Electrochemical power sources: fundamentals, systems, and applications : hydrogen production by water electrolysis / [1 ed.] 9780128194256, 0128194251

Citation preview

Electrochemical Power Sources: Fundamentals, Systems, and Applications Hydrogen Production by Water Electrolysis Edited by

Tom Smolinka Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany

Juergen Garche Ulm University, Ulm, Germany

Elsevier Radarweg 29, PO Box 211, 1000 AE Amsterdam, Netherlands The Boulevard, Langford Lane, Kidlington, Oxford OX5 1GB, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States Copyright © 2022 Elsevier B.V. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress ISBN: 978-0-12-819424-9 For Information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals Publisher: Joe Hayton Acquisitions Editor: Susan Dennis Editorial Project Manager: Andrea Dulberger Production Project Manager: Debasish Ghosh Cover Designer: Greg Harris Typeset by Aptara, New Delhi, India

Contents Contributors...........................................................................................................................................xv

CHAPTER 1 The importance of water electrolysis for our future energy system...............................................................................1 Franz Lehner, David Hart 1.1 Introduction...................................................................................................................1 1.1.1 Chapter structure.................................................................................................1 1.1.2 Hydrogen and water electrolysis........................................................................2 1.1.3 Hydrogen production and use today...................................................................2 1.1.4 Does hydrogen have a color?..............................................................................3 1.2 Motivation and key drivers for hydrogen in the future energy system.........................5 1.2.1 Historic interest in hydrogen: defossilization.....................................................5 1.2.2 The contemporary drive for hydrogen: decarbonization....................................6 1.2.3 Net zero versus defossilization versus 100% renewable energy supply.............9 1.2.4 Regional drivers for hydrogen..........................................................................10 1.3 Hydrogen in global energy future scenarios...............................................................11 1.3.1 Published energy scenarios with detail on hydrogen........................................11 1.3.2 Different net-zero strategies and what they mean for hydrogen.......................14 1.4 Water electrolysis in a net-zero future........................................................................19 1.4.1 Hydrogen by production pathway in net-zero scenarios..................................19 1.4.2 Required water electrolyzer capacity in net-zero scenarios.............................20 1.4.3 Are there limitations to solar and wind deployment?.......................................21 1.4.4 Green versus blue hydrogen—renewable versus fossil energy........................24 1.5 Summary and outlook to 2030....................................................................................25 1.5.1 Unprecedented drive for water electrolysis as a transition enabler..................25 1.5.2 Best use of green hydrogen from early water electrolyzer projects.................26 1.5.3 Deployment of water electrolysis in the 2030 timeframe.................................27 1.5.4 Green hydrogen as an accelerator of the energy transition...............................28 Abbreviations, terminology, units and conversions....................................................29 References...................................................................................................................31

CHAPTER 2 Fundamentals of water electrolysis...............................................37 Pierre Millet 2.1 Introduction.................................................................................................................37 2.1.1 Brief historical perspective...............................................................................37 2.1.2 The machinery..................................................................................................38

v

vi

Contents

2.2 The water electrolysis cell..........................................................................................39 2.2.1 Cell design........................................................................................................39 2.2.2 Role of electrolyte pH.......................................................................................40 2.2.3 Different electrolysis cells................................................................................41 2.3 Thermodynamics........................................................................................................43 2.3.1 Thermochemistry – ideality..............................................................................43 2.3.2 Thermochemistry – non-ideality.......................................................................47 2.3.3 Electrochemical thermodynamics.....................................................................48 2.4 Non-equilibrium thermodynamics..............................................................................49 2.4.1 Review of dissipation sources...........................................................................49 2.4.2 The hydrogen evolution reaction (HER)...........................................................50 2.4.3 The oxygen evolution reaction (OER)..............................................................52 2.4.4 I–V curves.........................................................................................................53 2.5 Cell efficiency.............................................................................................................55 2.5.1 Energy efficiency..............................................................................................56 2.5.2 Process energy efficiency..................................................................................57 2.5.3 Coulombic efficiency........................................................................................57 2.6 Conclusions.................................................................................................................58 Glossary......................................................................................................................58 Abbreviations..............................................................................................................58 Latin symbols..............................................................................................................59 Greek symbols............................................................................................................60 References...................................................................................................................60

CHAPTER 3 Thermochemical hydrogen processes...........................................63 Maximilian B. Gorensek, Claudio Corgnale, John A. Staser, John W. Weidner 3.1 Introduction.................................................................................................................63 3.2 Metal oxide water splitting cycles..............................................................................64 3.2.1 Generic metal-oxide cycle................................................................................64 3.2.2 Cerium oxide cycle...........................................................................................65 3.2.3 Nonstoichiometric perovskite-based solar thermochemical cycles..................68 3.3 Copper–chlorine process............................................................................................69 3.4 Sulfur-based process...................................................................................................72 3.4.1 Sulfur–iodine cycle...........................................................................................72 3.4.2 Hybrid sulfur cycle...........................................................................................74 3.5 Conclusions and outlook.............................................................................................77 References...................................................................................................................78

Contents

vii

CHAPTER 4 The history of water electrolysis from its beginnings to the present..................................................................................83 Tom Smolinka, Henry Bergmann, Juergen Garche, Mihails Kusnezoff 4.1 Introduction.................................................................................................................83 4.2 First developments......................................................................................................85 4.2.1 Electrochemical fundamentals..........................................................................89 4.2.2 Direct current generators..................................................................................89 4.2.3 Drive principles.................................................................................................90 4.3 Preindustrial time up to about 1900............................................................................91 4.3.1 Water electrolysis..............................................................................................93 4.3.2 Chlor-alkali electrolysis..................................................................................100 4.4 Alkaline water electrolysis in the 20th century........................................................105 4.4.1 Industrial commercialization until 1950.........................................................105 4.4.2 Large industrial deployment until 1980..........................................................112 4.4.3 Advanced alkaline water electrolysis.............................................................117 4.5 History of polymer electrolyte membrane water electrolysis...................................126 4.5.1 Military and space application as early driver................................................126 4.5.2 Beyond niche applications..............................................................................130 4.6 History of high-temperature steam electrolysis........................................................133 4.6.1 Pioneering high-temperature fuel cells...........................................................134 4.6.2 Progress in solid oxide electrolysis cells........................................................140 4.7 Recent past with focus on new markets for renewables energies.............................143 4.7.1 Early German research projects since 1980....................................................144 4.7.2 Selected international research projects until the early 2000s........................150 Abbreviations............................................................................................................152 References.................................................................................................................153

CHAPTER 5 Alkaline electrolysis—status and prospects................................165 Asif S. Ansar, Aldo S. Gago, Fatemeh Razmjooei, Regine Reißner, Ziqi Xu, Kaspar Andreas Friedrich 5.1 Brief history of water electrolysis.............................................................................165 5.2 Physical and chemical principles of electrolysis......................................................167 5.2.1 Main technologies of water electrolysis.........................................................167 5.2.2 Efficiency of an electrolyzer...........................................................................167 5.3 Principle of operation of an alkaline electrolyzer.....................................................168 5.4 Technical concepts of electrolysis—status and prospects........................................170 5.4.1 Key performance parameters..........................................................................170 5.4.2 Technical concepts of alkaline electrolysis—past and today.........................172

viii

Contents

5.5 Materials...................................................................................................................174 5.5.1 Separators.......................................................................................................175 5.5.2 Electrodes.......................................................................................................177 5.5.3 Operating conditions.......................................................................................181 5.6 Degradation effects in alkaline electrolyzers............................................................183 5.7 Anion exchange membrane water electrolysis.........................................................185 5.8 Description of technical plants.................................................................................188 5.8.1 Components of an alkaline electrolysis plant.................................................188 5.9 Alkaline electrolysis—future prospects....................................................................190 Abbreviations............................................................................................................192 References.................................................................................................................193

CHAPTER 6 PEM water electrolysis..............................................................199 Magnus S. Thomassen, Anita H. Reksten, Alejandro O. Barnett, Thulile Khoza, Kathy Ayers 6.1 General principle and cell layout..............................................................................200 6.1.1 Introduction.....................................................................................................200 6.1.2 Cell layout.......................................................................................................201 6.2 Cell and stack materials............................................................................................202 6.2.1 Electrocatalyst for oxygen evolution reaction and hydrogen evolution reaction...........................................................................................202 6.2.2 Membrane.......................................................................................................203 6.2.3 Bipolar plates..................................................................................................204 6.2.4 Current collectors............................................................................................205 6.3 Performance on cell and system level.......................................................................206 6.4 Degradation mechanisms and lifetime......................................................................207 6.4.1 Bipolar plates and current collectors..............................................................208 6.4.2 Catalysts and electrodes..................................................................................209 6.5 Electrolyte.................................................................................................................209 6.6 System aspects and operational experience..............................................................210 6.7 System configuration and design..............................................................................210 6.8  Modeling of polymer electrolyte membrane or proton exchange membrane electrolyzers..............................................................................................................210 6.9 Material level............................................................................................................212 6.9.1 Modeling of the oxygen evolution reaction mechanism.................................212 6.9.2 Conductivity of membrane.............................................................................212 6.10 Cell level.................................................................................................................. 213 6.10.1 Semiempirical modeling.............................................................................. 213 6.10.2 Mechanistic multiphase modeling............................................................... 214

Contents

ix

6.11 Stack and system level............................................................................................. 214 6.11.1 Stack modeling............................................................................................ 214 6.11.2 System modeling combined with renewable intermittent energy sources....... 215 6.12  Cost reduction potential of polymer electrolyte membrane or proton exchange membrane electrolyzers........................................................................................... 216 6.12.1 Polymer electrolyte membrane or proton exchange membrane ­electrolyzer cost development..................................................................... 216 6.13 Cost breakdown....................................................................................................... 217 6.14 Main actors and highlights of recent years.............................................................. 218 6.15 Outlook and new concepts....................................................................................... 219 References.................................................................................................................221

CHAPTER 7 High-temperature steam electrolysis..........................................229 Annabelle Brisse, Josef Schefold, Aline Leon 7.1 Introduction and general principle............................................................................230 7.2 Architecture of solid oxide cells...............................................................................231 7.3 Cell materials............................................................................................................232 7.3.1 Electrolyte.......................................................................................................232 7.3.2 Hydrogen electrode.........................................................................................233 7.3.3 Oxygen electrode............................................................................................233 7.3.4 Manufacturing.................................................................................................234 7.4 Stack components and designs.................................................................................234 7.4.1 Interconnect materials and coatings................................................................235 7.4.2 Sealing materials.............................................................................................235 7.5 Cell performance......................................................................................................236 7.5.1 Introduction: impact of SOFC/SOEC reversibility & limits of reversibility.....................................................................................................236 7.5.2 Testing set-up..................................................................................................238 7.5.3 Performance....................................................................................................239 7.5.4 Durability........................................................................................................241 7.6 Stack performance....................................................................................................250 7.6.1 Introduction.....................................................................................................250 7.6.2 Testing set-up..................................................................................................250 7.6.3 Performance and durability.............................................................................251 7.6.4 Cycling modes and pressurized operation......................................................254 7.7 Structural analysis of cells and stacks......................................................................256 7.7.1 Classical characterization techniques.............................................................257 7.7.2 Advanced imaging with synchrotron radiation...............................................259

x

Contents

7.8 High temperature steam electrolyzer system............................................................265 7.8.1 Key parameters indicators..............................................................................265 7.8.2 Balance of plant and system design................................................................266 7.9 From cell to system cost analysis............................................................................ 267 7.10 Summary and outlook on future development........................................................ 270 Abbreviations............................................................................................................270 References.................................................................................................................272

CHAPTER 8 Chlor–alkali electrolysis............................................................281 Hiroshi Ito, Akiyoshi Manabe 8.1 Introduction...............................................................................................................281 8.2 Brief history of the chlor–alkali industry..................................................................282 8.3 Overview of chlor–alkali technologies.....................................................................283 8.3.1 Diaphragm cell................................................................................................283 8.3.2 Mercury cell....................................................................................................285 8.3.3 Membrane cell................................................................................................286 8.4 Materials and electrochemistry of a membrane cell.................................................287 8.4.1 Membranes.....................................................................................................287 8.4.2 Anodes............................................................................................................288 8.4.3 Cathodes.........................................................................................................290 8.4.4 Cell design and operation...............................................................................291 8.5 System configuration of membrane cell.................................................................. 292 8.5.1 Overview of system configuration..................................................................292 8.5.2 Rectifying.......................................................................................................293 8.5.3 Brine processing.............................................................................................293 8.5.4 Chlorine processing........................................................................................294 8.5.5 Caustic soda processing..................................................................................294 8.5.6 Hydrogen processing......................................................................................294 8.6 Overview of the chlor–alkali industry with membrane cells....................................295 8.6.1 Main players...................................................................................................295 8.6.2 Task analysis...................................................................................................295 8.7 Future outlook and a new cathode concept...............................................................299 Acknowledgments....................................................................................................302 References.................................................................................................................303

CHAPTER 9 Seawater electrolysis................................................................305 Youri Gendel, Gidon Amikam, Paz Nativ 9.1 Introduction...............................................................................................................305 9.1.1 Hydrogen economy and seawater electrolysis................................................305 9.1.2 Thermodynamics of water electrolysis...........................................................307

Contents

xi

9.1.3 State-of-the-art water electrolysis techniques.................................................308 9.1.4 Water quality requirements for water electrolysis..........................................309 9.2 Seawater electrolysis.................................................................................................310 9.2.1 Seawater composition.....................................................................................310 9.2.2 Chlorine evolution reaction in seawater electrolysis......................................311 9.3 Chlorine-free seawater electrolysis for hydrogen production...................................314 9.3.1 Seawater electrolysis at very low current density...........................................314 9.3.2 Implementation of special catalysts to prevent Cl2 production in seawater electrolysis.......................................................................................315 9.3.3 Membrane-assisted chlorine-free seawater electrolysis.................................316 9.3.4 Introduction of elemental sulfur into seawater to avoid Cl2 evolution reaction...........................................................................................................316 9.3.5 Alkaline seawater electrolysis........................................................................317 9.4 Summary...................................................................................................................323 References.................................................................................................................323

CHAPTER 10 Economic considerations for hydrogen production with a focus on polymer electrolyte membrane electrolysis............................327 Alex Badgett, Mark Ruth, Bryan Pivovar 10.1 Introduction............................................................................................................. 328 10.2 Background.............................................................................................................. 330 10.2.1 Electrolyzer types........................................................................................ 330 10.3 Hydrogen markets................................................................................................... 333 10.4 Hydrogen production cost economic considerations............................................... 335 10.4.1 Current electrolysis cost projections............................................................ 336 10.4.2 Estimating the economics of hydrogen........................................................ 337 10.5 Basic hydrogen economics (100 level).................................................................... 338 10.5.1 Operations and maintenance costs............................................................... 338 10.5.2 Electricity costs............................................................................................ 338 10.5.3 Capacity factor............................................................................................. 338 10.5.4 Efficiency..................................................................................................... 339 10.5.5 Capital costs................................................................................................. 339 10.5.6 Case studies................................................................................................. 339 10.6 Hydrogen economics (300 level)............................................................................. 344 10.6.1 Electrolyzer capital costs............................................................................. 344 10.6.2 Operating lifetime........................................................................................ 346 10.6.3 Fixed operating and maintenance costs....................................................... 346 10.6.4 Electricity prices and capacity factor........................................................... 346

xii

Contents

10.7 Advanced hydrogen economics (500 level)............................................................. 352 10.7.1 Operating strategy and potential impacts on durability............................... 352 10.7.2 Sensitivity analysis...................................................................................... 356 10.7.3 Implications for the electric power sector.................................................... 358 10.8 Assumptions and knowledge gaps........................................................................... 358 10.9 Summary.................................................................................................................. 358 Funding sources........................................................................................................361 Acknowledgments....................................................................................................361 Conflict of interest statement....................................................................................361 References.................................................................................................................361

CHAPTER 11 Regenerative fuel cells..............................................................365 Juergen Garche, Tom Smolinka, Maria Assunta Navarra, Stefania Panero, Bruno Scrosati 11.1 Introduction............................................................................................................. 365 11.1.1 General principles........................................................................................ 365 11.1.2 Unitized and discrete stack configurations.................................................. 366 11.1.3 Types of regenerative fuel cell..................................................................... 368 11.2 Regenerative fuel cells based on proton exchange membranes technology............ 370 11.2.1 Cell materials and components.................................................................... 370 11.2.2 Main system components............................................................................ 375 11.2.3 Technical parameters................................................................................... 381 11.2.4 Developers of proton exchange membranes- unitized regenerative fuel cell systems.................................................................................................. 388 11.3 Other unitized regenerative fuel cell systems.......................................................... 391 11.3.1 Alkaline unitized regenerative fuel cell systems......................................... 391 11.3.2 Solid oxide unitized regenerative fuel cell systems..................................... 391 11.3.3 Unitized regenerative fuel cells with other redox pairs............................... 393 11.4 Applications............................................................................................................. 394 11.4.1 General......................................................................................................... 394 11.4.2 Military and space applications................................................................... 395 11.4.3 Terrestrial energy storage............................................................................. 396 11.5 Outline..................................................................................................................... 399 Abbreviations............................................................................................................399 References.................................................................................................................400

CHAPTER 12 New electrolyzer principles: decoupled water splitting................407 Avigail Landman, Avner Rothschild, Gideon S. Grader 12.1 Introductions............................................................................................................ 407

Contents

xiii

12.2 Electrolytic schemes for decoupled water splitting................................................. 409 12.2.1 Electrolytic water splitting with soluble redox mediators........................... 410 12.2.2 Electrolytic water splitting with solid auxiliary redox electrodes............... 418 12.3 Electrochemical–chemical cycles for decoupled water splitting............................. 424 12.3.1 Electrochemical—chemical water splitting with soluble redox couples..... 425 12.3.2 Electrochemical—chemical water splitting with solid redox electrodes..... 431 12.4 Decoupled photoelectrochemical and photocatalytic water splitting...................... 443 12.5 Summary.................................................................................................................. 447 Acknowledgment......................................................................................................449 Competing interests..................................................................................................450 References.................................................................................................................450

CHAPTER 13 Hydrogen storage���������������������������������������������������������������������455 Henrietta W. Langmi, Nicolaas Engelbrecht, Phillimon M. Modisha, Dmitri Bessarabov 13.1 Introduction............................................................................................................. 455 13.2 Physical storage....................................................................................................... 456 13.2.1 Compressed hydrogen storage..................................................................... 456 13.2.2 Cryogenic hydrogen storage........................................................................ 457 13.2.3 Cryo-compressed hydrogen storage............................................................ 458 13.3 Adsorption storage................................................................................................... 460 13.3.1 Metal–organic frameworks.......................................................................... 460 13.3.2 Carbon-based materials............................................................................... 461 13.3.3 Porous organic polymers............................................................................. 462 13.4 Chemical hydrogen storage..................................................................................... 463 13.4.1 Metal hydrides............................................................................................. 463 13.4.2 Liquid organic hydrogen carriers................................................................. 464 13.4.3 Ammonia..................................................................................................... 469 13.4.4 Formic acid.................................................................................................. 470 13.4.5 Methanol...................................................................................................... 471 13.5 Comparison of different storage technologies......................................................... 472 13.6 Large-scale and underground hydrogen storage...................................................... 473 13.7 Storage for mobile applications............................................................................... 474 13.8 Summary.................................................................................................................. 477 13.9 Suggestions for further reading............................................................................... 479 Abbreviations............................................................................................................480 References.................................................................................................................481 Index................................................................................................................................................... 487

Contributors Gidon Amikam Technion - Israel Institute of Technology, Faculty of Civil and Environmental Engineering, Haifa, Israel Asif S. Ansar German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Electrochemical Energy Technology, Stuttgart, Germany Kathy Ayers NEL Hydrogen, Wallingford, CT, United States Alex Badgett National Renewable Energy Laboratory, Golden, Denver, CO, United States Alejandro O. Barnett Hystar, Oslo, Norway; SINTEF Industry, Trondheim, Norway Henry Bergmann Anhalt University of Applied Sciences, Köthen, Germany Dmitri Bessarabov North-West University, HySA Infrastructure Centre of Competence, Faculty of Engineering, Potchefstroom, South Africa Annabelle Brisse European Institute for Energy Research, Emmy-Noether-Strasse, Karlsruhe, Germany Claudio Corgnale Greenway Energy, Aiken, SC, United States Nicolaas Engelbrecht North-West University, HySA Infrastructure Centre of Competence, Faculty of Engineering, Potchefstroom, South Africa Kaspar Andreas Friedrich German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Electrochemical Energy Technology, Stuttgart, Germany; University of Stuttgart, Institute of Building Energetics, Thermal Engineering and Energy Storage (IGTE), Stuttgart, Germany Aldo S. Gago German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Electrochemical Energy Technology, Stuttgart, Germany Juergen Garche Ulm University, Ulm, Germany Youri Gendel Technion - Israel Institute of Technology, Faculty of Civil and Environmental Engineering, Haifa, Israel

xv

xvi

Contributors

Maximilian B. Gorensek Savannah River National Laboratory, Aiken, SC, United States Gideon S. Grader Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa, Israel David Hart E4tech Sarl, Lausanne, Switzerland Hiroshi Ito National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan Thulile Khoza SINTEF Industry, Trondheim, Norway Mihails Kusnezoff Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Dresden, Germany Avigail Landman Department of Chemical Engineering, Technion - Israel Institute of Technology, Haifa, Israel Henrietta W. Langmi University of Pretoria, Department of Chemistry, Hatfield, South Africa Franz Lehner E4tech Sarl, Lausanne, Switzerland Aline Léon European Institute for Energy Research, Emmy-Noether-Strasse, Karlsruhe, Germany Akiyoshi Manabe De Nora Permelec Ltd., Fujisawa, Japan Pierre Millet Paris-Saclay University, ICMMO-Eriee, UMR CNRS 8182, Orsay, France Phillimon M. Modisha North-West University, HySA Infrastructure Centre of Competence, Faculty of Engineering, Potchefstroom, South Africa Paz Nativ Technion - Israel Institute of Technology, Faculty of Civil and Environmental Engineering, Haifa, Israel Maria Assunta Navarra Sapienza University, Rome, Italy Stefania Panero Sapienza University, Rome, Italy Bryan Pivovar National Renewable Energy Laboratory, Golden, Denver, CO, United States Fatemeh Razmjooei German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Electrochemical Energy Technology, Stuttgart, Germany

Contributors

Regine Reißner German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Electrochemical Energy Technology, Stuttgart, Germany Anita H. Reksten SINTEF Industry, Oslo, Norway Avner Rothschild Department of Materials Science and Engineering, Technion - Israel Institute of Technology, Haifa, Israel Mark Ruth National Renewable Energy Laboratory, Golden, Denver, CO, United States Josef Schefold European Institute for Energy Research, Emmy-Noether-Strasse, Karlsruhe, Germany Bruno Scrosati Elettrochimica & Energia, Rome, Italy Tom Smolinka Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, Germany John A. Staser Ohio University, Athens, OH, United States Magnus S. Thomassen SINTEF Industry, Oslo, Norway; Hystar, Oslo, Norway John W. Weidner University of Cincinnati, Cincinnati, OH, United States Ziqi Xu German Aerospace Center (DLR), Institute of Engineering Thermodynamics, Electrochemical Energy Technology, Stuttgart, Germany

xvii

CHAPTER

1

The importance of water electrolysis for our future energy system

Franz Lehner, David Hart E4tech Sarl, Lausanne, Switzerland

Chapter Outline 1.1 Introduction........................................................................................................................................ 1 1.1.1  Chapter structure..........................................................................................................1 1.1.2  Hydrogen and water electrolysis.....................................................................................2 1.1.3  Hydrogen production and use today................................................................................2 1.1.4  Does hydrogen have a color?..........................................................................................3 1.2  Motivation and key drivers for hydrogen in the future energy system..................................................... 5 1.2.1  Historic interest in hydrogen: defossilization....................................................................5 1.2.2  The contemporary drive for hydrogen: decarbonization......................................................6 1.2.3  Net zero versus defossilization versus 100% renewable energy supply................................9 1.2.4  Regional drivers for hydrogen.......................................................................................10 1.3  Hydrogen in global energy future scenarios....................................................................................... 11 1.3.1  Published energy scenarios with detail on hydrogen.......................................................11 1.3.2  Different net-zero strategies and what they mean for hydrogen........................................14 1.4  Water electrolysis in a net-zero future............................................................................................... 19 1.4.1  Hydrogen by production pathway in net-zero scenarios...................................................19 1.4.2  Required water electrolyzer capacity in net-zero scenarios...............................................20 1.4.3  Are there limitations to solar and wind deployment?.......................................................21 1.4.4  Green versus blue hydrogen–renewable versus fossil energy.............................................24 1.5  Summary and outlook to 2030........................................................................................................... 25 1.5.1  Unprecedented drive for water electrolysis as a transition enabler....................................25 1.5.2  Best use of green hydrogen from early water electrolyzer projects....................................26 1.5.3  Deployment of water electrolysis in the 2030 timeframe.................................................27 1.5.4  Green hydrogen as an accelerator of the energy transition...............................................28

1.1 Introduction 1.1.1  Chapter structure This chapter sets the scene for the potential role of water electrolysis in the global future energy system. Firstly, hydrogen, water electrolysis, and other hydrogen production routes are briefly introduced. Electrochemical Power Sources: Fundamentals, Systems, and Applications. DOI: https://doi.org/10.1016/B978-0-12-819424-9.00008-2 Copyright © 2022 Elsevier B.V. All rights reserved.

1

2

Chapter 1  The importance of water electrolysis for our future energy system

Secondly, historic and contemporary drivers for hydrogen are identified and discussed. In the third section, an overview of the role of hydrogen in publicly available global energy system models is provided, suggested options for decarbonization are analyzed, and the amount of hydrogen they need is compared. The fourth section discusses the drivers that determine the amount of water electrolysis needed in different energy futures and provides an overview of quantifications in public studies. This chapter then closes with developments in water electrolysis at the beginning of the 2020s and an outlook to 2030.

1.1.2  Hydrogen and water electrolysis While some hydrogen “deposits” exist on earth and could potentially be exploited as primary energy [10], the majority of the value hydrogen brings to the future energy system is in acting as a secondary energy carrier. Virtually any source of energy can be used to make hydrogen, and hydrogen can be used as a fuel in virtually any energy application. Beyond its pure energy value, hydrogen is or can be used as feedstock in the fertilizer and chemical industry, and as a reductant in steel making. Hydrogen is used in a closed cycle if it is made by splitting water into hydrogen and oxygen, and then later recombined with oxygen from the air to release energy and again form water. If the energy to split water comes from renewable sources, hydrogen is considered a renewable energy carrier, not depleting any of the planet’s energy resources. If hydrogen is emitted to the atmosphere unused (e.g., leakages, venting off), it acts as a secondary greenhouse gas (GHG) [11]. However, since there is an inherent economic incentive for operators to minimize gas losses, no major climate impact is to be expected from a potential future hydrogen infrastructure. As part of global efforts to address the climate crisis, the global energy system is undergoing a transition away from fossil fuels to low-carbon electricity (e.g., to solar and wind). In the first instance, this drives direct electrification of end uses (e.g., battery-electric cars instead of internal combustion engines; heat pumps instead of oil heating). However, there are applications hard or impossible to directly electrify (e.g., aviation, shipping, long-haul road transport, interseasonal energy storage, nonenergy feedstock uses). This is where water electrolysis comes into play, as it extends the reach of low-carbon electricity from wind and solar by using it to make molecule-based energy carriers and feedstock—via hydrogen. In addition to using this hydrogen directly as a fuel (e.g., in fuel cell vehicles, heating appliances), it can also be used to make synthetic natural gas and various synthetic liquid fuels. These include “drop-in fuels” that can directly replace conventional jet, diesel, and gasoline, as well as alternative fuels such as ammonia and methanol, which could, for example, be used as shipping fuels. In the remainder of this chapter, synthetic fuels (synfuels) and chemicals made with hydrogen from water electrolysis are referred to as e-fuels.

1.1.3  Hydrogen production and use today Despite its potential to play a key role in the energy transition, neither the concept nor the use of hydrogen as a fuel is anything new. In the 19th century, hydrogen was used in early combustion engines, and until the middle of the 20th century, it was a major component of town gas [3]. Hydrogen has also been produced and used at an industrial scale for more than a century in the production of ammonia fertilizers. More detail on the historical role of water electrolysis in this application can be found in Chapter 3.

1.1  Introduction

3

Hydrogen by demand Mt/a 6

Hydrogen by producon route Mt/a Coal

116 Mt/a (2018 esmate)

Ammonia (ferlisers)

25 Ammonia (industrial use)

16 4

Natural gas

52

Electricity/other

700°C for feasible oxidation reaction yields (i.e., hydrogen production rates) [1]. Recent studies examining the performance of ceria cycles doped with 5–15 mol% Fe, Co, No, and Mn elements appear to demonstrate enhanced hydrogen production rates, reaching values up to 25% higher than the corresponding values achieved for the undoped ceria cycle [8]. However, the main

3.2  Metal oxide water splitting cycles

67

drawback of the proposed approach was an increase in the oxidation reaction temperature, as was the case for the Zr-doped ceria cycle, requiring temperatures on the order of 800–1150°C [8]. Other metals have also been considered as possible dopant elements. The inclusion of tantalum or trivalent lanthanides (La, Sm, and Gd) to form binary oxides was investigated [9]. Ceria doped with tantalum showed high reducibility, with a reduction of the maximum operating temperatures, but structural evolution during thermal treatment resulted in the formation of a secondary phase that hindered the water dissociation reaction [9]. Doping with trivalent lanthanides was also tested, demonstrating improved thermal stability during cycling, with hydrogen production rates comparable to those obtained with ceria. The ceria-based cycles, including their modifications based on the introduction of suitable dopant materials, still require additional development to achieve satisfactory techno-economic performance for actual on-sun cycle testing and demonstrate the ability to produce hydrogen on a large scale. However, preliminary techno-economic feasibility studies have been carried out. Fig. 3.2 shows a simplified process schematic [6]. A comprehensive techno-economic analysis (and one of the few currently available in the literature) has been carried out recently showing an economic comparison between the ferrite cycle, with Ni dopant, and the ceria cycle, operating between 1400°C (reduction section) and 900°C (oxidation section) [10]. Results demonstrated higher STH efficiencies achieved with the ceria cycle (13.4%) than the ferrite cycle (6.4%), mainly due to the different kinetics and reaction yield [10]. This resulted in a baseline hydrogen production cost on the order of 14.7 $/kg (the referenced value is 13.06 €/kg in 2019 [10]), projecting a value of 7.5 $/kg (6.68 €/kg, in 2019) for the best-case scenario [10].

FIG. 3.2  Simplified ceria cycle schematic (Win is the high-temperature solar heat input, Wout is the exothermic heat of reaction to be removed).

68

Chapter 3  Thermochemical hydrogen processes

3.2.3  Nonstoichiometric perovskite-based solar thermochemical cycles Perovskite oxides (ABO3) have recently received a great deal of attention as potential materials for use in two-step thermochemical water-splitting cycles driven by concentrating solar heat. The mineral perovskite, CaTiO3, is the protonym for this class of compounds. A and B are typically cations having very different sizes (like Ca2+ and Ti4+), with the larger A cation coordinating with 12 oxide anions, whereas the smaller B cation coordinates with 6. What makes these materials interesting for water-splitting applications is that both the A and B cations can be partially substituted by one or more other cations. For example, lanthanum manganite, LaMnO3, can be doped with strontium, resulting in lanthanum strontium manganite (LSM), La1−xSrxMnO3, which is used as a cathode material in solid oxide fuel cells [11]. In fact, both cation positions can have one or more substitutions, as in the case of Ba0.5Sr0.5Co0.7Fe0.2Ni0.1O3, a perovskite developed for use as a low-temperature solid oxide fuel cell cathode material [12]. Because of this interchangeability, there is literally an endless variety of possible combinations of cation substitutions that can be used to fine-tune thermodynamic properties, in particular, the stability of specific perovskites with respect to a wide range of oxygen nonstoichiometries at equilibrium with different oxygen partial pressures at different temperatures. While traditional metal oxide cycles rely on shuttling metal oxidation states between a reduced and an oxidized condition (e.g., Ce (III) and Ce (IV) for the cerium oxide cycle), using water as the oxidizing agent, nonstoichiometric perovskite oxide cycles take advantage of perovskites’ ability to exist in stable form with different levels of oxygen vacancy at different temperatures and oxygen partial pressures. The two reactions that comprise these cycles can be written as:

ABO3−δ1 → ABO3−δ 2 +

δ 2 − δ1 O2 (3.6) 2

ABO3−δ 2 + (δ 2 − δ1 ) H 2 O → ABO3−δ1 + (δ 2 − δ1 ) H 2 (3.7)

Reaction (3.6) is the high-temperature step, in which a perovskite oxide with a lower level of nonstoichiometry is heated to release oxygen until its level of nonstoichiometry is at equilibrium with the oxygen partial pressure. This can take place at lower temperatures than those typically required for metal oxide cycles, making it an attractive alternative. The oxygen-depleted perovskite is then cooled to a lower temperature in an inert atmosphere and subsequently oxidized with steam (reaction 3.7), extracting oxygen from the water to decrease its nonstoichiometry and releasing hydrogen. The cycle is closed by heating the perovskite and repeating the process. A recent review of nonstoichiometric redox-active perovskite materials [13] provides a good overview of the current pursuit of their use for thermochemical water-splitting. It includes a graphical representation of one such water-splitting cycle using La0.6Sr0.4MnO3 (LSM(x = 0.4)) as the active material, with the equilibrium nonstoichiometry taken from Scheffe et al. [14] and the cycle adapted from Davenport et al. [15] (Fig. 3.3). As depicted in Fig. 3.3, this particular cycle would have the LSM cycling over a narrow range of nonstoichiometry, between δ1 = 0.03 and δ2 = 0.12. LSM (x = 0.4, δ1 = 0.03) would be heated with concentrating solar energy to 1400°C (1673 K), driving off enough oxygen in an endothermic process to increase the nonstoichiometry to δ2 = 0.12. The temperature would then be dropped to 1000°C (1273 K) under an inert atmosphere in a quench step. The resulting LSM (x = 4, δ2 = 0.12) would be oxidized with steam at the lower temperature in an exothermic process, releasing hydrogen and restoring LSM (x = 0.4, δ1 = 0.03).

3.3  Copper–chlorine process

69

FIG. 3.3  Equilibrium nonstoichiometry of LSM (x = 0.4) as a function of oxygen partial pressure between 873 and 1973 K [14]. Representation of two-temperature water-splitting cycle (reduction reaction at 1673 K (1400°C) D to A; quench to 1273 K (1000°C) A to C; oxidation reaction at 1273 K (1000°C) C to D) from Haeussler et al. [13] as depicted by Davenport et al. [15]. LSM, lanthanum strontium manganite.

The previously mentioned ability of the cations A and B to have multiple substitutions means that there is an endless variety of possible perovskite oxide formulations. Substitutions affect thermodynamic properties, opening the possibility of fine-tuning the composition to achieve the desired behavior. The viability of a nonstoichiometric perovskite as the basis for a water-splitting cycle depends on the energy needed to create or fill oxygen vacancies. Creating an oxygen vacancy in ceria, the typical baseline material against which nonstoichiometric perovskites are compared, requires about 5 ev/O atom [16]. Temperatures of at least 1550°C are needed to remove oxygen at that level. However, if the vacancy is created too easily, there will not be enough of a driving force to break the O–H bond and release hydrogen under the low-temperature steam oxidation step. The perovskite material La1−xSrxMnyAl1-yO3 (LSMA) has been flagged as representative of the lower limit of feasible oxygen vacancy formation energy for a thermochemical water-splitting cycle [16,17]. Depending on the values of x and y, about 1.5–2.5 eV/O atom are needed to create vacancies [18]. Consequently, materials with oxygen vacancy formation energies in the range of 2.5–5 eV/O atom are potential candidates for thermochemical water-splitting applications. Of course, the kinetics of the two reactions as well as the structural stability over the range of nonstoichiometry are important factors.

3.3  Copper–chlorine process Several copper–chlorine (Cu–Cl) thermochemical cycles exist, including the so-called five-step, four-step, and three-step processes [19]. Hydrogen production steps vary in the different processes. Some, but not all, of the Cu–Cl cycles possess an electrochemical step, and this step is different among the various cycles.

70

Chapter 3  Thermochemical hydrogen processes

The Cu–Cl process historically has received less attention than other thermochemical cycles, including the S–I process discussed in the next section, but the pace of research has increased in recent years. The Cu–Cl process possesses several advantages over other methods of hydrogen production, including lower environmental impact than traditional carbon-based processes and potentially greater efficiency than electrolysis. Compared to other thermochemical cycles, the maximum operating temperature is only around 550°C, significantly lower than, for example, the S–I cycle [20,21]. Thus, the Cu–Cl cycle does not rely on high-temperature next-generation nuclear reactors as a heat source. In that regard, the Cu–Cl process could be coupled to solar concentrators for process heat. One of the earliest reports of the Cu–Cl cycle occurred at the 2nd Information Exchange Meeting on Nuclear Production of Hydrogen [22]. Many of the earlier reports in the field focused on the fivestep process, in which H2 is generated in a high-temperature step at around 450°C [23–28]. In these cycles, Cu is generated electrolytically and participates in the high-temperature H2 production reaction. Electrolytic production of Cu adds another step to the process and requires the handling of solid copper metal [22].

Step1 : 2Cu(s) + 2HCl(g) → 2CuCl(molten ) + H 2 (g);450 ° C (3.8)



Step 2 : 4CuCl(aq ) → 2Cu(s) + 2CuCl 2 ; 30 − 80 ° C (3.9) Step3 : CuCl2 (aq ) + n f H 2 O(l) → CuCl2 ⋅ n h H 2 O(s) + ( n f − n h ) H 2 O



n f > 7.5,n h = 0 − 4; 30 − 80 ° C

(3.10)



Step 4 : 2CuCl2 ⋅ n h H 2 O(s) + H 2 O(g) → CuOCuCl2 (s) + 2HCl(g) + n h H 2 O(g) (3.11) n h = 0 − 4;375 ° C



Step 5 : CuOCuCl 2 (s) → 2CuCl(molten ) + 1/ 2O2 (g); 530 ° C (3.12)

The highest temperature step in the process is decomposition Step 5. At 530°C, it is considerably lower than the highest temperature step required in the S–I process. It is possible to reduce the five-step process to a three-step process with the same 450°C hydrogen-production steps, but the heat grade is increased substantially [19]. A discussion of the alternative Cu–Cl cycle in which hydrogen is generated electrolytically occurred at the 4th International Topical Meeting on High-Temperature Reactor Technology in 2008 [29]. In this process, the electrochemical production of Cu is obviated in favor of a CuCl/HCl electrolysis step that produces hydrogen electrochemically. In that sense, the three-step copper chlorine process may more closely resemble many of the other thermochemical cycles in which hydrogen is generated electrolytically. The electrochemical half-reactions for this process are as follows [30]:

Anode : 2CuCl + 2HCl → 2CuCl 2 + 2H + + 2e − (3.13)



Cathode : 2H + + 2e − → H 2 (3.14)

Electrolytic generation of hydrogen in this way simplifies the Cu–Cl system from a five-step process to a three- or four-step process without an increase in required heat grade. The CuCl/HCl electrolysis process, in which hydrogen is generated in the electrochemical step (vs the high-temperature production of hydrogen in the five-step Cu–Cl cycle) may ultimately require lower energy requirements and

3.3  Copper–chlorine process

71

operate at greater energy efficiency. For example, the five-step Cu–Cl process may require 500 kJ/mol H2 or more, while the three-step Cu–Cl process requires around 450 kJ/mol H2; the energy efficiency of the five-step process is perhaps as low as about 42% while efficiencies of >50% may be achievable with the three-step process, according to a recent energy analysis comparing the various Cu–Cl processes [31]. The electrochemical production of hydrogen can be carried out at near ambient temperatures (