Solar-Driven Green Hydrogen Generation and Storage [1, 1 ed.] 0323995802, 9780323995801

Table of contents :
Front Cover
Solar-Driven Green Hydrogen Generation and Storage
Copyright
Contents
Contributors
Preface
Acknowledgments
Chapter 1: Exploring the hydrogen evolution reaction (HER) side of perovskite-based materials during photoelectrochemical wa
1. Introduction
2. Photo(electro)chemical mechanism of catalyst: Perovskite oxide materials
3. Double perovskites as HER catalysts
4. Tailoring double perovskites
5. Nano-structural engineering
6. Effect of A-site cation doping
7. Effect of B-site cation doping
8. Effect of anion doping
9. Effect of oxygen vacancies
10. Prospect and summary
References
Chapter 2: Phosphorene-based functional nanomaterials for photoelectrochemical water splitting
1. Introduction
2. About phosphorene
2.1. Black, blue, and green phosphorene: Characteristics and recent advances
2.2. Stability in air/water
3. Phosphorene functionalized nanomaterials for the PEC water splitting
3.1. Hydrogen evolution reaction (HER)
3.2. Oxygen evolution reaction (OER)
3.3. Bifunctional
4. Challenges, gaps, and perspectives
Acknowledgment
References
Chapter 3: Polymer-based catalyst for photoelectrochemical water splitting
1. Introduction
2. Basic principles of PEC of water
2.1. Polymeric carbon nitride
2.2. Conjugated polymers and organic polymers
2.3. Heterostructures of polymers-based hybrids for overall water splitting
3. Conclusion
References
Chapter 4: Transition metal-based single-atom catalyst for photoelectrochemical water splitting
1. Introduction
2. Fundamental mechanism for water splitting reactions
2.1. Hydrogen evolution reaction (HER)
2.2. Oxygen evolution reaction (OER)
3. Advantages of single-atom catalysts (SACs)
4. Transition metal-based single-atom catalysts for PEC water splitting
4.1. Nickel-based single-atom catalysts for PEC water splitting
4.2. Copper-based single-atom catalysts for PEC water splitting
4.3. Cobalt-based single-atom catalysts for PEC water splitting
4.4. Palladium-based single-atom catalysts for PEC water splitting
5. Future perspectives and conclusion
Acknowledgment
References
Chapter 5: Clathrate hydrate as a potential medium for hydrogen storage application
1. Introduction
2. Clathrate hydrate structures specific to hydrogen hydrate
3. The thermodynamic aspect of hydrogen clathrate
4. Kinetic aspects of hydrogen clathrate hydrate
5. Storing hydrogen in the presence of THF and promoters with a tuning effect
6. Modeling of hydrogen clathrate hydrates
7. Conclusion and future direction
References
Chapter 6: Advanced carbon-based nanomaterials for photoelectrochemical water splitting
1. Introduction
2. Performance evaluation of electrocatalysts
2.1. Activity
2.2. Stability
2.3. Efficiency
3. Different carbon materials
3.1. Graphene
3.2. Graphitic carbon nitride
3.3. Carbon quantum dots
3.4. Fullerene
3.5. Carbon nanotubes
4. Enhancing the properties of carbon-based materials
4.1. Surface functionalization
4.2. Doping
4.3. Interface engineering
5. Conclusions and future outlook
References
Chapter 7: MXene-transition metal compound sulfide and phosphide hetero-nanostructures for photoelectrochemical water splitt
1. Introduction
2. Synthetic routes to MXene-based hetero-nanostructures
3. Photoelectrochemical water-splitting application
4. Conclusion
Acknowledgments
References
Chapter 8: Design and advances of semiconductors for photoelectrochemical water-splitting
1. Introduction
2. Principle of water-splitting
3. Photoelectrode materials
3.1. Pure metal oxides as semiconductors
3.2. Metal chalcogenides as semiconductors
3.3. Modified semiconductors and composites
3.4. Hybrid organic-inorganic semiconductors
4. Tandem reaction setup
5. Conclusion and outlook
References
Chapter 9: Dye-sensitized photoelectrochemical cells in water splitting
1. Introduction
2. Device architecture
2.1. Chromophores
2.2. Water oxidation catalyst (WOC)
2.3. Adsorbing group
2.4. Electrolyte
2.5. Semiconductor oxide
3. Working principle of DSPECs
4. Dye-sensitized photoanodes for water splitting cells
4.1. Photoanode materials
5. Dye-sensitized photocathodes for water splitting cells
5.1. Photocathode materials
6. Tandem DSPECs for water splitting
7. Conclusion and outlook
References
Chapter 10: Photobiological hydrogen production: Introduction and fundamental concept
1. Introduction
2. Fundamental concepts of photobiological hydrogen generation
2.1. Direct biophotolysis
2.2. Indirect biophotolysis
2.3. Photofermentation
3. Enzymes involved in photobiohydrogen generation
3.1. Hydrogenase
3.1.1. Classification of hydrogenase enzyme
3.1.2. [NiFe] Hydrogenase
3.1.3. [FeFe] Hydrogenases
3.1.4. [Fe] Hydrogenases
3.2. Nitrogenase
4. Modulation of factors affecting photobiological hydrogen production
4.1. Role of genetic modification
4.2. Role of metabolic modulation
4.3. Choice of substrates/nutrients
4.4. Co-culture and immobilization
4.5. Integrated systems
4.6. Photobioreactor design
5. Challenges and future prospects
6. Conclusion
Acknowledgments
References
Chapter 11: Biological hydrogen production driven by photo-fermentation processes
1. Introduction
2. Biological hydrogen production by photo-fermentation process
2.1. Direct bio-photolysis
2.2. Indirect bio-photolysis
3. Photo-fermentation process
3.1. Effect of substrate
3.2. Role of nitrogen source
3.3. Effect of light illumination
3.4. Effect of metal ions and minerals
4. Hydrogen production from waste
4.1. Hydrogen production from waste containing organic acid and sugar
5. Future perspectives
6. Conclusion
References
Chapter 12: Photobiological hydrogen production by microorganisms
1. Introduction
2. Different mechanism of production of photobiological hydrogen
2.1. Direct photolysis
2.2. Indirect photolysis
2.3. Photofermentation
2.4. Dark fermentation
3. Photobiological H2 production by hydrogenase and nitrogenase
3.1. Hydrogenases
3.2. Nitrogenases
4. Bioreactor systems in photobiological hydrogen production
5. Bioreactors incorporating cyanobacteria and green algae
6. Feedstocks for photobiological hydrogen production
7. Advantages, disadvantages, and challenges of using photobiological methods
7.1. Advantages
7.2. Disadvantages
7.3. Challenges
8. Conclusion
References
Further reading
Chapter 13: Photobiological production of hydrogen from biomass
1. Introduction
2. Mechanism of photo-biohydrogen production
2.1. Nitrogenase
2.2. Hydrogenase
3. Photobiological hydrogen production technologies
4. Direct biophotolysis
5. Indirect photolysis
5.1. Dark fermentation
5.2. Photofermentation
6. Microbial biomass as feedstock for biohydrogen production
7. Experimental conditions and approaches to enhance biohydrogen production
7.1. Genetic engineering of microorganisms to improve their hydrogen production capacity
8. Bioreactors for commercial biohydrogen production
9. Conclusion
References
Chapter 14: Challenges in scaling low-carbon hydrogen production in Europe
1. Hydrogen requirements in Europe to achieve net zero emissions
1.1. Global projections
1.2. Hydrogen's role in the net zero emissions (NZE) scenario
1.3. The strategy in the EU
2. Hydrogen production and use
2.1. Hydrogen applications
2.2. Centralized vs. distributed hydrogen production
2.3. Hydrogen valleys
3. Blue hydrogen
3.1. CO2 sequestration technology
3.1.1. Capture of CO2
3.1.2. Transportation
3.1.3. Storage
3.2. CCS facilities
3.3. Regulatory/political position in Europe of CCS
3.4. Viable blue hydrogen route
3.5. Challenges for blue hydrogen
4. Green hydrogen
4.1. Renewable energy sources
4.2. Transport and storage
4.3. Current projects
4.3.1. SINES Portugal
4.3.2. HyDeal Spain
4.4. Regulatory/political position in Europe
4.4.1. EU hydrogen strategy
4.4.2. Policies that incentivize the investigation and application of hydrogen as a source of energy in Portugal
4.5. Challenges for green hydrogen
5. Conclusions
References
Chapter 15: Photobioreactor for hydrogen production
1. Introduction
2. Photobioreactors for hydrogen production
2.1. Shaking flasks
2.2. Stirred tank
2.3. Horizontal tubular
2.4. Coiled tubular
2.5. Vertical tubular
2.6. Immobilization
2.7. Flat panel
3. Materials used for different components of the photobioreactor
4. Metals used for construction
4.1. Coating
4.2. Glass
4.3. Photobioreactors incorporating cyanobacteria
4.4. Photobioreactors which use green algae
References
Chapter 16: Thermochemical hydrogen production
1. Introduction
2. Thermochemical conversion of biomass into hydrogen
2.1. Pyrolysis and gasification
2.1.1. Biomass pyrolysis process
2.1.2. Biomass gasification process
2.1.3. Supercritical water gasification of biomass
2.1.4. Chemical looping gasification of biomass
2.2. Steam reforming process
2.2.1. Using biomass-derived feedstock
3. Conclusion
References
Chapter 17: Hydrogen production driven by nuclear energy
1. Introduction
2. Nuclear energy
3. Energy obtained from the nuclear energy
3.1. Electricity generation
3.1.1. Alkaline electrolysis
3.1.2. Proton exchange membrane electrolysis
3.1.3. Solid oxide electrolyzer
3.1.4. High-temperature electrolysis
3.2. Thermochemical process
3.2.1. Sulfur-iodine (SI) cycle
3.2.2. Integration of sulfur-iodine (SI) cycle to nuclear reactor
4. Life cycle assessment
5. Conclusion and future perspective
References
Chapter 18: Hydrogen production driven by seawater electrolysis
1. Introduction
2. Fundamentals of water splitting
3. Challenges for seawater electrolysis
4. Electrocatalysts for seawater electrolysis
4.1. Anode materials for saline water electrolysis
4.2. Cathodes for hydrogen evolution in seawater
5. Electrolyzer design for water splitting
6. Conclusions
Acknowledgment
References
Chapter 19: Prospects and challenges for the green hydrogen market
1. Hydrogen production and economy decarbonization
1.1. Green hydrogen
1.2. Water electrolysis
2. Challenges
2.1. Technical challenges
2.2. Infrastructure
2.3. Economics
2.4. Legal and policy framework
3. Hydrogen storage
3.1. Pressure tanks
3.2. Liquid hydrogen storage
3.3. Metal hydrides
3.4. Cryo-compressed storage
3.5. Liquid organic hydrogen carriers
4. Hydrogen distribution
5. Uses of hydrogen in industry
5.1. Ammonia production
5.2. E-fuels
5.3. Refineries
5.4. Iron and steel industry
6. Green hydrogen in the energy transition
7. Blockchains of green hydrogen
8. Hydrogen market
8.1. Policy making
9. Ongoing projects
9.1. The green pipeline project
9.2. The Iberdrola-Fertiberia project
9.3. REFHYNE-Green refinery hydrogen for Europe
9.4. ITEG-Integrating tidal energy into the European grid in the Orkney Islands
9.5. GreenH2Atlantic
10. Conclusions
References
Chapter 20: Hydrogen production from biomass gasification
1. Introduction
2. Hydrogen production from biomass gasification process
2.1. Biomass steam gasification
2.2. Supercritical water gasification (SCWG) of biomass
2.2.1. Thermophysical property of SCW
2.2.2. Contribution of water in SCWG
2.2.3. Application of catalysts
3. Conclusions
References
Chapter 21: Approach toward economical hydrogen storage
1. Introduction
2. Technologies available for hydrogen storage
2.1. Physical techniques for hydrogen storage
2.1.1. Compressed gaseous hydrogen storage technique
2.1.2. Liquid hydrogen technique
2.1.3. Cryo-adsorption of hydrogen using high surface area materials
Zeolites as adsorbent for hydrogen storage
Carbonaceous materials for hydrogen storage
Metal organic frameworks (MOFs) for hydrogen storage
Polymers for hydrogen storage
3. Chemical storage techniques
3.1. Inorganic chemical hydrides
3.1.1. Chemolysis of the metal hydrides (hydrolysis, aminolysis)
3.1.2. Thermolysis of metal hydride
3.1.3. Nanocomposites materials for hydrogen storage
Magnesium-based nanocomposite materials
Li-N-H system
Li-M-N-H (M=alkaline earth abundant metals)
Li-p-block elements-H-based hydrogen storage system
Lithium-carbon-hydrogen system
Lithium-aluminum-hydrogen system
3.1.4. Mixed systems containing p-block and alkaline earth metals along with LiH
3.2. Carbonaceous materials for hydrogen storage
3.3. Liquid hydride materials
3.3.1. Ammonia and related compounds
3.3.2. Formic acid and methanol
3.3.3. Liquid organic hydrogen carriers (LOHCs)
3.4. Alloys for hydrogen storage
4. Conclusions
References
Chapter 22: Power-paste hydrogen storage technologies
1. Introduction
1.1. Salient features of magnesium hydride
1.2. Shortcoming of magnesium hydride-based system
2. Hydrolysis of magnesium hydride
3. Techniques used for enhancing the hydrolysis rates
3.1. Addition of acids to the MgH2 solution
3.2. Addition of oxides, hydride, amides, and borohydride solution
3.3. Addition of metal halides (MClx and MFx)
3.4. Addition of carbon and metal sulfide-based materials
3.5. Addition metal alloys to the magnesium hydrides
3.5.1. Magnesium-based alloys with earth abundant metals
3.5.2. Magnesium-based alloys with rare earth metals
4. Conclusions
References
Chapter 23: Advanced nanomaterials for hydrogen storage
1. Introduction
2. Overview of H2 production techniques
3. Characteristics to improve hydrogen storage capacity
3.1. Kubas interaction
3.2. Hydrogen spillover
4. DFT study for the evaluation of nanomaterials for hydrogen storage
5. Hydrogen storage in nanomaterials
5.1. Metal hydrides
5.1.1. Mg-based H2 storage materials
5.2. Carbonaceous materials
6. Conclusions and future demands
References
Chapter 24: Application of hydrogen in various sectors
1. Introduction
2. Hydrogen economy
3. Hydrogen applications
3.1. Petroleum refining
3.2. Fertilizer industry
3.3. Food industry
3.4. Medical/pharmaceutical industry
3.5. Aviation
3.6. Power generation
3.7. Fuel cell technology
3.8. Metallurgical industry
3.9. Marine industry
3.10. Electronic industry
4. Conclusion
References
Chapter 25: Application of machine learning approach for green hydrogen
1. Introduction
2. Hydrogen production methods and types of hydrogen
2.1. Types of hydrogen
3. Water-splitting mechanism and role of catalysts
3.1. Water-splitting mechanism
3.1.1. HER and OER
3.2. Role of catalysts in water-splitting mechanism
3.2.1. Activity
3.2.2. Stability
3.2.3. Efficiency
3.3. Catalysts for hydrogen evolution reaction
4. Importance of various statistical and computational approaches in green hydrogen generation
4.1. Response surface methodology and artificial neural network
4.2. Response surface methodology (RSM)
4.3. Artificial neural network
5. Summary and outlook
References
Index
Back Cover

Citation preview

SOLAR-DRIVEN GREEN HYDROGEN GENERATION AND STORAGE

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SOLAR-DRIVEN GREEN HYDROGEN GENERATION AND STORAGE Edited by

ROHIT SRIVASTAVA Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

JAYEETA CHATTOPADHYAY Amity Institute of Applied Sciences, Amity University Jharkhand, Ranchi, India

DIOGO M.F. SANTOS Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal

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 © 2023 Elsevier Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-99580-1 For information on all Elsevier publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Charlotte Cockle Acquisitions Editor: Peter Adamson Editorial Project Manager: Aleksandra Packowska Production Project Manager: Anitha Sivaraj Cover Designer: Mark Rogers Typeset by STRAIVE, India

Contents Contributors Preface

1. Exploring the hydrogen evolution reaction (HER) side of perovskite-based materials during photoelectrochemical water splitting

xiii xvii

1

S.K. Tarik Aziz, Anwesha Banerjee, Tannu Kaushik, Sukanta Saha, and Arnab Dutta 1 2

Introduction Photo(electro)chemical mechanism of catalyst: Perovskite oxide materials 3 Double perovskites as HER catalysts 4 Tailoring double perovskites 5 Nano-structural engineering 6 Effect of A-site cation doping 7 Effect of B-site cation doping 8 Effect of anion doping 9 Effect of oxygen vacancies 10 Prospect and summary References

2. Phosphorene-based functional nanomaterials for photoelectrochemical water splitting

1 5 7 9 9 10 13 14 16 16 18

23

Sneha Lavate and Rohit Srivastava 1 Introduction 2 About phosphorene 3 Phosphorene functionalized nanomaterials for the PEC water splitting 4 Challenges, gaps, and perspectives Acknowledgment References

23 26 31 36 37 37

v

vi

Contents

3. Polymer-based catalyst for photoelectrochemical water splitting

41

Anuj Jain and Rohit Srivastava 1 Introduction 2 Basic principles of PEC of water 3 Conclusion References

4. Transition metal-based single-atom catalyst for photoelectrochemical water splitting

41 44 54 55

61

Shankary Selvanathan, Pei Meng Woi, and Rohit Srivastava 1 Introduction 2 Fundamental mechanism for water splitting reactions 3 Advantages of single-atom catalysts (SACs) 4 Transition metal-based single-atom catalysts for PEC water splitting 5 Future perspectives and conclusion Acknowledgment References

5. Clathrate hydrate as a potential medium for hydrogen storage application

61 64 68 70 81 82 83

87

Pawan Gupta and Rohit Srivastava 1 2 3 4 5

Introduction Clathrate hydrate structures specific to hydrogen hydrate The thermodynamic aspect of hydrogen clathrate Kinetic aspects of hydrogen clathrate hydrate Storing hydrogen in the presence of THF and promoters with a tuning effect 6 Modeling of hydrogen clathrate hydrates 7 Conclusion and future direction References

6. Advanced carbon-based nanomaterials for photoelectrochemical water splitting

87 89 92 93 94 97 99 99

103

Lokesh Sankhula, Sneha Lavate, and Rohit Srivastava 1 Introduction 2 Performance evaluation of electrocatalysts 3 Different carbon materials

103 106 108

Contents

4 Enhancing the properties of carbon-based materials 5 Conclusions and future outlook References

7. MXene-transition metal compound sulfide and phosphide hetero-nanostructures for photoelectrochemical water splitting

vii 119 121 122

129

Ranjit Mohili, N.R. Hemanth, Kwangyeol Lee, and Nitin K. Chaudhari 1 Introduction 2 Synthetic routes to MXene-based hetero-nanostructures 3 Photoelectrochemical water-splitting application 4 Conclusion Acknowledgments References

8. Design and advances of semiconductors for photoelectrochemical water-splitting

129 131 133 135 138 138

141

Sauvik Chatterjee and Sanjib Shyamal 1 Introduction 2 Principle of water-splitting 3 Photoelectrode materials 4 Tandem reaction setup 5 Conclusion and outlook References

9. Dye-sensitized photoelectrochemical cells in water splitting

141 142 145 152 154 155

157

Mahesh Dhonde, Prateek Bhojane, Kirti Sahu, and V.V.S. Murty 1 Introduction 2 Device architecture 3 Working principle of DSPECs 4 Dye-sensitized photoanodes for water splitting cells 5 Dye-sensitized photocathodes for water splitting cells 6 Tandem DSPECs for water splitting 7 Conclusion and outlook References

10. Photobiological hydrogen production: Introduction and fundamental concept

157 161 165 167 172 176 180 181

193

Nandini Mukherjee and Rohit Srivastava 1 Introduction

193

viii

Contents

2 Fundamental concepts of photobiological hydrogen generation 3 Enzymes involved in photobiohydrogen generation 4 Modulation of factors affecting photobiological hydrogen production 5 Challenges and future prospects 6 Conclusion Acknowledgments References

11. Biological hydrogen production driven by photo-fermentation processes

196 204 211 218 219 220 220

223

Bishnu Kumar Pandey, Sonam Mishra, Ravindra Dhar, and Rohit Srivastava 1 Introduction 2 Biological hydrogen production by photo-fermentation process 3 Photo-fermentation process 4 Hydrogen production from waste 5 Future perspectives 6 Conclusion References

12. Photobiological hydrogen production by microorganisms

223 224 226 229 231 232 232

237

Jutishna Bora, Shrayana Ghosh, and Ayooshi Mitra 1 2 3 4 5 6 7

Introduction Different mechanism of production of photobiological hydrogen Photobiological H2 production by hydrogenase and nitrogenase Bioreactor systems in photobiological hydrogen production Bioreactors incorporating cyanobacteria and green algae Feedstocks for photobiological hydrogen production Advantages, disadvantages, and challenges of using photobiological methods 8 Conclusion References Further reading

13. Photobiological production of hydrogen from biomass

237 238 243 246 247 248 251 253 253 257

259

Jutishna Bora, Sayantani Ghosh, Sulagna Das, and Sumira Malik 1 Introduction 2 Mechanism of photo-biohydrogen production

259 260

Contents

3 4 5 6 7

Photobiological hydrogen production technologies Direct biophotolysis Indirect photolysis Microbial biomass as feedstock for biohydrogen production Experimental conditions and approaches to enhance biohydrogen production 8 Bioreactors for commercial biohydrogen production 9 Conclusion References

14. Challenges in scaling low-carbon hydrogen production in Europe

ix 263 264 265 268 270 274 280 281

287

Carlos Rojas López, Lucia F. Perez Garces, Daniela Tepordei, Sebastian Púin Moreno, Biljana Šljukic, and Diogo M.F. Santos 1 Hydrogen requirements in Europe to achieve net zero emissions 2 Hydrogen production and use 3 Blue hydrogen 4 Green hydrogen 5 Conclusions References

15. Photobioreactor for hydrogen production

287 290 299 309 316 317

321

Nimmy Srivastava and Jayeeta Chattopadhyay 1 Introduction 2 Photobioreactors for hydrogen production 3 Materials used for different components of the photobioreactor 4 Metals used for construction References

16. Thermochemical hydrogen production

321 322 325 326 329

333

Priti Singh, Sushant Kumar, Nimmy Srivastava, Rohit Srivastava, and Jayeeta Chattopadhyay 1 Introduction 2 Thermochemical conversion of biomass into hydrogen 3 Conclusion References

333 334 341 343

x

Contents

17. Hydrogen production driven by nuclear energy

347

Hari Pavan Sriram Yalamati, R.K. Vij, and Rohit Srivastava 1 Introduction 2 Nuclear energy 3 Energy obtained from the nuclear energy 4 Life cycle assessment 5 Conclusion and future perspective References

18. Hydrogen production driven by seawater electrolysis

347 348 352 359 360 361

363

Lokesh Sankhula, Devendra Kumar Verma, and Rohit Srivastava 1 Introduction 2 Fundamentals of water splitting 3 Challenges for seawater electrolysis 4 Electrocatalysts for seawater electrolysis 5 Electrolyzer design for water splitting 6 Conclusions Acknowledgment References

19. Prospects and challenges for the green hydrogen market

363 364 366 367 373 374 377 377

381

Arcílio B.S. Semente, Catarina B. Madeira Rodrigues, Margarida A. Mariano, Miguel B. Gaspar, Biljana Šljukic, and Diogo M.F. Santos 1 Hydrogen production and economy decarbonization 2 Challenges 3 Hydrogen storage 4 Hydrogen distribution 5 Uses of hydrogen in industry 6 Green hydrogen in the energy transition 7 Blockchains of green hydrogen 8 Hydrogen market 9 Ongoing projects 10 Conclusions References

381 384 389 396 396 402 404 405 409 413 413

Contents

20. Hydrogen production from biomass gasification

xi

417

Priti Singh, Sushant Kumar, Nimmy Srivastava, Tara Sankar Pathak, and Jayeeta Chattopadhyay 1 Introduction 2 Hydrogen production from biomass gasification process 3 Conclusions References

21. Approach toward economical hydrogen storage

417 419 430 431

435

Prakash Chandra and Rohit Srivastava 1 Introduction 2 Technologies available for hydrogen storage 3 Chemical storage techniques 4 Conclusions References

435 436 442 457 457

22. Power-paste hydrogen storage technologies

463

Prakash Chandra and Rohit Srivastava 1 Introduction 2 Hydrolysis of magnesium hydride 3 Techniques used for enhancing the hydrolysis rates 4 Conclusions References

23. Advanced nanomaterials for hydrogen storage

463 465 466 476 480

483

Sneha Lavate and Rohit Srivastava 1 Introduction 2 Overview of H2 production techniques 3 Characteristics to improve hydrogen storage capacity 4 DFT study for the evaluation of nanomaterials for hydrogen storage 5 Hydrogen storage in nanomaterials 6 Conclusions and future demands References

483 485 486 490 492 501 501

xii

Contents

24. Application of hydrogen in various sectors

507

Sneha Lavate, Hari Pavan Sriram Yalamati, and Rohit Srivastava 1 Introduction 2 Hydrogen economy 3 Hydrogen applications 4 Conclusion References

25. Application of machine learning approach for green hydrogen

507 508 510 520 520

525

Amit Verma, Kanchan Rathore, and Rohit Srivastava 1 2 3 4

Introduction Hydrogen production methods and types of hydrogen Water-splitting mechanism and role of catalysts Importance of various statistical and computational approaches in green hydrogen generation 5 Summary and outlook References Index

525 527 527 532 537 541 545

Contributors S.K. Tarik Aziz Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, India Anwesha Banerjee Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, India Prateek Bhojane Department of Physics, University of Petroleum and Energy Studies, Dehradun, Uttarakhand, India Jutishna Bora Amity Institute of Biotechnology, Amity University Jharkhand, Ranchi; Amity Institute of Biotechnology, Amity University Kolkata, Kolkata, India Prakash Chandra Department of Chemistry, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Sauvik Chatterjee School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, India Jayeeta Chattopadhyay Amity Institute of Applied Sciences, Amity University Jharkhand, Ranchi, India Nitin K. Chaudhari Department of Chemistry, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Sulagna Das Amity Institute of Biotechnology, Amity University, Kolkata, West Bengal, India Ravindra Dhar Centre of Materials Sciences, University of Allahabad, Prayagraj, India Mahesh Dhonde Department of Physics, Medi-Caps University, Indore, Madhya Pradesh, India Arnab Dutta Department of Chemistry; Interdisciplinary Program in Climate Studies; National Center of Excellence-CCU, Indian Institute of Technology Bombay, Mumbai, India Miguel B. Gaspar Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Sayantani Ghosh Amity Institute of Biotechnology, Amity University, Kolkata, West Bengal, India

xiii

xiv

Contributors

Shrayana Ghosh Amity Institute of Biotechnology, Amity University Kolkata, Kolkata, India Pawan Gupta Department of Petroleum Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand, India N.R. Hemanth Department of Materials Science and Engineering, University of Washington, Seattle, WA, United States Anuj Jain Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Tannu Kaushik Interdisciplinary Program in Climate Studies, Indian Institute of Technology Bombay, Mumbai, India Sushant Kumar Amity Institute of Applied Sciences, Amity University Jharkhand, Ranchi, India Sneha Lavate Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Kwangyeol Lee Department of Chemistry and Research Institute for Natural Sciences, Korea University, Seoul, Republic of Korea Carlos Rojas Lo´pez Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Catarina B. Madeira Rodrigues Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Sumira Malik Amity Institute of Biotechnology, Amity University Jharkhand, Ranchi, India Margarida A. Mariano Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Sonam Mishra Centre of Materials Sciences, University of Allahabad, Prayagraj, India Ayooshi Mitra Amity Institute of Biotechnology, Amity University Kolkata, Kolkata, India Ranjit Mohili Department of Chemistry, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

Contributors

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Sebastian Pu´in Moreno Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Nandini Mukherjee Department of Chemistry, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India V.V.S. Murty Department of Physics, Govt. Holkar Science College, Indore, Madhya Pradesh, India Bishnu Kumar Pandey Department of Physics, SPM College, University of Allahabad, Prayagraj, India Tara Sankar Pathak Chemistry Department, Surendra Institute of Engineering and Management, Siliguri, West Bengal, India Lucia F. Perez Garces Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Kanchan Rathore Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Sukanta Saha Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, India Kirti Sahu Department of Physics, SNGPG College, Khandwa, Madhya Pradesh, India Lokesh Sankhula Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Diogo M.F. Santos Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Shankary Selvanathan Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia Arcı´lio B.S. Semente Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Sanjib Shyamal School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, India Priti Singh Amity Institute of Biotechnology, Amity University Jharkhand, Ranchi, Jharkhand, India

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Biljana Sˇljukic Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Nimmy Srivastava Amity Institute of Biotechnology, Amity University Jharkhand, Ranchi, Jharkhand, India Rohit Srivastava Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Daniela Tepordei Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal Amit Verma Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Devendra Kumar Verma Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi, India R.K. Vij Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Pei Meng Woi Department of Chemistry, Faculty of Science; University Malaya Centre for Ionic Liquids (UMCiL), Universiti Malaya, Kuala Lumpur, Malaysia Hari Pavan Sriram Yalamati Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

Preface Floods. Droughts. Blizzards. Wildfires. Extreme natural events are increasing in frequency due to the undeniable climate changes on our planet. Although some refuse to believe it, global warming is a proven fact related to our increasing greenhouse gas emissions. The world population keeps steadily increasing, and so do its energy needs. As our society is still heavily dependent on fossil fuels, their consumption and the associated carbon emissions will only decrease if we shift the energy paradigm. Scientists have raised their voices together to point in the right direction, and now governments, industry, and the general population are becoming aware that an energy transition is mandatory. One of the paths that have started being followed in most countries is that toward the Hydrogen Economy. It is based on the use of hydrogen as a clean energy carrier that will be able to decarbonize our society. Most people see it as an urgent approach to replace fossil fuels, but they must be made aware that more than 95% of the hydrogen we produce comes from fossil fuels. We can alleviate our global hydrocarbon dependency if we bet on green hydrogen, typically produced by water electrolysis using renewable energy sources. However, because of the low efficiency of water electrolysis, green hydrogen prices are still too high to compete with fossil fuels. Researchers worldwide are trying to develop new cell components (e.g., electrode materials, and membranes) that improve electrolysis efficiency and enable producing green hydrogen at a lower cost so that we can rely exclusively on renewable energy sources. The book Solar-Driven Green Hydrogen Generation and Storage discusses possible directions toward the Hydrogen Economy, including different approaches to hydrogen production and storage. In that sense, it comprises 25 chapters contributed by different authors who are actively involved in this field. Chapter 1, Exploring the hydrogen evolution reaction (HER) side of perovskite-based materials during photoelectrochemical water splitting, discusses the fundamental principles of solardriven water splitting and the structure and functional properties of perovskite materials. It describes up-to-date research progress on designing and developing perovskite-based catalysts for water electrolysis through solar energy. Chapter 2, Phosphorene-based functional nanomaterials for photoelectrochemical water splitting, discusses various approaches for the functionalization of phosphorene and its application in electrochemical and photoelectrochemical water splitting. Chapter 3,

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Preface

Polymer-based catalyst for photoelectrochemical water splitting, deals with different polymeric photocatalytic materials, as their electric and structural properties can be easily regulated and methodically tuned at the molecular level for efficient photoelectrochemical water splitting. Chapter 4, Transition metal-based single-atom catalyst for photoelectrochemical water splitting, provides a state of the art on the fundamental mechanisms, materials design, and other aspects that contribute to the high efficiency of photoelectrochemical water splitting using transition metal catalysts. Chapter 5, Hydrogen hydrate as a potential medium for hydrogen storage application, provides a comprehensive outline of the current state of work in the area of hydrogen hydrates related to the storage of hydrogen, including the structure and its characterization aspects, thermodynamic and kinetic studies, modeling studies, and recent developments. Chapter 6, Advanced carbon-based nanomaterials for photoelectrochemical water splitting, provides a comprehensive overview of carbon-based materials, such as graphene oxide/reduced graphene oxide, CNT, graphene, C60, and carbon quantum dots, in enhancing the performance of semiconductor photocatalysts for hydrogen production. Chapter 7, Mxene-transition metal compound sulfide and phosphide heteronanostructures for photoelectrochemical water splitting, promotes the state of the art of MXene-transition metal sulfide and phosphide hetero-nanostructures as potential candidates for photoelectrochemical water splitting and other solar energy applications. Chapter 8, Design and advances of semiconductors for photoelectrochemical water-splitting, discusses the journey of photoelectrochemical water splitting with a focus on H2 generation, the development of semiconductor materials for designing the PEs, the challenges in the field, and some prospective areas for advancement. Chapter 9, Dye-sensitized photoelectrochemical cells in water splitting, encompasses state-of-the-art DSPECs and various advancements in DSPECs’ functional architecture to improve their watersplitting efficiency and stability. The chapter further highlights the current challenges and prospects for the widespread adoption of DSPEC technology. Chapter 10, Photobiological hydrogen production: Introduction and fundamental concept, deals with photobiological hydrogen production through biophotolysis and photofermentation, and discusses the role of the hydrogenase and nitrogenase enzymes in hydrogen metabolism. Chapter 11, Biological hydrogen production driven by photo-fermentation processes, discusses the mechanism of hydrogen production from various microorganisms and the role of various factors affecting the photofermentation process. Chapter 12, Photobiological hydrogen production by microorganisms, explores the different mechanisms by which photobiological hydrogen can be produced by microorganisms and

Preface

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the advantages it brings to the economy and the environment. Chapter 13, Photobiological production of hydrogen from biomass, summarizes the findings on this topic and discusses the challenges, knowledge gaps, and future perspectives of commercialization. Chapter 14, Challenges in scaling low-carbon hydrogen production in Europe, intends to provide an overview to the reader of the current prospects and challenges involved in scaling low-carbon H2 production in Europe. Chapter 15, Photobioreactors for hydrogen production, highlights the current state-of-the-art photobioreactors for hydrogen production. Chapter 16, Thermochemical hydrogen production, presents the thermochemical processes used to produce hydrogen from biomass, which are well covered in the chapter. Chapter 17, Hydrogen production driven by nuclear energy, provides a broader insightful direction on the potential of nuclear energy to generate hydrogen for various applications. Chapter 18, Hydrogen production driven by seawater electrolysis, addresses the associated challenges in the design of electrolyzers and discusses future potential approaches that may yield highly active and selective materials for seawater electrolysis. Chapter 19, Prospects and challenges for the green hydrogen market, covers the production, storage, and distribution of green hydrogen as well as the economic factor of its value chain. Chapter 20, Hydrogen production from biomass gasification, deals with the difficulties and potential aspects of producing hydrogen through biomass gasification. These are examined to guide the crucial knowledge gaps that demand more research. Chapter 21, Approach toward economical hydrogen storage, provides several physical and chemical techniques for efficient hydrogen storage. Chapter 22, Power-paste hydrogen storage technologies, primarily focuses on the state-of-the-art powerpaste technique for hydrogen storage and transportation applications. Chapter 23, Advanced nanomaterials for hydrogen storage, offers an insight into solid-state hydrogen storage materials and future outlooks. Chapter 24, Application of hydrogen in various sectors, outlines the current research and applications of hydrogen in the processing and upgrading of hydrocarbon fuels, fuel cells, platform chemical synthesis, pharmaceuticals, aerospace and maritime uses, metallurgy, and electronics. Chapter 25, Application of machine learning approach for green hydrogen, accurately predicts the role of different photo/electrocatalysts on the dependent parameters through modeling approaches. It is believed that the proposed book would interest graduate students, researchers, academicians, and industrialists working in energy and environment, biomass conversion, sustainability, biofuels, and chemical industries. The book intends to show ongoing research that should gradually improve the efficiency of green hydrogen production and storage so that green

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hydrogen may soon become widely available at competitive prices. That will allow our society to reduce greenhouse gas emissions, fight climate change, and make fossil fuel-free countries more energy-independent.

Acknowledgments It is our immense pleasure to present this book on various possible directions toward the hydrogen economy, production, and storage through solardriven green pathways. We thank each one of the chapter contributors for their valuable involvement and continuous support in making the book successful. We take this opportunity to thank all of the reviewers for their valuable contributions to improving the quality, coherence, and content presentation of the book chapters. We would like to place on record our thanks to all Elsevier staff members including Aleksandra Packowska, Sathya Narayanan and K. S Anitha for their time-to-time guidance throughout the publication processes. We also thank all the research staff members for their continuous support that has helped us complete this book on time. There are many others who have helped us along the way and we are sure we are forgetting some. The people we could never forget are the ones who allowed us to write about them. We are humbled by the patience, generosity, and friendship shown to us by our characters (the word fails to do justice to their humanity). None of them knew what they were getting into when they signed on to this project. None of them ever showed any signs of resentment or impatience. Rohit Srivastava Jayeeta Chattopadhyay Diogo M.F. Santos

CHAPTER 1

Exploring the hydrogen evolution reaction (HER) side of perovskite-based materials during photoelectrochemical water splitting S.K. Tarik Aziza, Anwesha Banerjeea, Tannu Kaushikb, Sukanta Sahaa, and Arnab Duttaa,b,c a Department of Chemistry, Indian Institute of Technology Bombay, Mumbai, India Interdisciplinary Program in Climate Studies, Indian Institute of Technology Bombay, Mumbai, India c National Center of Excellence-CCU, Indian Institute of Technology Bombay, Mumbai, India b

1 Introduction Perovskites belong to a unique class of metal oxide materials that resemble the crystalline architecture of naturally occurring calcium titanium oxide or calcium titanate (CaTiO3). Calcium titanate was first discovered in the Ural mountain area of Russia in 1839 by Geologist Gustav Rose, and it was eventually named after Russian mineralogist Lev Perovski [1]. Perovskites typically constitute an ABO3 type cubic crystal lattice where an alkaline-earth or rareearth metal occupies the A-site with a coordination number of 12. On the other hand, the B-site is primarily occupied by transition metal with six oxide anions (BO6) positioned at the corners of an octahedron template. The threedimensional cubic crystal framework is constituted by eight corners sharing BO6 octahedra and an A-site cation stationed at the central position. Their structural and compositional versatility offers ample opportunities to create active sites poised for catalyzing small molecule activation reactions, which are vital for establishing a sustainable renewable energy-driven power landscape [2]. A new class of highly active, selective, and stable perovskite oxide catalysts has been designed in recent times by rational doping or substituting A (A2+) and B-cations (B4+) with an array of other metal ions to unravel novel combinations with unique reactivities. H2 molecule has emerged as one of the potential vectors for converting otherwise intermittent and dilute renewable energy resources to readily Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00010-8

Copyright © 2023 Elsevier Inc. All rights reserved.

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Solar-driven green hydrogen generation and storage

usable and reliable electricity. Currently, steam methane reformation (SMR) and coal gasification are responsible for the majority of industrial H2 production. However, those processes are classified as brown or grey H2 production due to the associated unavoidable CO2 production. Water electrolysis is regarded as a green H2 production process, which is primarily driven by platinum (Pt)-based catalysts anchored on the electrodes. Nevertheless, the worldwide implementation of such green H2 production has been impeded by the low abundance and high price of Pt. Perovskites are considered as one of the leading materials that can substitute the stateof-the-art Pt-catalysts for the electrocatalytic hydrogen evolution reaction (HER) [3,4]. The perovskites contain inexpensive, earth-abundant metals, and their structural flexibility, high stability, and tunable electronics underpin their direct applicability in catalyzing HER under both electrochemical and photochemical processes [3,4]. The flexible compositional framework of perovskite oxide materials can be easily tailored by introducing cationic and anionic dopants and spurring oxygen vacancies for increased catalytic activity. Primitive perovskite oxides evolve into ordered structures called “double perovskites” by partially substituting half of the A-site (or B-site) cations with other A0 metal cations (or B0 cations) of different size and charge to give rise to new variants with a general formula of AxA0 1xBxB0 1xO6 [5]. Moreover, perovskite oxides with enhanced and exotic properties can be prepared by restructuring the ABO3 framework into layers such as the Aurivillius phase (Bi2An1BnO3n+3), Dion Jacobson phase (AnBnO3n+1), and Ruddlesden-popper phase (An+1BnO3n+1) [5]. A broad spectrum of perovskites has been generated by substituting either at active A and B sites or both with metal cations in different valence states or varied transition metals and heavy metals. Consequently, the intrinsic activity enhancement of perovskites, such as electrical conductivity, electrochemically active surface area, more oxygen vacancies, number of active sites, and faster charge transport, can be achieved through strategic substitution, which directly impacts the resultant HER activity. The overall water splitting involves two half-cell reactions that occur concomitantly at two terminal electrodes: oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction (HER) at the cathode (Eqs. 1–3) [6]. Overall reaction: 2H2 O ! 2H2 + O2

(1)

OER: 2H2 O ! O2 + 4H+ + 4e E a ¼ 1:23 V vs NHE

(2)

HER: 4H+ + 4e ! 2H2 Ec ¼ 0 V vs NHE

(3)

Exploring HER side of perovskite-based materials

3

HER is a two-electron transfer reaction transpiring on a negatively charged electrode (cathode) surface to convert solution-phase protons (or water molecules) to gaseous H2. The reaction follows either the “Volmer-Heyrovsky” or “Volmer-Tafel” mechanism, where the first step (Volmer) is typically the rate-determining step [3,6]. In the Volmer reaction, proton (acidic medium) or H2O (alkaline medium) adsorbs on the active catalyst surface. Then, an electron transfer occurs between the electrode surface and adsorbed protons to eventually generate surface-bound hydrogen atoms. The variability of this reaction under acidic and alkaline media is mentioned as follows (Eqs. 4–9). Under acidic conditions: Step 1: M + H+ + e ! M  H ðVolmerÞ

(4)

Step 2: M  H + H+ + e ! M + H2 ðHeyrovskyÞ

(5)

or 2M  H ! 2M + H2 ðTafelÞ

(6)

Under alkaline conditions: Step 1: M + H2 O + e ! M  H + OH ðVolmerÞ

(7)

Step 2: M  H + H2 O + e ! M + H2 + OH ðHeyrovskyÞ

(8)

or 2M  H ! 2M + H2 ðTafelÞ

(9)

In the second step, H2 formation can follow two different pathways. In the Heyrovsky reaction, another electron transfers to the adsorbed H-atom is in tandem with the adsorption of another proton from the solution [3,6]. However, the Tafel reaction entails the combination of two adsorbed H-atoms on the surface of the electrode to release H2 gas [3,6]. The rate of hydrogen evolution reaction in perovskites is determined by the surface binding energy of the adsorbed hydrogen atom (H*) intermediate known as “Hydrogen adsorption free energy” (ΔGH) [2]. According to Sabatier’s principle of catalysis, an ideal catalyst binds the reaction intermediate(s) neither too strongly nor weakly; thus, adsorption energy typically hovers close to zero for such catalysts [7–10]. Binding energy of perovskites is found to be a function of electronic configuration on the catalyst surface. Therefore, the surface binding of distinct metal centers largely influences the perovskite-driven HER activity. The simulation studies reveal that the B-site substituted transition metals are the primary active adsorption and desorption centers originating the HER intermediates, while the A-site dopants subtly influence the catalytic activity [2,3].

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Solar-driven green hydrogen generation and storage

In certain instances, the negatively charged oxygen ions on the electrode-bound perovskite surface layer act as the reactive HER site instead of metal centers [3]. The electronic structural dynamics of perovskite oxides are primarily ruled by the B-site metal-oxygen bond on the surface layer exhibiting a BO5 coordination environment contrary to conventional octahedral BO6 symmetry observed in the bulk. The filling of frontier molecular orbitals (antibonding eg (σ*) and t2g (π*)), generated by the overlap of 3d-orbitals of B-site transition metal and oxygen’s 2p orbital, defines the basic features such as surface reactivity and stability of perovskites toward HER [2]. A-site dopants typically play a key role in determining the catalytic activity of perovskite oxides by altering the coordination symmetry and subsequent electronic configuration of B-site cations. HER performance significantly ameliorates the framing of larger A-site cations by prompting greater overlap of B-site cations and an oxygen atom [3]. This phenomenon accounts for an increase in energy of eg band center closer to Fermi level, thus promoting the transfer of eg electrons to the antibonding orbitals of adsorbed H species while increasing the electrical conductivity [2,3]. The position of the d-band center of the transition metal and p-band center of the oxygen relative to the Fermi level is the governing factor in this perovskite oxide-driven HER. A variation in size and charge of the A-site substituent influences the overall crystal structure, while stabilizing the valence state of B-site metals [2,3,6]. Subsequently, it leads to a change of structural tolerance factor called Goldschmidt tolerance factor (t), a parameter that explains the stability of perovskites as follows (Eq. 10): r ¼ ðr A + r O Þ=√2ðr B + r O Þ

(10)

where, rA ¼ ionic radii of A-site cation, rB ¼ ionic radii of B-site cation, and rO ¼ ionic radii of oxide anion. A-site dopants usually have larger radii than B-site cations owing to the distinctive metal type and the different valence states of cations [11]. Therefore, perovskites undergo lattice distortions to varying extents and induce changes in the crystal structure. Perovskites with a tolerance factor close to unity possess a stable and optimal cubic crystal lattice suitable for inducing catalytic HER. Solar-driven (i.e., photocatalytic) water splitting is a nonspontaneous and thermodynamically uphill reaction due to the positive standard free energy change (237.3 kJ/mol) required for its proper execution [12]. For an efficient solar water splitting, the band gap of the photocatalysts must be higher than the thermodynamic energy requirement of 1.23 eV. Here, the conduction

Exploring HER side of perovskite-based materials

5

band is required to be positioned negatively relative to the water reduction (H+/H2O; 0 V vs NHE). However, the valence band of the material needed to be active in a more positive potential region compared with the water oxidation potential (O2/H2O; 1.23 vs NHE). Catalytic systems exhibiting a band gap smaller than the energies of incident photons enhance charge separation and charge transfer following photoirradiation. Here, the optimum reactive sites for the conversion are ideally suitable for solar energy-induced water splitting. To date, numerous catalytic constructs have been developed mimicking the architectural blueprint of natural photosynthetic assembly for producing H2 and O2 from water-splitting reaction, presenting a profusion of innovations in this direction. In this context, inorganic semiconductors, namely chalcogenides, nitrides, and oxides, have been extensively studied for photocatalytic water splitting [5]. However, these materials absorb only in a narrow visible range due to their wide band gap, necessitating an extension for their photo-response range and the resulting visible-light-induced photocatalytic activity. The light-response range can be widened by explicitly tuning the electronic band structures by prudently modulating the elemental compositions. Perovskites appear to be the ideal choice in this aspect due to their favorable framework and flexibility toward varied elemental composition.

2 Photo(electro)chemical mechanism of catalyst: Perovskite oxide materials In a photocatalytic reaction, the primary step is the effective absorption of photons, which initiates the photoexcited electron-hole pair generation and separation, followed by charge carrier transfer to the photocatalyst surface for molecular transformation Fig. 1A. The semiconductor surface is irradiated with a light source of appropriate energy, so that incident photon energy is equal to or greater than the photocatalyst’s bandgap energy (hν > Eg). Apparently, a minimum band gap of 1.23 eV is essential for solar-driven water splitting, and thus corresponding maximum wavelength that photochemical materials can absorb is 1000 nm (formula Eg ¼ hc/ λ ¼ 1240/λ). The semiconductor photocatalysts primarily absorb in a narrow UV region, which constitutes only 5% of the natural solar spectrum due to their characteristic wide band gap. The photo-response of these semiconductor materials can be extended to visible (constitutes 45% of solar spectrum) and infrared regions (constitutes 50% of solar spectrum), spanning the broadband spectrum by incorporating exclusive band engineering.

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Solar-driven green hydrogen generation and storage

Fig. 1 (A) Mechanism and energy diagrams of photoexcitation in semiconductor-based photochemical water splitting for hydrogen generation, (B) The target of bandgap engineering for semiconductor photochemical water splitting, CBM, conduction band minimum, Eg, bandgap energy; VBM, valence band maximum.

Photoexcitation of electrons (e) from the valence band to the conduction band creates holes (h+) in the valence band. These photogenerated charge carriers must have enough energy to migrate to the catalyst surface to participate in the redox reaction. In the photocatalytic water-splitting process, water is split into H2 and O2 on the photocatalyst surface, wherein the electrons in the conduction band participate in the reduction reaction for hydrogen production, and holes are scavenged for water oxidation to produce oxygen. The second step charge recombination process significantly limits

Exploring HER side of perovskite-based materials

7

the apparent quantum yield (AQY) for the photocatalytic materials. Specific modification methods are required to improve the separation efficiency of charges, like embedding co-catalysts on the surface or combining them with a material with decent conductivity. Theoretically, the potential of the conduction band (CB) of an ideal photocatalytic system is more negative than water’s reduction potential, such that photogenerated electrons can reduce the water molecules to generate hydrogen (H2). Likewise, water gets oxidized by photogenerated holes to produce Oxygen (O2) when the potential of the valence band (VB) is more positive than oxygen potential. With the contentment of both fundamental conditions, the overall water splitting is attainable at higher rates and optimum overpotential. Subsequently, the corresponding bandgap energy should be >1.23 eV (1.6 eV. Herein, Fig. 1B illustrates the objective of bandgap engineering for semiconductors. Developing narrow-bandgap materials for expanding the light absorption range is essential for promoting solar water-splitting proficiency. The overall VB of the metal oxide semiconductors is composed of O 2p orbitals, and the metal cations determine the span of CB. An in-depth investigation of perovskite materials suggests that varying the p orbitals of the anions or the s orbitals of the p-block metal ions can change the VB position, thus inducing a narrow bandgap. Elemental doping can promote the absorption of visible light by wide-bandgap material. The VB of the semiconductor improves by replacing the oxygen ion with an anion, such as nitrogen (N) or sulfur (S), or phosphorous (P), which has a higher valence band potential.

3 Double perovskites as HER catalysts Double perovskites are derived from primitive perovskites with a general formula ABO3 by substituting half of the B-site cations with different B0 transition metal cations to originate the A2BB0 O6 units in a rock-salt type ordered arrangement [13]. Further, a double substitution of exactly half of the metal cations at A-site with another rare-earth metal and alkalineearth metals (A0 ) and B-site substituent with other transition metal elements

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Solar-driven green hydrogen generation and storage

leads to a doubling of formula unit AA0 BB0 O6 known as doubly ordered perovskites [13,14]. It is the advanced generation of simple perovskites with extended and enhanced features such as better flexibility and degrees of freedom and architectural and electronics versatility owing to four-metal cation substitutions. However, double perovskite (A2BB0 O6) is more prevalent for catalysis as the B-site cations govern their physical and chemical characteristics. A-site cations act as the electron donor to the BO6 unit. In the sublattice of double perovskites, metals in a higher valence state (+6 or +7) can be accommodated with tunable electronics and increased stability than simple perovskites. These outstanding characteristic features of double perovskites foster them as potential photo-electrocatalysts for improved catalytic performance for HER. Since the reporting of the pioneering work by Honda and co-workers, the era of photocatalytic HER converting solar energy into hydrogen and oxygen using semiconductor-based photocatalysts has emerged as one of the most promising methods for sustainable renewable energy conversion. During the HER process, the photocatalytic semiconductor electrode absorbs sunlight and generates charge carriers, or photogenerated e/h+ pairs, which move to the photocatalyst surface and perform redox reactions toward hydrogen production (H+/H2). Several semiconductor-based photocatalysts (such as TiO2, CdS, and g-C3N4) have been well explored as promising photocatalytic materials for enhancing solar energy absorption while diminishing the rapid charge carrier recombination, owing to their suitable forbidden gap with corresponding light absorption features. Oxide alternatives with a single perovskite structure have been widely surveyed, and most of them have been found to exhibit bandgaps equal to or even larger than TiO2 (3.2 eV), which limits their usage mainly in the ultraviolet region. In particular, the double perovskites have been recognized as having the narrowest bandgaps (2.0 eV) for Bi-based materials. Cox and co-workers reported a double perovskite oxide Ba2Bi3+Bi5+O6 (or BaBiO3, with a charge disproportionation of the Bi cation), where the ordering of Bi3+ and Bi5+ cations, due to the presence of 6s orbitals in the valence band, as compared to other oxide semiconductors [1], leads to reduced bandgaps. Later, Yan and co-workers depicted the effect of substitution of Bi with Nb leading to the formation of a Ba2Bi1.4Nb0.6O6 double perovskite, with an exciting bandgap of 1.6 eV and producing similar photocurrent densities of 0.2 mA/cm2 at 1.23 and 0 V vs NHE when used separately in a PEC device [2].

Exploring HER side of perovskite-based materials

9

4 Tailoring double perovskites With the development of heterogeneous material catalysts, perovskite oxides have garnered attention for their high efficiency and activity, structural flexibility, and great tunable properties. The general formula is ABO3, where A is usually a 12-coordinated alkaline-earth metal or rare-earth metal, and B is a transition metal with a coordination number of 6 [3]. Initially only simple perovskites were reported for intrinsic OER and HER activities. However, in recent years, double perovskites with the formula A2B2O6 have garnered attention with their promise of significant advantages to the physicochemical properties, affecting the stability, activity, and efficiency in relevant applications. Both of these perovskite oxides have been extensively investigated as water-splitting electrocatalysts and have demonstrated promising results for the oxygen evolution reaction (OER) but poor electrocatalytic HER activity. Albeit, the compositional and structural modification of perovskite oxides has led to uncovering their potential as active HER electrocatalysts in both acidic and alkaline conditions in recent times. Their HER activity depends on a multitude of factors, mainly: the effect of the type of cation at the A-site, the impact of the B-site cation type, the oxidation state of the B-site cation, anion doping, and oxygen vacancies.

5 Nano-structural engineering Downsizing the bulk double perovskite particles to the nanometer range increases the active surface area and the electrocatalytic activity. For example, PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) nanofibers with a large surface area demonstrated a substantial enhancement in alkaline ORR/OER activity [4]. Although it was traditionally hypothesized that the geometric effect alone affects the electrochemical activity, the recent findings also indicate the role of the electronic effect on the electrochemical activity of this perovskite structure. In addition, double perovskite micro/nanostructures require a large surface area, high porosity, and good connectivity between the nanosized components since the structures may experience coarsening or sintering during long-term catalytic reactions, especially under high-temperature conditions, causing catalyst degradation. Oxygen vacancy defects in perovskites present a nonstoichiometric ratio and are structurally stable. Several studies have explained the effect of oxygen vacancies in perovskites on water-splitting performance. The concentration

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Solar-driven green hydrogen generation and storage

Fig. 2 Schematic representation of the formation of oxygen vacancies: (A) aliovalent substitution and (B) postprocessing methods. Vo, oxygen vacancy [15].

and location of oxygen vacancies are critical and determined by how defects are introduced into the material. Zhao and co-workers studied the method of introducing oxygen vacancies [5] with cation substitution (cations of A-site) and nonstoichiometric composition. Fig. 2A highlights the scenario when a segment of the initially positioned rare-earth metal ion A in the perovskite template is replaced by a lower valence alkaline-earth metal ion A0 . While this substitution maintains the charge balance, it also creates oxygen vacancies by releasing a considerable amount of oxygen. As shown in Fig. 2B, in the ABO3 type perovskite, heat treatment under a low oxygen partial pressure (reducing atmosphere), known as the postprocessing method, leads to an increase in the lattice energy of perovskite oxides, which in turn causes the oxygen ions to become active.

6 Effect of A-site cation doping The A-site cation in perovskite oxides plays a vital role in their HER activity. Although the B-site transition metals are often regarded as the active sites, the A-site cations can indirectly influence the HER activity. Substitution of the A-site has led to enhanced electrical conductivity. Even though several perovskite oxides have been reported for their HER performance in acidic solutions, very few are known to show alkaline HER activity. Recently, Xu and co-workers demonstrated one of the first perovskite oxides Ba0.5Sr0.5Co0.8Fe0.2O3δ (BSCF) (also known for OER activity) as a suitable candidate for HER catalysis in alkaline media, where the A-site has been doped with praseodymium (Pr) [6]. With the enhanced activity, stability, and cost-effectiveness, Pr0.5BSCF has emerged as a promising candidate for a nonprecious-metal catalyst for alkaline HER. In the

Exploring HER side of perovskite-based materials

11

alkaline HER pathway, the water molecules are first adsorbed on the active sites, followed by their reduction into adsorbed hydrogen atoms and hydroxyl ions. Next, the desorption of the OH ions take place to regenerate the catalyst surface while evolving hydrogen molecules. It is hypothesized that the B-site transition metal center acts as the active site (cobalt ion center in this case). On doping 50 mol% of Pr3+, the surface states of Ba/Sr states get modified, partial oxidation of cobalt is altered, and the concentration of lattice oxygen increases. The high valent cobalt ion is proposed to enhance the HER performance by forming a robust electrostatic bond with the hydroxyl ions. At the same time, the relatively higher Lewis acidic O anions enable the adsorption of water (Lewis base) via Lewis acid-base interaction. Pr0.5BSCF exhibits a significant improvement in the HER activity with a low operational overpotential of 237 mV and a Tafel slope of 45 mV/dec. The effect of the optimum ionic radius of the A-site can be observed in several other perovskites. For example, a study of a series of electrocatalysts Ca2FeMnO6δ, CaSrFeMnO6δ, and Sr2FeMnO6δ by Hona et al. [7] demonstrates excellent water-splitting catalysts with unique bifunctional properties. These oxygen-deficient perovskites only differ in their A-site cations, leading to a variation in the crystal structures. The change in the ionic radii of the different A-site cations further leads to different arrangements of oxygen vacancies in the materials, giving rise to significant differences in electrical charge transport and electrocatalytic properties. In Ca2FeMnO6δ, the oxygen vacancies have an ordered arrangement and appear only in alternating layers, while they are disordered in CaSrFeMnO6δ and Sr2FeMnO6δ. The degree of oxygen deficiency for Ca2FeMnO6δ is δ ¼ 1, δ ¼ 0.57 for CaSrFeMnO6δ and δ ¼ 0.22 for Sr2FeMnO6δ. The HER activities of these three materials are studied in acidic (0.5 M H2SO4) and basic (1.0 M KOH) conditions. CaSrFeMnO6δ exhibited the best electrocatalytic performance among the three, with overpotential values of 0.31 and 0.39 V vs RHE in acidic and basic conditions, respectively, while operating at 10 mA/cm2 current density. The observed Tafel slopes were 157 mV/dec in the acidic condition and 163 mV/dec in the basic condition. The Fe and Mn sites are found to be responsible for improved electronic conductivity. This MdOdM bond system (M ¼ transition metal) favors the electron hopping mechanism, while the bond angle flexibility determines the electronic conductivity of the material. A greater MdOdM bond angle allows a better overlap between M 3d and O 2p orbitals, resulting in enhanced

12

Solar-driven green hydrogen generation and storage

conductivity. The 180 degrees bond angle and small (Fe-Mn)dO bond distance in CaSrFeMnO6δ leads to a better orbital overlap and excellent conductivity. Further, the synergistic effect between Ca/Sr on the A-site and Fe/ Mn on the B-site also leads to improved electrocatalytic activity. Another example of such an effect of the A-site on HER has been investigated in the highly active LnBaCo2O5+δ (Ln ¼ La, Pr, Nd, Sm, Eu, and Gd) perovskite electrocatalysts. In the series, LnBaCo2O5+δ illustrates one of the highest and most robust HER activity operating in a 1.0 M KOH solution at room temperature. This material has an overpotential value of only 156 mV and the smallest Tafel slope of 64.4 mV/dec while functioning at a current density of 10 mA/cm2 [8]. On changing the A-site cation, the ionic radius changes, leading to structural transformation. With an increase in the ionic radius of the lanthanides, the structure changes from orthorhombic to tetragonal to finally cubic, with a Goldschmidt tolerance factor close to unity. Due to the inductive effect of A-site on active B-site, the structural tolerance factor is correlated with the HER activity and stability. In the series, LaBaCo2O5+δ, with the largest A-site cation and a cubic structure with a tolerance factor of 1.009, shows the best HER activity. The electrocatalytic HER activity is further attributed to the active B-site valence state, i.e., the valency of cobalt ion, and the change in bond angles with a change in the A-site cation size. Corresponding XPS studies confirm the presence of highly oxidative Co4+ ions in the lanthanide perovskite catalysts. As the identity of Ln ions are altered from Gd to La, the Co valence is enhanced from 3.39 to 3.50 on the catalyst surface. The increased Co4+ ions can facilitate the hopping of polaron through the Co4+-O-Co3+ linkage. Such a transfer critically shifts the conduction mechanism from an insulating intermediate-spin Co3+ to metallic intermediate-spin Co4+ to further improve the HER activity [9,10]. According to the previous studies, the Co4+ ions may also aid the efficient adsorption of molecular H2O toward alkaline HER due to large electrostatic attraction. As a result, the HER activities of LnBaCo2O5+δ perovskites are enhanced with increasing Co valence for Ln ¼ Gd-La. Furthermore, the increasing size of the A-site cation increases the OdCodO bond angle, close to 180 degrees, thereby improving the electron cloud overlap, which leads to a shift in the position of the eg band center toward the Fermi level [11], making the eg electrons easily transportable to the antibonding orbitals of the adsorbed H species. This effect is obvious from the resultant overpotential values of this series that ranges from η10 ¼ 441 mV for EuBaCo2O5+δ to η10 ¼ 156 mV for LaBaCo2O5+δ in 1.0 M KOH solution.

Exploring HER side of perovskite-based materials

13

7 Effect of B-site cation doping The transition metals located at the B-sites of perovskite oxides act as the active sites in the HER. As a result, the substitution and doping of metals at this site lead to significant changes in the electrocatalytic performance. A single-phase perovskite oxide SrTi0.7Ru0.3O3δ (STRO) [12] has been developed, which demonstrates alkaline HER electrocatalysis while showcasing the influence of the B-site cation on the electrocatalytic performance. It shows remarkable HER activity with an overpotential as low as 46 mV at 10 mA/cm2 and a very small Tafel slope of 40 mV/dec when evaluated in 1.0 M KOH. These results indicate the presence of a facile HER pathway. This particular perovskite oxide maintains its HER activity for more than 200 h in an alkaline media. Combined theoretical and experimental results disclose that the partial substitution of Ti by Ru in SrTiO3 results in the formation of SrTi0.7Ru0.3O3δ, which leads to an increase in the electron density around the Ti4+ sites. These Ti4+ ions are subsequently reduced to Ti3+, while the Ru4+ ions get oxidized to Ru5+. This charge redistribution between the two ions induces an unusual superexchange interaction between adjacent Ti (III) and Ru (V) sites, resulting in an upgraded electrical conductivity. Additionally, the band gap of the SRTO decreases from 3.20 to 2.20 eV after doping the SrTiO3 perovskite with Ru ions, which leads to enhanced electrical conductivity by favoring the excitation of charge carriers to the conduction band. Further, keeping in mind the remarkable activity of molybdenumbased derivatives confirmed by prior studies, Zhang et al. have doped the B-site lattice of the perovskite oxide SrCo0.70Fe0.30O3δ with Mo to tune the HER activity and stability [13]. The resultant perovskite SrCo0.70Fe0.30xMoxO3δ displays an overpotential of 323 mV at 10 mA/cm2 and a Tafel slope of 94.21 mV/dec in 1.0 M KOH. The introduction of high valent Mo (VI) increases the oxidation state of the B-site cations, e.g., Co cations, which causes the coexistence of a Co3+/Co4+ couple proven to be beneficial for HER. These surface Co3+ and Co4+ ions tend to remain in an intermediate-spin state, favoring the Jahn-Teller distortion of the CoO6 octahedra [14,16,17]. Molybdenum doping is also found to cause an increase in the lattice oxygen species content, which promotes the OER half-reaction and boosts the HER activity. The experimental studies also reveal a slight increase in the BdO bond distance with an increase in Mo concentration, which

14

Solar-driven green hydrogen generation and storage

is optimum since it offers a bonding strength that is neither too weak nor too strong, in line with Sabatier’s principle. Along with a low overpotential and Tafel slope, the Mo-doped perovskite is durable for five hours and remains stable for up to 1000 cycles. It significantly improves the stability of perovskite in an alkaline solution by inhibiting the leaching of Sr, thus maintaining structural integrity and inducing stable electrochemical HER activity.

8 Effect of anion doping Some recent studies have proposed highly active anion-doped HER electrocatalysts where nonmetal elements (e.g., P, N, S, and F) have been incorporated into the perovskite lattice to modify their surface properties and electronic structures, thereby improving their catalytic activities. Gao and group have synthesized such a perovskite oxide, an N-doped Sr2Fe1.5Mo0.5O6δ at a temperature of 450°C (SFMON-450), by the ammonolysis of the precursor Sr2Fe1.5Mo0.5O6δ (SFMO) [18]. The partial substitution of O2 by N3 leads to the formation of an oxynitride perovskite, with a smaller band gap, a high concentration of oxygen vacancies, and high valence Fe ions (B-site metal cations), which aid the enhancement of catalytic activity. The SFMON-450 displays excellent HER activity in 1.0 M KOH, with an overpotential of 251 mV at a current density of 10 mA/cm2 and a Tafel slope of 138 mV/dec. It further shows superior electrocatalytic stability for a period of about 40 h. A phosphide-doped BSCF perovskite, Ba0.5Sr0.5(Co0.8Fe0.2)1xPxO3δ, shows an overpotential of η10  333 mV and a Tafel slope of 73.3 mV/dec in 1.0 M KOH for a composition containing x ¼ 0.05. These parameters are significantly better compared with the parent perovskite [19]. Here, the presence of the higher valent transition metals and greater content of oxygen vacancies result in the increased electrocatalytic HER activity analogous to the perovskite SFMON-450. Another example of anion-doped perovskite is La0.5Ba0.25Sr0.25CoO2.9δ F0.1 (LBSCOF), which results from a lower valent fluoride ion doping into the oxygen site of La0.5Ba0.25Sr0.25CoO3δ (LBSCO) [20]. It demonstrates high HER activity with an overpotential of η10  180 mV, compared with η10  240 mV for LBSC in 1.0M KOH, and a Tafel slope of 90 mV/dec. The introduction of fluorine lowers the oxygen vacancies but activates the lattice O sites by uplifting the O p band center. Further, the Gibbs free energy of adsorbed H (H*) formation (ΔGH⁎) becomes less negative (closer to zero) as compared to that of the undoped perovskite LBSCO during this doping.

Exploring HER side of perovskite-based materials

15

This factor also boosts the proton/electron transfer step and lowers the CodOO* bond desorption energy. In AB(ON)3-type perovskites, the presence of the N 2p orbital reduces the width of the forbidden band, which creates a new intermittent energy level higher than the original valence band (VB), composed of only O 2p orbitals. This is known as valence band engineering. Fig. 3 displays the refinement of several anoxic perovskites from the work of Domen and co-workers [21]. Here, the band structures of NaTaO3 and BaTaO2N, in which the top of the valence band of NaTaO3 is composed of O 2p orbitals. Once the N atom partially or completely replaces the O atom in the perovskite structure, a perovskite oxynitride with a higher VB position is formed without disturbing the conduction band (CB) of the original perovskite.

Fig. 3 (A) Schematic band structures of a metal oxide (NaTaO3) and metal (oxy)nitride (BaTaO2N), (B) The schematic diagram of cis and trans nitrogen order [22], (C) UV-visible diffuse reflectance spectra for (oxy)nitrides containing Ti4+, Nb5+, and Ta5+, with band structure of BaTaO2N by DFT calculations [21].

16

Solar-driven green hydrogen generation and storage

9 Effect of oxygen vacancies Along with the B-sites in perovskite oxides, the oxygen vacancies can also act as active sites for water adsorption and dissociation, which is an important step in the Volmer reaction of alkaline HER. In a study led by Togano et al., PrBaCo2O6δ (PBCO6δ) has been synthesized with different concentrations of oxygen vacancies (0  δ  0.85), such as PBCO6, PBCO5.9, PBCO5.8, PBCO5.7, PBCO5.5, and PBCO5.15. For PBCO5.8, where δ ¼ 0.2, the highest HER activity has been observed with an overpotential of η10 ¼ 240 mV and a Tafel slope of 60 mV/dec in 1.0 M KOH solution [23]. The change in the oxygen vacancies causes an increase in the Co valence leading to a strong metal-oxygen covalency. This gives rise to an enhanced charge transfer between the metal ions and the oxygen atom, thereby increasing the HER activity. However, the presence of high content of oxygen vacancies may lead to a decrease in the Co valence and a lower catalytic activity. Thus, the subtle adjustment of oxygen vacancy is critical for the optimized HER activity. It is further observed that oxygen vacancies can also be created by cation deficiency, e.g., a perovskite oxide Ba1xCo0.4Fe0.4Zr0.1Y0.1O3δ (B1xCFZY) with a small degree of barium deficiency (x ¼ 0.00  0.05) [24] leads to an increase in the oxygen vacancy concentration. It shows an overpotential of η10 ¼ 360 mV in 1 M KOH, a 30 mV improvement compared with that of the parent compound (η10  390 mV). In this case, the enhanced HER performance may originate from the increased oxygen vacancies, which generate coordinatively unsaturated active Fe/Co sites to improve the adsorption of reaction intermediates.

10 Prospect and summary The conversion of solar energy to chemical energy has emerged as a favorable way to sustainably convert renewable energy to electricity. The exceptional photochemical performance of the perovskite oxide materials has catapulted them as one of the leading candidates for establishing the solar energy harvest system under pragmatic conditions. However, the narrow excitation band for perovskites and their relative nonreactivity toward HER have raised questions on the prospective applications. However, rational doping of the perovskite leads to the formation of an array of novel materials that allows HER under unexplored areas (Table 1). The N-doped BaNb0.5Ta0.5O2N perovskite presents a nice example of this feature.

Table 1 The leading perovskites exhibiting HER under photoelectrocatalytic conditions. Sr no

Material

Band gap (eV)

Solvent

Light source

Gas evolution of H2 (μmol/h)

Ref.

2.9 3.5 2.9

Methanol/water blend Water Water Water Methanol/water blend Water Water Aqueous Na2CO3 Methanol Water NaSO3

300-W Xe lamp 400-W Xe lamp 420-nm LED 400-W Hg lamp 500-W Hg lamp – 300-W Xe lamp 400-W Xe lamp 300-W Xe lamp 350-W Xe lamp 350-W Xe lamp

936 36 – 138 92 148.3 – 280 3.7 4860 50

[25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

Perovskites

1 2 3 4 5 6 8 9 10 11

SrTiO3(300 nm and (b) 300 W Xe lamp with visible light filter (>420 nm). Polymer

Cocatalyst

OER (μmol h21)

Light source

Ref.

M-g-C3N4

RuO2 Co(II) in polymer None

(a) (b) (a) (b)

[62]

M-g-C3N4 PMDA-gC3N4 M-g-C3N4

11.4 (100) 1.2 (100) 13.0 (50) 7.7 (200) 5.0 (50) 7.2 (50) 2.4 (50) 6.8 (50) 3.6 (50) 27.4 (50) 25.1 (50) 13.0 (50) 28.0 (50) 34.0 (50) 9.0 (50) 14.0 (50) 7.0 (50) 12.5 (100) 16.5 (100) 22.5 (100) 24.0 (100) 7.0 (50) 22.6 (100) 5.9 (100) 0.6 (30) 1.0 (25) 1.6 (50) 2.5 (50) 1.1 (50)

(a) (a) (a) (a) (a) (a) (b) (b) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (a) (b) (a) (b) (b) (b) (b)

M-g-C3N4 M-g-C3N4 M-g-C3N4

PHI PTI

CTF-1 CTF-0 CTP-1 aza-CMP g-C40N3-COF sp2 c-COF

None Fe(OH)3 Cu(OH)2 Ni(OH)2 Co(OH)2 Mn(OH)2 Co3O4 CoN CoSx CoSe2 CoOx None None PtOx Co species Pt/Co species RuOx None Co species Co(OH)2 None Co species Co species

[63] [35] [64]

[65] [37] [38]

[46] [48]

[52] [53] [54] [51] [61] [58]

Reproduced with permission from Fang Y, Hou Y, Fu X, Wang X. Semiconducting polymers for oxygen evolution reaction under light illumination. Chem Rev 2022;122:4204–56. https://doi.org/10. 1021/acs.chemrev.1c00686.

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2.3 Heterostructures of polymers-based hybrids for overall water splitting It has been suggested that making a heterostructure by mixing 2D polymeric materials with MOs may be an ingenious solution to overcome the disadvantages of MOs for PEC water splitting. This is based on the idea that building a heterostructure can be accomplished by combining the two types of materials. For PEC water splitting, MOs-2D polymeric materials heterostructures at the nanoscale provide advantageous properties in five distinct ways. To begin, despite the fact that nanoscale MOs have a noticeably larger surface area than their bulk counterparts, they have a propensity to clump together into larger clusters that have a smaller surface area [66]. To this end, MOs may be uniformly distributed throughout the surface of 2D materials and used as an immobilization platform. Thus, agglomeration may be reduced by fostering more extensive surface interactions between MOs and 2D materials. Second, the efficiency of photo-generated charge separation may be enhanced by manipulating the energy mismatch between MOs and 2D polymeric materials [67]. In this way, the lives of charge carriers like electrons and holes may be extended due to their separation in space. Third, surface stresses may be affected by, for example, tuning as a result of the lattice discrepancy that arises at the intersection of 2D materials and MOs. This provides an alternative approach to extending the visible light spectrum. The fourth benefit is that 2D materials with strong water stability may be used to coat MOs that have a compromised structural integrity, so preventing further structural breakdown. Finally, although certain 2D polymeric materials may exhibit catalytic-active behavior, the vast majority of MOs are photoactive [57]. Photoactive 2D polymeric materials have a higher concentration of electron–hole pairs as a result of light absorption. In addition, they may serve as catalysts to quicken the surface reaction by decreasing the related activation energy. In contrast, polymeric 2D materials that do not engage with sunlight are restricted to catalytic roles. Together, these two materials may form a heterostructure, which has the potential to provide a photoelectrode with superior light absorption and catalytic activity compared to either material used alone [68]. In order to facilitate the photon excitation process, a new direct Z-scheme photocatalyst was created using 3D-CTP and 0D-MSQDs as a hierarchical honeycomb scaffold; the MSQD ornamentation on CTP expedites electron transfer and inhibits charge recombination. Because of these factors, the rate of H2 evolution exhibited a value of 1070 mol h1 g1, which is almost 8 times greater than the value

54

Solar-driven green hydrogen generation and storage

Table 2 Various heterostructures composite polymer photo catalyst for rate of hydrogen evolution. Photocatalyst

3D-CTP0D-MSQDs Pt/TiO2/rGO CN/NixP/RP Ni-doped ZnS-graphene 0.5% CO/SPI ZnO-ZnS/ graphene

Light source

Rate of H2 production 1

1

g

Ref.

300 W Xe lamp

1070 mol h

[69]

Solar stimulator Full-arc visible-light illumination 300 W mercury lamp

1075.68 μmol h1 g1 3.63 μmol h1 1.78 μmol h1 8683 μmol h1 g1

[72] [71]

300 W Xe lamp 300 W mercury lamp

127.2 mol g1 h1 1070 μmol h1 g1

[70] [74]

[73]

of pure 3D-CTP 129 mol h1 g1 [69]. Based on direct Z-scheme, nanocatalysts including Co3O4 and sulfur-doped polyimide (CO/SPI) have been successfully synthesized using a quick and environmental-friendly thermal treatment approach. The activity of the 0.5CO/SPI composite sample was the greatest, with an average rate of production of 127.2 mol g1 h1, which is about 13 times and 106 times greater than that of SPI and Co3O4, respectively [70]. Furthermore, an efficient CN/NixP/RP Z-scheme system was created. At a reaction temperature of 35°C, the CN/NixP/RP nanosystem demonstrated hydrogen evolution of 3.63 and 1.78 μmol h1 under full-arc and visible-light illumination, respectively [71]. In addition to the charge transfer mechanism, heterostructures fabricated from 2D materials are also referred to as vdW heterostructures. 0D-2D heterostructures, 1D-2D heterostructures, 2D-2D heterostructures, and 3D-2D heterostructures are all distinct types of vdW heterostructures (Table 2).

3 Conclusion Photocatalytic overall water splitting is an essential research topic in the field of renewable energy. The use of environmental-friendly methods to divide water has come a long way in recent years. Sustainable business models based on the circular economy rely on the development of unique and ecologically beneficial technologies, such as the production of highly effective photocatalytic materials for use in energy applications. In this chapter, we looked at a special case of PECs polymer-based photocatalysts. Due to their unique properties, they may be adjusted quickly to meet the needs of the system.

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55

A variety of techniques, including (i) band-gap engineering, (ii) the use of co-catalysts, and (iii) the construction of heterojunctions that include organic and/or inorganic semiconductors, have been used in recent years to generate photosensitive materials with enhanced photon and electron activity. As photocatalysts, they may be used alone or in tandem with other chemicals. Designs for a wide range of polymer photoelectrodes, including those that make use of hybrid architectures, multiple layers, and more, are described. These details provide light on photocatalytic water splitting using catalysts based on polymers.

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CHAPTER 4

Transition metal-based single-atom catalyst for photoelectrochemical water splitting Shankary Selvanathana, Pei Meng Woia,b, and Rohit Srivastavac a

Department of Chemistry, Faculty of Science, Universiti Malaya, Kuala Lumpur, Malaysia University Malaya Centre for Ionic Liquids (UMCiL), Universiti Malaya, Kuala Lumpur, Malaysia Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India b c

1 Introduction In response to the environmental calamity and energy-shortage issues, scientists are necessitated to pursue sustainable, clean, and renewable energy sources to replace exhaustible fossil fuels. Hydrogen is seen as a clean energy carrier for the future amid the world energy crisis, explicitly in this 21st century. Several research strategies are focused on the development of novel, cheap, clean, and efficient processes for the hydrogen production and conversely its utilization in fuel cells. Currently, there are few methods for producing hydrogen in the industrial scales, and among them, steam methane reforming from petroleum is one of the major processes. Yet, this conversion involved high energy consumption and generating substantial carbon emissions, which in return attributes to the global warming, despite the green initiatives behind. [1,2] Some other ideas for hydrogen production include the decomposition of ammonia, methanol, hydrogen sulfide, and methane; none of these represents a workable long-term solution for solar energy capture and utilization. In contrast, the process of hydrogen production via water splitting by electrochemical method is mild and only driven by electrocatalysts. Thus, the production process is classified as green and sustainable, with zero carbon emissions, as the only waste produced is water molecule. Although water splitting is a technology celebrated for its eco-friendly nature and ability to produce pristine hydrogen, the overall reaction obligates to a high overpotential value which translates into high consumption of electrical energy. Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00001-7

Copyright © 2023 Elsevier Inc. All rights reserved.

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By incorporating renewable energy as part of the production route, for instance, adapting solar energy, wind energy, waterpower, etc., to generate sufficient energy for the water electrocatalysis, such option of power generation approach may offer the answer to achieve a green and sustainable industry cycle. For example, the water produced by the combustion of hydrogen can be used to generate hydrogen, thus forming a virtuous cycle along the way. [3] This green initiative highlighted the potential of photoelectrochemical (PEC) water splitting, also known as PEC electrolysis, as an efficient and appealing approach to obtaining high purity hydrogen fuels and oxygen. [4] Ever since the first electrocatalyst reported by Fujishima and Honda in 1972, [5] numerous efforts and attempts have been made in the development of electrocatalyst for hydrogen production via photoelectrochemical water splitting. Generally, the water splitting process can be categorized into two half-reactions which is the hydrogen evolution reaction (HER) that occurred at the cathode. In this HER, the hydrogen ions, H+ or water, H2O molecules were reduced to the valuable hydrogen molecule, H2. The oxidation process happens on the anode electrode where the oxygen-evolution reaction (OER) converts water molecules and/or the oxygen-containing anions species into oxygen, O2 gas. [6,7] Both of these reactions are crucial for the overall efficiency of the water splitting process and were driven by the electrocatalysts that are vital to speed up the reaction kinetics for these redox reactions on cathode and anode by reducing the required overpotentials thermodynamically. Electrocatalysts have been extensively applied to accelerate and expedite the interfacial proton-electron charge transfer which occurred during the half-reactions on cathode and anode, respectively, and hence improve the photoelectrocatalytic performance of water splitting [8]. For the past few decades, commercially viable applications to produce hydrogen through water splitting have been greatly hindered by the slow and sluggish OER rate; therefore, it is imperative to develop highly efficient OER electrocatalysts to reduce the applied overpotential, which in return facilitate the overall process, and hence, improve the energy conversion efficiency. At present, noble metals continued as the most efficient electrocatalysts for PEC water splitting in both electrodes. For example, platinum-based materials exhibit the highest HER activity, while iridium, Ir and/or ruthenium, Ru group metals perform adequately for the OER. Nevertheless, such catalyst selections endure from scarcity and high price, which restricted their large-scale commercial applications (Fig. 1).

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V Hydrogen evolution reaction (HER)

+

–

Oxygen evolution reaction (OER)

Cathodic half reaction

Anodic half reaction

2H+ + 2e– ® H2

2H2O ® O2 + 4H+ + 4e–

Two-electron transfer reaction

Four electron transfer reaction

Best known catalyst: Platinum (Pt)

Water splitting

Best known catalyst: Iridium (Ir) or Ruthenium (Ru)

Fig. 1 Conventional catalyst for HER and OER.

Due to such shortcomings, substantial efforts to search for alternatives have been dedicated to the design and synthesis of earth-abundant electrocatalysts for water splitting, such as two-dimensional materials exploration using transition-metal chalcogenides, carbides, phosphides, and nitrides, which activates for HER [9,10], whereas for the OER, transition-metal hydroxides, oxides, phosphates, perovskites, sulfides, and selenides have been developed at the same pace [11,12]. These metal-based electrocatalysts which are mainly based on first and second row transition metals offer characteristics of cost-effective, stable, and highly active; hence, making the overall water splitting process becomes practically feasible as opposed to strong dependent expensive noble metals. Despite the advantages presented by such transition metals-based electrocatalysts, still scientists are not satisfied with the current stage. Numerous efforts were undertaken to minimize the cost of such electrocatalysts in terms of accessibility and price, while at same times, maximizing the use of metal atoms as driven by sustainability goals needs to be conducted at the same stride to realize the wider applications of such system. Some of the fundamental questions need to be answered, for example, the inhomogeneity of catalytic active sites due to deficiencies during the fabrication and processing methods, the complex interactions among metal atoms, and which active species involved need to be understood well in the quest for a new class of electrocatalysts. Small metal particles or isolated metal atoms usually exhibit exceptional size effects which is directly associated to their catalytic activity. It has been reported that rationalizing a catalyst from the microsized to nanosized, from the nanoparticle into the nanocluster assembly, or even as an isolated single atom could be

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exceptionally effective in achieving these targets for such heterogeneous electrocatalysts, namely maximizing the utilization and potential of such metal atoms efficiently. Therefore, catalysts with atomic metal species are projected to play a crucial role in enhancing the PEC performance. Along with this direction, single-atom catalysts, SACs, which have very distinctive electronic and geometric structures that can help to boost massto-atom and proton-coupled electron transfer during the photoelectrocatalytic process, have been proposed. [6,13] It should be highlighted that the catalytic center of a SAC contains not only the atomically dispersed metal atoms, but also the possibility of having a direct neighbor atom, potentially a different metal species, or functional groups, as their supports which serve as substrate and/or as promoter in accelerating the reaction kinetic of PEC water splitting process. [13,14] The emergence of SACs has helped to bridge the homogeneous and heterogeneous catalysts as one system where the connections in between the isolated metal atoms and support surfaces can be better identified and understood, thus allowing a more precise selection of metal species combination and substrate which works optimally by offering great impact on the chemical properties of the supported isolated metal atoms. By understanding the PEC mechanism, herein, we summarize the recent reported work on the fabrication, synthesis, and modification, as well as the catalytic performance evaluation of selected SACs based on transition metals for PEC water splitting. The advantages of such electrocatalyst systems were also discussed in the section.

2 Fundamental mechanism for water splitting reactions Before we can design and propose strategic SACs with excellent catalytic performance by considering the synergic effect in between the single atom catalytic center with its support, it is imperative to understand the fundamental mechanism of the reactions occurring on the electrode surfaces during the PEC of water splitting. The PEC water splitting mechanism is mainly divided into the two half-reactions, i.e., hydrogen evolution reaction (HER) which took place at the cathode and the oxygen evolution reaction (OER) which happens at the anode. In general, the overall PEC water splitting reaction involves three crucial steps, namely i) generation of photoexcited electron-hole pairs, ii) separation and relocation of the photogenerated charge carriers to the surface of the electrode, and iii) consumption of the photoexcited carriers by water redox reactions which reflect the water splitting reaction (Fig. 2). HER is generally noted as the simplest

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65

Fig. 2 Schematic diagram of the photoelectrochemical cell for solar water splitting.

electrocatalytic reaction which can be further elaborated by two theories, namely Volmer-Heyrovsky and Volmer-Tafel mechanisms that explain how this reaction can occur in solutions with different pH, namely acidic and alkaline solutions. On the other hand, for the OER, the reaction is deemed to be more complicated than the HER as it involves four-electron transfer in the process, thus explaining the involvement of multiple elementary steps in the electrochemical reactions. In this segment, the basic mechanisms for both HER and OER are elaborated and summarized.

2.1 Hydrogen evolution reaction (HER) As mentioned above, in general, Volmer-Heyrovsky and Volmer-Tafel are the two mechanisms which have gained acceptance for HER. This halfreaction of the redox reactions demonstrates high pH sensitivity which determined the starting material in the Volmer step. Within the acidic environment, the protons, H+, are reduced into H2 molecule, while in neutral and alkaline medium, two units of water molecules were reduced into two H2 gas molecules and the hydroxides, OH
, respectively. In both media, the reaction is initiated by the Volmer step, in which the H+ or H2O molecule is being reduced to produce the adsorbed H∗ on the catalyst surface. The subsequent step may occur via two different routes, namely the Tafel step and

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the Heyrovsky step, respectively. The HER mechanisms at different pH values are elaborated as follows: Acidic medium: H+ + e
! H∗ ðVolmer stepÞ

(1)

H∗ + e
+ H+ ! H2 ðHeyrovsky stepÞ

(2)

H∗ + H∗ ! H2 ðTafel stepÞ

(3)

2H+ + 2e
! H2 ðTotalÞ

(4)

Alkaline and neutral medium: H2 O + e
! H∗ + OH
ðVolmer stepÞ





(5)

H∗ + H2 O + e ! H2 + OH ðHeyrovsky stepÞ

(6)

H∗ + H∗ ! H2 ðTafel stepÞ

(7)

When the catalyst surface is exposed in acidic environment, two successive reaction steps were observed. Firstly, the adsorbed H* is formed when the H atom binds to the active sites on the catalyst, soon after that, the Volmer step or the discharge step took place (Eq. 1). Next, the adsorbed H* is combined with H and electrons (e
) to form H2 gas molecule. The process involved here is known as the Heyrovsky step or the electrochemical desorption reaction (Eq. 2). Additionally, two of the adsorbed H* on the active catalyst surface can also be rearranged into H2 gas molecule, which is in parallel to the Tafel step (Eq. 3). The overall HER process can be expressed by Eq. (4). Alternatively, under the alkaline solution, there is no proton in presence. Therefore, the HER reaction starts with the dissociation of H2O molecule to provide the adsorbed H* on the catalyst surface. This process requires the contribution of Volmer step (Eq. 5) and Heyrovsky step (Eq. 6), which is in accordance with the Tafel step (Eq. 7) that took place in the acidic electrolyte as stated before. The entire reaction equation is represented by Eq. 7. In some cases, the HER activity can be initiated in alkaline electrolyte, where both the H+ adsorption process and the hydroxyl, OH
, formation are needed to be counted against water separation, while at the same time preserving a reasonable H2 absorption rate to stimulate the water dissociation process. Presently, noble metal platinum remained as the best HER catalyst with the finest hydrogen adsorption energy in both media, while

Transition metal-based single-atom catalyst

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demonstrating the highest exchange current density. The activity of hydrogen evolution reaction in alkaline electrolyte is usually lower in performance as compared to the one in acidic medium which is mainly due to the sluggish water dissociation process that further restricted the reaction kinetic rate.

2.2 Oxygen evolution reaction (OER) Theoretically, OER consists of successive H+ and/or electron transfer steps which require the transmission of four electrons to take part in the reaction; such electrons are needed for the process of the O-H bond dissociation and the formation of the O-O bond in conjunction. Therefore, OER is typically known as unfavorable process thermodynamically and thus demands a high potential to overcome the kinetic energy restriction. Until now, OER is still the bottleneck to industrialize the water splitting process commercially. Similar to the HER mechanism, the reaction kinetics of OER is governed by the pH value of electrolyte. In alkaline environment, hydroxide groups are oxidized into H2O and O2 molecules, whereas in acidic and neutral media, the oxidation of two H2O molecules resulted in four-proton species, H+ and O2 molecules. The proposed OER mechanisms at different pH values are elaborated as follows: Acidic and neutral environments: H2 O ! OH∗ + H+ + e


(8)




OH∗ ! O∗ + H + e +

(9)


O∗ + H2 O ! OOH∗ + H + e

(10)

OOH∗ ! O2 + H+ + e


(11)

2H2 O ! 2O2 + 4H+ + 4e
ðTotalÞ

(12)

OH
! OH∗ + e


(13)

+

Alkaline medium:





OH∗ + OH ! O∗ + H2 O + e

(14)

O∗ + O∗ ! O2

(15)

O∗ + OH
! OOH∗ + e



(16)


OOH∗ + OH ! O2 + H2 O + e

(17)

4OH
! 2H2 O + O2 + 4e


(18)

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In acidic medium, the OER reaction begins with two H2O molecules being oxidized, and one O2 molecule is formed via the four-proton-coupled electron transfer step as depicted in Eq. (12). The adsorbed hydroxyl, OH*, is generated from H2O by losing an electron and proton simultaneously (Eq. 8). From this point onward, the adsorbed hydroxyl OH* loses a H+ and an electron, thus resulting in the formation of adsorbed oxygen (O*) on the surface of catalyst (Eq. 9). Interaction of adsorbed O* with H2O produces the hydroperoxide intermediate (OOH*) species as depicted in Eq. (10). After continuously loosing another H+ and an electron, the intermediate OOH* releases the O2 molecule, whereas in this alkaline medium, O2 molecule is produced by the transformation of hydroxyl, OH
, through the four-electron transfer steps, something distinctively different from the reaction happened in the acid electrolyte as mentioned previously. Furthermore, the H2O molecule is produced in the same process as shown by Eq. (18). The initial process kicks-starts in the alkaline medium is where the OH
produces the adsorbed OH* by freeing an electron (Eq. 13). Next, the generated adsorbed OH* interacts with the OH
to obtain O* by losing another electron (Eq. 14). O2 molecule is then produced via recombination of two adsorbed O* (Eq. 15). The other possibility of the reaction can be proceeded via the nucleophilic attack on O* by OH
to generate an intermediate (OOH*) (Eq. 16). The further proton-coupled electron transfer of OOH* then leads to the releasing of O2 molecule (Eq. 17).

3 Advantages of single-atom catalysts (SACs) The featured chemical structures and characteristics of SACs are substantially different as compared to those catalysts synthesized from nanoparticles and metal clusters in terms of metal distribution and catalytic activity. For example, the single or isolated metal atoms behave as unsaturated coordination atoms or single-atom catalytic sites, which are generally interpreted as the active centers that are responsible for many reactions. The enhancement of the strong metal-support interactions is achieved due to the well-defined geometric structures modulated by the rearrangement of electron configuration in between the atomic metal and the support, thus leading to an efficient electron transfer in the system that enhanced the intrinsic characteristic such as catalytic activity along with high selectivity toward target products. Furthermore, the metal atoms in SACs always exhibit a distinct energy-level distribution and

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69

exclusive band gap in between the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO-LUMO), owing to the quantum size effect via the combination of different metals and supports such as semiconductor, thus attributing to a discrete energy-level structure that can be activated in the visible light region. Most importantly, due to the atomic dispersion configuration in SACs, it is possible to achieve a theoretical efficiency of 100% in catalytic reactions. At the same time, the isolated single metal atoms and well-distributed active sites in SACs that exist in various coordination environments can significantly maximize the concentration of the homogeneous metal atom analogs, which in return minimize the amount of precious and noble metal needed for the synthesis of SACs, thus leading to the cost reduction in catalyst consumption. On top of that, the benefits possessed by the heterogenous and homogeneous catalysts can be realized at once with such SACs that taking advantages on the single atomic distribution on the catalyst supports. All these advantages will eventually give the SACs a superior activity, selectivity, and scalability for many catalytic reactions. Therefore, SACs are considered to have bright expectations for applications in electrochemical water splitting. As a matter of fact, quite a few SAC catalysts which are based on noble and precious metals such as platinum, ruthenium, and iridium have been diligently explored along the line. Such expensive noble metal-based SACs possess an enhanced mass-to-volume activity as compared to nanocluster or nanosized particle catalysts. At the same time, nonprecious metal-based SACs have also been studied broadly due to their being considerably cheaper in price and ease of accessibility. These earth-abundant nonnoble metal active sites can be altered owing to the strong metal-support interactions which could achieve compatible catalytic activities to that of catalysts based on conventional noble metals. Such nonprecious metals, which are mainly from the transition metals group, also display strong metal-support interaction that would ensure sturdy SACs, therefore making it possible to have these electrocatalysts being stable in acid or alkaline mediums. Likewise, the distinctive structure and coordination of single-dispersed active sites in SACs could provide a platform for prediction and identifying of those actual intrinsic properties comprised within the active sites [7]. Such data may provide further insights into their HER and OER catalytic mechanisms. Therefore, stunning progress has been dedicated by scientists to design and fabricate high-performance SACs for both HER and OER based on the earth-abundant transition metals.

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4 Transition metal-based single-atom catalysts for PEC water splitting For electrochemical water splitting, various transition metal-based SACs have demonstrated remarkable catalytic abilities, as shown in Table 1. During the inceptive stages, the electrocatalytic performance of these transition metal-based electrocatalysts could not surpass or coming near to its noble metal counterparts. However, gradual exploration of various transition metal compositions revealed that the electrochemical performance of the material can be optimized via two strategies: i) heteroatom doping mechanism and ii) morphological engineering. Since then, the venture to explore ideal transition metal-based electrocatalyst has led to a plethora of studies dedicated to synthesis of innumerable nanostructures, from simple architectures like nanorods, nanosheets, and nanowires to more intricate designs such as nanoflowers, nanobush, and nanothorns. The commonly used transition metals were nickel, cobalt, Table 1 Selected transition metal-based SACs for electrochemical water splitting reaction. Reaction

HER

Catalyst

Ni/GD Fe/GD Cu@MoS2 Co1/PCN Co-substituted Ru CoSAs/PTF-600 SACo-N/C

OER

A-Ni@DG w-Ni(OH)2 Ni-NHGF CoIr-0.2 Co-C3N4@CS Co-Fe-N-C Co-C3N4/CNT CoNi-SAs/NC SCoNC Co-Nx/C NRA A-CoPt-NC

Medium

0.5 M H2SO4 0.5 M H2SO4 1 M KOH 1 M KOH 1 M KOH 0.5 M H2SO4 1 M KOH 0.5 M H2SO4 1 M KOH 1 M KOH 1 M KOH 1 M KOH 0.1 M PBS 1 M KOH 1 M KOH 1 M KOH 1 M KOH 0.1 M KOH 0.1 M KOH 0.5 M H2SO4 1 M KOH

Overpotential

Ref. –2

0.088 V at 10 mA cm 0.066 V at 10 mA cm–2 0.0131 V at 10 mA cm–2 0.041 V at 10 mA cm–2 0.138 V at 10 mA cm–2 0.013 V at 11 mA cm–2 0.094 V at 10 mA cm–2 0.178 V at 10 mA cm–2 0.169 V at 10 mA cm–2 0.178 V at 10 mA cm–2 0.270 V at 10 mA cm–2 0.273 V at 10 mA cm–2 0.331 V at 10 mA cm–2 0.373 V at 10 mA cm–2 0.470 V at 10 mA cm–2 0.309 V at 10 mA cm–2 0.380 V at 10 mA cm–2 0.340 V at 10 mA cm–2 0.310 V at 10 mA cm–2 0.300 V at 10 mA cm–2 0.027 V at 10 mA cm–2 0.050 V at 10 mA cm–2

[15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

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and copper. Under this section, each of these types of SACs is discussed in certain detail in terms of their chemical properties and electrocatalytic activity. Conventionally, these SACs can be fabricated into water splitting electrocatalysts via formation of slurry in the presence of conductive binders, followed by drop casting onto the working electrode. However, the use of binder in the slurry, which is electrically insulating in nature, creates a hindrance between the electrocatalyst and the electrolyte. This barrier significantly affects the catalytic performance of the active material as its conductivity is being compromised. Moreover, in the drop cast method, the active material is unable to anchor its position strongly on the surface of the electrode, thus causing it to leach out into the electrolyte during the vigorous production of hydrogen or oxygen bubbles in the catalytic process. Additionally, it is difficult to control the regular arrangement of these nanostructures on the electrode surface. Thus, to overcome these major shortcomings, an alternative method was introduced where the electrocatalyst was synthesized in situ via electrodeposition in the presence of conductive substrates for fabrication of binder-free electrocatalysts.

4.1 Nickel-based single-atom catalysts for PEC water splitting Nickel-based materials are one of those potential transition metals to replace platinum-based materials due to their low cost and excellent HER catalytic performance. Atomically dispersed Ni-based materials have exhibited high photocatalytic efficiency toward hydrogen evolution from water splitting. Early in 2014, Wang and his team electrodeposited Ni-Mo alloy nanospheres on copper foam to be evaluated as electrocatalysts for HER [32]. In this study, two of the most fundamental parameters of electrodeposition, namely current density and time, were optimized and the resulting morphologies of the metal particles were analyzed. It was noted that when the current density was increased from 0 to 250 mA cm
2, the surface of the copper foam substrate appeared to be rougher and covered by irregular Ni-Mo nanospheres. The gaps in between these nanospheres grew wider with every increment of the current density, which helped to facilitate electrolyte diffusion during the reaction, hence enhancing its performance as an electrocatalyst. The evaluation of HER activity for all the electrodes revealed that up to 200 mA cm
2, the catalytic performance is gradually increased and had a slight decrease when it approached to the current density of 250 mA cm
2. The drop in this catalytic performance is presumably due to overcrowding of the nanoparticles. Apart from the reported Ni-Mo alloy, other alloys comprise Ni as one of

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the metals can also be fabricated to improve the HER catalytic activity. Hong et al. [33] synthesized Ni-tungsten, W alloy materials on a copper foil substrate by using the one-pot electrodeposition technique. By varying the ratio in between Ni2+ and W6+ in the electrochemical cell, various Ni-W alloys with different element ratios were obtained and characterized. Among them, Ni59W41 materials exhibited a better activity with overpotential of 122 mV in alkaline electrolyte which prepared from potassium hydroxide. This result exhibited that the alloying effect of Ni and Mo prompted the surface area to increase, thus offering more hydrogen active sites, which resulted in excellent HER activity. Using an electrodeposition strategy, Xue and the team [15] have constructed a single atom of Ni and iron, Fe on graphdiyne substrate, a carbon sheet with acetylenic linkages (Ni/Gd and Fe/GD). Here in this work, the Fe/GD shows an overpotential of 66 mV at 10 mA cm–2, as compared to 88 mV which recorded for Ni/GD for HER activity. Surprisingly, these values are superior as compared to other reported earth-abundant HER electrocatalysts such as those organometallic system, for example, Ni2P (110 mA cm–2) [34] and MoS2 (170 mA cm–2) [35]. In fact, the reported catalytic activity in this work is compatible to those favorably noble metal-based electrocatalysts such as Pt nanowires/single-layered Ni(OH)2 [36] as depicted in Fig. 3. Apart to be active for HER, the transition metal nickel atom has also been reported to be capable of catalyzed the OER as well. Yao et al. [21] reported the fabrication of an atomically dispersed Ni catalyst trapped on the defective graphene sheet (A-Ni@DG) via incipient wetness impregnation method, followed by acid leaching. Such carbon-based material as a substrate provides more efficient anchor sites of Ni atom via the strong charge transfer in between the metal atoms and the 2π antibonding state of the carbon atoms, thus reducing the level of agglomeration and promoting isolated distribution of Ni atom. The derived catalyst exhibits exceptionally good activity for OER, with an overpotential of 0.27 V at 10 mA cm
2 in the reaction. This performance exceeded the superior Ir oxide catalyst in the same reaction setup. On another literature, both theoretical calculations and experimental studies revealed that Ni metal which embedded in nitrogen-doped holey graphene frameworks (Ni–NHGF) to be highly active and stable for OER. [23] Electrochemical measurements with linear sweep voltammetry (LSV) revealed the overpotential of 0.331 V, which is smaller than those of Co–NHGF and Fe–NHGF that reported in the same work.

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Fig. 3 Electrochemical properties of Ni/GD and Fe/GD. (A) Polarization curves of (i) Pt/C, (ii) Fe/GD, (iii) Ni/GD, (iv) GDF, and (v) CC (inset: enlarged view of the linear sweep voltammetry (LSV) curves for Fe/GD and Ni/GD near the onset region). (B) Onset values and (C) overpotentials at 10 mA cm
2 of Ni/GD and Fe/GD (red square) along with other nonprecious single-atom HER catalysts (green circle) and several bulk catalysts (olivine triangle). (Reprinted with permission from Xue Y, et al., Anchoring zero valence single atoms of nickel and iron on graphdiyne for hydrogen evolution. Nat Commun 2018;9(1):1460. Copyright 2018 Springer Nature.)

4.2 Copper-based single-atom catalysts for PEC water splitting Although nickel-based materials have been exhaustively used for electrochemical applications in the last decade, copper-based substrates such as copper foam, copper foil, and copper mesh are being explored as alternative substitutes owing to their excellent electrical conductivity, high chemical stability, and lower preparation cost. Over the past years, despite copper oxides which derived from molecular Cu complexes or Cu salts via electrodeposition or conventional hydrothermal method were found to display interesting OER activity, most of them suffer from low activity and instability due to less available catalytic active sites, poor electrical conductivity, and ease of aggregation during the electrochemical process. The performance of copper-based OER catalysts can potentially be further improved by homogeneous distribution of cationic metals in metal–organic

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frameworks. Due to these, copper-based materials are commonly used as cocatalysts for HER instead of OER. Some reported works include fabrication of CuNi with different structures via the wet chemical etching method in a one-pot solution containing copper acetylacetone, nickel acetylacetone, and iron chloride hexahydrate mixtures as reported in the work by Li et al [37]. The team has carefully designed the synthesis temperature, time, and the amount of solvent to manipulate the fabrication of the unique edge-cut Cu@Ni nanocubes, edge-notched Cu@Ni nanocubes, and mesoporous Cu-Ni nanocages by varying the amount of each metal salt ratio in the process. The synergistic effect of copper and nickel, together with the hollow structure, has led to profoundly larger surface area-to-volume ratio, which give rise to the active sites, thus enhancing the intrinsic activity of HER. As a result, mesoporous Cu-Ni nanocage with an average size of 62 nm exhibited the optimum geometric activity with an overpotential of approximately 140 mV at 10 mA cm
2, with Tafel slope of 79 mV dec
1. In the subsequent stability test by running the same experiment for 2000 cycles, activity of mesoporous Cu-Ni nanocage was not significantly reduced and maintained its catalytic activity for a period of 8 hrs, which once again confirmed the stability of the prepared electrocatalyst. The copper single atom as HER catalyst on molybdenum sulfide (Cu@MoS2) was demonstrated by Ji et al. [16] MoS2 is another type of two-dimensional material that has attracted great interest as a promising nonprecious metal catalyst candidate to replace the precious metal-Pt catalysts for the HER. Yet, its catalytic efficiency is significantly restricted by its density of catalytic active sites due to the atomic arrangement. On top of that, MoS2 planar is adequately inert where surface modification is needed to improve its electronic conductivity properties. With the doping of Cu atoms, it can significantly enhanced the proton-electron transfer in between the metal atoms and the substrate, which is achieved by the one-pot solvothermal synthesis using the octahedral molybdenum oxide (MoO3) as the ideal starting material together with thioacetamide as the sulfur source, and urea as the weak reducer to obtain the high purity MoS2 flakes. Heteroatom of Cu from the copper(II) nitrate source was then doped and inserted into the sulfur, S layer and bonded with S atoms on the MoS2 sheet (Fig. 4A). The electrochemical behavior of Cu@MoS2 as shown in the volcanoshaped plot is significantly impelled by the mass of Cu atoms loaded, which is directly associated with the amount of thioacetamide consumed. This phenomenon is presumably due to the presence of unsulfurized MoO3 and thus inhibited the reduction process to take place during the HER, thus

Fig. 4 (A) Schematic diagram on formation mechanism of Cu@MoS2. (B) Electrochemical measurement of the prepared catalyst in HER. (a) Polarization curves of HER for Cu@MoS2, Pt/C, and MoS2. (b) Tafel plots of Cu@MoS2, Pt/C, and pristine MoS2. (c) Volcano-shaped plot of overpotentials at 10 mA cm
2 of Cu@MoS2 with different Cu(NO3)2 precursor loading. (d) Chronoamperometry measurements of Pt/C, pristine MoS2, and Cu@MoS2 at the current density of 10 mA cm
2. (Reprinted from Ji L, Yan P, Zhu C, Ma C, Wu W, Wei C, Shen Y, Chu S, Wang J, Du Y, Chen J, Yang X, Xu Q. One-pot synthesis of porous 1T-phase MoS2 integrated with single-atom Cu doping for enhancing electrocatalytic hydrogen evolution reaction. Appl Catal Environ 2019;251:87–93. Copyright 2019 Elsevier.)

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restricting unwanted reaction which could slow down the rate of reaction. As such, a modest overpotential of 131 mV at the current density of 10 mA cm
2, with the Tafel slope of 51 mV dec
1, was accomplished in this work. CuO which derived from different precursors that grown on the copper substrates tends to produce unique morphologies which typically at the same time possess higher conductivity attributed to its metal effect. For example, Lee et al. synthesized CuO nanoflakes derived from CuPPc (copper polymeric phthalocyanine) on copper foam as the electrocatalyst for water oxidation. [38] The monomer of the ionophore, 1,2,4,5-tetracyanobezene, was deposited on copper foil via a simple chemical vapor deposition technique which was subsequently calcined at higher temperatures to convert it into oxide form, CuO. At lower calcination temperature, the surface of the substrate appeared to be smooth and sparsely embedded with nanoparticles. As the temperature increased, a prominent morphology of thorn-like structure began to form where the surface becomes roughened and more active sites were exposed. The initial CuO nanothorns were transformed into thin nanoflakes that recorded the OER activity with an overpotential of 0.287 V at a current density of 10 mA cm
2 in the alkaline electrolyte.

4.3 Cobalt-based single-atom catalysts for PEC water splitting Dispersion of Co molecules on nonmetal supports in photoelectrocatalysis has gained great attention in recent years. For instance, Co supported on carbon materials such as graphene oxide, multiwalled carbon nanotubes, and conducting polymer can be used as effective electrocatalysts for water splitting [39–42]. Apart from electrocatalytic route, photocatalytic water splitting, which is driven by single atom photocatalysts, can drive the hydrogen economy on a lab scale. Such SACs of photocatalyst offer abundant active sites on the surface due to maximizing the atom dispersion through its unique electronic structures. To date, Co-based SACs have taken significant position in the photocatalytic water splitting, in which the single Co atoms assist in enhancing the light-harvesting supports as well as the catalytic efficiency in both HER and OER. One of the reported works by Xu and coworkers [43] discovered that monoatomic of Co which dispersed on the N-doped graphene (Co-NG) surface is capable to catalyze the photocatalytic water splitting which leads to H2 production effectively via the Volmer-Heyrovsky mechanism by just utilizing as little as 0.25 wt% of Co-NG loaded on the cadmium sulfide as the semiconductor to

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accomplish the hybrid photocatalyst system. This work which adopted inexpensive elements indicated that Co-NG can be deemed as an effective and stable cocatalyst to enhance the H2 release capacity on a light harvesting semiconductor constructed from quantum dots, thus offering new perceptions into the fabrication of high-performance photocatalysts for water splitting without the dependent on noble metals. During the electrochemical fabrication process of electrocatalyst, the presence of complexing and buffering agents is necessary particularly in the direct deposition of metals to ensure the atomic metals are strongly adsorbed and/or embedded on the surface. Sodium citrate and boric acid are one of those common complexing agents and buffering reagent which were used during the electrodeposition experiment, respectively, to improvise the surface loading to allow more active site utilization. In a study conducted by Cheng et al. [44], the orderly arranged nanosheet array of Co atoms was grown on copper foam using the mentioned chemicals. The recorded morphology images revealed an optimum weight-to-surface coverage by Co atomic metals were successfully embedded onto the copper support layer which displaying a huge amount of pore volume that is presumably related to the active sites for hydrogen and its intermediate species adsorption. Upon testing this highly porous network structure, an overpotential of 0.135 V for HER in 1.0 M of alkaline KOH electrolyte with a current density of 10 mA cm
2 was recorded. Alternatively, atomically dispersed Co sites on g-C3N4 were found to be favorable for PEC of both hydrogen production and oxygen reduction reaction [26,27,45]. Throughout the years, two-dimensional materials such as this graphitic carbon nitride (g-C3N4) have been regarded as a promising substrate material to offer binding sites or allow anchoring of the isolated well-dispersed metal atoms that active in photocatalysis, owing to its high stability, fascinating electronic properties, plentiful of coordination sites, and active in visible-light region. [46,47] In all the reported PEC water splitting system, the active species adsorption effect can be improved by single metal atoms supported on g-C3N4 via isolated dispersion, hence can be improvised as an efficient active site to boost the reaction kinetics rate by promoting faster electron transfer process. To date, much research has found that single or monoatomic transition metals from d-block can be anchored onto g-C3N4 for photocatalytic hydrogen evolution, which in return can effectively catalyze the PEC water splitting into H2 gas. For example, Yao et al. [17] reported their work in anchoring single-Co atoms on the carbon-based substrate where the team discovered that the atomic Co can

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be coordinated with the N atoms from the six-N cavity, thus forming Co-N4 sites in Co1/PCN. The bonded N donors accessing from the graphitic carbon nitride can enhance the electron density, while lowered the energy barrier or activation energy for formation of Co hydride intermediate, thus accelerating the hydrogen evolution reaction rates. Moreover, the doping of nonmetal phosphorus atoms on the g-C3N4 layered structure has maximized the photocatalytic efficiency of single atom Co for water splitting. As such, the Co1/PCN recorded HER activity with an overpotential at 0.138 V with a current density of 10 mA cm–2. The Tafel slope recorded for Co1/PCN electrocatalyst suggested the Volmer-Heyrovsky mechanism was involved in the HER pathway, which is further proven via theoretical density functional calculation (DFT). Subsequently, DFT calculations were conducted based on the recorded Tafel slope results as a measure to predict the HER catalytic cycle when the reaction is taking place in the alkaline medium. It was noticed that the molecular hydrogen can be released through the Heyrovsky reaction step which is initiated by the adsorption of H2O onto Co as represented as first step in the reaction, followed by the dissociation process which ended with adsorbed OH* and H* on the Co and the nearby N, as stated as second step. Hydrogen is then generated by having the proton from an adjacent water molecule to react with the first adsorbed H* in the calculation. Co-substituted Ru nanosheets that are active for HER in alkaline media were reported by Mao et al. [18] where the isolated cobalt atoms were dispersed into Ru lattice to achieve the Pt-free electrocatalyst. The synthesis was done via simultaneous reduction of two metal salts in the presence of glucose and citric acid hexahydrate as the reducing agents. The selfassembled hexagonal nanosheets array exhibits superior catalytic activity toward HER in alkaline media (1 M KOH), with a current density of 10 mA cm
2 and an ultralow overpotential of 0.013 V as depicted in Fig. 5. The results clearly demonstrated that a single substitution of Co can greatly enhance the HER activity, while the formation of a Co-Co bond in the alloy combinations (RuCo and RuCo2) would direct to a negative performance in the catalytic activity. The experimental results and density functional theory (DFT) calculations further revealed that the energy barrier of water dissociation by the mono-Co atom substituted Ru catalysts had remarkably reduced as compared with the pristine Ru and RuCo alloys, leading to superior performance for HER. A pyrolysis solid-state reaction method to fabricate a single atom Co-nitrogen-doped catalyst (SACo-N/C) by coordination between cobalt

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Fig. 5 (A) SEM and (B) TEM images of Co-substituted Ru; (C) liner sweep voltammetry at scan rate of 5 mV s
1 and (D) overpotentials of Ru/C, Pt/C, RuCo, and Co-substituted Ru at 10 mA cm–2. (Reproduced from Mao J, et al. Accelerating water dissociation kinetics by isolating cobalt atoms into ruthenium lattice. Nat Commun 2018;9(1):4958 with permission. Copyright 2018 Springer Nature.)

chloride hexahydrate and sodium thiocyanate compounds has also been reported [20]. Attributed to the decomposition of both starting materials at temperatures lowered than that of carbon layer deposition, Co-rich particles were able to grow faster and bigger in size, which are presumably capable of escaping or releasing itself from the encapsulation of carbon layers, thus forming the two-dimensional graphene structure that improved the accessibility to the active sites while at the same time enhanced the electron transfer ability. Furthermore, the Co-particles are also able to form coordination with nitrogen atoms without blockages by the carbon layer. HER and ORR were performed in acid and alkaline media, respectively, with the prepared catalyst using LSV setup. On another work by Zhi et al, [30] the A-CoN3S1@C electrocatalyst was constructed using the atomic exchange strategy and exhibits the half-waves potential at 0.901 V with

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exceptional durability for ORR under alkaline conditions, credited to the positive effect of S atoms which helps to optimize the electron distribution structure of the atomic CoN3S1 active sites. Gao et al. explored the in situ growth of cobalt on copper foam as an electrocatalyst for OER [48]. Initially, the metals were first electrodeposited on a flat surface of pure copper foam which significantly required a long deposition period up to 40 min. Using a similar mass loading, the electrodeposition of Co was done on a nanostructured copper foam in a much shorter time of 10 min. Prior to the deposition process, the copper foam was first chemically oxidized and subsequently reduced to obtain a uniformly formed nanowire. The morphology analysis of both electrodes revealed that under shorter deposition period, the surface displayed a more evenly distributed nanosphere featuring channel-like structures as compared to the rather larger particles and nonuniform Co particles for the fabrication using longer periods. This difference in morphology is reflected in their OER electrocatalytic performances as well where the second electrode, which is more evenly in nanostructured, exhibited the highest activity with an overpotential of 0.293 V while the other electrode had shown significantly lower activity with an overpotential of 0.320 V, both at same current density of 50 mA cm
2.

4.4 Palladium-based single-atom catalysts for PEC water splitting Besides the abovementioned nonprecious metals supports, Li et al. [45] reported the immobilization of Pd on the Cu-Pt nanorings (Pd/Cu
Pt NRs). In this work, the well-dispersed atomic Pd was supported on the metal-dual site using a minimal 1.5 atom% of noble Pd which was predoped on copper, as the electrocatalyst used in HER. The resultant Pd/Cu
Pt NRs showcase astonishing catalytic activity for hydrogen evolution reaction in contrast to those of electrocatalyst supported on Pd, Pd/Cu with a Cu single site, including the commercially available 20 wt% of Pt supported on carbon (Pt/C) which all reported in the same work. The obtained Pd/Cu-Pt NRs also possessed remarkable stability as compared to Pt/C despite when multiple scans of voltammetry cycles were performed in the acidic electrolyte medium. In addition, the recorded Tafel slope of 25 mV dec
1 is lower than that of commercially available system, Pt/C (28 mV dec
1), thus making it one of the best HER catalysts ever reported in the scientific community. The DFT computational calculation which performed in the same group has revealed the essential role of these neighboring Cu atoms in the support layer where such metals were believed to

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assist in balancing the interaction in between the single dispersed platinum atom-based active site and the adsorbed hydrogen atom, thus increasing the HER activity by offering more active sites for Pd atoms. Another reported work investigated the distribution of Pd atoms on the transition metal oxidemanganese oxide, (MnO2) nanowires, and carbon nanotubes (CNTs) (Pd/ MnO2-CNT), which is the active electrocatalysts for the oxygen reduction reaction. The electroconductivity was improved via the synergistic effect between the palladium atoms which embedded in the manganese oxide and the CNT, forming an interwoven structure which binds homogeneously together that leads to tremendous stability which is capable of undergoing a constant scan cycle despite a strong alkaline electrolyte [49]. With the development of computational chemistry, it is of importance to explore such advanced theoretical calculation methods using the development of quantum computing which are capable and conducive to the further identification of real active sites for such PEC reactions which further provide insights into the overall mechanism. Fan et al. [50] used spin-polarized DFT calculations to explore the catalytic performance of several palladium nanoclusters which systematically anchored themselves on the experimentally available defective MoS2 monolayer (Pdn/MoS2) and being manipulated as the electrocatalysts for both the oxygen reduction reaction and oxygen evolution reaction. Attributed to the strong interaction of Pd clusters with the dangling Mo atoms and the monovacancy S atoms on the surface, it led to a high stability of the studied Pdn/MoS2 materials. The study concluded that the high reactivity toward oxygen evolution and oxygen molecule activation was very much depends on the atomic size of the anchored Pd clusters which dispersed in those available vacancies offered by S atoms, that later is identified as the Pd2 cluster.

5 Future perspectives and conclusion In this chapter, the advances of SACs as photo- and electrocatalysts in PEC reactions for water splitting to produce hydrogen and oxygen were reviewed based on the recent published works. The main roles of single metal atoms in such photoelectrochemical catalysts are assessed in detail by focusing onto its morphologies and catalyst support system. It can be concluded that the interaction of single metal atoms with supports can affect the performance and catalytic efficiency, which is attributed to the formation of atomically well-dispersed active sites that could be embedded on the surface of substrate layer. For instance, the interaction of metal atoms with two-dimensional

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material supports such as g-C3N4 can alter the layered structure of such system, thus further influencing the behavior of photoelectrogenerated charge carriers by improving the conductivity. The discussed advantages of SACs highlighted the potential and practical applications in water splitting process. On the other hand, it also revealed the active sites and catalytic mechanisms of SACs for PEC have not been thoroughly explored to date, which greatly restricts the design and fabrication of such effective and stable SACs. Incidentally, advanced theoretical calculation methods, taking advantages of recent powerful quantum computing which are crucial for the insightful explanation of real active sites, that is, electron formation, adsorption, and desorption processes, should be one of the prime concerns. In addition, sophisticated in situ characterization methods should be developed and employed to monitor the structural changes of active sites under reaction conditions so that insightful information on the action of active sites can be provided. In return, details of the reaction mechanisms can be understood based on the gathered information from various characteristics, which, in return, will provide new information for this decisive step in revealing the whole PEC process for water splitting. To realize commercially viable applications of SACs in PEC water splitting in near future, appropriate synthesis strategies need to be adopted to fabricate more high-performance single atom PEC. For example, in the synthesis process, interaction in between metal atoms needs to be weakened to avoid agglomeration, whereas the interaction of those isolated metal atoms with the support needs to be strengthened as the measure to achieve the sufficient activity of SACs in practical. In addition, enabling technologies for sophisticated synthesis techniques such as using computer-aided design to explore how to maximize the use of metal precursors should be prioritized. At present, the studies of SACs in PEC water splitting are mainly focused on hydrogen production, with slight oversight on the electrocatalysts for OER, especially with the SACs fabricated from transition metals. Therefore, it is vital to have more SACs that actively drive both HER and OER simultaneously. Hence, such bifunctional electrocatalyst will expedite the practical realization of water splitting devices.

Acknowledgment The author would like to acknowledge the financial support from Universiti Malaya for SATU RU Grant (ST005-2022) and the IMRC/AISTDF/CRD/2018/000048 (ASEAN-India Collaborative R&D Scheme).

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air batteries. Adv Mater 2022;34(15):2110172. [29] Chen G, et al. Continuous nitrogen-doped carbon nanotube matrix for boosting oxygen electrocatalysis in rechargeable Zn-air batteries. J Energy Chem 2021;55:183–9. [30] Zhi Q, et al. Atomic CoN3S1 sites for boosting oxygen reduction reaction via an atomic exchange strategy. Nano Res 2022;15(3):1803–8. [31] Jia Y, et al. Identification of active sites for acidic oxygen reduction on carbon catalysts with and without nitrogen doping. Nat Catal 2019;2(8):688–95. [32] Wang Y, et al. A 3D nanoporous Ni–Mo electrocatalyst with negligible overpotential for alkaline hydrogen evolution. ChemElectroChem 2014;1(7):1138–44. [33] Hong SH, et al. High-activity electrodeposited NiW catalysts for hydrogen evolution in alkaline water electrolysis. Appl Surf Sci 2015;349:629–35. [34] Lei Y, et al. Increased silver activity for direct propylene epoxidation via subnanometer size effects. Science 2010;328(5975):224–8. [35] Voiry D, et al. The role of electronic coupling between substrate and 2D MoS2 nanosheets in electrocatalytic production of hydrogen. Nat Mater 2016;15(9):1003–9. [36] Yin H, et al. Ultrathin platinum nanowires grown on single-layered nickel hydroxide with high hydrogen evolution activity. Nat Commun 2015;6(1):6430. [37] Li Z, et al. Mesoporous hollow Cu–Ni alloy nanocage from core–shell Cu@Ni nanocube for efficient hydrogen evolution reaction. ACS Catal 2019;9(6):5084–95. [38] Lee H, et al. An organic polymer CuPPc-derived copper oxide as a highly efficient electrocatalyst for water oxidation. Chem Commun 2020;56(26):3797–800. [39] Xie L, et al. Molecular engineering of a 3D self-supported electrode for oxygen electrocatalysis in neutral media. Angew Chem Int Ed Engl 2019;58(52):18883–7. [40] Li X, et al. Carbon nanotubes with cobalt corroles for hydrogen and oxygen evolution in pH 0–14 solutions. Angew Chem Int Ed Engl 2018;57(46):15070–5. [41] Li H, et al. Convenient immobilization of cobalt corroles on carbon nanotubes through covalent bonds for electrocatalytic hydrogen and oxygen evolution reactions. ChemSusChem 2019;12(4):801–6. [42] Meng J, et al. Attaching cobalt corroles onto carbon nanotubes: verification of fourelectron oxygen reduction by mononuclear cobalt complexes with significantly improved efficiency. ACS Catal 2019;9(5):4551–60. [43] Luo M, et al. Shape and composition effects on photocatalytic hydrogen production for Pt–Pd alloy cocatalysts. ACS Appl Mater Interfaces 2016;8(32):20667–74. [44] Cheng C, et al. High-efficiency bifunctional electrocatalyst based on 3D freestanding Cu foam in situ armored CoNi alloy nanosheet arrays for overall water splitting. J Power Sources 2019;427:184–93. [45] Chao T, et al. Atomically dispersed copper–platinum dual sites alloyed with palladium nanorings catalyze the hydrogen evolution reaction. Angew Chem Int Ed 2017;56 (50):16047–51.

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CHAPTER 5

Clathrate hydrate as a potential medium for hydrogen storage application Pawan Guptaa and Rohit Srivastavab a

Department of Petroleum Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad, Jharkhand, India b Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

1 Introduction Hydrogen is present as complex molecules such as water or hydrocarbons. Hydrogen (H2) is considered tomorrow’s fuel, which can reduce carbon emissions. Therefore, developing hydrogen production and storage technologies has become a top priority for the world. India and other countries are considering purchasing green hydrogen to minimize their dependence on fossil fuels. According to the United States DOE, hydrogen storage targets are 6.0 wt% by 2010 and 9.0 wt% by 2015 [1]. Hydrogen storage at safe pressures and large quantities is critical for a hydrogen-based economy. Hydrogen poses an explosion risk when stored in the natural state, whereas compressed hydrogen gas density is very low. Numerous researchers are searching for low-cost hydrogen storage solutions. Storage of hydrogen in a solid state is safe and efficient for handling—for example, metal hydrides and carbon nanotube store hydrogen in a solid form. Storage in solid form also results in more energy density and may be friendly due to the reversible phenomenon of adsorption and desorption. Hydrogen hydrates are an exciting material to store hydrogen. At first, hydrogen storage in hydrate form seems to be good. As discussed earlier, cages can house multiple molecules in their structure. More than one hydrogen molecules are possible in a single cage, depending on the size of the cage. Mixing other gases with hydrogen is an amicable solution for a sustainable energy source. The amount of hydrogen that can be stored in the cages, which can accommodate 2–6 hydrogen molecules, is 3.77 wt%. Above 160 K, the Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00018-2

Copyright © 2023 Elsevier Inc. All rights reserved.

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hydrogen molecules rotate in the cages, while below 50 K, they get locked and do not move as determined neutron-scattering experiment. Typically, hydrogen, water, and other molecules such as methane and tetrahydrofuran (THF) form complex clathrate hydrate. It is a very optimistic statement that hydrogen hydrate could be formed under some circumstances. Considerable research efforts are required to enrich and improve the storage of hydrogen in cages. In this book chapter, we tried to present a comprehensive outline of the current state of work in the area of hydrogen hydrates related to the storage of hydrogen, such as structural information and their characterization aspects, thermodynamic and kinetic studies, and highlights on recent development. Potential challenges and prospects for further improving the capacity to store hydrogen and recommendation are highlighted. Fig. 1 shows the relative position of the hydrogen hydrate corresponds to other storage mediums [2].

Fig. 1 Hydrogen hydrate comparison with other storage mediums. (Adapted from Gupta A, Baron GV, Perreault P, Lenaerts S, Ciocarlan RG, Cool P, et al. Hydrogen clathrates: next generation hydrogen storage materials. Energy Storage Mater 2021;41:69–107.)

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2 Clathrate hydrate structures specific to hydrogen hydrate Table 1 shows numerous past works relevant to the hydrogen hydrate structure and methodology used to identify the structure and corresponding storage capacity. Hydrogen typically forms sII hydrate, containing 16 small dodecahedron (512) cages and 8 large (51264) hexakaidecahedron cages. Table 1 shows four hydrogen molecule hydrate forms that seem to exist under high-pressure and low-temperature conditions. Hydrogen clathrate structure II is known as compound 0, and 2-filled ice hydrates with hydrogen are known as compound 1 and compound 2. The sII clathrate hydrate needs pressures of 180–220 MPa and a temperature of approximately 300 K, and its structure contains 48 hydrogen and 136 water molecules in the unit cell [16]. The compound 0 clathrate has a trigonal quartz-like structure. The arrangement of water molecules in compound 0 is not the same as in ice structures [16]. Lokshin et al. have shown four hydrogen molecules resting in large cavities using neutron diffraction. The distance between two hydrogen mole˚ . In the case of solid hydrogen, the intermolecular distance is cules is 2.93 A ˚ at atmospheric pressure, and 4.2 K. Strobel et al. studied sH hydrates 3.78 A and found that the hydrogen can be stored in small cages of sH hydrate provided that some other molecules are resting in larger cages. They studied sH hydrate formation using different additives, such as methyl tert-butyl ether (MTBE), trimethylbutane (TMB), methylcyclohexane (MCH), and dimethylcyclohexane (DMCH) [17]. Duarte et al. too studied sH hydrates for phase equilibrium in the presence of additives, such as MCH, MTBE, and DMCH. DMCH showed higher stability than other additives [18]. Both the above studies predicted a significant increase in hydrogen storage by up to 40% compared to sII hydrates. sH hydrogen hydrate stability region is too high, with an approximate pressure range of 50–100 MPa and temperature range of 267–279 K. sII hydrogen hydrate stability region is also high with a pressure of 30 MPa and temperature range of 265–285 K. Semiclathrate is stable at low pressures less than 30 MPa and temperature range of 285–300 K and provides the possibility of storage at moderate atmosphere. In addition, new sH hydrate formers should also be found and developed to operate under moderate conditions for storing hydrogen. Florusse et al. found that THF can reduce the formation condition to 5.0 MPa and 279.6 K; however, the storage capacity is reduced. Hydrogen hydrate at maximum can store 5.3 wt% of hydrogen in sII [19].

Table 1 Summary of the literature showing the cage occupancy, storage capacity, and methodology followed for evaluation. Cage 512 Cage 51264

Authors

Observation

Mao et al.

Shown to form classical sII 2 structure

4

Patchkovskii et al. Sebastianelli et al.

Theoretically stable

3.96

2

1–5 hydrogen molecules accommodate in a large cage Jianwei Wang – 2 Paul H2 molecule migration 2 D. Gorman from the small to large cage if more than two occupations in 512 1 Papadimitriou Hydrogen occupation in et al. the cage is considerably influenced by the lattice constant The promoter should be in the small cavities to

4 –

4

Method and P & T

Storage capacity

3.8 wt% Raman, infrared, X-ray, and neutron diffraction studies 200 MPa, 234 K 2.5 MPa and 150 K – Quantum dynamics – at low temperature Raman spectroscopy – Molecular dynamic simulation

Numerical simulations/ Monte Carlo

–

Author/ Reference

Mao et al. [4]

Patchkovskii and Tse [5] Sebastianelli et al. [6] Wang et al. [7] Gorman et al. [8]

Papadimitriou et al. [9]

Brumby et al. –

accommodate more hydrogen molecules 2

Less than or equal to two H2

–

Mao and Mao Energy of 1.8 kWh/kg or 1.5 kWh/L

–

Leonardo del Rosso

Few cages, five Monte Carlo H2 molecules simulations 300 MPa, 225–250 K – Raman spectroscopy, 200 MPa, 263 K 5 77 K, and atmospheric pressure

–

Brumby et al. [10]

–

del Rosso et al. [11]

Mao and Mao 5.3 wt% of [12] hydrogen could be stored 200–300 MPa and 240–249 K Lokshin and 3.77 mass % of hydrogen Zhao [13]

Lokshin et al. Kinetics of hydrate can be – improved with the use of hexagonal ice powders Kumar et al. – –

–

77–273 K

–

Willow et al.

5–7

sII hydrogen hydrate – Lower pressure of 15–18 MPa Molecular dynamics 5.3 wt% simulations

The presence of larger molecules will enhance the stabilization

Kumar et al. [14] Willow and Xantheas [15]

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At present, as evident from the literature review, hydrogen hydrate cannot be implemented at a commercial scale. The literature regarding hydrogen occupancy has a discrepancy. Therefore, a concentrated effort and systematic research approach are needed to form hydrogen hydrates at lower pressures using novel additives.

3 The thermodynamic aspect of hydrogen clathrate The thermodynamics of hydrogen hydrate plays a crucial role in devising a strategy for reducing the formation condition of hydrogen hydrate. As the hydrate formation condition in the presence of hydrogen as a guest molecule is extreme, it is required to add promoters to bring down the formation condition to a moderate condition, thereby making the concept of hydrogen storage a real phenomenon. Many authors have tested many thermodynamic promoters to understand and collect the thermodynamic data fully. Other new hydrate promoters can also be tested using the same methodology. As can be seen in Fig. 2, four different hydrate systems are shown [20]. It can be seen that very high pressure and low-temperature conditions are required to form pure hydrogen hydrate. However, with specific additives

500 450 400

Pure hydrogen hydrate

Pressure (in MPa)

350 300 250 200 150 100

sH hydrogen hydrates

50 sII hydrogen hydrates

0 250

255

260

265

270

275

280

Semiclathrate hydrogen hydrates

285

290

295

300

305

310

Temperature (in K)

Fig. 2 Thermodynamic equilibrium condition of hydrate in different forms. (Adapted from Veluswamy HP, Kumar R, Linga P. Hydrogen storage in clathrate hydrates: current state of the art and future directions. Appl Energy 2014;122:112–32.)

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30

Pressure (MPa)

25 20 15 TBPB 3.0 mol% [134] TBPB 2.6 mol% [133] TMA 4.7 mol% [136] TMA 8.3 mol% [136] TBAC 3.23 mol% [134] TBAC 3.23 mol% [132] TBAN 3.0 mol% [135] TBAN 3.7 mol% [135] TBAF 1.8 mol% [129] TBAF 3.4 mol% [129]

10 5 0 275

280

285

290

295

300

305

Temperature (K)

Fig. 3 Thermodynamic equilibrium condition of hydrate in the presence of various promoters. (Adapted from Veluswamy HP, Kumar R, Linga P. Hydrogen storage in clathrate hydrates: current state of the art and future directions. Appl Energy 2014;122:112–32.)

like promoters, the pressure can be brought down to form hydrogen hydrate. Other additive classes include semiclathrate hydrate, which can lower the pressure to moderate conditions. Fig. 3 represents the thermodynamic conditions of various additive promoters which forms hydrate in the with hydrogen as guest gas [20]. The additives acts as promoters are TBPB (tetra n-butyl phosphonium bromide), TMA (trimethylamine), TBAC (tetrabutylammonium chloride), TBAN (tetrabutylammonium nitrate), and TBAF (tetrabutylammonium fluoride). TBAF forms hydrogen hydrate at a slightly moderate condition as compared to others. However, from hydrogen storage perspective, one must also look into the storage capacity in the presence of the additive by looking into the structure formed. The additive selected as a good promoter can also hamper the amount (in wt%) of hydrogen to be stored in the final hydrate structure formed.

4 Kinetic aspects of hydrogen clathrate hydrate The H2 charging time is a very important aspect of hydrogen storage (time for the hydrogen to form a stable structure). The kinetics of hydrate helps us understand the time to charge the hydrogen in the hydrate system.

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The hydrate formation rate helps us characterize the charging time of hydrogen to get it loaded in the clathrate structure. Kinetic studies of hydrogen are essential in determining the formation and dissociation of hydrogen hydrate, which determine the formation time and supply time of hydrogen for different applications. There are many challenges in the formation and dissociation rate of hydrogen. Inherently, the rate of hydrogen hydrate formation is low and can be boosted with the help of THF and other novel promoters additive. An excellent example is provided in the literature, which explains the importance of the rate of hydrogen hydrate process for commercial application. Davoodabadi and coworkers suggested the calculation, considering the storage capacity of 5 wt%, 50 g of hydrogen hydrate is required per day to meet the energy requirement of land transportation 0.02 kWh/km for a 50-km drive. Since the formation rate of hydrate is in the range 0.05 mol/h, the time to charge the battery would be as high as 54 h [21]. Therefore, kinetics plays an important role in commercializing hydrate based storage (hydrogen or any other hydrocarbon gas) technology. The challenge is to develop and research novel additives and technology to advance the kinetics of hydrogen hydrate.

5 Storing hydrogen in the presence of THF and promoters with a tuning effect Fig. 4 shows the structural placement of THF and guest gas hydrogen molecules. As can be seen, THF occupies the larger cage, and hydrogen gets the smaller cage to get itself accommodated. Of all the larger cages that are occupied, there would be little space left for more hydrogen to get placed. Fig. 5 shows the time evolution of THF as the hydrogen hydrate former [2]. With the increasing understanding, it is now clear that optimization has to be done to trade off the percentage of THF required for the hydrogen formation process and increase the storage capacity. Fig. 6 too explains the thermodynamic condition of hydrate of THF in the presence of hydrogen gas [20]. It is observed that, there exists anoptimum concentration of THF to be a good hydrogen hydrate former thermodynamically which is 5.6 mol%. In addition, a trade off has to be done to the amount of THF to be added to maximize the hydrogen storage in the clathrate structure. Table 2 shows the storage aspect of hydrogen in the presence of THF and other additives. It provides an insight into the work done in the presence of

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Fig. 4 Structural placement of THF and guest hydrogen molecule (red is hydrogen and blue is THF) (a) occurs at high pressure and (b) occurs at relatively moderate pressure and temperature conditions.

Fig. 5 Evolution of THF as the hydrogen hydrate former and tuning effect with corresponding pressure and temperature conditions. (Adapted from Gupta A, Baron GV, Perreault P, Lenaerts S, Ciocarlan RG, Cool P, et al. Hydrogen clathrates: next generation hydrogen storage materials. Energy Storage Mater 2021;41:69–107.)

THF as promoter. Also, pressure, and temperature conditions with the outcome and observation are presented. In sII pure hydrogen hydrate, the composition of (2H2)
(4H2)
17H2O exists, which translates to the 5 wt% [23]. The structure remains stable even if a small fraction of the large cage is occupied with THF [3,23].

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30

Pressure (MPa)

25

20 0.5 mol% THE [91] 1.0 mol% THE [91] 1.0 mol% THE [90] 2.4 mol% THE [90] 2.4 mol% THE [99] 2.7 mol% THE [91] 3.5 mol% THE [99] 5.3 mol% THE [104] 5.56 mol% THE [91] 5.6 mol% THE [78] 5.79 mol% THE [99] 5.9 mol% THE [91] 13 mol% THE [90]

15

10

5

0 274

276

278

280

282

284

286

Temperature (K)

Fig. 6 Thermodynamic phase equilibrium of hydrogen hydrate in the presence of THF showing an optimal concentration of THF is 5.6 mol%. (Adapted from Veluswamy HP, Kumar R, Linga P. Hydrogen storage in clathrate hydrates: current state of the art and future directions. Appl Energy 2014;122:112–32.)

Florusse et al. identified that small cages get filled by hydrogen molecules stably when the larger cages are occupied with a larger guest such as THF or any larger size guest. According to them, the conditions could be drastically reduced to near-ambient conditions of 5 MPa and 279.6 K by adding an additional “promoter” molecule such as tetrahydrofuran (THF) but at the cost of reducing the storage capacity to around 1.0 wt% [19]. It may be possible to leave some of the large hydrate cavities open for H2 occupancy but still, maintain reduced formation conditions. Tuning THF in the right amount will help in improving the storage of hydrogen. Above 2 mol% THF, larger cages are not available for hydrogen gas, and the storage capacity is limited to 2% by weight. As soon as the quantity of THF is optimized or tuned, hydrogen storage capacity can be taken up to 4%. However, if the concentration of THF is reduced below a threshold, the pressure has to increase to stabilize the structure. One thing to be noticed is that if the promoter is used (any promoter), they tend to shrink the storage capacity of the gas in the hydrate structure. Therefore, finding a suitable promoter and additive is challenging for hydrate storage applications in the form of hydrate at moderate formation conditions.

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Table 2 Storage aspect of hydrogen in semiclathrate hydrate. Additive and observation

Reference

THF and hexafluorophosphoric acid, 350–700 MPa Hydrogen placed at 91% of the small cages THF concentration (5.56 mol% to 0.1 mol%) at 12 MPa Showed a tuning effect THF concentration reduced (5.56 mol% to 0.5 mol%) at 13.8 MPa Storage capacity remains 1 wt% At 85 MPa and 277.15 K Hydrogen storage capacity of 1.05 wt% Ice in powder form and THF at 60 MPa and 255  2 K Storage capacity 3.77 wt% (observed) Hydrogen stored in a large cage depends on the concentration of THF Seeds of THF plus hydrogen in a nitrogen environment given rapid hydrogen hydrate formation. Hydrogen placed in a large cage of sII hydrate Hydrogen hydrate was formed in TBAB of 0.60 mol% and 4.0 mol% Stability of the hydrate improves with TBAB concentration Hydrogen/TBAB: Kinetics favorable at high pressure and high TBAB concentration Storage capacity 0.046 wt% (16 MPa, 281.15 K, 3.7 mol% TBAB) 0.031 wt% (16 MPa, 281.15 K, 2.6 mol% TBAB) Hydrogen/TBAB and hydrogen/TBAF hydrates at 13.0 MPa Small cavities are more in hydrogen/TBAB hydrates, but hydrogen storage was higher in hydrogen/TBAF hydrates

Udachin et al. [22] Lee et al. [23] Strobel et al. [24] Ogata et al. [25] Sugahara et al. [26] Grim et al. [27] Grim et al. [27]

Arjmandi et al. [28] Trueba et al. [29]

Trueba et al. [30]

6 Modeling of hydrogen clathrate hydrates Lunine and Stevenson developed a double occupancy model and allowed hydrogen and methane in the same cavity to predict the probability of hydrogen occupation in large and small cages for sII hydrate. They predicted that two hydrogen molecules occupy the same cavity with a 60% probability, whereas hydrogen and methane occupy a large cavity with a 20% probability [31]. Martin et al. developed a model for sH hydrates to predict the thermodynamics, storage capacity, and cage occupancy of hydrogen and additive molecules (such as MCH, MTBE, and DMCH). For sH, hydrogen storage capacity was predicted in the range of 0.85%–1.05% by weight [32]. Babaee et al. developed a thermodynamic model to study the performance of sH hydrate and additive molecules for predicting the storage of hydrogen,

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in the presence of alkanes, alkenes, alkynes, and cycloalkanes. They predicted storing 50%–57% of the volume by hydrogen [33]. A theoretical modeling-based study has been conducted to evaluate the thermodynamics of hydrogen hydrate using the solid solution theory of van der Waals and Platteeuw (vdW-P). The model was validated by constructing a phase diagram of pure ethane hydrate with cubic structure II. There are two types of hydrogen cubic structures, of which cubic structure two (CS II) was observed to be thermodynamically more stable than cubic structure one (CS 1). It was also verified that pure hydrogen hydrate with CS I structure could hold more hydrogen than CS II. CS I can hold 3.1 wt% of hydrogen and reach up to 4 wt% at a pressure of 400 MPa at a temperature of 250 K. It was found that when the pressure increased, the large cavities were able to hold one to three, even up to four molecules of hydrogen per cage. Ninety percent of small cavities were filled with single molecules of hydrogen. As the temperature increased, the amount of hydrogen molecules filled inside the cavities decreased. This new model was developed to better understand the guest gas behavior on host lattices [34]. Another model was created to understand hydrogen hydrate cage occupancy [35]. This prediction approach is based on the effective molecular size of hydrogen and its arrangement, as well as the stability of the hydrate phase. Initially, the hydrate phase stability was calculated with a modified version of the well-known van der Waals-Platteeuw (vdW-P) model. The hydrogen molecule was considered to be a spherical molecule to compute the effective volume per molecule of hydrogen. The hydrogen-to-hydrogen bond and its interactions in the hydrate cage were modeled using the Kihara potential and additivity rule. The accuracy of the data was compared with already available literature sources. The occupancy behavior of different cage structures according to the pressure for improving the storage efficiency of hydrogen molecules inside the hydrate structures was determined using the model. The storage efficiency of a small cage of the CS II structure improved (double occupancy) by increasing the volume of the cage at a considerably lower pressure. The model also confirms that the same was not true in the case of large cages [35]. A model was developed to predict hydrate formations containing hydrogen gas inside it. The fugacity computations in different phases were performed using the Patel-Teja equation of state, and the clathrate phase was computed using the Chen-Guo hydrate model. With tetrahydrofuran (THF) being a thermodynamic promoter by reducing the pressure to form a hydrate, the modeling was conducted in the presence of THF. The two-step

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99

hydrate formation mechanism and its complex quasichemical reaction were modeled using an expression proposed by Chen and Guo. This model was used to generate the hydrogen hydrate formation conditions, and the results were correlated with the results of eight different concentrations of hydrogen in methane and propane. The hydrogen hydrate behavior with respect to the hydrogen concentration was also studied using the model [36].

7 Conclusion and future direction High pressure and low temperature are the major hurdles in making hydrogen storage a realistic solution. Potential challenges and prospects for improving the capacity to store hydrogen and recommendation are discussed. As the hydrate formation condition in the presence of hydrogen as a guest molecule is extreme, it is required to add promoters to bring down the formation condition to a moderate condition, thereby making the concept of hydrogen storage a real phenomenon. There are other additive classes that form semiclathrate hydrate which can again bring down the pressure to moderate conditions. A trade-off is to be done to the amount of THF to maximize hydrogen storage. Therefore, finding a suitable promoter and additive is challenging for hydrate storage applications in the form of hydrate at moderate formation conditions. Kinetics of the hydrate formation has to play an important role in commercializing the technology of hydrate storage. The challenge is to develop and research the novel additives and technology to advance the kinetics of hydrogen hydrate. The fundamental work related to hydrate has grown over time with the effort of many researchers in the field. The structure can be known with much accuracy. The amount of gas entrapped can be calculated easily based on the structural information. The weight percentage of hydrogen may be improved by accommodating the promoter in small cages while hydrogen rests in larger cages. In addition, the binary hydrate concept could help in achieving the target. This knowledge will allow us to advance our understanding of clathrate hydrate dissociation metastability (anomalous selfpreservation), which could have exciting implications for energy storage and transportation at near-ambient conditions.

References [1] Zhao Y, Xu H, Daemen LL, Lokshin K, Tait KT, Mao WL, Luo J, Currier RP, Hickmott DD. High-pressure/low-temperature neutron scattering of gas inclusion compounds: progress and prospects. Proc Natl Acad Sci U S A 2007;104(14):5727–31.

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[2] Gupta A, Baron GV, Perreault P, Lenaerts S, Ciocarlan RG, Cool P, Mileo PGM, Rogge S, van Speybroeck V, Watson G, van der Voort P, Houlleberghs M, Breynaert E, Martens J, Denayer JFM. Hydrogen clathrates: next generation hydrogen storage materials. Energy Storage Mater 2021;41:69–107. [3] Sch€ uth F. Hydrogen and hydrates. Nature 2005;434(7034):712–3. [4] Mao WL, Mao HK, Goncharov AF, Struzhkin VV, Guo Q, Hu J, Hu J, Hemley RJ, Somayazulu M, Zhao Y. Hydrogen clusters in clathrate hydrate. Science 2002;297 (5590):2247–9. [5] Patchkovskii S, Tse JS. Thermodynamic stability of hydrogen clathrates. Proc Natl Acad Sci U S A 2003;100(25). [6] Sebastianelli F, Xu M, Bacic Z. Quantum dynamics of small H2 and D2 clusters in the large cage of structure II clathrate hydrate: energetics, occupancy, and vibrationally averaged cluster structures. J Chem Phys 2008;129(24), 244706. [7] Wang J, Lu H, Ripmeester JA. Raman spectroscopy and cage occupancy of hydrogen clathrate hydrate from first-principle calculations. J Am Chem Soc 2009;131 (40):14132–3. [8] Gorman PD, English NJ, MacElroy JMD. Dynamical cage behaviour and hydrogen migration in hydrogen and hydrogen-tetrahydrofuran clathrate hydrates. J Chem Phys 2012;136(4), 044506. [9] Papadimitriou NI, Tsimpanogiannis IN, Economou IG, Stubos AK. The effect of lattice constant on the storage capacity of hydrogen hydrates: a Monte Carlo study. Mol Phys 2016;114(18):2664–71. [10] Brumby PE, Yuhara D, Hasegawa T, Wu DT, Sum AK, Yasuoka K. Cage occupancies, lattice constants, and guest chemical potentials for structure II hydrogen clathrate hydrate from Gibbs ensemble Monte Carlo simulations. J Chem Phys 2019;150(13), 134503. [11] del Rosso L, Celli M, Ulivi L. Raman measurements of pure hydrogen clathrate formation from a supercooled hydrogen-water solution. J Phys Chem Lett 2015;6 (21):4309–13. [12] Mao WL, Mao HK. Hydrogen storage in molecular compounds. Proc Natl Acad Sci 2004;101(3):708–10. [13] Lokshin KA, Zhao Y. Fast synthesis method and phase diagram of hydrogen clathrate hydrate. Appl Phys Lett 2006;88(13), 131909. [14] Kumar R, Klug DD, Ratcliffe CI, Tulk CA, Ripmeester JA. Low-pressure synthesis and characterization of hydrogen-filled Ice Ic. Angew Chem Int Ed 2013;52 (5):1531–4. [15] Willow SY, Xantheas SS. Enhancement of hydrogen storage capacity in hydrate lattices. Chem Phys Lett 2012;525–526:13–8. [16] Qian GR, Lyakhov AO, Zhu Q, Oganov AR, Dong X. Novel hydrogen hydrate structures under pressure. Sci Rep 2014;4(1):1–5. [17] Strobel TA, Koh CA, Sloan ED. Water cavities of sH clathrate hydrate stabilized by molecular hydrogen. J Phys Chem B 2008;112(7):1885–7. [18] Duarte ARC, Shariati A, Rovetto LJ, Peters CJ. Water cavities of sH clathrate hydrate stabilized by molecular hydrogen: phase equilibrium measurements. J Phys Chem B 2008;112(7):1888–9. [19] Florusse LJ, Peters CJ, Schoonman J, Hester KC, Koh CA, Dec SF, Marsh KN, Sloan ED. Stable low-pressure hydrogen clusters stored in a binary clathrate hydrate. Science (New York, NY) 2004;306(5695):469–71. [20] Veluswamy HP, Kumar R, Linga P. Hydrogen storage in clathrate hydrates: current state of the art and future directions. Appl Energy 2014;122:112–32. [21] Davoodabadi A, Mahmoudi A, Ghasemi H. The potential of hydrogen hydrate as a future hydrogen storage medium. IScience 2021;24(1), 101907.

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[22] Udachin KA, Ltpkowski J, Tkacz M. Double clathrate hydrates with helium and Hydrogen. Supramol Chem 2006;3(3):181–3. [23] Lee H, Lee JW, Kim DY, Park J, Seo YT, Zeng H, Moudrakovskr IL, Ratcliffe CI, Ripmeester JA. Tuning clathrate hydrates for hydrogen storage. Nature 2005;434 (7034):743–6. [24] Strobel TA, Taylor CJ, Hester KC, Dec SF, Koh CA, Miller KT, Sloan ED. Molecular hydrogen storage in binary THF-H2 clathrate hydrates. J Phys Chem B 2006;110 (34):17121–5. [25] Ogata K, Hashimoto S, Sugahara T, Moritoki M, Sato H, Ohgaki K. Storage capacity of hydrogen in tetrahydrofuran hydrate. Chem Eng Sci 2008;63(23):5714–8. [26] Sugahara T, Haag JC, Prasad PSR, Warntjes AA, Sloan ED, Sum AK, Koh CA. Increasing hydrogen storage capacity using tetrahydrofuran. J Am Chem Soc 2009;131 (41):14616–7. [27] Grim RG, Kerkar PB, Sloan ED, Koh CA, Sum AK. Rapid hydrogen hydrate growth from non-stoichiometric tuning mixtures during liquid nitrogen quenching. J Chem Phys 2012;136(23), 234504. [28] Arjmandi M, Chapoy A, Tohidi B. Equilibrium data of hydrogen, methane, nitrogen, carbon dioxide, and natural gas in semi-clathrate hydrates of tetrabutyl ammonium bromide. J Chem Eng Data 2007;52(6):2153–8. [29] Trueba AT, Radovic IR, Zevenbergen JF, Kroon MC, Peters CJ. Kinetics measurements and in situ Raman spectroscopy of formation of hydrogen–tetrabutylammonium bromide semi-hydrates. Int J Hydrog Energy 2012;37(7):5790–7. [30] Trueba AT, Radovic IR, Zevenbergen JF, Peters CJ, Kroon MC. Kinetic measurements and in situ Raman spectroscopy study of the formation of TBAF semi-hydrates with Hydrogen and carbon dioxide. Int J Hydrog Energy 2013;38(18):7326–34. [31] Lunine JI, Stevenson DJ. Clathrate and ammonia hydrates at high pressure: application to the origin of methane on Titan. Icarus 1987;70(1):61–77. [32] Martı´n A´, Peters CJ. Hydrogen storage in sH clathrate hydrates: thermodynamic model. J Phys Chem B 2009;113(21):7558–63. [33] Babaee S, Hashemi H, Javanmardi J, Eslamimanesh A, Mohammadi AH. Thermodynamic model for prediction of phase equilibria of clathrate hydrates of hydrogen with different alkanes, alkenes, alkynes, cycloalkanes or cycloalkene. Fluid Phase Equilib 2012;336:71–8. [34] Belosludov RV, Zhdanov RK, Subbotin OS, Mizuseki H, Souissi M, Kawazoe Y, Belosludov VR. Theoretical modelling of the phase diagrams of clathrate hydrates for hydrogen storage applications. Mol Simul 2012;38(10):773–80. [35] Rasoolzadeh A, Shariati A. Hydrogen hydrate cage occupancy: a key parameter for hydrogen storage and transport. Fluid Phase Equilib 2019;494:8–20. [36] Ma QL, Chen GJ, Sun CY, Yang LY, Liu B. Predictions of hydrate formation for systems containing Hydrogen. Fluid Phase Equilib 2013;358:290–5.

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CHAPTER 6

Advanced carbon-based nanomaterials for photoelectrochemical water splitting Lokesh Sankhula, Sneha Lavate, and Rohit Srivastava

Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

1 Introduction The global energy scarcity and environmental pollution has been drastically increasing every year; however, many research groups across the globe have been focused on development of an alternative fuel to replace nonrenewable fossil fuels and to address the energy and environmental crisis. The industrialization and increase in population has increased the demand for clean resources for sustainable livelihood. There is an urgent need to develop eco-friendly, sustainable, and technologically potential techniques for adapting cleaner energy and to capture the toxic gases from atmosphere and generate useful chemicals. Hydrogen is the most abundant element in nature in the form of water and it has been considered as a potential fuel due to its higher heating value of 141.8 MJ/kg and it produces water and heat during combustion, which is also an additional benefit from environmental perspective. Hydrogen can be produced from natural gas (CH4) and coal by using steam reforming, partial oxidation, and coal gasification; however, the hydrogen produced from these processes can also generate toxic gases such as CO2, CO, SOx based on the feed material utilized. Hydrogen can also be produced by using water electrolysis process, where water molecules are split into hydrogen and oxygen molecules by utilization of electrical energy. The electrical energy required for electrolysis should be utilized from renewable resources like solar, wind, geothermal; thus, hydrogen produced by using this process can have high purity compared to other processes. The hydrogen production by using renewable resources and saline water is the research area on which many scientists have been Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00006-6

Copyright © 2023 Elsevier Inc. All rights reserved.

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focused for development of sustainable and cleaner fuel technologies. The hydrogen can be produced from water electrolysis by photoelectrochemical process which is one of the effective techniques for conversion of solar energy to chemical energy. The solar to chemical conversion can be achieved by three different techniques such as thermochemical water splitting, photobiological water electrolysis, and photocatalytic water splitting. Solar energy and water are one of the inexhaustible, cleanable, and sustainable resources on the earth. The splitting of water molecule was demonstrated by Fujishima and Honda using TiO2 anode and Pt cathode under UV light irradiation in photoelectrochemical cell to produce hydrogen and oxygen. There have been many research activities performed on semiconductor-based photocatalysts for efficient water electrolysis process; however, heterogeneous catalysts were being developed in recent years. There are some semiconductor-based catalysts such as TiO2, ZnO, CdS, Bi2O3, WO3, BiVO4, SnO2, Fe2O3 were developed and investigations were done for hydrogen evolution reaction; however, the oxidative and reduction reactions take place in concurrent manner. The photocatalytic electrolysis of water mechanism involves four steps, namely (1) generation of electron-hole pairs due to irradiation of light on photo-anode, (2) oxidation of water by photogenerated holes on the surface of photo-anode to produce O2 and H+, (3) photogenerated electrons were transferred to cathode by external circuit, and (4) reduction process of H+ by photogenerated electrons at cathode surface to produce H2. Water electrolysis process occurs only when the practical potential is higher than the minimum potential value to minimize system loss and overcome overpotential. The hydrogen production using photocatalytic water electrolysis can be classified into two types such as photochemical cell reactions and photoelectrochemical cell reactions (Fig. 1). Photochemical reactions were performed by introducing light energy to perform chemical reactions and produce hydrogen and oxygen. Liao et al. and Jiang et al. demonstrate the photochemical reactions by suspending photocatalysts in electrolyte solution; however, Jiang et al. explained some of the factors which are unable to control the photochemical reactions such as absorption of light by suspended particles, pH change during the process, and determining the instantaneous substrate concentration. The abovementioned factors affect the kinetic behavior of the reactions and also stated that kinetics of suspended particles is time-consuming. Photoelectrochemical reactions were studied as a modification to photochemical reactions where an electrical circuit is accompanied to supply electrical energy to

Advanced carbon-based nanomaterials for photoelectrochemical

Potential V vs NHE PH = 0

105

H+

Re

e– e– e– e– e– e– e–

H+/H2 0

hv ≥ Eg

Eg Exciton binding energy

Photoexcitation

tion

duc

CB

hv

H2

hv

1.23 O2/H2O

H 2O n

io dat

i

Ox

h+ h+ h+ h+ h+ h+ h+ VB

O2 Photo catalyst

Fig. 1 Mechanism of photocatalytic water splitting. (Reproduced with permission from Lokesh S, Srivastava, R. Advanced two-dimensional materials for green hydrogen generation: strategies toward corrosion resistance seawater electrolysis – review and future perspectives. Energy Fuels 2022;36(22):13417–13450. https://doi.org/10.1021/acs. energyfuels.2c02013.)

electrolytic cell. Liao et al. performed experiments in photoelectrochemical cell by depositing a thin film of photocatalyst on substrate to form photoanode for water electrolysis process; however, an external circuit is required for directing the photogenerated electrons to cathode for hydrogen production. The photocatalysts utilized in photocatalytic water splitting involve absorption of light energy to excite electrons from valence band (VB) to conduction band (CB) by leaving behind holes in valence band. The emergence of carbon-based nanomaterials, such as graphene, graphitic carbon nitride, carbon nanotubes, fullerene, has been intensively investigated due to their higher charge carrier mobility, low band gap, high efficiency, highly abundant, and stable. The carbon-based nanomaterials and semiconductors have gained major attention of researchers; however, Eder and Prato have performed intensive investigation on nanocarbonsemiconductor interface engineering for energy generation and environmental applications [1–4] and explained that photocatalytic reaction initiates only when the incident light has energy more than or equal to the band gap of photocatalyst [5]. The migration of electrons takes place from VB to CB by forming electron-hole pair and hole present in VB helps in oxidizing water of OH
at the surface to produce hydroxyl radical (OH⁎); however, it acts as powerful oxidizing agent for degrading organic pollutants [6]. The electrons present in CB can reduce H+ ions to H2 or can form superoxide

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radical anion (O
⁎ 2 ) by reacting with dissolved oxygen, hyperoxide radical ⁎ ( OOH) by reacting with H+ ions [5]. The radical species has potential to react with pollutants and mineralize pollutants into water and CO2 [6]. The major drawback of the process is lack of stability for photogenerated species, recombination capacity, thus loses absorbed energy in the form of heat, and leads to photocatalytic efficiency [7]. Therefore, heteroatom doping [8], noble metal doping [9], coupling with semiconductors [10], and nanocomposite formation with carbon-based materials such as graphene and g-C3N4 [11,12] have been adopted by researchers to enhance efficiency of photocatalysts. Hence, carbon-based materials with various morphologies have made substantial contribution for applications in materials chemistry.

2 Performance evaluation of electrocatalysts Electrochemical water splitting has to overcome the energy barrier resulting from high activation energy to form reaction intermediates and drives the electron transfer process at an optimum rate. Electrocatalysts play a prominent role in lowering the kinetic barrier but do not lead to any change in thermodynamic equilibrium. The performance of electrocatalysts can be determined on basis of important properties such as activity, stability, and efficiency [13–15].

2.1 Activity According to the Nernst equation, the thermodynamic potential required for electrolyzing water at a temperature of 25°C, pH ¼ 0, and 1 atm pressure can be calculated as 1.229 V, but due to the energy barrier, the actual potential is greater than the thermodynamic equilibrium potential. The excess potential required to initiate the electron transfer process is overpotential, which defines the catalyst activity [16]. The large overpotential is mainly due to the slow oxidation and reduction reaction kinetics at respective electrodes [17,18]. The following Eq. (1) gives the equilibrium potential for electrochemical water splitting process, where Eeq is potential under equilibrium conditions, T is the temperature in kelvin, F is Faraday constant, n is the number of moles of electrons, and [Ox] and [Red] are the concentrations of oxidation and reduction species: Eeq ðNHEÞ ¼ RT=nF log ½Ox=½Red

(1)

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107

As the electrode potential changes with pH, reversible hydrogen electrodes have been widely used as reference electrodes at 25°C as given by Eq. (2): Eeq ðRHEÞ ¼ Eeq ðNHEÞ + 0:059 pH ¼ 1:23 V

(2)

The overpotential value at a current density of 10 mA/cm2 should be small to be considered an effective electrocatalyst. Tafel slope and exchange current density are another two key properties to assess the activity of electrocatalyst from overpotential vs current kinetics represented by an equation η ¼ a+b log j, here η refers to overpotential and j is the current density. The linear correlation in the Tafel plot determines two important kinetic parameters that are Tafel slope b and exchange current density (jo) at zero overpotential. Tafel slope defines the electrochemical behavior between the catalyst and the reactants, where a lower slope implies good electrocatalytic kinetics corresponds to the fact that a small increase in overpotential leads to a large increase in the current density.

2.2 Stability In industrial applications, the stability of electrocatalysts is always a key parameter to evaluate the potential for their usage in long-term water electrolysis experiments. There are two different characterization techniques available for analyzing the stability of electrocatalysts namely chronoamperometry and chronopotentiometry. Both these experiments analyze the charge transfer process with time at constant potential or vice versa. The longer period for which potential or current is consistent, the better the stability of the catalyst. Another characterization technique is cyclic voltammetry, which calculates the amount of charge mobility by introducing several potential cycles, and usually requires more than 5000 cycles at a particular scan rate. Linear sweep voltammetry is also used to determine the overpotential deviation after performing cyclic voltammetry analysis, smaller the overpotential shift implies better electrocatalysts’ stability.

2.3 Efficiency The quantitative parameter that defines the efficiency of electron mobility for the electrochemical reaction is faradaic efficiency. This is the ratio of experimental hydrogen (oxygen) production to theoretical hydrogen (oxygen) production. Theoretical production rate can be obtained by the integral of chronoamperometric or chronopotentiometry analysis, whereas the

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experimental rate is obtained by downward displacement of water technique or gas chromatography method. Another useful parameter for describing the rate of reaction in terms of reaction active sites is the turnover frequency (TOF). The general number of reactant molecules that can be converted to desired products per active site of catalyst per unit of time is determined to find the efficiency of the catalyst.

3 Different carbon materials 3.1 Graphene The graphene has been considered as an important material for photocatalytic hydrogen generation due to their electrical, thermal, optical, and mechanical properties. The graphene mainly consists of densely packed sp2 hybridized carbon atoms to form an atomically thin layer of 2D hexagonal honeycomb-like structure [19]. The graphene-based carbon material possess ultrafast electron transfer (200,000 cm2 V
1 s
1), high specific surface area (2600 m2/s), and high thermal conductivity (5000 W/m K) due to pi-conjugated structure of graphene material [20]. The higher transparency, good elastic modulus, higher mechanical strength, and optical transmittance are additional advantages for graphene-based carbon material; however, these properties make it as potential material in different applications such as optical electronics, photosensors, and photocatalysis [21–24]. The advantage of zero band gap for graphene materials helps in oxidation reactions, so it can be combined with other semiconductors and metallic nanostructures to form composite materials for photocatalytic applications; however, it can also act as an excellent electron acceptor and transporter in nanocomposites. The deposition of pollutants on graphene surface is an additional advantage that accelerates the photocatalytic degradation of adsorbed pollutants [20]. There are several chemical and physical techniques for synthesis of graphene and its nanocomposites such as mechanical exfoliation, arc discharge method, oxidative exfoliation reduction, liquid phase exfoliation, chemical vapor deposition, epitaxial growth, unzipping of CNTs, substrate-free gas phase, total organic synthesis, and template route as shown in Table 1. The Hummers’ method of synthesis includes chemical oxidation of graphite flakes to form graphene oxide (GO). Graphene oxide contains carboxyl, epoxides, and hydroxyl groups which are covalently bonded to graphene sheet, which leads to electrical conductivity losses and thus limits their application. The presence of polar functional groups in GO is responsible for

Table 1 Advantages and limitations of graphene synthesis methods [25]. Sr. no.

Synthesis methods

1.

Mechanical exfoliation Arc discharge method Oxidative exfoliation reduction Liquid phase exfoliation

2. 3.

4.

5. 6.

7. 8.

9.

10.

Advantages

Limitations

Uniform and nanosize graphene thickness fragmentation

More reaction time consumption, high energy consumption for residue elimination Energy-intensive nature and high precision in process control Release of toxic gases (NO2), usage of expensive and toxic chemicals like hydrazine during reduction, less surface area, and poor electronic conductibility Produces defects on its edges and basal planes during sonication process, optimization of parameters to reduce defects is still lacking in the literature Low yield, usage of expensive precursors and chemicals

Low-cost graphene, fine graphene layers [2–10] synthesis Low synthesis temperature, high yield of GO

High concentration of graphene products, single-layer nanosheets can be synthesized

Unzipping of CNTs Chemical vapor deposition

The controlled thickness of graphene based on nanotubes

Epitaxial growth Substratefree gas phase Template route

Simple operation and production of high-quality graphene ( 420 nm) in Na2S/Na2SO3 aqueous solution. These experiments indicated that incorporation of organic ligands is necessary for formation of specific structures on carbon surface for increasing hydrogen production. Sordello et al. [82] demonstrated that COOH and NH2 functionalized graphene compounds help in shape controlling for TiO2 during hydrothermal synthesis of graphene-TiO2 photocatalysts. Fang et al. [83] synthesized thiolated graphene sheets by modifying graphene oxide with 1-cysteine and followed by reduction process using hydrazine solution. This photocatalyst exhibits a hydrogen production of 2.15 mmol/h in lactic acid solution in visible light spectrum (λ > 420 nm). There are other surface functionalizations of graphene oxide such as carboxylic acid functionalization with chloroacetic acid [80], and functionalization of triphenyl amine with diphenyl amino benzaldehyde [81] for better dispersion of photocatalysts.

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4.2 Doping The effective technique for improving electrical conductivity, charge carrier ability, electron mobility tuning, and electronic structure is to dope heteroatoms in carbon materials. Kwon et al. [85] examined the importance of metal cations such as Na+, Ca2+, Mg2+, and K+ in alkali metal chlorinedoped graphene and found that cations with low work functions lead to n-type doping by forming interfacial dipole complexes with oxidized functional groups on graphene, thus decreasing the conductivity and graphene work function. Teng et al. [86] have developed nitrogen-doped graphene oxide quantum dots by reacting graphene with ammonia at 500°C followed by oxidation through modified Hummers’ process. The electrochemical Mott-Schottky analysis revealed that N2-doped graphene oxide quantum dots show both p- and n- type conductivities that result in internal Z-scheme charge transfer. The photocatalytic splitting experiments of pure water by using N2-doped graphene oxide quantum dots under visible light (λ > 420 nm) revealed overall water splitting in the hydrogen and oxygen ratio of 2:1. The p-type conductivity of oxygen functionalized graphene oxide enhances hydrogen evolution reaction (HER), and n-type conductivity of N2-doped graphene oxide improves oxygen evolution reaction (OER). They have also synthesized surface intact N2-doped GO quantum dots by ultrasonic exfoliation of ammonia-treated GO nanosheets for enhancing hydrogen production in triethanolamine solution. LatorreSanchez et al. [87] demonstrated synthesis procedure of P-doped graphene by using thermal decomposition of H2PO4-modified alginate at 900°C under Ar atmosphere. This photocatalyst shows conversion of zero band gap graphene to semiconducting graphene (till 2.85 eV) and exhibits photocatalytic activity under UV and visible light spectrum. This exhibits hydrogen production rate of 282 μmol g
1 h
1 under UV light irradiation with triethanolamine solution and Pt as cocatalyst.

4.3 Interface engineering There are generally two types of interfacial effects for carbon-based nanomaterials such as Schottky junction formed between metallic carbon materials and semiconductor photocatalysts, whereas semiconductor heterojunctions such as p, n-junctions and Z-scheme heterojunction could form between semiconducting carbon materials and semiconducting photocatalysts [88,89]. The interfacial area between carbon materials and coupled semiconductors is mainly responsible for photoinduced charge transfer.

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The designing of photocatalysts with high interfacial contact area between carbon nanoparticles and semiconductor is important for charge separation efficiency. The theoretical study of density function theory (DFT) calculations [90] suggests that graphene acts as sensitizer for SrTiO3 [70] with a termination layer of TiO, where it serves as an electron shuttle for SrTiO3 [70] with a termination layer of SrO, while when reduced graphene oxide was coupled, a type II hetero junction could form at the interface, with negatively charged O atoms in the RGO as the active sites. As a result, an enhanced photocatalytic performance could be achieved by tuning the interface for efficient charge separation and optimal active sites. Cherevan et al. [1] synthesized CNT-Ta2O5 hybrid photocatalysts by in situ growth of Ta2O5 on multiwalled CNTs by hydrothermal method with Ta (OEt)5 as precursor solution. They have prepared two hybrid photocatalysts by varying amounts of CNT and Ta2O5 in the ratio of 1:4 (thick sample H1) and 1:2 (thin sample H2); however, these variations lead to different interfacial effects between CNT and Ta2O5. The hybrid H2 is more crystalline in nature than H1 hybrid and thus possesses few grain boundaries for enhanced interface charge transfer. The thin growth of Ta2O5 on CNTs results in the formation of tight Schottky junction in H2 hybrid that promotes better charge separation at interface, whereas the charge transfer in H1 hybrid should move a long distance due to tunneling effect; thus, it was less effective than H2. Besides the charge transfer, the tight and thin growth of Ta2O5 in H2 hybrid causes a large band gap with higher reduction potential of CB. The hydrogen production measurements of H1 hybrid and H2 hybrid are 520 and 1600 μmol/h under the illumination of UV light with Pt as cocatalysts and methanol as sacrificing agent.

5 Conclusions and future outlook The carbon-based nanoparticles are one of the most promising materials for photocatalytic water splitting applications and also degrading of organic pollutants. In this book chapter, we have discussed about importance of hydrogen as an alternate fuel to replace fossil fuels and utilization of solar energy for conversion of water molecules into hydrogen and oxygen fuels. The mechanism of photocatalysis for effective conversion of solar energy into chemical energy by incorporation of photocatalysts is also investigated in this chapter. The carbon-based materials, such as graphene/reduced graphene oxide, carbon quantum dots, fullerene, CNT, graphitic carbon nitride, are mainly used in photocatalytic applications and also used to synthesize semiconductor-based

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nanocomposites. The performance of photocatalyst can be analyzed by using different techniques such as voltammetry and stability in different mediums can be observed by using chronoamperometry and chronopotentiometry; however, the activity of photocatalyst can be enhanced by doping and surface functionalization, and interfacial engineering techniques have been discussed briefly. The carbon-based materials possess mechanical and chemical stability, better photocatalytic activity, and low cost; thus, these materials can be used for photocatalytic water electrolysis applications and also can be used for seawater splitting due to its higher stability in harsh environments. This book chapter helps many researchers for analyzing and resolving the issues regarding photocatalytic applications.

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CHAPTER 7

MXene-transition metal compound sulfide and phosphide hetero-nanostructures for photoelectrochemical water splitting Ranjit Mohilia, N.R. Hemanthb, Kwangyeol Leec, and Nitin K. Chaudharia a

Department of Chemistry, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India b Department of Materials Science and Engineering, University of Washington, Seattle, WA, United States c Department of Chemistry and Research Institute for Natural Sciences, Korea University, Seoul, Republic of Korea

1 Introduction Solar energy is a plentiful and decentralized natural resource that can outpace global human energy consumption using thermal, photovoltaic, and photocatalytic conversion technologies. Photoelectrochemical (PEC) water splitting enables the coupling of solar energy with overall water splitting to produce renewable hydrogen (2H2O!O2+2H2) from water on a grand scale [1]. The reaction is thermodynamically uphill at 298 Κ and 1 bar requiring 286 kJ/mol input of Gibbs free energy. The energy required to drive the reaction comes from the sunlight. Semiconducting materials have been widely researched for PEC water splitting since the discovery of photoassisted water splitting by n-type TiO2 single-crystal electrode under ultraviolet by bandgap excitation [2]. Band-gap excitation generates electrons and holes inside the semiconductor photocatalyst and these photoexcited carriers are transported to the surface-active sites, where they undergo surface redox reactions to form H2 and O2 (Fig. 1). Despite tremendous advances in the class of semiconductor-based materials, conversion efficiency such as solar-to-hydrogen (STH) still fails to meet the demands of modern applications.

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Fig. 1 Schematic representation of (A) PEC water splitting process (B) PEC water splitting mechanism in full cell system with Pt cathode, semiconductor photoanode. CB-conduction band; VB-valence band; Ef-fermi level; Eg-bandgap. ((A, B) Reproduced with permission from Zheng G, Wang J, Liu H, Murugadoss V, Zu G, Che H, Lai C, Li H, Ding T, Gao Q, Guo Z. Tungsten oxide nanostructures and nanocomposites for photoelectrochemical water splitting. Nanoscale 2019;11:18968–94. https://doi.org/10. 1039/C9NR03474A. Copyright 2019, Royal Society of Chemistry.)

Currently, noble metals such as Pt-, Ru-, and Ir-based compounds are associated with high intrinsic electrocatalytic activity making them suitable as electrocatalysts for hydrogen production [3]. However, the scarcity of noble metals associated with their high costs hinders the economical and large-scale hydrogen production through water splitting. On the other hand, various materials consisting of transition metal chalcogenides, phosphides, oxides, sulfides, and nitrides are explored as alternatives to noble metals [1,4]. Among them, transition metal phosphides (TMP) and sulfides (TMS) are widely investigated as promising nonnoble metal catalysts [5,6]. Photocatalysts loaded with TMPs are recognized for their ability to increase light absorption, provide large surface-active sites, lower the overpotential, promote rapid charge transfer, and enhance the photostability [5] whereas TMSs prove themselves as robust hydrogen-evolution photocatalysts owing to the plenteously exposed active sites for H adsorption and narrow bandgap [6,7]. Therefore, when both TMPs and TMSs are combined with suitable substrates, they could fully exploit their potential as efficient photocatalysts. Transition metal carbides, nitrides, and carbonitrides (MXenes) belong to a rapidly growing class of 2D materials with an empirical formula Mn+1XnTx (where, n ¼ 1, 2, or 3; Early transition metals M ¼ Sc, Ti, V, Cr, Zr, Nb, Mo, Hf, and Ta; X ¼ C and/or N; Surface terminations T ¼ -OH, O2, and F) and have aroused considerable interest as potential materials in energy storage and conversion applications [8,9]. MXenes are typically synthesized by selective etching of A (A ¼ Al, Zn, Ga, and Si) element from MAX (where M ¼ early transition metals listed above,

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A ¼ mostly group 13 or 14 elements, and X ¼ C and/or N) precursor materials using solutions like HF. The resulting surface terminations and metallic phases result in the hydrophilicity and electrical conductivity of MXenes. 2D MXenes have received great interest as viable photocatalysts or co-catalysts with their large surface area, excellent electrical conductivity (10,000 S cm1), hydrophilic nature, tunable bandwidth and structure, and sufficient catalytic sites, especially active nonedge atoms, which promote water-splitting reaction [2,10]. However, it is critical to develop low-cost MXene-based photocatalysts to achieve desired practical requirements such as high selectivity, enhanced light absorption, efficiency, stability, and earthabundant metal, and increased overall performance. Therefore, the construction of MXene-based heterostructures with TMPs and TMSs can help modify the electronic structure, which allows for optimal binding of redox sites while also suppressing photo-induced charge recombination and facilitating the surface reaction.

2 Synthetic routes to MXene-based hetero-nanostructures In general, the heterostructures of MXenes (Ti3C2) are prepared by etching of MAX with HF or in situ formed HF, followed by the deposition or growth of hetero-nanostructures consisting of phosphides or sulfides using suitable phosphorization or sulfurization process, respectively. The synthesis of binary heterostructures composed of MXenes and transition metal phosphides or sulfides has been achieved by centrifugation of transition metal salts/precursors with phosphides or sulfides of the main block elements in a sealed environment using suitable solvents. For example, the synthesis of CdS nanowires NWs)-Ti3C2Tx MXene by single-step electrostatic self-assembly was reported by Li et al., where the negatively charged Ti3C2Tx suspension was stirred with positively charged CdS NWs dispersed in deionized (DI) water by sonication [11]. Similarly, Chen et al. synthesized MoS2-CdS-MXene incorporating a narrow bandgap (2.4 eV) of CdS to explore the effect of MoS2 on the H2 evolution performance of CdS [12]. MoS2, on the other hand, is a viable electrocatalyst for H2 evolution and can be substituted for rare earth metal co-catalysts because of its availability and low price [13]. The proportion of MoS2 edges and the density of molybdenum vacancies are decisive variables in improving their catalytic activity. Due to their good electron mobility properties and promising co-catalyst behavior, MoS2-MXene hybrid composites have been also studied [13].

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Li et al. prepared Ti3C2@TiO2@MoS2 dual co-catalyst composites with an unusual 2D-2D-2D structure (Fig. 2A) [14]. The TiO2 nanosheets (NSs) were first grown on Ti3C2 MXene by an in situ method, followed by deposition of MoS2 NSs on the (101) and (001) highly active uncovered facets of TiO2 NSs using two-step hydrothermal process similar to the earlier reports [13,14]. Ti3C2Tx/CdS hybrids containing oxygen groups and free of noble metals are prepared using a mild solvothermal approach involving in-situ heterogeneous nucleation followed by plasma treatment [15]. The profuse oxygen-containing surface terminations and plasma treatment on the Ti3C2Tx surface are supposed to provide hydrophilicity. Ti3AlC2 and HF mixture was stirred, washed, dried, and stored in the DMSO solvent to obtain delaminated MXene. The conversion of Ti3C2 to Ti3C2Tx was

Fig. 2 Schematic representation of (A) synthesis of MoSx@TiO2@Ti3C2 and MoS2@TiO2@Ti3C2 composites by a hydrothermal approach. (B) Synthesis of Co2P@Ti3C2Tx composite by electrophoresis and phosphorization treatment. ((A) Reproduced with permission from Li Y, Ding L, Liang Z, Xue Y, Cui H, Tian J. Synergetic effect of defects rich MoS2 and Ti3C2 MXene as co-catalysts for enhanced photocatalytic H2 production activity of TiO2. Chem Eng J 2020;383:123178. doi:10.1016/j.cej.2019.123178. Copyright 2020, Elsevier. (B) Reproduced with permission from Lv Z, Ma W, Dang J, Wang M, Jian K, Liu D, Huang D. Induction of Co2P growth on a MXene (Ti3C2Tx)-modified self-supporting electrode for efficient overall water splitting. J Phys Chem Lett 2021;12:4841–48. https://doi.org/10.1021/acs.jpclett.1c01345. Copyright 2021, American Chemical Society.)

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achieved via the radiofrequency plasma technique followed by solvothermal preparation of CdS/Ti3C2Tx. A metalorganic framework (MOF) enwrapped with nanoporous carbons, carbides, sulfides, and phosphides may generate a photo electrocatalyst with highly accessible surface areas as suggested by Zhao et al. [16]. They prepared a ternary TiO2-Ti3C2CoSx heterostructure with controlled size by a simple solvothermal mixing process using ZIF-67 template and Ti3C2 as a starting material. A recent study by Yan et al. developed to grow a TMP nanosheet array on MXene nanosheets by topotactic evolution of transition metal-layered double hydroxide to produce a CoP/Ti3C2 heterostructure [17]. Likewise, Liu et al. fabricated CoP/Mo2CTx from Co(OH)F/Mo2CTx and NaH2PO2 through a hydrothermal and calcination process [18]. The electrophoretic deposition process has also been utilized to obtain TMP/ MXenes (Fig. 2B) [19]. Co2P@Ti3C2Tx electrocatalyst was reported by the electrophoretic deposition process in the Ti3C2Tx solution by using nickel foam and Pt plate as anode and cathode, respectively [20].

3 Photoelectrochemical water-splitting application Recent works reported on MXene as a co-catalyst for photoelectrochemical water splitting showed the appealing properties of MXene class of 2D materials in photo-electrocatalytic H2 gas evolution. The excellent electrical conductivity of MXene could be maintained even after functionalization with –O, –OH, and –COO-; thus, when coupled with other semiconducting and porous nanostructures, MXenes can exhibit a promising photoelectrocatalytic behavior. To formulate a viable photoelectrocatalyst, the semiconducting materials should have characteristics such as cost effectiveness, long-term photocatalytic stability, wide-spectrum absorption, transport efficiency, powerful oxidation, active surface sites, and hydrophilic functionalities. Transition metal compound sulfide- and phosphide-based hetero-nanostructures could exhibit a high separation efficiency, high porous morphology, as well as full exploitation of charge carriers, thereby positioning them as excellent photoresponsive materials. Recently, various TMD-MXene materials with desirable photoelectrocatalytic properties have been prepared to explore the synergy between the TMD hetero-nanostructures and MXene co-catalysts. For example, Zhao et al. prepared several samples of ternary TiO2-Ti3C2-CoSx composites (using various mass ratios of Ti3C2, TiO2, and 1%CoSx), exhibiting a remarkable improvement in photoelectrocatalytic performance [16].

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TiO2 has strong oxidation property and long-term photocatalytic stability, and introducing 1 at% ZIF-67-templated CoSx to the pristine TiO2 showed an improved optimal evolution rate. Furthermore, the addition of 0.5 wt% Ti3C2 on TiO2-Ti3C2 gave an optimal photoelectrocatalytic H2 production of 0.95 mol g1 h1 (5.8 times more than pure TiO2), owing to the high conductivity of Ti3C2. This work also demonstrated better dispersibility of TiO2 with ZIF-67-templated CoSx and lower relative photoluminescence (PL) intensity for TiO2- Ti3C2-CoSx. Similarly, Chen et al. obtained a CdS-MoS2-MXene catalyst with a remarkable H2 generation of 9679 μmol g1 h1 when exposed to visible light. This demonstrated a significant improvement by a factor of 251.3% over CdS-MoS2 and an apparent quantum efficiency (AQE) of nearly 27% at 420 nm [12]. Furthermore, this study has opened the prospects for designing efficient photoelectrocatalysts (ternary composites) using MXenes, which can promote charge carrier transport efficiency and optimize electrocatalytic activity in multicomponent catalyst systems. The challenging task that needs to be overcome is to study the bonding between these ternary components which eventually determines the stability of the photoelectrocatalysts in different electrolytic media. Two-dimensional Ti3C2 with abundant –O terminations has acceptable photocatalytic HER activity, but Ti3C2 NPs demonstrate limited H2 activity on exposure to visible light [21]. However, MXene NPs perform remarkably well as a co-catalyst on ZnS or ZnxCd1Xs as demonstrated by Ran et al. [21] CdS/Ti3C2 composite catalyst displayed a super-high photocatalytic H2 generation rate of 14.342 μmol g1 h1 and AQE of 40% at 420 nm along with increased short (t1), long (t2), and intensity-average (t) PL lifetimes [21]. These results depict that the addition of 2D Ti3C2 renders TMDs with efficient photoinduced interfacial charge diffusion under visible-light irradiation. Also, optimization of intimate coupling between Ti3C2 and CdS has been reported, which can be further extended to other promising TMD materials [21]. Moreover, p-type semiconductor NiS co-loaded CdS/NiS/Ti3C2 showed a surprisingly increased photocatalytic H2 evolution rate of 18,560 μmol g1 h1. Similarly, Xie et al. demonstrated Ti3C2Tx as a Janus co-catalyst that improves the photocatalytic performance of CdS by preventing Cd2+ leaching and charge carrier side reactions involving electron-hole recombination and the unwanted photocorrosion of CdS [22]. These unique 2D CdS/Ti3C2Tx sheet-onto-sheet heterostructures could be used for photoelectrocatalytic H2 evolution. Plasma treatment can enhance the number of active sites in the photocatalysts for

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photocatalytic H2 production [15]. The plasma treatment also creates a stable electron transfer channel, which inhibits photogenerated electron-hole pairs to recombine, without changing the morphology and compositions of the photoelectrocatalyst. Using plasma-treated Ti3C2Tx sheets, the Ti3C2Tx/ CdS hybrid was synthesized without a noble metal co-catalyst. The optimized Ti3C2Tx/CdS hybrid achieved an excellent H2 evolution of 825 μmol g1 h1 (Fig. 3B) and AQE of about 10% at 450 nm. Li et al. demonstrated 2D MXene/1D CdS NWs with excellent photoelectrocatalytic properties for integrated organic synthesis in which H+ is extracted from ethanol to produce H2 giving diethoxyethane an important by-product [11]. When compared to pristine CdS, the optimal photoactivity obtained was approximately 3.2 times higher (15.4 mmol g1 h1) and remained steady after four cycles, showing good reusability. Similar to the ternary composites, a unique structure of MoxS@TiO2@ Ti3C2 composite with Mo vacancies and double co-catalysts (Ti3C2 and MoS2) was studied by Li and the group. They found that the photocatalytic H2 generation was 193 and 6 times greater than pure TiO2 NSs and MoS2@TiO2@Ti3C2 composites, respectively [13]. The photocatalytic H2 production of MoS2@TiO2@Ti3C2 composites increased from 2128.3 μmol g1 h1 to 10505.8 μmol g1 h1.after introducing Mo vacancies. Moreover, Li et al. demonstrated Ti3C2 and MoS2 as efficient double co-catalysts on TiO2 NSs with rich highly active (001) surfaces (Fig. 3C) and obtained increased H2 production of 6425.297 μmol g1 h1 for Ti3C2@TiO2@MoS2 composites [14]. Such performance enhancement can be related to the incorporation of Mo vacancies and the formation of 2D-2D-2D heterojunction of Ti3C2@TiO2@MoS2 composites, where Mo vacancies in MoS2 can avoid charge recombination to enhance HER kinetics. Ti3C2MXene enhances the electronic conductivity and makes the electron transfer process more efficient [23]. Hence, both WO3, MoS2 and Ti3C2 as double co-catalysts enhance the photocatalytic performance of TiO2 [14].

4 Conclusion In conclusion, we discussed the emerging MXene-based composites as photoelectrocatalysts with appealing photoelectrochemical efficiency. MXenes with transition metal sulfide/phosphide can lead to outstanding visible light photocatalytic HER activity (Table 1). As presented in this chapter, MXene-based composites possess better electronic carrier transport and

Fig. 3 (A) Comparison of hydrogen evolution for hybrid composites with different weight percentages (wt%) of Ti3C2Tx named as CTx (x ¼ 0, 1, 2.5, 8) and NPCT1 for 1 wt% of Ti3C2. (B) Graphical representation showing PEC hydrogen evolution for different photocatalysts. SEM images of (C) Ti3C2, (D) Ti3C2@TiO2 composites (E, F) Ti3C2@TiO2@MoS2 composites with 15 wt% MoS2. ((A) Reproduced with permission from Yang Y, Zhang D, Xiang Q. Plasma-modified Ti3C2Tx/CdS hybrids with oxygen-containing groups for high-efficiency photocatalytic hydrogen production. Nanoscale 2019;11:18797–805. https://doi.org/10.1039/C9NR07242J. Copyright 2019, Royal Society of Chemistry. (B) Reproduced with permission from Chen R, Wang P, Chen J, Wang C, Ao Y. Synergetic effect of MoS2 and MXene on the enhanced H2 evolution performance of CdS under visible light irradiation. Appl Surf Sci 2019;473:11–19. https://doi.org/10.1016/j.apsusc.2018.12.071. Copyright 2019, Elsevier.(C–F) Reproduced with permission from Li Y, Yin Z, Ji G, Liang Z, Xue Y, Guo Y, Tian J, Wang X, Cui H. 2D/2D/2D heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity. Appl Catal B Environ 2019;246:12–20. https://doi.org/10.1016/j.apcatb.2019.01.051. Copyright 2019, Elsevier.)

Table 1 A summary of MXene-transition metal phosphide/sulfide photoelectrocatalyst performances. S No.

Photoelectrocatalysts

Synthesis method

H2 evolution rate (μmol g21 h21)

Quantum efficiency

References

1 2 3 4 5 6 7 8

TiO2-Ti3C2-CoSx CdS/NiS/Ti3C2 CdS-MoS2-MXene 2D CdS/Ti3C2Tx Ti3C2Tx/CdS MoxS@TiO2@Ti3C2 Ti3C2@TiO2@MoS2 MXene/1D CdS

Solvothermal Hydrothermal Hydrothermal Hydrothermal Solvothermal Hydrothermal Hydrothermal Hydrothermal

950 14,342 9679 – 825 10,505 6425 15,400

– 40.1% 26.7% – 10.2% – 4.61% –

[16] [21] [12] [22] [15] [13] [14] [11]

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increase the solar-to-H2 (STH) efficiency. MXenes work efficiently as co-catalysts due to their excellent electronic conductivity, high anisotropy of holes and electrons, and hydrophilic surface. Moreover, a large surface area with tunable functional terminations can alter the work function and carrier transport properties that offer a broad spectrum of electronic features. We believe that these promising results promote the state-of-art of MXenetransition metal compound sulfide and phosphide hetero-nanostructures as potential candidates for high selectivity, enhanced light absorption and stability, and efficient photoelectrochemical water splitting and open up opportunities for energy-related devices such as water fuel cells, water electrolyzers, and batteries.

Acknowledgments This study was financially supported by the Department of Science and Technology (DST) under a joint India-Korea bilateral project (INT/Korea/P-52) and by the Central Power Research Institute (CPRI), Bangalore (RSOP/21-26/GD/6). The authors gratefully acknowledge the financial support from Pandit Deendayal Energy University (PDEU) under the Start-up grant (ORSP/R&D/PDPU/2021/NC00/R0069).

References [1] Roger I, Shipman MA, Symes MD. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat Rev Chem 2017;1:0003. https://doi.org/ 10.1038/s41570-016-0003. [2] Huang K, Li C, Li H, Ren G, Wang L, Wang W, Meng X. Photocatalytic applications of two-dimensional Ti3C2 MXenes: a review. ACS Appl Nano Mater 2020;3:9581–603. https://doi.org/10.1021/acsanm.0c02481. [3] Li C, Baek J-B. Recent advances in noble metal (Pt, Ru, and Ir)-based electrocatalysts for efficient hydrogen evolution reaction. ACS Omega 2020;5:31–40. https://doi.org/ 10.1021/acsomega.9b03550. [4] Yi S-S, Zhang X-B, Wulan B-R, Yan J-M, Jiang Q. Non-noble metals applied to solar water splitting. Energ Environ Sci 2018;11:3128–56. https://doi.org/10.1039/ C8EE02096E. [5] Yang Y, Zhou C, Wang W, Xiong W, Zeng G, Huang D, Zhang C, Song B, Xue W, Li X, Wang Z, He D, Luo H, Ouyang Z. Recent advances in application of transition metal phosphides for photocatalytic hydrogen production. Chem Eng J 2021;405, 126547. https://doi.org/10.1016/j.cej.2020.126547. [6] Wang M, Zhang L, He Y, Zhu H. Recent advances in transition-metal-sulfide-based bifunctional electrocatalysts for overall water splitting. J Mater Chem A 2021; 9:5320–63. https://doi.org/10.1039/D0TA12152E. [7] Lee SL, Chang C-J. Recent progress on metal sulfide composite nanomaterials for photocatalytic hydrogen production. Catalysts 2019;9:457. https://doi.org/10.3390/ catal9050457.

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[8] Chaudhari NK, Jin H, Kim B, San Baek D, Joo SH, Lee K. MXene: an emerging twodimensional material for future energy conversion and storage applications. J Mater Chem A 2017;5:24564–79. https://doi.org/10.1039/C7TA09094C. [9] Hemanth NR, Kim T, Kim B, Jadhav AH, Lee K, Chaudhari NK. Transition metal dichalcogenide-decorated MXenes: promising hybrid electrodes for energy storage and conversion applications. Mater Chem Front 2021;5:3298–321. https://doi.org/ 10.1039/D1QM00035G. [10] You Z, Liao Y, Li X, Fan J, Xiang Q. State-of-the-art recent progress in MXene-based photocatalysts: a comprehensive review. Nanoscale 2021;13:9463–504. https://doi. org/10.1039/D1NR02224E. [11] Li J-Y, Li Y-H, Zhang F, Tang Z-R, Xu Y-J. Visible-light-driven integrated organic synthesis and hydrogen evolution over 1D/2D CdS-Ti3C2Tx MXene composites. Appl Catal Environ 2020;269, 118783. https://doi.org/10.1016/j.apcatb.2020.118783. [12] Chen R, Wang P, Chen J, Wang C, Ao Y. Synergetic effect of MoS2 and MXene on the enhanced H2 evolution performance of CdS under visible light irradiation. Appl Surf Sci 2019;473:11–9. https://doi.org/10.1016/j.apsusc.2018.12.071. [13] Li Y, Ding L, Liang Z, Xue Y, Cui H, Tian J. Synergetic effect of defects rich MoS2 and Ti3C2 MXene as co-catalysts for enhanced photocatalytic H2 production activity of TiO2. Chem Eng J 2020;383, 123178. https://doi.org/10.1016/j.cej.2019.123178. [14] Li Y, Yin Z, Ji G, Liang Z, Xue Y, Guo Y, Tian J, Wang X, Cui H. 2D/2D/2D heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity. Appl Catal Environ 2019;246:12–20. https://doi.org/10.1016/j.apcatb.2019.01.051. [15] Yang Y, Zhang D, Xiang Q. Plasma-modified Ti3C2Tx/CdS hybrids with oxygencontaining groups for high-efficiency photocatalytic hydrogen production. Nanoscale 2019;11:18797–805. https://doi.org/10.1039/C9NR07242J. [16] Zhao J-H, Liu L-W, Li K, Li T, Liu F-T. Conductive Ti3C2 and MOF-derived CoSx boosting the photocatalytic hydrogen production activity of TiO 2. CrstEngComm 2019;21:2416–21. https://doi.org/10.1039/C8CE02050G. [17] Yan L, Zhang B, Wu S, Yu J. A general approach to the synthesis of transition metal phosphide nanoarrays on MXene nanosheets for pH-universal hydrogen evolution and alkaline overall water splitting. J Mater Chem A 2020;8:14234–42. https://doi.org/ 10.1039/D0TA05189F. [18] Liu S, Lin Z, Wan R, Liu Y, Liu Z, Zhang S, Zhang X, Tang Z, Lu X, Tian Y. Cobalt phosphide supported by two-dimensional molybdenum carbide (MXene) for the hydrogen evolution reaction, oxygen evolution reaction, and overall water splitting. J Mater Chem A 2021;9:21259–69. https://doi.org/10.1039/D1TA05648D. [19] Lv Z, Ma W, Dang J, Wang M, Jian K, Liu D, Huang D. Induction of Co2P growth on a MXene (Ti3C2Tx)-modified self-supporting electrode for efficient overall water splitting. J Phys Chem Lett 2021;12:4841–8. https://doi.org/10.1021/acs.jpclett.1c01345. [20] Cai T, Wang L, Liu Y, Zhang S, Dong W, Chen H, Yi X, Yuan J, Xia X, Liu C, Luo S. Ag3PO4/Ti3C2 MXene interface materials as a Schottky catalyst with enhanced photocatalytic activities and anti-photocorrosion performance. Appl Catal Environ 2018;239:545–54. https://doi.org/10.1016/j.apcatb.2018.08.053. [21] Ran J, Gao G, Li F-T, Ma T-Y, Du A, Qiao S-Z. Ti3C2 MXene co-catalyst on metal sulfide photo-absorbers for enhanced visible-light photocatalytic hydrogen production. Nat Commun 2017;8:13907. https://doi.org/10.1038/ncomms13907. [22] Xie X, Zhang N, Tang Z-R, Anpo M, Xu Y-J. Ti3C2Tx MXene as a Janus co-catalyst for concurrent promoted photoactivity and inhibited photocorrosion. Appl Catal Environ 2018;237:43–9. https://doi.org/10.1016/j.apcatb.2018.05.070. [23] Zheng G, Wang J, Liu H, Murugadoss V, Zu G, Che H, et al. Tungsten oxide nanostructures and nanocomposites for photoelectrochemical water splitting. Nanoscale 2019;11:18968–94. https://doi.org/10.1039/C9NR03474A.

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CHAPTER 8

Design and advances of semiconductors for photoelectrochemical water-splitting Sauvik Chatterjee and Sanjib Shyamal

School of Materials Sciences, Indian Association for the Cultivation of Science, Kolkata, India

1 Introduction The pursuit of happiness is striving for a better life with an abundance of daily requirements and efficiency of investments. As humanity has progressed for the last one million years, it has tried to make their life better, smarter, and easier by the use of technology. The progress of technology was geared up as humans invented ways to ignite fire, make wheel, create electricity, etc. Now, we are at the juncture of life where it is the technology that sustains the civilization and creates an obvious requirement of energy. The most basic source of energy is fossil fuel which upon burning gives us energy and pollution. The exhaustive source of energy and the pollution created out of it raises concern to the scientific community to develop new technology for energy generation that is abundant, clean, and easy to produce. To capture the energy of sunlight and break water into hydrogen is one of the most serious as well as lucrative challenges of the modern scientific community of present time. The sun is the source of 105 TW energy per year—more than 104 times the average global requirement [1]. Hydrogen is a green fuel that produces 289 kJ mol
1 energy along with water and perhaps the only fuel that does not generate any environmentally harmful side products. Hydrogen hence can be considered as a form of energy that can be storable, easily transportable, and can be transformed to energy whenever required. By splitting water utilizing solar energy helps capture the energy in the form of H2 fuel (Fig. 1). The first documented effort of splitting water into hydrogen and oxygen was done by Honda–Fujishima in 1972 where they demonstrated photoassisted electrochemical watersplitting using n-type TiO2 single-crystal electrode [2]. Splitting of water, Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00007-8

Copyright © 2023 Elsevier Inc. All rights reserved.

141

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Solar-driven green hydrogen generation and storage

Fig. 1 Photoelectrochemical (PEC) water-splitting is an emerging technology for energy production in a greener and cost-efficient way.

water being a stable compound, is an uphill process with the requirement of 237 kJ mol
1 energy which is provided by band gap (BG) excitation leading to the formation of holes and electrons on the catalytically active sites. In this chapter, the basic principle of photoelectrochemical (PEC) watersplitting, the development of semiconductor materials from 1972 to the current state-of-the-art, and different directions of the strategic inventions will be discussed. This will help to get a better insight into the changes and modifications that were made in the course will be analyzed and the future scopes for the successful technology transfer.

2 Principle of water-splitting Water-splitting in presence of sunlight is the mimicry of the natural photosynthesis (NP) using man-made materials, where water is converted to oxygen (O2) and hydrogen (H2) and known as artificial photosynthesis (AP). The overall water-splitting is an “uphill reaction” involving large positive charge with Gibbs free energy (ΔG0 ¼ 237 kJ mol
1) or 1.23 eV as shown in Eq. (1)–(3). 2H 2 O ! O2 + 4H + + 4e



(1)

4H + 4e ! 2H 2

(2)

2H 2 O ! O2 + 2H 2

(3)

+

Therefore, the formation of solar fuels through water-splitting reaction is based on the involvement of the photogenerated charge carriers, i.e., electrons

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143

(e
) and holes (h+). The water-splitting reactions fall into three different categories: photocatalytic (PC), photoelectrochemical (PEC), and photovoltaicphotoelectrochemical (PV-PEC) systems [3]. There are three basic steps for photocatalytic water-splitting reaction using semiconductor photocatalysts: (i) absorption of photon (with energy greater than the semiconductor band gap) that leads to the formation of electron and hole pairs; (ii) separation and migration of the charge carrier to the active interface of the semiconductors; and (iii) chemical reaction between the charge carrier and water to produce O2 and H2 respectively, as shown in Fig. 2 [4]. For PEC water-splitting, a three- or two-electrode configuration cell adapted with a potentiostat is used for the application of small bias on electrode, where the working electrodes are made up of the photocatalysts deposited/attached on the conducting substrates. In 1968 for the first time, photoelectrochemical water-splitting was reported using TiO2 electrode in an aqueous solution under illumination conditions presented in Fig. 3A. The irradiated incident light on the electrode generates charge carriers, where holes at TiO2 surface oxidize water to form O2 and electrons travel to the Pt counterelectrode to form H2. There are three types of PEC cells, either of which or both electrodes have to be photoactive semiconductors. Similar to a Schottky junction, an equilibration is established at the interface as soon as a nonilluminated semiconductor electrode in an electrolyte solution comes in contact with a redox couple present in the same. The Fermi level of the semiconductor shifts to attain the same redox couple present in the solution. This possibly leads to the development of a space charge layer within a thin region in

H2O O2

H+/H2

ta

tio

n

(fs

~p

s)

e– e–

hv

ex ci

H2O/O2

+

h+ h+

CB VB

Ph

h

n

tio

ot o-

ina mb co e R – e ce rfa + Su h

yst

atal

Coc

h+

e–

+

ion

rat epa

S rge Cha C ocataly st

(ps~

ms)

H2

–

h e Bulk Recombinatio

n (ps~ms)

e–

H+

Surface Reaction

s (ms~ms)

Fig. 2 Fundamental principle of photocatalytic water-splitting on semiconductor.

(a) e–

e–

Anode

Cathode H2

Membrane H+

H2O OH

e–

e–

Photoanode Membrane H+

–

Photocathode H2 H2O

OH–

Electrolyte

OH–

(c) e–

e–

Photoanode Membrane H+

Photocathode H2 OH–

H2O

O2

O2

O2 OH–

(b)

Electrolyte

Fig. 3 (A) PV-PEC setup, (B) PEC setup with parallel illumination, and (C) tandem PEC/PEC setup.

OH–

Electrolyte

Design and advances of semiconductors

145

proximity to the surface of the semiconductor, where the electronic energy bands (valence and conduction) are bent upward or downward. This phenomenon is dependent upon the relation of the Fermi levels of the solution redox couple and the semiconductor. If the semiconductor is exposed to photons of higher energies compared to the band gap of semiconductor material, electron–hole pairs are formed and separated in the space charge layer (electrons for a p-type photocathode, holes for an n-type photoanode). PEC has an upside to photocatalyst system. There is no requirement for gas separation in photoelectrochemical setup as the formation site of hydrogen and oxygen is separated spatially at different electrode sides. PEC will be discussed in this chapter with a focus on different semiconductor materials which have been deployed for the development of photoelectrodes.

3 Photoelectrode materials Selection of photoelectrode materials is dependent upon several factors. Based on the physical properties like band gap, junction potential in the case of composite or heterojunction, and diode type the performance of electrode varies as varies the nature of electrode. Typically, the n-type materials as discussed earlier are used as photoanode, whereas the p-type semiconductors are used as photocathodes. In the photoanode, the water is split to generate oxygen. Hence, the valance band (VB) should lie higher than the O2/H2O redox potential (1.23 V vs. NHE at pH ¼ 0) for the successful evolution of oxygen. The bending of bands in the electrolyte-semiconductor interface can be a key for designing the appropriate semiconductor material for the desired performance. Overall, to design a semiconductor material for water-splitting, the following points need to be addressed. 1. Suitable band gap and band position. 2. Photostability. 3. Efficient charge carrier separation. 1. Suitable band gap and band position. 43% of total solar irradiation falls into the visible range of electromagnetic spectrum (400–800 nm). It is an essential criterion for the semiconductor material that this part of the spectrum is utilized for energy harvesting which will improve the efficiency of the process. The absorption range of a semiconductor depends upon the band gap of the material. The theoretical potential required for water-splitting is 1.23 VRHE at pH ¼ 0. But, in real situations, there are certain thermodynamic barriers of charge carrier transportation and

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surface reaction kinetics. The practical band gap requirement is ca. 1.8 eV which corresponds to absorption of 700 nm energy. Again, since the irradiation below 390 nm has very little contribution in the sunlight, the band gap above 3.2 eV will turn the semiconductor very inefficient for water-splitting purposes. Hence for optimization, the band gap range of 1.8–2.3 is the field to play. 2. Photostability. Photocorrosion is a major concern in the way to commercialization of the water-splitting technology. It is, in technical terms, decomposition of semiconductor materials upon exposure to solar irradiation under applied bias. The semiconductor, in the case of photoanodes, drives oxidation of the material itself instead of water. This happens when the anodic oxidation potential is placed above the VB potential. On the other hand, if the cathodic decomposition potential of the photocathode is placed below the conduction band potential, it will lead to self-decomposition of the semiconductor. In general metal sulfides, such as MoS2 and CdS, and metal oxides like BiVO4, ZnO, etc., suffer from anodic photocorrosion depending upon pH. Very slow cathodic photocorrosion is observed for materials like TiO2 even though the they have more positive anodic decomposition potential due to the thermodynamic stability. 3. Efficient Charge carrier separation: the PEC performance is highly dependent upon the charge separation and transfer. This event is generally driven by the electric field associated with the band bending in the surface space charge layer of the electrolyte-exposed side of the photoelectrodes. The nanometer to micrometer width of the space charge layer cannot ensure the charge separation of the whole film. Low charge separation and transfer efficiency is a major challenge in PEC process. To deal with this parameter, attempts have been made to extend the width of built-in electric field by using gradient doping. Changing the lattice dimension and removing the symmetry to achieve internal dipole moment is another methodology to create bulk band bending [5]. Band structure engineering by doping can be a very useful method. Nonmetal doping into metal oxides or doping colored centers into the material, etc., helps to narrow the band gap. The formation of pure crystal phases also leads to improve charge separation. Surface modifications such as creation of surface defects of O vacancies or having surface morphologies like nanobelt and nanowires can have sufficient influence on the charge diffusion length. The fine-tuning of surface features is also a very important part to control the charge separation and transfer [6].

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In the last four decades, innumerable semiconductor materials have been reported for their water-splitting applications. There have been various modifications made in order to match the band gap as well as resist photocorrosion. Different methodologies of synthesis of materials have been utilized to gain higher activity, reasonable band bending in the electrolyte semiconductor interface, etc.

3.1 Pure metal oxides as semiconductors By their own rights, metal oxides have established themselves as the most suitable semiconductor materials due to their high sustainability. Both pand n-type semiconductors are available among metal oxides. ZnO, TiO2, WO3, BiVO4, etc., are well-established semiconductors for water oxidation while Cu2O, p-doped Fe2O3, and CaFe2O4 are some metal oxides that efficiently show hydrogen evolution. To address sustainability and increase efficiency, the heterojunction of these metal oxides is used. Different surface morphologies and facet engineering are employed for the same semiconductor material. It has been observed that with the change in morphology of the semiconductor material, the efficiency varies a lot. Several methods of synthesis were employed to achieve different morphology. ZnO and TiO2 are the two most popular semiconductors explored widely for understanding these effects. A review by Han and Liu has demonstrated the changes in the activity of ZnO for PEC water-splitting with a change in synthetic methodology as well as the formation of composite [7]. It is pertinent to mention that making composite and heterojunction improves the PEC activity. T. Zhou et al. have discussed the role of morphology for TiO2/ZnO composite in increasing light trapping, light harvesting, and increased photostability [8]. BiVO4 has emerged as a promising n-type electrode as it is stable, neutral, nontoxic, and relatively cheap with a direct band gap of 2.4 eV making it a good reagent for the absorption of a wider spectrum of visible light [9]. It has low conduction band edge of 0.02 VRHE compared to other common photoelectrodes like WO3 and Fe2O3. This leads to the lower bias requirement to enhance the photoelectrons and increase the water reduction potential (0.0 VRHE) [10]. However, BiVO4 suffers from sluggish water oxidation kinetics and poor electron conductivity. The monoclinic scheelite form of BiVO4 shows a very high theoretical STH of 9.1% but for this being used as photoanode modifications are required to overcome the aforementioned limitations [11]. WO3 similarly has a very low band gap of 2.6 eV, capable of 12% solar irradiation capture, thus

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attracting attention for photocatalytic studies. But similar to other metal oxides, WO3 also encounters electron-to-hole recombination due to bad charge separation [12]. Crystal-facet-modified WO3 was prepared for the reduction of hole diffusion length, thereby reducing electron–hole recombination. Morphologies like nanorods, nanoflakes, and nanowires are proven to be good for the same purpose. Among all, hematites are found to have the optimal band gap of 2.1 eV which is also a lucrative option for their stability in an oxidative environment, natural abundance, and low cost. However, these also suffer from a very low absorption coefficient, low excited state lifetime, etc. Apart from these materials, there are several oxides and mixed oxides of AMO4 structures like CuWO4 (2.25 eV); perovskites like BiFeO3, BaFeO3, and SrFeO3 are found to be good catalysts for water-splitting and degradation. CuWO4 shows better photostability, however suffers from electron–hole recombination and poor conductivity. Metal tungstate with Zn, Mg, Ca, and Sr show BG higher than 3 eV and those can be used for UV light harvesting. It is due to photocorrosion of metal oxide based photocathodes there have been very limited examples of metal oxide semiconductors which meets the criteria for been explored as industrially viable electrodes. Cu2O and CuO are p-type metal oxides which are used as photocathodes. Cu2O, with a direct band gap of 2.0 eV, is an attractive material for PEC HER which can deliver a theoretical photocurrent density of 14 mA cm
2 and STH 18%. The low electrochemical stability of the Cu2O in an aqueous solution is the major limitation for the application. The reduction potential from Cu(I) to Cu(0) is 0.47 eV which falls in the potential range of watersplitting, thus making this as less stable during water-splitting. To protect this photocorrosion, several techniques have been employed, one of which is atomic layer deposition (ALD) of passivized metal oxides like ZnO, TiO2, Al2O3, etc. [13]. Similarly, CuO also a good choice as photocathode by virtue of it’s band gap and it is is anticipated for self-degradation due to photocorrosion under an aqueous solution. CaFe2O4 and CuRhO2 are two multinary metal oxides that have their conduction band and valance band edge positions straddling the electrolysis potential. CaFe2O4 has a major drawback of rapid recombination through its polycrystalline structure. Development of controlled morphology by using solution method and high-temperature annealing, Ida and coworkers demonstrated, can lead to an impressive 1 mA cm
2 photocathodic current density at
0.6 VAg/AgCl [14].

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3.2 Metal chalcogenides as semiconductors Transition metal dichalcogenides are also very well-explored semiconductor materials for photoelectrochemical water-splitting. MoS2, WS2, and Co0.85Se are some of the well-established catalysts that have shown very good performance for PEC HER. Density function theory (DFT) calculations have shown that the Gibbs free energy of hydrogen adsorption (ΔG) on the transition metal dichalcogenides is comparable to Pt, making them intrinsically active for HER [15]. However, most of the applications of this type of semiconductor have been carried out as a heterojunction with p-type support materials which will be discussed later part of this section. CdS is a good class of semiconductor with band gap 2.4 eV that is quite well explored for PEC HER. However, these also suffer the weak electron–hole separation and photocorrosion in aqueous media. CdxZn1
xS, CuGaS2, and CuInS2 are some mixed metal sulfides that have attracted the attention of various research groups owing to very low band gap and high light absorption coefficient. CuS-based metal thiolate framework (MTF-1) [16] and CuZnS4 are a few examples of sulfides that have been used for PEC HER as a stand-alone semiconductor electrode. MoS2, WS2, Ag2S, ZnS, etc., are n-type semiconductors capable of being used as photosensitizer due to their low band gap and high light absorption efficiency. However, the low charge separation and photocorrosion restrict their applications as independent semiconductor materials for PEC OER. These materials as part of heterojunction assembly showed very promising results by transferring electrons to the wide band gap semiconductors such as ZnO, TiO2, and SnO2.

3.3 Modified semiconductors and composites Band gap engineering by formation of heterojunction, like morphological modification, crystal engineering, etc., is a good strategy to deal with the drawbacks of the semiconductors. Addressing the issues like charge separation, photocorrosion, etc., has been done by the formation of multijunction of various semiconductor materials. Surface modification using doping and postsynthetic treatment like acid treatment, surface hydrogenation, etc., are employed to introduce band bending. Composite of two or more semiconductors showed good performance with respect to stability, charge separation, etc., and heterostructures such as WO3/BiVO4 and BiVO4/graphene were prepared for the enhancement of interfacial charge transfer (Fig. 4).

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Fig. 4 Schematic working principle of WO3/BiVO4 heterojunction.

Au electrodeposition on spin coated WO3/BiVO4 shows 0.11 mA cm
2 photocurrent at 1VAg/AgCl. While nanostructured piller morphology of WO3/BiVO4 developed by electrostatic spraying shows 3.3 mA cm
2 photocurrent at 0.6 VAg/AgCl, autocombusted BiVO4 nanoparticle over spin-coated WO3 shows 3.43 mA cm
2 photocurrent at 1.23 VRHE. By the formation of intrinsic and extrinsic defects, the electronic structure can be tuned. Mo and/or W doping and creation of oxygen vacancies help to modify the electronic properties of the semiconductor by changing the band bending. Mo-BiVO4 showed a significant enhancement of incident-photon-to-current efficiency (IPCE) value to reach near 40% at 450 nm wavelength [17]. Surface H-treated BiVO4 photoanode showed a photocurrent density of 3.5 mA cm
2 at 1.0 VAg/AgCl which is three times higher than the pristine BiVO4. Doping of Au to BiVO4 has shown an improvement in visible light absorption due to the suitable band gap energy over pristine BiVO4 [18]. TiO2 nanowires doped with N atom show the improvement of photocatalytic activity under the visible light region. Mo/C doping to TiO2 can considerably reduce the band gap below 1.1 eV leading to enhanced photoabsorption [19]. H-treated WO3 and TiO2 show intrinsic oxygen vacancies deduced by hydrogen treatment. It has also been observed that by changing the temperature of annealing, the optical property varies significantly. For TiO2, the optimum annealing temperature was found to be 350°C. The color changes from pale yellow to gray to black indicating the change in optical properties when the temperature is increased [20]. Though S doping is believed to be more challenging both in terms of synthesis and electronic interaction, S-doped TiO2 showed 4 mA cm
2 photocurrent density at 1.05 VAg/AgCl, twice as compared to

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N-doped TiO2. N doping in ZnO changes the photocurrent almost 25 times as compared to bare ZnO [21], whereas CN doping enhances the photocurrent further [22]. Similarly, in the cases of photoanodes, there are composite materials that have been reported to achieve better efficiency in terms of photocurrent, solar to hydrogen (STH) value, charge separation, etc. To eliminate the issue of photocorrosion of Cu(I) to Cu(0), ALD and surface passivation techniques are employed. ALD was found to be one of the most efficient ways of deposition of ultrathin film on metal oxide photocatalysts with good control over thickness. Paracchino group reported protective coating by ALD of ZnO/Al2O3/TiO2 over Cu2O grain followed by Pt surface decoration [13]. The photoactivity and stability of the photocathode were substantially increased compared to bare Cu2O. 7.5 mA cm
2 was achieved with the aforementioned composite and showed 100% faradaic efficiency after 1 h of usage. Cu2O has also been ALD coated by Al-doped ZnO and TiO2 followed by photodeposited RuOx which has shown 10 mA cm
2 photocurrent density with the stability of more than 50 h [23]. Perovskite-BiVO4, one of the benchmark materials for PEC HER, shows 20 mA cm
2 photocurrent density at 0 VRHE and over 90-h stability [24]. MoSxCly has shown an impressive current density of 43 mA cm
2 photocurrent density at 0 VRHE.

3.4 Hybrid organic–inorganic semiconductors Organic–inorganic hybrid materials have gained considerable attention nowadays for their exceptional performance in PEC water-splitting reactions. Certain metal organic frameworks (MOFs), covalent organic frameworks (COFs), etc., are reported for lower onset potential, photostability, and high photoabsorptivity. Ni-ZIF-8 MOF showed an improvement of IPCE value by 11% compared to its Zn analog. Heterojunction of ZnO-ZIF MOF shows PEC water oxidation. Triclinic ZnO NZO-1/ COF heterojunction shows 0.62 mA cm
2 photocurrent density at 0.2 VRHE under neutral pH where the COF acts primarily as photosensitizer; however, the crystalline nature of the COF helps to separate charges better [25]. In 2016, Gong and coworkers have reported La-MOF based on a pi-conjugated organic ligand which shows a facile photoinduced electron transfer process [26]. A 2D Co-MOF developed using mixed ligand was employed as photoanode for PEC water oxidation that showed 5.89 mA cm
2 photocurrent density at 2 VRHE [27]. MOF composite of

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Fig. 5 MOF-modified TiO2 photoanode for water oxidation. In the box: band diagram of TiO2/NH2-MIL-125 heterojunction.

BiVO4 with Co-MIm developed by in-situ growth of Co-MIm shows an enhancement of 240% photocurrent density at 1.23 VRHE as compared to pure BiVO4 [28]. A very stable heterojunction developed with NH2-MIL-125/TiO2, which has been used as photoanode that shows 1.63 mA cm
2 photocurrent density at 1.23 VRHE (Fig. 5).

4 Tandem reaction setup Integration of photocathode and photoanode is essential for direct watersplitting due to the limitation of individual electrodes. For example, TiO2 has a large suitable edge potential owing to the large band gap which in turn leads to limited light adsorption. Along with that, high overpotential value makes it extremely difficult to produce hydrogen. On the contrary, small band gap photoanodes like Si have suitable conduction band edge potential for HER, but the oxidative power is very little for successful water oxidation. This can be dealt by the introduction of tandem setup of suitable photoanode and photocathode to facilitate direct water-splitting. Back in 1975, Yoneyama and coworkers for the first time presented a PEC cell made of a p-GaP photocathode and TiO2 photoanode that was able to produce H2 and O2 under no applied bias [29]. Based on that work, in 1976, Nozik for the first proposed the theory of integration of photocathode and photoanode in order to develop a tandem structure to create chemical fuel from solar energy [30]. In 2004, Lewis and coworkers proposed an architecture to assemble photocathode and photoanode nanowires decorated on two sides

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of an ion exchange membrane for direct solar water-splitting [31]. The HER and OER catalysts were decorated on the electrodes to reduce the overpotential and ion membranes were used for the mechanical support for electrodes as well as for gas separation and safe operation (Fig. 3C). In the past 15 years, multiple designs and developments have been carried out in the field of tandem water-splitting. Liu et al. have developed a tandem setup using p-Si nanowires as photocathode and TiO2 nanowires as photoanode. IrOx and Pt were used as OER and HER cocatalysts, respectively. These setups did not include any ion separation membrane. The device showed stable and impressive photocurrent density under chopped light illumination and long-time continuous light illumination without the application of any external bias indicating the concept to be feasible for device fabrication. The yield was determined with gas chromatographic measurements of H2 and O2. BiVO4 has been explored for designing PEC/PEC tandem setup due to the lower band gap leading to greater photoabsorption. BiVO4/NiO was one of the early reported tandem setups, showed a faradaic efficiency of hydrogen generation 80% and stable for several hours, however showed very little STH conversion due to large band gap of NiO (3.6–4.0 eV) [32]. Cu2O, on the other hand, has a lower band gap of 2.1 eV and is suitable for use as a photocathode with the ability to absorb light below 600 nm wavelength. BiVO4/Cu2O tandem cell, reported by Sivula et al., shows an STH efficiency of 0.5% [33]. p-Si nanoarrays are by far better semiconductor photocathode for PEC/PEC tandem cells due to 1.1 eV band gap providing a whole spectrum of photoabsorption efficiency as discussed earlier. Compared to planer Si photocathode film, which exhibits poor onset potential for water reduction, p-type Si nanoarrays show better onset potential for HER because of the reduced transfer distance of the photoinduced electron to the bulk surface [34]. BiVO4/p-Si PEC tandem cell achieved 0.57% STH efficiency with 0.46 mA cm
2 photocurrent density. Co-Pi/Mo and Pt were used as cocatalysts for OER and HER, respectively, that show that the system reaches a stable photocurrent of 1.2 mA cm
2 after 3.5 h indicating the existence of instability which might be caused by the pH gradient developed near the electrodes or dissolution of Co-Pi. Composite photoelectrodes are developed for constructing tandem cells. Pt/CdS/CuGa3Se5/(Ag,Cu)GaSe2 electrode demonstrates an onset potential 0.9 VRHE and over 0.67% STH efficiency with NiOOH/ FeOOH/Mo:BiVO4 as anodic counter electrode. At 1.23 VRHE, 3.4 mA cm
2 photocurrent was recorded for the aforementioned system [35]. Several other PEC tandem cells were developed using hematite as a photoanode.

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5 Conclusion and outlook This discussion in this chapter so far can conclusively assert that the generation of hydrogen fuel by splitting water can be the future of power research as well as the development of green economy. However, water-splitting using semiconductors, though a very lucrative option from theoretical standpoint, has numerous obstacles. Each step of the process faces challenges that need to be addressed to make a more sustainable and efficient technology. For example, the metal oxide semiconductors have limited light absorption and poor electrical conductivity. As discussed in Section 3, several strategies are being employed to increase the electrical conductivity like doping with heteroatoms or creating self-defects. These in turn also helped in improving the optical property for increasing the visible light absorption. However, the efficiency of doping and control of the defect creation is still far from perfect. Creation of heterojunction has also addressed these problems as well as the issues related to quick charge recombination and improper band edge energies. It has been demonstrated by various groups that almost in every case, creating a proper composite of a particular band edge can improve photocurrent density, reduce onset potential, and provide photostability. Besides that, the exploration of new metal oxide semiconductors should also be a way forward. As we have seen that the multicationic oxides show good performance in PEC water-splitting as photoanode. Different combinations of the same can lead to higher efficiency in terms of photostability and photocurrent. Formation of monolith growth as well as postsynthetic modification for band gap engineering can help tackle the poor electrical conductance issue of the multicationic oxides. Extensive structural studies of metal oxides and composites are required for the understanding of the underlying mechanism. It is necessary to probe the reaction intermediates and charge dynamics on the surface of photoelectrodes under desired parameters. Spectroscopic and microscopic techniques are being developed for the said purpose. Theoretical studies also help understanding the reaction dynamics, transition state, etc. DFT studies of water-splitting by semiconductors help realizing the effect of doping in the band positions, the electronic transfers, the formation of transition state and their stability, etc., that will in turn lead to select more appropriate doping, defect, or junction site for a feasible water-splitting at varied conditions like pH, presence of different electrolytes and microbes, etc. Sulfides also suffer from various issues, photocorrosion being a major one in the case of photocathode. Several strategies like creation of heterojunction and application of protective

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layer can be applied to alleviate these ordeals. On successful handling of these problems, sulfides can be used as one of the most efficient materials as photoelectrode due to their excellent visible light absorptivity. In the wake of global energy crisis and pollution caused by fossil fuel burning, PEC water-splitting can emerge to be a very sustainable alternative. The rate of progress and exploration of the technology is very promising. Further advancement will offer the opportunity for fuel production by PEC water-splitting as a cost-efficient, clean, and abundant technology.

References [1] Hisatomi T, Kubota J, Domen K. Chem Soc Rev 2014;43:7520. [2] Fujishima A, Honda K. Nature 1972;238:37. [3] Hasija V, Raizada P, Hosseini-Bandegharaei A, Thakur VK, Van Le Q, Nguyen V-H, et al. ChemCatChem 2021;13:2965–87. [4] Thakur A, Ghosh D, Devi P, Kim K-H, Kumar P. Chem Eng J 2020;397:125415. [5] Hu Y, Pan Y, Wang Z, Lin T, Gao Y, Luo B, et al. Nat Commun 2020;11:2129. [6] Xu Y, Li A, Yao T, Ma C, Zhang X, Shah JH, et al. ChemSusChem 2017;10:4277–305. [7] Han J, Liu Z. ACS Appl Energy Mater 2021;4:1004–13. [8] Zhou T, Wang J, Chen S, Bai J, Li J, Zhang Y, et al. Appl Catal Environ 2020;267:118599. [9] Jia Q, Iwashina K, Kudo A. Proc Natl Acad Sci U S A 2012;109:11564–9. [10] Moniz SJ, Shevlin SA, Martin DJ, Guo Z-X, Tang J. Energ Environ Sci 2015;8:731–59. [11] Sivula K, Van De Krol R. Nat Rev Mater 2016;1:15010. [12] Wang GM, Ling YC, Wang HY, Yang XY, Wang CC, Zhang JZ, et al. Energ Environ Sci 2012;5:6180. [13] Paracchino A, Laporte V, Sivula K, Gr€atzel M, Thimsen E. Nat Mater 2011;10:456. [14] Ida S, Yamada K, Matsunaga T, Hagiwara H, Matsumoto Y, Ishihara T. J Am Chem Soc 2010;132:17343. [15] Li H, Tsai C, Koh AL, Cai L, Contryman AW, Fragapane AH, et al. Nat Mater 2016;15:48. [16] Chatterjee S, Shyamal S, Mukherjee M, Halder D, Chongdar S, Paul A, et al. ChemSusChem 2022. https://doi.org/10.1002/cssc.202200114. [17] Zhou M, Bao J, Xu Y, Zhang JJ, Xie JF, Guan ML, et al. ACS Nano 2014;8:7088. [18] Venkata Reddy C, Reddy IN, Ravindranadh K, Reddy KR, Shim J, Cheolho B. Appl Surf Sci 2021;545:149030. [19] Gai Y, Li J, Li S-S, Xia J-B, Wei S-H. Phys Rev Lett 2009;102, 036402. [20] Wang GM, Wang HY, Ling YC, Tang YC, Yang XY, Fitzmorris RC, et al. Nano Lett 2011;11:3026. [21] Yang XY, Wolcott A, Wang GM, Sobo A, Fitzmorris RC, Qian F, et al. Nano Lett 2009;9:2331. [22] Xie SL, Lu XH, Zhai T, Li W, Yu MH, et al. J Mater Chem 2012;22:14272. [23] Luo JS, Steier L, Son MK, Schreier M, Mayer MT, Gr€atzel M. Nano Lett 2016;16:1848. [24] Pornrungroj C, Andrei V, Rahaman M, Uswachoke C, Joyce HJ, Wright DS, et al. Adv Funct Mater 2021;31:2008182.

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[25] Chatterjee S, Bhanja P, Biswas D, Kumar P, Das SK, Dalapati S, et al. ChemSusChem 2021;14:408–16. [26] Gong Y-N, Ouyang T, He C-T, Lu T-B. Chem Sci 2016;7:1070–5. [27] Natarajan K, Gupta AK, Ansari SN, Saraf M, Mobin SM. ACS Appl Mater Interfaces 2019;11:13295–303. [28] Zhou S, Yue P, Huang J, Wang L, She H, Wang Q. Chem Eng J 2019;371:885–92. [29] Yoneyama H, Sakamoto H, Tamura H. Electrochim Acta 1975;20:341–5. https://doi. org/10.1016/0013-4686(75)90016-X. [30] Nozik A. Appl Phys Lett 1977;30:567. [31] Lewis NS. Nat Nanotechnol 2016;11:1010. [32] Tong L, Iwase A, Nattestad A, Bach U, Weidelener M, G€ otz G, et al. Energ Environ Sci 2012;5:9472. [33] Bornoz P, Abdi FF, Tilley SD, Dam B, van de Krol R, Graetzel M, et al. J Phys Chem C 2014;118:16959–66. [34] Chen Q, Fan G, Fu H, Li Z, Zou Z. Adv Phys: X 2018;3:1. [35] Kim JH, Kaneko H, Minegishi T, Kubota J, Domen K, Lee JS. ChemSusChem 2016;9:61–6.

CHAPTER 9

Dye-sensitized photoelectrochemical cells in water splitting Mahesh Dhondea, Prateek Bhojaneb, Kirti Sahuc, and V.V.S. Murtyd a Department of Physics, Medi-Caps University, Indore, Madhya Pradesh, India Department of Physics, University of Petroleum and Energy Studies, Dehradun, Uttarakhand, India Department of Physics, SNGPG College, Khandwa, Madhya Pradesh, India d Department of Physics, Govt. Holkar Science College, Indore, Madhya Pradesh, India b c

1 Introduction Global energy consumption is expanding rapidly due to population increase and economic development, with fossil fuels accounting for more than 80% of worldwide energy use [1,2]. Due to present and projected future usage, it is questionable if fossil fuel supplies will last for less than 100 years or millennia [3,4]. However, greenhouse gases such as CO2, produced when these fuels are burned, considerably influence climate change. It is critical to seek a substitute that is a renewable and carbon-free source of energy in order to fulfill global energy demand in an ecologically friendly manner. Renewable sources, including solar, wind, nuclear, geothermal, and biomass energy, are regarded as viable alternatives to hydrocarbons [5]. Out of various existing renewable energy sources, solar energy serves as the most accessible and long-lasting renewable energy source that has the ability to be scaled up to meet future energy needs. Thanks to Edmond Becquerel’s seminal work on the photoelectric effect, the research community has been using the notion of harvesting energy from the sun for almost two centuries. CO2 emissions need to be stabilized at an acceptable level, and carbon-free energy sources are essential for this. Energy is constantly being poured into Earth, and if properly harnessed, it would be more than enough to meet the demands of every living thing on the globe [4,5]. Devices that convert solar energy into electricity must have a high conversion efficiency and be made from low-cost, abundant resources on Earth if solar power is to be used on a large scale. Nonetheless, the input power should always be stored sustainably since solar energy is intermittent. The ability to transform solar energy into chemical energy that is stored in fuels addresses the constraints associated Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00005-4

Copyright © 2023 Elsevier Inc. All rights reserved.

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with renewable energy sources’ intermittency, which are hampered by concerns such as optimal storage and transportation. Photoelectrochemistry offers a promising alternative for producing carbon-neutral renewable fuels when combined with appropriate electrocatalysts. In the semiconductor depletion layer, light absorption causes the production and separation of electron-hole pairs. These separated carriers are transferred to the semiconductor active layer and the surface. The reverse of this can be observed in the case of p-type semiconductors [6]. Photogenerated carriers are captured by the electrolytes’ oxidizing and reducing agents, which aid in the reaction process. An electron and a hole are created when a photon is absorbed, resulting in a quantum yield of 1. The addition of protic species to the corresponding electrolytes can help alleviate some of the charge-transport restrictions currently seen in these systems. Similarly, photoanodes can produce a variety of renewable products by utilizing light-assisted photoelectrochemical processes [7]. For almost two billion years, plants have produced food in the presence of sunshine by storing chemical energy via the photosynthesis process. Therefore, liquid fuels can be created by capturing solar energy via similar pathways and replicating the photosynthesis process. Hydrogen, the simplest form of energy carrier, can be produced in a self-sufficient manner using photoelectrochemical cells (PECs) or electrolysis cells coupled to photovoltaic (PV) devices. With its tiny size and simplicity, hydrogen is becoming an increasingly significant player in the world’s energy economy. When combined with oxygen or utilized in a closed cell, hydrogen produces no emissions, and the process may be reversed if necessary. Thus, using solar energy to generate clean H2 to replace conventional fossil fuels is a feasible step toward establishing a sustainable society [8]. An ideal PEC comprises semiconductor materials that are both stable and appropriate for the PEC reaction to occur at the interface between these materials and a solution. In developing clean energy, the PEC hydrolysis powered by visible light creates oxygen and hydrogen fuels and turns them into electricity. The OER and HER can occur concurrently throughout the PEC cell’s reaction process. The photoanode undergoes the O2 oxidation process (2H2O!4H++O2+4e), whereas the cathode undergoes the H2 reduction reaction (2H++2e!H2). Research into hydrogen as a replacement for fossil fuels has increased in recent years. It is possible to produce and use hydrogen without releasing any carbon dioxide into the atmosphere, making it an attractive option for reducing emissions of greenhouse gases and protecting the environment. Hydrogen generation is very efficient and stands out among renewable

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Fig. 1 Different methods for H2 production. (Reproduced from Esmaili H, Kowsari E, Ramakrishna S. Significance of nanostructure morphologies in photoelectrochemical water splitting cells: a brief review. J Mol Struct 2021;1230:129856. https://doi.org/10. 1016/j.molstruc.2020.129856. with permission, Copyright 2021, Elsevier.)

energy sources [9–11]. A schematic of various methods for hydrogen production is shown in Fig. 1. Hydrogen’s specific energy is three times higher than gasoline, yet its combustion produces just water and energy, making it a clean and sustainable fuel [12]. Under typical conditions, the energy of the light photons (1.23 eV) must be running downhill in free energy to complete the entire water splitting reaction. Due to unfavorable catalytic intermediate states, resistances at electrolyte interfaces, solid-state junctions, and bulk losses, oxidation and reduction half-reactions have substantial overpotentials. As a result, semiconductor devices with relatively wide bandgaps (1.6 eV) are preferred for this application. However, bandgaps over 2 eV only permit the absorption of a tiny portion of the spectral region, leading to poor solar-to-hydrogen (STH) conversion efficiency. To date, no commercially viable and scalable PEC water splitting system has been produced employing semiconducting materials despite extensive research and development on the basic principles of the technology. The primary reason for this is that the utilized materials must simultaneously meet various standards. Fig. 2 illustrates the different solar-driven H2 production approaches.

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Fig. 2 Various solar-driven approaches for hydrogen production, (A) photocatalytic water splitting, (B) photoelectrochemical (PEC) water splitting, (C) photovoltaicelectrochemical (PV-EC) water splitting, (D) solar thermochemical (STC) water splitting, (E) photothermal catalytic (PTC) H2 production, and (F) photobiological (PB) H2 production [11]

In 1972, Honda and Fujishima validated the possibility of PECs for H2 production under UV light effortlessly [13]. The proposed device comprises an n-type TiO2 anode and a platinum (Pt) cathode in its simplest form. UV stimulation of TiO2 causes water to split into H2 and O2 in an acidic solution with a slight applied bias. TiO2 nanoparticles with oxidizing holes (h+) in their valence band (VB) have the potential to oxidize water on their surfaces. The conduction band (CB) electrons travel to the cathode during H+ reduction to H2. An external bias was needed to raise CB electron potential to reduce water. Single-material PECs can only absorb UV radiation; hence, they can only harness a negligible portion (10%) of the solar spectrum. Significant efforts have been made in tailoring the material’s bandgap in photocatalytic water splitting systems to fulfill the thermodynamic requirements while absorbing an additional solar spectrum [14–18]. Later in 1981, Gr€atzel

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and his coworkers came up with a photocatalyst system that used TiO2 nanoparticles with RuO2 as the WOC and Pt as the proton reduction catalyst (PRC) on the surface. Unlike in the suspension system, water reduction and oxidation take place on different electrodes. Further, the charge recombination significantly affects the conversion efficiency in suspension systems. Nevertheless, a PEC allows for the control of the Fermi level of the photoanode by an applied bias, allowing for the redox processes to be optimized at the appropriate potential. Water splitting is a common application for dye-sensitized photoelectrochemical cells (DSPECs), owing to hydrogen’s high combustion value and low environmental impact. The photochemical splitting of CO2 into carbon-containing fuels and oxygen and the interaction of N2 with water to create NH4 and O2 are both becoming increasingly popular [19–23]. As discussed in previous reports, PEC systems often exhibit underrated quantum yields due to uncontrolled charge recombination and have made significant efforts to address this issue [24–26]. Moreover, creating a semiconductor with a sufficiently small bandgap and appropriate energy levels for catalytic water splitting is inherently tricky. One method for extending the absorption of a wide-bandgap semiconductor is to use molecular photosensitizers attached to the semiconductor’s surface, as proven in dyesensitized solar cells (DSSCs) [27–29]. The employment of a molecularly engineered dye and a catalyst enables systematic alterations that can be made due to the versatility and relative ease of chemical synthesis. Many different materials are used in photoanodes and photocathodes in DSPECs; therefore, this chapter aims to provide an overview of how DSPECs operate as a whole. It also discusses the design of the devices, the operating mechanism, and the various materials used. In addition, tandem DSPECs are also explored, and the prospect of highly efficient DSPECs is highlighted.

2 Device architecture 2.1 Chromophores In DSPECs, the chromophore is a critical component. The chromophore must have a high absorption coefficient for the visible and near-infrared (NIR) range of light to be a viable option. Furthermore, the chromophore’s lowest unoccupied molecular orbital (LUMO) must be more negative than the n-type semiconductor’s CB edge for effective electron injection, and its highest molecular orbital (HOMO) must be positive relative to the WOC’s catalytic onset potential for oxygen evolution. In addition, two other crucial factors are photochemical and electrochemical stabilities. [Ru(bpy)3]2+

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exhibits several desirable properties as an ideal sensitizer, including strong absorption between 400 and 500 nm, a relatively high lifetime (600 ns), and a sufficiently positive Ru(III)/Ru(II) reduction potential for the effective water oxidation process. Despite all the favorable attributes of [Ru(bpy)3]2+, its instability in favorable environments for water splitting restricts its use [30,31]. To find a suitable alternative, porphyrin dyes [32,33] that are highly charged but have a narrow absorption range and organic dyes [34–36] with a high molar extinction coefficient have also been explored; however, their oxidization is again a critical impediment to their large-scale application [37]. Fig. 3 demonstrates various Ru-based chromophores used in DSPECs for water splitting.

Fig. 3 Various Ru-based chromophores utilized in DSPECs for H2 production [30].

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2.2 Water oxidation catalyst (WOC) Recent years have seen substantial development in the development of molecularly engineered WOCs based on Ru [38–47], Ir [48–50], Cu [51,52], and Co [53–55] complexes, as well as in the synthesis of these compounds. While initial research concentrated on high catalytic performance, long-term development requires low-cost, chemically stable, and effective catalysts for water oxidation and hydrogen production. For DSPECs, WOC must accrue four oxidizing equivalents per oxygen molecule, stimulate oxygen production, and be chemically inactive. Moreover, for viability, a catalyst must be superactive or self-repairing. Since the photoanode activity is the primary focus of most DSPEC research, Pt surfaces are frequently employed as the catalyst in cathodes for H2 generation. Due to its low overpotential need, platinum is an excellent catalyst for hydrogen production. Although Pt is a valuable element in its own right, its exorbitant cost makes it a poor choice when contemplating the long development of DSPECs. Pt can be replaced by hydrogenase enzymes, which are already being employed in biohybrid applications as a substitute for Pt [56,57].

2.3 Adsorbing group Adsorbing groups are crucial parts of the DSPECs assembly because they link the semiconductor with the photosensitizer or catalyst. According to general principles, three types of common adsorbing groups are added to DSPECs: the carboxylic, phosphonic, and silicic groups [58–64]. According to scientific literature, the silicic group is more stable in water-based fluids over a wide pH range (2-11). It has a poor synthesis and extraction process. While carboxylic and phosphonic groups are easy to synthesize, they are exclusively stable in acidic solutions [65–70]. Moreover, carboxylic groups have electrical coupling capabilities on semiconductors, enabling permissible electron exchanges. For this reason, they lend themselves better to synthesizing a photosensitizer for use in DSPECs. However, phosphonic groups may be used as both catalysts and photosensitizers in near-neutral electrolytes due to their electrolyte stability. The silicic group, which has poor electrical conductivity, is more advantageous for synthesizing electron donors WOC in DSPECs. As a result of their unique properties, the diverse adsorbing molecules are especially well suited to coadsorption and supramolecular DSPECs with complex assembly requirements.

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2.4 Electrolyte An electrolyte is a critical component of DSPECs assembly, and it essentially boosts the performance of DSPECs. An inorganic solution is a suitable choice for the function of electrolytes since it allows proton transport between the photoanode and photocathode. Light exposure causes a burst of photocurrent density (J1) at the outset that quickly decays to a steady state (J2), and this process can be performed many times. Many factors contribute to a drop in photocurrent density, including polarization effect, interfacial charge production, charge buildup and blocking in comparatively thick layers, and other factors [71,72]. Among these, one should not overlook the polarization impact, which arises due to the somewhat slower transport of electrons between the catalyst and the photosensitizer as the catalyst oxidizes, thereby contributing to increased solution acidity. Further, increasing overpotential for oxidative catalysis lowers the catalytic rate as the pH drops. Buffer solutions reduce electrode polarization by weakening the pH variations in the surrounding environment. In general, buffer solutions are used to reduce electrode polarization by minimizing local pH variations. While a buffer solution reduces electrode polarization, it introduces additional potential difficulties. The buffer anion has a beneficial effect on DSPECs in two ways: it can serve as a proton acceptor base, enabling ATP during OdO bond formation [51] and it can be absorbed on the electrode surfaces, preventing recombination of electrons and thus increasing the photoactivity [73]. The buffer solution has two aspects, just like a coin. There are also nucleophilic assaults by water and buffer anions on the surface of the semiconductor, followed by the removal of adsorbing groups [74]. As a result, the selection of buffer solution significantly impacts DSPECs’ photoactivities and stabilities. For DSPECs, the best way to achieve high photoactivity and long-term stability is still up in the air. The influence of the redox electrolyte on charge transport among photosensitizer and catalysts is another something that DSPECs should investigate further.

2.5 Semiconductor oxide When illuminated, the surface of the photoelectrode in a DSPEC must allow the formation of charge carriers to be collected at the counter electrode (CE). For DSPEC research, photoelectrodes are often made from TiO2, ZnO, or SnO2, whereas photocathodes are typically made from NiO. Since TiO2 has a higher positive CB potential and is easier to synthesize, it is commonly utilized to provide an extensive surface area base support for DSSCs

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and DSPECs. TiO2 is also stable in aqueous and non-aqueous phases, making it an excellent choice for DSSCs and DSPECs. When coupled with the appropriate catalyst, the photogenerated electrons in TiO2’s CB states have enough energy to promote H2 production. Numerous researches are devoted to optimizing the DSPECs’ performance for efficient hydrogen production [74–77]. In this regard, Sherman and coworkers have achieved exceptional sixfold high photocurrent using SnO2- and TiO2-integrated core-shell photoanodes in a DSPEC [78]. In a DSPEC system, the performance of photocatalytic water splitting or hydrogen generation is significantly impacted by charge recombination that takes place between the photoelectrode and the photosensitizer or photocatalyst. As a result, significant efforts are required to increase the performance of DSPECs and their global acceptance. Fig. 4 provides a comparative analysis between the bandgaps of several semiconductors and the redox potential in the water.

3 Working principle of DSPECs The electrochemical energy needed to split water into H2 and O2 molecules is generated by converting photons with energy more significant than the semiconductor bandgap [79]. The heart of this reaction is the formation of minority charge carriers in n-type and p-type semiconductors and the accompanying charge migration [80]. There are two types of

Fig. 4 A comparative analysis of bandgaps of various semiconductor materials [68].

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electrochemical processes that take place at the electrode surfaces: oxidation and reduction. The migratory minority charge carriers execute these reactions. One or more of the following occurs in a PEC: light absorption, creation of charges; separation of charges; transport of charges; and chemical reactions at electrode surfaces [81]. When illuminated, electrons are stimulated to the CB, causing hole formation in the valence band. Because of the inherent electric field in the depletion area, a fraction of these photogenerated charge carriers travel to the semiconductor/electrolyte interface to accomplish the necessary chemical process. The difference in potential between the Fermi level of the semiconductor and the water’s redox potential generates the electric field in the depletion area. In photoanode-based PEC systems, generated electrons travel via the external circuit to the CE, reducing water to produce hydrogen, thereby promoting the HER. However, valence band holes of the semiconductor holes in the valence band of semiconductors reach the semiconductor’s surface and oxidize water to generate molecular oxygen, thereby fueling the OER. 2H2 O $ 4H+ + 4e + O2 ": E 0 ¼ 1:23V ðoxidation at photoelectrodeÞ 

2H + 2e $ H2 ": E ¼ 0V ðreduction at photocathodeÞ +

0

(1) (2)

The remaining separated charge carriers proceed through a process called bulk recombination, in which they lose their energy thermally as phonons. This has the effect of lowering the photoelectrode’s overall activity [82]. A schematic representation of the working principle of a DSPEC is shown in Fig. 5. The research demonstrates that recombination of carriers and other overpotential losses can be reduced by utilizing hetero-atomic doping, tailorable nanostructures, and surface modification techniques [83,84]. However, present materials have not achieved the greatest STH conversion efficiency for an unaided photovoltaic (PEC) system. In addition, the water oxidation limits the performance of a PEC device due to its sluggish kinetics for OER and more significant energy requirement of 4 e- and 4 h+. Because of this, the total rate of PEC water splitting is controlled by the water oxidation process [85–88]. For an efficient DSPEC operation, the photoelectrode material must be capable of producing an effective photovoltage of 2.0 volts vs. reversible hydrogen electrode (RHE) [89]. As a result, selecting proper photoelectrode materials is critical in hydrogen production by PEC water splitting.

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Fig. 5 Schematic of working principle of a DSPEC. (Reprinted from Hamdani IR, Bhaskarwar AN. Recent progress in material selection and device designs for photoelectrochemical water-splitting. Renew Sust Energ Rev 2021;138:110503. https:// doi.org/10.1016/j.rser.2020.110503. with permission, Copyright 2021, Elsevier.)

4 Dye-sensitized photoanodes for water splitting cells Many different OER catalysts have been developed due to the relevance of the four-electron oxidation of water to oxygen in aqueous electrochemical systems. An array of colloidal and molecular catalysts have been investigated on similar electrode topologies since the first demonstration in a DSSC of visible light water splitting. Photoanodes made with different dyes and catalysts show exciting similarities in their electrochemical behavior. At first, it was hard to figure out how these two types of photoanodes could be similar. After being irradiated with simulated sunlight, the initial photocurrent densities ranged from 15–200 A/cm2 but quickly dropped to a fraction of their original value within a few seconds. Since much of the original photocurrent could be regained after briefly turning off the light, dye decomposition or desorption could be considered the primary cause of photocurrent degradation. As demonstrated by generator-collector studies, the faradaic oxygen evolution efficiency was nearly equal to unity despite the photocurrent’s transient character [90]. Further, the electrodes tested in non-aqueous iodide solutions delivered significantly higher current density and had very little photocurrent decay than those tested in aqueous solutions [91].

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Researchers conducted in-depth experiments on electron transfer processes concerning dye penetration, catalyst loading, adsorbing solvents, and input light intensity using electrodes made from sintered IrO2. Through photoinjection of electrons by dye molecules, holes spread over the surface and reach the IrO2 catalyst. Charge recombination and electron scavenging by Ir(IV) compete for the regeneration of reduced dye molecules. An electrochemical potential step approach and transient photovoltage measurements yielded cross-surface diffusion coefficient (Dapp), charge recombination (krecomb), and electron scavenging (kscav) rate constants. Notably, the solvent and circumstances employed to adsorb the dye greatly influenced the photocurrent for water splitting. Despite the high diffusion coefficient values, the electrodes were exposed to acidic, aqueous solvents, which resulted in limited photocurrent. In the first report of DSPEC, the chromophore and catalyst were chemically coupled before being loaded onto the semiconductor surface [92]. These features allow the chromophore/catalyst distance to be precisely controlled, as well as a separation between the catalyst and semiconductor surface to prevent electron-catalyst recombination processes. Nonetheless, complicated synthetic techniques are required for such chromophorecatalyst complex designs. For a DSPEC, the initial water splitting chromophore-catalyst assembly proved unsuitable: electron injection into TiO2’s CB occurred less than 5% of the time in the excited state of chromophore bridging ligand [93]. Several additional chromophore-catalyst combination configurations were also unsuccessful due to insufficient oxidizing power generated by the reduced chromophore to produce the RuV¼O form of the catalyst, a necessary intermediary for the creation of the first OdO bond [94,95]. The incorporation of carbene-based WOC into chromophore-catalyst assemblies allowed for the production of OdO bonds at the weakly oxidized RuIV¼O configuration of the catalyst, with added redox potential. Developing water splitting DSPECs employing a single-site WOC in the chromophore-catalyst combination was promising [97–99]. Since the discovery and introduction of [Ru(bda), (L)2] (bda: bipyridine-6,60 -dicarboxylate) into DSPEC assemblies by Sun and coworkers, significant improvements in DSPEC performance have been realized due to its low overpotential and rapid water oxidation, Fig. 6 [96,100–102]. For example, a Nafion overlayer was placed over a Ru(bpy)3-sensitized TiO2 thin film, which was then loaded with the catalyst, Fig. 7 [103]. However, a DSPEC breakthrough was obtained when a Ru(bpy)3 type of chromophore and a

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Fig. 6 Schematic illustration of a DSPEC TiO2-based photoanode incorporating Ru-based WOC and porphyrin chromophore [96].

Fig. 7 Schematic of a TiO2 photoanode-based DSPEC with RuP chromophore and complex 1+. (Adapted from Li L, Duan L, Xu Y, Gorlov M, Hagfeldt A, Sun L. A photoelectrochemical device for visible light driven water splitting by a molecular ruthenium catalyst assembled on dye-sensitized nanostructured TiO2. Chem Commun (Camb) 2010;46(39):7307-9. https://doi.org/10.1039/c0cc01828g, PMID 20686714, with permission from The Royal Society of Chemistry, copyright 2010.)

Ru-bda catalyst were loaded together on a mesoporous TiO2 thin film to produce photocurrent densities as high as 1.7 mA/cm2 and 14% IPCE at 450 nm, [104]. Numerous chromophore-catalyst combinations have been successfully coloaded onto DSPEC photoanodes using this approach [105–109].

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Youngblood and colleagues loaded chromophores and catalysts onto the semiconductor’s surface using a layer-by-layer strategy [110]. They developed the Ru(bpy)3 type of chromophores, which contained an anchoring phosphonated bipyridine ligand and a malonate-functionalized ligand for the colloidal IrO2.nH2O WOC nanoparticles. Meyer et al. [111] devised an analogous layer-by-layer technique for nanosized metal-oxide thin films based on prior Si and Au planar electrodes’ experiments [112,113]. Several DSPEC photoanode and photocathode designs employ this method to use phosphonate groups’ high affinity for valent cations like Zr(iv) [90,114–116]. In another report, a few nanometers thick film of an oxide (TiO2, Al2O3, etc.) is coated by atomic layer deposition (ALD) over a chromophore. Next, an anchoring group of metal-oxide type is used to load on water oxidation catalysts to this oxide layer. The prepared ALD oxide layer stabilizes and preserves the chromophore and facilitates the loading of the catalyst. The ALD layer-by-layer approach has seen considerable use in manufacturing DSPEC photoanodes [117–120]. Electropolymerization is another way that DSPEC photoanodes have been made. This method adds electropolymerizable groups (like vinyl groups) to the chromophore and the catalysts. During the electropolymerization process, these groups end up chemically linked [121,122]. In a different version of this method, the catalyst is just electropolymerized over a dye-sensitized photoelectrode to make a thin film. The catalyst molecules stay in the porous structure of the mesoporous thin-film electrode because the polymer is hard to dissolve in water [123]. Recently, hydrophobic linkages between extended alkyl chains have been used to form self-assembled bilayers (SABs) on an electrode surface to combine chromophores and catalysts [124]. These long alkyl chains serve as anchoring groups for chromophore and the water oxidation catalyst in this method, as depicted in Fig. 8. The outcome is an electrode that has been functionalized with both anchoring groups and extended alkyl chains. When the extended alkyl chains of the catalyst meet those of the chromophore, they form a SAB. The length of the alkyl chains determines the distance between different chromophores and catalysts, making this a simple method to use.

4.1 Photoanode materials In DSPECs, electrode materials serve a variety of essential roles. The majority of DSPEC advancements are attributable to the development of

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Fig. 8 Schematic of a self-assembled bi-layers (SABs) chromophore-catalyst assembly on a metal-oxide. (Reprinted with permission from Wang L, Polyansky DE, Concepcion JJ. Self-assembled bilayers as an anchoring strategy: catalysts, chromophores, and chromophore-catalyst assemblies. J Am Chem Soc 2019;141(20):8020-4. https://doi.org/ 10.1021/jacs.9b01044, PMID 31062973, Copyright 2019 American Chemical Society.)

Fig. 9 Various approaches for improving the performance of DSPEC’s photoanode.

semiconductors. As illustrated in Fig. 9, these techniques include doping cocatalysts, incorporating light-harvesting materials on semiconductors, employing composite materials in photoelectrodes, and modifying the surface morphology of semiconductors. In addition, they support chromophores and catalysts and may be used as a part of chromophore-catalyst

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integration schemes in many circumstances. The separation of charges and the collection and transportation of electrons also depend on electrode materials. For photoanodes used in DSPECs, mesoporous TiO2 thin films are the most reliable electrode materials. On the other hand, core-shell electrode materials have demonstrated their superiority in the recent decade. In 2013, DSPEC water splitting was demonstrated by using a revolving ring-disk electrode arrangement with a TiO2 core-shell photoelectrode on the disk and a conductive core (indium-doped tin oxide, ITO, and antimony oxide, ATO) for efficient electron collection and transport [125]. The chromophore-catalyst assembly was found to be fixed to the TiO2 thin film through the phosphonic groups on the chromophore. Oxygen created at the photoelectrode was identified and measured at the ring by introducing light from the bottom of the cell (Pt). Later, in a more traditional DSPEC arrangement, the same chromophore-catalyst combination was employed using a SnO2/TiO2 core-shell as the photoelectrode material, with similar results [126]. Using SnO2 instead of ITO as the core resulted in a fivefold enhancement in photocurrent density, reaching 1.97 mA/cm2 at a pH of 7. Additionally, Al2O3 or TiO2 overlayers were applied through ALD to preserve the cell’s anchoring groups, a perfect example of the multiple functions that electrode materials play in DSPECs. The utilization of [Ru(bda)(pic)2]-type WOC in conjunction with SnO2-TiO2 core-shell photoelectrode materials has led to substantial advancements in DSPECs [107,127–133]. Because of the difference in CB locations between TiO2 and SnO2, SnO2-TiO2 electrodes performed better than pure TiO2 electrodes in the first tests. Recombination of electrons injected into SnO2 should be significantly slowed because of the more significant positive CB. Meyer and colleagues found that oxidized chromophores might survive milliseconds on core-shell interfaces of SnO2-TiO2 core-shell systems [134]. Later, the same group identified a novel electronic state at the SnO2-TiO2 interface, which is slightly positive than both SnO2 and TiO2 states [135]. The effectiveness of these electrode materials in DSPECs and other applications demonstrates that discovering novel materials is not always necessary. Sometimes, inventive methods utilizing recognized resources can provide equivalent or even superior results.

5 Dye-sensitized photocathodes for water splitting cells The lack of suitable p-type semiconductor materials has impeded the advancement of photocathodes for DSSCs and DSPECs. Photocathodes

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made of DSPEC include semiconductors, chromophores, and catalysts, all of which are placed on a TCO glass substrate, as is the case with photoanodes. As a suitable wide-bandgap semiconductor (p-type) material, NiO has been a choice of researchers for photocathode material since its first report almost two decades ago [136]. However, NiO has been highlighted as one of its significant constraints due to its high trap density and limited hole mobility [137]. In order to passivate defect states and enhance NiO’s optical and electrical characteristics, researchers have turned to target atomic deposition (TAD) [138–140]. For instance, the addition of TAD of Al to DSSCs improves the VOC of NiO, increasing their performance by around three times [138]. In the case of DSPECs, surface -OH groups coupled with Ni vacancies in DSPECs operating in aqueous systems pose further problems. Because of this, proton-coupled charge transfer mechanisms in aqueous NiO photocathodes are degraded [141]. Sun and coworkers announced the first light-driven hydrogengenerating photocathode using a sensitized photocathode, as depicted in Fig. 10 [142]. An organic dye (triphenylamine) was attached to nanoshaped NiO using a cobaloxime molecular catalyst in solution. DSPEC was prepared using a similar photocathode and the cobaloxime catalyst attached to the NiO [143]. At a pH of 7, the cell achieved photogenerated current densities of 300 A/cm2 while maintaining an IPCE of 25% at 380 nm. In another report, Wu and his colleagues fabricated a dye-sensitized CE that

Fig. 10 Schematic of a first reported p-DSPEC and its corresponding transient current responses. (Adapted from Li L, Duan L, Wen F, Li C, Wang M, Hagfeldt A et al. Visible light driven hydrogen production from a photo-active cathode based on a molecular catalyst and organic dye-sensitized p-type nanostructured NiO. Chem Commun (Camb) 2012;48 (7):988-90. https://doi.org/10.1039/C2CC16101J, PMID 22143335, with permission from The Royal Society of Chemistry, copyright 2012.)

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was highly stable in acidic solutions [144]. There were two perylenemonoimide (PMI) acceptor groups, one on either side of the donor moiety, coupled to the donor moiety by oligo-3-hexylthiophene (OHT)conjugated π-linker groups (Fig. 11B). Thiophene linkers with hydrophobic hexyl groups shielded the anchors and NiO from acidic environments in which they were implanted, allowing the TPA donors to bind to NiO. The proton reduction catalyst was chosen as the cubane molybdenum sulfide cluster ([Mo3S4]4+) due to its superior stability in an acidic environment. Over 16 h of continuous operation (in 1.0 M HCL at pH 0), the cell produced more than 180 μA/cm2 of photocurrents with a faradaic efficiency of 49% for creating H2. Using ALD and a molecularly tailored perylene3,4-dicarboximide chromophore, Wasielewski and coworkers deposited a thick Al2O3 layer over a NiO film (PMI). The Al2O3 layer sheets protect the NiO from the aqueous solution while allowing extended charge separation lifetimes [145]. In a separate study, Meyar et al. utilized an ALD layer of Al2O3 over NiO film to link a Ru(bpy)3 chromophore to a Ni(L)2 proton reduction catalyst in the same manner as described for photoanodes.

Fig. 11 Different photocathode assemblies for H2 production. (Reprinted with permission from Click KA, Beauchamp DR, Huang Z, Chen W, Wu Y. Membrane-inspired Acidically stable dye-sensitized photocathode for solar fuel production. J Am Chem Soc 2016;138(4):1174-9. https://doi.org/10.1021/jacs.5b07723, PMID 26744766. Copyright 2016 American Chemical Society and Shan B, Brennaman MK, Troian-Gautier L, Liu Y, Nayak A, Klug CM et al. A silicon-based heterojunction integrated with a molecular excited state in a water-splitting tandem cell. J Am Chem Soc 2019;141(26):10390-8. https://doi.org/10.1021/jacs.9b04238, PMID 31244171. Copyright 2019 American Chemical Society.)

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Scientists have been searching for new materials to use in photocathodes because of the limitations of NiO as a semiconductor (p-type) material. To generate H2, Reisner and coworkers have employed the CuCrO2 material as a p-type semiconductor, which is delafossite-type material [146]. A phosphonated diketopyrrolopyrrole dye and a proton reduction catalyst Ni(L)2 were used to functionalize the semiconductor. When exposed to UV-filtered simulated solar irradiation, the CuCrO2 photocathode produced a photoinduced current of 15 μA/cm2 (at 0.0 V vs. RHE and pH 3) in a liquid electrolyte solution. After two hours of operation, the photocathode showed excellent stability and produced H2 with a turnover number of 126 for their Ni(L)2 catalyst. The CuCrO2-based system performed better than a NiO-based photocathode, although product production was constrained by poor dye adsorption and low catalyst loadings. A fivefold increase in loading was observed in a subsequent investigation using macropore structures based on inverted opal CuCrO2 [147]. In a recent report, Meyer et al. employed boron (B)-doped silicon as the p-type material. In their work, a thin Ti layer of 10 nm thickness over 18-μm-long Si nanowires was deposited via the physical vapor deposition technique. After that, using the ALD technique, a 3.0 nm layer of TiO2 was deposited onto the Si nanowires (Fig. 11A) [116]. The perylene-diimide (PDI) chromophores anchored to the p-type Si electrode were protected against photodegradation by this method. The Zr-bridged layer-by-layer method incorporated Ni(L)2 proton reduction catalysts. In the absence of any applied bias, the integrated photocathode could produce a photogenerated current density of roughly 1.0 mA/cm2 (vs. NHE). In contrast to proton reduction catalysts, CO2 reduction photocathodes have higher overpotentials, making them more difficult to design. Nonetheless, recent years have seen considerable progress on this front. As an example, Kumagai et al. developed a photocathode for reducing CO2 to CO by utilizing a Ru(ii)-Re(i) supramolecular combination on a NiO electrode [148]. While conducting trials in a DMF and triethanolamine complex in the ratio of 5:1 and an applied bias potential of 1.2 V vs. Ag/AgNO3, the photocathode performed 32 turns with an efficiency of 65% for CO during five hours of operations. The same Ru(ii)-Re(i) supramolecular complex was able to convert CO2 into CO with a faradaic efficiency of 68% on a CuGaO2 p-type semiconductor with an applied bias of 0.7 V vs. Ag/AgCl at ambient temperature [149]. Meyer and his colleagues developed an innovative photocathode approach using the binary P-N junction, a key component of the

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CO2 reduction process that produces formate more efficiently [150]. In order to absorb light and catalyze reactions, these photocathodes had a semiconductor p-n junction composed of nanowire arrays of GaN on silicon and surface-bound molecule assemblies. On average, the reduction efficiency of CO2 to formate was as high as 64% throughout the 20-h irradiation period, with photoinduced current densities as low as 1.1 mA/cm2.

5.1 Photocathode materials Since its first report as a photocathode in a DSSC, NiO has established itself as the promising wide-bandgap semiconductor (p-type) material for use in sensitized photocathodes [136]. As indicated before, this material’s fundamental constraints have been recognized as difficulties connected with higher trap densities and limited hole mobility. These two issues are particularly problematic for the material [137]. When operating DSPECs with aqueous solutions, additional problems might arise due to the locally generated electronic states concentrated on the surface –OH groups and related to vacancy sites for Ni. Therefore, proton-coupled charge transfer mechanisms impact the functionality of aqueous NiO photocathodes [141]. Alternative photocathode materials, including CuCrO2 [146,147] and CuGaO2 [149], have demonstrated greater potential than NiO, although their performance still lags behind the photoanode. For the p-type material, Meyer and coworkers employed boron-doped Si, which was protected by a 10-nm Ti layer and a 3.0-nm TiO2 layer, to bind chromophores together [116]. The NiL2 catalyst for proton reduction to H2 supplied a photocurrent density of roughly 1.0 mA/cm2 to the incorporated photocathode for H2 production under zero applied bias. Finding new materials that have the appropriate qualities might be intricate. Thus, an option that involves creatively combining existing components could be fruitful. For instance, using the “binary p-n junction” approach, Meyer and his colleagues created photocathodes that combine semiconductor p-n junctions with supramolecular assemblies to absorb light while also catalyzing reactions. During 20 h of irradiation, the photocathodes convert CO2 to formate at steady current densities of 1.1 mA/cm2 while maintaining faradaic efficiencies of 64%.

6 Tandem DSPECs for water splitting Research efforts have been devoted to using oxide semiconductors like TiO2, ZnO WO3, BiVO4 Fe2O3, and SnO2 to attain the highest achievable

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efficiencies of a tandem DSPEC while maintaining a lower cost and making the materials easier to produce. The most straightforward configuration for a tandem PEC comprises a photoanode containing an oxide semiconductor indicated already, which is then coupled to a photocathode and an electrolyte solution. Several studies have demonstrated such p/n-PEC tandem cell configurations, but these systems generally yield very low STH efficiencies. This is due to the photoanode’s huge bandgap energies and the photocathode’s poor charge transport properties. Theoretically, a tandem system comprised only of dye-sensitized photoelectrodes should provide limitless opportunities to alter each interface’s characteristics, except for synthetic or structural limits inherent to chemistry. In order to make solar fuels commercially viable, tandem DSPEC systems must be pursued since they give a path to achieving STH efficiencies close to their theoretical limits. Nevertheless, only a few DSPEC tandem systems have been documented thus far. DSPEC photoelectrodes are challenging to fabricate and study, requiring competence in material science, chemical synthesis, electrochemistry, reaction kinetics, and spectroscopy. In addition, the primary technological obstacles must be overcome to build stable electrode interfaces over the long term while simultaneously encouraging rapid charge carrier transfer and preventing unproductive charge carrier recombination. Fig. 12 illustrates a simplified view of a tandem DSPEC system. Fan and coworkers presented the first report on the n/p-DSPEC tandem configuration with a TiO2-coated photoanode and a NiO photocathode

Fig. 12 Schematic illustration of a tandem DSPEC device [151].

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containing coadsorbed 1, Ru(pdc)(pic)3 as WOC, and an H2 catalyst that included 1 and Co, respectively [65]. The tandem device reached a photogenerated current density of 12 μA/cm2 when the two photoelectrodes were coupled together with no bias applied. Sun and colleagues reported a lightdriven organic dye-sensitized tandem DSPEC. The photoanode contained an 8-μm-thin TiO2 layer, a triphenylamine dye, and a Ru-based WOC [143]. For proton reduction, a molecular Co-based catalyst was used in conjunction with a 1-μm-thin NiO film and a triphenylamine-based organic dye. Without bias, the cell could achieve water splitting photocurrent densities of 70 μA/cm2 when exposed to 100 mW/cm2 input power. A significant challenge is to improve the STH performance of the p/n-DSPECs by realizing higher photocurrent activity from dye-sensitized p-type photocathodes. The limitations of the p-type interface can be overcome by a tandem DSPEC composed of distinct n-type single-junction DSPEC and DSSC components. Moore and coworkers demonstrated one of the first examples of this tandem system [99]. They employed a SnO2 DSPEC coupled with a TiO2 DSSC to photochemically change hydroquinone (QH2) to hydrogen using a SnO2 DSPEC sensitized by a porphyrin dye. Sherman and colleagues presented a new technique for water splitting using tandem DSPEC devices [122]. The solar spectrum could be used more effectively thanks to the combination of water splitting DSPECs and DSCs, which reduce the requirement for an external bias, Fig. 13. A DSPEC with a SnO2/TiO2 core-shell electrode in conjunction with RuP2+ 2 chromophore and a Ru(bda) WOC comprised the entire tandem cell system. Electropolymerization was used to attach the chromophore and catalyst to the core-shell electrode. Series connections were made between a photoelectrode and a dark Pt cathode using either the N719 dye and I/I 3 mediator or the Co(bpy)3 mediator and D35 dye. Photocurrents of 40 μA/cm2 were measured in the tandem cell with an efficiency of 0.06% under simulated solar light. Recently, Wang and coworkers announced an enhanced system with an excellent STH efficiency of 1.5% [152]. This advancement was made possible by the continued evolution of tandem DSPEC systems. In this work, BnDT-FTAZ [153] and ITIC [154] polymers were employed as donor and acceptor layers, which showed significant absorbance in the wavelength range of 500 to 750 nm in a layered configuration with a SnO2@TiO2 core-shell photoanode that was sensitized by Ru(II) (bpy)2 dye. The tandem cell’s DSPEC component was completed by a Pt dark cathode, and it demonstrated a current density of 1 mA/cm2 over the course of an hour under

Fig. 13 A schematic illustration of a wired DSPEC system in series with a DSSC [122].

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simulated solar illumination of 1 sun condition. The tandem dye-sensitized water splitting efficiency has dramatically improved since the first tandem photocells were reported, demonstrating the promise of this type of photocell. A suitable replacement for semiconductor absorber-based systems is expected to be developed with further advancement. As a whole, lowering the cost of hydrogen generation is challenging to make new technologies more realistic. The quest for optimum PE materials, their corresponding device configurations, and the feasible integration with high-performance PV panels are all critical.

7 Conclusion and outlook Dye-sensitized photoelectrodes have been extensively studied as direct cathodes or anodes for creating solar fuels throughout the past few decades. Because of the rapid charge separation in the picosecond range and the slow return response in these systems, initial results were encouraging. However, the energy storage efficiency is much below the 10% mark in most cases. A molecular engineering technique is needed to design and synthesize novel dyes for solar fuel applications since the traditional design of new dyes for electricity-producing regenerative solar cells is not ideal. Factors that need to be considered include matching dyes’ energy levels, semiconductor characteristics, and their redox processes for fuel generation. In addition, creating appropriate interposed molecular or heterogeneous catalysts should also increase the electron transport rate between dye and electroactive species in the electrolyte. Chromophores usually employed in DSPEC research exhibit injection efficiency of less than 50% at pH levels of up to 8.0, where broad bandgap semiconductors such as TiO2 are commonly studied. Catalytic and chromophore stability is also reduced when the pH increases. Significant gains in DSPEC performance improvement and stability are possible at relatively low pH (1) when injection efficiencies are near 100%. For efficient H2 creation and release, most DSPECs require an applied bias. Combining DSPECs with DSSCs eliminates the need for a biasing potential and opens an avenue for CO2 reduction photocathodes, which generally function at higher potentials relative to proton reduction photocathodes. Additionally, in tandem devices, there is a great deal of potential for exploring the present high-performance hydrogen and oxygen evolution catalysts that can give a promising performance in tandem devices and innovative as well as robust catalysts. Tandem DSPECs have the highest possibility of achieving STH efficiencies that might eventually offer a viable

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and alternative method of delivering green fuel to society instead of carbonbased fossil fuels. Despite the inability of existing DSPEC systems to compete with established state-of-the-art semiconductor photovoltaic-based technologies, researchers are optimistic that these flexible and tunable interfaces may offer the tools to bring realizable efficiencies closer to theoretical limits. While significant challenges need to be addressed, including prolonged stability, robust alternative catalysts, and charge carrier recombination, tandem devices have shown improved STH efficiencies approaching 10% in only a few studies thus far. Solar fuel production could become more efficient with these systems if they continue to advance. They could also have other possible applications, such as the recent demonstration of lignin depolymerization using these systems [155]. There is great potential for DSPECs to play a crucial role in the energy future. However, the process of commercializing the device, in reality, may take a long time, and further efforts need to be given to the cause.

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CHAPTER 10

Photobiological hydrogen production: Introduction and fundamental concept Nandini Mukherjeea and Rohit Srivastavab a

Department of Chemistry, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India b

1 Introduction Tesla once said “If you want to find the secrets of the universe, think in terms of energy, frequency, and vibration.” Now even without being driven by the curiosity for revealing the secrets of universe, human civilization is compelled to think in terms of and more importantly in search of energy. The energy crisis is looming over us especially since the beginning of 21st century. Due to the overreliance and overuse of fossil fuels, we are close to exhausting the natural reserve for the same [1]. This has made researchers and policymakers to desperately strive for finding alternative energy sources that are renewable, cost-effective, energy efficient, and environmentally sustainable. Hydrogen (H2) in this context has attracted worldwide attention as a secondary energy carrier due to its high energy density and zero carbon emission. Energy content of hydrogen is 122 kJ/g, i.e., 2.7–3 times greater than the hydrocarbon fuels [2]. In its ionic form, hydrogen is not only the most abundant element in the universe, but also lightweight and non-toxic. The end product of hydrogen combustion is only water, making it a clean energy source [3]. According to the International Renewable Energy Agency (IRENA)’s roadmap analysis, by 2050 hydrogen will share 6% of total energy consumption [4]. Now each year, more than 120 million tons of hydrogen is produced, which equals 14.4 exajoules (EJ). Hydrogen market is experiencing 6%–10% growth every year and is estimated to reach over 191.80 billion USD in 2024 [5]. Still, more than 90% of this hydrogen is produced from coal and natural gas leading to further depletion of the fossil fuel reserve and higher carbon emission. Therefore, the world is now striving toward hydrogen production from renewable sources, which is yet to be

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Copyright © 2023 Elsevier Inc. All rights reserved.

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Fig. 1 Summary of different pathways for hydrogen generation and its multifaceted use. Adapted from Energy Technologies Area@Berkeley National Laboratory (Berkeley lab).

significant. India has recently launched its hydrogen mission to produce five million tons of green hydrogen per annum by 2030, while EU is gearing toward the production of 10 million tons of hydrogen from renewable energy by that time [6]. Fig. 1 summarizes the various pathways for hydrogen generation and possible uses of the hydrogen. The commercial aspects of hydrogen fuel suffer major challenges due to (i) lack of technical feasibility of large-scale renewable H2 production, (ii) economic incompetence compared to conventional fossil fuels, and (iii) storage and transport problems. Environmental sustainability and cost-effectiveness of hydrogen production can be achieved by technical and scientific breakthrough in the processing by involving renewable energy/feedstock like electrolysis of water, biomass gasification and pyrolysis, fermentation, biophotolysis, solar energy-driven thermochemical reactions, etc. In this context, photobiological hydrogen production involving microorganisms and solar energy can address some of these issues for future scale-up and commercialization of renewable hydrogen [7]. Photobiohydrogen (PBH) generation was first reported in a filamentous cyanobacterium by Jackson and Ellms in 1896 [8]. However, the revitalization of photobiological hydrogen production research happened in 1942 when Gaffron and Rubin reported that the green microalga Scenedesmus obliquus produces H2 in the dark at low rates post a dark anaerobic adaptation period, while the rate enhances when the microorganism is illuminated [9,10] Fig. 2. However, PBH has received more focused attention only in the past two decades as it involved mild biochemical reaction and no carbon footprint [7].

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Fig. 2 Simplified schematic diagram of dark fermentation. Adapted from Hallenbeck PC, Ghosh D, Trends Biotechnol 27 (2009) 287–297.

Photobiological H2 production is defined as any microbial process that needs light energy to extract electron from an electron-donating substrate (e.g., water in oxygenic photosynthesis; organic acids in anoxygenic photosynthesis), light energy-harvesting pigments (e.g., chlorophyll, bacteriochlorophyll), electron carriers/redox mediators to protons (e.g., ferredoxin, NADP, etc.), ATP, and catalysts (e.g., hydrogenase, nitrogenase enzymes) that combines electrons and protons to generate H2 gas [11]. Microorganisms that produce hydrogen with the assistance of direct/indirect solar energy include the microalgae, cyanobacteria, purple non-sulfur bacteria, and green sulfur or non-sulfur bacteria that involve different enzymes, and follow significantly different biochemical pathways to produce H2. While most microorganisms (e.g., green algae) use photosynthetically active radiation (PAR) of 400–700 nm, oxygenic photosynthetic bacteria can use 700–950 nm [12]. PBH generation is achieved in the following ways: (i) direct biophotolysis of water shown by microalgae and cyanobacteria in oxygenic condition (chlorophyll, 400–700 nm), (ii) indirect biophotolysis shown by cyanobacteria and microalgae under anoxic condition, and (iii) photofermentation [13]. This chapter will discuss the fundamental concepts of these processes, the enzymes involved, the factors affecting PBH generation, and insights into research approaches to overcome the limiting issues. Finally, the challenges for scale-up and commercialization of the PBH generation will also be critically reviewed.

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2 Fundamental concepts of photobiological hydrogen generation Hydrogen can be generated using raw materials like fossil fuels, biomass, and water [14]. Use of biomass and/or water as the proton/electron source would lead to net zero carbon emission and hence considered as sustainable approaches. Photobiological production of hydrogen by oxygenic and anoxic photosynthetic microorganisms offers the promise of generating clean carbon-free renewable energy using abundant natural resources, such as sunlight, water, and recyclable waste biomass [13]. Different microorganisms produce biohydrogen employing different mechanisms under different conditions. For instance, in the presence of oxygen, microalgae use light energy to produce hydrogen from water (direct biophotolysis), while cyanobacteria and microalgae under anoxic condition use photosynthetically generated carbohydrates as the energy source to produce hydrogen from water (indirect biophotolysis) [15]. Photobiological hydrogen generation can be classified into three categories: (i) direct biophotolysis, (ii) indirect biophotolysis, and (iii) photofermentation. Each of these processes is discussed below.

2.1 Direct biophotolysis Production of hydrogen from water by the use of solar energy is called biophotolysis (Table 1; Fig. 3). It results in the production of both molecular hydrogen (H2) and oxygen (O2). This approach may use either isolated cellular components or microalgae culture [17]. Direct biophotolysis is similar to the photosynthetic process, which is known to take place in algae and plants that contain light-harvesting pigments. It involves both photosystems I and II (PSI/700 and PSII/680) to produce strong oxidants capable of oxidizing water into oxygen, proton, and electrons or reducing equivalents [18]. The electrons, mediated via electron transport chain, then reduce ferredoxin with the help of ferredoxin oxidoreductase enzyme (FNR) (Fig. 3). Post this, the hydrogenase enzyme catalyzes the reduction of protons using the reduced ferredoxin as electron source, finally leading to hydrogen evolution. The microalgae Chlamydomonas reinhardtii is the best studied microorganism that shows direct biophotolysis process [19]. The reaction of direct biophotolysis can be described as: 2H2 O + light energy ! 2H2 + O2

(1)

The biochemical process is elaborated in the following section and also depicted in Fig. 3. The first step of photosynthesis in microalgae is the solar

Table 1 Summary of the general features, advantages, and disadvantages of the photobiological H2 generation processes [16]. Process

General features

Advantages

Disadvantages

Direct biophotolysis

Basic principle: In the presence of direct light source (like sun), photosynthetic pigmentcontaining microorganisms generate H2 from H2O involving a sequence of e transfer processes Microorganisms Green algae, cyanobacteria Reactions involved: 2H2O+Ferredoxin(oxidized) +light!4H++Ferredoxin(reduced)(4e) +O2 4H++Ferredoxin(reduced)(4e)$2H2+ Ferredoxin(oxidized) Basic principle: In separate cells/time period, usually sulfur-deficient (inactive PSII) microorganisms generate hydrogen from complex carbohydrates or pyruvate (which has formed utilizing water and solar energy) Microorganisms: Cyanobacteria and blue-green algae under anoxic condition Reactions involved: 6H2O+6CO2+light!C6H12O6+6O2 C6H12O6+2H2O!4H2+2CH3COOH +2CO2 2CH3COOH+4H2O+light!8H2+4CO2 N2+8H++Ferredoxin(reduced)(8e) +16ATP!2NH3+H2+Ferredoxin (oxidized)+16ADP+16Pi 8H++8e+16ATP!4H2+16ADP+16Pi

• H2 production directly from

• Oxygen accumulation inhibits

Indirect biophotolysis

H2O using abundant solar energy • simple cultivation process • concomitant CO2 removal from environment

• H2 production from H2O and sunlight • H2 production separate from O2 production (spatial or temporal or spatiotemporal separation) • Simultaneous N2 fixation for microbial growth

H2 production

• High-intensity light inhibits/ decreases energy efficiency of the H2 production • H2/O2 mixture formation leading to impure H2, and combustibility issue in case of large-scale production

• Consumption of H2 by uptake hydrogenase

• High energy demand for H2 production in case of nitrogen-fixing cyanobacteria • Presence of nitrogen substrate limits H2 evolution

Continued

Table 1 Summaryof the general features, advantages, and disadvantages of the photobiological H2 generation processes—cont’d Process

General features

Advantages

Disadvantages

Photofermentation

Basic principle: In the presence of light, microorganisms convert organic biomass into simpler organic or inorganic compounds Microorganisms: Photosynthetic bacteria and green algae Reactions involved: 12H2O+6CO2!C6H12O6+6O2 C6H12O6+6H2O!6CO2+12H2 (In green algae) CH3COOH+2H2O+light!4H2+2CO2 N2+8H++8e+16ATP!2NH3+H2+16ADP +16Pi (In photosynthetic bacteria) Basic principle: In the absence of light, microorganisms convert organic biomass into simpler organic or inorganic compounds (Fig. 2) Microorganisms: Fermentative bacteria Reactions involved: C6H12O6+6H2O!12H2+6CO2 Pyruvate+CoA!acetyl-CoA+formate Pyruvate+CoA+2Ferredoxin(ox)!AcetylCoA+CO2+2Ferredoxin(red)(2e)

• Photosynthetic bacteria

• Relatively lower light

Dark fermentation

absorb wider spectrum of solar energy during this process • Use various organic substrates from small organic acids to dark-fermentation effluent of waste matter • Recycling of waste biomass and bioremediation

• Formation of various useful • • • •

metabolites as by-products Ability to use various substrates (waste matter) Light-independent process Sustained H2 production Bioremediation

conversion efficiency

• Inhomogeneity of light distribution affects the process

• Problem of water pollution due to fermentation broth

• Requires C- and N-based nutrients for continuous growth • CO2 impurity in H2 gas • Low biohydrogen yield in the absence of light • Water pollution may arise from fermentation broth

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Fig. 3 Direct and indirect biophotolysis processes. The black bar denotes the lipid bilayer of the thylakoid membrane of photosynthetic microorganisms. The electron transfer steps are indicated by blue dashed arrows. Black dashed arrows denote multiple possible reactions. Abbreviations used: PS¼photosystem; PTOX¼plastid terminal oxidase; PQ¼plastoquinone; NDH¼NAD(P)H dehydrogenase; Cyt b6f¼cytochrome b6f complex; PC¼plastocyanin; Fd¼ferredoxin; FNR¼ferredoxin NAD(P)+ reductase; H2ase¼hydrogenase; FDP¼flavodiiron protein; ATPase¼ATP synthase. Adapted from Oh YK, Raj SM, Jung GY, Park S, Biohydrogen (2013) 45–65 and Akkerman MJ, Rocha JMS, Reith JH, Wijffels RH, Photobiological hydrogen production: photochemical efficiency and bioreactor design. In: Reith JH, Wijffels RH, Barten H (Eds.), Bio-methane and biohydrogen. Dutch Biological Hydrogen Foundation, The Netherlands (2003) 124–145 and Peltier G, Tolleter D, Billon E, Cournac L, Photosynth Res 106 (2010) 19–31.

energy capture by light-harvesting complex proteins (LHC proteins) [20]. This excitation energy then facilitates photolysis of H2O into H+, e, and O2 (Eq. 2). The photons absorbed by LHC-II (light-harvesting complex consisting of chlorophyll a, chlorophyll b, and carotenoids that are embedded within a protein framework in the thylakoid membranes having predominant interaction with PSII) then drives the linear flow of e along the electron transport chain in the following manner: PSII-LHCII supercomplex to plastoquinone (PQ) to cytochrome b6f (Cyt) to plastocyanin (PC) to PSI-LHCI supercomplex to ferredoxin (FD) [21–23]. The last step is mediated by ferredoxin-NADP+ oxidoreductase (FNR) enzyme. Finally, the e is used to reduce NADP+ to NADPH, which together with ATP is used in the subsequent biochemical processes to reduce CO2 to sugar, starch, and other biomolecules, i.e., biomass. While the e is being transported and used for CO2 fixation, the H+ released into the thylakoid lumen

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post photosynthetic H2O splitting creates an electrochemical H+ gradient across the thylakoid membrane. This induces ATP synthase-mediated ATP formation and transfer of excess H+ back to the stroma. Under normal aerobic photosynthetic condition, CO2 is reduced as carbohydrates, which are used for mitochondrial oxidative phosphorylation (respiration) and cell growth. Under anaerobic condition, however, the reduced ferredoxin acts as an e donor to hydrogenase enzyme, which drives the reversible recombination of stromal H+ and e to yield molecular H2 (Eq. 3). The stepwise reactions involved in the direct photolysis process can be expressed as: 2H2 O + light ! 4H+ + 4e + O2 

4H + 4e $ 2H2 +

(2) (3)

Direct biophotolytic H2 generation offers high theoretical photon conversion efficiency. The major technical bottleneck it suffers from is the high oxygen sensitivity of the hydrogenase enzyme. The hydrogenase enzyme is only active under hypoxic or anaerobic conditions, and it gets inactivated by the slow accumulation of O2 produced as a by-product of PSII’s function [24]. The photosynthetically produced O2 suppresses all the processes related to H2 generation including relevant gene’s expression, mRNA stability, and final enzymatic catalysis. While under hypoxic/anoxic condition, it provides 95% pure H2 gas stream, the H2 production under natural circumstances is only transient and of less economic value [25]. If the oxygen can be removed by inert gas purging of the reaction mixture or directing the consumption of the photosynthetically generated O2 by cell’s own mitochondrial respiration process leading to hypoxic condition, the efficiency of H2 production can increase. To date, the highest efficiency for biohydrogen production is reported for microalgae [26–28] and this is partly because of the high efficiency of the microalgal [FeFe] hydrogenase, about 100-fold higher than other types of hydrogenases (turnover rate: up to 104 H2 molecules/s) [29–31]. Some of the green microalgae that have been identified to possess [FeFe] hydrogenase are Chlamydomonas reinhardtii, Scenedesmus obliquus, Chlorococcum littorale, Chlorella fusca, and Platymonas subcordiformis [32]. Among these, Chlamydomonas reinhardtii is known to be the best studied species with respect to biophotolytic H2 production processes. The main advantage of direct biophotolysis is that there is no need for adding substrate as nutrients. Water serves as the primary electron donor

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required for the production of hydrogen gas. Solar energy and atmospheric CO2 are the basic inputs needed to grow the microalgae that contain hydrogenase enzymes. The optimization of direct biophotolysis process for pure and efficient H2 production involves controlling various factors, which will be discussed in Section 4.

2.2 Indirect biophotolysis Indirect biophotolysis is possible both in microalgae and in cyanobacteria. It involves two stages that are naturally separated either temporally or spatially. The first stage is the storage of the solar energy in carbohydrates in the form of chemical energy, a process that generates O2 as a by-product (Eq. 4). The second stage is the use of the carbohydrates as substrates for H2 production (Eqs. 5 and 6) (Fig. 3). The term “indirect” arises because here instead of H2O being the direct source of H+/e and sunlight being the direct source of energy, carbohydrates/biomass serve as the H+/e source, and the chemical energy of carbohydrate is used. The general reactions for indirect biophotolysis: First stage: 6H2 O + 6CO2 + light ! C6 H12 O6 + 6O2

(4)

Second stage: C6 H12 O6 + 2H2 O ! 4H2 + 2CH3 COOH + 2CO2

(5)

2CH3 COOH + 4H2 O + light ! 8H2 + 4CO2

(6)

In the second stage, i.e., H2 production process, pyruvate ferredoxin oxidoreductase enzyme leads to CO2 evolution by decarboxylation of pyruvate (generated after oxidation of the carbohydrate) to acetyl Co-A and H2 evolution via reduction of ferredoxin mediated by NADPH. In case of cyanobacteria, both hydrogenase and nitrogenase enzymes are involved in the H2 evolution process, while the microalgae depend entirely on hydrogenase for H2 production [33]. The advantage of nitrogenase enzyme over hydrogenase is the unidirectionality of the former. However, the nitrogenase reaction is less energy efficient as it requires 4 ATP per H2 production, i.e., less than half of the expected efficiency of that of hydrogenase-based indirect photolysis [34]. But both the enzymes are oxygen sensitive and require separation of the oxygen evolution and hydrogen production reactions. This is achieved spatially in case of filamentous

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cyanobacteria, which carry out the photosynthetic process/O2 evolution reactions in the vegetative cells and N2 fixation/H2 evolution processes in specialized cells called heterocysts (PSI functional but no PSII activity). The low ratio of heterocysts to vegetative (1:10) cells, however, limits the level of hydrogen production and needs further genetic engineering optimization. Though cyanobacteria are mostly filamentous and nitrogenfixing (e.g., genus Anabaena, Nostoc, Calothrix, etc.), they can also be non-nitrogen-fixing (e.g., Synechococcus, Synechocystis, etc.). In case of the microalgae, O2 evolution and H2 evolution are separated temporally, i.e., photosynthetic reduction of CO2 to carbohydrates/lipids happens during the day and H2 generation in the night. Thus, inhibition of hydrogenase enzyme by oxygen accumulation is suppressed during the night. The reaction for H2 evolution along with N2 fixation: N2 + 8e + 8H+ + 16ATP ! 2NH3 + H2 + 16ADP + 16Pi

(7)

The reaction for H2 evolution without N2 fixation: 8H+ + 8e + 16ATP ! 4H2 + 16ADP + 16Pi

(8)

2.3 Photofermentation Photofermentation by a diverse group of photosynthetic bacteria can occur under anaerobic condition. The most promising microorganism showing this process are the purple non-sulfur (PNS) bacteria. They use sunlight as the energy source and small organic molecules (usually organic acids like succinate, malate, lactate, etc.) as the main substrates to produce H2 and CO2. H2 generation by PNS bacteria, facilitated by the solar energy and small organic molecules as carbon source, involves the activity of nitrogenase enzyme under N2-deficient condition. PNS bacteria show high efficiency of H2 generation because of their (i) high substrate conversion efficiency and utilization of various substrates (organic acids, waste biomass), (ii) ability to function under anaerobic condition avoiding the oxygen sensitivity issue of the hydrogenase and nitrogenase enzymes, and (iii) ability to absorb and use both visible and NIR regions of the solar radiation (400–950 nm) [13]. In photofermentation process, the microorganism or vascular plants use the absorbed solar energy to produce ATP, which facilitates photocatabolic oxidative degradation of the organic substrate to extract high energy electron (Fig. 4). The es then get transferred to the plastoquinone (PQ) pool

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Fig. 4 Simplified schematic diagram of photofermentation. Adapted from Hallenbeck PC, Ghosh D, Trends Biotechnol 27 (2009) 287–297.

located at PSI and PSII (PSI assimilates the solar energy to generate the es). This step also involves the NADPH-plastoquinone oxidoreductase enzyme. The es, mediated by hydrogenase/nitrogenase enzyme, are then used to reduce ferredoxin [35]. The reduced ferredoxin finally donates the electrons to the protons to produce H2. Under high or moderate N2 partial pressure, N2 is converted to ammonium ion during this process. The organic substrates act as source of electron which through reverse flow is supplied to ferredoxin for its reduction [36]. The following Eq. (9) depicts the H2 generation by photofermentation process [37]. 16ATP + N2 + 16H2 O + 10H+ + 8e ! 16ADP + 2NH4 + + 16pi + H2 (9)

For this pathway, instead of water, the green algae gain electrons from heterotrophic fermentation and the catabolic reaction of the endogenous substrate. Electrons are separated from organic substrate products by photocatabolism. Light energy and oxidative carbon metabolism also play a role in the extraction of electrons. The electrons generated from the endogenous substrates then undergo the plastoquinone (PQ) pool, which is situated in Photosystem I (PSI) and Photosystem II (PSII). The transfer of electrons to the PQ pool is associated with nicotinamide adenine dinucleotide phosphate (NADPH)plastoquinone oxidoreductase (PQOR) in the chloroplast of different types of vascular plants [38]. In this process, PSI assimilates light energy and generates

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electrons. The redox potential of this electron can be elevated through the transfer of these electrons to the corresponding level of hydrogenase and ferredoxin (FDX) [35]. The dark and anoxic environment induces hydrogenase and helps to produce a high yield of biohydrogen [16]. Theoretically, stoichiometric conversion of organic acid substrates to hydrogen is possible in photofermentation process. But researchers have found that the light conversion efficiency and production volume are quite low, which according to some recent reports can be improved by combining sequential photo-dark fermentation [39–41]. A major advantage of photofermentation process over biophotolysis process is the use of organic waste matter, which certainly holds promise for renewable hydrogen generation with environmental sustainability. Another prominent class of photosynthetic bacteria called purple sulfur bacteria (PSB), anaerobic or microaerophillic in nature, also show photofermentation process. Instead of organic acids, they use reduced sulfur compounds like hydrogen sulfide as e donor. They can produce H2 and granules of elemental sulfur in the photofermentation process [42].

3 Enzymes involved in photobiohydrogen generation The enzymes involved in molecular hydrogen production by microorganisms are hydrogenase and nitrogenase. These enzymes are metalloproteins, i.e., they contain metal (transition metals) at their active site necessary for the catalytic conversion of proton to hydrogen. Hydrogenase enzymes can be further classified as [NiFe]-uptake hydrogenase, [NiFe]-bidirectional hydrogenase, [NiFeSe] hydrogenase, [FeFe] hydrogenase, and [Fe]-only hydrogenase (formerly known as Fe-S free or metal-free hydrogenase) [43]. [Fe]-only hydrogenase uses H2 for reduction of CO2 to form CH4 and it does not produce H2. [NiFe] and [FeFe] hydrogenases are the most commonly occurring hydrogenases in photosynthetic bacteria that are capable of hydrogen production. Hydrogenase enzymes are, however, oxygen sensitive and get reversibly or irreversibly inhibited in the presence of oxygen. Nitrogenase enzymes are mainly involved in N2 fixation, a process that is accompanied by simultaneous H2 evolution. There are three major types of nitrogenase enzymes known: Mo-based, V-based, and Fe-based; each produces different amount of hydrogen. Nitrogenase enzymes too are oxygen sensitive, and in contrast to hydrogenase enzymes, they require high amount of ATP consumption to produce H2. Also, the formation of NH3 or NH+4

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limits the stoichiometric H+ to H2 conversion. A major goal of researchers working in the field of biohydrogen production is driving the nitrogenase enzyme to yield higher amount of H2 in N2-limiting condition [44]. The following section elaborates the role of hydrogenase and nitrogenase enzymes for the H2 production.

3.1 Hydrogenase Hydrogenase (H2ase) enzymes are found in the stroma of cyanobacteria, thylakoids of microalgae, and periplasm of photosynthetic bacteria. These enzymes are either involved in hydrogen uptake or evolution process as depicted in the following reversible reaction: 2H+ + e $ H2

(10)

As mentioned earlier, there are various types of hydrogenase enzymes known, including hup-encoded [NiFe]-uptake hydrogenase; hoxencoded [NiFe]-bidirectional hydrogenase; [NiFeSe] hydrogenases where Ni-coordinated cysteine residues of [NiFe] hydrogenases are replaced by selenocysteine; [FeFe] hydrogenases; and [Fe]-only hydrogenases. Most well-known and studied among these are [NiFe] hydrogenases (found in bacteria and cyanobacteria) and [FeFe] hydrogenases (found in green microalgae and obligate anaerobic fermentative bacteria). [Fe]-only hydrogenase enzyme, found in methanogenic bacteria/archea, contains a mononuclear Fe-based active site but lacks the Fe-S cluster found in [FeFe] hydrogenases. [FeFe] hydrogenase enzymes are located in the chloroplast of green microalgae and catalyze the proton (H+) to hydrogen (H2) conversion under hypoxic photosynthetic conditions. These enzymes could be monomeric or dimeric with an average molecular weight of 50 kDa. They contain a 4Fe-4S cubane subcluster covalently attached to a 2Fe-2S subcluster together being recognized as a 6Fe-6S cluster or H-cluster (Fig. 5A). This H-cluster in green microalgae is responsible for 100-fold higher activity in proton reduction compared to other hydrogenases (cyanobacterial [NiFe] hydrogenase). The presence of oxygen, however, irreversibly inhibits these enzymes inhibiting the H2 production under oxygenic photosynthesis conditions in case of green microalgae. The strong bond between Fe atoms of the active site and O2 leads to this irreversible inhibition. These enzymes catalyze reversible reduction of H+ to H2 only in anoxic or hypoxic conditions where ferredoxin acts as an e carrier [13].

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Fig. 5 Active site structure of (A) [FeFe] hydrogenase and (B) [NiFe] hydrogenase, where X may be S, O, OOH, OH.

[NiFe]-uptake hydrogenases are present exclusively in photosynthetic nitrogen-fixing bacteria and cyanobacteria that also contain nitrogenase capable of producing H2 [22]. The [NiFe]-uptake hydrogenase or unidirectional hydrogenase uses this H2 as a source of high-energy e and the reaction catalyzed by them is: (11) H2 ! 2H+ + e [NiFe]-uptake enzymes contain two units: hupS and hupL, the small and large subunits, respectively. The small subunit hupS (30 kDa) comprising three 4Fe-4S cluster transfers electrons from hupL to electron-acceptor molecules involved in electron transport chain. The large subunit hupL (60 kDa) contains the Ni-Fe bimetallic active site where Ni is coordinated to four cysteine residues, out of which two act as bridging ligand to the Fe(CN)2CO unit (Fig. 5B). [NiFe]-bidirectional hydrogenases are found in cyanobacteria, and as indicated by their name, they are capable of either producing or consuming hydrogen according to the cellular redox condition. H2 production is found to sustain in dark, while subsequent irradiation leads to a short bust of H2 evolution followed by its uptake. These enzymes are sensitive to O2, CO, cell’s bioenergetics, metabolic flux, and light exposure limiting the rates and yield of H2 production. Nevertheless, with respect to large-scale H2 production with any commercial viability, it is worth to explore the genetic or substrate or environmental manipulation of the [NiFe]-bidirectional and [FeFe] hydrogenase enzymes among all. Since turnover for H2 generation is higher for [FeFe] hydrogenase, while [NiFe] hydrogenases suffer less from oxygen sensitivity, research direction could be aimed at increasing the oxygen tolerance of the former and manipulating the N2 partial pressure or light exposure of the latter to enhance H2 yield.

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3.1.1 Classification of hydrogenase enzyme Based on the transition metal ions and their composition, it is divided into three types: (1) [NiFe] hydrogenases, (2) [FeFe] hydrogenase, and (3) [Fe] hydrogenase. The flow diagram of classification of hydrogenases enzyme is shown in Fig. 6. 3.1.2 [NiFe] Hydrogenase Based on the molecular phylogeny, [NiFe] hydrogenases are described in four groups that are depicted in Table 2. Kaur-Ghumaan et al. [46] described the possible reaction mechanism of [NiFe] hydrogenase for the water splitting that is shown in Fig. 7. There are three states in the catalytic cycle of [NiFe] hydrogenase, namely, (1) Ni-SIa, (2) Ni-C, and (3) Ni-R. In order to come to an equilibrium state, these three states can be interconverted among each other by donating and accepting one electron or proton. In the first step of the cycle, the dihydrogen is attached to the Ni center. After that, the hydride species are formed due to the occurrence of either oxidative addition or base-assisted heterolytic

Fig. 6 Classification of hydrogenase enzyme. Table 2 Description of the [NiFe] hydrogenase groups [45]. S.No.

Group

Type of hydrogenase

1. 2. 3.

Group 1 Group 2 Group 3

4. 5.

Group 4 Group 5

Membrane-bound H2 uptake hydrogenase Sensory or regulatory hydrogenase and uptake hydrogenase F420-reducing, NAD(P)+-reducing, methyl viologen reducing, and bidirectional NAD(P)+-reducing hydrogenases Energy-converting hydrogenases The new actinobacterial hydrogenases

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Fig. 7 Catalytic cycle of [NiFe] hydrogenase for the hydrogen production [46].

cleavage of the H2 molecules. It is also proposed that the Ni center acts as a base because of the terminal cysteine attached to it and it is also responsible for the uptake of the proton. 3.1.3 [FeFe] Hydrogenases [FeFe] hydrogenase enzymes are mainly originated from anaerobic prokaryotes such as green algae, fungi, and ciliates and in some anaerobic eukaryotes. [FeFe] hydrogenases are mainly extracted from Desulfovibrio desulfuricans, Clostridium pasteurianum, and Chlamydomonas reinhardtii. The mostly researched [FeFe] hydrogenase are from green algae, Desulfovibrio desulfuricans and Clostridia pasteurianum. [FeFe] hydrogenase mostly exists in single subunit form and consists of an active metal center, i.e., H-cluster. Some of the [FeFe] hydrogenases are made up of multiple subunits and form heterotetramers. It can be used to catalyze the reduction reaction of the proton into hydrogen. Brown et al. developed a photocatalytic H2 production system [47]. In this system, the [FeFe] hydrogenase from Clostridium acetobutylicum enzymes encapsulate in CdS nanorods. Under the visible light, the turnover frequency is 380–900 H2 s1 and 20% quantum efficiency was obtained at 405 nm (Fig. 8). 3.1.4 [Fe] Hydrogenases [Fe] hydrogenase enzyme mainly exists in methanogenic archaea. The [Fe] enzyme participates in only one reaction step, i.e., the conversion of carbon dioxide along with H2 to methane. The methanogens are categorized into

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Fig. 8 Diagrammatic representation of [FeFe] hydrogenase from Clostridium acetobutylicum enzymes encapsulate in CdS nanorods [47].

five [NiFe] hydrogenases: (1) the membrane-associated energy-converting hydrogenase, (2) the cytoplasmic heterodisulfide reductase-associated hydrogenase, (3) the membrane-associated methanophenazine-reducing hydrogenase, and (4) the cytoplasmic coenzyme F420-reducing hydrogenase; and (5) one [Fe] hydrogenase. [Fe] hydrogenase enzymes function under specific condition, i.e., with low-nickel concentration. The [Fe] hydrogenase takes part as a catalyst in the reversible reduction of methenyltetrahydromethanopterin (methenyl-H4MPT+) and converts into a proton and methylene-H4MPT along with H2. The whole reaction occurs due to the transferring of hydride species to the proR position of the C14 a carbon of methylene-H4 MPT. Thus, [Fe] hydrogenase is also termed as “methylenetetrahydromethanopter in (methylene-H4MPT) dehydrogenase.” The [Fe] hydrogenase has a 38-kDa molecular weight, and it is a homodimeric protein with a specific ˚ . It contains two peripheral N-units, and it is condimension of 905040 A nected to a central globular C-terminal unit in a linear form that is shown in Fig. 9. The peripheral N-unit also is known as dinucleotide-binding domain, and it belongs to the Rossmann fold family. The C-terminal unit forms an inter-subunit helix bundle along with four α-helices [48].

3.2 Nitrogenase Nitrogenase enzymes are found in cyanobacteria, PNS bacteria, and green sulfur bacteria, i.e., in prokaryotic organisms [22]. The main function of these enzymes is to catalyze N2 fixation to produce NH3. The process, however, leads to the formation of H2 as a by-product according to the following irreversible reaction:

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Fig. 9 Overview of [FeFe]-hydrogenase diversity, structure, and mechanism of cluster activation and final maturation [50].

N2 + 8e + 8H+ + 16ATP ! H2 + 2NH3 + 16 ðADP + PiÞ

(12)

In the absence of N2, the reaction is as follows: 2e + 2H+ + 4ATP ! H2 + 4 ðADP + PiÞ

(13)

As evident from Eqs. (12) and (13), this process involves expenditure of large amount of energy in the form of ATP, precisely hydrolysis of 4 mol ATP per 1 mol H2 generation. N2ase enzyme is actually a two-protein complex: (a) dinitrogenase (commonly called as FeMo protein) and (b) dinitrogenase reductase (Fe protein) (Fig. 10). The dinitrogenase reductase is a homodimeric Fe-S protein of molecular weight of 65 kDa and transfers electrons from external e donor to the dinitrogenase complex. The dinitrogenase complex or MoFe protein, a α2β2 heterotetramer with a molecular weight of 230 kDa, catalyzes N2 to NH3 reduction in a stepwise manner. The MoFe cofactor is indicated in Fig. 6. NH3 production is associated with concomitant evolution of H2. Although Mo-nitrogenase is the most widely distributed among N2-fixing prokaryotes, the catalytic site of nitrogenase enzymes has been found to contain Fe or V too. Thus, based on the metal cofactor, there

Fig. 10 FeMo cofactor situated at dinitrogenase complex in the Mo-based nitrogenase.

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are three types of nitrogenases and they produce different stoichiometry of H2 production as described in the following reactions: Mo  nitrogenase : N2 + 8H+ + 8e ! 2NH3 + H2

(14)

Fe  nitrogenase : N2 + 21H+ + 21e ! 2NH3 + 7:5H2

(15)



V  nitrogenase : N2 + 12H + 12e ! 2NH3 + 3H2 +

(16)

The presence of N2 in the medium lowers the H2 yield by default. It had been predicted and observed experimentally that in the absence of N2, nitrogenase enzymes can exclusively produce H2 and continues to be an active area of research. However, the major problem with nitrogenase-based H2 production is its energy-demand lowering the energy efficiency of the overall H2 evolution process. The advantage of N2ase lies in its irreversible H2 generation, resulting in high-pressure H2 formation suitable for industrial-scale production. This process is also free from the issue of explosive H2/O2 mixture formation observed in photosynthetic bacteria involving hydrogenase enzyme [13].

4 Modulation of factors affecting photobiological hydrogen production Photobiological hydrogen production by microorganisms is affected by various factors, viz., oxygen sensitivity of hydrogenase, nitrogenase enzymes; expression of uptake hydrogenase in N2-fixing organisms; size of the light-harvesting antenna of pigments like chlorophyll or bacteriochlorophyll; alternate e consumption pathways; nutrients used; culture conditions; light intensity; O2/CO2 concentration, etc. Some of these factors can be modulated by genetic and/or metabolic engineering approaches to enhance overall PBH production efficiency (Table 3). The following section gives an overview of the factors that affect H2 evolution and insights to how they may be tuned to improve H2 production yield/rate.

4.1 Role of genetic modification (a) Reducing the antenna size of light-harvesting pigments for photosynthesis: Growth of photosynthetic microorganisms (green microalgae, cyanobacteria, photosynthetic bacteria) requires bright sunlight. As evolutionary survival strategies, light-harvesting antenna size in these organisms are large to ensure better absorption of sunlight even in the areas where it is limited. In the context of commercial

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Table 3 Limitations of and strategies to improve photobiological hydrogen production. Limiting issue

Potential strategies

Oxygen sensitivity of enzymes involved in H2 production

• Develop oxygen-tolerant enzyme through genetic engineering of existing strains

• Metabolic deprivation of proteins involved in PSII where O2 evolution reaction occurs

• Attain temporal separation by growing the

Hydrogen consumption

Low energy conversion efficiency

• • • • • •

Competitive e consumption pathway Nutrients requirement for fermentative bacteria

• •

microorganism (algae) in open pond during day and harvest at night anaerobically Purge inert gas Inactivate uptake hydrogenase enzyme Induce nitrogen fixation under N2-limting conditions Reduce the pigment antenna size by genetic modification Culture microalgae and photosynthetic bacteria together Introduce integrated systems, multiple pathways of H2 production Direct es (reducing equivalents) to hydrogenase/nitrogenase enzyme Use waste biomass that contains complex organic compound instead of procuring pure compounds

H2 generation, which involves high-density microbe culture and bright sunlight, larger antenna size leads to overabsorption of the light with subsequent loss of the excess solar energy as heat or fluorescence. Thus, the photosynthetic solar energy conversion becomes inefficient. This problem can be overcome by genetic modification of the photosynthetic apparatus to have reduced antenna size. Truncated antenna sizes in high-density green algal culture and PNS bacteria in the presence of bright sunlight have indeed been found to enhance H2 productivity by many folds in comparison with wild-type microalgae or PNS bacteria of the same species under same conditions. Pigment concentration reduction also enhances the penetration of light inside the culture and diminishes the solar energy wastage [49–52]. (b) Reducing oxygen sensitivity issue of hydrogenase and nitrogenase enzymes: Wildtype cyanobacteria and green microalgae produce oxygen from water involving PSII, thereby inhibiting hydrogenase and nitrogenase activities. Oxygen sensitivity is an inherent problem for photobio-H2 production

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especially under aerobic condition or open-system culture. This issue may be tackled by various ways such as lowering the photosynthesis/respiration ratio to establish hypoxic or anoxic condition, inactivating the D1 reaction center protein of PSII to slow down oxygen generation and induce prolonged H2 production [49,53]. (c) Inactivation of uptake hydrogenase enzyme: As discussed in Section 3, uptake hydrogenases, expressed in cyanobacteria and photosynthetic bacteria, actually consume and recycle the hydrogen produced during nitrogen fixation. Thus, expression of these enzymes reduces the overall molecular hydrogen’s yield. Various attempts have been made to inactivate or eliminate the uptake hydrogenase. For instance, in PNS bacteria, the deletion of hup gene responsible for hup uptake hydrogenase expression has been found to enhance H2 production. Site-directed mutagenesis of the uptake hydrogenase employing a suicide vector has also led to higher H2 evolution [49,54–56]. (d) Pathways that are competitive to H2 production, viz., nitrate assimilation, respiratory electron transport chain, and carbon fixation through Calvin-Benson cycle etc. are another limiting factor for high H2 yield. For example, polyhydroxybutyrate (PHB) synthesis pathway competes with H2 generation in PNS bacteria. Deletion of PHB synthase gene has been found to enhance the H2 production capacity of the microorganisms [13,57]. (e) Another genetic modification approach is to decrease the sulfate permease activity that leads to sulfur deprivation in chloroplast causing limited biosynthesis of D1 protein of PSII and creating hypoxic (anoxic in case of sealed culture) environment suitable for H2 production. The major problem still faced in the genetic engineering approach for improved H2 production is the lack of knowledge on the genome sequence of the microorganisms, lack of transformation technologies especially for microalgae. Chlamydomonas reinhardtii is an exception to this because its genetic system is thoroughly characterized and most genetic engineering approach has been studied using this species as a representative one [58–61].

4.2 Role of metabolic modulation While genetic modification if achieved would be better for more efficient, less labor/time-intensive process for photobiohydrogen generation, it has yet to be successfully realized at the commercial scale and more exploratory research is still to be carried out on different microorganisms. Metabolic

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modulation is an alternative/additional approach to control the H2 metabolism and other competitive pathways. (a) Sulfur deprivation has shown great promise in this regard. S-deprivation reduces the biosynthesis of PSII reaction center protein D1 that consists of S-containing amino acid methionine. Reduction of D1 protein concentration leads to photodamage of PSII and lowering of water-splitting ability of the system with concomitant lowering of O2 production. Rate of O2 production when dropped below that of mitochondrial respiration, hypoxic or anoxic condition is achieved. This enhances the expression and activity of hydrogenase. (b) O2 removal: O2 build-up in the culture medium inhibits the activity of hydrogen-producing enzymes necessitating the removal of O2 for large-scale H2-producing system. There are multiple approaches for the same such as use of inert gas like argon; addition of oxygen scavenging compounds like sodium sulfite; use of reducing agents like ascorbate, glucose oxidase, etc. [62]. (c) A comparatively new strategy for metabolic modulation is magnesium deprivation. This leads to decreased photosynthesis but enhanced respiration allowing sustained H2 generation [23].

4.3 Choice of substrates/nutrients For growth and H2 production, the cultured microorganisms need essential nutrients in optimum quantity. Metal ions like Fe2+ are crucial for [FeFe] hydrogenase or [NiFe] hydrogenases. Fe and Mo are also necessary for the respective metal cofactors of Fe or Mo(Fe) nitrogenases; V for the V-only nitrogenase. Fe also serves as the cofactor for many proteins/enzymes involved in the electron transport chain for photosynthesis as well as respiration, viz., cytochromes, ferredoxin, etc. Other metal ions like Mg2+ and Mn2+ also must be present in optimum amount for chlorophyll’s functions. The C/N ratio in the nutrients is another important factor as the nitrogenase activity strongly depends on the presence of N-containing substrates or ammonia. As described in the previous section, deficiency of nitrogen leads to better hydrogen production where nitrogenase enzyme is concerned. Nitrogenase enzyme is also sensitive to ammonium ions. Efficiency of nitrogenase enzyme toward the forward reaction of H2 production increases when concentration of ammonia salts present in the culture medium is less (Eq. 12). Thus, along with C/N ratio, the source of N is also important [13].

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4.4 Co-culture and immobilization Co-cultivation of two different algae strains or two photosynthetic bacteria or algae-bacteria together has also shown promise in enhancing the overall H2 production efficiency. One of the best combinations is the co-culture of microalgae and aerobic bacteria. This combination leads to efficient removal of O2 through enhanced bacterial respiration from the culture medium creating algal anaerobiosis condition. This in turn mitigates the oxygen sensitivity issue of the hydrogenase enzymes [23]. This approach further creates environment for bacterial fermentation and CO2 formation, which acts as C-source for algal growth [62]. Immobilization of the microorganisms in several non-toxic matrices (e.g., in calcium alginate matrix) allows protection from mechanical stress and could be a useful strategy for scale-up [63].

4.5 Integrated systems As discussed previously, different photobiological hydrogen production processes have their own advantages and limitations (Table 1). An approach to take advantage of each process into our stride is to design integrated systems so that the overall energy expenditure and substrate requirement decrease, wider range of solar spectrum is utilized, yield of H2 production increases, sustained evolution of H2 can be achieved, and the process is coupled with waste matter treatment. There are different approaches to create such integrated systems, the major two being (a) combining photo- and dark fermentation and (b) coupling two photobiological processes that absorb different wavelengths of the solar radiation. (a) Dark fermentation is mainly observed in anaerobic organisms, which are readily and widely available in waste matter like anaerobic compost, sewage sludge, and garden soils. The organic compounds present in these can act as substrates for dark fermentative hydrogen production. Integrating non-photosynthetic dark fermentative bacteria with photofermentative ones has shown promise in overall hydrogen generation efficiency [13,64–67]. In dark fermentation process, usually fermentation of sugar (e.g., glucose) yields H2, CO2, and small organic acids according to Eq. (17). The biomass resulted due to the dark fermentation serves as the growth medium for the photosynthetic bacteria that carry out photofermentation process (Fig. 7). Thus, the organic acids in the biomass are now used by the photosynthetic bacteria as substrates for photofermentation process to evolve hydrogen in the presence of light

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(Eq. 18). Integrating dark and photofermentation should theoretically yield 12 mol H2 from 1 mol of hexose involving small organic acids as the intermediates and connecting factor between the two processes. The reactions for the integrated system are described below: Dark fermentation : C6 H12 O6 + 2H2 O ! 2CH3 COOH + 2CO2 + 4H2 (17) Photofermentation : 2CH3 COOH + 4H2 O + light ! 4CO2 + 8H2 (18) In the dark fermentation stage, anaerobic fermentative bacteria consume and metabolize carbohydrates for their growth. Carbohydrate also acts as the electron source for proton to reduce the latter to H2 and itself get oxidized to CO2 and small organic acids. Complete conversion of carbohydrate to CO2 does not occur in the absence of light in this stage. 1 mol of hexose yields 4 mol of H2 theoretically in this process. The organic acids are secreted in the growth medium and act as substrates for the photosynthetic bacteria, which by using solar energy degrade the acids to CO2 and produce H2/N2 in anaerobic nitrogen-fixing conditions. This stage can yield 8 mol of H2 per 1 mol hexose [13]. Integrating the two processes diminishes the overall energy requirement of photosynthetic bacteria and enhances substrate to H2 conversion. Several researchers have reported improved H2 generation by this integrated system using waste matter like sewage sludge, food processing, or agricultural wastewater. The advantage of integrating dark fermentation in photobiological H2 production relies on the (i) simple reactor design, (ii) availability of anaerobic fermentative microbes and organic substrates in waste matter, and (iii) high rate and yield of H2 production. Since circumventing the oxygen sensitivity issue of the hydrogenase and nitrogenase enzymes still seems to be a distant goal, exploring the integrated system does offer immediate practical solution of PBH generation (Fig. 11). (b) Two photosynthetic hydrogen-producing microbial systems can also be combined in order to get better efficiency in H2 generation. One of the most promising combinations in this regard is designing photobioreactor with green microalgae/cyanobacteria and photosynthetic bacteria. Microalgae absorbs visible light (400–700 nm, 45% energy of solar radiation reaching the earth), while anoxic photosynthetic bacteria absorb near infrared light (700–950 nm, 25% energy of solar spectrum) of the solar radiation [12]. Thus, utilization of wider range of solar energy spectrum enhances the overall light conversion efficiency for photobiological H2 generation.

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Fig. 11 Simplified schematic of a two-component integrated biological system (dark and photofermentation) for H2 production. Adapted from Eroglu E, Melis A, Bioresour Technol102 (2011) 8403–8413.

Fig. 12 Simplified schematic of a three-component integrated biological system (photosynthetic green algae, photofermentative bacteria, and dark fermentative bacteria) for H2 production. Adapted from Eroglu E, Melis A, Int J Hydrog Energy 41 (2016) 12772–12798.

There is also possibility of 3-component system by combining dark fermentative bacteria with the two photosynthetic organisms leading to costeffective organic substrate generation for H2 evolution (Fig. 8). However, these processes are yet to be successfully performed in commercial scale (Fig. 12).

4.6 Photobioreactor design Photobioreactor design is a crucial aspect for attaining efficient H2 generation, especially with respect to large-scale production. The performance of a photobioreactor whether made up of individual system or integrated systems depends on several factors that can be classified as follows:

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(a) physicochemical parameters including pH, temperature, light intensity, total dissolved oxygen, CO2, shear stress due to agitation, source of carbon and nitrogen, C/N ratio; (b) physical parameters such as light source and penetration, geometry of the reactor, area-to-volume ratio (higher the better), temperature control system, materials used for photobioreactor design, gas exchange, mixing/agitation system, etc. Excellent comprehensive reviews are available on various aspects of photobioreactor design and control of multiple parameters to improve H2 generation performance. The readers are directed to [48,68].

5 Challenges and future prospects Hydrogen as a clean, renewable energy source is considered to be the “energy source for 21st century” by many of its advocates. However, there are challenges in almost every aspect of hydrogen generation, including the capital investment, production cost, sustained long-term production, storage, distribution, transport, safety, social acceptance, and overall efficiency, making it uncompetitive to fossil fuels, wind turbine, or photovoltaicsgenerated electricity. The advantage lies mainly in the theoretically high energy density and the lower environmental impact with respect to global warming. The most popular industrial method for H2 production till date is natural gas steam reforming [22]. In comparison with thermochemical, photoelectrochemical, or electrolytic production of H2, photobiological hydrogen generation seems a more distant goal in terms of large-scale or even small-scale commercial production. PBH generation is considered to be one of the most challenging areas of biotechnological research. However, introduction of in silico modeling and synthetic biology that incorporates changing the genomics, proteomics, and metabolomics for more efficient and tolerant microalga strain or engineered photosynthetic bacteria or cyanobacteria is expected to enhance their H2 production capacity [69]. Another important area that needs more comprehensive and recent case studies (laboratory scale and pilot scale) is the life cycle assessment (LCA), which quantitatively evaluates the use of resources and potential impact of any product on the environment that includes global warming, eutrophication, acidification, and human health hazards, etc. This evaluation process considers the full life cycle of the product starting from acquisition of raw materials to production, use, disposal, and/or recycling [22]. Based on many LCA analysis, hydrogen economy shows great potential in the aspect of environmental impacts. But the technological and economic barrier is

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limiting to the progress of industrial H2 production R&D. In contrast to chemical hydrogen generation, biohydrogen production has more challenges. According to El-Dalatony et al. solving the technological problem may cost 1.42 million USD [70]. This is counterproductive to the feature of higher energy density of hydrogen (1 kg H2 is equivalent to 3.79 kg gasoline) [71] since to compete with gasoline price (2.50 USD per GJ), H2 production cost should be 0.30 USD per kg [72,73]. In case of photobio-H2 production, the bioreactor design and microorganism system determine the primary cost for PBH. It has been observed that open-pond microalgal culture is cost-effective compared to custom-built bioreactor system [74]. Gholkar et al. recently conducted a study of a plant in India that cultivated microalgae in an open pond at the rate of 12,790 kg/h and produced 1239 kg of H2 per hour [75]. Estimated total capital investment was found to be 144.6 million USD, 11% of which was used to procure the gasifier, 76% of the total operating cost (7692 USD per year) was attributed to the microalgae cultivation, and rest was spent on utilities. Considering market price of H2 of 10 USD per kg H2 and a payback period of >3 years, the bio-H2 generation was deemed economically feasible. Many of such technoeconomic analyses do not take into account of the handling and storage expenditure. In addition, such analyses are often highly optimistic and disregard the environmental fluctuation factors like day light, temperature, humidity, weather, etc. A more thorough review and analysis of these aspects are needed to obtain a holistic view of the current landscape of photobiological H2 production. On an average, biological hydrogen production cost was found to be approximately fourfolds the cost of hydrogen from natural gas [76]. Review of current literature indicated that hydrogen production cost is estimated to be 10–20 USD per GJ, which is many folds higher than that of gasoline (0.33 USD per GJ) [77]. Future of photobiological hydrogen production thus depends on comprehensive assessment and mitigating the limitations in operation; cost of raw materials; non-uniformity of weather, light energy; requirement of land; better control of genetics and metabolomics of the microorganisms; co-culture; stability; biomass mixing, recycling of biomass, etc. [78–82].

6 Conclusion Out of the three photobiological hydrogen evolution techniques discussed in this chapter, each has its own promises and disadvantages in terms of feasibility for large-scale implementation. The fermentation processes offer

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better environmental sustainability since it involves recycling of biomass. But they suffer from low sunlight to H2 conversion efficiency and low yield. Biophotolysis, on the other hand, is less energy-intensive with respect to photoelectrochemical processes but has low experimental H2 yield (typically Mg2+>Cu2+>NH4+>K+ [34].

7.1 Genetic engineering of microorganisms to improve their hydrogen production capacity Various microbes, in their wild-type conformation itself, are capable of producing H2 in significant amount, as illustrated in Table 3. However, the production capacity is still limited to laboratory-scale studies and is not sufficient enough to produce biohydrogen at commercial scale [10]. The key obstacle in the path of adequate amount of hydrogen production, sustainably, is the O2 sensitivity of the hydrogen-producing enzymes (hydrogenase and nitrogenase). To overcome this limitation, different genetic engineering techniques have been carried out, some of which have been highlighted in Table 4. Among other microorganisms, Chlamydomonas reinhardtii has been utilized as a model organisms for studying different genetic modifications that might enhance the H2-producing capability. This microorganism could be grown in both dark and light conditions; where the various mutant strains could be generated that might survive in sulfur deprived conditions, to produce H2 by anaerobic fermentation [41].

Table 3 Hydrogen-producing wild-type microorganisms. Microorganism

Mode of operation

References

Calothrix sp. 336/3 Chlamydomonas reinhardtii Stm6 Scenedesmus obliquus Chlorella vulgaris Spirulina platensis Chlamydomonas MGA 161 Anabaena siamensis TISIR 8012 Clostridium pasteurianum (MTCC116)

Anaerobic fermentation Photofermentation Photofermentation Dark fermentation Anaerobic fermentation Dark fermentation Anaerobic fermentation Dark fermentation

[35] [35] [12] [12] [36] [12] [37] [38]

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Table 4 Metabolically and genetically engineered microorganisms with increased hydrogen-producing capability. Microorganism

Genetic modification

References

Chlamydomonas reinhardtii, D1 protein Chlamydomonas reinhardtii (miRNA1166.1) Chlamydomonas reinhardtii, miRNA-D1, psbA Chlamydomonas reinhardtii, Stm6 Chlamydomonas reinhardtii CC-124

Photogenic manipulation of an artificial microRNA Up-regulated expression of internal micro-RNAs under sulfur deprivation treatment Regulation of miRNA targeting psbA

[12]

Knockdown of IFR1 protein

[41]

Modification to enable growth under sulfur-deprived conditions to carry out anaerobic fermentation Knockdown of OEE2 gene

[35]

Reduced size of chlorophyll

[12]

Modification to enable growth under sulfur-deprived conditions to carry out dark fermentation Mutation of hydrogenase enzyme (Δhup Lmutant) to carry out photofermentation Modification to enable growth under nitrogen-deprived conditions to carry out anaerobic fermentation Modification to enable higher H2 production under anaerobic conditions Genetic engineering to produce DT transgenic green algal strain that produces H2 by photo-biocatalysis Modification of strictly anaerobic strain to carry out anoxygenic fermentation Mutation of hydrogenase enzyme (Δhup Wmutant) to carry out anoxygenic fermentation Modification to enable growth under immobilized conditions to carry out anoxygenic fermentation Modification to enable growth under nitrogen-deprived conditions to carry out anaerobic fermentation

[10]

Chlamydomonas reinhardtii Chlamydomonas reinhardtii Chlamydomonas reinhardtii Anabaena sp. PCC 7120 Anabaena sp.

Auxenoenochlorella protothecoides, IMM627 Chlorella spp. Hyda, CshydAc.DT Clostridium butyricum CWBI10 Nostoc PCC 7120,

Synechocystis sp. PCC 6803 Synechocystis strain M55, ndhB

[39]

[40]

[10]

[10] [42]

[10]

[28]

[12] [12]

[12]

[10]

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8 Bioreactors for commercial biohydrogen production H2 production at commercial scale can only become attainable if H2 can be generated efficiently at low costs. Industrial-level production of biohydrogen from microbial biomass can be carried out by culturing microorganisms under heavily monitored conditions, which include various parameters like pH, temperature, pressure, CO2 levels, and others as discussed above [28]. For this purpose, a photobioreactor (PBR) is necessary. The key factors that influence the H2 production rate and the quantity of H2 produced are the mode of operation and the design of the bioreactors. The latter of which further depends on various factors like agitation rate, supply of light energy, deepness of the tank, and others [43]. Generally, open ponds are employed for microalgal cultivation at commercial grade. However, H2 being a gas the same process cannot be used, as this does not provide adequate level of control of the above-mentioned factors [44]. For allowing improved process control, and for microalgae cultivation at large scale, different types of closed PBRs have been devised [45]. The source of the light that penetrates into the PBRs is either sunlight (in outdoor cultivation conditions) or other artificial sources (in indoor cultivation conditions) [46]. Other parameters of the PBRs are devised based on the characteristics of the microbial strain used [10]. The performance of such a bioreactor depends on different parameters like agitation rate, exchange of gases, penetration of light into the bulk of the media, temperature, and media-to-inoculum volume ratio [12]. Several types of bioreactors are employed for biohydrogen production (Figs. 6 and 7). These include flat-plate PBRs, tubular PBRs, vertical PBRs, anaerobic baffle reactors (ABRs), continuous stirred tank reactors (CSTRs), internal circulation anaerobic reactors (ICARs), open-air bioreactor systems, and up-flow anaerobic sludge bed reactors (UASBRs) [10,47]. The features of these PBRs are highlighted in Table 5. The flat-plate PBRs have been devised with an external control system that is able to track various parameters like optical density, temperature, pH, along with the yield of H2 [48]. The tubular PBRs contain long clear tubes that can be washed by taking them out. Inside these PBRs, the movement of the microbial biomass is controlled by air bridge devices, while an air system is utilized for the maintenance of CO2 levels within [12]. On the other hand, the vertical PBRs possess a water jacket covering and externally circulating cold water to control the temperature of the culture media inside the tank. Such tanks are aerated from the bottom [49]. The ABRs make the cascading utilization of organic matter to attain high H2 production due to biophase

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Fig. 6 Schematic representation of an ICAR [47].

Fig. 7 Diagrammatic representation of various PBR types for industrial biohydrogen production. (A) Flat-plate PBR, (B) Vertical or Column PBR, (C) Fence Tubular PBR, (D) Helical Tubular PBR, (E) CSTR, and (F) Open-air system [12,28].

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Table 5 Advantages and disadvantages of different PBRs. PBRs or culture systems

Advantages

Disadvantages

References

Flat-plate PBRs

Relatively cost efficient; large surface area for illumination; high H2 production rate; easy cleaning; good light penetration; low power demands

[48]

Tubular PBRs

Relatively costefficient; large surface area for illumination; high H2 production rate; best suited for outdoor culture conditions; easy control of all the parameters Low power demands; easy control of all the parameters; higher rate of transfer of mass; ensures proper mixing; provides lower shear stress; offers increased efficiency of photosynthesis Require lesser energy; offers higher H2 production capacities Ensures good mixing; high H2 production rate; easy control of all the parameters; large surface area for illumination; easy cleanliness and maintenance; computerized controls

Scaling up is difficult; complicated temperature control process; biomass from the media sticks to the reactor walls; applies hydrodynamic stress to some algal strains Biomass from the media sticks to the reactor walls and grows there; needs extensive land space; allows photoinhibition

Costly; small surface area for illumination; needs sophisticated materials for construction

[49]

Complicated maintenance

[50]

Issues with scale-up; used only for laboratory scale

[51]

Vertical or Column PBRs

ABR

CSTR

[12]

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Table 5 Advantages and disadvantages of different PBRs—cont’d PBRs or culture systems

Open-air Reaction Systems

Advantages

Disadvantages

References

Cost efficient; easy cleanliness and maintenance; require lesser energy inputs

Reduced control over culture variables; offers inadequate mixing and light as well as CO2 usage; offers difficulties in culturing algal species for longer durations; only a single culture can be utilized at a go; has issues with rapidly growing strains

[28]

partition [50]. However, the CSTRs are mostly used as dark fermentation biogas reactors, and are generally restricted to laboratory purposes for H2 production [51]. ICARs, on the other hand, enhance H2 production by monitoring the amount of volatile fatty acids, media alkalinity, and COD indicators [52]. The open-air systems are cost-efficient and also need lower energy inputs, but provide lowered control over culture conditions. However, the UASBRs check the pH of the influent media to maintain the system neutrality for carrying out anaerobic fermentation to accomplish stable and H2 efficient production [53]. H2 production may also be increased by combining PBRs with membraneaerated biofilm reactors (MABRs), where the H+ from the effluent media of the PBRs get attached to the membrane of the MABRs [47]. Various types of other such hybrid reactor systems may be used, depending on the reaction conditions. Recently, a novel baffle-type fiber bundle biofilm H2 reactor had been constructed to provide an enhanced rate of H2 production. It consisted of a photosynthetic biofilm joined directly to the diffuse fiber surface, and provided increased transmittance of light through it. A microfilm-translucent plate PBR has also been designed. It allows hydrogenation with a larger specific surface area containing engraved microchannels on the high light guide carrier surface [54]. Immobilization can serve as an alternative method for H2 production in a sustainable manner, keeping in mind some parameters like bead stability,

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Fig. 8 Schematic representation of a biomimetic leaf-like device for H2 production. [56].

bead diameter, and immobilization method in accordance with PBR [55]. In 2015, Das et al., developed an artificial entirely biomimetic leaf-like device for H2 production by direct photon-induced lysis of water due to diverting of photosynthetic pathways in immobilized microalgae resulting from anoxygenic conditions (Fig. 8). This device provides a higher yield of H2, as compared to other batch PBRs (Fig. 9A), due to its continuous nature (Fig. 9B), without replacing the microalgal culture [56]. Fig. 10 shows the working and setup of the device where the C. reinhardtii cells are cultured in suspension media in the presence of O2 and then harvested by centrifugation. The harvested cells are then mixed together with 5% sodium alginate and decanted uniformly over the synthetic fabric attached

Fig. 9 Diagrammatic comparison of working of (A) a Batch PBR with that of (B) a biomimetic leaf-like device with fabric-hydrogel-immobilized C. reinhardtii cells. [56].

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Fig. 10 Schematic representation of the setup of a biomimetic leaf-like device for H2 production. [56].

to a glass platform and spread uniformly over the fabric with the help of a sterile glass rod. 2% CaCl2 solution is used for polymerizing the alginate films. Researchers have also explored a multistage bioreactor system comprising four separate reactors operating sequentially (Fig. 11). The system has been incorporated with a microbial electrolysis cell for converting the organic acids, produced by DF, into H2 in a process that does not need light [19].

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Fig. 11 Schematic representation of a multistage hydrogen production unit. [19].

This integrated unit has shown a significant increase in the hydrogen yield and productivity rate for making high compact producers of energy for a future H2 economy. Advanced technologies and models of microalgal kinetics associated with different factors like hydrodynamics, light intensity, and mass transfer phenomena within the PBRs have been designed. These complex models, along with carefully curated algae-processing techniques and downstream steps, can be applied to adjust the overall culturing process to produce H2 [46]. Researchers have already devised several types of PBRs and other culture systems. Yet, there still exists the requirement of producing adequate volumes of microalgal biomass and coming up with economical and ecofriendly of mining the fuel from the cultured biomass [44].

9 Conclusion From the birth of the idea of using H2 as a clean and sustainable fuel to development of different methods and genetically engineered microbial strains for

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its production, scientists have come a long way. As highlighted in the chapter, different types of photobiological processes of microbial H2 production have been identified, alongside the development of advanced bioenergy strategies to extract and utilize the produced biohydrogen. These processes provide the added advantage of automatic product separation from the microalgal biomass, which makes the techniques more cost-efficient. Continuous research over the years has led to the development of various types of PBRs, which have increased the H2 yield significantly. Though, like with every other process, there lies the scope of improvement with the paradigm in photobiological production of biohydrogen as well. It is necessary to design techniques that will commercialize the production of H2 at minimum cost and increase its application to industrial purposes. To make this a reality, scientists are looking to develop a hydrogenase that is either O2-resistant or has a high respiration-to-photosynthesis ratio. This research continues along with the efforts to create genetically engineered organisms with O2-resistant H2-producing enzymes. Along with this, parallel studies are being carried out to (i) diminish or eradicate competing pathways, like CO2 fixation; (ii) enhance effective conversion efficiency of photosynthesis; and (iii) improve biosynthesis of starch. Multiple innovative processes include the production of H2 by a repetitive anaerobic digestion stage via direct employment of residual algal biomass as feedstock, or synthesis in mixed cultures by the utilization of integrated biosystems. On the other hand, metabolic modifications by depriving the culture of various important nutrients like nitrogen and sulfur, and other genetic alterations, along with interdisciplinary tactics like electrochemistry, cell immobilization on novel materials; application of nanotechnology; using wastewater as sustainable nutrient source for the production purpose; and many others are being employed either individually or in combination to yield commercially feasible levels of H2. Studies suggest that the production cost can be lowered by utilizing strains that do not necessitate sulfur starvation due to the presence of increased levels of saturation irradiance and a higher respiration-to-photosynthesis ratio as compared to the currently available strains of C. reinhardtii [57].

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CHAPTER 14

Challenges in scaling low-carbon hydrogen production in Europe
rez Garce
s, Daniela Tepordei, Sebastian Carlos Rojas López, Lucia F. Pe Púin Moreno, Biljana Šljuki
c, and Diogo M.F. Santos Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal

1 Hydrogen requirements in Europe to achieve net zero emissions The European Green Deal, which outlines the primary policy efforts for achieving net-zero greenhouse gas (GHG) emissions by 2050, was introduced by the European Commission (EC) in 2019. The targets represent the necessity for change in energy production and consumption for society to achieve net zero emissions. This results in a cut of emissions by at least 55% by 2030 [1], compared to the 1990 emissions, an achievement in which H2 plays a crucial role, as identified in the European Green Deal.

1.1 Global projections Under the net zero scenario, the global energy demand in 2050 will be around 8% lower [2] than it is today. Still, it supports a larger population of more than 2 billion people compared to today and an economy that is more than twice as large, further revealing this necessity to change. Achieving the target of net zero emissions by that year means a reduction of the emissions of all economic sectors, including areas where options are limited, such as heavy industry and long-distance transportation. In the case of H2, several projections have been made to understand its role in these reductions, as presented in Fig. 1. All these projections assume that the production of H2, which is currently gray H2 and based on fossil fuels, will be replaced by green H2 and complemented by blue H2, which is an alternative to the current one with the addition of carbon capture and storage (CCS). Looking at the H2 rate in the final energy demand, it can be seen that it shows significant values,

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Fig. 1 Projections for global hydrogen demand in 2050 [3].

meaning that it will play an essential role in the transition. However, this share of H2 in 2050, despite being notable, starts at almost nothing today (Fig. 1). In 2021, global H2 production reached 94 Mt [4], resulting in over 900 Mt CO2 emissions for that year. Less than 1 Mt (0.7%) was low-emission H2 production; from this already small value, only 35 kt was from electricity via water electrolysis.

1.2 Hydrogen’s role in the net zero emissions (NZE) scenario In the net zero emissions (NZE) scenario, the total H2 production is projected to reach over 500 Mt by 2050, taking into account electrolysis-based and blue H2, as seen in Fig. 1. The importance of these values in the whole scheme is given by their increasing share in the total final energy consumption, as seen in Fig. 2. Starting at less than 0.1% in 2020, this share is expected to reach 2% and 10% by 2030 and 2050, respectively. Notably, these percentages exclude on-site H2 production in the industry, increasing the shares to 1%, 4%, and 13% in the presented years [5].

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Fig. 2 Share of total final energy consumption by fuel, 2020–50 [5].

1.3 The strategy in the EU In July 2020, the European Commission (EC) released its H2 strategy to achieve the goals of the European Green Deal, focusing on renewable H2. For a better understanding, the targets presented in the strategy are broken down into three phases, as seen in Fig. 3.

Fig. 3 The EU hydrogen strategy [6].

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Until 2024, at least 6 GW of renewable H2 electrolyzers will be installed in the EU, which will increase to 40 GW by 2030, producing up to 1 Mt and 10 Mt of renewable H2 by 2024 and 2030, respectively [6]. For comparison, the global capacity of electrolyzers in 2021 was around 0.5 GW [7]. In mid2021, the EC presented the Fit For 55 legislative package to ensure that the EU policies align with the climate goals to meet its objectives. The package included 5.6 Mt of H2 from renewable sources, making sure to focus primarily on the production of green H2. However, this path has been subject to changes, with the EU presenting an increase in these targets. This year, the war in Ukraine has led the EU to substantially raise its H2 goals, publishing the REPowerEU, a plan outlining the EU’s path to energy independence from Russian fossil fuels by 2027. The necessity of a fast solution, combined with this level of ambition by the EU, is such that the action needs to overpass the other proposals which, until this moment, were already seen as ambitious. The REPowerEU presents a target of an increase from 5.6 Mt, in the previous proposal, the Fit for 55, to a new target of 20 Mt by 2030, hoping to replace 25–50 billion cubic meters of Russian gas. This value combines 10 Mt of domestic renewable H2 production in the EU. The remaining 10 Mt will be imported via three major corridors, the Mediterranean, the North Sea area, and Ukraine, when conditions allow. In addition, 120 GW of electrolyzers, on top of the 40 GW presented in past strategies, is to be installed to achieve a total of 180 GW by 2030 to aid in the domestic production of these H2 quantities. As seen in Fig. 4, these ambitious H2 numbers become even more evident when it comes to its planned use under RePowerEU, compared to its use under Fit For 55. Under RePowerEU, domestic production and imports of H2 in the form of ammonia and other derivatives were presented to have a large role in these 20 Mt, unlike in the initial plan. Other sectors planned to increase the use of H2 are industrial heat, petrochemicals, refineries, and transportation.

2 Hydrogen production and use While H2 production has inherent scalability challenges, the architecture to scale the process has generated significant debate around the need for such production to be local. Since the cost and complexity of transporting H2 and storing it for long-term consumption are significant, the feasibility and practicality of producing it locally, consuming it at a large scale, and producing it on demand deserve consideration. This section aims to create a framework

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Fig. 4 Hydrogen consumption projection, by sector, under the RePowerEU and Fit for 55 targets in the EU in 2030 [8].

for the discussion based on the application areas for H2 consumption. Based on the applications, the convenience of a distributed or centralized architecture will be discussed. Finally, it describes the concept of the H2 valleys, which occupy an important place in the different countries’ or regions’ H2 strategic plans.

2.1 Hydrogen applications While H2 has a diverse range of applications, one will focus on the areas that can generate a significant impact on decarbonization goals and have the potential to enable the H2 ecosystem growth in the next few years. The likelihood of H2 having such an impact depends on three things: • Is H2 a suitable replacement for the existing hydrocarbon consumption for a particular application? • Is there a pricing barrier to enable H2 adoption for a particular application? • What amount of H2 would such an application consume? Regarding the impact of H2 on applications compared to hydrocarbon alternatives, the Hydrogen Council, in its Hydrogen Insights 2021 report [9], shows the applications (considering the total cost for H2 production, distribution, and storage) most likely to have significant improvements (Fig. 5). Looking at the second requirement, the price barrier in H2 production for adoption, the same report from the Hydrogen Council provides an

Fig. 5 Hydrogen competitiveness per end-use application in 2030 [9].

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interesting view of the necessary H2 cost in 2030 to break even with the cost of conventional technologies (Fig. 6). Those estimates use a CO2 emissions credit of $100/ton. This seems to be the most interesting case for the European market. The third question is how much H2 will applications move if adopted in the market? The initial point is current consumption. Today, most applications of H2 are industrial. According to the International Renewable Energy Agency, IRENA, in its 2022 Green Hydrogen for Industry Report, the 2020 consumption was ca. 87 Mt, heavily concentrated in refineries and ammonia production [10] (Fig. 7). The expected low-carbon H2 consumption will depend on the conversion of current applications, depending on an economical replacement, and on the adoption of new applications, in particular mobility, fuels, and electricity production. The recently published U.S. Department of Energy (DOE) draft report for Hydrogen Strategy [11] is a good start for this analysis (Fig. 8).

Fig. 6 Required hydrogen production cost for breakeven with conventional solutions [9].

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Fig. 7 Industrial hydrogen consumption in 2020 [10].

Fig. 8 Scenarios showing estimates of potential clean hydrogen demand in key sectors of transportation, industry, and the grid, assuming hydrogen is available at the corresponding threshold cost [11].

The DOE numbers seem a little different from the ones presented in the Hydrogen Council report; part of the discrepancy is the available government incentives in the different economies, but the thought process is very interesting. The DOE report combined technical information with marketdriven information. For example, early adopters in truck transportation in the United States would start switching to FCEV with an H2 price

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(produced, delivered, compressed, and dispensed) of $5/kg, and 14% of medium and heavy trucks would adopt it if the price reached $4 for an expected demand of 5–8 Mt per year. The chart shows the expected (dark) and optimistic (light) adoption numbers. Another example is Sustainable Aviation Fuels; a total replacement in the United States by 2050 would require
6 Mt H2 per year to produce 4 billion gallons of power to liquid (PTL) fuels. 10–20 of steelmaking by 2050 would require 1–3 Mt per year, and ammonia plants could use an additional 4–5 Mt per year. 4–8 Mt is the estimate of H2 for storage needed to achieve 100% clean electricity production. Combining the three pieces of information, it is evident that the shortterm enabler for green H2 where the price would be competitive with today’s numbers, is in medium and heavy-duty vehicles: mid- and highrange trucks, bus fleets, trains, and the replacement of industrial H2 use with low-carbon alternatives.

2.2 Centralized vs. distributed hydrogen production A second aspect to consider when scaling H2 production is related to the locality of H2 production. Some of the large-scale applications of H2 presented before are inherently centralized, like the use in refineries, the production of ammonia, and the manufacturing of steel. On the other hand, some uses are highly distributed, like home use or personal vehicles, and others, like fleets of heavy trucks, are decentralized but need a small number of recharging or H2 distribution points. Domestic heating and personal use vehicles were the two most significant drivers for a high level of capillarity in the distribution network for H2. The 2022 IRENA report shows an updated cost of 1 MW of services in those two segments. It was clear that new technologies like heat pumps and district heating are now offering significant advantages for domestic heating (Fig. 9). The use case for personal use vehicles has shifted over time. The efficiency of the energy use (directly using electricity vs. using the electricity to produce green H2, transport, distribute, and then use in an FCEV) adds significant operational cost. On the other hand, significant challenges in refueling times, autonomy, or battery cost remain for electric vehicles. While the technical discussion is ongoing, the EV market has grown exponentially in the last 10 years, with thousands of refueling stations deployed [12] (Fig. 10). On the other hand, Hydrogen Mobility Europe, in its report,

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Fig. 9 Estimation of renewable electricity generation needed for 1 MWh by energy services [10].

Fig. 10 Global electric car stock, 2010–21 [12].

shows a little over 2000 FCEVs and 100 H2 refueling stations to be deployed by 2022 [13]. While fuel cells seemed like the path of choice a few years ago, and car manufacturers had an initial significant interest, the current market appears to favor electric vehicles. Without the personal mobility case and the emergent array of high-efficiency home heating solutions, the case for a pervasive

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Fig. 11 Green hydrogen policy priority [10].

distribution network has significant challenges, at least in the short term. In its latest reports, IRENA shows residential heating and urban vehicles as electrification priorities in the green H2 agenda (Fig. 11).

2.3 Hydrogen valleys The strategic plans for advancing the hydrogen agenda are now focusing on Regional Networks. In the United States, The DOE Draft Hydrogen Strategy targets “achieving large-scale, commercially viable deployment of clean hydrogen by matching the scale-up of clean hydrogen supplies with a concomitant and growing regional demand. Co-locating large-scale clean hydrogen production with multiple end-uses can foster the development of low-cost hydrogen and the necessary supporting infrastructure to jumpstart the hydrogen economy in important market segments. Regional hydrogen networks will create near-term and long-term jobs, increase tax revenues for regional economies, and reduce emissions” [11]. A hydrogen strategy focused on large-scale industrial applications and transportation fleets seems to fit the model of large H2 hubs or H2 valleys: clusters of large users like oil refining and ammonia producers, which are usually located in coastal zones with ports that facilitate the transport of natural gas or H2 from overseas at large scale. From those areas, H2 would be transported incrementally to other areas, initially in tanker trucks and later in regional pipeline infrastructure.

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This approach would not preclude the decentralized model, where smaller-scale production facilities would use distributed renewable energy production peaks to produce H2 for local industry or transportation. The Hydrogen Valley Platform (https://h2v.eu/hydrogen-valleys) is a global collaboration platform for all information on large-scale hydrogen flagship projects and aims to facilitate a clean energy transition by promoting the emergence of integrated hydrogen projects along the value chain as well as by raising awareness among policy makers. On this platform, the most advanced Hydrogen Valleys around the globe provide insights into their project development [14]. The H2 Valleys Platform has identified three archetypes according to the size of the infrastructure: Smaller-scale local mobility-centered H2 valleys (typically 1–10 MW of local electrolyzer capacity): this archetype may look like a small regional decarbonization effort, likely to support green transportation fleets or mobility valleys with a few refueling stations, which has been endorsed by a local community. Some of those have already achieved some commercial success. The H2 valley of South Tyrol in Italy is an excellent example of this type of valley. In this project, a 55 M€, approximately $58 M, investment is set up to produce 90 tons of H2 per year by 2026. The aim is to decarbonize the mobility sector (cars, buses, trucks) in the area. The H₂ production route is through alkaline electrolysis, using cylinders for storage and trucks for H2 transport [14]. Other examples cited in the report are the Zero Emission Valley Auvergne-Rh^ one-Alpes (FR) and the Hydro-spider project (CH). Medium-scale H2 valleys focusing on industrial decarbonization (typically 10–30 MW of local electrolyzer capacity): in this category, there are locations with a target application for H2, most likely a petrochemical industry already with the need, and that can be converted to low-carbon H2. Applications like vehicle fleets and other decarbonization initiatives can be developed around this main activity since the H2 cost will already be highly competitive. A prime example is the H₂ Proposition Zuid-Holland/Rotterdam project in the Port of Rotterdam. This project is expected to be completed in 2030, with an investment of more than 1000 M€ from public and private investment from partners like Air Liquide, Air Products, ExxonMobil, Port of Rotterdam, Proton Ventures, Shell, and Siemens. The project aims to produce more than 1 Mt of H2 per year using PEM electrolysis and steam methane reforming (SMR) with carbon capture, utilization and storage (CC(U)S). The uses include industry (chemicals, refineries), mobility (cars, buses, ships), and

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energy (gas-fired power plants, gas grid injection). Other examples are the Basque Hydrogen Corridor (ES) and HyNet North-West England (UK). Large-scale and ultimately export-oriented H2 valleys (typically 250–1000 MW, or even more, of local electrolyzer capacity): in this case, the anchor activity is present, and the hub has a low-carbon producing advantage already develop, but now the production exceeds the local need, and the focus is now on the coverage of regional or international markets. Large-scale green electricity generators or giga-scale farms that produce photovoltaic or wind farms that produce surplus electricity can generate competitive cost green H2 used as an energy transport mechanism. The massive project AquaVentus H2 is a critical piece of the German decarbonization strategy. Their direction is based on renewable electricity produced using offshore wind. The offshore wind farms will power electrolyzers also installed at sea on an industrial scale. The total electrolysis capacity is expected to reach 10 GW by 2035, enough to produce 1 Mt of green H2. The project is located near the German North Sea Island of Heligoland [15]. RWE, Shell, and Equinor will work together in a large subproject of AquaVentus, to build the first 300 MW offshore wind farm linked to electrolyzers to produce green H2 by 2028. The first step will carry out a detailed feasibility study for the subproject named AquaSector, which will provide initial indications of the conditions under which the large-scale offshore H2 park can be deployed. The green H2 is planned to be transported via a small pipeline called AquaDuctus, at first only to Heligoland. Once the project grows toward the 10 GW dimension, the pipeline will be extended to the German mainland [16]. Other large-scale H2 valley projects include NEOM (KSA), H2 Magallanes (CL), and Pilbara Hydrogen Hub (AU). Looking at the scale of the Valleys, the small ones will likely get their electrolyzer energy from the grid, which in turn is produced at a utility scale. Medium size already has a local H2 generation capability to support the local ecosystem but most likely will include a variety of H2 sources that complement an industrial use like petrochemicals. The large H2 valleys will use intensive renewable energy to produce green H2. The variety of sources and drivers for use lead us to concentrate on the two most likely types of H2 that will be used: blue and green H2.

3 Blue hydrogen Blue H2 is produced either by steam reforming or by autothermal reforming of natural gas, which produces CO2 as a by-product. To consider this H2 as

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blue, the CO2 emitted needs to be captured and stored using CCS, involving a set of stages to reduce the amount of CO2 released into the atmosphere. Over the past decades, CCS technologies have been used to decrease the amount of CO2 naturally occurring in natural gas (sweetening) to meet the target market and pipe specifications to produce liquefied natural gas (LNG). The technology then evolved to assist in the climate neutrality targets and was adapted to capture CO2 from large point sources, such as industrial processes like cement production, chemical, coal-power, or biomass plants. Nowadays, CCS plays a relevant role in H2 production as an energy vector, tackling the emissions associated with natural gas while providing a cost-effective pathway to meet the increasing demand for low-carbon H2 in various applications.

3.1 CO2 sequestration technology Carbon capture and storage (CCS) is the process of capturing CO₂ formed during power generation and industrial processes and storing it so that it is not emitted into the atmosphere. CCS technologies have significant potential to reduce CO₂ emissions in energy systems. Facilities with CCS can capture almost all the CO₂ they produce (some currently capture 80 or even 95%) [17]. Deploying CCS at a power plant or industrial facility generally entails three major steps, namely, CO2 capture, transportation, and storage, which will be further detailed. 3.1.1 Capture of CO2 There are currently three processes for capturing CO2 that allow its separation from the flue gases that result from burning fossil fuels: • Postcombustion is the process of absorbing CO2 in a suitable solvent that reacts with the CO2 and selectively removes it from the flue gas. • In precombustion, the fuel is first “gasified,” producing “syngas” (CO + H2). Steam is added to convert the CO to CO2 through the water-gas shift reaction, producing additional H2. The CO2 can then be removed. • Oxy-fuel uses pure oxygen (O2) for fuel combustion, rather than air. This results in a flue gas containing mainly CO2 and H2O, which can be more easily purified. The most mature capture technologies are first-generation amine chemical solvents (e.g., monoethanolamine) and physical absorption solvents (e.g., selexol, rectisol). However, physical solvents require significant electricity for compression of the source gas stream, to provide the elevated operational pressure that these physical solvent absorbents need [18].

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3.1.2 Transportation Pipelines are the most common method of transporting the large quantities of CO2 involved in CCS. There are already extensive networks of kilometers of pipelines worldwide that transport various gases, including CO2. Transport of CO2 by truck and rail is possible in small quantities. Trucks are used at some project sites, moving the CO2 from where it is captured to a nearby storage location. Given the large quantities of CO2 that would be captured via CCS in the long term, it is unlikely that truck and rail transport will be significant. Ship transportation can be an alternative option for many regions of the world. Shipment of CO2 already takes place on a small scale in Europe, where ships transport food-quality CO2 (around 1 Mt) from large point sources to coastal distribution terminals (Fact Sheet Global [19]).

3.1.3 Storage The selection of a geological site for storage must be done to meet three main conditions, namely, capacity (adequate pore volumes to store large amounts of CO2), injectivity (high permeability ensuring that wellhead pressures can be used to maintain desired injection rates), and containment. Competent cap rocks and sealing faults are necessary to ensure that the injected CO2 does not escape to the surface or leak into groundwater due to the lower density of the CO2 gas compared with reservoir fluids. The next geological storage points are most likely to meet the injection requirements [20]: Saline formations: Saline aquifer formations represent the best salted sink for the storage of CO2 among all geological options due to their enormous storage capacity. It is required that the aquifer be saline because this already makes it unsuitable for industrial, agricultural, and human purposes. Depleted oil and gas reservoirs: Characteristics required for a storage site are present in such formations and have been employed for geologic sequestration. A potential disadvantage, however, is that when hydrocarbon recovery was completed at these fields, the production wells were filled with mud and plugged with cement; therefore, they could lead to CO2 leakage if their integrity is not maintained. Enhanced oil recovery (EOR): CO2 is used for enhanced oil recovery (EOR) from mature fields. Between 50% and 70% of CO2 returns with the oil; however, this can be separated and re-injected into the hydrocarbon reservoir to minimize operational costs.

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3.2 CCS facilities There are currently 30 large-scale CCS facilities in operation globally, with 11 more under construction and 155 (2 suspended) in different stages of development. As shown in Fig. 12, the leadership is visibly led by North America, which accounts for 13 CCS projects in the United States and 5 in Canada. In contrast, there are five operating facilities in Europe: 2 in Norway, 1 in Hungary, and 1 in Iceland [21]. According to the independent Global CCS Institute (GCCSI), the current storage capacity is about 40–43 Mt CO2 per year. Considering all new CCS projects announced in the last year, this number is expected to increase to 244 Mtpa. However, to mitigate the hardest effects of climate change and support Europe’s path to a climate-neutral economy, the installed capacity of CO2 must grow from 244 Mt to thousands of Mt per year. Nowadays, operational facilities, on average, can inject just over 1 Mt CO2 per year [23]. As low-carbon H2 is playing an increasingly important role in the strategies of European countries who want to decarbonize key sectors like transport, industrial processes, and domestic heat, several CCS projects incorporate the production of H2 through SMR of natural gas, while capturing and storing the associated CO2, such as the Quest CCS facility in Canada, operating since the end of 2015, where 4 Mt CO2 had been captured and safely stored from the SMR process for H2 production. “Next wave” facilities based around CCS hubs and clusters have taken advantage of the fact that many emission-intensive facilities (both power and industrial) tend to be concentrated in the same areas (Fig. 13). Hubs and clusters significantly reduce the unit cost of CO2 storage through economies of scale and offer commercial synergies that reduce investment risk. They can play a strategically important role in climate change mitigation. Production of blue H2 also requires access to natural gas and pore space for the geological storage of CO2. Global resources for geological storage of CO2 are also more than sufficient for CCS to play its full role in H2 production—storage for CCS is abundant under any climate mitigation scenario for all applications in all industries (Fig. 14). To illustrate, in an extreme hypothetical case where all 530 Mt of clean H2 aimed to be produced in 2050 is blue H2, annual CO2 storage requirements would be just 7600 Mt. This compares to a global storage capacity measured in thousands of billions of tonnes [24]. Historically, most CO2 has been used for enhanced oil recovery (EOR), given the CCS industry was born out of EOR in the United States. These

Fig. 12 Current CCS facilities around the world [22].

Fig. 13 CCS Hubs and Cluster globally [22].

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Fig. 14 CO2 storage projects across storage types and geographies [23].

facilities showed that Mt CO2 injection rates at multi-Mt storage sites were possible. Today, deep saline formations are the most common type of CO2 storage reservoir across all storage facilities (over 150) at all stages of development, from operational through to early development phases. CO2 storage facilities targeting deep saline formations are most substantial in North America and the North Sea. Storage in depleted oil fields is becoming more common, for example, in the UK, Australia, and Southeast Asia [25]. Among all the long-term storage of CO2 in geological formations, together they are estimated to have a global storage capacity of 13,954 Gt CO2 [26]; therefore with current world energy-related CO2 emissions of about 40 Gt CO2/year, there is sufficient storage capacity for CCS to play a major role in emissions abatement.

3.3 Regulatory/political position in Europe of CCS One of the most significant barriers to the widespread deployment of CCS technologies is the high cost of the technologies. Although cost estimates vary widely, the greatest costs are typically associated with the equipment and energy needed for the capture and compression phases. Capturing the CO2 can decrease plants’ efficiency and increase their water use, and the additional costs posed by these and other factors can ultimately render a CCS project financially nonviable.

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As highlighted in the Intergovernmental Panel on Climate Change’s Special Report on Carbon Dioxide Capture and Storage, policies that increase demand and reduce costs will be needed to accelerate CCS development. Several different types of policies have the potential to bring down the costs of CCS and encourage research, development, and deployment, including carbon pricing policies, public investment and subsidies, and clean energy standards that credit companies generating electricity or other energy sources with CCS. In the United States, multiple enacted policies aid and encourage the use of CCS technology. National tax credits for carbon sequestration are created through Section 45Q of the Internal Revenue Code. On the other hand, the European Commission of target neutrality by 2050 is driving investment in CCS, through the EU Innovation Fund to invest in 11 CCS projects supplemented by individual member state policies: In Europe, Denmark announced €5 billion in subsidies for CCS, Norway announced NOK1 billion ($100 million) to support three large blue H2 projects, and the European Union’s Innovation Fund allocated €1.1 billion for seven innovation projects, of which four are intended to scale up CCS, after the first call for large-scale innovative clean-tech projects in 2021. The four projects including CCS technologies are a bio-energy facility (BECCS) in Sweden, a cement facility production in France, an H2 production facility in Finland, and a chemical plant (H2, ammonia, and ethylene production) in Belgium. In total, the Innovation Fund aims to allocate €25 billion toward low-carbon technologies by 2030.

3.4 Viable blue hydrogen route The projections of future clean H2 demand will exceed 500 Mtpa by 2050 compared to total H2 production today of approximately 120 Mtpa, including clean H2 production of only around 1 Mtpa [24]. There were announced 28 blue H2 projects in varying stages of development and two in operation. The production capacity of each of these facilities ranges from tens of Mt to hundreds of Mt of H2 per year. It was found in the latest reports of the International Energy Agency that most CCS projects were full-value chains incorporating a single CO2 capture plant with its dedicated CO2 compression, transport (usually, pipeline), and storage systems. Nowadays, networks or clusters are the predominant models for developing CCS projects, and this makes more economic sense if the

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facility also integrates H2 production. It could be said that sharing transport (pipelines, shipping, fleets) and storage infrastructures help to scale up the deployment of blue H2 projects with CCS networks as the risk is shared between the stakeholders, and large investment in H2 transport infrastructure will be required to deliver H2 to demand centers [27]. Given that H2 has been traditionally produced from natural gas and petrochemical plants, for these companies, the integration of CCS technologies is an upgrade in their value chain from a business perspective, extending their revenue from fossil fuels as a primary energy source. Therefore, most of the oil and gas companies, such as Shell, are developing newer technologies and retrofitting with CCS existing methane reforming plants and IGCC facilities to achieve very high capture rates (around 90%–95%) and taking advantage of their distribution infrastructure and mature utilization technologies for H2 production supply chains. However, for ramping up the blue H2, combined efforts from the Governments will be required to put in place supportive policies, involving private companies to build, own, and operate the clusters, and the financial sector to provide capital and economic backing.

3.5 Challenges for blue hydrogen One of the most significant barriers to the widespread deployment of H2 production with CCS technologies is high cost, typically associated with the equipment and energy needed for the capture and compression phases of CO2. Capturing CO2 is an energy and water-intensive process. Therefore, it can potentially decrease the net plant efficiency while increasing water usage for cooling purposes. The proportional impact on manufacturing costs also varies according to product value and carbon intensity. The cost-effectiveness of CO2 capture has three primary drivers, namely, CO2 concentration and purity, the mass flow rate of the streams, and the relative cost and market value. The costs of capturing CO2 from an ethanol or H2 plant are about $25 per ton of CO2. And that includes the cost of compression, transportation, deep injection, and monitoring [28]. The relative impact of implementing CCS varies significantly between subsectors. For example, in the cement industry (low market price), the production cost is estimated to increase by over 70%. In contrast, in the refining industry, where the value of the products is higher, the impact is estimated to be only 3%. Therefore, it is more likely that the refining subsector could bear a significant proportion of the CCS implementation costs while maintaining

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a competitive position in the market. Thus, integrating blue H2 projects in industrial clusters benefits the cost and market value of every end-use product or service within this network. In terms of storage capacity in geological deposits, it is worth mentioning that, even though the CO2 Storage Resource Catalogue Cycle 3 report for 2022 of The Oil and Gas Climate Initiative (OGCI) the storage resources were estimated at 13,954 Gt CO2, just over 577 Gt (or 4.1%) have been discovered, meaning they have been proven with subsurface data such as a well and seismic surveys. Unfortunately, only a tiny fraction of the total global storage resource base can be considered commercial resources—just 253 Mt CO2 (or 0.002%) [23]. Other challenges that H2 production involving CCS technologies can face concerning technical, economic, and environmental impacts are: 1. Research needs to be done concerning the long-term ability of storage sites to sequester carbon without significant leakages. In the meantime, tracking and monitoring of the behavior of CO2 in reservoirs should be mandatory, as preventive and control measures. 2. Governments need to take action in subsidizing the costs involved for the early movers, thereby encouraging more participation. For example, one of the most progressive CCS incentives is the carbon dioxide sequestration credit named 45Q in the United States, which provides a certain amount of monetary credit for each ton of CO2 that is permanently stored via usage, tertiary oil injection, or in geologic formations. Such initiatives should be tailored to the European community’s specifications. 3. CCS may be viewed as prolonging the role of fossil fuels in the economy. Further research is needed to understand better what the public thinks about it [29]. 4. CO2 storage is receiving increased attention internationally because some alerts have been aroused for the potential for seismic activity allegedly caused by underground injection. Promoting public awareness and education about CCS technologies is relevant for public acceptance. In summary, a successful H2 production project with CO2 storage would involve accurate site selection, characterization (storage and capacity estimation), and monitoring to avoid the risks of leakages through seals, faults, and abandoned wells. CCS makes economically more sense when combined with a utilization process to produce high-value chemicals and energy carriers such as H2 and ammonia to offset the high costs of capture operations [23].

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4 Green hydrogen Green H2 is achieved through an electrolysis process powered by renewable energies, such as wind or solar. Electrolysis uses an electric current to break down the water molecule into O2 and H2 at the electrode surface.

4.1 Renewable energy sources The three primary sources of carbon-free electricity are water, the wind, and the sun. Hydroelectricity is the primary source of renewable electricity worldwide. This technique transforms the power of water into electric current in hydropower plants installed on natural water courses or dams. In second place is wind power (onshore or offshore), which is produced by transforming the kinetic power of the wind into mechanical energy, and then electricity, using wind turbines. Finally, solar electricity finishes in third place, thanks to farms of photovoltaic panels that convert light into electricity. Renewable energy sources can then be used to produce renewable H2 (Fig. 15). Green H2 must be obtained by electrolysis powered by renewable energy sources, leading to the decomposition of water molecules into O2 and H2. The H2 molecule has a high energy density per unit mass, three times more than gasoline and 120 times more than lithium batteries [11]. In conventional electrolysis, O2 is formed at the anode while H2 is released at the cathode as a DC current passes through the water. An additive, such as salt, is used to increase conductivity [7]. The electrolysis uses an ion-selective membrane that allows specific ions to pass between the anode and cathode compartments, replacing the need for additives in the water and making the process more environmentally friendly. The result is H2 and O2 produced using “green” (decarbonized) electricity, with the recovered green H2 being compressed and stored. Thus, the first requirement to produce H2 is clean water, meaning that the production site requires access to this resource. Wastewater cannot be used as a resource without treatment, an opportunity for wastewater treatment plants to add value to the operation. For the electrolysis to take place, high conductivity is a requirement, which can be seen as an opportunity for seawater desalination plants.

Fig. 15 Green hydrogen production and use (adapted from [30]).

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4.2 Transport and storage Green H2, which is mainly produced by water electrolysis using renewable electricity, is one of the levers that will speed up the transition to a carbonneutral future: development of green mobility, decarbonization of largescale industrial uses of H2 (e.g., fertilizers, refineries, chemicals), improved integration of intermittent renewable energies into the energy system, massive storage of the surplus electricity generated. According to the U.S. Department of Energy [11], most of the H2 used in the United States is produced at or near where it is used, usually at large industrial sites. The necessary infrastructure to distribute H2 to the national network of filling stations necessary for the widespread use of fuel cell electric vehicles has yet to be developed. The initial deployment of vehicles and stations is focused on building these distribution networks, primarily in Southern and Northern California. The example of the United States can be taken as a scale distribution, due to the size of the country, the industry, and the market. In Portugal, the situation of the distribution could be easy to solve. Currently, H2 is distributed through three methods: Pipeline: This is the least expensive way to deliver large volumes of H2. In the case of Portugal, this is an advantage, because nowadays, the pipeline belongs to the gas system, which is a 1375-km-long onshore pipeline project operated by REN Gas pipelines. This gas pipeline, with a maximum diameter of 32 in., starts at Setubal (Portugal) and ends in Leiria (Portugal) [31]. However, the conditions that H2 needs to be transported are quite different from natural gas; thus, the infrastructure must be modified and adapted. High-pressure tubular trailers: Transporting compressed H2 gas by truck, railcar, ship, or barge in high-pressure tubular trailers is expensive and is used primarily for distances of 200 km or less, which could represent a way to inter-continent transportation. Liquefied hydrogen tanks: Cryogenic liquefaction is a process that cools down the H2 to a temperature where it becomes a liquid. Although this process is highly costly in terms of energy and money, it allows H2 to be transported more efficiently (compared to high-pressure tube trailers) over longer distances by truck, railcar, ship, or barge [11].

4.3 Current projects The EU Commission’s Hydrogen Strategy explores ways renewable H2 can help decarbonize the European energy sector. According to

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estimates by the commission and EU leaders, green H2 will play a key role in decarbonizing sectors where other alternatives might be unfeasible or more expensive. Therefore, the EU Commission created a hydrogen strategy, which was approved and adopted by the various EU parliaments across the continent. The strategy is basically a roadmap on how H2 can help decarbonize the EU economy in a profitable way. It is also in line with the European Green Deal and contributes to the post-COVID-19 economic recovery. The following two examples from the Iberian Peninsula are projects funded by the EU. 4.3.1 SINES Portugal Under the name GreenH2Atlantic, the renewable H2 production project in Sines will be developed by a consortium composed of 13 entities, including companies such as EDP, Galp, ENGIE, Bondalti, Martifer, Vestas Wind Systems A/S, McPhy and Efacec, and academic and research partners such as ISQ, INESC-TEC, DLR, and CEA, in addition to a public-private cluster, Axelera. GreenH2Atlantic was one of the three projects selected by the Horizon 2020—Green Deal Call to demonstrate the viability of green H2 production on an unprecedented scale. The €30 million grant will help finance the construction of the H2 plant, located in the coal-fired power plant area in Sines. The construction should start in 2023, and operation is expected to begin in 2025, subject to securing the necessary authorizations by the responsible authorities. The 100 MW electrolyzer will be composed of innovative, scalable, and fast-cycling 8 MW modules to overcome bottlenecks such as efficiency, size, lifetime, and flexibility. Other innovative features include the interface system composed of an advanced management system required to enable the project’s direct connection to a local hybrid renewable power plant (solar and wind). 4.3.2 HyDeal Spain HyDeal Spain will be the first industrial implementation of the HyDeal Ambition platform announced in 2021, which will supply renewable H2 to produce green steel, green ammonia, and green fertilizers. Recently, IRENA ranked the project as the world’s largest large-scale renewable H2 project. Lead sponsors include international steelmaking corporation ArcelorMittal, Spanish gas transmission system operator Enaga´s, Spanish

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chemical group Fertiberia, and Madrid-based H2 company DH2 Energy [32]. Production is scheduled to begin in 2025. The total installed capacity is expected to reach 9.5 GW of solar power and 7.4 GW of electrolyzers by 2030. ArcelorMittal and Grupo Fertiberia plan to purchase 6.6 Mt of renewable H2 over 20 years to produce steel, ammonia, and fertilizers. “HyDeal Spain is the first concrete implementation of the 1.5€/kg green H2 system announced in February 2021,” said Thierry Lepercq, chairman of the joint venture and spokesperson for HyDeal Ambition, adding that green H2 can now compete with coal, oil, and natural gas in both cost and volume [32].

4.4 Regulatory/political position in Europe 4.4.1 EU hydrogen strategy The goal of the European Commission communication on an H2 strategy for a climate-neutral Europe, adopted in 2020, is to accelerate the development of clean H2, ensuring its role as a cornerstone of a climate-neutral energy system by 2050. The strategy envisions a gradual trajectory to reach this goal, initially including blue H2 projects. Several key actions will be implemented throughout three strategic phases between 2020 and 2050. The strategy points to the existing status quo, concluding that H2 (and renewable H2) plays only a minor role in the overall energy supply today, with challenges in terms of cost competitiveness, the scale of production, infrastructure needs, and perceived safety (EU H2). Toward an H2 ecosystem in Europe, the roadmap to 2050 points out the different ways to produce H2, their associated greenhouse gas emissions, and relative. It describes green H2 as “renewable H2.” Thus, “Renewable H2” is H2 produced through the electrolysis of water (in an electrolyzer, powered by electricity) and with the electricity stemming from renewable energy sources. The full life-cycle greenhouse gas emissions of the production of renewable H2 are close to zero [1]. Renewable H2 may also be produced through the reforming of biogas (instead of natural gas) or biochemical conversion of biomass, if in compliance with sustainability requirements [1]. 4.4.2 Policies that incentivize the investigation and application of hydrogen as a source of energy in Portugal In a general context, numerous policies incentivize the development of H2, such as the recovery and resilience plan (PRR). For Portugal, there are the innovation calls from the Portuguese agency of innovation, and the roadmap

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for carbon neutrality 2050, among others. This section will focus on the National Energy and Climate plan 2030, and the National Hydrogen Strategy (EN-H2). The National Hydrogen Strategy (EN-H2) was approved by the Council of Ministers on 21 May 2020, and this document was available for public consultation from 22 May until 6 July 2020. The purpose of this consultation was to have a period of listening to society and of close dialogue with the leading players in the sector. The overall goal is to consolidate the main objectives of EN-H2, particularly regarding the targets to incorporate H2 into the various segments of the economy. The strategy mainly describes how green H2 can contribute to achieving the objective of carbon neutrality in 2050 in Portugal, ensuring that green H2 is produced exclusively from processes that use renewable energies. • To achieve carbon neutrality in 2050, it will be necessary to follow a long path that leads to a reduction between 85% and 90% in GHC emissions by that year. • The PNEC 2030 defines the policies and measures for the next decade to achieve carbon neutrality in 2050. • Achieving carbon neutrality and the 2030 targets means switching fast from fossil fuels and combining different technologies and energy vectors. H2, in this case, is fundamental for decarbonizing various sectors of the National economy. The National H2 strategy describes the main objective of introducing H2 as a sustainable pillar of the transition to a decarbonized economy. The EN-H2 gives a solid framework to the private sector and promoters with H2 projects to make possible the consolidation of these projects into a broader and more coherent strategy. The strategy also promotes an industrial policy around H2, defining a set of public policies that guide, coordinate, and mobilize public and private investments in projects of production, storage, transportation, and consumption of renewable gases in Portugal [33]. It is important to keep in mind that the H2 value chain includes three phases that comprise H2 production: (i) centralized and decentralized production; (ii) storage, distribution, and supply; and (iii) end of use. The strategy provides political and financial mechanisms to support the development of H2 in Portugal.

4.5 Challenges for green hydrogen The main constraints for scale production of green H2, including the costs of renewable energies, the inefficient storage of generated electricity, the

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storage of H2, and the significant cost of seawater desalination, lead to a lack of industrial demand for H2. Assuming all these barriers are “knocked down,” there is still a major issues to keep in mind—water demand. Within this chapter, the total future need for H2 in all applicable sectors is considered, including chemical synthesis, transportation, buildings and heating, and energy storage. The calculated H2 demand in the renewable future is 2.3 Gt per year. For the following example, it is assumed that H2 will be produced by water electrolysis powered by renewable energy [34]. In this sense, one can reduce the carbon emissions from the energy sector by up to 10 Gt. It is important to determine the feasibility of the amount of water that will be required for generating 2.3 Gt of H2 each year. This issue concerns the conservation community, stating that obtaining water for the economy will be too expensive or demanding on the water and energy requirements [34]. Based on the water splitting reaction for 1 kg of H2 produced, 9 kg of water is consumed. This further means that 20.5 Gt (or 20.5 billion m3/year) of fresh water is required to produce 2.3 Gt H2. Assuming that this water will not return to its original source, one can assume that this amount of water is consumed. Still, water electrolysis powered by renewable energy used to produce H2, consumes lower amount of water compared to the fossil fuel-based systems. Namely, it should be taken into account that the water requirement is very high when fossil fuels are used for primary energy production and power generation. In 2014, 251 billion m3 of freshwater was withdrawn for power generation, and energy production from fossil fuels, such as coal, oil, and natural gas, and 31 billion m3 was consumed as the water was used for cooling, mining, hydraulic fracturing, and refining [34]. In comparison with the 20.5 billion m3/year production by water electrolysis coupled with renewable energies, using fossil fuels as primary energy consumes 33% more water. Thus, this last assumption would suggest that producing H2 by water electrolysis would not be economically viable. Still, it is beneficial for freshwater conservation when directly compared to the traditional use of fossil fuels for power generation as primary energy production. Moreover, adding the process of desalination to the value chain can be game-changing. Namely, accessible freshwater makes up less than 1% of the planet’s water. Adding a desalination process increases the energy requirement of H2 production, but to a negligible extent. To desalinate water, reverse osmosis is commonly used, which requires 3.5–5 kWh of energy for each cubic meter of clean water produced [34]. The current efficiency of reverse osmosis is 50%; thus, it requires double the water quantity, with the advantage of seawater being superabundant. Assuming a global

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Fig. 16 Annual global water withdrawals and consumption by sector. (Reprinted with permission from Beswick RR, Oliveira AM, Yan Y. Does the green hydrogen economy have a water problem? ACS Energy Lett 2021;6(9):3167–9. https://doi.org/10.1021/ acsenergylett.1c01375. Copyright 2021 American Chemical Society.)

H2 demand of 2.3 Gt, this yields an additional 0.26–0.37 EJ of annual energy required to perform reverse osmosis for water electrolysis; this can be considered as not too high keeping in mind that the energy source is renewables, and the marginal price is zero [34]. Fig. 16 compares the global freshwater consumption of three sectors in 2014, namely, fossil fuel energy production and power generation, agriculture, and the implementation of a global hydrogen economy [34].

5 Conclusions The global projections for H2 and the targets set by the European Union appear to be somewhat ambitious and subject to several changes in a short period. Although original targets seemed ambitious, the war in Ukraine demonstrated the urgent need for reducing Russian gas and general hydrocarbon dependency, further increasing the E.U. goals. At this point, the United States and the European strategies seem to be converging toward the H2 valleys approach, where projects with a limited geographical scope create H2-driven “mini-economies” capable of generating economic incentives, jobs, and well-being for the citizens while the macro-goal of decarbonization advances.

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Looking at the scale of the Valleys, the small ones will likely get their electrolyzer energy from the grid, which is produced at a utility scale in another location and most likely with guaranteed green sources. Medium-sized valleys already have a local H2 generation capability to support the local ecosystem but most likely will include a variety of H2 sources that complement an industrial use, like petrochemicals. The large H2 valleys will use intensive renewable energy to produce green H2. If the renewable energy needed to produce green H2 is supplied, the next challenge would be water. Nevertheless, significant amounts of water are currently being used in fossil fuel exploitation. An alternative is using a desalination process to supply water in geographic areas where the resource is scarce. Indeed, supply must be doubled due to the current efficiency of desalination (reverse osmosis).

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[11] U.S. Department of Energy. DOE national clean hydrogen strategy and roadmap., 2022, https://www.hydrogen.energy.gov/pdfs/clean-hydrogen-strategy-roadmap.pdf. [Retrieved October 2022]. [12] International Energy Agency (IEA). Global EV Outlook 2022: Securing supplies for an electric future. IEA; 2022. https://www.iea.org/reports/global-ev-outlook-2022. [Accessed 17 October 2022]. [13] Element Energy. Hydrogen mobility report - emerging conclusions., 2020, https:// h2me.eu/wp-content/uploads/2021/01/H2ME_Emerging-Conclusions2020.pdf. [Accessed October 2022]. [14] Weichenhain U, Kaufmann M, Holscher M, Scheiner M. Going global: an update on hydrogen valleys and their role in the new hydrogen economy., 2022, https://www. clean-hydrogen.europa.eu/media/publications/update-report-hydrogen-valleys-andmission-innovation-hydrogen-valley-platform-was-published-22_en. [Retrieved October 2022]. [15] RWE. Hydrogen production in the North Sea: aquaventus. RWE Site; 2022. https:// www.rwe.com/en/research-and-development/hydrogen-projects/aquaventus. [Retrieved October 2022]. [16] Radowitz B. RWE, Shell and Equinor confirmed for large sub-project of giant AquaVentus H2 plan. Recharge; 2021. https://www.rechargenews.com/wind/rwe-shelland-equinor-confirmed-for-large-sub-project-of-giant-aquaventus-h2-plan/2-11044399. [Retrieved October 2022]. [17] Staib C, Zhang T, Burrows J, Gillespie A, Havercroft I, Rassool D, Consoli C, Liu H, Erikson J, Loria P, Nambo H, Wu Y, Judge C, Gebremedhin R. Global status of CCS 2021: CCS accelerating to net zero., 2021, https://www.globalccsinstitute.com/wpcontent/uploads/2021/10/2021-Global-Status-of-CCS-Report_Global_CCS_ Institute.pdf. [Retrieved November 2022]. [18] Cu
ellar-Franca RM, Azapagic A. Carbon capture, storage and utilisation technologies: a critical analysis and comparison of their life cycle environmental impacts. J CO₂ Util 2015;9:82–102. https://doi.org/10.1016/j.jcou.2014.12.001. [19] CCS Institute. Fact sheet: transporting CO2., 2018, https://www.globalccsinstitute. com/wp-content/uploads/2018/12/Global-CCS-Institute-Fact-Sheet_TransportingCO2-1.pdf. [Retrieved November 2022]. [20] Intergovernmental Panel on Climate Change. Carbon dioxide capture and storage., 2005, https://www.ipcc.ch/report/carbon-dioxide-capture-and-storage/. [Retrieved 11 November 2022]. [21] Bui M, Adjiman C, Bardow A, Boston A, Brown S, Fennell P, Fuss S, et al. Carbon capture and storage (CCS): the way forward. Energy Environ Sci 2018;11 (5):1062–176. https://doi.org/10.1039/c7ee02342a. [22] Page B, Turan G, Zapantis A. Global status of CCS 2019: targeting climate change., 2019, https://www.globalccsinstitute.com/wp-content/uploads/2019/12/GCC_GLOBAL_ STATUS_REPORT_2019.pdf. [Retrieved November 2022]. [23] Steyn M, Oglesby J, Turan G, Zapantis A, Gebremedhin R. Global Status of CCS 2022: ambition to action., 2022, https://status22.globalccsinstitute.com/wpcontent/uploads/2022/11/Global-Status-of-CCS-2022_Download.pdf. [Retrieved 11 November 2022]. [24] Siemenski A, Zapantis A, Advisory Committee for the Circular Carbon Economy: Keystone to Global Sustainability Series. Blue hydrogen 2021., 2021, https://www. globalccsinstitute.com/resources/publications-reports-research/blue-hydrogen/. [Retrieved November 2022]. [25] The Parliamentary Office of Science and Technology. CO2 capture, transport and storage. Postnote; 2009. https://post.parliament.uk/research-briefings/post-pn-335/. [Accessed November 2022].

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[26] International Energy Agency (IEA). Technology roadmap carbon capture and storage., 2013, https://www.iea.org/reports/technology-roadmap-carbon-captureand-storage-2013. [Retrieved October 2022]. [27] International Energy Agency (IEA). Technology perspectives energy 2020 - special report on carbon capture utilisation and storage: CCUS in clean energy transitions [Retrieved October 2022]; 2020. https://doi.org/10.1787/208b66f4-en. [28] Durusut E, Mattos A. Industrial carbon capture, usage and storage (CCUS): business models, report for the department for business, energy and industrial strategy., 2018, https://www.gov.uk/government/publications/industrial-carbon-capture-usageand-storage-ccus-business-models. [Accessed October 2022]. [29] Gonzales V, Krupnick A, Dunlap L. Carbon capture and storage 101. Resources for the future., 2020, https://www.rff.org/publications/explainers/carbon-capture-andstorage-101/. [Retrieved November 2022]. [30] Hydro-Electric Corporation. HydroTasmania. (2023). Hydrogen. https://www.hydro. com.au/clean-energy/hydrogen. [31] Offshore Technology. Portugal Gas System, Portugal. Offshore Technology; 2021. https://www.offshore-technology.com/marketdata/portugal-gas-system-gaspipeline-portugal/. [32] HyDeal. HyDeal Espan˜a. https://www.hydeal.com/copie-de-hydeal-ambition; n.d.. [Accessed 25 October 2022]. [33] Direc¸a˜o-Geral de Energia e Geologia (DGEG). National Strategy for Hydrogen., 2020, https://www.dgeg.gov.pt/en/transversal-areas/international-affairs/energy-policy/ national-strategy-for-hydrogen/. [Accessed October 2022]. [34] Beswick RR, Oliveira AM, Yan Y. Does the green hydrogen economy have a water problem? ACS Energy Lett 2021;6(9):3167–9. https://doi.org/10.1021/ acsenergylett.1c01375.

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CHAPTER 15

Photobioreactor for hydrogen production Nimmy Srivastavaa and Jayeeta Chattopadhyayb a

Amity Institute of Biotechnology, Amity University Jharkhand, Ranchi, Jharkhand, India Amity Institute of Applied Sciences, Amity University Jharkhand, Ranchi, India

b

1 Introduction Algae have been known since the early days to produce hydrogen by utilizing water and sunlight [1], and way before in the early 1940s, it was discovered that Chlamydomonas reinhardtii has the inherent ability to produce hydrogen under anaerobic circumstances [2]. The discovery of the photolysis process using a biological substrate led to an advancement in the research based on the biophotolysis process [3–6]. The conventional methods used for the production of hydrogen at the industrial level are highly costly, and the aim of researchers is to find a costeffective substitute. Microorganisms since ages have been known for their ability to produce hydrogen utilizing an organic substrate or water catalyzed by enzymes like hydrogenase and nitrogenase [7]. A bioreactor is an industrially essential device for diversified cell growth under controlled and monitored conditions. It is a closed system ranging from a small size of 10 mL to about 500,000 L used at the industrial level. Bioreactors using microorganisms for hydrogen production can be divided into two groups based on the reaction type for hydrogen production (Fig. 1). 1. Dark anaerobic reaction-based bioreactors:
Water gas shift reaction-based bioreactors
Fermentation-based bioreactors 2. Photobioreactors are made up of either tubes or plastic tanks and are used to grow photosynthetic microorganisms like algae which utilize light as one of the essential components [8,9]:
Using cyanobacteria
Using green algae

Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00025-X

Copyright © 2023 Elsevier Inc. All rights reserved.

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medium flow pump

cells hollow-fibers

H2

medium

Fig. 1 A hollow fiber bioreactor for continuous H2 synthesis by Chlamydomonas reinhardtii CC-503 cw92 [mt+] is depicted schematically. (Reproduced with permission from Markov, S.A. Hydrogen production in bioreactors: current trends. Energy Procedia 2012;29:394–400.)

2 Photobioreactors for hydrogen production Photobioreactors are used mainly for large-scale cultivation of algae [10–15], but recently, they have been explored for hydrogen production [16]. Various designs of photobioreactors have been developed for the same purpose and are currently being utilized for hydrogen production from green algae. The relationship between the bioreactor design and hydrogen production efficiency by algae has been studied by various researchers [17]. The photobioreactor design can be classified into seven major design groups as discussed elsewhere.

2.1 Shaking flasks Erlenmeyer flasks are the most basic type of bioreactor used for algae cultivation. Shaking as the agitation method has also been utilized for hydrogen production. This has been used in studies of cell age optimization for hydrogen production [18], investigations of the effect of light intensity on Chlamydomonas hydrogen production [19], and studies of Tetraselmis (Platymonas) hydrogen production [20]. The use of a culture vessel with movement as the agitation method has the disadvantage of making continuous gas collection more difficult.

2.2 Stirred tank Stirred tank (STR) reactors, in various forms, are a typical type of photobioreactor used for hydrogen production at the lab scale. The simplest systems are bottles with magnetic stir bars but no sensors, where extraction of samples from the bottles is required to track parameters other than hydrogen production during the experiment. The benefit of such systems is that they are inexpensive and

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simple to use, allowing for the study of a large number of parallel cultures. Equivalent bottles with sensors typically placed from the top or the side, allowing logging of many parameters inside the reactor, are also widely used. The magnetic stir bar or impeller is employed for agitation in the stir tank type of the bioreactor. Temperature can be adjusted using flowing water vests, water baths, heating or cooling coils, or fans. The advantages of such systems are that they are simple to operate on a small scale, that there are numerous alternatives for inserting a large number of sensors, that effective mixing makes the culture highly homogeneous, and that internal illumination is conceivable in particular forms. The downsides are the low surfaceto-volume ratio and high energy consumption, as well as the fact that they are problematic for both outdoor cultivation and upscaling. Dilution methods for prolonged hydrogen generation [21], starch metabolism and e-pathways during hydrogen production [22], and culture parameters during hydrogen production from Chlorella were all studied in cylindrical bottles with magnetic stirring [23]. Similar setups have been utilized using Erlenmeyer flasks and magnetic stirring [24–26]. Following a precultivation stage in an outdoor vertical tube sparging photobioreactor, cylindrical bottles were used for hydrogen production [27]. Simple bottles with impeller stirring are also sometimes used to produce hydrogen [28].

2.3 Horizontal tubular Because of its outstanding productivity, the horizontal tubular-type photobioreactor is a popular design for upscaled microalgae culture [29]. The horizontal tubes are normally relatively thin, offering a high surface-to-volume ratio, and also stacked in fence-like structures to give optimal illumination efficiency for the lowest use of the land area. Temperature regulation in these structures might be difficult at times. In horizontal tubular reactors, the culture is agitated by pumping it through the tubes. The disadvantages of employing this sort of reactor are the poor gas exchange and the substantial gas gradient along the tubes created by the majority of the gas exchange taking place in a separate chamber. Other drawbacks include high energy input and the formation of biomass in the tubes [11,12,29,30].

2.4 Coiled tubular A tubular coiled photobioreactor is comparable to a horizontal tube reactor in many ways [31]. Pumps circulate the culture and bubble with air/CO2, and the temperature can be regulated with water spraying or by utilizing heating or cooling coils. The benefits include a high surface-to-volume ratio

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and the fact that a conic helical shape can boost light harvesting efficiency per land area. The downsides of this reactor type are limited gas exchange, high shear stress, biomass accumulation in the tubes, and high energy input.

2.5 Vertical tubular Vertical tube photobioreactors are typically agitated by air sparging. Water spraying is the most popular method for temperature control in outdoor systems for this bioreactor type; however, heating-cooling coils or water jackets have also been utilized, with the latter being more common in lab-scale systems. Such a system has the advantages of efficient gas exchange, low shear stress, effective mixing, low energy consumption, and low cost. The disadvantages of outdoor gardening include a low surface/volume ratio and inadequate solar radiation capture. These designs are inconvenient for hydrogen production since culture agitation is dependent on either gas input, which dilutes the produced hydrogen, or gas circulation, which increases the possibility of leakage.

2.6 Immobilization Algae can be immobilized in a variety of matrices, including alginate beads, biofilm on solid surfaces, and alginate films. When compared to traditional liquid cultures, this unique structure has numerous advantages. The films can be made of a very thin layer of cells, resulting in an excellent surface-tovolume ratio and high cell density with a very short light path. There is no shear stress, the cells are protected within the gel, and the cells can be harvested easily. The fact that food availability may be quickly changed is a crucial element of this approach for hydrogen generation. The first demonstration of hydrogen production from immobilized microalgae was in a system in which the algae were immobilized on a glass fiber matrix [32]. Immobilization in alginate films created a protective matrix in which the algae produced hydrogen for an extended length of time, with continuous production for 90 days achieved by feeding limited amounts of sulfur [33]. Algae encased in alginate films were found to be more resistant to O2 inactivation of hydrogen generation, presumably due to slower O2 diffusion through the alginate matrix [34]. Algae immobilized in alginate films were also utilized to demonstrate greater hydrogen production throughout different light/dark phases [35] and when different C-sources and pH levels were used [36]. Hydrogen pressure has been demonstrated

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to reduce the efficiency of hydrogen production in immobilized cultures [37]. It has also been demonstrated that hydrogen may be produced successfully from immobilized cyanobacteria and other phototrophic microorganisms [38,39].

2.7 Flat panel Flat panel photobioreactors have numerous advantages and are extensively employed in lab-scale algae cultivation investigations; upscaled versions have also been utilized outdoors. The benefits of this sort of photobioreactor include a high surface-to-volume ratio and the ability to have a very short and evenly dispersed light route throughout the reactor. This bioreactor type can also be flexible in terms of light capture angle, and depending on the agitation mechanism utilized, a high mixing rate with minimal shear stress can be produced. A research using horizontal flat panel photobioreactors with magnetic stirring discovered that the hydrogen headspace concentration inhibited hydrogen production efficiency [37,40,41]. Tamburic et al. employed flat plate bioreactors for hydrogen synthesis with agitation by sparging with recirculated gas, avoiding hydrogen inhibition by utilizing a clever technique for isolating the hydrogen from the remainder of the gas phase [38]. Agitation method employed including sparging is the most popular approach while growing algae. Magnetic stirring, impeller stirring, baffles, and rocking motion are other approaches. Temperature control methods include an internal or external heating/cooling coil, water circulation in a separate surface compartment, and a heat exchanger on the reactor’s surface. The drawbacks of hydrogen production are connected to agitation methods. Agitation by sparging entails either dilution of the hydrogen gas or recirculation of the generated gas, both of which may increase the danger of leakage. Agitation by stirring, whether magnetic stirring or impeller, necessitates a large amount of energy.

3 Materials used for different components of the photobioreactor Various materials used for different components of the photobioreactor should fit under the criteria such as heat resistance, autoclavability, oxidation/corrosion resistance, and durability. Other secondary criteria are availability on the market, hydrogen leakage, weight, transparency when required, and potentially any harmful impact on microalgae cultures.

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4 Metals used for construction The interior frame is made of stainless steel, a strong and easily formed material. Temperature, pH, and salinity are all factors that influence the corrosion resistance of steel alloys (Table 1). When using a photobioreactor to cultivate marine strains, the salt mixed with heat during autoclaving has a substantial impact on corrosion resistance. The outer frames are mostly made of aluminum alloy 5052 H32, a lightweight material with enough strength for the job. This alloy is more corrosion resistant than other aluminum alloys. It is especially vital to pick a lightweight metal for the outside frame, as the inner frame’s steel alloy makes the reactor extremely hefty.

4.1 Coating Many materials widely utilized in the construction of photobioreactors have the ability to limit algal growth. In addition to many other benefits, a protective coating may provide shielding effects in this regard (Table 1). The inner frame, constructed of stainless steel with potentially dangerous elements Cr, Ni, and Mo, was coated with the fluoropolymer perfluoroalkoxy polymer resin (PFA) (Table 1), a material from the Teflon family. Color is present in many PFA resins. This is owing to the enhanced application convenience. Even though the composition of PFA resins is typically protected by IP rights and hence lacks composition statements, there are signs that these color components frequently contain particular metal oxides that, if released from the material, might potentially harm algae.

4.2 Glass The bioreactor’s glass windows were composed of fortified soda-lime-silica glass (Table 1, material 4), which proved resistant to autoclaving under physical stress induced by tension between the glass and the metal frame when placed in the frame. Standard soda-lime-silica glass, often known as window glass, was found to be insufficient due to a tendency to crack during autoclaving. Borosilicate glass is used to make the gas collection unit and the gas exhaust tube (Table 1). To reduce additional tension, the units are autoclaved separately and then installed on the metal connection. Current R&D efforts are focused on developing tubular PVC or plastic bag photobioreactors that incorporate cyanobacteria and green algae.

Table 1 Materials used in the construction of photobioreactors. Part

No.

Material

Advantage

Disadvantage

Inner frame

1

Stainless steel AISI 316L

– –

High strength Less corrosive than other SS alloys

– –

Inner frame coating/ coating of bolts

2

Primer: primer black 420–710 (Du Pont) Outer coating: PFA powder sparkling clear 532–7000 (Du Pont)

– – –

Inert Ease of cleaning Replaces lubricant for O-rings Protects steel from culture exposure, reducing the risk of corrosion Resistant to autoclaving Protects algae against metal exposure Prevents algae attachment Lightweight Sufficient strength Less corrosive than other aluminum alloys High transparency High strength

–

– – – Outer frame

3

Aluminum alloy 5052 H32

– – –

Glass windows

4

Fortified soda-lime glass

– –

– –

–

Heavyweight Alloy contains the potential toxic elements Cr, Ni, and Mo Potentially corrosive Sensitive to scratches

Strength sensitive jagged edges

to

Continued

Table 1 Materials used in the construction of photobioreactors—cont’d Part

No.

Material

Advantage

Bolts Gasket between glass sides and outer frame O-ring seal between inner frame and glass sides O-ring seal between inner frame and bolts, electrodes, and ports Septa of sampling ports

5 6

Stainless steel 316 Silicon rubber 40SH

– –

7

Viton FKM 70 shore (Eriks) Viton FKM 75 shore (Eriks)

9

Gas collection unit Exhaust tube for gas during the bubbling phase Inlet tube for air/ CO2 mixture Outlet tube for water displacement from gas collection unit

Disadvantage

–

High strength Protects against tension between glass and metal Hydrogen tight

–

–

Hydrogen tight

–

GR-2 silicon rubber (Supelco)

– –

10 11

Borosilicate glass Borosilicate glass

– – –

Hydrogen tight Self-sealing after needle penetration Low cost Transparent Transparent

12

Silicone peroxide, crosslinked (VWR)

– –

Transparent Very flexible

8

Needs replacement Needs replacement

frequent

–

Needs replacement

frequent

– –

Breakable Breakable

–

Cannot be exposed to hydrogen, due to high hydrogen penetration

frequent

Reproduced with permission from Skja˚nes K, Andersen U, Heidorn T, Borgvang SA. Design and construction of a photobioreactor for hydrogen production, including status in the field. J Appl Phycol 2016;28:2205–2223. https://doi.org/10.1007/s10811-016-0789-4. Epub 2016 Jan 27.

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4.3 Photobioreactors incorporating cyanobacteria Cyanobacteria are prokaryotic microorganisms that use plant-type O2-evolving photosynthesis. Cyanobacteria are classified as heterocystous or nonheterocystous. Under aerobic conditions, specialized cells called heterocysts are the location of H2 generation in heterocystous strains. Nitrogenase is a key enzyme in heterocystous cyanobacteria H2 synthesis. In comparison to green algae, water is indirectly involved in H2 synthesis by heterocystous cyanobacteria via a long chain of events, as shown in the scheme: Vegetative cells undergo reactions: H2 O ! photosystem II ! photosystem I ! ferredoxin ! ½CH2 On ðorganiccompoundsÞ Heterocysts undergo reactions: ½CH2 On ! photosystem I ! ferredoxin ! nitrogenase ! H2

4.4 Photobioreactors which use green algae Green algae may make H2 directly from water via a hydrogenase-catalyzed reaction, but only for a brief time under anaerobic conditions because hydrogenase is extremely sensitive to O2. H2 O ! photosystem II ! photosystem I ! ferredoxin ! hydrogenase ! H2 Because of the oxygen sensitivity of green algal hydrogen production, there have only been a few publications on photobioreactors containing green algae [41]. Green algae are one of the largest (with over 7000 species) and most diversified classes of algae in freshwater, but they also exist in marine habitats.

References [1] Benemann J. Hydrogen biotechnology: progress and prospects. Nat Biotechnol 1996;14:1101–3. [2] Gaffron H, Rubin J. Fermentative and photochemical production of hydrogen in algae. J Gen Physiol 1942;26:219–40. [3] Burgess SJ, Tamburic B, Zemichael F, Hellgardt K, Nixon PJ. Solar-driven hydrogen production in green algae. In: Laskin AI, Sariaslani S, Gadd GM, editors. Advances in applied microbiology, 75. San Diego, CA: Elsevier; 2011. p. 71–110. [4] Eroglu E, Melis A. Photobiological hydrogen production: recent advances and state of the art. Bioresour Technol 2011;102:8403–13.

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[5] Dubini A, Ghirardi ML. Engineering photosynthetic organisms for the production of biohydrogen. Photosynth Res 2015;123:241–53. [6] Gonzalez-Ballester D, Luis Jurado-Oller J, Fernandez E. Relevance of nutrient media composition for hydrogen production in Chlamydomonas. Photosynth Res 2015;125:395–406. [7] Benemann JR. The technology of biohydrogen. In: Zaborsky OR, editor. Biohydrogen. New York: Plenum Press; 1998. p. 19–30. [8] Janssen M, Tramper J, Mur LR, Wijffels RH. Enclosed outdoor photobioreactors: light regime, photosynthetic efficiency, scaleup and future prospects. Biotechnol Bioeng 2003;81:193–210. [9] Carvalho AP, Meireles LA, Malcata FX. Microalgal reactors: a review of enclosed system designs and performances. Biotechnol Prog 2006;22:1490–506. [10] Borowitzka MA. Commercial production of microalgae: ponds, tanks, tubes and fermenters. J Biotechnol 1999;70:313–21. [11] Molina E, Fernandez J, Acien FG, Chisti Y. Tubular photobioreactor design for algal cultures. J Biotechnol 2001;92:113–31. [12] Posten C. Design principles of photo-bioreactors for cultivation of microalgae. Eng Life Sci 2009;9:165–77. [13] Morweiser M, Kruse O, Hankamer B, Posten C. Developments and perspectives of photobioreactors for biofuel production. Appl Microbiol Biotechnol 2010;87:1291–301. [14] Chen C-Y, Yeh K-L, Aisyah R, Lee D-J, Chang J-S. Cultivation, photobioreactor design and harvesting of microalgae for biodiesel production: a critical review. Bioresour Technol 2011;102:71–81. [15] Olivieri G, Salatino P, Marzocchella A. Advances in photobioreactors for intensive microalgal production: configurations, operating strategies and applications. J Chem Technol Biotechnol 2014;89:178–95. [16] Dasgupta CN, Gilbert J, Lindblad P, Heidorn T, Borgvang SA, Skja˚nes K, Das D. Recent trends on the development of photo-biological processes and photobioreactors for the improvement of hydrogen production. Int J Hydrog Energy 2010;35:10218–38. [17] Akkerman I, Janssen M, Rocha J, Wijffels RH. Photobiological hydrogen production: photochemical efficiency and bioreactor design. Int J Hydrog Energy 2002;27:1195–208. [18] Kim JP, Kang CD, Sim SJ, Kim MS, Park TH, Lee D, Kim D, Kim JH, Lee YK, Pak D. Cell age optimization for hydrogen production induced by sulfur deprivation using a green alga Chlamydomonas reinhardtii UTEX 90. J Microbiol Biotechnol 2005;15:131–5. [19] Sim SJ, Kim JP, Park TH, Kim MS. The effective hydrogen production by light intensity control using green alga Chlamydomonas reinhardtii under the sulfur deprived environment. In: International hydrogen energy congress and exhibition—IHEC, Istanbul, Turkey; 2005. [20] Guan YF, Deng MC, Yu XJ, Zhang W. Two-stage photo-biological production of hydrogen by marine green alga Platymonas subcordiformis. Biochem Eng J 2004;19:69–73. [21] Laurinavichene TV, Tolstygina IV, Galiulina RR, Ghirardi ML, Seibert M, Tsygankov AA. Dilution methods to deprive Chlamydomonas reinhardtii cultures of sulfur for subsequent hydrogen photoproduction. Int J Hydrog Energy 2002;27:1245–9. [22] Chochois V, Dauvillee D, Beyly A, Tolleter D, Cuine S, Timpano H, Ball S, Cournac L, Peltier G. Hydrogen production in Chlamydomonas: photosystem II dependent and -independent pathways differ in their requirement for starch metabolism. Plant Physiol 2009;151:631–40. [23] Alalayah WM, Alhamed YA, Al-Zahrani A, Edris G. Influence of culture parameters on biological hydrogen production using green algae Chlorella vulgaris. Rev Chim 2015;66:788–91.

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[24] Hahn JJ, Ghirardi ML, Jacoby WA. Effect of process variables on photosynthetic algal hydrogen production. Biotechnol Prog 2004;20:989–91. [25] Timmins M, Thomas-Hall SR, Darling A, Zhang E, Hankamer B, Marx UC, Schenk PM. Phylogenetic and molecular analysis of hydrogen-producing green algae. J Exp Bot 2009;60:1691–702. [26] Volgusheva A, Styring S, Mamedov F. Increased photosystem II stability promotes H2 production in sulfur-deprived Chlamydomonas reinhardtii. Proc Natl Acad Sci U S A 2013;110:7223–8. [27] Geier SC, Huyer S, Praebst K, Husmann M, Walter C, Buchholz R. Outdoor cultivation of for photobiological hydrogen production. J Appl Phycol 2012;24:319–27. [28] Antal TK, Krendeleva TE, Laurinavichene TV, Makarova VV, Ghirardi ML, Rubin AB, Tsygankov AA, Seibert M. The dependence of algal H2 production on photosystem II and O2 consumption activities in sulfur-deprived Chlamydomonas reinhardtii cells. Biochim Biophys Acta Bioenerg 2003;1607:153–60. [29] Slegers PM, van Beveren PJM, Wijffels RH, van Straten G, van Boxtel AJB. Scenario analysis of large scale algae production in tubular photobioreactors. Appl Energy 2013;105:395–406. [30] Xu L, Weathers PJ, Xiong XR, Liu CZ. Microalgal bioreactors: challenges and opportunities. Eng Life Sci 2009;9:178–89. [31] Lindblad P, Christensson K, Lindberg P, Fedorov A, Pinto F, Tsygankov A. Photoproduction of H2 by wildtype Anabaena PCC 7120 and a hydrogen uptake deficient mutant: from laboratory experiments to outdoor culture. Int J Hydrog Energy 2002;27:1271–81. [32] Laurinavichene TV, Fedorov AS, Ghirardi ML, Seibert M, Tsygankov AA. Demonstration of sustained hydrogen photoproduction by immobilized, sulfur-deprived Chlamydomonas reinhardtii cells. Int J Hydrog Energy 2006;31:659–67. [33] Laurinavichene TV, Kosourov SN, Ghirardi ML, Seibert M, Tsygankov AA. Prolongation of H2 photoproduction by immobilized, sulfur-limited Chlamydomonas reinhardtii cultures. J Biotechnol 2008;134:275–7. [34] Kosourov SN, Seibert M. Hydrogen photoproduction by nutrient deprived Chlamydomonas reinhardtii cells immobilized within thin alginate films under aerobic and anaerobic conditions. Biotechnol Bioeng 2009;102:50–8. [35] Rashid N, Lee K, Mahmood Q. Bio-hydrogen production by Chlorella vulgaris under diverse photoperiods. Bioresour Technol 2011;102:2101–4. [36] Rashid N, Lee K, Jong-in H, Gross M. Hydrogen production by immobilized Chlorella vulgaris: optimizing pH, carbon source and light. Bioprocess Biosyst Eng 2013;36:867–72. [37] Kosourov SN, Batyrova KA, Petushkova EP, Tsygankov AA, Ghirardi ML, Seibert M. Maximizing the hydrogen photoproduction yields in Chlamydomonas reinhardtii cultures: the effect of the H2 partial pressure. Int J Hydrog Energy 2012;37:8850–8. [38] Tamburic B, Zemichael FW, Crudge P, Maitland GC, Hellgardt K. Design of a novel flat-plate photobioreactor system for green algal hydrogen production. Int J Hydrog Energy 2011;36:6578–91. [39] Leino H, Kosourov SN, Saari L, Sivonen K, Tsygankov AA, Aro E-M, Allahverdiyeva Y. Extended H-2 photoproduction by N2- fixing cyanobacteria immobilized in thin alginate films. Int J Hydrog Energy 2012;37:151–61. [40] Liao Q, Zhong N, Zhu X, Huang Y, Chen R. Enhancement of hydrogen production by optimization of biofilm growth in a photobioreactor. Int J Hydrog Energy 2015;40:4741–51. [41] Ghirardi ML, Zhang L, Lee JW, Flynn T, Seibert M, Greenbaum E, Melis A. Microalgae: a green source of renewable H2. Trends Biotechnol 2000;18:506–11.

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CHAPTER 16

Thermochemical hydrogen production Priti Singha, Sushant Kumarc, Nimmy Srivastavaa, Rohit Srivastavab, and Jayeeta Chattopadhyayc a

Amity Institute of Biotechnology, Amity University Jharkhand, Ranchi, Jharkhand, India Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India c Amity Institute of Applied Sciences, Amity University Jharkhand, Ranchi, India b

1 Introduction Overall, 96% of H2 generation technologies are based on nonrenewable resources, with coal gasification with a water gas shift (WGS) reaction coming in second after natural gas steam reforming (48%) and oil steam reforming (30%). Water electrolysis produces only 4% of H2 that is generated. Consequently, new sustainable processes produced from renewable sources must be created to reach the targets for reducing CO2 release and fossil fuel consumption. Currently, renewable hydrogen can be produced using a variety of methods and technologies as follows: (a) solar conversion: either thermal decomposition by solar-generated heat for hydrogen production or photolysis using electrochemical systems for hydrogen production; (b) electrolysis: splitting water into hydrogen and oxygen using electricity energy from one of the many renewable sources; and (c) biomass hydrogen: thermochemical or biochemical conversion. It is possible to produce hydrogen indirectly from renewable resources using electrolysis and electricity. The production of hydrogen using thermochemical processes has a reasonably obvious outcome. Finding thermochemical technologies is preferable for converting heat into electricity and then hydrogen through water electrolysis in several ways. The Energy Depot project results were published [1,2], and these were not exactly promising. However, the first H2 gathering, named THEME, which was held in Miami in 1972 and was coordinated by Dr. Nejat Veziroglu, attracted a lot of interest and enthusiasm. One of the potential methods that is frequently seen as water electrolysis’ rival is thermochemical water breakdown. According to the ratio of the higher heating value of hydrogen to the thermal energy input to produce Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00019-4

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electricity for electrolysis, net electrolysis efficiencies, for instance, are typically around 24%, whereas thermochemical hydrogen using nuclear heat can achieve heat-to-hydrogen efficiencies of up to about 50%. The three basic energy supply classes that can be employed to execute the hydrogen economy—fossil fuels, nuclear energy, and renewable energy sources—have been thoroughly compared. The renewable energy sources include energy from hydropower, wind, and solar energy systems, as well as systems for converting ocean thermal energy, geothermal energy, and solar energy (including biomass, photovoltaic, and solar thermal technologies) [3]. The investigations into alternative hydrogen-generating systems have also been conducted [4]. For instance, researchers have looked at the potential applications of bio-catalysis, photocatalysis, and bio-photocatalysis to produce hydrogen [5,6]. Numerous studies on the thermochemical hydrogen production process have been published [7], along with a thorough list of historical and contemporary activities [8]. Additionally, specific types of thermochemical hydrogen production, such as solar and nuclear [9,10], have been examined. The current chapter will only focus on the issues, challenges, and prospective on the thermochemical conversion of biomass into hydrogen.

2 Thermochemical conversion of biomass into hydrogen One of the most promising technologies is the thermochemical conversion of biomass, which can produce hydrogen for industrial and commercial uses from biomass in a way that is both economical and environmentally beneficial. This would be a significant advancement. Agricultural waste, lignocellulosic biomass, and energy crops, as well as their by-products like biogas, biooil, and biochar, can all be classified as fundamental groups of biomass materials that can be used as a feedstock to produce low-cost and lowcarbon hydrogen. A variety of cultivars that are available in each country and their growth cycles, as well as the various demands that each cultivar has for other uses, as well as the costs of harvesting, transporting, and pretreatment, are the main considerations when choosing biomass as a feedstock to produce hydrogen. Municipal bio-waste and agricultural residues are also viable options for low-sulfur energy sources. Because they do not directly compete with any food crops, the prospective energy crops are an excellent option for producing hydrogen. Both for remediation and to meet the need for bioenergy, large-scale cultivation of energy crops is required in contaminated soils. The physicochemical properties and morphological structures of

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various types of biomasses, and even the same types of biomasses obtained from various resources, might vary. These variations and irregularities in biomass can have a negative impact on conversion efficiency and end products. Crude biomass presents significant hurdles for the thermochemical conversion process due to its low bulk density, high oxygen content, and high levels of inorganic chemicals such as potassium, calcium, sodium, silica, phosphorus, and chlorine compounds. It is important to consider how producing hydrogen from biomass-derived fuel may affect the environment and human health. To encourage pyrolysis and gasification as catalytic effects, the intrinsic metal salt values in the biomass should be emphasized. Fig. 1 illustrates the many processes used to produce hydrogen from various feedstocks produced from biomass. The thermochemical process may convert a range of wet biomass and use the complete biomass-derived fuel, making it easier to produce hydrogen than chemical or biological methods. It also typically requires no chemical addition. The use of sorbent for in situ CO2 removal is a key factor in the thermochemical process’ improved hydrogen production, but studies show that adding more sorbent—more than the required amount—does not significantly enhance intensification performance [3]. Regardless of the numerous advantages of producing hydrogen from biomass, it is not currently being used on a commercial scale because of a number of difficulties related to the widely dispersed feedstock obtained from biomass and the general inadequacies of the thermochemical conversion technique. The issues, difficulties, and developments related to

Fig. 1 Hydrogen production from various biomass resources.

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the various types and characteristics of biomass and the environmentally friendly thermochemical conversion to hydrogen production are thus highlighted in this chapter and are anticipated to be helpful to researchers, students, industry players, and government officials. The hydrogen production from various biomass resources is shown in Fig. 1.

2.1 Pyrolysis and gasification Pyrolysis and gasification were initially investigated as extremely affordable technologies to generate reliable hydrogen generation from biomass. Numerous research groups investigated pyrolysis and gasification of biomass using steam, air/oxygen, CO2, or catalysis by using different devices [11–15]. The conversion of biomass was carried out at experimental temperatures of up to 1000–1400°C under solar irradiation to conserve energy and showed that employing solar energy is feasible. 2.1.1 Biomass pyrolysis process Biomass pyrolysis has been proposed as a fundamental technique in the bioenergy process to manufacture chemicals and fuels, and it serves as the primary subprocess in the thermochemical hydrogen generation and gasification. The in-depth principles and dynamics of biomass pyrolysis have been examined and explored [16,17]. Research has shown that biomass can be pyrolyzed at high temperatures to produce gaseous compounds [18,19]. However, the hydrogen content of pyrolysis-derived gaseous products is still too low from a commercial standpoint. Utilizing catalytic pyrolysis is one method for improving the hydrogen yield. According to Watanabe et al., basic sites in TiO2 cause glucose to isomerize into fructose, whereas acid sites promote further dehydration to 5-hydroxymethylfurfural (5-HMF) [20] Ca and Cr oxides appeared to function the best when Chen et al. investigated the impact of catalysts on fast pyrolysis [21]. Duman and Yanik investigated the effects of different char-based catalysts on hydrogen production from the steam pyrolysis of olive pomace in a two-stage axial bed reactor. The findings revealed that the steam had no impact on hydrogen production and that both Ni-impregnated and nonimpregnated biomass char catalysts can increase hydrogen production [22]. The process conditions should be high temperature and lengthy residence time, and the catalyst should be taken into account if the goal of pyrolysis is to optimize the hydrogen yield.

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2.1.2 Biomass gasification process It is important to highlight the potential of different gasification methods to create hydrogen to demonstrate abundance, cheap cost, and a variety of uses for biomass conversion. Several review articles on biomass gasification have recently been published; however, few of these concentrated on the general flaws and performance of process intensification [7–9]. For a plant with an expected hydrogen output of 139,700 kg per day and a cost of biomass in the range of $46–80 per dry ton, the hydrogen production cost is anticipated to be $1.77–2.05 per kg [23]. It is estimated that a typical route for biomass gasification and steam reforming (and/ or WGS) with a pressure swing adsorber system (PSA) requires 2.4 TJ of primary energy input per TJ of hydrogen [10]. Given that biomass gasification is a series of intricate thermochemical processes, it is implausible to divide the gasifier into various zones that carry out numerous simultaneous reactions. The steps and temperature variations involved in the gasification process are drying, pyrolysis, combustion, and reduction zones. Combustible gases like CO, H2, and CH4 are combined with noncombustible gases like CO2 and N2 to create the gaseous product. A good grade gas from the gasifier should have a high hydrogen and a low tar percentage for hydrogen production. A number of factors such as temperature, pressure, residence duration, equivalency ratio, biomass properties, and gasifier design can have an impact on the quality of gas produced by biomass gasification. Biomass that is air-gasified only yields syngas with a low H2 content and little heating. While the hydrogen yield in steam gasification is significantly higher than that in pyrolysis and air gasification, the overall efficiency of converting thermal energy to hydrogen can also reach up to 52%. Although biomass oxygen-rich air gasification is one efficient technique of producing medium heating value gas, its disadvantage—requiring a significant investment for oxygen production— hinders its widespread use. According to experimental research published in earlier literature [24], steam gasification methods based on a fluidizedbed reactor are likewise capable of producing MHV of 10–16 MJ N
1 m3 gas with a 30–60vol% H2 content. This is because there is no nitrogen from air gasification in the end products and because a homogenous WGS reaction can boost hydrogen generation early in the gasification process. However, this technique needs steam that is over a temperature of 700°C, and good-performing steam generators come at an additional cost.

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2.1.3 Supercritical water gasification of biomass In the last 10 years, there has been an increase in interest in the supercritical water gasification of agricultural wastes, leather wastes, switchgrass, sewage sludge, algae, manure, olive mill wastewater, and black liquor. The main product components of H2, CH4, CO2, and CO content are very low because CO reacts further by the WGS and methanation reaction, lowering the CO content. Anthracene and furfural are the main intermediates in supercritical water gasification. They are efficient in clean conversion of biomass, due to their unique chemical and physical properties, as reported by Jin et al. Some microscopic reaction mechanisms may provide information for reactor design and operating condition selection [25]. The experiments showed that at temperatures above 700°C, corn silage and clover grass can practically completely convert in supercritical water gasification, and hydrogen contents ranging from 26% to 57% were produced [26]. According to the report of Chen et al. on hydrogen production using sewage sludge gasification in supercritical water in a fluidized bed reactor, a carbon gasification efficiency of more than 40% was attained at 540°C and 25 MPa for a feedstock containing 2% sodium carboxymethyl cellulose [27]. In supercritical water gasification, the use of catalysts like Ni, Ru, Pt, and various alkali metal-based compounds can lower the temperature and pressure, which lowers the equipment and operating expenses [28]. High biomass conversion and high hydrogen yields were obtained from the production of hydrogen from the supercritical water gasification of oleic acid with a Ru/Al2O3 catalyst; without the catalyst, the majority of the oleic acid remained unconverted [29]. To investigate hydrogen yields, pinecone biomass was gasified in subcritical, near-critical, and supercritical water. The results showed that for the noncatalytic gasification of the pinecone, the supercritical water produced the gas products with the highest hydrogen concentration of 1.42 mmol g
1 and the highest heating value of 488 kJ N
1 m3. The greatest hydrogen yield was raised to 3.26 mmol g
1 at 30-wt% catalyst loading [30]. 2.1.4 Chemical looping gasification of biomass An innovative approach with low cost for chemical looping gasification (CLG) replaces the gasification agent (O2) with an oxygen carrier (OC), generating syngas from biomass feedstock. After using OC oxygen as a source of oxygen for combustion and gasification, the decreased OC can then be renewed in the air reactor. To improve the CLG conversion of biomass, Huang et al. used steam in addition to natural iron ore as an OC for the CLG of biomass char in a fixed-bed reactor. The results revealed that this

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combination of iron ore, steam, and redox processes produced a char gasification rate of 80% [31]. Wang et al. also studied the CLG mechanism of redox reactions involving biomass and OCs, and Gibbs free energy minimization techniques were used to conduct a thermodynamic analysis of the CLG of biomass by Mn2O3 OC for the formation of syngas [32]. The total dry concentration of CO and H2 was estimated to be 98.8% under the recommended conditions, and substantial carbon conversion was also attained [32,33]. When producing H2-rich syngas from bauxite residues in a fluidized bed reactor, Chen et al. researched the multifunctional iron-based OC and assessed the viability of autothermal CLG based on the OC roles as heat and oxygen transporters [34]. A concept for the CLG of biomass using natural hematite as the OC, rated at 25 kWth, was investigated [35]. In this case, 860°C was found to be the ideal gasification temperature, correlating to high carbon conversion and syngas yields. This was obtained by considering the impacts of temperature, the ratio of steam to biomass, and the mass percentage of hematite [35]. The analysis of the CLG of biomass in a fixed-bed reactor revealed that CO was produced more quickly than other components due to the partial oxidation of carbon and the rapid reaction and consumption of H2 by the lattice oxygen [O] in the OC [36]. Two fluidized-bed reactors were used to study the synthesis of hydrogen from agricultural biomass utilizing steam as the gasifying agent by CLG with in situ CO2 capture. According to the research, the produced gas had a 71% H2 content and almost 0% CO2 concentration. A temperature of 800°C could regenerate 40% CaCO3 to CaO [37]. In these investigations, two fluidized bed reactors connected by solid CaO are used to produce hydrogen from biomass gasification using a CLG. By employing in situ CO2 removal with CaO-based sorbent in the first carbonator, the gasification and WGS process can be improved and enhanced, and the amount of tar may be reduced. The obtained CO2 is discharged and gathered in the second regenerator. Dual fluidized-bed reactors were used in a constantly operating 20 kWth CLG demonstrated by Armbrust et al. [38]. The results showed that the H2 concentration of 84.4 vol% and a CO conversion of 76.4% were reached after the effects of the key parameters, such as temperature, looping ratio (LR), and space-time, were examined. Moghtaderi et al. carried out an experimental examination on biomass gasification for high-purity hydrogen production by CLP over a temperature range of 650–900°C and pressures up to 20 atm. The findings confirmed that high pure hydrogen and CO2 capture efficiency of 56.4% could be reached and that concrete and demolition debris could be employed as a source of sorbent for in situ CO2 removal [39].

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2.2 Steam reforming process 2.2.1 Using biomass-derived feedstock By pyrolysis and gasification, biomass can be converted into bio-oil or syngas, albeit the end products are typically employed as low-quality fuels due to their complicated chemical makeup and low hydrogen content. Several reforming techniques over catalysts, including steam reforming, dry reforming, autothermal reforming, partial oxidation, and chemical looping reforming, can produce high hydrogen yields. A schematic illustration of the synthesis of hydrogen from biomass using various reforming techniques is shown in Table 1 [32,40–43]. The most advanced industrial process, steam reforming, has the largest hydrogen production and the lowest operating

Table 1 Various synthetic approaches of hydrogen production from various biomass resources. Technology

Advantage

Disadvantage

Reference

Steam reforming

Most advanced industrial process, best H2/CO ratio, NO oxygen required, lowest operation temperature Environmental potential for CO2 emission

High water and energy consumption and CO2 emission

[40]

Low H2/CO ratio, high operating temperature and limited commercial experience Limited commercial experience

[41]

High operating temperature, a low H2/CO ratio, and a challenging handling approach Limited commercial experience

[42]

Dry reforming

Autothermal reforming

Partial oxidation reforming

Chemical looping reforming

Lower process temperature than partial oxidation and low methane slip Lowered desulfurization requirements, no need for a catalyst, and less methane slip High carbon capture and less energy consumption

[32]

[43]

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costs. The in situ hydrogen synthesis using steam reforming of liquid biomass-derived feedstock as the hydrogen carrier may have a benefits in future due to the challenges in hydrogen transportation and storage. Methanol, ethanol, and glycerol are some desirable hydrogen sources among all the carriers and have relatively high H/C ratios. The endothermic steam reforming reactions bring more difficulties and prospects for usage in the future because they can take place in the gas or aqueous phase. Recent advancements in research have improved the main pathway for producing renewable hydrogen as an effective candidate in the future global energy scenario using different types of biomass-derived feedstock and materials from Table 2 [35,37,44–59]. The production of hydrogen from the steam reforming of feedstock derived from biomass at temperatures higher than 400°C has been demonstrated thermodynamically, and some studies have shown that the increase in temperature and steam used favor the production of hydrogen, while high pressure significantly reduces hydrogen yield. Low-cost Ni-based catalysts were widely used in these studies [57–59]. The application of high temperature in the steam-reforming process possesses two effects: first, WGS can shift toward CO and H2O, reducing the H2 production; second, decomposition increases the conversion of feedstock generated from biomass [44]. The steam-to-carbon ratio (S/C) in steam reforming is a crucial variable since raising this ratio will enhance conversion while simultaneously lowering carbon formation. A high steam partial pressure may accelerate the steam gasification of carbon.

3 Conclusion Despite the numerous benefits of producing hydrogen from biomass, there are several obstacles related to the widely dispersed feedstock of biomass and general inefficiencies in conversion methods, which prevent them from being commercialized. The industrial application of thermochemical processes like pyrolysis, gasification, and steam reforming is highly promising. The thermodynamic limitations of the reversible WGS reaction constrain the H2 productivity and feedstock conversion of conventional thermochemical processes. The high-temperature hydrogen production systems used catalysts made of noble metals and nickel, which demonstrated both catalytic stability and thermal stability.

Table 2 Various types of biomass-derived feedstocks and obtained products. Feeding

Catalyst

Reactor and process condition

Product

Reference

Biomass-derived syngas or gasification Biomass-derived ethanol

Ni-based catalyst, Ir-Ni

Fixed-bed reactor at 800–950°C, 1 bar, 114,000 h
1

H2, CO2

[35]

Ni-based, Pt/Al2O3, Rh/Al2O3, Rh-Pt/ CeO2, Co3O4/ CeO2 Pt/Al2O3 monoliths, Ni/Ce-Zr

Fixed-bed reactor with atmospheric pressure and 500–800°C

H2, CO2, CO, CH4 Carbon conversion > 95%

[37,44–46]

A tube reactor heated by an electric oven or fixed-bed reactor with atmospheric pressures of 500–800°C, S/C ¼ 2–5 Fixed-bed reactor, 500–850°C, S/C ¼ 3

H2, CO2, CO, CH4 H2O

[47,48]

H2, CO2, CO, CH4

[49,50]

Bench-scale unmixed steam reformer, 600–800°C Packed bed reactor, 600°C

H2, CO2, CO, CH4

[51]

H2 > 95%

[52]

H2, CO2, CO, CH4 Carbon conversion > 90% H2, CO2, CO, CH4,H2O, Carbon conversion > 90% H2, CO2, CO, CH4, H2O

[53,54]

Bio-diesel

Bio-diesel by-product glycerol Sunflower oil

Ni-based catalyst

Waste cook oil

Ni-based catalyst with CaO sorbent Ni/Al2O3, Ni-Co/ MgO, LaKMnO3, Fe/olivine Ni/Al2O3, Co-Fe/ Al2O3, Ni/TiO2, Y-zeolite

Biomass-derived pyrolysis oil or model Biomass tars model

Biomass-derived organic mixture (methanol, ethanol, phenol)

Ni-based catalyst

Ni/Al2O3

Plug flow reactor under near atmospheric pressure (1.07 bar), a residence time of 0.68 s and 400–1000°C Fixed-bed reactor, gliding are reactor, fluidized-bed reactor

Fixed-bed reactor at 773–1073 K

[55–58]

[59]

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ugas N, Zakarauskas K, Stravinskas G, Grigaitien_e V. Comparison of steam reforming and partial oxidation of biomass pyrolysis tars over activated carbon derived from waste tire. Catal Today 2012;196:67–74. [43] Luo M, Yi Y, Wang S, Wang Z, Du M, Pan J, et al. Review of hydrogen production using chemical-looping technology. Renew Sustain Energy Rev 2018;81:3186–214. [44] Rossetti I, Compagnoni M, Finocchio E, Ramis G, Michele AD, Zucchini A, et al. Syngas production via steam reforming of bioethanol over Ni–BEA catalysts: a BTL strategy. Int J Hydrogen Energy 2016;41:16878–89. [45] Hajjaji N, Khila Z, Baccar I, Pons MN. A thermos environmental study of hydrogen production from the steam reforming of bioethanol. J Energy Storage 2016;7:204–19. [46] Nanda S, Rana R, Zheng Y, Kozinski JA, Dalai AK. Insights on pathways for hydrogen generation from ethanol. Sustain Energy Fuels 2017;1:1232–45. [47] Nahar G, Dupont V, Twigg MV, Dvininov E. Feasibility of hydrogen production from steam reforming of biodiesel (FAME) feedstock on Ni-supported catalysts. Appl Catal B 2015;168–169:228–42. [48] Martin S, Kraaij G, Ascher T, Wails D, W€ orner A. An experimental investigation of biodiesel steam reforming. Int J Hydrogen Energy 2015;40:95–105. [49] Wu G, Zhang C, Li S, Han Z, Wang T, Ma X, et al. Hydrogen production via glycerol steam reforming over Ni/Al2O3: influence of nickel precursors. ACS Sustain Chem Eng 2013;1:1052–62. [50] Adhikari S, Fernando S, Gwaltney SR, To SDF, Bricka RM, Steele PH, et al. A thermodynamic analysis of hydrogen production by steam reforming of glycerol. Int J Hydrogen Energy 2007;32:2875–80. [51] Dupont V, Ross AB, Hanley I, Twigg MV. Unmixed steam reforming of methane and sunflower oil: a single reactor process for H2-rich gas. Int J Hydrogen Energy 2007;32:67–79. [52] Pimenidou P, Rickett G, Dupont V, Twigg MV. Chemical looping reforming of waste cooking oil in packed bed reactor. Bioresour Technol 2010;101:6389–97. [53] Cui Q, Xu S, Zhou C. Steam reforming of bio-oil from coconut shell pyrolysis over Fe/ olivine catalyst. Energ Conver Manage 2017;41:40–7. [54] Mei Y, Wu C, Liu R. Hydrogen production from steam reforming of bio-oil model compound and byproducts elimination. Int J Hydrogen Energy 2016;41:9145–52. [55] Guan G, Kaewpanha M, Hao X, Abudula A. Catalytic steam reforming of biomass tar: prospects and challenges. Renew Sustain Energy Rev 2016;58:450–61. [56] Liu S, Mei D, Wang L, Tu X. Steam reforming of toluene as biomass tar model compound in a gliding arc discharge reactor. Chem Eng J 2017;307:793–802. [57] Kaisalo N, Simell P, Lehtonen J. Benzene steam reforming kinetics in biomass gasification gas cleaning. Fuel 2016;182:696–703. [58] Kuba M, Havlik F, Kirnbauer F, Hofbauer H. Influence of bed material coatings on the water–gas-shift reaction and steam reforming of toluene as tar model compound of biomass gasification. Biomass Bioenergy 2016;89:40–9. [59] Chitsazan S, Sepehri S, Garbarino G, Carnasciali MM, Busca G. Steam reforming of biomass-derived organics: interactions of different mixture components on Ni/ Al2O3 based catalysts. Appl Catal B 2016;187:386–98.

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CHAPTER 17

Hydrogen production driven by nuclear energy Hari Pavan Sriram Yalamati, R.K. Vij, and Rohit Srivastava

Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India

1 Introduction Energy demand is increasing with increasing population and economies. This energy demand is met 80% by using fossil fuels, which are the leading cause of the increase in the greenhouse gases such as carbon dioxide (CO2), methane(CH4), nitrous oxide (N2O), and ozone(O3). These gases are the reason behind the adverse climate change that is currently happening across the planet. To curb this, the governments of all countries have prioritized reducing carbon dioxide (CO2) emissions from energy production, transportation, and other industrial activities [1]. Renewable energy is the primary energy source that can help reduce greenhouse gas emissions. Still, this technology has a few limitations in replacing fossil fuels for the energy requirements, such as on-demand energy production, adaptation cost, and the cost of the energy produced from this technology. For energy needs on this planet, burning fossil fuels has created significant pressure on the earth’s natural system, especially over the last five decades; an estimated 350 GT of carbon emissions were released into the atmosphere during this period. An approximate 290 GT of carbon is from the burning of fossil fuels and the remaining 60 GT from the change of land use. Where 45% of carbon is still present in the atmosphere, the oceans and land areas assimilate 55% of carbon [2]. Switching to renewable energy is the best way to reduce carbon emissions and limit global warming to the preindustrial revolution, which is 2°C. The rapid switch to renewables for green energy is cost-intensive. The best and presently available solution for the transition is nuclear energy. But reducing the amount of carbon dioxide in the atmosphere will take a long time and effort with the help of carbon capture, storage, and renewables. Even though nuclear energy has a significant amount of sharing in the electricity generation, there is a decline in the newly built nuclear power Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00017-0

Copyright © 2023 Elsevier Inc. All rights reserved.

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plants, according to the International Energy Agency. This trend started after the Fukushima nuclear disaster triggered by an earthquake in 2011. Before this, nuclear energy had a share of 16.7% of total electricity requirements in Japan, dropping to 7.5% in 2011. Currently, only 6.6% of Japan’s electricity is being produced from nuclear energy [3]. The golden period of nuclear energy was during world war II, as the war funded the research and the development of nuclear-based missiles, bombs, and mini-scale nuclear reactors installed, replacing the traditional naval fuel in the ships. Atomic energy for electricity generation has increased due to the energy crisis in the 1970s, which led France to adopt this for their power needs. France is the only country to produce 70% of its energy from nuclear energy. This technology has its limitations, too, such as radiation and nuclear waste management. The Chernobyl disaster showed the world how dangerous atomic power could be. Still today, the effects of that mistake are visible. Most countries are phasing out the old and high-maintenance nuclear reactors to save money and avoid disasters from the old technology [4].

2 Nuclear energy In the search for carbon-free energy for our daily needs, nuclear power is one of the proven and old technology that doesn’t emit any greenhouse gases. According to the International Energy Agency (IEA), in the past 50 years, by using nuclear energy, over 60 gigatonnes (GT) of carbon dioxide (CO2) emissions have been reduced till now, which is 2 years’ worth of global energy-related carbon emissions [5]. In 2020, the global nuclear electricity generation capacity was at 95 TWh, around 10% of the world’s total energy production. Renewable energy has the next most significant share in electricity generation. In 2021, energy-related CO2 emissions are at 31.5 GT, with the highest ever concentration of CO2 at 412.5 PPM, which is 50% more compared with the period of the rise of the industrial revolution. Compared with 2019, in 2021, 400 MT less CO2 was emitted in the energyrelated emissions of around 1.4% [6]. The International Energy Agency (IEA) has cited that renewable and nuclear energy will play a crucial role in reducing energy-related carbon emissions, a primary industry for the carbon emissions on this planet. By increasing atomic energy production, in IEA 2015 report predicts there will be 15% fewer carbon emissions by 2050 [7]. Nuclear energy has been used for energy generation for the last seven decades. The first nuclear reactor went live in the United States at the

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Argonne National Laboratory in Idaho. Later, the United States, the United Kingdom, the Union of Soviet Socialist Republics, France, and Germany were the first to use nuclear technology commercially. Then, nuclear energy saw a golden period between 1970 and 1985; there was an increase of 350% from 80 reactors in 1970 to 360 reactors in 1985 as shown in Fig. 1. The United States generates the most electricity using nuclear energy compared with any other nation on this planet. Nuclear power provides around 19.6% of the electricity demand for the country. Even though France produces less electricity from atomic energy, 69% of the electricity demand is supplied by nuclear power stations. Thus far, France is the only country with significant power from non-carbon emitting fuel. And the government is steadily increasing their capacity over the last 30 years.

Fig. 1 The amount of nuclear energy generated in different countries annually [8].

On the other hand, Germany and the UK are decreasing their dependence on nuclear power as the existing power plants are not profitable or significant component failure or deterioration. In India, only 3.2% of the energy needs are fulfilled by nuclear energy, around 39.76 TWh. India currently has seven fully functional nuclear reactors, and 22 are under construction, in which two pressurized light-water moderated and cooled reactors (PWR), two boiling light-water cooled and moderator reactors (BWR), and 18 pressurized heavy-water moderated and cooled reactors (PHWR) [8]. According to International Energy Agency, renewable energy share

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for electricity generation is around 23.2% in 2019, which is around 6529.422 TWh. Hydro and wind energy are the major contributors to renewable energy in 2019, with 66.3% and 21.9%, respectively. Four hundred thirty-nine operating nuclear reactors are generating 2789.694 TWh of electricity as shown in Fig. 2.

Fig. 2 Amount of electricity generated through different sources in India [9].

India mainly relies on coal for its electricity generation, around 72.5% in 2020, whereas nuclear energy stands only at 2.7%. Hydral power has the following significant share, but this power generation heavily relies on seasonal rainfall, and on the remaining days, the power generation is negligible to none. Solar is the third primary source of electricity generation [10]. According to the India energy report 2005, India has the 5th largest coal reserves worldwide, proving to be a cheaper way to produce electricity. If the coal consumption is increased by 5% early, the coal reserves will last for another 40–50 years [9]. Currently, India is generating around 43 TWh of electricity through nuclear energy, which accounts for 10% of the electricity generation in India. The new reactors under construction will increase atomic power production slightly. These new power plants might not be able to replace or reduce the dependence on coal-based power plants, but they still might significantly help the grid to be more efficient. With the help of a fission reaction, energy is generated using radioactive materials such as uranium. The generated heat energy is then converted to electricity using steam turbines. There are five primary methods to generate hydrogen from nuclear power, (1) radiolysis, (2) electrolysis, (3) hightemperature steam electrolysis, (4) hybrid thermochemical water splitting,

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and (5) thermochemical water splitting as represented in Fig. 3. The first method uses nuclear radiation to split the water molecule into hydrogen and oxygen. In the second method, electricity produced from nuclear energy is used to split water into hydrogen and oxygen; in the third and fourth methods, the heat and the electricity from nuclear power are used to separate the water. In the fifth method, heat energy generated by nuclear power is used to split the water.

Fig. 3 Various methods to produce hydrogen from nuclear energy.

India has indigenously developed a heavy-water reactor at the Bhabha Atomic Research Center, Kalpakkam Mini Reactor (KAMINI). This is the only reactor in the world that uses Thorium and 233U as fuel; it is also called the fast breeder thorium reactor (FBTR). This nuclear reactor supplies process heat at 600–1000°C [11]. India has already started working toward green hydrogen production to replace the existing fossil fuel transportation and industrial sector with hydrogen and has released the road map for this plan. Bhabha Atomic Research Center has collaborated with Bharath Petroleum Corporation Limited to scale up the alkaline electrolyzer technology [12]. Significant factors influencing hydrogen production using nuclear energy are (1) the rate of production of Oil and Gas, (2) the Government policy toward carbon-free fuels, (3) the change in public behavior toward environment-friendly fuels, (4) the nation’s energy security policy depends less on the foreign fossil fuel supply chains, and (5) hydrogen production on a large scale and transportation costs significantly influence adapting hydrogen as fuel.

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Hydrogen production using atomic energy in India is possible as nuclear power plants require water to cool down the reactors, so almost all the plants in India have a considerable water resource. Adding equipment to India’s existing nuclear reactor infrastructure can produce hydrogen on a large scale when the reactors are not contributing electricity to the grid during less demand.

3 Energy obtained from the nuclear energy When controlled nuclear fission starts in the atomic reactor, heat energy is obtained when the atoms bombard each other. This heat energy is converted to electricity by superheating the water and converting it to steam. This highpressure and superheated steam is then passed through the turbine, generating electricity. So, two primary energy sources from nuclear power can be used to produce hydrogen: heat and electricity. When there is energy conversion, there will be an energy loss. Most energy loss in nuclear energy is in the form of heat. So, this waste of abundant heat energy can be used for hydrogen production, even after most of the atomic power is converted to electricity.

3.1 Electricity generation Generating power from atomic energy started in 1954 in Russia with a 5 MW capacity [13]. Currently, 413 GW of energy is being generated by nuclear energy worldwide and could be doubled to 812 GW by 2050 [14]. Unlike the coal power plant, once the nuclear reactor has started closing or switching off during the low demand hours is tough. The excess energy that the nuclear power plant generates can’t be stored. But it can be transformed into hydrogen by using electrolyzers for future purposes, such as generating electricity back utilizing the fuel cell in remote areas. The hydrogen generated with the help of electricity generated by nuclear power using an electrolysis process is called pink hydrogen. Three different electrolyzers are commercially available for large scale to convert water into hydrogen and oxygen. • Proton Exchange Membrane Electrolysis • Alkaline Electrolysis • Solid Oxide Electrolysis. 3.1.1 Alkaline electrolysis Alkaline electrolysis is the old, most accessible, simplest, and most suitable commercially developed method to produce hydrogen. Typically, these electrolyzers operate between 40°C and 90°C temperature at 3 MPa

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pressure. The recent most advanced electrolyzers are reaching an operating pressure of up to 12 MPa. In this method, water splits into H and OH
ions and OH
travel toward the anode from the cathode. At the cathode, hydrogen will evolve, and at the anode, oxygen will develop. Then, these gases are separated by a diaphragm, which is only permeable to the OH
ions and water. The most typical alkaline electrolysis electrolytes are either sodium hydroxide (NaOH) or potassium hydroxide (KOH) at 20–40 wt%. These electrolytes are corrosive and often decrease catalytic activity, damage the electrodes, and increase operational costs. Asbestos is a suitable diaphragm that divides the anode and cathode, but this material is not being used anymore due to safety regulations. Instead, composite materials based on ceramic and microporous materials such as polyethersulfone(PES) and glass-reinforced polyphenylene sulfide(PPS) compounds, can be used to separate gases. Metal electrodes are the best option for the gas evolution processes, but nickel is considered best due to its high activity and cheap availability. In 50% KOH electrolyte at 80°C, nickel has been reported to be around 1.1*10
4 A/cm
2 at the cathode and 4.2 *10
6 A/cm
2 as shown in Fig. 4 [5].

Fig. 4 General scheme and operation of an alkaline electrolysis cell [15].

3.1.2 Proton exchange membrane electrolysis General Electric developed the first proton exchange membrane in the 1960s to reduce the drawbacks of the alkaline electrolyzer [16]. PEM electrolyzers are the recent trend in electrolyzer systems. Platinum-black, iridium, ruthenium, and rhodium are used for electrode catalysts. Nafion membrane separates the electrodes, and the gases evolve at the respective electrodes. The water splits into protons and oxygen at the anode. This proton travels through the membrane toward the cathode and recombines to form hydrogen.

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The gases produced by the electrolyzer were separated by passing them through gas separators. Gas dehumidifiers reduce the water content in the gases coming from the electrolyzer. This additional process increases the purity of the gases [17]. The main parts of the PEM electrolyzer (Fig. 5) are the membrane electrode assembly, which includes the membrane and the electrode with the catalyst. These three were added and hot pressed to form the membrane electrode assembly. The bipolar plate helps with the conduction of electrons, connecting single cells and directing the flow inside the electrolyzer. The bipolar plate needs high thermal and electron conductivity, low gas permeability and high mechanical and corrosion resistance, which helps increase the electrolyzer’s efficiency. Generally, the bipolar plates are made with Titanium as it has high mechanical and corrosive resistance [16].

Fig. 5 Schematic diagram of PEM Electrolysis [15].

3.1.3 Solid oxide electrolyzer The solid oxide electrolyzer (SOE) is just solid oxide fuel cells working in reverse. Their working temperature is almost equivalent to the output temperatures of nuclear reactors. This makes it perfect to utilize the waste heat energy from the nuclear reactors or any other waste heat source to preheat the water to the operating temperatures of the SOE, which is around 700–1000°C. During this process, the water will convert to super-saturated steam, and this process is called high-temperature electrolysis (HTE), as shown in Fig. 6 [18]. Total electricity demand decreases at higher temperatures due to a rise in temperature, the ion conductivity of the electrolyte, and at the electrode surface, the rate of electrochemical reaction increases. The solid electrolyte in the SOEC helps reduce the lags in producing the hydrogen-like PEM electrolyzer.

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The efficiency of the solid oxide electrolyzer depends on the heat source. If the SOEC utilizes the electricity and the waste heat from the nuclear power plant simultaneously, the efficiency of the electrolyzer will be up to 60%. If SOEC uses solar energy for electricity and the waste of heat energy from the nuclear power plant, then much higher efficiencies of electrolyzer can be achieved.

Fig. 6 Schematic diagram of solid oxide electrolysis [15].

3.1.4 High-temperature electrolysis In this method, the electrolysis process occurs at higher temperatures than usual. The heat energy is provided to the electrolyte from additional sources such as geothermal and waste heat from nuclear power. With the help of electrolyzers, hydrogen and oxygen can be produced from water more efficiently than just electricity. Recently, concentrated solar energy for achieving high-temperature for the electrolyte. In this method, large convex mirrors focus the solar energy to a point a few meters away from the panels, where power will be transmitted to a heat transfer fluid that, in turn, increases the electrolyte temperature when passed through heat exchangers [19,20]. If this system is integrated into the hightemperature electrolyzer, the system’s efficiency would be much higher when compared to the integrated system with nuclear energy. As this is a separate system, it can also be used to run the turbine as the typical thermal power plant. Toshiba is developing a high-temperature electrolysis plant and integrating it into the nuclear power plant. In this concept, the right side of the nuclear reactor is for general power generation, and the left is for hydrogen production with high-temperature electrolysis units. In this concept design, the plant requires thermal and electrical energy, which will be supplied by nuclear power [21]. Fig. 7 shows the schematic diagram of the

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Power to grid

H2,H2O separator H2

H2O

Power for electrolysis

Make-up water

Heat exchanger

Electrical generator

H2O He Hightemperature heat exchanger

He Gas turbine He

Hightemperature steam electrolysis unit

HTGR H2O + H2

He

HP compressor

He

Intercooler

Recuperator Primary heat rejection He

He

LP compressor

Fig. 7 Schematic diagram of high-temperature electrolysis [22].

high-temperature electrolysis method where the waste energy of the nuclear reactor increases the efficiency of hydrogen production.

3.2 Thermochemical process Thermochemical water splitting requires heat energy to split water with one or more endothermic cycles. These cycles can be hybrid or heat-induced thermochemical cycles that require energy in the form of electricity, thermal, or photonic energy. Pure thermochemical processes require only heat and water to produce hydrogen and oxygen. Hybrid thermochemical methods require heat and electricity input to produce hydrogen and oxygen from water [23]. These systems can generate massive amounts of hydrogen when integrated with concentrated solar power or nuclear power plants, requiring tremendous amounts of heat energy for the reactions. There are so many different thermochemical cycles reported till now. But only a few processes are adapted for the commercial scale production of hydrogen and oxygen [24]. Few thermochemical cycles that are common for large-scale hydrogen production are • Sulfur-Iodine (S-I) cycle • Hybrid-Sulfur (HyS) cycle • Iron-Chlorine (Fe-Cl) cycle

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• Copper-Chlorine (Cu-Cl) Cycle • Magnesium-Chlor ide (Mg—Cl) cycle But the sulfur-iodine cycle is the most integrated cycle with the nuclear power plants for producing hydrogen and oxygen with the help of waste heat energy from the reactor. 3.2.1 Sulfur-iodine (SI) cycle In the 1970s, General Atomic [25] proposed sulfur-iodine (SI) cycle to integrate with the nuclear reactors to produce hydrogen by using the waste heat energy from the nuclear reactor, which is the most developed and reliable thermochemical process available. But the company used a solar thermal reactor for production [26]. In 1997, Japan Atomic Energy Agency (JAEA) produced 1 L/h of hydrogen on a laboratory scale. In 2004, 31.5 L/h hydrogen was produced continuously when high-temperature gas-cooled nuclear reactors supplied heat energy [27]. In Korea, similar studies are carried out by Korean Atomic Research Institute (KAERI); the plant produced 50 L of hydrogen per hour with the help of an electrodialysis cell and a membrane [28]. The first exothermic reaction of the sulfur-iodine (SI) cycle as shown in Table 1 is known as the Bunsen reaction. In this reaction, as shown in Fig. 8, in the presence of iodine and water, two immiscible acids are formed when iodine (I2), sulfur dioxide (SO2), and water (H2O) are used as reactants. In the second and third endothermic reactions, the acids formed in the first cycle will decompose to produce hydrogen and oxygen [29]. The remaining products are reintroduced back into the Bunsen reaction for decomposition. In the S-I cycle, oxygen is carried by the H2SO4, whereas the HI carries hydrogen. Table 1 Chemical reactions. Sno

Reaction Name

Reactions

Temperature (K)

1

Bunsen

293–393

2

H2SO4 Decomposition

3

HI decomposition

I2(s) + SO2(g) + 2H2O ! H2SO4(aq) + 2HI(aq) H2SO4 ! SO3 + H2O SO3 ! SO2 + 0.5O2 Overall reaction H2SO4(g) ! SO2(g) + H2O(g) + 0.5O2(g) 2HI(g) ! H2(g) + O2(g)

1073–1273

573–773

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Fig. 8 Schematics of the S-I process [24].

In the Bunsen Reactor, a spontaneous reaction caused by the iodine separates the acids into light and heavy phases; the lighter phase includes sulfuric acid (H2SO4) and water (H2O), and the denser phase includes HI, I2, and water (H2O). Before going to the second reaction, these two phases are separated in a liquid–liquid separator. The second reaction of the sulfur-iodine’s thermochemical cycle occurs in a flash tank and a decomposition reactor. The water starts to evaporate, and the sulfuric acid starts to concentrate due to the higher volatility of water than acids. SO3 and water vapor are produced when the concentrated sulfuric acid is heated and evaporated. When SO3 is further heated in the decomposition reactor, it decomposes to SO2. After condensation, H2SO4 is formed by water and undecomposed SO3 returns to the decomposition tank. And the residual water, O2 and SO2, goes back to the Bunsen reactor. The oxygen is separated from the mixture and stored before going to the Bunsen reactor. In the last reaction, the decomposition reactor, electro-electrodialysis reactor and distillation column play an essential role in separating the iodine and hydrogen from the hydrogen Iodide. The SO2 is removed from the HIx phase in the purification chamber, passing the gas through the concentration section. Direct distillation cannot be done due to limitations. Three recommended methods can be used to separate hydrogen and oxygen from HI. The first one is extractive distillation with phosphoric acid (H3PO4), which

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requires energy to recycle the H3PO4 and catalyst. Reactive distillation is the second method, where there is an adverse environmental effect by using this method. Electro-electrodialysis is the third method to separate gases. 3.2.2 Integration of sulfur-iodine (SI) cycle to nuclear reactor As high temperatures are required to decompose the H2SO4, integrating with the very high-temperature nuclear reactor (VTHR) is studied by HYdrogen THErmochemical Cycles (HYTHEC) and the European ReActor for Process heat, Hydrogen And ELectricity generation (RAPHAEL) projects [30]. These represent self-sustainable projects where the heat and the electricity supply to the S-I cycle is from the nuclear reactor. The high temperatures from the reactor are transmitted to the S-I cycle using the intermediate heat exchangers (IHX), as shown in Fig. 9. With the help of an intermediatory loop, the HIX takes part of the heat energy and leaves the rest for the Brayton cycle for electricity production.

Fig. 9 S_I Cycle coupling to a Nuclear Reactor [31].

4 Life cycle assessment The International Organization of Standardization provides the Life Cycle Assessment framework. ISO 14044:2006 helps identify opportunities to improve products’ environmental performance at various points in their life

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cycle and helps assist decision-makers in government and nongovernment organizations. The life cycle assessment (LCA) is calculated in five units, (1) global warming potential (GWP), (2) human toxicity potential (HTP), (3) acidification potential, (4) eutrophication potential (EP), and finally, (5) radiation. LCA produces hydrogen using high-temperature electrolysis, producing GWP of 2000 g CO2 and Acidification Potential (AP) of 0.15 g equivalent [32]. To improve the electrolysis potential, use of alternative materials and a 50% reduction in material requirements will reduce 25% in GWP and 30% in AP. The sulfur-iodine cycle for hydrogen production, GWP of 2900 kg CO2 equivalent, and AP, 17 kg SO2, are generated [33].

5 Conclusion and future perspective Every country is trying to produce hydrogen at a low cost with less pollution possible without burning any fossil fuel involved in the whole manufacturing cycle by using renewable energy such as solar, wind, and tidal energy. But, nuclear energy is one of the proven and highly efficient energy with fewer carbon emissions than conventional power plants since the first nuclear power plant was in the 1950s. Different types of nuclear power plants with various technologies operate at different temperatures and pressures. Additional systems can be integrated into these atomic reactors to utilize the waste heat energy for the thermochemical splitting of water to produce bulk hydrogen production. The electricity generated can be used to split water with the help of electrolyzers. High-temperature electrolysis systems can also be integrated into nuclear power plants by utilizing heat energy. The S-I cycle can be integrated into high-temperature nuclear power plants as the temperatures required for decomposing the H2SO4 is relatively high. Combining the S-I cycle can increase the efficiency of the whole atomic reactor system by producing hydrogen, oxygen, and electricity with the same nuclear fuel. Nuclear power plants require a lot of investment, but in current situations, producing hydrogen from renewables is costly. If the existing nuclear power plants are fitted with hydrogen production units, the overall efficiency of the nuclear power plant and the overall efficiency of the hydrogen production systems that are fitted to utilize the heat energy will increase. This helps in the additional production of hydrogen from the waste heat energy of the nuclear power plants.

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[19] Puig-Samper G, Bargiacchi E, Iribarren D, Dufour J. Assessing the prospective environmental performance of hydrogen from high-temperature electrolysis coupled with concentrated solar power. Renew Energy 2022;196:1258–68. [20] Wang L, Yuan Z, Zhao Y, Guo Z. Review on development of small point-focusing solar concentrators. J Therm Sci 2019;28(5):929–47. [Internet]. 2019 Aug 28 [cited 2022 Sep 17]; Available from: https://link.springer.com/article/10.1007/s11630019-1134-4. [21] Pursuing Next-Generation and Advanced Reactors with Enhanced Safety: Research and development j Nuclear Power j Toshiba Energy Systems & Solutions. [Internet]. [cited 2022 Nov 13]. Available from https://www.global.toshiba/ww/productssolutions/nuclearenergy/research/safety-reactor.html. [22] Keynote Lecture: High Temperature Heat Exchangers., 2005, https://www. researchgate.net/publication/254591751_Keynote_Lecture_High_Temperature_ Heat_Exchangers. [Accessed 11 November 2022]. [23] Oruc O, Dincer I. Assessing the potential of thermo-chemical water splitting cycles: a bridge towards clean and sustainable hydrogen generation. Fuel 2021;286:119325. [24] Mehrpooya M, Habibi R. A review on hydrogen production thermochemical watersplitting cycles. J Clean Prod 2020;275:123836. [25] Norman JH, Besenbruch GE, Brown LC, O’Keefe DR, Allen CL. Thermochemical water-splitting cycle, bench-scale investigations, and process engineering. Final report, February 1977-December 31, 1981, 1982. May 1 [cited 2022 Sep 17]; Available from: http://www.osti.gov/servlets/purl/5063416-Hhmrtj/. [26] Naterer GF, Dincer I, Zamfirescu C. Thermochemical water-splitting cycles. In: Hydrogen production from nuclear energy; 2013. p. 153–272. [Internet] [cited 2022 Sep 17]; Available from: https://link.springer.com/chapter/10.1007/978-1-44714938-5_5. [27] Kubo S, Kasahara S, Okuda H, Terada A, Tanaka N, Inaba Y, et al. A pilot test plan of the thermochemical water-splitting iodine–sulfur process. Nucl Eng Des 2004;233 (1–3):355–62. [28] Shin Y, Lee K, Kim Y, Chang J, Cho W, Bae K. A sulfur-iodine flowsheet using precipitation, electrodialysis, and membrane separation to produce hydrogen. Int J Hydrogen Energy 2012;37(21):16604–14. [29] Onuki K, Kubo S, Terada A, Sakaba N, Hino R. Thermochemical water -splitting cycle using iodine and sulfur. Energ Environ Sci 2009;2(5):491–7. [Internet]. [cited 2022 Sep 17]; Available from: https://pubs.rsc.org/en/content/articlehtml/2009/ee/ b821113m. [30] The RAPHAEL project., 2022, https://www.irsn.fr/EN/Research/Researchorganisation/Research-programmes/RAPHAEL/Pages/The-RAPHAEL-project5197.aspx. [Accessed 11 November 2022]. [31] Cerri G, Salvini C, Corgnale C, Giovannelli A, de Lorenzo MD, Martinez AO, et al. Sulfur–iodine plant for large scale hydrogen production by nuclear power. Int J Hydrogen Energy 2010;35(9):4002–14. [32] Solli C, Strømman AH, Hertwich EG. Fission or fossil: life cycle assessment of hydrogen production. Proc IEEE 2006;94(10):1785–93. [33] Utgikar V, Thiesen T. Life cycle assessment of high temperature electrolysis for hydrogen production via nuclear energy. Int J Hydrogen Energy 2006;31(7):939–44.

CHAPTER 18

Hydrogen production driven by seawater electrolysis Lokesh Sankhulaa, Devendra Kumar Vermab, and Rohit Srivastavaa a

Catalysis & Hydrogen Research Lab, Department of Petroleum Engineering, School of Energy Technology, Pandit Deendayal Energy University, Gandhinagar, Gujarat, India b Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi, India

1 Introduction The shortage of nonrenewable resources and environmental pollution have become a major concern of modern society across the globe. The importance of renewable resources such as solar, wind, geothermal, and tidal energy sources has gained major attention for the sustainable development of energy and environmental sectors [1]. There are many alternative fuels for addressing key future environmental and energy supply challenges, which include reformulated gasoline or diesel, methanol, ethanol, synthetic liquids like dimethyl ether made from natural gas or coal, compressed natural gas, and hydrogen [2]. Among the different alternative fuels available, hydrogen (H2) offers greater potential to deal with modern issues effectively. There are four basic sources of hydrogen production at an industrial scale, which include steam reforming of natural gas, oil, coal, and electrolysis. The majority of the hydrogen is produced by steam reforming of natural gas (CH4), which accounts to 48% of total hydrogen production across the world, and only 4% of hydrogen is produced from water-splitting technique. Hydrogen gas is the most attractive fuel for energy conversion and storage applications due to its higher gravimetric energy density (maximum heating value of 142 MJ/Kg and minimum heating value of 120 MJ/Kg at 25°C). The combustion of hydrogen in internal combustion engines and fuel cells produces water, so hydrogen is considered a cleaner fuel with almost zero emissions. The annual production of hydrogen is approximately 45 million metric tons, and its consumption is increasing by 6% every year [3]. The electrolysis of water is the most effective technique for the production of hydrogen as it generates highly pure hydrogen (99.999 vol%) when compared to other production methods without the emission of harmful gaseous pollutants [4]. The production of hydrogen from freshwater (deionized water) is another Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00013-3

Copyright © 2023 Elsevier Inc. All rights reserved.

363

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concern from an economical point of view; however, the usage of seawater is beneficial, but it causes many problems during the electrochemical process. Thus, H2 production through electrochemical water splitting by utilizing the most abundant saline water (96.5% of total earth’s water resources) rather than depending on deionized water could help in reducing the consumption of scarce freshwater around the world’s highly demanding areas such as arid zones, coastal areas, and islands [5]. The ions present in seawater get precipitated and block the electrode surface area and are also responsible for catalyst degradation due to local pH fluctuations at both electrodes. The chloride oxidation reactions also compete with the oxygen evolution reactions in terms of potential applied across the electrodes. The electrocatalysts also play an important role in energy consumption for hydrogen generation process and chloride-based corrosion resistance. The chloride ions move toward the metal surface and enhance the oxidation of metal surface. The research on electrocatalytic seawater splitting for the production of H2 and O2 fuels is at its initial stages but several works have been reported for better HER, OER, and water-splitting processes in natural seawater or alkaline seawater. In this chapter, we will discuss fundamental challenges for better HER, OER performance of different electrocatalysts in seawater, and corrosion inhibition mechanisms, which would be beneficial for many researchers to develop better electrocatalysts and electrolyzers at an industrial scale.

2 Fundamentals of water splitting The electrolysis of water consists of two half-cell reactions, HER at the cathode and OER at the anode, whereas HER is a two-electron process and OER is a multi-electron process. The redox reactions occurring in an electrochemical cell depend on the electrolyte present in the cell. The following reactions take place in acidic and alkaline mediums. Acidic medium Cathode : 2H+ + 2e
! H2 Anode : H2 O ! 2H+ +

1 O2 2

Basic medium Cathode : 2H2 O + 2e
! H2 + 2OH
Anode : 2OH
! H2 O + 2e
+

1 O2 2

Hydrogen production driven by seawater electrolysis

365

The redox reactions in a neutral electrolyte medium are quite similar to alkaline medium; however, the reactions at the anode are complex because of competition between chloride oxidation and oxygen evolution reaction. The Gibbs free energy (ΔG) for electrolysis of water is 237.2 KJ/mole corresponding to a potential of 1.23 V, but due to kinetic energy barriers and efficiency of electrochemical cell, it requires more than 1.23 V. Many factors limit the water-splitting process such as high activation energy, low ion, and gas diffusion rates, solution concentration, and electrolyte diffusion blockage leading to higher overpotential for the electrolysis process. The electrocatalysts play a prominent role in the water-splitting process to reduce the onset and overpotential for HER and OER reactions; thus, developing an efficient electrocatalyst for energy consumption and stability of the electrolyzer system is important for industrial applications. The classification of electrolyzer is shown in Fig. 1, and there are four different electrolyzers, which are used for hydrogen production.

(a)

+ – O2

DC Supply

(b)

H2 H2O O2

Anode

H2 OH-

Cathode

H2

½O2 H2O

Membrane Gas diffusion layer

H–

Gas diffusion layer

Alkaline electrolysis

Anode

Cathode Membrane

Energy Source

(c)

e– 2H2 + 4OH– → 4H2O + 4e–

O2 + 2H2O + 4e– → 4OH–

(d) Water

H2O H2+ H2O

O2+ H2O

Back diffusion

Oxygen

O2–

–

OH (H2O)n

Flow field

CL MPL

O2+H2O GDS

MPL CL

GDS

Flow field

Anode

AEM

Electroosmotic drag

H2+ H2O

Cathode

Hydrogen Cathode

Gas-tight electrolyte

Anode

Steam electrolysis

Fig. 1 Schematic of different electrolyzer systems (A) Alkaline electrolyzer, (B) PEM electrolyzer, (C) AEM electrolyzer [6], (D) Steam electrolyzer or high-temperature electrolyzer.

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Solar-driven green hydrogen generation and storage

3 Challenges for seawater electrolysis The main challenges for direct electrolysis of seawater include changes in local pH values, presence of bacteria/microbes and small particulates, chloride ion oxidation, and membrane utilized for separating the cathode and anode compartments. The presence of carbonates in saline water acts as buffers, but the concentration of carbonates is not sufficient for preventing changes in local pH values at the cathode and anode. The pH value near electrode surface changes could be in the order of 5–9 units from that of bulk seawater for a slightly buffer medium when its overall pH value is between 4 and 10, even at less current densities (9.5. The addition of supporting electrolytes can help in the stabilization of pH fluctuations. The most serious problem is caused due to the presence of chloride ions present in seawater (0.5–0.6 M NaCl). The equilibrium potential for OER vs normal hydrogen electrode (NHE) is higher than chloride evolution reaction by 130 mV [10], but OER is a four-electron oxidation process that requires a higher overpotential than two-electron chloride oxidation reaction with a kinetic advantage. The equilibrium potential of chlorine evolution does not depend on pH, but OER depends on the pH of the electrolyte. Selective OER over chlorine generation can be achieved in alkaline electrolytes to lower the onset potential of OER. However, the formation of hypochlorite is still competitive with OER with an onset potential of 490 mV more than that of OER, so highly active OER electrocatalysts are necessary for higher H2/O2 production at overpotential well below hypochlorite formation. The aggressive chloride ions may still corrode catalysts and substrates through metal chloride hydroxide formation reaction mechanisms, even with a highly active OER catalyst in alkaline electrolytes [11]. The metal corrosion process due to the presence of chloride ions in seawater includes dissolution of metal, and chloride is converted to hydroxide. The corrosion reaction mechanism is shown in flowing reactions (3), (4), and (5). Cl
+ 2OH
! OCl
+ 2e
E 0 ¼ 1:72 V
0:059⁎ pH vs NHE


4OH ! O2 + 2H2 O + 4e




⁎

E ¼ 1:23 V
0:059 pH vs NHE 0

(1) (2)




Adsorption of Cl by surface polarization M + Cl
! MClads + e


(3)

Hydrogen production driven by seawater electrolysis

367

Dissolution by further coordination MClads + Cl
! MClx


(4)

Conversion from chloride to hydroxide MClx
+ OH
! MðOHÞx + Cl


(5)

The development of electrodes that are corrosion resistant for splitting water into hydrogen and oxygen fuels is important for seawater electrolysis advancement to avoid relying on the costly desalination process. The chloride oxidation electrochemistry is so complicated, and it depends mainly on the value of pH, applied potentials, and temperature. if we fix the temperature at 25°C and the concentration of seawater to 0.5 M, a pourbaix diagram for aqueous chlorine chemistry is shown in Fig. 2A. when the pH value is below 3.0, the free chlorine evolution occurs predominantly in the chloride oxidation reactions. Hypochlorous acid formation takes place at pH values greater than 7.5. the chloride oxidation reactions at two extremes pH values are shown in the following reactions. CLER:
2Cl
! Cl2 + 2e
E0 ¼ 1:36 V vs SHE, pH ¼ 0 (6) Hypochlorite formation:


Cl
+ 2OH
! ClO
+ H2 O + 2e
E0 ¼ 0:89 V vs SHE, pH ¼ 14 (7) The competing chloride oxidations are thermodynamically unfavorable compared to OER and the difference between the standard electrode potentials increases with an increase in pH value until the hypochlorite formation, where it remains at the maximum value of 480 mV [12]. There is a debate among the researchers on choosing the direct seawater electrolysis and twostep seawater electrolysis for the production of green hydrogen in terms of economic benefits; however, the efficiency and cost of the desalination process conclude that capital and operating costs of two-step seawater electrolysis are only a fraction of single-step process.

4 Electrocatalysts for seawater electrolysis Recently, there are many electrocatalysts have been reported in the literature for better HER activity in natural seawater or simulated seawater (0.5–0.6 M NaCl) such as platinum-based alloys, transition metal-based

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Solar-driven green hydrogen generation and storage

Fig. 2 (A) The pourbaix diagram of an aqueous saline electrolyte and (B) representation of E vs pH for competing for OER and chlorine oxidation reactions [5].

carbides, nitrides, phosphides, and carbon-based materials. Table 1 describes different electrocatalysts which have better HER activity in seawater electrolyte medium. Platinum-based electrocatalysts are considered benchmarks for HER performance in acidic, basic, and neutral electrolyte mediums. The commercial Pt/C (20 wt%) has achieved a current density of 10 mA/cm2 at a lower overpotential of 120–300 mV depending on the experimental conditions. This catalyst exhibits a loss in the current density by 50% after continuous operation of 100 h by using the alkaline water electrolysis technique. The high cost and scarcity of platinum-based alloy catalysts limit large-scale operations (Table 2).

Table 1 Electrocatalysts for HER activity in seawater electrolyte medium. S.No.

Electrocatalysts

Electrolyte

Overpotential (mV) at 10 mA/cm2

Durability (h)

References

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

2.4% Pt @mh-3D Mxene VS2@V2C CoMoC/Mxene/NC NiCoP/NF 0.5Rh-GS1000 NiCoN/NixP/NiCoN PtMo0.1 Mo2C-MoP NPC/CFP-800 Fe-Co2P Ptat-CoP MNS CoMoP@C Co3Mo3C/CNT RuCo Pt-Ru-Mo Mn-NiO-Ni/Ni-F

Natural Natural Natural Natural Natural Natural Natural Natural Natural Natural Natural Natural Natural Natural Natural

280 444 (20 mA/cm2) 208–312 287 320 165 254.6 346 489 300 448 124 387 196 170

250 200 225 20 10 24 173 16 100 24 10 26 12 172 14

[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

seawater seawater seawater seawater seawater seawater seawater seawater seawater seawater seawater seawater seawater seawater seawater

Table 2 Electrocatalysts for OER activity in a seawater electrolyte medium. S.No.

Electrocatalyst

Electrolyte

Overpotential (mV) at 10 mA/cm2

Durability (h)

References

1. 2. 3. 4. 5. 6. 7. 8. 9.

NCFPO/C@CC S-(Ni,Fe)OOH CaFeOxjFePO4 FTO/NiO NiFe LDH NiCo-DEA CoFe LDH NiMoN@NiFeN CoSe1

0.1 M KOH+0.5 M NaCl 1 M KOH+0.5 M NaCl Phosphate buffered seawater 1 M KOH+0.5 M NaCl 0.1 M KOH+0.5 M NaCl Natural seawater Simulated seawater 1 M KOH+0.5 M NaCl Natural seawater

285 278 @ 100 mA/cm2 710 340 359 80 530 286 @ 100 mA/cm2 450

100 100 10 100 120 40 8 100 -

[28] [29] [30] [31] [9] [32] [33] [34] [35]

Hydrogen production driven by seawater electrolysis

371

The development of efficient OER electrocatalysts is important due to the completion of oxygen evolution and chloride oxidation or hypochlorite formation reactions during seawater electrolysis. The following aspects are considered for future investigation on OER electrocatalysts [36]. (1) The adsorption energy difference between catalytic surfaces and intermediates of OER and ClER (hypochlorite formation) by using density functional theory to study the mechanism involving seawater electrolysis. This also helps in designing an efficient and stable electrocatalyst for seawater electrolysis. (2) The natural seawater along with simulated seawater (0.5–0.6 M NaCl) should be used for studying the long-term development of electrocatalysts and electrolyzers. Natural seawater contains various 2+ + ions such as Na+, Mg2+, Cl
, SO2
4 , Ca , and K and microorganisms like microbes, and bacteria. These ions, microorganisms, and insoluble salts might degrade the catalyst and block the active metal sites; thus, we should develop a catalyst to sustain in this complex environment. (3) The faradaic efficiency of O2 should be calculated to determine the OER selectivity in seawater electrolyte medium at large current densities. However, the OER reaction is most favorable than ClER (or hypochlorite formation) according to thermodynamics, but OER is a four-electron transfer process and chloride oxidation is a twoelectron process. The sluggish nature of OER reaction kinetics compared with ClER is also to be considered. (4) Self-supported catalysts are more reasonable and efficient than powdered catalysts for seawater electrolysis. The highest current density of powdered electrocatalyst on rotating disc electrode is less than 50mA/cm2at the corresponding potential may not reach the threshold limit of ClER (or hypochlorite formation reaction). (5) An electrolyzer design with asymmetric electrolyte feed has already shown higher activity and selectivity of OER from separating simulated seawater with high pH electrolyte and reducing the catalyst degradation [37]. (6) Simple filtration techniques are necessary to precipitate some ions which might pose a problem during natural seawater electrolysis. The pretreatment of natural seawater with Na2CO3 precipitates Mg2+ and Ca2+ before electrolysis. The microbes and other insoluble salts can also be reduced by using the filtration technique and ensuring the higher activity and selectivity of the OER electrocatalyst.

372

Solar-driven green hydrogen generation and storage

4.1 Anode materials for saline water electrolysis There are many strategies for the selection of better OER-selective electrocatalysts during seawater electrolysis. The alkaline OER/ClER design criterion has been leveraged, which implies the maximization of thermodynamic potential between two catalytic processes by performing water splitting in alkaline conditions. Thus, the development of catalysts performing better in alkaline conditions provides the desired faradaic current at or below 480 mV. The development of catalysts that have selectivity toward the OER mechanism by binding OER intermediates to the active sites. The presence of chloride ion blocking layers next to the OER catalyst also prevents the diffusion of Cl- ions from the electrolyte to the catalyst surface. Alkaline design criteria are mainly based on thermodynamic and kinetic considerations because saline water is a non-buffered electrolyte; thus, additive is necessary for avoiding local pH changes during the electrolysis process. The competition between OER and ClER is very challenging due to the sluggish kinetics of OER, whereas alkaline conditions provide a large potential for a favorable OER mechanism. The above-mentioned factors help in achieving 100% OER selectivity in saline water electrolysis with an over potential lower than 480 mV at higher current densities. The electrocatalysts with selective OER active sites optimize the chemical adsorption of reactive OER intermediates. This strategy works in any pH value but catalysts with OER-selective active sites are also feasible for ClER. The thermodynamic limitations for OER-based electrocatalysts can be overcome by developing a protective layer of MnO, which repels the chloride ions migrating toward the metal surface.

4.2 Cathodes for hydrogen evolution in seawater The major concern for cathode material used for seawater splitting is longterm stability due to precipitation of different cations and blocking the active sites of the catalyst. The saline water and surface freshwater contain many cationic species like Ca2+, and Mg2+, which deposit on the cathode surface as hydroxides under reductive conditions, and losses in current density by 50% are reported after a short period of operation. The changing concentration of marine water resources defines a new challenge for electrolysis while focusing on specific impurities; thus, it would be beneficial for analyzing the standardized seawater composition. Solid impurities and microbial contaminants are absent in synthetic electrolyte medium. The utilization of membrane for the design of electrolyzer would be beneficial for improving the stability of electrodes and preventing impurities migration toward electrode surface.

Hydrogen production driven by seawater electrolysis

373

5 Electrolyzer design for water splitting The development of an electrolyzer for high-performance electrolysis to hydrogen and oxygen fuels at an industrial scale at an optimum cost [38]. There are four types of electrolyzers like proton exchange membrane electrolyzer (PEME), anion exchange membrane electrolyzer (AEME), alkaline water electrolyzer (AWE), and high-temperature water electrolyzers (HTWE) such as proton-conducting ceramic membrane electrolysis (150–400°C) and solid oxide electrolysis (800–1000°C). The proton exchange membrane electrolyzer (PEME) consists of solid acid polymer electrolyte (Nafion 117 membrane) in between the cathode and anode layers; however, water is fed to the anode side and gets oxidized to produce oxygen gas, and protons are transferred through the membrane and hydrogen gas is produced at cathode [5]. This electrolysis process leads to an increase in local pH value at the anode, so the oxygen evolution and chlorine evolution reactions may be competitive. The membrane is also susceptible to foreign impurities at the cathodic surface, which results in a decrease in proton conductivity. The deposition of calcium and magnesium salts at the electrode surface also acts as resistance to OER and HER process. The anion exchange membrane electrolyzer (AEME) is similar to a proton exchange membrane electrolyzer in which a proton exchange membrane is replaced by an anion exchange membrane. Water is fed to one of the electrodes H2 and OH
are formed at the cathode, whereas OH
ion is transferred to the anode side and oxidized to O2. The anion exchange membrane has the advantage of restricting the migration of unwanted species (Cl
); thus, sea water electrolysis can be performed by using this kind of electrolyzer; however, alkaline conditions are mostly preferred due to high oxygen selectivity. The alkaline water electrolyzer consists of two compartments separated by a porous diaphragm, which allows only OH
ions and restricts the passage of gas molecules. Alkaline electrolytes can be fed from both sides of the electrolyzer, where H2 and O2 are produced at the cathode and anode. The passage of Na+, Cl
, and H+ ions across the porous diaphragm poses a serious problem during seawater electrolysis. High-temperature water electrolyzers were classified based on the ions being transferred across the membrane such as ceramic membrane electrolysis (150–400°C) and solid oxide electrolysis (800–1000°C). In ceramic membrane electrolysis, water is fed at the anode and is oxidized to O2 and H+; however, H+ ions migrate toward the cathode, and H2 is liberated. Solid oxide electrolysis involves steam being fed at the cathode side to produce H2 and O2
, whereas O2
ion is transferred to the anode for the production of oxygen gas.

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Solar-driven green hydrogen generation and storage

The main advantage of HTWE is the utilization of high-temperature steam, which helps in improving reaction kinetics, and heat utilized for the electrolysis process is much cheaper compared with electricity. The major disadvantages of HTWE include high energy consumption, catalyst degradation, several limitations on materials and electrolyzer design, and lifetime. The simple filtration of natural seawater enhances the lifetime durability of electrolyzers during the seawater electrolysis process, however further reduces the risk of corrosion current collectors, separator plates made of titanium, graphite, and stainless steel. The chloroalkali industry provides better guidance for choosing materials suitable for harsh environments. Titanium has been used for all components that come in contact with chlorinated water, including supporting materials for anode and cathode catalysts. Photocatalytic water splitting: The following Table 3 gives different photocatalysts for seawater electrolysis, and the hydrogen production yield is also described in different electrolyte mediums [50].

6 Conclusions The electrolysis of saline water is one of the major concerns for the production of hydrogen and oxygen fuels due to the presence of various ions, microbes, and insoluble salts. These impurities form precipitates on the electrode surface and block the active sites of the catalyst; thus, removal of foreign ions and utilization of appropriate membrane also help in efficient seawater electrolysis. In this chapter, we have discussed the mechanism of electrolysis and corrosion at the electrode surface, different electrocatalysts responsible for dealing with saline water splitting, and photo electrocatalysts for seawater splitting. This chapter also gives brief information about various electrocatalysts with better HER and OER performance, and various electrolyzers for water-splitting mechanism. The problems caused due to the presence of chloride ions at cathode and anode surfaces are discussed. The transformation toward a decarbonized society requires careful considerations in cost reduction. In many cases, saline water electrolyzer technologies will need to have competitive capital and operational costs compared with electrolyzer technologies coupled with desalination and purification units. Although it remains unclear which of the electrolyzer technologies will be more suitable for saline waters, the operation at near-neutral pH (7–9) is preferable and AEM would most likely fulfill this requirement unless gas-phase electrolysis becomes competitive. This chapter would be beneficial for the research community to assess new materials for saline water splitting using standard criteria.

Table 3 Different photocatalysts studied in seawater and their hydrogen yield. S.No. Photocatalyst

Light (Intensity)

Electrolyte medium

1. 2.

NiO/Ni/La2Ti2O7 Pt/CdS/TiO2

UV VIS (>420 nm)

3.

Pt/TiO2

4.

Ti3+ self-doped Ti-O-Si

5.

(WS2)0.7/(C-TiO2)5/g-C3N4

6.

(WS2)0.7/(C-TiO2)5/g-C3N4

7.

SiO2/Ag@TiO2 core shell

8. 9.

0.3 wt% Ni/NaTaO3 NiS/ZnS1
x
0.5yOx(OH)y/ZnO

UV-VIS (>320 nm, 558 mW/cm2) 420 nm (3.15 mW/cm2) Solar (AM1.5G, 117 mW/cm2) 420 nm (9.584 mW/cm2) Solar (AM1.5G, 100 mW/cm2) UV-VIS VIS (>420 nm)

Natural seawater (pH¼8.5) Natural seawater+10 mM Na2S +2 mM Na2SO3 Natural seawater+1.09 M glycerol (pH¼7.7) Artificial seawater+10 vol% triethanolamine (pH 8.2) Natural seawater

10. 11.

(Ni-ZnO)@C nanoreactors Pt/Cd0.5Zn0.5S

UV-VIS VIS (>420 nm)

H2 yield (mmol/g h) References

0.696 1.86

[39] [39]

1.57

[40]

0.236

[41]

4.56

[42]

Natural seawater

0.249

[42]

Artificial seawater+20% v/ v glycerol (pH 7.99) 0.5 M NaCl+0.1 M glucose Natural seawater+24 g/L Na2S9H2O+5 g/L Na2SO3 Artificial seawater+2% methanol 0.5 M NaCl+0.05 M glucose (pH 12)

0.857

[43]

23.4 0.333

[44] [45]

5.110
3 0.183

[46] [47] Continued

Table 3 Different photocatalysts studied in seawater and their hydrogen yield—cont’d S.No. Photocatalyst

Light (Intensity)

Electrolyte medium

12.

Nanotube TiO2/Pt/Cd0.8Zn0.2S

UV-VIS (>395 nm)

13.

Nanotube TiO2/Pt/Cd0.8Zn0.2S

UV-VIS (>395 nm)

14. 15.

Cd0.25Zn0.75Se/CoP Nanorod ZnO/Pt/Cd0.8Zn0.2S

Solar (AM1.5G) UV-VIS (>395 nm)

Natural seawater+Na2S/ Na2SO3 (pH 6.8) Seawater+benzyl alcohol/acetic acid Artificial seawater Seawater+benzyl alcohol/acetic acid

H2 yield (mmol/g h) References

5.1

[48]

21.7

[48]

36.6 23.7

[49] [48]

Hydrogen production driven by seawater electrolysis

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Acknowledgment The authors would like to thank the Shell Energy India Private Limited (SEIPL) for providing the financial support.

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CHAPTER 19

Prospects and challenges for the green hydrogen market Arcílio B.S. Semente, Catarina B. Madeira Rodrigues, Margarida A. Mariano, Miguel B. Gaspar, Biljana Šljukic, and Diogo M.F. Santos Center of Physics and Engineering of Advanced Materials, Instituto Superior Tecnico, Lisbon, Portugal

1 Hydrogen production and economy decarbonization Nowadays, one of the most critical challenges is the decarbonization of the global economy. The key to meeting this challenge is to consistently expand the use of renewable energy sources like wind, solar, biomass, and hydro energy. Hydrogen appears to be an exceptionally promising energy carrier. Even though it is a colorless gas, it can be classified as green, blue, gray, brown or black, turquoise, purple, pink, red, and white. These color codes refer to the source or the process used to make hydrogen, and the most known, due to their more widely used production processes, are the green, blue, and gray hydrogen. Green hydrogen is generated by using electricity from renewable sources to split water molecules into hydrogen and oxygen; this process is called electrolysis. In this case, the greenhouse gas (GHG) emissions would stem only from the electrolyzer manufacturing process and the electricity supply (only in the case of grid electricity being used, i.e., for yellow hydrogen production). The blue hydrogen can be produced by steam methane reforming (SMR) using natural gas or coal gasification in plants with carbon dioxide (CO2) capture and storage (CCS). The process involves converting natural gas or coal into hydrogen and carbon dioxide. Regarding GHG emissions, hydrogen is only considered “blue” if the carbon footprint is lower than 36.4 g CO2/MJ produced. To restrain the carbon dioxide emission to the proposed levels, this gas is captured and stored, or reused, in plants with CCS. The gray hydrogen is produced exactly like the blue hydrogen, but there is no capture and storage of CO2. Presently, gray hydrogen is cheaper than blue or green hydrogen but also carries additional costs related to CO2 emissions [1]. Solar-Driven Green Hydrogen Generation and Storage https://doi.org/10.1016/B978-0-323-99580-1.00021-2

Copyright © 2023 Elsevier Inc. All rights reserved.

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1.1 Green hydrogen As mentioned, green hydrogen is an important piece of the energy transition and contributes to the economy’s decarbonization. As most of today’s hydrogen is gray, the carbon footprint is still increasing, even using hydrogen. On the other hand, green hydrogen is considered carbon-free hydrogen because the carbon footprint is very low or even zero, depending on the source of the electricity used in the production process. Use of green hydrogen is currently facing numerous challenges, including high cost across the entire value chain, from electrolysis to transport and fuel cells; lack of existing infrastructure for transport and storage (Fig. 1); high energy losses; and lack of value for the main benefit (e.g., lower GHG emissions) that green hydrogen can have. To make the hydrogen economy possible by 2050, its price must decrease from almost 4 $/kg to 1 $/kg (Fig. 1), below the production cost of gray hydrogen. It is predicted that in 2050 hydrogen will be the second decarbonization vector, with 13% of final energy demand, whereas the first will be direct

Fig. 1 Current and predicted cost ($/kg) of hydrogen production [2].

Prospects and challenges for the green hydrogen market

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Fig. 2 Indicative scenario for decarbonization in 2050 [3].

electrification, with 68% (Fig. 2). However, this will only happen if green hydrogen production volume becomes competitive [2]. In this regard, some prospects of this market type are important to be noted. First, some specialists defend that the commercial viability of green hydrogen production will increase because of decreasing prices of renewable energy coupled with the cost reduction of electrolyzers and increased efficiency due to technology improvements. This can reduce production costs of green hydrogen to $0.70–$1.60 per kg in most parts of the world by 2050. There were suggestions that green hydrogen production costs could achieve equality (if not superiority) with fossil fuels as early as 2025 [4]. So, it is clear that the production of green hydrogen has advantages and drawbacks. For example, one of its benefits is that it is an exceptional alternative to traditional fossil fuels. It can be a means to store energy and then generate power with zero carbon emission in fuel cells. Or it can be an alternative to hard-to-decarbonize industrial sectors currently dependent on nonrenewable sources, thus decreasing their carbon footprint. On the other hand, there are safety concerns because hydrogen is one of the most highly flammable and volatile substances and is colorless and odorless, making any leak detection almost impossible. The high cost is a considerable disadvantage, as well as its problematic transportation.

1.2 Water electrolysis Electrolysis is the method of choice to produce green hydrogen. It uses an electrical current to break water molecules into hydrogen and oxygen. Thus,

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if this electricity is obtained from renewable sources, there are no GHG emissions into the atmosphere. Electrolyzers can be divided into three types, depending on the transporter of the electrolyte: • Proton-exchange membrane (PEM) electrolyzer • Alkaline electrolyzer • Solid oxide electrolysis cell (SOEC) Proton-exchange membrane and alkaline electrolyzers are the most widely used for water electrolysis because they are mature technologies. The solid oxide electrolyzer is still in development and has an estimated price, for 2050, of less than 200 $/kW produced, which is two times more than the alkaline and PEM electrolyzers, with less than 100 $/kW (Table 1) [5,6]. The major component of the green hydrogen cost is the electricity supply. Auspiciously, this component’s cost decrease is already underway through the competitive deployment of renewables.

2 Challenges As mentioned above, despite hydrogen having great potential for the energy transition and decarbonization of multiple sectors, there are still barriers hampering green hydrogen from taking a bigger role in the global energy mix. The following sections discuss the technical and economic challenges that green hydrogen still face, as well as the required infrastructure and the legal and political framework.

2.1 Technical challenges Presently, the efficiency of fuel cell and electrolyzer systems is at a level that is not competitive for several electrical end-use applications. There are substantial energy losses when green hydrogen is produced by electrolysis, liquefied, compressed, converted to other carriers, transported, or used in fuel cells [7]. Namely, the production of green hydrogen from water electrolysis has an efficiency of 60%–70%, which means that about one-third of the electricity goes as waste heat. Hydrogen is then compressed and stored, losing another 10% of its energy, and used to produce electricity in fuel cells with an efficiency of 40%–55%. Thus, the overall efficiency from electricity to hydrogen and back to electricity is about 38% [8,9]. Consequently, this will demand considerable amounts of energy from renewable sources to feed green hydrogen electrolyzers, increasing the capital cost for energy production from renewables. The operating efficiency of

Table 1 Key performance indicators for four electrolyzer technologies today and in 2050 [5]. 2020

Cell pressure [bar] Efficiency (system) (kWh/kgH2) Lifetime (thousand hours) Capital costs estimate for large stacks (stack-only, >1 MW) (USD/kW) Capital cost range estimate for the entire system, >10 MW (USD/kW)

2050

Alkaline

PEM

AEM

SOEC

Alkaline

PEM

AEM

SOEC