Electrochemical Hydrogen Production from Water Splitting: Basic, Materials and Progress 9819944678, 9789819944675

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Shengjie Peng

Electrochemical Hydrogen Production from Water Splitting Basic, Materials and Progress

Electrochemical Hydrogen Production from Water Splitting

Shengjie Peng

Electrochemical Hydrogen Production from Water Splitting Basic, Materials and Progress

Shengjie Peng Department of Applied Chemistry Nanjing University of Aeronautics and Astronautics Nanjing, Jiangsu, China

ISBN 978-981-99-4467-5 ISBN 978-981-99-4468-2 (eBook) https://doi.org/10.1007/978-981-99-4468-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

As a clean, efficient, safe, and sustainable new energy source, hydrogen energy is attracting much attention in the process of addressing global climate change and accelerating the low-carbon transition, and related research has become a hot topic in the field of energy science. Nowadays, there is already fierce competition among countries around the world in the commercialization of hydrogen energy vehicles. We believe that hydrogen energy will shine like sunlight in every corner of the world and influence our lives in the future. Electrochemical Hydrogen Production from Water Splitting: Basic, Materials and Progress provides a systematic and comprehensive introduction to the fundamentals of hydrogen energy, hydrogen energy-related technologies and systems, and the environmental and economic impacts of hydrogen energy. Chapters 1 and 2 provide an overview of hydrogen energy and current status of its development, including its distribution, properties, forms, laboratory and industrial methods of hydrogen production (including electrolytic water splitting hydrogen production, thermochemical hydrogen production, fossil energy hydrogen production, solar energy hydrogen production, biomass hydrogen production, wind, ocean, hydroelectric and geothermal energy hydrogen production, nuclear energy hydrogen production, hydrogen-containing carriers hydrogen production, coupled hydrogen production, etc.). Chapters 3–7 deal in detail with hydrogen production from electrolysis of water, including alkaline water electrolysis, proton exchange membranes water electrolysis, anion exchange membranes water electrolysis, solid oxides electrolysis, and electrolysis of hydrogen from seawater, covering such aspects as reaction principles, process equipment, technical characteristics, catalyst development, and the development directions. Chapters 8 and 9 analyze the industrial storage and transport of hydrogen energy and the applications and systems engineering, as well as the challenges in development. This book is rich in content, combining theory with practice and reflecting the latest world achievements in hydrogen energy utilization and research, including a large number of research results published at home and abroad. It can be used by a wide range of scientific technologists engaged in the development and utilization of hydrogen energy and other energy sources. It can also be referred for technical v

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Preface

workers in power engineering, aerospace, transportation, chemistry, chemical engineering, physics, nuclear engineering, refrigeration, metallurgy, and those engaged in safety management, as well as for teachers and students of related disciplines in higher education institutions. The authors sincerely hope that the publication of this book will improve the readers’ understanding of hydrogen energy and make some modest contribution to the development of hydrogen energy by strengthening the hydrogen energy research teams. Nanjing, China

Shengjie Peng

Contents

1 Overview of Hydrogen Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Hydrogen Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 Hydrogen on Earth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.2 Hydrogen in the Universe . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.3 Hydrogen in Life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Properties of Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Physical Properties of Hydrogen . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Chemical Properties of Hydrogen . . . . . . . . . . . . . . . . . . . . . 1.3 Hydrogen Storage Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.1 Gaseous Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Liquid Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Solid Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Laboratory Preparation of Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Industrialized Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5.1 Hydrogen Production from Traditional Fossil Fuels . . . . . . 1.5.2 Hydrogen Production from Chemical Raw Materials . . . . . 1.5.3 Hydrogen Production by Electrolysis of Water . . . . . . . . . . 1.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 3 4 4 5 5 7 7 8 11 12 13 14 15 15 16 17

2 Current Status of Hydrogen Energy Development . . . . . . . . . . . . . . . . . 2.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Electrolysis of Water to Produce Hydrogen . . . . . . . . . . . . . . . . . . . . 2.2.1 Principle of Water Electrolysis . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2 Electrolyzed Water Hydrogen Production Device . . . . . . . . 2.2.3 Electrolyzed Water Performance Evaluation . . . . . . . . . . . . 2.2.4 Catalyst Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Thermochemical Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Two and Three-Stage Thermochemical Hydrogen Production Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

19 19 20 20 23 25 27 28 29

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2.3.2 Multi-stage Thermochemical Cycle for Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Hydrogen Production from Fossil Fuels . . . . . . . . . . . . . . . . . . . . . . . 2.4.1 Coal to Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2 Hydrogen Production from Natural Gas Reforming . . . . . . 2.4.3 Hydrogen Production from Methanol Steam Reforming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Solar Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Solar Thermochemical Hydrogen Production . . . . . . . . . . . 2.5.2 Photoelectrochemical Hydrogen Production . . . . . . . . . . . . 2.5.3 Photocatalytic Hydrogen Production . . . . . . . . . . . . . . . . . . . 2.5.4 Artificial Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.5 Biological Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . 2.6 Biomass Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7 Hydrogen Production from Wind, Ocean, Water and Geothermal Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7.1 Hydrogen Production from Wind Energy . . . . . . . . . . . . . . . 2.7.2 Hydrogen Production from Ocean Energy . . . . . . . . . . . . . . 2.7.3 Hydrogen Production from Hydraulic Energy . . . . . . . . . . . 2.7.4 Hydrogen Production from Geothermal Energy . . . . . . . . . 2.8 Hydrogen Production from Nuclear Energy . . . . . . . . . . . . . . . . . . . 2.9 Hydrogen Production by Hydrogen-Containing Carrier . . . . . . . . . 2.9.1 Reaction of Hydrogen Production from Methanol . . . . . . . . 2.9.2 Catalysts for Hydrogen Production by Steam Reforming of Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.3 Process of Hydrogen Production by Steam Reforming of Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Coupling with Hydrogen Production . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30 31 31 32 32 33 34 35 37 38 39 39 43 43 46 47 48 49 51 52 52 53 54 54

3 Alkaline Water Electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Electrolysis Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Prospects for Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57 58 60 62 66 68

4 Proton Exchange Membrane Water Electrolysis . . . . . . . . . . . . . . . . . . . 4.1 Electrolyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 The Catalysts for OER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Development Direction of OER Catalysts . . . . . . . . . . . . . . 4.2.3 Recent Research Progress of Acid OER . . . . . . . . . . . . . . . . 4.2.4 The Catalysts for HER . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 The Structure of the Proton Exchange Membrane . . . . . . . .

69 69 73 74 77 78 92 92 93

Contents

4.3.2 Influence of Proton Exchange Membrane on the Electrolyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 The Modification of the Proton Exchange Membrane . . . . . 4.4 Development Prospect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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93 94 95 97

5 Anion Exchange Membrane Water Electrolysis . . . . . . . . . . . . . . . . . . . 5.1 Electrolyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Evaluation Metrics for Electrocatalysts . . . . . . . . . . . . . . . . . 5.2.2 HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 OER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Anion Exchange Membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Membrane Electrode Assembly and Electrolyzer Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Current Challenges and Prospects for AEMWE . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

99 99 100 101 103 115 124

6 Solid Oxide Electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Electrolyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Air Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Cathode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Electrolyte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4 Other Constituent Materials of SOEC . . . . . . . . . . . . . . . . . . . . . . . . 6.5 SOEC and Stack Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Material Degradation in SOEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.7 Other Applications Using SOEC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.8 Technical Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

147 149 150 150 151 153 156 156 158 161 162 163

7 Hydrogen Production by Seawater Electrolysis . . . . . . . . . . . . . . . . . . . . 7.1 Chlorine Precipitation from Seawater Electrolysis . . . . . . . . . . . . . . 7.2 Special Electrodes to Mitigate Chlorine Precipitation . . . . . . . . . . . 7.2.1 Nickel-Based Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Other Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3 Hydrogen Production Equipment by Seawater Electrolysis . . . . . . 7.4 Difference Between Seawater and Freshwater Electrolysis . . . . . . . 7.4.1 pH Effects on Seawater and Freshwater Electrolysis . . . . . . 7.4.2 Requirements of Electrolytic Seawater Cells . . . . . . . . . . . . 7.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Current Status and Development Direction of Seawater Electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6.1 Current Status of Seawater Electrolysis Research . . . . . . . . 7.6.2 Development Direction of Seawater Electrolysis . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

167 167 171 172 174 177 177 187 189 190 193

130 139 140

194 194 199 200

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8 Storage and Application of Hydrogen Energy . . . . . . . . . . . . . . . . . . . . . 8.1 Storage and Transport of Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.1 High-Pressure Gas Cylinder . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.2 Liquid Hydrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.3 Comparison of Pressure Storage and Liquid Storage . . . . . 8.1.4 Hydrogen Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.5 Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1.6 Metal Hydrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Hydrogen Fuel Cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Brief History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.4 Types of Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Hydrogen Refueling Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 Hydrogen Refueling Station Process . . . . . . . . . . . . . . . . . . . 8.3.2 HRS Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Layout of Future Hydrogen Refueling Stations . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

203 203 204 207 209 210 214 216 218 218 219 220 221 224 225 226 228 232 233

9 The Challenge and Prospect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237

About the Author

Prof. Shengjie Peng (FRSC) received his Ph.D. degree in Nankai University (P. R. China) in 2010. Following a postdoctoral fellowship with Prof. Alex Yan and Prof. Seeram Ramakrishna in Nanyang Technological University and National University of Singapore, he is now working as a professor in Nanjing University of Aeronautics and Astronautics. His current research interests focus on development of rationally designed functional materials with finely tailored nanoscale architecture to tackle critical problems (such as energy density, power density, cycle and calendar life, safety, and cost) in diverse energy-related applications, including ORR, water splitting, batteries, fuel cells, as well as clean and renewable energy. To date, he has co-authored 170 peer-reviewed publications with over 12,000 citations and 59 H-index.

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Chapter 1

Overview of Hydrogen Energy

1.1 Hydrogen Distribution Energy is not only the material basis for the survival and development of human society but the key driving force of industrialized society. With the increase in global population, industrialization, and urbanization, energy demand is rising rapidly. At present, 85% of the world’s total energy consumption comes from non-renewable resources, such as coal, natural gas, and oil. Using fossil fuels to meet energy demand is rapidly leading to serious environmental problems worldwide, as well as economic problems and political crises. Key energy issues in the twenty-first century may include energy security, global population, and global warming mainly caused by carbon dioxide emissions. In addition, the supply and utilization of low-cost clean fuels are particularly important for global stability and peace, as energy plays a vital role in industrial and technological development around the world [1]. Exploring the possibility of sustainable energy is an effective means to solve the problem. Hydrogen energy is an important means to realize the green transformation of energy structure, and a vital support to achieve the goal of “carbon neutrality and carbon reduction”. Hydrogen is abundant and the most available renewable energy source. Hydrogen exists in nature as a chemical substance, and its molecular formula is H2 , which is not easy to obtain. It is usually in the form of a compound called hydride, which is negative or anionic, denoted as (H− ). Because the combustion of hydrogen only produces water vapor, it is considered the cleanest energy source. The direct production of industrial hydrogen comes from the steam reforming of hydrocarbons. In addition, other technologies include electrolysis and pyrolysis. If hydrogen is produced from renewable resources, it can be considered an appropriate solution to environmental problems. The advantages of hydrogen are mainly shown that the greenhouse gas emissions will be zero if renewable energy is used for production, and the energy density is high, between 120 MJ kg−1 (low calorific value) and 142 MJ kg−1 (high calorific value) [2].

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_1

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1 Overview of Hydrogen Energy

Fig. 1.1 The proportion of hydrogen energy in the world energy consumption terminal structure in 2050 [3]

At present, the world is faced with important problems such as fossil fuel shortage and pollution. Due to the increase in world population, the development of technology, and the improvement of living standards, the world’s energy demand has been increasing. These factors have led to world population transition, migration, hunger, environmental problems, deterioration of health and diseases, terrorism, natural resources problems, and wars. Therefore, it is very important to study the alternative energy strategy for future world stability, and the most important characteristic of alternative energy is its environmental compatibility. According to this characteristic, hydrogen energy is likely to become one of the most attractive energy carriers in the near future. The conversion from global fossil fuels to hydrogen will eliminate many problems and their consequences, and it is an ideal method to produce hydrogen from pollution-free sources. At the same time, many environmental advantages can thrive in the hydrogen economy, and the best ending of conversion to a hydrogen economy is to replace the current fossil fuel with clean hydrogen. Therefore, it can be called hydrogen environmental economy. Figure 1.1 shows the world energy consumption terminal structure in 2050 predicted [3]. At present, scholars at home and abroad have done a lot of research on hydrogen energy, including experimental and theoretical research. However, in order to speed up the process of implementing hydrogen economy, promote the replacement of fossil fuels with clean hydrogen, and alleviate energy and environmental problems, it is necessary to conduct in-depth research on a wider range of hydrogen energy.

1.1.1 Hydrogen on Earth Hydrogen energy refers to the energy released by hydrogen in the process of physical and chemical changes. Hydrogen energy is the chemical energy of hydrogen, which mainly exists in the form of combined state and is the most widely distributed

1.1 Hydrogen Distribution

3

substance in the universe. Hydrogen is widely distributed in nature, and water is the “warehouse” of hydrogen-the mass fraction of hydrogen in water is 11%. Although water exists in the hydrosphere (atmosphere and ocean) mainly in the form of H2 O molecules, in the deeper parts of the earth, the substance commonly called “water” is H in minerals, melts and fluids in the form of various species. Water on the earth rarely exists in the form of H2 O molecules, but it is most abundant in the form of hydrogen dissolved in various minerals inside the earth. A large amount of earth’s water is located in nominally anhydrous minerals, which may be equivalent to more than one mass of the earth’s oceans stored in the mantle in this way [4]. There is about 1.5% hydrogen in soil; Oil, natural gas, animals, and plants also contain hydrogen. In the air, there is not much hydrogen, accounting for about five-ten million of the total volume. In addition to the ubiquitous water in the hydrosphere, most of the “water” on the earth actually exists in the rock silicate minerals that make up the earth’s crust and mantle in the form of trace hydrogen and may also be stored in the metal core. Due to the abundance of hydrogen in the universe and its small size, it may exist in almost all stages from the earth’s atmosphere to the earth’s core. In silicate/carbonate melt or volatile fluid, H can contain hydrogen in the form of water H2 O, hydroxyl OH, hydrogen H2 , methane CH4 , hydrogen sulfide H2 S, and possibly more complicated high-pressure. Under the reducing conditions prevailing in some parts of the upper mantle, water may also exist in olivine, pyroxene, and garnet in the form of H2 molecules. According to the tectonic background characteristics of H2 distribution, it can be found that the content of H2 as high as 20% can also occur in sedimentary basins, while the content of H2 over 50% is mainly distributed in areas where deep faults are developed. First of all, the deep faults in the extensional background are generally channels for the upward migration of deep fluids, so there is a strong magmatic activity in areas where deep faults are developed or areas greatly affected by them, such as rift valley areas. Magmatic activity has strong H2 transport capacity, which can transport a large amount of hydrogen from the earth to the surface. Therefore, the rift basin, which is strongly influenced by magmatism, is the main area where high concentration of H2 will be found in the future [5]. At the same time, affected by the plate compression, the area with oceanic crust residue is also the main target area for the discovery of high-content H2 in the future.

1.1.2 Hydrogen in the Universe According to atomic percentage, hydrogen is the most element in the whole universe. In the atmosphere of the sun, hydrogen accounts for 81.75% by atomic percentage. In space, the number of hydrogen atoms is about 100 times larger than the sum of all other elements. In the earth’s troposphere atmosphere (12~15 km above the ground), the hydrogen content is 0~50 km in the earth’s stratosphere, and there is almost no hydrogen. Hydrogen accounts for 50% of the inner layer of the earth’s atmosphere 80~500 km, and 70% in the outer layer of the earth’s atmosphere over 500 km.

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1 Overview of Hydrogen Energy

The abundance of hydrogen in the solar photosphere is 25*1010 (based on the abundance of Si), which is 25,000 times that of silicon, and it is the most abundant element in the solar photosphere. According to calculation, hydrogen accounts for 92% of the total atoms of the sun and its planets and 74% of the atomic mass CH4 exists in the atmosphere of giant planets, and its quantity greatly exceeds that of hydrogen. In addition, a small amount of hydrogen is found in the atmosphere of Jupiter and Saturn. Giant planets are made up of cores surrounded by ice, and some of them are made up of highly compressed hydrogen. The two lightest elements, hydrogen, and helium are the richest elements in the universe.

1.1.3 Hydrogen in Life 90% of the elements in the universe and 63% of the elements in the human body are hydrogen. In the early days of the earth, the carbon dioxide concentration was very high, but it also contained a large amount of hydrogen, up to 40%. In such a high hydrogen and carbon dioxide environment, the possibility of organic compound growth and carbon-based life was increased. Most of the dry weight of living organisms are organic compounds, which are composed of compounds rich in carbon, hydrogen, oxygen, nitrogen, and phosphorus. Among more than 90 naturally occurring chemical elements, only about 30 are necessary for life. There are 81 kinds of elements in the human body, including 11 kinds of O, C, H, N, Ca, P, K, S, Na, Cl, and Mg, accounting for more than 99.95% of the human body mass, and the other 70 elements that make up the human body are trace elements. Oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus account for 61%, 23%, 10%, 2.6%, 1.4%, and 1.1% of human body mass respectively.

1.2 Properties of Hydrogen As a secondary energy, hydrogen energy is called the cleanest energy in the world. Compared with the harmful products such as SO2 and NO produced by conventional energy combustion, the combustion product of hydrogen energy is only water. The calorific value of hydrogen is 4.5 times that of coke. Under the same conditions, the heat released by hydrogen is higher. Hydrogen energy is regarded as the most promising clean energy in the twenty-first century. It is also a comprehensive solution to alleviate energy pollution, improve energy storage and conversion efficiency, and build a safe and efficient modern energy system. Therefore, it is very important for the future energy system. Hydrogen energy can be obtained by fossil fuel hydrogen production, solar energy hydrogen production, biological hydrogen production, and other ways. Meanwhile, hydrogen energy is the cleanest renewable energy, which is called the “ultimate energy”. The substances generated by its combustion will not cause any pollution

1.2 Properties of Hydrogen

5

to the environment. The physical and chemical properties of hydrogen are discussed below.

1.2.1 Physical Properties of Hydrogen Hydrogen is a colorless and tasteless gas, with a density of 0.09 g L−1 under standard conditions. It is the lightest gas, and the same volume is much lighter than air. In addition, when the pressure is 101 kPa and the temperature is − 252.87 °C, hydrogen can be changed into light blue liquid. At − 259.1 °C, it becomes a snow-like solid. At room temperature, the nature of hydrogen is very stable, and it is not easy to react chemically with other substances. When the volume fraction of hydrogen in the air is 4–75%, it may cause an explosion when it encounters a fire source. There are three kinds of hydrogen isotopes: Protium, with a proton in the nucleus and no neutron; Deuterium, there is a proton and a neutron in the nucleus; Tritium, it has one proton and two neutrons in its nucleus. The chemical formula of these three isotopes with water is H2 O, D2 O, T2 O and their relative molecular weights are 18.016, 20.032 and 22.048 respectively. Hydrogen is insoluble in water, so it is usually collected by drainage and gas collection. Hydrogen has the best thermal conductivity and combustibility. It is estimated that the thermal conductivity of hydrogen is about 10 times higher than that of most gases, and it is an excellent heat transfer carrier in the energy industry. In addition, hydrogen has good combustion performance and quick ignition and can replace fossil energy such as coal and oil as a good industrial raw material, which plays a role in energy conservation and environmental protection. At the same time, hydrogen is nontoxic and pollution-free. Hydrogen itself is nontoxic, and its leakage will not have adverse effects on the health of employees and surrounding residents. Other physical properties of hydrogen are shown in Table 1.1.

1.2.2 Chemical Properties of Hydrogen At room temperature, the chemical property of hydrogen is stable, and the stable chemical property is mainly determined by the strong covalent bond between the two hydrogen atoms that make up hydrogen. At high temperatures, hydrogen is highly active and can react with many substances except inert gas elements. The mixture with halogen or oxygen will react violently when ignited or illuminated; Generate metal hydride with metal at high temperature; It can also react with various metal oxides, metal halides and other salts. (1) Inflammability Hydrogen is flammable. Under the conditions of ignition or heating, hydrogen can easily react with various substances. When pure hydrogen is ignited, it can burn quietly, emitting light blue flame, releasing heat and generating water. If a dry and

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1 Overview of Hydrogen Energy

Table 1.1 Other physical properties of hydrogen

Project

Numerical value

Relative molecular mass

2.016

Melting point

− 259.2 °C

Boiling point, 101.325 kPa

− 252.76 °C

Critical temperature

− 239.97 °C

Critical compressive force

1.31 mPa

Critical volume

64.15 cm3 mol

Critical density

0.0314 g cm−3

Critical compression coefficient

0.305

Liquid density, at − 250 °C

0.067 g cm−3

Gas density (101.325 kPa,21.1 °C)

0.083 kg m−3

Solubility in water, at 25 °C

1.53 × 10−6

Gas viscosity, at 25 °C

88.05 × 10−7 Pa s

Thermal conductivity of gas, at 25 °C

0.17064 W mK−1

Self-ignition point

400 °C

Combustion heat, at 25 °C gas state

119,950.4 kJ kg−1

cold beaker is covered in the flame, water droplets can be seen on the wall of the beaker. The chemical reaction of combustion is as follows: 2H2 (g) + O2 (g) −→ 2H2 O(L)H∗298K = 285.8 (kJ mol−1 ) light

(1.1)

The concentration range at which hydrogen burns is 4–74%. If it is lower or higher than this concentration, it will not burn or explode even under high pressure. In an oxygen environment, the combustion concentration of hydrogen ranges from 4 to 94%. When the oxygen concentration is lower than 4%, even under very high-pressure conditions, the mixture of hydrogen and oxygen will not burn. The combustion of hydrogen only produces water and a small amount of ammonia, and does not produce substances that pollute the environment, such as carbon monoxide, carbon dioxide, hydrocarbons, lead compounds, and dust particles. Moreover, the water produced by the combustion of hydrogen can continue to be used to produce hydrogen energy, achieving the effect of repeated recycling. People can use this characteristic of hydrogen for diving, and they can also use these characteristics of hydrogen to design equipment for safely breathing hydrogen. (2) Reducibility Hydrogen has reducibility. The chemical property of hydrogen is active, and it reacts with oxygen to produce water, which is prone to combustion and explosion. Combustibility is also the embodiment of the reducibility of hydrogen, which is determined by the nature of hydrogen-reducing oxygen. Hydrogen can react not only with oxygen but also with oxygen in some compounds. For example, by passing hydrogen

1.3 Hydrogen Storage Forms

7

through hot copper oxide, red metallic copper can be obtained, while water is generated. In this reaction, hydrogen takes the oxygen from copper oxide and generates water; Copper oxide loses oxygen and is reduced to red copper, which proves that hydrogen has reducibility and is a good reducing agent. Hydrogen can also reduce other metal oxides, such as tungsten trioxide, ferroferric oxide, lead oxide, and zinc oxide. (3) Oxidation Hydrogen is not only reductive but also oxidizing. Hydrogen is a diatomic molecule covalently formed by hydrogen atoms, and each hydrogen atom can obtain one electron to form negative hydrogen ions. This situation can be seen in the reaction with strongly reducing metals, and its effect is similar to chlorine. In this kind of reaction, hydrogen is an oxidant, which can oxidize metals into metal ions. Strictly speaking, the product of the reaction between hydrogen and metal is hydride, which is characterized by strong reducibility and is very easy to react with water to release a large amount of hydrogen.

1.3 Hydrogen Storage Forms In physical storage technology, hydrogen can be stored by high-pressure gas hydrogen, liquid hydrogen, low-temperature compressed hydrogen, slurry hydrogen, and physical adsorption. Among them, compressed hydrogen and metal hydride are considered to be effective methods for small and medium-sized hydrogen storage, and low-temperature liquid hydrogen is an effective way for large-scale storage and transportation.

1.3.1 Gaseous Hydrogen The unit mass hydrogen storage density of gaseous hydrogen storage is 1.0~5.7%, and the hydrogen storage density at room temperature and 20 MPa is 17.9 kg m−3 . The electricity consumption per kilogram is only 2 kWh, and the storage and transportation efficiency is over 90%. The technology is mature and low in energy consumption and cost, but there are some problems such as low volume density and high long-distance transportation costs.

1.3.2 Liquid Hydrogen Hydrogen, the liquid obtained by cooling hydrogen, is a colorless and tasteless highenergy low-temperature liquid fuel. The normal boiling point of hydrogen in one

8

1 Overview of Hydrogen Energy

atmosphere is 20.37 K (− 252.78 °C) and the freezing point is 13.96 K (− 259.19 °C). Liquid hydrogen has certain particularity. Compared with low-temperature fluids such as Liquefied Natural Gas and liquid nitrogen, the temperature of liquid hydrogen is lower, and its thermal properties such as density, viscosity, and sound velocity are obviously different, especially from that of normal-temperature medium water. In addition, liquid hydrogen is flammable and explosive, which makes the experimental research of liquid hydrogen equipment risky and difficult [6]. Liquid hydrogen is a colorless, odorless, and transparent liquid liquefied from hydrogen. It is a mixture of para-hydrogen and normal hydrogen. Normal hydrogen and secondary hydrogen are two kinds of spin isomers of molecular hydrogen, which are caused by two possible coupling of nuclear spins of two hydrogen atoms. The nuclei of positive hydrogen have the same spin direction, while those of secondary hydrogen have the opposite spin direction. The magnetic moment of the secondary hydrogen molecule is zero, and the positive hydrogen molecule is twice that of proton. The chemical properties of secondary hydrogen and normal hydrogen are identical, but their physical properties are different, which shows that the ground state energy of secondary hydrogen is lower than that of normal hydrogen. At or above room temperature, the equilibrium composition of normal and secondary hydrogen is 75:25, which is called standard hydrogen or normal hydrogen. When the temperature is lower than normal temperature, the equilibrium composition of normal and secondary hydrogen will change, and the percentage of secondary hydrogen will increase. The positivesecondary conversion of gaseous hydrogen can only occur in the presence of catalyst, while the positive-secondary conversion of liquid hydrogen will occur spontaneously without catalyst, but the conversion rate is slow. The positive and secondary conversion of hydrogen is an exothermic reaction, and the heat released during the conversion is related to the temperature during the conversion. In order to reduce the evaporation loss of liquid hydrogen caused by the exotherm of conversion of normal and secondary hydrogen, the content of secondary hydrogen in all liquid hydrogen products should be at least 95%, that is, the normal hydrogen should be basically converted into secondary hydrogen by catalysis during liquefaction. Hydrogen is widely used, and it is a very important storage form of hydrogen energy. With the progress of space technology, hydrogen liquefaction technology, and its production scale have also developed rapidly, and its commercial application is gradually expanding and developing. In the 1960s, liquid hydrogen was mainly used for satellite launches. In the 1970s, it was used in metal processing, float glass production, chemical synthesis, and grease treatment. Since the 1980s, it has been widely used in space shuttles, powder metallurgy, and electronic technology. Since the 1990s, hydrogen fuel cell, as a kind of green and clean energy, has been widely concerned. Hydrogen-powered fuel cell vehicle technology has also been highly valued by governments, automobile companies, and energy companies [7]. Because the critical temperature and conversion temperature of hydrogen are low, the latent heat of vaporization is small, and its theoretical minimum liquefaction work

1.3 Hydrogen Storage Forms

9

is the highest among all gases, so it is difficult to liquefy. In the process of liquefaction. The catalytic conversion of secondary hydrogen is an exothermic reaction, and the amount of heat released varies with the reaction temperature. Different catalysts have different conversion efficiency. Therefore, what kind of catalyst is used in the liquefaction process and how to arrange the catalyst temperature level are very important for the production and storage of liquid hydrogen. At the temperature of liquid hydrogen, except helium, all other gas impurities have solidified, which may block the pipeline of liquefaction system, especially the choke part blocked by solid oxygen, which is easy to cause explosion. Therefore, the raw hydrogen must be strictly purified. Generally, three liquefaction cycles can be used to produce liquid hydrogen, namely throttling hydrogen liquefaction cycle, hydrogen liquefaction cycle with expander, and helium refrigeration hydrogen liquefaction cycle. Among these three basic liquefaction cycles, there are many different liquefaction cycles. Here, only one of them is selected for brief explanation. (1) Throttling the hydrogen liquefaction cycle Throttling cycle was put forward independently by Linde in Germany and Hampson in England in 1895, so it is also called Linde (or Hampson) cycle. Throttling cycle is the earliest gas liquefaction cycle used in industry, because of its simple device and reliable operation, it is widely used in small gas liquefaction cycle devices. Because of the low conversion temperature of hydrogen, throttling has obvious refrigeration effect when it is lower than 80 K. Therefore when throttling circulation is used to liquefy hydrogen, an external cold source (such as liquid nitrogen) must be used for precooling. Actually, only when the pressure is as high as 10–15 MPa and the temperature is reduced to 50–70 K, the liquid hydrogen can be obtained with an ideal liquefaction rate (24–25%). Throttling hydrogen liquefaction circulation process: gas hydrogen is compressed by a compressor, and then enters a liquid hydrogen tank after sub-heat exchange and throttling by a high-temperature heat exchanger, a liquid nitrogen tank, and a main heat exchanger. Part of liquefied hydrogen product is stored in the liquid hydrogen tank, and unliquefied low-pressure hydrogen flows back to the compressor for reheating. There are two differences between the process of the hydrogen liquefaction plant completed and put into operation in 1966 by Aerospace Industry Corporation 101 and the above process: first, in order to reduce the evaporation temperature of liquid nitrogen in the liquid nitrogen tank, a vacuum pump B is installed on the nitrogen steam pipeline; second, two positive and secondary hydrogen converters with ferroferric oxide catalyst are installed in the liquid nitrogen tank and the liquid hydrogen tank. When the hydrogen pressure is 13–15 MPa and the evaporation temperature of liquid nitrogen is about 66 K, the liquefaction rate of normal hydrogen can reach 25% (100 L h−1 ), while that of liquid secondary hydrogen (the concentration of secondary hydrogen is more than 95%) will decrease by 30%, that is, 70 L of liquid secondary hydrogen can be produced every hour. The liquid hydrogen produced by this device has basically met the needs of the development

10

1 Overview of Hydrogen Energy

Fig. 1.2 Basic form of simple Claude cycle [9]

and test of the first generation hydrogen–oxygen engine in China since it was put into operation in 1966 until it was decommissioned in 1980s. (2) Hydrogen liquefaction cycle with expander Claude of France first realized the air liquefaction cycle with piston expander in 1902, so the liquid cycle with expander is also called Claude liquefaction cycle. The theory proves that under the adiabatic condition, the compressed gas can expand by the expander and work externally, which can obtain greater temperature drop and cooling capacity. Figure 1.2 shows the basic form of Claude loop. This cycle combines gas liquefaction technology, partition cooling and expander. Among them, the expander, the core component, provides cold energy in the low temperature zone through isentropic expansion. Claude cycle with precooled liquid nitrogen is 50~70% more efficient than Linde–Hampson cycle with precooled liquid nitrogen, which is the foundation of large-scale hydrogen liquefaction plant currently in use [8]. Therefore, at present, the liquefaction cycle with expander is the most widely used in gas liquefaction and separation equipment. There are two kinds of expanders: piston expander and turboexpander. Piston expander is used for medium and high pressure system, and turboexpander is used for low pressure liquefaction system. A large-scale hydrogen liquefaction cycle with turboexpander is adopted in the U.S. liquid hydrogen plant with a daily capacity of 30 tons. The process consists of a double-pressure hydrogen refrigeration cycle with a turboexpander and a pressure of 4 MPa, and adopts two-stage pre-cooling of normal pressure (0.1 MPa) liquid nitrogen (80 K) and negative pressure (0.013 MPa) liquid nitrogen (65 K). In this cycle, most of the cooling capacity is provided by liquid nitrogen and cold nitrogen, and the cooling capacity below 65 K is provided by turboexpander in medium-pressure (0.7 MPa) circulating hydrogen system and two-stage throttling in high-pressure (4.5 MPa) circulating hydrogen system. During the whole liquefaction process, the raw material undergoes positive and secondary catalytic conversion at six temperature levels, and finally liquid hydrogen with secondary hydrogen concentration greater than 95% can be obtained.

1.3 Hydrogen Storage Forms

11

(3) Helium refrigeration hydrogen liquefaction cycle This cycle is also called the hydrogen liquefaction system of reverse Brayton cycle refrigeration. The hydrogen liquefaction system of reverse Brayton cycle refrigeration consists of two parts: helium refrigeration system and hydrogen system. This cycle uses helium as the refrigerant, and the helium refrigeration cycle provides the cooling capacity needed for hydrogen condensation and liquefaction. In the whole process, the working medium helium in the helium refrigeration system is firstly compressed, precooled by liquid nitrogen, then cooled by the heat exchanger step by step, and finally expanded in the helium turboexpander to lower the temperature. In the hydrogen system, compressed hydrogen is precooled by liquid nitrogen, and then cooled by cold helium in a heat exchanger to obtain liquid hydrogen. Helium refrigeration hydrogen liquefaction system is complex and consumes high energy, so it has not been widely used in large-scale hydrogen liquefaction systems [9].

1.3.3 Solid Hydrogen James Dewar developed a Dewar bottle with ingenious thermodynamic design, which surrounded the container with liquid nitrogen, cooled the compressed hydrogen to − 200 °C, and then expanded and further cooled the ultra-cold hydrogen. Then the hydrogen is led back to the container through the pipe, and this cycle is repeated again and again. In 1898, Dewar finally cooled down to obtain liquid hydrogen, and a year later, in 1899, solid hydrogen was first produced. Different from traditional hydrogen storage technology, solid hydrogen storage technology is a new and efficient indirect hydrogen storage method. The hydrogen storage principle is that solid hydrogen storage materials react with hydrogen to absorb hydrogen, and when certain conditions are provided by the outside world, the hydrogen storage reaction is reversed to release hydrogen. Different from the traditional direct hydrogen storage method, the solid hydrogen storage method has the obvious advantages of high energy density, large hydrogen storage capacity per unit volume, stable, safe and low cost of the generated compounds [10]. Among many solid hydrogen storage materials, metal hydride has great application potential because of its mature preparation technology, high hydrogen storage capacity, good hydrogen absorption and desorption platform, good reversible cycle performance and safety. The fields of heat pump, hydrogen compression, medical treatment, cogeneration of cold, heat and power, etc. have gradually become hot research fields of hydrogen storage technology [11]. Generally, hydrogen in metal hydride is stored in the alloy in the form of atoms, and hydrogen is released through diffusion, phase change, chemical reaction and other processes. The hydrogen absorption process can be approximately regarded as a reversible process of hydrogen release from metal hydride. The difference is that the former is an exothermic reaction, while the latter is an endothermic reaction. The hydrogen desorption process can be represented by the following formula [12]:

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1 Overview of Hydrogen Energy

y T M(s) + H2 ←→ MH y (s) + Q 2

(1.2)

In the formula, M(s) and MHy (s) are solid metal alloys and metal hydrides, and Q is the heat generated or absorbed in the reaction, that is, the reaction enthalpy. The output characteristics of the solid hydrogen storage hydrogen source system based on metal hydride are controlled by the hydrogen absorption and desorption characteristics of the hydrogen storage reactor, and the temperature, pressure and flow rate of hydrogen flow will change with the operating conditions of the solid hydrogen storage reactor. The hydrogen output from the hydrogen source system is used as the inlet fuel of hydrogen fuel cell, and its parameters such as temperature, pressure and flow rate greatly affect the dynamic characteristics of fuel cell [13].

1.4 Laboratory Preparation of Hydrogen Compared with the complex distribution of conventional energy and the difficult storage of secondary energy, hydrogen energy has a wide range of sources. It can be prepared by electrolysis technology, photolysis technology and fossil fuel byproduct processing technology, which is convenient for storage and transportation and suitable for large-scale energy storage. In 1865, based on experiments, Bekitov first determined the order of metal activity, including hydrogen, according to the mutual displacement ability between metal and metal ions, as well as the intensity of reactions between metal and acid, water, etc. Hydrogen can be replaced by the metal in front of it from dilute acid, and hydrogen can also replace the metal behind it from their salt solution, while the metal behind hydrogen cannot replace hydrogen from acid. That is to say, at that time, Bekhetov distinguished between active and inactive metals, and hydrogen was used as the standard. Of course, the early chemists were not strict in measuring the activity of metals. The accurate method should be to compare the activity of metals with the standard electrode potential of metals, and the standard electrode potential is also set to zero by hydrogen electrode. Metals with negative standard electrode potential are more active than hydrogen; The activity of metal with positive standard electrode potential is less than that of hydrogen. Generally, there are the following methods for preparing hydrogen in the laboratory: (1) Active metals react with water. The metals used include lithium, sodium, potassium, rubidium and calcium, usually sodium, preferably calcium. Magnesium can also be used to react with hot water. The reaction equation is as follows: Na + 2H2 O → 2NaOH+ H2

(1.3)

1.5 Industrialized Hydrogen Production

13

(2) Metal reacts with acid. Zinc is the most commonly used metal to react with acid to produce hydrogen, and iron was used before. Generally, hydrochloric acid is used as acid. If the metal contains impurities, it will produce toxic gases such as phosphine, arsine, or hydrogen sulfide. The higher the purity of the metal, the slower the reaction speed. At this time, a little copper salt can be added to the reaction system, and the generated zinc and copper will help the reaction move to the side where hydrogen is generated. (3) Metal reacts with strong alkaline. Aluminum can react with sodium hydroxide solution to generate sodium metaaluminate (NaAlO2 ) and hydrogen. (4) Reaction of metal hydride with water. LiH, CaH2 and LiAlH4 can be used to react with a controlled amount of water. The purity of hydrogen produced by this method is high, but the cost is high. (5) Laboratory-scale aqueous solution electrolysis, in which hydrogen ions (H+ ) gain electrons at the cathode of the electrolytic cell to generate hydrogen.

1.5 Industrialized Hydrogen Production Hydrogen can be produced from different raw materials such as water, coal, natural gas, biomass, hydrogen sulfide, borohydride by different methods such as pyrolysis, electrolysis and photolysis. Hydrogen production is mainly divided into six ways: reforming, gasification and partial oxidation, thermal catalytic cracking, biomass, pyrolysis and thermochemical decomposition, photolysis and electrolysis. In the past few decades, the preparation technology of hydrogen energy has been the focus of research in various countries, and remarkable achievements have been made. The conventional hydrogen production technology route mainly uses traditional fossil energy to produce hydrogen, and the world-wide hydrogen production mainly uses natural gas. At present, the most mainstream hydrogen production methods include methane steam reforming, methane autothermal reforming, electrolysis of water to produce hydrogen, extraction of industrial by-products or industrial waste residues, etc. [14]. Reforming, gasification and partial oxidation (POx ) are three commonly used methods to prepare hydrogen from fossil fuels in industry, and reforming is the main process route. These methods can also be used to treat renewable biomass for hydrogen production.

1.5.1 Hydrogen Production from Traditional Fossil Fuels Hydrogen production from fossil raw materials is produced by reforming coal, natural gas, oil and shale gas. At present, the technical route is very mature and the average price is relatively low. China is rich in coal resources. Coal or coal char is used as the raw material for making atmosphere from fossil raw materials. The gas mixture with H2 and CO as the main components is obtained through reforming reaction, and then

14

1 Overview of Hydrogen Energy

the finished hydrogen is produced through purification. In the world, natural gas, shale gas and other raw materials with methane water vapor as the main component are mainly reformed. Steam reforming of methane (SMR) is one of the most widely used technologies in the production of hydrogen from fossil fuels. Hydrogen production from steam reforming accounts for 50% of global hydrogen production [15]. It uses steam to catalytically convert light hydrocarbons (such as natural gas, propane, butane, petroleum, biogas, or landfill gas) [16]. The traditional steam reforming process of methane is shown in Fig. 1.3, which mainly includes raw material pretreatment, reforming reaction, steam replacement and hydrogen separation. Steam reforming process is a reversible endothermic reaction process, which needs to be carried out at high pressure and high temperature. At the same time, the reaction process is accompanied by CO steam transformation. Generally, Ni/Al2 O3 is used in the reaction, and additives are usually added to the catalyst to inhibit the carbon deposition reaction on the catalyst. After the substitution reaction, CO is converted into carbon dioxide and additional hydrogen to improve the hydrogen yield. The ideal reforming reaction formula is as follows: CH4 + H2 O → CO+3H2

(1.4)

In this reaction, some CH4 , H2 O and CO2 may exist in the synthesis gas due to the influence of external factors such as the operating temperature and pressure of the reformer. Subsequently, in the water vapor displacement reaction, CO and water vapor produce CO2 and more H2 under the action of catalyst. The reaction formula is shown below. CO + H2 O → CO2 + H2

(1.5)

In the final pressure swing adsorption process, carbon dioxide and other impurities are removed from the gas stream, and the rest is basically pure hydrogen. SMR is well developed, and there is limited space to achieve major technological breakthroughs or reduce costs. Although the traditional methane reforming technology has been widely used, it still has some limitations such as high reaction temperature and easy carbon deposition. In recent years, plasma methane reforming technology is also under research and development [17].

Fig. 1.3 Hydrogen production process of methane steam reforming [16]

1.5 Industrialized Hydrogen Production

15

1.5.2 Hydrogen Production from Chemical Raw Materials Chemical raw materials such as methanol are cracked under certain temperature and pressure under the action of catalyst to produce carbon-containing gases such as hydrogen and CO. And then CO2 is removed by pressure swing adsorption to obtain high-purity H2 . Methanol cracking technology process system is simpler and more stable than using fossil energy to produce hydrogen. The product gas contains no pollutants or harmful gases, which are especially suitable for small and mediumsized hydrogen production. However, the production cost is obviously affected by the price of methanol, and the cost of hydrogen production is significantly higher than that of fossil energy or industrial by-products. Moreover, the first two hydrogen production technologies are still based on fossil fuels and inevitably emit carbon dioxide, which obviously violates the original intention of green and sustainable.

1.5.3 Hydrogen Production by Electrolysis of Water Hydrogen production by electrolysis of water is the simplest hydrogen production method, in which positive and negative electrodes are inserted into water and direct current is applied. Hydrogen ions in water undergo a reduction reaction at the cathode to precipitate hydrogen, and hydroxide ions undergo an oxidation reaction at the anode to precipitate oxygen. According to different electrolytes, they can be divided into alkaline electrolysis, proton membrane electrolysis and solid oxide electrolysis. The latter two are the reverse reactions of proton membrane fuel cells and solid oxide fuel cells, and their technical level can’t reach the stage of large-scale commercial application. The technology and equipment of water electrolysis hydrogen production are simple, the process flow is stable and reliable, and the purity of hydrogen produced is extremely high, which can meet the demand of high-purity hydrogen without pollution. However, the disadvantage is high energy consumption, and the cost of hydrogen production is the highest in the field of industrial hydrogen production at present. The unit cost of hydrogen production is 4~5 times that of coal. And the scale is small, and the hydrogen production is generally less than 200 m3 h−1 . Using renewable energy such as wind energy, solar energy and tidal energy to provide the energy needed for electrolysis of water is one of the solutions to this problem. At the same time, it can effectively solve the problems of wind, light and water abandonment in China, and it is the best way to achieve zero hydrocarbon [18].

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1 Overview of Hydrogen Energy

1.6 Summary Hydrogen energy is a kind of green energy that can solve the future energy crisis and environmental pollution problems at the same time, and it is the trend of future energy development. With the increasing demand of hydrogen energy in social development, hydrogen production, as the upstream of the hydrogen industry chain, will also get rapid development. Selecting the technical route with economic advantages and reducing the cost of hydrogen production is the key to the popularization and use of hydrogen energy. At present, the use of fossil fuels, mainly natural gas, is still the most mainstream hydrogen production method, among which methane steam reforming is the most economical, but its shortcomings are quite obvious, the efficiency is not high, and a large amount of CO2 will be emitted at the same time. Furthermore, the use of fossil energy as raw materials is not sustainable after all, and it will produce new pollution. Therefore, vigorously developing carbon capture technology will be a solution to achieve the goal of low-carbon hydrogen production in the future hydrogen production industry. At present, it seems that it can support the huge demand of hydrogen energy in the future, and the hydrogen production method with stable source of raw materials should be electrolysis of water. Nowadays due to the high cost, water electrolysis in hydrogen energy preparation industry only accounts for about 4%, compared with other methods, which do not have competitive advantages. However, with the development of low-carbon economy in the future, it will play a more important role in the future hydrogen production industry. At this stage, the main bottleneck restricting the development of hydrogen production by electrolysis of water is how to reduce the consumption of electric energy. However, if considering using a large number of renewable energy such as wind energy and photovoltaic power which cannot be connected to the net in China every year as energy, we can greatly reduce the electricity cost for hydrogen production, promote the popularization and application of electrolyzed water technology, and effectively solve the problem of renewable power consumption. At the same time, the technology of hydrogen production by electrolysis of water is mature and the equipment is simple, if combined with renewable energy for power generation, it can greatly reduce the cost of hydrogen production electricity while solving the problem of renewable power consumption, which can be used as a stable source for a large amount of hydrogen energy demand in the future.

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Chapter 2

Current Status of Hydrogen Energy Development

2.1 Overview As an important raw material for chemical synthesis, hydrogen has been studied in China since the early 1960s. During this stage, hydrogen primarily served as an industrial product. With domestic energy structure transformation and social environmental protection consciousness enhancement, hydrogen energy property gets more attention. Hydrogen as a clean and efficient energy has become the hotspot in research of energy, at the same time in the downstream technologies such as electric cars and fuel-cell progress, also leading to the rapid development of the hydrogen industry. Therefore, the hydrogen energy industry in China will stride into a period of sustained and rapid development, and the role of hydrogen energy in promoting economic and social development will become more and more obvious. In the past decades, hydrogen energy preparation technology has been the focus of research in various countries and has earned remarkable achievements. At present, the most mainstream methods for hydrogen production include methane steam reforming, methane self-thermal reforming, water electrolysis for hydrogen production, extraction of industrial by-products or waste residues, and so on. From the perspective of sustainable development, renewable energy as the main primary energy supplier has become an inevitable trend of energy structure transformation. The storage and conversion of renewable energy based on large-scale electrolysis of water for hydrogen production is an ideal model to achieve efficient utilization of renewable energy and “carbon neutral” energy system. However, there are still many problems in renewable energy water electrolysis technology, which is difficult to achieve in a short period. In the long run, hydrogen production through water electrolysis with renewable energy is key to achieving the energy transition mediated by hydrogen energy, and it is also the main way for large-scale hydrogen production in the future. Other new technologies for hydrogen production from clean energy such as solar photolysis, biomass fermentation, thermochemical conversion, and thermochemical cycle can be used as effective supplements. At the same time, a variety of new technologies are widely used in the hydrogen production industry © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_2

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to improve the efficiency of hydrogen production and reduce energy consumption and carbon emissions, such as process enhancement technology, carbon capture, and storage technology. At present, the relevant reviews of hydrogen production technology mainly focus on the introduction of one or a limited number of hydrogen production methods. To further compare and evaluate the development of existing hydrogen production technology routes, nine common hydrogen production technologies are introduced in this chapter, which are water electrolysis hydrogen production, thermochemical hydrogen production, fossil energy hydrogen production, solar energy hydrogen production, biomass hydrogen production, wind energy hydrogen production, nuclear energy hydrogen production, hydrogen-loaded system hydrogen production, and geothermal energy hydrogen production.

2.2 Electrolysis of Water to Produce Hydrogen Electrolysis of water for hydrogen production is a relatively convenient method of hydrogen evolution. At present, about 4% of the hydrogen in the world is made by electrolysis of water. Electrolysis water hydrogen production technology is mature and the equipment is simple. Combined with renewable energy generation, it can greatly reduce the electricity cost of hydrogen production and solve the problem of renewable electricity consumption at the same time. It can be used as a stable source of large amount of hydrogen energy demand in the future. At present, the high cost of electrolysis is the most important reason to restrict the popularization and use of electrolysis water hydrogen production technology [1]. Therefore, the current research hotspot in the field of hydrogen production involves the use of renewable energy sources, namely wind and solar power generation, followed by electrolysis hydrogen production. This approach offers both technical feasibility and economic advantages and has the potential to significantly decrease the cost of hydrogen production.

2.2.1 Principle of Water Electrolysis The water-splitting can be divided into two half-cell reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). HER refers to the reaction in which water is reduced to H2 at the cathode, and OER refers to the reaction in which water is oxidized to O2 at the anode. One of the key barriers preventing water decomposition from being practically usable is the slow reaction kinetics of OER and HER due to high overpotential, a measure of kinetic energy impairment. Therefore, catalysis plays an important role in OER and HER. Efficient catalysts are required to minimize OER and HER overpotentials for efficient H2 and O2 production. The hydroelectrolysis reaction is composed of two half-reactions: oxygen release reaction (OER) in the anode cavity and hydrogen release reaction (HER) in the

2.2 Electrolysis of Water to Produce Hydrogen

21

cathode cavity. The following electrochemical reactions occur at the cathode and anode in different electrolytes: Acidic electrolyte solution: Cathode: 4H+ + 4e− → 2H2 ; E0 = 0 V

(2.1)

Anode: 2H2 O → O2 + 4H+ + 4e− ; E0 = 1.23 V

(2.2)

Alkaline electrolyte solution: Cathode: 2H2 O + 2e− → H2 + 2OH− ; E0 = −0.402 V

(2.3)

Anode: 4OH− → O2 + 2H2 O + 4e− ; E0 = 0.828 V

(2.4)

Overall reaction: 2H2 O → O2 + 2H2 ; E0 = 1.23 V

(2.5)

HER: In hydroelectrolysis, the reaction step is a key semi-reaction that produces hydrogen at the cathode and involves a two-electron transfer process. The mechanism of this HER is highly dependent on environmental conditions. For HER reaction in acidic medium, there are three possible reaction steps: H+ + e− → Had

(2.6)

H+ + e− + Had → H2

(2.7)

2Had → H2

(2.8)

The step is Volmer step to produce adsorbed hydrogen. The hydrogen evolution reaction can then be carried out by Heyrovsky step or Tafel step or both to produce H2 [2]. For HER reaction in alkaline medium. There are two possible reaction steps, namely Volmer step and Heyrovsky step [3], as shown in the following equation: H2 O + e− = OH− + Had

(2.9)

H2 O + e− + Had = OH− + H2

(2.10)

Balancing Had , hydroxyl adsorption (OHad ), and water dissociation in alkaline media is essential for HER activity.

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2 Current Status of Hydrogen Energy Development

OER: As mentioned earlier, OER is another key semi-reaction in water splitting. This reaction occurs at the anode, involving a four-electron transfer process, and requiring a very high overpotential compared with that of HER. OER is the main bottleneck to improve the overall efficiency of water splitting. Therefore, it is imperative to find efficient OER catalysts that can effectively reduce the kinetic limit. Significant progress has recently been made in understanding the mechanism of OER for the rational design of OER electrocatalysts. It is generally accepted that OER can proceed through two different mechanisms: conventional adsorbent evolution (AEM) and lattice oxygen-mediated mechanisms (LOM), which are discussed in the following two subsections. Reaction steps in OER absorbent evolution mechanism (AEM). For OER, AEM is commonly used to describe various reaction steps. In AEM, the reaction typically involves four coordinated proton and electron transfers, in which the metal center acts as the active site (M) to produce oxygen molecules from water in acidic and alkaline media [4]. The reaction pathway of alkaline OER includes the following steps: OH− + M → M−OH + e−

(2.11)

M−OH + OH− → M−O + H2 O + e−

(2.12)

M−O + OH− → M−OOH + e− /2M−O → 2M + O2 + 2e− M−OOH + OH− → O2 + H2 O + e− + M

(2.13) (2.14)

First, the hydroxide anion adsorbs on the metal active site to form M–OH. And then the M–OH deprotonates to form M–O. After that, there are two different ways to form O2 molecules. One method is to react M–O with OH− to form an M–OOH intermediate, which is deprotonated to produce O2 by regeneration of the active site. The other, which involves the binding and conversion of the two M–O species to O2 and the simultaneous regeneration of the M-active site, is thought to have a large activation barrier. The consensus for the mechanism of acidic OER is that it involves the same intermediates, such as M–OH, M–O, and M–OOH. For the electrocatalysis of OER, a detailed understanding of the binding strength of the reaction intermediates on the electrode surface is essential to improve the overall OER performance, as the binding strength is a key parameter controlling the reaction overpotential. The different reaction steps in OER as shown in Fig. 2.1, if the free energy gap of each basic step is maintained at 1.23 eV [5], no overpotential is required for OER. However, this ideal situation is almost impossible to achieve. The OER overpotential was determined by the rate determination step (RDS) from the step with the largest reaction free energy (ΔG) among the four steps. Based on a database of

2.2 Electrolysis of Water to Produce Hydrogen

23

Fig. 2.1 a Free energy of OER at U = 0 and U = 1.23 V over an ideal catalyst. b Relationship between *OOH and *OH adsorption energies on a series of oxide OER catalysts

different oxide catalyst models, a scaling relationship (linear correlation) was established in AEM according to the binding energies of these intermediates (M–OH, M–OOH, and M–O). The binding energies of the adsorbed M–OH and M–OOH exhibited a constant difference of 3.2 eV (ΔGHOO* − ΔGHO* ). HOO* and HO* are bound to the catalyst surface through a single bond with an adsorption configuration like the oxygen atom [6]. According to the scaling relationship, the minimum theoretical overpotential is 0.37 eV, which represents the difference between the constant difference in binding energy (3.2 eV) and the ideal value of 2.46 eV. Furthermore, since the second step (M–O formation) and the third step (M–OOH formation) are regarded as RDS in most OER catalysts, the difference between ΔGO* and ΔGHO* was used to predict a generic descriptor of OER activity. This is represented by the Sabatier volcano shape relationship, which is traditionally used to explain OER activity trends of metal oxides in acidic and alkaline media.

2.2.2 Electrolyzed Water Hydrogen Production Device The water electrolysis has been known since 1800 and the first water electrolyze device was designed in the same year, with collecting and measuring the generated oxygen and hydrogen. A complete water electrolyze consists of four parts: electrolyte, ion exchange membrane, cathode material, and anode material. Electrolyzes can be divided into unipolar (trough type) and bipolar (filter press). One way to increase the efficiency of the electrolysis process is to increase the electrode surface area and change its composition. There is a membrane between the electrodes that acts as an electrolyte that blocks electrons. These electrolytes are responsible for the dissociation of oxygen. Electrolytes can be mainly divided into

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2 Current Status of Hydrogen Energy Development

Fig. 2.2 System of alkaline electrolyze and PEM

acidic electrolytes and alkaline electrolytes. The alkaline electrolytes mainly use KOH (potassium hydroxide), NaOH (sodium hydroxide), and the acidic electrolytes are H2 SO4 (sulfuric acid). Electrolysis is carried out by a power source, which is necessary to dissolve ionic species, but which in turn does not participate in the reactions involved. This splitting of water occurs due to the electrical potential, which enables chemical reactions. The place where the reaction takes place is the electrolyze, and the most widespread electrolysis systems for electrolyzes are based on alkaline electrolyzes and PEM electrolyzes. Alkaline electrolysis is one of the state-of-the-art techniques and is a simple method for electrolytes to have basic characteristics. Electrolysis can also be performed with so-called polymer electrolyte membranes (PEMs), one of the most promising technologies for water splitting, offering higher efficiency and gas purity. However, little has been explored. Figure 2.2 [7] shows the system of alkaline electrolyze and PEM. For alkaline electrolyzes, water splitting is performed under alkaline conditions. Compared with the devices using acidic media, water splitting in alkaline media expands the choice of electrocatalysts to non-precious metals or metal oxides. However, the activity of HER in alkaline media is typically 2–3 orders of magnitude lower than in acidic media [7]. Therefore, the suitable design for different media and electrocatalysts with low loadings depends on the operating conditions of the water electrolysis cell. Currently, there are three main types of electrolysis techniques: (1) proton exchange membrane (PEM) electrolysis (2) alkaline electrolysis (3) high temperature solid oxide water electrolysis. Due to the high temperature, solid oxide water electrolysis requires high energy consumption. For PEM-based electrolyzers, water splitting is performed under acidic conditions and PEM is used. Compared with other conditions, this condition has some advantages, such as lower gas permeability and higher proton conductivity. It is characterized by high energy efficiency and fast hydrogen production. However, the requirement of acidic media restricts OER electrocatalysts to noble metal and noble metal oxide catalysts, which are state-of-the-art OER electrocatalysts in acidic media. PEM electrolyzers with

2.2 Electrolysis of Water to Produce Hydrogen

25

a thickness of 20–300 μm, have high proton conductivity and can operate at high pressures. In addition to these advantages, the electrolyte is thinner than alkaline electrolytes, and electrolysis is less expensive to run because PEMs can operate at high energy densities. However, the disadvantages are high component cost, low durability, significant corrosiveness, and acidity in the MW range, which also lead to higher costs for the battery. In addition, among the methods to generate hydrogen, researchers have used photoanode or photocathode for catalysis, where the first method acts on the oxidation of water to O2 , while the second method uses photoexcited electrons to reduce water to H2 . The choice between the models can infer that the energy consumption exceeds the cost of the catalytic reaction. Not only that, but the requirements for efficiency may be lower, depending on the method.

2.2.3 Electrolyzed Water Performance Evaluation The evaluation of electrocatalytic water splitting performance is based on several key parameters of activity, stability, and efficiency. The activity is characterized by overpotential (η), Tafel slope, and exchange current density (j0 ), which can be extracted from polarization curves. Stability is characterized by changes in overpotential or current density over time. According to the experimental results, the efficiency is characterized by the Faradaic efficiency and the flip frequency. The performance evaluation elements introduced in this section mainly include electrochemical reaction overpotential, Tafel slope, exchange current density, electrochemical impedance spectroscopy (EIS), stability, and Faradaic efficiency. Electrocatalytic reaction overpotential: Theoretically, the thermodynamic potential is 1.23 V at 25 °C and 1 atmosphere in any medium. However, due to the kinetic barrier of the reaction, water electrolysis requires a higher potential than the thermodynamic barrier (1.23 V) to overcome the kinetic barrier. From this, the concept of overpotential is derived, that is, the difference between the onset potential of the electrocatalyst to activate HER/OER and the standard electrode potential of HER/ OER. Overpotential is an important indicator for evaluating the activity of electrocatalysts. The polarization curve can be obtained by plotting the current density versus overpotential by linear sweep voltammetry (LSV). Generally, the overpotential value corresponding to the current density of 10 mA cm−2 is selected to compare the activity of different catalysts. High-performance electrocatalysts with lower overpotentials require less energy to achieve the same current density. Tafel Slope and Exchange Current Density: These are two additional parameters for assessing activity in terms of overpotential versus potentiodynamic relationship, expressed by the equation: η = a + blog j, where η is the overpotential and j is the current density. In the Tafel plot, the linear dependence yields two important kinetic parameters. One is the Tafel slope b and the other is the exchange current density j0 , which can be obtained by extracting the current at zero overpotential. A smaller value of the Tafel slope means that increasing the same current density requires a smaller

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2 Current Status of Hydrogen Energy Development

overpotential, which implies faster charge transfer kinetics [8]. The theoretical value of the Tafel slope is 2.303 RT/αF (R is the gas constant, T is the temperature, α is the electron transfer coefficient, and F is the Faraday constant). The experimentally measured Tafel slope is the apparent value for a multi-step reaction, which corresponds to the apparent electron transfer number, whereas most electrochemical reactions involve multiple electron transfers. Therefore, the mechanism and kinetics of some simple electrons transfer during electrochemical reactions can be judged by the Tafel slope. It is a common method to obtain the Tafel curve from the polarization curve, which is the Tafel slope. The exchange current density is the intersection of the extrapolated linear part of the Tafel curve and the X-axis, that is, the value of the current density when η is equal to zero [9]. The exchange current density is an inherent characteristic of electrode reaction, which is related to catalyst material, electrolyte concentration, and reaction temperature. The exchange current density reflects the size of the electron transfer ability and the difficulty of the electrode reaction. The intrinsic cause of the overpotential is the exchange current density. The exchange current density describes the intrinsic charge transfer equilibrium condition. Higher exchange current density means a higher charge transfer rate and lower reaction barrier. For electrode reactions with high exchange current density, only a small driving force (smaller external current density) is required to carry out the reaction. The electrode reaction with a small exchange current density requires a large driving force (larger external current density). Both the Tafel slope and the exchange current density are important parameters to describe electrode materials. Generally, materials with smaller Tafel slopes and larger exchange current densities have good electrocatalytic performance. Electrochemical Impedance Spectroscopy: The entire electrochemical reaction can be viewed as an impedance. In the EIS test, alternating currents of different frequencies are often input, and the current is measured to obtain the impedance of the system. Impedance in electrochemical reactions is a complex number, denoted by Zω , which consists of the real part ZRe and the imaginary step ZIm . Usually, an electrochemical process includes electric double layer and Faradaic reaction. It can be equivalent to the internal resistance RΩ (the internal resistance of the electrode and the electrolyte solution), the electric double layer capacitance CD (the ions that do not participate in the chemical reaction in the electrolysis), and the Faraday impedance Zf (the redox reaction occurs when the electron transfer occurs in the electrolyte solution. The circuit diagram composed of active ions), its Faraday process can be divided into charge transfer and material transfer process. Therefore, Zf can be further equivalent to the charge transfer resistance Rct and the Warburg impedance Zw . In the low-frequency region, the electrochemical AC impedance spectrum is a straight line, and in the high-frequency region, it is a semicircle with a center coordinate of (RΩ + Rct/2 , 0) and a radius of Rct/2 . Therefore, electrochemical AC impedance spectroscopy can effectively reflect the kinetics of electrochemical reactions and the interfacial reactions involved. The smaller the Rct , the faster the charge transfer, which often represents the good catalytic performance of the catalytic material.

2.2 Electrolysis of Water to Produce Hydrogen

27

Stability: Considering that most electrocatalyst materials are carried out under alkaline or acidic conditions, the good structure and stability of the materials are also important parameters to evaluate the catalytic performance and determine their application prospects. There are usually two methods for stability testing: First, cyclic voltammetry (CV). Second, chronoamperometry (CA) or chronopotentiometry (CP). The first approach is that the potential cycle is repeated within the region containing the HER and OER. After several cycles (generally more than 500 times), it can be seen intuitively from the voltammogram that the polarization curves of stable electrocatalyst materials do not change much before and after CV cycling. In addition, multiple LSV scans can also be directly performed to evaluate the stability of electrocatalyst materials. The smaller the increase in overpotential at the same current density, the better the stability of the electrocatalyst. Chronoamperometry (CA) or chronopotentiometry (CP) is the time-dependent trend of the potential (or current density) of an electrocatalyst at a constant current density (or overpotential). The corresponding current density at a constant potential in the CP must be greater than 10 mA cm−2 . A current density of 10 mA cm−2 is commonly used in CA because this value is the most used standard value in electrocatalysis, but some other current density values may be used in some other cases. The duration varies from a few hours to dozens of hours. The smaller the current density or potential change and the longer the duration, the better the stability. Faradaic efficiency: Faradaic efficiency refers to the efficiency with which electrons provided by an external circuit are transferred for the target reaction. Problems such as heat generated by the electrode reaction or by-products can occur with Faraday loss. To measure the Faradaic efficiency, it is generally necessary to calculate the theoretical gas yield and measure the actual gas yield. The theoretical gas yield can be calculated by galvanostatic or potentiostatic electrolysis. At the same time, the actual production of gas can be directly measured by gas chromatography (GC). The ratio of the actual production of gas to the theoretical production is the Faradaic efficiency. However, for most HER and OER catalysts, their Faradaic efficiencies are not much different, most of which are close to 100%, so unless there is a special need, Faradaic efficiencies are generally not mentioned.

2.2.4 Catalyst Stability The performance evaluation metric of electrocatalysts for electrolytic water splitting is not only an uphill reaction, as reflected by the positive value of ΔG (Gibbs free energy), but also has to overcome a significant kinetic hurdle. Catalysts play a crucial role in lowering the kinetic barrier. Stability is an important parameter to evaluate whether the catalyst is likely to be used in water splitting batteries in practical applications. There are two typical approaches to characterize the stability of electrocatalysts. One method is chronoamperometry (I-t curve) or chronopotentiometry (E-t curve), which measures the change in current with time at a fixed potential or the change in potential with time at a fixed current.

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For this measurement, the longer the test current or potential remains constant, the better the stability of the catalyst. To compare with different research groups, one usually sets the current density to be greater than 10 mA cm−2 for at least 10 h of experiments. Another method is CV, which measures current by circulating a potential, usually requiring more than 5000 runs at a scan rate (e.g. 50 mV s−1 ). LSV is commonly used to examine overpotential shifts before and after CV cycling at specific current densities. The smaller the overpotential change, the better the stability of the electrocatalyst. At present, in addition to noble metal catalysts, the most promising catalyst materials used in the electrolysis of water are transition metal catalysts. Due to the shortage of precious metal catalysts, easy deactivation, and high price, the development of transition metal-based catalysts is an important direction to reduce the cost of catalysts. However, the true stability of these catalysts is still debated: some researchers believe that transition metal-based catalysts have high stability, while others cite ions to demonstrate the rapid dissolution of similar catalysts, and it has been reported that many OER-active materials will dissolve during the reaction, which means that the highly active OER catalyst will be accompanied by the dissolution of the metal. Considering the preparation of catalytic materials for the application, the ability to maintain excellent and stable activity is the most important criterion in the research process.

2.3 Thermochemical Hydrogen Production Thermochemical hydrogen production is a process of producing hydrogen by heating chemical reactions. There are mainly hydrothermal chemical hydrogen production methods, which are mainly chemical processes in which water is decomposed into hydrogen and oxygen by heating different temperatures in a water system containing additives to make the system go through several different reaction stages. Thermochemical hydrogen production is the thermochemical decomposition of water, that is, water is decomposed with heat provided by primary energy, and its reversible efficiency is: η=

ΔH1−2 (TH + 273) − (TC + 273) · ΔG1−2 TH + 273

As shown in the equation: ΔH1-2 : Molar enthalpy difference from state 1 to state 2. ΔG1-2 : Moorgi’s free energy difference from state 1 to state 2. TC: Low temperature heat source temperature. TH: High temperature heat source temperature.

(2.15)

2.3 Thermochemical Hydrogen Production

29

Since this process only consumes water and a certain amount of heat, the added elements or compounds involved in the process can be regenerated and reused, and the entire reaction process constitutes a closed cycle system. Compared with the direct pyrolysis of water to produce hydrogen, each step of the thermochemical cycle of hydrogen production is carried out at a lower temperature (800–1000 °C), and problems such as energy matching, temperature resistance requirements of equipment and equipment, and investment costs are easier to solve, it may become the lowest energy consumption and the most reasonable hydrogen production process, and can also match the temperature level provided by high temperature nuclear reactors, which is easy to realize industrialization. The most ideal way is to combine with solar energy to become the cheapest hydrogen production process. After the 1960s, the United States, Japan, and European countries invested heavily and developed more than 100 thermochemical cycle processes. The efficiency of pyrolyzed water is much greater than the total efficiency of electrolyzed water (40%). But the direct pyrolysis reaction of water (i.e. one-step reaction). The reaction can only occur when the temperature is as high as 4300 K. At such a temperature, the device materials and the membrane materials for separating hydrogen and oxygen cannot work normally. When the temperature drops to about 2000 °C (flame temperature), H2 and O2 are burned again. In 1996, Fan Ke et al. proposed a method of combining several chemical reactions to complete thermal decomposition from the results of thermodynamic analysis, which is called the thermochemical cycle H2 method, or the thermochemical H2 method for short.

2.3.1 Two and Three-Stage Thermochemical Hydrogen Production Methods The simplest cycle is a secondary cycle, a cycle consisting of two main reactions. In 1976, Yasuo Tokuyong of the Hiratsuka Research Institute of Tomo Heavy Industries Co., Ltd. in Japan proposed a more practical secondary closed-circuit circulation system, and the reaction is as follows: Fe2 O3 + H2 O + 2SO2 → 2FeSO4 + H2

(2.16)

1 2FeSO4 → Fe2 O3 + 2SO2 + O2 2

(2.17)

The net result of the cycle can be derived from the reaction equation as the splitting of water into hydrogen and oxygen. The added Fe2 O3 and SO3 continue to be recycled.

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2 Current Status of Hydrogen Energy Development

The characteristic of the secondary circulation system is that there are fewer chemical unit operations, and it is the simplest circulation, so the cost is lower. But in addition to the above cycle, most of the secondary cycle reaction temperature is high, resulting in the cycle is not easy to achieve. So how should a secondary thermochemical cycle system be formed? Fan Ke et al. deduced the thermodynamic properties of the circulating substances in the secondary cycle through rigorous thermodynamic derivation. Those elements such as C, N, Se, S, and Cl that can form weak bond compounds with H2 or O2 should be selected as the circulating substances.

2.3.2 Multi-stage Thermochemical Cycle for Hydrogen Production To reduce the reaction temperature and find a thermochemical cycle H2 production system with higher energy conversion efficiency, three-stage, four-stage, and even multi-stage thermochemical cycle H2 production systems are produced. The sulfur-iodine cycle studied by the American GA Company, European Ispra Institute, and Japan Institute of Chemical Technology is a classic example of a threestep cycle. The energy conversion efficiency of the three-stage cycle has reached 31%, while the energy conversion efficiency of H2 produced by electrolysis is 24% or higher. This three-stage cycle hydrogen production method is enough to compete with the electrolysis method. If the separation supply and demand efficiency can be further improved, it is possible to further improve the energy conversion efficiency. Later, some scholars successively proposed a newer one-step hydrogen production cycle system with higher energy conversion efficiency. How to form a reasonable and economical thermochemical cycle H2 system? Y. Furatani et al. have made some suggestions. An ideal thermochemical H2 system should have the following conditions: (1) On the premise of ensuring a certain energy conversion efficiency, the fewer the number of reaction stages, the better, because the more stages, the higher the product cost. (2) For each step of the cycle, the reaction unit should have a fast reaction speed, no side reactions, and a high yield. (3) Substances with low prices and low corrosiveness should be selected as the circulating medium, and the recovery rate of using the circulating medium should preferably be above 99.9%. (4) The heat source used should be as cheap and convenient as possible, such as industrial preheating. (5) The units in the cycle should be arranged in descending order of temperature to make full use of energy.

2.4 Hydrogen Production from Fossil Fuels

31

2.4 Hydrogen Production from Fossil Fuels Fossil energy is a hydrocarbon or its derivatives. It is deposited from the fossils of ancient creatures and is a primary energy source. After the incomplete combustion of fossil fuels, toxic gases will be emitted, but they are essential fuels for human beings. With the rapid development of the petroleum refining industry and the petrochemical industry centered on the three major synthetic materials, the consumption of hydrogen is also increasing rapidly. Many organic synthesis industries, metallurgical industries, electronics industries, and fast-developing fuel cell vehicles urgently need a large amount of pure hydrogen, and hydrogen consumption has become the main cost of each hydrogen-using enterprise, which in turn determines the profitability of the entire enterprise. At present, the main methods of producing hydrogen from fossil energy include coal-to-hydrogen, natural gas-to-hydrogen, methanol-to-hydrogen, and so on.

2.4.1 Coal to Hydrogen At present, coal-to-hydrogen (coal gasification to hydrogen) dominates domestic hydrogen production. Coal to hydrogen is the combustion reaction of coal with oxygen, and then with water to obtain gaseous products with hydrogen (H2 ) and carbon monoxide (CO) as the main components, and then after desulfurization and purification, carbon monoxide continues to undergo a shift reaction with water vapor. More hydrogen is generated, and finally, a certain purity of product hydrogen is obtained through separation, purification, and other processes. The process of coal gasification hydrogen production technology generally includes main production links such as coal gasification, gas purification, carbon monoxide shift, and hydrogen purification. The core technology of coal-to-hydrogen production is to first convert coal into gaseous products through different gasification technologies and then to further convert it into high-purity hydrogen through separation processes such as low-temperature methanol washing [10]. The reaction equation is [11]: C + H2 O → CO + H2 − Q

(2.18)

CO + H2 O → CO2 + H2 + Q

(2.19)

The total reaction equation is: C + 2H2 O → CO2 + 2H2 − Q

(2.20)

Compared with the traditional hydrocarbon steam reforming hydrogen production process used in the petrochemical industry, the advantages of the domestic coal

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hydrogen production process are low raw material cost and large equipment scale; The disadvantage is that the equipment investment is large and the technology is slightly immature.

2.4.2 Hydrogen Production from Natural Gas Reforming Compared with coal-based hydrogen production, the production of hydrogen from natural gas has higher yield, lower processing costs, and less greenhouse gas emissions. Therefore, natural gas is the main raw material for hydrogen production abroad. Among them, steam reforming of natural gas is the most common method for hydrogen production. Industrially, nickel is used as a catalyst in the methane steam reforming process. The operating temperature is 750–920 °C and the operating pressure is 2.17–2.86 MPa. Higher pressure can improve conversion efficiency. The conversion reaction is endothermic, and heat is supplied by burning methane in a combustion chamber. The synthesis gas produced by steam reforming of methane converts carbon monoxide into carbon dioxide and additional hydrogen through a high and low temperature shift reaction. The final yield of hydrogen is related to the technical route adopted. To prevent the carbon deposition reaction in the methane steam reforming process, it is necessary to add excess water vapor during the reaction process. The essence of hydrogen production from natural gas is to replace hydrogen in water with carbon in methane, which acts as a chemical reagent and provides heat for the replacement reaction. Most of the hydrogen comes from water, and a small part comes from natural gas itself [12].

2.4.3 Hydrogen Production from Methanol Steam Reforming Compared with coal and natural gas, methanol has excess production capacity, abundant raw material resources, and is easier to store and transport. Therefore, methanol-reforming the hydrogen production process has been rapidly promoted in recent years. With the continuous improvement of the methanol hydrogen production process and catalyst, the scale of methanol-reforming hydrogen production continues to expand, and the cost of hydrogen production continues to decrease. It has become the preferred solution for medium-scale hydrogen production such as oil refineries. Hydrogen production via methanol steam reforming involves the conversion of methanol and water into a mixture of hydrogen, carbon dioxide, and trace amounts of carbon monoxide as well as methane under specific conditions of temperature and pressure. Methanol steam reforming for hydrogen production offers several advantages, including a low reaction temperature and ease of separation. The theoretical hydrogen yield per unit mass of methanol using this process is 18.8% (mass fraction), resulting in a higher amount of hydrogen produced than through direct methanol decomposition. Moreover, the product has a lower carbon

2.5 Solar Hydrogen Production

33

Table 2.1 Comparison of three methods for hydrogen production from fossil fuels Advantage

Disadvantage

Production of hydrogen scale

Coal to hydrogen Mature technology and low cost

High investment costs and high carbon emissions

Large scale preparation

Hydrogen production from natural gas

Mature technology, low cost, high hydrogen conversion rate

The application of natural gas in the chemical industry is severely restricted

Various scale preparations

Hydrogen production from methanol

Methanol raw material resources are abundant and easy to transport and store; hydrogen production reaction temperature is low, and separation is simple

Methanol raw material is a secondary energy product with a high cost

Small scale preparation

monoxide content. Therefore, the currently developed methanol hydrogen production technology mainly adopts the methanol steam reforming hydrogen production process. A comparison of the advantages and disadvantages of the three methods for hydrogen production from fossil fuels is shown in Table 2.1.

2.5 Solar Hydrogen Production Solar thermochemical hydrogen production is considered to be one of the most potent ways of sustainable energy utilization and has great strategic significance for promoting carbon peaking and carbon neutrality goals and alleviating energy and environmental crises. Although direct pyrolysis of water can achieve near-zero carbon emission hydrogen production, the problems of ultra-high reaction temperature and difficult separation of hydrogen and oxygen products make it difficult in large-scale hydrogen production. The solar thermochemical cycle indirectly decomposes water to produce hydrogen, reduces the temperature of direct pyrolysis water through the circulation of oxygen-carrying materials, realizes the step-by-step separation of hydrogen and oxygen products, and converts intermittent, fluctuating, and low energy flux density solar energy into stable, high-density solar energy. The chemical energy of hydrogen has received extensive attention and research. However, due to the high temperature, low efficiency, and poor economy of solar thermochemical cycle hydrogen production, the current research is still in the stage of theoretical analysis and experimental research, which hinders the further development of this technology. The development history and important progress of different cycle systems are reviewed around the solar thermochemical cycle for hydrogen production, and the main challenges faced by the thermochemical cycle are discussed and

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suggested, to provide new insights for the research and development of solar thermochemical cycles. With new ideas, it lays the foundation for the efficient, stable, safe, and large-scale production of solar fuels. According to the sixth assessment report of the Intergovernmental Panel on Climate Change (IPCC) of the United Nations, the concentration of greenhouse gases such as carbon dioxide, methane and nitrous oxide in the atmosphere exceeded the pre-industrial revolution levels by 47% and 156%, respectively. and 23%, the global surface temperature rise in the past 10 years is about 1.59 °C [13]. The burning of fossil fuels has a huge impact on the climate and environment and seriously threatens the survival and development of human beings. As a responsible major country, my country promises to the world in 2020 “30 carbon peaks, 60 carbon neutral” [14]. The country has accelerated the development and application of renewable energy represented by solar energy from a strategic level. Solar hydrogen production converts intermittent, unstable, low-energy–density solar energy into clean fuel chemical energy, which is one of the most potent ways for sustainable energy utilization [15].

2.5.1 Solar Thermochemical Hydrogen Production Compared with the traditional approach of hydrogen production from fossil fuels, solar hydrogen production is a relatively new technology that has emerged only in the last three to four decades. The research on solar hydrogen production mainly focuses on the following technologies: thermochemical hydrogen production, photoelectrochemical decomposition hydrogen production, photocatalytic hydrogen production, artificial photosynthesis hydrogen production, and biological hydrogen production. Solar thermochemical hydrogen production has garnered significant attention and research due to its noteworthy advantages. By utilizing concentrated solar thermal energy, water is split to generate hydrogen. This process boasts high theoretical efficiency and produces no emissions of greenhouse gases or harmful substances. Additionally, the energy and raw material sources required for this method are widely distributed. However, due to the ultra-high reaction temperature (at atmospheric pressure of 2500 °C,) and the difficulty of separation of hydrogen and oxygen [16], it is difficult for large-scale application of solar water direct pyrolysis [17]. Furthermore, the practical application of photoelectrochemical hydrogen production is still hindered by its low energy conversion efficiency. At present, more and more semiconductors can be used as photoanode materials. However, these semiconductors generally have wide band gaps, which limit their spectral absorption to the ultraviolet and visible regions. Infrared light accounts for about 50% of the energy of sunlight, so extending the spectral absorption range of materials into the infrared region can help greatly improve the efficiency of the device. Narrow-bandgap semiconductors have near-infrared absorption capabilities. However, the electron– phonon interaction in narrow-bandgap semiconductors leads to a shorter lifetime of photogenerated carriers, and a decrease in the concentration of photogenerated

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holes on the catalyst surface, which in turn reduces the probability of surface oxidation reactions. So far, the photoelectric conversion efficiency (IPCE) of near-infrared photoactive photoanode has been difficult to improve. Even so, the huge advantages of solar hydrogen production cannot be ignored. Through solar thermochemical cycling, the reaction temperature of direct pyrolysis of water can be reduced, and the hydrogen and oxygen can be separated step by step. Based on the above advantages, the solar thermochemical cycle of indirect water splitting to hydrogen production is regarded as one of the main energy utilization technologies for mankind in the future. Compared with other hydrogen production methods, the cost of hydrogen production from fossil energy is lower and the technology is more mature. Hydrogen production from fossil energy is dominated by natural gas reforming and coal gasification, accounting for 59% and 19% of the total hydrogen production respectively [18]. Natural gas reforming or coal gasification for hydrogen production requires heating a mixture of natural gas or coal and water (excess) at 800–1100 °C to produce a mixture of H2 and CO2 , and then obtain pure hydrogen through gas separation (such as pressure swing adsorption). The required high-temperature driving heat energy is usually obtained by burning part of natural gas or coal, so the carbon emission of this method is relatively high. Concentrated solar heating or integrated carbon capture technology can reduce carbon emissions, but hydrogen production costs will increase.

2.5.2 Photoelectrochemical Hydrogen Production A typical photoelectrochemical decomposition solar cell consists of a photoanode and a cathode. The photoanode is usually a photo-semiconductor material, which can be excited by light to generate electron–hole pairs. The photoanode and the opposite electrode (cathode) form a photoelectrochemical cell. In the presence of an electrolyte, the photoanode absorbs light and the electrons generated on the semiconductor band flow to the cathode through an external circuit, the hydrogen ions in the water accept electrons from the cathode to produce hydrogen gas. The semiconductor photoanode is the most critical factor affecting hydrogen production efficiency. The light absorption limit of the semiconductor should be moved to the visible light part as much as possible to reduce the recombination between photogenerated carriers and improve the lifetime of the carriers. The most studied photoanode material is TiO2 . TiO2 is used as a photoanode, which is resistant to light corrosion and has good chemical stability. However, it has a large band gap and can only absorb photons with wavelengths less than 387 nm. The main solution is doping and surface modification. Doping includes non-metal ion doping, metal ion doping, rare earth element doping, etc. For the water splitting reaction to occur, a minimum energy of 1.23 V is required. Now the most commonly used electrode material is TiO2 , which has a forbidden band gap of 3 eV. It is used as the anode of the solar photoelectrochemical hydrogen production system, which can generate 0.7~0.9 V voltage, so a certain bias voltage must be applied to make the water split. Since the commonly used

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methods of applying bias voltage in solar hydrogen production are: using solar cells to apply external bias voltage and using solar cells to apply bias voltage internally, solar photoelectrochemical water splitting for hydrogen production can be divided into one-step and two-step methods. The one-step method involves the preparation of catalytic electrodes directly on the two electrode plates of the solar cell, rather than extracting electrical energy from the solar cell and using it to split water into hydrogen and oxygen through a voltage drop. This method has been paid more and more attention in recent years in the case of the progress in the research of multijunction tandem solar cells (such as triple-junction tandem amorphous silicon solar cells). Since the open circuit voltage of the tandem solar cell can exceed the voltage required for the electrolysis of water, and the electrolyte can be light-transmitting, the reaction of electrolyzed water will be spontaneous in light. The advantage of this method is that the external circuit is eliminated and the energy loss is reduced, but the photochemical corrosion problem of the photoelectrode is relatively prominent, so the research focuses on the energy gap matching between cells, the selection of the anti-corrosion layer on the cell surface and the preparation of the device structure. The design of the catalytic electrode requires a low overpotential, good desorption, transparency to visible light, anti-corrosion, and low cost. Two-step photovoltaic water electrolysis is to carry out the photoelectric conversion and electrochemical conversion of solar energy in two independent processes so that several solar cells can be connected in series to meet the voltage conditions required for water electrolysis. Two-step hydrogen production has the following advantages: in the system, solar cells with high conversion efficiency and better electrochemical electrode materials can be selected respectively to improve the photoelectrochemical conversion efficiency; photochemical corrosion problems caused by the use of semiconductor electrodes can be effectively avoided. However, the two-step method needs to draw the current out of the battery, which consumes a lot of electric energy, because the electrolyzed water only needs a low voltage. When the current density is very high, the overpotential of the electrode is also increased. The photocatalytic principle is shown in Fig. 2.3. Fig. 2.3 Schematic diagram of photoelectric catalysis

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2.5.3 Photocatalytic Hydrogen Production In 1972, Japanese scholars Fujishima A and Honda K published the first experimental study on hydrogen evolution through photo-splitting water using TiO2 single crystal electrodes [19], which opened a new way to produce hydrogen [20]. It is considered to be the best way to produce zero-carbon hydrogen in the future. With the evolution of electrode electrolysis of water to the heterogeneous photocatalysis of semiconductor photocatalytic water splitting for hydrogen production and the successive discovery of photocatalysts other than TiO2 , the research on photocatalytic water splitting for hydrogen production has arisen, and great progress in the synthesis and modification of photocatalysts has been made. With the increasing development of hydrogen energy, countries and regions such as the United States, Europe, Japan, and China are continuing to promote the technology research and development of photo-splitting water for hydrogen production. Semiconductor TiO2 , transition metal oxides, layered metal compounds, such as K4 Nb6 O17 , K2 La2 TiO10 , Sr2 Ta2 O7 , etc., as well as catalytic materials that can utilize visible light, such as CdS, Cu-ZnS, etc., can catalyze water splitting under certain lighting conditions, thereby producing hydrogen. However, so far, the photolysis efficiency of water using catalysts is still very low, only 1–2%. The redox catalytic systems for photocatalytic water splitting that have been studied mainly including semiconductor systems and metal complex systems, among which the research on semiconductor systems is the most in-depth. Semiconductor photocatalysis is similar in principle to photoelectrochemical cells. The tiny photo-semiconductor particles can be regarded as micro-electrodes suspended in water. They work like photo-anode. The cells are so separated that even the cathodes are assumed to be on the same particle, and the reactions of water splitting into hydrogen and oxygen take place simultaneously. When the ultraviolet light less than 387 nm is irradiated to TiO2 , the electrons in the valence band absorb energy and then transition to the conduction band. Holes and electrons are generated in the valence band and conduction band respectively, and the water molecules adsorbed on TiO2 are strongly oxidized. The holes are oxidized to oxygen, and the generated hydrogen ions are transferred into the electrolyte and then reduced to hydrogen by electrons [21]. Compared with the photoelectrochemical cell, the photocatalytic reaction of semiconductor photocatalytic water splitting and dehydrogenation is greatly simplified, but the electron–hole pairs generated on the same semiconductor particle by photoexcitation are very easy to recombine. Therefore, to suppress the reverse reaction of hydrogen and oxygen and the recombination of electrons and holes generated by photo-excited semiconductors, electron donors can be added as hole scavengers to improve the hydrogen desorption efficiency. Many organic compounds in wastewater are good electron donors. If wastewater treatment is combined with photocatalytic hydrogen production, solar hydrogen production, and solar decontamination can be realized at the same time. Xi’an Jiaotong University is one of the earliest teams in China to start the research on solar photocatalytic water splitting for hydrogen production. It took the lead in

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Fig. 2.4 Schematic illustration of the “hydrogen farm” strategy for large-scale solar water splitting to produce hydrogen

establishing the first direct solar continuous flow large-scale hydrogen production demonstration system. The system has been running stably for more than 200 h. At the same time, the standard GB/T 26915-2011 “Energy Conversion Efficiency and Quantum Yield Calculation of Solar Photocatalytic Water Splitting Hydrogen Production System” was formulated. Li Can’s research team from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences has been exploring the demonstration of large-scale applications of solar hydrogen production. Drawing on the idea of large-scale planting of crops on farms, the team proposed and verified the “Hydrogen Farm Project” (HFP) (shown in Fig. 2.4) strategy for large-scale solar water splitting and hydrogen production based on powder nanoparticle photocatalyst system, the STH efficiency exceeds 1.8%, which is the highest value of the STH efficiency of photocatalytic water splitting based on powder nanoparticles reported in the world.

2.5.4 Artificial Photosynthesis Artificial photosynthesis aims to simulate the process of photosynthesis in plants and utilize sunlight to produce hydrogen. The specific process is as follows: first, the metal complex is used to decompose electrons and hydrogen ions in the water; then, the energy of the electrons is increased by the use of solar energy, so that it can photosynthesize with the hydrogen ions in the water to generate hydrogen. The artificial photosynthesis process is water electrolysis, except that solar energy is used instead of electricity. At present, only a small amount of hydrogen can be prepared in the laboratory, and the utilization rate of light energy is only 15–16%.

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2.5.5 Biological Hydrogen Production Certain algae and bacteria present in rivers, lakes, and oceans can act as bioreactors, utilizing water as a raw material and continuously producing hydrogen under the irradiation of sunlight. The physical mechanism of biological hydrogen production is the existence of enzymes related to hydrogen production in some organisms (light and organisms and fermentation bacteria), among which the main ones are nitrogenase and hydrogenase. The biological hydrogen production technology has outstanding advantages such as cleanliness, energy saving, and no consumption of mineral resources. As a renewable resource, organisms can replicate and reproduce themselves and can convert matter and energy through photosynthesis. At the same time, this conversion can obtain hydrogen through the catalysis of enzymes at room temperature and pressure. Photosynthetic organisms capable of producing hydrogen include photosynthetic bacteria and algae. At present, the most studied photosynthetic bacteria are prokaryotes such as Rhodospirillum rubrum and Rhodopseudomonas. The enzyme that catalyzes the hydrogen production of photosynthetic bacteria is mainly nitrogenase. Photosynthetic bacteria contain a photosynthetic system. When photons are captured and sent to the photosynthetic system, charge separation is performed, high-energy electrons are generated, and proteins are formed. Finally, H is reduced under the action of nitrogenase to generate H2 . Many algae (such as green algae, red algae, and cyanobacteria) are microorganisms capable of photosynthetic hydrogen production, and H2 metabolism is mainly carried out by hydrogenase. The most important thing in photosynthetic hydrogen production is to have sufficient sunlight. Therefore, it involves the rational design of the light concentrating system and the light extractor in the biological hydrogen production reactor. The prospect of biological hydrogen production is very good. At present, it is necessary to further clarify the physical mechanism of such biological and microbial hydrogen production, and cultivate efficient hydrogen production microorganisms, so that it is possible to make solar biological hydrogen production in practical technology.

2.6 Biomass Hydrogen Production Biomass energy is the energy provided by living plants in nature. These plants use biomass as a medium to store solar energy and belong to renewable energy. It is calculated that the energy stored by biomass is 2 times larger than the total energy consumption of the world. The earliest energy used in human history is biomass energy. Before the second half of the nineteenth century, firewood was the main energy source used by human beings. Currently, the more effective ways to utilize biomass energy are (1) Biogas production. It mainly uses urban and rural organic waste, straw, water, human and animal excrement to generate combustible gas methane through anaerobic digestion for living and production purposes. (2) Use biomass to produce alcohol. In the current world energy structure, the proportion of biomass energy is very small.

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Solar energy is converted into biomass energy through photosynthesis. It is then processed into gaseous hydrogen energy or liquid biodiesel, methanol, and so on. The hydrogen can be directly used in the fuel cell, and the generated water is reused by the organisms to generate new substances. Biomass can be turned into liquid fuel, which can supply engine output energy and CO2 . The generated CO2 is absorbed in the photosynthesis of organisms, and the CO2 generated during combustion is equivalent to the CO2 absorbed during its growth. Overall, biomass energy does not emit additional CO2 in the process of utilization [22]. The research and development of biomass energy mainly focuses on gasification, liquefaction, pyrolysis, solidification, and direct combustion of biomass. Biomass energy has made great progress since it was recognized by people. Scientists and engineers have been exploring what kind of method, what kind of process, what kind of equipment can make biomass energy better, and in what ways to use the released energy. Biomass energy cannot be directly used in modern industrial equipment as energy and normally needs to be converted, or converted into gaseous fuels, or converted into liquid fuels. Hydrogen is an important energy carrier, so biomass hydrogen is a natural thing to do. Figure 2.5 shows the main methods of biological hydrogen production. In general, the utilization of biomass energy mainly includes microbial conversion and thermal chemical conversion. Biomass hydrogen technology has outstanding advantages such as cleanliness, energy saving, and no consumption of mineral resources. As a renewable resource, organisms can replicate and reproduce themselves, and can also convert matter and energy through photosynthesis. This conversion system can obtain hydrogen through the catalysis of enzymes at room temperature and pressure. From a long-term and strategic point of view, using water as a feedstock and utilizing light energy to produce hydrogen from organisms is the most promising approach. Many countries are investing a lot of financial resources in the development and research of biological hydrogen production technology, to realize the transformation of this technology to commercial production at an early date.

Fig. 2.5 Biological hydrogen production method

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Microorganisms capable of producing molecular hydrogen during physiological metabolism can be divided into two main groups: photolytic hydrogen-producing organisms (green algae, cyanobacteria, and photosynthetic bacteria) and fermentative hydrogen-producing bacteria. The biological hydrogen production technology based on photolysis hydrogen production organisms was first proposed by Gavreau and Rubin and then this research has been carried out rapidly in many countries in the world. From the research results over the years, some experts have predicted that the hydrogen production capacity of cyanobacteria and photosynthetic bacteria is 1/1000 of that of green algae. Therefore, from the perspective of commercialization, hydrogen production by cyanobacteria and photosynthetic bacteria has not been studied [23]. The hydrogen productivity of green algae and the conversion efficiency of solar energy are still low. At the same time, due to many problems such as industrialized production equipment and light sources, the development of photolysis hydrogen production technology is restricted. Compared with photolysis biological hydrogen production technology, fermentation biological hydrogen production technology shows more advantages in many aspects: (1) The hydrogen production capacity and growth rate of fermentative hydrogen-producing bacteria are higher than those of photosynthetic hydrogen-producing bacteria; (2) Fermentation-based biological hydrogen production requires no light source, so the design, operation, and management of the reaction device are simple and convenient; (3) The raw materials for fermentation-based biological hydrogen production are widely sourced and cost-effective; therefore, the fermentation-based biological hydrogen production technology is better than the photolysis biological hydrogen production technology, and such is easier to achieve large-scale industrial production. Photolysis hydrogen-producing organisms Bacteria and green algae. This kind of organism can use the photosynthetic function in the body to convert solar energy into hydrogen energy, and the research on its hydrogen production mechanism is far more profound than that of nonphotosynthetic organisms. Both can photosynthetically split water to produce hydrogen, but the hydrogen production mechanism is different. Gaffron reported that the algae scenedesmus can photo-split water to produce hydrogen. In 1974, Benemann observed that Anabaena cylindrica (heterocyst species) could photolyze water to produce H2 and O2 . Although photosynthetic water splitting is an ideal way of hydrogen production, cyanobacteria, and green algae do not seem to be suitable as sources of hydrogen production. Due to the simultaneous release of oxygen during photosynthetic hydrogen production, in addition to the low efficiency of hydrogen production, the desorption of hydrogen is deactivated by oxygen. Miyamoto of Japan has used nitrogen-starved cells to conduct outdoor hydrogen production research under the condition of a continuous supply of oxygen, but the average conversion efficiency is only 0.2%. Asada also reported the nitrogen fixation and hydrogen production of hydrogen-absorbing enzyme-deficient strains under aerobic conditions. Finally, hydrogen production by hydrogenase can proceed smoothly, but the hydrogen production of this green algae is only 15% of the theoretical value after the transformation [24]. Hydrogen production by green algae under light and anaerobic

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conditions is caused by hydrogenase. The present study shows that under light conditions, the reducing power required by hydrogenase is water and endogenous organic matter (starch). The organic matter accumulated by green algae during photosynthesis during the day can also produce hydrogen through hydrogenase fermentation under dark conditions, but the hydrogen production efficiency is low. Cyanobacteria produces hydrogen. The hydrogen production of cyanobacteria is divided into two categories, one is hydrogen production catalyzed by nitrogenase, and the other is hydrogen production catalyzed by hydrogenase [25]. Nitrogenase produces hydrogen. Nitrogenase is inactivated in the presence of oxygen, producing hydrogen and releasing oxygen at the same time. The mechanism of nitrogen fixation and hydrogen release varies from species to species. Anabaena cylindrica is a filamentous aerobic nitrogen-fixing bacterium with two types of cells: vegetative cells and heterosexual cells. Vegetative cells contain photosystems I and II, which can perform photolysis of H2 and reduction of CO2 to produce O2 and reducing substances. The produced reducing substances can be transported to heterocytic cells through thick-walled pores as hydrogen donors for nitrogen fixation and hydrogen production in heterocytic cells. Heteromorphic cells only contain photosynthetic system I and have a thicker cell wall, which provides a local anaerobic or low oxygen partial pressure environment for heteromorphic cells, so that the process of nitrogen fixation and hydrogen release can be carried out smoothly. A unicellular oxygen-consuming nitrogen-fixing bacteria without heterocytic cells, its hydrogen production is also catalyzed by nitrogenase. Cells fix CO2 to store polysaccharides and release oxygen, while under dark anaerobic conditions, the stored polysaccharides are degraded into electron donors for nitrogen fixation and hydrogen production. In this way, cells produce hydrogen under alternating light and dark conditions. Hydrogenase production of hydrogen. There are relatively few studies on its hydrogen production. Oscillatoria limnetica is a kind of non-heterocytic facultative aerobic nitrogen-fixing filamentous cyanobacteria. Its hydrogen production process is catalyzed by hydrogenase, and the glycogen accumulated during photosynthesis during the day is exposed to argon or anaerobic conditions. Hydrolysis produces hydrogen. Spirulina platensis can produce hydrogen by hydrogenase in dark anaerobic conditions. The study also reported a reversible hydrogenase (reversible hydrogenase), but this type of enzyme is still controversial. Anaerobic photosynthetic bacteria. Compared with cyanobacteria and green algae, its anaerobic photosynthetic hydrogen release process does not produce oxygen. The process is simple, and the hydrogen production purity and efficiency are high. Since Gest first proved in 1949 that photosynthetic bacteria could utilize organic matter for photosynthesis and dehydrogenation, Japan, the United States, Europe, China and other countries have carried out a lot of research on it. Although photosynthetic dehydrogenation has the potential to produce hydrogen, the process is complex and precise. As a result, current research is primarily focused on screening or breeding highly active hydrogen-producing strains, optimizing and controlling environmental conditions to increase hydrogen production. The research is still at a laboratory or pilot level in terms of level and scale. Almost all purple non-sulfur bacteria produce hydrogen through photosynthesis catalyzed by nitrogen enzymes,

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which is different from cyanobacteria and green algae. Photosynthetic bacteria only provide ATP through photosynthesis, and not reducing power. In some species, reducing power is obtained by hydrogenase (HD) as a component of organic matter. Formate dehydrogenase (FDH), which is related to hydrogen production, catalyzes non-productive reactions and is repressed by O2 , NO− , and MB. Another reverse electron transfer occurs. Under the conditions of limiting nitrogen or providing hydrogen production, the electrons generated by the photosynthetic oxidation of organic matter are transferred to Fd for reduction [26], and the ferritin (nitrogenase reductase) of nitrogenase oxidizes it while accepting electrons from reduced Fd. Under the action of ATP and Mg2+ , ferritin is activated to form a reduced nitrogenase reductase-ATPMg2+ complex. The complex then transfers electrons to the ferritin of nitrogenase to make it an active nitrogenase. In the absence of a suitable substrate, nitrogenase uses H+ as the final electron acceptor to reduce it to produce the molecule H2 [27].

2.7 Hydrogen Production from Wind, Ocean, Water and Geothermal Energy 2.7.1 Hydrogen Production from Wind Energy Wind energy and solar energy are typical clean energy sources with the advantages of low pollution, low carbon emissions, renewability, and water conservation. Thanks to its favorable geographical location and topography, China boasts abundant reserves of wind and solar energy. In recent years, wind power generation and solar power generation have experienced rapid development. Wind energy is the kinetic energy generated by a large amount of air flowing on the surface (Fig. 2.6). Influenced by many factors, such as atmospheric circulation, surface topography, solar radiation, and air composition, atmospheric pressure difference occurs in different areas. In the horizontal direction, air flows from the high-pressure area to the low-pressure area to form the wind. China is rich in wind energy resources. It is estimated that the total reserves of wind energy resources in China are about 2.4 × 109 kW [28]. This is due to China’s location on the eastern side of the sub-continent, adjacent to the Pacific Ocean, which results in strong monsoon winds. There are many inland mountains and complex terrain, and the Qinghai-Tibet Plateau stands in the west, which changes the air pressure distribution and atmospheric circulation, and increases the intensity of monsoons in China. With abundant wind energy resources, China’s wind power industry has developed rapidly. British oil company BP and Ember, a British climate think tank, have counted the installed capacity and power generation data of wind power in various economies around the world by 2019 (the data of Germany will be updated to 2020). By the end of 2019, the installed capacity of wind turbines in my country reached 210.48 GW, far ahead in the world and more than double that of the second-placed United States [29]. Wind power generation has also increased rapidly in recent years, reaching

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Fig. 2.6 Technical diagram of hydrogen production by wind power generation principle of hydrogen production by wind energy

405.70 TWh. Generally, the site selection of wind turbines needs to meet several conditions: (1) Generally, the average annual wind speed is required to be above 6 m/s (60–70 m height) and above 5.8 m s−1 in mountainous areas; (2) The accumulated hours of wind speed of 3–25 m s−1 in a year are more than 2000 h (3000–5000); (3) The annual average effective wind power density is above 150 W m−2 ; (4) The average spacing of each machine is 4–6 times of the blade diameter; (5) The gridconnected conditions are good, and the wind farm is required to be no more than 20 km away from the connected grid; (6) Distance of more than 300 m from residential areas. Therefore, in general, the location of wind power generation will be at sea, or the plain or valley with strong wind and far away from residential areas. Therefore, hydrogen production from offshore wind power is one of the main forces of green hydrogen production in the future [30]. The total reserves of hydrogen production projects from electrolyzed water have reached 32 million kW, about half of which comes from offshore wind power. Among them, Germany, the Netherlands, Denmark, and other European countries have plans for hydrogen production from offshore wind power with a capacity of more than one million kilowatts. China’s offshore wind power is developing rapidly. According to the latest data released by the National Energy Administration, the newly installed capacity of wind power and photovoltaic power generation in China will reach 101 million kW in 2021, of which the newly installed capacity of wind power will reach 47.57 million kW. Offshore wind power has sprung up suddenly, and new installations have been installed throughout the year. The capacity of the wind turbine is 16.9 million kW, which is 1.8 times of the total scale built before. At present, the cumulative installed capacity reaches 26.38 million kW, surpassing Britain and ranking first in the world, which is close to half of the cumulative installed capacity of offshore wind power in the world. It is

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estimated that by the end of 2030, the cumulative grid-connected installed capacity of offshore wind power in China will reach 97 GW, and the cost of leveling electricity will be reduced by 46% compared with the level in 2021. At present, hydrogen production by electrolysis of water is mainly divided into four technical routes: alkaline electrolysis of water, proton exchange membrane (PEM) electrolysis of water, solid oxide electrolysis of water, and solid polymer anion exchange membrane electrolysis of water [31]. Among them, solid oxide electrolyzed water has the highest hydrogen production efficiency [32]. However, its working temperature is high (700~900 °C), and it is inconvenient to start and stop the electrolyzer. It is still in the initial demonstration stage, so it is not suitable for offshore wind power hydrogen production. The working temperature of solid anion exchange membrane electrolyzed water to produce hydrogen is low (40~60 °C), and it can be started and stopped quickly. It is still in the initial stage of laboratory research and development, and cannot be applied to offshore wind power to produce hydrogen in a short time [33]. At present, the technologies of hydrogen production from electrolyzed water that can be used in offshore wind power applications are mainly alkaline electrolyzed water and PEM electrolyzed water. Hydrogen production from alkaline water is a mature technology that has been fully industrialized, and its working temperature is moderate (70~90 °C), but its start-stop response time is long, and current density is low, which leads to the environmental pollution caused by alkali infiltration, and requires complex maintenance of alkaline fluid [34]. In addition, the output pressure of hydrogen production is low, and extra pressure is needed during storage and transportation, which weakens the advantage of lower initial investment cost to some extent. On the whole, alkaline water electrolysis technology is more suitable for land water electrolysis. PEM hydrogen production from electrolyzed water is a promising green hydrogen production technology in the future [35], and it has entered the preliminary commercial stage. Compared with that of alkaline hydrogen production from electrolyzed water, it has a lower working temperature (50~80 °C), faster start-up time, current density increased to 5 times, more flexible operation, which is conducive to rapid load change, and has good matching with offshore wind power with strong volatility and intermittence. Moreover, the electrolyzer has compact structure, smaller floor area, higher hydrogen output pressure, no corrosive medium pollution, and is safer and more reliable, so it is especially suitable for centralized or distributed electrolytic water hydrogen production at sea. The main bottleneck of hydrogen production from electrolyzed water by PEM lies in its cost and service life, because its electrodes, coatings and catalysts are mostly made of precious metals, the current price is still higher than that of alkaline electrolyzed water, and its service life is low. However, with the in-depth research, popularization, and application of noble metal catalyst, anode diffusion layer, bipolar plate, and other high-cost components of PEM electrolyzer, its cost is expected to drop rapidly. Due to the low storage and high cost of rare metal iridium in PEM electrolysis water technology, Liu et al. [36] focused on three common low-activity, high-stability low-iridium catalysts, namely heteroatom-doped iridium-based catalysts, perovskite-based iridium-based catalysts, and pyrochlore-based iridium-based catalysts. The relationship between the structural properties of the materials and the

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intrinsic properties of catalysis was studied. Ding et al. [37] aimed at the problem of the high cost of the anode diffusion layer and a bipolar plate of platinum-plated porous titanium plate commonly used in PEM electrolyzers, prepared iridium ruthenium mixed oxide coating on the surface of titanium fiber felt by coating-roasting reduction method, and applied it to the anode diffusion layer of PEM electrolysis. The effects of the optimal composition ratio of coating, thickness of titanium fiber felt and preparation method of membrane electrode assembly on the performance of the electrolyzer were analyzed, and the feasibility and effectiveness of noble metal oxide coating in reducing the cost of PEM electrolyzers were verified. Tajuddin et al. [38]. developed a low-cost, corrosion-resistant and high-stability non-noble metal catalyst instead of platinum-based catalyst. Using graphene-coated NiMo alloy as the non-noble metal catalyst electrode of PEM electrolyzer, the balance between cathode catalytic activity and chemical stability was achieved in acidic environment by adjusting the number of nitrogen-doped graphene layers to 4–8, which provided a new way for low-cost replacement of noble metal catalyst in PEM electrolyzer.

2.7.2 Hydrogen Production from Ocean Energy The total area of the earth’s oceans is about 360 million square kilometers, accounting for 71% of the earth’s surface area, and the average water depth is about 3795 m. The ocean contains more than 1.35 billion cubic kilometers of water. Therefore, if the seawater resources can be fully utilized, many energy problems existing in the current society will be solved. At present, the main methods of hydrogen production from ocean energy include tidal energy, wave energy, temperature difference energy, ocean current energy and so on. Tidal energy was first recognized and utilized by people. More than a thousand years ago in the Tang Dynasty, coastal residents in China used tidal power to grind millet. In the eleventh century, tidal mills on the west coast of Europe appeared and were brought to the New World of America. In 1600, the French built the first tidal mill in America on the east coast of Canada. There is still a 12th-century tidal mill in Safol, England, and it is still grinding millet for tourists to visit. In the mid-1950s, there was a climax of tidal energy utilization in China’s coastal areas, and more than 40 small tidal power stations and some hydraulic pumping stations were built. Due to various reasons, only the 40 kW tidal power station in Shashan, Zhejiang Province remains. Jiangxia Port Power Station in Yueqing Bay, Zhejiang Province is the largest tidal power station in China and the third largest tidal power station in the world. Since the 1980s, it has developed rapidly, and the micro tidal power generation device for navigation lights floating has become commercialized. The rear elbow buoy power generation device developed in cooperation with Japan has been exported abroad, and its technology has reached the international leading level. The shore-fixed wave power station developed on Dawanshan Island in the Pearl River Estuary is the first device with an installed capacity of 3 kW. Through the huge seawater fluctuation

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brought by the ebb and flow of the tide, the generator set is driven to rotate, to generate electricity, and then hydrogen is prepared by electrolytic water technology. This technology of using water energy to generate electricity and then using electric energy to electrolyze water can be truly pollution-free and environment-friendly. Hydrogen production from wave energy Wave energy that is similar to ocean energy refers to the kinetic energy and potential energy of ocean surface waves. The energy of a wave is directly proportional to the square of the wave height, the movement period of the wave, and the width of the wavefront. Wave energy is the most unstable energy in ocean energy, but it has the highest taste, the widest distribution, and high energy flow density. According to the survey of the World Energy Commission, the available wave energy in the world reaches 2 billion kilowatts, which is equivalent to twice the current world power generation energy. China has vast marine resources, and the theoretical storage of wave energy is about 70 million kilowatts, and the energy flow density of coastal wave energy is about 2~7 kW m−1 . The coastal areas of Zhejiang, Fujian, Guangdong, and Taiwan Provinces are rich in wave energy. Waves mainly refer to wind-induced waves. It can be said that the source of wave energy is solar energy. When the wind blows, water waves will appear on the calm water surface under the action of friction. With the increase of wind speed, the peak increases, and the distance between two adjacent peaks also increases gradually. When the wind speed continues to increase to a certain extent, the wave crest will break, and then a wave will be formed. Currently, the method for hydrogen production from wave energy involves wave power generation followed by the electrolysis of water to produce hydrogen. Wave power generation is one of the main ways to utilize wave energy. There are many devices for utilizing wave energy. At present, the research focuses on four kinds of devices that are considered to have commercial value, including oscillating water column devices, pendulum devices, oscillating float devices, and contraction channel devices. These devices are composed of three-stage energy conversion mechanisms. Among them, the primary energy conversion mechanism (wave energy capture device) converts wave energy into the mechanical energy of a carrier; The secondary energy conversion mechanism converts the primary energy into mechanical energy; the Three-stage energy conversion converts mechanical energy into electrical energy through a generator [39].

2.7.3 Hydrogen Production from Hydraulic Energy Hydraulic energy refers to other forms of energy, such as potential energy, caused by gravity drop. Hydrogen production from hydraulic energy is similar to that from ocean energy. Generally speaking, the gravitational potential energy of water is converted into the kinetic energy of a generator set, and the generator set rotates to generate electricity, and then electrolyzed water is hydrolyzed to produce hydrogen (Fig. 2.7). As China’s terrain shows a stepped downward trend from west to east, resulting in a big gap between east and west, China has great advantages in utilizing

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2 Current Status of Hydrogen Energy Development

Fig. 2.7 Scheme hydroelectric generator

hydraulic energy, such as the Three Gorges Dam of the Yangtze River, the largest hydropower station in the world, which has important responsibilities such as power generation, flood control and shipping [40].

2.7.4 Hydrogen Production from Geothermal Energy Geothermal energy, as an earlier energy except for ocean energy and wind energy, is green, low-carbon, and recyclable renewable energy. Driven by the 13th FiveYear Plan for Geothermal Energy Development and Utilization and other policies, China’s geothermal energy utilization scale has grown rapidly, and its technical level has been continuously improved. International exchanges and cooperation have become increasingly close, which is in the golden period of development. In the future, geothermal energy will occupy a more important position in the utilization of renewable energy in China. The earliest form of geothermal energy used in Chinese history is hot springs, which can be traced back to the pre-Qin period. The so-called geothermal energy is used to produce hydrogen and generate electricity, which is the process of converting geothermal energy into mechanical energy and then into electrical energy. The main areas of middle and deep geothermal power generation in China are Tibet, western Yunnan, and western Sichuan, which are rich in high-temperature geothermal resources. At present, the geothermal power generation projects completed and put into operation mostly adopt flash power generation technology and organic Rankine cycle technology. By the end of 2019 [41], the installed capacity of geothermal power generation in my country has increased from 27.28 MW at the end of the “Twelfth Five-Year Plan” to 49.08 MW, an increase of 21.8 MW. Affected by factors such as my country’s geothermal electricity price policy and equipment technology, some geothermal power stations built in the twentieth century have been shut down. At present, only the Yangyi Geothermal Power

2.8 Hydrogen Production from Nuclear Energy

49

Station and the Yangbajing Geothermal Power Station in Tibet continue to operate. Therefore, the overall growth is slow.

2.8 Hydrogen Production from Nuclear Energy Nuclear energy (or atomic energy) is the energy released from atomic nuclei through nuclear reactions, which conforms to Albert Einstein’s mass-energy equation E = mc2 , where E = energy, m = mass, and c = speed of light. Nuclear energy can be released through one of three nuclear reactions: (1) Nuclear fission, the fission of heavier nuclei to release nodule energy. (2) Nuclear fusion, where lighter nuclei fuse to release nodule energy. (3) Nuclear decay, is the release of energy during the spontaneous decay of atomic nuclei. At present, global energy has entered a new stage of development, and the world’s energy has accelerated its transformation to diversification, cleanliness, and low carbon. Nuclear energy and hydrogen energy are representatives of clean, efficient, and safe energy. The integration of nuclear and hydrogen energy will facilitate a clean energy production and utilization process, promoting high-quality economic development, and enabling carbon peaks and carbon neutrality to be achieved. At present, civil nuclear energy is mainly power generation. As fourth-generation nuclear energy systems and other related technologies continue to mature and become more widely implemented, the comprehensive utilization of nuclear energy, with a particular emphasis on hydrogen production, will play a crucial role in building a clean and low-carbon energy system. Hydrogen production by nuclear energy refers to hydrogen production by coupling nuclear reactor technology with advanced hydrogen production technology. Different reactor types can provide heat or electric energy required for hydrogen production in different temperature ranges (Table 2.2). At present, there are several ways to produce low-carbon hydrogen from nuclear energy: cold water electrolysis to produce hydrogen. Nuclear energy provides electricity for cold water electrolysis, and the hydrogen energy project of Hitsham Nuclear Power Station in the UK has studied this process. This process is available and has the lowest cost in the prior art, and has been verified on a small scale. The temperature of high temperature steam electrolysis is about 600–1000 °C, and its energy consumption is 1/3 less than that of cold water electrolysis, so it is expected to achieve higher efficiency. Low temperature can also improve the electrolysis efficiency. For example, the low temperature heat energy (150–200 °C) of the British pressurized water reactor supports steam electrolysis, which is proven to be feasible, and its efficiency is better than that of cold water electrolysis. Using the heat of 600–900 °C generated by AMR, water can be decomposed into hydrogen with high efficiency under the condition of using chemical catalyst [42]. The existing reactor cannot generate enough high temperature for this process, but the government is developing AMR to support this application. Nuclear waste heat provides high-temperature heat for steam reforming of fossil fuels to produce hydrogen, but it needs to be equipped with carbon capture and storage facilities.

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Table 2.2 Parameters of different reactor types and hydrogen production process Reactor

Outlet temperature/°C

Suitable technology

Qingshuidui

280–325

Brine electrolysis

Heavy water reactor

310–319

Supercritical water reactor

430–625

Electrolytic water and thermochemical cycle

Fast reactor

500–800

Electrolytic water, thermochemical cycle, methane steam reforming

Molten salt reactor

750–1000

Gas cooled fast reactor

850

Electrolytic water, steam electrolysis, thermochemical cycle, methane steam reforming

High-temperature gas cooled reactor

750–950

Many kinds of reactors can be coupled with the hydrogen production process, but from the point of view of hydrogen production, hydrogen production efficiency is closely related to working temperature. To obtain high hydrogen production efficiency, the reactor type with high outlet temperature should be selected. High-temperature gas-cooled reactors (outlet temperature 700~950 °C) and very high-temperature gas-cooled reactors (outlet temperature above 950 °C) are the most suitable choices. The new generation of nuclear hydrogen technology is summarized as high-temperature gas-cooled reactor and iodine–sulfur cycle hydrogen production. The feasibility study of the steam supply project of Lianyungang Chemical Industry Park [39], a domestic model project of 600 MW high-temperature gas-cooled reactor, has been completed [43]. 10 MW high-temperature gas-cooled reactor has been built and operated in China, and 200 MW high-temperature gas-cooled reactor commercial demonstration power station has been completed and put into operation in 2020. China National Nuclear Corporation and Tsinghua University have started the project implementation of 600 MW high-temperature gas-cooled reactor commercial power station, basically completed its standard design and evaluation, and started the site selection [44]. Comparison of three different hydrogen production technologies: According to the total green hydrogen (90%) smelting calculation, the demand for green hydrogen in a million-ton green hydrogen metallurgical steel plant is 100,000 nm3 h−1 , and the consumption index is 750 Nm3 ton−1 steel. Considering the following three hydrogen production schemes: (1) Hydrogen production from natural gas (NG). The cost of hydrogen production from natural gas is about 1.97 yuan Nm−3 (natural gas is calculated according to 2.5 yuan m−3 ), and there is a small amount of CO2 emission; (2) Hydrogen production by nuclear power. The cost of hydrogen production by electrolysis of nuclear water is calculated according to the on-grid electricity price of nuclear power of 0.43 yuan kWh−1 , plus other expenses (about 0.3 yuan Nm−3 ) such as depreciation, maintenance and transportation costs of hydrogen production stations, and

2.9 Hydrogen Production by Hydrogen-Containing Carrier

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the cost of hydrogen is close to 2.5 yuan Nm−3 ; (3) Wind power and photovoltaic hydrogen production. According to the calculation of wind power and photovoltaic electrolysis water hydrogen production cost and transmission cost (0.1 yuan Nm−3 ), when the low electricity price is 0.3363 yuan kWh−1 , the hydrogen production cost is 1.83 yuan m−3 . From the analysis of cost and resources, the current domestic price of wind power and photovoltaic power is about 0.35 yuan kWh−1 . With the progress of technology, the cost of wind power and photovoltaic power generation is expected to drop to 0.15~0.2 yuan kWh−1 , and the cost of hydrogen production by electrolysis of water will be further reduced. According to the report of China Net Finance on June 22nd, the comprehensive cost of green hydrogen in Baofeng Energy’s national demonstration project of hydrogen production from solar electrolyzed water is 1.34 yuan Nm−3 . With the continuous updating of technology and the investment of the company’s depreciation funds, the cost of hydrogen production from renewable energy can be reduced to 0.7 yuan Nm−3 , which is equivalent to the cost of hydrogen production from fossil energy, reaching the best level in the industry. Judging from the current situation, the cost of wind power and photovoltaic power generation is expected to drop below 0.2 yuan kWh−1 in the next few years, and the cost of hydrogen production from wind power and photovoltaic power may drop below 1.16 yuan Nm−3 . To be economically competitive, nuclear energy is only possible in the short term, such as biomass thermochemical hydrogen production and methane reforming hydrogen production, and thermochemical iodine–sulfur cycle (pyrolysis) hydrogen production and high-temperature solid oxide (electrolysis) hydrogen production can be planned as long-term technical research targets [45].

2.9 Hydrogen Production by Hydrogen-Containing Carrier As a colorless, odorless, and highly flammable gas at room temperature and pressure, hydrogen exists in most parts of the earth. Despite not being a highly reactive gas, hydrogen can readily form compounds with the majority of elements. There are millions of known hydrocarbons. In addition to the hydrogen production methods described above, methods for producing hydrogen can also be found in these hydrogen-containing carriers. The above content has briefly mentioned the method of preparing hydrogen from methanol vapor. This method is described in more detail below. At present, it is known that Johnson-Matthey Company started to prepare hydrogen from methanol steam in the laboratory in the 1970s [46], and the technology of methanol steam reforming for hydrogen production is relatively mature at present. Compared with other hydrogen production methods, methanol steam reforming hydrogen production has several obvious characteristics [47]: (1) If primary energy is used as raw material to prepare hydrogen, the preparation process needs to be designed at about 800 °C. This will make the preparation equipment run at a very high temperature, which will inevitably increase the

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investment. Besides, the steam for conversion, the heat source for preheating combustion air and purifying hydrogen, and the recovery and utilization of side steam must also be considered. Methanol is a secondary energy product from primary energy. The reaction temperature of reforming methanol to produce hydrogen only needs (200~300 °C), which is much lower than that of the hydrogen production process using primary energy as raw material. Compared with the same-scale natural gas or light oil conversion hydrogen production plant, the energy consumption of methanol steam reforming hydrogen production is only half of the former, so it is more suitable for small and medium-sized hydrogen production. (2) Methanol raw materials have high purity, do not require further purification treatment, mild reaction conditions, simple processes, and convenient operation. (3) The equipment of this hydrogen production process is detachable or mobile, which is easy to operate and flexible to carry.

2.9.1 Reaction of Hydrogen Production from Methanol According to the present research, there are three possible reactions for hydrogen production from methanol: methanol cracking, reverse water vapor shift reaction, and methanol steam reforming. Combined with the research contents, it is found that methanol steam reforming is a high-efficiency hydrogen production method. At present, copper catalysts are mainly used for hydrogen production from methanol steam reforming. At first, Pour [48] found that there was always CO in the reaction according to experimental phenomena, so he proposed a mechanism similar to methanol steam reforming, and methanol was cracked to generate CO and H2 at first, and then CO and water vapor reacted to generate CO2 and H2 . This explanation explains why CO always exists in products.

2.9.2 Catalysts for Hydrogen Production by Steam Reforming of Methanol At present, there are three kinds of catalysts for methanol steam reforming, namely, copper-based catalysts, nickel-based catalysts, and precious metal catalysts. 1. Copper catalyst The earliest catalyst for methanol steam reforming was the copper catalyst, the main catalyst was CuO, and the carrier was Al2 O3 or SiO2 . To reduce the CO content in the product gas, it is generally necessary to add ZnO, that is, its basic composition is CuO/ZnO [49]/Al2 O3 [50] or CuO/ZnO [51]/SiO2 [52].

2.9 Hydrogen Production by Hydrogen-Containing Carrier

53

X-ray photoelectron spectroscopy (XPS) and temperature program reduction (TPR) show that the addition of ZnO increases the dispersion of Cu on the catalyst surface, and it is considered that ZnO and Cu form Cu+ –O–Zn2+ , thus stabilizing Cu in the hydrogen production reaction of methanol steam reforming. Cu is considered the active center of the catalyst for MSR reaction [53]. In addition, many researchers directly use some special oxides as carriers in their research, including Cu/ZrO2 [54], Cu–Ni [55], Cu–CeO [56], Cu–Mn spinel [57], Cu–CeZrYOx [58], Cu–Zr–LaY [59] and Cu–Ce. 2. Nickel-based catalyst The basic composition of a nickel catalyst is NiO/Al2 O3 , in which NiO is the active component of the catalyst and Al2 O3 is the carrier of the catalyst. Its industrial preparation method is the same as that of industrial methane steam-reforming catalyst. Nickel-based catalysts have the advantages of good stability, wide application range, and low toxicity. The disadvantage is a high reaction temperature. Generally, when nickel catalysts are used, the reaction temperature is higher than 300 °C and the reaction pressure is generally below 3 MPa. However, when the reaction temperature rises, the CO content in the system increases and a certain amount of by-product methane exists. Precious metals are characterized by high activity, good selectivity, and stability. Precious metal catalysts for methanol steam reforming are generally Pt and Pd, and the carriers are generally Al2 O3 , SiO2 , TiO2, or ZrO2 .

2.9.3 Process of Hydrogen Production by Steam Reforming of Methanol The process flow of hydrogen production by steam reforming of methanol can be summarized as follows: methanol and desalted water are mixed according to a certain proportion, preheated by the heat exchanger, and then fed into a vaporization tower; vaporized methanol–water passes through the heat exchanger, then fed into the reactor for catalytic cracking and shift reaction in catalyst bed; the reactor contains about 74% hydrogen and 24% carbon dioxide; after heat exchange, cooling and condensation, it is fed into water washing tower; unconverted methanol and water are collected in tower bottom for recycling; and the top gas is sent to pressure swing adsorption device to purify hydrogen. According to the different requirements of product hydrogen purity and trace impurity components, the hydrogen purity can reach 99.9~99.999% by adopting four or more pressure swing adsorption processes. One mol methanol reforming requires 131 kJ of energy, of which 82 kJ is used for the gasification of liquid reactants. However, in actual operation, to improve the conversion rate of methane and reduce the content of impurity CO in the product gas, the ratio of water to carbon exceeding stoichiometric is generally adopted. In industrial operations, the water-carbon ratio is generally two or even higher.

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2 Current Status of Hydrogen Energy Development

2.10 Coupling with Hydrogen Production Taking photoelectric coupling hydrogen production as an example, photoelectric thermal coupling hydrogen production is a process of decomposing water to produce hydrogen by using the photothermal effect and photoelectric effect of sunlight. Licht [60] put forward and conducted a feasibility study on the thermodynamics of photo-thermal-electrochemical hydrogen production, in which part of the longwave light of sunlight was used to directly heat water, reduce the voltage required to decompose water, thus improving the hydrogen production efficiency. The key point of this theory lies in the collection and separation of spectra. Through spectral separation, the light intensity (heat) with a low band gap is used for heating, while the light intensity with a high band gap is used for irradiating semiconductors to produce a photovoltaic effect or photoelectrochemical effect. Licht also proposed that the conversion efficiency of the solar-coupled hydrogen production system was expected to reach 50% if sufficient light intensity, temperature, pressure, and photosensitizer band gap were provided under the condition that thermodynamics and photosensitizer (band gap) were satisfied. Previously, in the solar hydrogen production system, it was expected that the conversion efficiency could reach up to 30% at room temperature because the influence of low band gap photothermal on water decomposition voltage was not considered. This system mainly includes daylighting and light gathering and spectral analysis of low band gap (heat) and high band gap (electricity). Light with a low band gap is used to heat water to a certain temperature and pressure, while light with a high band gap causes photovoltaic effect or photoelectrochemical charge transfer. Different from the traditional regional solar collectors, this system can use a single solar collector, so this method provides an efficient way to utilize solar energy. Compared with the direct thermal decomposition method and thermochemical cycle method, the composite/coupled system overcomes some temperature limitations and absorbs the advantages of the photothermal method, photovoltaic method, and photoelectrochemical method for hydrogen production. Theoretically, it is not enough in thermodynamics if only the infrared part of sunlight is used to supply heat to traditional solar cells. Therefore, the influence of solar thermal effect is generally not considered when studying hydrogen production by photovoltaic method and photoelectrochemical method. However, the composite system utilizes the energy of all wavelengths of sunlight, thus improving the utilization efficiency of solar energy.

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Chapter 3

Alkaline Water Electrolysis

Electrochemical methods to produce hydrogen require electrical energy to facilitate the reaction. Green hydrogen can be produced electrochemically if the electrical energy is obtained from processes that benefit the environment, such as wind solar tidal energy, etc. Several electrochemical methods can be used to generate hydrogen, of which water electrolysis is the best known. Other electrochemical processes also have the potential to produce hydrogen, however, an important aspect of the production of green hydrogen from electrical energy is the clean technology of using electrical energy to produce hydrogen from low-value feedstocks. Troostwijik and Diemann first discovered the phenomenon of electrolysis in 1789 as one of the main landmarks of electrochemical hydrogen production [1, 2]. Another milestone was the idea that Faraday pointed out in 1834 that the mass of the reacting substance at an electrode was proportional to the circulating charge. In the military marine sector, nuclear-powered water electrolyzers were used to produce oxygen from water for deep-sea survival. The energy crisis of the 1970s again aroused more interest among scientists in electrolysis technology. By 1980, seven manufacturers around the world were producing ammonia using hydrogen mainly from alkaline electrolysis plants with capacities of 535–30,000 standard cubic meters per hour (electricity consumption in the range of 2–100 MW). By the end of the twentieth century, alkaline electrolysis (AWE) hydrogen production was the most mature and commercialized electrolytic hydrogen production technology [3], with MW-scale electrolysis plants being used commercially. In this chapter the focus is on the consideration of electrochemical hydrogen production from alkaline solutions, reviewing the basic principles of electrolysis and providing information on the types of electrolyte states, the selection, and design of electrolytic cells and electrode materials. Finally, it focuses on the integration of electrolytic cells and various renewable generators, with emphasis on green hydrogen production. In electrochemical reactions, the electrolysis of water is a process in which a chemical reaction takes place under the influence of an electric field generated by an electrode. Usually, the electrodes are solid metals or semiconductors. In addition to the electrodes, two types of charge conductors are required to form the external circuit © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_3

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(electron conductor) and the internal circuit (ion species conductor). The medium through which the ions conduct electricity is denoted as the electrolyte. In electrochemistry, electrical energy is the driver that provides the free energy required for the desired reaction to take place. Some electrochemical reactions are reversible, such as rechargeable batteries and reversible fuel cells/electrolyzer cells. Electrons are transferred directly from the electrodes to or from the molecules and atoms involved in the reaction. This is why redox (or redox) reactions take place at the electrodes. In a complete electrochemical electrolytic cell, an oxidation reaction and a reduction reaction will take place at two different locations for a complete reaction. Ionic species are exchanged through the electrolyte medium, while electrons are transferred through an external circuit. The electrolysis reaction is a non-spontaneous electrochemical reaction that requires the consumption of electrical energy to occur. The process of electrolysis of water (or steam) can be seen as a superposition of simultaneous or consecutive electrochemical reactions (half-reactions) occurring near the electrodes and the overall effect of splitting water molecules and separating gaseous products (hydrogen and oxygen). These electrodes are connected to a direct current (DC) power supply. The electrode connected at the positive end of the power supply is denoted as the anode, while the negative end is denoted as the cathode. In a typical water electrolysis process, oxygen is evolved at the anode, and hydrogen is produced at the cathode. Alkaline electrolysis is one of the leading candidates for the large-scale production of green hydrogen soon.

3.1 Electrolysis Cell Alkaline electrolyzers use an alkaline electrolyte with a pH greater than 7. In an alkaline electrolyte, the moving ion species are hydroxyl negative ions whereas the typical electrolyte solution is potassium hydroxide or sodium hydroxide. An alkaline electrolyzer for hydrogen production requires a comprehensive water treatment plant. The purity of water can be a major concern in water electrolysis. The presence of metal atoms in the water, such as calcium or magnesium, can cause reactions on the electrode surface that eventually lead to the formation of scaling, which reduces the active surface area and enhances the blockage of the diaphragm. The salinity of the water is also important. If the water contains sodium chloride or chloride ions from other sources, the chlorine will have a highly corrosive effect at the cathode. Commercial alkaline electrolyzers have efficiencies in the range of 60–70%, while advanced systems currently under development can exceed 90%. Commercial alkaline electrolyzers operate at temperatures of 80–200 °C and have a production capacity of between 500 and 30,000 Nm3 h−1 (or 0.5–40 MW per unit of LHV equivalent to the hydrogen produced). The concentration of electrolyte must be high enough to ensure good mobility of the ions; typically potassium hydroxide concentrations of 30% or more are required.

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Conventional AWE has some operational limitations, in particular, the maximum current density is usually limited to 0.45 A cm−2 (typically 0.2–0.4 A cm−2 ). This is because at higher current densities the resulting air bubbles flow upwards along the electrode surface by gravity, thus forming a continuous non-conductive air film over the entire electrode surface. Alkaline electrolyzers are developed and produced by companies such as NEL (Norway), MacPhy (France), ErreDue (Italy), and Enapter (Italy). In the new AWE electrolyzers, electrodes made up of a porous grid are pressed onto the diaphragm to reduce the ohmic resistance by reducing the spacing distance. This zero-gap configuration increases electrolytic efficiency [4, 5]. By using this new process, the current density of AWE systems can be increased to 2 A cm−2 [6, 7]. The water vapour electrolyzer can also avoid the “bubble evolution problem”, as in the case of the Russian company Centrotech [2]. In this electrolyzer, the water vapour is electrolyzed and the hydrogen produced is carried out with the water vapour, the hydrogen/water vapour mixture is dynamically circulated in the electrolyzer and thus there is no problem with bubbles. Anion exchange membranes with high thermal stability (especially those based on polyvinyl alcohol) can be used for water vapour electrolysis [8]. However, the water balance control of such systems is very complex, and improvements in sensors and control systems are particularly needed if water vapour electrolyzers are to be commercialized [9]. Intermittent fluctuations in renewable electricity increase the dissolution of Ni at the cathode, a problem that can be mitigated by coating the cathode with a thin layer of stabilizing active material, thus AWE can generally operate at between 15 and 100% of rated power. However, AWE has a long start-up time, requiring 30–60 min to restart after a shutdown [10]. As a result, alkaline electrolyzers perform relatively poorly with renewable energy sources that have fast fluctuating characteristics. At present, the alkaline electrolytic water hydrogen production technology in China is very mature, with a total of 1500– 2000 installations, mostly for the preparation of hydrogen for cooling in power plants. The single-tank scale of domestic equipment has reached international leading levels, with domestic equipment reaching a maximum of 1000 m3 h−1 (referring to the volume of hydrogen at 0 °C and standard atmospheric pressure, later the same) [11]. The energy efficiency (electricity-hydrogen) of commercial electrolyzers ranges from 55 to 0%. The analysis of electrochemical processes must take into account all aspects. The most fundamental is the thermodynamic analysis, which determines the driving force of the process based on the application of conservation laws (such as the first law of thermodynamics), thermodynamic conservation, conservation of charge, the thermodynamic equilibrium principle, and other special principles. The thermodynamic analysis must be accompanied by a kinetic analysis to study the reaction rate. In addition, the study of transport phenomena within the electrolytic cell is important and should not be neglected. Several other aspects can be understood from the material analysis, the economic analysis, and the environmental impact analysis. Kinetic and transport process analyses are relevant and focus on determining the relationship between the reaction rate (or product generation rate) and the operating conditions, as well as the relationship between the current and voltage consumed by the electrolytic cell. The kinetic analysis determines the relationship between the reaction rate at the electrode and the electrode overpotential, and the relationship

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between the current of ionic substances on the electrolyte and the potential on it. When the electrolyte assembly is more complex, including membranes or diaphragms or anodes/anodes, the kinetic analysis explains the overpotential between them and the matter and charge currents through them. For electrolytes in alkaline electrolyzers, there are two specific problems: (i) mass transport must be controlled to reduce the diffusion layer, and (ii) the inevitable bubbles on the electrode surface must be removed to try to avoid excessive increases in ohmic losses. Both of these problems can be reasonably solved by the use of an appropriate additive in the electrolyte. The additive is designed to reduce the surface tension, thereby increasing the diffusion mechanism at the electrode surface and accelerating bubble detachment. It should be noted that bubble detachment requires activation energy, which can be translated into a slight increase in overpotential. Typical variations in overpotential in commercial electrolytic cells are used in industrial applications. The ohmic loss is minimal at the electrodes and the overpotential is highest at the anode. It is also observed that ohmic losses in the electrolyte become significant at higher current density ranges. Some of the enhanced uses of alkaline electrolyzers are known for a variety of systems and applications. Magnetic fields are known to affect charge through the Lorentz force, which increases the convection mechanism in the electrolyte solution due to magnetohydrodynamic processes. If applied properly, magnetic fields can significantly reduce the concentration potential and reduce the ohmic losses of the electrolyte due to better charge transport. In a magnetically assisted electrolytic cell, the magnetic field is perpendicular to the electric field. The effect of a magnetic field on the convection current in an alkaline electrolytic cell. The magnetic field is applied by using permanent magnets placed on two opposite sides of the electrolytic bath to produce an upward Lorentz force. The effect of the Lorentz force is that there is a net convection current in the upward direction at the electrode surface, which helps to remove air bubbles. As a result, the concentration of overpotential is reduced and the ohmic resistance caused by the presence of air bubbles is reduced. The best electrode for magnetic field-assisted alkaline electrolysis appears to be a nickel-based electrode because of its good electrochemical activity and ferromagnetic behavior. When a magnetic field is present, nickel electrodes are used more favorably than platinum electrodes because their current density is increased by a factor of two compared to platinum. In addition, nickel is much cheaper than platinum; it, therefore, seems to be the electrode of choice for alkaline electrolysis baths.

3.2 Electrode Materials Typical electrode materials for cathodes in alkaline electrolyzers are Ni, Fe, Co, Zn, Pb, Pd, Pt, and Au; while Ni, Pt, Ir, Ru, Rh, titanium dioxide, and Co are used for the cathodes. Typically, commercial alkaline electrolyzers operate at current densities between 1 and 3 kA m−2 , while advanced alkaline electrolyzers operate at densities of 2–15 kA m−2 . One of the main problems with electrodes is the variation in

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time caused by the various electrochemical reactions and deposition processes that may occur on their surface. These processes may lead to deactivation of the electrocatalyst, and a decrease in current density, while the overpotential will increase. To develop suitable electrodes for AW electrolyzers, the material chosen must have high corrosion resistance, high conductivity, high surface area, and high catalytic effect and must be reasonably priced and have a long life such as nickel-based metals. Nickel foam electrodes are very representative nickel-based metal electrode materials. The true surface area of porous electrodes is 2–3 orders of magnitude larger than that of general flat electrodes. The large specific surface area reduces the hydrogen precipitation overpotential of the electrode, thus increasing the catalytic activity of the electrode. Nickel foam is a pure nickel porous material with a three-dimensional mesh structure, which has the advantages of a large specific surface area, low price, and easy preparation. The use of nickel foam as a substrate for spraying or electrodeposition of certain alloying elements gives the electrode excellent hydrogen production performance. Nickel-based alloy electrodes are mainly binary alloy electrodes and ternary alloy electrodes. The binary alloy electrodes mainly include Ni–Mo, Ni–Co, Ni–Zn, Ni– Fe, Ni–Cr, and Ni–W, etc. The ternary alloy electrodes mainly include Ni–Mo–Co, Ni–Mo–W, Ni–Mo–Cu, Ni–Mo–Fe, and Ni–Co–Zn, etc. Among them, Ni-Mo alloy has the advantages of good catalytic activity, corrosion resistance, wear resistance, low expansion coefficient, etc. It is also inexpensive, easy to prepare, and suitable for large-scale applications, and is an ideal cathode material for an alkaline filter-press water electrolyzer. Nickel-based non-metallic alloy electrodes mainly include Ni–S, Ni–P, and other binary alloys, as well as Ni–Co–P, Ni–Co–S, Ni–Mo–P, Ni–P–C, Ni– Zn–P, and other ternary alloys. The catalytic activity of the non-metallic binary alloy electrode is reduced because it is easy to change into crystalline alloy surface when working in high temperature solution. Therefore, a multicomponent alloy composed of Ni, S and P is considered as the electrode. The activity of hydrogen evolution, corrosion resistance and stability of this alloy have been improved to some extent. There are many electrode materials with oxygen evolution electrocatalytic activity. The early application of oxygen evolution electrode materials are some noble metals, which have high oxygen precipitation activity, but the price is high, and it is difficult for large-scale application. Therefore, it is necessary to develop anode materials with corrosion resistance, high strength, good electrical conductivity, high catalytic activity, and low price. At present, the development of oxygen evolution electrodes is mainly carried out in the following two aspects: (1) improvement of the anode by surface modification and alloying; (2) use of conductive organic polymers as anode materials. The above methods reduce the oxygen evolution overpotential to improve the oxygen evolution activity of the electrode. For nickel-based electrodes, deactivation occurs due to the formation of nickel hydride. Maintenance operations requiring the circulation of dissolved vanadium in the electrolyte can remove the nickel hydride and re-establish a clean surface electrode. Improvements in electrode performance are often obtained by doping their surfaces with components that may act as very active electrocatalysts. For example, electrodes in alkaline electrolyzers can be doped with Pt, Pd, Mo, Ru,

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and Ir. The structure of the electrode surface plays an important role. Nanostructured surfaces have recently been designed to obtain higher specific surface areas and better current densities. The electrodes have a flat geometry to enhance the contact area with the electrolyte, and most importantly, they do not require expensive catalysts (carbon steel coated with nickel is usually used). Due to these properties, alkaline electrolyzers can be built cost-effectively to achieve higher production capacity. An interesting technique for improving the efficiency of water electrolysis is to couple a multiphase electrocatalytic process occurring at the electrode surface with a homogeneous catalytic process in the electrolyte volume near the electrolyte– electrode interface. A net enhancement of hydrogen production is obtained by the action of the homogeneous catalyst in parallel with the multiphase process. As mentioned earlier, it is necessary to reduce the activation overpotential of the hydrogen and oxygen evolution reactions in order to increase the efficiency of the electrolytic system. This can be achieved by using electrocatalysts deposited on the electrode surface or as electrodes themselves. Electrocatalysts provide low activation pathways for specific electrochemical reactions and enable reactions to take place at high current densities. They do this by altering the reaction kinetics and even the mechanism by which the reaction takes place. A material is a good electrocatalyst, i.e. in order to achieve a low overpotential, the Tafel slope generated must be low, and/or the exchange current density must be high. The performance of the electrode depends not only on the composition of the catalyst but also on its surface area and microstructure. When the electrolysis process takes place, hydrogen and oxygen bubbles form on the surface of the electrode and only separate from the surface when they reach a certain size. This phenomenon reduces the effective active area of the electrode; the bubbles act as an electrical shield and increase the ohmic losses of the system. Therefore, concerning the composition of the electrodes, in addition to having the appropriate catalytic activity, they must also be porous, allowing electrolyte permeation pathways that facilitate the separation of bubbles. This separation also relies on the wettability of the electrodes, so that the hydrophilic electrode surface reduces the surface coverage of bubbles.

3.3 Membrane AWE electrolytic cell uses NaOH or KOH aqueous solution as electrolyte, which oxidizes to oxygen in anode water and reduces to hydrogen in cathode water. It has the advantages of simple operation and low production cost, but there are some problems such as large volume and weight, strong corrosion of alkaline solution. The diaphragm is one of the key components of the alkaline electrolytic cell, separating the product gases and avoiding the mixing of hydrogen and oxygen. Porous diaphragms based on asbestos were used for decades until the mid-1970s when they were banned because of their toxicity and high gas permeability. Subsequently, various types of alternative materials for diaphragms were developed. For example, Hydrogenics’ HySTA modular electrolyzer using inorganic ion exchange membranes IMET [4],

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63

which produce hydrogen with a purity of > 99.999%. The function of the electrolyzer diaphragm is to prevent the products generated at the two poles of the electrolyzer from mixing by physical or chemical means, without impeding the passage of current. Various porous materials such as ceramics, porous rubber, porous plastics, fiber fabrics (natural or synthetic), asbestos (asbestos paper, asbestos cloth, piles), modified asbestos diaphragms, are used to restrict the diffusion of ions and gases through the diaphragm by the physical or mechanical action of the pores. However the membrane is not selective for the passage of ions, and some people call this membrane “mechanical membranes”. The selective ion exchange membranes are called “chemical membranes”. At present, the most widely used asbestos diaphragms and modified asbestos diaphragms in the water electrolysis industry are both “mechanical membranes”, which are briefly described below. The ideal electrolytic diaphragm (mechanical membrane) should meet the following conditions. (1) To allow ions to pass through, but not gas molecules. (2) Large porosity to maintain low resistance. (3) A small average pore size to prevent the passage of gas bubbles and inhibit diffusion. (4) For uniform current distribution and high current efficiency, the physical and chemical properties of the material should be homogeneous. (5) Resistance to corrosion of the raw materials and products of electrolysis. (6) Adequate chemical stability to the operating conditions of the electrolytic bath such as temperature, pH, etc. (7) A certain degree of mechanical strength and rigidity. (8) Easy and inexpensive sources of raw materials, suitable for use in industry. (9) Easy to handle the waste after use. (10) Easy to make membrane process, and easy to implement industrialization. As can be seen from the above conditions, the choice of diaphragm material is very difficult. Asbestos is the only good material that can largely meet these demanding conditions. Therefore, until now, asbestos still dominates the diaphragm materials for diaphragm electrolyzers in China. In addition to basically meeting the above conditions, asbestos diaphragms have two incomparable advantages: firstly, their excellent hydrophilic properties; secondly, the negative charge and OH− concentration in its surface hydrated magnesium silicate has the function of inhibiting OH− counter-diffusion. Therefore, asbestos has long been used as the main raw material for water electrolysis tank diaphragms. However, in the course of production practice, it was gradually realized that due to the swelling and chemical instability of asbestos diaphragms themselves, pure asbestos diaphragms have the defect of severe swelling in specific operating environments, especially under high current loads, resulting in reduced mechanical strength of the diaphragm, much shorter service life and a significant drop in current efficiency. Chrysotile is a fibrous silicate of many types, the main one being chrysotile or white asbestos, whose structural equation is Mg3 Si2 O5 (OH)4 . Due to its alkaline structure, chrysotile cannot be used in acidic media; and in alkaline media, corrosion may also occur if the temperature rises high enough and the corrosion rate increases with the temperature. Therefore, when using

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chrysotile as a diaphragm, it is difficult to increase the efficiency of the electrolyzer by raising the solution temperature. This is because increasing the solution temperature not only increases the corrosion rate of the asbestos but also increases its mechanical deformation and reduces the mechanical resistance. Another issue worth considering is the toxicity of asbestos. Asbestos is known to be carcinogenic and to cause lung diseases, chronic respiratory diseases, lung cancer, stomach cancer, colon cancer, mesothelioma cancer, etc. As a result, many countries have banned the use of asbestos and related products, and China has also proposed to ban asbestos diaphragms in the next few years. For these reasons, asbestos is no longer the ideal diaphragm material and it is important to find a new material that can replace it for the development of the water electrolysis industry. One of the more mature modified asbestos diaphragms is the Teflon resin-modified asbestos diaphragm. Polytetrafluoroethylene (PTFE) resin-modified asbestos diaphragm is a new type of polyfluoroalkylenebonded asbestos diaphragm in which polytetrafluoroethylene resin is blended into the asbestos diaphragm and treated to improve the performance of the diaphragm compared to the common asbestos diaphragm. The membrane making process of this diaphragm follows the vacuum adsorption method of the pure asbestos diaphragm of caustic asbestos wool, using the operating technique of low vacuum thin adsorption, and fine workmanship. The diaphragm made has the advantages of thin thickness, uniform membrane layer, tight structure, and thorough ripening. Considering the tetrafluoroethylene resin-modified asbestos diaphragm under a microscope, it can be seen that the molten polytetrafluoroethylene resin envelops and bonds the asbestos fibers together. This action improves the corrosion resistance and mechanical properties of the diaphragm. It has been determined that the greater the proportion of resin in the PTFE resin-modified asbestos diaphragm, the greater the corrosion resistance, the higher the diaphragm breaking strength, and the lower the water permeability. Polytetrafluoroethylene resin-modified asbestos diaphragm process is simple and easy, and the film-making operation technology is non-toxic, can reduce asbestos pollution, low maturation temperature, low investment, fast results, and improve the performance of the diaphragm. The control of the amount of PTFE resin is important, and low content is not enough to improve the performance of the diaphragm, while a high amount can improve the strength of the diaphragm, but there are many disadvantages. If the amount of water-repellent PTFE resin is too high, the two valuable properties of asbestos diaphragms (good hydrophilicity and inhibition of OH− ion counter-diffusion) will be lost, and the high amount of PTFE resin will increase the cost of the diaphragm. Therefore, several measurements have been proposed to improve and enhance the hydrophilic properties of PTFE resin-modified asbestos diaphragms, such as using hydrophilic fluorine resins with ion exchange groups as reinforcing binders to improve the hydrophilic properties of the diaphragm.

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The main component of non-asbestos diaphragms is polyramix, which is a physical combination of a metal oxide particle and a polymer, a unique fibrous material known as “polymer/inorganic composite fiber”, or composite fiber for short. The polymers are usually homopolymers, copolymers, graft copolymers, or mixtures of them, and require chemical stability under electrolytic conditions. The polymers used include those containing fluorine or fluorine and chlorine, such as polyvinyl fluoride, polyvinylidene fluoride, polytetrafluoroethylene, F-46, PFA, polytrifluoroethylene, copolymers of trifluoroethylene and ethylene, etc. Polytetrafluoroethylene is the most widely used polymer. The inorganic particles used in the preparation of composite fibers should be fuse-resistant substances or mixtures thereof, which remain intact during the preparation of the composite fiber and at the same time show inertness to the polymer fiber matrix and do not react chemically in the composite fiber, but merely bond physically to the polymer. Suitable inorganic substances are oxides, carbides, borides, silicides, sulfides, nitrides, or their mixtures, also available are silicates (magnesium silicate and aluminum silicate), aluminates, silicate ceramics, metal alloy ceramics or their mixtures, metals or metal oxides. Oxytech started research on non-asbestos diaphragms in the early 1980s and has achieved some success in the application of non-asbestos diaphragms. The next objectives are: (1) to improve the performance of non-asbestos diaphragms and reduce electrical energy consumption, making them superior to polymer-asbestos-modified diaphragms; (2) to reduce the cost of making non-asbestos diaphragms. There is a shortage of asbestos in China, especially in the form of high-quality asbestos for water supply for electrolytic membrane production, which is imported from Canada and Zimbabwe at the great expense of foreign exchange every year. Because asbestos is a carcinogenic material, it is very difficult to produce, process, and reprocess. Although non-asbestos diaphragms have been developing for more than 20 years, they have not been used due to the poor wettability of PTFE and various other reasons. Polysulfone is a kind of membrane material used earlier and more widely and is also one of the hot spots in the research of membrane materials. Polysulfone resins are a class of polymer compounds containing sulfone groups and aromatic rings in the main chain, mainly bisphenol A-type polysulfone, polyethersulfone, polyethersulfone ketone, polyphenylene sulfide sulfone, etc. As can be seen from the structure, these materials have excellent oxidation resistance, thermal stability, and high-temperature melt stability because the S atom of the sulfone group is in its highest oxidation state and the sulfone group has benzene rings on both sides to form a highly conjugated system. In addition, polysulfone materials also have excellent mechanical properties, high-temperature resistance, acid and alkali resistance, bacterial corrosion resistance, good mechanical properties, inexpensive and easy-to-obtain raw materials, pH application range, and other advantages. Although polysulfone diaphragm materials have outstanding separation performance, there are shortcomings in performance. Polysulfone polymers such as polysulfone (PSF), polyethersulfone (PES), and other materials have excellent chemical stability, heat resistance, and mechanical strength, but there are certain disadvantages as diaphragm materials. For example, their hydrophilic properties are too poor, so the water flux of the diaphragm is low, and the anti-pollution properties are not ideal, affecting their

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application range and service life. Therefore, most modification work on polysulfone diaphragm materials has focused on improving their hydrophilicity, and the introduction of hydrophilic substances into them through blending is an effective way to improve the hydrophilicity of polysulfone diaphragm materials. Polysulfone is a diaphragm material with excellent performance and a wide range of applications. If better diaphragm materials can be developed by certain means, existing preparation techniques can be improved and new film production techniques can be developed, which will certainly lead to a wider application of polysulfone diaphragms in more fields and under more stringent conditions. Although many types of diaphragms have been produced so far, no diaphragm can completely replace asbestos diaphragms, the reason being, of course, the excellent hydrophilic properties of asbestos, which can inhibit OH− counter-diffusion and woven structure (with many pores). Organic diaphragms are not widely used because of their poor hydrophilicity. The hydrophilicity of the diaphragm can be improved by introducing hydrophilic substances by certain means without affecting its performance. If the hydrophilicity of organic diaphragms can be improved, there will be a good future and market for these diaphragms. Although non-asbestos diaphragms are made from a combination of inorganic materials and polymers, there have been no reports of inorganic materials alone being used to make diaphragms. Inorganic materials have the advantages of high-temperature resistance, corrosion resistance, strong mechanical properties, good chemical stability, etc. It is believed that inorganic diaphragms will also become one of the hot spots of research in future research.

3.4 Prospects for Development Hydrogen produced by the electrolysis of water, i.e. green hydrogen, is a promising alternative to fossil fuels. It emits no carbon dioxide and is used with little or no air pollution, thus offering a solution for decarbonizing industrial processes and economic sectors. A comparative study of the main water electrolysis technologies has been carried out. Different perspectives are presented on the advantages and disadvantages of the two commercial methods. AWE is currently the most environmentally friendly water electrolysis technology and is the best technology for hydrogen production in a large-scale industrial environment. However, it is crucial to improve the efficiency of AWE systems in terms of energy consumption and cost so that this electrochemical technology can compete with conventional energy production from fossil fuels. This chapter provides an analysis of the kinetics and thermodynamics of water electrolysis, pointing out the main developments and challenges by electrodes, electrolytes, membranes, etc. As previously mentioned, these variables rise due to certain aspects of the electrochemical cell composition, and therefore an overview of these variables is given. This concentrates on the best characteristics of the electrodes and the most suitable electrocatalysts for HER and OER. The electrolyte concentration and possible additives to be used are analyzed. By providing options for optimizing battery components and cell configurations, this

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chapter provides an opportunity for industrial systems to minimize energy input, particularly at high current densities, improving their efficiency, and to encouraging investment in the implementation of modern electrolysis systems. Even so, it is important to remember that, despite the recent advances highlighted, further research efforts are needed to build a future based on the hydrogen economy and renewable energy. China is already cost-competitive in alkaline electrolyzers, of which Tianjin Continental Hydrogen Production Equipment Co Ltd is the world’s leading supplier of alkaline electrolyzers, having delivered over 400 production plants since 1994. The most representative renewable energy hydrogen production project in China is the Guyuan wind power hydrogen production project (4 MW) invested by Hebei Construction & Investment New Energy Co. The demonstration project consists of a 200 MW wind farm, a 10 MW hydrogen production system (4 MW in Phase I and 6 MW in Phase II), and a hydrogen utilization system. According to the overall hydrogen energy industry plan of Hebei Province, part of the hydrogen will be used for industrial production to reduce the consumption of fossil energy such as coal and natural gas, while the other part will be used to build a network of supporting hydrogen refueling stations to support the development of clean energy powered vehicles in Hebei Province. Overall, a growing number of countries are undertaking pilot and initial commercial projects in the electrolysis of hydrogen from renewable energy sources, with a particular focus on scale and on improving the interactive performance of power systems. Projects have been developed to MW scale, but further research, production scale-up, and innovation in practice are needed to significantly reduce costs. As shown in Fig. 3.1 [12], for alkaline electrolyzers, equipment costs are mainly driven by the cost of core components such as electrodes and diaphragms. Over 50% of the cost component of the electrolytic stack in alkaline electrolyzers is related to electrodes and diaphragms, compared to 24% of the cost of membrane electrodes in the electrolytic stack of PEM electrolyzers. In alkaline electrolyzers, the bipolar plates represent only a small part of the cost of the electrolytic stack, whereas in PEM electrolytic stacks they represent more than 50% of the cost due to the simpler design of the bipolar plates in alkaline electrolyzers, simpler manufacture, cheaper materials (nickel plated steel) and the redesign of the electrodes and diaphragms to reduce costs. The auxiliary part of the alkaline electrolytic hydrogen production system, the lye circulation system, and the hydrogen after-treatment are more important for cost reduction.

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Fig. 3.1 Cost components of MW alkaline electrolytic cells [12]

References 1. M. Schalenbach, A.R. Zeradjanin, O. Kasian, S. Cherevko, K.J. Mayrhofer, Inter. J. Electro. Sci. 13, 1173–1226 (2018) 2. M. Balat, Int. J. Hydrogen Energy 33, 4013–4029 (2008) 3. A. Markandya, P. Wilkinson, The Lancet 370, 979–990 (2007) 4. P. Trinke, B. Bensmann, R. Hanke-Rauschenbach, Int. J. Hydrogen Energy 42, 14355–14366 (2017) 5. V.N. Kuleshov, N.V. Kuleshov, S.A. Grigoriev, E.Y. Udris, P. Millet, A.S. Grigoriev, Int. J. Hydrogen Energy 41, 36–45 (2016) 6. M. Schalenbach, O. Kasian, K.J.J. Mayrhofer, Int. J. Hydrogen Energy 43, 11932–11938 (2018) 7. N.V. Kuleshov, V.N. Kuleshov, S.A. Dovbysh, S.A. Grigoriev, S.V. Kurochkin, P. Millet, Int. J. Hydrogen Energy 44, 29441–29449 (2019) 8. Z.Q. Liu, C. Chen, X.T. Wang, J.H. Zhong, J.L. Liu, G. Waterhouse, Angew. Chem. Int. Ed. 60, 22043–22050 (2021) 9. I. Vincent, D. Bessarabov, Renew. Sust. Energy Rev. 81, 1690–1704 (2018) 10. Ø. Ulleberg, T. Nakken, A. Eté, Int. J. Hydrogen Energy 35, 1841–1852 (2010) 11. H. Lee, B. Choe, B. Lee, J. Gu, H.-S. Cho, W. Won, H. Lim, J. Clean. Prod. 377, 134210 (2022) 12. L.M. Pastore, G. Lo Basso, M. Sforzini, L. de Santoli, Renew. Sust. Energy Rev. 166, 112685 (2022)

Chapter 4

Proton Exchange Membrane Water Electrolysis

4.1 Electrolyzer The oxygen evolution reaction (OER) occurs at the anode and the hydrogen evolution reaction (HER) at the cathode of a proton exchange membrane electrolyzer [1–3]. Proton Exchange Membrane Water Electrolysis (PEMWEs) consist of membrane electrode assembly (MEA), bipolar plates (stone ground plates, metal plates, etc.), and end plates [4–6]. In this case, the membrane electrodes assembly of the PEMWEs is prepared by spraying the cathode and anode catalysts onto each end of the PEM using a spray gun and then covering the catalyst surface with a gas diffusion layer, thus forming a complete MEA [7]. The anode of PEMWEs undergoes OER, while the cathode undergoes HER [8]. The function of the bipolar plate is to conduct the reactant water and to export the product H2 /O2 [9, 10]. The anode plate is mainly Ti plate. The commonly used cathode plate is graphite, stainless steel, Ti, etc. When PEMWEs work, the reactant H2 O enters the flow channel of the anode plate and then enters the diffusion layer. The diffusion layer (such as carbon paper, Ti mesh, etc.) promotes the mass transfer of gas/liquid and the conduction of electrons. H2 O from the anode diffusion layer to the anode catalyst surface loses electrons to produce OER, oxygen, and H+ . H+ , carried by water molecules, passes through PEM to the surface of the cathode catalyst to obtain electrons and generate HER, releasing H2 [11]. The H2 /O2 produced at the cathode and anode of PEMWEs enters the bipolar plate flow channel after passing through the diffusion layer, and then exits with H2 O [12]. The reaction equations and total reaction equations for PEMWEs at the cathode and anode, respectively, are: Cathode : 2H+ + 2e− → H2

(4.1)

Anode : H2 O − 2e− → 1/2 O2 + 2H+

(4.2)

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_4

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Overall reaction : H2 O → H2 + 1/2 O2

(4.3)

The cathodic HER process is composed of the Tafel/Volmer or Heyrovsky/Volmer primitive steps: Tafel : H2 + 2∗ ↔ 2Had Heyrovsky : H2 +

∗

(4.4)

↔ Had + H+ + e−

Volmer : Had ↔∗ + H+ + e−

(4.5) (4.6)

where * is the catalyst’s active site and Had is the adsorbed hydrogen, where Volmer is the RDS of HER. Anodic OER is a typical four-electron transfer process with multiple reaction intermediates (*O, *OH, *OOH, etc.) and a more complex reaction pathway, often requiring a high overpotential [13]. Over the past decades, many authors have proposed different OER reaction mechanisms, two of which have received much attention: the traditional adsorbate evolution mechanism (AEM) and the lattice oxygen evolution (LOE) [14]: AEM mechanism: H2 O molecules are first adsorbed on the catalyst’s surface, and then a series of reactions are initiated [15]. These include charge-proton transfer processes, chemical bond breaking/formation processes, adsorption/desorption processes of surface oxide species, and finally, the production of O2 molecules. Man et al. proposed a four-proton-electron transfer pathway based on the results of DFT calculations [16]: H2 O+∗ → H+ + ∗ OH + e− ∗

∗

(4.7)

OH → H+ + ∗ O + e−

(4.8)

O + H2 O → H+ + ∗ OOH + e−

(4.9)

∗

OOH → H + O2 + e− +

∗

(4.10)

Here, * represents the active site of the catalyst. In the AEM process, the first H2 O molecule adsorbs on the catalyst’s active site to remove an H+ to form *OH, followed by the release of another H+ to form *O [17]. The second H2 O molecule then adsorbs on *O and releases an H+ to form *OOH, which is deprotonated to form O2 and released. Under standard conditions, the theoretical overpotential of the OER is 1.23 V. Under equilibrium conditions, the total reaction’s Gibbs free energy (ΔG) is 4.92 eV. Thermodynamically, although

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the ΔG of the total reaction is equal to the sum of the ΔG of each primitive step, the RDS of the whole reaction is the primitive step with the largest ΔGmax . In the ideal case, the ΔG of each primitive step under standard conditions is equal to 1.23 eV. However, this case does not take into account the kinetic energy barrier. In practical OER catalysis, there is usually a primitive step with a ΔG greater than 1.23 eV. This leads to a sizeable theoretical overpotential due to the different adsorption strengths of the different oxygen-containing reaction intermediates at the catalyst active site [18, 19]. Based on theoretical calculations, there have been concluded that between the binding energies of transition metal oxides [20]. Where the best fit for all points is: ΔG*OOH − ΔG*OH = 3.20 eV with a mean absolute error of 0.17 eV. In addition, the linear relationship between *O, *OH/*OOH can be expressed as ΔG*OH = a1ΔG*O + b1; ΔG*OOH = a2ΔG*O + b2. where a1 and a2 are close to 0.5, which is attributed to the *O double bond to the single bond of *OH/*OOH [21]. For different materials and even other crystalline planes, b1 and b2 are different, and it is related to the configuration of the catalytic site. For an ideal OER catalyst, the OER overpotential η = 0, and the binding energy relationship between *O and *OOH is then: ΔG*OOH − ΔG*OH = 2.46 eV. However, in practice, ΔG*OOH − ΔG*OH = 3.20 eV, which means that the overpotential minimum is: (ΔG*OOH − ΔG*OH )/2e − 1.23 V = 0.37 eV. According to the AEM mechanism, the binding energy of *OH and *OOH at the active site is an essential factor in the performance of the catalyst OER, and reducing the binding energy gap between the two reaction intermediate species *OH and *OOH is one way to improve the OER activity. Since there is a universal scalar relationship between the binding energies of *O, *OH, and *OOH, a relationship between ΔG*O − ΔG*OH or ΔG*O and the catalyst OER overpotential can be established. For surfaces where the oxygen binding energy is too strong, the OER activity is controlled by the formation of *OOH species, whereas for surfaces where the oxygen pooling energy is too weak, the OER activity is controlled by the oxidation of *OH. According to Sabatier’s principle, only catalysts with a moderate *O binding energy will produce the best OER activity [22]. For the AEM mechanism, there has been summarized the relatively unified understanding that (i) a four-proton-electron transfer process occurs on the catalyst surface and the OER activity is independent of the electrolyte pH on the relative RHE scale, (ii) the catalyst activity is dependent on the binding energy of the oxygen intermediate at the active site and there is a “volcano” type relationship; (iii) the binding energies of different reaction intermediates at the active site of the catalyst are correlated with each other [23]. Despite the current understanding of the AEM mechanism, there are still some questions to be resolved by researchers, such as the location of the active metal ion and the source of the difference in the pooling energy of oxygen intermediates. LOE reaction mechanism: In the OER process, the lattice oxygen of the catalyst can be directly involved in the precipitation of O2 molecules. However, there is still no consensus on the exact reaction pathway of the LOE mechanism. Based on DFT calculations, Stevenson et al. proposed that lattice oxygen can participate in the reversible formation of surface oxygen vacancies (Vo) [24]. kolpak et al. summarized the primitive steps in the LOE reaction process [25]:

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( ) OH → Vo + ∗ OO + H+ + e−

(4.11)

( ) ( ) H2 O + Vo + ∗ OO → O2 + Vo + ∗ OH + H+ + e−

(4.12)

∗

( ) ( ) Vo + ∗ OH + H2 O → HO−site∗ + ∗ OO + H+ + e− ( ) HO−site∗ + ∗ OO → ∗ OH + H+ + e−

(4.13) (4.14)

where the brackets indicate adsorbents calculated in the same super cell. The oxygen binding energy of different chalcogenide catalysts was investigated by theoretical calculations, which were used to establish the relationship between OER activity and oxygen binding energy. A “volcano” relationship between OER activity and oxygen binding energy was selected based on theoretical calculations [25]. According to the LOE mechanism, the OER performance is limited by ΔG = 1.4–1.6 eV, which leads to a minimum overpotential of 0.17–0.41 V. The results show that the highest OER activity according to the LOE mechanism is better than that of the AEM mechanism. Considering that the active site oxygen binding energy of the catalyst in the LOM mechanism is similar to that of the AEM mechanism, making the two processes co-existent and competitive [26]. They proposed that lattice O remains the most active catalyst in the AEM mechanism. DFT calculations show that the formation of O–O bonds between two lattice O’s is thermodynamically favorable and the RDS for LOE is deprotonation of the surface OH*. It is evident from both AEM and LOE OER mechanisms that some LOE radical steps are non-synergistic protonelectron transfer processes, leading to the observation that OER activity is dependent on electrolyte pH [27]. As mentioned above, lattice O is involved in the catalytic active site. In contrast, metal ions are not involved in the overall catalytic reaction process, which makes the LOE mechanism different from the AEM mechanism [28]. There is no unified understanding of the LOE mechanism as it has not been thoroughly studied. As shown in Fig. 4.1, based on the mechanisms proposed by different researchers, Shi et al. summarized the following features: (i) most LOE reaction processes are four non-synergistic proton-electron transfer processes; (ii) the OER activity of catalysts still depends on the oxygen intermediate binding energy at the active site; and (iii) oxygen vacancies are critical to the LOE reaction process [5]. In summary, the AEM mechanism can be compared with the LOE mechanism in terms of catalytic reaction pathways, active sites, reaction energy barriers, and mechanism characteristics: (i) the AEM mechanism performs redox reactions via cations, whereas the LOE mechanism performs redox reactions via anions; (ii) according to DFT calculations, the LOE mechanism has a lower reaction energy barrier compared to the AEM mechanism; (iii) The active site of the AEM mechanism is a coordinated unsaturated metal ion, whereas the active site of the LOE mechanism is a coordinated unsaturated oxygen ion; (iv) the OER activity corresponding to the AEM mechanism is not pH dependent, whereas the OER activity corresponding to the LOE mechanism

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Fig. 4.1 Proposed AEM (left) and LOM (right) for OER in the acidic environment [5]

is pH dependent; in addition to the above differences, both the AEM and LOE mechanisms involve a nucleophilic attack on oxygen (either lattice oxygen or adsorbed oxygen), which is the key to O2 evolution. Therefore, increasing the electrophilicity of oxide species can promote their OER activity. Furthermore, the degree of difficulty of the redox reaction between cations and anions in the OER potential interval determines their catalytic mechanism and significantly affects their OER catalytic activity.

4.2 Electrode Materials The overpotential of PEMWEs anode OER is high, and most metals form soluble high valence oxides in high potential and acidic corrosive environments. Currently, the main catalysts that meet the OER conditions in acidic media are Ir-based and Rubased materials. However, both Ir and Ru are noble metals with high costs. A common approach is to optimize the composition of noble metal catalysts to reduce the amount of noble metal and increase the intrinsic catalytic efficiency of the active sites [29]. During OER, the higher overpotential and strongly acidic corrosive conditions will lead to changes in the catalyst structure and therefore need to find a realistic active site under OER conditions. Modifying the active site usually introduces a foreign atom into the noble metal system. The heteroatom doping in the noble metal can be either a noble metal or a transition metal, thus changing the electronic configuration or geometry. This synergistic effect or modulation of the electronic/geometric structure

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results in the binding of OER reaction intermediates at the active site being able to proceed in a favorable direction for the response [30, 31].

4.2.1 The Catalysts for OER There are not many types of OER catalysts in acidic media; this book introduces the mainstream Ir-based and Ru-based catalysts, respectively, plus a brief introduction to some other less common types of catalysts [32]. Ir-based catalysts: The high stability of IrO2 in acidic media makes Ir-based catalysts the most widely used in PEMWEs. However, due to the complexity of the OER process, there are still several issues to be resolved for IrO2 catalysts. (i) The need to gain insight into the changes in IrO2 surface species during OER. (ii) The need to confirm the adequate depth at which OER occurs on the catalyst surface or in bulk. (iii) Due to the inevitable dissolution of precious metals during the OER process, a rational design of the catalyst structure is required to balance activity and stability. (iv) The need to clarify the effect of heteroatom doping on the catalyst’s active site and the impact on the OER process. There are different views on the chemistry of Ir(IV) oxides. It is essential to discern whether the particle surface or the whole bulk affects the catalyst activity during the OER process. Atmospheric pressure XPS was employed to perform in situ electrochemical tests on IrO2 nanoparticles under OER conditions, confirming that the chemical valence state of Ir changed from IV to V valence during the OER process. When IrO2 nanoparticles come into contact with water, they exhibit a mixed surface with oxide and hydroxide [33]. Hydroxide resurfacing and the discovery of V-valent Ir suggest a mechanism of *OOH deprotonation of Ir. At the same time, forming *OOH, V-valent Ir, requires a higher free energy of adsorption. In addition, the conversion of 0.7 nm Ir atoms to V-valent Ir during the OER reaction is considered the adequate depth at which Ir undergoes OER. IrOx (OH)y films were prepared on fluorine-doped tin oxide electrodes using a spin-coating technique. The nanostructures’ composition and crystallinity were controlled by varying the annealing temperature. One of the Ir sites in the IrOx (OH)y formed by annealing at 300 °C has an ultra-high intrinsic catalytic activity with an OER beyond that of amorphous Ir and crystalline Ir nanoparticles [34]. The surface composition of the catalysts is of great importance for the OER activity of Ir. Heteroatom doping is essential for enhancing the OER activity of Ir-based catalysts. IrCo hollow nanospheres with an ultrathin shell layer of only 11 atomic layers exhibited an overpotential of 284 mV at a current density of 10 mA cm−2 and only a 17 mV increase in overpotential after a 14 h stability test. A more substantial Jahn–Teller effect was observed in the CuO6 octahedron, resulting in longer Ir-O bonds. As the extended top O in the Cu–O bond distorts the equatorial O in the

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neighboring Ir site, the resulting O defect, located at the top O in the IrO6 octahedron, promotes the simplicity of the Ir-5d orbital (t2g 5 eg 0 ), thus lowering the energy of dz2 and raising the point of dxy . As a result, the eg orbital can be partially filled by an electron leap to the dz2 orbital. Based on DFT calculations, Cu0.3 Ir0.7 Oδ has a lower binding energy of 0.29 eV compared to IrO2 forming *OOH, which gives Cu0.3 Ir0.7 Oδ a smaller theoretical overpotential over a more comprehensive pH range [35]. The heteroatom-doped Ir atom enhances the OER activity of IrO2 by modulating the electronic configuration of the Ir atom (t2g and eg ). Inspired by the doping of Cu atoms in the Ir system, various types of Ir alloys have been summarized. By calculating the density of states (DOS) in the d-band, the binding energies of IrM alloys are in the order of Ir > IrNi > IrCo > IrCoNi, and their enhanced OER catalytic activity can be reflected by smaller Tafel slopes and higher transition frequencies (TOF). The IrW nano dendrites obtained by Guo et al. showed a significant increase in OER activity compared to Ir, achieving a current density of 10 mA cm−2 at a potential of 1.48 V. The DFT results showed that the W doping could optimize the binding energy of Ir to the reacting intermediate species, thus enhancing the OER performance. In addition, IrO2 can also improve the OER activity and stability when used in conjunction with Ti electrodes [36]. Ru-based catalysts: RuO2 theoretically has the highest activity for OER in a volcano-type curve, as it has the most moderate binding energy for oxygen species. However, the OER activity of the catalyst is influenced by many factors, such as pH, crystallinity, catalyst morphology/size, etc. Although RuO2 has the highest OER activity, Ru-based materials suffer from serious stability problems, especially during the OER reaction. At OER reaction potentials above 1.4 V, RuO2 dissolves due to excessive oxidation, which can be expressed as: RuO2 + 2H2 O → RuO4 + 4H+ + 4e−

(4.15)

Although metallic Ru has somewhat higher OER activity than RuO2 , it is less stable, and the dissolution rate of Ru is several orders of magnitude higher than RuO2 . Chorkendorff et al. found that RuO2 stability was highly sensitive to the pretreatment of its surface and examined the changes in the Ru electrode during OER by SEM and ICP, showing that at 1.5 V for 15 min electrolysis, 90% of Ru dissolved from the electrode. For the corresponding oxides, only 30% of the dissolution was detected. So that proper oxidation of Ru increased its stability [37]. Both in acidic and alkaline solutions, Cherevko et al. found that the order of activity and stability of Ru, RuO2 , Ir, and IrO2 remained the same, with the order of OER activity being IrO2 < RuO2 ≈ Ir < Ru and their metal dissolution rates being: Ru >> Ir > RuO2 >> IrO2 . However, RuO2 nanoparticles of 4~6 nm were shown to be more unstable [38]. Typically, studies of the RuO2 reaction mechanism have been carried out on the RuO2 (110) crystal plane. It is generally accepted that the OER process of RuO2 consists of four main steps, with the concerted transfer of four protons and electrons at the fivefold unsaturated Ru atom to produce *OH, *O, and *OOH reaction intermediates in turn. However, the RDS for RuO2 (110) is controversial. Ru on the (110)

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and (101) crystal planes can form five Ru–O bonds with O, producing a surfaceactive site towards the OER. In contrast, Ru on the (001) crystal plane and (111) crystal plane has only four Ru–O bonds, providing two vacant sites. According to DFT calculations, the number of Ru–O bonds per square nanometer is (110) < (111) < (101) < (001), which is consistent with the order of activity on each crystal face within two h of the occurrence of the OER. As shown in Fig. 4.2, introducing a second metal element plays an essential role in enhancing the OER activity of Ru. Man et al. synthesized various Ru-M alloys (M=Co, Ni, Cu, Cr, Re, Pd, Ir) and then tested their OER activity in acidic media. Normalizing the OER performance to the mass of Ru, Ru60 Co40, and Ru40 Co60 exhibited the highest mass activity, where 1.48 V corresponded to a mass activity of 200 mA mgRu −1 . In contrast, the pure Ru catalyst exhibited a mass activity of 100 mA mgRu −1 at this potential [37]. DeSario et al. proposed a general deposition method to obtain ultrathin RuO2 films on various 3D substrates (SiO2 or C-coated SiO2 ). The prepared RuO2 @SiO2 and RuO2 @C@SiO2 electrodes showed very high activity in acidic media, requiring an overpotential of 270 mV and 280 mV, respectively to achieve an OER current density of 10 mA cm−2 . By calculating the OER mass activity at 1.56 V for both electrodes, 20 mA mg−1 and 60 mA mg−1 were achieved, respectively. In addition, both electrodes can be operated at a constant current of 10 mA cm−2 for 100 h [32]. In addition, Cu atoms could be directly doped into the RuO2 lattice, and relative to undoped RuO2 , Cu DFT calculations showed that doping of RuO2 with low-valent Cu would lead to an increase in oxygen vacancies, which would trigger the production of unsaturated Ru ions on the surface, and the pband center of O would be closer to the Fermi energy level, thus increasing the OER activity of the catalyst [38]. In addition, doping other divalent metals such as Ni, Co, Fe, and Mn can also bring the p-band center of O closer to the Fermi energy level. In addition, Lin et al. synthesized Cr0.6 Ru0.4 O2 catalysts with low Ru content for the OER reaction. According to XAS analysis and DFT calculations, the electronic state of RuO2 was modulated by the doped Cr, resulting in a reduction of the electron density at the Ru site, the length of the Ru–O bond becomes shorter and the p-band center of O is closer to the Fermi energy level, leading to a higher OER activity [35]. Other types of OER catalysts: Ru and Ir precious metals, such as Rh, Pt, Pd, etc., can also be used as OER catalysts. However, the OER activity of Rh, Pt, and Pd alone is insufficient. The precious metals need to be modified to improve the OER activity further. Because Rh, Pt, Pd, etc., have a larger OER overpotential in an acidic medium, and the dissolution rate is faster than Ir and Ru, their OER performance is usually investigated in an alkaline medium [39]. In alkaline media, 2D Pt48 Pd40 Co11 NRs showed higher ORR and OER activities than Pt/C and Ir/C. In the process of OER reaction, oxides of Ni, Fe, Co, or hydroxyl oxides are precipitated on the surface of the catalyst, which significantly improved the TOF and OER activities of PtPdM NRs compared with PtPd NRs. However, as a sacrificial template, the increased OER activity of RhCu alloy is not derived from Cu but from Rh(III) oxide.

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Fig. 4.2 Three essential features of acidic OER electrocatalysts, including intrinsic activity, high current density operation, and long-term stability [6]

4.2.2 Development Direction of OER Catalysts In addition to significantly reducing the cost of anode catalysts, the development of highly active and stable low Ir and ultra-low Ir catalysts and non-Ir catalysts is the key to PEM water electrolysis for hydrogen production. Thus, strategies to prepare highperformance, durable, reliable, low-cost catalyst materials must be further explored with the following research directions [40]. (1) Doped type catalysts: A standard method for preparing high-performance, lowcost catalyst materials is to dope Ir with other noble or non-precious metals to form binary or ternary homogeneous eutectics, reducing the amount of Ir in the electrode and enhancing its dispersion [41, 42]. For example, it has been found that mixing RuO2 with IrO2 can maintain both the high activity of RuO2 and the high stability of IrO2 and its catalytic oxygen precipitation reaction (OER) activity is higher than that of pure IrO2 ; however, this method cannot fundamentally solve the problem of poor stability of RuO2 [43]. To further reduce the amount of Ir at the electrode and enhance its activity, some scholars have added inert non-precious metal oxides such as CeO2 , SnO2 , TiO2 , Nb2 O5 , TaO2 , Ta2 O5 and their mixtures to the electrode catalytic material. The addition of non-precious metal oxides is preferably limited to the range of 50~60%. If the addition ratio is too large, the activity of the catalyst will be reduced; moreover, the doping elements need to have a good match with Ir or Ru elements. Therefore, fewer doping species can be selected. (2) Supported type catalysts: Loaded catalysts avoid the problems of mismatching multiple elements in the bulk phase of doped catalysts and achieve a high degree of dispersion of the active catalytic center. However, under the demanding conditions of the PEM water electrolysis anode reaction, the carrier must have high electrical conductivity (conductivity > 0.01), excellent resistance to oxygen and

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acid corrosion, and long life (tens of thousands of hours or more). In addition, the carrier must have a large specific surface and a rich and adjustable pore structure to disperse the precious metal particles to a high degree and to ensure that water, gas, and other substances can diffuse sufficiently [44]. It isn’t easy to find high conductivity, large specific areas, and other advantages of high-performance carrier materials. More commonly used carriers are TiO2 , Nb-doped TiO2 , Sb-doped SnO2 , In-doped SnO2, and certain nitrides, borides, etc. Carbon materials have the advantages of high electrical conductivity and a large specific area. Still, they are not resistant to oxidation and have high potential corrosion, which limits their application in anode catalysts [45]. Therefore, synthesizing new corrosion-resistant, highly conductive, large specific surface area carriers is an important future research direction. (3) Core–shell catalysts: Although doped and loaded catalysts can reduce the amount of Ir to a certain extent, the dissolution of non-Ir metals or the degradation of the conductivity and corrosion resistance of the carrier during PEM water electrolysis leads to a continuous decline in the performance of the anode catalyst, thus limiting the practical application of doped elements and carriers. Ir is still by far the best anode catalyst active component [45]. At the same time, the electrocatalytic process is a surface reaction process, and only the active sites distributed on the catalyst surface can participate in the reaction. Therefore, anode catalysts can make full use of the core–shell structure, with a non-precious metal core and a precious metal such as Ir in the shell, which increases the chance of contact between the precious metal in the shell and the reactant while reducing the amount of precious metal such as Ir. Core–shell catalysts consist of two or more substances and are generally called “core–shell.” Thanks to the surface strain and electron-modulating effects (charge transfer between cores and shells), core–shell catalysts have unique physicochemical properties and synergies that enhance their stability and catalytic activity in the OER process [46].

4.2.3 Recent Research Progress of Acid OER Developing reservoir-rich electrocatalysts for OER capable of operating under acidic conditions remains challenging. Although many first-row transition metal oxides are highly competitive in alkaline media, most dissolve or become inactive at high proton concentrations [47]. Only noble metal catalysts, such as IrO2 , can maintain high activity and stability in acidic media. Recently, a promising treatment has been proposed combining two powerful strategies in one anode: (i) nanostructured OER catalysts incorporating reservoir-rich metals to maximize the active surface area; (ii) paraffinic oil and graphite powder made from a conductive, partially hydrophobic binder to support it. The researchers first synthesized carbonencapsulated Co (Co@C) nanoparticles by heat treatment of the metal–organic backbone precursor (ZIF-9) Co(bIM)2 (bIM = 2-benzimidazole). Oxidation was then

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Fig. 4.3 Scheme of the synthesis and processing protocol to obtain a Co3 O4 @C/GPO electrode [8]

carried out at low temperatures to achieve their complete transformation into Co3 O4 nanoparticles, which were covered by an organic skeleton-derived amorphous carbon coating (Co3 O4 @C). As shown in Fig. 4.3, High-resolution transmission electron microscopy revealed the presence of graphite-like nanostructures around the samples embedded with Co3 O4 nanoparticles [48]. The high activity and stability are better than other reported non-precious metal catalysts. In addition, the researchers quantitatively confirmed the high Faraday efficiency (> 96%) of these electrodes for OER by the amount of oxygen precipitated, with negligible involvement of other oxidation processes. This study opens up another avenue for developing fast and energy-efficient acid-mediated water oxidation electrodes [49]. Acid hydrolysis based on a proton exchange membrane electrolyzer is the most mature process for the preparation of green hydrogen; the anodic water oxidation process is the decisive step in the overall PEM water electrolysis compared to the 2 electrons transfer cathodic hydrogen precipitation half-reaction. Due to the harsh operating conditions of a highly acidic environment and high oxidation potential, the anode material is currently limited to Ir and Ru based noble metal oxides. Compared to the scarce IrO2 reserves, RuO2 is cheaper and has higher OER electrocatalytic activity but is not stable in actual water oxidation due to carrier corrosion and high valence Ru6+ /Ru8+ leaching. For RuO2 -catalyzed OER processes in acidic media, there are currently two main reaction pathways are widely accepted, namely the LOM and the AEM. In AEM, four synergistic proton-electron transfer steps occur at a single Ru site. Therefore, proper adjustment of the *O adsorption strength can significantly modulate the reactivity of OER and reduce the overpotential demand, thus enhancing OER stability. Although the exchange of catalyst lattice oxygen can reduce the OER reaction energy barrier in LOM, it inevitably generates soluble RuO2 (OH)2 and RuO4 species and accelerates catalyst corrosion

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[50, 51]. In this regard, developing low-cost, high-activity RuO2 -based anode materials through dispersion and electronic structure modulation while suppressing the LOM pathway to enhance electrocatalytic stability is a critical way to achieve stable ruthenium-based oxide-driven PEMWE operation. (i) Oxygen defects are created on TiO2 carriers by a simple H2 /Ar annealing treatment, followed by NaBH4 liquid phase reduction and secondary annealing to prepare highly dispersible RuO2 /D-TiO2 sizestabilized anodes. In particular, the oxygen vacancies within D-TiO2 and the heterojunction structure between RuO2 and D-TiO2 allowed the RuO2 /D-TiO2 composite catalyst to exhibit conductive behavior comparable to that of the carbon-loaded catalyst. (ii) The LOM mechanism for the destruction of RuO2 lattice structure and loss of active sites; and the RuO2 (110)/D-TiO2 has the lowest OER limiting potential (UL) compared to the defect-free TiO2 -loaded catalyst and the RuO2 (110) model surface, which theoretically reveals the reason for the high activity of the acidic OER. (iii) The RuO2 /D-TiO2 catalyst required an overpotential of only 180 mV to reach 10 mA cm−2 in the three-electrode OER test, and the electrolyzer tank pressure was only 1.84 V at 2 A cm−2 in the PEMWE monomer test, with an energy conversion efficiency of 68.1%; and stable electrolysis at 200 mA cm−2 . This work proposes a defect engineering strategy to prepare RuO2 /D-TiO2 composite oxide catalysts by introducing oxygen vacancies into the classical size-stabilized anode; electrochemical tests and DFT simulations (Fig. 4.4) confirm that the electronic structure modulation of the RuO2 active site by the D-TiO2 carrier. On the one hand, it reduces the charge transfer resistance between the interfaces. On the other hand, it reduces the thermodynamic limiting potential for the decisive step of *OOH generation, which significantly enhances the reaction rate of OER. It thus improves the catalytic activity and stability of the acidic oxygen precipitation reaction in a balanced manner. Given the wide range of applications of various transition metal oxides in renewable energy conversion and storage, this method may provide new ideas for designing more robust electrocatalytic materials. Although the oxygen production reaction in acidic electrolytes plays a significant role in electrochemical energy conversion, the slow reaction kinetics of the oxygen production reaction and the fragile stability of materials in acidic electrolytes lead to more substantial difficulties and challenges in the development of such materials [52]. The authors have developed a multilevel hybrid Bix Er2−x Ru2 O7 pyrochlore that can regulate both the orbital and spin-electron occupation states. It can thus control Bi 6s lone pair electron bonding interactions and transform asymmetric high spin states into symmetric low spin states. The atomically partially distorted BixEr2−x Ru2 O7 pyrochlore thus exhibits excellent long-term stable performance for up to 100 h in acidic electrolytes, with a current density of 10 mA cm−2 and an ultra-low overpotential (−0.18 V). In Fig. 4.5, the relationship between symmetry-breaking and OER performance was studied by preparing Bix Er2−x Ru2 O7 , an approach implemented by influencing the bonding interactions between Ru atoms and adjacent oxygen atoms via Bi 6s orbital electrons, leading to the formation of room temperature distorted hybridization. The introduced Er cation is insensitive to symmetry-breaking phenomena. Still, it can be used to control the extent of the atomic distortion situation in the Bix Er2−x Ru2 O7 sample when the atomic distortion effect is accompanied

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Fig. 4.4 Theoretical calculation and in situ electrochemical Raman spectra [15]. a Schematic representation on AEM and LOM paths in RuO2 -catalyzed OER. b Calculated free-energy diagrams at 1.5 V for RuO2 (110) (black line) and that supported on either TiO2 (blue line) or D-TiO2 substrate (red line). c Limiting potential diagram for AEM and LOM on model surfaces. d In situ electrochemical Raman spectra recorded on RuO2 /D-TiO2 during the potential sweep from 1.0 to 1.8 V in 0.1 M HClO4 , together with e potential dependence of integrated Ru−OOH band intensity

by the elimination of orbital simplicity, thus causing a change in the spin structure of the electron due to the broken symmetry structure [53]. This work provides a method for designing catalysts with atomic distortion and gains insight into how the kinetics of the OER reaction in an acidic electrolyte environment can be controlled by modulating the spin of the materials. Due to the poor stability of metal–organic skeletons in the acidic oxygen precipitation process, constructing metal MOF electrocatalysts for high-performance acidic oxygen precipitation reactions is essential but challenging (Fig. 4.6). The researchers have essentially resolved the structure of the anchored monometallic center, a subsquare planar Mn2 Cl2 coordination geometry, by single-crystal X-ray diffraction,

Fig. 4.5 Reaction mechanism design [21]. a The molecular orbital energy diagram for octahedral MO6 . b Schematic formation of energy splitting and electronic structure transformation, EF represents the Fermi level. c The RuO6 coordination polyhedron in different BERO samples with ordered, disordered and halfdisordered configuration. d Ru-4d orbital splitting for different symmetry. e Spin-resolved projected density of states (PDOS) for RuO6 coordination polyhedron in different BERO samples

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Fig. 4.6 Single-metal-site catalyst for an efficient acidic oxygen evolution reaction [23]

demonstrating its potential as a doubly accessible open metal center and its unusual semi-rigid character due to two rotatable charge-balanced Cl− anions. The optimized CoCl2 @Th-BPYDC exhibits not only excellent acidic OER activity comparable to that of commercial IrO2 catalysts but also high electrocatalytic stability, which is attributed to the robust, rigid Th-BPYDC backbone, the solid chelating coordination between the monometallic center and the bipyridyl N in the BPYDC ligand, and the accessible open monometallic center. More importantly, theoretical calculations and kinetic analyses show that the monometallic-centered catalysts exhibit an unusual semi-rigid character during the OER process, quite different from the well-known rigid monocentric motivations of MN4 . When linking and activating the reaction intermediates, the chlorine atoms are free to rotate, providing suitable geometrical space and thus significantly reducing the reaction energy barrier. Developing highly active and durable electrocatalysts for acidic oxygen precipitation reactions remains challenging due to the slow kinetics and severe catalyst dissolution of OER IV electron transfer reactions. XAS analysis and DFT calculations showed that lithium, as an electron donor, affects the electronic structure and lattice strain of RuO2 , with Ru–O 4d-2p hybridization weakening as the Ru–O covalent bond decreases; as the stable Li–O–Ru local structure formation, the valence state of Ru decreases (Fig. 4.7). Thus, the involvement of lattice oxygen and the solvation of Ru during OER is suppressed, improving the stability of RuO2 . DFT calculations show that the surface structural distortion caused by intrinsic lattice strain activates the dangling O atom near the Ru active site as a proton acceptor, thus stabilizing OOH* and improving the activity of RuO2 . This work presents an innovative strategy to tune the electronic structure and lattice strain simultaneously to design highly active and stable acidic OER catalysts with potential practical applications [54, 55].

Fig. 4.7 Structural and compositional characterizations [26]. a Schematic illustration of the preparation of lithium intercalated RuO2 . b RuO6 octahedron before lithium intercalation. c RuO6 octahedron after lithium intercalation. d Operando XRD of RuO2 during electrochemical lithiation under a constant current density of 10 mA g−1 , followed by 14 h relaxation. e Ex situ XRD patterns of the pristine RuO2 and the Lix RuO2 . f The HAADF-STEM images of the pristine RuO2 (left) and the Li0.52 RuO2 (right). g The high-resolution Li 1 s XPS of Li0.52 RuO2

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Developing non-precious metal catalysts with excellent activity and durability for OER in acidic media is essential for hydrogen electrolysis from water. Based on this, a cost-effective and stable bromine manganese oxide (Mn7.5 O10 Br3 ) catalyst has been reported that exhibits excellent OER activity in acidic electrolytes. As shown in Fig. 4.8, in the 0.5 M H2 SO4 solution, Mn7.5 O10 Br3 exhibited an OER overpotential of 295 ± 5 mV at a current density of 10 mA cm−2 . It maintained good stability (at least 500 h) at a current density of 10 mA cm−2 . This performance is superior to state-of-the-art noble metal-free catalysts and comparable to catalysts containing noble metals (such as IrOx or SrIrO3 ). In situ, Raman spectroscopy combined with density functional theory (DFT) calculations were used to investigate the catalytic activity and stability of different Mn-based materials (γ-MnO2 and Mn-OX, X=Cl, Br). Experiments and calculations show that tightly packed oxide surfaces are formed during the OER process due to the catalyst autoxidation process, resulting in excellent long-term stability and an active phase for the OER reaction. In addition, the presence of halogen ions provides the catalyst with enhanced electron transport capacity, further enhancing the OER activity. The design of acid-stable OER electrocatalysts with low noble metal content to facilitating slow kinetics is crucial for green hydrogen production. There has been reported a ruthenium-manganese (Ru-Mn) solid-solution oxide with oxygen vacancies (named Mn1−x Rux O2−δ , where x denotes the molar percentage of Ru atoms in all metals and 2 − δ represents the oxygen-derived vacancies generated from nonchemo-metric compounds), using an electron supply modulation strategy to stabilize the catalyst structure and accelerate OER kinetics. Among the synthesized catalysts, Mn0.73 Ru0.27 O2−δ has the best electronic form and enhanced conductivity, exhibiting a low overpotential of only 208 mV to achieve ten mA cm−2 current density in a 0.5 M H2 SO4 electrolyte, outperforming the benchmark RuO2 and most reported noble metal-based OER catalysts (Fig. 4.9). Experimental characterization confirms that the enhanced acidic OER activity is attributed to the acceleration of electrons from Ru to Mn atoms via O-atom bridges, particularly for Mn atoms adjacent to O vacancies, resulting in Ru active sites. Furthermore, density flooding theory (DFT) calculations indicate that the simultaneous introduction of Mn1−x Rux O2−δ for both Mn and O vacancies reduces the antibound spin state of the Ru d orbital. Thus the adsorption and desorption energy of the reactive oxygen intermediate at the Ru site favors a lower potential barrier for the transition from the O* to the OOH* intermediate, thereby accelerating the OER reaction kinetics. The electron supply regulation strategy described in this work can provide new concepts for designing highly active and stable solid-solution oxide catalysts by introducing secondary metal elements and generating oxygen vacancies. The results obtained can be further extended to other vital electrocatalytic processes. Hydrogen energy has a very high energy density of 2.7 times that of traditional fossil fuels. The conversion product is only water, making it the clean energy source with a tremendous potential to replace fossil fuels. The vigorous development of hydrogen energy will help to achieve carbon peaking, carbon neutrality, and the early realization of zero carbon emissions. Hydrogen production by water electrolysis is currently the most effective, reliable, and environmentally friendly way to produce

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Fig. 4.8 Evaluation of OER electrochemical activity [13]. a LSV curves of different catalysts at 1 mV/s scan rate with iR correction. b Electrochemical impedance spectra (EIS) at 1.40 V (set potential). The equivalent circuit is shown (Rs : series resistance; Rct : charge-transfer resistance). c Tafel plots of Mn7.5 O10 Br3 , Mn8 O10 Cl3 , and γ -MnO2 . d TOF calculated from the current density at an iR-corrected overpotential of 300 mV. e Chronopotentiometry curves of Mn7.5 O10 Br3 at 10 mA cm−2 (25 °C). f Chronopotentiometry tests of the Mn7.5 O10 Br3 oxide catalyst at 100 mA cm−2 in the PEM electrolyzer measured at 50 °C. Inset photo: PEM electrolyzer architecture

hydrogen. Among them, the PEM electrolyzer is a highly efficient electrolytic water device with advantages such as higher voltage efficiency, greater current density, and lower electrolyte resistance than conventional alkaline media electrolyzers. It is therefore considered to be a promising technology for hydrogen production. However,

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Fig. 4.9 Synthesis and structural characterization of Mn1−x Rux O2−δ [29]. a Schematic illustration of the synthesis process for rutile Mn1−x Rux O2−δ . b XRD patterns of Mn0.75 Ru0.25 O2−δ , Mn0.73 Ru0.27 O2−δ , and Mn0.69 Ru0.31 O2−δ . c HAADF-STEM image of Mn0.73 Ru0.27 O2−δ . d EDX elemental mapping images of Mn0.73 Ru0.27 O2−δ , including the Mn, Ru, and O elements. e Normalized XANES spectra of Mn0.73 Ru0.27 O2−δ and MnO2 at the O Kedge. f The R-space curve-fitting curves of Mn0.73 Ru0.27 O2−δ in the Mn K-edge. g Crystal structure of rutile Mn0.73 Ru0.27 O2−δ

its acidic OER process on the electrode involves a complex four-electron reaction process, resulting in slow reaction kinetics and large overpotentials. In contrast, the strongly acidic and oxidizing environment is highly prone to catalyst corrosion and affects stability. Therefore, there is an urgent need to develop new long-lasting acidic OER electrocatalysts, which can further enhance the application prospects for the decomposition of aquatic hydrogen in cutting systems. As shown in Fig. 4.10, a 0/2D nanocomposite structure of WC-loaded RuO2 nanoparticles (RuO2 -WC NPs) anchored on carbon nanosheets is reported. In addition, the lower noble metal Ru (4.11 wt%) loading exhibited vigorous acidic OER activity with an overpotential of only 347 mV at a current density of 10 mA cm−2 . The mass activity of 1430 A gRu −1 was also eight times higher than that of commercial RuO2 (176 A gRu −1 ). Theoretical calculations indicate a strong catalyst-support interaction between RuO2 and the WC substrate, with WC acting as a stable catalytic substrate

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to optimize the electronic structure around the Ru site, thereby reducing the adsorption energy of the active site to the reaction intermediate ligand. At the same time, the catalyst has the same excellent catalytic ability for hydrogen precipitation, with the acidic PEM total decomposition water electrolyzer constructed from RuO2 –WC NPs bifunctional electrocatalysts exhibiting good stability. The unique 0D/2D nanostructure rationally combines the WC substrate with noble metal oxides, providing a promising strategy for designing highly active and low-cost catalysts for application in acidic OWS. In summary, this work presents the first combination of a stable WC substrate with a noble metal oxide, RuO2 , and its application as a bifunctional electrocatalyst in an acidic condition. The excellent OER performance can be attributed to the catalyst-substrate interaction between RuO2 and the WC substrate. It optimizes the surrounding electronic structure of the Ru active site, giving it good adsorption energy for OER intermediates and thus reducing the reaction potential of the RDS. At the same time, the WC substrate contributes more electrons to the catalyst surface to protect the Ru active site from excessive oxidation during the acidic OER process. Notably, the WC substrate reduces the amount of RuO2 , resulting in a higher Ru utilization than commercial RuO2 and other Ru-based electrocatalysts. Based on the excellent OER and HER performance, a PEM electrolyzer constructed from bifunctional RuO2 -WC NPs (4.11 wt%) can produce hydrogen at a current density of 10 mA cm−2 with an electrolyzer voltage of only 1.66 V. These results suggest that WC is a suitable substrate for noble metal oxides, which can be used to trade-off weak catalytic activity with high cost, and provide an innovative guide to drive the practical application of acid hydrolysis. The design and synthesis of electrocatalysts for the OER are the keys to developing high-performance electrolytic water devices. Among these, polymer-based electrolyte membrane electrolyzers have received much attention in recent years. However, such electrolytic devices usually require the use of precious metal Ir-based electrocatalysts, hindering their commercial development. Therefore, an urgent need is to find OER electrocatalysts that are highly active, acid resistant, and rich in elemental reserves. There have been reported that embedding Mn into the lattice of Co3 O4 spinel can extend the catalyst lifetime from a few hours to hundreds or even thousands of hours (depending on the current density) in acidic systems while maintaining the catalyst activity. According to DFT calculations, the resulting spinel Co2 MnO4 has an optimal binding energy to the OER intermediate with an activation potential comparable to that of state-of-the-art iridium oxide catalysts (Fig. 4.11). The calculations also show that the dissolution thermodynamics of Co2 MnO4 is suppressed, which allows the trigger to operate stably for more than 1500 h at 200 mA cm−2 with pH = 1. Adding foreign metals to IrOx or IR-based perovskite oxides can effectively improve the activity of acidic OER by adjusting the electronic structure of Ir sites, but its stability is poor. In addition, strain engineering can also enhance catalytic activity, typically in the form of compressive or tensile surface strains in alloy nanostructures. However, achieving enhanced activity under lattice strain while maintaining catalytic

Fig. 4.10 Acidic OER activity of RuO2 –WC NPs in 0.5 M H2 SO4 [7]. a LSV curves of RuO2 –WC NPs, Ru–WC NPs, WC NPs, m-RuO2 /WC NPs and commercial RuO2 . b Mass activity of commercial RuO2 and RuO2 –WC NPs with different Ru amounts at 1.55 V (vs. RHE). c Comparison of η10 and mass activity employed in this work and other Ru-based electrocatalysts. d Tafel slope plots of RuO2 –WC NPs, Ru–WC NPs, WC NPs and commercial RuO2 . e Nyquist plots and f Linear relationships between capacitive current and scan rate of RuO2 –WC NPs, Ru–WC NPs and WC NPs. g Long-term chronopotentiometric and chronoamperometric curves of RuO2 –WC NPs at 10 mA cm−2 and 1.55 V, respectively. h The corresponding W and Ru dissolution rates of RuO2 –WC NPs measured by ICP-OES during the chronoamperometry tests shown in (g)

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Fig. 4.11 Long-term stability of Co2 MnO4 during the OER in acid [41]. a Time dependence of the electrochemical potential necessary to perform OER at 100, 200, 500 and 1,000 mA cm−2 geo in H2 SO4 (pH 1) and H3 PO4 (pH 1), respectively. The stabilities of Co3 O4 and γ-MnO2 in H2 SO4 (pH 1) are shown for comparison. b A comparison of the stability of Co2 MnO4 with Co3 O4 and other Earth-abundant OER catalysts reported in the literature

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stability has been challenging. As shown in Fig. 4.12, the authors propose for the first time a torsion-strained Ta0.1 Tm0.1 Ir0.8 O2−δ nanosatellite with many grain boundaries based on doping, thus achieving excellent activity in 0.5 M H2 SO4 , it showed a low overpotential of 198 mV at 10 mA cm−2 . Furthermore, the authors demonstrate for the first time that the principal torsional strain along GB between nanocrystalline domains of nanostructured catalysts can adjust the Ir-O bond length, thus optimizing the adsorption energy of oxygen intermediates and reducing the energy barrier of OER while maintaining excellent catalyst stability. The doping effect caused by substituting Ta and Tm for IrO2−δ also adjusts the active site’s electronic structure and optimizes the catalytic intermediate’s binding energy.

Fig. 4.12 Electrocatalytic properties of various Tax Tmy Ir1−x−y O2−δ nanocatalysts towards OER in 0.5 M H2 SO4 electrolyte [46]. a LSV curves of GB-Ta0.1 Tm0.1 Ir0.8 O2-δ and GB-IrO2-δ nanocatalysts versus those without GB. b LSV curves of different doped GB-Tax Tmy Ir1−x−y O2−δ samples versus C-IrO2 . c Mass activities of these nanocatalysts at η = 266 mV, showing the effects of both strain and doping on enhancing OER activities. d Chronopotentiometry curve of GB-Ta0.1 Tm0.1 Ir0.8 O2-δ nanocatalyst operated at 10 mA cm–2 (pink), and the Faradaic efficiency (black dots) during the 500 h test

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4.2.4 The Catalysts for HER In PEM water electrolysis for hydrogen production, the conditions of the cathode reaction are more demanding. Thus the electrolyzer requires more activity and stability of the cathode catalyst materials, which differ from the cathode catalysts for fuel cells. Currently, cathode catalysts for electrolyzers are mainly Pt/C catalysts, with Pt mass fractions ranging from 20 to 60%, which is nearly an order of magnitude lower than anode catalysts. Nevertheless, when PEM water electrolysis technology becomes famous on a large scale, the development of common Pt-content cathode catalysts is an essential direction for cost reduction, and the development of singleatom low Pt content catalysts, Pt alloy catalysts, and non-Pt catalysts has become an important research direction.

4.3 Membrane Proton Exchange Membrane (PEM) is a polymer electrolyte membrane. As can be seen from the name, the primary function is to transfer protons and, in addition, to isolate the fuel from the oxidizer and prevent direct chemical reactions caused by permeation. The most common membrane materials currently available in the market are perfluoro sulphonic acid proton exchange membranes, represented by Gore’s Gore Select enhanced proton exchange membranes, DuPont’s Nafion membrane series, Solvay’s Aquivion series of membranes, as well as American Dow, Japanese Aciplex and Flemion series of membranes. China’s leading membrane material companies are Shandong Dongyue Chemical, Jiangsu Kerun Membrane Material Co. Ltd., and Hancheng New Material Technology Co. Ltd. Among them, Comu Nafion™ series membranes have the advantages of low electronic impedance, high proton conductivity, good chemical stability, mechanical stability, and gas permeation resistance and are currently the most used proton exchange membranes for electrolytic hydrogen production. Ballard developed BAM3G, a partially fluorinated sulphonic acid-based proton exchange membrane. Inorganic components are introduced to prepare organic/ inorganic nanocomposite proton exchange membranes, which combine the flexibility of organic membranes with the good thermal, chemical stability, and mechanical properties of inorganic membranes. This has become a hot research topic in recent years. In addition, using inexpensive materials such as poly aryl ether ketone and polysulfone to prepare fluorine-free proton exchange membranes is also a trend in proton exchange membranes.

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4.3.1 The Structure of the Proton Exchange Membrane Perfluoro sulphonate membranes have the same structure, with a hydrophobic fluorocarbon backbone and hydrophilic sulphonate side chains. One of the more accepted principles of membrane operation is that the micro-phase separation structure between the hydrophilic and hydrophobic phases within the membrane causes the aggregation of hydrophilic clusters, forming proton transport channels. It is clear that the humidified environment, which directly affects the water uptake rate of the membrane, directly impacts the effectiveness of the proton transport channels. Perfluorinated materials have long dominated the market for proton exchange membranes. A common criticism is their high cost and high fuel permeability (mainly for liquid cells, e.g., methanol fuel cells, liquid flow cells, etc.). As a result, some fluorine-containing, non-fluorinated proton exchange membranes have emerged as the most promising membrane material to replace PFA membranes due to their low cost and low swelling. Still, they are not yet in mass production.

4.3.2 Influence of Proton Exchange Membrane on the Electrolyzer (1) The membrane resistance Proton exchange membranes significantly contribute to polarisation losses, costing approximately 5% of the stack, and are subject to high pressures (> 3 MPa), low load operation, and frequent start/stop operating conditions. (2) Durability of membrane material As the core component of a water electrolyzer membrane electrode, the proton exchange membrane conducts protons and isolates hydrogen from oxygen. It also supports the catalyst, and its performance directly determines the performance of the water electrolyzer, its service life, and the price of manipulation. The degradation of membrane materials is mainly due to the attack of hydroxyl radicals and peroxyl radicals located at the end of the main chain and the side chain ether bonds. In the long run, the membrane will degrade and the polymer binder in the catalytic layer. Perfluoro sulphonic acid membranes are more prone to failure during long-term operation due to mechanical damage, impurity contamination, and chemical degradation, resulting in reduced electrochemical performance. However, loss due to metal ion contamination is reversible, and treatment of Nafion 117 membranes with 0.5 M H2 SO4 revealed that PEMWE performance was unchanged after 7800 h of testing.

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4.3.3 The Modification of the Proton Exchange Membrane To reduce membrane cost and improve membrane performance, domestic and international research focuses on modified perfluoro sulfonic acid proton exchange membranes, organic/inorganic nanocomposite proton exchange membranes, and fluorine-free proton exchange membranes. Research on PFSA membrane modification has focused on three approaches: polymer modification, membrane surface etching modification, and precious metal catalyst deposition on the membrane surface. It has been found that thinner PFSA membranes can reduce polarization losses but also reduce the mechanical strength and gas permeation resistance of the membrane, affecting stability and gas product purity. The membranes were structurally strengthened and modified by introducing polypropylene-based polymers such as polyether ether ketone (PEEK), polysulfone (PSF), and polybenzimidazole (PBI), which are non-PSFA membranes with lower cost and good chemical mechanical stability. At a TPA content of 4.3%, the composite membrane exhibited the best diaphragm properties, including proton conductivity, water content, and ion exchange capacity. Organic–inorganic composite membranes are considered one of the most promising membrane modification methods due to their stable mechanical and chemical properties and low cost. However, few studies have focused on the durability degradation and deactivation mechanisms of organic–inorganic hybridization-based proton exchange membranes under high-temperature conditions. To this end, the article presents the first incorporation of muscovite mica (Mus) into PBI membranes. Mus’s surface-rich hydroxyl groups and hygroscopicity promote the formation of Mus-PA and Mus-PBI crosslinks, which enhance acid retention and proton transfer capabilities. The MusPA cross-linkage can improve the acid retention capacity of PBI/Mus composite membranes and thus mitigate catalyst degradation. Conventional high-temperature proton exchange membrane materials tend to lose proton carriers during operation, resulting in reduced membrane performance and limited long-term operational stability. In contrast, boron nitride (BN), commonly known as white graphene, has a similar graphitic laminar structure and is highly stable chemically. The authors successfully introduced BN into polyethersulfone (PES)/polyvinylpyrrolidone (PVP) composite membranes. BN nanosheets were well dispersed in the PES-PVP-BN composite membranes, while interaction forces with phosphoric acid (PA) improved proton conductivity. The power density of the composite membranes did not degrade significantly during 100 h operation, enhancing the long-term stability of the membranes. In summary, for the application characteristics of proton exchange membranes, the following requirements are generally imposed on proton exchange membranes: high proton conductivity, high mechanical strength and chemical stability, low scale change and fuel permeability, and appropriate cost. Some of the current modifications for proton exchange membranes are based on the above requirements for proton exchange membranes to improve membrane performance to reduce or achieve

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the level required for membrane applications. This summary explores the preparation of composite membrane materials by introducing fillers. Introducing functional fillers into polymeric membranes can enhance various membrane properties, such as proton conductivity, long-term operational stability, etc. However, there are some problems with the introduction of fillers, most of which are reported in the literature to be below 5%, which cannot significantly enhance the membrane performance. At the same time, the excessive introduction of fillers tends to cause aggregation, which not only blocks the proton transport channel but also goes against largescale preparation. Therefore, the researchers aim to explore suitable fillers with high dispersion and compatibility in the casting liquid to prepare a comprehensive proton exchange membrane material with outstanding performance in terms of high proton conductivity and long-term stability.

4.4 Development Prospect PEM water electrolysis for hydrogen production has entered the early stages of commercialization. Still, the bottlenecks that limit the large-scale development of the technology are the use of proton exchange membranes, which a few manufacturers monopolize, the service of precious metals for cathode and anode catalyst materials and the high energy consumption of electrolysis. As shown in Fig. 4.13, solving these problems is the key to further developing and promoting PEM water electrolysis for hydrogen production. Based on the idea of combining the advantages of alkaline and PEM water electrolysis, alkaline solid anion exchange (AEM) water electrolysis for hydrogen production using alkaline solid electrolytes instead of PEM has become a new direction. Compared to PEM water electrolysis, AEM water electrolysis uses a solid polymer anion exchange membrane as the diaphragm material, with a wider choice of membrane electrode catalysts and bipolar plate materials. Future breakthroughs in key materials such as anion exchange membranes and highly active non-precious metal catalysts are expected to significantly reduce electrolyzer manufacturing costs. At present, the cost of fuel cell systems is decreasing at a rate of around 20–30% per year due to the promotion of domestic substitution. As the market moves to a new level, PEM electrolyzers are expected to reduce the cost of electrolytic cells. In addition, the difficulty and high cost of producing PEM severely restrict the reduction of PEM electrolyzer production costs. In other words, the key to the marketization of PEM electrolyzers lies in the large-scale production of PEM. Today, due to the high production cost of the mainstream PFOS-based PEM, the difficulty of synthesizing monomers, and the deterioration of proton conductivity at elevated temperatures, PEM manufacturers are looking into new directions such as partially fluorinated PEM, fluorine-free PEM, and composite PEM to reduce costs and improve performance. The fluorine-free PEMs currently being developed are mainly based on sulfonated aromatic polymers, which are highly stable, inexpensive, and less difficult to process, but have a short service life and do not meet the needs of PEMVC.

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Fig. 4.13 Prospects for developing acidic OER, including innovative catalyst design and synthesis, visualizing in situ characterization, high current density operation and long-term stability, and PEMWE applications [6]

In terms of application promotion, the share of volatile renewables in the current power system is increasing and this trend will continue in the coming decades. Hydrogen from renewables is the only green, low-carbon way to produce hydrogen that increases grid flexibility and supports renewables’ development on a larger scale by transporting and distributing them over long distances. As medium hydrogen facilitates the spatial and temporal redistribution of renewable energy, it can help the power system to establish industrial links with industrial, building, and transport sectors that are difficult to decarbonize deeply and continuously enrich the application scenarios for hydrogen. This also opens up huge scope for the development of PEM water electrolysis hydrogen production technology.

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29. K. Wang, Y. Wang, B. Yang, Z. Li, X. Qin, Q. Zhang, L. Lei, M. Qiu, G. Wu, Y. Hou, Energy Environ. Sci. 15, 2356–2365 (2022) 30. Y.H. Lee, I. Song, S.H. Kim, J.H. Park, S.O. Park, J.H. Lee, Y. Won, K. Cho, S.K. Kwak, J.H. Oh, Adv. Mater. 32, 2002357 (2020) 31. J. Chen, P. Cui, G. Zhao, K. Rui, M. Lao, Y. Chen, X. Zheng, Y. Jiang, H. Pan, S.X. Dou, W. Sun, Angew. Chem. Int. Ed. 58, 12540–12544 (2019) 32. Y. Li, Y. Wang, J. Lu, B. Yang, X. San, Z.-S. Wu, Nano Energy 78, 105185 (2020) 33. L. Cao, Q. Luo, J. Chen, L. Wang, Y. Lin, H. Wang, X. Liu, X. Shen, W. Zhang, W. Liu, Z. Qi, Z. Jiang, J. Yang, T. Yao, Nat. Commun. 10, 4849 (2019) 34. M. Retuerto, L. Pascual, J. Torrero, M.A. Salam, Á. Tolosana-Moranchel, D. Gianolio, P. Ferrer, P. Kayser, V. Wilke, S. Stiber, V. Celorrio, M. Mokthar, D.G. Sanchez, A.S. Gago, K.A. Friedrich, M.A. Peña, J.A. Alonso, S. Rojas, Nat. Commun. 13, 7935 (2022) 35. N. Li, L. Cai, G. Gao, Y. Liu, C. Wang, Z. Liu, Q. Ji, H. Duan, L.-W. Wang, W. Yan, ACS Catal. 12, 13475–13481 (2022) 36. S. Chen, H. Huang, P. Jiang, K. Yang, J. Diao, S. Gong, S. Liu, M. Huang, H. Wang, Q. Chen, ACS Catal. 10, 1152–1160 (2020) 37. D. Chen, T. Liu, P. Wang, J. Zhao, C. Zhang, R. Cheng, W. Li, P. Ji, Z. Pu, S. Mu, ACS Energy Lett. 5, 2909–2915 (2020) 38. F.-F. Zhang, C.-Q. Cheng, J.-Q. Wang, L. Shang, Y. Feng, Y. Zhang, J. Mao, Q.-J. Guo, Y.-M. Xie, C.-K. Dong, Y.-H. Cheng, H. Liu, X.-W. Du, ACS Energy Lett. 6, 1588–1595 (2021) 39. Y. Zhu, J. Wang, T. Koketsu, M. Kroschel, J.-M. Chen, S.-Y. Hsu, G. Henkelman, Z. Hu, P. Strasser, J. Ma, Nat. Commun. 13, 7754 (2022) 40. Y. Jiang, H. Liu, Y. Jiang, Y. Mao, W. Shen, M. Li, R. He, Appl. Catal. B Environ. 324, 122294 (2023) 41. A. Li, S. Kong, C. Guo, H. Ooka, K. Adachi, D. Hashizume, Q. Jiang, H. Han, J. Xiao, R. Nakamura, Nat. Catal. 5, 109–118 (2022) 42. D. Liu, Y. Yan, H. Li, D. Liu, Y. Yang, T. Li, Y. Du, S. Yan, T. Yu, W. Zhou, P. Cui, Z. Zou, Adv. Mater. 35, 2203420 (2023) 43. S. Wang, S. Yang, Z. Wei, Y. Liang, J. Zhu, Y. Tang, X. Qiu, J. Mater. Chem. A 10, 25556–25563 (2022) 44. J. Gao, Y. Liu, B. Liu, K.-W. Huang, ACS Nano 16, 17761–17777 (2022) 45. T. Zheng, C. Shang, Z. He, X. Wang, C. Cao, H. Li, R. Si, B. Pan, S. Zhou, J. Zeng, Angew. Chem. Int. Ed. 58, 14764–14769 (2019) 46. S. Hao, H. Sheng, M. Liu, J. Huang, G. Zheng, F. Zhang, X. Liu, Z. Su, J. Hu, Y. Qian, L. Zhou, Y. He, B. Song, L. Lei, X. Zhang, S. Jin, Nat. Nanotechnol. 16, 1371–1377 (2021) 47. H.Y. Lin, Z.X. Lou, Y. Ding, X. Li, F. Mao, H.Y. Yuan, P.F. Liu, H.G. Yang, Small Methods 6, 2201130 (2022) 48. L. Ai, Y. Wang, Y. Luo, Y. Tian, S. Yang, M. Chen, J. Jiang, J. Alloys Compd. 902, 163787 (2022) 49. F. Zhao, B. Wen, W. Niu, Z. Chen, C. Yan, A. Selloni, C.G. Tully, X. Yang, B.E. Koel, J. Am. Chem. Soc. 143, 15616–15623 (2021) 50. A.A.H. Tajuddin, M. Wakisaka, T. Ohto, Y. Yu, H. Fukushima, H. Tanimoto, X. Li, Y. Misu, S. Jeong, J.-I. Fujita, H. Tada, T. Fujita, M. Takeguchi, K. Takano, K. Matsuoka, Y. Sato, Y. Ito, Adv. Mater. n/a, 2207466 (2022) 51. C. Liu, B. Sheng, Q. Zhou, D. Cao, H. Ding, S. Chen, P. Zhang, Y. Xia, X. Wu, L. Song, Nano Res. 15, 7008–7015 (2022) 52. Q. Dang, H. Lin, Z. Fan, L. Ma, Q. Shao, Y. Ji, F. Zheng, S. Geng, S.-Z. Yang, N. Kong, W. Zhu, Y. Li, F. Liao, X. Huang, M. Shao, Nat. Commun. 12, 6007 (2021) 53. Z. Shi, J. Li, J. Jiang, Y. Wang, X. Wang, Y. Li, L. Yang, Y. Chu, J. Bai, J. Yang, J. Ni, Y. Wang, L. Zhang, Z. Jiang, C. Liu, J. Ge, W. Xing, Angew. Chem. Int. Ed. 61, e202212341 (2022) 54. Y. Liu, L. Cai, Q. Ji, C. Wang, Z. Liu, L. Lv, B. Tang, H. Duan, F. Hu, H. Wang, N. Li, Z. Sun, W. Yan, ACS Energy Lett. 7, 3798–3806 (2022) 55. D. Simondson, M. Chatti, J.L. Gardiner, B.V. Kerr, D.A. Hoogeveen, P.V. Cherepanov, I.C. Kuschnerus, T.D. Nguyen, B. Johannessen, S.L.Y. Chang, D.R. MacFarlane, R.K. Hocking, A.N. Simonov, ACS Catal. 12, 12912–12926 (2022)

Chapter 5

Anion Exchange Membrane Water Electrolysis

5.1 Electrolyzer Anion exchange membrane water electrolysis (AEMWE) is an emerging technology that combines the advantages of proton exchange membrane water electrolysis (PEMWE) and conventional alkaline electrolysis to produce hydrogen efficiently and cost-effectively. Like PEMWE, the AEMWE cell has a “zero gap” configuration, which results in low cell resistance due to the small gap between the electrodes [1]. The cell consists of two end plates or bipolar plates on either side of the cell, followed by two gas diffusion layers (GDLs) used as collectors and a membrane electrode assembly (MEA) placed between the GDLs [2]. The MEA consists of an AEM with the anode and cathode catalysts deposited directly onto the membrane (catalyst coated membrane (CCM) method) or GDL (catalyst coated surface (CCS) method). The AEMWE design offers several operational advantages over conventional alkaline processes, such as improved process efficiency, higher current density operation, lower gas penetration, and differential pressure operation [2–6]. In the AEMWE, water and electrons combine at the cathode to produce hydrogen and hydroxide ions. The hydroxide ions then migrate through the AEM to the anode, where they are used to produce oxygen, water, and electrons. In this process, anion exchange ionomers (AEI) bind to the particles and guide OH− ions through the catalytic layer to form a particle-based catalytic layer. Figure 5.1 provides a diagram of a conventional alkaline AEM electrolyser. Similar to conventional alkaline electrolyzers, the AEMWE system operates in an alkaline environment allowing the use of low-cost, non-precious metal catalysts and cell materials. In addition, the AEM allows the use of low concentrations of alkaline electrolytes or distilled water [3, 4], both of which are more favourable than the high alkalinity of conventional alkaline processes. In AEMWE, membranes and ionomers typically consist of a hydrocarbon backbone and cationic group titles for hydroxide ion conduction [3, 4, 8–10], providing a more economical and environmentally friendly alternative to PEMWE perfluorinated membranes such as Nafion™,

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_5

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Fig. 5.1 Schematic representation of AEM electrolyzers [7]

which are costly [3, 4] and release hydrogen fluoride (HF) upon chemical decomposition [8, 11]. An important disadvantage of the AEMWE process is that it is still in the research and development (R&D) phase compared to PEMWE, while PEMWE is a well-established technology for small-scale operations [3, 4]. Overall, the AEM system is one of the most promising green hydrogen production technologies of the future compared to all water electrolysis systems. It is suitable for medium operating conditions. The use of low resistance battery configurations and inexpensive electrocatalysts and battery components can result in significant cost savings.

5.2 Electrode Materials One of the main advantages of AEM electrolysis is the ability to use platinum group metal (PGM)-free electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in separate reaction chambers. This reduces the capital cost of AEM water electrolysis. The challenge for current AEM catalyst development is to optimize the chemical composition, stability, and overall activity when integrated into an AEM system [12–14]. Electrocatalysts without PGMs typically have relatively low mass-specific activity compared to noble metal-based catalysts, which results in high catalyst loading and ohmic resistance losses on membrane electrode assemblies (MEAs). The research focused on the development of AEMWE electrocatalysts typically involves the preparation of catalyst powders. The catalyst powder can then be transformed into catalyst layers (CL) up to several tens of microns thick. The catalyst

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activity measured in powder form, i.e., before integration into the CL, may differ from the catalyst activity in the actual CL of the MEA. In some studies, thin film catalysts have also been deposited on solid and smooth electrode substrates, like gold foil. The latter may be valuable as model catalysts, but porous collectors that allow easy flow of reactants and reaction products are required for practical applications. Therefore, the preparation of HER and OER electrocatalysts for AEMWE tends to focus on powder catalysts. However, there has also been interested in depositing catalysts directly onto porous and high surface area collectors for use in MEA. Recent literature has shown that such designs can extend AEMWE operation to high current densities (> 5 A cm−2 ) [15].

5.2.1 Evaluation Metrics for Electrocatalysts Both HER and OER are heterogeneous reactions. Therefore, the electron transfer between reactants occurs at the electrode surface. Modification of electrocatalysts is usually aimed at lowering the energy barrier of the reaction, with lower overpotentials (η) and an overall increase in electrochemical activity being observed in electrocatalysis. The two main methods used to increase the activity of electrocatalysts are (1) increasing the number of active surface sites and (2) increasing the intrinsic activity of the catalyst [16]. One obvious strategy is to increase the electrochemical active surface area (ECSA) of the electrocatalyst. Many approaches have been developed to prepare nano-sized catalysts to maximize the ratio of surface to bulk atoms. However, studies in actual AEMWE cells are also required to confirm whether nano-sized catalysts (especially if they are not supported on a substrate) retain their high surface area advantage. In MEA, electrocatalysts need to form electronically conducting networks without impeding the flow of reactants and products. The exchange current density ( jo ) and the current density measured at a given η are indicators of catalyst activity, usually expressed as mass activity (current density per unit mass of catalyst) or intrinsic activity (current density per unit ECSA). Mass activity has practical significance. As mentioned earlier, intrinsic activity is a measure of actual catalysis of the material. Unfortunately, it is challenging to accurately measure ECSA values for many electrocatalysts other than platinum, especially in the case of high surface areas [17, 18]. Therefore, it is difficult to distinguish catalysts based on their intrinsic activity. Furthermore, there are inconsistencies in the measurement of catalyst activity. Data extracted for different electrolytes are usually reported as η per unit electrode area (cmgeom 2 ) at a specific current density. These values are very difficult to compare because the catalyst loadings on the electrodes may be different, and of course, different catalysts may have widely different ESCA values. Another metric is the turnover frequency (TOF) used in some studies as a function of the amount of H2 or O2 gas produced at a particular η, similar to the TOF equation for s−1 : (amount of gas produced at a particular η)/(F × ne × n), where F is the Faraday constant, ne is the number of electrons involved and n is the number of

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catalyst atoms. However, there are significant inconsistencies in calculating the TOF number, particularly in estimating the amount of gas produced and the number of catalyst atoms (n) used. For example, some authors use the total number of metal atoms of the catalyst, while others use the number of atoms on the surface of the catalyst. In some cases, the measured HER or OER current density is used as the amount of gas produced, while others measure the amount of gas produced directly. Therefore, the TOF values reported in the literature do not allow easy comparison between studies of catalyst performance. If the measurements are consistent, TOF values may be a useful engineering metric. However, for catalyst material studies, it seems preferable and easier to simply report the current density (per unit mass and, if possible, per unit ECSA of the catalyst) at a specific η value (and preferably for the same electrolyte) rather than TOF. This approach is consistent with a recent study by G. C. Anderson et al., who used an OER catalyst measurement scheme, reporting current densities measured at specific values of η [19]. The Tafel equation reflects the kinetic information and produces the Tafel slope value (b) as follows: η = a + b × log( j). The Tafel slope reveals information on the reaction mechanism. The Tafel slope needs to be determined at values of η above RT/F, i.e., at least above 45 ~ 50 mV, to ignore the contribution of the inverse reaction [20]. As j increases, it is favorable to produce smaller Tafel slopes, i.e., smaller increases in η and Ecell as the HER and OER rates increase (Ecell = Erev + Δan + Δcat + iRcell ). η values are catalyst-specific and indicate how the catalyst surface binds, interacts, and releases various reaction intermediates. Reaction kinetics depend on many experimental factors, including the nature and morphology of the catalyst and the final electrode. Catalytic activity is influenced by the bulk and surface properties of the catalyst. It is known that catalyst activity can be modulated by alloying and introducing shape and ligand effects [21]. Tafe plots are extrapolated to η of 0, i.e., to a potential equal to the standard potential, to obtain jo . Tafel slope values need to be obtained from steady-state measurements (e.g., constant current or constant potential experiments) as the Tafel slope depends on the surface coverage of the adsorbed intermediate species. Recent studies have extracted mechanistic and Tafel slope information from slow linear sweep voltammetry. Slow linear sweep voltammetry does not provide steady state conditions and therefore may produce incorrect values. Anantharaj et al. recently highlighted this and demonstrated in Fig. 5.2, which showed the results of iR-corrected [ΔmV/Δdec] slopes extracted at different scan rates for Co foils measured in 0.1 M KOH as an example [22]. The results reveal that the [ΔmV/Δdec] slope depends on a scan rate that varies between 45 and 90 mV dec−1 , whereas the actual Tafel slope value for this system extracted from the constant potential experiment is 60 mV dec−1 . In addition, the highest Tafel slope value that can be measured is 120 mV dec−1 (at 20 °C). Slopes above 120 mV dec−1 are not Tafel slopes, i.e., their values cannot be interpreted as following the Butler-Volmer behavior of the electrochemical reaction. The observed high 120 mV dec−1 slopes result from the changes in catalyst/electrode structure, which may be due to the formation of resistive oxides on the surface and/ or other changes in catalyst structure [20].

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Fig. 5.2 Demonstration of the erroneous impact of attempted Tafel slope measurements using slow-sweep voltage polarization, i.e., a nonsteady state method. (a) 100% iR drop corrected OER LSV curves. (b) Corresponding Tafel lines. (c) Tafel lines extrapolated to zero overpotential. (d) Plot of Tafel slopes of lower overpotential region and j0 against the scan rates used. The data are for a Co foil measured using a 0.1 M KOH electrolyte [22]

The development of catalysts is often focused on exploring a material with high electrocatalytic activity. However, the activities of catalysts do not always correlate with their lifetime. Therefore, it is also useful to assess other indicators of catalysts, such as the recently proposed S [23]. The value of S is the ratio between the amount of H2 or O2 gas given off and the amount of dissolved catalyst metal [23, 24]. The amount of gas given off is normalized using the ECSA value. The value of S appears to be a good indicator providing a comparative and balanced measure of catalytic activity and stability. However, care needs to be taken when measuring S-values as the ECSA of a catalyst can change during the measurement process. Other similar indicators may reflect the utilization and lifetime of the catalyst in the CL layer and MEA for AEMWE operating conditions.

5.2.2 HER Catalysts The kinetic pathway of HER typically follows either the Volmer-Heyrovskey or Volmer-Tafel mechanism [25]. Both mechanisms include water adsorption followed by hydrolysis (Volmer step, Eq. 5.1) and then dissociation of hydrogen to form H2 by

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chemical desorption (Tafel step, Eq. 5.3) or electrochemical desorption (Heyrovsky step, Eq. 5.2) [25]: The Volmer step: H2 O + e− + catalyst = catalyst − Had + OH− next the Heyrovsky step: H2 O + catalyst − Had + e− = catalyst + H2 + OH− or the combined Tafel step: 2catalyst − Had = catalyst + H2

(5.1)

(5.2) (5.3)

Had denotes the hydrogen intermediate. If the Heyrovsky, Tafel or Volmer reactions are rate determining steps (RDS) respectively, the Tafel slope can be observed at − 30, − 40, or − 120 mV dec−1 measured at 20 °C [25–27]. However, in the case of the − 120 mV dec−1 Tafel slope, it is not possible to distinguish the actual reaction route of the HER [26]. The energy barriers associated with each step play roles in determining catalytic activity. It has been proposed that HER current density correlates with the calculated metal surface hydrogen binding energy (HBE) [28] and that HBE has been shown to play a dominant role in HER activity [28–31]. HER is one of the most studied electrochemical reactions, but limited data are available in alkaline electrolytes compared to acidic conditions. HER activity decreases monotonically with increasing pH, which supports the theory that higher HBE inhibits catalytic activity [32]. Furthermore, HER occurs at a more negative potential than OER. Therefore, there are more stable materials available for HER than for OER. These less stringent conditions also provide a wider choice of electronically conducting and high surface area support materials for HER catalysts. Among the numerous studied systems, Pt and Pt-based catalysts exhibited the highest intrinsic HER activity in both basic and acidic electrolytes [33, 34]. Typical jo values for bulk and polycrystalline Pt measured in 0.1 M KOH were 0.62 ± 0.01 mA cmPt −2 . The HER kinetics of Pt in alkaline media was slowed down by 2 orders of magnitude compared to acidic media due to the additional water dissociation step [35]. Similarly, the Tafel slope of Pt was favorable (i.e., lower), − 30 mV dec−1 in acidic solutions compared to approximately − 120 mV dec−1 in alkaline solutions [28, 35]. In 0.1 M KOH, using smooth monometallic bulk catalysts, the order of extraction from HER measurements was as follows: Pt >> Pd > Ni > Fe ≈ Co > W > Cu > Au > Ag [28]. Figure 5.3a shows the exchange current density jo and HBE values following the path of the volcano diagram in alkaline electrolytes and the same trend in acidic media. The HER activities (measured as jo ) of these bulk metal electrodes show differences of up to 4 orders of magnitude. Further examination of the series of Tafel slopes (Fig. 5.3b) reveals that the slopes reported for this series range from − 90 to − 216 mV dec−1 , with only two catalysts, W and Pt, showing actual Tafel slopes of less than − 120 mV dec−1 . In the case of W, it is questionable whether HER was studied on the metal surface since in an aqueous solution the surface of tungsten would be covered with oxides which are difficult to reduce to the metal surface state in this electrolyte. The same approach applies to Ni, Fe, and Co catalysts studied, as

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Fig. 5.3 HER results measured for bulk, single-metal electrodes in 0.1 M KOH. a jo versus calculated HBE (ΔH) values revealing a Volcano-plot relationship. b Tafel slope values as reported. The horizontal line at − 120 mV dec−1 [shown in (b)] indicates the highest value a Tafel slope can display [28]

surface oxides form readily on these metals and their complete reduction to the metal surface state is challenging. In addition, hydride incorporation into metals such as nickel and palladium can further complicate HER activity measurements. Indeed, a recent study using ambient pressure X-ray photoelectron spectroscopy (XPS) has shown that the formation and transformation of Pt–H components and/or H inlay may occur on Pt surfaces under alkaline conditions [36]. In acidic conditions, the crystal orientation of the catalyst will affect its activity. The lower density and stepped surfaces of Pt are more active for HER [37]. The HER activity of Pt (110) exceeds that of Pt (100), while dense surfaces like Pt (111) have significantly lower HER activity [37]. For AEMWE applications, it is not practical to use single crystal electrodes. The experimental results show that in addition to increasing the ratio of surface to bulk atoms, adjusting the morphology of the catalyst and using nanoparticles can also change the intrinsic activity of the catalysts. The use and development of nano-structured and nano-engineered catalysts are important. Still, structural changes and aggregation of small, especially nano-sized, particles may occur during electrolysis, thus reducing the activity of the catalysts [38]. The high cost of Pt is an issue for large-scale applications. Accordingly, Pt nanoparticles less than 5 nm in size are commonly loaded on carbon black (e.g., Vulcan XC-72) and referred to as loaded Pt/C catalysts. These catalysts benefit from high ECSA and correspondingly high-quality activity. W. Sheng et al. carefully extracted the jo value and activation energy (Eact ) for the HER and H2 oxidation reactions (HOR) of bulk metal, polycrystalline Pt, and commercial 46 wt% Pt/C catalysts in KOH electrolyte [35]. Table 5.1 shows that the intrinsic exchange current density ( jo, intr ) and Eact values are essentially the same for the native metal Pt and 46 wt% Pt/C catalysts. Figure 5.4a shows a plot of the mass activity of the Pt/C catalysts versus the corresponding η values (i.e., all measured at 10 mA cmgeom −2 ), indicating that the expected jmass increases in an exponential manner with η. The latter was confirmed by plotting the same data as log10 of jmass versus η (Fig. 5.4b). Both plots show

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Table 5.1 Summary of Average HER/HOR Results for Polycrystalline Pt and Commercial Pt/C Catalysts [35] Catalysts Electrolyte

jo, intr a (at 21 ± jo,mass a (at 21 ± Eact (kJ mol−1 ) Tafel slopea (at 1.5 °C) (mA 1.5 °C) (mA 21 ± 1.5 °C) cmPt −2 ) mgPt −1 ) (mV dec−1 )

Pt(pc)b

0.1 M KOH 0.62 ± 0.01

/

28.9 ± 4.3

109

Pt/Cc

0.1 M KOH 0.57 ± 0.07

0.35 ± 0.05

29.5 ± 4

/

Measured at 21 ± 1.5 °C Polycrystalline bulk metal Pt c Commercial 46 wt% Pt/C (Tanaka Kikinzoku International, Inc.). Measured ECSA = 62 m2 g −1 Pt a

b

dispersion in the data, which can be attributed, at least in part, to experimental variation, as most studies were recorded using non-stationary polarisation curves and different scan rates. The purpose of Fig. 5.4b is to show the scatter in results reported in various studies rather than suggesting extracting the Tafel slope, which is not a valid way to use such data. Since the ECSA of Pt can be estimated using the charge generated by the adsorption and desorption of H (Hads/des ), it is necessary to measure the intrinsic activity of the Pt catalysts [39, 40]. For the Pt/C catalysts shown in Fig. 5.5, three sets of ESCA values are reported. Only one set of reported data allows estimation of its intrinsic HER activity at the same η (– 70 mV), indicating that the commercial Pt/C and homemade Pt nanowire (NW) catalysts have intrinsic activities of 0.88 and 1.4 mA cmPt –2 , respectively. The number of data points (measured under consistent conditions) was insufficient to draw conclusions and validate the activity values. However, the results of the study emphasize the need for appropriate and consistent measurements using Pt/C catalysts and establishing a valid baseline. The results reported for the various HER catalysts are discussed below. Combinations of Pt and Ni [41], such as alloy and Ni deposited on Pt, are thought to be able to exceed the HER activity of Pt in alkaline media [41–44]. There is a synergistic effect between Pt and Ni in favour of HER. This effect was demonstrated by S. Xue et al. using a model catalyst formed by growing ultrathin Ni(OH)2 clusters (and in subsequent work, thin NiFe(OH)2 with 15–20% surface coverage on Pt (111)) [41, 45]. Compared with bare Pt (111), the intrinsic activity of Ni(OH)2 clusters on Pt (111) is increased by eightfold, which occurs via the H spillover mechanism from Pt to Ni(OH)2 . The HER activity was further increased by adding cations such as Li+ to the electrolyte, thus promoting the formation of hydrogen intermediates. The same authors also deposited this cluster on a more practical Pt/C powder catalyst and also observed this synergistic effect. A schematic representation of the H-overflow effect is shown in Fig. 5.5a. In addition, the polarisation curves and η values for Pt/C, Ni(OH)2, or NiFe(OH)2 clusters on Pt/C catalysts (labeled Ni@Pt/C or NiFe@Pt/C, respectively) measured at 10 mA cmgeom –2 are shown in Fig. 5.5b and c. It can be seen that the NiFe(OH)2 clusters formed on the Pt/C powders show the highest HER activity. This is also the case for NiFe(OH)2 clusters formed on bulk Pt(111) crystals

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Fig. 5.4 Mass current density ( jmass ) for Pt/C catalysts reported in the literature versus the corresponding η value, both of which were measured at 10 mA cmgeom −2 . a The data follow an exponential-type relationship, which is confirmed by (b), which shows essentially the same as (a) but as a plot of η versus the log10 of jmass of the Pt/C catalysts

(Fig. 5.5d). Furthermore, the NiCo(OH)2 cluster on Pt(111) showed the lowest HER enhancement, lower than Ni(OH)2 and NiFe(OH)2 (Fig. 5.5d). Many studies have reported the synthesis of various forms of Pt and Ni based composite catalyst powders as catalysts with higher HER activity by introducing synergistic H spillover effects. In most cases, the mass activity and η values per mg of Pt were measured at 0.01 A cmgeom −2 as shown in Fig. 5.6a. Data for commercial Pt/C catalysts are also presented in the text. Several authors also reported intrinsic activity and ECSA values measured at η = 0.07 V. These results are summarized in Fig. 5.6b. Yin et al. [44] formed Pt nanowires and Pt nanoparticles (the latter formed by exfoliation of layered Ni(OH)2 ) on monolayer Ni(OH)2 flakes. At η = 0.07 V and 1 M KOH, the intrinsic activity (measured as j/Pt area) for Pt catalysts

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Fig. 5.5 a H-spillover mechanism and enhancement of HER activities created by various Ni(OH)2 type clusters deposited on (b, c) Pt/C and (d) bulk Pt (111) crystals. A NiFe(OH)2 cluster on Pt is used to demonstrate the H-spillover mechanism in (a), while Ni(OH)2 and NiFe(OH)2 clusters are deposited on Pt/C powder catalysts for the polarization curves and η values shown in (b) and (c), respectively. d Polarization curves for NiCo(OH)2 , Ni(OH)2 , and NiFe(OH)2 clusters deposited onto bulk Pt (111). The abbreviations NiCo@, Ni@, and NiFe@ for the NiCo(OH)2 , Ni(OH)2 , and NiFe(OH)2 clusters, respectively, are used in the graphs [45]

formed on monolayer Ni(OH)2 sheets were approximately 8 and 3 times higher than that of commercial Pt/C and home-made Pt-only nanowires, respectively. The single Ni(OH)2 layer provided a high specific surface area for the dispersion of the Pt catalysts. However, the possible contribution of a large number of Ni(OH)2 surface sites to the HER measurement was not considered in j cmPt −2 and cannot be completely excluded based on the reported measurements. The authors further reported that the combination of Pt nanowires with the monolayer Ni(OH)2 structure also improved the stability of the catalyst, although the stability experiments were carried out over a short period of 4000 s. Abbas et al. deposited Pt nanoparticles of 1.7 ~ 3.1 nm on nickel urchin-like structures, referred to as xPt@Ni-SP [46]. They reported that the HER mass activity per weight of Pt of xPt@Ni-SP catalyst in 1 M NaOH was up to 3.15 times higher than that of commercial 40 wt% Pt/C catalysts. The differences in intrinsic activity measured in A cmPt –2 were smaller. The 0.75Pt@Ni-SP catalyst showed the highest increased intrinsic activity of 1.3 times, while some xPt@Ni-SP catalysts had lower intrinsic activity than Pt/C catalysts. The use of nickel-based carriers may be beneficial to the long-term performance of the catalysts, as the authors reported higher stability of the xPt@Ni-SP catalysts than commercial Pt/C catalysts. The Tafel slopes

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Fig. 5.6 a Mass current activities per amount of Pt versus the corresponding η for various Pt–Ni catalysts, both measured at 10 mA cmgeom −2 . b Plot of the intrinsic activity per ECSA of Pt ( jint ) measured at η = 70 mV for two Pt-based and several Pt–Ni based catalysts. The blue diamonds represent Pt/C, the gray diamonds represent Ptx Niy alloys, and the orange circles represent the Pt nanosized catalysts with Ni(OH)2 in (a)

for all Ptx ( x > 0.5)@Ni-SP catalysts and Pt/C catalysts were in the range of −30 mV dec–1 , suggesting that the Volmer reaction is the rate determining step. However, the reported Tafel slopes were extracted from non-stationary polarisation curves. Chen et al. explored the deposition of Pt on honeycomb-like NiO@Ni thin film catalysts [43]. Ni thin films were actual Ni nanofoams that could also serve as collectors in MEA. The intrinsic HER activity per unit Pt surface area did not vary significantly among catalysts. Pt on a honeycomb NiO@Ni-nanofoam substrate was reported to have 15 times more HER activity per weight of Pt than commercial Pt/C catalysts. This increase may be due, at least in part, to H spillover effects. However, depositing the catalyst directly onto the collector can also improve mass activity by increasing catalyst utilization (in this case, depositing Pt on a honeycomb NiO@Ni film) compared to powder catalysts. Powder catalysts are often converted to electrodes using ionomers and/or binders, which may block the catalyst sites. Measurements of η at 10 mA cmgeom −2 showed a 40% increase in the overpotential of Pt on both the honeycomb NiO@Ni films and the Pt/C powder catalysts within 24 h. All these results suggest that the combination of finely dispersed Pt with nickel on a Ni(OH)2 layer having a high surface area may be an HER catalyst providing higher mass activity per equivalent of Pt. Therefore, it is not surprising that other metal additions (e.g., Fe and Co) have been explored. It has been mentioned that NiFe(OH)2 clusters deposited on Pt(111) crystals further can promote the HER activity, suggesting that Fe assists Ni in the water splitting step [45]. Fe increases the

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electrical conductivity and oxidation state of Ni in its vicinity. Co nanowires grown on a Ti network were decorated with Pt–Co alloys by Wang et al. [47] Only one Pt–Co catalyst exceeded the HER mass activity of commercial Pt/C catalysts, and only the less active catalyst showed stable catalytic activity over 50 h. However, as can be seen in Fig. 5.6, the Ptx Niy alloy particles had the highest HER activity among these types of catalysts. Wang et al. prepared different Pt–Ni nanowire catalysts by annealing, which consisted of various alloy phases such as Pt3 Ni4 , Pt3 Ni3 , Pt3 Ni2 , Pt3 Ni, and NiOx [42]. They reported that the HER mass activity of their Pt–Ni nanowire catalyst in 1 M KOH was 12 times higher than that of commercial Pt/C catalysts. Many interfaces between Pt3 Ni and NiOx were given higher mass activities during the optimized annealing process. The NiOx surface receives OH– generated in the H2 O cleavage reaction, and the nearby Pt sites receive Had and generate H2 . No intrinsic HER activity was measured and the HER onset potential was the same for all studied catalysts, including commercial Pt/C catalysts. Ru is another PGM that has received attention as a potential HER catalyst for acidic and basic electrolytes. The ~65 kcal mol−1 H-bond energy of Ru is similar to that of Pt [46]. While not as expensive as Pt, Ru is scarce. Therefore, Ru will be a viable candidate catalyst for large-scale AEMWE if affordable materials and routes for synthesizing catalysts and supports can be used to fabricate Ru catalysts with high HER activity and long-term stability. Recent activities in the development of Rubased HER catalysts for AEMWE have focused on the formation and anchoring of Ru as well as PtRu alloy nanosized particles on conductive carbon-based carriers. Highsurface-area carbon supports, such as phosphorus-carbon nanosheets and N−doped porous 2D carbon sheets, composed of repeating units such as C2 N structures, were synthesized to allow the anchoring of Ru-based particles [48]. Density functional theory (DFT) calculations have shown that Ru particles embedded in these C2 N and C2 N2 structures have reduced H-binding energies and that Ru and auxiliary carbon atoms act as catalyst sites [46–49]. In many of these studies, the HER activity and catalyst loading at the electrode appear to be given as the total catalyst mass, i.e., including Ru and other components such as the carrier. In addition, the measurements of ECSA are rare, probably due to the challenging nature of reliably extracting ECSA values for Ru-based catalysts. Electric doublelayer capacitance values and COads exfoliation measurements have been used to obtain ECSA information, but Ru could form many different oxides at low potentials, each yielding a different Cdl value, and COads only adsorb on metal surfaces [18]. Similarly, the Cu underpotential deposition method (Cuupd ) can be applied to catalyst sites in the metallic state, but not to oxides [18]. However, based on thinlayer catalyst measurements, some ruthenium-based catalysts showed promise for application, as shown in Fig. 5.7. Figure 5.7 shows a plot of mass activity versus corresponding η values (both measured at 10 mA cmgeom –2 ) for Ru-based and Pt/ C powder catalysts. Most of the results for the HER activity of the Ru catalysts are underestimated because the catalyst total weight was used for the mass activity calculations (where the Ru loading of many catalysts appears to be undetermined), while the Pt/C catalyst (black rhombus) and the loaded Pt1 Ru1.54 alloy (red cross) catalysts activity was calculated on a total precious metal weight basis. Figure 5.7

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shows that the mass activity of the Ru-based catalysts was as high as, if not higher than, that of Pt/C. The activity per total noble metal loading in the 2.5 nm Pt1 Ru1.54 alloy catalyst formed on phosphorus carbon nanosheets appeared to exceed that of the commercial Pt/C catalysts [50]. The 2.5 nm Pt1 Ru1.54 alloy catalyst prepared by Y. Li et al. outperformed Pt/C catalysts and homemade Pt catalysts loaded on phosphor carbon nanosheets [50]. The authors concluded that the observed enhanced activity of the Pt1 Ru1.54 alloy catalyst was attributed to the electronic interaction between the nano-sized Pt1 Ru1.54 catalyst and the phosphor carbon nanosheets, resulting in enhanced H2 O dissociation kinetics. Ru particles with an average size of 1.6 nm were dispersed in porous twodimensional carbon nanosheets made from repeating C2 N units by Mahmood et al. [48]. Figure 5.8 illustrates the formation and distribution as well as the embedding of Ru nanoparticles within the layers of the high surface area nanosheets. The authors used Cuupd , COads solvation voltammetry, and Hads/des charge to estimate the ECSA values. They reported that the number of active sites for Ru/C2 N was approximately 18% lower than for Pt/C2 N and Pt/C catalysts. Based on the number of active sites estimated from these three methods, the TOF (i.e., the intrinsic HER activity of Ru/ C2 N) per active site of the catalyst is ~1.7 times higher than the TOF of commercial Pt/C catalysts. It is assumed that the ECSA measurements for the Ru/C2 N catalyst reflect the Ru sites in the metallic state as previously described. There is only a small decrease in HER activity after 10,000 potential cycles between 0.2 and − 0.1 V in 1.0 M KOH compared to a reversible hydrogen electrode. No details are given about the electrochemical experiments, e.g., whether a Pt-free pair of electrodes with high surface area was used.

Fig. 5.7 Mass activities ( jmass ) versus the corresponding η values of various supported catalysts, namely, Ru nanoparticles (green crosses), a 2.5 nm Pt1 Ru1.54 alloy (red cross), and Pt/C (black diamonds). The jmass and η values are measured at 10 mA cmgeom −2 in 1 M KOH. The mass activities are measured in A/mg noble metal catalyst for the supported Pt1 Ru1.54 alloy and the Pt/ C catalysts, while in the case of the supported Ru catalysts, the mass activities are in mg per total catalyst, i.e., including the carbon support

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Fig. 5.8 Schematic of the synthesis to form nanosized Ru catalysts embedded within holey, two-dimensional carbon nanosheets made of repeating C2 N units [48]

Other studies have also focused on the dispersion of Ru on high surface area carriers. Lu et al. [49] grew Ru nanowires on N-doped carbon nanowires, and Zheng et al. [51] formed Ru particles with an average size of 2 nm in a C3 N4 matrix, while Xu et al. [52] used carbon carriers of unknown origin to form Ru particles with an average size of 1.5 by pyrolysis at 350 °C. These catalysts were close in mass activity

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to commercial Pt/C catalysts at comparable η values (Fig. 5.8). As mentioned above, the authors appear to have taken the total loading of the Ru catalysts (including the carbon carriers) as the catalyst loading, whereas the mass activity of Pt/C is per Pt metal. Recent studies have also involved Ru–Ni systems, which are typically concerned with the dispersion of Ru (at the nanoscale) on Ni(OH)2 type substrates, to exploit the fact that Ni-hydroxides can form two-dimensional high surface structures. Ding et al. [53] synthesized Ru–Ni nanoplates with dimensions of ~10–30 nm, and Chen et al. [54] synthesized laminated RuNi sheets on Ni nanofoam (RuNi-LMH). Both samples had lower η values measured at 10 mA cmgeom −2 compared to the Pt/C catalyst. Similar to the RuNi system, RuCo catalysts are also being investigated. A nitrogen-doped carbon-loaded Ru–Co alloy catalyst was prepared by an optimum annealing temperature of 600 °C with an η value of 34 (measured at 10 mA cmgeom −2 ), which is lower than the commercial Pt/C catalyst (49 mV). The total catalyst loading was approximately 0.255 mg cmgeom −2 , while the RuCo loading on carbon did not appear to be measured [55]. The addition of Co to Ru was intended to enhance the H* recombination step [55, 56]. Mao et al. synthesized cobalt-substituted ruthenium nanosheets and reported that the ~30 nm Co atoms distributed in the ruthenium lattice had comparable reaction kinetics (measured by TOF) to commercial Pt/C, Ru/C, and homemade RuCo alloy catalysts [57]. Detailed information on TOF number calculations and Pt/C catalyst loading does not appear to be given [58, 59]. In summary, the reported activity of ruthenium-based HER catalysts focuses on the dispersion and anchoring of ruthenium catalysts and shows promise for the development of thin-layer electrode studies. However, to fully understand these issues, a detailed analysis of these catalysts under AEMWE conditions and a determination of the Ru content in the catalysts are required. Actual Tafel slope measurements under steady-state conditions in the effective η region are also needed. Ru stability is an issue of concern for water splitting under alkaline conditions. It has been established that Ru catalysts under alkaline conditions are less stable over the OER potential range. Therefore, it is important to conduct thorough long-term stability measurements of these ruthenium-based HER catalysts under conditions that reflect real AEMWE operation, i.e., conditions involving intermittent periods and possible potential reversals. Ni is a metal with abundant reserves and is used as an HER and OER catalyst in conventional WE electrolyzers, making it a popular candidate to replace Pt or Ru based catalysts under alkaline conditions [60–62]. Ni metal exhibits good water absorption, but the hydrogen bonding energy of Ni metal is high, and the usual ratedetermining step is the H* recombination reaction [63, 64]. The HER activity of pure Ni catalysts is lower than that of Pt/C catalysts. For example, at 10 mA cm−2 , the η of metallic Ni is about 0.15 V higher than that of Pt/C catalysts [64]. The HER activity of metallic Ni also decreases with increasing the electrolysis time, which is usually attributed to hydride incorporation into the nickel lattice at the substrate and electrode surfaces [65]. Especially for catalysts with small grain sizes, they correspondingly have a large number of grain boundaries where preferential H2 adsorption occurs [58, 59]. Corrosion is another factor that ultimately reduces the HER activity during

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electrolysis, as are the changes in base concentration induced by OH− adsorption [60]. Ni metal surfaces can adsorb oxygen from the electrolyte and react to form NiO [66]. NiO is converted to NiOOH as it cycles to the positive potential region, causing the electrolyte to adsorb onto the surface and react with NiO to form NiOOH, further penetrating the catalyst and reducing the HER activity [66]. Ni-based hybrids are often used as HER catalysts in AEM electrolysis. In most studies, they were reported to be less active than PGM-based materials. Ni nanopowders with a catalyst loading of 2 mg cm−2 can be used as catalysts for hydrogen precipitation in AEM electrolysis systems with current densities up to 100 mA cm−2 at an applied voltage of 1.99 V and a temperature of 44 °C [67]. Ultra-low Ni/Pt particles were coated on carbon paper (13.2 μg cm−2 ) to make particle-based electrodes. The MEA prepared with Pt-Ni/CP-2 as the cathode electrocatalyst had an effective current density of 250 mA cm−2 at 1.9 Vcell (60 °C). This performance exceeded or was close to the highest activity reported previously when using a higher catalyst loading range of 3.1–80 mg cm−2 [68]. Nevertheless, the activity and stability of nickel metal is still relatively poor and needs to be combined with other materials such as transition metal based oxides, sulhur group selenides, nitrides and phosphides to improve performance [69]. The performance of these mixed Ni-based oxides was tested using commercially available materials such as ACTA 3030 (Ni/(CeO2 –La2 O3 )/C) as HER electrocatalysts in dilute K2 CO3 /KHCO3 electrolyte solutions (pH 10–11) with A-201 anionexchange membranes (Tokuyama) [70]. The catalyst loading on the cathode was considered to have a dramatic effect on the performance of the AEM system when the mixed CuCoOx oxides were immobilized on the anode at a loading of 36 mg cm−2 . By varying the Ni/(CeO2 –La2 O3 )/C loading from 0.6 to 7.4 mg cm−2 , output cell potentials between 2.01 and 1.89 V were obtained at 43 °C and 470 mA cm−2 . The cathode catalyst loading also affects the AC (AC/1 kHz) resistance at different current density values, but the reaction mechanism remains controversial due to the complexity of the system. It is also noteworthy that the stability of the prepared system was remarkable, with no drop in cell potential at a constant pressure difference of 3 MPa for 800 h. This demonstrates its potential value in terms of durable and efficient hydrogen production. The use of alloys is also a viable method to increase the HER activity of nickel [71]. NiMo alloy cathodes and NiFe alloy anodes were assembled and impregnated with elf-crosslinked quaternary ammonium polysulfone (xQAPS) membranes as the MEA [72]. The anodes and cathodes had a high catalyst loading of 40 mg cm−2 , which resulted in an AEM cell performance of 400 mA cm−2 at a cell voltage of 1.8 ~ 1.85 V under a pure water electrolyte at 70 °C. In another work, Ni0.9 Mo0.1 nanosheets were synthesized and tested, whose cathode performance was comparable to that of Pt nanoparticles in an AEM cell [13]. The spongy structure of NiMo alloys is thought to expose a higher specific surface area. NiMo/X72 in 1 M KOH reached 1 A cm−2 at 1.9 V, while Pt particles reached the same current density value at 1.8 V, suggesting a small performance gap between NiMo alloys and Pt-PGM catalysts (Fig. 5.9).

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Fig. 5.9 Current density–voltage polarization curves recorded for different electrolytes (1 M and 0.1 M KOH solution) and electrocatalyst types (NiMo alloy/Ir and Pt/C/Ir PGM-based materials) at 50 °C. a NiMo alloy with a catalyst loading of 5 mg cm–2 ; b Pt nanoparticles loaded on a porous carbon matrix at 1 mg cm–2 content. Both anode electrodes used 3 mg cm−2 of Ir-black [13]

In the above work, HER electrocatalysts for AEM were simply nickel particles, nickel-based substances, and nickel-doped carbon materials [73]. Although many PGM-free catalysts in half-cell assemblies have demonstrated significant HER activity, research into the design of HER electrocatalysts for AEM remains limited [74]. Although NiMo alloys currently possess excellent cathode performance in AEM electrolyzers, the ultimate level of AEM performance is still unknown. Considerable effort is still required to design HER catalysts for practical applications and operations.

5.2.3 OER Catalysts The OER process under alkaline conditions involves the consumption of OH– anions in several steps, as shown in the following equation, where M denotes the active catalyst [75]: M + OH− = MOH + e−

(5.4)

MOH + OH− = MO + H2 O(l) + e−

(5.5)

MO + OH− = MOOH + e−

(5.6)

MOOH + OH− = M + O2 (g) + H2 O(l) + e−

(5.7)

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Total reaction: 4OH− = O2 (g) + 2H2 O(l) + 4e−

(5.8)

The multiple four-electron reactions on the OER suggest a more complex mechanism and inherent retardation than the HER. The OER free energy diagram has a single step height of 1.23 eV and a total free energy change of 4.96 eV under standard conditions [76]. Changes in the binding energy of the reaction intermediate to the catalyst will alter the overpotential. The minimum η value for catalyst surfaces that strongly bind reaction intermediates is determined by the breaking of bonds between the reaction intermediates and the catalyst surface. Catalysts such as Mn, Co, Ir, and Ru oxides that bind tightly to reaction intermediates formed in the OER are expected to have a minimum η of 0.37 V [77]. According to equations, NiO and TiO2 are catalysts with weak binding energies for OER intermediates [77]. In AEM systems, cell performance is closely related to and highly dependent on OER. The alkaline pH of AEMWE is thought to provide a wide range of non-PGM OER catalysts, including transition metals such as Ni, Co, Fe, and Cu. However, this view is overly simplistic, as long-term stability is an issue for many TM catalysts and catalyst carriers. As a result, the number of materials suitable as OER catalysts and carriers is limited. Both IrO2 and RuO2 catalysts commonly used in PEMWE lack long-term stability under alkaline conditions. RuO2 is the least stable of the two oxides and the metal counterparts of the oxides, i.e., Ir and Ru metal electrode catalysts, exhibit poor stability [78]. The stability issue is demonstrated in the Pourbaix plots of Ni, Cu, Fe, and Cu, i.e., shown in Fig. 5.10, for four TMs of high concern as catalysts or catalyst components for AEMWE OER catalysts. The inset shows the voltage and pH ranges associated with AEMWE considering the different AEMWE feed modes, i.e., water and dilute electrolyte. These zones are pH∼7 for water, pH∼9 for KHCO3 and pH range 9–12 for dilute NaOH or KOH. The coloured areas indicate the corrosion zones of the corresponding elements. An important point to note is that the stability of these TMs depends not only on the potential region but also on the pH and therefore on the nature of the solution feed to the AEMWE cell. These diagrams show that the stability of Co and Ni is destroyed at pH ~ 7 but improved at pH above 9. NiO is known to have long-term stability at high pH and is used as an OER anode in conventional WEs. Copper is also considered to be a candidate catalyst in the pH range of ~9–12. The Fe schematic shown in Fig. 5.10 ignores the formation of passivated iron oxide and suggests that iron corrosion is likely to occur under AEMWE operating conditions. It should be noted that the Pourbaix diagrams are only a guide based on thermodynamic information. They do not include reaction kinetic information and require experimental verification of catalyst stability. In addition, the stability of a catalyst is influenced by its chemical and physical structure, including its physical dimensions. Nevertheless, Pourbaix diagrams provide insight and a guide to initial material stability. The harsh conditions of OER also severely limit the amount of stable carrier materials used for OER catalysts. Carbon carriers with high surface area and electronically conductive typically used for HER catalysts (e.g., Vulcan XC-72, graphite and possibly even CNT) are not suitable as carriers for OER catalysts as carbon is easily oxidised and consumed during the OER reaction. Non-carbon-loaded OER

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Fig. 5.10 Pourbaix diagrams of cobalt, copper, iron, and nickel in aqueous electrolytes at ambient pressure and 25 °C. The inset shows the voltage–pH range that an anode catalyst may experience in an AEMWE [78]

catalysts are therefore typical for AEMWE, although carbon-loaded TM OER catalysts (including graphene, organic frameworks and CNT carriers) have been used, mainly in shorter experimental time scales. As previously mentioned, the electronic conductivity of the carrier (or catalyst body) also plays a role in producing effective OER catalysts. Indeed, conductive carriers can reduce the conductivity limitations of some catalysts and, in the case of very thin (on the atomic layer scale) catalysts, the carriers can change the electronic properties and lattice constants [77]. Research on OER catalysts for AEMWE has been extensively developed over the last decade. Many synthesis conditions have been applied, and the size and structure of the catalysts are not always completely defined. In the case of OER catalysts, most OER catalysts studied for AEMWE applications are powder catalysts and the activity of newly prepared OER catalysts is usually tested in thin-layer electrode devices and/ or directly in individual AEMWE cells. In addition, benchmark catalysts need to be established. Several studies have reported OER activity comparisons with commercial iridium oxide catalysts, which are still considered to be the most advanced catalysts in terms of initial activity. However, the reported OER activity of iridium

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oxide catalysts varies considerably. This may be partly due to the different preparation methods and the different forms and properties of iridium oxide. Figure 5.11 shows transmission electron microscopy images of four different iridium oxides, demonstrating some of the possible differences. Ir-oxide powder catalysts are usually derived from the thermal synthesis of precursor salts (i.e., IrCl3 containing water). It is known that the annealing temperature affects the ECSA, the water content, and the electronic conductivity and crystallinity of the resulting oxide catalysts, with higher temperatures leading to more crystalline and denser oxides [79]. The catalytic activity of amorphous IrOx for OER is higher than that of crystalline and rutile IrO2 , which may be due to the more open structure of aqueous and amorphous IrOx compared to the dense structure of IrO2 . The stability of aqueous and amorphous IrOx is lower [23]. Hydrated and amorphous forms of IrOx can also be formed on the surface of Ir metals, which may be beneficial for OER activity [80]. Many commercial suppliers sell iridium oxides in the form of IrO2 or hydrated IrO2 ·H2 O. Researchers should develop a method to characterize the commercial Ir-oxides received to understand the type of Ir-oxide structure under study. At a minimum, the received Ir-oxide needs to be characterized, including X-ray photon spectroscopy (XPS) of the Ir and O regions, X-ray diffraction (XRD) patterns, and the redox chemistry of the received Ir-oxide examined by cyclic voltammetry (CV). It is rare for consistent baseline data on the OER activity of iridium oxide catalysts under alkaline conditions. In a recent study, Anderson et al. suggested a baseline controlled study and reviewed the published data [19]. A summary of the literature showed that the mass activity of two IrO2 powders from two different suppliers was approximately 11 and 60 A g−1 at 0.1 M KOH and a η of 0.35 V. More data was available for the 1 M KOH electrolyte, and the mass activity data for the IrO2 powder varied between 9 and 275 A g–1 at η = 0.35 V. Single crystal IrO2 (and RuO2 ) catalysts were investigated by Stoerzinger et al. [79]. They reported that at pH = 13, for both oxides, the (100) surface was intrinsically more active than the thermodynamically more stable (110) surface and correlated these OER activities with the density of unliganded metal sites on the crystal plane. The IrO2 and RuO2 single crystals used in the study are rutile forms of the oxides, forming thin layers and MgO substrates on (001) oriented SrTiO3 . All these features, i.e., possible differences in crystalline catalyst structure, light layers and potential substrate effects, may be responsible for some of the intrinsic OER activity differences observed between the catalyst powders and the single crystal films. The reported OER mass activity results for the iridium oxide powder catalysts show a large variation, as shown in Fig. 5.12. Figure 5.12 shows the measured mass activity and corresponding η values at 10 mA cmgeom –2 . The results illustrate the sluggishness of the OER as the η values required to achieve the same mass current density are significantly higher for the OER catalysts than for the HER catalysts. Furthermore, the exponential dependence of the mass activity on the η value is not significant. The slope dependence of most catalysts on η is zero (or very small). The exact reasons for this are not clear. It may be partly due to the different nature of the iridium oxide powder catalysts studied by different suppliers, including differences in ECSA values, or it may be that many of these results were obtained from slow scan

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Fig. 5.11 Transmission electron microscopy images for various Ir-based catalysts are as follows: a–c Ir particles, d–f nanosized Ir particles, g–i Ir black from Umicore, j–l amorphous IrOx from a TKK and b the rutile form of IrO2

polarisation curves rather than actual steady-state tests. One catalyst has significantly higher OER activity in the η range of 0.35–0.4 V (red arrows in Fig. 5.12a and IrOx (2)). This catalyst was reported as IrOx , which may indicate a non-crystalline form of iridium oxide [81]. The OER activity of this IrOx catalyst was extracted from a steady-state Tafel slope test, thus adding to the validity of these catalyst measurements. Figure 5.12a shows the results for another IrOx catalyst (red arrow and IrOx (1)) with an OER activity that falls within the wide range of other Ir-oxide catalysts.

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Fig. 5.12 Comparison of mass activities ( jmass ) reported for commercial Ir-oxide powder catalysts versus the corresponding overpotential (η). Both jmass and the corresponding η values were measured at 10 mA cmgeom –2 . b Enlarged version of (a) demonstrating the variability in the reported data for the lower η range. The majority of the Ir-oxides were reported to be IrO2 , except for two oxides that are referred to as IrOx , as indicated in (a) [82]

All these results point to the need to establish a baseline for selected catalysts based on their preparation methods and detailed physical and chemical characterization under appropriate steady state testing. The identification of the best performing OER catalysts is complicated by the lack of a true comparative study throwing the quality and intrinsic activity of a large number of well-characterized iridium oxide powder catalysts. Due to the challenges of accurately determining the ECSA values of many catalysts, the mass activity of catalysts is often reported, and the mass activity of catalysts is characterized by a high degree of practical relevance. However, it does not directly translate into intrinsic activity. High surface area nickel and nickel alloy catalysts have been of interest for conventional alkaline aqueous electrolytes operating on high concentration (in the 30 wt%

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range) KOH electrolytes, where the catalyst is formed directly on a nickel grid collector. Very little has been reported on methods for the synthesis of carrier-free and high surface area alloy powder OER catalysts. Indeed, the synthesis of high surface area carrier-free alloy powder catalysts required for OER catalysts in AEMWE is challenging. Notably, nickel alloys have shown promising intrinsic OER activity, which may be high especially when freshly prepared [83–85]. Nickel/iron (NiFe)-based materials have been shown to have significant OER activity in various applications [86, 87]. Their activity was evaluated in AEM cells. In controlled experiments with atomic ratios (Fe/Ni), NiFeOx hybrids were synthesized and transformed from a less crystalline character to a spinel phase with a higher Fe content. When evaluated in a single AEM cell, the Ni1 Fe1 oxide anode showed a high current density of 650 mA cm–2 at a temperature of 2 V below 50 °C [88]. However, it was less durable in the 500 h potential cycling test, losing more than 150 mA cm−2 in the first 400 h. NiFe-based layered double hydroxides (LDH) are excellent OER catalysts to accelerate the reaction [89]. Based on the inverse relationship between lateral size and effective surface area, a size reduction strategy is proposed to improve the performance of NiFe-LDH. It was hypothesised that an increase in the latter parameter would greatly increase the hydroxyl ions in the feed material. Another advantage is the reduction of the catalyst layer film thickness, which reduces the ohmic loss on the MEA. With this in mind, the single point spontaneous gel-deflocculation method (Fig. 5.13a) was used to prepare ultrafine NiFe-LDH particles with a lateral size of less than 10 nm [90]. The prepared NiFe-LDH had significant OER activity in both half-cells and single cells, exceeding IrOx , and required a low overpotential of 247 mV to reach 10 mA cm–2 compared to 281 mV for IrOx to reach 10 mA cm–2 . In the entire AEM cell, the anode electrocatalyst using ultrafine NiFe-LDH as the MEA (Fig. 5.13b) showed excellent performance, achieving a conversion efficiency of 74.7% at 80 °C and 1.0 A cm–2 with a cell voltage of 1.59 V. This performance was recognized as the highest of the MEAs decorated with PGM-free catalysts to date (Fig. 5.13c). To further investigate OER half-tank testing and full AEM electrolysis performance, various transition metal (Ni, Co and Fe)-based oxides/hydroxides were prepared using a surfactant-free hydrothermal method [81, 91, 92]. Interestingly, the NiFeOx Hy hydroxides showed the best OER performance in the half-cell but the worst activity in the AEM stack. This may be due to the poor electrical conductivity of NiFeOx Hy and the lack of liquid electrolyte penetration to its surface. Therefore, only a fraction of the surface active sites is involved in the reaction. Using the dry conductivity as a reliable indicator, the influence of these oxides/hydroxides on the performance of the three-electrode system and the AEM system can be better understood. The electrical conductivity has a positive effect on the tuning of the OER properties of these oxide/hydroxide materials. Intentionally anchoring Fe species on the NiCoOx surface dramatically increases the Fe/NiCoOx activity due to the enhanced electrical conductivity. Therefore, this finding may help to establish the relevance of the catalyst performance in conventional three-electrode systems to the actual AEM system through positive interactions at the oxide/hydroxide catalyst interface.

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Fig. 5.13 a, b Schematic illustration of NiFe-LDH synthesis mechanism and its application in AEM cells. c Polarization curves of membrane electrode assembly (MEA) using NiFe-LDH and IrOx as the anode catalysts [90]

The selection of earth-abundant OER catalysts is highlighted as an important factor in achieving AEM utilisation targets. Although Ni/Fe-based materials have been extensively investigated as OER catalysts, they have rarely been studied for AEM electrolysis due to their poor stability. Further efforts should focus on composition, structure and simple synthesis methods to improve the overall performance of real devices. CoCu-based catalysts are also of interest [93]. The use of Co makes the catalysts expensive, but according to the Pourbaix diagram, both Cu and Co may show the stability required for OER catalysts in AEMWE. Indeed, a single AEMWE cell using a commercial CuCoOx (Acta 3030) anode catalyst has an operating time of

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> 100 h [70, 94]. Cu itself is a poor OER catalyst, but its combination with CoOOH produces a catalyst with higher activity than Cu(OH)2 and CoOOH, however, the mechanism for the enhancement of OER activity is not known. Early studies using Cu-containing Co3 O4 catalysts (Cux Co3−x O4 , 0 ≤ x < 1) involved single AEMWE cell tests and OER catalyst loadings in 3 mg cm MEA area –2 [67, 95, 96]. The substitution of Co into the spinel Co3 O4 lattice reduced the Co3+/4+ redox reaction and the η of the OER to lower potentials. Promising AEMWE cell performance was demonstrated with a cell voltage of 1.8 V at 1 A cm–2 and Cu0.7 Co2.3 O4 was reported as the most active catalyst composition. Catalysts were prepared using a thermal decomposition method with reported average catalyst particle sizes in the 20–30 nm range, although larger particles may be present based on the transmission electron microscopy (TEM) images shown. Alternative synthetic routes may optimise catalyst particle size and increase ECSA, which in turn may lead to higher catalyst activity and AEMWE cell performance. Recent studies have explored the synthesis of high surface area, Cu-substituted Co3 O4 catalysts [92, 97, 98]. Karmakar and Srivastava synthesised Cu0.3 Co2.7 O4 nanochains [98]. The catalyst particles were minimized in the range of 10–26 nm. Jang et al. prepared Cu0.5 Co2.5 O4 and Co3 O4 catalysts using a low-temperature and pH-adjusted co-precipitation method [92]. They reported that Cu0.5 Co2.5 O4 particles with a size of 3–4 nm were the smallest and most efficient OER catalysts. The reported η value for the Cu0.5 Co2.5 O4 catalyst loaded with 10 mA cmgeom –2 and 0.5 mg cmgeom –2 in 1 M KOH was 285 mV. The Tafel slope for this catalyst was 79 mV dec–1 , which was also lower than the 98 mV dec–1 for the IrO2 catalyst, but higher than the Tafel slope of the Niz Fez−1 Ox Hy catalyst. The halfcell performance of the Cu0.5 Co2.5 O4 catalyst was tested on a nickel foam current collector electrode at 1.3 A cm–2 with a cell voltage of 1.8 V. The polarisation curve recorded at 10 mA cmgeom –2 decreased by ~ 15 mV after 2000 h. However, the catalyst loading on nickel foam was higher at 10 mg cm−2 and the current density used for stability testing was low. Therefore, the improvement over previous AMEWE cell tests appears to be minimal. The co-precipitation method used by Jang et al. may also have resulted in the formation of some larger particles [92], as it is a challenge to control particle size without stabilisers using co-precipitation. This method was used by Park et al. to synthesize nanostructured nickel foam collectors directly on the CuCo2 O4 catalysts [99]. A cell voltage of 1.8 V was achieved at 1 A cm−2 , exhibiting higher performance at higher current densities than commercial IrO2 powder catalysts, although the loading of CuCo2 O4 catalyst on nickel foam was much higher than IrO2 , at 23 and 4 mg cm–2 respectively. The result of Cux Co3–x O4 (0 ≤ x < 1) brings this system attention, but methods to form higher ECSA catalysts need to be found. Chalcogenides are another class of materials that have been extensively studied as OER catalysts in alkaline media. The general formula of the chalcogenide structure is ABO3 , where A and B are cations of different sizes. The chalcogenide catalyst consists of a rare alkaline earth metal at site A and 3d TM at site B. The variation in OER activity of chalcogenides correlates with eg orbital filling, indicating that the closer to the unified eg the higher the activity [100]. A decade ago, Ba0.5 Sr0.5 Co0.8 Fe0.2 O3−δ was reported to have high intrinsic OER activity [101]. but

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it was unstable under the oxidizing conditions associated with AEMWE. Further studies have shown that Ba2+ and Sr2+ are leached out, leaving a less active Fe−Co surface [102]. Recently, a bicubic chalcocite Pr0.5 Ba0.5 CoO3−δ with the highest OER activity and increased stability in chalcocite was reported [100]. According to computational studies, the position of the p-band center of O relative to the Fermi energy level can explain the different OER activities of chalcogenide catalysts [34, 101, 103]. Fermi energy levels close to and overlapping the p-band of O are associated with higher activity. Unfortunately, it is difficult to determine the exact binding energy of M–O due to surface changes caused by leaching and redeposition of metal cations during the OER process. Research has also focused on sulphur compounds, sulphides and phosphides as dopants for TM catalysts, as such materials have shown promising HER activity [104–106]. Many studies have reported that TM sulphides and phosphides are better OER catalysts than TM alone [104, 107–109]. Metal sulphides, phosphides, and nitrides are thermodynamically unstable under oxidizing conditions [78, 110]; therefore, these catalysts are expected to be oxidized to (oxy)hydroxides. However, detailed experimental support is lacking. Researchers have confirmed that oxide and hydroxide phases are formed on the surface, while the core (if any) is present as sulphides, phosphides, and sulphuric compounds [111, 112]. The nature of the resulting structures is likely to have enhanced catalytic activity due to the creation of defective sites or higher surface areas. This mechanism needs to be avoided in the manipulation and posthoc analysis.

5.3 Anion Exchange Membranes AEMs are key components that can be used in a variety of electrochemical devices, including fuel cells, electrolyzers, redox liquid flow cells, electrodialysis and the production of a wide range of value-added products through the electro-reduction of CO2 [10, 113–118]. AEMs consist of a polymer backbone and a nitrogen-containing cation [119–121]. The use of polymers with very high thermal and mechanical stability is most suitable for the synthesis of AEMs [120, 121]. Cationic sites that can transport hydroxide ions are responsible for ion conduction. AEMs with very high ionic conductivity are the best choice for AEMWE [119–121] The ionic conductivity of AEM can be improved by increasing the water content within the membrane [119– 121]. However, higher water content or water uptake can lead to mechanical instability by swelling the membrane [119–121]. Appropriate ion conduction channels contribute to high hydroxide ion transport. The formation of phase-separated block copolymers helps to regulate the spatial position between the hydrophilic nitrogencontaining cations and the hydrophobic polymer backbone [119–121]. Therefore, mitigation strategies can be developed between high ionic conductivity and low water uptake by forming appropriate ion high rate transport pathways. In addition,

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the conductivity of the hydroxide ion can be increased by increasing the positive charge density of the ion-conducting sites. However, due to the high hydrophilicity of AEM, higher positive charge densities will exacerbate hydroxide erosion and thus lead to degradation of AEM. Recently, researchers have systematically addressed the problems of low ionic conductivity, poor stability at high pH and high water absorption [119–121]. The structure of the polymer backbone and the position of the cation in the polymer structure largely influence the long-term stability of AEM under alkaline conditions. The most common degradation pathways for AEM include polymer backbone degradation, SN 2 substitution of cationic side chains, and Hoffman elimination [122– 124], polymer backbones with polysulfone and polyketide groups favour attack by hydroxide ions and undergo backbone degradation [125–129]. In addition, polyaryl ether backbones are also subject to hydroxide attack and backbone degradation under alkaline conditions, thus limiting their long-term stable use in alkaline devices [124, 130, 131]. This suggests that all hydrocarbon backbone polymers are useful. Preparation of AEM using a polymer consisting of all hydrocarbons eliminates the possibility of degradation through the polymer backbone [132–134]. The lack of β-hydrogen lends itself to avoiding Hoffman degradation. When cations are present in long alkyl chains, the barrier to Hoffmann elimination increases and the risk of degradation is minimized by cation elimination [132–134]. In addition, long alkyl chains reduce the electron-withdrawal and resonance effects of the benzene ring if benzene is present in the polymer chain [132–134]. Thus, the main degradation pathway is avoided. The type of cation present in the polymer backbone can also affect the overall performance of AEMWE. A detailed study of AEM with quaternary trimethylammonium, quinoline and phosphonium cations in the polymer backbone showed that polymers with quaternary trimethylammonium exhibited excellent performance in terms of long-term alkaline stability and mechanical properties [135]. Water management within the membrane is a key part of the AEM electrolyzer. High water uptake causes ion conductivity channels to overflow and leads to membrane softening. As a result, ionic conductivity and mechanical stability are reduced. For the mobility of the ions, it is necessary to have some water in the form of free water within the membrane. Therefore, it is necessary to balance the amount of free and bound water within the membrane to produce maximum activity [119–121]. With current advances in computational quantum chemistry, first-principles DFT can be used to explore the correlation between ionic conductivity, chemical stability and AEM degradation mechanisms, with some examples explored in the literature [11, 123, 136–140]. However, most studies tend to utilize commercially available membranes for electrical membrane processes, such as A-201 produced by Tokuyama in Japan [141]. Nevertheless, novel membrane materials are developed and tested, and they are expected to facilitate the development of hydrogen energy. To investigate the effect of stability on AEM, several different AEM chemicals were used in the testing of the AEM system. Polysulfone (PSF) was chosen as the polymer backbone, with different cationic groups grafted onto the adjacent side chains. The cationic groups included quaternary benzyl trimethyl ammonium (TMA+ ), quaternary benzyl quinoline (ABCO+ , aka 1-azabicyclo[2.2.2]-octane) and

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quaternary benzyl 1-methylimidazole (1 M+ ), as shown in Fig. 5.14 [142]. The chloromethylation and subsequent AEM formation based on the above reactants were characterized using nuclear magnetic resonance hydrogen spectroscopy (1H NMR). The stability of the PSF AEMs was tested at a constant current density of 200 mA cm–2 for 6 h. An increase in cell voltage was detected from 1.6 V to 2.4 V. Post-analysis of the PSF AEMs indicated that CO2 intrusion led to instability of the electrolyser system during short-term operation, and that the irreversible loss of performance could be mainly attributed to polymer degradation of the skeleton. The proximity of the cationic groups of the polymer backbone due to ether and quaternary carbon hydrolysis can easily lead to backbone degradation [141]. The addition of β-hydrogen spacers can mitigate Hoffman elimination and improve cationic stability. Several recent studies have reported that cationic stability in AEM can be significantly improved by cationic groups attached to the long alkyl side groups of the polymer backbone [124, 143–146]. This is a promising strategy for AEM water electrolyzer design. Kreuer et al. reported that N-helical cyclic quaternary ammonium salts (QA) showed great potential for improving basic stability, with 6-azoniaspiro[5.5]undecane (ASU) having the longest half-life of 110 h, superior to N,N-dimethylpiperidinium (DMP), N,N-d imethylpyrrolidinium (DMPy), 5azonia-spiro[4.4]nonane (ASN), tetrapropylammonium (TPA), and N-benzyl-Nmethylammonium (BMP) composites. This can be explained by the fact that the bound ring conformation in the six-membered spiro ring system creates a large transition state energy barrier for substitution and elimination reactions compared to methyl [147]. To introduce the N-helical cyclic QA cation into the polymer in a more direct manner, researchers have proposed a cyclic condensation strategy to anchor the

Fig. 5.14 Polysulfone based AEMs with quaternary benzyl ammonium and different imidazolium groups [142]

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spiral-centred QA cation to the polymer chain, as shown in Fig. 5.15. After NMR and SAXS characterization, the “spiral-zirconium” type AEM exhibited high thermal and basic stability. In a hydrogen isotope-labelled 1 M KOH electrolyte, the AEMs degraded at 80 °C for over 1800 h [148]. The activity of helical zircones in this study provides a facile way to prepare AEMs with high OH-conductivity and alkalic stability. Ponce-González and Zhuang et al. suggested that the use of benzyl-Nmethylpyrrolidine head groups to form pendant benzyl-QA-type AEMs should be considered [149]. They proposed a radiation grafting method to produce benzyllinked saturated heterocyclic QA head groups. The two resulting AEMs, ETFEg-poly(vinylbenzyl-N-methylpiperidinium)-QA and ETFE-g-poly(vinylbenzylN-methylpyrrolidinium)-QA, had excellent basic stability with an ion exchange capacity loss of only about 17–18%, a value 30% higher than the baseline benzyl trimethylammonium head-based functionalized AEM. In AEM fuel cells with PtRu/ C, Pt/C as anode and cathode, respectively, and PSF as ionomer, the efficiency of the benzyl-N-methylpyrrolidine head group reaches 980 and 800 mW cm−2 , respectively. Although most findings agree that the alkali instability originates from the positively charged N in the benzyl-position QA head group, extending the radiation grafting approach to prepare AEMs with non-benzyl-QA group remains challenging. Yan et al. also reported a series of poly(arylpiperidine) (PAP) AEM/ ionomers by grafting base-stabilized piperidine cations along a high molecular weight ether-free, rigid and hydrophobic aryl backbone [150]. The addition of piperidine cations to the aryl polymer backbone improved the balance between

Fig. 5.15 Synthetic process of spiro-ionenes (1 and 2 type) via cyclopolycondensation of 4BMB with BP and TMDP, separately [148]

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hydroxide conductivity (> 50 mS cm–1 ) and dimensional stability, resulting in high molecular weight PAPs that exhibit simultaneously state-of-the-art chemical stability, hydroxide conductivity and mechanical stability. The ionomer usually acts as a bridge and is a reliable conductor between the AEM and the active sites on the surface of the catalyst layer [151, 152]. The ion exchange capacity (IEC) is a key parameter used to assess the performance of ionomers, and it can be regulated by grafting a large number of ion exchange groups onto the polymer surface. Excessive ion-conducting groups can lead to high water uptake of the ionomer, leading to further dissolution of the ionomer in the electrolyte, especially during operation at elevated temperatures [153]. The interface between the ionomer and the polymer matrix is easily disrupted. Their interaction can increase the mechanical strength of the AEM, while also decreasing the mobility and total pore volume of the AEM chains [154]. Therefore, it is extremely urgent to find suitable crosslinkers to ensure high ionic conductivity and stability. Recently, the research team synthesized a series of trimethylammoniumfunctionalised polystyrene (in TMA-x, x is the molar percentage of quaternary benzylammonium) as ionomers (Fig. 5.16) [155]. The design of the ionomer binders was based on the following considerations. Firstly, the use of aliphatic groups in place of phenyl in the polymer matrix eliminates possible phenyl adsorption and the appearance of acidic phenyl species. Secondly, the use of TMA-x avoids using long non-ionic alkyl chains in the polymer backbone, which has been shown to significantly reduce the solubility of the polymer in solvents. Thirdly, the ammonium or amine groups in the polymer side chains facilitate the maintenance of a high pH of the electrolyte. The maximum IEC value obtained for hexamethyltrimethylammoniumfunctionalised Diels–Alder polystyrene (HTMA-DAPP) AEM reached a maximum value at 3.3 mequiv g–1 . In addition, HTMA-DAPP has a significant hydroxide conductivity of 120 mS cm–1 at 80 °C. The polyphenylene backbone can be effectively manipulated to control and optimise the molecular weight of HTMA-DAPP, which directly results in excellent mechanical strength (tensile stress > 20 MPa at 50 °C at ~90% moisture) [155]. After assembling HTMA-DAPP, Pt/Ru cathodes and IrO2 anodes into MEAs, the MEAs were tested in a pure water-fed all-AEM cell with a current density of 107 mA cm–2 at 60 °C and 1.8 V. After determining the optimum ionomer content, the anode NiFe catalyst was further explored. At the highest ion concentration, a high cell performance of 2.7 A cm–2 at 1.8 V was delivered in the newly constructed MEA. This level of activity can even compete with PEM electrolyzers using catalysts without PGMs. However, for industrial applications, the instability of the constructed AEM electrolyzer system remains to be proven. Tang et al. introduced Menshutkin reaction and olefin metathesis reaction to prepare anionically conductive poly(2,6-dimethyl-phenylene ether) PPO with bulky imidazolium cations as ionomers [156]. PPO-based AEMs have good mechanical properties (tensile stress > 20.8 MPa) and relatively low water absorption (about 0.13 wt%). The stabilizing properties of PPO are inherited from the imidazole groups, which show no degradation in 1 M NaOH electrolyte at 80 °C. In addition, the electrical conductivity of the PPO-based AEM was maintained after nearly 960 h of

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Fig. 5.16 The chemical structures: a trimethyl ammonium functionalized polystyrenes (TMA-x) ionomers; b hexamethyl trimethyl ammonium-functionalized DielsAlder polyphenylene (HTMADAPP) AEMs [155]

operation, and the current density did not drop. This may be related to the specific positions of the imidazolium cations (C2, C4, and C5) and the resulting cross-linked structure. This special function protects the cationic centers of PPOs from being attacked by H2 O and/or OH– . The battery of PPO-based AEMs has a peak output power of 173 mW cm–2 at 410 mA cm–2 and remains stable during operation at a constant 200 mA cm–2 . The lifetime of the cross-linked imidazole is 3 times that of the benzyltrimethylammonium functionalized PPO ionomer. In practice, many factors can affect the performance of ionomers, not only the cationic crosslinker but also the polymer matrix itself [152]. The type of solvent, the method of catalyst coating, the operating temperature and the nature of the catalyst can all determine the extrinsic properties of the ionomer [153, 157]. Before designing an ionomer, researchers should consider the compatibility of all components in the AEM system and the expected level of performance. The lack of highly conductive and alkaline stable AEMs and ionomers is a significant challenge for current AEM technology. The use of PGM as anode and cathode electrocatalysts increases the capital cost of electrochemical device fabrication. The requirement to develop PGMfree electrocatalysts that match the performance of PGM electrocatalysts is high. In an alkaline electrolyzer (AEL), CO2 in ambient air reacts with the circulating KOH solution to form K2 CO3 . The formed K2 CO3 precipitates in the pores of the gas diffusion layer, hindering ion transport and affecting the ionic conductivity and the

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amount of OH– reduction for the OER reaction. Therefore, the overall performance of electrolysis is degraded when an aqueous KOH solution is used as the electrolyte. In addition, in AEM water electrolysis, CO2 enters the inside of the membrane, reducing the membrane resistance and electrode resistance, and negatively affecting the electrolyzer performance.

5.4 Membrane Electrode Assembly and Electrolyzer Performance The MEA is a functional unit integrated by the AEM, ionomer, anode, and cathode. The membrane must provide mechanical stability during the compression of the MEA and transport layer and enable ion transport while suppressing gas and electron crossover. The catalyst layer can be thought of as the central interface layer in the MEA where all transport paths, including chemicals, ions, and electrons, need to converge at the center of the reaction, i.e., the catalyst particle surface. In this section, we first discuss how to design the optimum catalyst layer structure. This is followed by a discussion of possible ways to bind the catalyst layer to the membrane and fabricate the MEA. The catalyst layer is designed to achieve optimum conditions for the transport pathways of the species of interest. Here, electrons need to be transported via electron conduction pathways which are the catalyst particles and the metals of the collector. Typically, ions are transported through liquid and solid electrolytes, while the reactants are liquid water and gases transported through pores. These catalyst particles and transport pathways are intertwined, and this is something that must be considered in catalyst layer design, which goes beyond the concept of catalyst activity [158]. The catalyst layer design aims to find simple ways to fabricate porous structures that have a high density of reaction sites within the so-called three-phase (gas/solid/ liquid) boundary (Fig. 5.17) [159]. This is the three-dimensional region within the catalyst layer where the reaction takes place [159]. To obtain a large number of these sites, it is essential to integrate the catalyst into the MEA as far as the catalyst layer structure is concerned. In the case of a water-only feed, the anion exchange ionomer (AEI) is the sole OH– conductor, and the alkaline electrolyte feed helps to increase the conductivity of OH– . AEI needs to be integrated with the catalyst to provide high OH– conductivity without blocking the catalyst sites and to allow for catalyst layer porosity, thereby facilitating the escape of H2 and O2 products. AEI also acts as a binder to form a mechanically stable catalyst layer from the catalyst powder. In addition, the catalyst sites require electrons to be connected to the collector for the electrochemical reaction to occur. High electron conductivity of the catalyst and carrier is required, but electron conductivity alone does not directly produce the highest performance MEA, thus emphasizing the importance of fabrication methods and catalyst integration into the MEA [81].

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Fig. 5.17 Simplified schematic of the triple-phase (gas, liquid, and solid) boundary for the OER showing the catalyst particles (black) that are in direct contact with the current collector (shown as a gray bar in the figure). The OH− -conducting AEI acting as an electrolyte and often also as a binder is shown in blue. In an actual MEA, the catalyst particles form up to several-micrometerthick layers, and electronic conductions through the catalyst layer (from catalyst particle to adjunct catalyst particles) are needed [159]

It is essential that AEI is dispersed in a manner that achieves maximum catalyst utilisation and facilitates OH– transport from the cathode to the anode catalyst site via a continuous and highly conductive path. The OH– conductivity of the catalyst layer may be up to one order of magnitude lower than the equivalent AEM and is influenced by the curvature of the catalyst layer. The latter can be seen as an average deviation from the shortest possible connection to the travel time within the porous material [160]. Optimal AEI loading is typically between 5 and 20 wt%, depending on many factors, and must be evaluated experimentally [161]. Figure 5.18 shows an example of the effect of AEI loading on the characteristics and performance of various MEAs. For these MEAs, tested in a single cell, the lowest voltages were found at specific current density values when the AEI loading was 20 wt%. Nyquist (Fig. 5.18b) further shows that the high frequency resistance (HFR) and the resistance of the anode and cathode charge transfer reactions are the lowest when the AEI loading is 20 wt% [162]. The SEM images show a different pore structure with a secondary pore appearance at higher AEI loadings. The latter is thought to reduce cell performance. The behaviour of specific AEI (as a binder for catalyst particles) in terms of swelling and conductivity in the feed electrolyte at different pH values is crucial to understanding the effect of AEI on transport in the pores. Mayerhöfer et al. recently investigated the effect of 10 and 30 wt% AEI loading in the anode catalyst layer on the performance of AEMWE in water and 0.1 M KOH (as shown in Fig. 5.19a and

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Fig. 5.18 a Polarization curves and b Nyquist plots for AEMWE with different (10, 20, and 30 wt%) AEI loadings at 50 °C and c fieldemission (FE)-SEM images of the MEAs fabricated using the different AEI loadings. KOH (1.0 M) at 1 mL min–1 was fed to the anode and cathode [162]

b) [163]. The AEI used by itself, even at 30 wt%, was found to be unable to provide the required basic environment of OH− species for the OER catalyst sites without PGMs during pure water feed operation (Fig. 5.19c). However, when using a 0.1 M KOH feed, performance was improved by a factor of 20–45 at a cell voltage of 1.8 V. In addition, three-electrode scanning-flow cell (SFC) experiments were used to investigate the anode catalyst layer. The results showed that higher binder content blocked the catalyst sites and thus reduced the catalyst activity of the higher pH feeds (Fig. 5.19d). This effect was attributed to changes in film resistance and contact resistance due to the different swelling behaviour of the materials in the respective feed solutions. This suggests that the role of the catalyst layer binder may differ significantly depending on the feed solution. Some studies have also considered the concept that AEI is not required if an alkaline electrolyte feed is applied [160, 164]. For this case, the AEI may only act as a catalyst particle binder, while the liquid alkaline electrolyte may be an important OH− provider. AEIs also affect the pH and hydrophobicity of the catalyst layer. AEIs need to have appropriate chemical properties, including maintaining a pH that is conducive to catalyst chemical stability and hydrophobicity, as well as needing to be mechanically stable to prevent catalyst shedding. For example, the pK a of the QA cationic group or the conjugate acid of some AEIs is lower than that of KOH by about 10:15 [165]. In the case of eg nickel-based catalysts, the choice of KOH (or other liquid alkaline electrolytes) feed also seems to be preferred for stability reasons. High

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Fig. 5.19 SEM images of 2 mg cm–2 CuCoOx anode catalyst layers with a 10 wt% and b 30 wt% ionomer loadings. c Polarization curves of the single AEMWE cells for pure water (dashed lines) and for 0.1 M KOH (solid lines) feed. d iR-corrected linear sweep voltammograms of the CuCoOx anode catalyst layers with varying ionomer contents at pH 12.7 (solid) and pH 7 (dashed) of a 0.05 M phosphate buffer solution in a scanning-flow-cell measurement. A Pt loading of 0.5 mg cm–2 was used at the cathode

pH can also enhance OER kinetics, depending on the reaction sequence of the catalyst [166]. The chemical similarity between AEIs and AEMs allows for low interfacial resistance and similar swelling, which helps to prevent delamination of the catalyst layer from the AEM [8]. The functional groups and backbone structures of AEIs can be used to modulate the hydrophobicity and chemistry of the three-phase boundary, allowing for pH adjustment and alteration of H2 O availability at the catalyst site. Chemical groups of AEIs can react at both electrodes and have a negative impact [167]. The specific adsorption of QA cations and the interaction of benzyl groups with Pt can be reduced by using large rigid cations and non-rotatable phenyl groups [168], although unsubstituted phenyl groups in the polyaromatic hydrocarbon backbone still adsorb well onto Pt into the positive potential region [169]. The adsorption energy depends on the catalyst. In the case of benzene, the bimetallic surface (e.g., Mo, Ni, or Ru alloyed with Pt) may be lower [170, 171].

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In addition, catalyst site blocking side effects may occur, such as lowering the pH, which may lead to the dissolution of the TM catalyst [155]. This has been proposed for AEI containing phenyl in the main chain structure, which can be oxidized to acidic phenolic compounds (Fig. 5.20). DFT calculations show that phenyl adsorbed at parallel or lying positions on the electrode surface is most susceptible to oxidation. The potential at these two positions is up to 1.6 V (Fig. 5.21). The adsorption energy is determined by the surface: PtO2 (110) > IrO2 (110) > PtO (110) > IrO(110) > La0.85 Sr0.15 CoO3 (001) > La0.85 Sr0.15 CoO3 (111). The study also involved the use of polytetrafluoroethylene (PTFE) as a binder in the catalyst layer [173–177]. PTFE is a non-ionic monomer; therefore, in the absence of AEI, liquid alkaline electrolytes need to be injected to provide OH– conductivity. PTFE can play a role in tuning the hydrophobicity/hydrophilicity of the catalyst layer to avoid overflow and gas plugging at critical locations. The PTFE loading needs to be sufficient to act as a binder but only to prevent catalyst plugging and negative effects on the porosity of the catalyst layer [178]. The durability of AEI in pure water-fed AEMWE, particularly at high operating voltages of the anode, is considered a limiting factor. In a recent review, Li et al. [160] distinguished durability limiting factors for AEMWE with water and concentrated KOH feeds, including ionomer poisoning, ionomer shedding, and AEM instability. Alternative electrode fabrication methods without the use of AEI in the catalyst layer are increasingly reported, and such studies for AEMWE cells were recently reviewed by López-Fernández et al. [179]. The related AEMWE studies report performance measurements of at least 100 h. These include reports of uniform electrode designs where the catalyst layer is integrated into a single component in a GDL by growing the OER catalyst directly on a substrate (e.g., nickel foam) [177, 180].

Fig. 5.20 Phenyl oxidation of a benzyl trimethylammonium hydroxide and b polyaromatic AEI at OER potentials [172]

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Fig. 5.21 a Polarization curves of an AEM water electrolyzer before and after the 100 h test at 2.1 V at 80 °C. b 1 H NMR spectra of the anode AEI before and after the durability test. The inset in (b) is the expanded view of the oxidized phenol peak in the 1 H NMR spectra; * denotes other expected oxidation sites [172]

The CCS and CCM methods are commonly used to fabricate MEAs and assemble them into AEM cells for performance testing [181]. The CCM method has received considerable attention as it offers unique advantages over CCS. In the CCM preparation process, the electrocatalyst and binder are mixed to form a homogeneous ink, which is then applied to both sides of the AEM by spraying. Next, the AEM is placed between the GDL, and the AEM is then treated using either mechanical or thermal pressure. The thermal pressure method widely used in PEM cells is not a good option in the AEM system. The level of mechanical force and elevated temperature can cause irreversible damage to the membrane, even though it is supported

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by a metal-based electrode substrate (e.g., metal foam, titanium plates, and stainless steel mats) [182–184]. In the CCS method, catalyst inks are deposited directly onto GDLs and then sintered to give a gas diffusion electrode (GDE). The final MEA is obtained by hot pressing of sandwich GDLs or GDEs with AEM as the inner layer. Researchers have explored various strategies to improve the effectiveness of AEM systems, and work related to MEA design is discussed next. To develop new MEA devices, the concept of an integrated inorganic membrane electrode assembly (I2 MEA) for AEM electrolysis systems was proposed by Zhao et al. [185]. Inorganic magnesium–aluminum layered double hydroxides (Mg–Al LDHs) were used as ionic conductors, and the films were formed by a cold-pressing process. Subsequently, a PGM-free catalyst was coated on both sides of the film to form the I2 MEA. The development of the I2 MEA was guided by two objectives. The first was to avoid the complex and toxic preparation process of conventional AEM cells, which typically includes chloromethylation, bromomethylation, quaternisation, and alkalisation steps. The second was to maintain the good stability and high hydroxide ion conductivity required for AEM systems. A range of operating conditions and structural effects were explored during the establishment of the I2 MEA-based AEM. CuCoOx mixed oxide and Ni/(CeO2 La2 O3 )/C were used as the anode and cathode catalysts, respectively, with a loading of 40 mgcat cm−2 . Nickel foam and carbon paper were chosen as the anode and cathode counterparts for the GDLs. Titanium end plates with a circular single serpentine flow field were used to seal the I2 MEA and GDLs. Many operating conditions have been reported to significantly affect AEM performance, including film thickness, electrolyte type, and operating temperature (Fig. 5.22). Using the novel I2 MEA fabrication method, the AEM cell was able to achieve a peak current density of 208 mA cm−2 at a cut-off voltage of 2.2 V in a 0.1 M NaOH electrolyte at 70 °C. Remarkably, at a constant 80 mA cm–2 , the I2 MEA maintains remarkable stability after 600 h with a decay parameter of less than 100 μV h−1 . The excellent stability is associated with a complete integrated architecture that allows smooth diffusion of hydroxide ions. The design strategy of the I2 MEA is expected to be useful for other solid-state energy storage devices using electrolyte-based solid-state energy storage devices. To obtain more details about the structure and morphology of the catalyst layer prepared by the CCS method during the actual AEM electrolysis, Bessarabov et al. introduced the SEM technique to study the interface between the catalyst layer and the binder within the MEA [186]. The AEM electrolyzer used Sustanion, AEMION and A-201 as membranes with Nafion as the binder. NiFe2 O4 on nickel was used as the anode and NiFeCo on a stainless steel substrate as the cathode. The impedance values for Sustanion, AEMION, and A-201 MEA were 0.097, 0.120, and 0.133 Ω cm−2 at 60 °C in a 1 M KOH electrolyte. This performance indicates that the Sustanion membrane has a higher ionic conductivity despite being the thickest. The unique polymer resin morphology and boundary interactions at the interface may account for the low ionic resistance of the Sustanion membrane. However, the specific factors contributing to the difference in impedance remain unknown due to the complex structure of the MEA. The AEM cell performance of

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Fig. 5.22 a Crystalline structure of Inorganic Mg−Al layered double hydroxides (Mg−Al LDHs). Polarization curves of AEM electrolyzer based on I2 MEAs under different operating conditions: b membrane thickness; c electrolyte feed; d operating temperature [185]

the different MEAs was examined, and the results showed that the Sustanion-based MEA exhibited the most significant electrocatalytic performance at 60 °C, 0.1 M KOH at an available current density of 300 mA cm–2 . AEMION and A-201 did not perform as well as the Sustanion-based MEA. Post analysis of the Sustanion-based MEA under SEM showed that the catalyst layer on the anode and cathode adhered firmly to the membrane surface and maintained its original morphology, with no visible cracks on the surface. In contrast, the AEMION- and A-201 membranes had irregular and dark areas, indicating that the catalyst had been detached from the membrane surface. This result suggests that although the hot pressing method on CCS can improve cell performance and enhance the membrane-catalyst surface, in most MEAs there is a weak bonding between the catalyst layer and the AEM membrane. Bessarabov also reports that the delamination process in AEMION-based and A-201-based MEAs may be due to incompatibility with the Nafion ionomer and polymer resin, as previously reported in PEM fuel cells [187, 188]. Another possible reason for the difference in MEA performance is the difference in thermal and hydration expansion at the interface between the Nafion ionomer binder and the catalyst [189].

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The CCM approach offers significant advantages, including close contact between the catalyst layer and the polymer membrane, which typically results in higher MEA ionic conductivity compared to those fabricated by CCS. The technical advantages of CCM enable the resulting MEA to maintain the overall performance of the AEM cracking system at lower catalyst loadings [6]. However, the CCM approach can also lead to poor electrical contact between the MEA and the collector, which will require the development of a suitable polymeric binder for scalable applications. Spraying and deposition are commonly used to transfer a homogeneous catalyst ink onto the surface of the AEM [190–193]. Recently, N. Li et al. synthesised a series of piperidine-functionalised poly(2,6-dimethylphenylene oxide) as AEM and then used CCM to coat anodic Pt catalysts and cathodic AS-4 catalysts onto the MEA surface. The hydroxide conductivity of the resulting MEA was comparable to that reported for QA-based AEM, being 29.0 mS cm–1 at 20 °C. To further check the cell performance of CCM-MEA, a highly conductive MEA based on LSCPi film was prepared, achieving a 300 mA cm–2 at 1.8 V, 50 °C, with pure water as the feed. The LSCPi membrane was found to be stable, maintaining a conductivity of 98% after 560 h of operation at 80 °C in 1 M NaOH. After post-analysis, NMR confirmed that the piperidine cation functional group on the LSCPi membrane determined the lifetime of the AEMs [184]. MEA was prepared by electrodeposition of nickel on carbon paper (CP) for a relatively low nickel loading of 8.5 μgNi cm–2 and a homogeneous distribution of catalyst particles. The CCM-induced homogeneous catalyst distribution was also reported to be favourable for increasing the electrochemical surface area used for gas precipitation reactions [69]. Using Ni/CP electrodes as anode and cathode on the MEA, the rationally prepared MEA was able to deliver a current density of 150 mA cm–2 at an applied cell voltage of 1.9 V at 50 °C. The results show that electrodeposition can be an effective and simple tool to precisely control the roughness and catalyst loading of the final MEA. Despite the good cell performance of the Ni/CP-based MEA, there is a lack of stability testing of the MEA during operation. Park et al. [162] compared the cell performance of MEAs fabricated by the CCS and CCM methods. In this study, the MEA fabrication method, operating conditions, and several parameters affecting AEM performance were explored. Electrochemical impedance spectroscopy (EIS) was used to differentiate the effect of the MEA fabrication method on the performance of the resulting AEM. As shown in the polarisation curves in Fig. 5.23a, different MEA fabrication methods have different effects on the performance of the stacked AEM electrolyser. The performance of the CCMbased MEA (630 mA cm−2 at 1.9 V) is much higher than that of the CCS-based one (390 mA cm−2 at 1.9 V with hot pressing; less than 100 mA cm−2 at 1.9 V without hot pressing). In the CCS method, the thermocompression process is considered necessary to improve the performance of the MEA. The large performance gap between the CCM and CCS MEAs was well explained by the Nyquist diagram (Fig. 5.23b). The ohmic resistance of CCS without the hotpress process is much higher than that of CCM and CCS with the hot-press process, resulting in significantly lower electrolyzer performance. In contrast, the MEA based

5.5 Current Challenges and Prospects for AEMWE

139

Fig. 5.23 a Polarization curves, b Nyquist plots of MEA fabricated with CCM and CCS with and without hot-pressing treatment [162]

on the hot-pressing CCS method and the CCM-based method exhibited essentially the same ohmic resistance. This suggests that the hot pressing process is a prerequisite for good contact between the catalyst layer and the AEMs. In addition, when the CCS method is combined with the hot pressing process, it produces higher mass transfer resistance, as can be seen from the larger radius of arc values in the low-frequency region. Similarly, the MEA obtained from the CCM method has the lowest ohmic resistance in the high-frequency region, the lowest charge transfer in the medium-frequency region, and the lowest mass transfer resistance in the low-frequency region. Based on these results, the CCM method is considered the best MEA fabrication method for AEM water electrolysis systems. Interestingly, opposite trends in MEA performance between the CCS and CCM methods were reported by Mamlouk et al. [166]. The polarization curves recorded at different operating temperatures were compared by spraying the NiCo2 O4 catalyst directly on the GDL and the CCM method. To provide a current density of 100 mA cm–2 at 60 °C in a 0.1 M NaOH electrolyte, voltages of 1.65 V and 1.69 V were required for the GDL spraying and CCM methods, respectively. These performance values are superior to various non-precious metal catalysts reported in the literature, which typically require voltages of 1.7–1.9 V to achieve the same current densities [96, 194, 195]. Meanwhile, the excellent performance of GDL spray-based MEA at low base concentrations suggests that there is scope for further improvement if the NiCo2 O4 particle size can be reduced to approximately 10 nm.

5.5 Current Challenges and Prospects for AEMWE The AEM electrolyzer can be integrated with intermittent energy sources, which will significantly reduce the total cost of the battery/stack. In addition, the system offers great flexibility, including the ability to utilise hydrogen with or without the use of

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compressed hydrogen storage cylinders. The high purity hydrogen produced can be used directly in several applications such as fuel cells and chemical hydrogenation. Future directions for the development of AEMWE technology include: (1) The development of anionic conducting polymers with high ionic conductivity, ion selectivity, long-term alkaline stability, and less gas penetration as membranes and ionomers are the most important formulations achieved for the success of AEM technology. (2) To overcome the existing cost barriers associated with Pt-based catalysts, PGM-free anode, and cathode catalysts can be used to reduce costs without sacrificing much performance. (3) Simpler (one-step) and less costly synthesis using cheaper starting materials can also reduce the total cost of AEM. (4) Membranes with high ionic conductivity are ideal to minimise the effects of carbonation, which can mitigate the reduction in mobility during carbonation, and self-scavenging can occur quickly to aid cell decarbonization. (5) Cell performance stability will be another important aspect to consider. AEM electrolytes that can exhibit long-term stability over thousands of hours are considered ideal for future device manufacture. Therefore, the development of highly stable ionomers and membranes is most needed. (6) In AEM water electrolysis, CO2 intrusion should be minimized to achieve higher stability. Furthermore, CO2 contamination is known to reduce OER activity by lowering the concentration of OH− in the electrolyte. (7) The performance of distilled water, ultrapure water, and 0.1% K2 CO3 as the electrolyte should be matched to the performance of KOH as the electrolyte. Finally, it should be noted that laboratory-developed AEM electrolyzers have recently shown competitive performance over some commercial AEM electrolyzers, which means that there is much scope for further development of AEM electrolyzers.

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Chapter 6

Solid Oxide Electrolysis

Solid oxide electrolysis, a new type of efficient energy conversion device (700– 900 °C), converts raw material H2 O into H2 and O2 through an electrochemical reaction process at high temperatures, achieving efficient conversion of electrical and thermal energy to chemical energy. From a chemical reaction or energy conversion point of view, the high-temperature electrolysis of water for hydrogen production in solid oxide electrolysis cells (SOEC) is the reverse process of hydrogen reacting with oxygen to form water in solid oxide fuel cells (SOFC). As shown in Fig. 6.1 [1], when energized, the water molecules on the hydrogen electrode side diffuse to the “hydrogen electrode–electrolyte-hydrogen water vapor mixture” at the three-phase boundary (TPB) and decompose, producing adsorbed H and O. The two Hs combine to form H2 , which diffuses out of the hydrogen electrode and to be collected; the Os capture two electrons and form O2 . O2 diffuses through the oxygen ion conductor electrolyte to the anode and electrolyte interface. The O2 ion is oxidized, and the two electrons flow to the external circuit to complete the current course. The SOEC electrode reaction can be expressed as: Hydrogen electrode (cathode): H2 O + 2e− → H2 + O2− Oxygen electrode (anode): O2− → 1/2O2 + 2e− Total electrode reaction: H2 O → H2 + 1/2O2 . The total energy requirement (enthalpy change, ΔH) of the electrode reaction can be expressed as follows: ΔH = ΔG + TΔS. Where ΔG is the Gibbs free energy change, representing the total electrical energy input, ΔS is the entropy change, and TΔS represents the heat input. Figure 6.2 shows how the energy demand for water electrolysis varies with temperature [2]. The heat demand for water electrolysis increases significantly with increasing temperature, while the electrical demand decreases considerably without a significant increase in total energy demand. By taking advantage of this feature and selecting the appropriate operating temperature, SOEC high-temperature water electrolysis for hydrogen production can minimize the need for high-grade electrical energy while increasing the utilization of low-grade industrial waste heat. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_6

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Fig. 6.1 Operating principle of SOEC [1]

Fig. 6.2 Calculated energy demands for electrolytic H2 with varying temperatures [2]

The electrochemical process of high-temperature electrolysis of solid oxides for hydrogen production has thermodynamic and kinetic advantages over lowtemperature electrolysis. It has the advantage of reversible operation due to its wide feedstock adaptability and can be flexibly switched between electrolytic and fuel cell modes. It can be used as a highly efficient high-voltage or electrochemical energy storage device to convert electrical energy into chemical energy (hydrogen). It can also be operated in fuel cell mode to obtain electrical power through electrochemical reactions.

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6.1 Electrolyzer According to the type of electrolyte, SOEC can be divided into oxygen ionconducting SOEC (O-SOEC) and proton-conducting SOEC (H-SOEC). So far, research on SOEC has mainly focused on O-SOEC, in which oxygen ions are the charge carriers in the electrolyte. Some novel electrode materials with good performance and durability have been developed and used in O-SOEC [3]. Researchers have also developed models with different levels and operating conditions to study O-SOEC [4]. Furthermore, some large-scale O-SOEC stacks and systems have been fabricated and tested [5]. Compared to O-SOEC, the development of H-SOEC is still at an early stage. In recent years, H-SOEC have gradually gained the attention of researchers, mainly because they offer some unique advantages over O-SOEC, such as potentially low operating temperatures (773–973 K), relatively low activation energies, and easy gas separation. Figure 6.3 shows a schematic diagram of O-SOEC and H-SOEC [6]. Compared to O-SOEC, the most obvious difference of H-SOEC is that the electrolyte is a proton conducting instead of oxygen ion conducting, resulting in different working principles. When an external voltage is applied to the H-SOEC, protons are transported from the air electrode side to the fuel electrode side, where hydrogen is generated. In contrast, the charge carriers in the electrolyte of O-SOEC are oxygen ions. When a voltage is applied, oxygen ions migrate from the fuel to the air electrode side. Steam is fed to the fuel electrode in O-SOEC, but to the air electrode in H-SOEC. Therefore, in O-SOEC, the generated H2 requires an additional drying process on the fuel electrode side to obtain dry hydrogen. In contrast, in H-SOEC, dry, pure H2 can be produced directly at the fuel electrode side, simplifying the system and reducing operating costs. In addition, compressed hydrogen can be delivered directly by increasing the operating pressure on the fuel electrode side (electrochemical compression). The insitu utilization of electrochemical reduction can potentially increase the total energy efficiency and simplify the system compared to external compression. In addition to the ease of gas separation, H-SOEC can offer other advantages over O-SOEC. A significant advantage is that H-SOEC can operate at lower temperatures than O-SOEC due to the proton conductor’s higher ionic conductivity and lower activation energy at relatively low temperatures (673–973 K) [7]. Due to the lack of ionic conductivity of the electrolyte material at low temperatures, conventional O-SOEC is usually operated at high temperatures (between 973 and 1273 K) [8]. The high operating temperatures lead to increased costs for interconnecting and sealing materials. Therefore, when the operating temperature drops below 973 K, low-cost materials can be used to balance the device and interconnects, thus reducing the cost of the H-SOEC system [9]. Furthermore, it can be predicted by thermodynamics that lowering the operating temperature can shift the composition of the H2 O–CO2 co-electrolysis product (H2 –CO) to CH4 . Products containing a high percentage of CH4 have a higher volumetric energy storage density than hydrogen [10].

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Fig. 6.3 Schematic diagram of the two SOEC. a O-SOEC; b H-SOEC [6]

6.2 Electrode Materials The solid oxide electrolysis method simplifies the electrolysis system device. It allows a broader choice of electrodes, but it is not universal due to high-temperature environment requirements and high system material cost. Polymer membrane electrolysis is a promising method for reducing the reaction energy barrier, reducing electrical energy consumption, and improving hydrogen production efficiency. The preparation of membrane electrodes and the research of cost-effective exchange membrane materials are essential research directions. The traditional alkaline solution hydrogen production equipment is simple, and the technology is mature. At present, the research focus of this method is to reduce power loss.

6.2.1 Air Electrode Materials Only two materials can be used in highly oxidizing environments as SOEC anode materials: (1) precious metals, such as Pt and Au, and (2) conductive oxide mixtures. Like cathodes, the use of noble metals is excluded for cost reasons. As a result, only some conductive oxides are suitable materials for use as SOEC anodes. The most commonly used anode materials are mixed oxides with a chalcogenide structure, such as lanthanum strontium manganate (LSM). Similar to the Ni-YSZ metal-ceramic cathode, the thermal expansion coefficient of the LSM anode is close to that of the YSZ electrolyte (10.6–11.0 × 10−6 K−1 ). However, during operation, MnOx can diffuse from the LSM to the YSZ, causing La2 O3 or SrO to react with the YSZ to form poorly conducting La2 Zr2 O7 or SrZrO3

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[11]. This problem can be solved by over-doping the LSM with several percentages of MnOx so that it cannot react with SrO or YSZ [12]. Eguchi et al. measured the properties of J-V SOEC with zirconia or ceria-based electrolytes [13]. Experimental tests showed that Ce4+ could be reduced to Ce3+ . To avoid the reduction of Ce4+ to Ce3+ , the use of YSZ-SDC bilayer electrolyte was demonstrated to be effective in reducing the resistance of SOEC. During electrolysis, the working potential of SOEC with a Ni-YSZ cathode and a La0.6 Sr0.4 MnO3 (LSM) anode was lower than that of a Ni-YSZ cathode and a La0.6 Sr0.4 CoO3 (LSC) anode. However, when Pt was used as the cathode, the SOEC with the LSC anode showed better performance than the cell with the LSM anode. Recently, the electrochemical properties of La0.8 Sr0.2 MnO3 (LSM), La0.8 Sr0.2 FeO3 (LSF), and La0.8 Sr0.2 CoO3 (LSC) were compared for use in SOEC [14]. The potential of SOEC decreased sequentially with the anode material: LSM-YSZ > LSF-YSZ > LSC-YSZ. It was also found that the LSC-YSZ composite electrode exhibited stable performance degradation over 100 h due to the reaction between LSC and YSZ. As a comparison, LSF-YSZ showed rather good stability in the short term at temperatures below 1073 K. As can be seen from this study, the LSM-YSZ composite may not be the best material for SOEC anodes. Due to limited reports on SOEC electrodes, more research efforts are required to gain a basic understanding of the electrode’s electrochemical performance and to find ways to improve its long-term stability.

6.2.2 Cathode Materials The reaction at the SOEC cathode is the decomposition of water vapor to produce H2 , also known as the hydrogen electrode. Its primary role is to provide a site for the water vapor decomposition reaction and a channel for electron and ion transport. Therefore, in addition to meeting the general material requirements of SOEC, the cathode material should also meet the following criteria: (1) Stable structure and composition under high temperature and high humidity conditions, which is the most significant difference between SOEC and SOFC in terms of hydrogen electrode material requirements. (2) It must have a good electron conductivity and a high oxygen ion conductivity to ensure the transport of electrons and oxygen ions and a good catalytic activity for the decomposition reaction of water vapor. (3) It should have high porosity. The porous structure ensures the supply of water vapor required for electrolysis and the output of products while providing a transport path for electrons from the electrolyte/cathode interface to the linker material. In addition, if SOEC is used to electrolyze CO2 or CO2 /H2 O mixtures, the cathode material should also have good resistance to carbon build-up. The primary cathode materials commonly used as SOEC are metals, metal ceramics, and mixed conductivity oxides. Metal materials that can be used for SOEC, including Ni, Pt, Co, Ti, etc., are generally seldom used due to the disadvantages of poor compatibility with electrolyte materials, volatility, and expensive [15]. Ni/YSZ

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porous cermet is the preferred cathode material for high-temperature SOEC [16]. Ni is a suitable catalyst for reforming catalytic and hydrogen electrochemical oxidation reactions. The cost of Ni is lower than that of Co, Pt, Pd, etc., which is economical. YSZ is used as the matrix of Ni. Ni and YSZ do not fuse or interact with each other in a wide temperature range. After processing, they form an excellent microstructure, which can keep the material stable for a long time. At the same time, adjusting the appropriate Ni doping ratio can close its thermal expansion coefficient to that of the adjacent electrolyte layer. More importantly, a suitable cathode microstructure and material composition can obtain a lower internal resistance of the Ni/YSZ interface and improve its electrochemical activity for interfacial reactions. A porosity (volume ratio) of more than 30% favors the transport of reactant and product gases [17]. In addition, the YSZ framework can also inhibit the growth of Ni particles during the reaction process and make the cathode have good electron conductivity. Since the H2 O content of the SOEC feed gas is much higher than that of SOFC, using Ni-YSZ as a SOEC cathode poses some problems. Accorsi et al. [18] used (In2 O3 )0.96 (SnO2 )0.04 /YSZ/Ni-YSZ solid oxide electrolytic cells for hydrogen production. They found that after 1000 h of operation at 900 °C, the anode material did not change significantly, while cracks and evaporation of Ni were found in the cathode Ni-YSZ layer. Hauch et al. [19] investigated the effect of high humidity conditions (98% H2 O, 2% H2 ) on the performance of the hydrogen electrode. It was found that the degradation of hydrogen electrode performance was mainly due to the accumulation of Ni under high temperature and humidity conditions. Cells of the same material remained stable for 1000 h in fuel cell operation (98% H2 , 2% H2 O). In addition, Jensen et al. found that the hydrogen electrode “passivated” in the electrolytic mode of operation when using an LSM/YSZ/Ni-YSZ cell for hydrogen production at 750 °C and recommended that the Ni-YSZ electrode be activated with an anodic current before cell operation [10]. Høgh et al. [20] investigated the kinetics of Ni-YSZ electrodes in electrolytic mode and found that the degradation of the electrode performance was mainly due to the influence of impurities in the electrode material (Fig. 6.4). Trace impurities (S, Si, Na, etc.) in the material can generate passivation layers on the surface of Ni particles, at the Ni/YSZ interface, and at the electrode/electrolyte three-phase interface (TPB) (Fig. 6.4), reducing the active area for electrode reactions. Thereby reducing the hydrogen electrode performance. The impurities such as Si are mainly from the electrolyte material, while the source of sulphur cannot be determined yet. Ni can also form cermet materials with other electrolyte materials, such as SDC (Ce0.8 Sm0.2 O1.9 ), GDC (Ce0.8 Gd0.2 O1.9 ) and so on. Norikazu et al. [21] used the self-made Ni-SDC as the hydrogen electrode, YSZ/ScSZ as the electrolyte, and LSC (SDC as the transition layer) as the anode to form an electrolytic cell. A series of experiments were carried out under the conditions of 900 °C and 0.5 A cm−2 . The open circuit voltage of the electrolytic cell is 1.13 V.

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Fig. 6.4 Influence of impurities on the Ni-YSZ electrode [20]

6.3 Electrolyte Since conventional SOEC must operate at high temperatures due to the limited ionic conductivity of existing electrolytes, the primary consideration for reducing their operating temperature is the development of electrolyte materials with higher ionic conductivity at moderate temperatures [7, 22]. It has been tested that doped CeO2 [23] and La0.9 Sr0.1 Ga0.8 Mg0.2 O3−δ (LSGM) [24] can replace oxygen ion conductors due to their excellent ionic conductivity at moderate temperatures. However, hightemperature plasmonic conductors offer several advantages over oxygen ion conductors. Firstly, high-temperature plasmonic conductors show a higher ionic conductivity than oxygen ion conductors in the medium temperature range [7, 25]. Ni is the most commonly used hydrogen electrode for SOEC and SOFC, and proton conducting oxides are chemically compatible with Ni [26]. In contrast, LSGM reacts with Ni, making its employment still challenging [27]. Third, electrolytic cells with protonconducting electrolytes show adequate current efficiency. The primary requirement for electrolyte materials in solid oxide cells is that they exhibit good ionic conductivity and negligible electronic conductivity under operating conditions. High-temperature plasmonic conductors satisfy this requirement. Although doped CeO2 shows good ionic conductivity and chemical compatibility with most electrode materials, it is the most popular electrolyte material for medium-temperature SOFCs [28]. The high applied potential under SOEC operating conditions inevitably leads to the occurrence of the reduction of Ce4+ to Ce3+ , resulting in increased electron conductivity and reduced ion transport numbers [13, 29]. Therefore, the current efficiency of SOEC with doped CeO2 as the electrolyte can only reach a few percent [13]. In contrast, SOEC with proton-conducting electrolyte as the electrolyte has been shown to maintain a sufficiently high current efficiency (50–95%) even at high applied potentials [30]. In addition to the advantages of proton-conducting materials, SOEC systems using proton-conducting electrolytes show several advantages compared to SOEC using oxygen ion electrolytes. Firstly, for a SOEC with proton conduction, only pure and dry hydrogen is produced at the hydrogen electrode side, and no further gas separation is required [31]. Figure 6.5 shows the working mechanism of a SOEC with a protonconducting electrolyte [32]. Water (vapor) is fed into the air electrode side, which is electrochemically decomposed into oxygen and protons. The protons migrate through

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Fig. 6.5 Sketch of the working mechanism of proton-conducting SOEC [32]

the dense electrolyte layer to the hydrogen electrode side, combining with electrons to form H2 , the desired product. As the proton-conducting electrolyte membrane is impermeable to both oxide ions and molecular gases, only protons can pass through the electrolyte, thus producing a product only consisting of pure and dry hydrogen on the hydrogen electrode side. Secondly, cell reliability is better with proton-conducting SOEC. Ni is the most widely used hydrogen electrode in SOEC, [33]. Still, there is always a risk of Ni being oxidized because the vapor is generated on the hydrogen electrode side of the oxygen ion SOEC and the high vapor concentration tends to oxidize nickel particles [13, 34]. Oxidation of Ni by vapor has been reported to decrease cell performance [34]. In contrast, this oxidation does not occur in a proton-conducting SOEC because the moisture is generated on the air electrode side; thus, the Ni electrode is only exposed to dry H2 , leading to better electrode stability. Thirdly, reversible SOFC is more applicable to proton-conducting solid oxide cells. The solid oxide cell can be switched from fuel cell mode to electrolytic mode when a potential value above the open-circuit potential (OCP) of the cell is applied and switched back to fuel cell mode from electrolytic mode when a possible discount below the OCP is applied [35]. The whole device is known as a reversible SOFC (R-SOFC), capable of generating electricity and producing hydrogen in times of power shortage and storing electrical energy as chemical energy in times of power surplus. To optimize cell performance by reducing ohmic resistance, state-of-the-art solid oxide cells must use a thin film electrolyte in which the electrolyte layer is supported on an air or hydrogen electrode substrate [22, 36]. Electrochemical modeling studies revealed that the hydrogen electrode-supported cell configuration is the most favorable design for proton conduction cells to achieve high energy conversion efficiencies in both SOFC and SOEC modes, [37]. In contrast, the hydrogen electrode-supported cell configuration is more favorable in SOFC mode, and the air electrode-supported

6.3 Electrolyte

155

cell configuration is more advantageous in SOEC mode, implying that the cell configuration must be changed for the cell to be electrochemically optimized in both methods, which is practically unrealistic. In other words, the hydrogen electrodesupported configuration is the optimized cell configuration for proton-conducting R-SOFC operating in fuel and electrolytic modes. In contrast, an R-SOFC with an oxygen ion electrolyte must be subjected to high overpotentials in either fuel cell mode or electrolytic mode, regardless of the electrode support configuration chosen. Therefore, SOEC with a proton-conducting electrolyte would be promising for hydrogen production and renewable energy utilization. The first study on H-SOEC was reported in the early 1980s when Iwabara et al. [30, 38] demonstrated proton conduction in SrCeO3 -based oxides by measuring voltage experiments in gas cells and then fabricated electrolyte-loaded H-SOEC electrolyte layers of SrCe0.95 Sc0.05 O3 or SrCe0.90 Sc0.10 O3 (0.5 mm thick) and platinum (Pt) as symmetrical electrodes. The Faraday efficiency of the electrolysis has been demonstrated to be 50–95% in the electrolytic current range of 0.1–0.8 A cm−2 at 1173 K by measuring the rate of hydrogen precipitation. In addition to Sc doping, Y or Yb-doped SrCeO3 -based proton conducting oxides have also been used as electrolyte materials for H-SOEC, exhibiting similar electrolytic behavior. This was also found by Iwahara et al. [39]. The Faraday efficiency of electrolysis decreases with increasing operating temperature and oxygen partial pressure on the air electrode side, which may be due to an increase in the electron conductivity of the electrolyte [39, 40]. In 2008, Stuart et al. [41] reported that BaZr0.90 Y0.10 O3 was a better choice of electrolyte material for H-SOEC due to its higher proton conductivity than BaZr0.90 Y0.10 O3 . However, the stability of BaCe0.90 Y0.10 O3 in an atmosphere containing H2 O has not been considered. In 2009, Sakai et al. [42] reported that Pt electrodes showed poor activity in air and fuel electrodes for H-SOEC. In contrast, the application of Sr0.5 Sm0.5 CoO3 (SSC) in the air electrode and Ni in the fuel electrode can significantly reduce the electrode overpotential and thus greatly improve the performance of H-SOEC. Before 2010, all studies on H-SOEC were conducted on electrolyte-supported cells, resulting in high ohmic resistance of the electrolyte layer. The high ohmic resistance of the thick electrolyte layer limited the performance of H-SOEC and hindered the development and practical application of H-SOEC. In 2010, He et al. [43] first proposed and reported the Ni–BaCe0.50 Zr0.3 Y0.20 O3 (BCZY53) fuel electrode loaded H-SOEC with BCZY53 thin electrolyte layer and SSC-BCZY air electrode. HSOEC with a thin electrolyte layer exhibited better electrochemical performance than H-SOEC with a thick electrolyte layer. Electrolyte materials for H-SOEC must meet several requirements. For example, reasonable proton conductivity and negligible electron conductivity under operating conditions are the basic requirements for the electrolyte material. Furthermore, since the electrolyte layer is exposed to both a reducing atmosphere (fuel electrode side) and an oxidizing atmosphere containing high humidity (air electrode side), the electrolyte material is required to maintain chemical stability under dual atmosphere conditions. In addition, the electrolyte material should be chemically and physically compatible

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with common electrode materials such as nickel-metal ceramic fuel electrodes and chalcogenide type ceramic air electrodes. To date, the most widely studied H-SOEC electrolyte materials are ABO3 (A=Ba, Sr, Ca; B=Ce, Zr) chalcogenide oxides [8, 32, 44]. As mentioned previously, Iwahara et al. [30, 38, 39] used Sc-, Y- or Yb-doped SrCeO3 -based proton-conducting oxides as electrolyte materials for H-SOEC. Since it has been demonstrated experimentally and theoretically that BaCeO3 and BaZrO3 -based oxides exhibit better hydration capacity and higher proton conductivity than Sr/CaCeO3 and Sr/CaZrO3 -based oxides [44, 45], the former has been widely used as proton-conducting oxides in the past decades [46]. However, neither BaCeO3 nor BaZrO3 -based oxides meet all the requirements to be used as H-SOEC electrolytes. Despite the excellent proton conductivity and good sinterability of BaCeO3 -based oxides, thermodynamics [47] and experiments [48] have shown that BaCeO3 -based oxides are chemically unstable in an H2 O-containing atmosphere under typical operating conditions of H-SOEC. The high resistance and refractory nature of the grain boundaries of BaZrO3 -based oxides in H2 oxygen-containing atmospheres [49] have been shown to pose a significant challenge for the application of BaZrO3 -based oxides as H-SOEC electrolyte materials [50].

6.4 Other Constituent Materials of SOEC In SOEC, the interconnect material, also known as bipolar, has two main functions: one is to connect and conduct electricity between cells; the other is to separate the electrolyzed and fuel gases on the cathode side from the oxidizing gases on the anode side. Two main types of interconnect materials are currently used: LaCrO3 based ceramic materials and superalloy materials [51]. SOEC with tubular structures do not need to be sealed to each other, while those with flat designs require reliable sealing connections. The research on flat SOEC sealing materials mainly focuses on glass systems based on silicate, borate, and phosphate, mica glass sealing systems, glass–ceramic composite sealing systems, and ceramic composite sealing materials. In addition, there are pressure seals by an external force, high-temperature metal seals, and adaptive seals [52]. Compared with SOFC, the water vapor content in SOEC intake air is very large. Particular attention should be paid to the influence of high temperature and humidity when selecting sealing materials [53].

6.5 SOEC and Stack Configuration A single cell is the smallest unit of SOEC and can be in a tubular or planar configuration, as shown in Fig. 6.6a and b, respectively. Conventional SOEC cells are made in a cylindrical shape, such as High Operating Temperature Electrolysis (HOTELLY) cells and Westinghouse cells [54]. In tubular SOEC, vapor enters the tube and is

6.5 SOEC and Stack Configuration

157

Fig. 6.6 SOEC configurations. a Tubular SOEC (end view) and b planar SOEC

reduced to hydrogen and oxygen ions. Oxygen is extracted from the outer layer of the tubular SOEC. Compared with planar SOEC, tubular SOEC has higher mechanical strength and is easier to seal. Despite the considerable sealing length between the anode and cathode compartments, planar batteries have received increasing attention in recent years due to their better manufacturability. Hino et al. investigated and discussed the electrochemical properties of tubular SOEC and planar SOEC [55]. They found that the performance of planar SOEC was significantly better than that of tubular SOEC. This is because the gas species distribution on the planar SOEC is more uniform. Considering the above factors and the fact that planar cells are more accessible to mass production, the planar SOEC system configuration is advantageous and should be further investigated. In the literature, several new structures have been proposed to improve the performance of solid oxide fuel cells, such as the flat tube high power density solid oxide fuel cell (HPD-SOFC), the monolithic layer built solid oxide fuel cell (MOLB-SOFC) and the thin-walled solid oxide fuel cell [56]. In addition, more and more researchers are interested in single-chamber SOFCs and micro-SOFCs [57]. The single-chamber concept greatly simplifies the gas supply system, but the electrode material’s selectivity still needs improvement. Micro-SOFCs are expected to be used for small-scale applications. Since SOFCs use the same materials but are developed in the opposite

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direction to SOEC, the novel configurations of SOFCs described above may also apply to SOEC. More work is needed to evaluate their suitability for SOEC. To increase the hydrogen production rate, the active area of the electrolytic cell should be increased. As this task is challenging to achieve by increasing the size of individual SOEC alone, it is essential to connect a large number of individual cells to build the required stack. Large electrolytic stacks with tandem tubular SOEC have been tested and proven to be feasible [54c]. Stacks using planar SOEC and other new SOEC are also possible, but more work is needed to optimize stack performance.

6.6 Material Degradation in SOEC As mentioned earlier, long-term degradation is a significant issue for the viability of SOEC technology as a practical hydrogen production system. Several long-term degradation studies have been conducted, concluding that further improvements are required before commercialization. For example, an aging survey of metal-supported cells at DLR showed a degradation rate of 3.2% per 1000 h at 800 °C, –0.3 A cm−2 , and 43% RH steam at the fuel electrode [58]. AC impedance studies have shown enhanced polarisation resistance during electrolysis compared to fuel cell operation, mainly attributed to the hydrogen electrode (Ni-YSZ). Aging studies at Risø 850 °C, –0.5 A cm−2 , and 50% RH steam conditions for up to 1300 h showed a degradation rate of 2% [59]. The degradation was also observed through AC impedance and was mainly attributed to the Ni/YSZ electrode. The growth of Ni particles and the presence of Si impurities were also found by SEM to be possibly related to changes in electrolytic operating conditions [60]. A follow-up analysis of the INL 720 cell stack after 1080 h in the SOEC row was also reported [61]. In this case, the hydrogen electrode (Ni-YSZ) was in good condition, except for a small amount of silicon impurities from the seal. In some cases, a transition from 6ScSZ to a monoclinic crystalline phase was detected near the rim. The presence of Cr-doped Al2 O3 near the seal of the bipolar plate and cation diffusion at the oxygen electrode was also observed. Furthermore, as followed by other scientific teams, their biggest degradation problem is the delamination of the oxygen electrode due to the high oxygen partial pressure at the electrode/electrolyte interface [62]. Short-term degradation of SOEC cells is also expected, mainly when operating under extreme conditions such as high current densities or high vapor concentrations at the fuel electrode. Matsui et al. [34b] studied the effect of fuel humidity on the performance and stability of Ni-YSZ fuel electrodes at 1000 °C. For high vapor concentrations, they found that forming vapor or hydroxide layers on the metal-ceramic led to performance degradation. Microstructural studies confirmed significant changes in the Ni-YSZ microstructure, with running samples having two-thirds the TPB length of unrunning cermets. High electrolytic current densities (> –1 A cm−2 ) were also investigated by Knibbe et al. [63]. They found that the decrease in cell voltage was mainly attributed to the ohmic drop and that there

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159

was no direct relationship between the decline in polarisation resistance and current density/cell polarisation. Degradation was also observed through intergranular fractures near the oxygen electrode/electrolyte interface. They found an increase in the oxygen content across this grain boundary in porous regions. Similar findings were made on microtubular SOEC [64]. They found that when running at a current density above − 1.75 A cm−2 at 895 °C and 70% RH steam, significant degradation was observed in the electrochemical data, as confirmed by SEM micrographs, EDS analysis and Raman spectroscopy (Fig. 6.7). In agreement with the results of Knibbe et al. [63], they detected voids at the YSZ grain boundaries in the region close to the oxygen electrode, even producing large cracks in the electrolyte. The presence of excess oxygen near the degraded region of the oxygen electrode correlated with high pO2 at the electrolyte–electrode interface, in agreement with Virkar’s model [65]. Irreversible degradation of the electrolyte can occur due to electro-reduction of the YSZ and in some cases delamination of the oxygen electrode. Similar findings were reported by Schefold et al. [66] using YSZ planar cells. They observed that electronic conduction occurs in the YSZ electrolyte when the vapor conversion rate corresponding to the current operating density is higher than 100% or when the vapor supply is interrupted at a constant current.

Fig. 6.7 SEM micrographs showing different stages of damage for the same cell a general view of the cell; b the origin of the degradation at the YSZ grain boundaries; c cracking of the YSZ electrolyte and d delamination of the LSM–YSZ electrode [64]

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As mentioned previously, these findings were predicted by Virkar’s model [65]. The model suggests that electron conduction in the electrolyte plays a crucial role in determining the local oxygen chemical potential within the electrolyte. Under certain conditions, high pressures are generated in the electrolyte very close to the oxygen electrode/electrolyte interface, leading to delamination of the oxygen electrode. The model also found that the higher the electron conductivity of the electrolyte, the lower the tendency to form high internal pressures. Preliminary calculations showed that small changes in electron conductivity led to an order of magnitude change in the oxygen partial pressure. Therefore a small amount of electron conductivity through the electrolyte is beneficial to the stability of the material. A schematic of the change in electrical and oxygen chemical potentials can be observed in Fig. 6.8.

Fig. 6.8 a Schematic variations according to reference of electric and oxygen chemical potential through the electrolyte in the fuel cell mode (‘true’ steady state) [65]. The directions of the particle fluxes and the directions of ionic and electronic current are shown. b Schematic variations of electric potential and oxygen chemical potential through the electrolyte in the electrolyzer mode. Delamination along the electrolyte/anode (oxygen electrode) is likely in such a case

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In this regard, findings on the degradation of scandium oxide and ceria-doped zirconia (10Sc1CeSZ) under extreme SOEC conditions are also relevant [67]. This material has advantages over standard YSZ. During SOEC operation, degradation monitoring of Ce4+ ↔ Ce3+ transitions can be monitored by spatially resolved vibrational spectroscopy and Ce3+ electron Raman spectroscopy and Er3+ emission spectroscopy studies due to the presence of Ce4+ dopants that are reduced to Ce3+ . The reduction of the electrolyte was observed to occur near the Ni-YSZ electrode and then along the thickness of the electrolyte, and in some cases, was also associated with the phase transition of the electrolyte from cubic to rhombohedral. It was also observed that this degradation occurred when the cell was operated above 1.8 V. In summary, all these studies confirm that further microstructural improvements to existing materials and/or the development of new materials are required before SOEC devices can be commercialized.

6.7 Other Applications Using SOEC Besides hydrogen production, SOEC has been proposed for various fields in recent years [68]. Martinez-Frias et al. presented an interesting approach [69]. They offered a novel high-efficiency solid oxide natural gas-assisted steam electrolyzer (NGASE), in which natural gas reacts with oxygen produced in electrolysis, reducing the chemical potential of the electrolyzer, thereby minimizing power consumption. In this system, the oxygen produced during electrolysis can be consumed in a partial or full oxidation reaction with natural gas: CH4 → CO + H2 (Partially oxidized)

(6.1)

CH4 → CO2 + H2 O (Fully oxidized)

(6.2)

Their analysis concluded that by incorporating a heat recovery system into the plant, primary energy efficiency would be around 70%. In this area, Wang et al. [70] investigated Cu-CeO2 -YSZ and Pd-C-CeO2 -YSZ composites as anodes for NGASE and CO-assisted steam electrolysis. They found that the catalytic activity of the Cu composites was lower when exposed to CO or CH4 . They also found that the Pd composites had the highest catalytic activity, although the oxidation of CH4 on the anode was significantly lower than the theoretical value. Pati et al. [71] also demonstrated that a solid oxide membrane (SOM) electrolyzer could be used to produce hydrogen from steam using a solid carbon reductant in a liquid metal anode. They demonstrated that the energy required for hydrogen production could be effectively reduced by adding a solid carbon reductant to a liquid tin anode. The feasibility of hydrogen production from carbon and steam was also demonstrated by Lee et al. [72] They used a YSZ cell with a platinum electrode,

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which was shown to produce spontaneous carbon-free hydrogen during constant current operation and cogeneration during Galvano. As mentioned earlier, the potential advantages of SOEC lie in its chemical flexibility, such as the electrolysis of CO2 [73] and, perhaps more interestingly, the coelectrolysis of steam and CO2 to produce syngas. For example, renewable or nuclear energy could be used to generate the heat and electricity needed to decompose CO2 and H2 O. CO2 → CO + 1/2O2

(6.3)

2H2 O + CO2 → CO + H2 + 3/2O2

(6.4)

Today, research into co-electrolysis is very active and promising, especially when combined with renewable or nuclear energy, as it can be used to recycle CO2 into sustainable hydrocarbon fuels [74].

6.8 Technical Prospects Solid oxide (SOEC) cells operate at high temperatures (700–850 °C), and the kinetic advantages allow using inexpensive nickel electrodes. If using high-quality waste heat from industrial production (e.g., the energy input of 75% electricity + 25% heat in water vapor), system efficiency of SOEC (LHV H2 to AC) is expected to reach up to 85% soon and reach the EU 2030 target of 90% within 10 years. The SOEC electrolyzer feeds water vapor. If carbon dioxide is added, it can generate syngas (a mixture of hydrogen and carbon monoxide) and produce synthetic e-fuels (such as diesel and jet fuel). Therefore, SOEC technology is expected to be widely used in carbon dioxide recovery, fuel production, and chemical synthesis, which has been the focus of research and development in the EU in recent years. Another advantage of SOEC is reversibility, that is, the use of reversible fuel cells for the storage of renewable energy, which is also a long-term essential research and development topic in Europe and the United States. At present, the research on SOEC is still in its infancy. Whether it can achieve commercial mass production, it still needs to solve a series of problems: (1) Energy loss and cost problems, the polarization of oxygen electrodes, ohmic loss of electrolytes and cost of connector materials, etc.; (2) The life of the electrolytic cell, the performance degradation of the hydrogen electrode under high temperature and high humidity conditions, the stability of the sealing material and the thermal cycle stability of the stack, etc.; (3) The development of high-efficiency heat exchangers, the thermal management of hydrogen production systems, the utilization of waste heat; (4) Hydrogen safety issues. Despite the above problems, SOEC has shown its broad development prospects in the fields of energy and environment. To make this

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Chapter 7

Hydrogen Production by Seawater Electrolysis

7.1 Chlorine Precipitation from Seawater Electrolysis Hydrogen stands out as a highly promising future energy carrier capable of replacing fossil fuels and mitigating the adverse effects of global warming. Presently, over 80% of the world’s energy is derived from non-renewable sources, including oil, coal, and natural gas, which contribute significantly to CO2 emissions. However, the production of hydrogen through electrolysis using renewable energy sources presents a novel avenue for transforming the existing energy framework while ensuring longterm environmental sustainability. Traditionally, electrolytic water systems have primarily relied on distilled water with varying pH levels, which may not be an optimal approach in regions where freshwater resources are scarce, yet abundant seawater is available. From a practical perspective, direct seawater electrolysis offers significant advantages due to the Earth’s ample seawater reserves and its capacity to generate high-purity hydrogen. Compared to freshwater, seawater is an almost limitless raw material on our planet, emphasizing the crucial role of seawater electrolysis, particularly in arid regions where freshwater scarcity is a pressing concern. Moreover, seawater electrolysis also holds potential for desalination and salt production. The storage of renewable energy through accelerated chemical reactions emerges as an appealing solution to address the intermittent challenges faced by various alternative energy sources. Due to its impressive gravimetric energy density (142 MJ kg−1 ) and environmentally friendly nature, hydrogen is widely regarded as one of the most promising clean energy carriers. The electrolysis of water, which involves an efficient and stable oxygen evolution reaction (OER) at the anode, represents a clean and reliable method for hydrogen production at the cathode. However, if decomposed water is extensively utilized for large-scale energy storage purposes, issues related to water allocation arise when substantial amounts of purified water are required for fuel generation. Presently, prevailing electrolytic water-to-hydrogen technologies employ purified freshwater as the primary feedstock, employing appropriate acids, bases, or pH buffers as needed. Nevertheless, the extensive use of freshwater for hydrogen production through electrolysis poses a significant threat to the © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_7

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limited freshwater resources of our planet. Seawater, on the other hand, represents the most abundant electrolyte feedstock on Earth, constituting 96.5% of the total global water reserves, and holds great potential for electrolytic hydrogen production despite the challenges it presents, particularly regarding anode reactions. From a practical standpoint, direct seawater electrolysis offers remarkable advantages due to the abundance of seawater reserves on Earth and its ability to produce hydrogen of high purity. However, seawater electrolysis is a difficult problem because Cl− (~0.5 M) in seawater may compete with OER by oxidation on the anode and Cl− can strongly corrode the electrode. For example, under acidic conditions at pH = 0, the theoretical overpotential of OER (1.229 V vs. SHE) is 130 mV lower than that of chlorine precipitation reaction (ClER) (1.358 V vs. SHE), but OER is a 4electron transfer process while ClER is a 2-electron transfer process, and ClER has a kinetic advantage. Although chlorine (Cl2 ) is a high-value product, Cl2 is toxic and not easily stored and transported, and in practice plants generally make it on-hand, and the by-products of seawater electrolysis, Cl2 , can quickly exceed actual demand, hence the need to suppress ClER [1]. The involvement of chlorine in oxidation is a complex reaction that varies with reaction conditions such as pH, potential and temperature. Both ClER and OER lie at the heart of the large-scale electrochemical conversion step, which makes them important for renewable energy infrastructure. In chlorine precipitation, chloride ions are oxidized to Cl2 . Similarly, the oxygen precipitation process involves the oxidation of water to O2 with the release of protons. In order to compare the oxidation of chlorine and water, the corresponding reactions are shown below for two pH solutions: In a pH = 0 solution, ClER: 2Cl− → Cl2 + 2e− , E0 = 1.358 V versus SHE

(7.1)

Cl− + 2H2 O → HClO + H+ + 2e− , E0 = 1.482 V versus SHE

(7.2)

OER: 2H2 O → 4H + O2 + 4e− , E0 = 1.229 V versus SHE

(7.3)

In a pH = 14 solution, ClER: Cl− + 2OH− → OCl− + H2 O + 2e− , E0 = 1.570 V versus SHE OER: 4OH− → O2 + 2H2 O + 4e− , E0 = 0.401 V versus SHE

(7.4) (7.5)

Direct electrolysis of seawater faces the challenge of selectivity and stability of OER catalysts. Due to the presence of Cl− in seawater, depending on the pH of the electrolyte, the formation of ClER or hypochlorite may occur at the anode, where Eqs. (7.6) and (7.7) represent the reactions under acidic conditions (ClER) and alkaline conditions (hypochlorite formation), respectively.

7.1 Chlorine Precipitation from Seawater Electrolysis

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In an acidic environment: 2Cl− → Cl2 + 2e− , E0 = 1.358 V versus SHE

(7.6)

In an alkaline environment: Cl− + 2OH− → ClO− + H2 O + 2e− , E0 = 1.720 V−0.059 pH

(7.7)

As can be seen from Fig. 7.1, ClER or hypochlorite formation is competitive with OER and is thermodynamically not favored by OER because it usually requires higher overpotential and more energy consumption. In addition, chlorine generated under acidic conditions and hypochlorite generated under alkaline conditions are both dangerous and difficult to handle during seawater electrolysis. Therefore, the generation of ClER or hypochlorite during seawater electrolysis is undesirable and it is critical to control the splitting of seawater to be fully OER selective. Synthesizing catalysts with only OER active sites is quite difficult because surfaces active for OER intermediates are usually also applicable to ClER intermediates [2]. Controlling some aspects of the electrolytic environment, such as pH, can be more effective in achieving OER selectivity in seawater electrolysis than in developing OER-selective active catalytic surfaces. ClER is thermodynamically inferior to OER, and the potential difference becomes larger with increasing pH, reaching a maximum of ~ 480 mV between hypochlorite generation and OER at pH above ~ 7.5. Also, considering that OER is more favorable under high pH conditions, alkaline electrolytes with high pH are more suitable for seawater electrolysis. In order to ensure high selectivity of OER during alkaline seawater electrolysis, highly active OER catalysts are needed to provide large current density at small overpotentials and avoid hypochlorite formation process. It should be noted that when the pH is greater than 9.5, precipitates such as Mg(OH)2 and Ca(OH)2 are produced in the electrolyte, which in combination with other insoluble solids such as bacteria and microorganisms inherent in seawater, can block some of the active sites and lead to a decrease in catalytic performance [3]. Given that the overpotential of ClER (1.36 V vs. SHE) does not vary with pH like OER, inhibition of ClER can be achieved by increasing the alkalinity of the electrolyte. However, another reaction occurs in an alkaline environment, namely the hypochlorite production reaction. Hypochlorite generation still forms a competing reaction with OER, whose initial overpotential is 490 mV higher than that of OER. To prevent hypochlorite generation, the overpotential of OER catalysts at practical industrial application current density (~ 1 A cm−2 ) needs to be lower than the clathrate generation potential, which requires OER electrocatalysts with high activity that can be used at much lower than overpotential of hypochlorite horizons for high current operation to produce H2 /O2 at high rates. In addition to the selectivity problem, the presence of Cl− in seawater has a strong corrosive effect on the catalyst, especially on transition metal-based materials. Even for OER catalysts with high activity in alkaline electrolytes, chloride ions in seawater can corrode many catalysts and substrate materials through a metal chloride-hydroxide generation mechanism [4].

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Fig. 7.1 Pourbaix plot of artificial seawater (0.5 M NaCl electrolyte) at 25 °C [1]

Surface polarization adsorption of Cl− : M + Cl− → MClads + e−

(7.8)

Further coordination of dissolution: MClads + Cl− → MCl− x

(7.9)

Conversion from chloride to hydroxide: − − MCl− x + OH → M(OH)x + Cl

(7.10)

In order to avoid relying on expensive desalination processes, the development of corrosion-resistant electrodes for the decomposition of seawater into H2 and O2 is crucial for the development of seawater electrolysis technology, an undertaking that has been rather limited so far. Cl− is highly corrosive and it can react directly with these electron-deficient transition metals, changing the composition of the catalyst. Therefore, Cl− initiated corrosion can gradually dissolve transition metal-based catalysts, leading to catalyst poisoning and poor catalytic durability. Catalysts based on transition metal nitrides, phosphides and sulfides show good performance against corrosive Cl− in seawater. Changing the active metal to a higher valence state has also been shown to be an effective strategy to inhibit Cl− corrosion [5]. In addition, the corrosion problem can be solved by introducing a chlorine rejection layer. For example, Kuang and coworkers [6] reported an efficient OER catalyst for alkaline seawater electrolysis by growing NiFe hydroxides on NiSx and demonstrated that the derived polyatomic sulfate and carbonate-rich layers can repel chloride ions from seawater with high corrosion resistance.

7.2 Special Electrodes to Mitigate Chlorine Precipitation

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Direct use of seawater for electrolysis has some open problems. Natural seawater is a complex system including various inorganic ions (e.g. Cl− , Br− , SO4 2− , Na+ , K+ , Mg2+ , Ca2+ ) and marine microorganisms, which is very different from high purity distilled water. Among them, the concentration of chloride ions, the main component of seawater, is as high as ~ 3.5%. Seawater electrolysis consists of two main half-reactions: the hydrogen precipitation reaction (HER) at the cathode and the OER at the anode. It is difficult to evaluate the effect of all components on the catalytic reaction. Although a large amount of Cl− can act as a carrier to improve the conductivity of the electrolyte, the ClER that occurs will compete with the OER. Therefore, materials with high selectivity for OER are ideally preferred in seawater electrolysis. In addition, corrosive chlorine products (Cl2 , HClO or ClO) can damage the electrolyte environment and negatively affect the reaction system. Both electrode and catalyst materials are inevitably subjected to strong corrosion by complex chlorides and their lifetime will be severely shortened. Unlike conventional corrosion protection studies of marine materials, seawater electrolyzer materials should have high catalytic activity, selectivity and good corrosion resistance. Most of the reported active substances are unlikely to last long in strongly corrosive solutions. Competent catalysts often exhibit poor catalytic performance, which is far from the practical application of seawater electrolysis [7]. Therefore, the selection and design of competent materials for seawater electrolysis is important to deal with the damage of chlorine chemistry in brine systems.

7.2 Special Electrodes to Mitigate Chlorine Precipitation Based on the preceding section, the anodic reaction of electrolytic seawater necessitates careful consideration of high selectivity for the oxygen precipitation reaction while simultaneously enhancing catalyst efficiency and stability. Among the wide range of electrode catalysts, significant attention has been directed towards nickel metal-based catalysts. Nickel metal possesses favorable charge separation properties, abundant active sites, low oxygen adsorption energy for desorption, and minimal overpotential for the OER [8–11]. Furthermore, nickel metal can be subjected to various modifications to optimize its performance in electrolytic seawater oxygen precipitation reactions, including alloy formation to enhance catalytic properties and the creation of specialized morphological structures to expose active sites and impede the involvement of chloride ions in the reaction. Therefore, this chapter initially presents the notable advancements achieved in nickel-based catalysts within the realm of electrolytic seawater reactions. Subsequently, other electrocatalysts, distinct from nickel-based ones, are introduced for comparative and complementary purposes.

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7.2.1 Nickel-Based Catalysts Juodkazis et al. successfully prepared NiO thin layers characterized by a large electrochemically active specific surface area, exceptional corrosion resistance, and nearly 100% selectivity for oxygen precipitation reactions [12]. Scanning electron microscopy images clearly depict the layered and highly porous structure of the spray pyrolysis NiO films, indicating a high rate of adsorption and dissociation, thereby contributing to the oxygen precipitation reaction. Notably, the electrolysis process revealed that only approximately 1% of the charge reacted through the electrolytic cell to form H2 O2 , while the oxidation of chloride ions was completely inhibited. Consequently, NiO thin layers emerge as a promising class of catalysts for seawater decomposition, owing to their high electrochemical activity area and selective blocking capability against chloride ions at the anode [6]. Furthermore, the effectiveness of NiFe mixed oxides and hydroxides in the context of low OER overpotential and high stability has been experimentally validated [8]. Zhou et al. established a general design criterion for selective seawater electrolysis by analyzing the electrochemical competition between oxygen and chloride during the hydrogen production process [9]. The study substantiated the effectiveness of noble metal-free NiFe layered hydroxide (LDH) electrocatalysts in seawater. Importantly, the experimental results demonstrated that the presence of chloride ions did not compromise the activity, stability, and selectivity of NiFe LDH. Figure 7.2 illustrates the mechanism of the oxygen precipitation reaction involving NiFe LDH, wherein the synergistic effect between NiFe LDH and Fe hydroxide layers induced a reorganization of the catalyst’s electronic structure, leading to enhanced adsorption performance of OH intermediates. Under alkaline conditions (pH 13), NiFe LDH exhibited excellent applicability in the selective cracking of seawater for oxygen production. Selective experiments demonstrated the absence of chlorine precipitation, and the Faraday efficiency of OER reached approximately 100%. Characterization studies revealed that the presence of chloride and sodium ions in the alkaline solution promoted the electrochemical contact of the nickel oxidation center, thereby improving the catalyst’s electrochemical activity and facilitating the oxidation process. Kuang et al. developed a NiFe/NiSx /Ni multilayer electrode that exhibits remarkable activity and corrosion resistance for selective oxygen precipitation reactions in alkaline electrolytes containing chloride ions [7]. The activation of this electrode leads to the generation of negatively charged polyanions derived from the oxidation of the underlying nickel sulfide layer in alkaline solution. These polyanions effectively repel chloride ions present in seawater, thereby enabling selective oxygen evolution. Raman spectroscopy characterization of the outer NiFe hydroxide layer formed after activation revealed its high OER activity and corrosion resistance. During the 10-h electrocatalytic experiments, the Faraday efficiency surpassed 96%. In comparison tests, Ni-based electrocatalysts without the electrodeposited NiFe layer exhibited significantly lower activity and stability. Notably, the combined effect of the NiFe and NiSx layers resulted in excellent electrocatalytic activity under different conditions. In electrolysis experiments conducted using real seawater, the catalyst

7.2 Special Electrodes to Mitigate Chlorine Precipitation

173

Fig. 7.2 OER reaction mechanism of NiFe LDH [7]

achieved a current density of 400 mA cm−2 at 2.1 V, and its activity remained nearly unchanged throughout a 1000-h stability test. These findings provide a solid foundation for the large-scale application of NiFe/NiSx /Ni multilayer electrode catalysts in seawater electrolysis. Additionally, Yu et al. observed that the preparation of NiFebased metal nitride core–shell structures could create an active layer during the oxygen precipitation reaction, enhancing the catalyst’s OER selectivity in seawater [10]. The NiMoN@NiFeN catalyst exhibited high activity and stability at a current density of 50 mA cm−2 in seawater OER experiments. Moreover, the overpotential of the reaction displayed a decreasing trend with increasing temperature, indicating a positive correlation between electrode activity and temperature. Importantly, the hydrogen and oxygen precipitation ratio during the reaction was approximately 2:1, confirming the high selectivity of the anodic oxygen precipitation reaction.

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7.2.2 Other Catalysts Manganese oxide as an anodic electrocatalyst in alkaline environment is of high research value in terms of economic efficiency and oxygen precipitation performance [11]. The experiments of Fujimura also found that the addition of molybdenum contributed significantly to the stability of the Mn2 O3 electrocatalyst [13]. As shown in Fig. 7.3, the stability of Mn0.88 Mo0.12 O2.13 and MnO2 was tested at a current density of 1000 mA cm−2 , and the comparison showed that MnO2 without Mo modification shed more severely in a shorter electrolysis time, while Mn0.88 Mo0.12 O2.13 showed a greater improvement in stability, and its oxygen precipitation Faraday efficiency was maintained at 98.5% in the 1500 h test, which was much higher than other electrocatalysts. The use of different metals to form alloys with Pt is also one of the design ideas of anode catalysts for electrolytic seawater. Zheng synthesized PtPd, PtNi, PtCo and PtFe on pretreated Ti substrates by electrodeposition, and the synthesized alloys were in the form of nanoparticles [14]. XPS spectroscopy revealed that the orbital electrons between Pt and the alloying elements interacted, transferring the charge from the alloying elements to Pt and thus redistributing the electrons on the Pt surface. The charge transfer from the alloying element to Pt, which results in the redistribution of electrons on the Pt surface. The charge transfer resistances of different samples are in the following order: PtPd < PtNi < PtCo < PtFe < Pt. It can be seen that the alloying of Pt with metal can adjust the electronic structure of Pt and enhance the charge transfer process. The results of OER tests in seawater are in the opposite order of the transfer resistance arrangement, with the best performing PtPd alloy reaching a current density of 100 mA cm−2 at an overpotential of 900 mV with almost no decay

Fig. 7.3 SEM images of Mn0.88 Mo0.12 O2.13 and MnO2 after different time stability tests in simulated seawater environment [13]

7.2 Special Electrodes to Mitigate Chlorine Precipitation

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for 12 h. Ti-loaded PtM electrode has a promising application with high catalytic efficiency and good durability. Cobalt hydroxides have been widely used as efficient and stable catalysts for the electrolysis of water [15]. Generally, conventional cobalt hydroxide is in powder form and is produced by co-precipitation of cobalt salt and base in aqueous solution. This method is easy to operate, and the composition and nanostructure of the catalyst can be well controlled. However, powdered electrocatalysts can easily aggregate and fall off from the electrode during application [16]. Song et al. prepared a large amount of CoSn(OH)6 nanoparticles by co-precipitation, and then selectively dissolved tin hydroxide by electrochemical etching to form layered nanoporous Co(OH)2 , which can improve the electronic conductivity and enhance the adsorption and transport capacity of OH [17]. The outer layer of the catalyst has been practically transformed into an amorphous material composed of CoOOH, which serves as the electrochemically active site for the selective oxygen precipitation reaction. In addition, the substrate used to prepare the electrode also affects the performance of the catalyst. For example, the preparation of cobalt hydroxide electrocatalysts by electrodeposition using activated carbon cloth as a flexible substrate requires an overpotential of only 195 mV to achieve a current density of 10 mA‧cm−2 in an alkaline environment of 1 M KOH [18]. Another important approach to improve the OER performance of cobalt hydroxide is to construct nanostructures to expose more active sites. McAteer et al. prepared Co(OH)2 nanosheets by liquid-phase dissolution method, which exhibited excellent OER catalytic activity. They also analyzed the effect of the size of the nanosheets and the thickness of the electrode on the electrocatalytic activity, and the results showed that the smaller the nanosheet the better its performance. Bennett initiated the selective electrolysis of seawater to H2 /O2 in 1980. A MnO2 coating was applied to a dimensionally stable anode (DSA) by anodic electrodeposition [19]. The electrode’s unique porous MnO2 coating was able to release O2 from seawater and saturated NaCl solutions with 99% and 95% efficiency, respectively. It is also clarified that this selective catalytic reaction is attributed to the fact that the MnO2 coating on the electrode surface causes the CIER to be rapidly limited by mass transfer and its ultimate current density is reduced by 2–3 orders of magnitude, thus making the OER the dominant anodic reaction. However, the simple MnO2 coating layer will gradually fall off due to the increase of electrolyte temperature during electrolysis. Since 1997, many researchers have started to dope and modify manganese-based catalytic materials and gradually prepared manganese-molybdenum oxides [19, 20], manganese-tungsten oxides [20], manganese-molybdenum-tungsten oxides [21], manganese-molybdenum-iron oxides [22], and manganese-molybdenum-tin oxides [23], which have higher OER catalytic activity and are more stable. The addition of metallic elements such as molybdenum, tungsten, iron, and tin will improve the catalytic activity of the anode OER and enhance the stability of the electrode at the same time. Among them, Fujimura’s group investigated the conditions for the electrodeposition of manganese molybdenum oxide anodes and derived their optimal electrodeposition [24]. Manganese molybdenum iron oxides covered with IrO2 /Ti electrodes prepared by Abdel et al. in MnSO4 -Na2 MoO4 -FeNH4 (SO4 )2 solution by

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anodic deposition at 30–90 °C and pH = 12, with an OER efficiency of 100% when electrolyzed on 0.5 M NaCl solution at a current density of 100 mA cm−2 [22]. The incorporation of iron resulted in increased activity and stability of the catalyst at higher temperature conditions. In 2009, Surendranath et al. discovered another cobalt phosphate salt catalyst material with high OER activity [23]. Electrolysis of a 0.5 M NaCl solution containing potassium phosphate buffer using this catalytic material as an anode achieved an OER efficiency close to 100%. To achieve higher current densities in a typical liquid electrolytic cell would result in a sharp drop in anode pH, destroying the locally stable acid–base environment required for the cobalt phosphate salt catalyst material, even though carbonate and borate ions are present in seawater, their average concentrations are too low to act as proton acceptors at high currents. Therefore, buffer solutions or other additives (e.g., 0.5 M H2 SO4 , 1 M KOH) must be added to maintain a stable local pH, in addition to other methods (e.g., using diaphragm cells) to overcome the inhibitory effect of such local pH changes on catalyst activity. Considering this effect, in 2011, Esswein et al. tested the activity of cobalt phosphate and cobalt borate catalyst materials in natural seawater electrolytes containing potassium phosphate buffer (pH = 7), potassium borate buffer (pH = 9.2), and potassium hydroxide (pH = 14), respectively, and found that both cobalt phosphate and cobalt borate had high activity in neutral seawater [25]. However, the effect of the pH of the electrolyte solution was not considered in the previously reported work. In 2017, Cheng et al. investigated the effect of different electrolytes on CoFe-LDH as a seawater OER catalyst and found that the activity of CoFe-LDH was stronger in natural seawater than in NaCl simulated seawater solution due to the synergistic effect between the ions in natural seawater and the CoFe-LDH catalyst, a synergistic effect that was previously reported has not been mentioned [26]. In 2018, Hsu et al. published a report on the use of transition metal salts and basic cobalt carbonate conducting core (MHCM-z-BCC) as OER catalysts. The thin shell of MHCM in this core–shell nanostructure provided good catalytic activity, while the conductive inner core of BCC promoted efficient charge transfer. It has high stability in natural seawater electrolytes containing neutral buffers [27]. In 2019, Yu et al. prepared a three-dimensional core–shell nanostructured OER catalyst of NiMoN@NiFeN loaded on nickel foam based on the existing work on electrolytic freshwater in their group, using transition metal nitrides as the object of study, with the internal NiMoN nanorods having high conductivity to ensure fast and efficient charge transfer from NiMoN to NiFeN surface [28]. The external NiFeN nanoparticles generate an amorphous layer of NiFeOx and NiFeOOH in situ during the OER catalysis process. The amorphous film is not only the real active material of OER, but also prevents the corrosion of the electrode by chloride ions in seawater. This unique three-dimensional core–shell nanostructure provided a large specific surface area and abundant active sites, which exhibited better electrocatalytic activity and stability in the OER process of electrolysis of seawater. In the same year, Zhuang et al. used in situ etching to remove ZnO from the hollow nanobox structure of RuIrZnOx to form a bifunctional RuIrOx catalyst with high activity and stability, and the removal of ZnO significantly exposed the active sites and greatly improved the

7.3 Hydrogen Production Equipment by Seawater Electrolysis

177

atomic utilization of Ru/Ir with an electrochemically active surface area (ECSA) three times higher than that of commercial Ru–C/Ir–C electrodes [29]. The high activity was achieved in electrolytes with pH = 0–14. Combined with density functional theory (DFT) calculations and X-ray absorption spectroscopy (XAS) analysis, the introduction of Ir into RuOx significantly reduced the energy barriers of HER and OER and prevented the over-oxidation of Ru under acidic OER conditions, which greatly alleviated the stability problems of noble metal catalysts in the OER process.

7.2.3 Summary The current electrolysis of seawater for hydrogen production is still in the early development stage, and the biggest challenge is to develop catalytic materials that are stable and have high OER selectivity. At present, researchers have developed a variety of non-precious metal-based HER/OER electrocatalysts, but they also have their own problems and do not have the capability of large-scale industrial application. In recent years, more researchers have improved the stability and activity by further nano-engineering of catalysts, such as prepared multistage micro- and nano-catalytic materials in core–shell, two-dimensional sheet, cage, and feather shapes. In addition, we can improve the catalysts for electrolysis of seawater by drawing on the already more mature types of OER catalyst materials for electrolysis of freshwater, such as compounding oxides with nitrides, sulfides, phosphides, etc. Through surface and interface engineering, multi-component heterogeneous catalysts with excellent catalytic activity and stability were constructed to increase the number of active sites of oxide by heteroatom doping; The combination of theoretical calculation and experiment, as an auxiliary means to explore the real active sites, provides ideas for the continuous improvement of catalytic materials and the acceleration of the development of high-performance OER electrocatalysts.

7.3 Hydrogen Production Equipment by Seawater Electrolysis The preparation of hydrogen by electrolysis of seawater can be achieved in two ways (Fig. 7.4), one is the electrolysis of seawater after first desalination and dehybridization to form fresh water, and the other is the electrolysis of seawater directly. From the application point of view, in addition to the development of stable and efficient catalysts, suitable high-performance and low-cost seawater electrolyzers must be designed. Currently, two low temperatures (< 100 °C) electrolyzers, alkaline water electrolyzer and proton exchange membrane water electrolyzer, are more mature in

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7 Hydrogen Production by Seawater Electrolysis

Fig. 7.4 Two methods of hydrogen production by seawater electrolysis [30]

the commercial market; there are also two emerging technologies, low temperature anion exchange membrane water electrolyzer and high temperature water electrolyzer, of which high temperature electrolysis includes proton conductive ceramic electrolysis (150 ~ 400 °C) and solid oxide electrolysis (800 ~ 1000 °C). When these electrolyzers are used directly to electrolyze seawater, the complex natural composition of seawater can have an impact on the electrolysis. The main problems are physical or chemical clogging of ion exchange membranes and corrosion of metal components, e.g. Na+ , Mg2+ and Ca2+ ions in seawater can reduce the performance of the proton exchange membranes in proton exchange membrane water electrolyzers and high temperature water electrolyzers; Cl− , Br− , SO4 2− and other anions will adversely affect the membrane performance of low-temperature anion exchange membrane water electrolyzer, alkaline water electrolyzer and hightemperature water electrolyzer. Therefore, the development of stable diaphragms is an important challenge for direct electrolysis of seawater. The challenges associated with direct electrolysis of seawater differ from those of freshwater due to the intricate composition of seawater, as illustrated in Table 7.1. In addition to a wide range of dissolved inorganic salt ions, seawater contains various organic substances and impurities such as plastics, microorganisms, and dissolved gases. More than 80% of the 100 known elements can be found in seawater, and its composition is influenced by geographical location, weather patterns, and seasons, necessitating the use of different electrolyzers in conjunction with different sea areas. A recent investigation on direct alkaline seawater electrolysis revealed that catalyst deactivation occurred rapidly within a few hundred hours due to clogging of ion channels and the adhesion of insoluble materials from natural seawater onto the surfaces of ion exchange membranes and catalysts. Even with the use of pure water and NaCl additives, industrial long-term electrolysis remains challenging, as artificially prepared NaCl solutions do not replicate the complexity of seawater. The concentration of chloride ions in natural seawater is approximately 0.5 mol/ L, and during electrolysis, they can be oxidized to chloride gas and hypochlorite at the anode. This not only corrodes the metal electrode substrate but also leads

7.3 Hydrogen Production Equipment by Seawater Electrolysis

179

Table 7.1 Main components of seawater (g/kg) Anions Cl−

Cations 18.98

Na+

Inorganic salt compounds 10.56

CaSO4

1.38

2.65

Mg2+

1.27

MgSO4

2.10

HCO3 −

0.14

Ca2+

0.40

MgBr2

0.08

B−

0.06

K+

0.38

MgCl2

3.28

F−

0.003

Sr2+

0.01

KCl

0.72

B(OH)4 −

0.03

NaCl

26.69

SO4

2−

to catalyst deactivation. Although chlorine and hypochlorite are valuable industrial chemicals, their demand is considerably smaller compared to the growing global market for hydrogen energy. Moreover, their transportation costs are high, and regulations regarding the long-distance transport of liquid chlorine and hydrochloric acid have been implemented in China due to safety concerns. In well-established chloralkali processes, a saturated sodium chloride solution serves as the electrolyte, and direct use of seawater is avoided due to the stringent water quality requirements of the diaphragm. Alkaline electrolysis of seawater typically requires operating conditions with a pH greater than 7.5. This pH threshold is commonly adopted as a design criterion for contemporary seawater electrolysis anode catalysts due to the substantial potential difference between OER and chlorine evolution reaction (ClER). This potential difference can achieve a maximum of 480 mV in alkaline environments, contrasting with a mere 130 mV under acidic conditions. However, purification of seawater under high pH conditions is unavoidable because alkaline earth metals form precipitates directly and adhere to the electrode surface during electrolysis, thus gradually reducing the catalyst active area until deactivation. In addition, the concentration of hydrogen ions as well as hydroxide ions in seawater is very low, and their slow mass transfer rate during electrolysis makes the electrolysis efficiency low. The resulting local pH difference is not conducive to the thermodynamic changes of the hydrogen and oxygen precipitation half-reactions and may lead to the precipitation of alkali metal hydroxides, etc. Although carbonate in seawater can be used as buffer solution, its content is too low to suppress the local pH increase at cathode and pH decrease at anode. The above problems can be alleviated by adding buffers, acidbased solution, etc. to seawater, but this also increases the cost of water treatment. In contrast, only water is consumed in the electrolysis process using high-purity water, and the acid-based solution can be recycled in the system. Direct seawater electrolysis is challenging because the concentration of impurities increases during the electrolysis process, precipitation adheres to the electrode surface and is subject to corrosion by chloride ions, and the selectivity and durability of the catalyst is greatly impacted. With the large-scale development of wind power generation, randomness and volatility are no longer the main problems limiting its grid connection, and the lagging grid construction can no longer meet the rapidly expanding wind power development. Among them, wind power coupled with hydrogen production technology is one of the ideal ways to realize the full utilization of renewable energy

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7 Hydrogen Production by Seawater Electrolysis

and the conversion of wind energy to hydrogen energy. And offshore wind power hydrogen production is the best choice for large-scale development of hydrogen energy nowadays because of its huge wind resources available, not occupying land and abundant seawater resources. Wind power coupled with seawater desalination to produce hydrogen can not only supply electricity, but also water and also hydrogen fuel. It helps to solve the problem of fresh water supply in water-scarce island regions, and at the same time solves the problem of demand for electricity and fuel through renewable energy conversion. Large-scale wind power coupled with seawater hydrogen production not only improves its utilization efficiency, but also realizes the problem of “where does the hydrogen come from” in the current green energy transformation, which is of great strategic significance. According to China’s energy production plan, the total installed capacity of wind power and photovoltaic will reach 1200 GW in 2030. In order to adapt to the pattern of energy supply and energy demand in China, wind power generation for deep and distant sea will be the inevitable choice. Among the many electrolytic water technologies (Table 7.2), alkaline water electrolyzer has the advantages of low cost and high reliability, and is more suitable for hydrogen production from seawater than the proton exchange membrane electrolyzer, which requires the use of precious metal catalysts and ultra-high purity water feed. The traditional alkaline water electrolyzer has the disadvantage of being difficult to start and regulate quickly due to power fluctuations, while with the development of wind power technology, the ability of wind farm power control systems to respond quickly to frequency regulation can be achieved by improving power supply technology, making it much more adaptable to alkaline water electrolyzers. Currently, alkaline electrolyzers for freshwater applications often use nickel screens with sprayed nickel alloy catalysts for the cathodes and nickel foam for the anodes. The composition of seawater is very complex, especially the high concentration of chloride ions can corrode the metal substrate and lead to catalyst deactivation, and calcium and magnesium ions can also block ion channels and active sites by scaling during the electrolysis process. Therefore, electrolytic cells for seawater use must be treated for corrosion resistance and pre-treatment of seawater. A recent study has introduced an intriguing approach to directly decompose seawater without the need for a purification step, as depicted in Fig. 7.5. Researchers have ingeniously combined a two-step process to simplify electrochemical seawater splitting. The method involves passive forward osmosis, which facilitates the continuous separation of seawater with minimal efficiency loss. While this water decomposition technique offers a novel means of directly cracking seawater, it does exhibit certain limitations that impact its practical applicability. Firstly, the absence of a separator between the electrodes in this system results in the coexistence of hydrogen and oxygen, posing a potential explosion hazard. Additionally, an additional step is required to separate the generated H2 and O2 gases effectively. Secondly, due to the imperfect selectivity of positive osmosis and reverse osmosis membranes, there remains the possibility of chloride ions passing through the positive osmosis membrane and being oxidized at the anode. Thirdly, the concentration gradient across the forward osmosis membrane may induce the reverse flow of solutes, leading to the loss of phosphate buffers into seawater. This phenomenon can have implications

650 ~ 1000

Y2 O3 /ZrO2

Solid oxide electrolyzer

60 ~ 80 60 ~ 80

20 ~ 30% KOH

Alkaline water electrolyzer

Temperature/°C

PEM electrolyzer Perfluorosulfonic acid proton membrane

Electrolyte

Technique

100

67 ~ 82

62 ~ 82

Efficiency/%

Table 7.2 Comparison of electrolytic hydrogen production technologies

> 3.7

4.2 ~ 6.6

4.5 ~ 6.6

Energy consumption/ [(kW·h)·m−3 ]

Metal Ceramic Oxide

Platinum-based metals

Ferronickel

Catalyst

< 10,000

20,000 ~ 60,000

60,000 ~ 90,000

Lifespan/h

7.3 Hydrogen Production Equipment by Seawater Electrolysis 181

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7 Hydrogen Production by Seawater Electrolysis

Fig. 7.5 a Basic composition of the electrolytic cell. b Basic working principle of the forward osmotic water decomposition cell [31]

for both economic considerations and environmental impact. These shortcomings need to be addressed to enhance the feasibility and safety of the approach, thereby ensuring its viability for practical implementation. The neutral pH of seawater poses challenges for the use of Proton Exchange Membrane (PEM) or Anion Exchange Membrane (AEM) technologies, as they are not designed to function efficiently under such conditions. Furthermore, the presence of impurities in seawater can cause damage to membranes, necessitating additional purification steps. However, electrolysis under neutral pH conditions offers several advantages. Firstly, neutral electrolysis of water reduces safety concerns associated with strongly acidic or alkaline electrolytes, making it a safer option. Additionally, constructing electrolytic cells and catalysts with less expensive materials becomes feasible in neutral environments, as they are less prone to corrosion compared to acidic or alkaline media. The non-auxiliary membrane electrolyzer, which does not rely on auxiliary membranes susceptible to impurities, holds the potential for direct utilization in seawater electrolysis. Furthermore, the pH value of the electrolyte and the ionic properties do not significantly affect ionic conduction through the liquid electrolyte. As a result, the unassisted membrane electrolytic cell can be employed in environments with varying pH solutions. Figure 7.6 provides an illustrative example of an electrolytic cell without an auxiliary membrane, showcasing the potential of this technology. Despite the advantages of the unassisted membrane electrolytic cell, it is not without its drawbacks. One such limitation is the lower voltage efficiency at high working current densities, primarily due to solution IR loss. The distance between the electrodes in the unassisted membrane electrolytic cell is significantly greater than that in PEM electrolytic cells using Nafion membranes. Although the ionic conductivity of added electrolytes, such as concentrated H2 SO4 and KOH, is higher than that of PEM or AEM, it does not fully compensate for the impact of the increased distance. The larger electrode gap results in higher ohmic resistance as ions are

7.3 Hydrogen Production Equipment by Seawater Electrolysis

183

Fig. 7.6 Different types of electrolytic cell without auxiliary membrane [32]

transported, leading to higher ohmic voltage losses. To address these challenges, microfluidic unassisted membrane electrolytic cells offer a promising alternative, as depicted in Fig. 7.7. In this configuration, two parallel electrodes are positioned with a very narrow electrode gap, typically on the scale of micrometers, usually around a hundred micrometers. The evolved gas moves in close proximity to the corresponding catalyst surface, accompanied by the electrolyte. By utilizing a membrane-less electrolyzer, concerns regarding membrane instability due to impurities in seawater can be avoided. Additionally, the microfluidic system helps reduce ohmic losses and enhances catalyst efficiency due to the larger specific surface area in miniaturized systems. Consequently, a microfluidic membrane-less electrode enables focused research on selective electro-catalysts. If an electro-catalyst can function in alkaline chloride-containing electrolytes, such as seawater, with an overpotential of less than 480 mV, it can achieve greater selectivity in the OER. The surface decoration of catalysts with anions has demonstrated effectiveness in mitigating electrode corrosion. Therefore, a microfluidic membrane-less electrolyzer operating in an alkaline medium below the 480 mV overpotential, along with a chloride retarding coating, holds significant potential for direct seawater electrolysis. Offshore has abundant renewable energy sources such as wind and solar energy, which is an excellent place for in-situ hydrogen production from renewable energy sources. However, the commonly used electrolytic water hydrogen production technology mainly uses pure water as raw material, and the lack of pure water supply under offshore conditions often limits the application of electrolytic hydrogen production from offshore renewable energy sources. If seawater with complex composition is used as raw material and electrolysis is added with alkali to produce hydrogen, among them, Ca2+ and Mg2+ ions will generate Mg(OH)2 and Ca(OH)2 precipitates under alkaline conditions, leading to clogging, corrosion and efficiency decay of hydrogen production equipment. Under the actual hydrogen production operating voltage of 1.8–2.0 V, the chlorination reaction will compete with the oxygen precipitation reaction, reducing the Faraday efficiency of electrolysis, and generating highly corrosive OCl− ions, triggering the rapid decay of the catalyst performance of the oxygen precipitation electrode. Therefore, the complex ionic composition of seawater has become a key bottleneck in direct seawater electrolysis for hydrogen

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7 Hydrogen Production by Seawater Electrolysis

Fig. 7.7 Schematic diagram of microfluidic electrolytic cell without auxiliary membrane [33]

production (electrolysis after alkali addition). The current technical route of seawater hydrogen production generally adopts the route of desalination followed by alkaline electrolysis to produce pure water in situ, such as using seawater desalination technologies like distillation, reverse osmosis and electrodialysis. At present, there are more seawater electrolysis hydrogen production projects at home and abroad, as shown in Table 7.3. In general, the “wind power platform + electrolysis equipment” hydrogen production method is suitable for new offshore wind farms, which can realize large-scale distributed hydrogen production by setting up water electrolysis hydrogen production equipment on the wind turbine platform, and the product hydrogen is directly sent out through the pipeline. The “old platform retrofit + electrolysis equipment” method of hydrogen production is to be built near the decommissioned oil and gas platforms and existing oil and gas pipeline accessories to reduce the cost of hydrogen production. The “new offshore platform + electrolysis

7.3 Hydrogen Production Equipment by Seawater Electrolysis

185

equipment” hydrogen production method is suitable for wind farms far away from the shore, and the new offshore hydrogen production platform can reduce the power transmission loss. From the perspective of energy consumption and economics of seawater hydrogen production, although some calculations show that the energy cost of desalination is not high for the post-electrolysis hydrogen production route, the high equipment investment and installation conditions in offshore scenarios still greatly limit the implementation of offshore renewable energy hydrogen production projects. Based on this, the development of catalysts capable of direct electrolysis of seawater or hydrogen production from alkaline seawater has become a hot research topic in recent years. Among them, the most commonly used development strategy is to prepare oxygen precipitation catalysts with a functional layer of chlorine-resistant surface. For example, Yu et al. effectively improved the chlorine corrosion resistance and activity of the classical NiFeN oxygen precipitation catalyst by nitriding it, and the resulting NiFeN oxygen precipitation catalyst could achieve a large hydrogen production current of 1000 mA cm−2 with an applied voltage of 1.709 V in alkaline seawater after adding 1 mol KOH, and the catalyst could work stably at a current Table 7.3 Domestic and foreign seawater electrolysis hydrogen production technologies and projects Project

Project type

Dolphyn project, UK

Start year

Target

Main participating institution/ company

Wind platform 2016 + electrolysis plant

2026 Stand-alone hydrogen production

Environmental Resources Management

Deep purple project, Norway

Wind platform 2018 + electrolysis plant

Complete offshore trials by 2025, large-scale operation by 2031

TechnipFMC, The Research Council of Norway

JIDAI program

New platform electrolyzer

Commercialization by 2030

Det Norske Veritas

Tractebel proposal

New platform 2019 + electrolyzer

Development of 400 MW offshore wind hydrogen platform

Tractebel Engineering, Tractebel Ovenlick

Lhyfe, France

New platform 2017 + electrolyzer

2025 offshore wind

Lhyfe

AuqaVentus, Germany

New platform 2020 + electrolyzer

2030 10 GW offshore wind Rwe

Wind power fusion demonstration project, Qingdao

New platform 2020 + electrolyzer

Achieve 2 million kW offshore wind power by 2025

2015

Zhongneng Fusion Offshore Wind Turbine Co.

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7 Hydrogen Production by Seawater Electrolysis

density of 500 mA cm−2 for more than 100 h. The amorphous NiFe oxide layer and hydroxyl NiFe generated on the catalyst surface were found to be effective in resisting the corrosion of the chlorination reaction products during the oxygen precipitation process. In addition, MnOx was also found to be an effective antichlorine functional layer, and Vos et al. demonstrated that the MnOx layer on the surface of IrO2 reduced the chlorination activity of the catalyst by blocking the transport of Cl− to the Ir active site, thereby reducing the chlorination reaction and thus enhancing the performance of the catalyst in seawater oxygen precipitation [34]. Similar surface functional layers include Co–Bi, Co–P, Ni–S, etc., all of which can effectively improve the chlorine resistance of direct electrolytic hydrogen production in seawater. However, unfortunately, there is no real engineering application demonstration of seawater direct hydrogen production to verify the effectiveness of the above catalyst surface functional layers. In addition to adding a surface functional layer, another strategy to solve chlorine corrosion is more radical, which is to decouple the oxygen precipitation process from the hydrogen precipitation process in time and space, as shown in Fig. 7.8. First, the oxidation half-reaction (such as Ni(OH)2 oxidation to NiOOH) occurs through the decoupling agent loaded on the positive electrode, and the hydrogen precipitation half-reaction forms a pair to complete the electrochemical reaction, so that the seawater decomposes and obtains hydrogen, but no oxygen precipitation occurs. After the circuit is disconnected, the decoupling agent is reduced and regenerated spontaneously, and oxygen is generated at the same time. Finally, the whole water decomposition process is completed. It can be found that the whole process, the original oxygen precipitation reaction on the positive pole is replaced by the oxidation and reduction of the uncoupling agent, as long as its oxidation potential is lower than the chlorination potential, and the reduction of the uncoupling agent is reversible and has excellent electrochemical kinetics, so it can greatly avoid chlorination.

Fig. 7.8 Basic principle of decoupled electrolytic water for hydrogen production [34]

7.4 Difference Between Seawater and Freshwater Electrolysis

187

The same idea can be applied to the decoupling of hydrogen precipitation reactions to solve the problem of calcium and magnesium ion precipitation when the pH value of the hydrogen precipitation side changes. For example, when silicotungstic acid is used as the decoupling agent for hydrogen precipitation, its good electrochemical reversibility allows the H2 production rate to reach more than 30 times the theoretical hydrogen production rate of the conventional PEM electrolytic water system, showing great potential. At present, this decoupled hydrogen production idea is still in the laboratory research stage, and the authors believe that further research should focus on the preferential selection of the decoupling agent and the optimized design of the decoupled electrolyzer. Theoretically, any redox agent with redox potential between the hydrogen precipitation reaction and oxygen precipitation reaction potential can be used as decoupling agent, such as potassium hydroquinone sulfonate, phosphomolybdic acid, polymetallic oxides, etc. The overall efficiency of decoupled hydrogen production depends largely on the reversibility of the redox reaction of the decoupling agent, but the high reversibility is generally accompanied by poor stability, such as easy oxidation and decomposition at high voltage, which leads to the decrease of hydrogen production capacity. The capacity of hydrogen production decreases. In addition, since the decoupling agent is generally solid and loaded on the electrode, how to increase the load of decoupling agent per unit tank volume (or electrode volume) and improve the hydrogen production capacity in a single decoupling cycle is another challenge to be faced before the real application of this technology of decoupled hydrogen production.

7.4 Difference Between Seawater and Freshwater Electrolysis At this stage, the research on hydrogen production by freshwater electrolysis has achieved outstanding results. However, the research on electrolysis of seawater for hydrogen production is still in its infancy. Freshwater resources are scarce and exist mostly in the form of frozen or chemically synthesized water. On the contrary, seawater is abundant around the world, comprising 96.5% of the earth water resources. Electrolysis of seawater as an electrolyte for hydrogen production is a promising method. However, up to 3.5 wt% salt is present in seawater compared to freshwater. These metal salt ions participate in the competing electrochemical cathode HER reaction, which greatly limits the efficiency of water electrolysis [34, 35]. In addition, the presence of bacteria and microorganisms in natural seawater can lead to the formation of insoluble precipitates at the active sites on the catalyst surface, resulting in reduced HER performance. On the other hand, high concentrations of chloride ions (about 0.5 M) can block the active center of the catalyst. In addition, chlorine evolution reaction (ClER) can also occur as a competing reaction for anode OER (Fig. 7.9). The theoretical overpotential (1.23 V vs. SHE) of the four-electron transfer OER under acidic conditions at pH = 0 is 130 mV lower

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7 Hydrogen Production by Seawater Electrolysis

Fig. 7.9 Advantages and challenges of MW seawater electrolysis [37]

than that of the two-electron transfer ClER, resulting in faster kinetics [36]. Since the overpotential of ClER does not change with pH, the inhibition of ClER can be achieved by increasing the alkalinity of the electrolyte. However, the chlorate formation reaction takes place in an alkaline environment [10]. Therefore, in order to realize high-efficiency hydrogen production and industrial seawater electrolysis, it is necessary to systematically summarize and deeply understand the difference between freshwater hydrogen production and seawater hydrogen production and the related parameters that affect the performance. Water electrolysis is a thermodynamically inclined chemical process with a Gibbs free energy of about 237.2 kJ mol−1 [38]. The process of freshwater electrolysis consists of anode OER and cathode HER. HER is a two-electron transfer process, and OER is a four-electron transfer process, generating hydrogen and oxygen molecules through several proton and electron coupling steps, respectively. The overpotential is the excess potential above the total water splitting thermodynamic value of 1.23 V. It can be seen from formulas (7.11–7.14) that water can be decomposed in different pH environments. In acidic pH media: Anode : 2H2 O = O2 + 4H+ + 4e−

(7.11)

Cathode : 2H+ + 2e− = H2

(7.12)

Anode : 4OH− = O2 + 2H2 O + 4e−

(7.13)

In alkaline pH media:

7.4 Difference Between Seawater and Freshwater Electrolysis

Cathode : 2H2 O + 2e− = H2 + 2OH−

189

(7.14)

Compared to fresh water, seawater contains additional salt, which makes the electrolysis reaction more complicated. The effects of various cations and anions in seawater on electrolyzed water are discussed. Seawater shows high ionic conductivity due to the presence of up to 3.5 wt% salt [39]. Sodium ions (Na+ ), potassium ions (K+ ), calcium ions (Ca2+ ), magnesium ions (Mg2+ ), chloride ions (Cl− ) and sulfate SO4 2− account for more than 99% of the total salt in seawater. 3.5 wt% NaCl solution was the most commonly used electrolyte to simulate seawater in laboratoryscale studies. It has also been reported to introduce some Mg2+ , Ca2+ , K+ and SO4 2− to simulate natural seawater. Complex ions in seawater can increase the ionic conductivity of seawater. However, they also create complications in seawater electrolysis. For example, when H+ is consumed, the resulting OH reacts with Ca2+ and Mg2+ to form insoluble precipitates. Insoluble precipitates on the electrode surface may block the active sites and hinder further reactions. Compared to fresh water, the presence of chloride ions in seawater has an effect on both the cathode and the cathode [40]. Chloride ions on the cathode side can inhibit HER by blocking the active sites of the catalyst, leading to accelerated degradation of the cathode catalyst in the reaction. A large amount of chlorine or hypochlorite may be produced on the anode side because chloride ions may undergo reduction reactions that compete with OER. Equations (7.15) and (7.16) describe the reduction reaction of chloride ions at the anode [41]. In acidic pH media: 2Cl− = Cl2 + 2e−

(7.15)

Cl− + 2OH− = ClO− + H2 O + 2e−

(7.16)

In alkaline pH media:

The reduction reaction of chloride ion undergoes only two electrons, which may have faster kinetics than the four-electron transfer OER. In addition, it can be known from the above equation that the pH value of the electrolyte will affect the reduction of chloride ions, and the anode reaction in seawater electrolysis can be controlled by adjusting the pH value. According to the Pourbaix plot, the OER is more favorable at pH > 7.5 when the overpotential is lower than 480 mV. In this case, catalysts with overpotentials less than 480 mV are more prone to OER at the anode [1].

7.4.1 pH Effects on Seawater and Freshwater Electrolysis Due to the ion exchange and transport during the electrolysis of fresh water for hydrogen production, the local pH dynamics of the electrodes are changed. This

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7 Hydrogen Production by Seawater Electrolysis

pH change affects the stability and catalytic activity of the catalyst. Although the carbonate in seawater can act as a buffer, its capacity is not sufficient to prevent an increase in the local pH at the cathode and a decrease in the local pH at the anode [19]. Studies have shown that when the overall pH is between 4 and 10, the pH change near the electrode surface may be on the order of 5–9 pH units even in mildly buffered media at moderate currents (10 mA cm−2 ). Such drastic pH fluctuations can lead to catalyst degradation. The local pH increase near the cathode during seawater (non-artificial buffering) electrolysis can lead to precipitation of magnesium hydroxide (Mg(OH)2 ), which occurs at pH ~ 9.5, blocking the cathode. Stabilizing pH fluctuations may require the addition of supporting electrolytes [42]. In addition, the pH of seawater electrolysis for hydrogen production has always been the most important factor determining its working efficiency and stability. This is due to the presence of a large amount of chloride ions in seawater, resulting in electro-oxidation of chlorides. Chloride electrooxidation is a complex chemical reaction in which multiple reactions occur depending on pH, applied potential and temperature. For simplicity, if a temperature of 25 °C is considered and the total chloride concentration is fixed at 0.5 M (a typical chloride concentration in seawater). At pH below 3.0, the free chlorine precipitation reaction (ClER) is superior to other chlorine oxidation reactions. Hypochlorite generation may also occur at low pH and high anodic potential, but becomes the dominant reaction at pH 3–7.5. Hypochlorite generation occurs at pH above 7.5, which represents the pKa of hypochlorite. partial dissociation (i.e., chlorine dissolved in water) and disproportionation (i.e., hypochlorite ion subjected to higher temperatures) complicate the chemistry of the chlorine species. Also, there is a difference between freshwater and seawater hydrogen production in terms of OER [19]. In contrast to OER, competitive chlorine oxidation is thermodynamically unfavorable and the difference between standard electrode potentials increases with pH until the onset of hypochlorite formation, which remains at its maximum value ~ 480 mV. In other words, under alkaline conditions, water oxidation catalysts can exhibit kinetic overpotentials up to 480 mV without disturbing the chlorine chemistry.

7.4.2 Requirements of Electrolytic Seawater Cells Currently, two technologies, alkaline water electrolyzer (AWE) and proton exchange membrane water electrolyte (PEMWE), have proven to be the dominant low temperature (< 100 °C) water electrolysis technologies in the commercial market. Other emerging technologies include low temperature anion exchange membrane water electrolyzers (AEMWE), and high temperature electrolysis such as proton conductive ceramic electrolysis (~ 150–400 °C) and solid oxide electrolysis (~500–800 °C) [43]. These four configurations are shown in Fig. 7.10. These electrolytic techniques use ultra-pure deionized 18.2 MΩ-cm water or 20– 30% aqueous KOH solutions with contaminants at or below the ppm level. Such high levels of water purity are chosen to avoid complications associated with catalyst

7.4 Difference Between Seawater and Freshwater Electrolysis

191

Fig. 7.10 a Alkaline water electrolyte (AWE) operating as a 2-chamber cell in which a liquid alkaline electrolyte (typically 20–30% KOH) is pumped around both sides and a porous diaphragm allows hydroxyl ion (OH− ) migration while preventing gas crossover. b Anion exchange membrane water electrolyzer (AEMWE) with an OH− transport membrane. In this example, water is supplied to the cathode, but can also be supplied to the anode or both. c Proton exchange membrane water electrolyte (PEMWE) consisting of a solid acid electrolyte polymer sandwiched between the anode and cathode. In most cases, water is supplied to the anode only. d High temperature water electrolytes include proton conductive ceramic electrolytes (150 ~ 400 °C) and solid oxide electrolytes (800 ~ 1000 °C). Water evaporates and is transported as steam to the cathode to produce hydrogen, while the solid oxide or ceramic membrane transports oxygen to the anode [43]

operation, membrane operation, and general component degradation. The severity of the challenges associated with using impure water depends to some extent on the electrolysis configuration. PEMWE contains a solid acid polymer electrolyte (e.g. Nafion) between the anode and the cathode. In most cases, water is supplied only to the anode, where it is oxidized to form O2 and H+ . The protons then migrate through the PEM towards the HER catalyst (cathode). In this approach, the low pH medium provided by the PEM complicates the anode chemistry and the selectivity of the OER for the chloride oxidation reaction becomes challenging. In addition, the commonly used Nafion membrane acts

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as a cation transporter and is susceptible to retention and concentration of foreign ions, especially cationic impurities, resulting in reduced proton conductivity. The PEM can isolate certain impurities at the anode, however cationic species such as metal ions and Mg2+ will still migrate from the anode to the cathode. Water can be supplied to the cathode only, thus minimizing any interaction between the chloride ions and the anode. In this case, water migrates through the membrane where it is oxidized and protons are transferred back to the cathode for H2 production. However, the electrolyte and impurities still migrate to the anode to some extent. Due to the transport through the membrane, both configurations may lead to contact between the impurities and the cathode, possibly leading to metal or salt deposition. The AWE is a two-chamber cell separated by a porous diaphragm that allows OH− migration while preventing gas crossover. The liquid alkaline electrolyte is pumped to both sides of the cell where water is reduced to hydrogen and hydroxide at the cathode. OH− then migrates through the diaphragm to the anode formed by O2 . Diaphragm materials for AWE are reported to be physically stronger and less prone to clogging than membranes (PEM/AEM). PEM and AEM mostly block anions or cations, respectively, while both H+ , Na+ , OH− and Cl− can migrate through the diaphragm, which should be considered during system design. This may be problematic if it decreases the migration number of most mobile ions [44]. In AEMWE, the anion exchange membrane is sandwiched between the anode and cathode. Water can be supplied to the cathode, the anode or both sides. H2 and OH− are produced at the cathode, and OH− migrates through the membrane to the anode where O2 is produced. The membrane itself has the same limitation as AWE in that it causes harmful migration of anions such as Cl− , meaning that no matter where the electrolyte is injected, OH− and Cl− oxidation is a problem. The high working pH of AWE and AEMWE helps to reduce chloride oxidation, making them particularly suitable for the decomposition of brine [3]. High temperature water electrolytes include proton conducting ceramic electrolysis (150 ~ 400 °C) and solid oxide electrolysis (800 ~ 1000 °C). Water evaporates and is transported as steam to the cathode to produce hydrogen gas. The solid oxide or ceramic membrane selectively delivers O2 to the anode to form O2 . This configuration provides an opportunity to purify the water source (producing clean steam) before it reaches the catalyst and membrane. Thus, this technology has the potential to open a design window beyond other technologies studied for electrode materials. However, high temperature operation implies higher energy requirements and higher operating costs than competing technologies that typically operate below 100 °C. In addition, high temperatures limit the types of electrode materials and other electrolytic components required to meet the stability requirements for longterm operation. These challenges may hinder their potential installation in offshore facilities and large wind farms, making them more suitable for coastal installations [68]. Unlike freshwater electrolysis, problems common to all four configurations in seawater electrolysis include physical clogging of the catalyst or separator material by solid impurities, precipitates and microbial contamination. Therefore, a simple filtration of brine or low-grade water is essential to avoid membrane clogging. Membrane activity can be maintained through recovery procedures. For example, periodically

7.5 Summary

193

placing the electrolyte in an open circuit has been shown to restore some of the activity lost due to clogging of the membrane by chloride ions. Metal parts are also at risk of corrosion. For example, in PEM electrolyzers, the current collector and separator plates are usually made of titanium, graphite, or coated stainless steel, and the lifetime of these materials, especially in the presence of Cl− , should be carefully considered [45].

7.5 Summary To enable the electrolysis of impure or saline water, several considerations must be considered to ensure its feasibility. The selection of appropriate membranes is crucial for the construction of efficient electrolyzers using seawater or low-quality water without extensive purification or treatment. Conventional membrane and diaphragm technologies are vulnerable to foreign ion transport and clogging. However, the full understanding of the impact of these factors on system activity and lifetime is still lacking, making further investigation into the effect of impurities on membrane clogging highly valuable. At the anode, successfully splitting brine requires overcoming the competition between chloride chemistry and water oxidation. Achieving selectivity in oxygen precipitation is possible under alkaline conditions, as demonstrated in highly alkaline systems with a pH of approximately 13 containing NaCl. However, when transitioning to real seawater with high pH, additional challenges may arise regarding precipitation formation when the seawater is adjusted to a pH greater than approximately 10. By carefully considering the Pourbaix plot, there may be an opportunity to operate at pH levels around 8–9 while maintaining oxygen selectivity. In such cases, precise pH control becomes a critical task and may necessitate the use of a strong buffer. Other strategies for operating anodes and cathodes in impure water involve selective ion transport through membranes, the formation of penetration-selective materials or barriers on the catalyst surface, and the exploration of electrode materials with catalytic selectivity sites that support the desired reaction while minimizing side reactions and catalyst poisoning. Furthermore, the development of seawater electrolysis catalysts with high activity and selectivity is crucial to mitigate the influence of ions and impurities present in seawater.

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7.6 Current Status and Development Direction of Seawater Electrolysis 7.6.1 Current Status of Seawater Electrolysis Research Two methods of electrolysis of seawater were first proposed by Williams as early as 1975 [46]. One was to perform conventional electrolysis after desalination and de-hybridization of seawater to form distilled water. However, at that time, the desalination process of seawater was costly in terms of investment and the improper treatment of the removed impurities could cause environmental problems. The other is electrolysis based on natural seawater, which not only reduces the investment cost but also may recover a large amount of non-reactive metals present in seawater. But the problems of corrosion and bad electrochemical products in electrolytic process should be solved. Meanwhile Williams conducted experiments on direct electrolysis of seawater using a double carbon electrode and showed that H2 can be produced by direct electrolysis of seawater, and at a current density (90 mA cm−2 ) with maximum hydrogen precipitation efficiency (89%), O2 and Cl2 are produced simultaneously at the anode, and the Cl2 yield initially exceeds the O2 yield. Insoluble precipitates also form on and near the cathode electrode surface resulting in reduced device hardware lifetime. Afterwards, to solve the problems encountered by Williams in direct electrolysis of seawater, numerous researchers have studied HER/OER catalysts for electrolysis of seawater. So far, OER catalysts with higher performance for electrolysis of seawater have been found to include transition metal-doped manganese-based oxides, cobalt phosphate salts, nickel–iron composite layered hydroxides (NiFe-LDH), etc.; while HER catalysts mostly contain metal elements such as Co, Cu and Ni. Earlier studies of electrolytic seawater focused on the preparation of high performance OER catalysts for use at the anode, believing that the chemical reactions occurring at the cathode appear to have simpler requirements for catalyst materials because there are no competing reactions at the cathode where the reaction equilibrium potentials are closer as in the case of OER and ClER. However, the presence of interfering ions in natural seawater can cause cathode catalyst poisoning or accelerate catalyst degradation, resulting in higher operating voltages required for the electrolysis process and higher costs for hydrogen production. Therefore, the development of cathode HER catalyst materials will not only lead to the search for inexpensive nonprecious metal catalysts to replace the expensive metal Pt, but also to the discovery of more HER catalysts with better performance. Previous studies have shown that, unlike the anode chamber, there are few reactions that compete with HER in seawater electrolytes. However, the main problem of cathodic HER is the presence of impurities in seawater, which leads to active site blockage corrosion, low efficiency and poor stability [47]. The main electrocatalysts reported for seawater electrolytic HER are noble metal alloys, carbon-loaded noble metals, MXene based complexes, metal phosphides, metal oxides, metal nitrides, and hybrid electrocatalysts (Table 7.4).

7.6 Current Status and Development Direction of Seawater Electrolysis

195

Table 7.4 Summary of HER electrocatalyst performance Catalyst

Electrode

Electrolyte

Onset potential (mV)

Overpotential @10 mA cm−2 (mV)

Tafel slope (mV dec−1 )

Pt

TM

Seawater

151.80

285

45.8

Pt–Ru–Cr

TM

Seawater

129.89

256

45.7

Pt–Ru–Fe

TM

Seawater

125.92

248

45.2

Pt–Ru–Co

TM

Seawater

112.79

222

44.8

Pt–Ru–Ni

TM

Seawater

103.25

206

44.5

Pt–Ru–Mo

TM

Seawater

96.22

196

44.0

Pt/C

GCE

Seawater

185

PtNi5

GCE

Seawater

380

PtCr0.1

TM

Seawater

149.93

Ti/NiPt

TF

Seawater

230

111

Ti/NiAu

TF

Seawater

410

170

RuCo

TF

Seawater

253

107

RuCoMo1

TF

Seawater

354

NiRuIr-G

59 119 266.5

137

Seawater

80

0.5Rh-G1000

GCE

Seawater

340

Pt@mh-3D MXene

GCE

Seawater

280

VS2 @V2 C

GCE

Seawater (pH = 0)

148 (20 mA cm−2 )

h-MoN@ BNCNT

GCE

Seawater

NiCoP/NF

NF

Seawater

287

C-Co2 P

GCE

1 M KOH

30

Ru-CoOx

NF

Seawater (1 M KOH)

630 (100 mA cm−2 )

Mo5 N6

GCE

1 M KOH

94

Ni–SN@C

GCE

1 M KOH

28

Ni–SN@C

GCE

Seawater (1 M KOH)

23

NiCoN|NixP| NiCoN

NF

Seawater

165

Ni5 P4 / Ni2+δ Oδ (OH)2-δ

CC

Seawater

144

37 128

Note TM, titanium mesh; TF, titanium foil; PBS, phosphate buffer solution; GCE, glassy carbon electrode; NF, nickel foam; CC, carbon cloth; NiRuIr-G, graphene-loaded NiRuIr; 0.5Rh-G1000, Rh-loaded N-doped carbon nanosheets; mh-3D Mxene, multilayer hollow Mxene; BNCNT, boronnitrogen co-doped carbon nanotubes

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7 Hydrogen Production by Seawater Electrolysis

Precious Metal-Based Catalysts

Noble metals (e.g. Pt and Pd) and their alloys with high electrical conductivity, good stability and good catalytic activity are widely used HER catalysts nowadays. In the volcano diagram, Pt is located at the top and represents the most promising electrode material, superior to other noble metal-based electrocatalysts. However, the high cost hinders its large-scale application in industry. Alloying Pt with other transition metals is a good solution for cost reduction. In general, transition metals have three-dimensional orbitals. The alloying process of Pt changes the outermost electronic state of the Pt atom and affects the HER process. For example, alloying Pt with Fe, Co, Mo, Cr and Ni atoms can significantly reduce the overpotential and Tafel slope of the catalyst [48]. Furthermore, the addition of foreign atoms such as Fe, Co, Mo, Cr, and Ni effectively mitigated the effect of Cl2 on Pt, thus improving the stability of the catalyst. The Ti/PtMo electrode showed the highest catalytic activity and maintained an initial current density of 91.13% after 172 h of operation in real seawater [14]. PtNix alloys were also used to catalyze HER reactions in seawater cracked by adjusting the stoichiometric ratio of Pt/Ni, the final PtNi5 electrode showed the best catalytic performance with a minimum starting potential of 0.38 V and good stability over 12 h of operation. In addition to Pt, other noble metals such as Au, Ru and Ir have also been investigated for the electrolytic hydrogen production from seawater. Zhang et al. prepared NiAu and NiPt alloys on titanium foil at starting potentials of 230 mV and 410 mV, respectively, but the NiPt electrode still showed good catalytic activity for the HER process [49]. Niu et al. also prepared RuCo and RuCoMox alloys for seawater catalytic hydrogen production [50]. The titanium foil-supported RuCo and RuCoMox alloy electrodes required overpotentials of 387 and 550 mV, respectively, to achieve a current density of 10 mA cm−2 .

7.6.1.2

Carbon-Loaded Metal Catalysts

Carbon-loaded metal catalysts are receiving increasing attention due to their tunable surface physicochemical properties and simple synthesis process. Modifying the surface of metal particles using metal–carbon interactions is a good way to improve the catalytic activity. It is usually required to optimize the surface and interface of carbon-loaded metal catalysts to make the metal loading uniformly dispersed. Liu et al. successfully synthesized N/S co-doped carbon nanosheets loaded with Rh nanoparticles. This thin mesoporous nanosheet with a high specific surface area of 437.1 m2 g−1 allowed the Rh nanoparticles to be uniformly dispersed [51]. The doping of S enhanced the interaction between the Rh nanoparticles and the carbon carrier and promoted the electron deflection and transfer from Rh to the Rh-carbon interface. The results show that this Rh-based catalyst exhibits extremely high catalytic activity in seawater with a loading of only 0.5 wt%, which is comparable to the performance of commercial 20% Pt/C. The catalyst also provided a current density of 15 mA cm−2 with good stability over 10 h. With its high stability and electron mobility, graphene

7.6 Current Status and Development Direction of Seawater Electrolysis

197

is a promising carbon-based carrier for catalysts. The use of graphene as a carrier for noble metal catalysts ensures high catalytic efficiency, improves the stability of noble metals in chloride-rich electrolytes, and reduces the number of noble metals used. Sarno et al. synthesized a graphene-loaded nickel, ruthenium, and iridium-based nanocatalyst [52]. Ir elements ensured high hydrogen precipitation activity in acidic and seawater environments, enhancing the alloy’s stability. In addition, the different work-up of each metal in the alloy promotes the electron aggregation on the Ir surface, which further enhances the activity. In addition, the negative charge accumulated on Ir prevents the attack of chloride ions. The highly conductive graphene network stabilized the nanoparticles immobilized on it and reduced the charge transfer resistance on the electrode. In 0.5 M H2 SO4 , the overpotential remained at 0.06 V with a Tafel slope of 28 mV even after 11,000 cycles. after 250 cycles in real seawater, the Tafel slope of the sample was 48 mV dec−1 with a lower overpotential of 0.08 V and no significant loss of current density. The results indicate that the synthesized electrocatalysts have good HER performance in real seawater due to the presence of graphene carriers and synergistic alloying effect.

7.6.1.3

Metal Phosphide Catalysts

The electronegative P atoms in metal phosphide can trap positively charged protons and provide higher activity for HER at the same time. Metal phosphides can exhibit good electrical conductivity when the ratio of metal to phosphorus atoms is appropriate. CoMoP@C shows good catalytic activity, close to 20% Pt/C at pH = 0–1 and better than 20% Pt/C at high overpotential at pH = 2–14. The stronger proton absorption ability of the carbon shell layer effectively contributes to the performance of HER. Meanwhile, the carbon shell on the CoMoP core protected the catalyst from corrosion, agglomeration and poisoning in seawater. As a result, CoMoP@C showed excellent HER performance in real seawater. Lv et al. prepared porous plume NiCoP electrocatalysts on nickel foam using hydrothermal and phosphorylation methods [53]. Meanwhile, their porous structure and conductive substrate facilitate the increase of the specific surface area and expose more active sites, which also facilitate the release of the generated H2 . In addition, the synergistic effect of the three-dimensional pore structure, electronic effects and conductive substrate greatly improves the electrochemical stability. NiCoP/NF exhibits good activity and stability in seawater with a current density of 10 mA cm−2 and an overpotential of 287 mV. Recently, Tian et al. grew core/shell type PSS-PPy/Ni-Co-P HER electrocatalysts on copper foil in an orderly manner with Ni–Co–P as the core and pyridine-poly (sodium benzene sulfonate) hybrid polymer as the shell [54]. The electrical conductivity, hydrophilicity and electronic structure of Ni–Co–P were optimized using PSS-PPy. Another carbon-doped nanoporous cobalt phosphide C–Co2 P also showed excellent catalytic activity in a mixture of NaCl, MgCl2 and CaCl2 chlorides in an artificial alkaline seawater electrolyte. The electronegative carbon atom with small atomic radius tuned the electronic structure of Co2 P, weakened the Co–H bond and finally promoted the HER kinetics.

198

7.6.1.4

7 Hydrogen Production by Seawater Electrolysis

Metal Oxide Catalysts

Metal oxides are considered as an effective HER catalyst because of their diverse crystal structures, high abundance, high catalytic activity, and Pt-like structure in natural seawater, and high abundance and catalytic activity in natural seawater properties. However, there is still a long way to go for the industrialization of natural seawater. Mn–doped nickel oxide/nickel (Mn–NiO/Ni) as HER catalysts were prepared by pyrolysis of Mn–MOF/Ni–F precursors in an inert environment, where the Mn–MOF/Ni–F precursors are in an inert environment and nickel foam is used as a substrate to provide elemental nickel interaction with Mn–MOF [47]. Amal et al. designed carbon-based NiO/Ni for HER using a simple and controllable method, and found that the degree of HER and the degree of oxidation of Ni had a significant effect on the performance [55]. The doping of noble metal atoms in metal oxides can modulate the electronic structure of the oxides and also of the catalysts, where Ru is more cost-effective than other noble metals. Since the binding energy of Rh to hydrogen is similar to that of Pt, it is potentially advantageous and promising in HER electrocatalysis. In addition, the structure of metal oxide materials also plays a modulating role on their electrocatalytic performance. The arrangement of atoms in amorphous materials can provide a large number of exposed surfaces and defects [56]. Wu et al. constructed ultra-low Ru amorphous cobalt-based oxides Ru-CoOx / NF [57]. The amorphous structure provides more active sites for electrocatalytic reactions. In addition, the incorporation of Ru elements promotes internal charge transfer for better performance. These positive factors effectively drive its electrolysis under high current density conditions in seawater media.

7.6.1.5

Metal Nitride Catalysts

Metal nitrides show great potential in seawater electrolysis due to their high electrical conductivity and excellent corrosion resistance. In general, alloying, vacancy engineering, heterogeneous element doping, and interfacial engineering can further improve the catalytic efficiency of metal nitrides, thus compensating for the inherent defects of metal nitrides. Jin et al. synthesized atomic-scale Mo5 N6 nanosheets using a transition metal-catalyzed phase change approach combined with a twodimensional lateral growth method [36]. Compared with conventional nitrogendeficient metal nitrides, the two-dimensional Mo5 N6 nanosheets prepared by metalrich-nitrogen bonding exhibited excellent HER performance in natural seawater and remained stable at high currents for 100 h. The performance was much better than that of Pt/C benchmark labeled and other metal nitrides. The high activity of Mo5 N6 originates from its Pt-like electronic structure. Its stability comes from the high valence state of its Mo atom, which makes it relatively immune to active site poisoning by other ions in seawater. In addition, the electronic structure of the metal nitride can be adjusted by adjusting the proportion of nitrogen atoms in the metal matrix. The nitrogen content in metal nitrides is controlled by nitrogen enrichment processes and incomplete nitration processes. The purpose of the nitrogen enrichment process

7.6 Current Status and Development Direction of Seawater Electrolysis

199

is to embed additional nitrogen atoms in the metal nitride lattice, but this process is often only possible under high temperature and pressure. The incomplete nitridation process can promote the formation of metal/metal nitride interfaces, which usually have better electrical conductivity and electrocatalytic properties. Jin et al. synthesized nickel surface nitrides (Ni–SN@C) encapsulated in a carbon shell layer using an unsaturated nitridation process [58]. In contrast to conventional transition metal nitrides or metal/metal nitride heterostructures, no bulk nickel nitride phase with unsaturated Ni–N bonds was detected on the surface of Ni–SN@C. Ni–SN@C has a unique unsaturated surface Ni–N bond with metallic nickel as the main chemical component. The overpotential of Ni–SN@C catalyst in alkaline seawater was as low as 23 mV at a current density of 10 mA cm−2 . This strategy not only produced catalysts with desirable metal nitride properties, but also promoted the redistribution of the catalyst surface charge, resulting in good corrosion resistance and high HER activity in seawater electrolysis.

7.6.2 Development Direction of Seawater Electrolysis Seawater, being an abundant resource on Earth, holds the potential to address the energy crisis by serving as a source for hydrogen production through electrolysis. Despite the considerable efforts dedicated to seawater electrolysis technology in recent years, there is still significant room for improvement to achieve efficient hydrogen production. Seawater electrolysis is inherently more intricate than freshwater electrolysis due to the diverse range of cations and anions present in seawater. Extensive research has been conducted on various materials for seawater electrolysis, including metal oxides, metal nitrides, metal phosphides, metal borides, and precious metal alloy catalysts. However, most of the reported catalysts do not exhibit the desired catalytic activity and stability necessary for practical applications. Analysis of Table 7.4 reveals that the overpotentials of these catalysts are still relatively high. Furthermore, a majority of the electrolyte studies thus far have utilized artificial brine rather than real seawater, adding to the complexity of seawater electrolysis for hydrogen production. To achieve high-performance seawater electrolysis, researchers need to dedicate further efforts in the following directions: (1) Reaction pathways and active sites: Combining experimental and theoretical analyses can provide valuable insights into the reaction pathways and active sites of catalysts used in seawater electrolysis. The complexity of catalyst components in multi-metallic compounds and heterostructured catalysts necessitates systematic studies to understand their electrocatalytic behavior and guide the design of materials with desired structures and properties. (2) In situ characterization: To reveal the true active sites of catalysts, in situ characterization methods are crucial. The electrolysis process can induce surface oxidation or reconstitution of catalysts, potentially altering their active sites.

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7 Hydrogen Production by Seawater Electrolysis

In situ techniques such as X-ray absorption spectroscopy (XAS), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) can track the changes of intermediates during catalytic reactions, providing insights for the design of efficient catalysts. (3) High-performance electrocatalysts: Developing electrocatalysts with high activity and stability in seawater is essential. The presence of various cations and chloride ions in seawater can interfere with water splitting reactions. Optimizing the electronic structure of catalysts through alloying, vacancy engineering, heterogeneous element doping, and interfacial engineering can enhance catalytic performance. Hybrid catalysts combining different active materials can also be explored for high performance. (4) Advanced reactor design: While catalysts play a crucial role, considering the entire reactor system is equally important. Rational design of reactors dedicated to seawater electrolysis can enhance efficiency and address challenges specific to seawater electrolysis. Asymmetric reactor designs, such as separating the anode and cathode chambers, can facilitate Cl− diffusion and protect the anode catalyst, contributing to improved performance and durability. It is important to note that the current direct hydrogen production technology from seawater is still in the stage of development and verification. Challenges such as Cl2 precipitation, fouling, membrane contamination, and corrosion need to be addressed. Techno-economic assessments for direct electrolysis of seawater are lacking, and there is a need to accurately evaluate the costs and benefits of this technology. Additionally, the stability and efficiency of electrolytic catalysts and cells remain significant challenges. While there are no major technical obstacles in theory, practical issues in the production process, including process connection, storage, and transportation, still need to be resolved through demonstrations and further research. By addressing these areas, researchers can advance the field of seawater electrolysis and pave the way for high-performance systems capable of efficient hydrogen production.

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40. M.M. Ayyub, M. Chhetri, U. Gupta, A. Roy, C.N.R. Rao, Chem. Eur. J. 24, 18455–18462 (2018) 41. I. Sohrabnejad-Eskan, A. Goryachev, K.S. Exner, L.A. Kibler, E.J.M. Hensen, J.P. Hofmann, H. Over, ACS Catal. 7, 2403–2411 (2017) 42. (a) I. Katsounaros, J.C. Meier, S.O. Klemm, A.A. Topalov, P.U. Biedermann, M. Auinger, K.J.J. Mayrhofer, Electrochem. Commun. 13, 634–637 (2011); (b) M. Auinger, I. Katsounaros, J.C. Meier, S.O. Klemm, P.U. Biedermann, A.A. Topalov, M. Rohwerder, K.J.J. Mayrhofer, Phys. Chem. Chem. Phys. 13, 16384–16394 (2011) 43. S. Jiang, H. Suo, T. Zhang, C. Liao, Y. Wang, Q. Zhao, W. Lai, Catalysts 12, 123 (2022) 44. M. Schalenbach, W. Lueke, D. Stolten, J. Electrochem. Soc. 163, F1480 (2016) 45. S. Kumari, R. Turner White, B. Kumar, J.M. Spurgeon, Energy Environ. Sci. 9, 1725–1733 (2016) 46. P. Farràs, P. Strasser, A.J. Cowan, Joule 5, 1921–1923 (2021) 47. X. Lu, J. Pan, E. Lovell, T.H. Tan, Y.H. Ng, R. Amal, Energy Environ. Sci. 11, 1898–1910 (2018) 48. H. Li, Q. Tang, B. He, P. Yang, J. Mater. Chem. A. 4, 6513–6520 (2016) 49. Y. Zhang, P. Li, X. Yang, W. Fa, S. Ge, J. Alloys Compd. 732, 248–256 (2018) 50. X. Niu, Q. Tang, B. He, P. Yang, Electrochim. Acta 208, 180–187 (2016) 51. Y. Liu, X. Hu, B. Huang, Z. Xie, A.C.S. Sustain, Chem. Eng. 7, 18835–18843 (2019) 52. M. Sarno, E. Ponticorvo, D. Scarpa, Electrochem. Commun. 111, 106647 (2020) 53. Q. Lv, J. Han, X. Tan, W. Wang, L. Cao, B. Dong, ACS Applied Energy Materials 2, 3910–3917 (2019) 54. F. Tian, S. Geng, L. He, Y. Huang, A. Fauzi, W. Yang, Y. Liu, Y. Yu, Chem. Eng. J. 417, 129232 (2021) 55. E.C. Lovell, X. Lu, Q. Zhang, J. Scott, R. Amal, Chem. Commun. 56, 1709–1712 (2020) 56. S. Anantharaj, S. Noda, Small 16, 1905779 (2020) 57. D. Wu, D. Chen, J. Zhu, S. Mu, Small 17, 2102777 (2021) 58. H. Jin, X. Wang, C. Tang, A. Vasileff, L. Li, A. Slattery, S.-Z. Qiao, Adv. Mater. 33, 2007508 (2021)

Chapter 8

Storage and Application of Hydrogen Energy

8.1 Storage and Transport of Hydrogen The common isotope of hydrogen, H, contains one proton and one electron and has a relative atomic weight of one. In 1932, the preparation of a stable isotope, deuterium (D), with an atomic weight of 2 (1 proton and 1 neutron plus 1 electron) was announced. Two years later, an unstable isotope tritium (T) with an atomic weight of 3 (1 proton, 2 neutrons, and 1 electron) was discovered. Among them, tritium (T) is radioactive and has a half-life of 12.5 years. Deuterium atoms can be found one by one in 6000 ordinary hydrogen atoms. Tritium atoms are also present but in much smaller proportions. Since there is only one extranuclear electron in the hydrogen atom, all isotopes of hydrogen react together to form covalent molecules such as H2 , D2, and T2 , respectively. Hydrogen has different reaction characteristics from other elements. It appears in ionic compounds in the form of anion (H− ) or cation (H+ ) and can participate in the formation of covalent bonds together with electrons. Hydrogen molecule H2 can exist in different forms depending on the temperature and pressure. As shown in Fig. 8.1, hydrogen has a solid density of 70.6 kg m−3 at a low temperature of –262 °C. Hydrogen is a gas with a density of 0.089886 kg m−3 at 0 °C and a pressure of 1 bar. A small region from the triple point to the critical point at 253 °C shows hydrogen as a liquid with a density of 70.8 kg m−3 . Hydrogen at ambient temperature (298.15 K) is a gas and can be described by the van der Waals equation, namely: P(V) =

n2 nRT −a 2 V − nb V

(8.1)

Where P is the pressure, V is the volume, n is the number of moles, R is the gas constant (R = 8.314 J K−1 mol−1 ), T is the absolute temperature, a is the dipole interaction or repulsion constant (a = 2.476 × 10−2 m6 Pa mol−2 ) and b is the volume occupied by hydrogen molecules (b = 2.661 × 10−5 m3 mol−1 ). Due to the

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_8

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Fig. 8.1 Phase diagram of hydrogen. Liquid hydrogen exists only in the solidus and the region from the triple point at 13.9 K to the critical point at 32 K [1]

strong repulsion between hydrogen molecules, the critical temperature of hydrogen is low (Tc = 33 K). The fundamental significance of hydrogen storage is to reduce the huge volume of hydrogen. At ambient temperature and atmospheric pressure, one kilogram of hydrogen has a volume of 11 m3 . Increasing the density of hydrogen in a storage system, it can be done by compressing the hydrogen by doing work, lowering the temperature below a critical temperature, or reducing intermolecular repulsion through the interaction of hydrogen with another species. The second important criterion for hydrogen storage systems is the reversibility of hydrogen absorption and desorption. The standard excludes all covalent hydrogen-carbon compounds as hydrogen storage materials since hydrogen is only released when the hydrocarbon– hydrogen compound is heated to temperatures above 800 °C or when the carbon is oxidized. There are six methods of reversibly storing hydrogen at high bulk and gravimetric densities (Fig. 8.2). The following sections will focus on these methods and explain their advantages and disadvantages.

8.1.1 High-Pressure Gas Cylinder The most common storage system is a high-pressure gas cylinder with a maximum pressure of 20 MPa. New lightweight composite cylinders have been developed that can withstand pressures up to 80 MPa, so the bulk density of hydrogen can reach 36 kg m−3 , which is about half of the liquid state at a normal boiling point. Due

8.1 Storage and Transport of Hydrogen

205

Fig. 8.2 Six basic hydrogen storage methods and phenomena [1]

to the increased thickness of the pressure cylinder walls, the gravimetric hydrogen density decreases with increasing pressure. The wall thickness of a cylinder covered with two hemispheres is given by the following equation: dw Δp = do 2σV + Δp

(8.2)

Where dw is the wall thickness, do is the outer diameter of the cylinder, Δp is the overpressure of the material, and σv is the tensile strength of the material. The tensile strength of the material varies from 50 MPa for aluminum to over 1100 MPa for high-quality steel. In the future, the development of new composite materials has the potential to increase tensile strength to levels higher than steel at a material density less than half that of steel. Currently, most pressure cylinders use austenitic stainless steels (e.g. AISI 316 and 304 and AISI 316L and 304L above 300 °C to avoid carbon grain boundary segregation), and copper or aluminum alloys are largely unaffected by hydrogen at ambient temperature. Many other materials, such as alloys or highstrength steels (ferritic, martensitic, and bainite), titanium, and alloys, should not be used due to their tendency to embrittle [2]. Figure 8.3 shows the bulk density and wall thickness to outer diameter ratio of hydrogen in a stainless-steel pressure cylinder with a tensile strength of 460 MPa. The bulk density of hydrogen increases with pressure and reaches a maximum above 1000 bar, depending on the tensile strength of the material. However, the gravimetric density decreases with increasing pressure and the maximum gravimetric density is found at zero overpressure. Thus, in pressurized gas systems, an increase

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Fig. 8.3 Bulk density of compressed hydrogen as a function of gas pressure, including ideal gas and liquid hydrogen. For steel with a tensile strength of 460 MPa, the ratio of cylinder wall thickness to outer diameter is shown on the right. A schematic diagram of the pressure cylinder is shown in the inset [2]

in volumetric storage density is accompanied by a decrease in gravimetric density (Fig. 8.4). The safety of pressurized gas cylinders is a concern, especially in populated areas. Future pressure vessels are expected to consist of three layers: an inner polymer lining wrapped in carbon fiber composite material (stress-carrying component), and an outer layer of aramid material capable of withstanding mechanical and corrosion damage. The target the industry has set for itself is a 70 MPa cylinder with a mass of 110 kg, a gravimetric storage density of 6%, and a volumetric storage density of 30 kg m−3 . Hydrogen can be compressed using standard piston mechanical compressors. Slight modifications to the seal are sometimes required to compensate for the higher diffusivity of hydrogen. The theoretical work required for the isothermal compression of hydrogen is given by the following equation: (

p ΔG = RT ln p0

) (8.3)

In the formula, R is the gas constant (R = 8.314 J mol−1 K−1 ). T is the absolute temperature. p and p0 are the end pressure and the start pressure, respectively. In the pressure range of 0.1–100 MPa, the work error calculated with Eq. (8.2) is less than

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Fig. 8.4 Volumetric and gravimetric hydrogen storage densities of pressurized gases. Steel (tensile strength σv = 460 MPa, density 6500 kg m−3 ) Assuming composite material (σv = 1500 MPa, density 3000 kg m−3 ). Marks represent Dynetek’s pressure cylinders [2]

6%. Therefore, the isothermal compression of hydrogen at 0.1–80 MPa consumes 2.21 kWh kg−1 . In the actual process, the work consumption of compression is significantly higher since the compression is not isothermal. Metal hydrides can only be used to compress hydrogen in heat sources. When the final pressure exceeds 100 MPa, the compression ratio may be greater than 20:1. Despite the simplicity of the high-pressure cylinder technology and its largescale proven application in the laboratory, the relatively low hydrogen density and extremely high gas pressure in the storage system are the main drawbacks of this method.

8.1.2 Liquid Hydrogen Liquid hydrogen is stored in cryogenic tanks at an ambient pressure of 21.2 K. Due to the low critical temperature of hydrogen (33 K), liquid hydrogen can only be stored in open systems because no liquid phase exists above the critical temperature. At room temperature, the pressure in a closed storage system may increase to about 104 bar. The bulk density of liquid hydrogen is 70.8 kg m−3 , which is slightly higher than that of solid hydrogen (70.6 kg m−3 ). The challenges of liquid hydrogen storage are energy-efficient liquefaction processes and the insulation of cryogenic storage vessels to reduce hydrogen vaporization. A hydrogen molecule consists of

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two protons and two electrons. The combination of two electron spins results in a bound state only when the electron spins are antiparallel. Considering the exchange of the spatial coordinates of the two fermions (spin = 1/2), the wave function of the molecule must be antisymmetric. Therefore, according to the total nuclear spin (I = 0, antiparallel nuclear spin; I = 1, parallel nuclear spin), there are two groups of hydrogen molecules. The first group with I = 0 is called para hydrogen and the second group with I = 1 is called ortho hydrogen. Normal hydrogen at room temperature contains 25% para form and 75% ortho form. Orthogonal forms cannot be prepared in the pure state. Since these two forms of energy are different, their physical properties are also different. The melting and boiling points of para-hydrogen are about 0.1 K lower than those of normal hydrogen. At 0 K, all molecules must be in the rotational ground state. Liquefaction process: When hydrogen is cooled from room temperature (RT) to normal boiling point (nbp = 21.2 K), ortho-hydrogen is transformed from 75% equilibrium concentration at room temperature to 50% equilibrium concentration at 77 K– 0.2% equilibrium concentration at boiling point concentration. Self-transformation is a very slow activation process, with a half-life of transformation greater than one year at 77 K. The conversion reaction from ortho- to para-hydrogen is exothermic and the heat of conversion is also temperature dependent. At 300 K, the heat of transformation is 270 kJ kg−1 and increases with decreasing temperature, reaching 519 kJ kg−1 at 77 K. At temperatures below 77 K, the transformation enthalpy is 523 kJ kg−1 and is almost constant. The transformation enthalpy is greater than the latent heat of vaporization (HV = 451.9 kJ kg−1 ) at the boiling point of ortho and para hydrogen. If unconverted normal hydrogen is placed in a storage vessel, the enthalpy of transformation will be released in the vessel, causing the liquid hydrogen to evaporate. Many surface-active and paramagnetic species can catalyze the conversion from ortho to para hydrogen, for example, normal hydrogen can be adsorbed on charcoal, cooled with liquid hydrogen, and desorbed in an equilibrium mixture. If a highly active form of charcoal is used, the conversion may only take a few minutes. Other suitable ortho-para catalysts are metals such as tungsten, nickel, or any paramagnetic oxide such as chromium or gadolinium oxides. Invert nuclear spins without breaking hydrogen bonds (Fig. 8.5). The simplest liquefaction cycle is the Joule-Thompson cycle (Linder cycle). The gas is first compressed, then cooled in a heat exchanger, and then subjected to isenthalpic Joule–Thomson expansion through a throttle valve, producing some liquid. The cooled gas is separated from the liquid and returned to the compressor through a heat exchanger. The Joule- Thompson cycle is suitable for gases with a temperature inversion above room temperature, such as nitrogen. However, hydrogen heats up as it expands at room temperature. For the hydrogen to cool as it expands, its temperature must be below the inversion temperature of 202 K. Therefore, the hydrogen is usually pre-cooled with liquid nitrogen (78 K) before the first expansion step takes place. The free enthalpy change between gaseous hydrogen at 300 K and liquid hydrogen at 20 K is 11 640 kJ kg−1 . The theoretical energy (work) required to liquefy hydrogen from RT is Wth = 3.23 kWh kg−1 . The technical workload is about 15.2 kWh kg−1 is almost 40% of the high calorific value of hydrogen combustion.

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209

Fig. 8.5 Joule-Thompson cycle (Linder cycle). The gas is first compressed, then cooled in a heat exchanger, and then passed through a throttle valve, where isenthalpic Joule–Thomson expansion occurs, producing some liquid. The cooled gas is separated from the liquid and returned to the compressor through a heat exchanger [3]

The rate of vaporization of hydrogen gas from heat leaks in liquid hydrogen storage containers is related to the size, shape, and thermal insulation properties of the container. In theory, the best shape is a sphere because it has the smallest area-to-roll ratio and uniform stress and strain distribution. However, large-sized spherical containers are expensive due to the difficulty in manufacturing. Since the evaporation loss due to heat leakage is proportional to the surface-to-volume ratio, the evaporation rate decreases sharply with increasing tank size. Evaporation losses are typically 0.4% per day for a double vacuum insulated spherical Dewar, a tank with a storage capacity of 50 m3 , 0.2% for a tank of 100 m3 , and 0.06% for a tank of 20,000 m3 . Low-temperature quasi-hydrogen requires the use of materials that maintain good ductility at low temperatures. Austenitic stainless steels (e.g. AISI 316L and 304L) or aluminum and aluminum alloys (Serie 5000) are recommended. Polytetrafluoroethylene (PTFE, Teflon) and 2-chloro-1,1,2-trifluoroethylene (Kel-F) can also be used.

8.1.3 Comparison of Pressure Storage and Liquid Storage The weight and bulk density of hydrogen is strongly dependent on the size of the storage vessel, as the surface-to-volume ratio decreases with increasing vessel size. Therefore, only the upper limit is defined. The large amount of energy required for

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Fig. 8.6 Hydrogen densities for compressed, liquid, and solid hydrogen [3]

liquefaction, i.e. 40% of the upper calorific value, makes liquid hydrogen not an efficient energy storage medium. Furthermore, the continuous vaporization of hydrogen limits the possible applications of liquid hydrogen storage systems to situations where hydrogen is consumed in a relatively short period, such as air and space applications (Fig. 8.6).

8.1.4 Hydrogen Adsorption 8.1.4.1

Adsorption Phenomenon

The physical adsorption of gases on the surface is caused by weak van der Waals forces between the adsorbent and the adsorbent. The resulting gas–solid interaction consists of an attractive term and a repulsive term. The attractive term decreases with the 6th power of the distance between the gas and the substrate, and the repulsive term decreases with the 12th power of the distance between the gas and the substrate. This attractive interaction originates from the long-range forces generated by the fluctuations in the charge distribution of gas molecules and surface atoms, resulting in dipole images and dipole attraction. However, at very small distances, the overlap between the electron cloud of the gas molecule and that of the substrate is significant and the repulsive force increases rapidly. The combination of these two terms causes the potential energy curve to exhibit a shallow minimum at a distance of about one molecular radius from the gas molecules. This energy minimum usually corresponds to 1–10 kJ mol−1 . The small enthalpy change generated during the adsorption process is not enough to cause bond breakage and the gas is adsorbed in its molecular form. Physical adsorption usually occurs only at low temperatures

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211

Fig. 8.7 Langmuir isotherm types [3]

due to very weak interactions between adsorbent and adsorbent. Usually, there is no energy barrier to prevent molecules close to the surface from entering the physisorption trap. Therefore, the process is inactive and physical adsorption is characterized by fast kinetics. Due to the limited pore size of microporous adsorbents, gas molecules tend to form a simple monolayer on the solid surface even at temperatures close to the gas liquefaction temperature. This behavior can be qualitatively described by the Langmuir isotherm, also known as the Type I isotherm in the International Union of Applied Chemistry (IUPAC) classification (Fig. 8.7) [3]. The saturation of all adsorption sites (θ = 1) on the solid surface exhibits plateau characteristics on the isotherm. The Langmuir adsorption isotherm assumes that the surface is homogeneous and each adsorption site is equal, when all adsorption sites are occupied, the substrate surface is saturated and a monolayer has formed, and there is no interaction between adsorbed particles. In general, physical adsorption isotherms exhibit different shapes. According to the IUPAC classification, Type II and Type III describe adsorption on non-porous or microporous adsorbents with strong and weak gas–solid interactions, respectively. Type IV and type V adsorption isotherms show typical hysteretic cyclic capillary condensation adsorption, and type VI adsorption isotherms show stepwise multilayer adsorption. Physical adsorption hydrogen storage is usually applied between 77 K and room temperature. Since the temperature is much higher than the critical temperature of H2 , the multi-layer adsorption does not occur in the porous material, resulting in Itype or Henry-type isotherms. The highest volumetric hydrogen density for surface physisorption is limited by the liquid H2 density. Microscopically, the minimum distance between hydrogen molecules in the adsorbed monolayer is determined by

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the intermolecular distance in the liquid phase. Liquids are nearly incompressible, so a higher density means a chemical reaction with hydrogen and the formation of new phases. If the adsorption of hydrogen on the substrate is physical, then this interaction is nonspecific and the amount of hydrogen that can be stored depends mainly on the specific surface area of the adsorbent and operating conditions, such as hydrogen pressure and temperature. Therefore, materials with very high specific surface areas seem promising, at least in systems where cooling is not an issue. In recent years, significant progress has been made in understanding the adsorption phenomena of microporous surfaces through molecular simulations. The physical adsorption isotherms of carbon materials are studied by using the large canonical Monte Carlo simulation and the effective classical potential or using the Feynman path form combined with the Monte Carlo method to consider quantum effects [4]. To accurately model hydrogen adsorption at low temperatures, these quantum effects must be included. In the last case, hydrogen is considered a quantum fluid. The basic idea of Feynman’s path integral formalism is to study the possible paths for particles to move from one point to another [5]. In the case of the grand canonical Monte Carlo simulation, considering that the interaction between hydrogen molecules and carbon materials is simple physics, the adsorption isotherm can be described by the classical empirical potential. Chemical interactions are not considered in this type of calculation. The interaction between two particles (hydrogen–hydrogen and hydrogen-carbon) can be described by the Lennard–Jones potential (8.4). U(s) = 4ε

] [( ) σ 12 ( σ )6 − s s

(8.4)

where s is the distance, σ and ε are potential parameters, and the potential parameters of hydrogen–hydrogen and hydrogen-carbon interactions are different.

8.1.4.2

Chemical Absorption

In chemisorption, gas particles interact with the surface atoms of the adsorbent to form chemical bonds, a typical covalent feature. The enthalpy of chemisorption is larger than that of physical adsorption, on the order of 100 kJ mol−1 . Sometimes the value of adsorption energy is used to differentiate chemical and physical interactions with the adsorbent, but this criterion of 10 is not absolute and most commonly spectroscopic techniques are necessary to determine the species adsorbed on the surface and the bonds that occur type. It is difficult to distinguish strong physisorption from chemisorption without spectroscopic techniques. Potential energy curves of adsorbents close to the solid surface usually exhibit two energy extremums at different depths. One corresponds to physisorption farther from the adsorbent, which may be the precursor state of chemisorption. The other

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213

Fig. 8.8 Potential energy of adsorbent as a function of adsorbent distance [6]

occurs at shorter distances, with deeper minima, corresponding to genuine chemical interactions (Fig. 8.8). As the first step of chemisorption can be physisorption, the number of atoms chemisorbed can be determined by the number of molecules physisorbed. Chemisorption is a typical activation process, which means that when the adsorption is dissociative, an activation energy is required to break the chemical bonds in the gas molecules, or when the gas particles are close to the surface at a small distance, an activation energy is required to regenerate. Therefore, this type of adsorption requires higher temperatures and has slower kinetics than physical adsorption. For example, energy can be provided by high temperatures or by the activation of gas molecules. Due to the existence of dangling bonds on the solid surface, ideally, every atom can serve as an adsorption site for gas particles, and when all accessible sites are occupied, a monolayer is formed, resulting in saturation. This process is well described by the Langmuir isotherm, and in the case of directory chemisorption, the equilibrium value of the surface coverage is comparable to the square root of the pressure. Therefore, formula (8.4) is modified to formula (8.5). θ=

(KP)1/2 1 + (KP)1/2

(8.5)

To explain and predict the adsorption of hydrogen on carbon nanomaterials, many theoretical calculations have been performed, all of which consider the chemisorption of hydrogen atoms on the substrate. Monte Carlo simulation using classical potential is not sufficient to describe chemical processes such as bond formation and bond breaking that occur in chemisorption. To study the formation of chemical bonds between carbon and hydrogen atoms, many authors employ ab initio methods [7].

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8.1.5 Carbon Materials Carbon nanomaterials have the advantages of high specific surface area, micropores, low mass, and good adsorption properties, making them attractive candidates for hydrogen storage materials. In these materials, the carbon is in the sp2 hybrid state, each atom has a nonhybridized free p electron, perpendicular to the sp2 bond. Due to their extended pi-electron clouds, these materials have very versatile properties and are known as pi-electron materials. Activated carbon can be made from some different carbonaceous feedstocks, such as coconut shells, coal, or lignin, which have been carbonized and then activated by steam treatment and oxidation or chemical activation. This processing is responsible for the formation of porous structures with high specific surface areas (from 1000 to 3000 m2 g−1 ). So far, activated carbon is considered to be the best carbonaceous adsorbent based on the principle of physical adsorption. Since their discovery by Iijima in 1991, carbon nanotubes have aroused great interest in the scientific community [6]. Carbon nanotubes can be viewed as rolledup graphene sheets with an inner diameter of one to several times the diameter and a length of 10–100 μm. Usually, the ends of the nanotube are closed, and the cap-like structure is fullerene. These carbon nanotubes can be classified into singlewalled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWNTs). MWNTs consist of up to 50 graphitic layers, typically between 15 and 50 nm in diameter, with interlayer distances close to those of graphite (≈0.34 nm). SWCNTs consist of one layer and are therefore much thinner. According to the different rolling directions of the graphite flakes on the lattice vector, SWCNTs can exist in three types: zigzag, armrest, and chirality. Within these nanostructures, there are different potential sites for hydrogen adsorption. SWCNTs exhibit a large free volume inside the tube, in addition to that, the curvature of the graphene flakes and the channels between the tubes can be sites for new interactions with hydrogen. Storing large amounts of hydrogen in multi-walled carbon nanotubes between different concentric tubes seems impossible because the strong carbon–carbon bonds of the graphite sheets have to be stretched. In recent years, a new type of carbon nanostructure has been synthesized by catalytic decomposition of hydrocarbons. These fibrous materials are graphitic nanofibers (GNFs). Graphite nanofibers consist of graphite flakes stacked together in different orientations with interlayer distances similar to those of graphite. Depending on the angle of orientation, three different structures can exist: tubular, platelet, and chevron (Fig. 8.9). Many researchers have been spending time and effort finding new materials that can reversibly store large amounts of hydrogen at room temperature and moderate pressure. Storage media with these properties would be ideal for mobile applications because of the low energy losses involved during adsorption and desorption. Dillon et al. measured reversible hydrogen uptake by ultrasonic SWCNTs at room temperature at 0.01 wt% using TDS. The sample contained only 0.1 wt% nanotubes, so they estimated the hydrogen storage capacity of the pure nanotube sample to be

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Fig. 8.9 Carbon nanostructures obtained from different assembled graphene sheets: a graphitic nanofibers, b MWCNTs, and c SWCNTs [8]

around 5–10 wt%. High-power sonication was used to open the lids that closed the nanotubes, cutting the nanotube bundles into shorter segments, and allowing easier access for hydrogen to the interior and interstitial locations of the nanotube bundles. It was later shown that the high hydrogen adsorption could be attributed to the titanium particles at the tip used for sonication, rather than the nanotubes [8]. Liu et al. determined the hydrogen storage capacity of SWCNTs. They reported a storage capacity of 4.2 wt% for SWCNTs with a diameter of about 1.85 nm at room temperature and a storage capacity of 10 MPa. The material’s purity is to be 50%, and the authors claim that the storage capacity is reproducible [9]. However, it has never been replicated in any laboratory. Researchers from Japan performed volumetric measurements of different carbon materials for storing hydrogen at room temperature. The purified HiPCO (high-pressure CO conversion) SWCNTs had the highest absorption at 0.43 ± 0.03 wt% [10]. In 1998, Rodriguez et al. studied the extremely high hydrogen absorption rate of herringbone GNFs, reaching 67%. They determined the hydrogen pressure at a constant volume of the sample to be 11.2 MPa and measured a huge pressure drop within 24 h [11]. In this case, however, no laboratory was able to reproduce the results. Using the same measurement technique, Ahn et al. [12] found that for GNFs with a significant partial herringbone structure, the hydrogen storage capacity did not exceed a value of 0.2 wt%. Recently, the team of Rodriguez and Baker measured that the maximum adsorption capacity of herringbone GNFs was only 3.3–3.8 wt% after high-temperature hydrogen pretreatment at 700 °C [13]. In 1998, the Chinese Academy of Sciences group claimed that the hydrogen uptake of tubular GNFs was 10–13 wt%. This result was obtained after the nanofibers were boiled in hydrochloric acid at a pressure of 11 MPa. However, in another work, the same group reduced the

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hydrogen storage capacity of GNFs by a factor of two. So far, these results have not been confirmed by any other group [14]. Both the Canadian research group [15] and Hirscher et al. [16] reported that carbon nanofibers have very low hydrogen uptake values. At around 11 MPa, 0.7 and 0.1 wt% were obtained, respectively. The measurements of Ritschel et al. [17] and Tibbetts et al. [18] confirmed that the hydrogen storage capacity of GNFs was not significant or very low. Using thermogravimetric analysis, Strobel et al. [19] determined the hydrogen adsorption capacity of carbon nanofibers and activated carbon under high pressure. At a pressure of 12 MPa, they observed the maximum weight increase corresponding to hydrogen take-up of 1.6 wt% activated carbon and 1.2 wt% GNFs. Similar results were reported for tubular and herringbone carbon nanofibers. They hold about 1% by weight at room temperature and an H2 pressure of 12 MPa. A Spanish team from the University of Alicante performed adsorption measurements on activated carbon materials at room temperature using a mixed gravimetric and volumetric methods. They reported that the activated carbon obtained from anthracite had a specific surface area of 1058 m2 g−1 , and the highest value of hydrogen adsorption was close to 1% at 10 MPa [20]. The data on hydrogen uptake of carbon materials at room temperature are widely distributed. The reasons for these differences can be attributed to difficulties in measuring hydrogen uptake and large differences in sample quality. Unfortunately, it seems clear that all reproducible results refer to about 1% of the maximum storage capacity at 298 K, well below what is required for technical applications.

8.1.6 Metal Hydrides In chemistry, most metals have a certain affinity for reacting with hydrogen to form metal hydrides. However, the gravimetric storage capacity of these complexes is generally low due to the large difference in atomic mass between the metal and hydrogen atoms. In this regard, it is most sensible to use the lightest metals as storage materials, but these hydrides are usually the most stable and therefore require the most stringent conditions to release hydrogen again. The lightest metal lithium can form a lithium-ion liquid with 12.5 wt% hydrogen, but the dehydrogenation temperature of pure lithium is 944 °C [21]. When choosing a metal hydride, a trade-off is required between the dehydrogenation temperature and the mass fraction of hydrogen to be stored. MgH2 is an example that demonstrates this trade-off. This monometallic hydride is made from metal magnesium of moderate atomic weight (MW = 24.3), and the dehydrogenation temperature is elevated but moderate. MgH2 stores 7.7 wt% hydrogen and approximately 110 H2 L−1 and releases it at a temperature of 300 °C [22]. The advantage of Mg is that it is the fifth most abundant metal in the Earth’s crust, which lowers the price of Mg-based storage [23]. One of the biggest drawbacks of MgH2 is the slow hydrogenation reaction kinetics: hydride formation is a surface reaction, so an MgH2 layer first forms on the Mg metal part in the storage vessel. However, this layer is an important barrier to the further diffusion of hydrogen into

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217

the metal, which slows down further hydrogenation. The solution offered was to use smaller magnesium particles so that the hydrogen didn’t have to penetrate as deep into the metal, but research has shown that these small particles are less recyclable due to agglomeration. In the current study, there is a lot of focus on so-called intermetallic compounds described using the general formula ABx Hy , where a is a rare earth or alkaline (earth) metal and B is a transition metal. An example of a metal hydride used in transportation applications is the use of TiMn-like compounds. This is the so-called laves phase alloy [24]. A 2010 publication by Bevan described a canal vessel operating in the UK capable of storing 4 kg of hydrogen; the exact composition of the storage material was described as Ti0.93 Zr0.05 (Mn0.73 V0.22 Fe0.04 )2 . The practical hydrogen storage capacity of this material is about 1.4 wt%. Hydrogen could be pressurized in this material at 3 MPa and released by lowering the pressure and adding cooling water at 9 °C. However, the high cost of the system is also considered a disadvantage of this promising storage system [25]. Such metal hydride storage systems sealed in pressure vessels are also known as hybrid hydrogen storage vessels [26]. Another metal hydride under study is alanine; these are AlH4 − anions, of which NaAlH4 is an interesting compound because it contains aluminum and sodium, respectively, the most abundant in the earth’s crust first and fourth metals. Complex metal hydrides do not release all the hydrogen in one step, but when exposed to high temperatures, a cascade of reactions occurs. 3NaAlH4 → Na3 AlH6 + 2Al + H2

(8.6)

2Na3 AlH6 → 6NaH + 2Al + 3H2

(8.7)

Reaction (8.6) occurs in the temperature range of 185–230 °C, and reaction (8.7) occurs at 260 °C. It can be seen from the reaction equation that one hydrogen atom in each propionate combines with sodium to form NaH; The compound does not decompose at temperatures below 425 °C. Therefore, in practical applications, only 5.6 wt% of the theoretical 7.4 wt% of NaAlH4 is available for storage. NaAlH4 can store about 75 g H2 L−1 of which about 63 g L−1 can be used [27]. NaAlH4 itself is considered to be an irreversible hydrogen storage support, however, the addition of other metals as catalysts (titanium) shows that propionate species can indeed recover their hydrogen storage capacity from direct hydrogenation at elevated pressure. A second class of solid hydrogen storage materials is boron-based materials. The element is the fifth lightest element in the periodic system and is classified as a metalloid, which means it has both metallic and non-metallic properties. In the literature, the two most common forms of boron-based hydrogen storage materials are NaBH4 and NH3 BH3 . Both materials have a very high hydrogen capacity of 410 wt%; this is due to the lightweight of boron while still being able to store many hydrogen atoms. NaBH4, with a hydrogen capacity of 10.8 wt%, about 125 H2 L−1 has a high decomposition temperature (530 °C), but has the advantage of decomposing in

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hydrogen-releasing water according to the reaction: Without the weight of water, the hydrogen storage capacity of NaBH4 will reach 21 wt% [28]. However, due to the lower solubility of NaBH4 and lower solubility of NaBO2 (0.282 kg L−1 ), the addition of this water is not stoichiometric but must be added in excess. Furthermore, the formation of NaBO2 increased the pH of the reaction medium, thereby inhibiting further hydrolysis of NaBH4 . To counteract the self-inhibition of this reaction, catalysts, usually cobalt-based, have been developed. In terms of hydrogen release, NaBH4 has many hydrogen release advantages, but the reverse reaction is difficult to perform efficiently [29]. The boron-oxygen bond is very stable, so direct rehydrogenation is difficult. One method for rehydrogenation of the boron-oxygen bond is to mix it with MgH2 in a ball mill; however, this shifts the problem of rehydrogenation of boron to the hydrogenation of magnesium. Another boron compound generally regarded as a promising hydrogen storage material is NH3 BH3 . This compound has the highest hydrogen storage weight capacity, 19.4 wt%, with a volume close to 180 H2 L−1 . This is the highest hydrogen storage capacity among all non-CO2 hydrogen storage media. To release hydrogen from NH3 BH3 , two different strategies exist. One can choose to release hydrogen by supplying heat [30]. Like the above NaBO4 , the dehydrogenation of NH3 BH3 can also be achieved by the hydrolysis reaction in an acidic solution at room temperature [31]. As can be seen from the reaction equation, all hydrogen atoms stored in the NH3 BH3 compound can be released. In addition to acids, dehydrogenation can also be achieved by metal catalysts. It should be noted that the final products of the dehydrogenation reaction, B(OH)3 and NHBH (a polycyclic mixture of its oligomers) are very stable. The main limitation of using NH3 BH3 as an energy storage medium is the recovery of the product. The boron oxygen bond of the hydrolysis reaction is limited by the same regeneration reaction as above for NaBH. The stability of the boron-nitrogen bonds produced by the thermal decomposition of NH3 BH3 is also unsuitable for direct hydrogenation strategies, and several strategies have been described for the hydrogenation of these bonds, but only at very low conversion levels [32].

8.2 Hydrogen Fuel Cell 8.2.1 Introduction It is often said that fuel cells are a key technology of the twenty-first century. While this claim is controversial, fuel cells certainly are one enabling technology for the future hydrogen economy. Fuel cells use pure hydrogen and air to efficiently convert the chemical energy of the fuel directly into electricity and produce only water, eliminating all local emissions. The share of renewable energy from wind, water, and solar will further increase, but these resources are not suitable to cover the electricity base load because of their irregular supply. Therefore, electricity from

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renewable sources, combined with hydrogen production and fuel cells, is likely to be an important option for stationary and mobile power generation in the future. Fuel cells are not limited to hydrogen as a fuel, but they can also use various other fuels [33]. However, the use of carbonaceous fuels is associated with increased system complexity and carbon dioxide (CO2 ) emissions. In contrast, operating pure hydrogen fuel cells has the advantages of system simplicity and high system energy efficiency. In particular, the recently measured efficiency of hydrogen fuel cell vehicles is 1.5–2 times higher than that of hydrogen-powered internal combustion engine (ICE) vehicles. Therefore, from a practical and theoretical point of view, the energy conversion of fuel cells is most suitable for hydrogen as a fuel.

8.2.2 Brief History Fuel cells as an electrochemical energy conversion device to generate electricity from hydrogen and oxygen are credited to British lawyer and scientist William R. Grove. However, the fuel cell principle was discovered between 1829 and 1868 by Christian F. Schonbein, a professor at the University of Basel. After experimenting with hydrogen, oxygen, and platinum, Schonbein published the first paper on the fuel cell effect in December 1838 [34]. Grove, doing similar experiments with hydrogen, first mentioned his fuel cell experiments later in 1839 and reported on a working fuel cell in 1842 [35]. In the nineteenth century, fuel cells however did not achieve any technical significance in terms of electricity production. The lack of understanding of fundamental electrochemical processes and the lack of suitable materials hinders the development of powerful fuel cell devices. As a result, generators were invented in the 1860s to generate electricity more cheaply, and the technology was soon developed into megawatt-scale power plants. For mobile supply of power, batteries were developed. It wasn’t until the early 1960s that fuel cells were first used in space exploration. Hydrogen is used as fuel for propulsion and can therefore be used to generate electricity. A deep understanding of electrochemical processes and newly developed materials enable the development of reliable and high-performance fuel cell systems. Since weight is one of the most critical issues in spacecraft, the superior gravitational energy density of fuel cells compared to batteries paves the way for this application. All manned spacecraft today use fuel cells to generate electricity. In the 1980s, when ecological discussions about energy use and atmosphericrelated emissions (e.g., carbon dioxide) revived interest in efficient energy conversion systems. The main drivers of fuel cell development are stationery and transportation applications. Decentralized stationary power generation based on fuel cells can improve overall fuel efficiency due to the cogeneration of heat and power. The use of fuel cells may even lower capital costs for owners because it is easier and more efficient to distribute heat over a small area. However, natural gas will be the primary fuel for this application for the foreseeable future. Second, the growing focus in the transport sector on how to achieve sustainable mobility in the future has reignited interest

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in fuel cells as efficient energy converters in road transport. Hydrogen-fueled, fuel cell-based powertrains are expected to significantly improve tank-to-wheel efficiency compared to conventional internal combustion engines. The broad renewed interest in fuel cell technology has sparked an intensive search and development effort for better materials and advanced processes and production technologies. In the future hydrogen-based energy economy, fuel cells can play an important role as clean and efficient energy converters.

8.2.3 Principles Both batteries and fuel cells convert chemical energy directly into electricity, and both functions similarly. Batteries store chemical energy in the form of active electrode mass, such as metallic lead and lead oxides in lead-acid batteries, while chemical energy in fuel cells is provided in a continuous fashion, such as hydrogen and air. Batteries are devices for energy storage and conversion, fuel cells are just converters. This, at first glance, the slight difference between batteries and fuel cells leads to major consequences for the application of their technology in energy conversion systems. Batteries are compact but tend to be heavy, limiting their energy density. The battery cannot work continuously for a long time. Fuel cells are lighter and require a tank to store fuel, the size of which determines the run time and available energy. For smaller power demands (like flashlights, toys, etc.) or short-duration loads (like car starter batteries), the inherent disadvantages of labeled batteries are trivial. However, as performance and operating time increase, the small energy density of batteries is a considerable limitation, and fuel cells may be more suitable for the application since the size of the converter (fuel cell) and the size of the energy storage (tank) can be scaled independently. The basic structure and principle of all fuel cells are similar: the cell consists of two electrodes separated by an electrolyte. The electrodes are connected by an external circuit. The electrodes are exposed to a gas or liquid flow to provide fuel and oxidants (e.g., hydrogen and oxygen). The electrodes must be gas or liquid-permeable and therefore have a porous structure. The gas permeability of the electrolyte should be as low as possible. For fuel cells using acidic electrolytes, hydrogen is oxidized at the negative electrode (anode) according to the following formula. The formed protons enter the electrolyte and are transported to the cathode H2 → 2H+ + 2e−

(8.8)

At the positive electrode (cathode), oxygen reacts as follows: O2 + 4e− → 2O2−

(8.9)

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During these reactions, electrons flow in external circuits. Oxygen ions combine with protons to form water: O2− + 2H+ → H2 O

(8.10)

The product of this reaction is water, which is formed at the cathode of an acid fuel cell. It can form at the anode if an oxygen ion (or carbonate) conducting electrolyte is used, as in the case of high-temperature fuel cells or liquid alkaline fuel cells. The reaction product water must be removed from the cells. An important advantage of fuel cells is the selectivity of electrochemical reactions. In contrast to the combustion process, where the reaction is indirectly controlled by temperature and pressure-dependent rates, the electrochemical reaction is directly related to the cell voltage and is highly selective, i.e., NOx is not produced when air is used as the oxidant at the cathode. The fuel cell itself has no moving parts, so it makes almost no noise. In some fuel cell systems, the blower may generate lower noise levels, but generally, these systems are relatively quiet.

8.2.4 Types of Fuel Cells Fuel cells are generally classified according to their electrolytes (Fig. 8.10). Five types are currently being developed to a commercial level. All of these fuel cells work well on hydrogen. Depending on the nature of the electrolyte, the operating temperature is between 20 and 1000 °C. For fuel cells operating at lower temperatures (< 200 °C), thermal cycling is more dynamic and reliable. On the other hand, high-temperature fuel cells operate with reduced system complexity when using hydrocarbon feedstocks. Low-temperature fuel cells include alkaline fuel cells (AFC), polymer electrolyte fuel cells (PEFC), direct methanol fuel cells (DMFC), and phosphoric acid fuel cells (PAFC). High-temperature fuel cells operate at temperatures between 600 and 1000 °C and two different types are under development, molten carbonate fuel cells (MCFC) and solid oxide fuel cells (SOFC). This section describes all types to increase operating temperatures. Using pure hydrogen as fuel, low-temperature fuel cells have major advantages in dynamic operation and durability, so these types will be discussed in more detail than high-temperature fuel cells. A summary of all fuel cell types is shown and the reactants (pure hydrogen or carbonaceous fuel) are shown.

8.2.4.1

Alkaline Fuel Cell

Alkaline fuel cells use potassium hydroxide aqueous solution as the electrolyte and operate at temperatures ≤ 80 °C. Although noble metal electrocatalysts are used for space applications, the advantage of AFC is that it can also be achieved with nonprecious metal electrocatalysts such as silver and nickel enough reactivity. AFCs

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Fig. 8.10 Overview of different fuel cell types, temperature ranges, and reactants [35]

typically use pure hydrogen and oxygen (or carbon dioxide-free air) as fuel and oxidant because their electrolytes are sensitive to carbonate formation. However, the advantage of the KOH electrolyte used in AFC (typically at a concentration of 30–45 wt%) is that the oxygen reduction kinetics are much faster under alkaline conditions than in acidic electrolytes, making AFC a potentially efficient system. AFCs are ideal for applications in CO2 -free environments such as space. As such, AFC was used for the Apollo missions and the Space Shuttle program.

8.2.4.2

Polymer Electrolyte Fuel Cell

Polymer electrolyte fuel cells, sometimes also called SPEFCs (solid polymer electrolyte fuel cells) or PEMFCs (polymer electrolyte membrane fuel cells), use a proton exchange membrane as the electrolyte. PEFCs are low-temperature fuel cells that typically operate between 40 and 90 °C and therefore require noble metal electrocatalysts (platinum or platinum alloys on anode and cathode). PEFC is characterized by high power density and fast dynamics. Therefore, one prominent application area is automotive powertrains that require quick start-up. The known electrolyte in PEFC is the perfluorinated sulfonic acid membrane. These membranes include a polytetrafluoroethylene (PTFE)-based backbone, which is chemically inert in reducing and oxidizing environments, and side chains with sulfonic acid groups. On contact with humidity, these membranes undergo phase separation at the nanoscale, forming hydrophilic (ionized water) and hydrophobic (polymer backbone) phases. Proton transport occurs in the aqueous phase, similar

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to proton transport in aqueous solutions. Therefore, the conductivity of the dry film is lower. Therefore, water management in membranes is one of the main issues of PEFC technology. Factors affecting membrane water content are electroosmotic water resistance (each proton portion of its hydrated shell is also transported through the membrane) and the reverse transport of product water gradients between the anode and cathode.

8.2.4.3

Phosphoric Acid Fuel Cell

The advantages of phosphoric acid fuel cells are their relatively simple structure, thermal, chemical, and electrochemical stability, and low electrolyte volatility at operating temperatures (150–200 °C). These factors may aid early deployment in commercial systems compared to other fuel cell types. Polyphosphoric acid is usually stabilized in a silicon carbide-based matrix. The high concentration of acid increases the conductivity of the electrolyte and reduces corrosion of the carbon-supported electrodes. PAFCs require platinum-based noble metal electrocatalysts. PAFCs are mainly used in stationary power stations for distributed heating and power generation. The power plant, with a single module output of around 200 kW, has been installed worldwide to provide electricity, heat, and hot water to shopping malls or hospitals.

8.2.4.4

Direct Methanol Fuel Cell

Direct methanol fuel cells are a special form of low-temperature fuel cells based on PE technology. In a direct methanol fuel cell, methanol is electro-oxidized directly at the anode without the intermediate step of converting ethanol to hydrogen-rich gas. Since hydrogen is the focus of this contribution, this type is not discussed further.

8.2.4.5

Molten Carbonate Fuel Cell

Molten carbonate fuel cells use molten salt electrolytes of lithium carbonate and potassium carbonate and operate at about 650 °C. MCFCs promise high fuel-to-power efficiency and the ability to use coal-based fuels. Another advantage of MCFC is that due to the high operating temperature (600–700 °C), internal reforming is possible, as well as the use of waste heat in combined cycle power plants. High temperature significantly improves oxygen reduction kinetics, eliminating the need for precious metal catalysts. Molten carbonate (usually Li-K or Li-a carbonate) is stabilized in a matrix (LiAlO2 ), which can be supported by Al2 O3 fibers for mechanical strength. Molten carbonate fuel cell systems can achieve electrical efficiencies of at least 50% or as high as 70% when fuel cells are combined with other generators. MCFCs can use many different fuels and are not as susceptible to CO or CO pollution as

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low-temperature batteries. For fixed power, micro-battery controllers can play an important role in distributed generation.

8.2.4.6

Solid Oxide Fuel Cell

In solid oxide fuel cells, the most commonly used electrolyte material is yttriumstabilized zirconia (YSZ). YSZ exhibits sufficient oxygen ion conductivity for fuel cell applications at temperatures > 760 °C. SOFC is a simple two-phase gas–solid system, so it does not suffer from water management, catalyst layer immersion, or slow oxygen reduction kinetics. On the other hand, it is difficult to find suitable materials with the required thermal stability and matching thermal expansion coefficients to operate at such high temperatures. However, due to the high temperature of the cells and exhaust gas, an important advantage of SOFCs is the possibility of internal reforming of carbonaceous fuels and integration with other power generation systems. The combined system can achieve higher electrical efficiency. Siemens-Westinghouse achieved 60% efficiency with a combination of SOFCs and micro-turbines. Different SOFC concepts have been developed: flat plates offer the possibility of easier stacking, while tubular designs have fewer sealing problems. Monolithic panels and even single-chamber designs are considered and studied. The tubular design is probably the most famous. It was developed by Westinghouse Electric Company (now Siemens Power Generation) [36]. The first concept that Westinghouse pursued consisted of fuel cell tubes supported by air electrodes. Earlier, the test tube was made of calcium-stabilized zirconia, with active cellular components sprayed on it. Today, this porous support tube (PST) is replaced by a doped lanthanum manganate (LaMn) air electrode tube (AES), which increases the power density by about 35%. LaMn tubes were extruded and sintered to serve as air electrodes. Other battery components are deposited on the structure by plasma spraying. Hexis is developing a different type of SOFC design. The HEXIS (Heat Exchanger Integrated Stack) chimney concept can be used in small thermal power plants. In this case, metal interconnects are used as heat exchangers and bipolar plates.

8.2.5 Concluding Remarks Fuel cells can make a valuable contribution to the energy economy of the future and help build hydrogen-based energy systems. They increase the flexibility of generation and add options for many applications, such as distributed fixed generation, vehicle propulsion, and portable devices. Their main characteristic is their high electrical efficiency. The modularity of fuel cells makes the technology flexible because the required power can be easily met by varying the number of modules. The advantages and disadvantages of low and high-temperature fuel cells depend on the requirements of the application.

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8.3 Hydrogen Refueling Station The industrial development and economic growth of various countries have greatly stimulated the demand for energy and the environment [37]. Therefore, a consensus on building a resource-saving and environment-friendly society is gradually being reached around the world. Renewable hydrogen is considered ideal alternative energy in the future due to its advantages of cleanliness, environmental protection, zeroemission, and high energy density. Recently, the global marketization of hydrogen fuel cell vehicles (FCVs) has accelerated significantly [38]. As the link between the hydrogen consumption side (i.e. FCV) and the production side, hydrogen refueling stations (HRS) have been increasingly opened to the public [39]. From the perspective of the global hydrogen refueling station system, as of 2020, a total of 553 hydrogen refueling station systems were put into operation, and 107 hydrogen refueling station systems were put into operation in 2020. In addition, China built a total of 118 h (not including the 3 dismantled HRS). Among them, 101 h units have been put into operation, and 17 units are to be put into operation, with an operation rate of over 85%. As can be seen from the above data, many countries in the world are optimistic about hydrogen energy [40]. They have been actively carrying out the construction of HRSs and issuing corresponding support policies. This has accumulated a lot of valuable data and experience for the large-scale operation of the hydrogen refueling station system [41]. The “National Hydrogen Energy Roadmap” promulgated by the US Department of Energy explains the preparation, storage, transportation, and application of hydrogen energy from a macro perspective. It provides a feasible development path for the hydrogen energy industry [42]. However, there are many obstacles to the commercial application of hydrogen energy. Research shows that the construction of hydrogen refueling station infrastructure is one of the main obstacles [43]. This is because increased penetration of the fuel cell vehicle market will require a reduction in the cost of hydrogen [44] so that fuel cell vehicles can compete with alternatives such as internal combustion engine vehicles, hybrid vehicles, and plug-in electric vehicles [45]. The cost reduction of FCVs depends to a large extent on the technical operation of the fueling infrastructure [46]. Therefore, it is necessary to conduct indepth research on the structure and core technology of the hydrogen refueling station management system to realize its large-scale application [47]. HRS is a complex system with necessary hardware such as compressors, distributors, high-pressure storage vessels, chillers, heat exchangers, etc. The best combination of performance and cost can only be achieved if the entire system is wellintegrated and optimized. However, current studies are mainly focused on specific components or processes of HRS, and there is still a lack of uniform HRS design and construction guidelines developed for FCV bunkering requirements. Therefore, it is very important to formulate a reasonable process plan and select the appropriate equipment. In recent studies, HRS process design [48] and the selection of compression, storage [49], and filling technologies [50] have become the focus of research in the process of infrastructure construction.

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Based on the above research requirements to ensure better equipment utilization and plant performance, this chapter will review the plant categories, key equipment, and process flow of HRS. And then it will systematically analyze the configuration schemes of key equipment such as the hydrogen supply system, hydrogen pressure storage vessels compressors, and hydrogen dispensers, as well as the process optimization technology to maximize the operating efficiency and minimize the total cost of the HRS. Finally, the research in this field is summarized and prospected.

8.3.1 Hydrogen Refueling Station Process Before examining the key components and available processes of a hydrogen refueling station system, it is necessary to start with the definition and classification of a hydrogen refueling station system [51]. The following sections categorize and describe hydrogen refueling station management systems from four different perspectives.

8.3.1.1

Based on Hydrogen Production Location

According to hydrogen production technology and location, HRS can be divided into on-site HRS and off-site HRS. The former applies when the HRS is far from an external hydrogen source. They produce hydrogen at the power station to meet hydrogen demand [52]. Hydrogen sources typically contain methanol, petroleum, gasoline, diesel, natural gas, LPG (liquefied petroleum gas), or electricity [53], which can be used as an indicator for further classification by on-site HRS [54]. The latter prefers to use large-scale power generation technology (i.e. natural gas SMR) to produce hydrogen off-site. They deliver hydrogen to HRS from external central production units via pipeline trailers, liquid hydrogen container trucks, or hydrogen pipelines. The hydrogen can then be compressed, stored, and refilled into the FCV. In early hydrogen refueling infrastructure, researchers used tube trailers more often. In contrast, when transporting large-scale hydrogen, a more economical option is to use a pipeline system.

8.3.1.2

Based on Hydrogen Storage State

According to the hydrogen storage state, HRS can be divided into compressed gaseous hydrogen (CGH2 ) stations or liquid hydrogen (LH2 ) stations [55]. At the CGH2 station, hydrogen is stored in gaseous form in a hydrogen storage vessel and then re-injected into the FCV. For the LH2 station, hydrogen is stored in a liquid hydrogen storage vessel and refilled into the vehicle after gasification [56]. At present, China is dominated by CGH2 stations, while the United States and Japan are dominated by LH2 stations.

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Compared with CGH2 , LH2 has higher bulk density and larger storage capacity [57]. In terms of economics, the capital cost of a CGH2 station is very sensitive to the hydrogen transport distance. In the case of short-distance transportation, the capital cost of the CGH2 station has obvious advantages. But in the case of long-distance transportation, the CGH2 station requires additional compressors and pipelines to compress and store the gaseous hydrogen, so its cost will be significantly higher than that of the LH2 station. At the same time, LH2 station is usually suitable for occasions with long transportation distances and large transportation volumes [58]. Which is beneficial to the commercialization of fuel cell vehicles. However, LH2 has evaporation losses and high liquefaction energy consumption during application [59]. Liquid hydrogen infrastructure such as liquefiers and liquid hydrogen vessels requires large initial investments and high liquefaction costs. In addition, under the current technical and economic conditions, the construction of LH2 power plants is more difficult than that of CGH2 power plants [60].

8.3.1.3

Based on Construction Form

According to different structural forms, HRS can also be divided into fixed HRS and mobile HRS. Fixed HRS cannot be moved and are more commonly used [61]. The mobile HRS takes the vehicle’s working area as the service area and forms a small refueling network with the parent station. Compared with the fixed HRS, it has greater flexibility, a larger service radius, wider coverage, a stronger demonstration effect, and more convenient disassembly and assembly. They typically include skid-mounted HRS and mobile hydrogen refueling vehicles. The former integrates hydrogen production storage and filling devices, connecting pipelines and safety facilities into one or more skids, which can be moved as a whole. The latter integrates in-vehicle high-pressure hydrogen storage, transport, loading, self-pressurization, unloading, and filling functions, and is used in conjunction with a stationary HR. In the demonstration operation of HRS, the FCV program of Shanghai World Expo 2010 successfully operated a hydrogen supply network including a hydrogen purification facility, pipeline trailer, two stationary HRS, and two mobile HRS. The stationary HRS can directly fill 90 FCVs and 6 fuel cell buses. The mobile HRS can enter the Expo Park to refuel 100 fuel cell sightseeing vehicles, and go to the Expo HRS to replenish hydrogen.

8.3.1.4

Based on Construction Content

From the perspective of construction content, the hydrogen refueling station system includes an independent hydrogen refueling station system and a combined hydrogen refueling station system [62]. Under the realistic conditions of the high cost of hydrogenation equipment and unclear return expectations, some regions have begun to attach importance to the construction of joint HRSs. According to the type of energy, combined HRS can be further divided into oil-hydrogen combined station,

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gas-hydrogen combined station, and electric-hydrogen hybrid station [63]. In July 2019, Sinopec Foshan Zhangkeng Oil and Hydrogen Synthesis Station was officially completed and put into use [64]. This is the first hybrid power station in China that integrates the “three-in-one” energy supply of oil, hydrogen, and electricity, transforming the existing oil station into an oil-hydrogen combined power station with charging facilities.

8.3.2 HRS Equipment Most HRS consists of hydrogen production, compression, storage, refueling, and control systems. Figures 8.11 and 8.12 illustrate possible device configurations for gas HRS and liquid HRS, respectively [65]. We can see that the key equipment involved includes compressors, hydrogen storage containers, distributors, precooling devices, etc. The performance parameters of the above equipment determine the overall refueling capacity and hydrogen storage capacity of the HRS (Figs. 8.11 and 8.12) [66]. These devices are described in detail in this section.

8.3.2.1

Compressor

The hydrogen compression system is the core of the entire HRS and controls the HRS cost [67]. At present, hydrogen compression technology mainly includes mechanical compression and non-mechanical compression [68]. Among them, mechanical compressors are the most widely used type and are mainly divided into four categories, including piston compressors, diaphragm compressors, linear compressors,

Fig. 8.11 Equipment configuration of gaseous hydrogen refueling station [66]

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Fig. 8.12 Equipment configuration of liquid hydrogen refueling station [66]

and ionic liquid compressors. They both convert mechanical energy into gaseous energy by compressing gaseous hydrogen, resulting in higher hydrogen pressures. Piston compressors use an electric motor to drive a crank and connecting rod mechanism, which acts directly on the piston in the cylinder to achieve the suction and discharge processes [69]. They are typically used in HRSs with larger daily refueling volumes due to the larger compressed flow. At present, the piston compressor technology is mature and the cost is low, and it is the best choice for high-pressure hydrogen compression [70]. A diaphragm compressor is a special structure of a positive displacement compressor. The gas and hydraulic oil are isolated, so the discharged gas has high purity, good heat dissipation, and a large compression ratio. However, it is suitable for HRS substations with small daily refueling due to the smaller stroke and displacement. In recent years, diaphragm compressors have been widely used in demonstration HRS worldwide. American PDC company has launched a hydrogen diaphragm compressor, the exhaust pressure under the maximum suction pressure is 100 MPa, and the displacement is 3000 Nm3 /h. A linear compressor uses a piston directly connected to a linear motor and has a resonant spring system instead of a connecting rod-crank assembly. Compared to the above compressors, the layout of the entire system is simpler, resulting in significant cost savings. Another distinct advantage of linear compressors is that the piston and cylinder are separated by a gas-bearing system, resulting in oil-free operation, higher efficiency, and smaller physical size. Today, linear compressors are particularly suitable for cryogenic applications involving hydrogen and helium, cooling electronics, and domestic refrigeration [71]. The ionic liquid compressor uses an ionic liquid column to compress natural gas and hydrogen. The compression process is closer to an isothermal process because the cooling process of the ionic liquid can effectively remove the heat generated by the compression. Compared to piston compressors, energy consumption can be reduced by about 20% and no additional heat exchangers are required [68]. In addition, ionic

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liquid compressors have few moving parts and long service life, but high cost, long delivery time, high power consumption, and immature technology. On the other hand, there are also some non-mechanical hydrogen compression methods, such as cryogenic compression, metal hydride compression, electrochemical compression, and adsorption compression [72]. Cryogenic compression has the potential to meet the system’s gravimetric and volumetric capacity goals, as well as hydrogen loss goals. It can also reach pressure levels of 70 MPa, which is important for developing efficient HRS. Metal hydride compression is driven by reversible heat between hydrogen and metal/alloy/intermetallic compounds (IMCs). Compressors using this technology can use industrial waste heat or renewable energy instead of electricity. Therefore, they have better system economics and environmental friendliness than mechanical compressors. Electrochemical hydrogen compressors are innovative devices based on the same basic principles as proton exchange membrane fuel cells (PEMFCs) [73]. Their most important advantages are high efficiency, noiseand vibration-free operation. But its performance is affected by the reverse diffusion of gas through the membrane, especially at high pressures. Adsorption compressors use emerging hydrogen compression technology based on adsorption to reduce hydrogen storage pressure. They are used to compress gas streams with low heat by heat transfer between the compression vessel and the system environment [74]. They are usually used in small and medium-sized decentralized hydrogen refueling station management systems.

8.3.2.2

Hydrogen Storage Container

The hydrogen storage container is one of the core equipment of HRS, which largely determines the hydrogen supply capacity. For the FCV to reach a mileage of 400 km after a single hydrogen charge, the ideal onboard hydrogen storage pressure is 35– 70 MPa [75]. The hydrogen storage container usually adopts three storage pressures of low pressure (20–30 MPa), medium pressure (30–40 MPa), and high pressure (40– 75 MPa). Sometimes hydrogen tube trailers (10–20 MPa) are also used as primary storage facilities, constituting a quaternary storage method. Typically, the working pressure of the hydrogen storage vessel is 45 MPa for 35 MPa hours. The working pressure of the hydrogen storage container within 70 MPa hours is 87.5 MPa. Currently, there is a strong demand for cost-effective hydrogen storage systems. High-pressure hydrogen storage facilities have hydrogen storage and pressure buffering functions. The higher the working pressure, the greater the inflation pressure difference, and the shorter the inflation time of the FCV. However, the increase in working pressure will cause the compressor to start frequently. The hydrogen storage pressure vessel mainly adopts the seamless steel hydrogen storage vessel and the high-pressure hydrogen storage vessel of the composite winding structure. The former is mainly a 45 MPa large-capacity steel seamless hydrogen storage cylinder designed according to the requirements of ASME standards and TSG 21-2016. The main body is made of 4130X high-strength structural steel, and the nominal volume of a single cylinder is 0.895 m3 [76]. The latter is generally a large-volume multi-layer

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steel high-pressure hydrogen storage container, such as a 98 MPa buffer container with a volume of 1–20 m3 designed by Zhejiang University.

8.3.2.3

Hydrogen Charging System Equipment

To quickly complete the filling process, the equipment of the hydrogen filling system mainly includes high-pressure filling the pipeline, pneumatic stop valve and its auxiliary explosion-proof solenoid valve, electronic pressure regulator, air gun, temperature, and pressure sensor, mass flowmeter, monitor, sequential gas control panel and chip controller. Most of the above components are integrated into the dispenser. Essential features of the dispenser include overpressure protection, ambient temperature compensation, hose rupture protection, and sequential gas extraction to ensure automatic shut-off of the fueling process in special situations such as emergency shutdowns. The realization of its hydrogen refueling function is integrated with the refueling control device of the tanker and complies with the internationally recognized SAE J2601s standard.

8.3.2.4

Pre-cooling Device

The main requirements of drivers of hydrogen fuel cell vehicles are short hydrogen charging time and a high final state of charge. To meet these requirements, hydrogen precooling is necessary without exceeding the safe limits of the tank material [77]. The SAE J2601 protocol requires that the hydrogen should be pre-cooled to a temperature range of 33–40 °C before distribution to limit the temperature in the vessel onboard to below its maximum value (85 °C) during rapid filling. The cost of the pre-cooling unit (PCU) accounts for about 10% of the total equipment cost of HRSs [78]. In-depth research and analysis of its energy consumption, performance, and design optimization is necessary to achieve the greatest possible cost reduction for hydrogen refueling stations. A typical PCU in an HRS uses a thermal refrigeration cycle, which typically includes a hydrogen cooler, heat exchanger, and piping. The hydrogen in the storage vessel is piped into the heat exchanger and then into the distributor. At the same time, the cooling liquid in the cooler enters the heat exchanger and conducts sufficient heat exchange with hydrogen. Then, the hydrogen is returned to the distributor through the pipe, and the heat-exchanged cooling liquid is returned to the distributor cooler through the pipe. This forms a cycle to cool the hydrogen. Currently, T20 coil coolers are used in the PCU at the California State University Los Angeles Hydrogen Research and Fuels Facility [79]. In addition, a techno-economic and thermodynamic analysis of PCUs in HRSs was performed to optimize their cost and energy utilization [80].

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Process Flow

In the entire process flow of HRSs, hydrogen is usually produced and transported to the site, compressed to a certain pressure by a compressor, stored in a stationary highpressure vessel, and then quickly filled into the FCV through a hydrogen distributor. The specific process mainly depends on the hydrogen transportation/storage form and equipment configuration strategy [81]. The researchers mainly considered hydrogen distribution scenarios, including on-site electrolysis, on-site reforming, CGH2 trucks, LH2 trucks, and CGH2 pipelines [82]. The economic viability of on-site electrolysis or on-site retrofits is primarily limited by the cost of electricity and the availability of construction land. For the early stages of the small HRS market, the CGH2 truck or tubular trailer delivery method is considered a more economically viable option. The above distribution options require different configurations to store, package and distribute hydrogen. We can conclude that the process flow consists of a storage unit, a filling system, and a compressor for gas transfer or a pump/evaporator system for liquid transfer. When the fuel cell vehicle needs to be charged with hydrogen, the active hydrogen charging strategy of low-pressure, medium-pressure, and highpressure storage containers is adopted in sequence. Type IV shipboard hydrogen storage container needs to pre-cool hydrogen to 40 °C for fast refueling at about 1.7 kg/min [83]. If the HRS adopts the 4-stage gas storage method, the distributor first extracts the hydrogen from the tube trailer until the hydrogen pressure in the tube trailer reaches equilibrium with the onboard hydrogen storage container. Then, it is turned to low-pressure, medium-pressure, and high-pressure hydrogen storage vessels to supply hydrogen gas, respectively. When the pressure of the high-pressure storage container cannot fill the onboard container to the set pressure, the compressor starts. The hydrogen is compressed into the three-stage hydrogen storage container in the order of high, medium, and low pressure to prepare for the next hydrogen charging process. This step-by-step filling method is beneficial to reduce compressor consumption and improve filling efficiency.

8.3.3 Layout of Future Hydrogen Refueling Stations The construction of hydrogenation infrastructure is an important part of promoting the rapid commercialization of fuel cell vehicles. The scientific design of HRS process flow and the rational configuration of related equipment are the main factors that determine the technical and economic benefits of HRS. However, the current development of the hydrogen refueling station industry is still limited by its huge capital investment and the immaturity of related equipment technology. Therefore, it is necessary to continuously optimize and upgrade the equipment allocation strategy of the human Resource Center to reduce the asset investment cost and energy consumption as much as possible and ensure the best equipment utilization rate and working performance. Compared with the high cost and high energy consumption of LH2 , CGH2 on a tubular trailer is a more suitable hydrogen source choice at present.

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In addition to the design specification and operating parameter configuration of the hydrogen source itself, it should be further optimized for matching hydrogenation requirements and other systems such as storage, compression, and refueling. Although the three-stage hydrogen storage vessel is considered to be the configuration with the optimal number of stages, its volume ratio per stage has not yet reached the unanimously recognized optimal configuration. It is because the container volume ratio, storage pressure, and container switching mode all affect hydrogen utilization, energy consumption, and filling capacity. Different study boundary conditions and assumed parameters lead to different conclusions about the hydrogen storage configuration. In addition, existing research shows that the increase of the low-pressure volume ratio and the reduction of the high-pressure volume ratio are more conducive to the improvement of the economy of the hydrogen refueling station. Thermodynamic parameters such as compressor throughput and outlet pressure need to be considered in the compressor configuration. A reasonable match of compressor capacity and storage vessel capacity should be based on fueling needs across different vehicle classes and fueling frequencies. In this regard, the optimal compressor displacement, number of high-pressure vessels, and storage capacity at a specific HRS scale can be explored by modeling the entire hydrogen supply process. Hydrogenerator configuration is mainly based on empirical methods and queuing methods. The former ignores the actual fluctuation of the hydrogen lance occupancy rate, leading to deviations from practical applications. The latter contains models with different refueling service time probability distribution characteristics that require further optimization. In general, relevant scholars have done a lot of research on the specific equipment and technology of the hydrogen refueling station system, but there are few reviews on its configuration. Minimizing cost and maximizing equipment efficiency by optimizing parameter design and quantity configuration has become an urgent problem to be solved. Techno-economic configuration and matching optimization of key equipment such as hydrogen supply systems, compressors, high-pressure storage vessels, and distributors are expected to improve the performance of HRS. At the same time, considering the optimized layout, energy consumption, refueling capacity, and total cost of HRS, it is expected to provide a theoretical basis for the design and construction of HRS in the future.

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Chapter 9

The Challenge and Prospect

The number and size of hydrogen electrolysis projects have grown exponentially internationally in recent years, with most pre-2010 projects typically below 0.5 MW and projects in the range of 1–5 MW in 2017–2019. The scale of application has grown to megawatt level development projects, but further research, production scaleup, and innovation in practice are needed to achieve significant cost reductions. Research and development into new materials may have long-term results. However, in the short term, it is important to be able to produce high-performance, cost-effective PEMWE systems based on existing technology. Our argument is therefore based on the confirmation presented in this book that the operation of PEMWE at different temperatures and overpotentials translates into significant lifetime improvements. Strategies for improvement would be the following. (1) Use of ultra-thin, reinforced membranes with good mechanical strength and H2 cross-resistance: the thinner membranes compensate for the reduced proton conductivity at lower temperatures (higher ohmic loss), while the reduced fluoride release rate (FRR) translates into a longer membrane lifetime. (2) An upper limit on the OER overpotential is set to avoid significant catalyst degradation: clearly, higher activation losses (slower OER kinetics) occur at lower temperatures. (3) The coating on the porous transport layer (PTL) is used to reduce contact resistance and passivation. In addition, the morphology/pore structure of the PTL is designed to provide improved catalyst utilization and smooth support for the ultra-thin film. (4) Limiting electrochemical gas compression to a state below that of triggering catalyst or PTL degradation or pressure leading to significant hydrogen embrittleness in the PTL. Obviously, cross safety limits should not be exceeded. An additional benefit of working at lower overpotentials and temperatures is that a wider range of stable support and coating materials can further help to reduce the cost of the components. For example, Ir catalysts dispersed in stable support materials (e.g. carbides or nitrides of transition metals) can improve catalyst performance. PTL © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 S. Peng, Electrochemical Hydrogen Production from Water Splitting, https://doi.org/10.1007/978-981-99-4468-2_9

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and bipolar plates (BPP) can be made from stainless steel, while scarce and expensive metal coatings (e.g. Ir and Au) can be used, such as cost-effective Ti/Nb coatings. While there are many opportunities for advancing PEM electrolytic technology, the transition to commercial production is likely to involve a long timeline for three main reasons. Firstly, as previously mentioned, the interactions between components, interfaces, and manufacturing processes require the simultaneous development of multiple elements before new materials can be incorporated. Secondly, literature varies widely and standard tests are required to better match performance from lab to lab. Electrolytic cells have a very long life expectancy, which proves to be very challenging for new designs to meet these expectations. Therefore, the priority research area of electrolytic elements is to achieve the characterization of degradation mode and failure path. We need improved methods for predicting lifetime, particularly accelerated testing that does not require months of operation to understand changes in electrolyzer durability in response to design changes. Some initial efforts have shown promise in accelerating catalyst migration and other effects. To effectively accelerate degradation to predict long-term performance, the mechanisms must be understood. Such work is likely to be an iterative process, with initial test parameters defined as a start and modified with additional understanding. However, this opportunity represents the potential to reduce capital costs by over 50% and reduce operating costs through increased efficiency. In addition, PEM electrocatalysts need further research and development. For example, the reaction pathways and active sites of HER and OER catalysts during seawater electrolysis have been further defined by combining experimental and theoretical analyses: in addition to monometallic compounds, various polymetallic compounds and heterostructured catalysts have been extensively investigated as electrolysis catalysts. There is a trend to design catalysts consisting of multiple metal components by exploiting the synergistic effect of the multi-metal components. As catalyst components become more complex, it becomes increasingly difficult to identify electrocatalytic reaction pathways and active sites. Therefore, systematic studies based on theoretical analyses are necessary to guide the design of materials with desirable structures and properties. In situ characterization methods are used to reveal the true active site of the catalyst: many current electrocatalysts, such as metal oxides, phosphides, and nitrides, undergo surface oxidation or recombination in aqueous electrolysis, meaning that the true active site of the catalyst may change during the reaction. As each step in the catalytic reaction process is changing rapidly, we need to employ in situ characterization techniques to track changes in intermediates during the catalytic reaction process. If more in-situ techniques such as in-situ XAS, Raman, Fourier transform infrared spectroscopy and other new techniques are involved in the mechanistic study, it will provide clear principles and guidance for the design of efficient catalysts. Despite the rapid development of electrolytic water, the PEM electrolysis route to hydrogen production is still very expensive and difficult to scale up. Further investment by academia and industry in the development of low-cost, energy-efficient electrocatalysts is therefore essential for future market growth. With 1.2 billion people worldwide living in water-scarce regions, there is an opportunity to further develop energy-efficient and economically attractive desalination technologies. A

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great deal of effort has been made by researchers in the field of seawater electrolysis in recent years, but there is still much room for improvement in the area of high-performance hydrogen production from seawater electrolysis. Seawater electrolysis is more complex than freshwater electrolysis due to the presence of multiple cations and anions. Various materials such as catalysts for seawater electrolysis have been extensively investigated, but the catalytic activity and stability of most of the reported catalysts still do not meet the requirements of practical applications, and most of the electrolytes studied so far are artificial brines rather than real seawater. Therefore, more efforts are needed to achieve high-performance seawater electrolysis. Such as exploring the development of highly active and stable electrocatalysts in seawater and designing advanced seawater electrolysis reactors. Current research on the electrolysis of hydrogen from seawater is mainly focused on catalysts. To achieve electrocatalytic production of hydrogen, we need to consider the whole reactor, not just the catalyst. It is necessary to rationalize the design of the reactor to suit the particular seawater electrolysis. For example, an asymmetric reactor design is considered more promising, which includes alkaline water in the anode chamber and seawater in the cathode chamber. This design not only promotes the diffusion of Cl− to the anode but also protects the anode catalyst, which is of great importance in seawater electrolysis. PEM and AWE hydrogen technologies are still relatively expensive in terms of production and operating costs and investment in equipment compared to fossilbased hydrogen production. However, these two electrolysis technologies have great potential for cost reduction, given the rapid technological progress, the increased availability of the corresponding components, the huge hydrogen market demand, and the strategic deployment of energy. For solid oxide as well as anion-exchange membrane electrolysis technologies, cost reduction is relatively difficult, as only a few companies are working on their commercialization. In addition, many of its components are still at a laboratory scale and no original manufacturers have undertaken production and commercialization. Solid oxide electrolysis offers significant advantages in this respect. Due to its operation at high temperatures (700–900 °C), the efficiency of electrolytic high voltage can approach 100%. The technical characteristics of solid oxide electrolysis determine that it is very suitable for the large-scale, efficient preparation of hydrogen energy in coupling with primary energy sources that can provide both electrical and thermal energy. High-voltage technology is suitable for the distributed supply of cold and hot power and large stationary power stations. It is an indispensable green power generation technology for the future electricity market. The current development of solid oxide electrolysis technology applied to large-scale renewable energy storage technology routes includes mainly: gas-fired power generation (PtG), liquid power generation (PtL), and a combination of both. Namely, through solid oxide electrolysis, the energy from renewable energy is efficiently converted into H2 (or syngas) storage and the prepared H2 can enter the gas network or can be used for power generation; at the same time, the solid oxide electrolysis method can also communicate with the water network to achieve an efficient and optimal configuration of the entire energy network. The energy source for the solid oxide electrolysis method can be nuclear energy, various renewable

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energy sources, or various other high-temperature heat sources. A German company proposes the use of reversible Solid oxide cells (SOC) technology for the preparation of hydrogen for steelmaking. At the same time, waste gas from steel mills can be used for fuel cell power generation. Reversible SOC technology can be used as a medium for efficient and clean use of resources and energy. Certain methods can be used to connect these energy networks by using the medium as a substance for energy networks and as an energy exchange medium for electricity, heat, gases, etc. Compared to AWE or PEM electrolysis for hydrogen production, the development of solid oxide as well as anion-exchange membrane electrolysis technology has a long way to go. The cost of hydrogen production by electrolysis generally includes (i) equipment cost; (ii) energy cost (electricity); (iii) other operating costs; and (iv) raw material cost (water). Among these, the energy cost is the largest, typically 40–60% (AWE/ PEM) and even up to 80%, and is mainly driven by the energy conversion efficiency (i.e. electrolysis to hydrogen efficiency) factor, with the equipment cost being the next largest. According to IRENA (2020), the cost of hydrogen production decreases significantly when the electricity price is $20 MWh−1 ($0.13 kWh−1 ) compared to $65 MWh−1 ($0.42 kWh−1 ), and the decrease is significantly higher than the decrease in the cost of electrolyzer equipment (from $1000 kW−1 to $650 kW−1 ), i.e. the reduction in equipment costs does not compensate for the impact of high electricity prices. The hydrogen energy industry chain is relatively long and typically including high-pressure, storage, transport, hydrogen refueling, and end-use applications. It involves many fields such as chemicals, electricity, transportation, and automobiles. China’s hydrogen industry is growing rapidly and maintaining an upward trend year after year. China’s average annual growth rate continues to grow and it ranks first in the world in terms of production and demand. According to the market, the China Hydrogen Energy Alliance predicts that China’s hydrogen demand will reach 35 million tons by 2030. By 2050, the national demand for hydrogen will be close to 60 million tons. As can be seen, hydrogen energy is becoming more and more widely used in China and the market is expanding at a faster and faster rate. For the Chinese market, when the cost of hydrogen production drops below 20 RMB kg−1 , only then does electrolytic hydrogen production have some competitive advantage over fossil energy hydrogen production. For alkaline electrolyzers, the cost of equipment is mainly driven by the cost of core components such as electrodes and diaphragms. Over 50% of the cost component of the electrolytic stack in an alkaline electrolyzer is related to electrodes and diaphragms, compared to 24% of the cost of membrane electrodes in a PEM electrolyzer stack. In alkaline electrolyzers, the bipolar plates represent only a small part of the cost of the electrolytic stack, whereas in PEM electrolytic stacks they represent more than 50% of the cost due to the simpler design of the bipolar plates in alkaline electrolyzers, simpler manufacture, cheaper materials (nickel plated steel) and the redesign of the electrodes and diaphragms to reduce costs. The auxiliary part of the alkaline electrolytic hydrogen production system, the lye circulation, and the hydrogen after-treatment are more important for cost reduction. For PEM electrolyzers, the cost of the electrolytic stack equipment is

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mainly driven by the cost of core components such as bipolar plates. Bipolar plates account for approximately 53% of the cost of the electrolytic stack in PEM cells, mainly because they usually require Au or Pt coatings. Technological innovation plays an important role in enhancing the performance and durability of bipolar plates as well as reducing costs. Cheaper alternative materials are being investigated, such as the use of Ti coatings to keep their functional properties unaffected while reducing costs. The rare metal Ir is an important component of membrane electrode materials and in practice, although Ir accounts for less than 10% of the cost of the overall PEM electrolysis system, it can become a bottleneck in late-stage PEM electrolyzer production due to severe supply shortages. Water circulation and hydrogen posttreatment in the auxiliary components of PEM electrolysis hydrogen production systems are also important areas for cost reduction. Reducing the cost of green hydrogen requires not only government policies and incentives on renewable electricity, but also advances and breakthroughs in the development of key materials by researchers. This will facilitate the scaling up of production and thus reduce the cost of equipment. The cost of electrolytic hydrogen production equipment can be reduced in two ways. One is to start with electrolyzer design and single cell materials, using fewer key materials, especially higher cost of precious metals such as Pt and Ir, or replacing them with non-precious metals (Ni, Fe, etc.). Redesigning the electrolyte to achieve higher efficiency (lower cost of electricity), higher durability (longer life), and higher current density. This can be achieved by optimizing film thickness to reduce ohmic resistance (while also taking into account gas permeation issues) to improve electrolytic efficiency; and then, structural optimization of key components such as porous transport layer and bipolar plates, such as optimizing PTL structural parameters such as porosity, pore size, and thickness, and adopting a 3D grid structured flow field, to improve electrolytic cell performance and lifetime. The second is to improve application economics by increasing the scale of single-cell and plant production, reducing the cost per component by implementing a high throughput, automated manufacturing process. Increasing the size of a single cell can lead to economies of scale. Although the scope for increasing the size of a single tank is limited due to leakage, large module manufacturing constraints, mechanical instability of large modules, and maximum cell area limitations, strong economic effects can still be generated. A study by PlanDelyKad in Germany found that a 100 MW alkaline electrolyzer (costing e520 kW−1 ) was approximately 50% cheaper than a 5 MW electrolyzer (costing e1070 kW−1 ). However, the cost reductions from increased capacity diminish considerably beyond 10 to 20 MW. China has become the world’s number one producer of hydrogen and leads the world in industrial hydrogen production. China is expected to deploy about 500 GW of installed electrolytic hydrogen production capacity in 2060 and will be a major producer and user of hydrogen in the future hydrogen energy market. It is expected that by 2060, hydrogen energy will be widely used in transportation, energy storage, industry, and construction and that China’s hydrogen demand will rise from the current 30 million tons to about 130 million tons, an increase of more than 300%. In the future, hydrogen energy is expected to open up the penetration path of renewable electricity in transportation, industry, and construction end-use applications, gradually reducing the

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proportion of fossil energy consumption in these end-use areas. With breakthroughs in material and component preparation, system integration and other technologies, the green hydrogen energy production technology will develop towards extending the operational life, increasing the unit power, reducing safety risks and costs, etc. The key component materials will be localized, the unit power of hydrogen production will be increased to a 10 MW level and the system unit energy consumption will not be higher than 4 kWh m−3 . To realize the large-scale application of hydrogen energy, an in-depth research is still needed in the following areas. (1) Research the impact of new energy input on the electrolyzer and hydrogen production system to solve the problem of a high proportion of renewable energy to the grid. Under the random and fluctuating input of new energy, the changes in hydrogen and oxygen concentration and pressure of the hydrogen production system caused by the variable working conditions and frequent start-up and shutdown operation characteristics have put forward new requirements for the safe and stable operation of the equipment. There is a lack of international research on these aspects, and there is a lack of microscopic analysis and experimental research data on the impact of new energy input on electrolyzers and hydrogen production systems, and the compatibility and compatibility between electrolysis equipment and fluctuating power sources need to be improved. Therefore, the impact of new energy input on the electrolyzer and hydrogen production system (mainly AWE and PEM) needs to be studied in depth shortly, to promote the large-scale demonstration application of renewable energy electrolysis for hydrogen production. (2) Improving electrolyzer and system reliability and durability. At present, there is still a gap between the reliability and durability of Chinese electrolyzers and systems under full working conditions and at the international advanced level. The reliability and lifetime of the electrolyzer system are not only related to the electrolytic reactor but also depend on the supporting auxiliary equipment. Therefore, it is necessary to further strengthen the research on the reliability and durability of electrolytic tank products, promote the participation of electrolytic hydrogen production technology in the peak and frequency regulation of power grids and increase the interaction with power grids. (3) Improve the level of independent research of key materials and core components of electrolytic cells. From the analysis of the cost of green hydrogen, it can be seen that the cost of electrodes, diaphragms, bipolar plates, etc. accounts for a relatively high proportion, but at present, the gap between China’s R&D level on key materials and core components and foreign countries is large, and it relies heavily on imports and does not have the ability of mass production, which seriously restricts the scale development of China’s electrolytic hydrogen production industry. Therefore, it is urgent to strengthen the independent R&D level of key materials and core components, accelerate the formation of batch preparation methods with completely independent intellectual property rights, and fully realize the localization of key materials and core components. As a clean and non-polluting secondary energy source that can be stored for a long time, hydrogen energy can help achieve deep decarbonization in various fields

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such as industry, transportation, and construction. In particular, with the increasing proportion of renewable energy, the combination of electrolytic hydrogen production technology and renewable energy can maximize the amount of renewable energy consumed and smooth out its fluctuations, ensuring the safe and stable operation of the grid. Thus, the use of renewable energy for hydrogen production has a bright future for sustainable development.