Nanostructured Materials for Energy Related Applications [1st ed.] 978-3-030-04499-2, 978-3-030-04500-5

Table of contents :
Front Matter ....Pages i-xvi
Recent Trends in Nanomaterials for Sustainable Energy (D Durgalakshmi, Rajendran Saravanan, Mu. Naushad)....Pages 1-20
Recent In Situ/Operando Characterization of Lithium-Sulfur Batteries (Yan Yang, Yimin Zhu, Kumar Raju, Sheng Dai, Charl J. Jafta)....Pages 21-40
Recent Advances in Flexible Supercapacitors (Swati Jadhav, Vikash Chaturvedi, Manjusha V. Shelke)....Pages 41-72
Noble-Metal-Free Nanoelectrocatalysts for Hydrogen Evolution Reaction (Natarajan Thiyagarajan, Nithila A. Joseph, Manavalan Gopinathan)....Pages 73-120
Energy-Saving Synthesis of Mg2SiO4:RE3+ Nanophosphors for Solid-State Lighting Applications (Ramachandra Naik, Ramyakrishna Pothu, Prashantha S. C, Nagabhushana H, Aditya Saran, Harisekhar Mitta et al.)....Pages 121-143
Studies of Multi-walled Carbon Nanotubes and Their Capabilities of Hydrogen Adsorption (Edgar Mosquera, Mauricio Morel, Donovan E. Diaz-Droguett, Nicolás Carvajal, Rocío Tamayo, Martin Roble et al.)....Pages 145-162
Emerging Vertical Nanostructures for High-Performance Supercapacitor Applications (Subrata Ghosh, Tom Mathews, S. R. Polaki, Sang Mun Jeong)....Pages 163-187
Hydrogen Production Through Solar-Driven Water Splitting: Cu(I) Oxide-Based Semiconductor Nanoparticles as the Next-Generation Photocatalysts (Sanjib Shyamal, Ashis Kumar Satpati, Arjun Maity, Chinmoy Bhattacharya)....Pages 189-222
Application of Nanoparticles in Clean Fuels (Kumaran Kannaiyan, Reza Sadr, Vignesh Kumaravel)....Pages 223-242
Biomass-Derived Nanomaterials (Sebastian Raja, Luiz H. C. Mattoso, Francys K. V. Moreira)....Pages 243-270
Recent Progress of Carbon Dioxide Conversion into Renewable Fuels and Chemicals Using Nanomaterials (Harisekhar Mitta, Putrakumar Balla, Nagaraju Nekkala, Krishna Murthy Bhaskara, Rajender Boddula, Vijyakumar Kannekanti et al.)....Pages 271-293
Back Matter ....Pages 295-297

Citation preview

Environmental Chemistry for a Sustainable World

Saravanan Rajendran Mu. Naushad Subramanian Balakumar Editors

Nanostructured Materials for Energy Related Applications

Environmental Chemistry for a Sustainable World Volume 24

Series Editors Eric Lichtfouse, Aix-Marseille University, CEREGE, CNRS, IRD, INRA, Coll France, Aix en Provence, France Jan Schwarzbauer, RWTH Aachen University, Aachen, Germany Didier Robert, CNRS, European Laboratory for Catalysis and Surface Sciences, Saint-Avold, France

Other Publications by the Editors

Books Environmental Chemistry http://www.springer.com/978-3-540-22860-8 Organic Contaminants in Riverine and Groundwater Systems http://www.springer.com/978-3-540-31169-0 Sustainable Agriculture Volume 1: http://www.springer.com/978-90-481-2665-1 Volume 2: http://www.springer.com/978-94-007-0393-3 Book series Environmental Chemistry for a Sustainable World http://www.springer.com/series/11480 Sustainable Agriculture Reviews http://www.springer.com/series/8380 Journals Environmental Chemistry Letters http://www.springer.com/10311 Agronomy for Sustainable Development http://www.springer.com/13593

More information about this series at http://www.springer.com/series/11480

Saravanan Rajendran • Mu. Naushad Subramanian Balakumar Editors

Nanostructured Materials for Energy Related Applications

Editors Saravanan Rajendran Faculty of Engineering, Department of Mechanical Engineering University of Tarapacá Arica, Chile

Mu. Naushad Chemistry Department, College of Science King Saud University Riyadh, Saudi Arabia

Subramanian Balakumar NCNSNT University of Madras Chennai, India

ISSN 2213-7114 ISSN 2213-7122 (electronic) Environmental Chemistry for a Sustainable World ISBN 978-3-030-04499-2 ISBN 978-3-030-04500-5 (eBook) https://doi.org/10.1007/978-3-030-04500-5 Library of Congress Control Number: 2018968402 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

The scenario of necessity being the mother of scientific inventions has added criteria; the world’s growing population and increasing numbers of people aspire to higher standards of living. The world population has more than doubled in the last 60 years; in 2011 there were seven billion people on the planet. The United Nations Food and Agriculture Organization predicts that this growth will continue and in 2050 the population will reach nine billion. This rate of growth demands more food along with more and more energy, in a way sometimes to increase food supply we need energy. As prosperity continues to grow, we are eventually at a point where the demand for energy outstrips supply. Energy, being the crux for humankind survival, also finds its place in day-to-day applications. Many countries use electricity to heat their homes, power vehicles, light buildings, use electronic devices, run machines in industries, etc. This electricity comes from energy sources such as the burning of petroleum and other fossil fuels. Countries require a massive amount of fuel to generate enough electricity to meet demands. Fortunately, there is a solution to this problem: renewable energy. Unlike fossil fuels, renewable sources like solar energy, wind energy, and water power never run out. With a much lower impact on the environment, using renewable energy helps to protect our planet by significantly reducing the amount of carbon emissions that we produce. By using renewable energy sources, we also reduce our dependence on fossil fuel gas and oil reserves, which means that we can avoid the rising cost of energy bills and improve our energy security. The solution is clear, but what prevents its feasibility is a technology to aptly generate, process, or store these energies. As a boon the multidisciplinary field that is nowadays called nanotechnology is critical to overcome some of the technological limitations. Novel multifunctional nanomaterials offer impeccable improvements in all domains including energy system, such as generation, transportation, and storage of energy when compared to conventional systems. In a way, researchers are finding nanomaterials to increase the efficiency and store the energy of the conventionally produced energy, without much change to existing technology. On the other hand in

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the near future, a complete revamp of existing technologies is targeted with novel nanomaterials and nanotechnologies like advanced PV systems, biomass-derived energy, etc. Nanomaterials serve as energy carriers, absorbents, media for energy transfer, catalysts, converters, and energy pools or vessels for reactions. In all of these applications, the core technology to be developed is the preparation of novel nanomaterials with controllable sizes, shapes, and/or structures. In this book, an in-depth analysis is focused on particular nanomaterials and systems applicable for technologies such as clean fuel; battery; hydrogen generation, absorption, and storage; supercapacitors; and battery applications. It is aimed not only to exploit certain nanomaterials for technology transfer but also a wide knowledge on certain avenues such as biomass-derived nanomaterials, carbon dioxide conversion into renewable fuel chemicals using nanomaterials, etc. These are the areas with lacunae that demand more research and application. To conclude, as the president of the World Bank, Jim Yong Kim said “Ending poverty and ensuring sustainability are the defining challenges of our time. Energy is central to both of them.” If renewable energy was available to all, life would be made easier, the environment would be safer, and poverty would be lessened. With thoughts of global and personal care, we all need to be involved in making our homes more energy efficient with nanotechnology and switching to renewable energy sources to preserve our planet, our energy sources, and our wallets too. Arica, Chile Riyadh, Saudi Arabia Chennai, India

Saravanan Rajendran Mu. Naushad S. Balakumar

Acknowledgments

First of all, we thank the Almighty God who has been kind enough to bless us with good health while working on this book and for making this task a success. We would like to express our heartfelt gratefulness to the series editor, Eric Lichtfouse, for his immense knowledge, enthusiasm, inspiring discussions, and valuable suggestions which made us to engage in this book work successfully. We would like to extend our gratitude to the publisher, Springer, for accepting this book as part of the series Environmental Chemistry for a Sustainable World, to the contributing authors and reviewers for their esteemed work, and to all the authors, publishers, and other researchers for granting us the copyright permissions to use their illustrations. Although every effort has been made to obtain the copyright permissions from the respective owners to include citation with the reproduced materials, we would still like to offer our deep apologies to any copyright holder if unknowingly their right is being infringed. Saravanan Rajendran gratefully acknowledges financial support from the SERC (CONICYT/FONDAP/15110019), FONDECYT, Government of Chile (Project No.: 11170414), and School of Mechanical Engineering (EUDIM), University of Tarapaca, Arica, Chile. He would also like to extend his warmest thanks to Prof. Francisco Gracia (DIQBT, University of Chile), Prof. Lorena Cornejo Ponce (EUDIM, University of Tarapaca), and Prof. Rodrigo Palma (Director, SERC) for their constant support, moral guidance, enthusiasm, and immense knowledge which helped him to complete the task. Mu. Naushad would like to express his deep gratitude to the Chairman of the Department of Chemistry, College of Science, King Saud University, Saudi Arabia, for his valuable suggestions and constant inspiration. He is also thankful to the Deanship of Scientific Research at King Saud University for the support. S.B. thankfully acknowledges financial support from the National Centre for Nanoscience and Nanotechnology, University of Madras, Chennai. Saravanan Rajendran, Mu. Naushad, and S. Balakumar

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Contents

1

Recent Trends in Nanomaterials for Sustainable Energy . . . . . . . . . Durgalakshmi D, Saravanan Rajendran, and Mu. Naushad

2

Recent In Situ/Operando Characterization of Lithium-Sulfur Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yan Yang, Yimin Zhu, Kumar Raju, Sheng Dai, and Charl J. Jafta

3

Recent Advances in Flexible Supercapacitors . . . . . . . . . . . . . . . . . Swati Jadhav, Vikash Chaturvedi, and Manjusha V. Shelke

4

Noble-Metal-Free Nanoelectrocatalysts for Hydrogen Evolution Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Natarajan Thiyagarajan, Nithila A. Joseph, and Manavalan Gopinathan

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21 41

73

5

Energy-Saving Synthesis of Mg2SiO4:RE3+ Nanophosphors for Solid-State Lighting Applications . . . . . . . . . . . . . . . . . . . . . . . . 121 Ramachandra Naik, Ramyakrishna Pothu, Prashantha S.C, Nagabhushana H, Aditya Saran, Harisekhar Mitta, and Rajender Boddula

6

Studies of Multi-walled Carbon Nanotubes and Their Capabilities of Hydrogen Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Edgar Mosquera, Mauricio Morel, Donovan E. Diaz-Droguett, Nicolás Carvajal, Rocío Tamayo, Martin Roble, Vania Rojas, and Rodrigo Espinoza-González

7

Emerging Vertical Nanostructures for High-Performance Supercapacitor Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 Subrata Ghosh, Tom Mathews, S. R. Polaki, and Sang Mun Jeong

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Hydrogen Production Through Solar-Driven Water Splitting: Cu(I) Oxide-Based Semiconductor Nanoparticles as the Next-Generation Photocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Sanjib Shyamal, Ashis Kumar Satpati, Arjun Maity, and Chinmoy Bhattacharya

9

Application of Nanoparticles in Clean Fuels . . . . . . . . . . . . . . . . . . 223 Kumaran Kannaiyan, Reza Sadr, and Vignesh Kumaravel

10

Biomass-Derived Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 Sebastian Raja, Luiz H. C. Mattoso, and Francys K. V. Moreira

11

Recent Progress of Carbon Dioxide Conversion into Renewable Fuels and Chemicals Using Nanomaterials . . . . . . . . . . . . . . . . . . . 271 Harisekhar Mitta, Putrakumar Balla, Nagaraju Nekkala, Krishna Murthy Bhaskara, Rajender Boddula, Vijyakumar Kannekanti, and Ramachandra Rao Kokkerapati

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

About the Editors

Dr. Saravanan Rajendran has received his Ph.D. in Physics-Material Science in 2013 from the Department of Nuclear Physics, University of Madras, Chennai, India. He was awarded the University Research Fellowship (URF) during 2009–2011 by the University of Madras. After working as an Assistant Professor in Dhanalakshmi College of Engineering, Chennai, India, during 2013–2014, he was awarded SERC and CONICYT-FONDECYT postdoctoral fellowship (2014–2017) by the University of Chile, Santiago. He has worked (2017–2018) in the research group of Professor John Irvine, School of Chemistry, University of St. Andrews, UK, as a Postdoctoral Research Fellow within the framework of an EPSRC Global Challenges Research Fund for the removal of blue-green algae and their toxins. Currently, he is working as a Research Scientist, in the School of Mechanical Engineering (EUDIM), University of Tarapaca, Arica, Chile. He has published several international peer-reviewed journals and book chapters. His work interest includes nanoporous and nanomaterial-based catalysts for renewable energy and wastewater purification. Dr. Mu. Naushad is presently working as an Associate Professor in the Department of Chemistry, College of Science, King Saud University (KSU), Riyadh, Kingdom of Saudi Arabia. He obtained his M.Sc. and Ph.D. degrees in Analytical Chemistry from Aligarh Muslim University, Aligarh, India, in 2002 and 2007, respectively. He has a vast research experience in the multidisciplinary fields of analytical chemistry, materials chemistry, and environmental science. He holds several US patents, over 250 publications in the international journals of repute, 15 book chapters, and several books published by renowned international publishers. He has >5800 citations with a Google Scholar H-Index >47. He has successfully run several research projects funded by National Plan for Science, Technology, and Innovation (NPSTI) and King Abdulaziz City for Science and Technology (KACST), Kingdom of Saudi Arabia. He is the Editor/Editorial Member of several reputed journals like Scientific Reports (Nature), Process Safety and Environmental Protection (Elsevier), Journal of Water Process Engineering (Elsevier), and International Journal of Environmental xi

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About the Editors

Research and Public Health (MDPI). He is also the Associate Editor of Environmental Chemistry Letters (Springer) and Desalination and Water Treatment (Taylor & Francis). He has been awarded the Scientist of the Year Award (2015) by the National Environmental Science Academy, New Delhi, India, and Almarai Award (2017), Saudi Arabia. Dr. Subramanian Balakumar received a PhD in Materials Sciences from Anna University in Chennai, India. He is currently the Professor and Director for National Centre for Nanoscience and Nanotechnology, University of Madras, Chennai. He is Associate Editor for Chemical Papers (Springer) and Journal of Nanofluids. He is also a Fellow of the Royal Society of Chemistry, and his research interests are in the design and development of nanostructured multifunctional materials with emphasis on energy and environment, storage, electromagnetic shielding, and biomedical applications. He is a coauthor of 150+ research publications and wrote 3 book chapters in subjects related to nanotechnology, materials chemistry, and biosensors. He has ten patents or patents pending.

Contributors

Putrakumar Balla State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India Krishna Murthy Bhaskara Department of Physics, SVM Degree College, Gadwal, Telangana, India Chinmoy Bhattacharya Department of Chemistry, Indian Institute of Engineering Science & Technology (IIEST), Shibpur, West Bengal, India Rajender Boddula CAS-Key Laboratory of Nano-system and Hierarchical Fabrication, National Centre for Nanoscience and Technology, Beijing, China Nicolás Carvajal Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Prashantha S. C Research Center, Department of Science, East West Institute of Technology, Bengaluru, India Vikash Chaturvedi B101, Polymer and Advanced Materials Laboratory, Physical & Material’s Chemistry Division, CSIR-National Chemical Laboratory, Pune, India Sheng Dai Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Donovan E. Diaz-Droguett Instituto de Física, Pontificia Universidad Católica de Chile, Santiago, Chile Durgalakshmi D Department of Medical Physics, Anna University, Chennai, India Rodrigo Espinoza-González Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Subrata Ghosh Green Energy Lab, Department of Chemical Engineering, Chungbuk National University, Chungbuk, Republic of Korea xiii

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Contributors

Manavalan Gopinathan Department of Chemistry, National Chung Hsing University, Taichung, Taiwan Harisekhar Mitta State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India Swati Jadhav B101, Polymer and Advanced Materials Laboratory, Physical & Material’s Chemistry Division, CSIR-National Chemical Laboratory, Pune, India Charl J. Jafta Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Sang Mun Jeong Green Energy Lab, Department of Chemical Engineering, Chungbuk National University, Chungbuk, Republic of Korea Nithila A. Joseph Institute of Biomedical Sciences, National Chung Hsing University, Taichung, Taiwan Kumaran Kannaiyan Department of Mechanical Engineering, Texas A & M University at Qatar, Doha, Qatar Vijyakumar Kannekanti College of Chemistry, Key Laboratory Physics and Technology of Ministry of Education, Sichuan University, Chengdu, China Ramachandra Rao Kokkerapati Crystal Growth and Nano-Science Research Center, Department of Physics, Government Autonomous College, Rajamahendravaram, India Vignesh Kumaravel Department of Environmental Science, School of Science, Institute of Technology Sligo, Sligo, Ireland Arjun Maity DST/CSIR Innovation Centre, National Centre for Nanostructured Materials, Pretoria, South Africa Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa Tom Mathews Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam, Tamil Nadu, India Luiz H. C. Mattoso National Nanotechnology Laboratory for Agribusiness, Embrapa Instrumentação, São Carlos, SP, Brazil Francys K. V. Moreira Department of Materials Engineering – DEMa, Federal University of São Carlos – UFSCar, São Carlos, SP, Brazil Mauricio Morel Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Edgar Mosquera Departamento de Física, Universidad del Valle, Cali, Colombia

Contributors

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Nagabhushana H Prof. CNR Rao Center for Advanced Materials, Tumkur University, Tumkur, India Ramachandra Naik Department of Physics, New Horizon College of Engineering, Bengaluru, India Mu. Naushad Chemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia Nagaraju Nekkala Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India S. R. Polaki Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam, Tamil Nadu, India Sebastian Raja National Nanotechnology Laboratory for Agribusiness, Embrapa Instrumentação, São Carlos, SP, Brazil Saravanan Rajendran Faculty of Engineering, Department of Mechanical Engineering, University of Tarapacá, Arica, Chile Kumar Raju Energy Materials, Council for Scientific and Industrial Research, Pretoria, South Africa Ramyakrishna Pothu College of Chemistry and Chemical Engineering, Hunan University, Changsha, China Martin Roble Instituto de Física, Pontificia Universidad Católica de Chile, Santiago, Chile Vania Rojas Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Reza Sadr Department of Mechanical Engineering, Texas A & M University at Qatar, Doha, Qatar Department of Mechanical Engineering, Texas A & M University, College Station, TX, USA Aditya Saran Department of Microbiology, Marwadi University, Rajkot, Gujarat, India Ashis Kumar Satpati Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India Manjusha V. Shelke B101, Polymer and Advanced Materials Laboratory, Physical & Material’s Chemistry Division, CSIR-National Chemical Laboratory, Pune, India Sanjib Shyamal Department of Chemistry, Indian Institute of Engineering Science & Technology (IIEST), Shibpur, West Bengal, India

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Contributors

Rocío Tamayo Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile Natarajan Thiyagarajan Department of Chemistry, National Chung Hsing University, Taichung, Taiwan Yan Yang Collaborative Innovation Center for Vessel Pollution Monitoring and Control, Dalian Maritime University, Dalian, China Yimin Zhu Collaborative Innovation Center for Vessel Pollution Monitoring and Control, Dalian Maritime University, Dalian, China

Chapter 1

Recent Trends in Nanomaterials for Sustainable Energy Durgalakshmi D, Saravanan Rajendran, and Mu. Naushad

Contents 1.1 Energy and Its Future Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Nanostructuring for Sustainable Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Nanomaterials Fulfilling Energy Needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Low-Temperature Solid Oxide Fuel Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Hydrogen Production from Solar Light-Driven Photocatalytic Water Splitting Under Nanomaterial Co-catalyst . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.4 A Shift Toward Nanostructured Metal Oxides as Efficient Supercapacitors . . . . . 1.3 Nanomaterials from Biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 6 8 11 12 13 15 16 17

Abstract The consumption of energy in any form is inevitable in the present era, and is growing. Currently, most energy resources come from fossil fuels. However, the alarming situation of global warming has attracted increasing attention to the development of renewable energy resources. The future of energy is completely in the hands of green energy resources. Hydrogen fuel cells are one of the focus areas of green energy generation with zero emission. Energy storage must be low cost, be small in size, and have a higher storage capacity. Developments in the field of nanoscience and nanotechnology are creating a more reliable pathway toward energy generation and storage. The unique characteristic of nanomaterials can control the dimensionality of materials (e.g., zero-, one-, two-, and threedimensional), morphology, and composite formation, through which the desired electrical, electronic, and storage properties of materials can be achieved. This Durgalakshmi D (*) Department of Medical Physics, Anna University, Chennai, India e-mail: [email protected] R. Saravanan Faculty of Engineering, Department of Mechanical Engineering, University of Tarapacá, Arica, Chile M. Naushad Chemistry Department, College of Science, King Saud University, Riyadh, Saudi Arabia © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_1

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chapter provides a brief outline of the current state of energy and recent developments in fuel cells, batteries, and supercapacitors with the aid of nanotechnology.

1.1

Energy and Its Future Needs

The word energy was coined by the Greek philosopher Aristotle from the Greek word energeia, which is a metaphysics term signifying motion, action, work, and change. In 1842, Encyclopaedia Britannica offered a very brief description of energy as, “the power, virtue, or efficacy of a thing. It is also used figuratively denoting emphasis in speech.” Thus, the term energy is widely used in relation to work, food, power, and even health conditions. The first theoretical relations were introduced by Thomas Young in 1807, with energy being the product of the mass of a body and the square of its velocity; however, this was restricted only to kinetic energy. The science of energy was inventively active in the seventeenth century, from a viewpoint of understanding physics and employing engineering experiments, particularly steam engines by Isaac Newton (1642–1727) and James Watts (1736–1819). The theoretical principle of producing kinetic energy from heat with maximum efficiency was first introduced by Sadi Carnot (1796–1832). On the other hand, the interpretation of the evolution of CO2 and H2O from human and animal metabolism was first discovered by Justus Von Liebig (1803–1873). This provided the first step toward understanding the correlation of energy with oxidation and energy conversion. Yet another concept of oxidation and energy conversion was formulated by a German physician, Julius Robert Mayer (1814–1878), from his observations of blood oxidation in temperate regions. He extended the idea of energy conservation to all natural phenomena, including electricity, light, and magnetism. In the mid-nineteenth century, an English physicist established a correct value for the equivalence of heat and mechanical energy. This provided a wide understanding of thermodynamics and the birth of the first law of thermodynamics or the law of conservation of energy. A further refinement of energy principles was specified by understanding entropy in the closed system by Clausius in 1865 and this formulated the second law of thermodynamics. The third and final law of thermodynamics was initially formulated by Walther Nernst in 1906, who stated that at a near absolute zero (
273  C) temperature, the heat process comes to a stop. Albert Einstein’s concept of mass-energy equivalence (E ¼ mc2) in 1905 brought a fundamental extension to the first law of thermodynamics. This equation is the rule of thumb for so many scientific discoveries of energy applications. According to a nineteenth century physicist, energy is not an easily definable single entity; it is rather an abstract collective concept. The most commonly encountered forms of energy are thermal energy (heat), kinetic or mechanical energy (motion), electromagnetic energy (light), and chemical energy from fuels and foodstuffs (Harman and Harman 1982). The conversion of energy is a matter of

1 Recent Trends in Nanomaterials for Sustainable Energy

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Fig. 1.1 Energy conversion and its application devices

convenience enjoyed on a large scale, such as in batteries, which operate because of the conversion of chemical energy to electrical energy (Dunn et al. 2011). The efficiency of energy conversion is another major criterion to be noted when working with energy devices and applications (Arico et al. 2011). If the energy conversion is of low efficiency, then there is a high energy loss. Figure 1.1 shows the energy conversion from one form to another and its application devices (Smil 2017). Next to efficiency, power is the major factor that attract the attention all researchers and industrial product developers of energy components. Because the energy product that is to be used for a large number of cycles is either battery-based charging and discharging or power density distribution to the targeted region, the power is expected to be of the narrow constant region (Hadjipaschalis et al. 2009; Lahiri et al. 2002). In pre-industrial societies, the fuels needed for everyday activities predominantly came from places very close to the settlement. The energy transitions from coal to crude oil and natural gas include the growing prominence of electricity, which has profoundly changed the pattern of energy supply. Energy accounts for a growing share of the global value of international trade; it was about 8% in 2000 and increased to 17% by 2014 (Group 2014; www.iea.org). In 2025, the increase is

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expected to be twice as high as in 2014 (www.iea.org). In comparison with largescale flows of fossil fuels, the international trade in electricity is significant in only a limited number of sales or multinational exchanges. Even though with a higher prediction of fuel needs with available resources, common failures are unavoidable, similar to one of the proofs of the post-World War II global economic downturn in 2008–2009 (Gokay 2009; Dullien 2010). Even though some individual numbers come very close to actual performance, the link to resource availability is always missing. Compared with wind-power electricity and hydropower electricity, photovoltaics still makes a much smaller contribution; only 1% of all electricity generation in 2015 (Quaschning 2016). The efficiency of photovoltaic cells has risen from less than 5% during the 1960s to 25% for high-purity silicon crystals in the laboratory, but the field efficiency is only around 15% (Lincot 2017). The most efficient photovoltaic cells based on nanomaterial-based technology advances would be most welcome because of their relatively high-power densities. The increased need to control global warming will have a huge impact on the use of fossil fuels as a source of energy. A global agreement to cut global warming to less than 2  C was adopted by consensus on 12 December 2015 at the United Nations Climate Change Conference (Gao et al. 2017). Globally, we call for more efficient and environmentally friendly production cycles, which can be achieved by making use of the by-products and waste of energy production. Figure 1.2 shows that electricity production from renewable resources has been higher in the past decade. Similarly, Fig. 1.3 shows that access to electricity has been increasing to almost 90% of the population, with electricity production from oil, gas, and coal sources reaching

Fig. 1.2 Chart representing electricity production from renewable sources around the world from 1960 to 2017 (www.worldbank.org)

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Fig. 1.3 Chart representing electricity production from oil, gas, and coal sources, and the percentage of the world population that has access to electricity (www.worldbank.org)

saturation point. This clearly depicts that there is a greater shift toward energy from renewable resources to fulfil the electricity needs of the world population. Energy production and storage are two equally important criteria for all energy utilization sectors. Energy consumption that depends on the combustion of fossil fuels, the major source of energy widely chosen around the word, is forecast to have a severe impact on the environment and on economics in the future. Henceforth, there seems to be a major shift toward electrochemical energy production and storage that is more sustainable and more environmental friendly. Batteries, fuel cells, and supercapacitors are the major systems used for electrochemical energy conversion and energy storage devices (Bagotsky et al. 2015; Eftekhari and Fang 2017). In batteries and fuel cells, electrical energy is generated by conversion of chemical energy by redox reactions, where the anode and cathode are merely charge transfer media. In fuel cells, the active masses undergoing redox reactions are considered fuels, such as oxygen, hydrogen, and hydrocarbons, and are delivered from outside the cells. Hence, in fuel cells, energy storage and conversion are not included as part of the cells and are locally separated. The functioning of electrochemical-based supercapacitors, however, may not be delivered by redox reaction and it depends greatly on the orientation of electrolyte ions at the electrolyte interface creating an electrical double layer, leading to parallel movement of electrons in the external connecting wire, resulting in the energy delivery process. The power and energy capabilities are generally represented by the Ragone plot, as shown in Fig. 1.4. In the automobile industries, hybrid electrochemical power systems are already available on the market. In the near future, hybrid

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Durgalakshmi D et al. Disccharge Times 100 hr

60 min

36 sec

1000

0.36 sec

Specific Energy (Wh/kg)

Fuel Cells 100 Batteries 36 msec

10 1

Supercapacitors/ Ultracapacitors

0.1 Capacitors 0.01

1

10

102 103 105 104 Specific Power (W/kg)

106

107

Fig. 1.4 Simplified Ragone plot showing the energy storage delivery performance of various electrochemical conversion and storage systems (Thomas and Qidwai 2005; Kötz and Carlen 2000)

electrochemical power systems are expected to provide greater power and deliver large amounts of energy.

1.2

Nanostructuring for Sustainable Energy

For the past few decades, more research has been focused on studying the effect of size on altering the electrical properties of the materials. Figure 1.5 clearly shows the increasing trend in the number of articles published per year on nano applications in the field of fuel cells, batteries, and supercapacitors. Because of the large surface area, the use of nanomaterials as an anode and cathode will enhance the surface reactivity and hence the efficiency of energy products. More work is focused on nanomaterials obtained from biomass, which could be used as an anode/cathode material with the motivation of reducing the cost of manufacturing. Nanomaterial applications in fuel cells were of great interest in many countries and Fig. 1.6 shows the trend in the total number of documents published per country and territory from 2001 to 2017. In the field of nanomaterials, in addition to the surface area, interest in its property of dimensional confinement is also growing. By altering the synthesis process, both by top-down and bottom-up approaches, we can obtain zerodimensional (0D), one-dimensional (1D), two-dimensional (2D), and threedimensional (3D) nanoparticles (Fig. 1.7a), from which we can also alter the surface area, size, and kinetic properties of the material for energy devices (Tiwari et al. 2012). The work chart for the development of nanostructuring material for successful energy application is shown in Fig. 1.7b.

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Fig. 1.5 The total number of publications published per year on the topic of batteries, fuel cells, and supercapacitors. (Scopus publication details up to 2017)

Fig. 1.6 The total number of documents published from 2001 to 2017 per country/territory. (Scopus publication details up to 2017)

The energy conversions and storage mainly depend on the material used as anode and cathode; the next main factors are an electrolyte, applied potential parameters, and operating temperature. Fuel cells are ideal primary energy conversion devices for remote site location and have application, although an assured electrical supply is required for power generation and power distribution.

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Fig. 1.7 Schematic representation of (a) multiple dimensions of nanoparticles and their versatile energy applications and (b) the work chart for the development of nanostructuring materials for energy applications

Moving toward a system dominated by hydrogen is clearly consistent with the long-term decarbonization of the modern energy supply. Much scientific literature repeatedly implies that hydrogen might be a significant source of energy in the near future (Dincer and Acar 2015; Sharma and Ghoshal 2015; Hosseini and Wahid 2016). However, unlike methane, it is not present in huge reservoirs in the Earth’s crust, and energy is needed to produce it, from either methane or water. Energy from hydrogen has the property of zero-emission and also makes it an outstanding energy carrier; combustion with oxygen yields only water and the possibility of using it in fuel cells.

1.2.1

Nanomaterials Fulfilling Energy Needs

The energy content of the widely used fuels is as follows: hydrogen (120 MJ/kg), gasoline (44–45 MJ/kg), crude oils (42–44 MJ/kg), and lignites (12–20 MJ/kg) (www.world-nuclear.org). Thus, it could be clearly said that the amount of energy available from hydrogen fuel cells is very large. The quantity of chemical energy stored in hydrogen and several hydrocarbon fuels is significantly higher than that

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found in common batteries. There are many types of fuel cells, such as direct methanol fuel cells (DMFCs), alkaline fuel cells (AFCs), polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), and reversible fuel cells (RFCs) (O’Hayre et al. 2016; Li 2018; Sharaf and Orhan 2014). Each type of fuel cell has a unique set of processes and reactions to describe its operation, and each type of fuel cell has its own operating temperature, fuel requirements, and energy output. Overall, fuel cells can be roughly divided into low-temperature (operating temperature < 200  C) and high-temperature (operating temperature > 450  C). PEMFCs, AFCs, and DMFCs are low-temperature fuel cells, and PAFCs, MCFCs, and SOFCs are high-temperature fuel cells. Low-temperature fuel cells use alkaline or acidic electrolytes. The anode feed is typically pure hydrogen/gasoline/methanol/natural gas, whereas the cathode feed is oxygen/air. The overall reaction output is the formation of H2O and energy for most of the fuel cells. Apart from MCFCs and SOFCs, the fuel cells will have carbon composites as anode and cathode materials. Among the various materials that have been investigated with regard to low-temperature fuel cells, carbon materials are of great interest owing to their abundance, stability, efficiency, and relative environmental friendliness. In particular, the excellent chemical stability across a wide temperature range in either acidic or basic media makes carbon materials extremely attractive for use as electrodes in fuel cells. From the various allotropes of carbon, graphene emerges as an exciting novel material because of its high specific surface area (2630 m2/g), good chemical stability, good mechanical properties, and excellent electrical conductivity. Table 1.1 shows some of the properties of graphene compared with other carbon materials used for energy conversion and storage applications. These properties make graphene an excellent candidate for these functions. In recent years, more research has focused on the development of graphene-based materials on Table 1.1 Properties of carbon materials Properties Specific surface area (m2/ g) Thermal conductivity (W/mK) Intrinsic mobility (cm2/V s)

Young’s modulus (TPa)

Carbon nanotubes 1315

Activated carbon 1200

0.15–0.5

0.56

>3000 (multiwalled carbon nanotube) ~100,000

0.01

0.64

0.138

Fullerenes 5

0.4

Graphite ~10

Graphene 2630

~3000 (in-plane values) 13,000 (in-plane values)

~5000

1.06

~15,000 (in-plane values onto SiO2 surface) ~200,000 (freestanding) ~1.0

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Fig. 1.8 Illustration of carbon nanomaterials for lithium–sulfur batteries.(Xu et al. 2018)

macrostructural complexity, i.e., 0D (e.g., free-standing graphene dots and particles), 1D (e.g., fiber-type and yarn-type structures), 2D (e.g., graphenes and graphenebased nanocomposite films), and 3D structures for its application toward energy and storage applications (Shaari and Kamarudin 2017; Pandey et al. 2017; Bayer et al. 2014). Figure 1.8 illustrates the carbon nanomaterials used for Li-ion battery applications. However, to realize the expected full-scale practical application, both the quality and reproducible quantity of the electrode materials have to be improved further, in particular with regard to the development of the most desired structures tunable on a nano, micro, meso, or macro scale. In addition, nanostructured carbon materials, nanocomposites consisting of graphene-based and pseudocapacitive materials, i.e., graphene/conductive polymers, graphene/metal oxides or hydroxides, are promising for achieving the long-awaited requirement of both power density and high-energy density (Ke and Wang 2016; Ji et al. 2016). Therefore, there is a need for future exploration efforts on clarification of nanohybrid structures and control of the interfacial interaction between graphene and pseudocapacitive materials to improve the overall faradaic processes across the interface. The multifunctional or self-powered hybrid systems are of considerable interest for future development (Hwang et al. 2015). Recent pioneering work on the combination of flexible supercapacitors with other electronic and energy devices

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CO2

Fuel (H2, Alcohol)

Anode membrane Cathode

O2 H2 O

e– CO2

Fuel (H2, Alcohol)

e–

H2O Nanostructured materials (Anode)

Polymer electrolyte membrane

+

H

e–

O2

Nanostructured materials (Cathode)

H2O

Fig. 1.9 Schematic illustration of the workings of nanostructured material-based polymeric electrolyte membrane fuel cells (Martínez-Huerta and Lázaro 2017)

(i.e., solar cells, lithium-ion batteries, electrochromic devices, and nano-generators), as hybrid energy devices, also show a much more welcoming approach in industrial sectors.

1.2.2

Low-Temperature Solid Oxide Fuel Cells

Low-temperature solid oxide fuel cells are yet another important advancement brought about by nanomaterials-based technologies. To date, numerous fuel cell designs have been rigorously investigated, with PEMFCs and SOFCs representing two of the most important and most promising types of fuel cells for wide mobile and stationary use (Fan et al. 2018; Moreno et al. 2015). Figure 1.9 depicts the illustration of nanomaterials in PEMFCs. Compared with PEMFCs, SOFCs have greater electrical and overall energy efficiency (Wachsman et al. 2012). Lowering the operating temperature of SOFCs grants substantial technical and economic benefits, including use of cheap cell components such as stainless steel interconnects instead of expensive ceramic oxides, enabling rapid system start-up and shut-down, minimizing the possibility of the redox degradation of cermet anodes, extending the lifespan of the fuel cell, and broadening the areas of application. In an SOFC, a nanoscale thin film electrolyte can effectively reduce the ohmic resistance compared with a micrometric membrane (Litzelman et al. 2008; Joh et al. 2017). Furthermore, nanocrystalline thin films may significantly improve the ionic conductivity because of the large grain boundary (GB)/bulk ratio and interfacial lattice mismatch, which can also cause an unexpected changes in physical–electrochemical properties, such as high sintering temperature capacity and surface/

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interfacial super-conduction (Bowman et al. 2017; Pergolesi et al. 2015; Fan et al. 2018). Nanoscale electrolytes also promote the implementation of SOFCs for combined heat and power generation devices, such as micro-SOFCs, and this extends to automotive devices and even portable electronics (Fan et al. 2018). Nanopowder increases the triple-phase boundary (TPB) length of electrodes, which in essence has been proven to be an effective solution to improving the kinetics of the electrode reaction. The effect of the microstructure, including the porosity, composition, tortuosity, specific surface area, particle and pore size distribution, particle connection, ionic and electronic conductivity, electrode thickness, and interfacial bonding, becomes more evident, especially below 600  C (Fan et al. 2018; Forouzan et al. 2017). Moreover, extensive research into nanomaterials and composite materials integrated with advanced fabrication technologies has recently resulted in several breakthroughs, such as single-layer fuel cells and semiconductors for electrolyte application. Such advances significantly enrich the fundamental knowledge of fuel cells and accelerate the commercialization of fuel cell technologies.

1.2.3

Hydrogen Production from Solar Light-Driven Photocatalytic Water Splitting Under Nanomaterial Co-catalyst

The sun and wind are two major sources of renewable energy and of renewable hydrogen production. Photovoltaic water electrolysis may become more competitive as the cost continues to decrease with technology advancement; however, the extensive use of semiconducting materials with a narrow band gap may have a serious life-cycle environmental impact. In photocatalytic water/air purification, valence band holes are the key elements that induce decomposition of contaminants (Ni et al. 2007; Stewart 2009). The conduction band (CB) level is insignificant. On the other hand, when photocatalysis is applied to perform water-splitting for the production of hydrogen, the reducing CB electrons become important, as their role is to reduce protons to hydrogen molecules. The CB level should be more negative than the hydrogen evolution level (EH2/H2O) to initiate hydrogen production (Fig. 1.10) (Abe 2010). The above factors imply that photocatalytic water/air purification conditions might not be applicable to photocatalytic hydrogen production. Review and discussion of the state-of-the-art of photocatalytic hydrogen production technologies are thus beneficial. The development of a photocatalytic system that functions efficiently under visible light, representing almost half of the available solar energy on the surface of the earth, is therefore essential for the practical utilization of solar energy (Jing et al. 2010; Ismail and Bahnemann 2014). Taking into consideration the basic mechanism and the individual processes of photocatalytic water splitting, there are two keys to the development of a suitable high-efficiency semiconductor for the visible-light-driven photocatalytic splitting of water into H2 and/or O2: first, a

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a

13

b H2O

solar energy photocatalyst

(iii) hν

H2 + 1/2O2

H2

Potential – C.B.

+

H /H2

e–

e–

e

–

H

H+

(i) e– (ii) h

O2

recombination

+

H2O

+

H2O

H2

O2 /H2O V.B. +

h

+

h

+

e

–

O2

Fig. 1.10 (a) Mechanism of hydrogen production from solar light-driven photocatalytic water splitting and (b) schematics of the band structure for the production of hydrogen (Abe 2010)

photocatalyst should have a sufficiently narrow band gap (1.23 eV < Eg < 3.0 eV) to both harvest visible light and possess the correct band structure; second, photoinduced charges in the photocatalyst should be separated efficiently to avoid bulk/ surface electron/hole recombination (Ismail and Bahnemann 2014; Qu et al. 2013). Early work on TiO2 photoelectrochemical hydrogen production was reported by Fujishima and Honda (1972). Subsequently, scientific and engineering interest in semiconductor photocatalysis has grown significantly. During the past few decades, many photocatalysts have been developed, such as ZnO, TiO2, and CdS (Qu et al. 2013; Saravanan et al. 2013; Etacheri et al. 2015). However, various drawbacks, such as poor absorption ability (e.g., TiO2), the fast recombination rate of photogenerated electron–hole pairs, the high toxicity for human health, and the harm caused to the environment (e.g., CdS), greatly limit the photocatalytic efficiency and practical application for solar hydrogen conversion. Coupling with quantum dots of narrow band gap semiconductors has been proven effective at improving photoactivity in the visible spectrum.

1.2.4

A Shift Toward Nanostructured Metal Oxides as Efficient Supercapacitors

In general, nanostructured metal oxides, such as ruthenium oxide (RuOx), manganese oxide (MnOx), nickel oxide (NiOx), cobalt oxide (CoOx), iron oxide (FeOx), and titanium oxide (TiOx), can provide higher energy density in supercapacitor applications than conventional carbon materials(Maitra et al. 2017; Chen et al. 2013; Lokhande et al. 2016). They not only store energy such as electrostatic carbon materials, but they also exhibit electrochemical faradaic reactions between electrode materials and ions within appropriate potential windows. The schematic

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Durgalakshmi D et al. Separator Inner cavity

Tube spacing

Current collector

Current collector Anode

IL anion

ACNT

II. cation

Cathode

Fig. 1.11 Schematic of an electrochemical capacitor based on plasma-etched aligned carbon nanotube electrodes and an ionic liquid electrolyte (Lu et al. 2009)

representation of electrochemical capacitors based on CNTs is shown in Fig. 1.11, in which each of the constituent aligned tubes is connected directly onto a common current collector, maximizing the charge transport capability of the aligned carbon nanotube electrode. This ensures a combined charge capacity from all individual tubes and thus an enhanced energy density for the capacitor. In turn, the stored charge can be delivered rapidly through each individual tube of the electrode, resulting in a high power density for the capacitor. Of the transition metal oxides, RuOx has been the most extensively studied candidate owing to its wide potential window, highly reversible redox reactions, metallic type conductivity, remarkably high specific capacitance, good thermal stability, long cycle life, and high rate capability (Lee et al. 2010; Jian et al. 2016). As a typical pseudocapacitive material, the solution-based, binder-free synthetic approach of RuO2 films enabled an allsolid-state supercapacitor (Wang et al. 2014). The all-solid-state quality, combined with superior electrochemical performance, makes the RuO2-based device an excellent candidate for power sources in portable electronics (Wang et al. 2014, 2015). In another example, an all-solid-state supercapacitor with a Nafion solid polymer electrolyte membrane and a RuO2 combined Nafion ionomer electrode were used (Park et al. 2002; Kannan et al. 2008). The supercapacitor exhibits a relatively stable capacitance during 10,000 cycles of operation (about 30% loss). In comparison, MnOx is often considered a promising transition metal oxide for pseudocapacitors because of its high theoretical specific capacitance (1100–1300 F/g), low cost, environmental benignity, and as an abundant resource (Wei et al. 2011; Yu et al.

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2013). To overcome the drawback of poor ionic conductivity, MnOx is usually composited with CNTs, graphene, and conductive scaffold. Yuan et al. reported a highly flexible solid-state supercapacitor based on a hybrid structure of carbon nanoparticles (CNPs)/MnO2 nanorods using a PVA/H3PO4 electrolyte (Yuan et al. 2011). The device exhibited good electrochemical performance with an energy density of 4.8 Wh/kg at a power density of 14 kW/kg. Inspired by this work, Yang et al. developed a new kind of worm-like amorphous MnO2 nanowires (NWs) on textiles for high-performance flexible supercapacitors(Yang and Mai 2014). The flexible solid-state symmetric supercapacitors assembled with worm-like MnO2 electrodes exhibited a high energy density of 6.3 Wh/kg.

1.3

Nanomaterials from Biomass

To achieve high energy and power densities, electrical double-layer capacitance of energy storage materials must possess a large specific surface area available for ion electro-adsorption, pore size distribution optimized for the desired combination of energy and power characteristics, good electrical conductivity, and wettability by the electrolyte utilized in the device construction. Activated carbon is the oldest and most common type of porous carbon. The use of activated carbon in Egypt was described as early as 1550 BC (Cooney 1980). Zang et al. reported on the fabrication of silicon/nitrogen-doped carbon/carbon nanotube (SNCC) nano/micro-hierarchical structured spheres through a facile electrospray approach cost-effective for possible industrial applications, and placed great importance on the sustainable development of energy (Zhang et al. 2016) (Fig. 1.12). Industrial production of ACs in the USA started in 1913 (Baker et al. 1992). At present, nearly all commercial electrostatic double-layer capacitors utilize a large surface area. Activated carbon powers due to their well-developed manufacturing technologies, easy production in large quantities, relatively low cost, and great cycle stability. Petroleum coke, pitch, and coal used to be the most common precursors to commercial activated carbon production, but the decreasing availability of fossil fuels, the growing global demand for energy, and the increased awareness of the environmental impact of fossil fuel combustion led to activated carbon production from sustainable and renewable resources, such as nutshells, wood, starch, sucrose, cellulose, corn grain, banana fiber, coffee grounds, and sugarcane bagasse. The costs of some of the raw materials and the carbon yields of pyrolysis from these carbon sources depended on the availability and the percentage of the carbon source after synthesis (Wei and Yushin 2012).

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Fig. 1.12 Silica nanoparticle obtained from Rice husk doped with CNTs for possible energy applications (Zhang et al. 2016)

1.4

Summary

The advent in science and technology of “nano”-based materials provides a more prosperous shift in the performance of energy-based material development to greater efficiency at low cost. In this chapter, the introduction of energy into developments for acquiring energy from energy conversions are discussed in detail. This is an outline for the subsequent chapters of this book, which give a detailed conceptual review of emerging nanomaterials for energy applications. The wide range of topics for the discussion of energy materials is crystallized into nanomaterials for hydrogen evolution, which could be used for green fuel cells, and emphasized in detail. CO2 conversion into renewable fuel chemicals with nanomaterials is also discussed in depth. This is designed to encourage the need for a shift toward renewable energy resources worldwide. In addition to energy production, the advancement of nanomaterials in energy storage and their application as supercapacitors and batteries constitute another area of interest covered in this book. Acknowledgement One of the authors, D. Durgalakshmi, gratefully acknowledges DST-INSPIRE Faculty Fellowship under the sanction DST/INSPIRE/04/2016/000845 for their funding. R. Saravanan gratefully acknowledges financial support from the SERC (CONICYT/ FONDAP/15110019), FONDECYT, Government of Chile (Project No.: 11170414), and the School of Mechanical Engineering (EUDIM), Universidad de Tarapacá, Arica, Chile.

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Park K-W, Ahn H-J, Sung Y-E (2002) All-solid-state supercapacitor using a Nafion® polymer membrane and its hybridization with a direct methanol fuel cell. J Power Sources 109 (2):500–506 Pergolesi D, Roddatis V, Fabbri E, Schneider CW, Lippert T, Traversa E, Kilner JA (2015) Probing the bulk ionic conductivity by thin film hetero-epitaxial engineering. Sci Technol Adv Mater 16 (1):015001 Qu Y, Zhou W, Jiang L, Fu H (2013) Novel heterogeneous CdS nanoparticles/NiTiO 3 nanorods with enhanced visible-light-driven photocatalytic activity. RSC Adv 3(40):18305–18310 Quaschning V (2016) Understanding renewable energy systems. Routledge, London Saravanan R, Karthikeyan S, Gupta V, Sekaran G, Narayanan V, Stephen A (2013) Enhanced photocatalytic activity of ZnO/CuO nanocomposite for the degradation of textile dye on visible light illumination. Mater Sci Eng C 33(1):91–98 Shaari N, Kamarudin S (2017) Graphene in electrocatalyst and proton conducting membrane in fuel cell applications: an overview. Renew Sust Energ Rev 69:862–870 Sharaf OZ, Orhan MF (2014) An overview of fuel cell technology: fundamentals and applications. Renew Sust Energ Rev 32:810–853 Sharma S, Ghoshal SK (2015) Hydrogen the future transportation fuel: from production to applications. Renew Sust Energ Rev 43:1151–1158 Smil V (2017) Energy: a beginner’s guide. Oneworld Publications, Oxford Stewart L (2009) Tungsten trioxide and titanium dioxide photocatalytic degradations of quinoline. Iowa State University, Ames Thomas JP, Qidwai MA (2005) The design and application of multifunctional structure-battery materials systems. JOM 57(3):18–24 Tiwari JN, Tiwari RN, Kim KS (2012) Zero-dimensional, one-dimensional, two-dimensional and three-dimensional nanostructured materials for advanced electrochemical energy devices. Prog Mater Sci 57(4):724–803 Wachsman ED, Marlowe CA, Lee KT (2012) Role of solid oxide fuel cells in a balanced energy strategy. Energy Environ Sci 5(2):5498–5509. https://doi.org/10.1039/C1EE02445K Wang W, Guo S, Lee I, Ahmed K, Zhong J, Favors Z, Zaera F, Ozkan M, Ozkan CS (2014) Hydrous ruthenium oxide nanoparticles anchored to graphene and carbon nanotube hybrid foam for supercapacitors. Sci Rep 4:4452 Wang Y, Guo J, Wang T, Shao J, Wang D, Yang Y-W (2015) Mesoporous transition metal oxides for supercapacitors. Nano 5(4):1667–1689 Wei L, Yushin G (2012) Nanostructured activated carbons from natural precursors for electrical double layer capacitors. Nano Energy 1(4):552–565. https://doi.org/10.1016/j.nanoen.2012.05. 002 Wei W, Cui X, Chen W, Ivey DG (2011) Manganese oxide-based materials as electrochemical supercapacitor electrodes. Chem Soc Rev 40(3):1697–1721 World Bank Group (2014) World development indicators 2014. World Bank Publications, Washington, DC www.iea.org. https://www.iea.org/weo2017/ www.worldbank.org. https://data.worldbank.org/indicator/EG.USE.COMM.FO.ZS?end¼2015& start¼1960&type¼shaded&view¼chart www.world-nuclear.org. http://www.world-nuclear.org/information-library/facts-and-figures/heatvalues-of-various-fuels.aspx Xu Z-L, Kim J-K, Kang K (2018) Carbon nanomaterials for advanced lithium sulfur batteries. Nano Today 19:84 Yang P, Mai W (2014) Flexible solid-state electrochemical supercapacitors. Nano Energy 8:274–290 Yu Z, Duong B, Abbitt D, Thomas J (2013) Highly ordered MnO2 nanopillars for enhanced supercapacitor performance. Adv Mater 25(24):3302–3306

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Chapter 2

Recent In Situ/Operando Characterization of Lithium-Sulfur Batteries Yan Yang, Yimin Zhu, Kumar Raju, Sheng Dai, and Charl J. Jafta

Contents 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 General Electrochemical Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 In Situ/Operando Characterization Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 X-Ray Diffraction (XRD) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2 Small-Angle X-Ray Scattering (SAXS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3 Transmission Electron Microscope (TEM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4 Transmission X-Ray Microscopy (TXM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.5 X-Ray Fluorescence (XRF) Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.6 Atomic Force Microscope (AFM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Conclusion and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract To address the ever-increasing energy demand due to the increase in the population and at the same time depleting fossil fuel reserves, new energy storage systems, such as batteries, are being developed. Among various battery systems, Li-S batteries have comparatively high theoretical energy density and lower cost, making Li-S batteries very promising for next-generation secondary batteries. However, the current energy density and capacity decay with cycling of Li-S batteries do not fulfill the industrialization needs. To overcome the drawbacks of Li-S batteries and improve its performance, better mechanism understanding is necessary. A great deal of ex situ characterization research has been conducted making a lot of progress in the understanding of the working mechanism. Considering the complexity of the Y. Yang (*) · Y. Zhu Collaborative Innovation Center for Vessel Pollution Monitoring and Control, Dalian Maritime University, Dalian, China e-mail: [email protected] K. Raju Energy Materials, Council for Scientific and Industrial Research, Pretoria, South Africa S. Dai · C. J. Jafta (*) Chemical Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA e-mail: [email protected] © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_2

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intermediate lithium polysulfides and the inherent multi-electron reaction pathways, advanced in situ/operando techniques are highly required. In this chapter, recent progress of a few typical in situ/operando characterization techniques used in Li-S battery studies is reviewed. These techniques provide more in-depth understanding of Li-S batteries in different aspects and could be enlightening for other energy storage systems too.

2.1

Introduction

With the deteriorating environmental problems caused by traditional fossil energy consumption and the increasing demand for mobile devices, the development of new energy storage systems with high energy density is highly required. Among various promising candidates, lithium-sulfur (Li-S) batteries have attracted a great deal of attention due to its high theoretical capacity (1675 mAh g
1) and energy density (2600 Wh kg
1), abundant sulfur reserves, eco-friendliness, and low cost of sulfur (Peled et al. 1989; Manthiram et al. 2014; Peng et al. 2017). However, there are a few problems that need to be tackled before commercialization of Li-S batteries, such as the relatively low cycle life and power density (Bresser et al. 2013). A great deal of work has been done to improve the cell performance from different aspects, i.e., cathode materials, electrolytes, separators, and anodes (Cao et al. 2015; Zhang et al. 2015; Kang et al. 2016; Yao et al. 2011; Fang and Peng 2015; Pope and Aksay 2015). Great progress has been made; however, Li-S batteries still have not reached industrial needs (Urbonaite et al. 2015; Hagen et al. 2015). This is mainly because of the unclear electrochemical mechanism originating from the complex multistep and multi-electron reactions (Xu et al. 2015). Furthermore, polysulfides (Li2Sx, 4  x  8) have a high solubility in electrolytes and will constantly undergo disproportionation reactions in solution and form a variety of transient species during charge and discharge (Xiao et al. 2015; See et al. 2014). Thus, detecting and identifying these temporal intermediate species are very difficult by traditional characterization methods. Ex situ measurements have been performed by many researchers and achieved many enlightening results that helped mechanism understanding of Li-S battery systems (Xu et al. 2015). However, sometimes conflicting results were reported, which may mostly be caused by the change of these transient species during different posttreatment methods or cell contamination during the disassembling process. Thus, in situ/operando characterization methods are highly desired for more accurate and discerning analyses of the fast electrochemical reactions. This will allow for more in-depth mechanism understanding, which will better guide researchers to improve the capacity and power density decay of Li-S batteries. Up to now, many characterization methods have been employed in situ/operando to provide more insights on the complex electrochemical processes of Li-S batteries, for example, X-ray diffraction (XRD) (Lowe et al. 2014; Cañas et al. 2013), X-ray

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absorption spectroscopy (XAS) (Wujcik et al. 2015; Gao et al. 2011; Wujcik et al. 2017; Lowe et al. 2014; Gorlin et al. 2016), small-angle neutron and X-ray scattering (SANS and SAXS) (Jafta et al. 2017), UV-visible spectroscopy (UV-vis) (Gorlin et al. 2015; Patel and Dominko 2014), Raman spectroscopy (Hagen et al. 2013; Hannauer et al. 2015; Wu et al. 2015; Yeon et al. 2012), Fourier transform infrared spectroscopy (FT-IR) (Saqib et al. 2017a, b), nuclear magnetic resonance (NMR) (Patel et al. 2014; See et al. 2014; Wang et al. 2017a; Xiao et al. 2015), electron paramagnetic resonance (EPR) (Wang et al. 2015; Wujcik et al. 2016), transmission electron microscopy (TEM) (Kim et al. 2015; Yang et al. 2016), transmission X-ray microscopy (TXM) (Nelson et al. 2012; Sun et al. 2015; Risse et al. 2016; Lin et al. 2014), X-ray fluorescence (XRF) (Yu et al. 2015), and atomic force microscopy (AFM) (Lang et al. 2017; Lang et al. 2016). These techniques are combined in a single in situ/operando experiment or with ex situ methods to give complementary information on morphology, chemical compositions, and structure changes in both electrodes (cathode and anode), as well as the revolution process of transient polysulfides in the electrolytes. In consideration of the increasing number of in situ/operando studies and the important role they play in revealing the Li-S battery mechanism, in this chapter, we will review the recent progress of a few typical in situ/operando characterization techniques used in Li-S battery studies and mainly focus on visible in situ/operando techniques. This chapter will demonstrate characterization techniques by showing specific studies that were applied to illustrate how these in situ/operando characterization techniques promoted the understanding of the Li-S battery mechanism. Furthermore, these in-depth mechanism studies may guide the future cell design in all aspects, i.e., the construction of cathode materials, the protection of the lithium anode, and the selection of electrolytes, separators, and additive salts, which ultimately will result in a high-performance Li-S cells that have the potential to be industrialized. In order to better understand the complex mechanism revealed by in situ/ operando studies, we will firstly introduce the electrochemical reaction mechanism and the general characterization techniques used in Li-S batteries as a background.

2.2

General Electrochemical Characterization Techniques

A typical lithium-sulfur (Li-S) cell consists of a sulfur-containing cathode, a metal lithium foil anode, and sandwiched between them an organic electrolyte. Different from the lithium ion battery, with its intercalation mechanism, the Li-S battery mechanism is based on redox reactions. These, electroreduction and electrooxidation, reactions have been shown to be very complex (Zhang 2013; Barchasz et al. 2012). The reason being that the transformation between S8 and Li2S with the overall reaction, written as S8+16Li $ 8Li2S, is not a one-step reaction but consists of many intermediate reactions. Many polysulfide species form during S8 reduction due to (i) their Gibbs free energy of formation (ΔGf) that is so close to

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one another and (ii) their equilibrium chemical reactions such as chain growth that will promote the generation of different polysulfides. General electrochemical analyses are widely used to characterize the redox reactions. The most general technique to see that there are different polysulfides present during the reduction of S8 is the galvanostatic charge-discharge technique. With this, a constant current is supplied to the cell where it will either charge or discharge depending on the current direction. By discharging the cell, with a constant current, the S8 will be reduced, and a unique two-plateau discharge curve is observed. These two plateaus at ~2.3 V and ~2.1 V are related to the electrochemical reactions: S8 þ 2Liþ þ e
! Li2 Sn , 4  n  8 ð 2:3 VÞ þ




Li2 Sn þ Li þ e ! Li2 S2 and=or Li2 S ð 2:1 VÞ

ð2:1Þ ð2:2Þ

The reduction products at the end of discharge are Li2S and/or Li2S2 with the existence of Li2S2 still being debated. The dissolved intermediate polysulfide products formed will shuttle between the positive and negative electrodes during charge and discharge which will react with both the sulfur cathode and Li metal. This will cause deposition of reactive species forming passivating layers on the electrodes and ultimately increasing the impedance. This obviously affects the cell negatively. By means of the galvanostatic charge-discharge method, it is possible to investigate the shuttle mechanism. This was first done by Mikhaylik and Akridge (2004), where they have derived a shuttle equation to evaluate the shuttle behavior. The shuttle factor ( fc) is the ratio of charge current (Ic), the specific capacity of the upper plateau (qup), total sulfur concentration ([Stotal]), and the shuttle constant (ks). The specific capacity of qup is fixed at 419 mAh/g which is ¼ of the total theoretical capacity (Mikhaylik and Akridge 2004; Manthiram et al. 2014). fc ¼

k s qup ½Stotal  Ic

ð2:3Þ

Simulated charge profiles, with different fc values, are shown in Fig. 2.1. With fc reaching a value of zero, there is no shuttling of polysulfides (sharp increase in the voltage), which means that the system has an infinitely high current density, an infinitely small ks, or an infinitely low [Stotal]. An increase in fc > 1 results in the charge potentiogram to become horizontal without a sharp increase in the voltage, resulting in overcharge protection. Nevertheless, this would cause severe corrosion of the Li metal anode resulting in a decreased cycle life (Mikhaylik and Akridge 2004; Cheon et al. 2003; Lee et al. 2003). The galvanostatic charge-discharge, of course, allows for determining the overall performance of the cell and therefore also the energy and power density. At the end of charge and discharge, from Eqs. (2.1) and (2.2), it is shown that the end products are S8 for charge and Li2S and/or Li2S2 for discharge (the possibility of Li2S2 is cleared up in Sect. 2.3.1). The S8 have been found to exist in different solid phases, i.e., α-S8, β-S8, and amorphous S8. The Li2S is overwhelmingly regarded as

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0.99 2.5

0.9 1.00

voltage

0.5 0

1.01 2.3

4.0

2.1 0

2000 mAh/g

1000

3000

Fig. 2.1 Simulated potentiograms with different fc values. (Reproduced with permission (Mikhaylik and Akridge 2004). Copyright 2004, The Electrochemical Society)

a

b

3

300 A

G

250

2.6 A

Zlm (Ω)

Potential (V)

2.8

2.4 2.2

F C

B

B

D

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100

2 1.8

200

F

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D

0

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400

800

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Specific Capacity (mAhg–1)

1600

0

40

80

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ZRe (Ω)

Fig. 2.2 (a) The charge and discharge profiles with points where EIS was performed in the charge and discharge process and (b) the corresponding Nyquist plots. (Reproduced with permission (Yan et al. 2016). Copyright 2016, Wiley-VCH)

nanocrystalline, even though there is one study that claimed to have found amorphous Li2S (Waluś et al. 2013; Kulisch et al. 2014; Schneider et al. 2015; Waluś et al. 2015; Cañas et al. 2013; Lowe et al. 2014; Nelson et al. 2012). Electrochemical impedance spectroscopy (EIS) is a useful tool to indirectly detect the precipitation of solid phases that would cause an increased impedance. It is not possible to detect the crystalline phases of the solids with EIS. In a recent study, the dissolution and precipitation of nonconductive and insoluble Li2S were investigated, with discharging and in situ EIS. It is shown, in the Nyquist plots taken at the discharge state in Fig. 2.2, they exhibit two depressed semicircles, one in the high-frequency region and another in the medium-frequency region, followed by an inclined line at the low-frequency region. The high-frequency semicircle is related to the charge

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Fig. 2.3 SEM images of (a) the MWCNTs and (b) the carbon nanofiber paper. (Reproduced with permission (Zu et al. 2013). Copyright 2013, Royal Society of Chemistry)

transfer resistance, the medium-frequency semicircle is related to the formation of solid Li2S, and the inclined line at the low frequency is related to the Warburg diffusion. By fitting the Nyquist plots with suitable electric equivalent circuits, it would be possible to extract parameters that would allow for observing when the solid Li2S would precipitate. Combining this with operando XRD would give additional insights on whether there exist amorphous Li2S during the precipitation process. The precipitation is known to be a major cause of the deterioration of the cathodes. With charging it is known that S8 precipitates on the carbon surface and forms agglomerates, increasing in size with increasing amounts of electrolyte. The precipitated dead Li2S and S8 are shown to be the reason for the capacity fading (Yan et al. 2016). Therefore, it is important to choose a carbon matrix with the necessary voids to allow sulfur to precipitate and still have good electronic conduction by means of the carbon host. It has been shown that carbon nanotubes and fibers are suitable candidates as carbon matrix for lithium-sulfur batteries. This would also suggest that the macrostructure plays an important role in the electrochemical performance of the lithium-sulfur batteries. Several such types of binder-free conductive matrixes were explored in lithium polysulfide batteries as shown in Fig. 2.3 (Fu et al. 2013; Zu et al. 2013; Zu and Manthiram 2014).

2.3 2.3.1

In Situ/Operando Characterization Techniques X-Ray Diffraction (XRD)

Operando XRD is a very useful tool to probe the crystalline solid phases in lithiumsulfur batteries whereby the evolution of these species can be studied during the

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Fig. 2.4 Detection of S8 and Li2S from different in situ XRD studies, from top to bottom (red to gray) for references (Cañas et al. 2013; Waluś et al. 2013; Kulisch et al. 2014; Schneider et al. 2015; Waluś et al. 2015; Lowe et al. 2014), as summarized in (Zhang et al. 2017). (Reproduced with permission (Zhang et al. 2017). Copyright 2016, Wiley-VCH) (Color figure online)

charge and discharge processes. It is known that at the discharge state the sulfur will reduce to Li2S and at the charge state the sulfur will oxidize to S8 (Waluś et al. 2013). When preparing the cathode of the lithium-sulfur battery by impregnating the carbon matrix with sulfur, it is in an orthorhombic α-S8 phase. Reports are in agreement that as the battery is discharged (1st), the α-S8 phase will completely be reduced to polysulfides, during the ~2.3 V discharge plateau, and no sign of S8 is observed by XRD (Waluś et al. 2013; Waluś et al. 2015; Kulisch et al. 2014; Schneider et al. 2015; Cañas et al. 2013; Lowe et al. 2014; Nelson et al. 2012). During the first charge, it was found that the sulfur recrystallized in the allotrope monoclinic β-S8 and orthorhombic α-S8 (Waluś et al. 2013; Waluś et al. 2015; Kulisch et al. 2014; Schneider et al. 2015; Lowe et al. 2014). Different habits of these allotropes have also been identified with radiographic imaging (Risse et al. 2016), as discussed in Sect. 2.3.4. The transition from β-S8 to α-S8 was observed when the cell was rested for several hours (Waluś et al. 2013), while other reports found the transformation from α-S8 to amorphous sulfur (Schneider et al. 2015; Lowe et al. 2014). During discharge the solid phase sulfur will reduce to Li2S; however the onset DoD where Li2S will start to form is still a subject of debate. Not only are the answers different from different groups doing operando XRD but also differ among different techniques (i.e., NMR) (Huff et al. 2015; See et al. 2014). The periods during which crystalline S8 and Li2S were detected are summarized by Zhang et al. (Zhang et al. 2017) as seen in Fig. 2.4. Polysulfides are soluble in the electrolyte, which leads to rapid molecular reorientation, thus making them difficult to observe via XRD. However, Conder et al. (Conder et al. 2017) showed that it is possible to observe the formation and

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evolution of two broad peaks that are related to the polysulfides. This is made possible by using glass fiber to adsorb the polysulfide and therefore making them “visible.” It was experimentally proved, by synthetically making different Li2Sn polysulfides, that α-S8 and Li2S are the only stable crystalline phases (Cuisinier et al. 2013). However, in a recent study, it has been shown that crystalline Li2S2 does exist in a “solvent-in-salt” electrolyte system. This electrolyte which is a highly concentrated 7 M LiTFSI in DME/DOL solution is the reason why the Li2S2 crystalline phase appears. It is reported that this phase forms as a result of the chemical disproportionation reactions directly derived from high-order polysulfides and not from the reduction products of the stepwise electrochemical reduction process. Therefore counterintuitively, this does not form at the end of the discharge but at the end of the charge or beginning of the discharge (Paolella et al. 2016).

2.3.2

Small-Angle X-Ray Scattering (SAXS)

Small-angle X-ray scattering can be used to investigate the structural properties of the carbon/sulfur cathode. This is done, in brief, by illuminating the sample with X-rays, which will interact with electrons of the sample. This will provide information regarding the fluctuation of the electronic densities in the carbon/sulfur sample. Due 

to the carbon and sulfur having similar electron densities (C, 17:00  10
6 A
2 ; S, 

17:90  10
6 A
2 ), the carbon/sulfur system can be treated as a two-phase system. Therefore, the filling of the carbon pores with sulfur can be probed precisely by SAXS. The distribution of sulfur in a carbon matrix was studied by SAXS, showing the sulfur filling process, where the micropores are first filled then the mesopores with an increase in sulfur content (Petzold et al. 2016). The use of SAXS for operando measurements is not fully exploited. Taking advantage of SAXS for in situ/operando measurements can prove valuable for a better understanding of the Li-S battery mechanism. Each of these techniques (XRD and SAXS) has its own pros and cons, for example, XRD is able to detect the onset SoC and DoD precipitation of crystalline sulfur and Li2S but is unable to detect amorphous sulfur. SAXS, on the other hand, have the ability to detect the precipitation of sulfur in carbon pores but cannot distinguish between amorphous and crystalline phased sulfur. Therefore, by probing a sample simultaneously with XRD and SAXS, it would be possible to determine if the onset SoC precipitation includes amorphous sulfur and if this starts to precipitate earlier than the crystalline phased sulfur as previously observed with XRD. Suffice to say that by probing a sample with multiple techniques simultaneously can overcome one technique’s limitation with another’s ability, allowing to probe a broader spectrum of parameters. SAXS has the unique ability to identify, by means of contrast changes, if the solid phases of sulfur at the end of discharge and

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charge precipitate in the pores or on the surface of the carbon. This information can be used to direct the research on different architectures for the host carbon matrix.

2.3.3

Transmission Electron Microscope (TEM)

TEM can provide the visualized morphology and structure variations and give clear and solid proof on changes of the cathode and anode, such as the volumetric changes during lithiation, the formation of S nanoparticles, the striping and plating of lithium anode, and the formation of lithium dendrites. More importantly, TEM can give details in high resolution and on the nanoscale. Together with other techniques, such as selected area electron diffraction (SAED), electron energy loss spectroscopy (EELS), and energy-dispersive X-ray spectroscopy (EDS), TEM can give compositional information about the lithiated and delithiated phases, for example, Li2S and S8 phases, elemental content, and its distribution, respectively. Kim et al. (Kim et al. 2015) confined sulfur within 200 nm cylindrical inner pores of individual carbon nanotubes (CNTs) and monitored the electrochemical reaction through in situ TEM. During lithiation, the Li2S expanded into the empty pores rather than into the CNT walls, as seen in Fig. 2.5. A flat reaction front was found, indicating that the lithiation rate on the surface of the conductive carbon and at the center of the pore is comparable. Combining TEM with in situ electron diffraction, a direct phase transformation of S8 into nanocrystalline Li2S was found without any intermediates, thus suggesting the presence of electron pathways at the insulated Li2S/S8 interfaces. Besides their report, Yang et al. (Yang et al. 2016) also observed direct transformation of S8 into Li2S without detectable intermediates with the help of in situ TEM. It is worth mentioning that, in order to perform in situ TEM, they designed a solid-state Li-S nanobattery and used carbon-coated sulfur as pristine materials to protect S8 from evaporation under high-vacuum conditions and avoid irradiation damage during in situ TEM measurements. These results are different from the liquid Li-S batteries where polysulfide intermediates were involved, which are related to their unique in situ setups. In another in situ TEM experiment performed by Xu et al. (2014), they observed the formation of a solid Li2S crust on the S8 bulk instead of deeper lithiation, due to the preferential surface diffusion of lithium during discharge. The insulating nature of Li2S causes increased resistance, making the diffusion of Li+ ions from the interface into the bulk more difficult. Based on the above results, it seems, compared to electron transfer, the Li+ ion diffusion plays a key role in direct S8 to Li2S transformation; the existence of porous carbon around S8 nanoparticles would benefit the Li+ ion diffusion; and the use of nanosized sulfur particles with a high specific surface area may help improve the sulfur utilization during discharge. Recently, Li et al. (2017) and Wang et al. (2017a, b), using the cryogenic TEM technique, studied Li dendrites and the SEI on Li surfaces. The cryogenic TEM technique can avoid beam damage to metallic Li to some degree and thus can obtain

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Fig. 2.5 Selected images of the sample evolution during a typical lithiation process: (a–c) TEM images captured during lithiation of S nanoconfined in a CNT reaction vessel and (d–f) their corresponding EDP patterns; panels (a) and (d) correspond to the sample before the initiation and panels (c) and (f) after the completion of the electrochemical reaction. The darker area appearing at the bottom left corner of the CNT in panel (c) is a contaminant. (Reproduced with permission (Kim et al. 2015). Copyright 2015, Wiley-VCH)

more informative results regarding Li dendrites and SEI. This technique can be considered as a big improvement for in situ anode studies.

2.3.4

Transmission X-Ray Microscopy (TXM)

TXM uses X-rays as a detective source, compared to the electrons in electron microscopy. X-rays, depending on the energy, could have a larger penetration depth (mm scale) and is nondestructive to the sample. It can be used to provide 2D and 3D morphology in a macroscopic scale. Nelson et al. (2012) reported the use of operando XRD and TXM to investigate the structural and morphological changes during electrochemical reactions. They found that bulk soluble polysulfides remain trapped within the cathode matrix and the loss of polysulfides was not significant during the first discharge plateau. The recrystallization of sulfur is dependent on the preparation technique of the sulfur cathode, and crystalline Li2S does not form at every end of discharge process. Lin et al. (2014) adopted the operando TXM and studied extensive dimensional variations of S8 particles at different DoD and SoC, providing information about the dynamics of polysulfide dissolution and

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redeposition. They concluded that the dissolution rate of polysulfides has complex dependence on the Li content and that the polysulfide redeposition is nucleation limited, leading to a considerable aggregation of the active particles. These results indicate the importance of using a carbon cathode matrix and decreasing the size of sulfur nanoparticles. For lab-assembled Li-S batteries, carbon black and polymeric binder are always needed to better the electric conductivity and fragility of the cathode. However, the difficulty of distinguishing between carbon black and other carbon materials as well as the decomposition products of polymeric binder produced during charge and discharge may complicate the analysis of the cathode materials when studied by TXM. To create a simplified model system that is suitable for multidimensional operando measurements without the disturbance of carbon black and binder, Risse et al. (2016) used a cloth-like (1-mm-thick) commercial Kynol (ACN-157) carbon as cathode material. They developed a setup that can be used to perform multidimensional operando measurements by combining X-ray radiography and impedance spectroscopy while the cell is running. Li2S8 solution (0.1 M) was adopted as sulfur source and electrolyte (catholyte). Macroscopic S dendrites were found at the end of each charge process as seen in Fig. 2.6 (a and b). These appeared at ~2.4 V and are seen growing rapidly afterward. At the end of charge, these dendrites are then observed reaching lengths of up to 4 mm. These are the largest S dendrites reported thus far, which may originate from the use of soft Kynol carbon cloth and catholyte as sulfur source. The soft sponge-like Kynol material and rich amount of unreacted Li2Sx in the electrolyte may kinetically promote the rapid formation and growth of sulfur dendrites. However, these sulfur dendrites were found able to dissolve quickly at the beginning of each discharge process (3.0 V–~2.4 V). According to the characteristic crystal behavior of these sulfur dendrites, they can be assigned to stable α-sulfur (rhombic) in minor amount and metastable β-sulfur (monoclinic) in major amount (see Fig. 2.6a). No macroscopic Li2S crystals were found, which is in full agreement with other studies by operando XRD. In addition, X-ray radiography reveals non-wetted areas on the carbon cathode, even though ~100 μg of Li2S8 solution was added. Furthermore, these regions grow and reduce periodically during each discharge and charge, respectively, indicating they were driven by the changes made by the electrochemical reactions (see Fig. 2.6b). It is worth noting that the non-wetted areas are inversely proportional to the discharge capacity as seen in Fig. 2.6d, further proving the correlation with the variation of polysulfide species. Furthering on this work, Yang et al. (2017) studied the influence of cathode materials. Controlling the porous nanostructure of the cathode carbon matrix is necessary for such a study. This was achieved by synthesizing tunable N-doped reduced graphene oxide (rGO)/carbon monolith with high electrical conductivity, yield, and mechanical strength. The monolith was sliced into thin cylindrical pieces of ~0.8 mm thickness which was used directly as the carbon matrix in the Li-S coin cells. The cell showed high specific capacity (1558 mAh g
1 at 0.1 C), low overpotential (~0.13 V), and good cycling stability (>100 cycles). The in situ/ operando experiment was carried out using the same setup as was used by Risse et al. (2016) shown in Fig. 2.7. Sulfur dendrites in millimeter scale were also found

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Fig. 2.6 (a) X-ray images: Fully charged (I) and discharged (II) state of the Li-S cell of the 2nd cycle and fully charged state of the 1st cycle (III). Both insets (α, β) magnify the obtained macroscopic sulfur structures. Multidimensional operando analysis (c) of the first five cycles at 0.1 C. The vertical dashed and solid lines correspond to the end of charge and discharge, respectively. The radiography images on the left are taken at these lines (b). (d) The image analysis of the non-wetted areas (red) after each discharge step. The solid circles qualitatively follow the discharge capacity, while the open circles give the qualitative progression of the integrated pixel value of the red areas. (Reproduced with permission (Risse et al. 2016). Copyright 2016, Royal Society of Chemistry) (Color figure online)

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Fig. 2.7 Time-dependent evolution of surplus electrolyte ring between the separator and the lithium anode hole during the first charge/discharge cycle at 0.1 C, using a home-designed setup with vertical configuration. The thickness d of the observable ring (see coin cell sketch left upper corner) was measured every 20 s and is plotted in the upper right graph (red line) together with the charge/discharge curve. Representative points (A–H ) at different states of charge or discharge and the corresponding X-ray radiography images were listed and presented in state of charge (%). (Reproduced with permission (Yang et al. 2017). Copyright 2017, Elsevier) (Color figure online)

in this study, which substantiated the previous results. Furthermore, two other macroscopic phenomena were observed. A surplus electrolyte ring was observed between the separator and lithium anode hole, with its thickness shrinking and expanding during serial charge and discharge processes, respectively, which is also independent of different C rates (0.1 C and 0.5 C, four and five cycles, respectively). The driving force is mainly due to the concentration gradient of polysulfides as well as a combination of the other variations during electrochemical reactions, i.e., the volume change of polysulfides, the corresponding viscosity change of electrolyte solution, and the transport properties of polysulfide species. In addition to this, a high X-ray transmittance reaction front was observed as seen in Fig. 2.8, moving rapidly (~0.8 μm/s) from the edge of the lithium anode hole to the center at the end of every discharge step (started at ~2.1 V). The analysis of EIS results suggests that this process is mainly controlled by Li+ ion diffusion. A rapid increase of Li+ ions will lower the resistance and benefit the formation of Li2S (see Fig. 2.8). The rapid movement of Li+ ions mainly originates from the hierarchical

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Fig. 2.8 The left graph summarizes the parameters obtained by the EIS. From top to the bottom voltage-time curve, Warburg coefficient (black), solution resistance (red), and distribution of relaxation times can be seen. The inset letters in the voltage-time curve correspond to the radiography images on the right. A moving reaction front of high transmittance is observed in the images B–F; the corresponding states during discharge process at 0.1 C are shown in left graph, upper row (B–F). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article). (Reproduced with permission (Yang et al. 2017). Copyright 2017, Elsevier) (Color figure online)

nanostructure of the carbon cathodic material, which corresponds well with its high electrochemical performance. The in situ/operando imaging experiments are well suited for studying the evolution of S and Li2Sx (x ¼ 1, 2), due to their solid state. Recently, progress was made on the mechanism understanding of Li-S batteries where the study suggested that the main reason for the poor cycle life is electrolyte depletion along with Li2S layer formation rather than the polysulfide dissolution. To gain a better picture of these conclusions made, in situ/operando techniques were employed to reveal the Li anode changes during cycling. Sun et al. (2018) designed a facile homemade cell which is suitable for X-ray phase contrast tomography. Using this for in situ/ operando experiment, they found that the initial dense Li transformed into porous Li due to the inwardly growing dendrites during continuous charge and discharge processes. The process of Li dendrite growth correlates well with the overall cell performance degradation.

2.3.5

X-Ray Fluorescence (XRF) Microscopy

Besides normal transmission X-ray imaging techniques, XRF microscopy was also adopted. Yu et al. (2015) utilized in situ XRF microscopy to track the morphology and chemical state changes of the sulfur electrode. The view area is variable from micrometer to centimeter, providing both overview information in macroscopic and detail in microscopic. Combined with XAS, the dissolution and redistribution of the

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S8 were observed directly. Polysulfides were found to generate immediately after the discharge is initiated, part of which deposited and/or reacted with lithium metal, leading to a low coulombic efficiency and poor cycle life of the Li-S batteries. Their results suggested that by physical capsulation only, it is quite difficult to prevent the dissolution of polysulfides. They suggest integrated approaches are required.

2.3.6

Atomic Force Microscope (AFM)

AFM can provide high resolution of the surface morphology of samples. Lang et al. used a lithium polysulfide semiliquid battery and adopted in situ AFM to probe the dynamic evolution of Li2S2 and Li2S at the cathode/electrolyte interface (Lang et al. 2016, 2017). They found that Li2S2 nanoparticles appeared at the early stages of the reaction and its oxidization is incomplete, thus increasing upon cycling, which leads to the capacity fading of the Li-S cell.

2.4

Conclusion and Outlook

In summary, we have reviewed recent typical in situ/operando techniques and their advancements used in Li-S battery research, as summarized in Table 2.1. It is worth mentioning that in situ/operando imaging techniques have been developed from TEM to TXM, XRF, and AFM. Each method provides visualized mechanism information from nanoscale to macroscale. These in situ/operando techniques provided large amount of real-time information within running cells, which is irreplaceable and crucial for complex mechanism understanding. Current in situ/operando techniques are mainly based on single characterization method and can normally give information about one phase (solid or liquid) or electrode (anode or cathode). This also usually needs the assistance of ex situ measurement to provide supplementary information. Combining different characterization methods that can operate in situ/operando and provide multidimensional information simultaneously is an attractive and ongoing trend. For example, one could monitor the evolution of S, morphology changes, the variation of polysulfide species, the compositional change within electrolyte, and the changes in the reaction interfaces at the same time. However, up to now related reports are still rare, mainly due to the complexity in the design of in situ/operando setups and the difficulty in combination of different techniques. Furthermore, current in situ/operando setups are mostly homemade and differ from group to group, and some of them suffer from poor cell performance. These uncertain elements in in situ/operando setups will cause unnecessary disturbance and sometimes misleading results and thus erroneous mechanism understanding. Thus, standardized in situ/operando setups like coin cell with windows should be encouraged and promoted.

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Table 2.1 The different in situ/operando techniques discussed and their advantages Technique XRD

SAXS

TEM

TXM

XRF

AFM

In situ/operando technique advantages This technique allows for determining not only the solid phases of S but also the polysulfide phases which are not solid and their evolution during charge and discharge of the cell With this technique it is possible to probe the amorphous structure of the carbon and follow the sulfur filling of the micropores/mesopores or the lack thereof while the cell is operating. This technique is also able to determine the sulfur distribution in the different pore sizes This technique provides high-resolution image information as compared to TXM and XRF of samples during cell operation. Combined with SAED, it is possible to distinguish between different phases and how these change as a function of cell voltage. Combined with EELS and EDS, it is possible to follow the changes in the distribution of elements and the element contents on a specific area of the sample as a function of the cell voltage This technique has a high penetration depth and is nondestructive, allowing to observe changes within the bulk of the sample. It is possible to identify elements and follow the changes in distribution through the sample as a function of the cell voltage. It provides dynamic 2D and 3D images of a sample during operation It is possible to measure micrometer to centimeter areas of interest, detecting elements and their contents during cell operation. The technique is also able to detect trace amounts of elements in the sample The ability to measure surface morphology of the sample during operation of the cell

We believe more effort will be placed on the multidimensional in situ/operando investigation and its setup standardization. Accordingly, more in-depth mechanism understanding will be provided, which will shed light on the Li-S cell design and push forward its industrial application potential. Acknowledgments C. J. Jafta and S. Dai were supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division. Y. Yang and Y. M. Zhu thank the Xinghai Program (205000/02500512) and Collaborative Innovation Center for Vessel Pollution Monitoring and Control Seed Foundation (205000/017180517) of Dalian Maritime University for financial support.

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Xu R, Belharouak I, Zhang XF, Chamoun R, Yu C, Ren Y, Nie AM, Shahbazian-Yassar R, Lu J, Li JCM, Amine K (2014) Insight into sulfur reactions in Li-S batteries. ACS Appl Mater Interfaces 6:21938–21945. https://doi.org/10.1021/am504763p Xu R, Lu J, Amine K (2015) Progress in mechanistic understanding and characterization techniques of Li-S batteries. Adv Energy Mater 5:1500408. https://doi.org/10.1002/Aenm.201500408 Yan J, Liu X, Li B (2016) Capacity fade analysis of sulfur cathodes in lithium-sulfur batteries. Adv Sci 3:1600101. https://doi.org/10.1002/advs.201600101 Yang Z, Zhu Z, Ma J, Xiao D, Kui X, Yao Y, Yu R, Wei X, Gu L, Hu Y-S, Li H, Zhang X (2016) Phase separation of Li2S/S at nanoscale during electrochemical Lithiation of the solid-state lithium-sulfur battery using in situ TEM. Adv Energy Mater 6:1600806. https://doi.org/10. 1002/aenm.201600806 Yang Y, Risse S, Mei S, Jafta CJ, Yan L, Stöcklein C, Kardjilov N, Manke I, Gong J, Kochovski Z (2017) Binder-free carbon monolith cathode material for operando investigation of high performance lithium-sulfur batteries with X-ray radiography. Energy Storage Mater 9:96–104. https://doi.org/10.1016/j.ensm.2017.06.008 Yao ZD, Wei W, Wang JL, Yang J, Nuli YN (2011) Review of sulfur-based cathodes for high performance lithium rechargeable batteries. Acta Phys Chem Sin 27:1005–1016. https://doi.org/ 10.3866/Pku.Whxb20110345 Yeon JT, Jang JY, Han JG, Cho J, Lee KT, Choi NS (2012) Raman spectroscopic and X-ray diffraction studies of sulfur composite electrodes during discharge and charge. J Electrochem Soc 159:A1308–A1314. https://doi.org/10.1149/2.080208jes Yu XQ, Pan HL, Zhou YN, Northrup P, Xiao J, Bak S, Liu MZ, Nam KW, Qu DY, Liu J, Wu TP, Yang XQ (2015) Direct observation of the redistribution of sulfur and Polysufides in Li-S batteries during the first cycle by in situ X-ray fluorescence microscopy. Adv Energy Mater 5:1500072. https://doi.org/10.1002/aenm.201500072 Zhang SS (2013) Liquid electrolyte lithium/sulfur battery: fundamental chemistry, problems, and solutions. J Power Sources 231:153–162. https://doi.org/10.1016/j.jpowsour.2012.12.102 Zhang S, Ueno K, Dokko K, Watanabe M (2015) Recent advances in electrolytes for lithium-sulfur batteries. Adv Energy Mater 5:1500117. https://doi.org/10.1002/aenm.201500117 Zhang G, Zhang Z-W, Peng H-J, Huang J-Q, Zhang Q (2017) A toolbox for lithium-sulfur battery research: methods and protocols. Small Methods 1:1700134. https://doi.org/10.1002/smtd. 201700134 Zu C, Manthiram A (2014) High-performance Li/dissolved polysulfide batteries with an advanced cathode structure and high sulfur content. Adv Energy Mater 4:1400897. https://doi.org/10. 1002/aenm.201400897 Zu C, Fu Y, Manthiram A (2013) Highly reversible Li/dissolved polysulfide batteries with binderfree carbon nanofiber electrodes. J Mater Chem A 1:10362–10367. https://doi.org/10.1039/ c3ta11958k

Chapter 3

Recent Advances in Flexible Supercapacitors Swati Jadhav, Vikash Chaturvedi, and Manjusha V. Shelke

Contents 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Nanostructured Materials for Supercapacitor Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.1 Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.2 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1.3 Graphene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Pseudocapacitive Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2.1 Transition Metal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2.2 Conducting Polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Gel Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Classification of Flexible Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Carbon-Based Current Collectors for Flexible SCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Freestanding Flexible SCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Flexible Substrate-Based SCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Metal-Based Flexible Current Collector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.5 The Wearable Fibrous Flexible Electrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Flexible devices are shock absorbent, unbreakable, light in weight, smaller in dimension, and come at a lower cost. These exceptional properties of the flexible devices are catalyst for the continuous development of stretchable nanostructured materials to address the related challenges such as structure complexity. Similar to flexible electronic devices, the energy storage devices are also an essential part of such system and parallel work going on to develop the electrochemical flexible energy storage system such as flexible batteries and supercapacitors. In the flexible electronic devices such as microsensors, biomedical devices, and S. Jadhav · V. Chaturvedi · M. V. Shelke (*) B101, Polymer and Advanced Materials Laboratory, Physical & Material’s Chemistry Division, CSIR-National Chemical Laboratory, Pune, India e-mail: [email protected] © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_3

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wearable electronic suits, flexible supercapacitors (FSCs) can be easily embedded as an essential part working as a powerhouse. However, the technology is still at the early stage of lab research, and thus exploring novel approaches remains an urgent academic and industrial challenge. This chapter covers all the important parameters related to the energy storage mechanism and constructional details of flexible SC electrodes. The chapter includes a brief review on frequently used nanostructured electrode materials for all kind of SC electrodes based on recently reported literature. A separate section provides mechanistic insight into the role of gel electrolyte for solid-state device fabrication. The further detailed classification of FSCs has been provided on the basis of design schematic and fabrication process by highlighting the importance of different components responsible for mechanical flexibility. Cost-effective materials and construction processes and their electrochemical performances have been covered with technical discussion targeting to present and future design prospect of FSCs. Hence the chapter explores the ideas of paramount importance to meet the stringent requirements for the applications mentioned above, related to design and fabrication of stretchable electrodes to improve the performance of state-of-the-art SCs.

Abbreviations AC BC CD CF CNF CNT CV EDLC FSCs GCF GNF GO IS LED MWCNT NW PDMS PEDOT PErGO PET PMMA PPy PSS

Activated Carbon Bacterial Cellulose Charge-Discharge Carbon Fibers Carbon Nanofiber Carbon Nanotubes Cyclic Voltammetry Electric Double-Layer Capacitor Flexible Supercapacitors Graphene Cellulose Paper Graphene Nanofiber Graphene Oxide Impedance Spectroscopy Light-Emitting Diode Multiwalled Carbon Nanotubes Nanowires Polydimethylsiloxane (PDMS) Poly(3,4-ethylenedioxythiophene) Porous Electrochemically Reduced Graphene Oxide Poly(ethylene terephthalate) Poly(methyl methacrylate) Polypyrrole Polystyrene Sulfonate

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PTFE PVA PVDF rGO SCs SEM SSA SWCNT TEM XPS XRD

3.1

43

Polytetrafluoroethylene (PTFE) Polyvinyl Alcohol Polyvinylidene fluoride Reduced Graphene Oxide Supercapacitors Scanning Electron Microscopy Specific Surface Area Single-Walled Carbon Nanotubes Transmission Electron Microscopy Photoelectron Spectroscopy X-ray Diffraction

Introduction

The $ 6 trillion energy sector powers the rapidly growing global economy. It has been estimated that the world will need to double its energy supply by 2050. The ever-increasing global energy demand, the reduced availability of fossil fuels, and the rising environmental concerns pose severe challenges to human health and energy security, thereby revealing a growing need to develop new types of clean and sustainable energy conversion and storage systems, such as supercapacitors (SCs) and batteries. SCs or ultracapacitors are promising modern energy storage devices capable of delivering energy at a fast rate. Since its discovery in 1954 by General Electric engineers, several attempts have been made to rediscover the technology, but none was successful in market penetration. It was only during the mid-1990s that various technological breakthroughs allowed the rapid improvement in performance of SC (Pushparaj et al. 2007; Leif et al. 2011; Li et al. 2014). Currently the development of supercapacitor has gained high momentum because of their features of fast charge-discharge rate, high power density, and long operation life. Supercapacitors have relatively higher power densities and higher energy densities compared with batteries and conventional capacitors, respectively. Based on their different energy storage mechanisms, supercapacitors are divided into three major categories: electrochemical double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors. In EDLCs, electrical energy is stored by the electrostatic accumulation of charges due to adsorption over the porous electrode material. Electrochemical performances of EDLC electrodes are dependent on their electrical conductivity, pore structures, and specific surface area. Meanwhile, for pseudocapacitors, the energy storage is achieved through reversible and fast redox reactions. Hybrid capacitors are combinations of an EDLC or pseudocapacitor electrode (Gartia et al. 2012). Supercapacitors are becoming necessary for different electronic systems due to their extensive scope of application and ease in manufacturing. Various supercapacitor electrode materials are engineered in different ways to

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accommodate the application and required load performance. In recent years, with the introduction of multifunctional and portable electronic appliances such as wearable electronics suits, smart sensors, actuators, miniature biomedical device, smart garments, flexible touch screens (Meng et al. 2017; Zheng et al. 2016; Jost et al. 2014; Park et al. 2017; Gartia et al. 2012), and electronic newspapers, the fabrication of flexible SCs has become an emerging field. There is thus increasingly rising need for developing lightweight, flexible, and highly integrated SCs that can be folded, rolled up, or even remolded into other structures while retaining their electrochemical functions. Hence intensive research work is going on to improve the flexibility-related scientific understanding of the materials which are used as an electrode in electrochemical energy storage devices. Material scientists are increasingly enthusiastic about flexible energy storage devices and testing their potential applications in a variety of fields (Pushparaj et al. 2007; Leif et al. 2011; Li et al. 2014). The current chapter focuses on the recent and advanced trends in development of FSCs and relevant technological aspects. For the better understanding of the contexts, the basic knowledge of electrochemistry, electrode processes, and electrochemical energy storage is essential. The chapter will provide a categorical overview of the different construction techniques which were adapted to introduce mechanical flexibility in SCs during electrode preparation and also possess commercial application potential.

3.2

Nanostructured Materials for Supercapacitor Electrode

SCs store the energy in two ways, either by the formation of the electrical double layer also known as non-faradic process or by fast redox reaction on the electrode surface known as a faradic process. The SCs can be used as replacements as well as complements with batteries for electrochemical energy storage. The better understanding of mechanism and control over design had enabled adjustments to critical parameters such as appropriate pore distribution, surface redox reaction, kinetics of electrolytic ions, and ion diffusion in electrode surfaces. Nanostructured carbon is most widely used as EDLC electrode material, while transition metal oxides and conducting polymers are used as pseudocapacitive materials for SC electrode.

3.2.1

Carbon Materials

Porous carbon materials are well-known prospective electrode material for supercapacitors not only due to its easy availability and nontoxic nature but also due to parameters such as high specific surface area, excellent electronic conductivity, high chemical stability, and wide operating temperature range. Various carbon

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materials like activated carbon, graphene, porous carbon, and carbon nanotubes (CNTs) have been widely reported as electrode materials for EDLCs over the past years (Wang et al. 2016). Herein, we summarize the latest progress in the development of carbon materials for flexible supercapacitor.

3.2.1.1

Activated Carbon

Activated carbon (AC) can be produced by chemical, physical, or combination of physical and chemical activation processes from various types of carbonaceous materials like wood, coal, leaf, nutshell, etc. The physical activation of carbonaceous materials involves high-temperature treatment over the range of 700–1200
C in the oxidizing gas environment like steam, CO2, and air. The chemical activation is carried out at a lower temperature over the range of 400–700
C using activating agents such as sodium hydroxide, potassium hydroxide, zinc chloride, and phosphoric acid. Depending on the activation methods and the carbon precursors used, AC possesses numerous physiochemical properties with well-developed surface areas of up to 3000 m2/g. The porous structure of AC obtained using activation processes had a broad pore size distribution that consists of micropores (< 2 nm), mesopores (2–50 nm), and macropores (>50 nm) (Salele et al. 2016). Recently, the research work devoted to the use and improvement of the performance of AC in EDLCs is extensive (Sevilla and Mokaya 2014). Biomass-derived activated carbons have continued to attract attention due to the activation process that turns low-value waste into a useful activated carbon product. For example, Zequine et al. developed an efficient, flexible supercapacitor by carbonizing abundantly available and recyclable jute. The active material is synthesized from jute by a facile hydrothermal method, and chemical activation further enhances its electrochemical performance. It shows specific capacitance of 408 F/g at 1 mV/s using CV and 185 F/g at 500 mA/ g using charge-discharge measurements with excellent flexibility. The cyclic stability test confirmed no loss in the charge storage capacity of the electrode even after 5000 charge-discharge measurements. The fabricated device using carbonized jute shows promising specific capacitance of about 51 F/g and improved charge storage capacity over 60% on increasing temperature from 5 to 75
C (Zequine et al. 2017). In another study, Karnan et al. fabricated a device using the activated carbon derived from aloe vera by chemical activation. The fabricated device shows a high specific capacitance of 244 F/g with an energy density of 8.6 Wh/kg, and 126 F/g in ionic liquid results in a high energy density of 40 Wh/kg. The symmetric supercapacitor device fabricated using this activated carbon electrodes in ionic liquid could power a red LED for more than 20 min upon charging for less than 20 s (Karnan et al. 2016). Zhi et al. (2014) present an effective synthetic method that utilizes waste tires as the precursor to prepare the activated carbon electrodes by the pyrolysis and chemical activation processes, and the device exhibits a specific capacitance of 106 F/g (Zhi et al. 2014).

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Carbon Nanotubes

The discovery of CNTs plays a very crucial role in the field of science and engineering of carbon materials. A great deal of attention has been given to CNTs as supercapacitor electrode material due to its unique pore structure, excellent mechanical and thermal stability, and superior electrical properties. With catalytic decomposition of some hydrocarbons, and careful manipulation of various parameters, it becomes possible to control the crystalline structure and generate nanostructures in different conformations. CNTs can be categorized as single-walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs); both are used as electrode materials for supercapacitor. Hata et al. fabricated SWCNTs for supercapacitor, and the specific capacitance of the solid EDLC was estimated at 20 F/g from the discharge curves of cells charged at 2.5 V for a two-electrode system and corresponds to 80 F/g for a three-electrode system (Futaba et al. 2006).

3.2.1.3

Graphene

Graphene, known for its high surface area and pore size distribution as well as good surface exposure to electrolytes, has been recognized as an excellent electrode material for SCs (Purkait et al. 2018). Graphene-based supercapacitors have shown high power densities in the range of 90–160 Wh/kg. The specific surface area (SSA) of graphene is limited by the theoretical limit of 2630 m2/g, and most of the reported values for SSA are around 1500 m2/g. Meanwhile, AC materials have been reported with very high SSA of 3000 m2/g, and they are cheaper as compared to graphene. Instead of replacing graphene as primary electrode materials, the recent research on supercapacitors trend is taking a hybrid approach. As graphene is less likely able to improve the specific capacitance on its own, it is being composited with other nanomaterials such as CNTs (http://www.thegraphenecouncil.org/? page¼Supercapcitors). Li et al. (2014) combined CNTs and graphene to create high-performance and a low-cost SC. The hybrid device has advantages of both carbon nanomaterials such as high conductivity and SSA from graphene and stable integrated network by CNTs. The device shows the specific capacitance of 100 F/g, three times higher than the specific capacitance of a supercapacitor made by CNTs alone (Li et al. 2014). Yang et al. (2017) synthesized the nanoporous graphene by an annealing process in hydrogen. It demonstrates the high specific capacitance of 306.03 F/g and high power density of 41 kW/kg. The devices can retain almost 100% capacitance after 7000 charge-discharge at a current density of 8 A/g (Yang et al. 2017). Panmand et al. (2017) demonstrated a green approach for the synthesis of the high surface area (850 m2/g) and mesoporous perforated graphene from Bougainvillea flower. The synthesized perforated graphene sheets conferred a very high specific capacitance of 458 F/g and a high energy density of 63.7 Wh/kg at the power density of around 273.2 Wh/kg (Panmand et al. 2017).

3 Recent Advances in Flexible Supercapacitors

3.2.2

Pseudocapacitive Materials

3.2.2.1

Transition Metal Oxides

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As compared to the conventional carbon and polymer material, the metal oxides (MOs) provide higher energy density and better electrochemical stability, respectively, for the development of SCs. The MOs not only store energy like electrostatic carbon materials but also exhibit electrochemical faradic reactions between electrode materials and ions within appropriate potential windows. Nevertheless, the lower cost of production and use of a milder electrolyte also make MOs a feasible alternative as an electrode material for SC. The commonly used MOs are ruthenium dioxide (RuO2) (Arunachalam et al. 2018), nickel oxide (NiO) (Xiao et al. 2016), and manganese oxide (MnO2) (Huang et al. 2015). RuOx is the most extensively studied transition MO for the candidature of a SC electrode. RuOx possesses the properties such as high proton conductivity, excellent thermal stability, durable cyclic stability, high conductivity, and high rate capacity which makes it unique in the category (Wang et al. 2012). It contains three distinct oxidation states (within 1.2 V window), highly reversible redox reactions with a wide potential window. RuO2 is electrochemically produced using electrodeposition method for SC application. The electrodes were stable for a large number of cycles demonstrating a high specific capacitance of 498 F/g at the scan rate of 5 mV/s (Gujar et al. 2007). Kim et al. reported hydrous RuO2 prepared by electrostatic spray deposition for an electrochemical capacitor application. The specific capacitance of the sample is 510 F/g, which increased to a maximum value of 650 F/g and then decreased rapidly to 25 F/g due to decrease in structural water content by annealing (Kim and Kim 2006). Although amorphous hydrous RuO2 can provide an extremely high specific capacitance, the use of it for commercialization is limited due to high cost and environmental harmfulness. As an alternative approach, researchers have put significant effort into finding cheaper and environmentally friendly materials that exhibit electrochemical behavior similar to that of RuO2. These alternative materials include NiO, MnO2, and Fe3O4. In general, NiO is considered as an alternative electrode material for ES in alkaline electrolytes due to its environmental friendliness, facile synthesis, relatively high specific capacitance, and low cost. NiO nanomaterial is synthesized at different calcination temperatures using cetyltrimethylammonium bromide as surfactant via microwave method which shows nanoflake-like morphology with the maximum specific capacitance of 401 F/g at a current density of 0.5 mA/cm2. NiO nanoflakes retain 92% of the initial capacitance after 500 cycles (Vijayakumar et al. 2013). Xinhong et al. (2016) reported multishelled NiO hollow microspheres for high-performance supercapacitors. Among the multishelled hollow microspheres, triple-shelled NiO with outer single-shelled microspheres shows a remarkable capacity of 1280 F/g at 1A/g and still keep a high value of 704 F/g even at 20 A/g. The outstanding performances are attributed to its fast ion/electron transfer, high specific surface area, and large shell space. The specific capacitance gradually increases to 108% of its initial value after 2500 cycles, demonstrating its high stability (Xinhong et al. 2016).

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MnOx also has attracted significant attention and is considered a promising alternative material for supercapacitor applications. A particular interest is given to MnO2 as an electrode material for supercapacitors due to its low cost and excellent capacitive performance in aqueous electrolytes (Huang et al. 2015).

3.2.2.2

Conducting Polymer

The development of conducting polymers began in the early 1960s, and since then they are considered as a potential candidate electrode material for pseudocapacitors (Santino et al. 2017). These polymers are highly conducting and can store and distribute energy through electrochemical doping reactions, leading to high energy and power density. The conductivity of conducting polymers mainly arise through large conjugated bond system along the polymer backbone. They possess capacitive behavior through redox reactions that occur on the surface as well as throughout the entire bulk system. The redox processes are highly reversible as no phase transformations happen during the redox reactions. Different conducting polymers, including polyaniline (PANI), polypyrrole (PPy), poly(3,4-ethylenedioxythiophene) (PEDOT), and polyacetylene (PA), have been widely researched as supercapacitor electrode material due to easy production and low cost (Wang et al. 2016). Shi et al. reported graphene/PANI nanofiber composite for supercapacitor application. The devices based on this composite show capacitance of 210 F/g at a discharge rate of 0.3 A/g. Polypyrrole nanowires are prepared under mild condition with FeCl3 as an oxidant. PPy nanowire-based electrode shows a high specific capacitance of 420 F/g at 1.5 A/g. The as-prepared electrode can work well even after 8000 cycles at 1.5 A/g (Zhao et al. 2016).

3.2.3

Composite Materials

The composite materials have a significant reputation as SC electrodes because the individual material in the composites can have a synergistic effect. The performance of the material can be enhanced through minimizing particle size, enhancing the specific surface area, inducing porosity, preventing particles from agglomerating, facilitating electron and proton conduction, expanding active sites, extending the potential window, protecting active materials from mechanical degradation, improving cycling stability, and providing extra pseudocapacitance. Composite electrodes are integrated on carbon-based material with either metal oxides or conducting polymers, which in turn offers both physical and chemical charge storage mechanism together with a single electrode. As a result, the obtained composites can overcome the drawbacks of the individual materials and represent the advantages of all constituents. It is worth to point out that the reverse effects may also take place in the process of making composites. Consequently, there should be a compromise among the composition of individual materials and an optimized molar ratio of

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constituents for every composite material (Sevilla and Mokaya 2014; Xiao et al. 2016). Li et al. (2017a, b) reported nitrogen-doped activated carbon/reduced graphene oxide (rGO) composite prepared by pre-carbonization of the mixture of graphene oxide (GO) and nitrogen-doped AC and KOH activation of the pyrolysis products. It showed highest specific capacitance of 265 F/g and increased 17.8% compared with the AC (225 F/g) under a current density of 50 mA/g (Li et al. 2017a, b). The MOs are the primary sources that store the charge and the energy. The electro-activities of MOs contribute to high specific capacitance and high energy density of the carbon nanostructure-MO composite electrodes (Zhi et al. 2013). A ternary nanocomposite that consists of MnO2 nanorods and rGO sheets supported on PEDOT:PSS polymer is developed for supercapacitor applications, and it shows an enhanced specific capacitance of 633 F/g at a current density of 0.5 A/g and 100% stability up to 5000 charging-discharging cycles at 1 A/g (Hareesh et al. 2017).

3.3

Gel Electrolytes

Electrolytes are mainly responsible for the transportation of ions during chargedischarge in electrochemical energy storage devices. Liquid electrolytes (acidic, basic, and neutral medium) are most widely used in SCs due to fast ion diffusion and high mobility of ions. However, for commercial purpose organic electrolytes are more economic due to their higher voltage window and thermal stability. However, a number of challenges associated with the use of liquid or organic electrolyte, such as leakage, prevent its use for solid-state device fabrication. To counter such problems, solid electrolytes or quasi electrolytes were introduced. Later on quasi solid-state or gel electrolytes have become the central components of FSCs which affect the electrochemical properties, such as energy density, rate capability, thermal and cyclic stability. The advantages of SCs based on solid-state electrolytes over those based on conventional liquid electrolytes include easy and inexpensive packaging, simple fabrication steps, and no leakage of toxic electrolytes. Apart from this, solidstate electrolytes provide excellent mechanical stability, which allows successful assembly of different flexible and bendable SCs in various applications. In FSCs, solid-state electrolyte acts as a dual agent for the ionic conducting media and electrode separators. Three main types of solid-state electrolytes are used in FSCs, namely, ceramic electrolytes, gel polymer electrolytes, and polyelectrolytes. Among these, gel polymer electrolytes are extensively employed in SCs due to their relatively high ionic conductivity (104–103 S/cm under ambient conditions) (Dubal et al. 2018; Gao and Lian 2014). Gel polymer electrolytes are composed of a polymeric host (most commonly PVA), a solvent as the plasticizer and a conducting electrolytic salt (such as H3PO4, H2SO4, KOH, NaOH, KCl, NaCl); the polymer serves as a medium that swells in the solvent, and the ions travel through the solvent. Figure 3.1 depicts the diffusion of various electrolytes based on their ionic radius in graphene-based solid-state SC

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Fig. 3.1 Schematic illustration of ion diffusion in different electrolytes (https://doi.org/10.1039/ c4ra05553e)

electrodes (Chen et al. 2014). The study concludes that smaller H+ ion of H2SO4 electrolyte can easily be adsorbed by small pores in graphite resulting in high capacity. Polymer matrices are also commonly used to prepare gel polymer electrolytes for FSCs. Various polymers have been reported in literature to prepare gel electrolytes including PVA (Yu et al. 2012), poly(polyacrylate) (PAA), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly(ethylene glycol) blending poly(acrylonitrile) (PAN-b-PEG-b-PAN), polyacrylonitrile (PAN), poly (vinylidene fluoride) (PVDF), and poly(vinylidene fluoride-cohexafluoropropylene) (PVDF-co-HFP).

3.4

Classification of Flexible Supercapacitor

The conventional supercapacitor on the basis of different parameters such as charge storage mechanism electrode symmetry and electrolyte state can be classified into different categories: By charge storage mechanism: EDLC, pseudocapacitive SCs, hybrid SCs, composite hybrid SCs, and battery-type hybrid SCs By electrode symmetry: symmetric or asymmetric By electrolyte state: aqueous electrolyte-based SCs, gel electrolyte-based SCs, and ionic liquid electrolyte-based SCs

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In the conventional SCs, geometrical designs of the SCs were relatively easy and simple. With the increasing demand of portable and wearable electronics, the construction of flexible energy storage device has become a complex necessity. Electrode materials with nanostructures and micro-features have been reported as various components (electrode, current collector, packing shell, etc.) in numerous researches. The key of success behind the fabrication of FSC electrodes can be considered as construction of flexible electrodes. On the basis of physical forms and functional features, there are some excellent reviews that have been articulated to classify the flexible electrodes, but most of these articles emphasize on a particular prospect related to FSCs. Due to diverse parameters and complex geometrical consideration, it is always difficult to put forward a detailed classification of FSCs. It is also apparent that the actual development of FSCs is difficult to summarize in simple ways. Therefore, method to effectively categorize the advance FSCs has become necessary to hold systematic views and address important issues. In this subchapter, we have classified the FSCs by the component primarily responsible for the introduction of the mechanical flexibility in the SC device, and hence it can be categorized as follows, and the details are tabulated in Table 3.1. 1. 2. 3. 4. 5.

Carbon-based current collectors for flexible SCs Freestanding flexible SCs Flexible substrate-based SCs Metallic current collector-based flexible SCs Wearable fibrous flexible SCs

3.4.1

Carbon-Based Current Collectors for Flexible SCs

Carbon materials have excellent electrical conductivity, high specific surface area, excellent corrosion resistance, and low density, which have played an irreplaceable role in the development of supercapacitors. Carbon nanomaterials such as 1D carbon fiber and carbon nanotubes, 2D graphene, rGO, nanoribbon, carbon cloths and 3D hierarchical porous carbons, ACs, etc. are the promising electrode materials for SC electrodes. Most of these carbon materials possess high electronic conductivity as well as high mechanical strength due to their strong C-C bond networks. Hence much effort has been devoted to develop ultrathin planner sheets of different carbon materials which act as current conductor in stretchable and wearable flexible SCs. This subsection provides an overview of recent progress toward the development of advance flexible capacitor in which carbon-based materials have been used as flexible substrate for the electrochemical energy storage. Wu et al. reported CNT integrated carbon fibers (CF) as the flexible substrate for the growth of NiCo2O4 nanoneedle arrays by an in situ hydrothermal approach to obtain a NiCo2O4/CC flexible electrode for supercapacitor application. The NiCo2O4 nanoneedles with diameters of 40–50 nm formed on the hydroxylfunctionalized CC. The fabrication process of the NiCo2O4 nanoneedles on CC is shown in Fig. 3.2A.

569 mF/cm2 at 1 mA/cm2 6.3 F/cm3 at 10 mA/cm3

Na2SO4 PVA, H3PO4 gel electrolyte

Cotton fabric

KOHH2SO4

Graphene

CNT-RGO and polypyrrole on cotton fabric V2O5/SWCNT fiber For asym RGO/SWCNT

45.40 F/g 23.89 F/g

PVA-KOH

Fiber based

Fiber-based all solid Co3O4@MnO2 core-shell nanoarray Carbon fiber/graphene Electrochemically exfoliated 3D graphene

13.9 mF/cm2 at 0.1 mA/ cm2

KOH

PAN carbon fiber

1757 f/g at 2 mA/cm2 (sym) 134 F/g at 1 A/g (asym) Relative capacity enhancement by increasing voltage window 327 F/g at 1 A/g

PAN carbon fiber

PVAH3PO4

KOH

Carbon fibers

310 F/g at 0.8 A/g

Metal wire

KOH

90% at 10 mV/s after 3000 cycles

99% after 500 cycles without bending, 96.3% after bending 91% after 1000 cycles

93% after 10,000 cycles at 20 A/g 82% after 1000 cycles at 0.6 mA/cm2

93% after 20,000 cycles

138% after 5000 cycles at 7 mA/cm2 (sym)

84% after 3000 cycles at 2 A/g 93.6% at 8A/g after 20,000 cycles

91% after 2000 cycles

23 mF/cm2 402 F/g at 1 A/g

Stability 98% after 7000 cycles

Capacity 236 F/g at 5 mA/cm2

Polypyrrole and carbon

Pyridinic-nitrogen-doped nanotubular carbon arrays on carbon cloths NiCo2O4 decorated on PAN/ lignin-based carbon fiber

PVAH3PO4 KOH

Electrolyte PVA-KOH

Carbon cloth

Plastic, cloth, glass, metal, plank Co3O4 on graphene

CNT

Co3O4 on graphene

Current collector Steel mesh

Electrode material NiFe2O4

Table 3.1 Examples of materials and their electrochemical properties for FSCs reported in the literatures

Symmetric and asymmetric

Asymmetric

Symmetric

Asymmetric

Symmetric

Symmetric and asymmetric Symmetric

Symmetric

Symmetric

Symmetric

System Symmetric

Liang et al. (2017) Li et al. (2017)

Sari et al. (2017)

Tan et al. (2017) Niu et al. (2017)

Chang et al. (2017)

Lei et al. (2017)

References Bandgar et al. (2018) Cao et al. (2018) Xiong et al. (2018) Li et al. (2018a, b)

52 S. Jadhav et al.

Polyvinyl alcohol/ H3PO4 PVA/ H3PO4 gel electrolyte LiCl/PVA gel electrolyte Na2SO4

Carbon fabric

Gold

Carbon cloth

Graphene/CNFs/MnO2

Carbon nanoparticles/MnO2 nonfoods

MnO2/graphene nanosheets

Hydrogenated ZnO coreshell nanocables

Graphene/CNFs/MnO2

920 F/g at 3 A/g

95% after 2000 cycles

87.5% after 10,000 cycles

92% after 7000 cycles

267 F/g at 0.2 A/g

1260.9 F/g

97.3% after 10,000 at 1 A/g

6.1 mF/cm2

130 F/g

94.5% after 5000 cycles at 5 A/g 83% after 5000 cycles

81 F/g

Cu wire

Electrochemically reduced graphene oxide Graphene/MnO2 composite

412 F/g at 1 A/g

H 2SO4 -PVA gel electrolyte PVA/ H3PO4 Na2SO4

Gold-coated poly-imide substrates

97.3% after 10,000 cycles at 1 mA/ cm2 86% after 10,000 cycles

10.5 mF/cm2 at 0.1 mA/ cm2

PVAH3PO4

Carbon fabric

128 mF/cm2 at 1.8 mA/ cm2

91% after 5000 cycles, 90% after 800 bending cycles 80% after 5000 cycles

1015 F/g at 1 A/g

PVA-LiCl

PVAH2SO4

Carbon cloth

91.5% after 2000 cycles

121.5 mF/cm2 at 5 mV/s

Aluminum foil

H3PO4PVA

CNT film

Functionalized graphene hydrogels

Core-shell tubular graphene nanoflake-coated polypyrrole GNFs/PNTs Carbon nanoparticles/MnO2 nanorod hybrid

Adenine-based metalorganic framework-derived carbon Polyaniline MOF composite

Asymmetric

Symmetric

Symmetric

Symmetric

Symmetric

Symmetric

Symmetric

Symmetric

Symmetric

Symmetric

Symmetric

(continued)

He et al. (2013)

Yang et al. (2013)

Peng et al. (2013)

Purkait et al. (2018) He et al. (2013) Yuan et al. (2012)

Xu et al. (2013)

Yuan et al. (2012)

Qi et al. (2018)

Shao et al. (2018)

Li et al. (2018a,b)

3 Recent Advances in Flexible Supercapacitors 53

Reduced graphene oxide/ polypyrrole/cellulose hybrid papers Manganese dioxide@polyaniline

Reduced graphene oxide/polypyrrole/cellulose hybrid papers Ni foam

Carbon cloth

PVA/KOH gel electrolyte

PVA/ H3PO4 Cellulose fibers

Gold

NiO/MnO2 core-shell nanoflakes rGO-PEDOT/PSS

Carbon fabric

WO3-x@Au@MnO2 coreshell nanowires H-TiO2@MnO2//H-TiO2@C core-shell nanowires

Carbon cloth

Electrolyte KOH/PVA gel electrolyte PVAH3PO4 LiCl/PVA gel electrolyte. Na2SO4

Current collector Carbon cloth

Electrode material Ni-Co-Mn multicomponent metal oxides

Table 3.1 (continued)

129.2 F/g at 0.5 A/g

89% after 5000 cycles

95% after 10,000 cycles at 1 A/g 89.5% after 5000 at 2 mA/cm

448 mF/m2 at 10 mV/s 1.20 F/cm2 at 2 mA/cm2

89% after 2200 cycles

91.2% after 5000 cycles

110% after 5000 cycles

Stability 94% after 3000 cycles

204.3 F/g at 50 mV/s

139.6 F/g at 1.1 A/g

1195 F /g1 at 0.75 A/g

Capacity 1434.2 F/g at 2 mA/cm2

Symmetric and asymmetric

Symmetric

Symmetric

Symmetric

Asymmetric

Symmetric

System Asymmetric

Ghosh et al. (2017)

Xi et al. (2017) Liu et al. (2015) Wan et al. (2017)

Yuan et al. (2012) Xihong et al. (2013)

References He et al. (2017)

54 S. Jadhav et al.

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Fig. 3.2 Schematic representation of (A) formation processes of the NiCo2O4 nanoneedles on CC (https://doi.org/10.1007/s1085), (B) synthesis of NiCo2O4@CNF composites (https://doi.org/10. 1016/j.polymer.2017.10.051), (C) (a) synthesis process and formation mechanism of NTC supported on the carbon cloth, (b) an SEM image of ZnO NW templates grown on the carbon cloth. SEM and TEM images are showing the morphology evolution of NTC at different temperatures of 260
C (c), 500
C (d ), and 650
C (e) as the temperature increased (https://doi.org/10. 1039/C7NR07414J)

This hybrid electrode NiCo2O4/CC not only exhibits a high specific capacitance of 249.69 F/g but also exhibits favorable cycling stability of 63.3% retention after 1000 cycles at high mass loading (Wu et al. 2017). CF is also potential materials for flexible electrodes. However, the development of CFs is still restricted mainly due to the cost, which is attributed to the manufacturing process expense as well as the costly precursor, polyacrylonitrile (PAN), mainly derived from petroleum. With the purpose of cutting down the cost of CFs, some alternatives based on biomass-derived inexpensive precursors are drawing the attention of researchers. CFs derived from lignin (biomass) precursor could effectively decrease the cost of electrodes and maintain the flexibility of fibers. Lei et al. (2017) fabricated flexible electrodes composed of binary NiCo2O4 metal oxide nanostructures grown on the surface of PAN/lignin CNFs by a facile hydrothermal method without any toxic reagents presented in the scheme (Fig. 3.2B). It exhibited a specific capacitance of 134.3 F/g at current densities of 1A/g and possessed high energy density of 47.75 Wh/kg with a power density of 799.53 W/kg (Lei et al. 2017). Yang et al. reported pyridinic-nitrogen-doped nanotubular carbon (NTC) arrays grown on a carbon cloth for FCs. Pyridinic-nitrogendoped NTC arrays with multimodal pores in the wall are synthesized via a one-step

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template strategy using 1,3,5-triamino-2,4,6-trinitrobenzene as both carbon and nitrogen precursors, and ZnO nanowire arrays were grown on carbon cloths (Fig. 3.2C) as templates. The N-doped NTC yields a high specific capacitance of 310.7 F/g (0.8 A/g), a cycling retention ratio of 105.1% after 20,000 charge-discharge, respectively (Li et al. 2018a, b). Liang et al. fabricated high-performance FCs using cotton fabric which is tuned to have an optimal pore structure. The CNT/rGO hybrid is deposited on the tuned porous cotton fabric using a facile dip-coating method, and then PPy has grown on the fabric via chemical polymerization. Figure 3.3a illustrates the preparation scheme of the fabric electrodes and the asymmetric supercapacitor. The resulting asymmetric supercapacitor with a voltage

Fig. 3.3 (a) Preparation scheme of the fabric electrodes and asymmetric supercapacitor (https://doi. org/10.1039/C7RA08703A), schematic illustration of (b) interaction and carrier conduction between PANI and UiO-66 (https://doi.org/10.1016/j.jpowsour.2018.01.028), (c) synthesis procedure of HZM core-shell nanostructure on carbon cloth and the corresponding sample photos (https://doi.org/10.1021/nn306044d)

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window of 1.8 V exhibits a superhigh areal energy density of 0.26 mWh/cm2. Moreover, it possesses excellent stability under charge-discharge for 1000 cycles and under bending 100 times with an angle of 180
(Liang et al. 2017). Shao et al. reported flexible solid-state supercapacitor based on polyaniline and metal-organic framework (UiO-66) composites in which carbon cloth (0.3 mm thick) is used as a current collector. Figure 3.3b is the schematic illustration of interaction and carrier conduction between PANI and UiO-66. The resultant symmetric supercapacitor shows a favorable specific capacitance of 647 F/g at 1A/g and high cycling stability (91%) with a capacity retention after 5000 cycles. The bending test indicates that the obtained supercapacitor is flexible, and its performance is only decreased 10% after 800 bending cycles with a bending angle of 180
(Shao et al. 2018). Yang et al. designed and fabricated hydrogenated single-crystal ZnO@amorphous ZnO-doped MnO2 core-shell nanocables (HZM) on conductive carbon cloth presented in Fig. 3.3c. HZM serves as a lightweight and flexible current collector as SC electrodes, showing excellent performance such as areal capacitance of 138.7 mF/cm2 and specific capacitance of 1260.9 F/g. The working devices achieved very high total areal capacitance of 26 mF/cm2 and retained 87.5% of the first capacitance even after 10,000 charge-discharge (Yang et al. 2013). Zhang and Fu’s group have synthesized a series of NPC from the metal-organic framework of UiO-66 with different ratios of adenine coated on carbon nanotube film. The carbon nanotube film is used as a current collector and is conductive on one side, and the other side is non-conductive with insulating tape. The assembled optimum flexible SSC shows superior areal capacitance of 43.2 mF/cm2 at the scan rate of 5 mV/s. The device shows excellent foldability and cycling stability, which can retain 91.5% of the initial capacitance after 2000 cycles (Li et al. 2018a, b).

3.4.2

Freestanding Flexible SCs

Freestanding electrodes are generally binder-free and possess high mechanical strength. High conductivity and large capacitance are desirable for the preparation of the freestanding electrode materials. From these perspectives, nanostructured carbon nanomaterials have proven promising candidates owing to their excellent mechanical and electrochemical properties. High capacitance values have been achieved by making freestanding composites of carbon materials (fibers, nanotubes, and graphene) with transition metal oxides or conducting polymers. These composite materials have been composed in the form of fibers, film papers, or even in cloth with excellent tensile strength and mechanical flexibility. The essential factor for fabricating flexible SCs is the development of flexible electrodes with high capacitance and high electrical conductivity to ensure fast charge-discharge. For example, carbon nanotubes, graphene oxide, etc. have been deposited as inks on cellulose papers (Zhe et al. 2011), porous cotton (Fan et al. 2015; Hu et al. 2010), and synthetic polymer sponges. Conducting polymers like polypyrrole and polyaniline are deposited on soft and porous substrates via chemical

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or electrochemical polymerization. Despite their high flexibility and excellent ion accessibility, the electrical conductivity of these electrodes has been restricted by insulating properties of substrates used, affecting the performance of SCs. Also, the total weight of SC increased due to the use of insulating substrates, leading to a decrease of capacitance per unit weight. To resolve this problem, some researchers have prepared freestanding flexible electrodes (Liu et al. 2015). Weng et al. (2011) reported a simple and scalable method to fabricate graphene-cellulose paper (GCP) membranes shown in Fig. 3.4A. It shows the excellent mechanical flexibility, proper specific capacitance and power performance, and excellent cyclic stability with a specific capacitance of 120 F/g and retains >99% capacitance over 5000 cycles (Weng et al. 2011). Hu et al. (2010) described wearable power devices using textiles as with simple dipping and drying process using single-walled carbon nanotube (SWNT) ink shown in Fig. 3.4B. Such conductive textiles show outstanding flexibility and stretchability and demonstrate strong adhesion between the SWNTs and the textiles of interest. Supercapacitors made from these conductive textiles show high areal capacitance, up to 0.48F/cm2, and high specific energy.

Fig. 3.4 A(a) Illustration of structural evolution of GCP as the GNS loading increase, (b) comparison of CV curves at 2 mV/s for a flexible laminated poly-SC tested as normal and bent. (c) Photograph of a red LED lit by an in-series poly-SC with three units, (d ) photograph of a flexible inter-digital GCP poly-SC (https://doi.org/10.1002/aenm.201100312). (B) Porous textile conductor fabrication, (a) schematic of SWNTs wrapping around cellulose fibers to form a 3D porous structure. (b) Conductive textiles are fabricated by dipping textile into an aqueous SWNT ink followed by drying in an oven at 120
C for 10 min. (c) A thin, 10 cm  10 cm textile conductor based on a fabric sheet with 100% cotton and Rs of 4 Ω/sq. (d ) SEM image of coated cotton reveals the macroporous structure of the cotton sheet coated with SWNTs on the cotton fiber surface (https://doi.org/10.1038/srep15388)

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Liu et al. reported highly flexible, bendable, and conductive rGO-PEDOT/PSS films prepared using a simple bar-coating method and demonstrate the high capacitance of 448 mF/ cm2. The rGO-PEDOT/PSS composite films with GO loadings are prepared and assembled in an all-solid-state supercapacitor (SC) device as illustrated in Fig. 3.5a. Fuertes et al. reported a novel strategy to fabricate freestanding flexible hybrid papers made up of porous carbon particles combined with graphene sheets in which graphene acts as a scaffold that retains and interconnects the carbon particles. The synthesis procedure of the composite hybrid system is illustrated in Fig. 3.5b. It shows an excellent areal capacitance (103 mF/cm2) at current densities as high as 1400 mA /cm2 and can deliver a significant amount of energy (12 mWh/cm2) at high power densities (316 mW/cm2) (Ferrero et al. 2017). Wan et al. (2017) fabricated freestanding flexible SC device using hybrid paper electrode composed of reduced graphene oxide (RGO), polypyrrole (PPy), and cellulose. The in situ polymerization of pyrrole and chemical reduction of graphene oxide take place by using NaBH4. Figure 3.5c shows a schematic diagram of the fabrication of PPy/cellulose papers. The solid-state supercapacitor had a high areal capacitance of 0.51 F/cm2 at 0.1 mA/ cm2 and a high energy density of 1.18 mW h/cm3 (Wan et al. 2017). AC/rGO composite films fabricated via a facile vacuum filtration process are shown in Fig. 3.6a. The assembled supercapacitor was flexible, lightweight, cheap, and environmentally friendly and can achieve a superior energy density of

Fig. 3.5 Schematic illustration of (a) preparation process of rGO-PEDOT/PSS films and the structure of assembled supercapacitor devices (https://doi.org/10.1038/srep17045), (b) synthesis procedure (https://doi.org/10.1039/C6SE00047A), (c) fabrication of PPy/cellulose papers (https:// doi.org/10.1039/C6TA04844G)

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Fig. 3.6 Schematic illustration of (a) preparation procedure of AC/rGO films (https://doi.org/10. 1039/C7RA07459J), (b) (a) schematic illustration of the fabrication process of an N-CNFs/RGO/ BC paper electrode. (b) Photograph of the BC pellicle (https://doi.org/10.1021/acsami.6b11034)

16.2 mWh/cm2 at a power density of 100 mW/cm2 and 85% capacitance retention after 10,000 charging-discharging cycles. Moreover, 90% of the capacitance retained after 1000 bending cycles (Xu et al. 2017). Huang et al. reported nitrogen-doped carbon networks/graphene/bacterial cellulose paper electrode for freestanding SC. Figure 3.6b represents the schematic of the fabrication process of an N-CNFs/rGO/BC paper electrode. The fabricated paper electrode exhibits an ultrahigh areal capacitance of 2544 mF/cm2 (318 F/g) in the H2SO4 electrolyte with excellent cycling stability (100% retention after 20,000 cycles). Ma et al. (2016) explained flexible all-solid-state supercapacitor from large freestanding graphenePEDOT/PSS. The assembled device using rGO-PEDOT/PSS electrode could be bent and rolled up without any decrease in electrochemical performance. A relatively high areal capacitance of 448 mF/cm2 is achieved at a scan rate of 10 mV/s. The rGO-PEDOT/PSS composite films with various GO loadings were prepared and assembled in an all-solid-state SC device. The composite films displayed high flexibility, and the assembled device could be bent and twisted without impairing the integrity of the device (Ma et al. 2016).

3 Recent Advances in Flexible Supercapacitors

3.4.3

61

Flexible Substrate-Based SCs

Sometimes, flexible electrodes might suffer complex structural deformation and other safety issues during continuous operations. Plastic, polymer, and flexible metal-based substrate can be used as flexible support to prepare paper like stretchable, bendable, and portable FEs/FSCs to prevent these shortcomings. Flexible substrates have been also used to prepare 3D printed electrodes for printable stretchable SCs. Safety, compatibility, and reusability of these FSCs can make them useful for sophisticated applications such as electronic health monitoring and touchscreen display and advance sensing. Chang et al. introduced phosphate-doped PPy as an excellent active material for pseudocapacitance. In the presented scheme (Fig. 3.7a) (Chang et al. 2017)], PPy has been deposited over CFs embedded in melamine sponge. H3PO4/PVA is used as gel electrolyte for solid-state devising. The tandem arrangement of electrodes (PPy-CFs/ MS/PPy-CFs) helps to control the overall output by adjusting the combinations of the electrodes’ structural parameters. Metal wire is used as a current collector and PMMA substrate to combine the series of the electrode with mechanical flexibility.

Fig. 3.7 (a) The simple diagram for fabricating the single device containing a various number of equally divided ECs internally connected in series. (A) The diagram for the fabrication of device containing two equal units. (B) The devices containing three, four, and five equal units connected in series, respectively (https://doi.org/10.1016/j.electacta.2017.10.106), schematic illustration of (b) the structure of the flexible sticky-note supercapacitor (https://doi.org/10.1039/C7TA10756K), (c) stepwise preparation of flexible conducting substrate and solid-state supercapacitor device assembly using RuO2/CNO electrodes separated by a PVA/H2SO4 gel electrolyte (https://doi.org/10.1021/ acssuschemeng.5b01627)

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The device exhibits excellent electrochemical property as well as high mechanical flexibility. This arrangement provides the significant enhancement in capacity by an increment in the operating voltage window in series combination. The device performance is highly stable with 93% capacity retention after 20,000 cycles. The device performance does not change on applying mechanical stress or compression opening opportunities for flexible commercial supercapacitor. CNT array has been used to develop a new kind of supercapacitor electrode material which can be easily mounted/disassembled from the substrate depending upon the requirement. Cao et al. (2018) reported a new family of flexible and transferable sticky-note supercapacitors Fig 3.7b with repeatedly conductive stickiness has been developed by designing a novel kind of sticky aligned CNT array electrode. This kind of sticky electrode material can provide great convenience in the development of portable and wearable devices for the wide-ranging applications. The sticky-note supercapacitor exhibits high capacitance and can be quickly and repeatedly attached to various substrates including cloth, glass, paper, plastic, and metal. For up to 200 attaching/removing cycles on different substrates, the capacitance of the supercapacitor note has been well maintained at above 99% (Cao et al. 2018). Muniraj et al. (2016) prepared a composite of RuO.nHO nanoparticle with carbon nano-onions as an efficient electrode for solid-state flexible supercapacitor. Preparation of flexible conducting substrate and device assembly is shown in Fig 3.7c. RuO is known for high capacity, and carbon nano-onions provide cyclic stability for durable and enhanced electrochemical performance. Conducting carbon paper has been used as a current collector as well as the flexible substrate. For achieving the high mechanical stability in the conducting carbon fiber, it is coated with a thin layer of PDMS polymer which binds the carbon fiber by cross-linking using a curing agent. For the fabrication of solid-state device, PVA/H2SO4 is used as a gel electrolyte. The composite electrode material has a very high specific capacitance of 570 F/g at a current density of 1A/g in aqueous electrolyte. The flexible solid-state device shows a high specific capacitance of 305 F/g at 1A/g current density and high cyclic stability with 94.5% capacity retention after 4000 cycles. The device also represents significant resistance against mechanical deformation and retains almost 89% of initial capacity under mechanical strain in 4000 cycles (Muniraj et al. 2016).

3.4.4

Metal-Based Flexible Current Collector

As discussed in previous sections of the chapter, the carbon fabric and fibers are the most widely used current collectors in literature for FSCs. Though carbon-based current collectors exhibit excellent mechanical flexibility, their electronic conductivity is not always satisfactory. Some recent reports included metallic foils, metal wires, and metal-coated polymers as current collector in all-solid-state FSCs. Stainless steel mesh, Ni-foils, and Ti-foils have been used as highly conductive and

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flexible current collector in several research articles. These metal current collectors have high conductivity, but they may be a bit rigid for flexible electrode fabrication. Also metallic current collectors suffer poor stability in aqueous acidic or alkaline electrolytic medium. Therefore, mechanical flexibility along with good stability is highly desirable for metallic current collector in FEs/FSCs. In the scheme presented in Fig. 3.8a, stainless (Bandgar et al. 2018) steel mesh was used as the flexible substrate. Different nanostructures of NiFe2O4 like nanosheets, nanoflowers, and nanofeathers have grown over the substrate by using Pearson acid-base concept. The different morphologies of the NiFe2O4 sheet are achieved by changing the precursor of Ni2+ ions (NiCl2.6H2O for nanosheets, Ni (NO3).6H2O for nanoflowers, NiSO4.6H2O for nanofeathers), while the FeCl2.4H2O is used as a precursor for Fe2+ ions in all cases. The detailed electrochemical performances of different NiFe2O4 electrodes have been investigated in the work and nanosheet morphology showing best results among them. Flexible symmetric supercapacitor device is made by using PVA-KOH solid gel electrolyte. The flexible solid-state device using NiFe2O4 nanosheets as active electrode materials shows high specific capacitance of 236 F/g at an areal current density of 5 mA/cm2 and very high capacity retention of 98% after 7000 cycles. Conducting polymer composites with carbonaceous materials are frequently used as active electrode materials for supercapacitor. Figure 3.8b (Qi et al. 2018) demonstrates another such example where polypyrrole nanotubes (PNT) are decorated with graphene nanoflakes in core-shell structure. Studies suggest that graphene acts as surface protector and averts the dissolution of PNT into the electrolyte. For the preparation of the flexible device, the material coated with Al foil and PVA-LiCl is used as a gel electrolyte. The device was sealed by using Parafilm, and it shows an areal capacitance of 168–128 mF/cm2 at 0.18–1.8 mA/cm2 current density. The flexibility is tested under steady voltage scan rate, by bending the device electrode and only 2–5% decrement observed in the capacitance values. The material also shows high cyclic stability with 80% retention of initial capacitance after 5000 cycles with an areal input current density of 1 mA/cm2. Purkait and coworkers (Purkait et al. 2018) demonstrated the simple approach to prepare one-dimensional wirelike supercapacitor device which has likely use in smart textiles and wearable devices. The active electrode was prepared by electrochemical deposition of porous rGO (prGO) over copper foam (Cuf)/Cu wire which also acts as a current collector shown in Fig. 3.8c. For uniform deposition prGO on copper wire, the wire is modified by depositing copper foam using galvanostatic deposition, and then rGO is deposited over Cuf/Cu wire. PVA-H3PO4 has been used as gel electrolyte, and the PET was used to provide mechanical support and flexible platform for the demonstration of the device. The device was assembled by using two identical pErGO@Cuf/Cu electrodes on flexible PET sheet. The flexible device exhibits an excellent capacitance of 81F/g and very high stability with 95% retention of initial capacitance after 5000 cycles at 5 A/g input current density.

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Fig. 3.8 (a) Schematic of the formation mechanism of NiFe2O4 thin film nanosheet (nickel chloride salt), nanoflower (nickel sulfate salt), and nanofeather (nickel nitrate salt) morphologies employing Pearson acid-base concept (https://doi.org/10.1021/acsaem.7b00163), schematic representation of (b) synthesis of graphene nanoflake (GNF)-coated polypyrrole nanotube (PNTs) hybrid (GNFs/PNTs) and pictorial representation of an all-solid-state flexible supercapacitor device based on GNFs/PNTs (1:3) (https://doi.org/10.1039/C7TA11245A), (c) the overall design and process flow for the stepwise fabrication of 3DrGO@Cuf/Cu wire supercapacitor (https://doi.org/10.1038/ s41598-017-18593-3)

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3.4.5

65

The Wearable Fibrous Flexible Electrode

Growing demand for consumer electronics for small portable and wearable products such as smart clothing, electronic skin, and implantable medical devices has opened gates for non-planar one-dimensional fibrous FEs/FSCs. Recent reviews related to FSCs have given great importance to the one-dimensional SCs due to their potential applications in smart devicing. Traditional supercapacitors are planar which may prove bulky and rigid for making lightweight wearable devices. Fibrous FSCs can easily be integrated with small devices with excellent flexibility. Figure 3.9A represents the fabrication schematic and electrochemical performance of a wearable fibrous electrode synthesized by Lu et al. (2012). The highperformance flexible supercapacitor is made up of WO3–x@Au@MnO2 core-shell NWs synthesized on carbon fabric. The device exhibits excellent electrochemical properties including high capacity, cyclic stability, and high energy-power density. The specific capacitance of WO3–x@Au@MnO2NWs has been calculated at 1195 F/ g at a current density of 0.75 A/g by MnO2 mass. Moreover, the cyclic stability of the WO3–x@Au@MnO2 NW electrode is examined at 50 mV/s, and it shows 110% of its initial capacity after 5000 cycles. The study elaborates the role of Au nanoparticle film over WO3 which helps significantly reduce the internal resistance and gradually enhance the capacitive performance. The solid-state symmetric supercapacitor

Fig. 3.9 A(a) Schematic of the fabrication process for WO3–x@Au@MnO2 NWs, (b) optical photographs of the as-fabricated solid-state supercapacitor device. The bottom images demonstrate the high flexibility of the as-prepared device, (c) CV curves of a WO3–x@Au@MnO2 supercapacitor at different curvatures of 0
, 45
, 90
, 135
, and 180
between 0 and 3 V. (d ) Galvanostatic charge-discharge curves of WO3–x@Au@MnO2 supercapacitors at different current densities between 0 and 3 V, (e) light-emitting diode (LED) lighting demonstration, with the diode driven by two supercapacitors in series (https://doi.org/10.1002/adma.201104113). B(a) Schematic illustration of the fabrication process of the Ni wire/Co3O4@MnO2 nanowire array electrode, (b) the structure of the fiber-based flexible all-solid-state asymmetric supercapacitor and (c) of the energy storage mechanism of the hierarchical Ni wire/Co3O4@MnO2 nanowire array electrode. The electrochemical performance of the all-solid-state asymmetric supercapacitor: (d ) CV curves at different scan rates, (e) galvanostatic charge-discharge curves at a different current density (https:// doi.org/10.1039/C7TA07899D)

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device has been prepared by separating the two carbon fabric sheets immersed in PVA/H3PO4 gel electrolyte. The results are evidence of promising future electrode prepared by WO3–x@Au@MnO2 core-shell NWs, with high-performance electrochemical behavior (Lu et al. 2012). Niu et al. (2017) synthesized the Co3O4@MnO2 core-shell nanowire-based allsolid-state asymmetric supercapacitor shown in Fig. 3.9B along with electrochemical performance. Figure 3.9B represents (Niu et al. 2017) the Co3O4@MnO2 core-shell nanowire-based all-solid-state asymmetric supercapacitor. The Co3O4 nanowires are grown over Ni wire with hydrothermal synthesis route and then further MnO2 layer deposited with the subsequent hydrothermal reaction by using KMnO4 and precursor. The negative electrode was prepared by depositing the activated carbon, graphene oxide, and PTFE mixture over carbon fiber. PVA/KOH is used as gel electrolyte for the solid-state supercapacitor device. The device represents high areal capacitance of 13.9 mF/cm2 at 0.1 mA/cm2. It also retains 82% of its initial capacity after 1000 cycles at high input current density of 0.6 mA/cm2 Figure 3.10A, B (Li et al. 2017a, b) depicts the synthesis schematic and electrochemical performance of solid-state wearable, flexible supercapacitor. The device is

Fig. 3.10 (A) Schematic preparation representation of the all-solid-state flexible fiber supercapacitors based on V2O5-SWCNT hybrid fiber electrodes, (B) (a) XRD pattern, (b) GCD curves at a current density of 0.25 A/cm3, (c) the specific capacitance of each evaluated current density of V2O5 and (d ) V2O5-SWCNT fiber electrodes with different SWCNT amounts, XPS spectrum of V2O5-SWCNT-10 fiber electrode, (e) the mechanical property of V2O5 and V2O5/ SWCNT-10 fiber electrodes, and ( f ) SEM image of the knotted V2O5/SWCNT-10 fiber electrode (https://doi.org/10.1016/j.jpowsour.2017.10.031)

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made up of V2O5 nanobelt SWCNT composite fiber by the wet spinning method by Li et al. The fiber has been synthesized by mixing the V2O5 nanobelt suspension, with SWCNT suspension under vigorous stirring. Further, the homogeneous suspension injected into a coagulation bath is made up of CaCl2, ethanol-water solution using a syringe pump. The electrochemical performance of the V2O5-based fiber has been tested in a three-electrode system using the 1 M Na2SO4 solution as electrolyte under 0.3–1.3 V window. It shows high volumetric capacitance of 99 F/cm3 at a current density of 0.25 A/cm3. The allsolid-state symmetric device was prepared by using V2O5/SWCNT hybrid electrode by assembling in the configuration V2O5/SWCNT/V2O5/SWCNT electrode, and PVA/H3PO4 is used as a gel electrolyte. Electrochemical and flexible behavior is investigated by bending the electrode on different angles (0–180
) at a scan rate of 20 mV/s. The device shows almost 100% capacity retention after 5000 cycles without bending and 98% retention after 500 cycles on bending at a 90
angle. The asymmetric solid-state device has also been demonstrated with improved electrochemical behavior by using V2O5/SWCNT as a positive electrode, rGO/SWCNT as a negative electrode in V2O5/SWCNT/rGO/SWCNT, and PVA/H3PO4 as a gel electrolyte. The study boosts the scope of potential utilization of V2O5-based 1D electrode for wearable, flexible energy storage devices.

3.5

Conclusion

In this chapter we have covered the recent progress in utilization of nanostructured materials for the preparation of all-solid-state flexible supercapacitors. Polymer gel electrolytes have become a most crucial and inseparable part of FSC devices due to their excellent mechanical stability and high ionic conductivity. Further categorical classification of the FSCs on the basis of construction component provides easy insight of complex design parameters to summarize vast literature. Two-dimensional, sandwiched electrodes are most common for FSCs due to their high surface area and high packing efficiency. Carbonaceous or metallic current collectors can be used with sandwiched active electrode materials. Paperlike freestanding electrodes are made up of carbon materials and their composites with transition metal oxide, and conducting polymers have proven to be promising for preparation of 2D high-performance FSCs. Flexible substrates such as plastic and polymers are used frequently to provide mechanical stability in FSCs. Apart from planar SCs, 3D FSCs can be made by using activated carbon and their composites as electrode materials over metallic- or carbon-based current collectors. One-dimensional fiber-like FSCs are future prospect for smart and innovative energy storage, and hence lot of research is going on to develop this kind of devices. The categorization of FSCs is justified by detailed discussion of design parameters along with electrochemical performance. FSC device presents the potential for the energy storage application with increasing demands in various sophisticated applications such as consumer electronics,

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wearable smart fabrics, and medical implants. However, the FSCs are a relatively new field of electrochemical energy storage and contain major scope of further improvements. Overall FSCs are a reality in the near future for safe and environment-friendly energy storage. Acknowledgments Swati Jadhav is thankful to SERB for the National Postdoctoral Fellowship (PDF/2017/000388). Vikash Chaturvedi is thankful to DST Nano Mission project (GAP-314126) for funding. All the authors acknowledge the contribution of CSIR NCL for the facilities.

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Chapter 4

Noble-Metal-Free Nanoelectrocatalysts for Hydrogen Evolution Reaction Natarajan Thiyagarajan, Nithila A. Joseph, and Manavalan Gopinathan

Contents 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1 Volcano Plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Tafel Slope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Turnover Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.4 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.5 Faradaic Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Non-noble Electrocatalyst for Hydrogen Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1 Molybdenum-Based HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1.1 Molybdenum Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1.2 Molybdenum Diselenide (MoSe2) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1.3 Molybdenum Phosphide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1.4 Molybdenum Carbide and Boride . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Tungsten-Based HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2.1 Tungsten Sulfide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2.2 Tungsten Phosphide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2.3 Tungsten Nitride and Carbide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 Cobalt-Based HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3.1 Cobalt Chalcogenides and Borides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3.2 Cobalt Phosphide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4 Nickel-Based HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4.1 Nickel Chalcogenides and Borides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.4.2 Nickel Phosphide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.5 Iron-Based HER Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract The rapidly progressing global warming due to large carbon emission originating from increased consumption of fossil fuels has become a leading cause of N. Thiyagarajan (*) · M. Gopinathan Department of Chemistry, National Chung Hsing University, Taichung, Taiwan e-mail: [email protected] N. A. Joseph Institute of Biomedical Sciences, National Chung Hsing University, Taichung, Taiwan © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_4

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concern. To slow down global warming and to shift toward a sustainable path, the development of alternative renewable energy sources is inevitable. Hydrogen is one such source which is considered to be green. However, current hydrogen generation methods are both energy intensive and generate CO2 as by-product, and thus, developing efficient green methods is necessary. The generation of hydrogen through water splitting is a straightforward method. The evolution of several less expensive non-noble electrocatalysts in the recent past has fueled research efforts related to electrocatalytic hydrogen evolution. Some of these non-noble catalysts have exhibited excellent electrochemical activity and stability, and their performances have rivaled the bench mark catalyst “platinum.” Unlike Pt, whose prohibitive cost prevents large-scale usage, these catalysts can be produced in an affordable manner to be used in mass scale. This chapter reviews some of the basic catalyst evaluation parameters along with interesting results being achieved using catalysts composed of metal dichalcogenides, carbide, nitrides, and phosphides.

4.1

Introduction

Hydrogen gas has one of largest energy density, with the ability to hold 143 kJ/g, enabling it to store significant amounts of energy with relatively small content. This relatively large amount of energy is stored in the form of chemical bonds. The energy potential is realized through reaction of molecular hydrogen with dioxygen in the fuel cells with only water being generated as by-product, making it a clean energy process (Chen et al. 2017; Merki and Hu 2011). However, hydrogen is not readily available in the free form in earth and exists as a gas in the atmosphere only in tiny amounts – less than one parts per million by volume. Therefore, hydrogen is produced industrially through two major processes, namely, steam methane reforming (4.1) and coal gasification (4.2), which account for ~94% of overall hydrogen generation. Both of these processes rely heavily on fossil fuels as source which are nonrenewable and generate CO2, one of the greenhouse gases, as by-product. CH4 þ H2 O ! 4H2 þ CO2

ð4:1Þ

C þ 2H2 O ! 2H2 þ CO2

ð4:2Þ

Thus, the current production is clearly not a green process, though hydrogen is considered as a clean fuel source and the mere production method itself has become a source of CO2 generation, leaving a large carbon footprint. The majority of the hydrogen produced through these methods has been utilized for the refining of petroleum, hydrogenation of oils, and manufacturing of ammonia and as flushing gas in semiconductor industry to produce silicon wafers, as clean fuel in NASA’s space programs, etc. Further, intense efforts are ongoing in automobile industry to

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tap the potentials of fuel cell-powered vehicles, with the hydrogen being stored in the form of compressed hydrogen gas or hydrogen carriers (Turner 2004) (https://www. afdc.energy.gov/fuels/hydrogen.html). With already buses and cars being powered by hydrogen, albeit in lesser numbers, fuel cells are making headways and a potential “hydrogen economy” in future can be a reality. When fully succeeded, the demand for the hydrogen would increase manifold, and the production capacity has to be ramped up substantially. In light of this, the huge amount of hydrogen that is being stored in the form of water can be considered as a renewable source, and electrochemical water splitting offers one of the cleanest routes for the production of hydrogen (Zou and Zhang 2015). Current production of hydrogen through electrochemical route is quite minimal, which accounts for only 4% of overall hydrogen produced. Thus, the potential for electrochemical hydrogen production is enormous, and rightly so, it has been receiving much attention in the recent past (Scheme 4.1). The water splitting is composed of two half-cell reactions, viz., anodic oxygen evolution and cathodic hydrogen evolution reactions, respectively. The process is pH dependent, and the mechanisms differ slightly between acidic and alkaline mediums. In acidic solution reactions (4.3) and (4.4) take place: 4Hþ þ 4e ! 2H2 þ

2H2 O ! O2 þ 4H þ 4e

ð4:3Þ 

Scheme 4.1 Schematic representation of hydrogen generation process

ð4:4Þ

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Scheme 4.2 Overall water splitting process

In alkaline solution the reaction varies depending on the ions, as shown in (4.5) and (4.6), in which the primary source of hydrogen is the electroreduction of water itself (Scheme 4.2): 4H2 O þ 4e ! 2H2 þ 4OH

ð4:5Þ

4OH ! 2H2 þ O2 þ 4e

ð4:6Þ

The thermodynamic potential for water splitting is 1.23 V at 25  C and 1 atm, irrespective of the pH medium (Zou and Zhang 2015). The equilibrium Nernst potential for the hydrogen evolution is given by Eq. (4.7) which is pH dependent and shifts by 59 mV per unit change in pH. However, conversion of the potential to RHE (the reversible hydrogen electrode) negates the pH contribution, making it straightforward and more convenient from the point of practical consideration (4.8) (Zeng and Li 2015): EHER ¼ E0ðH2 =Hþ Þ 

RT a þ  ln H F PH1=2

ð4:7Þ

2

EHER ¼ 0:059  ðpHÞ V vs:NHE ¼ 0 V vs:RHE

ð4:8Þ

A closer look at the cathodic process can provide significant information about the overpotential and efficiency of the hydrogen evolution reaction. In general, the hydrogen evolution process can be explained by three fundamental reaction steps at various electrocatalysts in acidic media. The initial electrochemical hydrogen

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adsorption step called Volmer step (1). The kinetic rate constant depends on the applied potential during this electron transfer step: H3 Oþ þ e þ M $ MHads þ H2 O ðstep 1Þ where M represents the active site and MHads represents the chemisorbed H on the site of electrode. Subsequently, this step is preceded by either Heyrovsky (2) or Tafel step (3), or a combination of both Heyrovsky and Tafel steps is possible.

In Heyrovsky step, an electrochemical desorption occurs through the reduction of proton. MHads þ H3 Oþ þ e ! M þ H2 ðstep 2Þ Tafel step takes place through recombination (chemical desorption) of adjacent Hads. This step involves no charge transfer. 2MHads ! 2M þ H2 ðstep 3Þ

4.1.1

Volcano Plot

Since the hydrogen evolution proceeds through a two-step process, Sabatier principle can be applied to this process. According to Sabatier principle, a catalyst should

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have a moderate binding energy such that the binding energy of the reactant (atom or molecule) to the catalyst surface must be strong enough to produce reaction intermediates but should be weak enough for the products to leave the surface to allow the catalytic cycle to continue, as failing to desorb would result in poisoning of the active catalyst surface (Medford et al. 2015; Laursen et al. 2011). Regardless of the subsequent Tafel or Heyrovsky steps, H adsorption is the first common step, and therefore, the Gibbs free energy for hydrogen adsorption (ΔGH) is regarded to be one of the descriptors in the screening of hydrogen evolution catalysts. A Gibbs free energy value of adsorbed hydrogen close to zero (ΔGH  0), which would bind with the intermediate bond strength, is considered to be ideal for a catalyst to be optimal for hydrogen evolution process. Similarly, the exchange current density (J0) is an important kinetic parameter that represents the electrochemical reaction rate at equilibrium, where both the forward and backward reaction take place at similar rate. The exchange current density of Pt electrode is the highest of any catalyst for hydrogen evolution reaction. It is also important to note that the hydrogen evolution process on Pt electrode surface is nearly thermoneutral, one of the parameters that owes the highest activity of Pt over other metal catalysts at equilibrium potential (Nørskov et al. 2005). The Gibbs free energy of hydrogen adsorption (ΔGH) and the exchange current density (J0) form the basis for the generation of so-called volcanic plot predicting the activity of various catalysts for the hydrogen evolution reaction in acidic medium. This relation perfectly resembled the Sabatier principle, a wellknown paradigm of gas-phase heterogeneous catalysis (Zeradjanin et al. 2016). Parsons and Gerischer independently related the practical quantitative relationship between adsorption energies of Hads intermediates and exchange current (Io) (Parsons 1958; Gerischer 1958). Parsons used a Frumkin isotherm which led to a flat-topped volcano plot and gave a prediction that, at different sections of the volcano, different slopes can be expected. In the case of Gerischer, the maximum in exchange current was the same for the three partial reactions, while in the case of Parsons, the maximum in exchange current differs for different partial reactions (Zeradjanin et al. 2016). Further, Trasatti compiled the experimental data and presented the first predictive kinetic model for HER that resulted in a volcano plot composed of exchange current density and metal-hydrogen bond strength of polycrystalline metal systems (Trasatti 1972) (Fig. 4.1). Norskov et al. further related the reaction rate and the adsorption energy of intermediate and d-band center (Zeradjanin et al. 2016). Additionally, Santos et al. incorporated some of the missing parameters into their theoretical calculation process which indicated that a good catalyst for hydrogen evolution should exhibit ΔG ~ 0 for hydrogen adsorption, a d-band that extends across the Fermi level, and a strong coupling of this band to hydrogen (Santos et al. 2012).

4.1.2

Tafel Slope

This cathodic process, however, does not readily take place on a given electrode surface at equilibrium potential due to high activation energy barrier, ohmic potential

Exchange Current for H2 Evolution, -log i,/A cm–2

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Pt

3

Re Rh Ir Au

5

Ni Cu

Co Fe

W

7

Mo

Sn Bi Zn Ga

Ti

Ag

Nb

Pb

9 Ti

Ta

Cd In

30

70 50 M-H Bond Strength / kcal mol–1

90

Fig. 4.1 Volcanic plot composed of exchange currents for electrolytic hydrogen evolution and strength of intermediate metal-hydrogen bond formed during electrochemical reaction. Image reproduced with permission from (Trasatti 1972)

drop, and mass transfer resistance. Thus, an excess potential from the equilibrium potential, known as overpotential (η), needs to be supplied for the electrochemical reaction to proceed at an acceptable rate. η ¼ E  Eeq

ð4:9Þ

The reaction rate of an electrochemical process can be correlated to the current as follows: i0 ¼ nFk0 C

ð4:10aÞ

where k0 is the standard rate constant, F is Faraday’s constant, i0 is the exchange current, n is the number of electrons transferred, and C is the concentration. However, the exchange current density is more useful than k0, as it can be correlated to the equilibrium potential, i.e., overpotential, rather than formal 0 potential E0 . The electrochemical reaction kinetics at equilibrium, under the absence of mass transfer effect and with no appreciable difference between surface and bulk concentration, can be described using Butler-Volmer equation (Bard et al. 1980):

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  j ¼ j0 expαnFη  expð1αÞnFη

ð4:10bÞ

where α is the transfer coefficient, j is current density, and j0 is the exchange current density. At very low overpotential, where the overpotential is linearly correlated to net current density within a narrow potential range near Eeq, the Butler-Volmer equation can be reduced to:   RT η¼j nFj0

ð4:11Þ

At higher overpotential, one of the terms on the right side of Eq. (4.10b) becomes zero depending on the applied potential, and the Butler-Volmer equation is reduced to: η¼

RT RT ln ðj0 Þ  ln ðjÞ αF αF

ð4:12Þ

The current is related exponentially to the overpotential (4.13) or by Tafel equation as follows (4.14) (Bard et al. 1980): i ¼ a expη=b

ð4:13Þ

η ¼ a þ b logðjÞ

ð4:14Þ

where b and j denote the Tafel slope and current density, respectively. The first term on the right side in eq. 4.12 corresponds to “a” in eq. 4.14. In the Tafel process, the current response with respect to the applied potential is analyzed, which provides information associated with the rate-determining step. The Tafel slope would be different under different electrochemical conditions (Fletcher 2009). Since hydrogen evolution on different electrode catalysts proceeds through different reaction mechanism, Tafel analysis can be useful in elucidating the ratedetermining step and the possible mechanism. It can be noticed here that the exchange current density (j0) can be obtained from the intercept of the plot of η versus log j. Tafel analysis can be an indicator of electrocatalytic activity, i.e., it provides one of the descriptors j0 that defines the activity of a given catalyst through k0. A low J0 value indicates a slow electron transfer kinetics, and thus a substantial overpotential is needed to overcome the activation barrier (Fig. 4.2). It should be noted here that the Tafel form holds good when the reverse reaction is negligible (>1%), as it is an indicator of totally irreversible kinetics (Bard et al. 1980). The Tafel slopes corresponding to the three steps (1–3) in acidic medium exhibit different slope values. A slope value of ~120 mV per decade can be assigned to Volmer/proton-coupled electron discharge step. Similarly, a slope value of ~40 mV per decade and ~30 mV per decade can be assigned to electrochemical desorption/ Heyrovsky and chemical desorption or recombination reaction/Tafel steps, respectively. For example, on platinum electrode in acid solutions at low overpotentials,

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Fig. 4.2 (a) Schematic illustration of the HER energetics. (b) Schematic HER polarization curves on two different electrocatalysts with their onset overpotentials indicated. (c) Schematic Tafel plots on two different electrocatalysts with their Tafel slopes and exchange current densities indicated. Image reproduced and modified with permission from (Zeng and Li 2015)

chemical recombination or Tafel step is rate determining with a slope of ~30 mV per decade due to low surface coverage. Higher overpotential leads to surface saturation with Hads, leading to accelerated atom–atom recombination. Therefore, the Volmer step becomes rate determining with a measured slope value of ~120 mV per decade (Zeng and Li 2015). However, at higher surface coverage of Hads, a slope of ~120 mV per decade is observed for Heyrovsky step and must be evaluated carefully while defining the rate-determining step (Shinagawa et al. 2015). Thus, the Tafel slope is dependent on both applied potential and surface coverage. In addition, the Tafel slope is also structure sensitive (Markovića et al. 1996; Marković et al. 1997). The surface heterogeneity of the catalyst can alter the Tafel slope. For example, the presence of both S2 and S22 in the amorphous MoS2 catalyst might lead to difference in the adsorption of surface hydrogen (47 mV/dec), and insertion of chemical rearrangement between the Volmer and Heyrovsky step can give rise to a slope of 60 mV/dec (Shin et al. 2015). Therefore, attention must be paid while deducing the mechanism using Tafel slope.

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Turnover Frequency

Similar to exchange current density, the turnover frequency (TOF) is also a good indicator of catalyst performance. Though exchange current density value is an inherent measure of activity, it can be affected by electrode nature such as porosity and surface roughness. Thus, the TOF can be used systematically to evaluate the intrinsic activity of different catalysts. The TOF of heterogeneous catalytic hydrogen evolution processes can be estimated using normalized catalytic current value (with respect to total number of surface active site or electro-active surface area of catalyst). Here, the overall current observed is assumed to be associated with the hydrogen evolution process. The TOF at given potential can be calculated using the following equation (Shin et al. 2015): TOF ¼

I nFN

ð4:15Þ

where I is the normalized catalytic current at a given overpotential, n is the number of electrons transferred (n ¼ 2 for HER), and N is the number of electro-active surface area or total number of surface active site (Merki et al. 2011). As indicated, the N-value is obtained using different approaches. Electro-active surface area of catalyst: In case of Pt, the well-known underpotentially deposited hydrogen adsorption-desorption region can be used to estimate the active surface area. However, for materials that exhibit no such adsorption-desorption characteristics, either the reduction or oxidation signal corresponding to the active metal (Bonde et al. 2009) or the slope from the plot of scan rate vs. current obtained from the scan rate study of an active catalyst-modified electrode can be utilized to estimate the N-value (4.16) (Pintado et al. 2013): slope ¼

n2 F2 AN 4RT

ð4:16Þ

where R is gas constant, T is temperature in kelvin, and A is the geometric electrode area. Underpotential Deposition of Cu The active sites for proton reduction are also responsive to the Cu2+ reduction at an underpotential (Yang et al. 2015b). Thus, the charges exchanged during the oxidative stripping of the copper obtained in UPD can be used to estimate the active site density (Fig. 4.3). The charge under the oxidation curve can be readily calculated to give the surface coverage: Surface coverage ðΓÞ ¼ Q=nFA

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Fig. 4.3 Cyclic voltammograms of underand overpotentially deposited Cu. Reproduced and modified with permission from (Yang et al. 2015b)

where Q is the charge, n is number of electron corresponding to the redox process, and A is the electrode geometric area. The active site density can be obtained by multiplying the surface charge with Avogadro’s constant (6.023  1023). N-value calculation based on surface roughness (Shin et al. 2015): N ¼ Rf

 NA d

2

=M f Þ 3

ð4:17Þ

where NA and Mf are Avogadro’s number and formula weight of active catalyst, respectively, and Rf is the surface roughness factor defined as a ratio of the real surface area to the geometric area. For materials, like MoS2, where the activity originates from the particular active sites, the current is normalized to the total number of such active sites (Jaramillo et al. 2007). The TOF per site can be estimated using formula given below. The total number of active sites can be calculated using different approaches depending on the preparation method of catalyst (Ma et al. 2015a; Kibsgaard et al. 2014). When the number of active sites is not known with certainty, the surface area value determined using Brunauer-Emmett-Teller (BET) calculation may be useful (Popczun et al. 2013): TOF ¼

No:of total H2 turnovers=cm2geometric area No:of active sites=cm2geometric area

The total number of H2 turnovers can be calculated from the following formula (Zhou et al. 2016a): #H 2 turnovers      mA 1C s1 1 mol e1 1 mol H2 6:023  1023 H2 molecules ¼ j 2 cm 1 mol H2 1000mA 96845:3 C 2 mol e

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Turnover Number The turnover number can be calculated from the amount of product and catalyst. TON ¼ moles of gas=moles of catalyst

4.1.4

Stability

The stability of the catalyst during continuous operation is one of the critical parameters in the evaluation of a catalyst. Though efforts are being made to predict the activity of the catalyst, not many specific theoretical studies have been devoted toward the prediction of catalyst stability. Norskov et al. have screened several bimetallic catalysts for HER through computational DFT calculations and also predicted the stability of identified catalysts under the electrochemical environment (Futaba et al. 2006). The free energy change associated with the surface segregation, de-alloying and island formation, free energy of oxygen adsorption leading to surface poisoning and oxide formation, and corrosion of surface alloy of interest under acidic condition during catalysis is identified to provide a comprehensive stability profile of different catalyst (Fig. 4.4). The stability of the catalyst can be practically evaluated through an accelerated stability test by subjecting the catalyst-modified electrode to multiple cycling between appropriate potential windows. Also the change in potential at fixed current density or the current-time profile at a bias potential can be studied (Mathew et al. 2014; Shen et al. 2016b).

Fig. 4.4 Pareto-optimal plot of stability and activity of surface alloys for the HER. Image reproduced from (Futaba et al. 2006)

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Faradaic Efficiency

The faradaic efficiency can be calculated from the total amount of hydrogen produced (experimental yield), quantified using standard analytical techniques such as gas chromatography, and theoretical yield. The theoretical yield can be calculated based on the moles of electron passed into the solution during the electrochemical process: No:of moles of electron passed ¼

Q F

where Q is the charge which is a product of current I passed in ampere over the time. According to the stoichiometry of reaction as shown below, two electrons are needed to produce one mole of hydrogen. 2Hþ þ 2e ! H2 Thus, the theoretical yield would be: yieldtheory ¼

  Q 1 mol of hydrogen  F 2 mol of electrons

efficiency ¼

yieldexp  100% yieldtheory

The Tafel slope, TOF, stability, and faradaic efficiency, apart from the main descriptor, namely, hydrogen adsorption energy (ΔGH), can be evaluated readily from the electrochemical data and can be considered as good indicators of catalyst’s activity. Therefore, these parameters should assist in the evaluation of the performance of a catalyst and may form the basis in comparing the performances of different catalysts.

4.2

Non-noble Electrocatalyst for Hydrogen Evolution

The most active catalyst for hydrogen evolution process in acidic medium is platinum and its composites, although nickel is quite active in alkaline condition. Pt-type catalysts are more compatible with the conditions envisioned for photochemical water splitting, especially in direct photo-electrochemical approaches. (Merki et al. 2011). However, Pt is a scarce metal, and the forbidding cost makes it less viable for large-scale usage. Enzymes, such as hydrogenase and nitrogenase, can effectively catalyze the formation of molecular hydrogen. The active metal centers in these enzymes consist of Fe, Ni, and Mo (Hinnemann et al. 2005). Though efficient for HER process, these enzymes are inactivated by oxygen and are sensitive to temperature, pH, and other environmental factors (Cammack et al. 2015).

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Fig. 4.5 Nitrogenase FeMo cofactor (FeMoco) with three hydrogen atoms bound at the equatorial μ2S sulfur atoms (left). Hydrogenase active site with one hydrogen atom bound (middle). MoS2 slab with sulfur monomers present at the Mo-edge (right). Image reproduced with permission from (Hinnemann et al. 2005)

Nevertheless, understanding of the protein structure and mechanism aspect has helped to mimic them. For example, under-coordinated sulfur atoms at the edges of MoS2 have properties very similar to those of the active enzymatic centers, and this has been verified experimentally with MoS2 nanoparticles for hydrogen evolution (Hinnemann et al. 2005; Yan et al. 2014) (Fig. 4.5). The non-noble HER active metal exists in abundant quantity and is less expensive compared to Pt-group metals, thus suitable for mass-scale production of hydrogen evolution cathode. Recently, several non-noble metal-based catalysts such as metal nitrides, metal carbides, metal chalcogens, and metal phosphides have been explored for the electrochemical hydrogen evolution process (McEnaney et al. 2014). Some of the catalysts show interesting properties such as activity over a wide pH range, excellent turnover frequency, and enhanced stability under constant operation. The results, thus, clearly demonstrate the potentials of these materials as suitable alternatives for Pt-materials for the electrocatalytic hydrogen evolution process. The consistent efforts made to decipher the material and the origin of activity have led to more understanding of the catalytically active sites and enabled researchers to tune the activity by preparing designed nanostructures, by tuning the Fermi level, or by creating defects/vacancies (Jaramillo et al. 2007; Chen et al. 2013b; Li et al. 2015b). Although Ni-based materials like nickel oxides/hydroxides and Raney nickel show good activity under alkaline condition, transition metal dichalcogenides, phosphides, nitrides, and carbides have emerged as new stable and active catalysts especially under acidic condition and are briefly discussed in the following sections (Fig. 4.6).

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Fig. 4.6 (a) Crystal abundance of metals that are used for constructing HER electrocatalysts. (b) Strategies for improving the catalytic performance of 2D nanomaterials for HER. Image reproduced and modified with permission from (Zou and Zhang 2015; Chen et al. 2017)

4.2.1

Molybdenum-Based HER Catalysts

4.2.1.1

Molybdenum Sulfide

Molybdenum sulfide belongs to a large family of two-dimensional layered metal chalcogenide materials that have the general formula MX2, where M is a metal and X is a chalcogen (S, Se, or Te) (Lukowski et al. 2013). Computational study shows the close structural resemblance between nitrogenase active site and the MoS2 edge

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structure (Hinnemann et al. 2005). Structural studies of MoS2 indicate flat polygons of S-Mo-S trilayers either stacked in a graphite-like manner by weak van der Waals interactions in hexagonally packed structures or as single trilayers with a trigonalprismatic coordination depending on the preparation (Gołasa et al. 2014; Wang et al. 2013d). For single trilayers, two kinds of surface sites exist: terrace sites, which are on the basal plane, and edge sites, which lie at the edge of the nanoparticles. MoS2 has long been considered as not an active material for hydrogen evolution, as bulk MoS2 is a poor electrocatalyst. However, it has been used in the hydrodesulfurization (HDS) process, and it gained attraction following the report of successful application of MoS2 nanoparticles for HER. The active sites of MoS2 are believed to be the (1010) planes that are exposed on the edges, rather than the (0001) basal planes (Karunadasa et al. 2012; Jaramillo et al. 2007). Both highresolution scanning tunneling microscopy and theoretical calculations have revealed that the catalytic activity of MoS2 is derived from the Mo and under-coordinated sulfur edge sites, where dangling bonds are available for chemistry, while the basal plane remains catalytically not very active due to saturated atoms (Choi et al. 2013; Bonde et al. 2009; Min and Lu 2012) (Fig. 4.7). Further, the HER active site corresponding to the edges was experimentally shown by plotting the exchange current density versus the MoS2 edge length. Despite some scattered points, the overall results provided a straight line. The rate of reaction was found directly proportional to the number of edge sites, irrespective of particle size, and this confirmed the activity of edge sites. Further, exchange current density, calculated from the TOF, value of 7.9  106 A/cm2 lies close to precious Pt-group metals in the volcano plot (Jaramillo et al. 2007; Lukowski et al. 2013). Nanostructure gives rise to altered electronic behavior due to nanoscale crystal dimensions and altered physiochemical properties due to an increased ratio

Fig. 4.7 Atomic ball model (top view) showing a hypothetical, bulk-truncated MoS2 hexagon exposing the two types of low-index edges, the S-edges and Mo-edges. The Mo atoms (blue) at the Mo-edge are coordinated to only 4 S atoms (yellow). Image reproduced with permission from (Lauritsen et al. 2004)

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of surface atoms relative to bulk (Choi et al. 2013). Given the layered nature and high surface energy of the two-dimensional structure, MoS2 nanomaterials are prone to stacking through ππ interaction during the preparation process. This leads to decreased number of catalytically active edge sites on MoS2 and increased resistance for electron transfer and diffusion of reactant molecules resulting in substantial decrease in overall catalytic process (Min and Lu 2012). Therefore, the preparation method is one of the crucial parameters in obtaining a good catalyst. The MoSx nanomaterials are predominantly obtained by methods including exfoliation of bulk material, chemical vapor deposition (CVD), and solvothermal methods that exhibit different structural and morphological property (Yan et al. 2014; Benck et al. 2014; Merki and Hu 2011). Some of the preparation methods with interesting results are highlighted in the following section. MoS2 nanoparticles on a carbon support were first synthesized by Norskov et al. in an incipient wetness impregnation method with active metal loading of 9.7% Mo that exhibited good activity with an overpotential of 100–200 mV (Hinnemann et al. 2005). MoS2/rGO (reduced graphene oxide) hybrid was prepared by Dai et al. following one-step solvothermal reaction using (NH4)2MoS4 and hydrazine in an N, N-dimethylformamide (DMF) solution of mildly oxidized GO. The (NH4)2MoS4 precursor was reduced to MoS2 on GO, and the mildly oxidized GO transformed to rGO by hydrazine reduction. This catalyst exhibited a Tafel slope value of 41 mV/ decade (Li et al. 2011). Similarly, Wang et al. proposed a facile one-step solvothermal approach to produce a networked nanocomposite consisting of low crystalline MoS2 coated on carbon nanotubes (CNTs) (Yan et al. 2013). How the catalyst is affected by the number of layers in MoS2 was demonstrated by Cao et al. who prepared MoS2 thin films with defined layers on a glassy carbon by CVD process. The number of layers was controlled by adjusting the Mo precursor amount. MoS2 pyramid platelets were also grown under a similar process except by varying the pressure suitably. It was found that the catalytic activity, evaluated in terms of exchange current density, decreased by a factor of about 4.47 for the addition of each subsequent layer of MoS2 (Yu et al. 2014). Li et al. synthesized monodispersed molybdenum sulfide nanoparticles from bulk MoS2 through longtime ultrasonication, in DMF, followed by gradient centrifugation. The resulting nanoparticles exhibited small size and narrow distribution. The incubation of a clean Au electrode into this solution yielded a better catalytic performance over MoS2 drop-coated Au electrode (Wang et al. 2013c). Shaijumon et al. prepared a highly dispersed suspension of MoS2 quantum dot interspersed in few-layered MoS2 NSs (NSs) by liquid exfoliation of MoS2 flakes. This resulted in simple material extraction using liquid dispersion without significant aggregation of layers that could be scaled-up (Gopalakrishnan et al. 2014). By ball-milling of MoO3 and S microparticles at room temperature, followed by sulfurization at different temperatures, Wilkinson et al. prepared a nanostructured MoS2. Ball-milling provided nanosized powder with close contact, forming isolated microdomains for sulfurization during the annealing process. This led to quick sulfurization, limited inner oxide core, and diminished agglomeration of catalyst at low temperatures. An intrinsic improvement in the density of active sites is achieved

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through a higher ratio of perimeter-to-basal area. The catalysts exhibited larger BET surface area and higher density of active edge sites that resulted in higher catalytic activity (Wu et al. 2013). Besenbacher et al. synthesized highly active [Mo3S13]2 nanoclusters using a simple wet chemical process. Scanning tunneling microscopy showed seven bright protrusions arranged in a hexagonal pattern with one in the center representing the expected symmetry pertaining to the structure of a single [Mo3S13]2 cluster composed of three terminal S22, three bridging S22, and one central apical S. The as-prepared material showed improved TOF value over similar MoS2 catalysts, except UHV-MoS2 (3 s1 at η ¼ 0.2 V) (Kibsgaard et al. 2014). Though the performance of MoS2 is impressive, the material preparation requires sophisticated and/or energy-intensive preparation procedures. To overcome this difficulty in the material preparation, Hu et al. proposed MoS3/MoS2 preparation on a variety of substrates through electrodeposition. MoS3/MoS2 film, obtained by cycling in a 2.0 mM aqueous solution of (NH4)2MoS4, formed after five cycles and reached a plateau after 25 cycles. Film thickness and particle size can be controlled by modulating the deposition potential, time, and precursor concentration during electrodeposition. It also provides good electrical conductivity between the catalyst and the electrode substrate (Merki et al. 2011). Altering Electrical and Electronic Properties of MoS2 As indicated, the conductivity of MoS2 and exposure of more active edges are important to obtain enhanced catalytic activity. The conductivity has been increased by strategies such as converting the thermodynamically favored native 2H phase into a metastable 1T phase through Li intercalation. Unlike natural semiconductor MoS2 that has two SMoS layers built from edge-sharing MoS6 trigonal prisms, the metallic 1T polymorph consists of a single SMoS layer composed of edgesharing MoS6 octahedra (Lukowski et al. 2013). Cui et al. demonstrated that the layer spacing, oxidation state, and the ratio of 2H semiconducting to 1T metallic phase of MoS2 tuned by Li intercalation can correspondingly tune the catalytic activity of HER. The lower oxidation state of Mo and 1T metallic phase of MoS2 turns out to have better HER catalytic activities (Wang et al. 2013b). Jin et al. synthesized metallic 1T-MoS2 polymorph by exfoliating flowerlike MoS2 nanostructures that were prepared by CVD method. The relatively mild reaction conditions enabled high-density deposition of MoS2 nanostructures on a variety of substrates, including silicon/silicon oxide, glass, fluorine-doped tin oxide on glass, Mo foils, carbon paper, and graphite. The original trigonal-prismatic 2H-MoS2 was then converted into a metallic 1T-MoS2 polymorph by soaking them in n-butyllithium solution followed by exfoliation by reacting the intercalated lithium with excess water, which generated H2 gas and separated the 2D nanosheets (Lukowski et al. 2013). Similarly, Chhowalla et al. prepared metallic 1T-MoS2 from bulk MoS2 through lithium intercalation via a solvent-free method by reacting MoS2 powder with lithium borohydrate in a 1:2.5 ratio (Voiry et al. 2013a). Brinker et al. reported that Li-intercalated exfoliation to

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1T-MoS2 transformation rendered the normally inert basal plane to become active toward hydrogen adsorption by lowering the ΔGH from +1.6 eV in the 2H phase to +0.18 eV in the 1T phase which is comparable to 2H MoS2 edges on Au(111), one of the most active HER catalysts yet characterized. More importantly, DFT calculation indicated that when H coverage was >0.4, the stability of 1T phase would surpass that of 2H phase (Chou et al. 2015). Employing a different strategy from the Li-intercalation method, Xie et al. incorporated oxygen into MoS2 catalyst. Since molybdenum dioxide has a high electrical conductivity, the incorporation of oxygen into MoS2 was proposed to improve the intrinsic conductivity of the catalyst. The calculated density of states after oxygen incorporated into MoS2 slab exhibited much narrower bandgap of 1.30 eV with respect to that of the pristine 2H-MoS2 slab (1.75 eV), indicating more charge carriers and higher intrinsic conductivity. Further reduction in bandgap was observed with heavier oxygen incorporation. The result was attributed to the enhanced hybridization between Mo d-orbital and S p-orbital after oxygen incorporation. In addition, ΔGH on the edges of the oxygen-incorporated MoS2 exhibited smaller differential binding free energy than that of the pristine MoS2 indicating a lower overpotential would be required to drive the HER process in the former case. Thus simultaneous disorder engineering and oxygen incorporation would be an effective way to improve the catalytic activity of HER (Xie et al. 2013). Tuning the Fermi Level and Activating Basal Plane of MoS2 Tuning the Fermi level is also an effective way to improve the catalyst activity. Norskov et al. utilized first-principle calculations within density functional theory (DFT) to investigate the change in electronic and chemical properties of a singlelayer MoS2 adsorbed on Ir(111), Pd(111), or Ru(0001) that had minimal lattice mismatch with the MoS2 overlayer. The study revealed that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS2 layer, and the binding of hydrogen can be enhanced by 0.4 eV. A stronger H-S coupling enabled by the charge transfer from the substrate to the MoS2 overlayer and a stronger MoS2-metal interface by the hydrogen adsorption are attributed for the improved behavior. The results indicated that a suitable metal electrode surface could significantly affect the HER efficiency of the planar MoS2 overlayer (Chen et al. 2013b). In another study, the basal plane of monolayer 2H-MoS2 was activated by introducing sulfur (S) vacancies and strain. The S-vacancies acted as new effective catalytic sites in the basal plane, in which the gap states around the Fermi level allowed hydrogen to bind directly to exposed Mo atoms. The combination of strain and S-vacancies yielded optimal ΔGH ~ 0 eV (Li et al. 2016). Cobalt-Promoted MoS2 The addition of Co to WS2 and MoS2 often promotes catalytic activity in the hydrodesulfurization reaction (Topsøe et al. 1996). The addition of cobalt has been found to alter the morphology of MoS2 significantly. Cobalt-promoted MoS2 largely

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exists as truncated triangles exposing the S-edge (1110), while pristine MoS2 has a triangle form with less truncated dominant Mo-edge (1010) (Brorson et al. 2007; Lauritsen et al. 2007). The DFT studies carried out by Bonde et al. indicated that only the ΔGH values of S-edge are reduced to 0.10 eV for MoS2 (down from 0.18 eV), as cobalt incorporates itself into the S-edge on the MoS2. Thus, the MoS2 catalyst is expected to have an increased number of sites with high activity. Based on the DFT calculations, it is noted that the cobalt-promoted MoS2 (Co-Mo-S) should be a better catalyst, as Mo-edge (ΔGH ¼ 0.08 eV) is already active and the incorporation of Co onto S-edge (converted to more active) would result in creation of higher number of active sites at MoS2 (Bonde et al. 2009).

4.2.1.2

Molybdenum Diselenide (MoSe2)

Molybdenum diselenide has a similar structure to that of MoS2 in which the 2D layers are linked by van der Waals interaction (Wang et al. 2013a). Compared to MoS2, MoSe2 material is expected to be more electrically conducting due to the inherent conducting nature of Se, and the material has a narrower bandgap. Cui et al. demonstrated the electrocatalytic hydrogen evolution activity of mono- or few-layer MoSe2 (Kong et al. 2013b). Further, theoretical studies confirmed that the unsaturated Se-edges of MoSe2 are as active as S-edges in MoS2 and the Mo-edges were also found to be catalytically active as well, for the higher HER activity (Tsai et al. 2014) (Fig. 4.8). Theoretical calculations also revealed a lower ΔGH on MoSe2 edges, closer to thermoneutral, than that of MoS2 for the resulting higher coverage of hydrogen adsorption (Huang et al. 2015; Liu et al. 2016). Shu et al. performed systematic calculations to investigate HER activities and mechanisms on a series of MoSe2 catalytic structures from point defects to edges using density functional theory (DFT). It was shown that most of thermodynamically stable defects and edge configurations would have high HER activity, whereas the energetically unfavorable structures would exhibit a weak HER activity. This originates from the fact that the highly stable point defects, holes, and edges can moderately modify the electronic structures of MoSe2 facilitating the HER process, whereas the catalytic sites of the unstable MoSe2 structures have much stronger interaction with H atoms leading to weak HER activities. They also indicated that Volmer-Tafel pathway would be preferred on mono- and diselenide vacancies due to small space of Se vacancy (~1.7 Å) and Volmer-Heyrovsky mechanism is favored due to larger space (~3.3 Å) in the case of hole and edge structures (Shu et al. 2017). Catalyst morphology, size, and conductivity are the main factors affecting the catalytic performance of transition MoSe2. Therefore, different strategies were employed to tune them alone or in combinations. Some of the preparation methods and their effect on the catalytic activity are highlighted here. Wu et al. prepared a MoS2/MoSe2 NSs/graphene composite with a small overpotential of 70 mV (at 10 mA cm2), Tafel slope of 61 mV/decade, as well as excellent long-term durability (Xu et al. 2015). Similarly, perpendicularly oriented MoSe2/graphene hybrid nanostructure was grown on a graphite disk current collector. The resultant material maximized exposure of active edges to protons, facilitated

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Fig. 4.8 The structures and hydrogen adsorption free energies for MoSe2. Only the most stable edge configurations are shown. All values of ΔGH are shown for the final adsorbed hydrogen at the corresponding θH. Image reproduced with permission from (Tsai et al. 2014)

ions/electrolyte transport at the electrode interface, and minimized the agglomeration and restacking of NSs due to the perpendicular arrangement. It also improved electron transfer at the electrode interface owing to the presence of highly conductive perpendicular arrangement of GN. The catalyst exhibited a noticeable positive shift (0.1 V) in the onset potential and a lower overpotential of 0.159 V (at 10 mA/cm2), with a decreased Tafel slope (from 86 to 61 mV/decade) compared to bare MoSe2 NSs (Mao et al. 2015). Huang et al. prepared MoSe2 in 3D carbonized melamine foam in a solvothermal process using Se powder, dissolved in hydrazine, with a stoichiometric amount of Na2Mo4 in N,N-dimethylformamide. The enhanced HER catalytic performance, low-onset potential, and Tafel slope were attributed to interconnected 3D architecture that helps uniform deposition of MoSe2 NSs, leading to efficient and rapid ion and electron transport through the carbonaceous backbone (Huang et al. 2018). Tuning Electrical and Electronic Property Apart from the reduced sheet size and increased edge-exposed surface, the improved electron transfer between electrode and the active sites, induced by the phase transition from the natural semiconducting 2H phase to the metallic 1T phase, is crucial in enhancing the activity and decreasing the overpotential. From comparing only the exfoliated materials, it can be seen that WS2 and MoSe2 have outperformed MoS2 and WSe2 as they required a lower overpotential of about 0.35 V (vs. RHE) to generate 10 mA/cm2. WS2 experienced the most impressive

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improvement upon exfoliation, with an overpotential shift of about 500 mV. MoS2 resulted in an overpotential shift of about 300 mV. WS2 and MoSe2 exhibited largest percentage of T1 composition and consequently better performance (Ambrosi et al. 2015). Coleman et al. further compared activity of MoSe2, MoS2, MoTe2, WS2, WSe2, and WTe2 dispersions prepared by probe sonication in aqueous sodium cholate. Current density vs. overpotential plot of the dispersed TMDs and singlewalled CNTs (SWCNT) (10 wt%)/TMD composite films exhibited an interesting activity trend MoSe2 > WSe2 > WS2 > MoTe2 > WTe2 > MoS2 and MoSe2 > WSe2 > WS2 > MoS2 > MoTe2 > WTe2, respectively. MoSe2 was the best-performing catalyst in the group (Gholamvand et al. 2016). Shaijumon et al. exfoliated MoSe2 through electrochemical route in a 0.1 wt% aqueous lithium bis(trifluoromethane)sulfonimide electrolyte. The catalyst was attached to a working electrode through drop casting, self-assembly, or electrophoretic deposition. The acid-treated electrophoretically deposited MoSe2 demonstrated excellent electrochemical activity. Enhanced electrocatalytic activity is attributed to a combination of factors such as high edge-to-basal plane ratio, electrochemically active uncompensated chalcogenide edges, lithiation-induced new active sites, and enhanced conductivity due to coexisting MoO3-x (Damien et al. 2017). Yin et al. prepared 1T-MoSe2 NSs through a facile hydrothermal synthesis route using Na2MoO42H2O, Se, and NaBH4. Employing different molar ratios of NaBH4 and temperatures, the intrinsic catalytic activity, density of the active sites, and bulk conductivity were tuned. A synergistic modulation, that is, an increase in the degree of disorder, led to formation of more unsaturated Se active sites, while phase transformation from 2H to 1T enhanced the intrinsic catalytic activity (Yin et al. 2017). Deng et al. constructed a binder-free self-supported N-doped 1T-2H MoSe2 on vertical graphene (VG) composite arrays as an advanced electrocatalyst for HER. The N-MoSe2/VG nanoflakes exhibited a relatively low onset and overpotential with a Tafel slope of 49 mV/decade. The N-MoSe2/VG electrode also demonstrated enhanced long-term durability with no decay of its catalytic activity after 32 h (Deng et al. 2017). Liu et al. synthesized an edge-rich MoSe2/MoO2/Mo hybrid by CVD process. The as-prepared material exhibited an overpotential and onset potential of 142 mV (10 mA cm2) and 60 mV, respectively, with a small Tafel slope of 48.9 mV/decade. The enhanced activity was attributed to the increased active area and charge transport between the active MoSe2 and MoO2/Mo. The deposition of MoSe2 on the surface MoO2/Mo has decreased the contact resistance between the active catalyst and the conducting substrate (Jian et al. 2018).

4.2.1.3

Molybdenum Phosphide

Given that transition metal phosphides (TMPs) are interstitial alloys with good electrical conductivity and active for HDS reaction which relies on the reversible binding of hydrogen, molybdenum phosphate (MoP) is a suitable catalyst for HER process (Zhu et al. 2016). A variety of MoP catalyst have been prepared using

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different synthetic approaches. Both experimental and theoretical studies indicate that MoPs can efficiently catalyze HER. Xiao et al. performed DFT calculation to access the HER activity of Mo and MoP catalysts. The Mo-terminated surface on (001)-MoP has similar or even stronger H-binding than that of Mo, which excluded Mo as the active site. However, the investigation of P sites indicates a change in the ΔGH value from 0.36 eV to 0.54 eV, when H coverage increases from 1/4 ML to 1, implying that P could bond H at low coverage while desorbing H at high coverage. This behavior resembles the S-edges in MoS2 making it possible for P to behave like a “hydrogen deliverer” (Xiao et al. 2014). The direct synthesis of catalyst on carbon substrates such as carbon cloth or carbon paper is advantageous, as it avoids additional material casting step on working electrode surface. The addition of binder is needed to stably immobilize the catalyst on the electro surface leading to decreased active sites and at times affects diffusion of reactants and products. Sun et al. synthesized a binder-free MoP nanosheet (NS) on a carbon cloth (CC) in a two-step process. MoP2 NS/CC showed an overpotential of 58 mV (at 10 mA cm2) and a Tafel slope of 63.6 mV/decade suggesting a Volmer–Heyrovsky route with the Volmer step as the rate-determining step (Zhu et al. 2016). Similarly, Mu et al. prepared MoP on Mo foil (MoP2 NPs/Mo) in a two-step process, in which the Mo foil was anodized electrochemically followed by calcination of as-formed MoOx in the presence of sodium dihydrogen phosphate. The catalyst exhibited an overpotential of 143 mV (at 10 mA cm2) with a Tafel slope of 57 mV/decade (Pu et al. 2016). Chen et al. prepared MoP2 through a solidstate process using a mixture of molybdenum oxides and red phosphorus. The MoP2 NP-modified carbon paper fiber electrode exhibited an onset and overpotential of 38 mV and 121 mV (at 10 mA cm2) with a Tafel slope of 53 mV/decade. Despite lower conductivity than MoP, DFT studies in MoP2 indicated a slightly longer PH bond than MoP and lower energy barrier for transition state of hydrogen atom adsorption on the MoP2 slab of 0.93 eV than that of MoP (1.05 eV); these along with structural advantages such more diphosphorus active sites attributed for the improved catalytic activity over MoP (Wu et al. 2016).

4.2.1.4

Molybdenum Carbide and Boride

Similar to molybdenum dichalcogenides and phosphides, molybdenum carbide and boride were also demonstrated to have interesting HER property under electrochemical conditions. Metal borides can be alternative electrocatalysts because they are generally very stable both in acidic and basic solutions (Park et al. 2017)]. Hu et al. first demonstrated that molybdenum carbide and boride can exhibit excellent catalytic activities, in terms of onset potential, under acidic as well as basic conditions which is not very common for non-Pt-based catalysts. An activation process was observed for catalysts under these conditions, except for MoB at pH-14. They exhibited an onset potential of 100 mV and an overpotential between 210 and 240 mV (at 20 mA cm2). They also exhibited small Tafel slope values between 55 and 59 mV/decade at pH 0 and 14 (Vrubel and Hu 2012).

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Chen et al. prepared Mo2C from ammonium molybdate exploiting the support carbon material as the carbon source, instead of external gaseous carbon source, under an inert environment using a thermogravimetry-differential thermal analyzer (TG-DTA). This method avoided the usual drawback of char formation on the catalyst that blocks the pores and active sites in gas-phase synthesis. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy studies indicated a charge transfer from molybdenum to carbon on the carbon-supported Mo2C. The Mo-Mo bond length for obtained Mo2C/CNT and Mo2C/XC (2.98 and 3.29  A) was found to be different from that of pure Mo (2.73  A) and bulk Mo2C (2.97 and 3.04  A). The ratios of coordination number (NMo–C/NMo–Mo) were found to be higher than that of bulk. The electronic modification led to a relatively moderate Mo–H bond strength. An overpotential of 63 mV was observed to reach 1 mA cm2 of current density. The Mo2C/CNT and Mo2C/XC exhibited much smaller overpotential compared to Mo metal and bulk Mo2C, indicating a reduction in energy input for activating HER. The optimal Mo2C/CNT requires an overpotential of 152 mV to deliver 10 mA cm2 of current density. The enhancement in electrochemical activity is attributed to the unique effects of the anchored structure coupled with electronic modification (Chen et al. 2013a). Jothi et al. synthesized a nanocrystalline MoB2 in a single-step solid-state metathesis process by annealing a pellet made of high purity powders of anhydrous MoCl5 and MgB2 in a quartz tube under inert atmosphere. MoB2 exhibited an overpotential of 154 mV (at 10 mA cm2) and a Tafel slope of 49 mV/decade. DFT calculations showed several active sites in the nanocrystalline MoB2 with B-terminated MoB2 (100) surface being the most active. In addition, the B layer and the Mo/B mixed layers were found to be more active than the three Mo layers (Mo (110), Mo-terminated MoB2 (100), and (001) surfaces) indicating a strong dependence on B. Fokwa et al. synthesized four binary single-phase bulk molybdenum borides, namely, orthorhombic β-MoB, tetragonal Mo2B and α-MoB, and hexagonal MoB2 by arc melting. With increasing boron content, the B-B connectivity increased from 0D (isolated boron atoms in Mo2B) to 1D (zigzag boron chains in α-MoB and β-MoB) to 2D (graphene-like boron layers in MoB2). The catalyst exhibited a boron dependency for HER with activity trend of Mo2B < α-MoB < β-MoB < MoB2, and the Tafel slopes were found to be 128, 76, 84, and 75 mv/ decade, respectively (Park et al. 2017).

4.2.2

Tungsten-Based HER Catalysts

4.2.2.1

Tungsten Sulfide

Similar to MoS2, tungsten sulfide (WS2) is also widely considered for HER process due to similarity in the structural properties. WS2 has traditionally been used as heterogeneous catalyst in dehydrogenation and HDS reactions (Sobczynski et al. 1988, 1989; Seo et al. 2015; Brorson et al. 2007). The hydrogen evolution reaction

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on WS2 was studied long back by Tseung et al. (1985). DFT calculation for MoS2 indicates that HER predominantly occurs at the Mo-edge due to lower free energy of adsorption that exhibits a ΔGH value of 0.08 eV, while the S-edge has ΔGH value of 0.18 eV. In case of WS2, both the W-edge and S-edge are active; however, the ΔGH value was found to be 0.22 eV. Although, WS2 is expected to exhibit a lower activity compared to MoS2, co-catalyst promoted and carefully tuned WS2 can exhibit activity on par with MoS2. The addition of Co to WS2 improves the catalytic activity in dehydrogenation and HDS reaction through the formation of additional active sites (Sobczynski et al. 1988). DFT study indicated that only the ΔGH values of S-edge are reduced to 0.07 eV for co-catalyst containing WS2 (down from 0.22 eV), as cobalt attaches itself only into the S-edge on WS2, and thus new active sites are being created on WS2 (Bonde et al. 2009). WS2 predominantly forms closely stacked layers similar to graphite. WS2 that is typically prepared from WO3 precursor often exists in 0D or 1D structure, due to quasi-0D onion-like structures or nanotubes (NTs) of WO3, with decreased number of dangling bonds that is essential for catalytic activity (Wu et al. 2012) (Yang et al. 2013). Therefore, preparing two-dimensional nanosheets that would possess nanoscale lateral dimensions is critical to achieve a maximum ratio of edge-to-basal plane sites with either W- or S-edge termination (Choi et al. 2013). Wu et al. synthesized WS2 NSs by ball milling WO3 and S followed by sulfurization. An onset potential and overpotential of 60 mV and 150 mV (at 9.66 mA cm2) with a Tafel slope of 72 mV/decade were observed. The activity was found to be better than commercial MoS2 and WS2, revealing improved activity of ball-milled NSs. The improvement in activity was attributed to increase in exposed edge sites, especially improved edge-to-basal plane ratio, rather than to overall increase in BET surface area (Wu et al. 2012). Yang et al. synthesized WS2 NSs on rGO from tungsten chloride and thioacetamide precursors in a one-pot hydrothermal process. After annealing, the composite exhibited improved HER characteristics with ~150 mV and an overpotential of 300 mV (at 23 mA cm2, ~250 mV at 10 mA cm2) with a Tafel slope of 50 mV/ decade. Impedance study indicated a decreased resistance for the WS2/rGO nanocomposite, and the improved catalytic activity is attributed to enhanced charge-transfer characteristics between rGO and WS2 (Yang et al. 2013). Choi et al. synthesized uniform WS2 NSs by unzipping multi-walled WS2 NTs obtained through sulfidized WO3x. The unzipping was carried out by cup-horn sonication in 35% ethanol solution with ~20% yield. The method is more facile than Li intercalation due to nonhazardous nature and unaltered layer spacing and avoids additional material incorporation leading to direct activity measurement of obtained material. However, the method yields multilayered nanoflakes (Choi et al. 2013). Cheng et al. prepared ultrathin WS2 nanoflakes from WCl6 dissolved in oleylamine (OM)/1-decane and elemental sulfur dissolved in OM. The formation of ultrathin layer was confirmed from Raman analysis. An onset potential and overpotential of 100 mV and ~170 mV (at ~10 mA cm2) with a much lower Tafel slope of 48 mV/ decade were observed due to ultrathin morphology which resulted in more exposed edges. The importance of heat treatment to obtain improved activity was confirmed

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by subjecting the as-prepared material to HER process, while decreased HER activity is due to blocked active sites by surfactant molecules (Cheng et al. 2014). Joo et al. prepared a core-shell W18O49@WS2 nanorods (NRs) and WS2 NTs through partial and complete sulfidization process at an elevated temperature, respectively. X-ray adsorption and XPS characterization of W18O49@WS2 NRs and WS2 NTs indicated a higher oxidation state for W present on WO core, which exhibited a better catalytic performance compared to WS2 NTs. In addition, the catalysts were electrochemically treated to produce more defects and surface oxides that resulted in further improvement in activity (Seo et al. 2015). Pumera et al. deposited thin film of WS2.64 on a glassy carbon foam using cyclic voltammetry technique. The electrodeposition method represents one of the cleanest synthesis routes in terms of obtaining pure materials. It avoids the catalyst immobilization step that often requires the use of binder to support the catalyst on the electrode surface, and also it dispels the contribution from porosity and roughness in the observed catalytic activity. An overpotential of 494 mV (at 10 mA cm2) with a Tafel slope of 43.7 mV/decade was observed. The low Tafel slope value was attributed to adsorption and desorption of protons and hydrogen molecules based on ΔGH value and a slight difference in electron density of S22 and S2 sites, respectively (Tan and Pumera 2016). The effect of tuning the electrical conductivity on HER activity is well demonstrated for Mo-based HER catalysts. Li intercalation was found to convert 2H-MoS2 into 1T metallic phase. Voiry et al. prepared a stable strained 1T-WS2 using Liintercalation-assisted exfoliation of 2H-WS2. The STEM-HAADF study indicated a zigzag pattern and a large concentration of locally strained bonds, with two distinct W-W distances of 2.7 Å and 3.3 Å that are substantially different from value of 3.15 Å observed for pristine 2H-WS2. The exfoliated catalyst exhibited an onset potential of 80–100 mV with a Tafel slope of 60 mV/decade. The thermal treatment of monolayered WS2 thin film revealed a strong link of strain and 1T phase on the catalytic activity, as increasing temperature converted the 1T phase into 2H phase. The first-principle calculation indicated an energy barrier of 0.87 eV for the conversion of 1T into 2H. It was also demonstrated through DFT studies that ΔGH could come closer to thermoneutral when the strain was varied between 2 and 3% for the 1T phase, with no such change being observed for 2H phase, to indicate the key role of 1T phase (Voiry et al. 2013b).

4.2.2.2

Tungsten Phosphide

WP is a known HDS catalyst. The reversible binding and dissociation of H2 represent a possible mechanistic shared by both HDS and HER catalysts. However, WP has not been explored much, and therefore, studying the electrochemical HER activity of WP exhibits some interesting results. The activity is related to P which possesses lone pair electrons in 3p orbitals and vacant 3d orbitals, can induce local charge density, and accommodate the surface charge state (Yan et al. 2015). Sun et al. prepared WP NR arrays directly on carbon cloth (CC) through a two-step process in which WO3 NR arrays on CC (WO3 NAs/CC) were first

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grown hydrothermally, followed by phosphidation of WO3 NAs/CC. The material exhibited an onset and overpotential of 50 mV and 130 mV (at 10 mA cm2), with Tafel slope of 69 mV/decade in 0.5 M H2SO4. Interestingly, it exhibited activity under neutral as well as alkaline conditions with overpotentials of 200 mV and 150 mV (at 10 mA cm2), with Tafel slopes of 125 and 102 mV/decade, respectively (Pu et al. 2014a). Similarly, they also prepared WP2 submicroparticles (SMPs) through a temperature-programmed hydrogen reduction of air-calcined precursor obtained from (NH4)6H2W12O40xH2O, (NH4)2HPO4, and citric acid with mole ratio of 1:2:2. The presence of citric acid led to increased BET surface area. The material exhibited an onset and overpotential of potential of 54 mV and 161 mV (at 10 mA cm2), with Tafel slope of 57 mV/decade in 0.5 M H2SO4. Under neutral and alkaline conditions, overpotentials of 143 mV and 153 mV (at 2 and 10 mA cm2), with Tafel slopes of 92 and 60 mV/decade, respectively, were observed. Based on the XPS results that exhibited a slightly lower BE value for P compared to elemental P and a higher BE value for W compared to metallic W, a partial positive (δ+) and negative (δ) charge for W and P are assigned, and small transfer of electron density from W to P in WP occurred. Both hydrogenases and metal complex-based HER catalysts incorporate proton relay from pendant acid-base groups close to the metal center where hydrogen evolution occurs. Catalysts like CoP and MoP also feature pendant base P (δ) in close proximity to the metal center M (δ+). Thus a similar mechanism is invoked for the observed HER activities for WP catalysts (Xing et al. 2014). McEnaney et al. prepared amorphous WP nanoparticles by heating W(CO)6 and TOP in squalane. The catalyst had an overpotential of 120 mV (at 10 mA cm2) (McEnaney et al. 2014). Du et al. prepared a crystalline WP2 NR by sulfidizing (NH4)0.25WO3. Similar to other WP-based catalysts, this also exhibited pH-independent HER activity. In this study the phosphidation temperature was found to play a crucial role as lower temperature yielded incomplete phosphidation, while higher temperature predominantly produced monophosphite. The temperature also affected the morphology of the material. The higher metal oxidation state of W in WP2 that likely promotes charge distribution of P ligand leading to the proton to bind significantly weaker, compared to WP, along with enough P site to work as proton acceptor has been attributed to efficient activity of WP2 (Du et al. 2015).

4.2.2.3

Tungsten Nitride and Carbide

Tungsten nitride WN (W2N) has been used as HDS catalyst owing to their Pt-like characteristics. The similar electronic structure of W with Mo has also supported this speculation. Theoretical calculations predict that the hydrogen is inclined to adsorb on the metal nitride surface, making the reaction facile (Yan et al. 2015). Li et al. prepared a N-doped carbon-coated tungsten oxynitride NW array by calcinating WO3 nanoarray dip coated in homogenous ink containing dicyanodiamine and PVP in DMF along with additional amount of dicyanodiamine under Ar. The catalyst was active independent of pH. Improved conductivity, higher utilization of active area, diffusion of electrolyte and product facilitated by 3D arrangement, change in

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chemical inertness and electronic structure of C by N in NC, and the role of O in effectively regulating the electronic structure and improving the conductivity of catalyst were ascribed for the observed performance of the catalyst (Li et al. 2015a). In tungsten carbide (WC), the filling of the interstitial lattice positions of W by carbon atoms at the Fermi level of tungsten by carbon intercalation gives rise to “Ptlike” d-band electronic density states and shows Pt-like behavior in surface catalysis owing to their unique electronic structure (Shen et al. 2016a). They have been studied as efficient electrocatalysts in methanol oxidation and oxygen reduction. They are also electrochemically active for HER under acidic conditions but suffer from poor stability resulting from corrosion, especially at neutral and higher pH (Du et al. 2015). The incorporation of N into metal carbides was found to resist corrosion and modify the catalytic activity; however, a nitrogen-rich surface can be detrimental to HER activity (Chen et al. 2014). Chen et al. synthesized tungsten carbide–nitride on graphene nanoplatelets (GnP) by carburization of GnP with ammonium tungstate and aniline. The results indicated that the α-W2C is catalytically more active than δ-WC. Downshifting of the d-band center of tungsten in the presence of nitride compared to W2C is due to higher electronegativity of nitrogen than carbon. This downshifting of d-band center of the metal decreases the hydrogen binding energy and in turn favors electrochemical desorption of Hads, which is considered the origin of enhanced activity (Chen et al. 2014). Fan et al. prepared WC nanocrystals on a vertically aligned (VA) CNT by atomic hydrogen treatment on a sputter-coated W (50–150 nm) @ VA-CNT in a H2, H2O, and CH4 mixture at different temperatures. The higher current density observed for the WC-1050 sample was attributed to the high-quality carbide phase which has Pt-like electronic structure in combination with high surface area of the material (Fan et al. 2015). Shen et al. prepared WC nanocrystals intimately riveted to graphite felt fabrics on carbon NSs. The in situ carburization of phosphotungstic acid (PTA) loaded GF fabrics in the presence of methane transformed PTA into WC. XPS analysis revealed slightly more positive binding energy for W that was attributed to the charge transfer from tungsten to carbon atoms, due to strong coupling between tungsten carbide and carbon NSs. The catalyst was found to be active in acidic and basic conditions. The coupling interaction of C–W that could modify the electronic structure of tungsten and in turn the bonding strength of W–H and higher BET area were important for the observed activity (Shen et al. 2016a).

4.2.3

Cobalt-Based HER Catalysts

4.2.3.1

Cobalt Chalcogenides and Borides

Similar to molybdenum and tungsten, a variety of Co-based catalysts have been synthesized and tested, albeit in a lesser number. Co-embedded nitrogen-rich CNT (carbon nitride) was found to demonstrate a pH-independent HER activity (Zou et al. 2014). The incorporation of Co into MoS2 and WS2 was found to improve the HER

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process considerably by decreasing ΔGH value in both the cases. The Co was found to incorporate into the S-edges. Taking cue from this, Sun et al. electrochemically deposited CoS on FTO electrodes and annealed under Ar. The catalyst exhibited onset and overpotential of 43 and 397 mV (at 50 mA cm2) with a Tafel slope of 93 mV/decade, at neutral pH. It demonstrated a good activity for HER in seawater containing 1 M NaClO4 with an onset potential of 600 mV vs. NHE. Faber et al. prepared CoS2 micro- and nanowires via precipitation and sulfurization. The catalysts required an onset potential of 75 mV along with an overpotential of 158 and 145 mV (at 10 mA cm2) with a Tafel slope of 58 and 51.6 mV/decade for MW and NW, respectively, in 0.5 M H2SO4. The MW structure exhibited an improved stability due to facilitated release of bubbles, while increased surface lowered the overpotential (Faber et al. 2014). Similarly Se-based Mo and W-based catalyst showed comparably good activity, and the intrinsic conducting nature of Se improved the conductivity of these catalysts. Kong et al. compared the activities of a variety of metal sulfide and selenide catalysts (Kong et al. 2013a). They also prepared CoSe2 NPs on high surface area carbon fiber paper through selenization of CoO NPs. The catalyst obtained exhibited a metallic conducting nature (Kong et al. 2014) (Fig. 4.9). Cobalt boride was also found to be an active catalyst for HER over a range of pH. Gupta et al. prepared CoB using a chemical reduction method which was active in neutral pH. Both the theoretical calculation and experimental results showed an electron transfer from B to Co, thus enriching the d-band electron density of Co. This improves the electron-donating ability of CoB and enhances HER activity. A plot of pH vs. current density showed that pH 9.2 would be more suitable for HER using CoB (Gupta et al. 2015). Masa et al. prepared Co2B in a similar chemical reduction cum annealing process. Further incorporating this catalyst into nitrogen-doped graphene enhanced HER activity drastically due to improvement in charge-transfer process provided by the highly conducting nitrogen-graphene (Masa et al. 2016).

Fig. 4.9 Crystal structure of CoSe2 in cubic pyrite-type phase (left) and orthorhombic macarsitetype phase (right), in which Co and Se are displayed in orange and yellow, respectively. Image reproduced with permission from (Kong et al. 2014)

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Cobalt Phosphide

Cobalt phosphide (CoP) is another known HDS catalyst involved in reversible binding of hydrogen. Theoretical calculations such as DFT have been successfully applied in recent times to identify potential catalysts for a given process. This descriptor-based approach, using ΔGH, has been used to describe the trends in HER activity for transition metal alloys, TMD edge sites, and transition metal carbides. Jaramillo et al. calculated the steady-state hydrogen coverage on the most likely surface terminations of various transition metal phosphide catalysts using the Bravais-Friedel-Donnay-Harker crystal morphology algorithm (based only on geometry). The calculation showed that CoP binds hydrogen only slightly strongly and lies close to 0 eV (Kibsgaard et al. 2015). A wide range of CoP-based catalyst with varying Co to P ratio has been prepared, and partially charged natures (δ+/) of Co and P, similar to those of the hydride acceptor and proton acceptor in [Ni-Fe] hydrogenase, have been attributed to the enhanced activity observed for majority of the cases. Even low-temperature synthesized or electrodeposited catalyst exhibited this charge transfer, observed from XPS characterization, between Co and P. The CoP character was found be important for decreasing the onset and overpotentials. Preparation of porous materials with high surface area and decreased electrical and mass transfer resistance showed better activity for CoP-based materials (Fig. 4.10). Popczun et al. prepared CoP quasi-spherical nanoparticles that exhibited an overpotential of 95 mV (at 20 mA cm2) with a Tafel slope value of 50 mV/decade, in 0.5 M H2SO4 (Popczun et al. 2014). Sun et al. decorated CNT with CoP NCs. The hybrid exhibited an onset and overpotential of 40 mV and 122 mV (at 10 mA cm2) with a Tafel slope of 54 mV/decade in 0.5 M H2SO4. The XPS survey spectrum indicated binding energy (BE) corresponding to Co 2p3/2 at 779.1 eV, positively shifted from that of Co metal (778.1–778.2 eV), along with the P 2p3/2 BE of

Fig. 4.10 Crystal structures of (left) Co2Si-type Co2P and (right) MnP-type CoP. Unit cells are shown as dashed black lines. Image reproduced with permission from (Callejas et al. 2015)

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129.8 eV negatively shifted from that of elemental P (130.2 eV) suggesting a partial positive charge (δ+) for Co and negative charge (δ) for P, implying a transfer of electron density from Co to P resulting in covalent character. The hydrogenases use pendant bases that are proximate to the metal centers as active sites, and the CoP nanocrystal also features a similar pendant basic phosphorus center (δ) in close proximity to the cobalt metal center (δ+), where Co acts as the active center (Liu et al. 2014b). Similarly, Co2P NR prepared by solvothermal method and CoP NS arrays prepared by phosphidation of electrochemically precipitated α-Co(OH)2 on a Ti plate that exhibited a partial positive charge (Coδ+, 0 < δ > 2) and partial negative charge P (Pδ, 1 < δ > 0) (Huang et al. 2014a; Pu et al. 2014b). Du et al. fabricated CoP NTs via an anodic aluminum oxide (AAO) templateassisted synthesis by soaking AAO into cobalt chloride ethanol solution followed by vacuum drying and etching of AAO in dilute HF followed by phosphidation. The CoP NT exhibit an overpotential of 129 mV (at 10 mA cm2) and a Tafel slope of 60 mV/decade, in 0.5 M H2SO4 (Du et al. 2014). Jiang et al. prepared CoP nanostructures with different morphologies. Interestingly, the CoP NWs demonstrated an onset potential of 40 mV and an overpotential of 110 mV (at 10 mA cm2) and a Tafel slope of 54 mV/decade, in 0.5 M H2SO4. The increased surface area, lower resistance, and stacking of NW into a 3D arrangement that facilitates the bubble convection away from electrode surface along with the hydride- and proton-donor characteristics were attributed for the improved activity (Jiang et al. 2014). Callejas et al. compared the activity of Co2P with morphologically similar CoP catalyst. Co2P adopts the structure of Co2Si with edge-sharing CoP4 tetrahedra and CoP5 pentahedra resulting in nine-coordinate P atoms, while CoP adopts the MnP structure type consisting of face-sharing CoP6 octahedra and edge-sharing PCo trigonal prisms. The catalysts casted on Ti foil exhibited an overpotential of 75 and 95 mV (at 10 mA cm2) with Tafel slope of 50 and 45 mV/decade, respectively, for CoP- and Co2P-modified electrodes. The results indicated that the bulk structure and bonding may not significantly affect HER activity; both CoP and Co2P are intrinsically active HER catalysts irrespective of structure and composition, and both crystalline and amorphous catalyst can be quite active. However, the Co/P ratio may be crucial, as CoP has more Co-P character than Co2P which is identified through DFT calculation to exhibit a partially charged surface giving rise to better HER activity. A HER trend of increasing activity and stability with corresponding increase in metal-phosphorus bonding is indicated (Co < Co2P < CoP) (Callejas et al. 2015). Liu et al. prepared a porous CoP hollow polyhedron as a dual HER and OER catalyst. To obtain a porous morphology, a metal-organic framework (MOF, Co-ZIF 67) was utilized to obtain the final catalyst. The improved conductivity, active surface area, and lower oxide content over CoP NPs assisted the overall improvement in performance of the catalyst (Liu and Li 2016). Wang et al. deposited CoP2 NP on reduced graphene oxide sheets (rGO) by sonicating CoCl2 and rGO mixture followed by phosphidation of grounded freezedried materials in the presence of NaH2PO2. The XPS observation indicated a partial positive charge (δ+) and negative charge (δ), implying the transfer of electron

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density from Co to P as observed for CoP nanoparticles. The smaller size of NPs that exposes more active sites and improved conductivity of rGO provided a better activity for CoP2/rGO composite over CoP2 NPS. As a lower Co to P ratio yields a better activity, CoP2 is expected to provide a good activity (Wang et al. 2016). Zhou et al. prepared an interconnected urchin-like CoP microspheres in a two-step hydrothermal cum phosphidation process. The catalyst was active for HER under both acidic and alkaline conditions with an overpotential of 45 and 60 mV (at 10 mA cm2) and a Tafel slope of 49.3 and 49.1 mV/decade, in 0.5 M H2SO4 and 1 M KOH, respectively. This outperformed most of the CoP catalyst in terms of overpotential required to obtain a bench mark current density of 10 mA/cm2 (Zhou et al. 2016a). Similarly, Yang et al. prepared a 3D urchin-like CoP NCs in two-step process of hydrothermal synthesis (Yang et al. 2015a). Tian et al. improved the HER performance by hydrogenating the CoP catalyst. Hydrogenation was shown to improve the performances of photocatalysts, Li-ion batteries, super capacitors, etc. The hydrogenated catalyst showed structural changes for the CoxP nanoparticles on a nanometer scale, i.e., crystalline/disordered coreshell structures. The catalyst demonstrated an onset and overpotential of 20 and 110 mV (at 10 mA cm2) and a Tafel slope of 51 mV/decade, in 0.5 M H2SO4. The hydrogen treatment of catalyst decreased the catalyst resistance and energy barrier along with increased surface area (Tian et al. 2017). Advantages of self-supported catalysts include high stability, room for improving the architectural properties for facile charge carrier transfer and gas transport, avoiding catalyst immobilization step, and improved electrical conductivity. Therefore it is desirable to directly prepare self-supported catalyst on conducting platforms. Saadi et al. electrochemically deposited CoP on a Cu disk from a mixture of boric acid, NaCl, NaH2PO2, and CoCl2 at 1.2 V vs. saturated calomel electrode. The electrodeposited cobalt phosphide film characterized by XPS showed a significantly lower BE value than either the Co2+ or Co3+ species revealing the covalent nature of the cobaltphosphorus bond. The deposited CoP catalyst exhibited an overpotential of 85 mV (at 10 mA cm2), with a Tafel slope of 50 mV/decade, in 0.5 M H2SO4 (Saadi et al. 2014). Yang et al. assembled CoP NS on CC, in which Co was electrochemically deposited on CC and exposed to air to form oxide layer for subsequent phosphidation using NaH2PO2. The CoP NS/CC catalyst exhibited an overpotential of 49 mV (at 10 mA cm2) and a small Tafel slope of 30.1 mV/decade, in 0.5 M H2SO4. The electrochemical method yielded a high double-layer capacitance and high density of active sites (Yang et al. 2015b). Yang et al. prepared a porous CoP thin film on sputter-coated Au/Cr glass slide by electrodepositing Co metal followed by electrochemical anodization and phosphorous deposition using CVD. The anodization process created a porous Co-oxide film in a controlled manner, which is usually difficult to obtain due to water oxidation that prevents porous oxide formation (Yang et al. 2015d). Zhu et al. prepared a self-supported CoP mesoporous NR array on a porous Ni foam by electrodeposition from an electrolyte solution (pH ¼ 5) containing 25 mM CoCl2 and 0.5 M NaH2PO2) at potential of 0.8 V vs Ag/AgCl. In 1 M KOH, the CoP NR-modified electrode showed an onset potential close to 0 V and overpotential of 54 mV (at 10 mA cm2). The Tafel slope

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of 51 mV/decade was lower than that of Pt. It also exhibited considerably good activity in both acidic and neutral pHs. The direct deposition of catalyst on a conducting porous Ni foam has improved the conductivity and mass transport and facilitated faster release of bubbles (Zhu et al. 2015). Tian et al. prepared CoP nanowire (NW) arrays on CC exhibited an onset and overpotential of 38 mV and 67 mV (at 10 mA cm2), with a Tafel slope of 51 mV/decade, respectively. The intimate mechanical contact, inherent conductivity of CoP along with porous structure, and improved diffusion of electrolyte are attributed for the enhanced activity apart from the hydride- and proton-acceptor behavior that facilitates the HER (Tian et al. 2014a).

4.2.4

Nickel-Based HER Catalysts

Nickel finds application in processes such as catalysis, batteries. It has been used as a HER catalyst for a long time now, though it exhibits low efficiency The advent of nanostructured morphology of Ni has increased the electrochemical active surface area tremendously that in turn enhanced the HER activity. A wide range of Ni-based catalysts have been developed including layered Ni double hydroxide, Ni(OH)2incorporated Pt, Li-intercalated Ni-FeO, Nio/Ni, Ni-Mo, Ni-Fe-Mo, and Ni-Fe that efficiently catalyze HER under alkaline condition (Brown et al. 1982, 1984; Raj and Vasu 1990; Subbaraman et al. 2011; Nocera 2012; Gong et al. 2016). In the case of Ni(OH)2-incorporated Pt, H2O interacts with Ni(OH)2 through O, while H interacts with Pt surface leaving the H atom on a nearby empty Pt site, and the OH desorbs from Ni(OH)2 sites that outperformed Pt under alkaline condition. Though Ni demonstrates good catalytic activity under alkaline condition, it suffers corrosion under acidic condition. The incorporation of nitrogen into catalyst not only altered the Fermi level but also substantially increased the stability of the catalyst under acidic condition (Chen et al. 2012). The development of nickel chalcogenides, carbide, boride, and phosphide showed excellent catalytic activity, and some of them demonstrated improved stability under acidic condition.

4.2.4.1

Nickel Chalcogenides and Borides

Han et al. first demonstrated HER activity of electrodeposited Ni-S. The calculated activation energy, at 80  C for hydrogen adsorption, was found to be 27.5 kJ mol1 at low current density that was much lower than bulk Ni, Ni-Fe, and Ni-Mo-Cd catalysts. At a higher sulfur content, a mixture of Ni and Ni3S2 crystals was observed (Han et al. 2003). Feng et al. prepared a high-index faceted Ni3S2 (hkl; 210) NS array on Ni foam (NF) that is catalytically favorable owing to surface atomic structures (more atomic steps), although not very easy to stabilize because of higher surface energy. The NF was directly sulfurized in an autoclave to yield Ni3S2/NF. The catalyst exhibited an overpotential of 223 and 170 mV (at 10 mA cm2) in alkaline

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and neutral pH, respectively, for a catalyst loading of 1.6 mg cm2. However, it performed poorly under acidic condition. Compared to (001) surface where Ni was found to have a lower ΔGH value than S, the (210) surface possessed both active Ni and S sites as majority of the active sites were present at the steep edges resulting in under-coordination and sterically less hindered sites (Feng et al. 2015). Chung et al. later studied HER activity of NiS and Ni3S2 under acidic as well as alkaline conditions. The DFT calculation indicated negative adsorption values (eV) for both NiS and Ni3S2, and with NiS the value was found to be larger that led to stronger adsorption of H. Following a Volmer pathway, stronger adsorption resulted in better activity, and it was concluded that atomic configuration strongly affects the adsorption intensity and activity (Chung et al. 2015). Zhou et al. prepared a porous NiSe2 through direct selenization of Ni foam (NF) pretreated in acetic acid and polyvinylpyrrolidone. The catalyst required an overpotential of only 57 mV (at 10 mA cm2) with a Tafel slope of 43 mV/decade. Higher electrochemical active surface area obtained through pretreatment and the presence of Se monomer and dimer adsorbed on the surface which decreases the free energy of adsorption (ΔGH) to nearly ~0.1 eV have resulted in the enhanced activity (Zhou et al. 2017). Zeng et al. prepared a Ni-B-modified electrode through electroless deposition process. The catalyst exhibited an overpotential of 132 mV (at 20 mA cm2) with a Tafel slope of 112 mV/decade, in 1 M HClO4. It also showed good activity under both neutral and alkaline condition. The XPS study showed a positive shift in BE about 0.7 eV for the elemental boron in NiB, indicating a partial electron donation from B to Ni. This donation of electron to Ni prevented the oxidative corrosion of Ni under acidic condition and provided a superior stability. The formation of small particles exhibited better activity, however, with decreased stability (Zeng et al. 2016).

4.2.4.2

Nickel Phosphide

Nickel phosphide (Ni2P) is another known HDS catalyst. The supported Ni2P exhibits an extremely high HDS activity due to the complex role of P (Oyama 2003; Sawhill et al. 2003; Layman and Bussell 2004). Owing to the presence of P atom, Ni2P surface exhibits proton-acceptor (P sites have a negative charge of 0.07 e) and hydride-acceptor centers (Ni site), similar to hydrogenase, which exhibit a negatively charged proton accepting nonmetal site and a highly coordinated and isolated hydride-acceptor metal site that provides moderate bonding to hydrogen (Liu and Rodriguez 2005) (Fig. 4.11). The formation of Ni-P bonds results in a weak “ligand effect” leading to stabilization of Ni 3d levels and a Ni to P charge transfer. The number of active Ni sites decreases due to an “ensemble effect” of P. This causes weaker adsorption of H and makes addition of second atomic H exothermic on Ni2P, unlike pure Ni metal hollow sites that adsorb H strongly. The theoretical calculations show that Ni-P bridge site is the preferential site for H adsorption on Ni (001) surface after occupying Ni hollow sites. Once the Ni hollow sites have been occupied, the

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Fig. 4.11 Optimized structures for each step in a catalytic cycle for the HER on a H-poisoned Ni2P (001) surface (white, H; blue, Ni; purple, P). Image reproduced with permission from (Liu and Rodriguez 2005)

additional adsorbate has to interact with the less active Ni-P bridge site, and the removal of H2 from Ni2P(001) becomes less energy-consuming (ΔErls ~ 0.45 eV). Therefore, Ni2P (001) is considered to be the best possible synthetic catalyst that can mimic coordination or electron structures of active sites of hydrogenases for HER that match or replace Pt. Popczun et al. synthesized hollow and multifaceted Fe2P-type Ni2P NPs in a solvothermal process by heating nickel(II) acetylacetonate in 1-octadecene, oleylamine, and tri-n-octylphosphine (TOP). The Ni2P/Ti catalyst, prepared by annealing, needed an overpotential of 130 mV (at 20 mA cm2) with a Tafel slope of 81 mV/decade, in 0.5 M H2SO4 (Popczun et al. 2013). Similarly, Huang and Pu et al. deposited nickel phosphide (Ni12P5) and Ni2P NPs film on Ti substrate, respectively. XPS results indicated a partial positive and negative charge on Ni and P, revealing the presence of proton- and hydride-acceptor sites (Huang et al. 2014b); (Pu et al. 2014c). Laursen et al. prepared micron-sized Ni5P4 catalyst in a solvothermal process. The catalyst exhibited an overpotential of 23 mV and 49 mV (at 10 mA cm2) with a Tafel slope of 33 and 98 mV/decade, in 1 M H2SO4 and 1 M NaOH, respectively. Its activity was better than that of Pt under acidic condition. It exhibited an exceptional operational stability with no phase transformation. The NiP bond length was used as a descriptor, as electron transfer to the bridging proton at NiP bond is rate determining. The calculations indicated an increase in Ni-P bond length, while the

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Ni-Ni bond length at trigonal Ni site decreases, leading to an increased overlap of the filled Ni valence orbitals and hydrogen 1s orbital. Therefore, favorable electron transfer occurred due to increased covalent nature and accelerated the reaction at higher population of this intermediate. This in turn increases the second proton affinity and accelerates the overall reaction rate compared to Ni2P (Laursen et al. 2015). Similarly, Wang et al. prepared flowerlike Ni5P4 microballs on a Ti foil following a hydrothermal cum phosphidation method. XPS results indicated a partial positive and negative charge on Ni and P. The theoretical DFT calculation indicated, ΔGH value of 0.196 and 0.219 eV for P site of Ni5P4 and Ni2P, an energetically unfavorable interaction of P with hydrogen. However, the Ni site of Ni5P4 exhibited ΔGH value of 0.152 eV corresponding to a relatively higher interaction between Ni sites of Ni5P4 with hydrogen. The high exposure of (001) phase, improved conductivity, and good adhesion were attributed to enhanced activity of the catalyst (Wang et al. 2017). The preparation of porous high surface area catalyst can improve the catalyst-electrolyte interface. Ni-P catalysts with different morphology such as peapod-like Ni2P NPs, 3D hierarchically porous Ni-P polyhedron from MOF-74Ni precursor and Ni-P nanowires yielded high electrochemical active surface area with more exposed edges; better dispersion resulted in improved activity (Bai et al. 2015; Yan et al. 2017). Chung et al. prepared a short- and long-Ni-P NWs along with irregular NPs. A comparative study of these materials indicated a higher activity for long NWs over other materials; however, they all exhibited a similar Tafel slope value of 70 mv/decade. XPS and electron microscopy results indicated the formation of P-rich Ni2P with Ni3P_P-terminated edges that rapidly react with oxygen to form oxidized-P passivation layer. The calculations indicated that the tri-Ni (Nitri) would be the conceivable site for H adsorption [ΔGH values (0.41 eV), Ni-Pbridge (0.23 eV) and Ptop (0.42 eV)]. They indicated that the catalytically favored Ni-Pbridge is less stable compared to Nitri, and therefore, the actual site under electrochemical operation condition would be Nitri (Chung et al. 2016). Thus, efforts must be made to prepare Ni-P surfaces with exposure of Nitri to enhance the HER catalytic activity, although DFT calculation predicted Ni-Pbridge to be the most active site.

4.2.5

Iron-Based HER Catalysts

Iron is one of the most abundant transition metals available whose cost is much cheaper than even Ni or Co (Callejas et al. 2014). Fe-containing hydrogenase is an effective HER biocatalyst with turnover frequencies as high as 9000 moles of H2 per mole of hydrogenase per second in water at pH 7 and 30  C (Shima et al. 2008; Frey 2002). However, this enzyme is less stable in the presence of oxygen. Cubane-type Fe-S clusters can be used to produce H2 from weak organic acids; however, it is less stable in aqueous systems in the presence of oxygen (Di Giovanni et al. 2014). It is therefore possible to prepare different Fe-based systems that can be employed for

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large-scale applications in an affordable way. Iron phosphides is predominantly used as HER catalyst due to its improved performance. Xu et al. first reported the preparation of FeP NSs from Fe18S25-TETAH (protonated triethylenetetramine) NSs (an inorganic-organic hybrid). Diffusionmediated anion exchange between trioctylphosphine (TOP) and Fe18S25-TETAH transforms the hybrid into FeP NSs. The resultant product exhibited macro-size morphology with nanoporous properties under FTIR analysis (Xu et al. 2013). Liu et al. synthesized FeP NR arrays on a HCl-treated Ti foil (FeP NAs/Ti) via low-temperature phosphidation of a FeO(OH) NAs/Ti precursor. Stability and HER activity of the fabricated nanoarrays are due to good mechanical and electrical connection between FeP nanoarrays and HCl-treated Ti foil, inherent electrical conductivity of FeP facilitating fast electron transport and facile electrolyte diffusion through 3D porous configuration that effectuates superior active site utilization. A positive shift in BE from Fe metal to Fe 2p (706.9 eV–707.2 eV) and a negative shift from elemental P to P 2p (130.2 eV–129.3 eV) suggest that both Fe and P of FeP, respectively, have partially positive and partially negative charges, aiding electron transfer from Fe to P (Liu et al. 2014c). Schaak et al. synthesized FeP NPs that are active as effective electro- and photocatalysts in acidic and pH aqueous solutions for sustained hydrogen production. These nanoparticles at a current density of 10 mA cm2 exhibited overpotential of 50 mV and Tafel slope of 37 mV/decade in 0.5 M H2SO4 and 102 mV in 1 M phosphate-buffered saline. FeP nanoparticles deposited on TiO2 produced hydrogen at rates that approach Pt/TiO2. The advanced activity of FeP nanoparticles could be attributed to the increased surface area due to the smaller size of the particles (Callejas et al. 2014). FeP nanoparticles deposited on graphene sheets and carbon cloth that exhibited low-onset potentials of 30 and 19 mV with Tafel slope of 50 and 32 mV/decade. Electronic coupling between catalyst and carbon substrate, rapid electron transfer from the FeP to electrodes, and abundance of exposed catalytic sites were attributed to the observed activity (Zhang et al. 2014; Tian et al. 2014b). Similarly, binder-free FeP NR arrays on CC synthesized by Liu et al. also perform efficiently in neutral and alkaline pH (Liang et al. 2014). Sun et al. developed a nitrogen-doped carbon nanotube-supported FeP nanocomposites (FeP/NCNT) under low-temperature phosphidation of Fe2O3/NCNT precursor. The Fe and P act as the hydride-acceptor and proton-acceptor center, respectively, facilitating the HER (Liu et al. 2014a) (Table 4.1).

4.3

Conclusion

The advent of metal dichalcogenides, phosphides, nitrides, and carbides has substantially addressed the quest to find a suitable non-noble electrocatalyst for efficient hydrogen evolution process at lower overpotential. Though some of the material preparation methods require demanding conditions like ultrahigh vacuum, waterfree solvent system, and inert conditions to obtain catalysts with the expected

0.5 M H2SO4

0.5 M H2SO4

0.5 M H2SO4/ 0.1 M KOH PBS, pH-7

CoSe2/carbon fiber paper

CoP urchins

CoP2/reduced graphene oxide CoB

CoS2 nanowire

70

38 (KOH)

50

–

~75

–

105

–

127

70/88

137

145

106/152/130

4.9  1.4

15.1

–

–

0.5 M H2SO4/1 M PBS/1 M KOH 0.5 M H2SO4

65/68

75.8/53.7

–

1 M H2SO4 /KOH

WC@carbon nanosheets/ graphite felt fabrics WN

148

0.5 M H2SO4

WP2

100 –

0.5 M H2SO4

WS2/graphene

6

58

56

0.5 M H2SO4

MoC

830

–

102

119

0.1 M KOH

MoP nanosheets

360.2

–

100

–

0.5 M H2SO4

MoSe2

0.25

–

Current density (10 mA cm2) vs. RHE 78

78

0.5 M H2SO4

MoS2-1T

Exchange current density (μA cm2) vs. RHE –

Onset potential (mV) vs. RHE 50

179

Electrolyte 0.5 M H2SO4

Catalyst MoS2

Table 4.1 Comparison of catalytic performance of some of the non-noble HER electrocatalysts

92

50 (KOH)

46

40

51.6

65/128/128

61/72

52

43

41

63.6

53

47

Tafel slope (mV dec1) 53 Reference Zhang et al. (2015) Wang et al. (2013b) Qu et al. (2016) Zhu et al. (2016) Ma et al. (2015b) Zhou et al. (2016b) Du et al. (2015) Shen et al. (2016a) Li et al. (2015a) Faber et al. (2014) Kong et al. (2014) Yang et al. (2015a) Wang et al. (2016) Gupta et al. (2015)

110 N. Thiyagarajan et al.

Rugae-like FeP nanocrystals

FeP nanowires

0.5 M H2SO4/1 M KOH 0.5 M H2SO4

1 M HClO4/PBS, pH-7/1 M KOH 1 M H2SO4/KOH

NiB

Ni2P

0.5 M H2SO4

Porous NiSe2

– 680

–

200

– 50/100

612

–

34

45/221

132(HClO4)/194 (KOH) 140/250 (20 mA/cm2)

57

29.2

53/134

87/100

53(HClO4)

43

Zhou et al. (2017) Delidovich et al. (2016) Feng et al. (2014) Lv et al. (2016) Yang et al. (2015c)

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compositions; overall improvement in the activity justifies such preparation conditions. Other simple preparation methods such as electrodeposition, low-temperature hydrothermal, and chemical reduction of some of the materials highlight the versatility of available preparation methods. In addition, the as-synthesized catalysts could be modified into photocatalysts using relatively facile adaptations. The theoretical calculations supported by appropriate material characterization have improved the overall understanding of catalytic origination and ways to tune them to enhance their activity. The descriptor-based screening of potential catalyst using free energy of adsorption has assisted identification of several new catalysts. Incorporation of additives such as metals C, N, and B has increased the conductivity and improved the stability substantially. In case of metal dichalcogenides, higher edge-to-basal plane ratio, lower electrical resistance between electrode and catalyst, and tuning electronic properties of the catalyst were found to be highly useful in improving the activity of catalyst. The creation of defects on the basal planes of pre-synthesized catalyst showed a substantial increase in the HER activity. Catalysts composed of metal phosphides, carbides, and borides showed dependence on the catalyst composition; decreased contact resistance, higher surface area of catalysts, and electrode architecture-like 3D materials with porous structures demonstrated better activity. The formation of compact porous structure facilitates the proton transport and efficiently releases H2 bubbles that result in achieving higher current densities at lower overpotentials and exhibited exceptional stability. The demonstration of HER activity over a wide pH range and its ability to work under seawater to produce H2 clearly demonstrate the efficiency of some of the catalyst systems. The myriad of catalysts and the understanding reached from the studies would assist in designing more efficient catalysts in the future to meet the demand. Further development in material preparation and novel bimetal or alloys with lower cost can be expected to improve activity and make production of hydrogen more sustainable.

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Chapter 5

Energy-Saving Synthesis of Mg2SiO4:RE3+ Nanophosphors for Solid-State Lighting Applications Ramachandra Naik, Ramyakrishna Pothu, Prashantha S. C, Nagabhushana H, Aditya Saran, Harisekhar Mitta, and Rajender Boddula

Contents 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1 Nanophosphors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2 Magnesium Silicate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3 Rare Earth Ions Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.4 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.5 Combustion Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.6 Photoluminescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.7 Potential Applications of Photoluminescent Nanophosphors . . . . . . . . . . . . . . . . . . . .

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R. Naik Department of Physics, New Horizon College of Engineering, Bengaluru, India R. Pothu College of Chemistry and Chemical Engineering, Hunan University, Changsha, China Prashantha S. C (*) Research Center, Department of Science, East West Institute of Technology, Bengaluru, India e-mail: [email protected] Nagabhushana H (*) Prof. CNR Rao Center for Advanced Materials, Tumkur University, Tumkur, India e-mail: [email protected] A. Saran Department of Microbiology, Marwadi University, Rajkot, Gujarat, India H. Mitta State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India R. Boddula (*) CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing, China e-mail: [email protected]; [email protected] © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_5

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5.2 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.1 Stoichiometric Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.2 Synthesis of Undoped Mg2SiO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2.3 Synthesis of Rare Earth-Doped Mg2SiO4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.1 Formation of an Mg2SiO4 Nanophase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 Effect of Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.3 Energy Saving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.4 Mg2SiO4:RE3+ Photoluminescence Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Magnesium silicate (Mg2SiO4) doped with rare earth (RE3+) ions can be prepared using different methods. The combustion method is the most widely used technique because it saves time and energy compared with conventional solid-state reactions. Preparation of nanophosphors via the combustion method can be carried out using different fuels such as urea, oxalyldihydrazide (ODH), diformylhydrazine, and plant extracts. In this study, Mg2SiO4:RE3+ nanophosphors are prepared using the combustion method with ODH fuel, which is an energy-saving synthesis because the products are formed at a low temperature (350
C). Photoluminescence analysis is carried out with the prepared nanophosphors for solid-state lighting applications.

5.1 5.1.1

Introduction Nanophosphors

The word phosphor means “light bearer” in Greek; it appears in Greek myths in reference to the personification of the morning superstar Venus (Cao 2004). Phosphors are prepared by introducing an activator into a host material; the activator acts as a luminescent center, and the host can be any compound. Usually, lanthanide ions are used to act as the luminescent centers. Lanthanide ions cause discrete energy levels within the host; they are the center from which luminescence emanates. The energy states are situated within the band gap, such that the electrons are de-excited from the higher to the lower states radiatively. The phosphors may be in the form of either powder or a thin film. The phosphor substances are doped intentionally with exact impurities to obtain the favored wavelength. These phosphor powders and thin films are extremely important in the development and improvement of display technologies. Smaller-sized particles are essential for high-resolution images. Hence, there is a need for the construction and development of phosphor nanoparticles (NPs) with enhanced emission intensities. Phosphor particles that are of submicrometric size, with narrow particle distribution and spherical morphology, provide greater particle packing densities than commercial products (3–5 μm) and are therefore effective in the enhancement of luminescence efficiency (Noto 2011). Phosphors with enhanced properties are required for the development of new types of high-efficiency and high-resolution displays. Monodispersed crystalline fine

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Fig. 5.1 Schematic diagram of the comparison of energy levels in different materials

particles of high-efficiency phosphor materials are the key to the development of these devices. Phosphors must have narrow size distribution, fine size, spherical morphology, and non-aggregation to exhibit good luminescent characteristics (Rao 1994). Nanosized phosphors exhibit fascinating properties, such as extremely fast recombination time, an increase in the band gap because of the smaller particle size, and high quantum efficiency for photoluminescence (PL) (Hagenmuller 1992). Owing to their potential quantum confinement effect and low dimensionality, inorganic nanocrystals exhibit fascinating size- and shape-dependent properties. In general, the intrinsic properties of nanoscale materials are determined by their composition, structure, crystallinity, size, and morphology. Over the past few years, the synthesis of inorganic nanoscale materials with specific morphologies has been the focus of extensive studies in materials science. In particular, the development of nanostructured luminescent materials has also made a very positive contribution to systematic fundamental studies of synthesis and possible new applications (Fendler and Meldrum 1995; Lakshmi et al. 1997; Sun and Xia 2002; Cushing et al. 2004; Fernández-García et al. 2004; Wang et al. 2009). Figure 5.1 represents the schematic diagram of the comparison of energy levels in different materials, such as bulk, quantum wells, wires, and dots (NPs). It can be seen from the diagram that energy levels of bulk material are parabolic, a step model for quantum wells, a spike model for quantum wires, and discrete energy levels for quantum dots. Figure 5.2 shows a schematic diagram of the comparison of the bandgap of bulk and quantum dots. It is observed that bulk material has a lower band gap value and that energy levels are continuous. Nanoparticles have larger band gaps and more discrete energy levels owing to the various quantum confinement effects within it, which can be as small as only a few dozen atoms wide (a couple of nanometers). Quantum confinement is defined as confinement of the movement of the particles in one or more dimensions. When these dimensions are comparable with the de Broglie wavelength of the particle, quantum confinement effects take place and cause the band gap of the particle to increase.

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Fig. 5.2 Schematic diagram of the comparison of the bandgap of bulk and quantum dots (nanoparticles)

5.1.2

Magnesium Silicate

The silicate family is an attractive class of materials among inorganic phosphors and is used for a wide range of applications owing to its special properties, such as water and chemical resistance, and visible light transparency. In particular, inorganic nanophosphors with the incorporation of trivalent rare earth (RE) cations reveal major luminescence effects. Further, various vacancies and defects present in the host matrix result in different luminescence features (Prashantha et al. 2011). Enhanced electrical, luminescent, and optical properties of nanophosphors are caused by the quantum size effect, which is generated by an increase in the bandgap due to a decrease in the quantum allowed state and the high surface-to-volume ratio (Cho et al. 2010). Among the silicate family, the Mg2SiO4 (forsterite) host doped with RE ions exhibits some interesting applications, such as long-lasting phosphor, X-ray imaging, light-emitting display, and environmental monitoring. Forsterite, an important material in the magnesia–silica system and a member of the olivine family of crystals, has an orthorhombic crystalline structure in which Mg2 + occupies two non-equivalent octahedral sites: one (M1) with inversion symmetry (CI) and the other (M2) with mirror symmetry (CS). The material has some essential properties, such as a high melting point, chemical stability even at high temperatures, vast electrical and refractory characteristics, in addition to good mechanical

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properties, bioactivity, and biocompatibility (Mostafavi et al. 2013). Therefore, it has commercial applications in many industrial areas, e.g., electronics as insulators working at high frequencies (Sanosh et al. 2010), the refractory industry (Bos 2001), advanced technologies such as solid oxide fuel cells (Kosanović et al. 2005), biomedicine (Kharaziha and Fathi 2009), and luminescent technology (Lin et al. 2008).

5.1.3

Rare Earth Ions Doping

Rare earth (RE) ions have attracted much attention owing to the distinct electronic and optical characteristics arising from their 4f electrons. When RE ions are introduced into a suitable host, an efficient luminescent phosphor with a high quantum yield, narrow bandwidth, large Stokes shifts, converting unusable UV to useful visible light, and ligand-dependent luminescence sensitization (Wong et al. 2013; Krishna et al. 2014). In particular, the RE-doped luminescent materials are particularly attractive because of the increasing interest of the lighting industry, and as a result, the materials are quite useful in the solid state lighting (SSL) applications (Dorman et al. 2012; Han et al. 2012; Krishna et al. 2013). From an application point of view, it is obvious that a size reduction of NPs can significantly enhance the surface phenomena. For this reason, the study of optical properties of RE-doped nanophosphors synthesized at low temperatures compared with their bulk counterparts has encouraged this research more intensely. Highly luminescent nanophosphors with greater brightness can be prepared with the addition of alkali metal ions such as Li+ as co-dopants. Alkali metal ions remove the charge imbalance problem, increase the crystallinity, and enhance the emission intensity of nanophosphors because of having low oxidation states and distinct ionic radii (Su et al. 2008; Balakrishnaiah et al. 2011). This technique is employed in several systems, such as ZnB2O4:Eu3+, CaWO4:Eu3+, YBO3:Eu3+, YVO4:Eu3+, GdVO4:Yb,HO, and GdVO4:Yb,Er, to achieve impressive luminescence properties (Su et al. 2008; Yang et al. 2010; Balakrishnaiah et al. 2011; Mu et al. 2011; Chen et al. 2012; Gavrilović et al. 2015).

5.1.4

Synthesis

The efficiency of a nanophosphor depends on the host material and on improved synthetic routes. Pure and doped Mg2SiO4 NPs are synthesized via different chemical routes. Forsterite is preferably prepared using solution-based methods to obtain high chemical homogeneity and small crystallite size compared with the conventional solid-state reaction, which needs higher calcination temperatures to obtain phase-pure crystals. However, the synthesis of the pure and doped nanocrystalline forsterite with controlled particle size remains challenging. Therefore, many

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alternative synthesis techniques have been reported for the synthesis of forsterite, including the citrate–nitrate method (Gavrilović et al. 2014), the molten salt approach (Saberi et al. 2009), combined mechanical activation (Sun et al. 2009), the polymer precursor method (Tavangarian and Emadi 2010), flame spray pyrolysis (Martin et al. 1992), mechano-thermal synthesis (Tani et al. 2007), combustion synthesis (Kharaziha and Fathi 2009), mechano-chemical synthesis (Fathi and Kharaziha 2008), and sol–gel techniques (Fathi and Kharaziha 2009; Mostafavi et al. 2013). Forsterite can be synthesized at high temperatures for an extensive time period via a solid-state reaction process. This process generally produces powders with large grain sizes, and the final product is not homogeneous and has an unwanted MgSiO3 phase (Saberi et al. 2007). Therefore, synthesizing single-phase Mg2SiO4 NPs without an MgSiO3 and MgO (periclase) phase remains a challenging task for material scientists (Lin et al. 2006).

5.1.5

Combustion Synthesis

Combustion synthesis is also known as self-propagating high-temperature synthesis. To generate combustion, an oxidizer, a fuel, and the right temperature are required. The process makes use of highly exothermic redox chemical reactions between an oxidizer and a fuel. A redox reaction involves simultaneous oxidation and reduction processes. The term combustion covers flaming (gas-phase), smoldering (solid-gas), and explosive reactions. The preparation of forsterite via solid-state reactions usually requires a high temperature and long reaction time, whereas the solution combustion process with the proper selection of fuel is a rapid and precise process for achieving nano-sized compounds. It also reduces the crystallization temperature and prevents phase segregation during heating because it is not only implemented in minutes at a relatively low temperature, it also allows molecular-level mixing, a high degree of homogeneity, and uniform doping of trace amounts of RE and transition metal ions in a single step (Kosanović et al. 2005).

5.1.6

Photoluminescence

Luminescence was first observed in an extract of Ligrium nephiticiem by Monardes in 1565, but Sir G.G. Stokes in 1852 fully described the theoretical basis for the mechanism of absorption (excitation) and emission. Today, luminescence, in its various forms, is one of the fastest growing and most useful analytical techniques in science. Applications can be found in areas as diverse as materials science, environmental science, microelectronics, physics, chemistry, biology, biochemistry, medicine, toxicology, pharmaceuticals, and clinical chemistry. This rapid growth has only occurred in the past couple of decades and has been principally driven by the

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unique needs of the life sciences. The tremendous interdisciplinary appeal of luminescence techniques has resulted in a growing number of researchers desiring to quickly employ new and emerging luminescence techniques without the timeconsuming effort of becoming an expert in physical spectroscopy. Luminescence provides some of the most sensitive and specific analytical techniques, with the possible exception of radioactive labeling procedures. The advantages of emission techniques include high sensitivity, good selectivity, qualitative environmental information, a large linear quantitative range, and multidimensional information. The general term luminescence includes a wide variety of light-emitting processes that derive their names from the various sources of energy that power them. Photoluminescence, which includes fluorescence and phosphorescence, is one of many categories of luminescence. To illustrate the diversity of luminescence emissions, the following are some of the more commonly observed types of luminescence: (i) Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or a strong electric field. Electroluminescence is the result of the radiative recombination of electrons and holes in a material. Example: A gas-discharge lamp. (ii) Radioluminescence is the phenomenon by which light is produced in a material by bombardment with ionizing radiation such as beta particles. Example: A luminous radium watch; a mixture of radium and copperdoped zinc sulfide is used to paint the instrument's dials, giving a greenish glow. (iii) Triboluminescence is an optical phenomenon in which light is generated through the breaking of chemical bonds of a material when it is pulled apart, ripped, scratched, crushed, or rubbed. The word tribo is derived from the Greek language meaning “to rub.” Examples: A diamond may begin to glow while being rubbed (Mostafavi et al. 2013), certain types of sugar crystals. (iv) Sonoluminescence is the emission of short bursts of light from imploding bubbles in a liquid when excited by sound. (v) Chemiluminescence is the generation of electromagnetic radiation as light by the release of energy from a chemical reaction. Although the light can, in principle, be emitted in the ultraviolet, visible or infrared regions, those emitting visible light are the most common. It is the breaking of the chemical bonds that supplies the energy. (vi) Bioluminescence is the production and emission of light by a living organism. Its name is a hybrid word, originating from the Greek bios meaning “living” and the Latin lumen meaning “light.” Bioluminescence is a naturally occurring form of chemiluminescence. Fireflies, anglerfish, and other creatures produce the chemicals luciferin (a pigment) and luciferase (an enzyme). The luciferin reacts with oxygen to create light. The luciferase acts as a catalyst to

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speed up the reaction, which is sometimes mediated by cofactors such as calcium ions or adenosine triphosphate (ATP) (Naik et al. 2014a). Example: Light emitted by fireflies and glow-worms. (vii) Cathodoluminescence (ChL) is an optical and electromagnetic phenomenon in which electrons that have an impact on a luminescent material, such as a phosphor, cause the emission of photons that may have wavelengths in the visible spectrum. A familiar example is the generation of light by an electron beam scanning the phosphor-coated inner surface of the screen of a television that uses a cathode ray tube. ChL is the inverse of the photoelectric effect, in which electron emission is induced by irradiation with photons. (viii) Photoluminescence (PL) derives energy from the absorption of light energy (most commonly within the wavelength ranges of infrared, ultraviolet, or visible light). Photoluminescence is further divided into the categories of fluorescence, delayed fluorescence, and phosphorescence. Today, they are defined via the emission-based quantum mechanical mechanism for the orbital angular momentum multiplicity of the emitted electron (i.e., the singlet or triplet excited state). However, before the advent of quantum theory, PL was defined solely on the basis of empirical evaluation of the duration of an emission lifetime. A photoluminescent emission arises from the singlet electronic state. To the human eye, fluorescence is observed only when the exciting light source shines on the radiator. Phosphorescence Phosphorescence is defined as a photoluminescent process that originates from the triplet electronic state. Emissions from the triplet state are 10 to 10,000 times longer than those from fluorescence; therefore, to the naked eye, these radiators appear to emit after the excitation radiation is removed. Delayed Fluorescence Delayed fluorescence is a rare phenomenon whereby the electron responsible for the emission starts out in the singlet state, crosses over to the triplet state, but eventually returns to the singlet state before emission. The result is a singlet state emission with a much longer lifetime than normal. Photoluminescence is the emission of light that follows the absorption of photons by nanomaterials. Generally speaking, it is possible to distinguish between two forms of PL, namely fluorescence (spin-allowed emission of light from an electronically excited state) and phosphorescence (spin-forbidden emission of light from an electronically excited state). These two emission mechanisms can be schematically illustrated by the Jablonski diagram (Fig. 5.3), in which the excitation and the relaxation pathways are shown. Luminescent materials can be divided into different families, according to their chemical nature (Kirkwood 2005). Organic and coordination compound-based dyes are among the most popular classes of luminescent compounds. More recently, semiconductor NPs (quantum dots) and lanthanide-doped NPs have received great attention because of their remarkable luminescent properties.

5 Energy-Saving Synthesis of Mg2SiO4:RE3+ Nanophosphors for Solid-. . . Fig. 5.3 Simplified Jablonski diagram for generic luminescent materials, showing the possible pathways of excitation and relaxation

S3

S  Singlet state T  Triplet state A  absorpon P  Phosphorescence emission F  Fluorescence emission ic  internal conversion isc  inter system Crossing

E ic

S2

129

S1 isc

A

F

T1

P

S0 Ground Energy Level

Transition metal complexes constitute a fascinating class of phosphors because of their unique optical properties and because of the possibility of tuning such properties by making (small) changes in the ligands that are involved in the coordination of the metal ion (Lakowicz 2006).

5.1.7

Potential Applications of Photoluminescent Nanophosphors

Nanophosphor particles hold promise in the application of solid-state lighting devices. Since the blue light-emitting diode (LED) based on GaN was invented, it has become a promising SSL device that has great potential in lighting applications owing to great lighting efficiency and a low consumption of electrical energy (Wang et al. 2006). In particular, white light can be generated by combining a blue LED with yellow phosphor materials, which convert part of the blue LED emission. For lighting applications, white light-emitting diodes (WLEDs) can be generated with the aid of converting phosphor layers onto the surface of an LED. This principle is based on the absorption and re-emission of light (Campagna et al. 2007; Nakamura and Fasol 1997; Muthu et al. 2002; Muthu and Gaines 2003). Phosphor materials are deposited on top of an LED chip to convert part of the blue light in such a manner that via additive color mixing, white light is generated. Further, with the excitation of blue light, the phosphor layers can emit yellow light. The un-absorbed blue light, mixed with yellow light, can result in white light. Currently, phosphor-coating approaches are commonly used because of the lower cost and this yields a higher efficiency and much better light-distribution characteristics compared with a combination of separate LED chips emitting blue, green, and red light. Similarly, UV-LED chips lined with red, green, and blue phosphor layers may also generate

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white light. For white light, the coating on blue LEDs is more popular because of the greater energy conversion efficiency. Consequently, in an effort to achieve enhanced efficiency, yellow or green and red phosphors with strong blue absorption are needed. It is known that the efficiency of both WLEDs and UV-LEDs is mainly dependent on the efficiency of the phosphors. Color rendering of the phosphors is also important to obtain a spectrum close to sunlight and extremely efficient lightemitting phosphors are important components of WLEDs (Sasaki and Talbot 1999; Damilano et al. 2001). The following lists the few important applications of nanophosphors (Sato et al. 1996): • • • • • • • •

Digital data storage chips Good insulators Inexpensive and high-quality display systems Better cutting tools and future weapons Removal of pollution High-power magnets and batteries Variety of sensors and satellites Automobiles and aerospace components with better performance characteristics

Recently, the development of nanophosphors for WLEDs (Yokota et al. 2001; Park et al. 2017) and fingerprint detection has increased the number of applications in a variety of areas by enhancing contrast and increasing selectivity (Devakumar et al. 2017). The detection of latent fingerprints using nanophosphors with a wide range of articles of evidence represents a new model in criminalistics that originated in 1976 (Zhang et al. 2017). In forensic investigations, chemical substances are often used to make latent fingerprints visible using a conventional powdering technique. The powder-dusting technique involves superior adherence of particles to fingerprint ridges to provide contrast between the fingerprint profile and the background surface. To detect latent fingerprints, physical properties such as adhesion are very important, because selective adsorption of nanophosphors to the fingerprint ridges is vital. Also, an optical property of the prepared nanophosphors plays an important role in exhibiting their emission color when they are excited under UV light. The Judd–Ofelt theory introduced by B.R. Judd and G.S. Ofelt in 1962 to study the quantum efficiency of nanophosphors based on the intensities of 4f–4f transitions has become a focus in optical spectroscopy. The theory proves and supports applications of RE-doped materials in solid-state lasers, optical amplifiers, and phosphors for displays, solid-state lighting, upconversion, quantum cutting materials, and fluorescent markers (Champod et al. 2004).

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5.2 5.2.1

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Experiments Stoichiometric Calculations

The stoichiometry of the redox mixture for combustion is calculated based on the total oxidizing (O) and reducing (F) valencies of the oxidizer and the fuel, keeping O/F ratio unity, using the concepts of propellant chemistry with Eq. 5.1 (Seah et al. 2005). P Oxidizer all oxidizing and reducing elements in oxidizer P ¼ Fuel ð1Þ oxidizing and reducing elements in fuel The total oxidizing valency of Mg (NO3)2.6H2O 2 Mg ¼ +2 12 H ¼ + 12 12 O ¼ 24 2N¼ 0 Total ¼ 10

5.2.2

ð5:1Þ

The total reducing valency of oxalyldihydrazide (C2H6N4O2) 2C¼ +8 6H¼ +6 2O¼ 2 4N¼ 0 Total ¼ +12

Synthesis of Undoped Mg2SiO4

Undoped Mg2SiO4 is prepared via the low-temperature (350
C) solution combustion route. Stoichiometric amounts of magnesium nitrate (5.1282 g) and fumed silica (0.6008 g) are mixed with laboratory-prepared oxalyldihydrazide (ODH; 2.362 g) fuel, dissolved in a minimal quantity of double distilled water in a cylindrical Pyrex dish and mixed thoroughly using a magnetic stirrer for about 5 min. A Pyrex dish containing this solution is placed in a pre-heated muffle furnace maintained at 350  10
C. The solution boils, resulting in a transparent gel. The gel then forms white foam, which expands to fill the vessel. Thereafter, the reaction is initiated somewhere in the interior; a flame appears on the surface of the foam and proceeds rapidly throughout the entire volume, leaving a white powder with an extremely porous structure. The energy released from the reaction produces a temperature greater than 1200
C. The reaction is self-propagating and able to sustain a high temperature. The final product is ground into a fine powder using an agate pestle and mortar. The complete procedure is demonstrated in Fig. 5.4. A theoretical equation assuming complete combustion of the redox mixture used for the synthesis of Mg2SiO4 can be written as:

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Oxalyl dihydrazide (ODH)

Redox mixture

Rapid heating in a muffle furnace heated at 350 0C

Final product

Mg2SiO4 Fig. 5.4 Flowchart showing the preparation of Mg2SiO4 using the combustion method

2 Mg½NO3 2 þ SiO2 þ 2C2 H6 N4 O2 ½ODH ! Mg2 SiO4 þ 6 N2 þ 6H2 O þ 4CO2

5.2.3

ð5:2Þ

Synthesis of Rare Earth-Doped Mg2SiO4

Eu3+ (1–11 mol %)-doped Mg2SiO4 nanophosphors are prepared via the low-temperature (350
C) solution combustion route. Suitable amounts of magnesium nitrate (Mg (NO3)2.6H2O [Sigma Aldrich] and fumed silica (SiO2 [Sigma Aldrich] are mixed in stoichiometric amounts with laboratory-prepared ODH (C2H6N4O2) fuel and dissolved in a minimal quantity of doubled distilled water in a cylindrical Pyrex dish and mixed thoroughly using a magnetic stirrer for about 5 min. Eu3+ (1–11 mol %) dopant is added in the form of nitrate Eu(NO3)2:4H2O to the above combination. Details of the stoichiometric calculations of the redox mixtures (oxidizer and fuel) are provided (Venkatachari et al. 1995; Hehlen et al. 2013). The Pyrex dish containing this solution is placed in a pre-heated muffle furnace maintained at 350  10
C. Combustion takes place and the final products are obtained, as explained in Fig. 5.4. The same experimental procedure has been repeated for other compositions Eu3+ (3, 5, 7, 9, and 11 mol %) and for the remaining dopants (Sm, Tb, Dy, Cr, Fe).

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Fig. 5.5 Shimadzu X-ray diffractometer (PXRD7000)

5.3 5.3.1

Results and Discussion Formation of an Mg2SiO4 Nanophase

Powder X-ray diffraction (PXRD) is the most fundamental characterization technique for the structural identification of materials. It gives us important information such as the crystallinity of the material, lattice parameter, phase, average crystallite size, etc. In the present work, PXRD is carried out with a Shimadzu X-ray diffractometer; PXRD-7000 (Fig. 5.5) using Cu Kα radiation (λ ¼ 1.541 Å) operating at 50 kV and 20 mA. Data were collected with a counting rate of 2
per min. X-ray diffraction patterns were recorded from 20
to 80
for ground powder samples. Formation of undoped and doped Mg2SiO4 is confirmed using PXRD patterns, as shown in our reports (Prashantha et al. 2011; Sunitha et al. 2012a, b; Naik et al. 2014a, b, 2015, 2016). The as-formed sample prepared by low-temperature combustion synthesis using ODH as fuel shows a pure orthorhombic phase without any post-calcinations. This is attributed to the effect of the fuel and it confirms that fuel plays an important role in combustion synthesis. All the X-ray diffraction peaks of the samples at (0 3 1), (1 3 1), (2 1 1), (2 2 1), and (1 4 0), etc., were indexed and well-matched with JCPDS card No. 78–1371 with space group pbnm (No. 62) (Naik et al. 2017).

5.3.2

Effect of Doping

The effect of doping different dopants in the Mg2SiO4 host has been studied in detail. Doping Eu3+ (1–11 mol %) ions shifts the diffraction peaks to the lower angle side, which indicates that Eu3+ ions are strongly capped into the crystal lattice of

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Mg2SiO4. The Eu3+ ions doped into the Mg2SiO4 matrix cause expansion of the unit cell, resulting in tensile stress; as a result, the PXRD peaks shift toward the lower angle side (Naik et al. 2014a). The introduction of an activator (Eu3+) does not influence the crystal structure of the host matrix, but certainly modifies the lattice parameters owing to the difference in the ionic radius between the dopant and the substituted magnesium ion. Doping Sm3+ (1–11 mol %) ions in the Mg2SiO4 host have the result that full width at half maximum (FWHM) increases and its intensity decreases. Further, Sm3+ ion doping does not change the lattice structure significantly, but does modify the lattice parameters because of the difference in the ionic radius between the dopant and the substituted magnesium ions (Naik et al. 2015). Doping Tb3+, Dy3+ (1–11 mol %) ions in the Mg2SiO4 host have the result that, as the dopant concentration increases, the position of the main diffraction peaks shifts to the lower angle side and the PXRD intensity decreases, which indicates that Tb3+ ions are strongly capped into the crystal lattice of Mg2SiO4. The Tb3+ ions doped into the Mg2SiO4 matrix cause expansion of the unit cell, resulting in tensile stress. Consequently, the PXRD peaks shift toward the lower angle side (Naik et al. 2014b, 2016). The peak shift and line broadening in PXRD profiles arise because of the presence of microstrain in nanophosphors.

5.3.3

Energy Saving

To prepare Mg2SiO4 NPs, different methods are followed. One of the most efficient methods is the solution combustion method. Further, the solution combustion method involves preparation of NPs using various fuels, such as urea and ODH; Mg2SiO4 NPs prepared using urea require a high furnace temperature of nearly 500
C and a calcination temperature of greater than 800
C (Prashantha et al. 2011), whereas the same sample prepared by the same method, but using ODH fuel, needs a low furnace temperature of nearly 350
C and no calcination. The nanophase is formed without calcination for the as-prepared sample using ODH fuel. Therefore, energy can be saved if the material is prepared using ODH fuel with the combustion method (Naik et al. 2014a). The mechanism of applications of NPs prepared using the combustion method is shown in Fig. 5.6.

5.3.4

Mg2SiO4:RE3+ Photoluminescence Properties

Oxalyldihydrazide is used as a fuel to prepare undoped and doped (Eu3+ [1–11 mol %], Sm3+ [1–11 mol %], Tb3+ [1–11 mol%], Dy3+ [1–11 mol%]) Mg2SiO4 phosphors. Mg2SiO4:Eu3+ phosphor exhibits different emissions (within the range 550 nm to 750 nm) because Eu3+ corresponds to 5D0--7Fj (j ¼ 0,1,2,3,4) transitions upon excitation of 393-nm. The comparative PL Mg2SiO4:Eu3+ phosphors with and

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Fig. 5.6 Mechanism of applications of NPs prepared using the combustion method

without Li+ doping proved that Li+ ion doping enhances the emission intensity of the phosphors, with good crystallinity and charge compensation. Li+-doped red emitting phosphors are checked for latent fingerprint detection sensors on different surfaces. Therefore, the prepared Mg2SiO4:Eu3+ phosphor could be an ultimate choice for solid-state lighting and fingerprint detection. Mg2SiO4:Sm3+ phosphor exhibits emission spectra ascribed to 4G5/2- 6HJ (J ¼ 5/ 2, 7/2, 9/2, 11/2) transitions of the Sm3+ ion; the most intense emission of Sm3+ has been recorded for the transition 4G5/2–6H7/2. The estimated International Commis sion on Illumination (CIE) chromaticity coordinates (0.588, 0.386) are very close to the National Television System Committee standard value of red emission of this phosphor and correlated color temperature has been found to be 1756 K. Therefore, the Mg2SiO4:Sm3+ phosphors are promising materials in the red region for WLEDs and optical display system applications. The emission spectra of Mg2SiO4:Tb3+ are related to 5D4–7FJ (J ¼ 6, 5, 4, 3) transitions of the Tb3+ ion; the most intense emission of Tb3+ is registered for the transition (5D4–7F5) at 541 nm. The excellent green emission properties and the estimated CIE chromaticity coordinates suggest that the Mg2SiO4:Tb3+ phosphors might be promising materials in the green region for optical display system applications. PL spectra of Mg2SiO4:Dy3+ show three main peaks in the blue (magnetic dipole), yellow (forced electric dipole), and red regions. Further, the phosphor shows excellent CIE chromaticity coordinates (x, y); as a result, it is quite useful for display applications. The investigation of photocatalytic ability has shown that the activity of Dy3+-doped Mg2SiO4 NPs is greatly influenced by the Dy3+ dopant concentration. At 1 mol % Dy dopant concentration, superior photocatalytic activity is observed for the decolorization (98%) of rhodamine B. This photocatalytic enhancement is attributed to charge separation efficiency, dopant concentration, and a wide band gap. The present Dy3+-doped Mg2SiO4 (1 mol %) photocatalyst is easily recycled without any

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Table 5.1 Review of the literature Serial number 1 2 3 4 5 6 7 8 9 10 11 12 13

Sample Mg2SiO4: Eu3+ Mg2SiO4: Eu3+ Mg2SiO4 Mg2SiO4: Dy3+ Mg2SiO4: Eu3+ CdSiO3: Pr3+ CdSiO3: RE3+ CdSiO3: Sm3+ CdSiO3: Dy3+ Y2SiO5 Zn2SiO4: Eu3+ Zn2SiO4: Eu3+ ZrSiO4

Preparation technique Combustion

Calcination temperature 800
C

Polyacrylamide gel method Sol gel Combustion

900
C 800
C 800
C

Hassanzadeh-Tabrizi and Taheri-Nassaj (2013) Kharaziha and Fathi (2009) Lakshminarasappa et al. (2011)

Combustion

800
C

Mostafavi et al. (2013)

Combustion

800
C

Sunitha et al. (2012a, b)

Solid state

1100
C

Kuang et al. (2006)

Combustion

800
C

Manjunatha et al. (2013)

Solid state

1100
C

Fu et al. (2004)

Sol–gel Combustion

1200–1400
C 1000–1200
C

Huang and Yan (2006) Sunitha et al. (2013)

Solid state

1200
C

Yang et al. (2013)

Solid state

1687
C

Kaiser et al. (2008)

Reference Prashantha et al. (2011)

significant loss of photocatalytic activity, which is favorable for potential practical applications. The literature reveals that Mg2SiO4 doped with various dopant ions finds a wide range of applications. Normally, the formation of nanophase Mg2SiO4 or any silicate material requires a high temperature, as shown in Table 5.1. Prashantha et al. reported that NPs of Eu3+-doped Mg2SiO4 are prepared using a low-temperature solution combustion technique, with metal nitrate as a precursor and urea as fuel (Prashantha et al. 2011). The synthesized samples are calcined at 800
C for 3 h. The PXRD patterns of the sample reveal an orthorhombic structure with an α-phase. These phosphors exhibit a bright red color upon excitation by 256 nm of light and show the characteristic emission of the Eu3+ ions. The electronic transition corresponding to 5D0 ! 7F2 of Eu3+ ions (612 nm) is stronger than the magnetic dipole transition corresponding to 5D0 ! 7F1 of Eu3+ ions (590 nm). Enhancement of the PL intensity of Eu3+ has been observed because of the formation of different lattice sites in the host phosphor. Tabrizi et al. reported that a Mg2SiO4:Eu3+ nanopowder has been synthesized using a polyacrylamide gel method. Via this route, the gelation of the solution is achieved by the formation of a polymer network that provides a structural framework for the growth of particles (Hassanzadeh-Tabrizi and Taheri-Nassaj 2013). The

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densification of the powders has also been studied. An amorphous nanopowder is synthesized and crystallized to Mg2SiO4 after heat treatment via a solid-state reaction at a relatively low temperature of about 700
C. The powders prepared using the polyacrylamide gel method show better sinterability than those synthesized using the conventional sol–gel method. The relative density of the sample is 97% at 1500
C. Kharaziha and Fathi reported the sol–gel preparation, characterization, and bioactivity evaluation of forsterite nanopowder. Bioactivity evaluation is performed by immersing the forsterite powder in the simulated body fluid (SBF) and apatite formation on the surface of the immersed forsterite nanopowders has been investigated. Results have shown that the particle size of pure forsterite is 25–45 nm. During immersion in SBF, the dissolution rate of the forsterite nanopowder is higher than that of conventional forsterite powders and apatite is formed after soaking for 14 days (Kharaziha et al. 2009). Lakshminarasappa et al. reported ionoluminescence (IL) of nanocrystalline Mg2SiO4:Dy3+ pellet samples bombarded with 100 MeV Si+8 ions, with fluences within the range (1.124–22.480)  1012 ions cm2 (Lakshminarasappa et al. 2011). Two prominent IL bands with peaks at ~480 nm and ~580 nm and a weak band with a peak at ~670 nm have been recorded. The characteristic peaks are attributed to the luminescence center activated by Dy3+ ions due to the transitions 4F9/2 – 6H15/2, 6 H13/2, and 6H11/2. It is found that IL intensity initially decreases rapidly and then continues to decrease slowly, with further increases in ion fluence. The reduction in the IL intensity with an increase in ion fluence may be attributed to the degradation of Si-O (2ν3) bonds present on the surface of the sample and/or due to lattice disorder produced by dense electronic excitation under heavy ion irradiation. Mostafavi et al. reported nanocrystalline forsterite (Mg2SiO4) powder synthesized via a two-step combustion–calcination procedure. Mixtures of citric acid and glycine are used as fuel and nitrate ions are used as oxidizers. The samples are synthesized with different fuel to nitrate (F/N) molar ratios. Photoluminescence spectra show that the luminescent intensity of Mg2SiO4:Eu3+ particles is much higher than that of MgO:Eu3+ particles (Mostafavi et al. 2013). Various silicates have been studied by authors such as Sunitha et al., who reported a series of Pr3+ (1–9 mol %)-doped CdSiO3 nanophosphors prepared for the first time by the low-temperature solution combustion method using oxalyldihydrizide (ODH) as a fuel. The optical energy band gap (Eg) of undoped for Pr3+-doped samples is estimated from the Tauc relation, which varies from 5.15 to 5.36 eV. The thermoluminescence (TL) properties of the Pr3+-doped CdSiO3 nanophosphor has been investigated using γ-irradiation within the dose range 1–6 kGy at a heating rate of 5
C s1. The phosphor shows a well-resolved glow peak at ~171
C, along with a shouldered peak at 223
C on the higher temperature side. It is observed that TL intensity increases with the increase in Pr3+ concentration. Further, the TL intensity at 171
C is found to increase linearly with the increase in the γ-dose, which is highly useful in radiation dosimetry. The kinetic parameters, such as activation energy (E), frequency factor (s), and order of kinetics, are estimated using Luschik’s method and the results are discussed.

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Kaung et al. reported the effect of RE3+ as a co-dopant in the long-lasting phosphorescence CdSiO3:Mn2+ (RE ¼ Y, La, Gd, Lu). A longer orange emitting phosphorescence phenomenon has been observed in the co-doped CdSiO3:Mn2+, RE3+ phosphors after exciting with ultraviolet (UV) light. The luminescence properties, including PL spectra and TL spectra, in addition to the afterglow decay curves, have been studied. The results revealed that one of the origins of the improvement is due to non-equivalent substitution to produce more e-traps, and energy transfer from Gd3+ to Mn2+ to boost the performance of CdSiO3:Mn2+, Gd3+ (Kuang et al. 2006). Manjunatha et al. reported the effect of NaF flux on the crystallization behavior of CdSiO3:Sm3+ (1–7 mol %) nanophosphors using a low-temperature solution combustion method. The addition of NaF could lead to the formation of particle type morphology and the increase in wt% of NaF changes crystallinity and phase. The TL glow curves for 2–6 wt% NaF in CdSiO3:Sm3+ show a single, well-resolved glow peak at 126, 130, and 150
C respectively, but the sample prepared with 8 wt% of NaF flux shows two glow peaks at 148 and 220
C. With the addition of NaF flux TL intensity can be effectively enhanced (Manjunatha et al. 2013). Fu et al. reported on a novel white light-emitting long-lasting phosphor Cd1xDyxSiO3. The phosphorescence can be seen clearly with the naked eye in the dark, even after the 254-nm UV irradiation has been removed for about 30 min. All three emissions form a white light with CIE chromaticity coordinates x ¼ 0.3874 and y ¼ 0.3760, and the color temperature is 4000 K under 254-nm excitation. This phosphor is indicated to be a promising new luminescent material for practice application (Fu et al. 2004). Huang et al. reported that using RE coordination polymers with salicylic acid as precursors for the luminescence species YxGd2-xSiO5:Eu3+, composing the polyvinyl alcohol (PVA) as dispersing media, nanophosphors of YxGd2-xSiO5:Eu3+ (x ¼ 0.10, 0.25, 0.50, 0.75, 0.90) with different molar ratios of Y and Gd are synthesized by the sol–gel process. Both X-ray diffraction and scanning electronic microscopy show that these materials have a nanometric size of 100–200 nm (Huang and Yan 2006). Sunitha et al. reported structural characterisations, IL, and TL studies on the Zn2SiO4:Sm3+ (3 mol%) nanophosphor bombarded with swift, heavy ions within the fluence range 3.91  1012 to 21.48  1012 cm2. TL studies have been carried out for 3 mol% Sm3+ concentration in the fluence range 3.91  1012 to 21.48  1012 ions cm2. Two TL glow peaks at 152 and 223
C were recorded. The kinetic parameters (E, b, s), were estimated using Chen’s peak shape method. A simple glow curve structure (223
C), high resistance, an increase in TL intensity up to 19.53  1012 ions cm2, and simple trap distribution make the Zn2SiO4:Sm3+ (3 mol%) phosphor highly useful in radiation dosimetry (Sunitha et al. 2013). Yang et al. reported Zn2SiO4:xEu3+ phosphors synthesized by using tetraethyl orthosilicate (TEOS) solution as a silicate source. The samples are sintered at 1200
C for 2 h in air using solid-state sintering. The XRD results show that the prepared Zn2SiO4:Eu3+ phosphors have a small impurity phase of SiO2 when the Eu3 + concentration is less than x ¼ 0.005. The PL results demonstrate that the intensity

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of the emission spectra increases and reaches a maximum value at x ¼ 0.005, and then decreases because of the concentration-quenching effect (Yang et al. 2013). Kaiser et al. reported on the thermal stability of zircon (ZrSiO4). Based on annealing experiments with natural and synthetic raw materials of known grain size and impurity level in addition to single crystals, the temperature of the thermal dissociation of ZrSiO4 is assessed at 1673  10
C. ZrSiO4 decomposes via a solidstate reaction releasing SiO2 in the form of discrete metastable intermediate phases with superstoichiometric Si content (Kaiser et al. 2008).

5.4

Conclusions

A number of methods have been used for preparing nanomaterials. Among the wet chemical routes, the combustion technique is capable of producing the nanoscale powders of silicates (Mg2SiO4) at a low temperature in a short time. The combustion technique has emerged as an important synthesis technique for materials, being a very simple experimental set-up, providing a molecular level of mixing, a high degree of homogeneity, being time-saving, more economical, and providing an ultra-pure, larger surface area. The solution combustion synthesis may be a promising synthesis method for commercial needs. In comparison with other methods, the powders obtained using the combustion synthesis method are generally more homogeneous, less impure, and have larger surface areas than powders prepared using conventional methods.

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Chapter 6

Studies of Multi-walled Carbon Nanotubes and Their Capabilities of Hydrogen Adsorption Edgar Mosquera, Mauricio Morel, Donovan E. Diaz-Droguett, Nicolás Carvajal, Rocío Tamayo, Martin Roble, Vania Rojas, and Rodrigo Espinoza-González

Contents 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.1 Synthesis of Multi-walled Carbon Nanotubes (MWCNTs) . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Purification of the Grown MWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Sample Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Scaling Up MWCNTs by AACVD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.5 Hydrogen Storage in MWCNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Over the last decade, there has been a significant interest of the scientific community in the synthesis of carbonaceous materials due to its wide range of application, as well on the hydrogen storage problem. Since the discovery of carbon nanotubes by Iijima, carbon nanotubes have been one of the candidate nanomaterials for hydrogen storage. However, experimental studies on hydrogen storage capacity of carbon nanotubes are still very few, and the mechanism of how hydrogen is stored into carbon nanotubes and the factors affecting the adsorption remains still unclear. In this chapter, we describe in detail the synthesis, purification, structural characterization, and hydrogen adsorption capabilities of multi-walled carbon nanotubes (MWCNTs) obtained by an aerosol-assisted chemical vapor deposition (AACVD) E. Mosquera (*) Departamento de Física, Universidad del Valle, Cali, Colombia e-mail: [email protected] M. Morel · N. Carvajal · R. Tamayo · V. Rojas · R. Espinoza-González Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Santiago, Chile D. E. Diaz-Droguett · M. Roble Instituto de Física, Pontificia Universidad Católica de Chile, Santiago, Chile © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_6

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method and using low-cost raw materials. In our investigation, we found that the hydrogen adsorption capacity was strongly dependent on the chemical, structural, and morphological characteristics of the carbon nanotubes obtained and purified which depend on the starting materials used for the synthesis by AACVD. In addition, hydrogen storage properties of MWCNTs were studied using a quartz crystal microbalance (QCM). Values between 0.22 and 3.46 wt% of adsorbed hydrogen were reached depending on the exposure pressure at room temperature. The maximum adsorption capacity was obtained for a purified sample with specific surface area of 729.4
2.8 m2 g1 and average pore size of 22.3 nm.

6.1

Introduction

The research and development on carbon nanotubes (CNTs) with unique tubular structure (single- or multi-walled) have been very active over the last several years because of their remarkable chemical and physical properties (Suzuki 2013; Shah and Tali 2016). In addition, the increasing interest in the synthesis of CNTs is due to its versatility and functionality in a wide range of applications (Franklin 2013; Lin et al. 2012; Liu et al. 2010a, b; Reyhani et al. 2011; Wang et al. 2014). Due to these interesting properties and applications, reviews of the fabrication and their mass production have been reported (Kumar and Ando 2010; Iijima and Ichihashi 1993; Harris 1999; De Volder et al. 2013; Dai and Mau 2001). Since the discovery of CNTs by Iijima (1991), CNTs have been one of the candidate nanomaterials for hydrogen storage (Reyhani et al. 2011; Lin et al. 2012; Rafiee 2012; Sheng et al. 2012; Mosquera et al. 2014; Morel et al. 2015; Liu et al. 2010a, b; Cheng et al. 2001; Froudakis 2011). However, experimental studies on hydrogen storage capacity of CNTs are still very few, and the mechanism of how hydrogen is stored into CNTs and the factors affecting the adsorption remains still unclear (Liu et al. 2010a, b; Morel et al. 2015; Froudakis 2011). In view of the above, many research groups started to carry out theoretical and experimental studies (Rafiee 2012; Aboutabeli et al. 2012; Spyrou et al. 2013; Jeing et al. 2013) in this field, and noticeable progress has been made with the finality to fill the benchmark of 5.5 wt% (H2 gravimetric capacity) with a volumetric capacity of 40 g of H2/L, set by the US Department of Energy (DoE) for the year 2020, for hydrogen storage systems (Froudakis 2011; Morel et al. 2015). Despite the effort that has been made by the researchers to solve this demand, the solution has not been found yet. Previous studies reported that hydrogen storage capacity for carbon materials has been less than 10 wt% (Dillon et al. 1997; Liu et al. 2010a, b; Mosquera et al. 2014; Morel et al. 2015; Froudakis 2011; Deck and Vecchio 2006; Esconjauregui et al. 2009; Steiner et al. 2009; Liang et al. 2011; Hou et al. 2011; Xue et al. 2011). In addition, there is a big challenge of finding a material capable of storing very high amounts of hydrogen (at ambient conditions) as to make of this environmentally friendly fuel a fuel economically viable. On the other hand, it is believed that the

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growth method and synthesis conditions (Kumar and Ando 2010), purification (Reyhani et al. 2008), and structure (with high surface area and large free pore volume) (Froudakis 2011; Panella et al. 2005) of carbon-based architectures, such as CNTs, could have a strong effect on hydrogen storage capacity. In this chapter, we describe the synthesis, structural characterization, and hydrogen storage capability of multi-walled carbon nanotubes (MWCNTs) grown in our group by an aerosol-assisted chemical vapor deposition (AACVD) method and using low-cost raw materials.

6.2

Carbon Nanotubes

Carbon nanotubes (CNTs), such as single-walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs), can be formed by rolled up one-atom-thick sheets of graphene that are closed at each end by half of a fullerene molecule (Shah and Tali 2016). However, the formation of SWCNTs or MWCNTs is governed by the size (in order of nm) of metal catalyst. Typical metal nanocatalyst is Fe, Ni, and Co, due to their high solubility of carbon at high temperatures and high diffusion rate in these metals (Kumar and Ando 2010). In addition, the widely accepted growth mechanism depends on the catalyst-substrate interaction, respectively. When the catalystsubstrate interaction is weak, the mechanism is known as “tip-growth model” (Baker et al. 1972; Kumar and Ando 2010), while a strong catalyst-substrate interaction is known as “base-growth model” (Baker and Waite 1975; Kumar and Ando 2010). Generally, nanocatalysts are required to fabricate SWCNTs, whereas MWCNTs can be obtained without nanocatalyst. Hydrocarbons are used as precursors to grown CNTs at temperatures in between 600 and 1200  C. The synthesis of SWCNTs is obtained at higher temperatures (> 900  C), whereas MWCNTs are synthesized at temperatures less than 900  C. The morphology growth (straight or curved) of the CNTs depends on the molecular structure of the precursor. Linear hydrocarbons produce straight and hollow CNTs, instead, cyclic hydrocarbons produce curved and hunched CNTs (Nerushev et al. 2003; Morjan et al. 2004). However, by a proper selection of the carbon precursor, as well as the nanocatalyst and the growth rate, the yield and quality of the CNTs could be improved.

6.2.1

Synthesis of Multi-walled Carbon Nanotubes (MWCNTs)

MWCNTs were synthesized using an aerosol-assisted chemical vapor deposition (AACVD) method. The reaction was carried out in a homemade ultrasonic spray pyrolysis system. Approximately 2.0 g of camphor (as carbonaceous source), mixed with 10 mL of alcohol (isopropyl or ethanol), were placed in the ultrasonic nebulizer.

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Fig. 6.1 Powder of MWCNTs grown inside the quartz tube by our research group using AACVD method. (Morel et al. 2015)

Table 6.1 Experimental conditions in the synthesis of MWCNTs by AACVD method

Sample CNT11 CNT12 CNT13 CNT14 CNT15 CNT16 CNT17

Catalyst Ni Ni Ni Mineral magnetite (80%) Magnetite (90%) Mineral magnetite (98%) Mineral magnetite (78%)

Support – Zeolite Zeolite – Zeolite – Zeolite

Carrie gas N2 N2 Ar Ar Ar N2 N2

Pure nitrogen (N2) and argon (Ar) gas were used to transport the precursor mist generated in the atomization chamber to a horizontal quartz tube (length: 50 cm, diameter: 3.0 cm) inserted in a furnace. The catalyst (nickel, magnetite, or Chilean mineral magnetite) with or without zeolite (as support) was placed inside the furnace with 15 cm of heat zone. The mist (camphor mixed with alcohol) was pyrolyzed over a catalyst/support at 800  C under a flow of 1 L/min for 30 min. Afterward, the system was cooled naturally to room temperature (RT), and a blackened powder was obtained in the middle of quartz tube, which was collected for characterization without purification process (see Fig. 6.1). The experimental condition of synthesis (catalyst/support proportion and carrier gas) used is shown in Table 6.1 (Mosquera et al. 2014; Morel et al. 2015).

6.2.2

Purification of the Grown MWCNTs

Purification of MWCNTs was performed by a modified route using two acids (fluorhydric acid, HF, and chlorhydric acid, HCl) in order to determine the effect of purified MWCNTs for hydrogen storage. A schematic diagram of the purification process is shown in Fig. 6.2. First (Step 1), ~ 500 mg of MWCNTs were placed in a tubular furnace at 450  C at a rate of 15  C min1 for 1 h. Afterward, the system

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Fig. 6.2 Schematic diagram of a new purification process developed by our research group

was cooled down to room temperature using the own thermal inertia of the equipment. The sample was characterized using X-ray diffraction (XRD). The same amounts of MWCNTs were immersed in a HF solution with a volume ratio (HF/CNTs) of 1:3 and sonicated for 1 h (Step 2). Then, the MWCNTs were washed out with deionized water and centrifuged at 3500 rpm until the pH value of the solution became neutral (Step 3). The samples were then dried in an oven for 48 h in ambient (Step 4) and characterized using XRD. After, the samples was immersed in a HCl solution with a volume ratio (HCl:CNTs) of 1:3 for 24 h at room temperature (Step 5). The samples were washed out with deionized water and centrifuged until the pH value of the solution became neutral (Step 6). The samples were dried in a vacuum oven at 80  C for 1 h (Step 7).

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Sample Characterization

The crystalline structure and phase purity of the prepared and purified MWCNTs were determined using a Bruker D8 X-ray diffractometer (XRD, CuKα12 radiation) at room temperature (RT). Thermogravimetric analysis (TGA) of blackened powder was performed with a TA Instruments, TGA Q50, under nitrogen gas and heated from RT to 800  C at heating rate of 10  C min1. The surface area and the pore dimension of the MWCNTs were determined by N2 adsorption-desorption isotherms at 196  C in a Micromeritics ASAP 2010 equipment. Micro-Raman spectra were recorded at RT on a LabRam 010 from ISA to identify the structure and crystallinity of MWCNTs. A He-Ne laser with wavelength of 632.8 nm was used as the Raman excitation source. A scanning electron microscope (SEM, FEI Quanta 250) was used to characterize the morphology of synthesized MWCNTs. Structural characterization was done by high-resolution transmission electron microscope (HRTEM, Tecnai F20 FEG-S/TEM) operated at 200 kV. For gas adsorption measurements, as-prepared and purified MWCNTs were deposited at RT on top face of a microbalance quartz crystal (QC) used as support substrate. The powder dispersed in isopropyl alcohol was sonicated for 7 min. The suspension formed was deposited using a dropper onto the QC and then dried to room air at RT. Then, the QC was located in the head of a quartz crystal microbalance system (MDC, SQM-310) placed inside a vacuum chamber (pumped down to 7  106 Torr). A gate valve placed between the chamber and the turbo pump to isolate the chamber from the vacuum allowed pressurizing with hydrogen gas injecting it through a needle valve. The mass changes upon hydrogen adsorption were determined by in situ monitoring of the changes in the resonance frequency of the QC as function of time while the sample was exposed to hydrogen for 8 minutes. After H2 exposure, the chamber was again pumped down to 7  106 Torr; the process was repeated by injecting hydrogen until reaching a higher pressure. Increasing pressures between 3 and 110 Torr (measured with a capacitive gauge, Baratron from MKS instruments) were used in the different hydrogenation cycles for studying the pressure effect on the hydrogen storage behavior at RT of the MWCNTs. The relationship between the mass added to the QC due to the H2 adsorption by the MWCNTs and the shift in its resonance frequency, Δf, is represented by the Sauerbrey’s equation (Sauerbrey’s 1959; Lucklum and Hauptmann 2000; Mecea 2005; Mosquera et al. 2014; Morel et al. 2015; Souza et al. 2017): 2f 2 Δ f ¼ pffiffiffiffiffiffiffiffiffiffiffi Δm A ρμ where f is the QC resonant frequency, ρ is the density, μ is the shear modulus of the QC, and A is the area covered by the mass. This equation indicates that a negative variation of the resonance frequency is due to a mass gain. Details about the use of this equation and method for the determination of mass gained by a QC can be found in El Far et al. (2012).

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Scaling Up MWCNTs by AACVD

For synthesizing carbon nanotubes (CNTs) by AACVD, the use of metal catalysts, typically submicron or nanometer size, is necessary to catalyze CNTs’ growth. The catalysts are required to enable hydrocarbon decomposition at a lower temperature than spontaneous decomposition temperature of the hydrocarbon (Kumar and Ando 2010). However, it is difficult to compare our results with those reported, due to CVD method presents more sensitive parameters and conditions, such as temperature, flow, and gas carrier on CNTs fabrication. Comparing camphor (C6H16O) with the most commonly used CNT precursors, Kumar and Ando (2005, 2010) reported mass production of CNTs from camphor using CVD and established the growth condition of MWCNTs and SWCNTs using Fe-Co catalyst impregnated in zeolite support. MWCNTs were grown at a temperatures as low 550  C, whereas SWCNTs were grown at 900  C. These studies confirm that SWCNTs or MWCNTs can be selectively grown by proper selection of catalyst materials and their concentration (Kumar and Ando 2010). It was found that for the synthesis of MWCNTs using a mixture of camphor/ alcohol, the performance is dependent on the catalyst materials, gas carrier, and support used (Morel et al. 2015; Mosquera et al. 2014). Initially, we used a metal loading for nickel catalyzed reactions, but the yield was poor in comparison to using magnetite and mineral magnetite like catalyst. TEM and HRTEM images of the MWCNTs are shown in Figs. 6.3, 6.4, and 6.5. Therefore, these images clearly reveal that the formed products are nanotubes with average diameters between 20 nm and 200 nm, respectively. It can be seen from images (see Fig. 6.3) that numerous nanoparticles of the order of nanometers are adhered to the nanotubes. HRTEM images show that the interplanar spacing varies as well with the diameter of MWCNTs. The morphology depends on the catalysts and gas carrier used during the growth. In addition, some defects are present on the graphitic layers (see Fig. 6.4). Thus, there is a lower degree of crystallinity. In addition, with respect to Figs. 6.3 and 6.4, Fig. 6.5 shows two multiple-walled carbon nanotubes with distinct morphology and diameter. For Fig. 6.5 (a–c), the MWCNT (CNT17) shows a multi-wall of ~ 9 nm, which corresponds to ~ 27 concentric nanotubes. And in Fig. 6.5 (d–f), the sample CNT14 shows a straight nanotube with a multi-wall of ~ 4.3 nm that corresponds to ~ 13 concentric nanotubes. The experimental conditions of synthesis are shown in Table 6.1. Figures 6.6 and 6.7 show TEM and HRTEM images of two purified nanotubes, CNT12 and CNT15, respectively. Figure 6.6(a–e) shows images of straight MWCNTs (CNT12). It can be seen from Fig. 6.6(c and e) that there is no catalyst present at the tip of the nanotube, and it is found to be broken. Figure 6.6d clearly shows that the CNT12 sample presents ~ 48 concentric nanotubes. While in Fig. 6.7 (a–d), the MWCNTs (CNT15) present another morphology, due to that the sample was synthesized with magnetite at 90% and zeolite-like support. It clearly seems that the morphology of the nanotubes depends on the synthesis conditions (see Table 6.1). In addition, the sample CNT15 displayed ~ 115 concentric nanotubes.

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Fig. 6.3 TEM images of the MWCNTs produced by AACVD of a mixture camphor/alcohol. (a, c) Low-magnification TEM images of the CNT12. (b, d) Zoom of the dotted frame in (a) and (b), showing nickel particle on the tips of the CNTs

Also, there is no catalyst present into nanotubes, and some defects are observed on the graphitic layers. Thus, a lower crystallinity is observed, and the graphitic nature of MWCNTs has been preserved. Raman spectroscopy was performed to confirm the degree of crystallinity of the produced and purified nanotubes (see Fig. 6.8(a and b)). As shown in Fig. 6.8a, two distinct peaks were observed at about 1326 and 1580 cm1 corresponding, respectively, to the disorder of the graphite structure (D-band (ID)) and to the highfrequency E2g first-order mode of graphite structure (G-band (IG)), while, for purified MWCNTs (Fig. 6.8b), the characteristic D- and G-bands were observed at around 1345 and 1570 cm1, respectively. In addition, the purified MWCNTs present a shift of D-band and G-band frequency to higher and lower wave number with respect to raw MWCNTs, respectively. This shift could be attributed to an inter-tube van der Waals interaction (Singh et al. 2010; Morel et al. 2015). The intensity ratio (ID/IG) is indicative of the crystallinity of the nanotubes (Brown et al. 2001; Ferrari and Robertson 2001; Wang et al. 2010; Bao et al. 2012), where values closer to 1 indicate

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Fig. 6.4 (a–f) TEM images of CNT13 produced by AACVD. (b) The dotted frame in (a) with increased magnification. (d) and (f) zooms from the dotted frames in (c) and (e) show two as-grown CNTs

Fig. 6.5 (a, d) TEM images of the samples CNT17 and CNT14. (b, c) The dotted frames in (a) with increased magnification. Panel (f) zoom from the dotted frame in (e) shows the straight carbon nanotube

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Fig. 6.6 TEM images of the purified sample CNT12. (a, c) Low-magnification TEM images of the CNT12. (b–e) Zoom of the dotted frame in (a), (b), and (c), showing the tips and walls of the CNTs

the presence of a lower degree of crystallinity and more defects contained in graphene wall of the carbonaceous material (Belin and Epron 2005; Suzuki 2013; Cho et al. 2013; Mosquera et al. 2014). From the Raman spectra in Fig. 6.8 and Table 6.2, the intensity ratio ID/IG has been established; this is a signature of the defects contained in graphene walls and a low degree of crystallinity. In our samples, the Raman results showed amorphous carbon and crystalline graphitic-like forms. As well, it is clearly observed that for purified MWCNTs, ID/IG ratio decreases for CNT11, CNT14, and CNT15 and increases for CNT12 and CNT13 in comparison with unpurified carbon nanotubes. In addition, the Raman spectroscopy results are in good agreement with our TEM observation. Many researchers reported that defect sites in CNTs could adsorb hydrogen molecules in comparison to ideal hexagonal structures of CNTs (Reyhani et al. 2011). The samples CNT16 and CNT17 were not purified and Raman spectra not measured. Therefore, Raman intensity ratio, ID/IG, has not been reported in Table 6.2.

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Fig. 6.7 (a–d) TEM images of the purified sample CNT15. (b) The dotted frame in (a) with increased magnification shows the curved tubing. (c) and (d) zooms from the dotted frames in (a) and shows the wall and defects in purified MWCNTs

6.2.5

Hydrogen Storage in MWCNTs

Over the last years, hydrogen has been recognized as an ideal renewable energy carrier, but it has actually been used commercially due to the achievement of the benchmark of 5.5 wt% (hydrogen gravimetric capacity) and volumetric capacity of 40 g of H2/L, established by US Department of Energy (DoE) for the year 2020, for H2 storage systems (Froudakis 2011, Mosquera et al. 2014, Morel et al. 2015). In view of the above, this work has been carried out in order to study the H2 adsorption capacity of carbon nanotubes, grown by AACVD using low-cost raw material. The H2 adsorption capacity of the as-prepared and purified MWCNTs at room temperature has been determined by means of a quartz crystal microbalance using different pressures of H2 exposure. No pretreatments were carried on the samples for evaluating their performance regarding H2 adsorption.

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Fig. 6.8 Raman spectra of the (a) as-grown MWCNTs and the (b) purified MWCNTs using the purification process shown in Fig. 6.2. Here, the disorder (D-band) and crystallinity (G-band) of the carbon nanotubes are shown

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Table 6.2 Intensity ratio of Raman spectra of the MWCNTs ( not measured) Sample CNT11 CNT12 CNT13 CNT14 CNT15 CNT16 CNT17

Raman intensity ratio, ID/IG (without purified) 1.02 0.77 0.63 0.97 0.95 1.43 0.96

Raman intensity ratio, ID/IG (purified) 0.78 0.9 0.69 0.71 0.61 – –

Table 6.3 Hydrogen storage capacity of the MWCNTs ( not measured) Sample CNT11 CNT12 CNT13 CNT14 CNT15 CNT16 CNT17

H2 storage capacity (without purified, wt.%) 1.5 1.2 2.1 1.76 – 0.04 0.25

H2 storage capacity (purified, wt.%) 1.03 0.44 0.22 0.54 3.46 – –

Figure 6.9(a–c) and Table 6.3 show that the samples of MWCNTs exhibit different H2 adsorption behaviors. Here, the H2 adsorption capacity is strongly dependent on the chemical, structural, and morphological characteristics of the MWCNTs. In Fig. 6.9a, the sample CNT11 reached 1.4 wt% during the first exposure using just 3 Torr of H2, value that did not vary significantly with increased H2 pressure, where percentages between 1.2 and 1.5 were obtained. On the contrary, the sample CNT12 increased significantly between 3 and 20 Torr reaching a maximum of about 2.3 at 20 Torr (see Fig. 6.9a). Then, the adsorbed H2 dropped at higher pressures down to 1.2 at 43 Torr. The peak at 20 Torr is attributed to a sudden filling of the MWCNTs and voids generated among the agglomerated carbon nanotubes as well as to a stress effect of the first monolayer of adsorbed H2 leading to a higher value in the QC resonance frequency and, therefore, of mass gained (Wang et al. 1989). The sample CNT13 showed a gradual increase of the H2 amount adsorbed as pressure increased reaching a maximum value of about 2.1 at the highest exposure pressure of 43 Torr. On the other hand, in Fig. 6.9b, the sample CNT16 reached 0.41 wt% during the first exposure using just 5.7 Torr of H2, value that did not vary significantly up to 28 Torr. After this pressure the H2 storage capacity slightly drops down 0.04 wt% at 43 Torr. On the contrary, the sample CNT14 showed a notable dependence on the H2 exposure pressure, reaching a maximum value of about 1.76 at the highest exposure pressure of 44 Torr. In the case of CNT17 sample, the H2 adsorbed capacity increased significantly between 3 and 14 Torr

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Fig. 6.9 Hydrogen storage capacity at RT from as-prepared and purified MWCNTs exposed to various H2 pressures. A sample composed only of zeolite was used for comparison

reaching a maximum value of 1.42 at 14 Torr. At higher exposure pressure, the amount of adsorbed H2 dropped 0.25 wt% at 43.4 Torr. These weight percentages are comparable or higher than other reported values of H2 storage capacities at RT by non-pretreated/doped carbon nanotubes (Cheng et al. 2001; Hou et al. 2003; Banerjee et al. 2006; Lim et al. 2007; Liu et al. 2010a, b; Mosquera et al. 2014) (Fig. 6.9). In purified MWCNTs, the hydrogen adsorption capability of samples (CNT11, CNT12, CNT13, CNT14) is in agreement with previously reported by Mosquera et al. (2014) and Morel et al. (2015). Instead, the sample CNT15 showed a gradual increase of the H2 amount adsorbed as a pressure increases reaching a maximum value of about 3.46 at the highest exposure pressure of 100 Torr (see Fig. 6.9c). Finally, the last curve corresponding to a sample made of zeolite (Fig. 6.9a and b) indicates that the microporous aluminosilicate particles adsorb hydrogen, which increased as pressure increased. The porous character of the zeolite with pore diameter as small as 2 nm increases the specific surface area of whole sample, and the pore channels formed help to provide more adsorption sites for the hydrogen molecule. This would help to understand the better H2 adsorption capacity of the samples which contain zeolite particles, mixed with carbon nanotubes.

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Summary

In this study, MWCNTs were produced by AACVD using a camphor/alcohol mixture and Ni, Fe, and mineral magnetite as catalyst with or without zeolite as precursors. The identified characteristics of as-synthesized MWCNTs as well as hydrogen storage capacity were studied. TEM micrographs revealed that the formed MWCNTs have an average diameter in the order of nanometers and depend on synthesis conditions. In addition, some defects and a low degree of crystallinity are observed on the graphitic layers. From Raman spectroscopy we conclude that the results are in good agreement with our TEM observation. The H2 storage results showed that the samples grown using zeolite in the support together with a lower crystallinity of their carbonaceous structure exhibit higher values of adsorbed H2 (weight percentages), as well as a behavior dependent on the hydrogen pressure. The hydrogen adsorption capacity was strongly dependent on the chemical, structural, and morphological characteristics of the MWCNTs which in turn depend on the ratio of the catalyst and zeolite. Acknowledgments This research was partially funded by CONICYT (Grant no. ACT1117 and ID14I10124). We also acknowledge professor A. Cabrera from Physics Institute of the Pontificia Universidad Católica de Chile and facilities from Universidad de Chile for the provision of equipment and measurements for this research.

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Chapter 7

Emerging Vertical Nanostructures for High-Performance Supercapacitor Applications Subrata Ghosh, Tom Mathews, S. R. Polaki, and Sang Mun Jeong

Contents 7.1 7.2 7.3 7.4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basics of Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Importance of Vertical Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vertical Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.1 Carbon Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.2 Transition Metal Dichalcogenides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.3 Pseudocapacitive Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4.4 Composite Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.5 Asymmetric Supercapacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.6 Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract The foremost challenge in energy crisis management is to meet the everrising demand for the seamless supply of energy to the technology-driven twentyfirst century. The rising depletion of fossil fuels and environmental pollution impose an immediate need for green energy. This stimulated the fabrication of energy storage devices necessary for hybrid electric vehicles, portable electronics, and power grid systems. Supercapacitor is an important energy storage device for electric vehicle and portable electronics. However, architecting the electrode materials with suitable geometry is one of the major hurdles toward the development of energy storage devices with high-energy densities. In view of this, the current chapter focuses on the fabrication of binder-free emerging vertical nanostructures for the application as active supercapacitor electrodes. This chapter emphasizes the importance of vertical nano-architectures and critical points toward the rational S. Ghosh · S. M. Jeong Green Energy Lab, Department of Chemical Engineering, Chungbuk National University, Cheongju, Chungbuk, Republic of Korea T. Mathews (*) · S. R. Polaki Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Homi Bhabha National Institute, Kalpakkam, Tamil Nadu, India e-mail: [email protected] © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_7

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design of supercapacitor electrodes. A broad overview on the recent developments of vertical nano-architectures for supercapacitor electrode applications and the future directions in achieving efficient supercapacitor devices are highlighted.

7.1

Introduction

Limited reserve of fossil fuels, climate change due to global warming, and the everincreasing demand for energy oblige us to look for alternate sources of energy and fabrication of high-performance energy storage devices. An energy storage device should store the energy harnessed from various sources, especially solar, wind, tidal power, etc., and deliver the stored energy to operate hybrid vehicle, portable electronics, smart grid, etc. The supercapacitor, also often referred to as electrochemical capacitor or ultracapacitor, in the literature, is one of the promising energy storage devices with higher power density and longer cycle stability (Dubal et al. 2016; González et al. 2016). Developing a highly efficient supercapacitor device is crucial in mitigating the global energy challenge. The evolution of supercapacitor from Leyden jar to micro-supercapacitor is remarkable (Miller 2007; Qi et al. 2016; Zhang et al. 2017a). The advanced technology witnessed potential utilization of supercapacitor in real-world applications, such as regenerative braking in hybrid vehicles, smartphone, power baking grid, camera flash, and so on (Miller and Burke 2008; Simon et al. 2014). More than 20,000 hybrid buses and 60,000 stop-start technologies in the cars utilize supercapacitors worldwide (IDTech, 2014). However, the Ragone plot, demarcating the region of supercapacitor comparing to conventional batteries and fuel cells (Fig. 7.1a), indicates the essential necessity of improving its energy density.

Fig. 7.1 (a) Ragone plot for energy storage devices. (b) Scheme of improving energy density in supercapacitors

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The lower energy densities of supercapacitor devices compared to conventional batteries and fuel cell limit their full fledge utilization. Hence, globally the research attention is devoted toward improvement in energy density of supercapacitors. The energy density strongly depends on the capacitance and operating potential window through the relation Energy density, E ¼ 1=2 CV 2

ð7:1Þ

where C is the specific capacitance (in the unit of F/g or F/cm3), and V is the operating potential window of the supercapacitor device. Therefore, the possible ways to tackle the challenges are either increasing the capacitance or increasing the potential window. Former step can be achieved by architecting the electrode and improving the electrode/electrolyte interaction (Fig. 7.1b) (Hossain et al. 2017; Tiwari et al. 2012; Yu et al. 2015). The later can be achieved by using novel electrolytes stable over a wider potential window. In this case, an electrolyte can be aqueous or organic, and recently hybrid electrolytes are getting lot of attention (Balducci 2016; Ghosh et al. 2017a; Xia et al. 2017). It is suggested to the reader to track the existing review articles or literature in order to get an overview on electrolyte choice for supercapacitor applications. The importance of vertical nanoarchitectures with edge density, proper vertical alignment, and packing density, for efficient charge storage performance, is not yet reviewed. Hence, based on the existing literature, the present chapter discusses on nano-architecting of electrode surfaces for enhanced capacitance. In this chapter, we first introduce the basics of supercapacitor and their classification. Based on the criteria of supercapacitor electrode material, difficulties with nonaligned and agglomerated structure and evolution of vertical structures are discussed. In addition, the key parameters toward better charge storage performance are addressed. We then compare the state-of-art vertical nanostructures behavior to nonaligned and randomly distributed structures.

7.2

Basics of Supercapacitor

A supercapacitor device consists of two electrodes separated by a separator/electrolyte. The key criteria for improving the energy density of supercapacitor electrodes are depicted in Fig. 7.1b. Controlling these parameters will help in fabricating highperformance supercapacitor devices. Electrode materials are classified into two major categories, based on their charge storage mechanism: electric double-layer capacitor (EDLC) and pseudo-capacitors (PC). Charge storage mechanism of EDLC relies on the double layer formed at the electrode/electrolyte interface. The fast and reversible redox reaction in/at the electrode surface is responsible for charge storage in PC. Generally, capacitance is

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expressed in the unit of F/g. However, the unit F/cm2 is also used in case of a planar electrodes, especially for the electrodes with negligible weight such as thin films and coatings on the current collector. The specific capacitance (Cs) of the EDLC electrode materials can be expressed as C s ¼ EðA=d Þ

ð7:2Þ

where E is the electrolyte dielectric constant, A is the electrode surface area in contact with the electrolyte, and d is the distance of charge separation or distance between electrodes. In general carbon structures are extensively used as EDLC electrode (Béguin et al. 2014; González et al. 2016; Inagaki et al. 2010). The charge storage of EDLC is based on the electrostatic polarization of charge at the electrode surface. Stern model is the widely accepted one to explain the double-layer charge storage behavior. Taking into consideration the diffusion of electrolyte ions and polarization of counter electrolytic ions at the electrode surface, Stern proposed a model and is depicted in Fig. 7.2 (Béguin et al. 2014). On the other hand, the first redox kinetics or faradaic reaction at the electrode surface is the mechanism behind charge storage in the case of PC. Ox þ ne $ Red

ð7:3Þ

The specific capacitance of PC is expressed as Cs ¼

nF m:ΔV

ð7:4Þ

where n is the number of participating electrons in the redox reaction, F is the Faraday constant (9.648  104 Coulomb/mol), m is the active mass of the electrode material, and ΔV is the potential window. Owing to the Faradic reaction, a reversible Fig. 7.2 Stern model for electric double-layer formation mechanism

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molecular layer formation takes place at the surface of the electrode material without any chemical transformation (Liu, et al. 2017). Transition metal oxides/hydroxides (Shi et al. 2014), transition metal nitrides/oxynitrides (Ghosh et al. 2018a), and conducting polymers (Fong et al. 2017) are falling into this class. They can store ten to hundred times more charge than EDLC (Ghosh et al. 2018a).

7.3

Importance of Vertical Structure

A vertical structure comprises of non-agglomerated self-supported structures of high surface-to-volume ratio, leading to effective accessibility of electrolyte ions. A schematic of self-supported vertical structures and their role as energy storage electrodes is depicted in Fig. 7.3a–c. The different forms of vertical structures are nanowalls, nanotubes, nanosheets, nanowires, nanorods, nanoflakes, etc. Each vertical nanosheet can act as a nanoelectrode, whereas interspacing between the vertical sheets serves as ion reservoir (Ghosh et al. 2017a, c, Heller et al. 2005; Wang et al. 2008). The intersheet spacing between the vertical sheets and optimized pore formation by them are the main factors for the effective utilization of surface by the electrolyte ions (Fig. 7.3d).

Fig. 7.3 Schematic of ion movement through (a) vertical versus planar graphene (Reprinted with permission from Yoo et al. (2011) Copyright 2011, American Chemical Society), (b) aligned carbon nanotubes versus granular activated carbon (Reprinted from Inagaki et al. (2010) Copyright 2010, with permission from Elsevier), and (c) RuO2.xH2O nanotubular arrays (Reprinted with permission from Hu et al. (2006) Copyright 2006, American Chemical Society). (d) Scheme for adsorption of electrolyte ions with or without solvation to the surface of pores with different sizes (Reprinted from Inagaki et al. (2010) Copyright 2010, with permission from Elsevier)

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Fig. 7.4 (a–c) Morphology and (d–g) supercapacitive characterization of vertical, lateral, and planar graphene structure (Reprinted with permission from Zhang et al. (2016c) Copyright 2016, American Chemical Society)

The benefits of vertical structure are represented in Fig. 7.4 (Zhang et al. 2016a, b, c) by taking three morphological graphene structures: vertical, lateral, and planar. Besides the vertical orientation, the packing density of the vertical sheets also influences the charge storage performance. In addition, the sharp edges of the vertical structure enhance the current-carrying capability (Ghosh et al. 2018a, b, c; Zhang et al. 2016a, b, c). Approximately 4.2 times higher charge density, more than twofold enhancement in ion packing density with closer ion packing location and larger ion separation degree, was found for the edge-rich vertical structure with sharp edges compared to their basal and non-oriented counterparts (Yang et al. 2016a). The linear diffusion controlled the electrochemical property of planar electrode, whereas faster mass transport and stronger current density of edge electrode were attributed to the convergent diffusion mode (Yuan et al. 2013, Brownson et al. 2012). The molecular dynamics (MD) was employed to get insights of the enhanced charge storage performance of edge-rich carbon nanostructures (Yang, et al. 2017;

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Dive and Banerjee 2018; Bo et al. 2018). To show the impact of vertical structure, the comparative study was carried out between planar state and edge state. The edge states are generally corrugated containing sub-nanometer pores leading to the confinement effect (Bo et al. 2018). These edge states can be edge of single-layer graphene and edge of multi-layered graphene. In case of single-layer graphene, zigzag edges showed higher capacitance due to higher quantum capacitance compared to armchair edges (Bo et al. 2018). On the other hand, edges of multi-layered graphene are more effective in enhancing charge-storage capacity. Two prominent factors to achieve highest charge storage capacity are edge density and pitch. According to the density functional theory, in the case of edge-rich graphene, the tendency of charge accumulation at the graphene edges is much higher than that at planar counterpart owing to the quasi-localized πz states of edge sites (Yang et al. 2016a, b). In addition, atomic layer distance for the edge states is much higher than that of planar states. This enables the electrolyte much closer accessibility to the edges. Moreover, enhanced charge storage properties of edge states are attributed to the higher charge screening efficiency on edge electrode by water molecules and reduced electrical potential across double-layer dominant region (Yang et al. 2016a, b). The second factor is pitch. The pitch of nanotube was defined as distance between two nanotube surfaces. The pitch of nanotube is like interspacing between them. The MD simulations of N-methyl-N-propyl pyrrolidinium bis(trifluoromethanesulfonyl) imide ionic liquid were carried out considering carbon nanotube (CNT) with different pitch (Dive and Banerjee 2018). The result of MD suggested that the noninteracting double-layer formation is more effective in achieving higher amount of charge storage. In particular, the nanotube with pitch of 12 A0 formed an interacting double layer lead to reduction in capacitance. Whereas, noninteracting double layer occurred for the nanotube with pitch of 16 A0 resulted in high-density ion storage (Dive and Banerjee 2018). Important to note that this optimization depends on the nature of electrolyte being used. Hence, the principal objective of the supercapacitor research is nano-architecting the vertically oriented electrode surface with optimized edges and interspacing between the vertical sheets. An important point to mention is that the binder-free electrode materials are more advantageous since binder elements are generally found to reduce the power density as given by the following relation: Power density, P ¼

V2 4Rs

ð7:5Þ

where Rs is the equivalent series resistance, which is high when binder is used. In addition, direct growth of nanostructures on the current collectors provides better mechanical robustness and reduced capacitive loss at their interface (Choudhary et al. 2016). Therefore, a lot of attention has been dedicated toward fabrication of binder-free vertical nano-architectures and integration of current collector-free vertical nanostructures.

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Vertical Nanostructures Carbon Materials

Activated carbon is the extensively used supercapacitor electrode material for commercial purposes because of its large surface area. Recently, it has been observed that vertically aligned reduced graphene oxides with high packing density and opened edges give better diffusion of electrolyte ions (Yoon et al. 2014). High areal capacitance of 1.83 F/cm2 was obtained using them (Yoon et al. 2014). However, the fabrication process of vertically aligned reduced graphene oxides needs several steps like graphene oxide preparation from graphite powder, reduction of graphene oxides, oxidizing single-walled CNT, and rolling and cutting process (Fig. 7.5) (Yoon et al. 2014). Noteworthy to mention that the agglomeration and use of binder, or conductive additives limits the energy as well as power density of activated carbon-based electrode. In view of this, direct growth of carbon nanofiber and CNT on current collector was rapidly developed and used as electrode materials because of their excellent conductivity and high surface-to-volume ratio. Growing CNT on graphene demonstrated an excellent capacitance of 653.7 μF/cm2, which is

Fig. 7.5 Schematic of vertically oriented reduced graphene oxide fabrication (Reprinted with permission from Yoon et al. (2014) Copyright 2014, American Chemical Society)

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around seven times of that of planar graphene (Kim et al. 2012). This configuration of CNT on graphene minimizes the self-aggregation as well as the intrinsic resistance. It has been shown that the perfect alignment of CNT is significant in order to get better supercapacitor performance (Fedorovskaya et al. 2014). Since the random orientation has limited channels for the electrolyte ion diffusion and higher contact resistance. In case of nanotube, both the diameter and interspacing of nanotubes are found to have strong impact on the electrochemical properties (Fig. 7.6a–b). Based on the range of pore size distribution, the specific capacitance of electrode materials is dominated either by surface area, ion space constriction, or distortion of ion solvation shell (Fig. 7.6c) (Raut et al. 2012). Unfortunately, controlling the process parameters in chemical vapor deposition (CVD) to get well-aligned CNTs with optimum inter-nanotube spacing for effective utilization of electrolyte ion is very difficult. In addition, the catalysts, used during growth, on the tip of nanotubes block electrolyte ions’ access to the inner side of CNT. Hence researchers started concentrating on vertically aligned graphene nanosheets (VGN). Vertical graphene nanosheets are prepared by plasma-enhanced CVD (Ghosh et al. 2017a, b, c). With 70–80% porosity, VGN shows its effective utilization as supercapacitor electrode material (Lehmann et al. 2017, Sahoo et al. 2017). The charge storage mechanism of VGN in organic electrolyte (tetrabutylammonium tetrafluoroborate/ propylene carbonate) was explained by electrochemical quartz crystal microbalance and ac-electrogravimetry (Lé et al. 2018). It has been shown that anion plays major role in charge-storage kinetics compared to cation counterpart. Even, the strategy

Fig. 7.6 Dependency of specific capacitance on (a) CNT diameter, (b) CNT spacing, and (c) pore size. (Reproduced from Raut et al. (2012) Copyright 2012; with permission from Elsevier). (d) Schematic of an interface between the liquid phase (electrolyte) and gas phase (air). (e) Plot of extent of wetting of interior surface (Se) with inter-sheet distances (S) of vertical graphene. (Reproduced from Shuai et al. (2017) Published by the Royal Society of Chemistry)

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adopted in simultaneous functionalization of VGN in order to further enhance the charge storage performance via microwave-assisted doping using leaf extracts from bokchoy, chrysanthemum, and spinach is given in the literature (Seo et al. 2015). The enhanced performance of VGN was attributed to the simultaneous formation of nitrogen and oxygen functional group. These functional groups are well known for the redox reaction with aqueous electrolyte and serve as donor/acceptor sites. The nitrogen functional group shifts up the Fermi level and while oxygen functional group lowers it down. Therefore, a change in charge distribution takes place; however the dominating carriers are determined by the nitrogen to oxygen functional group ratio present on the surface of graphene. In addition, oxygen functional group helps in better wettability of the electrode (Sahoo et al. 2018a). The advantages of CNT and VGN are that they can be grown directly on the current collector using CVD process. Point to be noted that these vertically oriented carbon nanostructures are near super-hydrophobic in nature (Ghosh et al. 2018a, b, c). Hence, the rigorous effort has been devoted toward transforming it to hydrophilic via plasma treatment (Sahoo et al. 2018b), tuning inter-sheet spacing between vertical sheets (Shuai et al. 2017), controlling defects (Zhang et al. 2016a), and chemical activation (Ghosh et al. 2017c). However, the type of oxygen functional group present on the surface of carbon nanostructures is crucial to have higher charge storage capacity. In particular, hydroxyl and carbonyl groups are preferential to obtain higher redox contribution compared to carboxyl group (Sahoo, et al. 2018b). The accessible surface area and diffusion of electrolyte ion were found to have strong dependence on the capillary force at liquid/air interface and hence their charge storage performance (Fig. 7.6d–e) (Shuai et al. 2017). Apart from conventional oxygen and nitrogen functionalization, Li et al. fabricated high-performance supercapacitor based on co-doped carbon nanowires (CNW) (Li et al. 2017). They showed superior properties of carbon structures while doping with heteroatoms with different electronegativities due to synergistic effect. Nitrogen- and sulfur-doped structures were achieved by using electrochemical deposition of polymer nanowire on anodic alumina oxide template followed by high-temperature annealing and subsequent etching of AAO (Li et al. 2017). The possible configurations for nitrogen in graphitic matrix are pyridine, pyrrole, and graphitic nitrogen. The pyridinic nitrogen enhanced the ion attraction, pyrrolic nitrogen contributed the pseudocapacitance along with improving wettability, and graphitic nitrogen assisted the better electron-transfer kinetics. Meanwhile, the sulfur doping modified the surface characteristics of CNW (Li et al. 2017). In summary, vertically oriented carbon nanostructures used to give the impression of being ideal supercapacitor by virtue of their high surface area, excellent conductivity, better electrochemical stability, and easy functionalization ability. However, the capacitance value of these materials is limited to 17-556 F/g or 0.21350 mF/cm2 (Ghosh et al. 2018a, b, c). The wide range of capacitance value depends on the microstructures, pore size and volume, defects, geometry, edge density, wettability, inter-sheet spacings, etc.

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Transition Metal Dichalcogenides

In parallel to graphene-related materials, transition metal dichalcogenides (TMDs) have been studied as promising supercapacitor electrodes (Bissett et al. 2016). The TMDCs are expressed as MX2, where M is a transition metal (e.g., Mo or W) and X is a chalcogen atom (e.g., S, Se, or Te). Like in the case of graphene synthesis, the CVD techniques have been used to fabricate MoS2 nanowall electrodes (Soon and Loh 2007). The supercapacitor performance of MoS2 nanowalls with reactive basal edges was found to be comparable to that of CNT arrays but with fast discharge time. In addition, authors anticipated effective utilization of MoS2 nanowalls as supercapacitor electrodes if inter-wall spacing is adjusted to prevent electrostatic repulsion between adsorbed ions (Soon and Loh 2007). These chalcogenides are synthesized at high temperatures. However, material synthesis at low temperature is always advantageous in order to utilize them in flexible device. In view of this, room temperature chemical bath deposition has been adopted to fabricate the flake-like MoS2 structures (Karade et al. 2016). Depending on the nature of aqueous electrolytes used, MoS2 nanoflakes show double-layer mechanism in some electrolytes and redox behavior in NaOH as well as KOH electrolytes (Karade et al. 2016). The possible mechanisms behind charge storage performance of vertically oriented MoS2 are (i) inter-sheets double-layer charge storage, (ii) intra-sheet double-layer charge storage on individual atomic layers through diffusion into the basal edges, and (iii) redox kinetics on Mo transition metal center (Karade et al. 2016; Soon and Loh 2007). ðiÞ double layer formation : ðMoS2 Þsurface þ Xþ þ e $ ðMoS2  Xþ Þsurface ð7:6Þ ðiiÞ Faradic kinetics :

MoS2 þ Xþ þ e $ MoS2 X

ð7:7Þ

where, X+ is the protons (H+) or alkali metals ions. However, 2D transition metal dichalcogenides have H and T phase. Therefore, the abovementioned mechanism depends on the type of phase present. The charge storage mechanisms of H-MoS2 and 2H-MoS2 are EDL formation and redox kinetics at the Mo-edge, respectively. Whereas, T-phase MoS2 demonstrates pseudocapacitive behavior (Zhang et al. 2016a). It has been shown that 1T metallic phase of MoS2 is responsible for the superior performance owing to their higher electronic conductivity and hydrophilicity (Acerce et al. 2015). However, synthesizing MoS2 with 1T-metallic phase is quite challenging from its bulk counterpart. An easy and efficient solvothermal method for preparing petal-like metallic T-MoS2 has been demonstrated (Mishra et al. 2018). Impressively another TMD, WS2, core/shell structure anticipated to have great potential as a supercapacitor electrode. The hybrid structures were integrated through simultaneous oxidation/sulfurization of tungsten foil (Fig. 7.7) and demonstrated zero loss of capacitance even up to 30,000 charge-discharge cycles (Choudhary et al. 2016). The unique feature of the hybrid structure is that core, shell, and current collector are fabricated from a single material.

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Fig. 7.7 (a) Schematic of one-body array of core/shell nanowire supercapacitor electrode and (b) fabrication schematic of hybrid nanowire with corresponding micrographs, XRD spectra, as well as Raman spectra (Reprinted with permission from Choudhary et al. (2016) Copyright 2016, American Chemical Society)

It is not only the TMD structures, the hybrid structures obtained by depositing TMD layers onto other nanostructures are also demonstrated as promising supercapacitor electrodes for next-generation devices. For example, MoS2 nanoflake decoration over MWCNT exhibited around nine times enhancement in specific capacitance with a higher rate of capacitance retention compared to bare MoS2 (Karade et al. 2017). The capacitance values of WS2-RGO hybrids were found to be 2.5 and 5 times higher than that of bare RGO and WS2 electrodes, respectively. The enhancement was attributed to the increased conductivity, reduced diffusion length of electrolyte ion, and Na+ intercalation into the van der Waals gap of WS2 layers (Ratha and Rout 2013). The possible redox reaction mechanism in case of WS2 is similar to that of MoS2 (Eq. 7.7). The recent gradual shift of research attention from carbon materials to TMDs in energy storage application is significant. The growing interest here is due to their unique electronic properties, sufficient interlayer spacings, improved stability, and ability to serve as mechanical backbone for heterostructures. More importantly, 2D materials provide pseudocapacitance along with the double-layer capacitance. The capacitance of these materials depends on phase, geometry, interspacing, and the type of electrolyte used. Therefore, controlling the synthesis parameters can enhance the supercapacitor performance further. In addition, vertically oriented other TMDs like MoSe2, TiS2, and MoTe2 are yet to be explored as supercapacitor electrodes.

7.4.3

Pseudocapacitive Material

In contrast to the EDLC, metal oxides/hydroxides are of great interest as pseudocapacitor electrode materials. A variety of vertical nanostructures such as nanorods, nanowires, flake-like structure, and nanoplatelets (Fig. 7.8) are fabricated. Fe2O3 nanostructures (Nithya and Arul 2016), MnO2 nanoflakes (Jeong et al. 2016), TiO2 nanorods (Ramadoss and Kim 2013), and Mn(OH)2 nanosheets

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Fig. 7.8 Micrographs of (a) nanoplatelets, (b) nanorods, and (c) nanoflakes

(Yang et al. 2017) are the few examples of the well-demonstrated high-performance pseudocapacitor electrodes. Readers can also find out more vertical structures made up of transition metal oxides in the existing reports. Herein, the focus is mainly on the alignment and geometrical influence and their charge storage performance of vertical structures. The high performance in Mn(OH)2-based supercapacitor is attributed to the vertical alignment and large open network (Yang et al. 2017). The specific capacitance of TiO2 nanotube was found to be proportionally related to its aspect ratio (length/tube diameter) (Raj et al. 2015). The enhanced capacitance of TiO2 with higher aspect ratio was ascribed to the change in the carrier concentration, conductivity, and the defect states. It is also reported that the hollow core-shell structures have higher charge storage capability compared to solid nanowires or nanorods (Guan et al. 2012). Since the nanogap in hollow core-shell structures served as “ion reservoir” and created a spatial confinement for electrolyte ion between core and shell resulting in enhanced accessible area and shorter ionic transportation pathways. The enhancement in transition metal-based supercapacitor was also achieved by nanoparticle decoration, conformal coating by other oxide materials, nitridation, and so on. For example, nitridation followed by Nb2O5 coating on TiO2 nanotubes leads to significant enhancement in capacitance and is attributed to the enhancement of electronic conductivity by nitridation and conformal secondary coating maintaining the inter-nanotube spacing (Ozkan et al. 2017). However, it should be noticed that the conformal coating maintains the vertical alignment as well as the porous nature enabling easy accessibility of the electrolyte ions. In the case of nanoparticle decoration, enhanced capacity and electrochemical stability of MnO2 nanowires were attributed to the increased defect, surface area, electrical conductivity, and charge storage capacity after Au nanoparticle incorporation (Khandare and Terdale 2017). A considerable increase in performance has been observed in Ni(OH)2-based supercapacitor electrode upon Al and Co co-doping (Chen et al. 2014). In addition, recent trends are geared toward developing vertical structures of binary and ternary oxides (Samantara et al. 2018; Boruah and Misra 2016). Subsequently, advances in combining two or more active materials illustrated an improved surface morphology, enhanced electrical conductivity, and better charge-transfer kinetics and hence offer effective gateways toward high-performance charge storage (Fig. 7.9) (Balamurugan et al. 2017; Qorbani et al. 2015; Reddy et al. 2017).

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Fig. 7.9 (a) Schematic of charge storage for Co3O4/Co(OH)2 electrode (Reprinted with permission from Qorbani et al. (2015) Copyright 2015, American Chemical Society). (b) Schematic for the fabrication of hierarchical Cu1xNixS nanosheet arrays supported on a porous 3D Ni backbone by a two-step method (Reproduced from Balamurugan et al. (2017) with permission from the Royal Society of Chemistry)

Besides metal oxides and metal hydroxides, vertically aligned metal nitrides also garnered attention extensively as a supercapacitor electrode (Ghosh et al. 2018a). Vanadium nitride vertical structures were grown using CNT as replica, and microsupercapacitor based on VN nanotrees showed excellent performance with capacitance of 37.5 mF/cm2 (Ouldhamadouche et al. 2017). The surface oxidation of the metal nitrides was found to have an increased supercapacitor performance (Gray et al. 2017). In particular, electrochemical oxidation provides better performance due to the thicker oxide layer formation compared to the thermal oxidation process (Gray et al. 2017). In view of the perspectives on the nanostructure fabrication, layered double hydroxides – ternary spinel hybrids with cross-linked vertical structures – have drawn significant attention (Zhang et al. 2017c). In the crosslinked vertical structure, the double hydroxides provide multiple channels between the layers and also a stable support to ternary spinel for contributing towards both double-layer capacitance as well as pseudocapacitance. Whereas, the ternary spinel structures provide ultrafast transfer of electrons, plenty of capacitive reactive sites and an excellent transportation of electrolyte ions (Zhang et al. 2017c). The coreshell heterostructures are made up of layered double hydroxides and found applications as high-performance supercapacitors (Fig. 7.10) (Liang et al. 2018), where the layered double hydroxides serve as core and metal hydroxides acted as shell. Vertically oriented core-shell structures (NiCo-LDH@NiOOH) revealed much higher capacitance than the core and shell electrode materials alone. The superior performance was attributed to the synergetic effects of core and shell and the vertical nano-architecture. More impressively, NiCo-LDH@NiOOH structure is found to remain unchanged morphologically even after 10,000 charge-discharge cycles (Liang et al. 2018). Apart from aforementioned vertical nanostructures, conducting polymers are also found to be a suitable electrode material owing to its higher theoretical capacitance value (Song et al. 2018; Wang et al. 2010). For example, electrode made by in situ

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Fig. 7.10 Electrochemical performance of NiCo-LDH@NiOOH supercapacitor electrode. (Reproduced from Liang et al. (2018)) Copyright 2018; with permission from Elsevier)

polymerization of polyaniline exhibited high performance (Song et al. 2018). Although the specific capacitance of composite electrodes increased with concentration of aniline monomer, the reduced capacitance has been observed beyond a certain limit. The reduced storage capacity was attributed to the formation of disordered polyaniline nanowires (Song et al. 2018). The vertically oriented structure composed of wide range of materials including metal oxides, metal hydroxides, metal nitrides, and conducting polymers were explored for supercapacitor performance. The pseudocapacitor materials have disadvantages over electrochemical stability, mechanical stability, and conductivity. However, these materials provide much higher capacitance than the EDLC (Ghosh et al. 2018a). On the other hand, the excellent electrochemical stability and electronic conductivity of carbon nanomaterials have driven the research interest to develop novel nano-architectures of it to further enhance the performance. The problems associated with both type of electrode materials (EDLC and pseudocapacitor) are taken care by making hybrid nanostructures using them and are discussed in next section.

7.4.4

Composite Materials

As mentioned before, self-supported vertical carbon nanostructures not only provide the double-layer capacitance but also serve as mechanical backbone for the pseudocapacitor materials growth. The pseudocapacitor materials can be grown on the vertical carbon nano-architectures either conformally or by vertical deposition on the surface as well as on the edges of the vertical carbon structures. Conformal

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Fig. 7.11 (a) Schematic of fabrication, morphology, and corresponding Raman spectra of ZnO nanowires on aligned CNT (Reprinted from Al-Asadi et al. (2017) with the permission from AIP publishing). (b) Schematic and (c) picture of symmetric supercapacitor device composed of nitrogen-doped CNT and polyaniline (Reproduced from Malik et al. (2017) Copyright 2017; with permission from Elsevier). (d) Demonstration to power red LED by symmetric supercapacitor device (Reproduced from Malik et al. (2017), Copyright 2017; with permission from Elsevier)

coating of MnO2 on the vertical graphene showed 110 times enhancement in charge storage capacity (Ghosh et al. 2016). The ZnO nanowires grown on aligned CNT is an example of second type of heterostructure growth (Fig. 7.11a) (Al-Asadi et al. 2017). Around 12-fold enhancement has been observed in specific capacitance as well as corresponding energy and power density (Al-Asadi et al. 2017). These enhancements were due to the higher ionic accessibility, improved conductivity, and electrochemical stability. The charge storage capacity is the combination of both double-layer and pseudocapacitive contributions. It is also reported that surface activation of carbon materials by chemical treatment showed better nucleation and growth of the other structure (Rooth et al. 2009). For example, in oxygen functionalized carbon surface, oxygen functional group serves as bridging element between carbon and heteroatoms. Coupling polypyrrole conducting polymers into vertically oriented ZnO nanorods resulted in a high-performance supercapacitor electrode, where ZnO was used as template for growth (Sidhu and Rastogi 2014). Integration of carbon nanostructures, transition metal oxides, and conducting polymers was also adopted in order to improve the charge storage performance

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(Giri et al. 2014). It is reported that graphene-ZrO2 composite offers reactive sites for polyaniline growth owing to the electrostatic interaction between aniline ions and functional group. ZrO2 is found to accelerate the polymerization. The presence of ZrO2 also prevents collapse and aggregation of polyaniline nanofibers. The vertically aligned polyaniline nanofiber promotes the electron and ion kinetics as well as effective ion diffusion into the ZrO2 core (Giri et al. 2014). Even, core-shell freestanding and flexible electrodes were fabricated with nitrogen-doped vertically oriented CNT and polyaniline (Malik et al. 2017). Doping of nitrogen effected three times enhancement in specific capacitance, and incorporation of polyaniline boosted the performance of the hybrid electrode (Fig. 7.11c–d) (Malik et al. 2017). The composite materials showed the promising supercapacitor performance. However, the electrochemical properties of composite materials strongly depend on synthesis parameters. In particular, mass loading of heterostructure plays a significant role; negative impact can be seen beyond certain limit. Another important factor is that vertically oriented base materials should not be blocked by the second coating materials, otherwise electrolyte ion cannot access the base material and hence reduced performance. Therefore, a considerable research is still required to achieve the high-performance supercapacitor electrode.

7.5

Asymmetric Supercapacitor

Based on the electrode configuration, supercapacitors are classified as symmetric and asymmetric SC. For symmetric supercapacitor, as the name suggests, both the electrode materials are the same. The maximum operated potential window of symmetric supercapacitor is limited to around 1.5-2 V. To improve the energy density, maximizing potential window is more efficient (Eq. 7.1). Hence designing asymmetric supercapacitor became a hot topic in this field. The asymmetric supercapacitor is comprised of positive and negative electrodes (Fig. 7.12a) (Choudhary et al. 2017; Gund et al. 2015). The list of conventionally used electrode materials as positive and negative is shown in Table 7.1. However, it is noteworthy that the proper selection of pair electrodes is very essential in order to obtain highstorage performance in asymmetric configuration. The asymmetric supercapacitor features higher energy density and increased operating voltage compared to its symmetric counterpart (Dai et al. 2015). The additional potential window depends on the work function of the electrode materials as follows: E ¼ 1=F ðφ1  φ2 ÞN A þ ΔE 1 þ ΔE2

ð7:8Þ

where F is the Faraday constant, φ1 (φ2) is the work functions of positive (negative) electrode, NA is the Avogadro’s number, and ΔE1 (ΔE2) is the surface dipole potential of positive (negative) electrode. For the symmetric configuration, ΔE1 ¼ ΔE2 and φ1 ¼ φ2. In case of asymmetric configuration, the large work

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Fig. 7.12 (a) Schematic of an asymmetric supercapacitor with maximum operating voltage (Reproduced from Dai et al. (2015) Copyright 2015; Springer Nature). (b) Supercapacitive characterization of asymmetric supercapacitor made up with MnO2/PEDOT/PSS/CNT as positive and VN/@C NWAs/CNT fiber a negative electrode (Reproduced from Zhang et al. (2017b) Copyright 2017, with permission from American Chemical Society). (c) Microsupercapacitor in combination with series and parallel shows higher potential window and energy density. (Reproduced from El-Kady and Kaner (2013) Copyright 2013; Springer Nature)

Table 7.1 Positive and negative electrode materials for asymmetric supercapacitor Positive electrode (cathode) Carbon-based materials, MoS2, MOF, TiC (Mxene) RuO2, MnO2, NiO, Co3O4, V2O5, VOx, TiO2, CuO, CeO2, ZrO2, ZnO2 NiCo2O4, LiCoO2, LiMn2O4, NaMnO2, ZnCo2O4, CuCo2O4, Zn2SnO4/MnO2, (Cu,Ni)O Co(OH)2, Ni(OH)2, MnOOH, NiCo-LDH, CoMn-LDH, NiCoCu-oxyhydrate PANI, PPy TiN, Ni3N NiS, Ni3S2, MnS, CoSx, Co3S4, Co0.85Se, NiCo2S4

Negative electrode (anode) Carbon-based materials, TiC (Mxene) Fe2O3, Fe3O4, Mn3O4, V2O5, MoO3, MoOx, Bi2O3, ZnO2, InO2, TiO2, SnO2, WO3 LaMnO3, MnCo2O4, MnFe2O4, CoFe2O4 Ni(OH)2, FeOOH, Co(OH)xCO3

VN, FeN, WON, MON, MO2N Ni3S2

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Table 7.2 Electrode materials and their work function Materials ZrO2 MnO2 CoO SnO2 Cu2O NbN In2O3 VN/VO2 TiN TiO2 CuO

Work function (eV) 4 4.4 4.5 4.6 4.9 4.95 5 5.05–5.15 5.05–5.15 5.2 5.6

Materials Fe3O4 GaN MoO2 Co3O4 RuO2 ZnO Bi2O3 WO3 NiO V2O5 MoO3

Work function (eV) 5.8 4.1–6 6 6.1 6.1 3.9–6.3 6.4 6.65 6.7 6.85 6.9

function difference between electrode materials implies wider potential window. The work function of the conventionally used electrode materials is shown in Table 7.2. However, the work function of the materials depends on the chemical and structural factors like oxygen vacancy, cationic oxidation state, point defects, etc. (Greiner et al. 2012). Noteworthy, one should consider the charge balance between the electrodes (q+ ¼ q). The charge balance can be mass specific (Eq. 7.9) or area specific (Eq. 7.10). For mass-specific case, q ¼ C  V  m, whereas for area-specific condition, q ¼ C  V  A. Here, C, V, m, and A are specific capacitance (either in unit of F/g or F/cm2), potential window, mass, and electrode exposed area to the electrolyte. Hence, in order to fulfill the charge balance criteria, optimized mass (or area) ratio should follow: for mass specific,

mþ C  V  ¼ m Cþ  V þ

ð7:9Þ

for area specific,

Aþ C   V  ¼ A C þ  V þ

ð7:10Þ

There are several reports on asymmetric supercapacitor in literature. It has been reported that an aqueous asymmetric supercapacitor consisting of MnO2 and Fe2O3 nanostructured electrodes performed in the widest potential window of 3.2 V (Tomiyasu et al. 2017). In a study, asymmetric coaxial fiber-shaped supercapacitors fabricated by wrapping aligned CNT around vanadium nitride nanowire arrays displayed ultrahigh energy density (Fig. 7.12b) (Zhang et al. 2017b). Stacking cells in series and parallel combination enabled scale up to high operating potential window and energy density (Fig. 7.12c) (El-Kady and Kaner 2013).

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In summary, asymmetric supercapacitor can be promising alternatives to the conventional battery. However, choice of electrodes, charge balance, and electrolytes are the key factors to obtain high-performance supercapacitor. A list of electrode materials with their work functions is provided here to select the proper combination.

7.6

Summary and Outlook

Importance of vertical structures with controlled porous network for their commercial utilization as supercapacitor electrode material is addressed in this chapter. We summarized recent advances in the fabrication of different kinds of vertical structures comprising of carbon nanostructure, metal-based architecture, as well as conducting polymers for supercapacitor application. However, one should remember that the vertically aligned nano-architecture is insignificant when it is used for micro-supercapacitor application. In this scenario, nano-channeled and stacked nanosheets provide efficient pathways for ion diffusion parallel to the nanosheet planes (Chang et al. 2015; Mendoza-Sánchez and Gogotsi 2016). Although a plethora of research has been carried out from the beginning, the goal of improving energy density is not still satisfactory. Few aspects must be taken into consideration in order to enhance the charge storage performance. Primarily, the as-prepared selfsupporting vertical nanostructures should have good pore distribution, which in turn result in nonuniform diffusion path of electrolyte ions. Therefore, uniform pore size or uniform inter-vertical sheet distance is crucial and is a subject of research in order to provide an effective pathway for the electrolyte ions. Second, restacking of vertical sheets and their cross-linking also blocks the access of electrolyte ions to the full interior of vertical structures. Third, attention needs to be paid on mechanical property, electrical conductivity, and stability of as-prepared vertical nanostructures. Finally, interaction of electrolyte with the electrode plays a significant role in enhancing capacitance performance. For example, materials with pore size less than the hydrated electrolyte ions cannot contribute in charge storage. Hence, choosing proper electrolyte compatible with the electrode is a subject of concern. Future efforts should be focused toward pioneering novel concepts in nanomaterials architecture to improve the energy density of supercapacitor for real-world applications. Roll-to-roll fabrication of binder-free and self-supported electrode materials is expected to mitigate some of the problems associated with the energy storage and its delivery. With the rapid progress in the energy research field, it is anticipated that vertically oriented novel nanostructure-based supercapacitor devices will alleviate the global challenges, to a great extent, in energy storage and delivery. Acknowledgment Subrata Ghosh acknowledges financial support from Basic Science Research Program (2017R1D1A1B03028311) of the National Research Foundation of Korea. We are thankful to the anonymous reviewers for valuable suggestion and researchers for their significant contribution in the energy storage research.

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Chapter 8

Hydrogen Production Through SolarDriven Water Splitting: Cu(I) Oxide-Based Semiconductor Nanoparticles as the NextGeneration Photocatalysts Sanjib Shyamal, Ashis Kumar Satpati, Arjun Maity, and Chinmoy Bhattacharya

Contents 8.1 8.2 8.3 8.4

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoelectrochemical Water Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Simple Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4.1 Cuprous Oxide (Cu2O) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5 Complex Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.1 Copper Vanadates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.2 Copper Niobates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.3 Copper Tantalates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.4 Copper Delafossites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.4.1 Small Bandgap Cu(I) Delafossite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.5.4.2 Wide Bandgap Cu(I) Delafossite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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S. Shyamal · C. Bhattacharya Department of Chemistry, Indian Institute of Engineering Science & Technology (IIEST), Shibpur, Howrah, West Bengal, India e-mail: [email protected] A. K. Satpati Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai, India A. Maity (*) DST/CSIR Innovation Centre, National Centre for Nanostructured Materials, Pretoria, South Africa Department of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa e-mail: [email protected] © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_8

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Abstract Production of clean fuels like H2 using renewable sources such as sunlight, through photoelectrochemical (PEC) system, is one of the promising approaches. For large-scale applications of the PEC devices, the photocatalyst used should be of low cost, quite stable, and with high conversion efficiency for H2 production. This chapter describes the application of Cu(I)-based binary and ternary oxide photocatalysts toward solar H2 generation. Due to many advantages of Cu(I)-based oxides, including low bandgap energy, suitable band positions, high charge carrier mobility, and most importantly low cost and nontoxic nature, it has received significant attention in PEC water splitting reaction. Different synthetic routes, electrodeposition, atomic layer deposition, anodization, chemical vapor deposition, e-beam evaporation, pulsed laser deposition, sputtering, successive ionic layer adsorption and reaction, sol-gel, spray pyrolysis, thermal oxidation, etc., have been explored to obtain efficient Cu2O thin films. Employing suitable substrate offering better electrical connectivity facilitates the hole transport mechanism leading to improvement of water reduction process. Various co-catalysts have been identified, and application of different other compounds like metal oxides, carbon-based derivatives, etc. influences the separation of the photogenerated charge carriers, thereby enhancing the overall performance and stability of the materials.

8.1

Introduction

The developed countries must thank fossil fuel for their economic growth and advancement of human societies, with about 50% of total primary energy supply consumed by 10% of population residing in developed countries (Armaroli and Balzani 2011). In 2015 the primary energy consumption rate was 18 TW (5.67
1020J), out of which nearly 86% was derived from fossil fuel sources. Because the limited quantities of estimated remaining nonrenewable energy resources are 4
1023 J, therefore, fossil fuels cannot continue to meet our energy demands indefinitely. By 2040 the global population will likely have grown to ~9 billion, and with rapid economic and technological growth experienced by some countries particularly by China and India, the energy consumption rate projected to increase in the next few years from the present 15 TW to 27 TW by 2050 and to 43 TW by 2100 (Lewis and Nocera 2006). The depletion of natural resources attracts less attention than the environmental impact caused by the release of the greenhouse gases from burning of carbon-based fossil fuels, in particular CO2. It has been calculated that the increase in global mean temperature varies almost linearly in correlation with the rise in an atmospheric concentration of CO2 over hundreds of thousand years. Therefore, to fulfill the energy demand in a scalable and sustainable fashion, the energy conversion efficiency should be increased along with the increasing contribution of carbon-free energy sources into the total energy mixture. Therefore, the only way to meet the future energy demands maintaining the atmospheric CO2 concentration within the safe limit of 450 ppm is to focus on the various renewable energy resources and

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implementation of low-carbon technology in association with carbon capture and storage. We are fortunate that exploration of the renewable energy resources has begun in large scale, as presented in Fig. 8.1a–c, to meet up the global energy demand in contrast to the conventional sources such as oil, coal, gas, etc. The projected proportion of generation of renewable energy is 50% of total energy production by 2050 and 90% by 2100. Solar energy holds the largest potential capacity among all the different types of renewable sources. In spite of the planet receiving 174
103 TW of solar radiation, which is ~10,000 times higher than that of the total human society consumption per day (18 TW), the global energy demand can be fulfilled by harvesting the fraction of this solar energy (Tsao et al. 2006). To harness this alternative energy source, sunlight needs to be absorbed by the materials and converted into useable form of energy and storage. Semiconductors have the ability to absorb light directly from sun and convert it into electrical energy which can be either stored in batteries or used to split water or reduce carbon dioxide to chemical fuels. Solar photovoltaic, solar thermal, solar water heater like different technologies can convert energy from the sun for mankind as presented in Fig. 8.2. Photovoltaic cell, predominantly made by silicon and various thin-film materials, can convert sunlight directly into electricity which can be injected onto electric grid. Solar thermal technology converts the optical energy to thermal energy through low-cost, environmentally friendly devices to heat water or other fluids and can also power solar cooling systems. These technologies are implemented to convert solar energy to energy vector, electricity, or heat, which neither be easily stored and nor appropriate on a terawatt scale. Therefore, the alternative nature-inspired approach is more practical to convert directly solar energy into a chemical fuel which can be stored and later be used in demand. In natural photosynthesis, water and carbon dioxide are converted to hydrocarbon and oxygen using sunlight through Z-scheme mechanism and are presented in Fig. 8.3. Artificial photosynthesis mimics the natural one. Where the semiconducting materials are often selected as they can capture solar photon, then photo-excited electrons and holes are generated and migrated to their respective places (electron from valance band to conduction band) and drive chemical reaction at their surface. The lightest and most abundant element in the universe is “hydrogen” (H2). It is colorless, odorless, nontoxic, and most energy-dense fuel per mass. One pound of hydrogen holds 142 MJ, i.e., three times of the energy of the same amount of gasoline (http://www.fchea.org). Hydrogen can be transported and shipped like oil and natural gases. It can form water vapor, when burned or used in fuel cell. Because of these advantageous properties, hydrogen can be used as a fuel to run electric motors, cars, trains, ship, airplanes, and rocket in various forms. Now the major challenge is the production and storage of hydrogen through clean and cost-effective way toward the hydrogen economy. The production of hydrogen through “steam reforming” of gasoline or natural gas, releasing CO2 or CO, thus, possesses no advantage over direct use of these fuels.

a

Energy

Renewable

Recyclable

Non-Renewable

Nuclear

Solar

Oil

Wind

Coal

Wave

Natural gas

Tidal

Nuclear

Biomass Geo-thermal Hydro-electric

b

World Energy Consumption by Fuels (Btu) 2015

250

petroleum and other liquids 200

coal

natural gas

150 renewables 100 nuclear

50 0 1990

1995

2000

2005

2010

2015

2020

2025

2030

2035

2040

c

Fig. 8.1 (a) Different energy aspects including petroleum and other liquid fuels and renewable include hydropower, biomass, solar, geothermal, and wind. (b) World energy consumption in Btu (British thermal units) by different fuels and (c) proportion of different energy sectors like conventional (petroleum and other liquid fuels) with non-conventional or renewable (hydropower, biomass, solar, geothermal, wind, tidal, etc.) energy sources (https://www.eia.gov/outlooks/ieo/)

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Solar Energy

Mechanical Energy

Photovoltaic

Electricity

Heat

Photolysis of H2O

Thermolysis

Biomass

Conversion

H2

Fig. 8.2 Solar energy conversion path and technologies for H2 production

CH4 þ H2 O ! CO þ 3H2

ð8:1Þ

CO þ H2 O ! CO2 þ H2

ð8:2Þ

In recent years due to the growing concern about the climate change, effort has been made to develop the sustainable way to produce hydrogen. There are several ways for solar hydrogen production (Kudo and Miseki 2009): • Reforming of biomass • Electrolysis of water using a solar cell • Photocatalytic or photoelectrochemical water splitting (artificial photosynthesis) Due to certain ecological and land-use limitation, biomass reformation is not enough implemented for hydrogen production, whereas the water electrolysis technology with 70–80% efficiency is widely used. By coupling of water electrolyzer of ~80% efficiency and silicon photovoltaic cell of 15% efficiency, solar-to-hydrogen production efficiency can be reached to 12%. In a carbon-neutral economy, the present gasoline consumption of the USA would require the production of 0.34 million tons of hydrogen per day. With PV-electrolyzer of 12% solar-to-hydrogen efficiency, the panels would cover the surface of 5700 km2 just to replace the US consumption of gasoline. The land needed for installing the panels and the cost of silicon photovoltaics today are obvious drawbacks for solar photovoltaic electricity. Since the solar thermal system is able to generate heat, a concentrated solar power

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Z Scheme

2e–

P700 –2.0

Reducing P680

Potential (V)

–1.0

fd

H+

NADPH NADP+

2e–

Q PQ 0 2e +1.0

+2.0

Oxidizing 2e– H2O

–

Cit b/f

PC

P680

P700

PS I

2H+ + 1/2 O2 PS II

Fig. 8.3 Natural photosynthesis through Z-scheme mechanism

plant is able to continuously produce electricity, and electrolyzers coupled to solar thermal can therefore be operative at full capacity. Among the sustainable hydrogen production technologies, photoelectrochemical (PEC) water splitting is a promising method for conversion of solar energy to renewable clean hydrogen fuel. A PEC system can electrolyze water in presence of solar energy using a single semiconductor-based device, where the starting material is water in presence of sunlight and the outputs are highly clean hydrogen and oxygen gases. The “holy grail” of solar energy conversion and storage is the photo-electrolysis of water using semiconductors as both the light absorber and energy converter, to store solar energy in the simplest chemical bond: H2 (Bard and Fox 1995). The continuous effort has been made to search for the suitable semiconductor for PEC water splitting. For the ideal semiconductor, there are several key criteria: it can efficiently absorb light, generate enough photovoltage to sustain water splitting, remain stable during operation, and most importantly, it must be cheap and environmentally friendly to allow for large-scale implementation.

8.2

Photoelectrochemical Water Splitting

When the photons come from sunlight and hydrogen is produced through lightinduced electrochemical processes, the cell is termed as photoelectrochemical (PEC) cell. In 1972, Fujishima and Honda first demonstrated the photocatalytic water

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Bias

H2

O2 e–

e–

Sunlight

h+

Aqueous Electrolyte

Photocathode

Metal Anode

Ion permeable membrane

Aqueous Electrolyte

Fig. 8.4 Simplified diagram for a hydrogen evolution PEC cell

splitting in PEC cell using TiO2 as the photocatalyst (Fujishima and Honda 1972). A PEC cell consists of an anode and a cathode immersed in an electrolyte and connected by an external circuit and depicted in Fig. 8.4. The key feature is that one of the two electrodes is a semiconductor that can absorb photons from solar light and the other is typically metal. A p-type semiconductor photocathode reduces the proton to hydrogen, while oxygen is evolved at the metal anode. The reverse formation is also possible, with an n-type semiconductor photoanode oxidizing water to oxygen and hydrogen being produced at the metal cathode. If both electrodes are made up of light-absorbing materials, the cell is called tandem cell. When the photoelectrode absorbs photons having energy higher than its bandgap, some electrons get enough energy to transit from VB to CB generating electron-hole pairs. The photogenerated carriers can either recombine or be separated by the external electric field and diffuse to semiconductor-electrolyte interface, undergoing interfacial electron transfer process. The band bending occurs for both VB and CB edges at the surface of the semiconductor due to the electric field and is necessary to pass the free carriers at the respective electrodes. In acidic electrolyte, the photogenerated holes oxidize water molecules into oxygen and protons at anode surface (oxygen evolution reaction): 2H2 OðliqÞ þ 4hþ ! O2ðgasÞ þ 4Hþ

ð8:3Þ

The migration of hydrogen ions and electrons toward cathode occurs simultaneously through the electrolyte and external circuit, respectively. Hydrogen gas gets evolved at the cathode surface due to the reduction of protons by the electrons.

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4Hþ þ 4e ! 2H2ðgasÞ

ð8:4Þ

At high pH, the redox reaction can be written as: 4OH þ 4hþ ! 2H2 O þ O2ðgasÞ 

4H2 O þ 4e ! 2H2ðgasÞ þ 4OH



ð8:5Þ ð8:6Þ

So the overall water splitting reaction may be written as: 1 2hν þ H2 OðliqÞ ! O2ðgasÞ þ H2ðgasÞ ΔEo ¼ 1:23 V 2

ð8:7Þ

where ΔE0 is the standard potential of the electrochemical cell, which is independent on the pH. In a photoelectrochemical cell, the formation of gaseous hydrogen and oxygen takes place at different electrodes. This is helpful to prevent the reverse reaction between oxygen and hydrogen to form water. The formation of gaseous hydrogen and oxygen through the water splitting reaction is highly endothermic and endergonic. This means that the process associated with the positive Gibbs free energy changes (ΔG0 ¼ +237 kJ/mol), and hence the reaction is non-spontaneous (Latimer 1938), which is converted to potential difference between two electrodes using Nernst equation (ΔE0 ¼ 1.23 V).

8.3

Background

In addition to fulfill the aforementioned requirements for efficient hydrogen production and long-term stability, the materials employed for solar hydrogen production have to be earth abundant, cheap, and environmentally friendly. There are several photocathode materials explored for water splitting through PV cell. Due to the expensive production cost, these materials have limited use as a semiconductor. Memming et al. first developed Ga-based phosphide (p-GaP), a photocathode material with an indirect bandgap (2.25 eV) which is stable in aqueous environment (Memming and Schwandt 1968) (Fig. 8.5). It shows small photocurrent densities, due to the large minority carrier recombination, because of short carrier diffusion lengths relative to the depth of the visible light absorption in the film. The maximum photocurrents obtained for nanostructure p-GaP are 420 nm) 0.5 M Na2SO4, 400 W Xe arc lamp (>420 nm) 0.5 M Na2SO4, 400 W Xe arc lamp (>420 nm) 0.5 M Na2SO4, 100 mW/cm2

6.3

0.22 at 0.2 V/ SCE 0.1 at 0.6 V/Ag/ AgCl 0.4 at 0.35 V/ RHE 0.75 at 0.6 V/ SCE 1.5 at 0.6 V/SCE

0.5 M Na2SO4, 400 W Xe arc lamp (>420 nm) 1 M NaOH 100 mW/cm2

6.3 14

2.6 and 2.5 at 0.6 V/SCE 0.7 at 0.4 V/RHE

CuFeO2

1 M NaOH 100 mW/cm2

14

1.5 at 0.35 V/RHE

CuRhO2

1 M NaOH, 49.5 mW/cm2

14

1.0 at 0.9 V/SCE

CuNb3O8 CuNb8O21 α and β-Cu2Ta4O11 Cu3Ta7O19 and Cu5Ta11O30 CuFeO2

8.5.1

pH 11

6.3 12 6.5

References de Jongh et al. (2000) Mao et al. (2012) Paracchino et al. (2012) Li et al. (2014) Shyamal et al. (2015) Zhang et al. (2012) Yang et al. (2016) Zhang et al. (2013) Sahoo et al. (2015) Joshi et al. (2011) Joshi et al. (2012) Choi et al. (2013) King et al. (2016) Fuoco et al. (2012) Read et al. (2012) Prévot et al. (2015) Gu et al. (2014)

Copper Vanadates

Due to the interesting structural features and wide range of visible light absorption with favorable band energy position, copper vanadium oxides are gaining more attention for researchers. The crystal structure of the materials is Cu-coordinated tetrahedral geometry, and the bandgap energy varies from 1.2 eV to 2.7 eV. There are only two copper vanadate compounds known with Cu(I) cation, e.g., Cu3VO4

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and CuVO3. Other copper vanadates are formed with either Cu(II) or mixture of Cu (I) and Cu(II). The photoelectrochemical reduction of water to hydrogen using Cu3VO4 as a photocathode was reported by Sahoo et al. (2015). Solid-state reaction has been conducted to prepare the desired compound with proportional mixture of Cu2O and V2O5. Since the final product is obtained with black color, it absorbs most of the visible region of the solar spectrum with small bandgap energy 1.2 eV (Fig. 8.11). The polycrystalline Cu3VO4 film exhibited very small photocurrent 0.02 mA/cm2 in 0.5 M Na2SO4 electrolyte (pH 5.8) at 0.2 V/SCE under illumination of AM 1.5G for water reduction reaction. The significant enhancement of the photocurrent increased to 0.05 and 0.22 mA/cm2 when the film is heated in air at 300 and 350  C for 15 min. This enhancement of the photocurrent is preliminary due to the formation of Cu(II) on the surface through oxidation of Cu (I) which increases the meta-vacancies within the crystal and imports the better p-type conductivity.

8.5.2

Copper Niobates

Cu(I)-based niobate oxides have currently been investigated for their excellent photoelectrochemical performance and stability in aqueous environment. There are several Cu-Nb-mixed oxides, among them CuNbO3 (Sleight and Prewitt 1970), CuNb3O8 (Marinder et al. 1980), and Cu2Nb8O21 (Choi et al. 2013) have been studied for their photoelectrochemical and photocatalytic properties. Sleight first reported synthetic procedure of CuNbO3 with a black crystal at high temperature 1000–1200  C and high pressure 65 kbar, which consists of Cu(I) and Cu(II) in mixture (Steele and Heinzel 2001). Where Wahlstrom and Marinder successfully synthesized CuNbO3 crystal with only Cu(I) cation, they did not examine the PEC properties of the prepared materials. Recently the photoelectrochemical and photocatalytic properties of CuNbO3 have been reported by Maggard group (Joshi et al. 2011; Zoellner et al. 2016). They prepared the desired compound through solid-state reaction and found the cherry-red color of the product with indirect bandgap energy ~2.0 eV. The photoelectrode thin film of CuNbO3 was prepared over FTO substrate, and the as-prepared films annealed in air at different temperature 250–500  C for 3 h. With irradiation of solar-simulated light 400 W lamp with >420 nm UV cutoff filter in 0.5 M Na2SO4 solution at pH 6.3, the maximum photocurrent density of 0.1 mA/cm2 generated for the polycrystalline film is heated at 350  C. Beyond this optimum temperature, Cu(II) impurity phase generated and simultaneously decreased the photocurrent for water reduction. In case of CuNbO3 photocatalyst, the photocurrent for water reduction increases with the increase of the dark current also, while the increment of dark current can be decreased when Ta incorporated into CuNbO3 crystal, e.g., Cu(Nb1xTax)O3, maintaining the similar photocurrent for water reduction. However, in case of particles, the photocatalytic rate of H2 production decreases with the increase of the Ta substitution into CuNbO3 structure. When 1 g of suspended CuNbO3nanoparticle irradiates in visible light,

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142 μmol H2 is produced, which is decreased with the increase of the Ta into the CuNbO3 structure. In case of 25% Ta-substituted oxide, the amount of H2 production decreased to 67 μmol/g with the decrease of the Faradic efficiency. CuNb3O8 is another p-type Nb-based mixed oxide photocatalyst reported by Joshi et al. (2012). The CuNb3O8-mixed metal oxide was prepared by hightemperature solid-state reaction, where Cu2O and Nb2O5 are mixed stoichiometrically and heated 750  C for 24 h. The bandgap of the CuNb3O8 was found 1.26 eV (indirect) and 1.47 eV (direct) and confirms suitable absorption of entire visible light of solar spectrum. Thin films of CuNb3O8 were prepared on FTO and annealed at 450  C for 3 h under vacuum. The maximum water reduction photocurrent density achieved 0.4 mA/cm2 in 0.5 M Na2SO4 electrolyte (pH 6.3) at 0.35 V/RHE under illumination of 400 W lamp with >420 nm UV cutoff filter. When the polycrystalline film was held at an applied bias 0.156 V/RHE for 1 h under visible light irradiation, > 3 μmol/cm2 H2 gas evolved through water reduction with maximum Faradic efficiency 62%. Recently Choi et al. reported another interesting Cu2Nb8O11 copper niobate mixed oxide photocathode (Liu et al. 2010). They synthesized the complex compound into two different ways, either by solvothermal reaction of Li3NbO4nanoparticle and CuCl or by stoichiometric mixture of Cu2O and Nb2O5 in CuCl flux. The bandgap of the Cu2Nb8O11 oxide exhibits 1.43 eV (Fig. 8.11). Similar to the previous, thin films of Cu2Nb8O11 were prepared on FTO and annealed at 350  C for 3 h. The photocurrent density 0.25 mA/cm2 is observed in 0.5 M Na2SO4 (pH 12) under chopped visible light irradiation. When the as-prepared thin films are heated at 450  C, in air, the photocurrent significantly enhanced to 0.75 mA/cm2 due to the formation of Cu(II) at the surface.

8.5.3

Copper Tantalates

The family of the photocathodes including Cu2Ta4O11, Cu3Ta7O19, and Cu5Ta11O30 are the interesting materials for photoelectrochemical hydrogen production. Cu2Ta4O11 is prepared by mixing of Cu2O and Ta2O5 in a CuCl-mediated flux in a temperature range of 625–700  C. Depending upon the temperature, two forms of Cu2Ta4O11 are observed, α and β. The photocurrent density of the thin film of both phases showed 1.5 mA/cm2 at 0.6 V/SCE for water reduction reaction (King et al. 2016). Cu3Ta7O19- and Cu5Ta11O30-mixed metal oxides are prepared by molten salt flux method, where stoichiometric ratio of Ta2O5 and Cu2O is mixed and heated at 700 and 900  C for 24 h. Thin films are prepared on FTO at 400–500  C and followed by annealing in air at 350–550  C for 3 h. Fuoco et al. first investigate the photoelectrochemical properties of Cu3Ta7O19 and Cu5Ta11O30 as a p-type photoelectrode. The photocurrent density observed is 2.6 mA/cm2 in 0.5 M Na2SO4 (pH 6.3) under chopped visible light irradiation when the thin films are heated at 550  C (Fuoco et al. 2012). The band positions and the carrier concentration of these materials were investigated by Mott-Schottky measurement. The

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conduction band energies of Cu3Ta7O19 and Cu5Ta11O30 are at 1.28 and  1.54 V/ NHE at pH 6.3, which are low enough than the water reduction potential (0.372 V) for hydrogen production. Powder nanoparticles of these mixed oxides show the significant photocatalytic H2 generation under illumination, but the production rate relative is lower than copper niobate-mixed oxides. Kato et al. reported the photocatalytic rate of H2 production using platinum-decorated Cu5Ta11O30 for water reduction is 0.6 μmol/h in a methanol-water mixture (Kato et al. 2013). Cu3Ta7O19showed relatively low H2 production rate of 0.1 μmol/h, which can enhance to 2.6 μmol/h for Cu1.8La0.4Ta7O19.

8.5.4

Copper Delafossites

The general formula of Cu(I) delafossites is CuMO2, where M is the metal cation with +3 oxidation state (Cr, Fe, Co, Rh, Al, Ga, In, and Sc). The Cu(I)-based delafossites are wide range of visible light-active semiconducting materials and are promising for photoelectrochemical hydrogen production from water. Depending upon their light absorption possibility, they are classified into two categories: (a) small bandgap Cu(I) delafossite and (b) wide bandgap Cu(I) delafossite.

8.5.4.1

Small Bandgap Cu(I) Delafossite

CuFeO2, CuRhO2, and β-CuGaO2 belong to this family due to their relatively low bandgap energy, 1.36, 1.6, and 1.47 eV, respectively, and are capable to absorb large range of light absorption. Cathodic electrodeposition of CuFeO2 from nonaqueous (DMSO) bath containing the mixture of Cu(NO3)2 and Fe(NO3)3 on FTO was reported by Read et al. (2012). Potentiostatic electrodeposition was conducted at 0.3 V/Ag/AgCl followed by annealing the deposited film at 650  C for 1 h in Ar. In 2015 Prevot et al. also synthesized the CuFeO2 using sol-gel method on FTO substrate (Prévot et al. 2015). The water reduction performance in terms of photocurrent density achieved for pure CuFeO2 is very low enough, which is significantly improved by optimizing the thickness and increasing the carrier concentration through thermal intercalation of oxygen. Photocurrent up to 1.51 mA/cm2 in 1 M NaOH with O2 saturation was observed, while for Ar-saturated aqueous solution, the maximum photocurrent is 0.5 mA/cm2 at +0.35 V/RHE under one-sun illumination. The performance of the bare delafossite can be enhanced by applying the suitable oxide over layers and platinum catalyst. The CuFeO2/AZO/ TiO2/Pt shows the enhanced solar hydrogen production photocurrent of 0.4 mA/ cm2 at 0 V/RHE. Stability is the most important feature of CuFeO2delafossite; no significant loss of photocurrent is observed after 40 h under the operating condition. Recently, Jang et al. reported the oxygen intercalated CuFeO2 photocathode prepared by hybrid microwave annealing for efficient solar hydrogen production (Jang et al. 2016). The maximum photoelectrochemical water reduction photocurrent

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achieved is 1.3 mA/cm2 in Ar-purged 1 M NaOH solution at +0.4 V/RHE under 100 mW/cm2 for hybrid microwave annealing compared to that of 0.62 mA/cm2 for thermal annealing. Further loading of NiFe double hydroxide/reduced graphene oxide electrocatalyst on hybrid microwave annealing post-treated sample exhibits high photocurrent 2.4 mA/cm2 in Ar-purged 1 M NaOH solution at +0.4 V/RHE under 100 mW/cm2 for solar hydrogen production. Gu et al. reported the “selfhealing” p-type CuRhO2 delafossite materials for photoelectrochemical hydrogen production under solar irradiation (Gu et al. 2014). The CuRhO2-mixed oxide has been prepared by solid-state method. Faradic efficiency of ~80% for H2 production was reported at 0.2 V over potential in an air-saturated solution under visible light illumination, while in Ar-saturated solution, H2 production decreases with time leading to the formation of Cu(0) at the electrode surface. A maximum photocurrent of 1.0 mA/cm2 in 1 M NaOH solution (pH 14) at 0.9 V/SCE under 49.5 mW/cm2 illumination is achieved in air, and the electrode is stable for 8 h in operation condition. It is very interesting that the material regenerates up to 95% in terms of photocurrent when it is heated 100  C for 1 h in air. This feature makes this material a significant candidate for solar conversion of hydrogen. Omata et al. first reported wurtzite-type β-CuGaO2 photocathode material synthesized by ion exchange of Na+ ion in β-NaGaO2 with Cu(I) in CuCl flux. This is a potentially efficient lightharvesting material due to its low bandgap energy 1.47 eV.

8.5.4.2

Wide Bandgap Cu(I) Delafossite

Delafossite CuCrO2 has been extensively investigated as a potential material for solar hydrogen production due to its p-type semiconducting nature and long-term stability. It has direct and indirect bandgap energy of 3.0 and 1.3 eV, respectively; therefore, it can absorb wide range of the solar spectrum. Several groups have studied CuCrO2 for its application in H2evolution. Saadi et al. first reported that powder CuCrO2 can generate H2 from water in the presence of hole scavengers under visible light irradiation (Saadi et al. 2006). They synthesized delafossite CuCrO2 in two different ways: (i) chemical evaporation followed by annealing and (ii) solid-state method. CuCrO2 evolve significant amount of 2.2 ml H2 gas in 1 M KOH with 0.025 M S2 under three tungsten lamp with light intensity 200 W each, which significantly enhances when p-CuCrO2 is coupled with n-Cu2O. Ma et al. reported photoelectrochemical properties of CuCrO2 prepared via solid-state method for H2 production (Ma et al. 2014). They have also reported that the PEC performance of CuCrO2 increases with increasing annealing temperature for H2 production. The thin films are prepared by depositing CuCrO2 on FTO substrate by electrophoresis method followed by calcinations at 400  C for 2 h. Under visible light irradiation, CuCrO2 film generated cathodic photocurrent, and the current density increases with increasing annealing temperature. Maximum photocurrent observed for 1100  C annealed CuCrO2 of 0.1 mA/cm2 in 0.5 M Na2SO4 under white LED illumination of 100 mW/cm2. Photocatalytic H2 generation is also conducted with suspended CuCrO2particles in water/ethanol mixture, while ethanol acts as a hole scavengers. The maximum amount of 5 μmol/g H2 gas produced for

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CuCrO2, but the amount significantly increases when platinum co-catalyst decorated over CuCrO2 and it gives 8 μmol/g for 3 h of reaction time under 500 W Xe-Hg lamp. Structural formation of delafossite materials also affects their photocatalytic performance for H2 production. Gao and co-workers reported mesoporous delafossite CuCrO2 with better performance than the bulk CuCrO2 in terms of PEC activity (Zhang et al. 2013b). A simple nanocasting method has been used to synthesize mesoporous CuCrO2nanoparticle. For bulk CuCrO2 the amount of H2 generated is ~ 2 μl/h when 5 mg of nanoparticles is suspended in 15 ml of basic 0.025 M Na2S solution under white light illumination, while for mesoporous CuCrO2, the amount of evolved H2 gas increased to ~ 14 μl/h under the same experimental condition. Furthermore, the incorporation of Mg in to CuCrO2 can also enhance more than threefold of the H2 generation activity than non-doped CuCrO2 in 15 ml of basic 0.025 M Na2S solution under visible light illumination for 7 h. This is due to the better electronic connectivity of the semiconductor with Mg doping, which prevents the photogenerated charge carrier recombination. The H2 production efficiency of delafossite CuCrO2 can also be increased with the formation of p-n heterojunction. Zong-yuan et al. reported that when p-type CuCrO2 coupled with wide bandgap n-type semiconductor e.g. ZnO and WO3, the PEC activity significantly enhanced for water splitting H2 production (Zong-yuan et al. 2013). CuCrO2 has been synthesized by semi wet method followed by calcinations at two steps at different temperature at N2. The p-n junction has been prepared by grinding of p-CuCrO2and n-ZnO or WO3 with their stoichiometric and finally calcinations. Very small amount of H2 gas has been generated when CuCrO2 particles are suspended in water under 250 W high-pressure Hg lamp. Changing the irradiation source to Xe lamp, no H2 has been detected. While the CuCrO2-ZnO or CuCrO2-WO3 composite particle shows significant improvement for water splitting H2 generation in bath source of irradiation. CuAlO2 a p-type delafossite photoelectrode with large bandgap energy can also generate H2 from water using sunlight. Recently Choi et al. (2017). CuAlO2has been synthesized via electrodeposition of Cu(II) and Al(III) on FTO substrate in aqueous and DMSO solvent, followed by annealing in air and Ar. The optimized CuAlO2 electrode exhibited the Faradic efficiency for H2 production of ~70% at +0.3 V/RHE in 0.1 M KOH solution under 100 mW/cm2 light illumination. The PEC performance of the optimized CuAlO2 significantly enhanced to ~100% Faradic efficiency upon the addition of sacrificial hole scavengers (sulfide and sulfite). Further, the deposited CuAlO2 over ~150 nm Au coated FTO substrate, in absence of hole scavengers, shows enhanced PEC H2 production efficiency by three fold than bare CuAlO2, with improve the Faradic efficiency to ~100% at +0.3 V/RHE. Electrocatalytic role of CuAlO2 delafossite nanoparticle has been reported by Ahmed et al. For the first time, the nanoparticle of CuAlO2 with an average size of 20 nm has been synthesized by ultrasonic method followed by annealing at 850  C in N2 (Ahmed and Mao 2015). The maximum current density of 64 mA/cm2 in 0.5 M KOH at 1.2 V/AgAgCl is observed for H2evolution reaction from cyclic voltammogram. Photoelectrochemical hydrogen production using delafossite CuGaO2 electrode has been reported by Lee et al. (2014b). The semiconducting material has been synthesized by high-temperature solid-state reaction of CuO and

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Ga2O3 with 2:1 molar ratio. The material shows direct and indirect transition with bandgap energy 3.6 and 2.7 eV. When CuGaO2 electrode is subjected to irradiation of light from 300 W Xe(Hg) arc lamp (light intensity, 1.5 W/cm2) in 0.1 M KH2PO4 aqueous solution, with an applied potential of 0.5 V/SCE, H2 gas evolved from the surface of the electrode, and cathodic photocurrent also generates. During the continuous exposure of light up to 8 h, > 175 μmol of H2 gas is generated from the electrode.

8.6

Conclusion

In this chapter we discuss about the renewable and clean production of solar fuels for the mankind which would solve the serious issue of global energy crisis and resolve the environmental impact from the use of fossil fuel. Photocatalytic production of H2 from direct splitting of water using a suitable semiconductor is an important way to serve this purpose. Enormous research effort has been made to find the suitable photocathode material for solar hydrogen production from water. Even though no such material has been identified for large-scale implementation, some materials have shown that have been discussed to have very good potential and can partially satisfy most of the requirements for a good photocatalyst for solar water splitting hydrogen production. Among them Cu(I)-based oxides have some favorable characteristics such as (i) p-type semiconductivity, (ii) small bandgap with wide range of visible light absorption, (iii) suitable band potential for H2 production from water, (iv) readily converted to Cu(II) which can import better electrical conductivity, and most importantly (v) nontoxic, low cost, and earth abundant for the potential application in H2 production. Most of the Cu(I)-based binary and mixed metal oxides absorbs wide range of visible light due to their favorable bandgap energy (from 1.2 to >3 eV) and suitable band positions which make these materials efficient for photoelectrochemical H2 production. As presented in this chapter, several researches have been worked to improve the H2 production efficiency of this material through different ways. Doping of Cu(I) oxide with the foreign elements and bandgap and band position engineering, lowering the charge carrier recombination through the formation of p-n heterojunction, and loading of suitable co-catalyst and incorporation of hole-selective layer and electron-transporting layer are some of the ways that have been tried by the researchers to enhance the efficiency. Photoelectrochemical stability is the major issue for Cu(I) oxides. During PEC operation, self-reduction of Cu(I) to Cu(0) occurs at the surface. Several research report the surface-protecting layer (Al-doped ZnO and TiO2) and formation of heterojunction with stable semiconductors that have been effectively used for Cu2O. Cu(I)-based mixed metal oxides are extensively investigated for their variable bandgap with varying conduction and valance band energies. Due to the high carrier concentration and open-circuit voltage, Cu(I)-based mixed metal oxides have great future for photoelectrochemical H2 production from direct splitting of water using renewable solar light.

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Chapter 9

Application of Nanoparticles in Clean Fuels Kumaran Kannaiyan, Reza Sadr, and Vignesh Kumaravel

Contents 9.1 Need for Clean Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 Metal Particles as Fuel Additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2.1 Key Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.3 Nanofuel Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4 Influence of Nanoparticles on Fuel Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.1 Thermophysical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.2 Spray (Atomization) Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.3 Evaporation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.4 Ignition Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.4.5 Combustion and Emission Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.5 Closing Remarks/Future Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Across the globe, the economic developments, especially in developing countries, have led to a constant increase in energy demand in the last two decades or so. Such rapid progress comes at a price of having everlasting damage to the environment. This has placed immense responsibility on the scientific community to find a sustainable way to produce clean energy and at the same time safeguard our planet. Thanks to the advent of nanoscience/nanotechnology, researchers are

K. Kannaiyan (*) Department of Mechanical Engineering, Texas A & M University at Qatar, Doha, Qatar e-mail: [email protected] R. Sadr Department of Mechanical Engineering, Texas A & M University at Qatar, Doha, Qatar Department of Mechanical Engineering, Texas A & M University, College Station, TX, USA e-mail: [email protected] V. Kumaravel Department of Environmental Science, School of Science, Institute of Technology Sligo, Sligo, Ireland © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_9

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actively engaged in utilizing the so-called nanomaterials, i.e., metal or metal oxide particles at the nanometer scale, to produce clean energy in a sustainable way. Today, the application of nanomaterials covers a wide spectrum from water purification to clean energy production to energy storage to healthcare drug delivery to the aviation industry. This chapter focuses on one such application where nanoparticles are utilized to derive clean energy. With the advent of nanotechnologies, the metal/metal oxide nanoparticles (MNPs) have received enormous interest as fuel additives from the scientific community in the recent past. Several research efforts have been made to understand its role in enhancing the liquid fuel performance and eventually achieving the broader objective of producing environmentally clean fuels. In this section, the influence of dispersion of MNPs in liquid fuels on their thermophysical characteristics and, in turn, the evaporation, ignition, combustion, and emission performance are discussed. Current developments, challenges ahead, and future opportunities are highlighted.

9.1

Need for Clean Fuels

Over the last two decades, there have been increasing concerns about the environmental impact of the usage of fossil fuels (like diesel, gasoline, and jet fuels for different modes of transportation) and the sustained supply of those fuels in the future (at a reasonable price) due to dwindling oil resources. These two factors are crucial because it is perceived by the policy makers and scientists alike that those fuels will continue to serve as the lifeline for the global community in the near future (Agarwal 2011). It is true that there has been a significant interest in using environmentally friendly “electric” energy for transportation by several nations. However, there are two critical factors, required infrastructure and economic viability, that will determine its success. It is worth pointing out that in many countries with abundant oil resources, fossil fuels are used to generate electricity, and therefore, it is not fully independent. In view of the above facts, a great deal of effort has been devoted to find sustainable alternative, clean, and efficient fuels. This goal can be achieved through different ways like finding alternative fuels from different feedstocks (Blakey et al. 2011; Fyffe et al. 2011; Kannaiyan and Sadr 2014a, b), improving the fuel performance (Khalife et al. 2017; Khond and Kriplani 2016; Mehta et al. 2014; Saxena et al. 2017), developing efficient engines through new technologies (Graham et al. 2014; Kyprianidis and Dahlquist 2017; Lee 2010), etc. In engines, the fuel has a greater role than just power generation as shown in Fig. 9.1. The focus of this section is on “improving the fuel performance.” This section will start with a brief discussion on the background, followed by a discussion on the methodologies employed, and the performance enhancements achieved, and close with its potential future prospects.

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Fig. 9.1 Different roles played by fuel in engines. (General Electric (GE90) aviation engine, source: https://en.wikipedia.org/wiki/General_Electric)

9.2

Metal Particles as Fuel Additives

The concept of mixing high energetic (i.e., containing high energy stored per unit volume) metal particles as fuel additives with hydrocarbon liquid fuels has been around for a while now (Baek and Cho 1999; Beloni et al. 2008; Peleg and Timnat 1982; Roy Choudhury 1992; Wong and Lin 1992). The key driving factor for adding metal particles to liquid fuels is to enhance the amount of energy released during combustion within the fixed volume of the combustor. This method of enhancement is well established, and several advancements have been in progress with solid fuels that are typically used in the field of propulsion (Shalom and Gany 1991). However, it is still in its nascent stage with respect to liquid fuels. Initially, the metal particles with a high energy density (i.e., amount of energy stored per unit volume) in the size range of a few micrometers were added to liquid fuels. The positive effect of the fuel additives was negated by the increase in fuel ignition delay (Beach et al. 2006; Hunt and Pantoya 2005) (i.e., increase in the time required to ignite the fuel droplets with metal particles). This adverse effect on the fuel ignition delay was attributed to the insufficient surface area-to-volume ratio of the micrometer-sized metal particles, and hence, the size of the particles has to be reduced further to overcome this issue (Dreizin and Schoenitz 2015; Mehta et al. 2014; Yetter et al. 2009). With the arrival of nanotechnologies, the metal particles in the size range of a few nanometers (typically, particle diameters less than 100 nm, see Fig. 9.2) are produced. As a result, a whole new class of fluids, called as nanofluids, received a significant attention as potential heat transfer fluids because of their reported anomalous thermophysical properties (Ganvir et al. 2017; Prasher et al. 2005; Puliti et al. 2012; Raja et al. 2016; Sarit et al. 2007; Sergis and Hardalupas 2011; Sharma et al. 2016; Taherian et al. 2018; Trisaksri and Wongwises 2007). Design of nanofluids

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Fig. 9.2 Comparison of micrometer (μm)- and nanometer (nm)-sized aluminum particles. Scale: 1000 nm ¼ 1 μm (SEM pictures from the Internet (a) NanoAmor, USA, http://www.nanoamor. com/, and (b) Jiangsu-Tianyuan Metal power Co., China, http://www.aluminum-powders.com/)

provides the opportunity to adjust the thermophysical properties of the fluid with minimum or no requirement to alter the mechanical components of the fluidic systems. The high surface area and the potential to store surface energy make nanometer-sized particles more attractive than their micrometer-sized counterparts (Yetter et al. 2009). Consequently, a renewed focus on nanometer-sized particles as fuel additives has led to several investigations to understand its influence on the fuel thermophysical properties, evaporation, combustion, and emission performance. Metal nanoparticles (MNPs) such as iron (Fe) (Gan et al. 2012), boron (B) (Karmakar et al. 2011), nickel (Ni) (Ma et al. 2011), aluminum (Al) (Aly et al. 2011), and aluminum oxide (Al2O3) (Sonawane et al. 2011) dispersed in liquid fuels (called nanofuels) have been investigated. The addition of MNPs to liquid fuels has a catalytic effect on the fuel performance like enhanced burning rate, short ignition delay, and lower emissions (Khalife et al. 2017; Khond and Kriplani 2016; Mehta et al. 2014). The reduction in the size of metal particles from micrometer to nanometer scale has enabled to overcome the issue of long ignition delays and achieve higher burn rates (Pivkina et al. 2004).

9.2.1

Key Objectives

In any combustor, the liquid fuel has to break up into fine droplets (called as “fuel atomization,” which increases the fuel surface area for faster evaporation), then the liquid fuel has to evaporate to form fuel vapor, the fuel vapor has to mix with the oxidizer, and then the air-fuel mixture has to ignite before the combustion process to produce the needed thrust. All these processes have to happen within a short fuel residence time, which is typical of the order of a few seconds, inside the combustor. Therefore, all the precombustion processes such as atomization, evaporation, mixing, and ignition play a crucial role in the latter combustion and emission processes. Under these circumstances, when MNPs are added to the liquid fuels at varying particle concentrations, it will alter the fluid properties such as density,

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viscosity, surface tension, etc. The change in these properties will affect the fuel atomization characteristics, which will, in turn, have an effect on the fuel distribution inside the combustor, and eventually, on the fuel combustion and emission characteristics. As a result, the influence of MNPs addition on all of those processes has to be investigated to gain a better understanding of its effect individually.

9.3

Nanofuel Preparation

The nanoparticles of interest are mixed with liquid fuels at different weight concentrations as shown in Fig. 9.3. In the literature, typically, the weight concentrations of MNPs are varied from 0.1 to 10 wt. % (weight percentage) depending upon the application. The MNPs must be well dispersed in the liquid fuel to obtain a homogeneously dispersed, stable nanofuel. From the literature, it is evident that the MNPs tend to agglomerate (i.e., the formation of a group of two or more nanoparticles) due to the interparticle attractive forces. The agglomeration of MNPs results in an increase in the size of the cluster particles, which are no longer in the nanometer range (20%) dispersed in liquid fuels, called as gelled fuels, were reported to alter the sheet breakup methodology (von Kampen et al. 2007). For a fixed Fig. 9.9 Spray development

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Fig. 9.10 Change in liquid sheet breakup length (L) due to the addition of MNPs. (a) Liquid fuel without dispersing MNPs. (b) Liquid fuel dispersed with MNPs at 2 wt. %. Here, “y” is the axial distance from nozzle exit, “Dn” is the nozzle exit diameter, and “r” is the radial distance (Reproduced with permission from Kannaiyan and Sadr (2017))

combustor length, this will be beneficial because the fuel can be disintegrated into fine droplets at a shorter distance and the fuel will have more time/space for other processes related to power generation inside the combustor. Hence, it is essential to study the influence of nanoparticles on the spray characteristics at elevated ambient conditions that are relevant to actual engines to account for the evaporation characteristics (Khond and Kriplani 2016). The author has been investigating the influence of MNPs on the atomization characteristics of liquid fuels at atmospheric and elevated ambient conditions.

9.4.3

Evaporation Process

Once the bulk fuel is disintegrated into fine droplets, they are exposed to high pressure and temperature conditions inside the combustors. The fuel droplets have to undergo a phase change from liquid to vapor to generate fuel vapor, which will mix/react with the oxidizer depending upon the combustion mode. Therefore, it is important to understand the influence of nanoparticles on the fuel droplet

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Fig. 9.11 Evaporation rate of kerosene fuel with and without the dispersion of Al MNPs at different weight concentrations and temperatures. (Reproduced with permission from Javed et al. (2013b)). “d” is the droplet diameter at time, t, and “d0” is the initial droplet diameter. Here, “OA” represents surfactant-oleic acid, and “NP” represents nanoparticles

evaporation process. The classical theory that describes the evaporation rate for fuel droplets is the d2-law, where d represents the fuel droplet diameter with respect to time (Turns 2000). Several studies have investigated the evaporation rate of nanofuel droplets and reported that they follow the classical evaporation mechanism (Ghamari and Ratner 2017; Javed et al. 2013b), while others have reported they did not follow the classical mechanism (Javed et al. 2014, 2015a, b). Furthermore, in some cases, the evaporation rate was enhanced (Gan and Qiao 2012a; Ghamari and Ratner 2017; Mehta et al. 2014) (see Fig. 9.11), whereas in some cases it was decreased (Gan and Qiao 2011b; Javed et al. 2013a, b). The difference in nanofuel droplet evaporation characteristics was attributed to the occurrence of the nanoparticle agglomeration, which in turn depends on the nanoparticle concentration and ambient conditions. The nanoparticle agglomeration was reported to occur at a timescale that is similar to the droplet lifetime, and hence, the agglomeration of nanoparticles was reported to nullify the increase in evaporation rates in some occasions (Gan et al. 2012; Gan and Qiao 2012b; Javed et al. 2014). All these studies suggest that evaporation is influenced by the nanoparticles. However, it varies depending on the concentration and conditions.

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Ignition Process

Following evaporation, the fuel vapor mixes with air, and the air-to-fuel mixture ratio has to be within the fuel flammability limits. The mixture will ignite, with the help of an igniter or under compression, when it is within the lower and upper bounds of the fuel flammability. As mentioned earlier, the metal particles in the size range of micrometers result in higher ignition delay (Hunt and Pantoya 2005) and higher ignition temperature (Trunov et al. 2006) when compared to those of nanometer-sized particles. The nanoparticles were reported to enhance the ignition probability of conventional fuels like diesel (Tyagi et al. 2008), and the MNPs can play a catalytic role in enhancing the ignition process by reducing the ignition temperature (Ma et al. 2011). Furthermore, an increase in ambient pressure and temperature tends to decrease the nanofuel ignition delay; however, beyond a critical pressure, the ignition delay increases again (see Fig. 9.12) (Kim et al. 2016). During nanofuel preparation, surfactants are added to obtain stable, homogenous nanofuel. However, the boiling characteristics of the base fuel and surfactants are different. As a result, the lower boiling fluid forms bubbles inside the fuel droplet and leads to the so-called “micro-explosion” phenomena, which in turn leads to disruptive combustion of nanofuel droplets (Gan et al. 2012).

Fig. 9.12 Effect of MNPs on the ignition delay of kerosene fuel at different pressures (Reproduced with permission from Kim et al. (2016)). Here, OA represents surfactant-oleic acid, and NP represents nanoparticles

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Combustion and Emission Performance

The influence of micrometer- and nanometer-sized aluminum particles on the combustion phenomena of n-decane and ethanol droplets was investigated (Gan and Qiao 2011a). In the case of nanofuel droplets, five distinctive stages of combustion such as preheating, classical combustion, micro-explosion, surfactant flame, and aluminum droplet flame were identified. Whereas in fuel droplets dispersed with micrometer-sized Al, only the first three stages were observed. The reason for different combustion mechanism between those fuel droplets was attributed to the different agglomeration mechanisms involved with nanometer-sized and micrometer-sized particles. In the former, the agglomerates were identified as porous, uniformly sized spherically shaped, while in the latter, the agglomerates were densely packed with the impermeable shell. This difference is reasoned to have influenced their evaporation and in turn their combustion process. Later, the same authors investigated the effect of particle loading, i.e., dense and diluted concentrations in hydrocarbon fuels, and found that the particle agglomeration mechanism influenced the combustion mechanism between the dense and dilute concentrations of MNPs (Gan et al. 2012). The MNPs added in hydrocarbon fuels were reported to undergo a morphological change in their structure during the nanofuel combustion and result in complete oxidation of MNPs to their respective oxides of MNPs (Karmakar et al. 2011). The combustion details of MNPs have been studied and reported in the literature for aluminum (Basu and Miglani 2016; Dreizin and Schoenitz 2015; Ermoline et al. 2013; Julien et al. 2014; Levitas et al. 2014), magnesium (Corcoran et al. 2015), copper oxide (Gumus et al. 2016), aluminum oxide (Gumus et al. 2016), nickel (Abraham et al. 2016), graphite oxide (Ooi et al. 2016), iron (Mandilas et al. 2016), and boron (Shariatmadar and Pakdehi 2016; Wang et al. 2017; Yu et al. 2018). For example, the combustion nature of n-decane droplet dispersed with boron MNPs at 5 wt.% is shown in Fig. 9.13. The burning rate (i.e., the rate at which the fuel droplets are oxidized or burned to end products; it is typically expressed as length over time) of nanofuel droplets increases with MNP concentration and starts to decrease beyond a certain concentration (Ghamari and Ratner 2017) (see Fig. 9.14). The decrease was attributed to the agglomeration of MNPs. In fact, during the combustion of nanofuels, it was hypothesized that the MNPs no longer remain in the nanometer scale due to the agglomeration and that was reported to be the limiting factor for reducing the burning times with size reduction (Chakraborty and Zachariah 2014). The combustion of such droplets was also associated with instabilities (Miglani et al. 2014). With the dispersion of nanoparticles, the flame temperatures increase due to the presence of high energetic metal nanoparticles (Acharya et al. 2012; Julien et al. 2014). Furthermore, the metal particles at nanometer scale dispersed in liquid fuels tend to increase the flame speed/burning rate much higher than those of pure fuels and fuels dispersed with micrometer-sized metal particles (Guerieri et al. 2018; Julien et al. 2014; Levitas et al. 2014).

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Fig. 9.13 Combustion of n-decane droplet dispersed with boron MNPs at 5 wt. %. (Reproduced with permission from Gan et al. (2012))

The MNPs dispersed in liquid fuels have been tested extensively in practical engines (Arul Mozhi Selvan et al. 2014; Bidita et al. 2014; Khond and Kriplani 2016; Mandilas et al. 2014; Mandilas et al. 2016; Ooi et al. 2016). The addition of nanoparticles decreases the exhaust gas emissions such as NOx, SOx, unburnt hydrocarbons, particulate matters, and CO when tested at engine-relevant conditions (Bidita et al. 2014; Khalife et al. 2017; Khond and Kriplani 2016; Khorramshokouh et al. 2016; Mehta et al. 2014); for example, see Fig. 9.15. Even at the fundamental level, the dispersion of MNPs in liquid fuels has been demonstrated to reduce the CO and NOx emissions when compared to pure liquid fuel combustion (Mehregan and Moghiman 2014).

9.5

Closing Remarks/Future Prospects

With the advent of nanotechnology, there has been a renewed interest on the high energetic metal nanoparticles as fuel additives for hydrocarbon liquid fuels. However, the dispersion of nanoparticles even at low weight concentrations affects the thermophysical, atomization, evaporation, ignition, combustion, and, eventually, the

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Fig. 9.14 Effect of MNPs on the burning rate. (Reproduced with permission from Ghamari and Ratner (2017)). Here, GNP represents graphene nanoparticles

Fig. 9.15 Graphic illustration of the effect of MNPs on the emission performance of conventional fuel. (Reproduced with permission from Khalife et al. (2017)). The number of studies is indicated by the numbers

fuel emission characteristics as briefly discussed here. Nevertheless, the effect of nanoparticles seems to depend on various parameters such as nanoparticle type, size, concentrations, and operating conditions like ambient pressures and temperatures. The literature clearly demonstrates the prospects of using metal/metal oxide nanoparticles in the process of deriving clean energy from hydrocarbon fuels.

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Application of MNPs in practical engines can help in reducing the toxic pollutants emitted when compared to those emitted while burning pure convectional fuels. Although the prevailing literature helps to advance the knowledge of nanofuel and its future prospects, they are still in early stage, and further research is warranted from all aspects of fuel to truly understand the potential benefits of nanoparticle dispersion in liquid fuels.

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Chapter 10

Biomass-Derived Nanomaterials Sebastian Raja, Luiz H. C. Mattoso, and Francys K. V. Moreira

Contents 10.1 10.2

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nanocellulose (NC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Cellulose Nanofibrils (CNF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Cellulose Nanocrystals (CNC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Bacterial Cellulose (BC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Starch Nanocrystals (SNCs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.1 Modifications of SNCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3.2 SNC-Polymer Nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Carbon Nanostructures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.1 CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.2 Carbon Dots (C-Dots) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4.3 Silica NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Concluding Remarks and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract The arising of global energy crisis and tremendous risk of climate change has drastically endangered the survival and development of human society, which certainly demand for modern technologies that require high-performance materials with superior properties. In this context, nanotechnology has emerged as a powerful tool for the scientific community to the design and development of engineered materials. In particular, nanomaterials derived from biomass (plants or plantbased) resources are gaining much attention by governments, industries, and academia owing to their low-cost, environmental compatibility, and replacement capability for petroleum-derived products for energy and environmental applications. S. Raja (*) · L. H. C. Mattoso National Nanotechnology Laboratory for Agribusiness, Embrapa Instrumentação, São Carlos, SP, Brazil e-mail: [email protected] F. K. V. Moreira Department of Materials Engineering – DEMa, Federal University of São Carlos – UFSCar, São Carlos, SP, Brazil © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_10

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Meanwhile, it is desirable that the development of novel materials from the natural resources that meet our daily needs and at the same time environmental-friendly in nature. This chapter provides a comprehensive understanding for obtaining renewable nanomaterials from natural biomass precursors for energy and environmental applications. Three different nanomaterials (i) cellulose nanostructures, (ii) starch nanocrystals (SNC), and (iii) carbon nanostructures along with their recent advancement in energy and environmental applications are figured out.

10.1

Introduction

Energy is inevitable to all living organisms, ranging from daily life to advanced science and technology. With ever-increasing demand and continuously deteriorating environmental issues, energy has become a bottleneck and is hampering the development of modern society. Unfortunately, the current energy supply systems rely on the limited nonrenewable fossil fuels. Furthermore, these systems are interrelated and interdependent with environment. Thus, fossil fuels result in the production of carbon dioxide (CO2) and other greenhouse gases causing global warming. To build a sustainable future, it is necessary to choose energy sources from non-fossil based and at the same time reliable, affordable, and unlimited. Thus, it is indispensable to explore energy sources from most abundant, natural, and renewable resources to replace traditional fossil fuel sources and attain more energy-efficient technologies to overcome increasing energy demands as well as to mitigate the environmental issues. Biomass is an organic material that comes from agricultural residues and forest byproducts, which are renewable sources of energy. The worldwide production of biomass is estimated approx. 150–200 billion metric tons a year (Balat and Ayar 2005; Gandini et al. 2016; Chen et al. 2018). Generally, energy is produced from biomass by simply burning biomass waste, which affords low efficiency of energy along with serious air pollution. Thus, it is desirable to convert biomass into a novel material for superior properties with high performances. Nanometer dimension of biomass has received noteworthy attention in recent years, in energy-related applications. Figure 10.1 shows the illustration of biomass from various sources. Nanotechnology is typically described as the material that possesses at least one dimension in the nanometer scale of 1–100 nm and is considered to be a promising technology in order to overcome many challenges in the modern society. On the application of nanotechnology, the design and development of novel materials with controlled sizes, shapes, porosities, crystalline phases, and structures are of utmost importance for breakthroughs in several sustainable energy technologies. Likewise, the fabrication or extraction of nanoscale materials from sustainable and renewable resources extends the promising potentials of nanotechnology (Mohammadinejad et al. 2016).

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Agricultural crops & residues

Forestry crops & residues

Industrial residues

Biomass

Sewage sludge

Animal residues

Municipal solid wastes

Fig. 10.1 Illustration of biomass from various sources

10.2

Nanocellulose (NC)

Cellulose is a fibrous, semicrystalline, most abundant, natural, and renewable biopolymer resource on earth and is widely present in various fibrous crops and agroindustrial wastes (Shaghaleh et al. 2018) with a total production about 1011–1012 tons per year (Hoeng et al. 2016). Cellulose is derived from D-glucose units, which condense through β-(1 ! 4)-glycosidic bonds (Fig. 10.2). It displays unbranched straight chain characterized by their high hydrophilicity, chirality, biodegradability, and large capacity for chemical modification. Also, cellulose forms an ordered structure, high thermal stability, and high crystallinity and is especially useful due to the existence of intra- and intermolecular hydrogen bonds. The structure of cellulose comprises cell walls, bundles of cellulose molecules (C6H10O5) as microfibrils in the form of stabilized or elongated by hydrogen bonds. Cotton contains the highest percentage of cellulose (> 95%) among various sources such as plants, algae, marine creatures, and bacteria. A single cotton fiber (thickness, 20–30 nm) consists of superfine fibrils having diameter in the range of 2–20 nm and is affected by many factors such as source, climate, soil, etc. (Habibi et al. 2010; Siqueira et al. 2010; Menon et al. 2017). Table 10.1 summarizes the cellulose contents of various sources. The nanometer dimension of cellulose is commonly described as nanocellulose (NC). NC has broadened the appeal of nanotechnology to consumers and opens up new markets for “renewable nanomaterials.” NC possesses remarkable physical properties, flexible surface chemistry, and low thermal expansion in addition to transparency, high elasticity, and anisotropy. Furthermore, many attractive features of NC, such as their inherent renewability, sustainability, high strength, large specific surface area, and nanoscale dimension, offer several applications including polymer reinforcement, nanocomposite formulations, biosensing, food and

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Fig. 10.2 Structural representation of cellulose with cellobiose repeating units

Table 10.1 Percentage of cellulose present in various sources of plants Source Cotton Eucalyptus Sisal fibers Boro variety Wheat straw Rice straw Flax fibers Hemp fibers Achira fibers Cassava peel

Cellulose content (%) 95–97 76 65.5 89.6 43 71 72 68 19.1 17.8

References Chen (2014) Theng et al. (2015) Megiatto Jr (2006) Saito et al. (2013) Alemdar and Sain (2008) Nasri-Nasrabadi et al. (2014) Bledzki et al. (2008) Thygesen et al. (2005) Andrade-Mahecha et al. (2015) Widiarto et al. (2017)

nutraceuticals, and tissue engineering scaffolds (Moon et al. 2011; France et al. 2017; Golmohammadi et al. 2017; Khan et al. 2018; Rajinipriya et al. 2018; Xue et al. 2017). Figure 10.3 shows the diagram of various cellulose nanostructures from biomass. Moreover, the high density of hydroxyl groups on the surface of NC endows for easy modifications with various functional groups (namely, carboxyl, sulfate ester, amine and aldehyde groups, etc.), small organic molecules, nanoparticles, and grafting with polymers (Espino-Pérez et al. 2014; Chen et al. 2015; Lin and Dufresne 2014; Nechyporchuk et al. 2016; Habibi 2014). Although most of the studies on NC have centered on polymer reinforcement and biomedical applications, the contemporary research has begun to address the functionalization and engineering of NC toward energy and environmental applications. By utilizing its structural and chemical features, cellulose can be assembled into various nanoscale configurations in order to design and develop energy-related devices. In this association, several comprehensive reviews have been published along this direction (Liu et al. 2010; Mohammed et al. 2018; Hoeng et al. 2016; Chen et al. 2018; Julkapli and Bagheri 2017; Wang et al. 2017a, b, c, d). Thus, this section provides a fundamental understanding for obtaining renewable NC from natural biomass precursors and their recent advancement in energy and environmental applications.

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Biomas

Nanocellulose

Cellulose nanofibrils (CNF)

Cellulose nanocrystals (CNC)

Bacterial cellulose (BC)

Fig. 10.3 Illustration of various nanostructures of cellulose from biomass

10.2.1 Cellulose Nanofibrils (CNF) Cellulose nanofibrils (CNF) are otherwise known as microfibrillated cellulose or nanofibrillated cellulose in the literature. Firstly, Turbak et al. (1983) introduced CNF by high-shear mechanical approach from various sources including cotton, wood, and annual plants. Nevertheless, wood is considered to be the main source for the production of CNF (Zhu et al. 2016). In this connection, CNF isolated from other natural resources such as curaua (Correá et al. 2010), Eucalyptus kraft pulp (Tonoli et al. 2012), and cotton (white and colored) fibers (Teixeira et al. 2010) were reported from our laboratory. Moreover, the following renewable resources, such as sisal, flax, hemp, grass, sorghum, barley, sugar cane, pineapple leaf fibers, banana rachis, soy hulls, algae, bacterial cellulose, kenaf stem, swede root, wheat straw, carrots, empty fruit bunches, potato pulp, branch bark of mulberry, bagasse, rice straw, chardonnay grape skins, stems of cacti, coconut husk, bamboo, pea hull fiber, and industrial bio-residues, have also been utilized for the extraction of CNF (Mohammadinejad et al. 2016). The sources endow size, morphology, and an aspect ratio (250) of CNF. In the process of extraction, the mechanical treatment causes the longitudinal cleavage of cellulose fibers to obtain CNF. Grinding, microfluidization, and homogenization are the common processes in mechanical disintegration. Since this approach consumes more energy and several passes of cellulose through the apparatus, other pre-treatment approaches such as enzymatic hydrolysis, chemical purification, TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated surface modification, etc. have been explored (Abe et al. 2007; Kargarzadeh et al. 2012; Isogai et al. 2011) to facilitate the extraction of CNF. The high aspect ratio, superior physical properties, and entangled structure enable CNF as a potential nanomaterial for energy-related applications.

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10.2.2 Cellulose Nanocrystals (CNC) The polymeric chains of cellulose microfibrils are organized in the elongated nanocrystals with small diameters in structures called cellulose nanocrystals (CNC), a unique and promising natural material extracted from various natural resources (Reid et al. 2017; Islam et al. 2018; Tang et al. 2017). According to the literature, CNC are also reported as nanocrystalline cellulose, cellulose nanowhiskers, and cellulose crystallites or crystals. Native cellulose consists of amorphous and crystalline regions, and the amorphous regions have lower density compared to the crystalline regions; hence the cellulose fibers were subjected to harsh acid treatment in order to break up the amorphous regions to release the individual crystallites. CNC are extracted via acid hydrolysis of cellulose fibers under controlled conditions, wherein the presence of amorphous region is completely hydrolyzed to yield highly crystalline CNC. Various mineral acids such as sulfuric acid, hydrochloric acid, and phosphoric acid can be used for hydrolysis. These plant-derived CNC exhibit lateral dimensions of 5–70 nm and lengths ranging from 100 to 250 nm. Like CNF, the properties of CNC depend on various factors such as sources, reaction time, temperature, and types of acids used for hydrolysis. On the account of its remarkable physical properties, special surface chemistry, NC offers wide range of applications. Moreover, biocompatibility, biodegradability, and negligible toxicity favor CNC as a prominent candidate in biomedical field (Domingues et al. 2014).

10.2.3 Bacterial Cellulose (BC) BC is yet another category of nanocellulose produced by microbial fermentation process with high aspect ratio (Picheth et al. 2017; Jang et al. 2017). Major microbes, namely, Acetobacter, Agrobacterium, Alcaligenes, Pseudomonas, Rhizobium, and Sarcina produce BC. In comparison to CNF, CNC, these are mainly isolated from higher plant cellulose sources, fabricated biotechnological processes from low-molecular weight carbon sources (e.g., D-glucose). This process involves the following steps: (i) cultivation of bacteria in aqueous nutrient media; (ii) BC extracted as exopolysaccharide at the interface to the air; and (iii) producing a thick gel consisting of interconnected 3D porous BC nanofiber network. The obtained BC contains  99% water, called pellicle. In addition, type of bacterial strain, additives of culture medium, condition and cultivation, and the drying process are the key factors to control the BC nanofiber network. BC has also been utilized as raw material in the preparation process of CNF and CNC. On the account of highpurity cellulose and web-like entangled structure, it can be used to integrate with active materials for energy harvesting and energy storage devices.

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10.2.4 Energy Applications Long-term performance and good reliability of crystalline silicone dominate 80% of the solar panel industry today. In terms of cost and intrinsic insulating property, cellulose can be utilized as transparent substrate for solar energy harvesters. Moreover, the high surface area and good charge-transporting properties of NC play a crucial role in absorbing photons and converting them into electricity. High optical transparency and high optical haze are the desired factors for solar cell substrate in order to enhance the photon absorption (Zhu et al. 2014; Fang et al. 2014; Wang et al. 2017a, b, c, d). However, the organic solar cell with traditional paper substrate exhibited low transmittance, as a result of low power conversion efficiency, although they offer high haze (Barr et al. 2017). In this context, NC could act as a potential candidate as substrate for solar cells owing to their smaller sizes than the wavelength of visible light, which could attribute for high transparency with significant scattering along the light transporting direction. In the past decades, intensive research has been performed along this direction. Hu et al. (2013) developed a solar cell based on a highly transparent NC paper deposited on conductive materials such as ITO, carbon nanotubes (CNT), and silver nanowires (AgNWs). As a result, the organic bulk of heterojunction solar cell demonstrated 0.4% power conversion efficiency (PCE). NC-based recyclable solar cell was achieved by Zhou et al. (2013) using CNC paper substrate with polyethylenimine-modified Ag electrode. It resulted in an enhanced power conversion efficiency (PCE) of 2.7% with recyclability. The circuit voltage and fill factor values are almost similar to ITO/glass-based solar cells. This device exhibits attractive technology for sustainable and environmental production, though the PCE is lower than ITO/glass-based solar cells. Fang et al. (2014) demonstrated excellent optical transparency ( 96%) and ultrahigh haze ( 60%) based on CNF on ITO glass. From this investigation, it is observed that the haze effect enhanced the incident angle independency of solar cell device while laminating a piece of TEMPO-oxidized CNF paper on an ITO glass. Nogi et al. (2015) developed a transparent conductive paper from cellulose and AgNWs. A good hydrophilic affinity of cellulose with AgNWs offers good foldability and stable resistance, even after folding cycles up to 20 times. PCE of 3.2% can be reached using fabrication of solar cells with this conductive paper. Ha et al. (2014) fabricated solar cells based on transparent paper with GaAs, in which transparent paper showed angle-independent behavior at all wavelengths, decreased the light reflectivity, and increased the PCE from 13.55% to 16.79%. Recently, Costa et al. (2016) explored 1.4% and 0.5% of power conversion efficiency based on CNC and CNF as substrates, respectively, in inverted organic solar cells. Similarly, energy storage devices such as supercapacitors and batteries have been proven to be the most effective energy storage systems (Larcher and Tarascon 2015). Nevertheless, classical electrodes are composed of graphite and inorganic rare metals, which are neither renewable nor sustainable. To replace the above, biomass-derived materials are regarded as promising alternatives due to their intrinsic properties along

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with their eco-friendliness, natural abundance, high mechanical strength and flexibility, and versatility to integrate with other functional materials. Generally, biomassderived nanomaterials have been utilized as binders, separators, and electrode materials in the energy storage devices. Many efforts have been devoted to develop sustainable energy storage devices with high performance from biomass-derived nanomaterials, in particular, NC (Dubal et al. 2018; Chen et al. 2018; Gao et al. 2017a, b; Julkapli and Bagheri 2017; Wang et al. 2017a, b, c, d). Supercapacitors (SC), also known as ultracapacitors, have been considered as promising energy storage devices owing to their high power density, outstanding pulse charge-discharge performance, superior lifetime, simple principles, and low maintenance cost (Dubal et al. 2018; Wang et al. 2017a, b, c, d; Zhang et al. 2015). Charge storage mechanism divides supercapacitors into two categories: (i) electrical double-layer capacitors (electrostatic charge is at the electrode surface) and (ii) pseudo-capacitors (capacitance arises from the reversible redox reaction at the potential). In order to achieve sustainable energy, biomass-derived electrode materials play a crucial role in supercapacitors (Herou et al. 2018). In this context, several attempts have been explored to utilize NC as electrode material to achieve supercapacitors with high performances. High volumetric energy density of electrode is crucial for miniaturization of supercapacitors. Qi et al. (2018) fabricated a symmetric supercapacitor by coating CNF on graphite papers to enhance the thickness of MnO2 layers for attaining high volumetric specific capacitance. The supercapacitors displayed an extremely high volumetric energy density of 10.6 mWh/cm3 at a power density of 0.11 W/cm3. Also, it was found that the performance was superior to that of MnO2-based symmetric and asymmetric devices that were reported earlier. Jiang et al. (2017) reported a novel fabrication of a composite electrode via incorporation of graphene oxide (GO) flakes and PEDOT: PSS conducting polymer within the bacterial cellulose (BC) matrix. The obtained electrode exhibited excellent electrochemical performance of 373 F g1 at 1 A g1 and cycling ability of approx. 85% capacitance retention over 1000 cycles. Flexible free-standing film electrode with excellent redox reversibility and cyclic stability was achieved by Li et al. (2016) through the deposition of polypyrrole (PPy) on the surface and inside the CNF/multi-walled carbon nanotubes (MWCNTs) film. The electrochemical performance of the hybrid CNF/MWCNTs/PPy nanocomposite electrode resulted in an elevated specific capacitance of 288 F g1 at a scan rate of 5 mV S1. Wang et al. (2015a, b, c) demonstrated a CNF/PPy (polypyrrole)-based composite electrode by introducing quaternary amine groups on the surface of CNF prior to PPy polymerization. The supercapacitor exhibited high gravimetric capacitance of 127 F g1 and volumetric capacitance of 122 F cm3 with high current densities of 300 mA cm-2 at 33 A g1 cycling ability. Lithium-ion batteries (LIBs) have emerged as the main power source in automobile and portable electronic industries as energy storage devices. In addition, LIBs are popular rechargeable energy storage devices that are widely used in consumer electronic products. This is mainly because of their high energy density, large output

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voltage, appreciable lifetime, and benign operation. LIBs are mainly composed of anode, cathode, and electrolyte. In principle, Li ions move from the negative electrode to the positive electrode during discharge and back when charging, through electrolyte. Considering factors such as low-cost, high density, and environmentfriendliness, the current technology demands novel materials with high performances. Similarly, sodium-ion batteries (NIBs) have received considerable attention as desirable candidates for energy storage devices nowadays. Features such as abundancy, inexpensive, and eco-friendliness have brought sodium (Na) as energy storage devices. Both LIBs and NIBs are similar in working principle. However, Na possesses larger radius (approx. 55%) than Li; thus most of the materials do not possess host framework. On the account of its radius, it is also possible for structural changes while extraction. Based on this concept, NC-derived materials have received considerable interest in NIBs. NC could be integrated with active materials to further develop electrodes for LIBs and NIBs because of their superior physical and structural properties. Zhang et al. (2016) demonstrated binder-free anode material composed of BC, three-dimensional (3D) MoS2 nanoleaves, and carbon nanofibers. As-prepared nanocomposite was utilized as anode material and current collector for LIBs without any binder or conductive additive. From this investigation, a high reversible charge capacity of 935 mA h g1 is achieved at 0.1 A g1, and capacity of 581 mA h g1 is maintained after 1000 cycles at 1 A g1 with negligible decay. Li et al. (2015a, b) demonstrated CNF as a green dispersant to efficiently disperse 2D materials such as boron nitride (BN) and molybdenum disulfide (MoS2) in aqueous solution and utilized the nanocomposite as anode materials for sodiumion batteries. Further, the authors also demonstrated TEMPO-oxidized CNF as dispersant for molybdenum disulfide (MoS2) with carbon nanotubes (CNT) 2D materials. It is reported that, the as-prepared CNF/MoS2/CNT composite films could act as a flexible electrode for NIB anode. The discharge capacity of this system achieved 147 mA h g1, and the coulombic efficiency from the first and third cycle was 43.8% and 89.7%, respectively. The NC-derived materials and their electrochemical performances are listed in Table 10.2.

10.3

Starch Nanocrystals (SNCs)

Starch is another vastly abundant renewable resource on earth. Alongside cellulose, starch is currently one of the most promising renewable polymers for developing engineered nanomaterials. It is produced worldwide at an approximate extent of 85 million tons per year, of which about 10.7 million tons are produced in Europe. This implies that starch could supply various technological sectors as it is sustained by a well-orientated industry. Starch is composed of two polymers: amylose and amylopectin. Both are made up of D-glucopyranose units linked by α(1 ! 4) glycosidic bonds at the main chain and α(1 ! 6) glycosidic bonds at the branch points. Microstructurally, starch is

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Table 10.2 Electrochemical performances of NC-derived materials reported in the recent literatures Substrate Material Supercapacitors (SC) Carbon BC/NiS aerogel

Carbon nanofiber

N-doped BC

Carbon nanofiber

N-doped BC

Hybrid aerogel film

NFC

Lithium-ion batteries (LIBs) Carbon BC/MoS2 nanofiber Carbon BC/Fe2O3 aerogel Carbon BC/KOH nanofiber Composite / CNF/SiNPs CNT Carbon BC/carbon aerogel aerogel Sodium-ion batteries (SIBs) Spinifex hard CNF carbon Porous carbon

CNC

Capacitance

Cycling stability capacity

References

Energy density 21.5 W h kg1 Power density 700 W kg g1 Energy density 36.3 W h kg1 Power density 800.2 W kg1 Energy density 0.1 mW h cm2 in KOH 0.31 mW h cm2 in H2SO4 Power density 27.0 mW h cm2 in KOH Energy density 28.4 uW h cm2 Power density 9.5 mW cm2

1000 cycles

Zuo et al. (2017)

2500 cycles

Lai et al. (2016)

10,000 cycles

Ma et al. (2016)

1000 cycles

Zheng et al. (2015)

935 mA h g1 at 0.1 A g1

581 mA h g1 1000 cycles 754 mA h g1 100 cycles 857 mA h g1 100 cycles 1840 mA h g1 100 cycles 359 mA h g1 100 cycles

Zhang et al. (2016) Wan et al. (2015) Wang et al. (2015a, b, c) Wang et al. (2015a, b, c) Wang et al. (2014a, b)

–

Gaddam et al. (2017)

–

Zhu et al. (2017a, b)

410 mA h g1 at 1 A g1 1948.5 mA h g1 3200 mA h g1 Discharge: 797 mA h g1 Charge: 386 mA h g1 at 75 mA h g1 386 mA h g1 at 20 A g1 300 mA h g1 at 100 A g1 340 mA h g1 at 100 A g1

formed by granules with unlike polymorphic arrangements involving amylose and amylopectin (Haaj et al. 2016). Amylopectin chains are arranged perpendicular to the granule surface, with their branch points distributed periodically along the main chain, forming amorphous and crystalline lamellae with thickness between 100 and 400 nm and periodicity of about 10 nm. Yet, amylose chains are arranged randomly

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at the starch granule surface in a single helical configuration or interacting with amylopectin by hydrogen bonding as double helices (Haaj et al. 2016). Dufresne et al. (1996) reported that the starch microstructure could be disrupted by acid hydrolysis to form nanocrystals in an analogous way as CNCs are formed. These starch nanocrystals (SNCs) consist of stacked platelet-like nanostructures with geometrical dimensions, in general, below 50 nm (Angellier et al. 2004; Putaux et al. 2003). The potential of SNCs in several applications is now attracting much interest due to the possibility of applying them as renewable and nontoxic-engineered nanomaterials (Mukurumbira et al. 2017). The most common approach used to prepare SNCs was the acid hydrolysis using H2SO4 (Putaux et al. 2003). The process is similar to that of acid hydrolysis of cellulose that forms CNCs requires less concentrated acidic solutions and lower temperatures, and it is simple to be controlled. However, it is time-consuming process (it usually takes 3–7 days) with low yield of SNCs (Angellier et al. 2004). Recently, Dai et al. (2018) combined ball milling with acid hydrolysis to produce SNCs. It was possible to shorten the conventional acid hydrolysis time from 5 to 3 days and achieved 19.3 wt. % of yield. Hao et al. (2018) reported that the enzymatic pre-treatment of waxy potato starch using glucoamylase could also be used to shorten the acid hydrolysis time ensuring SNC suspensions with improved stability. Previously, Amini and co-workers reported that combining H2SO4 acid hydrolysis with sonication treatment produces SNCs within 45 min and yields of 21.6% (Amini and Razavi 2016). All these efforts contribute to the challenge of the green scaled-up production of SNCs.

10.3.1 Modifications of SNCs SNCs have an intrinsic potential for applications in several fields because of their biocompatible, edible, fully biodegradable, and nontoxic nature compared to most inorganic engineered nanomaterials. SNCs have also been chemically functionalized to extend their application potential. Cai et al. (2017) reported the synthesis of fluorescently labeled SNCs (FL-SNCs) for biomedical and food applications as biomarkers and biosensors. The synthesis of the FL-SNCs consisted of anchoring reactive amino groups on the SNC surface through silanization with 3-aminopropyl triethoxysilane (APTES) with further attaching of fluorescein isothiocyanate (FITC) groups. FL-SNC displayed a higher fluorescence intensity and better photostability. Cellular studies performed with human hepatoma cell line (HepG2) and revealed that the biocompatible character of SNCs was preserved after surface functionalization and that the FL-SNCs could be easily internalized by human cells. SNCs were functionalized with polyurethane following a suitable lab-scale methodology for biodiesel production purposes. The polyurethane-attached SNCs presented better performance than that of bulk polyurethanes, reducing acid value and contaminants of crude biodiesel at a larger extent. This could

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enormously contribute to reduction in water consumption and wastewater treatment in the biodiesel production sector (Desai et al. 2017). Application of SNCs as nanoreinforcement could be effective for polar and watersoluble polymer, but for nonpolar polymer the surface hydrophobicity of SNCs must be improved. Wei et al. (2016) hydrophobized SNCs using hexadecyltrimethoxysilane (HDS). The contact angle of the HDS-modified SNCs increased with increasing HDS content onto the SNC surface. This enhanced the dispersibility of the SNCs in acetone and n-hexane, which could be extended to predict a good dispersibility of HDS-modified SNCs in nonpolar polymers. Zhou et al. (2016) explored hydrophobization of SNCs via cross-linking reaction with citric acid. The citric acid-cross-linked SNCs exhibited good affinity to solvents with low polarity such as dichloromethane, which has attributed to the increased surface roughness of the SNCs induced by the cross-linking process. Proper application of SNCs also requires the control of the SNC stability over time when dispersed in aqueous medium (Wei et al. 2014). Approaches relating to enhance this aspect of SNCs include the chemical modification of the SNC surface with cross-linking agents and esterification groups among others. Dual modification of SNCs with cross-linking agents and esterification groups was recently proposed. Ren et al. (2016) used sodium hexametaphosphate (SHMP) as a cross-linker to stabilize aqueous SNC suspensions. It is revealed that the structure of waxy maize SNC is not disrupted by SHMP but ensured colloidal stability to the SNC suspension for 72 h only due to surface chemical modification (Ren et al. 2012). Dodecenyl succinic anhydride (DDSA), 2-octen-1-ylsuccinic anhydride (OSA), and acetic anhydride (AA) were used to ensure surface hydrophobicity to SNCs, which could be important for their use as reinforcing filler in nonpolar polymers. The hydrophobicity of the SNCs was more effective when the SNCs were previously cross-linked with SHMP, which was confirmed by the better dispersion of the functionalized SNCs in nonpolar solvents (Ren et al. 2016). Amphiphilic corn SNCs with sizes of 0–100 nm were synthesized by graft copolymerization of styrene on SNCs in aqueous emulsion medium (Song et al. 2008). The amphiphilic SNCs dispersed well in polar and nonpolar solvents comprising water, toluene, and dichloromethane mixtures, which was explained by the different conformation variation of the hydrophobic polystyrene side chains and the hydrophilic starch backbone. Such amphiphilic SNCs present a good potential as nanoreinforcements for non-polyolefin polymer such as natural rubber, poly(lactic acid), and poly(ε-caprolactone) (PCL). The enhanced reinforcing effect of SNCs-gpolystyrene was already demonstrated for natural rubber (Wang et al. 2014a, b).

10.3.2 SNC-Polymer Nanocomposites Starch nanocrystals have been considered as promising nanoreinforcements for polymers intended to food packaging materials and other applications, especially due to their natural and biocompatible characters (Scaffaro et al. 2017; Yu et al. 2008;

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Kristo and Biliaderis, 2007; Viguie et al. 2007). The number of studies dedicated to assessing the reinforcing efficiency of SNCs on polymer matrices has been increasing year by year. Table 10.3 summarizes the most recent achievements. Poly(vinyl alcohol) (PVOH)-SNC nanocomposites were obtained from hydrolysis of rice starch with 3.2 M H2SO4 at 40  C for 5 days to obtain the SNCs which were further added to PVOH at loadings of 0–10 wt.%. The nanocomposites were also loaded with 8 wt. % silver nanoparticles (AgNPs) to provide some antimicrobial property to the PVOH-SNC nanocomposites (Kumar et al. 2018). The addition of rice SNCs up to 8 wt.% increased the tensile strength and elasticity of PVOH due to strong chemical interactions. The SNCs increased the Tg of the PVOH films, and AgNPs improved the physical properties of the nanocomposites (Kumar et al. 2018). Poly(ε-caprolactone) (PCL) was loaded with SNCs obtained from regular cornstarch by acid hydrolysis at 40  C using 3.2 M H2SO4 for 5 days. The SNCs were also acetylated to improve their affinity to the PCL matrix (Xu et al. 2018). PCL-SNC composites were prepared by first solubilizing both components in CHCl3 solutions, then casting/evaporation, and finally hot pressing for obtaining films with controlled thickness. The addition of 1 wt. % SNC induced a considerable increase in tearing strength and reduction in the oxygen transmission rate (OTR) of the PCL matrix. The acetylated SNC induced a far higher improvement in these parameters which was ascribed to the higher chemical compatibility with PCL. A

Table 10.3 Recent achievements in SNC-reinforced polymer nanocomposites

Polymer Poly(vinyl alcohol)

SNC content (wt.%) 0–10

Poly (ε-caprolactone)

0–3

Regular corn starch Soy protein isolate Pea starch

0–2

Amaranth protein

0–12

Natural rubber latex

0–20

0–45 0–9

Processing method Casting/ evaporation

Property modifications Increase in tensile strength and elasticity; increase of Tg

Casting/ evaporation/ hot pressing Casting/ evaporation Hot pressing Casting/ evaporation Casting/ evaporation

Increase in tensile strength with 1 wt. % SNC; reduction of OTR; improvement of creep resistance Increase in tensile strength up to 6 wt. % SNC; reduction of WVP Increase in tensile strength and toughness; reduction of OP Increase in tensile strength up to 6 wt. % SNC; reduction of WVP Increase in tensile strength; reduction of WVP

Casting/ evaporation

Increase in tensile strength and elasticity; increase of WDC

References Kumar et al. (2018) Xu et al. (2018) Ren et al. (2017) Zhu et al. (2017a, b) Li et al. (2015a, b) Condés et al. (2015, 2018) Rajisha et al. (2014)

WVP water vapor permeability, OTR oxygen transmission rate, OP oxygen permeability, WDC water diffusion coefficient, Tg glass transition temperature

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composite film with better performance for packaging applications was obtained in this case by adding low amounts of eco-friendly SNCs (Xu et al. 2018). Conductive nanopapers based on soy protein isolate (SPI) were obtained using 1 wt. % sheet-like graphene (RGO) as conductive filler. Waxy SNCs obtained from acid hydrolysis at 40  C using 3.2 M H2SO4 for 5 days were tested as nanoreinforcements for these conductive SPI nanopapers (Zhu et al. 2017a, b). Mixed SPI/RGO/SNC powder was obtained by freeze-drying and further hot-pressed to obtain the conductive paper sheets. Addition of waxy SNCs at 15 wt. % increased tensile resistance and toughness of the SPI nanopapers; also the combination of SNCs and RGO substantially reduced the oxygen permeability of the SPI nanopapers by up to 50 wt. % (Zhu et al. 2017a, b). All starch-made nanocomposites are an interesting class of SNC-reinforced nanocomposites because the same biopolymer is combined both as a nanoreinforcement and matrix to produce all biodegradable, nontoxic materials. Recently, SNCs were produced from waxy starch using an optimized acid hydrolysis and further cross-linked with sodium hexametaphosphate or glutaraldehyde for improved water suspension stability (Ren et al. 2017). These cross-linked SNCs were tested as nanoreinforcements for glycerol-plasticized cornstarch films obtained by casting/evaporation. Tests involving different relative humidifies showed that the mechanical resistance of the cornstarch films was increased by the reinforcing effect induced by the cross-linked SNCs in comparison with the unmodified ones. The water vapor permeability of the films was substantially reduced with addition of only 1% cross-linked SNC. All these effects were attributed to the higher stability of the SNC suspensions when the nanostructure was cross-linked, which further improved their dispersion within the starch matrix (Ren et al. 2017). Waxy SNCs prepared by 3.2 M H2SO4 solution at 40  C for 7 days were used as filler at contents of 0–9 wt. % to reinforcing pea starch films plasticized with glycerol. Maximum increase of tensile strength was achieved at waxy SNC content of 6 wt. %, but water vapor permeability reduced gradually with increasing waxy SNC content (Li et al. 2015a, b). Compared to starch films produced by melt processing, waxy SNCs also induced a remarkable reinforcing effect, even in the presence of glycerol at distinct levels, which was explained by the establishment of hydrogen bonding not only between the SNCs but also between the SNCs and the thermoplastic starch matrix, with some contribution of crystallization effect (Angellier et al. 2006). Maize and waxy SNCs with sizes below 200 nm were prepared by acid hydrolysis using 3.2 M H2SO4 solution at 40  C for 5 days and further incorporated into glycerol-plasticized amaranth protein isolate films produced by casting. The addition of SNC slightly increased tensile strength of amaranth protein isolate films, but reduced their water vapor permeability, particularly at loadings superior to 6 wt.%. The reinforcing effect of SNCs was superior to that obtained with starch granules (Condés et al. 2018). The botanical origin of SNCs influenced the reinforcement effect on protein isolate films, mainly by disulfide bonds for waxy maize SNCs and by hydrogen bonds for the normal maize. These nanocomposites were fully biodegradable as expected (Condés et al. 2015).

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Potato SNCs were obtained by acid hydrolysis using 36 N H2SO4 at 40  C for 5 days, which were further incorporated into latex of natural rubber (NR-latex) films at 0–20 wt.% of loadings by the casting/evaporation technique. The homogenous dispersion of the potato SNCs and their ability to form a 3D network into the NR-latex matrix were the key parameters for determining the remarkable improvement of mechanical properties of the NR-latex films. On the other hand, the water diffusion coefficient of NR-latex films increased with potato SNC content due to the hydrophilic character of the SNCs (Rajisha et al. 2014). Previously, SNC-SNC interactions (SNC obtained from waxy starch) were also found to be determining the mechanical properties of NR-latex. Furthermore, adding only 10 wt. % waxy SNC in NR-latex caused a stiffness effect comparable to that obtained with 26.6 wt. % carbon black (CB), which is utilized as the conventional filler for natural rubber (Angellier et al. 2005). Overall, the potential of SNCs as an engineered nanomaterial, in particular as a nanoreinforcement, has begun to be explored. Upcoming research on new processing methods and testing the SNC polymer nanocomposite properties under different conditions will result in new improvements and unveil the complete potential of SNCs for widespread applications.

10.4

Carbon Nanostructures

Carbon is a versatile and most abundant universal element in the form of several allotropes with unique properties. Graphite, amorphous carbon, diamond, and fullerenes are the well-known forms of carbon. In the past decade, several low-dimensional carbon forms were discovered, which include fullerene, carbon nanotubes (CNTs), and graphene. These promising materials have attracted academia and industrial sectors owing to their prodigious physiochemical properties, such as high conductivity, excellent chemical stability, high specific area, controllable porosity, and dense electroactive sites. Thus, carbon materials play a prominent role in various fields including energy, environmental science, public transportation, and aerospace. Carbon makes a vital bonding with hydrogen and oxygen atoms and yields carbohydrates and biomolecules. Furthermore, carbon associates with nitrogen, sulfur, and phosphorous to produce other biomolecules such as lipids, alkaloids, antibiotic, amino acids, lignin, chitin, fat, alcohols, and so on. In carbon fixation, plants consume CO2 from the environment and build biomass, as in Calvin cycle (carbon respiration). Thus, biomass (plant or plant-derived) is considered as the key source for carbon nanostructures (Fig. 10.4). As mentioned earlier, the major resources of biomass include agricultural crops and their residues, wood and wood wastes, energy crops, municipal and animal wastes, aquatic plants, and algae (Gao et al. 2017a, b). The hierarchical structure and periodic pattern of some special nanoarchitectures bequeath them with unique functionalities, which include anti-reflection, super-hydrophobicity, structural

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Fig. 10.4 Pictorial representation of Calvin cycle

H2O

ATP

O2

NADP+

NADPH

ADP + P

Calvin Cycle

CO2

Sugars

coloration, and biological self-assembly (Zhou et al. 2011). However, the fundamental challenge of biomass is to improve the performance by augmenting its structure in order to broaden their application to the next level. In addition, the fundamental understanding of conversion mechanisms is necessary to obtain highperformance feedstock to resolve energy and environmental challenges. Generally, carbon nanomaterials are obtained by three major synthetic methods from renewable biomass, namely, physical activation, chemical activation, and hydrothermal carbonization (HTC), based on the experimental conditions and conversion mechanisms (Gao et al. 2017a, b). Biomass-derived nanomaterials have been demonstrated as potential candidate in various fields including wastewater treatment, catalyst support, air pollution control, energy conversion, hydrogen storage, and energy storage. This chapter emphasizes mainly on carbon nanotubes (CNTs), carbon dots (CDs), and silica nanoparticles (SiNPs) from biomass along with their recent findings in energy applications (Fig. 10.5).

10.4.1 CNTs CNTs are cylindrical in shape with one or more layers of graphene, classified as single-wall carbon nanotubes (SWCNT) and multi-wall carbon nanotubes (MWCNT), with open or closed ends. Perfect CNTs are defined as all the carbon bonded with hexagonal lattice, except at their ends. The diameters of SWCNT and

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Biomass

Carbon nanostructures

Carbon nanotubes (CNTs)

Carbon dots (Cdots)

Silica nanoparticles (Silica NPs)

Fig. 10.5 Illustration of various nanostructures of carbon from biomass

MWCNT possess in between 0.8–2 nm and 5–20 nm, respectively. The diameter of MWCNT can also exceed 100 nm. The length of the CNTs ranges from less than 100 nm to several centimeters. A superior electrical, mechanical, optical, and thermal property of CNTs offers numerous applications. Meanwhile, the application of CNT mainly depends on their structural features such as diameter, length, walls, chiral angle, etc. CNTs are produced from various methods, in which chemical vapor deposition (CVD) is the dominant one for the large scale (Mubarak et al. 2014). CNTs have been considered as an inevitable carbon nanomaterial in nanotechnology. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are significant for energy conversion system such as batteries, fuel cells, and watersplitting technologies. In this context, Wang et al. (2017a, b, c, d) developed nestlike CNT framework from chlorella by encapsulating cobalt NPs through pyrolysis process. The prepared CNT showed excellent performance as a bifunctional catalyst for ORR and OER, in which the half-wave potential of ORR was positively shifted by 40 mV compared to that of commercial Pt/C catalyst. More recently, cobalt/nitrogen-doped CNTs (Co-NCNTs) were developed from carbonization of chitosan by Zhang et al. (2018). It is revealed that the Co-NCNTs displayed superior ORR catalytic activity than commercial Pt/C catalysts due to the presence of large Co-N-C catalytic sites. Zhu et al. (2012) reported MWCNs from bamboo by CVD method in the presence of ethanol vapor and Mg2SiO4. Further, it is ascribed that the calcium silicate acts as an effective catalyst for the nucleation and growth of CNTs at 1200–1400  C, which is confirmed through transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). In addition, the specific surface area and adsorption capacity of CNTs were reported as 655 m2 g1 and 0.35 cm3 g1, respectively. One pot synthesis of CNTs was explored by Dubrovina et al. (2014) through pyrolysis of cellulose acetate (CA) cross-linked polyisocyanate with silica and NiCl2 as template and catalyst, respectively. It is stated that the role of the CA in generation

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of CNTs may be to combine closed macropores in the template during CO2 evolution in cross-linking reaction, while silica particles are responsible for the mesopores.

10.4.2 Carbon Dots (C-Dots) Carbon dots (C-dots) are a novel class of carbon nanomaterials that encompass discrete, quasi-spheroidal NPs with sizes below 10 nm. C-dots were first introduced by Xu et al. (2004) during the purification process of SWCNTs fabricated by arc discharge method. Since then, much attention has been paid toward this fascinating nanomaterial. So far, various precursors such as polymers, fossil materials, organic materials, and biomass materials have successfully been utilized to synthesize C-dots. Nevertheless, C-dots from biomass resources are of special attention not only in terms of environmental concern but also for commercialization purposes. Typically, C-dots are divided into several subgroups, namely, carbon nanoparticles, graphene quantum dots, amorphous carbon dots, graphitic carbon quantum dots, and polymer dots. In general, carbon dots have been coined as C-dots due to their spherical-like carbon-based objects in shape. The C-dots are considered as superior materials than organic dyes and semiconductor quantum dots in terms of excellent photostability, outstanding water solubility, robust chemical inertness, high resistance to photobleaching, tunable fluorescence emission and excitation, and facile surface modification. Further, the remarkable biological properties, namely, biocompatibility, excellent selectivity, and low toxicity render C-dots as potential candidates for drug delivery, bioimaging, and biosensor applications. The rich photochemical and photoluminescence properties endow C-dots as efficient catalysts and active additives in energy devices (solar cells, water-splitting, batteries, and supercapacitors). The synthetic methods for C-dots can be roughly classified into “top-down” and “bottom-up” approaches (Gao et al. 2017a, b). Typically, top-down methods include laser ablation, arc discharge, electrochemical oxidation, acidic exfoliation, and MW/hydrothermal synthesis. Bottom-up methods include hydrothermal, solvothermal, and MW synthesis. In comparison with top-down approach, bottom-up synthesis has emerged as successful method due to its inexpensive nature and simple operational steps, along with the possibility of tuning the properties by controlling the molecular size and shape (Lim et al. 2015), while other routes require complex equipment and treatment processes. This chapter covers the recent breakthroughs of C-dots in energy and environmental applications. Briscoe et al. (2015) reported C-dots from waste materials such as chitin, chitosan, and glucose by solvothermal approach. They demonstrated that the as-synthesized C-dots can be utilized to sensitize ZnO nanorods to visible light for use in solid-state nanostructured solar cells. Further, it is reported that the performance of the devices has shown to depend on the functional groups on the surface of the C-dots. The C-dots devices from chitin and chitosan combined layer exhibited

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highest PCE of 0.077%. Further, the performance could be improved potentially by coating thicker layer without increasing the series resistance or by increasing the visible light absorption. Xu et al. (2016) demonstrated C-dots from protein-rich biomass precursor – soybeans by two-step carbonization process. In the first step, nitrogen doping was obtained at low temperature, while hierarchical porous carbon with interconnected microstructure was obtained at high temperature in the second method. As a result, the C-dots were utilized to construct high-performance supercapacitors which exhibited a higher surface area of 1663.1 m2 g1 along with high specific capacitance of 337.3 F g1in 6 M KOH activating agent at 1 A g1 and good stability.

10.4.3 Silica NPs Silica is a very promising material and offers widespread applications in metallurgy and semiconductor industries (Liu et al. 2013). After the evolution of nanotechnology, it has received high interest toward modern technologies such as photonics, nanoelectronics, energy harvesting, energy storage, and biotechnology. In addition, silica NPs offer many key chemical applications such as separation, sensing, sorption, and catalysis. The various forms of silica NPs include fumed silica, silica gel, colloidal silica, and silica aerogel. Generally, silica NPs produced from silicon alkoxide precursor is most effective to attain controlled particle size, morphology, and porosity; however, their main drawbacks are the cost and sustainability. Also, this traditional silica NPs synthesis is centered on carbothermal reduction process, which requires multistep reaction route, intensive energy, high temperature, high pressure, and high acidity and is eco-hazardous. Thus, it is highly desirable to seek an alternative approach with less cost and beneficial to environment. Rice (Oryza sativa) is the second-largest crop species produced worldwide (7.0  108 metric tons per year); and 1.2  108 tons of rice husks (RHs) are produced per year across the globe. Rice husk is considered to be cost-effective and nonmetallic precursor for producing silica NPs, among other agricultural bio-resources. The enormous amounts of RH biomass offer an opportunity to produce nanostructured silica in large scale for industrial applications. Nevertheless, the current applications of silica NPs are limited to fertilizer additive, fuels, and landfill or paving materials, because of their tough, abrasive nature, great bulk, high ash content, and low nutritive properties. Also, it is difficult to dispose RHs as it causes pollution. Thus, it is desirable to make RHs as value-added properties. Moreover, the silica within the RHs naturally exists in the form of NPs. Thus, RHs are natural reservoirs for silica NPs since the rice absorbs silicic acid from the soil and accumulates around the micro-compartments of cellulose. The recent findings prove the significance of biomass-derived NPs in energy and environmental applications, which is described here. Bose et al. (2018) demonstrated a one-pot solution-processed low temperature microwave synthesis of silica NPs from cheap and readily available RHs. The

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as-synthesized NPs exhibited approx. 4.9 nm diameter and excellent monodispersity and water dispersibility and high photostability and pH stability along with high quantum yield (approx. 60%). Further, the silica NPs offered solution as well as solid-state luminescence properties with green luminescence, which resulted in white light color coordinate (0.31 and 0.27) closer to that of pure white light coordinate (0.33 and 0.33). These silica NPs can be utilized as a phosphor material to produce white light LEDs and lasers. It facilitates further to use in solar cells and printable electronics. Carbon-incorporated silica NPs have employed acid pre-treatment and controlled calcination of RHs biomass (Wang et al. 2017a, b, c, d). Further, they investigated that the formation of C-O-Si in the structure of the amorphous silica attributed to the photoluminescence (PL). They proposed a mechanism for PL that could be based on the localized energy levels introduced by the doping of carbon. This investigation is an important guidance for researchers to optimize the experiments further to develop high-performance PL materials for practical applications. Athinarayanan et al. (2015) explored irregular biogenic silica NPs (approx. 10–30 nm) from rice husk as a precursor by a pressurized condition; the inorganic impurities of rice husks removed by the acid pre-treatment have further induced the hydrolysis of organic substances; pre-treatment residues were calcinated at different temperatures. Mao et al. (2018) demonstrated amorphous silica NPs (approx. 20–30 nm) with high surface area from biomass fly ash through sol-gel method. This investigation revealed that the synthesized silica NPs possess spherical, monodisperse particle with diameter of 20–30 nm in addition to high specific surface area of 608 m2 g1. Besides, this method could also be useful to solve the environmental issues due to large amount of fly ash waste each year.

10.5

Concluding Remarks and Future Perspectives

The development of plant or plant-based engineered nanomaterials has been increasing steadily in recent years. Applications of cellulose nanostructures, starch nanoparticles, and carbon-based nanostructures are on the rise and will expand rapidly because synthetic methods are being improved as well as new synthetic routes are being discovered. These nanomaterials could also be combined in an assortment of ways to develop innovative systems with outstanding electrical, thermal, and mechanical properties, energy-conversion efficiency, and biocompatibility properties, in comparison with their pure, isolated forms. These promising nanomaterials represent a more sustainable alternative to the nonrenewable products, especially due to the tendency for a green carbon-based economy. Thus, biomassderived nanomaterials have certainly been turned as promising choices for food, biomedical, energy, and agricultural applications. However, there are some concerns about the development of these novel nanomaterials, in particular the limited scalability of the production methods, their

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unsolved self-tendency to agglomeration, and possible impacts to the environment. Thus, intense investigation has been focusing toward these materials to address the above issues and make them as a potential candidate for the benefit of the mankind. Acknowledgments The authors thank Embrapa, FAPESP (Proc. No. 2015/00094-0; Proc. No. 2017/22017-3), MCTI/SISNANO, REDEAGRONANO, DEMa/UFSCar, and CNPq for the financial support.

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Chapter 11

Recent Progress of Carbon Dioxide Conversion into Renewable Fuels and Chemicals Using Nanomaterials Harisekhar Mitta, Putrakumar Balla, Nagaraju Nekkala, Krishna Murthy Bhaskara, Rajender Boddula, Vijyakumar Kannekanti, and Ramachandra Rao Kokkerapati

Contents 11.1

Electrocatalytic CO2 Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2 Challenges for Electrocatalytic CO2 Conversion Using Nanomaterials-Based Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2.1 Significance of Electrocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.2.2 Nanostructured Metal-Based Electrocatalysts . . . . . . . . . . . . . . . . . . . . . . 11.1.2.3 Classification of Nanostructured Metal Catalysts . . . . . . . . . . . . . . . . . . 11.1.2.4 Nanostructured Cu-Based Electrocatalysts for CO2 Conversion . . 11.1.3 Reaction Parameters for Electrochemical CO2 Conversion . . . . . . . . . . . . . . . . . . . 11.1.3.1 Effect of Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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H. Mitta (*) · P. Balla State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India e-mail: [email protected] N. Nekkala Catalysis Division, CSIR-Indian Institute of Chemical Technology, Hyderabad, India K. M. Bhaskara Department of Physics, SVM Degree College, Gadwal, Telangana, India R. Boddula CAS-Key Laboratory of Nano-System and Hierarchical Fabrication, National Centre for Nanoscience and Technology, Beijing, China V. Kannekanti College of Chemistry, Key Laboratory Physics and Technology of Ministry of Education, Sichuan University, Chengdu, China R. R. Kokkerapati Crystal Growth and Nano-Science Research Center, Department of Physics, Government Autonomous College, Rajamahendravaram, India © Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5_11

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11.1.3.2 Influence of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.3.3 Effect of the Electrolyte Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Photocatalytic Reduction of CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.2 TiO2-Based Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 g-C3N4-Based Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.4 Miscellaneous Nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract Currently, global emissions of carbon dioxide (CO2) have caused serious issues with environmental contamination and global warming. Thus, for the sustained development of a clean society, the highly efficient conversion of CO2 into renewable liquid fuels and chemicals via greenery and novel chemical processes is very desirable. This chapter provides an overview of recent developments in efficient nanomaterial-based catalysts, which have led to several discoveries in the catalytic conversion of CO2 into desired liquid fuels and chemicals. Various technologies are also summarized, such as photochemical and electrochemical CO2 conversion. These two processes have received great attention because their prospective pathways can diminish the amount of atmospheric CO2. In this chapter, recent research advances on the nanostructure catalysts and the general parameters for CO2 conversion. Further increases in the selective formation of various desired reductive products, such as HCOOH/HCOO
, CO, CH3OH, CH3CH2OH, CH4, and C2H4, are also presented. An understanding of these topics is necessary to design novel nanomaterial catalysts for the efficient and selective reduction of CO2.

11.1

Electrocatalytic CO2 Conversion

11.1.1 Introduction There is increasing interest in deriving the significant platform molecules to produce fuels and chemicals from renewable resources. This is mainly due to the increase in fossil feedstock prices together with the concerning increase in greenhouse gases linked to global warming. Renewable sources such as solar, wind, and geothermal energy are further utilized for power generation; most importantly, the transport and chemical industries need an alternative source of fossil fuels. Moreover, the industrialized nations are largely dependent upon natural gas and petroleum, which is expressed in the high consumption rate for these convenience fuels. Nevertheless, the consumption of the various fossil fuels will be disproportionate to their availability. The present trend in research into the utilization of CO2 in fuels and chemicals is predominantly directed toward using CO2 more effectively as an alternative energy source to replace valuable natural gas and oil. Currently, technological improvements are in development to minimize undesirable emissions. The CO2 concentration has increased from 280 ppm in the 1800s to 385 ppm now, and it

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will reach nearly 600 ppm in 2100 (Zhang et al. 2017), thus resulting in increased desert formation and extinction of species. Furthermore, energy storage is of major significance. Electrical plate would absorb a definite amount of renewable electricity, but there is a limit. The plate should have a certain baseline that assures dependability of the power supply, whose baseline supply is typically realized using a vast amount of fossil fuel, and nuclear power, which cannot be easily switched off and on, but must be run continuously. The possible renewable sources should be being linked to the electrical plate; the problem of electricity oversupply will eventually become a reality. CO2 conversion into other useful carbon materials, forming a sustainable recycling system, is one possible route to reducing climatic hazards caused by the increasing CO2 concentration. A long-proposed method of energy storage and transformation is through CO2 conversion into useful molecules such as methane, HCOOH, and CO. So far, several methods have been developed, including conversion of CO2 into other carbon compounds. These methods include chemical reforming, photochemical, biological, mineralization, thermo-electrochemical (Gong et al. 2017). Electrocatalytic CO2 conversion is the most important as it can be investigated even under mild conditions. It is a highly controllable reaction step with higher faradaic efficiency (FE), because it can be driven by renewable energy/integrated into renewable energy systems. Interestingly, the CO2 conversion reaction is very attractive, i.e., renewable electricity is used to electrolyze water, which can convert through H2 and O2 into a valuable bi-product. The H2 produced will subsequently react with CO2 via the reverse water–gas shift reaction and generate syngas. In the CO2 electroreduction reaction, water acts as a source of protons, and CO2 hydrogenation in an electrochemical cell occurs at the cathode to generate liquid compounds such as HCOOH, CH4, and CH3OH. Then, like conversion technology, suppose it is implemented on a bulk scale; this would regenerate the emission of CO2 into fuels, having an extreme effect on the global carbon balance. Another method is to produce CO from CO2 conversion via the electrochemical process. However, this approach is not attractive as thermo-catalytic CO2 conversion into CO is much more efficient with regard to throughput and compactness. Then again, the efficiencies of electrochemical cells used for CO2 conversion to CH3OH and other liquid chemicals such as HCOOH remain relatively poor. In the progress evolution of highly efficient metal-based catalysts and new advanced technology, electrochemical cells will be designed in the future to obtain high current density under precise operating conditions. Initially, presentation of electrochemical CO2 conversion received renewed attention because of the rapid increase in the price of fossil fuels. So far, there has been extreme interest in this area, which represents a novel strategic pathway through CO2 conversion into fuels and chemicals, including important feedstocks such as HCOOH and CH3OH. Several methods of electrochemical CO2 conversion have been rapidly developed in the past decade (Jones et al. 2014; Zhang et al. 2018). Among all the processes studied, electrocatalytic reduction appears to be the most feasible as it can be conducted in ambient conditions whilst having a highly controllable reaction step and relatively high conversion efficiency and can be driven by renewable energy/ integrated into renewable energy systems. So, electro CO2 reduction system is

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attractive because (1) it sequesters carbon from the atmosphere, (2) it provides an energy storage solution for intermittent renewable sources with high energy density and (3) it can be used to produce industrial chemicals and fuels. The third point is particularly interesting because as society decreases its dependency on conventional fossil fuels, a void will be left for the materials and petrochemical industries. However, if petroleum feedstock can be derived from CO2 conversion, then the feedstock could potentially be produced anywhere and directly implemented into conventional downstream industries. However, every CO2 conversion process is faced with the ultimate challenge of an extremely stable molecule. Therefore, every CO2 conversion process is great. First CO2 is a highly stable molecule, very difficult to activate. Activating the CO2 molecule into its useful state by reduction requires not just significant energy input, such as temperature or pressure, but also adjusting through an extremely active catalyst is needed. Second, CO2 solubility in aqueous environments required for electrochemistry is relatively low. These limitations present fundamental challenges associated with efficient novel catalysts, electrochemistry, and electrochemical cell engineering.

11.1.2 Challenges for Electrocatalytic CO2 Conversion Using Nanomaterials-Based Catalysts 11.1.2.1

Significance of Electrocatalysts

With regard to electrochemical CO2 conversion into liquid fuels and chemicals, highly efficient electrocatalysts have to develop industrial applications. Generally, an electrocatalyst participates in the electron transfer reaction on the electrode and facilitates the electrochemical CO2 reduction reaction that takes place on the electrode surface, as shown in Fig. 11.1 (Benson et al. 2009). For efficient electrocatalysis, both processes have to be accelerated at the same time. Thermodynamically, it must be a good combination between the redox potential for the electron transfer and the catalyzed electrochemical CO2 reduction reaction. Preferably, an optimal electrocatalyst have to operate near the thermodynamic potential of the occurring electrochemical reaction. In the search for an optimal electrocatalyst, various catalytic materials are typically screened for their redox potentials, as well as for the electron transfer rate, chemical kinetics, and current efficiencies. To distinguish, all the characteristics of highly efficient catalysts have to be considered. Fig. 11.1 Analogous electrocatalyst CO2 conversion provides electron transfer through an electrode surface and in an electrolyte solution (Benson et al. 2009)

Cat Electrode

Vapplied

Substrates kcat

kh ne-

ne-

Catn+

E0(Catn+/0)

Products

E0(products/substrates)

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The electrochemical process is rather complex, as it has bearing on both electrical processes and chemical kinetics. Direct electrochemical CO2 conversion occurs on the surface of the electrode, which requires large overpotentials, i.e., the difference between redox potentials of reactant and products along with the applied voltage. Large overpotentials result in less conversion efficiencies. Both thermodynamic and kinetic parameters must be considered in order to overcome this restriction, at least to a definite extent. With aim to minimize the overpotential, the electrocatalysts conventional potential must be well matched with the redox potentials of the species involved in the electrochemical reaction. Moreover, the rate constant for the electrocatalyst reduction (kh) and reaction rate constant (kcat.) have to be high at the applied voltage. In contrast, herein it is necessary to distinguish between redox catalysis and chemical catalysis. For redox catalysis, the electrocatalyst acts as an electron transfer agent, which means that electrons pass through the electrode and the reactant. Moreover, the chemical reaction speeds up is due to the highly efficient number of electrons circulating. In addition, it can also succeed through an improvement of the electrode morphology. In case of chemical catalysis, an electrocatalyst pass through more intimate interaction with chemical species undergoing chemical transformations. Therefore, an efficient novel catalyst/electrode assembly should easy both mechanisms significantly lowering the overpotential and the activation energy of the electrochemical reaction. Differentiating between homogeneous and heterogeneous electrochemical reduction has been classified; the two approaches should have different reaction mechanisms. This could require various electrochemical cell designs. In the homogeneous approach, electrocatalysts are usually distributed in the electrolyte solution, which can diffuse on the electrode surface. So far, different transitions of metal-based complexes, including macrocyclic, bipyridine or phosphine ligands, have been applied as typical homogeneous electrocatalysts because their remarkable coordinative structure and active sites are strongly co-coordinative with CO2 molecules, but there are many disadvantages, such as the preparation method, less reduction activity, and toxic effects. Hence, heterogeneous catalysts have been developed. They can behave as electrocatalysts, or another catalytic material that can be immobilized on the porous electrode surface. In addition, they can be synthesized very easily, have less toxicity, no recovery issues, and better electrocatalytic performance. Finally, heterogeneous electrocatalysts are arguably more suited to bulk scale industrial applications because of the long-term stability and the ease of separation of the desired product. Therefore, the metal, either in bulk form or as supported forms (e.g., Cu, Ni, Fe, Sn, Ti), used as catalysts, makes metallic electrodes using a single-element polycrystalline form. In more recent studies, new types of efficient heterogeneous electrocatalysts have drawn attention to advanced nanomaterials, including noble and non-noble metal-like nanoparticles (NPs), nanowires, nanotubes, and core–shell structures.

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Fig. 11.2 Plausible mechanism pathways for electrochemical CO2 conversion in an electrolyte solution using various metal-based electrocatalysts. (Reproduced with permission Jones et al. 2014)

O

coordinating

O

non-coordinating CO2

+

e-

O

O

H2O

H2O O

O-

O

OH OH-

H Hg, Pb, Bi, etc.

e-

O

O H OH-

eOH-

O C

CO Au, Ag, Zn, etc.

eO

11.1.2.2

C°

H+ e-

Cu higher products

Nanostructured Metal-Based Electrocatalysts

In previous studies, new catalysts have been required to enhance the desired product selectivity and the FE of the electrocatalytic activity in CO2 conversion, which simultaneously decreases the overpotential. Concerning these issues, nanotechnology has exploited several new strategies for designing efficient CO2 conversion over electrocatalysts for the last few decades (Fig. 11.2). Plausible mechanisms of different metal-based electrocatalysts are used for CO2 conversion in electrolyte solution. The reason for nanostructure metal-based samples attracted attention in this area, as this type of nanomaterial has an incredibly large surface, structural fine tuning, and mixed metals, alloys, and heteroatom doping have tremendous conductivity, are less costly, and are environmentally friendly. At present, a huge achievement is being made regarding novel nanostructure metal-based electrocatalysts used for CO2 conversion. More recently, various new types of heterogeneously structured electrocatalysts have significantly increased CO2 conversion, including metallic nanomaterials, which have exhibited greatly enhanced performance compared with normal catalysts.

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Table 11.1 The performance of nanostructured non-noble metal-based catalysts in electrocatalytic CO2 conversion Experimental parameters
0.80 V (vs RHE)

Electrolyte 0.10 M NaHCO3


1.2 V vs RHE
0.18 V vs RHE

0.1 M KHCO3 0.1 M LiClO4


1.2 V vs RHE

0.1 M KHCO3

H2 (~80%), CO (~10%), and CH4 (minor)

0.1 M KHCO3 0.1 M KHCO3 0.1 M KHCO3 05 M KHCO3

CO (91.2%)

Au NPs embedded on functional GO ribbon


0.89 V vs RHE
1.49 V vs RHE
0.77 V vs RHE
0.87 V vs RHE

Oxide-derived nanostructure Au

0.05 V vs ag/AgCl

CO (~82%)

Au-Cu bimetallic NPs Sn-quantum sheets graphene nanostructured Sn


0.73 V vs RHE.
1.8 V vs SCE
1.8 V vs SCE

0.5 M KHCO3 or NaHCO3 0.1 M KHCO3 0.1 M NaHCO3 0.1 M NaHCO3

Cathode catalyst Au NPs

Nanoporous In-Sn Ag-Pd-nanodendritemodified Au nanoprisms Au-Cu bimetallic NPs Pd NPs Trigonal-Ag NPs Au-Cu NPs

11.1.2.3

Main products, faradaic efficiency CO (0.7 and 0.8 mL/(h. cm2), H2 (2.0  0.5 mL/ (h.cm2) HCOO
(78.6%) CO (85–87%)

CO (96.8%), HCOO
CO (~87%) and H2 CO (80%)

CO (52%) and H2 (20%) Hydrocarbon (~91%) HCOO
(93%)

References Trindell et al. (2017) Dong et al. (2017) Shan et al. (2017) Mistrya et al. (2017) Gao et al. (2015) Liu et al. (2017) Kim et al. (2017a, b) Rogers et al. (2017) Kim et al. (2017a, b) Kim et al. (2014) Lei et al. (2016) Zhang et al. (2014)

Classification of Nanostructured Metal Catalysts

Supported catalysts are usually prepared by loading active species onto carbon paper or glass carbon electrodes. Normally, the as-prepared catalysts are powder samples that are obtained using a co-precipitation method or a wet-chemical reduction method. By tuning the reaction conditions, the morphology, composition, well-controlled size, and oxide-derived (OD) surfaces exhibit a great influence on CO2 conversion. In early studies, metals can be classified into three distinct groups, which are differentiated based on reaction paths and the formation of desired products: i) for producing HCOOH such as Sn, Cd, Hg, Pb, Tl, and Bi, ii) specific CO-producing metals such as Au, Ag, and Zn, and iii) Cu, in its own group owing to its ability to produce low-range hydrocarbons (Table 11.1).

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In addition, other metals such as Al and Ga (except Pd) exhibit less electrocatalytic activity because of the formation of CO and thus, low FE was obtained. Finally, CO selectivity toward the ratio of HCOOH/HCOO
on the metal catalysts was in order: Au > Ag > Cu > Zn> > Cd > Sn > In> Pb > Tl (Hori et al. 1994). Zhu et al. (2013) reported that various monodispersed Au NPs show maximum FA of CO selectivity (90%). Sun et al. (2017) observed the effects of surface modifications of Au NPs over carbon nanotubes, whereas formation of CO could be improved (97.6%) with stable surface-active sites. Recent research in the electrochemical CO2 conversion of Au-, Pd-, and Sn-based nanostructure electrocatalysts has employed different parameters, such as particle sizes, surface modification, and reaction conditions. However, the development of this engineering involves high costs, low FE, and poor selectivity of the hydrocarbons (Kuhl et al. 2014). Generally, the high price of the noble-based electrocatalysts has prevented the progress of their extensive applications. Moreover, for the hydrogen (H2) development reaction, a competitive reaction consequently often results in less selectivity for CO2 conversion.

11.1.2.4

Nanostructured Cu-Based Electrocatalysts for CO2 Conversion

To understand these issues, extensive research efforts have been made in developing nanostructure electrocatalysts for CO2 reduction. However, compared to immobilized molecular catalysts and metallic electrocatalysts, Cu is a highly attractive and promising avenue. It has been demostrated that nanostructure copperbased electrocatalysts, alloying, surface modification, etc., play an important role in the electrocatalytic activity and selectivity toward CO2 conversion. In a recent study Cu was the only metal individually grouped, because it is more favorable for producing alcohols, aldehyde, and various hydrocarbons. The product distribution apparently depends on the nature of the metal catalysts involved, such as neither the bulk nor the surface structure of the catalysts (Kas et al. 2015). Moreover, much research and great attention have been invested in investigating a novel Cu for CO2 conversion. For example, the study of the morphology-based Cu electrode that provides high electrocatalytic activity and selectivity in CO2 conversion is reported. Interestingly, the main products such as CH4 and CH2CH3 have the lowest FE on commercial Cu electrodes (Kung et al. 2017; Jeon et al. 2018). In addition, an enhanced yield of methane and ethane was observed through in situ deposition of the Cu electrode (Wei et al. 2014). The size of nanoparticles is of crucial importance because, similar to thermocatalysis, there is a strong size-activity relationship. Generally, the sizedependent reactivity correlates with several factors: (1) quantumsize effects, (2) the presence of a high density of low-coordinated atoms, (3) excess electronic charge, (4) interactions between NPs, and (5) band alignment with the support electrode. In particular, in CO2 reduction reactions, the particle size is known to determine the exposed amount of active low-coordinated surface sites, which facilitates the adsorption and stabilization of key intermediates. For example, a prominent size-dependent activity and selectivity of electrocatalytic reduction of CO2 was identified over Cu

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nanoparticles with sizes ranging from 2 to 15 nm. As the particle size decreased, a dramatic increase in current density and CO selectivity was observed, while product selectivity’s for hydrocarbons, such as CH4 and C2H4, were relatively reduced at smaller particle sizes (Garcia et al. 2017). Therefore, the results demonstrate that electrochemically reduced oxide-derived Cu outperforms either polycrystalline copper or reduced copper from copper oxide via hydrogen at a high temperature (Li et al. 2014a, b). It is suggested that the small copper NPs showed high surface area which is the key factor to improve the electrocatalytic activity. In this report, discussions are presented from a study on the industrial applications of the electrocatalytic reduction techniques of nanostructure copper-based catalysts and its relation to their activity in converting CO2 into fuels and chemicals. The first part of this discussion concerns the selection process, in which there are competitive reaction conditions and renewable energy systems (Table 11.2). Obviously, the existence of an appropriate electrocatalytic reduction is a primary requisite for a successful application, i.e., nanomorphology on the adsorption of reaction intermediates and their dynamics during the CO2 conversion reaction. In view of the previous research on the Cu-supported electrocatalysts such as Cu nanostructures, Cu nanowires, metal–organic frameworks, and Cu-based alloys used to improve the CO2 electrochemical reduction (in Table 11.2).

11.1.3 Reaction Parameters for Electrochemical CO2 Conversion 11.1.3.1

Effect of Temperature

Generally, CO2 conversion routes greatly depend on various parameters such as electrode potential, electrolyte solution, reaction temperature, pressure, and CO2 concentration. Hori et al. (1994) and Hua et al. (2010) investigated the product selectivity that is reliant on reaction temperatures using a Cu cathode. Hori et al. (1994) reported that when the temperature is increased from 0  C to 40  C, CH4 production decreases from 65% to 5% (FE), with a Cu electrode. In addition, the selectivity of C2H4 and H2 increased significantly. Mizuno et al. (1995) have also investigated In, Pb, and Sn electrodes being used for CO2 conversion. With the In electrode, HCOOH FE increases to 100% temperature between 20 and 60  C, but for the Sn electrode, the HCOOH FE decreases with increasing temperature and an FE of 99.2% can even be obtained at 20  C. For the Pb electrode, FE to formic acid of 100% was obtained at 60  C. Therefore, it is indicated that temperature plays an important role in the selectivity of the desired product.

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Table 11.2 The electrochemical activity CO2 conversion with various copper materials used in the nanoparticles, nanowires, composite electrocatalysts, their reaction conditions, and selectivity Cathode catalyst Cu NPs Cu NP-based electrode

Copper-hydride nanoclusters Nanocrystalline copper Nanostructured Cu2O-derived copper Nanopore-modified copper Cu NPs into MOF Carbonsupported Cu NPs Cu NPs on PyN-rich GO Small Cu particle on nanocarbon

Experimental parameters
1.1 V (vs ag/AgCl)
1.8 V vs Ag/AgCl
2.4 vs g/AgCl
0.81 V vs RHE
0.3 V vs SCE
1.0 V vs RHE

Electrolyte 0.1 m KHCO3 0.1 M KHCO3

0.1 M KHCO3 0.1MKOH 0.1 M KHCO3

Main products, faradaic efficiency C2H4 (44%) or CH4 (2%) CH4 (12.1%)

HCOOH (>80%), CO and H2 (minor) CO, Et-OH (42.9%), Ac-OH (13.6%), n-Pr OH (0.6) C2H4 (32.1%), and Et-OH (16.4%)

Tang et al. (2017) Li et al. (2014a, b) Zhou et al. (2016) Peng et al. (2017) Kung et al. (2017) Baturina et al. (2014) Li et al. (2016) Marepally et al. (2017)


1.3 V (vs RHE)
0.82 V vs RHE
1.1 V vs RHE

0.1 M KHCO3 0.1 M NaClO4

Ethylene (35%), CH4(1%) H2 (66), HCOO
(28%) and CO (1%) H2 and C2H4 (22–25%) and CO and CH4 (13%)


0.9 V vs RHE -2 V vs Ag/AgCl

0.5 M KHCO3 0.5 M KHCO3

C2H4 (~19%)

Cu NPs rGOsnanocomposite


0.4 V vs RHE

0.1 NaHCO3

Prism-shaped Cu


1.1 V (vs RHE)
0.5 V (vs RHE)
1.1 V vs RHE
0.6 V vs RHE
1.25 V vs RHE

0.1 M KHCO3 0.1 M KHCO3 0.5 M NaHCO3 0.1 m KHCO3 0.1 M KHCO3

C2H4 (27.8%)


1.4 V vs RHE

YSZ

CO (74.29%)

Copper nanowires Sponge type of Cu Cu nanowire 5-fold twinned copper nanowires NixCu1-x/ Nb1.33Ti0.67O4

References Kas et al. (2015) Garcia et al. (2017)

HCOO
(27.5%), CH3OH (47.4%), and C2H5OH (0.6), Ac-OOH (23.9), isopropanol (0.3%). CO (~12%), HCOOH (~40%) and CH4 (~25%)

CO (29%), HCOOH (25%) and EtOH, C2H4 (minor) C2H4 (32.3%) and C2H6 (29.1%) CO (~50%) and HCOOH (~30%) CH4 (55%) and CO, C2H6 (5%)

Hossain et al. (2017) Jeon et al. (2018) Cao et al. (2017) Dutta et al. (2017) Ma et al. (2015) Li et al. (2017a, b) Wei et al. (2014)

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Influence of Pressure

To improve CO2 conversion, product selectivity is significantly dependent on the solubility of CO2 in the solution. When pressurized, the CO2 during the reaction increases the conversion rate and FE of products as well. Pd, Hg, and In electrodes exhibited high FE of HCOOH up to 100% under high pressure, with no CO formation (Todoroki et al. 1995).

11.1.3.3

Effect of the Electrolyte Solution

Typically, electrochemical reduction has an influence on the nature of the electrolyte solution, which provides a couple of electrons and protons. As per previous studies, the mono-bivalent cations possess higher electrochemical CO2 conversion, which is expressed to the order of Mg+2 ¼ Ca+2 > Ba+2 > Li+ > Na+. The halide anions are also readily adsorbed onto the electrode surfaces, increasing the capture capacity of CO2 and suppressing the adsorption of protons. Nakata et al. (2014) and Wu et al. (2012) have described the importance of electrolyte solution in electrochemical reduction. To investigate, the non-aqueous solvents of organic liquids are utilized to obtain higher CO2 conversion. However, methanol is the best organic solvent in electrochemical CO2 conversion. Therefore, the basic properties of the methanol act as a protic solvent, which provides hydrogen atoms for the formation of H2/or hydrocarbon. Interestingly, CO2 and methanol should both be miscible with each other. Kaneco et al. (2002) have also investigated the use of methanol as an electrolyte with various salts such as LiCl, LiBr, LiClO4, LiI at a low temperature.

11.2

Photocatalytic Reduction of CO2

11.2.1 Introduction Since the invention of photocatalysis, numerous reports have been published in the literature, mainly concerned with photocatalyst synthesis and evaluation in various applications such as photocatalytic (PC) water splitting to H2 and PC CO2 conversion to hydrocarbons (Nakata et al. 2012; Kudo and Miseki 2009; Ryu 2010). Solar energy is a nonfinite source of energy. Utilizing the infinite solar energy to which CO2 is reduced in fuels and chemicals such as methanol and methane is a promising approach to addressing the global warming problem and energy crisis (Nakata et al. 2012; Hashimoto et al. 2005; Etacheri et al. 2015). Basically, there are two types of photosystems for CO2 conversion. The first is a PC system made up of simple devices and accessible photocatalysts, which utilize a mixture of photocatalyst particles and solvent used for the reduction of dissolved CO2 (Nakata and Fujishima et al. 2000). In this kind of system, the driving force for CO2 conversion is entirely

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Light source

CB Ebg

A

VB

h

+

Pa t

c

fa ur

mb co + + -

e er

S

Photo absorption and electron-hole pair generation.

+

-

hD

Pat hB

Pat

hC

Pat

+ h

n

donor+

oc

acceptor

es

s

xid

n

io

at

donor

pr

s

es

oc

e

tio

or pt

uc

ce

ed

+ - ++ on mbinati o c re e Volum

-

ac

R

hv

-

hv

on

ti ina

pr

O

Semiconductor catalyst

Fig. 11.3 Mechanism and possible pathways for photoinduced reduction and oxidation processes on the surface of the photocatalyst (Linsebigler et al. 1995)

solar energy, and lighter hydrocarbons and a few other organic compounds such as CH3OH, CH4, and HCHO, HCOOH, and CO are produced. The other is the photoelectrochemical cell (PEC) system, which is composed of a semiconducting photoelectrode and a counter electrode (Ryu 2010). The semiconducting photoelectrode harvests light to produce charge carriers and accomplish a half-cell reaction to reduce CO2 on the photocathode. The PC phenomenon, possible pathways for photoinduced reduction, and oxidation action on the photocatalyst surface are illustrated in Fig. 11.3. (Linsebigler et al. 1995). It is well known that PC CO2 conversion involves three key steps. In the first step, the incident photons possessing energy equal to/higher than the band gap energy of the photocatalyst are absorbed to generate electron (e-1)–hole (h+) pairs. In the second step, these resultant charge carriers transit independently in different pathways to the photocatalyst surface and are trapped at the edges of the conduction band. This acts as a reduction site for electron acceptors, whereas the holes being trapped at the edge of the valance band serve as oxidizing centers for donor species. This means that the photo-generated electrons could reduce CO2 into fuels, whereas the holes could oxidize H2O into O2. In competition with the charge transfer of generated charge carriers to adsorbed species, there are several chances to recombine radiatively or nonradiatively. Thus, suppressing the recombination is a crucial step in controlling the reactions for higher yields. So far, the semiconductor materials have accounted for catalytic photoreduction of CO2 including metal oxides, nitrides g-C3N4, and metal sulfides have been noted to be active (Inoue et al. 1979; Xie et al. 2013; Li et al. 2012; Sato et al. 2010; Suzuki et al. 2011; Zhou et al. 2011; Guo et al. 2013; Yan et al. 2013; Park et al. 2012; Zhang et al. 2012: Liu et al. 2012a, b; Chaudhary et al. 2012; Liu et al. 2011; Arai

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et al. 2011), even though the scientific community is far away from discovering efficient viable devices. CO2 conversion into hydrocarbons such as CH4 and CH3OH, is not superior, i.e., 10 mmol of product per hour per gram of photocatalyst. Specifically, photoactivity mainly depends on the ability of the photocatalyst to absorb light, the type of photocatalyst and its morphological features, the composition of the reaction medium, and adsorption of the reactants on the surface of the photocatalyst. To achieve a highly efficient charge separation and transportation, for example, nanostructured photocatalysts with various morphologies have to be developed.

11.2.2 TiO2-Based Nanomaterials Miscellaneous semiconductive metal oxides such as titanium oxide (TiO2), zinc oxide (ZnO), cadmium sulfide (CdS), zirconium oxide (ZrO2,) iron oxide (Fe2O3), silicon carbide (SiC), tungsten oxide(WO3), and gallium phosphide (GaP) have been employed for PC CO2 conversion to produce formic acid, formaldehyde, methanol, and methane (Roy et al. 2010; Huff and Sanford 2011). Considering the availability, stability, and optical/electronic features, TiO2 nanomaterials appear to be good candidates and have attracted more attention to PC CO2 conversion. In this discussion, the correlation between the catalytic properties of engineered TiO2 nanocatalysts and CO2 conversion yields using solar energy is highlighted. Naturally occurring TiO2 has three types of crystal phases: anatase, rutile, and brookite. Thermodynamically, the anatase and brookite crystal phases of TiO2 are highly metastable, and can be changed to the stable rutile phase by thermal annealing. PC CO2 conversion occurs in the anatase, rutile, and mixed phases, which have been investigated widely because the surface characteristics and crystallinity can have a significant influence on efficiency. Li et al. (2014a) reported producing CO and CH4 by PC CO2 conversion over TiO2 with anatase, rutile, and brookite phases (Liu et al. 2012). The brookite phase exhibited the highest CO2 conversion because of it has a higher number of oxygen vacancies than the other phases. Xu et al. investigated the performance of the TiO2 nanosheets and nanocuboids on PC CO2 conversion (Xu et al. 2013). TiO2 nanosheets, having 95% (100) facets, show the highest CH4 formation (5.8 ppm/gcat h) compared with TiO2 cuboids, which have 53% (100) facets (1.2 ppm/gcat h). Later on, Yu et al. (2014) reported TiO2 nanosheets with 58% (001) facets, resulting in the highest CO2 conversion (1.35 μmol gcat h). Xu et al. (2015) also examined TiO2 cubes with an anatase phase that are exposed with various facets such as (100) and (001). TiO2 cubes have 75% (100) facets and 25% (001) facets, showing a CH4 evolution rate of 4.56 μmol gcat h and a CH3OH evolution rate of 1.48 μmol gcat h. Koci et al. (2009) reported the influence of the efficiency of the PC conversion of CO2 on the crystal size (4.5–29 nm) of anatase TiO2 NPs. The highest CO2 conversion rate (0.308 μmol gcat h) was observed for TiO2 NPs measuring 14 nm, which may be due to the competing effects of optimal particle size on the absorption of light,

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Table 11.3 Photoreduction of CO2 over various TiO2-based catalysts Products (mmol/gcat h
1) 11.74 CH4 18.67 HCOOH 29.88 C2H5OH 160 CH4

Photo catalyst Multi-wall carbon nanotubes supported TiO2

Reductant CO2 saturated H2O solution

Light source 15 W UV lamp (365 nm)

Cu and Pt NPs N-doped TiO2 nanotubes Pt/TiO2 nanotubes

Water vapor

Sunlight

Water vapor

4.8 h
1 CH4

Titania nanotubes/nanorod

Water vapor

Cu-TiO2 nanorod

Water

300 W Hg lamp (365 nm) 100 W Hg lamp (365 nm) 8 W VA

Ag-TiO2 nanorods

Water

2.6 CH4

0.5 wt% Cu TIO2 nanoflower films

CO2 saturated H2O solution

8 W UVA lamp (365 nm) UV Hg lamp and xenon lamp

Pt-doped reduced graphene oxide/TiO2 nanocomposites NiO/In2O3 promoted TiO2 nanocatalysts 1.5 Wt% CuO/TiO2 nanorods

Water vapor

Pt-Cu2O TiO2 nanocrystals

CH4 CH4

1.8 CH4

1.7 CH4

H2O reductant. CO2 and H2O vapor

500 W Hg lamp UV light (365 nm)

Water vapor

UV light (300 nm
400 nm)

240 CH4 36.18 CH3OH 79.13 C2H5OH 1.4 CH4

Reference Xia et al. (2007)

Varghese et al. (2009) Zhang et al. (2009) Vijayan et al. (2010) Tan et al. (2012) Kong et al. (2013) Liu et al. (2015) Tan et al. (2015) Tahit et al. (2016) Cheng et al. (2017a, b, c)

Zhuo et al. (2017)

charge-carrier dynamics, and scattering efficiency. Hao et al. (2002) synthesized TiO2 powders with crystal sizes ranging from 8 to 15 nm. These powders have shown relatively high PC performance compared with commercial TiO2, with a particle size of 150 nm. Hence, It is strongly recommended that the optimized size of NPS should be synthesized for high efficient CO2 conversion. This means that the efficiency of the TiO2 material is strongly affected by their structure and morphology. Hence, it is suggested that crystal facets of photocatalysts might be carefully designed to make the photocatalyst highly effective. Recent studies on photoreduction of CO2 over various TiO2-based catalysts are summarized in Table 11.3.

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Facile combining of photogenerated charge pairs and a high-energy band gap (3.2 eV) of pure TiO2 results in poor efficiency in CO2 conversion to hydrocarbons. Noble metal (Ag, Pd, and Pt) deposition on TiO2 is an effective solution to enhance the PC activity of TiO2. Jian et al. reported that an Ag-modified TiO2 (2 wt%. Ag/TiO2) nanocomposite prepared using a spray pyrolysis method exhibits much higher photoactivity toward CH4 production compared with pure TiO2 (Jian et al. 2011). Ag/TiO2 nanocomposites with a rutile phase exhibited higher activity for CO2 conversion under visible light than bare TiO2, owing to the localized surface plasmon resonance (LSPR) effect of the Ag NPs. The highest CO and CH4 production rates are about 72 μmol g
1 and 6.8 μmol g
1 respectively (Xudong et al. 2017). Pt-loaded nano-sized (010) dominant TiO2 anatase rods with (010) facet have shown superior PC activity in generating CH4 from CO2 at H2O vapor compared with TiO2 nanocrystals (P25). Nanosized noble metal (Pt or Pd)-deposited TiO2 nanofibers (Pt or Pd/TiO2 NFs) are synthesized using a wet impregnation method (Anjana et al. 2016). Then, CdSe quantum dots (QDs) are decorated onto the metal TiO2 NFs. Pd-coated TiO2 NFs exhibited enhanced performance compared with Pt-coated TiO2 NFs in the production of CH4. In the presence of CdSe, Pt-coated TiO2 NFs have shown the highest selectivity of CH3OH (90 ppmg
1 h
1). In the recent past, transition metal-doped TiO2 catalysts have been utilized for PC CO2 conversion to CH4. A mo-doped TiO2 nanotube catalyst with a 1.32% Mo/Ti ratio shows a CH4 production rate of 0.48 μmol gcat
1 h
1(Nguyen et al. 2015). By the doping of transition metals such as V, Cr, and Co onto TiO2 NPs, the light absorption property was remarkably enhanced (Ola et al. 2015). Among the catalysts under study, 1 wt% Co-doped TiO2 NPs show the highest production rate of 26.12 μmol gcat h for CH3OH. Copper (1.5 wt%)-TiO2 nanorod thin films have shown higher CH3OH and CH3CH2OH yields, 36.18 mmol/gcat h and 79.13 mmol/ gcat h, in the photoreduction of CO2 in optofluidic planar reactors (Cheng et al. 2017a, b, c). The highly efficient activity of Cu–TiO2 nanorod thin film is attributed to the doping of Cu2+ ions and one-dimensional nanostructure, which enhance the photon transfer limitations.

11.2.3 g-C3N4-Based Nanomaterials Metal-free graphitic carbon nitride, known as g-C3N4, has received enormous attention since it was discovered and has rapidly become a hotspot in various fields, including PC CO2 conversion, because of its availability, facile synthesis, physicochemical stability, and electronic structure, and because it has a band gap (2.7 eV) (Niu et al. 2014). g-C3N4 is a conjugated metal-free semiconductor that can achieve the direct transformation of infinite solar source to chemical energy. Still, practical applications are limited owing to its small specific surface area, lower electronic conductivity, high rate of recombination of photo-generated charge carriers, and lack of light absorption (above 460 nm). A number of solutions, such as specially structured nanocomposites, e.g., g-C3N4 nanofibers, nanotubes, nanosheets, and

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nanosphere-based nanocomposites, doping hetero-metal atoms (B, N, P, and S, etc.), depositing noble metals (Pt, Au), or constructing heterojunctions, have been employed to promote PC activity. g-C3N4, with a helical rod-like morphology, enhances CO2 conversion to CO with a rate about 20 times the bare g-C3N4 (Zheng et al. 2014). This enhanced rate is attributed to efficient charge transfer throughout the helical structure and the recombination rate of the inhibited charge carrier. On the other hand, decoration of metal oxides on g-C3N4 may be another suitable approach to improving CO2 conversion activity. g-C3N4/ZnO nanocomposite results in better CO2 conversion activity than bulk g-C3N4. Brookite TiO2/g-C3N4 nanocomposite, in which g-C3N4 nanodots with a mean size of2.8 nm are uniformly distributed on brookite TiO2 nanocube surfaces, exhibits a great adsorption tendency of reactants for CO2 conversion. g-C3N4 nanosheets with an ordered mesoporous structure are obtained using the nanocasting approach (Li et al. 2017b). Later on, the CdIn2S4/g-C3N4 nanosheets are synthesized via a facile hydrothermal method. The nanocomposites obtained show superior PC performance and high chemical stability in photochemical CO2 conversion to methanol with H2O under visible light (420 nm) irradiation. Copper (3 wt%)modified graphitic carbon nitride (Cu/g-C3N4) nanorods efficiently reduce CO2 to methane (109 μmol gcat
1 h
1) in the photo-induced CO2 water system. Significantly enhanced reduction over Cu/g-C3N4 nanorods is mainly due to a hindered photogenerated charges recombination rate over a polymeric structure (Tahir et al. 2017). Cu-modified g-C3N4 and TiO2 nanocomposites have been investigated for CO2 photoreduction with water over UV and visible light. The high yield of the desired products has been observed to be about 2574 and 5069 mmol/gcat of CH3OH and HCOOH respectively. In addition, under visible light, enhanced CO2 conversion efficiency is mainly attributed to fostering carrier charge separation (David et al. 2017).

11.2.4 Miscellaneous Nanomaterials The main CO2 conversion product of the novel catalyst Co-doped MoS2 NP is methanol, the yield of which reaches 35 mmol at 350 min. Indium oxide (In2O3) nanobelts covered by a 5-nm carbon layer with Pt as a co-catalyst and H2O as a reductant (Fig. 11.4), which provides an improved PC CO2 conversion to CO and CH4, yielding a CO evolution rate of 126.6 and a CH4 rate of 27.9 μmol h
1. Recently Yu et al. (2016) have synthesized carbon layer-coated cuprous oxide (Cu2O) mesoporous nanorods on Cu foils (CCMNRs) through a facile chemical oxidation and subsequent carbonization method. The schematic diagram in Fig. 11.4 shows PC CO2 conversion over CCMNRs under visible light. The optimized catalyst achieves 2.07% quantum efficiency for CH4 and C2H4 at a wavelength of 400 nm, and activity remained at 93%, even after six photoreduction cycles.

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Ocean of electrons

CO2+H2O

CH4, C2H4

- -

-

-

-

-

-

-

CB VB

Protective Layer + +

H2O

+

+ Cu2O nanorod Carbon Layer Cu2O nanoparticle

O2

Fig. 11.4 Schematic diagram of the proposed reaction process of CO2 conversion over coated cuprous oxide mesoporous nanorods under visible light (Yu et al. 2016)

11.3

Conclusions

Most forecasts of prospective energy supply and demand show a projected shortfall in liquid hydrocarbons, which had become relatively severe by about the turn of the century. Considerable research is currently being initiated to evolve processes that will produce ecologically acceptable liquid fuels, gases, and chemicals from advanced CO2 conversion technologies. CO2 conversion involves several engineering and chemical stages that together extract the fuels and chemicals. Owing to the molecular composition, the structure of the nanomaterials explored, thermodynamics, and kinetics. The development of low-price catalysts, therefore, may enhance the economic competitiveness of catalytic CO2 conversion to liquid fuels/chemicals. Various CO2 conversion applications such as photochemical and electrochemical processes are reported and reviewed. The present investigation explores advanced nanomaterials containing noble/ non-noble metal catalysts (Pd, Au, Sn, In, Cu, Ni, etc.), and acceleration of electrochemical and photochemical CO2 conversion reactions. The main objective is for improving the desired product selectivity and understanding thoroughly the structure and morphological features of the nanomaterials. On the other hand, nanostructured Cu catalysts, being relatively inexpensive compared with noble metals, have been assumed to be suitable materials. They show a relatively attractive performance in electrochemical CO2 conversion; hence, they may provide a large active catalyst area per unit of electrode and show greater dispersion of the catalyst. In this chapter, it is hoped that prospective interest will be taken in the development of highly efficient Cu-supported electrocatalysts for CO2 conversion by tuning the morphology and composition of the nanostructure. Further trends in the field of

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electrochemical CO2 conversion should focus on rationally designed Cu-based catalysts, which will be in great demand. The primary investigation into renewable energy opportunities utilizing PC CO2 conversion to hydrocarbons has been discussed. The recent development of nanoscale structures in particular offers a large specific surface area, inhibition of the recombination rate, and improved charge separation. The NP size, crystal phases, and surface morphologies of TiO2 have been optimized to achieve conversion efficiency. Moreover, the deposition of metal oxides and doping of hetero atoms on the crystal lattice of TiO2 has been conducted from the enhancement of PC activity. The recent progress made in g-C3N4-based nanomaterials used for PC CO2 conversion is discussed in detail. Acknowledgements We acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for financial support.

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Index

A Advanced nanostructure materials, 279 Aerosol assisted chemical vapor deposition (AACVD), 147, 148, 151–155, 159 Atomic force microscopy (AFM), 23, 35, 36

B Batteries, vi, 3, 5–7, 9–11, 16, 22–36, 43, 44, 50, 104, 105, 130, 164, 165, 182, 191, 249–252, 259, 260 Biomass, vi, 6, 15, 45, 55, 192, 193, 244–262

CO2 conversion, vi, 16, 272–288 Combustion, 5, 8, 15, 126, 131–139, 225–228, 230, 232, 234–237 Composite layers, 205 Conducting polymers, 44, 48, 57, 63, 67, 167, 176–178, 182, 250 Copper (Cu), 53, 63, 64, 82, 83, 104, 127, 133, 198, 199, 201, 204, 206–213, 235, 275, 277, 279, 280, 284–287 Cu(I) oxide photocatalyst, 199

D Dichalcogenides, 86, 109, 112, 173, 174 C Camphor, 147, 148, 151, 152, 159 Carbides, 86, 95, 96, 99, 100, 102, 105, 109, 112, 283 Carbon dots (C-dots), 258, 260–261 Carbon materials, 9, 10, 13, 31, 44–47, 51, 57, 67, 96, 146, 170–172, 174, 178, 257, 273 Carbon nanostructures, 49, 168, 172, 177, 178, 182, 259 Carbon nanotubes (CNTs), 9, 14–16, 26, 29, 30, 45, 46, 51–53, 56, 57, 62, 89, 96, 100, 102, 109, 146, 147, 151, 153–158, 167, 169–173, 176, 178–181, 206, 249, 251, 252, 257–259 Catalyst, 12, 46, 77, 127, 147, 171, 196, 226, 258, 273 Characterization techniques, 23, 24, 26–35, 133 Clean energy, 43, 74, 237

E Electrochemical, 6, 22, 43, 75, 164, 194, 250, 273 Electrochemical energy storage, 5, 44, 49, 51, 68 Energy, 2, 22, 43, 74, 122, 155, 164, 190, 224, 244, 272 Energy harvesting, 191, 248, 261 Energy storage, v, 5–7, 15, 16, 22, 43, 44, 49, 51, 65, 67, 164, 167, 174, 182, 194, 248–251, 258, 261, 273 Environment, v, vi, 4, 5, 9, 12–15, 22, 43, 45, 47, 59, 68, 84, 85, 96, 124, 126, 127, 146, 190, 191, 194, 196, 207, 209, 214, 224, 244, 246, 249, 251, 257, 258, 260–263, 276 Evaporation, 29, 200, 202, 212, 226, 228, 231–236, 255–257

© Springer Nature Switzerland AG 2019 R. Saravanan et al. (eds.), Nanostructured Materials for Energy Related Applications, Environmental Chemistry for a Sustainable World 24, https://doi.org/10.1007/978-3-030-04500-5

295

296 F Flexible supercapacitor, 10, 15, 43–68 Free standing electrode, 58, 67, 179, 250 Fuel cells, 5–9, 11, 12, 16, 74, 75, 125, 164, 165, 191, 259

G g-C3N4, 197, 282, 285, 286, 288 Gel electrolytes, 49, 50, 52–54, 61–63, 66, 67

H Hole transport mechanism, 214 Hydrogen evolution, 12, 16, 74–112, 195, 199, 205–207, 212, 213 Hydrogen storage, vi, 146–148, 150, 155, 157–159, 258

I In situ, 22–36, 51, 59, 100, 176, 278

L Liquid fuels chemicals, 192, 225–228, 231, 232, 235, 236, 238, 287

M Metal oxides/hydroxides, 10, 13–15, 44, 47, 48, 54, 55, 57, 67, 167, 174, 176–178, 198, 202, 207, 210, 214, 237, 282, 283, 286, 288 Multi-wall carbon nanotubes (MWCNTs), 9, 26, 46, 146–159, 174, 250, 258, 284

N Nanocellulose (NC), 99, 248–252 Nanofuels, 226–229, 233–235, 238 Nanomaterials, v, vi, 2–16, 46, 47, 51, 57, 87, 89, 128, 139, 146, 177, 182, 246, 284 Nanoparticles (NPs), 6, 8, 29, 31, 35, 62, 65, 86, 88, 89, 95, 99, 101, 102, 104, 107–109, 122–126, 128, 134–136, 151, 175, 199, 201, 204, 205, 209–211, 213, 224–238, 246, 259–262, 275, 277, 278, 280, 283–285, 288 Nanophosphors, 123 Nanostructured materials, 10, 11, 67, 123, 279

Index Nanostructures, 13–15, 31, 33, 44, 46, 49, 51, 55–57, 63, 86, 88–90, 92, 103, 105, 164, 176, 196, 246, 247, 253, 256, 258–262, 277, 280, 283, 285, 287 Nitrides, 86, 99, 100, 109, 167, 176, 177, 181, 206, 251, 282, 285, 286 Non-noble metals, 86, 275, 277, 287

O Operando, 22–36

P Phosphides, 86, 94, 95, 98, 99, 102–109, 112, 196, 283 Photochemical, 85, 260, 273, 286, 287 Photoelectrochemical hydrogen production, 13, 199, 201, 207, 210–214 Photoluminescence (PL), 123, 126–129, 134–138, 260, 262 Properties, 6, 8, 9, 11, 28, 33, 45–47, 49, 52, 57, 58, 62, 65, 66, 86, 88–91, 93–96, 104, 109, 112, 122–125, 128–130, 134–139, 146, 168, 169, 171, 172, 174, 179, 182, 191, 197, 198, 209, 210, 212, 225, 226, 228–231, 244, 245, 247–249, 251, 255, 257, 259–262, 281, 283, 285 Purification, 12, 147–149, 156, 247, 260

R Renewable, v, vi, 4, 5, 12, 15, 16, 75, 155, 190–192, 194, 214, 244–247, 249, 251, 253, 258, 276, 288

S Separation of photogenerated charge carriers, 195 Silica nanoparticles (SiNPs), 252, 258, 261–262 Simple and complex oxides, 198, 207 Small angle X-ray scattering (SAXS), 23, 28, 29, 36 Solid-state light application, 122–139 Spray, 47, 126, 147, 200, 231, 285 Starch, 15, 251, 253–257, 262 Structural study, 88 Suitable co-catalyst, 214 Supercapacitors (SCs), vi, 5–7, 10, 13–16, 43–68, 104, 164–182, 249, 250, 252, 260, 261

Index Surface area, 6, 9, 12, 15, 29, 43–48, 51, 67, 82, 83, 90, 97, 99–106, 108, 109, 112, 139, 147, 150, 158, 166, 170–172, 175, 199, 205, 225, 226, 231, 245, 249, 259, 261, 262, 285, 288

T TiO2, 13, 109, 174, 175, 180, 181, 195, 197, 202–206, 211, 214, 283–286, 288 Transmission electron microscopy (TEM), 23, 29, 30, 35, 36, 55, 150–155, 159, 259 Transmission X-ray microscopy (TXM), 23, 30–36 2D materials, 87, 174, 251

297 V Vertical nanostructures, 171

W Water splitting, 12, 13, 75, 76, 85, 190–214, 259, 260, 281 Wearable electronic devices, 44, 51

X X-ray diffraction (XRD), 22, 26–28, 30, 31, 36, 66, 133, 138, 149, 150, 174, 202 X-ray fluorescence (XRF), 23, 34–36