Chem. Int. Edit., 53, 6407, 2014. 182. Zhaoqian, W., Xiao, H., Shunlian, N., Xiongwu, K., Chen, S., Supported Heterostructured MoC/Mo2C Nanoribbons and Nanoflowers as Highly Active Electrocatalysts for Hydrogen Evolution Reaction. ACS Sustain. Chem. Eng., 7, 8458, 2019. 183. Michalsky, R., Zhang, Y.-J., Peterson, A.A., Trends in the hydrogen evolution activity of metal carbide catalysts. ACS Catal., 4, 1274, 2014. 184. Yu, L., Zhou, H.Q., Sun, J.Y., Qin, F., Yu, F., Bao, J.M., Yu, Y., Chen, S., Ren, Z.F., Cu nanowires shelled with NiFe layered double hydroxide nanosheets as bifunctional electrocatalysts for overall water splitting. Energy Environ. Sci., 10, 1820, 2017. 185. Xiong, K., Li, L., Zhang, L., Ding, W., Peng, L., Wang, Y., Chen, S., Tan, S., Wei, Z., Ni-doped Mo2C nanowires supported on Ni foam as a binder-free electrode for enhancing the hydrogen evolution performance. J. Mater. Chem. A, 3, 1863, 2015. 186. Wan, C. and Leonard, B.M., Iron-Doped Molybdenum Carbide Catalyst with High Activity and Stability for the Hydrogen Evolution Reaction. Chem. Mater., 27, 4281, 2015. 187. Fan, H., Yu, H., Zhang, Y., Zheng, Y., Luo, Y., Dai, Z., Li, B., Zong, Y., Yan, Q., Fe-Doped Ni3C Nanodots in N-doped carbon nanosheets for efficient hydrogen-evolution and oxygen-evolution electrocatalysis. Angew. Chem. Int. Edit., 56, 12566, 2017. 188. Maldonado-Hódar, F.J., Moreno-Castilla, C., Rivera-Utrilla, J., Hanzawa, Y., Yamada, Y., Catalytic graphitization of carbon aerogels by transition metals. Langmuir, 16, 4367, 2000. 189. Chen, Z., Wu, R., Liu, Y., Ha, Y., Guo, Y., Sun, D., Liu, M., Fang, F., Ultrafine Co nanoparticles encapsulated in carbon-nanotubes-grafted graphene sheets

180  Materials for Hydrogen Production, Conversion, and Storage as advanced electrocatalysts for the hydrogen evolution reaction. Adv. Mater., 30, 1802011, 2018. 190. Lin, H., Liu, N., Shi, Z., Guo, Y., Tang, Y., Gao, Q., Cobalt-Doping in molybdenum-carbide nanowires toward efficient electrocatalytic hydrogen evolution. Adv. Funct. Mater., 26, 5590, 2016. 191. Chen, Y.Y., Zhang, Y., Jiang, W.J., Zhang, X., Dai, Z., Wan, L.J., Hu, J.S., Pomegranate-like N,P-Doped Mo2C@C nanospheres as highly active electrocatalysts for alkaline hydrogen evolution. ACS Nano, 10, 8851, 2016. 192. Wang, D., Liu, T., Wang, J., Wu, Z., N,P(S) Co-doped Mo2C/C hybrid electrocatalysts for improved hydrogen generation. Carbon, 139, 845, 2018.

7 Dark Fermentation and Principal Routes to Produce Hydrogen Luana C. Grangeiro1, Bruna S. de Mello1,3, Brenda C. G. Rodrigues1,3, Caroline Varella Rodrigues1,2, Danieli Fernanda Canaver Marin2, Romario Pereira de Carvalho Junior2,4, Lorena Oliveira Pires1, Sandra Imaculada Maintinguer2,4, Arnaldo Sarti1,2,3 and Kelly J. Dussán1,2,3* Department of Engineering, Physics and Mathematics, Institute of Chemistry, São Paulo State University-UNESP, Av. Prof. Francisco Degni, 55 – Jardim Quitandinha, Araraquara, São Paulo, Brazil 2 São Paulo State University (UNESP), Bioenergy Research Institute (IPBEN), Av. Prof. Francisco Degni, 55 – Jardim Quitandinha, Araraquara, São Paulo, Brazil 3 Center for Monitoring and Research of the Quality of Fuels, Biofuels, Crude Oil, and Derivatives – Institute of Chemistry – CEMPEQC, São Paulo State University (UNESP), Araraquara, SP, Brazil 4 UNIARA – University of Araraquara. SP, Brazil 1

Abstract

Interest in biohydrogen (bioH2) production from dark fermentation (DF) has increased due to green routes involving reusing by-products, wastewater, and residues from agroindustry. Moreover, bioH2 as an energy carrier of the future leads to clean combustion with the formation of a single product (water) and also releases 242 kJ mol−1 or 121 kJ g−1 energy per mass unit. As a result, it could be transformed into electrical energy using a fuel cell or an internal combustion engine. However, several studies state that the yield of bioH2 production in anaerobic reactors by dark fermentation (DF) is still low when compared to the yields of conventional hydrogen processes and technologies such as water electrolysis CH4 reform, and gasification coal, among others. Therefore, in the literature, different anaerobic technologies have been investigated, for example, changing the conventional systems to high-rate reactors and studies on the pre-treatment of inoculum, types of substrates, and genetic modifications of hydrogen-producing microorganisms. Therefore, this chapter shows the principal biochemical routes and main types of *Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (181–224) © 2023 Scrivener Publishing LLC

181

182  Materials for Hydrogen Production, Conversion, and Storage reactors used in wastewater-fed bioH2-producing systems. Finally, essential recommendations are highlighted. Keywords:  Biohydrogen, dark fermentation, wastewater, residues, anaerobic reactors, biochemical routes

7.1 Introduction Hydrogen is undoubtedly the energy carrier of the future. Different from fossil fuels, hydrogen has a low environmental impact especially regarding its clean combustion with the formation of only water as a byproduct (thus the zero-carbon footprint). Moreover, this clean fuel presents a higher energy content than other fuels such as methane, syngas, and natural gas. As a result, many researchers argue that H2 will substitute fossil fuels as the center of energy production worldwide, acting as the primary energy source for electricity generation, central heating fuel, and transportation fuel for trucks, ships, and airplanes [1]. Therefore, H2 is a promising alternative for long-term energy storage while associated with solar and wind renewable energy systems [2]. The biggest challenge involved in using H2 as an energy carrier is developing sustainable and low-cost production. H2 can be produced by various methodologies, for example, water electrolysis, photolysis, thermolysis, catalytic oxidation, pyrolysis, and biolysis [3]. However, one of the most common large-scale ways to obtain hydrogen is still based on fossil fuels by extracting H2 by breaking the bonds between hydrogen and carbon through a highly endothermic process such as steam-methane or natural gas reforming [4]. Water electrolysis is a method that is also usually adopted whose main limitations are low conversion efficiency and high electrical requirements [5]. One clean and promising alternative is hydrogen production through biological pathways. The so-called biohydrogen (bioH2) can be obtained by dark and photo fermentation, direct and indirect bio-photolysis. The bio-photolysis or water-splitting photosynthesis is a route to produce bioH2 by acting on oxygenated photosynthetic microorganisms, such as cyanobacteria and green microalgae, using only water and in the presence of light [6]. Nevertheless, among these biological systems, dark fermentation is by far the most sustainable and attractive method due to the use of organic waste as a substrate, integrating waste treatment and clean energy production simultaneously [7]. Dark fermentation consists of an organic compound degradation process by anaerobic bacteria to generate bioH2 without light [8]. A great

Dark Fermentation to Produce Hydrogen  183 variety of organic waste and residue can be utilized as substrates to produce bioH2 in DF process such as vinasse from ethanol production, crude glycerol, dairy waste, agricultural and fruit waste, biomass waste materials, and waste activated sludge [9–12]. Besides, many studies have focused on analyzing the microorganisms and their metabolic routes aiming to improve bioH2 production [13–15]. Thus, hydrogen-producing microorganisms commonly found in DF systems can be categorized into two major groups: strict anaerobic heterotrophs (e.g., Clostridium beijerinckii, Clostridium butyricum, micrococci, methanobacteria) and heterotrophic facultative anaerobes (e.g., Escherichia coli, Bacillus coagulans, Enterobacter aerogenes, and Citrobacter intermedius) [16–19]. The drawbacks of dark fermentation include the low yield of bioH2 associated with the generation of secondary metabolites such as organic acids (acetic acid, lactic acid, among others) and solvent as ethanol, low substrate conversion efficiency, thermodynamic limitations, and the presence of CO2 in the gas produced, requiring further separation. However, the major problem of bioH2 production from dark fermentation is the lack of studies on a pilot or industrial scale [20] as most studies are carried out on a bench-scale in batch, semi-continuous or continuous bioreactors [21–24]. Many alternatives have been proposed over the years to overcome the low yield of bioH2 production. One of them is the project of new bioreactors, for instance, the continuous multiple tube reactor [24], improvement of operational conditions (pH, HRT, H2 partial pressure) [14, 25] in the overall system and startup strategies [9], as well as pilot-scale studies [20] that various researchers have focused on. Biohydrogen is a promising energy carrier to change the world energy landscape from fossil fuels to clean and sustainable energy. However, there is room for further studies. In this chapter, biohydrogen production from various organic substrates, such as fruit and dairy waste and glycerol from Brazilian agro-industries is critically and comprehensively summarized. Moreover, it presents the principal metabolic pathways for anaerobic conversion of some types of residues and the principal microorganisms involved in the process are briefly introduced and discussed. In addition, advances in bioreactor configurations, biomass immobilization and operational condition optimization are presented.

7.2 Biohydrogen Production from Organic Waste Hydrogen is an attractive fuel for presenting high energy content (141,9 MJ kg−1) when compared to diesel (45 MJ kg−1), gasoline (47 MJ kg−1) and CH4

184  Materials for Hydrogen Production, Conversion, and Storage (50,02 MJ kg−1), for example [26]. Furthermore, the procedure to obtain H2 can be considered a clean energy source, as it generates a unique product (water) during combustion [27]. However, H2 is considered an essential intermediate in the anaerobic digestion of organic matter, as described below [28]. Anaerobic digestion involves some biological steps, such as hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Hydrolysis initially disintegrates and solubilizes proteins, carbohydrates, and lipids into simpler derivatives by a microbial enzymatic reaction. The following two steps are then carried out by chemoorganotrophic microorganisms, which obtain energy by fermentation using amino acids, saccharides, or various forms of volatile fatty acids, such as electron donors. Methanogenic microorganisms use acetic acid and H2 as the primary electron donor to produce CH4 and CO2, stabilizing all organic matter [29]. Therefore, to obtain high rates of H2, the anaerobic digestion process in the acidogenic phase needs to be interrupted. Favorable operating conditions for acidogenic bacteria should be established while the activity of H2 consuming microorganisms (methanogenic archaea and homoacetogenic bacteria) should be avoided. Some pretreatments with environmental inoculum have been applied to inhibit hydrogen consuming bacteria such as heat, freezing, acid, basic, among others [22, 30, 31]. Secondary metabolites of biotechnological and aggregate values, besides hydrogen, are generated during anaerobic digestion, such as volatile organic acids (acetic, butyric, propionic, caproic, lactic, valeric), 1,3 propanediol (1,3 PD) and some biofuels (ethanol, butanol, methanol, which are produced in the acidogenic and acetogenic stage) [32]. These generated co-products depend on the type of substrate. 1,3-PD has several applications in the textile, cosmetic, cleaning, fragrance, resin, and personal care industries, among others. It can be used as a monomer to produce polytrimethylene terephthalate (PTT), polyurethane, and polyesters. The world market demand for this product was considered 1.46 million tons in 2014 and is expected to increase to 2.25 million tons by 2022 [33]. The 1.3-PD market growth is boosted by the increased demand for PTT. In the past, 1,3-PD was produced from acrolein or ethylene oxide through chemical processes. However, these methods have several disadvantages, including their high costs and the need to operate under high pressures [34]. Recently, the economical and ecologically correct nature of 1,3-PD microbial biosynthesis has made this approach more viable and preferable to chemical synthesis, which requires expensive catalysts and leads to toxic intermediates [33]. Acetic acid is currently used in industries as a precursor to produce polymers and plastics, among others. It can be obtained by the carbonylation

Dark Fermentation to Produce Hydrogen  185 process using fossil fuels as feedstock. However, acetic acid can be generated by lignocellulose as raw materials instead of oil. Butyric acid is used in the chemical, pharmaceutical, and food industries in flexible plastics with high resistance to heat, cold, and light; to enhance butter flavor; to substitute for antibiotics, among others. Alternatively, butyric acid can be utilized as a precursor of butanol and it is obtained from propylene, a compound from petroleum in a high-energy demanding synthesis. In addition, butyric acid can be generated from sugar fermentation, such as glucose, sucrose, and xylose by Clostridium species [35]. Propionic acid is used as an essential chemical intermediate in the synthesis of vitamin E, artificial fruit flavors, fragrances, and perfumes. Its salts are widely used as food and feed preservatives. Currently, almost all industrial propionic acid is produced through petroleum-based chemical synthesis with an annual production capacity of approximately 180,000 tons in the United States [36, 37]. Lactic acid corresponds to natural organic acid with broad applications in food, cosmetics, textiles, and pharmaceuticals. Nowadays, it stands out as one of the most potential monomers, which can be transformed into chemical products such as pyruvic acid, acrylic acid, 1,2-propanediol, 2,3-pentanedione, lactate esters, and polylactic acid. The latter is biodegradable and is used in applications of medical, packaging, and electronics. This acid can be obtained by two routes: chemical synthesis or fermentation. Currently, lactic acid can be produced by fermenting sugars such as glucose and sucrose [38, 39]. Ethanol is largely produced in Brazil from sugarcane fermentation and, so far, is the only fuel capable of meeting the growing world demand for low-cost renewable energy with reduced polluting power. In addition, gaseous emissions from burning ethanol are in the order of 60 % lower, compared to emissions from burning gasoline [40]. Butanol can be utilized as a solvent in cosmetics, drugs, or even vitamins [41]. It has gained significant attention because of its environmental benefits, such as high energy content (29 MJ L−1) and low vapor pressure [42]. In addition, this biofuel is more similar to gasoline in its properties (corrosion, viscosity, boiling point, and octane index) [43]. Various agro-industrial wastes are helpful as substrates in anaerobic digestion, such as crude glycerol, dairy waste, fruit waste, and vinasse from sugar and ethanol production. Nevertheless, this chapter explores the relevance of applications of crude glycerol from biodiesel production, dairy waste, and fruit waste from Brazilian agro-industries to biohydrogen production.

186  Materials for Hydrogen Production, Conversion, and Storage

7.2.1 Crude Glycerol Biodiesel production has gained attention, mainly due to the depletion of global oil sources and environmental disorders [18]. Biodiesel is considered a green fuel for transportation. In addition, biodiesel is considered a green fuel for transportation. It can be one of the principal potential alternatives for sustainable environmental energy, widely accepted in the energy market [44]. Various feedstocks can be used for biodiesel production: vegetable oils, animal fats, or used cooking oils (UCO) from restaurants, homes, and public establishments [45]. UCO is gaining prominence because it is responsible for the reduced cost of biodiesel production. Furthermore, it is a cheap raw material and is easily obtained [46]. The transesterification reaction is the common way to obtain this biofuel, producing two main products: methyl esters (biodiesel, about 90 wt %) and crude glycerol (CG) (a valuable byproduct, about 10 wt %) [44, 45]. Therefore, for each 100 kg of biodiesel produced, 10 kg of CG is obtained [46]. Approximately, 66.7 billion pounds of biodiesel are produced per year, generating 5.87 billion pounds of crude glycerol [47, 48]. The CG is composed of glycerol, alcohol, water, soap, salts, free fatty acids, methyl esters, heavy metals and mono-, di- and triglycerides [49– 51]. All these impurities in the CG make it difficult for the main industries that could reuse them as pharmaceutical and food products [49–51]. The purification cost of CG is expensive on a small scale and it can be as high as 50.85 $/kg while the market price of pure glycerol is 1–15 $/kg [47, 52]. The dramatic drop in glycerol prices (from 0.55$/kg to 0.11$/kg) makes this component waste instead of a product [45, 46, 50, 51]. In addition, its disposal is considered harmful to the environment due to the impurities present in CG [53]. The increased production of CG is becoming an alarming situation in terms of high availability and low market consumption. Therefore, utilization of glycerol is required to maintain the sustainability of the biodiesel industry and the harmony between the supply and demand of glycerol through the circular economy [48, 52]. Bioconversion of CG is an attractive way to remediate large quantities of this waste and increase its value, providing benefits for the environment and improving the economic viability of biodiesel production [53]. As an easily degradable carbon source, CG can be used to generate biomass and microbial products using an anaerobic process [47, 54]. Moreover, the CG can be obtained at low prices and can be stored at room temperature for a long time [55].

Dark Fermentation to Produce Hydrogen  187

7.2.1.1 Dark Fermentation of Crude Glycerol to Biohydrogen and Bio Products Many microorganisms utilize glycerol in the respiratory process. The microbiological fermentation of CG has been studied using pure cultures, for example, Citrobacter freundii, Clostridium pasteurianum, Clostridium butyricum, Enterobacter agglomerans, Entrobacter aerogenes, Klebsiella pneumoniae, Klebsiella aerogenes, and Lactobacillus reuteri [18, 19]. In general, there are two pathways for glycerol metabolism: oxidative and reductive. In a reductive pathway: first, the glycerol dehydratase (GDHt, dhaB) is used to generate 3-hydroxypropionaldehyde; second, the 3-hydroxypropionaldehyde is reduced by 1,3-propanediol dehydrogenase (PDDH, dhaT), and finally, the 1,3-propanediol is produced [18, 56] (Figure 7.1). In the oxidative route: the glycerol is primarily transformed in dihydroxyacetone through glycerol dehydrogenase (GDH, dhaD), which is further phosphorylated through dihydroxyacetone kinase (DHAK, dhaK). Then, the phosphorylated product is metabolized by glycolysis to produce metabolic intermediates and final products of fermentation, such as propionic, lactic, butyric, acetic and formic acids, alcohols such as n-butanol, 2,3-butanediol, ethanol, and also gases as carbon dioxide and hydrogen (Figure 7.1). Methane can be generated by converting formic acid, acetic acid, H2 and CO2, obtained by fermentation through the action of methanogenic archaea [18]. The two pathways comprise a functionally important process for intracellular carbon balance while providing energy and small molecules for cell growth [57]. The H2 production through dark fermentation of CG is mostly dependent on microbial physiological capacities. Strains of H2-producing bacteria could be used in pure cultures, including Klebsiella sp. and Enterobacter sp., types of facultative anaerobes, and Clostridium sp. such as the strict anaerobes [15]. However, the CG contains several complex compounds and impurities. All these materials can make the action of microorganisms difficult in the process; therefore, an alternative to overcome this limitation is to use mixed cultures. Mixed consortia are able to use mixed substrates, combining enzymatic systems of various microorganisms with distinct synergy. As a result, it can allow a better conversion of GC with a narrower spectrum of selected products and also decrease production costs [18]. H2 production during the CG fermentation is usually accompanied by generating other byproducts, as can be seen in the following equations [18, 19]. The generation of highly reduced final products is accompanied by high H2 yields [10].

188  Materials for Hydrogen Production, Conversion, and Storage Crude Glycerol (CG)

Glycerol GDHt

GDH

3-hydroxypropionaldehyde PDDT

Biomass

Dihydroxyacetone ATP ADP

NADH NAD+

Dihydroxyacetone-P

1,3-propanediol (1) (2) (3) (4)

Pyruvate

H2 (1) (2) (3) (4)

NAD+ NADH

Lactate (3)

Acetate

Acetone

(1) (2) (3) (4)

(2)

Ethanol (1) (2) (3) (4)

Butanol (2)

Butyrate

2,3-Butanediol

(2)

(1) (4)

(1) Klebsiella (2) Clostridium (3) Lactobacillus (4) Enterobacter

Figure 7.1  Metabolic pathways for anaerobic conversion of crude glycerol to biohydrogen and bioproducts. Source: Adapted from Wischral et al. [56].

C3H8O3 + H2O → CH3COOH (acetic acid) +CO2 +3H2

2 C3H8O3 → C4H8O2 (butyric acid) + 2CO2 + 4H2

(7.1) (7.2)

2 C3H8O3 → C4H10O (butanol) + 2CO2 + H2O + 2H2

(7.3)

C3H8O3 → C2H6O (ethanol) + CO2 + H2

(7.4)

The adopted metabolic routes and the generation of metabolites during the anaerobic process of glycerol will be the responsibility of the microbial

Dark Fermentation to Produce Hydrogen  189 community, which can direct the NADH fluxes for the formation of the products, depending on the needs of the bacteria present in the reactors, in addition to the composition and concentration of CG. These pathways can occur concomitantly in the same microorganism [58–60]. Thus, 1,3-­propanediol and H2 correspond to the principal products that can be generated during glycerol bioconversion. However, for 1 mol of 1,3-propanediol to be generated, 1 mol of H2 is consumed, thus reducing the yield of H2 with the production of 1,3-propanediol [46]. According to Vivek et al. [61] several microorganisms such as Klebsiella, Citrobacter, Clostridium, Lactobacillus, and Enterobacter, for example, are reported as producers of 1,3-propanediol, which have important applications in polymer manufacture, through glycerol. Studies have reported that impurities present in CG may favor the generation of 1,3-propanediol [62]. In addition to impurities, CG corresponds to a residue rich in vitamins and other nutritional factors that can be used by microorganisms, resulting in an increase in metabolism, favoring the reduction pathway route [62]. Given these circumstances, some strategies have been adopted to favor H2 generation, such as reducing the concentrations of impurities present in CG through acid pre-treatment to remove soap or even promoting methanol evaporation by autoclaving the CG at 65°C for 15 min at 121°C [22, 63]. The most important advantage of using CG for H2 generation by biological processes is that it will improve the overall profit of biodiesel manufacturing factories, making the biodiesel industry more sustainable and cost-effective with the concomitant effluent treatment and energy recovery [22, 63]. During the anaerobic digestion of CG, thermal energy can be obtained that is converted into heat or electricity by fuel cells and applied in the biodiesel industry. Thus, it substitutes and avoids dependence on other non-renewable [55, 63]. This fact may promote the production and application of biofuels, which produce gains for the environment.

7.2.2 Dairy Waste The population and economic growth, combined with improving the quality of life, has led to an increasing global demand for food. The dairy sector is one of the most relevant components of the food industry, taking a vital role in the worldwide economy, reaching 718.9 billion dollars in 2019 with a global production of 852 million tons of milk [64]. However, the dairy industry is one of the main sources of waste generation, such as wastewater and cheese whey [65]. Furthermore, dairy products have high perishability with a consequent short shelf life. After the expiration date, these products are not considered fit for consumption,

190  Materials for Hydrogen Production, Conversion, and Storage resulting in significant waste stream generation [66]. The current expired dairy product management is followed by using a small portion for animal feed and most of it is sent to landfills [67, 68]. Although dairy waste does not contain toxic substances, its disposal in landfills is a real threat to the environment as it is characterized by a high organic content, easily biodegradable by microorganisms responsible for greenhouse gas emissions [69]. Several studies demonstrate the adverse effects caused by food waste disposal and highlight the need to convert waste into resources using sustainable techniques and processes [70, 71]. Dark fermentation (DF) has been considered an appropriate alternative for organic waste management (mainly carbohydrate-rich ones) with the advantage of clean energy generation and it has gained widespread attention in recent years [72]. Dairy waste contains a substantial amount of carbohydrates, mainly lactose, which is useful as a suitable substrate for biohydrogen obtention from DF [65].

7.2.2.1 Dark Fermentation of Dairy Waste to Biohydrogen and Bioproducts A proposed metabolic pathway of dairy waste as the main carbon sources for H2 production is described. Proteins, carbohydrates, and lipids compose dairy waste. Therefore, they can be degraded in amino acids, monosaccharides, and long-chain fatty acids. Next, anaerobic consortia bacteria mainly formed by Clostridium, Enterobacter, Lactobacillus and Megasphaera genera can consume these carbon sources and generate other aggregate value co-products (ethanol, acetate, butyrate, propionate) and H2 in dark fermentation of dairy waste [73] (Figure 7.2). Stavropoulos et al. [74] investigated different values of pH (4.0, 4.5, 4.7, 5.0, 5.3, 5.7) to establish the optimal pH in the DF for bioH2 obtention from a mixture of end-of-life dairy products (DPs). The study of a CSTR operated under mesophilic conditions (37°C), HRT of 6 days identified the maximum hydrogen yield (HY) and productivity (HPR) (0.84 mol H2 mol−1 carbohydrates consumed and 0.76 L. Lreactor−1 d−1, respectively) at pH 5.0, while the highest soluble carbohydrate degradation (98.7 %) was achieved at pH 4.7. The principal acids produced were butyric acid, acetic, and lactic acid. Castelló et al. [75] evaluated the performance of a CSTR reactor to produce H2 using raw cheese whey (without pretreatment) as a substrate with an organic loading rate (OLR) of 30 g COD L−1 d−1 and HRT of 24h. The reactor operated at 30°C with controlled pH of 5.5 for 30 days. The H2 production varied significantly, decreasing after 17 days of operation.

Dark Fermentation to Produce Hydrogen  191

Dairy Waste

Proteins

Carbohydrates

Lipids

Amino acids

Monosaccharides

Long Chain Fatty Acids

Pyruvate

Ethanol

Butyrate

Lactate

Acetate

Propionate

H2

(1)(2)(3)

(1)

(3)

(1)(2)(3)(4)

(4)

(1)(2)(4)

(1) Clostridium (2) Enterobacter (3) Lactobacillus (4) Megasphaera

Figure 7.2  Metabolic Pathways for anaerobic conversion of dairy waste to biohydrogen and bioproducts. Source: Adapted from Hassan et al. [73].

192  Materials for Hydrogen Production, Conversion, and Storage The maximum HY and HPR were 0.9 mol of H2 mole−1 of lactose consumed and 0.8 LH2 L−1d−1, respectively. The main acids produced were also butyric, acetic, and lactic acid. Both studies reported acid lactic as an intermediate metabolite, implying the coexistence of the H2-producers’ bacteria (HPB) (e.g., Clostridium and Enterobacter) with lactic acid bacteria (LAB), which are naturally present in wastewaters, mainly in dairy ones. LAB includes microorganisms of genus Lactobacillus, Lactococcus, Streptococcus, Pediococcus, and Leuconostoc. This group of bacteria is characterized as homofermentative when lactate is the only product of glucose fermentation and heterofermentative, when the products include lactate, acetate, CO2, and ethanol [76]. In general, the H2 yield depends on the combination of fermentation pathways that take place in the biological reactor. Castelló et al. [75] and Stavropoulos et al. [74] considered that there may have been a decrease in H2 productivity caused by substrate competition between HPB and LAB. This hypothesis is frequently approached in studies on bioH2 production [77, 78]. However, LAB plays a controversial role in dark fermentation since both studies also emphasized a tendency of lactic acid metabolization with a consequent increase in hydrogen production. A synergy between these two groups of lactate bacteria is inferred as an additional substrate. Recently, the number of publications involving hydrogen production from lactate has increased and the results demonstrate the hydrogen-­ producing route with positive interactions between LAB and HPB [13, 79]. Various species of bacteria, especially from the Clostridium genus present the potential to use lactate as a carbon source that characterizes them as lactate-fermenting, hydrogen-producing bacteria (LF-HPB). Some LF-HPB species (e.g., Clostridium neopropionicum) can produce H2 from lactate as a sole carbon source and others (e.g., Clostridium beijerinckii) using acetate as a co-substrate, which act as an oxidizing agent [79]. Ohnishi et al. [80] evaluated and confirmed the feasibility of producing H2 from lactate as a sole carbon source. The research showed the maximum H2 yield (0.43 mol mol−1 lactate) using a non-pretreated microflora from acid slurry and 0.4 mol mol−1 lactate from single culture (Megasphaera elsdenii). Moreover, acetate and propionate were obtained as the main products of both inoculums. García-Depraect et al. [11] studied the metabolism and microbiota using intermediate analyses at DF from mixed wastewater. The anaerobic batch reactors were operated at 35°C with pH 5.5. In this study, the cooperation between LAB and LF-HPB was evident, because during the initial phase of operation, there was a relative abundance of the Lactobacillus and Acetobacter genera leading to generation of lactic and acetic acid, without

Dark Fermentation to Produce Hydrogen  193 H2 production. Subsequently, there was the consumption of acetic and lactic acids with simultaneous H2 and butyric acid generation and relative abundance of Clostridium genus. The maximum hydrogen yield (HY) and productivity (HPR) were 100 N mL L−1 h−1 and 1200 N mL Lreactor−1, respectively. A positive interaction between the LAB and HPB resulted in bioH2 obtention from the degradation of lactate and acetate. Results obtained from the literature demonstrated that the use of organic waste which naturally contains lactic acid bacteria, as dairy waste, is feasible to produce H2 due to the interaction between the anaerobic bacteria during the process. However, it is a complex process and several factors (biotic and abiotic) have been influenced during the dark fermentation.

7.2.3 Fruit Waste Biohydrogen production from agro-industrial solid waste and wastewater is considered highly advantageous because its materials are abundant, biodegradable, and cheap. In addition, the recovery of agro-industrial waste has aggregate value and it can solve environmental problems and bring economic benefits [7]. Demand for fruits has increased significantly in recent years because of population growth and changing dietary habits. In 2019, China (22%), mainland China (22%), India (9%), Brazil (4%), the United States of America (2%), and Mexico (2%) were the biggest fruit producers, according to FAO [81]. The main production is oranges and bananas in Brazil, with 16.7 and 6.8 thousand tons per year, respectively [82]. There is a fresh (or natural) consumption of these fruits, but most are for industrial processing, around 47%. The waste that is produced in the field and fruit processing at various stages of the production chain must be considered. These residues can be solids, such as peel and seed, or liquids, such as washing water from industrial processing [83]. Most agro-industrial fruit waste is lignocellulosic materials (second generation biomass, which is the most abundant bio-waste on earth) that can also be used for H2 production; however, their complex structure often requires pretreatment and/or hydrolysis to make them more bioavailable. Among them, co-products are generated, which can negatively interfere in the fermentation, such as acids, furan derivatives and phenolic monomers from hydrolysis from hemicellulose and decomposition of lignin The compound 4-hydroxybenzoic acid, followed by 5-hydroxymethylfurfural affected the H2 production. It is important to know about the degree of inhibition of these compounds better to consolidate the use of lignocellulosic materials as renewable substrates.

194  Materials for Hydrogen Production, Conversion, and Storage

7.2.3.1 Dark Fermentation of Fruit Waste to Hydrogen and Bioproducts The biological hydrogen production from fruit waste can be done through self-fermentation by autochthonous bacteria. Several bacteria have been reported as H2 producers from agro-industrial fruit waste. Furthermore, the presence of natural fruit microflora can lead to less needs for inoculum addition and contributes to hydrogen production [84]. Camargo et al. [85] isolated Enterococcus casseliflavus from in natura citrus pulp and bagasse (processed to obtain a residue, as in industrial plants). The authors (op. cit.) evaluated the H2 production in anaerobic reactors filled with peptone cellulose solution (PCS) modified medium, separately with 2 g L−1 of glucose, fructose, sucrose, xylose, starch, cellobiose, cellulose, and lactose, besides citrus peel waste itself. In auto fermentation, higher H2 production (13.9 mmol H2 L−1) was obtained, followed

Citrus peel waste

Limonene

Cellulose

Hemicellulose

Lignin

Disaccharides

Cellobiose

Phenols

Monosaccharides Pyruvate

H2

Acetic acid

Acetyl-CoA

Acetaldehyde

Lactic acid Butyryl-CoA

Malonyl-CoA

Butyric acid

H2

Propionic acid

Ethanol

Figure 7.3  Metabolic pathways for anaerobic conversion of citrus peel waste to hydrogen and bioproducts. Source: Adapted from Camargo et al. [85].

Dark Fermentation to Produce Hydrogen  195 by xylose (10.3 mmol H2 L−1). Despite its lignocellulosic composition, the residue was rich in readily assimilable monomers, which may have contributed to the higher production of H2. However, from cellulose, as the only carbon source, lower H2 production was observed (4.3 mmol H2 L−1), probably because Enterococcus casseliflavus was not efficient in hydrolysate β-ponds between glucose molecules. During the hydrogen production from the citrus peel waste, volatile fatty acids (VFAs) were also formed, for example, acetic, butyric, propionic, and lactic acids, and ethanol. Then, they proposed the main metabolite pathway for its conversion (Figure 7.3). Mazareli et al. [86] evaluated effects of pH, temperature, headspace volume, percentage of inoculum, and carbohydrate concentration in the anaerobic production of BioH2. Anaerobic batch reactors were filled with autochthonous bacteria from banana waste and PCS medium to compose the working volume, as only pH, temperature, and headspace showed influence in the bioH2 production. The higher production (38.08 mL) was obtained at pH 7.5, 44°C, 40% of headspace, 15 % of inoculum, and carbohydrate concentration of 15 g L−1. The acids produced were acetic, butyric, and lactic, besides ethanol. In that essay, Clostridium was the predominant genera, followed by Lactobacillus, with 39.88 and 38.02 %, respectively. They also suggested the metabolic pathways for H2 and organic acids by autochthonous microbial biomass (Figure 7.4). However, usually, the use of sludge from an anaerobic reactor is indicated due to the variability and abundance of H2 producing bacteria from this inoculum, in which some pretreatment may be applied to increase the H2 production through the enrichment of these bacteria and the inactivation of the H2 consumers, such as methanogenic archaea. Turhal et al. [84] evaluated the bioH2 production from melon and watermelon mixture by dark fermentation. The production was improved from 80.62 (only with natural microflora) to 351.12 mL H2 Lreactor−1 h−1 when using 10 % (v/v) of seed sludge from a local anaerobic wastewater treatment plant after heat pretreatment. They also concluded that hydrogen production rose by increasing the substrate concentration due to higher initial total sugar content. Torquato et al. [12] demonstrated the potential for bioH2 production from different effluents and the biological potential of anaerobic sewage sludge used as inoculum. An increase in the biogas production was observed with the additions of either wastewater, achieving 85.3 and 13.4 mmol H2 L−1 when applying 1 L of the wastewater and vinasse, respectively. Furthermore, the studied effluents exhibited unique energetic reuse viewpoints: 24 and 4 MJ m−³, respectively. Concerning the microorganisms involved, the morphology results were characteristics of

196  Materials for Hydrogen Production, Conversion, and Storage Banana waste Glucose

Sucrose

Fructose

D-Fructose

Glucose-1-P Glucose-6-P

Fructose-6-P Fructose-1-P Fructose-1,6-diP glyceraldehyde-3P

Ribulose-5-P Acetyl-CoA

dihydroxyacetone-P

Pyruvate butyric acid

acetic acid H2 ethanol

lactic acid

Figure 7.4  Metabolic pathways for anaerobic conversion of banana waste to hydrogen and bioproducts Source: Adapted from Mazareli et al. [86].

hydrogen-producing bacteria, especially in Clostridium species. Therefore, biohydrogen production from the fruit industry integrates management and mitigation of waste with an alternative for the local energy supply. Moreover, their results showed the possibility of applying various types of waste generated at different stages of the fruit utilization by the agroindustry, with first- and second-generation waste. Montoya-Pérez et al. [87] analyzed the influences of initial substrate concentration, initial pH, and type of nutrient on bioH2 production from the fermentation of pineapple core waste from UASB wastewater treatment plant. Their best results in anaerobic batch reactors were achieved at an initial pH of 5.5, substrate concentration of 5 gglucose L−1, and using a nutrient formulation based on magnesium, iron, zinc, and sodium. Considering these conditions, a 5 L bioreactor was used to scale up, then a maximum yield of 1.54 mol H2 molglucose−1 was acquired, showing the possibility of using the pineapple waste as a renewable source for hydrogen production. Silva et al. [88] tested the anaerobic digestion of citrus wastewater with three types of inocula (Clostridium Acetobutylicum ATCC 824; Clostridium Beijerinckii ATCC 10132, citrus anaerobic consortium). At mesophilic

Dark Fermentation to Produce Hydrogen  197 conditions, anaerobic batch reactors were operated for 51 h in triplicate, fed with two different carbon sources: glucose (10.7 g COD L−1) and citrus wastewater (10.0 g COD L−1), at initial pH 7.0, under static mode. The reactors fed with glucose showed a bioH2 production only in the presence of pure cultures [inocula 1 (36.8 mmol L−1) and 2 (64.1 mmol L−1)]. While in the reactors operated with citrus wastewater, production was: 21.2; 15.7; and 37.6 mmol H2 L−1 with inocula 1, 2, and 3, respectively. The authors also produced ethanol from all inocula tested in the reactors fed with glucose. The characterization of the citrus anaerobic consortium showed a dominance of Gram + bacilli that grew in specific media to anaerobic H2-generating bacteria (UFC mL−1) such as Clostridium sp. (3 × 105), Lactobacillus sp. (4 × 105) and Streptococcus sp. (5 × 104), indicating a great diversity of anaerobic hydrogen-producing bacteria present in agro-­ industrial wastewater. As can be seen, some fruit residues may have inhibitory compounds for biological hydrogen production. A co-substrate can be applied to alleviate this problem, in addition to bringing other benefits capable of overcoming barriers such as a lack of buffering capacity and nutrient limitation. Soltan et al. [89] mitigated the inhibitory effects of phenolic compounds and/or metals in mixed fruit peel (which included oranges, tangerines, bananas, pomegranates, and mangoes in equal weights) by integrating them with paper mill sludge in a 30/70 proportion. This caused the hydrogen production to increase 3.01 times, reaching 366.2 mL, when using pretreated and concentrated anaerobic sludge from the thickener chamber of a wastewater treatment system. The experiments were conducted in anaerobic batch reactors with a working volume of 150 mL, 50 mL inoculum, and 100 mL substrates, and operated at 35°C and 6.0 initial pH. They also did an energetic and economic feasibility analysis, considering the need for heating and energy potential, which resulted in maximum net energy of 32.2 kJ kg−1 feedstock, representing an economically viable solution. Nevertheless, the biological hydrolysis of fruit waste, such as lignocellulosic biomass, can limit biohydrogen production. Therefore, some pretreatment protocols can be used to enhance the separation of the lignin and hemicellulose fractions, which reduces the crystallinity of the cellulose and increases the surface area of the materials. Physical, chemical, and biological treatments can improve the degradation process, system performance, and hydrogen obtention [8]. Thus, Saha et al. [90] investigated the pretreatment of some fruit waste with dilute acid acetic aiming to maximize the bioavailability of fermentable biomass components. The optimized conditions (0.2 M acetic acid, 100°C, 1 h) at 10% substrate loading provided an enhanced sugar recovery from banana peel (99.9%),

198  Materials for Hydrogen Production, Conversion, and Storage pineapple waste (99.1%), grape pomace (98.8%), and orange peel (97.9%). Besides, the efficacy of the acid pretreatment was confirmed by scanning electron microscopy (SEM), which indicated rupture in the biomass structure. Furthermore, a minimal loss of fermentable sugars was noted, as suggested by Fourier transform infrared spectroscopy (FTIR) analysis. This use of fruit without industrial processing as a substrate is also important and has been studied, as it can solve other environmental problems related to management and disposal. For example, there is rotten fruit, seeds and peel in supermarkets, for example, pieces that are not usually consumed by people. Akinbomi et al. [91] explored improving hydrogen production from various fruit wastes, such as apples, bananas, and grapes. Amongst them, apples had the most improved cumulative hydrogen production of 504 mL gVS−1, followed by a fruit mixture, 513 mL gVS−1, with 20% of orange and 80% shared equally by the other fruit. These results were obtained from a continuous fermentation system at 55°C and hydraulic retention time (HRT) of 5 days. The predominance in the production of acid acetic and acid butyric occurred for all fruits. It is essential to highlight that hydrogen generation from various fruit waste, residues from its industrial use, and second-generation lignocellulosic biomass can occur in different metabolic pathways benefitting the generation of secondary metabolites or subproducts, as volatile fatty acids and biofuels, mainly ethanol and butanol.

7.3 Anaerobic Systems Historically, anaerobic systems have been developed since the late 19th century. These systems were created for the treatment of waste and biological semi-solids such as animal manure, domestic waste, and sludge stabilization of primary and secondary effluent [92, 93]. They present the hydraulic retention time (HRT) equal to the solids retention time (SRT). As a result, these systems are characterized as low-rate [94, 95]. The reduction of the (HRT) in this conventional system decreases the number of biological solids inside the reactor as induced by the biomass washout in the effluent stream [96–98]. However, the maintenance of biological activity is necessary for high values of HRT regardless of the rate of growth of microorganisms, whether fast or slow. Due to this, these low rate configurations were considered limited systems when applied to anaerobic processes involving highly concentrated residues [94, 95]. From the 1970s, anaerobic contact reactors were developed to treat liquid effluents, such as industrial wastewater. These reactors are associated

Dark Fermentation to Produce Hydrogen  199 with higher cell retention levels and they give rise to better interaction between the biomass and the substrate at low HRTs [99–102]. The new high-rate processes allow the separation of HRT from the SRT using three basic mechanisms of biomass immobilization: (i) formation of highly sedimentable granules (auto-immobilization), combined with structures that allow the biogas separation and the sedimentation of granular sludge, (ii)  adhesion of microorganisms in support materials which can be fluidized due to the application of appropriate ascension velocities, and (iii) retention of sludge aggregates and biofilm formation on the fixed support inside of a reactor [94, 96, 100, 102]. The high-rate reactors were developed because extensive volumes of effluent must be treated, and optimally designed bioreactors can decrease the treatment time while increasing the treatment efficiency resulting in high yields of energy. In addition, these configurations are considered robust, simple, and can reduce the cost of the bioprocess, besides making it more efficient and suitable for environmental protection [96, 103, 104]. However, anaerobic high–rate reactors were firstly used in methane production after these configurations were used and modified for the bioH2 production from different industrial wastewaters [104, 105]. Thus, different reactor configurations have been studied in the literature to increase the hydrogen yields and to improve the volumetric production rates of this biogas [106]. According to Oliveira et al. [23], the bioH2 production in fermentative processes are related to several parameters and one of the most important ones is the reactor configuration. Therefore, several studies reported different types of reactor configurations applied in acidogenic systems [23, 96, 107–109]. Examples of these reactors are up-flow reactors and sludge blanket (UASB – up-flow anaerobic sludge blanket reactor), fixed bed reactors (APBR – anaerobic packed-bed reactor) [110, 111], and fluidized bed reactors (AFBR – anaerobic fluidized-bed reactor [112–115]. It is not simple to define the type of reactor configuration suitable for the energy bioprocess. Optimization and studies are needed. In the literature, the most frequently used reactor is the batch operating mode due to easy operation and control, but for large-scale operations continuous production is used due to practical engineering reasons [116]. To obtain continuous and stable hydrogen production, appropriate operational strategies need to be adopted for each type of reactor. Several studies have shown that the main challenge of acidogenic systems is to decrease the instability on hydrogen production that could be related to the OLR of food-to-microorganism ratio (F/M), which could reach values that decrease or increase hydrogen production [24, 117]. Many types of

200  Materials for Hydrogen Production, Conversion, and Storage research to understand this food-microorganism relation can be found, in other words, many types of research about the effect of shock organic loads to treat wastewater and produce bioenergy on different types of reactors [118–124]. To sum up, different configurations of reactor are used in hydrogen production; both suspended cell reactors and immobilized cell reactors. Immobilized cell reactors have biomass that is adhered to the surface of the support material, forming a biofilm. This reactor is a robust configuration compared to systems with suspended cells to separate the liquid and solid phases more efficiently and supports the application of high organic loading rates (OLR) [23, 108, 125]. Thus, the variety of materials can be used as support material. In cases where the reactor with suspended cells is similar to a uniform reactor, it does not have support material for the microorganism adhesion. The literature presents several works on the types of materials used as support material in immobilized cell reactors. The most commonly used are activated carbon and rock particles, polyurethane foam, plastic, and ceramic rings [96, 126–128]. Examples of this type of configuration are Anaerobic packed-bed reactors – APBR [129–131], and Anaerobic fluidized-bed reactors – AFBR [77, 132, 133]. Reactors with suspended cells are a type of uniform reactor. They do not have support material for microorganism adhesion. An example of this type of reactor is an Upflow anaerobic sludge blanket reactor (UASB). Therefore, research was conducted considering the last 20 years to verify the principal reactor configurations used in hydrogen production. After a data collection study, it was found that the two types of reactor configuration that were most used in hydrogen production were the CSTR and anaerobic packed-bed reactor APBR. The use of UASB is observed and the least used configuration was CTAD and ABR. Since 2015, the CMTR configuration has been applied in dark fermentation as a new type of reactor to improve the hydrogen yield and control of biomass inside the reactor. These studies are presented in Table 7.1. The CSTR is the most widely used technology. It is a continuous agitation tank that promotes a more efficient homogeneous mixture, thus leading the system to lower resistance to mass transfer and obtaining operating parameters in various circumstances [151]. In a CSTR, the rate of microbial growth is controlled by the hydraulic retention time (HRT), equal to the cell retention time (CRT). For this system to work better, the HRT parameter must be greater than the maximum growth velocity of the  bacteria as smaller values would cause a dilution and, consequently, the system’s washout [106].

Dark Fermentation to Produce Hydrogen  201

Table 7.1  Study of data collection over the last 20 years on the types of reactor configuration most used in hydrogen production. Characteristics of wastewater

Operating conditions

Energy efficiency (HY)

Reference

CSTR (170 L)

Molasses (sugarcane) COD = 128 g L−1

T = 30–37°C HRT = 7 d (time of batch reactor)

8.4 L H2 L−1 vinasse

[134]

CTAD (4 L)

Synthetic wastewater (glucose) COD = 20 g L−1

T = 35°C OLR = 52–416 mmol glucose m−3d−1 HRT = 0.25–2 d

33.3–711 mmol H2 mol−1 glucose

[135]

CSTR (1.2 L)

Bean curd

T = 35°C OLR = 52–416 mmol glucose m−3d−1 HRT = 10 h

2.54 molvH2 mol−1 hexose

[136]

UASB (3 L)

Rice winery COD = 14–36 g L−1

T = 20–55°C HRT = 2–24 h

1.37–2.14 mol H2 mol−1 hexose

[137]

ANSBR (5 L)

Synthetic wastewater (sucrose) COD = 20 g L−1

T = 35°C OLR = 40–120 kg COD m−3d−1 HRT = 4–12 h

0.6–2.6 mol H2 mol−1 sucrose

[138]

UASB (3 L)

Synthetic wastewater (sucrose) COD = 20 g L−1

T = 35°C HRT = 4–24 h

53.5 mmol H2 d−1

[139]

Type of reactor

(Continued)

202  Materials for Hydrogen Production, Conversion, and Storage

Table 7.1  Study of data collection over the last 20 years on the types of reactor configuration most used in hydrogen production. (Continued) Characteristics of wastewater

Operating conditions

Energy efficiency (HY)

Reference

UASB (8 L)

Citric acid COD = 15–21 g L−1

T = 35–38°C OLR = 10–75 kg COD m−3d−1 HRT = 8–48 h

0.84 mol H2 mol−1 hexose

[140]

CSTR (3 L)

Cheese COD = 86.3 g L−1

T = 55°C OLR = 21, 35 and 47 kg COD m−3d−1 HRT = 1, 2 and 3.5 d

5–22 mmol H2 g−1 COD

[141]

ASBR (7 L)

Tequila COD = 29.9–30.5 g L−1

T = 35°C OLR = 18–30 kg COD m−3d−1 HRT = 24 h

44.4–57.6 mL H2 g−1 DQO

[142]

CSTR (2 L)

Cassava COD = 61.9 g L−1

T = 55°C OLR = 3 kg SV m−3 d−1 HRT = 3 d

69 mL H2 g−1 SV

[143]

ABR (24 L)

Tapioca COD = 16.3 g L−1

T = 32.3°C OLR = 16.15 g COD L−1d−1 HRT = 3–24 h

5.7–18.7 mL H2 g−1 COD

[144]

APBR (3.75 L)

Cheese whey COD = 25 g L−1

T = 30°C OLR = 22–37 kg COD m−3d−1 HRT = 24h

1.1 mol H2 mol−1 lactose

[145]

Type of reactor

(Continued)

Dark Fermentation to Produce Hydrogen  203

Table 7.1  Study of data collection over the last 20 years on the types of reactor configuration most used in hydrogen production. (Continued) Characteristics of wastewater

Operating conditions

Energy efficiency (HY)

Reference

APBR (2.3 L)

Stock + Molasses (sugarcane) COD = 35.2 g L−1

T = 55°C OLR = 54.3–108 kg COD m−3d−1 HRT = 24, 8 h

−1

0.3–1.4 mol H2 mol CH

[146]

APBR (2.3 L)

Stock + Molasses (sugarcane) COD = 35.2 g L−1

T = 55°C OLR = 84.2 kg COD m−3d−1 HRT = 10, 2 h

1.6 mol H2 mol−1 CH

[147]

APBR (2.3 L)

Stock + Molasses (sugarcane) COD = 35.2 g L−1

T = 25°C OLR = 36.4 kg COD m−3d−1 HRT = 24 h

0.6 mol H2 mol−1 CH

[148]

CMTR (1 L)

Synthetic wastewater (sucrose) COD = 2–4 g L−1

T = 25°C OLR = 24 g COD L−1d−1 HRT = 2–4 h

0.4–0.5 mol H2 mol−1 CH

[149]

CMTR (1 L)

Cassava COD = 55 g L−1

T = 25°C OLR = 24 g COD L−1d−1 HRT = 4 h

0.73 mol H2 mol−1 CH

[150]

CSTR (2 L)

Synthetic water (lactose)

HRT = 6–24 h

0.86 mol H2 mol−1 lactose

[14]

Type of reactor

(Continued)

204  Materials for Hydrogen Production, Conversion, and Storage

Table 7.1  Study of data collection over the last 20 years on the types of reactor configuration most used in hydrogen production. (Continued) Type of reactor

Characteristics of wastewater

Operating conditions

Energy efficiency (HY)

Reference

APBR (2.3 L)

Sugarcane vinasse COD = 20–30 g L−1

T = 55°C OLR = 82.4 kg COD m−3d−1 HRT = 7.5 h

1.6–3.0 mol H2 mol CH−1

[125]

CSTR (2L)

Synthetic wastewater COD = 3–10 g L−1

T = 37°C HRT = 169 h (operating a batch anaerobic reactor)

0.3–1.4 mol H2 mol−1 CH

[21]

ANSTBR (1.9 L)

Sugarcane vinasse COD = 19.5–49 g L−1

T = 55°C OLR = 40–120 kg COD m−3d−1 TDH = 6–24 h

0.2–15.6 mmol H2 g−1 CH

[111]

CMTR (1 L)

Synthetic wastewater (sucrose) COD = 2–4 g L−1

T = 25°C OLR= 24–48 g COD L−1d−1 HRT = 2 h

0.48–0.51 mmol H2 g−1 CH

[24]

ANSTBR

Sugarcane molasses COD = 7308 g O2 kg−1 molasses

T = 55°C OLR = 60 g COD L−1 d−1 HRT = 4 h

1.18 mol H2 mol CH−1

[23]

Abbreviations: CSTR = continuously stirred-tank reactor, CTAD = chemostat-type anaerobic digester, UASB = Upflow anaerobic sludge blanket reactor, ANSBR= anaerobic sequencing batch reactor, ABR = anaerobic baffled reactor, ASBR = anaerobic sequencing batch reactor, APBR = anaerobic packed-bed reactor, CTMR= continuous multiple tube reactor, ANSTBR = anaerobic structured bed reactor. Operating conditions: COD = chemical oxygen demand, T = temperature, OLR = organic loading rate, HRT = hydraulic retention time, HY = hydrogen yield.

Dark Fermentation to Produce Hydrogen  205 The UASB is very used for effluents from different wastewater. Studies report that the UASB configuration allows the application of volumetric organic loads between 4–15 kg DQO m−3 d−1 [100, 152]. High organic loads can damage the system causing suspended biomass washing. The greatest challenge in the UASB reactor is the occurrence of granules, that is, after inoculation of the reactor, the biomass needs to develop in the form of granules for the bacteria to multiply and the anaerobic process to occur [100, 153–155]. Although these reactors do not have support material, their structure is simple, easy to operate, and they have little biomass loss [154, 156]. Studies report that the major advantage of this reactor configuration, when compared to anaerobic reactors such as a fluidized bed system, is that it needs fewer investment requirements. A disadvantage of this system is that it has a long startup time and needs a sufficient amount of granular seed sludge for faster startups [96, 134]. Carrillo-Reyes et al. [9] argue that in the UASB reactor, long periods of 10–12 hours are necessary for hydrogen production to occur when the reactor is inoculated with granular sludge. However, the hydrogen production takes 70 hours when the reactor is a startup with disaggregated grains. Studies such as Kotsopoulos et al. [157] reported the HY equal 2.47 mol H2 mol−1 hexose to a period of HRT equal to 27 hours in an UASB (0.22 L). In a study carried out by Ferraz Júnior et al. [148], the best material used in an APBR fed with sugarcane vinasse is PEBD compared to expanded clay because it is a material that has low wear susceptibility. When compared to UASB, APBR allows the application of organic loads from the order of 20 kg DQO m−3 d−1 due to the ability to adhere biomass to the support material [152]. However, the main limitation of this type of configuration is the clogging of the bed in long-term operations which can be caused by the accumulation of suspended solids, resulting in operational failures such as the formation of preferential paths and dead zones [100]. Works such as Camiloti et al. [158] and Mockaitis et al. [159] reported 98% porosity in structured bed reactors with polyurethane foam as support material. The ASTBR is used in acidogenic systems to produce hydrogen from vinasse to avoid operational problems such as bed clogging and biomass washout during the application of high hydraulic and organic loads [108]. ASTBR are reactors with material supports and operate as follows: the flow of wastewater is fed into the reactor by an upward path (1), it passes through a region of material support (2) when the biofilm is formed by immobilized cells. The liquid fraction of the reactor containing microorganisms can be collected by an effluent outlet (3); finally, the

206  Materials for Hydrogen Production, Conversion, and Storage biogas produced is collected on the gas outlet (4) [141, 160]. The difference between APBR and ASTBR is the type of material used as support and the form of this material is organized.

7.3.1 Continuous Multiple Tube Reactor Considering the challenges of biological hydrogen production associated with the control of cellular concentration within reactors to achieve stable long-term bioenergy production, the continuous multiple tube reactor (CMTR) is a configuration that stands out as a proposed unit of work to produce bioenergy without supporting material. The proposal is to use the inside of the tubes as support material and area for the development of biofilm, characterizing the new configuration as a simple and inexpensive reactor of immobilized cells when compared to fixed bed reactors (APBR, AFBR, ASTBR). Moreover, the structure of this reactor configuration can provide a larger surface area for fixing biomass in related to conventional tubular reactors without support material. It is because the CMTR presents a reaction region formed by a set of small-diameter parallel tubes with internal grooves adapted as a worm thread [24, 149, 150]. The CMTR is a configuration based on the structuring of parallel tubes that was investigated to overcome maintenance difficulties and instability problems in the H2 production associated with the lack of biomass control in the system. This configuration presents a reaction zone that consists of several small diameter parallel tubes. These tube zones provide a high length/ diameter ratio and consequently improve control over the surface speed of the flow in the “bed” providing better control over the SOLR. Moreover, the CMTR comprises an intermediate configuration between a single tube reactor without a bed and a fixed packed bed since the grooves in the inner walls of the tubes facilitate the adhesion of microorganisms to the surface. The CMTR was presented by Gomes et al., [149] and its differential is the ability to provide a larger contact surface for fixing the biomass eliminating the need for support material and at the same time allowing its continuous and controlled discharge due to the high surface speed and also assist in maintaining the specific organic load rate (SOLR) [24, 149, 150]. Some researchers, such as Anzola-Rojas et al. [161], report that SOLR is a significant factor of continuous and stable hydrogen production, as it is observed that high loads of SOLR can affect the conversion of substrates and changes in the metabolic routes of microorganisms in acidogenic systems. Therefore, the CMTR is proposed as a new approach to process control the SOLR function.

Dark Fermentation to Produce Hydrogen  207 The performance of CMTR in hydrogen production has been underexplored in the literature since 2015. Some factors being investigated at the present moment are: hydraulic retention time (HRT), support material in the outlet chamber, different concentrations of the synthetic medium, and cassava wastewater in terms of COD (chemical oxygen demand), feeding strategies [149], and the accumulation and attachment of biomass inside these reactors [24, 149, 150]. These studies addressed CMTR tests with twelve tubes (CMTR-12) in dark fermentation of synthetic effluent based on sucrose and cassava effluent, and related biomass accumulation data inside the reactor through the control of OLR, SOLR and low yields of hydrogen. Given the difficulty of maintaining H2 production in the CMTR for long periods, Gomes et al. [149] suggested increasing the internal roughness of the tubes, as a strategy to promote greater biomass retention in the reactor, and thus reach SOLR values closer to the range recommended in the literature. In this scenario, Trevisan et al. [24] increased the roughness of the inner part of the tubes to improve the CMTR biomass control capacity. The influence of HRT and substrate concentration on continuous H2 production were evaluated from sucrose-based synthetic wastewater. The hypothesis put forward by Gomes et al. [149] on the potential of the CMTR was proven since the continuous H2 production was recorded for periods of approximately 100 days, higher than the maximum periods reported in many studies: 25 days [150], 30 days [75], 30 days [162], and 38 days [110]. Figure 7.5 shows the first structure reported by Gomes et al. [149] tested with synthetic wastewater and Gomes et al. [150] tested with real wastewater (cassava). The CMTR on those studies was used a bench-scale (1 L). It was manufactured in three main parts: both inlet and outlet chambers were constructed of tubular acrylic (inner diameter of 80 mm), the reactional region has 12 polyvinyl chlorides (PVC) tubes (inner diameter of 12 mm and length of 680 mm) to allow biomass attachment. Modification studies have been developed, and the CMTR-10 and CMTR-18 have similar construction characteristics [24, 150, 151]. They also included three compartments and the principal differences of CMTRs modified are the number and material of tubes on those reactors. At CMTR-10 the reactional region consists of 10 polyamides (PA) tubes with a diameter of 12 mm. The CMTR-18 comprises 18 polyamides (PA) tubes with a diameter of 12 mm. Both present inlet and outlet chambers made of tubular acrylic. Moreover, the internal surface of the tubes was adapted with internal grooves as worm thread to improve the biomass adherence. Some studies are reported on the CMTR-10 and CMTR-18. A characterization

208  Materials for Hydrogen Production, Conversion, and Storage Biogas Outlet

Sludge Outflow

Tubes

A

A

Section AA

Waste Input

Figure 7.5  The constructive characteristics of the CMTR reactor with 12 tubes. Source: Adapted from Gomes et al. [149].

study of the flow pattern of the CMTR-10 and CMTR-18 reactors through ­stimulus-response hydrodynamic experiments demonstrated that both CRMT reactors exhibited similar hydrodynamic profiles and presented a predominantly tubular flow pattern. Characterization of the hydrodynamics of novel reactor configurations is fundamental to assess their performance and to identify possible obstacles associated with operational and design failures. Another study with the CMTR-18 showed the production of hydrogen from synthetic wastewater based on sucrose. In this study, it was observed that the values o ​ f average conversion of sucrose (ERCH %) were equal to 23.54% ± 14.14% and the average hydrogen yield (HY mol H2 mol CH−1) obtained was 0.002 ± 0.003 mol H2 mol CH−1. The CMTR-18

Dark Fermentation to Produce Hydrogen  209 was not capable of producing hydrogen and this production was unstable in the long term. Therefore, biomass control during the dark fermentation process is crucial to acquire high bioH2 yields. Considering this, a CMTR is a reactor system that has been proposed as a work unit for H2 production and biomass control inside the reactor. It is important to mention that the first operations with the CMTR-12 were characterized by instability and short periods of hydrogen production, with maximum yields observed that did not exceed 0.6 mol H2 mol CH−1. Moreover, unstable H2 production has been observed frequently in studies with biological routes of hydrogen [149, 163, 164]. These results demonstrate that studies on the CMTR should be developed considering its ability to promote H2 production for long periods and also to try to promote more stable production profiles. Despite these aspects that still require studies at the CMTR, this reactor has shown to be promising in terms of not requiring a cleaning operation to dispose of biomass and/or starting a new production cycle, which facilitates operations with a shorter period of time. Because of that, it is necessary to propose more scientific research at the bench level to test this configuration and verify, for example, whether the increase in the number of tubes can influence biohydrogen production as it allows an increase in the reactor’s surface area and can offer more space for biomass adherence.

7.4 Conclusion and Future Perspectives DF is a biological process applied to biohydrogen production, and it is essential to highlight the role of the choice of substrate and reactor configuration for optimizing bioH2 production. Furthermore, microorganisms and their metabolic pathways perform a significant role in bioH2 production. The biochemical routes of microorganisms commonly found are strict anaerobic heterotrophs (e.g., Clostridium beijerinckii, Clostridium butyricum, micrococci, methanobacteria) and heterotrophic facultative anaerobes (e.g., Escherichia coli, Bacillus coagulans, Enterobacter aerogenes, and Citrobacter intermedius). Due to this great diversity of microorganisms, biohydrogen is produced. Moreover, other metabolites such as volatile organic acids (acetic, butyric, propionic, caproic, lactic, valeric), 1,3 propanediol and some biofuels (ethanol, butanol, methanol, which are produced in the acidogenic and acetogenic stage) are co-products generated and their amount depends on the type of substrate.

210  Materials for Hydrogen Production, Conversion, and Storage Several studies for DF about pre-treatment of the substrates, inoculum enrichment, genetic modifications of the hydrogen-producer microorganisms have been developed to improve hydrogen yield. However, many solutions, such as the construction of new anaerobic systems or innovation of the existing models, are necessary to optimize biohydrogen production and its application on an industrial scale. The great potential of developing biohydrogen production technologies is known worldwide. These technologies can reduce environmental impacts in the generation and use of energy, increase energy security, improve the use of natural resources, develop local areas in terms of labor and income generation, and diversify the energy matrix. In addition, it can affect the development of hydrogen cells that revolutionized the use of vehicle fuel, changing the entire automotive sector. However, to make bioH2 production reality, it is now up to public policymakers to distinguish government actions and create incentives for the economy, production, logistics development, and hydrogen systems.

Acknowledgements The authors acknowledge the support of the Ph.D. fellowship awarded to Luana C. Grangeiro, Bruna S. de Mello and Brenda C. G. Rodrigues awarded by CAPES.

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222  Materials for Hydrogen Production, Conversion, and Storage production from cassava flour wastewater in a continuous multiple tube reactor. Int. J. Hydrogen Energy, 41, 8120, 2016. 151. Ren, N., Cao, G., Wang, A., Lee, D.J., Guo, W., Zhu, Y., Dark fermentation of xylose and glucose mix using isolated Thermoanaerobacterium ihermosaccharolyticum W16. Int. J. Hydrogen Energy, 33, 6124, 2008. 152. Moletta, R., Winery and distillery wastewater treatment by anaerobic digestion. Water Sci. Technol., 51, 137, 2005. 153. Wiegant, W.M., Claassen, J.A., Lettinga, G., Thermophilic anaerobic digestion of high strength wastewaters. Biotechnol. Bioeng., 27, 1374, 1985. 154. Jhung, J.K. and Choi, E., A comparative study of UASB and anaerobic fixed film reactors with development of sludge granulation. Water Res., 29, 271, 1995. 155. Syutsubo, K., Harada, H., Ohashi, A., Granulation and sludge retainment during start-up of a thermophilic UASB reactor. Water Sci. Technol., 38, 349, 1998. 156. Seghezzo, L., Zeeman, G., Van Lier, J.B., Hamelers, H.V.M., Lettinga, G., A review: The anaerobic treatment of sewage in UASB and EGSB reactors. Bioresour. Technol., 65, 175, 1998. 157. Kotsopoulos, T.A., Zeng, R.J., Angelidaki, I., Biohydrogen production in granular up-flow anaerobic sludge blanket (UASB) reactors with mixed cultures under hyper-thermophilic temperature (70°C). Biotechnol. Bioeng., 94, 296, 2006. 158. Camiloti, P.R., Mockaitis, G., Rodrigues, J.A.D., Damianovic, M.H.R.Z., Foresti, E., Zaiat, M., Innovative anaerobic bioreactor with fixed-structured bed (ABFSB) for simultaneous sulfate reduction and organic matter removal. J. Chem. Technol. Biotechnol., 89, 1044, 2014. 157. Mockaitis, G., Pantoja, J.L.R., Rodrigues, J.A.D., Foresti, E., Zaiat, M., Continuous anaerobic bioreactor with a fixed-structure bed (ABFSB) for wastewater treatment with low solids and low applied organic loading content. Bioprocess Biosyst. Eng., 37, 1361, 2014. 160. Niz, M.Y.K., Etchelet, I., Fuentes, L., Etchebehere, C., Zaiat, M., Extreme thermophilic condition: An alternative for long-term biohydrogen production from sugarcane vinasse. Int. J. Hydrogen Energy, 44, 22876, 2019. 161. Anzola-Rojas, M.D.P., Da Fonseca, S.G., Da Silva, C.C., De Oliveira, V.M., Zaiat, M., The use of the carbon/nitrogen ratio and specific organic loading rate as tools for improving biohydrogen production in fixed-bed reactors. Biotechnol. Rep., 5, 46, 2015. 162. Andreani, C.L., Tonello, T.U., Mari, A.G., Leite, L.C.C., Campaña, H.D., Lopes, D.D., Rodrigues, J.A.D., Gomes, S.D., Impact of operational conditions on development of the hydrogen-producing microbial consortium in an AnSBBR from cassava wastewater rich in lactic acid. Int. J. Hydrogen Energy, 44, 1474, 2019.

Dark Fermentation to Produce Hydrogen  223 163. Kyazze, G., Martinez-Perez, N., Dinsdale, R., Premier, G.C., Hawkes, F.R., Guwy, A.J., Hawkes, D.L., Influence of substrate concentration on the stability and yield of continuous biohydrogen production. Biotechnol. Bioeng., 93, 971, 2006. 164. Manssouri, M., Rodrigues, J.A.D., Ratusznei, S.M., Zaiat, M., Effects of organic loading, influent concentration, and feed time on biohydrogen production in a mechanically stirred AnSBBR treating sucrose-based wastewater. Appl. Biochem. Biotechnol., 171, 1832, 2013.

8 Catalysts for Electrochemical Water Splitting for Hydrogen Production Zaib Ullah Khan1,2, Mabkhoot Alsaiari3,4, Muhammad Ashfaq Ahmed2, Nawshad Muhammad5, Muhammad Tariq6, Abdur Rahim1* and Abdul Niaz7 Interdisciplinary Research Centre in Biomedical Materials, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan 2 Department of Physics, COMSATS University Islamabad, Lahore Campus, Lahore, Pakistan 3 Empty Quarter Research Unit, Department of Chemistry, College of Science and Art in Sharurah, Najran University, Sharurah, Saudi Arabia 4 Promising Centre for Sensors and Electronic Devices (PCSED), Advanced Materials and Nano Research Centre, Najran University, Najran, Saudi Arabia 5 Department Dental Materials, Institute of Basic Medical Sciences, Khyber Medical University, Peshawar, KP, Pakistan 6 National Centre for Excellence in Physical Chemistry, University of Peshawar, Peshawar, Pakistan 7 Department of Chemistry, University of Science and Technology, Bannu, KP, Pakistan 1

Abstract

Inside the transportation business, hydrogen is a promising and ecologically benign energy transporter and can replace fossil fuels. At this time, there is no commercially feasible, environmentally friendly, wide area, economical, and highly efficient hydrogen production method. Solar-powered water-splitting thermochemical cycles might be the advanced way to generate free hydrogen through CO2. The way is globally beneficial because it just uses water and solar energy. The applications and uses of water splitting automation in the role of non-polluting as well as longterm methods of producing hydrogen from solar energy provide a lot of advantages lately sparked as you’ve gotten a lot of attention as a potential energy vector. Water splitting with thermal, electrical, photonic, and biological energy is a simple way to make hydrogen from renewable and nonrenewable resources. Thermal energy *Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (225–248) © 2023 Scrivener Publishing LLC

225

226  Materials for Hydrogen Production, Conversion, and Storage from fossil fuel steam reforming/gasification is used to create the bulk of hydrogen. because nonrenewable resources will eventually be exhausted. Although the majority of inexhaustible resources might be put to use to make up power for water splitting, pricing remains a concern. This review focuses on hydrogen production and the use of electrochemical water splitting, the electrocatalysts foundations for water splitting, as well as the characteristics of a good electrocatalyst for hydrogen. Keywords:  H2 evaluation reaction (HER), O2 evaluation reaction (OER), electrochemical water splitting (EWS), nanoparticles (NPs), single-atom catalysts (SACs)

8.1 Introduction Water splitting via electrochemistry stores energy as hydrogen and oxygen equivalents and might be used to scale up renewable energy storage. The broad use of such energy storage, on the other hand, will be assisted by abundant and readily available water. Water splits down into hydrogen and oxygen through the chemical process of water splitting as shown in Figure 8.1.

2 H2O → 2 H2 + O2



The technology breakthrough that is both efficient and cost-effective in water splitting can modify a hydrogen providence depend on green hydrogen. The technique of water splitting happens during photosynthesis, although no hydrogen is produced. The hydrogen fuel cell works by reversing the water-splitting process [1]. According to experts, platinum is farthermost the effective catalyst for the creation of hydrogen gas by breaking up water molecules. Researchers from Brown University recently published a study that explains why platinum is so successful — and it’s not for the reasons you would expect. His finding, which was published in ACS catalysis, assists in the resolution of

+ O2 2H2O

2H2

Figure 8.1  Water electrolysis, a kind of water splitting, is seen in this diagram [1].

Electrochemical H2O Splitting for H2 Production  227 a nearly century-old scientific problem, according to the authors. It might also help with the creation of new hydrogen-producing catalysts that are both less costly and more plentiful than platinum. This might help reduce fossil fuel emissions in the long run. This is possible if you’re clever enough to figure it out how to manufacture hydrogen inexpensively and effectively, offers up a slew of new options for chemicals and fossil-free fuels, said Andrew Peterson, a senior author of the research and an associate professor at Brown University’s School of Engineering. “In fuel cells, hydrogen can be conjugate with CO2 and nitrogen to produce fuel and fertilizer. The first step in creating new catalysts is to figure out what makes platinum so special for this reaction. We’ll need a less expensive catalyst to make water splitting a viable source of hydrogen” [2]. Goldilocks’s binding energy has been credited for a long time with platinum’s prosperity. Ideal catalysts attach to interacting particles in the center, but not too loosely or too tightly. The particles which are too loosely bound create problems initiating the reactions. If you secure them too tightly, the particles will cling to the catalyst’s surface, making a reaction more difficult to complete. Most scientists think that platinum’s hydrogen binding energy exactly balances the two components of the water-splitting process and that it is this characteristic that makes platinum so good [3]. However, as Peterson points out, there were reasons to dispute the accuracy of that photograph. In comparison to platinum, molybdenum disulfide (MoS2) has high binding energy but is a far worse water-splitting catalyst. This suggests, according to Peterson, that required energy isn’t the full story. Investigate the situation, the two of them utilized a novel technique they developed to analyze the water-splitting reaction on platinum catalysts by simulating electron and atom behavior in electrochemical reactions. According to the research, they settle under the platinum’s crystalline surface layer, where they continue to be passive spectators. The hydrogen atoms that do participate in the process are considerably less tightly connected than “Goldilocks” energy. They float above the platinum atoms, free to interact and generate H2 gas, rather than nesting in the lattice. The ability of hydrogen atoms on the surface to move freely, according to experts, is what makes platinum so reactive [4]. “The ‘Goldilocks’ binding energy isn’t an accurate design idea for the high area,” Peterson stated in a statement on the research. “The way forward, we believe, is to develop catalysts that convert hydrogen into this extremely mobile and reactive state.” At Brown University, Peterson’s lab uses computer simulations to design and develop novel materials for catalysis and other applications. As a result of this, the team plans to begin searching for new alternatives to platinum.

228  Materials for Hydrogen Production, Conversion, and Storage The most promising approach for creating high-purity hydrogen looks to be water electrolysis, and the essential power outcome for the electrolysis techniques could come from renewables (e.g. solar or wind) [5]. To accomplish effective large-scale electrolysis, materials that are abundant and steady are necessary to establish activity. Due to different demands linked to the environment’s acidity-alkalinity and the operating temperature, the practical challenges for the manufacture of electrocatalysts differ for the three electrolysis methods. The basic characteristics for a high-performance electrocatalyst are as follows: (i) the reaction (hydrogen or oxygen evolution) has a low intrinsic over potential; (ii) high active surface, allowing for easy access to reactants as well as quick extraction of products (ions, gases, and liquids); (iii) electrical conductivity (providing electron pathways); (iv) stability of electrochemistry (particularly for high-­temperature electrolysis); (v) stability of electrochemical reactions (i.e. not rusted when exposed to strong overpotentials) [6]. There are several advantages to using hydrogen as an energy carrier. Hydrogen is plentiful and may be generated using environmentally friendly methods or nonrenewable resources, and both fuel cells and internal combustion engines can benefit from it because oxidation produces just water; it has a minimal environmental effect and a three-fold greater gravimetric energy density fuels based on liquid hydrocarbons (although, it is worth noting that its low volumetric energy density present safety difficulties with its pressurized storage) [7]. Hydrogen as a source of energy, on the other hand, necessitates a substantial investment in infrastructure and finance. Water electrolysis can be used to make agglutinative fuels that can be utilized in existing infrastructures, which is a more viable solution to the energy problem. The electrons necessary for water splitting will come from renewable energy sources. CO2 will be collected from a wide range of sources and regenerated using renewable hydrogen (alternate water–gas–shift reaction) to produce synthetic fuels (Fischer–Tropsch synthesis) in a parallel process [8]. Hydrogen may be used as a reducing agent in several chemical industries, including crude oil refining and upgrading, as well as ammonia synthesis. Global hydrogen output was 50 Mt per year in 2015, with the majority of it originating from fossil sources. However, this also includes CO2 production. Various technologies for renewable hydrogen generation and photoelectrochemical water splitting have been developed as part of

Electrochemical H2O Splitting for H2 Production  229 the goal of a sustainable future. Water electrolysis is alone commercially accessible developed technology among them [9]. Due to economic constraints, hydrogen produced by electrolysis from sustainable energy sources accounts for just 4% of total hydrogen production nowadays (i.e. scarcity of low-cost renewable energy systems, a high initial investment cost, and a high energy input need). This scenario is projected to alter very soon as a result of the European Energy Directive, which sets a target of renewable energy sources meeting 14 percent of energy demand by 2020. Hydrogen synthesis by electrolysis of water utilizing renewable sources of energy is anticipated to perform a significant part in a green energy economy [10].

8.2 Water Splitting and Their Products Water is one of the most plenteous, inexpensive, and limitless beginnings on the planet, and it may serve as a clean and environmentally beneficial source. As a result, water splitting is a very advantageous hydrogen generation source and a long-term energy supply [11]. Using water electrolysis, you can store energy as hydrogen gas. In the electrolysis of water, two half-cell processes are involved [12]. The catalyst for each half-cell reaction determines the total energy efficiency of water electrolysis. As a result, substantial labors have been performed to create extremely active HER and OER electrocatalysts. The establishment of a meaningful metric for quantitatively measuring the activity of a certain catalyst is critical to the effective selection of the best catalyst [13]. Water splitting (chemical process) breaks down water into oxygen and hydrogen:

2H20 →2H2 + 02 A technical innovation that is both efficient and cost-effective in water splitting might enable a hydrogen economy based on green hydrogen. In photosynthesis, a word form of water splitting occurs, but no hydrogen is created. The hydrogen fuel cell is based on the reversal of water splitting [5].

8.3 Different Methods Used for Water Splitting 8.3.1 Setup for Water Splitting Systems at a Basic Level Fujishima and Honda performed, in 1972, photocatalytic water splitting demonstrated in a photo-electrochemical cell (PEC) for the first time [14].

230  Materials for Hydrogen Production, Conversion, and Storage V

O2

H2

e¯

e¯

H2

H2O H+ O2

H+ electrolyte

Figure 8.2  Shows a schematic design of a photo-electrochemical cell [16].

The PEC that was used to carry out the reaction is schematically illustrated in Figure 8.2. There are four basic phases in the procedure: H+ is converted to H2 by electrons generated by light on the cathode surface, Photogenerated holes on the surface of photo-anode oxidize water to generate oxygen and hydrogen and photo-generated electrons are delivered to the cathode via an external circuit [15].

8.3.2 Photocatalysis Table 8.1 shows the summary of the main methods for photocatalyst used for the water splitting reactions. Under typical circumstances, the free energy change for the process of transforming one molecule of water to hydrogen and half oxygen is 237.2 kJ/mol which translates to E according to the Nernst equation. The theoretical efficiency is calculated in the given equation in which we happened to convert solar energy to chemical energy, as shown in the equation:



η = Jg μexϕconv/S where ϕconv = Quantum yield for absorbed photons and μex = Excess chemical potential created by light absorption Jg = Absorbed photon, S = Total incident sun irradiance (mW cm−2) [17].

Electrochemical H2O Splitting for H2 Production  231

Table 8.1  Major photocatalysts synthesis methods for overall water splitting applications. Cocatalyst loading

Photocatalysts

Preparation method

Examples

Metal oxides

Molten salt (Flux) Solid state reactions Ammonia precipitation Calcination under controlled atmosphere

SrTiO 3:Al, SrTiO3: Rh,Sb, La2Ti2O7:Ba, NaTaO3, Ga2O3:Zn, BiYWO6, Bi1−xInxV1−xMoxO4 MxOy

Impregnation Photodeposition

Examples NiOx, CoOx, IrO2, RuO2, Rh2−yCryO3

Metal (oxy) nitrides

Thermal nitridation of → metal oxides using NH

Ge3N4, TaON:ZrO2, (Zn0.18Ga0.82)(N0.82O0.18) , LaMg1/3Ta2/3O2N, CaTaO2N, Ta3N5, LaScxTa1−xO1−2xN2−2x, GaN:Mg/InGaN:Mg

Impregnation Photodeposition

RuO2, Rh2−yCryO3

Metal-free photocatalysts

Thermal polymerization Electrochemical

g-C3N4, C-dot/g-C3N4

232  Materials for Hydrogen Production, Conversion, and Storage Table 8.2  The major electrolysis methods [6]. Low Temperature Electrolysis

Operation principles

High Temperature Electrolysis

Alkaline (OH·) electrolysis Proton Exchange (OH+) electrolysis Oxygen ione (O2-) electrolysis Liquid Polymer Electrolyte Membrane Solid Oxide Electrolysis (SOE) + + Conventional H - PEM H - SOE O2- - SOE Co-electrolysis Solid alkaline e¯+ e¯+ e¯+ e¯+ e¯+ e¯+ H2O/ O2 H2 H2O H2O H2O O2 O2 CO2 O H 2 2O O2 OH¯ H H + + 2 2 2 O H O2 H2 O2 H H2 H2O OH¯ H /CO 2

Charge carrier OHTemperature 20-80ºC Electrolyte liquid Anodic 40H¯ 2H2O + O2 + 4e¯ Reaction (OER) Anodes

OH-

H+

20-200ºC 20-200ºC solid (polymeric) solid (polymeric) 2H2O + O2 + 4e¯

Ni > Co > Fe (oxides) Perovskites: Ba0.5Sr0.5Co0.8Fe0.2O3.6, LaCoO3

Ni-based

Cathodic 2H2O + 4e¯ 2H2O + 4e¯ 4OH¯ + 2H2 4OH¯ + 2H2 Reaction (HER)

H+

500-1000ºC solid (ceramic)

2H2O 4H+ + O2 + 4e¯ 2H2O IrO2, RuO2, IrxRu1-xO2 Supports: TiO2, ITO, TiC 4H+ + 4e¯

2H2

4H+ + 4e¯+ O2

Perovskites with protonic-electronic conductivity 4H+ + 4e¯

2H2

O2-

O2-

500-1000ºC 750-900ºC solid (ceramic) solid (ceramic) O2-

/ O2 + 2e¯

1 2

O2-

/ O2 + 2e¯

1 2

LaxSr1-xMnO3 + LaxSr1-xMnO3 + Y-Stabilized ZrO2 Y-Stabilized ZrO2 (LSM-YSZ) (LSM-YSZ) H2O + 2e¯

H2O + 2e¯ H2 + O2H2 + O2- CO2 + 2e- CO + O2-

Pt/C Ni-YSZ Ni-YSZ Ni-cermets Subst. LaCrO3 perovskites MoS2 68-82% up to 100% up to 100% near-term laboratory scale laboratory scale laboratory scale demonstration Applicability commercial commercialization combination of compact design, low capital cost, Advantages relatively stable, mature alkaline and H+-PEM fast response/start-up enhanced kinetics, thermodynamics: + direct production lower energy demands, low capital cost of syngas electrolysis high-purity H2 technology corrosive electrolyte, low OH- conductivity high cost polymeric mechanically unstable electrodes (cracking), gas, permeation, membranes; in polymeric Disadvantages safety issues: improper sealing slow dynamics acidic: noble metals membranes Improve C deposition, miocrostructural changes in the Reduce noble-metal durability/reliability; Improve electrolyte Challenges microstructural electrodes: delamination, blocking of utilization change electrodes and Oxygen Evolution TPBs, passivation Cathodes

Ni alloys

Efficiency

59-70%

Ni, Ni-Fe, NiFe2O4

8.3.3 Electrolysis In electrolysis, the diaphragm primarily inhibits ionic conduction, but it can also be a cause of decreased efficiency owing to undesired gas penetration. It not only hinders overall performance but also poses several safety concerns. Gas crossover may also happen in PEM electrolyzers’ polymeric membranes, albeit to a lower level. In this situation, the membrane thickness can help to reduce gas permeation. Even though ceramic electrolytes prevent gas penetration, safety concerns emerge in solid oxide electrolysis (SOE). At high operating temperatures, good sealing between the anodic and cathodic chambers is difficult to establish over time, and the danger of abrupt braking is not insignificant. Below, Table 8.2 shows the major characteristics of the main electrolysis methods.

8.4 Principles of PEC and Photocatalytic H2 Generation According to Equation 8.1, the usual Gibbs free energy change for water splitting into H2 and O2 is 1.23 eV per electron transported. As a result, a semiconductor with a band of more than a gap of 1.23 eV, in water splitting into hydrogen and oxygen, at the same time suitable band edge locations that well overlap splitting of water potential are required as a whole (Figure 8.3) [18].



H 22O

1 ΔG = +237.3kJ mol -1-1 H 2(g) 2(g) + O2(g) 2(g), D 2

(8.1)

Electrochemical H2O Splitting for H2 Production  233 (a)

e¯

Bias

(ii) e¯ (iii) + H /H2 H+

H2

(b)

e¯

(iii) (ii) + e¯ e¯ V/NHE H 0

(ii)

H+/H2 (i) O2/H2O

e¯ H2

O2/H2O (iii)

(i) h+ h+

O2

1.23

h+ h+ (ii)

(iii) H2O

Figure 8.3  The basic principles and procedures for (a) photoelectrochemical hydrogen production and (b) photocatalytic hydrogen production from water splitting [18, 19].

8.5 Electrochemical Process for Water Splitting Application 8.5.1 Water Splitting Through Electrochemistry Water is one of the most plenteous, inexpensive, and limitless beginnings on the planet, and it may serve as a clean and environmentally beneficial source. As a result, water splitting is a very advantageous hydrogen generation source and a long-term energy supply [20]. An aqueous electrolyte, a cathode, and an anode are used in electrochemical water splitting. When a potential difference is supplied between the two electrodes, at the anode, oxygen gas is created, whereas, at the cathode, hydrogen gas is produced (Figure 8.4) [21], owing to the many-step procedure and energy buildup in all phases, when compared to the hydrogen evolution reaction and kinetically sluggish OER water electrolysis process with a significant surplus potential. Moreover, because OER follows a distinct process in acidic and basic media, it is greatly influenced by the electrolytic solution’s pH. Hydrogen and oxygen are created in acidic and neutral media by oxidizing two molecules of water at a potential of 0.404 V. When the voltage is set 1.230 V in a basic medium, the OH group was converted to O2 and H2O. As can be seen from the above, in an alkaline environment, the process operates more smoothly than in an acidic one [21].

8.6 Different Materials Used in Water Splitting 8.6.1 Water Oxidation (OER) Materials The reaction’s excess potential which is larger than the thermodynamic minimum indicates energy wasted. Catalysts can decrease this activation

234  Materials for Hydrogen Production, Conversion, and Storage (a)

(b)

H2

HER catalysts

membrane (diaphragm)

O2

In acidic solution: 2H2O

O2 + 4H+ + 4eˉ

Eº = -1.23 V

4H+ + 4eˉ

2H2

Eº = 0.00 V

2H2O

2O2 + 2H2

Eº = -1.23 V

In alkaline solution:

OER catalysts

4OHˉ

O2 + 2H2O + 4eˉ

Eº = -0.40 V

4H2O + 4eˉ

2H2 + 4OHˉ

Eº = -0.83 V

2H2O

2H2 + 2O2

Eº = -1.23 V

Figure 8.4  (a) Water splitting electrocatalytic cell; (b) electrocatalytic reactions [21, 22].

H2O + 2(h+)

H2

(OER) Oxygen evolution reaction

Energy / eV

(+)

2H+ + 2e¯

1/2 O2 + 2H+ (HER) Hydrogen evolution reaction

Ecb Eg

qEº(H+/H2) hv

(-)

Evb

1.23 eV Energy band gap qEº(O2/H2O)

semiconductor

liquid

Figure 8.5  For total water splitting (under acidic circumstances), O2 evolution reaction (OER), and H2 evolution reaction (HER); excellent semiconductor material for water splitting at its surface under light, with absolute energy scales [21].

barrier and speed up this reaction’s slow rate are desperately needed [23]. Figure 8.5 shows that the OER reaction is most typically thought of under alkaline settings; however, there is a lot of interest in finding OER catalysts that are acidic stable conditions so that the various water splitting architecture’s components can work together. As a result, researchers are working to determine the exact atomic structure of materials used as photoelectrodes in PEC structures that take up light and eventually track to OER and HER by separating charges [24].

Electrochemical H2O Splitting for H2 Production  235

8.6.2 Developing Materials for Hydrogen Synthesis However, extremely active photoelectrodes for PEC are necessary to absorb light and create hydrogen efficiently [25]. The search for earth-friendly materials that may convert light into charge effectively and quickly is gaining traction. A result of this is a significant increase in the study of metal oxides and other semiconductor materials of HER catalysts, i.e., metal phosphides and composite materials. Also, high surface area and form features of nanostructured materials can be exploited [26].

8.6.3 Material Stability for Water Splitting Water splitting requires material stability, to protect catalysts and photoelectrodes from the hostile aquatic environment; we use thin coatings. There must be no pinholes or flaws in the coating that could allow chemicals to penetrate and erode the sub-layers. To create thin layers with few or no imperfections, atomic layer deposition is becoming more and more popular [27]. For OER, HER, and other processes, the intrinsic stability of light electrode materials is also being explored from a fundamental standpoint [28].

8.7 Mechanism of Electrochemical Catalysis in Water Splitting and Hydrogen Production Electrochemically splitting pure water needs a considerable quantity of input electrical energy due to its resistance of 18 Mcm. Tap water and saltwater, on the other hand, have resistances of 5 and 20 cm, respectively, allowing for low-energy water electrolysis. Electrolysis of tap and saltwater is inefficient due to the presence of numerous redox-active impurity ions that waste a portion of the input energy for side reactions [29]. Half-cell reactions in acidic circumstances are described in Equations (8.2) and (8.3) and half-cell reactions in basic conditions are described in Equations (8.4) and (8.5).

4H+ +4e− → 2H2  E° = 0.0V

(8.2)

2H2O → O2 + 4H+ +4e−  E° = +1.23V

(8.3)

4H2O +4e− → 2H2 + 4OH− → E° = −0.828V

(8.4)

4OH− → O2 + H2O +4e− E° = +0.401V

(8.5)

236  Materials for Hydrogen Production, Conversion, and Storage

8.7.1 Electrochemical Water Splitting with Cheap Metal-Based Catalysts Extraordinary activity and nonprecious metals of all kinds have made a substantial contribution to the development of new energy sources, abundant availability, and simplicity of access. Researchers have invested a lot of time and effort in developing and using TM-based catalysts for EWS up to now. Fortunately, both catalyst production and study into reaction mechanisms have progressed significantly. In this part, the anionic elements organize and display freshly developed TM-based catalysts for EWS.

8.7.2 Catalysts with Only One Atom Single metallic atom materials have gotten a lot of interest as prospective catalysts for EWS because of their specific properties, enormous activity, and well-defined active regions [30, 31]. In reality, high-surface-free-­ energy atoms seldom survive on their own, necessitating the employment of appropriate supporters (e.g. metal sulfides, hydroxides, and g-C3N4) through a multitude of interactions, disseminate and maintain those metal (a)

MoS2

Pristine 2H MoS2 Cobalt nanodisks Sonication

SA Co on distorted 1T MoS2

Leaching Co NDs-MoS2 (c)

(b)

SA Co-MoS2

CN

400 ºC Ionothermal synthesis

N NC

N

Co

N N

CoSAs/PTF-400

CN

C N Co

0

500 ºC, 600 ºC 1 775 779

791 800

Energy (eV)

CN

Co-TPPCN

Ionothermal synthesis

CoSAs/PTF-500, 600 Current Opinion in Green Sustainable Chemistry

Figure 8.6  A common design for H2 evolution by electrocatalytic method. (a) The SA Co D-1T MoS2 manufacturing process is delineated schematically. (b) STEM-HAADF picture of SA Co D-1T MoS2, displaying the evident connection between SA Co D-1T MoS2 (dark cyan) and pure 2H MoS2 (wine). HRTEM and EELS spectra of SA Co D-1T MoS2 are presented (scale bar: 1 nm). SA, Single-atom, in inset figure [20].

Electrochemical H2O Splitting for H2 Production  237 active spots [32]. Another key way for creating ideal atomic site catalysts is to tune the interaction of active metal atoms. Single-atom catalysts (SACs) that were TM-based have been developed and recently showed exponential EWS performance in cost of structural stability and catalytic activity [33]. Qi et al. [34] published a paper in 2019. The increased HER actions were linked to the synergistic effect of the molybdenum and sulfur of the D-1T MoS2 support controlling the mode of H2 binding at the interface according to further experimental and DFT calculations [34]. Choosing a distinct coordination situation is described in Figure 8.6c [35]. These examples show how to fine-tune the structure and function of single-atom coordination environments, which could be employed in large-scale water electrolyzers in the future.

8.7.3 Electrochemical Water Splitting Using Low-Cost Metal-Free Catalysts Catalysts without metals have several specific benefits, including earth availability, economic effectiveness, pH resistance, and environmental friendliness [36, 37]. Nonetheless, employing various designs, numerous metal-free materials, primarily carbon materials, have been turned into extremely active electrocatalysts for EWS [38, 39]. Doping and the creation of specialized defect configurations are the most successful ways for further controlling carbon compounds’ catalytic activity for EWS [39]. P doping

+2

-O 2

OH

H C

H C O P

-2H2

g din

+2O

Bon

H

+2H+

-2H2O

+2H+ Current Opinion in Green and Sustainable Chemistry

Figure 8.7  A potential OER pathway with a pentagon defect on functional atoms’ C–O–P=O (OH)2 bonding (left). After the C–P bonds in the C3–P=O group are broken, the HER pathway is applied to the functional atoms in the freshly generated pentagon defect (right) [20].

238  Materials for Hydrogen Production, Conversion, and Storage greatly boosted the OER and HER activity of graphite layers, according to the findings. The defects formed by the breakdown of the C3–P double bond oxygen species were the operative functional sites for HER, according to their DFT calculations [39]. Phosphorene, in addition to the carbon compounds mentioned above, is gaining popularity as an electrocatalyst due to its excellent conductivity, direct bandgap, and characteristics. Several Phosphorene-based catalysts have been observed as excellent HER and/or OER results. Doping and flaw engineering can raise the activity of phosphorene in the same manner that they can improve the activity of carbon materials [40]. Furthermore, in DFT calculations by Xue et al. [41], the catalytic activity of OER would boost on a local P2O5-like surface, and OER performance was largely reliant on local phosphorene oxidation, was discovered. Phosphorene’s low air/water stability is a critical issue that has to be researched further (Figure 8.7) [20].

8.8 Water Splitting and Hydrogen Production Materials Used in Electrochemical Catalysis 8.8.1 Metal and Alloys Aside from single-atom catalysts (SACs), further available nanoparticles (NPs) and TM nanoclusters are other viable EWS catalysts [42]. As taking an example, Du et al. [43] found heteroatom doping appears to be a promising option for augmenting the catalytic activity of TMs in addition to putting active components on conductive supports [44]. According to theoretical research, charge redistribution produced by doping on the Ni surface may influence Ni metal’s electronic structure. To increase the activity of catalysts and the lifespan of transition metals, alloying is a sensible technique. Hundreds of TM-based alloys, notably those based on Fe, Ni, and Co, have demonstrated good EWS performance thus far. Hsieh et al., for example, created a NiF-Mo alloy [45]. In basic media, the couple showed considerable HER and OER activity, exceeding both the Pt/C and IrO2 couples. Hither, a few crucial things to bear in mind while creating effective alloy catalysts: (1) Building multimetal alloys with the right arrangements and nanostructures and (2) Using multimetal alloys in conjunction with low-dimensional conductive substances or various electrically active materials (i.e. hydroxides, sulfides).

Electrochemical H2O Splitting for H2 Production  239

8.8.2 Metal Oxides/Hydroxides and Chalcogenides Metal oxides/hydroxides with desirable traits such as diverse chemical compositions, simple synthesis methods, adaptable high activity, and physicochemical properties have piqued attention in the splitting of water, particularly OER as shown in Figure 8.8. To get the requirements of a successful electrochemical method, Gao et al. [46] designed an integrated method for a composite electrocatalyst that rationally targeted multiple metal oxide components. The Co-Cu-W electrocatalyst shows excellent stability, good faradic current, and low overpotentials for splitting of water in a basic medium.

8.8.3 Metal Carbides, Borides, Nitrides, and Phosphides Different anions can easily alter the electrical characteristics of the metal centers since they are directly coordinated to them [47]. It’s fascinating to (a)

Structure formation Chemical stability

Mixed metal oxide catalyst

Low overpotentials High conductivity High durability O2

H2 [SiW11O39]8Co

H E R

Cu

OER/HER Electrical activity conductivity

O E R

Copper foam electrode

rich chemical composition various lattice structures

abundant defect sites

boosting catalysts

Metal chalcogenides

tunable electron transition

Catalysts Application doping strategy

sy

n

th esi s

heterostructure optimize structure

pro m

g

unique electronic structure

various morphology

Water splitting

poor conductivity

modified strategies

limited stability limited activity

em s

antages dv

n isi

a

(b)

bl pro

Figure 8.8  Electrochemical water splitting methods based on transition metal oxides/ sulfides. (a) A diagram showing a material design and manufacturing procedure for mixed metal oxide electrodes. (b) Metal chalcogenides are used in the water-splitting process [20].

240  Materials for Hydrogen Production, Conversion, and Storage learn that TM-based materials can have a variety of electronic and crystal structures, as well as catalytic activity in EWS applications [48]. Heteroatom doping is a general method for the enhancing catalytic performance of transition metal borides, transition metal phosphides, and transition metal nitrides by changing the atomic ratio (transition metal carbides) [49]. Freshly, researchers have been concentrating their efforts on creating heterostructures by combining one electro-active material with additional nanomaterial [50]. Several assets (e.g., activity and stability) of distinct components are inherited by these sophisticated hybrids, resulting in increased performance [51]. An electro deposition-annealing nitration process was used to create NiCo-nitride/NiCo2O4-supported graphite fibers. Hybrid materials Co4N@nitrogen-doped carbon and CoP/Ti3C2 MXene are two examples [40] that also show pleasing performance for EWS.

8.9 Uses of Hydrogen Produced from Water Splitting 8.9.1 Water Splitting Generates Hydrogen Energy Figure 8.9 shows that the reverse reaction is simple to conduct, water splitting is a thermodynamical sort of spontaneous process that needs external energy, this is an uphill response. Hydrogen may be made fast from several renewable and non-renewable energy sources. Combining two or more kinds of energy can also generate hydrogen. Some of the most Thermal Energy

Electrical Energy

Energy driven water splitting

Biochemical Energy

Photonic Energy

Figure 8.9  Thermal, electrical, biological, and photonic energy, as well as mixtures of these energies, are used in various energy-driven water splitting systems [52].

Electrochemical H2O Splitting for H2 Production  241 popular hybrid energy systems are thermal, electrical, electrical, photonic, and photonic, biological. Under nuclear (mostly alpha) radiation, water can also be divided into hydrogen and hydroxyl radicals. Chemically reactive radicals, like superoxide (HO2) and peroxide, recombine to generate a serial publication of highly reactive chemicals (H2O2) [52].

8.9.2 Photoelectrochemical (PEC) Water Splitting This is a long-term technical approach that might result in very little, if any, greenhouse gas emissions. The PEC water splitting method changes solar energy directly to chemical energy by using semiconductor materials in the form of hydrogen. Panels (akin to solar panels), electrode systems, and ­slurry-based particle systems are all examples of PEC reactors, each with its own set of benefits and drawbacks. Panel systems have attracted the most attention thus far due to their similarities to existing solar panel technology.

8.9.3 Thermochemical Water Splitting In thermochemical water splitting, high temperatures are combined with chemical processes to generate H2 and O2 from water. This is a long-term technical method that could lead to low or negligible greenhouse gas emissions in the future. In thermochemical water-splitting devices, high-temperature heat is utilized to power a succession of chemical processes which generate H2. Each cycle recycles the substances utilized in the method, resulting in a closedloop in which hydrogen and oxygen are produced only from the water.

8.9.4 Biological Water Splitting As part of their metabolic activities, specific photosynthetic bacteria use light energy to generate hydrogen from water, a method for producing photobiological hydrogen because hydrogen developing enzyme systems must overcome their innate sensitivity to oxygen. Researchers work on a unique approach that algal cells are cycled between photosynthetic and non-­ photosynthetic states using a metabolic switch and non-­photosynthetic development phases (sulfur deprivation).

8.9.5 Fermentation A clostridium consortium capable of directly digesting hemicellulose to hydrogen is also being sought by researchers. Another area of research is finding effective cellulolytic bacteria. For Clostridium thermocellum, for

242  Materials for Hydrogen Production, Conversion, and Storage example, it is possible to convert crystalline cellulose to hydrogen directly. Once a theoretical account cellulolytic bacterium has been found, its genetic manipulation abilities, including antibiotic sensitivity and ease of hereditary alteration, will be assessed. To improve hydrogen generation, Future fermentation research at NREL will focus on developing mutants that are unable to produce waste acids or solvents.

8.9.6 Biomass and Waste Conversions Biomass is pyrolyzed to produce a liquid product (bio-oil) containing a variety of components that can be separated into useful compounds and fuels, including hydrogen. NREL researchers are currently concentrating their efforts on creating hydrogen from biomass pyrolysis products via catalytic reformation. Reforming pyrolysis streams and processing and investigating fluidizable catalysts are two special research areas.

8.9.7 Solar Thermal Water Splitting Extremely high temperatures are needed for the generation of hydrogen via thermochemical reaction cycles. Solar-driven thermochemical systems with high temperatures and fluxes offer a one-of-a-kind way of producing environmentally acceptable hydrogen. High reaction rates result in exceptionally rapid response rates at these high temperatures, substantially improving output rates and more than compensating for the solar resource’s periodic nature.

8.9.8 Renewable Electrolysis Photovoltaic, wind, biomass, hydro, and geothermal energy are examples of renewable energy sources that could provide our country with clean, long-term electricity. Renewable energy sources, on the other hand, are intrinsically variable, necessitating the use of energy storage. Researchers at the NREL are looking at the challenges of using renewable energy sources to produce hydrogen using water electrolysis. To minimize capital costs and enhance performance, NREL investigates design alternatives for integrated electrolysis systems.

8.9.9 Hydrogen Dispenser Hose Reliability NREL uses automated robots at the Energy Systems Integration Facility to simulate field conditions for fast examination and cycling of 700 bar

Electrochemical H2O Splitting for H2 Production  243 hydrogen dispensing tubes, to cut costs and enhance reliability and safety. Watch a video of a robot twisting and bending a pipe to feed H2 to a fuel cell vehicle’s onboard storage tank, simulating the repetitive stress of a human doing the same. Mechanical, temperature, and pressure stress testing was performed on new and old hydrogen distribution pipes.

8.10 Conclusion Electrochemical water splitting is a better method for generating huge amounts of little quantities of hydrogen time with the greatest cleanliness while minimizing the environmental impact. The world’s energy demand is increasing, yet the Earth’s fossil fuel reserves, oil, coal, and natural gas, to name a few, remain the principal sources of energy. Water is a crucial component in water splitting driven by various energy sources since it is the most abundant resource on Earth and also a key source of hydrogen. Water electrolysis now accounts for 4% of all hydrogen generated worldwide. In recent research, solar energy-driven hydrogen generation has gotten greater attention. Direct water splitting in the presence of sunshine can provide an endless source of clean fuel for several applications. Catalytic water splitting has been the focus of recent research, notably electrocatalysis, to improve hydrogen generation efficiency. Photocatalysis includes processes such as photocatalysis, thermochemical cycling, and enzymatic reactions to significantly enhance hydrogen generation efficiency in practical applications.

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9 Challenges and Mitigation Strategies Related to Biohydrogen Production Mohd Nur Ikhmal Salehmin1, Ibdal Satar2 and Mohamad Azuwa Mohamed1,3* Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia 2 Department of Food Technology, Faculty of Industrial Technology, Universitas Ahmad Dahlan (UAD), Umbulharjo, Yogyakarta, Indonesia 3 Department of Chemical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, Bangi, Selangor, Malaysia 1

Abstract

Although many studies related to biohydrogen have been reported, the productivity of biohydrogen production remains low. To achieve its implementation at an industrial scale, higher productivity is critical. Research on various bioreactor configurations and factors influencing hydrogen production has also been extensively investigated for mass production. Therefore, a review of the prevailing issues associated with bioreactor operation and the recent advancement in alleviating the challenges of biohydrogen production will be discussed in this chapter. Four challenges have been identified, namely, physical, biological, chemical, and economical, which enhancement strategies for improving biohydrogen productivity accompany each challenge. Keywords:  Biohydrogen, bioreactor, biomass washout, bioaugmentation, inhibitors, techno-economic analysis

9.1 Introduction The global energy demand is primarily met by non-renewable fossil-based fuels, which produce a range of toxic gases during combustion, such as carbon dioxide, carbon monoxide, and sulphur dioxide. The production and *Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (249–276) © 2023 Scrivener Publishing LLC

249

250  Materials for Hydrogen Production, Conversion, and Storage emission of these harmful gases pollute the environment. This situation leads to progressive research efforts seeking an alternative fuel that should be produced by renewable sources, which is eco-friendly. In this regard, anaerobic digestion fulfills the requirement which adopting the waste-towealth concept. Anaerobic digestion involves a sequential biochemical reaction conducted by five groups of different microorganisms. The digestion process commenced with the microbial decomposition of organic matter into smaller derivative molecules with concurrent biohydrogen production and acid, which eventually transformed into methane and CO2. Moreover, bioreactor configuration and design factors are vital parameters influencing the microbial growth environment [1]. A favorable microbial growth could increase the biohydrogen production efficiency for a long-term operation and could withstand a shock load. Biohydrogen production is frequently carried out in a continuous stirred tank reactor (CSTR). Many studies have intensified the focus on reactor optimization and operational control for a different type of substrate containing glucose [2, 3], sucrose [4], starch [2, 5], and cellulose [6]. It should be noted that the biomass contained in the CSTR is in suspended form. At low hydraulic retention time (HRT), the biomass is susceptible to being washed out and tends to mix with liquor concentrate, causing a reduction in biohydrogen production. It has been suggested that self-granulated biomass can be employed, resulting in doubled biomass retention time more significant than the HRT effect, supporting the slowly growing microorganisms favoring biohydrogen production [7]. Biohydrogen production can also be increased with the addition of different inoculums. Murugan et al. achieved a maximum hydrogen production of 566.44 mL/L by inserting Acinetobacter junii-AH4 inoculum into industrial wastewater [8]. An alternative to granular form, biomass can also be immobilized in biofilms consist of pristine and mixed cultures [9, 10]. This method has improved substrate degradation, biomass retention time, and biohydrogen production [11–13]. Load shock and heat shock treatment in the early operation also has been suggested could suppress methanogen growth, thus enhances biohydrogen production [14]. Notably, adjustment to operational control promotes the enhancement of biohydrogen production through anaerobic fermentation. The challenge of utilizing biohydrogen in real application relies on its competitiveness with the existing fuel sources regarding fuel price and efficiency. The determination of fuel price is based on the working capital and capital fixed cost, which will be elaborated further in the next

Mitigation Strategies in Biohydrogen Production  251 subchapter. Research on biohydrogen production has now been geared towards improving production efficiency through process optimization and the exploitation of byproducts into a valuable compound supporting the circular biohydrogen economy [15, 16]. In this chapter, four prevailing limitations of biohydrogen production from anaerobic digestion typically processed in bioreactors have been categorized into physical, biological, chemical, and economic (Figure 9.1). The reported enhancement strategies for each limitation will be discussed accordingly.

Categories

Issues Temperature control

Physical

HRT maintenance End product inhibition Membrane fouling

Start-up issue Biological

Challenges in biohydrogen production

Biomass washout

pH regulation issue Nutrient availability Chemical

End product inhibition Unwanted by-product formation

High energy consumption Economy

High operating cost

Figure 9.1  Prevailing challenges in biohydrogen production.

252  Materials for Hydrogen Production, Conversion, and Storage

9.2 Limitation and Mitigation Approaches of Biohydrogen Production Biohydrogen production refers to the biological activities in a bioreactor system to produce hydrogen from organic substrates. Hydrogen is an exciting energy carrier and is expected to play an essential role in the future due to zero pollution and high energy content [17]. However, biohydrogen production technologies are still in their infancy. Various limitations and constraints in the bioreactor, such as design and configuration, operation parameters, performance stability, substrate utilization and microbe used, and cost operation, are still not entirely resolved [18]. From this point forward, only prevailing challenges and their mitigation strategies related to biohydrogen production in a bioreactor, involving physical, biological, chemical, and economic, will be discussed.

9.2.1 Physical Issues and Their Mitigation Approaches 9.2.1.1 Operating Temperature Issue and Its Control Studies showed that temperature must be controlled during biohydrogen production, because the temperature variation affects productivity. Biohydrogen production at an industrial scale is performed at mesophilic conditions (20–45°C). Working in mesophilic conditions offers simplicity on bioreactor handling and low cost due to low energy consumption. However, the biohydrogen yield at mesophilic conditions is comparably lesser than that at thermophilic conditions. For instance, biohydrogen production at thermophilic conditions achieved 1.03 mol H2/mol [20], while 0.78 mol H2/mol at mesophilic conditions [19]. Increasing temperature to thermophilic conditions tends to improve biohydrogen yield. However, rising in temperature may also reduce biohydrogen production. Lee et al. [21] reported that the higher temperature might cause denaturation on the enzyme; consequently, the biomass growth or granule formation is inhibited. Additionally, the main drawback of thermophilic conditions for bio­ hydrogen production is high-cost operation and high energy consumption. In terms of cost, the mesophilic process is preferable than the thermophilic when the bioreactors are applied at an industrial scale.

9.2.1.2 Hydraulic Retention Time (HRT) and Optimization Hydraulic retention time (HRT) can be defined as the total time required by a unit of a substrate to be approached by the microorganism in a bioreactor

Mitigation Strategies in Biohydrogen Production  253 to produce the bioconversion of interest [21]. HRT is generally associated with the reaction rate (related to the concentration and type of substrate), appropriate cells (single- or mixed cells), and operating temperature. In the dark fermentation system, the optimum HRT for biohydrogen production is obtained around a few hours to one day [23–25], while biomethane production lasts for a few days [22]. Biohydrogen production is incompatible at the longer HRT because the metabolic route changes from acidogenic to methanogenic. The longer HRT of 72 hours leads to reduce biohydrogen generation. Also, other research showed that the biohydrogen production from olive mill effluent was low at the longer HRT [23]. This observation might be due to the washout phenomena on active biomass in the bioreactor [21]. In general, biomethane production is reduced with reducing HRT (i.e., 2–10 h) attributed to the poor growth of methanogenic [24]. A study reported that by reducing the HRT to seven-fold (7 days to 1 day) resulted in the 30-fold increment of biohydrogen [23]. Therefore, one of the strategies to solve low biohydrogen production is by reducing the HRT.

9.2.1.3 High Hydrogen Partial Pressure – Implication and Overcoming the Issue Inhibition and reduction of biohydrogen generation are caused by the accumulation of hydrogen in a liquid phase (refers to the high partial pressure) [25–27]. Thermodynamically, proton reduction to hydrogen is unfavorable at the high hydrogen concentration in a liquid phase. As a result, oxidation of hydrogen to proton is preferable and simultaneously reduces hydrogen production. In addition, biohydrogen production could also be reduced under high hydrogen partial pressure due to the conversion of long-chain fatty acids to hydrogen and acetate [28]. Moreover, the rise in hydrogen partial pressure may lead to metabolic route shift to lactic acid, acetic, acetone, and/or butanol with the reduction of hydrogen yield [29, 30]. Lowering hydrogen partial pressure to 20% with an interval of 2 hours led to improved biohydrogen production efficiency and yield of 54% and 202.15 mL, respectively, compared to the control [31]. Approaches such as increased headspace volume [27], continuous-­ release gas [32], and inert gas (i.e., N2 or CO2) sparging or stripping vacuum can be employed [33]. In general, the approaches used are based on the type of reactor and its application. The stirring of substrate and culture is the most straightforward approach to reduce the hydrogen partial pressure. Also, the hydrogen partial pressure can be reduced by eliminating the hydrogen pressure in the liquid phase and gas through the N2 sparging.

254  Materials for Hydrogen Production, Conversion, and Storage An automated control of hydrogen partial pressure can also be executed by installing a hydrogen sensor inside the bioreactor to bring down the pressure equal to ambient pressure, as demonstrated by Das et al. [34].

9.2.1.4 Membrane Fouling Issues and Solutions Another crucial issue in the anaerobic bioreactor is membrane fouling. The accumulation of foulants causes the membrane fouling to form a crust on the membrane surface; consequently, blocking pore, reducing membrane permeability, and declining flux density [35–37]. This issue may increase the operational cost as a whole. Generally, membrane fouling is affected by three aspects; operational parameters, physicochemical, and biological (Table 9.1). Type of fouling can be classified into biological fouling (biofouling), organic and inorganic fouling [53]. Biofouling referred to the fouling condition caused by the growth of microbe cells and deposited on the surface Table 9.1  Some typical parameters influencing membrane fouling. Parameters

Properties

Ref.

Operational

Hydraulic retention time (HRT)

[21, 38, 39]

Temperature

[40]

pH

[41]

Organic loading rate (OLR)

[42, 43]

Shear rate

[44]

Backwash

[45]

Flocculant/Coagulant

[46]

Chemical cleaning

[47, 48]

Physical scouring

[47, 48]

Salinity

[49]

Biomass

[38, 50, 51]

Bacteriophages

[48, 51]

Cell wall hydrolysis

[48]

Type of activated sludge

[49, 52]

Physicochemical

Biological

Mitigation Strategies in Biohydrogen Production  255 or pore membrane. Microbial growth tends to form biofilm on the membrane surface, which contribute the bioreactor performance. In a bioelectrochemical system, a high microbe density on the membrane surface tends to block the protons transportation from anode to cathode, leading to membrane fouling and reducing bioreactor performance [54]. On the other hand, organic fouling is caused by the deposition of substances, for instance, protein, polysaccharides, humic, and various organic [45]. Meanwhile, inorganic fouling is caused by the accumulation of chemical elements, e.g., magnesium, calcium silica aluminium, silica, and inorganic and organic biological compound on the membrane [46]. Chemical precipitation formed via the interaction of anions and metal cations could slit through the membrane pores, eventually contributed to the inorganic fouling. Numerous techniques such as modification of bioreactor operation system involving period of the filtration cycle, backwashing, flow rate tuning have been used to minimize the fouling on the membrane. Moreover, chemical cleaning, intensified aeration (for aerobic reactor), ultrasonic irradiation, membrane pre-treatment and membrane surface modification are also considered as alternative techniques to mitigate membrane fouling [55]. The conventional approaches (i.e., membrane backwashing and relaxation) can also be adopted to minimize fouling on the membrane and improve the bioreactor performance. Of the traditional techniques, the sonification could break down biofilm into micro fragments, which then entering membrane pores; thereby, the sonification is only practical for complex fouling conditions [56]. Membrane fouling can also be treated using chemical cleaners such as hydrochloric acid, ethylene diamine tetraacetate (EDTA), citric acid, nitric acid, hypochlorite, and NaOH. Bear in mind that the chemical technique to treat the membrane may cause damage in membrane performance and reduce its lifespan [53]. Activated carbon and zeolite can eliminate colloids and soluble compounds to reduce fouling on the membrane [57]. Antifouling agents such as polymerizable bi-continuous microemulsion (PBM) can be used to prevent fouling in ultrafiltration membranes. In addition, nanoparticles, for instance, titanium oxide (TiO) and zinc oxide (ZnO), carbon nanotubes (CNTs), nanosilver, graphene, and fullerene can also be utilized as antifouling agents given their high stability against microbial activities [58]. Among these nanoparticles, CNTs shows the best performance to prevent fouling membrane [59, 60].

256  Materials for Hydrogen Production, Conversion, and Storage

9.2.2 Biological Issues and Their Mitigation Approaches 9.2.2.1 Start-Up Issue and Improvement Through Bioaugmentation A continuous biohydrogen production can be guaranteed by overcoming the start-up issue of microbial culture in the bioreactor. Start-up remains a significant issue associated with the time needed to achieve stable performance. A review study has identified parameters responsible for alleviating the start-up issue, including bioreactors’ design, the acclimation of hydrogen-producing microbes population, and shift over fermentation mode [61]. Formulating fermentation media, including food to microbe ratio control, pre-determined culture supplement, and the reducing agent has also been proven to overcome the start-up issue, consequently improving bioreactor performance and increasing the biohydrogen yield [62]. Substrate concentration, composition, and the microbes’ metabolic properties also influence the whole biohydrogen production when harvested from a mixed culture. One of the practical strategies to achieve an improved start-up culture and high substrate conversion efficiency is through bioaugmentation strategies by inoculating preferred microbial strains to the existing microbial population in the fermentation media [63]. Through bioaugmentation effort, the inoculation of single or mixed native microflora with C. acetobutylicum [64], acidogenic consortia [65], and hydrogen-producing microbes [66] has been demonstrated to enhance production efficiency. In the same effort, bioaugmentation has also been proven effective to improve bioreactor performance [67], regaining the start-up of a bioreactor [68], and shielding the indigenous microbial population from harmful effects during fermentation [63]. Cumulatively, these encouraging impacts are beneficial to improve biohydrogen production.

9.2.2.2 Biomass Washout Issue and Solution Through Cell Immobilization Biohydrogen fermentation under continuous operation mode is susceptible to biomass washout during severe fermentation environment, short retention time, high organic loss, and shear stress due to stirring/mixing [69]. In the long run, such a condition deteriorates bioreactor performance and reduces productivity. Although the conventional recycling technique purposely maintains an adequate microbial population for hydrogen production, biomass washout is inevitable in short HRT. Hence, cell immobilization is a promising strategy as it offers cell stability and favors continuous

Mitigation Strategies in Biohydrogen Production  257 fermentation modes, economical recycling and recovery cost, improved fermentation productivity, and low-cost downstream processing [70]. Karel et al. characterized cell immobilization as the physical entrapment of cells occupied in a space with a specific desired catalytic activity that remained to function [71]. Additionally, because biohydrogen production is sensitive to oxygen, cell immobilization can provide an oxygen-free condition. Immobilization is also insusceptible to strain contamination which is critical for an extended period of biohydrogen fermentation under oxygen-free conditions [69]. Literature survey indicated that various supporting materials had been employed to entrap cells for biohydrogen production, including synthetic polymer of polydimethylsiloxane (PDMS) [72], bamboo stems [73], alginate, and polyvinyl alcohol [74]. These studies have demonstrated that cell immobilization is reliable to increase biohydrogen production compared to the control experiment.

9.2.3 Chemical Issues and Their Mitigation Approaches 9.2.3.1 pH Variation and Its Regulation Reaching an optimum pH regulation and redox conditions is challenging, especially for the continuous biohydrogen generation system. Significant low pH suppressed the metabolic activity of biohydrogen-producing microbes. A pH-dependent dark fermentation with pig manure and glucose as substrates for biohydrogen generation is well-controlled at the targeted pH by varying the organic loading rate [75]. It was reported that sodium hydroxide could be used for controlling pH reduction due to the presence of volatile fatty acid as the byproduct in a dark fermentation [76]. Thus, the production cost allocated for purchasing sodium hydroxide consumption can be saved. Besides, the methanogenic effluent can be recycled to minimize the pH uncertainty and regulation as this effluent exhibited basic pH of 7–8 [77]. Additionally, previous research indicated that using immobilized microbial cells could increase hydrogen production by using medium pH regulation during dark fermentation biohydrogen generation [78]. However, the enhancement of hydrogen production by the cell immobilization is subjected to the reactor configuration improvement.

9.2.3.2 Limiting Nutrient Loading and Optimization Nutrients such as inorganic compounds, carbon, nitrogen, and phosphate are accountable for cell growth and biohydrogen generation. During the metabolic activity, the substrate that provides carbon for energy will be

258  Materials for Hydrogen Production, Conversion, and Storage converted into biohydrogen. Moreover, nutrient concentration also affects biohydrogen generation, either increasing or decreasing biohydrogen production [55, 79, 80]. An ideal concentration of phosphate and nitrogen sources is critical for the growth of the biohydrogen-producing microbes to enhance hydrogen yield [81, 82]. Logan et al. found that high nitrogen and phosphate concentrations can induce ammonia production, leading to the increment of suspension of compounds and CO2 concentration [83]. These events affect biohydrogen-­ producing microbe’s growth activity, thereby lowering the quantity of hydrogen production [83]. A study observed that when ± 30% phosphate concentration was introduced of the nominal value, the hydrogen production rate plummed to 40% [81], indicating the existence of optimum concentrations to yield better performance. Besides nitrogen and phosphate, microelements such as iron, nickel, magnesium, and sodium are also critical for biohydrogen fermentation, especially for inducing enzyme secretion in microorganisms, activating enzymes for catalyzing substrates into biohydrogen, and the transportation of biohydrogen out of microorganism systems [84]. The presence of iron ions assists in synthesizing enzymes and encouraging the catalytic activity of hydrogen fermentation. The combination of sulphur and iron can function as a protein transporter which also helps in metabolic alteration of the enzyme hydrogenase. Ferrum propagates the microorganism’s growth rate; hence, high substrate consumption and hydrogen production rate can be increased accordingly [85]. However, ferrum outside the optimal value impedes biohydrogen production [86]. Micronutrients such as nickel facilitate electrons in transporting the hydrogenase, catalyzing hydrogen production. Meanwhile, the Ni-Fe co-catalyst hydrogenase can act as an electron donor, rendering substrate reduction to the proton, thus enhancing biohydrogen production by activating the enzymatic function [87, 88]. Magnesium is another micronutrient needed for cell growth, found in microbial cell membranes, which function as co-factors for enzyme synthesis. Studies suggested that magnesium stimulates enzymes catalyzing the metabolic bioconversion [89, 90]. Overall, an optimum amount of limiting nutrients is crucial for micro­ organism’s health and hydrogen fermentation.

9.2.3.3 Inhibitor Secretion and Its Control Biohydrogen production, microbial growth, and resporulation prevention are depending on, but are not limited to, the initial concentration of the substrate. For a process demanding high efficiency, a higher organic

Mitigation Strategies in Biohydrogen Production  259 loading rate is required. However, a higher substrate concentration does not necessarily promise biohydrogen fermentation, which is restricted by the accumulation of volatile fatty acid (VFA), pH alteration, change in hydrogen partial pressure, and dissolved particles in the system. These issues can cause substrate degradation and metabolic disorders upon micro­ organisms [91–93]. Additionally, a study indicated that ethanol influenced microbial growth rate, while format and acetate influenced biohydrogen production [94]. Therefore, the optimization of substrate concentration and its processing needs to be conducted, which is as critical as removing the inhibitory compounds using cell dialysis or recycling, to name a few. Furan is an inhibitor produced during substrate pre-treatment. Literature ascertained that compounds derived from furan (catalyzed by enzymes) could influence glycolysis (a fermentation step), destroy DNA integrity, alter metabolic pathways, and reduce cell growth rate [95–97]. Crushing lignin through the pre-treatment step allows the secretion of phenolic compounds, which is detrimental to the cell membrane by increasing its permeability, thus damaging the membrane. This pre-treatment causes seepage of potassium, phosphate, or proteins, disturbing the cell’s metabolic process, for instance, cell activity, microbial evolution, and fermentation pathways. As a result, biohydrogen generation was hampered, as documented in several studies [98–100]. These inhibitory compounds can be minimized during the pre-treatment step by applying various biological, physical, chemical, or other detoxification treatments in the hydrolysate, such as enzymatic, evaporative, alkaline, carbon and activated carbon, respectively [101–103]. Lin et al. eliminated the inhibitory compounds produced from the dark fermentation process by using sodium borohydride [104]. A study suggested that biohydrogen production can be enhanced by preparing detoxifying agents by mixing activated carbon, chitosan, sludge powder, and sodium alginate with calcium chloride solution [105]. Ammonia is another inhibitor produced during dark fermentation that impedes biohydrogen generation in anaerobic digestion. Ammonia is synthesized by decomposing a nitrogen-rich compound such as protein, urea, nitrate, and food waste. High concentrations of ammonia and free ammonium ions deter biohydrogen production [106–108]. The presence of free ammonia could lead to cell membrane destruction and the combination of free ammonia with a proton to produce ammonium ions which cause intracellular pH variation, followed by the ammonium ion accumulation due to reverse activity of antiporter, thereby constraining microbial metabolic activity [25]. Moreover, high ammonia concentration has also been proven to alter the dark fermentation metabolic pathway that produces soluble metabolites,

260  Materials for Hydrogen Production, Conversion, and Storage reducing biohydrogen production [109]. This study also confirmed that the acclimation of microflora in dark fermentation could also lessen the ammonia inhibition [109]. Several reports have also suggested that several procedures such as microflora acclimation, substrate dilution, pH and temperature control, optimization of C: N ratio, and immobilization by zeolites can lessen ammonia inhibition in anaerobic fermentation [110, 111].

9.2.3.4 Byproduct Formation and Its Exploitation Another problem arising during biohydrogen fermentation is the production of byproducts causing high acidity effluent on inefficient substrates conversion. The accumulation of mixed soluble acids such as VFA lowers the pH of the medium, affecting the microbial viability, thus diminishing hydrogen production. Several studies reported that only 30% of organic matter was consumed after the acidogenic stage, even under optimal operating conditions [15, 16, 19]. Jiang et al. found that the production of VFA at low pH (acid) decreased the biohydrogen production dramatically by 10 times compared to high pH (alkali) because of lesser hydrolytic enzyme activity and the inhibitory nature of acid medium towards microbial viability [112]. Of the total fermentation product, the produced VFA comprises 25% acetic acid, 50% butyric acid, and 15% propionic acid at neutral pH. The commonly generated byproduct during hydrogen production is acetate and butyrate, which the latter produced more at low pH [3]. Studies suggested that recycling waste carbon fractions from acidogenic effluent can improve fuel generation, thus curtailing the consequences of the environmental and economic issue if it is directly discharged to water streams [113–115].

9.2.4 Economic Issues and Ways to Optimize Cost Operational energy efficiency and economic analysis are the key factors in transforming technology into commercialization. Total capital investment is a critical measure to assess the transformation, which fixed capital and working capital cost must first be considered. Fixed capital costs include the necessity of the selected type and the capacity of the equipment. Meanwhile, the working capital cost needs to be determined based on the manufacturing cost and income, which can be calculated annually. Noting that biohydrogen is acquired through the waste treatment process, the wastewater does not entail further treatment to comply with effluent specifications. Annual income includes the profits from solid waste trade that can be used as fertilizer or livestock bedding,

Mitigation Strategies in Biohydrogen Production  261 Table 9.2  Summarization of the prevailing challenges in biohydrogen production and the reported enhancement strategies. Challenges Physical

Detail of challenges

Enhancement strategies

Ref.

Achieving an optimum temperature

A shift from mesophilic to thermophilic culture mode

[19]

Achieving an optimum hydraulic retention time

Shorter HRT inhibited methanogens’ growth and their metabolic process

[124]

Longer HRT could prevent biomass washout

[21]

Shorten the HRT from 7 to 1 day resulted in a 30-fold enhancement of biohydrogen yield

[23]

Decreasing hydrogen partial pressure to 20% with 2 hours interval time resulted in improved biohydrogen production efficiency and yield

[125]

Reducing hydrogen partial pressure during dark fermentation increased biohydrogen production

[31]

Zeolite and activated carbon removed colloids and soluble compounds to lessen fouling on the membrane

[24]

Carbon nanotubes exhibited high performance to prevent fouling membrane

[64, 126]

ZnO and TiO2 improved membrane hydrophilicity and curbed microbial activity

[127, 128]

High hydrogen partial pressure

Membrane fouling

(Continued)

262  Materials for Hydrogen Production, Conversion, and Storage Table 9.2  Summarization of the prevailing challenges in biohydrogen production and the reported enhancement strategies. (Continued) Challenges Biological

Detail of challenges Slow start-up culture

Biomass washout

Enhancement strategies

Ref.

Inoculating the fermentation media with bio augmented co-culture of Klebsiella Pneumoniae (native facultative) with Clostridium (an anaerobe bacteria)

[129]

Bioaugmenting native acidophiles with three acidophilic strains (Pseudomonas stutzeri, Lysinibacillus fusiformis, and B. subtilis) treating food wastewater in anaerobic sequencing batch reactors

[63]

Immobilizing microbes using alginate, polyvinyl alcohol etc.

[74]

Immobilizing cells using synthetic polymer, polydimethylsiloxane (PDMS) and resulted in an improved biohydrogen generation rate [91]

[72]

Using bamboo stems as supporting carriers for immobilizing cells

[72]

(Continued)

Mitigation Strategies in Biohydrogen Production  263 Table 9.2  Summarization of the prevailing challenges in biohydrogen production and the reported enhancement strategies. (Continued) Challenges Chemical

Detail of challenges

Enhancement strategies

Ref.

Achieving an optimum pH medium

Maintaining pH below six which favors the growth of biohydrogen producing microbes

[130]

Maximum production of biohydrogen was achieved at a pH of 5.5 and reduced to a pH of 4.5

[131]

Adjusting pH using potassium hydroxide, sodium hydroxide, calcium hydroxide as buffering agents [103,104]

[132, 133]

Applying low nitrogen and phosphate concentrations to prevent biohydrogen degeneration due to a rise in CO2 concentration, compound dissolution, and ammonia production

[83]

Introducing iron, sodium, nickel, and magnesium to induce enzyme production, which converts the substrate to biohydrogen

[84, 134]

Loading iron accelerates the microorganism’s growth rate, thus consuming more substrate, increasing substrate oxidation and hydrogen production rate [130]

[85, 135]

Adding up magnesium stimulates enzymes accountable for bioconversion metabolism

[136]

Determining nutrient loading

(Continued)

264  Materials for Hydrogen Production, Conversion, and Storage Table 9.2  Summarization of the prevailing challenges in biohydrogen production and the reported enhancement strategies. (Continued) Challenges

Detail of challenges

Enhancement strategies

Ref.

Combining Ni2+, Fe3+, Mo6+ ions in media composition, threefold hydrogen can be produced compared to control

[137]

Using alkali, evaporation, activated carbon, and biocatalysis to eliminate the inhibiting compound

[101, 138, 139]

In the dark fermentation process, sodium borohydride was applied to eliminate the generated inhibitory compounds

[104]

Formulating a detoxification agent containing calcium chloride, activated carbon, sodium alginate, powdered sludge, and chitosan increases biohydrogen production

[105]

Utilizing acclimated microflora substrate dilution, optimized C:N ratio, pH and temperature, to inhibit ammonia production in anaerobic fermentation

[110, 111]

Unwanted byproduct formation

Reducing HRT and Increasing the organic loading rate

[140]

Switching operation mode from batch to continuous at the early process

[141]

High operating cost

Selecting proper substrate and process for biohydrogen fermentation because it highly affects the production cost

[118, 120, 122, 123]

Inhibitor production

Economy

Mitigation Strategies in Biohydrogen Production  265 biomethane, biohydrogen, and charges incurred for waste treatment services. The net current value, internal reimbursement rate, and payback time are defined as profit from system operation. The net current value represents the total project cost throughout the process. The internal reimbursement rate is the interest rate at zero net present value, where a more significant internal cost of return is anticipated. Additionally, the payback time is the time required to recover the capital costs invested at the beginning of operation [112, 116, 117]. An economic investigation for biohydrogen production dealing with the mixture of food waste and corn flour in the two-phase fermentation discovered that the production cost was considerably low at $0.19/N.m3 [118]. As suggested, the profit can be amplified by taking advantage of eliminating waste disposal [118]. Ljunggren et al. showed that the use of barley straw waste reduced hydrogen production, which was burdened by the additional cost for maintaining pH, thus increasing production costs to $583.5/GJ [119]. When potato steam skins were used as substrates, the considerably high capital and nutrient costs reduced the economic value of the produced biohydrogen, as reported by Ljunggren & Zacchi [120]. Meanwhile, when substrates such as sugar beet molasses were used in twostage dark fermentation and photolysis, the biohydrogen production cost increased significantly, attributed to the dominating expenses of capital and operating cost [121]. In other studies, the cost of a reactor to perform single-stage algal bio photolysis was estimated to be 50 per/m2; meanwhile, 80% of the total cost was allocated for fixed capital costs [122]. Akkerman et al. estimated the cost of hydrogen production at $15/GJ; thereby, biohydrogen production cost from the photolysis process is in the range of $10–20 per GJ [123]. Based on the review above, the production cost is also greatly influenced by the type of substrate and processing technique for biohydrogen production. For a quick reference, a summarization of challenges and their mitigation strategies in biohydrogen fermentation is tabulated in Table 9.2.

9.3 Conclusion and Future Direction High reliance on depleting fossil-based fuel and improper management of solid waste are two main reasons causing the unceasing emissions of greenhouse gases. It remains a serious environmental concern considering the exponential energy demand by the ever-increasing human population and lifestyle. The extraction of biohydrogen from waste through anaerobic fermentation is foreseen to solve the energy

266  Materials for Hydrogen Production, Conversion, and Storage and environmental issues through hydrogen economics by adopting the waste-to-wealth concept. As simple as it sounds, the bioconversion of waste to biohydrogen production through anaerobic digestion remained challenging in terms of the accumulation of inhibitory compounds, a low substrate conversion rate that influences the bio-catalytic pathway, and the marketability of biohydrogen production. Literature suggested that the pre-treatment and combined process, for instance, the employment of two-stage anaerobic digestion, can improve the energy extraction efficiency owing to the concurrent production of hydrogen and methane. Hence more energy from waste materials can be harvested. Furthermore, the recovery of valuable byproducts accumulated in the effluent during biohydrogen fermentation contributes to the circular economy of biohydrogen production. Moreover, researchers unanimously agreed that an enhanced biomass conversion to biohydrogen could also be realized by applying nanocatalyst during fermentation. The integration of anaerobic digestion and electrochemical reaction in microbial electrolysis cells (MECs) would enhance hydrogen production, given that extra hydrogen can be produced through fermentation and electrochemical activity. Genetic modification on hydrogen-producing microorganisms was also proposed to improve biohydrogen production. Most importantly, the price and efficiency of the fuel that is competitive with the current fossil-based fuel would quicken the transition from technology to commercialization. Research on biohydrogen production, storage, and utilization should be simulated under optimal parameters and configuration near practical conditions, which could also improve the adoption rate of hydrogen fuel into practicality.

Acknowledgements The authors would also like to acknowledge the financial support from the Universiti Kebangsaan Malaysia under Research University Grant (Project code: DIP-2020-011).

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276  Materials for Hydrogen Production, Conversion, and Storage non-sterile continuous stirred tank reactor augmented with. Clostridium butyricum. Int. J. Hydrog. Energy, 36, 8697, 2011. 130. Nath, K. and Das, D., Modeling and optimization of fermentative hydrogen production. Bioresour. Technol., 102, 8569, 2011. 131. Ruggeri, B., Tommasi, T., Sanfilippo, S., Ruggeri, B., BioH2 & BioCH4 through anaerobic digestion: from research to full-scale applications, Springer, Verlag London, 2015. 132. Kim, S.-H., Cheon, H.-C., Lee, C.-Y., Enhancement of hydrogen production by recycling of methanogenic effluent in two phase fermentation of food waste. Int. J. Hydrog. Energy, 37, 13777, 2012. 133. Sivagurunathan, P., Gopalakrishnan, K., C-Y, L., Enhancement of fermentative hydrogen production from beverage wastewater via bioaugmentation and statistical optimization. Curr. Biochem. Eng., 1, 92, 2014. 134. Karadag, D. and Puhakka, J.A., Enhancement of anaerobic hydrogen production by iron and nickel. Int. J. Hydrog. Energy, 35, 8554, 2010. 135. Lee, D.-Y., Li, Y.-Y., Oh, Y.-K., Kim, M.-S., Noike, T., Effect of iron concentration on continuous H2 production using membrane bioreactor. Int. J. Hydrog. Energy, 34, 1244, 2009. 136. Wang, X.J., Ren, N.Q., Sheng Xiang, W., Qian Guo, W., Influence of gaseous end-products inhibition and nutrient limitations on the growth and hydrogen production by hydrogen-producing fermentative bacterial B49. Int. J. Hydrog. Energy, 32, 748, 2007. 137. Lin, C.-Y. and Shei, S.-H., Heavy metal effects on fermentative hydrogen production using natural mixed microflora. Int. J. Hydrog. Energy, 33, 587, 2008. 138. Bellido, C., Bolado, S., Coca, M., Lucas, S., González-Benito, G., GarcíaCubero, M.T., Effect of inhibitors formed during wheat straw pretreatment on ethanol fermentation by Pichia stipitis. Bioresour. Technol., 102, 10868, 2011. 139. Chang, A.C.C., Tu, Y.-H., Huang, M.-H., Lay, C.-H., Lin, C.-Y., Hydrogen production by the anaerobic fermentation from acid hydrolyzed rice straw hydrolysate. Int. J. Hydrog. Energy, 36, 14280, 2011. 140. Davila-Vazquez, G., Cota-Navarro, C.B., Rosales-Colunga, L.M., Leon Rodríguez, de, A Continuous biohydrogen production using cheese whey: Improving the hydrogen production rate. Int. J. Hydrog. Energy, 34, 4296, 2009. 141. Kim, D.-H., Kim, S.-H., Ko, I.-B., Lee, C.-Y., Shin, H.-S., Start-up strategy for continuous fermentative hydrogen production: Early switchover from batch to continuous operation. Int. J. Hydrog. Energy, 33, 1532, 2008.

10 Continuous Production of Clean Hydrogen from Wastewater by Microbial Usage P. Satishkumar1, Arun M. Isloor1,2* and Ramin Farnood3 1

Membrane and Separation Technology Laboratory, Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Mangalore, India 2 Apahatech Solutions, Science and Technology Entrepreneur’s Park, National Institute of Technology, Karnataka, Surathkal, Mangalore, India 3 Department of Chemical Engineering and Applied Chemistry, University of Toronto, Ontario, Canada

Abstract

Biohydrogen production from wastewater is a prominent way to address escalating global energy demand and alarming environmental pollution. The need for renewable, sustainable, economic, and environment-friendly pathways for energy generation is fulfilled by biohydrogen evolution. Wastewaters contain a vast array of organic contents, as well as microbes and are a suitable source for bioreactors. Treatment of wastewaters with hydrogen-generating bacteria significantly aids its purification process by reducing chemical oxygen demand with simultaneous hydrogen generation. Among the various methods that are available for hydrogen production from microbes, photo fermentation, dark fermentation, and microbial electrolysis cells are discussed thoroughly. Continuous hydrogen generation systems are most suitable for large scale commercial production. Uniform product quality is obtained in the case of continuous systems. Microbial electrolysis cells have been found to yield exceptionally good hydrogen purity. A variety of factors that affect hydrogen evolution in all the techniques are reviewed in detail. Keywords:  Biohydrogen, photo fermentation, dark fermentation, microbial electrolysis cell, wastewater treatment, continuous hydrogen production

*Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (277–318) © 2023 Scrivener Publishing LLC

277

278  Materials for Hydrogen Production, Conversion, and Storage

10.1 Introduction Energy predicament and pure water shortage are the most concerning issues in the world today. The rapid growth of the human population is escalating the global energy demand [1]. Nonrenewable fossil fuels solely cannot meet this demand in the future. Development of alternative, sustainable, and clean energy production earned prime importance as the burning of fossil fuels threatened the world with acute environmental pollution [2]. With the onset of industrialization and urbanization, enormous amounts of wastewaters are generated every day [3–6]. Direct discharge of industrial wastewaters to natural water bodies causes ecological disturbance [7–9]. Limited availability of pure water generates the inevitable need for wastewater treatment [10–12]. Membrane separation is one of the efficient ways of water purification [13–16]. However, the production of alternative energy sources like hydrogen from wastewater can account for both the energy crisis and water purification. Hydrogen can act as an efficient fuel with a calorific value of 142 KJ/mol. Hydrogen fuels do not contribute to global warming, unlike fossil fuels. Other characteristic physical properties of hydrogen are listed in Table 10.1 [17]. Table 10.1  Distinctive physical properties of hydrogen. Reprinted from [10], Copyright 2016, Elsevier. Properties

Values

Gas density

0.08 kg m−3

Boiling point

20.3 K

Heat of vaporization

444 kJ kg−1

Flammability range

4%–75% (in air)

Liquid density

71 kg m−3

Lower heating value (mass)

120 MJ kg−1

Lower heating value (liquid, volume)

8960 MJ m−3

Diffusivity in air

0.63 cm2 s−1

Ignition temperature in air

585°C

Ignition energy

0.02 MJ

Flame velocity

270 cm s−1

Production of Clean Hydrogen from Wastewater  279 Hydrogen production from natural gas, oils, and coal are energy-­ intensive and not eco-friendly. Hence low energy demanding, renewable, and sustainable biohydrogen production methods are encouraged [18]. Biohydrogen production involves a large array of microorganisms such as cyanobacteria, fermentative bacteria, photosynthetic bacteria, etc. Varieties of techniques are used for biohydrogen generation such as photofermentation (PF), dark fermentation (DF), microbial electrolysis cell (MEC), etc. [19]. Nitrogenase enzymes and light sources play a crucial role in photofermentation while hydrogenase enzymes are responsible for anaerobic dark fermentation. An electrochemical set up with a small power supply yields H2 with an outstanding purity in MEC.

10.2 Wastewater for Biohydrogen Production Vast assortments of wastewaters are accessible for biohydrogen production and are listed in  Table 10.2 [20]. Among which few are rich in Table 10.2  Organic content present in vast variety of wastewaters. Reprinted from [13], Copyright 2016, Elsevier. Substrate

pH

COD (g/L)

Carbohydrates (g/L)

Total solids (g/L)

BWW

2.6–3.4

760–900

660–750

N.A

Sugar beet juice

4.4

57.7

25.1

N.A

Distillery WW

4.5

120

N.A

25

Distillery WW

3.6

34.8

N.A

40.4

Dairy WW

7.2

25.4

N.A

2.34

Cheese whey WW

4.6

86.8

N.A

54.1

Cassava WW

3.59

32

N.A

12.18

Cassava WW

5.52

52.3

9.6

38.2

Sugar vinasses

N.A

99.6

N.A

N.A

Tequila vinasses

N.A

40.0

N.A

N.A

Raw plastic WW

N.A

191.2

N.A

N.A (Continued)

280  Materials for Hydrogen Production, Conversion, and Storage Table 10.2  Organic content present in vast variety of wastewaters. Reprinted from [13], Copyright 2016, Elsevier. (Continued) Substrate

pH

COD (g/L)

Carbohydrates (g/L)

Total solids (g/L)

Treated plastic WW

N.A

7.3

N.A

N.A

Aircraft’s toilet WW

N.A

5.4

N.A

N.A

Herbal WW

5.8

16.6

N.A

13.15

Sago processing WW

4.6

11.6

0.6

8.9

Olive mill WW

4.7

130.1

N.A

75.5

Textile WW

7.8

N.A

23.8

N.A

Apple processing WW

4.3

9

N.A

N.A

Potato processing WW

6.4

21

N.A

N.A

Confectionary WW

4.0

20

N.A

N.A

Brewery WW

N.A

2.4

N.A

0.62

Vinnase WW

3.8

88.4

N.A

69.1

Glycerin WW

N.A

36.75

N.A

N.A

Rice mill WW

5.1

18.6

N.A

49.14

Sweet sorghum extract

7.5

19.35

19.0

N.A

Palm oil mill effluent WW

4.5

100

24

720

Tofu processing WW

4.9

43.4

18.3

N.A

Soft-drink WW

8.0–12.3

1.6–3.4

N.A

N.A

Cheese whey WW

4.68

122

103.4

126.8

Tapioca WW

4.56

16.36

N.A

14.3

WW – Wastewater, N.A. – Not available.

Production of Clean Hydrogen from Wastewater  281 carbohydrates (involving glucose, sucrose, etc.), proteins, hydrogen production, repressive compounds, etc. The majority of the wastewaters are industrial discharges. Table 10.2 indicates the presence of organic contents in significant amounts in most of the wastewaters and the same can be utilized as feed for hydrogen producing bacteria. Biohydrogen production from wastewater is a two-way beneficiary process involving the purification of wastewater and simultaneous generation of clean energy. Other advantages of using wastewater as a source for continuous hydrogen production from microbes involve simple pretreatment, high amount of dissolved organic contents, low viscosity, and less need for nutrients [21].

10.3 Photofermentation Photosynthetic purple non-sulfur (PNS) bacteria act upon various carbon sources present in wastewater such as glucose, volatile fatty acid (VFA)s, sucrose, lactic acid, etc., to evolve H2. Light energy and anoxygenic conditions were mandatory for hydrogen generation [22]. PNS bacteria demand less energy for the production of hydrogen when compared to algae which break down water molecules to produce hydrogen [23]. Nitrogenase enzymes play an important role and are responsible for photofermentation by PNS bacteria [24]. Specifically, nitrogenase enzymes containing molybdenum (Mo) play a prominent role in catalyzing hydrogen evolution (Equation 10.1). This photofermentation proceeds in the absence of oxygen and a limited supply of nitrogen [25]. In the presence of nitrogen, ammonium formation takes place (Equation 10.2) and higher ammonium concentration retards hydrogen production[26]. H2 utilizing hydrogenase enzymes reduces hydrogen yield. Hence the development of mutant PNS bacteria with limited hydrogenase was encouraged to get a good H2 yield [27].

In presence N2 : N +210 NH H 2 + 16(10.1) ADP Inofabsence of2 N : 8H H++ + 88ee−−++16 4 ATP → 24H ++ 4+ADP 2 4

In presence of N 2 : N 2 + 10 H + + 8e − + 16 ATP → 2 NH 4+ + H 2 + 16 ADP (10.2) e of N 2 : N 2 + 10    H + + 8e − + 16 ATP → 2 NH 4+ + H 2 + 16 ADP Diagrammatic representation of photofermentative H2 evolution by PNS bacteria from wastewater is shown in Figure 10.1.

282  Materials for Hydrogen Production, Conversion, and Storage

Light Source Hydrogen

Wastewater

H+ Organic Acids H+

H+

Nitrogenase PNS Bacteria H+

Partially treated wastewater

Figure 10.1  Diagrammatic representation of photofermentative hydrogen production by purple non sulfur bacteria from wastewater.

Light energy is essential since the reaction showed a positive change in Gibbs free energy (Equations 10.3 and 10.4) which depicts nonspontaneity of the reaction.

resence of N 2 : N 2 H3+COOH + 8e − ++162 ATP H 2 2+  ΔG 16 ADP1 = +104 KJ (10.3) CH H2O → 24 NH H2 +4+ 2+ CO 2 + 10

esence of N 2 : N 2 C + 10 H +6H + 812eO−6++16 ATP ADP 2 = +3.2 KJ (10.4) 6H O → 212NH H24+++6HCO   ΔG 2 +216 2 The low-cost, solar-powered bioH2 production has its own hurdles such as intensity variation in different weathers and less light availability in different climatic conditions [28]; 4000–9000 lux intensity artificial light source of mercury tungsten lamps are used to enhance H2 production. However, exposing PNS bacteria to very high light intensity hampered their hydrogen production ability [29]. The different PNS bacteria that are widely used in photo fermentation involve Rhodopseudomonas sp., Rhodobacter sp., Rhodospirillumrubrum, etc. The portrayal of Rhodobacter sp. using scanning electron microscopy (SEM) is shown in Figure 10.2 [30]. Hydrogen yield from photofermentation is relatively high when compared to dark fermentation (DF). Another emphasizing factor is that wastewater purification ability in terms of reduction in chemical oxygen demand (COD) by PNS bacteria is very high in relation to DF [31]. However large-scale hydrogen production from wastewaters utilizing PNS bacteria is not economical due to high cost [32]. The low catalytic activity of nitrogenase and complexities involved in photobioreactors were

Production of Clean Hydrogen from Wastewater  283

15kV X20,000

1µm 261102

Figure 10.2  Physical morphology of Rhodobacter sp. obtained from scanning electron microscopy. Reprinted from [23], Copyright 2015, Elsevier.

hampering this process. However, the incorporation of inorganic nano additives such as TiO2, SiC, and ZnO significantly enhanced the hydrogen production rate up to 18% [33].

10.3.1 Continuous Photofermentation In continuous systems, controlled dilution rate helps to maintain log phase bacterial cell concentration for prolonged periods while in a batch system with the onset of stationary phase hydrogen production halts. Eroglu and group members made a comparative study of batch and continuous system hydrogen production by Rhodobacter sphaeroides O. U. 001 [32]. The batch system exhibited a hydrogen production rate (HPR) of 2 ml H2 L−1 h−1 while the continuous system with the intake of 100 ml fresh medium at regular intervals of 100 hours showed an enhanced hydrogen production rate of 20 ml H2 L−1 hr−1. Ghasem et al. reported hydrogen production of 7.2 mmol h−1 from Rhodospirillumrubrum ATCC 10801 in a continuous reactor with a flow rate of 0.65 ml min−1 [34]. Excellent H2 production of 80 ml L−1 h−1 was reported by Tsygankov et al. utilizing Rhodobactercapsulatus B10 in a continuous photobioreactor with a dilution rate of 0.06 h−1 [35]. The hydraulic retention time of 24 hours is most suitable for continuous biohydrogen production [36]. Baffle photobioreactors were appropriate for continuous biohydrogen production and they showed a peak HPR of 7.3 mmol L−1 h−1. From the literature, it was evident that hydrogen yield obtained from continuous mode photobioreactors was much greater than batch mode. Along with various reactor controlling parameters, continuous H2 generation from a variety of PNS bacteria and substrates are listed in Table 10.3 [37].

284  Materials for Hydrogen Production, Conversion, and Storage

Table 10.3  Various microbes and substrates utilized in continuous hydrogen generation via photofermentation. Reprinted from [30], Copyright 2009, Taylor & Francis. Maximum H2 rate (ml/L/h)

Yield %

Culture volume (ml)

Cell density (g/L)

Light source

Light intensity

Microorganisms

Substrates

Reactor type

R. sphaeroides O.U.001

Sugar refinery wastewater +malate

Continuous (HRT 80d)

1.0

-

400

1.40

N. S.

200 W/m2

R. sphaeroides O.U.001

Sugar refinery wastewater +malate

Continuous (HRT 32d)

3.0

-

400

0.97

N. S.

200 W/m2

R. sphaeroides RV

VFAs from fermentation of fruit and vegetable wastes

Continuous (HRT 25 h) (8–10 d)

48.0

-

1000

0.48

Tungsten

10 k lux

R. rubrum

Lactate

Continuous (HRT 74h)

65.0

64.5

1000

3.0–3.5

Tungsten

300 W/m2

Mixed phototrophic sludge culture

Synthetic wastewater (acetate + butyrate +ethanol)

Continuous (HRT 25h) (30d)

17.4

12.0

450

3.10

Tungsten

90–150 W/ m2

(Continued)

Production of Clean Hydrogen from Wastewater  285

Table 10.3  Various microbes and substrates utilized in continuous hydrogen generation via photofermentation. Reprinted from [30], Copyright 2009, Taylor & Francis. (Continued)

Microorganisms

Substrates

Reactor type

Maximum H2 rate (ml/L/h)

Yield %

Culture volume (ml)

Cell density (g/L)

Light source

Light intensity

R. sphaeroides RV

Succinate + (Porous glass support)

Continuous

310.0

55.0

-

11.2

300

300 W/m2

R. sphaeroides GL-1

Lactate + (Polyurethane Foam support)

Continuous (HRT43.5 h) (35 d)

210.0

86.0

200

-

300

-

R. capsulatus

Acetate + propionate

Continuous (HRT 72 h) (20 d)

21.0

45.9

1500

0.54

N. S.

-

R. capsulatus

Acetate + propionate + butyrate

Continuous (HRT 72 h) (10 d)

17.0

45.0

1500

0.45

N. S.

-

R. capsulatus

Effluent fromacidogenic H2 production reactor

Continuous (HRT 72 h) (10 d)

12.5

40.0

1500

0.38

N. S.

-

286  Materials for Hydrogen Production, Conversion, and Storage

10.3.2 Factors Affecting Photofermentation Hydrogen Production 10.3.2.1 Inoculum Condition and Substrate Concentration Log phase and early stationary phase were appropriate to produce hydrogen using PNS bacteria [38]. Polyhydroxyl butyrate formation occurs because of the alteration in metabolic pathways when bacteria growth proceeds for a prolonged time. Pretreatment of wastewater has usually been done to avoid any contents which slow down hydrogen production in photobioreactors. Different contents which have been found in brewery wastewater are shown in Table 10.4 [28]. Seifert and coworkers used thermal pretreatment for this brewery wastewater at 95°C for 45 minutes and it remarkably improved hydrogen production. Androga et al. studied the influence of carbon to nitrogen ratio upon hydrogen production rate using acetate and glutamate [39]. It showed that with an increase in C/N ratio hydrogen production rate will decrease. Surfeit biomass accumulation brings down an adequate amount of light from reaching the growth system. Lessened light intensity hinders ATP generation and in turn hydrogen production. Kim et al. studied the effect Table 10.4  Constituents of brewery waste water. Reprinted from [21], Copyright 2010, Elsevier. Parameters

Value

Ca (mg/L)

37.2

Fe (mg/L)

1.04

Mg (mg/L)

95.8

N (%)

0.669

C (%)

36.74

H (%)

6.98

S (%)

0.045

pH

4.71

COD (g O2/L)

202

N-NH4+ (mg/L)

95.8

Production of Clean Hydrogen from Wastewater  287

H2 Yield (mol H2/mol lactate)

3

2

1

H2 yield

0 5

10

15

20

25

30

Lactate concentration (mM)

Figure 10.3  Effect of lactate concentration on hydrogen yield. Reprinted from [33], Copyright 2012, Int. J. Hydrog. Energy, Elsevier.

of lactate concentration on continuous hydrogen generation using PNS bacteria Rhodobacter sphaeroides KD131 [40]. Hydrogen production is trivial when lactate concentration was 5 mM and the highest hydrogen yield (HY) of 2.3 mol H2/mol lactate was obtained at 20 mM lactate concentration (Figure 10.3). Lu and coworkers studied substrate effect on hydrogen formation in a baffled continuous photo fermentative reactor utilizing PNS bacteria HAU-M1 [41]. At substrate concentration of 20 g/L peak H2 production rate was found to be 202.64 ± 8.83 mol/m3/d. With an increase in organic loading rate, HPR also increased.

10.3.2.2 Carbon and Nitrogen Source Hydrogen production and growth of the bacterial cell depends on the carbon source. A large variety of carbon sources were utilized in photofermentation by PNS bacteria. Carbon sources include acetic acid, butyric acid, malic acid, propionic acid, succinic acid, glucose [42], molasses [43], olive mill wastes [44], brewery wastewater [45], paper mill effluent, agricultural wastes, and discharges from dark fermentation [46]. Different carbon sources vary in their ability of electron transfer in a variety of metabolic pathways of PNS bacteria. Lactic acid and malic acid were the foremost carbon sources in H2 production by PNS bacteria [47]. Rhodopseudomonascapsulatus JP91 used

288  Materials for Hydrogen Production, Conversion, and Storage glucose as a carbon source and exhibited HY of 75% [48]. Acetate acted as a carbon source for Rhodopseudomonaspalustris P4 and was successful in producing HY of 60%–70% [49]. As we discussed earlier NH4 + ions at higher concentrations slow down nitrogenase activity though nitrogen is required in little amount for cell synthesis. Glutamate acts as a good nitrogen source since PNS bacteria quickly consume it and its hampering effect on nitrogenase was very less [47]. Production cost increased by the use of glutamate since it is expensive.

10.3.2.3 Temperature The activity of enzymes and metabolism of bacterial cells are very much sensitive to alteration in temperature. Very high as well as very low temperatures retard hydrogen production efficiency of PNS bacteria and hence maintenance of optimum temperature is crucial to get good HY. A constant water flow of required temperature is kept around the photobioreactor to act like a thermostat and to maintain the desired temperature inside the reactor. Rhodobacter sp. works at its best in the temperature range of 31°C–36°C [50]. Optimum temperature extent for Rhodopseudomonaspalustris CQK 01 is found to be between 27°C and 32°C [51]. The most suitable temperature range for the majority of PNS bacteria in photofermentation is reported to be 30°C–40°C [19].

10.3.2.4 pH pH plays a significant role in providing optimum condition for bacterial cell and nitrogenase activity. Extreme acidic, as well as basic pH values, curb the proton movement and hamper ATP generation [52]. Insufficient ATP production retards the rate of hydrogen production and yield also. Different strains of PNS bacteria show variation in their optimum pH value (from 4 to 10); however, the majority of them show the best hydrogen production rate at pH 7 [53]. Many of the PNS bacteria such as Rhodospirillum sp., Sulfurospirillum sp., Rhodobacter sp., and Rhodopseudomonas sp. exhibited good HY at pH 7.

10.3.2.5 Light Intensity Appropriate usage of light enhances the hydrogen production ability of the reactors. Appreciable HY only by the absorption of sunlight is possible in photobioreactors. However, artificial light sources are preferred and used extensively to overcome drawbacks of sunlight such as intensity variation

Production of Clean Hydrogen from Wastewater  289

ml of Hydrogen evolved/vessel

and seasonal changes. A variety of artificial light sources were utilized in photofermentation, which includes optical fibers, light-emitting diodes (LED), halogen lamps, neon tubes, and fluorescent lamps [54]. Among these LED light sources were the most preferred ones. The reasons were their precise wavelengths, less heat generation, and low electrical consumption, and they were lightweight and can fit into any reactor [31]. The intensity of incident light varies when it passes through bacteria culture as per Beer-Lambert’s law. The proximity of the light sources was also important since light energy diminishes exponentially with distance. The ratio of the amount of energy released by the combustion of produced hydrogen to the energy of the incident light is termed as light energy conversion efficiency. Miyake and Kawamura reported maximum conversion efficiency of 7.9% with 50 Wm−2 xenon lamps utilizing Rhodobacter sphaeroides 8703 [55]. Sasikala et al. studied the effect of light intensity on hydrogen production with Rhodobacter sphaeroides O. U. 001 [56]. This study revealed that hydrogen production increased with an increase in light intensity and it attained saturation beyond 500 lux light intensity at 30°C (Figure 10.4). Adessi and coworkers used natural light sources involving light and dark cycles for photofermentation [29]. A decrease in dissolved H2 content was observed at night and during morning few hours are required to re-establish the same. Hence utilization of artificial light sources was recommended at night for continuous hydrogen production with better yield. Low light flux of only 5.85 W m−2 is used in continuous hydrogen production (CHP) by Rhodospirillumrubrum ATCC 10801 while a very high light flux of 250 W m−2 is used in CHP by Rhodobactercapsulatus B10 [31].

3·0 2·4 1·8 1·2 1·6 0·2 0

1000

2500 5000 7500 10000 Light intensity (Lux)

Figure 10.4  Variation of hydrogen generation in photofermentation by Rhodobacter sphaeroides O. U. 001 with increase in light intensity. Reprinted from [49], Copyright 1991, Elsevier.

290  Materials for Hydrogen Production, Conversion, and Storage (a) Tungsten lamps

Tubular loop Water bath

(b) Water bath Tubular loop

Figure 10.5  Images of continuous hydrogen production by tubular photobioreactor using artificial light source (a) and sunlight (b). Reprinted from [50]. Copyright 2018, Elsevier.

Palamae and group members used both sunlight and tungsten lamps of 10 Klux in combination for CHP (Figure 10.5) and were successful in producing hydrogen at a rate of 36 ml H2 L−1 medium h−1 [57]. Usage of light intensity depends on the strain of bacteria utilized, photobioreactor design, cell concentration, etc.

10.3.2.6 Immobilization Immobilization helps in the retention of cells for continuous operation and cells were prevented from washout even at low hydraulic retention time [58]. Immobilized cells work for longer periods in exponential growth. Zhu and group members immobilized Rhodobacter sphaeroides in agar cells to produce hydrogen from tofu factory wastewater [59]. The HY was found to be 1.9 ml/ml wastewater with a hydrogen production rate of 2.1 L h−1 m2. Rhodospirillumrubrum bacterial cells were immobilized in agar along with a microporous membrane by Planchard et al. for photofermentative hydrogen production from [60]. Fibler and coworkers

Production of Clean Hydrogen from Wastewater  291 immobilized Rhodopseudomonaspalustris DSM 131 PNS bacteria in agar, alginate gel, and agarose to obtain a very high hydrogen yield of 88% from benzoate [58]. Reverse micelles were utilized to immobilize chloroplasts and Halobacteriumhalobium  organelles by Singh et al. and it resulted in the escalation of hydrogen production rate [61]. Immobilization of Rhodobacter sphaeroides O.U.001 in transparent porous glass beads for enhanced HY was attempted by Basak and Das [54].

10.4 Dark Fermentation Dark fermentation is a prominent and well studied technique for the production of biohydrogen. A large array of substrates including organic contents of wastewater can be utilized in this technique and microorganisms produce hydrogen in the absence of oxygen and light sources [62]. Dark fermentation is a more pragmatic approach for biohydrogen production on a large scale. Expansive ranges of microorganisms are utilized in this technique among which leading ones are few thermophilic bacteria, Enterobacter sp, Clostridium species, Bacillus sp, etc. [1]. The chief enzyme in the case of dark fermentation is hydrogenase and is responsible for hydrogen production by merging H+ ions and electrons [63]. Carbohydrates are major and favored substrates for hydrogen production in dark fermentation. Ubiquitous carbohydrate sources such as starch and glucose contain glucose and its isomers as subunits. Dark fermentation of glucose follows different pathways and it depends on the kind of byproducts obtained along with molecular hydrogen production. Equation 10.5 represents the acetic acid pathway in which the highest molecular hydrogen yield can be observed [24].

of N 2 : N 2 + 10 H C+ 6+H812eO− 6++162 ATP + H 2 ++162 ADP H2O → 2 NH CH34+COOH CO2 + 4 H2  ΔG = −206 KJ

(10.5)

There is no need for external energy like light sources in this technique since the change in Gibbs free energy is negative. Equations 10.6, 10.7, 10.8, and 10.9 correspond to butyric acid, propionic acid, lactic acid, and ethanol fermentation pathways, respectively [20].

+ of N 2 : N 2 + 10 H + 8e − + 16 + H2CH ADP + 2 CO2 + 2 H2 C HATP O → 2 NH CH34+CH COOH 2 + 216 6 12 6

(10.6)

N 2 : N 2 + 10 H + C + 8e − + 16 + H 2 ++ 16CH ADP HATP O → 2 NH CH34+COOH CH2COOH + CO2 + H2 (10.7) 6 12 6 3

292  Materials for Hydrogen Production, Conversion, and Storage

presence of N 2 : C N 2 + 10 H + + 8e − + 16 + H 2 + 16 ADP HATP O → 2 NH CH34+CHOHCOOH 6 12 6

(10.8)

C6 H12O6 + 2 H 2O → 2 CH 3CH 2OH + 2 HCO3− + 2 H + (10.9)

From the above equations, it is clear that propionic acid pathways yield less hydrogen than butyric acid pathways while ethanol and lactic acid pathways fail to produce any hydrogen from glucose. In all the cases obtained H2 yields were less than theoretical yields since bacterial cells consume some amount of glucose for their growth. H2 evolution by dark fermentation of wastewater is shown in Figure 10.6.

10.4.1 Continuous Dark Fermentation Continuous mode dark fermentations are less reported as to compare to batch mode. The cause was batch mode exhibited a higher H2 yield than continuous mode. However, for large-scale and long-run hydrogen production plants continuous mode is more suitable than batch mode. The reason for this includes uniform product quality and enhanced yield than the batch mode in the long term. Azbar and group members carried out dark fermentation of cheese processing wastewater in continuous stirred tank reactors and the average hydrogen production rate was found to be 2.5 L/L/day [64]. For an organic loading rate of 35 g COD/L/day, a peak hydrogen yield of 9 mmol/ g COD was observed. In a pilot scale, dark fermentation reactor peak HPR of 5.57 m3 H2/m3 reactor/day was reported by Ren et al. using molasses as a carbon source [65]. The highest HPR was CO2 Dark Condition

H2 Gas Separator

Wastewater

H+ Organic Acids H+

Hydrogenase

H+

Anaerobic Bacteria H+

Partially treated wastewater

Figure 10.6  Schematic representation of hydrogen generation from dark fermentation.

Production of Clean Hydrogen from Wastewater  293

Table 10.5  Wastewaters utilized in hydrogen generation via continuous dark fermentation with various reactor parameters. Reprinted from [13], Copyright 2015, Elsevier. Substrate concentration (g COD/L)

HRT (h)

Reactor mode

Range studied

Optimal

pH

Range studied

Sweet sorghum extract

CSTR

9.89–20.99

17.50

5.5–5.7 35

Palm oil mill effluent

ASBR

20

-

Condensed molasses

CSTR

40

Condensed molasses

ICBR

Tofu processing wastewater Desugared molasses

Wastewater

Optimal

HY (mol/ molsugar)

HPR (L/L/d)

12

-

0.74

2.93

6.5–7.0 37

36–96

72

0.34 L H2/g COD

6.7

-

5.5

37

0.5–8

0.5

2.02/10−3 mol H2/g COD

14.04

40

-

5.5

37

0.5–8

0.5

1.26/10−3 mol H2/g COD

7.60

MBR

43.4

-

5.5

60

2–8

4

1.45

19.86

UASB

16.7

-

5.0–6.0 55

24

-

269.5 ml-H2/gsugar

4.5

Temperature

Molasses

CMISR

8–24

24

N. A

36

N. A

-

N. A

6.53

Sugarcane juice

CSTR

25

-

6.0

37

4–36

4

1.00

2.29 (Continued)

294  Materials for Hydrogen Production, Conversion, and Storage

Table 10.5  Wastewaters utilized in hydrogen generation via continuous dark fermentation with various reactor parameters. Reprinted from [13], Copyright 2015, Elsevier. (Continued) Substrate concentration (g COD/L)

HRT (h)

Reactor mode

Range studied

Optimal

pH

Range studied

Soft-drink wastewater

UAPBR

1.94

-

6.0–6.4 25

Cheese whey

UASB

20

-

Molasses

CSTR

8–32

24

Molasses

CMISR

8–32

Tapioca wastewater

ABR

Desugared molasses

Optimal

HY (mol/ molsugar)

HPR (L/L/d)

0.5

-

3.5

9.6

5.5–5.9 22–25

6–24

6

0.30

8.64

N. A

35

6

-

N. A

7.46

32

-

35

6

-

76.36 mmol/ mol COD

7.59

16.15

-

9.0

32.3–33.8

3–24

6

13.40 ml-H2/gCOD

0.83

UASB

16.7

-

5.6

55

16

-

132 ml-H2/ g-VS

5.6

Tofu processing wastewater

CSTR

20

-

5.5

35

6–24

8

N. A

1.73

Sweet sorghum

ASBR

30

-

5.0

30–33

24

-

1.6

3.2

Wastewater

Temperature

(Continued)

Production of Clean Hydrogen from Wastewater  295

Table 10.5  Wastewaters utilized in hydrogen generation via continuous dark fermentation with various reactor parameters. Reprinted from [13], Copyright 2015, Elsevier. (Continued) Substrate concentration (g COD/L)

HRT (h)

Reactor mode

Range studied

Optimal

pH

Temperature

Range studied

Dephenolized olive mill wastewater

PBR

38.79

-

7.03

35

Molasses

CSTR

8

-

Sugary wastewater

CSTR

6

-

Beet sugar wastewater

CSTR

12–18

18

Beverage WW

ICBR

20

-

Wastewater

Optimal

HY (mol/ molsugar)

HPR (L/L/d)

24–168

24

-

7.0

4.3–4.4 35

4–10

5

N. A

7.47

4.5

4–12

5

N. A

3.45

4.1–4.5 35

8

-

N. A

10.8

5.5–6.5 35

15.8

1.5

1.5

55.4

35

CSTR – Continuously stirred tank reactor; UASB – Up flow anaerobic blanket reactor; CMISR – Continuous mixed immobilized sludge reactor; ASBR – Anaerobic sequencing batch reactor; ICBR – Immobilized cell bioreactor; MBR – Membrane bioreactor; ABR – Anaerobic packed bed reactor.

296  Materials for Hydrogen Production, Conversion, and Storage observed when the concentration of ethanol was close to that of acetate. Produced biogas contained 40% to 52% of hydrogen and the remaining percentage was of CO2. Biohydrogen generation from domestic wastewater and food processing has been studied by Ginkel and coworkers [66]. It showed 60% H2 in the produced biogas along with 5% to 11% elimination of COD. Peak HPR of 1119 ml H2 L−1 h−1 was observed for batch mode dark fermentation while continuous systems were successful to bring HPR up to 7600 ml H2 L−1 h−1 [1]. Comprehensive varieties of wastewaters utilized in continuous mode dark fermentative reactors for H2 generation are listed in Table 10.5 [13].

10.4.2 Factors Affecting Hydrogen Production in Continuous Dark Fermentation 10.4.2.1 Start-Up Time The minimum time required to attain a steady reactor operation is termed start-up time. In continuous mode operations for large scale hydrogen production, great care must be taken to establish a proper startup. The nature of the microbes utilized, their acclimatization, and the design of the bioreactor affect the start-up time [67]. Guo and coworkers showed that the addition of L-Cysteine significantly reduces the start-up time and simultaneously assists in H2 production increment [68]. The addition of 0.5 g/L L-cysteine enhanced HPR by 23.7%. However, working costs escalated by the addition of L-cysteine. Quick acclimatization and steady H2 production has been achieved by monitoring the feed to bacteria ratio. In biohydrogen production from coffee drink wastewater, the addition of 1-week-old CSTR discharge to seed sludge notably lessened the startup time [69].

10.4.2.2 Organic Loading Rate With the increase in organic loading rate, fermentative hydrogen production also increases, and later it diminishes after crossing a threshold value. When OLR had risen from 3.11–68.21 kg COD/m3/d, biohydrogen generation from molasses increased and later declined with further increase in OLR from 68.21–85.57 kg COD/m3 reactor/d [65]. The reason for the falloff in the hydrogen yield at very high organic loading was mainly due to shifts in the metabolic pathways and formed byproducts which hampered the hydrogen production. Reyes et al. reported an effective method to surge HPR with a concurrent reduction in methane formation by increasing

HPR (mol H2/L-d) HY (mol H2/mol sucrose)

Production of Clean Hydrogen from Wastewater  297 5 Sucrose concentration 20 HRT 12 2 h

4

40 g COD/L

3 2 1 1000 800 600 400 200 0

0

50

100 150 OLR (g COD/L-d)

200

250

Figure 10.7  Variation of hydrogen yield and hydrogen production rate with increase in organic loading rate. Reprinted from [64], Copyright 2012, Elsevier.

organic loading from 20–30 g COD/L/day in a cheese whey dark fermentation [70]. In a study carried out by Azbar and group members, hydrogen yield was found to be 6, 9, and 3 m mol/g COD at different OLR of 21, 35, and 47 g COD/L/day, respectively [64]. Chen and coworkers found that an OLR of 66.7 g COD/L/d resulted in the highest HPR in a recycled continuous dark fermentation mode. Variation of hydrogen yield and HPR rate with increase in OLR is depicted in Figure 10.7 [71]. A remarkable increase in hydrogen formation rate was observed from 2.8 L H2/L/d to 28.47 L H2/L/d by enhancing the OLR from 55.4 g COD/L/d to 138.64 g COD/L/d in a continuous stirred tank reactor with a fixed HRT of 6 hours [72].

10.4.2.3 Hydraulic Retention Time Hydraulic retention time is a prominent factor that affects hydrogen production in continuous dark fermentation reactors. Non-hydrogen producing bacteria are eliminated from the reactor by lowering HRT and by keeping persistent substrate concentration. The majority of hydrogen forming bacteria including Clostridium sp. favors short HRT since it yields maximum hydrogen in the log phase. With the onset of the stationary phase, alcohol generation pathways will be dominant [73]. The reason behind shortening HRT was, methane- and propionic acid-forming bacteria cannot sustain dilution effect, but sturdy hydrogen-forming bacteria survive. However, too short HRT results in a wash-out of all the cells, and

298  Materials for Hydrogen Production, Conversion, and Storage

Table 10.6  Effect of hydraulic retention time on continuous hydrogen generation in dark fermentation with various substrates. Reprinted from [70], Copyright 2015, Elsevier.

HRT

Strategies adopted to increase HY/ HPR

HY (mol H2/mol substrate)

HPR (L/L-d)

Ctrl

Aft

Ctrl

Aft

Range studied

Optimal range

Pilot scale bioreactor

4–8

6

Decreasing HRT and increasing OLR

N. A

3.84

N. A

28.9

Starch

Continuouslystirred tank reactor

2–12

4

Constant OLR and gradual decrease of HRT

N. A

1.5

N. A

11.37

Glucose

Continuouslystirred tank reactor

2–12

6

Constant OLR and gradual decrease of HRT

N. A

1.53

N. A

14.4

Cheese whey

Up-flow anaerobic sludge blanket

6

-

Gradual OLR increase and constant HRT

N. A

N. A

N. A

1.12

Cheese whey

Up-flow anaerobic sludge blanket

6

-

Gradual OLR increase and constant HRT

N. A

N. A

N. A

0.11

Substrate

Reactor type

Sucrose

(Continued)

Production of Clean Hydrogen from Wastewater  299

Table 10.6  Effect of hydraulic retention time on continuous hydrogen generation in dark fermentation with various substrates. Reprinted from [70], Copyright 2015, Elsevier. (Continued)

HRT

Strategies adopted to increase HY/ HPR

HY (mol H2/mol substrate)

HPR (L/L-d)

Ctrl

Aft

Ctrl

Aft

Range studied

Optimal range

Continuouslystirred tank reactor

12–18

12

Decreasing HRT from 18 h to 12 h to reduce propionate concentration

0.5

.83

N. A

N. A

Glucose

Continuouslystirred tank reactor

6–12

6

Reducing HRT from 8 h to 6 h eliminated the propionate producing bacteria

N. A

1.88

N. A

7.77

Glucose

Continuouslystirred tank reactor

5–10

5

Decreasing HRT and pH

N. A

1.1

N. A

6.7

Substrate

Reactor type

Wheat starch co-product

(Continued)

300  Materials for Hydrogen Production, Conversion, and Storage

Table 10.6  Effect of hydraulic retention time on continuous hydrogen generation in dark fermentation with various substrates. Reprinted from [70], Copyright 2015, Elsevier. (Continued)

HRT Range studied

Optimal range

Strategies adopted to increase HY/ HPR

HY (mol H2/mol substrate)

HPR (L/L-d)

Substrate

Reactor type

Ctrl

Aft

Ctrl

Aft

Sucrose

Continuouslystirred tank reactor

2–12

4

HRT optimization under reduced pressure conditions of 380 mmHg

N. A

2.03

N. A

30.45

Cheese whey

Up-flow anaerobic sludge blanket

3–6

3

Organic shock load effect by decreasing HRT and influent substrate concentration

N. A

N. A

N. A

0.32

Sucrose

Continuouslystirred tank reactor

2–12

6

Constant OLR and decrease HRT with the recycle ratio of 0.2

N. A

3.88

N. A

20.4

Production of Clean Hydrogen from Wastewater  301 system operation fails. Zhang and coworkers reduced HRT from 8 hours to 6 hours and it amplified hydrogen yield from 1.6 mol H2/mol glucose to 1.9 mol H2/mol glucose [74]. In a study of continuous H2 production from wastewater, Krupp et al. claimed HRT of 14 to 15 hours showed peak H2 yield, and further shortening of HRT to 12 hours diminished hydrogen yield [75]. Optimum HRT was not only restricted to a few hours and may extend to a few days. An example of such a case was reported in a continuous H2 evolution experiment from cheese wastewater, in which HRT of 1, 2, and 3.5 days exhibited hydrogen yield of 5, 15, and 22 m mol/g COD [64]. Cheng and group members studied the effect of HRT on hydrogen yield and H2 formation rate. It showed a peak HPR of 1770 mL H2 L−1 h−1 with diminished HY (2.74 mmol H2 g−1 starch) at an HRT of 30 minutes. The same setup exhibited a maximum HY of 5.06 mmol H2 g−1 starch with lessened HPR (700 ml H2 L−1 h−1) at an HRT of 12 hours [76]. Effects of HRT on an expansive range of H2 forming continuous mode dark fermentation systems are listed in Table 10.6 [77].

10.4.2.4 Temperature Temperature is a crucial factor for the growth of microbes and H2 production as well since activity of enzymes was greatly affected by a change in temperature. Continuous dark fermentation can be carried out at a wide temperature range involving mesophilic temperature (25–40°C), thermophilic temperature (40–65°C), utmost thermophilic temperature (65–80°C) or hyperthermophilic temperature (>80°C) [23]. The majority of the continuous dark fermentation reactions were carried out at mesophilic temperature. In continuous dark fermentative hydrogen generation from coffee wastewater, non-hydrogen-producing bacteria are found to disrupt the operation of the reactor and to avoid this problem high temperature reaction conditions are utilized [78]. Elevated reactor temperature diminished the hydrogen solubility and increased the hydrolysis to improve hydrogen production. Another reason to opt for high temperature had been found to be the better thermodynamic feasibility of hydrogen forming reactions [79]. Lin and group members reported that even a small temperature increment of 5°C altered the H2 generation rate by 25% [80]. In a study of cellulose dark fermentation, it was found that the optimum temperature of 80°C resulted in a maximum hydrogen yield of 3.43 mol H2/mol cellulose [81]. The temperature of dark fermentation reactors had risen only after assuring that the energy outcome from improved hydrogen yield was more than the input thermal energy.

302  Materials for Hydrogen Production, Conversion, and Storage

10.4.2.5 pH pH acts as a prominent parameter in determining reaction pathways and it showed a remarkable effect on the activity of enzymes. The most appropriate pH value, which leads to the H2 generation pathway in the case of Clostridium sp. was found to be 5.5, and alteration of this pH sets off non-hydrogen-forming reactions [21]. Optimal pH value varies according to the nature of microbes utilized and the kind of substrate selected. Majority of H2-forming reactions in dark fermentation yield acetate and butyrate as byproducts. Acidic pH tends to favor H2 generating pathways while a pH value of 7 or above (basic conditions) upshot the unwanted propionic acid formation. The pH range of 6 to 7.5 was found to be conducive for hydrogen consuming bacteria, while hydrogen evolving bacteria showed better performance at pH below 6 [82]. Methanogenesis is of great concern in dark fermentation since it markedly brings down H2 production. Lowering pH was an efficient way to limit methane and propionic acid formation. Attempts in this direction are listed in Table 10.7 [77]. In the long run of the bioreactor, acidic byproducts gradually build up and tend to reduce the reactor pH. Decrement in pH affects hydrogen evolution. Hence maintenance of appropriate pH is essential to get an excellent H2 yield.

10.4.2.6 Immobilization Immobilization technique is used to prevent wash-out of useful H2 evolving bacteria under very low HRT. To maintain a sufficient concentration of useful bacteria even at short HRT, several methods were employed, among which cell embedment, self granulation, and the addition of plastic carriers were prominent. The mode of operation of these methods involves accretion, surface adhesion, and entrapment [77]. Wu and coworkers added 100 ppm calcium ions to a fluidized bed bioreactor containing activated carbon immobilized cells. Added calcium ions aid in microbial cell agglomeration and it appreciably improved the H2 formation rate up to 1.22 L/L/d at an HRT of 2 hours [83]. Granulation was of foremost importance in immobilization. Exceptionally well H2 production rate (15 L/h/L) was observed even at very low HRT of 0.5 hours in continuously stirred tank reactors with sucrose substrate. The reason behind this was the incorporation of silicone immobilized sludge which assisted in granulation and prevention of useful cell wash-out [84]. In a study carried out by Lin and group members it was reported that H2 evolving bacterial cells were immobilized by

Production of Clean Hydrogen from Wastewater  303

Table 10.7  Effect of pH on continuous hydrogen generation in dark fermentation. Reprinted from [70], Copyright 2015, Elsevier.

pH

Strategies adopted to increase HY/ HPR

HY (mol H2/mol substrate)

HPR (L/L-d)

Range studied

Optimal range

Ctrl

Aft

Ctrl

Aft

Up-flow anaerobic sludge blanket reactor

4.5–5.5

5.5

Reducing pH 4.5 to suppress the methanogenesis

N. A

2.6

N. A

1.72

Cheese whey

Up-flow anaerobic sludge blanket reactor

4.5–5.63

5.0

Reducing pH to control methanogenesis

N. A

N. A

N. A

0.31

Sugar beet refinery wastewater

Continuouslystirred tank reactor

4.2–6.0

4.2–4.4

Reduce pH below 4.5 to reduce the HPr accumulation

N. A

N. A

1.69

2.96

Molasses

Continuouslystirred tank reactor

4.0–5.2

4.0–4.4

Proposed ethanol type fermentation is more suitable than propionic type fermentation

N. A

N. A

0.71

11.97

Substrate

Reactor type

Glucose

304  Materials for Hydrogen Production, Conversion, and Storage ethylene vinyl acetate polymer and it resulted in a good HPR of 1.80 H2 L/h/L [85].

10.5 Microbial Electrolysis Cell Microbial electrolysis cell (MEC) is a combination of bio-oxidation of microbes and an electrochemical setup. It was termed a bioelectrochemical system in which a variety of microorganisms disintegrate the organic matter and subsequent electrochemical reactions lead to hydrogen evolution. The foremost advantage of MEC over photofermentation and dark fermentation was found to be the extensive range of organic substrates that can be utilized in biohydrogen generation. Only the organic materials containing abundant carbohydrates are favored in dark fermentation while protein and a vast variety of organic acids are utilized in MECs [86]. Other reasons for choosing MECs over conventional techniques like photo and dark fermentations were their ability to produce excellent H2 yield and exceptional purity of the evolved hydrogen.

10.5.1 Mechanism of Microbial Electrolysis Cell The different components involved in the electrochemical setup of the MEC are portrayed in Figure 10.8 [87]. Organic materials entering into

Power supply CO2

e¯

e¯

H2

H+

Bacteria

Anode

Membrane Cathode

Figure 10.8  Diagrammatic representation of microbial electrolysis cell function. Reprinted from [8], Copyright 2010, Taylor & Francis.

Production of Clean Hydrogen from Wastewater  305 the anodic chamber had broken down by microorganisms present on the anode. Upon disintegration, anode respiring bacteria handover the electrons from substrate to electrode [88]. Protons proceed across the membrane and upon reduction, transform into molecular hydrogen. The movement of electrons from one end of the electrode to the other was not spontaneous since the reduction potential of protons at the cathode is less than anode reduction potential. External voltage is applied to carry out the cell reaction however the applied voltage is too tiny and is less than 1.2 V needed for electrolysis of H2O [86]. Equations 10.10 and 10.11 show the chemical reactions involved at anode and cathode, respectively, when acetate was used as substrate. The overall reaction of the cell was represented by Equation 10.12 [17]. Positive change Gibbs free energy proclaims nonspontaneity of the cell reaction and the need for an external source.



CH 3COO − + 3 H 2O → 8 H + + HCO3− + CO2 + 8 e − (10.10)

+ In presence 8 of N 2 : N 2 + 10 H + + 8e − H + 16 +ATP 8 e− → 24 NH H2 4+ + H 2 + 16 ADP (10.11)



CH 3COO

3 H 2O

4 H 2 HCO3

CO2 DG

54.8 KJ mol

1



(10.12) Cation exchange membranes were employed in two chamber MEC systems to restrict fuel and bacteria movement from anodic to the cathodic compartment. Another salient function of the membrane was to assist in pure hydrogen generation. Membrane-less single chamber MEC was also developed [89] in which membrane resistance issue was resolved and remarkable enhancement in hydrogen production rate was also observed. However, the main drawback in single chamber MEC was the consumption of evolved H2 by bacterial culture.

10.5.2 Wastewater Treatment and Hydrogen Production Microbial electrolysis cells tend to lower the COD of wastewater with simultaneous generation of pure hydrogen. Heidrich and group members treated urban wastewater with pilot scale MEC for a long period of 1 year. It showed a notable reduction in COD by 33% and an appreciable H2evolution rate of 0.6 L/L/d [90]. Bio hydrogen productions from an expansive range of wastewaters utilizing MECs are arranged in Table 10.8 [19].

306  Materials for Hydrogen Production, Conversion, and Storage

Table 10.8  Wastewaters utilized in hydrogen generation via microbial fuel cell. Reprinted from [12], Copyright 2021, MDPI. Study duration (days)

MEC capacity (L)

Columbic efficiency (%)

COD removal (%)

Temp ( ̊C)

H2/CH4 production rate (L/L/d)

Domestic WW

≥ 365

100

41.2

33

1–22

0.007

Domestic

≥ 730

2

9–30

80

20

0.006–0.045

Domestic WW

149

120

55

34

16.6

0.015

Domestic WW

≥ 730

2

10–94

85

20

0.045

Domestic WW

35

0.2

38–65

76

30

0.3

Municipal WW

120

120

43

43.6

3.7–19.4

0.003–0.004

Municipal WW

> 100

130

28

5.9–25.4

-

0.031

Substrate/ WW

100

0.028

15–52

73.5–100

23

0–0.94

Effluent source

(Continued)

Production of Clean Hydrogen from Wastewater  307

Table 10.8  Wastewaters utilized in hydrogen generation via microbial fuel cell. Reprinted from [12], Copyright 2021, MDPI. (Continued) Study duration (days)

MEC capacity (L)

Columbic efficiency (%)

COD removal (%)

Temp ( ̊C)

H2/CH4 production rate (L/L/d)

Effluent/ WW

28

0.028

60–90

-

25

0.1

Industrial WW

-

0.028

7–12

85–89

30

0.8–1.8

Molasses WW

25

0.025

83.6–95

-

9

0.72–1.69

Piggery WW

-

0.72

9–30

48

-

0.095

Swine WW

15

0.028

29–70

19–72

30

0.8–1.0

Effluent source

308  Materials for Hydrogen Production, Conversion, and Storage

10.5.3 Factors Affecting Microbial Electrolysis Cell Performance 10.5.3.1 Inoculum The movement of electrons from the substrate to an electrode is one of the crucial aspects of MECs and it was done by microorganisms named exoelectrogens or anode respiring bacteria. Ample varieties of exoelectrogens were found in wastewaters like domestic wastewater and anaerobic sludge as well. A vast assortment of electrogenic microbes are arranged in Table 10.9 [87].

10.5.3.2 pH Change in pH affects the reaction pathway and in turn product efficiency in MECs. pH showed an inverse relation with electromotive force of the cell and hence with single unit increase in pH anode potential reduces by −0.059 V. At pH 9 Liu and group members observed a notable H2 generation rate of 0.55 L/L/d and an excellent COD removal of 76% [91]. Basic pH is found to be more conducive for bacterial growth and it facilitates electron movement in case of exoelectrogens. From a study carried out by Nimje and coworkers, it was found that pH increased from 7 to 9 uplifted electrochemical activities [92].

10.5.3.3 Temperature As microbial activities are highly sensitive to temperature change, a remarkable rise in the hydrogen evolution was observed with temperature elevation from 23°C to 28°C; 30°C–35°C has been found to be the appropriate temperature for anode respiring bacteria [93]. However, even at an extremely low temperature of 4°C MEC exhibited a notable H2 yield of 2.66 mol H2 mol−1 acetate [94].

10.5.3.4 Hydraulic Retention Time Hydraulic retention time remarkably affects the COD reduction value and H2 formation rate in a MEC. Carrera et al. studied the effect of HRT on COD removal efficiency, biohydrogen production, and energy efficiency

Production of Clean Hydrogen from Wastewater  309 Table 10.9  Electrogenic microbes used in microbial electrolysis cell. Reprinted from [80], Copyright 2016, Elsevier. Electrogenic microorganisms

Substrate

Rhodopseudomonaspalustris DX-1

Volatile acids, yeast extract, thiosulfate

Ochrobactrumanthropi YZ-1

Acetate, lactate, propionate, butyrate, glucose, sucrose, cellobiose, glycerol, ethanol

Acidiphilium sp. strain 3.2 Sup 5

Ferric iron Ferrous iron

Rhodoferaxferrireducens, Citrobacter sp. SX-1

Glucose; Citrate, glucose, lactose, sucrose, acetate, glycerol

Shewanellaputrefaciens MR-1, IR-1, SR-21

Lactate, pyruvate, acetate, glucose

Shewanellaoneidensis MR-1

Lactate

Klebsiellapneumoniae strain L17, Enterobacter cloacae

Glucose, starch; cellulose

Aeromonashydrophila KCTC 2358

Acetate

Aeromonas sp. strain ISO2–3, Geobacteraceae

Glucose; acetate

Geobactermetallireducens, Geobactersulfurreducens

Acetate

Desulfobulbuspropionicus

Pyruvate, acetate

Propionibacteriumfreudenreichii ET-3

Acetate, lactate

Arcobacterbutzleri strain ED-1

Sodium acetate

Clostridium beijerinckii, Clostridium butyricum EG3

Starch, glucose, lactate, molasses; glucose

Firmicutes thermincola sp. strain JR

Acetate

Geothrixfermentans, Gluconobacteroxydans

Acetate; glucose

310  Materials for Hydrogen Production, Conversion, and Storage as well. On shortening HRT from 10 to 4 hours, the COD removal efficiency of the MEC markedly dropped from 79% to 64% while the reverse effect was observed upon HPR. Hydrogen formation rate was enhanced from 37.6 ml/d to 89.4 ml/d when HRT was lowered from 10 to 4 hours [95]. Energy consumption had diminished with the decrement of HRT.

10.5.3.5 Applied Voltage An external power supply is essential to generate biohydrogen from MECs. The optimal applied voltage range for the majority of the MECs was 0.2 to 1.4 V. When the applied voltage was more than 2 V, it led to O2 production with voltage below 0.6 V volts, resulting in the build-up of electrons at the anode instead of the cathode [93]. However excess of the applied voltage may lead to electrolysis of water. Applied voltage has a prominent effect on the development of microbes and their activity. When the applied voltage goes below 0.3 V then it creates turbulence in the system.

10.6 Conclusions Wastewater exhibited great potential for biohydrogen generation. Hydrogen producing bacteria and electrogenic bacteria modified an energy consuming wastewater treatment to renewable energy production technique. To date, only a few experiments have been done on continuous photofermentation. However, for industrial scale production of biohydrogen, continuous systems are favored, however not matured enough. Dark fermentation exhibited a low substrate conversion rate and its effluents were reused as substrates in photobioreactors. Bioreactors demand large reactor volumes and it can be minimized by improving the hydrogen production rate. Genetic modifications of the microbes lead to improvement in hydrogen generation. Both photo and dark fermentation processes are of low cost while microbial electrolysis cells demand a small energy source with an expensive cell setup. The expansive range of organic substrates found in wastewaters can be utilized in microbial electrolysis cells. It yields pure hydrogen and eliminates the need for costly gas separation. Continuous hydrogen production from microorganisms has been found to be an optimistic approach in resolving issues of wastewater treatment and clean energy generation.

Production of Clean Hydrogen from Wastewater  311

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11 Conversion Techniques for Hydrogen Production and Recovery Using Membrane Separation Nor Azureen Mohamad Nor1, Nur Shamimie Nadzwin Hasnan1, Nurul Atikah Nordin1, Nornastasha Azida Anuar1, Muhamad Firdaus Abdul Sukur1 and Mohamad Azuwa Mohamed1,2* Department of Chemical Sciences, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, UKM, Bangi, Selangor, Malaysia 2 Fuel Cell Institute, Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia 1

Abstract

Energy and the environment are of global concern, stimulating worldwide research on clean, efficient, and sustainable energy technologies. Hydrogen and electricity are expected to be the two dominants energy carriers for the provision of enduse services when non-fossil based energy sources dominate the world’s energy sources. Concurrently, water splitting for hydrogen production may play a vital technology covering a substantial share of global energy needs. The combination of water splitting for hydrogen evolution with the rising membrane technology in producing high hydrogen gas purity makes the effective implementation of hydrogen as a renewable energy source economically and environmentally sustainable. This chapter discussed the conversion technique for hydrogen production via various water splitting systems and the hydrogen recovery using several types of membrane separation. Keywords:  Renewable energy, hydrogen production, water splitting, photocatalytic, membrane separation

*Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (319–342) © 2023 Scrivener Publishing LLC

319

320  Materials for Hydrogen Production, Conversion, and Storage

11.1 Introduction Renewable hydrogen is an exciting option to facilitate energy transition in the current global energy framework and according to the new commitments to reduce greenhouse gas emissions. Hydrogen is an alternative fuel that has very high energy content by weight. It is locked up in enormous quantities in water, hydrocarbons, and other organic matter [1]. Hydrogen can be produced from diverse, domestic resources, including fossil fuels, biomass, and water electrolysis with wind, solar or grid electricity. Hydrogen is considered the energy carrier for the future because of its clean and flexible conversion into different forms of energy, such as heat via combustion or electricity via fuel cells [2]. Traditionally, hydrogen is produced from natural gas steam reforming from fossil fuels making the effective implementation of hydrogen economically and environmentally unsustainable. Concurrently, solar energy and, more concretely, water splitting for hydrogen production may play an important technology that can cover a substantial share of global energy needs [3]. Nowadays, many researchers have developed various green energy systems for efficient evolution of hydrogens such as water splitting driven by two electrodes electrolysis of water, photoelectrochemical cell, photocatalytic hydrogen generation, and photovoltaic-photoelectrochemical cell. Although water splitting is acknowledged as a rising technology in producing hydrogen, the recovery of the high purity hydrogen has substantially become an issue. The development of high efficiency and cost-­ effective technologies for the recovery of high purity hydrogen is a key to extend the hydrogen fuel technologies [4]. Nowadays, immense intensive efforts have been carried out to explore membrane technology as an economically viable approach in hydrogen purification and separation for the hydrogen energy system. The membrane can be best defining as a selective barrier that allowing specific molecules/ions to pass through it while blocking other unwanted particles [5]. The selection of a suitable membrane is crucial in determining the success of producing high hydrogen purity. Various membranes have been widely applied for hydrogen recoveries, such as polymeric, porous, dense metal, and ion conductive membranes. However, the selection of the type of membranes depends on the physical states of the membrane materials and the type of molecules that need to be separated. Therefore, this chapter has been constructed to describe several conversion techniques for hydrogen evolution by water splitting and a glimpse of the several types of membrane separation for hydrogen recovery.

Hydrogen Production Techniques and Recovery  321

11.2 Conversion Technique for Hydrogen Production The evolution of hydrogen in developing high-energy green hydrogen using water splitting has attracted significant interest from academia and industry. Electrochemical water splitting has been considered as one of the most promising approaches to store renewable electricity in the form of hydrogen fuel. Water splitting refers to the chemical reaction where the water molecule is broken down into oxygen and hydrogen. There are various conversion techniques for hydrogen production by implementing the water splitting concept, such as the particulate system of photocatalysis, photoelectrochemical, photovoltaic–photoelectrochemical, and electrolysis which are discussed in the next following chapter.

11.2.1 Photocatalytic Hydrogen Generation via Particulate System Solar-driven hydrogen generation from the water via particulate photocatalysts is the most cost-effective and eco-friendly technique. It has been widely studied for solar fuel production, including photocatalytic water splitting for H2 production and CO2 photoreduction using semiconductor photocatalysis [6]. Research has gained an interest in exploring the photocatalytic water splitting technology to produce H2 since Fujishima invented it in 1972 [7]. In photocatalytic water splitting, single photocatalyst particles evolve hydrogen and oxygen, reducing solution resistances and concentration overpotentials, and using water as a reactant solution [8]. The reactions of water-splitting are summarized below.

2H+ + 2e- → H2  Reduction half reaction 2H2O + 4H+ → O2 + 4H+  Oxidation half reaction 2H2O → 2O2 + 2H2  Overall water splitting Generally, the photocatalytic hydrogen evolution processes with sacrificial reagents have been performed in tubular reactors with a compound parabolic concentrator [9]. However, the external irradiation type reactor, either a gas-closed circulation system or a gas-flow system, is required to evaluate wavelength-dependent gas evolution and quantum yield if the light intensity is irregular [10]. Water photocatalytic decomposition on semiconductor photocatalysts generates hydrogen and oxygen in three steps; (i) the energy-band-gap photon absorption to accelerate electrons from

322  Materials for Hydrogen Production, Conversion, and Storage the valence band (VB) to the conduction band (CB), resulting in electron/ hole (e/h+) pairs, (ii) photoexcited carriers are dissociated into free carriers, which then migrate to the active sites on particle surfaces to accumulate, and (iii) the initiation redox processes with these charges to generate hydrogen and oxygen with cocatalysts [11] as shown in Figure 11.1. The production of more H2 and O2 per unit irradiation interval implies a reliable photocatalytic overall water splitting reaction [12]. Furthermore, surface modification techniques for photocatalysts have been established, including cocatalyst loading, surface morphological management, surface modification, and surface phase junctions to improve charge separation and transfer by one-step excitation overall water splitting [10]. Recent study advancements in photocatalytic water splitting have indicated an increase in photocatalytic activity employing modified-­ photocatalyst compared to single photocatalyst [13]. Table 11.1 shows some comparison of photocatalytic activity using various photocatalyst for producing hydrogen that have been reported previously. Liu et al., (2015) have reported on fabrication of zinc indium sulfide with graphitic carbon nitride (ZnIn2S4-g-C3N4) by varying the loading of graphitic carbon nitride (g-C3N4) for efficient visible light photocatalytic H2 evolution. The optimal content of g-C3N4 was about 40 wt.% corresponding to 953.5 µmol/ hg of H2 produced which was about 1.91 times higher than that of pure

H+ e-

C.B.

H2

Eg

V.B.

h+

PHOTOCATALYST

O2

H2O

Figure 11.1  Model of band gap energy for photocatalytic water splitting reaction.

Hydrogen Production Techniques and Recovery  323 Table 11.1  A comparison of photocatalytic activity of hydrogen production using different photocatalysts. BET surface area (m2/g)

H2 production (µmol/hg)

Ref.

Photocatalyst

Light source

g-C3N4

300 W, Xenon (A.M. 1.5G)

26.5

108.2

[17]

ZnO BT-CCN@ ZnO

500 W, Xenon

4.50 11.51

35.8 441.4

[15]

UiO-66 Pd/UiO-66

300 W, Xenon (λ > 420 nm)

791.61 838.96

90.0 9100.0

[18]

CNO

300 W, Xenon (λ > 400 nm)

58.0

88.6

[19]

ZnO Ca-doped ZnO

300 W, Xenon (λ > 420 nm)

55.74 88.39

17.37 426.43

[20]

ZnIn2S4 ZnIn2S4-gC3N4

300 W, Xenon (λ > 420 nm)

66.9 29.8

500.0 953.50

[14]

NH2-UiO-66 NH2-UiO66/g-C3N4

300 W, Xenon (λ > 420 nm )

989.8 727.6

76.0 167.0

[21]

C3N4-TiO2

250 W, visible light source

135.0

1042.0

[22]

WO3-CoS2

300 W, Xenon (λ > 420 nm )

14.828

4423.0

[23]

CdS/Cu7S4/gC3N4

300 W, Xenon (λ > 420 nm)

30.0

3570.0

[24]

ZnIn2S4 photocatalyst [14]. In order to achieve one-step excitation total water splitting using visible light, a single photocatalyst band gap must be between 1.23 eV and 3.1 eV. In contrast, the visible-light photocatalysts can be directly used to split water in the Z-scheme [10]. Mohamed et al. (2019) discovered the heterojunction of bio-template C-doped g-C3N4 on the C, N co-doped ZnO (BT-CCN@ZnO) performed a higher rate of hydrogen

324  Materials for Hydrogen Production, Conversion, and Storage evolution at 441.4 µmol/hg compared to pure ZnO at 35.8 µmol/hg due to the excellent charge carrier separation achieved by the direct Z-scheme heterojunction structure, which prevents photoinduced electron-hole pairs from recombining compared to its single component [15]. Simultaneously, the addition of methanol acts as a sacrificial reagent that would retain the holes, preventing electron-hole pairs from recombining and increasing charge transfer at the photocatalyst/solution interface [16]. It indicates that the lower band gap, high number of active sites, and efficient charge separation efficiency of photocatalyst enhanced the performance for photocatalytic activity in producing the amount of H2.

11.2.2 Photoelectrochemical Cell (PEC) Photoelectrochemical (PEC) water splitting was discovered by Fujishima and Honda in 1970 while searching for new renewable and clean energy to fulfill the energy demand of population growth and industrial development. Their investigation found that TiO2, with a bandgap of 3 eV, can absorb photons and produce sufficient potential to split water into hydrogen, H2 and oxygen, O2. The obtained H2 is used to store energy from the sun and is called solar fuels [25]. Recent progress reported that PEC water splitting on a tantalum nitride photoanode had achieved 2.5% for solar-to-hydrogen (STH) conversion efficiency. Unlike particulate photocatalyst water splitting, photocatalysts are developed on a conductive substrate as a photoelectrode, and small voltages are applied as a bias to operate water splitting [26]. In this photocatalytic water splitting reaction, once sunlight touches the photoelectrode’s surface, the bandgap excitation occurs and subsequently produces electrons and holes inside the semiconductor photoelectrode. These electrons and holes are split up and move towards the surface of the semiconductor. When both arrived at the surface, they will be trapped in the surface active sites and finally catalyzed by water reduction and oxidation reactions [27–30]. One of the fundamental components of the PEC water splitting system is a photoelectrode that absorbs photons and uses them to generate electrons and holes. After the electron–holes pair broke up by the spacecharge field, both will move to a semiconductor-liquid interface. The holes will participate in the oxygen evolution reaction on n-type photoanode (OER). In contrast, electrons will participate in a hydrogen evolution reaction (HER) on a p-type photocathode (Figure 11.2) [31]. Thus, H2 and O2 will be produced at different electrodes, and gas separation is unnecessary. Fluorine-doped tin oxide (FTO) and indium-doped tin oxide (ITO) are commonly used as substrates for both photoelectrodes in the PEC system.

Hydrogen Production Techniques and Recovery  325 Photoanode

Photoelectrodes in PEC System

• Protection layer • Light absorber • Substrate

Similarities

Oxygen evolution reaction (OER)

Differences

Photocathode

• Protection layer • Light absorber • Substrate

Hydrogen evolution reaction (HER)

Figure 11.2  Types of photoelectrodes in PEC system.

The excellent performance of photoelectrode materials is determined by two criteria which are optical function and catalytic function. For the optimum optical function, the photoelectrode material must absorb maximum energy from the visible region of the solar spectrum. At the same time, photoelectrode with optimum catalytic function can conduct hydrogen evolution reactions and oxygen evolution reactions splendidly. Thermodynamically, the ideal semiconductor must possess a bandgap above the water-splitting reaction potential of 1.23 V vs. RHE. The suitable band gaps for ideal photoelectrode should be between 1.6–2.5 eV so that the semiconductor can provide extra energy to defeat the overpotential and other losses [32]. Other criteria must be possessed by ideal photoelectrode are i) can generate enough voltage to split water after irradiation, ii) possess bandgap that active under visible light irradiation, iii) position of band edges is always straddling with a redox potential of water, iv) stable in water medium for an extended period, v) can facilitate charge-transfer to cross semiconductor/electrolyte interface, and vi) environmentally friendly, economical and facile to process [33]. Table 11.2 shows the comparisons of the photoelectrode structure towards photoelectrochemical water splitting efficiency that was reported previously.

11.2.3 Photovoltaic-Photoelectrochemical Cell (PV-PEC) PV-PEC water splitting system is a combination of high efficiency photovoltaic solar cells system with water electrolysis. This system shows excellent performance in various aspects for solar energy conversion compared to the PEC system alone. However, this system demands a very high cost

326  Materials for Hydrogen Production, Conversion, and Storage Table 11.2  Comparison of different photoelectrodes structure used in photoelectrochemical water splitting. Morphology/ crystal structure

Method of fabrication

Electrolyte

Photocurrent (J mA−1 cm−2)

IPCE

Ref.

Compact porous walls, sponge

Anodization

1 M H2SO4

5 at 1.5 V vs. SCE

66% at 350 nm

[34]

Vertically oriented nanosheets/ monoclinic

Hydrothermal

0.1 M Na2SO4

2.3 at 1.2 V vs. Ag/AgCl

40% at 400 nm

[35]

Nanoflakes/ monoclinic

Solvothermal

0.5 M Na2SO4

0.88 at 1.0 V vs. Ag/ AgCl

85% at 300 nm

[36]

High porosity

Metal-organic chemical vapor deposition

0.1 M NaOH

4.79 at 1.23 V vs. RHE

Nanoflake arrays/ monoclinic

Hydrothermal

0.1 M Na2SO4

Nanowires

Hydrothermal followed by thermal annealing

1.0 M NaOH

0.9 vs. at 0.68 V vs. RHE

[39]

Flower-like spheres/ Hierarchical porous nanostructure

Template synthesis

0.5 M Na2SO4

−0.15 at −0.2 V vs. RHE

[40]

3D sponge-like microporous

Solvothermal

0.35 mol L−1 Na2SO3 and 0.25 mol L-1 Na2S

Crystalline nanoparticles

Hydrothermal

1 M KOH

Nanoporous structure

Conventional wet chemical etching

[37]

55% at 360 nm

1.05 at −1.5 V vs. Ag/ AgCl

[38]

47.6% at 400– 460 nm

[41]

1.28%

[42]

20%

[43]

Hydrogen Production Techniques and Recovery  327 to operate water splitting. Weaknesses in PEC water splitting, such as low light absorber and unstable semiconductors, which have a risk of corrosion and are hard to match the semiconductor band-edge energies to HER and OER, do not exist for PV-PEC water splitting system. In this system, the energy levels of bandgap in PEC cells no longer relate to water redox since PV takes a leading role in supply potential. Thus, there are no more restrictions in selecting suitable materials [44]. Three strategies have been suggested to combine the photovoltaic system with electrolytic water splitting components, including integrated PEC devices, partially integrated PEC devices, and non-integrated PEC devices [45] and are illustrated as in Figure 11.3. Khaselev and Turner (1998) had constructed the first monolithic PV-PEC devices in 1998 by combining gallium indium phosphide (GaInP2) with gallium arsenic (GaAs) in one tandem cell. The construction recorded more than 10% of STH conversion efficiency [46]. STH conversion efficiency increases up to 8.1% when tungsten oxide with bismuth vanadate (WO3/BiVO4) photoanode was combined with GaAs/InGaAsP to create an integrated photoelectrochemical catalysis device [26]. The efficiency of this hybrid system is already beyond the requirements for commercial hydrogen production. However, the manufacturing of this system is complex and costly. The cost for the hydrogen production process by this system is also more expensive than the production of the fuel by fossil.

11.2.4 Electrolysis Electrolysis is one of the standard methods to produce clean hydrogen from the water-splitting process. It involves splitting a water molecule into hydrogen and oxygen individual gas by using electricity, as shown in Figure 11.4.

(b)

Cathode

H2

Anode

O2 Electrocatalyst

Cathode

Anode

Cathode

Anode

Electrocatalyst

PV

H2

eˉ

Electrocatalyst

(a)

O2

eˉ

Electrocatalyst

PV

H2 Electrocatalyst

O2

Electrocatalyst

eˉ

(c)

Figure 11.3  Three designs of PV-PEC system. (a) Integrated PEC. (b) Partially integrated PEC. (d) Non-integrated PEC.

328  Materials for Hydrogen Production, Conversion, and Storage Mainly, this process will take place in an electrolyzer. Two conducting materials, known as electrodes, are placed in the conducting electrolyte and connecting them electronically. Both electrodes will experience a different reaction in the electrochemical cell, where reduction will occur at the cathode while oxidation at the anode. The current supply in the cell flows in electrons form and ions at the electrodes and electrolyte, respectively, to separate the electrodes. The electrolysis mechanism begins with the flow of electrons from the negative terminal of the power supply to the cathode, where the hydrogen ions receive electrons to produce the hydrogen atoms. The hydrogen ions would generally move towards the cathode, while the hydroxide ions move towards the anode [47]. The reaction that occurred at the surface of electrodes is as below:



1 Anode H 2O → O2 + 2H + + 2e − 2



Cathode   2H+ + 2e− → H2



1 Overall reaction H 2O → H 2 + O2 2

+

– e¯

Anode

Cathode

e¯

e¯

O2

H2

H2O

Figure 11.4  Schematic diagram of an electrochemical water-splitting system.

Hydrogen Production Techniques and Recovery  329 Recent progress in hydrogen production via electrolysis reaction has shown an increasing trend in the research publication, and most of the studies are from China in the field of energy. However, it has been reported that the water-splitting process only resulted in 4% of industrial hydrogen production globally due to the economic issue where higher cost was required in the electrolysis process [48]. Hence, various modifications have been employed to enhance the performance and hydrogen production via electrolysis involving acid or base electrolyte and electrocatalyst utilization. Ni@Ni(OH)2 core-shell structure formation has improved the electrocatalytic activity for oxidation and reduction reaction of water [49]. Besides, the development of bimetallic multiphase Ni-Fe-selenide nanosheet electrocatalyst resulted in the outstanding hydrogen and oxygen evolution rate, 56 mV and 342 mV, respectively, together with good durability [50]. Furthermore, the hydrogen and oxygen collected are 1.02 and 1.90 mmol/h, respectively, which meet almost 100% of Faradaic efficiency. This is due to the rich active sites available for the reaction to occur and their metallic properties, which promotes the electrons transfer and thus contributes to the enhanced performance in electrolysis. Another study on the 3D hierarchical porous Co and S modified Ni microsphere array shows current densities of 10 and 20 mA/cm2 at 1.57 and 1.63 V, respectively [51]. In addition, this electrocatalyst can also retain the electrocatalytic activity for over 100 h. This superior performance may be attributed to the electrocatalyst’s high electrical conductivity and its ability to optimize the binding energies for water dissociation and hydrogen adsorption/desorption on the electrocatalyst due to the synergistic effect of Co and S in the compound. The most recent development on the combination of two major hydrogen production technologies, methane steam reforming and water electrolysis, forming a hybrid system, demonstrated that about 82.1% of hydrogen efficiency was achieved with 4.2% more hydrogen was produced than the conventional system. In summary, these modifications would create a new path for developing an abundant and low-cost catalyst for electrochemical water splitting to produce green hydrogen.

11.3 Hydrogen Recovery Using Membrane Separation (H2/O2 Membrane Separation) Gas separation using membrane technology is rapidly becoming a mainstream separation technology for industrial gas separation. It is one of the most recent advanced technologies and is recognized as a viable and economical unit operation. It offers lower overall gas processing costs than

330  Materials for Hydrogen Production, Conversion, and Storage conventional separation techniques such as chemical and physical processes. Membrane gas separation also offers relatively simple operational procedures, process flexibility, and low energy consumption. The membrane is also known for its capability to separate gas of different size shapes, polarity, and simplicity in its design. Hydrogen separation membranes may be classified into several categories: polymeric membranes, porous membranes, dense metal membranes, and ion-conductive membranes, which have been discussed in the following content.

11.3.1 Polymeric Membranes Polymer membrane technologies have been improving over the years in terms of cost and usability. The usage of hydrogen as a chemical fuel also helps with the advancement of membrane technology, with researchers rushing to fabricate better polymeric membranes in terms of better H2/ O2 separation. A dense polymer membrane structure has been used considerably for various gas separation processes [52]. Hydrogen permeation through the membrane is achieved through a solution-diffusion mechanism where permeants are dissolved and then diffuse through the membrane down a concentration gradient. This mechanism typically involved three steps which are firstly the penetrants dissolving into the membrane. Then it diffuses through the membrane, and lastly, it desorbs on the permeate side of the membrane. The permeation and selectivity of the ­hydrogen/ oxygen by solution diffusion mechanism rely on the solubility and diffusivity of penetrants in the membrane [53]. Notable improvements mainly advance the current membrane situation for gas separation towards the membrane materials and fabrication technology. The breakthrough of polymeric materials has opened up the technology to fabricate polymeric membranes for hydrogen purification. There is a lot of polymer material that can be used for developing a membrane for hydrogen separation. It depends on the operating conditions that the membrane will be used [54]. A suitable membrane for any application has some features that need to be identified. Membrane with higher porosity will provide higher contact surface area for gas or water separation. More than that, a suitable membrane also should have higher polymer strength in terms of elongation, high burst, and collapse pressure, as good polymer flexibility is also one of the critical properties to determine the strength of the membrane [55]. Apart from those features mentioned, the most significant membrane property that dragged attention from many fields is

Hydrogen Production Techniques and Recovery  331 the fabrication cost of the membrane. Naderi et al. (2019) reported using a dual-layer hollow fibre membrane using sulfonated polyphenylsulfone/ polybenzimidazole for hydrogen purification at elevated temperatures. The selective layer of polyphenylsulfone/polybenzimidazole leads to an increment in H2 permeant [56]. Meanwhile, Li et al. (2018) reported on applying Nafion membrane as a compartment cell separator between H2 evolution from the photocatalytic process and O2 production by phenol oxidation [57]. The increment of 20% of H2 evolution was reported in this study. Marino et al. (2018) also used the Nafion membrane as a separator between H2 and O2 evolution through water splitting [58].

11.3.2 Porous Membranes The porous membrane is made out of carbon, ceramic, and metal that can separate hydrogen from oxygen by allowing hydrogen molecules to pass through the pores membrane, acting as a molecular sieve while leaving behind larger molecules that cannot pass through the membrane pores with its efficiency depends on pore sizes and its distribution [55]. A thin membrane with no defect is hard to fabricate to achieve a high level of hydrogen separation. Supported liquid membrane (SLM) is a thin liquid barrier saturated into porous supports favoring specific molecules’ selective route. SLM has flux compared to the solid membrane as gas can easily diffuse through liquid rather than solid [59, 60]. Figure 11.5 (a) illustrated the SLM that is used to separate hydrogen and oxygen after photocatalytic water splitting. 1H, 1H, 2H, 2H perfluoro-1-octanol (PFO) is a perfluorocarbon (PFC)-based SLM impregnated in porous alumina support. PFC has unique properties, including hydrophobicity that repels anion, low vapor pressure, high solubility for gases due to the membrane being l­iquid-based, and excellent chemical and thermal stability [61]. Flow rates and temperature inlet of the hydrogen and oxygen mixture reduces the membrane lifespan because water was lost more rapidly from the support but did not affect its separation performance. Overall, PFO had an average H2/ O2 selectivity. Figure 11.5 (b-c) shows the data of PFO in the gas separation. Another PFC-based SLM, perfluorotributylamine (PFTBA), supported on porous alumina, is used to compare different PFCs. SLM can give different outcomes on H2/O2 separation [62]. Temperature and gas flow rates affect the performance of the membrane by a little while still reduced the lifespan of the membrane.

332  Materials for Hydrogen Production, Conversion, and Storage (a) Feed

Porous support Liquid membrane

Permeate

(b) 10-7

(c) 1000

Temperature / ºC 16

100

10-10 10-11

10

H2

10-12

O2 selectivity

10-13

1 0

20

40 60 80 Total H2-O2 partial pressure / kPa

100

25

50

75

100

50 75 Flow rate / cm3min-1

100

Temperature Flow rate

14 Rate of liquid loss / 104 g min-1

10-9

Maximum H2/O2 selectivity

Permeance / mol m-2 s-1 Pa-1

10-8

12 10 8 6 4 2

25

Figure 11.5  (a) Supported liquid membrane (b) H2 and O2 permeance and maximum H2/ O2 selectivity at different partial pressures and (c) rate of liquid loss at different conditions. Reproduced from ref. [62] with permission from Elsevier.

11.3.3 Dense Metal Membranes Membrane technology has been developed and can be utilized in various applications with many benefits such as low energy consumption, continuous operation, and the chance to merge with other separation technologies. In general, a membrane is a barrier that separates the gas mixture from any unwanted components according to the nature of the gas and the interaction between the gas and the membrane. Dense metal or heavy metal refers to the metallic element with high density and high atomic mass and numbers. Several defects on the lattice structure, including vacancies, dislocations, as well as the metal and hydrogen interaction, would influence the overall process. Metal such as Fe, Nb, Ta, and V with body-centered cubic (BCC) lattices displays high hydrogen permeability. At the same time, metal such as Ni and Pd with face-centered cubic lattices also exhibits good hydrogen permeation. In the case of hydrogen recovery using a dense metal membrane, a few mechanisms have been developed as follows: • Higher pressure is required for the hydrogen molecule to diffuse over the surface of the membrane;

Hydrogen Production Techniques and Recovery  333 • The hydrogen molecule will dissociate and being adsorbed over the surface of the metal, followed by the diffusion of hydrogen atoms in the lattice; • At lower pressure, the hydrogen atoms desorbed from the surface of the metal and then recombined to form molecules; • Finally, the diffusion of hydrogen molecules takes place over the surface of the membrane at lower pressure. The hydrogen selectivity is high due to reaction mechanisms that include the conduction free electrons and a distinct catalytic surface to allow hydrogen dissociation on the raw feed streamside. In contrast, the recombination of protons and electrons takes place on the other product side. Therefore, other molecules such as O2, CO, CO2, CH4, or N2 could be inhibited. The dense and no defect present in the metal membrane have shown a boundless selectivity of hydrogen. One of the most common dense metals used as a membrane is palladium due to its high permeability and selectivity for hydrogen recovery [63, 64]. However, due to its high cost, various alternatives have been developed. The incorporation of Ta in the Pd-based membrane enhances the stability and increase the permeability, and Ta also possesses a very high melting point, hence making it a suitable addition to decreasing the use of noble metal. Besides, Pd-coated tubular vanadium also demonstrated a highly permeable and high selectivity toward hydrogen [65, 66]. In summary, these modifications would create a new path for developing abundant and low-cost catalysts for hydrogen recovery.

11.3.4 Ion-Conductive Membranes Ion conductive membrane is a semipermeable membrane generally made from ionomers and designed to conduct ions while acting as a barrier to other substances. This ion-conducting membrane can be divided into the proton-conducting membrane or anion conducting membrane. The essential function of the ion-conducting membrane in the hydrogen production system is as a separator of the hydrogen and oxygen evolved at the photo-anode and cathode surface, respectively, and provides continuity for the transfer of ions (protons or hydroxides) between the two cell compartments. Ion conducting membranes can be made from either pure polymer membranes or composite membranes. The performance of the ion-­conducting membrane is usually tested based on the ion conductivity, gas permeability, physicochemical properties (water uptake, IEC, swelling), and membrane stability (thermal, chemical, mechanical) [67].

334  Materials for Hydrogen Production, Conversion, and Storage Ion  conducting membrane having high ion conduction properties is in strong demand for the hydrogen production system. Typically, the ion conductivity of the membrane depends on various factors such as membrane structure, water uptake, ion exchange capacity, and functional group for ion carriers. It is believed that ion transportation required water as an assisted medium. Increased membrane water uptake could enhance ion mobility and develop an ion-conducting pathway in the membrane. In hydrogen evolution, the ion-conducting membrane was developed to be applied in the PEC cell system. The most and common commercially available proton-conducting membranes (PEM) are fluoropolymer (PFSA) and Nafion from DuPont [68]. Nafion has high thermal stability, distinguished ionic exchange capacity, and proton conductivity. PEM can allow protons to pass through while being nonelectronically conductive itself, and Nafion-based PEM is the best example [69]. Nafion has excellent chemical resistance, good mechanical properties, and is highly durable. Thus, it is stable when it undergoes radical degradation and peroxide ions (H2O2). However, Nafion is expensive and cannot operate smoothly at a temperature above 80°C as it will decrease its proton conductivity performance [70]. In contrast, the ion-conducting membrane acts as a solid electrolyte for ion transportation medium and H2/O2 separation. Amano et al. (2016) reported using Nafion membrane as a separator in PEC cell to test the

Carrier gas + H2O Photoanode e-

Counter electrode Pt/C

UWC

Carrier gas + H2O

H2

O2

H+

UWR

h+

Polymeric membrane

OH-

Reference Electrode Pt/C H2

Figure 11.6  Configurations of the membrane photoelectrode assembly utilizing either proton or anion conductive membranes. Reproduced from ref [72] with permission from Elsevier.

Hydrogen Production Techniques and Recovery  335 performance of rutile titanium dioxide (TiO2) thin layer as a photoanode in PEM-PEC cell. It shows that the Nafion membrane exhibit an excellent membrane separator for hydrogen evolution [71]. Figure 11.6 shows the configurations of the membrane photoelectrode assembly utilizing either proton or anion conductive membranes proposed by Zafeiropolous et al. (2018) [72]. The study used the commercial Nafion 117 membrane as a proton-conducting membrane and low-density polyethylene as an anion conducting membrane. Other polymers like polyethersulfone (PES) also have been reported to replace Nafion membrane in the PEM-PEC system. The manufacturing cost is also less costly than Nafion because the aromatic polymer ring is cheaper to produce and can deliver enough physicochemical properties [73].

11.4 Conclusion As discussed above, there are a few conversion techniques of hydrogen production for water splitting; photocatalytic hydrogen generation, PEC, PV-PEC, and electrolysis. Significant advancements in different H2 generation technologies from renewable resources such as biomass and water are being studied to reduce fossil fuel usage. However, materials, composition, chemical stability, system design, and fabrication procedures demand significant effort. Developing H2 from various fuel sources and techniques may enable every world region to self-supply energy. Membrane separation techniques (for O2, H2, and CO2) can enhance fuel conversion efficiency and minimize the negative environmental impacts of energy production. Gas separation membranes have been acknowledged as an effective and environmentally acceptable solution for obtaining hydrogen from biomass systems. Organic and inorganic membranes have been explored as hydrogen separation membranes, and currently, inorganic membranes attracted more attention in high conditions. Four membrane types have been established and explored for H2 separation, recovery, and purification: polymeric membranes, porous (ceramic, carbon, metallic) membranes, dense metal membranes, and ion-conductive membranes. Membranes are beneficial for increased productivity, selectivity, and resistance to hydrocarbons and other chemicals. Hence, nanotechnology can increase the endurance and stability of polymers, notably in self-healing polymer structures and integrity. However, this involves specifying the optimum operating temperature of the membrane material for integration into a process, determining surface exchange kinetics (for catalyst layers and electrodes) with gases, and establishing long-term material stability in real-world working settings

336  Materials for Hydrogen Production, Conversion, and Storage (temperature, pressure, contaminants). Any use of pollution wastes from industries and low-cost renewable biomass from animals or plants as sacrificial electron donors in water splitting systems is recommended.

Acknowledgements The authors would also like to acknowledge the financial support from the Ministry of Higher Education (MOHE) Malaysia under the Fundamental Research Grant Scheme (FRGS) (Project code: FRGS/1/2020/STG04/ UKM/03/2).

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12 Geothermal Energy-Driven Hydrogen Production Systems Santanu Ghosh and Atul Kumar Varma* Coal Geology and Organic Petrology Laboratory, Department of Applied Geology, Indian Institute of Technology (Indian School of Mines) Dhanbad, Jharkhand, India

Abstract

The present investigation reviews the applications of geothermal energy to produce hydrogen and their influences on the environment and future economy. Although economically viable hitherto, the use of fossil fuel resources has been targeted to alleviate due to increasing concerns about environmental pollutions and global warming. Also, due to declining conventional energy resources, there is an urge to boost the applications of renewable energy resources, like wind energy, solar power, hydro energy, as well as geothermal energy. These have minimal adverse environmental effects and hence, are considered as green energy. Geothermal energy, although restricted in tectonically active regions, has been increasingly employed to produce hydrogen, electricity, to supply cooling, space heating, and in many other appliances. This chapter presents geothermal energy-­ driven hydrogen production employing core geothermal power plants, integrated geothermal and solar systems, multigeneration systems, and other advanced technologies. The applications of different working fluids, driving units, thermodynamic efficiencies, amount and cost of hydrogen production, and their environmental impacts are illustrated sequentially. This chapter may opine although several advancements are made in these systems, an array of further technological augmentations are required for producing hydrogen on an industrial scale as a principal energy carrier in the upcoming hydrogen era. Keywords:  Geothermal energy, hydrogen production, hydrogen liquefaction, organic rankine cycle, working fluids, electrolysis, economy, environmental impacts

*Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (343–396) © 2023 Scrivener Publishing LLC

343

344  Materials for Hydrogen Production, Conversion, and Storage

Abbreviations $ United States Dollar (currency) € Euro (Currency) µW Microwatt AMIS Abatement of mercury and hydrogen sulfide Cost of energy required for driving the procedure CE Cost of production facility CF Cost of hydrogen energy CH Cost for maintenance CM Costs of operation CO Carbon dioxide CO2 Cost of production CP Cost of hydrogen storage CS Cost of transportation CT Water cost CW EES Engineering Equation Solver e- Electron g gram GJ Gigajoule GPP Geothermal powerplant GW Gigawatt h Hour H2 Hydrogen H2O Water HOT ELLY High-temperature steam electrolysis K Potassium kg kilogram km kilometer kW kilowatt kWh kilowatt-hour Liquid hydrogen LH2 m Meter MW megawatt MWth megawatt thermal O2 Oxygen Hydroxyl ion OH- ORC Organic Rankine Cycle PEM Proton Exchange Membrane PV Photovoltaic

Geothermal Energy in Uprising Hydrogen Era  345 RTV Rankine-Trough-Vapor s Second TH Thorium U Uranium VTR Vapor-Trough-Rankine

12.1 Introduction Hydrogen is deliberated as the utmost profuse element, followed by helium, and hydrogen gas is a colorless and non-toxic gas. In the near future, hydrogen may replace fossil fuels and is anticipated to play a critical function in energy production and distribution. Hydrogen is getting popularity as one of the potential energy carriers to shift towards a hydrogen-based economy from a fossil-fuel-based economy. As of now, fossil fuels are deliberated as the principal energy resources as well as energy carriers. However, fossil fuels have lots of adverse effects on the environment. Moreover, fossil fuel resources are non-renewable, and hence, their supply is limited. These concerns are forcing the world to look for non-renewable energy resources as they are green and their supply is relatively unlimited. These renewable energy resources incorporate solar power, hydropower, wind energy, and geothermal energy [1–4]. Hydrogen is synthesized from an array of methodologies using conventional fossil fuels, such as steam reforming process of low molecular weight hydrocarbons, partial oxidation reaction of high molecular weight hydrocarbons, coal gasification, methane decarburization, etc. [5–17]. The renewable energy-based hydrogen production systems include photolysis, water electrolysis, thermochemical water-splitting, fast pyrolysis, gasification, and biomass acid hydrolysis with aqueous phase reforming, biomimetic photosynthesis, dark fermentation, biophotolysis, photofermentation, microbial electrolysis, etc. [18–28]. Water electrolysis offers the most common path for producing hydrogen as this can be easily integrated with renewable energy resources [29, 30]. Electrolysis involves a non-­ spontaneous procedure driven by a power supply from external sources to the system [31]. The applications of renewable energy resources require significant development in infrastructural, technical, and economic aspects to augment the pace of their utilization [32]. Meanwhile, storing the produced energy efficiently for future applications is the most crucial concern about using non-renewable energy resources [33]. Hydrogen production from non-renewable resources may serve as a possible solution to alleviate

346  Materials for Hydrogen Production, Conversion, and Storage this issue [34–36]. Hydrogen production utilizing non-renewable energy resources offers a promising pathway towards the hydrogen-based economy and green future because of cost-effective and clean methods for producing hydrogen [37–40]. Among the other green renewable resources, geothermal energy is being widely used in the production of hydrogen as this energy barely depends on ambient conditions [41] and can deliver almost stable and constant load at throughout the year [42, 43]. Geothermal energy originates from the interior of the earth. The radioactive decay of the radioactive isotopes (K40, Th232, U235, and U238) in the earth’s lithosphere accompanied with the residual heat from planetary accretion event source the geothermal energy [44]. Rocks and subsurface water usually absorb heat from the magma floating in the earth’s mantle, and the hypersaline water that surfaces in the forms of steams or hot springs serves as the geothermal energy resources. Geothermal energy is applied to produce hydrogen, generate electricity, hydrogen liquefaction, and storage, space heating, cooling, direct heating, domestic and industrial purposes, agriculture, honey processing as well as milk pasteurization processes, etc. [45–59]. Geothermal energy is considered as a clean and green method of producing electricity and supplying thermal energy. Geothermal powerplants (GPPs) use high-temperature geothermal resources (or high enthalpy reservoirs) to generate electricity with negligible air pollution. The most common GPP types include flash steam, dry steam, and binary types [60–63]. Geothermal energy-driven hydrogen production mostly includes the application of the proton exchange membrane (PEM) electrolyzers. This electrolysis unit splits water into hydrogen and oxygen. The geothermal water temperature entering into the electrolyzers influences hydrogen production, power requirement, and operation cost [62]. Hydrogen production driven by geothermal energy is turning into the primary energy sector in the countries having an abundant supply of the geothermal energy [64]. Moreover, hydrogen production plants are combined with other power production systems to develop cogeneration units, which provide two kinds of outputs, i.e., hydrogen and electricity [65]. Therefore, the insertion of a hydrogen production system augments the overall efficiency of a powerplant. Conventional GPPs do not entirely consume the heat carried by the geothermal fluids. This remaining heat can be extracted from the geothermal fluids prior to its reinjection back to the subsurface of the earth. The electrolysis method for hydrogen production offers a promising solution to recover the excess heat [66]. Thus, the geothermal energy can be perfectly integrated with hydrogen production from environmental, economic, and thermodynamic points of view. The electrolysis process

Geothermal Energy in Uprising Hydrogen Era  347 requires electricity that can be supplied by the GPPs. On the other hand, the remaining heat from the geothermal fluid leaving the powerplants can be utilized to preheat the water arriving at the electrolysis unit [62]. This work offers a comprehensive review of the production of hydrogen utilizing the geothermal energy resource. This review encompasses the suitable technologies required for hydrogen production and the parameters that influence these technologies, like types of working fluid, flow rate, temperature of the geothermal fluid, etc. The thermodynamic and economic analyses of these hydrogen production systems reported by several authors have been illustrated. Further, the influences of multi-generation systems on enhancing the overall efficiency of the hydrogen production plants are discussed. Moreover, the costs of the hydrogen production utilizing these technologies as well as their augmentations to alleviate the production costs are noted down in detail. Besides, the environmental concerns regarding using the geothermal fluids and their plausible solutions are penned down in this review.

12.2 Hydrogen – A Green Fuel and an Energy Carrier Hydrogen is deliberated as a potential carrier of energy, which is easy to be stored and shipped. Also, hydrogen can be utilized as a fuel, and it can even be converted into electrical energy within fuel cells [67]. Hydrogen is a green fuel, which has hardly any adverse effects on the environment and can replace fossil fuel as the primary energy carrier in the near future. Hydrogen has many advantages to be used as a green fuel, which were reported by several scientists [68–71]. Hydrogen can be produced from water, a widely available source, and also from hydrocarbons and non-­ hydrocarbons. Besides, it is environmentally benign as its utilization includes direct oxidation, which produces water. Combustion of hydrogen in air yields a small amount of nitrogen oxide. However, that can be limited by improving the design of the engine [71]. Hence, hydrogen is the recyclable fuel where hydrogen is oxidized to water, and then that water is used as the source to manufacture the hydrogen again. Moreover, hydrogen is storable in different forms in large amounts and shippable through conventional transport systems. Hydrogen can be transported over long distances employing conventional pipelines with losses lesser than that acquired for transportation of electricity through electric wires [67]. It is used as a chemical feedstock in the refinement process of metallic ores, upgradation of tars and heavy oils, and in shipment [72, 73], and it is also used in different commercial and residential applications [74]. Additionally, hydrogen

348  Materials for Hydrogen Production, Conversion, and Storage energy systems include several synergisms. When hydrogen is used as a carrier of energy in a system, it can satisfy the other demands of the system [67]. On the other hand, hydrogen has some disadvantages as well. It is a bit costly to manufacture hydrogen to be used as a carrier of energy in comparison to conventional fossil fuel systems. Hydrogen storages have lower densities of energy storage compared to gasoline storages on a volume and mass basis, which may adversely influence the application of hydrogen in automotive industries as a fuel. Besides, hydrogen may seep from containment containers because of its small molecular size and low density [67]. Further, Cox and Williamson [75] reported that some alloys became embrittled in the presence of hydrogen. Hydrogen also has hazardous properties [75]. In 1937, at Lakehurst in New Jersey, the Hindenburg dirigible accident led to coining the term “Hindenburg Syndrome”, which denotes the fear of something that involves any role of hydrogen [67]. Meanwhile, hydrogen is also a safer fuel compared to methane, gasoline, coal, natural gas, etc. Further, hydrogen is safer than gasoline and methane in terms of flame emissivity, exploration energy, and flame temperature. As it is non-toxic, the leak of hydrogen hardly imparts any adverse effects on the environment. Also, it dissipates quickly, minimizing the risk of an explosion or any other mishaps [76, 77]. Hence, concerning the adverse environmental issues arising from the uses of fossil fuels as well as continuous depletion in fossil fuel resources, the world requires evolving from the fossil fuel age to the age of hydrogen. The world will climb to the age of hydrogen once the conventional fossil fuel systems turn too costly to recover in economic means and/or there are significant transformations in energy production systems [67]. Hydrogen can be yielded from renewable energy resources at a low cost [30], which is aimed to be the principal energy carrier in the age of hydrogen. Scot [70] implied that the hydrogen era, once established, will be extant for a long time in the near future.

12.3 Production of Hydrogen 12.3.1 Fossil Fuel-Based Hydrogen is produced by several methods depending on the uses of energy sources. These methods involve pyrolysis [78, 79], thermolysis [80, 81], reforming [82, 83] and electrolysis [84, 85], with heating, electricity, and refining form the base of these methods [62]. Hitherto, hydrogen

Geothermal Energy in Uprising Hydrogen Era  349 is produced dominantly from fossil fuel sources [86–89]. Following the works of several scientists [6–9, 12, 14, 16], the primary methods of yielding hydrogen are listed below: • Partial oxidation reaction of high molecular weight hydrocarbons (coal and oil). • Gasification of coal [11, 13, 15, 17]. • Steam reforming process of low molecular weight hydrocarbons (natural gas, naphtha, etc.) • Methane decarburization [5, 10]. Hydrogen is dominantly manufactured from methane through steam methane reforming. Also, other fossil fuels like coal, gasoline, propane, etc., are also employed in steam reforming to produce hydrogen. This method offers less pollution for obtaining hydrogen from fossil fuel resources [90]; also, the efficiency of this process peaks up to 70%–80% [30]. The steam methane reforming process employs a catalyst to generate a reaction between high-temperature steam and methane, producing hydrogen, carbon dioxide, and carbon monoxide. Subsequently, the catalyst, steam, and carbon monoxide react to generate more carbon dioxide and hydrogen. After that, the impurities and carbon dioxide are removed, to obtain the pure hydrogen. The hydrogen produced by using fossil fuels is called as the grey hydrogen, but from this process, a large amount of carbon dioxide is produced, which has a significant global warming potential and thus, imparts adverse effects on the environment. Meanwhile, if the carbon dioxide is captured and stored in this reaction process, the produced hydrogen is called as the blue hydrogen. Additionally, the production of hydrogen from biomass and fossil fuels at high temperature involves the gasification process [91], and the gasification temperature depends on the energy sources, like coals and other carbonaceous materials [62]. In this method, the presence of steam or oxygen is mandatory in most scenarios. The gasification of biomass at temperature >374°C is termed as the supercritical water gasification [92]. This process yields hydrogen gas and carbon dioxide.



CO + H2O ⇋ CO2 + H2#

(12.1)

12.3.2 Non-Fossil Fuel-Based Photolysis method uses sunlight directly to separate hydrogen and oxygen from water [21]. Meanwhile, this process is less commonly applied,

350  Materials for Hydrogen Production, Conversion, and Storage and scientists are on their way to look for potential catalysts to enhance this method [62]. On the other hand, water-splitting is usually conducted by electrolysis [93], and this method utilizes either of two pathways, (a)  proton exchange membrane route (PEM; Figure 12.1) [26, 48], and (b) alkaline electrolytic pathway [28]. The proton exchange membrane pathway needs input electricity to activate the splitting procedure and depends reasonably on the temperature of water inflowing the electrolyzer. On the other hand, the alkaline electrolysis method requires a strong base as an electrolyte [62]. PEM pathway:

1 H 2O + Heat electricity → H 2 + O2 2



(12.2)

Alkaline electrolysis:

2H2O + 2e− → H2 + 2OH−#

+

(12.3)

−

Membrane O O

H+ H+ H+

H

H H H

H+ H+

H+ O H H

O H H

Anode

H+

Cathode

Figure 12.1  Working principle of a proton electron membrane (PEM) electrolyzer (following Karapekmez and Dincer [47]; reuse of this figure is permitted by Elsevier and Copyright Clearance Center; License Number: 5042511369352).

Geothermal Energy in Uprising Hydrogen Era  351



1 2OH − → O2 + H 2O + 2e − # 2

(12.4)

The water electrolysis may be the most common pathway for producing hydrogen because of its easy integration with renewable energy resources, like wind, solar, and water [29, 30]. Besides, thermochemical water-splitting that comprises a sequence of reactive pathways is a significant method for the large-scale production of hydrogen [20, 22–25]. This process allows thermal decomposition of water at lower temperatures (850°C–1000°C) compared to that requisite for direct water decomposition to produce hydrogen and oxygen (>2500°C) [67]. The high temperature required for this process may come from nuclear power plants [6, 94–99] and/or from solar thermal plants [100, 101] and/or their integration in a combined energy system [102]. The copper–chlorine cycle and the sulfur–oxygen-­ iodine cycle that require temperature of ~550°C and ~1000°C, respectively, are the two main examples of such systems [67]. Hydrogen production from biological processes includes fast pyrolysis, biomass acid hydrolysis with aqueous phase reforming, and gasification [27]. Synthetic or biomimetic photosynthesis [18], dark fermentation, biophotolysis, photofermentation, microbial electrolysis [19], etc., methods were also proposed for hydrogen production, but these have limited applications due to technological barriers until now [67]. Hydrogen generated from renewable energy resources like geothermal energy, solar power, wind energy, etc., becomes free from any pollutant and is, therefore, called green hydrogen. Hence, green hydrogen is the most efficient component in the emerging age of hydrogen in the near future. Further, among all the renewable energy resources, geothermal energy is deliberated as the most reliable and stable source as it is mostly independent of ambient conditions [41]. This energy is a zero–CO2 energy source and can supply an almost constant and stable load during the day compared to the solar and wind energies [42, 43]. A carrier for heat transfer is required to carry the subsurface geothermal energy to the surface [103, 104]. This heat transfer mechanism involves two-step process; at first, the heat is transferred in the hot rocks through conduction, followed by the delivery of hot water to the surface through the convective heat transfer mechanism [105]. The leading geothermal power plants include flash steam, dry steam, and binary types [60–63]. The flash steam and dry steam plants directly use the geothermal fluid on the basis of prevailing conditions and constraints. The availability of the geothermal fluid only as steam triggers the use of the dry steam cycle. Flash cycles are either single or double, or multiple based

352  Materials for Hydrogen Production, Conversion, and Storage on number of separators used in the process [62]. Binary plants do not use the geothermal fluid directly; instead, these work on the basis of triggering an organic ranking cycle (ORC) when a low-grade geothermal source is used [106]. These powerplants include regenerator and recuperator as the internal heat exchangers. These heat exchangers exchange the heat between geothermal fluid that excites the turbine, and that excites the pump [62]. Besides, a flash-binary cycle (Figure 12.2; [62]) can be used to enhance these geothermal power plants, in which the binary cycle constitutes the bottoming cycle and the flash-steam forms the topping cycle [107, 108]. According to the IEA [109], the worldwide production of power from geothermal energy was 14 GW in 2017, which is anticipated to augment up to 17 GW in 2023. A brief account of geothermal energy and its occurrences is illustrated in the following section, which is followed by the discussions regarding hydrogen production using geothermal energy.

Geothermal fluid Water Produced gas Electricity ORC fluid

Generator

Flash separator Steam turbine

ORC turbine

Condensor

Generator Pump

Heat exchanger

Water

Production well

Water preheater

Electrolyzer

H2 O2

Injection well

Figure 12.2  Flash-binary geothermal cycle for hydrogen production (following Mahmoud et al. [62]; reuse of this figure is permitted by Elsevier and Copyright Clearance Center; License Number: 5042501405896).

Geothermal Energy in Uprising Hydrogen Era  353

12.4 Geothermal Energy 12.4.1 Introductory View Geothermal energy, as it goes by its name (geo (earth) and thermé (heat)), denotes the energy within the subsurface of the earth. The earth is built up with four major layers, and these are inner core (plasma behaving like a solid), outer core (liquid), mantle (comprises magma), and crust (continental and oceanic), and each of these layers has unique physical state and chemical composition. The earth’s mantle is mostly made up of magma, the hot-molten or semi-molten rocks. The temperature of the mantle ranges from ~1000°C near the earth’s crust to ~3500°C near the mantle-core boundary. The flow of heat from earth’s warm interior to the surface is represented by the geothermal gradient. It is defined as the increase in temperature with an increase of depth below the surface of the earth [44]. It peaks at the mid-oceanic ridges and lowers at the subduction zones [44] with an average value of 25°C/km depth. Earth’s internal heat or geothermal energy is derived from a combination of heat originated through radioactive decay of radiogenic isotopes (uranium, thorium, and potassium) and residual heat from planetary accretion [44]. The radioactive moieties (K40, Th232, U235, and U238) are found usually in the earth’s lithosphere. The decay of radiogenic K40, U235, U238, and Th232 isotopes produces heat around 29.17, 568.70, 94.65, and 26.38 µW/kg, respectively [110]. Now, the earth’s uppermost mantle and crust form the lithosphere; both oceanic and continental lithosphere are broken into tectonic plates, which are irregularly shaped, massive slabs of solid rocks floating over the magma. Magma reaches close to the earth’s surface near the edges of these tectonic plates resulting in volcanism. Rocks and subsurface water usually absorb heat from the magma, and the rocks and water found deeper within the earth’s subsurface have a high temperature (Figure 12.3, [111]). Hence, hyperthermal water existing deep below the earth’s surface generally forms geothermal energy resources. This hot water comes out on the surface as hot springs (geysers) or steams. Wells are usually dug into the underground reservoirs of geothermal energy to access the hot water and steam, which are then employed to drive turbines integrated to electricity generators to produce power. Geothermal energy is the primary form of energy and is a renewable energy source. This energy is used for direct heating, space heating, and cooling, agriculture, aquaculture [112], generating electricity [45, 49], honey processing, and milk pasteurization processes [50]. The geothermal energy can be used continuously at any day of a year, at any time of a day, as it is not intermittent like wind and solar energies.

354  Materials for Hydrogen Production, Conversion, and Storage

Recharge area

Geothermal well

Hot spring/ steam vent

Cold meteoric water Hot fluids

Impermeable caprocks Reservoir

Conductive heat flow Impermeable rocks

Geothermal energy

Magmatic intrusion

Figure 12.3  Geothermal energy field (following Barbier [111]; reuse of this figure is permitted by Elsevier and Copyright Clearance Center; License Number: 5061240401821).

12.4.2 Types and Occurrences The geothermal energy is not feasible in all countries and can be harnessed easily in the areas that are close to geothermal hot spots [113] or in the tectonically active regions. The geothermal reservoirs are discriminated on the basis of the temperature of the area and geological properties [114]. Hence, geothermal energy resources are classified into two groups: (a) high-enthalpy and (b) low-enthalpy reservoirs [115]. The high enthalpy reservoirs are characterized by temperature above 200°C and found at a depth of around 1000 m, while the low enthalpy reservoirs are characterized by temperature below 150°C and are encountered at the shallower depth [116]. The countries those are dominantly working in the geothermal energy aspects include Iceland, New Zealand, Kenya, Philippines, Mexico, Italy [116], United States, Hungary, Turkey, and China. In India, geothermal resources are marked on the earth’s surface in the forms of hot springs and geysers, and the temperature of these resources vary from 32°C–97°C [115]. Geological Survey of India had identified ~340 hot springs clustering in the northeast-south west trending

Geothermal Energy in Uprising Hydrogen Era  355 Himalayan province that is stretched to the Son-Narmada-Tapti lineament, Andaman-Nicobar island, west continental coast, Delhi fold belts, and Gondwana grabens [116]. In India, the hot springs are classified into seven regions (a) Sahara Valley (Aamby Valley, Pune), (b) Son-NarmadaTapti lineament, (c) Chhumathan (Himalaya), (d) Godavari Basin, (e) Puga, (f) Mahanadi Basin, and (g) West coast [117]. These hot springs are extended from Jammu and Kashmir in the north to Tamil Nadu in the south and Gujrat in the west to Arunachal Pradesh in the east. The production potential of these geothermal provinces is around 10,600 MW [116]. Besides, Singh et al. [116] reported that the geothermal provinces in India are divided based on the resource temperature. The moderate enthalpy reservoirs (100°C–200°C) are found in Puga-Chhumathan, Beas, Parbati, and Satluj of the Himalayas. In these areas, these resources are linked to the granite intrusive bodies. In the Son-Narmada-Tapti Lineament zone, west coast of Maharashtra, Rajgir-Monghyr in Bihar, Tatapani in Madhya Pradesh, and Eastern Ghats of Odisha; the moderate enthalpy reservoirs are linked with the tectonic lineaments. Moreover, these moderate enthalpy geothermal resources are encountered in the rifts and grabens of the Gondwana Basins and quaternary and tertiary sediments at the west coast of the Cambay Basin [116, 118, 119]. The low enthalpy reservoirs (420

(NH4)2SO3

40.18

[141]

TaON

Ni(OH)2

Vis light >400

Methanol

31.5

[142]

CdS

B-Ni(OH)2

300 W Xe lamp

NaOH & Ethanol

35000

[143]

CdS-TiO2

CNT-Ni(OH)2

300 W Xe >420

Lactic acid

1187

[144] (Continued)

604  Materials for Hydrogen Production, Conversion, and Storage

Table 19.1  Summary of the previously (last 10 years) reported Ni and Ni based co-catalyst for hydrogen production [89, 135–179]. (Continued) Photocatalyst

Co-catalyst

Light source (nm)

Sacrificial reagent

H2 production rate (µmol h−1 g−1)

Reference

Red P

Ni(OH)2

300 W Xe >400

Methanol

33

[145]

SnO2

Pt/Ni(OH)2Ni2O3

300 W Xe >420

Ethanol

10.8

[146]

WO3/g-C3N4

Ni(OH)2

300 W Xe >420

TEOA

576

[147]

ZnIn2S4

Ni(OH)2

300 W Xe >420

TEOA

8350

[148]

CdZnS

Ni(OH)2

300 W Xe >420

Na2S+Na2SO3

3774

[149]

HNb3O8

Ni(OH)2 (1 mol%)

300 W Xe >420

TEOA

5943

[150]

P25

Cu(OH)2Ni(OH)2(8:2)

100 W lamp (UV)

Ethanol

220000

[151]

Cd0.3Zn0.7S

Ni(OH)2/Pt

450 LED

ethanol

27000

[152]

CdS(NRs)

Ni3N

300 W Xe lamp>420

water

62000

[153]

g-C3N4

Ni (7.4%)

300 W Xe lamp

TEOA

4000

[154] (Continued)

Progress on Ni-Based as Co-Catalysts for Water Splitting  605

Table 19.1  Summary of the previously (last 10 years) reported Ni and Ni based co-catalyst for hydrogen production [89, 135–179]. (Continued) Photocatalyst

Co-catalyst

Light source (nm)

Sacrificial reagent

H2 production rate (µmol h−1 g−1)

Reference

g-C3N4

Ni

500 W Xe lamp

TEOA

161

[155]

S-doped g-C3N4

Ni (5nm)

300 W Xe >420

TEOA

2021.3

[156]

S- doped g-C3N4

Ni

150 W Xe lamp

TEOA

3628

[157]

EY/MOF-5

Ni (9nm)

300 W Xe >400

TEOA

302200

[158]

CdS

Ni@C (core/ shell)

300 W Xe >420

Lactic acid

76100

[85]

ZnO@ZnS

Ni Foam

350 W Xe lamp

Na2S+Na2SO3

5806

[86]

CdS

Ni2P

LED irradiation≥420

-

267.85

[159]

CdS

NiCoP/NiCoPi

300 W Xe >420

Lactic acid

80800

[160]

C-ZrO2/g-C3N4 Eosin Y

Ni2P

300W Xe lamp >400

TEOA

10040

[161]

g-C3N4

Ni2P (3%)

300 W Xe lamp

TEOA

1051

[162] (Continued)

606  Materials for Hydrogen Production, Conversion, and Storage

Table 19.1  Summary of the previously (last 10 years) reported Ni and Ni based co-catalyst for hydrogen production [89, 135–179]. (Continued) Photocatalyst

Co-catalyst

Light source (nm)

Sacrificial reagent

H2 production rate (µmol h−1 g−1)

Reference

g-C3N4

C,N Codoped Fe2P/Ni2P

300 W Xe >420

TEOA

138100

[121]

TiO2 Ti3AlC2

NiCoP (1wt %) Ni2P

300 W Xe lamp Xe lamp (AC 35W Ballast)

Methanol Methanol+water

1540 13000

[163] [164]

Cd0.5Zn0.5S

Ni2P

300 W Xe >420

Na2S+Na2SO3

1312

[165]

MIL-125-NH2

Ni2P

300 W Xe >420

TEOA

894

[166]

TiO2 WO3/g-C3N4

NiS(7wt%) NiS

300 W Xe lamp 5 W LED

Lactic acid TEOA

698 2929.1

[167] [168]

TiO2

NixSy

500 W Xe lamp

Cellulose soln

3022

[169]

Cd0.5Zn0.5S

NiS (0.6%)

300 W Xe lamp >420

TEOA

7160

[102]

g-C3N4

NiS-RGO

300 W Xe lamp

TEOA

393

[170]

g-C3N4

NiS/CNT

UV light irradiation

TEOA

521

[171] (Continued)

Progress on Ni-Based as Co-Catalysts for Water Splitting  607

Table 19.1  Summary of the previously (last 10 years) reported Ni and Ni based co-catalyst for hydrogen production [89, 135–179]. (Continued) Sacrificial reagent

H2 production rate (µmol h−1 g−1)

Reference

300 W Xe lamp >420

CH3OH+H2O

35.8

[172]

FeNiS

300 W Xe lamp

TEOA

1152

[173]

CdSe/CdS

NiS

500 W Hg >400

-

1200000

[174]

CdS QDs

NiS

500 W Hg >400

-

74.6

[175]

CdS (NRs)

NiS (5mol %)

300 W Xe >420

Na2S,Na2SO3

1131

[176]

CdS (NRs)/ZnS

NiS

300 W Xe >420

Na2S,Na2SO3

574

[177]

Zn0.5Cd0.5S

NiS-RGO

AM 1.5,100 mW/ cm2

Na2S,Na2SO3

7514

[108]

Zr(IV)based on the MOFs

NiS

390 nm LED lamp

-

4800

[178]

g-C3N4

Ni-Ni3C@C

300 W Xe lamp

TEOA

260.1

[179]

Photocatalyst

Co-catalyst

Light source (nm)

MgAl-LDH

NiS

g-C3N4

608  Materials for Hydrogen Production, Conversion, and Storage included the in-built electric field for the easy transfer of the charge for excellent hydrogen generation. Li et al. synthesized the multi-functional electrocatalyst to make the water-splitting process favorable for kinetics and to ease the photocatalytic process, lowering the H2 production overpotential [132]. Zhang et al. studied NiB effective co-catalyst for H2 production when integrated with the g-C3N4 [133]. Recently, Li et al. prepared nickel alloy with the bimetallic ratio 2:3 of Co and Ni with the 1% ZnI2S4 (ZIS) by the simple hydrothermal route and the reduction process [134]. 2D Nanosheet of CoNi shows an effective photocatalytic method by accelerating the charge migration and inhibited the combination of photogenerated charge carriers electron and holes. The hydrogen production of this photocatalytic system reaches 100.1 µmol h−1 at >420 nm, accompanied by ascorbic acid used as the sacrificial reagent, which is 4.3 times stronger than the nude ZnI2S4 (ZIS). Some previously (last 10 years) reported Ni and Ni containing co-catalyst for hydrogen production is summarized in Table 19.1 [89, 135–179].

19.6 Conclusion and Future Perspective Over the past few decades, the demand for the development of semiconductor-based photocatalyst continuously increased with high efficiency. Enhancing photocatalyst is well recognized by loading the proper co-­catalyst at the acceptable and suitable level. In this chapter, we briefly discussed different semiconductors like TiO2, CdS, ZnS, and metal-free semiconductors such as polymeric g-C3N4 , RGO, QDs, and so on. There are composites for appropriate band gap, which could be suitable for hydrogen production. Herein we mainly focused on the recent development of the various Ni containing co-catalyst such as including metallic Ni, a different form of its oxide (NiO, NiOx, Ni(OH)2, etc.), different forms of sulfides and phosphide and nickel-based molecular complexes, and other Ni-based co-catalyst, semiconductor-based photocatalyst for hydrogen production. This way, photocatalyst Ni(OH)2/CdS is given at 420 nm of visible light irradiation with a QE of 66.1%. The same trend when the metallic Ni-doped on the CdS at the optimal condition provides the QE of 48% at 447 nm, whereas Ni-based molecular co-catalyst, TON of 600,000 have been reported under the optimal state of CdSe incorporated with the molecular co-catalyst Ni2+ -DHLA. So the nickel-based co-catalyst provided a large surface area, stability, and electrical conductivity, so it is used in the various catalytic industries, but we mainly focused on hydrogen production.

Progress on Ni-Based as Co-Catalysts for Water Splitting  609 However, many outstanding inspiring achievements in this research field but although go-ahead for exploring this field for the practical application. However, various kinds of Ni-based photocatalyst exhibited significant attention to solar-driven photocatalytic performance for the hydrogen evolution. Still, they need some modification for photocatalytic hydrogen generation. Many disputes carry on in this field for the development of the Ni containing co-catalyst that efficiently influenced the size of Ni nanoparticle, morphology, chemical composition, and amount of loading, despite a lack of understanding of the tuning between their different structure and their ability for H2 production. Ultimately, some new strategies to design the construct that can have good water solubility, good tolerance for the photocorrosion, excellent stability, and optimize the loading content to adopt the simple process for synthesis that would be commercially adoptable and non-toxic for the environment too. Moreover, composite photocatalyst will be a well-designed structure to form the duel, ternary structure, and some core-shell structure playing the critical route for the hydrogen evolution from the water splitting. In this field, more practical and theoretical investigations should reveal other intrinsic properties to provide the scientific insight for maximizing the photocatalytic activity for the co-catalyst.

Author Declaration A. M. has major contribution with the help of K. C. M. in writing this book chapter along with drawing the Figures and Tables, taking the copyright permission etc.

Acknowledgment A. M. and K. C. M. are thankful to Indian Institute of Technology (Indian School of Mines) Dhanbad, India, for providing research fellowship.

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618  Materials for Hydrogen Production, Conversion, and Storage 114. Li, Z., Chen, X., Shangguan, W., Su, Y., Liu, Y., Dong, X., Sharma, P., Zhang, Y., Prickly Ni3S2 nanowires modified CdS nanoparticles for highly enhanced visible-light photocatalytic H2 production. Int. J. Hydrog. Energy, 42, 6618, 2017. 115. Cao, S., Chen, Y., Wang, C.J., He, P., Fu, W.F., Highly efficient photocatalytic hydrogen evolution by nickel phosphide nanoparticles from aqueous solution. Chem. Commun., 50, 10427, 2014. 116. Zhu, Q., Qiu, B., Du, M., Xing, M., Zhang, J., Nickel boride cocatalyst boosting efficient photocatalytic hydrogen evolution reaction. Chem. Eng. J., 57, 8125, 2018. 117. Li, Y., Jin, Z., Wang, H., Zhang, Y., Liu, H., Effect of electron-hole separation in MoO3@ Ni2P hybrid nanocomposite as highly efficient metal-free photocatalyst for H2 production. J. Colloid Interface Sci., 537, 629, 2019. 118. Wang, Y., Li, Y., Cao, S., Yu, J., Ni-P cluster modified carbon nitride toward efficient photocatalytic hydrogen production. Chin. J. Catal., 40, 867, 2019. 119. Liu, E., Jin, C., Xu, C., Fan, J., Hu, X., Facile strategy to fabricate Ni2P/g-C3N4 heterojunction with excellent photocatalytic hydrogen evolution activity. Int. J. Hydrog. Energy, 43, 21355, 2018. 120. Guo, X., Cao, J., Guo, M., Lin, H., Chen, Y., Chen, S., Excellent visible light photocatalytic H2 evolution activity of novel noble-metal-free Ni12P5/CdS composite. Catal. Commun., 119, 176, 2019. 121. Xu, J., Qi, Y., Wang, C., Wang, L., NH2-MIL-101 (Fe)/Ni (OH) 2-derived C, N-codoped Fe2P/Ni2P cocatalyst modified g-C3N4 for enhanced photocatalytic hydrogen evolution from water splitting. Appl. Catal. B Environ., 241, 178, 2019. 122. Hu, T., Dai, K., Zhang, J., Zhu, G., Liang, C., Noble-metal-free Ni2P as cocatalyst decorated rapid microwave solvothermal synthesis of inorganic-organic CdS-DETA hybrids for enhanced photocatalytic hydrogen evolution. Appl. Surf. Sci., 481, 1385, 2019. 123. Cao, S.W., Yuan, Y.P., Barber, J., Loo, S.C., Xue, C., Noble-metal-free g-C3N4/ Ni (dmgH) 2 composite for efficient photocatalytic hydrogen evolution under visible light irradiation. Appl. Surf. Sci., 319, 344, 2014. 124. Caputo, C.A., Gross, M.A., Lau, V.W., Cavazza, C., Lotsch, B.V., Reisner, E., Photocatalytic hydrogen production using polymeric carbon nitride with a hydrogenase and a bioinspired synthetic Ni catalyst. Angew. Chem. Int. Ed., 53, 11538, 2014. 125. Dong, J., Wang, M., Li, X., Chen, L., He, Y., Sun, L., Simple Nickel-Based Catalyst Systems Combined With Graphitic Carbon Nitride for Stable Photocatalytic Hydrogen Production in Water. ChemSusChem, 5, 2133, 2012. 126. Han, Z., Qiu, F., Eisenberg, R., Holland, P.L., Krauss, T.D., Robust photogeneration of H2 in water using semiconductor nanocrystals and a nickel catalyst. Science, 338, 1321, 2012. 127. Lei, J.M., Luo, S.P., Zhan, S.Z., Wu, S.P., A nickel (II) complex of S, S'-bis (2-pyridylmethyl)-1, 2-thioethane, a cocatalyst for photochemical driven

Progress on Ni-Based as Co-Catalysts for Water Splitting  619 hydrogen evolution from water under visible light. Inorg. Chem. Commun., 95, 158, 2018. 128. Zhang, W., Hong, J., Zheng, J., Huang, Z., Zhou, J., Xu, R., Nickel–thiolate complex catalyst assembled in one step in water for solar H2 production. J. Am. Chem. Soc., 133, 20680, 2011. 129. Han, Z., Shen, L., Brennessel, W.W., Holland, P.L., Eisenberg, R., Nickel pyridinethiolate complexes as catalysts for the light-driven production of hydrogen from aqueous solutions in noble-metal-free systems. J. Am. Chem. Soc., 135, 14659, 2013. 130. Ghiat, I., Boudjemaa, A., Saadi, A., Bachari, K., Coville, N.J., Efficient hydrogen generation over a novel Ni phyllosilicate photocatalyst. J. Photochem. Photobiol. A, Chem., 382, 111952, 2019. 131. Chen, Z., Gong, H., Liu, Q., Song, M., Huang, C., NiSe2 nanoparticles grown in situ on CdS nanorods for enhanced photocatalytic hydrogen evolution. ACS Sustainable Chem. Eng., 7, 16720, 2019. 132. He, K., Xie, J., Liu, Z.Q., Li, N., Chen, X., Hu, J., Li, X., Multi-functional Ni3 C cocatalyst/gC3 N4 nanoheterojunctions for robust photocatalytic H 2 evolution under visible light. J. Mater. Chem. A, 6, 13110, 2018. 133. Zhu, Q., Qiu, B., Du, M., Xing, M., Zhang, J., Nickel boride cocatalyst boosting efficient photocatalytic hydrogen evolution reaction. Ind. Eng. Chem. Res., 57, 8125, 2018. 134. Li, Z., Wang, X., Tian, W., Meng, A., Yang, L., CoNi bimetal cocatalyst modifying a hierarchical ZnIn2S4 nanosheet-based microsphere noble-metal-free photocatalyst for efficient visible-light-driven photocatalytic hydrogen production. ACS Sustainable Chem. Eng., 7, 20190, 2019. 135. Huerta-Flores, A.M., Torres-Martínez, L.M., Moctezuma, E., Ceballos Sanchez, O., Enhanced photocatalytic activity for hydrogen evolution of SrZrO3 modified with earth abundant metal oxides (MO, M = Cu, Ni, Fe, Co). Fuel, 181, 670, 2016. 136. Qian, X.B., Peng, W., Shao, Y.B., Huang, J.H., Nickel oxide/Au porous nanobelts: Synthesis and visible light-driven photocatalytic hydrogen production. Int. J. Hydrog. Energy, 43, 2160, 2018. 137. Yue, X., Yi, S., Wang, R., Zhang, Z., Qiu, S., Synergistic effect based NixCo1–x architected Zn0. 75Cd0. 25S nanocrystals: An ultrahigh and stable photocatalysts for hydrogen evolution from water splitting. Appl. Catal. B Environ., 224, 17, 2018. 138. Ma, X., Cui, X., Zhao, Z., Melo, M.A., Roberts, E.J., Osterloh, F., E., Use of surface photovoltage spectroscopy to probe energy levels and charge carrier dynamics in transition metal (Ni, Cu, Fe, Mn, Rh) doped SrTiO3 photocatalysts for H 2 evolution from water. J. Mater. Chem. A, 6, 5774, 2018. 139. Nsib, M.F., Naffati, N., Rayes, A., Moussa, N., Houas, A., Effect of some operational parameters on the hydrogen generation efficiency of Ni-ZnO/PANI composite under visible-light irradiation. Mater. Res. Bull., 70, 530, 2015.

620  Materials for Hydrogen Production, Conversion, and Storage 140. Yu, J., Wang, S., Cheng, B., Lin, Z., Huang, F., Noble metal-free Ni (OH)2 –gC3 N4 composite photocatalyst with enhanced visible-light photocatalytic H 2-production activity. Catal. Sci. Technol., 3, 1782, 2013. 141. Mao, L., Ba, Q., Jia, X., Liu, S., Liu, H., Zhang, J., Li, X., Chen, W., Ultrathin Ni (OH) 2 nanosheets: a new strategy for cocatalyst design on CdS surfaces for photocatalytic hydrogen generation. RSC Adv., 9, 1260, 2019. 142. Chen, W., Chu, M., Gao, L., Mao, L., Yuan, J., Shangguan, W., Ni (OH)2 loaded on TaON for enhancing photocatalytic water splitting activity under visible light irradiation. Appl. Surf. Sci., 324, 432, 2015. 143. Vamvasakis, I., Papadas, I.T., Tzanoudakis, T., Drivas, C., Choulis, S.A., Kennou, S., Armatas, G.S., Visible-light photocatalytic H2 production activity of β-Ni(OH)2-modified CdS mesoporous nanoheterojunction networks. ACS Catal., 8, 8726, 2018. 144. Wang, J., Wang, Z., Zhu, Z., Synergetic effect of Ni(OH)2 cocatalyst and CNT for high hydrogen generation on CdS quantum dot sensitized TiO2 photocatalyst. Appl. Catal. B Environ., 204, 577, 2017. 145. Shi, Z., Dong, X., Dang, H., Facile fabrication of novel red phosphorus-CdS composite photocatalysts for H2 evolution under visible light irradiation. Int. J. Hydrog. Energy, 41, 5908, 2016. 146. Chen, S., Chen, X., Jiang, Q., Yuan, J., Lin, C., Shangguan, W., Promotion effect of nickel loaded on CdS for photocatalytic H2 production in lactic acid solution. Appl. Surf. Sci., 316, 590, 2014. 147. He, K., Xie, J., Luo, X., Wen, J., Ma, S., Li, X., Fang, Y., Zhang, X., Enhanced visible light photocatalytic H2 production over Z-scheme g-C3N4 nanosheets/ WO3 nanorods nanocomposites loaded with Ni(OH)x cocatalysts. Chin. J. Catal., 38, 240, 2017. 148. Li, S., Dai, D., Ge, L., Gao, Y., Han, C., Xiao, N., Synthesis of layer-like Ni (OH)2 decorated ZnIn2S4 sub-microspheres with enhanced visible-light photocatalytic hydrogen production activity. Dalton Trans., 46, 10620, 2017. 149. Xu, Y., Gong, Y., Ren, H., Liu, W., Li, C., Liu, X., Niu, L., Insight into enhanced photocatalytic H2 production by Ni (OH) 2-decorated ZnxCd1–xS nanocomposite photocatalysts. J. Alloys Compd., 735, 2551, 2018. 150. Xia, Y., Chen, W., Liang, S., Bi, J., Wu, L., Wang, X., Engineering a highly dispersed co-catalyst on a few-layered catalyst for efficient photocatalytic H2 evolution: A case study of Ni (OH)2/HNb3 O8 nanocomposites. Catal. Sci. Technol., 7, 5662, 2017. 151. Majeed, I., Nadeem, M.A., Hussain, E., Waterhouse, G.I., Badshah, A., Iqbal, A., Nadeem, M.A., Idriss, H., On the synergism between Cu and Ni for photocatalytic hydrogen production and their potential as substitutes of noble metals. ChemCatChem, 8, 3146, 2016. 152. Markovskaya, D.V., Kozlova, E.A., Gerasimov, E.Y., Bukhtiyarov, A.V., Kozlov, D.V., New photocatalysts based on Cd0. 3Zn0. 7S and Ni (OH)2 for hydrogen production from ethanol aqueous solutions under visible light. Appl. Catal. A Gen., 563, 170, 2018.

Progress on Ni-Based as Co-Catalysts for Water Splitting  621 153. Sun, Z., Chen, H., Zhang, L., Lu, D., Du, P., Enhanced photocatalytic H 2 production on cadmium sulfide photocatalysts using nickel nitride as a novel cocatalyst. J. Mater. Chem. A, 4, 13289, 2016. 154. Kong, L., Dong, Y., Jiang, P., Wang, G., Zhang, H., Zhao, N., Light-assisted rapid preparation of a Ni/gC 3 N 4 magnetic composite for robust photocatalytic H 2 evolution from water. J. Mater. Chem. A, 4, 9998, 2016. 155. Bi, L., Xu, D., Zhang, L., Lin, Y., Wang, D., Xie, T., Metal Ni-loaded g-C3N4 for enhanced photocatalytic H2 evolution activity: The change in surface band bending. Phys. Chem. Chem. Phys., 17, 29899, 2015. 156. Sun, C., Zhang, H., Liu, H., Zheng, X., Zou, W., Dong, L., Qi, L., Enhanced activity of visible-light photocatalytic H2 evolution of sulfur-doped g-C3N4 photocatalyst via nanoparticle metal Ni as cocatalyst. Appl. Catal. B Environ., 235, 66, 2018. 157. Vu, M.H., Sakar, M., Nguyen, C.C., Do, T.O., Chemically bonded Ni cocatalyst onto the S doped g-C3N4 nanosheets and their synergistic enhancement in H2 production under sunlight irradiation. ACS Sustainable Chem. Eng., 6, 4194, 2018. 158. Zhen, W., Ma, J., Lu, G., Small-sized Ni (1 1 1) particles in metal-organic frameworks with low over-potential for visible photocatalytic hydrogen generation. Appl. Catal. B Environ., 190, 12, 2016. 159. Cao, S., Chen, Y., Wang, C.J., He, P., Fu, W.F., Highly efficient photocatalytic hydrogen evolution by nickel phosphide nanoparticles from aqueous solution. Chem. Commun., 50, 10427, 2014. 160. Zhao, Y., Lu, Y., Chen, L., Wei, X., Zhu, J., Zheng, Y., Redox dual-­cocatalystmodified CdS double-heterojunction photocatalysts for efficient hydrogen production. ACS Appl. Mater. Interfaces, 12, 46073, 2020. 161. Xu, J., Gao, J., Qi, Y., Wang, C., Wang, L., Anchoring Ni2P on the UiO-66-NH2/ g-C3N4-derived C-doped ZrO2/g-C3N4 Heterostructure: Highly Efficient Photocatalysts for H2 Production from Water Splitting. ChemCatChem, 10, 3327, 2018. 162. Yu, T., Si, Y., Lv, Z., Wang, K., Zhang, Q., Liu, X., Wang, G., Xie, G., Jiang, L., Cd0. 5Zn0. 5S/Ni2P noble-metal-free photocatalyst for high-efficient photocatalytic hydrogen production: Ni2P boosting separation of photocarriers. Int. J. Hydrog. Energy, 44, 31832, 2019. 163. Liu, W., Shen, J., Liu, Q., Yang, X., Tang, H., Porous MoP network structure as co-catalyst for H2 evolution over g-C3N4 nanosheets. Appl. Surf. Sci., 462, 822, 2018. 164. Tasleem, S., Tahir, M., Zakaria, Z.Y., Fabricating structured 2D Ti3AlC2 MAX dispersed TiO2 heterostructure with Ni2P as a cocatalyst for efficient photocatalytic H2 production. J. Alloys Compd., 842, 155752, 2020. 165. Peng, S., Yang, Y., Tan, J., Gan, C., Li, Y., In situ loading of Ni2P on Cd0. 5Zn0. 5S with red phosphorus for enhanced visible light photocatalytic H2 evolution. Appl. Surf. Sci., 447, 822, 2018.

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20 Use of Waste-Activated Sludge for the Production of Hydrogen Hülya Civelek Yörüklü1, Bilge Coşkuner Filiz2 and Aysel Kantürk Figen3* Department of Environmental Engineering, Yildiz Technical University, İstanbul, Turkey 2 Department of Metallurgical and Materials Engineering, Yildiz Technical University, İstanbul, Turkey 3 Department of Chemical Engineering, Yildiz Technical University, İstanbul, Turkey 1

Abstract

Hydrogen became an energy and energy carrier to ensure universal access to affordable, reliable, and modern energy services within the 2030 Sustainable Development Goals targets. It must consider the environmental effects of new generation technologies that increase energy efficiency and provide clean energy. To provide more efficient energy and save the environment, waste-to-hydrogen is a concept emphasizing hydrogen production from waste materials such as biomass waste, municipal solid waste, sewage sludge, packaging and plastics, and recovered fuels. Carbon-rich and nitrogen efficient solid waste such as activated sludge recovered by digestion, fermentation, combustion, gasification, and pyrolysis are well-known sludge-to-energy recovery strategies for producing biogas, fuel gas, biofuel (syngas, bio-diesel, bio-oil). This chapter focuses on the potential use of waste-activated sludge in hydrogen, strategies of waste-activated sludge to hydrogen energy, potential, and prospects. Keywords:  Waste, activated sludge, hydrogen production, thermochemical, biological

*Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (625–654) © 2023 Scrivener Publishing LLC

625

626  Materials for Hydrogen Production, Conversion, and Storage

20.1 Introduction Wastewater treatment plants (WWTPs) are designed to convert inorganic or organic pollutants into the sludge to reach acceptable final effluent quality for safe disposal. In these plants, it is critical to achieve optimum treatment efficiency with the lowest initial and operating costs and meet discharge standards with the least amount of sludge formation. Biological wastewater treatment plants, including a biological tank that converts organic and inorganic pollutants into biological waste, are the best option to meet all these criteria. Carbon and nitrogen oxidation and phosphorus removal were carried out in this method, also known as the activated sludge process and preferred in domestic wastewater treatment for years. In an aeration tank, microorganisms utilize dissolved pollutants such as, carbon and nitrogen, and convert them to new microorganisms. However, the concentration of the microorganisms must be kept stable to avoid operational problems and achieve a stable treatment. Thanks to a settling tank (final clarifier) placed just after the aerobic tank, microorganism concentration in the aeration tank could be kept constant by wasting the microorganisms produced, called waste-activated sludge (WAS). A diagram of a conventional WWTP is given in Figure 20.1. Solids accumulated in primary settling and final clarifier tanks are called “sludge”. Each sludge has different characterization and names. The sludge collected in the primary settling tank has a highly volatile organic content called raw or primary sludge. The final clarifier sludge is microbial biomass consisting of different species such as bacteria, protozoa, and fungi which have low biodegradability and are called biological or activated sludge. These sludges, which come from primary settling and final clarifier tanks having two different characteristics, are generally mixed. This new sludge mixture obtained is easier to manage via biological methods [1].

Coarse and fine screens Grit chamber Influent wastewater

Primary settling

Primary sludge

Aeration tank

Final clarifier

Return activated sludge

Figure 20.1  The diagram of a conventional wastewater treatment plant.

Effluent wastewater

Waste Activated Sludge (WAS)

Waste-Activated Sludge to Produce Hydrogen  627 Recent studies show that 10 million tons of dry, 20 million tons, and 49 trillion liters of sludge are produced yearly in Europe, China, and the United States, respectively [2, 3]. Moreover, this value increases each year with economic and population development, which indicates the requirement for recent technologies to reduce the cost of sludge management [4]. The waste sludge consisting of organic, inorganic, and microorganisms has an unstable, volatile, odorous, and sometimes toxic characteristic. Therefore, different biological, physical, and chemical treatments are performed to stabilize the organic content of this sludge to enhance effluent quality and/or make sludge management feasible. Anaerobic digestion is one of the most preferred biological methods used for the sludge stabilization approach. This approach can recover energy via combusting biogas produced, and secondary sludge may be valorized as fertilizer. Anaerobic digestion is also a cheaper method compared to physical and chemical methods. Sludge stability can be enhanced by applying pretreatment to the sludge to disrupt the microbial cell wall and help to release microorganisms’ extracellular and intracellular substances. In addition, pretreatment shortens the stabilization time and can make digestion easier. Waste-to-energy strategies are today’s preferred waste management methods because they influenced waste reduction, resource valorization, and energy generation. Waste activated sludge is a suitable alternative for energy production with a high organic content of up to 59%–88% (w/v). Organic content is composed of carbon (50%–55%), oxygen (25%–30%), nitrogen (10%–15%), hydrogen (6%–10%), phosphorus (1%–3%), and sulfur (0.5%–1.5%) [5]. Minerals such as quartz, calcite, and Microline, elements such as Fe, Ca, K, and Mg, and heavy metals such as Cr, Ni, Zn, Pb, and Hg were also discovered in sludge [6]. However, solid content is quite low at 5%, and the rest consists of water. Many sludge-to-energy recovery options have been studied in recent years, including biogas, fuel gas, and biofuel production (syngas, bio-diesel, and bio-oil). In Figure 20.2, different energy sources can be produced using WAS and their production methodologies. Anaerobic digestion is the generally used method for sludge stabilization in WWTPs due to operating and is economically feasible. In this biological process, which is carried out under anaerobic conditions, organics in the waste activated-sludge transforms to biogas, a mixture of CH4, CO2, and trace gases such as H2S and CO. Biogas produced can be turned into electrical energy by combustion in turbines. However, sludge to biogas reaction time takes a few weeks, which is long and causes high reactor

628  Materials for Hydrogen Production, Conversion, and Storage

Biogas (CH4, H2, CO2 etc)

Pyrolysis

Gasification

Combustion

Supercritical Gasification

Biochar

Bio-oil

Ash

Biohydrogen

Anaerobic digestion

Waste Activated Sludge (WAS)

Sludge

Flue gas (CO2,NOx, SOx etc)

Figure 20.2  Potential WAS-to-energy pathways and their products.

volume requirements. In addition, the organic substance remaining in the effluent is also high due to a low conversion yield of 40%–70%. In contrast to biological processes, thermochemical conversion methods such as combustion, pyrolysis, and gasification occur in a quite short time, changing between seconds and minutes. One of the most significant advantages of thermal procedures over biological processes is removing organic content, with around 80% [7]. However, the water content of the sludge needs to be low in thermochemical processes. Otherwise, the amount of heat required and operational costs increase. Even though energy requirements get attention worldwide, environmental pollution caused by carbon-based energy sources is another challenge to overcome, using carbon-based fuels such as fossil fuels or methane causes greenhouse gas production such as CO2, CO, SOx. In recent years, with the awareness of all of those, energy production strategies are established throughout ecological renewable energies. All authorities and studies have gravitated to create and develop zero-carbon energy sources such as solar, ocean, hydrogen (H2), and geothermal energy. Due to support zero-carbon energy production, waste-to-energy methodologies are also focused on zero-carbon fuel, hydrogen, instead of carbon-based [8]. Biohydrogen production technology, which is a relatively new technology compared to other biofuels, take notice of since it is a clean energy source that is free of carbon, nitrogen, sulfur, and other pollutants. Moreover, the final product of hydrogen after combustion gas is only vapor. Therefore, WAS is also one of the most promising biomasses for hydrogen gas production, with high bonded-hydrogen content and biodegradability.

Waste-Activated Sludge to Produce Hydrogen  629 In the present chapter, sludge-to-energy recovery strategies such as dark fermentation, photo-fermentation, microbial-electrolysis, gasification, pyrolysis, and supercritical water gasification for producing biohydrogen are explained. The effect of reaction parameters on pathways and yield of the process is discussed. Especially pretreatment methods to increase hydrogen production yield are given in detail. Suggestions for future processes have been made in order to eliminate existing obstacles, reduce sludge management costs, and ensure efficient energy conversion.

20.2 WAS to Hydrogen Production 20.2.1 Biohydrogen Production Biohydrogen is biological hydrogen generated by some specific microorganisms through their metabolism. Biological hydrogen production can be carried out via two concepts: (i) anaerobic hydrogen-producing bacteria such as some Clostridium species, Escherichia coli, and Enterobacter aerogenes, and (ii) specific photosynthetic microorganisms, containing different algae and photosynthetic species. Biological hydrogen generation can be classified as photo-dependent (photo-fermentation) or photo-­ independent (dark fermentation), depending on the microbial species used [9]. On the other hand, according to bio-hydrogen production efficiencies, fermentative H2 production microorganisms are more advantageous than photohydrogen production microorganisms [10]. Biological hydrogen production can be carried out using different substrates such as organic wastes, wastewater, agricultural wastes, and waste-activated sludge. Among these sources, activated sludge, which has a high organic content and is formed in high amounts in wastewater treatment plants, is a hard-to-manage waste for wastewater treatment plants and a low-cost substrate for biological hydrogen production [11].

20.2.1.1 Dark Fermentation Dark fermentation is carried out under dark anaerobic conditions to produce hydrogen by some anaerobic bacteria utilizing organic substances in the dark. Dark fermentative microorganisms can convert different types of organic substances into hydrogen with the help of different enzymes such as nitrilase or hydrogenase. Even though biomass-to-biohydrogen efficiency is still lacking [9], studies are focused on specific strain enrichment,

630  Materials for Hydrogen Production, Conversion, and Storage microorganisms acclimation, and optimizing reactor and operating conditions to overcome this obstacle [10]. WAS contains a high concentration of protein, glucose, and lipids in either extracellular polymeric substances or internal cells. Due to its complex structure, pretreatment can be applied for multi-structure components to easy-to-degrade monomers. This step is called the hydrolysis step, which can take place naturally or with the help of pretreatment. Hydrolysis is regarded as the rate-limiting step in dark fermentation [12]. Following hydrolysis, hydrogen is produced directly or indirectly through a sequence of hydrogen producer microorganisms such as Enterobacter sp. and Clostridium sp. Dark fermentation is the most widely used and well-­understood method for bio-hydrogen production from WAS [11]. Metabolic glucose degradation pathways and biohydrogen production during dark fermentation are given in Figure 20.3. According to Figure 20.3, hydrogen can be produced through two pathways after pyruvate formation, a crucial glycolysis intermediate. First, conversion of pyruvate to acetyl-CoA occurs via decarboxylation with reduced ferredoxin, which donates electrons to protons [13, 14]. Hydrogen is also produced during this reduction process. Second, even though species often used for bio-hydrogen production use this route for hydrogen generation [15], hydrogen can be produced via formate cleavage.

Waste Activated Sludge (WAS) Glucose 2 NAD+ + 2H+ 2 NADH 2 NADH

2 NAD+ + 2H+

2 Pyruvate 2 Formate 2H2 2 CO2

4 NADH

2 Lactate

2 Fdox 2 Fdred

Hydrogenase

2 CO2

2 NAD+ + 2H+

Propionate

4H+ 2 H2

2 Acetyl-CoA

2 NADH 2 NAD+ + 2H+

4 NAD+ + 4H+

2 Ethanol

2 NADH

2 H2

2 Acetate

Butyryl-CoA

Butyrate

Figure 20.3  Dark fermentation pathways and biohydrogen production.

Waste-Activated Sludge to Produce Hydrogen  631 A high amount of NADH is produced during glycolysis due to the usage of NADH in electron transport. NADH and H+ are generally oxidized to NAD+ to keep the NADH/NAD+ ratio stable. If NADH oxidation is slower than NADH production, an excess amount of NADH and H+ will unavoidably exist in the reactor. Fermentative bacteria produce hydrogen with electrons from the oxidation of excess NADH to maintain the NADH/ NAD+ ratio at a proper level [16, 17]. Even though it’s a high potential, a limited number of studies were carried out on biohydrogen production from WAS due to the low yield. There are several reasons behind this low yield, such as inefficient substrate characteristics for hydrogen-producing microorganisms (e.g., low C/N ratio) and hard-to-break down organic substances limiting microorganisms to utilize [18]. As a result of these restrictions, most studies were focused on obstacles to overcome for efficient WAS-to-H2 production. Applying different pretreatment methods to break down the complex content of the substrate is an option to enhance biohydrogen production efficiency. Since organic fraction is mainly found in microbial cells, pretreatment helps to lyse the cell membrane of WAS and extract organic compounds. As a result of the extraction of organic content, hydrogen-­ producing microorganisms can utilize organic fraction in a much shorter time and convert to hydrogen with a high yield. Table 20.1 lists some pretreatment methods applied to WAS and their effect on H2 production. C/N ratio optimization is also a support mechanism to enhance bio-­ hydrogen production efficiency. The addition of different carbon sources, Table 20.1  Comparison of dark fermentation based hydrogen production from WAS by the various pretreatments. Pretreatment

Effect on H2 production yield

References

Ozone + ultrasound

60.88% enhancement

[19]

Freezing in the presence of nitrite

5.5–13.4 times higher than control

[20]

Alkaline

2.5 times higher than the control

[21]

Sodium citrate + ultrasound

604.2% enhancement

[22]

Alkaline + thermal

0.44 m3/(m3 ∙d) enhancement

[23]

Sodium citrate + thermal pretreatment

346.9% enhancement

[24]

632  Materials for Hydrogen Production, Conversion, and Storage such as food wastes and agricultural wastes, to WAS to increase the C/N ratio was investigated by different researchers. These studies were conducted not only for using co-substrate to increase the C/N ratio but also to increase the yield by applying pretreatment methods. Yang et al. (2017) studied the biohydrogen production potential of raw forestry wastes, having high carbohydrate content, and WAS mixture in batch scale [25]. Yard waste addition to WAS helped increase the hydrogen yield 3.60 times, compared to sole sewage sludge. Zhou et al. (2013) observed an increase in biohydrogen production yield in their study carried out with WAS and food wastes [26]. Kim et al. (2013) investigated the effect of both pretreatment and food waste addition on biological H2 production efficiency and concluded that both procedures yielded positive results [27].

20.2.1.2 Photofermentation Photofermentative hydrogen production is a method in which hydrogen is produced by photofermentative bacteria using light (as an energy source) and soluble organic materials (as an electron donor). Purple nonsulfur bacteria are generally used for this method [17]. Photofermentation bacteria can produce hydrogen by transferring electrons produced in the photosystem (PS), which supply energy for phosphorylation. Then, with the help of degraded organic matters act as reducing power, hydrogen production is completed under nitrogenase catalysis. Some photo-fermentative species can even use complex sugars as substrates [28, 29]. The hydrogen production pathway via photofermentation is given in Figure 20.4 [30, 31]. Despite photo fermentation microorganisms that can utilize whole soluble organics having a low molecular weight (e.g., volatile fatty acids and alcohols), there are certain limitations in practice. Due to purple nonsulfur bacteria’s poor light conversion ability, constant enlightenment is necessary to assure reaction efficiency. Although natural sunlight usage is an inexpensive solution in continuous systems, its unstable circumstances obligate an artificial illumination which causes cost increment [32, 33]. The design, construction, and operation of photobioreactors are complex and costly due to various constraints such as a large surface area and high transparency requirement [17, 34]. In addition to the use of photo-fermentation alone, it is also used in combination with dark fermentation. Combining photo and dark fermentation to produce hydrogen offers several benefits over using one method alone. It is possible to use photo and dark fermentation microorganisms combined in two-stage and multistage systems. Dark fermentation effluent contains small organic acids and alcohols like acetic acid, ethanol, and butyric acid,

Waste-Activated Sludge to Produce Hydrogen  633 Waste Activated Sludge (WAS)

Glucose NAD+ NADH

2 Pyruvate NAD+ 2CO2

NADH

Acetyl-CoA Sunlight TCA Cycle

NADP

PS

NAP(H)

e¯ ADP

ATPase

ATP

e¯ H+ Nitrogenase

H+

H2

Figure 20.4  Photo-fermentation pathways and biohydrogen production.

a suitable substrate for photosynthetic bacteria. The use of both combined systems can enhance biohydrogen generation [10]. However, the darkish appearance of the first step effluent hinders photosynthesis efficiency in the second step due to causing low light penetration [35]. Since WAS is dark-colored biomass containing high suspended solids, its light transmission is very low. This prevents the use of waste-­activated sludge in photosynthesis-based studies. Besides, WAS contains a high amount of complex organic matter. The hydrogen production yield of WAS in photo-fermentation is relatively low due to the low degradability of complex organic substances in photo-fermentation. Due to these obstacles, studies on hydrogen production efficiency by the photo-fermentation method in WAS are few. Hydrogen production by photo-fermentation from WAS is often studied combined with dark fermentation. The study of Jeong et al. (2007), in which waste-activated sludge was fed to a photofermentation reactor just after the acid fermentation reactor as a substrate,

634  Materials for Hydrogen Production, Conversion, and Storage concluded that a synergistic effect was seen, and hydrogen production yield increased up to 50.1 ml/g T-VFA [36]. Hydrogen production yield can reach 7.8 mmol in sequential dark fermentation-photofermentation reactors. Pretreatment of waste activated sludge also positively affected hydrogen production yield in sequential biohydrogen production systems [37]. Zhao and Chen (2011) studied the enhancement of photofermentative hydrogen efficiency fed with WAS dark fermentation liquid as substrate by using nano-TiO2 [38]. The addition of 100 mg/L nano-TiO2 increased biohydrogen yield by 46.1%, which shows the positive impact of nanoparticle addition to biosystems.

20.2.1.3 Microbial Electrolysis Cell Microbial electrolysis cells (MECs) are a kind of microbial fuel cell that uses electrons generated from organic substances by exoelectrogen microorganisms for hydrogen synthesis. MECs were seen as promising technologies for renewable H2 production while also serving as a wastewater treatment process. In MECs, substrate oxidized to electrons (e−), protons (H+), and carbon dioxide by exoelectrogenic microorganisms at the anode chamber. The produced protons pass to the cathode chamber directly by diffusion through the membrane. The electrons remained at the anode chamber transmitted to the cathode by a circuit with an applied potential [17]. Biohydrogen production with MECs has many advantages, such as (i) offering a cheaper H2 production cost than the typical energy requirement for water electrolysis, (ii) no expensive metal requirement in anode chamber due to being a self-sustaining microbial biocatalyst, (iii) higher biohydrogen conversion rates than that could achieve conventional biohydrogen production systems due to minimization of electron loss during biological processes, (iv) no gas purification is required due to relatively pure H2 production in cathode chamber, (v) providing both waste/wastewater treatment and bioenergy production, and (vi) different valuable products, such as CH4 and ethanol, can also be produced by microorganisms in the anode chamber [39–43]. It is critical to comprehend the parameters affecting MECs’ performance to achieve maximum yield. Many biochemical and environmental factors such as microorganism species used and their metabolism, electron transfer in the anode chamber, electron acceptor in a cathodic chamber, the material of electrodes, membrane type, biodegradability easiness of substrate, temperature, and pH are the most important factors among the numerous factors affecting MEC yield [43].

Waste-Activated Sludge to Produce Hydrogen  635 The number of studies carried out by feeding waste activated sludge to MECs is limited due to the pretreatment requirement of WAS to increase biodegradability. Even though MECs can reach higher yields than conventional systems, large-scale MECs are not preferred due to high reactor costs and slow H2 production rates [44]. In Table 20.2, examples of studies carried out by feeding waste-activated sludge to MECs are summarized.

20.2.2 Thermochemical Hydrogen Production The use of sewage sludge as fuel for thermochemical processes such as gasification and pyrolysis which may substantially reduce the volume of the separated waste byproduct within the main treatment plant is an alternate and creative method to traditional land spreading. The thermochemical hydrogen or hydrogen-rich gas production based on the transformation of sewage sludge includes three main approaches: pyrolysis, gasification, and combustion. Also, more specific methods have been used, such as hydrothermal gasification and supercritical water gasification (SCWG). Before the thermochemical hydrogen production, the moisture content of feedstock was reduced to eliminate the extra energy requirement during the thermochemical process by pre-process. Mainly, pyrolysis process performed under inert atmosphere without air to get bio-oil, flue gas, and char. Bio-oil obtained from the process could be used for synthesis, conversion or upgrading to new materials; reforming, source of energy in boiler and engines. Gasification process required a partial air/steam/O2/CO2 combination to get syngas, char, and tar. The process out put-tar can be used in turbines and as a starting material for reforming. Syngas also can be used for synthesis, conversion or upgrading to get new products as well as energy source in turbine, engine, fuel cell or boiler. Contrary to pyrolysis,

Table 20.2  Hydrogen production from MEC systems fed with WAS. Pretreatment method

H2 production yield

References

Ultrasound

120 ml H2/g VSS/d

[45]

Chemical (alkaline)

15.08 ± 1.41 mg H2/g VSS

[46]

Chemical (SDS)

8.5 mg H2/g VSS

[47]

Chemical (alkaline)

0.027–0.038 m3 H2/m3/day

[48]

Chemical (FNA)

1.44 ml/g VSS

[49]

636  Materials for Hydrogen Production, Conversion, and Storage combustion process is performed under excess air, which resulted in ash, flue gas, and heat for the boiler [50]. Technology readiness level (TRL) for thermochemical hydrogen production process of WAS is determined by taking commercial viability for gasification, pyrolysis, and hydrothermal treatment are 6–7, 5, and 6, respectively [51]. Sewage sludge requires adequate treatment to reduce impurities and pollutants while also recovering resources and energy. Thermochemical processes have the potential to be economically feasible in the long run, given the low cost, high quality, and several types of products that they may generate, as well as the recovery capacity. Energy recovery from WAS improves environmental consequences by reducing resource use [52]. Hydrogen is produced together with other gases such as CH4, CO2, CO, and C2H6 throughout the process. Due to another gas component in biogas, evolved hydrogen gas is required to further purification processes such as steam reforming, water-gas shift, etc. The sustainability of the feedstock and elimination of greenhouse gas emissions make hydrogen production based on this process promising. Thermochemical processes enable higher conversion, yield, and efficiency compared to biological processes. Also, the thermochemical process is easier to control to get target production. Thermochemical processes are appealing because they reduce sludge volume, destroy dangerous bacteria, produce a wide range of products, and, most significantly, recover substantial energy. Together with these advantages, the generally thermochemical process required to use catalysts that still the studies have been going on to improve the performance of systems. The use of catalysts and additives to optimize thermochemical process products’ quality and quantity is becoming prominent [53].

20.2.2.1 Pyrolysis Pyrolysis is a thermochemical process based on the degradation of feedstocks into the fuel during heating under a non-oxygen atmosphere. The pyrolysis process is performed in a low to medium temperature range, such as 300°C to 900°C, depending on the type of material. During the pyrolysis process, sewage sludge thermally decomposed at about 400°C–600°C under inert atmospheres such as N2 or CO2. This thermochemical process produce bio-based oil, char, and gas (containing CO, H2, etc.) by reducing the pollution [53–56]. The thermal analysis of dried sewage sludge shows that three main weight losses occurred during heating at different temperature regions: (i) the first reaction is endothermic with 5–10 wt.% weight loss that related

Waste-Activated Sludge to Produce Hydrogen  637 to the evaporation of absorbed water in sludge occurs at 180°C–200°C, (ii) the second reaction is exothermic which is maximum weight loss step with 40–70 wt.% and main reaction related with the decomposition of volatile compounds; (iii) the third reaction is endothermic with 9–40 wt.% loss corresponding to the decomposition of organic and inorganic content at 300°C–700°C. Therefore, based on thermogravimetric analysis of dried sewage sludge, the maximum hydrogen production could be obtained at a range of 600°C –700°C [57, 58]. The hydrogen produced from pyrolysis process is one of the primary gaseous products with a composition of 30–36.04 vol. %. The pyrolysis of sewage sludge occurred as given in equations:



WAS s

inert atm.

H2 g

CO2 g

CO g

Cn H m g

char s

tar s



(20.1)

The gaseous products that contain humidity and methane required subsequent process to get pure hydrogen gas.



1   Cn H m( g ) →  nH 2O( g ) + n  CO( g ) + n + m H 2( g )  2 

(20.2)



n  CO( g ) +   nH 2O( g ) → nCO2( g ) + H 2( g )

(20.3)



Pyrolysis is divided into three categories based on the heating rate and residence duration. Different types of this process are classified into slow-, fast-, co-, ­vacuum-, flash-, and hydropyrolysis. The main pyrolysis process is affected by WAS residence period, its size, heating conditions [53]. The hydrogen yield of the pyrolysis is based on several factors: temperature, pressure, time of the process, source of sewage sludge, type of catalyst, etc. The characteristic features of sewage sludge, such as moisture and carbon content, directly affected produced biogas content. The factors are coming from the process parameters such as heating program, gas flow rate, and temperature also change the composition of evolved gas. Higher moisture content, temperature (~900°C), and faster heating rates than 5°C/min, long residence time resulted in higher hydrogen gas content of biogas. The addition of agents could shorten the pyrolysis rate of sewage sludge. Besides, the source of sewage sludge is effective for hydrogen yield. The humidity of materials is an essential factor. The higher the water content of sewage

638  Materials for Hydrogen Production, Conversion, and Storage sludge, the higher the hydrogen concentration (>35 vol%) obtained at high-­temperature pyrolysis. The pre-operations improve the hydrogen yields and conversions of sewage sludge [53, 59]. Slow pyrolysis, commonly termed carbonization or torrefaction, is characterized by low heating rates and a more extended residence period. Optimizing of structural properties of WAS and milling properties of torrified char, carbonization, or torrefaction are usually performed at low temperatures of less than 400°C. Thus, slow pyrolysis or carbonization yields more char despite yielding lower bio-oil and gas [56, 60]. Fast pyrolysis operated at conditions compared to the slow pyrolysis process. The characteristic conditions are the shorter residence time, faster heating rate, and higher operating temperature as 400°C–600°C than slow pyrolysis. Therefore, besides the biogas produced, they are generally preferred for the production of bio-oil [53, 56, 61]. The gaseous product obtained from these processes is mainly composed of H2, CO, CH4, C2H4, CO2, C3H8, and C4H10. In addition, higher temperature (700°C–800°C) results in higher biogas production that has a 24.7 MJ/kg heating value [62, 63]. Co-pyrolysis is an alternative approach for the improvement of quality and quantity of products by addition of another feedstock such as microalgae [64], wood waste [65], lignite [66], etc. This approach is based on the more volatile matter in the feedstock composition, resulting in a high yield of biogas production. The additional feedstock consists of cellulose, hemicellulose, and lignin decomposes at different temperature regions, and the amount of ash obtained from the process decreased. This process also provides a high yield (35 %) of hydrogen. The nitrogen content of feedstocks could be increased by evolving NOx, NOin, and bio-gas [67]. Catalytic pyrolysis is another approach for improving the yield and decreasing the time and energy consumption of the process. The pyrolysis process consists of three-steps: (i) conditioning as before pyrolysis, (ii) pyrolysis, and (iii) reforming of volatiles. The operation conditions and content of the catalytic material significantly affect the content of products. The impurities such as nitrogen, sulfur, and chlorine compounds could be decreased by using a proper catalyst [53]. Several metals, metal oxide, and zeolite type catalysts have been investigated, such as Al2O3, Fe2O3, CaO, TiO2, ZnO, Ni/Al2O3. It was reported that Ni/Al2O3, red mud, Fenton’s reagent, and CaO improved the hydrogen yield of the process. Catalysts enhance the hydrogen yield and purity by improving moisture content and transformation of nitrogen and phosphorus [68, 69]. HZMS-5 with only SiO2 content type zeolite also reported high bio-gas yield. Zeolites affect deoxygenation, denitrogenation, and selectivity of the pyrolysis process.

Waste-Activated Sludge to Produce Hydrogen  639 Their large pore size and 3D structure provide the formation of intermediates to generate clean gas [70]. Also, their combinations with metal affect the reaction pathway of pyrolysis reactions. Sewage sludge char and its combinations with metals were also reported to have a catalytic effect on the pyrolysis process. It is one of the process outputs, and its low cost can make it favorable as a catalytic material in the future. Besides, there are several obstacles to improving it using catalytic material [53]. Several different types of reactors have been reported at different operating modes such as fixed, fluidized, spouted, bed reactors, etc. [71]. The type of reactor affect the heating methods, residence time, scale-up, gas flow rate, and content of products. Conical spouted bed reactor produced gas in 3–11.4 wt.% yield [72], rotating cylinder reactor enhanced the yield up to 23.3 wt.% [73], and sand bed type reactor enables gas production between 17.3 wt.% to 28.9 wt.% [74]. The fluidized type reactors were reported at different gas yields 7.35–27.9 wt.% and 6.5–10.3 wt.% [75, 76]. This process also has some drawbacks such as the heavy metal content of the sewage sludge, high energy consumption for decreasing the moisture content, and long time requirements for the drying during the drying process deoxygenation. In addition, Denitrogenating could occur based on the characteristic sewage sludge. The type of reactor and content of the catalysts have limitations. Catalysts have deactivation due to formed coke during the process. Therefore, using the process to get commercial products still has limitations [53].

20.2.2.2 Gasification Gasification is another thermochemical process combination of drying and pyrolysis (devolatilization), releasing volatiles and leaving solid residue as a char. In that process, a small amount of char and ash is obtained while high amount of combustible gases (H2, CO, CO2, and light hydrocarbons) is obtained under reductive atmospheres such as air, oxygen, steam or carbon dioxide, or their mixtures [3, 77–82]. To produce a high yield of combustible light gases, further gas–gas and gas–solid reactions involving the gasifying agent and the results of drying, pyrolysis, and cracking are permitted to take place in a gasifier. As a result, gasification is an extension of the pyrolysis process. The most common method for gasifying sewage sludge is to use air as a gasifying agent [83, 84]. Generally, the process is performed under positive pressure and high temperature (500–1400°C). The gases produced from gasification are

640  Materials for Hydrogen Production, Conversion, and Storage hydrogen, methane, carbon monoxide, nitrogen, and carbon dioxide [59, 85]. Generally, gasification of sewage sludge consisted of four processes: (i)  drying, (ii) pyrolysis/devolatilization, (iii) oxidation, and (iv) reduction/gasification (Figure 20.5) [4]. In the dryer, water content up to 15 wt.% of sludge decrease between 70°C–200°C. The next step is pyrolysis that performed at 350°C–600°C and led to the thermal decomposition of sludge. In the oxidation step, formed solid residue and volatiles are exothermally burned up to 1100°C. The reduction as a last step, solid residue is converted into carbon monoxodie and hydrogen based on reactions given below [86, 87]. The gasification of char is frequently the rate-limiting phase in the gasification of sewage sludge: the devolatilization stage is a rapid process, but the reforming of volatiles is considerably faster than the gasification of char [88]. The gasification of sewage sludge occurred as given in equations; drying and devolatilization (20.4), tar cracking (20.5), gas-gas reactions such as decomposition (20.6), partial oxidation (20.7), hydrogen oxidation (20.8), CO oxidation (20.9), methane oxidation (20.10), dry reforming (20.11), dry methane reforming (20.12), steam reforming (20.13), steam methane reforming (20.14), water-gas shift reaction (20.15), methanation (20.16), gas-solid reactions such as carbon oxidation (20.17), partial oxidation (20.18), Boudouard reaction (20.19), steam gasification (20.20), and hydrogasification (20.21) as given below. The homogeneous gas-phase reforming of volatile (R8-R13) reactions affects the evolved gas composition. The hydrogen gas produced from the partial oxidation of hydrocarbons (20.7), methane oxidation (20.10), dry reforming of hydrocarbons (20.11), dry methane reforming (20.12), steam reforming of hydrocarbons (20.13), steam methane reforming (20.14), water-gas shift reaction (20.15), and steam gasification (20.16). The hydrogen yield of the gasification process could be improved by further application of evolved gasification gas composition. Generally, the steam reforming process is used for this aim at a high temperature (800–1500°C) [53, 59].  

WAS( s ) →  char( s ) + volatiles  (water + light   gases + primary  tar ) (20.4)

Dryer

Gasification reactor

Figure 20.5  Gasification steps of sewage sludge.

Condenser

Reduction

Waste-Activated Sludge to Produce Hydrogen  641



Primary tar s

light gases secondary tar  



(20.5)



Secondary  tar →  C +   H 2 +  light  hydrocarbon  



   n  m Cn H m +   O2  →  nCO +   H 2  2  2



H 2 + 0.5 O2  →   H 2O



CO + 0.5 O2  →  CO2



CH 4 + 0.5 O2  →  CO + 2 H 2

(20.10)



   m Cn H m + nCO2  →  2nCO +   H 2  2

(20.11)



CH 4 + CO2  →  2CO + 2 H 2



   n +m H Cn H m + nH 2O  →  nCO +   2  2



CH 4 + H 2O  →  CO + 3H 2



CO + H 2O  →  CO2 + H 2



CO + 3H 2  →  CH 4 + H 2O



C + O2  →  CO2

 

 

 

 

 

 

 

 

(20.6) (20.7)

(20.8) (20.9)

(20.12)

(20.13)

(20.14) (20.15) (20.16) (20.17)

642  Materials for Hydrogen Production, Conversion, and Storage  



C + 0.5 O2  →  CO



C + CO2  →  2CO



C +   H 2O  →  CO + H 2  



C +  2 H 2  →  CH 4  

 

 

 

(20.18) (20.19) (20.20) (20.21)

The reaction kinetics and gasification process are affected by several factors such as the origin of sewage sludge, type of reactor, temperature, time, reductive medium, catalysts, etc., that strongly affect the hydrogen yield. The Auger reactor, plasma reactors, and reactors similar to gasification’s reactors have been used for the process. Selecting suitable reactors is essential to controlling the agglomeration of ash and its complex problems [3, 53]. The moisture content of sewage sludge is a critical factor, and the production of high-quality syngas is required to use dried sewage sludge. The plasma type gasification reactors can use wet sewage sludge as a feedstock to produce gas by reducing pollutants. The selection of the drying process affects the particle size that affected the gasification yield. Circulating fluidized bed reactors have been reported to produce dried sludge with high particle sizes compared to bubbling fluidized and fixed bed reactors. Homogeneity of the particle improves heat and mass transfer during the process that increased the yield. Long residence time and slow flow rates also have a positive effect on the yield. The air to sewage sludge ratio is also another critical factor that affected the hydrogen yield. The lower air:sludge ratio results in higher H2 and CO content, the higher ratio of air resulted in high CO2 content in the gas. It is reported that 0.2–0.35 air to sewage sludge ratio resulted in optimized yields [89, 90]. The steam ratio to sewage sludge (S/WAS) is very critical to get hydrogen in obtained bio-gas. In addition to other operating parameters, the optimum S/WAS: 0–2 is important in WAS gasification for the production of hydrogen [91]. The gasification temperature and pressure are other factors that affect the products due to change in the reactions. At high temperatures, the amount of obtained gas increases while the solid products such as tar and char quantity decrease. High hydrogen production is related to methane decomposition, steam reforming, and hydrocarbon reforming reactions. Together with this, too high a temperature increases the ash content of

Waste-Activated Sludge to Produce Hydrogen  643 the obtained products. The catalyst is another factor that affects hydrogen gas production and efficiency [3, 53, 92]. Although the hydrogen yield of the gasification process is high, the process is carried out at higher temperatures compared to pyrolysis. High energy consumption is carried out due to harsh process conditions. Several obstacles have been reported to improve generating hydrogen [53, 59]. The co-gasification approach is also used for the gasification process. Mixing the sewage sludge with other biomass such as wood [90], paper sludge, coal [93], etc., could increase the yield and decrease the pollutant in the solid product. The catalyst is also another critical parameter that changes the syngas composition. Bio-char, activated carbon, metal-based catalysts, minerals such as dolomite, olivine, etc., have been used for this aim. By use of catalysts highest hydrogen content of syngas is reached up to 36 vol. % and yield of 99%. Tar in the outlet gas, high moisture content, high N and S content of sewage sludge, and high inorganics are the main obstacles of the process. Significantly, selecting the right equipment to get a high yield and selectivity based on the composition of sewage sludge is critical. High moisture content required using high energy consumption causes plugging in systems and decreases heat and mass transfer during the process [90]. The N and S content of the sewage sludge is higher than the biomass, which could lead to environmental pollution. These containers could be converted into ammonia (NH3), hydrogen sulfide (H2S), hydrogen cyanide (HCN), and other oxides (NOx, N2O, SOx), which could lead to environmental pollution [94, 95].

20.2.2.3 Super Critical Water Gasification Supercritical water gasification is another type of gasification of dewatered sewage sludge that operated in a moisture environment above supercritical conditions. These revealed promising and unique technology for sludge treatment and hydrogen generation. Special conditions of the process improve the effectiveness of the process compared to pyrolysis and gasification [96]. Supercritical conditions required specific equipment to performed operation (temperature >374°C and pressure > 220.64 bars) [97]. Generally, the process operates between 400°C and 600°C under pressurized atmosphere and higher residence time of feedstock. Supercritical conditions improved the solubility of the sewage sludge and decreased the miscibility of organic compounds in the material composition. Tarfree and gas-rich products (H2, CO, CO2, and CH4) are produced fast via

644  Materials for Hydrogen Production, Conversion, and Storage this process. Also, with the high moisture content (with a water content greater than 90%) and high organic compound content of sewage sludge, sewage sludge treatment in supercritical water gasification is an attractive approach [98, 99]. The supercritical water gasification of sewage sludge occurs in three reactions: steam reforming, WGS, and methanation. Therefore, the sludge plays different roles as reactant or catalysts based on the stage as given in reactions below: air  .

Steam reforming: WAS( s ) +   H 2O( g )  →   H 2( g ) + CO( g )    H 2( g ) + CO2( g ) WGS reaction: CO( g ) +   H 2O( g )  →

(20.22)



(20.23)



   H 2O( g ) + CH 4( g ) Methanation reaction: CO( g ) +  3H 2( g )  →



(20.24)

Similar to another thermo-chemical process, sewage sludge and operation conditions affect the yield and products composition. The origin of sludge, the composition of feed, temperature, pressure, time, and catalytic material play a crucial role at that point. High temperatures promote evolving of CO2 and H2. Low sludge input is suggested to get higher conversion of feed and pressure shows a complex relationship with the process. The use of catalytic material was increased conversion and hydrogen yield, while it makes the operation conditions moderate compared to the un-­catalytic process. Several materials have been used as noble-metals, non-noble metals, and alkali metals (KOH, NaOH, NaCO3, etc.) in the form of metal, salt, heterogeneous or homogeneous catalysts [100–105]. The use of catalysts also enables valuable products [106–108]. Similar to other processes, the high moisture of sewage sludge affect the gasification products. The byproducts of the process can decrease combustion efficiency and blocking the fuel lines. During the co-gasification process, the quality of the syngas improves. The use of a catalyst can be increased up to 51 vol.%. The nitrogenous compounds evolved in gas content are dangerous for the environment. By use of proper catalytic material, nitrogenous and sulfurous content of the product can be decreased. The coke formation on the surface of catalysts required a regeneration process within a periodic time. Agglomeration of ash and formation of clinker inner side of reactor and system are significant obstacles to operating [53, 109].

Waste-Activated Sludge to Produce Hydrogen  645

20.3 Conclusion Remarks Numerous publications in the literature discuss the critical role of energy recovery strategies that should play in a circular economy model, given its capacity to produce energy from organics, manures, and a wide range of WAS. The high organic content of WAS makes it an ideal candidate as a primary energy feedstock for energy recovery of several industries. There is a lot of research dealing with the increase of energy recovery efficiency. Several parameters, especially non-polluting characteristics, and energy recoverability, etc., impact the choice of preferred hydrogen production technologies. Biological (photo-fermentation, dark-fermentation, microbial electrolysis) and thermochemical (pyrolysis, gasification, and supercritical gasification) processes consist of the objective of sustainable goals target and are suitable in certain aspects. Although biological hydrogen production is a promising process for sustainable development, significant challenges are transforming fossil fuel-based economy to ­biohydrogen-based economy. Due to the low yield of biohydrogen production of the process, more engineering researches and developments are required to provide practical and economical solutions. Reutilizing or reusing WAS as a feedstock leads to a decreased unit cost of hydrogen production and the feasibility of biohydrogen energy commercialization. The origin of the WAS effect pretreatment steps are suggested for higher biohydrogen yields. The C to N ratio of the WAS is an essential criterion for reutilization by photo or dark fermentation. The photofermentation process is generally limited by low nitrogenase activity. The limitations will be overcome by improving nitrogenase expression levels and using the higher catalytic turnover enzyme. Although the total biohydrogen production from photo-fermentation was predicted to be greater than that from dark fermentation, the reaction rate was found to be inadequate. Although thermochemical processes have improved the hydrogen yields, remarkable CO2 emissions must be taken into consideration. Hydrogen production from WAS is a promising approach; clean energy can be achieved by the proper combination of highly efficient hydrogen production and CO2 capture technologies. Controlling the operating conditions and applying the CO2 capture procedure is also necessary to promote this technologies for future applications. Besides, the biological process provides economic benefits; it requires more process time to get hydrogen than the thermochemical process. When the economic and technological constraints in the sectors are overcome, waste as a feedstock and the production of clean energy will become more common. It will be offered for public use.

646  Materials for Hydrogen Production, Conversion, and Storage The  implementation of hydrogen from WAS and CO2 capture systems looks like a potential solution for the future energy policy.

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Waste-Activated Sludge to Produce Hydrogen  653 97. Lee, I.G., Nowacka, A., Yuan, C.H., Park, S.J., Yang, J.B., Hydrogen production by supercritical water gasification of valine over Ni/activated charcoal catalyst modified with Y, Pt, and Pd. Int. J. Hydrog. Energy, 40, 12078, 2015. 98. Weijin, G., Zizheng, Z., Yue, L., Qingyu, W., Lina, G., Hydrogen production and phosphorus recovery via supercritical water gasification of sewage sludge in a batch reactor. Waste Manage., 96, 198, 2019. 99. Gong, M., Zhu, W., Xu, Z.R., Zhang, H.W., Yang, H.P., Influence of sludge properties on the direct gasification of dewatered sewage sludge in supercritical water. Renew. Energy, 66, 605, 2014. 100. Yan, B., Wu, J., Xie, C., He, F., Wei, C., Supercritical water gasification with Ni/ZrO2 catalyst for hydrogen production from model wastewater of polyethylene glycol. J. Supercrit. Fluids, 50, 155, 2009. 101. Zhang, L., Champagne, P., Xu, C.C., Supercritical water gasification of an aqueous by-product from biomass hydrothermal liquefaction with novel Ru modified Ni catalysts. Bioresour. Technol., 102, 8279, 2011. 102. Xu, D., Wang, S., Tang, X., Gong, Y., Guo, Y., Zhang, J., Wang, Y., Ma, H., Zhou, L., Influence of oxidation coefficient on product properties in sewage sludge treatment by supercritical water. Int. J. Hydrog. Energy, 38, 1850, 2013. 103. Xu, Z.R., Zhu, W., Gong, M., Zhang, H.W., Direct gasification of dewatered sewage sludge in supercritical water. Part 1: Effects of alkali salts. Int. J. Hydrog. Energy, 38, 3963, 2013. 104. Seif, S., Fatemi, S., Tavakoli, O., Bahmanyar, H., Hydrogen production through hydrothermal gasification of industrial wastewaters using transition metal oxide catalysts. J. Supercrit. Fluids, 114, 32, 2016. 105. Wang, Y., Zhu, Y., Liu, Z., Wang, L., Xu, D., Fang, C., Wang, S., Catalytic performances of Ni-based catalysts on supercritical water gasification of phenol solution and coal-gasification wastewater. Int. J. Hydrog. Energy, 44, 3470, 2019. 106. Sawai, O., Nunoura, T., Yamamoto, K., Supercritical water gasification of sewage sludge using bench-scale batch reactor: advantages and drawbacks. J. Mater. Cycles Waste Manage., 16, 82, 2014. 107. Amrullah, A. and Matsumura, Y., Supercritical water gasification of sewage sludge in continuous reactor. Bioresour. Technol., 249, 276, 2018. 108. Adar, E., Ince, M., Bilgili, M.S., Supercritical water gasification of sewage sludge by continuous flow tubular reactor: A pilot scale study. Chem. Eng. J., 391, 123499, 2020. 109. Okolie, J.A., Nanda, S., Dalai, A.K., Berruti, F., Kozinski, J.A., A review on subcritical and supercritical water gasification of biogenic, polymeric and petroleum wastes to hydrogen-rich synthesis gas. Renew. Sust. Energy Rev., 119, 109546, 2020.

21 Current Trends in the Potential Use of the Metal-Organic Framework for Hydrogen Storage Maryam Yousaf1, Muhammad Ahmad1, Zhi-Ping Zhao1*, Tehmeena Ishaq2 and Nasir Mahmood3† School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, P.R. China 2 Department of Chemistry, The University of Lahore, Sargodha Campus, Sargodha, Pakistan 3 School of Engineering, RMIT University, Melbourne, Victoria, Australia

1

Abstract

The establishment of a hydrogen (H2) economy is necessary to meet the energy crisis and clean environment standards faced by humanity. However, the scientific challenge in commercializing H2 is the non-availability of efficient and cost-­effective storage methods. Currently employed methods for H2 storage are cryogenic and compression, which are not only expensive but also have serious safety issues. Metals-organic frameworks (MOFs) are considered as a potential candidate for reliable hydrogen storage due to their open structure, higher surface area, porosity, and pore volume which is an outcome of a novel hybrid combination of their inorganic and organic moieties. MOFs can adsorb H2 through weak van der Waals force of attraction i.e., physisorption with rapid desorption kinetics. While the spillover mechanism provides high-density H2 storage upon combining with the carbon materials. The effect of various factors, i.e., surface area, pore volume and size, open metal center, catenation, and ligand functionalization over H2 storage capacities of MOFs have been discussed in this chapter. Moreover, recommendations to overcome existing shortfalls in MOFs have been given to make them suitable for commercial H2 storage. This book chapter will

*Corresponding author: [email protected] † Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (655–680) © 2023 Scrivener Publishing LLC

655

656  Materials for Hydrogen Production, Conversion, and Storage provide fundamental knowledge to design advanced functional MOF materials for industrial-scale H2 storage. Keywords:  Metal-organic framework, hydrogen storage, doped MOFs, hybrid of MOFs, spillover mechanism, linker functionalization

21.1 Introduction Rapid industrialization and globalization have raised World’s energy demands leading towards serious energy and environmental crisis [1]. To meet increasing energy demands fossil fuel consumption has rapidly increased (56% by 2040) that has not only fastened fossil fuel depletion but have also increased environmental pollution. Globally used primary energy sources are coal, natural gas, and oil [2]; burning of these sources is continuously emitting greenhouse gases especially carbon dioxide (CO2) with its level raise to >400 ppm (in the year 2020) that is in turn creating anthropogenic global warming [3]. Moreover, fossil fuel resources (non-­ renewable resources) are depleting continuously e.g., oil supply can only last for 41 years, whereas energy demands are continuously rising [4]. The simplest solution to both problems is the replacement of non-­ renewable energy with clean and renewable energy like hydrogen (H2) that is considered to play an important role in developing a sustainable energy system by 2050 [5, 6]. However, its low energy density by volume limits the availability of a cost-effective and efficient H2 storage system that hinders the establishment of a “Hydrogen economy”. Therefore, the establishment of an efficient H2 storage system is the most important challenge for the scientific world in the 21st century [7, 8]. Presently used H2 storage technologies are cooling/cryogenic storage; the boiling point of H2 is 20 K that required an expensive insulated system to control lower temperature (−253°C) for extensive cooling of H2. Another method is compressed storage at atmospheric pressure that is not possible due to the low density of H2 (i.e., 0.08988 k g m-3). Therefore, high pressure of up to 700 bar is required in larger tanks for compressed storage of H2 gas. Both these methods are not only costly but also have safety issues like maintaining (−40°C to −60°C) such temperature during transportation. Moreover, compression at higher pressure reduced the volumetric energy density of H2 to 5.6 MJ/L at 298 K that is much lower than the gasoline energy density of 32.4 MJ/L [9, 10]. Therefore, an alternative route for the storage of H2 gas is storage over the solid materials than storing by compression or cooling [11]. H2 gas storage

Metal-Organic Framework for Hydrogen Storage  657 over solid materials takes place by physisorption (binding of H2 by van der Waals forces) or chemisorption (formation of a new compound upon atomic hydrogen strong bonding). Physisorption is more advantageous than chemisorption due to the rapid kinetics of adsorption/­desorption of H2 and the complete reversibility of H2 uptake. No doubt, larger quantities of H2 can be stored by the chemisorption but its release needs higher temperature with less uptake release. Therefore, physisorption is a more appropriate way of storing H2 over solid materials. Owing to the type of physisorption interaction most commonly employed H2 storage solid materials are metal-organic frameworks (MOFs), activated carbon, porous polymers, and zeolites. Among different adsorbents, MOFs have gained huge attention as H2 storage materials since 2003 after the pioneering work of Yaghi and co-workers reported for adsorption of H2 over highly porous MOFs [12, 13]. MOFs are crystalline materials with exceptional properties like tunable internal surface area, pore size, larger external surface area (up to 7140 m2 g−1) with ultrahigh porosity (up to 90% free volume), which makes it superior over other frequently used porous materials like zeolites and carbon nanostructures for H2 storage [14, 15]. Essential parameters required for H2 storage are favorable thermodynamics, gravimetric and volumetric storage capacity [16], fast kinetics with excellent reversibility, low desorption temperature, longer recyclability, and low cost to meet the US Department of Energy (DOE) 2020 targets, i.e., 6.5 wt% and 50 g/L usable H2 storage capacity at 233–358 K temperature and 5–12 bars pressure. Targets set by DOE emphasized R&D to stimulate research on H2 storage [17, 18]. This chapter will discuss the opportunities of using MOFs for H2 storage, their mechanism and the possible structural modification (Figure 21.1) to enhance their H2 storage capacity [19]. It will also explain the factors affecting the H2 storage capacity of MOFs and how to overcome these challenges. Further, recommendations for future work on this unique class of materials are also given to make them suitable for H2 storage for realworld application.

21.2 Structure of MOFs MOFs are the coordination frameworks formed by the co-ordinate covalent bonding between the positively charged metals ions (secondary building unit (SBU)) and negatively charged organic linker (Figure 21.2a). Unsaturation sites, i.e., Lewis sites in the SBU are among the binding sites for H2 gas [21]. The topology of the MOFs is directed by the well-directed

658  Materials for Hydrogen Production, Conversion, and Storage

Linker functionalization catenation

pristine MOF

MOF for H2 storage H2 doping

Open metal sites

H2 HH H metal H H H C H H

hy b

rid

H

MOFs

MOFs

H

MOFs

spill over

Figure 21.1  An overview of strategies that can be employed for the enhancement in the H2 storage capacity of MOFs. Images reprinted with permission from [20, 42, 46, 57, 58], Copyrights American Chemical Society.

(a) Mn-INA-1

(b) HO

O

HO

O

F

Functionalization

Mn-PAA-1

HO

Mn+2 Co+2 O

Cd

+2

HO

N

4-Pyridineacrylic acid (PAA) Cd-PAA-1

Zr4+

Al3+

Cr3+

Co-FINA-2

Cd-FINA-1

O

O

Mn-FPAA-1

Octahedron

Cuboctahedron

Triangle

Trigonal prism

MOF-5(Zn)

UiO-66

MIL-53(AI)

MIL-101(Cr)

Pore size: 8 Ā BET: 881 m2·g-1

Pore size: 7 & 2.1 Ā BET: 1500 m2·g-1

Pore size: 29 & 34 Ā BET: 4100 m2·g-1

F

Functionalization Co-PAA-1

Zn2+

N

3-Fluoroisonicotinic acid (FINA) Isoreticulation

Cd-INA-1

N

Isonicotinic acid (INA) Isoreticulation

Co-INA-2

HO

O OH Organic ligand: terphthalic acid

Mn-FINA-1

Co-FPAA-1

N

3-Fluoro-4-pyridineacrylic acid (FPAA) Cd-FPAA-1

Pore size: 11 Ā BET: 950 m2·g-1

Figure 21.2  (a) Different secondary building units and organic linkers used in the synthesis of MOFs. Reprinted with permission from [53], Copyright 2016, American Chemical Society. (b) Combination of one linker with different metal nodes to form different kinds of MOFs. Reprinted with permission from [28], Copyright 2019, MDPI.

Metal-Organic Framework for Hydrogen Storage  659 connecting points of SBUs with which organic ligands directionally get binds to form MOFs with a specific topology [22]. This organic coordinated framework forms highly porous materials with a larger surface area and permanent porosity that can be easily tuned without changing their underlying topology [23]. For instance, an organic linker can be combined with different SBU to form various combinations of MOFs (Figure 21.2b). Further, by changing the symmetry, and length of carbon chains in organic ligands pore size of MOFs can be easily tuned [24]. Longer chain organic linkers provide more adsorption sites and storage space for H2 gas storage. Moreover, the pore surface can be modified by the functionalization of the organic linkers with different functional groups that can further enhance the binding sites for the gas adsorption followed by storage [25]. The general structural arrangement of SBU and organic linkers in MOFs is presented in Figure 21.2a. Structural modification of MOFs can be easily done to enhance their H2 storage capacity and stored H2 can be converted into electricity through the proton exchange membrane cells. H2 storage capacity of MOFs is modeled using polynomial neural networks (GMDH-PNN) pressure, surface area, and adsorption enthalpy as input variables, and H2 storage capacity as the output variable. H2 storage capacity of MOFs mainly varies with the pressures and surface area and the H2 uptake capacity of MOFs can be determined by using Equation 21.1 [26]:



wt %

mass of the H 2 100 mass of MOF mass of the H 2



(21.1)

MOFs could be used for clean energy storage because of their high H2 storage capacity than the compression method but needs modification for practical and sustainable use [27]. H2 uptake capacity of MOFs can be modified by applying different strategies, i.e., increasing surface, functionalization of linkers, using open metal centers, making interpenetrated frameworks, post-synthetic modification by forming heterostructure with semiconductors, doping of MOFs, nanoparticles incorporation, and metal substitutions to achieve enhanced functionality and stability for H2 gas storage.

21.3 Mechanism of H2 Storage by MOFs H2 is stored in the MOFs mainly through weak van der Waals force/ London dispersion, i.e., physisorption with the linkers and connectors.

660  Materials for Hydrogen Production, Conversion, and Storage Such storage has the advantage of the high reversibility of adsorbed H2 molecules. The chemical nature of organic linkers and pore structure decides the adsorption of H2, where linkers with π-electrons show higher adsorption than without π-electrons; however, there is no clear correlation exist (Figure 21.3a). The higher the physisorption binding energy of MOFs, the higher will be the H2 storage. Physisorption of H2 is related to the surface area of the material, larger surface area favors more adsorption of H2, that is, MOFs having larger surface areas with low densities are attractive materials for H2 storage. By increasing surface area and pressure H2 storage capacity of a specific MOF can be increased [29, 30]. However, for practical application, MOF should adsorb and desorb H2 at room temperature that can be achieved by the “spillover” mechanism that offers higher adsorption/desorption at ambient temperatures. Physisorption binding is considered as a week binding of H2 molecules (carried out at 77 K) than the binding of H atoms at room temperature created by H2 dissociation catalyst (chemisorption). Mechanistically spillover occurs through i) chemisorption of H2 over the metal catalyst surface (energy required for chemisorption of H2 is 0.8–1.8 eV), ii) dissociation of H2 into H atoms over the metal catalyst, iii) migration of H from a metal catalyst to the support surface, and iv) finally diffusion and desorption at substrate surface (Figure 21.3b) [31]. The chemisorption of H2 can be attained by the phase nucleation process. Moreover, MOFs surface offered less energy barrier, i.e., 1.05–2.16 eV for the diffusion of H over the support surface that favors more adsorption of H2 at room temperature.

(a)

(b) H2

H

HH Pt AC

H

H MOF

z x

H

MOF

MOF

y

Figure 21.3  (a) H2 adsorption sites over M(bdc)(ted)0.5 linker. Reprinted with permission from [30], Copyright 2010, American Chemical Society. (b) Description of spillover mechanism for the H2 storage. Reprinted with permission from [31], Copyright 2006, American Chemical Society.

Metal-Organic Framework for Hydrogen Storage  661

21.4 Strategies to Modify the Structure of MOFs for Enhanced H2 Storage 21.4.1 Tuning the Surface Area, Pore Size, and Volume of MOFs The surface area played an important role in H2 adsorption. In fact, the rate of H2 adsorption by MOF is directly proportional to its surface area. The theoretical and experimental data have shown that at a higher pressure and low temperature (77 K), a linear increase in wt% of H2 uptakes is observed with an increase in MOFs surface area due to the availability of a large number of adsorption sites. That is consistent with Chahine’s rule (for microporous carbons) that every 500 m2/g adsorb 1 wt% H2 gas. However, H2 gas interaction with the surface of adsorbent is low, but the presence of pores with the diameter equivalent to the kinetic diameter of H2 molecule (2.89A°) could enhance the interaction of H2 with adsorbent surface showing the involvement of pores volume and size in H2 uptakes/storage along with larger surface area. Gravimetric and volumetric densities of H2 storage are directly proportional to the surface area of MOF [32]. MOF-5 (BDC-1,4-benzenedicarboxylate) was the first MOF to be studied for the H2 storage in 2003 with a wt% capacity of up to 7.1 at 40 bar and 77 K [33]. Further, Yang et al. prepared MOF-5 with varying crystallinity, i.e., pristine (P) MOF-5 and crystalline (C)-MOF-5. Despite having pores of similar sizes C-MOF shows higher H2 uptake capacity than P-MOF due to larger pore volume 1 nm. A similar role of pore volume was also observed in interwoven MOF-5 with and without coupling with multiwalled carbon nanotubes [34]. However, according to Snurr et al., both surface area and pore volume are important for H2 gas adsorption. Their Grand Canonical Monte Carlo (GCMC) simulations study revealed three H2 adsorption regimes, i.e., higher, medium, and lower pressure regimes where H2 adsorption is related to pore volume, surface area, and heat of adsorption of MOFs, respectively [35]. Later, Yang et al. correlated the H2 uptake of Cu3(BTC)2 with the Langmuir/BET surface area and pore volume [36]. Similarly, Xia et al. also performed GCMC simulation to study the H2 adsorption behavior of two aluminium-based MOFs, i.e., MOF-519 (Al8(OH)8(BTB)4(H2BTB)4) and MOF-520 (Al8(OH)8(BTB)4(-HCOO)4) having same framework topology. They suggested a correlation between structure and adsorption capacity of MOFs having the same topology but different H2 storage capacity. Different H2 storage capacity is attributed to

662  Materials for Hydrogen Production, Conversion, and Storage the different surface area (MOF-19 = 2400 m2g−1, MOF-520 = 3290 m2g−1) and pore volume (MOF-19 = 0.94 cm3/g, MOF-520 = 1.28 cm3/g). At lower pressure, MOF-19 shows higher H2 storage (3.94 wt%) at linkers oxygen atoms due to larger isosteric heats. While at higher pressure, MOF-520 shows higher H2 storage (7.8 wt%) that shows the involvement of higher surface area and pore volume of MOF-520 with adsorptive sites near aluminium oxide at 298 K and 130 bar. These findings confirmed the involvement of surface area, pore volume, and heat of adsorption in the H2 storage by MOFs that can be tailored to enhance the MOFs H2 storage capacity [37]. The surface area and pore volume of the MOFs can be increased either by increasing the length of linkers, by using mixed linkers, or both to enhance the H2 adsorption capacity of MOFs. Based on recent studies, a correlation is found between H2 uptake capacity and variation in the length of ligand that ultimately changed the interacting surface area and pore volume. The longer the ligand length, the larger will be the surface area and pore volume for the adsorption of H2 gas. For instance, at 77 K and 70 bar, MOF-177 {Zn4O (1,3,5-benzenetribenzoate)2} with a surface area of 4500 m2g−1 possess the capacity to store 9.9 wt% H2 [35]. However, upon expansion of MOF-177 with 4,4′,4″-[benzene-1,3,5-triyl-tris(benzene-4,1-diyl)] tribenzoate (BBC) linker to form MOF-200 [(Zn4O)3(BBC)2] increased surface area (4530 m2g−1) and H2 uptake capacity to 14.0 wt% at 77 K and 80 bar. The surface area and pore volume were further enhanced to 6240 m2g−1 which increase the H2 uptake capacity to 15.0 wt% at same conditions by using mixed linkers in MOF-210 [(Zn4O)3(BTE)4(BPDC)3] where BTE and BPDC correspond to 4,4′,4″-[benzene-1,3,5-triyl-tris(ethyne-2,1-diyl)]tribenzoate and biphenyl-4,4-dicarboxylate, respectively [38]. Similarly, another MOF (DUT-32, Zn4O(bpdc)(btctb)4/3) was synthesized by using ditopic (4,4′-biphenylendicarboxylic (bpdc2−)) and tritopic (btctb3−) mixed ligands linkers. Usage of di and tritopic ligands enhanced the surface area 6411 m2g−1 that ultimately increased H2 uptake to 14.2 wt% (Figure 21.4) [39]. In addition to the surface area and pore volume, another contributing factor in the H2 uptake capacity of MOF is the pore size. Yan et al. studied series of NbO-type MOFs, i.e., NOTT-112 and NOTT-116 (where NOTT represents Nottingham) with the same topology and cuboctahedral cages but have different organic linkers. NOTT-112 with lesser surface area (3800 m2g−1) than NOTT-116 (4664 m2g−1) shows higher adsorption of H2 (7.07 wt.%) than the NOTT-116 (6.4 wt.%) at 77 K and 50 bar. These results show that along with pore volume, pore size also plays a substantial role in the adsorption of H2 [7]. Further, Montes et al. fabricated Ni/ IRMOF-74 with larger pore size material and modified it by incorporating Li crown. The resultant Li/Ni/IRMOF-74 nanohybrid revealed enhanced

cis

trans

200 175 150 125 100 75 50 25 0

H2

50 40 30 20 10

0

20 40 60 80 100 120 p/bar

H2 uptake/g L-1

H2 uptake/mg g-1

Metal-Organic Framework for Hydrogen Storage  663

0

Figure 21.4  Diastereomeric secondary building subunits in DUT-32 with physisorption isotherm at 77 K for H2 adsorption (Colors: green – cluster, blue – tritopic linkers and orange – ditopic linkers). Reprinted with permission from [39], Copyright 2014, Royal Society of Chemistry.

H2 absorption and storage capacity at ambient temperature due to stronger H2-MOF linkage with an enthalpy value of −15.0 kJ/mole which was credited to enhance electrostatic interaction due to Li+ ion incorporation within the Ni/IRMOF-74 shell. Thus, it is a promising nominee for the H2 storage at the ambient condition of pressure and temperature. Density Function Theory (DFT) calculations revealed that Li+ ions inside the Ni/IRMOF-74 shell offer promising absorption sites for the H2 molecules which provide optimal H2-MOF interaction and results in improved H2 absorption volume at standard temperature value which is a prerequisite for the H2 storage application [40]. Hence, along with surface area, pore volume and size, changing the chemistry also impacts the H2 storage capacity.

21.4.2 Enhancement in Unsaturated Open Metal Sites Another potential contributing factor that promotes the H2 storage capacity of MOFs is the availability and alignment of the exposed unsaturated metal sites which act as binding sites for H2. On contrary, fully saturated metal sites will offer less tendency to adsorb H2. The presence of open metal sites offered H2 storage at ambient conditions. Open unsaturated metal sites can be incorporated in MOFs by removing/desolvating the coordinated solvent molecules with the metals that in turn enhance the adsorption capacity of MOFs. The strength of MOFs and H2 interaction can be represented in terms of the isosteric heat of hydrogen adsorption [7]. First studied MOF for H2 storage with unsaturated exposed metal sites was M3[(M4Cl)3(BTT)8]2 {M-BTT; BTT3− = 1,3,5-benzenetristetrazolate; M = Mn, Fe, Co, Cu, Cd} (Figure 21.5a). Among different metals, Mn-MOF shows the highest H2 uptake at ambient conditions

664  Materials for Hydrogen Production, Conversion, and Storage (a)

(b)

3.33 Å

IV C N I

Mn

D2 2.27 Å

C N CI

III

Mn

Cl

II

Figure 21.5  (a) Tritopic ligand H3BTT molecular structure with partially occupied extra framework open Mn2+ ion sites (orange spheres), Reprinted with permission from [42], Copyright 2006, American Chemical Society. (b) Mn3[(Mn4Cl)3(BTT)8(CH3OH)10]2 crystal structure portion with intra- (orange) and extra-framework (red) open metal sites for H2 adsorption. Reprinted with permission from [43], Copyright 2007, American Chemical Society.

(room temperature and 90 bar), i.e., 0.94 wt% and 7.9 g/L with an adsorption enthalpy of 10.1 kJ/mol. Fe-MOF, another member of this MOF family, possesses strong interaction with the MOF 11.9 kJ/mol enthalpy of adsorption. However, partial activation of these MOFs limits their application for H2 uptake [41, 42]. Dinca et al. fabricated and evaluated various [(Mn4Cl)3(BTT)8(CH3OH)10]2-based MOFs labeled as M[(Mn4Cl)3(BTT)8(CH3OH)10]2 with various metal ions like Cu+, Li+, Co2+, Cu+, Cu2+, Ni2+, and Zn2+. The fabricated microporous MOFs with exchanged metals were stable and revealed superior H2 storing capability with wt% of 2.0%–2.29% at 900 torr and 77 K (Figure 21.5 b) [43]. There have been reports about second metal incorporation enhanced the H2 storage capacity of Ni MOFs. For example, Yang et al. fabricated Mg/ Ni@CH hybrid from trimesic acid/Ni-based MOF (TMA/Ni MOF) and NiC as precursor via ball milling and demonstrated that two X-ray diffraction (XRD) peaks were detected in the fabricated Mg/Ni@CH nanohybrid which were ascribed to hydrogenation of Mg2Ni and Mg with enthalpy value of −52.30 and 75.80 kJ/mole for H2, respectively. It was revealed that the hydriding or dehydriding rate of MgH2 is considerably enhanced by adding Ni MOF. The values of activation energy for H2 absorption/ desorption were considerably decreased with NiC. Furthermore, stable H2 absorption/desorption capability and kinetics remained constant till 25 cycles of the hydriding/dehydriding process which was credited to the carbon enveloped array of the nanohybrid which makes it a more stable configuration [44].

Metal-Organic Framework for Hydrogen Storage  665 In addition to variation in metal centers incorporation of structural isomers can also vary the H2 absorption/desorption capacity of MOFs. For instance, Kapelewski et al. reported the H2 absorption capacity for M2(dobdc) with the incorporation of meta liner of dobdc that is 4,6-dioxido-1,3-benzenedicarboxylate linker (m-dobdc4−). An increase in binding enthalpy 0.4–1.5 kJ/mol was observed due to a change in ligand field symmetry near the metal site. Among different used metals highest H2 binding was achieved for Ni2(m-dobdc) form with a usable volumetric capacity of 11.0 g/L and 23.0 g/L upon varying temperatures between −75°C and 25°C at 100-5 bar pressure (Figures 21.6a,b) [10]. Hence, enhanced H2 storage capacity at ambient conditions can be achieved by varying metals, desolvating the metal centers and by use of linker structural isomers that will change the symmetry of the metal site.

21.4.3 MOFs with Interpenetration

25

2.0 (b)

Ni2(dobdc)

1.0

10

0.5

5 0

d 0

20

40

60

P (bar)

80

100

0.0

25

−75ºC −50ºC 20 −40ºC −25ºC 0ºC 25ºC 15 50ºC 75ºC 100ºC

2.0

Ni2(m-dobdc)

1.5 1.0

10

0.5

5 0

Total H2 Uptake (wt%)

1.5

Total H2 Uptake (wt%)

−75ºC −50ºC 20 −40ºC −25ºC 0ºC 25ºC 15 50ºC 75ºC 100ºC

Total H2 uptake (g/L of crystal)

(a)

Total H2 uptake (g/L of crystal)

Low-pressure adsorption of H2 is more favorable with MOFs having pore size close to the kinetic diameter of H2 as compared to MOFs with larger pore size. Therefore, controlling the pore size is also a crucial factor to enhance the H2 adsorption/storage capacity of MOFs. A reliable approach of modulation of pore size is catenation or interpenetration that involves the simultaneous intergrowth of different frameworks with controlled pore size and with enhanced adsorption sites per unit volume [45]. Initially, PCN-6 {Cu6(H2O)6(TATB)4; TATB-4′, 4″-s-triazine-2,4,6-triyl-tribenzoate) MOF was studied for the effect of interpenetration over H2 uptake capacity (Figures 21.7a,b). Interpenetrated PCN-6 at 77 K and 1.01 bar

b 0

20

40

60

P (bar)

80

100

0.0

Figure 21.6  H2 adsorption isotherms represented in terms of volumetric and gravimetric capacity, representing H2 uptake of (a) Ni2(dobdc) and (b) Ni2(m-dobdc) upon addition of meta linker taken at different temperature and pressure. Reprinted with permission from [10], Copyright 2018, American Chemical Society.

666  Materials for Hydrogen Production, Conversion, and Storage (a)

(b)

Figure 21.7  A visual representation of the effect of interpenetration over the pore size of PCN-6 MOF. PCN-6 without interpenetration (a) and with interpenetration (b). Reprinted with permission from [46], Copyright 2006, American Chemical Society.

showed 40% higher value, i.e., 1.90 wt% of H2 gravimetric uptake than the non-interpenetrated PCN-6 (1.35 wt%) [46]. Later on, Kim et al. studied the effect of interpenetration over H2 storage capacity on MOF-5, prepared through pH adjustment of reaction mixture around 4. Gravimetric and volumetric H2 capacities of interpenetrated MOF-5 at 77 K and 1 bar pressure were 2.0 wt% and 23.3 g/L compared to non-interpenetrated MOF-5, i.e., 0.7 wt% and 7.9 g/L, respectively, attributed to the smaller pores of interpenetrated MOF-5. Further, with the increase in pressure to 100 bar interpenetrated MOF-5 shows 2.8% H2 uptake that is less than non-interpenetrated MOF-5 (10%). A decrease in interpenetrated MOF-5 H2 uptake with increasing pressure is attributed to the decreases in Langmuir surface area from 4400 to 1130 m2/g due to the interpenetration that shows at higher pressure the H2 uptake is proportional to the Langmuir surface area which is higher in the case of non-­ interpenetrated MOF-5 [47] (Figures 21.8a,b). Hence, the structure is not only determining factor for the H2 storage capacity, it also depends on the working conditions. Recently, Xicohtencatl et al. compared the volumetric H2 storage capacity of interpenetrated (CFA-7) and non-interpenetrated (MFU-4l) MFU-4 MOFs families. Interpenetrated structure of CFA-7 has a smaller pore diameter of 11.71 Å than the MFU-4l (18.56 Å) due to intergrowths. Thus, CFA-7 showed two times higher volumetric H2 storage capacity than the MFU-4l at 77 K [48].

Metal-Organic Framework for Hydrogen Storage  667 (a) Amount adsorbed/gL-1

7 (b) 6 5 4 3 2 1 0

0

20

40 60 Pressure/bar

80

100

Figure 21.8  (a) Crystal structure of two different types of pores (0.76 nm and 0.6 nm). (b) Amount of H2 adsorbed. Reprinted with permission from [47], Copyright 2011, American Chemical Society.

Further, Pachfule et al. reported Cd-ANIC-1 ([Cd(ANIC)2]; ANIC = 2-amino-isonicotinic acid) and Co-ANIC-1([Co(ANIC)2]) isostructural amino-functionalized MOFs that displayed comparable significant H2 uptakes at 1 bar and 77 K, i.e., 1.84 wt% and 1.64 wt%, respectively, as the interpenetration pore size was comparable to the H2 molecule size [49]. Hence, highlighting that interpenetration not only tunes structure but also helps in modifying pore diameter. Later, Li et al. also studied the experimental and theoretical effect of twofold interpenetration and the imidazole-functionalized ligand over the H2  uptake of  UPC-501 (with Zn4O(COO)6 SBU) and it shows higher H2 uptake (2.96 wt %) at 0.1 MPa and 77 K. This capacity was highest than any other Zn MOF reported in the literature that confirmed the role of interpenetration in the pore size reduction with the enhanced H2 uptake [50].

21.4.4 Linker Functionalization of MOFs The H2 storage capacity of the MOFs can also be enhanced by increasing the adsorption sites of MOFs for H2 through introducing polar functional groups over the organic linker. This not only changes the polarity of the linker but also polarize the metal nodes, thus altering the polarity of whole MOFs and ultimately enhancing the binding interaction of H2. The addition of some polar functional groups like –F, Br, –NH2, –COOH, –CF3, and –OH will dramatically change the polarity of whole MOFs that will ultimately vary the binding of H2 with MOFs irrespective of binding with the negative or positive part of the MOFs [51]. For instance, Omary et al. incorporated –CF3 groups in the MOF and observed a hysteretic volumetric H2 sorption (41 kg/m3) at 77 K and 64 bar in FMOF-1 [Ag2[Ag4-Tz6]

668  Materials for Hydrogen Production, Conversion, and Storage n {Tz; 3,5-bis(trifluoromethyl)-1,2,4-triazolate}. Incorporation of the –CF3 group makes the cavities of FMOF-1 hydrophobic that shows a two-step H2 adsorption profile [20]. According to Zlotea et al., incorporation of –CF3 groups in MIL-53 (Fe) blocked pore contraction in MIL-53(Fe) (close pore structured MOF) and enabled it to adsorb 0.6 wt% of H2 (Figures 21.9a,b) [52]. Similarly, Banerjee et al. studied the effect of fluorination of linkers over the H2 adsorption capacity of MOF. FINA (3-fluoro-4-pyridinecarboxylic acid) and FPPA (trans-3-fluoro-4-pyridineacrylic acid) linkers with different metal ions (Co2+, Mn2+ and Cd2+) were studied, where fluorinated Co2+MOFs showed a positive effect of fluorination over linker with 1.61 wt% H2 adsorption at ambient conditions than nonfluorinated Co2+MOFs (0.54 wt%) [53]. Furthermore, Zhang et al. also studied that functionalization with fluorine depending upon their position and number have a remarkable effect on the H2 adsorption capacity of MOFs, showing controlled incorporation is required to obtain enhanced results (Figure 21.10a,b) [54]. Similarly, the effect of other functional groups (isocyanates, amides, nitro, etc.) incorporation in linker is also studied with enhanced H2 adsorption capacity of MOF depending upon the extent of functionalization and position along with the relative effect over the polarity of MOF. Interestingly, Szilagyi et al. described that functionalization can introduce new adsorption sites or can only modified existing ones depending upon the functional group used through thermal-desorption spectroscopy. Simultaneously, the introduction of different ligands through the push-and-pull effect of the linkers enhanced the H2 adsorption capacity at the positive/cationic nodes [55]. Thus, these studies have shown that the H2 adsorption capacity of MOFs can be improved through functionalization; however, the type, concentration, position and electronegativity are key factors to keep in mind.

21.4.5 Hybrid and Doping of MOFs In addition to the structural modifications, the H2 storage capacity of MOFs can also be enhanced by making its hybrid and composites with the different metals, semiconductors and graphene materials (graphene oxide (GO)) that directly impact the surface area, porosity, loading capacity, and controllability of MOFs. Formed composites of MOFs can adsorb H2 either through chemisorption and/or physisorption depending upon the composite forming material with the MOF. For example, incorporation of rGO in Zr-MOF increased the surface area of Zr-MOF from 1116 m2g−1 to 1480 m2g−1 and modified the pore size which resulted in increased H2 adsorption from 1.4 wt % to 1.8 wt% through physisorption [56]. Similarly,

Metal-Organic Framework for Hydrogen Storage  669

(a)

F HO

F

O

F

F

HO

F

N

F

F

F

N

Mn+2 = Co+2 Cd+2

Isoreticulation

F F

Isoreticulation

O

F

Functionalization

F F F

HO

Functionalization

F

1.0

0.8 0.6 0.4 0.2

1.4

Co-FPAA-1 Co-PAA-1 H2 Adsorbed (wt%)

1.2

Mn-PAA-1 Mn-FPAA-1 H2 Adsorbed (wt%)

H2 Adsorbed (wt%)

N

F

(b)

0.0 0.0

O

F

F

F

1.0

HO

F

N

1.2

F

O

0.8 0.6 0.4 0.2

0.2

0.4 0.6 Pressure (bar)

0.8

1.0

0.0 0.0

1.2

Cd-PAA-1 Cd-FPAA-1

1.0 0.8 0.6 0.4 0.2

0.2

0.4 0.6 Pressure (bar)

0.8

1.0

0.0 0.0

0.2

0.4 0.6 Pressure (bar)

0.8

1.0

Figure 21.9  (a) Structural representation of MOF with and without isoreticulation and F groups. (b) H2 adsorption isotherms at 77 K. Reprinted with permission from [53], Copyright 2016, American Chemical Society.

670  Materials for Hydrogen Production, Conversion, and Storage

N N

COOH Ni(NO3)2·6H2O

+R

COOH

Solvent

N

-H (TKL-101) -NH2 (TKL-102) -NO2 (TKL-103) 4-F(TKL-104) 3-F(TKL-105) 3,6-(F)2 (TKL-106) (-F)4 (TKL-107)

R=

Functional groups

TKL-104 @ 77 K TKL-105 @ 77 K TKL-106 @ 77 K TKL-107 @ 77 K

200

2.0 1.6

150

1.2

(wt%)

N N

(b)250

N

H2 adsorbed (cm3g-1)

(a)

100 50 0 0

200

400

0.8 TKL-104 @ 87 K TKL-105 @ 87 K TKL-106 @ 87 K 0.4 TKL-107 @ 87 K 0.0 600 800 1000

Absolute Pressure (mmHg)

Figure 21.10  (a) Synthesis route of TKL MOFs and (b) H2 adsorption isotherms at 77 K and 88 K. Reprinted with permission from [54] Copyright 2013, Nature.

Metal-Organic Framework for Hydrogen Storage  671 Panchariya et al. prepared carbon-MIL-101 hybrid composites through in situ hydrothermal carbonizations of glucose. Carbon-MIL-101 hybrid composites show 11% higher adsorption of H2 than the parent MIL-101 at ambient conditions (Figure 21.11a) [57]. MOFs composite can also be made with another MOF to enhance the H2 storage capacity. For instance, by designing the core-shell of ZIF-8@ZIF-67 vice versa through the seeded growth method, an enhanced H2 storage capacity of 2.03 and 1.69 wt % was observed than the parent ZIF-8 and ZIF-67 at ambient conditions (Figure 21.11b) [2]. Another way to enhance the H2 storage capacity of MOFs is through the introduction of alkali or alkaline earth metal ions (Li+, Na+, Ca+2, and Mg+2, etc.). Interestingly, this strategy can also modify the MOF with dual function to produce and store H2 through simultaneous hydrogenation

(a)

H2 uptake capacity (wt%)

Carbon-MIL-101 composites

1.7 1.6

MIL-101

1.5 1.4 1.3 1.2

Increasing carbon content

(b)

ZIF-8

0.2

0.4 0.6 1.8 Pressure (bar)

1.0

0 1.2

1.0 0.5 0.0

ZIF-67

2

1.5 Cn757n25-ZIP-8

0.5

Core-Shell ZIF-8@ZIF-67

ZIP-8+ZIF-67

4

2.0

ZIP-67@ZIP-8

8 6

H2

H2 H2

10

ZIF-8 ZIF-8@ZIF-67 ZIF-67@ZIF-8 ZIF-47 ZIF-8+ZIF-67 Langmuir Model

1.0

0.0 0.0

H2

H2 Storage (wt%)

H2 Storage (wt%)

1.5

H2

H2

ZIP-8

H2

H2 2.0

H2

ZIP-8+ZIF-67

H2

H2 Storage (mmol/g)

H2

ZIF-67

Figure 21.11  (a) Enhancement in H2 adsorption and storage capacity upon formation of C-MIL-101 hybrid than parent MIL-101, and (b) the formation of core-shell ZIF-8 vice versa. Reprinted with permission from [57] (a) [2] (b), Copyright 2019 (a), 2017 (b), American Chemical Society.

672  Materials for Hydrogen Production, Conversion, and Storage and de-hydrogenation of CO2 and formic acid using ZIF-8@Pd1Ag2@ZIF-8 core-shell structure proposed by Wen et al. Through control of the spatial arrangement of metals within MOFs structure its catalytic activity and selectivity for H2 production/storage can be controlled (Figures 21.12a,b) [58]. Similarly, Zhou et al. reported the fabrication of Pt@ZIF-8/GO composite through the facile liquid impregnation method. Highly dispersed and small-sized (3.8 nm) Pt nanoparticles upon intimate connection with the ZIF-8/GO receptor successfully increased H2 storage capacity by 2.2 times than pristine ZIF-8 at 298 K and 10.0 bar through spillover mechanism [59]. Furthermore, combining MOFs with the noble metals, e.g., Pd and Pt that tend to homolytically dissociate H2 can create a spillover effect that offers higher adsorption/desorption at ambient temperatures. Physisorption binding is considered as a week binding of H2 molecules (carried out at 77 K) than the binding of H atoms at room temperature created by H2 dissociation catalyst. H2 storage capacity of SNU-3, IRMOF-1, IRMOF-89, MIL-100, and HKUST-1 has been reported to be increased by using the spillover method. However, binding sites involved in H2 storage during this method are still not detectable but it is suspected that the presence of impurities increased the adsorption of H2 [12]. Hence, confirming that the incorporation of foreign atoms and/or materials not only change the mechanism of H2 storage but also enhance the capacity of the composite structure. Similarly, Chen et al. studied the effect of coating of Pd over HKUST-1 and investigated charge migration from Pd to HKUST-1 at their interface, which resulted in an enhanced H2 storage capacity from 0.5 H/Pd to 0.87 H/Pd at 1 bar and 303K (Figure 21.13). Pd nanocubes alone can adsorb less H atom i.e. 0.11 H/Pd atom because Pd has only unfilled band above (a)

Zn2+ H N

(b)

ZIF-8 growth

1a

N Hmin

OH

step1

ZIF-8 core

H H

step2

1b

HH O

O

step3

HO

H

O

O

H

1c

HCOO– + H2O step4

1d

1e

(B) Ts1c/1d Relative energy / kcal/mol

Pd1Ag2 loading

Shell growth

Pd1Ag2@ZIF-8

ZIF-8@Pd1Ag2@ZIP-8

Pd22 Ts1a/1b

Pd11Ag11

30.9

15.8 1a 0.0 0.0

11.9

39.8

20.2

1b -2.6 1.5

1d 18.0

1c -23.3 -20.3

1e -14.1 -17.1

Figure 21.12  (a) A synthesis scheme of ZIF-8@Pd1Ag2@ZIF-8 core-shell structure and (b) the possible mechanism of adsorption of H2 followed by conversion to fuel. Reprinted with permission from [58], Copyright 2019, Nature.

Metal-Organic Framework for Hydrogen Storage  673

HKUST-1

Pd@HKUST-1

O2e¯

Cu2+

e¯

e¯

Cu2+

e¯

Pd nanocubes H2

Pd e¯

5s EF Pd+δ

4d

HKUST-1 e¯

Cu 3d

Cu 4sp O 2p

Cu 3d O 2p

Figure 21.13  Scheme of charge transfer mechanism from Pd to HKUST-1 and changes in electronic structure at Pd@HKUST-1 interface upon H2 adsorption. Reprinted with permission from [32], Copyright 2018, Nature.

Fermi level, while in Pd@HKUST-1 electrons from Pd 4d are transferred to HKUST-1 which creates more vacancies for H2 and resulted in two times (from 27% to 74%) enhanced H adsorption capacity [32]. Furthermore, Ma et al. synthesized Ni-based MOF via a solvothermal approach and combine it with MgH2 (MgH2@Ni/MOF) to demonstrate ambient temperature hydrogenation. It was revealed that kinetics (41.50 ± 3.70 and 144.70 ± 7.80 kJ mol−1 H2 for absorption or desorption correspondingly) and thermodynamics (−65.70 ± 2.10 and 69.70 ± 2.70 kJ mol−1 H2 for absorption or desorption correspondingly) of Mg/MgH2 in the MgH2@Ni/MOF nanohybrid was enhanced significantly. Meanwhile, the Ni-MOF skeleton acts as an “aggregation blocker” to inhibit the agglomeration of Mg/MgH2 nanocrystals in hydriding and dehydriding phases, encompassing the excellent H2 cycling constancy. The enhanced kinetics and thermodynamics of MgH2@Ni/MOF were ascribed to the synergy between Mg2Ni/Mg2NiH4 and nanoconfined Mg/MgH2 [60]. Doping of borophene metals like Li, Na, K and Ca also followed the spillover mechanism for H2 uptake. Na and K doped borophene with hole pattern of S1 were not suitable for H2 uptake due to unfavorable energy of adsorption as indicated by DFT calculations. While Li and Ca doped

674  Materials for Hydrogen Production, Conversion, and Storage borophene showed gravimetric H2 uptake that varied from 8–14 wt.% and 7–9 wt.% depending upon the hole pattern [61, 62]. Position of dopant also played an important role over adsorption uptake, e.g., Yuan et al. studied doping of Yttrium (Y) through doping over porous graphene sheets on single and multi-sides. Doping of porous graphene on both sides adsorbed two times more H2 than single-sided doping that results in enhanced H2 storage [63].

21.5 Conclusions and Future Recommendations To establish hydrogen economy, an increasing research focus toward MOFs for H2 storage is observed. The well-designed microporous crystalline structure of MOFs provides a greater opportunity to uptake and store H2 due to its mixed organic and inorganic structure. Most of the MOFs show appreciable gravimetric H2 storage at ambient conditions (77 K, >20 pressure). However, only few have the capacity to show valuable volumetric H2 storage at ambient conditions. Currently, most of the work is done with powder form; densification of powder may also affect the H2 storage capacity of MOFs. Hence, evaluation of H2 storage capacity in monolithics of MOFs must be the future research focus of the researchers. Further, for enhancing H2 storage capacity at ambient conditions, porosity, catenation, open metal sites, pore volume, and BET surface area of MOFs play an important role. By varying these properties of MOFs, its H2 storage capacity has been altered/controlled according to the requirements and achieved high heat of adsorption (up to 35 kJ/mole). The incorporation of noble metals, alkali, and alkaline metals can efficiently enhance the H2 storage capacity of MOFs through a spillover mechanism. However, the mechanism and impact of these strategies need to be understood to develop simpler and effective MOFs modification techniques. It will be more appropriate to find out the replacement of noble metals/metals in terms of carbon materials that will play a significant role in enhancing the H2 storage capacity of MOFs. In future research, MOFs structure must be designed by considering the balance between volumetric and gravimetric H2 storage capacities in MOFs at ambient conditions. Moreover, theoretical study must be done in synergy with the experimental work to predetermine the complexities involved in H2 storage by MOFs. Further, meeting the DOE requirement for usable H2 storage by MOFs is still challenging and need a stable, recyclable, and low-cost MOFs catalyst for commercial-scale use to reduce the carbon footprints.

Metal-Organic Framework for Hydrogen Storage  675

Acknowledgement The authors acknowledged the National Natural Science Foundation of China (No. 21736001), (No. 22050410281) and the National Key Research & Development Program of China (No. 2021YFC2101203, No. 2021YFC2101202) for support. The authors also acknowledged the ViceChancellor fellowship scheme at RMIT University.

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676  Materials for Hydrogen Production, Conversion, and Storage of strategies for the development of solid-state adsorbents for vehicular hydrogen storage. Energy Environ. Sci., 11, 2784, 2018. 10. Kapelewski, M.T., Runcevski, T., Tarver, J.D., Jiang, H.Z., Hurst, K.E., Parilla, P.A., Ayala, A., Gennett, T., FitzGerald, S.A., Brown, C.M., Record high hydrogen storage capacity in the metal–organic framework Ni2(m-dobdc) at near-ambient temperatures. Chem. Mater., 30, 8179, 2018. 11. Ahmed, A., Liu, Y., Purewal, J., Tran, L.D., Wong-Foy, A.G., Veenstra, M., Matzger, A.J., Siegel, D.J., Balancing gravimetric and volumetric hydrogen density in MOFs. Energy Environ. Sci., 10, 2459, 2017. 12. Bakuru, V.R., DMello, M.E., Kalidindi, S.B., Metal-organic frameworks for hydrogen energy applications: advances and challenges. Chem. Phys. Chem., 20, 1177, 2019. 13. Lee, C.C., Chen, C.I., Liao, Y.T., Wu, K.C., Chueh, C.C., Enhancing efficiency and stability of photovoltaic cells by using perovskite/Zr-MOF heterojunction including bilayer and hybrid structures. Adv. Sci., 6, 1801715, 2019. 14. Garg, A., Almasi, M., Rattan Paul, D., Poonia, E., Luthra, J.R., Sharma, A., Metal-Organic Framework MOF-76(Nd): Synthesis, Characterization, and Study of Hydrogen Storage and Humidity Sensing. Front. Energy Res., 8, 604735, 2021. 15. Konnerth, H., Matsagar, B.M., Chen, S.S., Prechtl, M.H., Shieh, F.K., Wu, K.C., Metal-organic framework (MOF)-derived catalysts for fine chemical production. Coord. Chem. Rev., 416, 213319, 2020. 16. Liao, Y.T., Nguyen, V.C., Ishiguro, N., Young, A.P., Tsung, C.K., Wu, K.C., Engineering a homogeneous alloy-oxide interface derived from metal-­ organic frameworks for selective oxidation of 5-hydroxymethylfurfural to 2, 5-furandicarboxylic acid. Appl. Catal. B- Environ., 270, 118805, 2020. 17. Yu, X., Tang, Z., Sun, D., Ouyang, L., Zhu, M., Recent advances and remaining challenges of nanostructured materials for hydrogen storage applications. Prog. Mater. Sci., 88, 1, 2017. 18. Jorissen, L., Hydrogen and fuel cells: Fundamentals, technologies and applications. Edited by detlev stolten. Angew. Chem. Int. Ed., 50, 9787, 2011. 19. Han, S.S., Mendoza-Cortes, J.L., Goddard III, W.A., Recent advances on simulation and theory of hydrogen storage in metal–organic frameworks and covalent organic frameworks. Chem. Soc. Rev., 38, 1460, 2009. 20. Yang, C., Wang, X., Omary, M.A., Fluorous metal–organic frameworks for high-density gas adsorption. J. Am. Chem. Soc., 129, 15454, 2007. 21. Kalmutzki, M.J., Hanikel, N., Yaghi, O.M., Secondary building units as the turning point in the development of the reticular chemistry of MOFs. Sci. Adv., 4, eaat9180, 2018. 22. Butova, V.V., Soldatov, M.A., Guda, A.A., Lomachenko, K.A., Lamberti, C., Metal-organic frameworks: structure, properties, methods of synthesis and characterization. Russ. Chem. Rev., 85, 280, 2016.

Metal-Organic Framework for Hydrogen Storage  677 23. Bon, V., Senkovska, I. et al., Metal-Organic Frameworks, in: Nanoporous Materials for Gas Storage, K. Kaneko, and F. Rodriguez-Reinoso, (Eds.), pp. 137–172, Springer, Singapore, 2019. 24. Deng, H., Grunder, S., Cordova, K.E., Valente, C., Furukawa, H., Hmadeh, M., Gandara, F., Whalley, A.C., Liu, Z., Asahina, S., Kazumori, H., O’Keeffe, M., Terasaki, O., Stoddart, J.F., Yaghi, O.M., Large-pore apertures in a series of metal-organic frameworks. Science, 336, 1018, 2012. 25. Deng, H., Doonan, C.J., Furukawa, H., Ferreira, R.B., Towne, J., Knobler, C.B., Wang, B., Yaghi, O.M., Multiple functional groups of varying ratios in metal-organic frameworks. Science, 327, 846, 2010. 26. Kassaoui, M.E., Lakhal, M., Abdellaoui, M., Benyoussef, A., El Kenz, A., Loulidi, M., Modeling hydrogen adsorption in the metal organic framework (MOF-5, connector): Zn4O (C8H4O4)3. Int. J. Hydrog. Energy, 45, 33663, 2020. 27. Jiao, L., Seow, J.Y.R., Skinner, W.S., Wang, Z.U., Jiang, H.L., Metal–organic frameworks: Structures and functional applications. Mater. Today, 27, 43, 2019. 28. Rocio-Bautista, P., Taima-Mancera, I., Pasan, J., Pino, V., Metal-Organic Frameworks in Green Analytical Chemistry. Separations, 6, 33, 2019. 29. Kuc, A., Heine, T., Seifert, G., Duarte, H.A., H2 Adsorption in Metal-Organic Frameworks: Dispersion or Electrostatic Interactions? Chem. Eur. J., 14, 6597, 2008. 30. Nijem, N., Veyan, J.F., Kong, L., Li, K., Pramanik, S., Zhao, Y., Li, J., Langreth, D., Chabal, Y.J., Interaction of Molecular Hydrogen with Microporous Metal Organic Framework Materials at Room Temperature. J. Am. Chem. Soc., 132, 1654, 2010. 31. Li, Y. and Yang, R.T., Hydrogen Storage in Metal–Organic Frameworks by Bridged Hydrogen Spillover. J. Am. Chem. Soc., 128, 8136, 2006. 32. Chen, Y., Sakata, O., Nanba, Y., Kumara, L.S.R., Yang, A., Song, C., Koyama, M., Li, G., Kobayashi, H., Kitagawa, H., Electronic origin of hydrogen storage in MOF-covered palladium nanocubes investigated by synchrotron X-rays. Commun. Chem., 1, 61, 2018. 33. Rosi, N.L., Eckert, J., Eddaoudi, M., Vodak, D.T., Kim, J., O Keeffe, M., Yaghi, O.M., Hydrogen storage in microporous metal-organic frameworks. Sci., 300, 1127, 2003. 34. Yang, S.J., Jung, H., Kim, T., Im, J.H., Park, C.R., Effects of structural modifications on the hydrogen storage capacity of MOF-5. Int. J. Hydrog. Energy, 37, 5777, 2012. 35. Frost, H., Duren, T., Snurr, R.Q., Effects of surface area, free volume, and heat of adsorption on hydrogen uptake in metal–organic frameworks. J. Phys. Chem. B, 110, 9565, 2006. 36. Yang, H., Orefuwa, S., Goudy, A., Study of mechanochemical synthesis in the formation of the metal–organic framework Cu3(BTC)2 for hydrogen storage. Micropor. Mesopor. Mat., 143, 37, 2011.

678  Materials for Hydrogen Production, Conversion, and Storage 37. Xia, L. and Liu, Q., Adsorption of H2 on aluminum-based metal-organic frameworks: A computational study. Comput. Mater. Sci., 126, 176, 2017. 38. Furukawa, H., Ko, N., Go, Y.B., Aratani, N., Choi, S.B., Choi, E., Yazaydin, A.O., Snurr, R.Q., O’Keeffe, M., Kim, J., Yaghi, O.M., Ultrahigh porosity in metal-organic frameworks. Science, 329, 424, 2010. 39. Grunker, R., Bon, V., Muller, P., Stoeck, U., Krause, S., Mueller, U., Senkovska, I., Kaskel, S., A new metal–organic framework with ultra-high surface area. Chem. Commun., 50, 3450, 2014. 40. Montes-Andres, H., Orcajo, G., Mellot-Draznieks, C., Martos, C., Botas, J.A., Calleja, G., Novel Ni-IRMOF-74 postsynthetically functionalized for H2 storage applications. J. Phys. Chem. C, 122, 28123, 2018. 41. Sumida, K., Horike, S., Kaye, S.S., Herm, Z.R., Queen, W.L., Brown, C.M., Grandjean, F., Long, G.J., Dailly, A., Long, J.R., Hydrogen storage and carbon dioxide capture in an iron-based sodalite-type metal–organic framework (Fe-BTT) discovered via high-throughput methods. Chem. Sci., 1, 184, 2010. 42. Dinca, M., Dailly, A., Liu, Y., Brown, C.M., Neumann, D.A., Long, J.R., Hydrogen storage in a microporous metal– organic framework with exposed Mn2+ coordination sites. J. Am. Chem. Soc., 128, 16876, 2006. 43. Dinca, M. and Long, J.R., High-enthalpy hydrogen adsorption in ­cation-exchanged variants of the microporous metal–organic framework Mn3 [(Mn4Cl)3 (BTT)8 (CH3OH)10]2. J. Am. Chem. Soc., 129, 11172, 2007. 44. Yang, J., Zhang, K., Ma, Z., Zhang, X., Huang, T., Panda, S., Zou, J., Trimesic acid-Ni based metal organic framework derivative as an effective destabilizer to improve hydrogen storage properties of MgH2. Int. J. Hydrog. Energy, 46, 28134, 2021. 45. Broom, D.P., Webb, C., Fanourgakis, G.S., Froudakis, G.E., Trikalitis, P.N., Hirscher, M., Concepts for improving hydrogen storage in nanoporous materials. Int. J. Hydrog. Energy, 44, 7768, 2019. 46. Ma, S., Sun, D., Ambrogio, M., Fillinger, J.A., Parkin, S., Zhou, H.C., Framework-catenation isomerism in metal–organic frameworks and its impact on hydrogen uptake. J. Am. Chem. Soc., 129, 1858, 2007. 47. Kim, H., Das, S., Kim, M.G., Dybtsev, D.N., Kim, Y., Kim, K., Synthesis of phase-pure interpenetrated MOF-5 and its gas sorption properties. Inorg. Chem., 50, 3691, 2011. 48. Balderas-Xicohtencatl, R., Schmieder, P., Denysenko, D., Volkmer, D., Hirscher, M., High volumetric hydrogen storage capacity using interpenetrated metal–organic frameworks. Energy Technol., 6, 510, 2018. 49. Pachfule, P., Chen, Y., Jiang, J., Banerjee, R., Experimental and computational approach of understanding the gas adsorption in amino functionalized interpenetrated metal organic frameworks (MOFs). J. Mater. Chem., 21, 17737, 2011. 50. Li, F.G., Liu, C., Yuan, D., Dai, F., Wang, R., Wang, Z., Lu, X., Sun, D., Ultrahigh Hydrogen Uptake in an Interpenetrated Zn4O-Based Metal– Organic Framework. CCS Chem., 3, 1005, 2021.

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22 High-Density Solids as Hydrogen Storage Materials Zeeshan Abid1*, Huma Naeem1, Faiza Wahad1, Sughra Gulzar1, Tabassum Shahzad2, Munazza Shahid3, Muhammad Altaf1† and Raja Shahid Ashraf1‡ Department of Chemistry, Government College University Lahore, Katchery Road, Lahore, Pakistan 2 Department of Physics, Government College University Lahore, Katchery Road, Lahore, Pakistan 3 University of Education, Bank Road Campus, Lahore, Pakistan

1

Abstract

The hydrogen economy is an envisioned future in which hydrogen fuel will phase out hydrocarbons. This magnificent transition will not only resolve climate issues but will also guarantee sustainable energy systems. Although the hydrogen production technologies are well established, a lack of reliable storage methods restricts their practical applications. Present hydrogen storage techniques such as compressed gas and liquefied hydrogen have severe disadvantages compared to hydrocarbons, especially in volumetric terms. Among the developing hydrogen storage technologies, high-density solids are a great choice as they fulfill the criteria of being lightweight, flexible, portable, and controllable hydrogen carriers. This chapter introduces some of the most prominent high-density solids that are anticipated as a key enabler for the hydrogen economy. Keywords:  High-density solids, hydrogen storage, metal borohydrides, metal alanates, metal amides, amino boranes

*Corresponding author: [email protected] † Corresponding author: [email protected] ‡ Corresponding author: [email protected] Inamuddin, Tariq Altalhi, Sayed Mohammed Adnan and Mohammed A. Amin (eds.) Materials for Hydrogen Production, Conversion, and Storage, (681–706) © 2023 Scrivener Publishing LLC

681

682  Materials for Hydrogen Production, Conversion, and Storage

22.1 Introduction The present energy systems across the world rely on a hydrocarbon economy where energy requirements are primarily met with fossil fuels. The excessive consumption of hydrocarbons leads to the emission of greenhouse gases and harmful air pollutants, which pose a great threat to our planet’s environment and people [1]. Unclean burning, however, is not the only issue associated with the hydrocarbon economy; its unsustainability is a separate debate. A fast depletion ratio of hydrocarbon reserves speaks for their unreliability as a future fuel and thus necessitates the demand for reliable alternative energy resources [2]. The hydrogen economy is proposed as a cleaner and reliable alternative to the hydrocarbon economy. Hydrogen (H2) can be produced from water using renewable energy such as solar and wind and releases only water vapor upon combustion as compared to the hydrocarbons which result in emissions and particulates. These features of hydrogen have encouraged researchers to deem hydrogen economy as an envisioned future in which hydrogen will phase out hydrocarbons. The use of hydrogen fuel will not only limit global warming and decarbonize the energy sector, but will also enable long-term storage and long-distance transportation of renewable energy [3, 4]. One reason for this advantage is low energy density by volume of hydrogen which is 1/4 that of gasoline. Moreover, the energy content of hydrogen is 33.33 kWh/kg which is much higher than that of methane (13.9 kWh/kg) and gasoline (12.4 kWh/kg) [5]. Despite these promising advantages, hydrogen faces some safety concerns that hinder its immediate practical usage. Due to its high ignition mix range with air, it poses a danger of explosion in the enclosed space if it comes in contact with air. The sensitivity of this issue requires proper storage techniques that guarantee its safer handling and transportation. Reliable storage of hydrogen is also important from the aspect of ­hydrogen-based zero-emission vehicles that require its onboard storage. At present, hydrogen is stored in cylinders or tanks in the form of compressed gas or cryogenic liquid for industrial or space program applications. It takes extreme pressure and temperature conditions to attain and maintain these storage states. In particular, a pressure of 350–700 bar is required for compressed form or the temperature of hydrogen is reduced to −253°C to attain its liquefied state. The overall cost of these conditions adds to the hurdles disabling the widespread applications of hydrogen as a fuel [6, 7]. An alternative and increasingly popular way of hydrogen storage is the solid hydrogen storage method by using high-density solid materials [8].

High-Density Solids as Hydrogen Storage Materials   683 Hydrogen can be stored in these materials under certain conditions by physical adsorption or chemical absorption and then released on demand by pyrolysis once it is transported to its consumption destination. The major challenges for these materials include hydrogen storage capacity and unfavorable or extreme desorption conditions. According to the US Department of Energy, the ideal system gravimetric capacity goal is 6.5 wt.% between −40°C and 60°C [9]. This goal is practically achievable by materials with high theoretical density i.e., ≥10 wt.% with optimization and enhancement techniques. These materials include metal borohydrides, alanates, amides, amino boranes, and their derivatives. Considering the space limitations, only prominent high-density materials are highlighted in this chapter.

22.2 Metal Borohydrides Metal borohydrides are inorganic materials denoted as M(BH4)n where M is commonly an alkali metal cation bonded to [BH4]− complex anion comprising a boron atom bonded to four H2 atoms through covalent bonds. Metal borohydrides are known for high gravimetric H2 densities. These materials have long been used as strong reducing agents in organic synthesis. A high energy density and energy content has extended their applications to energy storage [10]. A few examples of metal borohydrides as H2 storage materials are briefly discussed below.

22.2.1 Lithium Borohydride Lithium borohydride (LiBH4), consists of a Li+ ion surrounded by four [BH4]− in a tetrahedral geometry. A unit cell of LiBH4 comprises four molecules (Figure 22.1) and possesses a point group of Pnma. LiBH4 has grabbed huge attention as an excellent H2 carrier compound possessing a high H2 storage mass of 18.5 wt.% [11]. A high theoretical H2 density of 13.8 wt.% is released upon desorption of LiBH4. In terms of energy requirements, the desorption of H2 from LiBH4 requires low pressure and a high temperature, typically, in the range of 270°C–492°C. On the other hand, regeneration of LiBH4 from its decomposed species (LiH and B) is also an energy-intensive process. It requires >600°C temperature and150 bar H2 pressure to enable a reversible H2 absorption reaction [13]. Apart from the extreme reaction conditions, the dehydrogenation processes of LiBH4 raises safety concerns due to the

684  Materials for Hydrogen Production, Conversion, and Storage

Figure 22.1  Unit cell model of LiBH4 at room temperature [12].

release of B2H6, which is classified as a toxic material. The dehydrogenation reactions are usually presented as below:



3 LiBH 4  → LiH + B +   H 2 2



1 LiH  → Li + B + H 2 2

In view of these limitations, researchers have been focusing to improve the reversibility of the reactions and bring conditions under control from extreme to mild. One common practice is to dope LiBH4 with additives that catalyze the desorption reaction, optimize its kinetics and deliver stable intermediate phases. These additives consequently lower the temperature required for desorption and increase desorption H2 pressure. Among the several additives used for the modification of LiBH4, metal hydrides hold great significance and offer promising thermodynamic control of the reactions. Researchers have demonstrated lowering of energy requirements, increase in H2 percentage, and improvements in the reversibility of the reactions. Apart from the metal hydrides, metals alone have been used as catalyst to improve H2 carrying potential of LiBH4. Metals work

High-Density Solids as Hydrogen Storage Materials   685 Table 22.1  Examples of modifications in LiBH4 materials from literature. Material

Improvement/optimization focus

Reference

Li4Al3(BH4)13

Thermodynamics

[14]

2LiBH4–MgH2

Desorption conditions

[16]

LiBH4–Fe3O4@rGO composites

Dehydrogenation temperatures and hydrogen release capacity

[17]

2LiBH4–Li3AlH6

Dehydrogenation properties

[18]

2LiBH4–Al composite

Reversibility

[19]

LiBH4–3TiO2

Absorption/reversibility

[20]

LiBH4–SiO2

Reversibility

[21]

LiBH4–0.2MgCl2–0.1TiCl3

Thermodynamic properties

[22]

6LiBH4–CaF2

Hydrogen storage reversibility

[23]

LiBH4@PHCNSs Composites

Hydrogen release capacity

[24]

by weakening the ionic bonds present in LiBH4 and distorting its structural stability, consequently lowering the desorption temperature [14]. Metal halides present another modification pathway that increases the H2 storage capacity of LiBH4 by replacing borohydride anion with halogen atoms leading to reduced dehydrogenation temperature. Metal oxides are also used to produce stable Li-containing oxides and metal borides that contribute to the H2 storage potential of LiBH4 [15, 16]. Researchers have also demonstrated the benefits of nanosizing to control and optimize the H2 storage capacity of LiBH4. LiBH4 nanoparticles show dramatic improvement in the thermodynamics of desorption and absorption kinetics [14, 17]. Table 22.1 presents some examples of modifications reported for the enhancement of LiBH4 materials.

22.2.2 Sodium Borohydride Sodium borohydride (NaBH4) in its solid form exists as α, β, and γ polymorphs. At room temperature, it assumes a NaCl-type structure where four H2 atoms are bonded with [BH4]− anion through covalent bonds and stabilized with Na+ as a counter cation. This stable phase is known to be

686  Materials for Hydrogen Production, Conversion, and Storage α-NaBH4, whereas at a pressure of 6.3 GPa, it undergoes a phase change and transforms into tetragonal β-NaBH4 having space group P421c. Further increase in pressure (8.9 GPa) leads to most stable orthorhombic phase with space group Pnma (γ-NaBH4). Figure 22.2 shows polymorphs based on the crystallographic data derived from literature [25, 26]. NaBH4 possesses a high H2 storage mass density of 10.6 wt.%. When heated at 390°C, NaBH4 completely decomposes into sodium hydride, boron, and H2 with 7.9 wt.% H2 release [18]. Further heating up to 440°C leads to the complete decomposition of NaH into Na and H2 with 2.7 wt.% H2 release. Overall, the desorption process is predicted to release a 10.6 wt.% H2 [27]. Although the desorption reactions require a high temperature and pressure, the requirements are even harsher for absorption reactions. In theory, the H2 absorption process of NaBH4 requires a temperature as high as 550°C–700°C and H2 pressure in the range of 30–150 bar. The desorption reactions of NaBH4 are expressed below:



3 NaBH 4  → NaH + B +   H 2 2

α-NaBH4

β-NaBH4

γ-NaBH4

Figure 22.2  Polymorphs of NaBH4.

High-Density Solids as Hydrogen Storage Materials   687 Table 22.2  Examples of modifications in NaBH4 materials from literature.



Material

Improvement/optimization focus

Reference

LiAlH4–NaBH4

Dehydrogenation properties

[18]

3NaBH4–YF3

Reversibility

[28]

NaH–B–0.05TiF3

Absorption and desorption of hydrogen

3NaBH4–PrF3

Kinetics

[30]

NaBH4–FGi composites

Kinetics and thermodynamics

[31]

NaBH4/Ca(BH4)2

Hydrogen desorption

[32]

NaBH4@Fe composites

Hydrogen storage reversibility

[33]

1 NaH  → Na + H 2 2

High thermodynamic stability and irreversibility of reactions restrict the potential of NaBH4 as a hydrogen storage material. Hence, huge attention has been given to improve the physical performance and reversibility of NaBH4 (Table 22.2) [28]. Metal hydrides and rare earth fluorides additives have demonstrated positive effects on desorption performance. Nanoengineering has emerged as a successful strategy to enhance the hydrogen storage potential of NaBH4. Several studies have proved a significant shift in desorption kinetics of nanoscale NaBH4 [29–31].

22.2.3 Potassium Borohydride Potassium borohydride (KBH4) has a theoretical H2 storage capacity of 7.4 wt.%. Given the high weight density, KBH4 as H2 carrier suits the utility where weight density is not a focus [34]. The desorption process of KBH4 is expressed as below:



KBH 4  → K + B + 2 H 2

The feasibility of this reaction has been found around 700°C and 0.1 MPa H2 pressure, indicating that KBH4 do not qualify as reversible hydrogen

688  Materials for Hydrogen Production, Conversion, and Storage Table 22.3  Examples of modifications in KBH4 materials from literature. Material

Improvement/optimization focus

Reference

KBH4–ZrCl4

Dehydrogenation kinetics

[34]

LaH3–MgH2–KBH4

Dehydrogenation temperature

[37]

LiBH4–0.275KBH4

Conducive physical properties

[38]

0.682NaBH4–0.318KBH4

Desorption conditions

storage materials [35]. However, doping of KBH4 with additives has shown improvement in their reversibility performance (Table 22.3). Among the various performance enhancement techniques, homogenous melting of KBH4 in the mixture of borohydrides of alkali and alkaline earth metals enables new avenues in the field of energy applications [36].

22.3 Metal Alanates Metal alanates refer to metal hydrides with the general formula M(AlH4)n where metals can be lithium and sodium etc. Owing to their high weight density, metal analates are renowned compounds for H2 applications. The dehydrogenation reactions of these compounds occur through multi-step reactions. M(AlH4)n yield H2 violently upon hydrolysis at room temperature. The reversible reaction from the products of hydrolysis is known to be poor. The two most prominent metal alanates are lithium alanate and sodium alanate.

22.3.1 Lithium Alanate Lithium alanate or lithium aluminium hydride (LiAlH4) exhibits a distinct electron-rich character, where Li centers are surrounded by tetrahedra and one H2 atom from each of tetrahedra is bonded to Li. Figure 22.3 presents the crystal structure of LiAlH4 showing Li atoms surrounded by AlH4 tetrahedra. LiAlH4 exhibits stability at ambient conditions with no toxicity and a high theoretical H2 storage capacity of 10.5 wt.%. At high pressure, it undergoes phase transition resulting in a beta-LiAlH4 structure which is also considered a promising H2 storage material. In general, LiAlH4 has received great attention as a suitable material for fuel cell feedstock [39].

High-Density Solids as Hydrogen Storage Materials   689

Figure 22.3  Crystal structure of LiAlBH4.

When heated at 150°C, about 5.3 wt.% H2 is released leaving Li3AlH6 and Al as decomposition products [40]. Further decomposition of Li3AlH4 at 160-210°C to LiH and Al yields 2.6 wt.% H2. In the final step, LiAl combines with Al to release 2.6 wt.% H2 as the temperature exceeds 350°C. The desorption reactions of LiAlH4 comprise three steps as represented below:



3LiAlH 4  → Li3 AlH 6 + 2 Al + 3H 2



3 Li3 AlH 6  → 3LiH + Al + H 2 2



3 3LiH + 3 Al  → 3LiAl + H 2 2

The regeneration of LiAlH4 from its decomposed products is not as convenient and practical as the original synthesis process. A facile reversible H2 absorption process to improve the reversibility of LiAlH4 is therefore a focus in the ongoing hydrogen storage materials research [41]. For this purpose, metal hydrides have been extensively used as additives that

690  Materials for Hydrogen Production, Conversion, and Storage Table 22.4  Examples of modifications in LiAlH4 materials from literature. Material

Improvement/optimization focus

Reference

LiAlH4–MgH2

Thermodynamics, Hydrogen storage capacity

[39]

LiAlH4–LaFeO3

Reaction kinetics

[41]

LiAlH4– BaFe12O19

Hydrogen release capacity

[42]

2LiAlH4–LiNH2

Reversible hydrogen storage

[44]

NaNH2–LiAlH4

Reversible hydrogen storage

[45]

2LiAlH4– Mg(BH4)2

Hydrogen release capacity

[46]

LiAlH4 + n-Ni mixture

Hydrogen release capacity

[47]

LiAlH4–TiCl4

Hydrogen release capacity

[48]

LiAlH4–NiCl2

Dehydriding kinetic performance

[49]

promise thermodynamic benefits and facilitate desorption reactions. Similar advantages have been demonstrated by the addition of metals and metal oxides as additives [42]. In addition, metal halides are also shown to improve decomposition processes [43]. Table 22.4 enlists the modifications made by scientists by adding materials with LiAlH4.

22.3.2 Sodium Alanate Sodium alanate or sodium aluminium hydride (NaAlH4) is analogous to lithium alanate in its chemical composition which comprises separated metal cations and tetrahedral AlH−4 anions. In terms of crystal packing, NaAlH4 is isostructural with calcium tungstate (CaWO4). NaAlH4 is considered a promising hydrogen storage material due to a H2 storage mass density of 7.4 wt.% [50]. The dehydrogenation of NaAlH4 comprises two steps. In the first step, NaAlH4 is decomposed into Na3AlH6 and Al at 150°C releasing 3.7 wt.% H2. Further rise in the temperature up to 200°C causes the decomposition of Na3AlH6 into NaH and Al with 3.0 wt.% H2 release [51]. The decomposition reactions steps are expressed below:

High-Density Solids as Hydrogen Storage Materials   691 Table 22.5  Examples of modifications in NaAlH4 materials from literature. Material

Improvement/optimization focus

Reference

MgH2–NaAlH4

Hydrogen storage capacity

[50]

NaAlH4–TiCl3–CeCl3

Hydrogen storage capacity

[52]

NaAlH4–MgH2–LiBH4

Hydrogen storage capacity

[54]

2NaAlH4–Ca(BH4)2

Kinetics and thermodynamics

[55]

NaAlH4–Ti/Fe

Hydrogen desorption and absorption

[56]

NaAlH4+Zirconium

Reversibility

[57]

NaAlH4–TiCl3

Kinetics

[58]



3NaAlH 4  → Na3 AlH 6 + 2 Al + 3H 2



2 Na3 AlH 6  → 6NaH + 2 Al + 3H 2

A large variety of additives have been used to improve the reversibility and lower enthalpy change of NaBH4. The addition of metal hydrides leads to a notable enhancement in the desorption and absorption abilities of NaBH4. Metal hydrides have also shown improvements in the reaction kinetics and hydrogen storage potential of NaAlH4 (Table 22.5). Another major area of modification is the use of metal-organic framework templates to produce NaAlH4 nanoclusters [52]. Several examples of this strategy suggest significant improvement in the desorption kinetics of NaAlH4 [53].

22.4 Ammonia Boranes Ammonia borane is the simplest boron-nitrogen-hydride compound with a chemical formula H3NBH3. H3NBH3 molecule is isoelectronic with ethane. Different crystal structures of H3NBH3 have been presented at different temperatures. At 200K, the crystal system is demonstrated to belong to an orthogonal system. With an increase in temperature to 225 K, it is shown to be orthogonal and subsequently changes to the tetragonal system at high temperatures. Figure 22.4 shows illustrated crystal structure of ammonia borane derived from reported crystal data [59].

692  Materials for Hydrogen Production, Conversion, and Storage

Figure 22.4  Illustration showing part of the crystal structure of ammonia borane.

Table 22.6  Examples of modifications in H3NBH3 materials from literature. Material

Improvement/optimization focus

Reference

NH3BH3–CuCo/MgO

Dehydrogenation process

[60]

NH3BH3–Ni crystal

Dehydrogenation processes

[64]

NH3BH3–ruthenium(II) complexes–PNP-pincer ligands

Hydrogen release capacity

[65]

NH3BH3–NanoNickel

Kinetics

[66]

NH3BH3–Ni0.88Pt0.12

Kinetics

[67]

H3NBH3 are valued for their potential to release hydrogen in large volumes at a low temperature of about 120°C. Metallic H3NBH3 in particular have a high H2 content with mild dehydrogenation conditions owing to their high H2 mass density of 19.6 wt.% [60]. The dehydrogenation of H3NBH3 can be achieved through different methods, including thermal decomposition, hydrolysis, and alcoholysis. Thermal decomposition or pyrolysis of H3NBH3 leads to a high H2 mass density through a multi-step desorption process. At about 120°C, H3NBH3 is decomposed to [NH3BH3]n releasing 6.5 wt.% H2. A slight increase in the temperature (150°C) further decomposes the product into [NHBH]n releasing 6.9 wt.% H2. The decomposition of [NHBH]n into BN requires a temperature as high as 500°C to release 7.3 wt.% H2 [61, 62]. All the reaction steps are shown below:

High-Density Solids as Hydrogen Storage Materials   693



nNH 3BH 3  → [NH 2 BH 2]n + nH 2



[NH 2 BH 2 ]n  → [NHBH 2 ]n + nH 2



[NHBH ]n  → nBN + nH 2

Given the complexity and stability of decomposition products, regeneration of H3NBH3 is not possible without a proper materials management strategy. The end products are digested by strong acids followed by reduction and ammoniation to enable regeneration of H3NBH3. Besides the extreme reaction conditions, the problems faced by H3NBH3 dehydrogenation processes include the release of B3N3H6 which is a poisonous gas. These issues reduce the H2 capacity of NH3BH3 to only one-third of its theoretical value. Several modifications have been suggested by researchers to manage the abovementioned issues [63]. Adding metallic compounds and nanomaterials to H3NBH3 have shown improved dehydrogenation processes as well as H2 release potential [64] (Table 22.6).

22.5 Metal Amides Metal amides or metal azanides are represented by formula M(NH2)n where M refers to different metals like Li, Mg, Na, etc. These compounds have earned a wide perception as effective H2 carrying compounds owing to an exceptional hydrogen density. A few prominent major metal amides are briefly discussed below are briefly discussed below and the modifications made with their structures are given in Table 22.7.

22.5.1 Lithium Amide Lithium amide (LiNH2) appears as a white solid with a tetragonal crystal structure as shown in the Figure 22.5. The theoretical H2 mass density of LiNH2 is 8.7 wt.%, achieving recognition as attractive choice for hydrogen storage applications [68]. The dehydrogenation of LiNH2 occurs through a multi-step reaction. It has been demonstrated that the pure LiNH2 is decomposed into Li2NH and NH3 at 200°C with almost no H2 evolution. On the other hand, LiNH2– LiH composites, with a theoretical H2 capacity of 6.5 wt.%, are decomposed into Li2NH with an evolution of H2 between the temperature range

694  Materials for Hydrogen Production, Conversion, and Storage

Figure 22.5  Unit cell model of LiNH2.

of 150°C–269°C. The release of NH3 was negligible and well under control by temperature optimization. LiNH2–2LiH composites, with a theoretical H2 capacity of 12.5 wt.%, show different dehydrogenation behaviour than the former compounds. The dehydrogenation products of LiNH2–2LiH are Li3N and H2. H2 evolution is found to start below 170°C attaining a maximum at 245°C, whereas no release NH3 is detected during the process [69]. The reactions presenting the decomposition of LiNH2, LiNH2–LiH and LiNH2–2LiH are expressed below:



2 LiNH 2 → Li2 NH + NH 3



LiNH 2 + LiH  → Li2 NH + H 2



LiNH 2 + 2 LiH  → Li3 N + 2 H 2

Similar to the materials discussed earlier, several additives including metals [70], metal oxides, metal hydrides [71, 72], and metal halides [73] have been used to increase the hydrogen storage proficiency of these materials.

22.5.2 Sodium Amide Sodium amide (NaNH2) is described as a salt-like material with tetrahedral geometry and crystallization as an infinite polymer. Figure 22.6 shows

High-Density Solids as Hydrogen Storage Materials   695 Table 22.7  Examples of modifications in metal amides from literature. Improvement/optimization focus

Material

Reference

2NaNH2–NaBH4

Dehydrogenation and desorption properties

[29]

2LiNH2–MgH2–0.05Mg(BH4)2

Hydrogen desorption and absorption stability

[68]

2LiNH2–MgH2–0.1Li3AlH6

Hydrogen release capacity

[70]

2NaNH2–3MgH2

Hydrogen release capacity

[71]

LiBH4–NaNH2

Thermodynamics

[72]

LiNH2–MgH2–AlCl3

Dehydrogenation processes

[73]

Mg(NH2)2–2LiNH2

Hydrogen absorption and desorption kinetics

[74]

LiNH2–LiBH4

Hydrogen release capacity

[75]

LiNH2– Ni, Fe, Co metals–1 mol % TiCl3

Hydrogen release capacity

[76]

unit cell model of sodium amide derived from the crystal data published in the literature [77]. The H2 mass density of NaNH2 is 5.1 wt.%, which reduces its potential as a high-performance hydrogen storage material. The dehydrogenation reaction of NaNH2 is demonstrated to occur at a temperature higher than its melting point leaving Na, a small amount of NH3 and H2 as decomposition products. The reaction is expressed below.



1 NaNH 2 → Na + N 2 + H 2 2

Researchers have demonstrated that the hydrogen storage performance of several NaNH2 composites exceeds the performance of parent material. In particular, 2NaNH2 composites with metal hydrides (2NaNH2–3MgH2) and metal borohydrides (2NaNH2–NaBH4 and LiBH4–NaNH2) showed an improved H2 storage density values [78].

696  Materials for Hydrogen Production, Conversion, and Storage

Figure 22.6  Unit cell model of NaNH2.

22.6 Amine Metal Borohydrides Amine metal borohydrides are a product of ammonia and metal borohydrides featuring both [NH3] and [BH4] species in the same material. The general formula of amine metal borohydrides is M(BH4)n·xNH3 where M represents many different metals, including Li, Al, Mg, Ca, etc. A brief overview of these materials is as follows. The enhancement in their activity by addition of metal compounds is given in Table 22.8.

22.6.1 Amine Lithium Borohydrides Lithium borohydrides (LiBH4) conveniently take up NH3 to form amine lithium borohydrides LiBH4.xNH3 (x = 1, 2, 3). Among many different variants, only LiBH4.NH3 is stable without NH3 atmosphere. Moreover, in

High-Density Solids as Hydrogen Storage Materials   697 addition to its lowest NH3 ratio (44 wt.%), LiBH4.NH3 exhibits a high theoretical H2 carrying density of 18 wt.%. LiBH4.NH3 is therefore the preferred choice for hydrogen storage applications. The heating of LiBH4.NH3 at 40°C results in the release of 40 wt.% NH3. As the temperature reaches 160°C, the compound completely decomposed into LiBH4 and NH3 [79]. The reaction is expressed as below:



LiBH 4 .NH 3 → LiBH 4 + NH 3

With further heating in the range of 160–250°C, H2 was released reaching a maximum of 4 wt.% at 300°C under the NH3 atmosphere. The earlier release of NH3 is attributed to a weaker bond between Li and N as compared to N–H and H–B bonds. Despite great hydrogen storage capacity, LiBH4. xNH3 compounds are because of excessive NH3 release. Several modifications have been suggested in the literature to control the NH3 evolution during decomposition and increase hydrogen storage performance [80].

22.6.2 Amine Magnesium Borohydrides Amine magnesium borohydride (Mg(BH4)2·xNH3, x = 1, 2, 3, 6) compounds are also renowned materials for hydrogen storage applications. The theoretical H2 densities of these compounds increase with the increase of NH3 content and range between 14.1 and 19.4 wt.%. The decomposition temperatures of Mg(BH4)2.xNH3 were found to decrease as the NH3 content of compounds increased [81]. The decomposition process starts with the release of NH3 at 90°C–230°C followed by sharp suppression of NH3 and simultaneous incline of H2 evolution as the temperature reaches 500°C. The decomposition reactions are shown below:



Mg ( BH 4 ).6NH 3 → Mg ( BH 4 ).2 NH 3 Na + 4 NH 3



Mg ( BH 4 ).2 NH 3 → Mg 3 B2 N 4 + 2 BN + 21H 2

Similar to their lithium analogues, Mg(BH4)2.xNH3 are of limited use due to the release of NH3 [82]. Researchers have shown the use of additives and nanomaterials to suppress the NH3 release and improve dehydrogenation processes [83].

698  Materials for Hydrogen Production, Conversion, and Storage Table 22.8  Examples of modifications in amine metal borohydrides M(BH4)n ·xNH3 from literature. Material

Improvement/optimization focus

Reference

ZnCl2–2LiBH4·NH3

Hydrogen release capacity

[79]

Mg(BH4)2·6NH3

Thermodynamic optimization, hydrogen release capacity

[81]

AlCl2–5LiBH4·NH3

Hydrogen release capacity

[84]

Ca(BH4)2·NH3–LiBH4

Dehydrogenation processes

[85]

Al(BH4)3· 4NH3–LiBH4

Hydrogen release capacity

[86]

Al(BH4)3· 6NH3–Mg(BH4)2

Dehydrogenation processes

[87]

Mg(BH4)2· 2NH3–2NaAlH4

[82]

22.6.3 Amine Calcium Borohydrides Amine calcium borohydrides (Ca(BH4)2·xNH3, x= 1, 2, 4, 6) have retained a continued interest as hydrogen storage materials for a long period. Their theoretical H2 densities range between 12.7 and 15.1 wt.% depending upon the NH3 content. The decomposition of Ca(BH4)2.xNH3 involves three stages. At first, Ca(BH4)2·4NH3 decomposes into Ca(BH4)2·2NH3 at 87°C under an inert atmosphere [85]. Subsequently, as the temperature rises to 162°C, Ca(BH4)2·2NH3 leaves behind Ca(BH4)2·NH3, which finally decomposes into Ca(BH4)2 at 230°C [88]. NH3 is released alongside all byproducts in three stages. The following equation shows all three steps.



Ca( BH 4 )2 .4 NH 3 → Ca( BH 4 )2 .2 NH 3 + 2 NH 3



Ca( BH 4 )2 .2 NH 3 → Ca( BH 4 )2 .NH 3 + NH 3



Ca( BH 4 )2 .NH 3 → Ca( BH 4 )2 + NH 3

Ca(BH4)2·2NH3 releases around 11.3 wt.% H2 if heated at extreme temperature (500°C). Hence, the desorption processes of Ca(BH4)2·2NH3 require modification to make H2 a predominant product instead of NH3.

High-Density Solids as Hydrogen Storage Materials   699 Literature reports have suggested doping of Ca(BH4)2·2NH3 with borohydrides and metal-based additives to improve the H2 evolution [85, 89].

22.6.4 Amine Aluminium Borohydrides Amine aluminium borohydride (Al(BH4)3·xNH3, x= 2, 3, 4, 5, 6) compounds find great importance as H2 careers due to high theoretical H2 densities of 17.1–17.3 wt.%. Similar to the family compounds, Al(BH4)3·6NH3 also releases NH3 on decomposition at 60°C–180°C. On the other hand, it released about 11.8 wt.% H2 when heated to 300°C [86]. Al(BH4)3·xNH3 suffers low H2 release and impurity issues that require significant research solutions [90]. Nanoparticles and double-metal cations have demonstrated a notable boost in the H2 purity and storage performance.

22.7 Conclusion Hydrogen fuel has many advantages over presently dominating fossil fuels. However, current hydrogen storage technologies do not offer adequate storage of hydrogen that is both safe and sustainable. High-density solids have a great potential to serve as ideal hydrogen storage materials. Nevertheless, the hurdles in their widespread application are variable densities, high desorption temperatures, irreversibility, and thermodynamic inflexibility. Recently, huge attention has been paid to the enhancement of high-density solids. Doping of additives, nanoengineering, and nanosizing approaches have shown great promises to resolve the challenges and improve the high storage performance of high-density solids.

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Index

α-glucosidase, 511 β-oxidation, 511 Abatement of mercury and hydrogen sulfide, 368 Absorption, 681 Acetogenesis, 439, 440, 441, 454, 511 Acetotrophic, 440 Acidification, 482 Acidogenesis, 439, 441, 442, 448, 453, 511 Acidogenic, 249, 252, 256 Acidophiles, 258 Adenosine triphosphate (ATP), 448, 457 Alkaline electrolytic pathway, 346 Alkaline pretreatment, 482 Alternate energy resource, 42 Alternative fuels, 431 Amine aluminum borohydrides, 695 Amine calcium borohydrides, 694 Amine lithium borohydrides, 692 Amine magnesium borohydrides, 693 Amine metal borohydrides, 692 Amino boranes, 679 Ammonia, 428 Ammonia boranes, 687 Ammonia-water absorption refrigeration cycle, 352 Anaerobic, 623, 624, 625 Antifouling agents, 251

Antiporter, 449 Apparent quantum efficiency, 580 Aqua splitting technology, 101 Aromatic biopolymer, 40 Aseptic, 444 Band gap, 319 Binary plants, 348 Bioaugmentation, 245, 252 Biocathode, 523 Biochemical, 66, 81 Biochemical routes, 179, 180, 207 Biofouling, 250 Biohydrogen, 508 Biohydrogen production, 277 Biological water splitting, 239 Biomass, 39 Biomass and waste conversions, 239 Biomass gasification, 420 Blue hydrogen, 345 Borides, 237 Bottoming cycle, 348 C-catalyst, 577 Carbon based composite of MOFs, 665–666 Carbon composite, 152, 159 Carbon dioxide sequestration, 377 Carbon felt, 518 Carbon nanotubes, 523 Catalysts with only one atom, 234 Catalytic conversion, 39 Catalytic conversion of lignin, 43

707

708  Index Catalytic hydrodeoxydation, 39 Catalytic hydrodeoxydation (HDO) of lignin, 46 Catalytic hydrogenolysis of lignin, 45 Catalytic transformation involving hydrogen, 44 Centralized biomass gasification, 375 Centrifugation, 482 Chalcogenides, 236 Chemical stability, 426 Chemical treatment, 44 Chemical treatment methods, 482 Chlor-alkali cell, 368 Cobalt doped molybdenum carbide (Co/MoC), 162 Cobalt-nickel selenide on porous nickel foam, 160 Cogeneration units, 342 Colloids, 251, 257 Concentrated photovoltaic/thermal system, 363 Concentrated solar thermal H2 production, 99 Conduction, 347 Conduction band, 318 Conduction band (CB), 585 Continuous dark fermentation, 288 Continuous photofermentation, 279 Continuous stirred tank reactor (CSTR), 246 Convective heat transfer, 347 Conventional halide perovskite, 543 Copper-chlorine cycle, 374 Cosmic reactor, 102 CPV, 74, 77 CPV based H2 generation, 107 Crystalline g-C3N4, 401 Dark fermentation, 287, 464, 511 Decarbonization, 103 Dehydrogenation reaction, 679 Digestate, 438, 439 Dry ice, 377 Dry stream, 347

Dual fluid organic ranking cycle, 356 Dual-stage fermentation, 514 Economic assessments, 375 Effect of interpenetration over H2 storage, 661 Electricity, 342 Electricity generation, 429 Electrocatalyst, 325 Electrochemical, 63, 64, 66, 67, 68, 69, 70, 71, 72, 75, 76, 77, 317 Electrochemical method, 99 Electrochemical properties, 426 Electrochemical water splitting (EWS), 224 Electrogenic microorganisms, 305 Electrolysis, 63, 66, 67, 71, 72, 73, 74, 75, 77, 226, 229, 316, 317, 323, 325, 341, 419 Electrolyzer, 374 Electrolyzers, 99 Energy, 365 Energy carrier, 344 Energy crisis and hydrogen production, 652 Energy resources, 678 Energy storage, 431 Environmental aspects, 377 Exchange current density, 150 Exergy, 365 Exoelectrogens, 518 Facultative anaerobic bacteria, 511 Faradic efficiency, 150 Fermentation, 239, 621, 625, 626, 627, 628, 629, 630, 641 Ferredoxin, 514 Flash stream, 347 Flash-binary cycle, 348 Food processing waste, 420 Fossil fuels, 418 Fossil-free fuels, 225 Free nitrous acid, 482 Freezing, 467

Index  709 Fuel cell vehicles, 432 Fuel cells, 430 Functionalization of linkers of MOFs, 664 Fusion, 426 Future energy carrier, 508 g-C3N4 morphology, 398 g-C3N4 thin nanosheets, 398 g-C3N4-based photocatalysis, 397 Gamma irradiation, 467 Gasification, 345, 418, 621 Geothermal energy, 349 Geothermal fluids, 342 Geothermal gradient, 349 Geothermal powerplants, 342 Geothermal reservoirs, 350 Glycerol, 181, 183, 184, 185, 186, 187 Glycolysis, 255, 446, 447, 451 Grand Canonical Monte Carlo (GCMC) simulations, 657 Graphene, 523 Graphite fibers, 518 Graphite plates, 518 Graphitic carbon nitride, 413 Green fuel, 343 Green hydrogen, 347, 430 Green technology, 508 Green waste management, 509 Grey hydrogen, 345 H2 evaluation reaction (HER), 224 H2 storage, 653 Halide perovskite, 543 Heating, 431 Heterogeneous photocatalysis mechanism, 395 Heterogenous photocatalysis, 575 Heterojunction, 320 Heyrovsky reaction, 148 High density materials, 679 High-enthalpy, 350

Homoacetogen, homoacetogens, 443, 454, 455 Homoacetogenesis, 517 Homogenous photocatalysis, 574 HOT ELLY, 375 Hot springs, 349 Hybrid system, 97 Hydraulic retention time, 246, 248, 293, 250, 521 Hydro wind energy, 375 Hydrocracking, 427 Hydrodeoxydation (HDO), 44, 49 Hydrofining, 427 Hydrogen, 341 Hydrogen claude refrigeration unit, 370 Hydrogen cost, 375 Hydrogen dispenser hose reliability, 240 Hydrogen effectiveness, 99 Hydrogen energy, 237 Hydrogen evolution reaction, 147 Hydrogen fuel, 677 Hydrogen liquefaction, 370 Hydrogen production, 413, 677 Hydrogen storage, 371, 667, 678 Hydrogen sulfide, 368 Hydrogen-based economy, 341 Hydrogenotrophic, 440, 443, 444, 454 Hydrolysis, 439, 440, 445, 446, 450, 457 Hydrolytic microbes, 515 Hydrophilicity, 257 Hydroxides, 236 Hyperthermal water, 349 Immobilization, 286 Inlet temperature, 369 Inoculum condition, 282 Inoculums, 246 Internal heat, 349

710  Index Kalina cycle, 365 Land subsidence, 378 Lead-free halide perovskites, 544 Light intensity, 284 Lignin, 39, 255 Lignin biosynthetic pathway mechanism, 43 Lignin extraction, 43 Lignin pyrolysis oil, 44 Lignin reductive depolymerization, 44 Lignin separation process, 43 Lignin valorization, 41 Lignin-based pyrolysis, 43 Lignin-carbohydrate complexes (LCC), 43 Lignin conversion, 39 Lignin-derived bio-oil, 45 Lignin-derived fuels, 44 Lignocellulose biomass, 40 Linde-Hampson cycle, 371 Lithium alanates, 684 Lithium amides, 689 Lithium borohydride, 679 Load shock, 246 Low-enthalpy reservoirs, 350 Magma, 349 Mantle, 349 Mass flow rate, 352 Material stability, 425 Maximum power point tracking, 98 Mechanism of HER, 118 Mechanism of ectrochemical catalysis, 233 Metal alanates, 684 Metal amides, 689 Metal and alloys, 236 Metal borohydride, 679 Metal carbides, 237 Metal doped photocatalysis, 402 Metal oxides, 236 Metal-free catalysts, 235

Methane decarburization, 345 Methanogen, 246, 249, 253, 257 Methanogen inhibitor, 523 Methanogenesis, 439, 440, 441, 442, 444, 454, 511 Methanogens, 440–445, 450 Methanol, 429 Methods of production of hydrogen using WAS, 463 Microbial electrolysis cell, 300, 466, 514 Microbial electrolysis cells, 630, 262 Microflora, 252, 256, 260 Mixotrophic acetogens, 444 MOFs and noble metals hybrid for H2 storage, 668 Molybdenum sulphide (MoS2), 160 Multi-flash cycle, 365 Multi-flash geothermal power plant, 365 Multi-junction solar cells, 106 n-type, 68, 71, 72 Nano titanium dioxide, 500 Nanoparticles (NPs), 224 Ni, Ru, and Pd-based catalysts, 45 Niobate-based oxide perovskite, 541 Nitrides, 237 Nobel metal catalysts, 151 Noble metal catalyst (such as Pd, Pt, and Ru), 50 Nuclear power, 375 O2 evaluation reaction (OER), 224 Open metal sites in MOFs for H2 storage, 659 Organic loading rate, 292 Organic rankine flash cycle, 372 Organic ranking cycle, 348 Organosolv lignin, 45 Overpotential, 115, 150 Oxide perovskite, 538 Oxidoreductase, 514

Index  711 p-type, 68, 71, 72 Palladium based catalysts, 153 Palladium nanocluster/Cerium dioxide composite, 154 Palladium-silver-aluminium alloy, 154 Palladium/Carbon nitride composite, 153 Parabolic concentrator, 317 Parabolic trough solar collectors, 366 Partial oxidation reaction, 345 Partial pressure, 482 Pebble-bed nuclear reactor, 375 PEC, 66, 68, 71, 72 Perovskite materials for hydrogen production, 535 Perovskite structure, 537 Petroleum refineries, 427 Phenolic compounds, 46, 255 Phenolic monomers, 47 Phosphides, 237 Photo electrocatalysis, 228 Photo fermentation, 465 Photo-fermentation, 512 Photoanode, 320 Photobiological water splitting, 572 Photocatalysis, 3, 7, 16, 20, 317, 318 Photocatalytic, 316, 317, 318 Photocatalytic system, 551 Photocatalytic water splitting, 396, 573 Photocatalytically hydrogen evolution by water splitting, 574 Photocathode, 320 Photochemical, 64, 66, 67, 69, 78, 83, 89 Photoelectrochemical, 238, 317 Photoelectrochemical system, 552 Photoelectrochemical water splitting, 227 Photoelectrode, 320, 321, 331 Photoelectrolysis, 4, 7, 13, 18, 21 Photoexcited, 318 Photofermentation, 277 Photoinduced, 320 Photolysis, 261

Photon, 317 Photonic energy, 238 Photoreduction, 317 Photovoltaic, 323 Photovoltaic solar, 419 Photovoltaic system, 367 Photovoltaic thermal, 98 Photovoltaic-electrocatalytic system, 555 Photovoltaic-photoelectrochemical, 317, 321 Physical treatment methods, 467 Physisorption and spillover, 656 Pinch point temperature, 364 Platinum, 224 Platinum based catalysts, 151 Platinum-nickel nanocages, 153 Polyhydroxyalkanoates, 500 Polymer electrolyte membrane fuel cell, 372 Polymer semiconductor, 421 Polymerization, 424 Porous g-C3N4, 400 Potassium borohydride, 683 Pretreatment, pretreatments, 439, 443–446, 450, 451, 454–457 Proton exchange membrane, 346 Proton-exchange membrane fuel cell, 98 Purple non-sulfur bacteria, 512 PV, 65, 71, 72, 73, 74, 75, 76, 77, 78 Quantum dots (QDs), 586 Quantum yield, 317, 579 Quintuple flash system, 365 R114, 366 R123, 365 R245fa, 365 Radiogenic isotopes, 349 Rankine-trough-vapor, 367 Reactor configuration, 523 Recyclable fuel, 343 Reinjection, 378

712  Index Renewable electrolysis, 240 Renewable energies, 419 Renewable energy, 341 Renewable fuel, 40 Replacing existing hydrogen, 430 Reserve osmosis desalination unit, 356 Resource temperature, 351 Resporulation, 254 Reversibility, 681 Sabatier principle, 119 Sacrificial electron donor, 594 Sacrificial reagent, 577 Semiconductor/g-C3N4 heterojunction, 403 Semipermeable, 329 Single platinum atoms, 152 Single-atom catalysts (SACs), 224 Single-stage reactor, 441, 442, 453, 454 Sodium alanates, 686 Sodium amide, 690 Sodium borohydride, 681 Sodium chloride, 368 Sodium citrate, 500 Sol-gel method, 422 Solar, 63, 65, 66, 76, 79, 82, 83, 91 Solar cracking, 103 Solar cracking of methane, 104 Solar electrolysis, 374 Solar energy, 367 Solar integrated ammonia fuel cell, 368 Solar thermal, 105 Solar water splitting, 550 Solar-geothermal energy, 367 Solar-to-hydrogen conversion, 3, 7 Solid oxide, 66, 73, 75, 77, 79 Solid retention time, 482 Solventogenesis, 448 Stability, 117 Start-up culture, 252, 258 Steam, 365 Steam reforming process, 345

Sulfur–oxygen-iodine cycle, 347 Supercritical gasification, 624, 625, 631, 639, 640 Supercritical water gasification, 345 Supported liquid membrane (SLM), 327 Sustainable energy, 677 Synergistic effect, 594 Synthesis of perovskite, 549, 547, 546, 549, 546, 545 Tafel reaction, 148 Tafel slope, 116, 150 Tantalate-based oxide perovskite (ATaO3), 540 Thermal decompositional properties of lignin, 43 Thermal stability, 425 Thermal treatment, 467 Thermo-chemical, 640 Thermochemical, 64, 66, 78, 79 Thermochemical method, 101 Thermochemical water splitting, 239, 347, 572 Thermodynamic modelling, 365 Thermoelectric generator, 366 Thermogravimetric analysis (TGA), 43 Thermolysis, 100 Thermolysis process, 101 Thermophilic bacteria, 482 Thermophysical characteristics, 365 TiO2, 8–16 Titanate-based oxide perovskite (ATiO3), 538 Topping cycle, 348 Transition metal carbides, 161 Transition metal chalcogenides, 160 Transition metal oxide, 3, 7, 23–28 Transition metal phosphides, 156 Transition-metal catalysts, 50 Transmembrane, 448 Triclocarban, 500

Index  713 Turbine, 365 Turnover frequency, 117, 150 Turnover frequency (TOF), 580 Turnover number (TON), 580 Two-stage reactor, 441, 442, 452–454 Ultrasonication, 467 Utilization of biomass, 43 Valence band, 318 Valence band (CB), 585 Vapor-Trough-Rankine, 367 Vinasse, 181, 183, 193, 199, 202, 203 Volatile fatty acid (VFA), 255, 439, 440, 448, 459 Volmer reaction, 147

Washout, 445 Waste-activated sludge, 509, 621–631 Wastewater, 179, 180, 187, 190, 191, 193, 194, 195, 196, 197, 198, 199, 200, 201, 202, 203, 205, 206, 275, 622, 625, 630 Water electrolysis, 146 Water splitting, 6, 23, 419 Wind method, 419 Working fluids, 365 Yield of hydrocarbons, 50 Z-scheme, 319 Zeotropic mixtures, 366 Zero–CO2 energy, 347

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