Nanomaterials for Electrocatalysis 9780323857109

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Nanomaterials for Electrocatalysis

Micro and Nano Technologies Series

Nanomaterials for Electrocatalysis Edited by

Thandavarayan Maiyalagan Department of Chemistry, SRM Institute of Science and Technology, Kattankulathur, India

Mahima Khandelwal Department of Materials Science and Engineering, Korea University, Seoul, South Korea

Ashok Kumar Nadda Department of Biotechnology and Bionformatics, Jaypee University of Information Technology, Waknaghat, India

Tuan Anh Nguyen Microanalysis Department, Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

Ghulam Yasin Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-323-85710-9 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Matthew Deans Acquisitions Editor: Simon Holt Editorial Project Manager: Gabriela Capille Production Project Manager: Prasanna Kalyanaraman Cover Designer: Greg Harris Typeset by Aptara, New Delhi, India

Contents Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv Preface ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix

PART 1 Introduction CHAPTER 1 Nanoelectrocatalysis: An introduction ..................... 3 Ghulam Yasin, Shumaila Ibraheem, Rashid Iqbal, Anuj Kumar and Tuan Anh Nguyen 1.1 Introduction .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 3 1.2 Construction and characterization of nanostructures . ... .. .. .. ... .. 4 1.3 Efficient electrocatalysis enabled by nanostructures . ... .. .. .. ... .. . 4 1.3.1 Low-dimensional nanostructures . .. ... .. .. .. ... .. .. ... .. .. .. .. 4 1.3.2 2D nanostructures .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .. 5 1.3.3 3D nanostructures .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .. 6 1.4 Conclusion ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. 6 References .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .6

CHAPTER 2 2D hybrid electrocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Rashid Iqbal, Shumaila Ibraheem, Mohammad Tabish, Adil Saleem, Anuj Kumar, Tuan Anh Nguyen and Ghulam Yasin 2.1 2.2 2.3 2.4 2.5

Introduction .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. . 11 Graphene-based electrocatalysts .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. 12 Graphene nonmetallic composites .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 13 Graphene-metallic composites .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 15 Conclusion ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. . 16 References . .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. . 17

CHAPTER 3 MXene-based nanomaterials for electrocatalysis .... 23 Anuj Kumar, Charu Goyal, Sonali Gautam, Shumaila Ibraheem, Tuan Anh Nguyen and Ghulam Yasin 3.1 Introduction .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. . 23 3.2 Structural and electronic properties . .. .. ... .. .. .. ... .. .. .. ... .. .. .. .24 3.2.1 Structural properties . .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... 24 3.2.2 Electronic properties . ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. 25 3.3 Engineering of MXene-based nanomaterial .. ... .. .. ... .. .. .. ... .. .27 3.3.1 HF etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 3.3.2 Lewis acidic etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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3.3.3 Water-free etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30 3.3.4 Treatment with alkali . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.3.5 Electrochemical etching .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 31 3.3.6 Chemical vapor deposition method .. .. .. ... .. .. .. ... .. .. .. .. 31 3.4 Applications in electrocatalysis .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. . 31 3.4.1 Oxygen reduction reaction .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .. 35 3.4.2 Oxygen evolution reaction .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... ..35 3.4.3 Hydrogen evolution reaction . ... .. .. ... .. .. .. ... .. .. .. ... .. .. 39 3.4.4 CO2 reduction reaction .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. . 40 3.5 Summary and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42 References . .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. . 42

PART 2 Nanomaterials for Electrocatalytic reactions such as ORR, OER and HER CHAPTER 4 Transition metal nanoparticles as electrocatalysts for ORR, OER, and HER . . . . . . . . . . . . . . . . . 49 Dinh Chuong Nguyen, Thi Luu Luyen Doan, Duy Thanh Tran, Nam Hoon Kim and Joong Hee Lee 4.1 Introduction .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. . 49 4.2 Synthesis methods of the TM nanoparticle-based catalysts . .. .. ..50 4.2.1 Hydrothermal method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50 4.2.2 Solvothermal method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 4.2.3 Chemical reduction method ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. 53 4.2.4 Electrochemical deposition method .. .. .. ... .. .. .. ... .. .. .. . 55 4.2.5 Other synthetic methods .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 56 4.3 Structure and properties of TM nanoparticle-based catalysts . .. ..57 4.3.1 Substrate-free TM nanoparticle-based catalysts .. ... .. .. .. . 57 4.3.2 Carbon substrate-assisted TM nanoparticle-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.3.3 Metallic substrate-assisted TM nanoparticle-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 4.4 Applications of TM nanoparticle-based catalysts toward the ORR, HER, and OER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.4.1 ORR applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66 4.4.2 HER applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68 4.4.3 OER applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69 4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 References . .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. . 72

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CHAPTER 5 Transition metal chalcogenides-based electrocatalysts for ORR, OER, and HER . . . . . . . . . . . . . . . . . 83 Tenzin Ingsel and Ram K. Gupta 5.1 Introduction .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. . 83 5.1.1 Overpotential (η) .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1.2 Tafel plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 5.1.3 Faradaic efficiency . ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 86 5.1.4 Stability .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. 86 5.2 Synthesis of metal chalcogenides . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... 87 5.2.1 Solvothermal .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87 5.2.2 Chemical vapor deposition .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. . 88 5.2.3 Other methods .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 88 5.3 Transition metal chalcogenides-based electrocatalysts for OER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 5.4 Transition metal chalcogenides-based electrocatalysts for ORR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 5.5 Transition metal chalcogenides-based electrocatalysts for HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 5.6 Transition metal chalcogenides-based multifunctional electrocatalysts .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. 103 5.7 Conclusion and outlook .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. . 105 Acknowledgment .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 106 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 106

CHAPTER 6 Metal-organic framework-based electrocatalysts for ORR, OER, and HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111 Muhammad Rizwan Sulaiman and Ram K. Gupta 6.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 111 6.2 MOF-based electrocatalysts for ORR . ... .. .. .. ... .. .. .. ... .. .. .. . 116 6.2.1 MOF-derived nitrogen-doped carbon-based electrocatalysts for ORR . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... . 117 6.2.2 MOF-derived nonprecious metal-based electrocatalysts for ORR . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... . 120 6.3 MOF-based electrocatalysts for OER . ... .. .. .. ... .. .. .. ... .. .. .. . 124 6.3.1 MOF-derived metal-free materials for OER electrocatalyst . .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 124 6.3.2 MOF-derived nonprecious metal-based OER electrocatalyst . .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. 126 6.4 MOF-based electrocatalysts for HER . ... .. .. .. ... .. .. .. ... .. .. .. . 130

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6.4.1 MOF-derived metal-free carbon-based material for HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .131 6.4.2 MOF-derived NPM-based electrocatalyst for HER . .. ... .132 6.4.3 Metal carbide, phosphides, and chalcogenides . .. ... .. .. .. 133 6.5 MOF-based multifunctional electrocatalysts ... .. .. ... .. .. .. ... .. 136 6.5.1 MOF-derived OER/ORR bifunctional electrocatalysts . .. 136 6.5.2 MOF-derived HER/OER bifunctional electrocatalysts . .. 137 6.5.3 MOF-derived HER/ORR bifunctional electrocatalysts . .. 137 6.5.4 MOF-derived HER/OER/ORR trifunctional electrocatalysts . .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. . 138 6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 139

CHAPTER 7 Heteroatom-doped graphene-based electrocatalysts for ORR, OER, and HER . . . . . . . . . . . . . . . 145 Xun Cui, Likun Gao, Yingkui Yang and Zhiqun Lin 7.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 145 7.2 Overview of graphene and heteroatom-doped graphene-based materials .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. . 147 7.2.1 Graphene . .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. 147 7.2.2 Heteroatom-doped graphene-based materials .. ... .. .. .. .. 149 7.2.3 Synthesis of heteroatom-doped graphene-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151 7.3 Heteroatom-doped graphene-based materials as electrocatalysts for ORR, OER, and HER .. .. .. ... .. .. .. ... .. . 152 7.3.1 Heteroatom-doped graphene-based materials for ORR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .152 7.3.2 Heteroatom-doped graphene-based materials for OER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .156 7.3.3 Heteroatom-doped graphene-based materials for HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .158 7.4 Summary and perspective .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. 160 Acknowledgments . ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. . 161 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 161

CHAPTER 8 Metal-containing heteroatom doped carbon nanomaterials for ORR, OER, and HER . . . . . . . . . . . . . . . . . . 169 Álvaro García, Jorge Torrero, María Retuerto and Sergio Rojas 8.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 169 8.2 M/N/C catalysts for the ORR .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

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8.3 Synthesis of highly active M/N/C catalyst for the ORR . .. .. ... . 177 8.3.1 Fe/N/M catalysts derived from metal-organic frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177 8.3.2 Fe/M/N catalysts from sacrificial templates . .. ... .. .. .. ... 178 8.3.3 Fe/N/C catalysts derived from PANI . .. ... .. .. .. ... .. .. .. .. 180 8.3.4 Fe/N/C catalyst from porous organic polymers as precursors . ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. ..181 8.3.5 Other strategies for obtaining highly active M/N/C catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .181 8.4 Assessment of ORR performance of M/N/C catalysts .. .. ... .. .. 181 8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .185 8.5.1 Mössbauer spectroscopy . .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... . 186 8.5.2 X-ray photoelectron spectroscopy ... .. .. .. ... .. .. .. ... .. .. .189 8.5.3 X-ray absorption spectroscopy .. ... .. .. .. ... .. .. .. ... .. .. .. 191 8.5.4 Transmission electron microscopy .. .. .. ... .. .. .. ... .. .. .. . 194 8.6 Metal-containing heteroatom-doped carbon nanomaterials for OER and HER reactions . .. ... .. .. .. ... .. .. .. ..195 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 200

CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211 Anuj Kumar, Shashank Sundriyal, Charu Goyal, Tribani Boruah, Dipak Kumar Das, Ghulam Yasin, Tuan Anh Nguyen and Sonali Gautam 9.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 211 9.2 Engineering and effective strategies for modification of MOFs 212 9.2.1 Modification of MOFs by doping . .. .. .. ... .. .. .. ... .. .. .. . 212 9.2.2 MOF-derived materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .214 9.2.3 MOF-based composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217 9.3 Applications of MOFs-based materials in fuel cells . .. .. ... .. .. . 220 9.3.1 MOFs for electrocatalytic ORR . .. ... .. .. .. ... .. .. .. ... .. .. 221 9.3.2 MOFs for hydrogen production .. .. .. .. ... .. .. .. ... .. .. .. .. 223 9.4 Conclusion and future prospects . .. .. ... .. .. .. ... .. .. .. ... .. .. .. ...229 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 229

CHAPTER 10 LDH-based nanostructured electrocatalysts for hydrogen production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237 Mohammad Tabish, Shumaila Ibraheem, Muhammad Asim Mushtaq, Rashid Iqbal, Tuan Anh Nguyen and Ghulam Yasin

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10.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 237 10.2 Construction of TM-LDH nanostructures . .. ... .. .. .. ... .. .. .. ... .238 10.2.1 Bottom-up approaches .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. . 238 10.2.2 Top-down approaches . ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... 240 10.3 Carbon nanomaterial-based TM-LDH nanohybrids . .. .. .. ... .. . 240 10.4 Electrocatalytic application for hydrogen production .. .. .. ... .. .241 10.5 Conclusion ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. ..245 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 246

CHAPTER 11 MOFs-derived hollow structure as a versatile platform for highly-efficient multifunctional electrocatalyst toward overall water-splitting and Zn-air battery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251 Lei Zhang, Yuan-Xin Zhu and Guang-Zhi Hu 11.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 251 11.2 Brief classification of hollow structures based on their geometrical configuration . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... . 252 11.2.1 Single-shelled hollow structures . . . . . . . . . . . . . . . . . . . . . . . . . . 252 11.2.2 Multishelled hollow structures . .. .. .. ... .. .. .. ... .. .. .. ... 253 11.2.3 Other complex hollow structures . . . . . . . . . . . . . . . . . . . . . . . . . .254 11.3 Active regulation strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 11.3.1 Active site assembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 11.3.2 Electronic structure effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 11.3.3 Single-atom catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .260 11.3.4 Defect chemistry . .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. . 262 11.3.5 Synergistic catalysis . .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .262 11.4 Conclusions and perspectives .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. .. 265 Acknowledgments . ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. . 265 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 265

PART 3 Nanomaterials for Electrochemical Nitrogen reduction reaction (NRR) CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Xue Zhao and Guangzhi Hu 12.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 273 12.2 Non-noble metal-based metal catalysts .. .. ... .. .. ... .. .. .. ... .. .. 274 12.2.1 Mo-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274 12.2.2 Fe-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277

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12.2.3 Ti-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .279 12.2.4 Bi-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282 12.2.5 Co, Ni-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 12.2.6 Other non-noble metal metal-based catalysts .. .. ... .. .. . 284 12.3 Non-metal-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 12.3.1 B-based NRR catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 12.3.2 N-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 12.3.3 O- and S-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 12.3.4 P-based catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Competing interests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .291 Acknowledgments . ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. . 291 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 291

CHAPTER 13 Noble metals-based nanocatalysts for electrochemical NNR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 Jing Li, Zihao Ye and Weiwei Cai 13.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 299 13.2 Ru-based NRR catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .300 13.2.1 Single-atom Ru-based NRR catalysts . ... .. .. .. ... .. .. .. . 300 13.2.2 Supported Ru-based NRR catalysts . .. .. ... .. .. .. ... .. .. ..301 13.2.3 Ru-based alloy catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 13.3 Au-based NRR catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 13.3.1 Au catalyst nanostructure adjusting .. .. ... .. .. .. ... .. .. .. .306 13.3.2 Supported Au-based NRR catalysts ... .. .. .. ... .. .. .. ... . 307 13.3.3 Au-based alloy NRR catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .308 13.4 Other noble metal-based NRR catalysts .. .. ... .. .. .. ... .. .. .. ... . 310 13.4.1 Pd-based NRR catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310 13.4.2 Pt-based NRR catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311 13.5 Conclusions and prospects . .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. .. 312 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 313

CHAPTER 14 Electrochemical NRR with noble metals-based nanocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 Mohd Khalid, Mohammad Rafe Hatshan, Ana Maria Borges Honorato, Bijandra Kumar and Hamilton Varela 14.1 14.2 14.3 14.4 14.5

Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 317 NRR mechanism . .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... . 318 Types of the electrochemical cell for NRR ... .. .. ... .. .. .. ... .. .. 320 Electrolytes for NRR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322 NRR based on noble metals .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ..323

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14.6 14.7 14.8 14.9

NRR based on Au nanocatalysts .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .324 NRR based on Ru nanocatalysts . .. .. .. ... .. .. .. ... .. .. .. ... .. .. ...326 NRR based on Pd nanocatalysts .. .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 327 Conclusions and outlook .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. 328 Acknowledgments . ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. . 329 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 330

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Zehui Yang, Quan Zhang and Shenglin Xiao 15.1 Introduction .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. . 335 15.2 Transition metal oxides-based electrocatalysts ... .. .. .. ... .. .. .. . 336 15.2.1 Titanium oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336 15.2.2 Chromium oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338 15.2.3 Manganese oxides .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. .. 338 15.2.4 Iron oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 340 15.2.5 Nickel-based oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341 15.2.6 Niobium oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .342 15.2.7 Other transition metal oxides .. ... .. .. .. ... .. .. ... .. .. .. ...342 15.3 Transition metal sulfides-based electrocatalysts ... .. .. .. ... .. .. ..346 15.3.1 Molybdenum sulfides .. .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. 346 15.3.2 Iron sulfides . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. . 348 15.3.3 Other transition metal sulfides . .. .. ... .. .. .. ... .. .. .. ... .. 350 15.4 Transition metal nitride-based electrocatalysts ... .. .. .. ... .. .. .. . 351 15.5 Transition metal phosphides-based electrocatalysts .. ... .. .. .. ...353 15.5.1 Cobalt phosphides .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... . 353 15.5.2 Nickel phosphides .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. . 355 15.5.3 Iron phosphides ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ..355 15.6 Transition metal carbides-based electrocatalysts . ... .. .. .. ... .. .. 357 15.6.1 Mxene-based electrocatalysts .. .. ... .. .. .. ... .. .. .. ... .. .. 357 15.6.2 Molybdenum carbides-based electrocatalysts .. ... .. .. .. .357 15.7 Metal-free electrocatalysts . ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. . 358 15.7.1 Boron-doped carbon .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. . 358 15.7.2 Nitrogen-doped carbon .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... 360 15.7.3 Fluorine-doped carbon . .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. 360 15.7.4 Sulfur-doped carbon .. .. ... .. .. .. ... .. .. .. ... .. .. ... .. .. .. . 362 15.7.5 Black phosphorus . .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... 363 15.8 Conclusion ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. ..363 References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 364

Contents

PART 4 Nanomaterials for Electrochemical CO2 reduction reaction CHAPTER 16 Nanomaterials for electrochemical reduction of CO2 : An introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373 Anuj Kumar, Ghulam Yasin and Tuan Anh Nguyen References . .. .. .. ... .. .. .. ... .. .. ... .. .. .. ... .. .. .. ... .. .. .. ... .. .. .. 375 Index ...... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377

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Contributors Tribani Boruah Northeast Hill University (NEHU), Umshing Mawkynroh, Shillong, Meghalaya, India Weiwei Cai Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Xun Cui School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, United States; Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education and Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan, China Dipak Kumar Das Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India Thi Luu Luyen Doan Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea Likun Gao School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, United States; Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, Harbin, China Álvaro García Grupo de Energía y Química Sostenibles Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain Sonali Gautam Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India Charu Goyal Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India Ram K. Gupta Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States Mohammad Rafe Hatshan Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia Ana Maria Borges Honorato Department of Materials Engineering, Federal University of São Carlos, São Carlos, SP, Brazil

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Guangzhi Hu Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, School of Chemical Science and Technology, Yunnan University, Kunming, China Shumaila Ibraheem Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Tenzin Ingsel Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States Rashid Iqbal Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Mohd Khalid Institute of Chemistry of São Carlos, University of São Paulo, São Carlos, SP, Brazil Nam Hoon Kim Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea Anuj Kumar Nano-Technology Research Laboratory, Department of Chemistry, GLA University, Mathura, Uttar Pradesh India; College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China Bijandra Kumar Department of Math., Comp. Sci. and Eng. Technology, Elizabeth City State University , Elizabeth City, NC, United States Joong Hee Lee Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea; Carbon Composite Research Center, Department of Polymer-Nano Science and Technology, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea Jing Li Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Zhiqun Lin School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, United States Muhammad Asim Mushtaq State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China

Contributors

Dinh Chuong Nguyen Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea Tuan Anh Nguyen Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam María Retuerto Grupo de Energía y Química Sostenibles Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain Sergio Rojas Grupo de Energía y Química Sostenibles Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain Adil Saleem Institute for Advanced Study, Shenzhen University, Shenzhen, Guangdong, China; College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Muhammad Rizwan Sulaiman Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States Shashank Sundriyal Advanced Carbon Products Department, CSIR-National Physical Laboratory, New Delhi, India Mohammad Tabish State Key Laboratory of Electrochemical Process and Technology for Materials, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China Jorge Torrero Grupo de Energía y Química Sostenibles Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain Duy Thanh Tran Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea Hamilton Varela Institute of Chemistry of São Carlos, University of São Paulo, São Carlos, SP, Brazil Shenglin Xiao Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences Wuhan, Wuhan, PR China Yingkui Yang Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education and Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan, China

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Zehui Yang Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences Wuhan, Wuhan, PR China Ghulam Yasin Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China Zihao Ye Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China Lei Zhang School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui, PR China Quan Zhang Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences Wuhan, Wuhan, PR China Xue Zhao Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, School of Chemical Science and Technology, Yunnan University, Kunming, China Yuan-Xin Zhu School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui, PR China

Preface The distinct depletion of fossil fuels and ever-growing human dependence on the energy-based usages has triggered a stern decline in ordinary energy reservoirs and also worsened our environment. It is expected that the world will requisite to duple its energy sources to endure the universal economy progression by 2050. Subsequently, there is an imperative need than ever to discover the use of renewable, clean, and bounteous energy sources. In this trend, an encouraging progress is the usage of sustainable energy fonts to transform molecules (i.e., nitrogen, carbon dioxide, and water) in the atmosphere into valued products (i.e., ammonia, hydrocarbons, and hydrogen) through electrocatalysis technologies. Indeed, a number of innovative energy conversion and storage systems, for instance, rechargeable metal-air batteries, water electrolysis, and fuel cells have broadly been considered. Remarkably, these techniques vastly reliant on the chain of electrochemical reactions, comprising the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), CO2 reduction reaction (CO2 RR), and nitrogen reduction reaction (NRR), etc. Nonetheless, the efficacy of these reactions intensely depends on the synthetic structure and properties of the catalysts used. The modern advancement in nanomaterials has opened up the innovative avenue by building nanostructures for proficient energy storage and conversion. This book credibly focuses on the enduring and advances in the nanotechnology and development of nanomaterials for advanced electrocatalysis. Of particular attention, metal-based and metal-free nanomaterials have been cost-efficiently designed into various electrocatalysts with efficient energy storage/conversion capabilities. The consequent innovations in building the competent nanostructures unlocked a novel era in electrochemistry and material science. Ever since then, transition-metal-, and carbon-based nanomaterials with distinctive surface/size-reliant electrochemical possessions have been revealed to be beneficial in electrocatalysis, and marvelous development has been accomplished in emerging nanomaterials for proficient energy storage and conversion technologies. This is a blistering field wherein a substantial extent of literature has been promptly engendered with several publications ongoing to upturn annually. So, it is very significant to cover the utmost latest advances in this field in a well-timed mode. This book compacts with the fundamentals, synthesis methods, and wide range applications of these metal-based and metal-free nanostructures. So as to cover the multistructured meadow of such variety, transition-metal, noble-metal and carbonbased nanomaterials for energy technologies deliver a pool of chapters transcribed by top scholars who have been keenly employed in associated fields, and the script has been distributed into different parts. This book is anticipated to cover all the different types of transition-metal based (e.g., transition metal oxides, hydroxides, and chalcogenides), noble-metal based, metal-free (e.g., carbon- and graphenebased, heteroatom-doped carbon), and hybrid nanomaterials for electrocatalysis. The

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novel synthetic modes, characterization, different structures, and their applications of above-mentioned nanomaterials and their multifunctional products are comprehensively discussed. The overhead tactic will let the booklovers to first understand the scientific fundamental information of electrocatalysis and then encompass the basic awareness to the synthesis, development, and application of nanomaterials in practical technologies. The readers who are fresh in this field will be motivated to learn fundamental illustrations that may well deliver a vivid devotion earlier to stern reading. Meanwhile, the updated citations in all chapters ought to facilitate readers to rapidly analyze the stimulating field with evidence on the hottest advances. So, this book “Nanomaterials for Electrocatalysis” is an indispensable reference on nanomaterials for energy storage and conversion systems to researchers, engineers, teachers, scientists and students in the field of materials science, nanotechnology, and electrochemistry. Academic specialists can use this book to swiftly review the up-to-date advances to widen their understanding of nanomaterials for electrocatalysis emerging innovative technologies for energy storage and conversion systems. At last, we want to express our earnest gratitude to Gabriela Capille and their coworkers at Elsevier for their generous and tolerant support through the accomplishment of this book. We would also like to acknowledge the entire chapter authors, collaborators, and our associates who contributed to the book. Last, but not the least, we are be the grateful to our families for their perpetual tolerance, love, and unremitting support. Shumaila Ibraheem, Ghulam Yasin Institute for Advanced Study College of Physics and Optoelectronic Engineering Shenzhen University, Shenzhen, Guangdong, China [email protected], [email protected]

PART

Introduction

1

CHAPTER

Nanoelectrocatalysis: An introduction

1

Ghulam Yasin a, Sehrish Ibrahim b, Shumaila Ibraheem a, Rashid Iqbal a, Anuj Kumar c and Tuan Anh Nguyen d a

Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, b College of Life Science and Technology, Beijing University of Chemical Technology, Beijing, China, c Nano-Technology Research Laboratory, Department of Chemistry, GLA University, Mathura, Uttar Pradesh India, d Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam

1.1 Introduction With the depletion of fossil fuels, the mandate for energy resources and rising ecological issues, the advancement of sustainable and renewable energy conversion and storage technologies with low cost and remarkable efficiency has regarded more decisive than ever [1–7]. However, integrating these energy devices in our everyday necessitates remains a great challenge since the competent catalyst materials are needed for all electrocatalysis systems. As a result, a thorough and systematic understanding of the mechanism of electrochemical processes is inextricably linked to the development of energy conversion and storage technologies, such as electrolyzers (water splitting) [8], fuel cells [9, 10], and batteries [6, 11]. In these applications, materials used as electrodes enable the dual purpose: they act as potential catalysts through reducing energy barrier for the overall electrochemical reactions, while also boosting the electron charge transfer rate on the electrode surfaces [12]. For that reason, the inclusive study focused on the development of definite surfaces in the strongly regulated environments is regarded as an esteemed technique for the essential understanding of electrochemical reactions, such as, oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER), nitrogen reduction reaction (NRR), and CO2 reduction reaction (CO2 RR) [13–17]. Therefore, the characteristics of electrode materials, for example, composition, surface structure, and morphology are imperative to control with corresponding to electrochemical conditions for efficient and high-performance electrocatalysis. Electrocatalysts in the practical applications are typically made of expensive precious metals that are employed in the form of nanoparticles, allowing for substantial material utilization [12]. For instance, spherical-shaped nanoparticles can have more than 1000 times larger total surface area as compared to their microparticles Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00017-4 Copyright © 2022 Elsevier Inc. All rights reserved.

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counterparts with the same mass and material. However, the nanodimensions add complication and raise a slew of new questions to be answered with the purpose to evaluate, utilize, and comprehend the impact of specific physical parameters such as defects, lattice mismatch, low-coordinated atoms, and surface strain, etc. [18]. Accordingly, rational technique in the development of nanocatalyst material needs projection of aimed characteristics for the nanoscale system. For example, in research associated to the effect of particle size, the morphology and composition of nanocatalyst material would be uniform [19], because, these have direct relationship between the functional properties and physical characteristics of nanoparticles. In this chapter, we briefly summarize the latest advances in the construction, characterization of advanced nanostructures, and their applications in the electrocatalysis for nextgeneration energy conversion and electricity storage technologies.

1.2 Construction and characterization of nanostructures The fundamental of nanoscience research is the well-organized fabrication of nanomaterials and characterization at nanoscale empowers us to explicitly associate the structural possessions with the chemical, physical, and biotic properties of nanomaterials. The end product must be consistent in shape, scale, and chemical composition, which is a basic prerequisite for nanomaterial synthesis. Many new synthetic methodologies for producing high-grade nanoparticles, nanowires, nanorods, nanocubes, and many other nanostructures using metals, metal oxides, and semiconductors have recently been created [20–25]. Likewise, to classify nanomaterials, a vast combination of methods is commonly used due to the high chemical and spatial resolution necessities. Table 1.1 lists the most generally used nanomaterial characterization procedures. Several of them, from Table 1.1, have been established and practically use to characterize the possessions of nanomaterials under employed situations and deliver the molecular-level information for additional activity optimizations.

1.3 Efficient electrocatalysis enabled by nanostructures 1.3.1 Low-dimensional nanostructures The low-dimensional (L-D) nanomaterials could approximately be classified into two systems: mainly, zero-D (0-D) and linear-D (1-D). Nanomaterials that hold sphere-shaped or approximately round forms are usually considered as particulate nanomaterials, and the rectilinear arrangement signifies nanomaterials with 1-D, such as nanoribbons (NRs), nanofibers, nanowires (NWs), nanocables, and nanotubes (NTs). Furthermore, nanomaterials with irregular morphology that do not emanated in the rectilinear and particulate systems also can be classified into L-D nanomaterials, such as nanoboxes, nanonails, nanocages, nanoframes, nanostars, nanobridges,

1.3 Efficient electrocatalysis enabled by nanostructures

Table 1.1 Generally employed characterization techniques for nanostructures. Technique name

Properties/Characteristics

X-ray diffraction (XRD) UV-vis-nIR spectroscopy X-ray photoelectron spectroscopy (XPS) Photoluminescence spectroscopy (PL) Chemisorption, physisorption Atomic force microscopy (AFM) Scanning electron microscopy (SEM) Transmission electron microscopy (TEM) Scanning tunneling microscopy (STM) Small angle X-ray scattering (SAXS)

Crystal structure Light absorption and scattering Chemical composition Light emission Surface area Shape, size, and work function Shape, and assembly structure Size, shape, and crystallinity Shape, size, and surface structure Distinctive distances of partially ordered nanostructures Chemical composition

Near-edge X-ray absorption fine structure (NEXAFS) energy dispersive X-ray analysis (EDX) Extended X-ray absorption fine structure (EXAFS) X-ray emission spectroscopy (XES) Ultraviolet photoelectron spectroscopy (UPS)

Chemical composition Chemical composition and bonding environment Electron band gap Electron valence band

dendritic, and nanomultipodic nanostructures that are promising electrode materials for electrocatalysis.

1.3.2 2D nanostructures 2D nanomaterials have acknowledged significant research exertion due to their unique chemical and physical properties inventing from their fascinating morphological and electronic properties [26–32]. The 2D nanomaterials could be engaged in a diversity of research extents due to their inimitable dimensional constructions. Unambiguously, 2D ultra-thin nanostructures with single-/few-layered wideness are supposed to be a sort of superlative electrocatalyst in numerous substantial electrocatalytic methods, meanwhile their abridged thickness effects in a momentous improvement of catalytic activities equated with their analogous multilayered and bulk complements. The benefits of ultrathin 2D nanostructures as electroactive catalysts comprise the following: (1) A cumulative available surface area by dropping their width clues to convenient interaction between imported molecules and active centers; (2) practically all internal surface atoms are visible to the exterior, expediting the development of extra defects to produce further active edges and coordination unsaturated metal active sites, which might aid as active sites in electrocatalysis that can decrease the combative energy barrier; and (3) the dropping of the wideness of the 2D nanostructures can enhance the conductivity of catalyst materials.

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1.3.3 3D nanostructures Generally, three-dimensional (3D) nanomaterials have an assimilated design made of L-D or nanoscale subunits, comprising 2D nanosheets, 1D NWs or NTs, and/or 0D NPs. Countless struggles are essential to disclose the consequence of material building on reaction and transportation kinetics to mend the structural design of 3D ordered nanostructures. Motivation could be imitative from environment, which is occupied by operative graded structures, such as petals, flowers, branches, trees, and sea urchins having collections of spines on their surface. When these assemblies are imitated and created on a nanoscale, the physical assets can be moderated accurately, giving occasions to advance their performance for numerous applications. 3D nanostructures are considered as vastly appropriate as catalysts in electrochemical energy-conversion and storage devices. Commonly, the fundamental kinetics of an electrocatalyst is accompanying with the nanomaterial of the active stage and the sustenance, though the inclusive properties are also pretentious by the morphological design.

1.4 Conclusion The rising demand to develop new viable and sustainable energy sources needed for our today modern society and to be used in the near future has prompted the scientific researchers to look for alternative and clean energy technologies owing to the environmental concerns and rapid reduction of conventional fossil fuels. The present advancements in nanoscience and nanotechnology have enabled the effective route for the construction of advanced nanostructures for efficient electrochemical energy production and electricity storage systems, which are more selective, efficacious, and long-term technologies. Therefore, one of the most competitive and dynamic research fields of chemistry, materials science, and energy applications is the construction of advanced nanomaterials with excellent activity and functional sturdiness for highperformance electrocatalysis processes. Significant innovative findings are already obtained for nanostructures and nanomaterials that are catalytically active for HER, OER, ORR, HOR, NRR, and CO2 RR. In conclusions, the unique design and costeffective construction of advanced nanostructures and functional nanomaterials are anticipated to further boost the efficiency of electrocatalysis and to decrease the cost of green energy generation and storage technologies, enabling them more appealing in addressing the global environmental concerns and energy crises.

References [1] G. Yasin, M. Arif, T. Mehtab, X. Lu, D. Yu, N. Muhammad, M.T. Nazir, H. Song, Understanding and suppression strategies toward stable Li metal anode for safe lithium batteries, Energy Storage Mater 25 (2020) 644–678. https://doi.org/10.1016/j.ensm. 2019.09.020.

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[2] T. Mehtab, G. Yasin, M. Arif, M. Shakeel, R.M. Korai, M. Nadeem, N. Muhammad, X. Lu, Metal-organic frameworks for energy storage devices: batteries and supercapacitors, J Energy Storage 21 (2019) 632–646. https://doi.org/10.1016/j.est.2018.12.025. [3] G. Yasin, M. Arif, T. Mehtab, M. Shakeel, M.A. Mushtaq, A. Kumar, T.A. Nguyen, Y. Slimani, M.T. Nazir, H. Song, A novel strategy for the synthesis of hard carbon spheres encapsulated with graphene networks as a low-cost and large-scalable anode material for fast sodium storage with an ultralong cycle life, Inorg Chem Front 7 (2) (2020) 402–410, doi:10.1039/C9QI01105F. [4] D. Yu, A. Kumar, T.A. Nguyen, M.T. Nazir, G. Yasin, High-voltage and ultrastable aqueous zinc–iodine battery enabled by N-doped carbon materials: revealing the contributions of nitrogen configurations, ACS Sustainable Chem Eng 8 (36) (2020) 13769–13776, doi:10.1021/acssuschemeng.0c04571. [5] N. Muhammad, G. Yasin, A. Li, Y. Chen, H.M. Saleem, R. Liu, D. Li, Y. Sun, S. Zheng, X. Chen, H. Song, Volumetric buffering of manganese dioxide nanotubes by employing ‘as is’ graphene oxide: an approach towards stable metal oxide anode material in lithiumion batteries, J Alloys Compd 842 (2020) 155803. https://doi.org/10.1016/j.jallcom. 2020.155803. [6] G. Yasin, M.A. Khan, W.Q. Khan, T. Mehtab, R.M. Korai, X. Lu, M.T. Nazir, M.N. Zahid, Facile and large-scalable synthesis of low cost hard carbon anode for sodium-ion batteries, Results Phys 14 (2019) 102404. https://doi.org/10.1016/j.rinp.2019.102404. [7] S. Ullah, G. Yasin, A. Ahmad, L. Qin, Q. Yuan, A.U. Khan, U.A. Khan, A.U. Rahman, Y. Slimani, Construction of well-designed 1D selenium–tellurium nanorods anchored on graphene sheets as a high storage capacity anode material for lithium-ion batteries, Inorg Chem Front 7 (8) (2020) 1750–1761, doi:10.1039/C9QI01701A. [8] R. Subbaraman, D. Tripkovic, K.-C. Chang, D. Strmcnik, A.P. Paulikas, P. Hirunsit, M. Chan, J. Greeley, V. Stamenkovic, N.M. Markovic, Trends in activity for the water electrolyser reactions on 3d M(Ni,Co,Fe,Mn) hydr(oxy)oxide catalysts, Nat Mat 11 (6) (2012) 550–557, doi:10.1038/nmat3313. [9] M.K. Debe, Electrocatalyst approaches and challenges for automotive fuel cells, Nature 486 (7401) (2012) 43–51, doi:10.1038/nature11115. [10] S.M. Haile, Fuel cell materials and components, The Golden Jubilee Issue Selected topics in Materials Science and Engineering: Past, Present and Future, edited by S. Suresh, Acta Materialia 51 (19) (2003) 5981--6000. https://doi.org/10.1016/j.actamat. 2003.08.004 [11] H. Wang, L. Sheng, G. Yasin, L. Wang, H. Xu, X. He, Reviewing the current status and development of polymer electrolytes for solid-state lithium batteries, Energy Storage Mater 33 (2020) 188–215. https://doi.org/10.1016/j.ensm.2020.08.014. [12] Y. Kang, P. Yang, N.M. Markovic, V.R. Stamenkovic, Shaping electrocatalysis through tailored nanomaterials, Nano Today 11 (5) (2016) 587–600. https://doi.org/10.1016/j. nantod.2016.08.008. [13] C. Wang, C. Li, J. Liu, C. Guo, Engineering transition metal-based nanomaterials for high-performance electrocatalysis, Mater Rep: Energy 1 (1) (2021) 100006. https:// doi.org/10.1016/j.matre.2021.01.001. [14] A. Kumar, G. Yasin, V.K. Vashistha, D.K. Das, M.U. Rehman, R. Iqbal, Z. Mo, T.A. Nguyen, Y. Slimani, M.T. Nazir, W. Zhao, Enhancing oxygen reduction reaction performance via CNTs/graphene supported iron protoporphyrin IX: a hybrid nanoarchitecture electrocatalyst, Diamond Relat Mater 113 (2021) 108272. https://doi.org/10.1016/j. diamond.2021.108272.

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CHAPTER 1 Nanoelectrocatalysis: An introduction

[15] M. Nadeem, G. Yasin, M. Arif, H. Tabassum, M.H. Bhatti, M. Mehmood, U. Yunus, R. Iqbal, T.A. Nguyen, Y. Slimani, H. Song, W. Zhao, Highly active sites of Pt/Er dispersed N-doped hierarchical porous carbon for trifunctional electrocatalyst, Chem Eng J 409 (2021) 128205. https://doi.org/10.1016/j.cej.2020.128205. [16] A. Kumar, G. Yasin, R.M. Korai, Y. Slimani, M.F. Ali, M. Tabish, M. Tariq Nazir, T.A. Nguyen, Boosting oxygen reduction reaction activity by incorporating the iron phthalocyanine nanoparticles on carbon nanotubes network, Inorg Chem Commun 120 (2020) 108160. https://doi.org/10.1016/j.inoche.2020.108160. [17] M.A. Mushtaq, M. Arif, X. Fang, G. Yasin, W. Ye, M. Basharat, B. Zhou, S. Yang, S. Ji, D. Yan, Photoelectrochemical reduction of N2 to NH3 under ambient conditions through hierarchical MoSe2@g-C3N4 heterojunctions, J Mater Chem A 9 (5) (2021) 2742–2753, doi:10.1039/D0TA10620H. [18] Z.L. Wang, T.S. Ahmad, M.A. El-Sayed, Steps, ledges and kinks on the surfaces of platinum nanoparticles of different shapes, Surface Sci 380 (2) (1997) 302–310. https:// doi.org/10.1016/S0039-6028(97)05180-7. [19] C. Wang, D. van der Vliet, K.-C. Chang, H. You, D. Strmcnik, J.A. Schlueter, N.M. Markovic, V.R. Stamenkovic, Monodisperse Pt3Co nanoparticles as a catalyst for the oxygen reduction reaction: size-dependent activity, J Phys Chem C 113 (45) (2009) 19365–19368, doi:10.1021/jp908203p. [20] G. Yasin, M. Arif, M.A. Mushtaq, M. Shakeel, N. Muhammad, M. Tabish, A. Kumar, T.A. Nguyen, Chapter Nine - Nanostructured anode materials in rechargeable batteries, in: H. Song, R. Venkatachalam, T.A. Nguyen, H.B. Wu, P. Nguyen-Tri (Eds.), Nanobatteries and Nanogenerators, Elsevier, USA, 2021, pp. 187–219. [21] G. Yasin, N. Muhammad, A. Kumar, M. Tabish, M.U. Malik, M.T. Nazir, D. Liu, T.A. Nguyen, Chapter Eleven - Nanostructured cathode materials in rechargeable batteries, in: H. Song, R. Venkatachalam, T.A. Nguyen, H.B. Wu, P. Nguyen-Tri (Eds.), Nanobatteries and Nanogenerators, Elsevier, USA, 2021, pp. 293–319. [22] S. Ibraheem, S. Chen, J. Li, W. Li, X. Gao, Q. Wang, Z. Wei, Three-dimensional Fe, N-decorated carbon-supported NiFeP nanoparticles as an efficient bifunctional catalyst for rechargeable zinc–O2 batteries, ACS Appl Mater Interfaces 11 (1) (2019) 699–705, doi:10.1021/acsami.8b16126. [23] S. Ibraheem, S. Chen, J. Li, Q. Wang, Z. Wei, In situ growth of vertically aligned FeCoOOH-nanosheets/nanoflowers on Fe, N co-doped 3D-porous carbon as efficient bifunctional electrocatalysts for rechargeable zinc–O2 batteries, J Mater Chem A 7 (16) (2019) 9497–9502, doi:10.1039/C9TA01964B. [24] S. Ibraheem, S. Chen, L. Peng, J. Li, L. Li, Q. Liao, M. Shao, Z. Wei, Strongly coupled iron selenides-nitrogen-bond as an electronic transport bridge for enhanced synergistic oxygen electrocatalysis in rechargeable zinc-O2 batteries, Appl Catal, B 265 (2020) 118569. https://doi.org/10.1016/j.apcatb.2019.118569. [25] S. Ibraheem, X. Li, S.S.A. Shah, T. Najam, G. Yasin, R. Iqbal, S. Hussain, W. Ding, F. Shahzad, Tellurium triggered formation of Te/Fe-NiOOH nanocubes as an efficient bifunctional electrocatalyst for overall water splitting, ACS Appl Mater Interfaces 13 (9) (2021) 10972–10978, doi:10.1021/acsami.0c22573. [26] A. Jabbar, G. Yasin, W.Q. Khan, M.Y. Anwar, R.M. Korai, M.N. Nizam, G. Muhyodin, Electrochemical deposition of nickel graphene composite coatings: effect of deposition temperature on its surface morphology and corrosion resistance, RSC Adv 7 (49) (2017) 31100–31109, doi:10.1039/C6RA28755G.

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[27] G. Yasin, M. Arif, M. Shakeel, Y. Dun, Y. Zuo, W.Q. Khan, Y. Tang, A. Khan, M. Nadeem, Exploring the nickel–graphene nanocomposite coatings for superior corrosion resistance: manipulating the effect of deposition current density on its morphology, mechanical properties, and erosion-corrosion performance, Adv Eng Mater 20 (7) (2018) 1701166. https://doi.org/10.1002/adem.201701166. [28] G. Yasin, M.A. Khan, M. Arif, M. Shakeel, T.M. Hassan, W.Q. Khan, R.M. Korai, Z. Abbas, Y. Zuo, Synthesis of spheres-like Ni/graphene nanocomposite as an efficient anticorrosive coating; effect of graphene content on its morphology and mechanical properties, J Alloys Compd 755 (2018) 79–88. https://doi.org/10.1016/j.jallcom.2018.04.321. [29] G. Yasin, M. Arif, M.N. Nizam, M. Shakeel, M.A. Khan, W.Q. Khan, T.M. Hassan, Z. Abbas, I. Farahbakhsh, Y. Zuo, Effect of surfactant concentration in electrolyte on the fabrication and properties of nickel-graphene nanocomposite coating synthesized by electrochemical co-deposition, RSC Adv 8 (36) (2018) 20039–20047, doi:10.1039/ C7RA13651J. [30] G. Yasin, M.J. Anjum, M.U. Malik, M.A. Khan, W.Q. Khan, M. Arif, T. Mehtab, T.A. Nguyen, Y. Slimani, M. Tabish, D. Ali, Y. Zuo, Revealing the erosion-corrosion performance of sphere-shaped morphology of nickel matrix nanocomposite strengthened with reduced graphene oxide nanoplatelets, Diamond Relat Mater 104 (2020) 107763. https://doi.org/10.1016/j.diamond.2020.107763. [31] G. Yasin, M. Arif, T. Mehtab, M. Shakeel, M.A. Khan, W.Q. Khan, Chapter 14 - Metallic nanocomposite coatings, in: S. Rajendran, T.A. Nguyen, S. Kakooei, M. Yeganeh, Y. Li (Eds.), Corrosion Protection at the Nanoscale, Elsevier, USA, 2020, pp. 245–274. [32] M. Tabish, M.U. Malik, M.A. Khan, G. Yasin, H.M. Asif, M.J. Anjum, W.Q. Khan, S. Ibraheem, T.A. Nguyen, Y. Slimani, M.T. Nazir, Construction of NiCo/graphene nanocomposite coating with bulges-like morphology for enhanced mechanical properties and corrosion resistance performance, J Alloys Compd 867 (2021) 159138. https:// doi.org/10.1016/j.jallcom.2021.159138.

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CHAPTER

2D hybrid nanoarchitecture electrocatalysts

2

Rashid Iqbal a, Shumaila Ibraheem a, Mohammad Tabish c, Adil Saleem a,b, Anuj Kumar d, Tuan Anh Nguyen e and Ghulam Yasin a a

Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, b College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, c College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China, d Nano-Technology Research Laboratory, Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India, e Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam

2.1 Introduction The increase in energy utilization and use of fossil fuels which resulted in an environmental pollution, to cope this problem it is necessary to use of renewable and environmentally friendly ways to meet the energy demands [1–9]. The new evolving energy production ways and methods such as hydrogen energy, lithium-ion battery, nuclear, solar, and wind energy, etc., have fascinated increase in interest over the past decade. Among which, electrocatalysis involves the hydrogen evolution (HER), oxygen evolution reaction (OER), N2 reduction reaction (NRR), and oxygen reduction reaction (ORR), plays a vital role for the approaches used for energy conversion as well as storage [10–22]. There are number of great efforts have been made in search of low cost, highly selective, stable, and efficient catalyst. Among them, twodimensional (2D) monolayer structure-based materials have got great importance due to multi-atomic π –π conjugate structure [10, 23, 24], have exceptional conductivity, large surface area, mechanical and electrical properties, which regarded them as an excellent candidate for electrocatalysis. Numerous methods have been used to modify graphene and composited it with nanoparticles or various other 2D materials like MoS2 , WS2 , covalent organic framework, metal organic framework, etc. Herein, we summarize the development and design, synthetic strategies involved in various electrocatalysts like, ORR, HER, OER, NRR, and CO2 RR [25, 26]. The hybrid architecture of graphene may include surface defects, heteroatom doped metal atoms, and mixture of various other carbon containing amorphous or crystalline materials (Fig. 2.1). This chapter will reasonably elaborate the potential trends and issues for hybrid nanoarchitecture materials based on graphene and various other 2D materials.

Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00006-X Copyright © 2022 Elsevier Inc. All rights reserved.

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CHAPTER 2 2D hybrid nanoarchitecture electrocatalysts

4H+

e-

C

2H2O

e-

e-

O2

e-

H

Intrinsic defect regulation

Doping of nonmetallic element

Graphene

Graphene-based composites

Graphene-based single-atom catalyst

FIGURE 2.1 Graphene-based electrocatalysts is controlled through introducing defects, nonmetallic elements, into graphene system. Reprinted with permission from Ref. [27]. Copyright 2018 Nature. Reprinted with permission from Ref. [28]. Copyright 2018 American Chemical Society. Adapted from Ref. [29]. Copyright 2019 Wiley-VCH. Adapted from Ref. [30]. Copyright 2018 Tsinghua University Press and Springer-Verlag GmbH Germany.

2.2 Graphene-based electrocatalysts Graphene has been used in broad range of electronic and energy applications [31–34], its novel 2D layered structure attract many researchers and have remained widely considered in the recent past [35–41]. In its pristine state, most 2D materials do not exhibit any chemical activity. Graphene has high electrical conductivity and huge surface area which make it a promising active material in field of catalysis. but the major drawback is its poor catalytic activity. However, their properties can be reduced and by use of different dopants [42–45], generating defects [46, 47], and by transition metals (graphene-based single-atom catalyst). Transition metal dichalcogenides, such as molybdenum sulfides and selenides, are considered to be effective catalysts for the hydrogen evolution reaction (HER) as well as CO2 reduction reaction. Therefore, introduction of transition metal sulfides into graphene has been used for HER electrocatalysts. Molybdenum disulfide (MoS2 ),

2.3 Graphene nonmetallic composites

the 2D sheet of vertically stacked S-Mo-S interlayers, has fascinated excessive concentration as a possible HER electrocatalyst owing to its extremely responsive atomic active sites uncovered at the edges. Apart from the reactive active sites, it is also prejudiced by numerous supplementary aspects, for instance the charge transfer and ion transport of the cathode. To avoid these hindrances, a 2D nanoarchitecture was prepared involving of very thin MoS2 nanoplatelets decorated on to the surface of graphene sheets (MoS2 /G). At low applied overpotential of −0.15 V, a current density of 10 mA cm−2 was achieve with a structural integrity and long-lasting stability even after testing in acidic solution. Nickel-doped into the FeNixS2 -RGO by 42 mV at a current density of 10 mA cm−2 in 0.5 M H2 SO4 due to faster electron transfer mechanism and enhances its electrochemical kinetics and advance the initial stability of the electrocatalyst compare to pure FeS2 or pure NiS2 . In particular, a cathodic current density of over 100 mA cm−2 at an overpotential less than 200 mV were attained by introduction of optimized addition of MoSe2 flakes on single walled carbon nanotubes (SWCNTs).

2.3 Graphene nonmetallic composites Most promising material in this category is carbon nitride, Si, organic compounds, and black phosphorus. But, due to poor conductivity these materials show sluggish performance and cyclic stability, which affect the industrial use of these materials. Scientists solve this issue making composite of the abovementioned nanomaterials with graphene. For instance, polynitrogen material increased the consideration owing to the high energy density and environmentally benign. Black phosphorus fewlayered exfoliated nanosheets is extensively useful in the optoelectronics, electronics, catalysis, and energy storage [48] owing to its excellent carrier mobility, tenable electronic structure, huge surface area with active lone-pairs, and entirely uncovered surface atoms [49, 50]. Yet, black phosphorus is not very stable at the room temperature and readily gets oxidized [51], that is why it is typically hybridized or mixed with graphene nanomaterials to guarantee the long-lasting steadiness. Yu et al. [52] mixed exfoliated black phosphorus and N-doped graphene (NG) by means of the electrostatic interface to produce a very thin black phosphorus nanosheet with NG (Fig. 2.2A). Due to the coherent interface fabrication, the interaction of black phosphorus with Ndoped graphene composite efficiently modified electronic structures to advance the inherent stability and electrochemical performance. Experimental characterization and DFT calculations showed that the electron transfer between the interfaces of black phosphorus and nitrogen-doped graphene might efficiently advance adsorption energy during OER and HER intermediates. Similarly, carbon nitride with graphitic structure (g-C3 N4 ) with the advantages of modifiable, low-cost, chemical structure, excellent environmentally benign and chemical stability has established growing attention in catalysis fields. In view of the characteristic sluggish conductivity of g-C3 N4 that confines it as electrocatalysis;

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CHAPTER 2 2D hybrid nanoarchitecture electrocatalysts

Bulk BP

EBP

(C)

--- - -Sonication --Self-assembly

EtOH/H2O

O ER

(A)

HE R

14

+

+

+

+

+

+

Bifunctional HER/OER catalyst

NG

(B) +

H

+ e-

H2

(D)

H2

H2O

+

H

ee-

C O

+ e-

H2 H2

e-

N H

H

+

+ e-

H2 H2

e-

H+ + e-

Porous C3N4 nanolayers@N-graphene film

(E)

(G)

(F)

N

1 2 6 7 5 3 8 4

1

Se

0.32

4

2 3 5

1

8 6

0.34

7

S

8

3 5 6

7

0.34

0.06 e

0.05 e

4

2

0.1 e

0.17

0.175

0.175

0.01

0.01

0.01

FIGURE 2.2 (A) Representation for exfoliation scheme of bulk BP and construction of EBP@NG. Reprinted with permission from Ref. [52]. Copyright 2019 American Chemical Society. (B) Porous C3 N4 nanolayers@N-graphene films as electrocatalyst for HER. Reprinted with permission from Ref. [53]. Copyright 2015 American Chemical Society. (C) TEM images of S-heterocyclic/rGO. (D) Different side of views of the molecular packing amongst linear rGO layers and conjugated polymers. (E–G) Mechanism study for electrocatalytic active sites on the heterocyclic structures for ORR. Reprinted with permission from Ref. [29]. Copyright 2019 Wiley-VCH.

numerous scientists have been made to enhance electrocatalytic activity and electrical conductivity by combining graphene with g-C3 N4 . Qiao et al. [53] designed a 3D cross blended layers by purposely mixing 2D porous N-doped graphene sheets with g-C3 N4 nanolayers for HER application (Fig. 2.2B). It is revealed that the electrocatalytic performance was strictly associated to the 2D chemical structure, plenty of uncovered active sites, well-ordered synergetic coupling, porous structure, and N-doping, which could efficiently advance electrocatalytic activity. Subsequently, Qu et al. [54] enhanced manufacture method of graphene-g-C3 N4 composites are fabricated by mesh-on-mesh graphitic-g-C3 N4 @graphene (g-CN@G MMs) for extremely effective

2.4 Graphene-metallic composites

hydrogen evolution. The exclusive mesh assembly and robust coupling results in gCN@G MMs with vigorous electron transfer ability and plentiful active sites, which was the chief motive for the enhanced activity. In current study, it displays that the without metal-based polymers can also be practical in the catalysis by merging with graphene nanomaterials. For instance, the linear conjugated polymers (LCPs) reported by Yang et al. [29], constructed a graphene-LCPs composites, the structure contained of N-, Se-, or S-heterocycles (Fig. 2.2C, D). Electrochemical results and DFT studies in Fig. 2.2E–G discovered that the S-heterocyclic polymer demonstrated unresolved ORR electrocatalytic performance related by means of Se-heterocycles and N-heterocycles. The interaction among S-heterocyclic and graphene-encouraged electron density reshuffle so as to upsurge the performance and generate a huge quantity of active centers.

2.4 Graphene-metallic composites Transition metal compounds, considering transition metal oxide [55], transition metal phosphide [56, 57], transition metal sulfides [58], and transition metal selenide [59] show a significant role in catalysis and energy application. Graphene-metallic mixtures, particularly transition metal-graphene hybrids nanoarchitecture have been useful in catalysis lately due to their high electrochemistry performance and robust synergetic effect. The plenty functional groups and huge specific surface area on the exposed surface of graphene deliver enough bonding sites for the expansion of metallic compounds, enabling the distribution of nanoparticles. In the meantime, the robust linking bonds among TMC and graphene can be built over interfacial engineering, indorsing the charge transfer [60]. In another study, the TMC composite with graphene specifically heteroatom doped-graphene encourages a consequence that advances the electronic structural network of TMC to upsurge the electrocatalytic performance [60]. Xie et al. [58] stated fabrication of graphene/cobalt sulfide an interfacial hybrid structure (NS-G/CoS2 ) to attain outstanding efficacy in electrochemical production of ammonia. X-ray absorption near edge spectroscopy showing a robust interfacial interaction built between NS-G/CoS2 . As a consequence, NS G/CoS2 exhibited the minor transfer of charge resistance and high electrocatalytic performance. A comparable robust nanoarchitecture was constructed by Lu et al. [60]. Monolayered mesoporous Co3 O4 layers sturdily joined with N-rGO exfoliated nanosheets displayed captivating bifunctional electrocatalytic performance of ORR and OER. Furthermore, Wang et al. [62] considered CoFe alloy nanoparticles blended with N-doped 3D carbon matrix. The above composited structure further scrambled with reduced graphene oxide (rGO) nanosheets (CoFe/N-GCT) for ORR and OER. The conclusion of experimental results displayed that transfer of electron and diffusion of oxygen were sponsored by 3D carbon structures fashioned by means of rGO and CNTs, and the N-doping could advance the electrocatalytic performance of the Co-Fe alloy. In the meantime, the in situ classification established that the electrocatalytic performances of OER and

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CHAPTER 2 2D hybrid nanoarchitecture electrocatalysts

ORR were delivered by Co and Fe species, correspondingly. CNTs and graphene as promoter better-quality charge and mass transmission, and suitable electronic network of above-mentioned alloy to increase the performance. The electrocatalytic procedure is frequently escorted by means of the corrosion and dissolution of the electrocatalyst due to harsh electrolyte, consequently undergoes deprived stability [64]. In another study, carbon-coated treatment is an auspicious approach to guard the electrocatalyst from dilapidation [65]. Subsequently, carbon shells are susceptible to diminishing the aptitude of transfer of mass and transport of ion, electrocatalysts uncovered by graphene could efficiently resolve this issue. Ohto et al. [61] constructed a composite with a N-doped graphene uncovering Ni-Mo alloy by mean of CVD method for OER activity. The Ni-Mo alloy with various covering quantity of graphene was manufactured to examine the effect of dissimilar alloy coverage by graphene. DFT simulation studies investigated that the alloy electronic network could be effectually attuned by N-doped graphene to decrease the Gibbs free energy of the intermediate’s species adsorption. Above and beyond, covering of electrocatalyst by N-doped graphene could efficiently lessen the dilapidation of the alloy and advance the life of electrocatalysts. Nevertheless, the in situ growth of graphene on the external surface of electrocatalyst by means of CVD method typically needs repetitive and complex stages. In another study, Chen et al. [63] planned a N-doped graphene in few layers uncovered nonprecious ternary alloys for OER and HER performance by heat-treating MOF. Throughout pyrolysis procedure, CN- of MOF was aided equally by nitrogen and carbon resources for the in situ procedure of N-doped graphene few layers on the coated on the surface of alloys. The growth of N-doped graphene layers could efficiently advance the transfer of electron in the nanomaterials; nonetheless correspondingly regulate the alloy material electronic network. Furthermore, the metal amounts in ternary alloy might also be altered deliberately to attain the enhanced activity of alloys. Guo et al. [66] created core-shell structured Fe-Ni alloy@N-doped carbon nanocages composited with nanosheets of N-doped graphene for OER. Equally, the graphene nanosheets and carbon shells delivered multichannels for oxygen release, diffusion of electrolyte, and supplementary active sites for electrocatalytic reaction. Consequently, the electrocatalyst exhibited outstanding OER electrocatalytic stability and activity.

2.5 Conclusion In overall, graphene can efficiently boost the electrical conductivity of the electrocatalyst and decrease the electron movement resistance. The active sites that bond with graphene can efficiently advance the electrocatalytic performance, the tolerance, and stability in the catalysis system. Consequently, there are countless requirements to discover the appropriate nanomaterials with electrocatalytic performance to couple with graphene and captivating full benefit of coupling role and synergistic effect among graphene and other materials. Ultimately, the electrocatalytic activity of graphene-based nanomaterials is mostly reliant on the electronic structure regulation

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among the catalytic site and heteroatoms, the graphene electronic coupling, and the enhancement of conductivity. Consequently, novel advancement mechanisms should be well thoughtout for development of an optimistic character of graphene in electrocatalysis as abundant as conceivable, such as, the graphene with a magic-angle superlattice structural network has been extensively considered for the application of superconductivity since the modification of electronic interactions and behavior. If the graphene with magic-angle shows distinct electronic structure traits can be practical in the application of electrocatalysis, it will be a new and innovative study course, indorsing the expansion of electrocatalysis materials.

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CHAPTER

MXene-based nanomaterials for electrocatalysis

3

Anuj Kumar a, Charu Goyal a, Sonali Gautam a, Shumaila Ibraheem b, Tuan Anh Nguyen c and Ghulam Yasin b a

Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India, b Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, c Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam

3.1 Introduction During the recent couple of years, considerable attempt has been dedicated to achieve clean, safe, and sustainable energy sources for the energy production to meet the continually rising global energy demand due to fast depletion of existing fossil fuels. In this perspective, the electrocatalytic reactions like oxygen, nitrogen, and CO2 reduction reactions (ORR, NRR, and CO2 RR, respectively), and oxygen and hydrogen evolution reactions (OER and HER, respectively) can be regarded as the key to address the sustainable demand of energy [1,2]. However, these electrocatalytic reactions possess extremely sluggish kinetics at electrode surfaces, involving a large overpotential and display major efficiency loss. Noble metals like Pt, Ru, and Irbased materials are quite effective for such electrocatalytic reactions. Nonetheless, low abundance and high price of these materials have created several constraints for their large-scale applications [3]. Therefore, the search for low cost and reliable electrocatalysts for these key electrocatalytic reactions is a potential area of research for the development of sustainable energy resources. In this regard, MXene-based nanomaterials are the one of the potential electrocatalysts for H2 , O2 , N2 , and CO2 involving electrocatalytic reactions. MXenes (transition metal carbonitrides, carbides, and nitrides) are the modish members of the 2D nanomaterial groups [3], having general formula Mn+1 Xn Tx (where n = 1, 2, 3), where M, X, and Tx signifies an early transition metal, C/Natom, and surface functionalities like −O, −OH, −F groups [4]. MXene has typical structures M2 XTx, M3 X2 Tx , and M4 X3 Tx, where n layers of X components are covered with n+1 layers of M [5]. In 2011, the first MXene (Ti3 C2 Tx ) was prepared and more than 19 varieties of MXenes have been produced since then and many more MXenes are likely to come. MXene is a class of compounds that can easily be combined with various materials such as oxides, polymers, and carbon nanotubes to modify the MXene Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00008-3 Copyright © 2022 Elsevier Inc. All rights reserved.

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characteristics more effectively for different applications. MXene-based nanomaterials have been found to be potential materials, exhibiting excellent performance in energy conversions and storage devices. Exceptional electrical conductivity of MXenes with hydrophilic surface confirm their outstanding candidature as electrocatalysts in electrocatalytic reactions like oxygen and CO2 reduction reactions (ORR and CO2 RR, respectively), oxygen and hydrogen evolution reactions (OER and HER, respectively). In this chapter, we highlighted the MXene-based nanomaterials for H2 , O2 , and CO2 involving electrocatalytic reactions, covering the research work carried out on MXenes from their early to recent stage, including preparation, properties (mechanical, electrical, and magnetic properties), and electrocatalytic performance.

3.2 Structural and electronic properties High degree of adaptability with their layered anatomy and 2D morphology, MXenes can effortlessly form composites with other fabrics, possessing high conductivity and excellent electrochemical activity. The stacking arrangement is the another crucial factor which governs overall stability to the molecular framework of MXenes. The different chemical compositions of MXenes give many interesting mechanical, electronic, magnetic, and electrochemical properties.

3.2.1 Structural properties The general crystal geometry of MXene has a close-packed hexagonal structure, similar to the corresponding MAX phase, where, M atoms are arranged in a closepacked structure, and the octahedral position is engaged by X atoms. The neighboring layers are coupled together with van-der-Waals forces which are same as that in 2D materials like graphene [6]. MXenes is usually prepared in aqueous solution in presence of acid fluoride. As a result, the MXenes surface is engaged by a mixture of –OH, –O, and –F functional groups. Recent computational studies indicated that the surface functional groups exert expressive impacts on the properties of MXenes. For instance, Hu et al., utilized Bader charge examination and thermodynamic estimations to systematically study the chemical properties of functionalized ending groups MXene [7]. The prepared materials showed the stability in the order of Ti3 C2 O2 > Ti3 C2 F2 > Ti3 C2 (OH)2 > Ti3 C2 H2 > Ti3 C2 , which was responsible for the splitting of the largely degenerated 3d orbitals of surface of Ti atoms. In another research, Fu et al. [8] systematically studied various –F, –Cl, –H, –O, and –OH functional groups incorporated MXene-based nanomaterials to figure out the influence of functional groups on their stability, mechanical properties as well as electrocatalytic activity. The results suggested that the oxygen-functionalized Ti3 C2 MXene has better thermodynamic stability and durability than other functional groups containing Ti3 C2 MXene because of critical charge transfer from inner bonds to the external surface of the material. Wang et al. [9] conducted the morphological investigation using scanning transmission electron microscope (STEM) to study the surface of Ti3 C2 Tx at atomic level.

3.2 Structural and electronic properties

The findings indicated that surface functional groups, like –OH, –F, and –O were unevenly arranged on the surface of Ti3 C2 Tx and tend to occupy the top position of central Ti-metal. Karlsson et al. [10] observed single-layer and double-layer Ti3 C2 using STEM-EELS with aberration correction, and revealed the inherent defects and complexes of the lamellar coating and TiOx adsorbent. In another investigation, Sang et al. noticed that the various point defects in Ti3 C2 monolayer nanosheets using STEM. Hope et al. used 19F and 1H nuclear magnetic resonance (NMR) experiments to quantify the functional groups on the surface of Ti3 C2 Tx, and observed that the proportion of various surface terminations largely depended upon the fabrication methods of MXene [11].

3.2.2 Electronic properties The application of MXenes in electrocatalytic reactions broadly depends on their excellent electronic properties. Recently, computational investigations have been conducted to study the influence of various M, X, and surface functionalities upon the electronic behavior of MXenes. The results suggested that due to presence of several transition metals, the electronic properties of MXene lied between metals and semiconductors [12]. However, several MXenes, having early transition metals like Mo, W, and Cr possessed topological insulating behavior [3,5]. Moreover, the surface treatment can also modify the electronic structure of pure MXene. For instance, Fredrickson et al. used DFT calculations to examine the structure and electronic properties of Ti2 C and Mo2 C in layers with various functional groups in aqueous phase [13]. In addition, the out-of-plane lattice parameters of MXene also depend on the functional surface groups and water retention. When the applied potential was zero, the bulk MXene (Ti2 C and Mo2 C) was functionalized with an O-monolayer. However, regardless of the lower applied potential, pure MXene was unstable. The alteration in functional group from O moieties to H moieties at Ti2 C surface can promote the transition of metal-insulator under the action of applied electric potential. In another study, it was found that bare MXene like Ti3 C2 showed metallic properties, however, functionalization of Ti3 C2 with different terminal groups such as –OH, –F, and –I can produce semiconducting features with precise band gaps. This observation also showed that the M layer had significant impact on the electronic properties of the produced material. Although Ti3 C2 Tx is metallic, Mo-containing MXene has semiconductor properties. Wang et al. [14] noticed that based on DFT calculations, Ni2 N-MXenes had inherent semimetallic properties. The electronic behavior of MXene has been related to its nanostructure. For example, Yenyashin et al. [15] supposed that hydroxylated Ti3 C2 nanotubes had metal-like properties. Zhao et al. [16] reported different electronic properties of Ti3 C2 nanoribbons from MXenes nanosheets. Some experiments were conducted to examine the electronic properties of MXene. However, only some of the electronic behavior of MXene like Ti2 CTx , Ti3 C2 Tx, and Mo2 CTx are achieved through experiments. For instance, Halim et al. [17] estimated the electronic conductivity of Ti3 C2 Tx and Mo2 CTx films. Further, Lipatov et al. [18] showed the electronic properties of Ti3 C2 Tx monolayer

25

(D)

(C)

(B)

CHAPTER 3 MXene-based nanomaterials for electrocatalysis

(A)

26

3.2 Structural and electronic properties

flakes. Lai et al. [19] observed excellent electronic properties of 2D-Ti2 CTx . Further, the theoretical calculation and experimental outcomes showed that MXenes has excellent electronic behavior and is a favorable candidate material for electrochemistry, electrocatalysis, and energy storage.

3.3 Engineering of MXene-based nanomaterial The syntheses strategies are the key in deciding the intrinsic properties of MXene [20]. Moreover, their synthesis conditions can directly be responsible for their properties and functionalities. Basically, there are two methods for the synthesis, wet-etching method, and nonetching method [3]. In wet-etching method, Mn Xn−1 (where n = 2, 3, 4) is extracted by removal of atomic layers from their counterpart [21] (Fig. 3.1A [i–iii]). Until now, different sorts of etchants like, HF, HCl-LiF, tetramethylammonium hydroxide (TMAOH), NH4 HF2 , and NaOH have been created and some novel manufactured courses have been investigated [22]. On the other hand, chemical vapor deposition (CVD) can also be utilized for the preparation of MXene under nonetching method [23]. For the preparation of MXene, many wet-etching methods have been described, such as HF etching, Lewis acidic etching, electrochemical etching, alkali treatment, and water-free etching.

3.3.1 HF etching First, the reported Ti3 C2 MXene was prepared in 2011. The Ti3 AlC2 MAX-phase prepared from highly reactive Al with fluoride-based hydrated solution, has finite techniques for the preparation of miscellaneous MXenes. The HF-etching reaction ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− FIGURE 3.1 (A) (i) Elements in periodic tables for the creation of MAX phases, (ii) different type of structures of MAX and their corresponding MXene phases, (iii) M3 X2 monolayer from top look, (iv) The representation of single layer molecular structure M3 X2 T. Reproduced with permission [29]. Copyright 2021, Wiley-VCH. (B) (i) Ti3 SiC2 MAX phase is submerge in Lewis molten salt of CuCl2 at 750°C. (ii) Diagrammatic representation of synthesis of Ti3 C2 Tx MXene. (ii) Gibbs free energy mapping (700°C) giving information about the selection of Lewis acid Cl salts on the basis of the electrochemical redox potentials of A-layered elements in MAX phases (x axis) and molten salt cations (y axis) in Cl melts [30] Copyright 2021, Springer Nature. Reproduced with permission [28]. Copyright 2018, Wiley-VCH. (C) (i) The reaction between Ti3 AlC2 and NaOH water solution at various conditions. (ii) Under various hydrothermal temperatures and NaOH concentrations, principal products for 24 batches of samples have been prepared. Reproduced with permission [30]. Copyright 2021, Wiley-VCH. (D) (i) Schematic representation of the Mo2 CTx :Co structure. (ii) Schematic illustration of preparation approach of the Cox Mo2−x C/MXene/NC catalyst. Reproduced with permission [31]. Copyright 2021, Wiley-VCH.

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of Ti3 AlC2 phases can be given by following reactions [24]: Ti3 AlC2 + 3HF = AlF3 + 3/2H2 + Ti3 C2

(3.1)

Ti3 C2 + 2H2 O = Ti3 C2 (OH)2 + H2

(3.2)

Ti3 C2 + 2HF = Ti3 C2 F2 + H2

(3.3)

The etching condition depends on the structure of MAX phases, particle size, and atomic bonding of MAX phases. The formation of a particular type of MXene depends on the temperature of etching process, concentration of the etchant (HF), and time taken of the reaction. Stronger etching with longer etching time can be obtained by increasing M numbers and n numbers in Mn+1 Cn Tx [20]. Moreover, to acquire isolated MXene sheets from the multilayer MXene after etching of HF, regular intercalation followed by delamination measures are needed [25]. For this purpose, by using sonication method, organic molecules like dimethyl sulfoxide (DMSO) and tetra-alkylammonium compounds like tetrabutylammonium hydroxide (TBAOH) and tetramethylammonium hydroxide (TMAOH) have been used for growing interlayer spacing of multi-layered MXene. The use of HF directly in the formation of MAX or MXene in these techniques may cause lack in safety and environmental conditions due its corrosive nature [26]. Since then, MXene are specifically prepared by etching of Alayer by hydrated solution having fluoride ions, like LiF+HCl mixture or HF in MAX phases. It is noticed that in situ HF is formed by fluoride ion and strong acids which is used in selective etching of a layer’s atoms and drive to intercalation of water and cations like Li+ , Na+ , K+ , and NH4 + in between MXene layers. Therefore, spacing between the layers of MXene increases, and results in sluggish interaction between the layers. It is noticeable that the nature and size of the MXene can be influenced by the amount of strong acid and fluoride salt. Further, minimally intensive layer delamination method (12 M LiF/9 M HCl) can also be used to produce single-layer or multilayers having larger lateral size and lesser defect [18].

3.3.2 Lewis acidic etching Li et al. found that Ti3 AlC2 reacts with ZnCl2 Lewis acidic molten salt at 550°C form Ti3 ZnC2 MAX phase by replacement reaction techniques. By enhancing the MAX:ZnCl2 ratio, Ti3 ZnC2 could be converted into Ti3 C2 Cl2 MXene [27]. Further, Huang et al. found that in the MAX phase, higher redox potential Lewis acid cations could impressively oxidize and etch A-layer atoms with lower redox potential [28]. This etching effect of molten Lewis acid salts gives a novel and practicable way to synthesize MXenes via fluoride-free chemical method. Thus to obtain particular MXene materials, relevant Lewis acid could be chosen to etch diverse MAX phases (Fig. 3.2B[i, ii]). According to this selection range, the preparation strategies would also differ. Other example of this method is the treatment of Ti4 AlN3 MAX phases in mixture of molten fluoride salt having weight ratio as LiF:NaF:KF = 29:12:59 at 550°C to synthesize Ti4 N3 under argon atmosphere [32]. The etching process can be finished

(A)

(B)

3.2 Structural and electronic properties

29

30

CHAPTER 3 MXene-based nanomaterials for electrocatalysis

inside a generally brief time of 30 minutes. Since, Tin Nn−1 has lower stability than Tin Cn−1 , it dissolves in the fluoride-based acid etchant. Therefore, it requires short reaction time. After that, washing with H2 SO4 and DI water and delamination in the TBAOH solution are also needed processes. Due to delamination of Ti4 N3 , its crystallinity decreases. Apart from this, TiO2 phase can be seen in the endmost product. Yet, this method has many limitations such as (1) high temperature and consumption of energy is required in the etching process; (2) lower crystallinity and less pure MXene are formed; (3) vacancies and defects are also obtained.

3.3.3 Water-free etching The use of water as a solvent may cause decrease in MXene performances. For instance, lithium-ion batteries and sodium ion batteries having organic electrolytes can be affected even by small amount of water. Natu et al. described that the polar organic solvent and ammonium dihydrogen fluoride mixture could be used to etch MAX phase and acquired F-terminated MXene in the absence of water [33]. Ti3 C2 TZ prepared in propylene carbonate made an electrode which showed better performance than water-etched MXene. These techniques can be used in some water-sensitive applications like storage of energy, polymer composites, and supports for quantum dots.

3.3.4 Treatment with alkali Mostly, MAX phase have acidic atoms which cannot be etched by HF-related methods as these methods are only applicable for MAX phase having amphoteric and alkaline elements. Li et al. prepared fluorine-free methods by hydrothermal process in which NaOH is used as a etchant to synthesize Ti3 AlC2 Ti3 C2 Tx (where T is OH or O) from Ti3 AlC2 [30]. This concluded that at 270°C in a 27.5 M NaOH solution, Ti3 AlC2 can produce OH- and O-terminated multilayer Ti3 C2 Tx by particular eviction of Al from Ti3 AlC2 (Fig. 3.1C(i, ii)). This result indicates that high concentration ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− FIGURE 3.2 (A) (i) Side and top views of optimized geometry of Ti3 C2 O2 , Ti3 C2 T2 -31/1, Ti3 C2 T2 -16/16, Ti3 C2 T2 -1/31, and Ti3 C2 F2 , (ii) Free energy diagram of ORR intermediates on Pt/v-Tin+1CnT2 (n = 1-3, T= O or F), (iii) Volcano plot for ORR potentials (U) as a function of G OOH∗ on various surfaces, (iv) Schematic electron transfer on different Pt/v-Tin+1CnT2 (n = 1-3, T = O or F) surfaces with ORR intermediates. Reprinted with permission [40]. Copyright 2021, American Chemical Society. (B) (i) Synthetic strategy for FeNC/MXene hybrid nanosheets, SEM images (ii) of the pristine MXene, (iii) FeNC/MXene-1, (iv) CV plots of the pristine MXene, FeNC/MXene-1 and Pt/C catalysts in N2 - or O2 -saturated 0.1 M KOH solution at 50 mVs−1 , (v) LSV curves of catalysts in O2 -saturated 0.1 M KOH solution at 50 mVs−1 with a rotating rate of 1600 rpm. Reprinted with permission [41]. Copyright 2021, Royal Society of Chemistry.

3.4 Applications in electrocatalysis

of NaOH and high reaction temperature can dissolve large amount of aluminum hydroxide (oxide) in NaOH and inhibit the oxidation of Ti elements. This was the first time when basic nature etching has been used for the preparation of MXene. These methods had some drawbacks due to high temperature and pressure hydrothermal conditions. Consequently, Geng et al. used organic alkali etching method to prepare MXene in which etchant was TMAOH. In this experiment, Al was hydrolyzed by TMAOH and formed Ti3 C2 MXene which is terminated by Al(OH)4 − and embedded by TMA+ [34]. TMAOH-etched Ti3 C2 prepared from the above method has variable properties compared to the results obtained by HF and NaOH etching.

3.3.5 Electrochemical etching Chemical etching essentially relies upon the higher chemically dynamic behavior of M-Al bond than M-C bond which incorporates the charge movement from target material to other material [35]. Pang et al. gave a thermal method of etching to produce fluoride-free MXenes like V2 CTx , Cr2 CTX , and Ti2 CTx having carbon black additive and porous substrate of carbon fiber cloth-based 3D composite electrode [36]. The etching efficiency of the MXene was improved by the dilute hydrochloric acid etching agent. Further, Feng et al. revealed the particular electrochemical etching of Al layers from Ti3 AlC2 utilizing an fluid electrolytes comprising 1 M ammonium chloride (NH4 Cl) and 0.2 M TMAOH [37]. This electrochemical etching strategy empowers a high production of single or multilayer Ti3 C2 MXene with an enormous average lateral dimension. The produced Ti3 C2 can be utilized as an electrode for all-solid state supercapacitors that can convey a high areal capacitance of 220 mFcm−2 . This is the most suitable and nontoxic method for the preparation of MXene.

3.3.6 Chemical vapor deposition method Aside from the wet-etching techniques, CVD gives a suitable way to prepare excellent MXene [38]. Ren et al. prepared ultrathin α-Mo2 C 2D-crystal having up to 100 μm lateral size by using CVD method. In this experiment, methane was used as a carbon source and substrate was double metal foils that is, a Cu sheet on a Mo sheet [23]. The MXene single crystal obtained from CVD method has a huge domain size and less density of defects in comparison to wet-etching-grown MXene. Besides this, ultrathin transition metal carbides like WC and TaC can also be synthesized by CVD method. There is still no indication of CVD prepared MXene monolayer. Therefore, further improvements are still essential for the development of MXene monolayer (Table 3.1).

3.4 Applications in electrocatalysis In this section, we sum up current advancement on the uses of MXene-based materials in electrocatalysis, including HER, OER, ORR, NRR, and CO2 RR.

31

32

Synthesis techniques Wet etching

Applicable MAX (formed MXene)

Reagents

Representative SEM/optical images

Properties

Characteristics of technique

HF etching

Approx. Mxenes

HF

(1) Concertina-like structure having more –F, –OH, and, –O terminations (2) tiny flake size with plentiful imperfections

Merits: High selectivity and sufficient yield; Demerits: plentiful imperfections in the products; precarious activity

Altered HF-based acid etching [1]

Ti3 AlC2 (Ti3 C2 ), Ti2 AlC (Ti2 C), Mo2 GaC (Mo2 C)

LiF/NaF/KF +HCl or NH4 HF2

(1) Large spacing between layers with high –O termination (2) Large flake size with less imperfections

Merits: Less imperfections and precarious; high flake size Demerits: High temperature and longer time for etching

(continued on next page)

CHAPTER 3 MXene-based nanomaterials for electrocatalysis

Table 3.1 Various synthesis approaches and the properties of the produced MXene [31,32].

Table 3.1 (continued)

Molten salts etching [24]

Ti4 AlN3 (Ti4 N3 ), Ti2 AlC (Ti2 C)

HF-free etching [6,7]

Ti3 AlC2 (Ti3 C2 )

Reagents LiF + NaF + KF

NaOH/TMAOH/ TMAOH+NH4 Cl

Representative SEM/optical images

Properties

Characteristics of technique

(1) Concertina-like structure with TiO2 phase (2) tiny flake size with multiple voids and imperfections

Merits: Prepare MXene, which is instability in HF solution Demerits: Low crystallinity at high temperature

(1) In case of NaOH etchant: Structure with highly arranged layers especially with –OH, –O terminations, and slight TiO2 phase; tiny and thick flakes

For NaOH etchant: Merits: no –F termination with less precarious Demerits: high conc. and high temperature.

(continued on next page)

3.4 Applications in electrocatalysis

Synthesis techniques

Applicable MAX (formed MXene)

33

34

Synthesis techniques

Applicable MAX (formed MXene)

Reagents

Representative SEM/optical images

Properties (2) In case of TMAOH etchant: Swell Ti3 C2 layers terminated with Al(OH)4 and sandwiching TMA+

Nonetching

CVD [8]

Mo2 C

CH4 and Cu/Mo foils

(1) High lateral sizes of good-quality crystals of MXene, (2) Highly pure and ordered with immensely low imperfections.

Characteristics of technique For TMAOH etchant: Merits: very competent etching Al; easily delaminate; no –F termination; Demerits: Al(OH)4 − termination may impact the potential applications Merits: Limited thickness of MXene crystals, less imperfections; Demerits: Exhausting synthesis protocols, less efficient preparations

CHAPTER 3 MXene-based nanomaterials for electrocatalysis

Table 3.1 (continued)

3.4 Applications in electrocatalysis

3.4.1 Oxygen reduction reaction Although ORR plays a crucial role in fuel cells and rechargeable MABs, its practical application is limited because of its poor kinetics at electrode surfaces [39]. Nevertheless, the development of inexpensive and high ORR performer electrocatalysts is still an important area for the production of energy regeneration and storage technologies. Liu et al. [40] simulated many Pt/v-Tin+1 Cn Tx heterostructure using DFT calculations. As shown in Fig. 3.2A, the ORR performance of F-terminal MXene is expected to be better than that of O-terminal MXene. However, due to the weaker chemical bond of the F terminal group of MXene, its stability may be lower. Various MXene-based materials have been studied to improve ORR function in a similar way. Yangyang Wen et al. [41] prepared a new hybrid FeNC/MXene nanosheet for the first time, which was studied by the pyrolysis of iron-ligand complex and MXene nanosheet (Fig. 3.2B[i]). The structural and morphological characteristics (Fig. 3.2B[ii, iii]) showed that a thin and permanent FeNC coating firmly adheres to the surface of MXen and forms a hybrid with excellent conductive substrate and many electrocatalytic active sites in the substrate. The electrochemical measurements showed that FeNC/MXene hybrid nanosheets exhibited excellent electrocatalytic performance, and their half-wave potential was 25 mV higher than Pt/C analogs (0.814 V vs. RHE). More importantly, after continuous testing for 20,000 seconds, this hybrid has excellent durability at only 2.6% decay, which is much better than the 115.8% degradation in performance of Pt/C (Fig. 3.2B[iv,v]). This work not only proves the promising prospects of FeNC/MXene hybrid nanoplates for ORR, but most importantly, it provides new insights for the rational design of base metal catalysts using MXene scaffolds. For example, a FePc/Ti3 C2 Tx hybrid was fabricated from the dimethylformamide solution through a simple self-assembly process [42]. Due to the presence of Ti3 C2 Tx , a series of characterization techniques confirmed the obvious delocalization of Fe3d electrons and the spin state transition of Fe(II) ions. This catalyst possessed less local electron density and high spin Fe(II) center, which helps to adsorb and reduce oxygen at the active site of FeN4 . Further, the optimized hybrid shows a low halfwave potential (−0.886 vs. RHE). And the ORR activity of the catalyst was also twice and five times higher than that of pure FePc and commercially available Pt/C. Zhang et al. [43] reported a novel Co/N-CNT@Ti3 C2 Tx hybrid prepared using an in situ advanced approach. The obtained catalyst exhibited excellent catalytic ORR performance, possessing weak onset potential (0.936 V vs. RHE), and the half-wave potential (0.815 V). These properties were credited to the sturdy interfacial interaction and transfer of electron in the compound.

3.4.2 Oxygen evolution reaction OER is the key reaction that occurs in electrochemical water splitting and MABs; however, its slow kinetics with high overpotential makes it necessary to find highly efficient catalysts. The noble metal oxides like RuO2 and IrO2 are highly efficient electrocatalysts for OER, despite that, their low abundance and high cost has hindered

35

(B)

CHAPTER 3 MXene-based nanomaterials for electrocatalysis

(A)

36

3.4 Applications in electrocatalysis

their practical application. MXene-based materials have been recognized as one of the highly OER active catalysts. For example, in the presence of Ti3 C2 Tx and urea, layered nanohybrid FeNi-LDH/Ti3 C2 Tx was prepared by coprecipitation of Ni2+ and Fe3+ ions. This material indicates better OER activity in terms of lower overpotential of 298 mV at 10 mA cm−2 and low Tafel slope value of 43 mV dec−1 . This performance can be explained by the obvious charge transfer between Ti3 C2 Tx and FeNi-LDH for strong phase interaction and electronic interaction. This interaction and adhesion improves the conductivity and stability, and also apparently promotes the FeNi-LDH redox process [44]. For instance, Zhao et al. [47] prepared the MXene/MOF hybrid material (Ti3 C2 Tx CoBDC) using a method promoted by mutual diffusion reaction (Fig. 3.3A[i–iv]). This hybrid material showed wonderful OER activity, displaying a low overvoltage of 410 mV @ 10 mA cm−2 with 48.2 mV dec−1 Tafel slope value, which can be credited to the well-marked interaction between the CoBDC layer and the Ti3 C2 Tx nanosheets, which enables fast charge/ions migration. The presence of Ti3 C2 Tx metal nanosheets not only inhibits the aggregation of the porous CoBDC layer, but also enhances charge/ions migration. In another study, CoNi-ZIF-67 @ Ti3 C2 Tx was fabricated using a simple coprecipitation reaction [48]. Due to the presence of Ti3 C2 Tx , the CoNi-ZIF-67 particles become smaller and the moderate oxidation of Co/Ni elements increases, so that the catalyst has excellent OER performance by means of low over potential (275 mV) and Tafel slope value (65.1 mV dec−1 ) (Fig. 3.3A[v, vi]). Further, a flexible film was prepared by self-assembly of graphite carbonitrides (g-C3 N4 ) and titanium carbide (Ti3 C2 ) layers (Fig. 3.3B [i–iv]). The prepared hierarchically porous films showed high OER activity, displaying a low overvoltage of 420 mV @ 10 mA cm−2 with 420 mV dec−1 Tafel slope value [45]. The obtained good OER activity of this material was attributed to the Ti-Nx catalyst pattern that acts as an electroactive site (Fig. 3.3B[v–vii]). Tan et al. [46] fabricated a hybrid material, SNiFe2 O4 @ Ti3 C2 @ NF, and found excellent electrocatalytic OER activity, indicating a lower 270 mA overvoltage and a slight Tafel slope of 46.8 mV dec−1 at a current density of 20 mA cm−2 . Delightfully, metal organic frameworks (MOFs) and their

←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− FIGURE 3.3 (A) (i) Preparation strategy for Ti3 C2 Tx-CoBDC hybrid for OER, (ii) SEM image of an accordion-like Ti3 C2 Tx structure, (iii) HRTEM image of a typical Ti3 C2 Tx nanosheets, (iv) Side-view of TEM image on the edge of a Ti3 C2 Tx nanosheets, (v) Scheme for a rechargeable ZABs, (vi) Charge and discharge polarization curves of the rechargeable ZABs based on the Ti3 C2 Tx−CoBDC+Pt−C and the IrO2 +Pt−C configurations, (vii) Charge and discharge cycling curves of the rechargeable ZABs at a current density of 0.8 mA cm−2 . Reprinted with permission [47]. Copyright 2021, American Chemical Society. (B) (i) Fabrication of the porous TCCN film, (ii) SEM, (iii) EDS elemental mapping, (iv) HRTEM images, (v) Polarization curves, (vi) Tafel plots, (vii) Chronoamperometric response at a constant potential of 1.65 V (E j=10 of TCCN) [45]. Copyright 2021, Wiley-VCH.

37

(H) (G) (F)

(C)

(D)

(E)

(B)

CHAPTER 3 MXene-based nanomaterials for electrocatalysis

(A)

38

3.4 Applications in electrocatalysis

derivatives have also been strongly hybridized with MXene nanosheets to obtain better OER functionality.

3.4.3 Hydrogen evolution reaction Hydrogen being green energy, may be considered as one of the most potential resources to address energy problems across the globe and its production via electrochemical water splitting is a possible sustainable strategy. Although Pt-based materials are outstanding as far as HER is concerned, their low abundance and high cost limit large scale industrial application [49]. The fabrication of MXene-based nanomaterials as HER electrocatalysts has created huge attraction because of their fascinating mechanical and electronic properties. To assess the possibility of use of MXene-based catalysts in electrocatalytic HER, several experiments along with their theoretical computation have also been performed. For instance, Seh et al. [50] carried out theoretical computation as well as experimental assessment on MXene-based catalysts for electrocatalytic HER for the first time. The Mo2 CTx :Co MXene was prepared, having cobalt substituted on molybdenum site and observed tremendous HER performance [31] (Fig. 3.1D[i–ii]). Theoretical calculations indicated the promising nature of Mo2 CTx as HER electrocatalyst. Further, Gao et al. [51] performed DFT calculations on O-terminated 2D MXenes like Ti2 C, V2 C, Nb2 C, Ti3 C2 , and Nb4 C3 and shown to have different Gibbs free energies for Hads on active site (GH∗ ) under different analyses, Ti3 C2 O2 in particular displayed the lowest GH∗ (Fig. 3.4A). Cheng et al. [52,53] has reported to carry out DFT calculations on Cr2 CO2 MXene and on the basis of calculated GH∗ at different hydrogen coverages indicated that the modification of surface functionalities of MXene can boost their HER performance. Experimental results suggested Mo2 CTx to require 189 mV overpotential to reach 10 mA cm−2 current density (Fig. 3.4B), justifying that Mo2 CTx is superior to Ti2 CTx catalyst. Moreover, DFT studies have shown that the base surface of Mo2 CTx can serve as the active sites for HER, which is significantly differs from the widely studied mechanism of the 2H-MoS2 phase. Jiang et al. [52] reported O-functionalized ultrathin Ti3 C2 Tx as HER electrocatalyst indicates 190 mV overpotential at 10 mA cm−2 ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− FIGURE 3.4 (A) Gibbs free energy plots for HER with Ti2 CO2 , V2 CO2 , Nb2 CO2 , Ti3 C2 O2 , and Nb4 C3 O2 , (B) Polarization curves Ti3 C2 Tx (F:Ti = 0.28), Mo2 Ti2 C3 Tx (F:Mo = 0.05), Mo2 TiC2 Tx (F:Mo = 0.04), Mo2 CTx (F:Mo = 0.02), and 20% Pt/C, (C) Preparations of N-Ti2 CTx nanosheets, (D) Polarization curves of Ti2 CTx with different nitridation degrees, pristine-Ti2 CTx, TiN, and Pt/C. Reproduced with permission [55]. Copyright 2018, Royal Society of Chemistry. (E) Gibbs free energy plots for HER on various MXene nanoribbons. (F) Free energy of the Tafel reaction for HER on d the edges of Ti3 C2 , (G) (Ti, Nb)C MXene nanoribbons, respectively. Reproduced with permission [55]. Copyright 2018, Royal Society of Chemistry. (H) Polarization curves of Ti3 C2 flakes, Ti3 C2 nanofibers, and Pt/C. Reproduced with permission [37]. Copyright 2018, American Chemical Society.

39

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CHAPTER 3 MXene-based nanomaterials for electrocatalysis

current density, which was better than that of untreated Ti3 C2 Tx . In a similar way, a lavish F-terminated Ti2 CTx material was synthesized and found that this material was shown to have excellent HER performance, exhibiting 170 mV overpotential at a current density of 10 mA cm−2 [54]. Yun et al. [55] prepared N-Ti2CTx hybrid MXene by nitridation of 2D Ti2CTx at high temperature using NaNH2 (Fig. 3.4C). As shown in Fig. 3.4D, when the current density is 10 mA cm−2 , the obtained N-Ti2CTx exhibits higher HER catalytic performance with an overvoltage of 215 mV, which is more than three times that of the original Ti2CTx (645 mV). Further, the impact of nanostructures on HER catalytic activity of MXenes was by Yang et al. [37]. They constructed 12 types of MXenes nanoribbon models and evaluated the role of MXenes nanoribbon edges in HER catalysis. Ti3C2 nanofibers and (Ti,Nb)C solid solutions showed an excellent HER performance, displaying low free absorption energy (close to 0 eV) and small Tafel barrier, respectively, lower than 42 and 0.17 eV (Fig. 3.4E–G). Yuan et al. have reported Ti3C2Tx MXene nanofibers [56]. The resulting nanofibers showed enhanced HER activity, with a slight overvoltage of 169 mV at 10 mA cm−2 (Fig. 3.4H), and a Tafel slope of 97 mV dec−1 .

3.4.4 CO2 reduction reaction Enormous scope anthropogenic carbon dioxide emissions are causing genuine ecological issues, such as global warming and species extinction. The electrocatalytic conversion of CO2 into valuable chemicals and energy rich molecules fuels has pulled a lot of research interest due to the ecofriendly nature of this strategy [57]. The DFT calculations were applied to examine the electrocatalytic CO2 RR activity of MXenesbased materials. For instance, Chen et al. [58] conducted calculations via DFT to study the CO2 RR activity of various MXenes, having –OH terminal groups and found that among various MXenes, Sc2 C(OH)2 was shown to have superior activity with a limiting potential of –0.53V. This extraordinary performance can be credited to the high reactivity of the H atom in the terminal –OH attached MXene, forming a stable intermediate which reduced the required overvoltage (Fig. 3.5A–E). It is reported that the IV-VI series of MXenes exhibited excellent CO2 RR performance, where Cr3 C2 and Mo3 C2 MXenes were considered as highly active electrocatalysts to convert CO2 into CH4 specifically [59]. The author considered favorable candidates for the selective conversion of CO2 to CH4 . Further, it was suggested that the production of OCHO∗ and HOCO∗ radicals take place simultaneously during the hydrogenation step as rate determining step of conversion of CO2 to CH4 . According to the calculation results of the minimum path energy, the conversion process CO2 →CH4 on pure Cr3 C2 and Mo3 C2 requires 1.05 and 1.31 eV overvoltage. Nevertheless, functional groups like –O or –OH on MXenes (Mo3 C2 ) require very little energy consumption. Further, Handoko et al. [60] noticed that W2 CO2 and Ti2 CO2 as M2 XO2 -MXene are expected to be potential materials for CO2 RR due to their low potential and high selectivity, followed by ∗ HCOOH

0.0

0.80 eV *CO+H O(g) 0.63 eV 2 *(H)CHO 0.27 eV CO2(g) slab slab-H *(H)COOH

slab-2H

–0.5 –1.0

one H in slab goes away as a part of C[H]H3(g)

–1.5 0

1

(B)

2

0.4

*(H)H

0.2

C O

0.0

–0.2

slab

H 2(g) slab -0.01 eV

0 1 2 Protone-electorn transferred

H

0.0 0.4

0.0

–0.4 0

6

7

8

4

6

8

10

12

14

16

18

0.4 +

HCOOH slab 0.80 eV CO2(g) CO2(g)+H2(g) slab

2

Reaction coordinate (Å)

+

0.8 0.4

0.2

slab-2H

(C) Sc

slab-H 0.34 eV

0.4

–0.2

-0.86 eV *(H2)O+C[H]H3(g) -1.02 eV H2O(g) -0.92 eV -0.30 eV slab-3H slab *(H)OH slab-H

3 4 5 Protone-electorn transferred

+

(E)

-0.34 eV slab *OCHO -0.01 eV slab 0 1 2 Protone-electorn transferred

(F) 0.0

–0.4

–0.8

0

2

4

6

8

10

12

14

Reaction coordinate (Å)

FIGURE 3.5 Gibbs free energy plots for (A) CH4 , (B) H2 and (C) HCOOH and H2 (via ∗ OCHO rout) on Sc2 C(OH)2 . Deformation charge density of (D) CO2 , ∗ (H)COOH, and ∗ (H)CHO. Energy barriers for (E) ∗ (H)COOH → ∗ CO, (E) ∗ CO → ∗ (H)CHO. In (D), ∗ (H)COOH → ∗ CO is the potential-limiting step. In (F), no transition state like ∗ CHO. Reproduced with permission [58]. Copyright 2021, American Chemical Society.

3.4 Applications in electrocatalysis

Reaction free energy (eV)

-0.18 eV *(H)OC[H]H2

slab-H slab

(D)

+

Energy (eV)

0.5

+

Energy (eV)

1.0

+

Reaction free energy (eV)

Reaction free energy (eV)

(A)

41

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CHAPTER 3 MXene-based nanomaterials for electrocatalysis

formation pathway which is more energy-favorable as compared to ∗ CO formation route due to extra stability of HCOOH intermediate via O-termination of MXene. However, no experimental data is reported to support these theoretical hypotheses.

3.5 Summary and outlook As a newly 2D material, MXenes has great potential toward the electrocatalytic O2 , H2, and CO2 involving reactions for the development of renewable energy systems. Many high performance MXene-based catalysts have been produced with different inherent properties like exceptional metallic conductivity, high surface chemistry, and exclusive morphology. In this chapter, we have described various strategies to enhance the electrocatalytic activity of MXene-based electrocatalysts toward ORR, OER, HER, and CO2 RR. The terminal functional groups like –O, –OH, and –F and metal sites like Ti, V, Mo, and Nb can act as sites of catalytic activity, as has been confirmed by theoretical and experimental investigation. First, adjusting the chemical composition of the surface of MXene is a favorable approach to enhance the electrocatalytic performance of MXenes-based materials. Second, the fabrication of nanocomposite with other active ingredients like nanoparticles, monoatomic and different 2D materials is an additional effective approach to enhance the electrocatalytic performance of MXene-based materials. The functional groups present on the surface of MXenes, make MXenes to form easy and strong interactions with other active ingredients. Many metal MXenes have been reported with improved carrier transport behavior and their 2D structure prevents these materials to aggregate. Although the use of MXene-based electrocatalysts has achieved initial success, there are still many problems to be solved. For example, theoretical calculations and experimental methods must be used to predict and synthesize the newer MXene. The ORR, OER, HER, and CO2 RR electrocatalytic functioning of MXene-based materials is still largely theoretical, so experimental studies are needed to verify theoretical calculations. In addition, great endeavors have been made to form MXene catalysts based on electrocatalysis, but facts have proved that it is difficult to clarify the corresponding catalytic mechanism. Therefore, more advanced research (such as in situ microscopy and spectroscopy) and theoretical approaches are needed to encourage the justifiable composition of MXene-based catalysts.

References [1] A. Kumar, V.K. Vashistha, D.K. Das, Recent development on metal phthalocyanines based materials for energy conversion and storage applications, Coord Chem Rev (2020) 213678. [2] Y. Zhong, et al., Rational design of copper-based electrocatalysts and electrochemical systems for CO2 reduction: from active sites engineering to mass transfer dynamics, Mater Today Phys (2021) 100354.

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PART

Nanomaterials for Electrocatalytic reactions such as ORR, OER and HER

2

CHAPTER

Transition metal nanoparticles as electrocatalysts for ORR, OER, and HER

4

Dinh Chuong Nguyen a, Thi Luu Luyen Doan a, Duy Thanh Tran a, Nam Hoon Kim a and Joong Hee Lee a,b a

Department of Nano Convergence Engineering, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea, b Carbon Composite Research Center, Department of Polymer-Nano Science and Technology, Jeonbuk National University, Jeonju, Jeonbuk, Republic of Korea

4.1 Introduction Recently, society has faced two formidable challenges, of climate change and the global energy crisis, which are both associated with natural resource usage. The emission of pollutant gases, such as CO2 , CO, NOx , SO2 , and NH3 , due to the combustion of coal, petroleum, natural gas, etc., generates serious consequences for the environment, causing global warming and leading to natural disaster, while also negatively affecting health, contributing to various medical diseases [1,2]. Additionally, the exhaustion of natural resources owing to massive mining and the growth in energy demand may result in a global energy crisis [3,4]. Consequently, the exploration and development of alternative energy sources to meet the requirements of cleanness, renewability, sustainability, and cost-effectiveness has been considered one of the best solutions to overcome natural resource-related drawbacks. Alternative energy sources, such as wind, thermal, solar, and hydrogen energy, can be converted and stored via energy conversion and storage technologies, such as the metal–air battery, water splitting technology, and fuel cell, which are emerging as promising candidates for large-scale energy applications to reduce the dependence on fossil fuel usage [5]. Regarding the principal mechanisms of the above technologies, some reversible processes that occur during their operation are the oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER). In this context, ORR and OER are two half-reactions of metal–air batteries, HER and OER are two half-reactions of water splitting systems, and ORR is a cathodic reaction in fuel cells [6,7]. All of the ORR, HER, and OER reactions have been demonstrated to occur through multistep electron transfer processes, which cause energy accumulation at each step. Such energy accumulation makes their kinetics sluggish, resulting in reduced actual energy efficiency of the related energy systems. Therefore, diverse electrocatalysts for ORR, HER, and OER have been studied intensively in recent years to lower the Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00010-1 Copyright © 2022 Elsevier Inc. All rights reserved.

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onset potential/overpotential to promote the overall energy conversion performance of such energy systems. Although numerous research efforts have developed different catalyst categories, and employed them for catalyzing ORR, HER, and OER, the expected performances of those catalysts have not been achieved [8]. To address this, nanotechnology has provided a new perspective for material fabrication for the catalysis fields. Since nanotechnology has been developed, nanoelectrocatalysts for ORR, HER, and OER have been prepared, and have shown desirable catalytic activity and durability. Initially, electrocatalysts based on nano noble metals, such as Pt, Rh for HER and ORR, or noble metal oxides, such as RuO2 /IrO2 for OER, demonstrate excellent catalytic activities in all pH conditions [9,10]. Unfortunately, the disadvantages of noble metal-based electrocatalysts, such as their high cost, limited resources, insufficient stability, and low conductivity, have hindered their large-scale application. Earth-abundant transition metal (TM)-based electrocatalysts overcome such drawbacks, and show desirable catalytic performance in ORR, HER, and OER catalysis [11]. Various nano TM-based electrocatalyst categories have been synthesized, and have attracted considerable interest in the electrochemical reaction, owing to their exciting physicochemical properties, as well as catalytic behavior, as compared to their bulk counterparts [12]. Accordingly, much research effort has been devoted to the development of ORR, HER, and OER catalysts based on TM, such as TM oxides, TM phosphides, TM sulfides, TM nitrides, TM carbides, and others. The large surface area, high surface-to-volume ratio, capacity of anchorage/dispersal on high-conductivity substrates, and easy functionalization of the surface via simple approaches make the catalytic activity of TM nanoparticle-based catalysts better than that of other catalyst categories [13,14]. The earth-abundant TM nanoparticle-based catalysts can be classified according to their structural characteristics into three main categories, of substratefree TM nanoparticle-based catalysts; TM nanoparticle-based catalysts supported on carbon-based substrates (graphene [Gr], carbon nanotubes [CNTs], etc.); and TM nanoparticle-based catalysts supported on metallic substrates (metallic threedimensional [3D] foams, metallic two-dimensional layers/sheets, and metallic onedimensional rod/wires), as represented in Fig. 4.1. Inspired by the excellent advantages of TM nanoparticle-based catalysts for electrochemical reactions, this chapter summarizes important aspects of this field. From this perspective, we will describe effective synthesis methods of TM nanoparticlebased catalysts, and then their structural and physicochemical properties, and potential applications for ORR, HER, and OER.

4.2 Synthesis methods of the TM nanoparticle-based catalysts 4.2.1 Hydrothermal method The hydrothermal method has great advantages for fabricating high-quality large crystals, as well as controlling the composition of catalyst products [15,16]. It is one

4.2 Synthesis methods of the TM nanoparticle-based catalysts

FIGURE 4.1 Different types of TM nanoparticle-based catalysts for their applications toward ORR, HER, and OER.

of the most popular methods for synthesizing TM nanoparticle-based catalysts so far. Yang et al. reported the successful fabrication of metal phosphide (Co2 P) nanoparticles (NPs) anchored on N-doped carbon originated from spirulina (Co2 P/NC) via an effective hydrothermal reaction [17]. In this work, an important precursor containing Co metal NPs and carbonaceous substrate was obtained initially by the hydrothermal treatment of a mixture originated from dried spirulina power and CoCl2 .6H2 O at 180°C for 10 h. The authors demonstrated that the hydrothermal condition was beneficial to the transformation of spirulina into carbonaceous material and the adsorption of metal ions. Following that, the Co2 P/NC was fabricated by annealing the precursor mixture at a reasonable temperature. Recently, Shen et al. also reported the utilization of a facile hydrothermal method to prepare FeIr alloy NPs supported by nickel foam (NF) by keeping a mixture solution of IrCl3 .xH2 O, FeCl3 , and ethylene glycol with NF in a Teflon-lined autoclave at 180°C for 16 h [18] (Fig. 4.2). Li et al. successfully fabricated trimetallic oxyphosphide (FeCoMo P–O) NPs through

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CoCI2 Hydrothermal treatment

Spirulina suspension (containing phosphous)

Carbonization treatment

Metal phosphide Metal

Conductive carbon

Biomass-bassed carbon

FIGURE 4.2 Schematic of the hydrothermal process for the fabrication of FeIr NPs/NF [18].

a simple one-step hydrothermal reaction [19]. In a typical procedure, FeCl3 ·6H2 O, CoCl2 ·6H2 O, urea, NaH2 PO4 ·H2 O, and Na2 MoO4 ·2H2 O were dissolved in deionized (DI) water under stirring for 10 min, followed by pouring into a Teflon-lined autoclave for hydrothermal treatment at 180°C for 10 h. The obtained FeCoMo P–O NPs were cleaned with ethanol. The morphology of NPs was demonstrated to strongly depend on the growth environment during the hydrothermal process. Proper control of the size, shape distribution, and crystallinity of NPs has been adopted by varying the certain experimental parameters of the hydrothermal reaction, such as temperature, time, type of solvent, surfactant, and precursor [15]. For example, Yang et al. investigated the influence of hydrothermal conditions, including reactant material, temperature, and reaction time, on the formation of Bi4 Ti3 O12 NPs [20]. He found that the crystal structure of Bi4 Ti3 O12 NPs was dependent on the reactant salts. In addition, by increasing the reaction temperature or time, the size of the Bi4 Ti3 O12 NPs was increased.

4.2.2 Solvothermal method In addition to hydrothermal methods, the solvothermal method is commonly used to synthesize many different types of catalysts. Like hydrothermal synthesis, the solvothermal reactions are carried out in sealed container at a temperature above 100°C; however, the reaction solvent is not water, but rather ethanol, dimethylformamide (DMF), benzyl alcohol, butanol, etc. Bao et al. reported the fabrication of ruthenium–cobalt (Ru1 Co2 ) NPs via a solvothermal reaction using benzyl alcohol as reaction medium [21]. They performed the synthesis in a simple manner by maintaining a mixture solution containing Ru (III) acetylacetonate and Co(II) acetylacetonate as precursor, polyvinylpyrrolidone as surfactant, and benzyl alcohol as solvent, at 170°C for 5 h. Similarly, Li et al. developed MoS2 NPs through a simple solvothermal process using DMF solvent [22]. Interestingly, the authors found that when the procedure was synthesized without the use of Gr substrate, the MoS2 NPs were formed

4.2 Synthesis methods of the TM nanoparticle-based catalysts

FIGURE 4.3 Schematic of the solvothermal process for the fabrication of FeP NPs supported on Gr [26].

as 3D-like particles of different sizes. Whereas with the introduction of Gr, the MoS2 was grown as nanosheets (NSs) uniformly distributed over the Gr surface. Previous studies also evidenced that the change of reaction conditions led to some modification of the synthesized catalyst properties. Fominykh et al. synthesized NiO NPs using nickel(II) acetylacetonate as the reactant, and tert-butanol as the reaction medium [23]. A solvothermal reaction was conducted at 200°C for 12 h. At low reaction temperature and short time, there was no NiO NPs formation. However, when the reaction time was increased, NiO NPs were formed with the increase of nanoparticle size, but without change of the crystalline phase. Other studies also reported the synthesis of TM nanoparticle-based catalysts via the solvothermal approach, such as Fe-doped NiO NPs [24], PdCu3 NPs [25], FeP NPs supported on Gr [26] (Fig. 4.3), Pt–Co bimetallic supported on commercial carbon black [27], and FeP supported on candle soot [28]. Although such hydrothermal and solvothermal methods have been extensively applied, their limitations due to low uniformity, insufficient repeatability, employment of toxic solvent, requirement of good quality seeds, and low control of the growth process were also challenges [15].

4.2.3 Chemical reduction method The chemical reduction method has been recognized as a simple and highly efficient approach for the preparation of TM nanoparticle-based catalysts. Liu et al. reported a facile and effective coreduction method to grow CoFe2 O4 NPs on CNTs substrate [29]. The CoFe2 O4 NPs was synthesized by heating a mixture solution containing

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CHAPTER 4 Transition metal nanoparticles as electrocatalysts

HO

HO

HO

OC

CO OH

HO

O

OH

OC

CO OH

Aniline

CoFe2O4

HO

OC

CO OH

Aniline

O

O

OH-1

2H2O+O2+4e-

OH-1

OH-1

OH-1

OH-1

APS

OH-1 OH-1 OH-1

MWCNTs

4OH-1 Co2+,Fe3+

PANI-MWCNTs

N2H4.H2O CoFe2O4/PANI-MWCNTs FIGURE 4.4 Schematic of the chemical reduction process for the fabrication of CoFe2 O4 NPs supported on CNTs [29].

FeCl3 .6H2 O and CoN2 O6 as metal precursors, hydrazine hydrate as reducing agents, and CNT as substrate at 120°C for 4 h (Fig. 4.4). The product was collected by centrifugation, cleaned with DI water and ethanol, and dried at 50°C for 12 h. Li et al. synthesized FeB2 NPs via a simple chemical reduction process, by using the LiBH4 as a reducing agent for reducing Fe2+ into FeB2 NPs [30]. In another study, Suryanto et al. fabricated a Janus Ni–Fe NPs (Ni–Fe NPs), using an effective chemical reduction at room temperature [31]. In this case, Na(oleate) solution was added to a mixture solution of Ni(NO3 )2 .6H2 O and FeCl2 .4H2 O, followed by the addition of hexane with stirring at 400 rpm for 1 h. A series of Ni ferrite NPs, including Fe3 O4 , Ni0.2 Fe2.8 O4 , Ni0.4 Fe2.6 O4 , Ni0.6 Fe2.4 O4 , and Ni0.8 Fe2.8 O4 , were also synthesized by a reduction method using hydrazine hydrate as reducing agent [32]. The pH environment of this reaction process was governed at pH = (9–10) via NH4 OH solution. The change of molar ratios of metal precursor (Ni2+ : Fe2+ : Fe3+ ) led to the changed composition of products. A simple and low-cost chemical reduction method has been conducted by using liquid ammonia solution as a reducing agent for the synthesis of the NPs precursor, followed by thermal treatment process under CH4 condition to obtain Mo2 C and W2 C NPs [33]. Calderon et al. successfully fabricated Au, Ag, and Pt NPs by a facile chemical reduction synthesis [34]. In this synthesis procedure, metal precursors, including HAuCl4 .xH2 O, H2 PtCl6 .6H2 O, and AgNO3 , chemically reacted with reductants that included NaBH4 and C19 H42 BrN. Furthermore, Lv et al. developed a new method for the co-reduction of Ni(C5 H7 O2 )2 and HAuCl4 .3H2 O with the use of C18 H35 NH2 as a reducing agent to synthesize

4.2 Synthesis methods of the TM nanoparticle-based catalysts

NiAu NPs [35]. Ethylene glycol and formaldehyde were applied as reducing agents for the synthetic procedure that involved two steps of carbon-supported Pt and Pd NPs [36]. In this case, the ethylene glycol was initially added to the mixture of carbon source and H2 PtCl6 or H2 PdCl4 . After the initial reduction reaction was finished, the formaldehyde was added to the solution to obtain particle products. By annealing at 80°C for 12 h, carbon-supported Pt and Pd NPs were formed. Huang et al. reported the successful synthesis of Ni–B NPs supported on Vulcan carbon via a two-step chemical reduction–annealing approach using NaBH4 as reducing agent, and NiCl2 ·6H2 O as metal source [37].

4.2.4 Electrochemical deposition method Electrochemical deposition is a common method for preparing various transition metal NPs on various conductive substrates. Tian et al. reported a facile one-step electrochemical deposition method of fabrication of Ni–Co–S–P NPs grown on carbon cloth (CC) [38]. In this case, the electrodeposition process was conducted in threeelectrode systems using saturated calomel electrode (SCE) reference electrode, Pt foil counter electrode, and cleaned CC as working electrode by cyclic voltammetry (CV) measurement in the potential window of (0.5–1.1) V for 10 cycles. The electrolyte solution contained NiCl2 .6H2 O, CoCl2 .6H2 O, thiourea, and NaH2 PO2 . In another approach, WS2 NPs confined by Gr film (Gr–WS2 ) were designed by potentiodynamic deposition [39]. In this approach, the author conducted the electrodeposition in the potential range (−0.1 to 0) V for 30 cycles in a three-electrode system assembled from Ti plate, Ag/AgCl, and graphite rod as working, reference, and counter electrodes, respectively. The electrolyte solution involved WS4 2− and graphene oxide, in which WS4 2− played the role of precursor for cathodic discharge. Seo et al. deposited TaOx NPs on carbon black using electrochemical deposition technology at a constant potential of −0.5 V (vs. Ag/AgCl) for 10 s, followed by heating under H2 atmosphere [40]. MoSx NPs anchored on Gr was fabricated by a one-step electrodeposition method at room temperature using a three-electrode configuration, with a glassy carbon electrode (GCE) as working electrode, Ag/AgCl reference electrode, and graphite rod counter electrode in the potential range (-1.2 to 0.5) V [41]. P–Co NPs were electrodeposited on the surface of NF at applied potential of -1.6 V (vs. RHE) in a three-electrode cell [42]. Recently, Ni–P NPs were also electrodeposited on the NF by CV technology at the potential range (−0.3 to −0.1) V with a scan rate of 10 mV s−1 for 15 cycles [43]. In another work, Liu et al. reported electrodeposition integrated with pyrolysis technologies in air atmosphere and NH3 atmosphere to fabricate a novel catalytic material containing NiCo-nitride NPs supported on NiCooxides/graphite fibers (Fig. 4.5) [44]. In this case, NiCo–hydroxide precursor was initially electrodeposited on graphite fiber at a constant potential of -1.0 V (vs. SCE) for 8 min, followed by annealing in air to form NiCo–oxides/graphite fibers. The final step was an annealing process under NH3 atmosphere to generate NiCo–nitride NPs/NiCo–oxides/graphite fibers. A series of NbOx , ZrOx , and TaOx NPs were also deposited on carbon black via electrodeposition technology at a constant potential

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Graphite fibers

Electrodeposition

NiCo-hydroxides

NiCo-oxides

Annealing

NiCo-nitrides/NiCo-oxides

Annealing in NH3

NiCo2O4

CoN/Ni3N

FIGURE 4.5 Schematic of the electrodeposition process for the fabrication of NiCo–nitrides NPs supported on NiCo2 O4 nanoflake/graphite fibers [44].

for 10 s, using carbon black as working electrode [45]. Furthermore, electrochemical deposition was used to fabricate other NPs, such as CoOx NPs supported on CNTs [46], Co-doped NiSe2 NPs [47], Au NPs [48,49], and Ag NPs [50].

4.2.5 Other synthetic methods In addition to the above-mentioned methods, TM nanoparticle-based catalysts were also synthesized by other approaches. The sol–gel method is defined as a chemical solution deposition, and has been recognized as a facile technology for the preparation of TM nanoparticle-based catalysts. The disadvantages of this method are that its chemistry process is relatively complex, owing to the formation of different reactivity of intermediates and modifying components, as well as possessing many reaction parameters. Peng et al. reported the synthesis of CoP NPs encapsulated in porous carbon substrate by using the sol–gel method and a consequent pyrolysis–oxidation– phosphorization method [51]. During the sol–gel process, a mixture solution was obtained by mixing alcohol solution of CoCl2 .6H2 O as metal precursor into a solution of urea and citric acid monohydrate. After the solvent was evaporated from that mixture solution, a gel phase was generated. Shen et al. successfully synthesized NiCu NPs shelled in graphitic carbon by a sol–gel process for the fabrication of NiCu NPs combined with an ambient-pressure chemical vapor deposition method for shelling the carbon layer [52]. In this method, a sol phase was obtained by dissolving nickel (II) nitrate hexahydrate and copper (II) nitrate trihydrate into a mixture of Tween, acetic acid, HCl, and ethanol. The gel phase was then obtained after the evaporation of ethanol, followed by calcination at 500°C in air for 5 h to form

4.3 Structure and properties of TM nanoparticle-based catalysts

NiCu alloy NPs. Fe2 N NPs supported on nitrogen-doped reduced graphite oxide [53], Pd NPs supported on reduced graphite oxide [54], and La0.4 Sr0.6 Ni0.5 Fe0.5 O3 NPs [55] were also prepared by effective sol–gel technologies. In addition, the microwaveassisted method has attracted considerable attention for NPs synthesis, and serves as a cost-effective technology, owing to the sufficient production of required heat for the synthesis process within a short time. For example, Zheng et al. developed an effective microwave-assisted method to synthesize WO3 NPs supported on carbon black [56]. In a specific procedure, W powder was dispersed into a mixture solution of H2 O2 , 2-propanol, and water, followed by the addition of carbon powder. After ultrasonication to form uniformly dispersed ink, a microwave-assisted synthetic procedure with 10 s on / 10 s pause for four times was conducted for the ink. In another research effort, Cabello et al. reported the synthesis of anatase–TiO2 NPs via a fast sol–gel method, followed by microwave–hydrothermal technology [57]. In this work, the microwave–hydrothermal process was performed from a colloidal solution involving titanium isopropoxide, polyethylene glycol, and H2 SO4 at 240°C for only 20 s. The authors found that fast nucleation occurred under microwave condition, leading to the formation of a colloidal suspension. The TiO2 NPs were then collected by centrifuge, and washed with water and ethanol several times. Furthermore, Nong et al. reported the designs of IrNix metallic NPs through a polyol method, using 1,2-tetradecandiol as reducing agent, and oleic acid and oleylamine as capping agents [58]. Nong et al. also employed a polyol method to fabricate IrOx core–shell NPs with desirable activity and high durability [59]. On the other hand, Gholamrezaei et al. reported the use of an ultrasonic method for the design of SrMnO3 NPs [60]. In a specific procedure, two reactant solutions were prepared separately, including an oxidation solution of KMnO4 and KOH, and a solution of metal precursor (Sr2+ and Mn2+ ). After metal precursor solution was mixed drop-by-drop into the oxidation solution, the mixture was sonicated for 30 min. Finally, the precipitate was collected and cleaned several times, followed by calcination to form SrMnO3 NPs.

4.3 Structure and properties of TM nanoparticle-based catalysts 4.3.1 Substrate-free TM nanoparticle-based catalysts In recent years, many different types of TM nanoparticle-based catalysts have been developed by varying the synthesis conditions, such as reactant concentration, growth temperature, growth time, reaction agents, and metal precursor. Such catalysts have demonstrated interesting morphological and structural features along with specific physicochemical properties, leading to desirable catalytic behavior. Initially, TMbased catalysts have been designed based on monometallic precursors. Gupta et al. successfully developed Co-Mo-B NPs with pure phase, excellent porosity, and high quality, through a chemical reduction approach (Fig. 4.6A) [61]. The authors demonstrated that the as-synthesized Co-Mo-B NPs had large specific surface area, which was superior to the previously reported boride NPs, and possessed optimal

57

58

(B)

(C)

(D)

(E)

(F)

FIGURE 4.6 TEM images of (A) Co-3Mo-B NPs [61], (B) NiSe2 NPs [62], (C) Nix Se NPs [63], (D and E) FeP NPs [64], and (F) STEMHAADF image of a single Ni–Fe NPs [31].

CHAPTER 4 Transition metal nanoparticles as electrocatalysts

(A)

4.3 Structure and properties of TM nanoparticle-based catalysts

adsorptive ability, thereby showing outstanding catalytic activity and durability for water splitting. Likewise, Liang et al. also reported a novel catalyst for HER based on NiSe2 NPs [62]. The NiSe2 NPs was prepared, and was shown to be well-crystallized with octahedral structure (particle size of around [100–150] nm), as seen in Fig. 4.6B. The surface area of the NiSe2 NPs was relatively higher, which was thus recognized as the main factor contributing to its high catalytic activity. As a result of numerous research efforts, various monometallic nanoparticle catalysts containing TM have also been successfully developed, such as Nix Se NPs (0.5 ≤ x ≤ 1) (Fig. 4.6C) [63], NiO NPs [23], FeP NPs (Figs. 4.6D,E) [64], CoP2 NPs [65], Co3 O4 , CoO, and Co [66]. In order to further improve the catalytic activities of TM nanoparticlebased catalysts, the combination of two or more metals into one nanostructure has emerged as an effective strategy, and has so far been intensively studied in the catalysis field. Synergistic effects between metal components in NPs can regulate electronic structure and catalytic behavior toward optimizing catalytic efficiency. Suryanto et al. developed Ni–Fe NPs catalysts with very small size, thus enlarging surface contact between reactant/electrolyte and NPs (Fig. 4.6F) [31]. Importantly, the catalytic performance of Ni–Fe NPs was higher than those of single Ni NPs and Fe NPs, due to the formation of Ni–O–Fe bridge at the interface of Ni and Fe2 O3 parts, which modified the adsorption ability of Ni–Fe NPs to intermediate H atoms. In another approach, Fichtner et al. reported the designs of Ptx Pr alloy NPs through cathodic corrosion technology [67]. The authors demonstrated that the improved catalytic activity of Ptx Pr alloy NPs was due to the presence of surface defects, which caused the formation of highly active concave surface sites, and a strained structure. Some promising alloy NPs were also reported by other studies, such as Ru1 Coy NPs [22], PdCu3 NPs [25], Nix Fe3-x O4 [32], and FeCoMo P–O NPs [19]. Core–shell structures of TM nanoparticle-based catalysts have also been widely employed in the catalysis field. This is because the core–shell structures not only provide the advantages of random mix alloys structure, they also have the ability to provide more catalytically active sites, and to enhance electronic conductivity [68]. For example, a core–shell NPs comprising Co as a core part and Ir as a shell part, denoted as Co@Ir, was developed by Dongliang Li et al. as an efficient electrocatalyst [69] (Figs. 4.7A–C). It was demonstrated that the Ir shell layer could maximally expose the electrochemical catalytically active sites and hence is intriguing and desirable. Dutta et al. also successfully fabricated a unique core–shell structured Fe3 O4 @Nix P nanoparticle catalyst to boost OER performance [70] (Fig. 4.7D). In this structure, the Fe3 O4 core played an important role in supporting the OER activity of the Nix P shell. While in other studies, various core–shell catalysts were reported, such as Zhang et al. with core–shell NPs of Ni/FePt [71], Mazumder et al. with core/shell Pd/FePt NPs [72], and Gan et al. with dealloyed Ptx Ni1−x core–shell NPs [73]. Furthermore, TM nanoparticle-based catalysts have been constructed with hollow structure, which demonstrated strongly hydrophilic surface, having the Kirkendall effect, and high surface area. Recently, Lv et al. developed a hollow structured nanoparticle catalyst of Ni–Fe diselenide [74]. Wang et al. also proposed a novel nanostructured catalyst of hollow Ru–RuPx –Cox P NPs [75] (Figs. 4.7E–G).

59

60

(B)

(D)

(C)

(E)

(F)

(G)

FIGURE 4.7 (A) TEM image Co@Ir NPs/NC; (B) HR-TRM image of Co@Ir NPs/NG, (C) EDS elemental mapping of Co and Ir in the Co@Ir NPs/NC [69]; (D) TEM image of Fe3 O4 @Nix P NPs [70]; (E and F) TEM images, and (G) EDS mapping images of Ru–Rux P–Cox P hollow NPs [75].

CHAPTER 4 Transition metal nanoparticles as electrocatalysts

(A)

4.3 Structure and properties of TM nanoparticle-based catalysts

4.3.2 Carbon substrate-assisted TM nanoparticle-based catalysts As is known, the ratios of surface and volume of nanoparticle catalysts greatly contribute to their catalytic efficiency. The low surface/volume ratio, meaning poor exposure of active sites, induces decrease in the catalytic activity of the catalysts, and vice versa. With regard to the substrate-free TM nanoparticle-based catalysts, most of the previous studies demonstrated the agglomeration phenomenon of NPs occurring on these catalysts during wet synthetic routes, leading to significant reduction of the surface/volume ratios, and thus lower catalytic performance than that expected [76]. To overcome this drawback, the promising strategy of dispersing/anchoring catalysts on a highly conductive substrate has been critical, and has received much interest in recent years. Give their excellent electrical conductivity, chemical/mechanical stability, tailored surface chemistry, and variety in structure, carbon materials have been emerging as ideal substrates for catalyst support [77]. As a result, much effort has been devoted to designing and constructing TM nanoparticle-based catalysts supported on carbon-based substrates, including zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and three-dimensional (3D) carbon. The use of N-doped carbon hollow spheres, as a 0D carbon material, to support Fe2 O3 NPs for catalyzing the ORR has been reported by Xiao et al. [78] (Figs. 4.8A and B). They discovered that the aggregation phenomenon of Fe2 O3 NPs was well prevented via anchoring on the unique hollow structured N-doped carbon sphere, leading to maximizing the utilization efficiency of the accessible active sites of the Fe2 O3 NPs. Meanwhile, the introduction of Fe2 O3 NPs could boost the catalytic activity of the substrate through improved adsorption ability to intermediates during the catalysis process, as well as the reduced formation energy of the intermediate products. Pan et al. also proposed an effective strategy for improving the catalytic properties of Ni2 P NPs by dispersing Ni2 P NPs on carbon nanospheres [79]. It was found that the good dispersion of Ni2 P NPs over the substrate surface without the aggregation phenomenon favored the exposure of more active sites for the catalysis process. Additionally, the presence of carbon nanosphere backbone significantly created a large surface area for the hybrid, thereby enhanced its catalytic activity. In another case, Lu et al. successfully developed an electrocatalyst of CoSe2 NPs hybridized with N-doped carbon polyhedral framework [80]. In this structure, by encapsulation of CoSe2 NPs into N-doped carbon polyhedra, the disadvantages of CoSe2 NPs, such as easy aggregation and oxidation, were addressed, leading to the enhancement of its activity and stability under catalysis condition. Lu et al. also reported the hybridization of MnO/Co NPs with porous graphitic carbon polyhedral framework [81]. The homogeneous distribution of MnO/Co NPs within this framework provided extra interaction support for the catalyst, thus improving the catalytic behavior and long lifetime. The use of CNTs as 1D carbon substrate to support catalysts has also been recognized as an effective approach to develop high potential nanoparticle catalysts, with remarkable improvement of the utilization efficiency. Liu et al. reported welldispersed CoFe2 O4 NPs on polyaniline-multi-walled CNTs [29] (Figs. 4.8C and D).

61

(D)

(G)

(F)

(J)

(H)

(K)

–

(I)

(C)

FIGURE 4.8 (A) TEM image, and (B) EDS mapping images of Fe2 O3 NPs/N-doped carbon hollow sphere [78]. (C) and (D) TEM images of CoFe2 O4 /CNT [29]. (E) Simulated structure of MnPx @MoPy supported on N,P–Gr, (F) SEM image, and (G) and (H) TEM images of MnPx @MoPy /N,P–Gr [91]. (I) Simulated structure of NiFeP NPs supported on 3D Fe,N-decorated carbon, (J) SEM image, and (K) TEM image of NiFeP/3D Fe,N-decorated carbon [98].

CHAPTER 4 Transition metal nanoparticles as electrocatalysts

(E)

(B)

62

(A)

4.3 Structure and properties of TM nanoparticle-based catalysts

Loading polyaniline on the surface of CNTs was the important factor, which provided more active chambers to grow uniform CoFe2 O4 NPs with average particle size of 6.22 nm, without the formation of cluster. In recent time, Wu et al. introduced Fedoped CoSe2 NPs encapsulated in N-doped bamboo-like CNTs [82]. The SEM image revealed that the pure CoSe2 NPs without CNTs substrate tended to agglomerate to cluster. In contrast, by encapsulation of the bamboo-like CNTs, the aggregation of NPs would be inhibited – in addition, the framework also acted as a conductive network to enhance the charge transfer ability of the hybrid catalyst. The development of catalysts based on hybridization of NPs and CNTs has also been informed by Wu et al. with Fe–S–CoP NPs/CNTs [83], Kazakova et al. with Fe/Co/Ni mixed oxide NPs/multiwalled CNTs [84], Qin et al. with Ni/NiM2 O4 (M = Mn or Fe)/N-doped CNTs [85], and Wang et al. with NiSe2 NPs/carbon nanotube networks [86]. As an alternative to CNTs, carbon fibers (CFs) have also served as a promising candidate for supporting TM nanoparticle-based catalysts. Several studies have recently proposed highly efficient electrocatalysts content of NPs catalysts on CFs substrates, such as peapod-like Co(Sx Se1-x )2 NPs grown on CFs [87], Fe2 O3 NPs wrapped in N-doped corncob-derived CFs [88], FeP and Fe3 O4 NPs attached on N,P-doped microporous CFs [89], and Ni/NiO NPs/CFs hybrid [90]. Currently, 2D-layered Gr is a perfect electrocatalyst support, especially for NPs, owing to its excellent electrical conductivity and charge mobility, extremely large specific surface area, and specific mechanical and electrical natures. In this context, Nguyen et al. successfully tried enhancing the catalytic ability of MoPx @MnPy heteronanoparticles by attaching the MoPx @MnPy onto N,P-codoped Gr [91] (Fig. 4.8E). SEM and TEM images in Figs. 4.8F–H clearly reveal the uniform distribution of NPs over the substrate surface. There is no aggregation of NPs, instead forming small active metal NPs over the N,P-codoped Gr substrate, which was demonstrated to generate remarkable adjustment of surface chemistry, resulting in increasing the number of electroactive sites to accelerate the catalysis process. Chuong et al. also proposed a hybrid of Fe2 O3 NPs encapsulated by MoS2 embedded on Ndoped Gr [92]. In this hybrid, N-doped Gr served as substrate for layered MoS2 to prevent the restacking phenomenon, as well as significantly regulating the electronic structure of MoS2 , creating an effective network. The Fe2 O3 NPs were then grown on this network, leading to their higher electron mobility and enlarged distance of hole diffusion, improving the catalytic property. Inspired by the advantages of Gr, a series of hybrid catalysts have also been explored, such as CoOx NPs/B,N-doped Gr [93], NiSe2 NPs@N-doped Gr [94], MnCo2 O4 /N–Gr [95], FeCoMoS NPs/N–Gr [96], and CoP NPs/reduced graphene oxide [97]. Moreover, the 3D carbon-based materials have also been employed as substrate for the fabrication of nanoparticle catalysts. For example, Ibraheem et al. synthesized a hybrid catalyst of NiFeP NPs on 3D Fe,N-decorated carbon framework [98] (Fig. 4.8I). SEM and TEM images (Figs. 4.8J and K) show the fine distribution of NiFeP NPs on the surface of 3D framework, forming a mixed two-phase material. Such structure well prevented the agglomeration of NiFeP NPs alone, endowing the obtained catalyst with enlarged interfacial contact and high specific surface area

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CHAPTER 4 Transition metal nanoparticles as electrocatalysts

for efficiently catalyzing the OER and ORR. In another effort, Li et al. reported a uniform dispersion of Ni–Ni3 P NPs onto the surface of N, P-doped carbon on 3D Gr framework [99]. This 3D carbon substrate was demonstrated to have excellent conductivity, large surface area, with rough surface and hollow structure, which were beneficial to anchoring active NPs toward enhancing surface contact and transmission of the active NPs catalyst. Some new hybrids based on the combination of NPs with 3D carbon-based substrates have also been suggested by Zhang et al. with carbon dots–Co9 S8 NPs/N, S-codoped carbon matrix [100], Chen et al. with hollow CoO NPs/N, S-codoped porous carbon [101], Yang et al. with CoP2 @3D N-doped porous carbon sheet network [102], Kong et al. with CoSe2 NPs/carbon fiber paper [103], and Huo et al. with Fex Ni2-x P NPs/3D interconnected porous carbon [104].

4.3.3 Metallic substrate-assisted TM nanoparticle-based catalysts In addition to the carbon-based substrates, metallic materials are also selected for supporting TM nanoparticle-based catalysts. The combination of catalysts with metallic substrates is not only an effective route to suppress the aggregation and dissolution, but is also beneficial to the electrical conductivity of the obtained catalysts, owing to the chemistry coupling effect between components. Previous studies proved that metallic materials in the form of 1D nanorod- or nanowire-like structure are interesting candidates for supporting catalysts. This can be attributed to the excellent charge transfer ability of this 1D structured material [105]. Liu et al. showed that the hybridization of Pd NPs with Mo2 C nanotubes improved the reaction kinetics, activity, and stability of the developed catalyst [106]. TEM images in Figs. 4.9A and B clearly indicate the formation of Pd NPs on the surface of Mo2 C nanotube. The EDS mapping in Fig. 4.9C includes C, Mo, and Pd signals. The lattice fringe of 0.226 nm corresponding to the Pd phase was also revealed in the HR-TEM image (Fig. 4.9D). Kuma et al. demonstrated remarkable catalytic performance improvement by the integration of Pt NPs and TiO2 nanorods [107]. In other works, such as Co3 O4 NPs embedded on MnO2 nanorods [108], Ag NPs supported on MnO2 nanorods [109], and Ag NPs grown on Ag2 WO4 nanorods [110], significant enhancement of catalytic activities could also be achieved by hybridizing the catalyst and 1D metallic materials. In a similar approach, improved efficiency during catalyzing the electrochemical reactions was obtained by growing the TM NPs on highly conductive 2D metallic substrates. Li et al. synthesized a hybrid catalyst based on CoS2 NPs grown on CuS NSs through an electrodeposition–hydrothermal synthesis route [111]. The unique morphology of this material was proved by SEM and TEM analyses (Figs. 4.9E– G), which showed that the surface of CuS NSs was decorated with uniform CoS2 NPs. The existence of Co, Cu, and S signals in EDS mapping images demonstrated the successful formation of CoS2 NPs/CuS material (Fig. 4.9H). The large interfacial contact area and synergistic effects of CoS2 and CuS were the main contributors to improving the electrocatalytic behavior. Chala et al. fabricated an advanced bifunctional electrocatalyst comprising Ag NPs anchored on NiRu-layered doubled hydroxide

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

(J)

(K)

(L)

FIGURE 4.9 (A and B) TEM images, (C) EDS mapping images, and (D) HR–TEM image of Pd/Mo2 C catalyst [106]. (E and F) SEM images, (G) TEM image, and (H) EDS mapping images of CoS2 /CuS catalyst [111]. (I–K) SEM images, and (L) FFT pattern of FeNi–P/NF [117].

4.3 Structure and properties of TM nanoparticle-based catalysts

(A)

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CHAPTER 4 Transition metal nanoparticles as electrocatalysts

(LDH) NSs, and further compared the catalytic performance of the proposed Ag NPs/NiRu LDH catalyst, pure NiRu LDH, and Ag NPs alone; the experimental and theoretical results displayed the superiority of the Ag NPs/NiRu LDH over the others [112]. The use of 2D metallic materials to assist the synthesis of TM nanoparticlebased catalysts was successfully reported for the development of Co3 Mo alloy NPs vertically aligned molybdenum oxide NSs [113], CoP NPs grown on WSe2 NSs [114], Co NPs combined with VN NSs [115], and Pt NPs dispersed on TiS2 NSs [116]. Very recently, a large number of highly active electrocatalysts based on TMs were also obtained by growing their corresponding NPs on 3D metallic substrates. A unique 3D nanostructured catalyst based on the growth of Fe–Ni–P NPs onto the surface of 3D NF was proposed by Zhang et al. [117] (Figs. 4.9I–L). Benefiting from the 3D structure of NF facilitating electrolyte diffusion/penetration, interconnection of the NPs with each other for reducing the diffusion distance and abundant exposure of active surface sites, and direct attachment of NPs on the NF surface for promoting electron/charge transfer, the obtained catalyst showed extraordinary catalytic activity. Shen et al. reported FeIr alloy NPs supported on NF, which showed favorable catalytic activity due to the synergistic effect between the merits of its components [18]. In this regard, other catalysts, such as P–Co NPs/NF [42], FeIr NPs/NF [18], and Co/Co2 P NPs/NF [118], have also been developed, and exhibit interesting catalytic activities.

4.4 Applications of TM nanoparticle-based catalysts toward the ORR, HER, and OER 4.4.1 ORR applications The ORR is one of the important electrochemical reactions in the energy conversion and storage technologies. The promotion of ORR performance through using efficient electrocatalysts is necessary to achieve the expected performance of related energy systems. Numerous research efforts have focused on designing and constructing catalysts by using TM NPs for the ORR catalysis. Jiang et al. evidenced the high catalytic performance of FePd/Pd NPs toward the ORR catalysis with half-wave potential of 0.88 V and mass activity of 99.7 mA mg−1 Pd (0.9 V vs. RHE), which were superior to the values of the commercial Pt/C catalyst [119]. Zhang et al. demonstrated a simple strategy to develop Ni/FePt core/shell NPs for catalyzing the ORR with high specific activity and mass activity of 1.95 mA/cm2 and 490 mA/mgPt at 0.9 V (vs. RHE), respectively [71]. The electrochemical measurement results also revealed the excellent half-wave potential of Ni/FePt core/shell NPs, which is around 0.913 V (vs. RHE). Chuong et al. reported a novel electrocatalyst through ultrasmall Fe2 O3 shelled by layered MoS2 directly grown on N-doped Gr substrate [92] (Fig. 4.10). The developed catalyst exhibited high activity for the ORR in alkaline condition in the order of N-doped Gr < MoS2 /N-doped Gr < Fe2 O3 @MoS2 /N-doped Gr, with an onset potential of 0.9476 V (vs. RHE), half-wave potential of 0.8546 V (vs. RHE),

4.4 Applications of TM nanoparticle-based catalysts toward

(A) Nanoparticle growth

α-Fe 2O

3 @M

α-fe2O3

MoS2 flakes

Graphene oxide

(B)

oS2/N

GNS

(C)

NGNS

MoS2

α-Fe2O3 NPs

α-Fe2O3 NP

100 nm

(D) 0

(E)

J (mA cm−2)

−2 −4 −6 Commercial Pt/C

−8 0.4

α-Fe2O3@MoS2/NGNS

0.6 0.8 E/V (vs RHE)

1.0

Current percentage (%)

200 nm

90 Retention −96.1%

60

Retention −81.4%

30 Pt/C

0

α-Fe2O3@MoS2/NGNS

0

10000 20000 Times (s)

30000

FIGURE 4.10 (A) Schematic of the formation of α-Fe2 O3 @MoS2 /N-doped Gr. (B) SEM, and (C) TEM images of α-Fe2 O3 @MoS2 /N-doped Gr. (D) Polarization curves of α-Fe2 O3 @MoS2 /N-doped Gr and commercial Pt/C catalyst. (E) Electrochemical stability for α-Fe2 O3 @MoS2 /N-doped Gr and Pt/C [92].

and a current retention of 96.1 % after long-term operation. In another study, Xiao et al. reported Fe2 O3 NPs embedded on N-doped carbon hollow microsphere to boost the ORR performance [78]. This catalyst showed remarkable activity for the ORR, consistent with four-electron transfer pathway and optimal onset potential and half-wave potential; in particular, the current density of the catalysts was 18.5% better than for the Pt/C. Wu et al. grew Co1.67 Te2 NPs on carbon black as ORR catalyst that

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CHAPTER 4 Transition metal nanoparticles as electrocatalysts

Table 4.1 Development of TM nanoparticle-based catalysts for the ORR.

ORR catalysts Porous Pt NPs Pd/FePt NPs MnO/Co NPs/PGC Ni/NiMn2 O4 /N-CNTs Ni/NiFe2 O4 NPs/N-CNTs Co3 O4 NPs/ MnO2 NRs Ag NPs/Ag2 WO4 NRs Co9 S8 NPs/N,S-doped carbon matrix N-CoCuMnOx NPs Nix Se NPs

Onset potential (V vs. RHE)

Half-wave potential (V vs. RHE)

Tafel slope (mV dec−1 )

Reference

0.85 − 0.95 0.084 0.038 −0.10 0.89 −

71 0.7 0.78 0.71 0.59 −0.19 0.66 0.84

− − 69 89 51 − 67.3 76.2

[121] [72] [81] [85] [85] [108] [110] [100]

0.92 −0.17

− −

67.3 49.3

[122] [109]

NRs, Nanorods; PGC, porous graphitic carbon.

outperformed the state-of-the-art Pt/C catalyst [120]. Table 4.1 lists the other ORR catalysts based on transition metal NPs, along with some important indications of onset potential, half-wave potential, and Tafel slope.

4.4.2 HER applications The development of electrocatalysts toward HER for green hydrogen (H2 ) production is very important for the clean energy industry in the near future. Efficient electrocatalysts can accelerate the reaction kinetics of the HER, promoting the evolution of H2 gas during the HER process, and thus enhancing the efficiency of green H2 production. The fabrication of HER electrocatysts based on the transition metal NPs can meet the requirements of cost effectiveness, high catalytic activity, and good stability. Suryanto et al. introduced an effective electrocatalyst for the HER based on Ni– Fe NPs prepared by a facile thermal reaction appoach [31] (Figs. 4.11A and B). The oustanding electrocatalytic activity of the catalyst with very low overpotential of 100 mV to gain current density of 10 mA cm−2 and small Tafel slope of 58 mV dec−1 was attributed to the formation of Ni–O–Fe bridge, which facilitated the electron/charge transfer (Figs. 4.11C and D). Very recently, V-doped CoP NPs were sucessfully synthesized, and used as catalysts for the HER [123]. The V-doped CoP NPs catalysts displayed high HER performance with an overpotential of 235 mV at 10 mA cm−2 , very high double-layer capacitance of 60.5 mF cm−2 , and the ability to evolve H2 gas over 10 h continuous catalysis. Li et al. prepared active catalyst containing Ni–Ni3 P NPs supported on N,P-doped 3D Gr [99]. When used as cathodic electrode, the catalyst showed superior catalytic performance with low overpotential of 113 mV at 20 mA cm−2 . In another case, Wu et al. proposed Co/Fe– P NPs supported on CNTs as good electrocatalysts for the HER [83]. The Co/Fe–P

4.4 Applications of TM nanoparticle-based catalysts toward

(A)

Ni-O-Fe bridge

Precursor mixture (aqueous)

Hexane

H2

NiOx

2 OH–

2 H2 O

Nickel Iron Oleate

Phase transfer

Thermal reduction

Ni

FeOx

e e FeOx

Ni

jGSA/mA cm–2

(111)

Ni/Fe-NP Ni NP Ni+Fe alloy NP

–40

Fe NP

–60

No iR-correction

–80

20 nm 2 nm

(311)

–100 –1.0

O

Fe NP 186 mV

H dec–1

0.3

Ni-Fe-NP

–20

Fe

(D)

20% Pt/C

Overpotential (i/N)

(C)

(B)

0

Ni-Fe alloy NP 236 mV dec–1 0.2

Ni NP 167 mV dec–1 Ni/Fe NP 212 mV dec–1

0.1

Ni-Fe NP 58 mV dec–1 20% Pt/C 50 mV dec–1 –0.8 –0.6 –0.4 –0.2 Potential (V vs RHE)

–0.0

0.0

0.6

0.7 0.8 0.9 1.0 log(jGSA/mA cm–2)

1.1

FIGURE 4.11 (A) Schematic of the formation of Ni–Fe NPs and the HER process on the Ni–γ –Fe2 O3 interface in alkaline medium. (B) TEM image of Ni–Fe NPs (Inset: selected area diffraction pattern of Ni–Fe NPs). (C) Polarization curves, and (D) tafel plots of Ni–Fe NPs and a set of counterparts [31].

NPs/CNTs only required overpotential of around 96 mV to reach a current density of 10 mA cm−2 , which was due to the chemical coupling effects of its components. Additionally, a novel material of Pt3 Ti NPs supported on Ti3 C2 Tx NSs was used as efficient electrocatalysts for the HER that needed a small overpotential of 32.7 mV to generate a current density of 10 mA cm−2 , and a low Tafel slope of 32.3 mV dec−1 [124]. A promising HER catalyst was developed based on the growth of nickel hydr(oxy)oxide NPs over the MoS2 nanosheet surface [125]. The expected activity of the obtained catalyst for the HER was demonstrated by the generation of a current density of 10 mA cm−2 , by using a very low overpotential of 73 mA cm−2 . Table 4.2 lists various electrocatalysts for the HER that were also developed by using TM NPs.

4.4.3 OER applications Developing highly active electrocatalysts for the OER reaction can reduce the overpotential required to generate an expected current, and thus significantly improve the performance of related energy technologies. Wang et al. succesfully fabricated NiFe NPs as superior electrocatalysts toward the OER in alkaline conditions [126]. These catalysts exhibited extraordinary performance with the OER activity of 226 and 263 mV at 10 and 100 mA cm−2 in alkaline electrolytes, respectively. An advanced electrocatalyst based on Co@Ir core–shell NPs encapsulated by N-doped porous carbon (Co@Ir/NC) was prepared from metal−organic frameworks including

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Table 4.2 Development of TM nanoparticle-based catalysts for the HER.

HER catalysts

Overpotential at 10 mA cm−2 (mV vs. RHE)

Tafel slope (mV dec−1 )

Reference

FeIr NPs/nickel foam FeCoMo P-O NPs FeB2 NPs NiSe2 NPs FeP NPs Ni2 P NPs/CNSs FeP/Fe3 O4 NPs/N,P-doped CFs CoP NPs/ RGO Co3 Mo NPs/molybdenum oxide NSs CoP NPs/WSe2 NSs

16.6 111 61 170 50 92 149 168 68 163

49.02 54.5 87.5 31.1 65 46 73 57 61 76.5

[18] [19] [30] [62] [64] [79] [89] [97] [113] [114]

CNSs, carbon nanospheres; RGO, reduced graphene oxide.

fabrication of the ZIF-67 nanocrystals, pyrolyzed ZIF-67 to Co/NC, and deposition Ir shell over the Co/NC surface, for the catalysis of water splitting process [69] (Fig. 4.12A). The unique core@shell structure and the synergistic effects between Co core and Ir shell resulted in excellent cataltyic performance with an overpotential of 280 mV, small Tafel slope of 73.8 mA dec−1 , and good stability under OER condition (Figs. 4.12B–E). Lu exploited a OER catalyst based on the CoSe2 NPs encapsulated on N-doped carbon polyhedra, which only needed potential of 1.59 V (vs. RHE) to gain a current density of 10 mA cm−2 [80]. In other study, a highly efficient electrocatalyst for the OER with a low overpotential of 260 mV at 10 mA cm−2 and superior stability was developed using CoSe2 –FeSe2 heteronanoparticles supported on reduced graphene oxide [127]. A novel OER catalyst containing Co NPs anchored on VN NSs was also developed, and showed remarkable activity with overpotential of 320 mV at 10 mA cm−2 , favorable kinetics with small Tafel slope of 55 mV dec−1 , and long lifetime [115]. Moreover, Table 4.3 lists various electrocatalysts using TM NPs that were fabricated recently for catalyzing the OER process.

4.5 Summary This chapter highlights recent remarkable advances in the field of TM nanoparticlebased catalysts involving substrate-free catalysts, carbon material-assisted catalysts, and metallic substrate-assisted catalysts. Different synthetic approaches have been developed toward generating many different TM electrocatalysts. TM nanoparticlebased catalysts with cost-effectiveness, high surface/volume ratio, and large specific surface area showed high potential to enhance the reaction rate and performance of the electrochemical reactions ORR, HER, and OER. With the support of the highconductivity substrates, such as carbon-based materials, and metallic substrates, the

4.5 Summary

15 10 5 0 1.3

(B)

E/ V vs RHE

Optimized 7.5 mL OAm 0.16 mL OA Au@CO NCs

Co(acac)2 TBAB

1.7

Au 1.6

V 7m 14

c de

–1

c

1

3.75 mL OAm 3.75 mL OA 20 nm

7.5 mL OA Au aggregations

50 nm

10

i/ mA cm–2 disk

1.60 Merged Au NCs

1.8

V de 60 m –1 Co 3O 4 V dec –1 Au + m 9 5 c Co 3O 4 V de 60 m Co 3O 4 Au @

(E)1.65 E/ V vs RHE

20 nm

1.7

–1

1.5

20 nm

1.6

1.5

E/ V vs RHE

1.8

Au@Co NCs + Co NCs 20 nm

Au NCs

1.4

(D) 1.9

7.5 mL OAm

4

4

Co3O4

Co

o3 O

Au

185 °C

Co3 O

Au

20

Au@C

TBAB, 110°C

air

i/ mA cm–2 disk

Co (acac)2

25

Au + Co 3 O4

(C)

(A)

–1

0 A g Ic Ir@i= 1

1.55

Au@Co3O4 @i= 10 A g–1Au@Co

3O4

1.50 1.45 1.40

0

50

100

150

time/ min

200

250

300

FIGURE 4.12 (A) Schematic of the formation of Co@Ir NPs/NC; (B) LSV curves, (C) Tafel plots and (D) EIS of the Co@Ir NPs/NC and its counterparts. (E) Stability test of the Co@Ir NPs/NC [69].

Table 4.3 Development of TM nanoparticle-based catalysts for the OER.

OER catalysts

Overpotential at 10 mA cm−2 (mV vs. RHE)

Tafel slope (mV dec−1 )

Reference

Ru1 Coy NPs NiO NPs CoFe2 O4 NPs/CNTs Fe3 O4 @Nix P NPs Ru@IrOx NPs Fe-doped CoSe2 NPs/CNTs Ag NPs/NiRu LDHs IrO2 NPs/NiFe LDHs Ag NPs/MnO2 NRs Co3 O4 NPs/Fe@C2 N NSs

240 325 314 260 282 330 310 270 432

54.4 40 30.69 43 69.1 74 33 59 94 103

[21] [23] [29] [32] [128] [82] [112] [129] [109] [130]

LDHs, layered double hydroxides; NRs, nanorods.

71

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CHAPTER 4 Transition metal nanoparticles as electrocatalysts

critical disadvantages of the TM nanoparticle-based catalysts, including agglomeration/dissolution phenomena during the catalytic process and low conductive nature, can be well solved, thereby significantly improving the catalytic activities. Irrespective of the promising catalytic behavior of the TM nanoparticle-based catalysts, their practical performance for the ORR, HER, and OER is still lower than that of the expected performance, which limitation must be overcome to meet the demand for scale-up of the related enegy technologies from the laboratory to industrial scale.

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CHAPTER

Transition metal chalcogenides-based electrocatalysts for ORR, OER, and HER

5

Tenzin Ingsel and Ram K. Gupta Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States

5.1 Introduction Energy that powers transportations, communications, and industrial manufacturing enriches our lives by supporting social and economic growth. On the other hand, the rapid increase in global energy consumption and strong dependence on nonrenewable hydrocarbon-based sources for energy has raised environmental concerns and called for the diversification of energy sources for consumption. Nonrenewable energy sources like fossil fuels fulfill the majority of the global energy demand but are responsible for the enhanced greenhouse effect and global warming. Diversifying energy sources and reducing our dependence on fossil fuels for a sustainable future require expanding renewable energy such as wind, solar, and hydroelectric power. Electrochemical water splitting is deemed a notable green technique for clean hydrogen and oxygen gas production [1–3]. Fig. 5.1 depicts the application of renewable resources and conventional energy sources in the electrochemical water splitting process to generate hydrogen as a clean fuel. Water splitting technology can be coupled with renewable intermittent energy sources like wind, sunlight, and tides to produce and store clean energy. Conventional resources such as coal and biomass could be used to provide the required potential for this process. Currently, industrial hydrogen production mostly employs steam methane reforming and coal gasification pathways which strongly depend on fossil fuels. However, one of the most significant limitations in actualizing a widespread practical usage of water electrolysis technology is its high cost. The electrochemical water splitting could be ideally performed by applying 1.23 V but in practice, a much higher potential is required to generate clean fuels. The extra potential is called overpotential which adds the cost to the process. Electrocatalysts are the materials that can reduce the overpotential and make this process more economical. Three main reactions, known as hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER), are involved in the water-splitting Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00005-8 Copyright © 2022 Elsevier Inc. All rights reserved.

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FIGURE 5.1 Hydrogen fuel generated from electrolyzer with inputs from electrical energy generated from renewable sources and chemical fuels [4].

process. The outcome of these reactions is the generation of hydrogen and oxygen gases at the cathode and anode, respectively [5–7]. 2H2 O + Energy → O2 + 2H2

(5.1)

The reactions involved in electrochemical water splitting depend on the nature of the electrolyte. Below are the reactions of HER, OER, and ORR process in acidic and alkaline conditions. The ORR is a multielectron reaction that goes through more than one path [8,9]: In an acidic electrolyte HER: 2H+ + 2e− → H2

(5.2)

OER: 2H2 O → O2 + 4H + + 4e−

(5.3)

ORR: O2 + 4H+ + 4e− → 2H2 O

(5.4)

O2 + 2H+ + 2e− → H2 O2

(5.5)

H2 O2 + 2H+ + 2e− → 2H2 O

(5.6)

5.1 Introduction

In alkaline electrolyte HER: 2H2 O + 2e− → H2 + 2OH−

(5.7)

OER: 4OH− → O2 + 2H2 O + 4e−

(5.8)

ORR: O2 + 2H2 O + 4e− → 4OH−

(5.9)

O2 + H2 O + 2e− → HO2 − + OH−

(5.10)

H2 O + HO2 − + 2e− → 3OH−

(5.11)

The thermodynamic potential for water splitting does not depend on the electrolytes used in this process. However, in practice, a higher potential than 1.23 V is required for this process. The extra potential is needed to overcome the activation barriers for both anodic and cathodic processes as well as to encounter solution and contact resistance. Therefore, the current research focusses on reducing the overpotential of this process to make the electrochemical water splitting process more efficient. Currently, noble and rare-earth metal-based compounds show efficient electrocatalytic activities to reduce the overpotential but these compounds are not cost-effective either. Pt and Ru/Ir-based compounds possess higher electrocatalytic properties for HER, OER, and ORR, however, these materials are not cut out for industrial application because of their high price and rarity [10]. Therefore, an existing need to explore and study earth-abundant inexpensive, efficient electrocatalysts such as transition metal oxides, hydroxides, carbides, nitrides, phosphides, and sulfides. Electrocatalysts possessing multifunctionality as a catalyst have positive implications for their suitability for applications in electrolysis cells, metal-air batteries, and synthetic solar reactors, among other electrolysis technologies [10–14]. Transition metal oxides have made their mark as efficient electrocatalysts for OER in alkaline media, where some of them have even surpassed the electrocatalytic performance and activity of IrO2 . Transition metal chalcogenides (TMCs) like MoSx are heavily studied as electrocatalysts for HER in acidic and basic medium. Additionally, metal-organic frameworks (MOFs)-based compounds and new nanomaterials are gaining attention for electrocatalysis applications. Numerous nonprecious metalbased electrocatalysts such as Co, Ni, Fe, W, and Mo have been widely studied for both OER and HER process [15]. Transition metal chalcogenides, in general, have different compositions and crystal structures with unique electronic systems. Their properties can be tuned by creating defects and by modifying morphology. Among TMCs, metal sulfides garner a lot of attention because of their optimizable metal-sulfur coordination environment, good electrical conductivity, among many other desirable properties. Before discussing the TMCs for the water-splitting process, understanding evaluation parameters for what makes a good electrocatalyst could help. Some of the important electrochemical parameters for an electrocatalyst are provided.

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5.1.1 Overpotential (η) Overpotential is loosely defined as the additional potential required in an electrochemical reaction due to kinetic hindrances. An overpotential required to generate a current density of 10 mA cm−2 is a relevant value for comparison when electrocatalysts are being screened for their catalytic activities. Therefore, logically, an electrocatalyst that requires a low overpotential is highly desirable.

5.1.2 Tafel plot Tafel plot describes the reliance of steady-state current on overpotential. A Tafel plot can be obtained from the polarization curve like a linear sweep voltammogram. At steady-state, the overpotential is related to the logarithm of current density (j), and the linear portion of the Tafel plot is fit to the Tafel equation: η = a + blogj

(5.12)

where b is the Tafel slope. When evaluating an electrocatalyst, it is desirable to have a small Tafel slope. A Tafel slope indicates the voltage requirement to fasten the reaction rate by a factor of ten.

5.1.3 Faradaic efficiency Faradaic efficiency indicates the efficiency of the participated electrons in an electrochemical process. For example, in HER, the ratio of experimentally quantified hydrogen molecules to the theoretical hydrogen quantity estimates the faradic efficiency of the HER process.

5.1.4 Stability It is crucial to evaluate how stable the electrocatalysts are under different media. Good electrocatalytic activities and structural stability are some of the highly desired properties of a material for practical applications. One way to assess their stability is to observe the current output with time. When measuring stability, it is a good practice to set the current density higher than 10 mA cm−2 for some time longer than 10 hours [9,16]. Fig. 5.2 depicts some of the important properties of a material for electrocatalytic applications. Now with a brief understanding of the requirement for a good electrocatalyst, let’s look at some common strategies employed by investigators to enhance the electrocatalytic activities. Some of these common strategies undertaken are: (1) Doping/defect engineering, (2) phase transition engineering, (3) hybridization or composite formation, (4) utilization of conductive support, and (5) morphology manipulation. From here onwards, this chapter will strive to shed light on the properties of transition metal chalcogenides-based electrocatalysts by giving examples of

5.2 Synthesis of metal chalcogenides

FIGURE 5.2 Desirable properties of an electrocatalyst. “Adapted with permission [15]. Copyright (2015) The Royal Society of Chemistry.”

the effects of materials manipulation that are reflected in materials electrocatalytic activities.

5.2 Synthesis of metal chalcogenides Transition metal chalcogenides are known for high electrocatalytic activities because of their controllable morphology and plentiful active sites. Transition metal chalcogenides can be synthesized in different ways such as using top-down and bottom-up approaches [17]. Under the top-down and bottom-up general synthesis route, various methods are included.

5.2.1 Solvothermal Solvothermal is a chemical reaction in a solvent at a temperature above the boiling point of solvent and pressures above 1 bar. Reaction temperature higher than the solvent’s boiling point increases the pressure built up in an autoclave promoting enhanced crystallinity. Solvothermal is a bottom-up synthesis type where the desired nanosheets can be obtained in high yield onto the substrate or in solution. This is

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one of the most widely used routes in the wet chemical methods through which metal chalcogenides of different phases, defects, and shapes can be acquired. In this synthesis process, the temperature, concentration of the precursor, surfactants, and solvent must be monitored closely to have a controlled experimental condition. Various types of precursors such as metal hydroxides, oxides, metal-organic frameworks are used to vary the morphology, crystallinity, defects, and catalytic activities. The solvothermal process has some disadvantages such as longer reaction time, unpredicted, and complicated mechanisms [18].

5.2.2 Chemical vapor deposition Chemical vapor deposition is an example of a bottom-up synthesis approach. In this process, to obtain the desired thin film, a substrate is subjected to one or multiple volatile precursors. The precursors end up getting decomposed on the surface of the substrate. This synthesis type is used to grow diverse 2D nanosheets because of considerable merits like controllable thickness, high crystallinity, and multiheterostructure engineering, among many other advantages [17,18]. For example, two general routes are available for the synthesis of MoS2 . In the first one, the Mo-based precursor can be deposited on a substrate and later be subjected to decomposition or sulfurization to acquire MoS2 . In the second route, molybdenum and sulfur precursors in their gaseous form can be reacted to form MoS2 on a given substrate. Some of the substrates used for transition metal chalcogenide synthesis are mica, sapphire, and quartz [17].

5.2.3 Other methods Other standard transition metal chalcogenide synthesis methods mentioned multiple times in literature include solid-phase chemical synthesis, self-assembly, ion exchange, and hot injection methods. Solid-phase chemical synthesis requires simple equipment and process, making it a low-cost approach that can provide a rapid reaction rate. However, the products attained are generally of low quality and nonuniform morphology. Nucleation and kinetics are the two processes or stages involved in solidphase synthesis. Overall, if the activation energy of nucleation and growth activation is compared, the activation energy of nucleation is higher. Therefore, when the nucleus is formed at the active centers like internal defects and sites with the absence of symmetry, the nanomaterials can grow and expand quickly [18]. The self-assembly method is a widely used synthesis method in different areas of application. Through this method, the end product acquired can be of ordered structure with the help of electrostatic interactions, van der Waals interaction, and hydrogen bonding among the crystals at the nano level. The ion exchange method is known for its high efficiency and selectivity with a quick reaction rate. Cation and anion exchange methods are part of the ion exchange synthesis. These approaches induce strain, or defects, or nanointerfaces that could work as an advantage in their

5.3 Transition metal chalcogenides-based electrocatalysts for OER

electrocatalysis process. The hot injection method generally requires the rapid introduction of the reactants into a hot solution that contains surfactants like oleic acid, oleyl amine, etc. Advantages of this method include nanomaterials with high crystallinity and uniform size and morphology. On the other hand, this method results in low-yield products where high temperature is generally needed in this strategy to convert metal precursors to metal sulfide. Additionally, the long-chained surfactants used in this process are challenging for removal [18]. All the transition metal chalcogenide synthesis processes mentioned above have their advantages and disadvantages, and depending on the area of application, suitable methods can be used.

5.3 Transition metal chalcogenides-based electrocatalysts for OER Transition metal chalcogenides based electrocatalysts have been explored for watersplitting applications for the past few decades. Some of them even surmount stateof-the-art OER electrocatalysts like ruthenium and iridium oxides, however, there is still enough room to further improve the electrocatalytic activities of TMCs [19]. One of the strategies discussed above, namely doping of an electrocatalytic material, is generally carried out to improve material’s electronic conductivity, increase the electrochemically active surface area, and optimize the adsorption/desorption energetics of the intermediates formed during HER and OER processes, among many other reasons. Iron doping has shown a significant increase in catalytic properties of nickel sulfide [20]. The OER catalytic activity of Ni3 S2, Fe-doped Ni3 S2 , and the state-ofart rare OER catalyst IrOx was compared in terms of their measured overpotential and Tafel slopes. The Fe-doped Ni2 S3 based electrocatalyst outperformed IrOx and pristine Ni2 S3 by requiring an overpotential of 214 mV, which is about 70 mV and 130 mV lower than in pristine Ni2 S3 and IrOx , respectively. The Tafel slopes of the Fe-doped Ni2 S3 , Ni2 S3 , and IrOx were 42, 82, and 107 mV dec−1 , respectively, with the Fe-doped Ni2 S3 showcasing better OER kinetic process. It was suggested that with the introduction of Fe dopant in Ni2 S3 , the surface of this sulfide-based catalyst was changed to Ni-Fe oxyhydroxide during the OER process, which in turn resulted in improved OER activity compared to pristine Ni2 S3 . Additionally, Fe doping in Ni2 S3 resulted in improved electrochemically active surface area than the Ni2 S3 , indicating more sites are available for electrocatalytic activities. Surface anion decoration is another effective method to improve the electrocatalytic activities. The surface anion plays a major role in tuning electronic structure without affecting the crystal structure and electrochemical surface area and hence provides better water splitting efficiency. Nitrogen anion decorated Ni3 S2 showed improved electrocatalytic activities by displaying a lower onset OER overpotential and enhanced OER kinetics [21]. Another anionic dopant, phosphorous, when introduced into the (Ni, Fe)3 S2, showcased a stronger density of state near the Fermi level when compared to pristine (Ni, Fe)3 S2 , which suggests enhanced electroconductivity

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CHAPTER 5 Transition metal chalcogenides-based electrocatalysts

FIGURE 5.3 Phase transformation of CoTe2 through phosphorous anion-doping. “Adapted with permission [23]. Copyright (2019) American Chemical Society.”

known to be beneficial to both OER and HER processes [22]. The phosphorous doped (Ni, Fe)3 S2 not only showed a significant improvement in catalytic activities but also showed very durable performance surpassing state-of-the-art electrocatalysts such as IrO2 . With the introduction of anionic dopant material, electrode materials’ enhanced electrical characteristics were observed, allowing changes in the electrochemical system that favors enhanced OER activity [21,22]. Structural-phase transitions of metal sulfide and selenide-based electrocatalysts were carried out to optimize the adsorption energy of intermediates generally to reveal more readily available active sites and enable fast charge-transfer electrochemical processes. Phase alteration of 2H-MoS2 from a semiconducting nature to a conducting nature (1H-MoS2 ) is a well-known example of a type of strategy utilized in enhancing MoS2 ’s electrocatalytic activity. The structural-phase transition of a transition metal ditelluride for its OER activities was performed [23]. The cobalt ditelluride (CoTe2 ) underwent a polymorphic transformation from a hexagonal to an orthorhombic phase; such manipulation was enabled by phosphorous anion-doping (Fig. 5.3). The cobalt ditelluride-based electrocatalyst with the orthorhombic phase exhibited the lowest OER overpotential (241 mV at 10 mA cm−2 ) and Tafel slope (46 mV dec−1 ) when compared to CoTe2 composites with hexagonal phases. Composites of transition metal chalcogenides are explored for improved OER electrocatalytic activities, such as graphene oxide and transition metal chalcogenides, for favorable synergistic effects toward their OER catalytic activity. Conductive materials like graphene oxide, in general, can help reveal more active sites and enable accelerated electron transport in metal chalcogenides. For example, MoS2 nanoparticles embedded over reduced graphene oxide and graphene oxide were synthesized to obtain more exposed catalytically active edge sites and enhanced electrocatalytic activities [24]. Fig. 5.4 showcases a TMC and carbon composite material, copper zinc tin sulfide with oleyl amine-functionalized graphene oxide (OAm-GO/CZTS), for its OER properties [24]. The morphology of graphene oxide nanosheet and that of the CZTS metal sulfide nanoparticles changed to a TMC-carbon composite with well-defined nanospheres. The graphene oxide nanosheets were wrinkled and agglomerated due to a high surface area while CZTS consists of nanospheres with

5.3 Transition metal chalcogenides-based electrocatalysts for OER

(A)

(B)

(C)

FIGURE 5.4 Morphology of OAm-GO/CZTS (A) FE-SEM image of GO, (B) FE-SEM of pure CZTS NPs size of 100−200 nm, and (C) FE-SEM image of OAm-GO/CZTS composite size of 200−250 nm (yellow arrows denoted graphene sheets and red arrows denote CZTS NPs). “Adapted with permission [24]. Copyright (2019) American Chemical Society.”

an average diameter of 100–200 nm. Nanospheres of OAm-GO/CZTS composite with 200–250 nm diameter can be seen where CZTS nanoparticles were enfolded in graphene oxide. The synthesized OAm-GO/CZTS composite required the lowest OER overpotential (1.36 V at 10 mA cm−2 ) and the lowest Tafel slope (91 mV dec−1 ) (Fig. 5.5). The electrode material faced the lowest charge transfer resistance among all the other electrocatalysts and was highly stable for more than 16 hours [24]. Carbonaceous and metal materials are generally hybridized to reveal more active sites, enhance their electronic conductivity, and prevent aggregation. In a study, three-dimensional carbon paper/carbon tube/cobalt-sulfide sheets (CP/CTs/Co-S) was synthesized to obtain a multifunctional electrocatalyst with high catalytic activity and stability toward both OER and HER processes [25]. Such higher catalytic activities of CP/CTs/Co-S, compared to that of the carbon paper/cobalt-sulfide sheets, were credited to its improved electron transport, more exposed active sites, and enhanced discharge of gaseous molecules. The carbon tube connected to the carbon paper provides an excellent pathway for the electrons to travel without much hindrance. When constructing an electrode, it is important to select suitable substrate material or conductive support. Commonly used supports are based on materials like titanium, nickel, glassy carbon, and platinum. Different substrate geometries are also under investigation, such as foils, discs, and foams [26]. To understand the effect of conductive support on an electrocatalyst’s catalytic properties, nickel selenide was electrodeposited on different substrates and characterized for its electrocatalytic properties [27]. Substrates such as gold-coated glass (Au-glass), gold-coated-silicon (Au-Si), glassy carbon (GC), nickel foam (Ni-foam), fluorine-doped tin oxide coated glass (FTO), and indium tin oxide coated glass (ITO) were used in the study. As shown in Fig. 5.6, conductive substrates affect the OER performance of the electrocatalyst, where nickel selenide electrodeposited on nickel foam delivered the lowest OER overpotential (270 mV) at a current density of 10 mA cm−2 . The high surface area of the nickel foam makes it more probable for the electrolyte to access the nickel

91

90 80 70 60 50 40 30 20 10 0

GO OAm-GO CZTS OAm-CZTS GO/CZTS OAm-GO/CZTS

(iii)

(v) (iv)

1.36 V

(vi)

0.5

1.0

1.5

2.0

(v)

2.5

1.6

140 mV dec-1

1.4

0.2

0.3

–50 (i)

(ii)

Rct=76 Ω

–0 0

100

(iv)

(iii)

Ret= 125 Ω

200 Z'/Ω

GO OAm-GO CZTS OAm-CZTS GO/CZTS OAm-GO/CZTS

300

400

30 20

j (mA cm–2)

(vi)

40 j (mA cm–2)

Ret= 250 Ω

Ret= 155 Ω

0.4

(i) 0.5

0.6

0.7

3.5

50

Ret= 398 Ω

Ret= 200 Ω(v)

(ii)

0.8

Log (j[mA cm–2])

(C)

–100

OAm-GO CZTS OAm-CZTS GO/CZTS OAm-GO/CZTS

91 mV dec-1

E (V vs. RHE) –150

142 mV dec–1

(iii)

1.8

1.2

(iv)

144 mV dec–1

2.0

(B)

160 mV dec–1

2.2

(ii)

(i)

2.4

(A) η(V vs. RHE)

j (mA cm-2)

CHAPTER 5 Transition metal chalcogenides-based electrocatalysts

-Z''/Ω

92

(D)

3.0 2.5

1.0 M KOH η-1.11V

2.0 1.5 1.0 0

200 400 600 800 1000

time (min)

1st Cycle 1000th Cycle

10 0 0.4

0.6 0.8 1.0 E (V vs. RHE)

1.2

FIGURE 5.5 (A) Polarization curves, (B) Tafel slopes, (C) Nyquist plots, and (D) durability test of OAm-GO/CZTS. “Adapted with permission [24]. Copyright (2019) American Chemical Society.”

selenide catalyst during OER. In terms of current density, Ni3 S2 electrodeposited on Si substrate, glassy carbon, and Au-coated glass delivered the highest current density [27]. As mentioned earlier, transition metal chalcogenides are a famous group of electrode materials for the HER process. However, it is crucial to explore transition metal chalcogenides that work toward HER and OER. This, in turn, would help in constructing bifunctional or multifunctional electrocatalysts to carry out excellent overall water-splitting. In the next few sections, transition metal chalcogenides-based electrocatalysts for ORR and HER will be discussed.

5.4 Transition metal chalcogenides-based electrocatalysts for ORR Oxygen reduction reaction has a rather complicated mechanism. Depending on the electrode and electrolyte used, the oxygen reduction reaction can carry on through a

5.4 Transition metal chalcogenides-based electrocatalysts for ORR

120

j / mA cm–2

100

Ni3Se2-Au/Si Ni3Se2-GC Ni3Se2-Ni-foam* Ni3Se2-Au/glass Ni3Se2-FTO Ni3Se2-ITO

80 60 40 20 0

10 mA cm–2 1.0

1.2

1.4

1.8

1.6

2.0

2.2

E/ V vs. RHE

FIGURE 5.6 Polarization curves of Ni3 Se2 electrodeposited on different substrates. “Adapted with permission [27]. Copyright (2015) The Royal Society of Chemistry.”

(A)

(B)

D

G

A F

C

E

End-on configuration 2e-

4e-

B

H2O2

H2O

Side-on configuration 4eH2O

FIGURE 5.7 (A) Adsorption of O2 molecule on probable adsorption sites of CoPd2 Se2 , (B) end-on and side-on adsorption configuration of O2 molecule. “Adapted with permission [29]. Copyright (2020) American Chemical Society.”

direct four-electron pathway or two-step two-electron route. An electrocatalyst that can promote ORR for the two-electron pathway can be used in hydrogen peroxide synthesis. An electrocatalyst that can selectively follow the direct four-electron ORR pathway is desirable for advanced energy conversion devices [28]. In literature, two types of O2 adsorption models, the Pauling and Yeager model, are discussed (Fig. 5.7) [29]. In the Pauling model or end-on mode, oxygen adsorbs on the catalyst surface through a single bond. In the Yeager model or side-on manner, one oxygen molecule interacts with two catalyst sites. Some investigators report that in the Yeager model, the adsorption of oxygen produces water, whereas, in the Pauling model, both water

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FIGURE 5.8 Polarization curve of ORR and OER showcasing the relevance of parameters like onset potential and overpotential from E° (1.23) for ORR and OER and half-wave potential for ORR. “Adapted with permission [32]. Copyright (2017) American Chemical Society.”

and hydrogen peroxide can be formed. For obtaining high energy efficiency, the direct four-electron reduction pathway to water is preferred. In recent years, different ORR electrocatalyst materials have been investigated: Metal oxides, carbons, perovskites, and spinels, among many others. Much work has been done on ORR electrocatalysts in decreasing their required overpotential and improving the electron transfer kinetics. In most of the reports, ORR electrocatalyst activity is usually discussed in terms of parameters like the onset potential, halfwave potential, and durability of electrocatalysts (Fig. 5.8). Mo4.2 Ru1.8 Se8 was among the first transition electrocatalyst to show ORR performance on par with that of the benchmark platinum electrode material. Ruthenium-based chalcogenides, Ru-X are well-known ORR catalysts where Ru is usually known to act as the active site for ORR [29–31]. Platinum is of high cost, rare, and does not possess sufficient long-term stability, which hinders its widespread commercial use. Transition metal chalcogenides are potential alternatives to costly platinum as high-performance electrocatalysts for ORR. Previous ORR-related studies have produced a general activity trend, such that Mx Sy > Mx Sey > Mx Tey (M = Co, Ni, Fe, Mn, etc.,) and Cox Xy > Nix Xy > Fex Xy . It is commonly known that cobalt and ferrous metal centers have their redox potential similar to that of reducing oxygen to water [33]. As per the trend, transition metal

5.4 Transition metal chalcogenides-based electrocatalysts for ORR

sulfides are high performing ORR electrocatalysts. They are usually known for high activity toward HER. A study was done on palladium sulfides’ ORR catalytic activities consisting of different compositions (Pd16 S7 , PdS, and Pd4 S). Among all the other palladium sulfide electrocatalysts, Pd4 S nanoparticles proved their superior catalytic activity toward ORR by showcasing high half-wave potential, high onset potential, and low Tafel slopes. Such excellent electrocatalytic properties of Pd4 S are attributed to its ideal oxygen adsorption free energy. Definitive structure-activity relation of transition metal chalcogenides as ORR electrocatalysts have not been explained; however, few transition metal chalcogenides and their composites have been reported for being efficient, low-cost ORR electrocatalysts [34]. According to the general ORR activity trend and prominence of cobalt chalcogenide-based electrocatalysts in literature, they exhibit high ORR activity and durability. In a study, the electrodeposition of cobalt sulfide on a carbon cloth support was carried out, followed by rapid low-temperature annealing. The cobalt sulfide that was thermally treated displayed higher ORR onset (0.85 V vs. RHE) than the cobalt hydroxide (0.7 V vs. RHE) and unannealed cobalt sulfide (0.79 V vs. RHE). Additionally, annealed cobalt sulfide delivered a current density of −1.51 mA cm−2 at 0.2 V, which is higher than that of the cobalt hydroxide (−0.76 mA cm−2 ) and unannealed cobalt sulfide (−0.95 mA cm−2 ). Such superior or improved electrochemical performances in the thermally treated cobalt sulfide can be attributed to its highly interconnected nanosheet morphology with higher specific surface area, improved crystallinity, and hence, superior intrinsic electrocatalytic activity [35]. One report indicated that the cobalt disulfide with pyrite structure displayed activity toward ORR comparable to that of chalcogenide groups like CoSe2 and Rux Sey that are known as highly active ORR electrocatalysts [36]. In another investigation, electrochemical lithium tuning of cobalt disulfide was carried out to improve the ORR activity. Such manipulation resulted in an enhanced surface area, a morphological transformation where more grain boundaries were established in the cobalt sulfide structure. These grain boundaries are theorized for being active sites for ORR. Electrochemically tuned cobalt disulfide showcased enhanced ORR activities experimentally when its electrochemical properties were compared to that of pristine cobalt disulfide [33]. Up to here, we have discussed mostly binary metal sulfides and selenides for ORR. Ternary metal chalcogenides are a fantastic group of materials suitable for various electrochemical applications due to their distinctive properties, such as the two constituent metal elements’ synergistic effect and rich redox reaction sites. Ternary metal sulfide like nickel-cobalt sulfide (NiCo2 S4 ) shows unique electrocatalytic activity because it can assume different morphologies like nanotubes, hollow spheres, nanowires, nanosheets, among many others. As can be seen in Fig. 5.9, nickel-cobalt sulfide hollow spheres (NiCo2 S4 HSs) architecture was constructed through an ionexchange strategy. NiCo2 S4 HSs electrocatalyst was compared regarding their ORR performances to its counterparts: nickel sulfide nanoparticle (NiS NPs), cobalt sulfide nanoparticle (CoS NPs), and nickel-cobalt sulfide nanoparticles (NiCo2 S4 NPs). NiCo2 S4 HSs electrocatalyst showcased superior ORR performance. NiCo2 S4 HSs required higher ORR half-wave potential (0.80 V) and lower Tafel slopes (48.6 mV

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CHAPTER 5 Transition metal chalcogenides-based electrocatalysts

Thiourea

DMF

EG

Bubbles

Cobalt precursor

Co2+ Ni2+

S2–

Coordination Decomposition Cobalt precursor

Co9S8

NiCo2S4 HSs

FIGURE 5.9 Schematics of the mechanism of formation of NiCo2 S4 hollow spheres. “Adapted with permission [37]. Copyright (2018) American Chemical Society.”

dec-1 ) when compared to its counterparts [37]. Another ternary metal chalcogenide, tungsten cobalt selenide (W-Co-Se) electrocatalyst, had been explored for its activity toward ORR. Even though tungsten and cobalt are inactive toward ORR, selenium played a vital role in the electrocatalyst’s activity [38]. The effect of nitrogen and phosphorous elemental doping on the ORR activity in MoS2 monolayer was analyzed using DFT computations. The study revealed that nitrogen-doped MoS2 showcased a low-energy barrier of 0.25 eV and an overpotential of 0.67 V toward ORR catalytic activity. The N-doping of MoS2 seems like a strategically better option than P-doping because N-doped MoS2 revealed higher catalytic activity with favorable computed energy barriers and lower overpotential toward ORR than P-doped MoS2 . From computational studies, N-doped MoS2 seems to display its superior catalytic activity comparable to well-known ORR catalysts like Pt catalyst, Co-N co-doped graphene, and N-doped carbon materials [39]. Note that there were not many experimental studies on heteroatom doping in transition metal chalcogenides for their ORR activities. Next, composites of carbon and transition metal chalcogenides for their activity toward ORR will be discussed. In recent years, composites of transition metal chalcogenide and carbon materials have attracted much interest in catalysis and the energy storage field. When a carbon material is utilized as a support, it can strongly affect the catalyst’s morphology, structure, and activity. Generally, the carbon support material can enhance the surface area for uniform dispersion of catalyst material, enhance conductivity, and make the catalyst relatively more stable in an acidic or alkaline environment. An advantage of the doped carbon material is that they exhibit enhanced electrocatalytic activity instead of pristine carbon material. Favorable interaction between nitrogen-doped carbon and the chalcogenide typically yield electrode material with excellent electrocatalytic activity toward ORR and OER. In general, cobalt-based carbon composites are an attractive material for electrocatalysis where a cobalt sulfide and nitrogen-doped carbon composite had delivered excellent ORR activity and stability, similar to that of a platinum carbon catalyst. A hybrid electrocatalyst, two-dimensional Co9 S8 /carbon

5.4 Transition metal chalcogenides-based electrocatalysts for ORR

nanosheets with rich active sites, was constructed where the composite electrocatalyst was synthesized from its cobalt oxide-based precursor, polyvinyl pyrrolidone, and sodium sulfide. The polyvinyl pyrrolidone acted as the source of nitrogen and carbon. The electrocatalyst delivered an onset potential of 0.89 V (vs. RHE) and a half-wave potential of 0.77 V (vs. RHE). The catalytic material was surrounded by a graphitized carbon layer, which protects the catalyst from external stressors, prevents corrosion, and enhances the catalytic activity [40]. Other cobalt-based composite electrocatalysts like cobalt selenides are discussed in the literature. In two different investigations, cobalt selenide and carbon composites were synthesized via conventional heating and microwave-assisted heating. The CoSe/C and CoSe/N-doped rGO seem to have comparable electrochemical performances [41,42]. The cobalt selenide obtained from microwave heating was put in the microwave oven with all the electrocatalyst precursors at the oven’s full power for 3 minutes. Then the carbon substrate was then added to the active catalyst. The onset potential achieved by the CoSe/C composite was 0.823 V vs. SHE for the ORR process [41]. In the cobalt selenide prepared by high-temperature pyrolysis, the N-doped reduced graphene was prepared through pyrolysis at high temperature (1000°C under Ar.), and the CoSe active catalyst was dispersed on the surface of the carbon material. The composite was further heated under Ar. at 600°C for 4 hours with a ramp rate of 10°C/min. The resultant electrocatalyst (CoSe/N-doped rGO) obtained an onset potential of 0.92 V vs. RHE and a half-wave potential of 0.86 V vs. RHE [42]. Group-six transition metal-based selenide and carbon composites like molybdenum disulfide (g-MoSe2 ) and tungsten disulfide (g-WSe2 ) have been studied for their ORR activity and stability. When these transition metal dichalcogenide carbon nanocomposites g-MoSe2 and g-WSe2 were compared to MoSe2 and WSe2 , there was a significant improvement in the current density and the onset potential delivered. Such enhancement in the performances and electrode’s excellent stability can be attributed to its combination with reduced graphene oxide [43]. Ferrous sulfidebased nanostructured Fe3 C/FeS composite was obtained by pyrolyzing Prussian blue-PEDOT. The electrode displayed an onset potential of 0.91 V vs. RHE, a halfwave potential of 0.76 V, and a Tafel slope of 84 mV dec−1 . The PEDOT nanotube was found to be uniformly covered by Prussian blue (PB). With that, nitrogen from PB and sulfur from PEDOT enabled heteroatom contribution, which helped pyrolyzed PB-PEDOT achieve high ORR activity and stability. Such results indicate that metal sulfides like FeS can be used as an active component for ORR [44]. In another study, iron disulfide-based Co@N-doped graphitized carbon was synthesized through a hydrothermal process, as shown in Fig. 5.10. The resultant electrocatalyst (Co@DNC@FeS2 -0.5) displayed excellent ORR catalytic activity with an onset potential of 0.942 V vs. RHE, charge transfer resistance of 4.06 , and half-wave potential of 0.846 V vs. RHE. The charge transfer resistance of Co@DNC@FeS2 0.5 is lower than of the Pt/C (20 wt.%), which indicates that the electrocatalyst can promote rapid charge transfer kinetics and Faradaic process for ORR. This electrocatalyst’s unique core-shell structure encouraged the synergistic effect of FeS2

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CHAPTER 5 Transition metal chalcogenides-based electrocatalysts

FIGURE 5.10 Schematic representation of the synthesis of Co@DNC@FeS2 . “Adapted with permission [46]. Copyright (2020) American Chemical Society.”

and cobalt species for enhanced ORR activity. In a similar study, but with bimetallic Co and Mg sulfide, supported on carbon nanotubes, a favorable ORR process was achieved (Fig. 5.11). Such results can be attributed to multiple factors such as the carbon nanotube promoting the quick transfer of electrons and the Mg ions doping activating the active sites of the cobalt disulfide and carbon nanotube composite [45]. In the next section, the HER activities of the transition metal chalcogenides will be discussed.

5.5 Transition metal chalcogenides-based electrocatalysts for HER Hydrogen fuel has been advocated by many as alternative energy to lessen the global environmental and energy crisis. Investigators in academia and industry are equally diligent in pursuing low cost, earth-abundant, effective electrocatalysts that can fasten the hydrogen evolution rate. In hydrogen evolution reaction, acidic mediums’ reaction steps are well-known, whereas in alkaline medium is still unclear. Three HER steps in an acidic medium are:

+

Volmer step: H + + e− → Hads

(5.13)

Tafel step: 2Hads → H2

(5.14)

Heyrovsky step: Hads + H+ + e− → H2

(5.15)

The reduction of H in the Volmer step leads to an adsorbed hydrogen (Hads ) on the electrode’s surface. The next step after the Volmer could progress either through Tafel

5.5 Transition metal chalcogenides-based electrocatalysts for HER

(A)

Mg

(N

O

3) 2 ·6

O 2 ·4H c) 2 (A Co

H

2O

Sulfur powder

hydrothermal

annealing

(B) R OR

Oxygen intermediate

Oxygen Absorption

R OE

e–

e–

O2

OH·

(Co, Mg)S2 nanoparticles

e–

CNTs Outer Wall

CNTs Inner Wall

FIGURE 5.11 Schematic representation of the synthesis of (Co, Mg)S2 @CNTs nanoparticles and electrocatalytic reaction happening at the surface of the catalyst. “Adapted with permission [45]. Copyright (2020) American Chemical Society.”

step or Heyrovsky, or both. Since the adsorbed hydrogen is involved in all the HER steps, the Gibbs free energy of hydrogen adsorption (GH ) is accepted as the indicator of HER electrocatalytic material’s performance [9]. The importance of low-cost electrocatalyst for constructing potential large-scale electrocatalyst-assisted energy conversion technologies was discussed in the previous section. Transition metal chalcogenides are appealing options for electrocatalysis. Fig. 5.12 showcases transition metal and chalcogen elements from the periodic table and common polymorphs of transition metal dichalcogenides, which are part of transition metal chalcogenides [47]. A widely known HER electrocatalyst, MoS2 , a transition metal dichalcogenide in its bulk form, proved nonreactive toward hydrogen evolution reaction a few decades ago. Since then, with nanotechnology and materials science advancements, smart strategies for manipulating electrocatalysts like MoS2 have been explored and applied. Likewise, let’s look at examples of approaches or tactics used to synthesize transition metal chalcogenide in promoting enhanced HER performances [9]. Doping is a commonly used technique in materials science to change a material’s electrical properties. Numerous studies and work can be found in scientific journals about the merits and demerits of different dopants regarding how they affect electrocatalytic properties. However, it is challenging to construct a database indicating

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CHAPTER 5 Transition metal chalcogenides-based electrocatalysts

(A) H

He

MX2 M = Transition metal X = Chalcogen

Li

Be

Na

Mg

3

4

5

6

7

8

9

10

11

K

Ca

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Rb

Sr

Y

Zr

Nb

Mo

Tc

Ru

Rh

Cs

Ba La-Lu

Hf

Ta

W

Re

Os

Fr

Ra Ac-Lr

Rf

Db

Sg

Bh

Hs

B

C

N

O

F

Ne

12

Al

Si

P

S

Cl

Ar

Cu

Zn

Ga

Ge

As

Se

Br

Kr

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

Ir

Pt

Au

Hg

Ti

Pb

Bi

Po

At

Rn

Mt

Ds

Rg

Cn

Uut

Fl

Uup

Lv

Uus

Uuo

(F) (B)

(C)

(D) Distorted 1T

1T

1H

A B A

A B C

3R

(E) 2H

Side view

FIGURE 5.12 (A) Transition metals on the left and the three chalcogen elements on the right highlighted. “Adapted with permission [48]. Copyright (2013) Springer Nature” (B–F) Polymorphs of transition metal dichalcogenides. “Adapted with permission [49]. Copyright (2015) Royal Society of Chemistry.”

the electrocatalytic performance trend, influenced by the dopant type. Therefore, a study has built volcano curves with first-principle calculations theoretical studies to understand the general direction of dopant’s effect on a metal chalcogenide like NiS2 . The volcano curves of numerous transition metal-doped in NiS2 in their different crystal planes were studied [50]. The free energy of hydrogen adsorption (GH ) and current [log(io )] variables provide insights on the influence of transition metal doping on nickel disulfide’s HER functioning. Fe dopant dominates the volcano curve in (100) and (110) NiS2 planes by appearing at the top of the volcano curves. Fe dopant is enabling reduced free energy of hydrogen adsorption (GH∗ ) requirement in (100) NiS2 , comparable to that of Pt (111) with a free energy of −0.01 eV [50]. The use of different sulfur sources affects the sulfidation of metal chalcogenides. Sulfur precursor choice becomes a determining factor in controlling the crystal’s structure, morphology, and size. When the nickel sulfide electrocatalysts were prepared to utilize different sulfur precursors to understand the HER performances

5.5 Transition metal chalcogenides-based electrocatalysts for HER

Microwave heating 1-dodecanethiol

Thiourea

MW 100 W

MW 100 W

b

b

a

c

Orthorthombic Ni9S8 (o-Ni9S8)

a

Hexagonal NiS (h-NiS)

O O Ni O O

S Ni

c

S Ni

Ni(acac)2

Current Density (mA/cm2)

0 o-Ni9S8 BCA o-Ni9S8 ACA h-NiS BCA h-NiS ACA

–10 –20 –30 –40 –50 –60

Improved electroactivity agter stability tests

–70 –80

–0.4

–0.3

–0.2

–0.1

0.0

0.1

0.2

0.3

Potential vs RHE (V)

FIGURE 5.13 Schematics of h-NiS and o-Ni9 S8 synthesis and their polarization curves before and after chronoamperometry tests. “Adapted with permission [51]. Copyright (2020) American Chemical Society.”

of nickel sulfides, the electrode treated with thiourea generated o-Ni9 S8, and the electrode treated with 1-dodecanethiol generated h-NiS, as shown in Fig. 5.13. The h-NiS displayed a lower overpotential (163 mV @10 mA cm−2 ) and smaller Tafel slope values (89.3 mV dec−1 ) than the o-Ni9 S8 ; this could be attributed to h-NiS’s highly crystalline structure with its large crystallite sizes [51]. In another effort, the amount of commercially available reductant, NaBH4, was varied in solid-phase reduction reaction to tune the concentrations of sulfur vacancies in MoS2 and WS2 and see their electrocatalytic properties. As can be observed in Fig. 5.14, with the NaBH4 solid-phase reaction treatment, the MoS2 seems to be experiencing lattice expansion along the c-axis (5.15–7.12 Å). Different electrode samples were synthesized in different mass ratios of NaBH4 to MoS2 , namely, 0.5, 1, 2, and 3. Among these electrocatalysts, the piece with NaBH4 :MoS2 (1:1) delivered the lowest overpotential (238 mV @10 mA cm−2 ), lowest Tafel plot value (57 mV dec−1 ), and the largest

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(A)

NaBH4 Annealing H2S Na2S Mo atom

NaBH4 powder

S atom

S vacancy Na+

BH4–

(B)

Pristine

Reduced

Annealing S vacancy region

1M HCl

Stirring

(C)

(002)′ NaBH4-MoS2

Intensity

102

(002) MoS2 11

12 13 14 15 2 Theta (degree)

16

FIGURE 5.14 (A) Schematics representing desulfurization of MoS2 . (B) Bulk MoS2 is going through solid-phase reduction. (C) XRD spectra of MoS2 before and after getting treated with NaBH4 . “Adapted with permission [52]. Copyright (2019) American Chemical Society.”

electrochemically active surface area (1.65 mF cm−2 ). In this particular sample, the atomic mass ratio of sulfur to molybdenum was 1.81, lower than that of the pristine MoS2 . The same solid-phase reaction strategy was applied to WS2 to remove the sulfur atom from the basal plane of WS2 to perform atomic plane-vacancy engineering. The double-layer capacitance of the desulfurized WS2 improved by 200 times compared to pristine WS2 [52]. On the other hand, composites, being made up of two or more components with different physical and chemical properties, are exciting materials with unique electrocatalytic properties. There have been multiple discussions about enhanced electron transfer due to sulfide electrode material and carbon substrate interactions.

5.6 Transition metal chalcogenides-based multifunctional electrocatalysts

Additionally, carbon substrates contribute toward the stability of electrocatalyst in acidic and alkaline media, and their physical and chemical structures can be altered with smart design. Different types of carbon components have been produced for applications in conductive support. Widely used carbon materials for conductive support are carbon nanotubes (CNTs), graphene oxide (GO), carbon fiber paper (CFP), and porous carbon materials [53]. In a NiO/NiS2 electrocatalyst, when CFP conductive support was utilized, it not only reduced the required HER overpotential, it helped provide better mechanical strength [54]. Fig. 5.15 depicts the usage of carbon cloth in CoMoS, W-doped NiO/NiS2 , and P-doped CoMoS. For the construction of bimetallic sulfide CoMoS electrocatalyst, polyoxometalate was used as a template to obtain ample interface for active sites favorable for improved electrocatalytic properties. In that CoMoS electrode, carbon cloth was employed as a conductive substrate, and the HER overpotential of 36 mV @10 mA cm−2 and Tafel slope of 56 mV dec−1 were recorded. Few reasons for its excellent HER catalytic performances are that the catalysts have well-dispersed and uniform cobalt promoter on the molybdenum sulfide edges with carbon cloth-facilitated fast charge transfer [55]. In a similar yet different study, phosphorous was incorporated in CoMoS anchored on carbon cloth substrate. Phosphorization of the CoMoS was carried out to substitute a few sulfur atoms for enhanced catalytic activity and durability. The resultant HER overpotential value was 66 mV at 10 mA cm−2 with a Tafel slope of 60.1 mV dec−1 [56]. In general, impurities in metal sulfides are inevitable where metal oxide constituents can be observed in the electrodes. If proper advantage can be taken of metal oxide and sulfide composites for applications in electrocatalysis, there are numerous merits to be gained because of their heterogeneous structure favorable for defects and active sites. A molybdenum-based W-doped metal oxide, metal sulfide, and CNT composite (MoS2 /MoO2 /CNT) displayed low overpotential requirement and high electrochemically active surface area. The electrode exhibited an extremely low overpotential of 62 mV for HER at 10 mA cm−2 and a high double-layer capacitance of 217.9 mF cm−2 . Such electrochemical properties can be attributed to many factors, including the porous structure of CNT conductive support and favorable interactions between the metal oxide and sulfide. In short, composites of dissimilar properties can help realize electrodes with unique electrocatalytic properties [57].

5.6 Transition metal chalcogenides-based multifunctional electrocatalysts Bifunctional electrocatalysts for the overall water splitting are explored in the hopes to fabricate efficient technologies for the conversion and storage of clean energy. For an efficient water-splitting system, electrodes that are highly active toward both HER and OER are required. On the other hand, bifunctional electrocatalysts that are active toward OER and ORR are important for fuel cells and metal-air battery systems. Following are some examples of bifunctional electrocatalysts for HER/OER

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(A)

(NH4)6[Co2Mo10]

CoMoS

Mo Co S O N C H

Co2+-NH4+ exchange

24h 210°C

In-situ Co3[Co2Mo10]

growth

(B) Hydrothermal Reaction

Sulfuration

Anneal

Carbon sheet

(C)

Carbon cloth

W doped NiO/NiS2

W doped NiO

Hydrothermal

Phosphorization

process

process

CoMoS/CC

P-CoMoS/CC

FIGURE 5.15 (A) CoMoS anchored on carbon cloth. “Adapted with permission [55]. Copyright (2020) Elsevier”, (B) W-doped NiO/NiS2 on carbon sheet. “Adapted with permission [54]. Copyright (2020) Elsevier”, and (C) P-doped CoMoS constructed on carbon cloth. “Adapted with permission [56]. Copyright (2017) American Chemical Society.”

and OER/ORR. Cobalt sulfide and carbon composite electrode showcased an overall water splitting potential of 1.74 V @ 10 mA cm−2 in a two-electrode system. The electrode showed good durability during electrolysis for more than 7000 seconds, with minimal degradation [25]. Another nickel-based electrode material, iron-nickel

5.7 Conclusion and outlook

60

immersion

selenization

Current density (mA cm–2)

NiC2O4·2H2O nanosheet NiSe2 nanowrinkle

Ni foam

50 40

NiSe2/Ni//Nise2/Ni NiC2O4/Ni//NiC2O4/Ni Ni foam//Ni foam

30 20 1.64 V

10 0 1.2

1.82 V 1.87 V

1.4

1.6

1.8

2.0

Potential (V)

FIGURE 5.16 Growth of NiSe2 on nickel foam and LSV plots of NiSe2 /Ni//NiSe2 /Ni, NiC2 O4 /Ni//NiC2 O4 /Ni, and Ni foam//Ni foam-based electrolyzer in 1 M KOH at a scan rate of 5 mV s−1 . “Adapted with permission [58]. Copyright (2017) American Chemical Society.”

sulfide, went through phosphorous doping. The phosphorous doping augmented the electrode’s free energy of the water/hydrogen, enhanced the electrochemically active surface area and its conductivity. This nickel-based electrolyzer exhibited 1.5 V @ 10 mA cm−2 with minimal degradation during a 15-hour test [22]. A nitrogen decorated nickel-based electrolyzer was fabricated to understand its water splitting efficiency. The electrolyzer system achieved 10 mA cm−2 current density at a potential of 1.48 V [21]. A CoS electrocatalyst anchored on carbon cloth proved efficient towards OER and ORR functionality [35]. 3D porous NiSe2 grown on Ni foam through a two-step process showed bifunctional behavior (Fig. 5.16) [58]. The electrolyzer fabricated using NiS2 required only 1.64 V to provide a current of 10 mA cm−2 for overall water-splitting with outstanding electrochemical stability. Another cobalt-based electrocatalyst, Co9 S8, and carbon hybrid provided good electrocatalytic performance in OER and ORR, benefitting from its two-dimensional sheet morphology and abundant active sites [40]. A NiCo2 S4 , binary metal sulfide, exhibited high activity toward ORR and OER, owing to the synergistic effect of Ni and Co and the high surface area hollow morphology of NiCo2 S4 [37]. In summary, bifunctional transition metal chalcogenide electrocatalysts are highly sought-after materials for energy-related applications.

5.7 Conclusion and outlook Transition metal chalcogenides make cost-effective electrode material with high activity toward HER, OER, and ORR. Multiple studies confirm the value in the usage of transition metal chalcogenides in areas of catalysis and electrochemistry. This chapter has attempted to bring together concepts on the fundamentals of OER, ORR, and HER; how transition metal chalcogenides make suitable electrocatalysts;

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and common tactics undertaken by investigators in transition metal chalcogenides electrocatalytic enhancement. There is room for improvement in developing controlled synthesis and characterization of transition metal chalcogenide electrocatalyst. On top of that, transition metal chalcogenides, like any other electrocatalyst, have unclear “structure effect” relationships. Overall, transition metal chalcogenides have become an essential part of electrochemical research over the years and have experienced tremendous progress.

Acknowledgment Authors wish to thank Ms. Anjali Gupta from Pittsburg High School, Pittsburg, Kansas for their help in drawing Fig. 1.

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CHAPTER

Metal-organic framework-based electrocatalysts for ORR, OER, and HER

6

Muhammad Rizwan Sulaiman and Ram K. Gupta Department of Chemistry, Kansas Polymer Research Center, Pittsburg State University, Pittsburg, KS, United States

6.1 Introduction Everyday work in our daily life requires some sort of energy whether it is kinetic, potential, or in some other form. Even thinking about something takes a bit of energy. Despite having a fraction of the total weight of our body, the brain consumes the largest fraction of about 20% of the total oxygen metabolism of the body [1]. Yet it is the most efficient system compared to the supercomputers ever made by the human and utilizes only 20 watts of energy. The most advanced supercomputer “Frontier” is being fabricated by the AMD and CRAY with the collaboration of the US Department of Energy that would have a computational speed of around 1.5 exaflop calculations per second and is expected to be fabricated by 2021 [2]. Hence, the power required to run this massive supercomputer is estimated by the US Department of Energy to be around 30 MW [3]. With the development of the supercomputers, we would be able to achieve the brain’s computing efficiency which is around 1 exaflop (1 billion-billion) calculations per second. However, attaining energy efficiency is still a challenge that can be addressed by the advancement in material science that made us capable of fabricating sophisticated processors having billions of transistors installed in it. One way to achieve this constraint is to look for the advanced materials that can generate or store energy and fulfill the energy need for upcoming advanced instruments. In an attempt to tackle present-day growing energy needs and to focus on challenges related to today’s energy system, the world has observed a reform intending to lessen the reliance on traditional petroleum fuels. Precisely, the focus has moved toward green and sustainable energy through chemical and electrical interconversion. Green energy is an excellent alternative to traditional exhaustible petroleum-based energy for many reasons. Fast diminishing fossil fuel reservoirs, fluctuating prices, and its adverse impact on the environment suggest us to focus on renewable energy. Renewable energy is the energy cultivated by greener means such as solar, wind, and tidal. It looks pretty promising to shift toward green energy and get rid of conventional Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00007-1 Copyright © 2022 Elsevier Inc. All rights reserved.

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FIGURE 6.1 Three major routes for the production of hydrogen gas. “Adapted with permission [5]. Copyright (2015) The Royal Society of Chemistry.”

energy but, its utilization and the consistency of the source are restricted. For instance, wind energy cannot be harvested in the absence of wind. The same goes for solar energy as it can never be produced in the nonappearance of the sunlight. These discrepancies in energy generation are addressed by using supercapacitors, batteries, and other energy-related devices. Nowadays, storing energy as hydrogen has proved to be a promising strategy for longstanding energy storage. In this consideration, electrochemical water splitting fuel cell is a proficient way to generate hydrogen at a low cost. On combustion, hydrogen gas offers a gravimetric energy density of about 120 MJkg−1 which is around three times better than gasoline (44 MJkg−1 ), which is considered as one of the ideal and the cleanest energy carrier because of superior combustion efficiency, clean exhaust products, nontoxicity, and storable characteristics [4]. The amount of hydrogen gas produced depends upon the chemistry of the electrocatalyst used for the water splitting. Hydrogen is one of the least abundant gases on the earth. Unlike natural gas and oil, hydrogen itself is not energy, but a suitable energy carrier that can be stored and transported. Annually, around 500 billion cubic meters of H2 gas is produced globally [5]. Around 95% of overall hydrogen gas is being generated by steam-reforming, coal gasification, and partial oxidation of petroleum products, as shown in Fig. 6.1. These practices give rise to serious environmental hazards due to CO2 emissions and limited natural reserves restrict widespread use of these processes [6]. However, only 4% of the total hydrogen is produced by water splitting. On comparison of these three hydrogen synthetic routes, only water splitting provides hope for clean energy with zero CO2 emission. Water, the most abundant compound on earth, solely comprise of H2 and O2 . So, splitting water to produce hydrogen is the most favorable and

6.1 Introduction

FIGURE 6.2 Schematic of an electrolyzer.

sustainable route to produce clean energy at low temperatures. Water splitting can be done by two different direct methods, such as photocatalysis and electrocatalysis that make the process more efficient. Photocatalysis is a chemical process in which light, usually sunlight, can be used directly to change the rate of any chemical reaction with the help of a catalyst. The catalyst facilitates absorbing the light and speeds up any chemical process [7]. Electrocatalysis is the chemical reaction that involves oxidation and reduction reaction when voltage is applied by direct transfer of electrons in the presence of electrocatalysts [8]. Electrocatalysis is more favorable as it can work with solar, wind, and other forms of renewable electricity sources. Since water splitting through electrocatalyst requires electricity, it is asserted that hydrogen can directly be generated by solar, wind, or other renewable sources acquired electricity. These approaches, that is, solar/wind technologies, to generate electricity are praised globally. This might create an excellent infrastructure for water-splitting technology. It might be able to fix problems related to the irregularity and the uncertainty associated with wind energy. For example, at night, the little requirement leads to wastage of wind energy which can be easily stored as hydrogen by water splitting. A water-splitting system comprises of three main parts: (1) The cathodic, (2) anodic, and (3) electrolytic, as shown in Fig. 6.2. The electrolyte is an aqueous solution, which can be acidic, alkaline, or neutral, and helps in the transportation of ions or protons. The electrodes are usually layered with the catalysts that provide aid for the water dissociation reactions. On applying voltage, water breaks into hydrogen and oxygen from their respective electrodes. Thus, the electrochemical water dissociation process undergoes a couple of half-cell reactions called the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Another reaction, known as the heart of fuel cells (FCs) and metal-air batteries (MABs), is the oxygen

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reduction reaction (ORR). OER is a vital part of numerous clean energy-generating systems, for example, overall water splitting, metal-air batteries, fuel cells, etc. It occurs at the anodic compartment of the cell and is a little more intricated contrary to the HER. It includes four proton/electron-coupled transfers reaction to produce oxygen molecules, which makes the reaction kinetically slow and ultimately lowers the performance of the overall water splitting. It is highly affected by the pH of the solution, and completely changes the reaction paths that are observed in acidic and alkaline conditions. OER includes various basic steps and adsorbed intermediates. In acidic and neutral solutions, two water molecules oxidize to produce four protons and one O2 molecule. However, in basic or alkaline solution, OH− groups oxidize to provide H2 O and O2 molecules. The chemical equations for OER in both conditions are as follows: Acidic/neutralsolution : 4OH− → O2 + 2H2 O + 4e− +

−

Inalkalineorbasicsolution : 2H2 O → O2 + 4H + 4e

(6.1) (6.2)

On the other hand, ORR is an essential reaction, which takes place in the cathodic compartment of the cell. The ORR can take place by four-electron transfer reaction or by two-electron transfer reaction, but usually, these two types of electron transfer reactions occur simultaneously. The chemical equations representing the four-electrons transfer reaction in the different media are shown below: Alkalinestate : O2 + 2H2 O + 4e− → 4OH−

(6.3)

Acidicstate :O2 + 4H+ + 4e− → 2H2 O

(6.4)

The oxygen reduction reaction involves two-electron transfer that can be represented by the following chemical equation in the acidic and alkaline state: Alkalinestate : O2 + H2 O + 2e− → HO2 − + OH−

(6.5)

HO2 − + H2 O + 2e− → 3OH−

(6.6)

Acidicstate :O2 + 2H+ + 2e− → H2 O2

(6.7)

+

−

H2 O2 + 2H + 2e → 2H2 O

(6.8)

In many instances, four-electron transfer ORR is favorable because of the generation of hydrogen peroxide intermediate in the two-electron transfer process, which may damage the parts of the cell. Hence, the efficiency of an oxygen reduction catalyst is critically based on the number of electrons transferred. Since ORR involves various oxygen-based intermediates adsorbed on the electrocatalyst surface, the performance of the electrocatalyst significantly relies on the way active sites interact with the O2 containing intermediates. The interaction ability can be enhanced considerably by the surface modification and the composition structure of the electrocatalyst. Unlike ORR, HER involves the adsorption of the hydrogen atom and requires electrocatalysts that can readily adsorb hydrogen atom and generate hydrogen.

6.1 Introduction

On the other hand, the hydrogen evolution reaction takes place on the cathodic side of the cell. In basic conditions, the reaction path of HER is still uncertain. However, In acidic conditions, the reaction may undergo in three steps. The first step is known as the Volmer step, whose equation is as follows: H+ + e− → Had . In the Volmer step, one proton combines with an electron at the cathodic surface, and an adsorbed hydrogen atom (Had ) is formed. Once the Had is formed in the Volmer step, the hydrogen evolution reaction proceeds by the Tafel step or the Heyrovsky step or by both of them taking place simultaneously. The Tafel step involves the addition of two Had atoms to generate an H2 molecule (2Had → H2 ). In the Heyrovsky step, adsorbed hydrogen (Had ), a proton (H+ ), and an electron (e− ) combine to give an H2 atom (Had + H+ + e− → H2 ). However, the formation of Had is a critical step because of its involvement in every step irrespective of the reaction path. Therefore, Gibb’s free energy of Had (GH ) is an important parameter in selecting electrocatalysts for HER. For instance, a platinum metal electrode has almost zero GH value and therefore proved to be the best hydrogen evolution catalyst. If Gibb’s free energy is positive and large, the Volmer step becomes easier because the adsorbed hydrogen atom strongly tied on the surface of the electrode. Still, the following Tafel and Heyrovsky steps become challenging. Whereas, large and negative Gibb’s energy of Had leads to the tenuous interaction between the electrode surface and the Had atom, which significantly slows down the Volmer step and ultimately decrease the evolution reaction rate. So, the best possible nonplatinum hydrogen evolution catalyst should have excellent surface characteristics and also has Gibb’s free energy of Had close to zero. These gas involving reactions can be affected greatly by the activity of the electrocatalyst. Currently, platinum or platinum-based compounds and IrO2 are considered as state-of-the-art catalysts for all the three reactions. But, because of the inadequacy and high price of noble metals, nonprecious metal-containing electrocatalysts become the center of interest to substitute the costly precious-metal and to scale up the energyrelated processes. A good electrocatalyst must undergo fast mass and electron transfer and has triphase enriched active sites. By considering these points, carbon-based materials having exceptional electrical conductivity and controllable porosity can produce a promising electrocatalyst by rational design to attain higher OER, ORR, and HER activities. In recent times, carbon materials are widely studied because of their multifunctional properties, good structural characteristics, and intrinsic activity. Carbon materials itself showed excellent activity for HER, OER, and ORR [9–11]. Carbon materials are commonly prepared by exfoliation of graphene, pyrolysis of materials having carbon, and polymerization of small molecules. Recently, metal-organic frameworks (MOFs) are gaining popularity. MOFs have appeared as an important development to bring the advancement in proficient materials for energy-related applications. MOFs are made up of systematically structured and adjustable organic linkers and metal ions or groups of metal ions that provide uniformity and well-defined active-sites conformation. They can be readily linked with various functional materials, that is, graphene to produce a multifunctional composite. They possess remarkable porosity,

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FIGURE 6.3 Schematic of a proton exchange membrane fuel cell.

structural flexibility, crystallinity, and modifiable functionalities. The increased porosity provides access to the inner active sites, encapsulation of reactants, and promotes the kinetics because of the clear passage and host-guest interaction. To achieve the appropriate pore size, shape, and surface group different metal sites and ligands are chosen rationally. MOFs allow the integration of other elements such as second metals or heteroatomic doping into the system to improve the catalytic activity and durability. The pronounced structural and compositional superiority of MOFs offers the great possibility to fabricate competent electrocatalysts for HER/OER/ORR. This chapter will cover the design and fabrication approaches for the electrocatalysts used for HER, OER, and ORR.

6.2 MOF-based electrocatalysts for ORR The ORR is an essential reaction for many fuel cells, for instance, proton-exchangemembrane fuel cells (PEMFCs) and batteries such as metal-air batteries. These energy options are reliable and clean means of energy with zero-emission, fast refueling, and low cost. In PEMFCs, hydrogen is fed as a fuel that gets oxidize on the anodic side producing protons and electrons. These electrons are transmitted to the cathodic compartment using the external circuits, and protons are transferred through the permeable membrane. At the cathodic side, oxygen gets reduced, yielding electric energy by completing the electron cycle at an ideal potential of 1.23 V with water and heat as the only by-product, which makes the reaction clean, as shown in Fig. 6.3. Unfavorably, oxygen activation, O-O bond breakage, and oxide withdrawal slow down the ORR requiring rigorous catalytic activity. After years of experimental

6.2 MOF-based electrocatalysts for ORR

investigation for the best ORR catalysts, Pt and Pt-based materials are proven to be the superior catalyst for ORR. At this time, ORR is catalyzed by the platinum nanoparticles endorsed on the carbon surface or other platinum loaded materials. However, the enormous cost of Pt limits the widespread adaptivity of PEMFCs. As per the strategic analysis report, Pt coating on the electrodes in PEMFCs costs about $11.24 per kW, which is over 20% of the total cost, and Pt consumptions alone cost around $10 per kW in the USA [12]. Therefore, determining efficient processes for the employment of Pt on the electrode surface with maintaining the catalytic performance is more than a challenge. Currently, different methods to achieve this goal have come up overtime, which include decreasing the size of Pt nanostructures, alloying, and designing Pt-loaded precise nanostructures [4,13]. Despite these advancements, these Pt-based ultrafine nanostructures show a significant inclination toward CO poisoning of Pt, carbon corrosion, deformation, and conglomeration of nanoparticles during the electrochemical processes, followed by the adverse deactivation and instability during the long run. Additionally, the sophisticated synthesis route of the catalysts makes the process more expensive. Considering the lesser price of the nonprecious metals (NPMs), that is, iron (Fe), nickel (Ni), cobalt (Co), the cost of any energy conversion and storage system, including PEMFCs, can be significantly reduced. At present, the activity of NPMs-based electrocatalysts is lesser than the Pt-based electrocatalysts by around one digit. But, the catalytic performance of NPM-based electrocatalysts can be improved significantly by coupling NPMs with rational designing and strategies of MOFs. Moreover, the target for achieving extended ORR activity from NPMs-based electrocatalysts is not vague since the living organisms are the perfect example of an efficient ORR catalytic activity. In any living organism, various enzymes, including cytochrome C oxidase and laccase, help in reducing the O2 molecules which accept the electrons from the food and producing energy. These enzymes contain NPMs as active sites and exhibit exceptionally minimized overpotential for the ORR compared to other artificial electrocatalysts. It shows that a remarkable enhancement in the ORR activity using NPMs-based electrocatalysts is not an unreasonable objective and can be achieved by appropriate designing techniques and strategies of the metal-organic framework. The ORR performance is usually evaluated by measuring onset potential (Po ) and half-wave potential (P1/2 ) through linear sweep voltammetry (LSV) test. The electrocatalyst is spread out on the rotating disk electrode, and the test is carried out in an O2 saturated solution. Recently, various kinds of MOFs based materials were studied such as nitrogen-doped carbon, nonprecious metal-based nanoparticles, and metal-free N-doped. These materials are discussed in the next section.

6.2.1 MOF-derived nitrogen-doped carbon-based electrocatalysts for ORR There are various types of N2 doping in carbon-based materials, such as pyrrolic nitrogen, pyridinic nitrogen, graphitic nitrogen, oxidized nitrogen, etc., as illustrated

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FIGURE 6.4 Representation of different types of N doping configuration. “Adapted with permission [17]. Copyright (2017) American Chemical Society.”

in Fig. 6.4. The catalytic activities of nitrogen-doped carbons are believed to be improved by the action of a nitrogen atom to activate neighboring carbon atoms. The active sites can exhibit dissimilar electrocatalytic performance under varying situations, and greatly relies on a variety of N-doping. For instance, researches show that in acidic condition, pyridinic nitrogen acts as a working spot for ORR [14], but it catalyzes OER in alkaline conditions [15]. Whereas, graphitic nitrogen behaves like an active spot for ORR in the basic electrolyte [15], and for HER in acidic electrolyte [16]. The arrangement of the nitrogen atom in ligands or other nitrogen sources can influence the N-doping configuration. Additionally, MOF-derived electrocatalysts usually consist of inexpensive transition metals and their oxides. These electrocatalysts are utilized extensively for ORR because of their potential composition of heavily active catalytic sites. Metal and metal oxides in the MOF promote activity and carbon offers conductive support to the structure. On the other hand, the metal and metal oxides in the MOF undergo conglomeration and decomposition during practical fuel cell operations and lower the practical long-run usability [18]. This obstacle can be resolved by fabricating MOF-derived metal-free electrocatalysts. Therefore, low boiling point metals, including Zn, Al, and Mg, are used to prepare MOF precursors. For instance, MOFs comprise Zn metal have been widely employed to fabricate MOF derived metal-free electrocatalysts for ORR. These include MOF-5, zeolitic imidazolate framework-7 (ZIF-7), ZIF-8, ZIF-67, etc. The removal of Zn-metal can be hurdled by the transformation of Zn into ZnO at high-temperature carbonization because of the high boiling temperature of ZnO that is 2360°C in comparison with Zn, which is 908°C. However, the formation of carbon atoms throughout the carbonization process reduces undesirable ZnO into Zn, which can be easily separated from MOF by evaporation and leave behind a porous structure with increased pore size and specific area. With the elimination of Zn from the MOF, the problem related to conglomeration and dissolution can be resolved. Furthermore,

6.2 MOF-based electrocatalysts for ORR

(A)

(C)

(B)

(D)

FIGURE 6.5 (A and B) Scanning electron microscope images of CIRMOF-3-950 at different magnification, (C) shows different N functionalities content in CIRMOF-3-600 and CIRMOF-3-950, and (D) reveals the percentage of D and G band in CIRMOF-3-600 and CIRMOF-3-950. “Adapted with permission [19]. Copyright (2015) Elsevier.”

the number of pores, pore size, and specific area can be enhanced by metal evaporation during pyrolysis. For the enhanced ORR activity, heteroatoms can be inserted into the carbon matrix from direct MOF’s ligands or external precursor. Currently, carbon-based materials, obtained from MOF, are doped with nitrogen. Fu et al. produced N-doped MOFderived hierarchically porous nanomaterial for ORR by pyrolysis of N-containing isoreticular MOF-3 (IRMOF-3) [19]. It was observed that porosity, surface functionality, pore-volume, and catalytic activity can be controlled by varying carbonization temperatures. At 600°C, the carbonization of MOFs resulted in the ZnO-containing carbon material, which later purified by the HCl etching process. Whereas, at 950°C, the author obtained a metal-free carbonized IRMOF (CIRMOF)-3-950 with a large specific area of about 553 m2 g−1 and superior porosity with the pore volume of about 0.342cm3 g−1 , as shown in the Fig. 6.5A and B. Furthermore, the CIRMOF-3-600 and

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CIRMOF-3-950 showed dissimilar nitrogen functionalities. Compared to CIRMOF3-600, CIRMOF-3-950 exhibited higher content of quarternary-N and offered highly ordered graphitic structures, which resulted in enhanced electrical conductivity and the electrocatalytic activity of CIRMOF-3-950 as seen in Fig. 6.5C and D. With appropriate N-doping, CIRMOF-3-950 showed comparable ORR performance as of Pt/C catalysts having improved stability and methanol crossover effect. Similarly, Wu et al. worked on N-doped metal-free carbon material derived from ZIF-8. NH3 activation was carried out to modify nitrogen configuration [17]. The ZIF-8 was carbonized at 900°C to get N-containing porous carbon (NPC-900) followed by the further activation by NH3 at different temperatures to obtain N-containing hierarchical porous carbon material (NHPC-900-Y), here Y symbolized for the temperature of NH3 activation. The NPC-900 prepared by direct carbonization displayed an N rich content with the larger part of pyridinic nitrogen and pyrrolic nitrogen. However, after the NH3 activation, the NHPC-900-1000 showed a large majority of graphitic nitrogen with a decrease in pyridinic nitrogen and pyrrolic nitrogen. On the evaluation, NHPC900-1000 outperformed NPC-900 with the Po of 960 mV and P1/2 of 840 mV in 0.1 M KOH. This outcome suggests that the ORR activity depends on graphitic nitrogen more than pyridinic and pyrrolic nitrogen in this system. The ORR electrocatalytic performance of the carbon-based compounds can be further improved via the introduction of other heteroatoms (N, P, S, and B) as dopants. The improved electrocatalytic activity is considered to be initiated from the difference in the electronegativity between the doped atom and carbon atoms. This electronegativity variations affect the charge and spin density of the carbon [20]. Furthermore, it is noteworthy that the quantity and species of dopant can influence this change considerably and affect the catalytic activity accordingly. Song et al. stated the N, S, codoped heteroatom electrocatalyst for ORR with NH3 and urea as the heteroatom dopants [21]. The N, S codopants were proved to be effective active sites having improved ORR performance in comparison with the activity of individual S and N doping. Qian et al. reported Zn-containing MOF with linkers having N and B to produce N, B codoped electrocatalysts for ORR. The synthesized electrocatalyst worked quite impressive for OER as well [22]. The enhanced catalytic activity was obtained due to the high porosity and the inclusion of heteroatom doping, which provide improved active sites for catalysis.

6.2.2 MOF-derived nonprecious metal-based electrocatalysts for ORR Owing to the higher prices and insufficient reserves of precious metal, nonprecious metals-containing electrocatalysts have been extensively researched and displayed an exceptional activity. In NPM electrocatalysts, the metal sites (Fe, Co) are believed to perform a significant part in active site formation. However, the precise knowledge about the active sites is uncertain. The focal point of the debate is whether these metals are taking part as the working sites or just engage with the working sites

6.2 MOF-based electrocatalysts for ORR

(A)

(B)

OH–

O2

e–

H2O Diffusion

O2

Activity/Conductivity

+ H2O +

Porosity

e– N, Fe doped

Removal

OH–

100 nm

Porosity

Oxygen reduction reaction in alkaline solution

(C)

(D) Current density (mA/cm2)

0

100 nm

2 4 6 8

MIL-88B-NH3 Pt/C CNPs

10 12 0.2

0.6 0.8 0.4 Potential (V vs. RHE)

1.0

FIGURE 6.6 (A) Graphical concept of ORR using efficient MOF-derived nanoparticles in alkaline condition, (B and C) TEM Images of MIL-88B-NH3 and MOF-derived nanoparticle, and (D) LSV curves of MIL-88B-NH3 , MOF-derived nanoparticles, and Pt/C at 1600 rpm. “Adapted with permission [25]. Copyright (2014) American Chemical Society.”

of the doped carbon material [23]. The previous works suggest that metal-nitrogen fragments inserted into the carbon material are the active sites with preferable oxygen adsorption and oxygen dissociation [24]. MOF-derived NPM catalysts must be designed to have exceptionally rich active sites with a suitable metal, nitrogen, and carbon composition, and a considerably porous structure. Large pore volume and high specific area assist in quick mass transfer and oxygen diffusion on the active sites, which can be achieved by the pyrolysis process. For instance, Tang et al. prepared spindle-like metallic Fe-based MIL-88B-NH3 nanoparticles (Fig. 6.6A) with controlled size and shape having a diameter of ∼50 nm and length of ∼140 nm as shown in Fig. 6.6B [25]. After the pyrolysis, the structure of the carbonized nanoparticles was well stable (Fig. 6.6C).

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Whereas, the structure of larger microstructures of MIL-88B-NH3 were collapsed with smaller particles on the surface. The difference in the morphologies implies that the carbonized nanoparticles are promising for the structural stability as compared to the microstructures during pyrolysis. The characterization of the MOF nanoparticles showed that most of the nitrogen converted into pyridinic-N and quaternary-N, which significantly improved catalytic performance along with the conductivity of nanoparticles. Due to well stable porous structure, Fe and N-containing active spots, and the pyridinic-N/quaternary-N, MOF-based nanoparticles outperformed conventional Pt/C. On evaluation, the sample showed Po of 1.03 V and P1/2 of 0.92 V, as shown in Fig. 6.6D. The MOF-based nanoparticles were applied as electrodes in direct methanol fuel cells (DMFC), which generated 22.7 mWcm−2 of power density, which is about 60% more than commercial Pt/C. Similarly, other Fe-based compounds and composites such as oxide and carbide have been researched extensively for their good ORR performance. For example, Hou et al. prepared Fe/Fe3 C composite nanoparticles obtained from prussian blue [26]. The nanocatalysts displayed a good P1/2 of about 0.83 V in 0.1 M KOH solution. On a similar note, other metal-based nanoparticles, such as alloys, phosphates, carbide, and oxides, can be produced by MOF pyrolysis. One of the most studied metal other than iron is cobalt-containing nanoparticles for ORR electrocatalysts. Metallic Co, Co-based compounds, and their composites have shown an excellent activity toward ORR. Pyrolysis of Co-containing MOF is one the easiest method to produce Co-based nanocatalysts. For instance, Li et al. studied MOF-derived metallic Co nanoparticles dispersed on the honeycomb-like carbon structure [27]. First, ZIF-67 was grown on CoAL layered double hydroxide (LDH). Second, the material was carbonized at the temperature of 800°C under nitrogen to obtain a 2D structured LDH@ZIF-67800, as displayed in Fig. 6.7A. During pyrolysis, the LDH structure was maintained. Whereas, the ZIF-67 structures were collapsed to provide porosity to the structure. Co nanoparticles of ∼11 nm in size were also formed during pyrolysis. The SEM and TEM images of the samples are displayed in Fig. 6.7B and C. The material LDH@ZIF-67-800 demonstrated an excellent activity towards ORR with a Po of 0.94 V and a P1/2 of around 0.83 V in 0.1 M KOH electrolyte as shown in Fig. 6.7D. The highly dispersed active sites of Co nanoparticles on the electrocatalysts surpassed Pt/C electrocatalytic activity in similar conditions. Zhou et al. studied cobalt phosphate-based nanostructures dispersed on carbon framework [28]. This cobalt-based electrocatalyst was obtained from Co-based MOF, having phosphonate-containing ligand and was named as Co3 (PO4 )2 C-N/rGOA. The structure shows the uniform incorporation of cobalt phosphate on the carbon sheet. The combined effect of cobalt phosphate and the nitrogen atom enhanced the ORR performance of the electrocatalyst. The LSV test of the electrocatalyst was performed in alkaline electrolyte, which showed a P1/2 of 0.837 V that is higher compared to platinum-based catalyst (Fig. 6.8A). The ORR LSV curves evaluated at various electrode rotational speeds (Fig. 6.8B) showed a good electron transfer number of around 4 at 0.60–0.75 V. From all the works referred above, it is clear that MOFderived Co-containing nanocatalysts exhibit promising catalytic activity toward ORR.

6.2 MOF-based electrocatalysts for ORR

(A)

Pyrolysis

MelM Co(NO3)2

Carbon-based framework

CoAI-LDH@ZIF-67

CoAI-LDH

(C)

(B)

(D)0 j ( mA cm–2)

–1

500 nm

CoAl-LDH-800 ZIF-67-800 Pt/C LDH@ZIF-67-800

–2 –3 –4 –5 –6

1 m

300 nm

0.2

0.4

0.6

0.8

1.0

E (V vs. RHE)

FIGURE 6.7

0

(A)

Current density ( mA cm–2)

Current density ( mA cm–2)

(A) Synthesis procedure of honeycomb-like carbon-based framework, (B) SEM image of LDH@ZIF-67-800, (C) STEM image of LDH@ZIF-67-800 electrocatalysts, and (d) linear sweep voltammetry curves of CoAl-LDH-800, ZIF-67-800, Pt/C, and LDH@ZIF-67-800 electrocatalysts. “Adapted with permission [27]. Copyright (2016) John Wiley and Sons.”

–1 –2 –3

PNrGO Co3(PO4)2C-N/rGOA Pt/C

–4 –5 –6 –7 0.2

0.4

0.6

0.8

Potential (V vs. RHE)

1.0

0

(B)

–1 –2 –3 –4

400 rpm 625 rpm 900 rpm 1225 rpm 1600 rpm 2025 rpm

–5 –6 –7 0.2

0.4

0.6

0.8

1.0

Potential (V vs. RHE)

FIGURE 6.8 (A) LSV plot of MOF-derived material with cobalt phosphate for ORR performance in O2 -saturated 0.1 M KOH solution at 1600 rpm and a scan rate of 5 mVs−1 , and (B) LSV plot at the various rotational speed. “Adapted with permission [28]. Copyright (2016) Royal Society of Chemistry.”

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In conclusion, ORR electrocatalysis is an essential implication of MOF-derived electrocatalysts. Nitrogen heteroatom doping, nanomaterials having Fe and Co are the active sites for ORR. The modifiable structure and evenly dispersed active sites on the MOF-derived electrocatalysts are the crucial characteristics to provide enhanced oxygen reduction activity.

6.3 MOF-based electrocatalysts for OER With the development of the various energy-associated devices and processes, particularly electrochemical water splitter and metal-air batteries, the need for effective and economical oxygen evolution electrocatalysis has been intensified. Just like ORR, OER is a sophisticated process involving a four-electron transfer process having various fundamental reaction steps. The part that determines the rate of reaction may not be the same for OER as of ORR because OER is the opposite reaction of ORR. Owing to these reasons, the class of active electrocatalytic materials for ORR and OER are generally different. Additionally, the performance of electrocatalysts is evaluated by measuring the onset potential and overpotential (P10 ) that is needed to generate a current density of 10 mAcm−2 . In this portion, we will discuss MOF-derived contents that are promising for OER.

6.3.1 MOF-derived metal-free materials for OER electrocatalyst The OER electrocatalysis is carried out in a stricter environment than ORR since OER takes place at an elevated oxidation potential that can lead to oxidation of carbon material. Additionally, OER involves bubble formation, which can trigger the structural disintegration of carbon. Generally, for the preparation of metal-free carbon electrocatalysts obtained from MOF, Zn is used extensively owing to its low boiling point, which is evaporated during pyrolysis at high a temperature. The metals and their oxides assist in catalyzing the graphitization process of carbon, which provides structural stability and conductivity to the material [29]. Without any metals and their oxides in the metal-free carbon materials, low graphitization of carbon occurs, which leads to poor stability in OER. Because of that reason, there are few reports on OER carbon materials that do not involve any metal, and most of them are doped with heteroatoms to improve their catalytic performance [30,31]. Lei et al. fabricated MOFderived N-doped metal-free carbon material by simple ZIF-8 carbonization. The obtained material was further processed by the cathodic polarization treatment (CPT) for different periods such as 2, 4, 6, and 8 hours to modify the outer functional group of the carbonized material denoted as ZIF-8 C2∼C8 [31]. The XPS characterization was performed to determine the quantity of N and C=O functional groups at varying CPT time. As the treatment time increases, the total content of oxidized N increased along with the increase in other oxygenated functionalities, as shown in Fig. 6.9A and B. After 4 hours of treatment, the N and C=O content tends to decrease. Consequently, ZIF-8-C4 displayed excellent OER performance against other samples (Fig. 6.9C)

6.3 MOF-based electrocatalysts for OER

pyridinic-N graphitic-N

(B) 100

pyrrolic-N oxidized-N

Abundance, %

40 20

(C) 10 8 j, mA/cm2

C2

C0

6

C4 C6 Samples

40 20 C0

(D) 5

C0 C2 C4 C6 C8 IrO2

4 2 0 1.0

60

0

C8

O=C-O

C2

C4 C6 Samples

Pyridinic-N C=O

4 3

C8 900 800

250 200

700

η10 , mV

60

0

C-O H2O

80

Abundance, at%

Abundance, %

80

C=O N-O

2

150

600

1

500

100

0 1.2 1.4 1.6 1.8 Potential, vs. RHE, V

2.0

0

2 4 6 8 Treatment time, hr

400

Tafel slope, mV/dec

(A)100

50

FIGURE 6.9 (A) Nitrogen and (B) oxygen abundance for different compositions in ZIF-8 based carbonaceous material, (C) LSV characterization of ZIF-8 C0∼C8 and IrO2 for comparable OER activity study in 0.1 M KOH solution, and (D) effect of the abundance of pyridine-N and C=O on OER catalytic efficiency. “Adapted with permission [31]. Copyright (2018) Royal Society of Chemistry.”

with a Po of 1.71 V to achieve 10 mAcm−2 in alkaline electrolyte. The effect of pyridinic N and C=O quantities in enhancing the OER performance of ZIF-8-based electrocatalysts can be verified in Fig. 6.9D. Besides nitrogen heteroatom doping, MOF-derived materials doped with different heteroatoms in addition to nitrogen were also investigated. The co-doped MOF materials offered improved OER catalytic activity. The dual-doping creates a variety of active sites having diverse catalytic activities. Hence, even regulation of the active sites and rational control of the pore size can result in high OER activity. Qian et al. worked on Zn-based MOF-derived material dual-doped by B and N [22]. The pyrolysis process helped in obtaining the even distribution of N and B under the H2 /Ar atmosphere at different temperatures. The best sample showed a low Po of 1.38 V. However, an increased P10 of 1.8 V was observed in alkaline electrolyte. On further

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increasing the electrolyte concentration to 6 M KOH, the overpotential dropped to 1.55 V, and a reduction in current density was also observed. These findings displayed enhanced intrinsic catalytic performance of co-doped MOF materials. Although the loading amount of uniform active sites and stability needs to be better to accomplish increased current density.

6.3.2 MOF-derived nonprecious metal-based OER electrocatalyst MOF-derived metal-incorporated carbon materials mostly contain nonprecious metals because of their low cost and broad availability. Carbon materials obtained from the MOF precursors containing metals exhibited high catalytic performance along with excellent stability for OER compared to the metal-free carbon materials. Furthermore, the surface oxidation of certain metal-containing carbon materials (oxides, nitrides, chalcogenides, and phosphides) can significantly improve OER catalytic activity [32]. Thus, MOF-derived OER electrocatalysts mostly contain metals as active sites. Here, we present various types of metal-containing MOF-derived materials for oxygen evolution reaction.

6.3.2.1 MOF-derived nonprecious metal oxides and hydroxide-based electrocatalysts Nonprecious transition-metal oxides and hydroxides were investigated widely for OER because of their low cost and anticorrosive nature. The multivalent oxidation states of the metal oxides and hydroxides such as M+2 , M+3 , and M+4 are considered active spots for OER [33,34]. For this reason, MOF-derived metal oxides and hydroxide carbon materials were also studied. For the metal oxides, oxidation states, structure, composition, the binding energy of O2 to the surface, and metal ions electron number perform a significant part in OER catalytic performance [35]. Jian et al. reported 1D, 2D, and 3D MOF-derived tertiary metal oxide-incorporated carbon composite material (MOx /C), having Co, Ni, and Cu as the metal sites for enhanced OER activity [36]. Fig. 6.10A represents the schematic diagram for the preparation of 1D, 2D, and 3D MOx composite supported on the conductor base. 1,3,5-benzenetrioxylate (BTC), 2,5-dihydroxyterephthalate, (DHTP), and 1,4-benzenedioxylate (BDC) were used as a precursor to producing 1D, 2D, and 3D materials by pyrolysis process, respectively. SEM image of 2D Co3 O4 -CBDC shows nanosheets like structure (Fig. 6.10B). LSV was performed to determine the OER activity. The 2D Co3 O4 CBDC outperformed other electrodes with a P10 of 208 mV compared to 285 mV and 313 mV of P10 for 1D and 3D Co3 O4 materials, respectively. The reason for this is the transformation of 2D Co3 O4 -CBDC into opened network morphology, having interlinked nanosheets of 30 nm of thickness and a high surface area of 181.9 m2 g-1 . 2D oriented morphology and the addition of carbon speed up the production of OH∗ intermediate from the adsorbed water molecules and boosting the 1st step of OER. Thus, assisting in the rapid break down of ∗ OH and increases OER activity.

6.3 MOF-based electrocatalysts for OER

(B)

(A) Annealing

F

N2

MO

1D

M-

1D

/C

MO

2

200 nm Annealing N2

F

MO

2D

M-

2D

/C

MO

2

Annealing

OF

N2

M

3D

M-

3D

/C 2 MO M = Co, Ni, and Cu

(C) 150

1 um

(D)

Co3O4C-NA

Overpotential (V)

J (mA cm–2)

Co3O4-NA

100

MOF

O2 bubbles

50 10 mA

MOF 142 mV dec–1

IrO2/NA

0.4

0.0 Co3O4C-NA

cm–2

70 mV dec–1

0 1.0

Co3O4-NA 123 mV dec–1

1.2

1.4

1.6

E vs. RHE (V)

1.5

0.0

1.0 0.5 Log [J (mA cm–2)]

1.5

FIGURE 6.10 (A) schematic diagram of different dimensional MOF-derived metal oxides based electrocatalyst, (B) SEM image of 2D Co3 O4 -CBDC with an inset image of Co-BTC, (C) LSV curves, and (D) Tafel slopes of Co3 O4 -C-NA and other samples in 0.1 M oxygen-saturated KOH electrolyte at a scan rate of 0.5 mVs−1 (Inset image of Fig. (c) shows O2 bubbling on the surface of Co3 O4 -C-NA. “Figure A and B are adapted with permission [36]. Copyright (2018) American Chemical Society.” and “Figure C and D are adapted with permission [37]. Copyright (2014) American Chemical Society.”

Furthermore, increased OER activity can be achieved by porous and open network morphology, which also helps in the rapid evolution of the O2 bubble. MOF-derived 3D Co3 O4 -C porous composites (Co3 O4 -C-NA) with accessible nanowire networks were developed by Ma et al. for competent OER electrocatalyst [37]. The design strategy followed the growth of Co-naphthalenedicarboxylate templates on the Cu foil hydrothermally. The precursor then transformed into Co3 O4 and C composite by pyrolysis under nitrogen. The composite Co3 O4 -C-NA displayed good catalytic activity with 1.52 V of P10 and a Tafel slope of 70 mVdec−1 . The catalytic activity even surpassed the activity of IrO2 -C (grown on Cu film having equal mass level as of Co3 O4 -C array), carbon-neutral Co3 O4 -NA (produced by calcining Co3 O4 -C-NA

127

CHAPTER 6 Metal-organic framework-based electrocatalysts

(A)

800 °C

Fe

Acid etching ZIF-8/NF Zn(NO3)2·6H2O 2-methylimidazol CH3OH

300

 = 230 mV

1.3

FeOOH/NPC-NF

(C) 0.30

200

0 1.2

Electrodeposition

NPC-NF

NF blank NPC-NF FeOOH-NF FeOOH/NPC-NF

100

3+

ZIF-8 NPC FeOOH

1.4 1.5 E (V vs. RHE

1.6

Overpotential (V)

(B)

j / mA cm–2

128

0.24

–1

c

0.27

V

1 10

de

m

–1

33.8

mV

dec

0.21 0.18 0.15 0.5

FeOOH-NF FeOOH/NPC-NF

2.0 1.0 1.5 Log [ j (mA cm–2)]

2.5

FIGURE 6.11 (A) Synthetic procedure of FeOOH-NPC-NF, (B) LSV, and (C) Tafel slope of FeOOH-NPC-NF and other control samples. “Adapted with permission [38]. Copyright (2018) John Wiley and Sons.”

in the air), and the pristine as displayed in Fig. 6.10C and D. Co3 O4 in Co3 O4 -CNA offers essential catalytic activity while the carbon contributed with a large active surface area, improved charge/mass transfer, and smooth O2 liberation. Hence, by rational structural design and pyrolysis, highly catalytic active OER electrocatalyst was produced from MOF oxide-carbon composite. On the other hand, the low stability of metal hydroxides at elevated temperatures makes it quite challenging to produce MOF-derived metal hydroxides by the direct pyrolysis process. Commonly, MOF-derived metal hydroxides or oxyhydroxides are obtained by some essential postpyrolysis treatment. Ferric oxyhydroxide (FeOOH) incorporated N-doped carbonized composite was prepared by Li et al. [38]. Fig. 6.11A illustrated the synthetic procedure of the electrode. The ZIF-8 was first grown on Ni-foam to get the enhanced surface area. Then, the composite was pyrolyzed at 800°C in an N2 atmosphere and then undergo an acid etching process to eliminate Zn. Finally, FeOOH was electrodeposited on the composite to obtain FeOOH-containing N-doped porous carbon composite (FeOOH@NPC-NF). In OER testing, the composite showed excellent performance as a catalyst with an overpotential of 230 mV for achieving 100 mAcm−2 and a low Tafel slope of 33.8 mVdec−1 , as shown in Fig. 6.11B and C. The N-doped carbon material does not directly take part in catalytic activity. However, it plays a pivotal role in providing enhanced electrical conductivity, electrolyte diffusion, and to boost the vulnerability of the active spots.

6.3 MOF-based electrocatalysts for OER

6.3.2.2 MOF-derived nonprecious metal chalcogenides-based electrocatalysts for OER Chalcogens are the elements of group 8A in the periodic table. But mostly, the word chalcogen is used to represent sulfides, selenides, tellurides, and polonides. Metal chalcogenides consist of at least one chalcogen anion and one or more of these metallic cations. Metal chalcogenides are one of the widely studied material for OER electrocatalyst in recent years. Various researches revealed that during the OER process, the surface anions of these metal chalcogenides are susceptible to be substituted by oxy (O2− ) and hydroxyl (OH− ) ions [32,39]. This oxygen integration process on the catalyst is called surface regulation and can further increase the OER activity [40]. Hence, competent OER electrocatalysts can be designed by using metal chalcogenides. For designing the OER electrocatalyst through MOF, the precursor must comprise of the respective atoms that are required to generate the anion of the metallic compound. For instance, Co9 S8 is considered to provide excellent electrocatalytic activity [41]. Wu et al. proposed Co9 S8 containing MOF-derived material for improved OER electrocatalyst [42]. CdS was used as the source for S and ZIF-67 as a template to produce Co9 S8 nanoparticles embedded in carbon nanofiber. The supportive CdS nanowires were used to grow ZIF-67, which was later converted to Co nanoparticles by pyrolysis. The CdS was reduced to Cd utilizing reductive carbon material, which later evaporated during the pyrolysis and provided S for Co9 S8 embedded N, S co-doped carbon nanofiber. Fig. 6.12A and B represented the TEM images of the electrode prepared by pyrolysis at 850°C (Co9S8 -NSCNFs). The morphology of the electrocatalyst shows the distribution of a large number of voids along with the Co9 S8 nanoparticles. The porous structure ensures fast electron/mass transfer, electrolyte diffusion, and gas liberation. The electrocatalyst displayed excellent activity towards OER with P10 of 302 mV. An excellent Tafel slope of 54 mVdec−1 was reported, which even surpasses the activity of commercial RuO2 with an equal level of loading as shown in Fig. 6.12C and D. The rational morphological designing is an essential step and could regulate the catalytic performance of the MOF-based carbon material. In this work, CdS and ZIF-67 template produced a synergistic effect on morphology that resulted in exceptional OER activity.

6.3.2.3 MOF-derived nonprecious metal composites for OER In metal-containing composites for OER, metal sites interfacing different materials exhibit a different electronic configuration than the sites present in bulk. Consequently, metal-containing hybrid materials are considered to have improved OER activity. MOF-derived multimetal-based composite was prepared by Hong et al. [43]. The electrocatalyst contains cobalt phosphide, ferric phosphide, ferric carbide supported on the carbon material. For this reason, Fe-Co-containing a dual metallic MOF precursor was used, followed by the P-doping at 300°C. On evaluation, the electrocatalyst showed a P10 of 362 mV in 1 M KOH.

129

CHAPTER 6 Metal-organic framework-based electrocatalysts

(A)

(B)

Co9S8

void

0.34 nm

void

m 9n 0.1 1) 1 d (5

Carbon layer 100 nm

=

2 nm

2 nm

d = 0.17 nm (440)

(D) 80 Co9S8/NSCNFs-750 Co9S8/NSCNFs-850 Co9S8/NSCNFs-950 RuO2 GCE

60 40 20 0 1.2

1.4 1.5 1.6 1.3 Potential (V vs. RHE)

Overpotential (V vs. RHE)

(C) Current density (mA cm–2)

130

1.7

0.36 Co9S8/NSCNFs-750 Co9S8/NSCNFs-850 Co9S8/NSCNFs-950 RuO2

0.34 0.32

104 mV dec–1

–1

60 mV dec

0.30 0.28

54 mV dec–1 61 mV dec–1

0.26 –0.2

0.2 0.6 1.0 1.4 Log (Current density, mA cm–2)

FIGURE 6.12 (A and B) TEM images of Co9 S8 -NSCNFs-850 at different resolutions, (C) LSV plots, and (D) Tafel slope of electrocatalysts in 1 M KOH electrolyte. “Adapted with permission [42]. Copyright (2018) John Wiley and Sons.”

Similarly, Co-Mo dual metal-based carbon material was fabricated by Huang et al. [44]. In this study, Co-Mo-based MOF was pyrolyzed in a nitrogen environment to obtain CoOx -MoC N-doped carbon material. The electrocatalyst displayed excellent activity toward OER having 330 mV overpotential in 1 M KOH. Thus, utilizing different metal-based compounds in the precursor can produce a multimetal composite for enhanced OER activity. In conclusion, OER occurs in a harsh environment and requires excellent stability and increased activity. By rational designing of MOFbased electrocatalysts, high catalytic activity can be achieved.

6.4 MOF-based electrocatalysts for HER Hydrogen is considered as the cleanest source of energy and is one of the rare gas on earth, while water is the most abundant compound on earth. Therefore, generating

6.4 MOF-based electrocatalysts for HER

H2 by the water splitting is a proficient and environmentally friendly approach [45]. The ideal potential for HER is 0 V against reversible hydrogen electrode (RHE.) However, HER activity estimation is based on the onset potential, overpotential, and the Tefal slope. HER is the two-electron transfer reaction where adsorption of hydrogen intermediates [H∗ ] takes place at the active spots. So, the active spots are selected based on H2 adsorption energy (GH∗ ), as discussed earlier. In this section, the application of MOF for HER is addressed.

6.4.1 MOF-derived metal-free carbon-based material for HER In recent years, numerous carbon-based electrocatalysts have been reported with upgrading HER activities. Yet, very few works were done on MOF-based N-doped electrocatalysts without any metals [5]. For HER, the material’s catalytic activity is measured by the Gibbs free energy (GH∗ ) of the H2 intermediates formed during the reaction. The higher the proximity of (GH∗ ) to zero, the higher H∗ adsorption and, in turn, higher HER activity [9]. For example, carbon-based active sites in pure graphene or other carbon structures show high GH∗ value, which results in low H∗ adsorption and lowers HER activity. A recent study shows that N2 contamination in a carbon material upgrades the electronic morphology of the graphene framework. However, various experiments and density functional theory (DFT) studies show that the isolated N-doping is not enough to reduce GH∗ significantly to achieve higher HER activity [16]. Therefore, reports indicate that N-doped carbon materials could not achieve HER performance close to that of precious-metal. For this reason, Lin et al. carried out multi-N doping to a six-carbon atom ring along with Cu assistance to modify the C-C bond and achieved two adjacent graphiticN in the carbon structure [46]. Cu-benzene-1,3,5-tricarboxylate (Cu-BTC) was used as a precursor for producing metal-free multi-N-doped carbon-based electrocatalyst. The synthetic route involves the carbonization of the MOF precursor at ∼600°C in the presence of N2 , followed by two solvothermal reactions. These solvothermal reactions were performed to remove Cu and to dope the carbon material with N by the action of ammonium hydroxide concurrently. The samples were named as S-1 and S2 after 1st and 2nd solvothermal processes, respectively. The obtained materials were passed through the various cycles in cyclic voltammetry to modify the structure of nitrogen in the material and denoted as S-1-cycle and S-2-cycle. Moreover, a control sample (S/FeCl3 ) was also prepared by etching Cu in Cu/C composite with the help of 0.1 M FeCl3 . The electrochemical characterization of the S-2 after 8,000 cycles outperformed other samples with the P10 of just 57 mV, and the Tafel slope of 44.6 mVdec−1 was recorded, suggesting enhanced HER performance. The improved HER activity is the response of the transformed structure of the N-doped carbon by cyclic voltammetry cycles in the presence of Cu. The Cu site present in the material only assisted in modifying the morphology of the material without participating in the H∗ adsorption. Hence, the electrocatalyst can be classified as metal-free. Similarly, another N-doped carbon-based HER electrocatalyst was prepared by Lei et al. [31]. The electrocatalyst was designed by the carbonization of ZIF-8,

131

132

CHAPTER 6 Metal-organic framework-based electrocatalysts

followed by the cathodic polarization treatment for varying time to obtain a range of materials. ZIF-8-C6 indicated the best HER activity with a P10 of 155 mV and the Tafel slope of 54.7 mVdec−1 in a 0.5 M H2 SO4 solution. The material discussed here for HER is different from the material discussed in section 6.3.1, which indicates distinct requirements for both activities.

6.4.2 MOF-derived NPM-based electrocatalyst for HER NPM-based electrocatalysts can be prepared with the help of the MOF technique. The electrocatalysts display high HER performance comparable to the activity of Ptbased electrocatalysts. In this section, commonly studied NPM carbon-based HER electrocatalysts will be discussed that have been derived by the MOF technique.

6.4.2.1 Single-atoms-based metal sites The single-atom dispersion into the carbon materials provide induced HER performance along with superior stability in acidic condition and increase the application efficiency of metal. Recently, MOF techniques are employed to produce highly dispersed single-atom active sites and accomplished high HER activity [47]. Fan et al. introduced the first MOF-derived metallic single-atoms dispersed carbon material for HER electrocatalyst [48]. The fabrication procedure involves the pyrolysis of nickel-MOF, followed by acid leaching. The obtained material was electrochemically activated to achieve single atomic-level isolated Ni sites spread on the carbon material denoted as A-Ni-C. The electrochemical characterization of the sample displayed superior HER activity with the Po of −34 mV. The Tafel slope of 41 mVdec−1 was also recorded and found analogous to commercially available Platinum catalysts. MOF-derived single-atom metallic site for alkaline HER electrocatalysis was also studied. Chen et al. worked on atomically-dispersed W atoms electrocatalyst for HER [47]. The electrocatalyst was denoted as W-SA and displayed excellent activity for HER with superior stability with P10 of 85 mV. The sample showed no noticeable deterioration even after 10,000 CV cycles in the potential range of 0 to −0.15 V. Zhao et al. prepared sulfur assisted N-doped single Co atomic sites-containing (Co-SAC) electrocatalysts from ZnCo-biMOF [49]. The same sample was prepared without the assistance of sulfur and named as Py-ZIF. On evaluation, the samples Co-SAC and PyZIF offered similar P10 of 260 mV in 0.5 M H2 SO4 . Both electrocatalysts displayed no apparent shift in polarization curves even after 2000 cycles, referring to excellent HER stability. Additionally, the Tafel slope of 80 mVdec−1 and 84 mVdec−1 was recorded with Py-ZIF and Co-SAC, respectively.

6.4.2.2 Nonprecious metals and metal alloys-based active sites As discussed in section 6.4.2.1, the nonprecious metal-based single-atom active spots can produce highly efficient electrocatalysts for HER. Similarly, nonprecious metals and metal alloys have also displayed enhanced catalytic activity. Metal and metal alloys nanoparticles can be simply fabricated by carbonization of MOF precursors at high temperatures. During pyrolysis, the metal ions are reduced by reducing

6.4 MOF-based electrocatalysts for HER

agents, such as carbon, and produce high HER activity. Different metals and synthetic routes have been employed to investigate their activity. Xu et al. reported N-doped Ni nanostructures uniformly embedded in graphitic carbon (G-carbon) [50]. The synthetic procedure involves the pyrolysis of Ni-containing MOF precursor [Ni2 (1,4 benzene dicarboxylic acid)2 -(triethylenediamine)] at 800°C under N2 atmosphere. Electrochemical characterizations show increased electron transfer from Ni nanostructures to G-carbon, which reduced the H attraction for Ni and enhanced H affinity for C, resulting in improved HER activity. The low P10 of 205 mV with a Tafel slope of 160 mVdec−1 was observed for Ni/NC-800 in alkaline solution. The sample showed good stability with insignificant decay after 1000 cycles. The combinations of some metal species have been widely studied and believed to have excellent HER performance. The Volcano Curve presented by Norskov et al. explained that some metals, such as Ni, Co, W, Mo, and Fe, exhibit high H∗ adsorption. Whereas, other metals, for example, Cu, Zn, Al, and Ag possess low H∗ adsorption [51]. Therefore, the combination of such metals can produce adequate GH∗ and demonstrate maximum HER performance. Kuang et al. applied this approach to fabricate Co and Cu bimetallic nanoparticles-based N-doped C material [52]. The synthetic procedure involves on the spot growth of ZIF-67 dodecahedra on Cu(OH)2 nanowires with subsequent carbonization at 800°C under argon atmosphere to produce CuCo/NC. The CuCo/NC needed a P10 of 145 mV in 0.5 M H2 SO4 , which is much lower than the corresponding single metal electrocatalysts such as Cu/NC > 450 mV and Co/NC 380 mV. A Tafel slope of 79 mVdec−1 was recorded along with inappreciable current decay after around 8 hours in 0.5 M H2 SO4 .

6.4.3 Metal carbide, phosphides, and chalcogenides MOF derived nonprecious metal-based carbon composites consist of two parts. First, the low-cost metallic part mainly consists of nonprecious metals, which include Mo, W, Co, Ni, Fe, Cu, Zn, and Mn; the other is the nonmetallic part such as B, C, N, S, and P, etc. Each class exhibit different H∗ attraction. Thus, HER performance can be improved by modifying the composition of the material and obtaining moderate GH∗ . Therefore, various metals and nonmetal have been reported for increased HER activity. Mo, Co, and W containing chalcogenide, nitride, and carbide have been widely studied and known for exhibiting excellent HER performance. Xu et al. reported N-doped tungsten carbide nanostructures encapsulated in porous carbon (WC/NPC) [53]. The encapsulation of W(CO)6 was carried out by vapor adsorption treatment (VAT) using a Zn-containing zeolitic metal azolate framework followed by the pyrolysis process, which gives WC nanostructures encapsulated in the carbon material. Electrochemical characterization revealed excellent HER performance of the WC/NPC with high stability. The sample required a P10 of only 51 mV and a Tafel slope of 49 mVdec−1 (Fig. 6.13A and B). A potentiostatic test was done to check the stability at 50 mV for 5 hours leading to a nonobvious change in the LSV curve (Fig. 6.13C).

133

CHAPTER 6 Metal-organic framework-based electrocatalysts

200 150

–20 –30 –40

overpotential: 51 mV WC@NPC W@NPC NPC 20% Pt/C

η (mV)

0 –10

(C) WC@NPC W@NPC 191 mV dec

–1

20% Pt/C

100 50

22 mV dec–1

0

49 mV dec–1

–50 –50 –0.30 –0.25 –0.20 –0.15 –0.10 –0.05 0.00 –0.4 –0.2 0.0 0.2 0.4 0.6 0.8 Potential (V vs. RHE)

lg|j|

WC@NPC

10 0

8 6

j (mA/cm2)

(B)

Current density (mA·cm–2)

(A) Current density (mA-cm–2)

134

4 2

–20 –40 –60 –80 –0.3

0 0

WC@NPC initial WC@NPC after

1

–0.2 –0.1 Potential (V vs. RHE)

2

3

4

0.0

5

Time (h)

FIGURE 6.13 (A) LSV curves at 5 mVs−1 in 0.5 M H2 SO4 , (B) Tafel slope, and (C) current-time curve of WC/NPC (inset HER LSV curves pre- and poststability test). “Adapted with permission [53]. Copyright (2017) American Chemical Society.”

Metal chalcogenides, particularly metal sulfide, selenides, and tellurides, contain metallic cations and negative S/Se anions, which contributes to strong ionic bonding. Additionally, partial filling of d orbitals provides metal chalcogenide the H∗ adsorption capability [54]. Furthermore, the negative S/Se/Te ions create charge polarization, which allows the localization of the partial negative charges at the center of the Se/S/Te structure. This localization increases the affinity of protons at the anionic surfaces and improves HER performance [55]. Among different metal chalcogenides, metal sulfides show semiconductive nature and exhibit lower conductivity than metal Se/Te [56]. However, these issues can be effectively solved by fabricating metal sulfide/C nanocomposites. Huang et al. reported Co-containing bimetallic sulfides (Mx Co3-x S4 ) with three different metals (M), including Zn, Ni, and Cu [57]. The synthetic procedure involves the sulfidation of ZIF precursor expressed as M-Co-ZIF at 350°C under the flow of nitrogen. The electrochemical characterizations indicated the higher performance of all Mx Co3-x S4 as compared to Co3 S4 ·Zn0.3 Co2.7 S4 polyhedra demonstrated the higher HER activity than Co3 S4 with a P10 of only 85 mV and the Tafel slope of 47.5 mVdec−1 , indicating the superiority of the alloying approach (Fig. 6.14A and B). DFT study identified that the inclusion of another metal in Co3 S4 shrank the bandgap and also improved the adsorption of H∗ by optimizing GH∗ (Fig. 6.14C). Similarly, NiSe embedded N-doped carbon nanocomposite was produced by heating Ni-MOF and selenium vapors under the flow of argon at 600°C. On evaluation, a P10 of 123 mV was recorded in 0.5 M H2 SO4 with the Tafel slope of only 53 mVdec−1 [58]. Similar to metal chalcogenides, metal phosphides have been extensively studied for their comparable HER performance. The superior HER activity of metal phosphides is due to the three main advantages. First, the structure of metal phosphides are typically triangular, which provides numerous unsaturated atoms at the surface and results in increased intrinsic catalytic activity. Second, in contrast to metal chalcogenides, metal phosphides contain dual active sites, which means both metal and phosphorus atoms can take part in the HER, improving HER activity [59]. Above all,

6.4 MOF-based electrocatalysts for HER

–20

(A)

(B) 85.3 mV dec–1

Cu0.30Co270S4

–40 –60

0.3

no catalyst Co3S4

Overpotential (V)

–2

Current density (mA-cm )

0

Ni0.30Co2.70S4 Zn0.30Co2.70S4 Pt/C milled Zn0.30Co2.70S4

0.2

Zn0.30Co2.70S4 Co3S4

47.5 mV dec–1

Pt/C

0.1

30.4 mV dec–1

–80 –100 –0.8 –0.7 –0.6 –0.5 –0.4 –0.3 –0.2 –0.1

0.0

0.0 0.0

Potential (V)

(C) Free energy (V)

0.0

–0.2

H+ + e –

H*

0.5

1.0 1.5 –2 log (J (mA cm ))

2.0

2.5

1/2 H2

Pt/C Zn-doped

–0.4

Ni-doped Pristine Cu-doped

–0.6 Reaction coordination

FIGURE 6.14 (A) LSV curve, (B) Tafel slope of Zn0.3 Co2.7 S4 , and other control samples, and (C) diagram of HER free energy at the equilibrium potential of Co3 S4 and various bimetallic-doped Co3 S4 . “Adapted with permission [57]. Copyright (2016) American Chemical Society.”

the higher electronegativity of phosphorus atoms induces partial negative charges on them, which helps in trapping the protons during HER [60]. Taking advantage of these characteristics, Chen et al. prepared Co-containing FeP nanotubes [61]. The CoFecontaining MIL-88B precursor was passed through solvothermal, calcination under air, phosphatization processes under Ar at 100°C, 500 °C, and 300°C, respectively (Fig. 6.15). On evaluation, Co-Fe-P outperformed pristine FeP in different pH range with the overpotential of 66 mV in 0.5 M H2 SO4 , 138 mV in 1 M phosphate buffer solution, and 86 mV in 0.5 M KOH. The sample showed excellent stability even after thousand CV cycles in 0.5 M H2 SO4 . In conclusion, different MOF-derived carbon materials for HER electrocatalysts having excellent activity have been proposed. Some state-of-the-art electrocatalysts showed a P10 of less than 100 mV in an acidic electrolyte. In alkaline media, H2 O and OH− are the typical reacting groups, which have lower conversion kinetics to H∗ than the conversion of H+ to H∗ in acidic media [62]. Therefore, HER activities are usually superior in acidic conditions than in the alkaline electrolyte.

135

136

CHAPTER 6 Metal-organic framework-based electrocatalysts

Fe Fe/Co P O

+

Fe3+ Co2+

+

1,4-BDC

100 °C

Air, 500 °C

Air, 300 °C

Solvothermal

Calcination

Phosphidation

CoFe MIL-88B

CoFeOx

Co-Fe-P

FIGURE 6.15 Schematic diagram of the synthetic approach of Co-Fe-P characterization. “Adapted with permission [61]. Copyright (2019) Elsevier.”

6.5 MOF-based multifunctional electrocatalysts With the development of new energy devices, the need for multifunctional electrocatalysts has been intensified. The use of OER/ORR in metal-air batteries, HER/OER in water electrolyzer, etc. are a few examples of the application of bifunctional electrocatalysts. Since all three reactions have their distinct conditions to take place, careful designing and fabrication can produce one electrocatalyst with multifunctional capabilities. Here, we will discuss some of the multifunctional electrocatalysts.

6.5.1 MOF-derived OER/ORR bifunctional electrocatalysts The rechargeable metal-air batteries are a promising alternative for the world’s energy crises and deteriorating environmental conditions. In metal-air batteries, OER and ORR carry out during the charging and discharging processes, respectively [63]. Hence, efficient OER and ORR activities are essential in metal-air batteries to lower the charge and discharge overpotentials. The activity of OER/ORR relies on the adsorption energy of different intermediates. However, theoretical analysis of the OER/ORR mechanism indicates that different intermediates offer different adsorption energies and are related linearly with each other, which in turn, impedes the individual active site to attain maximum OER/ORR activity [64]. Therefore, careful strategies and proper catalyst selection play an important role in altering scaling relations. The performance of a bifunctional electrocatalyst is broadly measured as the potential difference (E) between the OER overpotential and ORR half-wave potential. Besides the scarcity, precious metals exhibit monofunctional OER and ORR activity, which makes nonprecious metals the best alternative for bifunctional electrocatalysts. Among the other carbon-based materials, Co-containing active materials are

6.5 MOF-based multifunctional electrocatalysts

frequently studied and showed excellent bifunctional activity, which is due to the different oxidation state of Co during OER and ORR [65]. Hou et al. worked on Co-containing N-doped carbon nanotubes decorated on carbon cage for OER/ORR, having different amounts of induced Fe ions [66]. The increment in the Fe ions results in the formation of open-cage superstructures having the surface covered with CoFe-containing carbon nanotube, hence increasing the surface area. Due to this reason, CoFe20-CC displayed outstanding bifunctional activity with the P1/2 of 0.86 V in 0.1 M KOH, the overpotential of 1.52 V in 1.0 M KOH, and the E of 0.66 V. The upgraded catalytic performance of the sample CoFe20-CC is due to the improved surface area and the synergy between the Co and Fe. The same electrocatalyst was also examined as an electrode for zinc air-battery. The sample displayed superior potential gap of around 0.96 V and cyclic efficiency of 54.3% which later improved to the potential gap of around 0.84 V and cyclic efficiency of 58% after 400 cycles. The efficiency of the battery was increased owing to the enhanced infiltration of the electrolyte into the electrode throughout the cycling process.

6.5.2 MOF-derived HER/OER bifunctional electrocatalysts The HER/OER bifunctional electrocatalysts are mainly employed for water splitting applications. Low HER and OER overpotentials are generally required to diminish the energy intake of the electrolyzer. The performance of HER/OER electrocatalysts is commonly evaluated by onset potentials or overpotential gap between HER and OER. Mostly, metal-based MOF-derived materials are widely studied for HER/OER bifunctional electrocatalysts. Recently, Yan et al. fabricated a range of cobalt-nickel bimetallic phosphide nanotubes referred to as Cox Niy P (x and y represent the molar ratio of Co and Ni) [67]. The fabrication procedure involves the oxidation reaction of Co-Ni bimetallic MOF precursors having various Co/Ni molar ratios at 350°C to produce Cox Niy O, followed by the phosphorization at 300°C in N2 to obtain Cox Niy P. On evaluation, the sample Co4 Ni1 P displayed a remarkable electrocatalytic performance for HER and OER in an alkaline solution. The low P10 of 129 and 245 mV were recorded for HER and OER in 1.0 M KOH solution, and the Tafel slope of 61 mVdec−1 was noted. Moreover, an electrolyzer was fabricated using Co4 Ni1 P electrodes as anode and cathode. The electrolyzer achieved 10 mAcm−2 at 1.59 V in alkaline conditions, indicating competitiveness to Pt and RuO2 electrocatalysts. The durability test revealed the superior stability of the Co4 Ni1 P electrocatalyst; even after 50 hours of nonstop functioning, no significant deactivation was noted. The sample Co4 Ni1 P outperformed other control samples owing to its greater surface area, dispersion of the active spots, and highly porous nanotubular structures.

6.5.3 MOF-derived HER/ORR bifunctional electrocatalysts Contrary to HER/OER and OER/ORR, HER/ORR misses the potential application for its bifunctional catalysts. Due to this reason, very few HER/ORR bifunctional

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electrocatalysts derived from MOF have been investigated, and individual performance of HER and OER are evaluated. For example, Wang et al. produced Cu3 P nanoparticles from MOF precursor, enclosed in N, P co-doped carbon shell [68]. On HER evaluation, the sample Cu3 P/NPPC-650 showed a minimum P10 of 89 mV in acidic solution, and the Tafel slope of 76 mVdec−1 was recorded. For ORR, the reduced P1/2 of 0.78 mV was recorded. The performance of the sample was much comparable to the commercial platinum catalyst. The HER/ORR electrocatalysts did not apply in a device for a proper combined evaluation of the performance because of undefined application. Still, they can be used individually in water splitting and metal-air batteries.

6.5.4 MOF-derived HER/OER/ORR trifunctional electrocatalysts The electrocatalysts exhibiting all three catalytic activities such as HER, OER, and ORR can potentially be employed in water electrolyzers, metal-air batteries, or a selfsustaining water splitting system, which is fabricated by combining water splitter and the metal-air battery. However, for such electrocatalysts, optimizing all three activities at once is quite a challenge. Generally, the design strategy for HER, OER, and ORR trifunctional electrocatalysts involves the employment of materials that displayed the bifunctional activities. Therefore, instead of metal-free carbon-based catalysts, inexpensive transition metal-containing carbon electrocatalysts were widely studied for trifunctional electrocatalysts. Recently, Zhang et al. worked on Ni-Fe-containing N-doped carbon nanostructures (NiFe-NPC) using Ni foam and Fe salt [69]. The synthesis procedure involves the pyrolysis of Ni-Fe containing ZIF precursor in the N2 environment at 900°C, with subsequent cooling to 200°C in air. After the pyrolysis, NiFe-NPC displayed the morphology of disintegrated particles interconnected by many nanotubes. NiFeNPC electrocatalysts exhibited a good P1/2 of 0.869 V. A low P10 of 306 mV for OER and 150 mV for HER were observed when tested in alkaline solution and acidic solution, respectively. A water electrolyzer and zinc-air battery were also constructed using the NiFe-NPC electrocatalysts. The water electrolyzer produced 10 mAcm−2 at 1.60 V, whereas, zinc-air battery displayed a specific capacitance of 750 mAhgZn −1 with a superior energy density of 990 Wh kgZn −1 , and outstanding stability of charge/discharge potential even after 295 thousand seconds.

6.6 Summary The advancement in energy-related technologies significantly depends upon the improvement in the electrocatalysts for HER, OER, and ORR. Metal-organic frameworks provide a great technique that promotes the development of different carbonbased materials having innovative morphology and improved performance. The pyrolysis process was frequently adopted to prepare most carbon-based electrocatalysts with different structural advantages. Pyrolysis modifies the organic part of the

References

MOF into carbon material that provides high conductivity to the structure. While the metallic part of MOF vaporizes completely or partially resides in the structure and contributes to the structural porosity. The structure of the electrocatalysts can be altered by rationally design the MOF precursors or by the pyrolysis. Similarly, the composition of electrocatalysts can be enhanced and modified by the proper selection of the precursors, by introducing additional precursor, or by various treatment processes following the pyrolysis. These techniques have been successfully adapted, as discussed in this chapter, to obtain favorable morphology with promising HER, OER, and ORR activity.

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Heteroatom-doped graphene-based electrocatalysts for ORR, OER, and HER

7

Xun Cui a,b, Likun Gao a,c, Yingkui Yang b and Zhiqun Lin a a

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, United States, b Key Laboratory of Catalysis and Energy Materials Chemistry of Ministry of Education and Hubei Key Laboratory of Catalysis and Materials Science, South-Central University for Nationalities, Wuhan, China, c Key Laboratory of Bio-based Material Science and Technology of Ministry of Education, Northeast Forestry University, Harbin, China

7.1 Introduction Electrocatalytic redox reactions are the foundation of diverse emerging electrochemical energy conversion technologies [1–5]. Thereinto, the electrochemical conversions between oxygen (O2 ), hydrogen (H2 ), and water (H2 O), such as oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER), have garnered longstanding and considerable interests, for their supreme importance in many clean and sustainable energy processes, such as hydrogenoxygen fuel cells and water electrolysis [6–10]. As schematic illustrated in Fig. 7.1, a water electrolytic cell combined with a hydrogen-oxygen fuel cell result in an idea water energy cycle [11]. The clean H2 which produced from water electrolysis by HER and OER could further serves as the fuel source to generate electricity through the hydrogen oxidation reaction (HOR) and ORR in a hydrogen-oxygen fuel cell. Particularly, the ever-growing demands of clean energy and rising global warming caused by carbon dioxide (CO2 ) emissions as well as the environmental pollution issues render such electrochemical processes as attractive for renewable energy generation. In order to achieve large-scale practical commercialization of these energy technologies, low-cost and high-efficiency electrocatalytic materials for ORR, OER, and HER are required to decrease the activation energy barriers for these sluggish electrochemical reactions and increase the energy conversion efficiency [12–15]. To attain satisfactory performance of these gas-involving electrocatalytic reactions (i.e., ORR, OER, and HER), the electrocatalytic materials should be able to guarantee the rapid charge transfer and fast mass transport, and provide rich threephase interfaces for surface electrocatalytic reaction [16]. In this regard, graphene and Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00013-7 Copyright © 2022 Elsevier Inc. All rights reserved.

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(A)

(B)

FIGURE 7.1 Schematic representation of clean and sustainable energy related applications, that is, OER and HER in water electrolytic cell (A), and ORR in hydrogen-oxygen fuel cell (B). (Reproduced from Ref. [11] with kind permission of © 2014 Elsevier).

its derivatives (i.e., heteroatom-doped graphene), with large electrical conductivity, excellent mechanical strength, tunable electronic structure, and high specific surface area, can be promising alternative materials to create high-efficiency electrocatalysts for implementation in fuel cells and water electrolysis [17–20]. Currently, the mosteffective active electrocatalytic materials for ORR, OER, and HER are still noble metal-based compounds (e.g., Pt/C for HER and ORR, RuO2 and IrO2 for OER, etc.) [21–24]. As a result of the scarcity and high price of aforementioned noble metalbased materials, a variety of noble metal-free electrocatalysts were studied and recognized as potential candidates for ORR, OER, and HER [25–27]. In the past decade, the outstanding features of pristine graphene (one-atom-thick layer with sp2 hybridized carbon atoms in a hexagonal arrangement) have been fully understood and wellacknowledged through massive investigations [28–30]. Though the lack of intrinsic bandgap and the electrochemical inertia largely restrict the practical use of pristine graphene in electrocatalysis, numerous approaches for surface/structural modification can be employed to open the bandgap in graphene and tailor the electronic structure and electrochemical activity, effectively making the insert surface electrocatalytically active [31, 32]. As previously verified, the electrocatalytic performance of graphene can be significantly improved by microstructure control, molecular doping, as well as the combination with other metal-based nanoparticles via forming strong electroniccoupling interfaces [33–35]. Other than aforementioned approaches, doping graphene with various heteroatoms (e.g., nitrogen (N), sulfur (S), boron (B), phosphorous (P), etc.), that is, substituted or covalently bonded the graphitic carbon atoms with foreign atoms, is also a practical strategy to cause the electron modulation and tune the electronic properties of the catalysts [36–38]. Indeed, the improved electrocatalytic activity of heteroatomdoped graphene for ORR, OER, and HER have been widely explored in the past few years. It also has been demonstrated that the co-doped graphene (with at least two

7.2 Graphene and heteroatom-doped graphene-based materials

types of heteroatoms) yields even higher electrocatalytic activity, which probably can be ascribed to the synergistic effects [39–41]. Whereas, the current performance is still very challenging for large-scale applications, which makes it crucial to investigate the reaction mechanisms and gain more insightful understanding into the intrinsic origins of improved electrocatalytic performance and subsequently guide the exploration of more efficient heteroatom-doped graphene-based electrocatalytic materials. In this chapter, we first give an overview of graphene and heteroatom-doped graphenebased materials, covering the properties, origin of electrocatalytic activity, and various advanced synthetic strategies. Then the most recent advances regarding heteroatomdoped graphene-based electrocatalytic materials for ORR, OER, and HER are then performed. Particular attentions are paid to the structure-property correlations and intrinsic origins of improved electrocatalytic activity. At the end, future perspectives regarding the challenges and opportunities awaiting this research area are briefly proposed and discussed.

7.2 Overview of graphene and heteroatom-doped graphene-based materials Graphene, which was first prepared through a “Scotch tape” approach in 2004, has attracted massive research attention in various research areas since then due to its unique structure and outstanding physicochemical features [42]. In 2010, the Nobel Prize in Physics was awarded to both Andre Geim and Konstantin Novoselov for their path-breaking contributions to the graphene [43]. Generally, pristine graphene with two-dimensional (2D) monolayer and multiatomic π -π conjugated structure possesses unique physicochemical characteristics in terms of optical, electronic, thermal, mechanical, and chemical properties that are different from other forms of carbon materials [44, 45]. Ever since this fascinating material was successfully prepared, the possibility of doping heteroatoms into the carbon lattices of graphene opens up the opportunities for the exploration and design of novel materials with similar geometry while unique chemical composition, electronic structure, and electrocatalytic property. Up to now, heteroatoms including N, S, B, P, etc., have been incorporated into graphene by single, binary, or even ternary doping, resulting in the generation of a new kind of heteroatom-doped graphene-based material and thus displaying enhanced electrocatalytic performance toward ORR, OER, and HER [46– 49]. In this section, we will briefly present the structures and characteristics of the pristine graphene and heteroatom-doped graphene-based materials.

7.2.1 Graphene Graphene is a single layer (i.e., monolayer) of carbon atoms arranged in a 2D honeycomb lattice (Fig. 7.2A). It is in fact an allotrope of carbon in the shape of a flat plane of sp2 -hybridized carbon atoms with a covalent bond (carbon-carbon) length of

147

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CHAPTER 7 Heteroatom-doped graphene-based electrocatalysts

(A)

(B)

FIGURE 7.2 (A) The structural model and (B) aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) image of defect-free graphene (scale bars: 5 Å). (Reproduced from Ref. [51] with kind permission of © 2011 Nature Publishing Group).

1.42 Å (Fig. 7.2B) [50]. In an idea graphene sheet, each carbon atom is covalently bonded with three nearest neighbors by three σ -bonds and donates one electron to the conduction band, leading to the formation of a delocalized network of π electrons over the whole sheet [42]. Graphene has attracted world-wide attention over the last few years due to its unprecedented optical, electrical, thermal, and mechanical characteristics that are close to or even exceed those highest record ever achieved in any other materials. Graphene is a zero-gap semiconductor with ultrahigh carrier mobility (˜10000 cm2 V−1 s−1 at relativistic speed of ˜106 m s−1 ) at room temperature due to its overlapped valence and conduction bands, signifying excellent electrical conductivity and high probability to be used as catalyst supports or catalyst itself [42]. As a result of its high optical transparency of ˜97.3%, graphene is almost optically transparent in the visible region [52]. Significantly, it has been claimed in the literature that graphene is the thinnest (one atom thick) and lightest (˜0.77 mg m−2 ) while strongest material (Young’s modulus of 1.0 TPa and intrinsic strength of 130 GPa) discovered to man [50]. Graphene also possesses extremely high thermal conductivity (˜5 × 103 W m−1 K−1 ) and very high thermal stability [29, 53]. Moreover, the ultrahigh ratio of lateral to thickness of graphene results in an extremely high specific surface area of ˜2630 m2 g−1 [54]. The 2D geometrical configuration of graphene with the desired high surface area may possess high adsorption capacity to the reactants and/or reaction intermediates via strong π electronic interactions, allowing to be used in the catalysis [55]. Furthermore, chemical doping/modifying graphene with heteroatoms is a practical strategy to modulate the electronic structure and tailor the chemical reactivity, thus giving rise to new functions and affording new heteroatom-doped graphene-based materials for diverse applications. Considerable attempts have been made to the development of heteroatom-doped graphene-based materials, focusing on taking full advantage of their excellent properties in their application toward electrocatalysis.

7.2 Graphene and heteroatom-doped graphene-based materials

Table 7.1 A comparison of size, electronegativity, valence electron number, and electronic configuration of different dopants for graphene. Element

N

S

B

P

C

Van der Waals radius (pm) Electronegativity Valence electron number Electronic configuration

155

180

180

195

170

3.04 5

2.58 6

2.04 3

2.19 5

2.55 4

1s2 2s2 2p3 1s2 2s2 2p6 3s2 3p4 1s2 2s2 2p1 1s2 2s2 2p6 3s2 3p3 1s2 2s2 2p2

7.2.2 Heteroatom-doped graphene-based materials Generally, the intrinsic catalytic activity of an electrocatalyst is basically determined by its adsorption/desorption behavior toward the reactants and key reaction intermediates involved in the electrocatalytic reaction. To pursue highly active electrocatalysts, the engineering of electronic properties is required to optimize the binding energies of reactants and key intermediates on the electrocatalysts [56]. As aforementioned, pristine graphene is actually inert for almost of all the electrocatalytic reactions, including ORR, OER, and HER. Incorporating heteroatoms into the graphene lattice has been regarded as one of the most effective approaches to alter the local electronic structures of graphene and produce active sites for electrocatalysis, thus improving the reaction activity of system and enhancing the electrocatalytic performance [57]. Fundamentally, the tuning of electronic properties is closely related to the introduced electron-withdrawing/electron-donating effects and structural defects caused by the foreign dopants [58]. The dopants can also influence the spin density and atomic charge distribution and increase the number of active sites of graphene, thus enhancing the electrocatalytic activity significantly [59]. Notably, these heteroatoms will also influence the density of state near the Fermi energy level and subsequent the electronic conductivity of heteroatoms-doped graphene-based materials. However, depending on the type and concentration of dopants, the electronic/electrocatalytic properties may be aggravated. Therefore, an advisable balance must be made between the increased mobility and disrupted conduction pathways caused by the defects. Among different types of heteroatoms, N, S, B and P have already been identified as the most suitable elements for graphene doping as a result of their similarities with the carbon atoms, as shown in Table 7.1 [20]. The most commonly adopted and widely investigated dopant in the heteroatomsdoped graphene materials is N due to its similar size (in terms of Van der Waals radius) in comparison with that of carbon. The significantly higher electronegativity of N dopant (3.04) than that of carbon (2.55) causes an apparent polarization in the graphene network, hence modifying its physicochemical properties [60]. There are generally several N-bonding configurations in the graphene, including pyridinicN, pyrrolic-N, quaternary-N, and N-oxides of pyridinic-N as shown in Fig. 7.3A

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CHAPTER 7 Heteroatom-doped graphene-based electrocatalysts

(B) 0.8 (A)

Pridinic N (398.6eV) Quaternary N (401.3eV) + Nitrile N Oxidized N (399.5eV) (402~405eV) + Pyrrolic N + (400.5eV)

0.4 0 E-EFo (eV)

150

−0.4

Ab initio TB

−0.8 −1,2 −1.6 −2

M

Г

K

M0

5 10 DOS (u.a.)

15

FIGURE 7.3 (A) Schematic representation of different N-bonding configurations in graphene (Reproduced from Ref. [61] with kind permission of © 2013 John Wiley & Sons, Inc.), (B) Ab initio (black lines) and tight-binding (green lines) band structures (left) and density of state (right) of N-doped graphene (Reproduced from Ref. [63] with kind permission of © 2013 American Chemical Society).

[61]. The pyridinic-N (sp2 hybridization) refers to N atoms that bonded with two neighboring carbon atoms at the edges or defects of graphene network and donates one π electron to the system. The pyrrolic-N (sp3 hybridization) bonds to two carbon atoms and contributes two π electrons to the system, which is incorporated into a fivemembered heterocyclic ring. The quaternary-N (also called graphitic-N) is doped into the graphene by substituting one carbon atom within a hexagonal ring. In addition, N-oxides of pyridinic-N is bonded with two carbon atoms and one oxygen atom [62]. By N doping, the density of state near the Fermi energy level will be suppressed by substitutional N, leading to bandgap opening between the conduction band and valence band (Fig. 7.3B) [58, 63]. B (2s2 2p1 ) element with only one less valence electron to the neighboring carbon (2s2 2p2 ) is highly amenable for graphene doping. Generally, the homogeneous Bsubstituting is easier to be achieved in comparison to the in-plane N-doping as a result of the lower induced strain energy [64]. Since the B dopants form sp2 hybridization in the graphene lattice, the planar structure of graphene can be reserved. However, the slightly larger bond length of B-C (˜1.50 Å) than that of C-C (1.42 Å) in the graphene lattice, resulting in minute alter for the lattice parameters of B-doped graphene [65]. B-doping induces a charge polarization between the electron-withdrawing B atoms and the neighboring carbon atoms. B-doping also introduces larger density of state near the Fermi energy level and provides more holes to the valence band of graphene, leading to both the increased carrier concentration and improved conductivity [58, 66]. Unlike the other dopants, S element has an electronegativity (2.58) very close to that of carbon (2.55), which leads to negligible polarization of the S-C bonds in

7.2 Graphene and heteroatom-doped graphene-based materials

FIGURE 7.4 The geometrical model of P-doping configuration. The gray and pink spheres represent the carbon and P atoms, respectively. The bond distances are in Å. (Reproduced from Ref. [70] with kind permission of © 2013 Elsevier Publishing Group).

S-doped graphene. The extremely larger bond length of the S-C bond (1.78 Å) as compared to the C-C bond (1.42 Å) can easily create defect sites in graphene network. Several S-bonding groups including sulfate, sulfide (thiophene), and sulfonate groups presenting in S-doped graphene have been well demonstrated [58]. Notably, the sulfide-rich S-doped graphene possesses higher conductivity and faster charge transfer ability than that of sulfonate-rich S-doped graphene. In addition, S dopant also renders graphene a higher density of spin, edge strain, and charge delocalization than that of N dopant due to its larger size and greater polarizability compared with N dopant [67]. Theoretical investigation has revealed that the thiophene-like structure formed in the S-doped graphene has a positive effect on the electronic properties of graphene [68]. Although P is an element of the nitrogen group and often exhibits similar chemical properties to the N element because of the same valence electron number, however, P-doping generally produces more structural defects in the network due to the larger size and additional orbital of P (3s2 3p3 ) when compared with N. The electronegativity of the P element (2.19) is obviously smaller than that of the N element (3.04), thus the charge polarization caused by P dopant is opposite to that of the N dopant and favors the sp3 -orbital configuration [69]. P-doping is energetically more favorable than N-doping due to its lower formation energy [20]. The larger P-C bond length (1.77 Å) than that of C-C sp2 bond (1.42 Å) combined with the difference in bond angles drives P to protrude from the graphene network, leading to a pyramidal-like bonding configuration with three carbon atoms (Fig. 7.4) [70].

7.2.3 Synthesis of heteroatom-doped graphene-based materials Numerous strategies have been proposed for the preparation of heteroatom-doped graphene-based materials. These heteroatom doping strategies can be broadly classified into in situ doping approach and post-treatment approach. Generally, the in situ doping approach that simultaneously performs graphene preparation and the heteroatom incorporating contains ball milling and chemical vapor deposition (CVD), while the post-treatment approach includes wet chemical method, high-temperature thermal annealing, plasma treatment, and arc-discharge method. Recent published

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CHAPTER 7 Heteroatom-doped graphene-based electrocatalysts

literature has carefully summarized and reviewed the preparation of heteroatomdoped graphene-based materials [19, 20, 31]. Herein, in this chapter, we only present a comparative list of the most commonly used synthesis method covering the doping precursors, doping element, and the major advantages/limitations, as shown in Table 7.2.

7.3 Heteroatom-doped graphene-based materials as electrocatalysts for ORR, OER, and HER The thermodynamic equilibrium potentials of ORR/OER and HER are 1.23 V (vs. RHE) and 0 V (vs. RHE), respectively, where theoretically the electrochemical reactions take place. Nevertheless, all of aforementioned electrochemical reactions (i.e., ORR, OER, and HER) cannot occur at their thermodynamic equilibrium potentials, because of the extra energy input should be provided to overcome the reaction energy barriers. As a consequent, the kinetic rates of these electrochemical reactions are greatly dependent on the applied potentials. The extra potential required besides the thermodynamic equilibrium potential to accelerate these electrochemical reactions is defined as overpotential (η). Therefore, high-performance electrocatalysts are required to reduce energy consumption (i.e., decrease the η) and improve energy conversion efficiency. The typical electrochemical linear sweep voltammetry (LSV) curves of ORR, OER, and HER are illustrated in Fig. 7.5. Obviously, a small η is highly desirable to deliver a large electrocatalytic current density. The potential to obtain a current density of 10 mA cm−2 (Ei=10 ) is a widely adopted parameter to describe the electrocatalytic activity of both OER and HER. The half-wave potential (E1/2 ) is the most important parameter to indicate the electrocatalytic activity of ORR. The electrocatalytic activity of heteroatom-doped graphene for ORR, OER, and HER have already been widely studied by both theoretical and experimental means in the last several years. It has been well acknowledged that the origin of the electrocatalytic activity for heteroatom-doped graphene-based materials can be ascribed to the modulation of electronic properties of the extensive conjugated sp2 -sp2 network and/or the delocalized π -orbital electrons in the graphene plane with the presence of heteroatom dopants. Particularly, it is suggested that the incorporated heteroatoms can perturb the electronic structure of the graphene lattice in graphene, resulting in partially positively or negatively charged sites without substantially compromising the electronic conductivity. In this section, we will briefly present the representative examples of the heteroatom-doped graphene-based materials for ORR, OER, and HER applications.

7.3.1 Heteroatom-doped graphene-based materials for ORR Due to the sluggish reaction kinetics of ORR, the development of high-efficiency electrocatalytic materials is one of the most critical issues to boost the overall

Method

Precursors

Doping element

Major advantages/limitations

Ref.

Ball milling

CH4 N2 O/graphite Sulfur/graphite CH4 /NH3 H3 BO3 /polystyrene Sulfur/C6 H14

N (3.15 at%) S (4.94 at%) N (8.9 at%) B (4.3 at%) S (0.6 at%)

[71, 72]

Hydrazine/graphene oxide (GO) Urea/GO GO/NH3 H2 S/GO Ionic liquid/GO BCl3 /GO N2 /GO CS2 /graphene Graphite/NH3 Graphite/B/B2 H6

N (4.5 at%) N (10.13 at%)

Large scale production; simple operation/Doping occurs only at edges Simultaneous growth and doping; beneficial to doping level control/Complex process and high energy consumption; toxic waste gases generation Low energy consumption; large scale production/toxic liquid waste generation Beneficial to doping level control; wide choices of dopant precursors /High energy consumption

[78, 79, 80, 81]

Low energy consumption; rapid preparation/Low yield Large scale production/Low doping level; High voltage or current involved

[82, 83]

CVD

Wet chemical method

High-temperature thermal annealing

Plasma treatment Arc-discharge method

CVD, chemical vapor deposition.

N (8 at%) S (1.2–1.7 at%) P (1.16 at%) B (0.88 at%) N (2.51 at%) S (0.3 at%) N (1 at%) B (3.1 at%)

[73, 74, 75]

[76, 77]

[84, 64]

7.3 Heteroatom-doped graphene-based materials as electrocatalysts

Table 7.2 Summary and comparison of doping techniques for the preparation of heteroatom-doped graphene-based materials.

153

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CHAPTER 7 Heteroatom-doped graphene-based electrocatalysts

OER

overpotential

0 V vs. RHE HER

overpotential

ORR

1.23 V vs RHE

diffusion limiting current

FIGURE 7.5 Typical polarization curves for ORR, OER, and HER. (Reproduced from Ref. [85] with kind permission of © 2021 Elsevier Publishing Group).

performance of fuel cells. In the last several years, heteroatom-doped graphene-based materials have drawn massive interest as a result of their remarkable electrocatalytic performance toward ORR. As discussed above, the heteroatoms may exist in the doped graphene in different configurations, producing X-C as the electrocatalytic active sites for ORR. Compared with B-, S-, or P-doped graphene, the configurations of N are relatively complex, as four types of N forms (quaternary-N, pyridinicN, pyrrolic-N, and N-oxides of pyridinic-N) may concurrently present in N-doped graphene as a result of the comparable size of N and carbon atoms and the strong covalent bonds between them. Dai et al. first prepared and reported N-doped graphene as an electrocatalyst for ORR in 2010 (Fig. 7.6A) [62]. In this work, the N-doped graphene displayed good electrocatalytic activity, long time stability, and high tolerance to the methanol crossover effect and CO poison effect (Fig. 7.6B–D). The steadystate electrocatalytic current density of N-doped graphene electrode is significantly larger than that of Pt/C catalyst in the alkaline solution (Fig. 7.6B) [62]. The activity enhancement of graphene with N doping was also observed in other studies [86, 87]. It was proposed that both pyridine-N and pyrrolic-N in the N-doped graphene play important roles toward ORR. In addition to N-doped graphene, various heteroatomdoped graphene-based materials such as B-doped graphene, S-doped graphene, and P-doped graphene, synthesized through different strategies, were also explored as ORR electrocatalysts [80, 88, 89]. For example, Vineesh et al. synthesized the Bdoped graphene via a high-temperature thermal annealing method, which displayed an obviously enhanced ORR performance compared with that of the pristine graphene (Fig. 7.7A and B) [88]. Relevant theoretical study attributes the enhanced ORR performance to the positively charged B atoms which can interact with the negatively charged O atoms, thus resulting in chemisorption [90].

7.3 Heteroatom-doped graphene-based materials as electrocatalysts

FIGURE 7.6 (A) TEM image and corresponding electron diffraction pattern of N-doped graphene prepared via CVD. (B) LSV curves of graphene, N-doped graphene and Pt/C for ORR. (C) Chronoamperometric responses of Pt/C and N-doped graphene at −0.4 V. (D) Chronoamperometric responses of Pt/C and N-doped graphene to CO at −0.4 V. The arrows in (C) and (D) indicate the addition of 2% (w/w) methanol and 10% (v/v) CO into the systems, respectively. (Reproduced from Ref. [62] with kind permission of © 2010 American Chemical Society).

To further improve the ORR performance, the co-doping of graphene with two or more different kinds of heteroatoms that enables more complex electrical modifications of graphene was then proposed to take full advantage of the synergistic effect between different heteroatoms. Among various heteroatom dopants, N and B are acknowledged as the best codopants due to their similar size and different valence electron number. The strong interactions between the N atoms and neighboring B atoms ensure the N atoms occupying suitable sites for the improvement of electrocatalytic performance. Besides, the N,B-codoping also enables the activation of the electrocatalytically inert sites in corresponding single-doping configurations. Accordingly, the free energy barriers for O2 adsorption and the subsequent reduction steps can be considerably reduced, thus resulting in the enhanced ORR activity. For example, Jin et al. prepared the N,B-codoped graphene with urea, boric acid, and polyethylene

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CHAPTER 7 Heteroatom-doped graphene-based electrocatalysts

FIGURE 7.7 (A) HR-TEM image and corresponding electron diffraction pattern of B-doped graphene. (B) LSV curves of graphene, B-doped graphene and Pt/C for ORR. (Reproduced from Ref. [88] with kind permission of © 2015 Wiley-VCH.) (C) TEM image of N,B-cooped graphene. (D) LSV curves of various N,B-cooped graphene-based materials for ORR. (Reproduced from Ref. [91] with kind permission of © 2014, American Chemical Society).

glycol as the precursors through a high-temperature thermal annealing treatment [91]. The obtained N,B-codoped graphene displayed the superior electrocatalytic activity compared with that of commercial-available Pt/C catalyst as a result of the synergistic effect between N and B (Fig. 7.7C and D). Similarly, N,P-codoped graphene and N,Scodoped graphene were also explored and exhibited good catalytic activity toward ORR [92, 93].

7.3.2 Heteroatom-doped graphene-based materials for OER The OER plays a critical role in electrochemical water-splitting and rechargeable metal-air batteries due to its sluggish reaction kinetics. Generally, high-performance electrocatalysts are required to accelerate the kinetic rate and lower the overpotential. Heteroatom-doped carbon-based materials have been demonstrated to be efficient as

7.3 Heteroatom-doped graphene-based materials as electrocatalysts

FIGURE 7.8 (A) TEM image of NDGs-800. (B) Raman spectra of NDGs prepared at different temperatures. (C) High-resolution N 1s XPS survey spectrum of the NDGs-800. (D) LSV curves of different samples for OER. (Reproduced from Ref. [96] with kind permission of © 2018, American Chemical Society).

electrocatalysts for catalyzing OER [94, 95]. Wang’s group reported the construction of pyridinic-N dominated doped defective graphene (NDGs) through a wet chemical route followed by an annealing process [96]. The as-prepared NDGs-800 exhibited the largest proportion of pyridinic-N (47.9%) and rich structural defects in graphene as revealed by the XPS and Raman characterizations (Fig. 7.8A and B). Accordingly, the NDGs-800 displayed an excellent electrocatalytic activity with a low overpotential of 0.28 V for OER (Fig. 7.8C and D). Density functional theory (DFT)-based simulation results indicated that the synergetic effect between quadri-pyridinic N-doped carbon sites and the vacancy defects could significantly enhance the electrocatalytic OER performance. The dual-doping approach has also been applied to improve the electrocatalytic OER performance. Li et al. reported the synthesis of N,P-codoped graphene/carbon nanosheets (N,P-GCNS) via the high-temperature pyrolysis of a dried hydrogel composed of GO, polyaniline (PANi), and phytic acid (PA) [97]. The as-prepared N,P-GCNS displayed outstanding electrocatalytic activity toward OER with an onset potential of 1.32 V and a current density of 70.75 mAcm−2 at an applied potential

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of 1.9V. The superb electrocatalytic performance is simultaneously ascribed to the synergistic effects between the N and P dopants, the fully exposed active sites, the high conductivity, and the hierarchical porous structure for sufficient contact and rapid transfer of mass. In addition, the heteroatom-doped graphene, in fact, has been widely employed as a conductive support with high specific surface area for the incorporation of transition metal-based nanomaterials [31, 98, 99]. It has been proven that the performance of aforementioned heteroatom-doped graphene-based hybrids strongly correlates to the type of the metal-based nanostructures, indicating a dual-active-center mechanism of which the metal sites act as the main contributor and heteroatom doped-carbon species act as additional centers for OER. An appropriate integration of heteroatomdoped graphene with transition metal-based nanostructures can deliver a large number of electrocatalytic active sites and ensure a high-efficiency electron and mass transfer, which can lead to increased electrocatalytic activity toward OER.

7.3.3 Heteroatom-doped graphene-based materials for HER The widely employed metal-based HER electrocatalyst generally suffer from the intrinsic corrosion and oxidation sensitivity in acidic solution. To this end, metal-free carbon-based materials, such as graphene derivatives, provide attractive candidates to metal-based electrocatalysts due to the high accessibility of the constituent elements and the lower cost. However, the electrocatalytic performance of metal-free carbonbased materials is generally inferior to that of traditional metal-based electrocatalysts. Herein, we provide a brief overview of the heteroatom-doped graphene-based HER electrocatalysts. The doping of graphene with B is a promising approach to boost their HER activity. Sathe et al. developed a wet chemical synthetic strategy to prepare B-doped graphene and studied its HER performance [100]. They realized the controllable substitution of carbon atoms with B dopant through a commercially available borylation reagent (BH3 -THF). Compared with the pristine graphene, the B-doped graphene electrocatalyst exhibited a lower onset potential of 0.2 V and a smaller Tafel slope of ˜99 mV dec−1 (Fig. 7.9). Besides, this B-doped graphene possessed an excellent durability with negligible loss of the current density after 18 hours continuous testing under acidic condition. Although the doping of heteroatoms into graphene can undoubtedly enhance its intrinsic electrochemical activity, however, the electrocatalytic performance is still unsatisfactory for the single heteroatom-doped graphene. Recently, numerous studies have revealed that the codoping of graphene with two different dopants could generate additional synergetic coupling effects, which can activate more neighboring carbon atoms to upgrade their electrocatalytic activity [101]. For instance, Zhang et al. employed a template-free strategy to synthesize N,P-codoped three-dimensional (3D) porous graphene through the high-temperature pyrolysis of the mixture of melaminephytic acid supramolecular aggregate (MPSA) and GO [102]. The constructed 3D

7.3 Heteroatom-doped graphene-based materials as electrocatalysts

FIGURE 7.9 (A) TEM image of B-doped graphene, (B) LSV curves of B-doped graphene (B-SuG), defective graphene (DeG), and GC electrode, inset shows the corresponding Tafel plots. (Reproduced from Ref. [100] with kind permission of © 2017, Royal Society of Chemistry).

FIGURE 7.10 (A) TEM image of MPSA/GO-1000. (B) LSV curves of various samples for HER in 0.5 M H2 SO4 . (Reproduced from Ref. [102] with kind permission of © 2016, Wiley-VCH).

porous structure comprised of abundant ultrathin N,P-codoped graphene nanosheets (Fig. 7.10A) favored the fast transport of both charge and mass. As a result, the champion sample prepared at 1000°C (MPSA/GO-1000) showed the significantly improved HER activity in an acidic solution with a small onset potential of only 60 mV (60 mV higher than that of Pt/C) (Fig. 7.10B). In addition, Ito et al. reported the synthesis of N,S-codoped 3D graphene through a CVD approach [103]. The best sample (NS-500) displayed a superior HER performance with an operating potential of 0.28 V to achieve the current density of 10 mA cm−2 and a Tafel slope of 80.5 mV dec−1 , which are comparable to the best Pt-free HER electrocatalyst (2D

159

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MoS2 ). A similar study reported by Zhou et al. also supported the superiority of the dual-doped graphene for the improvement of HER activity [104]. Significantly, Qiao et al. explored the effect of doping graphene with either one or two elements based on both experiments and DFT-based calculations [105]. They claimed that the most active electrocatalytic sites are those carbon atoms adjacent to the dopant atoms in both single-doped and double-doped graphene materials. Based on their study, it is predicted that tailoring the doping level and the surface area of the graphene-based materials will afford a metal-free heteroatom-doped graphene-based material with activity rivaling the metal-containing benchmarks.

7.4 Summary and perspective Heteroatom doping endows graphene with various new physicochemical properties, making the heteroatom-doped graphene-based materials promising candidates for electrocatalytic energy conversion due to their high electrical conductivity, unique electronic properties, and high specific surface area. In this chapter, we present an overview of the properties, origin of electrocatalytic activity, and various advanced synthetic strategies in graphene and heteroatom (i.e., N, B, S, and P)-doped graphenebased materials. Besides, the most recent advances regarding electrocatalytic applications of heteroatom-doped graphene-based materials toward ORR, OER, and HER have been briefly introduced through enumerating typical examples. The heteroatom (i.e., N, B, S, and P)-doping of graphene could effectively modify the electronic properties and construct the electrocatalytic active sites by inducing charge and spin densities on the neighboring carbon atoms, accordingly influencing the adsorption and desorption properties for reactants, key intermediates, and products on the surface of heteroatom-doped graphene, boosting those electrocatalytic processes such as ORR, OER, and HER. Despite the tremendous progress has been obtained in the field of heteroatomdoped graphene materials for electrocatalytic ORR, OER, and HER, there are still some challenges. (1) Various doping configurations of heteroatoms play different roles during the electrocatalytic process, however, the heteroatom-doped graphenebased materials usually containing several doping configurations simultaneously. The controllable preparation of the heteroatom-doped graphene-based materials with only a single doping configuration is highly desirable to explore the specific effect of a particular doping configuration on the electrocatalytic activity. (2) Despite the currently reported electrocatalytic performance of heteroatom-doped graphene-based materials is extremely close to that of noble metal-based electrocatalysts, it is still a great challenge to further improve their electrocatalytic activity. (3) The reaction mechanisms are still inconclusive in many electrocatalytic systems as a result of the complex doping configurations and competence of several types of active sites. Therefore, with the development of new controlled synthetic technologies, the reaction

References

mechanisms need to be further investigated. (4) The current synthesis approaches generally require either high temperature or harsh conditions, which hampered the largescale production of heteroatom-doped graphene materials. Therefore, it is highly desirable to develop a facile, green, and scalable preparation strategy for the synthesis of heteroatom-doped graphene-based materials. In summary, more theoretical and experimental work is still needed for the rational engineering of heteroatom-doped graphene-based materials for future electrochemical energy devices.

Acknowledgments We would like to acknowledge the financial support from the National Natural Science Foundation of China (Grant 52003300).

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Metal-containing heteroatom doped carbon nanomaterials for ORR, OER, and HER

8

Álvaro García, Jorge Torrero, María Retuerto and Sergio Rojas Grupo de Energía y Química Sostenibles Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain

8.1 Introduction Electrochemical devices such as fuel cells, electrolyzers, and batteries are the technologies of choice replace combustion engines. The last years have witnessed tremendous advances in these technologies and electric vehicles based on advanced batteries (BEV) or fuel cells (FCEVs) are commercially available now. A number of studies discussing the technology of choice are available in the literature [1–3]. In addition, the recognition that green hydrogen, that is, the hydrogen obtained from renewable electricity, can be applied in several key sectors including transportation, industry (steel, cement…) and heating has triggered the interest in the production of hydrogen from water electrolysis with renewable electricity [4]. A deeper market penetration of electric vehicles, either BEV or FCEV ones, is limited by the economics, scarcity, and ill distribution on Earth of the critical raw materials (CRM) used in the electrodes, in particular of the platinum group metals (PGM) used as electrodes in low-temperature proton exchange membrane fuel cells (PEMFCs) and proton exchange membrane water electrolyzers (PEMWEs). Fuel cells are devices that generate a potential difference (work) by facing two redox reactions, namely the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR). These reactions only proceed at measurable rates in the presence of catalysts, especially the latter. The most active catalysts for the ORR in acid electrolyte are based on PtNi or PtCo nanoparticles deposited onto high surface area carbon. Pt is a scarce element, ill distributed in the Earth and consequently very expensive and subjected to strong price fluctuations. It is estimated that around 91% of the 69,000 metric tonnes reserves of Pt are located in South Africa [5], which clearly indicates that the market of Pt is captive, and any disruption in Pt distribution may result in severe price fluctuations posing a threat for FCs and related markets. Although the Pt loading in MEAs has decreased significantly in the last years reaching values of 0.37 mgPGM cm−2 in the Toyota Mirai or even down to 0.125 mgPGM cm−2 in lab scale bench, the cost of Pt accounts to ca. 42% of the cost of an 80 kW PEMFC stack for light duty vehicles [6]. More importantly, the cost of the PGM Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00012-5 Copyright © 2022 Elsevier Inc. All rights reserved.

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used in the catalysts does not dependent on manufacturing volume, which gives little room for decreasing the price of PEMFCs based on PGM catalysts. This feature has triggered the interest in the search for platinum group metal free (PGM-free) catalysts to replace PGM catalysts in PEMFCs. From the catalytic point of view, the performance of FCs is limited by the sluggish kinetics of the ORR, so most efforts to replacing Pt (or PGM) catalysts have been devoted towards the development of highly active and durable PGM-free catalysts for the ORR. Since, in principle, PGMfree catalyst can be significantly cheaper than Pt-based ones, their (in principle) lower ORR activity can be compensated by using more catalyst in the membrane electrode assembly (MEA). However, using high catalyst loadings on MEA result in very thick catalytic layers, which result in severe mass transport limitations. It has been estimated that the volumetric activity of PGM-free catalysts should be no less than 1/10th of the Pt activity, see Gasteiger et al. for further details [1]. In the last decade, an intense activity has been developed to designing PGM-free catalyst with an ORR performance comparable to that of Pt-based ones, and currently the so-called M/N/C catalysts are the best positioned PGM-free catalysts for fuel cell applications. In the coming sections, we describe the evolution of M/N/C catalysts in the last decades, the main synthetic routes and the characterization techniques usually employed for their characterization, including the evaluation of the ORR activity. Finally, Section 8.6 of this chapter introduces the recent studies dealing with M/N/C catalysts for the oxygen evolution reaction (OER) and for the hydrogen evolution reaction (HER).

8.2 M/N/C catalysts for the ORR By the time this chapter is written, the so-called M/N/C catalysts are the most likely candidates to replacing PGM catalysts in the cathode of low-temperature PEMFCs. The term M/N/C identifies a family of catalysts based in transition metal atoms (M) Fe, Co, Mn…, but mostly Fe, coordinated to several nitrogen atoms (ideally 4) within a carbon matrix. The structure is akin to that of Fe atoms in Fe-phthalocyanine or Fe-porphyrin. The catalytic properties of Nx -metal chelates for the ORR have been studied for almost a century now. The activity of phthalocyanines for the decomposition of H2 O2 was studied by A. Cook as early as 1938 [7], who reported that iron-phthalocyanine was the most active one. This work inspired R. Jasinski who in 1964 reported a thorough study about metal phthalocyanines as cathode catalysts in fuel cells and reported the ORR activity of Co-based phthalocyanine in alkaline electrolyte [8]. Next, H. Jahnke explored the ORR activity of Fe-phthalocyanine-carbon mixtures in acid electrolyte. Nevertheless, although a number of studies concerning the understanding of the activity of these catalysts appeared in the literature [9], the lack of stability of the iron centers, especially in acid electrolyte, which resulted in the complete loss of their activity, was clearly identified as a major impediment for their use in fuel cells [9]. This observation resulted in a large hiatus in the interest of these types of catalysts for the ORR. However, in 1973, Alt et al. reported that the thermal treatment in inert atmospheres of mixtures of the Nx -metal chelates

8.2 M/N/C catalysts for the ORR

with carbon resulted in catalysts with higher activity and durability for the ORR. A detailed survey of the most significant advances in this period can be found in several references [10,11]. One of the main findings in this period was that mixing Nx -metal chelates with carbon followed by a thermal treatment resulted in materials with higher activity and durability for the ORR. It was also recognized that the durability depended on the electrolyte. Whereas the durability during the ORR in acid electrolyte was compromised with the catalysts pyrolyzed at low temperature, the catalysts obtained after pyrolysis at temperatures as low as 400°C display high ORR activity in alkaline electrolyte [12]. At this point is important to remark that the ORR activity of Fe/N/C catalysts is highly dependent on the pH, and their ORR activity in alkaline electrolyte compares well with that of Pt/C, being significantly lower in acid electrolyte. This is because, at high pH, due to the specific adsorption of OHad on the electrode surface, the reactions proceeds via an outer-sphere electron transfer mechanism [13,14]. On the other hand, the ORR in acid electrolyte follows an inner-sphere electron transfer mechanism, that is, it is surface specific. Therefore, the challenge is to design highly performing (high activity and durability) ORR catalysts in acid electrolyte. Features such as whether the Nx -metal chelate structure could resist the thermal treatment or not, the relevance of the metal center in the ORR activity, and even the actual involvement of the Nx -metal ensembles in the electrocatalytic process, have been thoroughly discussed in the early literature [15] and continue to be discussed nowadays. Early studies ascribed the ORR activity of pyrolyzed Nx-Co chelates to metallic Co phases. This is because neither N x -metal nor N atoms were detected by x-ray absorption spectroscopy, or other spectroscopic techniques, in the catalysts subjected to thermal treatments above 600°C [16,17]. Detailed time-of-flight secondary ion mass spectroscopy (TOF-SIMS), X-ray photoelectronic spectroscopy (XPS), and transmission electron microscopy (TEM) analyze of a series of Cobased catalysts synthesized by pyrolysis of Co-phthalocyanine/carbon at different temperatures revealed the presence of metallic Co and Co-phthalocyanine moieties in the samples synthesized at low temperature; however, only metallic Co particles were detected after pyrolysis at high temperatures [16–18]. As a consequence, the ORR activity was ascribed to such Co particles [16]. Since the ORR activity was ascribed to metallic Co particles, it was hypothesized that the Nx -chelate fragments that remain after the pyrolysis could facilitate electron transfer to O2 [19]. Similar results were obtained with Fe-N4 chelates (phthalocyanines, tetraphenyl-porphyrins) [20,21]. Detailed TEM studies revealed, however, that the Co particles were actually wrapped within several graphite layers [17]. This observation, explained the otherwise unexpected stability of iron or cobalt metallic particles in acid electrolyte, but jeopardizes the proposal that such graphite-wrapped Co particles can be the active sites for the ORR. Therefore, their participation in the ORR needed to be reconsidered. The observation that Nx -Co or Nx -Fe fragments do not resist the high temperature pyrolysis but were needed to produce highly active and stable catalysts opened the way to use more simple metal precursors. Thus, M/N/C catalysts were obtained by pyrolysis of two individual precursors, one for iron (or the transition metal of choice), typically metallic salts and one for nitrogen (for instance N-containing polymers such

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as polypyrrole, polyaniline, or polyacrylonitrile, PAN), instead of using metal-N4 macrocycles as precursors. The pyrolysis of these precursors, usually impregnated onto carbon supports, resulted in the formation of catalysts with Fe-Nx ensembles that displayed high ORR activity. For instance, iron oxides were subjected to thermal treatments under H2 or NH3 atmosphere at temperatures around 600°C, followed by further thermal treatments in the presence of acetonitrile as nitrogen precursors [22–24]. The catalysts obtained after the second pyrolysis displayed high ORR activity. It was observed that in order to obtain materials with ORR activity, Fe and N should be present during the synthesis of the catalyst [25]. Despite Fe-Nx sites were not unequivocally detected in the pyrolyzed catalysts, this observation pointed out to the fact that N-containing moieties should participate in the ORR catalysis, or at least are needed to obtaining highly active catalyst. In line with the results obtained using Nx -macrocycles, the catalysts obtained from the pyrolysis of individual precursors also displayed metallic particles encapsulated within carbon layers, especially when the amount of the metallic precursor was high. This observation suggested that the fraction of Fe-Nx sites on the final catalysts would be determined by the number of C-Nx moieties available for the coordination of the metal rather than by the amount of metal precursor used during the synthesis. In fact, Fe loadings as low as 0.2 wt.% resulted in Fe/N/C catalysts with high ORR activity, which is not increased by increasing the amount of iron during the synthesis. The optimum loading would depend on the nature of the precursors (N- and Fe-containing ones) used for the synthesis of the catalyst. In the last years, the number of Fe/N/C catalysts reported in the literature has increased dramatically. M/N/C catalysts have been synthesized using a plethora of Fe-precursors, N-precursors, and different carbon matrixes, reaching performances as high as 150 mA cm−2 at 0.8V in H2 /air MEA with Fe/N/C catalyst obtained from pyrolysis of Zn-metal organic framework (Zn-MOF) precursors, see Osmieri et al. [26] and references therein. The most active families of catalysts reported to date have been obtained by (1) pyrolysis of metal organic frameworks, especially ZIF-8 (basolite Z-1200), or its precursor (imidazole salts) in the presence Fe, Co or Mn salts [27,28] with the possible addition of further nitrogen precursor during the pyrolysis such as 1,10-phenantroline, melamine or polyacrylonitrile-polymetacrylic acid copolymer, the latter resulting in the highest performing MEA reported to date (2) pyrolysis of polyaniline, carbon black, cyanamide, and iron salt precursors [29] and (3) using porous templates such as SiO2 ,and (4) porous organic polymeric frameworks. The nature of the active site(s) for the ORR in acid electrolyte was controversial for some time. Two types of catalytic sites were proposed, one formed at low pyrolysis temperature of around 500–600°C (LT), and one formed at high temperatures, above 800°C (HT). The spectroscopic data collected from the LT pyrolyzed catalysts revealed the presence of FeN4 ensembles, resulting from the partial decomposition of the metalloporphyrin used as precursors, the LT active site was assumed to be a dimeric complex [30,31] similar to the face-to-face form of cobalt porphyrin. ON the other hand, evidences of the presence of Fe or Co were not detected in the catalysts pyrolyzed at high temperature, thus the HT site was proposed to contain N, C, and

8.2 M/N/C catalysts for the ORR

O fragments, which were observed by ToF-SIMS, with the catalytic cycle involving the participation of N ions [32]. It should be noted, however, that iron or cobalt particles wrapped within several carbon layers were present the catalysts pyrolyzed at high temperatures and observed by TEM spectroscopy. Clearly, the very presence of such graphite layers jeopardizes the idea that such particles could be the real active sites for the ORR. This end was further clarified by Dodelet et al. [25] who synthesized catalysts at 1000°C (the HT regime) using iron and/or N precursors (vinyl ferrocene or acetonitrile, respectively) and found that the material obtained displayed ORR activity only if both precursors were used. XPS studies of Fe-based catalysts indicated an interaction of pyridinic-nitrogen and the iron atom and time-offlight SIMS detected the evolution of FeNx Cy + ions and suggested that the family of FeN2 C4 + ions correspond to the HT sites (labeled as FeN2 ) whereas FeN4 Cy+ ions characterize the LT sites (labeled FeN4 ) [33,34]. In this work it was observed that the ORR activity correlated well with the time-of-flight SIMS intensity of the family of ions labeled FeN2 C4 + , displaying a structure similar to that of 1,10-phenanthroline, in which two pyridinic N atoms coordinate one Fe ion. Later on, it was proposed that the most active sites for the ORR actually display an Fe-N2+2 configuration in which an iron (II) ion is coordinated by four pyridinic nitrogen atoms attached to the edges of opposed graphene planes delimiting a micropore in the carbon [35]. This structure is consistent with previous findings that the active sites are hosted in micropores [36] and the observation of the fingerprint signals for FeN4 motifs in XAS and 57 Fe Mössbauer studies [37]. The proposed structures are depicted in Fig. 8.1. Deeper analyses with a combination of advanced characterization techniques, especially 57 Fe Mössbauer spectroscopy revealed the presence of several types of FeNx moieties in the pyrolyzed catalysts, including Fe coordinated by 4 pyrrolic N atoms (FeN4 ), FeN2+2 , and N-FeN2+2 ···Nprot /Cx in which iron is coordinated to pyridinic nitrogen atoms in different layers [37,38]. N-FeN2+2 ···Nprot /Cx is also referred to as N-FeN2+2 ···NH+ [39]. In addition to Fe-Nx species, pyrolyzed Fe/N/C catalysts also contain several iron inorganic species, including Fe carbides or nitrides, metallic iron, and/or iron oxides nanoparticles. Due to their different coordination and electronic state, Fe-Nx sites display different ORR activities. Since the binding of molecular oxygen to the Fe-Nx site involves the d-orbitals of the metal, in particular of the 3dz 2 orbital, the occupancy of such orbitals would determine the actual involvement of each Fe-Nx ensemble in the ORR [40]. Thus, Fe-N4 and N-FeN2+2 ···Nprot /Cx are expected to display ORR activity because they display half-filled and empty 3dz 2 orbitals, respectively, and therefore they can adsorb molecular oxygen. By contrary, by displaying a completely filled 3dz 2 orbital, FeN2+2 would not be active for the ORR [37]. The occupancy of the 3dz 2 orbital is not the only descriptor of the catalytic activity. For instance, the ORR can be also determined by the protonation of the N atoms. Thus, Herranz et al. [41] postulated that the protonated form of this site, N-FeN2+2 ···NH+ , displays high ORR activity, whereas the neutralized from of this ensemble is characterized by low ORR activity. The formation of basic N groups, which are prone to be protonated, is typical for the catalysts that have been pyrolyzed under NH3 atmospheres [42]. These catalysts usually display high initial activity,

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Possible structure of the catalytic site: FeN4+2/C about 13 A

Crystallite 1

N

N Fe

N

N

N N Fe N

Crystallite 2

N

Micropore FeN4/C 2.46 A

N

N Fe

Particle

about 30 nm

FeN2/C 36

FIGURE 8.1 Proposed structures of Fe/N/C catalytic sites, including FeN2+2 /C, FeN4 /C and FeN2 C. Reprinted with permission from [35].

which declines rapidly under operation. Note that depending of the synthesis protocol for the Fe/N/C catalysts, namely the nature of the precursors, the amount of iron, the thermal treatment, and the acid washing, the number of Fe-Nx and iron inorganic species found in the pyrolyzed Fe/N/C catalyst might vary significantly from up to 10 iron species [39] to two or three iron species found in the purest catalysts [43–45]. In summary, Fe/N/C catalysts obtained from thermal treatments of Fe and N precursors contain different types of FeNx and Cy Nx sites with different roles in the ORR. Atanassov et al. [30,46,47] claimed that Fe-Nx sites would be responsible for the total reduction of O2 to H2 O either through a direct 4e− pathway or via a 2+2 mechanism whereby O2 is reduced to H2 O2 , which is further reduced to H2 O. In addition, metal free CNx ensembles promote the 2e− pathway, namely O2 to H2 O2 on N-pyrrolic, N-graphitic, N+ or N-protonated and H2 O2 to H2 O on N-pyridinic. In addition to the actual nature of the Fe/Nx ensemble, it is reasonable to assume that the activity of Fe/N/C catalysts would be also influenced by the electronic properties of the carbon basal plane. Early studies by revealed that contrary to pure M-N macrocycles, the catalysts obtained after pyrolysis do not display a proper square planar Fe-N4 geometry. Later on, in situ Fe K-edge XAS confirmed that the

8.2 M/N/C catalysts for the ORR

FeN4 ensembles are integrated within the carbon basal plane, displaying in-plane (OH-Fe(III)-N4 ), or out-of-plane (Fe(II)-N4 ) geometries [48,49]. The actual geometry depends on the potential of the Fe(III)/Fe(II) redox pair of the FeNx moiety, which is in turn affected by the p-electron delocalization in the carbon basal plane. The p-electron density is affected by the presence of defects in the carbon basal plane, since the presence of defects would disrupt the p-electron delocalization. Thus, the electron density in the Fe(II)-N4 moiety would be higher in low defect carbon. This would result in a too strong binding of the oxygen intermediates onto the metal center. Conversely, a too defective carbon would lead to a weak adsorption of oxygen onto Fe(II)-N4 . According to the Sabatier principle, this view supports the idea that the defective nature of the carbon matrix should be optimized to optimize the O2 binding energy of the oxygen intermediates onto Fe-Nx ensembles, At the time this chapter is written, the most active Fe/N/C catalysts for the ORR display two different iron species, namely high-spin FeN4 C12 moiety and a lowor intermediate-spin FeN4 C10 moiety. These species are referred to as S1 and S2, respectively, and are characterized by two doublets in the 57 Fe Mössbauer spectra, namely D1 with quadrupole splitting values between 0.9 and 1.2 mm s−1 and D2 with quadrupole splitting values between 1.8 and 2.8 mm s−1 [50]. Whereas both species display high initial ORR activity, only S2 remains active after 50 hours of operation, during which time S1 becomes oxidized forming iron oxides hence being inactive for the ORR. It is therefore concluded that FeN4 C10 is the iron species responsible for the long-term ORR activity. This view is further supported by a recent N-XAS study of the durability of Fe/N/C catalysts during the ORR which identifies that Fe-Nx -pyridinic species are more durable than Fe-Nx -pyrrolic ones during the ORR measured in RDE configurations [51]. The durability of the M/N/C catalysts is a major issue for the wider implementation of M/N/C catalyst at commercial level, since PGM-free catalyst-based PEMFCs can only operate over hundreds of hours, falling short to fulfill current DOE’s targets of 5000 hours for transportation applications [52]. As stated above, pioneering studies already reported that catalyst durability increases significantly upon thermal treatment of the catalysts. Whereas some catalysts display high durability during RDE testing, their stability is seriously compromised when tested in MEA configuration even for the best performing PGM-fuel cell reported to date by Zelenay et al. exhibiting stability for 700 hours at constant voltage of 0.40 V [53]. Several sources of catalyst deactivation are considered, including demetalation, carbon oxidation either surface oxidation via Fenton reactions, nitrogen corrosion, active site poisoning, carbon bulk oxidation and deprotonation or ion adsorption of N groups and flooding of the micropores where the active sites are located, see Fig. 8.2. Demetalation of FeN4 sites in micropores is considered a major source of catalyst deactivation, being responsible for the rapid initial performance decay of Fe/N/C fuel cells [54]. Also, poisoning of Fe atoms by fluorine ions leads to a severe decreasing of the ORR activity [55]. On the other hand, Choi et al. concluded that at high potentials the main source of degradation is the destruction of FeNx Cy active sites that takes place via carbon oxidation at potentials higher than 0.9 V [56]. Although carbon corrosion can take place at low

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FIGURE 8.2 Main degradation routes of Fe/N/C catalysts during the ORR. Reprinted with permission from [52].

potentials (C + 2H2 O CO2 + 4H+ + 4e− E0 = 0.207 V), it is a sluggish reaction and it can be avoided by controlling the upper potential of the device [57], or by using highly graphitic carbon matrixes such as graphene nanoplates. As demonstrated by in situ FTIR, graphitic carbons such as graphene nanoplates and carbon nanotubes display superior stability at high potentials than less graphitic carbon materials such as carbon Vulcan [58]. A major cause of degradation of Fe/N/C catalysts during the ORR is thought the oxidation of carbon through a Fenton reaction with the H2 O2 generated by an incomplete reduction of O2 in fuel cell, although this view was disputed by Zhang et al. [59] who reported that the main source of degradation is the partial oxidation of the carbon that resulted in the generation of oxygen-surface groups which increase the hydrophilicity of the catalyst hence leading to micropore flooding. The surface oxidation of the carbon has been also claimed to decrease the ORR activity by weakening of the O2 -binding [57]. As for the fate of the FeNx Cy ensembles during the ORR, as shown above, operando 57 Fe Mössbauer studies reveal that Fe atoms in FeN4 C12 moieties oxidizes to ferric oxide during PEMFC operation, whereas Fe atoms in FeN4 C10 ones remain stable during operation [45]. This observation is in good agreement with a recent study of the activity and stability of a Fe/N/C

8.3 Synthesis of highly active M/N/C catalyst for the ORR

from the point of view of the N atoms by N-XAS. The authors concluded that Npyridinic species are more stable than N-pyrrolic ones during the ORR studied in RDE configuration, observing the formation of iron oxides after the ORR [51].

8.3 Synthesis of highly active M/N/C catalyst for the ORR In Section 8.2 we showed that M/N/C catalysts can be synthesized by thermal treatment(s) of a transition metal precursor (including oxides, acetates, nitrates, chlorides of Fe, Co, and/or Mn) and a nitrogen precursor mixed or not with a carbon matrix (carbon Vulcan, nanotubes, graphene, microporous carbon…). In order to improve catalyst stability during the ORR, and to prevent leaching of nonstable iron species during MEA operation that would result detrimental for proton transportation, it is recommended to remove the nonstable iron phases after the first heat-treatment. This is usually done by acid leaching in a concentrated acid solution of HCl or H2 SO4 at temperatures between 40 and 60°C during several hours. The solid obtained should be rinsed with distilled H2 O and subjected to a further thermal treatment. The number and nature of the precursors reported in the literature is incommensurable, so in this section we focus on the synthesis that yields the most active ORR catalysts.

8.3.1 Fe/N/M catalysts derived from metal-organic frameworks Metal-organic frameworks (MOFs) are crystalline porous compounds with large surface areas up to few thousand m2 g−1 in which metal ions or clusters are connected by organic ligands. In principle, features such as their low electrical conductivity would be against the use of MOFs as candidates to be used as electrocatalysts. However, MOFs (or in particular ZIF-8 MOF) has proven to be an ideal starting material for the synthesis of highly active Fe/N/C catalysts, since once subjected to pyrolysis treatment a conductive porous carbon matrix, with optimum textural properties are obtained. There is a wide range of MOFs in the literature for plenty of applications, mostly as adsorbents. Among those, zeolitic imidazolate framework-8 (ZIF-8), where a Zn(II) ion is coordinated by nitrogen atoms of imidazole ligands, is the best choice to be used as precursor for synthesizing high activity electrocatalyst for the ORR. Dodelet et al. [27] were the first who reported the utilization of ZIF-8 to prepare a highperformance Fe/N/C cathode catalyst in fuel cells. They mixed commercial ZIF-8 with 1,10-phenathroline and Fe(II) acetate in a solution of ethanol and deionized water, then the dried slurry was ball-milled during 3 hours at 400 rpm in a steel vial with chromium balls previously sealed in a glove box. The resulting powder was heattreated at 1050°C in Ar, and a second time at 950°C under NH3 . During the thermal treatment, Zn is removed from the structure (the boiling point of metallic Zn is 907°C) yielding a highly porous carbon structure in which FeNx ensembles are hosted in a nitrogen-containing microporous carbon. Fig. 8.3 depicts the synthetic process for a ZIF-derived Fe/N/C catalysts.

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FIGURE 8.3 Illustration of the synthesis of Fe/N/C using ZIF-8 precursors and 1,10-phenontraline. Characterization details showing the formation of different N-species. Reproduced with permission from [48].

Another strategy to get a ZIF-8 like-structure support is to use the ZIF-8 precursors, namely 2-methylimidazole and zinc nitrate, instead starting from ZIF-8 itself. This is because ZIF-8 is an expensive compound, and during the pyrolysis step up to 90% of the initial material may be loss. Attempts to scaling-up the synthesis of Fe/N/C from MOFs have been made, resulting in a catalyst yield of ca. 80% [48].

8.3.2 Fe/M/N catalysts from sacrificial templates Usually, catalysts obtained from the pyrolysis of an organic precursor of nonwelldefined structure result in catalysts with low surface areas and as a consequence a low-site density, hence low-ORR activity. Therefore, an approach to obtaining highly active ORR catalysts would be to produce catalysts with high surface area. Furthermore, it is also desired to get a controlled morphology of the structure and the porosity helps the accessibility of the reactants to the active sites. The use of the sacrificial template methods (SSM) is a suitable strategy to reach these benefits. There are two types of sacrificial templates, hard templates and soft templates. Silica is the most typical example of a hard template; it is stable at high temperatures, and it can

8.3 Synthesis of highly active M/N/C catalyst for the ORR

be etched by treatments at high, very high, or very low pH values, using NaOH or HF, respectively. Atanassov et al. have explored this approach extensively, using fumed silica as template with excellent results [60]. To be successful, the synthesis based on SSM requires several features, namely: (1) The selected precursor should contain enough carbon atoms to allow the formation of an 3D open-frame structure, (2) the carbon-nitrogen ratio of the precursors should be adequate, around 4:1 in order to enhance the density of Fe-Nx active sites in the final catalyst, and (3) the volatility of the organic molecules is crucial in the selection of the precursor. Since the Fe-Nx active sites are formed at high temperatures, a low-volatile precursor is preferred in order to withstand the high pyrolysis temperature of the synthesis. A precursor able to polymerize making a complex with the transition metal is a good choice. For example, Atanassov et al. selected carbendazim as precursor because it displays a suitable C:N ratio of 9:3 and a melting point of 302–307°C. They dispersed the carbendazim and the iron precursor on SiO2 particles followed by a first heat treatment in a nitrogen atmosphere. Then, the silica template is removed by a strong acid treatment using a concentrated solution of HF as etching agent. This treatment also eliminates the unstable iron phases and iron oxides present in the catalyst. Finally, the material is subjected to a second heat treatment in a reactive flow of NH3 , in order to remove volatile HF species and to introduce surface defects. Ordered mesoporous silicas such as SBA-15 are gaining attention as template due to its periodic, uniform and tunable pore structure. Joo et al. [61] developed a simple method to get an ordered mesoporous carbon-metal catalyst in which a metalloporphyrin is grounded with SBA-15 in a 1:1 mass ratio followed by a high temperature treatment under inert atmosphere and silica etching with HF. The resultant catalyst exhibits a high surface area around 1500 m2 g−1 with a high density of Fe-Nx active sites. Fig. 8.4 shows a synthesis scheme to obtain metal-doped ordered mesoporous catalyst. Soft templates are a more environmentally friend alternative than hard templates, which, at time of the removal of silica, hazardous reagents like NaOH or HF must be used. Hayakawa et al. pioneering the use of the soft-template method for obtaining mesoporous Fe/N/C catalyst [62]. Block copolymers can lead the self-assembly process of the precursor molecules acting as a soft-template, but it is essential that the precursor of choice have selective affinity for one segment of the block polymer. Pluronic F127 (PEO-PPO-PE triblock polymer) is an excellent compound for synthesizing highly ordered mesoporous polymeric frameworks. Usually, for obtain M/N/C catalyst via soft-template strategy, the ordered mesoporous carbon framework is synthesized before add the selected metal by impregnation. For instance, Nabae et al. mixed oligo amic acid, resol and F127 as N/C source, cross-liner, and soft template respectively, in ethanol and dimethylformamide solvents. After the solvent evaporation, a first high temperature heat treatment in nitrogen is done. The soft-template is removed by temperature effect (350°C) yielding in a high-density carbon structure, due to a synergic shrink effect between C and N source with the soft-template, which also, compare to hard templates results in harder carbon structures. Once, ordered mesoporous carbon support is ready, iron is introduced by impregnation. Then, to form the active sites, they pyrolyzed the sample in a reactive

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FIGURE 8.4 Scheme of the synthesis of Fe/N/C catalysts with sacrificial templates and characterization details. Reprinted with permission from reference [61].

atmosphere (NH3 ), and the resulting powder is submitted to a strong acid leeching and pyrolyzed it again to obtain the final Fe/N/C catalyst.

8.3.3 Fe/N/C catalysts derived from PANI Zelenay et al. demonstrated that polyaniline (PANI) is a suitable including secondary nitrogen sources such as cyanamide, melamine, or urea as examples [29,63]. First, aniline is introduced in a 0.5–1.5 M HCl solution under magnetic stirring followed by the addition of an iron precursor, for instance FeCl3 , and the optional nitrogencontaining organic compound. Once, the compounds are dissolved, ammonium persulfate is added as oxidant to the solution to catalyze aniline polymerization. The mixture is stirred at room temperature during at least 4 hours. Usually, PANI-like catalysts are supported on previous treated carbon such as Black Pearls or Ketjen Black. These carbons are pretreated with nitric acid at 80–90°C under reflux for 6–8 hours. The pretreated carbon is ultrasonically dispersed and then mixed in the polymer

8.4 Assessment of ORR performance of M/N/C catalysts

solution. The solution can be heated up to 80°C and stirred during 2 days until it forms a foam/tar-like. The final product is dried overnight and ball milled or grounded in a mortar to get a powder before be submitted to a first pyrolysis at 900°C. Next, an acid leeching with H2 SO solution at 90°C is done with a subsequently second pyrolysis at same conditions.

8.3.4 Fe/N/C catalyst from porous organic polymers as precursors Porous organic polymers (POPs) can be used as excellent precursors to get high activity electrocatalysts without the need for incorporating further templates or supports during the synthesis of M/N/C. There are a myriad of monomers and cross-linking reactions to obtain high surface organic polymers containing diverse functional groups. When a metal-nitrogen macrocyclic compound, such as phthalocyanines or porphyrin like-structure, is employed in a 3D polymerization, the resulting POP would display a dense population of metal-N4 sites. Yu et al. [64] synthesized Fe/N/C POP catalyst with high ORR activity starting from a simple material to create an iron porphyrin which polymerized to a complex Fe-POP, which was subjected to thermal treatments to obtaining the actual catalyst. Covalent triazine frameworks (CTFs) are a kind of promising organic-porous polymers obtained by the trimerization of aromatic nitriles compounds. CTFs are Nrich materials, thermally stable with a high-surface area with micropores able to host M-N4 active sites. The use of CTFs to obtain NPMC catalysts is a novel approach and it is still under development [65]. Typically, CTFs are prepared via ionothermal synthesis in which an aromatic nitrile precursor is mixed with ZnCl2 in a Pyrex ampoule. The ampoule is sealed under vacuum and heat-treated to 400–600°C during 40 hours. To remove the Zn excess of the resultant powder it is leached with diluted HCl or heated at higher temperatures.

8.3.5 Other strategies for obtaining highly active M/N/C catalysts Finally, another simple and effective method is to mix nitrogen-metal organic molecules such as Fe-phthalocyanine or metal porphyrin rings with a carbon support (graphene, Vulcan…) in a stainless-steel vessel and proceeds to ball milling procedure [58]. The resulting powder is submitted to pyrolysis-acid washing- pyrolysis steps above mentioned yet. This route, if not subjected to pyrolysis, the materials display high ORR activity in neutral [66].

8.4 Assessment of ORR performance of M/N/C catalysts The ORR is a multielectron process involving several steps and intermediate species. As stated above, ORR may proceed via four- or two-electron transfer in aqueous acidic medium. The most relevant reactions pathways and their thermodynamic electrode

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potentials in acidic medium are shown below: O2 + 4H+ + 4e− → H2 OE0 = 1.229 V

(8.1)

O2 + 2H+ + 2e− → H2 O2 → E0 = 0.695 V

(8.2)

H2 O2 + 2H+ + 2e− → H2 OE0 = 1.776 V

(8.3)

The direct pathway, which is the desired one, involves four electrons in a direct reduction of O2 to H2 O. The serial pathway proceeds through the formation of H2 O2 as an intermediate species, which can be further reduced to H2 O resulting in a 2+2 electron pathway. The assessment of the catalytic performance (ORR activity selectivity and durability) can be done from two approaches, using a rotating ring disk electrode (RRDE) set up or in MEA configuration. The first approach is simpler and it allows a fast screening of catalyst performance. The latter approach is the most desired one, since is presents the actual performance of the catalyst in a fuel cell environment. Recommended practices and protocols for the assessment of the ORR with RRDE can be found in several papers [67–70]. RRDE experiments allow measuring the ORR activity of Fe/N/C catalysts, which are reported in terms of kinetic current density (j kin ) or mass activity (ig ) (see references above) and to determine the fraction of H2 O2 formed during the ORR, that is, to determine the contribution of the two-electron vs. four-electron pathways during the ORR. Although the actual protocol for the measurement of the ORR with RRDE is out of the scope of this chapter (but it is well described in refs above) we would like to bring readers’ attention to the observation that the fraction of H2 O2 detected during the ORR with Fe/N/C catalysts may depend on the catalyst loading on the electrode, so the assessment of the ORR reaction pathway should be conducted by using different catalyst loadings on the electrode, see Fig. 8.5 [63]. Pure kinetic density, mass-specific activity, onset potential and half-way potential are the most commonly reported metrics to benchmark Fe/N/C catalysts. However, the most suitable descriptor of the intrinsic kinetic parameter of an electrocatalyst is the turn over frequency (TOF); that is, the number of electrons exchanged per active site per time unit. In order to determine TOFs, it is necessary to quantify the number of active sites at the catalyst surface (site density, SD). The TOF and the kinetic current density are correlated by Eq 8.4         jkin,mass Ag−1 = TOFE electronsite−1 s−1 × SD siteg−1 × e Celectron−1

(8.4)

Note that since current density (j) is a function of the potential, the TOF is also dependent on the potential, so it is needed to report the potential at which the TOF is calculate (TOFE ). As observed from Eq X, the determination of the TOF is straightforward once the SD is known. However, this is not an easy task, and authors have determined the SD from different approaches, including the quantification of the fraction of Fe atoms in the configuration expected to render the active sites by 57 Fe Mössbauer spectroscopy or from the integration of the N 1s core-level peak ascribed to FeNx sites (at B.E. around 399.5 eV) assuming that all FeNx sites display ORR

8.4 Assessment of ORR performance of M/N/C catalysts

FIGURE 8.5 Disk and ring currents recorded during the ORR in 0.05 M H2 SO4 at 900 rpm with an Fe/N/C prepared by thermal treatment in NH3 at 950°C. Reproduced with permission from [71].

activity. However, those approaches either are not surface sensitive or are not suitable to discriminate between FeNx ensembles with iron atoms in different oxidation states, coordination, or spin state. Two main approaches for the quantification of Fe active sites (SD) are currently under development. The first one is an ex-situ approach that involves the chemisorption of CO at cryogenic temperatures (−80°C) [63,72]. The second is the in situ electrochemical nitrite adsorption [73]. The former relies on the selective adsorption of CO onto ionic Fe species at cryogenic temperatures.

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Once the sample is degassed at 600°C, pulses of a known volume of CO are passed through the sample until saturation. The total volume of CO (or moles of CO, nCO ) chemisorbed onto the iron ions can be directly correlated with the content of surface iron by assuming a Fe:CO stoichiometry 1:1. The second one is an in situ approach that relies on the strong, specific adsorption of nitrite onto Fe ions resulting in Fen+ (NO)x species that are stripped in a five-electron reduction process forming NH3 . The stripping charge (Qstrip ) is used to quantify the Fe-Nx sites. Thus, the mass-based Fe surface site density, SDmass (NO2 − ) can be calculated from:    site g−1 = SDmass NO− 2

Qstripp × NA nstrip × F × mcat

(8.5)

A drawback of this approach comes from the observation that nitrite adsorption leads only to a partial blockage of the iron active sites, of ca. 70%. Therefore, the TOFE is calculated by considering the kinetic densities of the poisoned and nonpoisoned experiments. unpoisoned poisoned   jkin,mas − Jkin,mass × NA  − T OF electron site−1 s−1 = SDmass NO2 × F

(8.6)

Although the number of studies reporting the assessment of the SD of iron sites is still very low, this approach is very promising for the assessment of the ORR activity of Fe/N/C catalysts. The assessment of the ORR performance of Fe/N/C catalysts with RRDE is a fast-screening, reliable approach for benchmarking novel catalysts. However, it is recommended to assess the performance of catalysts in a single-unit PEMFC. The basic component of a single-unit PEMFC are the bipolar plates which are the electron conducting plates and provide the pathways for reactants (H2 , O2 or air) distribution, the gas diffusion layer (diffusion of electrons and gas to the electrodes) and the membrane electrode assembly (MEA), which consists of a proton-conducting membrane (typically Nafion) onto which catalysts are deposited by hot-pressing anode and cathode against each side of a commercial membrane. Anode and cathode are fed by humidified hydrogen and air/oxygen, respectively. The actual performance of the fuel cell would be affected by features such as catalyst loading on the anode and cathode, ionomer to catalyst ratio, thickness of the electrode, gases flow and stoichiometry, relative humidity, pressure and temperature, so it is necessary to provide this information for the correct assessment of catalysts. Fig. 8.6 shows illustrates the complete characterization of a series of ZIF-derived Fe/N/C catalysts for the ORR, including RRDE measurements of the ORR activity and H2 O2 production, and MEA performance measured in H2 -air and H2 -O2 . The number of publications reporting activity data either or both in RRDE in MEA configurations is very large and the complete revision out of the scope of this work. Several excellent reviews are available in the literature [26,74]. In general, the reported performances have shown and upward trend, from ca. 10 to 30 mAcm−2 at 0.8 V in H2 /air fuel cells in the last decade to more than 150 mAcm−2 at 0.8 V in H2 /air fuel

8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts

FIGURE 8.6 Upper panel. (A and B) RRDE polarization curves and H2 O2 production for different ZIF-derived catalysts. Fuel cell performance of the 1.5Fe–ZIF catalyst: (C) H2 –O2 and (D) H2 –air. Anode: 0.2 mgPt cm−2 Pt/C; H2 flow rate 200 sccm, 1.0 bar H2 partial pressure; cathode: ca. 4.0 mg cm−2 , 200 sccm gas flow rate, 1.0 bar total partial pressure of gas flow; membrane: Nafion 211; cell: 80 °C, 100% RH, 5.0 cm2 MEA electrode area. Reproduced with permission from [28].

cells reported in the 2020s. This feature is perfectly illustrated in Fig. 8.7 extracted from reference [26].

8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts As shown above, pyrolyzed Fe/N/C catalysts contain multitude of phases, including isolated single transition metal atoms coordinated to nitrogen atoms, nanosized particles including metallic, oxides, carbides and nitrides, N-protonated species, NCx fragments and graphitic layers, so their accurate characterization is only possible by combining the information obtained from several characterization techniques. This section gives a brief overview of the use of 57 Fe Mössbauer spectroscopy Xray diffraction (XRD), transmission electron microscopies coupled with analytical techniques (HR-TEM, STEM, HADAAF, EDS, EELS, SAED), X-ray absorption

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FIGURE 8.7 Evolution with time of the performance of Fe/N/M-based MEAs in H2 /air and H2 /O2 . With permission from [26].

(XAS, Fe, and N edges), and X-ray photoelectronic spectroscopy (XPS) for the characterization of pyrolyzed Fe/N/C catalysts.

8.5.1 Mössbauer spectroscopy 57

Fe Mössbauer spectroscopy is a powerful technique for the determination of the oxidation state(s) as well as the electronic spin-state, and the chemical environment of the iron atoms in iron compounds and complexes. It also allows to quantify the content of the iron species in a material. 57 Fe Mössbauer spectra are usually performed at room temperature, resulting in information such as the basic hyperfine parameters of the iron species in Fe/N/C such as the isomer shifts, quadrupole interactions, and in some cases the hyperfine fields. These parameters are intrinsic to the iron species under study. The isomer shift (δ in mm s−1 ) depends directly on the s-electron densities but may be influenced indirectly via shielding effects of p-, d- and f-electrons. The

8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts

FIGURE 8.8 57

Fe Mössbauer spectra of pyrolyzed Fe/N/C catalyst under Ar or Ar+NH3 and assignment of the species. For the correct interpretation of references in the figure, please refer to the original work. Reproduced with permission from [39].

isomer shift values give information on the oxidation state, spin state, and bonding properties such as covalency and electronegativity. The electric quadrupolar splitting (EQ in mm s−1 ) gives information on the oxidation state, the spin state, and the local symmetry of the Mössbauer atom. Due to the relatively high natural abundance of the Mössbauer active 57 Fe, around 2.7%, it is possible to obtain spectra working at room temperature with relatively low amounts of sample. However, in some cases the information is not complete, since different iron species may display very similar parameters. In order to avoid this feature, it is recommended to record the spectra at low temperatures, allowing to obtain crucial insights into the nature of the iron sites and their hyperfine interactions. 57 Fe Mössbauer was used since the early days for the study of Fe/N/C catalysts, see [75–78], confirming that pyrolyzed Fe/N/C catalysts contain several iron species within their structure. The typical parameters for the analysis of the 57 Fe Mössbauer spectra have been reported in several works [79–82]. Depending on the actual nature of the sample, up to 10 different iron species have been identified. For instance, Kramm et al. recorded the 57 Fe Mössbauer spectra of different Fe/N/C catalysts subjected to a first heat treatment in argon (Ar) at 1050°C and a second heat treatment in ammonia at 950°C (Ar + NH3 ) [82], see Fig. 8.8. Those

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catalysts were considered the state-of-the-art catalysts for the ORR and their synthesis was reported in previous works [83,84]. The actual nature of the FeNx sites, including the oxidation and spin state of the iron atoms, can be deduced from the isomer shift and quadrupole splitting energy of the doublets. In the absence of strong magnetic fields, the typical isomer shift and quadrupole splitting energy for various FeNx specie are: Fe(II)-N4 (D1: low spin, δ iso = 0.30 mm s−1 and EQ = 0.86 mm s−1 ), Fe(II)-N4 in Fe-phthalocyanine environments (D2: δ iso = 0.27 mm s−1 and EQ = 2.56 mm s−1 ), N-Fe(II)N2+2 (D3: high spin δ iso = 0.98 mm s−1 and EQ = 1.94 mm s−1 ), NFe(III)N4 (D4: low spin δ iso = 0.32 mm s−1 and EQ = 1.52 mm s−1 ), and Fe(III)N4 with axial ligands coordinated to Fe atoms (D5: intermediate spin δ iso = 0.34 mm s−1 and EQ = 3.92 mm s−1 ). These values are similar to those reported by the same group in [81]. Although similar species were found in the 57 Fe Mössbauer of many other pyrolyzed Fe/N/C catalysts, different isomer shift (δ iso ) and quadrupole splitting (EQ ) values have been reported, and in fact the assignment of some of those doublets is controversial. For instance, doublets D1 (with isomer shifts between 0.30 and 0.45 mm s−1 and quadrupole splitting of 0.90–1.25) have been assigned to an Fe(II) zero spin (S = 0) species, but also to Fe(III) in high spin (S = 5/2). Likewise, D2 (with isomer shifts between 0.30 and 0.45 mm s−1 and quadrupole splitting between 2.0 and 2.8 mm s−1 ) has been assigned to a distorted Fe(II) in low spin, Fe(II) medium spin (S = 1), Fe(III) in low spin (S = 1/2) or Fe(III) medium spin (S = 3/2). In order shed light into the interpretation of the 57 Fe Mössbauer spectra of Fe/N/C catalyst, Mineva et al. [80] conducted density functional theory (DFT) studies to calculate the quadrupole splitting (EQ ) of several FeNx structures in different oxidation states and spin states. They modeled two ferrous clusters, namely FeN4 C10 and FeN4 C12 subgroups, corresponding to iron phthalocyanine and iron porphyritic structures. Comparing the calculated and experimental EQ values they concluded that the D1 (EQ = 0.94 mm s−1 ) and the D2 (EQ = 2.25 mm s−1 ) signals in the 57 Fe Mössbauer spectra of Fe/N/C catalysts can be ascribed to Fe(III)N4 C12 sites with high spin and Fe(II)N4 C10 sites with low spin, respectively. According to the authors, the D1 signal characterizes surface species, that are prone to react with the exposed O2 from air, and therefore are the main responsible for the ORR species. Conversely, D2 may be assigned to either (1) D1 species in the bulk of the catalyst that are not accessible to air (or the electrolyte) or (2) FeN4 surface species with at Fe(III)/Fe(II) redox potential high enough as to make Fe(II) species stable under air exposure at open potential. As for the actual iron phases responsible for the ORR activity, it is assumed that by displaying a FeN4 architecture, D1, D2, and D3 sites would be involved in the ORR. Kramm et al. observed a nice correlation between the iron content related to D2 and D3 species with the ORR activity [81], and concluded that they are involved in the ORR. However, they postulated that the species responsible for the D2 would lack ORR activity. This is because D2 characterizes FeN4 or FeN2+2 species in an intermediate spin state, and that due to the full occupation of the 3dz 2 orbital they would not be able to adsorb O2 . Therefore, D1 (S = 0 and empty 3dz 2 orbital) and

8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts

FIGURE 8.9 Fe Mössbauer spectra of a fresh and used Fe/N/C catalyst showing the appearance of Fe2 O3 clusters. Model of sites S1 and S2. Reproduced with permission from [45].

D3 (S = 2 and half occupied 3dz 2 orbital) species would be the sites responsible for the ORR activity. Recently, Li et al. [85] studied the stability of the FeNx sites during the ORR using in situ and ex situ 57 Fe Mössbauer, see Fig. 8.9, using a MOF-derived catalysts, first reported in [84]. The 57 Fe Mössbauer analysis of the fresh catalyst at 5 K reveals the presence of two doublets, D1 (δ iso = 0.34 mm s−1 and EQ = 0.94 mm s−1 ) and D2 (δ iso = 0.34 mm s−1 and EQ = 0.94 mm s−1 ) ascribed to high-spin O-Fe(III)N4 and low-spin or medium-spin Fe(II)N4 sites. The relative areas of each species are 64% and 36% respectively. After a linear sweep voltammetry program from open circuit voltage (OCV) to 0.8 V and back to OCV, a further doublet, D3 (δ iso = 2.25 mm s−1 and EQ = 2.84 mm s−1 ) is found in the spectrum and ascribed to high-spin FeO particles, with the relative area of the D1, D2 and D3 being 15, 73, and 12%, respectively. This result clearly indicate that the D1 is not stable during the ORR, while D2 is stable.

8.5.2 X-ray photoelectron spectroscopy XPS is a widespread technique to study solid catalysts, and it has been widely used to determine the oxidation state, environment and relative abundance of surface iron and nitrogen species in Fe/N/C catalysts. XPS is a surface technique based on the photoelectronic effect, spectra are obtained by irradiating the sample with a beam of X-rays which results in the emission of electrons from the elements of the material.

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Fe 2p

HT-1,2 HT-1,4 2HT-1,2 2HT-1,4

N 1s

HT-1,2

N 1s

2HT-1,2 2HT-1,4

HT-1,2 Fe 2p3/2

Fe 2p1/2

HT-1,4

730 720 710 Binding energy / eV

700

408

404 400 396 Binding energy / eV

392

408

404 400 396 Binding energy / eV

392

FIGURE 8.10 XP spectra of the Fe 2p and N 1s core-level regions of a series of pyrolyzed Fe/N/C catalysts. The deconvolution of the N 1s region into several species is shown, with permission from [65].

Since only electrons emitted from the surface can reach the detector, the information obtained from XPS is surface sensitive. XPS spectra represent the number of the ejected electrons at a given binding energy (BE). The binding energy can be deduced from the kinetic energy of the ejected electrons from the photoelectric effect equation (Ebinding = Ephoton - Ekinetic +), where Ephoton is the energy of the X-ray source and is the work function. Each element displays a set of characteristic XPS peaks that correspond to the electron configuration of the electrons and the number of detected electrons in each peak is directly related to the concentration of the species under study. For the characterization of the N and Fe surface species on Fe/N/C by XPS, high resolution N 1s and Fe 2p spectra should be collected. Fig. 8.10 shows typical Fe 2p and N 1s core-level spectra of a pyrolyzed Fe/N/C catalyst. In addition, C 1s and O 1s core-level spectra are also recorded. The N 1s peak comprises information of the Nx -C and Nx -Fe species present in the catalyst, so the correct analysis of this peak is of paramount importance for establishing appropriate structure-activity correlations. The analysis is usually conducted by deconvoluting the N 1s peak into several peaks, by using a combination of Gaussian-Lorentzian line shapes of a given full width at half maximum (FMWH). It is very important to be consistent and to deconvolute the spectra into components displaying identical (or very similar) widths, especially when analyzing spectra from the same sample. In general, N 1s core-level spectrum of Fe/N/C catalysts comprise; pyridinic-N (398.3–398.8 eV), Fe-Nx (399.5–399.8 eV), different types of nitrogen coordinated to transition or hydrogenated N (400.6– 400.9 eV), pyrrolic-N (401.5–402 eV), and graphitic-N (402.1–403 eV). In some

8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts

cases, a peak at lower binding energy of 398.0 eV is observed and assigned to imine, cyano or nitrile groups. In addition, a low BE peak from Fex N species can be also observed in the spectra. These assignments have been obtained from the analysis of the XPS spectra of model molecules (pyrrole, pyridine, Fe-phthalocyanine, etc.) and from density functional theory calculations of core-level binding energy shifts [86–89]. The analysis of the Fe 2p core-level region may be more complicated, in part due to the low content of iron in Fe/N/C catalysts. The following peaks might be observed, a peak for metallic iron at ca. 707 eV, which may be indicative of the presence of metallic iron clusters or iron carbides. The presence of the latter species can be confirmed from the analysis of the C 1s core-level region. However, it not unlikely that metallic iron or iron carbides are detected by bulk-sensitive techniques such as XAS, XRD or 57 Fe Mössbauer, but not by XPS. This is because, especially in the acid washed catalysts, metallic iron or iron carbides particles are usually wrapped within several graphite layers, therefore out of the detection range of the XPS analysis. A further Fe 2p corelevel peak, with the Fe 2p3/2 peak at ca. 709–713 eV characterizes Fe oxidized species, and it can be therefore assigned to Fe(II)-Nx species or to Fe(II) and/or Fe(III) atoms in iron oxides. The spectrum of iron oxides display shake-up satellite peaks for Fe2+ (at ca. 715 eV) or Fe3+ (at ca. 719 eV), so the lack of such satellite peaks can be indicative of the presence of Fe-Nx moieties, although the presence of Fe3 O4 cannot be ruled out [90].

8.5.3 X-ray absorption spectroscopy XAS requires a combination of high intensity and exceptional energy resolution, demanding synchrotron radiation. Each element has a characteristic set of excitation energies, usually in the range of keV. The different excitation energies (edges) of each element depend on which core electrons are excited (K-, L- and M- edges). Excitations of the 1s electrons occur at the K-edge, excitations of electrons from the 2s occur at the L1 -edge, excitations from the 2p½ occur at the L2 -edge, excitations from the 2p3/2 occur at the L3 -edge, etc. In the case of the K-edge the most intense features are due to 1s→p states and in the case of L2 and L3 -edges are transitions 2p→d states [91]. The wide range of energy (0.1–100 keV) needed to analyze the different edges of the different atoms (for instance, Fe and N atoms) involves the need of soft X-ray synchrotron radiation (10 eV to 10 keV energy range) and hard X-ray synchrotron radiation (ca. energies between 10 and 120 keV, or wavelengths of 0.10 and 0.01 nm), so usually different synchrotron instruments have to be used. In general, XAS study the valence state of a selected element, its electronic configuration, and its environment, including the atoms (ligands) coordinated to such element, as well as the number of these ligands and their distances to the element. A great advantage of XAS is that it does not require long-distance order between the ions that are under study, being very useful for the study of single-atom catalysts. XAS is a technique widely used to determine the presence of single atom M-Nx moieties and in some cases it is able to differentiate the architecture of such moieties [92]. Usually the XAS regions analyzed on M/N/C catalysts are the transition metal edges,

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FIGURE 8.11 (A) XANES spectra of a catalyst pyrolyzed at different temperatures compared to Fe phthalocyanine as reference. The data were taken in situ at 0.1 V to ensure the Fe2+ oxidation state of Fe inside FeNx moieties. (B) corresponding EXAFS spectra. Reproduced with permission from [49].

for instance, Fe K-edge (E ˜7120 eV). However, in the last years also the edges of the light atoms are being analyzed. In particular, it is very promising the study of the Near Edge X-ray Absorption Fine Structure (NEXAFS) at the N K-edge (E ˜ 400 eV), to determine the environment and architecture of N ions coordinated to Fe. In general, in a XAS spectrum there are different regions to consider. (1) Xray absorption near-edge structure (XANES). Just for clarification, XANES and NEXAFS have the same meaning but, historically, NEXAFS is used for soft X-rays and XANES for hard X-rays. The XANES edge of the M cations (usually the K-edge) gives information about the oxidation state of the absorbing ion with high sensitivity. The oxidation state depends on the position of the XANES edge. As the valence of an element increases, the edge position shifts to higher energy. In Fig. 8.11, several Fe single-atom catalysts are compared with Fe phthalocyanine. The position of the edge is the same, indicating a similar Fe2+ oxidation state in the catalysts than in Fe phthalocyanine [93]. A very useful tool for the characterization of single-atom catalysts using XANES is to fit the XANES experimental data with calculated models, trying to solve the configuration-architecture of M single-atoms in sites such as the ones in M/N/C moieties. For instance, in reference [92] several architectures are fitted to the data, with different N coordination and different axial positions occupied. In the XANES region of the transition metal K-edge spectra there is another region interesting to explore. This region is the XANES pre-edge that appears at lower energy than the edge itself (see Fig. 8.11A, in the Fe K-edge appearing at ca. 7115 eV). It gives information about the geometry (close coordination environment) of the M inside the M-Nx moieties. In the case of Fe phthalocyanine, a pre-edge appears because the Fe atoms are located in square-planar FeN4 sites (see Fig. 8.11A). In the M/N/C catalysts this pre-edge usually does not appear or it is much less pronounced due to the

8.5 Physicochemical characterization of pyrolyzed M/N/C catalysts

(A)

(B)

S* Intensity / a.u.

Initial Sample After Acid AST After Alkaline AST

396

400

404

408

412

Photon energy / eV

416

Intensity / a.u.

V*

396

Fe-Pyrrol N-Pyrrol Fe-Pyrid N-Pyrid

398

400

Initial Sample After Acid AST After Alkaline AST

402

404

406

Photon energy / eV

FIGURE 8.12 (A) N K-edge NEXAFS signal of a Fe/N/C catalyst (black) and of the catalyst after ORR in acid (red line) and alkaline (blue line). (B) p-resonance region of the N K-edge NEXAFS spectra, where the resonances corresponding to the pyridine-like and pyrrolic-like configurations, both with and without Fe atoms, are marked. Reproduced with permission from [98].

occupation of some of the axial empty positions of the square planar MN4 moieties, or to a bending of the moieties [95]. The same level of detail achieved to understand M environment with XAS has not been reached for the identification of N environment on M/N/C catalysts. In the last years N K-edge NEXAFS has been found very useful to understand N coordination. As it was previously detailed in the XPS characterization section, M/N/C catalysts prepared by thermal treatments display N atoms in different configurations, typically graphitic, pyrrolic and pyridinic with and without coordination with M. The ORR activity and stability of the different N-configurations is controversial [96,97]. As an example, Fig. 8.12 shows the N K-edge spectra of a Fe/N/C catalyst and the same catalyst after ORR reaction in acid and alkaline media [98]. The regions corresponding to ∗ and σ ∗ resonances describe the typical spectroscopic features in N K-edge spectra. The pi-resonance region can be explained by a variety of N chemical configurations, which can be associated to pyridinic, pyrrolic, quaternary, and graphitic N species [96–102]. From that references N-peaks of the ∗ region can be assigned. The peaks at ca. 398.7, 399.6, 400.5, and 401.2 eV have been assigned to N-pyridinic, FeNx -pyridinic, N-pyrrolic, FeNx -pyrrolic, respectively. As shown in Fig 8.12A, the intensity of the N signal of the used catalyst is lower than that of the initial, indicating the loss of N species during the ORR in both media, either coordinated to Fe or not. Also, by using this technique, it is possible to evaluate the degradation of the different type of N sites over the ORR reaction in both media.

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The EXAFS region appears at higher energies (beyond the edge) than the XANES region. Different species of an element display differences in the fine structure beyond the edge. The white line and the oscillations differ between the species and the oscillations can be isolated and Fourier transformed. The Fourier transform (FT) of the EXAFS region is usually presented in k-space or in R-space. It gives direct information about the radial distances (R) of the absorbing atom. For instance, the radial distances of Fe-N in FeNx moieties appear around 1.6 Å. Usually, this radial distance is compared with Fe phthalocyanine that only present such Fe-N distances (see Fig. 8.11B) [93]. The assignment of the peak to Fe-N species should be taken cautiously, since this radial distance can also account to the presence of Fe oxides, since Fe-N and Fe-O scatterings are similar. However, Fe-O also leads to high intense features between 2.5 and 3.0 Å, unless Fe oxide particles are too small (nanoparticles) so these features at higher radial distances are too weak. There are many publications about Fe/N/C single-atom catalysts in which XAS has been used to characterize the catalysts [47,103–106]. In some of these works only the radial distance around 1.6 Å was observed, indicating that only FeNx moieties are contained in the catalysts [105]. Also XAS studies have been performed on M/N/C with other transition metals [95,107]. For instance, in the case of a NiNx based catalyst, when analyzing the Ni k-edge, the Ni-N backscattering interactions appear at distances of around 1.44 Å, which are slightly shorter, but again very similar, than the Ni–O bond distances of 1.65 Å in NiO6 octahedra of NiO. Also the presence of metallic Ni will show radial distances at ca. 2.18 Å, corresponding to Ni-Ni distances in bulk metallic Ni [95].

8.5.4 Transmission electron microscopy TEM instruments provide imaging, diffraction, spectroscopic, tomographic, and holographic information at single-atom resolution. The most outstanding novel features are based on the aberration correctors and monochromators that have increased the resolution to less than an angstrom [108]. In general, TEM provides information about: (1) The morphology: size and shape of particles and their distribution. (2) The crystal structure by using electron diffraction (ED) and selected area electron diffraction (SAED). (3) The atomic structure: High-resolution TEM (HR-TEM) and highresolution scanning transmission electron microscopy (HR-STEM) are two different imaging modes that allow for direct imaging of the atomic structure of the catalysts. The image theories for these two modes are different: HR-TEM is by phase contrast while HR-STEM is by atom scattering (Z-contrast). Both are powerful tools to study solid catalysts on the atomic scale. HR-STEM is essentially a high-angle annular dark-field (HAADF) technique in which the heavy atoms scatter stronger (appearing brighter) than the light atoms. (4) The composition: There are two techniques to elementally identify atoms, and in the most advanced microscopes these techniques have single-atom sensitivity: Electron energy loss spectroscopy (EELS) and energy

8.6 Metal-containing heteroatom-doped carbon nanomaterials

dispersive X-ray spectroscopy (EDX). The differences and advances of both EELS and EDXS were compared in detail by Isaacson and Johnson [109]. EDX is the routine and standard technique for qualitative and quantitative elemental analysis. EELS has intrinsically better signal/noise ratios than EDX, which is an advantage especially for light elements (e.g., B, C, N, O, and P). EELS has become the leading analytical technique in HR-STEM. Using the aberration correctors, the sensitivity to low-energy X-rays has been improved and, as a result, EDX is now able to solve atomic maps in crystalline samples with spatial resolution of 2 Å [110]. (5) Electronic information: EELS can provide information about the chemical bonding and oxidation state of the ions in a catalyst, by analyzing the energy distribution of the transmitted electrons that have interacted with the core electrons. Advanced STEM has been used to determine single atoms in M/N/C catalysts. Li et al. [111] observed Fe and N atoms in carbon nanotube-graphene layers. Using ADF-STEM they observed the single Fe atoms located preferentially on the edges of the graphene structure exfoliated from the outer wall of the carbon nanotubes. And by EELS mapping, they showed that Fe and N atoms were often adjacent to each other, with possible chemical bonding between them. In 2017 Chung et al. [112] moved one step forward and, using ADF-STEM imaging, EELS analysis and DFT calculations, proposed FeN4 in graphene as the ORR active sites. Carbon-embedded nitrogen-coordinated iron (FeN4 ) moieties were directly visualized. Also, a higher concentration of Fe was observed on edges and steps. EELS revealed the tendency for Fe and N atoms to be co-located in the catalyst, directly suggesting coordination between Fe and N. With these results they were able to correlate a higher ORR activity with a higher density of atomic Fe sites. However, they revealed that a further increase in the Fe content produce a decrease in the ORR activity, due to a gradual loss of atomically dispersed Fe, forming Fe clusters, and finally Fe-rich nanoparticles. Fig. 8.13 illustrates the use of microscopy techniques for the identification of isolated metal atoms at atomic level resolution within N-doped graphene-like materials, and of Fe3 C particles wrapped within several graphite layers. In 2018, a series of monodispersed atomic transition metals (Fe, Co, Ni) embedded in N-doped graphene with a common MN4 C4 moiety where reported. The moieties where directly observed by TEM imaging [95]. Several recent publications have reported HAADF and ADF images of single-atom M/N/C catalysts [28,107,114– 116].

8.6 Metal-containing heteroatom-doped carbon nanomaterials for OER and HER reactions This section addresses briefly the most recent developments in the area of metalcontaining heteroatom-doped carbon nanomaterials for the OER and for the HER. Electrolyzers are devices that generate H2 and O2 by H2 O-electrolysis, preferably

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CHAPTER 8 Metal-containing heteroatom doped carbon nanomaterials

FIGURE 8.13 Upper panel (old central panel). High-resolution TEM images showing the direct visualization of singe atomic metals of Ni, Fe and Co in a 2D graphene lattice. Reproduced with permission from [113]. Bottom panel TEM image and SAED pattern of a Fe3 C particle surrounded by graphite layers showing dislocations reprinted from reference [65].

8.6 Metal-containing heteroatom-doped carbon nanomaterials

FIGURE 8.14 Representation of polarization curves of different “catalysts” for anode and cathode reactions in PEMWEs with state-of-the-art (SoA)PGM catalysts.

using decarbonized-renewable electricity. The overall reaction (Eq. 8.3) of the electrolysis process in acidic media is the sum of the two half-cell reactions, as shown in the equations below: OER (Anode) 2H2 O → O2 + 4H + + 4e−

(8.7)

HER (Cathode) 4H + + 4e− → 2H2

(8.8)

Overall Reaction 2H2 O → O2 + 2H2

(8.9)

As shown in Fig. 8.14, the kinetics of the OER are slower than the HER even with SoA PGM catalysts, namely Pt in the cathode and Ir-oxides in the anode. Similar to the ORR, PGM are the only suitable catalysts for acid water electrolysis [117]. Due to the very fast kinetics of Pt for the HER, the amount of Pt in the electrode is very low. Ir oxides are the SoA anode catalysts in PEMWEs [118]. Finding active, durable, and abundant nonexpensive catalysts is one fundamental topic of research for the electrochemical community [119,120]. In recent times, metal-containing heteroatom doped carbon nanomaterials have been proposed as alternative catalysts to replacing SoA PGM-based catalysts. Note that these catalysts are similar to those described for the ORR above, so the synthetic routes and their characterization display the same principles that have been described in previous sections. As shown in Fig. 8.14, the kinetics of the OER are slower than the HER, so the greater technological interest is to find PGM-free catalysts for the anode reaction. The catalysts for the OER are mostly based on co or Ni single atoms, especially in acid electrolyte, although Fe-based catalysts have also been studied for the OER

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CHAPTER 8 Metal-containing heteroatom doped carbon nanomaterials

(A)

Ni2+ +

Annealing

Leaching A-Ni@DG

Ni@DG

DG

(B)

(C)

1 Pm

BF

C

DF

Ni

(E)

1 nm

2 nm

(G)

0.5 nm

1 nm

0.5 nm

(I) 5

Ni foil Ni@DG A-Ni@DG 8350 8400 Photon Energy (eV)

Magnitude of FT [k2x(k)]

(H)

8300

(D)

(F)

Intensity (a.u.)

198

8450

Ni foil Ni@DG A-Ni@DG

4

(J) ExpData Fit Di-vacancy

3

D5775 Perfect

2 1 0

0

1

2

3 R (A)

4

5

6

8300 8320 8340 8360 8380 8400 Photon energy (eV)

FIGURE 8.15 Upper panel: Characterization of Ni single atom catalysts (synthesis, STEM-EDX mapping, HADDF-STEM images, bright field STEM images, Ni k-edge XAS spectra). Bottom panel: HER and OER activities reported in 0.5 M H2 SO4 and 1 M KOH, respectively. Reprinted with permission from [125].

8.6 Metal-containing heteroatom-doped carbon nanomaterials

(B)100

Pt/C DG Ni@DG A-Ni@DG

0

−10

10

−20

TOF (s-1)

Current density (mA/cm2)

(A)

−30 −40

Trend

MoS/xNCNT MoSx -G * * [Mo*3S13]2- * MoSx

1

* * Mo2C MoxC/Ni

0.1

−50

*

NiMo MoP

Co-NG(eincle storm)

−60 −0.4

−0.3

−0.2

−0.1

0.0

0.01

Potenial (V vs RHE) 60

Current density (mA/cm2)

50

10

30 20

Trend

1

10

Ni0.2Fe0.1OOH * N/C-NiO NiO * CoOx@Au * * CoMn LDH OOH Au-Ni0.75Fe*0.25 * NiFe LDH@CNT * P-MnCo2O4

0.1

0

(E)

300

100

40

1.2

Ni2P

50 100 150 200 250 Overpotenial (mV vs RHE)

(D)

Ir/C DG Ni@DG A-Ni@DG

TOF (s-1)

(C)

* Ni2P/RGO

1.3

1.4

1.5

1.6

1.7

200

Potenial (V vs RHE)

0S

5S

10 S

* NiFe/NF

250 300 350 400 450 Overpotenial (mV vs RHE)

15 S

500

20 S

FIGURE 8.15 Continued

in alkaline. Co-coordinated framework porphyrin with graphene hybridization was designed as the precursor to fabricate single-atom Co/Nx /C for OER [121]. Graphene is a beneficial support due to its high surface area, high electronic conductivity and good stability. The catalyst gives an overpotential of 470 mV (1.7 V) at 10 mA cm−2 (overpotential at 10 mA cm−2 , η10 ), value selected as the descriptor for OER activity [122]. The η10 is only slightly smaller than 410 mV required for Ir/C. Another method to prepare Co single-atom catalysts is using MOF derived from the ZIF-67 as precursors (“sacrificed-template” method) [123]. Atomically dispersed Co-Ni sites in

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CHAPTER 8 Metal-containing heteroatom doped carbon nanomaterials

N-carbon have been also reported as ORR/OER catalysts, with η10 1.57 V [124]. An atomically dispersed Ni catalyst on defective graphene was synthetized by a wetness impregnation method that is active to HER and OER, see Fig. 8.15. The η10 in alkaline media is 1.5 V. The stable OER performance demonstrates robust immobilization of Ni on the substrate [125]. Porous carbon-anchored Fe single-atom electrocatalyst with bifunctional OER/ORR activity in alkaline media have been reported [126]. A self-template strategy was used for the synthesis. The catalyst presents a η10 of 1.68 V, a Tafel slope of 114 mV dec−1 , and stability during 3000 CV cycles up to 1.8 V, with a negative shift of 20 mV. There are other examples of metallic single-atom catalysts for OER [95,127]. Although the catalysts shown above display reasonable OER activity, a major drawback of carbon-based materials for the OER is the lack of stability of carbon under the severe oxidizing environment of the OER, that is, highly acidic environment and potentials around 1.6–2.0 V. Regarding the HER reaction, there are several publications about active heteroatom doped carbon nanomaterials. Fei et al. fabricated atomically dispersed Co on N-doped graphene by subjecting graphene oxide and cobalt salts to pyrolysis in an NH3 atmosphere [128]. Other Co single atom catalysts have been reported as HER active in acid media [107]. Also reports with Ni single atoms have been published as active catalysts for HER in acid [125,129]. Fan et al. [130] used Ni-MOFs as precursors for Ni single atoms on graphitic carbon through pyrolysis, acid etching, and subsequent electrochemical activation. Xu et al. synthesized hierarchical graphitic porous carbon architectures with atomically dispersed FeN doping through pyrolysis and subsequent acid etching of a MOF composite precursor [131].

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CHAPTER

Metal-organic frameworks for the electrocatalytic ORR and HER

9

Anuj Kumar a, Shashank Sundriyal b, Charu Goyal a, Tribani Boruah c, Dipak Kumar Das a, Ghulam Yasin d, Tuan Anh Nguyen e and Sonali Gautam a a

Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India, b Advanced Carbon Products Department, CSIR-National Physical Laboratory, New Delhi, India, c Northeast Hill University (NEHU), Umshing Mawkynroh, Shillong, Meghalaya, India, d Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, e Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

9.1 Introduction In order to continue with sustainable global growth, the human appetite for energy is rapidly growing and becoming difficult to gratify due to excessive use of conventional energy sources creating environmental issues, including climate transformation, increasing tendencies of glaciers to melt resulting rise in sea levels [1,2]. Migration of approach on strong reliance on fossil fuels toward development of renewable, green, safe and sustainable energy and conversion technologies is the outcome of intense attention in the energy research community [3,4]. Out of all nonconventional energy resources [5], hydrogen is assumed to be one of the front-line energy carriers [6]. In this context, hydrogen fuel-cells [7] coupled with water electrolyzers [8] can be executed as the sustainable energy conversion cycle [9]. However, this model although efficient but is associated with several challenges like hydrogen storage, poor electrode kinetics, designing of membrane and catalysts layer assemblies including material cost, which discourage its commercial applications [10]. In this chapter, our primary aim and focus are dedicated to electrocatalysis in fuel cells to address the poor electrode kinetics as well as hydrogen production for the development of fuel cells technologies. In fuel cells, cathodic reaction, that is, oxygen reduction reaction (ORR) rate is extremely poor which essentially demands the use of suitable, efficient electrocatalyst [11]. Although, Pt-based materials were found to be successful to address this issue but their cost is again an unavoidable issue [12,13]. In this regard, sufficient reports are available related to cost-effective ORR electrocatalysts such as metal oxide, metal phosphides, metal sulfides, single atom catalysts, etc. However, further development with these catalysts is restricted due to their poor understanding of actual active sites Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00014-9 Copyright © 2022 Elsevier Inc. All rights reserved.

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and mechanisms [14]. Molecular catalysts like MN4 -types systems (porphyrin, corrin and phthalocyanines and their derivatives) are also tested and have been found to be potential ORR models with well-known active site and mechanisms but still have the limitation of conductivity and durability [15–17]. Therefore, the construction of durable molecular systems with significant conductivity is demand of the hour and metal-organic frameworks (MOFs)-based materials are a potential candidate in this context. MOFs are the organic and inorganic hybrid materials with adjustable poresized porosity, desirable conductivity and chemically modifiable structure and these unique qualities make MOFs material of choice for energy conversion as well as storage devices [18]. The recent advancements in the field of nanotechnology and available latest characterization techniques have opened new dimension for further development of modified MOFs-based materials with manageable size and morphology, responsible for their catalytic applications. The noteworthy properties of MOFs-based materials have attracted remarkable attention and investment toward these innovative materials for fuel cells [19,20]. In this chapter, we highlighted the engineering and effective strategies to fabricate MOFs-based materials for electrocatalytic applications in Section 9.2. In Section 9.3, we discussed the importance of MOFs-based materials for ORR electrocatalysis as well as hydrogen production. Further, in Section 9.4, we have discussed the challenge and future prospective on MOFs-based materials and their use in fuel cell technologies.

9.2 Engineering and effective strategies for modification of MOFs MOFs are highly porous materials with good chemical and thermal stability, however, pristine MOFs are limited with number of limitations including poor conductivity and varying structural tailor ability [21]. Therefore, it is desirable to modify the structure of MOFs by intercalating various materials into it or by adopting different strategies as shown in Fig. 9.1. This results in the enhancement of physiochemical properties by introducing various categories of dopants into the voids of MOF cluster, by forming MOF-derived materials and synthesizing various composites of MOFs [22].

9.2.1 Modification of MOFs by doping The introduction of various types of dopants within the vacant space of MOFs cluster such as incorporating metal oxides, metal cationic species, carbon nanotubes (CNTs) [23], conducting polymers, etc. has been implemented to alter or improved various properties of MOF material. The principle behind this designing approach is that the vacant pores are loaded with different kinds of guest species that provides an opportunity to coordinate with secondary building units (SBU) of the MOFs skeleton via in-situ synthesis or postsynthetic modification techniques.

9.2 Engineering and effective strategies for modification of MOFs

GRAPHENE

TCNQ

MNPs

IODINE POLYMERS

MOFs

FIGURE 9.1 Schematic illustration of modification of MOFs through various designing approaches.

9.2.1.1 Redox-active molecules doped MOFs Designing of conducting pathways of network within the metal organic framework are implemented by introducing different redox active moieties such as ferrocene, tetracyanoquinodimethane (TCNQ), and iodine into the porous voids. TCNQ displays high electron affinity and behaves as an excellent redox active molecule [24]. Through π -π interaction, it has the capability to coordinate with metal centers, resulting in the formation of charge transfer complexes. It has been reported that Talin et al. [25] synthesized thin film of Cu3 (BTC)2 onto the silica-coated silicon wafers that later transferred to saturated solution of TCNQ and they observed a tremendous increment in the electrical conductivity of the resultant sample. This enhancement in the activity was attributed to the guest TCNQ particles that play a significant role in coupling of metal ions within the MOFs structural unit by acting as a bridge. Furthermore, iodine also considered as wonderful redox active molecule that can be incorporated as a guest molecule into the host MOF cluster to improve the oxidative and electronic properties of MOFs [26–28]. The mechanism of this approach suggests that the iodine doping within the microspores of MOFs facilitates intermolecular interaction with the pi-electron cloud of aromatic organic ligands. This results in the development of charge carrier within the skeletal framework which contributes toward the molecular transitions of charges within the nanochannels. This iodine doping technique can be performed by simply

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CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER

immersing the MOFs into the dopant sample or exposing the sample into the iodine vapor to get the resultant iodine doped MOFs holding excellent properties [29]. Another well-known organometallic compound that can be considered as a redox active species is ferrocene that act as an excellent mediator upon its incorporation into the microspores of MOFs via a charge hopping transport mechanism and delivers excellent improvement in the conductivity by eliminating charge injection barrier. In addition, electronic coupling can be enhanced between metal nodes to a signified level by adopting such methodologies. However, ferrocene is not considered as a suitable dopant as compared to other redox active molecules like TCNQ because of some of the limitations. As per the literature, it has been found that the conductivity induce by ferrocene is approximately around 109 S cm−1 . This is attributed because of the comparable larger structure of ferrocene to the pore size of MOF and these results in loading of only one ferrocene molecule per pore of a MOF [30]. Hence, this limits its application boundary.

9.2.1.2 Metallic species doped MOFs The modification of MOFs can be done by introducing various types of metallic species that can be categorize as metallic nanoparticles, metallic nanoclusters, and cationic molecules. Loading of metallic nanoparticles into the pores of MOFs skeletal increases the properties of MOFs to a significant level. Various synthetic strategies have been implemented to incorporate metallic species such as metal nanoclusters within the framework to enhance the conducting properties of MOFs. However, utilization of such modification as well as the use of such methods both supports the charge transport via tunneling mechanisms and help to maintain the intrinsic porosity of the framework. For example, RbOH and d-cyclodextrin react together to produce RbCD-MOF which was allowed to immersed in AgNO3 solution dissolved in acetonitrile solvent allowing the incorporation of Ag nanoparticles into the pores of MOF skeleton [31]. Interestingly the conductivity of the resultant material enhanced to a desired level by increasing the intensity of light at a temperature of about 352K. As per the report the improvement was due to the transport of electrons within the nanochannels of MOFs assisted by light and that could be utilized as photodetector.

9.2.2 MOF-derived materials Tremendous efforts have been made to alter MOFs into inorganic functionalized fabricated materials in order to increase its application range especially for electrocatalytic purpose. Such classes of materials are categorized under MOF-derived materials that convert a parent MOF into variety of materials including mainly nanoporous carbons, metals-oxides, metal-hydroxides, and many more functionalized entities along with their nanocomposites [32–35]. A wide variety of functionalized materials holding incomparable morphological properties can be synthesized within the micrometer and nanometer scales [36]. In addition, MOF structural units can be utilized as precursors

9.2 Engineering and effective strategies for modification of MOFs

Zn2

MOF growth

Pyrolysis

Exfoliation

RT 1h

1000°C 4h,Ar

KOH, sonication thermal activation

Zn2 Precursors for MOF-74-Rod

MOF nanorods (MOF-74-Rod)

Carbon-nano rods (CNrod)

Graphene nano ribbons (GNrib)

FIGURE 9.2 Synthesis of MOF-74-Rod, carbon nanorods and graphene nano-ribbons. Reproduced with permission [37]. Copyrights 2021 Nature Publishing groups.

substrate as well as template support for derived materials that is categorized under self-template synthesis and this certain factors serve as remarkable aspects for the development of MOF-derived materials with outstanding performance. MOF-derived approach demonstrated two notable characteristics. First, the combining effect of higher porosity of MOFs precursor as well as the removal of organic species during the transformation process leads to the formation of derived materials with highly porous structure. This results in the development of sample cavities over the surface of MOF-derived materials that contribute to the architecture of porous or hollow structures. Second, under appropriate conditions MOFs could be easily converted to inorganic functionalized materials because of its thermodynamic stability and also one can manipulate as well as organized the desired derived structures. However, lowerdimensional derived materials are also been reported in the literature. Evidently, an unusual approach has been adopted in order to synthesize one dimensional carbon nanorods and two-dimensional graphene nanoribbons [37] via heating rod-shaped MOF precursor (MOF-74-Rod) (Fig. 9.2).Nevertheless, in order to get the desired functionalized materials; it is quite challenging to alter the precursor MOFs configuration especially their metal centers [38]. In addition, the postsynthetic modifications or MOFs materials belonging to certain families that possess open structural framework might permit the alteration of specific metal centers to some extent and contributes in eliminating the issues [39,40]. Carbon-based materials are considered as a suitable inorganic functionalized material derived from parent MOF because of their exceptional porosity [41–44]. However, the synthesis approach of these materials involves pyrolysis of MOFs under inert atmospheric conditions. It is typically monitored by an acid-leaching procedure that could be avoided if the vaporization of metallic species takes place during the pyrolysis and finally results in the formation of porous carbon material with higher specific surface area [41,42]. In addition, the optimization of pore size of MOF derived carbon materials could be done successfully by adding other carbon sources into the pore-network of MOFs (Fig. 9.3).

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O

O

FA introduction

O O HO

O

OH OH O

O OH O O

O O

OH OH

OH

Bare ZIF-8

FA polymerization O

O

Carbonization O

O O

5 nm

O O

O

O

OH OH

OH OH

Porous carbon

FIGURE 9.3 Synthesis of porous carbon by carbonization of ZIF-8 with infiltrated furfuryl alcohol (FA). Reproduced with permission [42]. Copyrights 2021 ACS Publishing groups.

Alternatively, metal oxides can also be produced through the pyrolysis synthesis method where MOFs precursor materials are heated in the presences of air [44,45]. Moreover, other functionalized materials can also be obtained as a derivative of MOFs by their treatment with secondary substrate under appropriate reaction phase (gas or liquid phase). With this multipurpose approach one can easily synthesize hydroxides [46,47], phosphates [48], sulfides [49–51], phosphides [52,53], and selenides [54] along with their nanocomposites. However, as compared to other available inorganic precursors, MOFs holds a quality that it can be easily converted into different functional materials in well-defined manner. Even though most of the MOF-derived materials are found in powdered form however, self-supporting films could also be designed via the accumulation of MOFs on the proper substrates, such as porous graphene films, carbon cloths, and metal foils. Meanwhile, MOF-derived materials are widely applied as electrocatalysts. However, the catalytic efficiency can affect by altering the chemical arrangements. For instance, Huang et al. [55] reported that by controlling the chemical configuration of MOFderived materials, Zn0.3 Co2.7 S4 hollow particles demonstrate excellent hydrogen evolution reaction (HER) activity. In addition, by reducing the size of metal-active species to an appropriate size like single atom or ultrafine nanoparticles, then a greater number of active sites could be assessable. In fact most of the MOF-derived materials could be utilized for catalyzing more than one reaction. In other report, Xia et al. [56] reported, N-doped carbon nanotubes (NCNTs) composed hollow particles derived from ZIF67 that displays improved ORR performance and high durability as compared to their commercial Pt/C catalyst counterparts in presence of alkaline electrolyte. However,

9.2 Engineering and effective strategies for modification of MOFs

similar performance has been delivered for the OER. The outstanding electrocatalytic performance and stability was attributed to their ordered hollow structure, larger specific surface area, and strong graphitic nature of NCNTs.

9.2.3 MOF-based composites The poor conducting behavior and comparatively lower stability of MOF cluster result in limiting the range of application in various fields. Therefore, in order to overcome these drawbacks various designing approaches have been implemented to design and fabricate composites of MOFs with improved properties. This can be done by linking MOFs with conductive species such as conducting organic polymers, graphene, carbon nanotubes (CNTs), MXene type metal carbides/nitrides, and metal oxides, etc. [57].

9.2.3.1 MOF@ organic polymer-based composites Modification of MOFs by designing composites of polymer with MOFs is gaining recent interest among the researchers. In these approach polymeric materials plays a very significant role in developing MOF composites with improved properties such as enhancement in conductivity that leads to increase their range of applications among various sectors including catalysis. Some of the major polymers that are being utilized are PANI/MOF, PABA/MOF, etc. (PANI = polyaniline; PABA = Poly (3-aminobenzoic acid)). Among various literature reports, Dang et al. [58] synthesized a MOF@polymer composite which had been employed for electrocatalytic OER reactions. In this approach, they synthesized a bimetallic MOF (CoNi-NDC) via solvothermal method that grown on a layer of polyaniline polymer (PANI) that had been deposited onto nickel foam (NF) current collector to yield CoNi-NDC/PANINF electrode. Further, it has been reported that with a requirement of lower potential (η10 = 353 mV) these hybrid electrode could be utilized as a durable electrocatalyst following fast kinetics. Hence, the enhancement in the activity was attributed to the charge transfer from MOF to NF effectively in presences of conducting polymer.

9.2.3.2 MOF@ conducting carbon material-based composites MOFs are widely modified by incorporating activated carbon, graphene oxide, and carbon nanotubes (CNTs)-based conducting carbon materials, as they hold excellent specific surface area with oxygen rich functional entities onto their surface. Owing to these characteristics, carbon-based materials are categorized as best candidates for designing composites of MOFs and hence we are able to design MOF @conducting carbon material-based composites possessing incredible catalytic activity. As per the literature, plenty of research has been reported where graphene was utilized as carbonbased materials in order to design MOF@graphene composites holding excellent charge transfer characteristics that had been utilized for various electrochemical applications. Furthermore, as per the study of mechanism in such composites, graphene not only act as a template for the growth of MOF material but also at the same time it significantly contributes in eliminating aggregation issues. For instance, Xie et al.

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[59] reported an excellent grapheme-loaded Nickel-MOF composite with enhanced catalytic property where they designed the Ni-MOF composite by loading 10 mg of 3D graphene with MOF and found incredible increment in the activity. However, the increment of activity was attributed to the presence of strong bond between Ni-MOF with completely exposed structure. This is being dispersed via ultrasonication along with the development of synergistic interaction between MOF and graphene. Interestingly, the oxyfunctional moieties present on the surface of carbon materials plays a very significant role in strengthening the structure of the composites via formatting a metal coordination sphere through coordination with metal atom present on the MOFs skeleton. Therefore, all these reported works display that carbon materials not only contribute in strengthening the structural framework by coordinating with metal atoms even at harsh reaction parameter but also help in increasing catalytic activity to a desired level.

9.2.3.3 MOF@MXene composites The conducting 2D layered materials also came into picture for the modification of MOFs with desired properties. Among the 2D materials, MXenes are considered as wonderful candidates for designing MOFs composites. These are generally categorized under transition metal compounds such as metal carbides, nitrides, and carbonitrides and are relatively considered as the youngest member in the family of 2D conductive material. Because of the unique atomic spatial arrangements, desirable flexibility of structural framework, displaying wonderful conducting power along with effortless processability, all these incredible features declared 2D layered conducting carbon materials (MXene) as the most suitable candidates for designing the appropriate MOF-conducting carbon-based composites. In addition, these materials are also utilized as conducting template surface to fabricate inorganicorganic composites holding intrinsic conductivity. In one such report by Wen et al. [60] where they incorporated MXene within a bimetallic MOF (CoNi-ZIF-67) in order to synthesize a hybrid composite (CoNi-ZIF-67@Ti3C2Tx) (Fig. 9.4) with desirable electrocatalytic activity via coprecipitation synthesis strategy. On further examination of electrocatalytic activity, it has been revealed that the resultant hybrid composite displays better catalytic power as compared to standard IrO2 and MOF itself. The increment in electrocatalytic behavior was attributed to the following benefits such as - (1) the development of synergistic interactions between the MOFs and MXene results in enhancement of conductivity, (2) by the introduction of MXene moieties within the MOFs structural skeletal the oxidation state of metals increased (3) smaller MOF particle size along with enhanced electrochemically active surface area (EASA), etc.

9.2.3.4 MOF@ metal chalcogen composites As per the literature many experimental work has been reported on the utilization of metal-oxide nanoparticles (MNP) for the development of electro-catalytically active materials. However, due to their smaller particle size possessing large surface area leading to the formation of aggregates and as a result they are not considered

9.2 Engineering and effective strategies for modification of MOFs

HF MAX phase

Etching

Ti3C2Tx

mixing

2-methylimidazole

Ni2+

stirring CoNi-ZIF-67@Ti3C2Tx

Co2+

FIGURE 9.4 Designing scheme of CoNi-ZIF-67@Ti3 C2 TX. Reproduced with permission [60]. Copyrights 2021 MDPI Publishing groups.

as a suitable material for extended period of application. Therefore, it has been illustrated that MOF materials not only serve as a template support for these metallic oxide nanoparticles but can also form desired composites under appropriate reaction conditions as per the requirement for certain applications. Thus, the composite so formed displayed excellent improvement in the rate of catalytic efficiency due to the synergistically involvement of multimetallic character within the framework. Evidently, in one of the works reported by Gao et al. [61] they designed Ni-MOF composite via loading Fe2 O3 nanoparticles with appropriate quantity onto the surface of Ni-MOF where the noncoordinating hydroxyl group linked with Ni-MOF results in the generation of Fe2 O3 @Ni-MOF composite through the fast surface reaction between phenol and iron.

9.2.3.5 Metal/metal oxide supported surface-coordinated MOF thin films (SURMOFs) Different approaches have been developed in order to modify MOFs composites. In general, carbon-based conducting species and metallic nanoparticles are widely being used for the fabrication of MOFs structural frameworks and are being widely utilized for electrocatalytic purpose. However, it has been observed that these materials possess some of the drawback that leading to limit the catalytic performance. After proper analysis, it has been reported that the reduction in the performance was attributed to the deposition of materials onto the surface of electrode. This may result in leaching later on and also leads to the masking of vacant pores thereby decreasing efficiency of

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CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER

the material. Therefore, in order to overcome these challenges, various methodologies have been adopted such as direct growth approach, layer by layer installation or electrochemical deposition of MOFs materials onto the surface of highly porous metal foam or metallic plates are on great demand. Interestingly, owing to the various potential benefits, porous conducting metallic foams are being utilized as a support for the growth of MOFs. This leads to the development of increased number of active sites over the surface along with the reduction of charge transfer route delivering efficient electrocatalytic conductivity [62]. In addition, these metallic foams also provide an opportunity to fabricate variety of porous materials holding incredible benefits by linking with MOFs because of their inherent spongy consistency.

9.3 Applications of MOFs-based materials in fuel cells Molecular catalyst in homogeneous catalysis has many leads over heterogeneous catalysis, most remarkable being well-defined catalytic intermediates which can be separated, analyzed to assess their mechanism during catalysis [63]. However, poor recoverability and reusability of catalysts are the main disadvantages in homogeneous catalysis [64]. Moreover, limitation of solubility of molecular catalysts requires the use of organic solvents during catalysis [65]. Besides, the active sites of molecular catalysts are effective when available in diffusion layer around the surface of electrode which acts as the electrons source to regenerate the active site of the catalyst continuously to undergo interaction with target molecule [66]. Therefore, such type practical construction of energy conversion devices is very challenging. In contrast, in heterogeneous catalysis, the active site of a heterogeneous catalyst can also be regenerated to proceed the electrocatalysis regularly [67]. However, interaction of active sites with target molecules such as O2 , H2 and CO2 , is the decisive step in heterogeneous catalysis because of involvement of various electron transfer steps under energy barriers, where, the sluggish step can be hurried up utilizing an appropriate electrocatalyst [68]. The immobilization of molecular catalysts onto electrode surface [69], resulting heterogeneous structure is advantageous due to their better stability, accessibility in aqueous media, regeneration of active sites during catalysis through redox processes, and follow the inner-sphere electron transfer mechanism [70]. Moreover, molecular catalysts have discrete energy levels which could be correlated with their electronic or electrochemical and catalytical properties. The systematic incorporation of organic and inorganic building blocks results the having redox-active metals, redox-active organic linkers, host–guest interactions and charge-transfer in terms of e-addition, removal, and transport. Additionally, the availability of adequate active sites, regular channels as well as high surface area of MOFs make them contenders for heterogeneous electrocatalysts for various electrocatalysis [71]. Moreover, their chemically amendable pore structure, pore size, and channel environment promote MOFs to become model templates for coated active guest molecules for electrocatalysis. Moreover, MOFs provide the opportunities to add additional active species with well-regulated size, morphology, constituent, and

9.3 Applications of MOFs-based materials in fuel cells

location of active sites [72]. Besides, the possibility of synergistic effects among all the functional moieties would increase the electrocatalytic activity of the MOFbased materials and reasonable permeability of MOFs pores can promote quick mass transport during the electrocatalysis [73]. Along with the selectivity is initiated by substrate specificity which all together makes MOFs an ultimate contender for electrocatalysis [74]. In this section, we have highlighted the importance of MOFsbased materials in various electrocatalytic applications.

9.3.1 MOFs for electrocatalytic ORR Due to higher bond strength of O-O bond in dioxygen molecule requiring higher breaking energy, resulting ORR process sluggish in nature at cathode in fuel cells. Oxygen being paramagnetic possesses two unpaired e- in π ∗ orbitals, therefore, any entering e- will occupy π ∗ molecular orbitals, resulting decrease in O-O bond strength, that is, ORR. The ORR may proceed through 2e- (at low potential, resulting H2 O2 , indirect pathway) and 4e- (at high potential, resulting H2 O, direct pathway), involving multisteps electron transfer as well as oxygen-intermediates such as OOH∗ , O∗ , and OH∗ [75]. Therefore, efficient designing of catalysts rest on how effectively the binding energy of O-species can be accommodated on the active sites of the catalysts. A range of Mn, Fe, Co, and Cu-based MOFs have been reported to be effective ORR electrocatalysts in alkaline [13,76,77]. Chain et al. [78] conducted a systematic theoretical study on four Ni3 (HITP)2 analogs (Co-THT, Ni-THT, Co-CAT, and Ni-CAT) to investigate their ORR activity using DFT calculation (Fig. 9.5A[i–iv]). The optimization of O-intermediates on central metal active site of all catalysts suggested the systematic decrease in adsorption strength followed by Co-THT > Co-CAT > Ni-THT > Ni-CAT order. Further, the ionization potential and HOMO-LUMO gap measurements indicated that both TCo and TS active sites were capable to catalyze 2e− and 4e− ORR spontaneously, however, 4e− pathway was extremely energetically beneficial (Fig. 9.5A[v, vi]). Among all studied catalytic models, Co-CAT exhibited the highest ORR performance in terms of −0.86 eV formal potential (Fig. 9.5A[iii]). Zhong et al. [79] developed a highly conjugated 2D-MOF (PcCu-O8 -Co) incorporating the cobalt-bis(dihydroxy) molecules (CoO4 ) and CuPc (Fig. 9.5B[i]). Fig. 9.5B(ii, iii) showed the TEM image, indicating homogeneous distribution of 2D MOF layers. The prepared 2D MOFs exhibited unique features exposing maximum number of active sites along with improved e- transfer capacity. Further, the prepared PcCu-O8 -Co 2D MOF-CNTs composite displayed exceptional ORR performance displayed as high formal potential (0.83 V vs. RHE), number of e- transferred (3.93), and high kinetic current density (5.3 mA cm−2 ) in alkaline media (Fig. 9.5B[iv,v]). The in-situ Raman spectro-electro-chemistry and catalytic tests along with theoretical modeling indicated that Co-moieties were the ORR active sites. Additionally, this catalyst was used to fabricate cathode electrode in ZAB and showed higher power density (94 mW cm−2 ) as compared to traditional Pt/C catalysts (78.3 mW cm−2 ).

221

222

Me

Metal

(i)

Ni-THT

−0.185

X

(B)

−0.203

0.194

(i)

0.266

Metal

X

X Co-CAT

Me tal

(ii)

Metal = Co, Ni X = S, O

l Meta

Ni-CAT

−0.483

−0.456

0.665

0.680

20 nm M=Co, Fo, Cu, Ni

(iii) O/S M

(iv)

O2 N2

4

(110)

(iv)

(ii)

6

2 0

−2 −4

0.0 0.2 0.4 0.6 0.8

1.0

1.2

E (V vs. RHE)

*OOH

−1.24 eV

*H2O2

−0.10 eV

−3

−0.79 eV

−2.86 eV

*O + H2O

−1.46 eV

−1.50 eV

*OH + H2O−0.71 eV

−4 −5

(v)

Co-THT

2H2O

−0.56 eV −0.48 eV

Reaction coordinate

0 −1 −2

O2

*H2O

(vi)

−1.11 eV −2.01 eV

*O + H2O −1.74 eV

−3 −0.77 eV

−4 −5

TCo Pb(III)

−0.97 eV −0.37 eV −1.02 eV *OOH

Co-CAT

*OH + H2O −0.86 eV 2H2O −0.88 eV

Reaction coordinate

(v)

(vi)

PcCu-O8Co/CNT 0

PcCu-(OH)8/CNT Pc-O8-Co/CNT

−2

−2

−4

−4 −6 0.0

Initial After 5000 cycles After 10000 cycles

0

j(mA cm−2)

−2

TCo Ts

−0.55 eV

j(mA cm−2)

−1

O2

Relative energy (eV)

Relative energy (eV)

0

−6 0.2

0.4 0.6 0.8 E (V vs. RHE)

1.0

0.0

0.2

0.4 0.6 0.8 E (V vs. RHE)

1.0

FIGURE 9.5 (A) (i) Structures of Co-THT, Ni-THT, Co-CAT, and Ni-CAT, (ii) Mulliken charge distributions on Co-THT, Ni-THT, Co-CAT, and Ni-CAT, (iii) The energy diagrams for Co-THT and Co-CAT for ORR. Reprinted with permission [78]. Copyrights 2021 ACS Publishing group. (B) (i) Structure of PcCu-O8-M, (ii) TEM images of PcCu-O8-Co, (iii) CV and (iv) LSV curves at different scan rates (v) LSV curves before and after 10000 cycles recorded on PcCu-O8-Co/CNT in O2-saturated 0.1 M KOH, (vi) the proposed ORR reaction mechanism. Reprinted with permission [79]. Copyrights 2021 Wiley-VCH Publishing Groups.

CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER

X

Co-THT

(iii)

tal

j(mA cm−2)

(A)

9.3 Applications of MOFs-based materials in fuel cells

In another work, a new concept was introduced to design efficient MOFs-based ORR electrocatalysts [80]. The concept is founded on Zr-linkers to improve the chemical and electrochemical permanency (Fig. 9.6A). Fig. 9.6B and C showed the HRTEM image of PCN-226 indicating a rectangular pore packing with d200 = 18.32 Å, and 16.18 Å. Interestingly, this strategy successfully incorporated high density of redox active sites and improved the ORR kinetics as well as ORR activity (Fig. 9.6D and E). Further, this catalyst exhibited promising performance in rechargeable ZABs, with high power density (133 mW cm−2 ). This work can be served as a logical inspiration to design the MOFs-based electrocatalysts incorporating the chainbased linkages for the further enhancement in ORR performance. On the other hand, MOFs are not enough conductive which is require for the facile electron transfer process during the electrocatalysis. In order to improve the conductivity of the MOFs-based electrocatalysts, Loh et al. fabricated a composite incorporated MOFs with graphene and found that the prepared composite exhibited efficient ORR activity achieving 3.83 electrons involvements during ORR [81]. A. Fateeva et al. designed an Al-based MOF using Co(III) porphyrinic linkers. This MOF exhibited excellent ORR activity which was originated from cobalt (III) porphyrinic centers in acidic media [82]. Further, Morris et al. also reported a porphyrin incorporated MOFs architecture as excellent ORR electrocatalyst [83].

9.3.2 MOFs for hydrogen production Hydrogen is one of the important fuels in fuel cell technologies. Therefore, the reaction yielding hydrogen as a product should be given similar importance like other reactions such as HOR and ORR for the development of fuel cell technologies. Although, sluggish in nature, HER generates hydrogen and is therefore expected to be mass distributions of hydrogen-powered fuel cells technologies in the years to come by introducing suitable catalysts. Hydrogen evolution can be achieved electrochemically and photo-chemically.

9.3.2.1 Electrocatalytic HER Hydrogen production through water splitting electrochemically is carried out either in acidic or in basic medium. In order to avoid higher corrosion prevalence in acidic media, research in the field is shifted in alkaline media to perform HER. Low overpotential and stability along with lower value of Tafel slope is essential to be a catalyst with excellent catalytic HER performance like Pt-based materials. However, Pt-based materials do not have wide applications due to high cost and less abundancy of Pt metal. In this context, many electrocatalysts like transition metal phosphides, sulfides, and their alloys [84–88] have been evaluated with the limitations of uncertainty toward the identification of real active sites precisely responsible for the electrocatalysis. Moreover, MOFs-based materials have also been tested for hydrogen production by electrocatalytic water splitting with impressive outcomes.

223

224

(D)

ZrO7 Infinitive zigzag chain

COCH

COCH

HOOC

j(mA cm−2)

0 7.2A × 4.8A

−1 N2 O2

−2

c

0.2

a

0.4

0.6 0.8 1.0 E(V vs RHE)

COCH

a

c

(C)

-1 2nm

1.0 0.8

0.6

20 nm

5 nm

1.4

(E) E(V vs RHE)

i

(B)

1.2

−75.5 mV dec−1

−58.9 mV dec−1 −77.6

−72.0 mV dec−1

mV de

c −1

−4260 mV dec−1

Pt/C PCN-226(Co) PCN-221(Co) PCN-222(Co)

−5.0

−4.5

−1352 mV dec−1 −1413 mV dec−1

−4.0 −3.5 −3.0 log|j(A cm−2)|

−1539 mV dec−1

−2.5

−2.0

FIGURE 9.6 (A) A truncated molecular model of PCN-226, (B and C) HRTEM images of PCN-226, (C) CV curves of PCN-226 recorded in 0.1 M KOH. (D) Tafel slopes of the PCN-based prepared samples. Reprinted with permission [80]. Copyrights 2021 ACS Publishing Group.

CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER

(A)

9.3 Applications of MOFs-based materials in fuel cells

Qin et al. [89] developed a 2D MOF film incorporating Co-dithiolene moieties, indicating its excellent HER in acidic medium [90] due to Co-dithiolene moieties. Raoofet et al. [91] reported a MOF derived nanoporous C/Cu material. This material displayed well HER activity in acidic medium, showing lower Tafel slope (34.0 mVdec−1 ), and high current density (1.2 mAcm−2 ). Nivetha et al. synthesized a meso-Cu-BTC-MOF and the electrocatalytic HER studies on this MOF-based electrocatalyst showed excellent performance in terms of overpotential of 89.32 mV, onset potential of 25 mV, and low Tafel slope value of 33.41 mVdec−1 with high current density and long-term durability in alkaline medium. Monama et al. [92] reported a Pd-supported CuPc/MOF composite (Pd@CuPc/ MOF) via a chemical reaction between MOF and CuPc followed by Pd plating (Fig. 9.7a[i]). The electrochemical HER performance of this composite was investigated by CV and Tafel measurements. The Tafel slope value and transfer coeficient was found to be 176.9 mV dec−1 and 0.67, respectively, indicating the fast HER kinetics (Fig. 9.7A[ii–iv]). Moreover, the prepared composite shoed better HER performance in terms of high activity and lower onset potential as compared to parent MOF. The systematic HER mechanism for these materials is shown in Fig. 9.7A(v). Further, Hod et al. [93] reported MOF Film incorporating Ni-S moieties with significant porosity as shown in Fig. 9.7B(i). The fabricated hybrid MOF-Ni-S showed high HER performance as compared to Ni-S free MOF in terms of lower kinetic overpotential with 10 mA cm−2 current density (Fig. 9.7B[ii, iii]). The HER activity enhancement for hybrid MOF-Ni-S as compared to Ni-S free MOF can be attributed to the chemical environment modification of MOF by Ni-S moieties.

9.3.2.2 Photocatalytic H2 production Usually, hydrogen production photochemically requires photosensitizers, to create electron–hole pairs by harvesting light and desirable catalysts for H2 generation via photogenerated electrons. And MOFs-based materials possess both photosensitizer and stable catalyst moieties which are suitable for long-term HER photochemical process. MOFs incorporated with functional groups containing organic linkers, nanoparticles, MNn-types complexes (like porphyrin, phthalocyanines and their derivatives) and polyoxometalates are considered to be efficient materials for the photochemical H2 production. Additionally, MOFs-based materials possess unique advantages for specifically photochemical H2 production [94–96]; (1) porphyrin and amino groups can be incorporated into MOFs to tune their light-adsorption properties, (2) MOFs can be exploited as host or substrate to incorporate photoactive molecular moieties to show stable and recyclable photocatalysis, (3) proper porosity of MOFs ensure monodispersion active moieties, thus avoid the breakdown and deactivation of active moieties via multimolecular conduits, (4) porosity and large surface area of MOFs provide close contact between MOFs and moieties incorporated in MOFs to ensure the e-transfer and proper charge separation, and (5) more than one active moieties can be introduced into a single MOF in order to exploit the synergistic effect between the active moieties incorporated to bring enhanced photocatalytic action.

225

226

(A) (i) Synthesis scheme of the composite, (ii–iii) SEM.TEM image. (iv) The log-log plot of the absolute value of the peak current vs scan rate and (v–vii) peak current as a function of square root of scan rate for MOF, Pd@MOF and Pd@CuPc/MOF composite in 0.1 M (vii) mechanisms for hydrogen evolution reaction using Pd@CuPc/MOF composite (˜2.0 10−4 mol.L−1 ) at 0.10 Vs−1 in the presence 0.300 mol.L−1 H2 SO4 and 0.1 mol.L−1 TBAP/ DMSO electrolytic system on Au electrode. Reproduced with permission [92]. Copyright 2021, RSC Publishing Group. (B) (i) Schematic representation of NU-1000’s crystal structure, (ii) J–V curves. (iii) Tafel plots. The listed values of 182 and 188 mVdec−1 for the HER Tafel slopes of the bare FTO and FTO_Ni-S samples refer to the low current density region only. The slopes clearly are larger at higher current densities. Reproduced with permission [93]. Copyrights 2021 Nature Publishing Group.

CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER

FIGURE 9.7

9.3 Applications of MOFs-based materials in fuel cells

(A) −O

O

O

(B) O−

Visible Light

NH HN N O

O− O−

y x

O

e−

e− Pt Z

H2O

Z y

H+

y

FIGURE 9.8 (A) TCPP porphyrinic linker and crystal structure of the Al-PMOF viewed down [001], [100], and [010] directions, respectively. (B) The photocatalytic hydrogen production reaction involving the Al-PMOF, colloidal platinum, and sacrificial EDTA. Reproduced with permission [97]. Copyright 2021, Wiley-VCH Publishing Group.

Rosseinsky et al. in 2012 [97] incorporated a free-based mesotetra(4-carboxylphenyl) porphyrin as linkers with Al as central metal to build MOF-architecture (AlPMOF) as shown in Fig. 9.8A. This MOF exhibited all the light absorption bands in visible region, indicating its potential toward photocatalysis. The photocatalytic activity of this MOF was studied in MOF/EDTA/Pt system, where Pt and EDTA acted as HER active site and e- supplier, respectively (Fig. 9.8B). Further, azobenzene moieties were amalgamated on the ligands to get a dye-like ligand-based MOF, which displayed the signals in visible region to produce hydrogen using Ag as cocatalyst [98]. Moreover, the incorporation of costly metal nanoparticles may initiate the photocatalytic activity in three different ways; (1) metal nanoparticles (MNPs), 0D material, possess localized surface plasmon resonance (LSPR) and under the visible light irradiation can produce excited electrons, displaying adsorption band in visible region, (2) MNPs is capable to capture electrons from MOFs to initiate charge separation vindicating the recombination of electron–hole, and (3) precious metals can reduce the energy barriers of reaction intermediate, endowing HER [99]. For instance, Pt NPs were amalgamated inside or on the UiO-66-NH2 by means to produce Pt@UiO-66-NH2 and Pt/UiO-66-NH2 systems, respectively (Fig. 9.9A). The TEM images of Pt@UiO-66-NH2 and Pt/UiO-66-NH2 are shown in Fig. 9.9B and C. The ultrafast short-lived absorption and time-resolved photoluminescence studies indicated that Pt@UiO-66-NH2 system exhibited charge separation more efficiently as compared to Pt/UiO-66-NH2 , which can be attributed to the shorter e- transfer distance from UiO-66-NH2 to Pt NPs. Therefore, Pt@UiO-66-NH2 displayed higher

227

CHAPTER 9 Metal-organic frameworks for the electrocatalytic ORR and HER

(A)

(B)

O

HO

NH2

Zr4+ +

Pt NPs

solvothermal method

OH

O

Pt NPs

UiO-66-NH2

in situ MOF growth

Pt/UiO-66-NH2 20 nm

(C)

H2

Pt@UiO-66-NH2

(D) 700

H2O

50 nm 350

600

Pt/UiO-66-NH2

500

UiO-66-NH2

H2 production rate (Pmol/h)

Pt@UiO-66-NH2

H2 production rate (Pmol/g)

228

300

Pt@UiO-66-NH2

(E)

Pt/UiO-66-NH2

250

400

200

300

150

200

100

100 0 0.0

0.5

1.0

1.5

Irradiation time (h)

2.0

2.5

50 0

run 1

run 2

run 3

run 4

FIGURE 9.9 (A) Schematic illustrations of the synthesis of Pt@UiO-66-NH2 and Pt/UiO-66-NH2 , with the photocatalytic hydrogen production process over Pt@UiO-66-NH2 being highlighted. TEM images of (B) Pt@UiO-66-NH2 and (C) Pt/UiO-66-NH2 . (D) The photocatalytic hydrogen-production rates of UiO-66-NH2 , Pt@UiO-66-NH2 , and Pt/UiO-66-NH2 . (E) Recycling performance comparison between Pt@UiO-66-NH2 and Pt/UiO-66-NH2 . Reproduced with permission [100]. Copyright 2021, Wiley-VCH Publishing Group.

HER photocatalytic activity with significant stability and recyclability as compared to Pt/UiO-66-NH2 (Fig. 9.9D and E) [100]. Leng et al. [101] fabricated a porphyrin-based MOF by controlling release of metal ions with an extraordinary In(OH)3 . The prepared porphyrin-based MOF showed high stability toward photocatalytic HER. In this MOF, In ions in addition to forming Inoxo chains, also form metalloporphyrin. The results indicated that the OOP In(III) ions getting detached from porphyrin moieties under light, slowing down back e- transfer process, resulting in the improvement of electron−hole separation and photocatalysis. Further, Songet et al. [102] reported Cu(II)-based MOF with high stability toward photocatalytic HER.

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9.4 Conclusion and future prospects To conclude, among all the reported efficient electrocatalysts such as single atom electrocatalysts, metal oxide, metal phosphides, metal sulfides, etc., MOFs have been recognized as one of the impressive catalysts and a front linear as potential material for the development of energy sectors. Usually, large surface area, extensive network of pore structure, variety of organic linkers, easier synthesis of protocol and probability to introduce more than one functional moieties make MOFs as a most extensively used electrocatalysts for fuel cells. MOFs with high crytallinity, porosity, and significant surface area are produced by appropriate choice of suitable organic linkers and metal centers. Moreover, several redox active sites can be incorporated on MOFs to facilitate the energy conversion and storage applications. MOFs-based materials could be used effectively for oxygen reduction reaction in fuel cells as well as for hydrogen production via electrochemical and photochemical processes. Because hydrogen production is also equally important for the development of fuel cells technologies. In spite of tremendous improvements in MOFs-based electrocatalysts, several drawbacks are there like limited conductivity, chemical and mechanical stability, and durability due to their decomposition or degradation under electrolytic media which are needed to be addressed for practical application of MOFs in such technologies. There are several aspects like (1) conductivity; the intrinsic electrical conductivity is necessarily key parameter which decides the MOFs electrocatalytic activity. Generally, nanoengineering and incooporation of MOFs into thin films leads to enhanced eand charge transportation, resulting improved conductivity. Moreover, the inclusion of redox-active moieties in MOFs can initiate charge transfer, resulting enhancement in intrinsic electrical conductivity of MOFs, (2) chemical and mechanical stability and durability; for the practical applications of MOFs in energy devices, thermal, mechanical, and chemical stability and durability of MOFs should be considered. MOFs in general become short lasting due to decomposition, resulting breakdown of framework when get exposed to electrolytic environment. In this context, the choice of suitable organic linkers and metal nodes and their incorporation to construct MOFs can show higher thermal and mechanical stability in electrolytic media. For instance, ZIF-9, MIL-101, UiO-66, PCN-601, and so on have been designed, synthesized and studied with improved their thermal and mechanical stability.

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CHAPTER

10

LDH-based nanostructured electrocatalysts for hydrogen production

Mohammad Tabish a, Sehrish Ibrahim b, Shumaila Ibraheem c, Muhammad Asim Mushtaq d, Rashid Iqbal c, Tuan Anh Nguyen e and Ghulam Yasin c a

State Key Laboratory of Electrochemical Process and Technology for Materials, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, China, b College of Life Science and Technology, Beijing University of Chemical Technology, Beijing, China, c Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, d State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China, e Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Viet Nam

10.1 Introduction Electrochemical conversion systems are greatly emphasized owing to their energy conversion and storage potentials [1–10] with continuous use of small fossil fuel resources and environmental degradation [4, 11–15]. Electrocatalysts are important in these electrochemical conversion processes because they optimize reaction performance, speed up the reaction rate, and improve reaction selection and sensitivity [16–21]. However, rare metal-based materials are commonly used electrocatalysts for practical and commercial applications such as Ru-, Ir-, and Pt-based materials. Effective catalysts that mainly consist of low-cost nonrare metals are being discovered to substitute such expensive rare electrocatalysts for sustainable economic growth and industrialization [22, 23]. In recent times, electrocatalysis has been thoroughly investigated based on the non-noble metal, particularly oxides, phosphides, nitrides, sulfides, borides, and layered double hydroxides (LDHs) [24, 25]. Following these active electrocatalysts, LDHs have been very beneficial in terms of their excellent physicochemical properties, variant synthesis technologies, versatile structure composition, and remarkable electrocatalytic activities and are one of the most progressive electrocatalysts for hydrogen production [25, 26]. In modern times, 2D transitionmetal-based (TM)-LDH-nanosheets have been of keen interest to researchers because of the low cost, high catalytic activity, stability, and plentiful precursors [27]. As the synthesis process is easily adoptive and inexpensive, the majority of TM LDHs are nanomaterials that have dynamic tunability and adaptable structure character (such as host layer and interlayer anions). Depending upon these characteristics, TM LDH nanosheets are a favorable energy development catalyst. Numerous experiments Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00002-2 Copyright © 2022 Elsevier Inc. All rights reserved.

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and theoretical studies have demonstrated the excellent catalytic efficacy of the TM LDH nanosheets for the electrochemical conversion of hydrogen evolution reaction (HER) [28]. Furthermore, the electrocatalytic behavior of certain altered TM LDH nanosheets is analogous to electrocatalysts based on noble metals [23, 25].

10.2 Construction of TM-LDH nanostructures The peculiar structure of the TM-LDHs reveals a high layer charge density, making it very difficult to prepare TM LDH nanosheets with exfoliation techniques (in the top-down method) than those of other layered materials. The most commonly used method of exfoliation is fluid exfoliation, often followed by ultrasonication. Over time, various top-down methods such as plasma etching have been explored for TM LDH nanosheets synthesis [29], which can significantly reduce the time required for preparation. Excepting top-down technique, another way has been obliged to get TM LDH nanosheets namely bottom-up technique, with the presence of layer growth inhibitors without the presynthesis of bulk LDHs, for example, solvothermal or hydrothermal techniques, microemulsion methods, and coprecipitation methods. In addition, the direct bottom-up TM LDHs are synthesized by mechanical forces like microchannel reactor [30] and pulsed-laser ablation [23, 31–33].

10.2.1 Bottom-up approaches Based on the comparatively simple approach, bottom-up methods often pay attention to the presence of layer growth inhibitor and mechanically supported synthesis which is very responsible for the development of nanosheets of TM LDH with required dimensions, for example, coprecipitation, hydrothermal or solvothermal methods, and microemulsion technique. To formulate the nanosheets of TM LDH, the growth limitation is provided by the microemulsion method in the form of a “microreactor” environment [34]. The typical reverse microemulsion consists of water, oil phase and surfactant (occasionally with cosurfactant), for example, Zhao et al. synthesized NiTiLDH nanosheets by employing the microemulsion method as shown in Fig. 10.1 [35]. (1) In the water droplets, urea, and water salts were mixed; (2) LDH was started to generate after the hydrolyzation of urea; (3) The produced NiTi-LDH monolayer nanosheets were influenced under the isooctane environment; (4) After washing, the nanosheets were transferred into the water. To develop the NiTi-LDH nanosheets sodium dodecyl sulfonate (SDS, surfactant) and butanol (cosurfactant) behaved like the additive/stabilizer in isooctane (oil phase). However, the removal of surfactants from TM LDH nanosheets requires an extensive and difficult purification procedure. Owing to this problem, a new method has been proclaimed namely the surfactant-free microemulsion method [34], which is entirely relied upon the cosurfactant, oil phase and oil to water ratio. Yang et al. fabricated NiCo–LDH nanosheets as the ultrathin material by utilizing 2D graphene oxides (GO) as a precursor (Fig. 10.2) [36]. (1) The GO surface was adsorbed with nickel and cobalt as source material, (2) The oxidation of Co2+ to Co3+

10.2 Construction of TM-LDH nanostructures

(A)

(B)

Isooctane

Water

SDS Butanol

Ni2+ Ti4+

Urea

OH+

(D)

(C)

FIGURE 10.1 Schematic reflection of the growth of nanosheets of monolayer-NiTi-LDH in microemulsions: (A) Urea and metal salts dissolve in water droplets; (B) urea hydrolysis causes the growth of LDH; (C) in isooctane, monolayer LDH nanosheets form; (D) after cleaning, nanosheets are moved into the water surrounding [35].

Ni2+

Free Nucleation

Co2+

4.6 Á Free Growth

(i)

8.1 Á

Bulky NiCo-OH Plates

D- and E- phase

(ii)

+

In-situ Oxidation Co2+ −e−

Graphene Oxides (GO)

Co2+

+

-COOH, -OH -CH(O)CH-

8.1 Á Guided Growth

+

+e− CO32-, H2O

LDH-phase on Graphene

NiCo-LDH-G Nanosheets

FIGURE 10.2 Schematic representation of the growth process of NiCo-LDH-G nanosheets. (i) The absence of GO produces rigid and bulky NiCo-OH sheets of ∼4.6 Å; (ii) After the addition of GO, the NiCo-LDH nanosheets have a complex structure in the form of ultrathin NiCo-LDH-G nanosheets possessing the basal gap of ≈8.1 Å [36].

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could be occurred due to the excess of GO functional groups (e.g., –CH(O)CH–, –C– OH and –COOH) and (3) A simple coprecipitation method was used to create NiCo– LDH nanosheets. The inclusion of GO with a multidimensional hierarchical structure and significant surface area exhibited a very complex structure with a basal gap of ∼8.1 Å, and also showed a 2D flake-like structure. Subsequently, the addition of GO tends to construct 1.7–1.8 nm thick NiCo-LDH through controlled propagation.

10.2.2 Top-down approaches The top-down approach consists of the exfoliation process, which is the extensively used method to develop the TM LDH nanosheets. Firstly in 1999, Adachi-Pagano et al. utilized this technique to develop nanosheets with the help of ultrasonication [37]. The delamination of the LDH sheets was facilitated by the large basal spacing between the interlayers due to the existence of ions (anionic surfactants), moreover, the dispersant reduced the interactions between the host layer and interlayer which made it possible to delaminate the LDHs. Mostly solvents, for example strongly polar solvents (acrylate, alcohols, water, and formamide) [38] and mild polar or nonpolar solvents (e.g., toluene or CCl4 ) are being utilized as the dispersing agent for the exfoliation of LDHs [39]. A suitable solvent is essential to obtain successful LDH delamination. Researchers sometimes used butanol or nonpolar solvents (toluene or CCl4 ) as dispersants when the interlayer ions were substituted with dodecyl sulfate to increase the gap and cease the contact between the layers. While, for the intercalation of glycine, CO3 2− , NO3 − , or ClO4 , formamide is often chosen to disperse the LDHs (Fig. 10.3) [40]. Otherwise, water is the most commonly consumed dispersant of LDHs for the intercalation of lactate than organic solvents [40].

10.3 Carbon nanomaterial-based TM-LDH nanohybrids Carbon nanomaterials (CNMs) have a wide range of applications in electrochemical conversion reactions due to their excellent electronic conductivity, large surface area, thermal, and structural stability [41]. However, TM LDH nanosheets are constrained in catalytic efficiency by the comparatively low electronic conductance and reduced electrical active surface area (ECSA) due to nanosheets agglomeration. For that reason, researchers combined CNMs with TM LDHs to fix this issue. CNMs, comprising 0D carbon nanodots (CNDs)/carbon quantum dots (CQDs), 1D carbon nanotubes (CNTs), and carbon nanofibers (CNFs), 2D graphene and 3D carbon networks, are frequently utilized to amend TM LDHs [23]. Researchers were also drawn to the association between 1D CNFs and TM LDHs: (1) CNFs can serve as a current collector and can create electrodes without binders in combination with TM LDHs. It has not only lessened the electronic transfer resistance owing to the binder’s immobilization but also prevents the ECSA decrement, (2) CNFs polymerize several nanosheets of TM LDH and allow vertical growth over the surface which eventually avoids the stacking of TM LDH nanosheets, (3) Electron transfer can be facilitated by CNFs due to high electronic conductivity in between TM LDHs and CNFs, and (4)

10.4 Electrocatalytic application for hydrogen production

FIGURE 10.3 The commonly used approach for LDH-CO3 exfoliation in formamide. (A) For exfoliation, LDH-CO3 is converted into LDH-NO3 using ion-exchange; (B) With integrating ultrasonication, multistep ultrasonic treatments, and shaking LDH-CO3 is directly exfoliated [40].

A potent matrix with adequate physicochemical stability for TM LDHs is provided by CNFs, and a porous framework is found due to the larger size macropores. For example, Yu et al. produced a vertically positioned NiCo–LDHs/CNFs assembling (Fig. 10.4) [42]. Likewise, nanosheets stacking obstruction and the excellent electron conductance, the 2D graphene is employed as the base to host the TM LDHs. Yang et al. suggested a closely linked graphene and FeNi LDH as an outstanding electroactive material for the first time in 2014 [43]. Jia et al. produced a bifunctional electrocatalyst for HER and OER using a face-to-face NiFe–LDHs/defective graphene composite (Fig. 10.5) [44].

10.4 Electrocatalytic application for hydrogen production Hydrogen energy was considered as an ideal energy carrier due to different qualities (e.g., clean energy, high energy density, and carbon-free). The main source of hydrogen is primarily based on water and hydrocarbons. In recent times, hydrogen is mostly generated in hydrocarbon distillation, but it is an energy-intensive process

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FIGURE 10.4 The NiCo-LDH-nanoarrays and NiCo-LDH-microspheres preparation is depicted schematically in the presence and absence of carbon fiber papers (CFPs), respectively [42].

that emits greenhouse gas. The generation of hydrogen through water splitting is the most auspicious technique; also it is an environmentally friendly process and gives a high level of purity. The popular commercial HER Pt electrocatalysts however are very uncommon and costly, which raises hydrogen production costs. Researchers are looking for advance and efficient electroactive material with valuable sources to supersede Pt, sustain high hydrogen evolution, and increase economic profits. TMbased materials are thought to be ambitious candidates. Pure TM LDH nanosheets have deprived electrocatalytic HER output due to the low rate of water dissociation (Volmer step) in alkaline conditions. Fortunately, HER with improved performance can be obtained by utilizing modified TM LDH nanosheets [23]. Wang et al. [45] used the electrodeposition of CeOx nanoparticles on NiFe LDH nanosheets to create a novel and spatial electrode system (NF@NiFe LDH/CeOx) with self-supporting metal oxide/hydroxide and possessing the moderate oxygen vacancies. Besides, the NiFe LDH/CeOx interface will exploit the oxygen vacancy at different times of electrodeposition. Demonstrating the oxygen vacancies at the surface is due to a group of positive charge that leads to the difference in potentials between the electrons of NiFe LDH and CeOx, which is studied by density functional theory (DFT) calculations. Consequently, oxygen vacancy engineering can be employed to ameliorate the HER activity of NiFe LDHs. The overpotential of 154 mV is attained at 10 mA cm–2 current density, while the overpotential of 267 mV is obtained upon 20 mA cm–2 current density toward HER for NF@NiFe LDH/CeOx in alkaline solution. In the 1.0 M KOH electrolyte, it can be exerted as an operative bifunctional electrocatalyst for overall water splitting with a potential of 1.51 V and a current density of 10 mA cm–2 , which significantly performs better as compared to the state-of-the-art RuO2 and Pt/C (Fig. 10.6). Besides, Ren’s group [46] announced the new hierarchical electrode for effective overall water splitting, which comprises Cu@CoFe LDH core-shell nanostructure (Fig. 10.7). At 10 mA cm−2 , the tailored composite provides low overpotentials of 171

(A)

Exfoilated Nife LDH nanosheet

Defective graphene

30 20 10

v

0.25 0.20 0.15

S 10 10 10 -N Ir/C NG DG NG DH @ @ S@ iFe L NS NS -N HN HH D D D eL eL eL NiF NiF NiF

NiFe LDH-NS@DG10 NiFe LDH-NS@NG10 NiFe LDH-NS@G10 NiFe LDH-NS Ir/C

0 1.0

0

0.30

Current density (mA/cm2)

40

Overpotential (V)

Current density (mA/cm2)

50

(C)

0.35

1.1

1.2

1.3

1.4

1.5

Potenial (V vs RHE)

1.6

1.7

−10

iv

−20 −30

iii

ii

i

−40 −50 −60 −0.6 −0.5 −0.4 −0.3 −0.2 −0.1

0.0

0.1

Potenial (V vs RHE)

FIGURE 10.5

243

(A) Schematic demonstration of the construction of NiFe LDH-NS@DG nanocomposite. (B) Different electrocatalysts’ OER LSV curves in 1 M KOH solution with inset of overpotential needed at 10 mA cm−2 current density. (C) HER linear sweeping voltammetry curves of different electrocatalysts (i) Pt/C; (ii) NiFe LDH-NS@DG10; (iii) NiFe LDH-NS@NG10; (iv) NiFe LDH-NS@G10; (v) NiFe LDH-NS) in 1 m KOH solution [44].

10.4 Electrocatalytic application for hydrogen production

(B) 60

Reconstruction NiFe LDH-NS@DG

Exfoliation & Self-assembly

NiFe-CO3LDH

CHAPTER 10 LDH-based nanostructured electrocatalysts

(A)

CeOx electrodeposition

Hydrothermal

NF

NF@NiFe LDH

NF@NiFe LDH/CeOx

(C) 1.7

120

NF@NiFe LDH/CeOx

1.6

Pt/C // RuO2

100

Potential (V)

(B) Current density (mA cm2)

244

80 60 40

1.51 V 1.63 V

20

NF@NiFe LDH/CeOx

1.5 1.4 1.3 1.2 1.1

0 1.2

1.4

1.6

Potential (V)

1.8

2.0

1.0 0

2

4

6

Time (h)

8

10

FIGURE 10.6 (A) NF@NiFe LDH/CeOx catalyst preparation is schematically depicted. (B) HER curves of Pt/C–RuO2 and NF@NiFe LDH/CeOx, (C) chronopotentiometric curve at 10 mA cm–2 current density [45].

and 240 mV for the HER and OER, respectively, as well as Tafel slopes of 36.4 and 44.4 mV dec−1 for the HER and OER, respectively, owing to their intelligent structure. Surprisingly, the conversion efficiency is excellent, since just 1.681 V is needed for 10 mA cm−2 , which is just 60 mV higher as compared to the threshold of IrO2 (+)/Pt(−) electrodes. Furthermore, the tailored composite electrodes have significantly greater longevity than the threshold after 48 hours of testing. This appropriate configuration of hierarchical core-shell nanoarchitectures provides a straightforward method for fabricating advanced water-splitting catalysts (Fig. 10.8). As a result, developing an effective electrode material that powers the HER with high efficiency is quite appealing. In general, the high current procedures of the electrocatalysts are required for the commercialization of water splitting. Yu et al. [47] reported the simple and scalable method to construct a stable Cu@NiFe LDH electrocatalyst carrying a 3D core-shell structure for enhanced overall water splitting performance. The catalyst displays exceptional HER efficiency in an

10.5 Conclusion

FIGURE 10.7 The preparation steps of Cu@CoFe LDH [46].

alkaline electrolyte, as well as serving as a flexible electrode for effective overall water splitting, thanks to its smart structure. Considering the catalyst as a bifunctional electrode, the voltage of 1.54 V is consumed to generate 10 mA cm−2 current density, and 100 mA cm−2 is achieved at 1.69 V with outstanding stability, which is well exceeding the benchmark of IrO2 (+)//Pt(−) electrodes for overall water splitting. This core-shell 3D electrocatalyst makes important progress in research into largescale useful water electrolysis.

10.5 Conclusion TM LDHs have highlighted huge importance to the researchers in the electrocatalytic field owing to their flexible structural characteristics, adjustable elemental composition, facile formulation approaches, and abundant raw materials resources. Herein, we categorically summarized the synthesis approaches, modification with other materials, unique structures and electrocatalytic applications of TM LDHs in detail. First, the development methods were discussed in detail. Second, the modifications with other materials such as carbon-based nanomaterials and their effect on the atomic and crystal structure and growth were studied. Finally, the application in the field of electrolysis and the production of hydrogen energy with their chronoamperometric curves were explained in depth. Succinctly, it was found that the LDH-based heterostructures delivered outstanding electrocatalytic efficiency.

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FIGURE 10.8 (A) Schematic illustration of the fabrication procedures of the self-standing 3D core–shell Cu@NiFe LDH electrocatalysts. (RT is the abbreviation for room temperature.). (B) Polarization curves for overall water splitting with the Cu@NiFe LDH electrode as both the anode and cathode at a scan rate of 2 mV s−1 . (The benchmark electrodes of IrO2(+)//Pt(−) are tested the same way.) (C) HER and overall water splitting performance of Cu@NiFe LDH conducted in 1 M KOH [47].

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CHAPTER

11

MOFs-derived hollow structure as a versatile platform for highly-efficient multifunctional electrocatalyst toward overall water-splitting and Zn-air battery

Lei Zhang a, Yuan-Xin Zhu a and Guang-Zhi Hu b a

School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan, Anhui, PR China, b Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, Yunnan University, Kunming, PR China

11.1 Introduction In recent years, clean and sustainable energy conversion and storage technologies, such as Zn-air battery (ZAB) and electrocatalytic overall water splitting (EOWS), have attracted extensive attentions due to their advantages of environmental friendliness, high efficiency, and recyclability. Among these two catalytic systems, hydrogen evolution reaction (HER), oxygen evolution, and reduction reaction (OER and ORR) can be regarded as the most important fundamental operations, but their slow kinetics and high overpotentials greatly reduce the overall efficiency of energy conversion and storage devices [1–3]. Although the catalytic efficiency can be improved with the help of precious metal-based materials, commercial Pt/C catalyst is only suitable for ORR and HER processes, while RuO2 is only effective for OER [4]. In addition, as a scarce resource on the earth, their high cost is also a disadvantage restricting their wide technological applications [5]. Therefore, it is still an important and challenging task to construct multifunctional electrocatalysts based on cheap elements for the simultaneous catalysis of ORR, HER, and OER [6]. As a new type of crystalline porous materials, metal-organic frameworks (MOFs) possess clear and flexible crystal structure, high designability and tailorability, as well as ultrahigh porosity and specific surface area, thus providing advantaged conditions for the designs and constructions of MOFs-based functional materials [7]. Compared with the pristine MOFs, MOFs-derived materials (e.g., carbon-based materials, metal sulfides, phosphides, and carbides) can inherit the porous characteristics of MOFs to a great extent [8]. Moreover, with the structural and composition advantages of MOFs, Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00004-6 Copyright © 2022 Elsevier Inc. All rights reserved.

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the active sites in the derived materials can be precisely controlled [9]. Therefore, MOFs-derived materials show attractive application prospects in the fields of various energy conversion and storage devices [10]. Especially in recent years, MOFsderived hollow structures have become the important research fields in chemistry and materials science. Specifically, the inner and outer walls of the functionalized nanoshell can be used to carry the active sites and give them shortened transfer paths of mass and charge [11]. Sufficient internal space endows the catalyst with higher specific surface area [12]; multiple exposed metal active sites can facilitate the rapid contact of the catalytic substrate, all of which are conducive to the electrocatalytic reaction involved in ORR/OER/HER processes [13]. In this chapter, the latest progress on the synthesis strategy and multifunctional electrocatalytic performance regulation of MOF-derived hollow structures is introduced systematically. First, we classify the MOF-derived hollow structures in term of their geometrical configuration. Then, we summarize the regulate strategy of multifunctional activity toward EOWS and ZAB devices. Finally, the future development directions of MOF-derived hollow structures as multifunctional electrocatalysts are prospected. We believe that this report can afford some guiding significances for the rational designs and precise constructions of efficient multifunctional catalysts in the future.

11.2 Brief classification of hollow structures based on their geometrical configuration 11.2.1 Single-shelled hollow structures In terms of the number of the shell layers, MOF-derived hollow structures can be easily divided into two types: Single-shelled and multishelled nanoarchitectures [14]. Generally, hollow structures with single shell can be obtained by simply calcining the MOFs precursor at high temperature [15]. For example, our group developed a “active site assembly/nanoscale hollowing” strategy to construct N-doped carbon nanoboxes encapsulated with CoP in interior IC layer and (Co-Fe)P in outer CC layer (namely Co-P@IC/(Co-Fe)P@CC hollow nanocube; here, IC denotes imidazole group derived carbon and CC denotes the cyanide derived carbon) as a highly-efficient bifunctional catalyst for EOWS, employing the ZIF-67@Co-Co/Fe Prussian blue analogue (PBA) as the reaction precursor (Fig. 11.1) [16]. Under the current density of 10 mAcm−2 , the corresponding overpotentials of OER, HER and EOWS were only 174, 53 and 230 mV, respectively. Benefiting from the integrated assembly of Co-P and Fe-P active sites, the electronic structure of the catalyst surface was effectively controlled, which could make it possible to optimize the adsorption and desorption process of intermediates on the active sites. Moreover, the hollow and porous nature not only could boost the electrochemically active surface area of the resulted catalyst product, but also accelerated the electrolyte to promptly enter the inside of the electrocatalyst, and the interior produced bubble could also be rapidly removed, and

11.2 Brief classification of hollow structures

FIGURE 11.1 Schematic illustration of the construction of Co-P@IC/(Co-Fe)P@CC electrocatalyst.

thus maximized the catalytic centers of the electrocatalyst. Deng et al. synthesized an N-doped carbon hollow nanocube embedded with atomically dispersed binary CoNi sites by pyrolysis treatment of polydopamine coated Co-Ni MOFs [17]. With the help of synergistic effect of rich single-atom active site and dual-atom structure in the porous conductive carbon framework (denoted as CoNi-SAs/NC), the obtained composite catalyst delivered excellent ORR/OER bi-functional catalytic abilities with a decent half-wave potential of 0.76 V for ORR and a low overpotential of 340 mV at 10 mAcm−2 toward OER, better than that of many reported non-noble-metal-based electrocatalysts and even noble metal Pt/C-IrO2 catalysts.

11.2.2 Multishelled hollow structures Multishelled hollow structures take nanoparticles as the basic building units, and the porous shells arranged from the outside to the inside can physically divide them to form a number of relatively independent spaces [18]. This kind of nanostructure is not only easy to control the material transport, but also can give each space relatively independent characteristics according to the demand, thus becoming a new functional material with great application prospects [19]. Compared with traditional single-shelled type, multishelled hollow structures have larger effective specific surface area, higher loading capacity, and unique space-time order, which has broad application prospects in the fields of electrochemical energy storage, solar energy conversion, electromagnetic wave-absorption, catalysis, adsorption, sensing, drug release, and so on [20]. Inspired by these, the researchers shifted their attentions

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FIGURE 11.2 (A) Schematic illustration of the preparation process of Co/N-codoped double-shelled nanocages and (B) possible formation mechanism of yolk-shelled ZIF-67@polydopamine-3.

to the fabrications of high-quality multishelled hollow structures, and a number of robust catalysts were successfully designed and prepared. For instance, Liu et al. proposed a simple and effective construction strategy toward Co/N-codoped doubleshelled nanocages with excellent ORR/OER bifunctional performances [21]. To be specific, the polydopamine coating was firstly introduced onto the surface of ZIF67 nanocube, leading to the formation of corresponding core-shell precursor. During the subsequent high temperature carbonization, polydopamine was first decomposed and formed a relatively solid carbon shell, followed by ZIF-67 carbonization process (Fig. 11.2). Due to the existence of outer carbon shell, the stress distribution was uneven, which could destroy the inner core, thus forming the desired multishelled hollow structures. As a result, the resulted catalyst product exhibited highly-efficient ORR/OER activities (ORR: half-wave potential of 0.827 V and limited diffusion current density of 5.54 mAcm−2 ; OER: overpotential of 401 mV at 10 mAcm−2 ), thereby showcasing a type of promising ORR/OER electrocatalyst.

11.2.3 Other complex hollow structures In recent years, the design and synthesis of complex hollow nanostructures has become an important research field in chemistry and materials science. Complex

11.2 Brief classification of hollow structures

FIGURE 11.3 Schematic showing product morphology acquired from spatially confined and nonspatially confined pyrolysis.

hollow nanostructures can further improve the electrochemical performance of hollow materials to meet the growing energy demand by adjusting their external geometry, chemical composition, shell structure units, and internal spatial structure [22]. According to the complexity of external geometric structure, this type of hollow structure can be divided into particle-in-box and carved type [23]. These hierarchical structures can effectively integrate the advantages of hollow nanostructures, and significantly improve the packing density of active materials, thus effectively improving the power/energy density and active sites of electrochemical devices [24].

11.2.3.1 Particle-in-box Usually, under some harsh reaction conditions (such as strong acid, strong alkali, and other strong corrosive media), with the continuous electrocatalytic reaction, the active components in the catalyst are easily excessively corroded or poisoned by the surrounding media due to lack of effective active site protection, which will lead to the serious deterioration of the electrode performance [25]. Therefore, how to realize the optimization of electrocatalytic activity and stability through the precise design and functional assembly of the electrocatalytic materials at micro/nanoscale is still a key scientific problem that needs to be solved as soon as possible. In view of this, our team used the surface polymerization coating method to realize the successful introduction of polydopamine shell layer onto the surface of Ni3 [Fe(CN)6 ]2 ·H2 O nanocube, thus forming the expected core-shell structure [26]. Subsequently, the “spatially confined top-down” synthesis strategy was used to convert the inside Ni3 [Fe(CN)6 ]2 ·H2 O core into N-doped graphene coated Ni-Fe alloy “chainmail catalyst,” while the outside polyadopamine layer was carbonized into N-doped carbon shell, thus a case of “particle-in-box” OER/HER electrocatalyst was prepared (Fig. 11.3). However, without the protection of the polyadopamine coating, Ni3 [Fe(CN)6 ]2 ·H2 O cubes could

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break into nanoparticles during high-temperature treatment, and then agglomerate obviously, leading to the increase of the size of “chainmail catalyst” nanoparticles. Compared with the agglomerated and large-size “chainmail catalyst”, the “particle-inbox” catalyst exhibited excellent catalytic activity and stability for HER and OER, and EOWS. On the one hand, the N-doped graphene layer could be in situ coated on the surface of Ni-Fe alloy nanoparticles, thus protecting the Ni-Fe alloy core and enhance its catalytic activity and stability. On the other hand, the external porous carbon shell might provide a guarantee for the further optimization of the catalytic activity and stability of the internal active components. In addition, this unique composite nanostructure not only greatly improved its catalytic performance, but also provided material basis and device model for the construction of other composite catalytic systems. Although polydopamine-assisted confined pyrolysis can effectively avoid the agglomeration of nanoparticles, however, the polydopamine-derived carbon does not exhibit electrocatalytic activity, and the real active center is the nanoparticles located in the core. Therefore, how to promote the reactants to pass through the shell quickly and contact with the active center in the core section efficiently, as well as the rapid escape of bubbles in the inner cores, is particularly important for improving the reaction rate of electrolyzed water. In addition, as for the active sites of catalytic reaction, the number of nanoparticles in the inside core usually needs to be appropriate. Too many particles will lead to the decrease of available electrocatalytic active area due to the accumulation/agglomeration of each other, while too few particles mean that there are not enough active sites. Therefore, we developed an “ammonia etching/thermal phosphorization” technology, which could simultaneously realize the construction of holes in the shell and the optimization of the number of nanoparticles in the core [27]. Experimental results showed that the overpotentials for OER was only 203, 242 and 254 mV at 10, 100 and 250 mAcm−2 , respectively, better than those of commercial RuO2 catalyst. This work not only used cheap transition metals to synthesize efficient OER electrocatalysts, but also showed that the catalytic performance of metal phosphides could be optimized by modulating the mass/charge transfer channels of electrocatalytic reaction and the number of catalytic active sites in the core section, thus providing a new strategy for rational design of nanocomposite catalysts in the future.

11.2.3.2 Carved hollow structure Because of their high specific surface area, decent surface to volume ratio and enhanced atom utilization efficiency, carved hollow structures can provide more electrochemically active sites and larger contact area with electrolyte [28]. More importantly, the thin and permeable shell structure can also greatly accelerate the transfer process of electrons and ions [29]. Because of these proposed advantages, many efforts have been devoted to the constructions of carved hollow structures, which could serve as robust electrocatalysts for enhancing the electrocatalytic efficiency. PBA materials are characterized by low-cost, easy synthesis, inherent open frame structure, and

11.3 Active regulation strategy

B A i.Self-assemble

H2O,24 h

ii.Etching urca,100 °C

iii.Phosphating

OE

R

N2,300 °C

HE R

FIGURE 11.4 Schematic illustration of the preparation of the carved hollow structures.

designable and controllable components. It can be easily transformed into nanoframeworks by suitable etching technology, thus providing an ideal structural platform for subsequent chemical conversion to corresponding carved hollow structure [28]. For example, Peng research team proposed a simple strategy to fabricate Co-FeP carved hollow structures as an advanced electrocatalyst for electrocatalytic water splitting (Fig. 11.4) [30]. Specifically, the introduced urea molecules could be used as both structure directing and etching agents for the transformation from Co-Fe PBA solid nanocubes to the final craved hollow structures, thereby affording the possibility to prepare expected metal phosphides catalyst with open structure by hightemperature phosphorization technology. The structural and compositional features resulted from the Co-Fe PBA carved precursor including enhanced active area, low transfer resistance of mass and charge, as well as facilitated bubble release, might account for the promoted electrochemical performances of the as-obtained catalyst toward both the HER and OER.

11.3 Active regulation strategy 11.3.1 Active site assembly For electrochemical water splitting, if different catalysts are used to make working electrodes in the same electrolyzed water device to respectively catalyze HER and OER reactions, it will not only lead to technical problems such as increased cost and manufacturing complexity, but also the possible pollution in the catalytic process of the two electrodes, which is not conducive to the in-depth understanding of the relationship between material structure and performance [31]. Although the above problems can be overcome to a certain extent by using bifunctional electrocatalysts,

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FIGURE 11.5 Schematic diagram of fabrication process for C-(Fe-Ni)P@PC/(Ni-Co)P@CC.

most bifunctional catalysts usually deliver excellent electrocatalytic activity for one half-reaction, while the catalytic activity for another half-reaction is usually unsatisfactory, thus showing moderate EOWS performance [32]. In addition, the OER and HER processes are heterogeneous catalytic reactions involving gas/liquid/solid phases. Therefore, in the process of catalyst design, on the one hand, it need to consider how to introduce active sites with high activity and high stability [33]; on the other hand, the maximum exposure of active sites and the construction of material transport channels also have an important impact on the overall activity of the catalyst [34]. As a unique nanostructure, porous hollow structure can use the inner/outer wall of its shell to carry active sites, and its porous characteristics also give the material high active area, low electrolyte penetration resistance, as well as rapid charge transfer, and gas release behavior [35]. Inspired by this, our research team designed and developed a novel “active site assembly-pore design” strategy. Specifically, pre-prepared Ni3 [Co(CN)6 ]2 ·12H2 O porous hollow nanobox was employed as the reaction precursor and template [36]. With the help of coating and coordination ability of functional polydopamine, the desired polydopamine-metal layer were successful introduced onto the surface of Ni3 [Co(CN)6 ]2 ·12H2 O nanobox. After thermal phosphorization, (Fe-Ni)P@PC/(Ni-Co)P@CC catalyst was produced (PC denotes polydopamine derived carbon). The synthesis process was shown in Fig. 11.5. Experimental results showed that the shell of the hollow nanobox consisted of two parts: The inner layer of (Ni-Co)P@CC and the outer layer of (Fe-Ni)P@PC.

11.3 Active regulation strategy

Due to its unique structure and composition advantages, the nanobox exhibited excellent catalytic activity. Under the current density of 10 mAcm−2 , the overpotentials toward OER and HER processes were only 251 and 142 mV respectively. If the (Fe-Ni)P@PC/(Ni-Co)P@CC porous nanobox was used both as cathode and anode, the applied voltage for EOWS was only 1.63 V at 10 mA cm−2 . Further studies revealed that the outside (Fe-Ni)P@PC component was the OER active sites, while the inside (Ni-Co)P@CC was the HER active site. Therefore, the OER and HER active components could be integrated together with the help of the favorable transfer channel design for mass and charge. As a result, a novel and robust catalyst with hollow and porous structure was successfully prepared, which could greatly reduce the overpotential of water splitting, and improves the electrocatalytic efficiency. Moreover, this synthesis idea could provide some new insights for the design of other heterogeneous catalysts.

11.3.2 Electronic structure effect Recently, researchers have attempted to determine the basic relationship between the electronic structure of material and their electrochemical property [37]. Because the conductivity and adsorption strength of intermediate (equivalent to reaction barrier) can be used as direct indicator to reflect the electronic structure of catalyst (usually determines the reaction kinetics to a certain extent), the electronic structure has been identified as an important descriptor to explain the corresponding catalytic behavior. It is worth noting that the optimal OER activity of perovskite oxide have been predicted by Yang’s group according to the molecular orbital principle [38]. Xie’s research team also emphasized the modulation of charge and spins order of 2D ultrathin solid, and clarified the relationship between structure and property [39]. These pioneering studies make the electronic structure optimization of materials become the focus of catalytic activity control. Spin order, band structure, and electron state density can essentially characterize electronic behavior. Combined with advanced operating characteristics and powerful simulation methods, clear electronic structure images can be obtained by detecting and simulating these characteristics. Therefore, these methods can provide a platform for understanding the electronic structure adjustment induced by atomic structure. The derived principle in turn shows the guiding significance of designing effective catalysts for EOWS and ZAB. For example, Li et al. designed and developed an advanced and highly-efficient ORR/OER/HER trifunctional catalyst of CoSA-Co9 S8 -HCNT through a simple selfsacrificing method (Fig. 11.6) [40]. The presynthesized ZnS nanorod was employed as template and precursor to afford sulfur source, thus producing hollow carbon nanotube with hierarchical porous structure. Experiments demonstrated that the resulted CoSACo9 S8 -HCNT product adopted hollow nanotube structure, with integrated single cobalt atom and Co9 S8 nanoparticle. The rechargeable liquid ZAB with CoSACo9 S8 -HCNT catalyst delivered a boosted power density of 177.33 mWcm−2 and improved stability. In addition, the water splitting activity of CoSA-Co9 S8 -HCNT revealed that it required only 1.59 V to afford 10 mAcm−2 . Density functional theory

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FIGURE 11.6 (A) Schematics of the preparation of CoSA-Co9 S8 -HCNT tri-functional catalyst; (B) TEM image (inset is SAED pattern); (C) HRTEM image; (D) HADDF-STEM image (single Co atoms are highlighted by orange circles); (E) HADDF-STEM and corresponding elemental mapping images of CoSA-Co9 S8 -HCNT (SA denotes single atom and HCNT is hollow carbon nanotube).

(DFT) calculation confirmed that the interaction between Co9 S8 and CoN4 sites could improve the electronic configuration of the catalytic active sites to synergistically reduce the reaction barrier. Therefore, this work provided an effective method to excellent trifunctional catalyst as a substitution to noble-metal-based catalyst.

11.3.3 Single-atom catalyst Compared with traditional catalyst, single-atom catalyst have completely exposed atoms, so the number of active sites can be significantly increased; In addition, its low coordination and unsaturated state features, as well as enhanced metal support interaction can also improve the intrinsic activity of the active sites [41]. On the other hand, high utilization of metal atoms can effectively reduce the cost of catalyst. However, due to its large specific surface energy, the resulted single atom is easy to migrate and agglomerate, thus there are many challenges in the synthesis of single-atom catalyst.

11.3 Active regulation strategy

FIGURE 11.7 (A) Schematic illustration of the synthesis of EA-Co-900; (B) TEM image of EA-Co; (C) SEM, (D) TEM (inset: corresponding SAED pattern), (E) Aberration-corrected BF-STEM, (F) HAADF-transition metals through its abundant STEM images and (G) corresponding EDX element mapping of EA-Co-900.

Carbon materials derived from MOFs possess high surface area, porous structure, and rich nitrogen content, which are regarded as excellent carriers for anchoring metal atoms [42]. For instance, Zhao et al. proposed a simple high-temperature calcination strategy of ellagic acid (EA)-metal MOFs materials to fabricate hollow N-doped carbon nanotube embedded with space-confined single-atom (Fig. 11.7) [43]. The advantaged factors resulted from the well-dispersed single-atom sites, as well as hollow and porous nature endowed the as-obtained electrocatalysts boosted catalytic activities and stabilities toward ORR/OER processes in ZAB with a high power

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density of 73 mWcm−2 at 65 mAcm−2 and decent durability of 110 hours at 20 mAcm−2 . Guided by DFT calculations, Chen et al. developed a MOF-derived hollow carbon nanocage confined with Co1 -N3 PS active moiety, which exhibited excellent ORR performance in alkaline medium including top-ranking reaction kinetics, enhanced current density, ultrasmall Tafel slope of 31 mVdec−1 , and outstanding half-wave potential of 0.92 V, surpassing many reported non-noble-metal ORR catalysts and commercial Pt/C product [44]. In addition, the ZAB device based on Co1 -N3 PS active moiety delivered improved battery performance and long-term charge/discharge durability, which proved its great potential application prospect. According to the designed control experiment and DFT calculation, the key role of the optimal electron density of the Co active center caused by the unique coordination of N, P and S atoms in the significant improvement of ZAB performance was successful confirmed. This work improved the scientific understanding of the relationship between electron density at atomic sites and ZAB performance, which might provide a new method for the rational design and optimization of catalysts.

11.3.4 Defect chemistry Defect is a very important research object in material science. Many properties of crystal materials, such as optical, electrical, magnetic, mechanical and thermal properties, are closely related to defect structure [45]. Defects are common in the preparation and application of materials, which can be used not only as active sites to achieve the preparation of specific catalyst materials, but also to promote the occurrence of catalytic reactions. Defect engineering is also an effective way to control the surface physical/chemical properties of catalytic materials [46]. The existence of defects will significantly change their electronic structure and chemical properties, promote the formation of new physical and chemical properties or strong synergistic effect, thus optimizing the catalytic performance of electrocatalysts. For example, Lei et al. prepared a bifunctional oxygen electrocatalyst, namely defects enriched hollow porous Co-N-doped carbons embedded with ultrafine Co-Fe alloy, through the simple calcination of ZIF-67-Fe ions@SiO2 precursors (Fig. 11.8) [47]. The resulted catalyst exhibited promoted electrochemical efficiency and durability with a small overpotential of 320 mV for OER at 10 mAcm−2 and an outstanding half-wave potential of 0.887 V for ORR, better than those of most recent reported bifunctional catalysts. When assembled into a rechargeable ZAB device, it also displayed excellent stability for over 200 hours and a high-power density of 152.8 mWcm−2 . This research project has pointed out a new way to improve the electrocatalytic performance from the perspective of defect engineering.

11.3.5 Synergistic catalysis Besides the active site assembly, electronic structure modulation, single-atom strategy and defect chemistry method, the robust synergistic catalysis stemming from different hetero-units in one hollow structure have been confirmed as an effective approach to

11.3 Active regulation strategy

C

GraP-N

Co

Pyrr-N

Fe

SiO2 layer

2-Methylimidazole

Pyri-N

O2

Fe2

OH

ZIF-67 layer SiO2 Coating

N N H

Adsorbed Fe2-

Pyrolsis & leaching

N N H

Pyrolsis

Co(NO3)2

ZIF-67

ZIF-67@SiO2

ZIF-67@SiO2@Fe

CoFe-Co@PNC

FIGURE 11.8 Illustration of the preparation process of the CoFe-Co@PNC catalysts.

boost the catalytic activity toward EOWS and ZAB. The multicomponent heterostructure electrocatalysts usually deliver greatly promoted electrochemical performances compared to the single-component ones, which may be ascribed to that each component in the multicomponent heterostructure electrocatalyst may serve as the catalytic center to improve the involved catalytic process [48]. In addition, various components in the interfacial structure can also synergistically boost the reaction kinetics on each active site and electron reconfiguration [49]. Therefore, in recent years, the researches in this field have attracted extensive attentions. Xu’s research team proposed a red phosphorus-assisted one-step thermal phosphorization route to prepare Co, N, and P multidoped carbon nanocages confined with Co2 P nanoparticles, employing pre-prepared ZIF-67 nanocubes as precursor (Fig. 11.9) [50]. Benefitting from the synergistic effect between hierarchical porous carbon materials with multiheteroatom doping properties and highly-dispersed Co2 P nanoparticles, the obtained catalysts showed excellent OER, HER, and ORR electrocatalytic performances, and also delivered outstanding electrocatalytic performances in EOWS and ZAB applications. Yan et al. developed a method for large-scale preparation of high efficiency and multifunctional electrodes toward to HER/OER/ORR processes [51]. To be specific, Ni-BTC (BTC is trimesic acid) was firstly obtained by hydrothermal method, and then the solution of Ni-BTC and graphene oxide was mixed by vacuum filtration to obtain the film. Then, the resulted film was treated at high temperature in an inert atmosphere to obtain a freestanding 3D heterostructure film. In the process of high temperature, Ni-BTC could produce carbon nanotubes and stitch reduced graphene oxide nanosheets, thus forming the expected self-supporting structure. Experimental results and theoretical calculations showed that the synergistic effect of nitrogen doped carbon and nickel nanoparticles enhanced the electrocatalytic activity of the films. At the current density of 10 mAcm−2 , the overpotentials of HER and OER were 95 and 260 mV respectively. For ORR process, the half-wave potential was 0.875 V. The multifunctional thin film electrode showed excellent performances in EOWS and ZAB, indicating the practical application potential of the flexible self-supporting electrode.

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FIGURE 11.9 (A) Schematic illustration of synthesis of Co2 P/CoNPC; (B and C) SEM images of ZIF-67; (D) SEM, (E) TEM (inset shows particle size distribution of Co2 P NPs), (F) HR-TEM images, (G) SAED pattern, and (H–K) elemental mapping images of Co2 P/CoNPC (CoNPC is Co, N, and P codoped carbon).

References

11.4 Conclusions and perspectives The present chapter has proposed an overview of the latest achievements in the designs and constructions of MOFs-derived hollow structure as a versatile platform for highly-efficient multifunctional electrocatalyst toward EOWS and ZAB devices. Thanks to its ultrathin and porous shell structure, the hollow micro/nanomaterials can provide more active sites for electrocatalytic reaction, favorable transfer channels of mass and charge, as well as convenient surface/interface control, thus facilitating the improvement of performance. However, despite the remarkable achievements, there are still some urgent problems needed to be solved in the future research work. (1) HER/OER/ORR processes are chemical reactions involving gases participation, including gas “overflow” OER and HER reactions and gas “absorption” ORR reaction. For the gas “overflow” reaction, if the bubbles fail to be removed from the electrode surface in time, the bubble film will greatly reduce the effective catalytic area, increase the electrolyte diffusion resistance and polarization effect, and eventually lead to the increase of energy loss. On the other hand, ORR reaction involving gas “absorption” usually faces the “submergence” of catalytic channels. Due to the poor solubility of O2 in electrolyte, it is difficult to give full play to the electrocatalytic activity. Therefore, how to design a reasonable transport channel to ensure the high contact of active sites in the inner/outer wall and enhance the transport capacity of substances and charges in the electrochemical charge/discharge process is particularly important for the improvement of catalytic performance. (2) For the hollow structure catalyst reported at present, its excellent activity and stability are limited to the laboratory conditions; the performance at the factory scale needs further study. (3) The relationship between the composition, structure, adjacent environment of the active sites, and the electrocatalytic performance of the catalysts still needs to be further explored. Therefore, we hope that this chapter can provide necessary guidance and reference for the development of high-performance electrocatalyst in the future.

Acknowledgments This chapter was financially supported by National Natural Science Foundation of China (No. 21975001), National Natural Science Foundation of China-Yunnan Joint Fund (U2002213), and the Double Tops Joint Fund of the Yunnan Science and Technology Bureau and Yunnan University (2019FY003025).

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PART

Nanomaterials for Electrochemical Nitrogen reduction reaction (NRR)

3

CHAPTER

Noble-metals-free catalysts for electrochemical NRR

12

Xue Zhao and Guangzhi Hu Institute for Ecological Research and Pollution Control of Plateau Lakes, School of Ecology and Environmental Science, School of Chemical Science and Technology, Yunnan University, Kunming, China

12.1 Introduction For a long time, noble metal-based materials have played an important role in various chemical reactions, such as electrochemical hydrogen evolution reaction (HER), electrochemical carbon dioxide reduction reaction (CO2 RR), electrochemical methanol oxidation reaction (MOR), and electrochemical oxygen evolution reaction (OER). In the electrochemical nitrogen reduction reaction (NRR), the standard reaction potential from N2 to NH3 is 0.092 V vs. RHE, so the electrochemical reaction potential of NRR mostly overlaps with HER. More importantly, N2 has very poor water solubility and is difficult to activate (the thermodynamic energy of N2 adding the first proton H is as high as 410 kJ mol−1 ), so the relatively easy HER becomes the most serious side reaction in NRR [1]. Similar to noble metal elements, most non-noble metal-based metal elements in transition metals (TMs) have empty d-orbital that are not occupied by electrons. These metals can effectively couple empty and occupied d-orbitals with N atoms to form N-M (metal) bonds to capture and activate N2 . Specifically, the unoccupied d-orbital in TMs can accept the lone pair of electrons from N in N2 , and the independent single electron on the d-orbital in TMs can inversely contribute to the π feedback bond in N2 (antibond orbital), the formation of “acceptance-reverse donation” approach achieves the purpose of weakening the N≡N triple bond [2]. On the one hand, legumes in nature use their own nitrogenases to continuously convert atmospheric N2 into their own organic nitrogen (nitrides), which inspires researchers in nitrogen fixation. The key site in nitrogenase is iron-/molybdenum-based cofactor, and N2 is transformed into a usable form in plants in the form of “N2 + 6H+ + nMgATP + 6e- → 2NH3 + nMg-ADP + nPi” [3–5]. In addition, E. Skúlason, J. G. Howalt and J. H. Montoya have used theoretical calculations to predict the NRR performance on various metal materials. The “volcano map” model drawn shows that most TMs have the ability to catalyze NRR [6–8]. In other words, the catalyst for catalyzing NRR can come from noble metal materials or non-noble metal materials, and the key to high catalytic activity lies in how to control the electronic structure and morphology of the corresponding material. Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00015-0 Copyright © 2022 Elsevier Inc. All rights reserved.

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

12.2 Non-noble metal-based metal catalysts 12.2.1 Mo-based catalysts As one of the active sites of nitrogenase, Mo element is widely used in electrochemical NRR. Mo-based materials are often used as NRR catalysts in the form of zero-valent Mo or doped with other elements (such as Mox Sy , Mox N, Mox C). For most metal-based materials, metals are usually active sites for electrocatalytic reactions, and exposing these active sites as much as possible is beneficial to increase the electrochemical reaction rate. Hui et al. anchored zero-valent Mo singleatom (Mo0 /DGY) on graphite diacetylene, achieving ammonia yield rate of up to 145 μg h−1 mgcat. −1 and Faraday efficiency of up to 21% in 0.1 M Na2 SO4 , with only slight current density attenuation after 100 hours of potentiostatic polarization testing [9]. Density functional theory (DFT) calculation results show that GDY uses its strong electron enrichment atmosphere to anchor zero-valent Mo through p-d coupling. The reversible and fast electron transfer between Mo and C1 not only inhibits the HER side reaction, but also contributes to the thermodynamically favorable N2 hydrogenation rate. Ling et al. anchored Cu, Pd, Pt, and Mo single-atoms on the carbon doped with N element, and determined by DFT calculation that Mo has the strongest binding ability with N2 when loaded on carbon (binding energy is −1.19 eV), the corresponding NRR overpotential is only 0.24 V vs. RHE. In this work, the coordination effect plays a key role. The state where the Mo atom is coordinated with only one N atom and two C atoms is most conducive to the progress of NRR, because the change of desorption free energy of NH3 generated on the catalyst is only 0.47 eV (Fig. 12.1) [10]. Similarly, the researchers achieved effective NRR in an alkaline environment (0.1 M KOH) by anchoring Mo single-atom (SA-Mo/NPC) on porous carbon doped with N element [11]. After SA-Mo/NPC runs continuously for 50,000 seconds, its NRR activity almost maintains the initial state, which is due to the stable anchoring of Mo single-atom and the advantages of 3D hierarchical structure of porous carbon. It is clear from these studies that both single-atomization of metal sites (to increase the exposure of active sites) and increasing the specific surface area of the support can increase the NRR catalytic activity of Mo-based materials. The crystal plane orientation can also affect the NRR activity of Mo metal. Yang et al. studied the influence of Mo materials with (110) crystal plane orientation and (211) crystal plane orientation on NRR [12]. Among them, Mo materials with (110) crystal plane orientation have much higher NRR activity than (211) crystal plane orientation. This is because the adsorption energy of N on the (110) crystal plane is lower than that of H. Defects (or vacancies) are one of the factors that affect the affinity of Mo-based materials for NRR. Chu et al. doped Mo into SnS2 not only to adjust the electronic structure of SnS2 , but also to promote the spontaneous generation of sulfur vacancies (Sv) [13]. When Mo doped into SnS2 , the valence of Mo-SnS2 nanosheets decreased, indicating that the doping changed the electronic structure of SnS2 , which resulted in

12.2 Non-noble metal-based metal catalysts

FIGURE 12.1 (A) Top view and side view of M1 -N1 C2 structure; (B and C) Side-on (B) and end-on (C) after M1 -N1 C2 adsorbed N2; (D–F)The evolution diagram of N2 on M1 -N1 C2 under different applied potentials, including alternate hydrogenation (D), distal hydrogenation (E) and adsorption states of each intermediate state (F). Reproduced with permission. [10] Copyright 2018, American Chemical Society.

a decrease in the crystallinity of the material and at the same time induced electron enrichment. Before and after Mo doping, the S/Mo ratio in Mo-SnS2 decreased from 1.88 to 1.66, indicating that a large amount of Sv was produced in Mo-SnS2 , which was confirmed by DFT calculations. In theoretical calculations, the original SnS2 hardly adsorbs N2 at the central site or the edge of Sn, while the Sv site in Mo-SnS2 can significantly change the bond length of the N≡N triple bond (Fig. 12.2). Mobased materials in bonded form can also catalyze NRR. Sun et al. prepared defect-rich MoS2 nanoflowers (DR MoS2 ) and studied its NRR catalytic performance in detail [14]. Compared with nondefective MoS2 , defect-rich DR MoS2 is easier to activate N2 . Specifically, N2 is easily adsorbed on bare Mo atoms at rich defect edges, forming

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

FIGURE 12.2 (a and b) HRTEM images of SnS2 (a) and Mo-SnS2 (b), where illustrations A and B are IFFT images recorded by in b image; (c–e) The optimized structures of N2 when adsorbed at SnS2 (central Sn site) (c), SnS2 (edge Sn site) (d) and Mo-SnS2 -Vs (Vs site) (e). Reproduced with permission. [13] Copyright 2020, Royal Society of Chemistry.

Mo-N bonds and N=N triple bonds with bond lengths of 1.97 eV and 1.18 eV, respectively, the inert N2 has been activated at this time. Obvious depletion of electrons was found in the two N atoms of N2 , and electron aggregation appeared around the N-Mo bond and N-H bond. The d-band center of the defect site in MoS2 is closer to the Fermi level than MoS2 as a whole, which is the reason for the strong interaction between N2 and Mo at the defect site. In order to further clarify the role of MoS2 in NRR, Zhang et al. drew the energy distribution diagram of NRR on MoS2 by using DFT calculation from the perspective of electronic structure [15]. DFT results also show that the edge Mo sites are more conducive to N2 adsorption than the MoS2 plane. Among them, the deformed charge density isosurface shows that a large amount of positive charges are enriched around Mo atoms, which are beneficial to the activation of N2 (with lone electron pairs). On these foundations, it is meaningful to further optimize the NRR catalytic performance of Mo-based catalysts from the aspect of electronic structure, of which doping is a more common approach. For example, after the defect-rich MoS2 is doped with N element, the conductivity of the material is significantly improved.

12.2 Non-noble metal-based metal catalysts

Here, the introduction of the N element forms a Mo-N bond and therefore accelerate the electron transfer rate in the material [16]. The hybridization of B, C, N, and P elements with Mo can also control the electronic structure of Mo-based materials. Among them, the three-coordinated Mo in the form of Mo-C3 has a strong adsorption effect on N2 , and the N vacancies generated by the MoN2 nanocrystals are filled into N2 through the Mo-N3 bond to enhance the NRR activity [17, 18]. Sun et al. designed Mo single-atom model materials with various ligands, and theoretically calculated the NRR catalytic activity when P, B, N, S, and C were used as Mo single-atom ligands. The simulation results of Mo-PC2 , Mo-PB2 ,and Mo-BC2 are the best, the NRR overpotential is all lower than 0.60 V [19]. Among them, the BC2 ligand can raise the d-band neutrality of Mo to the Fermi level to achieve high NRR selectivity, so Mo-BC2 has more application value. Metal doping can also control the electronic structure of Mo-based catalysts. In the report of Zhang et al., after doping Co atoms in MoS2 , the energy barrier of NRR (∗ NH → ∗ NH2 ) on MoS2 decreased from 1.62 eV to 0.94 eV, which is due to the lattice distortion of MoS2-x caused by the doping of Co [20].

12.2.2 Fe-based catalysts Fe, which is one of the active sites of nitrogenase, is widely used as a catalyst for NRR like Mo. Fe-based catalysts often exist in the form of oxides in NRR, and zero valent iron is reported less frequently, one of the reasons is that zero valent iron is easily oxidized into the form of oxidation state. For zero-valent Fe, Li et al. investigated the evolution behavior of N2 at the Fe-N3 site on graphene in detail from the aspect of theoretical calculation, where the local magnetic moment formed by highly polarized Fe-N3 is conducive to the adsorption of N2 , thus weakening the triple bond of N≡N [21]. Some vacancies of lattice defects are often formed in graphene, and the system does not show magnetism when Fe atoms are embedded in these single vacancies. When N3 (or N4 ) sites are formed in N-doped graphene, the entire system is magnetic after anchoring Fe. The Fe-N3 (or Fe-N4 ) site constructed by this strategy is conducive to the smooth progress of NRR. In the experiment, Lu et al. obtained the carbon material catalyst (ISAS-Fe/NC) with Fe-N4 after annealing ZIF-8 impregnated with Fe elements. The ammonia yield rate and Faraday efficiency of NRR driven by ISAS-Fe/NC are as high as 62.9 μg h−1 mgcat. −1 and 18.6%, respectively [22]. DFT calculations show that electrons on the 3d orbital of Fe atoms can be transferred to the 2p orbitals of N atoms, and thus the bond length of the N≡N triple bond increases from 1.098 Å to 1.134 Å (Fig. 12.3). For Fe atoms on N doped carbon support, the coordination (or confinement) effect of the support improves the exposure of Fe site and enhances its stability, which is the best way to design the stable presence of zero-valent Fe-based NRR catalyst. Structural control can also change the NRR catalytic performance of zero-valent Fe-based materials. In the report of Chu et al., after Fe atoms are introduced into CeO2 , CeO2 changes from a crystalline state to a partially crystalline state and generate a large number of oxygen vacancies, thereby

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

FIGURE 12.3 (A) Schematic diagram of the preparation of ISAS-Fe/NC; (B and C)The optimized structure (B) and charge density difference (C) of N2 adsorbed on ISAS-Fe/NC. (D) PDOS for ISAS-Fe/NC, ISAS-Fe/NC-N2 and N2 configurations, where the Fermi level is set at 0 eV. Reproduced with permission. [22] Copyright 2019, Elsevier B. V.

increasing the exposure of active sites and enhancing the electron transport efficiency [23]. Compared with Ce atoms with an atomic radius of 2.7 Å, the atomic radius of Fe atoms is only 1.26 Å, so Fe doping into CeO2 causes lattice distortion and thus changes the crystal phase state of CeO2 . Ov caused by the distortion of the CeO2 lattice can produce Ce3+ -Ce3+ pairs, which is the key to enhance NRR and inhibit HER. In addition, some researchers have simulated Fe-Mo dual active sites in nitrogenase to achieve effective electrochemical NRR, such as FeMo@NG and Mo3Fe3C [24, 25]. The reason why Fe-Mo bimetallic catalyst is superior to single metal catalyst is that FeMo dimer has geometric effects, such as lattice distortion caused by the difference in atomic radius, which activates nearby metal atoms, making them the active sites of NRR.

12.2 Non-noble metal-based metal catalysts

The study of iron oxide catalysts in NRR is more extensive, which is related to the poor HER and high stability of such materials. For most iron oxides (such as Fe2 O3 , Fe3 O4 or their mixtures), Ov is widespread. The Ov present on the surface (or inside) of the iron oxide can accommodate (or adsorb) N2 and provide electrons to the antibonding orbital of N2 to achieve the purpose of activating inert N2 [26]. Although Ov in iron oxide plays an important role in the NRR process, some other mechanisms need to be further explored. For example, in the reports of Suryanto et al. [27] and Liu et al. [28], the NRR activity of Fe3 O4 /Fe composites is generally higher than that of Fe2 O3 alone, so there is a synergy of other factors besides the influence of Ov. It is clear that some specific substances as dopants or carriers can change the catalytic activity of Fe2 O3 , such as carbon nanotubes (CNT), graphene (GO), and other transition metal elements [29-35]. For CNT and GO, they can not only serve as a good electron transport carrier to promote the smooth progress of electrondependent NRR, but also greatly disperse and anchor metal active sites. Transition metals have abundant d-electron structures and atomic size effects. The doping of transition metal heteroatoms can cause changes in the coordination environment in the material, and then trigger changes such as lattice distortion and atomic defects. These highly unstable sites often become active centers for N2 adsorption and activation.

12.2.3 Ti-based catalysts Skúlason et al. used theoretical calculations to evaluate the possibility of NRR on various transition metal surfaces, and the results showed a “volcano map” shape. Although Fe and Mo are highly active (at the top of the volcano map), the HER on their surface is more obvious [6]. Compared with the Rh, Ru, Pt, Ir, Co, and Ni atoms on the right leg of the volcano diagram, the adsorption of N atoms on some of the early transition metals (such as Sc, Ti, Zr, and Y) on the left leg of the volcano diagram is stronger than that of H atoms (Fig. 12.4). Ti-based materials are widely present in nature, and they play an important role in the fields of electrocatalysis and photocatalysis. The d-orbital of Ti element includes a large number of empty orbitals, which can effectively accept the lone pair of electrons from N2 , and the two lone electrons on its own d orbital can be donated to the antibonding orbital of N2 in the reverse direction. Due to the poor electrical conductivity, TiO2 almost has no HER activity, which provides the possibility for the modification of TiO2 for NRR. In the report of Zhang et al., TiO2 deposited on the Ti plate can induce a large amount of Ov. The existence of these Ov can effectively fix N2 and inject electrons into its anti-bonding orbital to weaken the N≡N triple bond [36]. Similarly, Qin et al. adjusted the Ti-C concentration and Ov content in the pyrolysis product C-TixOy/C by changing the temperature of the pyrolysis MIL-125(Ti) to achieve the activation of N2 [37]. Among them, the introduction of Ov and the formation of Ti-C bonds are the keys to achieve catalytic NRR. Furthermore, the researchers used electrospinning and pyrolysis technology to prepare Ti-based catalysts (TiC/C NF) on carbon nanofibers, and verified the excellent

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

FIGURE 12.4 Combined volcano diagrams (lines) for the flat (black) and stepped (red) transition metal surfaces for reduction of nitrogen with a Heyrovsky type reaction, without (solid lines) and with (dotted lines) H-bonds effect. The data points are the DFT values of −G for a given reaction step. Reproduced with permission. [6] Copyright 2012, Royal Society of Chemistry.

catalytic performance of TiC/C in NRR. The DFT calculation results show that the energy barrier when two N atoms in N2 are simultaneously adsorbed on the TiC surface is as low as 0.88 eV, and the subsequent hydrogenation is carried out by an enzymatic mechanism [38]. For TiC, more research has focused on a new type of catalyst. The two-dimensional MXene (Ti3 C2 Tx ) nanosheets with a layered structure not only provide a large number of Ti sites for the NRR, but the anchoring of Ti on C can also ensure the long-term stability of the catalyst. In the report of Wang et al., Ti3 C2 Tx has NRR catalytic activity, in which the active site comes from the exposed Ti at the edge of Ti3 C2 Tx instead of the MXene base surface, which laid the theoretical foundation for MXene to realize NRR [39]. Nevertheless, the NRR catalytic activity of MXene still needs to be improved, because the number of exposed Ti sites on the edge of Ti3 C2 Tx is limited and a large amount of F elements remain in traditional preparation methods. The presence of an inert F element reduces the electrical conductivity of the material, which is detrimental to electron-transport-dependent NRR. Based on this, Li et al. used a top-down strategy to prepare fluorine-free Ti3 C2 Tx with a particle size of 50– 100 nm and achieved an ammonia yield rate of 36.9 μg h−1 mgcat. −1 [40]. With the

12.2 Non-noble metal-based metal catalysts

FIGURE 12.5 Schematic diagram of the adsorption and activation of N2 on Zr4+ doped TiO2 . Reproduced with permission. [42] Copyright 2019, Nature Publishing Group.

same idea, the Ti-based MXene obtained by improving the preparation strategy of Ti3 C2 Tx to avoid the residue of the F element also achieves efficient NRR [41]. In titanium-based catalysts, the presence of Ti3+ helps to improve its performance in catalyzing NRR. Cao et al. first learned through DFT calculation that the bi-Ti3+ (a pair of Ti3+ ) pair formed on anatase TiO2 can become the active center of N2 adsorption, and further experiments verified this prediction [42]. Specifically, they doped TiO2 with Zr4+ with d-electron configuration, the introduction of Zr4+ with large atomic size caused the distortion of TiO2 crystal form, which triggered a large number of Ti3+ combination and formed a large number of OV at the same time (Fig. 12.5). In the experiment, the ammonia yield rate and Faraday efficiency of NRR catalyzed by Zr4+ modified TiO2 reached 8.90 μg h−1 cm−2 and 17.3%, respectively. Similar results were also reported by Sun et al., who achieved the electrochemical conversion of N2 to NH3 using nanowires of TiO2-x with self-doped Ti3+ [43]. DFT calculations show that Ti containing Ov has a stronger adsorption capacity for N2 . The

281

CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

'G*NNH(eV)

(A)

(C)

* +N2 + H+ + eo + NNH2

H+

H2

H2O

N2

3.0

HER-favoured

2.8 2.6

e−

e−

e−

e−

)

) (1

11

04 (1

(D)

Au

Bi

Bi

Bi

(B)

(1

(0

10

12

)

)

2.4

+K+

2.8

NH4+

K+ (hydrated)

ENRR-favoured

+K+ 2.6 +K+

2.4

e−

04

)

e−

(1

) 10 (1 Bi

Bi

(0

12

)

e−

Bi

'G*NNH(eV)

282

FIGURE 12.6 (A) The free energy of N2 transformed into ∗ NNH on Bi(012), Bi(110), Bi(104) and Au(111) crystal planes; (B) The free energy changes from N2 to ∗ NNH on Bi(012), Bi(110) and Bi(104) crystal planes with (without stripes) or without (with stripes) K+ ; (C) In acidic solutions without K+ , protons can be easily transferred to the catalyst surface, so HER will dominate; (D) In a solution containing K+ , K+ prevents the transfer of protons to the surface of the catalyst, and nitrogen will be preferentially adsorbed, thereby increasing ENRR. Reproduced with permission. [44] Copyright 2019, Nature Publishing Group.

bond length of N-Ti4 c formed between Ti4 c coordinated with only four O atoms and N atoms is shorter than the bond length of N-Ti5 c formed between Ti5c (coordinated with 5 O atoms) and N atoms.

12.2.4 Bi-based catalysts Late transition metal bismuth-based materials also play an important role in NRR. Related research suggests that the 6p orbital of Bi partially overlaps with the 2p orbital of N, and this interaction promotes the activation of N2 . Specifically, Chen et al. prepared Bi-nanocrystals doped with K+ and used for catalyze NRR, and the ammonia yield rate and Faraday efficiency driven by this Bi-based materials were greatly improved, even reaching 1–2 orders of magnitude of other reported NRR catalysts (Fig. 12.6) [44]. In addition to the p orbitals covering each other, they believe that the introduction of alkali metal K+ can regulate the proton transfer in NRR and stabilize the intermediates produced. The morphology and structure control engineering can

12.2 Non-noble metal-based metal catalysts

also affect the activity of Bi-based materials to catalyze NRR. Qiao et al. prepared Bi-inlaid two-dimensional nanomaterials by in situ electrochemical reduction using BioI nanosheets as precursors, and the NRR activity of BiNs was at least 10 times that of Bi nanoparticles [45]. On the one hand, Raman spectroscopy and electric double layer capacitance test results show that Bi NS exhibits a small size and thin thickness, which can greatly expose the active sites. In addition, the interlayer Bi-Bi bond of Bi NS is shorter than the inner Bi-Bi bond, indicating that the two-dimensional flake structure promotes the delocalization of p-orbital electrons, which is beneficial for electron transport-dependent NRR. In other reports, the strategy of constructing dendritic Bi-based materials [46] or loading flake Bi2 O3 on a two-dimensional carrier FEG [47] also achieved effective NRR. Defect engineering also affects the NRR performance of Bi-based materials. Lv et al. prepared an amorphous Bi4 V2 O11 /CeO2 hybrid through electrospinning and calcination procedures, where the relative content of Ce and Bi determines the degree of amorphization of this hybrid material [48]. As already discussed, the existence of Ov can improve the efficiency of NRR. On the one hand, the introduction of CeO2 promotes the amorphization of Bi4 V2 O11 and thus produces a large amount of Ov, which promotes the capture and further activation of N2 . In addition, the band alignment between CeO2 and Bi4 V2 O11 facilitates the transfer of electrons, promotes the production of NRR intermediates, and finally converts them into ammonia. In another report, Wang et al. prepared defect-rich Bi nanosheets (Bi2 O3 ) by means of plasma bombardment, in which Bi2 O3 contained a large number of defect-rich Bi (110) sites [49]. Compared with conventional Bi(110), the Gibbs free energy of N2 forming ∗ N2 H intermediates on Bi(110) with defects is lower. The Gibbs free energy as low as 0.89 eV indicates that the defective Bi(110) is closer to the volcanic summit of the NRR catalytic activity volcano map than Pt and Pd, and more importantly, the binding ability of Bi based materials to H atoms is weaker. These all hint at the unlimited potential of Bi-based materials in NRR.

12.2.5 Co, Ni-based catalysts Immediately after Fe element (the electron arrangement is [Ar]3d6 4s2 ), the electron arrangement of Co and Ni is [Ar]3d7 4s2 and [Ar]3d8 4s2 , respectively, so the NRR activity similar to Fe can be achieved. Generally, Co-based materials are often used as NRR catalysts in combination with elements such as N, P, and S. In the report of Qin et al., after high-temperature annealing treatment of imidazole zeolite frameworks (ZIFs), the catalyst Co-Nx-C for NRR was obtained, and the ammonia yield rate and Faraday efficiency of NRR driven by it reached 37.6 μg h−1 mgcat. −1 and 17.6%, respectively [50]. Further XAFS and DFT calculations show that Co-N3 -C when x is 3 is most conducive to the progress of NRR, and the most favorable evolution path is NN→∗ NN→∗ NNH→∗ NNH2 →∗ N→∗ NH→∗ NH2 →∗ NH3 →NH3 . Among them, Co-N3 -C is an active center that promotes N2 adsorption, which reduces the free energy of the reaction rate-decision step and inhibits the HER side reaction.

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

Li et al. used doping engineering to grow graphitic diyne nanosheet arrays (GDY/Co2 N) in situ on the surface of Co2 N nanowires grown on carbon cloth, achieving up to 219.72 μg h−1 mgcat. −1 ammonia yield rate and a Faraday efficiency of 58.6% [51]. The surface of GDY is rich in nonuniformly dispersed charges, which can improve the electrical conductivity of the material when covered on Co2 N. In addition, GDY interface bonding with unique p-electron properties promotes the obvious electronic activity of NRR in the interface area. The P-doped Co material also realizes the electrochemical reaction of N2 to ammonia. Guo et al. obtained CoP hollow nanocages (CoP HNC) assembled from CoP nanosheets after introducing a P source during the annealing treatment of the metal-organic framework ZIF-67 [52]. The reasons why CoP HNC can achieve catalytic NRR are: (1) The two-dimensional layered structure exposes more surface reaction sites; (2) The Co atom can transfer electrons to the P atom to make itself a positive site that can accept the electron-rich N2 attack, and the P atom with rich electrons can anchor the proton of the water molecule and supply to N2 adsorbed by the Co site. The method of doping S or N into Co is also considered as a strategy to optimize the activity of NRR catalysts. Chen et al. developed an interface engineering strategy to construct N and S co-doped Co-based materials (Co-N/S-C), in which metal sulfide and graphene are interface confinement carriers [53]. The ammonia yield rate and Faraday efficiency of NRR catalyzed by Co-N/S-C materials are as high as 25.0 μg h−1 mgcat. −1 and 25.9%, respectively. This result is achieved due to the strong coupling interface chemical bond, which acts as an electron transport channel and thus accelerates the reaction kinetics of NRR. Ni-based materials have also become candidates for NRR catalysts. Zhao et al. obtained the Ni-doped BCN heterostructures (BCN/Ni) after annealing the metal boron cluster organic polymer framework materials, in which the ammonia yield rate and Faraday efficiency of NRR driven by BCN/Ni were 16.72 μg h−1 cm−2 and 13.06%, respectively [54]. They believe that the charge separation and polarization in the heterojunction materials synergistically enhance the NRR catalytic activity of Ni-based materials. Specifically, Ni atoms can transfer their electrons to the carrier BCN to obtain partially positively charged Ni sites. The positively charged Ni sites can not only adsorb electron-rich N2 , but also repel the attack of protons to achieve the purpose of catalyzing NRR and inhibiting HER. Other Ni-based materials such as Fe-Ni LDH [55], N-NiO/CC [56] and Porous Ni [57] can also catalyze NRR, but their electronic/structure still needs to be further optimized to enhance their NRR activity. Ni-based materials are excellent HER catalysts, so how to rationally use catalyst preparation such as interface control, heteroatom doping, morphology control and defect engineering to stimulate the NRR potential of this cheap metal is still a topic worthy of in-depth discussion.

12.2.6 Other non-noble metal metal-based catalysts The research scope of non-noble metal-based metal catalysts in transition metals is very wide, which depends on their unique d-electronic properties. In addition to the

12.3 Non-metal-based catalysts

Table 12.1 The performance of some non-noble metal-based metal catalysts in NRR. Electrolyte

Potential vs. RHE

V-TiO2 VN NSs

0.5 M LiClO4 0.1 M HCl

−0.4 V −0.5 V

VN NPs

0.05 M H2 SO4

−0.1 V

VN NW/CC

0.1 M HCl

−0.3 V

mVOx -rGO Cr3 C2 @CNF Cr2 O3

0.1 M Na2 SO4 0.1 M HCl 0.1 M Na2 SO4

−0.35 V −0.3 V −0.9 V

Cr2 O3 nanofiber CrO0.66 N0.56

0.1 M HCl 1 mM H2 SO4

Mn3 O4 Cu/PI-300 Cu-TiO2 /CP Nb3 O7 (OH)/CFC Nb2 O5 nanofiber NbSe2 nanosheet SnO2

0.1 M Na2 SO4 0.1 M KOH 0.5 M LiClO4 0.1 M Na2 SO4 0.1 M HCl 0.1 M Na2 SO4 0.1 M Na2 SO4

−0.75 V 2.0 V (cell voltage) −0.8 V −0.3 V −0.55 V −0.4 V −0.55 V −0.4 V and −0.45 V −0.7 V

Sn/SnS2 SnS2 @Ni

0.1 M PBS 0.1 M Na2 SO4

−0.8 V −0.5 V

PdPb nanosponges Pd3 Pb

0.1 M HCl 0.1 M Na2 SO4

Catalyst

NH3 yield

FE (%) Ref.

17.73 μg h mgcat. 8.40×10–11 mol s–1 cm–2 3.30×10–10 mol s–1 cm–2 2.48×10–10 mol s–1 cm–2 18.84 μg h–1 mgcat. –1 23.9 μg h–1 mgcat. –1 4.96×10–11 mol s–1 cm–2 28.13 μg h–1 mgcat. –1 8.9×10–11 mol s–1 cm–2 11.6 μg h–1 mgcat. –1 12.4 μg h–1 cm–2 21.31 μg h–1 mgcat. –1 622 μg h–1 mgcat. –1 43.6 μg h–1 mgcat. –1 89.5 μg h–1 mgcat. –1

15.3 2.25

[58] [59]

6.0

[60]

3.85

[61]

16.97 8.6 6.78

[62] [63] [64]

8.56 6.7

[65] [66]

3.0 6.56 21.99 39.9 9.26 13.9

[67] [68] [69] [70] [71] [72]

2.17

[73]

6.5 10.8

[74] [75]

−0.05 V

1.47×10–10 mol s–1 cm–2 23.8 μg h–1 mgcat. –1 9.17×10–10 mol s–1 cm–2 37.68 μg h–1 mgcat. –1

5.79

[76]

−0.2 V

18.2 μg h–1 mgcat. –1

21.46

[77]

–1

–1

widely involved Mo, Fe, Ti, Bi, Co and Ni, V, Cr, Mn, Cu, Nb, Sn and Pb also have the ability to catalyze NRR, and the specific performance is shown in Table 12.1.

12.3 Non-metal-based catalysts Metals usually become the active centers of electrochemical reactions, but some nonmetal atoms also exhibit some catalytic activity. Many nonmetallic materials have shown excellent catalytic performance in HER, OER, ORR, and CO2 RR [78,79]. Although the d-orbital of transition metal (TM) is beneficial to the adsorption and

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

activation of N2 , it can also form M-H with proton H, which not only hinders the catalytic site of NRR, but also causes serious HER side reactions. In addition, metalbased catalysts are easily affected by the external environment, such as being oxidized, reduced, or changed in structure. The stable chemical bonds between nonmetal atoms in nonmetal-based materials can effectively solve the stability problem of the catalyst, and the HER side reaction of such materials is not as active as that of metal-based materials. Since most nonmetal-based catalysts are derived from the combination of other heteroatoms and C, only B-, N-, O-, S- and P-based catalysts are discussed here.

12.3.1 B-based NRR catalysts For elements with atomic numbers lower than 18, there is usually no combination of real and empty orbitals to achieve symmetric σ and π bonds, but they may show similar phenomena of the interaction of transition metals with N2 . For example, the sp2 empty orbital of element B on the B-based material can accept electrons from the σ orbital of N2 , and the electrons on the p orbital occupied by electrons can be donated back to the π ∗ orbital of N2 , thereby activating N2 . Of course, when the B atom exists in the form of sp3 , it can also achieve the same mechanism as capturing N2 in the case of sp2 [80]. As we all know, due to the lack of electrons, the B atom has the properties of Lewis acid, which is advantageous for activating the electron-rich N2 Lewis base. Zheng et al. obtained BC3 material with NRR catalytic activity after doping B element in graphene [81]. Among them, the introduction of B element does not change the two-dimensional structure of graphene composed of sp2 C, which ensures the maximization of the reaction interface. The doping of element B will deprive the electrons in graphene, thus promoting the adsorption of N2 on the electron-deficient graphene. Specifically, the electronegativity of B is lower than that of C, and thus the doping of B element breaks the uniformly distributed electron density of graphene, causing the carbon ring structure of graphene to be far from the equilibrium state due to the obvious differentiation of electron density. In addition, the negatively polarized O oxygen atoms are easily combined with the electron-deficient B atoms, blocking the adhesion of proton hydrogen and thus inhibiting HER. Qiu et al. used a top-down strategy to convert commercial bulk B4 C into B-doped graphene quantum dots, which also achieved good NRR activity [82]. Chen et al. also obtained a catalyst BNG-B with NRR catalytic activity by modifying graphene [83]. During the heat treatment, boric acid first reacts with ammonia to form a stable B-N bond, which tends to be embedded in graphene. DFT calculations show that the free energy of adsorption for ∗ H (G∗ H) of the B-N pair site on BNG-B is far from the optimal value, reaching 0.65 eV, indicating that BNG-B is inert to HER. Even so, B-N pairs are only distributed on the edges of graphene, the carbon atoms in the graphene with HER activity still exist (Fig. 12.7). For comparison, they prepared a B- and N-separated material (BNG-S) in a two-stage process, first doping graphene with B atoms, and then doping it with N atoms. Compared with BNG-B, BNG-S catalytic NRR performance is only slightly

12.3 Non-metal-based catalysts

(A)

C

B 1

(C) 3.4

N

1

2

1

3.2 2

3.8

3 3

2.8

'G*N2H(eV)

2

3

2.2 2.0

6 5

1

(B)

C-1 B-1 N-1 BN-1

1.6

4

3

(D) EP

DOS (a.u.)

EP

2

EF

Free Energy (eV)

BN

2.0

−0.0

*NNH2

0.5

−1.0 −5.0 −2.5 E-EF (eV)

1 2 3* C

1 2 3* B

1 23 N

1 2 3 4 5 6* BN

*NHNH

1.0

0.0

Adsorption

*N2H

1.5

−0.5

−7.5

1.8

N2

*NHNH2

*N+NH3

N2H4 *NH2 *NH

NH3

Reaction Pathway

FIGURE 12.7 (a) Schematic diagram of BNG-B model used for calculation, where the gray, pink, red, blue and green balls represent C, B, O, N and H atoms respectively; (B) Except for C-3, B-3, and BN-6 that cannot be bound to N2 H substances by chemical adsorption, G∗N2H calculated for different configurations; (C) The density of states of C-1, B-1, N-1 and BN-1 active sites (edge C atoms), where the red dotted line represents the Fermi level; (D) calculated free energy diagram of N2 reduction in the BN-1 system, where the inset is the geometric structure of BN-1, and the active site is highlighted by the dotted red circle. Adapted with permission. [83] Copyright 2019, Wiley-VCH.

improved. Detailed studies have shown that the energy level of C in BNG-B is far away from the Fermi level (0 eV), and doping with B or N heteroatoms alone will not significantly affect the energy level of C in graphene. On the contrary, the energy level of the B-N pair is closer to the Fermi level than that of C, N and B alone, which is an internal factor for BNG-B to achieve more effective NRR catalytic performance. Sun et al. studied in detail the behavior of unsupported B4C nanosheets to catalyze NRR, and the final ammonia yield rate and Faraday efficiency achieved were 26.57 μg h−1 mgcat. −1 and 15.95%, respectively [84]. N2 is first adsorbed on B4 C(110) in the form of distal association, and the subsequent hydrogenation process conforms to the enzymatic mechanism, that is, the alternate hydrogenation pathway. Among them, except for the rate determination step (∗ NH2 − -∗ NH2 → ∗ NH2 + ∗ NH3 ) which

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

needs to provide an energy barrier of 0.34 eV, the rest of the hydrogenation steps are easy to implement. Before the rate determination step, the hydrogenation pathway has accumulated a potential barrier of up to 2.7 eV, while the desorption energy barrier of ∗ NH3 on B4 C (110) is only 1.73 eV, so the NRR process is easy to perform. Another report by the Sun et al. further clarified the evolution of N2 in B4 C [85]. N2 is adsorbed on the three adjacent B sites on B4 C, and the N=N triple-bond bond length is stretched by 0.02 Å, where a large amount of charge is accumulated at the junction of N2 and the three B sites. Although the desorption of ∗ NH3 to NH3 is difficult, increasing the ammonia concentration near the catalyst can reduce the energy barrier for the desorption of ∗ NH3 to NH3 . B element has characteristics such as intrinsic electron deficiency and partial overlap with the 2p orbital of N so that it can effectively activate N2 . For this element with the potential to catalyze NRR, its maximum catalytic activity should be actively explored, for example, by using advanced preparation techniques to modulate a Bbased catalyst with high NRR catalytic activity and capable of inhibiting HER.

12.3.2 N-based catalysts We know that during the activation process of N2 , it can not only share its own lone pair of electrons, but also accept from outside electrons (injected into the ∗ π antibond orbital). Generally, N-based NRR catalysts containing only N elements do not exist. The N element is usually used as a doping element to impart catalytic activity to other substances, such as carbon nanotubes, metallic elements (to form M-N nitrides), and nonmetallic elements (such as B, C, and P). Liu et al. used metal-organic framework ZIF-8 as the precursor to obtain porous N-doped carbide (NPC) after pyrolysis [86]. Temperature-programmed desorption of N2 (N2 -TPD) test shows that the pyrolysis temperature can affect the ability of NPC to adsorb N2 . NPC treated at 750°C has the strongest N2 adsorption capacity, 850°C has weaker N2 adsorption capacity, and 950°C treatment has the weakest N2 adsorption capacity. The difference in N2 adsorption capacity mainly comes from the different contents of pyridine nitrogen, pyrrole nitrogen, and graphite nitrogen in NPC materials treated at different temperatures. Pyridine nitrogen and pyrrole nitrogen are favorable forms to promote NRR, while graphitic nitrogen has almost no catalytic ability for NRR, as is the DFT calculation result (Fig. 12.8). In another report, the researchers constructed a nanospikes-like N-doped carbon material and achieved an ammonia yield rate as high as 97.18 ± 7.13 μg h−1 cm−2 [87]. The nanospikeslike structure can induce the tip electric field effect, and the local electric field can effectively capture N2 and realize the subsequent multielectron transfer hydrogenation step. Defects are also an important factor affecting the NRR catalytic performance of N-doped materials. In the report of Lv et al., the polycondensed melamine was pyrolyzed again to controllably obtain an N-doped carbon material (PCN-NV4 ) rich in nitrogen vacancies (NV). Compared with PCN without NV, PCN-NV4 has higher NRR activity [88]. DFT calculation results show that N2 can be activated at the

12.3 Non-metal-based catalysts

(A) 10 h

pyrolysis

ZIF-8

(B)

NPC

(C)

(D)

500 nm

500 nm

(E)

(F)

(G)

10 nm

10 nm

−2

4

U=0V U = −0.7 V

−2

e -) *NH 2 -NH 2 +2 (H + + e) *NH 2 +NH 3 + H+ + e-

4(H + +

*NH -NH 2 +3 (H + +

+ e)

e -) 6(H + +

*N= NH +

0

H+

1

+ e)

2

e -)

3

−1

Reaction coordinate

10 nm

*N− =N +

2NH3

Free energy (eV)

+ e-

e -)

+ H+ 3

NH

+ 2( + H +

−1

*NH 2+

2

NH *NH 2-

5(H +

6(H + +

4(H + + e) *NH -NH 2 +3 (H + +

*NH =NH +

*N= NH +

*N− =N +

+ 6( + H

+ e)

e -)

2

+ e)

e -)

3

0

(I)

U = -0.7 V

+ 6(

U=0V

N2

Free energy (eV)

4

N2

(H)

1

500 nm

*NH =NH +

methylimidazole

5(H +

Zn2+

2NH3

Reaction coordinate

FIGURE 12.8 (A) Schematic illustration of NPC preparation; (B–D) SEM images of NPC-750 (B), NPC-850 (C), and NPC-950 (D); (E–G) TEM images of NPC-750 (E), NPC-850 (F), and NPC-950 (G); (H and I) Free energy diagram for ammonia synthesis on an NPC with (H) pyridine nitrogen and (I) pyrrole nitrogen. Reproduced with permission. [86] Copyright 2018, American Chemical Society.

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CHAPTER 12 Noble-metals-free catalysts for electrochemical NRR

binuclear end (two connected C atoms) near NV, which benefits from NV’s promotion of space electron transfer. Vacancies (defects) often have distorted charge distribution characteristics, not only in NV. For example, Mukherjee et al. obtained an N-doped carbon material with a large number of C defects after pyrolyzing ZIF-8, where the carbon vacancy sites near pyridine N can adsorb N2 and cleave the N≡N triple bond [89]. It should be noted that when N-based materials are used in NRR, their catalytic mechanism should be clearly defined. On the one hand, N2 may be converted to NH3 on N-based materials through the MvK pathway, that is, the N in NH3 is supplied by the N constituting the catalyst, and the missing N in the catalyst is supplied by N2 . In addition, a variety of in-situ tests and isotope tracking experiments should be used to verify whether the detected NH3 comes from the conversion of N2 on the catalyst surface via the MvK mechanism or other pathways, rather than the possible residual NH3 in catalyst or the electrochemical decomposition of catalyst.

12.3.3 O- and S-based catalysts Some reports have pointed out that O or S as a dopant achieves the improvement of catalytic NRR performance under the same conditions. Wang et al. doped oxygen into sodium gluconate to prepare a two-dimensional catalyst material (O-G) that can be used as a NRR catalyst. The ammonia yield rate of NRR under O-G drive reached 21.3 μg h−1 mgcat. −1 [90]. Here, the doping of O atoms can replace the C atoms in graphene to form C-O, C=O and O=C-O structures. After the O atom replaces the C atom, electron migration and polarization will occur between O-C, causing the C atom to be positive and the O atom to be negative. N2 with a lone electron pair is easily adsorbed on the positively charged C atom, and this phenomenon is more obvious in graphene with C=O and O-C=O. Another similar study by the team also used the above strategy [91]. In the report of Wu et al., O-free doped carbon nanotubes have poor NRR activity, while the material obtained after hollow carbon microtubes doped with O (O-KFCNT) can catalyze NRR, in which the ammonia yield rate reached 25.12 μg h−1 mgcat. −1 [92]. In DFT calculation, the active site in O-KFCNT is O atom instead of C atom. Although only the carbon support was changed from graphene to carbon nanotubes, the catalytically active sites were changed from C sites to O sites. There will be a certain difference between the DFT calculation model and the actual structure of the catalyst, but this also shows that the difference of carrier can affect the active center of the NRR. The doping of S atoms into the metal-based catalyzing agent often forms Sv (S vacancy), which is the reason for enhancing the catalytic NRR activity. S doping into nonmetallic materials also uses a similar principle to improve the NRR catalytic performance of nonmetallic materials. Tian et al. introduced N and S heteroatoms into graphene, which greatly stimulated the NRR activity of graphene [93]. The codoping of N and S results in the distortion of the graphene structure, resulting in a large number of defect (vacancy) sites, which activates the inert N≡N triple bond and promotes the electron transfer of NRR.

References

12.3.4 P-based catalysts P and N belong to the same main group in the periodic table, they have similar electron configurations. Some reports have shown that P-based materials can also catalyze NRR. Inspired by “like dissolves like,” Zhang et al. used fully exfoliated black phosphorus nanosheets as a catalyst for NRR, where the ammonia yield rate was as high as 31.37 μg h−1 mgcat. −1 [94]. DFT calculation results show that the HUMO and LUMO electron densities in FL-BP NSS are mainly distributed at the serrated edge. The asymmetric electron distribution on the nanosheets is the reason for the high NRR catalytic activity, N2 is easily converted to NH3 at the serrated edge. However, the poor conductivity of pure black phosphorus will affect the electron transfer in the NRR process. Based on this, Liu et al. chose SnO2-x as the electron transport conductor. After P forms a P-Sn coordination bond with Sn in SnO2-x , black phosphorus is anchored in SnO2-x nanotubes (BP@SnO2-x ), the ammonia yield rate of NRR derived by modified black phosphorus is increased to 48.87 μg h−1 mgcat. −1 [95].

Competing interests The authors declare no competing interests.

Acknowledgments This work was financially supported by National Natural Science Foundation of China-Yunnan Joint Fund (U2002213), the National Key R&D Program of China (2019YFC1804400) and the Double Tops Joint Fund of the Yunnan Science and Technology Bureau and Yunnan University (2019FY003025).

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13

Noble metals-based nanocatalysts for electrochemical NNR

Jing Li, Zihao Ye and Weiwei Cai Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, China

13.1 Introduction Ammonia is an important chemical raw material that plays an important role in the fields of industry and agriculture production and energy storage/conversion. Currently, the industrial synthesis of ammonia mainly relies on the traditional Haber– Bosch process. Because this technology needs to be carried out at high temperature and pressure (300–500◦ C, 150–200 atm), it not only consumes plenty of energy but also emits a lot of greenhouse gases. Therefore, in the context of the increasingly prominent energy crisis and environmental problems, it is urgent to develop a new method for the efficient synthesis of ammonia under mild conditions. Compared with the Haber–Bosch process, electrocatalytic nitrogen reduction reaction (NRR) can theoretically produce ammonia under low temperature and pressure. Since the raw materials for NRR are water and nitrogen, it brings opportunities for the realization of green ammonia synthesis under mild conditions. In recent years, electrocatalytic NRR has attracted great attention and related research reports have shown a rapid growth trend. However, electrocatalytic nitrogen reduction to produce ammonia is thermodynamically slow due to the extremely high stability of the N≡N triple bond and the slow adsorption of nitrogen. At the same time, selectivity of nitrogen reduction reaction and the rate of ammonia production would be greatly reduced due to the existence of hydrogen evolution competition reaction in aqueous electrolytes. As a result, the biggest challenge in the study of electrocatalytic NRR at room temperature and pressure is to increase the kinetics and selectivity of ammonia production simultaneously. Thanks to the rapid development of nanotechnology in recent decades, there is a significant increment on literatures of new and efficient electrochemical NRR catalysts since 2015. Some great results have been reported in improving the efficiency of ammonia synthesis (especially the Faraday efficiency). As a common sense, size, morphology, crystallinity and active site type of the nanocatalysts would all significantly affect the catalytic activity. Currently, most of the NRR experimental research work focuses on the following types of catalysts, including noble Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00003-4 Copyright © 2022 Elsevier Inc. All rights reserved.

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metal-based catalysts, transition metal-based catalysts, and metal-free catalysts. In order to overcome the kinetics limit of nitrogen activation and improve the reaction selectivity of NRR under environmental conditions, adjusting on crystal plane [1, 2], particle size [3, 4], and crystallinity [5, 6] on the catalysts of heteroatom doping [7–9] were widely carried out. Among the various candidates, precious metal-based catalysts, including Pt, Pd, Au, Ru, etc., have exhibited good electrocatalytic activity in other electrochemical reactions [10–13]. In recent years, the precious metal-based catalysts have attracted the attention of NRR researchers and the precious metal-based NRR catalysts would be reviewed by being catalogued on Ru-based catalysts, Aubased catalysts, and other precious metal catalysts in this chapter.

13.2 Ru-based NRR catalysts Ru is a precious metal with valence states varying from −2 to +8, which makes it widely applied in many fields, including catalytic hydrogenation, CO conversion and synthesis of hydrocarbon reforming [14–17]. Since Ru and Fe belong to the VIII group and Fe-based catalysts are state-of-art catalysts toward ammonia synthesis via the Fischer–Tropsch process, the ruthenium catalyst is therefore considered as the alternative ammonia synthesis catalyst [18]. Therefore, it is of great significance to study the performance of the Ru-based catalysts toward the electrochemical NRR. However, since the reverses of Ru in nature are relatively small and the price is relatively high, it is hence necessary to consider maximizing the atomic utilization rate and reducing the amount of Ru as much as possible. In order to achieve the above targets, single-atom Ru catalysts, supporting Ru catalysts, and Ru-based alloys were studied as NRR catalysts.

13.2.1 Single-atom Ru-based NRR catalysts Ru is considered to be the second generation NH3 synthesis catalyst, and its N2 reduction potential is calculated to be lower than Fe. Due to the uniformity of the catalytic active sites, the isolated metal atoms dispersed on the support have a low coordination environment of metal atoms and maximum metal utilization effectiveness. Tao et al. [19] encapsulated Ru single atoms in N doped porous carbon by a coordination assist strategy and further synthesized a Ru@ZrO2 /NC catalyst by adding ZrO2 to inhibit undesired HER. The Ru@ZrO2 /NC catalyst therefore achieved an ammonia synthesis rate of 3.665 mgNH3 h−1 mgRu −1 in a 0.1 M HCl electrolyte at 0.21 V and a Faraday efficiency of 21% at 0.17 V. In order to explore the influence of ZrO2 in the ruthenium catalyst, the FE of ammonia under different voltages and the average ammonia production rate were compared. The significantly enhanced NNR activity after adding ZrO2 indicated that ZrO2 would improve the NRR activity by inhibiting HER, which can also be confirmed by the DFT calculation. This method provides us with a means to increase the activity of NRR by adding ZrO2 to inhibit

13.2 Ru-based NRR catalysts

HER that competes with NRR, which opens up a broader prospect for ruthenium single atom in the field of electrochemical reduction of nitrogen. Geng et al. [4] also synthesized Ru-based single-atom catalyst via fixing Ru single atoms on nitrogen-doped carbon (Ru SAs/NC) by pyrolyzing the Ru-containing derivatives of ZIF-8. It can be confirmed by the characterizations that individual Ru atoms are uniformly distributed and the C, N, and Ru elements are evenly distributed as desired. In a 0.5 M H2 SO4 solution, Ru SAs/NC showed an ammonia synthesis rate of 120.9 μgNH3 mgcat −1 . h−1 at −0.2 V, which is twice the performance of Ru NPs/NC. Faraday efficiency of 29.6%, which is also much higher than the FE of Ru NPs/NC, can also be achieved. The DFT calculation indicated that the main reason for the high performance of the Ru SAs/NC catalyst is the relatively strong bonding strength between N2 molecules and ruthenium atoms. The dissociation of N2 was therefore became the rate determining step of the NRR reaction on this catalyst. At the same time, the Ru single atom decreases the activation energy of N2 dissociation and Ru SAs/NC can therefore promote the dissociation of N2 into nitrogen atoms, making the NRR reaction easier to proceed. This work provided guidance on fabricating uniformly dispersed monoatomic materials and opens up a pathway for the study of single atom catalysts toward NNR.

13.2.2 Supported Ru-based NRR catalysts By combining the properties of Ru atoms and the substrate materials, the supported Ru-based catalysts can improve the electrochemical NRR performance of metal Ru. At the same time, the invitation of the supporting materials can significantly reduce the cost the Ru-based catalysts. As a metal-organic framework (MOFs), ZIF provides inherent advantages and usable active sites for catalysis. Further, it is also suitable for being used as a substrate to support metal atoms/nanoparticles to improve electrocatalytic efficiency and reduce the costs. Zhang et al. [20] prepared a Rudispersed nitrogen-doped carbon skeleton catalyst by dispersing Ru in ZIF-8 for carbonization. The Ru atoms can be facilely anchored on the N-doped carbon skeleton and its structure can stabilize the ruthenium atoms and prolong the life time of the catalyst. In 0.1 M KOH electrolyte, high content of Ru in the catalyst can achieve higher ammonia production and FE by comparing the performance of catalysts with different Ru contents. As a result, the ammonia synthesis rate at −0.4 V can reach 16.68μgNH3 mgcat −1 . h−1 and the FE at −0.3 V is high to 14.23%. This catalyst promotes NRR activity through the following three aspects: (1) the nitrogen-doped carbon skeleton has great electron conductivity, dispersibility and plenty Ru anchoring sites; (2) ZIF-8 retains its original hydrophobic structure to inhibit HER; (3) the abundant pyridine nitrogen can firmly adsorb N2 molecules. This work therefore provided a feasible method for designing highly dispersed noble metal electrocatalysts for nitrogen reduction reactions. In addition to traditional carbon supports, metal oxides, represented by TiO2 , have the advantages of low cost, high abundance and nontoxicity, and can therefore be

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(A)

(B)

(C)

(D)

(E)

FIGURE 13.1 (A) Preparation procedure of Ru/TiO2 -Vo. (B) HRTEM image of Ru/TiO2 -Vo with a scale bar of 5 nm. (C) EDS elemental mapping of Ru, Ti and O in Ru/TiO2 -Vo. (D) Yield rate of NH3 with different catalysts at given potentials at ambient conditions. (E) Faradaic efficiency of different catalysts at various applied potentials.

used as ideal supports for NRR catalysts. Cheng et al. [21] proposed a synthesis method of Ru nanoparticle catalyst supported on TiO2 rich in oxygen vacancies (Ru/TiO2 -Vo). Due to the successful construction of oxygen vacancies (Fig. 13.1A), Ru NPs can be well dispersed on the TiO2 -Vo carriers according to the high relation TEM image (Fig. 13.1B), HAADF-SEM image and corresponding elemental maps (Fig. 13.1C). In 0.1 M KOH solution, the highest ammonia producing rate is 2.11 μgh−1 cm−2 at 0.15 V (Fig. 13.1D and E). According to DFT calculation, the mechanism and rate-determining steps of NRR on the catalyst are determined as:

13.2 Ru-based NRR catalysts

∗N2 +1/2H2 →∗N+ +∗NH. The existence of oxygen vacancies changes the electron state of Ru4 . Other than the electron effect of TiO2 -Vo, the presence of oxygen vacancies may change the reaction pathway and reduce the overpotential for NRR. This work focused on the effect of oxygen vacancies in the substrate on NNR and therefore providing a new viewpoint to improve the performance of metal catalyst toward NNR. However, the FE of the catalyst is relatively low (less than 1%) and the mechanism has to be further studied. Because the d-electron orbitals of noble metals are not fulfilled, the surface of noble metals can easily adsorb reactants, which is important to the formation of intermediate “active compounds” with high catalytic activity to produce an important catalyst material. MXene is a new type of two-dimensional inorganic material, composed of transition metal carbides, nitrides or carbonitrides, with a thickness of several atomic layers, and is widely used in the field of super capacitors and batteries. Moreover, MXene has the characteristics of high specific surface area, high conductivity, and high catalytic activity. Liu et al. [22] replaced the T in the Ti3 C2 Tx compound by MXene and dispersed Ru on Ti3 C2 (MXene) by impregnation. The Ru@Ti3 C2 (MXene) catalyst was hence highly performed toward NRR. The electrochemical NRR measurements were carried out in 0.1 M KOH solution. The ammonia synthesis rate and FE of Ru@Ti3 C2 (MXene) can reach 2.3 μmol h−1 cm−2 and 13.13%, respectively, at −0.4 V. More importantly, the Ru@Ti3 C2 (MXene) catalyst exhibited great durability during the 24 hours continues ORR. However, Ti3 C2 Tx support is prone to chemical degradation after being oxidized to TiO2 and the durability of the catalyst in practical application is still an issue that needs to be solved.

13.2.3 Ru-based alloy catalysts We already know that HER is the undesired competitive reaction during the electrochemical reduction of nitrogen, H2 will therefore cover the catalyst surface to prevent N2 from binding to the active site and inhibit NRR if the kinetics of HER on the catalyst is greater than the NRR. It is therefore investable to inhibit HER for the boosting of NRR performance of the new developed catalysts. However, H atom is also the reactant of both NRR and HER and inhibiting HER also decays the NRR activity to a certain extent. To this concern, it is reasonable to separate the active sites that bind N2 and H atoms via alloying the metallic catalysts. HER activity can be therefore reduced due to the lack of adjacent H atoms. The coverage N2 on active site can be therefore improved and the NRR activity can be subsequently enhanced. Due to relatively low H atom adsorption energy, Pt is an ideal metal to engineer such an alloying system with other metals. Revanasiddappa et al. [23] dispersed the RuPt alloy (1:1) on the Vulcan XC-72 carbon support (RuPt/C) as an electrocatalyst to catalyze NRR. At 0.023 V, the ammonia synthesis rate of 1.08 × 10−8 gNH3 s−1 cm−2 and Faraday efficiency of 13.2% were obtained in 1.0 M KOH solution at 0.023 and 0.123V, respectively. This NRR performance is better than that of both Pt/C and Ru/C. It can be therefore indicated that this improved NRR performance is resulted from the

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synergistic effect of Ru atoms and Pt atoms in RuPt/C. H atoms are generated on Pt sites and nitrogen atoms are adsorbed on Ru sites to dissociate nitrogen atoms. The specific equations are as follows: 6Pt + 6H2 O + 6e → 6Pt-Had + 6OH− 2Ru + N2 → 2Ru-Nad Rusur − Nad + Ptsur -3Had → RuPt +NH3 (Overall reaction) In this work, the idea of separating the active sites for generating H atoms and adsorbing N2 is applied for catalyst design to improve the performance of NRR and the alloying strategy is an effective pathway to achieve this target. Different from the above-mentioned PtRu alloy with the atomic ratio of 1:1, Zhang et al. [24] introduced low-concentration Pt atoms into Ru nanowires through atomic engineering. Due to successful employing of Pt in the Ru88 Pt12 nanowires with an atomic ratio of Ru:Pt = 88:12 (Figs. 13.2A), tensile strain effect was introduced according to the lattice structure analysis of Ru88 Pt12 (Figs. 13.2B–F). In 0.1M KOH solution, the NRR activity of Ru88 Pt12 nanowires is much higher than that of both Ru and Pt nanowires in a certain voltage range (Figs. 13.2I and J). At −0.2 V, Ru88 Pt12 nanowires achieved the highest ammonia synthesis rate of 47.1 mg gcat −1 . h−1 and the highest Faraday efficiency of 8.9%. Theoretical calculations show that the d band center of Ru atoms rises due to the presence of Pt atoms, which reduces the adsorption barrier of the intermediate product NNH on the Ru surface. Selectivity of N2 adsorption and the stability of the N2 H intermediate can therefore be improved. In other words, the authors applied the method of atomic engineering to dope a small amount of Pt atoms in Ru nanowires to improve the NRR performance. At the same time, the content of Pt atoms should not be too high, otherwise platinum clusters would be formed, which would increase undesired HER and decrease the NRR activity. Above all, the atomic modification strategy they used can be used to precisely design other nanocatalysts for various applications. In this section, we reviewed the Ru-based catalysts with different synthesis strategies to boost the NRR activity and to reduce the cost. It can be found that the ammonia synthesis rate and FE of Ru single-atom are much higher than other Rubased catalysts. However, NRR performance difference among various Ru singleatom catalysts is still huge, mechanism of NRR on Ru single-atom catalyst should be in-deep studied to guide the further development of novel Ru-based catalysts.

13.3 Au-based NRR catalysts Au was once recognized as an inert material for catalysis. With the development of nanotechnology, it was gradually discovered that Au can also achieve great catalytic activity toward many chemical reactions when the Au particle size is smaller than

(A)

(E)

Average yield (Pg h–1 mg–1)

30

20

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(J) 1.4

(H)

1.6 1.8 2.0 Diameter (nm) 2.2 Å

c b

40 30 20 10

a

– fcc [110]

–0.4 –0.3 –0.2 –0.1 Potential (V vs RHE)

0.0

10

Ru88Pt12 Ru Pt

8 6 4 2 0 –0.5

–0.4 –0.3 –0.2 –0.1 Potential (V vs RHE)

FIGURE 13.2 (A) A schematic diagram of the structure of a Ru88 Pt12 nanowire. (B–D) TEM images and the diameter distributions of Ru88 Pt12 nanowires (n¼ 100). (E–G) Atomic resolution HAADF images of Ru88 Pt12 nanowires. (H) A model of the atomic arrangement of the fcc structure viewed along the [110] direction. (I) NH3 production rates of Ru88 Pt12 , Ru, and Pt nanowires at different potentials. (J) Corresponding faradaic efficiencies.

0.0

13.3 Au-based NRR catalysts

(G)

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Ru Pt

–0.5

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Ru88Pt12

50

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(I)

Faradaic efficiency (%)

10 nm

Pt

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Ru

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3–5 nm. Besides the wide application in plenty chemical reactions [25–28], catalytic performance of Au-based nanocatalysts were also studied as potential catalysts for NRR. In order to boost the NRR activity of Au-based catalyst, adjusting the nanostructure and nanoparticle supporting and nanoscale alloying are the most commonly used strategies.

13.3.1 Au catalyst nanostructure adjusting Adjusting the nanomorphology of metal catalysts to expose more active sites is a commonly used strategy for performance boosting of metal catalysts. It was reported that nanomorphology of Au nanoparticles with dodecahedral large stellar [29] and higher index surfaces can improve the NRR activity. The catalyst was synthesized via the electrochemical two-step method in the choline chloride-urea-based deep eutectic solvent. For the same dodecahedral large star-shaped gold nanocrystals (Au GSD NCs) catalyst, hollow structure was design to make both the inner and outer surfaces available for NRR catalysis. It is a common sense that face-centered cubic (fcc) metal (Au, Pt, Pd, etc.) nanocrystals (NCs) are surrounded by high refractive index surfaces (HIFs). Due to the high density of atomic steps and curvatures, the catalyst can provide highly active sites toward NRR. The corresponding theoretical study pointed out that the intermediates involved in NRR bind more firmly to the surface sites on the steps, rather than to the sites on the flat terraces. Based on the above two points, the NRR performance of Au catalyst can be significantly improved. In 1.0 mM HCl solution, the NH3 yield and FE of Au GSD NCs at −0.4 V are 49.96 μg cm−2 h−1 and 28.59%, respectively. This performance is higher than the catalysts with Au nanosphere structure. According to the DFT calculation, the high index surface can provide abundant active sites for NRR. The rate-determining step on the high index surface of Au is the transfer of proton-electron to N2 to generate N2 H∗ (N2 + ∗ + H+ + e→N2 H∗ ), which takes up two adjacent active sites of Au. The Au GSD NCs thereby exhibited have very-high NRR activity and this work hence provided a feasible pathway to improve the NRR performance. In addition, Tan et al. [30] synthesized Au nanoparticles (AuNPs) with a high index via an improved seed-mediated strategy. A single nanocrystal oriented along the (110) direction show that there are various types of steps and multiple exponential surfaces on the gold surface, which confirm that the successful construction of high-index facets on Au nanoparticles. In 0.1 M Li2 SO4 solution, AuNPs exhibited an ammonia synthesis rate of 9.22 μg h−1 cm−2 and a Faraday efficiency of 73.32% at −0.3 V vs. RHE potential. It can be found from the DFT calculation that the rate-determining step of NRR on AuNPs is the first step of hydrogenation to form ∗ NNH reaction (∗ NN + e+ H+ →∗ NNH). At the same time, the catalytic reaction intermediates of NRR can remain stable on the high-index facets. Because the high-index facets reduced the free energy of the rate-determining step, the NRR reaction is easier to proceed. In addition, it has been found that the high-index facets can effectively inhibit the adsorption of H atoms for HER while can improve the adsorption of NNH, which

13.3 Au-based NRR catalysts

is a key intermediate of NRR. This is the reason of the ultrahigh Faraday efficiency of this catalyst. Since the NRR activity of Au catalysts toward the electrochemical NRR has been confirmed and he NRR on the Au surface follows an association mechanism where the N2 triple bond breaks and the hydrogenation of the nitrogen atom occurs simultaneously. The better NRR catalytic activity of Au than on other electrocatalyst surfaces is due to its multifaceted gold surface, which consists of various active sites for N2 adsorption and reduction. In addition to adjusting the morphology of Au nanoparticle, it is also possible to hollow out the middle part of the Au nanoparticle to obtain a hollow Au. Both inner and outer surfaces can be used for NRR simultaneously generate NRR to improve the NRR activity. Nazemi et al. [31] used AgNC as a template to prepare hollow Au nanotubes (AuHNCs) using galvanic replacement technology. In 0.5 M LiClO4 electrolyte, the ammonia synthesis rate of 3.9 μg h−1 cm−2 and the Faraday efficiency of 30.2% were obtained at −0.5 and −0.4 V vs. RHE potentials, respectively, for AuHNCs. Besides the increased reaction area due to the expose of both inner and outer surfaces, the cavity of AuHNCs also increases the residence time of N2 , which is beneficial for the reaction to be proceed sufficiently to increase the NRR rate. Above all, although the Au nanocatalysts have achieved considerable Faraday efficiency, the yield is relatively low and needs to be improved.

13.3.2 Supported Au-based NRR catalysts An ideal electrocatalytic NRR system has to meet the following requirements: (1) The active sites should have high accessibility for N2 ; (2) The active sites can effectively weaken the triple NN bond; (3) Mass transfer and electron conductive channels for protons and electrons transportation for the construction of triple-phase interfaces for the active sites. In order to achieve the above three requirements for the Au NRR catalysts, Liu et al. [32] engineer a net-like structure Ti3 C2 substrate for Au supporting using an ultrasonic reduction process inspired by the spider web structure in nature. Au particles as the active center were located on the nodes of the net-like structure Ti3 C2 substrate and therefore obtained the Au/Ti3 C2 catalyst for NRR. The process of gold nanoparticles adsorbing N2 is regarded as the predation process of spiders on the net, which enhances the adsorption of N2 molecules. At the same time, the Ti3 C2 network can enhance the interaction between the Au nanoparticles and the N2 molecules, the NN triple bond was therefore weakened and making N2 easier to dissociate and accomplish the reaction. In 0.1 M HCl electrolyte, the Au/Ti3 C2 catalyst was electrochemically measured and an ammonia yield of 30.06 μg h−1 mg−1 and a Faraday efficiency of 18.34% were achieved at −0.2 V vs. RHE. The DFT calculation revealed that the presence of oxygen on Ti3 C2 substrate is the key to the synthesis of the catalyst. Au nanoparticles can be fixed by oxygen atoms and then aggregate to form clusters. At the same time, the long bond between N and Au makes it easier for H to be interconnect to N2 ∗ , which is the rate-determining step of

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NRR. In other words, this work creatively linked the process of “capturing” N2 by the catalyst for electrocatalytic NRR. The application of "net" construction for the target molecules provides us a promising way to develop highly active NRR catalysts.

13.3.3 Au-based alloy NRR catalyst It has been confirmed that Au has the properties required for electrocatalytic NRR, but a single component of Au has the disadvantages of low available active site density and poor selectivity. It is reasonable to indicate that the catalytic NRR performance of Au can be effectively improved through alloying and structural adjusting. Alloying is the most facile strategy to adjust the electronic structure of Au to change the adsorption and activation of N2 on the catalytic active sites. Liu et al. [33] synthesized a series of Au-based bimetallic nanocatalysts (AuCu, AuAg, AuPd, and AuRu) for electrochemical NRR performance study. Among which, Au1 Cu1 with an atomic ratio of 1:1 exhibited the best NRR activity. In the 0.05 M H2 SO4 solution, the maximum ammonia synthesis rate obtained at −0.2 V is 154.91 μg mgcat −1 h−1 , and the maximum Faraday efficiency is 54.96%. In this catalyst, the synergistic effect between Au and Cu modulates the electronic structure of Au and reduces the free energy barrier of the decisive step simultaneously. The catalyst therefore simultaneously achieved a relatively high ammonia synthesis rate and Faraday efficiency at the same time. The introduction of another metal into the Au lattice will change the electronic structure, thereby regulating the free energy of nitrogen species on the catalyst. It has been widely reported that phosphorus has weak hydrogen adsorption, and its abundant valence electrons can provide abundant places for nitrogen activation. In addition to gold catalysts, palladium is also used to combine with other active metals to form alloys to obtain high NRR performance. As re result, Wang et al. [34] synthesized one-dimensional metal-nonmetal alloy AuPdP nanowire catalysts (AuPdP NWs) from a solvothermal method (Fig. 13.3A). The HAADF-STEM image (Fig. 13.3B) of the AuPdP NWs and its corresponding EDS element maps (Fig. 13.3C–F) as well as the line-scan profile (Fig. 13.3G) show that the gold, palladium, and phosphorus elements are evenly distributed throughout the nanowire. In 0.1 M Na2 SO4 solution, the electrochemical NRR performances of AuPdP NWs under different voltages were studied and displayed in Fig. 13.3H and I. Especially at −0.3 V, the ammonia synthesis rate was calculated to be 18.78 μg mgcat −1 h−1 , and the Faraday efficiency is 15.44%. For AuPdP metal-nonmetal alloy with three kinds of potential active sites, the adsorption of hydrogen species mainly occurs on Pd; P enhances the adsorption and reaction kinetics of N2 because of its abundant valences; Au is the active site where the nitrogen reduction reaction occurs. This work took the advantages of P of weakened hydrogen adsorption and abundant valences. And NRR performance of Au active sites was therefore improved due to the employment of both P and Pd. The adding nonmetal elements for alloying provided us a new strategy to improve electrocatalytic NRR activity of noble metal catalysts. From the electrochemical performance of the various Au-based nanocatalysts listed above, it can be understood that the overall FE of the Au catalysts is higher

13.3 Au-based NRR catalysts

(A)

(B)

(C)

(D)

(E)

(F)

(G)

(H)

(I)

FIGURE 13.3 (A) Schematic diagram for illustrating the preparation of the AuPdP NWs. (B) HAADF-STEM image of a single AuPdP nanowire, and (C-F) its corresponding EDX elemental mapping images and (G) line scan spectra. (H) UV–vis absorption spectra of electrolytes after color development and (I) NH3 yields and faradic efficiencies of the AuPdP NWs at selected potentials.

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than that of the Ru catalysts. The FE of some Au-based catalyst can even reach 70%, indicating that the practical application of Au for NRR. Unfortunately, the ammonia yield of the Au-based catalysts is not satisfied mainly due to that the side reactions in the NRR process are serious and this requires more scientific work on catalyst synthesis and surface engineering.

13.4 Other noble metal-based NRR catalysts Among all the noble metals, Ru and Au have already exhibited relatively good performance in NRR. The other noble metals, represented by Pt and Pd, were seldom studied as NRR catalysts due to the excellent HER activity of both Pt and Pd. Whatever, it is been widely recognized that great HER activity is double-edged sword for NRR since great HER activity often means high NRR kinetics but poor NRR selectivity. As a result, both Pd and Pt have been studied as NRR catalyst via enhancing the NRR selectivity.

13.4.1 Pd-based NRR catalysts Compared with pristine Pd catalyst, Pd-based alloys have exhibited better NRR activity because that the alloying process changes the electronic structure of the Pd metal. Pang et al. [35] used a facile two-step dealloying method to obtain PdAg alloy material for NRR catalysts. First, Pd1 Ag2 Al97 was used as the precursor alloy and was treated with NaOH solution. The Pd1 Ag2 Al97 was therefore dealloyed to obtain Pd1 Ag2 . The Pd1 Ag2 alloy obtained in the first step was then treated with 0.1 M Fe(NO3 )3 solution. Trivalent iron ions oxidized metallic Ag to Ag ions to obtain nanoporous Pd1Ag1 alloy for the second step of dealloying. The finally produced Pd1 Ag1 has a hyperbolic structure and the morphology characterization of the catalyst revealed that Pd and Ag elements are evenly distributed in the Pd1 Ag1 alloy, which confirms the successful synthesis of the Pd1 Ag1 alloy. In 1 M KOH electrolyte and at −0.2 V, Pd1 Ag1 showed an ammonia yield of 24.1 μg mgcat −1 h−1 . This catalytic NRR performance is much better than PdAg alloys with other atomic ratios as well as the corresponding single metal catalysts. The hyperbolic structure of the catalyst is supposed to provide abundant low-coordination surface atoms as active sites and the hierarchical porous structure provides high-efficiency reactant accessibility and fast electron transfer process. By considering the synergistic effect of Pd and Ag on adsorption energy of N2 and related intermediates as well, great NRR catalysis performance can be therefore achieved for Pd1 Ag1 alloy. Since this work is inspired the high catalytic activity of PdAg alloy in the high-temperature solid-phase electrochemical synthesis of NH3 , it provided us a new point view for seeking for efficient electrochemical NRR catalyst at ambient temperature.

13.4 Other noble metal-based NRR catalysts

Alloying with other transition metals that are not active to hydrogen species (such as Ag and Cu) has been considered to be a promising method to improve the performance of Pd toward NRR. At the same time, change in the electronic structure can also enhance the adsorption and activation of N2 molecules. Both NRR activity and selectivity can be therefore improved. Other than the alloying with transition metals, the incorporation of nonmetallic elements (nitrogen, phosphorus, boron, sulfur, etc.) into catalysts is also an attractive strategy, which can change the electronic structure and adjust the energy barrier for the adsorption and activation of the reactants. Based on the above two theoretical foundations, Wang et al. [36] synthesized threedimensional Pd-Ag-S porous nanosponges (Pd-Ag-S PNSs) via a two-step method by using NaBH4 reduction reaction and then adding sulfur-containing ethanol solution. The successful doping of sulfur into 3DPd-Ag-S PNSs as well as the alloying was confirmed before the electrochemical measurements. In 0.1 M Na2 SO4 solution, PdAg-S PNSs have an ammonia yield of 9.73 μg mgcat −1 h−1 and a Faraday efficiency of 18.41% at −0.2 V. The unique network nanostructure, which provides a convenient channel for the reactants and exposes more active sites, is important for the relatively high ammonia yield of Pd-Ag-S PNSs while the introduction of S element changes the electronic structure of the catalyst, making it easier for nitrogen to adsorb and activate.

13.4.2 Pt-based NRR catalysts Generally, surface of Pt nanoparticles tends to adsorb hydrogen atoms instead of N2 molecules, especially at negative potentials. In the NRR process, the excessive adsorption of hydrogen atoms can promote HER. However, the high H coverage on Pt surface means occupy most of the Pt active sites, which is also active for N2 activation, are occupied. The NRR activity and selectivity would be therefore greatly inhibited and the study of Pt in the NRR field is very few due to the above reasons. However, the uniformity of catalytic active sites, the maximum metal utilization efficiency and improved electronic properties of single atom metal catalysts can significantly change the catalytic property of Pt, thereby potentially providing new opportunities for the application in NRR field. Hao et al. [37] synthesized Pt single atom catalyst by loading Pt atoms on WO3 nanoplates (Pt SAs/WO3 ) and the isolated Pt sites were used to efficiently fix nitrogen during the NRR process. As desired, Pt SAs/WO3 exhibited an ammonia synthesis rate of 342.4 μg h−1 mgPt−1 and a Faraday efficiency of 31.1% in 0.1 M K2 SO4 solution −0.2 V. Importantly, this NRR activity is much higher than the NNR activity of Pt NPs/WO3 with Pt nanoparticles as active sites. DFT calculations revealed that the NRR catalytic mechanism of the Pt SAs/WO3 catalyst. The conversion of N2 to NH3 follows the alternative hydrogenation pathway. The isolated Pt active sites in the special positively charged Pt-3O structure can chemically adsorb and activate N2 molecules. In addition, the isolated Pt sites on the WO3 nanoplates can effectively inhibit the HER and greatly promote the NRR. This work was a breakthrough research on Pt-based catalysts toward NRR and provided the NRR researchers an effective method to inhibit the HER reaction.

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13.5 Conclusions and prospects In summary, electrochemical ammonia synthesis via NRR provides a green and sustainable method to replace the traditional Haber–Bosch process to synthesize ammonia, which relieves the pressure on the increasingly serious environmental problems. Among the various electrochemical NRR catalysts, noble metal catalysts have received increasing attention because of their relatively great activity and good stability. The currently synthesized noble metal catalysts mainly include singleatom catalysts; heteroatoms introduced into alloys, noble metal-based catalysts, and improved metal nanoparticle structures. Among them, ruthenium single atom has a relatively high ammonia yield and FE is the most applied and improved Au nanostructures have very high FE and have a relatively large potential. For Pdbased catalysts, although alloying with other metal or nonmetal materials has shown great improvement on the NRR catalytic activity, the overall performance of the Pd-based catalyst still cannot stratify the practical requirements. Single-atom Pd catalyst can be synthesized and studied for NRR application. Similar to Ru-based catalyst, the single-atom Pt catalyst also displayed relatively high NRR performance, which can be said to be a relatively big breakthrough. However, it can be concluded from the previous reported works that there are still many problems with noble metal-based catalysts. The most serious of which is that almost none of the noble metal-based catalyst can achieve high ammonia yield and FE at the same time. For the further researches on noble metal-based NRR catalysts, there are following prospects: 1. It seems that single-atom catalysts can isolate the active sites and hence inhibit the HER catalysis. By considering the decreased cost as well, it is valuable to develop more single-atom noble metal catalysts for NRR application. 2. There is a correlation between the loading of noble metal and the number of active sites in the catalyst while the number of active sites is directly related to the performance of NRR, so it is more important to find a way to maximize the utilization of noble metal atoms as much as possible. On the other hand, there will be a competitive reaction of HER, in where adsorption of H atoms is important, and other side reactions in the electrolyte. The NRR reaction in the neutral and alkaline electrolytes, where proton concentration is low, can be studied. 3. In order to suppress the occurrence of competitive reactions and side reactions to improving the performance of NRR, combination of noble metal active sites, especially the single atom ones, and the cost effective catalytic active sites should be carried out. 4. There are still many unclear points about the mechanism of noble metal catalysts toward electrochemical NRR, more theoretical studies should be taken for the NRR on noble catalysts before any big breakthrough in the field of noble metal electrocatalytic NRR.

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14

Mohammad Khalid a, Mohammad Rafe Hatshan b, Ana Maria Borges Honorato a, Bijandra Kumar c and Hamilton Varela a a

b

Institute of Chemistry of São Carlos, University of São Paulo, São Carlos, SP, Brazil, Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia, c Department of Math., Comp. Sci. and Eng. Technology, Elizabeth City State University, Elizabeth City, NC, United States

14.1 Introduction Electrochemical methods have long been studied for the production of value-added chemicals and converting chemical energy into electrical energy. Green NH3 synthesis is a potential energy vector for carbon-neutrality. Currently, NH3 being a largest manufactured (∼200 million tons per year) and perhaps the most important chemical in the world, is synthesized by Haber–Bosch method, of which ∼80% is utilized in fertilizer industries to sustain the living things on the earth and rest (∼20%) used as a feedstock for chemical industries and medicines [1–3]. To overcome the thermodynamic limitations of this reaction, the electrosynthesis of NH3 via N2 – H2 O chemical reaction with a zero-carbon emission is a well-thought-out alternating clean energy carrier to the fossil fuel and could solve the problem of hydrogen storage and transport. Considering the potential use in fertilizers and sustainable energy source, NRR is a key model electrochemical reaction for NH3 generation. The Haber–Bosch process for NH3 synthesis uses steam reforming hydrogen as a main source of energy, which makes this process extremely energy intensive by consuming around ∼1–2% of the global annual energy output, accompanied by liberating a huge amount of greenhouse gas CO2 (∼420 Mt year−1 ) in the atmosphere [4–8]. Even if the steam reforming hydrogen is replaced by water electrolyzer, a substantial energy is still required to produce hydrogen from water splitting to initiate the reaction. If the electrochemical synthesis of NH3 is powered by renewable energy source like solar, wind, and geothermal then it could be a vital approach to store carbon-free energy source in the form of NH3 as a long-term energy carrier, possessing high hydrogen content of about 17% by weight thus it is a possible hydrogen storage solution [9]. It can be easily liquefied at room temperature only under pressure of 8–10 bar and can be stored in existing infrastructure unlike hydrogen fuel. By electrochemical

Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00011-3 Copyright © 2022 Elsevier Inc. All rights reserved.

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approach, the energy consumption not only can be lowered compared to Haber– Bosch method but also can be a promising strategy to alleviate the issue of gradually increasing CO2 level in the environment. Indeed, the transformation of dinitrogen into ammonia (N 2 + 6H + + 6e − → 2 NH3 ) would be a game-changing solution to the counterpart of Haber–Bosch method. However, the sluggish reaction kinetics of NRR dealing with competitive HER is the main hurdle in electrosynthesis of NH3 . To this viewpoint, several catalysts have been designed in recent years, unfortunately, none of them met with ideal performance toward NRR. Generally, all catalysts exhibit poor faradaic efficiency and rate conversion. In fact, biological process through enzyme (nitrogenase) can also transform N2 into NH3 , but this natural biological process cannot meet the requirements of large-scale industrial NH3 production [9]. The ideal NRR activity of the catalyst can only be apprehended if the rational design with certain functionality is developed using low-cost preparation approach. Therefore, the exploration of an alternative method for NH3 synthesis is crucially important. Inspired by the blooming research on the energy conversion and storage reactions such as ORR (oxygen reduction reaction), OER (oxygen evolution reaction), and HER (hydrogen evolution reaction), the NRR has become a hot topic for “green” NH3 synthesis. In this context, the early attempt was made by E.E. Van Tamelen et al. [10] in dated back of 1969 via electrolysis of dinitrogen using aluminum anode and nichrome cathode in a solution of 1,2 dimethoxyethan (glyme), titanium tetraisopropoxide, naphthalene, tetrabutylammonium chloride, and aluminum isopropoxide. Later, Sclafani et al. [11] proposed a new strategy to nitrogen reduction by applying iron as cathode in aqueous electrolyte under ambient pressure. Similarly, Furuya and Yoshiba [12] used a gas diffusion electrode modified by Fe-phthalocyanine in several electrolyte solutions using Pt as an anode. Since then, many precious metals (Pt, Ru, Pd, Au, etc.) [13–19], nonprecious metals (Fe, Ni, Co, Mo, Zr, etc.) [20–24], and carbon-based materials [25–27] have been identified to show good activity toward NRR. In particular, noble metal-based catalysts have demonstrated good performance along with selectivity, owing to high electrical conductivity, fast charge transfer and reaction kinetics, which make them exceptionally interesting materials for NRR electrocatalysis. Nevertheless, the large amount of success in electrosynthesis of NH3 has not been accomplished until recent years, when structural design engineering of catalysts and advanced instrumentation techniques have been developed to acquire high activity, selectivity, and stability. This chapter summarizes the recent development on electrocatalysis of NRR based on advanced noble metals (e.g., Au, Ru, and Pd) based nanocatalysts.

14.2 NRR mechanism Almost all energy-related catalytic reactions use catalyst to accelerate the rate of reaction via several intermediate steps. Unlike, Pt and IrO2 /RuO2 , which serve as the state-of-the-art catalysts for the oxygen reduction, oxygen evolution, and hydrogen evolution reactions, are unable to electrocatalysis of NRR in order to explore the

14.2 NRR mechanism

mechanism. NRR is a multistep reaction process which goes through adsorption, hydrogenation, and desorption pathways depending on the electrode properties. It is generally accepted that during NRR, the generated protons/electrons simultaneously reduced and hydrogenated the dinitrogen molecules. However, the activation of dinitrogen at catalyst surface is very difficult to accomplish hydrogenation, due to the strong triple bonds between two nitrogen atoms (>8.5 eV) [13]. The NRR in aqueous solution involves proton coupled 6-electron transfer process which produces two molecules of NH3 per dinitrogen molecule or a 4-electron side reaction of hydrazine formation, resulting in a high overpotential with low selectivity [28]. N2 (g) + 6H+ + 6e− 2H3 (g)Eo = 0.0577 V vs RHE During the electrochemical reduction of dinitrogen, initial step takes place by fast electron transfer to adsorb N2 ad-molecule on the catalyst surface, with or without the assistance of a proton, as follows [29]. N2 (g) + H+ + e− N2 H(g)Eo ≈ − 3.2 V vs RHE N2 (g) + e− ↔ N2 − (aq)Eo ≈ − 4.16 V vs RHE The negative potentials indicate the intrinsic high-energy barrier for the initial hydrogenation step, which make NRR thermodynamically very sluggish to reduce dinitrogen molecules. For an ideal electrocatalyst, all the intermediate redox steps should have the same equilibrium potential equal to the overall reaction potential to minimize the activation energy barriers, unfortunately, the adsorbing ability of these intermediates cannot be modified separately. The bottom line for initiating reaction at fast rate is that the electrocatalyst should bind the intermediate atoms and molecules with moderate strength, neither too strong nor too weak. If the binding is too weak, the catalyst will be unable to activate the reactants, on the other hand, if the binding is too strong, the as-formed products cannot be desorbed from the surface of the catalyst, and catalyst can be poisoned by the intermediates [30]. During electrocatalysis, the dinitrogen bond cleavage follows two important pathways such as dissociative pathway and associative pathway. The main difference between dissociative and associative mechanisms is the way in which the hydrogenation of nitrogen adsorbed species occurs. In dissociation pathway the bond cleavage occurs onto the surface of catalyst prior to the hydrogenation, leaving two adsorbed N-atoms, which then undergo hydrogenation independently and are released NH3 from the catalyst surface. Associative pathway happens upon the breaking of N2 bond together with hydrogenation step, where only one nitrogen atom in the dinitrogen molecule is coordinated with the catalyst surface site. Overall, this step is followed by the consecutive protonation steps with the formation of different adsorbed NHx (x = 1–3) intermediates until NH3 molecule is formed, as represented in Fig. 14.1A [31]. However, the mechanism proposed above is still debatable and depends on the catalyst properties which can show different mechanism process with different catalysts. By considering this possible NRR pathway, a combined volcano plot was

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(A) Dissociative pathway

(B)

FIGURE 14.1 (A) General principle of dissociative and associative mechanism. Reproduced with permission [31], Copyright © 2017 Elsevier, and (B) NRR volcano plot for metal surfaces. Reproduced with permission [32] from PCCP Owner Societies, Copyright © 2012, Royal Society of Chemistry.

established using density functional theory (DFT), as represented in Fig. 14.1B [32]. The Ru, Rh, Mo, Ir, Fe identified the most active metals for enhancing the sluggish kinetics of NRR. However, the faradaic efficiency for NRR may be affected with highly competitive HER in the cathodic potential region [24, 32, 33].

14.3 Types of the electrochemical cell for NRR Typically, the electrochemical synthesis of NH3 is conducted in three types of electrochemical cells (Fig. 14.2) [24]. The Fig. 14.2A shows a type of cell where both anode and cathode are immersed into the same electrolyte in single compartment. In this electrochemical cell, the nitrogen gas is bubbled near to the cathode for the reduction of N2 into NH3 and the same time OER takes place at anode. As the NRR is carried out in a single compartment, the produced gases such as hydrogen, ammonia, and oxygen pass together from the outlet of the cell. Additionally, it is also impossible to stop diffusion of NH3 to be oxidized at anode without barrier, resulting in a low faradaic efficiency and production rate. In the type of cell, as represented in Fig. 14.2B, the anode and cathode are kept in two different compartments separated by Nafion membrane and generally it is called H-type cell, is often used for NRR analysis [13–16]. In this kind of cell, the cathode and reference electrodes are kept in one compartment and bubbled with N2 gas to be reduced into NH3 , while the anode is kept in another compartment bubbling with Ar gas, thus, the produced NH3 can be prevented to diffuse in anode side. Nevertheless, the both electrodes are kept in two different compartments and the liquid electrolyte between both compartments is separated by membrane which creates large resistance between the anode and cathode,

(C) (B) (A)

(A) Single compartment electrochemical cell, (B) Double-compartment electrochemical cell, and (C) Ion exchange membrane based electrochemical cell.

14.3 Types of the electrochemical cell for NRR

FIGURE 14.2

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which is inevitable. The Fig. 14.2C consists the electrochemical cell with three parts comprising bipolar plates, catalyst coated substrate, and ion conductive membrane (proton/or anion exchange membranes) which provide proton/hydroxyl ion during NRR. Therefore, this type of electrochemical cell conditions reflects some realistic operation for NRR electrocatalysis. The N2 gas is passed in the cathode side for N2 reduction and water is supplied to anode side for OER. Compared to single and double-compartment type cells, the ion conductive membrane-based cell is a compact system, which is useful for reducing the resistance between two electrodes and thus shows facile electrocatalytic reaction. However, this design faces some obstacles, for example, the produced ammonia during electrochemical process can interact with proton exchange membrane leading to underestimated NH3 rate. While, in case of anion exchange membrane, the degradation of the membrane and the leaching of quaternary ammonium cations, is unavoidable.

14.4 Electrolytes for NRR In general, the ion conducive membrane cell system shows some advantages over liquid electrolyte based single- and double-compartment cell systems, due to its compact architecture which shows relatively low resistance and as a result it improves the rate formation of ammonia. As it is well accepted that the pH of electrolyte plays critical role in electrochemical NRR by reducing the concentration of hydrogen ion in the electrolyte can be improved the performance of NH3 production, while suppressing the proton stealing HER. For example, Chen et al. [34] found that the same catalyst produced different yields of NH3 at different pH level of the electrolytes; 0.5 M KOH (pH 13.7), 0.5 M KHCO3 (pH 9.4), 0.25 K2 SO4 (pH 7), and 0.25 KHSO4 (pH 0.6) revealed NH3 yields of about 1.06 × 10−11 mol cm−2 s−1 , 7.87 × 10−12 mol cm−2 s−1 , 6.63 × 10−12 mol cm−2 s−1 , and 3.88 × 10−12 mol cm−2 s−1 , respectively. For another example, Kong et al. [35] prepared γ -Fe2 O3 catalyst and compared its NRR activities in double-compartment cell comprising an aqueous electrolyte of 0.1 M KOH, and in anion exchange membrane (AEM) cell system. The comparative NRR investigation resulted both mass- and area-normalized NH3 (0.95 μg h−1 mg−1 and 0.45 × 10−13 mol s−1 cm−2 ) with AEM, which was almost three times higher than that of as-obtained in double-compartment cell with aqueous 0.1 M KOH electrolyte (0.29 μg h−1 mg−1 and 0.14 × 10−13 mol s−1 cm−2 ). However, the faradaic efficiency was found much lower (0.044 %) with AEM compared to the aqueous 0.1 M KOH electrolyte (1.9 %). This might be due to the influence of HER and low solubility of N2 in liquid electrolyte leading to low faradaic efficiency for NRR. On the other hand, the nonaqueous electrolyte improves the solubility of N2 as well as suppresses the competitive HER due to the stronger adsorption capacity for nitrogen and the negligible content of water for proton supply in the electrolyte. For instance, Zhou et al. [36] used ionic liquid electrolyte ([P6,6,6,14][FeAP]) under ambient conditions, which demonstrated 60% faradaic efficiency, much higher than those reported in the aqueous electrolyte. In another study, conducted by

14.5 NRR based on noble metals

10−12

0.10 0.08 0.06

10−13

0.04 0.02

10−14

Pt

Ir

Pd

Ru

Au

(B)

0.12

0.00

0.6

PR at −0.2 V FE at −0.2 V

10−12

0.4 10−13

0.2

10−14

10−15

Pt

Ir

Pd

Ru

Au

Faradaic efficiency (%)

PR at −0.2 V PR at −0.4 V FE at −0.2 V FE at −0.4 V

Ammonia production rate (mol cm−2 s−1)

10−11

Faradaic efficiency (%)

Ammonia production rate (mol cm−2 s−1)

(A)

0.0

FIGURE 14.3 (A) NH3 production rate and faradaic efficiency on noble metal catalysts tested in PEM for 1 hour at −0.2 V and −0.4 V, and (B) NH3 production rate and faradaic efficiency on noble metal catalysts tested in AEM for 1 hour at −0.2 V. Adapted from [15], Copyright © 2017, Electrochemical Society.

Suryanto et al. [37] using hydrophobic fluorinated aprotic electrolyte, achieved 32% faradaic efficiency with suppressed HER. However, the rate production of NH3 in nonaqueous electrolyte was found lower than those obtained in the aqueous electrolyte. This is because of the lack of mass transfer of protons. Nevertheless, the poor ionic conductivity of nonaqueous electrolyte causes the high overpotential for NRR electrocatalysis.

14.5 NRR based on noble metals Recent years have witnessed a dramatic increased research interest in NRR electrocatalysis due to its environmentally friendly route to synthesis of NH3 at low energy consumption. Considering the requirement for the production of NH3 through NRR, the highly active and selective catalysts are crucial. Current key challenges for NRR electrocatalysts are as follows: improvement in energy efficiency by reducing overpotential; increasing selectivity by enhancing faradaic efficiency; durability of the catalyst and reducing cell operating cost. Besides the widely reported catalysts, noble metals demonstrate fairly good activity and selectivity toward NRR compared to the non-noble-based catalysts, and therefore they have recently attracted much attention, especially tuning their morphological designs which ideally possess unique electronic features, resulting in high-mass activity and selectivity [36–41]. Nash et al. [15] used a series of noble metals loaded on Vulcan XC-72 (Au/C, Pt/C, Pd/C, Ir/C, and Ru/C) and investigated their NRR activities using proton/and anion exchange membranes (PEM/AEM) cell systems. They calculated the rate production of NH3 in PEM electrolyzer higher than those in AEM electrolyzer, as represented in Fig. 14.3A and B. The faradaic efficiency was observed slightly higher in AEM than the PEM electrolyzer. Nonetheless, the competitive HER was inevitable

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for all tested catalysts in both PEM and AEM electrolyzers. Similarly, Kishira et al. [42] tested the catalysts Pt/C-, Pt-Ru/C-, Ru-C, Ru- and Ag-Pd loaded on carbon paper in proton conducting solid electrolyte (a mixture of CsH2 PO4 and SiP2 O7 in ratio of 1:0.5) at 220°C for electrosynthesis of NH3 in presence of H2 and steam as proton sources. They found that the faradaic efficiency almost the same in both cases of proton sources H2 and steam. But the rate of NH3 production was slightly higher (10−10 mol s−1 cm−2 ) in H2 proton source with Pt/C and Ru-loaded carbon paper. As can be observed from the studies of Nash [15] and Krishna [42], the bulk form of noble metals or simply dispersed in carbon possess low density of active sites and poor selectivity, however, this can be improved by reducing particle size, alloying them with other metals by manipulating the morphological design and crystal phase especially of Au, Ru, and Pd metals.

14.6 NRR based on Au nanocatalysts Among the noble metals, Au is widely studied metal for NRR electrocatalysis, due to its inert nature and weak hydrogen adsorption capacity which make Au attractive for NRR electrocatalysis. Considering the inertness of Au toward the HER, various Au catalysts were prepared by rational design of different nanostructures and modifying the electronic properties in order to improve N2 adsorption on catalyst surface. For example, Wang et al. [43] synthesized atomically dispersed Au1 on graphitic carbon nitride (g-C3 N4 ) and tested for NRR in two compartment cell system using cation exchange membrane separator in 5 mmol L−1 H2 SO4 aqueous solution. Due to the uniform catalytic active sites and unique electronic feature of catalyst, showed high faradaic efficiency of 11.1% for NH4 + formation with rate of 1305 μg h−1 mgAu −1 under ambient conditions, which was roughly 22.5 times as high as that was achieved by the counterpart Au nanoparticles. Using the same strategy, Liu et al. [44] employed Ti3 C2 to anchor Au nanoparticles (Au/Ti3 C2 ) by using ultrasonic reduction process. The best performance of the catalyst, they measured at −0.2 V NRR with rate production of NH3 (30.06 μg h−1 mg−1 ) and a high faradaic efficiency of 18.34%. Bao et al. [18] synthesized crystallite Au nanorods with tetrahexahedral structure toward electrocatalytic NRR. Nevertheless, the catalyst represented very low yield, faradaic efficiency, and selectivity for NH3 . Later, the same research group prepared Au nanoparticles anchored on CeOx -RGO (a-Au/CeOx -RGO) by coreduction method at room temperature, where CeOx transforms crystallite Au nanoparticles into amorphous phase and reduced graphene oxide acted as a substrate for facile anchoring of Au nanoparticles [45]. The as-synthesized amorphous a-Au/CeOx -RGO achieved higher faradaic efficiency (10.10%) and NH3 yield (8.3 μg h−1 mg−1 cat. ) at −0.2 V vs. RHE, which was higher than the crystalline counterpart catalyst (c-Au/RGO) and also performed well under harsh temperature and pressure. In another study, Shi et al. [19] synthesized Au clusters decorated on TiO2 substrate (AuO-Ti) and used as NRR electrocatalyst. The catalyst was able to generate NH3 with rate of 21.4 μg h−1 mg−1 without hydrazine formation and showed faradaic efficiency

14.6 NRR based on Au nanocatalysts

(B)

ZIF-8

H2O

N2

H2

NH3

H2O

H2

NH3 H2O

40

40

30

30

20

20

10

10

Faradaic efficiency (%)

H2O

N2

NH3 yield (ug.h*−1 *cm−2)

(A)

0 0 −1.0 −0.9−0.8 −0.7−0.6−0.5−0.4 Potential (V vs RHE)

FIGURE 14.4 (A) Demonstration of nitrogen reduction reaction on NPG@ZIF-8 electrocatalyst, and (B) NH3 production rate and Faradaic efficiency at various potential on NPG@ZIF-8 electrocatalyst. Reproduced with permission [49], Copyright © 2019 Wiley-VCH Verlag GmbH & Co. KGaA Weinheim.

8.11% at −0.2 V in 0.1 M HCl at room temperature. Very recently, Tan et al. [46] synthesized the high-index faceted Au nanoparticles by seed-mediated method. They found a record faradaic efficiency of 73.32 with high-rate production of NH3 with rate of 9.22 μg h−1 cm−2 at −0.3 V vs. RHE in 0.1 M Li2 SO4 aqueous solution. Such high selectivity of Au nanoparticles was further investigated by theoretical calculations and revealed that the high-index faceted surfaces of Au nanoparticles propose greater adsorption of NRR intermediates (∗ NHH) and significantly hinder the adsorption of ∗ H intermediate for competitive HER. Similarly, Wang et al. [47] designed Au in flowerlike structure, and employed for NRR, which achieved the faradaic efficiency of 6.05% with 100% selectivity and almost negligible production of unwanted hydrazine. This indicates that the process of NRR can be controlled by the structural design of catalyst and active site density. As the conductivity and porosity play crucial role in enhancing the catalytic process, Wang et al. [48] deposited a thin film of Au with 400 nm size on Ni foam (pAu/NF) by a micelle-assisted electrodeposition method. The as-synthesized catalyst material showed excellent NRR activity with high production rate of NH3 (9.42 μg h−1 cm−2 ) and high faradaic efficiency of 13.36% without detection of hydrazine at −0.2 V vs. RHE in 0.1 M Na2 SO4 at ambient conditions. Hydrophobic metal organic framework (zeolitic imidazolate framework-8, ZIF-8) was used to immobilize nonporous gold particles (NPG@ZIF-8) in order to weaken HER and retard the reactant diffusion (Fig. 14.4A), where NPG acted as a core and ZIF-8 was a hydrophobic outer shell. The as-designed electrocatalyst demonstrated highest faradaic efficiency of 44% (at −0.6 V vs. RHE) with excellent rate of NH3 production (28.7 ± 0.9 μg h−1 cm−2 ) at −0.8 V vs. RHE outcompeting the traditional gold nanoparticles and NPG (Fig. 14.4B) [49]. In another latest work of Zhang et al. [50] where they located Au nanoparticle in hydrophobic carbon fiber paper (Au/o-CFP). The as-designed catalyst facilitates three-phase contact points

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(gas N2 , liquid electrolyte, and solid Au nanoparticles) where N2 molecules come in contact with catalyst surface directly while suppressing HER. As a result, the catalyst showed NH3 yield of 40.6 μg h−1 mg−1 at −0.3 V vs. RHE and faradaic efficiency 31.3% at −0.10 V vs. RHE in 0.1 M Na2 SO4 electrolyte. Incorporation of other metals with Au nanoparticles can also be beneficial to enhance the chemical affinity between surface atoms of catalyst and the adsorbed N2 molecule, decreasing the activation energy for the reduction of dinitrogen. For example, Liu et al. [51] prepared a series of bimetallic electrocatalyst coupling Cu, Ag, Pd, and Ru metals with Au (CuAu, AgAu, PdAu, and RuAu) and examined their NRR activity in detail. Among them, the bimetallic CuAu displayed better performance toward NRR with highest faradaic efficiency of 54.96% and NH3 production rate (154.91 μg h−1 mg−1 cat ) at −0.2 V vs. RHE compared to those reported Au nanocatalysts. The high performance of CuAu electrocatalyst was ascribed to the synergy between Au and Cu via modulating the electronic structure and further changing the binding affinity of adsorbed N atoms on the catalyst. From the above discussion, one can be observed that the Au has unique properties required for NRR electrocatalysis, but the bulk form of Au as a single component shows poor activity and selectivity unless it is modulated in unique design structure or made in alloy form with other metals. Similarly, Ag nanocubes were deposited on Au electrode and protected by thin film of ZIF-71 to prevent the diffusion of water molecules. Thus, prepared catalyst (Ag-Au@ZIF) exhibited the faradaic efficiency ∼18% and rate formation of NH3 ∼1 × 10−11 mol s−1 cm−2 at −2.9 V vs. Ag/AgCl, much higher than the sample prepared without ZIF-71 (Ag-Au) in the electrolyte of THF containing 0.2 M LiCF3 SO3 with 1% ethanol [52].

14.7 NRR based on Ru nanocatalysts Ru is a catalyst for Haber–Bosch method; it has also been recognized as a good catalyst for NRR due to its appreciable N2 adsorption energy and low overpotential. According to the volcano plot as represented in Fig. 14.1B, the Ru is located almost at peak position together with Rh and Ir, suggesting its favorable NRR electrocatalytic activity, thus, it should hold the promise for significant NRR electrocatalysis in ambient conditions. In this context, Zhao et al. [53] synthesized Ru nanoparticles with average size of 3.65 nm via hydrothermal method. The, as-prepared Ru nanoparticles were modified on carbon paper (Ru NPs/CP) with loading of 0.1 mg cm−2 and used as a NRR electrocatalyst in two-compartment cell separated by Nafion 115 membrane employing aqueous 0.1 M HCl electrolyte. The electrocatalyst showed a large NH3 formation rate of 24.88 μg h−1 mg−1 cat. and faradaic efficiency of 0.35% at −0.15 V vs. RHE. Nevertheless, the rate and faradaic efficiency both were obtained significantly low due to the hindrance of preferable hydrogen ion adsorption on catalyst surface. In another work [54], Ru nanoparticles were decorated on oxygen vacant TiO2 (Ru/TiO2 -Vo) using solvothermal process. The uniformly anchored Ru nanoparticles with average size of 2.3 nm onto the oxygen vacant TiO2 were tested for NRR in

14.8 NRR based on Pd nanocatalysts

two-compartment cell using the electrolyte of N2 -saturated 0.1 M KOH. The catalyst displayed NH3 rate formation of about 2.11 μg h−1 mg−1 and faradaic efficiency of 0.72% between the potential range of −0.05 to −0.3V. The first principal calculation revealed that the oxygen vacancies induce the electron-rich Ru clusters, resulting in the improvement of NRR activity. Wang et al. [55] deposited Ru nanoparticles (2–5 nm in size) on carbon fiber paper by oleate-assisted decomposition/reduction method and directly used as working electrode for NRR analysis in 10 mM HCl electrolyte. The catalyst revealed rate production of NH3 near to 9 × 10−12 mol s−1 cm−2 at −0.1 V and faradaic efficiency 5.4% at 0.01 V without detection of side product hydrazine. Li et al. [56] synthesized high surface area ultrathin nanosheets of Ru, which achieved a high-rate formation of NH3 (23.88 μg h−1 mg−1 ) at −0.2 V almost double than the counterpart of Ru nanoparticles. This study suggested that the NRR activity is highly dependent on morphological design structure of the electrocatalyst. Recently, Tao et al. [57] developed Ru single atoms confined in nitrogen doped porous carbon and tested its nitrogen reduction activities using carbon paper as a substrate in a gastight double compartment cell separated by Nafion 117 membrane in 0.1 M HCl electrolyte solution. The as-synthesized electrocatalyst demonstrated the rate formation of NH3 more than 3.6 mgNH3 h−1 mg−1 Ru at −0.21 V vs. RHE, not only that, when Ru single atoms protected by ZrO2 , it effectively suppressed HER and showed faradaic efficiency up to 21% at 0.17 V vs. RHE. The first principal calculation revealed that the Ru sites with oxygen vacancies were the main active sites that allowed ∗ NNH adsorption on catalyst surface, while retarding the ∗ H intermediate. Similarly, Yu et al. distributed Ru single atoms in graphitic carbon nitride (Ru SAs/g-C3 N4 ). This catalyst showed the excellent NH3 rate production (23.0 μg mgcat −1 h−1 ) and faradaic efficiency as high as 8.3% compared to the bulk Ru counterpart. “Density functional calculations revealed that the high performance of catalyst Ru SA/g-C3 N4 originates from a tuning of the d-electron energies from that of the bulk to a single-atom, causing an up-shift of the d-band center toward the fermi level” [58].

14.8 NRR based on Pd nanocatalysts Noble metal palladium (Pd) has also attracted much attention for NRR electrocatalysis, due to its outstanding electronic features for photo-/and electrocatalytic activities. For instance, Wang et al. [59] developed Pd nanoparticles on carbon black via polyol reduction method. The as-synthesized catalyst (Pd/C) yielded NH3 rate around 4.5 μg mg−1 Pdh−1 demonstrating the faradaic efficiency of about 8.2% at 0.1 V vs. RHE in phosphate buffer solution. As it is perceived that the bi-metal based catalysts show enhanced catalytic performance compare with one metal-based catalyst, due to the synergistic effect and modification in surface electronic features of the catalyst, which improve the interaction between reaction intermediates and catalytic active sites. Wang et al. [60] reported a tripod PdRu bimetallic catalyst, which achieved 37.23 μg h−1 mg−1 cat. NH3 with faradaic efficiency of 1.85% at −0.2 V. However, due to the competitive HER, the performance of PdRu tripods

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CHAPTER 14 Electrochemical NRR with noble metals-based nanocatalysts

(B)3.0 CO OH

CO OH

O HO

CO OH

HO

O

Metal ions

O

HO

O

O

GO

O

Cu PD

OH

O H

Stirring (Ar gas) Adding Metal Salts

HO O

CO

OH

H O O C

O HO

2.5

HO

HO

HO

2.80

2.0

HO

HO

NH3 yield (Pg h−1 mg−1cat.)

(A) CO OH

328

1.34

1.5

1.17 0.88

1.0

Stirring (Ar gas)

PdxCu1−x/rGO

0.63

0.55

0.5

Adding Reducing Agents

0.0

O

/rG

2

Cu x 2 Pd x

O u x2 Pd O /rG Cu/rG d x2C P

Pd

Cu

FIGURE 14.5 (A) Schematic representation of PdCu/rGO composite, and (B) NRR activity of PdCu/rGO with comparison Pd/rGO, Cu/rGO, Pd, and Cu counterparts. Reproduced with permission [61], Copyright © 2018, WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

gradually reduced toward NRR. Similarly, Shi et al. [61] synthesized amorphous PdCu nanoclusters anchored on reduced graphene oxide (rGO) by simultaneous reduction of graphene oxide, using the precursors of Cu and Pd in the mixture of tannic acid and NaBH4 , as illustrated in Fig. 14.5A. The researchers claimed that the amorphous PdCu nanoclusters exchange the electron transfer between rGO providing additional active sites and assist the dangling bonds to support abundant N2 adsorption. As a result, the optimized catalyst (Pd0.2 Cu0.8 /rGO) showed excellent NRR activity under ambient conditions, yielding 2.80 μg h−1 mg−1 cat. at −0.2 V vs. RHE, much higher than monometallic counterparts, as represented in Fig. 14.5B. Following the same strategy, Wang et al. [62] developed bimetallic PdRu nanorod assemblies. Benefitting to their synergy of bi-metals and branched-shaped structure showed superior NRR performance under ambient conditions. When, the as-synthesized PdRu nanorod catalyst tested for NRR in 0.1 M HCl electrolyte it achieved NH3 rate of 34.2 μg h−1 mg−1 cat. at −0.2 V vs. RHE. This performance of catalyst was associated with its unique morphological branched design, which not only provided abundant active sites but also improved the utilization efficiency of the catalyst. In another study, Liang et al. [63] synthesized PdO/Pd heterostructure confined in carbon nanotubes for electrosynthesis of NH3 . The interface between metal and metal oxide enhances the active sites density for NRR, while inhibiting the competitive HER. As a result, the Pd phase preferentially adsorbed N2 molecules and PdO makes a stable α-PdH and activates the N2 by proton transfer to facilitate the NRR.

14.9 Conclusions and outlook In this chapter, we have summarized the latest NRR electrocatalysts especially focusing on Au, Ru, and Pd-based materials owing to their relatively low binding affinity with hydrogen atoms compared to other noble metal-based catalysts such as Pt and Ir. The aforementioned discussion highlights that the versatile strategies that have

Acknowledgments

been adopted to design unique morphological structured catalyst for NRR. Despite the remarkable progress, the electrochemical synthesis of ammonia faces several fundamental challenges, because of low NH3 yield and poor faradaic efficiency mainly due to the chemical inertness of N2 molecules and fierce competitive proton stealing HER. To achieve high catalytic properties from a catalyst, an effective surface engineering methodology is prerequisite. To this quest, feasible rational design and controllable preparation approach should be the central strategy for the development of NRR catalysts and to mitigate the major issues of faradic efficiency and rate production of ammonia. Recently, some researchers have come up with a unique strategy by employing nitrate molecules in place of inert N2 , which provides a promising way for electrosynthesis of NH3 by reduction of nitrate at ambient conditions [64]. However, tremendous efforts are required to assess whether other nitrogen components can be selective and active in different electrolyte medium. More strategies need to be established in this direction, including the use of variety of nitrogen containing reagents. Compare to the wide range of Au, Ru, and Pd-based nanocatalysts for NRR, other noble metals have also been proven theoretically good for NRR, and therefore, their alloy with unique design structure should also be experimentally explored. Majority of noble metal-based NRR electrocatalysts are synthesized in the bulk form, therefore, the atomic level designed structures are highly desirable which not only will provide high atomic efficiency but also reduce the price of noble metal-based catalysts to significantly by improving the NRR catalytic properties. As the electrocatalytic NRR performance closely related to the crystal plane of metals due to the variation of adsorption strength of N on metal surface, thus, noble metals with different crystal planes are highly anticipated to be utilized in NRR electrocatalysis. Advances in the field of electrosynthesis of NH3 go hand-in-hand with the development of characterization techniques. The combination of theoretical calculation and operando in-situ investigation using advanced techniques such as XAFS, Raman spectroscopy, and micro-FTIR will open new avenue to realize the active site and its role in reaction process to develop state-of-the-art NRR electrocatalyst. The electrosynthesis of NH3 is still in nascent stage but the continuous progress will certainly enrich the field and expand our understanding for NRR electrocatalysis.

Acknowledgments This study was financed in part by the São Paulo Research Foundation (FAPESP) under the Grant No. 2017/00433-5 and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. Authors also acknowledge the financial support under projects of FAPESP grant nos. 2019/22183-6 and 2013/16930-7.

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Electrochemical NRR with noble metals-free catalysts

15

Zehui Yang a, Quan Zhang a and Shenglin Xiao a a

Sustainable Energy Laboratory, Faculty of Materials Science and Chemistry, China University of Geosciences Wuhan, Wuhan, PR China

15.1 Introduction Ammonia (NH3 ) is one of the most significant nitrogen sources, which is the basic chemical for the modern society. The traditional synthesis of ammonia relies on the Haber–Bosch process using H2 and N2 as reactants with large energy consumption, which is overly dependent on fossil energy triggering environmental pollution [1]. Therefore, it is an urgent demand for an environmentally friendly and sustainable method to produce ammonia. It is a great innovation for the ammonia industry to allow the reaction equilibrium of ammonia synthesis not to be limited by the thermodynamic equilibrium. The emergence of electrocatalytic ammonia synthesis at mild condition (room temperature and atmospheric pressure) and utilization of water as hydrogen source has attracted more attention in recent years [2]. The system could be powered by solar, wind, and nuclear energies [3]. Recently, plenty of reports indicate that efforts have been made to design and manufacture electrochemical nitrogen reduction reaction (NRR) electrocatalysts. To obtain a high-performance electrocatalysts for electrocatalytic nitrogen fixation, the previous studies mainly focused on noble metal catalysts including Ru, Pt, Ir, Au, Pd, and their alloys [4,5]. In recent years, in addition to single metal catalysts, more than two noble metals were invited to form bimetallic 3D porous nanostructures to achieve high catalytic activity and ammonia yielding under acidic, neutral, and alkaline conditions [6]. The electrocatalysts have advantages of high temperature and corrosion resistance; while, the scarcity of precious metal resources seriously hindered the large-scale application in the field of ammonia electrocatalytic nitrogen fixation [7]. Therefore, cost-effective electrocatalysts without noble metals for ammonia synthesis have been developed as alternative, including transition metals and heteroatom doped carbon-based electrocatalysts. Transition metals (such as Mo, Fe, Co, Ni, Mn, Ti, Cr, Ce, etc.) and their oxides, sulfides, nitrides, carbide, and phosphides have shown excellent catalytic activities in electrochemical ammonia synthesis [8,9]. In this chapter, we will introduce the NRR performance of these nonprecious metal electrocatalysts.

Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00009-5 Copyright © 2022 Elsevier Inc. All rights reserved.

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CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

(B) 3

8 2

6 4

1

2 1

(C)

2

3

4 5 6 7 8 Cycle number

9 10

0

−4 −6 −0.5 V −0.6 V −0.7 V

−8

−10 −12

0

4

−0.8 V −0.9 V −1.0 V

12 16 Time (h)

(D)

TiO2

1.21 eV TiO2 after electrolysis

6 4 2 Binding energy (eV)

0

20

24

TiO2

g = 2.002

1.03eV 8

8

Intensity (a.u.)

0

0 −2

j (mA cm−2)

Rate of NH3 formation Faradaic efficiency

10

Faradaic efficiency (%)

rNH3 (x10−11 mol s−1 cm−2)

(A)

Intensity (a.u.)

336

TiO2 after electrolysis 330

335

340

345 350 B (mT)

355

360

FIGURE 15.1 (A) Cycling stability tests of TiO2 /Ti at -0.7 V vs. RHE. (B) Time-dependent current density curves of TiO2 /Ti at a series of potentials for 24 hours in N2 -saturated 0.1 M NaSO4 . (C) Room-temperature ESR spectra and (D) valence band XPS spectra of pristine TiO2 and TiO2 after electrolysis. Reproduced with permission from Ref. [10]. Copyright 2018, American Chemical Society.

15.2 Transition metal oxides-based electrocatalysts 15.2.1 Titanium oxides As a rich and nontoxic substance, titanium dioxide (TiO2 ) has become one of the potential candidates in photo(electro)catalysis due to the great chemical stability. Due to the excellent performance of TiO2 , many researches widely studied its nanostructure and heteroatom doping to drive the electrochemical N2 fixation. Zhang et al. [10] successfully synthesized TiO2 nanosheets array on the Ti plate (TiO2 /Ti) as promising electrocatalyst for catalyzing N2 to NH3 . TiO2 /Ti electrocatalyst performed a high ammonia production rate of 9.16 × 10−11 mol s−1 cm−2 and an excellent Faradaic efficiency (FE) of 2.50% at −0.7 V vs. RHE with great selectivity and long-term durability at ambient conditions shown in Fig. 15.1A and B. As shown in Fig. 15.1D, oxygen vacancies (VO ) were introduced and enhanced the surface disorder of TiO2 after electrochemical NRR to boost the adsorption and activation

15.2 Transition metal oxides-based electrocatalysts

of N2 during N2 fixation electrocatalysis. Zhang et al. [11] synthesized a TiO2 nanoparticles-reduced graphene oxide composites (TiO2 –rGO) served as a superior non-noble-metal NRR electrocatalyst to effectively promote the electrochemical conversion of N2 to NH3 at ambient condition. TiO2 –rGO electrocatalyst exhibited a −1 at −0.90 V vs. high FENH3 of 3.3% and NH3 production rate of 15.13 μg h−1 mgcat. RHE and performed preferable long-term durability in neutral electrolyte. Han et al. [12] prepared high-active TiO2 by a solvothermal process followed by calcination for electrochemical N2 fixation at room temperature and pressure. This method was efficient to engineer the surface oxygen vacancies of TiO2 ; as a result, its catalytic activity for the NRR was also adjusted. Vo-rich TiO2 displayed a higher FENH3 of 5.3% in a wide potential range from −0.02 to −0.17 V vs. RHE and a high NH3 −1 production rate of 3.0 μg h−1 mgcat. at −0.12 V vs. RHE than the pristine TiO2 in 0.1 M HCl aqueous solution. DFT calculations revealed that Vo on the surface of TiO2 was beneficial to reduce the free energy for the rate determining step of NRR process. Wan et al. [13] synthesized boron-doped TiO2 microspheres (B-TiO2 ) as a low-cost NRR electrocatalyst for ammonia synthesis due to the formation of oxygen vacancies induced by boron dopants. In 0.1 M Na2 SO4 solution, the electrocatalyst −1 and FENH3 of 3.4% at −0.8 V achieved a higher NH3 yield rate of 14.4 μg h−1 mgcat. −1 ; FENH3 : vs. RHE than the TiO2 without B doping (NH3 yield rate: 5.4 μg h−1 mgcat. 2.2%), proving the enhanced selectivity toward electrochemical N2 fixation. Zhao et al. [14] firstly found the nonmetal phosphorus cation-doped TiO2 nanorods (P-TiO2 ) electrocatalyst synthesized by hydrothermal method performed preferable electrochemical NRR performance with a higher FENH3 of 12.26% and a boosted −1 at −0.30 V vs. RHE and outstanding stability in NH3 yield rate of 23.05 μg h−1 mgcat. 0.1 M LiClO4 aqueous conditions. DFT calculations revealed that phosphorus cation substitution was beneficial to form Vo and surface Ti3+ served as active site for NRR. Similarly, Qin et al. [15] demonstrated a carbon-doped TiO2 catalyst (C-Tix Oy /C) by one-step pyrolysis of MIL-125(Ti). Due to the doping of carbon atoms into Vo, the formed Ti-C bonds lowered the energy barrier at the potential-determining step of NRR process; as a consequence, C-Tix Oy /C displayed a high performance toward NRR with a superior FENH3 of 17.8% and an excellent NH3 yield rate of 14.8 μg h−1 −1 mgcat. at −0.40 V vs. RHE in 0.1 M LiClO4 solution. Yang et al. [16] prepared isolated yttrium atoms anchored on porous carboncoated TiO2 via a simple method. The single yttrium site could effectively promote the conversion of N2 to NH3 at ambient condition. Due to the lower free energy of ratedetermining step on yttrium atoms via DFT calculation, the electrocatalyst exhibited −1 at −0.22 V vs. superior FENH3 of 11.2% and high NH3 yield rate of 6.3 μg h−1 mgcat. RHE. Cao et al. [17] reported the Zr4+ -doped TiO2 achieved outstanding performance of electrocatalytic N2 reduction. The larger ionic size boosted the formation of Vo by a strained effect and displayed superior FENH3 of 17.3% and NH3 yield rate of 8.90 μg h−1 cm−2 at −0.45 V vs. RHE than those of un-doped TiO2 or Ce4+ -doped TiO2 at ambient aqueous conditions. Wu et al. [18] synthesized iron-doped TiO2 (Fe-TiO2 ) electrocatalyst to achieve an excellent catalytic property of electrocatalytic nitrogen fixation. Fe-TiO2 electrocatalyst displayed a higher FENH3 of 25.6% and a larger NH3 production rate of 25.47 μg h−1 mgcat −1 at −0.40 V vs. RHE than

337

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CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

pristine TiO2 . Meanwhile, the electrocatalyst also showed superb electrochemical and structural stability. Meng Zhang [19] employed conductive and elastic TiO2 nanofibrous aerogels (NAs) by directional assembly of flexible TiO2 nanofibers followed by lithium reduction. The unique hierarchical structure of TiO2 NAs generated abundant Vo. Significantly, TiO2 -NAs exhibited exceptional catalytic performance for nitrogen fixation with a high FENH3 of 20.3% and a large NH3 yield rate of 4.19 × 10−10 mol s−1 cm−2 and outstanding long-term durability. DFT calculations displayed these extra abundant Vo could boost the N2 adsorption and activation on Vo-containing surface to accelerate the nitrogen fixation efficiency. Fang et al. [20] reported Vorich TiO2 /Ti3 C2 Tx electrocatalyst through a one-step ethanol-thermal Ti3 C2 Tx Mxene strategy for NRR catalysis. TiO2 /Ti3 C2 Tx electrocatalyst exhibited excellent NRR performance with a higher FENH3 of 16.07% at −0.45 V vs. RHE and NH3 yield −1 at −0.55 V vs. RHE than pure Ti3 C2 Tx nanosheets and rate of 32.17 μg h−1 mgcat. commercial TiO2 in 0.1 M HCl aqueous conditions. Selectivity was also enhanced due to the synergy of Ti3 C2 Tx and TiO2 nanoparticles to lower NRR energy barrier of rate-limiting step in NRR catalysis.

15.2.2 Chromium oxides Zhang et al. [21] synthesized multi-shelled hollow Cr2 O3 microspheres (MHCMs) as non-noble metal electrocatalysts for N2 fixation via a simple method. As displayed in Fig. 15.2A–C, MHCMs deposited on carbon paper electrocatalyst (MHCMs/CP) −1 ) achieve a high FENH3 (6.78%) and a large NH3 producing rate (25.3 μg h−1 mgcat. at −0.9 V vs. RHE with excellent selectivity and long-term stability in 0.1 M Na2 SO4 solution. DFT calculations in Fig. 15.2D suggested that the NRR process of MHCMs occurred on Cr2 O3 (110) surface by both distal associative and partially alternative routes. Du et al. [22] obtained Cr2 O3 nanofiber by calcining polyacrylonitrile/chromium acetate nanofiber in air as an excellent non-noble-metal electrocatalyst for artificial N2 -to-NH3 conversion in acidic electrolyte. Due to the enhanced conductivity and engineered surface oxygen vacancy in Cr2 O3 electrocatalyst, it exhibited −1 ) with strong a high FENH3 of 8.56% and NH3 formation rate (28.13 μg h−1 mgcat. electrochemical durability under ambient conditions. Similarly, Xia et al. [23] loaded Cr2 O3 nanoparticles on reduced graphene oxide (Cr2 O3 -rGO) as a nonprecious metal electrocatalyst for N2 fixation with outstanding selectivity and structural stability in acidic electrolyte. In consideration of the boosted conductivity and faster ∗ NNH formation, Cr2 O3 -rGO electrocatalyst displayed a large NH3 production rate (33.3 −1 ) at −0.7 V vs. RHE and a high Faradaic efficiency (7.33%) at −0.6 V μg h−1 mgcat. −1 , 1.38%). vs. RHE, outperforming its Cr2 O3 counterpart (13.9 μg h−1 mgcat.

15.2.3 Manganese oxides Wu et al. [24] synthesized that Mn3 O4 nanocubes electrocatalyst served as highperformance electrohydrogenation of the N2 -to-NH3 fixation in neutral electrolyte. The as-prepared electrocatalyst exhibited a high FENH3 of 3.0% and a larger NH3

15.2 Transition metal oxides-based electrocatalysts

FE (%)

10.76

12.4 7.95

−1.0

−0.9 −0.8 E (V vs. RHE)

VNH3 (µg h−1 mg−1 ) cat.

20 15 10 5 0

3

1

2

3 4 Cycle number

5

0.91

1.06

−1.1

−1.0 −0.9 −0.8 E (V vs. RHE)

0

−0.7

(C) 25

4.14

4

1 −1.1

4.99

5

2

5

8 7 6 5 4 3 2 1 0

(D)

1.0 0.5

FE (%)

VNH3 (µg h−1 mg−1 ) cat.

11.0

10

0

6.78

7 6

20 15

8

(B)

25.3

25

G/ eV

(A)

0.0

*NNH

NH3 *NNH2

*NH2NH2

*NHNH *N *NN *NHNH2 (N2) *NH

NH3

−0.5 −1.0 −1.5 −2.0

−0.7

(2NH3)

NH3

*NH2

*NH3

Reaction coordinates

FIGURE 15.2 (A) VNH3 and (B) FEs of NRR for MHCMs/CP catalyst at different potentials. (C) Cycling test of MHCMs/CP at −0.9 V vs. RHE. (D) Free-energy profile of N2 electro-reduction on Cr2 O3 (110) surface. Published with permission from Ref. [21]. Copyright 2018, American Chemical Society.

production of 11.6 μg h−1 mgcat −1 at -0.80 V vs. RHE and performed satisfactory long-term durability in 0.1 M Na2 SO4 aqueous solution. This work offers us the rational design of Mn oxide-based materials for electrochemical NH3 synthesis. Wang et al. [25] deposited MnO particles on Ti mesh (MnO/TM) as an efficient nonnoble-metal electrocatalyst for N2 fixation to NH3 in neutral condition. The MnO/TM electrocatalyst exhibited a high FENH3 of 8.02% and a superior NH3 production of 1.11 × 10−10 mol cm−2 s−1 at −0.39 V vs. RHE and performed excellent longterm durability in 0.1 M Na2 SO4 electrolyte. DFT calculations indicated that MnO (200) surface performed a lower adsorption energy for N than that of H accelerating the transformation of ∗ N2 to ∗ N2 H in NRR process. Zheng et al. [26] designed well-defined one-dimensional MnO nanocrystals encapsulated into carbon nanofibers (MnO-CNF) by an electrospinning method for electrocatalytic reduction of N2 in neutral electrolyte. In 0.1 M Na2 SO4 aqueous solution, MnO-CNF electrocatalyst −1 with a FENH3 of 1.52% at −1.25 V achieved a NH3 yield of 35.9 μg h−1 mgcat. vs. RHE. Enhanced selectivity toward electrochemical N2 fixation with high electrochemical durability was due to the promoted electrical conductivity and the synergetic effects between CNFs and MnO.

339

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CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

Chu et al. [27] synthesized Mo-doped MnO2 nanoflowers (Mo-MnO2 NFs) catalyst via a one-step hydrothermal strategy for the electrochemical N2 fixation in neutral electrolyte. Owing to the introducing Mo dopant to boost the N2 adsorption on Mo-MnO2 surface and enhance the conductivity of MnO2 , the prepared MoMnO2 NFs electrocatalyst recorded a larger NH3 production rate of 36.6 μg h−1 −1 mgcat. at -0.5 V vs. RHE and a higher FENH3 of 12.1% at −0.4 V vs. RHE than undoped MnO2 NFs in 0.1 M Na2 SO4 aqueous solution. The electron-deficient Mo with protons emerged the remarkable electrostatic repulsion and retarded the binding of Lewis acid H+ prohibiting the HER. Meanwhile, nonmetallic dopants (B, N, P, etc.) could also induce the formation of Vo to boost the NRR process. Chu et al. [28] recently also designed B-doped MnO2 nanosheets grown on carbon cloth (BMnO2 /CC) via a simple one-step hydrothermal way for the NRR process by combined experimental and theoretical investigations. Due to the B-doping modulating the charge distribution around MnO2 and inducing abundant Vo using the structural engineering, B-MnO2 /CC electrocatalyst performed NH3 yield rate of 54.2 μg h−1 −1 mgcat. at -0.4 V vs. RHE and FE of 16.8% at −0.2 V vs. RHE with long-term durability in 0.5 M LiClO4 aqueous electrolyte. DFT calculation indicated that the cooperation of B dopants and rich Vo active sites lowered the reaction energy barrier to improve the conductivity and boosted the intrinsic NRR activity at ambient conditions. Wang et al. [29] reported MnO particles loaded on Ti mesh (MnO/TM) as an efficient electrocatalyst for electrochemical N2 -to-NH3 conversion in neutral media. MnO/TM electrocatalyst exhibited a high FENH3 of 8.02% and a large NH3 yield of 1.11 × 10−10 mol s−1 cm−2 at −0.39 V vs. RHE with great selectivity and long-term cyclic stability. DFT results indicated that MnO/TM was more preferential to NRR than HER, proved by smaller adsorption energy of N (GN∗ , −2.20 eV) than that of H (GH∗ , −1.56 eV) on MnO (200) surface. Zhang et al. [30] obtained a Vo -rich MnO2 nanowire array on Ti mesh (MnOx NA/TM) electrocatalyst by hydrothermal method for the efficient electrocatalytic NRR with excellent long-term electrochemical durability. The prepared MnOx NA/TM electrocatalyst displayed excellent electrochemical NRR performance with a higher FENH3 of 11.40% and a larger NH3 yield rate of 1.63 × 10−10 mol cm−2 s−1 at -0.50 V vs. RHE than pure MnO2 NA/TM (2.3 × 10−11 mol cm−2 s−1 and 1.96%) in 0.1 M Na2 SO4 aqueous solution. The DFT calculations illustrated that the active center on the Vo -abundant MnOx surface possessed stronger electronic interaction with N2 molecule.

15.2.4 Iron oxides Liu et al. [31] prepared a spinel Fe3 O4 nanorod on Ti mesh (Fe3 O4 /Ti) as a great nonnoble-metal NRR electrocatalyst for electrochemical NH3 synthesis. The Fe3 O4 /Ti electrocatalyst obtained a robust FENH3 (2.6%) and NH3 production rate (5.6 × 10−11 mol s−1 cm−2 ) at −0.40 V vs. RHE and performed outstanding long-term durability in 0.1 M Na2 SO4 solution. Suryanto et al. [32] reported a core-shell α-Fe@Fe3 O4 nanorods grown on carbon fiber paper (CFP) for electrochemical N2 fixation to NH3 using aprotic fluorinated solvent-ionic liquid mixture as the electrolyte. The prepared

15.2 Transition metal oxides-based electrocatalysts

electrocatalyst achieved high-performance NRR with a high FENH3 of 32% and a large NH3 production rate of 3.71 × 10−13 mol s−1 cmECSA −2 at −0.65 V vs. RHE in aprotic fluorinated solvent. The DFT calculations explicated that α-Fe (110) was an active surface to be pivotal to N2 activation and the core-shell structure boosted the charge transfer. Meanwhile, the use of hydrophobic fluorinated aprotic electrolyte effectively limited the availability of protons; as a consequence, the competing HER was suppressed. Hu et al. [33] synthesized a Fe/Fe3 O4 electrocatalyst by oxidizing Fe foil for catalyzing NRR process in 0.1 M phosphate buffer solution (PBS). The prepared Fe/Fe3 O4 electrocatalyst performed NRR with a high FENH3 of 8.29% and appreciable NH3 yield of 0.19 μg cm−2 h−1 at -0.30 V vs. RHE in 0.1 M PBS electrolyte, which was more excellent than Fe, Fe3 O4 , and Fe2 O3 nanoparticles, implying Fe/Fe3 O4 was an efficient electrocatalyst for N2 fixation. The high selectivity was enabled by an enhancement of the intrinsic NRR activity as well as an effective suppression of the HER activity. Additional comparative experiments have been performed to further understand the effect of chemical state and composition of Fe-based electrocatalysts on their NRR selectivity. Xiang et al. [34] deposited Fe2 O3 nanorods on carbon paper (Fe2 O3 /CP) as superior electrocatalyst for NH3 synthesis under neutral conditions. Fe2 O3 /CP electrocatalyst obtained a FENH3 of 0.94% and an exceptional NH3 producing rate of 15.9 μg h−1 mgcat −1 at -0.80 V vs. RHE; also, a robust durability under recycling measurement in 0.1 M Na2 SO4 electrolyte was achieved. Zhu et al. [35] studied FeOOH quantum dots loaded on graphene sheet (FeOOH QDs-GS) for electrochemical N2 fixation. The prepared FeOOH QDs-GS hybrid displayed a promising NH3 production rate (27.3 μg h−1 mgcat .−1 ) and a high FENH3 (14.6%) at −0.4 V vs. RHE with outstanding selectivity and long-term stability during the electrolytic process in aqueous media.

15.2.5 Nickel-based oxides Chu et al. [36] reported NiO nanodots loaded on graphene (NiO/G) by a selfpropagating combustion method as a superb electrocatalyst for N2 -to-NH3 conversion in aqueous media. The as-prepared NiO/G electrocatalyst displayed a high FENH3 of 7.8% and NH3 producing rate of 18.6 μg h−1 mgcat −1 at −0.70 V vs. RHE; also performed good selectivity and long-term durability in 0.1 M Na2 SO4 solution. The DFT results indicated that NiO was the fundamental active center for N2 fixation and the nanodot structure enabled the exposure of more active sites for substantially exploring the NRR activity of NiO. And it was further revealed that the distal associative route was the energy-favorable pathway with ∗ N2 →∗ NNH being the ratedetermining step. Wang et al. [37] employed the heteroatom nitrogen-doped way to boost the NiO activity and prepared the nitrogen-doped NiO nanosheet array on carbon cloth (NNiO/CC) catalyst for electrochemical N2 fixation in neutral media. The N-NiO/CC electrode performed a higher FENH3 of 7.3 % and a larger NH3 production of 22.7 −1 −1 at −0.50 V vs. RHE than undoped NiO/CC (3.6%, 9.2 μg h−1 mgcat. at μg h−1 mgcat.

341

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CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

−0.50 V vs. RHE) in 0.1 M LiClO4 solution. DFT calculations illustrated N-doping method could finely modulate the electronic structure of NiO to upraise d-band center and debase the energy barrier, promoting the NRR activity. Furthermore, Chen et al. [38] synthesized N-doped porous carbon coated NiO array on graphite paper (NC@NiO/GP) as NRR electrocatalyst to boost the conductivity and stability of NiO. The experimental result exhibited a high FENH3 of 30.43% and a large NH3 producing −1 at −0.20 V vs. RHE. Due to the protection of N-doped rate of 14.02 μg h−1 mgcat. porous carbon layer, N-C@NiO/GP electrocatalyst performed high electrochemical and structure durability. Meanwhile, 15 N2 as the feeding gas was also employed to verify the source of generated ammonia, suggesting that the reaction followed a Marsvan Krevelen mechanism. As a typical defect active site in transition metal oxides, Vo greatly affected the NRR performance of electrocatalyst. Li et al. [39] employed a facile plasma technique to form Vo in the NiO nanosheet array on carbon cloth (P-NiO/CC) for N2 fixation. Due to more abundant plasma-induced Vo , P-NiO/CC electrocatalyst exhibited high-performance NRR with a robust FENH3 of 10.8% and −1 at −0.50 V vs. RHE in 0.1 M a superior NH3 production rate of 29.1 μg h−1 mgcat. Na2 SO4 solution.

15.2.6 Niobium oxides Han et al. [40] prepared crystallized Nb2 O5 nanofiber by electrospinning method as an efficient NRR electrocatalyst. The Nb2 O5 nanofiber exhibited an excellent FENH3 −1 of 9.26% and a promising NH3 production of 43.6 μg h−1 mgcat. at -0.55 V vs. RHE in 0.1 M HCl solution with outstanding selectivity and long-term durability. DFT calculations indicated the corresponding active center of N2 fixation was Nb2 O5 (181) facet. Furthermore, Huang et al. [41] reported NbO2 nanoparticles as NRR electrocatalyst with a higher NRR FENH3 of 32% at −0.60 V vs. RHE and a better −1 at −0.55 V vs. RHE than Nb2 O5 in N2 NH3 production rate of 11.6 μg h−1 mgcat. saturated 0.05 M H2 SO4 solution. The results show that Nb4+ cation effectively promoted N2 adsorption and subsequent activation for NRR. Wu et al. [42] designed an ultrafine Nb3 O7 (OH) nanoparticles grown on carbon fiber cloth (Nb3 O7 (OH)/CFC) by vapor-phase hydrothermal method for electrochemical N2 fixation. The prepared Nb3 O7 (OH)/CFC electrocatalyst performed an ultrahigh FENH3 of 39.9% and a −1 at −0.55 V vs. RHE in 0.1 M superb NH3 production rate of 622 μg h−1 mgcat. Na2 SO4 solution. The experimental results indicated that the in situ electrochemically converted NbO from Nb3 O7 (OH) during the NRR was the catalytic active center. With a reaction time over 30 minutes, the formed NbO active sited further converted to low active oxygen-containing niobium nitride leading to the decrement in NRR performance.

15.2.7 Other transition metal oxides Han et al. [43] synthesized MoO3 nanosheets via a one-step hydrothermal reaction as efficient NRR electrocatalyst. The MoO3 nanosheets displayed a FENH3 of 1.9%

15.2 Transition metal oxides-based electrocatalysts

−1 and a striking NH3 production of 29.43 μg h−1 mgcat. at −0.50 V vs. RHE in 0.1 M HCl solution with boosted selectivity and electrochemical durability. The DFT calculations indicated that the outermost Mo atoms could promote N2 adsorption as the active sites. Wang et al. [44] reported reduced graphene oxide as the supporter for MoO2 nanoparticles (MoO2 /RGO) for electrocatalytic ammonia synthesis. Due to the stronger electronic interactions with ∗ N2 H and donated more electrons from active Mo sites to ∗ N2 H, the prepared MoO2 /RGO electrocatalyst presented an outstanding selectivity and NRR performance with a high FENH3 of 6.6% and NH3 production rate −1 at −0.35 V vs. RHE in 0.1 M Na2 SO4 solution. Zhang et al. of 37.4 μg h−1 mgcat. [45] designed Vo-rich and Vo-poor MoO2 nanosheets electrocatalyst by chemical vapor deposition method for electrochemical N2 fixation. The results indicated that the Vo-rich MoO2 nanosheets electrocatalyst performed outstanding NRR with a −1 at a low higher FENH3 of 8.34% and a larger NH3 production of 12.20 μg h−1 mgcat. −1 −1 potential of −0.15 V vs. RHE than Vo-poor MoO2 (3.66 μg h mgcat.) in 0.1 M HCl aqueous solution. The presence of Vo was propitious to the chemical adsorption for N2 and proton transfer step by selective stabilization of N2 H∗ and destabilization of N2 H2 ∗ . Liu et al. [46] employed reduced graphene oxide as carrier to obtain ZnO quantum dots hybrid electrocatalyst (ZnO/RGO) for electrochemical N2 fixation. Due to the largely exposed active sites of ultrafine quantum dots, ZnO/RGO achieved superior NRR performance with a high FENH3 of 6.4% and an exceptional NH3 −1 production of 17.7 μg h−1 mgcat. at -0.65 V vs. RHE in 0.1 M Na2 SO4 solution. The DFT results revealed that the electronic coupling of ZnO with RGO boosted the charge transfer and depressed activation-energy barrier for rate-determining step of N2 fixation. Similarly, Wang et al. [47] deposited In2 O3 nanoparticle on reduced graphene oxide (In2 O3 /RGO) electrocatalyst for NRR, which exhibited a high FENH3 of 8.1% −1 at -0.60 V vs. RHE with excellent and a boosted NH3 production of 18.4 μg h−1 mgcat. selectivity and stability in 0.1 M Na2 SO4 solution. Chu et al. [48] supported CoO quantum dots on reduced graphene oxide (CoO-QD/rGO) to achieve the electrochemical N2 fixation. CoO-QD/RGO electrocatalyst presented a high FENH3 of 8.3% and −1 at -0.60 V vs. RHE. Through a superior NH3 producing rate of 21.5 μg h−1 mgcat. alternating metal oxide quantum dots, Chu et al. [49] supported SnO2 quantum dots on reduced graphene oxide (SnO2 /RGO) for NRR at ambient condition, which −1 performed a high FENH3 of 7.1% and a larger NH3 production of 25.6 μg h−1 mgcat. at −0.50 V vs. RHE with excellent selectivity and long-term durability in 0.1 M Na2 SO4 solution. Furthermore, Liu et al. [50] doped fluorine into SnO2 mesoporous nanosheets supported on carbon cloth (F-SnO2 /CC) electrocatalyst for NRR, which presented superior NRR performance and excellent durability with a higher FENH3 of −1 at -0.45 V vs. RHE than 8.6% as well as NH3 producing rate of 19.3 μg h−1 mgcat. SnO2 /CC in 0.1 M Na2 SO4 solution shown in Fig. 15.3A and B. DFT calculations indicated that F atoms induced alternation of the electronic structure of SnO2 with improved conductivity and increased positive charge on active Sn sites, leading to the lowered reaction energy barriers and boosted NRR activity in Fig. 15.3C. The density of states analysis in Fig. 15.3D explained that F-doping could enhance conductivity

343

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

24

12

(B)25

10

20

8

15

6

10

4

5

2

8

16

6 4

8

2 0

−0.65

(C)2

−0.55

PDS 0

N2

−0.45

−0.35

Potential (V vs. RHE)

*N2

−0.25

0

0

SnO2 F-SnO2

*N2H *NNH2

10

1

2

3 4 Cycle number

5

0

6

(D)

FE %

(A)

FE %

344

SnO2

*N

*NH

*NH2

F-SnO2

NH3

−2

Reaction coordinate

−10

−5

0 5 Energy (eV)

10

FIGURE 15.3 (A) Experimental testing of NH3 yields (columns) and FEs (stars) (B) Cycling test of F-SnO2 /CC for six cycles of chronoamperometric measurement. (C) Free energy diagrams of SnO2 and F-SnO2 though the distal pathway at zero applied energy. (D) Total density of states (TDOS) of SnO2 and F-SnO2 . Reproduced with permission from Ref. [50]. Copyright 2019, American Chemical Society.

of SnO2 to facilitate the electron transport during NRR process due to a definite zero band gap of F-SnO2 . Meanwhile, Xu et al. [51] developed La2 O3 nanoplate loaded on the carbon paper (La2 O3 /CP) as NRR electrocatalyst. Due to the active center of La atoms on the surface, La2 O3 /CP electrocatalyst achieved a high FENH3 of 4.76% and a large NH3 −1 production rate of 17.04 μg h−1 mgcat. at −0.80 V vs. RHE with excellent selectivity in 0.1 M Na2 SO4 solution. Similar to the above, Xu et al. [52] also prepared Vorich CeO2 nanorod as NRR electrocatalyst showing a higher FENH3 of 3.7% and a −1 at -0.40 V vs. RHE than the pristine boosted NH3 production of 16.4 μg h−1 mgcat. −1 ) in 0.1 M Na2 SO4 solution. Cheng et al. CeO2 electrocatalyst (2.1%; 5.4 μg h−1 mgcat. [53] explored rare-earth Sm2 O3 electrocatalyst via a microwave-based hydrothermal method for N2 fixation. Due to donating more electrons from active Sm centers sites to ∗ N2 H, Sm2 O3 catalyst exhibited a robust FENH3 of 11.5% and NH3 production of −1 at −0.60 V vs. RHE with great selectivity and cyclic stability in 0.1 27.2 μg h−1 mgcat. M Na2 SO4 solution. Fu et al. [54] developed Vo-rich Ta2 O5 nanorods for nitrogen reduction in acidic electrolyte. As shown in Fig. 15.4A and B, the Vo-rich Ta2 O5

15.2 Transition metal oxides-based electrocatalysts

(A)

(B)18

10

10

16

8

Faradic Energy (%)

8 12

6 8

4 4

0

−0.6

(C)

−0.7 −0.8 −0.9 Potential (V vs. RHE)

−1.0

Intensity (a.u.)

Defect Ta2O5 Annealed Ta2O5 Commerial Ta2O5

3320

3360 3400 Magnetic field (Gauss)

12 6 9 4

6

2

3

0

0

(D)

Faradic Energy (%)

15

2

1

2

3 Cycle number

4

5

0

1.381 Å

3440

FIGURE 15.4 (A) The yields of NH3 and FENH3 of defect Ta2 O5 nanorods at different potentials. (B) Consecutive recycling tests of defect Ta2 O5 nanorods at −0.7 V vs. RHE. (C) Low-temperature EPR spectra. (D) The adsorption geometry of N2 on the O-vacancy site of the Ta2 O5 (001) surface. Published with permission from Ref. [54]. Copyright 2019, American Chemical Society.

−1 nanorods a considerable FENH3 of 8.9% and NH3 yield rate of 15.9 μg h−1 mgcat. at -0.80 V vs. RHE with outstanding selectivity and long-term stability in 0.1 M HCl electrolyte. Fig. 15.4C suggested rich O-vacancies were observed in Ta2 O5 nanorods. DFT results shown in Fig. 15.4D implied that the active site was Vo coordinated with two Ta atoms. CuO as a common transition metal oxide has been widely employed in various fields duo to the high stability, environmentally friendly and great supporter. Wang et al. [55] reported that CuO nanoparticles (NPs) deposited on reduced graphene oxide (CuO/rGO) was a robust electrocatalyst for electrocatalytic ammonia synthesis by a microwave-assisted solvothermal method. Due to the boosted conductivity and exposing more active sites, the prepared CuO/rGO exhibited a high NH3 yield of 1.8 × 10−10 mol s−1 cm−2 and high FE of 3.9% at −0.75 V vs. RHE in 0.1 M Na2 SO4 electrolyte. The DFT results understood NRR catalytic mechanism and further indicated that CuO (111) was the active site for NRR absorbing the nitrogen molecule and

345

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CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

lowering the free energy barrier of the rate-determining step (∗ N2 →∗ NNH) in NRR process.

15.3 Transition metal sulfides-based electrocatalysts 15.3.1 Molybdenum sulfides Zhao et al. [56] designed MoS2 /C3 N4 electrocatalyst via in situ formation during pyrolysis for electrochemical N2 fixation in neutral medium. Due to the strong interactions between MoS2 and C3 N4 to form Mo-N coordination as the high-performance active site, MoS2 /C3 N4 electrocatalyst achieved great selectivity and structural stability with a higher FENH3 of 6.87% associated with a larger NH3 production of −1 at −0.50 V vs. RHE than pure MoS2 (2.85% and 6.41 μg h−1 19.86 μg h−1 mgcat. −1 −1 mgcat.) and MoS2 /C3 N4 without Mo-N coordination (3.81% and 9.45 μg h−1 mgcat. ) in 0.1 M Na2 SO4 electrolyte. Chu et al. [57] fabricated 2D/2D MoS2 /C3 N4 by in situ growth of sheet-like MoS2 on C3 N4 nanosheets using a solvothermal approach for electrocatalytic conversion of N2 to NH3 . Two-dimensional (2D)/2D interfacial engineering was an effective method to design powerful electrocatalysts due to intimate face-to-face contact of two 2D materials facilitating the strong interfacial electronic interactions. As exhibited in Fig. 15.5A–C, the prepared MoS2 /C3 N4 −1 performed a FENH3 of 17.8% and NH3 production rate of 18.5 μg h−1 mgcat. at −0.30 −1 −1 V vs. RHE than the C3 N4 or MoS2 (3.9%; 11.3 μg h mgcat.) in 0.5 M LiClO4 aqueous solution. DFT calculations shown in Fig. 15.5D and E revealed that the interfacial charge transport from C3 N4 to MoS2 could enhance the NRR activity of MoS2 /C3 N4 by promoting the stabilization of the key intermediate ∗ N2 H on Mo edge sites of MoS2 and concurrently decreasing the reaction energy barrier. Meanwhile, MoS2 /C3 N4 rendered a more favorable ∗ H adsorption free energy on S edge sites than on Mo edge sites of MoS2 , thereby protecting the NRR-active Mo edge sites from the competing HER and leading to a high FENH3 . Liu et al. [58] employed reduced graphene oxide as supporter for anchoring the MoS2 nanodots to obtain MoS2 NDs/RGO non-noble metal electrocatalyst for conversion of N2 to NH3 at ambition conditions. Owing to the good dispersion of the MoS2 NDs and the strong interaction, MoS2 NDs/RGO electrocatalyst achieved a high FENH3 of 27.93% at −0.35 V vs. RHE and a large NH3 production rate of −1 at −0.75 V vs. RHE with remarkable structural stability in 0.1 16.41 μg h−1 mgcat. M Na2 SO4 electrolyte. Furthermore, the work also employed the X-ray photoelectron spectroscopy (XPS) to illustrate the strong C-S-C bridging bonds on MoS2 NDs/RGO surface to boost the NRR reaction kinetics rather than simple physical composition. Zhang et al. [59] deposited MoS2 nanosheet array grown on carbon cloth (MoS2 /CC) for high-performance N2 fixation at ambient conditions. Due to the formation of the N-Mo bond and the impaired N-N triple bond, the prepared MoS2 /CC performed a high FENH3 (1.17%) and a remarkable NH3 producing rate (8.08 × 10−11 mol s−1 cm−2 ) at −0.50 V vs. RHE in 0.1 M Na2 SO4 electrolyte. Furthermore,

20

20

20

16

18

10

5

2

0

−0.5 −0.4 −0.3 −0.2 Potential (V vs. RHE)

Free energy (eV)

(D)

*N2H 0

N2

MoS2

MoS2/C3N4

Mo1 (MoS2) Mo1 (MoS2/C3N4)

PDS

1

0

C3N4

*N *NNH2

*N2

*NH

NH3

−1

PDS

*NH2

(E) Free energy (eV)

0

−0.6

10 10

10

4

0

15

5

0

1

0.2

0.0

−0.2

2

3 4 5 6 Cycle Number

7

0

Mo-edge (MoS2) H+ + e-

S-edge (MoS2)

1/2 H2

S-edge (MoS2/C3N4)

−0.4

−2

Mo-edge (MoS2/C3N4)

Reaction coordinate

−0.6

Reaction coordinate

FIGURE 15.5 (A) Obtained NH3 yields and FEs. (B) NRR performances of MoS2 , C3 N4 , and MoS2 /C3 N4 under identical conditions. (C) NH3 yields and FEs at −0.3 V. (C) Free energy diagrams of the distal NRR pathway on Mo1 and Mo1h sites. (D) Free energy diagrams of ∗ H (G∗ H ) on Mo edge and S edge sites of MoS2 and MoS2 /C3 N4 . Reproduced with permission from Ref. [57]. Copyright 2020, American Chemical Society.

15.3 Transition metal sulfides-based electrocatalysts

5

FE, %

12

20

20

15

15 10

(C)

FE (%)

(B) MoS2/C3N4 20

FE, %

(A)

347

348

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

MoS2 /CC still achieved favorable NRR activity in acid condition with good stability, where strong hydrogen evolution occurs. Li et al. [60] designed diatomic boron doped single-layer MoS2 (B2 @MoS2 ) as electrocatalyst for N2 fixation. Compared to single boron anchored MoS2 , the B2 @MoS2 effectively activated the inert N2 and promoted N2 reduction to NH3 via the enzymatic mechanism; moreover, B2 @MoS2 showed much better electrocatalytic activity than B@MoS2 , as reflected by the significantly reduced overpotential (0.02 V vs. 0.30 V) and the much lower activation barrier (1.24 eV vs. 2.84 eV). Chu et al. [61] reported amorphous MoS3 with enriched sulfur vacancies for the ammonia synthesis, which obtained a higher FENH3 of 12.8% and a larger NH3 −1 yielding rate of 51.7 μg h−1 mgcat. at −0.30 V vs. RHE than the crystalline MoS2 1 −1 catalyst (3.9%; 11.3 μg h mgcat.) in 0.5 M LiClO4 aqueous solution. DFT calculations revealed S vacancies became the active site of catalysis to inhibit the HER and activate N2 . Similar to construct sulfur vacancies, Zeng et al. [62] introduced nitrogen and sulfur vacancies to obtain various content of sulfur vacancies N-doped MoS2 nanoflowers for electrochemical N2 fixation. Due to N-doping creating rich sulfur vacancies and generating Mo-N active sites, the N-doped MoS2 delivered a robust −1 at −0.30 V FENH3 of 9.14% and a superior NH3 production of 69.82 μg h−1 mgcat. −1 −1 at −0.40 V vs. RHE than the un-doped N-MoS2 catalyst (0.83%; 8.19 μg h mgcat. vs. RHE) in 0.1 M Na2 SO4 aqueous solution.

15.3.2 Iron sulfides Fe is a metal element with stable chemical properties and low price. Spectroscopic studies showed that Fe is a necessary transition metal atom for the binding of N2 in nitrogenase. Du et al. [63] prepared FeS2 with S vacancies via one-step hydrothermal strategy. Fig. 15.6A and B showed that FeS2 electrocatalyst displayed excellent catalytic NRR performance with a high NH3 yielding rate (37.2 μg h−1 −1 mgcat. ) and a considerable FENH3 (11.2%) with long-term stability at −0.5 V vs. RHE, outperforming other NRR electrocatalysts. According to experimental results, a preferable associative distal pathway was proposed. DFT calculations in Fig. 15.6C and D indicated Fe atoms act as the active sites for N2 -to-NH3 process in distal pathway rather than on S atoms on FeS2 electrocatalyst. Zhao et al. [64] employed DES (polyethylene glycol 200/thiourea) as a shape-controlling agent to obtain the nanostructured Fe3 S4 nanosheets catalyst for electrochemical N2 -to-NH3 conversion using one-step solvothermal method. Due to strong adsorption energy on the Fe3 S4 surface, the prepared Fe3 S4 nanosheets exhibited a high NH3 yield rate (75.4 μg −1 ) and FENH3 (6.45%) at −0.4 V vs. RHE with remarkable stability under h−1 mgcat. long-term electrolysis at room temperature and atmospheric pressure. Xiong et al. [65] adopted H2 S-plasma treatment on the Fe foam to obtain Fe foam supported FeSx (FeSx /Fe) electrode for electrochemical N2 fixation. The prepared self-supported electrode exhibited a higher FENH3 of 17.6% and a larger NH3 production rate of 4.13 × 10−10 mol cm−2 s−1 at −0.30 V vs. RHE than Fe2 O3 (2.2%; 2.71 × 10−11 mol cm−2 s−1 ) in 0.1 M KOH aqueous solution. The outmost FeSx converted to the

15.3 Transition metal sulfides-based electrocatalysts

(A)

(B)

32

12

40

10

32

8

24

6

16

4

8

2

0 2

(C)

0 −0.4

−0.5

−0.6

−0.7

−0.8

NH3 yield

0V −0.45 V 6(H++e)

1

H++e

2(H++e)

5(H++e) 6(H++e)

NH3

3(H++e) NH3

0

8

24

6 16

4

8

2

0

1

4(H++e)

−1

(D)

3

2

*N N *N NH *N NH2 *N

*NH

S-site

1 3(H++e) NH3

5(H++e) 6(H++e)

*NH2 *NH3

Reaction coordinate

NH3

−3

*NH3

H++e

4(H++e) 2(H++e)

−2

N2

0

5

Distal

0

−2

4

2

−1

−3

3

Cycle number (n)

PDS

Fe-site

12 10

Potential (V vs. RHE) Distal

FE

FE (%)

NH3 yield FE

FE (%)

40

N2

*N N *N NH *N NH2 *N

NH3

6(H++e)

*NH

*NH2 6(H++e) NH3

Reaction coordinate

FIGURE 15.6 (A) Tested NH3 yields and FEs. (B) Corresponding NH3 yield and FE of FeS2 with five cycles at −0.5 V vs. RHE. Reaction pathway for NRR with (C) Fe and (D) S atoms as the active sites. Published with permission from Ref. [63]. Copyright 2020, American Chemical Society.

mackinawite FeS after NRR process. DFT calculation clearly stated that the NRR took place on the FeS surface and the well-suited Fe-Fe distance on the mackinawite FeS surface. Huang et al. [66] promoted iron thiophosphite nanosheets (FePS3 ) for electrochemical N2 fixation via Co incorporation using a salt-template method. Due to Co doping boosting the electrocatalytic activity of Fe-edge centres and conductivity, Co-FePS3 nanosheets achieved a boosted FENH3 of 3.38% and NH3 yielding rate −1 at −0.40 V vs. RHE with outstanding selectivity and good of 90.6 μg h−1 mgcat. durability in 0.1 M KOH electrolyte. Wang et al. [67] introduced Mo to develop FeMo3 S4 nanorods electrocatalyst for N2 fixation. As shown in Fig. 15.7A, due to its surface-exposed and low-coordinate Fe3c sites, the prepared electrocatalyst performed an excellent FENH3 of 19.2% and −1 at −0.30 V vs. RHE with a remarkable NH3 production rate of 65.3 μg h−1 mgcat. long-term stability for 20 hours in 0.1 M KOH electrolyte. Fig. 15.7B displayed that the NMR spectra also confirmed a typical doublet chemical shift of 15 NH4 + using 15 N2 as feed gas. The Gibbs free energy diagrams (Fig. 15.7D) and the HER activity

349

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

(A)

80

25

(B) 1

60

15 40 10 20

(C)

JN-H = 72 Hz

20

FE (%)

350

1J

N-H =

15N 2

52 Hz

14N

2

Ar

5

0

−0.6

−0.5 −0.4 −0.3 Potential (V vs. RHE)

0.5

−0.2

0

6.8

7.0 7.2 Chemical shift (ppm)

7.4

0.0 −0.5 −1.0 −1.5

(D) 1

0

0

2

RDS(0.72) *

*N2

−1

4

6

8

Alternating Distal

*NNH *NHNH *NNH2

10 Time (h)

12

14

16

18

20

1.5

(E)

1.15 1.0 0.72

*N

*NH2NH2 NH3 *NHNH2 *NH2 *NH

0.52

0.5 G*N2(Fe3C)

0.0

G*H(Fe3C)

G*H(Mo8C)

−0.11

−2

−0.5

Reaction coordinate

FIGURE 15.7 (A) Tested NH3 yields and FEs of FeMo3 S4 nanorods. (B) 1 H NMR measurements using 14 N2 , 15 N2 and Ar as feed gases. (C) Chronoamperometry test for 20 hours at −0.3 V vs. RHE. (D) Reaction pathway for NRR with Fe3c at zero potential. (e) Gibbs free energies of ∗ H adsorption on Fe3c (G∗ H [Fe3c ]), ∗ N2 adsorption on Fe3c (G∗ N2 [Fe3c ]), RDS energy barrier on Fe3c (GRDS [Fe3c ]), and ∗ H adsorption on Mo8c (G∗ H [Mo8c ]). Reproduced with permission from Ref. [67]. Copyright 2020, American Chemical Society.

(Fig. 15.7E) revealed that FeMo3 S4 was more conducive to activate the NRR and retarding the HER, rendering a high NRR activity and selectivity.

15.3.3 Other transition metal sulfides Ma et al. [68] developed defect-rich WS2-x nanosheets electrocatalyst for electrocatalytic conversion of N2 to NH3 under neutral conditions. Due to the abundant sulfur vacancies and more exposed active sites to boost the adsorption and activation of N2 ,

15.4 Transition metal nitride-based electrocatalysts

WS2-x nanosheets displayed a higher FENH3 of 12.1% and a larger NH3 generating −1 at -0.60 V vs. RHE than the precursor WO3 with oxygen rate of 16.38 μg h−1 mgcat. −1 ) and commercial WS2 (0.35%; 3.63 vacancies (WO3 : 3.8% and 7.69 μg h−1 mgcat. −1 −1 μg h mgcat.) in 0.1 M Na2 SO4 solution. Xu et al. [69] ZrS2 nanofibers with a sulfur vacancy (ZrS2 NF-Vs) behaved as an efficient electrocatalyst for ambient N2 reduction to NH3 with excellent selectivity. The ZrS2 NF-Vs catalyst performed superior NRR performance with a high FENH3 of 10.33% at −0.30 V vs. RHE and a −1 at −0.35 V vs. RHE with superior larger NH3 producing rate of 30.72 μg h−1 mgcat. structural and performance stability in 0.1 M HCl solution. DFT calculations reveal that the introduction of sulfur vacancies facilitated the adsorption and activation of N2 molecules. Wang et al. [70] designed Sb2 S3 nanorods with S-vacancies (Sv -Sb2 S3 ) electrocatalyst for electrochemical N2 fixation. Due to the abundant S-vacancies, Sv Sb2 S3 electrocatalyst achieved a high FENH3 of 3.75% at −0.30 V vs. RHE and −1 at −0.40 V vs. RHE with a boosted NH3 production rate of 10.85 μg h−1 mgcat. preferable stability for 24 h in 0.1 M Na2 SO4 solution. As the DFT result shown, the existence of abundant S-vacancies altered an electron-deficient environment and modulated the electronic delocalization to improve the conductivity of electrocatalyst and activate the N≡N bond for NRR. The heteroatom doping is also an effective approach to tailor the electronic structures of electrocatalyst for improved electrocatalytic activity. Chu et al. [71] designed Mo-doped SnS2 (Mo-SnS2 ) nanosheets grown on for electrochemical N2 to NH3 at ambient conditions. Due to the abundant S-vacancies and the creation of Mo-Sn-Sn trimer catalytic sites, Mo-SnS2 /CC achieved good NRR performance with FENH3 −1 of 20.8% at −0.40 V vs. RHE and NH3 production rate of 41.3 μg h−1 mgcat. at −0.50 V vs. RHE with superior cycling stability in 0.5 M LiClO4 solution. Compared with metal doping and single catalytic active center of single metal, bimetallic sulfides possess different valence states of metals in M-S bonds to provide abundant active sites for enhanced nitrogen fixation activity. Li et al. [72] prepared CuCo2 S4 supported on multiwalled carbon nanotubes to obtain the CuCo2 S4 /MWCNT electrocatalyst for NH3 electrochemical synthesis. The CuCo2 S4 /MWCNT performed a −1 at −0.65 V vs. high FENH3 of 8.7% and a NH3 output rate of 137.5 μg h−1 mgcat. RHE with high selectivity and long-term durability in 0.1 M Na2 SO4 solution. Liu et al. [73] constructed FeNi2 S4 /NiS heterointerfaces supported on carbon nanofibers electrocatalyst for NRR catalysis, which exhibited a superior NRR performance with −1 at −0.65 V a high FENH3 of 28.64% and NH3 generation rate of 128.40 μg h−1 mgcat. vs. RHE in 0.1 M KOH solution. The interfacial engineering strategy of FeNi2 S4 /NiS heterointerfaces exposed more crystal planes combination of NiS (101) and FeNi2 S4 (311) for enhancing the N2 adsorption and accelerating reaction kinetics to boost the NRR activity.

15.4 Transition metal nitride-based electrocatalysts Transition metal nitrides (TMNs) as functional materials have become an interesting material for electrochemical nitrogen fixation application due to their good thermal

351

Electrochemical workstation

(A)

Rererence Working electrode e-electrode

Membrance

Cathode H2O

N2

NH3

H+

N2

O2

MoN NA/CC

(B)

0

j (mA cm−2)

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

−15

Counter e- electrode

N2 Ar

−30 −45

Anode

−0.4 −0.8 E (V vs. RHE)

−1.2

Graphite Ag/AgCl plate

00

3.0 1.5

2.4

1.0

1.8 1.2

0.5

0.6 0.0

0.0

−0.7 −0.6 −0.5 −0.4 −0.3 −0.2 −0.1 E (V vs. RHE)

Faradaic efficiency (%)

2.0

3

1.0

2 0.5

1 0

1

2

3 4 5 6 Cycle number (n)

7

0.0

Faradaic efficiency (%)

(D) NH3 yield (× 10−10 mol S−2 cm−1)

(C) NH3 yield (× 10−10 mol S−1 cm−1)

352

FIGURE 15.8 (A) Schematic diagram for electrocatalytic NRR. (B) LSV curves of MoN-NA/CC recorded in N2 -saturated (green line) and Ar-saturated (orange line) 0.1 M HCl. (C) NH3 yields and FEs at a series of potentials. (D) NH3 yields and FEs under different N2 flow rates at −0.3 V vs. RHE. All experiments were conducted at room temperature and ambient pressure. Published with permission from Ref. [74]. Copyright 2018, American Chemical Society.

mechanical properties, excellent conductivity, chemical corrosion resistance and the diversity of compound. Zhang et al. [74] designed self-supported MoN nanosheets array on carbon cloth (MoN-NA/CC) catalyst for electrochemical N2 fixation in acid medium. Fig. 15.8A exhibited the nitrogen reduction test device using H-type cell separated by proton exchange membrane in 0.1 M HCl solution. As shown in Fig. 15.8B–D, MoN-NA/CC electrode displayed an excellent NH3 yield rate of 3.01 × 10−10 mol s−1 cm−2 and FENH3 of 1.15% at −0.3 V vs. RHE with excellent selectivity and remarkable catalytic and structural stability under long-term electrolysis in 0.1 M HCl solution. Furthermore, DFT results revealed that MoN NA/CC preferred to split inert N≡N triple bond with activated surface at the end of the reaction via MvK mechanism due to the lowered kinetic energy barrier of replenishment of N-vacancies. Yang et al. [75] developed Mo-vacancy-rich MoN nanocrystals embedded in N-doped hierarchical porous carbon framework (MV-MoN@NC) electrocatalyst for

15.5 Transition metal phosphides-based electrocatalysts

electrochemical conversion of N2 to NH3 . Due to the introduced Mo vacancies on MoN surface to lower the energy barrier, MV-MoN@NC electrocatalyst achieved a −1 at high FENH3 of 6.9% and a promising NH3 producing rate of 76.9 μg h−1 mgcat. −0.20 V vs. RHE with superior NRR durability and a long-term operation in 0.1 M HCl solution. Notably, the isotopic experiments using 15 N2 as the feed source confirmed the NRR process of MV-MoN@NC followed MvK mechanism, in which a surface N was reduced to NH3 firstly and N-vacancy was replenished by adsorbed N2 subsequently. Xiao et al. [76] studied the VN3 embedded into graphene (GVN3 ), graphene and fluorographene (GF-VN3 ) for electrochemical N2 fixation using DFT analysis. The results indicated that the NRR on G-VN3 was relatively optimal, GH-VN3 possessed the excellent selectivity for NRR; while, GF-VN3 was excluded originated from the distinct charge transfer between the substrate and the NRR intermediates. Du et al. [77] analyzed vanadium(III) nitride, niobium(III) nitride, and Nb4 N5 electrocatalysts for electrochemical N2 -to-NH3 conversion. The experimental results in Fig. 15.9A and B showed their capability to catalyze the electrosynthesis of NH3 and propensities of VN and Nb4 N5 to release lattice nitride in a catalytic process leading to the formation of NH3 in acidic, neutral and alkaline solutions. The study in Fig. 15.9D confirmed the compulsory requirement for the implementation of reliable testing and analysis procedures for the assessment of the catalytic properties of materials for NRR. This result was interpreted in terms of irrecoverable losses of active sites or N vacancies on the nitride surface, which were the origin of the ammonia measured. Jin et al. [78] employed ammonia to etch Na2 W4 O13 precursor to obtain a novel rich nitrogen-vacancies-engineered ultrathin 2D layered W2 N3 (NVW2 N3 ). Due to the electron loss induced by the nitrogen vacancies to reduce the free energy barrier in NRR determining step, NV-W2 N3 displayed a remarkable NH3 −1 production of 11.66 μg h−1 mgcat. (3.80 × 10−11 mol cm−2 s−1 ) and an excellent FENH3 of 11.67% at -0.2 V vs. RHE with great stability under long-term electrolysis in 0.1 M KOH solution. The DFT results indicated that the existence of nitrogen-vacancy could boost the charge transfer process and activate the N≡N bond to lower the free energy change of the determining step (N2 →∗ NNH).

15.5 Transition metal phosphides-based electrocatalysts 15.5.1 Cobalt phosphides Transition metal phosphides (TMPs) have recently attracted attention in electrocatalytic conversion of N2 to NH3 due to their intrinsic conductivity, metallic nature of the lattice structure and the coordinately unsaturated active sites. Gao et al. [79] designed self-standing CoP3 nanoneedle arrays on carbon cloth (CoP3 /CC) catalyst for electrochemical N2 fixation. The CoP3 /CC electrode exhibited a superior FE of 11.94% and a remarkable NH3 yield rate of 3.61 × 10−11 mol s−1 cm−2 at −0.20 V vs. RHE with excellent durability during electrolysis in 0.1 M Na2 SO4 solution. The higher phosphorus content contributed to higher nitrogen fixation activity; thus, CoP3

353

after CVA

−0.5 V vs. RHE

−0.1 V vs. RHE

−0.6 V vs. RHE

−0.3 V vs. RHE

“adsorbed” NH3

= Nb or V

(C)

=N NH3

15 10

catalytic

tic taly -ca n o n

5 N2

0 140 120 100

(B): Nb4N5700/CC

after CVA

−0.5 V vs. RHE

−0.3 V vs. RHE

“adsorbed” NH3

−0.4 V vs. RHE

80 60 40 20 0

e-

e-

e-

e-

e-

(D) NH3 yield / mmol molcat−1

NH3 yield / mmol molcat−1

(A): VN600

300

N2-saturated 0.05 M H2SO4

250 200 150 100 −0.6 V to + 0.3 V vs. RHE

50

−1 V to 0 V vs. RHE

0 Experiment timeline

0

10

20 30 Time / h

40

50

FIGURE 15.9 Changes in the amount of NH4 + produced at different stages of experiments for the electro-reduction of N2 - (green background) and Ar-saturated (gray background) 0.05 M H2 SO4 solutions undertaken with the same (A) VN600 or (B) Nb4 N5 700 /CC working electrode. (B) Reaction model of catalyst for NRR. (D) Evolution of NH4 + amount generated during continuous registration of CV (v = 0.020 V s−1 ) within the −1 to 0 V vs. RHE (black) and −0.6 to +0.3 V vs. RHE (red) ranges using Nb4 N5 700/CC electrodes in N2 -saturated 0.05 M H2 SO4 . Reproduced with permission from Ref. [77]. Copyright 2019, American Chemical Society.

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

NH3 yield / mmol molcat−1

354

20

15.5 Transition metal phosphides-based electrocatalysts

nanoneedle possessed the lowest free energy barrier for N2 to NNH conversion. Guo et al. [80] designed 3D hierarchical cobalt phosphide hollow nanocage (CoP-HNC) electrocatalyst via solvothermal conversion for electrochemical N2 fixation. Due to an inhomogeneous surface with charge-separated sites and rich surface active sites from 3D hierarchical structure, CoP-HNC electrocatalyst exhibited a great FENH3 of 7.36% at low potential of 0 V vs. RHE and an increased NH3 production at −1 at −0.4 V vs. RHE with a increased overpotential reaching of 10.78 μg h−1 mgcat. high selectivity for ammonia.

15.5.2 Nickel phosphides Yuan et al. [81] obtained Ni2 P nanoparticles supported on N, P co-doped carbon nanosheets (Ni2 P/N, P-C) electrocatalyst for electrochemical reduction of N2 into NH3 under acid, neutral, basic conditions. Ascribing to the strong electronic interactions and the strong support effect, the prepared Ni2 P/N, P-C electrocatalyst achieved outstanding NRR performance with a high FENH3 of 17.21% at −0.2 V vs. RHE −1 at −0.2 V vs. RHE in 0.1 and a larger NH3 production rate of 34.4 μg h−1 mgcat. −1 M HCl electrolyte, FENH3 of 22.89% and NH3 generation rate of 57.2 μg h−1 mgcat. −1 −1 in 0.2 M PBS and FENH3 of 19.82% and NH3 yielding rate of 90.1 μg h mgcat. in 0.1 M KOH, respectively. DFT calculation indicated that the N, P-C substrate was regarded as an electronic storage medium that regulated the electronic distribution of Ni2 P/N, P-C. N2 was chemically adsorbed at the Ni site, playing a vital role in inhibiting the adsorption of H and promoting the adsorption and activation of N2 molecules. Guo et al. [82] firstly synthesized the Fe-doped Ni2 P with interconnected porous nanosheets electrocatalyst for electrocatalytic NH3 synthesis. Owing to the Fe atoms doping to optimize the active center and the electron configuration, the obtained Fe0.4 Ni1.6 P2 achieved the best performance with a high FENH3 of 7.92% and a large −1 at –0.3 V vs. RHE and with a long-term NH3 producing rate of 88.51 μg h−1 mgcat. durability.

15.5.3 Iron phosphides Zhu et al. [83] prepared a P-rich FeP2 nanoparticle-reduced graphene oxide hybrid (FeP2 -rGO) electrocatalyst for nitrogen reduction reaction. FeP2 -rGO electrocatalyst −1 at displayed a higher FENH3 of 21.99% and NH3 generating rate of 35.26 μg h−1 mgcat. −1 ) under ambient −0.40 V vs. RHE than FeP-rGO hybrid (8.57%; 17.13 μg h−1 mgcat. conditions, which was derived from the decreased HER catalytic activity, higher N2 adsorption energy and a larger number of active sites than FeP. Luo et al. [84] prepared Mo-doped iron phosphide (Mo-FeP) nanospheres via thermal-assisted phosphorization for electrochemical conversion of N2 to NH3 at ambient conditions using FeZIF precursor reacted with Na2 MoO4 to form Fe2 (MoO4 )3 -Fe(OH)3 intermediates as shown in Fig. 15.10A. The prepared Mo-FeP still maintained the nanosphere morphology and 3D porous structure. As shown in Fig. 15.10B–D, due to the high

355

356

Room

NH2

H2O Na2MoO2

85°C

12

4

8

3

FE (%)

N2

6

Faradaic efficiency

5 Fe-ZIF

PVP

7

NH4+ yield

16

Temperature

2-MIM

20

2 4

NaH2PO2 350°C-Ar Mo-FeP

(C)

Mo-FeP FeP

6.0 FE (%)

0

1

Fe2(MoO4)3-Fe(OH)3

(D)

7.5

1 2

3 Cycle number

4

5

0

14 Mo-FeP FeP

12 10

4.5

8

3.0

6 4

1.5

2 0.0 −0.6

−0.5 −0.4 −0.3 Potential (V vs. RHE)

−0.2

0

−0.6

−0.5 −0.4 −0.3 Potential (V vs. RHE)

−0.2

FIGURE 15.10 (A) Synthetic route of Mo-FeP nanosphere. (B) Cyclic test of Mo-FeP at −0.3 V vs. RHE. Tested NH3 yields (C) and FEs (D) of Mo-FeP and FeP at different potential. Published with permission from Ref. [84]. Copyright 2020, American Chemical Society.

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

(B)

(A)

15.6 Transition metal carbides-based electrocatalysts

surface area, synergistic effect and rich exposed active sites, Mo-doped FeP deposited on carbon paper (Mo-FeP/CP) displayed a high-performance NRR activity with a −1 at −0.3 V vs. RHE) associated with larger NH3 production rate (13.1 μg h−1 mgcat. −1 ; 1.7 %) FENH3 (7.49%) at −0.2 V vs. RHE than the pristine FeP/CP (5.0 μg h−1 mgcat. in 0.1 M HCl solution. Meanwhile, Mo-FeP/CP electrode showed excellent selectivity as well as remarkable structural stability under long term electrolysis. The Mo dopant to FeP could activate N2 molecules and improve the adjacent Fe sites to stabilize the ∗ NNH for electrochemical reduction of N2 into NH3 .

15.6 Transition metal carbides-based electrocatalysts 15.6.1 Mxene-based electrocatalysts Transition metal carbides (TMCs) possess unoccupied d orbitals and can show similar adsorption behaviors for electron-enriched adsorbents in the theory of d orbital. When transition metal is integrated with carbon to form transition metal carbides, its sp hybridized state will transfer to the surface center to hybridize with its d state as well as with the S state from carbon, causing excess occupied orbitals, which could provide more electrons to the π orbitals of N2 . Two dimensional layered transition metal carbides (Mxene) are promising electrocatalysts for N2 capture and reduction. Due to the decrease in activation barrier, Spontaneous N2 adsorption on Mxene nanosheets can induce the extension of N≡N triple bond and promote the catalytic conversion of N2 to NH3 . Shao et al. [85] employed a first-principles study for electrochemical N2 fixation based on Mxene. The calculation results indicated that Mo2 C and W2 C are effective and promising for N2 -fixation due to their exothermic reactions and relatively low reaction energy in the endothermic steps. Luo et al. [86] studied the electrocatalytic N2 fixation performance of the Mxene (Ti3 C2 Tx ) nanosheets by combining experiment and DFT calculation. Due to abundant exposed edge sites and a metal host with poor HER activity, Ti3 C2 Tx nanosheets achieved a FENH3 of 5.78% at −0.20 V vs. RHE and a large NH3 production rate of 0.53 μg h−1 cm−2 at −0.50 V vs. RHE with great durability under aqueous solution. Zhao et al. [87] synthesized Ti3 C2 Tx (T= F, OH) Mxene nanosheets for electrochemical N2 fixation under ambient conditions. Due to N2 chemisorbed on Ti3 C2 Tx experiences elongation/weakness of the N-N triple bond in the rate-limiting step of NRR, the electrocatalyst performed a −1 at −0.40 V high FENH3 of 9.3% and a robust NH3 producing rate of 20.4 μg h−1 mgcat. vs. RHE with excellent electrochemical and structural stability under 0.1 M HCl electrolyte.

15.6.2 Molybdenum carbides-based electrocatalysts Cheng et al. [88] designed molybdenum carbide nanodots embedded into ultrathin carbon nanosheets (Mo2 C/C) electrocatalyst via NaCl salt template strategy for efficient N2 -to-NH3 conversion. Due to the abundant nitrogen adsorption active sites and

357

358

CHAPTER 15 Electrochemical NRR with noble metals-free catalysts

the unique electronic structure, the prepared Mo2 C/C on a hydrophilic carbon cloth −1 exhibited a high FENH3 of 7.8% and a large NH3 yielding rate of 11.3 μg h−1 mgcat. at –0.30 V vs. RHE with long-term durability under 0.5 M Li2 SO4 aqueous solution; while, changing the hydrophilic carbon cloth to hydrophobic counterpart, great proton restraint did not benefit the NRR process since the appropriate proton donor was necessary and attributed to the high proton affinity of Mo2 C/C. Ren et al. [89] synthesized Mo2 C nanorod for efficient electrocatalytic conversion of N2 to NH3 under ambient conditions. Owing to enhanced bonding interactions with the anchored Mo atom, the electrocatalyst displayed a high FENH3 of 8.13% and a superior NH3 generating rate of −1 at –0.30 V vs. RHE with excellent long-term cyclic stability under 95.1 μg h−1 mgcat. 1 M HCl aqueous solution.

15.7 Metal-free electrocatalysts 15.7.1 Boron-doped carbon As a promising alternative to metal based electrocatalysts, heteroatom-doped carbons have attracted great interest due to their economic feasibility, environmentally friendly and noncorrosive properties, as well as unique physical and chemical properties. Owing to high conductivity and large specific surface area, carbon based materials have become electrocatalysts with ideal activity and stability. In addition, their catalytic activity can be accelerated by heteroatom doping, which can introduce more defects while adjusting the electronic structure. Specifically, the introduction of heteroatoms (B, N, F, P, and S) with different electronegativity can induce charge transfer in carbon materials, thus promoting the adsorption of reactants on electron deficient sites. Yu et al. [90] designed two-dimensional boron-doped graphene (BG) for electrochemical conversion of N2 to NH3 at ambient conditions. Due to the Bdoping providing electron-deficient boron sites and leading redistribution of electron density, BG at a doping level of 6.2% exhibited great NRR activity with a higher NH3 production rate of 9.8 μg h−1 cm−2 as well as FENH3 of 10.8% at −0.5 V vs. RHE than other B-doping level BG in 0.05 M H2 SO4 aqueous electrolyte. Furthermore, DFT results indicated that the B-doping graphene as a Lewis base provided a strong binding site and boosted the N2 adsorption; moreover, the BC3 structure could lower the free energy barrier in NRR process. Xiao et al. [91] synthesized boron and nitrogen dual-doped carbon nanospheres (BNC-NS) catalyst via hydrothermal method and carbonization afterward for electrochemical conversion of N2 to NH3 at ambient conditions. Due to the introduction of the secondary boron heteroatom inhibiting the HER and promoting N2 adsorption, BNC-NS electrocatalyst achieved −1 and a great NRR activity with a superior NH3 production rate of 15.7 μgNH3 h−1 mgcat. remarkable FENH3 of 8.1% at −0.4 V vs. RHE in 0.05 M H2 SO4 electrolyte shown in Fig. 15.11A and B. Meanwhile, stable electrocatalyst activity and excellent selectivity were also achieved for the BNC-NS electrocatalyst. Fig. 15.11C revealed the NMR and UV methods to accurately determine the NH4 + concentration. Furthermore,

15.7 Metal-free electrocatalysts

(A)

20

10 BNC-NS NC-NS

8

15

6

(B) 10 15

8 6

10

10 4

4 5

2 0 12

−300 −350 −400 −450 −500 Potential / V vs. RHE

(C)

Faradaic Efficiency NH3 yield rate

10

0 20

5 2 0

2

4

6 8 Time / h

(D) 14NH + 4

15

10

4

Intensity

8 6

0

10

15NH + 4

5 2 0

15N 2

NMR UV Detection method

0

6.8

as feeding gas

7.1 7.2 6.9 7 Chemical shift / ppm

7.3

FIGURE 15.11 (A) FEs and NH3 yield rates of the NC-NS and BNC-NS electrocatalysts at different potentials. (B) The stability test for BNC-NS catalyst at −0.4 V vs. RHE. (C) FEs and NH3 yield rates according to the NMR and UV detection methods. (D) NMR spectra of (14 NH4 )2 SO4 (black line), (15 NH4 )2 SO4 (blue line) and the NRR product of the use of 15 N2 as a feeding gas (red line). Reproduced with permission from Ref. [91]. Copyright 2020, Royal Society of Chemistry.

the superior NRR catalytic activity was confirmed by isotope test (Fig. 15.11D). Tang et al. [92] designed B, O-dual doped carbon microspheres (B, O-CMS) via a facile hydrothermal method using nitrogen-free raw materials for electrochemical N2 fixation. Due to abundant microporous structure and unique electronic structure as the main active sites of the B doping, B, O-CMS electrocatalyst performed a considerable FENH3 of 5.57% and a high NH3 generating rate of 19.2 μg h−1 cm−2 at −0.25 V vs. RHE with great stability in 0.1 M HCl aqueous solutions. Chang et al. [93] reported a series of boron carbonitride (BCN) via tuning B/N Lewis acid pairs for electrocatalytic conversion of N2 to NH3 in ambient conditions. DFT calculations

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indicated that boron-enriched BCN clearly presented stronger bonding interaction and was relative lower than nitrogen-enriched BCN for the energy of each step. The experimental results of B-enriched BCN nanomesh exhibited a high FENH3 of 9.87% and a robust NH3 producing rate of 41.9 μg h−1 cm−2 at −0.60 V vs. RHE with great durability in 0.05 M Na2 SO4 aqueous solutions.

15.7.2 Nitrogen-doped carbon Hu et al. [94] reported a N-doped porous carbon nanoparticles (NCNPs) from zeolitic imidazolate framework (ZIF) precursors via silica-assisted synthetic method for electrocatalytic synthesis of NH3 . Due to the mesopore-abundant structure with an enlarged specific surface area, NCNPs performed a high FENH3 of 7.42 % at −0.10 V −1 at −0.30 V vs. vs. RHE together with NH3 generation rate of 7.22 μg h−1 mgcat. RHE with excellent selectivity and remarkable structural stability during long-term electrolysis in 0.1 M HCl aqueous electrolyte. Liu et al. [95] synthesized N-doped porous carbon (NPC) via zeolite ZIF pyrolysis as a cost-effective nonmetal electrocatalyst for ammonia synthesis from electrocatalytic N2 reduction under ambient conditions. N content and species were tuned to enhance N2 chemical adsorption and N≡N cleavage. The resultant NPC electrocatalyst for NRR activity was effective for N2 fixation to ammonia with high ammonia producing rate (1.40 mmol g−1 h−1 at −0.9 V vs. RHE, Fig. 15.12A and B) and exhibited outstanding stability and great selectivity. As shown in Fig. 15.12C and D, the temperature-programmed desorption of N2 (N2 -TPD) results indicated that NPC-750 with high N content and N species promoted the N2 adsorption for N2 fixation. DFT calculation results in Fig. 15.12E and F revealed that the high pyridinic and pyrrolic N contents of NPC were conducive to its excellent activity for NRR process. Ren et al. [96] employed different ammonium salts to prepare various N-doped porous carbons with B, F, P, and S as the secondary heteroatoms doping for electrocatalytic ammonia synthesis. Originating from the promotion effect of another element to N-doped carbon for the electrocatalytic NRR, N, B co-doping of porous carbon performed the highest FENH3 of 10.58% together with NH3 generating rate of 16.4 μg h−1 cm−2 at −0.20 V vs. RHE with excellent long-term durability. Due to the B doping, N-doped porous carbon presented higher content of pyridinic-N and promoted the formation of B-N bonds to lower energy barrier for NRR and modified the intrinsic spin structure between different types of the doped heteroatoms. Meanwhile, in situ FT-IR findings implied that N, B dual-doped porous carbons followed the associative mechanism during NRR process.

15.7.3 Fluorine-doped carbon Fluorine (F), as the most negative nonmetallic element (3.98), is also adopted in doping engineering strategy. Liu et al. [97] designed F-doped 3D porous carbon

(A)

N2

1.5

Ar

1.0

0.5

0

0

−0.7 V

−0.9 V

0.0

−0.5 V

2.0

NH3 production rate

(B)

1.5

1.2

1.0

0.8

0.5 0.0

−1.1 V

0.4

0 1 2 3 4 5 6 7 8 9 10

0.0

Cycle number

(D)

(C)

1.6

Current efficiency

Current efficiency (%)

2.0

Production rate (mmol g−1 h−1)

Production rate (mmol g−1 h−1)

15.7 Metal-free electrocatalysts

graphite N pyrrolic N pyridinic N

12

NPC-950

N content (at.%)

TCD signal (a.u.)

15

NPC-850

NPC-750

50

100

6 3 0

200

150

9

NPC-750 NPC-850 NPC-950

NPC-1

Temperature (°C)

(F) U = −0.7 V

+ e) + e-

+2

2 -NH 3 H+

*NH

2 -NH 2

*NH

+ e)

(H +

+ e) +3 (H + 2

+4

(H + *NH -NH

+ e)

*NH =NH

(H +

6(H +

+5 NH

0

*N=

1

+ e)

2

+ e)

3

−1

−1

U=0V

*N=− H+

2NH3

Free energy (eV)

+ e) (H +

*NH

2

+N

H H+ 3

+ e) (H +

2 -NH 2+2

*NH

+ e)

*NH

-NH

2

+3

+ e) (H +

+4 (H + *NH =NH

+ e) 6(H +

+ e) (H +

+5 NH *N=

0

*N=− H+

1

+ e-

U = −0.7 V

3 2

4

U=0V

N + 2 6

Free energy (eV)

4

N + 2 6 (H +

(E)

2NH3

−2

−2 Reaction coordinate

Reaction coordinate

FIGURE 15.12 (A) Ammonia production rates of NPC-750 at −0.5 to −1.1 V vs. RHE. (B) Corresponding NH3 yield and FE of NPC-750 with 10 cycles at −0.9 V vs. RHE. (C) N2 -TPD profiles of NPCs. (D) Contents of pyridinic, pyrrolic, and graphitic N in NPCs. Free energy diagram for ammonia synthesis on NPC with (E) pyridinic and (F) pyrrolic N. Published with permission from Ref. [95]. Copyright 2018, American Chemical Society.

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frameworks (F-doped carbon) electrocatalyst for electrochemical N2 -to-NH3 conversion by pyrolyzing the mixture of UiO-66 and poly(tetrafluoroethylene). The obtained F-doped carbon displayed an ultra-high FENH3 of 54.8 % at −0.20 V vs. RHE and a comparatively remarkable NH3 production rate of 197.7 μg h−1 mgcat −1 at −0.30 V vs. RHE in 0.1 M HCl aqueous electrolyte, following with great structural stability under cycling electrolysis. Due to F doping provided more Lewis acid site, the competing HER activity was more inhibited and nitrogen activation reaction was promoted with relative to pristine carbon, resulting in enhancing selectivity of the electro-reduction of N2 into NH3 . Zhang et al. [98] rationally designed boron, nitrogen and fluorine ternary-doped carbon (BNFC) via a facile carbonization of cigarette butts saturated with the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate. Due to the B, N, F heteroatom synergistic doping, BNFC-800 performed an excellent NRR activity −1 and a high FEs of 14% at −0.4 V with an utmost NH3 yield rate of 41 μgNH3 h−1 mgcat. vs. RHE in 0.05 M H2 SO4 solution. Furthermore, it also exhibited an excellent NH3 production (39 μgNH3 h−1 mgcat .−1 ) and a large FEs (16%) at −0.4 V vs. RHE with outstanding selectivity and long-term stability in 0.1 M NaOH solution. The boosted N2 chemisorption was derived from heteroatoms-doping into the carbon and the stronger binding ability of N2 on the BNFC-800 surface. Nitrogen atoms in BNFC were recognized for adsorption of hydrogen ions and the neighboring B or F heteroatoms offer Lewis acid sites for chemisorption of N2 . DFT calculation indicated that synergetic effect of B, N and F ternary doping improved the N2 -binding capability and the energy barrier for the limiting step (N2 ∗ →NNH∗ ) was lowered.

15.7.4 Sulfur-doped carbon Xia et al. [99] prepared sulfur-doped graphene (S-G) for electrochemical conversion of N2 to NH3 . Due to carbon atoms close to the substituted sulfur atoms as the more active sites, the prepared S-G deposited on carbon paper (S-G/CP) achieved a greater −1 at −0.6 V vs. RHE and a higher FENH3 of NH3 producing rate of 27.3 μg h−1 mgcat. −1 ; 0.52%) in 11.5% at −0.5 V vs. RHE than the nondoped G/CP (6.25 μg h−1 mgcat. 0.1 M HCl solution. Chen et al. [100] synthesized a sulfur dots-graphene (SDG) metal-free electrocatalyst for NH3 synthesis. Due to the lower charge transfer resistance for NRR kinetics, the synthetic SDG loaded on carbon paper (SDG/CP) −1 and a remarkable exhibited a superior NH3 yielding rate of 28.56 μg h−1 mgcat. FENH3 of 7.07% with long-term durability in 0.5 M LiClO4 solution. Wang et al. [101] designed sulfur-doped three-dimensional graphene (S-3DG) for electrocatalytic conversion of N2 to NH3 . Due to the high electron transfer capacity and stable physicochemical properties of 3DG, S-3DG electrocatalyst achieved a high FENH3 of 7.72% and a large NH3 production rate of 38.81 μg h−1 cm−2 at −0.60 V vs. RHE with great long-term durability. Chu et al. [102] adopted S dopants into graphitic carbon nitride (C3 N4 ) to fill the nitrogen vacancies to obtain the S-NV-C3 N4 for electrochemical N2 fixation. Owing to the filled S dopants performing more optimized

15.8 Conclusion

adsorption of NRR intermediates and a significantly reduced energy barrier, S-NVC3 N4 presented a high FENH3 of 14.1% and a large NH3 producing rate of 32.7 μg h−1 cm−2 at −0.40 V vs. RHE with exceptional durability for 20 hours.

15.7.5 Black phosphorus Black phosphorus (BP) is a type of layered crystal with monolayers stacked together through interlayer van der Waals interactions. Moreover, BP has many characteristics in favor of nitrogen fixation due to anisotropic lattice structure and weak H absorption to restrain the competing HER process. Zhu et al. [103] prepared boron phosphide nanoparticles via a vacuum-seal method for electrochemical conversion of N2 to NH3 . The obtained BP nanoparticles deposited on carbon paper (BP/CP) exhibited −1 and a superior FENH3 of a remarkable NH3 production rate of 26.42 μg h−1 mgcat. 12.7% with great structural stability at −0.60 V vs. RHE in 0.1 M HCl solution. Moreover, DFT results indicated that P on BP surface further activated the N≡N bond and boosted the B-N bond to lower the free energy barrier resulting in more exposed active sites N2 -to-NH3 fixation process. Zhang et al. [104] acquired few-layer orthorhombic black phosphorus nanosheets (FL-BP NSs) metal-free electrocatalyst for electrocatalytic NH3 synthesis via a facile liquid exfoliating method. Due to the rich nitrogen-adsorption active sites and facilitated chemisorption of N2 molecules, FL-BP NSs loaded on carbon fiber (FL-BP NSs/CF) displayed a remarkable NH3 −1 production rate (31.37 μg h−1 mgcat. ) and a superior FENH3 (5.07 %) at −0.60 V vs. RHE with superior long-term cyclic stability in 0.01 M HCl aqueous solution. DFT result explained that the electron redistribution on the FL-BP NSs surface activated molecular orbitals of the catalyst and boosted the N2 adsorption and N≡N cleavage for electrochemical N2 -to-NH3 conversion.

15.8 Conclusion It is of great significance for social development to carry out ammonia synthesis route under ambient temperature and pressure. It has great development potential to employ sustainable energy to actualize electrochemical synthesis of ammonia, once the breakthrough will become a milestone in the field of energy technology innovation. At the moment, some progress has been made in the field of electrocatalytic ammonia synthesis, but the overall efficiency is still insufficient. The main reason is lack of efficient electrocatalysts. The development of NRR electrocatalysts with high Faraday efficiency, high catalytic activity, excellent selectivity and stability is the key to achieve efficient electrocatalytic ammonia synthesis. The design strategy of electrocatalyst mainly focuses on two aspects: improving surface activity and promoting intrinsic activity. Reasonable control of catalyst size, morphology and exposed crystal surface can effectively increase the number of active sites and improve the utilization of active sites to boost the NRR performance. The intrinsic activity of the electrocatalyst depends on the electronic structure of the material. Defect

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engineering including vacancy and heteroatom doping and stress engineering are common strategies to promote the intrinsic activity due to the modified electronic configuration of the electrocatalyst, resulting in the change of the atomic arrangement, which adjusts the adsorption strength of the material for NRR intermediates and optimizes the surface charge transfer performance to enhance the intrinsic activity of the electrocatalytic active site. Therefore, the development of electrocatalysts suitable for NRR via those strategies is of great significance to achieve electrochemical ammonia fixation under mild conditions.

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Nanomaterials for Electrochemical CO2 reduction reaction

4

CHAPTER

Nanomaterials for electrochemical reduction of CO2: An introduction

16

Anuj Kumar a, Ghulam Yasin b and Tuan Anh Nguyen c a

Department of Chemistry, GLA University, Mathura, Uttar Pradesh, India, b Institute for Advanced Study, College of Physics and Optoelectronic Engineering, Shenzhen University, Shenzhen, Guangdong, China, c Institute for Tropical Technology, Vietnam Academy of Science and Technology, Hanoi, Vietnam

The development of renewable, green, safe, and sustainable energy conversion technologies are essential for replacement of our strong reliance on fossil fuels, which create serious environmental issues including climate change and rise in sea levels due to greater tendency of melting of glaciers, etc. [1,2]. Electrocatalytic CO2 reduction reaction [3], water electrolysis [4], fuel cells (FCs) [5], and metal-air batteries (MABs) [6] can be considered as the advanced energy conversion systems to fulfill the demand of efficient, green, renewable, and sustainable energy systems. However, such next-generation energy devices demand deep understanding of the basic principles, ideas, and electrocatalytic mechanisms, which involve H2 /O2 /CO2 electrocatalytic reactions such as CO2 reduction reaction (CO2 RR), oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER) [7]. Particularly, the rise of CO2 concentration from 270 to 400 ppm amounting to an increase of near about 45% during the last century and if suitable inhibitory actions are not taken, is likely to raise upto 570 ppm by the end of the 21st century. Global warming is the result of high concentration of CO2 ; therefore, successful electrochemical conversion of CO2 to usable energy rich molecules like methanol, ethanol, formic acid, etc. will bring the end of the dangerous carbon cycle. In this context, fixation of CO2 into organic fuels will definitely cut-off the concentration of atmospheric CO2 , thereby, minimizing “green-house effect” with stabilization of sustainability of the energy cycle. A few developments related to transformation of CO2 into usable carbon material have been made through the electrochemical, enzymatic conversion, chemical reforming, photocatalysis, mineralization, etc. [8, 9]. Among them, electrochemical CO2 RR generates CO as the main product having useful industrial applications in Monsanto, hydroformylation, and Fischer–Tropsch catalytic processes. In addition, various products can be formed via the transfer of nH+ /ne- pairs to C/O-atoms of CO2 [10]. Usually, at neutral pH and −0.53 V vs. SHE, the 2e- CO2 RR pathway results the formation of ∗ COOH as main intermediate Nanomaterials for Electrocatalysis. DOI: https://doi.org/10.1016/B978-0-323-85710-9.00001-0 Copyright © 2022 Elsevier Inc. All rights reserved.

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during the capturing of H+ /e− pair by CO2 and then ∗ CO and water resulted and eventually ∗ CO desorption is the final step [11]. However, CO2 being chemically inert compound due to strong C=O bond, and its electroreduction is awfully sluggish as supported by high anodic potential (−1.90 V vs. NHE) because of large reorganization energy of linear CO2 molecule [12]. Whereas, other nH+ /ne− steps need lower energy near at pH-7.0, forming energy-rich molecules like CO, CH3 OH, HCOOH, CH4 , C2 H4 , etc. but still these steps need to be initiated using suitable electrocatalysts. Hence, overall, the design strategy involving CO2 RR depends on the products desired at the end. Therefore, the fabrication of highly active novel CO2 RR electrocatalyst with significant durability is most demanding for the development of CO2 RR-based technologies. In this regard, nanomaterials can be considered as the most potential contender due to their exceptional mechanical, optical, electronic, and redox properties [13]. Nanomaterials have observed tremendous development over the past decade. Nanomaterials with unique properties and large surface area generated many path breaking discoveries in the field of catalysis as well as electrocatalysis [14]. However, there is a need to formulate facile strategies to impose the nanomaterial’s structures in multidimension comprising the various nano-objects with tunable shapes and sizes like quantum dots, nanoparticles particles, nanorods, nanowires, nanosheets, etc. Based on the dimensions, nanomaterials can be classified into four categories, namely, zero dimensional (0D), one dimensional (1D), two dimensional (2D), and three dimensional (3D), having own significant property and advantages in terms of their dimensions [15]. Nanomaterials possessing spherical or almost spherical shape are 0D materials, generally known as nanoparticle (NPs). A number of reports are available on modified NPs, having various exceptional electrocatalytic activities, different from spherical NPs. On the other hand, nanomaterials, having rod-like morphology can be considered as 1D NPs with length at least three times longer than their diameter. 2D nanomaterials on the other hand possess sheet-like morphology with the horizontal size more than 100 nm and thickness less than 5 nm. Whereas, 3D is an integrated nanoarchitecture consisting of 0D/1D/2D nano-objects aligned in a controlled ordered fashion [16, 17]. The fundamental knowledge of tuning the size, shape, and composition of nanomaterials in material science can open numerous practical applications in the field of energy conversion and storage technologies [18]. To utilize high mass activity to advantages, it is necessary to make materials at nanoscale to improve surface-tovolume ratio to impart greater active sites to get involved in catalytic reactions. For example, nanosized particles can offer surface area 1000 times higher than microsized particles, catalytic performance of NPs depends on surface area of facets on the crystal, although size reduction brings adverse effects like increased charge transfer resistance. However, it will not be wise to conclude reasonably about the relation between physical characteristic and performance of nanomaterial [19, 20]. In recent years, nanomaterials have been successfully utilized to fabricate the electrode with outstanding performance toward electrolytic CO2 RR, playing the dual

References

role of electron transfer booster as well as to minimize the energy barriers during electrocatalytic reactions [21]. To date, a number of nanomaterials consisting of metals/metal oxides, alloys, carbides, and MOFs have been utilized as CO2 RR electrocatalysts. Usually, Au, Pt, Ni, and Ag-based nanomaterials showed significantly CO2 RR activity, but less abundance and high price of these meals further limited their applications at large scale [22]. In addition, several metal oxides-based nanomaterials such as TiO2 , RuO2 , and IrO2 NPs have also been found to be potential CO2 RR electrocatalysts [23, 24]. However, these oxides-based nanomaterials showed low efficiency, high cost, poor stability, and low current exchange density toward CO2 RR electrocatalysis. Therefore, the fabrication of low cost, highly active, and durable CO2 RR electrocatalyst is still an obstacle to be solved for the development CO2 RRbased technologies.

References [1] A.M. Omer, Energy, environment and sustainable development, Renewable Sustainable Energy Rev 12 (2008) 2265–2300. [2] A. Qazi, F. Hussain, N.A. Rahim, G. Hardaker, D. Alghazzawi, K. Shaban, K. Haruna, Towards sustainable energy: a systematic review of renewable energy sources, technologies, and public opinions, IEEE Access 7 (2019) 63837–63851. [3] W. Zhang, Y. Hu, L. Ma, G. Zhu, Y. Wang, X. Xue, R. Chen, S. Yang, Z. Jin, Progress and perspective of electrocatalytic CO2 reduction for renewable carbonaceous fuels and chemicals, Adv Sci 5 (2018) 1700275. [4] J. Chi, H. Yu, Water electrolysis based on renewable energy for hydrogen production, Chin J Catal 39 (2018) 390–394. [5] A. Kumar, V.K. Vashistha, D.K. Das, Recent development on metal phthalocyanines based materials for energy conversion and storage applications, Coord Chem Rev (2020) 213678. [6] D. Yu, A. Kumar, T.A. Nguyen, M.T. Nazir, G. Yasin, High-voltage and ultrastable aqueous zinc–iodine battery enabled by N-doped carbon materials: revealing the contributions of nitrogen configurations, ACS Sustainable Chem Eng 8 (2020) 13769–13776. [7] Z. Pu, T. Liu, I.S. Amiinu, R. Cheng, P. Wang, C. Zhang, P. Ji, W. Hu, J. Liu, S. Mu, Transition-metal phosphides: activity origin, energy-related electrocatalysis applications, and synthetic strategies, Adv Funct Mater 30 (2020) 2004009. [8] Y. Zhong, S. Wang, M. Li, J. Ma, S. Song, A. Kumar, H. Duan, Y. Kuang, X. Sun, Rational design of copper-based electrocatalysts and electrochemical systems for CO2 reduction: from active sites engineering to mass transfer dynamics, Mater Today Phys (2021) 100354. [9] T. Gao, A. Kumar, Z. Shang, X. Duan, H. Wang, S. Wang, S. Ji, D. Yan, L. Luo, W. Liu, Promoting electrochemical conversion of CO2 to formate with rich oxygen vacancies in nanoporous Tin oxides, Chin Chem Lett 30 (2019) 2274–2278. [10] M. Koper, B.R. Cuenya, Electrochemical CO2 reduction, Res Needs 19 (2019) 19–27. [11] M. Aresta, A. Dibenedetto, E. Quaranta, State of the art and perspectives in catalytic processes for CO2 conversion into chemicals and fuels: the distinctive contribution of chemical catalysis and biotechnology, J Catal 343 (2016) 2–45.

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[12] S. Shyshkanov, CO2 chemical transformations using metal-organic frameworks, EPFL (2020) 1–145. [13] H.-J. Zhu, M. Lu, Y.-R. Wang, S.-J. Yao, M. Zhang, Y.-H. Kan, J. Liu, Y. Chen, S.-L. Li, Y.-Q. Lan, Efficient electron transmission in covalent organic framework nanosheets for highly active electrocatalytic carbon dioxide reduction, Nat Commun 11 (2020) 1–10. [14] R. Paul, Q. Dai, C. Hu, L. Dai, Ten years of carbon-based metal-free electrocatalysts, Carbon Energy 1 (2019) 19–31. [15] J.N. Tiwari, R.N. Tiwari, K.S. Kim, Zero-dimensional, one-dimensional, twodimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices, Progr Mater Sci 57 (2012) 724–803. [16] X. Wang, Z. Cheng, G. Zhang, B. Wang, X.-L. Wang, H. Chen, Rich novel zerodimensional (0D), 1D, and 2D topological elements predicted in the P 6 3/m type ternary boride HfIr 3 B 4, Nanoscale 12 (2020) 8314–8319. [17] X.-Y. Duan, M.-L. Wei, From one-dimensional, two-dimensional to three-dimensional entangled architectures with polythreading feature: synthesis, structures, and properties, Crystal Growth Des 17 (2017) 1197–1207. [18] E. Pomerantseva, F. Bonaccorso, X. Feng, Y. Cui, Y. Gogotsi, Energy storage: the future enabled by nanomaterials, Science 4 (2019) 366. [19] V. Mlinar, Engineered nanomaterials for solar energy conversion, Nanotechnology 24 (4) (2013) 042001. [20] Y. Zhu, L. Peng, Z. Fang, C. Yan, X. Zhang, G. Yu, Structural engineering of 2D nanomaterials for energy storage and catalysis, Adv Mater 30 (2018) 1706347. [21] W. Li, X. Zhan, X. Song, S. Si, R. Chen, J. Liu, Z. Wang, J. He, X. Xiao, A review of recent applications of ion beam techniques on nanomaterial surface modification: design of nanostructures and energy harvesting, Small 15 (2019) 1901820. [22] H. Zhang, X. Liu, Y. Wu, C. Guan, A.K. Cheetham, J. Wang, MOF-derived nanohybrids for electrocatalysis and energy storage: current status and perspectives, Chem Commun 54 (2018) 5268–5288. [23] A. Liu, X. Liang, X. Ren, W. Guan, M. Gao, Y. Yang, Q. Yang, L. Gao, Y. Li, T. Ma, Recent progress in MXene-based materials: potential high-performance electrocatalysts, Adv Funct Mater 30 (2020) 2003437. [24] A. Dutta, F. Bizzotto, J. Quinson, A. Zana, C.E. Morstein, M.A. Rahaman, A.C. López, M. Arenz, P. Broekmann, Catalyst development for water/CO2 co-electrolysis, CHIMIA Int J Chem 73 (2019) 707–713.

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

A

E

Active site assembly, 323 Alkali, treatment, 30 Ammonia, 145, 273 Au-based nanocatalysts, 150, 158 AuPdP nanowire catalysts, 158

Electrical active surface area (ECSA), 52 Electrocatalysis, 113 applications in, 31 Electrocatalysts, 49 application of, 373 graphene-based, 239f Electrocatalytic application for hydrogen production, 55 overall water splitting, 317 redox reactions, 111 Electrochemical cell, 212 conversion systems, 49 deposition method, 256 devices, 169 etching, 31 methanol oxidation reaction, 169 methods, 211 water dissociation process, 114 water splitting, 83, 323 Electrolyzers, 195, 113f Electron energy loss spectroscopy (EELS), 194 Electronic information, 195 Electronic structure effect, 329 Electrospinning and pyrolysis technology, 173 Energy dispersive X-ray spectroscopy (EDX), 195 Energy-related catalytic reactions, 212

B B-based NRR catalysts, 179 B-doped graphene electrocatalyst, 122 Bismuth-based materials, 175 Black phosphorus (BP), 363 usage of, 238 Boron-doped carbon, 358 Bottom-up approaches, 50

C Carbon materials, 326, 341 nanomaterials, 52 nanotubes, 338 substrate-assisted TM nanoparticle-based catalysts, 259 Carved hollow structure, 323 Catalyst nanostructure adjusting, 152 Cathodic reaction, 335 Chemical reduction method, 255 Chemical vapor deposition (CVD), 88, 117, 27 method, 31 Chromium oxides, 277 Co-coordinated framework porphyrin, 197 CO2 reduction reaction, 40, 373 Covalent triazine frameworks, 181 Cyclic voltammetry (CV), 256

D D-electron orbitals, 150 Density functional theory (DFT) calculations, 56, 169, 274 2D hybrid electrocatalysts graphene-based electrocatalysts, 238 graphene-metallic composites, 240 graphene nonmetallic composites, 238 Dimethylformamide (DMF), 254 Direct pathway, 182 Doping, usage, 99 3D porous nanostructures, 274

F Faradic efficiency, 86 Fe-based catalysts, 171 Fe-Mo bimetallic catalyst, 171 Fenton reactions, 175 Fe-P active sites, 319 Fluorine-doped carbon, 360 Fourier transform (FT), 194 Fuel cells (FC), 113 role, 35

G Gas-involving electrocatalytic reactions, 111 Gibb’s free energy, 115 Glassy carbon electrode (GCE), 256 Graphene, 113, 114 based electrocatalysts, 238 based materials, 113

377

378

Index

metallic composites, 240 nonmetallic composites, 238

H Haber–Bosch method, 145, 211, 217 Heteroatom-doped graphene, 121 based materials, 114, 117, 118, 119 HF etching, 27 High resolution scanning transmission electron microscopy (HR-STEM), 194 Host–guest interactions, 349 Hydrogen energy, 55 Hydrogen evolution reaction, 39, 170, 113, 57, 83, 237, 238, 251, 317, 373 activity, 342 application of, 265 MOF-based electrocatalysts for, 130 metal carbide, phosphides, and chalcogenides, 133 metal-free carbon-based material, 131 NPM-based electrocatalyst for HER, 132 transition metal chalcogenides-based electrocatalysts for, 98 Hydrogen gas, production of, q12f Hydrogen oxidation reaction (HOR), 169, 111, 373 Hydrogen production, 355 Hydrothermal method, advantages of, 253

I Imidazole zeolite frameworks (ZIFs), 177 Iron oxides, 279 phosphides, 355 sulfides, 286

L Layered double hydroxides (LDHs), 49 Lewis acidic etching, 28 Lewis acid site, 362 Linear conjugated polymers (LCP), 241 Linear-D (1-D) nanomaterials, 374 Linear sweep voltammetry (LSV) test, 117 Low-dimensional (L-D) nanomaterials, 374

M Magnetic moment, 171 Manganese oxides, 279 Melaminephytic acid supramolecular aggregate (MPSA), 124 Membrane electrode assembly (MEA), 184 Metal-air batteries (MAB), 113 Metal-based catalysts, 145 Metal-based catalyzing agent, 182

Metal-containing heteroatom-doped carbon nanomaterials, 195 Metal-free electrocatalysts boron-doped carbon, 358 Metallic substrate-assisted TM nanoparticle-based catalysts, 260 Metal organic frameworks (MOF), 39 based electrocatalysts, 130 metal carbide, phosphides, 130 metal-free carbon-based material, 131 NPM-based electrocatalyst for HER, 132 based electrocatalysts for OER, 124 metal-free materials, 124 nonprecious metal-based, 126 based electrocatalysts for ORR, 116 nitrogen-doped carbon-based electrocatalysts, 117 nonprecious metal-based electrocatalysts, 120 based multifunctional electrocatalysts, 136 Metal-organic frameworks (MOFs), 117 catalysts, 169, 170 composites, 343 electrocatalysts, 352 materials, 169, 170, 335 carbon-based composites, 345 hollow structures, 318 materials, 318, 341, 342 hydrogen production, 355 Metal-oxide nanoparticles (MNP), 346 Molybdenum carbides-based electrocatalysts, 362 Molybdenum sulfides, 286 Mössbauer spectroscopy, 185 Multielectron process, 181 Multishelled hollow structures, 319 Multishelled nanoarchitectures, 319 MXene-based nanomaterials applications in electrocatalysis, 31 CO2 reduction reaction, 40 hydrogen evolution reaction, 39 oxygen evolution reaction, 35 oxygen reduction reaction, 35 electronic properties, 25 engineering of, 27 chemical vapor deposition method, 31 electrochemical etching, 31 HF etching, 27 Lewis acidic etching, 28 treatment with alkali, 30 water-free etching, 30 properties of, 32 structural properties, 24

Index

N Nanomaterials, 374 2D, 374 3D, 374 Nanoparticles (NP) based catalysts, 181 based materials, 182 doped carbide, 181 doped carbon nanotubes, 342 doped graphene, 240, 320 spherical-shaped, 373 Nanostructures characterization techniques for, 5 construction and characterization of, 4 electrocatalysis enabled by, 4 2D nanostructures, 5 3D nanostructures, 6 low-dimensional nanostructures, 4 N-doped porous carbon nanoparticles (NCNPs), 360 Ni-based materials, 177 Nickel-based oxides, 280 Nickel-MOF composite, 345 Nickel phosphides, 291 NiCo-LDH-nanoarrays, 54f NiCo–LDH nanosheets, 50 Niobium oxides, 283 Nitrogen reduction reaction (NRR), 145, 237, 3 catalytic performance, 170 electrocatalysis, 212 electrolytes for, 213 electrocatalysts, 286 mechanism, 212 Nitrogen vacancies (NV), 181 NNH reaction, 155 Non-metal-based catalysts, 178 Non-noble metal-based metal catalysts, 178 Nonprecious metal (NPM), 117

O Oxygen reduction reaction (ORR), 35, 113, 83, 111, 169, 211, 237, 251, 35, 373 activity, 325 application of, 262 electrocatalysts, 335 MOF based electrocatalysts for, 116, 124 metal-free materials, 124 nitrogen-doped carbon-based electrocatalysts, 117 nonprecious metal-based, 126 nonprecious metal-based electrocatalysts, 120

transition metal chalcogenides-based electrocatalysts for, 89, 92

P PANI-like catalysts, 180 Particle-in-box, 320 PBA materials, 323 P-based catalysts, 184 Pd-based alloys, 158 Pd nanocatalysts, 217 Phosphate buffer solution (PBS), 280 Polycondensed melamine, 181 Polydopamine-assisted confined pyrolysis, 322 Porous organic polymers (POPs), 181 Proton-exchange-membrane fuel cells (PEMFC), 116 Pt-based NRR catalysts, 288

R Raman characterizations, 119 Redox-active molecules, 340 Rotating ring disk electrode (RRDE), 182 Ru-based alloy catalysts, 150 Ru-based NRR catalysts, 147, 149 Ru nanocatalysts, 217 Ru nanoparticles, 217

S Sacrificial template methods (SSM), 178 Saturated calomel electrode (SCE), 256 S-based catalysts, 182 Scanning transmission electron microscope (STEM), 24 “Scotch tape” approach, 113 S-doped graphene, 115 Secondary building units (SBU), 338 Shell structure units, 320 Single-atom catalyst, 326 Single-shelled hollow structures, 319 Single walled carbon nanotubes (SWCNT), 238 Soft templates, 179 Sol–gel method, 256 Solid-phase chemical synthesis, 88 Solvothermal, 87 method, 254, 255f Substrate-free TM nanoparticle-based catalysts, 257 Sulfur-doped carbon, 362 Synergistic catalysis, 327

T Tafel plot, 86 Temperature-programmed desorption, 181

379

380

Index

Tetracyanoquinodimethane (TCNQ), 340 Tetramethylammonium hydroxide (TMAOH), 27 Ti-based catalysts, 172 Titanium-based catalysts, 175 Titanium oxides, 274 Top-down approach, 50 Transition metal nanoparticles, 169, 290 applications, 262 chemical reduction method, 254 electrocatalysts, 286 electrochemical deposition method, 255 hydrothermal method, 252 metal oxides, 283 nanoparticle-based catalysts, 252 nitrides, 290 solvothermal method, 253 titanium oxides, 274 structure, 257 metallic substrate-assisted, 260 substrate-free, 257 types of, 253f Transition metal chalcogenides (TMC), 357, 85 Faradic efficiency, 86 for HER, 98 multifunctional, 103 overpotential, 86

stability, 86 synthesis of, 87 chemical vapor deposition, 88 other methods, 88 solvothermal, 87 Tafel plot, 86 Transmission electron microscopy (TEM), 194

W Water-free etching, 30 Water splitting system, 113 technology, 83

X Xray absorption near-edge structure (XANES), 192 Xray diffraction (XRD), 185 X-ray photoelectron spectroscopy, 189

Z Zeolitic imidazolate framework (ZIF), 118, 360 Zero-D nanomaterials, 374 ZIF-67 carbonization process, 319 ZIF-67 nanocubes, 327 Zn-air battery (ZAB), 317