Energy, Environment and New Materials: Volume 2 Hydrogen Storage for Sustainability 9783110596281, 9783110596236

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Marcel Van de Voorde (Ed.) Hydrogen Storage for Sustainability

Also of Interest Ethics in Nanotechnology Emerging Technologies Aspects Marcel Van de Voorde, Gunjan Jeswani (Ed.),  ISBN ----, e-ISBN ----

Ethics in Nanotechnology Social Sciences and Philosophical Aspects Marcel Van de Voorde, Gunjan Jeswani (Eds.),  ISBN ----, e-ISBN ----

Handbook of Nanoethics Gunjan Jeswani, Marcel Van de Voorde (Eds.),  ISBN ----, e-ISBN ----

Nanoscience and Nanotechnology Advances and Developments in Nano-sized Materials Marcel Van de Voorde (Ed.),  ISBN ----, e-ISBN ----

Hydrogen Storage for Sustainability Volume II Edited by Marcel Van de Voorde

Volume Editor Professor Dr. Dr. h. c. mult. Marcel Van de Voorde University of technology DELFT (NL) Rue du Rhodania, 5, BRISTOL – A, App. 31 3963 CRANS – MONTANA Switzerland

ISBN 978-3-11-059623-6 e-ISBN (PDF) 978-3-11-059628-1 e-ISBN (EPUB) 978-3-11-059431-7 Library of Congress Control Number: 2021933279 Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2021 Walter de Gruyter GmbH, Berlin/Boston Cover image: Petmal/iStock/Getty Images Plus Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck www.degruyter.com

Hydrogen will play a decisive role in future attempts to solve the challenges in connection with global warming. This requires enormous technological and political attempts. Professor Gerhard ERTL, Nobel Prize in Chemistry 2007 Hydrogen is often called the missing link of the energy transmission but equally important it is also a unique opportunity to create the necessary prosperity in our society Bart BIEBUYCK Executive Director EU Research Programme FCHJU (Fuel Cell and Hydrogen Joint Undertaking)

Series editor preface The decarbonization of the energy system is critical to reach the European climate objectives for both 2030 and 2050. In this respect, the “European Green Deal” put forward the need to ensure a smart integration of renewable energy sources, energy efficiency, and other sustainable solutions such as carbon capture. The “European Green Deal” also recognized the production of hydrogen, hydrogen storage, hydrogen networks, and the utilization of hydrogen as one of the important technology options that can ensure that the EU industry remains at the technology forefront and delivers breakthrough technologies in key industrial sectors. The book provides compiling reviews on these aspects from internationally recognized researchers, industrialists, and government agencies, and assembles topclass contributions. The topical scope of the book is broad, ranging from hydrogen production, storage of hydrogen, and the multiple applications but also the development of many new materials and new technologies and innovations will be necessarily developed in many fields of physics, chemistry, biology, and mechanical engineering. Common to all the books in the series is that they represent top-notch research and are highly application-oriented, innovative, and relevant for industry. The titles of the volumes in the series Hydrogen Technologies for Sustainable Economy are: – Hydrogen Production and Energy Transition – Hydrogen Storage for Sustainability – Utilization of Hydrogen for Sustainable Energy and Fuels They fall in the topics of energy, environment, and new materials but mobile applications such as automobiles, air, and space transport are becoming very popular. The book series appeals to a wide range of readers with backgrounds in physics, chemistry, biology, and engineering but also for those interested in renewables energy, environment, economy, and industrial safety. Students at universities to scientists at institutes and technologists in industrial companies may also have a great interest in this new hydrogen energy source. Ever since hydrogen was introduced many years ago it will greatly change our lives for the next generations. Developments are planned for many areas, which will result in a new “hydrogen economy”; in short, hydrogen technology is a hot topic! Marcel Van de Voorde, November 2020

https://doi.org/10.1515/9783110596281-202

Hydrogen Technology – Innovations and Applications Volume  Hydrogen Production and Energy Transition ISBN ---- e-ISBN ----

Volume  Hydrogen Storage for Sustainability ISBN ---- e-ISBN ----

Volume  Utilization of Hydrogen for Sustainable Energy and Fuels ISBN ---- e-ISBN ----

About the series editor Marcel Van de Voorde: Prof. Dr. Ing. ir. Dr. h. c., has many years’ experience in European Research Organizations, including CERN- Geneva and the European Commission research. He was involved in research, research strategies and management. He is emer. professor at the University of Technology, Delft (NL), holds multiple visiting professorships and is doctor honoris causa. He has been a member of numerous Research Councils and Governing Boards: e.a. CSIC (F), CNR (I), CSIC (E), NIMS (JP), of science and art academies, of the Science Council of the French Senate and the National Assembly, in Paris, and Fellow of multiple scientific societies. He has been honored by the Belgian King and received an award for European merits in Luxemburg by the former President of the European Commission. He is author of multiple scientific and technical publications and books.

Thanks to my wife for the patience with me spending many hours working on the book series through the nights and over weekend. The assistance of my son Marc-Philip related to the complex and large computer files with many sophisticated scientific figures is also greatly appreciated. Marcel Van de Voorde

https://doi.org/10.1515/9783110596281-203

Contents Volume II: Hydrogen technology: innovation and applications Series editor preface

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Volume editor: Marcel Van de Voorde List of contributors (for Volume II)

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Paolo Ciambelli, Marcel Van de Voorde Hydrogen: Present Accomplishments and Far-Reaching Promises

Forewords Louis Schlapbach Foreword 9 Alexander Wokaun Foreword 15

Extended Introductions Pierre Etienne Franc Hydrogen: why the times to scale have come Ad van Wijk Hydrogen key to a carbon-free energy system Paula Abreu Marques, Ruud Kempener The European hydrogen strategy 105 Andreas Züttel Introduction to the hydrogen books Václav Bartuška Geopolitics of hydrogen

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Volume II: Hydrogen storage for sustainability Romano Giglioli 1 Overview for hydrogen storage

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Barbara Thijs, Maarten Houlleberghs, Lander Hollevoet, Gino Heremans, Jan Rongé, Johan A. Martens 2 Hydrogen fueling the future: introduction to hydrogen production and storage techniques 159 Mieczyslaw Jurczyk, Marek Nowak 3 Materials overview for hydrogen storage

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Jean-Marc Bassat 4 Survey of SOFC cathode materials: an extended summary Ankur Jain, Shivani Agarwal, Takayuki Ichikawa 5 Ammonia: a promising candidate for hydrogen economy

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Tom Depover, Kim Verbeken 6 Hydrogen diffusion in metals: a topic requiring specific attention from the experimentalist 247 Marek Nowak, Mieczysław Jurczyk 7 Nickel metal hydride batteries

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Zhao Zhang, Xianda Li, Omar Elkedim 8 Methods of preparing hydrogen storage materials Mieczyslaw Jurczyk, Marek Nowak 9 RE–Mg–Ni hydrogen storage alloys

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Dina Lanzi, Cosma Panzacchi, Christian Coti, Donatella Barbieri, Pierpaolo Ferraro, Francesco Maria Augusto Ghidoni, Matteo Scapolo, Sara Vassallo 10 Hydrogen storage 347

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Felipe Rosa, Alfredo Iranzo 11 An overview of technological research needs for a successful hydrogen economy deployment 375 Marcel Van de Voorde, Paolo Ciambelli Conclusions and Recommendations: “The Future of Hydrogen” Index

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Volume I: Hydrogen production and energy transition Paolo Ciambelli, Marcel Van de Voorde Hydrogen: Present Accomplishments and Far-Reaching Promises List of Contributors (for Volume I) Volume Editor: Marcel Van de Voorde

Forewords Louis Schlapbach Foreword Alexander Wokaun Foreword

Extended Introductions Pierre Etienne Franc Hydrogen: why the times to scale have come Ad van Wijk Hydrogen key to a carbon-free energy system Paula Abreu Marques, Ruud Kempener The European hydrogen strategy Andreas Züttel Introduction to the hydrogen books Václav Bartuška Geopolitics of hydrogen Volume I: Hydrogen production and energy transition Gaetano Iaquaniello, Emma Palo, Annarita Salladini 1 An overview of today’s industrial processes to make hydrogen and future developments’ trend

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Volume I: Hydrogen production and energy transition

Paolo Ciambelli 2 Catalytic autothermal reforming for hydrogen production plant to distributed energy system Oscar Daoura, Maya Boutros, Franck Launay 3 An overview of recent works on Ni silica-based catalysts for the dry reforming of methane Maria Mikhail, Jacques Amouroux, Maria Elena Galvez, Stéphanie Ognier, Patrick Da Costa 4 CO2 hydrogenation by plasma-assisted catalysis for fuel production: power-to-gas application Alberto Giaconia, Massimiliano Della Pietra, Giulia Monteleone, Giuseppe Nigliaccio 5 Development perspective for green hydrogen production Long Han, Qinhui Wang 6 Hydrogen production from biomass pyrolysis Qinhui Wang, Long Han 7 Gasification of biomass and plastic waste Martin Paidar, Karel Bouzek 8 Water electrolysis as an environmentally friendly source of hydrogen Nicolas Grimaldos-Osorio, Kristina Beliaeva, Jesús González-Cobos, Angel Caravaca, Philippe Vernoux 9 Electrolysis for coupling the production of pure hydrogen and the valorization of organic wastes Stefano Campanari, Paolo Colbertaldo, Giulio Guandalini 10 Renewable power-to-hydrogen systems and sector coupling power-mobility Paolo Ciambelli, Maria Sarno, Davide Scarpa 11 Photoelectrocatalytic H2 production: current and future challenges Dimitrios A. Pantazis 12 Biological water splitting

Volume I: Hydrogen production and energy transition

Gunther Kolb 13 Fuel processing for fuel cells and energy-related applications Cheng Yi Heng, Susu Nousala 14 Emergent-based well-being design for a hydrogen-based community: social acceptance and societal evolution for novel hydrogen technology Giuseppe Ricci, Maurizio Dessi, Marco Tripodi, Paolo Fiaschi, Roberto Palmieri, Luca Eugenio Basin, Thomas Pasini, Alessandra Guarinoni 15 Eni’s projects in Italy for hydrogen production Marcel Van de Voorde, Paolo Ciambelli Conclusions and Recommendations: “The Future of Hydrogen”

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Volume III: Utilization of hydrogen for sustainable energy and fuels Paolo Ciambelli, Marcel Van de Voorde Hydrogen: Present Accomplishments and Far-Reaching Promises List of Contributors (for Volume III) Volume Editor: Marcel Van de Voorde

Forewords Louis Schlapbach Foreword Alexander Wokaun Foreword

Extended Introductions Pierre Etienne Franc Hydrogen: why the times to scale have come Ad van Wijk Hydrogen key to a carbon-free energy system Paula Abreu Marques, Ruud Kempener The European hydrogen strategy Andreas Züttel Introduction to the hydrogen books Václav Bartuška Geopolitics of hydrogen

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Volume III: Utilization of hydrogen for sustainable energy and fuels

Volume III: Utilization of hydrogen for sustainable energy and fuels Gabriele Centi, Siglinda Perathoner 1 Applications of hydrogen technologies and their role for a sustainable future Tobias Christoph Brunner 2 Perspectives of hydrogen in trucks Katsuhiko Hirose 3 Hydrogen for transport Laurent Allidières 4 Introduction to hydrogen energy: from applications to technical solutions Luigi Crema, Matteo Testi, Martina Trini 5 High-temperature electrolysis: efficient and versatile solution for multiple applications Luca Sementa, Fabio R. Negreiros, Alessandro Fortunelli 6 The use of hydrogen in ammonia synthesis, and in oxygen and carbon dioxide catalytic reduction – the reaction mechanisms Michel Noussan 7 The potential of hydrogen passenger cars in supporting the decarbonization of the transport sector Massimo Prastaro 8 The hydrogen as a fuel Urs Cabalzar, Christian Bach, Stefan Hiltbrand, Patrick Stadelmann 9 Hydrogen refueling of cars and light-duty vehicles Thomas Von Unwerth 10 Fuel cells for mobile applications Jens Mitzel, K. Andreas Friedrich 11 Hydrogen fuel cell applications

Volume III: Utilization of hydrogen for sustainable energy and fuels

Christophe Coutanceau, Marian Chatenet, Deborah Jones, Gael Maranzana 12 Materials for proton-exchange fuel cell for mobility and stationary applications Ciro Caliendo, Paola Russo, Paolo Ciambelli 13 Hydrogen safety: state of art, perspectives, risk assessment, and engineering solutions Giuseppe Ricci, Laura Prosperi, Maurizio Dessì, Marco Tripodi 14 Hydrogen application in Eni: from industrial application to power generation Marco Chiesa, Alessio Zolla 15 Hydrogen for mobility Paul E. Dodds, Daniel Scamman, Paul Ekins 16 Hydrogen distribution infrastructure Henning Zoz, Tejas Bopardikar 17 Power to gas to fuel – P2G2F® Paolo Ciambelli, Marcel Van de Voorde Conclusions and Recommendations: “The Future of Hydrogen”

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List of contributors Marcel Van de Voorde University of technology DELFT (NL) Rue du Rhodania, 5, BRISTOL – A, App. 31 3963 CRANS – MONTANA Switzerland [email protected] Paolo Ciambelli University of Salerno and Narrando srl Via Giovanni Paolo II, 132 84084 Fisciano Italy [email protected] Louis Schlapbach ETH & Empa, Zurich Switzerland [email protected] Alexander Wokaun ETH Zurich Switzerland [email protected] Pierre Etienne Franc Hydrogen Energy World Business Line, Air Liquide Hydrogen Council Secretary Air Liquide, France, 75 quai d’Orsay Paris France [email protected] Ad van Wijk Department Process and Energy Faculty of Mechanical, Maritime and Materials Engineering. TU Delft Leeghwaterstraat 39 2628 CB Delft The Netherlands [email protected] Andreas Züttel Laboratory of Materials for Renewable Energy (LMER) https://doi.org/10.1515/9783110596281-207

Institute of Chemical Sciences and Engineering (ISIC) Basic Science Faculty (SB) École polytechnique fédérale de Lausanne (EPFL) Valais/Wallis Energypolis Rue de l’Industrie 17, CP 440 CH-1951 Sion Switzerland and Empa Materials Science and Technology, Dübendorf Switzerland [email protected] Dina Lanzi Head Hydrogen Technology Development Snam S.p.A. Business Unit Hydrogen Piazza Santa Barbara 7 20097 San Donato Milanese(MI) Italy [email protected] Cosma Panzacchi Executive Vice President Business Unit Hydrogen Snam S.p.A. Business Unit Hydrogen Piazza Santa Barbara 7 20097 San Donato Milanese(MI) Italy [email protected] Christian Coti Head Giacimenti Snam S.p.A. Business Unit Asset Italia - Stoccaggi Via Libero Comune 5 26013 Crema (CR) Italy [email protected] Donatella Barbieri Reservoir Engineer Snam S.p.A.

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Business Unit Asset Italia - Stoccaggi Via Libero Comune 5 26013 Crema (CR) Italy [email protected] Pierpaolo Ferraro Reservoir Engineer Snam S.p.A. Business Unit Asset Italia - Stoccaggi Via Libero Comune 5 26013 Crema (CR) Italy [email protected] Francesco Maria Augusto Ghidoni Hydrogen Technology Development Expert Snam S.p.A. Business Unit Hydrogen Piazza Santa Barbara 7 20097 San Donato Milanese(MI) Italy [email protected] Matteo Scapolo Reservoir Engineer Snam S.p.A. Business Unit Asset Italia - Stoccaggi Via Libero Comune 5 26013 Crema (CR) Italy [email protected] Sara Vassallo Hydrogen Business Development Expert Snam S.p.A. Business Unit Hydrogen Piazza Santa Barbara 7 20097 San Donato Milanese(MI) Italy [email protected] Paula Abreu Marques European Commission Directorate General for Energy DM24 4/138 B-1049 Brussels Belgium [email protected]

Ruud Kempener Directorate General for Energy European Commission DM24 4/138 B-1049 Brussels Belgium Vaclav Bartuska Ambassador-at-Large for Energy Security Ministry of Foreign Affairs Loretanské nam. 5 118 00 Praha 1 Czech Republic [email protected] Romano Giglioli Università di Pisa – DESTEC Dipartimento di Ingegneria dell’Energia, dei Sistemi, del Territorio e delle Costruzioni Largo Lucio Lazzarino 56122 – PISA Italy [email protected] Barbara Thijs COK-KAT division, Department of Microbial and Molecular Systems KU Leuven Celestijnenlaan 200 F P.O. box 2461 B-3001 Heverlee Belgium [email protected] Maarten Houlleberghs COK-KAT division, Department of Microbial and Molecular Systems KU Leuven Celestijnenlaan 200 F P.O. box 2461 B-3001 Heverlee Belgium [email protected] Lander Hollevoet COK-KAT division, Department of Microbial and Molecular Systems KU Leuven Celestijnenlaan 200 F P.O. box 2461

List of contributors

B-3001 Heverlee Belgium [email protected] Gino Heremans COK-KAT division, Department of Microbial and Molecular Systems KU Leuven Celestijnenlaan 200 F P.O. box 2461 B-3001 Heverlee Belgium [email protected] Jan Rongé COK-KAT division, Department of Microbial and Molecular Systems KU Leuven Celestijnenlaan 200 F P.O. box 2461 B-3001 Heverlee Belgium [email protected] Johan A. Martens COK-KAT division, Department of Microbial and Molecular Systems KU Leuven Celestijnenlaan 200 F P.O. box 2461 B-3001 Heverlee Belgium [email protected] Mieczyslaw Jurczyk Poznan University of Technology Institute of Materials Science and Engineering Poznan Poland [email protected] Marek Nowak Poznan University of Technology Institute of Materials Science and Engineering Poznan Poland [email protected]

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Jean-Marc Bassat ICMCB – CNRS 33600 PESSAC cedex France [email protected] Ankur Jain School of Applied Sciences Suresh Gyan Vihar University Mahal Road, Jagatpura Jaipur – 302017 Rajasthan, India Shivani Agarwal Department of Physics JECRC University IS 2036-2039 Ramchandrapura Industrial Area Vidhani, Sitapura Extension Jaipur – 303905 Rajasthan, India Takayuki Ichikawa Graduate School of Advanced Science and Engineering Hiroshima University Higashi-Hiroshima Hiroshima 739-8527 Japan Tom Depover Research group Sustainable Materials Science Department of Materials Textiles and Chemical Engineering Ghent University (UGent) Technologiepark 46, B-9052 Ghent Belgium [email protected] Kim Verbeken Research group Sustainable Materials Science Department of Materials Textiles and Chemical Engineering Ghent University (UGent) Technologiepark 46, B-9052 Ghent Belgium [email protected]

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Zhao Zhang North University of China School of Energy and Power Engineering No. 3, Xueyuan Road, Taiyuan, China 030051 Xianda Li Dept. Materials Science Xi’an Technological University No.2 Xuefuzhonglu Roa Weiyang District, Xi’an Shaanxi Province, 710021 China Omar Elkedim FEMTO-ST, MN2S, Université Bourgogne Franche-Comté Site de Sévenans, 90010 Belfort cedex France [email protected] Felipe Rosa Iglesias Laboratory of Engineering for Energy and Environmental Sustainability

Department of Energy Engineering ETSI Sevilla. Camino de los Descubrimientos s.n. 41092 Seville Spain [email protected] Alfredo Iranzo Laboratory of Engineering for Energy and Environmental Sustainability Department of Energy Engineering ETSI Sevilla. Camino de los Descubrimientos s.n. 41092 Seville Spain [email protected]

Paolo Ciambelli, Marcel Van de Voorde

Hydrogen: Presents Accomplishments and Far-Reaching Promises Despite the realistic analysis of the waving trend of the enthusiasm for hydrogen in the last 50 years, starting from the oil crisis in the 1970s to announce the future of a hydrogen economy, today the driving force of the global warming issue and a favorable convergence of interests by different stakeholders support the role of hydrogen as zero-carbon fuel much strongly than before. After the Paris Agreement on climate in 2015, many countries produced national road maps and collaboration projects involving hydrogen in order to reduce greenhouse gas emissions and achieve deep decarbonization. Pioneering countries in investing huge financial resources are Japan and Germany. To reach this goal, it is expected that hydrogen could be produced from renewable sources, consumed with no pollution, and universally used, for example, in transportation, energy storage, residential and industrial applications, and high-grade heat production. In the last 2 years, IEA [1] and WEC [2] reports confirmed the “unprecedented momentum for hydrogen,” the European Union (EU) has increased the financial support to research and innovation on hydrogen, and new investment plans are frequently announced, often at a gigawatt scale. At January 2021 Linde Company announced it will build, own and operate the world’s largest (24 MW) Proton Exchange Membrane electrolyzer plant in Germany The most concrete proof of this renewed interest is the publication by the EU Commission of two just published documents, inside the EU’s Green Deal and its Covid recovery plan, which assign to hydrogen a fundamental role to contribute to reach a climate-neutral and zero-pollution economy in 2050 [3, 4]. Even more surprising was the China’s President Xi Jinping announcement (September 2020) of pledge to achieve carbon neutrality before 2060, taking into account that China is the largest carbon emitter (28.6% share of global carbon emissions in 2018) and energy consumer. The target of European Green Deal (55% reduction in emissions by 2030 with reference to 1990 and carbon neutral by 2050) is mostly based at medium term on electrolyzers (40 GW at 2030). It is not an easy challenge, if one takes into account at least two main aspects: first, hydrogen today accounts for only a small fraction of the energy mix, while its contribution is expected to reach at least 15% in 30 years; second, more than 95% of 70 million tons of hydrogen yearly consumed for industrial purposes (mostly ammonia, methanol, and oil refining) is today still produced from fossil fuels (grey hydrogen from natural gas in Europe), giving a huge contribution to annual carbon dioxide release, close to 100 million tons. Therefore, the decarbonization goal requires a full change to cleaner hydrogen, especially green H2 (water, wind, and sun related). There is a need to decrease the cost of renewable hydrogen, although there is a continuous https://doi.org/10.1515/9783110596281-001

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Paolo Ciambelli, Marcel Van de Voorde

improvement in this direction. Even more critical is the availability of all renewable energies necessary to produce all H2. Therefore, storage, especially on a medium-tolong term is a critical issue to be considered. The only solution is to produce H2 in remote areas where this can be cost-competitive, but here the problem is the transport from these regions. In terms of use, in the first phase, as indicated in the cited H2 strategy, this can be just the substitution of grey H2 with cleaner sources, but in the longer term (beyond about 2025) the issue is the development of new uses of H2, from mobility to industrial uses, in competition with the direct use of renewable electrical energy. There is thus the need of a full change in the energy system in relatively short term, requiring an impressive effort for this system transformation, which can be realized only in a concerted effort which catalyzes the transformation. With respect to the first aspect, the time to achieve the full change to green hydrogen, a realistic and objective analysis shows that in the short-to-medium term it will be necessary to resort to other forms of low-carbon hydrogen (blue hydrogen), to reduce emissions from actual hydrogen production, and to support the parallel and future growth of renewable hydrogen. Along this intermediate step, up to 90% of CO2 emissions can be captured during the production of grey hydrogen and stored in adequate places, for example, empty gas fields. However, the cost of going over 90% increases significantly, while it is possible that at 2030 the cost of hydrogen from electrolyzers could be competitive with that from methane. Therefore, the necessity of this step caused, as expected, criticism by influential international associations, such as WWF, European Environmental Bureau, Friends of the Earth Europe, and also from part of the scientific community, fearing that maintaining the blue option at least until 2030 could compromise the EU action on climate. There are alternatives, however, which can be considered, even maintaining the use of fossil fuels as a primary source. In fact, a better and more economical alternative to achieve the same goal of decreasing H2 production carbon footprint instead of producing CO2 and then sequestrating and storing it (carbon capture and storage (CCS)) is to intrinsically decrease its production. By using electrically heated reactors for steam reforming of methane, it is possible to achieve a reduction in CO2 emissions above 50–60%. Producing H2 from cracking of biomethane is another better possibility with respect to the use of CCS. Really, even if the priority of the EU is to develop green hydrogen, that is, hydrogen produced through the electrolysis of water powered by electricity stemming from renewable sources (wind and sun), it appears that it will be necessary to profit from intermediate opportunities for producing at least blue hydrogen, taking into account that they could be available in shorter time, contributing to progressively move toward the full change to the really green hydrogen. The European Clean Hydrogen Alliance, bringing together industry, national and local public authorities, civil society, and other stakeholders, aims at bringing together renewable and lowcarbon hydrogen production by 2030. It is obvious that to reach the final objective of decarbonization, a joint effort by the different stakeholders involved with different roles (industry, electricity, mobility,

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infrastructure building, government, research, and education) must be planned. This requires well-coordinated policies at national and European levels. The publication of the EU Commission indicates three time steps and relevant actions, starting from now, to achieve the goal through an investment agenda for the EU: – Boosting demand for and scaling up production – Designing an enabling and supportive framework: support schemes, market rules, and infrastructure – Promoting research and innovation in hydrogen technologies. With respect to the financial aspect, the reference to the next-generation EU plan will strongly support these actions. A regulatory framework for the hydrogen market will be assessed. Because of the complexity of the technology and the wish to achieve a breakthrough in the field, there is a great interest for a deep international collaboration between the United States, China, Canada, and EU countries, and some international organization between universities, research institutes, and industries. In addition, the EU, the International Energy Agency, and the countries themselves give hydrogen energy a top priority in their energy research and development plans. Therefore, with respect to the invitation of the Commission to all stakeholders from their specific point of view as to how to help support the fundamental role of hydrogen in a decarbonized economy, we offer this series of books as a small contribution. The series consists in three books treating, respectively, the three major areas of (i) processing technologies, (ii) storage technologies, and (iii) application technologies. The essential role of hydrogen for a sustainable development through decarbonization as fuel is analyzed and discussed in industry, in mobility (from cars to trains, to ships, to airplanes), for heating and cooling buildings, and in electricity demand and supply. Starting from the current, fossil-dependent, production processes, intermediate steps such as biomass and waste sources are presented, to arrive to updated progress in renewable hydrogen mostly based on water splitting and renewable energy. The second book deals with hydrogen storage. Together with an overview on technologies, most of the chapters are devoted to innovative materials for batteries and fuel cells, and others to stationary storage. The third book treats the whole range of applications in industry (power to gas to fuel, carbon dioxide conversion) and mobility (vehicles and refueling stations). One chapter is dedicated to a very critical issue: hydrogen safety and risk assessment, including the case of mobility. A review is available with respect to new materials, components, designs for the installations and facilities for the future, as well as for the production, storage, and multiple applications. Without materials research, design, and development it would be impossible to think on the new hydrogen technology applications. However, it is necessary to really integrate all R&D developments in a single vision to achieve fast the transition. This book series contributes to give this unitary vision.

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The pillars of the three books are “Role of Hydrogen Energy and Hydrogen Economy” reflected in the characteristics of the advanced technologies as well as their sustainability, that is, their implications on economy, society, including safety and public acceptability, and environment-friendly technologies. In addition, hydrogen technologies offer ways of handling some critical problems that current societies face, ways that could be preferable to other ways of coping with the problems. We invited several authors from academy, research centers, companies, and authorities to write a chapter keeping in mind mostly two keywords: research and technology. Therefore, you will find different contents, from basic research to innovation, to realizations, to scenarios, to safety and regulation aspects. All three books have an Introduction and are closed by Conclusions and recommendations. Their content typically includes chapters on the “State of Existing Technologies” and chapters on research and innovation in relevant fields from a theoretical, applied, innovative, and industrial point of view. Moreover, the current state of standardization and safety in the field of systems and devices for production, storage, transport, measurement, and use of hydrogen is analyzed in some chapters. The treatment of this aspect, a critical one for commercialization of hydrogen, is also required as an effect of cooperation of countries and continents on the roll out of hydrogen infrastructures, such as refueling stations or distribution networks.

Processing technologies In this book, most of the potential hydrogen processing technologies have been described in detail: the improvements of actually operating systems toward completely new and flexible techniques. Therefore, from various chapters the itinerary from grey to blue to green hydrogen is covered. Attention has been given to the theoretical aspects, thermodynamics, process calculations, and modeling approaches and reports of multiple successful new pilot systems.

Storage technologies This book focuses on new developments of hydrogen storage technologies with great attention to new advanced materials. Tools for the development of new materials focus on synthesis, kinetics, and thermodynamics and application of nanoscale hydrogen storage materials with status on existing technologies and perspectives. An impressive range of nanomaterials has been investigated: solid-state, metal, intermetallic, chemical, advanced carbon, and complex hydrides. Nanomaterials for hydrogen storage achieve an optimum compromise between having the hydrogen too weakly bonded to the storage material, resulting in a low storage capacity, and too strongly bonded to

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the storage material, requiring high temperatures to release hydrogen. For the storage of electric energy, nanostructured materials for positive and negative electrodes are used for advanced next-generation batteries.

Application technologies This book presents the current state of hydrogen economy with the focus on applications in the automotive technology and industry, aiming to reply to some key questions: Where are we in the implementation of hydrogen economy? What are the perspectives? Which are the directions to develop this field? This book provides compiling reviews on these aspects from internationally recognized researchers, industrialists, and government agencies. The mobile applications such as automobiles, air, and space transport are becoming very popular with a wide number of different types of fuel and electrolysis cells well described in the book. Large attention is being given to materials science and technologies with focus on the development of new materials for specific applications, as well as structured and functional materials, including research and innovations on electrolytes. Functional materials are key for advances in energy research in preparation of energy carriers from renewables, energy storage, and efficient energy conversion. Nanostructured materials offer high potentials, provided that they are tailored to exactly the right size of the nanometer scale. Catalysts for efficient energy conversion are a crucial enabling factor, but there is a need to avoid the use of critical raw materials such as noble metal catalysts in fuel cells. Hydrogen is a chemical widely used in various applications including ammonia and methanol production, oil refining, and energy. Hydrogen is widely regarded as an ideal energy storage medium, due to the ease with which electric power can convert water into its hydrogen and oxygen components through electrolysis and can be converted back to electrical power using a fuel cell. Power to gas is the conversion of electricity to a gaseous fuel such as hydrogen to be injected in methane distribution network. Via underground storage, hydrogen is stored in caverns, salt domes, and depleted oil and gas fields. Challenges and requirements for car industry are discussed presenting a status on existing technologies, particularly hybrid systems. Specific attention is devoted to mobility application discussing efficiency, technological development, and demonstration projects for fuel cell vehicles such as cars, heavy-duty vehicles, trains, and ships. Strictly related to mobility is the relation between vehicles and refuel stations, discussed in some chapters. One more critical aspect connected to automotive mobility is the safety analysis, risk assessment for both infrastructures and transport of persons or dangerous goods.

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The state of the art and development of portable applications is also presented. Some chapters gave an overview of the research and development of new components for hydrogen designs, and of newly developed monitoring and control equipment to assure reliable and efficient operation. Because of the great interest in “Hydrogen Technology” worldwide, the authors participating are university professors, research managers, industrialists, and government agencies from industrialized countries: China, Japan, the United States, Australia, Africa, South America, and many European countries. Focus is given to modernization of existing technologies to completely new developments. Developments are planned in many areas, which will result in a new “Hydrogen Economy” in the next decades given from the progresses in “Hydrogen Technology.” Breakthroughs are expected, and efforts are planned so that the new “Hydrogen Technology,” which has been impossible in the past, will finally become a reality.

References [1] [2]

[3] [4]

IEA, The Future of Hydrogen, IEA, 2019, Paris. https://www.iea.org/reports/the-futureofhydrogen Innovation Insights Brief – New Hydrogen Economy – Hype or Hope? Word Energy Council, 4 June, 2019. https://www.worldenergy.org/assets/downloads/WEInnovation-Insights-BriefNew-Hydrogen-Economy-Hype-or-Hope.pdf Powering a climate-neutral economy: An EU Strategy for Energy System Integration, COM/ 2020/299, Brussels, 8 July 2020. A hydrogen strategy for a climate-neutral Europe, COM/2020/301, Brussels, 8 July 2020.

Forewords

Louis Schlapbach

Foreword Overcoming the initiated climate crisis is the greatest challenge of humanity. Human activities like power generation and mobility, industrial processing, and building, and maintenance of infrastructure need to become carbon neutral, that is, to transform into a sustainable energy economy with a closed carbon cycle. Our attitude in handling of energy and materials must change from “just consume” to using renewable sources in a circular economy. Hydrogen is a key component in the sustainable handling of energy, materials and chemicals, and processing. Is the production of the three new volumes Hydrogen Production and Energy Transition: i) Synthesis and Processing ii) Storage and Transport iii) Hydrogen Applications timely? Yes, it is, as “Climate change is now detectable from any single day of weather at global scale” [1], and as the crucial function of hydrogen in all major low carbon energy technologies is well established, innovative solutions are needed leading us to a Hydrogen Based Low Carbon Society. The global acceptance that we are approaching directs us to serious challenges relative to our handling of energy and environment and thus of climate. All three subfields of the book series have gained a lot of international priority in science, technology, economics, and politics. Over the last decennies, it was shown that on a lab scale, hydrogen-based steps are able to make significant contributions to almost all non-nuclear energy technologies for main use in buildings, for mobility of persons and transport of goods including air and space, for industrial manufacturing and processing, as well as for communication and handling of big data. Hydrogen technology and economy covers production by traditional steam methane reforming and coal gasification and water splitting by electrolysis, transport and storage by high-pressure pipelines and tanks for gaseous hydrogen, by cryotechniques for liquid molecular hydrogen, and solidstate storage by reversible hydride formation, and finally the use and applications in thermal, mechanical, chemical, electrical, and electrochemical processes. Apart from space technology, wave-like ups and downs of enthusiasm and budgets for research, development, and innovation (R&D&I) characterized the chances for a major technoeconomic breakthrough. There are clear signs for a turning point: For those who want to set up climatefriendly energy technologies, closed hydrogen cycle and closed carbon cycle are central topics based on renewables. Concerning hydrogen, we approach asymptotically what Jules Verne was dreaming about in his “L’Ile mystérieuse” stating that “water will be the coal (fuel) of the future.” https://doi.org/10.1515/9783110596281-002

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After the Kyoto Protocol, 187 countries signed and ratified the Paris 2015 Climate Agreement COP21 for limiting greenhouse gas emissions. A Global Hydrogen Council was founded in 2017 in Davos by Toyota, Air Liquide, and Linde. IEAs 2019 statement by Director Fatih Birol, “Hydrogen is today enjoying unprecedented momentum; the world should not miss this unique chance to make hydrogen an important part of our clean and secure energy future,” supports the turning point. The European Academies Science Advisory Council EASAC published in September 2020 its strong Hydrogen and Synthetic Fuel support paper on the implementation of the EU Hydrogen Strategy on production and use of renewable hydrogen. Japan acts as a pioneering country in science, technology, and politics on hydrogen [Japanese Hydrogen Gamble, NATURE 591, 25 March 2021]: Tokyo Olympics 2020 (postponed to 2021) runs a hydrogen-based mobility system. JR East tests hydrogen FC trains. Fukushima Hydrogen Energy Research Field will soon operate a 10,000 kW hydrogen production facility. Many public R&D&I laboratories in PRC China focus their work on innovation in energy-efficient technologies for enhancing resource recycling and energy efficiency [2]. Reduced promotion of battery-electric cars in favor of methanol and hydrogenpowered cars is a crucial example. Hydrogen fuel cell pioneering car manufacturers in Japan and South Korea not only entered the car market (Toyota’s Mirai and Hyundai’s Nexo) but also have advanced plans for extensions to buses and trucks as hydrogen fuel cell electric vehicles including a 1,000 vehicle fleet in Switzerland. In Europe, Germany set a goal with the recent decision to push hydrogen-powered mobility. Fast recharging is a main advantage compared to battery-powered electric vehicles. Cost reductions by almost two orders of magnitude have been realized. Already in 2018, Germany started the commercial service of its Cordia iLint hydrogen fuel cell trains over 100 km rail. Hydrogen technology for marine applications based on H-fuel cells with metal hydrides or methanol reformers as providers of hydrogen are a hot navy topic [3]. The potential to transform nonmilitary transport of goods by open sea navigation – still exempted from Paris COP15 agreement – from today’s polluting heavy oil to sustainable hydrogen-based powering corresponds to around 4% of global CO2 emissions. We are used to transport goods over very large distances, for example, coal from Australia to Japan or crude oil from South America to everywhere. Why do we not (yet) transport solar power in the form of synthetic liquid hydrocarbons from Australia or other parts of the southern hemisphere to consumers further north, with the additional advantage of seasonal storage? Near-future energy scenarios will – in addition to the naturally used solar power – rely on decreasing amounts of fossil fuels (decarbonization) and classic fission-type nuclear energy and on rapid raising of different types of renewables (solar radiation, hydropower, wind power, biomass conversion, etc.) and some geothermal energy. The relative importance of the different contributing technologies is not yet fixed. However, it is clear that after the energy production and conversion, storage, especially seasonal storage to bridge the lower solar radiation of

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the winter period, will be the great challenge. In view of today’s low-cost solar and wind power, the old idea to use fission-based nuclear heat for the coproduction of electricity and hydrogen is reconsidered “to provide an outlet for the nuclear power that is now sold at a loss” [4]. Hydrogen (H) is a present of nature: with only one proton (zero neutrons) and one electron per hydrogen atom with strong (13.5 eV) binding energy, hydrogen has the highest energy per weight or mass of all chemical energy carriers. It is a carbonfree synthetic fuel; when burnt with oxygen, water vapor is produced and no CO2. Nature has quasi-nonlimited amounts of hydrogen chemically bound in water (H2O). Hydrogen atoms are not stable, so they combine to molecular H2. The challenge is that molecular hydrogen (H2) is a gas at room temperature. Cryogenic temperatures allow liquefaction, and additional very high pressure leads to solidification. For compact applications around room temperature densification of H2 gas, for example, compression by up to three orders of magnitude is needed. Important, still challenging options are the transformation of pure hydrogen into a hydrogen-rich synthetic fuel, a hydrocarbon or ammonia, preferentially in the liquid state. The extraordinary potential of hydrogen is first of all a strong driver for R&D&I for energy technologies but secondly for materials science and as a crucial chemical for semiconductor industry, metallurgy, and petroleum refinement. The following short list of selected recent achievements illustrates the progress and topicality of the themes. The publication “How the energy transition will reshape geopolitics; path to a low-carbon economy will create rivalries, winners and losers” by Goldthau et al. [5] elaborates societal impact by hydrogen energy and climate politics. Grolms [6] analyzed economic challenges of the realization of a sustainable hydrogen economy. The handling of safety and risks of new energy technologies requires reliable sensing. Based on nanostructured sensing devices, sensitive and fast response detectors were developed to handle hydrogen as a highly flammable, colorless, and odorless gas [7, 8]. Promising R&D&I work aims at larger scale synthesis of hydrogen by water splitting and at the production of synthetic liquid hydrocarbon fuels, both using solar, thermal, or photonic energy. Photo- or photoelectrochemical water splitting, solar thermal CO2 reduction, and CO2 hydrogenation reactions are steps in the direction of solar fuels. Hydrogen is always involved in an intermediate step. Laboratories like the Joint Center for Artificial Photosynthesis, an US-DOE Innovation Hub established in 2010 or GREEN at the National Institute for Materials Science in Tsukuba, Japan, started early as pioneers. Pham et al. [9] reviewed solar water splitting heterogeneous interfaces. Various hydrogenation reactions of CO2 based on intermediate H2 storage are well described by Bhanage and Arai [10]. The European H2020 project “From sun to liquid-fuels from concentrated sunlight” with a leading team at ETH Zurich is developing a solar thermochemical technology for synthetic hydrocarbon at large scale and competitive costs. Solar

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radiation is concentrated by an array of heliostats and efficiently absorbed in a solar reactor that thermochemically splits H2O and CO2 and converts via hydrogen to syngas, which is subsequently processed in a Fischer–Tropsch process into hydrocarbon fuels. Solar-to-syngas energy conversion efficiencies exceeding 30% can potentially be realized, thanks to favorable thermodynamics at high temperature and utilization of the full solar spectrum [11]. Such solar-based carbon-neutral fuels – produced in more than lab-scale quantities – will make aviation and maritime transport sustainable. Hydrogen-dominated materials remain a hot topic in materials science and technology. First of all, we refer to reversibly formed solid hydrides of metallic elements and compounds and their surfaces for hydrogen gas storage, as electrodes for rechargeable batteries, and as sources of atomic hydrogen, for example, in catalytic reactions [12]. An example is superconductivity of materials transporting electricity with zero resistivity and expulsing magnetic fields. At low temperature and very high pressure, pure molecular hydrogen gas transforms first into liquid (21 K) and then solid molecular hydrogen; under still higher pressure a transition to atomic metallic hydrogen, supposed to be superconducting, is expected (see, e.g., Dias et al. [13] and references therein). Rather than using pure hydrogen, it was demonstrated recently that the incorporation of hydrogen atoms into suitable solids (hydrogen compounds) induces superconductivity. Newest results report superconductivity in lanthanum hydride at 250 K under high pressure [14, 15]. Research will open more and attractive solutions, technology development will make it economically feasible, and the society will make its choice, hopefully respecting needs of mankind and nature. The well-advanced hydrogen technologies continue to offer great challenges for materials science and technology and are promising fields for young scientists and engineers with entrepreneurial spirit, for this and coming generations. And, on a medium term, it will be profitable for investors.

References [1] [2] [3] [4] [5]

[6] [7] [8]

Sippel S, et al. Nat Clim Chang 2020, 10, 35. “Clean energy in China”, Nature Spotlight and Advertisement. Nature 2020, 584(S1). Marine Forum 5, 2019, ISSN 0172–8547 “Could hydrogen bail out nuclear power?”, Kramer D, Physics Today, August 2020, p. 20. Goldthau A, Westphal K, Bazilian M, Bradshaw M. How the energy transition will reshape geopolitics; path to a low-carbon economy will create rivalries, winners and loosers. Nature 2019, 569, (29). Grolms M. Realizing a sustainable hydrogen economy. Adv Science News, Sept 2018. Gao M, et al. Small 2018, 14 (10), 1703691. Nugroho FA, et al. Metal-polymer hybrid nanomaterials for plasmonic ultrafast hydrogen detection. Nat Mater 2019, 18, 489.

Foreword

[9] [10] [11]

[12]

[13] [14] [15]

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Pham TA, et al. Review Modelling heterogeneous interfaces for solar water splitting. Nat Mater 2017, 16, 401. Bhanage BM, Arai M, eds. Transformation and utilization of carbon dioxide, Springer Series Green Chemistry and Sustainable Technology, 2014 (ISBN 978-3-642-44987-1) Marxer D, Steinfeld A, et al. Solar thermochemical splitting of CO2 into separate streams of CO and O2 with high selectivity, stability, conversion, and efficiency. Energy Environ Sci 2017, 10, 1142. Kim S, et al. A complex hydride lithium superionic conductor for high-energy-density allsolid-state lithium metal batteries. Nat Commun 2019, 10, 1081; Mohtadi, R., Orimo, S. “The renaissance of hydrides as energy materials” Nature Rev. Mater. 2, 16091 (2017); L. Schlapbach, A. Zuettel, “Hydrogen-storage materials for mobile applications”, Nature 414, 625 (2001); L. Schlapbach, ed., “Hydrogen in Intermetallics” I & II, Springer Series Applied Physics, 63 & 67 (1988, 1992). Dias RP, Silvestra IF. Science 2017, 355, 715, and references therein. Drozdov AP, et al. Nature 2019, 569, 528. Pickett W, Eremets M, Phys Today, May 2019, 52.

Alexander Wokaun

Foreword Why does hydrogen play such an eminent role as an energy carrier? From a chemist’s point of view, the answer is clear and unambiguous: it offers the best ratio of chemical binding energy to weight. Hydrogen consists of one electron, involved in chemical bonds, and one nucleon, the proton. By comparison, lithium features 1 binding electron per 7 nucleons,1 and sodium contains 1 binding electron per 23 nucleons. Consequently, dihydrogen offers the highest value of stored energy per weight: its heat of combustion (lower heating value) amounts to 120 MJ/kg, and rises to 142 MJ/kg with condensation of the produced water (higher heating value). This value is unsurpassed by any other chemicals. Besides, hydrogen offers many advantages: As we shall discuss, it may be produced by a variety of routes, is useful and required for a plethora of applications, and can be converted to other forms of energy by clean and efficient routes. Of course, there are also some disadvantages: For hydrogen as a light gas, the energy stored per volume is low (11 MJ/m3), as compared to methane (36 MJ/m3). For energy storage, hydrogen must therefore be compressed or liquefied, and pipeline transport needs to move larger volumes. As a reactive gas, hydrogen requires safe handling with adequate precautions. Fortunately, appropriate technical solutions are available to overcome these difficulties, such that society and industry can take advantage of hydrogen’s eminent properties. Henry Cavendish discovered the element of hydrogen in 1766. Soon thereafter in the eighteenth century, the light gas hydrogen was already used for lifting gas balloons, and its use for aerial transportation continued until the twentieth century. Unfortunately, this early career found an undeserved end with the accident of the “Hindenburg” zeppelin, although later it was established that the disastrous fire was due to burning of the aircraft’s skin, not of the hydrogen. The rising chemical industry continued to use hydrogen as a clean reducing agent, albeit in small quantities in the nineteenth century.

Hydrogen in the chemical industry The volume of hydrogen required and used increased drastically at the beginning of the twentieth century, mainly for two applications. The first is the chemical synthesis of nitrogen fertilizers for agriculture, when the need of the growing world population

1 Naturally occurring lithium is a mixture of two isotopes: 7Li with an abundance of 92.5% and 6Li with an abundance of 7.5%. https://doi.org/10.1515/9783110596281-003

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could no longer be covered from mineral or organic sources. Catalyzed ammonia synthesis (N2 + 3H2 → 2NH3) fixes nitrogen from air. This is accomplished by the famous Haber–Bosch process. The technical chemist Fritz Haber laid the foundations by establishing the reaction conditions of high pressure and moderate temperature, and chemical engineer Carl Bosch realized the reactors for large-scale production. The decisive role of the catalyst was elucidated by Nobel prize winner Gerhard Ertl, and search for advanced catalysts is still ongoing to afford ammonia synthesis at milder reaction conditions of temperature and pressure. Part of the ammonia is subsequently oxidized to yield nitric acid. Without the availability of ammonium nitrate, the so-called green agricultural revolution and the associated crop yield increases would definitely not have been possible. Today, the production of hydrogen for ammonia synthesis accounts for more than 2% of the global energy needs. The second large application of hydrogen is oil refining. Two of the associated processes rely on hydrogen, that is, the removal of sulfur from crude oil by hydrotreating, and the conversion of heavy (large) molecular weight fractions to lighter compounds by hydrocracking (breaking of carbon–carbon bonds with hydrogen). The amount of hydrogen used for these purposes matches the global volumes of transportation fuels (gasoline, diesel, and kerosene); hence, refineries handle very large volumes of hydrogen which is often transported in pipeline networks. In the early days of the Haber–Bosch ammonia synthesis, hydrogen was obtained as a side product of the chlorine–alkali electrolysis (2NaCl + 2H2O → Cl2 + H2 + 2NaOH), with chlorine as the main product. As the demand for hydrogen increased and natural gas became available, the steam reforming of the latter became the predominant source. In fact, the steam reforming of methane (overall reaction2 CH4 + 2H2O → CO2 + 4H2) is still the highest volume and least expensive production route of hydrogen, amounting for 48% of global use, followed by hydrogen from coal and oil. Together, these fossil sources still account for 96% of the hydrogen produced, which is termed brown hydrogen in the literature. Clearly, the production of hydrogen as an energy carrier in a future sustainable energy system cannot be based on fossil fuels, and hence we shall return to the production based on renewable energies, after briefly discussing another use of hydrogen.

Space applications Hydrogen played a pivotal role in NASA’s Apollo and Apollo-Soyuz space projects. The carrier rockets used in the launches between 1961 and 1975 were propelled by

2 The overall reaction consists of steam reforming (CH4 + H2O → CO + 3H2) and water gas shift (CO + H2O → CO2 + H2).

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hydrogen. For example, the fuel of Saturn II rocket consisted of an astounding quantity of 1,000,000 L of hydrogen, and 331,000 L of oxygen, both in liquefied form. Today, heavy-duty carriers like the Ariane rocket series use a combination of solid-state propellant boosters and hydrogen/oxygen engines, and carry more than 100 tons of fuel. Interestingly, the Apollo mission already featured another use of hydrogen: Electricity on board of the lunar command module was provided by fuel cells, and similarly the supply of the International Space Station is furnished by alkaline fuel cells (cf. section on transportation).

Hydrogen production by renewable energy Solar energy, wind, and hydropower are the renewable energies used for the production of so-called green hydrogen. There are three basic routes to achieve this aim. The first avenue harnesses renewable electricity and uses the latter in electrolysis to produce hydrogen and oxygen. The invention of electrolysis by Nicholson and Carlisle dates back to 1800. In the past three decades, tremendous advances have been achieved with respect to efficiency, power density, and cost reduction. There are three main variants, the choice among which depends on the application: – alkaline electrolysis is the industrial benchmark, usually the least expensive option for applications requiring semicontinuous hydrogen production; – high-temperature electrolysis is related to solid oxide fuel cells (see below3); it uses steam as the reactant, and the efficiency is increased by the possibility to use waste heat as secondary input; – polymer–electrolyte membrane electrolyzers excel by high current densities and fast switching times, and are therefore well matched to photovoltaic or wind energy sources with their inherently varying power; rapid start-up is possible as a lengthy warm-up phase of high-temperature electrolyzers is not required. At this point, it is appropriate to mention briefly another variant nicknamed blue hydrogen. The latter is again produced using (nearly) CO2-free electricity, but this time generated either by nuclear power or by coal/gas-fired power plants where CO2 is removed from the flue gases by carbon capture and sequestration. The second pathway uses concentrated solar power. The latter may again either be used to provide renewable electricity, or for the direct production of hydrogen using thermochemical cycles. A variety of cycles has been tested, and at present, the most promising candidates appear to be based on perovskite-type materials. In the high-temperature step, lattice oxygen is released from these materials, while in the subsequent low-temperature step the desired reduction of reactants takes place:

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The now oxygen-deficient perovskite is exposed to water vapor or water/CO2 mixtures, whereby either hydrogen or synthesis gas (H2/CO mixtures) are produced. The third route is termed “photocatalytic water splitting.” A suitable semiconducting material, or a sandwich of two judiciously chosen semiconductors, is doped with cocatalysts on either side, and exposed to sunlight. If the band gaps of materials and their surface chemical composition are adequately positioned in energy, hydrogen and oxygen gases are produced directly on opposite sides of the device, without the intermediacy of electricity generation. With respect to maturity, photocatalysis is still investigated at the laboratory scale, and hence considerably less advanced with respect to technological readiness, as compared to the abovementioned photovoltaics/electrolysis combination. Yet, a community of researchers considers photocatalysis as an option with highest long-term potential. In general, it mimics the photosynthesis process of nature in which water and CO2 are reduced; this is why one research team designated its realization as an “artificial leave.”

Energy storage using hydrogen Decarbonization of electricity generation is a high-level target in the European Union and worldwide. As the potential for an increase in hydroelectric power generation is limited on the continent, further substantial increases in photovoltaic generation and wind energy are elements of many national strategies. The intermittent generation due to these sources poses a well-recognized problem. While in summer, the peak of insolation may coincide with the maximum demand for air conditioning in warmer countries, photovoltaic electricity is not available in the later evening and night hours. The partially complementary profile of wind energy cannot yet be fully exploited due to a lack of high-voltage north–south transmission lines. Consequently, there is a need to store electricity on a daily, weekly, and seasonal timescale. Demand side measures (shifting consumption to match available power) and sectoral transfer to transportation are valid, but only partial responses that will likely not be sufficient. Batteries provide storage of up to megawatt-hours of electricity over timescales of hours or days. Chemical energy storage potentially offers large volume solutions of up to terawatt-hour quantities, operated during periods when an excess of renewable power in the grid cannot be absorbed otherwise, with the option of storing the produced chemicals over timescales of months and seasons. Conversion of the electrical energy to chemical energy stored in the form of hydrogen is the most efficient solution for long-term storage; this process is termed “power to gas” in the literature. Electrolyzers with efficiencies exceeding 70% are available for this purpose, and are being installed at increasing power ratings and decreasing cost.

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For longer term storage of the produced hydrogen, three strategies may be followed. Hydrogen could be injected directly into the natural gas grid with its inherently large storage capabilities. While the hydrogen content of the transported gas is presently limited to 2%, most pipelines could be safely operated with up to 10% hydrogen if regulations were adapted. Where direct injection is not feasible, conversion of hydrogen to methane is an option (cf. section on synthesis). Third, tests are ongoing to realize dedicated hydrogen storage caverns, for example, by repurposing salt caverns previously used as natural gas reservoirs.

Transport applications The decarbonization of the transportation sector is likely the most recalcitrant problem on the pathway to an energy system with “zero” CO2 emissions. The difficulty of replacing the standard fossil fuels by CO2-free fuels increases in the order from motorbikes, passenger cars, heavy-duty vehicles, to ships and airplanes. A wave of vibrant interest in hydrogen fuel cells for transportation started around 1990. In the fuel cell process, the reverse of electrolysis, hydrogen and oxygen (air) react electrochemically to produce electricity on board, which is then used to power an electric motor propelling the vehicle. As early as 1839 and 1845, respectively, Christian F. Schoenbein and William R. Grove had invented the principle of the fuel cell, but the technology was for long considered as too expensive for cars, and only used for space and military applications.3 Tremendous progress in increasing the fuel cell power per weight and volume, in decreasing the amount of required platinum, and in driving down the cost gave rise to the expectation that fuel cell passenger cars would be on the road by 2005. However, several challenges (mainly lifetime and cost) need to be addressed, and the development took until 2015 when the first fuel cell cars were sold to the general public. In the meantime, another form of electric propulsion had made headways, that is, the direct use of electricity in battery electric vehicles. This was made possible by advances in lithium ion battery technology – increasing the energy and power per weight, providing adequate safety, and decreasing the cost per stored kilowatt-hour to an unprecedented level. As of present, battery electric vehicles are on the market with ranges around 300 km, and premium models with ranges above 500 km. The consequence was a heated debate about the pros and cons of fuel cell and battery technologies, at times resembling a war of faith, with various countries and automotive companies taking different bets on the winning option. On the scientific

3 Polymer electrolyte fuel cells are predominantly used for transportation, as they are capable of rapid start-up and fast power-level changes. High-temperature solid oxide fuel cells, usually fueled with reformed natural gas, are suited for stationary combined heat and power applications.

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level, this was accompanied by careful life cycle analyses, emphasizing the importance of the source of electricity (for both), the materials used in the manufacturing of batteries or fuel cells, the weight increase of the car due to aggregates, the “refueling” time, the storage of the fuel on board, and the available utilities in summer and winter. Today, a consensus is slowly arriving that there is not an either/or decision between the two options but that the two technologies complement each other for defined purposes. For (lighter) vehicles mainly used in short-distance driving, battery electric vehicles offer unbeatable advantages due to the efficiency of charge/discharge cycles, albeit at the price of longer charging time. On the other hand, for heavy vehicles with longer driving distances per day, and the need for fast refueling, propulsion by fuel cells is the preferred option. Heavy-duty trucks and buses are prime candidates to be equipped with fuel cell aggregates. Delivery truck and bus fleets offer the advantage that they often depart from a central location, where the refueling infrastructure can be built up close by and thereby profit from predictable turnover. Several companies are investing in the development of fuel cell trucks. Particularly noteworthy is the concept of a Swiss consortium that focuses on the parallel buildup of fuel cell truck fleets and hydrogen fueling stations located close to their home base.

Synthesis of transportation fuels and base chemicals Two issues need to be addressed in the context of using hydrogen for transportation. First, the buildup of a hydrogen refueling infrastructure covering a country’s complete area is expensive, while a well-organized distribution network for liquid fuels already exists. Second, fueling by hydrogen for the most energy-demanding applications, such as large airplanes for long-distance flights, appears close to impossible, as here the high energy density of liquid fuels is irreplaceable. The synthesis of liquid fuels starting from hydrogen and CO2 offers a solution for aircraft and other heaviest duty applications. The simplest liquid fuel that can be produced by catalyzed synthesis is methanol. The advantages of methanol have been recognized early in the discussion around hydrogen, such that even the term “methanol economy” had been proposed. Methanol is a an important intermediate in chemical industry – today being produced from fossil sources in very large volumes, and used for a variety of applications, including the methanol-to-gasoline process. The latter might become an option for producing gasoline from renewables. A further option for production of a liquid fuel is Fischer–Tropsch synthesis. The basic technology was developed early in the twentieth century and is used to produce diesel-like hydrocarbon fuels from coal. Researchers improved the catalysts

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that promote the formation of carbon–carbon bonds and yield long-chain saturated hydrocarbons from synthesis gas (CO + 2 H2). With suitable modifications, the Fischer– Tropsch-type catalysts can be adapted to synthesize liquid hydrocarbons from hydrogen and CO2, a subject that received renewed and increasing interest from the late 1980s onward. For all of these options, the question about the source of CO2 remains to be answered. In the first option, CO2 is separated from the flue gases of fossil power plants, incinerators, cement plants, or steel production (carbon capture, storage, and use). The carbon moiety is thereby still of fossil origin but the emissions of the plant are “reused,” and the fossil transportation fuel is thereby replaced. The purist option is the extraction of CO2 from the ambient air (direct air capture), a cycle being closer to CO2 neutral, except for the energy expended for the capture. It offers the advantage that the synthesis plant can be erected wherever renewable hydrogen is available, not requiring the proximity of a flue gas separation facility. Another potential source of carbon is biomass. The combustion of biomass for power generation, combined with CO2 separation from the flue gases, has been proposed as a “negative emission” technology (biomass energy carbon capture and sequestration). If, instead of storing the separated CO2 underground, the latter is used for the synthesis of liquids, the overall cycle ending with combustion of the synfuel becomes CO2 neutral, as the biogenic carbon had before been extracted from the atmosphere by photosynthesis. At this point, we should mention another option, the synthesis of methane from H2 and CO2, which is also termed “power to gas” in a broader sense. It is legitimate to ask when this technology can make a meaningful contribution while, at the same time, hydrogen is produced in large volumes from fossil methane in today’s chemical industry. The answer requires careful argumentation: The target is the storage of “excess” renewable electricity. If the large volume of hydrogen, produced by electrolysis from this electricity, cannot be used in short time nor fed into the gas grid, then the production of CH4 offers a valuable solution. A prime application of the soproduced biogenic methane is the fueling of natural gas cars, thereby reducing their tailpipe emissions close to zero. Further, European directives target a content of 10% of renewable gas in the natural gas grid, and the methanation of CO2 will be needed to achieve this value. An attractive option for “power to methane” is a combination with the fermentation of biogas. The raw fermentation gas contains roughly equal amounts of CH4 and CO2, and conventionally the latter must be separated and released to the atmosphere before the biogas can be injected into the grid. If instead, the CO2 in the raw biogas stream is converted to methane using renewable hydrogen, the overall yield can be increased (technically by ~60%) while the separation step becomes superfluous. Widening the scope of syntheses from H2 and CO2, we focus on the feedstock for chemical industry. Presently, the base organic chemicals used in industry – olefins, aromatic compounds, and others –are almost exclusively derived from fossil fuels,

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mainly from natural gas and oil. The point has often been made that rather than burning oil for heating and transportation, one should save oil reserves as a resource for chemical industry as the chemical feedstock is difficult to replace. However, for a system targeting “zero” carbon emissions, other sources for feedstock are required. Biomass (“naturally regrowing raw materials”) can offer a partial replacement of the fossil base, provided that competition with food production can be avoided. For a larger volume, the synthesis of base chemicals and intermediates from H2 and CO2 provides a sustainable solution. The production of methanol, mentioned above, is a first option. Further, a plethora of small molecules exists that can be synthesized from the CO2 building block, and suitable routes have been developed and are available. In this way, renewable hydrogen opens a viable path to decarbonize the chemical industry.

Role of hydrogen in energy systems integration Sector coupling is key to achieving the goals of the energy strategies of many European countries, to reach their emission targets. The concept implies that the sectors of energy provision, industry, services, households, and transportation are no longer advanced separately but are considered as a connected entity and controlled together to reach a global optimum with minimum emissions. Examples of system integration have been amply discussed and have meanwhile become accepted concepts. For example, individual houses become prosumers that import and export electricity. The electricity grid and transportation may be coupled such that excess renewable electricity is used for charging electric vehicles, while the latter vehicles feed electricity stored in their batteries to the grid in periods of high demand (vehicle to grid). “Smart grids” implement demand side management by load shifting. The operational control of such a “smart” sector-coupling network has been termed “energy hub.” As one of key characteristics, these energy hubs, typically at the community or city quarter level, share a network of grids where not only electricity but also heat, cooling fluids, and gases circulate and can be exchanged. In such multienergy carrier grids, hydrogen will undoubtedly have to play an important role.

The future role of hydrogen Technology development often proceeds in “waves,” a concept proposed by Cesare Marchetti in other context. Reviewing the discussion of the preceding sections, one could discern the following phases that each implied a surge in the importance and use of hydrogen:

Foreword

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23

the first wave started with the massive need for hydrogen in the chemical industry, for fertilizer production, and refining; the second wave involved the highly visible use of hydrogen in space missions; the third wave, in the 1990s, was characterized by widespread enthusiasm for hydrogen in transportation and energy industries, such that hydrogen was considered the unique energy carrier of the future, and scientist formulated the vision of the hydrogen society.

Today, we are in the middle of a fourth, more realistic, yet more powerful phase that will engender a massively increased production and use of hydrogen. We no longer suppose a single solution and one unique predominant energy carrier, but realize that interconnected grids of electricity, heat, hydrogen, and other gases are required to achieve system integration and the needed coupling of sectors of our society and economy.

Conclusions and recommendations –

–

–

–

–

Hydrogen is an indispensable energy carrier in the future energy system.As was demonstrated in the previous sections, hydrogen is urgently needed for energy storage, in transportation, as well as for the synthesis of renewable liquid fuels and feedstock for the chemical industry. European and global targets for energy efficiency, renewable energy shares, and decarbonization, and the associated climate protection goals, cannot be reached without hydrogen. For large-scale penetration, hydrogen technologies must become competitive in the energy markets. Therefore, research and development should focus on innovations that drive down costs. At the same time, governments need to agree on an adequate price for CO2 emissions. Several measures (CO2 tax and emission trading systems) have been discussed elsewhere. In order to reach competitiveness, initial support is required to mature the associated technologies. In particular, the entire chain from academic research via industrial development to pilot and demonstration should be financially encouraged by science policy. Hydrogen must become an integral part of teaching at institutions of higher education, in particular, at the universities of technologies. These institutions should also offer courses of continued education for practicing engineers and industrial leaders. Researchers are to be encouraged to reach out in communication to governments, to industry, to insurance companies, and to the public, to enter into a dialogue in the spirit of partnership, and to inform stakeholders about the advantages and

24

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Alexander Wokaun

unique properties of hydrogen, including the safety aspects. The financial support of such a dialogue process must be considered an integral part of public funding. Implementations at industrial scale are required to demonstrate to societal public the safe handling of hydrogen, according to the established standards. Capacity markets are an adequate measure to valuate hydrogen as a storage medium for intermittent renewable electricity (photovoltaics and wind energy). In transportation, strategies should aim for a concomitant build-up of hydrogen supply infrastructure and hydrogen demand, in order to avoid expensive upfront investments. The various variants of power-to-X technologies should be further advanced to increase the efficiencies of each step subsequent to hydrogen production. Innovative combined approaches such as coelectrolysis and coproduction of H2 and CO are to be pursued. Power to methane is a powerful option to store hydrogen by injection of CH4 into the national gas grids. This technology is required, in addition to biogas, in order to meet the European targets of renewable shares in the circulating and dispensed gas. In particular for air transportation, the synthesis of liquid fuels based on renewables should become a high-level target for research and development. Without the availability of these synthetic fuels, the decarbonization goals of transportation cannot be reached. The sectors of the economy must be coupled by implementing “smart” energy systems, including multi-energy carrier grids. Chemical industry should prepare for a change from fossil to renewable resource base. This constitutes a major challenge in view of the very large associated mass flows. Europe’s energy strategies and its decision to establish a CO2-neutral energy system by 2050 make it a prime mover to advance and demonstrate hydrogen technologies. The determinedness of Europe with respect to these goals provides a strategic advantage compared to the United States and China, countries that have partially other priorities. In view of Japan’s strategy of direct governmental financial support to industry, the European Union should act to secure its competitive advantage and thereby open up sustainable energy markets of the next decades.

In summary, hydrogen is thus truly indispensable to reach Europe’s high-level goals. It has a vital role to play on our ambitious climb toward a sustainable energy system. It offers the unique advantages of high energy per weight, long-term storability, pollution-free convertibility to other forms of energy, and flexibility in application. The quantity of hydrogen to be employed in society and the global energy system is only limited by the available renewable energy used for its generation, as the only other resource required for its production is water. Within a future-oriented, multipronged

Foreword

25

energy system, our societies should be strongly encouraged to exploit this unique potential of hydrogen as one truly renewable energy carrier.

Further Reading For scholarly quotations on the individual aspects and technologies, the reader is kindly referred to the respective chapters where a large number of technical references are given. Here we restrict ourselves to a very few general references and compendia. [1] [2] [3]

Sperling D. Future Drive. Island Press, Washington D.C., 1995. Rivkin J. The Hydrogen Economy. Blackwell, Oxford, 2002. Olah GA, Goeppert A, Surya Prakash GK. Beyond Oil and Gas: The Methanol Economy. Wiley-VCH, Weinheim, 2006. [4] Ball M, et al. (eds.). The Hydrogen Economy: Opportunities and Challenges. Cambridge University Press, Cambridge, 2009. [5] Wokaun A, Wilhelm E. Transition to Hydrogen. Cambridge University Press, Cambridge, 2011. [6] Corbo P, Migliardini F, Veneri O. Hydrogen Fuel Cells for Road Vehicles. Springer, London 2011. [7] Kopernikus initiative, project. P2X (Power-to-X), https://www.kopernikus-projekte.de/ projekte/p2x. [8] Kopernikus initiative, project. SynErgie, https://www.kopernikus-projekte.de/synergie. [9] Stolten D, Scherer V (eds.). Transition to Renewable Energy Systems. Wiley-VCH, Weinheim, 2013. [10] Sherif SA, Goswami DY, Stefanakos EK, Steinfeld A (eds.). Handbook of Hydrogen Energy. CRC Press, Boca Raton, 2015. [11] Cox KE, Williamson KD (eds.). Hydrogen: Its Technology and Implication. CRC Press, Boca Raton, 2017. [12] Kober T, et al. Power-to-X: Perspektiven in der Schweiz. Swiss Competence Centers for Energy, 2019.

Extended Introductions

Pierre Etienne Franc

Hydrogen: why the times to scale have come Hydrogen technologies have been around for centuries. In the early decades of the eighteenth century, many engineers developed their first engines to use this magic molecule. However, the human history of energy utilization has – to now – chosen different routes to heat, move, and industrialize. A well-known graph shows, in a simplified way, that the history of our relationship with energy seems like the one of a quest to reduce its carbon content (even though it never was a conscious move). Each step of civilization seems to mean one level of carbon less, toward pure hydrogen-based solutions. Each step involves a further reach in technological knowledge to meet higher energy needs. We started our quest with wood, and then shifted to coal. The discovery of oil brought a fuel with far better energy density, boosting the transport revolution. Natural gas came next – getting us closer to hydrogen – but this shift to gas sources required us to master new storage, transportation, and distribution techniques. At the same time, each step has enabled a further sophistication of the materials and feedstocks used, shifting the use of fossil resources from a pure energy source to a far broader source of materials for building, equipment, and everyday life provisions for our needs.

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But the hydrogen economy, the hydrogen society, has not yet arrived. This long awaited next step into a hydrogen-driven energy model, where only electrons and hydrogen become the vectors of energy generation, storage, transport, and distribution, is not here yet. The key technologies needed to achieve such a hydrogen-based system are known, mastered. They still have their complexities and challenges, which are broadly discussed in this book and some are significant. But the issue of taking that next step is not technology related. It all depends on our goals.

Why can’t we say this time, therefore, that hydrogen is no longer the future, but the present? What has fundamentally changed to build conviction from the key hydrogen players and supporters? As noted, technologies have a major impact to progress in all key fields – power electronics, materials chemistry, thermodynamics, silicon technologies, miniaturization technologies, and digital – all enable performance improvements in the production, storage, distribution, and use in fuel cells of hydrogen-based solutions. Many solutions that exist today to store and transport high pressure or liquid hydrogen safely and competitively (heavy storage, supercritical transportation, leakage detection and leak preservation, safety measures, and control systems), to boost electrolysis performance (footprint, capex, usage of raw and precious materials in the catalyst and electrodes), and to capture carbon – all fields of global technological progress that enable the use of hydrogen in places, at densities and for public uses – could not have been considered when Cavendish, Rivaz, and other key scientists of the time first discovered hydrogen’s properties. In a way, hydrogen technologies take a full benefit of everything being developed around them, including the huge electrification wave currently under development in the automotive world. But more significantly, to reach the scale necessary to make a change, hydrogen technologies needed to serve a proper goal, one which outweighs all challenges and enables whole sectors to grasp it properly. This required three following steps: 1. Designing a shared vision against a major challenge. 2. Developing concrete and tangible proofs of the viability of hydrogen-based solutions. 3. Developing a concerted, coordinated multilateral ambition. This is what has been achieved over the past decade.

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The vision The vision, at present, results from the widely accepted challenge of fighting climate change. Whatever the economic options are considered, our world will not continue its current course of growth and energy use without significant life-threatening harm to our societies. Either through different growth models or a change in economic wealth patterns and goals, we need to shift to low-carbon energy sources, as renewable as possible, and this has to happen now. And it does. The share of renewable power in primary production continues to grow; new energy capacity additions are now higher from renewable sources than from fossil ones, and the levelized cost of electricity (LCOE) of those new sources of energy are in many places competitive with fossil base sources. Primary generation of renewable sources is on its way, but its transport, distribution, and use in downstream consuming sectors (transport, industry, district heating) are now the key challenges. Many analyses of future energy systems have now clearly demonstrated that hydrogen is an essential vector to enable a below 2 °C scenario. A key vector to store and transport vast amounts of renewable intermittent energy in places or times where they are in surplus to places and times where they are in need. A key vector to enable the continued use, for several decades, of fossil fuels that are as clean as possible, by using only the hydrogen part and sequestering the carbon through carbon capture and use or sequestration technologies. A key vector to decarbonize transport, alongside battery electric solutions, especially for large and intensive mobility needs. A key vector to decarbonize the use of feedstocks (by synthesis of gases) or fuels in high-energy-intensive industries. A key vector (in some geographies) to develop highly efficient low-carbon stationary heat and power generation solutions for district heating needs. These views have led several different organizations to converge on the view proposed by the Hydrogen Council, that hydrogen could represent up to 18% of final energy demand by 2050 and help achieve 20% of the CO2 reduction target of the planet. In other words, hydrogen is now increasingly seen as an unavoidable part of any future clean energy system. This vision, proposed by the coalition of CEOs on the Hydrogen Council, followed initial views pushed by leading countries on the matter – Germany, California, Japan. Recently, strong measures have also been adopted by South Korea, China, Australia, the European Union, and several Middle East countries. The International Energy Agency (IEA), the ETC, the International Renewable Energy Agency (IRENA) and Bloomberg, to name the most prominent consultants/commentators on the energy field, have all acknowledged the need to boost the deployment of hydrogen technologies if we are serious about energy transition.

1

Enable large-scale renewables integration and power generation

3

2

Act as a buffer to increase system resilience

Distribute energy across sectors and regions

Enable the renewable energy system

7

Serve as feedstock, using captured carbon

Help decarbonize building heating and power

Decarbonize industry energy use

5

6

Decarbonize transportation

4

Decarbonize end uses

The 7 roles of hydrogen in the energy transition

32 Pierre Etienne Franc

Hydrogen: why the times to scale have come

33

This has also been possible because not only technologies but also implementations have progressed significantly over the decade. Back in 2010, a couple of hundred cars from several original equipment manufacturers (OEMs) were nicely demonstrating that it is possible to drive a hydrogen car. Stations were tested here and there, but nobody was seriously thinking yet that electrolysis could become a key node in the future energy system of the world. Ten large demonstration projects for buses were started in several cities. First forklift applications were considered in the United States. Now, technology deployment is catching up in all fields, and series have started to reach scale. The year 2020 saw more than 20,000 vehicles on the road, more than 500 stations up and running, several thousand trucks and buses, 30,000 forklifts in use every hour of the year, several GW of electrolysis capacity under construction and tens of GW of projects in development. The first large-scale trials for the use of hydrogen in power plants, in steel-making processes and of the first-liquid hydrogen carriers have been launched. The whole vision of a hydrogen-enabled energy system is being backed by a significant wave of deployments, experimentations, and tangible operations. Finally, this has also worked because not only have we seen a vision been built and the first tangible operational demonstrations being developed but also there is a clear alignment of views on the remaining key hurdles that the industry, jointly with policy makers, has to clear to make this real. First safety. Hydrogen, as any energy carrier, is dangerous. It needs to be handled with proper care and correct operating procedures. This includes stringent equipment, process, and behavioral rules that will have to pervade throughout the general public – so in a far broader and more public space than was the case when it was confined to industry-related topics. The more the technology spreads in its potential and use, the more players – large and small – will touch it, and the more chances we face for handling and safety issues. For this to be addressed properly, regulations have to be developed and harmonized between countries and a

2015

8

2020

10

Potential demand for hydrogen in a +2 °C Scenario (in EJ)

2030

14

2040

28

2050

13%

12%

14%

21%

29%

12%

78

5

6

Power generation, buffering

7

New feedstock (CCU, DRI) Existing feedstock uses

Building heating and power

18% of final energy demand

Transportation

1

Industry energy

4

Tomorrow’s use for the energy transition will unlock ten times bigger market

34 Pierre Etienne Franc

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Hydrogen: why the times to scale have come

Hydrogen mobility markets: Ready to scale TODAY

Ferries 1 T/day

Cruise ships 10 T/day

Material handling vehicles 100 kg/day per site

Trucks 100 kg/day per truck

Buses 20 kg/day per bus Trains 150 kg/day per train

Airplanes Applications

Individual cars 100-200 kg/day per station

Drones

Bicycles & scooters

very stringent program of training and education of all energy related players and OEMs will have to be developed. This is by far the most critical of the challenges ahead and it has to be tackled rapidly as many SMEs and new actors are entering the field. Education and normalization is to be developed to help them properly capture the challenges of this molecule in their activities and to benefit

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from competent resources to grasp those safely. But it is no different an issue than that which arose with the growth of oil-based activity, then gas, and now battery and electric uses. We are starting a new wave, developing uses of hydrogen technologies in much larger spaces for far more numerous applications, and we need to prepare the population for it. Second – low-carbon shift. Hydrogen technologies have to provide for a lowcarbon hydrogen source. If not, the hydrogen revolution will not happen, as it will not serve our common goal with respect to using clean energy vectors. Clean hydrogen technologies exist, but they are, by definition, more expensive today than fossil-based methods. In total, 95% or more of hydrogen produced today is based on natural gas, coal, or oil with an associated 10–20 kg CO2 produced/kg of hydrogen. The end game is to provide a kg of hydrogen at the point of use for less than 1 kg CO2. This is a huge technical and economic challenge. It is also the place where hydrogen reconciles all players, as it is the ultimate high-energy molecule if we are able to supply it without associated carbon. Developing the right methodology and goals for low-carbon hydrogen production, to be shared between geographies with agreed metrics and measurement methods, with a tracking system enabling hydrogen to be traded with its CO2 footprint and thus to contribute to decarbonizing energy in each geography is a key topic. Second, new disruptive technologies for low carbon or zero-carbon hydrogen production are still likely to emerge in coming decades and we shall see many future developments which could further accelerate the shift to hydrogen systems. In that context and in the light of the above, European 2021–2027 Research & Development program needs to continue to support Hydrogen technologies and be further strengthened, next to the need for broad deployment support schemes. Last, low-carbon hydrogen requires a low-carbon supply chain, and thus requires strong investment upstream with renewable sources, or with low-carbon energy sources to optimize the energy cycles and the capex cost of those solutions. Similarly downstream, as the book will show, because often only 10% of the hydrogen value chain relates to production, while up to 80% relates to the conditioning, transport, and distribution side where emissions and energy spending happen as well. Optimization of the supply chain solutions – on site, gas pipes re-purposing, liquid solutions, other hydrogen rich vectors as transportation means – will all have to be revisited over the coming decade to develop the best techno-economic compromise to enable safe, reliable, and competitive low-carbon pathways to hydrogen at the point of use. Third, cost. Hydrogen production through fossil means is relatively mature today, with a landed cost of ~ 1.5 €/kg (50 €/MWh). But the whole value chain goes from production, in a clean way, to the transition of all downstream processes to enable the use of hydrogen at the point of use. The gap to competitiveness with longstanding and mature incumbent technologies is significant. In each part of the value chain, efforts still have to be made to halve the cost of hydrogen-related technologies. In certain applications, this will still not be enough to make hydrogen competitive with incumbent fossil solutions, but in others, it will. The Hydrogen Council developed a strong fact-based

Hydrogen: why the times to scale have come

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analysis of 33 applications enabling hydrogen to cover 60% of world emitting sources. Of those, half can achieve competitiveness in the coming decade if the proper cost roadmaps are put in place. Those call for further progress in technologies, but primarily for scale deployments. This is especially the case for fuel cell technologies, electrolysis, and network development for hydrogen distribution. It is, however, widely acknowledged by all players that a pathway to cost competitiveness exists for most key transport applications, many heavy heat-intensive processes, and several district heating applications. Last but not least, this will not work without a paradigm change in the way industry and policy makers’ act and play. To date, the world has benefited from the dynamics of a very competitive environment, in which the economics of scale and the role of the free market has driven a fierce struggle to cost- and value-based solutions to the point that the satisfaction of individual needs comes with only a marginal cost to volume-based production. Digitalization, massive marketing efforts, and supply demand elasticity of production tools have enabled this. But, we are reaching the point of no return in terms of resource absorption, saturation of needs, deterioration of environment, livestock, and resources. The marginal cost of “things” brings with it the risk of the whole planet’s destruction! The next phase will require a paradigm shift to a more resilient and clean energy growth environment, for which hydrogen is one of the essential tools. It means more cooperation to reach scale, by sharing risks between industry players, strong alignment of development and deployment strategies between industry verticals, and a very stringent and sustainable regulatory and financial support scheme from the policy makers to ensure that efforts made will be valued over time. This also calls for different approaches to performance metrics from the financial markets and new accounting standards, to enable large industry players to commit significant amounts of investment to collaborative schemes, with adjusted returns at start. This would acknowledge the cost of developing competitive solutions using so many disruptive yet critical technologies as hydrogen to scale. What is a stake is a first, not only in the diversity of fields touched, but also in its criticality for the planet. In this context, it is of paramount importance that a comprehensive review of the current state of hydrogen technologies is put forward as proposed in this book, which would also help design properly the key expected pillars of the future European Research Program for the years to come. This will both explain to the public where we stand and what is ahead of us, and also enable opinion leaders, policy makers, investors, and industry leaders to see the amount of progress achieved already toward the successful realization of a clean – and hydrogen-based – energy system. For the European citizen, we have here a chance to prepare Europe early enough, not only to master and use the future key clean energy vector for its energy needs (it being coming from fossil or renewable sources) but also by making such moves early enough to also provide a new source of technologies, growth, and jobs

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for the continent. Energy policies for Europe need to match the famous tryptich of sovereignty, competitiveness, and sustainability. Hydrogen, by proposing a versatile source of energy as feed, will enable Europe further to build energy independence, as a way to use all possible sources of energy, including locally produced with its strong renewable resources. It creates a further element of network integration and interconnection between southern and northern parts of Europe, as possible feed for a clean hydrogen production and the central and continental Europe as a strong energy consumer. Last, starting early, the phases of deployments should help master the key technologies and reach competitiveness earlier than other geographies. As we said in a book done on the matter,1 hydrogen also provides Europe with another way to provide guidance to the world with a better policy for sustainability and climate change, and therefore provides a strong alignment with Europe’s political agenda in the global geopolitics.

1 Hydrogen, La transition énergétique en marche, PE Franc / P. Mateo, Preface from Pascal Lamy.

Ad Van Wijk

Hydrogen key to a carbon-free energy system Abstract: Hydrogen has a key role to play as a carbon-free energy carrier alongside electricity. Hydrogen can be transported worldwide by ship and pipeline and can be stored underground in large volumes. This makes it possible to deliver cheap renewable energy, especially solar and wind, cost efficiently at the right time and place to the customers. Next to this systemic role, hydrogen is important to decarbonize energy use in hard to abate sectors in industry, mobility, electricity balancing, and heating. Future hydrogen systems will have similar characteristics as present day natural gas systems. Large-scale multi-GW renewable hydrogen production plants at good resources sites will produce a minimum of 1 million tonnes hydrogen. Hydrogen infrastructure can be realized by re-using the gas infrastructure, pipelines, and salt cavern storage, without major adaptations. As a transition, hydrogen produced from fossil fuels at the resource sites with Carbon Capture and Storage directly in the field below, can bring low-carbon hydrogen volume in the system. Such an approach can establish a fast, cheap, and reliable transition to a sustainable energy system, whereby hydrogen will fully replace natural gas, coal, and oil. The conversion technology used today is based on combusting technologies: boilers, furnaces, engines, and turbines. These combustion technologies can be easily and are fast adapted to combust hydrogen. In future, however, combustion technologies will be replaced by electrochemical conversion technologies including heat pump technologies. These technologies offer the promise to be cheaper, moreefficient with no harmful emissions to the air, land, or water. A smart symbiosis between electricity and hydrogen as zero-carbon energy carriers with electrochemical and heat pump technologies, will establish a clean, costeffective, reliable, fair, and circular energy system.

1 The role of hydrogen in a carbon-free energy system Hydrogen today (2020) is produced from natural gas and coal, emitting CO2 to the air. The use of hydrogen today is especially as a feedstock in the chemical and petrochemical industry. However, in a future zero greenhouse gas emissions, reliable, affordable, and fair energy system, hydrogen needs to play a more important role. Hydrogen is, like electricity, a carbon-free energy carrier, with a high mass energy density. The role of hydrogen in a future sustainable energy system is therefore to transport and store large volumes of energy and to decarbonize energy use in hard-to-abate energy sectors.

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Ad Van Wijk

1.1 Worldwide energy and hydrogen consumption 1.1.1 At present, hydrogen is mainly used as feedstock, representing 2% of primary energy consumption About 90% of the world’s primary energy consumption in 2016, amounting to 556 EJ or 155,000 TWh, is fossil energy: oil, gas, and coal. This fossil energy is transported around the world by ship or pipeline, and then converted into a useful energy carrier, electricity, gasoline/diesel, or a gas. The conversion to a useful energy carrier is often associated with energy losses. For example, the efficiency of a modern gasfired power plant is about 60%, which implies that 40% of the energy is lost as heat. These useful energy carriers are then distributed and used in houses, cars, factories, and so on. Energy is used in many parts of our modern life: For heating and cooling houses and buildings; for electricity production to power equipment, appliances, and lighting; for transport by vehicles, ships, or planes; and for industry where processes require high temperature of heat and steam. Moreover, fossil energy is also a feedstock, from which chemical products such as plastics or fertilizers can be made. The final energy consumption represents the energy use by companies, houses, cars and is the primary energy consumption minus conversion losses in power plants, refineries, and so on. A distribution of 2014 global final energy consumption [1] is given in the Fig. 1.

Fig. 1: Final energy consumption worldwide [1].

At present, hydrogen is mainly produced from natural gas and coal and primarily used as a feedstock to produce chemical products, ammonia (the main component of fertilizers), and methanol. And hydrogen is used in refineries to desulfurize oil and in the production of gasoline and diesel. The primary energy input by gas and coal is about 3.200 TWh, representing roughly 2% of worldwide primary energy

Hydrogen key to a carbon-free energy system

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consumption. Figure 2 presents the energy balance for worldwide hydrogen production and consumption [2]. At present almost all of the hydrogen is produced and used at or nearby chemical and petrochemical sites.

1.2 Solar and wind potential 1.2.1 In 1 h time the earth receives from the sun more energy than total worldwide energy demand The question is whether all the world’s fossil energy use can be replaced by renewable energy. From all renewable resources, solar and wind have by far the largest potential, therefore the question could be rephrased, “is it possible to generate worldwide energy use, by solar energy systems and wind turbines. The answer is simple: that is no problem. The potential of solar and wind energy is very large [1].” If worldwide energy demand, 155,000 TWh, had to be produced with solar PV systems only, it would require a surface area covering about 10% of Australia (see Fig. 3) or 8% of the Sahara Desert [1]. The Sahara Desert is about 9.2 million km2 in size, which is more than twice the area covered by the European Union. The global wind energy potential is also very high. In a scenario where the entire worldwide energy demand that would be produced with wind turbines would only require an area of 1.5% of the Pacific Ocean, see Fig. 3. It should be noted that surface, however, is used to a limited extent, with one large floating wind turbine at every kilometer. So there is more than enough space to produce all the necessary energy for the whole world with solar and wind. This is even the case with an increasing global population and rising prosperity level.

1.3 Development solar and wind levelized cost of electricity 1.3.1 At good resource locations, solar LCOE will be about 1 $ct/kWh and wind LCOE about 2 $ct/kWh before 2030 The development of electricity production cost by solar and wind energy, expressed in the LCOE, Levelized Cost of Electricity, has seen a steep decrease over the past 10 years. In the past couple of years, LCOE for onshore wind and solar PV at the best resource locations, are even below low cost electricity generation by fossil fuels, as showed in Fig. 4 [3]. Recent Tenders for 2 GW Solar PV in Abu Dhabi and 1 GW solar PV in Portugal has resulted in prices respectively of USD 1.35 cents/kWh [4] and Euro 1.12 cents/kWh [5]. Also wind onshore tenders have shown prices between USD 2 and 3 cents/kWh, for example in Mexico [6].

2 Mtoe

By-product hydrogen

48 Mt H2 of which 80%), these electrochemical storage systems based on hydrogen would be “nonsense.” It is different, however, if the comparison is made on an economic level, in particular, if one thinks of electrical energy storage systems with many equivalent hours of accumulation, such as weekly or seasonal accumulations. In these cases, some studies report economic estimates carried out on large-sized accumulations that show a possible economic convenience in the use of electrochemical accumulations based on hydrogen.

156

Romano Giglioli

AC electric system

Hidrogen base storage system DC electric system

94 p.u. power

16 p.u. power

oxi-fuel cell

40 p.u. power

plant aux systems optimistic round trip of system

H O storage fuel cell

electrolizer

chemical storage H

H storage

O

H O

LO storage

37%

electrochemical storage

Fig. 1.3: Simplified diagram of an electrochemical storage system of electricity based on hydrogen.

Further reading [1] [2]

[3] [4] [5] [6]

[7] [8] [9]

Møller KT, Jensen TR, Akiba E, Li H-W. Hydrogen – A sustainable energy carrier. Prog Nat Sci: Mater Int 2017, 27(1), 34–40. Compendium of Hydrogen Energy, Volume 2: Storage Transportation and Infrastructure. Woodhead Publishing, 2016, https://www.sciencedirect.com/book/9781782423621/compen dium-of-hydrogen-energy. Walker I, Madden B, Tahir F. Hydrogen supply chain evidence base. Prepared by Element Energy Ltd for the Department for Business, Energy & Industrial Strategy November 2018. Staffell I, Scamman D, Abad AV, Balcombe P, Dodds PE, Ekins P, Shah N, Ward KR. The role of hydrogen and fuel cell in future energy systems. Energy Environ Sci 2019, 12(2), 463–491. The Role of Hydrogen and Fuel Cells in Future Energy Systems. Staffell I, Dodds PE (eds.), 2017, H2FC SUPERGEN, London, UK. Compendium of Hydrogen Energy, Volume 2: Storage transportation and infrastructure. Woodhead Publishing, 2016, https://www.sciencedirect.com/book/9781782423621/compen dium-of-hydrogen-energy. David M, Ocampo-Martínez C, Sánchez-Peña R. Advances in alkaline water electrolyzers: A review. J Energy Storage June 2019, 23, 392–403. Bhandari R, Trudewind CA, Zapp P. Life cycle assessment of hydrogen production via electrolysis - a Review. J Clean Prod December 2014, 85, 151–163. Driving clean energy foreword: Power to gas application, McPhy,: https://mcphy.com/en/ your-applications/power-to-gas/?cn-reloaded=1, available on line June 2020.

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[10] Gorre J, Ortloff F, Van Leeuwen C. Production costs for synthetic methane in 2030 and 2050 of an optimized Power-to-Gas plant with intermediate hydrogen storage. Appl Energy November 1, 2019, 253, 113594. [11] Gotz M, Lefebvre J, Mors F, McDaniel Koch A, Graf F, Bajohr S, Reimert R, Kolb T. Renewable power-to-gas: A technological and economic review. Renewable Energy January 2016, 85, 1371–1390. [12] Andersson J, Grönkvist S. Large-scale storage of hydrogen. Int J Hydrog Energy 2019, 44(23), 11901–11919. [13] Rivard E, Trudeau M, Zaghib K. Hydrogen storage for mobility: A review. Materials (Basel) 1973, 12(12), published in 2019. [14] National Renewable Energy Laboratory (NREL). Hydrogen Station Compression, Storage and Dispensing Technical Status and Costs, published for U.S. Department of Energy, https:// www.nrel.gov/docs/fy14osti/58564.pdf

Barbara Thijs, Maarten Houlleberghs, Lander Hollevoet, Gino Heremans, Jan Rongé, Johan A. Martens

2 Hydrogen, fueling the future: introduction to hydrogen production and storage techniques 2.1 Introduction: hydrogen and the energy transition In today’s economy, fuel is the main energy vector and most fuels contain carbon atoms, which turn into carbon dioxide when the fuel is used in a combustion process. Fuel is part of any future energy scenario but the chemical nature and production processes of different fuels vary to minimize their CO2 footprint. In its roadmap to 2050, the International Renewable Energy Agency (IRENA) estimated that in a scenario where a 70% reduction of greenhouse gas emissions is accomplished by 2050, fuels will still represent a 50% share of the total energy consumption [1]. Hydrogen is well suited as a renewable fuel, since it can be produced from renewable energy sources and its combustion product, water, is one of the only chemicals without restrictions. It is a versatile fuel for use in many sectors such as transportation by road and rail over land, over water, and possibly also over air. It can be used to generate heat and electric power using fuel cells, and it is a base chemical for industrial production. It is estimated by IRENA that renewable hydrogen will represent an energy equivalent of 19 EJ by 2050 [1]. Color codes are used to grade hydrogen according to the involvement of carbon in its production process (Fig. 2.1). Today, hydrogen is mainly produced from fossil fuels with CO2 as the by-product released into the atmosphere (grey hydrogen). A significant part of the CO2 by-product may be captured and stored underground in empty gas and oil fields (carbon capture and storage, CCS) (blue hydrogen). A variant of blue hydrogen is obtained by methane pyrolysis. In this process, carbon atoms of methane are converted to elemental carbon (turquoise hydrogen). Hydrogen can also be produced without involving carbon atoms in the production process, using water as a source of H-atoms instead of hydrocarbons. In such a process, water molecules are split into hydrogen and oxygen molecules using renewable energy sources such as solar, wind, or

Acknowledgments: JAM acknowledges the Flemish Government for their long-term structural funding (Methusalem). JR and MH acknowledge the Flemish FWO for a postdoctoral fellowship and a PhD grant, respectively. The authors acknowledge the Flemish FWO for project funding (CO2PERATE, No. HBC.2017.0692, and CATCO2RE, FWO S004118N) and VLAIO for Moonshot funding (ARCLATH, No. HBC.2019.0110, and P2C, No. HBC.2019.0108). https://doi.org/10.1515/9783110596281-010

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hydropower (green hydrogen). Hydrogen can also be produced by the conversion of biomass, using newer technologies such as thermochemical water splitting [2] or it can be recuperated as a by-product from chemical processes such as the chlor-alkali process.

Fig. 2.1: Types of hydrogen [3].

Europe has unique assets that could allow it to take a pole position in the future hydrogen economy. It has a clear political vision and ambition, world-class companies dealing with different aspects of the hydrogen economy, and a creative scientific community. Due to its flexibility and versatility, producing renewable hydrogen could help to decarbonize other sectors in the EU, on top of the energy sector. For a widespread availability of hydrogen, innovation in hydrogen production and storage infrastructure will be needed. In addition, the cost must be lowered to make hydrogen fuel a convenient and economically viable energy vector. Enabling the hydrogen economy will require large initial investments for upscaling to reduce production and storage costs. Hydrogen roadmaps have indicated that investments in the European hydrogen industry of 60 billion EUR [4] by 2030 and 180–470 billion EUR [5] by 2050 could result in the employment of 1 million people, while achieving a market size of 150 and 630 billion EUR, respectively. Investing in renewable energy and green hydrogen production provides a way for Europe to increase its own energy production, reducing its dependency on imported fossil fuels.

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Technological solutions to this grand challenge are in the making. For production as well as transportation and storage, there are many options, and depending on the application, different choices could be made. This chapter provides an overview of mature and emerging technologies and their relevance at different production scales. Three approaches can be distinguished: (i) centralized, large-scale production of hydrogen in Europe; (ii) decentralized, small-scale production of hydrogen, directly at the point of use; and (iii) a stranded scenario, where renewable energy is produced at remote locations in other continents. Production, transportation, and storage concepts in each of these approaches are different.

2.2 Centralized hydrogen production scenario 2.2.1 Hydrogen production technologies Centralized hydrogen production refers to large-scale installations (GW scale) that produce hydrogen at a relatively low cost by economy of scale. Most of the current hydrogen demand (115 Mt/yr) is concerned with industrial applications such as petroleum refining and petrochemistry, ammonia production, and methanol synthesis [6]. These centralized hydrogen production facilities are mostly built near chemical production plants and connected via hydrogen pipelines. Currently, almost the entire hydrogen production is still fossil-fuel based [6]. However, to meet the objectives of the Paris agreement and to fulfil the Sustainable Development Goals, hydrogen producers will have to reduce their carbon emissions. This will promote the production of blue or green hydrogen. In addition, many nations have recognized the pivotal role of carbon-neutral hydrogen production in their future energy scenarios [6]. Hydrogen could provide solutions to applications and sectors that are hard to decarbonize with electrification [7]. In addition, hydrogen is recognized as an essential component in providing energy security in the future renewable energy mix [8]. In the Fuel Cells and Hydrogen Joint Undertaking’s (FCH JU) hydrogen roadmap for Europe, it is estimated that hydrogen production in Europe could increase from the current 325 TWh to 2,250 TWh by 2050 [4]. In 2050, this will represent a 24% share of the global energy demand, consisting primarily of green and partially blue hydrogen [4]. It will, therefore, be required to not only retrofit current fossil-fuel based hydrogen production plants with CCS, but also to build new centralized green hydrogen production facilities at GW scale, to serve future hydrogen demands.

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2.2.1.1 Steam reforming coupled with CCS Roughly 60% of the current global hydrogen supply originates from dedicated hydrogen production facilities [6]. The major inputs are natural gas (71%) and coal (27%) [6]. The other 40% of hydrogen supply is produced as a by-product in the chemical industry, where fossil fuel is the major energy input [6]. Currently, hydrogen production from fossil fuels is mostly categorized as grey hydrogen, as less than 0.4% of hydrogen is produced with CCS [6]. Most installations are centralized and located near industrial sites that are the main consumers. In Europe, most of the currently consumed hydrogen originates from natural gas that is produced with steam methane reforming (SMR) without CCS [4]. According to the Hydrogen Analysis Production Models (H2A) developed by the National Renewable Energy Laboratory (NREL), a typical centralized hydrogen SMR production plant in the United States produces roughly 380 ton/day at a cost of 1.15 $/kg [9]. Adding CCS to centralized SMR plants is estimated to drive up hydrogen production costs from 1.15 to 1.56 $/kg [9]. The highest cost component is the feedstock price. The International Energy Agency (IEA) confirms this hydrogen pricing from SMR in the United States and acknowledges the important role of feedstock price [6]. In Europe, the hydrogen price from SMR was estimated to be roughly 1.5 and 2 €/kg with CCS due to higher feedstock prices [6]. Due to the relatively low cost of CCS in centralized production facilities and the avoidance of CO2 emissions that will become increasingly expensive in the future, large-scale CCS plants are gaining traction in Europe such as the Northern Lights [10] project in Norway and the Porthos [11] project in the Netherlands. The geographical location of the hydrogen production facility is an important aspect to consider in view of the compromise between the local industrial needs and the availability of CO2 storage capacity. At locations with a high hydrogen demand but difficult CO2 storage, green hydrogen production from water electrolysis could be a suitable alternative. Green hydrogen production is also the more sustainable option in the long term, since SMR with CCS cannot guarantee 100% CO2 capture [6].

2.2.1.2 Centralized green hydrogen production Currently, less than 0.4% of the hydrogen production of the world is produced by renewable energy [6]. In the long term, hydrogen production from renewable energy via water electrolysis will likely take over from SMR with CCS. This will require the scaling up of the electrolysis technology to reduce capital costs while also leveraging the low electricity prices provided by solar or wind energy. H2A models of NREL estimate a current hydrogen cost for centralized PEM electrolysis at 4.53 $/kg [9]. Most of this cost is related to the required electricity input (4.15 $/kg). Therefore, low electricity prices will be required to be able to compete with SMR. According to the forecast of the Hydrogen Council reports, due to the decreasing electricity costs and the development of larger

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scale electrolyzers, water electrolysis using offshore wind energy in Europe will gradually result in a hydrogen cost drop from 6 to 2.5 $/kg in the period 2020–2030 [12]. At this price level, green hydrogen could become competitive to grey and blue hydrogen. Water electrolysis technologies are currently in the MW scale and will require substantial upscaling, since Europe has already ambitioned 15 to 40 GW by 2030 [5, 6]. The IEA also acknowledges the important role of plants at industrial ports to drive down the costs by shifting to clean hydrogen production at a large scale [6].

2.2.1.3 Water electrolysis technologies Hydrogen is produced from renewable electric power by means of water electrolysis. In this electrochemical reaction, water molecules are split into their constituents, hydrogen and oxygen gas, by applying a potential above the thermodynamic minimum of 1.23 V. Two major commercial water electrolysis technologies exist, namely, alkaline water electrolysis (AWE) and proton exchange membrane water electrolysis (PEMWE) (Fig. 2.2) [13–15]. Both technologies have their specific advantages and disadvantages. Currently, AWE seems to be more beneficial for a centralized application with constant power input and it has the lowest capital expenditure, while PEMWE is better in coping with an intermittent power supply that demands a fast start-up/shutdown procedure [16]. In addition, two other water electrolysis technologies are being developed as well: solid oxide water electrolysis (SOWE) and anion exchange membrane water electrolysis (AEMWE). Both these technologies are still in the R&D phase and are not yet mature for commercialization compared to AWE and PEMWE.

Fig. 2.2: Four different water electrolysis setups: alkaline water electrolysis (AWE), proton exchange membrane water electrolysis (PEMWE), solid oxide water electrolysis (SOWE) and anion exchange membrane water electrolysis (AEMWE).

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2.2.1.3.1 Alkaline water electrolysis (AWE) AWE is the most mature commercial water electrolysis technology available, allowing hydrogen production up to the MW scale [17]. It is operated in concentrated alkaline solution and uses a porous diaphragm between both electrodes to separate the product gases. A major advantage for AWE is the possibility to use non-noble metal-based electrocatalysts, which resist corrosion only at high pH values [15]. Most commonly, steel or nickel alloy-plated steel materials are used as electrodes [17]. In the past decades, advances have been made to improve the performance of alkaline water electrolyzers, including the discovery of new types of diaphragms. By reducing pore sizes and gas cross-over, a zero-gap configuration is allowed that minimizes ohmic losses and thus increases conversion efficiency [13]. To further increase the efficiency, advances are made in operating at higher temperatures [18, 19], or optimizing electrocatalytic materials to improve reaction kinetics [13]. Commercially, raney Ni is mainly used as a catalyst; however, materials with higher activity have been developed [13]. These materials mainly consist of alloys of Ni, Fe, Co and/or Mo [13, 20]. Most recently, diaphragm-less electrolysis operation has been proposed to further bring down the capital cost and increase the current density [21]. Based on this concept, current densities of up to 3.5 A/cm2 and hydrogen purity of 99.83% have been obtained [22]. 2.2.1.3.2 Proton exchange membrane water electrolysis (PEMWE) PEMWE utilizes an acid membrane to separate electrode compartments. It is permeable to protons and water but very limited to hydrogen and oxygen gas. This limited permeability for gases allows membranes to have thicknesses as low as 20 μm while producing hydrogen gas at high purity and at elevated pressures [23]. Since high ion conductivity is provided by such membrane positioned in a small gap between the electrodes, PEMWE has low ohmic losses, resulting in higher energy efficiency and higher hydrogen productivity compared to AWE. At similar efficiency, commercial PEMWE units reach current densities up to 2 A/cm2 compared to 0.4 A/cm2 for AWE [14, 24, 25]. This results in a more compact design. Due to the low gas cross-over rate through the membrane, PEMWE is able to work at differential pressures over the electrode compartments [26]. In addition, it also allows for a more dynamic operation with a fast start and a broad load flexibility [16]. A major drawback, however, is the corrosive acid environment. Because of this aggressive acid electrolyte, noble metal electrocatalysts, titanium-based bipolar plates and current collectors need to be used [20]. Most commonly, Pt, Pd, Ir, and Ru metals are used as catalyst materials [23]. Hence, currently, PEMWE, has a higher capital investment cost (1,100–1,800 €/kW) compared to AWE (500–1,400 €/kW) [6]. Another drawback of PEMWE is the sensitivity towards cation exchange in the membrane. Inorganic cations introduced with the water feed such as sodium and calcium will occupy cation exchange sites instead of protons, thus increasing ionic and charge transfer resistances [27]. Therefore, very pure

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water, which is commonly produced with reverse osmosis, is required for operating a PEMWE [28]. To further improve durability and lower the capital expenditure, research and development is needed to bring innovation in membranes that have enhanced chemical and thermomechanical durability. Developing alternative electrocatalysts for the oxygen evolution reaction (OER) to substitute the expensive Ir or Ru chemical elements is another challenge [13, 29]. Furthermore, finding alternatives to titanium-based current collectors and bipolar plates is crucial to enable this technology, since these are currently the most expensive components [30]. 2.2.1.3.3 Solid oxide water electrolysis (SOWE) SOWE is operated at much higher temperatures compared to AWE and PEMWE, and steam is fed instead of liquid water. By operating at higher temperatures, the energy efficiency can be better since thermodynamically a lower electric potential has to be applied to produce hydrogen [31]. For example, the required potential according to thermodynamics to split water is lowered from 1.23 to 0.96 V by increasing the temperature from 25 to 800°C [32]. In addition, higher temperatures also improve reaction kinetics of water splitting, resulting in lower additional kinetic overpotentials that have to be applied [31]. To provide ion conduction, this technology uses a solid oxide electrolyte that transports oxide anions from cathode to anode [31]. Long-term operation tests have been performed and SOWE units were found to be relatively stable when operating at moderate current density of ca. 1 A/cm2 [32, 33]. Operating at higher current densities requires higher potentials that cause increased degradation in both anode and cathode electrodes [34, 35]. Durability at higher current density could be improved by finding more active electrodes and thus lowering the required overpotentials [36]. Increasing the operating pressure is another approach to reduce the operating potential by decreasing the cell resistance [35, 37, 38]. To mitigate the degradation phenomena that are observed in SOWE, some research groups are using proton-conducting solid electrolytes [39]. These electrolytes have enhanced ion conductivity, which can lower the required temperature to 600°C [39]. Moreover, with this concept, pure and dry hydrogen gas can be produced, while other technologies produce wet hydrogen gas that necessitates a downstream drying operation [39]. Despite the lower electrical energy demand of SOWE compared to AWE and PEMWE, SOWE requires much more heat compared to AWE and PEMWE to reach the high reaction temperature. Part of this heat can be delivered from ohmic losses and overpotentials, and from an external heat supply such as waste heat [31]. SOWE technology is less appropriate when frequent start-up and shutdown required to cope with intermittent power supply from renewable energy sources, since SOWE would require the heat up of the installation at every start-up or to keep it heated even when it is not being used [40]. In this regard, this technology is more suited in a centralized context with constant power supply.

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2.2.1.3.4 Anion exchange membrane water electrolysis (AEMWE) By implementing an alkaline anion exchange membrane as solid electrolyte in AEMWE, it is possible to combine the strengths of AWE and PEMWE: (i) Minimized gas cross-over [41], (ii) short dynamic response time, and (iii) use of less costly non-noble metals for current collection and electrocatalysis [17, 20]. During water splitting, the anion exchange membrane (AEM) conducts hydroxides, which are produced at the cathode and consumed at the anode (Fig. 2.2) [42]. Another advantage of using an AEM as solid electrolyte is that, since it provides OH− and water transport between the electrodes, no highly concentrated alkaline electrolyte is needed. Operation with water without added dissolved electrolytes is also possible. Despite all the advantages of the AEMWE concept operating on pure water, it is a relatively new research field with limited experience in practical applications [24, 43–56]. The first implementation of an AEM without alkalinity was published in 2012 [43]. The initial performance was similar to established AWE systems. Later reports in literature have shown that AEMWE with non-noble metal catalysts is able to compete with PEMWE, as an efficient operation is feasible and current densities of up to 1 A/cm2 have been obtained [51]. To further advance in the field, long-term stability appears to be the main challenge as most reports have shown significant performance loss over time [24, 53, 55–57]. Long-term stability is also the main bottleneck in the more mature AEM fuel cells field [58]. Another major challenge is to optimize the interface between the electrocatalytic layers and the AEM. Ionic contact should be maximized at this interface, which can be improved by adding electrolytes such as KOH, K2CO3 or KHCO3 [45–49].

2.2.2 Grid technologies for hydrogen distribution There are several options for hydrogen distribution, but for larger volumes, pipelines are the method of choice.

2.2.2.1 Blending of hydrogen in natural gas grid In the short term, blending hydrogen into the existing natural gas grid is a way to distribute it at a limited capital investment cost. The European gas infrastructure is well-spread across the continent with 2.2 million km of pipeline. With the currently available infrastructure, the capacity for energy transmission through gas pipelines is significantly higher than by the electricity grid [59]. In the past, gas mixtures of hydrogen and light hydrocarbons with a high share of hydrogen have been distributed through a gas grid with success. In the nineteenth and twentieth centuries, the United States, United Kingdom, and Australia used “town gas.” Originally discovered by the Dutch inventor J. P. Minckelers, this

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is a gas mixture manufactured from coal or oil that contains 30–60% hydrogen [60]. Today, some regions with limited availability of methane, such as Hawaii, still employ hydrogen-rich gas blends [59]. Natural gas and hydrogen have significantly differing characteristics in terms of calorific value, density, flame speed, and combustion properties. This leads to a need for pipes and burners with different material requirements. Due to the very small molecular size of hydrogen, it has a five times higher diffusion rate through the pipe walls compared to methane. However, studies have shown that the leakage of hydrogen is limited to approximately 0.0005–0.001% of the total volume transported. Another potentially important problem with gas blends containing large shares of hydrogen is embrittlement, which causes degradation of the mechanical properties of pipes, such as surface cracking and crack propagation, leading to pipe failure [61]. To avoid complications, the concentration of H2 in the existing gas grid needs to be limited to 5–15% by volume [62]. When gas is used for heating and cooking, hydrogen concentrations of 5–20% can be handled without modification of existing burners [59]. The maximum volume concentration of hydrogen that can be added to natural gas may differ from location to location and, therefore, needs to be assessed on a case-by-case basis. Besides burner applications, a hydrogen/natural gas mixture can be used to supply pure hydrogen. In this case, the consumer needs to be equipped with separation and purification technologies to extract hydrogen from the gas blend [62].

2.2.2.2 Hydrogen gas grid A pure hydrogen gas grid is needed when hydrogen becomes an important energy vector. Such a grid does, however, require investments for retrofitting or replacing existing pipelines, which are often made of steel, to noncorrosive and nonpermeable materials such as polyethylene [59]. End user applications such as gas boilers will then need to be adapted as well [59]. A pure hydrogen grid would also offer possibilities for storage as a small increase in the operational pressure of the grid would translate into a significant increase in the overall hydrogen storage capacity of the system. Another advantage is that pure hydrogen can be used to feed a fuel cell, providing a more efficient way of converting the stored energy, compared to the combustion of hydrogen gas blends. Today, a 550 km long network of hydrogen pipelines is operated by Air Liquide and Linde AG in the Benelux and Germany to transport hydrogen between chemical plants for use as chemical feedstock [63]. However, this is still a very small grid compared to the 2.2 million km of natural gas pipelines in Europe [59]. The amount of hydrogen that can be delivered is also limited by the small diameter of the pipelines and is, for now, too low to supply entire residential areas [63].

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2.2.3 Large-scale hydrogen storage technologies 2.2.3.1 Compression, liquefaction, and cryo-compression of hydrogen Hydrogen can either be stored in the gas phase, through compression, or in the liquid phase under cryogenic conditions. Compression and liquefaction are the only two types of hydrogen storage technologies that are currently employed at a large scale [64–66]. A compressed hydrogen storage system consists of two main components: the storage vessel and the compressor required to achieve the desired storage pressure. Because of the demanding material requirements and the high operating costs, large amounts of gaseous hydrogen are usually not stored at pressures higher than 100 bar in stationary, above-ground vessels [67, 68]. As the storage pressure is limited, so is the achievable hydrogen storage density – at 20 °C and pressures between 100 and 300 bar, the density of hydrogen gas varies from 7.8 to 20 g/L. This low density results in large storage volumes and, consequently, high investment costs [68]. In addition, energy losses during compression can be as high as 15% of the amount of energy stored. The density of hydrogen can be substantially increased through liquefaction. This technique allows hydrogen storage densities of up to 70 g/L to be attained at 1 bar. Liquefying hydrogen is expensive and both time and energy consuming, as the boiling point of hydrogen at 1 bar is as low as −253 °C. Almost 40% of the stored energy is lost in the storage process as a result of the low temperature requirements and the need to regularly boil-off the evaporated hydrogen to prevent pressure build-up inside the cooled storage vessel. Despite its obvious drawbacks, liquefaction is still the solution for industrial, large-scale storage of hydrogen because of its very high hydrogen storage density (Fig. 2.3). Globally, the installed hydrogen liquefaction capacity is approximately 355 ton H2/day [68]. Cryo-compressed hydrogen storage combines properties of both hydrogen compression and liquefaction systems. The technology was developed to minimize boil-off losses from liquefied hydrogen storage while at the same time enhancing the hydrogen storage density (up to 80 g/L) [65]. The hydrogen is stored in an insulated vessel that can withstand cryogenic temperatures (−253 °C) and high pressures (at least 300 bar). The high-pressure resistance of the storage vessel minimizes the evaporative losses of liquefied hydrogen as the pressure inside the vessel is allowed to reach higher values before boil-off is required [68]. High hydrogen storage densities and improved longterm storage efficiency render cryo-compression an interesting alternative for largescale, long-term hydrogen storage. There is ongoing research to study and evaluate the full potential of this technology and demonstrate its applicability in automotive applications [65].

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Fig. 2.3: Liquid hydrogen storage tank at the Kennedy Space Center [69] (photo credit: Tom Fawls).

2.2.3.2 Underground storage of hydrogen in salt caverns The concept of storing gas underground in impermeable, geological formations originated from the need to supply gas to consumers during periods of high seasonal demand. An underground gas storage facility is capable of injecting and withdrawing gas during periods of peak demand. Today, depleted gas/oil reservoirs, aquifers, and salt caverns (with volumes up to 700,000 m3) are already used for the underground storage of natural gas. Compared to surface gas tanks, underground storage allows storage of larger volumes of gas at high pressures (up to 200 bar), resulting in hydrogen storage densities of around 15 g/L. Additional advantages include the relatively low construction costs, low leakage rates, fast injection and withdrawal rates, and minimal risks of hydrogen contamination. The main drawback of underground storage is its limited applicability, as only few regions provide the appropriate geological prerequisites [70]. It was recently demonstrated that salt caverns can be used for the underground storage of large quantities of hydrogen. At present, there are three underground hydrogen storage facilities operational worldwide [70, 71]. Combining the underground storage of hydrogen with the on-site production of green hydrogen could allow an effective use of hydrogen as a chemical energy vector in “power-to-gas” energy schemes (Fig. 2.4) or as a chemical feedstock [72]. In a recent study, Caglayan et al. mapped the on- and offshore salt caverns and bedded salt deposits across Europe and evaluated their technical hydrogen storage potential to be 84.8 PWhH2, rendering it a viable technology for large scale hydrogen storage [73].

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gas combustion unit

electricity

RENEWABLE

H2O

electricity

H2

wind energy solar energy electrolyser

injection withdrawal

H2

salt caverns

Fig. 2.4: Salt cavern facility for hydrogen production and gas storage. Reprinted from Ref. [70]. Copyright (2012), with permission from Elsevier.

2.2.3.3 Centralized production of chemical hydrogen carriers Instead of storing hydrogen in its gaseous or liquefied state, it can also be stored by triggering a chemical reaction in other molecules thus forming chemical hydrogen carriers. At the place and time of use, the reverse chemical reaction can be performed, releasing the stored hydrogen. Examples of hydrogen carriers are carbon-containing compounds such as methane, methanol and dimethyl ether, and nitrogen-containing compounds such as ammonia. The main reason for using these carriers instead of pure hydrogen is their beneficial physical properties for storage. Storage and transportation of liquid ammonia is less complex due to the lower pressures (9–10 bar) at which ammonia is in its liquid form at room temperature [74]. In this state, ammonia has a hydrogen storage density of 129 g/L. Ammonia, as a hydrogen carrier, is also attractive in terms of safety. It is lighter than air, so if a leak occurs, ammonia quickly dissipates to the upper atmosphere, a property shared with hydrogen. Ammonia has also a very strong smell and is detectible at concentrations of only 20–50 ppm, which is well below the harmful limit of 300 ppm. Furthermore, it is considered nonflammable when transported due to its very narrow flammability range of 15–28%. Procedures for safe handling of large quantities of ammonia are established and there is a widespread infrastructure for transportation of liquid ammonia by train, truck, or pipeline [74]. Carbon-containing hydrogen carriers include a wide range of compounds. With some of these carriers, hydrogen recovery is accompanied by CO2 emission (formic acid, methanol, dimethyl ether, Fischer–Tropsch (F–T) fuels, etc.). With other carriers, hydrogen is released without CO2 emission (N-ethylcarbazole, naphthalene, (dibenzyl)toluene, phenazine, and ethylene glycol). When the carrier is a liquid at ambient temperatures and pressures, it is referred to as a liquid organic hydrogen

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carrier (LOHC). These LOHC carriers store hydrogen with high gravimetric and volumetric capacities, while minimizing health and safety risks [75]. For example, dimethyl ether, liquefied at a low pressure of 5–6 bar, methanol, and formic acid have hydrogen storage densities of 91, 96, and 52 g/L respectively. LOHCs have properties similar to crude oils, thus requiring little to no change to the existing fuel handling infrastructure, rendering them extremely suitable for long-distance transportation. In LOHCs such as N-ethylcarbazole and naphthalene, hydrogen can be repeatedly loaded and unloaded via reversible, catalytic (de)hydrogenation cycles [76, 77]. As an example, the reversible storage of hydrogen in dibenzyltoluene is shown in Fig. 2.5.

Fig. 2.5: Schematic representation of the reversible storage of hydrogen using dibenzyltoluene (H0-DBT)/perhydrodibenzyltoluene (H18-DBT) as liquid organic hydrogen carrier (LOHC) molecule. Reprinted with permission from Ref. [78].Copyright (2017), American Chemical Society.

High temperatures and pressures are required for these catalytic processes to proceed, leading to a low efficiency of the regeneration process that liberates the hydrogen gas. Additionally, most reactions require a catalyst from the platinum metal group, making it very expensive. Development of less expensive, highly selective, and stable catalysts, which are active at lower pressures and temperatures, will be a key to success in industrial applications [77]. 2.2.3.3.1 Carbon dioxide utilization CO2 is a chemical building block. Most of its utilization pathways involve the use of hydrogen, and, in this way, it can be considered a hydrogen carrier. There are several chemical reaction routes available where the hydrogen carriers are synthesized

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either from CO2 and water (“direct synthesis”) or from H2 and CO/CO2 (“two-step synthesis”) (Fig. 2.6, Tab. 2.1) [2].

Fig. 2.6: Routes for the synthesis and use of carbon-containing hydrogen carriers.

Tab. 2.1: Overview of synthesis routes for the production of hydrogen carriers [79, 80]. Hydrogen carrier

Direct synthesis

Two-step synthesis

CH

Electrochemical CO2 reduction CO2 + 8 H+ + 8 e− ! CH4 + 2 H2 O

Sabatier–Senderens reaction CO2 + 4 H2 ! CH4 + 2 H2 O

CHOH

Electrochemical CO2 reduction CO2 + 6 H+ + 6 e− ! CH3 OH + H2 O

Hydrogenation CO2 + 3 H2 ! CH3 OH + H2 O Syngas-to-methanol conversion CO + 2 H2 ! CH3 OH

CHO

Electrochemical CO2 reduction CO2 + 2H+ + 2e− ! CH2 O2

Hydrogenation CO2 + H2 ! HCOOH

CHO

Synthesis through methanol 2 CO2 + 6 H2 ! C2 H6 O + 3 H2 O

Mixture of hydrocarbons

Fischer–Tropsch ð2n + 1Þ H2 + n CO ! Cn H2n + 2 + n H2 O

NH

Electrochemical N2 reduction N2 + 6H+ + 6e− ! 2 NH3

Haber–Bosch N2 + 3 H2 ! 2 NH3

CO2 conversion can be performed wherever a concentrated source of CO2 is present. For the process to be renewable, this can either be an industrial point source of CO2 off-gas from biological origin or CO2 captured from atmospheric air at a direct air-capture facility. If hydrogen or syngas are available at these facilities, or if there is an energy supply for an electrolyzer, the two-step synthesis routes might be beneficial due to their technology readiness. On a smaller scale, if a more modular system is required or if hydrogen is not available, plants might opt for direct synthesis routes (Fig. 2.6). Here, the various routes will be briefly discussed in terms of their technology readiness and demonstration status in Europe. Considering the diversity of options, this discussion will be limited to the production of methane, methanol, dimethyl ether, formic acid, and F–T fuels (Tab. 2.1).

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2.2.3.3.2 Methanation of CO2 The catalytic methanation of CO2, known as the Sabatier–Senderens reaction, dates back to the nineteenth century. This process has a high technology readiness level (TRL) of 8–9 [81]. The reaction is typically performed on Ni-supported metal oxides and is strongly exothermic. To drive up the CO2 conversion, the reaction is performed in several stages with intermediate cooling. Götz et al. estimated a capital investment (capital expenditure, CAPEX) cost for a CO2 methanation plant that produces methane at 311 Nm3/h to be in the range of 35.8–38.8 million EUR. This includes the cost of an electrolyzer for H2 production, which is approximately 77% of the CAPEX [79]. There are several renewable power-to-methane demonstration projects currently up-and-running. In Europe, they are mostly located in Germany, Denmark, and the Netherlands. For example, since 2013, Audi has an operational power-to-gas facility producing 325 Nm3/h or 6 MW (in terms of energy) of renewable methane from wind power [79]. In Falkenhagen, Germany, a demonstration plant has been constructed where methane is produced from wind energy, with CO2 from a bio-ethanol plant. Since March 2019, an average of 1,400 Nm3 of synthetic methane is produced per day and injected in the natural gas grid [82]. 2.2.3.3.3 Fischer–Tropsch (F–T) F–T processes were developed at the beginning of the twentieth century and have been widely operational ever since. Depending on whether renewable syngas production routes are used or not, F–T using CO2 has a TRL of 5–9 [79]. The reaction conditions can be tuned toward synthesis of organic alkane compounds with different molecular weights and composition, including sulfur-free diesel, gasoline, alcohols, and alkenes. The majority of the operational plants use fossil fuels as their carbon resource. For a plant employing renewable energy and CO2, the German Environment Agency estimated a capital investment cost of 308 million EUR for a low-temperature F–T production process producing 97,000 tons of liquid fuel per year [79]. F–T technology could be of great importance for the synthesis of renewable kerosene. To this purpose, research is being carried out to increase the selectivity toward C10–C16 hydrocarbons [83]. As an example, the Horizon 2020 KEROGREEN project aims at the production of sustainable aircraft-grade kerosene through F–T synthesis [84]. 2.2.3.3.4 CO2 hydrogenation to methanol In the conventional methanol synthesis process, CO2 is converted to CO through the reverse water–gas shift reaction, followed by the hydrogenation of CO to methanol over CuZnO-based catalysts [85]. This technology is well established, with about 40 million tons of methanol produced per year. The reaction can also proceed in a single-step process, with the same catalyst carrying out both reactions, leading, however, to lower

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productivities. Research efforts are still being made to develop highly efficient catalysts for this direct methanol synthesis from CO2 [85]. Methanol synthesis from CO2 has reached a TRL of 6–7 [79]. An early adopter of methanol production from renewable H2 is Mitsui Chemicals Inc., producing 100 tons of methanol per year since 2010. In Europe, a renewable methanol plant was installed in Iceland, where CO2 captured from a nearby power plant is combined with renewable H2. This facility has been producing around 4,000 ton of methanol per year since 2012 [79, 85, 86]. In Antwerp, Belgium, a power-to-methanol demonstration plant is expected to be operational by the end of 2022, producing 8,000 tons of methanol per year from waste CO2 and sustainably produced H2 [87]. 2.2.3.3.5 CO2 hydrogenation to formic acid Different from methanol, CO2 hydrogenation to formic acid is a technology that is low in maturity and is mostly on a lab and pilot scale, limiting the TRL to 3–5 [79]. The reaction has been shown to proceed on homogeneous Rh- or Ru-complex catalysts in either water or organic solvents. Hydrogenation toward formic acid is currently challenged by the limited performance despite the use of expensive catalysts. Currently, the process is far from being economically viable [79]. 2.2.3.3.6 Dimethylether synthesis Dimethylether (DME) is commonly synthesized in a two-step process from methanol over an acid catalyst. This technology is widely used in chemical industries and is a technologically mature synthesis route. DME can also be synthesized from syngas in a 1-step process on a hybrid- or bifunctional catalyst. This route gained interest more recently, with the development of the bifunctional catalyst being at the core of the research [88]. Demonstration plants are located mostly in Asia. DME is considered a leading alternative to petroleum-based fuels for the domestic fuel market. Volvo is considering DME to be a viable alternative to diesel for heavy-duty trucks and buses, emitting far less soot and NOx gases. Plans to commercialize DME-powered trucks have been announced [88]. 2.2.3.3.7 N2 hydrogenation to ammonia Today, ammonia (NH3) is one of the most important manmade chemicals used in fertilizer production, pharmaceutical production, organic synthesis, synthetic fibers, and also more recently for energy storage and conversion [89]. Of these applications, fertilizer production is by far the most important, responsible for 79% of the ammonia consumption [90]. The fossil fuel-based Haber–Bosch process, as the world’s current major source of ammonia, accounts for 90% of the annual production [91]. Natural gas is by far the most economical feedstock for the Haber–Bosch process, achieving the lowest energy consumption and requiring the lowest investment.

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The prospect of continued and growing use of this process for future supplies of ammonia creates serious environmental concerns, as ammonia synthesis constitutes ca. 1.5% of global CO2 emissions from fossil fuels sources (the global average is 2.86 tons of CO2 per ton of ammonia, and 1.6 tons of CO2 per ton of ammonia in the most efficient plants) [92]. Exacerbating this issue, the sluggish kinetics of the reaction between N2 and H2 requires elevated temperatures (500°C) and pressures (>200 atm). Combined with the consumption of large amounts of methane-derived H2, the overall process is highly energy-demanding. As a result, approximately 2.5% of anthropogenic energy is consumed [93]. In order to keep feeding the growing world population, the production of ammonia is expected to increase, along with its CO2 emissions. In addition to the current applications of NH3, it has been proposed as an environmentally friendly, carbon-free energy carrier. However, in order to fulfil this purpose, ammonia needs to be produced without emitting vast amounts of CO2. The simplest way to achieve carbon-free ammonia production is by substituting the methane-derived hydrogen with green or blue hydrogen, produced through electrolysis. This idea is not new. Until the 1960s, the majority of Europe’s ammonia was produced in Norway, based on H2O electrolysis using hydropower [94]. In the 1960s, the increased fertilizer demand and the abundant availability of cheap natural gas resulted in the replacement of H2O electrolysis by steam-methane reforming [94]. The energy cost of the electrified Haber–Bosch process is mainly determined by the energy-efficiency of the H2O electrolyzer. As discussed in Section 2.2.1.3, alkaline electrolysis (AWE, TRL 8–9 [95]) is the most mature and common technology. A 60% efficient AWE allows ammonia production at an energy cost of around 0.675 MJ/mol NH3 [96–100], or a total energy efficiency of 50%. Proton exchange membrane water electrolysis (PEMWE) can reach higher efficiencies and deliver hydrogen at higher pressures. With 75% efficient PEMWE (TRL 6–7 [101]), operating at 80 bar, the energy cost of ammonia production decreases to 0.54 MJ/mol, leading to a total efficiency of 63% [96, 102, 103]. Solid oxide water electrolysis (SOWE, TRL 3–5 [104]) is carried out at an elevated temperature. SOWE offers unique benefits when used in combination with the electrified Haber-Bosch process. As the reaction from N2 and H2 to ammonia is exothermal, waste heat is available and can be employed by the SOWE process [96]. With 80% efficient SOWE, an energy consumption of 0.51 MJ/mol and a total efficiency of 67% can be reached [96, 104]. Today, the cost of green ammonia, produced by an electrified Haber–Bosch process, is between 1.5 and 2 €/kg [105]. The traditional fossil fuel-based Haber–Bosch process has a significantly lower cost of 0.40–0.45 €/kg and is almost entirely determined by the price of natural gas [106]. However, the rapidly decreasing price of electrochemically produced hydrogen has the potential to bridge the price gap between green and fossil fuel-based Haber–Bosch ammonia production. As an additional advantage, the electrified Haber–Bosch process can be run at a much smaller scale than the fossil-fuel based process. While the fossil fuel-based

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process is typically installed at capacities ranging from 300,000 to 500,000 ton/year, the production cost of the electrified process is stable for a large scale as well as an intermediate production capacity of 3,000 ton/year, which can still be categorized as relatively centralized facilities. The production cost strongly increases for lower production capacities [105, 107]. The electrified Haber–Bosch process is well suited for ammonia production in centralized facilities due to the high TRL of 8–9 and the low CAPEX of the ammonia synthesis loop [96, 105].

2.2.3.4 Reconversion of chemical hydrogen carriers to hydrogen In technologies where carbon- and nitrogen-based chemical compounds are used for hydrogen storage and transportation, efficient ways of regaining hydrogen are essential. Ammonia can be decomposed back to N2 and H2 by increasing the temperature and running it over a catalyst bed. Ruthenium-based catalysts have been developed for this purpose. The best Ru catalysts are sufficiently active from temperatures of about 300°C, but at this temperature the maximal NH3 decomposition conversion is limited to a few percentages. To achieve nearly complete ammonia decomposition, a temperature of 500°C is required [108, 109]. The major disadvantage of Ru catalysts is their high cost. More recently, sodium amide catalysts have been developed as a noble-metal-free alternative, which approach the performance of ruthenium catalysts [109]. Hydrogen can be recovered from molecules like methane, methanol, synthetic diesel, or gasoline using several established technologies involving steam (catalytic steam reforming), oxygen (partial oxidation), or both consecutively (auto thermal reforming). The selectivity towards hydrogen can be further increased by catalytic water-gas shift reactions. The resulting product is a mixture of hydrogen and CO2 [2]. Steam reforming is the most widespread technology for the generation of hydrogen from light carbohydrates. This technique was discussed in Section 2.2.1.1 as a method to synthesize blue hydrogen. It is usually performed at large facilities, but development of small-scale facilities has gained interest due to the increasing need in hydrogen fuel cells. These carbon- and nitrogen-containing compounds are not only useful as hydrogen carriers, but they can also be used in several other applications. For carboncontaining compounds, this is the concept of carbon capture and utilization (CCU). Firstly, they can be used locally in the industrial plants, either for their energy content or for their chemical properties. Ammonia can be used for fertilizer production and nitric acid synthesis, methanol is an important feedstock for polymer synthesis, formic acid is used as a food additive, and so on. A second scenario is the application in the transportation sector. Fuels such as methanol, dimethyl ether, and F-T fuels can be used with relatively little modification to the existing spark ignition engines. Owing to their high energy density, these fuels are especially suitable for heavy-duty transport such as trucks, buses, boats, and airplanes. It is likely that

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heavy-duty long-distance transport will remain dependent on liquid fuels such as biofuels, e-liquids, biomethane-based LNG or CNG and hydrogen, while the aviation sector will require a mix of e-jet fuels and synthetic kerosene [110]. These fuels can be distributed through the current gasoline, diesel, or LPG fueling stations. This is different from H2, where distribution requires the building of new hydrogen fueling stations. As an alternative to spark ignition engines, liquid fuels can be converted to electrical energy in direct fuel cells. A car equipped with a direct methanol fuel cell requires a tank of 41 L for a 1,000 km driving range. In comparison, for a hydrogen powered car, for a range of 1,000 km, a tank of 153 L at 350 bar is currently needed. A third application is the use of CCU chemicals for domestic purposes. With the rise of renewable energy, it is likely that buildings will be equipped with electric heating, air conditioning, cooking, lighting, etc. In summer, a sufficient supply of renewable energy can be expected. However, in winter, fuels might be necessary to ensure the energy supply. Liquid renewable CCU fuels transported by trucks and stored locally provide an interesting option. Assuming an energy consumption between 3,000 and 7,000 kWh during the winter season, a methanol or formic acid storage tank of between 0.6 and 4 m3 would suffice. A fully developed hydrogen network to individual houses would be obsolete. When carbonaceous fuels are used for transport or for domestic purposes, these are diffused sources of CO2. If they cannot be captured locally, emissions need to be compensated elsewhere through direct air capture of CO2.

2.3 Decentralized hydrogen production scenario 2.3.1 Hydrogen production technologies Decentralized hydrogen production refers to a smaller scale production (MW scale or smaller) for consumers that require hydrogen but are at a remote location from a centralized plant. When hydrogen is not produced locally, it would need to be delivered by compressed or liquid hydrogen tube trailers or by a hydrogen grid. However, this increases costs. With time, local green hydrogen production and storage are expected to become affordable in more and more cases compared to central production and distribution. In future energy scenarios, hydrogen will gain more importance in novel applications such as in mobility, heating applications, and as feedstock in new industrial applications [4]. Some of these applications will not require large amounts of hydrogen and could be powered by a decentralized hydrogen production unit. For example, hydrogen fueling stations, depending on the scale, require variable amounts of hydrogen. In the US, already 7 of the 45 fueling stations have onsite water electrolysis [111]. Onsite SMR only accounts for 2 of the 45 refueling stations [111]. SMR does

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not scale down very well compared to water electrolysis. H2A models of NREL estimate a current hydrogen cost for distributed SMR of 1.5 $/kg for a production unit of 1.5 ton/day [9]. This cost is still low compared to the current estimated price for distributed PEM water electrolysis (4.98 $/kg) [9]. However, it becomes very difficult for decentralized SMR production units to capture and store CO2 efficiently and at low cost [112]. In addition, hydrogen pricing of both these technologies is still mostly dependent on feedstock price, making water electrolysis more interesting at locations with low electricity costs and high gas prices.

2.3.1.1 Small-scale electrolysis Energy input for water electrolysis could be delivered by an electricity grid or by direct connection to renewable energy sources. If connected to the grid, hydrogen produced by water electrolysis is only green if the electricity on the grid is renewable. This is currently still not the case, but will become so in the future. Grid-based operation could allow a higher utilization of the water electrolysis unit, which results in a lower capital expenditure [113]. However, if the grid is mainly based on renewables, low cost electricity on the grid will be mostly available at periods when renewable electricity is also generated locally. Since using the grid adds extra costs and results in efficiency losses by power converters, in some cases, it could be more interesting to directly connect renewable energy sources to the water electrolysis unit [114]. This is the case for solar hydrogen generators that directly produce hydrogen gas from sunlight illumination.

2.3.1.2 Solar hydrogen generation This concept of solar hydrogen generator is still at the lower TRL level, but researchers have analyzed the potential of direct coupling of solar cells with electrolysis units and were able to achieve high solar-to-hydrogen efficiencies [115]. Other researchers are even looking at integrating solar cells or photo-absorbers with hydrogen (HER) and OER catalysts in monolithic device concepts [116, 117]. Although monolithic device concepts are more integrated, they are yet to achieve performance and price levels comparable to PV electrolysis. It is claimed that, in the best case scenario, monolithic device concepts could only compete with, but not beat PV electrolysis [118, 119]. Compared to water electrolysis units, solar hydrogen generators require larger surface areas to reach a similar productivity. Therefore, it is imperative that lowcost materials are used to be competitive with grid-based electrolysis [113]. In addition, the device should allow a dynamic response to cope with the intermittent nature of solar irradiation. In this regard, using an AEM-based solar hydrogen

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generator could provide a valuable alternative to the current day grid-powered AWE or PEMWE [113]. In Europe, large areas required for solar hydrogen generators could become accessible by integrating the land that is used for agriculture with energy production as demonstrated in the concept of agrivoltaics [120]. This concept is based on the principle that some crops are tolerant to partial shading of solar panels or solar hydrogen generators [120]. In addition, installation of these panels can also reduce water consumption by resulting in less evapotranspiration during the summer and in drought conditions [120].

2.3.1.3 Water vapor electrolysis Starting with the area required for solar energy capture, estimations have shown that outdoor air contains sufficient water that can be consumed during water splitting [114]. Therefore, recent research is examining this novel concept to produce hydrogen from water vapor at electric potentials and current densities obtained from solar cells [121–123]. The first demonstration of the potential of air-borne water vapor, as feed in a solar hydrogen generator, dates back to 2014 [124]. In this approach, water was captured from the atmospheric air to produce hydrogen gas. After oxidation of the hydrogen gas in fuel cells or burners, water would be released back to the atmosphere, closing the water cycle. The device concept of a solar hydrogen generator can be drastically simplified. This type of device would be able to operate without the need for pumps, water purification, reservoirs, etc. and offer a means to disentangle the energy-water nexus [114, 124, 125]. Engineering challenges, when operating with liquid water systems, such as corrosion and frost are also avoided. The use of non-noble metals and AEM is an option [121]. In the laboratory, a 15% solar-to-hydrogen efficiency with sunlight and water vapor as input has been reported [121].

2.3.2 Small-scale hydrogen storage technologies Many of the storage technologies discussed below have shown their merit for storing hydrogen in small-scale laboratory set-ups and pilot units. Application at larger scales is still not within reach.

2.3.2.1 Physical adsorption of hydrogen Hydrogen storage via adsorption (also referred to as hydrogen physisorption) exploits physical van der Waals interactions between molecular hydrogen and (porous) materials with a large specific surface area [68]. Such materials include porous carbon-

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based materials [126, 127], carbon nano-tubes (CNTs) [128], metal-organic frameworks (MOFs) [129, 130], porous polymeric materials [131] and zeolites [132]. Because of the weak nature of these van der Waals interactions, however, elevated hydrogen pressures (10–100 bar) and temperatures as low as 77 K are required to achieve significant hydrogen storage densities up to 10 wt% and 20–50 g/L, potentially limiting its practical application [68, 133, 134]. At larger scales, this technology also suffers from lower volumetric/gravimetric hydrogen storage densities because of the nonideal packing of the carrier material and the need for a thermal management system, which is indispensable for mitigating (local) heat generation arising from the exothermic adsorption of hydrogen. The weight of the carrier material and the pressure-resistant storage vessel further decrease the gravimetric storage capacity of these systems, limiting widespread adoption of this technology for large-scale use [135, 136].

2.3.2.2 Chemical sorption of hydrogen Hydrogen can also be stored by chemically bonding it to metallic carrier materials, including Li, Be, Na, B, Al, Mg, Ti, La, and their alloys. During initial loading, hydrogen (exothermally) adsorbs at a metal center, dissociates to form two hydrogen atoms, and is inserted into the lattice structure of the metallic carrier, which is then referred to as a metal hydride (e.g., MgH2). These chemical bonds are much stronger than the van der Waals interactions governing the physical adsorption of hydrogen, allowing hydrogen storage densities as high as 14 wt% and 90 g/L (theoretically rivaling liquid and cryo-compressed hydrogen) at moderate temperatures and pressures [68, 133]. Stronger bonding, however, also implies more energy is needed to load and release the hydrogen. Hydrogen loading is typically carried out under pressure (10–150 bar) at temperatures varying from 20 to 300°C, depending on the composition of the metal hydride used. The stored hydrogen is easily released at the point of use by re-heating the system to 40–350°C [68, 135]. Despite its high storage capacity, there are a few issues slowing down the development of metal hydrides for larger scale application. Packing problems and the complexity of storage tanks suited for proper heat management significantly lower the hydrogen storage capacities achievable in practice. Also, the kinetics of (de-)hydrogenation can be relatively slow, even at elevated temperatures, making the (un)loading process fairly lengthy. Incorporation of noble metals (such as Ti and La) has been shown to boost kinetics, but at the expense of the overall cost of the intermetallic hydride [135]. Most of these limitations are, however, mitigated when used in limited scale, rendering hydrogen storage in the form of metal hydrides ideal for small-scale applications [133]. The gravimetric and volumetric performance of different hydrogen storage technologies are collected in Fig. 2.7 [133, 137, 138]. This figure is composed based on actually reported data.

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Fig. 2.7: Volumetric and gravimetric hydrogen storage capacity of different technologies.

Theoretical values can be higher. The compressed and liquefied hydrogen storage systems perform relatively well in gravimetric capacity, but are less performant in volumetric capacity. This is because the reservoirs are filled only with the light hydrogen gas. It is the weight of the reservoir that limits the gravimetric capacity. Metal hydrides and adsorbents have rather low gravimetric capacity and perform similarly to storage tanks in volumetric capacity. LOHCs are on the upper side of the volumetric capacity. The liquid fuels produced from CO2 and/or N2, and hydrogen (formic acid, dimethyl ether, methanol, LNG, and ammonia) outperform the other storage systems, but catalytic technology is needed to liberate H2 from these carriers.

2.3.2.3 Decentralized production of chemical hydrogen carriers Electrochemical reduction of CO2 or N2 offers a way to synthesize chemical hydrogen carriers, such as methanol, ethanol, methane, formic acid, or ammonia, directly from water and CO2 or N2 on a small, decentralized scale (Tab. 2.1). In contrast to traditional chemical processes such as methanation, F–T, hydrogenation of CO2, or Haber–Bosch, the electrochemical approach allows the chemical hydrogen carriers to be synthesized on a much smaller scale. In combination with small scale reconversion to hydrogen, this approach enables a decentralized hydrogen production and storage.

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2.3.2.3.1 Electrochemical CO2 reduction Electrochemical reduction of CO2 can be used to synthesize several products, including methanol, ethanol, CO, CH4, and formic acid. The selectivity of the reaction towards one product can be tuned by using different electrocatalysts and electrolytes. The main advantage of electrochemical reduction is that it can proceed directly from H2O at ambient temperatures and pressures, instead of needing H2. However, this technology is still in its infancy, at a low TRL of 3–5 [79]. Due to the strong stability of CO2, a large overpotential is needed for a one-electron reduction of CO2 to the activated CO2 radical intermediate. Therefore, catalysts design is focused on stabilizing the reaction intermediate or circumventing this oneelectron reduction path [85]. Formic acid is often discussed as the CO2 reduction product with the highest likelihood to become economically viable. The reaction only requires two electrons and can proceed with relatively low energy and high selectivities [139]. According to a techno-economic analysis performed by Spurgeon et al., electrochemical reduction of CO2 to formic acid could become cost-competitive with its commercial bulk price, considering an increase of the current density as opposed to the current state of the art [140]. Distillation of the formic acid product from the liquid electrolyte represents a major economic and environmental cost, shifting research efforts toward semi- or fully gas-phase electrochemical systems [141]. Demonstrations are currently at a pilot-scale level. In Canada, Mantra Energy Alternatives Ltd. has demonstrated a CO2 electrolyzer for formate production, with a capacity of 100 kg of formate per day [142]. In Europe, DNV GL developed a semipilot size electrolyzer for CO2 reduction utilizing renewable energy, electrochemically reducing approximately 1 kg of CO2 per day [143]. 2.3.2.3.2 Electrochemical NH3 synthesis Direct electrochemical ammonia synthesis from H2O and air offers a way to circumvent the high temperatures and pressures needed for the Haber–Bosch process. One of the challenges related to this approach is to achieve a sufficient selectivity towards ammonia, which is limited by the competing hydrogen evolution reaction, forming H2 as the by-product. This selectivity is the main factor determining the energy cost of the electrochemical approach. Electrochemical ammonia synthesis becomes competitive with the Haber-Bosch process when the selectivity to ammonia of the electrocatalyst exceeds 30% [144]. In recent years, electrocatalysts with excellent selectivity toward ammonia have been developed, reaching selectivities up to 66% [145]. However, these electrocatalysts have a limited activity, reaching current densities of a few mA/cm2 at best [146–148]. In comparison, commercial alkaline water electrolyzers achieve current densities of 100–300 mA/cm2 [102]. For electrochemical ammonia synthesis to

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become economically viable, the achieved current densities need to increase at least 1 to 2 orders of magnitude. 2.3.2.3.3 Plasma-enabled ammonia production Plasma provides another way to circumvent the extreme conditions of the Haber– Bosch process. Just like the Haber–Bosch synthesis loop, the plasma reactor also converts H2 and N2 into ammonia, but instead of heating the entire reactant mixture, the electrons of the ionized gas are accelerated by an electric field due to their low mass. A thermal nonequilibrium is created between the electrons and the gas molecules. When the electrons collide with the gas molecules, new molecules are formed [2, 149]. This way, a high reactivity can be obtained at relatively low bulk temperatures. As this reaction is exothermal, the relatively low bulk temperatures that can be maintained in a plasma reactor become increasingly attractive [149]. Unfortunately, reported results show a trade-off between energy consumption and high yields [150, 151]. A high yield (>10%) is accompanied by a very high energy cost (>80 MJ/mol) [152]. At a relatively low energy cost of 2 MJ/mol, the obtained ammonia is very diluted (1%) is 18.6 MJ/mol [153], without taking the energy cost of hydrogen production into account. As the electrified Haber–Bosch process requires only 0.67 MJ/mol, including hydrogen production, a drastic decrease in energy cost is required for plasma-synthesized ammonia to become economically viable.

2.4 Stranded hydrogen production scenario Globally, more and more large-scale solar or wind farms are emerging. According to the European Commission’s long-term decarbonization strategy, Europe will need between 230 and 450 GW of offshore wind by 2050 to meet the CO2 emission reduction stipulated in the Paris Agreement [154]. Large photovoltaic plants are arising quickly in Europe, the largest comprising more than 1.4 million solar panels, with a total capacity of 500 MW (Núñez de Balbao, Spain). At these solar and wind farms, large amounts of renewable energy are produced that need to be transported over significant distances, either over sea or over land. Today, such large energy farms are typically directly connected to the electricity grid. When the share of these intermittent energy sources exceeds a critical limit, the energy of these farms will have to be used for producing other energy carriers, such as hydrogen. Additionally, it makes sense to place these green hydrogen production plants where there is enough space and where renewable energy is abundantly available. For example, in the Atacama Desert in Chile, there is a large potential for wind

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energy, and it has the highest yearly solar irradiation in the world. Compared to Spain, the same solar farm in Chile would have a 33% higher energy yield. Compared to Belgium, the energy yield in Chile is 140% higher [155]. The cost savings due to cheap renewable energy may outweigh the increased cost for transportation from these remote locations. Importing solar energy from places in the southern hemisphere during winter also helps to overcome the issue of seasonal intermittency.

2.4.1 Transport of hydrogen from stranded locations The 70 g/L hydrogen storage density attainable with liquefaction has made it the technology of choice for transporting and delivering hydrogen, both across the road and overseas (Fig. 2.8). In addition, compressed hydrogen gas tube trailers are also used for transporting hydrogen by road (Fig. 2.9). Cryogenic tank trucks can carry up to 5,000 kg of hydrogen, which is more than five times the maximum capacity of gas tube trailers. As liquefied hydrogen is transported in specialized vessels under atmospheric pressures, cryogenic tankers represent less safety hazards than gas tube trailers, which usually transport hydrogen pressurized at 200 bar. Hydrogen distribution through pipelines is available to specific industrial markets, but its reach is currently limited to those specific regions [136]. A finer hydrogen grid linking centralized and decentralized production facilities with small and large consumers will likely be needed by 2050.

Fig. 2.8: Left:Kawasaki heavy industry concept design for liquid hydrogen carriers. Right: Cryogenic trailer for transporting liquid hydrogen. Reprinted from Ref. [136]. Copyright (2019), with permission from Elsevier.

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Fig. 2.9: Hydrogen gas tube trailer (Linde) containing gaseous hydrogen compressed at 200 bar. Reprinted from Ref. [156]. Copyright (2013), with permission from Elsevier.

2.4.2 Transport of chemical hydrogen carriers from stranded locations In-situ production of liquefied methane, ammonia, methanol, formic acid and other chemical hydrogen carriers could be an interesting route for trans-oceanic transport. Liquid, carbon-containing hydrogen carriers strongly resemble crude oil. A large network of pipelines, boats, trucks, and trains is therefore already present for long-distance transport. The storage and transport of ammonia is also well explored and widely established. Today, liquefied ammonia is transported across the world via ship, train, pipeline, and truck. In the United States, ammonia is used directly as a fertilizer. For that reason, a large network of ammonia pipelines and over 10,000 ammonia storage sites are spread across the country. Large ports across the world have ammonia storage tanks with capacities exceeding 100,000 tons [157]. Ammonia is, therefore, ideal for transportation of large quantities of chemically stored energy across the world.

2.5 Conclusion Hydrogen-based energy is technologically on-track for reaching the target of CO2neutrality by 2050. Productions at different scales (centralized, decentralized and stranded) are complementary and all three scales are needed to make hydrogen a game changer. There are plenty of technological approaches for producing, distributing, and using hydrogen. Some approaches are still scientifically challenging, and upscaling of technologies in the second part of the TRL scale often meets hurdles and challenges. But given the multitude of alternatives, a viable hydrogen economy is within reach.

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Mieczyslaw Jurczyk, Marek Nowak

3 Materials overview for hydrogen storage 3.1 Introduction Hydrogen is an ideal energy carrier. Methods such as high-pressure gas or liquid cannot fulfill storage goals. Chemical or physically combined storage of hydrogen in alloys, intermetallic, or complex hydrides has potential advantages over other storage methods [1–12]. Increasing application of hydrogen energy is the only way forward to meet the objectives of the Department of Energy (DOE), United States, that is, reducing greenhouse gases and increasing energy security [13]. Currently, the hydrogen fuel R&D subprogram focuses on early-stage research and development (R&D) to reduce the cost and improve the reliability of technologies used to produce and store hydrogen [13]. The subprogram is divided into two categories: (i) hydrogen production R&D and (ii) hydrogen storage R&D. Hydrogen storage projects focus on material-based hydrogen storage R&D, including the discovery, design, synthesis, and validation of metal hydrides, metal-organic frameworks, and other innovative concepts that have the potential to reach high energy densities as well as favorable kinetics and thermodynamics. Storing hydrogen in solids is widely thought of as one of the most promising solutions to the issue of safe, compact, and affordable hydrogen storage for use as energy carrier [11]. In the last 50 years, metal hydrides (MH) with adapted properties for applications, such as high gravimetric and volumetric capacities, reversibility near room temperature and atmospheric pressure, fast kinetics, low cost, and environmental friendliness have attracted attention. The pressure and temperature of hydrides can be tuned to near-ambient conditions by adapting their chemical composition. This allows much better safety than compressed or liquid hydrogen. The main drawback of hydrogen absorption in solids is its low weight capacity, which is detrimental to mobile applications (Fig. 3.1). A large number of the hydrogen storage materials have been studied till now. The first review on metal hydrides was written by Buschow et al. in 1982 [14]. Presently, many reviews are available on this topic [3, 4, 6, 8–10, 15–29]. This chapter focuses on the current status and the future applications of hydrogen storage materials. Additionally, ongoing research on new hydrogen storage materials/nanomaterials is presented. Finally, an overview of nanotechnology and its implications to hydrogen society is presented. The discussion includes a range of different types of available hydrogen storage materials and their technical applications.

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Fig. 3.1: Different types of hydrogen storage methods.

3.2 AB2-, AB5-, AB- and A2B-, and A2B7-type alloys Studies in metal–hydrogen grew rapidly in the 1970s. As a result, many hydride alloys were studied and developed. In the early 1980s, AB2-type Ti/Zr-V-Ni-M systems and rare-earth-based AB5-type Ln-Ni-Co-Mn-Al alloys were investigated. These systems obtained long-life hydrogen storage electrode alloys, but Ti/Zr system has a higher capacity. In the late 1980s and early 1990s, nonstoichiometric AB2 and AB5 systems with improved performance such as higher capacity and long cycle life were developed. Some example compounds include LaNi5H6 and Mg2NiH4, which have hydrogen densities currently reported (Table 3.2). Now, there is renewed interest in hydrogen storage materials, both conventional metal hydrides and new materials such as carbon nanostructures [30–35] and novel compounds [7, 18, 36, 37]. Hydrogen storage nanomaterials can be produced by many methods, one of which is mechanical alloying (MA). This method consists of repeated fracturing, mixing, and cold welding of a fine blend of elemental particles, resulting in size reduction and chemical reactions. MA has been recently used to produce nanocrystalline, nonequilibrium metal hydride materials. It has already been observed that the kinetic barriers in nanocrystalline hydrides should be lower than in coarse-grained materials. Moreover, the MA process increases diffusion channels and shortens diffusion paths for H-atoms. There are numerous possibilities for the design, synthesis, and control of the properties of nanostructured, multicomponent hydrogen storage materials, and nanocomposites fabricated by the application of the MA process. The process permits the control of microstructural properties and the nature of the base alloy or composite. The availability of large amounts of specifically tailored nanostructured metal-based powders is crucial for the successful development of new energy materials for hydrogen storage.

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Several factors (crystal structure, microstructure, crystallite size, purity of the produced nanomaterials, and electronic structure) have shown their influence on the final properties such as hydride stability, hydrogen storage capacity, thermodynamics and kinetics of hydrogenation–dehydrogenation processes, reversibility and hydrogen storage capacity, solid-phase/gas and solid-phase/electrolyte solution systems, and, finally, on electrochemical properties. When the metal storage material is in the form of nanoparticles, there are two advantages – one related to the kinetics and the other to the thermodynamics. The kinetics aspect is obvious; with nanoparticles, the uptake and release kinetics become much faster than with the more commonly used microcrystalline particles. The second aspect concerns the possibility of controlling the thermodynamic properties by tuning particle size. Since the thermodynamic properties of sufficiently small particles change due to, for instance, surface energy and elasticity/plasticity effects, the dissociation pressure may be varied for a given material by using particle size as a tuning parameter. In the last few years, interest in the study of nanostructured materials has been increasing at an accelerated rate, stimulated by recent advances in materials synthesis and characterization techniques and the realization that these materials exhibit many interesting and unexpected properties with several potential technological applications. For example, hydrogen storage nanomaterials are the key to the future of the storage and batteries/cell industries [17, 28, 29]. Recently, it has been shown that MA of TiFe, ZrV2, LaNi5, or Mg2Ni is effective in the improvement of the initial hydrogen absorption rate due to the reduction in the particle size and the creation of new clean surfaces [29, 38–41]. Tab. 3.1: Examples of intermetallic hydrides. Type of alloy

A-site element

B-site element

Alloy

Structure

AB

Mg

Ni, Cu

MgNi, TiNi

Cubic, MoSi, or TiNi type

AB

Ti

Cr, V, Ni, Fe, Mn

TiFe, ZrNi

Cubic, CsCl- or CrB-type

AB

Zr, Ti, Hf

Cr, V, Ni, Mn, Fe, ZrV, ZrMn, Co, Cu, Zn TiMn

Laves phase, hexagonal or cubic

AB

La, Ca, Y, Ce, Nd, Pr,Sm

Ni, Co, Mn, Al, Fe, Cu

CeNi, YFe

Hexagonal, PuNi type

AB

La, Ca, Y,Ce, Nd, Ni, Co, Mn, Al, Pr, Sm Fe, Cu

LaNi

Hexagonal

AB

La, Ca, Y,Ce, Nd, Ni, Co, Mn, Al, Pr, Sm, Mg Fe, Cu

YNi, ThFe

Hexagonal, CeNi type

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Independently, La–Mg–Ni compounds with A2B7-type structure are already being used in novel Ni–MHx batteries [42]. Due to low production costs and high discharge capacities (above 300 mAh/g), La–Mg–Ni system A2B7-type alloys are considered to be the most promising candidates as negative electrode materials of Ni–MHx rechargeable battery. Proper engineering of microstructure by the usage of unconventional processing techniques will lead to advanced nanocrystalline intermetallics representing a new generation of metal hydride materials [17, 38–41, 43–47]. The group of metallic hydrides is increased in number if, in addition to binary systems, one also includes hydrides based on intermetallic compounds of transition metals. A wide set of alloys composed of rare earth (RE) with nickel (AB5 type), and alloys of zirconium and vanadium with nickel (AB2 type), as well as titanium and magnesium with nickel (AB or A2B type) was offered for use as hydrogen storage materials (Tab. 3.1). In all these alloys, component A is the one which forms the stable hydride. Component B performs several additional functions: – It can play a catalytic role in enhancing the hydriding-dehydriding kinetic characteristics. – It can alter the equilibrium pressure of the hydrogen absorption-desorption process to the desired level. – It can also increase the stability of the alloy, preventing dissolution or formation of a compact oxide layer of component A. For solid-state hydrogen storage, the following characteristics are expected for a system to be adopted: – Favorable thermodynamics: that, is the storage and release should take place without much heat requirement – The kinetics of adsorption and desorption should be fast enough to be useful for applications on hand – The extent of storage should be sufficiently large enough so that it can be adopted for mobile applications – The material employed should withstand a sufficient cycle number for both adsorption and desorption – The material of choice should have sufficient mechanical strength and durability – The system chosen should be capable of a good heat transfer medium LaNi5, ZrV2, and TiFe phases are familiar materials that absorb large quantities of hydrogen under mild conditions of temperature and pressure. These types of hydrogen forming compounds have recently proven to be very attractive as negative electrode material in rechargeable nickel-metal hydride batteries [28, 29, 45]. Magnesium-based hydrogen storage alloys are also possible candidates for electrodes in Ni–MH batteries [17, 29, 46, 47]. The main example of the AB5 class alloy is LaNi5 with a hexagonal, CaCu5-type structure containing three octahedral and three tetragonal sites per elemental cell

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unit [22, 25]. LaNi5 forms two hydrides: one with low hydrogen content (α phase, LaNi5H03) and the other with high hydrogen content (β phase, LaNi5H5.5); they differ from each other significantly (about 25%) in the specific lattice volume. The discrete lattice expansion during conversion from the hydrogen solution phase (α) to the hydride phase (β) promotes the crumbling of the alloy particles in hydriding–dehydriding cycles. In the AB5-type alloy, both La and Ni can be replaced by other elements (Mm, Ce, Pr, Nd, Zr, Hf→La, and Al, Si, Cr, Mn, Fe, Co, Cu, Zn→Ni) [23, 28, 47–49]. Substitutions of A in AB5 (La1–yMyB5) by Zr, Ce, Pr, Nd decrease the unit cell volume and improve activation. On the other hand, the use of Mm instead of La reduces the alloy costs. Substitution of B in AB5 (La(Ni1–xMx)5 by M = Co, Cu, Fe, Mn, Al) by Ni is indispensable to prevent the decrease of the amount of absorbed hydrogen. Additionally, Co decreases the volume expansion upon hydriding and retards an increase of the internal cell pressure, but increases the alloy costs. Substitution of Co by Fe allows cost reduction without affecting cell performance and decreases decrepitation of the alloy during hydriding. Al increases hydride formation energy. Mn decreases equilibrium pressure without decreasing the amount of stored hydrogen. V increases the lattice volume and enhances hydrogen diffusion [28]. Additionally, additions to B in AB5 {(La,Mm)(Ni,M)5–xBx M = Co, Cu, Fe, Mn, Al; B = A1, Si, Sn, Ge, In, Tl} was studied. The metals Al, Si, Sn, and Ge minimize corrosion of the hydride electrode. Ge-substituted alloys exhibit facilitated kinetics of hydrogen absorption/desorption when compared to Sn-containing alloys. In, Tl, and Ga prevent the generation of gaseous hydrogen [28]. In non-stoichiometric AB5±X alloys (A = La; B = (Ni, Mn, Al, Co, V, Cu)), additional Ni forms separate finely dispersed phase. In MmB5.12, the Ni3Al-type second phase with high electrocatalytic activity is formed. Alloys poor in Mm are destabilized, and the attractive interaction between the dissolved hydrogen atoms increases [28]. Furthermore, a partial replacement of the A and B components significantly changes the AB5 alloy microstructure, for example, Mn facilitates nucleation [3]. Also, as reported by Ovshinsky et al. [50], the stoichiometry of an alloy influences its durability in the long-term hydriding-dehydriding cycles. Independently, it was found that the respective replacement of La and Ni in LaNi5 by small amounts of Zr and Al resulted in a prominent increase in the cycle lifetime without causing significant decrease in capacity [51]. A typical AB5-type alloy consists of at least seven different metals, for example, La0.64Ce0.36Nd0.46Ni0.95Cr0. 19Mn0.41Co0.15 (hydralloy F) [52]. ZrV2 and TiFe alloys crystallize in the cubic MgCu2 and CsCl structures and at room temperature, they absorb up to 5.5 and 2 H/f.u., respectively. Nevertheless, the application of these types of materials in batteries has been limited due to slow absorption/ desorption kinetics, in addition to a complicated activation procedure. The properties of hydrogen host materials can be modified substantially by alloying to obtain the desired storage characteristics, for example, proper capacity at a favorable hydrogen pressure. Significant improvements of the AB2-type alloys have been achieved by additives

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contributing to changes both in the unit cell volume and in the electronic structure, and, as a consequence, strongly affecting the stability and stoichiometry of the related hydrides [52, 53]. A serious problem in the Zr-containing alloy system is the formation of a compact ZrO2 layer on the sample surface. Owing to the low electrical conductivity of this oxide, the transport of hydrogen into the catalytic subsurface layer is impeded. This disadvantage in ZrV2 can be eliminated by substitution, in which Zr is partially replaced by Ti, and V is partially replaced by other transition metals (Cr, Mn, and Ni). But the hydrogen storage capacity of ZrV2 alloy decreases with increasing additive content. For example, in ZrM2-type alloy (M = V, Cr, Mn, Ni), an increase of the V content increases the maximum amount of absorbed hydrogen. Ni substitution decreases the electrochemical activity of an alloy. Composite alloy, a mixture of ZrNi2 with RNi5 (RE = rare earth element), shows improved characteristics when compared to the parent compounds. In stoichiometric AB2-type alloy, the distribution of A and B elements on the A and B sites is crucial for high hydrogen storage capacity; the over-stoichiometric alloy, in which some of the V atoms move from B to A sites, shows very high capacity. Substitution of A in AB2 by A = Zr + Ti: Zr contributes to an increase of the amount of stored hydrogen and induces the formation of the C15-type desired phase structure (Zr), increases the equilibrium pressure of hydrogen, and decreases the electrochemical capacity (Ti). Amount of C15 phase decreases with increasing x in TixZr1–xNi1.1V0.5Mn0.2Fe0.2; at x = 0.5 it is pure C14 phase, at x = 0.75 it is 13% cubic bcc phase + 87% C14 phase; bcc phase absorbs more hydrogen than the C14 hexagonal phase. On the other hand, in substitution of B in AB2 the Si, Mn-substituted Zr0.8Ti0.2(V0.3Ni0.6M0.1)2 alloys have C14 Laves phase structure, but the Co, Mo-substituted alloys form Cl5 Laves phase structure. Additionally, Mn enhances the activation of an alloy during chemical pretreatment and increases discharge capacity. Co addition leads to the longest cyclic lifetime. Cr addition reduces the discharge capacity but extends cyclic lifetime; Cr controls the dissolution of V and Zr. In nonstoichiometric alloys, AB2 ±x (Zr0.495Ti0.505 V0.771Ni1.546, Ti0.8Zr0.2Ni0.6) increases the Ni content and decreases V-rich dendrite formations. Typical AB2-type alloys for hydrogen storage are multicomponent and multiphase systems. For example, a chemical composition of multicomponent AB2-type alloy is Zr0.55Ti0.45V0.55Ni0.88Cr0.15Mn0.24Co0.18Fe0.03 (patent Ovonic Battery Company) [28]. The hydrogenation behavior of AB2- and AB-type alloys is strongly influenced by their composition and stoichiometry as well as by the microstructure of the final material and the presence of impurities [52, 53]. Among the different types of hydrogen forming compounds, Ti-based alloys are among the promising materials for hydrogen energy applications [20, 54–57]. For example, the TiFe alloy, which crystallizes in the cubic CsCl-type structure, is lighter and cheaper than the LaNi5-type alloys and can absorb up to 2 H/f.u. at room temperature. It is a nontoxic material. Nevertheless, the application of TiFe material has been limited due to poor absorption/desorption kinetics in addition to a complicated

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Tab. 3.2: The characteristics of some hydrogen storage alloys. Type Alloy

Structure Lattice parameters (nm)

Hydride

Plateau width Δ(H/M)

Desorption pressure (MPa)

Desorption temperature (K)

TiFe

B(CsCl)

a = .

TiFeH ~ TiFeH ~



. .

 

TiCo

B(CsCl)

a = .

TiCoH.

.

.



(P)

a = . c = .

MgNiH

.

.



(Fddd)

a = . b = . c = .

MgH

.

.



C

a = .

TiCr.H.

.

.



C

a = . c = .

TiCr.H.

.

.



TiMm. C

a = . c = .

TiMn.H.

.

.



ZrCr

C

a = . c = .

ZrCrH

.

.



ZrMn

C

a = . c = .

ZrMnH

.

.



ZrV

C

a = .

ZrVH. ZrVH.

.

 Pa



AB CaNi

(CaCu)

a = . c = .

CaNiH

.

.



LaNi

(CaCu)

a = . c = .

LaNiH



.



MmNi* (CaCu)

a = . c = .

MmNiH







AB

AB MgNi MgCu

AB

TiCr.

* Mischmetal.

activation procedure. To improve the activation of this alloy, several approaches have been adopted. For example, the replacement of Fe by some amount of transition metals to form the second phase may improve the activation property of TiFe. After activation, the TiFe reacts directly and reversibly with hydrogen to form two ternary hydrides, TiFeH (orthorhombic) and TiFeH2 (monoclinic), each of which is a distorted form of the bcc structure of the unhydrided alloy. Also, excess Ti in TiFe, that is, Ti1+xFe enables the alloy to be hydrided without the activation treatment. On the other hand, ball milling of TiFe is effective for the improvement of the initial hydrogen absorption rate, due

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to the reduction in the particle size and the creation of new clean surfaces [58]. Pd substitution in TiFe1–xPdx increases both the lattice constant and the catalytic activity but decreases the plateau pressure. Magnesium-based alloys have been extensively studied during the last years, but the microcrystalline Mg2Ni alloy can reversibly absorb and desorb hydrogen only at high temperatures [58, 59]. Substantial improvements in the hydriding-dehydriding properties of Mg-type metal hydrides could be achieved by the formation of nanocrystalline structures (Tab. 3.3) [15, 17]. It was found that the electrochemical activity of nanocrystalline hydrogen storage alloys can be improved in many ways, by alloying with other elements [29] or by ball milling the alloy powders with a small amount of nickel or graphite powders. For example, the surface modification of nanocrystalline hydrogen storage alloys with graphite by ball milling leads to an improvement in both discharge capacity and charge-discharge cycle life. Recently, it has been demonstrated that the use of palladium and MWCNTs offers many opportunities in tailoring hydrogen sorption properties of metal hydride [60]. The catalytic effect of Pd and MWCNTs and MA process on Ti2Ni alloys hydrogen storage properties have been studied in detail [60]. Tab. 3.3: Dissociation enthalpy ΔH and hydrogen storage capacity of Mg-based hydrogen materials. ΔH (kJ/mol H)

Strategy for improvement properties

Material

Synthetic method

Alloying

MgNiH

Mechanical alloying



.

Mg–Cu–H

Hydrogen–plasma–metal reaction

.

.

(Mg.Ca.) Induce furnace melting Ni



.

Mg.In.

Ball milling



.

MgH–.TiH Ball milling





MgH–C

Ball milling

.

.

LaMg– LaNi

Mechanical milling

.

.

Mg nanowires Vapor-transport approach

.

.

γ-MgH (. wt%)

Ball milling

.



γ-MgH (. wt%)

Electrochemical synthesis

.

.

Nanostructuring

Metastable phase

H capacity (wt%)

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Among many studied metal hydrides, La–Mg–Ni-based alloys were paid much attention in recent years, as they are candidates for hydrogen storage materials and also for negative electrode materials of Ni–MHx secondary batteries [61]. It is well known that the substitution of La with other rare earths and/or Ni with other transition metals changes the electrochemical performance of the A2B7 materials [62, 63]; hence, understanding the effect of the different elements on the phase compositions and properties of these alloys is crucial. The following (La,Mg)Ni3, (La,Mg)2Ni7 and (La,Mg)5Ni19 phases are commonly seen in the La–Mg–Ni-type alloys, which are composed of the [A2B4] and [AB5] subunits alternatively stack along the c-axis in different patterns [42]. Studies on the electrochemical behavior of the La2Ni7-type alloy revealed that both the (La,Mg)5Ni19 and the LaNi5 phase have catalytic effects on the fast discharge of this alloy [42]. Transitional elements can affect the hydrogen absorption/desorption plateau pressure of hydrogen storage alloys and influence their electrochemical properties. For example, Co metal is often added to La–Mg–Ni-based alloys. The formation of LaNi5 phase over (La,Mg)Ni3 phase improved the La–Mg–Ni–Co alloys’ cycle stability [64]. Additionally, the substitution of Co with Fe facilitated the formation of (La,Mg)5Ni19 and LaNi5 phases over (La,Mg)2Ni7 phase in the La0.74Mg0.26Ni2.55Co0.65–xFex (x = 0–0.4) system [65]. Adjusting the transition metal compositions in the La–Mg–Ni–M alloys can effectively improve the electrochemical hydrogen storage performances of the alloy electrodes [8, 66]. Further improvement of hydrogenation properties of this system may be achieved by encapsulation of material particles with a thin amorphous nickel coating [67, 68]. Modification of La1.5Mg0.5Ni7 particle surface with electroless, 1-μm thick Ni-P coating deteriorates electrode corrosion behavior, which can be ascribed to the preferential dissolution of Mg during deposition process, worsening of layer continuity, and increase of surface porosity. Recently, the effect of substitution La by Mg [69], La by Gd, and Ni by Co [70] on electrochemical and electronic properties of the La1.5Mg0.5Ni7 alloy was studied. The experimental characterization was accompanied by the density functional theory (DFT) calculations. The most promising electrochemical properties were identified for La1.25Gd0.25 Mg0.5Ni7 and La1.5Mg0.5Ni6.5Co0.5 alloy electrodes. The analysis of theoretical valence bands was focused on the van Hove singularity observed close to the Fermi level. The presence of this narrow peak is believed to be the origin of variations in electrochemical properties for the systems with Gd and Co substitutions.

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3.3 Alanates and borohydrides Intense interest has developed in low weight complex hydrides such as alanates [AlH4], amides [NH2], imides, and borohydrides [BH4] (Tab. 3.4) [7]. In such systems, hydrogen is often located at the corners of a tetrahedron. The alanates and borates are especially interesting because of their lightweight and the capacity for a large number of hydrogen atoms per metal atom. Borates are known to be stable and decompose only at elevated temperatures. Alanates are remarkable due to their high storage capacities; however, they decompose in two steps, upon dehydriding.

Tab. 3.4: Overview of characteristics of some alanates and borohydride. Material

H capacity (wt %)

Dehydrogenation temperature (K)

Dissociation enthalpy (kJ/mol H)

NaAH

.

– (I step) > (II step)

 (I step, . mass% H)  (II step, . mass%H)

Ca(AlH) 

.

 (I step)  (II step)

− (I step. . mass% H)  (II step. . mass% H)

LiAlH

.

– (I step) – (II step)

− (I step, . mass%H)  (II step, . mass% H)

Mg(AlH)

.

– (I step) – (II step)

 (I step,  mass% H)  (II step. . mass% H)

Ca(BH)

.





NaBH

.



− to −

LiBH

.



−

Mg(BH)

.

–

−. to −

Some of the lightest elements in the periodic table, for example, sodium, lithium, boron, and aluminum, form stable and ionic compounds with hydrogen (Tab. 3.4) [6, 71]. The hydrogen content reaches values of up to 18 mass% for LiBH4. However, such compounds desorb the hydrogen only at temperatures from 80 to 600 °C. The decomposition temperature of NaAlH4 can be lowered by doping the hydride with TiO2. The use of complex hydrides for hydrogen storage is challenging because of both kinetic and thermodynamic limitations [69]. On the other hand, sodium aluminum hydride, NaAlH4, could be a possible candidate for application as hydrogen storage material due to the theoretically reversible hydrogen storage capacity of 5.6 wt% and low cost [72, 73]. But the complex aluminum hydrides are not considered as rechargeable hydrogen carriers due to irreversibility and poor kinetics. By using appropriate transition or rare-earth metals as

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catalysts, the complex hydrides can be made reversible. Upon doping with proper titanium compounds, the dehydriding of aluminum hydrides could be kinetically enhanced and can maintain reversibility under moderate conditions in the solid state. Lithium alanates are very attractive for hydrogen storage due to their high hydrogen content. The total hydrogen content is 10.5 and 11.2 wt% for LiAlH4 and Li3AlH6, respectively. But LiAlH4 composition is an unstable hydride, which decomposes easily and cannot be re-hydrogenated [7]. So, the commercialization of lithium-based compounds is hindered by their slow kinetics and high-temperature absorption and desorption. It was reported that lithium nitrides can store up 5.5 wt% of hydrogen after ball milling of the LiNH2 and LiH mixtures with the TiCl3 as a catalyst [6]. Independently, Hu and Ruckenstein reported a reversible hydrogen capacity of 3.10 wt%, which increases with the number of cycles due to improved porous structure of this material [74]. Boron is also an interesting element for hydrogen-storage technologies (Tab. 3.3). For example, LiBH4 has a gravimetric hydrogen density of 18 wt% [75, 76]. At temperatures greater than 470 °C, hydrogen desorbs from LiBH4. It was found that this compound releases hydrogen in different reaction steps and temperature regimes. The low-temperature desorption releases only 0.3 wt% of hydrogen, and the high-temperature phase releases up to 13.5 wt%. LiBH4 can reversibly store 8–10 wt% of hydrogen at temperatures of 315–400 °C after the addition of MgH2 with 2–3 mol% TiCl3. Besides alanates, nitrides and borohydrides, lithium–beryllium hydrides (Li3Be2H7) are a new group of metal hydrides for hydrogen storage [76]. They show high reversible hydrogen capacity with more than 8 wt% at 150 °C, but these materials are toxic. Mobile applications in combination with hydrogen fuel cell systems require sustainable storage materials that contain large amounts of hydrogen. It has been shown that light metal hydrides show high potential for reversible hydrogen storage applications [77]. The hydrogen content reaches values of up to 9.3 wt% for magnesium alanate, Mg(AlH4)2. While the hydrogen content of these compounds is high and desorption kinetics seems to be promising, the absorption kinetics has to be improved. Additionally, it has already been observed that the kinetic barriers in nanocrystalline hydrides should be low when compared to coarse-grained materials [43]. The search for new materials with a high hydrogen content and sufficiently fast reaction kinetics of absorption and desorption at moderate temperatures continues.

3.4 Carbon materials Carbon materials created great expectations in the field of hydrogen storage. Compared to absorption, adsorption of hydrogen on carbon materials is observed to be more favorable in terms of storage capacity. An overview of hydrogen adsorption on

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activated carbon and different allotropes of carbon like graphite, carbon nanotubes, and carbon nanofibers was presented [78]. The experimentally measured maximum hydrogen storage capacity of activated carbon, graphite, single-walled nanotubes, multiwalled nanotubes, and carbon nanofibers at room temperature are 5.5, 4.48, 4.5, 6.3, and 6.5 wt%, respectively. Thermal treatments and metal doping on carbon nanostructures are observed to be useful for improving hydrogen storage capacities, but higher storage capacities can be obtained at cryogenic temperature and higher pressure. The total hydrogen storage on the best-activated carbon at 298 K is 16.7 g H2/L and 37.2 g H2/L at 20 and 50 MPa, respectively [79]. Kubas-type and liquid organic hydrogen carriers (LOHC) are other possible chemical hydrogen candidates that are being widely studied [77, 80, 81]. In the manganese hydride, Kubas-type interaction, gravimetric energy density of reported 10.5 wt% and 23.64 MJ/L at 120 bar and ambient temperature were observed [80]. This hydride showed no sign of degradation after 54 cycles. Furthermore, the reaction is thermally neutral. Adsorption is triggered by pressure variation, eliminating the need for a temperature management system. The Kubas-type interaction is a lowstrength chemical bond occurring with transition metals. LOHC apply the concept of hydrogenating and dehydrogenating chemical compounds to store hydrogen [81]. The advantage of storing hydrogen in this manner is the ability to use existing infrastructure, such as tankers and tanker trucks. For example, dodecahydro-N-ethylcarbazole has been widely studied [77]. When dehydrogenated, the substance becomes N-ethylcarbazole. It holds a theoretical maximum of 8.5 wt% hydrogen. This corresponds to approximately 7 MJ/L, which is quite comparable to that of liquid hydrogen.

3.5 Conclusion Onboard hydrogen storage for transportation applications continues to be one of the most technically challenging barriers to the widespread commercialization of hydrogen-fueled light-duty vehicles. The US DOE Office of Energy Efficiency and Renewable Energy (EERE), Fuel Cell Technologies (FCT) Program’s hydrogen storage activity focuses primarily on the applied R&D of low-pressure, materials-based technologies to allow for a driving range of greater than 500 km while meeting packaging, cost, safety, and performance requirements to be competitive with comparable vehicles in the market place. The DOE has established different targets for on-board hydrogen storage systems, including the minimum “gravimetric” and “volumetric” capacity and the reversibility of the charging/discharging processes [82] as shown in Fig. 3.2. For the years 2020 and 2025s, the storage system should have a gravimetric capacity of 1.5 kWh/kg

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Fig. 3.2: Volumetric and gravimetric hydrogen density of some selected hydrides.

(4.5 wt% of H2) and 1.8 kWh/kg (5.5 wt% of H2) as well as a volumetric capacity of 1.0 kWh/L (0.030 kg H2/L) and 1.3 kWh/L (0.040 kg H2/L), respectively. As the solidstate hydrogen storage option has been observed to be more reliable in terms of safety and transportation, several research groups worldwide are focusing on the development of a suitable solid-state hydrogen storage system to achieve the projected target. To meet the goals for hydrogen storage materials, the recent work aims at combining the latest developments in the metal hydride field with novel concepts for tailoring materials properties. Leading expertise in the field of complex hydride synthesis, synthesis and functionalization of nanostructured carbon, nanoparticle coating, structural characterization, and computational methods will be used to achieve a fundamental understanding combined with considerable practical progress in the development of novel nanostructured materials for hydrogen storage [83]. The target materials are nanocomposites consisting of hydride particle sizes in the lower nanometer range, which are protected by a nanocarbon template or by self-assembled polymer layers to prevent agglomeration. Thus, there is potential to lower working temperature and pressure, to enhance the reversibility, and to control the interaction between the hydride and the environment, leading to improved safety properties. The composites can be synthesized out of novel complex hydrides with very high hydrogen content and nanocarbon templates. Alternatively, hydride colloids will be coated in a layer-by-layer self-assembling process of dedicated polymers. Computational methods should be used to model the systems and predict optimal materials/size combinations for improved working parameters of the systems.

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[45] Hong K. The development of hydrogen storage alloys and the progress of nickel hydride batteries. J Alloys Comp 2001, 321, 307–313. [46] Gasiorowski A, Iwasieczko W, Skoryna D, Drulis H, Jurczyk M. Hydriding properties of nanocrystalline Mg2-xMx Ni alloys synthesized by mechanical alloying (M= Mn, Al). J Alloys Comp 2004, 364, 283–288. [47] Jurczyk M, Okonska I, Iwasieczko W, Jankowska E, Drulis H. Thermodynamic and electrochemical properties of nanocrystalline Mg2Cu-type hydrogen storage materials. J Alloys Comp 2007, 429, 316–320. [48] Zuttel A, Chartouni D, Gross K, Spatz P, Bachler M, Lichtenberg F, Folzer A. Relationship between composition, volume expansion and cyclic stability of AB5-type metal hydride electrodes. J Alloys Comp 1997, 253, 626–628. [49] Hu WK, Lee H, Kim DM, Jeon SW, Lee JY. Electrochemical behaviors of low-Co Mm-based alloys as MH electrodes. J Alloys Comp 1998, 268, 261–265. [50] Ovshinsky SR, Fetcenko MA, Ross J. A nickel metal hydride battery for electric vehicles. Science 1993, 260, 176–181. [51] Pan H, Ma J, Wang C, et al.. Studies on the electrochemical properties of MINi4.3-xCoxAl0.7 hydride alloy electrodes. J Alloys Comp 1999, 293–295, 648–652. [52] Majchrzycki W, Jurczyk M. Electrode characteristics of nanocrystalline (Zr,Ti)(V,Cr,Ni)2.41 compound. J Power Sources 2001, 93, 77–81. [53] Wojcik G, Kopczyk M, Młynarek G, Majchrzycki W, Beltowska-Brzezinska M. Electrochemical behavior of multicomponent Zr-Ti-V-Mn-Cr-Ni alloys in alkaline solution. J Power Sources 1996, 58, 73–78. [54] Schlapbach L, Zuttel A. Hydrogen-storage materials for mobile applications. Nature, London 2001, 414, 353–358. [55] Smardz K, Smardz L, Jurczyk M, Jankowska E. Electronic properties of nanocrystalline and polycrystalline TiFe0.25Ni0.75 alloys. Phys Stat Sol a 2003, 196, 263–266. [56] Zaluski L, Zaluska A, Ström-Olsen JO. Nanocrystalline metal hydrides. J Alloys Comp 1997, 253–254, 70–79. [57] Jurczyk M, Jankowska E, Nowak M, Wieczorek I. Electrode characteristics of nanocrystalline TiFe-type alloys. J Alloys Comp 2003, 354, L1–L4. [58] Zaluski L, Zaluska A, Ström-Olsen JO. Hydrogen absorption in nanocrystalline Mg2Ni formed by mechanical alloying. J Alloys Comp 1995, 217, 245–249. [59] Gennari FC, Castro FJ, Andrade-Gamboa JJ. Synthesis of Mg2FeH6 by reactive mechanical alloying: formation and decomposition properties. J Alloys Comp 2002, 339, 261–267. [60] Balcerzak M, Jakubowicz J, Kachlicki T, Jurczyk M. Effect of multi-walled carbon nanotubes and palladium addition on the microstructural and electrochemical properties of the nanocrystalline Ti2Ni alloy. Int J Hydrogen Energy 2015, 40, 3288–3299. [61] Balcerzak M, Nowak M, Jurczyk M. Hydrogenation and electrochemical studies of La-Mg-Ni alloys. Int J Hydrogen Energy 2017, 42, 1436–1443. [62] Ma S, Gao M, Li R, Pan H, Lei Y. A study on the structural and electrochemical properties of La0.7−xNdxMg0.3Ni2.45Co0.75Mn0.1Al0.2 (x = 0.0–3.0) hydrogen storage alloys. J Alloys Compd 2008, 457, 457–64. [63] Li Y, Han S, Li J, Zhu X, Hu L. The effect of Nd content on the electrochemical properties of lowCo La–Mg–Ni-based hydrogen storage alloys. J Alloys Compd 2008, 458, 357–362. [64] Liu Y, Pan H, Gao M, Li R, Sun X, Lei Y. Investigation on the characteristics of La0.7Mg0.3 Ni2.65Mn0.1Co0.75-x (x = 0.00–0.85) metal hydride electrode alloys for Ni/MH batteries. J Alloys Compd 2005, 387, 147–153. [65] Hao J, Han S, Li Y, Hu L, Zhang J. Effects of Fe-substitution for cobalt on electrochemical properties of La-Mg-Ni based alloys. J Rare Earths 2010, 28, 290–294.

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[66] Zhang F, Luo Y, Sun K, Wang D, Yan R, Kang L, Chen J. Effect of Co content on the structure and electrochemical properties of La1.5Mg0.5Ni7−xCox (x = 0, 1.2, 1.8) hydrogen storage alloys. J Alloys Compd 2006, 424, 218–224. [67] Dymek M, Nowak M, Jurczyk M, Bala H. Encapsulation of La1.5Mg0.5Ni7 nanocrystalline hydrogen storage alloy with Ni coatings and its electrochemical characterization. J Alloys Compd 2008, 749, 534–542. [68] Dymek M, Nowak M, Jurczyk M, Bala H. Electrochemical characterization of nanocrystalline hydrogen storage La1.5Mg0.5Ni6.5Co0.5 alloy covered with amorphous nickel. J Alloys Compd 2019, 780, 697–704. [69] Werwiński M, Szajek A, Marczyńska A, Smardz L, Nowak M, Jurczyk M. Effect of substitution La by Mg on electrochemical and electronic properties in La2-xMgxNi7 alloys: A combined experimental and ab initio studies. J Alloys Compd 2018, 763, 951–959. [70] Werwiński M, Szajek A, Marczyńska A, Smardz L, Nowak M, Jurczyk M. Effect of Gd and Co content on electrochemical and electronic properties of La1.5Mg0.5Ni7 alloys: a combined experimental and first-principles studies. J Alloys Compd 2019, 773, 131–139. [71] Bogdanovic B, Brand RA, Marjanovic A, Schwickardi M, Tölle J. Metal-doped sodium aluminum hydrides as potential new hydrogen storage materials. J Alloys Comp 2000, 302, 36–58. [72] Jensen CM, Zidan R, Mariels N, Hee A, Hagena C. Advanced titanium doping of sodium aluminum hydride segue to a practical hydrogen storage material. Int J Hydrogen Energy 1999, 24, 461–465. [73] Sandrock G, Gross KJ, Thomas G. Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanates. J Alloys Comp 2002, 339, 299–308. [74] Hu YH, Ruckenstein E. Hydrogen storage of Li2NH prepared by reacting Li with NH3. Ind Eng Chem Res 2006, 45(1), 182–186. [75] Fakioglu E, Yürüm Y, Veziroglu TN. A review of hydrogen storage systems based on boron and its compounds. Int J Hydrogen Energy 2004, 29, 1371–1376. [76] Zaluska A, Zaluski L, Ström-Olsen JO. Lithium–beryllium hydrides: the lightest reversible metal hydrides. J Alloys Comp 2000, 307, 57–166. [77] Rivard E, Trudeau M, Zaghib K. Hydrogen storage for mobility: A Review. Materials 1973, 2019(12). [78] Mohan M, Sharma VK, Kumar EA, Gayathri V. Hydrogen storage in carbon materials – A review. Energy Storage 2019, 1, e35. [79] Jordá-Beneyto M, Suárez-García F, Lozano-Castelló D, Cazorla-Amorós D, Linares-Solano A. Hydrogen storage on chemically activated carbons and carbon nanomaterials at high pressures. Carbon 2007, 45, 293–303. [80] Morris L, Hales JJ, Trudeau ML, Georgiev P, Embs JP, Eckert J, Kaltsoyannis N, Antonelli DM, Embs JPP. A manganese hydride molecular sieve for practical hydrogen storage under ambient conditions. Energy Environ Sci 2019, 12, 1580–1591. [81] He T, Pei Q, Chen P. Liquid organic hydrogen carriers. J Energy Chem 2015, 24, 587–594. [82] DOE Technical Targets for Onboard Hydrogen Storage for Light-Duty Vehicles (Accessed June 22 at https://www.energy.gov/eere/fuelcells/doe-technical-targets-onboard-hydrogen-stor age-light-duty-vehicles) [83] Novel Nanocomposites for Hydrogen Storage Applications (Accessed June 22 at https://cor dis.europa.eu/project/id/210092)

Jean-Marc Bassat

4 Survey of SOFC cathode materials: an extended summary 4.1 General requirements – electronic conductors The cathode, or oxygen electrode, is a porous layer where the reduction of the oxygen molecules to oxide ions (O2−, ORR) takes place: 1=2

O2 þ 2e ! O2

Consequently, a high level of electronic conductivity, σe, of the cathode material is required (σe ~ 100 S/cm at the operating temperature). Moreover, the microstructure (especially the porosity) of the electrode is a key parameter to ensure the access of gaseous oxygen to the surface in between the cathode and the electrolyte, where O2− species are available. Let us summarize here the general requirements for solid oxide fuel cell (SOFC) cathode: – High electronic conductivity – High catalytic activity towards oxygen reduction – Thermal expansion coefficient (TEC) comparable to that of other SOFC components – Low chemical reactivity towards other materials used in the SOFC – High thermal stability – Optimized microstructure, low cost materials, and mechanical strength – Because the SOFC is operated at high temperatures for long periods, it should be able to withstand repeated thermal cycling In the case of electronic conductors, the ORR occurs at the interface between the electrode, the electrolyte, and the gaseous phase, and then on the triple contact lines (labeled TPB for triple phase boundaries). Their number has to be as high as possible. To increase the length of the triple contact lines, composites electrodes formed with an electronic conductor (the cathode material) and an ionic conductor (possibly an electrolyte type conductor) are often prepared. It is mandatory that both phases form two percolating networks. Pure electronic materials were first studied, but rapidly MIEC (mixed ionic and electronic conductors) materials as well as composites were considered. This extended summary follows this plan. With respect to electronic conductors, the strontium-substituted lanthanum manganite (LSM) [1], with very high electronic conductivity (thanks to the mixed valence of manganese, Mn3+/Mn4+ with the formation of electronic holes), has been largely studied (including SOEC operations) in combination with YSZ, also in the form of

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composites (see, for instance, [2–4]). LSM was, for long, the classical SOFC cathode material, at high operating temperatures (900–1,000 °C).

4.2 MIEC materials Unlike pure electronic conductors, materials with MIEC properties [5, 6] allow the incorporation of oxide ions directly in the network and their diffusion occurs through the material. In this case, the effective area of the cathodic reaction is extended with respect to the first situation; hence, the reaction is delocalized at the overall surface of the cathode, improving the cell performance. The use of cathode materials with MIEC properties has been largely developed over the last few years. There are mainly three kinds of MIEC materials: i) Oxygen-deficient compounds with perovskite-type structure, ii) double perovskites, and iii) oxygen over-stoichiometric materials with Ruddlesden– Popper-type structure.

4.3 Oxygen-deficient MIEC materials 4.3.1 Perovskites Among the different materials with the perovskite structure (Fig. 4.1) identified as MIEC conductors, the strontium-substituted cobaltites La1–xSrxCoO3–δ (LSC) have a high ionic conductivity and good catalytic properties with respect to the oxygen reduction reaction (ORR) [7–9]. However, their TEC coefficients are very high with respect to YSZ or CGO. Using the ferro-cobaltites, La1–xSrxFe1–yCoyO3–δ (LSFC), a good compromise is obtained between the thermomechanical constraints and the ionic properties [10–13]. In the LSFC materials, the electronic conductivity arises thanks to the strontium to lanthanum substitution leading to the mixed valencies, Fe3+/Fe4+ and Co3+/Co4+. Another consequence is the formation of oxygen vacancies in this 3D network; the under stoichiometry δ being dependent on the x value as well as on the oxygen partial pressure. The higher δ and best ionic conductivity (arising from migration of oxygen in the vacant network) values are obtained for Co-rich materials. Another studied composition as cathode material is La0.6Sr0.4Fe0.8Ni0.2O3–δ (LSFN). In Tab. 4.1, the electrical conductivity, TEC, the tracer oxygen diffusion (D*, which is directly related to the ionic conductivity) and surface exchange (k*, which characterizes the ability of gaseous oxygen to be reduced at the surface of the material) coefficients of these perovskite materials are compared with the traditional La1–xSrxMnO3–δ (LSM). D* and k* values of these MIEC perovskites are higher than those of LSM.

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Fig. 4.1: Oxygen-deficient perovskite structure.

High TEC values [14–16] compared to those of LSM are evidenced. Another disadvantage of these materials is that they easily form insulating phases such as La2Zr2O7 and SrCoO3 at the interface with the YSZ electrolyte, which further degrade the performance of the cell. Tab. 4.1: Comparison of electrical conductivity, TEC, tracer oxygen diffusion (D*) and surface exchange (k*) coefficients of some MIEC perovskite materials with LSM. Composition

σe at  °C S/cm

TEC, α (− K−)

D* at  °C (cm/s)

k* at  °C (cm/s)

LSC

–, []

– []

− (x = .)  × − (x = .) []

− (x = .)  × − (x = .) []

LSCF ()

 []

 []

 × − []

 × − []

LSFN ()

 []

 []

 × − []

 × − []

LSM

 []

– []

− (x = ) []

 × − (x = ) []

Ba0.5Sr0.5Co0.8Fe0.2O3–δ (BSCF) can also be used as SOFC cathode materials [23]. Its electronic conductivity is high (~ 30 S/cm at 600 °C) [24], the oxygen diffusion properties interesting (D* ~ 5.10−7 cm2/s and k ~ 10−5 cm/s at 600 °C) [25], and the oxygen under stoichiometry high (δ ~ 0.3 at RT) [26]. However, this material is highly sensitive to the presence of CO2. Formation of strontium and barium carbonates at the grains’ surfaces is observed, which is detrimental, in particular to the oxygen diffusion properties [27, 28].

4.3.2 Double perovskites Several materials belonging to a family directly derived from the perovskite have recently been studied as O2− – SOFC cathodes; this family is labeled “doubled perovskites”

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with general formulation AA’B2O5+δ (A = rare earth, A’ = alkaline earth metal, B = transition metal). Despite the usual formulation of “5 + δ,” these materials can be also considered as oxygen-deficient. A characteristic of such phases is the ordering of the rare earth and alkaline earth metal layers along the (001) axis, leading to a doubling of the c parameter with respect to the perovskite with cubic symmetry [29]. As a result of this cationic ordering, the oxygen vacancies are mainly located in the rare-earth layer (AOδ), and for an oxygen stoichiometry value δ ~ 0.5, they are as well-ordered along the a-axis, leading to a doubling of the b parameter [30]. To our knowledge, only cobalt and manganese allow the formation of these compounds. Moreover, the ordering of the cationic layers is induced by a large difference between the rare earth and the alkaline rare earth metal radii; therefore, barium is mainly used, as also rare earths with small radii. With respect to applications, main studies are focused on cobaltites. Taskin et al. [31–33] performed the first studies on the MIEC properties of these materials. The most studied compositions are GdBaCo2O5+δ and PrBaCo2O5+δ [32, 34, 35]. However, substitutions (Co–Fe, Co–Ni, and Ba–Sr) enable slight improvement of the electrode performance [36–38]. Currently, the oxygen diffusion values are still under debate; regarding electronic conductivity, a higher value is obtained for PrBaCo2O5+δ [29, 39].

4.4 Oxygen – over-stoichiometric MIEC materials: the Ruddlesden–Popper (RP) series An+1MnO3n+1 Compounds belonging to the RP series have the general formulation An+1MnO3n+1. A is a lanthanide or alkaline-earth metal and M is a transition metal. With respect to MIEC properties, nickelates have been more investigated than cobaltites, for instance. The structure can be described by the intergrowth of octahedral layers (perovskite type) with AO layers (NaCl type), where n is related to the number of octahedral layers between the AO layers. Figure 4.2 shows the first three terms of the RP series, which are the only known ones at least for the most used transition metals. When n = ∞, the perovskite structure is described.

4.4.1 Ln2NiO4+δ (Ln = La, Pr, and Nd) With respect to the RP class of materials, the nickelate series Lnn+1NinO3n+1 where Ln = La, Pr, and Nd, have gained significant attention in recent years, especially n = 1 phases, as alternative cathode materials [34, 39–43]. It has been established that Ln2NiO4+δ (Ln = La, Pr and Nd) with K2NiF4 structure exhibits a large range of oxygen over-stoichiometry. The reported mean values of oxygen over-stoichiometry are 0.16 for La2NiO4+δ [44, 45], 0.22 for Pr2NiO4+δ [46, 47], and 0.25 for Nd2NiO4+δ [48], then

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Fig. 4.2: Schematic representation of the different structures in the Ruddlesden–Popper series An+1MnO3n+1, n = 1, 2, 3, ∞: : A cation, : M cation, : oxygen.

being dependent on the rare-earth size. The oxygen over-stoichiometry δ value is also dependent upon the synthesis condition for a given composition [49, 50]. The additional (interstitial) oxygen is located in the Ln2O2 rock-salt interlayer. Moreover, the NiO6 octahedra is elongated along the c-axis; hence, three kinds of oxygen are finally distinguished: Oapical, Oequatorial, and Ointerstitial (Fig. 4.3).

Fig. 4.3: Schematic representation of the Ln2NiO4+δ structure: : nickel, : Ln, : oxygen (including interstitial oxygen) localized in the Ln cations tetraedra (one is shown).

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Thanks to the oxygen over-stoichiometry, a mixed valence (Ni2+/Ni3+) appears. The excess oxygen ions and the charge-compensating Ni3+ ions (electronic holes) stabilize the K2NiF4 structure by reducing i) the intrinsic charge separation between the electropositive Ln2O2 and electronegative NiO2 layers and ii) the structural strain due to the misfit between the Ln2O2 and NiO2 layers. The Goldschmidt tolerance factor: ðrLn + rO Þ t = pffiffiffi 2*ðrNi + rO Þ where rLn, rO, and rNi are the effective ionic radii of Ln3+, O2−, and Ni2+/3+, respectively, helps to predict the stability domains of the various structural varieties (either tetragonal or orthorhombic) of the K2NiF4 structure. The oxygen diffusion (D*) and surface exchange (k*) coefficients of these nickelates are among the highest of the known values, especially at intermediate temperatures (about one order magnitude larger than that of conventional perovskites in an intermediate temperature range (600 < T °C < 800)). For example, the reported D* values at 600 °C are 1.5 × 10−8, 2.5 × 10−8, and 7 × 10−9 cm2/s for La2NiO4+δ, Pr2NiO4+δ, and Nd2NiO4+δ, respectively [41, 43, 51]. The ionic conductivity of these materials is directly related to D* value; hence, the ionic conductivity is the maximum for Pr2NiO4+δ [52]. The favored mechanism is probably of the intersticialcy type, involving both apical and interstitial oxygens [53, 54]. Compared to the perovskite materials, the ionic conductivity of the materials considered here has a strong 2D character (about 100 times higher in the (a, b) plane compared to the c-axis direction) [55–57]. The corresponding k* values at 600 °C are 1 × 10−6, 5 × 10−7, and 10−7 cm/s for La2NiO4+δ, Pr2NiO4+δ, and Nd2NiO4+δ, respectively [43]; these characteristics are associated with good electrocatalytic activity and hence the ORR. Finally, thanks to the mixed valence of nickel and to the oxide-ion conductivity, Ln2NiO4+δ materials are mixed ionic and electronic (O2−/e−) conductors, exhibiting good electro-catalytic properties with respect to the ORR. Hence, these materials are suitable for cathodes in SOFC applications. Up to now, to our knowledge, the best electrochemical result, that is, the lowest polarization resistance (Rp = 0.1 Ω cm2 at 600 °C), was obtained for Pr2NiO4+δ (in symmetrical half-cell configuration Pr2NiO4+δ//GDC//YSZ [58]). The power density is also maximum, 400 mW/cm2 for Pr2NiO4+δ electrode with anode-supported cells [58]. However, chemical stability is an issue for Pr2NiO4+δ material [59–61], while La2NiO4+δ and Nd2NiO4+δ are stable [61, 62] in the 700–900 °C temperature range for up to 72 hours.

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4.4.2 LnnNin+1O3n+1 (Ln = La, Pr, and Nd) The electronic conductivity of the upper-terms (n = 2, 3) of the RP series involving nickel is generally increased with respect to the term n = 1 [63–65]. Then, these compositions also attract interest for potential application as cathode for intermediatetemperature (IT)-SOFCs (see, e.g., [66, 67]). Among the n = 2 members, Ln3Ni2O7–δ (Ln = La, Pr, Nd), till date, only La3Ni2O7–δ could be prepared in a phase-pure form, while Pr3Ni2O7–δ and Nd3Ni2O7–δ have been observed only as disordered intergrowths in the corresponding n = 3 members [65]. Several studies have investigated the electrode performance of RP-type lanthanum nickelates with conflicting results. Using impedance spectroscopy on symmetric cells with La0.9Sr0.1Ga0.8Mg0.2O3–δ as electrolyte, Amow et al. [68] found the areaspecific resistance (ASR) in the temperature range of 500–900 °C following a trend La4Ni3O10–δ < La3Ni2O7–δ < La2NiO4+δ. This trend is consistent with that of the cell performance of Lan+1NinO3n+1/SDC/Ni–SDC cell configurations [69]. However, such a trend was not observed in a comparative study on symmetric cells by Woolley et al. [70]. Sharma et al. [67] have shown that La3Ni2O7–δ would be a better cathode material than La4Ni3O10–δ based on the ASR data of symmetric cells using GCO (Ce0.9Gd0.1O2–δ) as the electrolyte. Increasing the order of RP nickelates promotes the electrical conductivity and the long-term stability at 600–800 °C [68, 69, 71, 72]. In a paper by Vibhu et al. [71], an anode-supported (Ni-YSZ//YSZ) single cell including GDC//Pr4Ni3O10+δ co-sintered electrode showed a maximum power density of 1.60 W/cm2 at 800 °C and 0.68 W/cm2 at 700 °C. In a recent paper by Song et al. [73], an increase of the electronic p-type conductivity of the RP phases with the order parameter n was observed. Moreover, the RP phases display remarkable similarity in their values for the oxygen surface exchange coefficients kchem, despite differences in structural order and the type of lanthanide ion. Their oxygen self-diffusion coefficients, Ds, calculated from the corresponding values of Dchem, using data of oxygen non-stoichiometry, are found to profoundly decrease with the order parameter n. Note that significant oxygen hypo-stoichiometry is found in the higher order RP phases in contrast to the oxygen hyper-stoichiometry found in the n = 1 members. Finally, apart from material composition, again the microstructure of the cathodes plays a key role in their performance [71].

4.5 Composites (prepared by infiltration) As the ionic conductivity of the ionic conductors (YSZ, Gd doped ceria . . .) is larger than that of the MIECs, the use of composite MIEC/electrolyte is still of interest as shown, for instance, by Laberty et al. [74] who measured the high power density for

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a composite cathode made of La2NiO4+δ and Sm0.2Ce0.8O1.9 (SDC). SOFC developers have, for many years, used composite materials to improve the performance of the anode; however the use of composite cathodes is a more recent innovation. Dusastre et al. [75] wrote a full review paper. The usual preparation route of composites is by ball milling the two materials in an appropriate ratio, prior to their coating (by screen-printing, for example) and sintering on the electrolyte. However, among the fabrication techniques of the composite electrodes, infiltration of a porous ceramic backbone by a solution of cation salts has recently raised growing interest in the SOFC community. This interest is due to the fact that infiltrated cathodes display lower polarization resistances and better robustness to long-term ageing. With this technique, only the backbone is deposited by screen-printing and sintered at high temperature. The catalyst is then infiltrated into the porous backbone and generally annealed at lower temperature. Detailed overviews of electrodes made by infiltration are reported in two reviews from Vohs and Gorte [76], and Ding et al. [77]. More precisely, two strategies are developed: 1) Infiltration of an ionic conductor in a porous backbone made of cathode material, and 2) the infiltration of a (MIEC) material in a porous backbone made of ionic conductor. The second strategy is the more used one.

4.5.1 Infiltration of an ionic conductor in a porous cathode backbone This route was applied to LSM, mainly using infiltration of LSM by GDC by San Pin Jiang [78, 79]. Several others studies evidenced the efficiency of this approach when infiltrating SDC in LSM [80–85]. Sc-doped SDC and Pr-doped ceria were infiltrated in YSZ/LSM [86, 87], while LSM was infiltrated by doped Bi2O3 [88]. MIEC backbones such as LSCF by SDC have also been infiltrated [89]. Table 4.2 sums up the main results using this approach, including the infiltration using metallic catalysts.

4.5.2 Ionic conductor backbones infiltrated with electronic/MIEC conductors The second strategy is the opposite of the first one. The ionic conductor backbone is sintered at high temperature in the first step prior to the infiltration and annealing of the cathode material in a separate step performed at limited temperature. This strategy avoids the chemical reactivity between the two materials. For instance, Armstrong et al. measured very high power densities on a YSZ-based cell infiltrated with LSC: 2.1 W/cm2 at 800 °C [92].

4 Survey of SOFC cathode materials: an extended summary

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Tab. 4.2: Summary of the electrochemical performances obtained when infiltrating a cathode material with an ionic conductor or metallic catalyst. Backbone

Infiltrated material

Refs

Measurement T° (°C)

Performance before infiltration

Performance after infiltration

LSM

GDC

[]



Rp = . Ω·cm

Rp = . Ω·cm

LSM

GDC

[]



Rp = . Ω·cm

Rp = . Ω·cm

LSM

SDC

[]



Rp = . Ω·cm

Rp = . Ω·cm

[]



–

Pmax =  mW/cm

[]



Pmax =  mW/cm

Pmax =  mW/cm

[]



Pmax =  mW/cm

Pmax =  mW/cm

[]



Rp =  Ω·cm

Rp = , Ω·cm

YSZ/LSM

LSM-SDC

[]



Rp =  Ω·cm

Rp = , Ω·cm

YSZ/LSM

Ce.Pr.O–δ

[]



Rp =  Ω·cm

Rp = . Ω·cm

LSM

(Y.Bi.)O

[]



Rp = . Ω·cm

Rp = . Ω·cm

LSCF

SDC

[]



Rp = . Ω·cm

Rp = . Ω·cm

LSCF

LaNiO+δ

[]



Rp = . Ω·cm

Rp = . Ω·cm

YSZ/LSM

Pd

[]



Pmax = . W/cm

Pmax = . W/cm

The polarization resistance measurements, Rp, performed on symmetrical cells and the power densities on complete cells are given. Tab. 4.3: Summary of the electrochemical performances obtained when infiltrating ionic conductors. Backbone

Infiltrated phase

Refs

Measurement T° (°C)

Performance after infiltration

GDC

LSC

[]



Rp = . Ω·cm

YSZ

LSF

[]



Rp = . Ω·cm

GDC

LSCF

[]



Rp = . Ω·cm

YSZ

LSC

[]



Pmax = . W/cm

YSZ

LSM

[]



Pmax = . W/cm

LSGM

LSM

[]



Pmax = . W/cm

SDC

Sm,Sr,CoO–δ

[]



Pmax = . W/cm

The polarization measurement resistances, Rp, performed on symmetrical cells and the power densities on complete cells are given.

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A summary of several important results obtained using this strategy is given in Tab. 4.3. As a final and recent result [98, 99], praseodymium nitrate with (un-expected) MIEC properties was infiltrated into Gd doped ceria (GDC) backbone and fired at 600 °C to form a composite oxygen electrode Pr6O11/GDC. Electrochemical measurements show very low polarization resistance, Rp = 0.028 Ω cm2 at 600 °C. A single cell made of a commercial Ni-YSZ/YSZ half-cell and of the infiltrated cathode is able to deliver a maximum power density of 825 mW/cm2 at 600 °C.

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Ankur Jain, Shivani Agarwal, Takayuki Ichikawa

5 Ammonia: a promising candidate for hydrogen economy 5.1 Introduction The increasing energy demand all over the world has boosted up energy-related research activities in the last few decades. The current energy infrastructure depends on the use of fossil fuels that needs to be replaced by alternative methods because of several issues associated with the use of fossil fuels. The major issues include the limited availability and greenhouse gases emission [1]. Hydrogen, the simplest element of the periodic table, has the potential to work as alternative fuel. It can efficiently store and deliver energy, without the emission of any greenhouse gases. Moreover, it has two to three times higher energy density than conventional fuels [2] as shown in Tab. 5.1. Tab. 5.1: Energy density of different fuels [2]. Fuel

Hydrogen

Lower heating value Higher heating value (LHV) kJ/g (HHV) kJ/g .



Natural gas

.

.

Gasoline

.

.

Diesel

.

.

Biodiesel

.

.

Coal

.

.

In spite of the wonderful features of hydrogen as energy carrier, it is far from the projected targets due to several challenges associated with its production, distribution and storage. Among these challenges, hydrogen storage is a key point in establishing hydrogen economy. Several methods including conventional gaseous storage and storage in liquid form have been adopted for hydrogen storage. However, these methods are bulky, unsafe and energy inefficient. The storage of hydrogen in materials has been proposed as an efficient method. Several materials have been explored so far [3–10] in order to achieve US-DOE targets as shown in Tab. 5.2 [11]. Several materials including MgH2 [7], alanates, borohydrides [8], and amides [10] have shown high capacities according to the targeted values. However, they could not fulfill the other requirements, for example, they exhibited poor thermodynamics and/or kinetic characteristics. Recently, ammonia (NH3) has attracted attention as a potential hydrogen storage medium https://doi.org/10.1515/9783110596281-013

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[12–16] due to its high volumetric density of 107.3 kg H2 per cubic meter as well as high gravimetric density of 17.8 wt%. In addition, ammonia has several advantages over the direct use of hydrogen. These include low cost, already established infrastructure, and easy handling, which make it the perfect hydrogen carrier. The first report on the use of NH3 as energy vector appears in literature by Green [17], during the early 1980s. However, NH3 could not be established as a practical hydrogen carrier so far due to several issues related to its efficient production, namely, cracking to produce onboard hydrogen and high toxicity. Nevertheless, several attempts are being carried out to improve its properties and make it an efficient hydrogen carrier as well as a source of hydrogen for fuel cell. This chapter will describe the basic properties of ammonia and its potential for H2 storage. In addition, it will describe the recent progress of different aspects of NH3, such as production, storage, cracking, and application in fuel cell. Tab. 5.2: US-DOE targets for hydrogen storage system [11]. Target 

Ultimate target

Gravimetric density kWh/kg system (kg H/kg system)

. (.)

. (.)

Volumetric density kWh/L system (kg H/kg system)

. (.)

. (.)

 ()

 ()

Cost $/kWh ($/kg H)

5.2 Basic properties of ammonia as fuel Ammonia is a simple compound of nitrogen and hydrogen, and usually exists in colorless, gaseous form in atmospheric conditions. The basic properties are given in Tab. 5.3. It is clear from the table that NH3 has low vapor pressure of 0.86 MPa at room temperature (20 °C) and hence it can be liquefied easily under mild condition. This property allows easy and inexpensive storage of ammonia in contrast to hydrogen, where more complex and bulky vessels are needed. Ammonia also shows promise when comparing its performance with the other common fuels (Tab. 5.4). The energy density of NH3 is very similar to that of compressed natural gas (CNG) or methanol, but it is much higher than hydrogen. In addition, it has low explosion limits in air in comparison to hydrogen. The specific energy cost of ammonia is also very less when compared to other fuels, on the basis of HHV of the fuel. The only problem in using ammonia in vehicular and/or other applications is its high toxicity. However, recent developments of NH3 storage in solid form have already overcome this issue to a great extent [18]. Thus, in the light of above facts, ammonia can be established as an

5 Ammonia: a promising candidate for hydrogen economy

227

Tab. 5.3: Basic properties of ammonia [12, 13]. Molecular formula

NH

Molecular weight

.

Liquid density

. g/L

Melting point

 K

Boiling point

 K

Critical point

 K, . MPa

Critical density (ρc)

. g/mol

Auto-ignition temperature

 K

Vapor pressure

. MPa

Tab. 5.4: Various parameters of ammonia and other fuels [14, 16]. Fuel/storage system

Density Vapor IDLH (kg/m) pressure (ppm) P, at  K (MPa)

Specific Specific volumetric energy cost US$/GJ cost US$/m

Apparent toxicity P,  K/ IDLH

Energy density GJ/m (storage tank pressure, MPa)

 . × 

. ()



.



.

Hydrogen



–

–

–

. (.)



.

Gasoline



.





. (.)



.

Methanol



. ,

.

. (.)



.

LPG



CNG

Liq. Ammonia

Mg(NH)Cl

–

–

–

 (.)



.



–

–

–

. (.)



.



−



.

–

–

–

. × 

efficient, safe, and cost-effective fuel or hydrogen carrier. To establish the ammoniabased hydrogen economy, the following issues need to be addressed: (i) ammonia production, (ii) ammonia storage, (iii) H2 production from NH3 cracking, and (iv) its use in fuel cells. These topics will be reviewed in the following sections.

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5.3 Ammonia production

Fritz Haber 1868–1934

Carl Bosch 1874–1940

The use of ammonia as a fuel/hydrogen carrier is well supported through the established infrastructure of ammonia production. Industrial ammonia production is being carried out using the well known Haber–Bosch process, which is performed at 300–500 °C under 20–35 MPa [17, 19]. The industrialization of NH3 production using the above method was first established by Bosch after a theoretical proposal of the above process by Haber [20–23], in the early twentieth century. The discovery has not only benefited science and industry but has also contributed to society and civilization. The reaction with its thermodynamic parameters is given as follows: 3 H2 + N2 ! 2 NH3 ðΔH = − 46 kJ=mol, ΔS = − 99.35 J= kmolÞ

(5:1)

The reaction seems to proceed at low temperature due to mild thermodynamic parameters; however, the presence of highly stable N≡N bond in N2 with high bond energy of 945 kJ/mol creates a significant activation barrier [20, 24, 25]. Due to this activation barrier, the synthesis process needs high pressure and temperature conditions as mentioned above. To achieve a high NH3 yield with a reasonable production rate, it is essential to balance the reaction conditions, that is, temperature, pressure, and reaction rate. Since the reaction rate is directly dependent on temperature, it increases with temperature up to a certain point; however, it starts decreasing with further increase in temperature due to the dissociation of NH3 at high temperature, thus resulting in low equilibrium NH3 concentration. To optimize all these conditions, several catalysts have been developed in last century. Alwin Mittasch [26] developed a multicomponent catalyst system based on iron and suggested that a mixed catalyst based on fused iron can be a better catalyst when compared to the single element catalyst. This fused iron-based catalyst has proven so effective that industrial NH3 synthesis still uses this hypothesis based on mixed component catalyst. The magical catalyst developed by Mittasch contains a mixture of Fe3O4, Al2O3, CaO, and K2O. The mechanism of this magical

5 Ammonia: a promising candidate for hydrogen economy

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performance has been studied over decades [26–29], and the requirement of an electron-donating material is found essential for N2 cleavage through the back donation process. This back donation process for N2 cleavage has been studied in detail for several transition metals by Rao et al. [26]. It suggested that there is an electron donation to the transition metal through the occupied bonding orbital (π) of N2 and a back donation of charge into the antibonding (π*) orbital. This type of bonding reduces the activation barrier of N2 dissociation; however, this alone is not enough for efficient N2 cleavage. It can be enhanced by the addition of electron donors such as K2O, which can further weaken the N≡N bond [27, 28]. Fe3O4 as a precursor for this magical catalyst was believed to be the best until the discovery of Fe1–xO-based catalyst in 1986 [30]. The presence of more exposing (111) and (211) planes in Fe1–xO makes it a better catalyst, as these planes are the planes of high catalytic activity in ammonia synthesis [31]. In addition, the higher catalytic activity of the Wustite catalyst is also believed to be due to the presence of higher lattice micro strains or more defects on the surface, which facilitate chemical reaction as active sites. Another interesting but relatively less-explored catalyst for NH3 synthesis is ruthenium. It was first proposed in 1917 by Mittasch with the conclusion that it has lesser catalytic activity than its rival, that is, an Fe-based catalyst. This conclusion suppressed the research activities for Ru-based catalyst for almost 50 years. In 1969, Sudo et al. [32] prepared a catalyst system consisting of alkali metal (Na or K) as electron donor, transition metal (Fe, Ru, Co, or Os) as electron acceptor, and an electron carrier material such as graphite. Interestingly, these systems have shown good catalytic activity for NH3 synthesis and have ignited the interest of researchers into Ru-based catalyst. In 1972, a very high catalytic activity could be achieved for Ru-based catalyst by Aika et al. [33–35], when they used K as metal promoter and C as catalyst carrier with this catalyst. The effect was not limited to K-based oxides only, but also applied to other alkali and alkaline metal oxides such as Cs oxide or Ba oxide [33–36]. However, these oxides do not have a small work function, resulting in lesser electron donation power. On the contrary, the alkali metals themselves exhibit small work function and thus have strong electron donation power. Due to this fact, metals are more efficient catalysts to facilitate NH3 synthesis under mild conditions, in comparison to their oxides [36]. At the same time, alkali metals have low melting point, and hence they are quite unstable. They can easily evaporate at reaction temperature and/or can react with ammonia to form amides [37]. These shortcomings of metal or their oxides make the search of a stable electron donor material with low work function, high melting point, and less reactivity with H2, N2, or NH3 important. The search of this material ended with the discovery of a stable electride (C12A7:e−) in 2012 [38]. This electride was first synthesized from 12CaO·7Al2O3 in 2003 and found stable at room temperature, in contrast to other organic electrides that easily decompose around 230 K. The unique band structure of this material gives rise to low work function with high chemical stability. The coexistence of these two incompatible properties makes this compound

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suitable as an electron donor for N2 cleavage. It is observed that Ru/C12A7:e− has almost one magnitude higher catalytic activity than that of Ru–Cs /MgO catalyst (Fig. 5.1). Table 5.5 shows a comparison of various Ru-loaded catalysts and suggests the superiority of Ru/C12A7:e− over other catalysts. The N≡N bond dissociation is considered as the rate limiting step for conventional catalysts supported NH3 synthesis. However, in the case of Ru/C12A7:e−, the strong back donation from C12A7:e− to π* orbitals of N2 molecule through Ru facilitate easy N2 dissociation, and thus it does not remain a rate limiting step. For conventional catalysts, a simultaneous H2 dissociation covers the Ru surface by H adatoms rather than N adatoms, which hinders the efficient NH3 synthesis, even with the increase in the pressure. In contrast, some of the H adatoms on Ru move in to the cages of C12A7:e− through reversible reaction H0 + e− ↔ H−, resulting in the availability of sufficient N2 cleavage sites on Ru surface, and efficient NH3 formation without H poisoning can proceed as shown in Fig. 5.2 [39].

Fig. 5.1: TOFs for high-pressure ammonia synthesis over 1 wt% Ru/C12A7:e− and 6 wt% Ru-Cs/MgO [Reproduced from [38] with permission from Nature Publishing Group].

Even though Ru-based catalysts have shown great promise, their implementation at industrial level could not be realized owing to their high cost. This limitation necessitates a cheap and efficient alternative. A new category of alloys of cobalt and molybdenum has been developed on the basis of theoretical calculation and volcano-shaped curve of catalytic activity versus nitrogen adsorption energy of metal (Fig. 5.3). It was found that Fe, Ru, or Os lead other individual elements in

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231

Tab. 5.5: Catalytic performance of Ru catalysts on various supports [40]. Catalyst Ru/CA:e

−

Ru loading (%)

NH formation (μmol/g h)

.

,

TOF (s−) .

Ea (kJ/mol) 

−



Ru–Cs/MgO

.

,

. × 

Ru–Ba/AC Ru/CA:O−

. .

, 

. × − . × −

 

Ru/CaO

.



. × −



Ru/γ-AlO

.



. × −

–

Fig. 5.2: Proposed reaction mechanism of ammonia synthesis over Ru/C12A7:e− (Reproduced from [40] with permission from American Chemical Society).

terms of catalytic activity; however, when the metals are mixed together, for example, Co–Mo alloy showed highest catalytic activity [41–44]. A further enhancement in the catalytic activity of Co–Mo–N system could be achieved by using Co–Mo nanoparticles decorated on CeO2 support [45]. To reduce the cost and enhance the efficiency, industrial NH3 synthesis is coupled with H2 production. Hydrogen is produced by the gasification of natural gas to CO and H2, which is then allowed to react with water and nitrogen to synthesize NH3. The ammonia industry has become mature enough with a 150 million tons/year production in 2015 that is projected to increase to 200 million tons /year by 2020. The energy and reaction pressure has tremendously decreased to 27 GJ and 10–15 MPa from the starting values of 78 GJ and 100 MPa in the early twentieth century. NH3 production is increasing day by day; however, the increasing population worldwide and recent possibility of NH3 application in the energy sector demands the development

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Fig. 5.3: Calculated turnover frequencies for ammonia synthesis as a function of the adsorption energy of nitrogen (reproduced from [41] with the permission from American Chemical Society).

of new and efficient ways to produce NH3. In this connection, several alternate methods have been explored. Electrochemical ammonia production is a very efficient method and can be helpful in fulfilling the anticipated gap between the rapid growth in ammonia demand and the industrial production by the conventional Haber–Bosch process [46–49]. In addition, it can reduce the energy input by 20% when compared to the Haber–Bosch process. The electrochemical reaction for NH3 synthesis, depending on the use of proton- conducting electrolyte or oxygen ion-conducting electrolyte (Fig. 5.4), can proceed as follows: Proton-conducting electrolyte 3 H2 O ! 3=2 O2 + 6 H+ + 6 e− ðAnode acidic conditionÞ

(5:2)

6 OH− ! 3 H2 O + 3=2 O2 + 6e− ðAnode base conditionÞ

(5:3)

+

−

N2 + 6 H + 6 e ! 2NH3 ðCathode acidic conditionÞ

(5:4)

N2 + 6 H2 O + 6 e− ! 2 NH3 + 6 OH− ðCathode base conditionÞ

(5:5)

Oxygen-conducting electrolyte Anode:

3O2 − ! 3=2 O2 + 6e−

(5:6)

Cathode:

N2 + 3 H2 O + 6 e− ! 2 NH3 + 3 O2 −

(5:7)

Thus, overall reaction is N2 + 3 H2 O ! 3=2 O2 + 2 NH3

(5:8)

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Fig. 5.4: Schematic of solid state ammonia synthesis (SSAS) process for proton-conducting reactor cell (left) and oxygen ion-conducting cell (right) (reproduced from [46] with the permission from Springer).

The electrochemical synthesis can be divided into four categories on the basis of working temperature that depends on the use of different electrolytes: (i) liquid electrolyte-based system which operates at ambient temperature (ii) molten salt electrolyte-based system which operates at 300–500 °C (iii) composite electrolyte composed of solid electrolyte and melting salts that can operate at 300–700 °C (iv) solid electrolyte for wide range of room temperature to 700–800 °C. Electrochemical synthesis of NH3 through electrolysis of water has the advantage of being carbonfree as it does not use any fossil fuel as in the Haber–Bosch process, which usually needs steam reforming of fossil fuels for H2 supply. This is also important from the economic point of view, as the cost of NH3 production through the Haber–Bosch process includes a large fraction of H2 purification. The hydrogen produced from the steam reforming contains CO, water vapor, oxygen, and sulfur, all of which are poisonous to the catalysts [19, 50]. This issue does not arise in the case of electrochemical synthesis, as it uses a solid electrolyte that has a selective ionic membrane and allow only protons to reach the cathode. The high electrical energy required for water electrolysis is a big drawback associated with this method and has an effect on the economic feasibility of electrochemical synthesis of NH3. However, this can be controlled and improved by coupling it with renewable energy such as solar or wind energy as the electricity source. Another alternative for NH3 synthesis occurs in nature – natural enzymes called nitrogenases that can reduce N2 to NH3 by the following reaction [51]: N2 + 8 H+ + 8 e− + 16 ATP ! 2 NH3 + H2 + 16 ADP + 16 Pi

(5:9)

where ATP is adenosine triphosphate, ADP is adenosine diphosphate, and Pi represents inorganic phosphate (a mixture of HPO42– and H2PO4− ions). This reaction requires 244 kJ/mol NH3 energy, which is better than the amount required for the

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industrial process [51]. Nitrogenases are a type of enzymes that are a combination of two proteins, one of which mediates the electron transfer for the reduction of N2, while the other facilitates the actual N2 fixation according to the above equation. The most effective nitrogenases are FeMo nitrogenases that have two fused ironsulfur clusters. Recently, another approach to NH3 synthesis has been proposed by Jain et al. [52] through the hydrolysis of nitrides as follows: Li3 N + H2 O ! 3 LiOH + NH3

(5:10)

The reaction is found to be strongly dependent on the reaction rate and temperature. It can be proceed well below 100 °C, which eliminates the need of fossil fuels.

5.4 Ammonia storage Since ammonia has special physical and chemical properties, it needs special attention for storage and transportation, in terms of design and engineering of on-board storage system. The high vapor pressure, high thermal expansion coefficient, high toxicity, and high reactivity with container material are some of the issues associated with the current storage and distribution systems of liquid NH3. Currently, ammonia is being stored at 0.1 MPa and −33 °C in insulated tanks for large quantities up to 50,000 tons. For small quantities up to 1,500 tons, ammonia is stored under pressure in SS chambers [53]. To handle these issues of liquid ammonia, the storage of ammonia in solid form is desirable, and can be achieved by the use of metal ammine salts and amine metal borohydride [53, 54]. These metals have good volumetric hydrogen density as can be seen from Tab. 5.6, which makes them ideal as ammonia storage material. It can be seen that Mg(NH3)6Cl2 has volumetric H2 density of 109 g/L comparable to that of 108 g/L for liquid ammonia. A number of metallic salts (MXm) can absorb ammonia through the following reaction: MXm + nNH3 $ MðNH3 Þn Xm

(5:11)

where M = Mg, Ca, Cr, Ni, or Zn and X = Cl, Br, SO4. Among all these, MgCl2 attracts a lot of attention due to its high gravimetric and volumetric hydrogen densities, that is, 9.19 wt% and 109 g/L, as well as its low vapor pressure [55–57]. The desorption reaction of hexa-ammine complex of MgCl2 proceeds through the following steps: MgðNH3 Þ6 Cl2 ! MgðNH3 Þ2 Cl2 + 4 NH3 ð440 KÞ

(5:12)

MgðNH3 Þ2 Cl2 ! MgðNH3 ÞCl2 + NH3

ð575 KÞ

(5:13)

MgðNH3 ÞCl2 ! MgCl2 + NH3

ð675 KÞ

(5:14)

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Tab. 5.6: Mass and volumetric density of different H2 storage material to store 10 kg H2 [18]. Storage material

Mass (kg)

Volumetric density (g/L)

NH (liquid)





Mg(NH)Cl









LaNiH





MgNiH













H (liquid)

LiAlH Li(NH)BH

The thermodynamic parameters using P–C isotherms were estimated as ΔH = −58 ± 6 kJ/mol, ΔS = −150 ± 20 J/K mol and ΔH = −64 ± 1 kJ/mol, ΔS = −97 ± 2 J/K mol for Mg(NH3)6Cl2 and Mg(NH3)Cl2 formation [57] respectively. It suggests that the formation of mono or di-amine complex at room temperature is suppressed due to high kinetic barrier, and direct formation of Mg(NH3)6Cl2 takes place at room temperature. The mono and di-amine complex could be formed only at 373 and 573 K, respectively. Another interesting amine complex is Ca(NH3)8Cl2, which possesses the second highest density in the group, that is,, 9.78 wt%. The desorption temperature of this complex is lower than the Mg complex (Fig. 5.5), but at the same time it possesses high vapor pressure of 0.07 MPa which is one order of magnitude lower than that of liquid ammonia [55]. It has been pointed out recently by Zhang et al. [18] that the equilibrium plateau pressure for ammonia sorption has a close relationship with the Pauling electronegativity of the cation and/or the electronegativity difference between cation and anion element of metal halides, as can be understood from Fig. 5.6. It is depicted that the lower vapor pressure can be obtained for a cation of smaller electronegativity among the metal halides having the same anion. It is also noted that a material having a larger electronegativity difference between anion and cation (>2.3) cannot absorb ammonia, whereas the plateau pressure is found to decrease with the decrease in electronegativity difference for smaller values ( Li > Ba > Ca [71]. Another approach to produce H2 from NH3 was proposed by Vitse et al. [72] in 2005. They proposed electrolysis of ammonia as a possible method, which occurred through the oxidation of NH3 in an alkaline medium at anode and reduction of water at cathode, as follows: 2 NH3 + 6 OH− ! N2 + 6 H2 O + 6 e− , Eo = − 0.77 V=SHE

(5:21)

6 H2 O + 6 e− ! 3 H2 + 6 OH− ,

(5:22)

Eo = − 0.82V=SHE

This process is an energy-economic process as it consumes only 1.55 Wh/g of H2 as compared to 33 Wh/g of H2 from H2O [73]. The thermodynamics of the above reaction is quite favorable, but the kinetics is very slow, thus making it difficult to realize the above reaction without a catalyst. To enhance the kinetics, an understanding of the reaction mechanism is essential. Several studies have taken place [74–76], and they suggest that the ammonia oxidation occurs through (i) the ammonia adsorption on catalyst surface, (ii) ammonia decomposition into adsorbed intermediate such as N, NH, or NH2, and (iii) reaction of these intermediates to form N2H2,ad, N2H3,ad, N2H4,ad, which finally reacts with OH− to form N2. It is important to note that NH and NH2 are active among the adsorbed intermediates, whereas, Nad acts as poison. According to the above mechanism, Pt was found to be the best catalyst as it avoids the formation of Nad (it is formed only at very high potential with Pt). Recently, David et al. [77] proposed an alternate method for cracking ammonia using sodium amide, which eliminates the need for any catalyst. They achieve a superior performance of Na/NaNH2 system when compared to Ni- and Ru-catalyst-supported NH3 cracking, when the following two reactions take place under NH3 flow at 530 °C: NaNH2 ðsÞ ! NaðsÞ + 1=2 N2 ðgÞ + H2 ðgÞ

(5:23)

NaðsÞ + NH3 ðgÞ ! NaNH2 ðsÞ + 1=2 H2 ðgÞ

(5:24)

Running these two reactions concurrently, 99.2% decomposition efficiency could be achieved for the chemical decomposition of NH3 as follows: 2 NH3 ðgÞ ! N2 ðgÞ + 3 H2 ðgÞ

(5:25)

5.6 Ammonia application in fuel cell Ammonia can be used in fuel cells directly. The use of NH3 in PEM fuel cells (PEMFC) needs external ammonia decomposition due to its low temperature operation. In most PEMFC, purification of H2 is needed, as even a small trace of remaining ammonia is

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poisonous to Nafion membrane [78, 79]. A number of studies have focused on the effect of ammonia on the performance of PEM fuel cells [80–84], and it has been suggested that the presence of even 30 ppm ammonia for 1 h is enough to degrade the performance of cell; however, a recovery in the performance could be achieved after the exposure of neat H2 for 18 h [80]. The recovery in the presence of neat H2 is directly dependent on the exposure time and amount of NH3. The use of ammonia in alkaline fuel cells is somehow better due to the good tolerance power of aqueous alkaline electrolyte towards ammonia [85]. To make ammonia fuel compatible with high temperature applications, researchers investigated solid oxide fuel cells (SOFC). Ammonia can decompose in SOFC, thus eliminating the need for an external unit for decomposition. NH3-fed SOFC can be divided in to two categories: (i) oxygen ion conducting electrolyte SOFCs (SOFC–O) that use oxygen ion-conducting electrolyte and (ii) proton ion-conducting electrolyte SOFC (SOFC–H). The reaction for SOFC–O using yttria-stabilized zirconia (YSZ) proceeds as follows [86]: 2 NH3 + 5 O2 ! 2 NO + 3 H2 O + 10 e −

(5:26)

2 NH3 + 3 NO ! 5=2 N2 + 3 H2 O

(5:27)

To eliminate NO formation in the above reaction, SOFC–H is beneficial. It undergoes the following anodic reaction by using BaCeO3 and BaZrO3 as electrolyte [87, 88]: 2 NH3 − 6 e − ! N2 + 6 H +

(5:28)

This type of SOFC has better efficiency than the oxygen-conducting electrolyte SOFC [89]. In addition to it, SOFC are considered to be promising among NH3 fuel cells from an economy point of view, as these can be operated with non-precious metals such as Ni.

5.7 Conclusions and future prospects Ammonia is the latest efficient storage medium for hydrogen. Due to its high hydrogen content and established technology, it has shown the possibility of leadership among other hydrogen storage materials. Even though it possesses several interesting properties and shows attractive numbers as a fuel, several issues and barriers have to be overcome before the commencement of ammonia-mediated hydrogen economy. Carbon-free ammonia production is one of the key issues, as the industrial production of ammonia involves steam reformation of fossil fuels. In order to keep the clean nature of hydrogen fuel, a CO2-free production technology needs to be developed. Electrochemical production of NH3 through the electrolysis of water, similar to natural synthesis of ammonia by some enzymes (nitrogenases), can be a key technology in achieving the goal. The safety concerns associated with liquid ammonia due to high toxicity and

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high vapor pressure is another serious issue to be solved. Recent developments in solid state storage of ammonia in amine salts and borohydrides could solve this problem to a great extent. Desorption of ammonia from these materials can easily be controlled by the variation of pressure and temperature. The search for new materials with better properties is warranted in the near future to commercialize this method for use in the transportation sector. Efficient hydrogen generation from ammonia is the third aspect, which should be considered before adoption of NH3 as a hydrogen carrier. H2 generation is currently being done using catalytic decomposition in a reverse Haber–Bosch process. Several catalysts have been developed so far; however, Ru is the winning candidate followed by Ni-based catalyst. Electrochemical NH3 decomposition is also under development, which can be an important technology for the direct use of ammonia in PEM fuel cells, which are very sensitive even to small traces of residual ammonia. As a concluding remark, developing NH3 as a hydrogen carrier has the potential to replace fossil fuel infrastructure, and an efficient ammonia hydrogen economy can be established to meet future energy demand.

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[56] Elmøe TD, Sørensen RZ, Quaade U, Christensen CH, Nørskov JK, Johannessen T. A highdensity ammonia storage/delivery system based on Mg(NH3)6Cl2 for SCR–DeNOx in vehicles. Chem Eng Sci 2006, 61, 2618–2625. [57] Aoki T, Miyaoka H, Inokawa H, Ichikawa T, Kojima Y. Activation on ammonia absorbing reaction for magnesium chloride. J Phys Chem C 2015, 119, 26296–26302. [58] Sullivan EA, Johnson S. The Lithium Borohydride–Ammonia System P–C–T Relationships and Densities. J Phys Chem 1959, 63, 233–238. [59] Aoki T, Ichikawa T, Miyaoka H, Kojima Y. Thermodynamics on ammonia absorption of metal halides and borohydrides. J Phys Chem C 2014, 118, 18412–18416. [60] Cheddie D. Ammonia as a hydrogen source for fuel cells: A review. In: Minic D, (ed.), Hydrogen Energy – Challenges and Perspectives. IntechOpen. 2012, 333–362. [61] Yin SF, Xu BQ, Zhou XP, Au CT. A mini-review on ammonia decomposition catalysts for on-site generation of hydrogen for fuel cell applications. Appl Catal A: General 2004, 277, 1–9. [62] Hill AK, Torrente-Murciano L. Low temperature H2 production from ammonia using rutheniumbased catalysts: Synergetic effect of promoter and support. Appl Catal B 2015, 172–173, 129–135. [63] Hill AK, Torrente-Murciano L. In-situ H2 production via low temperature decomposition of ammonia: Insights into the role of cesium as a promoter. Int J Hydrogen Energy 2014, 39, 7646–7654. [64] Li L, Zhu ZH, Yan ZF, Lu GQ, Rintoul L. Catalytic ammonia decomposition over Ru/carbon catalysts: The importance of the structure of carbon support. Appl Catal A 2007, 320, 166–172. [65] Zhang H, Alhamed YA, Al-Zahrani A, Daous M, Inokawa H, Kojima Y, Petrov LA. Int J Hydrogen Energy 2014, 39, 17573–17582. [66] Boudart M, Djega-Mariadassou G. Princeton University Press, Princeton, NJ, 1984, 98. [67] Bradford MCJ, Fanning PE, Vannice MA. Kinetics of NH3 decomposition over well dispersed Ru. J Catal 1997, 172, 479–484. [68] Prasad V, Vlachos DG. Multiscale model and informatics-based optimal design of experiments: Application to the catalytic decomposition of ammonia on ruthenium. Ind Eng Chem Res 2008, 47, 6555–6567. [69] Prasad V, Karim AM, Arya A, Vlachos DG. Assessment of overall rate expressions and multiscale, microkinetic model uniqueness via experimental data injection: Ammonia decomposition on Ru/γ-Al2O3 for hydrogen production. Ind Eng Chem Res 2009, 48, 5255–5265. [70] Ganley JC, Thomas FS, Seebauer EG, Masel RI. A priori catalytic activity correlations: The difficult case of hydrogen production from ammonia. Catal Lett 2004, 96, 117–122. [71] Wang SJ, Yin SF, Li L, Xu BQ, Ng CF, Au CT. Investigation on modification of Ru/CNTs catalyst for the generation of COx-free hydrogen from ammonia. Appl Catal B 2004, 52, 287–299. [72] Vitse F, Cooper M, Botte GG. On the use of ammonia electrolysis for hydrogen production. J Power Sources 2005, 142, 18–26. [73] Simons EL, Cairns EJ, Surd DJ. The Performance of Direct Ammonia Fuel Cells. J Electrochem Soc 1969, 116, 556–561. [74] Gerischer H, Mauerer A. Untersuchungen Zur anodischen Oxidation von Ammoniak an PlatinElektroden. J Electroanal Chem Interfacial Electrochem 1970, 25, 421–433. [75] Vidal-Iglesias FJ, Solla-Gullon J, Feliu JM, Baltruschat H, Aldaz A. DEMS study of ammonia oxidation on platinum basal planes. J Electroanal Chem 2006, 588, 331–338. [76] Vidal-Iglesias FJ, Solla-Gullon J, Perez JM, Aldaz A. Evidence by SERS of azide anion participation in ammonia electrooxidation in alkaline medium on nanostructured Pt electrodes. Electrochem Comm 2006, 8, 102–106.

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Tom Depover, Kim Verbeken

6 Hydrogen diffusion in metals: a topic requiring specific attention from the experimentalist Abstract: The presence of hydrogen in a metal microstructure can lead to an unpredictable failure. Hydrogen generally degrades the mechanical performance of a material. This is known as hydrogen embrittlement or hydrogen-induced degradation. Hydrogen diffusion inside the metal plays a crucial role during the failure mechanism. This chapter deals with the experimental evaluation of hydrogen diffusion by electrochemical permeation experiments. The hydrogen–metal interaction is first introduced in detail, including concepts such as hydrogen adsorption, absorption, transport, trapping, and finally embrittlement, elaborating on the most important mechanisms that govern hydrogen embrittlement. Then, the experimental permeation technique to assess hydrogen diffusion through a metal is introduced, including the experimental setup and conditions, the theoretical Fick’s diffusion laws, and the determination of the hydrogen diffusion coefficient. The role of stresses on hydrogen diffusion is further elaborated based on experimental data that were obtained by expanding the experimental setup with an external proof ring to apply a controlled constant load to the sample of interest. Elastic stresses cause an increase in the hydrogen diffusion coefficient due to an expansion of the crystal lattice, while plastic stresses reduce the hydrogen diffusivity due to the nucleation and generation of lattice defects induced by plastic deformation such as dislocations and vacancies. This chapter concludes with some examples of research performed at the research group of the authors, indicating the key role of hydrogen diffusion.

6.1 Introduction: hydrogen and metals Limited fossil-fuel resources, concerns about nuclear power, and increasing public and political awareness on climate change, define energy-related challenges that trigger scientists to find ecological solutions for an eco-friendly future. Hydrogen gas (H2) is an attractive candidate as a replacement for fossil fuels, since its combustion only Acknowledgments: TD holds a postdoctoral fellowship via the Research Foundation – Flanders (FWO, grant 12ZO420N). The authors also wish to thank the Special Research Fund (BOF), UGent (BOF15/ BAS/062 and BOF01P03516). The authors also acknowledge the technicians and staff working at the Department of Materials, Textiles and Chemical Engineering, UGent, for their help with the experiments and/or sample preparation. Special thanks go to Dr. Van den Eeckhout (UGent) for the research done in this field. https://doi.org/10.1515/9783110596281-014

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generates water and no greenhouse gasses are emitted. Despite these advantages, hydrogen has a negative public reputation due to catastrophic incidents in the past, for instance, the crash of the airship “Hindenberg” at Lakefield, New Jersey on 6 May 1937. In this case, hydrogen was ignited, whereupon the zeppelin exploded. Apart from the high flammability of hydrogen gas, the interaction between elementary hydrogen and metals is an important and relevant research topic that garnered increased attention from both the academic and industry over the past decade. The particular detrimental effect of atomic hydrogen on a material’s integrity is known as the hydrogen embrittlement (HE) phenomenon or hydrogen-assisted cracking (HAC). The presence of hydrogen in a material can lead to reduced ductility, toughness, and ultimately poor tensile strength, potentially causing unpredictable failure. A broader term that covers all hydrogen-damage-related issues such as HE and hydrogen-induced cracking (HIC) is hydrogen-induced degradation (HID). This phenomenon was reported for the first time in 1875 by W. H. Johnson [1] when he described: “This change is at once made evident to anyone by the extraordinary decrease in toughness and breaking strain of the iron so treated, and is all the more remarkable as it is not permanent, but only temporary in character, for with lapse of time the metal slowly regains its original toughness and strength.” This harmful effect of hydrogen became a topic of interest to metallurgists, chemists, and physicists in the decades following the 1920s. Interest also revived recently. So far, much research has been carried out and several mechanisms have been proposed to explain HE, but there is still no consensus among the scientists. Comprehension of the hydrogen–material interaction is important due to the recent importance of the hydrogen-based economy. This has resulted in a considerably more attention being provided to the metallurgical aspect of the clean energy transition toward hydrogen. In May 2020, a letter [2] addressed to the European Commission was signed by 90+ Hydrogen Europe CEOs from multiple sectors, for example, energy, aerospace, automotive, (petro-)chemical, material industry, transport, infrastructure, and technology. As members of Hydrogen Europe, the European hydrogen and fuel cell association, they wrote this letter to the European Commissioners stating that “160+ industry members are coming together to support the idea of a Clean Hydrogen Alliance.” These companies are of diverse sizes and represent the entire hydrogen value chain, from production to transport, distribution, and the final end-use of hydrogen. They claim that “through concrete and dedicated action, and with the European Commission support, they will deliver considerable climate action already by 2030 and hence contribute to Europe’s green recovery plan through the use of hydrogen and hydrogen technologies.” They believe that the deployment of clean hydrogen can massively contribute to the European Green Deal [3]. To become the world’s first climate-neutral continent by 2050, Europe needs carbonfree energy, increased energy efficiency, and deep decarbonization of industry, transport, and building. These solutions depend on large-scale hydrogen applications. The European industry is engaged in a common vision to promote hydrogen

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as an enabler of a zero-emission society. As a clean energy vector, a fuel, and a feedstock, clean hydrogen has the potential to accelerate the decarbonization of our energy system and of our industrial production pathways. Through their letter, they wish to re-emphasize the potential of hydrogen technologies to achieve climate neutrality by 2050. As an eco-friendly energy carrier, hydrogen can indeed enable an energy revolution toward renewable and sustainable energy systems. However, one major challenge for the future is energy storage. Green power production is expected to significantly increase over the coming years. This will include high fluctuations between supply and demand, which can be responded by converting the surplus electricity into hydrogen, that is, the excess green energy coming from solar and wind energy can be converted into hydrogen gas. Hence, the energy can be stored in the existing gas grid or in salt caverns, potentially replacing natural gas for energy supply to housing, industry, and transport. Major efforts are currently ongoing into the development of the hydrogen economy. Understanding the fundamentals of HID is thus required for numerous upcoming material developments, which will aid in establishing this future hydrogenbased economy. This hydrogen economy would be one solution to the problem of climate change, which is a consequence of CO2 emissions. Decarbonizing of the gas grid by the replacement of domestic supply with hydrogen and using hydrogen as a fuel in the automotive industry are two examples to illustrate the potential beneficial impact on the climate change that the world is currently facing [4]. Moreover, material scientists have developed materials such as high-strength steels to satisfy the high structural and economic requirements of the industry. For example, steels with increased strength levels are necessary to meet the targets on CO2 emission through reduction in vehicle weight. These are currently available to some extent to the automotive sector. However, to avoid safety issues, these materials cannot be used when the HID problem is not fully understood or solved [4]. Additionally, these advanced high-strength steels, which are excellent candidates to fulfill the abovementioned requirement, are prone to HE [5, 6]. Hydrogen might originate from the production process, product assembling, and finishing, or from service environment exposure. For example, electrodeposition processes could be accompanied by hydrogen production [7]. Also, the offshore industry encounters difficulties that come along with hydrogen. When cathodic protection is applied to control the corrosion processes of the steel, hydrogen is produced and can be absorbed by the material [8]. Besides, in wind turbines, the rolling contact fatigue life of bearing steels is severely limited due to hydrogen. Repair of these contacts is extremely expensive and difficult due to the location of the construction. The presence of hydrogen at these locations may originate from the use of long-life lubricants which contain certain additives to extend the turbine maintenance intervals. However, these additives contribute to the lubricant decomposition and hydrogen generation [9]. In this chapter, general aspects of the hydrogen/steel interaction will be introduced to establish the HE phenomenon. Further, a literature overview is included

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about the hydrogen permeation technique and the diffusion laws of Fick. Finally, the outlook indicates the significance of this research, together with some recently obtained research findings.

6.2 Hydrogen–metal interaction In general, the HID phenomenon is controlled by three main factors. First, there is the amount of atomic hydrogen present in the microstructure. To enter the material, the hydrogen is first adsorbed at the metal surface and then absorbed in the material, a process that will be explained later. The second factor is the microstructure of the metal. It plays a decisive role in the hydrogen transport through the metal lattice. Microstructural defects such as grain boundaries, dislocations, second phase particles, and other metal lattice imperfections affect both the hydrogen diffusion and trapping behavior, and thus the degree of the HID. Finally, there is the driving force of diffusion, which can be a result of an external applied load. For example, stresses introduced in the microstructure will affect the diffusion and local solubility of hydrogen through and in the lattice.

6.2.1 Hydrogen entry The mechanical properties of metals or alloys are, amongst others, affected by the diffusion of atomic hydrogen through the material. The importance of the hydrogen diffusivity is nicely demonstrated in the work of Depover et al. [10]. They observed a higher degree of embrittlement with increasing hydrogen diffusion coefficient, although a lower amount of hydrogen was present. This lower hydrogen content was obtained by decreasing the material’s carbon amount, that is, hydrogen trapping sites. As such, hydrogen was able to diffuse faster to critical regions and damage was induced more rapidly. Nevertheless, before hydrogen diffusion and damage can take place, it needs to enter the microstructural matrix. The first step of the entry mechanism is the hydrogen adsorption at the metal surface. There are two possible adsorption mechanisms: Chemisorption, which is the adsorption from the gas phase, and, secondly, electrochemical adsorption – adsorption through electrolysis. In the gaseous phase, hydrogen is present as molecular hydrogen (H2), which is too large to enter the metal via the surface [11]. In that case, the hydrogen molecule needs to dissociate. Gaseous charging depends on the pressure, temperature, time, and gas concentration. With the use of thermodynamic laws such as Sievert’s law, the hydrogen content and distribution can be determined. The second method is charging via electrochemical adsorption. Hydrogen protons (acid media) or water molecules (alkaline media) present in the electrolyte are

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converted at the cathodic surface to atomic hydrogen. By applying an appropriate current or potential, the following reduction reactions are initiated (Volmer’s reaction): H+ + e− ! Hads

(6:1)

H2 O + e − ! Hads + OH −

(6:2)

Once hydrogen is adsorbed, the atom may be transferred across the metal surface into the absorbed state in the subsurface layer (eq. (6.3)). However, hydrogen entering the metal passes through the same adsorbed state on the metal surface, leading to hydrogen evolution. So, parallel to hydrogen absorption, two different hydrogen recombination reactions can take place. The detachment of adsorbed hydrogen atoms from the metal surface can occur by either chemical desorption (eq. (6.4), Tafel reaction) or electrochemical desorption (Heyrovski’s reaction). The electrochemical desorption reaction differs in the acid and alkaline mediums, presented in eqs. (6.5) and (6.6), respectively. Generally, both recombination reactions occur simultaneously, often, one of them predominating. The reactions at the metal–solution interface in the aqueous solution are schematically illustrated in Fig. 6.1. The recombination reaction that is favored depends on the composition of the charging solution, the condition of the metal surface, and the surface hydrogen coverage, which is related to the applied current or potential. It was suggested by several authors [12–14] that the hydrogen evolution reaction is followed by predominantly chemical desorption at low over-potentials and by mainly electrochemical desorption at high over-potentials. More information concerning the kinetics can be found in specialized literature [15–17]: MHads ! MHabs

(6:3)

MHads + MHads ! H2 + M

(6:4)

+

−

MHads + H + e ! H2 + M ðacid mediaÞ

(6:5)

MHads + H2 O + e − ! H2 + OH − + M ðalkaline mediaÞ

(6:6)

Fig. 6.1: Reaction processes occurring at the metal charging surface, based on [18].

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In the offshore industry, an important hydrogen source is the applied impressed current cathodic protection (ICCP) system. Cathodic protection is used to prevent constructions from corrosion. The structure, which requires protection, is forced to act as a cathode. In this way, the steel is prevented from oxidation and mainly reduction reactions occur at the metal surface, while oxidation reactions take place at the anode. The highly conductive anodes, required for an ICCP system, usually have a more positive potential than the protected structure. Moreover, they must have a low solubility and a high resistance against impact, abrasion, or vibrations. The anodes are thus anodically stable noble metals or materials that form a conducting stable oxide film on their surface [19]. Figure 6.2 schematically shows how the potential of iron is pushed down from the corrosion potential, Ecorr, to Eʹ by applying a current equal to (|Iʹc|–Iʹa). At Ecorr, the anodic reaction rate of the metal will be higher than the rate Iʹa at a potential equal to Eʹ. In other words, corroding processes at the metal are minimized. However, at Eʹ, the external-applied current must be sufficient to sustain the total cathodic reaction, and thus both the oxygen reduction and the hydrogen evolution reaction [20]. Normally, the oxygen reduction reaction is the dominant one in aqueous environments, but at large negative potentials, water reduction becomes more relevant. Parts of the construction that are subjected to these large negative potentials are thus overprotected and originate from the heterogeneous current distribution of the ICCP system. Atomic hydrogen uptake in the metal hence takes place at these overprotected sites during ICCP.

Fig. 6.2: Polarization diagram showing corrosion, cathodic protection, and the role that hydrogen evolution plays. The dotted line represents the total cathodic current due to hydrogen evolution and oxygen reduction [20].

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6.2.2 Hydrogen transport Hydrogen is able to diffuse through the microstructure when absorbed in the material. The two main diffusion models that dominate are substitutional and interstitial diffusion. For substitutional diffusion, vacancies are needed in the metal lattice and atoms basically switch positions with the vacancy. In the case of interstitial diffusion, the dissolved atom diffuses more rapidly as it is not held by the lattice. This is the main mechanism when hydrogen diffuses through the metal. The hydrogen atom is sufficiently small and is therefore able to make interstitial jumps. Even at room temperatures, hydrogen can diffuse through the metal lattice. Temperature, chemical composition, and crystal structure are three important parameters that can alter the diffusivity and solubility [21]. The diffusion coefficient D (m2/s) expresses the diffusivity in a material and can be calculated for hydrogen by performing hydrogen permeation experiments. The permeation technique and the corresponding theory it relies on are elucidated in Section 6.3. As hydrogen transport happens via the interstitial sites of the lattice, the diffusion rate and the solubility will depend on the metallic structure. In Fig. 6.3, tetrahedral and octahedral interstitial sites for a face-centered cubic (fcc), hexagonal close-packed (hcp), and body-centered cubic (bcc) crystal are illustrated. The most common structure for steel at room temperature is bcc, the ferritic structure. It has an open lattice configuration, and shows a high diffusion rate and low solubility. Some specific steels such as transformation-induced plasticity steel contain an fcc austenitic phase at room temperature. This structure is a closer-packed lattice structure compared to bcc and can be perceived as a reservoir for hydrogen as the hydrogen diffusivity is very low when the solubility is high. Hydrogen prefers the tetrahedral sites for bcc and the octahedral sites for fcc lattices [22]. The martensitic structure is mostly body centered tetragonal (bct); however, Olden et al. [23] mentioned a rising tendency of hexagonal martensite formation with increasing carbon content. Both structures are closer packed

Fig. 6.3: Interstitial sites (octahedral – O and tetrahedral – T) in fcc, hcp, and bcc lattices [24].

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than bcc, leading to a diffusion rate between ferrite and austenite. Similar to the fcc lattices, hydrogen prefers the octahedral sites. Another way for hydrogen to move through the lattice is via dislocation displacements [25]. In this way, hydrogen is able to move faster and more hydrogen atoms can be transported in one moment. The binding energy between the atom and the dislocation is higher compared to a normal interstitial lattice site. So, when dislocations are moving in the matrix, the hydrogen can be dragged along, resulting in a much faster diffusion through the lattice. Although the binding energy between a hydrogen atom and a dislocation is stronger compared to a normal interstitial lattice site, it is still weakly trapped and, therefore, detrimental to the mechanical properties of the metal [26, 27]. A more detailed explanation concerning hydrogen trapping sites is given in the next section.

6.2.3 Hydrogen trapping Besides interstitial lattice sites, there are other locations in the microstructure where hydrogen can be present. These other sites include lattice imperfections, crystal defects such as dislocations, grain/phase boundaries, second phase particles, vacancies, and voids. At these localized regions, hydrogen has a residence time that is considerably longer compared to a normal interstitial lattice site. Consequently, lattice imperfections will delay hydrogen diffusion through the material. In the extreme case, some microstructural heterogeneities can act as a sink and retain hydrogen even during a thermo-mechanical loading. Generally, lattice imperfections that slowdown hydrogen movement are called hydrogen trapping sites [28, 29]. Possible hydrogen trapping sites present in a steel matrix are illustrated in Fig. 6.4. When hydrogen is found in these lattice imperfections or traps, it is in a lower energy state compared to its normal interstitial position. This is due to the deepening of the associated potential well (cf. Fig. 6.5). The desorption activation energy, EA, is the energy needed for a hydrogen atom to escape from the trap. A higher temperature or an applied stress can provide the required energy. Based on EA, trapping sites are generally divided in reversible and irreversible traps. ED is the activation energy that is needed for hydrogen diffusion through a perfect lattice. If the hydrogen atom is found in an interstitial place next to a trap, a saddle point, it will preferentially diffuse to the trap as this corresponds with a lower amount of energy (ES < ED). Finally, the trap binding energy, EB, depends on the trap characteristics. Hydrogen in reversible traps is in equilibrium with the hydrogen in the lattice. These reversible types of traps show rather low desorption activation energies. Hence, it is easy for a hydrogen atom to leave the trapping site again and to diffuse further through the metal lattice. When hydrogen is trapped irreversibly, the probability to escape is very low. The desorption activation energy barrier between reversible and irreversible traps is, rather arbitrary, assumed to be about 60 kJ/mol [30–33].

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Fig. 6.4: Possible hydrogen trapping sites in a material on the (a) atomic scale and (b) microscopic scale [29].

Fig. 6.5: Potential energy of hydrogen in interstitial and trapped position.

Nevertheless, the reversible behavior of a trap strongly depends on the temperature, the trap occupancy, and the interaction time [31]. Dislocations [27, 31, 34] and grain boundaries [32, 33, 35] are considered to be reversible traps although

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different activation energies are reported in literature (Tab. 6.1). Hydrogen is trapped and released in a relatively short time frame, even at room temperatures. Therefore, reversible traps play an important role in the HID phenomenon, as they serve as storage points for diffusible hydrogen and provide hydrogen to fracture-initiation sites [36]. Irreversible traps, on the other hand, hold hydrogen for a much longer time and hence they might decrease the materials’ susceptibility to HID [29, 35]. Examples are retained austenite [37, 38] and carbides [32, 33, 39–41]. Beeler and Johnson [42] performed some theoretical analysis and found that the binding energy for a hydrogenvacancy complex increased with cluster size. However, an upper limit of 77.2 kJ/mol exists for clusters of more than six vacancies. More information concerning trapping sites – trapping activation energies – can be obtained by performing thermal desorption spectroscopy (TDS) analysis. Although the analysis procedure to obtain the corresponding activation energies linked to the deconvoluted peaks in the TDS spectrum is not straightforward, TDS results provide valuable information concerning the trapping behavior of the material. Beside TDS, the permeation technique can also be used [43]. In this chapter, the permeation technique to determine the corresponding diffusion coefficients is elaborated. Some literature data on activation energies is given in Tab. 6.1. Tab. 6.1: Activation energies for different trapping sites. Trapping site Lath martensite Lath martensite Microvoids Vacancies HV–HV Vacancies HV–HV Dislocations Dislocations Dislocations Dislocation Grain boundary Grain boundary Retained austenite Retained austenite TiC (incoherent) TiC (semi-coherent) TiC (carbon vacancy inside carbide ( 0.2 exp − π 4

(6:14)

(6:16)

C, hydrogen concentration, dimensionless time τ = (Dt)/L²; t, time; L, sample thickness; D, diffusion coefficient; J, the measured permeation flux, and J∞, the steady-state permeation flux.

6.3.2 Diffusion coefficients Nowadays, numerous ways to calculate the hydrogen diffusion coefficient are available, of which some are explained in this section. It is important to note that diffusion coefficients, experimentally defined from the Fick’s solution, will be labeled as apparent diffusion coefficients (Dapp), as the data processing does not consider phenomena such as surface effects and trapping mechanisms. When performing electrochemical permeation tests, the diffusion coefficient could be determined by using the steady state current density (Iss), measured at the exit surface of the specimen. Figure 6.10 shows a schematic representation of a permeation transient, where the steady state current density is indicated. Since the permeation current is associated with the oxidation of hydrogen atoms and I=F*J

(6:17)

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Fig. 6.10: A schematic representation of the experimentally recorded permeation transient. The steady-state current density (Iss) and the amount of diffused hydrogen (Qt) are indicated.

where F is Faraday’s constant (s · A/mol1), the hydrogen permeation flux J (mol/m²·s) can be replaced by the recorded permeation current density I (A/m2). One way to determine the value of D is by fitting the normalized experimental transient curve with eq. (6.9) or eq. (6.10). Another method, called the “time-lag” method, is probably the most well known. This method involves the integration of the permeation curve with respect to time to obtain the amount of hydrogen which is diffused through the sample in a certain time period (Qt ), with Qt indicated in Fig. 6.10. In the case of potentiostatic conditions, at the entry side and for t ≫ L²/(6·Dapp), the following expression is obtained [88]:   Dapp C0 L2 (6:18) t− Qt = L 6Dapp For t ≫ L²/(6 · Dapp), the extrapolation of the linear part of the curve of Qt (t) intercepts the x-axis at the time lag [85]: tlag =

L 6Dapp

(6:19)

where tlag is called the “time lag” (s), L is the sample thickness (m), and Dapp the apparent diffusion coefficient (m²/s). It corresponds to the time needed for the hydrogen flux to reach 62.9% of its steady-state value. The time lag offers, in good approximation, an expression to calculate Dapp . If the same procedure is followed in the case of galvanostatic boundary conditions, tl corresponds to the time necessary to reach 61.7% of the steady-state value and is given by [85] tlag =

L 2Dapp

(6:20)

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In addition to the “time lag,” other points of the permeation transient may be used to determine Dapp. This can be done by the following mathematical relation derived from Fick’s solutions (cf. Tab. 6.2): Dapp =

L2 Mtx

(6:21)

L is the specimen thickness (m), tx is the time (s) where the normalized flux ðJtx =J∞Þ equals x, and M an adimensional constant which depends on the charging method and the chosen time value tx . M is determined by using τ = ðDtÞ=L2 and D = L2 =ðMtÞ in the Fick’s solutions (cf. Tab. 6.2) and by plotting the normalized flux with respect to the M value. As such, M is known for every value of the normalized hydrogen flux. Calculated values for M at different points of the normalized charging transient for the CC and CF conditions are given in Tab. 6.3. Until now, no work has been published that demonstrates which kind of polarization method is most convenient to provide reproducible permeation experiments. A constant cathodic current carries with it the advantage that the hydrogen coverage, which is linked to hydrogen evolution and absorption mechanisms, is controlled [89]. Tab. 6.3: M-values at different moments of the normalized permeation flux (J/J∞). M

Potentiostatic

Galvanostatic

. . . . .

. . . . .

J/J∞ . . . . .

Another method to determine the apparent diffusion coefficient was developed by Frappart et al. [84]. This technique, called the “Regime 1 technique,” left out the steady-state permeation current since this value could be affected by surface evolution or trapping processes during the first permeation transient. The method allowed one to verify that Dapp was coherent with the apparent diffusion coefficient calculated with other methods. It was based on the beginning of the transient, where Dapp was determined by fitting the experimental results to a simple mathematical eq. (6.18), which was related to eq. (6.13): ∂lnð jÞ 1 L2 =− + ∂t 2t 4Dapp t2

(6:22)

Dapp could depend on the evolution of the surface state and on trapping processes. In the research by Frappart et al. [84], the diffusion coefficients for a permeation

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experiment were calculated by both the methods at 10% of the steady state current and the “Regime 1” method. Regardless of the technique, the value of Dapp was similar. This indicated that the influence on the steady state value by trapping phenomena and surface changes could be ignored in determining Dapp.

6.3.3 Diffusion through a multilayer system In the previous section, no trapping or barrier effects were considered. In practice, hydrogen must diffuse through a multilayer system that consists of layers with different hydrogen diffusion coefficients and solubilities. Metal coatings, for example, can be used as barriers, as they strongly reduce the hydrogen permeability. These coatings reduce the HE susceptibility and most common ones are made of cadmium, platinum, nickel, and copper [90]. On the other hand, metal coatings can also be used to favor hydrogen absorption in steels by increasing the hydrogen activity. For this purpose, palladium (Pd) is the most common element used in the electrochemical permeation technique. Besides enhancing hydrogen absorption, it also impedes the oxidation of the metal surface. Many researchers plate a Pd layer at the entrance and/or exit side of a permeation membrane to avoid surface phenomena associated with the presence of oxide layers. Zakroczymski et al. [91] investigated the Pd layer at the entrance side and concluded that palladium impedes the hydrogen entry when compared to an uncoated iron membrane. The influence of the Pd layer on the hydrogen diffusion at the exit surface was considered by Manolatos et al. [92, 93]. They stated that a Pd layer was required at the detection side to ensure the reliability of a permeation experiment. Although it is well known that palladiums avoid corrosion of the steel and enhances the hydrogen oxidation at the exit side, defects and/or oxides at the metal/Pd interface are easily introduced during the plating process. These heterogeneities will affect the hydrogen diffusion in an uncontrolled manner, which is not desirable. Several authors [84, 94, 95] carried out permeation experiments without a Pd layer and concluded that palladium deposition was not necessary. The major risk of its absence is the incomplete oxidation of hydrogen, but at low fluxes and in bcc steels, this is not a problem. This was also proven by Frappart et al. [89] who obtained comparable diffusion coefficients with and without a Pd layer, confirming that hydrogen diffusion is controlled by the bulk material. Passive films and natural air-grown oxides are shown to be barriers for hydrogen permeation [96, 97]. The diffusion of hydrogen through the layer is determined by the nature of the oxide. In a permeation experiment, air-formed oxides are present at the entrance side while a passive layer is formed at the exit surface. During the permeation experiment, the natural film on the cathodic surface is reduced. In alkaline solutions, the reduction occurs partially while in acid media, the film reduction is fast [91, 98]. Distortions in the recorded permeation current can sometimes be associated with the evolution of the hydrogen concentration with time at the entrance surface [99].

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In the absence of a Pd layer, a stable passive layer must be created at the exit surface to prevent the surface from corroding. Although researchers agree on the barrier effect of the passive layer, there is no agreement yet on the stability during permeation. Casanova et al. [95] stated that the passive layer formed in a de-aerated NaOH solution was perfectly stable. This implied that the hydrogen charge transfer took place at the iron–oxide interface and that hydrogen migrated through the film as a proton [100]. Other authors claimed that the diffusion of hydrogen atoms reduced the film, and lowered its thickness and oxygen content [101]. Generally, the entrance and exit surface states of the metal membrane are very important factors to take into account when interpreting the obtained permeation data. Comparing results based on permeation experiments of different authors is, therefore, very complex when the experimental procedures differ. Vecchi et al. [102–104] pointed out the complexity of modeling a permeation experiment and highlighted the importance of the entry and exit surface states. This should, according to their work, be incorporated when modelling the overall hydrogen transport through a metal membrane. In the next section, the influence of stresses on the hydrogen diffusivity is considered.

6.4 The influence of stresses on the hydrogen diffusivity When steel is used in structural applications, it experiences different levels of stress during its lifespan. Plastic deformations typically occur during production, whereas elastic stresses are ubiquitous in most mechanical applications. If the hydrogen diffusivity increases with these stresses, the critical concentrations necessary for hydrogen-induced damage will be reached sooner. Moreover, when a crack is already present in the microstructure, during crack propagation, the crack tip could be under plastic strain while the regions away from the crack tip could be subjected to elastic stresses. Therefore, it is important to study the influence of both elastic and plastic stresses on the hydrogen transport through the material. This can be performed by permeation experiments combined with a tensile loading device. Zhao et al. [105] performed hydrogen permeation tests combined with a slow strain rate tensile test in which stress was applied to obtain a constant strain rate. Fig. 6.11 shows the stress as well as the permeation current density measured at the exit side of the sample. In the first part of the curve (a–b), an increased elastic tensile stress resulted in a higher permeation current density. On the one hand, this could be correlated with a higher diffusion coefficient. On the other hand, applied stresses could increase the hydrogen evolution reaction rate, leading to a higher hydrogen uptake [106]. However, this increasing current density does not extend for the entire elastic range. At point b in Fig. 6.11, the permeation current density starts to decrease. The authors claimed that a phenomenon called micro-plasticity caused

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4.5

d

4.0 Rt0.5

c

3.5

800 730 700 e 625 600 500

3.0 400 364 300

b 2.5 2.0

Stress / MPa

Permcation current density / µA.cm-2

a transition from a pure elastic deformation state to an elastic–plastic coexisting state. As such, lattice defects were created before reaching the yield stress. The increasing permeation current density at stresses higher than the yield stress (c–d) could be attributed to the enhanced hydrogen transport by dislocation movement.

200 a

1.5 0

1

2

3 4 Time / 104s

5

6

100 7

Fig. 6.11: Stress (dotted line) and hydrogen permeation current density (solid line) versus time [105].

A recent study considered the effect of a constant load on the hydrogen diffusion characteristics studied by electrochemical permeation experiments. Different loads, both in the elastic and plastic regime, are applied on the sample during the permeation experiment when hydrogen is migrating through the material. To measure the effect of stresses on the hydrogen diffusion process, a new permeation set-up was designed in such a way that it could be instrumented around an external proof ring. This permeation set-up is schematically represented in Fig. 6.12. Van den Eeckhout et al. [107] demonstrated for dual phase steel by in situ permeation experiments under an applied mechanical load that elastic stresses resulted in an increase in the hydrogen diffusion coefficient. Due to the volume increase of the unit cell, interstitial positions were enlarged and hydrogen diffusion was facilitated. This statement was verified by studying the concentration dependence of the diffusion coefficient, as a higher hydrogen uptake in the subsurface of the specimen with increasing strain could also be responsible for the increased diffusion rate. When the imposed stress was equal to the macroscopic yield stress – 100% condition (cf. Figure 6.13) – the increasing amount of hydrogen traps due to the generation of dislocations compensated the increase in the diffusion coefficient resulting from the lattice expansion. When the load further increased toward the plastic regime, the hydrogen diffusivity dropped further due to the additional formation of defects such as dislocations and vacancies, upon increased mechanical plastic deformation. As such, the hydrogen diffusivity decreases while the solubility of the material increases

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[108–110]. The corresponding apparent diffusion coefficient versus. the applied load is presented in Fig. 6.13.

Fig. 6.12: Schematic representation of the electrochemical permeation setup (left) adjusted to allow mechanical loading by a proof ring (right). CE = counterelectrode, RE = reference electrode, WE = working electrode (steel) [107].

Fig. 6.13: Apparent diffusion coefficients for the dual phase steel under constant elastic stress (left) and under constant applied stress in the plastic regime (right), % of the yield strength is shown with both with 0% (unloaded condition) as reference [107].

6.5 Some recent research at Ghent University In this section, the importance of hydrogen diffusion will be demonstrated based on different research findings. Generally, the presence of hydrogen is known to deteriorate the mechanical properties of steel. As such, there is an increased tendency for crack formation leading to a possible unforeseen failure of the construction.

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Nonetheless, a critical hydrogen concentration is required to initiate and propagate a crack in the microstructure. The susceptibility of the material to hydrogen-induced damage depends, thus, on the amount of hydrogen that is able to diffuse to specific, for example, stressed regions in the microstructure. Since hydrogen is attracted to these regions, the required concentration for crack propagation can be reached more easily. However, next to the hydrogen concentration, the ability to reach the critical zones is a very important parameter that cannot be ignored. This ability is described by the hydrogen diffusion coefficient of the material. Depover et al. [111] visualized the impact of hydrogen diffusion by studying the fracture surfaces of an in situ charged tensile specimen of dual phase steel. The calculated hydrogen diffusion distance, depending on the applied crosshead deformation speed, matched perfectly with the observed transition on the fracture surface between hydrogen-induced brittle and ductile fracture features. The prominent role of the hydrogen diffusivity was further demonstrated by in situ hydrogen charged tensile tests where the effect of hydrogen on the ductility of lab cast Fe–C alloys was measured. Generally, the ductility loss was more significant for a lower crosshead deformation speed, evaluated by in situ tensile testing [5, 10] and in situ bending testing [112]. In that case, the hydrogen had more time to diffuse and the critical concentration for crack initiation was reached earlier, increasing the HE sensitivity. Furthermore, the synergetic effect of both the hydrogen content and hydrogen diffusivity was demonstrated when different microstructural constituents were compared [10]. More specifically, two bainitic alloys with different carbon content of 0.2% and 0.4% were considered in [76]. Even though the higher amount of carbon led to a higher hydrogen concentration in the material, a lower embrittlement degree was obtained at lower crosshead deformation speed. This was attributed to the lower diffusion coefficient of the material, which contained 0.4% C. This lower diffusivity decreased the hydrogen ability to reach the critical zones, and crack initiation and propagation were postponed. At a higher crosshead deformation speed, the 0.4% C steel showed a larger susceptibility to HE. In that case, only limited diffusion took place and, therefore, the total amount of hydrogen played a more prominent role in the observed embrittlement. This work nicely demonstrated that the link between the amount of hydrogen and the susceptibility to HE is impossible without also incorporating the hydrogen diffusion coefficient of that specific microstructure into the discussion. Furthermore, when further increasing the strain rate toward dynamic conditions (450 and 900 s−1) to evaluate crash impact velocities, the impact of hydrogen was significantly reduced due to the inability for hydrogen to diffuse to critical regions during the dynamic test. Consequently, it is of high importance to consider the hydrogen diffusion coefficient when new materials are being developed with the purpose of decreasing the susceptibility to HE. Furthermore, the role of hydrogen diffusion in the hydrogen trapping evaluation by TDS cannot be neglected either. When slow diffusion materials such as pure fcc metals or even UNS S32205 duplex stainless steels containing a 50–50% ferrite

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(bcc) and austenite (fcc) are considered, experimental hydrogen trapping assessment is complicated. Claeys et al. [113] demonstrated that hydrogen trapping cannot be distinguished from experimental TDS data, as hydrogen diffusion through austenite is the rate-determining step during the desorption process. An average activation energy of 43.4 kJ/mol for hydrogen diffusion was determined. Similar findings were obtained with a finite-element model, including the heterogeneous hydrogen properties of the two phases present in this duplex stainless steel. When modifying the phase fractions of austenite and ferrite in this material, Cauwels et al. [114] found that the loss of ductility was larger for the sample with a higher ferrite fraction. This was correlated to the higher hydrogen content and the increased hydrogen diffusion coefficient, both contributing to the increased susceptibility. Apart from the impact of stresses on hydrogen diffusivity (cf. Section 6.4), hydrogen may also accumulate at hydrostatic stress fields ahead of, for example, a notch or a crack tip. Locally increased hydrogen concentrations can thus be present, causing hydrogen-related damage phenomena. Depover et al. [115] evaluated the influence of the local hydrostatic stress state and the resulting local hydrogen concentration on the HID of dual phase steel. Different tensile specimen geometries, with and without the presence of a notch, were selected to induce variable hydrostatic stress concentration gradients upon mechanical constant loading. Hydrogen charging was done simultaneously with constant loading to allow coupling with numerical results. The maximal concentration of hydrostatic stress and the related hydrogen concentration was located in the front of the notch tip. When inspecting the fracture surface of the samples, the present alumina inclusions in this material showed significant embrittlement in the region where hydrostatic stress and hydrogen concentration peaked. Moreover, hydrogen-embrittled fish-eyes were found surrounding these inclusions, only at these specific regions of increased hydrostatic stress.

6.6 Outlook: what future research is needed? In order to realize appropriate materials to establish the hydrogen economy, dedicated expertise toward materials science and hydrogen-related characterization will be an absolute prerequisite. In order to make a head start, a fundamental approach to the challenge is needed, combined with demonstrated expertise in the development of experimental set-ups. As the hydrogen/material interaction is very specific, lack of competence in dedicated experimental set-ups will most likely give rise to distorted results. Moreover, the complete picture of the interaction between hydrogen and a material should be determined, since HID entails a complex combination of many different aspects. They include the hydrogen charging procedures to introduce hydrogen in the material, the hydrogen diffusion phenomena, the interaction of hydrogen with microstructural features resulting, for example, in hydrogen

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trapping, the impact on the mechanical performance investigated by in situ tests, and the impact of applied or present stresses. These aspects along with their point of attention are now addressed in more detail. Firstly, the charging method needs to be carefully selected and controlled. Both electrochemical [5, 116, 117] and gaseous hydrogen charging [118, 119] can be used to induce atomic hydrogen into a metal microstructure. Both charging procedures can, however, lead to HIC or blister formation at the surface of the sample, which needs to be avoided at all costs in order to perform reliable and trustworthy experiments. Still, many questions arise on the possible comparison between electrochemical hydrogen charging under galvanostatic or potentiostatic conditions and gaseous H2 charging at elevated pressures or temperatures. Hence, considerable efforts are needed to establish the correlation between both charging procedures. Combined with other experimental work, as discussed below, this will allow developing fast(er) screening methods via electrochemical methods to generate relevant test results for H2 applications within the hydrogen economy. Secondly, a critical evaluation of the behavior of a specific material when in contact with hydrogen is key to an increase in our understanding and to be able to engineer more hydrogen-resistant alloys in a next step. Having appropriate materials at our disposal is clearly needed and requires important research efforts. As elaborated in this chapter, electrochemical hydrogen permeation testing will yield the required data on hydrogen diffusion. Moreover, the role of stresses [107], and local hydrostatic stresses due to increased material triaxiality should be incorporated in the hydrogen diffusion determination as well [115]. This can be carried out experimentally, as mentioned above, or based on finite element modeling. Local electrochemical techniques such as scanning kelvin probe microscopy [120, 121] will help in understanding the role of specific microstructural defects. The successful application of a suitable coating to prevent hydrogen entry needs to be further explored as well. Both metallic coatings and polymers might be an option. Furthermore, TDS is a very important tool to assess the available hydrogen trapping sites in a material. The desorption activation energies of different trapping sites can be linked to microstructural defects. Thorough understanding of suitable trapping sites in a ma terial will yield opportunities for an improved material design. Improvements in material design, among other, can be by modifying the chemical composition, thermomechanical processing, appropriate heat treatments, and post-processing. As with hydrogen permeation, a dedicated experimental methodology is also extremely important [27, 32, 49, 113]. This experimental technique can be combined by numerical finite element modeling [51, 53], while the theoretical understanding based on the density functional theory will be very relevant as well [58, 122, 123]. Visualizing hydrogen trapping, though very challenging, might also provide further confirmation on effective trapping, for instance, by precipitates. However, the lack of spatial resolution in conventional analysis methods such as hydrogen microprinting, tritium autoradiography, and secondary ion mass spectroscopy, complicate the determination of

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hydrogen trapping sites linked to nano-sized precipitates. Nevertheless, atom-probe tomography can be used for this purpose, for which some successful results on deuterium trapped at precipitates have been reported [57, 124, 125]. Thirdly, apart from the interaction of hydrogen with the material microstructure, including properties such as the hydrogen diffusion and trapping, detailed comprehension of the resulting mechanical degradation phenomena is important. Especially, the identification of the acting deformation and embrittlement mechanisms will facilitate the development of more performant materials. This requires different types of in situ mechanical test methods. In situ slow strain rate tensile testing allows a quick comparison between reference mechanical data from tests performed in air with tests done after specific hydrogen pre-charging [5]. Continuous charging during the test is very relevant in this context to avoid undesired consequences of hydrogen desorption and, correspondingly, controlling the amount of hydrogen in the sample during mechanical testing. Constant load tests or linearly increasing stress tests can provide complementary information [126, 127]. For more brittle microstructures, conventional tensile testing will not give useful results as hydrogen will lead to premature failure before plastic deformation occurs. Therefore, in situ bending experiments allow evaluating the sensitivity of these materials to environmental hydrogen-assisted degradation [112]. While tests to evaluate the impact of hydrogen on the material’s fracture toughness by in situ single edge notched tension testing, in situ fatigue tests to test the effect of cyclic stresses allow completing the picture for the most frequently used length scales. When considering mechanical testing, performing tests over different scale lengths provide an important added value. On the one hand, scaling down to the microstructural level by in situ micromechanical testing inside the scanning electron microscope results in very local information on crack initiation and subsequent crack propagation [38, 128]. On the other hand, the current understanding should be expanded to large-scale testing to better assess the structural integrity of real structures, in combination with hydrogen. Next steps in research should focus on closing the gap between the different scale lengths, allowing better correlations between the different test methodologies, while considerable efforts in the development of test methodologies, especially in the larger length scale range are needed. Finally, all the hydrogen related test methodologies discussed should be complemented with microstructural characterization, since microstructural aspects play a crucial role. For example, interrupted in situ mechanical tests followed by investigation of the sample by scanning electron microscopy – electron backscattered diffraction – allows identifying the active deformation mechanisms, as such elucidating the effect of hydrogen on these mechanisms [129–132]. Besides, fractographic evaluation allows characterizing the fracture surfaces and the presence of brittle features among other things. Other examples include the visualization of hydrides in hydrideforming materials via X-ray diffraction or TEM to characterize carbide type, size, distribution and morphology, as carbides are often considered as efficient hydrogen

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traps; yet, their efficiency depends on these characteristics [32, 52, 133]. The role of hydrogen on lattice defects such as dislocations or stacking faults close to the surface of bulk samples can be elucidated by electron-channeling contract imaging [133–136]. In conclusion, the hydrogen economy will not be realized without having appropriate materials. In order to be able to design those materials, an understanding of all aspects of the hydrogen/material interaction is an absolute prerequisite and considerable research efforts are still required. The reward for those efforts will be major breakthroughs in material development that will significantly boost the hydrogen economy.

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Marek Nowak, Mieczysław Jurczyk

7 Nickel metal hydride batteries 7.1 Introduction Nickel metal hydride (Ni–MHx) batteries have been commercially available since 1990 and have gained a market in many applications. Ni–MHx batteries are an extension of the nickel-cadmium battery technology, where an electrode made of a hydrogen-absorbing material was used instead of the cadmium electrode. The result of this replacement was the elimination of cadmium, without significantly changing the design of the batteries. These batteries were a big improvement over the then-common nickel–cadmium batteries (Ni–Cd) as they had higher energy density, longer cycle life, better tolerance for overcharge/over-discharge, and better environmental compatibility [1–15]. Table 7.1 shows a comparison between Ni–Cd and Ni–MH batteries, and Tab. 7.2 compares the key design features of different battery chemistries. Tab. 7.1: Comparison between Ni–Cd and Ni–MH batteries. Ni–Cd

Ni–MH

Gravimetric energy density (Wh/kg)

–

–

Cycle life (to % of initial capacity)

,

–

Overcharge tolerance

Moderate

Low

High-rate performance

Excellent

Good

Operating temperature

Good low-temperature performance

Good high-temperature performance

Memory effect

Yes

Moderate/negligible

Impact on environment

Uses toxic cadmium

Minimal environmental problems

7.2 The hydride electrode and Ni–MHx battery At the negative electrode, a metallic alloy (M) is used. Generally, the reaction of the metal hydride formation is as follows [4]: M + x=2H2 $ MHx ðsÞ where MHx is the hydride of metal M.

https://doi.org/10.1515/9783110596281-015

(7:1)

Fe

H

Zn

Zn

Zn

Zn

Li

Nickel iron

Nickel hydrogen

Nickel zinc

Silver zinc

Zinc bromine

Zinc air

Lithium ion

Na

  – (depends on MH)      ,   

. . . . . .

LixCoO

O

Bromine complex

AgO

. . . . . .

PC or DMC w/LiPF Beta alumina Beta alumina

KOH

ZnBr

KOH

KOH

KOH

NiOOH

NiOOH

KOH

KOH

KOH

HSO

Theoretical specific energy (Wh/kg)

Voltage (V)

Electrolyte

NiOOH

NiOOH

S

H (as MH)

Nickel metal hydride

NiOOH

Sodium sulfur

Cd

Nickel cadmium

PbO

NiCI

Pb

Lead–acid

Positive electrode

Sodium Nickel Na chloride

Negative electrode

Battery system

Tab. 7.2: Comparison of the key design features of different battery chemistries.

>

>





















Practical energy density (Wh/L)

























Practical specific energy (Wh/kg)

High-temperature battery, safety, low power Electrolyte

High-temperature operation, low Power

Safety issues, calendar life Cost

Low power, limited cycle life, bulky

Low power, hazardous components, bulky

Very expensive, limited life

Low cycle life

Cost, high pressure Hydrogen, bulky

Heavy, high maintenance

Cost

Toxic materials, maintenance, cost

Heavy, low cycle life, toxic materials

Major issues

282 Marek Nowak, Mieczysław Jurczyk

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283

Some alloys can be charged and discharged electrochemically. Equation (7.2) shows the electrochemical charging and discharging reactions: M + xH2 O + xe− $ MHy + OH−

(7:2)

The process of hydrogenation of metals with molecular hydrogen is a metal-gas interaction and can be described as a multistep process. The steps involved in the hydrogenation are: – transport of gaseous hydrogen to the metal surface, – adsorption of hydrogen molecules and the dissociation of H2 into H-atoms, – hydrogen diffusion into the bulk of the material, – formation of the metal hydride. The electrochemical reaction of a Ni–MHx cell can be represented by the following half-cell reactions. During charging, the nickel hydroxide Ni(OH)2 positive electrode is oxidized to nickel oxyhydroxide NiOOH, while the alloy M negative electrode forms MH by water electrolysis. The reactions on each electrode proceed via solid state transitions of hydrogen. The overall reaction is expressed only by a transfer of hydrogen between alloy M and Ni(OH)2: Nickel positive electrode: NiðOHÞ2 + OH − = NiOOH + H2 O + e−

(7:3)

Hydride negative electrode: M + H2 O + e− = MH + OH−

(7:4)

NiðOHÞ2 + M = NiOOH + MH

(7:5)

Overall reaction:

In sealed Ni–MHx cell, the M electrode has a higher capacity than the Ni(OH)2 electrode, thus facilitating a gas recombination reaction. During an overcharge situation, the MH electrode is charged continuously forming hydride, while the Ni electrode begins to evolve oxygen gas according to eq. (7.6): Nickel positive electrode: 2OH − = H2 O + 1=2O2 + 2e −

(7:6)

Hydride negative electrode: 2M + 2H2 O + 2e − = 2MH + 2OH −

(7:7)

2MH + 1=2O2 = 2M + H2 O

(7:8)

Actual (nonideal) case: H2 O + e − = 1=2H2 + OH −

(7:9)

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Marek Nowak, Mieczysław Jurczyk

2MH + 1=2O2 = 2M + H2 O

(7:10)

The oxygen diffuses through the separator to the MH electrode, and there it reacts chemically, producing water (eq. (7.10)) and preventing a pressure rise in the cell. During the over-discharge process, hydrogen gas begins to evolve at the Ni electrode (eq. (7.11)). The hydrogen diffuses through the separator to the MH electrode, and there it dissociates to atomic hydrogen by a chemical reaction (eq. (7.12)), followed by a charge transfer reaction (eq. (7.13)), ideally causing no pressure rise in the cell: Nickel positive electrode: 2H2 O + 2e− = H2 + 2OH−

(7:11)

Hydride negative electrode: H2 + 2M = 2MH

(7:12)

2OH − + 2MH = 2H2 O + 2e − + 2M

(7:13)

7.3 Electrode materials for Ni–MHx batteries In the Ni–MHx battery, the MHx electrode should be capable of reversible storing of hydrogen and should also exhibit a negligible self-discharge. To date, several classes of metal alloys that reversibly absorb hydrogen have been tested. They are AB5, AB2, RE–Mg–Ni, AB, A2B, and alloys based on V and Mg. Each alloy presents a different crystal structure. A few archetype materials of these classes are shown in Tab. 7.3. Tab. 7.3: Classification of intermetallic system for hydrogen storage. Properties

AB

AB

AB

AB

AB

Specific example

LaNi

MgNi

TiFe

TiMn

LaNi

Structure

Hexagonal

Cubic

Cubic

Hexagonal or cubic Rhombohedral

Hydrides

LaNiH

MgNiH TiFeH

Temperature

Room temp − °C

Near room temp Near room temp

Near room temp

Storage capacity (wt%)

.

.

.

.

TiMnH

.

LaNiH

The intermetallic compounds are ordered stoichiometric compounds, typically formed from two metallic components A and B. The component A easily forms stable hydrides, and the component B does not form stable hydrides, but it performs several additional functions. The components A and B can be completely or partially

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substituted by elements of similar size and chemical properties. Substitution of component A or B by other elements changes the properties of the alloy and allows designing its features. The effect of the chemical composition of the alloys on their properties is shown in the Tabs. 7.4 and 7.5 [2, 4–6, 10, 14, 16]. Tab. 7.4: The properties of AB5-type alloy electrodes in Ni–MHx battery and their chemical composition. Composition

Elements and their influence on the properties

Substitutions of A in AB

La–xMxB: Zr, Ce, Pr, Nd improve activation, high-rate discharge and cycle life, but increase the self-discharge owing to a higher dissociation pressure of the metal hydride

Substitutions of B in AB

A(Ni–zMz):A = La, Mm; M = Co, Cu, Fe, Mn, Al; Co decreases the corrosion rate and improves the cycle life of the electrode, especially at elevated temperature ( °C), but increases the alloy costs. Substitution of Co by Fe allows cost reduction without affecting cell performance, decreases decrepitation of alloy during hydriding. Al increases hydride formation energy, prolongs cyclic life. Mn decreases equilibrium pressure without decreasing the amount of stored hydrogen. V increases the lattice volume and enhances the hydrogen diffusion. Cu increases high rate discharge performance

Additions to B in AB

A(Ni,M)–xBx: A = La, Mm; M = Co, Cu, Fe, Mn, Al; B = A, Si, Sn, Ge, In, Tl. The metals Al, Si, Sn, and Ge minimize corrosion of the hydride electrode. Ge-substituted alloys exhibit facilitated kinetics of hydrogen absorption/ desorption in comparison with Sn-containing alloys. In, Tl, Ga increase overvoltage of hydrogen evolution (prevent generation of gaseous hydrogen)

Non-stoichiometric alloys

AB±x:A = La, Mm; B = (Ni, Mn, Al, Co, V, Cu). Additional Ni forms separate finely dispersed phase. In MmB. the NiAl-type second phase with high electrocatalytic activity is formed. Alloys poor in Mm are destabilized and the attractive interaction between the dissolved hydrogen atoms increases. Second phase (CeNi), which forms very stable hydride, is present in MmB.. When (–x) < . the hydrogen gas evolution during overcharge decreases

Mixture of two ABtype alloys

Electrode performance can be improved by mixing of two alloys characterized by various hydrogen equilibrium absorption pressures

The most common A-type substitutional elements are rare earth (RE) metals. The effect of the RE elements is largely based on the change of the unit cell volume of the MHx electrode. A decrease in the unit cell volume will lead to an increase in the plateau pressure. In the LaNi5 alloy, the largest effect is known for Ce and Sm [6]. These findings have largely been confirmed for the AB3 and A2B7 alloys, as well [17].

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Tab. 7.5: The properties of the AB2–, AB–, and RE–Mg–Ni-type alloy electrodes in Ni–MHx battery and their chemical composition. Composition

Elements from periodic table and their influence on the properties

ZrM-type alloy (M = V,Cr, Mn, Ni)

Zr(VxNi–x):An increase of the V content increases the maximum amount of absorbed hydrogen. Ni substitution decreases the electrochemical activity of an alloy. Composite alloy, mixture of ZrNi with RNi (R = rare earth element), shows improved characteristics in comparison with the parent compounds

Over stoichiometric ZrB-type alloy

ZrNi.Mn.V.Cr.Xx (X = La or Ce; x = .): La and Ce improve the activation behavior of alloy during chemical pretreatment and increase discharge capacity ZrV.Ni.: The over-stoichiometric alloy, in which some of the V atoms move from B to A sites, shows very high capacity

TiFe-type alloy

TiFe–xPdx: Pd substitution increases both the lattice constant and the catalytic activity, decreases the plateau pressure

Vanadium-based alloys

VTi(Ni–xMx), VTiNixMy: M = A, Si, Mn, Fe, Co, Cu, Ge, Zr, Nb, Mo, Pd, Hf, Ta. The addition of Co, Nb and Ta improves cycling durability. Alloys with Hf (or Nb), Ta and Pd show higher discharge capacity. M = Hf improves the high rate capability. TiNi phase exhibits high electrocatalytic activity

Substitution of A in AB

A = Zr + Ti:TixZr–xNi., V.Mn.Fe.:Electrodes without Ti, or a very low Ti content, exhibit excellent cycling and electrochemical stability. Electrodes with Ti:Zr atomic ratio : display higher storage capacity than that with Ti:Zr = :, and higher electrochemical activity. Amount of C phase decreases with increasing x; at x = . it is pure C phase, at x = . it is % cubic bcc phase +% C phase; bcc phase absorbs more hydrogen than the C hexagonal phase

Substitution of B in AB

Zr.Ti.(V.Ni.M.), Ti.Zr.Ni.V.Mn.Cr.: The Si, Mnsubstituted alloys has C Laves phase structure. The Co, Mosubstituted alloys form Cl Laves phase structure. Mn enhances the activation of an alloy during chemical pretreatment and increases discharge capacity. Co addition leads to the longest cyclic lifetime. Cr addition reduces the discharge capacity but extends cyclic lifetime; Cr controls the dissolution of V and Zr

Nonstoichiometric alloys AB ±x

Zr.Ti.V.Ni., Ti.Zr.Ni.V.Mn., ZrV.NiM: Increasing the Ni content C-type Laves phase preserves an overstoichiometric alloy and discharge capacity increases on increasing the amount of Ni decreases V-rich dendrite formations

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Tab. 7.5 (continued) Composition

Elements from periodic table and their influence on the properties

MgNi-based alloys for electrodes

The use of magnesium in hydrogen storage applications is motivated by its high hydrogen storage capacity as a pure compound (nominally . wt%). It is also an inexpensive and readily available element Ternary alloy: MgNiM (M = Mn, Cu, Fe): Ni substitution with Zn increases the deterioration rate. Substitution of Fe, W, Cu, Mn, Cr, Al, or C instead of Ni decreases both the deterioration rate and discharge capacity. Ni substitution by Se, Cu, Co, or Si decreases both the discharge capacity and cycling life

RE–Mg–Ni-based alloys for Ternary La–Mg–Ni hydrogen storage alloys with composition electrodes (La.Mg.Ni) form a new class of the materials for the negative electrodes in Ni–MHx batteries. The electrochemical discharge capacity of such alloys reaches  mAh/g, which is % greater than that of the commercial AB-type based electrodes. Mg replaces Lain La–Mg–Ni hydrogen storage alloy favorably changes the thermodynamics of the metal–hydrogen interactions allowing improved performance of the advanced metal hydride battery electrodes Substitution of A and B in RE–Mg–Ni

Apart from reports indicating positive role of RE substitutions by other RE on hydrogenation properties, there are also notifications that substitution effect is negligible. For example, Gd was already used to improve the hydrogen storage performance and electrochemical properties of the hydrogen La–Mg–Ni-based alloy electrodes. Co, Mn, Al, and some other elements are used as substitutes for Ni in La–Ni-type system to improve the electrochemical performance of MHx electrodes

B-type additives (Co, Cu, Fe, Mn, Al) are relevant for improving the alloy properties such as: enhancing the corrosion stability, decreasing the plateau pressure, increasing the hydrogen storage capacity, and improving the kinetics of the charge and discharge processes [2, 4]. Currently, AB5 and AB2 alloys are used as positive electrodes in commercial battery cells [1–4, 6, 9, 10]. The AB5 alloys are often intermetallic compounds that contain a RE metal in the composition: La, Ce, Nd, Pr, Y or their mixture, and Ni. The AB2 alloys are often intermetallic compounds of Zr, Ti, or V [1, 4, 5]. Microcrystalline hydride materials have been prepared by arc or induction melting and annealing [3, 4, 13]. The arc melting technique provides two main advantages over other melting processes: great versatility in terms of the types of materials and limited reactivity during melting. The disadvantages of this process are low efficiency and hazardous preparation conditions, as well as (in the case of multicomponent

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alloys) multiple remelting or prolonged annealing at high temperatures. On the other hand, the induction melting process is used for large-scale alloy production. However, low storage capacity by weight or poor absorption-desorption kinetics, in addition to the complex activation procedure, have limited the practical use of microcrystalline metal hydrides. Substantial improvements in the hydriding-dehydriding properties of the metal hydrides could possibly be achieved by the formation of nanocrystalline structures by nonequilibrium processing techniques such as mechanical alloying (MA) or high-energy ball milling (HEBM) [18–24]. To date, various performance parameters of the Ni–MHx batteries such as electrochemical hydrogen storage capacity and cycle life have been investigated and compared with theoretical hydrogen storage capacity (see Tab. 7.6). It is noteworthy that the electrode made from the LaNi5 phase reaches its maximum capacity (360 mAh/g) in the first cycle, but the discharge capacity decreases quickly in the cycles that follow [25, 26]. The properties of the hydrogen host material can be substantially modified by alloying in order to obtain the desired storage characteristics, for example, proper capacity at a favorable hydrogen pressure. For example, it was found that a partial respective replacement of Ni in LaNi5 by small amounts of Al resulted in a prominent increase in the cycle life without causing much decrease in the capacity [27]. Aluminum is believed to concentrate on the grain boundaries, and, in connection with the segregated La forms, the porous oxide layer that protects the material from further corrosion in the KOH electrolyte. On the other hand, cobalt contained in the alloys guarantees long cycle life of the negative electrode [4]. These electrodes usually obtain their maximum capacity within a few charge-discharge cycles, without any special pre-treatment. The substitution of Mn in the LaNi5 alloy revealed its more brittle character, resulting in a shorter cycle life. Generally, in the transition metal sublattice of the LaNi5-type compounds, substitution with Mn, Al, and Co has been found to offer the best compromise between high hydrogen capacity and good resistance to corrosion [4]. Tab. 7.6: Hydrogen storage capacity and theoretical discharge capacity for a few alloys. AB alloys

Hydrogen storage capacity (H/f.u.)

Temperature (K)

Hydrogen pressure (bar)

Theoretical discharge capacity (mAh/g)

LaNi

.







LaNi.Mn.

.







LaNi.AI.

.







LaNiCo

.







LaNiCu

.







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Tab. 7.6 (continued) AB alloys

Hydrogen storage capacity (H/f.u.)

Temperature (K)

Hydrogen pressure (bar)

Theoretical discharge capacity (mAh/g)

ZrV

.



99.5%) in a mixture of 50% Ar and 50% H2 in an atmosphere of 0.1MPa. The arc plasma was generated in a voltage of 25 V and a current of 200–300 A. A mixture of Mg and Cu nanoparticles in a 2:1 molar ratio was then immersed in acetone and later mixed by ultrasonic

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Fig. 8.11: Schematic diagram of the equipment for synthesis of nanoparticles by HPMR; (1) arc melting chamber, (2) water-cooled copper hearth, (3) tungsten electrode, (4) heat exchanger, (5) particle collector, (6) vacuum pump, (7) gas circulation pump, and (8) DC source.

homogenizer for 30 minutes. The metal mixture was pressed into pellets under a pressure of about 75 MPa for 30 s. The compressed pellets were crushed into pieces for alloy synthesis. Thereafter, Mg2Cu alloys were prepared via two different routes. The first method involved annealing to 673 K under argon atmosphere (0.1 MPa), while the other involved annealing under hydrogen atmosphere (4.0 MPa) [47].

Hydrogen Content (wt%)

2.5 b

2 1.5

c

e

d

a 523 K b 548 K c 573 K d 598 K e 623 K a

1 0.5 0

0

200

400

600

800

1000

Time (Sec) Fig. 8.12: Hydrogen absorption curves of the nanostructured Mg2Cu alloy under a starting hydrogen pressure of 3.0 MPa at 523, 548, 573, 598, and 623 K.

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In TEM observation, the average particle size of Mg2Cu prepared by HPMR in argon was about 300 nm and in high pressure hydrogen atmosphere, it was about 100 nm. Figure 8.12 shows the hydrogen absorption of the obtained Mg2Cu alloy annealed under hydrogen atmosphere. It shows that the sample prepared in hydrogen can absorb more hydrogen without any activation process at 523, 548, 573, 598, and 623 K under a starting hydrogen pressure of 3.0 MPa.

8.2.6 Vapor deposition Chemical vapor deposition (CVD) is a chemical process that can produce high purity solid materials. Take magnesium fiber produced by this method as an example [48]. The apparatus consists of an electric muffle furnace and a reactor tube (illustrated in Fig. 8.13). The synthesis was under 5 MPa hydrogen gas using Mg heated at 900 K. After the reactor was cooled naturally, it was detached from the gas line and transferred to a glove box filled with purified Ar. The synthesized sample was found to be pure MgH2. After measuring the hydrogen absorption properties, they analyzed the absorption kinetics of Mg powder and Mg fiber. The result indicated that the initial reaction rate of the Mg fiber was higher than that of the Mg powder. When temperatures are higher and pressures are lower, the reaction is close to the ideal nucleation and growth model, which results in the generation of hydride nuclei on the Mg surface and a dimensional spread.

Fig. 8.13: Scheme of CVD reactor.

Carbon nanotube is another ideal hydrogen storage materials, as nanostructured materials have unique features such as surface adsorption, inter- and intra-grain boundaries, as well as bulk absorption [49, 50]. The study on hydrogen storage of carbon

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nanotubes reported by Dillon et al. [51] lead a worldwide research interest on carbonaceous materials. Microwave plasma-enhanced chemical vapor deposition is a well-established method to grow carbon nanotubes [52]. These can be used to produce hydrogen storage materials, with their advantage of processing at lower temperatures.

8.2.7 Thermal plasma processing As a method for processing powder and nanopowder, thermal plasma synthesis is a relatively new method. This is similar to the thin film deposition, in concept. The momentum of this technique is not only increasing in research, but also in industrial applications. The temperatures in the thermal plasma processes are sufficiently high to vaporize the starting material. The super saturation of vapor species forces the particle to condense, resulting in the production of extremely fine particles by homogeneous nucleation [53]. Çakmak et al. [54] carried out a study on synthesizing Mg-Ti with two different methods: MM and plasma synthesis. The starting powders were Mg (99.5%) and Ti (99.5%) with average particle sizes (d50) of 47 and 32 μm, respectively. The volume ratio of Mg and Ti was 1:9. MM was carried out in a planetary ball mill under argon atmosphere with 1 wt% graphite added as an anti-sticking agent. The milling used 15 mm stainless steel balls and the ball-to-powder ratio was 10:1. The milling speed was 700 rpm and was programmed for a 30 min rest after every 30 min milling. The thermal plasma processing of the powder mixture was carried out in an RF system that consists of a plasma torch, an RF generator, a powder feeding system, reaction chamber connected to a filter system, and a vacuum pump. The plasma torch used a five turn induction coil, a water-cooled ceramic tube with 36 mm inner diameter, and another ceramic tube with a 40 mm diameter that separated plasma gas from the sheath gas. Plasma gas was a mixture of Ar (15–23 slpm) and He (5–8 slpm) and sheath gas was Ar mixed with H2 (4 ~ 5 slpm). Mixed powders were fed with a carrier gas He (6 slpm) and injected into the Ar-He plasma via a water-cooled stainless steel probe. The probe was positioned axially into the torch down to the first turn of the coil. The water-cooled reaction chamber was cylindrical in shape 200 mm in diameter and 1005 mm in length and contained viewing ports for in-situ process observations. The system and the reaction chamber were maintained at a pressure of 0.09 MPa. Where necessary, quenching was done by feeding He gas of 60 slpm at a position ~70 mm below the fifth (last) turn of the coil. The amount of powder that was processed in each experiment was typically 15 g. As a result, powders were collected from the wall of the reaction chamber instead of the filter system. In comparison, it is concluded that 1) MM yields large Mg agglomerates with embedded Ti fragments uniformly distributed within the agglomerates, 2) Mg

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agglomerates that arise as a result of MM are made of coherently diffracting volumes of small size, and 3) Plasma processing yields extremely small Mg powders of less than 100 nm. Both Mg and Ti did not dissolve into each other and plasma processing yields were relatively defect-free crystals.

8.2.8 Spark plasma sintering SPS is also known as field-assisted sintering technique or pulsed electric currentsintering technique. Pressure Pulsed DC power supply

Graphite punch

Powder

Vacuum chamber

Pressure

Graphite die

Fig. 8.14: Spark plasma sintering (SPS).

As shown in Fig. 8.14, the pulsed DC current directly passes through the graphite die and the powder compact. Unlike the conventional hot pressing, where heat is provided by external heating elements, the heat is generated internally in SPS. Thus, the heating or cooling rate is very high, which shortens the sintering process. This fast speed ensures that during the densifying process, the powders are able to maintain nanosize or nanostructure while avoiding coarsening that accompanies standard densification routes. Pei et al. [55] used SPS to prepare a laves phase-related BCC solid solution alloy. V35(Ti, Cr)51(Zr, Mn)14 was prepared by two methods: Arc melting and SPS. For the arc melting method, the alloy elements were arc melted under argon atmosphere based on the nominal chemical composition. For the SPS preparation, V35(Ti, Cr)65 BCC solid solution alloy and ZrMn2 Laves phase-alloy were mixed together evenly in the molar ration of 86:14, and then synthesized by SPS at a temperature of 1000 °C and pressure of 40 MPa. After a close inspection of the microstructures of both alloys and an examination of the hydrogen storage properties, they found that SPS could preserve the alloy with a large hydrogen storage capacity, similar to the BCC-solid solution single

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phase alloy, and maintain good activation and hydrogen absorption kinetic properties in the meantime. They explained that the formation of sintered interface between the Laves phase and the BCC-solid solution during SPS allowed the Laves phase to contribute its intrinsic effects to the activation and kinetic properties of the alloys. Therefore, It is concluded that SPS is an efficient method for the preparation of Laves phase-related BCC-solid solution alloys [56].

8.3 Summary Among the methods we have introduced above, it can be found that many advanced materials were synthesized under nonequilibrium conditions. Rapid solidification from the liquid state, mechanical alloying, vapor deposition, and plasma processing, have all been receiving special attention due to their ability to produce meta-stable phases that undergo an “energizing and quenching” procedure (Fig. 8.15 [28]).

Fig. 8.15: The basic concept of “energize and quench” to synthesize nonequilibrium materials.

It should be noted that most advanced material synthesis methods improve the activation and cycling properties of hydrogen storage materials, as hydrogen storage capacity is inherently limited by the theoretical capacity of the material itself [57]. Nowadays, new methods for the preparation of hydrogen storage materials are developing towards in several directions: 1) Modifying and improving existing processing method as there are many parameters and conditions to adjust in the synthesis process that can influence the results significantly, 2) combining the existing methods in the synthesis process, for example, CS and MM [58], and 3) coating [59]. Work on new methods for developing hydrogen storage materials has never stopped and there is a continuous research to improve and discover. Firstly, new materials which may

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need special preparation method are being proposed as suitable hydrogen storage candidates. Secondly, different methods for synthesizing hydrogen storage materials bring novel physical and chemical properties, which favorably improve the general hydrogen storage performance.

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[18] Wu Y, Han W, Zhou SX. Microstructure and hydrogenation behavior of ball-milled and meltspun Mg-10Ni-2Mn alloys. J Alloys Compd 2008, 466, 176. [19] Lin HJ, Ouyang LZ, Wang H, Liu JW, Zhu M. Phase transition and hydrogen storage properties of melt-spun Mg3LaNi0. 1 alloy. Int J Hydrogen Energy 2012, 37, 1145. [20] Li SL, Chen W, Luo G, Han XB, Chen DM, Yang K, Chen WP. Effect of hydrogen absorption/ desorption cycling on hydrogen storage properties of a LaNi3. 8Al1. 0Mn0. 2 alloy. Int J Hydrogen Energy 2012, 37, 3268. [21] Zhong C, Chao D, Chen Y, Wang W, Zhu D, Wu C. Electrochim Acta 2011, 58, 668. [22] Riabov AB, Denys RV, Maehlen JP. Synchrotron diffraction studies and thermodynamics of hydrogen absorption-desorption processes in La0.5Ce0.5Ni4Co. J Alloys and Compd 2011, 509, S844. [23] Joubert JM, Paul-Boncour V, Cuevas F. LaNi5 related AB5 compounds: structure, properties and applications. J Alloys Compd 2020, 862, 158163. [24] Spodaryk M, Gasilova N, Züttel A. Hydrogen storage and electrochemical properties of LaNi5-xCux hydride-forming alloys. J Alloys Compd 2019, 775, 175. [25] Rożdżyńska-KiełbikB, IwasieczkoW, Drulis H. Hydrogenation equilibria characteristics of LaNi5-xZnx intermetallics. J Alloys Compd 2000, 298, 237. [26] Liang G, Huot J, Boily S. Hydrogen storage in mechanically milled Mg-LaNi5 and MgH2-LaNi5 composites. J Alloys Compd 2000, 297, 261. [27] Deng C, Shi P, Zhang S. Effect of surface modification on the electrochemical performances of LaNi5 hydrogen storage alloy in Ni/MH batteries. Mater Chem Phys 2006, 98, 514. [28] Suryanarayana C. Mechanical alloying and milling. Progress Mater Sci 2001, 46, 1. [29] Zaluski L, Zaluska A, Ström-Olsen JO. Hydrogen absorption in nanocrystalline Mg2Ni formed by mechanical alloying. J Alloys Compd 1995, 217, 245. [30] Simicic MV, Zdujic M, Jelovac DM. Hydrogen storage material based on LaNi5 alloy produced by mechanical alloying. J Power Sources 2001, 92, 250. [31] Zhang Z, Elkedim O, Zhang M. Systematic investigation of mechanically alloyed Ti-Mg-Ni used as negative electrode in Ni-MH battery. J Solid State Electr 2018, 22, 1669. [32] Zhang Z, Elkedim O, Ma YZ. The phase transformation and electrochemical properties of TiNi alloys with Cu substitution: Experiments and first-principle calculations. Int J Hydrogen Energy 2017, 42, 1444. [33] Liang G, Boily S, Huot J, Van Neste A, Schulz R. Mechanical alloying and hydrogen absorption properties of the Mg-Ni system. J Alloys Compd 1998, 267, 302. [34] Shang CX, Bououdina M, Song Y, Guo ZX. Mechanical alloying and electronic simulations of (MgH2+M) systems (M= Al, Ti, Fe, Ni, Cu and Nb) for hydrogen storage. Int J Hydrogen Energy 2004, 29, 73. [35] Zhang Z, Elkedim O, Balcerzak M. Structural and electrochemical hydrogen storage properties of MgTiNix (x= 0.1, 0.5, 1, 2) alloys prepared by ball milling. Int J Hydrogen Energy 2016, 41, 11761. [36] Bai Y, Wu C, Wu F, Yang J, Zhao L, Long F, Yi B. Thermal decomposition kinetics of light-weight composite NaNH2-NaBH4 hydrogen storage materials for fuel cells. Int J Hydrogen Energy 2012, 37, 12973. [37] Hydriding combustion synthesis for the production of hydrogen storage alloy. J Alloys Compd 1997, 252, L1. [38] Wakabayashi R, Sasaki S, Saita I, Sato M, Uesugi H, Akiyama T. Self-ignition combustion synthesis of TiFe in hydrogen atmosphere. J Alloys Compd 2009, 480, 592. [39] Yasuda N, Wakabayashi R, Sasaki S, Okinaka N, Akiyama T. Self-ignition combustion synthesis of TiFe1-xMnx hydrogen storage alloy. Int J Hydrogen Energy 2009, 34, 9122.

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[40] Zhu Y, Liu Y, Gu H, Li L. Structural and hydriding/dehydriding properties of Mg-La-Ni based composites. J Alloys Compd 2009, 477, 440. [41] Liu X, Zhu Y, Li L. Hydriding and dehydriding properties of nanostructured Mg2Ni alloy prepared by the process of hydriding combustion synthesis and subsequent mechanical grinding. J Alloys Compd 2006, 425, 235. [42] Li J, Cheng S, Zhao Q, Long P, Dong J. Synthesis and hydrogen-storage behavior of metalorganic framework MOF-5. Int J Hydrogen Energy 2009, 34, 1377. [43] Wada N. Preparation of fine metal particles by means of evaporation in helium gas. Jpn J Appl Phys 1967, 6, 553. [44] Wada N. Preparation of fine metal particles by means of evaporation in xenon gas. Jpn J Appl Phys 1968, 7, 1287. [45] Wada N. Preparation of fine metal particles by the gas evaporation method with plasma jet flame. Jpn J Appl Phys 1969, 8, 551. [46] Liu T, Shao H, Li X. Synthesis and characteristics of Ti-Fe nanoparticles by hydrogen plasmametal reaction. Intermetallics 2004, 12, 97. [47] Shao H, Wang Y, Xu H, Li X. Preparation and hydrogen storage properties of nanostructured Mg2Cu alloy. J Solid State Chem 2005, 178, 2211. [48] Matsumoto I, Asano K, Sakaki K, Nakamura Y. Hydrogen absorption kinetics of magnesium fiber prepared by vapor deposition. Int J Hydrogen Energy 2011, 36, 14488. [49] Baburaj EG, Froes FH, Shutthanandan V, Thevuthasan S. Interfacial Chemistry and Engineering Annual Report, Pacific Northwest National Laboratory. Oak Ridge, Tenn, USA, 2000. [50] Schulz R, Boily S, Zalusky L, Zaluka A, Tessier P, Ström-Olsen JO. Nanocrystalline materials for hydrogen storage. Innovations Metal Mater 1995, 529. [51] Dillon AC, Jones KM, Bekkedahl TA, Kiang CH, Bethune DS, Heben MJ. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997, 386, 377. [52] Costa PM, Coleman KS, Green ML. Influence of catalyst metal particles on the hydrogen sorption of single-walled carbon nanotube materials. Nanotech 2005, 16, 512. [53] Suresh K, Selvarajan V, Mohai I. Synthesis and characterization of iron aluminide nanoparticles by DC thermal plasma jet. Vacuum 2008, 82, 482. [54] Çakmak G, Károly Z, Mohai I, Öztürk T, Szépvölgyi J. The processing of Mg-Ti for hydrogen storage; mechanical milling and plasma synthesis. Int J Hydrogen Energy 2010, 35, 10412. [55] Pei P, Song XP, Liu J, Zhao M, Chen GL. Improving hydrogen storage properties of Laves phase related BCC solid solution alloy by SPS preparation method. Int J Hydrogen Energy 2009, 34, 8597. [56] Froes FHS, Suryanarayana C, Russell K. Synthesis of intermetallics by mechanical alloying. Mater Sci Engin A 1995, 192, 612. [57] Sakintuna B, Lamari-Darkrim F, Hirscher M. Metal hydride materials for solid hydrogen storage: a review. Int J Hydrogen Energy 2007, 32, 1121. [58] Zhu Y, Yang Y, Wei L, Zhao Z, Li L. Hydrogen storage properties of Mg-Ni-Fe composites prepared by hydriding combustion synthesis and mechanical milling. J Alloys Compd 2012, 520, 207. [59] Rao D, Lu R, Xiao C, Kan E, Deng K. Lithium-doped MOF impregnated with lithium-coated fullerenes: A hydrogen storage route for high gravimetric and volumetric uptakes at ambient temperatures. Chem Commun 2011, 47, 7698.

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9 RE–Mg–Ni hydrogen storage alloys 9.1 Introduction A large number of hydrogen storage materials have been studied to date [1–12]. Among these, magnesium-based materials have been intensively investigated as promising candidates, owing to their high hydrogen storage capacity (7.6 wt%), low cost, low weight, and safety aspects [13]. The advantages of Mg-based materials, however, are countered by their slow kinetics and a relatively high operating temperature. The search for new hydrogen storage materials, both conventional metal hydrides and new materials such as novel compounds with high hydrogen contents and sufficiently fast reaction kinetics of absorption and desorption at moderate temperatures [14, 15] continues. Recently, ternary microcrystalline (RE–Mg)2Ni7 (RE = rare earth metals) compounds have been studied as their hydrogen storage properties are superior to the corresponding binary ABn (n = 2–5) compounds [16–27]. La–Mg–Ni-based compounds with light and not-so-expensive elements emerged as the most promising negative electrode materials for Ni–MHx batteries [28]. The crystal structures of La2–xMgxNi7.0 (x = 0.3–0.6), RE–Mg–Ni, La4MgNi19, La0.7Mg0.3Ni2.8Co0.5-H2, and Ce2Ni7H4.7 materials have been discussed [21, 29–33]. Conventionally, microcrystalline hydride materials have been prepared by arc or induction melting and annealing. However, either low storage capacity weight wise or poor absorption-desorption kinetics in addition to a sophisticated activation procedure have limited the practical use of metal hydrides. Substantial improvements in the hydriding–dehydriding properties of metal hydrides could possibly be achieved by the formation of nanocrystalline structures by nonequilibrium processing techniques such as mechanical alloying (MA) or high-energy ball milling (HEBM) [34–36]. As a nonequilibrium processing method, MA can be used to produce large quantities of materials at a relatively low cost (Fig. 9.1). The MA method consists of repeated fracturing, mixing, and cold welding of a fine blend of elemental particles, resulting in size reduction and chemical reactions. It has already been observed that the kinetic barriers in nanocrystalline hydrides are lower compared to coarse-grained materials. Moreover, the MA process increases the diffusion channels and, naturally, shortens the diffusion paths for the H-atoms. There are numerous possibilities for the design, synthesis, and control of the properties of nanostructured, multicomponent hydrogen storage materials and composites fabricated by the application of the MA process [13]. Nanocrystalline materials exhibit properties that are quite different from both crystalline and amorphous materials due to the structure, in which extremely fine grains are separated by, what some investigators have characterized as “glass-like” disordered grain boundaries. The generation of new metastable phases or materials https://doi.org/10.1515/9783110596281-017

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Fig. 9.1: Scheme for the synthesis of RE–Mg–Ni alloys by mechanical alloying.

with an amorphous grain boundary phase offers a wider distribution of available sites for hydrogen and, thus, totally different hydrogenation behavior. The mechanism of amorphous phase formation by MA results from a chemical solid state reaction, which is believed to be caused by the formation of a multilayer structure during milling [37]. By making metal storage material in the form of nanoparticles, two advantages can be gained, one related to the kinetics and the other to the thermodynamics [13, 38, 39]. The kinetics aspect is obvious; with nanoparticles, the uptake and release kinetics become much faster than it is in the case of the more commonly used microcrystalline particles. The second aspect concerns the possibility of controlling the thermodynamic properties by tuning the particle size. Since the thermodynamic properties of sufficiently small particles change due to, for instance, surface energy and elasticity/plasticity effects, the dissociation pressure for a given material may be varied by utilizing particle size as a tuning parameter. The common elements used to substitute the La in La–Mg–Ni-based alloy system include Pr, Nd, Gd, among others [16, 26]. It has been found that RE partially substituted for La was beneficial in terms of improving the electrochemical properties of the La–Mg–Ni-based alloy electrodes. The authors expect that the addition of Co, Mn, and Al can strengthen the antioxidation and anti-corrosion abilities of the alloy, for it will form a compact oxide film on the surface of the alloy electrode. To confirm this, a systematic investigation was carried out on the effects of the addition of Co, Mn, and Al on the structures and electrochemical hydrogen storage kinetics of the La–Mg–Ni electrode alloys.

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The materials were produced by MA and/or HEBM under argon atmosphere. The chemical composition was optimized in order to obtain the best electrochemical properties (replacement of La by Pr, Nd, Gd and Ni by Co, Mn, Al is essential to improve the stability of the electrode materials in the 6 M KOH solution) and a reduction in price of the hydrogen storage materials (mainly through the replacement of the RE elements with Mg). Moreover, the authors expect better anti-oxidation and anti-corrosion properties of the materials after the addition of Co, Mn, and Al.

9.2 Phase diagram of La–Mg–Ni system The introduction of Mg into the AB2−5-type rare earth-based hydrogen storage alloys facilitates the formation of the (La,Mg)Ni3 phase with the rhombohedral PuNi3-type structure or the (La,Mg)2Ni7 phase with the hexagonal Ce2Ni7-type structure (Fig. 9.2, Tab. 9.1). These phases have long-periodic one-dimensional superstructures, in which the AB5 unit (CaCu5-type structure) and the AB2 unit (Laves structure) are rhombohedrally or hexagonally stacked, with a ratio of n:1 along the c-axis direction, consequently resulting in a greater hydrogen storage capacity [19]. The presence of Mg in the LaNi3 phase and in the La2Ni7 phase prevents the amorphization of their hydride phases, leading to a greater electrochemical capacity.

Fig. 9.2: La–Mg–Ni phase diagram at 500 °C.

The formation of the (La,Mg)Ni3, (La,Mg)2Ni7, and (La,Mg)5Ni19 phases was studied and the formation temperatures were 792, 877, and 930 °C, respectively [40]. The corresponding peritectic reactions were:

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LaNi5 phase + Liquid phase ! ðLa,MgÞ5 Ni19 phase

(9:1)

ðLa,MgÞ5 Ni19 phase + Liquid phase ! ðLa,MgÞ2 Ni7 phase

(9:2)

ðLa,MgÞ2 Ni7 phase + Liquid phase ! ðLa,MgÞNi3 phase

(9:3)

Tab. 9.1: Phases in the La–Mg–Ni system. Phase

Space group

AB

 F 43m

AB

 R3m, P63=mmc

AB

 R3m, P63=mmc

AB

 R3m, P63=mmc

AB

 R3m, P63=mmc

AB

P6=mmm

The peritectic reaction temperatures increased with the increasing [AB5]/[A2B4] ratios for both the binary and ternary phases.

9.3 Thermodynamic and electrochemical properties of (RE,Mg)2Ni7 The thermodynamic and electrochemical properties of (RE,Mg)2Ni7 hydrogen storage materials were found to heavily depend on the alloy components, stoichiometric ratio, and their microstructure. In the case of the (RE,Mg)2Ni7 system, the AB3.0- and AB3.5-type alloys exhibit greater discharge capacity and better electrochemical kinetics. The optimum compositions mainly contain metallic elements of La, Mg, Ni, Co, Mn, and Al. The annealing treatment significantly increases the discharge capacity, improves the cyclic stability, and enhances the high rate dischargeability. The optimized annealing temperature falls between 850 and 950 °C. Investigations indicate that the pulverization of the alloy particles and the oxidation/corrosion of active components during cycling are the two main factors responsible for the fast capacity degradation of the RE–Mg–Ni-based alloy electrodes, and the degradation progress is divided into three consecutive stages – pulverization, La oxidation, as well as oxidation-passivation. Consequently, a decrease in the pulverization of the alloy particles and an increase in their anti-oxidation/corrosion ability are necessary for the cyclic stability improvement. Co and Al produce these effects in the RE–Mg–Ni-based electrode alloys. The influence of selected elements on the properties of the RE–Mg–Ni-based alloys has been summarized in Tab. 9.2.

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Tab. 9.2: The influence of selected elements for the properties of RE–Mg–Ni-based alloys. Element Properties RE

Increase the unit cell volume Improve the plateau properties Increase the discharge capacity, easy activation and good high-rate dischargeability Poor cyclic stability due to the corrosion of La and large unit cell expansion rate

Mg

Eliminate the amorphization of hydrides Decrease the unit cell volume and the stability of hydride Increase the discharge capacity, the high-rate dischargeability and the cyclic stability

Ni

The smallest atomic radius among all transition metals Form unstable hydride (ΔH = − kJ/mol for NiH.) Indispensable element because of its high electrocatalytic activity Form intermetallics, decreases the Me-H bond strength to a suitable level Sensitive to corrosion and oxidation during cycling – forms Ni(OH) Extensively high content leads to decrease in the discharge capacity

Co

Increase the lattice parameters and cell volumes Decrease the plateau pressure and the hysteresis of hydrogen absorption and desorption Does not form unstable hydrides (–ΔH = +  kJ/mol for CoH.) Increase the hydrogen storage capacity Improve effectively the cyclic stability due to the decrease of cell volume change and the increase of the surface passivation Increase the electrochemical kinetics

Al

Increase the lattice parameters and cell volumes Form unstable hydride (ΔH = − kJ/mol for AlH., ΔH = − kJ/mol for AlH) Decrease the plateau pressure, increase the plateau slope and reduce the hydrogen storage capacity Improve significantly the cyclic stability due to the formation of a dense oxide film Decrease the maximum discharge capacity and the high-rate discharge ability

The crystal structure of the binary RE2Ni7 compound is size-dependent. The Ce2Ni7-type structure is stable for larger RE-atomic radii, and the Gd2Co7-type structure is preferred for smaller M-atomic radii. In the case of medium-sized RE-atomic radii, both structures coexist [20]. The structure stabilities of the Ce2Ni7 or Gd2Co7 type in multicomponent (RE,Mg)2Ni7 compounds were studied. Based on the (La1.66Mg0.34) Ni7 compound, Ce, Pr, Nd, Y, Sm, and Gd are used as smaller substitutes for La in order to change the average A-atomic radius. Additionally, Ni is partially replaced with Co and Al to increase the average B-atomic radius. Nickel has the smallest atomic radius out of all the transition metals. The cast and annealed La1.5Mg0.5Ni7 hydrogen storage alloys exhibited a multiphase microstructure, such as the Gd2Co7-, Ce2Ni7-, PuNi3-, CaCu5-, and MgCu4Sntype phase (Fig. 9.3) [41]. The annealing treatment results in the evolution of the phase structure from multiphase to double phase. After the annealing treatment at

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800 °C, the CaCu5- and MgCu4Sn-type phases rapidly disappeared. Additionally, with the increase of the annealing temperature, the abundance of the PuNi3-type phase decreased and disappeared after heat treatment at 900 °C. At the same time, this annealing treatment led to the formation of a large amount of the Ce2Ni7 phase. The Gd2Co7 phase was mainly formed from the cooling process of the molten metallic solution. The (La1.66Mg0.34)Ni7 alloy with the Ce2Ni7-type structure can absorb and desorb hydrogen under moderate conditions [23, 42]. Its hydride formation enthalpy is approx. −31.4 kJ/mol H2, which is close to −30 kJ/mol H2 for the LaNi5 − H2 system. La–Mg–Ni (PuNi3-type) alloy electrodes suffered from a serious degradation of capacity during the charge/discharge cycles. In order to improve the electrochemical performance of the PuNi3-type alloy electrodes, a partial replacement of nickel by transition metals or lanthanum by misch metal was conducted and studied systematically, but the cyclic stability of the alloy electrodes did not improve effectively [43, 44]. Kohno et al. studied the hydrogen storage properties of the ternary La2MgNi9, La5Mg2Ni23, and La3MgNi14 system alloys [17]. Within 30 cycles, the La5Mg2Ni23 alloy electrode showed a perfect cyclic stability. The discharge capacity for this alloy was 410 mAh/g.

Fig. 9.3: The influence of annealing temperature on phase abundance in microcrystalline La1.5Mg0.5Ni7 alloy prepared by induction melting followed by different annealing treatments for 24 h.

Recently, the properties of the La0.63RE0.2Mg0.17Ni3.3Co0.3Al0.1 (RE = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Y, Sc) alloys prepared by induction melting followed

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by the annealing treatment at 900 °C for 8 h [19] were studied. The alloys consisted mainly of the La2Ni7 and LaNi5 phases. The substitution of La with RE metals was favorable for the formation of the Ce2Ni7-type phase. Electrochemical experiments showed that all alloy electrodes exhibited good activation characteristics, the discharge capacity improved with the substitution of La with Ce–Sc, and that the La0.63RE0.2Mg0.17Ni3.3Co0.3Al0.1 alloy had the maximum discharge capacity (400.6 mAh/g). Additionally, the cyclic stability of the alloy electrodes was improved by substitution with Ce–Sc at the La sites, and the capacity retention rate at the 100th cycle was increased by 23.6% for the Gd-substituted alloy. The Ybsubstituted alloy had excellent high rate dischargeability (HRD900 = 92.84%). All experiments implied that the alloy electrode with RE = Gd had excellent overall electrochemical properties. The positive impact of the RE substitution on the cycle stability of the alloy is attributed to the decrease of the cell parameters caused by such a substitution. Additionally, pulverization and oxidation of the alloy during the charging–discharging cycle are the reasons for the capacity decay of the electrode alloy. The cycle stability of the studied alloy increases with the RE substitution. The Ce2Ni7-type phase was the main phase of all the alloys. The experimental results indicate that the cyclic stability of the Ce2Ni7-type phase is much better than that of the Gd2Co7-type phase. Additionally, the use of other rare earths in place of lanthanum in (La,Mg)2Ni7 benefited the formation of the A2B7-type phase, the lattice parameters of the Ce2Ni7-type phase (in the range of 0.53–0.54 nm3) influenced the electrochemical properties, and the rare earth element substitution increased the electrochemical discharge capacity and improved the cycling stability except for the Yb-substituted alloy. The Y substitution exhibited the maximum discharge capacity (400.6 mAh/g), and the capacity retention rate at the 100th cycle of Gdsubstituted alloy increased by 23.6%. The discharge capacities of the Co-substituted La–Mg–Ni system were studied, too [45]. All the La1.5Mg0.5Ni7−xCox (x = 0, 1.2, 1.8) electrodes can be activated during free cycles and have discharge capacities above 390 mAh/g. These electrodes were prepared from a mixture of alloy and carbonyl nickel powders in the weight ratio of 1:2. Nevertheless, the cyclic stability of the hydrogen storage materials becomes worse with the increase in cobalt concentration. Recently, the effects of cobalt content and thermal treatment on the electrochemical behavior of La0.7Mg0.3Ni2.45–xCo0.75+xMn0.1Al0.2 (x = 0, 0.15, 0.3) electrodes have been reported [46]. An enhancement in the cyclic stability of the electrodes was observed in the function of both concentration of Co and annealing temperature. The values of C100/Cmax were 65.5% and 80.5% (C100 is the discharge capacity after 100th charge/discharge cycles, and Cmax is the maximum discharge capacity) before annealing (as-cast) and after annealing (1,173 K/8 h) for La0.7Mg0.3Ni2.15Co1.05 Mn0.1Al0.2, respectively. On the other hand, when Co amount increased from x = 0 to x = 0.3, the discharge capacity of the alloy electrodes decreased.

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The Mn and Al contents on the alloys phase structures and properties of the La–Mg–Ni system were investigated, as well [47, 48]. The partial substitution of Ni by Mn in RENi2.6–xMnxCo0.9 (x = 0–0.9) on their phase composition and microstructure was studied [47]. In the alloys synthesized by induction melting, the following main phases were detected: (La, Ce)2Ni7 (Ce2Ni7-type structure), (Pr, Ce)Co3 (PuNi3type structure), and (La, Pr)Ni5 (CaCu5-type structure). The hydrogen-storage capacity reached (1.04 wt%) for x = 0.45. On the other hand, the discharge capacity of the electrodes initially increased from 205 mAh/g (x = 0.0) to 352 mAh/g (x = 0.45) and finally decreased to 307 mAh/g for x = 0.9. Li et al. studied the properties of La0.7Mg0.3Ni2.55–xCo0.45Alx (x = 0–0.4) synthesized by casting and rapid quenching [48]. Multiphase samples, consisting of (La, Mg)Ni3, LaNi5, and LaNi2 were formed. With an increase of Al content in the alloy, the discharge capacities steadily decrease, while their cycle stabilities significantly increase. Additionally, the rapid quenching process deteriorates the capacity but improves the cycle stability. The substitution of Ni with Co, Mn, Fe, Al, and Cu in La2MgNi9 decreases the hydrogen storage capacity, but, at the same time, increases the hydride stability [49]. Additionally, both the discharge capacity and the high-rate dischargeability of the electrodes decrease; however, the cycling stability of the substituted compositions improves. The effects of the partial Ni replacement by Fe, Mn, and Al on the microstructures and electrochemical properties of La0.7Mg0.3Ni2.55–xCo0.45Mx (M = Fe, Mn, Al; x = 0, 0.1), synthesized by melt spinning, was studied by Zhang et al. [50]. The amount of the LaNi2 phase formed in the samples was strongly correlated with the Al and Mn contents in synthesized compounds. Significant refinement in the as-quenched samples was observed after the substitution of Al and Fe for Ni. Finally, rapid quenching markedly enhances the cycle stabilities of the samples. The published reports suggest that the kinetics of hydrogen absorption and desorption in the nanostructured hydrogen storage alloys can be improved due to large specific surface area and hence, short hydrogen diffusion pathways [51]. It was demonstrated that MA is a powerful method for the synthesis of hydrogen storage nanopowders [34, 36, 52–54].

9.4 (La,Mg)2Ni7-type alloys synthesized by mechanical alloying The innovation in our research is the combination of the method of production (MA) with the optimization of the chemical composition of the (RE-Mg)2Ni7-type materials, which has not been tried in the past and is very promising. This combination of MA and annealing is useful for the preparation of MHx electrode materials. Both the

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production method and the optimization of the chemical composition may significantly change the microstructure and electrochemical properties.

9.4.1 The effect of magnesium The La2–xMgxNi7 alloy powders (x = 0, 0.25, 0.5, 0.75, 1) were prepared by the MA and annealing processes [27]. The behavior of the MA process has been studied by X-ray diffraction. The MA La–Mg–Ni alloys were heat-treated at 700 °C for 0.5 h under high purity argon to form crystal structures (Fig. 9.4). It was found that all of the samples had a multiphase structure. The main phase of all the materials was LaNi5. This phase has hexagonal structure crystallizing in the P6/mmm space group. In materials with Mg content from x = 0 to x = 0.5 the La2Ni7 phase was detected [27]. This minor phase has hexagonal structure crystallizing in the P63/mmc space group, which was obtained by stacking two AB5 slabs with a single A2B4 [17, 54, 55].

Fig. 9.4: XRD spectrum of MA and annealed La1.5Mg0.5Ni7 alloy.

The results of the electrochemical measurements of the La2–xMgxNi7 alloys have been presented in Fig. 9.5. The most important data from the electrochemical measurements have been summarized in Tab. 9.3. The maximum discharge capacity

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increases with increasing Mg content to reach the highest value for x = 0.5. The highest obtained discharge capacity equaled 248 mAh/g. Any further increase in the Mg content caused a huge decrease in the discharge capacity. The same trend was observed in the PCT tests. The discharge capacities of all the studied materials were degraded with the charge/discharge cycles. The reason for this may be partial oxidation of the electrode material or the formation of the stable hydride phases. The degradation of the discharge capacity in the La–Mg–Ni alloys originates in the forming and thickening of the Mg(OH)2 and La(OH)3 surface layers [56]. These oxides hinder the hydrogen atoms from diffusion in or out in alkaline solutions. Another reason can be the expansion and contraction of the cell volume in the process of hydrogenation and dehydrogenation, leading to the pulverization of the alloy particles [57]. Despite a decrease in the capacity of all materials, a partial substitution of La with Mg causes an increase in the cycle stability of the electrodes (with the exception of x = 0.75). The best cycle stability was obtained for the material where x = 1. It was owing to the phase composition of this material. LaMgNi7 is mostly composed of the LaNi5 phase. The LaNi5 phase has a much higher electrochemical cycle stability than the (La, Mg)2Ni7 phase [58].

Fig. 9.5: Influence of Mg on the discharge capacity and cycle stability of La–Mg–Ni nanostructured alloy.

The La1.5Mg0.5Ni7 alloy, characterized by the best discharge capacity, was used to check the impact of the amount of the Ni addition on the electrochemical properties. In contrast to the content of Ni (10 wt%) commonly used in the authors’ laboratory, they used a 300 wt% addition of Ni. The addition of a much higher Ni content caused an obtainment of an electrode that was characterized by a much better cycle stability (improved from 60 to 92%), while maintaining the same maximum discharge capacity. The

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hydrogen storage capacity of the La–Mg–Ni alloys increased with the Mg content to reach a maximum (1.53 wt%) for the La1.5Mg0.5Ni7 alloy.

9.4.2 The effect of rare earth elements Besides the indispensable Mg element, the common elements used to substitute La in the La–Mg–Ni-based alloy system also include other REs. Recently, MA with subsequent annealing has been applied to produce the La1.5–xGdxMg0.5Ni7 (0 ≤ x ≤ 1.5) alloys [59]. Figure 9.6 presents the XRD spectrum of the La1.25Gd0.25Mg0.5Ni7 alloy after 48 h of MA and additional heat treatment at 850 °C for 0.5 h under argon atmosphere. The mass fractions of the phases in the La1.5–xGdxMg0.5Ni7 alloys were established by the Rietveld refinement of the XRD data in the 2θ range between 20° and 80°. The abundance of the A2B7-type phase decreased from 92.3% (La2Ni7 alloy) to 60.0% (Gd15Mg0.5Ni7 alloy). The electrochemical properties of the synthesized material have been investigated. For example, at the 50th cycle, the La1.5Mg0.5Ni7 material showed a much lower reversible electrochemical capacity than the La1.25Gd0.25Mg0.5Ni7 alloy. Additionally, a partial substitution of La with Gd improved the kinetics of the hydrogen absorption. On the other hand, the stability of the electrochemical discharge capacity increased with the increasing value of Gd up to x = 1.0. However, a significant reduction in the discharge capacity was observed for the Gd content above x = 0.25. From the application point of view, only the La1.25Gd0.25Mg0.5Ni7 alloy showed great potential in the future application as electrode material in the Ni–MHx batteries.

Fig. 9.6: The XRD spectrum of the La1.5–xGdxMg0.5Ni7 alloy after 48 h of MA and additional heat treatment at 850 °C for 0.5 h in argon.

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The results of the electrochemical measurements of the La1.5–xGdxMg0.5Ni7 alloys (x = 0, 0.25, 0.5, 1, 1.5) have been presented in Fig. 9.7. The most important data from these studies have been summarized in Tab. 9.3. In general, all the electrode materials displayed the maximum capacities at the third cycle. The highest obtained discharge capacity for the La1.5Mg0.5Ni7 electrode equaled 304 mAh/g.

Fig. 9.7: Discharge capacities as a function of cycle number of electrodes prepared with La1.5–xRExMg0.5Ni7 alloys with their cyclic stabilities.

The cycle stability of the La1.0Gd0.5Mg0.5Ni7 and La0.5Gd1.0Mg0.5Ni7 electrodes increased. In the case of La2–xMgxNi7, the best cycle stability was observed for x = 1 [27]. The reason for that is the phase composition of this material. The major phase LaMgNi7 material is LaNi5, which has a much higher electrochemical cycle stability compared to the (La, Mg)2Ni7 phase [59]. The best capacity retaining rate after the 50th cycle was obtained for the La1.25Gd0.25Mg0.5Ni7 alloys. The co-existence of the La2Ni7 phase with the LaNi5 phase increased the electrode stability [59]. Additionally, the influence of the amount of Pr or Nd in the La–Mg–Ni alloy on the electrochemical properties of the (La,Mg)2Ni7-type materials was studied [60]. It was observed that chemical modification by Pr and Nd could affect the kinetics of the hydrogen absorption process and the maximum hydrogen storage capacity. Moreover, the stability of the electrochemical discharge capacity during cyclic work of the (La,Mg)2Ni7-type alloys was improved by a substitution of La with the Pr or Nd atoms (Tab. 9.3, Fig. 9.7).

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9.4.3 The effect of transition metals The transitional elements can affect the hydrogen storage alloys and influence their electrochemical properties. Therefore, they are often used to substitute Ni, and understanding their effects on the phase evolutions of the La–Mg–Ni-based alloys is of great significance.

Fig. 9.8: The XRD spectrum of the La1.5Mg0.5Ni6.85Al0.15 alloy after 48 h of MA and additional heat treatment at 850 °C for 0.5 h in argon.

Recently, the A2B7-type La–Mg–Ni–M-based (M = Al, Mn) alloys have been synthesized by MA and annealing [61]. Figure 9.8 shows the powder X-ray diffraction patterns of La1.5Mg0.5Ni6.85Al0.15 compound. The synthesized material is composed of the La2Ni7 phase crystallizing with the hexagonal and rhombohedral symmetries. The basic La1.5Mg0.5Ni7 composition contains, additionally, 8 wt. % of LaNi5. In La1.5Mg0.5Ni6.85Al0.15, the AB3-type phase (space group: R-3 m) is formed. Additionally, traces of La2O3 in all La1.5Mg0.5Ni7–xMx (Al and Mn) samples, except La1.5Mg0.5Ni6.85Al0.15, are observed. In La1.5Mg0.5Ni6.8Al0.2, MgO is present. The above-mentioned oxide phases are likely to be formed during the MA process. No pure La, Mg, Ni, Al, or Mn elements are found in the collected XRD patterns. The mean crystallite sizes of produced powders were 37–46 nm, according to XRD analysis. The electrochemical properties of the synthesized materials have been studied. At the 50th cycle, the La1.5Mg0.5Ni6.85Al0.15 material shows a much higher reversible electrochemical capacity than the La1.5Mg0.5Ni7 alloy. The stability of the electrochemical discharge capacity increases with the increasing value of Al and Mn up to x = 0.2 and 0.3, respectively. However, a significant reduction in the discharge capacity was measured for the Al and Mn content above x = 0.25 and 0.5, respectively.

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From the utilization point of view, only the La1.5Mg0.5Ni6.80Mn0.20 alloy shows great potential as hydrogen storage material in future applications.

Fig. 9.9: Discharge capacities as a function of cycle number of electrodes prepared with La1.5Mg0.5Ni7–xMX alloys with their cyclic stabilities.

The most important data from these studies have been summarized in Tab. 9.3 and Fig. 9.9. In general, all the electrode materials exhibited the maximum capacities at the third cycle. The highest obtained discharge capacity for the La1.5Mg0.5Ni6.85Al0.15 electrode equaled 328 mAh/g. Due to partial oxidation of the electrode material or the formation of stable hydride phases, the discharge capacities of all the studied electrode materials were degraded during the charge-discharge cycles. The degradation originates in the formation of the Mg(OH)2 and La(OH)3 surface layers. The layers not only decrease the surface electrocatalytic activity but also prevent the diffusion of hydrogen into the electrode. Another reason may be the pulverization of the electrodes due to the expansion and contraction of the cell volume in the hydrogenation and dehydrogenation cycles. The cycle stability of the La1.5Mg0.5Ni7–xMx (Al x = 0.10, 0.15, 0.20 and Mn x = 0.2, 0.3) electrodes increased. In the case of La2–xMgxNi7, the best cycle stability was observed for x = 1 [27]. The reason for that is the phase composition of this material. The major phase LaMgNi7 material is LaNi5, which has much higher electrochemical cycle stability compared to the (La, Mg)2Ni7 phase [22]. The best capacity retaining rate after the 50th cycle was obtained for the La1.5Mg0.5Ni7–xAl0.2 alloy. It was clearly proved that a constituent change (Ni with Al or Mn) resulted in an increased cycle stability of the MHx electrodes. The effect of the Co content on the electrochemical properties of the mechanically alloyed (La,Mg)2Ni7-type alloys was studied. For example, a partial substitution of Ni with Co in La1.5Mg0.5Ni7 could improve the electrochemical properties of

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9 RE–Mg–Ni hydrogen storage alloys

Tab. 9.3: The electrochemical properties of some of (RE,Mg)2Ni7-type alloys. La1.5Mg0.5Ni7

La0.25Pr1.25Mg0.5Ni7

La1.25Nd0.25Mg0.5Ni7

La1.25Gd0.25Mg0.5Ni7

La1.5Mg0.5Ni6.5Co0.5

La1.5Mg0.5Ni6.85Al0.15

La1.5Mg0.5Ni6.7Mn0.3

Alloy

Discharge capacities at  cycles (mAh/g)















Discharge capacities at  cycles (mAh/g)

–















/

/

/

/

/

/

C

/Cmax

× %

the (La,Mg)2Ni7-type system. All the alloys exhibited excellent performance and obtained their maximum discharge capacities after three charging–discharging cycles. A partial substitution of Ni with Co resulted in an increased cycle stability of the electrodes. The transitional elements can affect the hydrogen absorption/desorption plateau pressure of the hydrogen storage alloys and influence their electrochemical properties. For example, Co is often added to the La–Mg–Ni-based alloys. The formation of the LaNi5 phase over (La,Mg)Ni3 phase improved the cycle stability of the La–Mg–Ni–Co alloys [62]. Additionally, the substitution of Co with Fe facilitated the formation of (La,Mg)5Ni19 and LaNi5 phases over the (La,Mg)2Ni7 phase in the La0.74 Mg0.26Ni2.55Co0.65–xFex (x = 0–0.4) system [63]. On the other hand, it was found that the addition of Si resulted in a decrease of the (La,Mg)2Ni7 phase and an increase of the LaNi5 phase owing to the preference of Si to form the LaNi5 phase in the Si added La– Mg–Ni–Co-based electrode alloys [64, 65].

9.5 Encapsulation of La–Mg–Ni-type nanocrystalline hydrogen storage alloy with Ni coatings A 20–50 μm fraction of the nanocrystalline La1.5Mg0.5Ni7 alloy particles was encapsulated with the 0.29 μm thick magnetron-sputtered Ni [66],. The electrochemical charge/discharge multicycling of the powder composite electrodes was carried out. The corrosion protection of the La1.5Mg0.5Ni7 nanomaterial by the magnetron-sputtered Ni films depends on the average film thickness. The relatively thick (0.29 μm)

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sputtered Ni film limits the corrosion degradation and stabilizes the exchange current density of the H2O/H2 system (Fig. 9.10). The particle modification by Ni encapsulation does not affect the hydrogen diffusivity. The effective diffusion coefficient of hydrogen for the La1.5Mg0.5Ni7 nanocrystalline material is close to 2 × 10−10 cm2/s, irrespective of the surface modification.

Fig. 9.10: The cross-section of glass sphere with deposited magnetron-sputtered Ni layer (left) with its surface (right).

Additionally, a partial substitution of Ni with Co in the La1.5Mg0.5Ni7 nanocrystalline alloy increases the electrode discharge capacity (up to 14%) and enhances its activation [67]. On the other hand, the Co addition deteriorates the corrosion degradation behavior, yet the corrosion resistance can be improved (≈30%) by the application of an amorphous nickel coating. Additionally, the Co-modified La1.5Mg0.5Ni7 nanocrystalline alloy exhibits a 15% greater exchange current density in the H2O/H2 system and improves the hydrogen diffusivity compared to the reference, nanocrystalline Co-free La1.5Mg0.5Ni7 hydrogen storage material. The La2Ni7-type alloy with nanostructure covered with amorphous nickel has good capacity and excellent activation understood as better sorption kinetics than that of the reference LaNi5 alloy. Although the corrosion resistance and the life cycle of the Co-modified material is slightly worse than that of the Co-free reference, it can be successfully improved by thin amorphous Ni layers deposited by magnetron sputtering [67]. Though the Gd-modified La1.5Mg0.5Ni7 nanocrystalline alloy reveals a 10% lower exchange current density in the H2O/H2 system compared to the parent alloy, this property can be considerably improved (by >50%) by Ni particle encapsulation [68]. The hydrogen diffusivity of the Gd-modified material is faster compared to the parent material but is slowed down by the Ni coating.

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9.6 Electronic properties in the La2–xMgxNi7 alloys In order to optimize the selection of the intermetallic compounds for the Ni-MHxtype battery application, a better understanding of the effect of each alloy constituent on the electronic properties of the material is crucial [15]. Semi-empirical models have shown that the energy of the metal hydrogen interaction depends both on the geometric as well as electronic factors [69–72]. The effect of the substitution of La with Mg on the electronic properties in the La2–xMgxNi7 phases was studied [71]. The synthesized samples have a multiphase character and consist mainly of Ce2Ni7-type and Gd2Co7-type structures with minor contributions of LaNi5 and La2O3. As mentioned earlier, the maximum discharge capacity for the La1.5Mg0.5Ni7 sample was identified. This composition was a subject of the XPS investigations covering the valence band and reference samples of La, Mg, Ni, and La2Ni7. The main contribution from the Ni electrons to the valence band was established. The observed narrowing of the La1.5Mg0.5Ni7 valence band in respect to La2Ni7 is apparently caused by an increase in the lanthanum oxide phase concentration. The ab initio analysis was intended to investigate the energetic stability of the La2–xMgxNi7 phases and to provide information on the La1.5Mg0.5Ni7 valence band. The Gd2Co7-type phases are only slightly more stable than the Ce2Ni7-type ones for both the La2Ni7 and La1.5Mg0.5Ni7 phases. For the modeling of the Mg substitution in the La1.5Mg0.5Ni7 phases, coherent potential approximation was used. The Mg atoms prefer to occupy the La 4f1 sites for the Ce2Ni7-type phase and the La 6c2 positions for the Gd2Co7-type phase. The stability analysis was followed by the valence band investigations (primarily to interpret the experimental XPS spectra). They confirmed that the La1.5Mg0.5Ni7 valence band consisted mainly of the contribution from the Ni electrons. The Mg substitution in place of La in La2–xMgxNi7 only slightly depopulated the valence band. A small shift of the Fermi level might affect the conductivity of the material. The effect of Gd and Co content on the electronic properties of the La1.5Mg0.5Ni7 alloys was also investigated [72]. Partial substitutions resulted in an increase of the cycle stability of the metal hydride electrodes. Two optimal compositions (La1.25 Gd0.25Mg0.5Ni7 and La1.5Mg0.5Ni6.5Co0.5) in terms of electrochemical properties were subsequently investigated by the X-ray photoelectron spectroscopy. Experimental analysis of the valence band was further extended by the density functional theory calculations. Analysis of the theoretical valence bands focused on the van Hove singularity observed close to the Fermi level. The presence of this narrow peak is suggested as the origin of the variations in the electrochemical properties for the systems with the Gd and Co substitutions. The example calculations of the Co site preferences in the La1.5Mg0.5Ni6.5Co0.5 phase indicated a relation between the type of the occupied site and the position of the van Hove peak.

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9.7 RE–Mg–Ni-based alloy electrodes A lot of RE–Mg–Ni-based electrode alloys have been synthesized for the Ni–MHx batteries. A few of them have been shown in Tab. 9.4 [19–27]. Tab. 9.4: Some RE–Mg–Ni-based alloy electrodes with their structures and discharge capacities. Alloy

Structure

Discharge capacity (mAh/g)

LaMgNi

PuNi-type



La.Mg.Ni.

(La,Mg)Ni



Ml.Mg.Ni. (Ml means La-rich mish metal)

amorphous



La.Mg.Ni.

CeNi-type, LaNi, LaMgNi



La.Mg.Ni.

(La,Mg)Ni, LaNi



La.Mg.Ni

(La,Mg)Ni, LaNi



La.Mg.Ni.Co .

PuNi, CeNi



Ml.Mg.Ni.Co.Al.

LaNi, LaNi



La.Mg.Ni.Mn .Co.Al.

(La,Mg)Ni, LaNi



La.Nd.Mg.Ni.Co.Al.

LaNi, LaNi, LaNi



La.Mg.Ni.–% Ti.Zr. V.Cr .Ni.

(La,Mg)Ni, LaNi, LaNi, bcc



9.8 Conclusions Recently, research was directed to the new generation hydrogen storage (La, Mg)2Ni7 materials. Many different methods of synthesis of nanostructured hydrogen storage materials are available. The mechanical processes include MA or HEBM. MA is an effective process to produce (La–Mg)2Ni7 alloys with reduced crystallite sizes and fresh surfaces. Great hydrogen capacity, moderate hydrogen equilibrium pressure, as well as light and less expensive elements makes them remarkable from the economics point of view. Microstructural La–Mg–Ni compounds with the A2B7-type structure are already being used in novel Ni–MHx batteries. The La–Mg–Ni-system A2B7-type alloys are considered to be the most promising candidates as negative electrode materials for the Ni–MHx rechargeable battery owing to their higher discharge capacities (above 300 mAh/g) and lower production costs.

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The nanocrystalline metal hydrides offer a breakthrough in terms of practical applications. The authors expect that Mg-containing hydride nanomaterials modified by partial La and Ni substitutions, with particles encapsulated with thin catalytic films will appear to be highly efficient ones in both future Ni-MHx batteries and efficient stationary hydrogen storage systems. The electrochemical properties of these materials can be improved by the introduction of metastable phases and the formation of nanocrystalline structures. It can be achieved through the application of a nonequilibrium processing technique, for example, mechanical milling/alloying. The effect of the different metals on the phase compositions as well as properties of this system was studied. In the La–Mg–Ni-type system, various crystalline phases could be formed, among which (La, Mg)Ni3, (La, Mg)2Ni7, and (La, Mg)5Ni19 were observed. They are composed of the [A2B4] and [AB5] subunits alternatively stacking along the c axis. It seems that the increase in the number of components in the hydrogen absorbing alloys has become a major development trend in improving hydrogen storage properties. It is believed that element substitution and nanostructuring are effective methods for improving the overall properties of the hydrogen storage alloys. Generally, the improvement of the properties of the La–Mg–Ni–M-based hydrogen storage alloys discussed in this chapter is a function of the phase composition and final microstructure of the synthesized hydrogen storage material. In the case of the La–Mg–Ni series hydrogen-storage alloys, a partial replacement of lanthanum with smaller RE, and nickel with some transition metals, can improve the final properties of the A2B7-type phase. An additional increase of hydrogenation properties of these hydrogen storage materials can be established by encapsulation of alloy particles with thin amorphous nickel coating.

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La0.7Mg0.3Ni2.45–xCo0.75+xMn0.1Al0.2 (x = 0.00,0.15,0.30). Int J Hydrogen Energy 2007, 32, 3387–3394. Jiang L, Li G, Xu L, Jiang W, Lan Z, Guo J. Effect of substituting Mn for Ni on the hydrogen storage and electrochemical properties of ReNi2.6–xMnxCo0.9 alloys. Int J Hydrogen Energy 2010, 35, 204–209. Li BW, Ren HP, Zhang YH, Dong XP, Ren JY, Wang X. Microstructure and electrochemical performances of La0.7Mg0.3Ni2.55–xCo0.45Alx (x = 0–0.4) hydrogen storage alloys prepared by casting and rapid quenching. J Alloys Compd 2006, 425, 399–405. Liao B, Lei YQ, Chen LX, Lu GL, Pan HG, Wang QD. A study on the structure and electrochemical properties of La2Mg(Ni0.95M0.05)9 (M = Co, Mn, Fe, Al, Cu, Sn) hydrogen storage electrode alloys. J Alloys Compd 2004, 376, 186–195. Zhang YH, Li BW, Ren HP, Cai Y, Dong XP, Wang XL. Cycle stabilities of the La0.7Mg0.3 Ni2.55–xCo0.45Mx (M = Fe, Mn, Al; x = 0, 0.1) electrode alloys prepared by casting and rapid quenching. J Alloys Compd 2008, 458, 340–345. Varin RA, Czujko T, Wronski ZS. Nanomaterials for Solid-State Hydrogen Storage. Springer Science+Business Media, LLC, 2009. Jurczyk M, Smardz L, Makowiecka M, Jankowska E, Smardz K. The synthesis and properties of nanocrystalline electrode materials by mechanical alloying. J Phys Chem Sol 2004, 65, 545–548. Majchrzycki W, Jurczyk M. Electrode characteristics of nanocrystalline (Zr,Ti)(V,Cr,Ni)2.41 compound. J Power Sources 2001, 93, 77–81. Jurczyk M, Jankowska E, Nowak M, Jakubowicz J. Nanocrystalline titanium type metal hydrides prepared by mechanical alloying. J Alloys Comp 2002, 336, 265–269. Chu HL, Qui SJ, Tian QF, Sun LX, Zhao Y, Xu F, et al. Effect of ball-milling time on the electrochemical properties of La-Mg-Ni-based hydrogen storage composite alloys. Int J Hydrogen Energy 2007, 32, 4925–4932. Li Y, Han D, Han S, Zhu X, Hu L, Zhang Z, et al. Effect of rare earth elements on the electrochemical properties of La-Mg-Ni-based hydrogen storage alloys. Int J Hydrogen Energy 2009, 34, 1399–1404. Dornheim M, Doppiu S, Barkhordarian G, Boesenberg U, Klassen T, Gutfleich O, et al. Hydrogen storage in magnesium-based hydrides and hydride composites. Scr Mater 2017, 56, 841–846. Zhang F, Luo Y, Wang D, Yan R, Kang L, Chen J. Structure and electrochemical properties of La2–xMgxNi7.0 (x=0.3–0.6) hydrogen storage alloys. J Alloys Compd 2007, 439, 181–188. Nowak M, Balcerzak M, Jurczyk M. Hydrogen storage and electrochemical properties of mechanically alloyed La1.5–xGdxMg0.5Ni7 (0≤x≤1.5). Int J Hydrogen Energy 2018, 43, 8897–8906. Balcerzak M, Nowak M, Jurczyk M. The influence of Pr and Nd substitution on hydrogen storage properties of mechanically alloyed (La,Mg)2Ni7-type alloys. J Mater Eng Perform 2018, 27, 6166–6174. Nowak M, Balcerzak M, Jurczyk M. Effect of substitutional elements on the thermodynamic and electrochemical properties of mechanically alloyed La1.5Mg0.5Ni7–xMx alloys (M= Al, Mn). Metals 2020, 10, 578. Liu Y, Pan H, Gao M, Li R, Sun X, Lei Y. Investigation on the characteristics of La0.7Mg0.3 Ni2.65Mn0.1Co0.75–x (x = 0.00–0.85) metal hydride electrode alloys for Ni/MH batteries. J Alloys Compd 2005, 387, 147–153. Hao J, Han S, Li Y, Hu L, Zhang J. Effects of Fe-substitution for cobalt on electrochemical properties of La-Mg-Ni based alloys. J Rare Earth 2010, 28, 290–294. Zhang YH, Chen LC, Yang T, Xu C, Ren HP, Zhao DL. The electrochemical hydrogen storage performances of Si-added La-Mg-Ni-Co-based A2B7-type electrode alloys. Rare Metals 2015, 34, 569–579.

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[65] Yanghuan Z, Ying C, Baowei L, Huiping R, Zhonghui H, Dongliang Z. Electrochemical Hydrogen Storage Characteristics of the as-cast and Annealed La0.8–xNdxMg0.2Ni3.35Al0.1Si0.05 (x = 0–0.4) Electrode Alloys. Rare Met Mater Eng 2013, 42, 1981–1987. [66] Dymek M, Nowak M, Jurczyk M, Bala H. Encapsulation of La1.5Mg0.5Ni7 nanocrystalline hydrogen storage alloy with Ni coatings and its electrochemical characterization. J Alloys Compd 2018, 749, 534–542. [67] Dymek M, Nowak M, Jurczyk M, Bala H. Electrochemical characterization of nanocrystalline hydrogen storage La1.5Mg0.5Ni6.55Co0.5 alloy covered with amorphous nickel. J Alloys Compd 2019, 780, 697–04. [68] Dymek M, Nowak M, Jurczyk M, Bala H. Electrochemical behaviour of a nanostructured La1.25Gd0.25Mg0.5Ni7 hydrogen storage material modified with magnetron sputtered nickel. J Electrochem Soc 2019, 166, 1393–1399. [69] Szajek A, Jurczyk M, Rajewski W. The electronic and electrochemical properties of the LaNi5, LaNi4Al and LaNi3AlCo systems. J Alloys Compd 2000, 307, 290–296. [70] Jurczyk M (ed). Handbook of Nanomaterials for Hydrogen Storage. Pan Stanford Publisher, 2018. [71] Werwinski M, Szajek A, Marczynska A, Smardz L, Nowak M, Jurczyk M. Effect of substitution La by Mg on electrochemical and electronic properties in La2–xMgxNi7 alloys: A combined experimental and ab initio studies. J Alloys Compd 2018, 763, 951–959. [72] Werwinski M, Szajek A, Marczynska A, Smardz L, Nowak M, Jurczyk M. Effect of Gd and Co content on electrochemical and electronic properties of La1.5Mg0.5Ni7alloys: A combined experimental and first-principles studies. J Alloys Compd 2019, 773, 131–139.

Dina Lanzi, Cosma Panzacchi, Christian Coti, Donatella Barbieri, Pierpaolo Ferraro, Francesco Maria Augusto Ghidoni, Matteo Scapolo, Sara Vassallo

10 Hydrogen storage 10.1 Role of storage: key element in the energy transition Currently, natural gas is considered the ideal ally of renewable energy in the transition to a decarbonized and low-emission energy system. In fact, for the same amount of energy produced, it generates a lower quantity of carbon dioxide than other fossil sources (25–30% less than petroleum products and 40–50% less than coal). In addition, natural gas is becoming a renewable source thanks to the ever-increasing contribution of biomethane and hydrogen [24, 25]. In addition to the clear environmental advantage, the use of hydrogen and biomethane could allow the exploitation of existing infrastructure, ensuring significant economic savings and a widespread presence in the territory. The key to making hydrogen the ideal and clean carrier to support the energy system is the development of large-scale, low-cost energy storage capacities. Currently, large-scale hydrogen storage is potentially the best economic opportunity for energy storage; the alternatives, in particular, traditional batteries are characterized by higher costs. To date, underground storage of hydrogen in salt caverns costs 1 identifies a gas that compresses less than the ideal one and a value of z < 1, a gas that compresses more. The behavior of hydrogen and methane – the main component of natural gas – is described in the following chart, where the yellow line identifies the trend of z for hydrogen, and the blue line, the evolution of methane. It is immediately noticeable that the difference between the two gases is considerable and increases as the pressure rises (Fig. 10.5). T = 50 ºC

1.2

xH2 = 0%, xCH4 = 100% xH2 = 20%, xCH4 = 80%

Z-factor (–)

1.1

xH2 = 100%, xCH4 = 0%

1

0.9

0.8

0

50

100 Pressure (bar)

150

200

Fig. 10.5: Variation of the z factor when a mixture of methane and hydrogen varies in composition.

10.2.3.2 Dissolution The solubility of hydrogen and methane at equal thermodynamic conditions is very similar. Considering a pressure of 100 bar and a temperature of 50 °C (323.15 K), the solubility coefficient is equal to about 0.07 mol/kg of water for hydrogen, while it is equal to approximately 0.08 mol/kg of water for methane. The loss of a small quantity of hydrogen (1.5–2.2%) injected during the first storage cycle by dissolution in the fluid already present in the rock (formation fluid) and in the cap rock is, therefore,

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predictable. The extent of this phenomenon is, however, limited and by virtue of the condition described above, comparable to what happens with natural gas.

10.2.3.3 Diffusion The gaseous mixture is contained in the porous matrix of the cap rock, which constitutes the storage. The permeability of these rock formations to hydrogen is comparable to that of natural gas and is, therefore, not considered critical. However, there is another mechanism by which hydrogen could escape through the cap rock and that is diffusion. This phenomenon is controlled by the solubility and saturation of gas in water. Although the solubility of hydrogen and methane is similar, the concentration of hydrogen in the formation water would be zero at the beginning of the storage operations. The loss of a small percentage of hydrogen by diffusion in the cap rock is anticipated and, this quantity is not expected to be significant because it would be a very slow phenomenon, similar to what takes place for natural gas [18].

10.2.3.4 Geochemical and microbiological reactions The hydrogen molecule is highly reactive and can react with a wide variety of chemical compounds. The most common reagents are sulfates, carbonates, and oxides, which are found within the formation and cap rock. Generally, these reactions are not activated at the typical reservoir temperatures (40–50 °C); however, there are microorganisms that can act as catalysts. The main reactions that can be activated in the reservoir by the effect of bacteria are the following: – Methanogenesis 4H2 + CO2 ! CH4 + 2H2 O 3H2 + CO ! CH4 + 2H2 O – Acetogenesis 2CO2 + 4H2 ! CH3 COOH + 2H2 O – Sulfate reduction SO4 2 − + 5H2 ! H2 S + 4H2 O – Iron reduction 3Fe3 + 2 O3 + H2 ! 2Fe 2 + 3 O4 + H2 O

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Several international studies [12, 21] have highlighted that metabolic processes are one of the main aspects to be investigated in the design of an UHS. This is also due to the fact that these microbial activities are highly site-specific. In fact, these reactions are affected by the microbial environment present in the storage by the mineralogy of porous rocks (e.g., presence or absence of sulfates) and by the composition of the stored gases (e.g., presence or absence of CO2 and H2O). In order to establish the compatibility of a given site with the hydrogen storage, microbiological communities present in the reservoir as well as mineralogical, petrographic, and hydrochemical characteristics of the rock and reservoir fluids must be studied [6, 17, 22].

10.2.3.5 Well and surface facilities The wells used for storage, similar to those completed for the primary production of hydrocarbons, are constituted by a series of elements that act as hydraulic barriers, preventing gas from reaching the surface in an undesired or uncontrolled way. In particular, the following elements can be identified: – Wellhead seals – Threaded connections of the production tubing – Packers – Cementation behind the production casings The hydrogen storage systems in salt caverns have already been operational for decades, therefore, the experience acquired on well materials and construction schemes can be used for those types of storage – depleted and aquifers – not yet used for hydrogen. Therefore, the realization of new wells does not present significant issues, while for retrofitting of existing infrastructures, site-specific analysis are required, taking into account the materials to be used. For example, steel which constitute the production tubing and the wellhead may be subject to hydrogen embrittlement, which are, however, excluded from this literature for a gaseous mix with low hydrogen percentages (

Pressure (bar)



–

–

Confidential

Operator First year of storage 



Table 10.2: Gas mixtures with hydrogen content (10–60%) storage solutions currently tested and type of sites. Typology

% H

Pressure (bar)

Bad Lauchstädt, Germania

Salt cavern

–



Kiel, Germania

Salt cavern

–

–

Beynes, Francia

Aquifer



Ketzin, Germania

Aquifer



Lobodice, Repubblica Ceca

Aquifer





Diadema, Argentina

Depleted field





Standards are not yet defined for evaluating the potential of a given salt deposit for the construction of a storage cavity and the criteria used depend on what the authors consider. In general, however, four criteria are used: – Adequate vertical thickness and lateral extension, which affect the size of the cavity to be created. Likewise, adequate salt thickness is required above, below, and at the sides of the cavity to ensure stability and gas tightness. Typical dimensions of a single cavity vary from 105 to 106 m3 [10]. – Adequate depth, which then affects the pressure at which the storage facility will be operated. The pressure range is defined in relation to the lithostatic load acting on the cavity; the maximum value established is 80% [9] of the lithostatic load and the minimum value at 24%. These values have been defined empirically and come from the experience of managing natural gas storage. Exceeding the maximum value could result in fracturing of the cavity walls, as a result of the pressure acting on them. The lower limit, on the other hand, is necessary to guarantee the stability of the cavity and maintain the performance of injection and delivery of the gas. In Germany, the cavities were created at depths between 500 and 2,000 m and extended up to 400 m in height. Depending on the cavity depth, they are operated up to a maximum of 200 bar. Another operational constraint imposed by the

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Fig. 10.9: Schematic representation of the distribution of saline deposits in Europe.

lithostatic load, and therefore by the depth, is the emptying/filling speed of the cavity. Too rapid operations could in-fact lead to non-optimal redistribution of the lithostatic loads with consequent problems of integrity. Finally, the depth, together with the withdrawal/injection reservoir cycles also determines the temperature, which in turn influences the mechanical stress to which the cavity walls are subjected. – Low content of insoluble compounds such as anhydrites, clays, and gypsum. During the creation of the cavity by leaching, the insoluble compounds are deposited at the bottom of the cavity, reducing its net volume available for storage. – Location of the deposit. The operations required to create the cavity by leaching produce significant quantities of brine (highly salty water), which, if not properly disposed of, can have an environmental impact. It is therefore necessary to establish a disposal area near the place chosen for the construction of the cavity. This disposal area can be a sea, deep saline aquifers, or industrial activities for which brine is a raw material.

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10.2.3.7 Main recommendations for storage in salt caverns The factors of calorific power, compressibility and impact on well, and surface facilities, previously considered, can be generalized for storage in saline cavities. During the construction phase, it is necessary to consider that the possibility of realizing saline cavities is strictly conditioned by the quality of the salts and by the structural characteristics of the deposit, whose presence at a national level must be verified on a case-by-case basis. An exploratory phase is, therefore, necessary to identify the salt deposits suitable for creating the cavity. In depleted oil and gas fields, this analysis has already been done and it is not necessary to repeat it. In addition, the data collected during the entire primary exploitation phase provides a deep knowledge of the structure. Finally, the creation of the cavities leads to the production of considerable quantities of brine (up to 8 times the volume of the cave to be built), which must be properly disposed of: the economic evaluation of this phase is fundamental and can affect the economic feasibility of storage. In the operational phase, this type of storage is characterized by great flexibility in injection/withdrawal operations, which allows replacing the gas stored completely several times during the year at high flow rates. At the same time, it must be taken into account that the stored volumes are generally of orders of magnitude lower than what is possible in aquifers and in depleted reservoirs. Finally, it should be emphasized that no metabolic processes using hydrogen take place in salt caverns: brine with high concentrations of dissolved salts are in-fact environments that prevent bacterial proliferation. These processes are, instead, still under analysis for depleted and aquifers storage.

10.2.3.8 Lined rock cavern LRC storage is a recent technique. It was used for the first time in Sweden in 1997 and only few field applications exist. This technique involves storing the gas in artificial caves, supported and lined with steel and concrete structures. The aforementioned Swedish example, at Skallen, has a volume of approximately 40,000 m3 and operating pressures between 10 and 230 bar. It is designed to support frequent operations of filling/emptying. Other examples of this technique are the storage at Haje in Czech Republic and at the Jurong Island in Singapore. This type of storage requires mine excavation operations and a careful coating of cavity walls in order to make them gas-tight. In addition, the volumes of gas stored are several orders of magnitude lower than those of other types of underground storage. At the same time, they represent a valid alternative to surface storage as their visual impact is practically zero and they also allow for the recovery of existing infrastructures such as abandoned mines.

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Fig. 10.10: Schematic model of a cavern used for storage.

10.3 Conclusions and recommendations Hydrogen could play a fundamental role as an energy vector in the transition path from fossil to renewable sources. The latter are characterized by strong fluctuations in supply which generally do not match with the typical fluctuations in demand. With this premise, storage contributes significantly to the stabilization of the energy market and security of supply. Large-scale storage, in particular, is potentially the best economic opportunity for energy storage. This is also because other options, such as batteries, involve higher costs. There are several large-scale storage options for hydrogen, both on the surface and at the underground level. In particular, at the underground level, we can distinguish two different types of storage linked to the nature of the rock: – Storage in geological units with a porous rock matrix: Depleted oil or gas reservoirs and aquifers – salt cavities and LRCs In the development of a new storage site, the choice between the various types listed above must necessarily take into account the availability of suitable geological structures, the needs of injection and withdrawal, and the market demand. Regarding storage in underground geological units with a porous matrix, this technology has not yet been exploited for hydrogen storage, which is why studies and in-depth analysis are necessary. In particular, the site-specific conditions

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related to mineralogy, petrophysics, and microbiology characteristic of each reservoir in relation to mixtures of methane and hydrogen at different percentages must be analyzed, up to the presence of pure hydrogen. If these studies confirm the possibility of storing hydrogen in these formations, they would be able to meet the different needs of the market and of end-users, characterized by high flexibility of management and volumes so as to ensure the safety of the energy system, as demonstrated with natural gas storage. With regard to salt caverns, different international experiences proved their compatibility with hydrogen storage. At the same time, this type of storage is characterized by certain limits concerning, for example, storable gas quantities and national scale availability of salt deposits suitable for the creation of cavities to be used as storage.

10.4 Above ground storage solution 10.4.1 Physical based 10.4.1.1 Compressed gas Hydrogen storage in compressed gas cylinders is the most common above ground storage system today. P range (bar)

– bar

Capacity (kg)

Modular

Temperature

Ambient conditions

Capex

 €/kg H

Volumetric density

.– kg H/m (function of P)

Typically, hydrogen is stored in steel or composite materials cylinders, at a pressure between 200 bar and 300 bar in industrial applications, while higher pressures of up to 700 bar can be reached in the hydrogen refueling stations for mobility applications. The maximum pressure that can be reached with today’s technologies is up to 1,000 bar. The main advantage of increasing the storage pressure is that the hydrogen volumetric density increases as well, so that a higher amount of energy can be stored in the same volume. In Fig. 10.11, the volumetric density trend is shown as a function of the storage pressure. As it can be observed, at an atmospheric pressure, the H2 density is 0.08 kg/Sm3, while by increasing the pressure, it reaches a value of 20 kg/Sm3 at 300 bar and 40 kg/Sm3 at 700 bar.

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Considering the costs of a storage system, the capital cost of the pressure cylinders is around 500 €/kg of hydrogen stored and this reference value can be found both in literature and in common market products quotations. The gas cylinders generally have an infinite design life and need periodic retesting, after a given number of years of operation, which is function of the cylinder-specific material. H2 density (Kg/Sm3) 50 40 30 20 10 0

0

100

200

300 400 Pressure (bar)

500

600

700

Fig. 10.11: Hydrogen density as function of pressure (T = 25 °C).

The compressed hydrogen gas storage is a modular solution and is easily scalable. Typically, hydrogen is stored in bundles of cylinders; each bundle has a capacity of up to 100 kg of hydrogen. In order to increase the storage capacity, different bundles can be assembled, generally in containerized solutions, in order to reach the required capacity. The main drawback of this approach is the increase of the storage footprint, which can become consistent. In order to provide an order of magnitude, a bundle of cylinders with a capacity of 90 kg of hydrogen at a pressure of 200 bar has a footprint of about 5 m2. When high volumes of hydrogen storage are needed and there is not enough ground available, the technology providers also offer solutions where the containers can be stacked one on top of the other, thus halving the system footprint. It is important to notice that, given the high pressure of the storage system, a gas compression is needed upstream of the storage. This is an element to be kept in mind, since compression has an impact on the total costs (both capital and operative costs) of the hydrogen storage system. In addition, these costs increase with the outlet compression pressure when inlet pressure is a constant. The inlet pressure has an impact on the compression cost too. In fact, a lower inlet pressure requires a higher compression energy, and thus a higher cost. Another element to be taken into account is the power consumption for compression. From literature review, an energy consumption of 2,4 kWh/kg of hydrogen is estimated [26] for a pressure increase from 15 bar to 200 bar. As for the costs, the energy consumption increases with increase in the hydrogen pressure at the outlet of the compressor and with decrease in the inlet pressure of hydrogen.

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10.4.1.2 Liquid hydrogen LH2 is a valid option for hydrogen storage and distribution. In this case, a higher energy density can be reached with respect to the compressed gas; in-fact LH2 has a density of 70 kg/m3 at atmospheric pressure.

P range (bar)

 bar

Capacity (kg)

,

Temperature

− °C

Capex

,–, €/kg H

Volumetric density

 kg H/m

On the other hand, the liquefaction process requires high energy consumption, given the extremely low boiling point of hydrogen, −253 °C at atmospheric pressure. It is in fact estimated that hydrogen liquefaction has an energy consumption between 10 and 20 kWh/kg; however, it is foreseen that this value will decrease with modern and larger plants, always remaining above the minimum theoretical value of 2.88 kWh/kg at 20 bar. Another issue that needs to be considered is that when LH2 is stored in dedicated double-tick vessels, a certain amount of energy is lost due to the boil-off effect, which is the phenomenon related to the evaporation of a certain amount of hydrogen in time. From literature, it is estimated that when the insulation systems of the tanks are optimal, the boil-off rate is around 0.1%/day of storage [27]. With respect to the existing liquefaction plants, the global liquefaction capacity today is around 350 ton/day [29] and the plants are mainly located in the United States and Canada, with the largest plant having a capacity of 35 ton/day. In order to have an estimation of the costs related to this technology, according to the US Department of Energy [28] the total cost for hydrogen liquefaction plants range from $50 million to $800 million for a capacity of 6,000 kg/day and 200,000 kg/day, respectively. Finally, considering the different field of application of LH2, today the major one is related to the space technologies, where LH2 has been used for decades as fuel. More recently, LH2 is gaining a larger interest in the mobility application, especially in the medium-long distances.

10.4.2 Material based 10.4.2.1 Liquid organic hydrogen carrier (LOHC) The LOHC technology is based on liquid organic hydrogen carriers, an organic oillike substance that binds hydrogen chemically. The chemical storage of hydrogen

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in the LOHC makes it possible to store it under ambient pressure (p = 1 bar) and at a normal temperature (T = 20 °C). Another advantage is that the stored hydrogen is not volatile and, therefore, cannot self-discharge. The LOHC technology enables safe and efficient high-density hydrogen storage of an easy-to-handle oil, thus eliminating the need for pressurized tanks for storage and transport. The application of the technology is based on a two-step cycle: (1) Loading of hydrogen (hydrogenation) into the LOHC molecule (i.e., hydrogen is covalently bound to the LOHC) (2) Unloading of hydrogen (dehydrogenation) after transport and storage

P range (bar)

– bar

Capacity (kg)

 kg/h

Temperature

Ambient conditions

Capex

– €/kg H

Volumetric density

 kg H/m LOHC

The LOHC can be charged and discharged with hydrogen as often as needed. The key barrier for this technology application is related to the cost of LOHC’s hydrogenation and de-hydrogenation. Today, these processes are still very expansive, hindering the large-scale diffusion and application of the solution. The technology is young and as on date, the market has experienced few tests in the operation of this storage solution.

10.4.2.2 Metal hydrides Metal hydrides consist of chemical solutions that are able to store limited percentage quantities of hydrogen, exploiting electro-chemicals powers between molecules. Today, this is the most compact way to store hydrogen (more dense than LH2), but due to the yet low development level, it is currently used only for small-scale applications. P range (bar)

–

Capacità (kg)

– kg

Temperature

– °C

Capex

, €/kg H

Volumetric density

 kg H/m

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The phenomenon of hydrogen embrittlement results from the formation of interstitial hydrides. Hydrides of this type form according to either one of two main mechanisms. The first mechanism involves the adsorption of dihydrogen, followed by the cleaving of the H–H bond, the delocalization of the hydrogen’s electrons, and finally the diffusion of the protons into the metal lattice. The other main mechanism involves the electrolytic reduction of ionized hydrogen on the surface of the metal lattice, also followed by the diffusion of the protons into the lattice. The second mechanism is responsible for the observed temporary volume expansion of certain electrodes used in electrolytic experiments. The most common solutions in the market are: MgH2, NaAlH4, LiAlH4, LiH, LaNi5H6, TiFeH2, ammonia borane, and palladium. Similar to LOHC, their main weaknesses are related to the % of H2 they can store (often less than 20%) and the cost of H2 loading and unloading (sometimes not negligible). Some are easy-to-fuel liquids at ambient temperature and pressure, whereas others are solids which could be turned into pellets. Different R&D projects are today trying to identify catalysts that are able to lower the activation power, guaranteeing more efficient and effective solutions for H2 storage. This is the key challenge for guaranteeing the use of this technology in large application opportunities in the market. Research studies are currently focusing on the application of metal hydrides that provide low reactivity (high safety) and high hydrogen storage densities: the most interesting solutions are lithium hydride, sodium borohydride, lithium aluminum hydride, and ammonia borane.

10.5 Conclusions Above ground hydrogen storage solutions should be taken into account when a limited amount of H2 has to be stored: For example, for mobility applications or using them as buffer between hydrogen production and transportation, and its final usage. These solutions are considerably more expensive than underground systems, while guaranteeing flexibility and portability. Compressed gas represents the mostly utilized solution as of date, but several technologies are coming up. The main hurdle to be overcome is the cost and the energy needed for the storage system’s load and unload (by compressing or liquefying H2 or inducing a chemical reaction). A second success factor is presented by the volumetric density. These solutions have to guarantee portability and low occupancy. R&D is currently striving at minimizing the volumetric density while minimizing energy and costs for charging/discharging, and guaranteeing safety and security.

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AA.VV. Enciclopedia degli idrocarburi. Treccani, 2005. AA.VV. Underground Storage of Natural gas and LPG. United Nations, New York, 1990. Ahluwalia RK et al., Argonne NL, 2019, “System Analysis of Physical and Materials-Based Hydrogen Storage” Ahmed T. Reservoir Engineering Handbook. Gulf Professional Publishing, 2018. Bünger U, Michalski J, Crotogino F, Kruck O. Large-scale underground storage of hydrogen for the grid integration of renewable energy and other applications. In: Compendium of Hydrogen Energy. 2016, 133–163, Doi: 10.1016/B978–1–78242–364–5.00007–5. Buzek F, Onderka V, Vancura P, Wolf I. Carbon isotope study of methane production in a town gas storage reservoir. Fuel 1994, 73(5), 747–752. Caglayan DG, Weber N, Heinrichs HU, Linßen J, Robinius M, Kukla PA, Stolten D, Hide details. Technical potential of salt caverns for hydrogen storage in Europe. Int J Hydrogen Energy 2020, 45(11), 28 February 2020. Doi: 10.1016/j.ijhydene.2019.12.161. Chierici GL (2004). Principi di ingegneria dei giacimenti petroliferi. Crotogino F. Larger scale hydrogen storage. In: Storing Energy. 2016, 411–429, Doi: 10.1016/ B978–0–12–803440–8.00020–8. Crotogino F. Traditional bulk energy storage – Coal and underground natural gas and oil storage. In: Storing Energy. 2016, 391–409, Doi: 10.1016/B978–0–12–803440–8.00019–1. Dake LP. Fundamentals of Reservoir Engineering. Elsevier, 1978. DBI-GUT. (2017) The effects of hydrogen injection in natural gas networks for the Dutch underground storages. Final report (Commissioned by the ministry of Economic Affairs), report. Caglayan DG, Weber N, Heinrichs H, Linßen J, Robinius M, Kukla PA, Stolten D. Technical potential of salt caverns for hydrogen storage in Europe. Int J Hydrogen Energy 2020, 45(11), 28 February 2020, 6793–6805. Donadei S, Schneider GS. Compressed air energy storage in underground formations. In: Storing Energy. 2016, 2016, 113–133. Doi: 10.1016/B978–0–12–803440–8.00006–3. IEA. World Energy Outlook 2019. IEA, Paris, 2019. https://www.iea.org/reports/worldenergy-outlook-2019. Katz DL, Coats. Underground Storage of Fluid. Ulriks Books, Inc, 1968. Kleinitz W, Bӧhling E, (2005): Underground gas storage in porous media – operating experience with bacteria on gas quality. SPE Europec/EAGE Annual Conference (SPE 94248), Madrid. Krooss B. Evaluation of Database on Gas Migration Through Clayey Host Rocks. Study Performed for the Belgian National Agency for Radioactive Waste and Enriched Fissile Material (ONDRAF-NIRAS). RWTH Aachen, 2008. Marcogaz Guidance. “Injection of Hydrogen/natural gas admixtures in Underground Gas Storage (UGS)” 8th May 2017. Panfilov M. Underground and pipeline hydrogen storage. In: Gupta R B (ed.), Compendium of Hydrogen Energy. Elsevier, Chapter 4, 2016, 92–116. Reitenbach V, Albrecht D, Ganzer L. Influence of Hydrogen on Underground Gas Storage – Literature Study. DGMK Publikation, 2014. Smigai P, Greksak M, Kozanova J, Buzek F, Onderka V, Wolf I. Methanogenic bacteria as a key factor involved in changes of town gas in underground reservoir. FEMS Microbiol Ecol 1990, 73, 221–224. Speight JG. Recovery, storage, and transportation. In: Natural Gas. 2019, Elsevier BV, 149–186. Doi: 10.1016/b978-0-12-809570-6.00005-9.

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[24] Timme VM, Daan P, Jenny C, Rik W, Mir GUR, Wieke H, 2018, “How gas can help to achieve the Paris Agreement target in an affordable way” Gas for Climate, Project number: SISNL17592. [25] Wolf E. Large-scale hydrogen energy storage. In: Electrochemical Energy Storage for Renewable Sources and Grid Balancing. 2015, 129–142. Doi: 10.1016/ B978–0–444–62616–5.00009–7. [26] Fuel Cell and Hydrogen Joint Undertaking (2017): Study on early business cases for H2 in energy storage and more broadly power to H2 applications – Final report. [27] Andersson J, Gronvist S. Large-scale storage of hydrogen Int. J. Hydrogen Energy, 2020 44 11901–19. [28] DOE Hydrogen and Fuel cell Program Record. (2019): Current status of hydrogen liquefaction costs. [29] Large-scale Liquid Hydrogen Production and Supply Advancing H2 Mobility and Clean Energy [Internet]. 2019 Sep 27. Available from: https://lngfutures.edu.au/wp-content/uploads/ 2019/10/Cardella-U.-Large-Scale-Liguid-H2-Production-and-Supply.pdf. [30] Stormont John C., Ph.D., P.E. Evaluation of Salt Permeability Tests. Solution Mining Research Institute; 28th February 2001.

Felipe Rosa, Alfredo Iranzo

11 An overview of technological research needs for a successful hydrogen economy deployment Abstract: This chapter presents an overview about technological aspects that need to be deeply investigated to accelerate the hydrogen economy deployment and to bring into the society its benefits in terms of sustainability, job creation, energy system resiliency, energy efficiency, and competitiveness. Since the 1990s, several steps have been taken to reach the actual status of hydrogen technologies, presenting non homogeneous scenarios in most representative countries/regions, but with a homogeneous opinion in favor of these technologies and the potential roles to achieve said benefits. The situation of countries is presented in brief, emphasizing the most important planned actions. A list of actions is presented, as considered by the authors, grouping actions in: transport related activities (5 lines), energy related activities (11 lines), overarching activities (1 line), and cross-cutting activities (3 lines). The chapter ends with the benefits that a hydrogen economy would bring, in relation to the recommendations of the G20 for the establishment of a decarbonizing system. Keywords: Technology research needs, hydrogen economy, sustainability, energy system, hydrogen technologies, decarbonization

11.1 Introduction Preserving energy independence is a strategic aspect supported by most Western countries, and aspects such as increasing energy efficiency, promoting indigenous energy production, increasing the diversification of primary energy sources, and having more resilient energy infrastructures, are common tasks, and countries have been working on specific tasks during the last decade. Logically, most of the measures are not being taken in isolation by countries – there is a growing tendency to implement multinational measures to increase synergies between countries; an example of this is the European Green Deal to move toward an economy based on climate neutrality. At the global level, several agreements can be analyzed that fix, in a slow but steady way, the path to a decarbonized society: – The Paris Climate Conference (COP21, 2015) established an action plan to limit global warming. About 195 countries signed a global climate agreement. https://doi.org/10.1515/9783110596281-019

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– At the European Union level, a number of measures were taken (2016) to increase energy efficiency, promote energy-neutral buildings, promote renewable energy sources, redesign energy system, and last but not the least, strengthen the security of electricity supply. – In 2017, electric energy storage began to gain importance, different innovative solutions were analyzed, and hydrogen-based solutions began to be identified. These solutions began to be implemented with the use of space application power plants. – Similarly, the European Commission proposed updating the EU Gas Directive to harmonize EU rules, opening markets to other operators and encouraging competition. Hydrogen is seen as one of the candidates to decarbonize the EU gas network and increase energy system resilience. Similarly, in November 2017, clean mobility measures were adopted, and an action plan was proposed for the trans-European deployment of alternative fuel infrastructures, including hydrogen as one of the clean fuels for transport. – In 2018, within the International Maritime Organization, the targets for maritime transport emissions reductions until 2050 were fixed at a minimum of 50%. – On 18 September 2018, the energy ministers discussed how hydrogen, as an energy vector in the immediate future, can help achieve key economic objectives of the Union. Finally, the European Green Deal is noteworthy as a determined commitment by the European Commission to lead the way toward climate neutrality by investing in technological solutions, including hydrogen and fuel cells.

11.2 Overview of international initiatives A review of most representatives’ national initiatives is presented here. This review does not try to present an exhaustive situation; rather, it gives a technological view of ongoing initiatives of most representatives [1].

11.2.1 China In 2016, as a part of its 5-year plan, China put into practice, a Fuel Cell Technology Roadmap, which set long term targets for the development of fuel cells and hydrogen production, mainly for the automotive sector. This roadmap expects that more than 1,000 HRS will be deployed by 2030, with hydrogen produced, mainly, from renewable energy sources, to fuel more than 1 million electric fuel cell vehicles (FCEV). A joint research center has been established in Beijing between Toyota and Tsinghua University. The following strategy is set out in a statement from the

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Ministry of Industry and Information Technology (MIIT): “Hydrogen fuel cell vehicles are suitable for large and long distance commercial vehicles. We believe that hydrogen fuel cell vehicles and pure electric vehicles will coexist and complement each other for a long time to meet people’s transportation and travel needs.”

11.2.2 Europe At the European level, the H2020 program has been operational until 2020, and is under review for its continuation [2]. In addition, national programs are available, with the most ambitious being those in Germany and the United Kingdom. Germany approved its Hydrogen and Fuel Cell Technology Program by incentivizing the use of public HRS and automotive applications. The first hydrogen-powered commercial train also began operations, and there is an increased interest in the use of steam reforming Stations. It is worth considering hydrogen applications for the manufacture of steel. In the UK, there are plans to promote local regional scenarios, where there will be a strong emphasis on the use of hydrogen, including domestic use, as a way to show added value of a hydrogen economy. Norway has signed a strategic Agreement with South Korea in the field of hydrogen technology.

11.2.3 United States of America The American initiative on H2 and fuel cell opened up hopes for a more sustainable energy policy and, in the short term, would fix a safer path to the goals of a low-carbon energy future. Currently, there are several initiatives underway: Nikola Motor Company is renewing its truck fleet by replacing heavy diesel trucks with hydrogen fuel cell trucks. The US Department of Energy [3] reports that a significant number of public hydrogen stations will be opened. In addition, a coalition of companies has established an ambitious roadmap for infrastructure development and the revision of regulations in the energy sector.

11.2.4 Japan The concept of an economy based on hydrogen and fuel cell has been the subject of numerous studies in Japan, since 1993. There are several fuel cell experiences in operation over the past 30 years, which have influenced Japanese society that sees this technology affecting self-sustenance of the system, while diversifying and strengthening its national energy infrastructure to move toward a clean and secure energy

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future. Japan aims to reach a target of almost 1 million FCEVs on its roads by 2030 and accelerate the construction of 320 HRS by 2025. Business models are being proposed for the use of hydrogen in residential areas.

11.2.5 South Korea An ambitious roadmap for South Korea was established in January 2019. Main objectives were to set the basis for a hydrogen society based on hydrogen refueling stations (HRS), fuel cell vehicles (FCVs), and industrial and domestic fuel cells. Some specific targets were fixed – more than 6 million FCV and the construction of more than 1,000 refueling stations covering a major part of the country by 2040. As an intermediate step, it is expected than more than 600,000 FCV will be on the road by 2030. The final idea is of a hydrogen economy that would drive economic growth and turn South Korea into a society powered by green energy. Important companies are increasing business in FCVs (Hyundai Nexo) and hydrogen fuel cell power plants (Daesan Green Energy).

11.3 Identification of technologies Technologies can be classified according several criteria. For this overview, the criteria used by the European FC&H2 funded program [2] is adopted. Hydrogen technologies will be classified as a function of their final use, namely: – Transport- related activities (technologies associated with transport) – Energy- related activities (technologies associated with energy) – Overarching activities (technologies associated with general aspects) – Cross-cutting activities (technologies associated with other technologies)

11.3.1 Transport-related activities Hydrogen and fuel cell technology is considered to have great potential and play a fundamental role in the future zero-emission mobility, without compromising on the way vehicles are currently refueled (refueling time and driving range). The suitability of hydrogen powertrains based on fuel cells varies among the different transport modes, that is, land, sea, and air transport. It must be noted that nearly half of the global energy demand for transport is derived from light-duty vehicles, and the number of passenger cars is expected to increase from 1 to 2.5 billion by 2050, worldwide. Nevertheless, it is in the heavy-duty transport sector that the suitability and economic feasibility of fuel cell powertrains is becoming more promising in the short-term (fuel

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cell buses and trucks, and also trains). In the light-duty sector, it is expected that battery vehicles will prevail for urban transport, short distances, and smaller vehicles, whereas fuel cell electric vehicles will dominate for longer distances. Research needs in the transport sector are mainly focused on achieving the necessary cost reduction of the whole system, including not only the fuel cell stack but also the hydrogen tanks and other elements of the Balance of Plant. Ensuring fuel cell durability is also a major aspect requiring research efforts. Cost reductions will be achieved by a combination of the economies of scale derived from the increase in the number of units produced per year and technology development driven by research efforts, which are still necessary to promote the deployment of the technology in the market [4].

11.3.2 Energy-related activities Electrolysis of water seems to be an important technology in the pathway to integrate renewable energy sources in the global energy system with a very high potential in regard to decarbonizing the energy market, in general, and, specifically, the gas energy sector. To be able to reach that goal, electrolysis must advance in terms of feasibility and performance increasing electrolyzer’s scale and, also, specific technical advances in electrolysis at high temperature and co-electrolysis of water and carbon dioxide. An item that is specifically challenging is reversible electrolysis. A specific and detailed research program should be conducted to study new materials and improve design concepts, especially for PEM electrolysis. These new materials, together with novel architectures and improved associated manufacturing processes will also be useful for reversible electrolyzer-fuel cell developments. Hydrogen injection into the natural gas infrastructure is happening in several countries, but in others, is the day is still far away, so regulatory efforts should be made to harmonize and expand its use, as also expand the sale of hydrogen in hydrogen refueling stations. As for stationary fuel cells, a huge potential for improvement is directly related to high temperature fuel cells or high temperature electrolyzers, including their integration with high temperature thermal power systems. Technologies to monitor and diagnose their status online are critical to detect degradation of components, limitation of the life of components, and the whole system. A basic research line seems to be necessary to understand physicochemical mechanisms that contribute to performance limitations in MEA poisoning of catalyst and life of the cell. When applied to transport applications, the former aspect is critical for PEM technology especially due to operation under high dynamic modes. For maritime transport applications, when the fuel cell is not operating in highly dynamic mode, the use of appropriate MW scale, considering durability, performances, and compatibility with maritime conditions should be investigated.

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11.3.3 Overarching activities Deployment of hydrogen technology should be conducted, in its initial steps, for the creation of local/regional hydrogen communities, where demonstration of value chains and the full and smart integration of FCH technologies in different sectors and applications should be good indicators that can drive and address the deployment of technologies. Decarbonization of energy is a key goal of developed/developing countries; in this sense, green hydrogen can help in meeting the CO2 reduction targets of the industrial and transport sectors. In this context, the demand for hydrogen of high quality, purity, and adequate pressure is steadily increasing. Proton conducting ceramic cells are a promising alternative to the manufacture and purification of hydrogen for both energy and transport applications.

11.3.4 Cross-cutting activities Transversal activities are very important to support and enable the activities undertaken within the former categories and to ease the transition from the ready-technology status to the market of fuel cell and hydrogen technologies. As in all technological fields, normative research is necessary to fix standardized protocols that allow the deployment of technology in a safe way and with common standards among countries. One aspect considered strategic for many countries is the need to investigate the hydrogen mixture in the natural gas network and its impact. Safety, sustainability, and recyclability issues need to be properly conducted to bring public awareness and acceptance of fuel cells and hydrogen technologies. Without them, market implementation and consolidation of these applications can be a challenge.

11.4 Overview of research needs in key technologies 11.4.1 Transport-related activities 11.4.1.1 Development of automotive MEAs with improved durability and reduced cost The membrane electrode assembly (MEA) is the core of the unit cell or SRU (single repetitive unit) and therefore the core of the fuel cell stack. Proton conduction and anode and cathode electrochemical reactions are taking place within the MEA, along with the associated charge, species, and heat transport. In order to reduce costs and materials in automotive fuel cell stacks, future MEAs must increase their power

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density (despite their reduced platinum loading). Single cells within the stack will need to become thinner and lighter. Both durability and cost are key pillars in the research needs for next generation MEAs. Efforts required in the technology are focused on the following areas: – Reducing the platinum loadings and seeking new catalysts and supports, yet with improved activity and durability. – Development of new materials and designs, where the range of operating temperature is also an issue under consideration, as sub-zero temperature conditions are common in many countries deploying fuel cells for transportation. – Beginning of life (BOL) performance is not the only parameter to focus on for the development of novel MEAs, as durability targets range between 5,000 and 7,000 h for light-duty vehicles, up to over 20,000 h in buses, and 30,000 h in trains. – Fundamental research aimed at achieving a much deeper understanding of charge, mass, and heat transport within the MEA. This will be needed for identifying the phenomena affecting the performance limitations and consequently guide the design progresses toward the next generation of MEAs, GDLs,, and bipolar plates, that shall allow for breakthroughs in performance and operation strategies.

11.4.1.2 Fuel cell stacks for automotive applications The fuel cell stack involves not only the connection in series of the single cells or SRU, but also reactants supply through the gas manifolds, heat transfer issues, anode and cathode welding of bipolar plates, and others. Stacks for automotive applications must fulfill strict requirements in terms of power density (gravimetric and volumetric), cost, performance, durability, robustness in cold and hot operation, start-up/shutdown, and transients in driving cycles, all of which require research activities in order to progress toward more compact and highly robust and durable fuel cell stacks: – Research is focused on new disruptive stack concepts involving new architectures, new materials and new processes, aiming at reducing stack sizes and costs. – Such targets must be fulfilled while increasing performance and ensuring durability. – A better understanding of the degradation phenomena at stack level is also required, in order to achieve the durability and lifetime needed for heavy-duty trucks, buses, and trains. Such applications are particularly challenging as the driving cycles and road duty cycles are significantly aggressive.

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11.4.1.3 Hydrogen tanks for fuel cell electric vehicles Autonomy range is the major end-user expectation to be fulfilled by the hydrogen storage tank. Current state-of-the-art tanks for hydrogen in gas form are cylindrically shaped, with up to 700 bar compressed hydrogen. Several tanks are packed within the automobile in order to allow for an increase in the autonomy of the vehicle, as in the Toyota Mirai. However, the achievement of further driving ranges needs the development of a new generation of hydrogen tanks, where research areas being considered include the following: – Novel and innovative concepts for high pressure (and high storage volume) tanks and vessels that need to be integrated and packed into a rectangular shape for battery spaces, while still complying with the strict type-approval regulations and certifications. – Both volumetric and gravimetric capacity need to be increased for extending the driving range, while also considering the strict space constraints within vehicles.

11.4.1.4 Liquid hydrogen on-board storage tanks On-board storage of liquid hydrogen (LH2) was developed by BMW in the first decade of 2000, although it was finally disregarded, despite the fact that storage density was as high as twice the value achieved by compressed hydrogen at 700 bar. However, the expectations on the development of fuel cell powertrains for heavyduty transport are increasing the need for further research on LH2 on-board storage, as such applications require significantly larger capacities (60–100 kg H2 and even over 100 kg H2), with a much higher utilization (over 100,000 km/year). LH2 is also a fuel candidate being considered for segments of the waterborne sector. Research activities are involving key aspects such as: – Optimization of the heat insulation, while keeping gravimetric storage capacity. – Mechanical and safety requirements are also driving research efforts.

11.4.1.5 Fuel cell systems for the propulsion of aerial passenger vehicle Air transport is also becoming a candidate for future use of hydrogen powered propulsion systems. This is however particularly challenging, due to the effects of high altitudes and the consequent reduced O2 concentration and partial pressure levels, in addition to the high gravimetric and volumetric power density required. Specific activities requiring further research efforts are therefore focused on the following issues: – The effects of reduced O2 concentration and partial pressure on the system performance require a deeper understanding in order to achieve optimized designs.

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– The performance dependency on altitude will affect the design, not only of the fuel cell stack but also of BoP components such as the air compressor or the hydrogen storage. – Research is also involving modular architectures to better adapt systems to the different power requirements.

11.4.2 Energy-related activities 11.4.2.1 New electrolyzers based on anion exchange membrane The production of hydrogen using anion exchange membrane water electrolysis introduces, as a fundamental advantage, the replacement of the electrocatalysts based on noble metals by a low cost of transition metal ones. However, it is still a developing technology and requires remarkable advances, before it can be used to obtain hydrogen at a competitive price with improved efficiency, robustness, membrane stability, handling and cost. The fundamental deficiencies of the technology are motivated by the low ion conductivity of the membrane as well as the low thermal and chemical stability (it is only stable from pH 12). The specific points subject to research are, among others: – ionic conductivity of the membrane – stability of the membrane (thermal and chemical) – optimization of chemical composition and catalyst activity, conductivity – improvements in cell design

11.4.2.2 Catalyst development for better economics of LOHC In an economy based on renewable energy and hydrogen sources, large-scale energy storage is needed. Among energy storage systems, those based on chemical hydrogenation and dehydrogenation methods using liquid organic hydrogen carriers are available. Being liquid under environmental conditions, they exhibit adequate properties for transport, handling, and storage through well known processes. The energy efficiency of the systems depends mainly on aspects such as energy integration and the proper selection of catalysts for chemical hydrogenation and dehydrogenation process. The technical feasibility of LOHC has not been demonstrated on a sufficient scale for massive use, so the scale factor needs to be increased in order to ensure its economic viability. To address its use at higher levels and demonstrate its economic viability, the following are proposed: – Development of catalysts adapted to the LOHC that allow achieving a high performance of transformation and energy

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– Energy integration as followed in conventional large-scale hydrogen generation technology – Economy of scale

11.4.2.3 Solid oxide electrolysis integration with renewable heat and power Water electrolysis is an appropriate route for hydrogen production that can be used for energy storage by integrating with renewable energy sources. If we consider high-temperature electrolysis, it is especially suitable for integration with solar thermal concentration energy, presenting high transformation yields. This performance can be increased when some of the energy input is made in the form of high-temperature solar thermal energy and the rest in the form of electricity, as dictated by the laws of thermodynamics. Due to the intermittent nature of the energy resource, a steam storage system is needed, minimizing the thermal cycles of the system. In addition, an energy integration system is required for process optimization. As a result of the dynamic operation, steam temperatures will change by introducing thermal stress into the system. From the above, changing operating conditions necessitates the following: – A detailed analysis of the various scenarios needs to be carried out – Operation and operating conditions evaluated and changes in the energy performance of the system carried out – Studies of the durability of the system need to be foreseen

11.4.2.4 Diagnostics and control of SOE High-temperature electrolyzers offer hydrogen at a competitive price from renewable energy sources, both electricity and high-temperature solar thermal energy. In the previous section, the challenge arising from its integration has already been highlighted, with the most harmful effects for its integrity being the thermal stress derived from variable loads with the consequent degradation. In addition, SOE technology offers advantages since reversible operation is possible, working as an electrolyzer or as a fuel cell, or electrolyzing steam and carbon dioxide. While this is an advantage, it also has a disadvantage: accelerated degradation as a result of changes in modes of operation. The control system that will monitor its operation under transient conditions, or during changes in operating modes, should also analyze the effect of the speed changes on the operating and counteract the nocive effect on the stack and system. In summary, the control system shall:

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– ensure a controlled change in operation regime modifications – identify the most suitable operating points for proper performance, ensure stability, and meet maintenance requirements

11.4.2.5 Flexi-fuel stationary SOFC Operation of solid oxide fuel cell (SOFC) with natural gas reduces the amount of CO2 significantly. As a result of the implementation of power-to-gas systems, the percentage of H2 in natural gas will be variable and fluctuating as a result of operation conditions. The operation of the SOFC under a variable gas composition is challenging if the following are desired: – Obtain a high electrical efficiency – Achieving high quality surplus thermal current – Have a long service life

11.4.2.6 Underground storage of hydrogen Production of hydrogen seems to be a useful way to introduce renewable energies into the energy sector, especially industry and automotive. To accomplish this, an effective storage media should be provided with minimum expectation of energy losses. Among these media, underground hydrogen storage system has proved its technical feasibility. A few geological places, depleted natural gas fields, and salt caverns are good potential candidates for hydrogen storage, but a few issues and challenges need to be properly addressed: – Geochemical and microbiological reactions in the subsurface. – Benchmark with natural gas in relation to the mobility, dissolution, and diffusion characteristics. – Cyclic analysis of the behavior under high charge and discharge rates. – Dynamic studies about variable profiles of renewable power and hydrogen production rates(input-output).

11.4.2.7 Offshore production of renewable hydrogen Offshore wind energy has evolved from a secondary energy source to a global resource through the increasing capacity of turbines of more than 10 MW and an overall project-level capacity of more than 600 MW. The conditions of these marine sites demand rugged and reliable products capable of withstanding environmental factors such as wind, waves, and salt in significant quantities. Globally, the main

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challenge facing the offshore wind energy industry is also seen in offshore electrolyzers, and includes: – Development of an electrolyzer module able to withstand that saline environment – Design of a compact unit to get high specific hydrogen production per wind turbine – The ability to have a long duration life when connected directly to an intermittent variable renewable power supply – Safety and accessibility for maintenance

11.4.2.8 CHP via reversible solid oxide cell Reversible solid oxide cells have great potential in stationary electricity generation, as they have the ability to operate in two different modes: electrolysis and fuel cell mode. In order to achieve a reversible polygeneration, the main challenges to be met are: – Effective thermal integration between the fuel cell and the thermal system – Rapid transition between different operating modes – A more efficient and lower cost gas processing system – Increase in system efficiency in different operating modes – Reduction of investment costs to deliver economical and attractive solutions

11.4.2.9 Injection of hydrogen into high-pressure gas networks Requirements for decarbonizing the economy mean that contaminated emissions must be reduced in the medium (and short term). This implies increasing energy production from renewable energy sources, reducing heating/cooling emissions, reducing emissions from the transport sector, and integrating and easing the energy market. Hydrogen from renewable resources can play an important role in this context and requires that it must be transported from where it is produced to where it is consumed, using widely developed gas infrastructure. However, for the gas system to be able to operate in a mixed hydrogen and natural gas environment, it must be analyzed in a systematic way: – Careful modifications in operating and maintenance conditions, materials, and mixing values – Harmonize technical and legal aspects – All technical specifications must be defined, based on know-how and experimentation at the level of laboratory and pilot installations

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11.4.2.10 Compact CHP fuel cell system for domestic use Fuel cells produce heat when generating electricity, so they are of particular interest to integrate heat and energy (CHP) and combined cooling (CCHP) applications. These systems offer high energy conversion efficiency and therefore the potential to reduce specific fuel costs consumption and CO2 emissions. These systems will be powered in the long term with hydrogen, but in the short term, the most feasible operation is with natural gas, so it will be necessary to develop and integrate a fuel processor that will operate together with the fuel cell (high-temperature PEMFC) In order to achieve the implementation of this technology, the following should be addressed: – Thermal integration of the fuel cell with the fuel processor, in case of PEMFC, and recovery of the maximum amount of heat from the fuel cell – Achieving compact design and equipment with high specific power density for use in residential and commercial environments

11.4.2.11 Thermochemical hydrogen production from concentrated sunlight Thermochemical water splitting uses a set of chemical reactions to produce hydrogen and oxygen from water. It requires a high temperature source, and as potential candidates, concentrated radiation from solar power plants can be used (or even from the waste heat of nuclear power reactions). This technology needs to be properly developed from low TRL and offers a very high potential with low or no greenhouse gas emissions. Intensive efforts should be made in to solve major problems as listed below: – Find out technical solutions for material properties – Work intensively on reactor designs and fluids consumption and handling – Achieve high overall efficiency at the level of the solar receiver, reactor unit, and the heat recovery.

11.4.3 Overarching activities 11.4.3.1 Hydrogen communities Specific locations (as island) have a great RES potential, which cannot be properly exploited due to the small size and weakness of its isolated non-interconnected electrical island system. The recent increase in RES production is already initiating curtailment of RES. The promotion of H2 technologies is a key instrument that will allow having energy storage systems and manageable loads for grid balancing in high RES penetration scenarios. It will also make it possible to dispose of H2 as an

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energy vector to use RES in the decarbonization of the road transport sector and maritime transport in the future. In these specific places, the complete value chain of H2 from production can be covered – from distribution, storage, and end-use. The deployment of hydrogen technologies in these singular scenarios will help boosting the reindustrialization and generating new low carbon economies.

11.4.4 Cross-cutting activities 11.4.4.1 Multi-fuel hydrogen refueling stations (HRS) Use of H2 for transport application will be the last target for full decarbonization of the automotive sector. To reach this ambitious objective, an extensive variety of fuels can be made available for the final users: Compressed hydrogen (at several pressures range), LH2, natural gas blended with H2 (at several compositions), and so on. Multi-fuel hydrogen refueling stations could be potentially considered, giving rise to several issues that should be considered: – Safety standards for considering zoning and risk assessment – Leakages characterization that can be anticipated from hydrogen dispensing technologies and their effects (fire, explosion)

11.4.4.2 Standards for passenger ships As previously stated, the International Maritime Organization has set targets for reducing maritime transport emissions by a minimum of 50% by 2050. In order to achieve this long-term objective, a regulatory framework on the use of hydrogen and alternative hydrogen-based fuels for water transport should be developed. Risks and design must be identified to guarantee safety in its use. Aspects to be investigated include, among others: – Regulatory framework, with detailed SWOT analysis – Definition of knowledge for risk assessment in the maritime environment – Proposals for changes to current regulations

11.4.4.3 Design guidelines, recycling and LCA of technologies for FCH products The concept of circular economy is being imposed on the development and manufacture of products so as to minimize the environmental impact of these affecting the economics of the process positively. The fledgling FCH product market cannot

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be alien to this trend, so it is imperative to minimize the environmental impact throughout its life cycle, from design to recycling. An open list of actions needed to be investigated, that can be grouped into: – Development of guidelines – Definition of guidelines – Evaluation of LCA assessment

11.5 Final remarks The long-term objectives of industrialized countries are to achieve a reduction in greenhouse gas levels, increase in the percentage of renewable energy sources in terms of primary energy, and enable increased energy efficiency in industry and buildings. To achieve these overall objectives, most countries support their energy policies on the following specific objectives: – Guarantee of energy supply and resilient energy infrastructure – Energy efficiency – Decarbonize the economy – Research, innovation, and competitiveness Of these objectives, energy storage and the definition of new energy vectors are considered priorities. Hydrogen, as a new energy vector, can play a leading role and can be an agent in achieving, in the long term, the set targets. The recommendations of the G20, in relation to the hydrogen economy, for the establishment and safe deployment of these technologies and their progressive development as part of the incoming energy system include the following: 1. Consider hydrogen as part of long-term energy system. The key actors will be national, regional, and local administrations. The industrial sectors, with a strong energy load, can apply hydrogen in: refining of oil and chemical industry, iron and steel sectors, long distance transport of goods and passengers, construction, generation and storage of energy. 2. Promote and setup hydrogen in fields that have achieved a suitable TRL and still have high cost of implementation or operation. It is to be expected that, with the economy of scale, the necessary cost reduction will be achieved. 3. Define and implement financial mechanisms to reduce the economic risks arising from early investors. 4. Encourage and support R&D as a means to increase process performance and cost reductions. National R&D programs or coordinated programs between nations are essential to establish the lines of action, promote synergies, reduce risks and attract private capital for innovation. 5. Develop and harmonize all regulatory aspects.

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6. Define, establish and support a true international cooperation program to facilitate the exchange of standards and good practices. 7. Leverage existing infrastructures to facilitate transition, minimize investment, increase investor confidence, and reduce costs (use of industrial ports, gas infrastructure, captive support fleets and industrial corridors, and sea routes for international hydrogen trade).

References [1] [2] [3] [4]

Thomas JM, Edwards PP, Dobson PJ, Owen GP. Decarbonising energy: The developing international activity in hydrogen technologies and fuel cells, J Energy Chem, 2020. www.fch.europa.eu www.energy.gov Rajalakshmi N, Balaji R, Ramakrishnan S. “Recent developments in hydrogen fuel cells: Strengths and weaknesses”, Elsevier BV.

Marcel Van de Voorde, Paolo Ciambelli

Conclusions and Recommendations: “The Future of Hydrogen” The book series at hand analyzes the current state of affairs for hydrogen and offers guidance on its future development. Hydrogen is currently enjoying unprecedented momentum, with the number of policies and projects around the world expanding rapidly. Together with renewable electricity, hydrogen constitutes the major energy vector substituting the use of fossil fuels, and its large-scale introduction could be the point of no-return in transformation to a sustainable and climate change responsible for society and economy. The combination of serious concerns about our changing climate, which are aggravating every month, and the economic emergency triggered by the pandemic provide the hydrogen industry with a unique opportunity to participate fully in the ongoing energy transition. Japan and South Korea were forerunners in designing development plans based on hydrogen, but in the latest months Europe has clearly implemented a strategy to be at the forefront in the world for the energy transition, assigning to hydrogen a fundamental role in the race to reach a climate-neutral and zero-pollution economy in 2050. Several European countries have now initiated strategies aiming at promoting their competitive advantage (wind energy in Northern Europe, photovoltaics (PV) in Southern Europe; usage and deployment in industry and transportation in Northern and Central Europe; diverse methods of decarbonization in the Netherlands and Scandinavia). China’s President Xi Jinping announced last September the objective of a pledge to achieve carbon neutrality before 2060, and it remains to be seen whether China will really intend to overcome its persistent hesitation (as presently the largest carbon emitter), for a policy addressed to decarbonize their coalbased hydrogen production. On the other hand, China currently gives Asia a head start in the upcoming electrical mobility market based on batteries or hydrogen. Moreover, the major energy players in the Middle East are now looking very seriously at hydrogen as a potential carrier of fossil fuels decarbonization, subject to development of carbon sinks, whether through capture and sequestration (carbon capture and storage) or through storage and reuse (carbon capture and utilization, CCU). Australia has the same approach, and projects are already well underway with its main Asian energy partners. Finally, the United States is currently a home to the world’s largest pool of fuel cell electric vehicles and forklift trucks (more than 30,000 already operational), with a lead from the US pioneering state, California. But political differences stand in the way of a nationwide scale-up. Yet, the biggest companies active in relevant technologies, and some of the most advanced and valuable start-ups, are developing on the American https://doi.org/10.1515/9783110596281-020

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soil. All that is missing for the rest of the industrial world to embark is, finally, in a unified and proactive policy on hydrogen’s silent and virtuous revolution that is presently emanating from the United States. The book series provides an extensive and independent survey of hydrogen that lays out where things stand now; the ways in which hydrogen can help to achieve a clean, secure, and affordable energy future; and how we can proceed to harvest its full potential. As the ultimate goal is the realization of a revised, much more comprehensive concept of the “hydrogen economy”, the overview is not only limited to technical aspects, to allow scientists, company managers, policy makers, and society as well to better understand this fast-moving area in terms of technologies but also of possibilities, thereby offering a privileged glimpse into the future. The world should not miss this unique chance to make hydrogen an important part of our clean and secure energy future. Where is today’s starting point to achieve cost reduction, hydrogen usage adoption in new sectors, such as transport, buildings, and power generation, and shifting to clean, hydrogen-based industrial processes? Clearly, these three aspects are strictly related. Supplying hydrogen to industrial users is now a major business around the world, and the demand for hydrogen has grown more than threefold since 1975 and continues to rise (about 70 million tons in 2019). Hydrogen use today is dominated by industry, namely, oil refining, ammonia production, methanol production, and steel production. However, it is almost entirely supplied from fossil fuels: about 6% of global natural gas use being currently the primary source of hydrogen production, accounting for around three quarters of the annual global dedicated hydrogen production, and about 2% of global coal go to hydrogen production. As a consequence, this production of hydrogen (termed grey H2) is responsible for CO2 emissions of around 830 million ton/year, equivalent to CO2 emissions of a country such as Germany. A range of technical and economic factors influences the production cost of hydrogen from natural gas, with gas prices and capital expenditures being the two most important ones. Fuel costs are the largest cost component, accounting for between 45% and 75% of production costs. Low gas prices in the Middle East, Russia, and more recently North America (shale gas) give rise to some of the lowest hydrogen production costs. Gas importers like Europe, Japan, Korea, China, and India have to contend with higher gas import prices, resulting in higher hydrogen production costs. In contrast to the hydrogen production from fossil fuels, today less than 0.1% of global dedicated hydrogen production comes from water electrolysis, since producing hydrogen from low-carbon energy is costly at the moment. With declining costs for renewable electricity, in particular from solar PV and wind, interest is growing in electrolytic hydrogen, and there have been several demonstration projects in recent years. In particular, building electrolyzers at locations with excellent renewable resource conditions could become a low-cost supply option for hydrogen, even after taking into account the transmission and distribution costs of transporting hydrogen from (often remote) renewable locations to the end-users.

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The cost of producing hydrogen from renewable electricity could fall 30% by 2030 as a result of declining costs of renewables and the scaling up of hydrogen production. This forecast supports the position that the energy transition should be strongly accelerated toward the production of “clean” (green) hydrogen from water as hydrogen source (water splitting process), and solar PV and wind as renewable energy drivers. Even more ambitious and integral is the view that the acceleration should be directed right away to the production of the green hydrogen by “artificial photosynthesis,” that is, directly from solar energy without the intermediate production of renewable electricity. The last mentioned option of direct hydrogen production is pursued in research laboratories around the world at a much lower technological readiness level; similarly, the ramping up of renewable electricity production will still require time. Consequently, it appears mandatory to invest today in producing so-called blue H2. One pathway for the latter is driving electrolyzers for water splitting with a CO2-free electricity mix that may include nuclear energy. A dedicated local electricity generation from renewables or nuclear power offers an alternative to the use of grid electricity for this production route of hydrogen. The second approach to produce blue hydrogen starts from fossil fuels, coupled to carbon sequestration. This strategy opens up a way to enable still the use of fossil fuels and constitutes an effective and presently available positive step paving the way for the transition to a “hydrogen economy.” The associated momentum appears to outweigh concerns that the latter approach would result in a delay of implementing green hydrogen. A parallel discussion deals with storage/transportation of H2 (pressurized, liquefied, or mixed with natural gas via gas pipelines). Indeed, the distribution as a mixture with methane, with the highest potential in multifamily and commercial buildings, particularly in dense cities, can be seen as an effective solution or a way to promote the co-use of hydrogen and natural gas, while longer term prospects could include the direct use of hydrogen in hydrogen boilers or fuel cells. It is worth remembering that an alternative solution, strongly sustained in Japan, is the use of H2 carriers (ammonia or liquid organic hydrogen carriers, for example), which could really enable a world-scale “hydrogen economy” available to all countries. What is missing is a real debate about pros and cons, sustained by independent studies. These should address in detail the costs, especially providing predictions of their future changes, discuss technology barriers, and assess how fast they could be overcome. With reference to transport, the competitiveness of hydrogen fuel cell cars depends on fuel cell costs and the accessibility of refueling stations, while for trucks the priority is to reduce the delivered price of hydrogen. Shipping and aviation have limited lowcarbon fuel options available and represent an opportunity for hydrogen-based fuels. In power generation, hydrogen is one of the leading options for medium- and long-term storage of renewable energy, and hydrogen and ammonia can be used in gas turbines to increase power system flexibility. Co-firing of ammonia in coalfueled power plants could also help reduce CO2 and pollutant emissions.

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Referring to the future of hydrogen a great focus on safety issues is necessary, not only about the actual safety level of systems but also about its perception in the public eyes, mostly relying on training, education, and public awareness. The risks related to hydrogen do not change in nature with its wider distribution but the rapidly widening reach will generate significant demand for skills, projects, and players who will need to acquire know-how rapidly, which certainly requires more specific “hydrogen energy” training courses at universities and technical colleges. It is critical to set and harmonize rules and norms to ensure that equipment needed to roll out hydrogen technology is operated to the strictest safety standards. This book series did not aim at giving definite answer to the various questions mentioned. Rather, it presents the state of the art, accounts for advanced research, outlines development perspectives, and proposes road maps toward the “hydrogen economy.” It provides clues to analyze all aspects dealing with the future of hydrogen and should thereby support decision makers in their quest which scientific and technological aspects should be investigated, which opportunities exist for companies, or which political strategies should be chosen. Practical opportunities at near term can be summarized as follows: 1. The time is right to tap into hydrogen’s potential to play a key role in a clean, secure, and affordable energy future. Scale up technologies and bring down costs to allow hydrogen to become widely used. Pragmatic and actionable recommendations provided to governments and industry will make it possible to take full advantage of this increasing momentum. 2. Hydrogen can help tackle various critical energy challenges. Decarbonize a range of sectors, including long-haul transport, chemicals, and iron and steel industry, where it is proving difficult to achieve substantial emission reductions. This achievement can also help improve air quality and strengthen energy security. 3. Hydrogen is versatile. Wide variety of pathways and fuels being available to produce hydrogen, including renewables, nuclear, natural gas, coal, and oil. Hydrogen can be transported as gas by pipelines or in liquid form by ships, much like liquefied natural gas. It can be transformed into electricity and methane to power homes and feed industry, as well as into fuels for cars, trucks, ships, and planes. 4. Hydrogen can enable renewables to provide an even greater contribution. H2 as the leading option for storing energy from renewables. Hydrogen appears to be promising to become the lowest cost option for storing electricity over days, weeks, or even months. Hydrogen and hydrogenbased fuels can transport energy from renewables over long distances – from regions with abundant solar and wind resources to energy-hungry cities thousands of kilometers away.

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5. New technologies are becoming available. Impressive advances of solar PV, wind, batteries, and electric vehicles. These successes have shown that policy and technology innovation has the power to build global clean-energy industries. The versatility of hydrogen is attracting stronger interest from a diverse group of governments and companies. Support is coming from governments that both import and export energy, from cities, as well as renewable electricity suppliers, industrial gas producers, electricity and gas utilities, automakers, oil and gas companies, and major engineering firms. Investments in hydrogen can help foster new technological and industrial development in economies around the world, creating skilled jobs. 6. Hydrogen can be used much more widely. Hydrogen penetrating into all sectors of the economy, enabling their coupling. Today, hydrogen is used mostly in oil refining and for the production of fertilizers. For it to make a significant contribution to clean energy transitions, it also needs to be adopted in sectors where it is almost completely absent at the moment, such as transport, buildings, and power generation. 7. International cooperation is vital to accelerate the growth of versatile, clean hydrogen around the world. Governments working together to scale up hydrogen in a coordinated way. This would help to spur investments in factories and infrastructure that will bring down costs and enable the sharing of knowledge and best practices. Trade in hydrogen will benefit from common international standards. As the global energy organization that covers all fuels and all technologies, the International Energy Agency(IEA) will continue to provide rigorous analysis and policy advice to support international cooperation and to conduct effective tracking of progress in the years ahead. Clean, widespread use of hydrogen in global energy transitions faces several challenges: – Hydrogen is almost entirely supplied from natural gas and coal today. Hydrogen is already in use at large industrial scale all around the world but its production is responsible for substantial CO2 emissions. – Producing hydrogen from low-carbon energy is costly at the moment. The cost of producing hydrogen from renewable electricity could fall 30% by 2030, as a result of declining costs of renewables and the scaling up of hydrogen production. – The development of a hydrogen infrastructure is slow and holding back widespread adoption. Hydrogen prices for consumers are highly dependent on how many refueling stations are available, how often they are used, and how much hydrogen is delivered per day. Tackling this issue is likely to require planning and coordination that brings together national and local governments, industry, and investors.

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– Regulations currently limit the development of a clean hydrogen industry. Government and industry must work together to ensure that existing regulations are not unnecessary barriers to investment. Trade will benefit from common international standards for the safety of transporting and storing large volumes of hydrogen, and for tracing the environmental impacts of different hydrogen supplies. Key recommendations to help governments, companies, and others to seize this chance to enable clean hydrogen to fulfill its long-term potential: 1. Establish a role for hydrogen in long-term energy strategies. European national and regional governments can guide future expectations. Companies should also have clear long-term goals. Key sectors include refining, chemicals, iron and steel, freight and long-distance transport, buildings, power generation, and energy storage. 2. Stimulate commercial demand for clean hydrogen. Clean hydrogen technologies are available but costs remain challenging. Policies that create sustainable markets for clean hydrogen, especially to reduce emissions from fossil fuel-based hydrogen, are needed to underpin investments by suppliers, distributors, and users. By scaling up supply chains, these investments can drive cost reductions, whether from low-carbon electricity or fossil fuels with CCU (carbon capture and utilization). 3. Address investment risks of first-movers. New applications for hydrogen, as well as clean hydrogen supply and infrastructure projects, stand at the riskiest point of the deployment curve. Targeted and time-limited loans, guarantees, and other tools can help the private sector to invest, learn, and share risks and rewards. 4. Support R&D to bring down costs. Alongside cost reductions from economies of scale, R&D is crucial to lower costs and improve performance, including investigations of fuel cells, hydrogen-based fuels, and electrolyzers (the technology that produces hydrogen from water). Government actions, including use of public funds, are critical in setting the research agenda, sharing risks, and attracting private capital for innovation. 5. Eliminate unnecessary regulatory barriers and harmonize standards. Project developers face hurdles where regulations and permit requirements are unclear, unfit for new purposes, or inconsistent across sectors and countries. Sharing knowledge and harmonizing standards are key, including norms for equipment, safety rules, and certification of emissions from different sources. Hydrogen’s complex supply chains imply that governments, companies, communities, and the civil society need to consult regularly. 6. Engage internationally and track progress. Enhanced international cooperation is needed across the board but especially on standards, sharing of good practices, and cross-border infrastructure. Hydrogen

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production and use need to be monitored and reported on a regular basis to keep track of progress toward long-term goals. It is clear that the hydrogen technology will reach great challenges in future with focus on: i) promotion of research, development, and new technologies; ii) support of governments with respect to infrastructure, regulations, and uprising new companies; iii) impact and encouragement of the society to accommodate the emerging renewable energy world.

Index 400 billion euros –EU spending on fossile fuels each year 107 A and B components of hydrides based on intermetallic compounds 161, 162, 252 above-ground storage 351 acetogenesis 362 activation barrier 196 additional (interstitial) oxygen 217 adsorbent materials 147 adsorption –carbon nano-tubes 144 –metal-organic frameworks 144 –zeolites 144 Africa’s First World War (1996–1997) 106 alanates 166, 167, 168 Algeria 108 alternative fuel infrastructures 331 aluminum hydride 153 amine-based 150 amine metal borohydride 203 ammonia 191, 193, 194, 195, 203, 207, 208, 211, 212 –cracking 208 –production 194, 196, 201, 212 –storage 194, 203 An+1MnO3n+1 216 April 20, 2020, 108 aquifers 358 arc melting 270, 271, 275, 281, 282, 286 artificial intelligence 104 automotive 332 automotive applications 332, 337 Azerbaijan 108 back donation process 197 Balance of Plant 334 balancing electricity supply and demand 67 battery 99, 106 bipolar plates 337 Beginning of life (BOL) performance 337 beyond the EU borders 104 big data 104 blending hydrogen 78 blue hydrogen 128, 129, 130, 140, 141 Bolivia 108 BoP components 338

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borohydrides 166, 167, 168 brine 366 Brunei 108 capacity (C) 138 capillary threshold pressure 357 caprock 353 Carbon Capture and Storage 49 carbon free 140 carbon free energy carrier 49 carbon materials 168, 169 carbon-neutral 151 carriers 137 catalysts 336 composite hydrogen storage alloys 262 CCS 128, 129, 130 CCU 141, 142 Ce0.9Gd0.1O2–δ 219 cell design 339 CERN 105 charge–discharge reaction 250, 251, 255, 258 chemical hydrogen carriers 136, 141, 145, 148 chemical synthetic 269 Child labour 106 China 103, 106 CHP 343 circular economy 345 clean fuels 331 climate neutrality 330 CO2 reduction 335 Coal Gasification 54 cobalt 106 co-electrolysis 334 Colombia 108 colour of hydrogen 58 combustion engines 52 combustion synthesis 269, 276, 277, 279 complex hydride 269, 275 composite cathodes 220 composite hydrogen storage alloys 259, 260 compressed hydrogen 337, 338, 344 compressibility 360 cost 336, 337 cost reduction 334 Covid-19 103 Cryo-compressed 135

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Index

cryogenic processes 142 Cushing, Oklahoma 108 Cushion gas 357 Darcy equation 356 data 104 decarbonisation 335, 344 decarbonized society 331 degradation 335, 337, 340 degrees of development 154 dehydrogenation 147 demand for oil 108 Democratic Republic of Congo 106 dependency on technologies 103 depleted reservoirs 352 diffusion 361 direct reduction of iron ore 77 direct use 149 dissolution 361 driving cycles 337 driving range 334, 337, 338 durability 335, 336, 337, 340 economy of scale 334, 339, 346 efficiency 357 Electrek magazine. 104 electric vehicles 146 electride 198 electrocatalytic activity and hence the ORR 218 electro-chemical conversion technologies 49 electrochemical synthesis 202 electrolysis 56, 334, 335, 339, 342 electrolyzer 340, 342 electronegativity 205, 206 Elon Musk 104 embrittlement 362 energy 330, 334 –carrier 192 –efficiency 330, 345 –independence 330 –infrastructure 330, 345 –market 334, 342 –production 330 –resource 340 –sector 333, 341 –storage 106, 340, 344, 345, 349, 368 –storage systems 339 –system 330, 334, 345

–system comparison 63 –vector 331, 344, 345 EU’s strategic sovereignty 105 European Commission 331 European Green Deal 221, 330 European industrial capacities 103 European Union 331 experimental methodology 238 Facebook 104 FC&H2 333 fuel cells 331 Fischer-Tropsch process 77 forms 137 fossil fuels 192, 202, 203, 212 fuel cell 78, 193, 211, 333–337, 340, 342, 343 –buses 334 –durability 334 –powertrains 334, 338 –stack 334, 336, 337, 338 –vehicles 332, 333 Fuel Cell Technology Roadmap 332 future hydrogen system 49, 76 gas infrastructure 342 gas turbines 78 gates 139 GDLs 337 geopolitical risks 103 Google 104 Great African War (1998–2003) 106 gray hydrogen 127, 129 green energy 333 green hydrogen 128, 129, 130, 136, 142, 147, 335 greenhouse gas 345 H 2020 programme 332 Haber–Bosch 149 Haber–Bosch process 196, 201, 202, 208, 213 heavy-duty 337, 338 heavy-duty transport 334 high energy ball milling 291, 304 high-temperature electrolysis 340 high temperature electrolyzers 335, 340 high temperature fuel cells 335 high temperature thermal power systems 335 human rights 106

Index

Hurricane Irma 104 hydrides 114 –chemical hydrides 114 –metal hydrides 114 hydriding combustion synthesis 278, 279 hydrogen 49, 102, 331, 333, 334, 335, 336, 337, 339, 341, 342, 343, 344, 345, 346, 362 –absorption 223 –absorption properties 270, 283 –adsorption 222 –backbone 75 –boilers 77 –carrier 193, 194, 196, 207, 213 –communities 335 –diffusion 220 –economy 221, 330, 333, 345 –embrittlement 220 –energy 103 –fuel cell trucks 332 –infrastructure 49 –mixture 336 –permeation 222 –plasma metal reaction 281, 282, 283 –powered propulsion systems 338 –powertrains 334 –production 339, 342 –production cost 59 –refueling stations 335 –service stations 333 –stations 333 –storage 159, 160, 161, 162, 164, 165, 166, 167, 168, 169, 338, 341 –storage material 269, 270, 275, 276, 277, 279, 280, 284 –storage tank 337 –Strategy 105 –system cost 59 –systems 105 –tanks 334, 337 –technology 330, 335, 344 –transport 224 –transport cost 61 –trapping 225 hydrogen/material interaction 240 Hydrogen and Fuel Cell Technology Program 332 hydrogen-assisted cracking 227

hydrogenation 147 Hydrogen Europe 221 hydrogen-induced cracking 329 Hyundai 333 income from oil 108 induction melting 270, 271, 272, 273 infiltration 219 infrastructure 141, 333 insoluble compounds 366 integrate renewable energy sources 334 intermetallic hydrides 158, 160, 161 Ionic conductor backbones infiltrated with electronic/MIEC conductors 220 Iraq 108 Iron reduction 362 Japan 333 Kazakhstan 108 large-scale energy storage 339 large-scale storage 349, 368 leaching 364 Levelized Cost of Electricity 51 Libya 108 lifetime 337 light-duty vehicles 334 light-duty 334 lined rock cavern 367 linepack 144 liquefaction 134, 135, 143, 147 liquid hydrogen 78, 338, 344 liquid organic hydrogen carriers 339 Lithium-ion 106 Ln2NiO4+δ (Ln = La, Pr and Nd) 216 LNG 144 load-leveling 145 LOHC 136, 137, 145, 339 LSM 213 magnesium hydride 153 management 146 manufacturing methods 159, 165 mass density 141 MEA 336, 337 MEA poisoning 335 mechanical alloying 291, 292, 304

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402

Index

mechanical milling 269, 274, 275, 278, 279, 284, 285 melt spinning 270, 271 membrane 339 membrane electrode assembly (MEA) 336 metal ammine salts 203 metal hydride 144 metal organic framework 279, 280, 281 methanogenesis 361 Mg 291, 294, 297 Mg based alloys 162, 165 microstructural characterization 239 Middle East 108 MIEC 213, 214 Ministry of Industry and Information Technology 332 morality 107 Multi-GW renewable hydrogen production plant 49

plasma Gasification 56 plasma synthesis 270, 284 platinum loading 336 porosity 354 porous and permeable rock 353 Post-Covid recovery 103 power density 336, 337, 338 power-to-gas 341 Pr2NiO4+δ 218 Pr4Ni3O10+δ 219 Pr6O11/GDC 222 PrBaCo2O5+δ 216 primary energy sources 330 primary source 140 privacy 104 proton conducting ceramic cells 336 pyrolysis 54

N2 cleavage 197, 198, 199 nanocrystalline materials 291, 292, 301, 302 natural gas 341, 343 natural gas network 336 natural gas system 65 negative rate for oil 108 Ni -MH – nickel metal hydride batteries 249, 251, 252, 255, 258, 259, 261, 263 Nikola Motor 332 nitrogenases 202, 203, 212 nuclear source 148

rare earth 296, 298 rare earth metals 106 raw materials 105 real maturity 154 refueling stations 333, 334 refueling 334 reliance on oil and gas producers 107 RE -Mg -Ni-based alloy 261, 291, 292, 294, 295, 303 renewable energy 98, 100, 339, 341 renewable energy sources 331, 340, 342, 345 renewable hydrogen exporting countries 74 renewable synthetic fuels 77 reversible electrolysis 335 reversible electrolyzer 335 reversible solid oxide cells 342 round trip 155 Russia 108

Oapical, Oequatorial, and Ointerstitial 217 offshore wind energy 341 on-board storage 338 oxygen-deficient compounds 214 oxygen over-stoichiometric materials 214 pandemics 108 Paris Climate Conference 331 peak flow 357 peak-shaving 145 performance limitations 337 permeability 354 phase structure 293, 295, 303 phases of La–Mg–Ni system 293 photo-electrochemical water splitting 57 photovoltaic 99 pipelines 129, 133, 134, 147, 148

quantum computing 104

Sabatier and Senderens 151 safety 336, 342, 344 saline deposits 363 salt caverns 143, 352 salt cavern storage 49 Saudi Arabia 108 science and research capabilities 105 sealed Ni -MHx batteries 258, 259

Index

security implications of our energy transition 103 self-ignition combustion synthesis 277, 278 SMR 129, 130, 142 SOE technology 340 SOFC 341 solar energy systems 51 solar hydrogen generator 143, 144 solar power plants 343 solid oxide fuel cell (SOFC) 341 solvothermal 280, 281 source-use process 139 South Korea 333 space requirements 71 spark plasma sintering 270, 286, 287 specific energy 142 steam reforming 332 stack 336, 337 stationary fuel cells 335 Steam Methane Reforming 53 steam reforming 148 storage 138, 339, 341, 344, 345 storage capacity 338 storage systems 112 struggle for resources 105 sulfate reduction 362 Super Critical Water Gasification 55 sustainable energy system 49 synthesis of liquid fuels 150 synthetic hydrocarbon 102 system costs 63 system efficiency 63 systemic rival –EU – China 106

The Hydrogen Cycle 79 thermal desorption spectroscopy 238 thermochemical water splitting 343 thermolysis 149 TOP 20 oil and gas economies 107 toxicity 193, 194, 203, 213 Toyota 332 transition metals 299, 301, 304 transport 330, 333 Tsinghua University 332 types of hydrogen storage alloys 252, 253, 256 U.S. Department of Energy 333 underground hydrogen storage 341 underground storage 135, 136, 351 United States 103, 108 US National laboratories 105 Václav Havel 109 value chains 335 vapor deposition 270, 283 Venezuela 108 viscosity 355 volume factor of the gas 355 water electrolysis 338 –AEMWE 130, 132, 133 –AWE 130, 131, 132, 133, 140, 143 –PEMWE 130, 131, 132, 133, 140, 143 –SOWE 130, 132, 140 wells 362 wind energy 105 working gas 357 XRD 272, 273, 275, 276, 277, 278, 279

Tesla cars 104 Tesla, Apple, Alphabet, Dell, and Microsoft –lawsuit 106

403

zero-emission mobility 334