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Advances in Energy Materials: New Composites and Techniques for Future Energy Applications [1 ed.]
 1774912562, 9781774912560

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ADVANCES IN ENERGY MATERIALS

New Composites and Techniques for

Future Energy Applications

ADVANCES IN ENERGY MATERIALS

New Composites and Techniques for

Future Energy Applications

Edited by Iuliana Stoica, PhD

Ann Rose Abraham, PhD

A. K. Haghi, PhD

First edition published 2024 Apple Academic Press Inc. 1265 Goldenrod Circle, NE, Palm Bay, FL 32905 USA

CRC Press 2385 NW Executive Center Drive, Suite 320, Boca Raton FL 33431

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© 2024 by Apple Academic Press, Inc. Apple Academic Press exclusively co-publishes with CRC Press, an imprint of Taylor & Francis Group, LLC Reasonable efforts have been made to publish reliable data and information, but the authors, editors, and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors, editors, and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged, please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www.copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identification and explanation without intent to infringe. Library and Archives Canada Cataloguing in Publication Title: Advances in energy materials : new composites and techniques for future energy applications / edited by Iuliana Stoica, PhD, Ann Rose Abraham, PhD, A.K. Haghi, PhD. Names: Stoica, Iuliana, editor. | Abraham, Ann Rose, editor. | Haghi, A. K., editor. Description: First edition. | Includes bibliographical references and index. Identifiers: Canadiana (print) 20230445624 | Canadiana (ebook) 20230445659 | ISBN 9781774912560 (hardcover) | ISBN 9781774912577 (softcover) | ISBN 9781003346074 (ebook) Subjects: LCSH: Renewable energy sources—Research. | LCSH: Power resources—Research. | LCSH: Energy storage— Materials. | LCSH: Nanostructured materials. Classification: LCC TJ808.6 .A38 2024 | DDC 621.042028/4—dc23 Library of Congress Cataloging-in-Publication Data

CIP data on file with US Library of Congress

ISBN: 978-1-77491-256-0 (hbk) ISBN: 978-1-77491-257-7 (pbk) ISBN: 978-1-00334-607-4 (ebk)

About the Editors

Iuliana Stoica, PhD Department of Polymer Materials Physics, “Petru Poni” Institute of Macromolecular Chemistry, Romania Iuliana Stoica, PhD, is a Scientific Researcher in Physics at the Romanian Academy, “Petru Poni” Institute of Macromolecular Chemistry, Department of Polymer Materials Physics. She received her PhD from the Department of Polymer Physics and Structure of the Romanian Academy at the same institute. She joined a postdoctoral fellowship program at Politehnica University of Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Bioresources and Polymer Science. Her area of scientific activity is focused on characterization of a wide range of polymers, copolymers, polymeric composites, and polymeric mixtures. She has been an author or coauthor for over 95 papers in peer-reviewed ISI journals, and has contributed several book chapters on polymer and materials science. She has also been a member of the organizing and program committees of several scientific conferences. She has reviewed a number of prestigious journals in the field of polymer science. Ann Rose Abraham, PhD Sacred Heart College (Autonomous), Thevara, Kochi, Kerala, India Ann Rose Abraham, PhD, is currently an Assistant Professor at the Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, Kerala, India. Dr. Ann received her MSc, MPhil, and PhD degrees in Physics from School of Pure and Applied Physics, Mahatma Gandhi University, Kerala, India. Her PhD thesis titled “Development of Hybrid Mutliferroic Materials for Tailored Applications.” She has expertise in the field of condensed matter physics, nanomagnetism, multiferroics, and polymeric nanocomposites, etc. She has research experience at various reputed national institutes like Bose Institute, Kolkata, India, SAHA Institute of Nuclear Physics, Kolkata, India, UGC-DAE CSR Centre, Kolkata, India and collaborations with various international laboratories. She is a recipient of a Young Researcher Award in the area of physics and Best Paper Awards–2020, 2021, a prestigious forum for showcasing

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About the Editors

intellectual capability. She served as assistant professor and examiner, at the Department of Basic Sciences, Amal Jyothi College of Engineering, under APJ Abdul Kalam Technological University, Kerala, India. Dr. Ann is a frequent speaker at national and international conferences. She has a good number of publications to her credit in many peer-reviewed high impact journals of international repute. She has authored many book chapters and edited more than 10 books with Taylor and Francis, Elsevier, etc. A. K. Haghi, PhD Coimbra University, Portugal A. K. Haghi, PhD, has published over 250 academic research-oriented books as well as over 1000 research papers published in various journals and conference proceedings. He has received several grants, consulted for several major corporations, and is a frequent speaker to national and international audiences. He is founder and former editor-in-chief of the International Journal of Chemoinformatics and Chemical Engineering, published by IGI Global (USA) as well as the Polymers Research Journal, published by Nova Science Publishers (USA). Professor Haghi has acted as an editorial board member of many international journals. He has served as a member of the Canadian Research and Development Center of Sciences and Cultures (CRDCSC) and the Research Chemistry Centre, Coimbra, Portugal. Dr. Haghi holds a BSc in urban and environmental engineering from the University of North Carolina (USA), an MSc in mechanical engineering from North Carolina A&T State University (USA) and an MSc in applied mechanics, acoustics, and materials from the Université de Technologie de Compiègne (France), and a PhD in engineering sciences from Université de Franche-Comté (France).

Contents

Contributors.............................................................................................................ix

Abbreviations .........................................................................................................xiii

Preface ................................................................................................................... xix

1.

Polymer Nanocomposites with Smart Behavior and

Their Applicability in Energy Applications..................................................1

Andreea Irina Barzic and Raluca Marinica Albu

2.

Magnetorheological and Electrorheological Properties of

Smart Polymer Systems and Their Energy-Related Applications............25

Raluca Marinica Albu and Luminita Ioana Buruiana

3.

Metal-Organic Frameworks: Emerging Porous Materials for

Energy Applications......................................................................................47

Mirela-Fernanda Zaltariov

4.

Carbon Nanomaterials for Energy Applications........................................77

M. V. Santhosh, R. Geethu, R. Divya, N. Ragesh, and Sam John

5.

Carbon Nanotubes: Application in Energy Harvesting and Storage..... 115

Sneha Mathew, Binila K. Korah, Anu Rose Chacko, and Beena Mathew

6.

Piezoelectric Materials and Their Configurations for Energy-Harvesting Applications................................................................139

Kavya Ravindran, V. T. Johnson, C. J. Rosemary, B. Jayasree, and Pius Augustine

7.

Light-Energy Harvesting Using Two-Dimensional Transition

Metal Dichalcogenide MoS2 .......................................................................163

Vidhya Sivan, E. P. Jijo, Taniya Tomy, and Pius Augustine

8.

The Smart Chromogenic Hydrated WO3 for

Energy-Storage Applications .....................................................................185

M. Manuja, Pius Augustine, and Gijo Jose

9.

Carbon Quantum Dots for Electronics Energy Applications .................209

Richa Roy, Anu Rose Chacko, Thomas Abraham, K. G. Ambady, and Beena Mathew

Contents

viii

10. Nano- and Smart Materials in Solar Energy,

Conversion, and Storage ............................................................................239

Athira Maria Johnson, Arjun Suresh P., Greeshma Sara John, K. A. Naseer, N. V. Unnikrishnan, and Arun Kumar K.V.

11. Application of Cement and Gypsum-Based Composite

Materials in Modern Constructions for Energy Saving ..........................267

Dmitro Starokadomsky and Maria Reshetnyk

12. Hydrodynamic Efficiency of a Wave Energy Converter in Intermediate Water by Changing the Geometrical Shape of a

Submerged Object: A Numerical Approach.............................................289

Deepak Kumar Singh and Pradip Deb Roy

13. Carbon Nanomaterials for Energy Applications:

Energy Storage and Conversion ................................................................305

Arya Vijayan and Rony Rajan Paul

Index .....................................................................................................................341

Contributors

Thomas Abraham

Catholicate College, Pathanamthitta, Kerala, India

Raluca Marinica Albu

“Petru Poni” Institute of Macromolecular Chemistry, Laboratory of Physical Chemistry of Polymers, Iasi, Romania

K. G. Ambady

Department of Special Education, National Institute for the Empowerment of Persons with Intellectual Disabilities, Secunderabad, Telengana, India

Pius Augustine

Material Research Laboratory, Sacred Heart College (Autonomous), Thevara, Kochi, India Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, India Material Research Centre, Indian Institute of Science, Bangalore, India

Andreea Irina Barzic

“Petru Poni” Institute of Macromolecular Chemistry, Laboratory of Physical Chemistry of Polymers, Iasi, Romania

Luminita Ioana Buruiana

“Petru Poni” Institute of Macromolecular Chemistry, Laboratory of Physical Chemistry of Polymers, Iasi, Romania

Anu Rose Chacko

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

R. Divya

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

R. Geethu

Department of Basic Science and Humanities, School of Engineering and Technology, Karukutty, Ernakulam, Kerala, India

B. Jayasree

Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, Kerala, India

E. P. Jijo

Department of Physics, St. Berchmans College, Changanassery, Kottayam, Kerala, India

Greeshma Sara John

Department of Physics, CMS College (Autonomous), Kottayam, Kerala, India Nanotechnology and Advanced Materials Research Centre, CMS College (Autonomous), Kottayam, Kerala, India

Sam John

Department of Chemistry, St. Berchmans College (Autonomous) Campus, Mahatma Gandhi University, Kottayam, Kerala, India

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Contributors

Athira Maria Johnson

Department of Physics, CMS College (Autonomous), Kottayam, Kerala, India Nanotechnology and Advanced Materials Research Centre, CMS College (Autonomous), Kottayam, Kerala, India

V. T. Johnson

Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, Kerala, India

Gijo Jose

Research and Postgraduate Department of Physics, St. Berchmans College (Autonomous), Changanasserry, Kottayam, Kerala, India

Binila K. Korah

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Arun Kumar K. V.

Department of Physics, CMS College (Autonomous), Kottayam, Kerala, India Nanotechnology and Advanced Materials Research Centre, CMS College (Autonomous), Kottayam, Kerala, India

M. Manuja

Research and Postgraduate Department of Physics, St. Berchmans College (Autonomous), Changanasserry, Kottayam, Kerala, India

Beena Mathew

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Sneha Mathew

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

K. A. Naseer

Department of Physics, Farook College (Autonomous), Kozhikode, Kerala, India

Arjun Suresh P.

Department of Physics, CMS College (Autonomous), Kottayam, Kerala, India Nanotechnology and Advanced Materials Research Centre, CMS College (Autonomous), Kottayam, Kerala, India

N. Ragesh

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Rony Rajan Paul

Department of Chemistry, CMS College, Kottayam, Kerala, India

Kavya Ravindran

Material Research Laboratory, Sacred Heart College (Autonomous), Thevara, Kochi, Kerala, India

Maria Reshetnyk

National Natural History Museum, NAS, Ukraine

C. J. Rosemary

Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, India

Pradip Deb Roy

Department of Mechanical Engineering, NIT, Silchar, Assam, India

Richa Roy

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Contributors

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M. V. Santhosh

School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

Vidhya Sivan

Material Research Laboratory, Sacred Heart College (Autonomous), Thevara, Kochi, India

Deepak Kumar Singh

Department of Mechanical Engineering, NIT, Silchar, Assam, India

Dmitro Starokadomsky

Chuyko Institute of Surface Chemistry, National Academy of Sciences (NAS) of Ukraine, Ukraine Institute of Geochemistry, Mineralogy & Ore Formation NAS, Ukraine

Taniya Tomy

Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, Kerala, India

N. V. Unnikrishnan

School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala, India

Arya Vijayan

Central Institute of Petrochemicals Engineering and Technology, (CIPET)- IPT, Kochi, Ernakulam, Kerala, India

Mirela-Fernanda Zaltariov

Department of Inorganic Polymers, “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania

Abbreviations

ACNFs ACNTAs Ag AIN Al ALD asMEG-O ASCQDs Au BCP BE BT CB CC CCG CCT Cd CFE CNDs CNF/G CNTs CQD CPs cPU CRI CS CSLs CTML Cu CV CVD DBS DMFCs DMSO

activated carbon nanofibers aligned carbon nanotube arrays silver aluminum nitride aluminum atomic layer deposition activated spherical microwave-expanded GO anti-solvent carbon quantum dots gold bathocuproine binding energy barium titanate conduction band carbon cloth chemically converted graphene correlated color temperature cadmium carbon fiber electrode carbon nanodots carbon nanofiber/graphene carbon nanotubes carbon quantum dots coordination polymers carboxylated polyurethane color rendering index chitosan catalyst support layers charge transfer material layer copper cyclic voltammetry chemical vapor deposition dodecylbenzenesulfonate direct methanol fuel cells dimethyl sulfoxide

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DP DSSCs DWCNTs ECL EDLCs EDLCs EGaInPs EH EQE ER ETL ETL EVs FAPbI3 FCCVD FDSSC FRF FRFP FTO GCE g-CQDs GNS GO GQDs HAMA HER Hg HGF HKUST HOMO HPAs HTL IVCT ITO ITQMOF LC LCST LED

Abbreviations

dicumyl peroxide dye-sensitized solar cells double-walled CNTs electrochemical luminescence electric double-layer capacitors electrochemical double-layer condensers eutectic gallium indium energy harvesters external quantum efficiency electrorheological electron transport layer efficient electron transport layer electric vehicles formamidinium lead iodide floating catalyst chemical vapor deposition flexible DSSC frequency response function frequency response function for power variables fluorine-doped tin oxide glassy carbon electrode green carbon quantum dots graphene nanosheets graphene oxide graphene quantum dots methacrylated hyaluronic acid hydrogen evolution reaction mercury holey graphene framework Hong Kong University of Science and Technology highest occupied molecular orbitals heteropolyacids hole transport layer inter valence charge transfer mechanisms indium tin oxide Institute of Chemical Technology of Metal-Organic Networks liquid crystal lower critical solution temperature light-emitting diodes

Abbreviations

LIB LSBs LSCs LSPR LUMO MAF MAI MAPbI3 MBE MCBF MCs MCP m-CQDs MEA MEMS MEP mesoMOF MFCs MFP MHP MILs MMS MNP MOFs MoS2 MR multi-DOF MW MWCNTs NBT NCNTs NFC NP NS NTSC OER OPV ORR OSC

xv

lithium ion batteries lithium-sulphur batteries luminescent solar concentrators localized surface plasmon resonance lowest unoccupied molecular orbitals metal-networks azolate methyl ammonium halide methylammonium lead iodide Molecular Beam Epitaxy multicolor bandgap fluorescent microcapsules microporous coordination polymer multimode-emissive CQDs membrane electrode assembly microelectromechanical multifunctional efficiency parameter mesoporous metal-organic networks microbial fuel cells metalloprotein-type networks metal halide perovskite Materials of the Lavoisier Institute methyltrimethoxysilane metallic nanoparticles metal-organic frameworks molybdenum disulfide magnetorheological multi degree-of-freedom molecular weight multiwalled CNTs sodium bismuth titanate nitrogen-doped carbon nanotubes porous CNF composites nanoparticles nanosheets National Television System Committee oxygen evolution reaction organic photovoltaics oxygen reduction reaction organic solar cells

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PAA PANI Pb PCE PCN PCNs PDs PDMS PEG PEH PEMFCs PEMFC PEN PEO PGM PIBs PLA PLD PMMA PMII PNIPAM POMLs PPy PR-NBE-T-CQDs PS PSCs Pt PU PVA PVD PVDF PVP PX PZT QDs RFL RGO SATS-GO

Abbreviations

polyacrylic acid polyaniline lead power conversion efficiency porous coordination network porous carbon nanosheets polymer dots polydimethylsiloxane polyethylene glycol piezoelectric energy harvesters polymer electrolyte membrane fuel cells proton exchange membrane fuel cell polyethylene naphthalate polyethylene oxide Pt group metals potassium-ion batteries polylactic acid pulsed laser deposition poly(methyl methacrylate) propylimidazolium iodide poly(N-isopropylacrylamide) polyoxometalates polypyrrole pure red narrow bandwidth emission triangular CQDs polystyrene perovskite solar cells platinum polyurethane poly(vinyl alcohol) physical vapor deposition polyvinylidene fluoride pyrrolidone poloxamer 407 lead-zirconium-titanate quantum dots reflective reduced graphene oxide sulfonated poly(arylene thioether sulfone)-grafted graphene oxide

Abbreviations

SBUs SCC SCPs Se S-CQD/PANI SCs SERB SHFB SHFL SIB SMES SPCs SPNs SPR SWNTs Ta TCO TMDs TMO TPEF UV-IL VACNTs VA-SWNTs W-CQDs WLEDs WPU ZIB ZIFs ZIF ZMOF 0D

xvii

secondary building units superconducting coil supercapacitors selenium sucrose-derived carbon quantum dots/polyaniline supercapacitors Science and Engineering Research Board super-hydrophobicity super-hydrophilicity sodium ion batteries superconducting magnetic energy storage smart polymer composites smart polymer nanocomposites surface plasmon resonance single-walled carbon nanotubes tantalum transparent conductive oxide transition metal dichalcogenides transition metal oxides two-photon excited photoluminescence UV-imprint lithography vertically aligned carbon nanotubes vertically aligned carbon nanotube arrays white-emitting CQDs white light-emitting diodes waterborne polyurethane zn-ion batteries zeolitic networks zeolite-type imidazolate network zeolite-type metal-organic networks zero-dimensional

Preface

This new title illustrates one of the most integrated research-oriented books on the topics of materials for energy with a wide spectrum of applications to solve alternative energy issues. Contents of this new book helps integrate the diverse disciplines in order to suggest key solution to resolve complicated challenges and reviews the state-of-the-art for postgraduate students and scientists interested to solve complex issues in global energy problems. This book reports advanced methods in preparation of nanoscale energy materials with explanations of the structure and properties. It highlights current developments in energy sector from materials angle and new techniques. This research-oriented book will find a great interest to a wide readership in several energy-related fields of materials science and engineering. The first chapter is devoted to polymer nanocomposites with smart behavior and their applicability of in energy applications. Magnetorheological and electrorheological properties of smart polymer systems and their energy-related applications are represented in the 2nd chapter. Chapter 3 of this new book highlights metal–organic frameworks emerging porous materials for energy applications. In Chapter 4, the authors studied the energy applications carbon nanomaterials. Chapter 5 evaluates the applications of carbon nanotubes in energy harvesting and storage. Recent trends and developments in piezoelectric materials and its configurations for energy harvesting applications are reviewed in Chapter 6. Chapter 7 of this book is dedicated to light energy harvesting using twodimensional transition metal dichalcogenide MoS2. The smart chromogenic hydrated WO3 for energy storage applications is evaluated in Chapter 8. Carbon quantum dots for electronics energy applications are studied in Chapter 9. A comprehensive overview on the role of nano and smart materials in solar energy, conversion, and storage is represented in Chapter 10. Application of cement and gypsum-based composite materials in modern constructions for energy saving is described in Chapter 11.

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Preface

In Chapter 12, we have represented the hydrodynamic efficiency of wave energy converter in intermediate water by changing the geometrical shape of a submerged object. In the last chapter of this book, carbon nanomaterials for the energy applications, energy storage, and conversion is studied in detail. This new title is an excellent reference source, which can be considered as an updated research-oriented book dedicated to materials for energy. It illustrates latest developments in advanced materials for efficient energy applications, including smart materials, nanomaterials, porous materials, piezoelectric materials, membrane materials, and composite materials. It represents the most comprehensive review of the state-of-the-art for postgraduate students and researchers.

CHAPTER 1

Polymer Nanocomposites with Smart Behavior and Their Applicability in Energy Applications ANDREEA IRINA BARZIC and RALUCA MARINICA ALBU “Petru Poni” Institute of Macromolecular Chemistry, Laboratory of Physical Chemistry of Polymers, Iasi, Romania

ABSTRACT The need of diversifying the material’s functionality is filled by intelligent behavior, which arises from the peculiarities of the macromolecular structure. This chapter aims to present the state of art in the area of polymersbased nanomaterials of practical importance in energy applications. Reinforcement of polymers with particles of variable dimensions can be regarded as a promising alternative for attaining of materials with upgraded properties. In the first part, molecular modeling is used to project the conformational properties of certain smart polymers under the influence of different temperatures. Then, some smart properties, like self-cleaning, self-healing, and actuation of polymer composites are discussed, in regard to their involvement in devices aiming toward energy-harvesting and energy storage. These aspects are reviewed in this chapter, emphasizing the importance of the material structure, composition, and properties explored for the practical use.

Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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1.1 INTRODUCTION The domain of material science has been devoted to expand and/or upgrade the performance of the functional and engineering products of interest in actual and future technologies. To accomplish such goals, scientists have analyzed the intelligence of nature and made considerable efforts to include some of its principles in the material’s design.1 So, after a long investigation of components from the living systems, they noticed specific reactions which are fast or reversible when triggered by environmental factors. The developed approaches in this direction have conducted to a new class of compounds with smart behavior, which is reflected in peculiar functions that derive from actuation, sensing, or control.2 Such extraordinary properties are encountered in carbohydrates, proteins, or nucleic acids, which are principally natural polymers.3 The synthesis routes were adapted to imitate such biomacromolecules into a diversity of structures that are trying to correspond to the standards enforced by industrial and scientific applications.4–8 The continuous search to increase the product’s functionality has been aided by introducing the concepts of the nanotechnology to produce smart polymer nanocomposites (SPNs).9 This can be performed by embedding particular fillers or additives into a matrix that contains stimuli-sensitive components. Regarding reinforced polymers, the literature presents three prime forms of them, namely intercalated, flocculated, and exfoliated composites.10 The research conducted in the past decades is focused on joining these trends in order to obtain products with increased smartness, which can be able to respond to more complex demands from several industrial sectors. Therefore, reinforced polymers with intelligent features have an enormous practical importance in applications, such as sensors, stretchable electronics, actuators, self-healing surfaces, aircraft, and other systems.4,9 All of these applications are imposing careful attention on a set of properties that can be controlled via means of the material’s structure and sometimes through the processing parameters. The intelligent behavior of an SPN is reflected in the manner in which the chemical or physical characteristics of the material can be adjusted, at the desired level, with the help of an internal or external factor (called stimulus). The nature of the latter is another relevant aspect that dictates the practical solutions for electro-technological issues, which could facilitate the advances in medical diagnosis, 3D printing, or sophisticated sensors.11 Moreover, a relatively recent tendency was to develop multistimuli-sensitive devices based on SPNs, where the multicomponent material reacts under the

Polymer Nanocomposites with Smart Behavior

3

influence of several stimuli. In this case, the resulted properties are affected by the features of the polymer matrix and the kind of the embedded nanofillers.12 As a consequence of these research directions, there has been a tremendous interest in utilizing the SPNs with multistimuli-sensitive properties for many of the actual products used in our daily life. Scientific concerns on the topic of smart polymers and their composites are still growing as indicated in the augmentation of the number of articles and book manuscripts which were published in the last 10 years.13–22 On the other hand, particle-loaded polymers are extremely important for energy production or storage. Thus, they are analyzed and implemented in devices that work using solar radiations, thermoelectric effects, or energy produced based on vibration.23–26 Introduction of polymer composites as covers23 or semiconducting layers24 in photovoltaic cells has been a great success. Concerning the thermoelectric power generation, these materials are introducing some benefits, like the lack of toxicity, ease of processing, and suitability for wide-area devices. The incorporation of magnetic fillers in polymers is ideal for making magnets within the harvesters, which are operating based on vibration-based energy. Particularly, inserting ceramic particles in polymers has been demonstrated to be a feasible route to fabricate electrostatic energy storage devices.25,26 This chapter is collecting from literature, the most significant case studies concerning polymer composites with intelligent behavior with potential use in the energy sector. Molecular modeling is performed to understand some of these systems in terms of molecular mechanics energies. Based on the latest advances, the state of art concerning the preparation, properties, and particular applications of SPNs in energy production or storage, is reviewed. 1.2 MOLECULAR MODELING OF SOME SMART POLYMERS In regard to the smart polymers, a particular importance is attributed to the thermo-sensitive ones since temperature is an easily controlled factor. Among the studied structures, polyacrylic acid (PAA) and poly(methyl methacrylate) (PMMA) are the of outmost interest and for this reason in this chapter, molecular modeling analyses are focused on these two materials. Before starting the design of any smart polymer composite, it is essential to know first the matrix behavior in various conditions. For instance, it is desirable to elucidate how temperature influences the conformational features, which can be quantified through typical energy parameters. In this context, using the

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Advances in Energy Materials

molecular modeling software, it is possible to project the geometric shape of the smart macromolecule having pre-established structural units at the lowest energy. The minimized conformation in vacuum by molecular mechanics is further subjected to molecular dynamic routine. This is performed to estimate molecular mechanics energies, namely: Kinetic (EKIN), potential (EPOT), and total (ETOT).27,28 The procedure of molecular mechanics for thermo-sensitive structures is accomplished at various temperatures, ranging between 300 and 400 K, to examine the influence of the chemical structure changes on the pursued energy parameters. EKIN energy of a polymer structure denotes the kinetic influence of the atoms along the main and side chains. Hence, this parameter is linked to the macromolecular degree of rigidity, which along with steric effects is generating on the overall form of the polymer chain at the lowest energy. On the other hand, the EPOT parameter contains information on the interactions among the chain atoms and the external potential. These energy parameters are computed for the selected smart polymer structures and the attained simulation data are depicted in Figures 1.1–1.3.

FIGURE 1.1 The molecular geometry of a chain (18 structural units) of polyacrylic acid (PAA) and poly(methyl methacrylate) (PMMA).

FIGURE 1.2 The profile and time-dependence graphs concerning the values of kinetic, potential and total energies (kcal/mol) of a single chain (18 structural units) of the polyacrylic acid (PAA) at three temperatures.

Polymer Nanocomposites with Smart Behavior 5

FIGURE 1.3 The profile and time-dependence graphs concerning the values of kinetic, potential and total energies (kcal/mol) of a single chain (18 structural units) of the poly(methyl methacrylate) (PMMA) at three temperatures.

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Polymer Nanocomposites with Smart Behavior

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According to the images comprised in Figure 1.1, the selected macromolecular structures display different shapes of their optimized geometrical conformations. This is caused by the chemical peculiarities that render variations in the chain flexibility. The PAA has smaller side groups comparatively to the bulky ones that form the pendant hanging to the backbone of PMMA. This aspect enables a bit higher chain packing for PAA owing to lower steric hindrances. However, PMMA contains a supplementary bending point in its structure, which additionally affects the torsion angles along the bonds. So, it can be presumed that PMMA has a larger degree of freedom for rotation around single and double bonds. To check this, the energy profiles are computed, where it can be noticed that the values of both EKIN and EPOT energies are fluctuating until they reach equilibrium values. Based on these profiles, the kinetic and potential contributions to the total energy of PMMA and PAA are extracted, as illustrated in tridimensional plots from Figures 1.2 and 1.3. The simulation data show a higher kinetic energy values for PMMA than those of PAA, proving the larger degree of freedom for the first compound. When performing the simulation at distinct temperatures, the conformational aspects and ascribed energy parameters are slightly changed. The imposed simulation temperatures (300, 350, and 400 K) determine a more pronounced movement of the constituting atoms for both PMMA and PAA. Regardless of the distinct flexibility of these smart polymers, the values of EKIN are increasing more with temperature for PMMA than in the case of PAA, as seen in the energy profiles from Figures 1.2 and 1.3. Also, the total energy is ranging in the same manner as the kinetic energy for both polymers.29 1.3 SMART POLYMER NANOCOMPOSITES AND THEIR USE IN ENERGY APPLICATIONS The outstanding properties of smart polymers can be utilized to advance the performance of the current composites by rendering additional functionality that can be tuned through external or internal factors. Such multicomponent materials are gaining interest in energy applications. In the following chapter sections, certain intelligent features are reviewed in relation to their role in energy-based devices. The principal uses in the energy-working systems of the reinforced polymers with self-cleaning, self-healing, sensing, and actuation properties, are illustrated in Figure 1.4. The most significant case studies are described below with emphasis on the most recent solutions for

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upgrading the materials properties and how these aspects are impacting the actual operability of the devices used in the energy industry for harvesting or storage purposes.

FIGURE 1.4 The main uses in energy industry of the smart polymer composites (SPCs) having either self-cleaning, self-healing, sensing, or actuation features.

1.3.1 FILLED POLYMERS WITH SELF-CLEANING PROPERTIES The property of surface self-cleaning represents the capacity of a material to clean itself in the absence of sensing or even an actuation factor. This special property is generally accomplished by means of super-hydrophobicity (SHFB) or super-hydrophilicity (SHFL). Polymer materials having the aforementioned feature are of great use in solar cells, especially for shielding purposes since soiling of the cover layer might produce enormous losses because solar radiation cannot travel in totality of the active zone. Syafiq and co-workers30 reviewed the methodologies used to produce selfcleaning materials of photovoltaic cells. They mention that there are chemical approaches that rely on SHFB, mechanical procedures that by brushing, blowing, and ultrasonic vibration are enabling dirt removal or electrostatic methods that clean the solar panel. In the next paragraphs, some relevant reports on this topic are briefly reviewed. In their work, Nayshevsky et al.31 described the anti-soiling mechanisms generated by the accumulation of dew on solar cells. For this, they are synthesizing fluoropolymer coatings having high surface hydrophobicity.

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However, this might cause optical losses at the cover/transparent conductive oxide (TCO) interface. To overcome such issues, Barzic et al.23 have reinforced a fluoropolyimide with ferrous sulfide so that the mismatch between refractive properties at each layer interface would be considerably reduced. The polymer composites display high water contact angles and to increase their adhesion to the TCO material, plasma exposure has been applied on one side of the shielding composite. In this way, the cover of the solar cell displays high hydrophobicity on one side and good compatibility with TCO on the other. Other recent studies were focused on attaining polymer composites with SHFB by more complex procedures.32–50 Gong and He32 made such materials by embedding silica with distinct hydrophylicity in polydimethylsiloxane (PDMS), which are processed by the spray-coating method. The interaction between this inorganic polymer and silica nanoparticles leads to a rough structure that is characterized by strong hydrophobicity (water contact angle exceeds 150°). These composites display outstanding stability and are keeping their surface features even after UV exposure, mechanical abrasion, alkali/acid immersion, or water contact. Moreover, the prepared surfaces are proving to be adequate for antifouling and self-cleaning purposes. Ibrahim et al.33 included silica in polystyrene (PS) to increase its SHFB. The wettability tests prove that the composites become non-wettable after reinforcement and the surface properties are constant in time. Chen et al.34 used natural polymers to obtain SHFB surfaces, which also display increased transparency (slightly above 90%). For this goal, they employ oxidized cellulose nanofibrils and polysiloxanes and noted the appearance of a “pearl-necklace-like” structure of inorganic macromolecule fibers deposited on the cellulosic sheet. Additional silanization applied to these materials determine a pronounced water-repellency (~71%) and great toughness (~119%). It is shown that the preparation methodology leads to materials with self-cleaning, but also lightmanagement. These characteristics are generating an increase of about 11% in the global conversion efficiency of the energy made the photovoltaic system. Such results confirm that the losses caused by dirt accumulation are avoided by the special surface properties of the hybrid materials. It can be stated that the approach proposed by Chen et al.34 brings insightful ideas on future design of sustainable functional energy panels. Sutar and collaborators35 constructed SHFB coatings based on hybrid polymer composites. They use PS as a matrix, which was combined with polymethylhydrosiloxane solutions in chloroform. The approach regarding the coating samples involves some precise steps, namely homogenization of the inorganic polymer solution, the

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addition of aluminum oxide dispersion under stirring and finally pouring the PS solution. Afterward, multiple dipping is performed to check the influence of the number of deposition sheets on the surface properties. The metallic particles from the composite display flake-type morphology, which in the presence of these two polymer structures leads to the occurrence of aggregated random particles (when the solutions are in the casting stage). This aspect favors the formation of hierarchical rough architectures on the surface of the samples. The Si–H and CH3 groups from the silane-based polymer are employed for the surface adaptation of hydrophilic alumina filler. In a similar report,36 it was demonstrated that combining PS with Al2O3 rendered upon electrospinning fibrous coatings with SHFB features. The methodology used by Sutar et al.35 produces multi-scale hierarchical roughness since the association of these materials determines the aggregation and appearance of closely packed particles at the surface of the support. The durability experiments, including the liquid jet impact, adhesive tape, and abrasion, prove a remarkable stability of the composite layer. The best SHFB (contact angle above 170°) is attained after the four-time dip coating of the composite on the support, showing outstanding self-cleaning performance. Cully et al.37 have tried to make materials that are imitating the hierarchical structure of the lotus. For this goal, fast self-assembly is mixed with UV-imprint lithography (UV-IL) at the nanometer scale. Distinct acrylate compositions comprising acrylated silica filler, but also an acrylated silicone surfactant, are utilized. During the coating preparation, the selected surfactant is moving toward the polymer surface and hence is lowering its surface hydrophilicity. This effect is further enhanced by the incorporation of the acrylated silica filler, which tunes the surface roughness. The most significant improvement in hydrophobicity is attained for the UV-IL printed lotus coatings employing the acrylate composition with the smallest viscosity. The insertion of silica in the largest amount converts the surfaces into SHFB ones. The mechanical experiments indicate that the prepared materials display hardness at micron level above 400 MPa and are also wear-resistant. These features are ideal for making covers for solar cells. Subramaniam and Shanthi38 achieved composites with elevated hydrophobicity by the addition of methyltrimethoxy silane (MMS) in PMMA or PS. The composite solutions are deposited on supports via a spin-coating route. The wettability test reveals a contact angle larger than 110° for all the resulted coatings. Surface energy is estimated with the help of the Hamaker constant, rendering a lower value for the systems containing PS. The dust residues that reach the sample surface are removed after contact with the water droplet, which supports the expected self-cleaning characteristic.

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In a recent review, Sarkın et al.39 have analyzed the possible approaches to improve the efficiency of solar cells by using anti-reflection and selfcleaning layers. They discussed the problems arising from sunlight reflection even when the cover surface is clean and mentioned that compounds like silica, metal oxides (of Ti or Zr), MgF2, and Si3N4 are highly adequate for the preparation of anti-reflection layers. Concerning the self-cleaning property, fabricating of SHFB surfaces remains the most applicable approach. For this aim, materials like Si3N4, Al2O3, and TiO2 are highly effective. Coatings made by multi-layering are efficacious when the scope is to accomplish proper surface adhesion and durability. The work of Cherupurakal et al.40 represents one of the most recent investigations on SHFB polymer coatings with anti-reflective and self-cleaning capacities for solar cells. They tried to address one of the most challenging aspects, namely the durability of SHFB and reflective (RFL) features. For this, they have selected several factors (SHFB, RFL, transparency, and durability) to describe the performance of the coatings on the solar cell efficiency. So, based on these aspects, they compare several polymers and related composites with special attention on the structural, chemical, and mechanical characteristics. Among the polymers that render materials with high SHFB, they are mentioning: • PS reinforced with ZnO,41 modified nanosilica,42 manganese doped zinc oxide43; • PDMS loaded with silica nanoparticles32,44,45 or nanotubes46 • Polyurethane (PU) acrylic colloidal suspension filled with fluorofunctionalized SiO2,47 graphene48; • Fluorinated polymers31 filled with ferrous sulfide,23 SiO249 and Nb2O5.49 In the case of PS-based coatings, the biggest SHFB is recorded for silane-functionalized nano-SiO2 inserted in PS (contact angle exceeding 160°) and transparency larger than 92%.42 Regarding the PDMS systems, it is reported that silica nanotubes led to the highest SHFB (contact angle of 165°), while the transparency is similar to that of glass.46 Materials based on fluorinated polymers lead to even better results. Fluorinated ethylene propylene50 coatings display a contact angle of 166°, combined with elevated transparency of 96%. All these properties are essential for maximizing the solar energy contribution of solar cell panels when the goal is to avoid soiling and blistering issues. The future research in this area should explore ways to prepare coatings based on double-side polymers with anti-RFL properties.

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1.3.2 FILLED POLYMERS WITH SELF-HEALING PROPERTIES The property of self-healing of a material arises from its ability to repair itself after suffering any damage. Energy storage devices (i.e., batteries or capacitors) are gaining increasing interest in manufacturing flexible or stretchable products. The main problem here resides in the fact that periodic deformation, together with electrochemical depletion, generates irreversible mechanical failure. This is seen in the formation of cracks, perforation, and delamination that are inducing undesired degradation or sometimes safety concerns. Analogous problems are noticed in energy production systems, like solar cells, where such defects might lead to light scattering and thus the conversion efficiency is lowered. In the next paragraphs, some important breakthroughs in the area of polymer-based materials with this sort of intelligent behavior are depicted with emphasis on their use in energy applications. In view of making smart composites for energy applications, it is important to mention the report of Yang et al.,51 which addresses the problem of electrical damage (i.e., aging and breakdown) in polymers and explains how self-healing can solve such issues. In high-power electronics, macromolecular compounds are used as dielectric components and insulators. The destruction of insulation property is caused by the appearance of defects, which are spreading as dendric hollow cracks (at micro-scale) up to the point where the breakdown and collapse of the entire dielectric are noted. To avoid the electrical treeing, the self-healing insulation material must fulfill certain demands: • the polymer must display dielectric features similar to those of common polymer insulators, in terms of breakdown strength, dielectric loss, and resistivity; • the material needs to be responsive to a wide length scale of defects; • the healing process should be considerably faster than the damaging one. The challenges on making dielectric polymers with self-repairing ability are involving conventional approaches, such as: • extrinsic self-healing: Fluid healing substances introduced within the matrix are delivered to the destroyed zones by capillary effect; • intrinsic self-healing: Several healing cycles of molecular scale and resolves the damage in the presence or absence of external energy input. This is often encountered in non-covalent systems.

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There are several procedures of self-repairing for electrical damage, namely51: • re-making of multiscale destruction through the defect-aimed superparamagnetic heating; • in situ electroluminescence; • anionic polymerization; • repairing electrical breakdown in polymer thin layers. Mai et al.52 examined the suitability of self-repairing materials for energy storage in agreement with chemical features, namely hydrogen linkages, electrostatic forces, and borate ester bonds. The re-construction process can be made via capsules or by capillaries or hollow channels to keep the repairing agents. A solution could be to insert methyl methacrylate inside porous polypropylene fibers having hollow features. An alternative method is to include epoxy or cyanoacrylate adhesive into glass. In contrast to these extrinsic self-healing strategies, the intrinsic ones presented by Mai et al.52 are focused on reversible dynamic linkages that may be imparted in covalent and noncovalent dynamic interactions. Moreover, the sensitivity of hydrogen linkages can be exploited in the preparation of self-healing coatings, and for this, the following polymers can be used: Carboxylated polyurethane (cPU), PAA, poly(vinyl alcohol) (PVA), and ureido pyrimidinone-derived polymers. Lin et al.53 prepared a physical hydrogel by using poly(acrylic acid-co-stearyl methacrylate) loaded with Fe3+ for polyaniline (PANI) supercapacitors (SCPs). By means of the hydrogen bonding and ionic interactions, the resulted material presents adequate mechanical features (extensibility beyond > 2400%), high ionic conduction (>30 S/m). The composite displays an outstanding energy density of about 19 Wh/kg at power density around 0.7 kW/kg, combined with the self-healable ability (~86% ability of retention after repairing), aging persistence, and cycling stability. Dai et al.54 prepared a hydrogel electrolyte for SCPs from poly(acrylic acid-co-acrylamide) and CoCl2 particles. The forces related to hydrogen bonding acting intra- and inter-molecularly are acting along with the metal-coordination among the metal ions and carboxyl groups to produce the network that withstands variable deformations. Their device keeps its capacitance at the remarkable performance (~90%) after several cycles of deformation (folding, twisting, and pressing) owing to self-repairing properties. The electrolytes containing Fe3+ and Co2+ can be mostly implemented in low-voltage storage systems since the electrochemically less stable metal ions could be reduced. For highvoltage devices (batteries), ions with appropriate electrochemical stability such as Ca2+ are preferable.55 Zhao et al.56 reported that PVA/chitosan (CS)

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composites with numerous hydrogen bonds can be a good candidate as smart solid electrolyte for electrochemical capacitors. The composite gel exhibits high mechanical strength and is capable of fast healing upon completely broken. The storage device presents a high capacitance above 389 mF/ cm2, which is maintained after repeatable bending. Furthermore, PVA/CS composite could reconnect the damaged capacitor with almost complete capacitance retention. Park et al.57 have obtained flexible and self-healing from cPU filled with nickel flakes, and eutectic gallium indium (EGaInPs). Such conducting materials can be employed as electrodes and joined with graphene platelets and ionic-liquid electrolytes. The composite presents 94% capacity retention after successive stretching. The electrochemical self-healing is revealed, arising from complexation-modulated redox property of EGaIn under the action of a ligand, which increases the reversion of the Faradaic reaction of Ga. Self-repairing is accomplished in the affected zones that are electrically recovered by the flow of liquid inorganic phase and mechanically restored through by the interfacial hydrogen bonding of the polymer. The composite conductor presents conductivity of almost 2480 S cm−1 that has the wide stretching ability at 700% strain. It also recovers after mechanical damage, having an electrical healing efficacy of 75%. Yang et al.58 have used furan-modified polydopamine to reinforce a polyurethane structure. In this way, they obtain phase-change composites characterized by recyclable and self-repairing features. The results indicate an excellent self-healing property (93.1%) and elevated photo-thermal conversion efficiency (~88%). The used particles, which act as cross-linkers and photo-thermal counterparts, are embedded in the maleimide-ended polyurethane via a Diels–Alder (DA) procedure. Sunlight exposure experiments show that the filler insertion considerably increases the storage efficiency and photo-thermal conversion. This is facilitated by the sample’s elevated solar-to-thermal energy conversion ability induced by the filler. Given the reinforced materials’ structural peculiarities, the self-repairing under nearinfrared radiation influence is possible. Moreover, the restored samples are capable to maintain the initial mechanical characteristics and thermal energy storage property. Such composites present a remarkable potential for use in the areas of solar-to-thermal energy conversion and storage. In regard to borate ester bonds, the idea starts from the fact that boronic acid leads to dynamic borate ester bonds by means of the tetrahedral shape having the electron-rich diols.59 The stability of these linkages is affected by the pH value. Pan and colleagues60 have grafted PAA with PVA and mixed

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it with borax and KCl to produce hydrogel electrolytes for intelligent electrochemical capacitors. The borax and diols from the PVA are able to create to borate ester linkages when the level of the pH is basic. This determines the desired self-healable characteristic. The capacitor enables a capacitance of ~85 F/g, while keeping almost intact its primer capacity upon several cutting–healing stages. 1.3.3 FILLED POLYMERS WITH OTHER SMART PROPERTIES There are other smart properties of loaded polymers which could be utilized in energy applications. Cazacu and collaborators61 have synthesized composites based on a siloxane-derived polymer and titania/hydrophobized silica particles and studied the actuation with the purpose of energy harvesting. The permittivity of the samples is ranging between 3 and 5 from 1 to 106 Hz. The actuation is reflected in the conversion of electrical energy in mechanical work generating huge strains. Under electrical voltage, these composites are driven by compressive forces that enlarge the sample along the in-plane direction. A non-contact measurement is done to evaluate the linear displacement at micro/nano-scale along the cross-section. The displacement is augmented by the voltage in a non-linear manner. The actuation is upgraded when the titania amount in the composite increases. The energy harvesting data indicate an impulse electrical voltage in the range of 6–20 V for a dynamic force comprised between 10–1 and 100 Kgf, being beneficial for energy harvesting uses. Khuyen and co-workers62 explored the multifunctional features of polypyrrole (PPy) mixtures with dodecylbenzenesulfonate (DBS) in the presence or absence of polyethyleneoxide (PEO). They analyzed the actuation-sensingenergy storage triple functionality of these composites. The examination of the response of these samples in a solution of several salts shows that all of the actuating, sensing, and charge storage properties are, insensitive of the electrolyte, but more powerful for the PPy-PEO/DBS. For the obtained composites, the reaction energy and the strain changes adjust and sense the electrical and chemical regimes. By utilizing connecting wires, it is possible to control the actuation, capacitance, and sensing properties. In other words, when placing the composites in electrolyte solutions they react at the same time as an actuator (strain variation), an electrical sensor (flowing current), a supercapacitor (specific capacitance modification), and a chemical sensor (specific cation).

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Fredi et al.63 have elaborated a strategy to optimize the balance between mechanical performance and thermal energy storage. For this scope, they employ multifunctional composites that are made from filler and a phase-changing material. The latter is known to diminish the mechanical characteristics of the host. In any case, a multifunctional material could be more advantageous concerning the mass saving in regard to two mono-functional counterparts performing the same functions individually. The materials are derived from epoxy and carbon laminates, where the energy storage is facilitates microcapsules (MCs) having paraffinic nucleus and melamine-formaldehyde outer layer. The MCs enhance the matrix viscosity and reduce the mechanical performance of host laminate. The multifunctional efficiency parameter (MEP) is estimated by cumulating the structural (linked to elastic modulus) and the thermal storage efficiency (related to specific melting enthalpy). An adequate mass saving is obtained when MEP values are supraunitary. Such results are also noted for other composites, like starch/wood in PEG. To enhance MEP, it is desirable to shift it down to the single phase scale and to improve the composite design and carefully select the raw materials. Wen and collaborators64 have explored the possibility to improve the energy density of PMMA-based materials for the purpose of making hightemperature capacitors. They prepare composites by doping PMMA with a conjugated polymer with a fluorinated structure. At 40 vol % loading, the composite presents an upgraded energy density than the neat counterparts. In conditions of 90°C and 350 MV/m, the energy density is almost 9 J/cm3, while efficiency is of 77%. At his level of reinforcement, the sample has discharge times around 15 μs, which are maintained in the studied temperatures domain. These composites also have elevated breakdown strength, excellent energy storage density, and rapid discharge speed. Munusamy et al.65 have analyzed the practical potential of PMMA as a thermal phase change component for solar water heater uses. The main idea is that such smart materials are capable to soak and deliver a significant quantity of heat while phase-changing in a restricted temperature interval. The resulted materials are obtained by covering myristic acid with PMMA to address the issue of leakage responsible for tank corrosion. To enhance the PMMA ability to preserve myristic acid during phase variation, the polymer was blended with dicumyl peroxide (DP) and thus the system cross-linking occurs. This approach determines dense and uniform sample with no surface splits or any other defects. At 0.1% of DP, the leakage is fully removed and this compound does not affect the value of latent heat of melting and freezing of samples.

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Baştürk and Kahraman66 have studied phase-changing materials derived from PAA grafted with fatty alcohol achieved by esterification of PAA using octadecanol and docosanol. The enthalpy corresponding to heating and freezing phase change is essential for the quantification of the heat storage performance. It is observed that with augmentation of the n-alkyl side chains, the melting temperature and enthalpy are larger. The same tendency was noted for freezing phase change enthalpy. The materials display adequate phase change characteristics and render a proper working temperature interval. The melting and freezing temperatures of the grafted PAA polymers are significantly lowered in regard to those of pure fatty alcohols. The latent heat enthalpy of heating is varying between 112 and 122 J/g; meanwhile, one of the freezing cycles is ranging from 117 to 126 J/g. Also, thermal analyses confirm the elevated heat stability of the prepared materials, together with the big latent heat storage capacity. Kozhunova et al.67 fabricated aqueous microgel based on PAA and poly(Nisopropylacrylamide) (PNIPAM). For rendering redox-active features, the system was modified with an amino-containing free radical, which was linked to the functional groups of PAA, resulting in amide groups. The interpenetrated network microgel is analyzed by cyclic voltammetry, which indicates that at 14% of amino-containing free radical, the material maintains the electroactive characteristics and owns a reversible redox response. The attained samples display the redox response at 0.56 V against Ag/AgCl with a steady capacity of 2.5 mAh/g. Additional adjustment of the sample with conductive species might considerably augment the capacity. This allows the reduced-viscous catholyte to enhance its capacity level over 100 mAh/L. Furthermore, the resulted microgel is able to render small viscosity solutions even at temperatures between 30 and 40°C. The eco-friendly materials are excellent candidates for flow batteries electrolytes having large volumetric energy density. Other applications might include redox mediators or nanoreactor systems. Herrmann et al.68 reviewed the polyoxometalates (POMLs) connection to conductive macromolecular systems in regard to their use in energy conversion, storage, and nanostructured sensors. For instance, electrodes based on protonated POMLs (named heteropolyacids, HPAs) are displaying acidic stability, large proton mobility, and big redox chemistry—all these recommending them as parts for supercapacitors. Yamada et al.69 linked HPAs on a Nafion proton-exchange membrane and include it in a capacitor system. The large reversible specific capacitance of 112 F/g and energy density of 36 J/g, were attained when the voltage was of 0.8 V. Asymmetric supercapacitors

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that involve POM/conductive polymer materials were prepared by Suppes et al.70 They reinforced PPy with phosphomolybdic acid to make electrodes for supercapacitors that rendered energy density of 14 J/g and a capacitance slightly higher than 30 F/g. Long-term decomposition of the composite caused by oxidation was remarked. This was reflected in the great loss of capacitance upon several cycles. POMs can be useful for charge separation and transport in solar cells or photoelectrochemical devices. In a related investigation, Sun et al.71 proved that POML anions might act as negative charge-trapping points in the solar device. The achieved composite relied on cobalt phthalocyanine as photosensitizer, anions as electron acceptors introduced in a conductive copolymer. The presence of POML determines a larger photovoltaic effect in regard to the reference lacking it. The analyses reveal that by fitting the electronic characteristics of photosensitizer, electron traps, and conductive matrix, it is suitable for improving the performance of photovoltaic devices at the molecular scale. This opens fresh perspectives in producing photovoltaic panels with more efficient electrical conversion capacities. In the last paragraph, the smartness of the polymer composites from the selected studies on energy applications might not be the main concern, but future research in this domain should expand by accounting materials with single/multi-functionality to advance the current technologies. The future outlook should account for the development of novel SPCs by means of approaches like blending and interpenetrating network structures.72–74 1.4 CONCLUSIONS Smart macromolecular compounds are capable of suffering substantial modifications when triggered external factors are acting on them. In particular cases, before preparing a composite, it is imperative to evaluate the conformational and other molecular mechanics energy-related parameters. Molecular modeling of two relevant thermo-sensitive polymer structures (PMMA and PAA) is performed and the structural peculiarities are analyzed as a function of temperature. The distinct chain flexibility of the selected macromolecular compounds is reflected in the kinetic and total energy values. Representative works described here reveal that smart polymers that possess self-leaning or self-healing properties can be used in energy applications. Also, some case studies on sensing and actuation of SPCs are included. Their use in the energy sector comprises the following: Supercapacitors, covers for solar cells, highvoltage batteries, redox mediators, nano-reactor systems, or components for thermal energy storage. The electrical and thermal performances of the

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composite are residing in both matrix and additive properties, but also the interactions between the counterparts are able to adjust the desired characteristics. Even if solid achievements have been reported, there are still lots of aspects that can be improved in the future in terms of material properties and processing. ACKNOWLEDGMENT This work was supported by a grant of the Ministry of Research, Innovation and Digitization, CNCS/CCCDI–UEFISCDI, project number TE 83/1.09.2020 within PNCDI III (code PN-III-P1-1.1-TE-2019-1878). KEYWORDS • • •

polymer composites molecular modeling energy applications

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22. Zhang, H.; Liu, J.; Shi, F.; Li, T.; Zhang, H.; Yang, D.; Li, Y.; Tian, Z.; Zhou, N. A Novel Bidirectional Fast Self-responsive PVA-PNIPAM/LimCsnWO3 Composite Hydrogel for Smart Window Applications. Chem. Eng. J. 2022, 431, 133353. 23. Barzic, A. I.; Albu, R. M.; Stoica, I.; Hulubei, C. New Shielding Covers based on Transparent Polyimide/ferrous Sulfide Composites that Reduce Optical Losses in Solar Cells, Compos. Sci. Technol. 2022, 218, 109140. 24. Nguyen, B. P.; Kim, T.; Park, C. R. Nanocomposite-based Bulk Heterojunction Hybrid Solar Cells. J. Nanomater. 2014, 1–20. 25. Barzic, A. I.; Soroceanu, M.; Rotaru, R.; Doroftei, F.; Asandulesa, M.; Tugui, C.; Dascalu, I. A.; Harabagiu, V. Cellulose Derivative/barium Titanate Composites with High Refractive Index, Conductivity and Energy Density. Cellulose 2022. DOI: https:// doi.org/10.1007/s10570-021-04343-2 26. Rotaru, R.; Peptu, C.; Harabagiu, V. Viscose-barium Titanate Composite for Electromagnetic Shielding. Cell. Chem. Technol. 2016, 50, 621–628. 27. Tomar, L.; Tyagi, C.; Kumar, P.; Choonara, Y. E.; Toit, L. D.; Pillay, V . Poly (PEGDMA-MAA) Copolymeric Micro and Nanoparticles for Oral Insulin Delivery: A Molecular Mechanistic Revisit. Int. J. Pharmacol. Pharm. Technol. 2012, 1, 62–67. 28. Hulubei, C.; Albu, R. M.; Lisa, G.; Nicolescu, A.; Hamciuc, E.; Hamciuc, C.; Barzic, A. I. Antagonistic Effects in Structural Design of Sulfur-based Polyimides as Shielding Layers for Solar Cells. Sol. Energy Mater. Sol. Cells 2019, 193, 219–230. 29. Pearlman, D. A.; Case, D. A.; Caldwell, J. W.; Ross, W. S.; Cheatham, T. E.; DeBolt, S.; Ferguson, D.; Siebel, G.; Kollman, P. AMBER, A Package of Computer Programs for Applying Molecular Mechanics, Normal Mode Analysis, Molecular Dynamics and Free Energy Calculations to Simulate the Structural and Energetic Properties of Molecules. Comput. Phys. Commun. 1995, 91 (1–3), 1–41. 30. Syafiq, A.; Pandey, A. K.; Adzman, N. N.; Rahim, N. A. Advances in Approaches and Methods for Self-Cleaning of Solar Photovoltaic Panels. Sol. Energy 2018, 162, 597–619. 31. Nayshevsky, I.; Xu, Q. F.; Barahman, G.; Lyons, A. M. Fluoropolymer Coatings for Solar Cover Glass: Anti-soiling Mechanisms in the Presence of Dew. Sol. Energy Mater. Sol. Cells 2019, 110281. 32. Gong, X.; He, S. Highly Durable Superhydrophobic Polydimethylsiloxane/silica Nanocomposite Surfaces with Good Self-cleaning Ability. ACS Omega 2020, 5, 4100–4108. 33. Ibrahim, S.; Labeeb, A.; Mabied, A. F.; Soliman, O.; Ward, A.; Abd-El-Messieh, S. L.; Abdelhakim, A. A. Synthesis of Super-hydrophobic Polymer Nanocomposites as a Smart Self-cleaning Coating Films. Polym. Compos. 2016, 38, E147–E156. 34. Chen, S.; Song, Y.; Xu, F. Highly Transparent and Hazy Cellulose Nanopaper Simultaneously with a Self-Cleaning Superhydrophobic Surface. ACS Sustain. Chem. Eng. 2018, 6 (4), 5173–5181. 35. Sutar, R. S.; Nagappan, S.; Bhosale, A. K.; Sadasivuni, K. K.; Park, K.-H.; Ha, C.-S.; Latthe, S. S. Superhydrophobic Al2O3–Polymer Composite Coating for Self-cleaning Applications. Coatings 2021, 11, 1162. 36. Radwan, A. B.; Abdullah, A. M.; Mohamed, A. M. A.; Al-Maadeed, M. A. New Electrospun Polystyrene/Al2O3 Nanocomposite Superhydrophobic Coatings; Synthesis, Characterization, and Application. Coatings 2018, 8 (2), 65. 37. Cully, P.; Karasu, F.; Müller, L.; Jauzein, T.; Leterrier, Y. Self-cleaning and Wearresistant Polymer Nanocomposite Surfaces. Surf. Coat. Technol. 2018, 348, 111–120.

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38. Subramaniam, A.; Shanthi, J. Spin-coated Polymer Composite Hydrophobic Surfaces with Self-cleaning Performance. Mater. Res. Express 2019, 1–8. 39. Sarkın, A. S.; Ekren, N.; Sağlam, Ş. A Review of Anti-reflection and Self-cleaning Coatings on Photovoltaic Panels. Sol. Energy 2020, 199, 63–73. 40. Cherupurakal, N.; Mozumder, M. S.; Mourad, A.-H. I.; Lalwani, S. Recent Advances in Superhydrophobic Polymers for Antireflective Self-cleaning Solar Panels. Renew. Sust. Energ. Revi. 2021, 151, 111538. 41. Hooda, A.; Goyat, M.; Gupta, R.; Prateek, M.; Agrawal, M.; Biswas, A. Synthesis of Nano-textured Polystyrene/ZnO Coatings with Excellent Transparency and Superhydrophobicity. Mater. Chem. Phys. 2017, 193, 447–452. 42. Hooda, A.; Goyat, M.; Kumar, A.; Gupta, R. A Facile Approach to Develop Modified Nano-silica Embedded Polystyrene Based Transparent Superhydrophobic Coating. Mater. Lett. 2018, 233, 340–343. 43. Faraz, M.; Ansari, M. Z.; Khare, N. Synthesis of Nanostructure Manganese Doped Zinc Oxide/polystyrene Thin Films with Excellent Stability, Transparency and Superhydrophobicity. Mater. Chem. Phys. 2018, 211, 137–143. 44. Park, E. J.; Sim, J. K.; Jeong, M.-G.; Seo, H. O.; Kim, Y. D. Transparent and Superhydrophobic Films Prepared with Polydimethylsiloxane-coated Silica Nanoparticles. RSC Adv. 2013, 3, 12571–12576. 45. Wang, P.; Chen, M.; Han, H.; Fan, X.; Liu, Q.; Wang, J. Transparent and Abrasionresistant Superhydrophobic Coating with Robust Self-cleaning Function in Either Air or Oil. J. Mater. Chem. 2016, 4, 7869–7874. 46. Zhang, L.; Xue, C.-H.; Cao, M.; Zhang, M.-M.; Li, M.; Ma, J.-Z. Highly Transparent Fluorine-free Superhydrophobic Silica Nanotube Coatings. Chem. Eng. J. 2017, 320, 244–52. 47. Wong, W. S.; Stachurski, Z. H.; Nisbet, D. R.; Tricoli, A. Ultra-durable and Transparent Self-cleaning Surfaces by Large-scale Self-assembly of Hierarchical Interpenetrated Polymer Networks. ACS Appl. Mater. Interfaces 2016, 8, 13615–13623. 48. Hou, T.-F.; Shanmugasundaram, A.; Nguyen, B. Q. H.; Lee, D.-W. Fabrication of Surface-functionalized PUA Composites to Achieve Superhydrophobicity. Micro Nano Syst. Lett. 2019, 7, 1–6. 49. Kim, M.; Kang, T.-W.; Kim, S. H.; Jung, E. H.; Park, H. H.; Seo, J.; et al. Antireflective, Self-cleaning and Protective Film by Continuous Sputtering of a Plasma Polymer on Inorganic Multilayer for Perovskite Solar Cells Application. Sol. Energy Mater. Sol. Cells 2019, 191, 55–61. 50. Vüllers, F.; Gomard, G.; Preinfalk, J. B.; Klampaftis, E.; Worgull, M.; Richards, B.; Hölscher, H.; Kavalenka, M. N. Bioinspired Superhydrophobic Highly Transmissive Films for Optical Applications. Small 2016, 12, 6144–6152. 51. Yang, Y.; Dang, Z.; Li, Q.; He, J. Self-healing of Electrical Damage in Polymers. Adv. Sci. 2020, 2002131, 1–21. 52. Mai, W.; Yu, Q.; Han, C.; Kang, F.; Li, B. Self-healing Materials for Energy-storage Devices. Adv. Funct. Mater. 2020, 30 (24), 1909912. 53. Lin, Y.; Zhang, H.; Liao, H.; Zhao, Y.; Li, K. A Physically Crosslinked, Self-healing Hydrogel Electrolyte for Nano-wire PANI Flexible Supercapacitors. Chem. Eng. J. 2019, 367, 139–148. 54. Dai, L.-X.; Zhang, W.; Sun, L.; Wang, X.-H.; Jiang, W.; Zhu, Z.-W.; Zhang, H.-B.; Yang, C.-C.; Tang, J. Highly Stretchable and Compressible Self-healing P(AA-co-AAm)/

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CoCl2 Hydrogel Electrolyte for Flexible Supercapacitors. ChemElectroChem 2019, 6, 467. 55. Li, L.; Fang, Y.; Vreeker, R.; Appelqvist, I.; Mendes, E. Reexamining the Egg-box Model in Calcium-alginate Gels with X–ray Diffraction. Biomacromolecules 2007, 8 (2), 464–468. 56. Zhao, J.; Ji, G.; Zhang, X.; Hu, R.; Zheng, J. Preparation of a High Strength, Rapid Self-healing Composite Gel and Its Application in Electrochemical Capacitor. Polymer 214, 2021, 123372, 1–9. 57. Park, S.; Thangavel, G.; Parida, K.; Li, S.; Lee, P. S. A Stretchable and Self-healing Energy Storage Device Based on Mechanically and Electrically Restorative Liquid-metal Particles and Carboxylated Polyurethane Composites. Adv. Mater. 2018, 1805536, 1–10. 58. Yang, S.; Du, X.; Deng, S.; Qiu, J.; Du, Z.; Cheng, X.; Wang, H. Recyclable and Selfhealing Polyurethane Composites Based on Diels-alder Reaction for Efficient Solar-tothermal Energy Storage. Chem. Eng. J. 2020, 125654, 1–11. 59. Cash, J. J.; Kubo, T.; Bapat, A. P.; Sumerlin, B. S. Room-temperature Self-healing Polymers Based on Dynamic-Covalent Boronic Esters. Macromolecules 2015, 48 (7), 2098–2106. 60. Wang, Z.; Tao, F.; Pan, Q. A Self-healable Polyvinyl Alcohol-based Hydrogel Electrolyte for Smart Electrochemical Capacitors. J. Mater. Chem. A 2016, 4 (45), 17732–17739. 61. Cazacu, M.; Ignat, M.; Racles, C.; Cristea, M.; Musteata, V.; Ovezea, D.; Lipcinski, D. Well-defined Silicone–Titania Composites with Good Performances in Actuation and Energy Harvesting. J. Compos. Mater. 2013, 48 (13), 1533–1545. 62. Khuyen, N. Q.; Kiefer, R.; Zondaka, Z.; Anbarjafari, G.; Peikolainen, A.-L.; Otero, T. F.; Tamm, T. Multifunctionality of Polypyrrole Polyethyleneoxide Composites: Concurrent Sensing, Actuation and Energy Storage. Polymers 2020, 12 (9), 2060. 63. Fredi, G.; Dorigato, A.; Fambri, L.; Pegoretti, A. Evaluating the Multifunctional Performance of Structural Composites for Thermal Energy Storage. Polymers 2021, 13, 3108, 1–15. 64. Wen, F.; Zhu, C.; Lv, W., Wang, P.; Zhang, L.; Li, L.; Wang, G.; Wu, W.; Ying, Z.; Zheng, X.; Han, C.; Li, W.; Zu, H.; Yue, Z. Improving the Energy Density and Efficiency of the Linear Polymer PMMA with a Double-bond Fluoropolymer at Elevated Temperatures. ACS Omega 2021, 6 (50), 35014–35022. 65. Munusamy, Y.; Shanmugam, S.; Shi-Ying, K. Development of Form Stable Poly(methyl methacrylate) (PMMA) Coated Thermal Phase Change Material for Solar Water Heater Applications. IOP Conf. Ser.: Earth Environ. Sci. 2018, 140, 012008. 66. Baştürk, E.; Kahraman, M. V. Thermal and Phase Change Material Properties of Comblike Polyacrylic Acid-grafted-fatty Alcohols. Polym. Plast. Technol. Eng. 2017, 57 (4), 276–282. 67. Kozhunova, E. Y.; Gvozdik, N. A.; Motyakin, M. V.; Vyshivannaya, O. V.; Stevenson, K. J.; Itkis, D. M.; Chertovich, A. V. Redox-Active Aqueous Microgels for Energy Storage Applications. J. Phys. Chem. Lett. 2020, 12, 1–5. 68. Herrmann, S.; Ritchie, C.; Streb, C. Polyoxometalate – Conductive Polymer Composites for Energy Conversion, Energy Storage and Nanostructured Sensors. Dalton Trans. 2015, 44 (16), 7092–7104. 69. Yamada, A.; Goodenough, J. B. Keggin-type Heteropolyacids as Electrode Materials for Electrochemical Supercapacitors. J. Electrochem. Soc. 1998, 145, 737–743.

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70. Suppes, G. M.; Cameron, C. G.; Freund, M. S. A Polypyrrole/phosphomolybdic Acidpoly(3,4-ethylenedioxythiophene)/phosphotungstic Acid Asymmetric Supercapacitor. J. Electrochem. Soc. 2010, 157 (9), A1030–A1034. 71. Yang, Y.; Xu, L.; Li, F.; Du, X.; Sun, Z. Enhanced Photovoltaic Response by Incorporating Polyoxometalate into a Phthalocyanine-sensitized Electrode. J. Mater. Chem. 2010, 20 (48), 10835–10840. 72. Ratna, D.; Karger-Kocsis, J. Recent Advances in Shape Memory Polymers and Composites: A Review. J. Mater. Sci. 2008, 43, 254–269. 73. Fong, K. D.; Wang, T.; Kim, H.-K.; Kumar, R. V.; Smoukov, S. K. Semi-interpenetrating Polymer Networks for Enhanced Supercapacitor Electrodes. ACS Energ. Lett. 2017, 2 (9), 2014–2020. 74. Alizadeh, N.; Broughton, R. M.; Auad, M. L. Graft Semi-interpenetrating Polymer Network Phase Change Materials for Thermal Energy Storage. ACS Appl. Polym. Mater. 2021, 3 (4), 1785–1794.

CHAPTER 2

Magnetorheological and Electrorheological Properties of Smart Polymer Systems and Their Energy-Related Applications RALUCA MARINICA ALBU and LUMINITA IOANA BURUIANA “Petru Poni” Institute of Macromolecular Chemistry, Laboratory of Physical Chemistry of Polymers, Iasi, Romania

ABSTRACT Materials with intelligent behavior represent an actual topic that significantly contributes to the progress of futuristic technologies, including in the energy sector. This chapter is devoted to describing the properties of some polymerfilled micro/nanofillers, starting from the solution phase. The rheological properties of such materials can give relevant information on the correlation between system composition and a sample response to shear deformation. An additional accent is made on the electrorheological and magnetorheological properties of such multicomponent systems. Molecular modeling studies are also performed to understand the phenomena that lie at the basis of the interaction of polymer and the reinforcement agent. Furthermore, the physical properties, which emphasize the smart behavior of the polymerbased materials, are also reviewed. In the final part of the chapter, some applications in the energy sector are briefly presented. 2.1 INTRODUCTION Polymer materials are more and more involved in a wide category of applications, from daily technologies (i.e., electronics and automobiles) to aerospace Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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(i.e., components for aircraft) and biomedicine (i.e., medical equipment and diagnosis devices).1–3 In the context of energy issues at gla obal scale, the research communities are working on finding novel solutions that are based on renewable sources. To further advance the performance of the actual energy systems the production techniques and the properties of the used materials must be improved.4,5 The literature presents a variety of polymer structures that can be used in the construction of components for energy harvesting or storage devices.6–8 As a function of the electrical, thermal, and mechanical characteristics, the materials can be used as insulators or conductors. However, in other practical situations, it is highly desirable to enhance the functionality of the material.9 Therefore, the polymer properties can be adjusted by the introduction of particles with complementary features.10 Polymer micro-/nanocomposites have emerged as a superior class of engineering plastics with sophisticated characteristics.11 In addition to this, if the material’s smartness is a key feature in the pursued application, a careful selection must be made among the available polymer structures with emphasis on those that are known to respond to specific stimuli.12 Among the latter ones, it is worth mentioning the following main categories12–14: • •

External stimuli: Temperature, light, electric field, and magnetic field; Internal stimuli: Solvent, pH, and (bio)chemical recognition.

The first category of stimuli is largely utilized owing to the ability to set and control the parameter in a facile manner in comparison with the second type of stimuli. This opens perspectives to implement smart materials involving physical factors in numerous technical solutions. The fundamental concern is to ascribe attention to the correlation between compound structure and the desired intelligent property. Smart polymer composites (SPCs) could be made by using a matrix that is characterized by one or more common intelligent properties, like shape memory, self-cleaning, sensing, self-repairing, self-heating, energy harvesting, or storage.13,14 When fabricating a polymer-based product, a primary aspect that needs huge consideration is the processing route, which essentially is accomplished from solutions. All the processing stages involve the deformation of the polymer fluid and its response is paramount for the definitive features (morphology, electrical conductivity, heat resistance, wettability, etc.) in the solid phase.15,16 The properties of the polymer solutions are affected by the chemical structure and the concentration domain.17–19 When working in semi-dilute and concentrated ranges, rheology is one of the most appropriate methods of analysis.20 This technique examines the deformation of fluid in

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situations in which they react with the plastic flow rather than deforming elastically as a result of an externally imposed mechanical force. This branch of physics enables investigation of the flow charts’ evolution with the imposed shear rate.21–23 Furthermore, the oscillatory experiments can offer indispensable information on the balance between the viscous and elastic properties of the polymers in the solvent environment.24–26 The rheological functions are changing with the variation of some factors, such as solvent quality, polymer concentration, and applied shear.24–26 The thermodynamic characteristics of the solvent dictate the level of chain expansion when surrounded by the solvent molecules.19 The backbone rigidity is reflected in the slope of viscosity curves, but also in the transition point from viscous to elastic flow.24 In the shear field, a flexible macromolecule is more easily aligned along the deformation force. When the solution is concentrated enough so that the polymer coils are entangled, the overall polymer alignment under shearing is less pronounced.27 Polymer reinforcement with particles under micron dimensions further produces modification in the flow behavior. Loading a polymer beyond the percolation point generally is reflected in the occurrence of a particle network within the solved matrix.28 The storage modulus is highly sensitive to such phenomena and is often employed to characterize polymer composites in the fluid phase.29 Aside from the aforementioned aspects, when the macromolecular compound owns smart features, the rheological functions can additionally range with the applied stimulus.30–32 One of the most common responsive factors is temperature.33 Rheology allows performing experiments at variable temperatures and hence to discern the microstructure changes under established environmental conditions. Moreover, when studying the flow properties in the presence of either a magnetic or electric field, the measured parameters are suffering specific variations.34,35 So, these sub-branches of rheology enable to discover new insights that are fruitful for the development of intelligent electrorheological (ER) and magnetorheological (MR) materials.34,35 Besides the mechanical shearing, the external fields are able to create an additional anisotropy on the flow behavior and, in this manner, the fluid properties can be tuned. Starting from the literature background on the described topic, the chapter presents the solution behavior of some SPC systems. First, the rheological aspects involved in monitoring the material performance are discussed. The actual advances regarding the rheological, ER, and MR properties of SPCs are reviewed since in most applications, including in the energy sector, the materials are acting under the influence of electromagnetic fields. Studying these aspects from the solution phase would provide an essential

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point of view in the future progress of smart polymer films and coatings with increased functionality. For additional understanding of the solution behavior of SPCs, molecular modeling of specific systems is performed. Hence, based on the simulations certain parameters relevant to applications are extracted and analyzed for the selected systems. The simulated systems are constructed based on several SPC structural units, filler particles, and solvent molecules. The chapter continues with depicting the most significant reports on the impact of rheological behavior under various factors, such as temperature, pH, electric field, magnetic field, etc. The discussions are made in relation to the physical properties of particular interest for applications, including in energy production or storage devices. 2.2 RHEOLOGICAL TECHNIQUES USED IN THE ANALYSIS OF SMART POLYMER SYSTEMS Polymer-filled micro/nanofillers and their special properties have received a special interest due to their technological applications, especially in the energy sector. The peculiar properties can arise, on the one hand, from the polymer that represents the matrix and, on the other hand, from the micro/nanofiller which denotes the discontinuous phase. In this way, one may obtain a material that combines the characteristics of each component. Rheological polymerbased fluids, which are responsive to electric or magnetic fields, are named electro-responsive ER and magneto-responsive MR fluids. The principle underlying the ER or MR phenomenon arises from electrostatic polarization or magnetization of embedded components in the polymer fluid, which under the imposed external field are changing the way that they are structured. Hence, ER and MR polymers can be considered a category of smart materials whose rheological functions are adjusted by physical stimuli. For a better understanding, a scheme regarding the rheological techniques used in studying reinforced polymer fluids with smart behavior in presence of the electrical or magnetic field is presented in Figure 2.1. In the following part of the chapter, the advances in rheology techniques (in the absence or presence of an electric/ magnetic field) for the examination of some polymer systems are reviewed. 2.2.1 RHEOLOGY OF SMART POLYMER SYSTEMS The rheological properties of smart polymers that raise interest for energy applications are briefly reviewed. The focus is directed towards the most

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recent studies on this topic, particularly the thermo-responsive systems since in energy applications temperature is a relevant factor that dictates the device performance.

FIGURE 2.1 Rheological techniques applied in the analysis of reinforced polymer fluids with smart behavior dictated by an electrical or magnetic field.

Misra et al.36 have employed stress-controlled rheology to analyze the behavior of aqueous suspensions of poly(N-isopropylacrylamide) (PNIPAM) particles. The temperature sweep experiments are done at constant strain amplitude and angular frequency. The temperature responses of the shear moduli are recorded at three maximum swelling ratios. At the same volume fraction and under lower critical solution temperature (LCST), the storage modulus of prepared suspensions is larger with a reduction in maximum swelling ratio. At this temperature, the elastic counterpart is greater than the viscous one (G′ > G″), which reveals that the sample can be regarded as a viscoelastic solid in such conditions. As the temperature is bigger, both shear moduli are lowered. The difference between G′ and G″ denotes the level of the relative solidity of the viscoelastic suspension. From these data is noted that the relative solidity becomes reduced upon increasing the temperature, while in the vicinity of LCST this aspect seems to be negligible.

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Therefore, the sample is less rigid close to the LCST. In the case of the soft PMMA particles, the loss modulus prevails over the storage one near the LCST, revealing the liquid-like behavior of the sample. The explanation arises from the sharp collapse in the dimensions of thermosensitive polymer particles close to the LCST. The temperature at which the rheological moduli are lowered is evaluated as being the LCST of the aqueous system. As the temperature is exceeding the value of LCST, a progressive reduction in the moduli is remarked. The forces among the polymer particles in the suspensions vary from attractive to repulsive at the LCST. The enhancement of the rheological moduli at a temperature beyond the LCST is ascribed to an augmentation of the attractive interactions between the polymer particles in the system. As a consequence, gel networks are produced which manifest as larger shear moduli at the respective temperatures. During the heating and cooling of the analyzed suspensions having the largest swelling ratios, it is noticed that the shear moduli follow similar paths during these processes applied to suspensions of the stiffer polymer particles. In any case, the viscoelastic moduli of soft PMMA particles tend to diverge from each other close to LCST while cooling. Such behavior could be ascribed to the variations in the polymer chains’ packing ability owing to their larger thermosensitivity and deformability. Hirun et al.37 have investigated the temperature-dependent properties of hydrogels made from poly(acrylic acid) (PAA) and poloxamer 407 (PX). The amount of inserted PAA in the system containing 20% PX is affecting thermosensitive micellization and gelation. A higher quantity of PAA favors the occurrence of more PX micelles with bound PAA. The intermicellar packing is influenced by the thermosensitive macromolecules and their existence tuned the gelation temperature. The prepared systems display disorder-to-order transition while heating. With the addition of PAA, the ordered sample structure displays a prevailing body-centered cubic phase. The recorded critical strain is low revealing a typical gel system with powerful interactions. Also, rheological tests show a less rigid sample structure in the presence of PAA. Conley and collaborators38 studied the impact of the structure of interpenetrating microgels on their shear flow abilities. They also used PNIPAM and prepared gels capable of deforming and compressing. By means of frequency-dependent oscillatory tests, they evaluate the viscoelastic properties, especially the dissipative behavior along a wide concentration regime. The performed analyses indicate that tiny particles seem to act qualitatively distinctly and might be overpacked up to such a level without showing the expected augmentation in the storage shear modulus.

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Abbadessa et al.39 attempted to attain novel material properties starting from multi-component hydrogels which can suffer phase separation. The emphasis is on the impact of phase separation on the mechanical characteristics and tridimensional printing ability of hydrogels. The latter is composed of copolymers based on polyethylene glycol (PEG), methacrylate hyaluronic acid (HAMA), and a methacrylate polymer. The hydrogels display hydrophilic HAMA inner zones distributed in a continuous medium with greater hydrophobicity. The elastic modulus, yield stress, and viscosity present higher values when more HAMA is introduced in the system. These aspects are allowing establishing an adequate processing range for the 3D printing ability of the gels, which is ideal in many applications. In a recent report, da Silva et al.40 analyzed graft copolymers having brush-like architectures. They are made of PEG methacrylate copolymerized with thermosensitive reactants which impart LCST to the material. The conducted rheological studies on concentrated solutions reveal all samples present narrow zones of high viscosity near their respective cloud points. Such changes are reflected in shear moduli, especially in the loss one. This could be interpreted as a clue for the appearance of particles that redound to the viscosity of the sample, while no elastically active interactions (like interparticle bridging) are noted.41 Therefore it can be assumed that the main chain comprising mainly thermosensitive units is collapsing beyond LCST with PEG parts stabilizing it. However, it is not capable to produce a percolating network inside the system so that bulk elasticity could be imparted. The temperature ascribed to the phase transition is affected by the components’ chemical structure, morphology, end-groups, and polymerization routes, so by this means it can be adjusted for applications of interest.42 2.2.2 MAGNETORHEOLOGY OF SMART POLYMER SYSTEMS The MR polymer suspensions can be directed through the action of an external magnetic field. Such fluids are made of micrometric or nanometric particles of magnetizable compounds, which are dispersed in a polymeric liquid. All physical and mechanical features are modified as a consequence of the applied field and are virtually instantaneous and can be reversed after cessation of the field influence.43 Hence, these materials display smart behavior so that their rheological functions (i.e., yield stress) can vary considerably under exposure to a magnetic field. Generally, the principle can be described as follows: Magnetic field is producing the magnetization of the dispersed fillers, which therefore experience attractive interactions,

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leading to the appearance of filler structures, which resist the flow. Thus, when magnetic forces are employed in the gap between two plates, an almost infinite “string” of particles oriented parallel to the direction of the magnetic field occurs. The MR material can comport similarly to a semisolid depending on the field intensity, while when the latter is null again the material is behaving as a fluid again.44,45 The formation of magnetic particle structures inside the polymer fluid can be monitored via viscosity changes as a function of shear rate magnitude. The MR effect and sedimentation stability represent essential aspects that dictate the performance of an MR fluid. Among the influencing parameters, one can mention: Volume fraction of inserted filler, its size, and the viscosity of the carrier fluid. A big ratio of embedded filler to carrier liquid in the system could point out enhanced magnetic properties, which in turn are known to vary with the magnetic filler density within the polymer fluid.44,45 As a function of the carrier matrix in the absence of a magnetic field, these polymer systems can be classified into fluids, elastomers, and gels, which have a wide range of applications in various domains, as follows: – MR fluids: Medical devices of disease diagnosis, thermal conduction, and sound conveyance precision machining.46–49 – MR elastomers: Sensors, adaptive vibration absorbers or isolators, micromachines, and energy transducing systems.50–52 – MR gels: There are few reports on this subject, but they are utilizable as vibration isolators or controlled delivery systems.53–55 Researchers have attended to the rheological characteristic of MR materials and revealed that the flow tendency is turned from the Newtonian fluid into a semi-solid-like one within seconds.56 Under the magnetic influence, the microstructure of the polymer fluid becomes more ordered.57 Such materials with smart behavior reveal Bingham fluid characteristics under a magnetic field, with maximum yield stress exceeding 100 kPa, while the apparent viscosity magnitude is modified by shifting the strength of the external field. All of these specific features come with a considerable drawback reflected in particle sedimentation. To attain particle redispersion into the matrix after their clustering, a solution could be to include surfactants.58 Thus, MR fluids are often used fillers slightly larger than colloidal ferrofluid particles (dimensions around 15 nm), for achieving greater stability in the non-magnetic matrix. Such particles are preferred to those having micrometer magnitude owing to the Brownian motion and Van der Waals force. Thus, ferrofluids characterized by a suspension of ferromagnetic nano-inclusions in a carrier

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matrix seem to be intensively examined in past years.59–61 Ferrofluids preserve their liquid-like behavior regardless of the magnetic field magnitude. Therefore, these materials cannot have high magnetic flow tension. To prevent the nanometer particle assembly, the latest investigations are focused on magnetic material with a fibroid or rod-like shape.62 Ferromagnetic particles characterized by core-shell structure and chemically adapted with several polymers are also studied.63 This type of structure display many benefits, such as decreased particle density, generating static electrical repulsion increasing through neighboring particles, enhancing the stability and dispersibility of the fluid. In addition, the anti-oxidation is improved owing to the polymer layer placed on the surface of the particles.64 Other types of MR fluids are composed of several sorts of nanometer fillers, such as nanotubes, carbonyl iron particles, organic clays, Co-γ-Fe2O3, or CrO2, which are dispersed into conventional fluid with the purpose of upgrading the matrix properties. Elastomer materials based on polymers are magnetic soft materials that respond to an external magnetic field, thus appearing as deformation or mechanical stress. Magnetic polymer gels are a sub-category of magnetic elastomers, exhibiting flexible cross-linked macromolecules with magnetizable particles and a swelling phase. Magnetic particles covered with thin polymer sheets (under 1 μm) are investigated due to their specific features that are utilizable in pharmaceutics, cosmetics, gene delivery, and magnetic resonance imaging.65,66 Moreover, when such magnetic microspheres present thermo-responsive behavior they exhibit relatively rapid magnetic separations, which are adequate for cell separation, enzyme immobilization or clinical diagnosis. The intelligent properties of polymer-based magnetic materials are explored in the design of magnetic biosensors. In this field are pointed out several aims, among them being67,68: • identification and development of new physicochemical processes and novel materials with advanced magnetic properties; • designing and manufacturing integrated devices for magnetic microsensors; • development of integrated magnetic chips for genic expression analysis; • evaluation of the magnetic screening for molecular genetic diseases using new diagnostic technics based on magnetic detection. Regarding magnetic nanoparticles, the latest reports are focused on the improvement of the magnetic, optic or mechanical performance by reducing the dimensions and adaptation of the composition at the surface

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level. Thus, alternative preparation methods of nanoparticles with new geometries, combined with augmented crystallinity and narrow distribution, are reported.69 In this way, the surface properties are more easily tuned. This enables the realization of nanoparticles having predetermined properties because the transport of the electrons through the multiphase material is depending on the interfacial features through which the charges are moving. As a matter of fact, chemisorption and self-assembling of some organic molecules placed on the nanoparticles’ surface are affecting the magnetotransporting properties. These modifications could vary the chemical reactivity for detection, physical protection, optical activity, etc. On the other hand, such tiny magnetic fillers represent a tremendous challenge for the sensor’s sensing characteristics since the magnetic moments of the particles are very low given their limited volume, relatively large surface, and great thermal activation of their magnetic moments.70 There are several reports dealing with the flow behavior of magneticresponsive composites.34,71–78 Vshivkov et al.34,71 studied the flow behavior of some paramagnetic systems containing polymers. They prepared aerosol suspensions in which PEG or polydimethylsiloxane (PDMS) is incorporated. The systems display a thinning behavior in the applied shear interval owing to disruption of the original structure. The magnetic field does not significantly influence the viscosity of the suspensions of the PEG-aerosil when the magnetic lines are along or orthogonal in regard to the rotor rotation axis. For PDMS-aerosil samples, the viscosity is enhanced although is not varying under magnetic force action. Augmentation of the viscosity of polymer systems in the presence of such external forces is reported for cellulose ether with moderate chain rigidity in solutions.72–76 Given the interactions occurring among the diamagnetic macromolecules and their associates with the magnetic forces, the impact of the field could reflect itself in a peculiar context: The particles should be anisodiametrical and their volume must exceed a critical point, while the surrounding environment should have small viscosity. The clusters of aerosil filler connected to PDMS chains could correspond to these conditions; hence the viscosity of this system in the field is raised in accordance with the latter condition. When introducing ferroparticles in PDMS and PEG systems, similar behavior is noticed.34 The magnetic field favors iron filler aggregation, so that one may observe a larger viscosity at low shear rates. Further increase in shearing produces aggregate destruction and filler orientation—all these aspects are decreasing the viscosity. The variation of suspension viscosity with the percent of iron filler in the magnetic field indicates an increase in viscosity upon reinforcement.

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Also, the magnitude of the viscosity in the transverse field seems to be larger in comparison with that measured longitudinally. The relative viscosity of suspensions depends on the concentration of inorganic magnetic particles so an enhancement of the viscosity with reinforcement is caused by stronger orientation and aggregation under magnetic forces. In this case, the transversally measured viscosity of the suspensions is twice the value of the longitudinal one. On the other hand, diamagnetic polymer systems are also investigated,77 particularly for cellulose ethers in liquid crystal (LC) form. Lyotropic solutions of cellulose ethers display, under a magnetic field, some large domains in the course of orientation and even orientation of strips of a certain height. The cellulosic chains follow the direction of the magnetic field with their long axis disposed along the force lines. This orientation is linked to the molecular diamagnetic anisotropy of cellulosic materials. Therefore, supramolecular particles appear, particularly near the LC phase transition and viscosity grows. The cellulosic derivative solutions exhibit non-Newtonian flow properties. The original structuring of polymer fluids is disrupted the associates are taking the course of shear flow direction. Under the magnetic field, the internal friction of isotropic fluids is larger, which could be related to the molecular diamagnetic anisotropy. The reduction of viscosity under the magnetic field of anisotropic fluids is favored by the orientation of chains and supramolecular units of anisotropic polymer fluids in the magnetic field and additionally by the diminishment in the particle dimensions. The relative viscosity plot versus concentration presents a maximum and might help elucidate the impact of the magnetic field on the orientation chains in solutions. The initial enhancement in the relative viscosity with the addition of polymer in the system is linked to a larger number of magnetically sensitive chains and supramolecular units. This is a situation that produced the strengthening of the orientation processes under external forces. In any case, with the additional increase of polymer amount, the raise in the density of the unsteadiness network of entanglements starts to impede the overall alignment and the effect of the field on the rheological features of solutions is less pronounced.78 2.2.3 ELECTRORHEOLOGY OF SMART POLYMER SYSTEMS The ER properties of polymer systems have been the topic of many reports.35,79–85 According to Liu,35 an ER fluid is made of semiconducting or highly polarizable components which have the role of electro-responsivity.

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The ER systems include a broad category of materials ranging from inorganic to organic macromolecular compounds and inorganic/organic composites loaded with fillers of variable dimensions or forms. A variety of polymers (with prevalent conductive characteristics) have been combined with metallic particles, clays, mesoporous materials, and carbon-based particles to enhance the ER performance, as expected from the synergistic features of each component included in the system.79 When loading with clay polyaniline (PANIL) and polypyrrole, the flow behavior is different in that upon intense shearing, the shear stress is lowered at first and then becomes higher beyond a critical shear rate. Extraordinary flow dependencies where a minimum appears could be attributed to the microstructural adaptation of the ER fluid under mechanical force. Other ER fluids based on PANIL and barium titanate (BT) are studied under several distinct electric field strengths. The shear stress is augmented gradually with the shear rate while no field is acting on it, so a constant viscosity plateau is noted. When turning on the electric field and its strength is enhanced, greater shear stress is remarked, regardless of the shear rate magnitude. Shear flow characteristics of the PANIL/BT fluids can be fitted by the Bingham relation. The latter can depict a steady shear response for ER liquid with yield stress where the sample is ranging from solid-like to fluid-like at the null shear rate edge. Moreover, there are other models that describe the shear flow of ER fluids, such as the six-parameter model constitutive equation of state35 elaborated by Cho–Choi–Jhan seen in equation (2.1): = τ

τy

 1 + η∞ 1+  β 1 + (t1 γ ) ( t  2 γ) α

  γ 

(2.1)

where τ is the shear stress, γ˙ denotes the shear rate, τy is a function of electric field strength, η∞ is the viscosity at an infinite shear rate, α is linked to the decrease in the stress, t1 and t2 are time constants, while β is an exponent with subunitary values. This approach explains even better the shear stress graphs and also covers the stress reduction in the small shear rate zone for PANIL/BT dispersions. The first factor is related to the shear stress properties at small shear rates and the second one reflects the shear stress variation at a large shear rate zone. The particle chains, occurring under electric forces, are perturbed under flow. When the shearing level is big enough, the particle chains could be smashed and the particles cannot own sufficient time to realign themselves in the presence of electric forces. Also, the reduction of shear stress when shearing is more intense is revealed to take place only under dc electric fields.

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The shear stress augmentation could be caused by the fibrillar structure of particles resulting upon exposure to the electric field. When the shear field is imposed on the ER fluid, the fibrils start to disrupt and reform many times, as a function of the level of shearing and the forces acting among the particles from the fibrils. The “trembling” shear tendency could be also remarked for a chitosan-derived ER fluid together with a novel rheological equation of state.86 Liu et al.80 evidenced distinct electro-responsive effects in polymerized ionic liquids. They show that even if the same ions are found in the ionic liquid and its polymerized counterpart, they present differences in ER features. In the presence of electrical forces, the ER effect on the polymerized ionic liquid overcomes that observed for the monomeric counterpart by two orders of size. When the temperature is augmented, the ER effect of polymerized ionic liquid is noted to remain almost unchanged, whereas the monomeric ionic liquid displays powerful temperature dependence. Another work on the topic is that of Zhao and collaborators.82 They have analyzed both anionic and cationic poly(ionic liquid)s and their ER properties. The shear stress of both fluids is enlarged under electric field exposure and decreased after its cessation, revealing a rapid response and adequate reversibility. When the intensity of electric fields is bigger, the shear stress of both fluids is larger, but to a higher degree for the cationic counterpart. When no field is applied, the viscosity of both ER samples is small and close owing to similarities in particle concentration and form. Both of the suspensions display pseudoplastic behavior. This may be the result of interparticle interaction and loose network structure in concentrated fluids. At 3 kV/mm, the flow behavior of cationic fluid has a Newtonian region and then climbs after that. The flow properties of the anionic sample also reveal a stable plateau in the same shear rate interval, after which the shear stress is lowered to the minimum. The rheological characteristics of ER fluids are generally affected by the alteration of the particle chains, which in turn are controlled by the electrostatic forces and the shear field-generated hydrodynamic interactions. At low shearing, the particle chains are not much affected and ER samples behave similarly to plastic solids having yield stress owing to the low hydrodynamic influence, while electrostatic effects are prevailing. In such situations, the polarization level of particles is essential since it dictates the measure of electrostatic force. When applying more intense shearing, the chains are disrupted by large hydrodynamic interaction, but they are rebuilt via electrostatic interactions. As a result, the shear stress−shear rate graph has a plateau reflecting the balance among these forces. So, the polarization rate

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of particles is paramount in the rate of reconstruction of chains. Hence, the flow behavior of both anionic and cationic poly(ionic liquid)s are ascribed to their peculiarities in dielectric polarization features. Moreover, when electric forces are acting on the samples, the shear stress quickly becomes larger and the suspensions have yield stress. This is attributed to the electric field-generated occurrence of particle chains. However, the level of yield stress is distinct for anionic poly(ionic liquid)s and cationic ones. More explicitly, these are slightly higher for the cationic fluid, denoting a bigger electric field-generated solidification level for this sample. The magnitude of the current density of the anionic sample is far smaller and presents reduced power consumption, but higher ER effectiveness. Another relevant work in this field is that made of He and collaborators,84 which studied the influence of the molecular weight (MW) on the electro-responsive of some poly(ionic liquid)s attained by freeradical polymerization. Rheological tests involved measurement of the static yield stress (denoting electric field-induced solidification) and dynamic yield stress. These two parameters are lower when the molecular weight is small. When MW is under 30,000, the magnitude of the values of static and dynamic yield stress displays a fast climb and is augmented at larger MW. If the latter is bigger than 30,000, the ER effect becomes saturated. In other words, ER effect of poly(ionic liquid)s is receptive to MW, when the molecular weight of poly(ionic liquid)s is below a particular value, but the ER effect is less affected by it when MW reaches a threshold. This behavior can be explained by considering the conductivity and the dissolution ability of the analyzed fluids. As corroborated with dielectric spectroscopy experiments, it is shown that for MW under 30,000, the conductivity of the sample is larger and the dissolution ability is reduced, rendering in a raised interfacial polarization and decreased leaking conduction. This is responsible for the enhancement of ER effect. When the level of MW exceeds 30,000, the conductivity and the dissolution characteristics exhibit insignificant changes, which are also in agreement with the saturation of interfacial polarization, leaking conduction, and observed ER effect. 2.3 ENERGY-RELATED APPLICATIONS OF SPCs SPCs are opening fresh perspectives in many applicative areas, including in the energy domain. For instance, in some situations, the material is subjected to the action of electric or magnetic forces and even under variable temperatures. The ER and MR properties are relevant for understanding the phenomena that help to advance the operability and reliability of the current devices.

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There are certain approaches for electrical storage that have been developed over the years for chemical, magnetic, or electrical energy storage. These are including87–89: • • • •

Batteries Fuel cells Superconducting magnetic energy–storage (SMES) systems Electrostatic/electrochemical capacitors or supercapacitors

MR materials can be used in SMES. This means that they can accumulate energy under a magnetic field generated by the flow of direct current from a superconducting coil (SCC) that is cryogenically chilled to a temperature under the superconducting critical one. After charging the SCC, the current is not decaying and the magnetic energy is stored. The latter could be delivered back to the network through the coil discharging. In their incipient phase, these systems were designed as massive energy–storage rings ~1000 MW, analogous in capacity to storage hydropower plants. Thévenot et al.90 studied the responsive behavior of reinforced polymers under a magnetic field. They revealed that magnetic actuation offers special capabilities since it might be spatially and temporally tuned, while it could be handled externally to the system, affording a non-invasive way to the remote control. Based on the activation mode, they proposed three categories of magnetic responsive composites: • Ability to be deformed during the action of a magnetic field; • Capacity of the possibility of magnetic guidance (to move toward a desired area; • The possibility to employ magnetic induction for thermoresponsive polymer systems actuation in energy-related systems. Yanik et al.91 studied the applicability of magnetic conductive polymer filled with graphene in energy storage uses. The prepared composites display superconducting properties and are utilized for electrode manufacturing with conductive ink and copper foil. Supercapacitor cells are made of a separator and electrolyte. Cyclic voltammogram data and scan rate results indicate adequate charge-discharge characteristics and large specific capacitance values of around 255 F/g. This means that the magnetic filler raises the capacitance by almost 12%. The energy density of these materials must be further enhanced to be used in lithium–ion batteries. This could be done by tuning the ratio of the magnetic particles and other parameters (dimension and type). It is also proposed that the supercapacitors could be examined under a magnetic field. The latter is able to align the magnetic filler in the system and this might upgrade the uniformity within the device, leading to

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improved charge transportation. This might offer a solution for increasing the specific capacitance. Xia et al.92 have studied suppressing the self-discharge of supercapacitors by means of ER effect of composites based on liquid crystals. The decay rate of device potential and leakage current is diminished by 80%. A nanosized triboelectric generator is employed for charging the device and a larger charging efficiency is attained. Wang93 and Kausar89 reported the use of composite polymer matrices for energy–storage purposes. The energy density of a multiphase material can be enhanced as a function of the dielectric constant and dielectric breakdown strength of the overall material. In addition to this, the size and polarizability of the used filler and the introduced amount in the polymer are essential factors for storage purposes. Koo et al.94 accomplished multifunctional electrochromic systems that enable energy accumulation. The materials are obtained by incorporation of WO3·H2O in chitosan and these phases are connected via a chemical cross-linking approach. The solid samples present raised electrical conduction and Li–ion diffusivity, combined with large electrochemical activity and stability. All these lead to excellent electrochromic behaviors reflected in rapid switching speeds, elevated coloration efficiency and also long-cycling retention. The composites exhibit remarkable energy–storage features with a raised specific capacitance (~154.0 F/g) and a steady rate capability. Singh and co-workers95 have debated the utility of hydrogels in energy storage. Their report includes the synthetic approach of hydrogels, starting from classical routes to modern ones, and discusses the role of conductive polymers in this domain. Also, they describe the utility of graphene hydrogels and hydrogel electrolytes in the manufacturing of supercapacitors. Among the presented hydrogels, one can mention graphene/PANIL/MnOx, reduced graphene oxide/PANIL, carbon nanotubes/ PANIL, folic acid-PANIL hybrids, and polypyrrole/nanocellulose. On the other hand, pyrrolidone (PVP)-derived hydrogels doped with Li3V2(PO4)3 are useful for making Lithium–ion batteries.95 Wang et al.96 have synthesized a hydrogel electrolyte derived from poly(vinyl alcohol), PAA, and KCl and attained self-healable materials for smart electrochemical capacitors. The grafting of PAA is paramount for achieving a greater self-healing ability, mechanical performances, and salt-toleration of the electrolyte because chemical modification of PAA considerably softens the coagulation of poly(vinyl alcohol) macromolecules due to the action of the inorganic salt. These materials present large ionic conductivity and self-repairing does not impose an external factor, aspects that recommend hydrogels for flexible and smart energy–storage systems.

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2.4 CONCLUSIONS Smart polymer materials represent one of the key elements in the future of material science. The reinforcement of macromolecular compounds with intelligent behavior renders additional features that expand the functionality of the final material. In many applications, the processing stage involves first the solution preparation hence the rheological behavior is essential for understanding the response to deformation, temperature, and other factors. Moreover, in energy-related uses, the smart material is under the action of electric or magnetic fields. The change in rheological functions under such external fields is reviewed based on the most recent reports on the topic. At the end of the chapter, some basic applications in energy–storage systems are shortly reviewed. ACKNOWLEDGMENT This work is dedicated to the annual anniversary of “Petru Poni” Institute of Macromolecular Chemistry of Romanian Academy. KEYWORDS • • • • •

polymer composites rheology smart behavior electromagnetic fields energy uses

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59. Sheikholeslami, M. Application of Darcy Law for Nanofluid Flow in a Porous Cavity Under the Impact of Lorentz Forces. J. Molec. Liquids 2018, 266, 495–503. 60. Sheikholeslami, M. New Computational Approach for Energy and Entropy Analysis of Nanofluid Under the Impact of Lorentz Force Through a Porous Media. Comput. Methods Appl. Mech Eng. 2019, 344, 319–333. 61. Sheikholeslami, M.; Mahian, O. Enhancement of PCM Solidification Using Inorganic Nanoparticles and an External Magnetic Field with Application in Energy Storage Systems. J. Clean. Prod. 2019, 215, 963–977. 62. Lopez-Lopez, M.; Vertelov, G.; Bossis, G.; Kuzhir, P.; Duran, J. New Magnetorheological Fluids Based on Magnetic Fibers. J. Mater. Chem. 2007, 17, 3839–3844. 63. Ghobashy, M. M. In-situ Core-shell Polymerization of Magnetic Polymer Nanocomposite (PAAc/Fe3O4) Particles via Gamma Radiation. Nanocomposites 2017, 3 (1), 42–46. 64. Park, B.; Fang, F.; Choi, H. Magnetorheology: Materials and Application. Soft. Matter. 2010, 6, 5246–5253. 65. Farah, F. H. Magnetic Microspheres: A Novel Drug Delivery System. World J. Pharm. Pharm. Sci. 2017, 6, 93–112. 66. Ganguly, S.; Margel, S. Design of Magnetic Hydrogels for Hyperthermia and Drug Delivery. Polymers 2021, 13, 4259–4281. 67. Kim, S.-E.; Tieu, M. V.; Hwang, S. Y.; Lee, M.-H. Magnetic Particles: Their Applications from Sample Preparations to Biosensing Platforms. Micromachines 2020, 11 (3), 302. 68. Wood, D. K.; Ni, K. K.; Schmidt D. R.; Cleland, A. N. Submicron Giant Sensors for Biological Applications. Sens. Actuators A Phys. 2005, 120, 1–6. 69. Di Paola, C.; D’Agosta, R.; Baletto, F. Geometrical Effects on the Magnetic Properties of Nanoparticles. Nano Lett. 2016, 16 (4), 2885–2889. 70. Xiao, Y.; Du, J. Superparamagnetic Nanoparticles for Biomedical Applications. J. Mater. Chem. B 2020, 3, 1–14. 71. Vshivkov, S. A.; Galyas, A. G.; Oznobikhin, A. Y. The Effect of a Magnetic Field on the Rheological Properties of Iron-aerosil-glycerol Suspensions. Colloid J. 2014, 76, 292–299. 72. Vshivkov, S. A.; Rusinova, E. V.; Galyas, A. G. Phase Diagrams and Rheological Properties of Cellulose Ether Solutions in Magnetic Field. Eur. Polym. J. 2014, 59, 326–332. 73. Vshivkov, S. A.; Soliman, T. S. Phase Transitions, Structures, and Rheological Properties of Hydroxypropyl Cellulose–Ethylene Glycol and Ethyl Cellulose – Dimethylformamide Systems in the Presence and in the Absence of a Magnetic Field. Polym. Sci. Ser. A 2016, 58, 499–505. 74. Vshivkov, S. A.; Rusinova, E. V.; Galyas, A. G. Effect of a Magnetic Field on the Rheological Properties of Cellulose Ether Solutions. J. Compos. Biodegradable Polym. 2014, 2, 31–35. 75. Vshivkov, S. A.; Soliman, T. S. Effect of a Magnetic Field on the Rheological Properties of the Systems Hydroxypropyl Cellulose – ETHANOL and Hydroxypropyl Cellulose – Dimethyl Sulfoxide. Polym. Sci. Ser. A 2016, 58, 307–314. 76. Vshivkov, S. A.; Rusinova, E. V.; Galyas, A. G. Effect of a Magnetic Field on the Rheological Properties of the Rheological Properties of Cellulose Ether Solutions. Polym. Sci. Ser. A 2012, 54, 827–832. 77. Kulichikhin, V. G.; Golova, L. K. Liquid Crystalline State of Cellulose and Cellulose Derivatives. Khim. Drev. 1985, 3, 9–27. 78. Xu, Y.; Liao G.; Liu, T. Magneto-sensitive Smart Materials and Magnetorheological Mechanism. In Nanofluid Flow in Porous Media; Kandelousi, M. S.; Ameen, S.; Akhtar M. S.; Shin, H.-S.; InTech: Croatia, 2019.

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79. Liu, Y. D.; Choi, H. J. Electrorheological Fluids: Smart Soft Matter and Characteristics. Soft. Matter 2012, 8 (48), 11961. 80. Liu, Y.; Wang, B.; Dong, Y.; Zhao, X.; Yin, J. Distinctly Different Electro-responsive Electrorheological Effect in Low Molecular Weight and Polymerized Ionic Liquids: Rheological and Dielectric Relaxation Study. J. Phys. Chem. B 2018, 122 (50), 12184–12193. 81. Lu, Q.; Han, W.; Choi, H. Smart and Functional Conducting Polymers: Application to Electrorheological Fluids. Molecules 2018, 23 (11), 2854. 82. Zhao, J.; Lei, Q.; He, F.; Zheng, C.; Liu, Y.; Zhao, X.; Yin, J. Interfacial Polarization and Electroresponsive Electrorheological Effect of Anionic and Cationic Poly(ionic liquids). ACS Appl. Polym. Mater. 2019, 1, 2862–2874. 83. Dong, Y. Z.; Seo, Y.; Choi, H. Recent Development of Electro-responsive Smart Electrorheological Fluids. Soft. Matter 2019, 15, 3473–3486. 84. He, F.; Xue, B.; Lei, Q.; Liu, Y.; Zhao, X.; Yin, J. Influence of Molecular Weight on Electro-responsive Electrorheological Effect of Poly(ionic liquid)s: Rheology and Dielectric Spectroscopy Analysis. Polymer 2021, 234, 124241. 85. Wang, Y.; Yuan, J.; Zhao, X.; Yin, J. Electrorheological Fluids of GO/graphene-based Nanoplates. Materials 2022, 15, 311. 86. Ko, Y. G.; Choi, U. S.; Chun, Y. J. Trembling Shear Behavior of a Modified-chitosan Dispersed Suspension Under An Electric Field and Its Model Study. Macromol. Chem. Phys. 2008, 209 (9), 890–899. 87. Chatzivasileiadi, A.; Ampatzi, E.; Knight, I. Characteristics of Electrical Energy Storage Technologies and their Applications in Buildings. Renew. Sustain. Energy Rev. 2013, 25, 814–830. 88. Dunn, B.; Kamath, H.; Tarascon, J.-M. Electrical Energy Storage for the Grid: A Battery of Choices. Science 2011, 334, 928–935. 89. Kausar, A. Green Nanocomposites for Energy Storage. J. Compos. Sci. 2021, 5, 202. 90. Thévenot, J.; Oliveira, H.; Sandre, O.; Lecommandoux, S. Magnetic responsive polymer composite materials, Chem. Soc. Rev. 2013, 42 (17), 7099. 91. Yanik, M. O.; Yigit, E. A.; Akansu, Y. E.; Sahmetlioglu, E. Magnetic Conductive Polymergraphene Nanocomposites Based Supercapacitors for Energy Storage. Energy 2017, 138, 883–889. 92. Xia, M.; Nie, J.; Zhang, Z.; Lu, X.; Wang, Z. L. Suppressing self-discharge of supercapacitors via electrorheological Effect of Liquid Crystals. Nano Energy 2018, 47, 43–50. 93. Wang, Q., Zhu, L. Polymer Nanocomposites for Electrical Energy Storage. J. Polym. Sci. Part B: Polym. Phys. 2011, 49 (20), 1421–1429. 94. Koo, B.-R.; Jo, M.-H.; Kim, K.-H.; Ahn, H.-J. Multifunctional Electrochromic Energy Storage Devices by Chemical Cross-linking: Impact of a WO3·H2O Nanoparticleembedded Chitosan Thin Film on Amorphous WO3 Films, NPG Asia Mater. 2020, 12, 1–12. 95. Singh, R.; Veer, B. Hydrogels: Promising Energy Storage Materials. ChemistrySelect 2018, 3 (4), 1309–1320. 96. Wang, Z.; Tao, F.; Pan, Q. A Self-healable Polyvinyl Alcohol-based Hydrogel Electrolyte for Smart Electrochemical Capacitors. J. Mater. Chem. A 2016, 4 (45), 17732–17739.

CHAPTER 3

Metal-Organic Frameworks: Emerging Porous Materials for Energy Applications MIRELA-FERNANDA ZALTARIOV Department of Inorganic Polymers, “Petru Poni” Institute of Macromolecular Chemistry, Iasi, Romania

ABSTRACT This chapter covers the impact of the development of metal-organic frameworks (MOFs), as emerging innovative materials, for energy applications. MOFs are a class of porous crystalline materials built up from the interconnection of organic linkers (ligands) and metal nodes (metal ions or clusters). The smart choice of these organic and inorganic synthons, the in situ and postsynthetic-generated functionalities/templates and the exerted control on their spatial arrangement (framework architecture) confer chemical diversity and allow the fine-tuning of their intrinsic porosity and accessible surface area. This controllable structure-to-function relationship, together with their remarkable structural and chemical versatility, has resulted in the evaluation of these porous coordination polymers (PCPs) in many energy storage and conversion applications referring to H2 storage and production, CO2 reduction, solar and fuel cells, hybrid supercapacitors, anode, cathode, and electrolyte for lithium-based batteries, etc. 3.1 INTRODUCTION The chemistry of coordination compounds is a very important chapter of current research, the basic studies in the field being laid in 1893 by Werner, Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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who described the structure of octahedral metal complexes and the necessary conditions for assigning the coordination number and the oxidation state of metals. This theory has emerged from the observation that, the transition metal ions form chemical bonds not only with anionic species to neutralize their charge, but also with additional species that have at least one donor atom in their structure, namely ligands. Thus, understanding the preferred coordination geometry of the metal ions, the rational synthetic methodologies for coordinating ligands were possible, which allowed the obtaining of new compounds with interesting structures and properties. The first reports in the field appeared in 1964 by contribution of Bailar,1 who described the porous coordination polymers (PCPs) with large internal surface, which, together with the Prussian blue complexes, [Fe4 [Fe(CN)6]3, and Hofmann clathrates are known to have reversible sorption properties.2 After almost 20 years, in 1983 and 1990, Hoskins and Robson3 laid the basic principles of metal-organic structures. In their work, they described stable, crystalline, microporous solids, built up of organic molecular blocks (ligands), and metal ions, which possess ion exchange, gas sorption, catalytic properties, and allow the introduction of functional groups by postsynthetic procedures. After about a decade, the obtaining of two representative metal-organic structures (MOFs), MOF-5 (Zn4O(bdc)3, bdc = 1,4-benzenedicarboxylate)4 and HKUST-1 (Cu3(btc)2, btc = 1,3,5-benzentricarboxylate)5 supported the early development of this domain, mainly due to their robust porosity. Shortly thereafter, another representative MOF appeared, MIL-101 (Cr3(bdc)3, bdc = 1,4-benzenedicarboxylate) with high stability.6 Since 2002, Ferey reported rigid and flexible porous MOFs: MIL-4720 and MIL-5321/MIL-88, respectively.7 The concept of isoreticular chemistry8 became known in 2002 for a series of zinc dicarboxylates, which extended to other materials by using different mixed ligands [M2(dicarboxylate)2(diamine)] (M = Zn, Cu). In 2002, the family of MOFs has expanded to imidazole-type compounds, also known as imidazole-based zeolitic networks (ZIFs).9,10 Recently, the science of porous solid materials has become one of the most intensively studied areas, these materials being known especially due to their applicability in various fields, such as adsorption, separation and purification, catalysis, etc.11,12 Metal-organic structures (MOFs) are organic–inorganic hybrid materials with infinite size, constructed of organic ligands, and network nodes (metal ions or clusters).13 Due to the lack of a generally accepted definition during the development of this new type of hybrid material, parallel names were used (Figure 3.1). Among them, the “term porous coordination polymer”

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(PCP)14 is the most commonly used, followed by the “porous coordination network” (PCN).15 Other names include: Microporous coordination polymer (MCP),16 zeolite-type metal-organic networks (ZMOF),17 zeolitetype imidazolate network (ZIF), metalloprotein-type networks (MFP),18 metal-networks azolate (MAF),19 and mesoporous metal-organic networks (mesoMOF).20 According to the traditional zeolite science, some researchers have also used an acronym depending on the laboratory in which such materials were obtained, such as the series: Materials of the Lavoisier Institute (MILs),21 Hong Kong University of Science and Technology (HKUST),22 Institute of Chemical Technology of Metal-Organic Networks (ITQMOF). Omar M. Yaghi, who introduced the acronym MOF in 1999 and chosed to represent the free volume of the pores by yellow “sphere”,23 proposed a clear distinction between the term “MOF” and “coordination polymer”, based on the valence bond and the bond energy.24 It is clear that, in the last decade, the rapid development of this field has been promoted mainly by the observation of the interesting properties and possible applications of this type of porous materials. Moreover, the structural flexibility of MOFs combined with other unique characteristics had contributed to their differentiation from traditional porous solids.25–27 It should also be noted that MOFs, as porous materials, have the largest known areas per gram.28 Being a relatively new field, the complexity of the composition and structures is constantly increasing, new applications of these compounds being evaluated. One of the most important motivations for MOF research is due to their porosity, regularity, flexibility/rigidity, and structural variety, so they are also considered advanced porous materials. Compared to the traditional porous inorganic materials (zeolites) and activated carbon, the number of possibilities to combine organic and inorganic units to obtain a porous material is amazing, supported by the large number of published works on this type of compound in the last years. Based on the ISI Web of Science (retrieved January, 2022), 72,922 MOFs articles have been reported (Figure 3.1) in which 13,516 (about 19%) of them refer to application in energy, 5,839 (8%) refer to application of MOFs in H2 storage and production and 2,116 (3%) refer to MOFs for hybrid capacitors and battery applications. In addition to the adsorption properties,29,30 MOFs can also have many other unique properties and can be useful in areas such as magnetism30 and luminescence.31 In this way, metal-organic networks fall into the family of porous materials with specific performance properties compared to previously reported porous materials (zeolites, mesoporous silica, and activated carbon).12

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FIGURE 3.1 The evolution of the assigned names of MOFs (metal-organic frameworks), the main fields of application and the publication dynamics for the last 30 years according to the ISI Web of Science (January, 2022).

3.2 STRUCTURAL AND CHEMICAL VERSATILITY OF MOFS This class of crystalline hybrid materials, formed by the interaction of metal centers or clusters (connectors) with organic ligands (linkers), offers a unique chemical versatility due to the possibility of network design, permanent porosity, and large surface area. The variety of metal ions and organic ligands offers an impressive number of structural motifs and endless possibilities for combination (Figure 3.2). In addition, the possibility of postsynthetic modification adds an additional dimension to the structural variability of MOFs.32

FIGURE 3.2 Self-assembly process of metal ions and organic ligands generating 1D, 2D, or 3D metal-organic frameworks.

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The connectors are frequently represented by transition metals, the alkaline and alkaline-earth ones showing certain inactivity. The main factors that effectively influence the ability of connectors to form coordination compounds are their electronic structure, ionic radius, and their charges. Establishing a series of metal ions depending on the affinity to form coordination compounds is difficult, as it also depends on the nature of the donor atom in the structure of the linkers, the type of metal ion and the stability of the new formed bonds, the coordination bonds, which also depend on the nature and number of the other atoms to which the donor atom is bound.34 The main characteristics of the connectors are the coordination number and the geometric coordination sphere. Depending on the metal and its oxidation state, the coordination number may vary, with the formation of different geometries, which may be linear, T- or Y-shaped, tetrahedral, plane-square, trigonal-bipyramidal, octahedral, trigonal-prismatic, pentagonal–bipyramidal, and corresponding distorted shapes.35 Connectors in the structure of MOFs have the advantage of providing control over the valence angles and the coordination number. In addition to metal ions, in the last period, the Secondary Building Units (SBUs) have a special role in establishing of metal complexes with extended structures. SBUs are essential for designing MOFs and in building a rigid network. These units are discrete molecules of metal carboxylates connected by polytopic chelating ligands. In MOF-5, tetrahedral zinc oxide clusters are connected by the benzenedicarboxylate anion (bdc) to form a continuous, cubic neutral network with the structure Zn4O(bdc)3.4 Any polydentate molecule containing functional groups that can coordinate metals can act as an organic ligand: Organic acids, amines, amides, amino acids, heterocyclic compounds, phosphines, etc.34 According to the number of donor atoms in the structure, the ligands can be classified into: – Bidentate ligands: Dicarboxylic acids (oxalic, malonic, succinic, and aromatic acids), hydroxyacids (glycolic, lactic, salicylic, and p-hydroxybenzoic acids), phenolic acids, and amino acids (amino acetic and α-aminopropionic), α-dioxime, β-diketones, aliphatic organic diamines: Ethylenediamine, propylenediamine, butylenediamine, heterocyclic diamines (condensed pyridine bases): α, α′-dipyridyl, 1,10-phenanthroline, hydrazine derivatives, resulting from the condensation of hydrazine with carboxylic acid esters or acid chlorides, semicarbazide (NH2-CONH-NH2), thiosemicarbazide (NH2-CS-NH-NH2), thiosemicarbazone (NH2-CO-NH-N=CR1R2), Schiff bases. – Tridentate ligands: Those in the group of amines, phosphines, amino acids, or macrocyclic compounds (macrocyclic polyethers),

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iminodiacetic acid (HOOC-CH2-NH-CH2-COOH), diethylenetriamine, Schiff bases, etc. – Tetradentate ligands: May be homogeneous if the donor atoms are of the same species and mixed when they coordinate through donor atoms of different species: Schiff bases, tetracarboxylic acids, linear and cyclic tetraamines, porphyrins; and mixed: Complexions (nitrilotriacetic acid), dithioamines, etc. – Pentadentate ligands, hepta-, octa-, decadentate: Are those in the group of complexions or macrocyclic polyethers whose central cavity consists of rings containing more than five nitrogen, oxygen, or sulfur donor atoms.34 The most used ligands for obtaining extended metal-organic networks are those with O and N donor atoms in the structure, aromatic and aliphatic polycarboxylic acids, and heterocyclic nitrogen compounds.46 Another class of ligands, used in the construction of coordination polymers, is represented by bridging ligands. This class included Halogens (F, Cl, Br, and I), which can be found in the coordination networks with organic ligands; the anions CN– and SCN– which also can be presented in the coordination complexes with different geometries, such as linear, in [M(CN)2]-.47–52 The tendency of metal ions to form complex combinations increases with an increasing affinity for electrons and the basicity of the ligand, and the tendency of neutral molecules or ions, which possess at least one lone pair of electrons, to function as ligands increases with their affinity for protons or with their basic strength.53 Therefore, the formation and stability of coordination compounds are mainly influenced by the nature of the donor atom in the ligand structure, the increased basicity of the ligand, and the existence of at least one lone pair of electrons. To these, a special influence of physical factors and the presence of non-covalent intermolecular interactions, the nature of the solvent, the reaction temperature, the pH, the counterions, and the concentration of the reactants must be noted (Figure 3.3). The synthesis of MOFs can be solvothermal, in closed devices, at high pressure, below the boiling temperature of the solvent or nonsolvothermal, performed at the boiling temperature of the solvent and ambient pressure. Temperature variation influences not only the final structure of MOFs, but also the morphology of crystals through the direct action on crystallization kinetics. The pH of the reaction in the synthesis of MOFs can be changed by changing the protonation degree of the organic ligands and by the charge balance in the final product.54–55

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FIGURE 3.3 The main factors that influences the preparation and dimensionality of MOFs; some unpublished representative results.

All blocks included in the coordination networks, the hydrophilic or hydrophobic groups, have specific interactions with solvents, depending on their polarity. The solvent can have a coordination role or can function as a “guest” molecule, which leads to obtain cavities in the structure. Solvent molecules can also co-crystallize by performing physical interactions such as hydrogen bonds with the acceptor groups in the MOF structure and can generate new coordination topologies by evaporation, such as activated structures. The counter ions are present in the structure when neutral ligands are used and can influence the environment of the central metal ion (coordination number). At the same time, depending on their chemical structure, they can participate in the realization of intermolecular physical interactions, they can make bridges or they can have the role of “guest” molecules (Figure 3.3). The stoichiometry of the reaction system has a key role in establishing the final architecture of the coordinated compounds, implicitly the dimensionality: 1D, 2D, or 3D. The architecture of metal complexes is mainly determined by the central metal ion. Thus, in 1D structures the metal ions and the organic ligands alternate “indefinitely”, in 2D structures, three or four ligand molecules coordinate around the central metal ion, (for example, at a ligand molar ratio: 2:1 metal, the metal center is coordinated by four ligand molecules which allow the structure to expand in two directions). 3D structures can be constructed with metal ions with a large coordination

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number, which allows for obtaining tetrahedral or octahedral network nodes56–57 (Figure 3.3). The formation of coordination compounds is directly controlled by the coordination bond. Coordinative bonds are formed by donating a pair of electrons from atoms that have non-participating electrons in the structure of the ligand (Lewis base) to the metal ion (Lewis acid). To these, the electrostatic interactions between the positively charged metal ion and the negatively charged or polarized ligand donor atom can be considered. The energy of such interactions can reach about 50 kJ/mol.57–59 Weaker interactions also influence the formation of coordination polymers: – Hydrogen bonds are defined by Steiner as D – H ••• A type interactions, in which D – H is a proton donor to A (acceptor). The distance D ••• A can be in the range 1.5−2.2 Ǻ for strong hydrogen bonds of type O – H ••• O/N (with angle D – H ••• A in the range 140–180°) and 2.0–3.0 Ǻ for weak hydrogen bonds C – H ••• O/N (with angle D – H ••• A in the range 120–180°). The energy of such interactions varies between 15 and 40 kJ mol−1.60–64 – π–π interactions may have a predominant character during the formation of coordinative bonds. This type of interaction frequently occurs in coordination compounds with heterocyclic aromatic ligands. Aromatic–aromatic interactions involve a face-to-face alignment, these being the sum of the contributions of electrostatic, van der Waals interactions, and the repulsions and charge transfers, the aromatic rings being preferentially oriented in an optimal way to minimize the forces of rejection and increase attraction. The energy of these interactions is estimated at 5–10 kJ/mol.65 – Metal-to-metal interactions. The energy of these bonds is about 5 kJ/ mol for the Ag–Ag interaction.66 – Metal–aromatic ring interactions can form if a metal ion accepts π electrons from unsaturated molecules. The energy of this type of bond is not precisely determined but is considered to be about 5–10 kJ/mol59 (Figure 3.3). In addition to solvo- and nonsolvothermal methods, other nonconventional methods have been developed and studied: By using microwaves and ultrasound or mechanochemical methods, the resulting products being in the form of crystalline particles with small dimensions and different morphologies. Variations in the composition and reaction parameters or the use of additives or microemulsions may also influence the size and the shape of crystals.67–69

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3.3 STRATEGIES IN SYNTHESIS OF MOFS There is a constant concern for the synthesis of new metal-organic networks that present new topologies and open structures with an area as large as possible. The aim was to find new materials with high gas storage capacity, especially for hydrogen.68 The field of MOFs is thus a particularly advantageous alternative from the perspective of three important characteristics, such as design (a), porosity (b), and structural flexibility (c). (a) Design In the design of MOFs, the process of self-assembly of metal ions or clusters with various polytopic ligands is the key factor in obtaining structural diversity and targeted physical and chemical properties in the solid state. Combining several design strategies aimed at obtaining isoreticular networks,70 incorporating reactive groups in the structure of MOFs in the process of synthesis/postsynthetic functionalization,71–72 protection (by introducing hydrophobic groups)/deprotection (opening of metal sites) approaches,73,74 the use of ligands with coordination sites consisting of different atoms or groups of atoms,75 crystalline hybrid structures approach76,77 contributed to the achievement of outstanding performance of MOFs especially in gas absorption and storage, separation, CO2 capture, etc. Also, the possibility of intentionally creating defects, the opening of metal sites and the engineering of crystal surfaces and internal interfaces are other strategies to create MOFs with improved properties. The design of MOFs for industrial applications and their processing as films has become very important, especially the processing of MOFs as thin films, with application in the field of sensors78 and membranes.79 The idea of making MOFs as thin films came from a closely related field of the zeolite films, in particular from the application of direct syntheses and in situ production methods. These new areas refer to the well-known materials and techniques for preparing thin films, which have recently become very important, as evidenced by the large number of publications.80–81 Knowing the possible topologies, the functionality of polydentate organic ligands, the metal coordination geometry, and the synthesis conditions of the building blocks help us to understand and to direct the synthesis strategies. The main objective in the design of MOFs is to establish the synthesis conditions that lead to inorganic building blocks without the decomposition of organic ligands. At the same time, the crystallization kinetics must

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be known to facilitate nucleation and crystal growth. Different MOFs can be obtained from the same reaction mixtures by varying the reaction time, reaction medium, temperature, and the applied method: Microwave, mechanochemistry, ultrasound, or conventional heating. Strategies related to the morphology, shape and size of the crystal, thin films, membranes are also required, to which is added the possibility of activating MOFs, which is absolutely necessary for postsynthetic functionalization, in order to reduce the number of side reactions that can occur. Changes in the structure of crystalline solid-state materials can be achieved by changes in chemical composition, functionality, and molecular dimensions. The insolubility of extended structures requires a one-stage assembly. Thus, in order to design extended structures with a practical accuracy similar to organic syntheses, the building blocks must have the necessary attributes for assembly in the desired structure, the synthesis must be adapted to the use of building block derivatives in order to obtain structures with the same architecture, but with different functionalities and sizes, and the products must have high crystallinity. From this perspective, design of isoreticular structures is an appropriate strategy for the development of new structures with improved properties, especially porosity, by changing the size of the carboxylic ligand. This method has resulted in low-density mesoporous structures.70 (b) Porosity The regular distribution of pores in porous solids is important for the adsorption processes of the guest molecules: Solvents,82–83 gases,84–87 biologically active principles, or drugs.88–89 The pores of such structures adopt a regular periodic architecture due to their intrinsic crystallinity. The structural relationships between adsorbed molecules and host cavities, the position of host molecules in host networks and the influence of guest molecules on the structure of porous channels are key factors influencing the adsorption process and the physical or chemical properties of metal-organic networks. In addition, the adsorbed molecules in the network cavities form molecular assemblies that lead to unique network properties that cannot be achieved in their absence. The shape and size of the pores are influenced by the structure of the ligands (their rigidity or flexibility and length) and the presence of SBUs with a major role in the extension of the network and in its structural stabilization. From this perspective, obtaining isoreticular structures based on rigid ligands is an accessible method to obtain large porous surfaces.70 In the case of the use of

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flexible ligands, the phenomenon of structural interpenetration prevents the obtaining of large porous surfaces.90–93 In this case, the activation of the pores by the influence of some external factor (temperature and pressure) leads to innovative structures with high sorption capacities.94–95 (c) Flexibility Recent studies on the dynamics of MOFs indicated that they are much more flexible than initially thought, with a particular emphasis on the rational selection of organic ligands with accessible shape, functionality, structural flexibility, and symmetry. Carboxylate ligands are widely used in the synthesis of functional MOFs due to their coordination versatility, structural diversity, and high thermal stability.96 The presence of oxygen atoms with a high electron density, high affinity for protons, and low ionization energies of fully occupied molecular orbitals allows the coordination of metal ions from d and f blocks, obtaining networks with various topologies.97,98 Carboxylate ligands can coordinate metal ions or act as a proton donor to make supramolecular structures.99 These abilities are influenced by the nature of the organic groups to which the carboxylic groups are attached. Depending on these, ligands with rigid (aromatic, terephthalate-spacer units in the structure70) or flexible (aliphatic: malonate,100 citrate, adipate,101,102 1,4-cyclohexandicarboxylate,103 1,2,3,4,5,6-cyclohexanehexacarboxylate,104–106 1,2,4,5-cyclohexane tetracarboxylate,107 or aromatics, bis (oxy) isophthalate96,108 with CH2− and (aliphatic spacers) CH2)2−) are known. Another category of flexible ligands is that of the nitrogen heterocyclic compounds, especially those derived from triazole and benzimidazole, with flexible alkane spacers in the structure. The presence of nitrogen atoms leads to various ways of coordination to metal ions and provides various structural motifs, and on the other hand, they can act as acceptors in the formation of hydrogen bonds, which makes them good candidates for building supramolecular networks.109–112 Interesting are also the structures obtained by combining these two categories of ligands (carboxylate type and triazolebased).109,113,114 The flexibility of MOFs can be also be considered as the result of the various ways of coordinating ligands (mono-, bi-, and polydentate) and of their structural conformation.115 Also, a flexibility due to the rotation of the ligands in order to achieve the coordination geometry of the metal ion in the self-assembly process or due to the structural rearrangements that appear in response to the guest molecules, can be noted. MOFs are known as dynamic networks, which show the “breathing” phenomenon.116–119 This phenomenon can be described as a reversible contraction and expansion of the structure

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after the application of an external stimulus, without producing irreversible changes in the structure of the MOF.120 There are a relatively small number of dynamic arrays based on carboxylic ligands that have this feature. The best known are those with the structural formula M3O(H2O)2X (dicarboxylate)3] based on carboxylic ligands such as fumarate (MIL-88a), terephthalate (MIL-88b), 2,6-naphthalenedicarboxylate (MIL-88c), and 4-4′-biphenyldicarboxylate (MIL-88d) (where M = Cr (III) or Fe (III)), which has a strong “breathing” effect with the pore increasing in volume of 270%. Dynamic structural transformations in flexible MOFs can occur when guest molecules are removed or reabsorbed while maintaining network integrity during this process (Figure 3.4). The degree of increasing the pore size depends on the molecules adsorbed in the synthesis process. From this point of view, Ferey demonstrated a 30% reversible pore size expansion when MIL-53 (Cr) MOF is dehydrated at moderate temperatures.118,119 The unique flexibility of this MOF is mainly due to the connectivity between the network ligands and the metal oxide cluster. The O–C–O bond between the carboxylated ligand and the metal centers allows a certain degree of rotation. Beside this, the flexibility of the carbon-phenyl bond which has a certain degree of rotation, lead to a complexity of how the network adjusts during expansion and contraction.120 At the same time, structural flexibility can induce the selective adsorption of molecules, manifesting performance in separation, a difficult objective to achieve in rigid MOFs.121 The conformational flexibility of MOFs is designed from synthesis, by the rational choice of carboxylic, heterocyclic nitrogen ligands with flexible spacers in the structure or by combining them in a desired arrangement.100–114

FIGURE 3.4 Schematic representation of the evolution of the MOFs generations as a function of their stability and crystallinity and the dynamic structural transformations that occur in flexible MOFs.

3.4 MOFS FOR ENERGY APPLICATIONS After more than 20 years of the research in the field of synthesis and structural properties (porosity and surface area) of MOFs, their applicability in

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energy storage and conversion, fuel cells, batteries, or supercapacitors had a real impact on chemistry and materials science research communities and publications (Figure 3.5).

FIGURE 3.5 The publication dynamics of “MOFs for energy applications” in the last 30 years according to ISI Web of Science (January, 2022).

MOFs proved an excellent porosity which recommended them suitable in direct or as precursors and/or substrate materials for energy applications, particularly for electrochemical processes.122 Their hybrid organic–inorganic nature allowed the exploration of their potential as templates for highly porous and functionalized derivatives such as carbon, metal, or composite materials to produce electrode materials. The advantages of utilization MOFs in energy applications are: – Simple synthesis, structural characteristics in terms of pore size, volume, accessible surface area and porosity designed from the careful choice of ligands and metal ions/metal clusters, some physicochemical properties (electric conductivity, thermal stability, heteroatom doping content) making them ideal templates for nanoporous carbons.123 – The pore size and the permanent porosity achieved by adequate selection of the ligands and metal centers constitute the premise for the generation of active materials; – Bimetallic and mixed MOFs could provide rich redox reactivity due to an enhanced charge transfer between the metal centers;

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– The adjustable pore size creating well-defined internal channels can efficiently adsorb and trap molecular species; – The inner surface of MOFs can be activated supporting the interaction with specific guests and increasing the immobilization ability; – The rigid MOFs can lead to volume expansion during the redox reactions, while the flexible MOFs the breathing phenomenon can induce the increase of free void surface; – MOFs are precursors for carbon-based materials possessing meso- and macropores, tunable and convenient morphology and composition, which can be used as transport paths and reservoirs of energy storage or electrolytes. Incorporating of other materials into their porous structure or decorating their surface with active species can be also used to improve the storage capacity. – The conductivity and capacity of MOFs can be improved after pyrolyzis process favoring the electron transport, in pure state their conductivity being poor; – MOFs are precursors for metal oxides ensuring high surface area for the active centers, high porosity, and tunable composition for superior electrochemical properties and mass transport, functioning as battery-type materials.124 The main disadvantages of MOFs for energy applications are related to their poor electrochemical conductivity, network flexibility, thermal, mechanical, and chemical stability in moisture, acidic, and base media due to the nature of bonds in the structure of MOFs and size distribution of pores together with ion insertion steric hindrance. In general, the chemical stability can be improved by adequate ions and ligands, such as high-valent metals Zr4+, Al3+, Fe3+, and oxygen-donor ligands, either by direct synthesis or by postsynthetic procedures: Metal-ion metathesis or ligand exchange,123 making their design possible by controlling their structure, morphology, dimensionality, pore size, and properties. Thus, MOFs derivatives, as nanoand microstructured materials could be used in electrochemical energy conversion to produce H2 and O2 by water splitting, O2 and carbon dioxide reduction reactions with impact for a sustainable energy conversion and storage and diminishing the greenhouse gases.125 The assembly of MOFs or MOFs templates with various functional species, such as carbon nanotube, graphene, metal nanoparticles, and oxides has demonstrated remarkable potential in photo-induced H2 generation and proton conductivity. MOFs nanocrystals (ZIF-67, polyoxometalate frameworks) proved to be used in supercapacitor, by coating postsynthetic procedure with carbon, polypyrole,

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or polyaniline, showing high performance. UiO-66-NH2 embedding Pt nanoparticles showed a high efficiency in photocatalytic H2 production provided promising potential application in solar energy conversion.126 For solar cell applications, the arrangement of photoactive species is the key factor. Thus, the doping with iodine and triiodine electrolyte, immobilization of Pd(II) on MOFs (Zn-SURMOF 2), fabrication of thin layers MOFs on the conductive substrate are the main strategies to improve the semiconductive properties of MOFs which are mandatory for application in photovoltaic solar cells.127 MOFs with specific functionality and uniform distributed metal ions and high porosity can be useful in the adsorption of electrolytes improving the ionic conductivity or can host sulfur in LiS batteries, while their derived materials can be applied as anode materials in Li-ion, LiS, or LiO2 batteries and supercapacitors. Even though there are numerous studies regarding their tested performance, the current limits for their applicability at the industrial level are due to their poor stability and low conductivity.128 3.4.1 MOFS FOR HYDROGEN STORAGE AND PRODUCTION The first reference regarding the applicability of MOFs in gas storage belongs to Kitagawa in 1997. Later, Yaghi group reported on the utilization of MOFs for hydrogen storage at 77 K. The highest hydrogen uptake of 17.6 wt% was mentioned for MOF-210 with a large surface area at 77 K and 80 bar. The H2 storage has been found to depend on MOF composition, BET area, porosity, pore volume, temperature, pressure, MOF stability in moist media, and structural stability of the network during the activation process (removal of solvents). Since then, the efforts of the researcher were directed toward the increase of hydrogen storage capacity at a suitable pressure (even if high pressure is required for ambient H2 storage) and ambient temperature useful for automobile applications. To improve the H2 uptake, some strategies have been applied: Doping MOFs with Pd with a positive impact on the hydrogen adsorption energy at low pressure, with Li or other electropositive metals or with Pt on carbon catalysts which promotes the adsorption of atomic hydrogen instead of molecular hydrogen or by introducing of open metal sites, postsynthetic metal substitution, mixing with semiconductors.129 The evolution of hydrogen by water splitting in the presence of a photocatalyst was reported in 2009, when a Ru-based MOF catalyst {Ru2(bdc)2}n (bdc = 1,4-benzenedicarboxylic acid) was used together

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with a Ru-based photosensitizer under irradiation of visible light, with a quantum yield of 4.82% and a turnover number of 8.16. The strategy of impregnation of MOFs with noble-metal nanoparticles or the implantation of some functional active species in MOF network proved to enhance the evolution of hydrogen.122 In H2 production, MOFs can be used as pure photocatalysts, photosensitizers, semiconductors, or as support for loading metal nanoparticles, with some advantages over the conventional photocatalysts: – Their rational design allows the choice of the most appropriate linkers and metal centers to obtain a large cavity and pore size; – The porosity and high surface area promotes the diffusion and mobility of proton and sacrificial agent through channels increasing the photocatalytic performance; – The particularly short-charge transport pathway to active sites in MOF structure. The main difficulties of H2 production systems based on MOFs are related to their instability during the photocatalytic reaction (aqueous media) and separating problems. The use of metals with high oxidation states Zr4+, Ti4+, Al3+, and Cr3+ increased the network stability, but the H2 production efficiency remained low. A feasible method proved to be the doping with non-metal or metal ions.130 3.4.2 MOFS FOR CO2 REDUCTION The first reported MOF for CO2 reduction was UiO-67, a highly stable porous structure that contain dispersed catalytic Rhenium(I) tricarbonyl complexes in the network. Porphyrin-based MOFs catalyst, amino-functionalized MOFs (NH2-MIL-125(Ti)), and heterogenous MOFs-based catalysts showed excellent efficiency in CO2 reduction at high temperature and high CO2 pressure, while Zr-MOFs exhibited the highest catalytic activity among all MOFs.127 3.4.3 MOFs FOR SUPERCAPACITORS AND BATTERIES Supercapacitors or electrochemical capacitors can be classified in two categories. The first one is the class of carbon materials such as Carbon nanotubes, graphene, mesoporous carbon, etc. These materials are used to

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stock up electrical energy on their inner surface through electrochemical double-layer capacitors by means of electrostatic forces. Another category, represented by transition metal oxides and conductive polymers, is known as pseudo-capacitors based on fast and reversible surface redox reactions. Their remarkable performance, consisting of in power density and energy storage ability, can be achieved in materials with high conductivity and large electrolyte-accessible surface area. MOFs incorporating redox active centers and large surface can store charges through reversible redox reactions or by physisorption of electrolyte ions on their inner surfaces. Other approaches recommend the utilization of MOFs as metal oxide precursors and porous carbon-based materials with accessible internal surface area promoting the diffusion of ions and enhanced conductivity. Thus, the Co3O4 structure obtained from the calcination of Co-based MOFs at 450°C for 2 h in air proved a specific capacitance of 208 F/g of a current density of 1A/g and retention of 97% after 1000 charge/discharge cycles in potassium hydroxide solution with a concentration of 6 M. The super capacitive behavior of MOFs in non-aqueous and aqueous electrolytes was reported for Co-MOFs: Co8-MOF-5-based electrode, where Zn ions were partially substituted by Co ions and electrode composition was obtain by mixing carbon black with polytertrafluoroethylene and Co-MOF-71, respectively. Low-electrical conductivity and the stability of the frameworks during redox processes are the main factors affecting the supercapacitive performance of MOFs.123,131 Recently, MOFs also have been investigated as anode, cathode, and electrolyte materials for Lithium-ion batteries and fuel cells. The redox active metal centers in their structure can serve as redox active sites in electrochemical processes. Hollow porous MOFs including Zn, Mn, Ni, and Co metals and templates were tested for their electrochemical performance to increase the reversible capacity and Li-ion diffusivity in conversion reactions, ensuring the long-term cyclability of Li ions. Metal-air batteries and fuel cells showed a real potential in energy technology applications. Fe-based porphyrin loaded on pyridine-functionalized graphene with large porosity and fast charge transfer showed high catalytic activity in electrocatalytic oxygen reduction reaction and could be used as cathode in alkaline methanol fuel cells. The main difficulties in application MOFs crystals as oxygen reduction electrocatalyst are due to their structural instability, which can affect the pore stability during the carbonization processes, and to the low conductivity, which diminishes the electron transfer process. Highly active electrocatalysts based on MOFs derivatives (MOFs precursors and MOF composite precursors) can be achieved by increasing the intrinsic activity

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and the mass density of the active sites, the electron transfer rate, and their durability. The high activity against oxygen reduction reaction was proved for Co, Fe, Cu, and Ni active sites in combination with heteroatoms (N, S, P, B)-based ligands.132 3.5 PERSPECTIVES In spite of promising results, the stability of MOFs in humidity conditions has been recognized as one of the key limitations for large-scale application in practical applications. Prototypical materials such as MOF-5 (containing tetrahedral Zn(II) SBUs) or HKUST (Cu(II) SBUs connected by 1,3,5-benzenetricarboxylate units) are prone to degradation with moisture due to the hydrolysis of weak metal-oxygen coordination bonds leading to the replacement of linkers with water molecules. This problem can be solved by direct synthesis of hydrophobic MOFs or by endowing the framework with stronger coordination bonds for more robust SBUs by using basic nitrogen-donor ligands or highly charged metals such as Zr(IV) or Ti(IV), often by metal-ion exchange metathesis. MOFs crystals can be efficiently protected from hydrolytic degradation by one-step encapsulation in hydrophobic coatings of carbon, polydimethylsiloxane (PDMS), or organic polymers. These examples highlight the importance of finding simple, general alternatives that permit stabilizing water-sensitive MOFs at high humidity levels while preserving their original structure and porosity, preferably at a single crystal level.133 In contrast with MOFs reported in the literature, the use of ligands containing phenyl-, or diphenylsilane units in the structure is an original approach to design novel MOFs with conformational flexibility (V-shaped ligands). It is expected that the presence of the silane spacers in such structures to give them some properties of interest such as Solubility (useful in processing the obtained materials), hydrophobicity (of interest for moisture stable MOFs), flexibility (“gate-opening pressure” MOFs of interest for gas storage). Among others, it is expected that Si–C bond longer in the linker (about 1.90 Å) as compared with C–C (around 1.5 Å) to allow a larger flexibility in structure and properties. There is a small number of publications in which silicon-containing carboxylic acids are used to prepare MOFs.134–138 In this context, our efforts have been directed toward the development of strategies for obtaining MOFs based on silane/ siloxane ligands which will provide both flexibility and hydrophobicity (due to dimethylsilane or dimethylsiloxane sequences) (Figure 3.6).139–142 Beside these characteristics, their intrinsic hydrophobicity allowed their

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application as heterogenous catalysts in alkaline decomposition of hydrogen peroxide and in photodecomposition of azo dyes (Congo Red) and some drugs (Doxorubicin) under sunlight, in ambient conditions,143,144 or as homogenous precursor catalysts in the electrochemical reduction of protons (for hydrogen production).

FIGURE 3.6 Approaches in the design of stable and hydrophobic MOFs based on original silane/siloxane-based ligands developed by author.

3.6 CONCLUSIONS Metal-organic frameworks (MOFs) and MOFs-based derived materials (metal oxide and carbon materials precursors) have developed as a new generation of advanced functional materials possessing unique characteristics and remarkable performances. To date, a variety of MOFs has been obtained with tailorable structures and composition, morphologies, porosity, and functionality. In this chapter, an overview of the evolution of MOFs structures and properties for energy applications was presented. MOFs materials showed promising perspectives as hydrogen storage devices, photo-catalysts for hydrogen production and CO2 reduction, capacitive and battery-type electrode resources. The main challenges for energy applications still remained

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the design of MOFs with specific structural characteristics to enhance their thermal, mechanical and physicochemical stability, and conductivity. Specific approaches to reach them by tuning the structure and functionality of ligands and the redox active metal ions, the pore size and specific surface area, the postsynthetic functionalization and structural modifications, doping with heteroatoms have been developed. A high electrical conductivity, the open channels in the structure or mixing MOFs with conductive materials are required for their use as supercapacitors to allow rapid electron transfer to the active site and efficient ion transport. For efficient, stable, recyclable and strong photocatalysis of hydrogen production, and carbon dioxide reduction, it is necessary to incorporate active particles in MOFs structure. As an emerging class of crystalline organic–inorganic porous materials, more rational and common approaches to control the structure-properties relationship will lead to enhanced performances of MOFs for more durable, eco-friendly energy storage, and conversion applicability. KEYWORDS • • • •

metal-organic frameworks porous materials energy applications MOFs-based derivatives

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110. Jiang, H.; Ma, J. F.; Zhang, W. L.; Liu, Y. Y.; Yang, J.; Ping, G. J.; Su, Z. M. Metal– Organic Frameworks Containing Flexible Bis(benzimidazole) Ligands. Eur. J. Inorg. Chem. 2008, 745–755. 111. Li, S. L.; Lan, Y. Q.; Ma, J. Q.; Ma, J. F.; Su, Z. M. Formation of Three New Silver(I) Coordination Polymers Involving 1,2-Phenylenediacetic Acid via the Modulation of Dipyridyl-Containing Ligands. Cryst. Growth Des. 2010, 10, 1443–1450. 112. Hao, J. M.; Zhao, Y. N.; Yang, R.; Cui, G. H. Self-assembly of Three d10 Metal Coordination Polymers Based on a Flexible Bis(2-methylbenzimidazole) and Dicarboxylate co-Ligands. J. Mol. Struct. 2014, 1070, 58–64. 113. Jiang, L.; Li, Z. X.; Wang, Y.; Feng, G. D.; Zhao, W. X.; Shao, K. Z.; Sun, C. Y.; Li, L. J.; Su, Z. M. Structures and Fluorescence Properties of Two Novel Metal-Organic Frameworks Based on the Bis(2-benzimidazole) and Aromatic Carboxylate Ligands. Inorg. Chem. Comm. 2011, 14, 1077–1081. 114. Zhou, Z. Y.; Liu, W. N.; Liu, F. F.; Fan, X. J.; Zhan, X. L.; Zhuang, L. D.; Ma, G. Z.; Cai, Y. P. Two 2-D 4-Connected Lanthanide Coordination Framework Based on Benzimidazole-5,6-dicarboxylate and Acetate Mixed Ligands. Inorg. Chem. Comm. 2010, 13, 1580–1584. 115. Zhang, W. H.; Dong, Z.; Wang, Y. Y.; Hou, L.; Jin, J. C.; Huang, W. H.; Shi, Q. Z. Synthesis, Structural Diversity and Fluorescent Characterisation of a Series of d10 Metal–Organic Frameworks (MOFs): Reaction Conditions, Secondary Ligand and Metal Effects. Dalton Trans. 2011, 40, 2509–2521. 116. Yang, C.; Wang, X. P.; Omary, M. A. Crystallographic Observation of Dynamic Gas Adsorption Sites and Thermal Expansion in a Breathable Fluorous Metal–Organic Framework. Angew. Chem. Int. Ed. 2009, 48, 2500–2505. 117. Volkringer, C.; Loiseau, T.; Guillou, N.; Ferey, G.; Elkaim, E.; Vimont, A. XRD and IR Structural Investigations of a Particular Breathing Effect in the MOF-type Gallium Terephthalate MIL-53(Ga). Dalton Trans. 2009, 2241–2249. 118. Mellot-Draznieks, C.; Serre, C.; Surble, S.; Audebrand, N.; Ferey, G. Very Large Swelling in Hybrid Frameworks: A Combined Computational and Powder Diffraction Study. J. Am. Chem. Soc. 2005, 127, 16273–16278. 119. Millange, F.; Serre, C.; Ferey, G. Synthesis, Structure Determination and Properties of MIL-53as and MIL-53ht: the first CrIII hybrid inorganic–organic microporous solids: CrIII(OH)·{O2C–C H –CO2}·{HO2C–C6H4–CO2H}x. Chem. Commun. 2002, 822–823. 6 4 120. Serre, C.; Bourrelly, S.; Vimont, A.; Ramsahye, N. A.; Maurin, G.; Llewellyn, P. L.; Daturi, M.; Filinchuk, Y.; Leynaud, O.; Barnes, P.; Ferey, G. An Explanation for the Very Large Breathing Effect of a Metal–Organic Framework during CO2 Adsorption. Adv. Mater. 2007, 19, 2246–2251. 121. Li, J. R.; Sculley, J.; Zhou, H. C. Metal–Organic Frameworks for Separations. Chem. Rev. 2012, 112, 869–932. 122. Wang, H.; Zhu, Q.-L.; Zou, R.; Xu, Q. Metal-Organic Frameworks for Energy Applications. Chem. 2017, 2, 52–80. 123. Ajdari, F. B.; Kowsari, E.; Shahrak, M. N.; Ehsani, A.; Kiaei, Z.; Torkzaban, H.; Ershadi, M.; Eshkalak, S. K.; Haddadi-Asl, V.; Chinnappan, A.; Ramakrishna, S. A Review on the Field Patents and Recent Developments over the Application of Metal Organic Frameworks (MOFs) in Supercapacitors. Coord. Chem. Rev. 2020, 422, 213441. 124. Wang, D.-G.; Liang, Z.; Gao, S.; Qu, C.; Zou, R. Metal-Organic Framework-Based Materials for Hybrid Supercapacitor Application. Coord. Chem. Rev. 2020, 404, 213093.

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143. Racles, C.; Zaltariov, M. F.; Iacob, M.; Silion, M.; Avadanei, M.; Bargan, A. SiloxaneBased Metal–Organic Frameworks with Remarkable Catalytic Activity in Mild Environmental Photodegradation of Azo Dyes. Appl. Catal. B 2017, 205, 78–92. 144. Racles, C.; Zaltariov, M.-F.; Silion, M.; Macsim, A.-M.; Cozan, V. Photo-Oxidative Degradation of Doxorubicin with Siloxane MOFs by Exposure to Daylight. Environ. Sci. Pollut. Res. 2019, 26, 19684–19696.

CHAPTER 4

Carbon Nanomaterials for Energy Applications M. V. SANTHOSH1,2, R. GEETHU1, R. DIVYA2, N. RAGESH2, and SAM JOHN3 Department of Basic Science and Humanities, SCMS School of Engineering and Technology, Karukutty, Ernakulam, India

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School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India.

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Department of Chemistry, St. Berchmans College (Autonomous) Campus, Mahatma Gandhi University, Kottayam, India

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ABSTRACT In this chapter we have reported the most recent applications of carbon nanotubes for different energy devices and energy sectors. The main objective is to introduce practical solutions for pollution control of our living environment. 4.1 INTRODUCTION Greenhouse gas emission that leads to global warming and air pollution is the major threat that is facing the mankind as a result of worldwide consumption of nonrenewable energy resources (like fossil fuels). It is high time to focus the research on production of renewable energy and storage systems. In this context, supercapacitors, solar cells, fuel cells, and lithium-ion batteries are the major candidates that can ensure sustainability and energy stability for all aspects of life. Extensive researches are going on to use carbon nanomaterials Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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with the intention to improve the efficiency of renewable energy production and storage systems as mentioned above. Carbon is one of the most abundant materials found on earth and its allotropic forms like carbon nanotubes (CNTs), graphene, fullerene, carbon nanofibers, etc. offer ease of production and possess excellent physical, chemical, and electrical properties.1–3 Carbon nanomaterials have been explored for photovoltaic devices due to their unique optoelectronic properties like superior capacity toward photon absorption and generation of charge carriers, high stability, material abundance, good transparency, and high conductivity. Moreover, carbon nanomaterials are lighter and cheaper than conventional solar cell materials and favors flexible photovoltaic devices. For energy storage systems like supercapacitors and lithium-ion batteries, carbon nanomaterial offers their ability to store charges with good electrical and mechanical properties, favorable pore size distribution and excellent specific surface area. In fuel cell, carbon nanomaterials generally function as catalyst supporting materials mainly due to their good thermal conductivity, high resilience, and exceptional surface area.1–3 Carbon nanotubes (CNTs) are introduced to the world by a Japanese physicist Sumio Iijima in 1991. They are very small-sized (nanometer) carbonbased material possessing tube-like (hollow cylinder) appearance. Depending upon its winding they are of different types, single-walled CNTs (SWCNTs), double-walled CNTs (DWCNTs), and multiwalled CNTs (MWCNTs).4,5 Andre Geim and Konstantin Novoselov, Russian-born scientists were the discoverers of graphene, a 2D sheet structure containing sp2 hybridized tightly packed carbon atoms. Fullerene was discovered by Harold W. Kroto, Richard E. Smalley, and Robert F. Curl in 1985 (Novel Prize in 1996).6 4.2 CARBON NANOMATERIALS FOR SOLAR CELL APPLICATION Solar energy is the most reliable source of energy and has the potential to solve all the issues regarding world energy crisis. The device that converts solar energy or light energy to electrical energy is called a solar cell or photovoltaic device. The main advantages of solar cells are: • • • • •

Clean green energy; environment friendly Source of energy—SUN—The Ultimate Source Can produce scalable energy (mW to MW) They have no moving parts and hence require little maintenance Decentralized

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• Silent, produce no noise at all • Direct production of electricity • Low cost for operation and maintenance compared to other renewable energy systems. Research in the field of photovoltaics initiated many years back using Si materials and currently is going intensively with wide varieties of materials. Depending upon the material, solar cell could be classified as follows (Highest efficiency reported while writing this article11): 1. Silicon-based solar cells (26.6 ± 0.5%) 2. Thin-film chalcogenide solar cells (CdTe: 22.1 ± 0.5%; CZTSSe: 12.6 ± 0.3%) 3. Dye-sensitized solar cells (12.25 ± 0.4%) 4. Organic solar cells (17.35 ± 0.2%) 5. Perovskite solar cells (25.2 ± 0.8%). Intensive researches are going on to enhance the device efficiency and different types of materials are incorporated to enhance stability, charge transport, reduce cost, etc. Carbon nanomaterials are one among them. As mentioned in the introduction part, these carbon nanomaterials are of different types such as carbon nanotubes, carbon nanofibers, carbon nanodots, graphene, fullerene, etc. And each material plays different roles in different types of solar cell. A brief description about the role of each carbon nanomaterial in different solar cells with the support of reported research works regarding the same in the literature is given below. 4.2.1 CARBON NANOMATERIALS IN SI-BASED SOLAR CELLS Silicon solar cells are the most common and highly commercialized solar cells fabricated using crystalline silicon or polycrystalline silicon materials. Even though Si-based solar cells are the highly commercialized one yet, its cost is unaffordable for a common man. This led to the innovations and research on developing other low cost materials suitable for solar cell application. As a result, wide varieties of materials possessing high potential for solar cell application were developed. A way to reduce the material cost of Si solar cell is to incorporate nanomaterials in the cells. One among the most common nanomaterial used in Si-based solar cell is carbon nanotube (CNT). Better stability, higher carrier mobility, tunable conductivity and band gap, and flexibility are the advantages of CNT to use in Si-based solar

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cells.12 A brief review of the heterojunction fabrication using CNTs and Si is given below. In the initial stage of this work, attaining Schottky device due to the formation of metal–semiconductor junction was reported by many researchers. CNTs were chosen as counterpart of silicon nanowire to form a junction and obtained a rectifying behavior leading to the conclusion of metal–semiconductor Schottky formation.13 An ohmic behavior was reported by Kawano et al. for a device with multiwalled carbon nanotube sandwiched between two silicons.14 Utilizing SWCNT for junction formation with Si, Schottky contact was reported by Behnam et al. in 2008.15 They explained the transport mechanism and electronic properties exhibited by junction formed between CNT and Si. Other similar reports on heterojunction between DWCNT and Si were given by Wei et al. and Jia et al.16,17 Tzolov et al. reported this heterojunction using vertically aligned CNTs.1,18 First report on photovoltaic device using CNT–Si heterojunction by Wei et al. in the year 2007 yielded device efficiency of 1.31%.19 In course of time, different works on CNT–Si heterojunction were reported. Improvement in device performance by combining CNTs with organic and inorganic materials was also carried out. Enhancing areal density was the aim behind the incorporation of organic materials in CNTs/Si heterojunction.20 Fan et al. used pristine SWCNT and composite film of SWCNT and PEDOT:PSS to fabricate solar cell, and enhancement in efficiency from 6.1 to 10.2% was achieved. But the device stability was very low.21 Another organic material used was polyaniline (PANI) which yielded an efficiency of 7.4%.22 To enhance the stability of these devices some other insulating polymer materials were introduced such as polymethyl methacrylate (PMMA),23 polydimethylsiloxane (PDMS),24 and polystyrene (PS).25 Along with stability this layer also aids in absorption of light. These types of devices attained an efficiency of 11.5%.25 Stability and carrier mobility of inorganic materials help them to act as hole-transporting materials in CNT/Si heterojunction. CNT/Si heterojunction with highest efficiency of 17% was reported by Wang et al. in 2015 by incorporating a metal oxide layer (MoOx). Apart from transport of charge carriers, metal oxide layer functions as an antireflective coating layer as well as carrier dopant for single-walled CNTs.26 Another report revealing better efficiency of solar cell was reported recently by Hu et al. It explained the synthesis of small-bundle SWCNTs for the better photovoltaic performances. Floating catalyst chemical vapor deposition (FCCVD) was the method chosen to synthesize low-resistive, highly transparent and high quality SWCNT. They attained photovoltaic power conversion efficiency of 14.2%.27

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4.2.2 CARBON NANOMATERIALS IN THIN-FILM CHALCOGENIDE SOLAR CELLS Thin-film solar cells can also be called as second generation solar cells and they make use of semiconducting thin-film materials possessing suitable optoelectronic properties for fabricating solar cells. Basic structure of a thin-film solar cell comprises of a transparent conducting oxide as the back contact (ITO/FTO), n-type buffer layer, p-type absorber layer, and a top metal contact. Commonly used absorber materials for thin-film chalcogenide solar cell are copper indium gallium selenide, copper indium gallium sulfide, copper zinc tin sulfide, copper indium selenide, copper indium sulfide, cadmium telluride, gallium arsenide, etc. Comparing to other type of solar cells, application of carbon nanomaterials are not so established. However, graphene founds application in thin-film solar cells as contact material. Graphene seeks contact material application in thin-film solar cell because of its high carrier mobility, flexibility, and high transparency. Bi et al. reported a CdTe device with graphene as the contact electrode. The device utilized ZnO layer in contact with graphene and yielded an efficiency of 4.17%.28 The device was modified with a 3D-structured graphene and yielded a device with enhanced PCE of 9.1%.29 Combination of Cu nanowire with graphene as back contact was reported by Liang and coworkers for CdTe device and the efficiency obtained was 12.1%.30 Single-walled CNTs have higher charge carrier mobility than materials used for thin-film solar cells.31 They possess high optical absorption in the infrared and near-infrared regions. Moreover, SWCNT and MWCNT possess higher chemical stability than many thin-film absorber materials.32,33 These properties of CNT made the researchers to utilize them in thin-film solar cells. But the output obtained was lower when compared to the theoretically suggested one. Simulation works are reported regarding its capability of CNT in CZTS devices, but not yet achieved for practical devices. 4.2.3 CARBON NANOMATERIALS IN DSSC-BASED SOLAR CELLS Dye-sensitized solar cells (DSSCs) is a category of thin-film solar cells possessing advantages like cost effective, easy to develop, less negative environmental impacts, etc.34,35 Researches in the field of DSSC started two decades back. Basic layer of DSSC consists of working electrode, dye, electrolyte, and counter electrode. Sharma et al. describe DSSC as “an assembly of working electrode soaked with a sensitizer or a dye and sealed

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to the counter electrode soaked with a thin layer of electrolyte with the help of a hot melt tape to prevent the leakage of the electrolyte.”36 The function of a counter electrode is to convert triiodide into iodide, to complete operation cycle in the electrotyle.37 Highly precious Pt electrodes were used conventionally in DSSC because of its fast electron transferring nature and amazing catalytic ability. But comparing to the cost of Pt material required for the device, efficiency offered by the device using Pt is low. Hence, replacing Pt with a suitable electrode was the primary aim of the researchers to reduce the cost of DSSC. Thus, the use of carbon nanomaterials in DSSC emerged. 4.2.3.1 CARBON NANOTUBES IN DSSC The cost-effective nature and the catalytic activity for I3– reduction made CNT a potential candidate to be a counter electrode in DSSC. Ma et al. describes that CNT–TiO2 combination enhance electron diffusion coefficient and lifetime of electron. Also, CNT possesses the capability to increase PCE in DSSC on addition to electrolyte.37 Research works on platinum–CNT combination like Pt/MWCNT, Pt/ SWCNT, and CNT–Pt were reported by Lee et al. in 2010, Xiao et al. in 2011, and Im et al. in 2012 respectively shows enhanced charge transfer and electron life time.38–40 CoS/multiwalled carbon nanotubes was another way of incorporation of CNT as counter electrode in DSSC. This was reported by Lin et al.,41 whereas Peng et al. reported a nanocomposite of polypyrrole and MWCNT for DSSC electrode.42 Another combination with polyaniline was also reported and revealed a good catalytic activity.43 In another context, CNT functions as anode when combining with TiO2, by enhancing photocatalytic and photoelectric conversion efficiency. Reports by Yu et al. (CNT/TiO2), Muduli et al. (TiO2-MWCNT), and Yen et al. (MWCNT-TiO2 nanocomposite) are the examples.44–46 Addition of CNT to electrolyte was reported by Hong et al. to increase the conversion efficiency of the device. The report describes the single-walled CNTs and mutiwalled CNTs for the said application. Many more research works are also reported in this field.47–49 4.2.3.2 CARBON NANOFIBERS IN DSSC Carbon nanofibers (CNFs) owing to its high conductivity and electrochemical stability seek its application in the field of DSSC as cost-effective electrode.50,51

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Many reports are seen in the literature dealing with the studies regarding replacing platinum (Pt) with CNF and modifying CNFs’ structure to attain efficiency as that of devices using Pt electrode. Joshi et al. initially chose CNFs as counter electrode for DSSC but the performance was lower. Highly porous structure of CNFs and its composites were developed, favorable for the 1-D conducting path for I–/I3– ions.52 Studies by Saranaya et al. focused on preparing (Fe–Ni)-embedded CNFs using electrospinning. The authors claim that the device efficiency was 4.7%, which is comparable to that obtained for standard platinum electrode. This could be attributed to the enhanced charge transfer rate, improved electrochemical stability, electrocatalytic activity, etc. of the device utilizing (Fe–Ni)-embedded CNFs.53 Similar works dealing with replacing Pt electrodes by CNF are reported by Yousef et al. in 2013, Aboagye et al. in 2015, Mohamed et al. in 2016, etc.54–56 Park et al. reported an electrospinned hollow activated CNF as DSSC counter electrode. The authors claimed that thus fabricated device performance was comparable with that of the Pt electrode.57 4.2.3.3 CARBON NANODOTS IN DSSC Carbon nanodots are, as its name suggest, nanosized carbon particles (200,000 cm−2 V−1 s−1 at an electron density of 4 × 109 cm−2), exceptional transmittance (>97%), large specific surface area (2600 m−2 g−1), etc.67–72 The application of graphene in DSSC varies as a photoanode, semiconductor, and sensitizer (because of its good absorption). Zhang et al. explains the cathode application of graphene due to its high electrocatalytic activity and excellent conductivity.72

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First report on graphene-based solar cell in which graphene acts as transparent conducting layer instead of FTO was reported by Wang and coworkers in 2008.73 Application of graphene as counter electrode for DSSC by Huang et al. reported an efficiency of 4.25%. They claimed that the value is comparable to the device using FTO.74 As a semiconductor, graphene/TiO2 used as interfacial layer between FTO and nanocrystalline TiO2 film was reported by Kim et al.75 The device with this interfacial layer performed better, yielding an efficiency of 5.26% (without interfacial layer it was 4.89%). The authors confirmed improved adhesion of FTO and TiO2 nanocrystalline layer due to the lower roughness of interfacial layer.75 Chen et al. reported an enhancement in device efficiency from 5.80 to 8.13% on incorporating graphene as photoelectrode.76 Graphene as a sensitizer is reported by Zhang et al., and was possible because of its strong absorption profile.77,78 Cathode application of graphene is also unavoidable. Apart from using graphene alone as cathode for DSSC, a combination of graphene with metal, carbon nanotube, polymer, etc. for DSSC cathode are reported. 4.2.4 CARBON NANOMATERIALS IN ORGANIC SOLAR CELLS (OSC) Organic photovoltaics (OPV) or organic solar cells utilize semiconducting π-conjugated polymers for fabricating solar cells. These π-conjugated polymers were developed in 1977 by Alan G. MacDiarmid, Alan Heeger, and Hideki Shirakawa and they won Nobel Prize in 2000. Discovery of C-60 by Kroto et al. in 1985 and report on “transfer of electron from conjugated polymer to C-60 molecule by photoexcitation on composite of conjugated polymer and C-60” by Sariciftci et al. in 1992 are the major stepping stones in the field of OPV. First heterojunction in this field was reported by Sariciftci in 1993. From then onwards intensive research works are going on to develop more efficient and stable OPVs. As in all cases, different carbon materials were included in OPVs to enhance the performance. Among them fullerene is the most important one. Fullerene can be called as “Star of Photovoltaics” (taken from title of a review article by Collavini et al.79). 4.2.4.1 FULLERENE IN OSC Efficient charge transfer is the highlight feature of fullerene and its derivatives, to be an efficient counterpart of organic photovoltaics. Charge mobilities of these material are ~10–4–10–3 cm2/(V s)–1.80 Electron-accepting ability along

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with the ability to slow down charge recombination in dark make the material more attractive in OPV.81 Apart from C60 fullerene (pentagons-12, hexagons-20), C70, C74, C76, C78, C80, C82, and C84 are the higher fullerenes.82 Fullerene in its pure form is not suitable for organic solar cells due to its poor solubility and less absorption, and hence it was modified in different ways like aryl group modifications, alkyl chain length modifications, end (ester) group modifications, etc. These modifications, the detailed development of different fullerene derivatives, and its applications in solar cells were described by Ganesamoorthy et al.82 Yu and coworkers introduced PC61BM with highly soluble nature which was used as an acceptor. Besides, it exhibits good electron mobility, stability, and low-lying LUMO levels.83 An example of modification in PC61BM was reported by Kim et al. in 2014 and the authors explained the development of NC61BM, AC61BM, and PyC61BM; and was then applied as the acceptor in a bulk heterojunction with P3HT as the donor. The power conversion efficiencies obtained for the devices were 3.80% for PC61BM, 4.09% for NC61BM, 1.14% for AC61BM, and 1.95% PyC61BM.84 In 2016, Huang et al.85 synthesized methoxylated 1,4-[60] fullerene bis-adducts (1a–1k), and were applied as acceptors with P3HT and PtB7. They possessed higher LUMO levels compared to PC61BM than the standard PC61BM and used it as acceptors in BHJ-OSCs with P3HT and PTB7 polymers as donors. A 20% enhancement in efficiency (PCE~5%) compared to PC61BM was contributed by 1e and 1i samples, and all others yielded an efficiency between 2–3.4%.85 In our report in 2015 and 2019, hybrid solar cells were fabricated using ZnO as the electron transport layer, and P3HT:PC61BM as active layer. Those devices were fabricated in open air condition, without using glove box and they exhibited a PCE of 1.62%.86,87 Most recently, Wagalgave and coworkers reported a bulk heterojunction using new organic semiconductors indoline and naphthalene diimide. The PCE of the devices when blended with PC71BM was 6.71%.88 4.2.4.2 GRAPHENE IN OSC High specific surface area of graphene material (already mentioned in section 4.2.4.) along with its strength, and inertness towards O2 and water vapor find its way to be a part of organic solar cell.89 Graphene plays different roles in OPVs. One among them is graphene as window electrode reported by Wu et al.90 The device exhibits PV parameters of Jsc, Voc, FF, and η of 0.36 mA/cm2, 0.38 V, 0.25%, and 0.29%. This device possesses configuration as graphene/

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copper phthalocyanine (CuPc)/C60/bathocuproine (BCP)/Ag. Even though efficiency was lower than device using ITO, its cost was very low compared to ITO device. Liu et al.91 reported the potential of functionalized graphene as a new acceptor. Device configuration was ITO/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/P3HT/graphene/LiF/Al.91 Graphene as a stability enhancer, when PCBM is doped with graphene, is reported by Bi et al. CNT also possess application in the field of OSC. Recently, Subhramanyam92 reported a work dealing with different applications of CNT in bulk heterojunction. Basic device configuration was ITO/PEDOT:PSS/ P3HT:PCBM/Al. Modifications were done by (1) replace PEDOT:PSS by CNTs/PEDOT:PSS composite, (2) replace P3HT:PCBM by CNTs/ P3HT:PCBM composite, and other by both 1 and 2. Devices utilizing CNT performed better and yielded a better power conversion efficiency as well as stability.92 4.2.5 CARBON NANOMATERIALS IN PEROVSKITE SOLAR CELLS One among the new generation solar cells is Perovskite solar cells (PSCs). It’s only about one decade that these perovskite material finds application in photovoltaics. Perovskite is a type of mineral and its structure represents a compound or anything possessing ABX3 form. Perovskite-based solar cells exhibit a high PCE even in its blooming stage and its PCE increases to 25.2% (at the time of writing) within a few years.93 Along with this, utilization of incident energy (~70%) is higher for PSC type solar cells.94,95 Basically a PSC comprise of a transparent electrode, electron transport layer, perovskite layer, hole transport layer, and a metal electrode. 4.2.5.1 CNT IN PSC Carbon nanotube, because of its advantages like stability, p-type nature, etc. is the best suitable material to be a hole transport in PSC. Mechanical flexibility of CNTs enable them to be part of flexible PSCs. Comparing to other hole-transporting materials like polymer or small molecules, CNTs does not require doping due to its mechanical and chemical stability.96 Snaith and his group conducted a series of studies based on perovskite solar cells with and without single-walled carbon nanotubes (SWCNTs) and results showed that charge extraction is faster in the device containing nanotubes.

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Also polymer wrapping SWCNTs enhance the potential of material to be functional as effective hole trapping layer.97–100 Jeon et al. reported a work in which CNTs were replaced for metal and metal oxide electrodes, so that they function as both cathode and anode for PSC.101 4.2.5.2 GRAPHENE IN PEROVSKITE Recently, in Greece (June 2020), worlds first graphene-based perovskite solar farm was established. The device yields a maximum electrical capacity of 261 Wp having area of 4.5 m2 with nine panels (graphene–perovskite). Mixed triple cation lead halide perovskite is the active layer of the device and it has graphene and two-dimensional materials like MoS2 to enhance the stability and performance.102 As mentioned earlier, exceptional properties of graphene materials, high charge carrier mobility, high optical transparency, and high flexibility, etc. enable this material to play different roles in perovskite solar cells. Reports showed that graphene have the potential and capability to act as (1) carrier transporting materials, (2) conducting electrodes, and (3) stability enhancer. Graphene as Carrier Transporting Material: Agresti et al. in 2016103 reported solar cell efficiency of 18.19% for device utilizing graphene-doped mesoporous TiO2 as an electron transporting photoelectrode. The device structure was FTO-coated glass substrate/compact TiO2/graphene-doped mesoporous TiO2/MAPbI3/graphene oxide/Spiro-OMeTad/gold. A better charge carrier transfer was demanded as a reason for device efficiency. Snaith et al.104 exploited mixture of nanoflakes of graphene and TiO2 nanocomposite to yield 15.6% device efficiency. The device efficiency in its pure form that is, without TiO2 was only 10%. Mixture of nanoflakes of graphene and TiO2 nanocomposite was treated as the electron selective contact layer (ESCL). Another device with configuration glass/fluorine-doped tin oxide (FTO)/TiO2/ graphene-SrTiO3/MAPbI3/Spiro-OMeTAD/Ag was reported by Wang et al.105 This device exhibited an efficiency of 10.49%, whereas the device without graphene had an efficiency of 6.85%. Enhancement in the transfer rate of electron was claimed as the reason for this betterment in the efficiency.105 Graphene as conducting electrode: You et al.106 was the first to report the application of graphene as a conductive electrode for PSC. Graphene was produced by chemical vapor deposition method and the device utilizing this graphene yielded an efficiency of 12.37%. Application of graphene as bottom anode was reported by Choi et al.107 A double layer graphene with the help of MoO3 achieved an efficiency of 17.1%.107 Then this structure was

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reproduced on a flexible substrate leading to the device with configuration (PEN/-graphene/MoO3/PEDOT:PSS/MAPbI3/C60/BCP/LiF/Al) possessing efficiency of 16.8% without any strain. Efficiency of the device retains within 90% of initial value even after bending 1000 times.108 This opens another application of graphene as conductive electrode. Extension hole of diffusion length by using graphene anode doped with gold chloride (AuCl3) was also reported.109 Graphene as Stabilizer Material: Stability of perovskite solar cell is the major factor that hinders it from commercialization. The moisture content causes the perovskite material to decompose. This could be overcome by supplying additional layer atop perovskite layer or by doping. Graphene is the best remedy to solve this issue because of its hydrophobic nature.110 As a result, the interaction between the perovskite layer and the moisture could be eliminated. Also Bi et al. reported that the graphene layer over perovskite is able to block halide ion diffusion from perovskite to top electrode.111 Jeong et al. reported ability of graphene to prevent halide diffusion from perovskite to electrode as well as it prevents metal-induced degradation. Along with this chemical stability, those devices exhibit mechanical stability too.112 Recently, Mahmoudi et al. reported a highly stable PSC using Perovskite/ NiO-graphene photoactive composite and NiO interface layer, yielding a higher current density of 25.9 mA/cm2 and PCE of 20.8%. Even though the device without the above said composite shows good output, stability was a major issue. After incorporating Perovskite/NiO-graphene photoactive composite and NiO interface layer, the device exhibits a good thermal, air and photo stability for 310 days by about 97–100%.113 Bi et al. and Hu et al. reported the enhancement in the device stability while utilizing graphene layer.110,111 4.2.5.3 FULLERENE IN PSC Fullerene and its derivatives are easy solution processable and have an excellent electron-transporting property. They possess appropriate energy level alignment with perovskite.114,115 These qualities made them a suitable candidate as ETM, stabilizer, electron acceptors, interfacial modification layer, cathode buffer layer, etc. Recently, Ahmad et al. reported a new fullerene derivative PCBC6 for flexible PSC (on polymer foil) as an n-type material and the device yielded power conversion efficiency of 18.4% (active area of 1 cm2).116 First report

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on fullerene C-60 and its derivatives used as ETMs for PSC was by Jeng et al. (in 2013)117 resulting in an efficiency of >3%. In PSC, fullerene was used as ETM, interface modification of ETMs or as additives118–121 so as to suppress the hysteresis by passivating charge traps. This provides the device a better performance. Also as in the case of graphene and CNTs, here fullerene acts as a stabilizer by blocking moisture from perovskite layer.122 Apart from PC61/71BM, other fullerene derivatives like DMEC60 and DMEC70 are used in PSC as ETMs. Devices utilizing DMEC60 and DMEC70 yield higher efficiency and better stability than PC61/71BM-based devices.123,124 Tian et al. first tested and reported the possibility of PC61BM-dimer, (D-C60) to act as ETM in inverted PSCs and yielded a device with enhanced efficient and improved stability compared to PC61BM device. Photoluminescence studies and electrochemical impedance measurements reveal that better electron extraction and transportation along with surface passivation is the reason for enhanced performance.125 Interfacial modification using fullerene is a best method to suppress hysteresis by decreasing nonradiative recombination which further led to enhancement in solar cell performance.126–128 4.3 CARBON NANOMATERIALS IN SUPERCAPACITOR APPLICATION World is in search of ecofriendly technologies for generating and storing energy and that can ensure sustainable growth of mankind. Supercapacitors are outstanding candidates for future energy storage device as it features long life cycle, environment friendliness, high energy density compared to normal capacitor, excellent power density, and reliability compared to battery.1–3,129,130 Similar to other capacitors, supercapacitors consist of two solid/porous electrodes separated by an ion-permeable membrane (separator) and an electrolyte ionically connecting both electrodes. When the plates are charged up, an opposite charge forms on either side of the separator, creating an electric double layer. Unlike normal capacitor, supercapacitor stores charges at the interfacial region between the electrolyte solution and the electrode. Owing to unique hierarchical structure, excellent electrical, mechanical, and thermal properties, high specific surface area, and remarkable chemical stability, carbon nanomaterials like carbon nanotubes (CNTs), graphene, mesoporous carbon, carbon nanofibers, and carbon nanomaterial composites have been widely studied as efficient electrode materials in supercapacitors.1–3,129,130 Schematic representation of a supercapacitor cell is shown in Figure 4.1.131

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FIGURE 4.1

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Schematic representation of a supercapacitor cell.

Among various carbon nanomaterials, CNTs are prospective candidates for supercapacitors owing to their outstanding electric properties, nanoscale texture and porosity, remarkable specific electrical conductivity, low cost, surface area, and chemical stability. Compared to conducting polymers, CNT-based supercapacitors possess relatively lower energy density and lesser specific capacitance. Hence, there are many attempts to modify the carbon nanotubes by conducting polymers. High yield was obtained for PEDOT/ MWCNTs composite prepared by in situ chemical oxidation polymerization by Bai et al. Energy density of grafted composite was 11.3 Wh/kg and presented 85% retention capability after 1000 cycles.132 Nitrogen-doped carbon nanotubes (NCNTs) were synthesized by carbonization of polyaniline-coated CNTs and as a supercapacitor electrode material its capacitance in 6 mol/L aqueous KOH solution increased from 107 to 205 F/g as the N-doped layer

Carbon Nanomaterials in Supercapacitors.

Electrode

Growth process

Electrolyte

Specific Specific Energy and surface area capacitance power density

Retention capability

Ref.

11.3 Wh/kg, 5000 W/kg

85% after 1000 cycles

132

17 Wh/kg,

96.4% after 1000 133 cycles

1 Carbon nanotubes (CNTs) PEDOT/MWCNT

In situ chemical polymerization

1 M LiClO4

46 m2 g–1

79 F/g

SWNT

Water-assisted chemical vapor deposition

1 M Et4NBF4/ propylene Carbonate

1250 m2 g–1

160 F/g at 4V

24 kW/kg

Nitrogen-doped carbon nanotubes (NCNTs)

Carbonization of 6 mol/L KOH polyaniline-coated CNTs

Spherical particles of N-doped CNT

Emulsion-assisted 1 M H2SO4 evaporation

Vertically aligned carbon nanotubes (VACNTs)

LPCVD

1 M H2SO4, 1 V

79 F/g

Aligned carbon nanotube arrays (ACNTAs)

CVD

(Et)4 NBF4/PC, 2.5 V

83 F/g

Vertically aligned carbon nanotube arrays (VA-SWNTs)

WACVD

ALD coating of Al2O3 and top electrode

23 mF/cm3 at 20 Hz

0.01–0.13 Wh/ kg

114 m2 g–1

205 F/g

92.8–97.1% after 134 1000 cycles

215 F/g at 0.2 A/g

>99% after 1500 135 cycles 1.1 ± 0.3 Wh/kg, 75% after 8.6 ± 4.1 500 cycles MW/kg 57% after

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137

1000 cycles 138

Carbon cloth (cc)/CNTs. MPECVD

0.5 M Na2SO4

724.8 m2 g–1

225 F/g

28 Wh/kg

87% after 10,000 139

cycles

CNT treated with HNO3 Arc discharge

H2SO4

430 m2 g–1

104 F/g

8 KW/kg

140

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Electrode

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Growth process

Electrolyte

Specific Specific Energy and surface area capacitance power density

Retention capability

Ref.

348 F/g at 0.2 A/g

120% after 3000 cycles at 10 mV/s

141

524 m2 g–1

150 F/g at 0.1 A/g

100% up to 500 cycles

142

2 Graphene Modified EMIMBF4/AN Hummers’ method

Graphene nanosheets (GNSs)

Oxidation and rapid heating processes

Holey graphene framework (HGF)

Conjugating holey EMIMBF4/AN graphene sheets

1030 m2 g–1

298 F/g at 1 35 Wh/kg A/g

95% over 20,000 143 cycles

Activated graphene

Chemical activation

EMIMBF4/AN

2400 m2 g−1

166 F/g at 5.7 A/g

70 Wh/kg, 250 kW/kg

97% after 10,000 144 cycles

N-doped graphene

Fluorination followed by annealing in ammonia

1 M TEABF4

280 F/g at 20 A/g

48 Wh/kg, 800 kW/kg

95% after 5000 cycles

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B-doped graphene nanoplatelets

Reduction of graphene oxide

6 M KOH solution

466 m2 g–1

200 F/g at 0.1 A/g

~5.5 Wh/kg, ~10 kW/kg

95% after 4500 cycles

146

Graphene PTFE binder Suspending GO KOH (1 V) sheets in waterhydrazine hydrate reduction

705 m2 g–1

135 F/g

Activated spherical microwave-expanded GO (asMEG-O)

3290 m2 g–1

174 F/g

Aerosol spray drying

30 wt% KOH in H2O

[EMIM][TFSI] 3.5 V

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74 Wh/kg, 338 kW/kg

~94% over 1000 148 cycles

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Reduced graphene oxide (rGO) using hydrobromic acid

(Continued)

Electrode

Growth process

Electrolyte

Chemically converted graphene (CCG)-water as spacer

Chemical conversion from graphite

H2SO4 (1 V)

Specific Specific Energy and surface area capacitance power density 215 F/g

8 Wh/kg, 414 kW/kg

Retention capability

Ref.

97% over 10,000 cycles

149

3 Carbon nanofibers (CNFs) Activated carbon nanofibers (ACNFs)

Electrospun

KOH (0.9 V)

1230 m2 g–1

173 F/g at 10 mA/g

82% at 800°C

150

CNFs on 3D nickel foam

Thermal CVD growth

2 M Li2SO4 salt

500 m2 g–1

1.2 F/cm2

89.8% after 3000 cycles

151

Porous CNF composites Electrospun (NFCs)

6 M KOH (1 V)

302 m2 g–1

126.86 F/g

N2-doped porous CNFs Template

6 mol/L of KOH

562.51 m2 g–1 202 F/g

7.11 Wh/kg, 7–8 kW/kg

81.7% over 3000 cycles

153

290 m2 g–1

204.9 F/g

7.76 Wh/kg, 186 kW/kg

Stable within 4000 cycles

154

1496 m2 g–1

171 F/g at 10 mV/s

3.3 Wh/kg, 4.2 kW/kg

74% at 0.1 A/g, 85% at 25 A/g

155

120 F/g at 0.1 V/s

95% at 1 V/s

156

305 F/g at 6.3 Wh/cm3, 3 73.5 mA/cm 1085 mW/cm3

93% after 10,000 cycles

157

N,P-codoped CNF Pyrolysis of 2M H2SO4 (1 V) networks from bacterial bacterial cellulose cellulose immersed in dopant solution

10–17 Wh/kg, 0.4–20 kW/kg

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4 Hybrid carbon nanomaterials CVD

6 M KOH

Graphene/CNT

Sequential self-assembly

1 M H2SO4

SWNT/N-rGO fiber

Hydrothermal process

PVA/H3PO4, 1 M H2SO4

396 m2 g–1

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Mesoporous carbon spheres/rGO

(Continued)

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TABLE 4.1 Electrode

Growth process

VACNTs/CNFs Carbon nanofiber/ graphene (CNF/G) CNFs-bridged porous carbon nanosheets (PCNs)

Electrolyte

Specific Specific Energy and surface area capacitance power density

Retention capability

Ref.

Heat treatment EMIMBF4 (4 V) followed by HNO3

950 m2 g–1

146.8 F/g

97% after 20,000 cycles

158

Ultrasound-assisted 6M KOH (0.8V) preparation

480 m2 g–1

183 F/g

92% after 4500 cycles

159

1037 m2 g–1

261 F/g

97.6% after 10,000 cycles

160

6 M KOH

70 Wh/kg, 8.8 kW/kg

20.4 Wh/kg, 90 W/kg

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thickness decreased, which was much higher than the capacitance of 10 F/g for the pristine CNTs.134 Aligned carbon nanotube arrays (ACNTAs) (lengths up to 150 µm) can be fabricated on metallic alloy (Inconel 600) substrates by pyrolysis of iron (II) phthalocyanine (FePc) in the presence of ethylene (C2H4). As-synthesized samples were used directly as electrode materials for supercapacitors and the specific capacitance obtained at 1000 mV/s scan rate was about 57% (47 F/g) of that obtained at 1 mV/s rate (83 F/g).137 Dense and entangled CNTs directly grown on the carbon cloth (CC) can be employed as supercapacitor electrode that shows high retention capability of 87% after 10,000 cycles, high energy density of 27.8 Wh/kg, and specific capacitance of 210 F/g.139 Graphene is also emerging as a promising candidate for supercapacitors applications due to its unique characteristics such as good electrical conductivity and high specific surface area.141 Graphene nanosheets (GNSs) with narrow mesopore distribution can be mass-produced from natural graphite via the oxidation and rapid heating processes to be used as electrode material for electric double-layer capacitors. They show excellent rate capability and reversibility at high scan rates in electrochemical performances. GNS electrode with specific surface area of 524 m2/g maintained a stable specific capacitance value of 150 F/g under specific current of 0.1 A/g for 500 charge/discharge cycle.142 Holey graphene framework (HGF) and chemically activated graphene (Table 4.1) are also exhibiting excellent electrochemical properties as electrodes in supercapacitors.143,144 N-doped graphene synthesized by fluorination followed by annealing in ammonia shows (electrolyte-1 M TEABF4) specific capacitance of 280 F/g at 20 A/g, energy density of 48 Wh/kg, and power density of 800 kW/kg. The device showed retention capability of 95% after 5000 cycles.145 Graphene PTFE binder electrodes fabricated by suspending GO sheets in water followed by hydrazine hydrate reduction show specific capacitance of 135 F/g using KOH (1 V) electrolyte.147 Chemically converted graphene (CCG) (water as spacer) from graphite as electrode shows remarkable specific capacitance of 212 F/g using H2SO4 (1 V) electrolyte. Calculated energy density is 8 Wh/kg and power density is 414 KW/kg. Moreover, the device exhibited remarkable retention capability of 97% over 10,000 cycles.149 Carbon nanofibers (CNFs) also showed great prospective as an efficient supercapacitor material with promising electrochemical performance such as exceptional cyclic stability and comparatively high specific capacity.153 Activated carbon nanofibers (ACNFs) prepared through electrospinning exhibit excellent specific surface area of 1230 m2/g. At 10 mA/g, the device exhibits specific capacitance of 173 F/g and retention capability of 82% at 800°C.150

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CNFs on 3D nickel foam, porous CNF composites (NFCs), and N2-doped porous CNFs show remarkable electrochemical performance (Table 4.1), ensuring their capability as electrode for supercapacitors.151–154 N,P-codoped CNF networks from bacterial cellulose synthesized by pyrolysis of bacterial cellulose immersed in dopant solution show specific capacitance of 204.9 F/g in 2 M H2SO4 (1 V) electrolyte. Energy density was 7.76 Wh/kg, power density was 186 kW/kg, and the performance was stable within 4000 cycles.154 Hybrid carbon nanomaterials prepared by combining carbon nanomaterials with distinct structures exhibit synergetic effects for electrochemical performance of supercapacitors. Hybrid film of self- assembled graphene/ carbon nanotube prepared by sequential self-assembly exhibits nearly rectangular cyclic voltammogram even at an exceptionally high scan rate of 1 V/s with an average specific capacitance of 120 F/g.156 Vertically aligned carbon nanotubes (VACNTs/CNFs) prepared through heat treatment followed by reaction with HNO3 show remarkable specific capacitance (146.8 F/g), energy density (70 Wh/kg), and power density (8.8 kW/kg). Retention capability was observed to be 97% after 20,000 cycles.158 Hybrid carbon nanomaterials like carbon nanofiber/graphene (CNF/G), CNFs/bridged porous carbon nanosheets (PCNs), and mesoporous carbon spheres/rGO show promising electrochemical performance offering immense possibilities in future.155,159,160 4.4 NANO-CARBON MATERIALS FOR FUEL CELLS Conversion of chemical energy into electrical energy can be attained effectively using fuel cell, with high energy conversion efficiency, low emission threat, and power density. Fuel cells are one of the promising candidates to crack upcoming energy crisis and environmental concerns.161 In hydrogen fuel cells, fuel (hydrogen) is fed to the anode and air is fed to the cathode and electrochemical reactions take place at the two electrodes. A catalyst at the anode splits H2 into protons and electrons. The electrons generated go through an external circuit to cathode, producing a current flow.162 Electrolyte aids the transport of the generated protons to cathode side from anode side, resulting in the production of water and heat with the help of O2 and the electrons.163 Platinum metal, generally used catalyst in fuel cells, is extremely costly and scarce, which is the root cause that hinders it from commercialization.164 Numerous attempts have been made to enhance the fuel cell efficiency by utilizing the superior catalytic properties of carbon

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nanomaterials like CNTs, graphene, and other forms of carbon.161 Schematic of a hydrogen fuel cell is shown in Figure 4.2.165

FIGURE 4.2

Schematic of a hydrogen fuel cell.

Jha et al. employed single-walled carbon nanotubes (SWNTs) with varying degrees of functionalization for catalyst support layers (CSLs) in polymer electrolyte membrane fuel cells (PEMFCs). Total Pt load of the constructed device was 0.06 mg Pt/cm2 (much less than 0.125 mg Pt/cm2—US DOE 2017 technical target for total Pt group metals (PGM) loading).166 Girishkumar et al. utilized SWCNTs support and platinum catalyst for realizing a MEA (membrane electrode assembly) for H2 fuel cells. They deposited films of SWCNTs along with usual platinum black using electrophoretic deposition on a carbon fiber electrode (CFE). Comparing CFE/carbon black (CB)/Pt electrodes, maximum power density of the device using CFE/SWCNT/Pt electrodes as both the anode and the cathode was ~20% better.167 All-electrochemical three-step method was employed for PtRuNi/ MWCNTs electrode preparation by Zhao et al. It was observed that PtRuNi/ MWCNTs electrode displays good performance in direct methanol fuel cells (DMFCs).168 Pt and Pd sequentially electrodeposited gold nanoparticles

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loaded carbon nanotubes were utilized for the electrocatalytic study of methanol, ethanol, and formic acid oxidations by Saipanya et al. They observed that the PtPdPt/Au-CNT catalyst is efficient in methanol oxidation, improves ethanol oxidation, and successfully catalyzes formic acid oxidation.169 Cu/CuxO and Pt nanoparticles supported on MWCNTs, synthesized using H2 as reductant (Cu/MWCNT-H2) were used (Sheng et al.) as electrocatalysts for the reduction of nitrobenzene in a half-cell set-up. The stability of the said electrocatalyst tested by 1000 cycles of cyclic voltammetry (CV), showed that the activity does not decline over time. Electrochemical reduction of nitrobenzene over the present electrocatalyst in acidic ethanolic medium led to 44% conversion with an overall selectivity towards azoxybenzene of 82%.170 Matsumoto et al. replaced Pt/C anode with Mo2C/CNT electrode, a correct contender of non-Pt anode material used for PEFC. Mo2C/CNT electrodes functioned at higher overvoltage related to Pt-based electrodes.171 Glassy carbon electrode (GCE) having MWCNTs/SnO2 coating of nanocomposite can be exploited as an anode material in Microbial fuel cells (MFCs)172. The MWCNTs-SnO2/GCE exhibited the superior electrochemical performance as related to MWCNT/GCE, bare GCE anodes ensuring it as an appropriate anode material for the MFCs.172 Highly dispersed Ru-decorated Pt nanoparticles were deposited onto N-doped MWCNTs employing atomic layer deposition (ALD). Catalysts were tested toward the electrooxidation of CO and methanol. It was observed that the catalyst decorated with five ALD Ru cycles was of highest activity (Johansson et al.).173 CNT–sponge composite synthesized by coating a sponge with CNTs can be used for microbial fuel cell electrodes. This electrode offers low internal resistance, better stability, adjustable and even macroporous structure, and enhanced mechanical properties. CNT–sponge attained higher current density, a real power density, and volumetric power density maximum of 2.13 mA/cm2 in a glucose medium (1g/L), 1.24 W/m2, and 182 W/m3 respectively.174 Nitrogen-doped graphene nanoplatelets can be employed as catalyst support for oxygen reduction reaction in proton exchange membrane fuel cell. Fuel cell was created with Pt-loaded N2-doped graphene nanoplatelets (Pt/N-G) and the results have been compared with platinum-loaded graphene nanoplatelets (Pt/G). The power density of Pt/N-G (440 mW/cm2) is better than that of Pt/G (390 mW/cm2), and improved electrical conductivity might be the reason.175 Cu2O nanoparticles/reduced graphene oxide (Cu2O/rGO) were used as a cathode catalyst microbial fuel cells. The device delivered an output voltage of 0.223 V, coulombic efficiency of 92.5%, outstanding ORR catalytic activity, and promoted oxygen diffusion to surface of cathode.

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Work emphasized on potential applications of Cu2O/rGO as great catalytic active and antibacterial cathode catalyst, as a substitute for commercial Pt/C for power generation.176 Lee et al. reported the preparation of filler for proton exchange membrane fuel cell (PEMFC) using highly sulfonated polymergrafted graphene oxide. The composite exhibited better proton conductivity compared to pure SPAES, and the sulfonic acid groups in sulfonated poly(arylene thioether sulfone)-grafted graphene oxide (SATS-GO) could enhance the compatibility among the filler and matrix polymer.177 Dong et al. selected plain graphene, oxidized graphene, and graphene oxide (GO). They studied the catalytic performance of these graphenes in MFCs and effect of oxygen comprising functional groups. Oxidized graphene possessed good catalytic performance for H2O2 production. Oxidized graphene air-cathode will enable instantaneous waste water treatment, power output, and H2O2 generation in microbial fuel cell.178 Air-cathodes in MFCs using a novel activated carbon/graphene membrane prepared by phase inversion exhibit excellent conductivities and performances. They concluded that cathodes prepared using graphene can generate electricity and high quality sewage with low cathode biofouling.179 4.5 NANO-CARBON MATERIALS FOR LITHIUM ION BATTERIES (LIBS) Lithium-ion batteries (LIBs) continue growing in popularity as a promising energy storage technology due to their high volumetric energy, high power density, minimal self-discharge property, nearly zero-memory effect, high open circuit voltage, and long lifecycle.1,180 They are the essential components in portable electronics, electric vehicles (EVs), military, and aerospace applications. In Li-ion batteries, cathode is an intercalated lithium compound and graphite is typically used as anode and the electrolyte in between these electrodes vary from one type of battery to another. On charging up the battery, a few lithium ions from cathode transfer to anode through the electrolyte and remain on the anode. On discharging, these lithium ions reach back cathode through electrolyte. In both cases, electrons flow in the outer circuit is in opposite direction to that of lithium ions.181–183 Schematic representation is show in Figure 4.3.184 Li batteries using graphite, silicon anodes and Li3V2(PO4)3, LiFePO4, LiCoO2, LiMn2O4 as cathodes result in damage of above said electrode because they undergo subsequent expansion and contraction.180 The challenges raised by conventional electrodes can be

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dealt with carbon nanomaterials such as graphene and CNTs that has high electrical conductivities, good specific surface areas, high electron mobility, less resistive heating and with minimum volume expansion, and good stability.1

FIGURE 4.3

Schematic diagram of Li-ion battery.

Targeting to improve reversible capacity and cycle stability of Li-ion batteries, Tao et al. prepared Co3O4 nanorods/GNS (graphene nanosheets) nanocomposites through one-spot solvothermal method. Even after 40 cycles at a current density of 100 mAg−1 and 1000 mAg−1, they are retaining approximately 1310 mAh g−1 and 1090 mAh g−1 of capacity respectively, which are attributed to the excellent electronic conduction pathway and ion diffusion offered by these structures.185 Bak et al. used microwave-assisted hydrothermal method to develop a high rate lithium-ion batteries using LiMn2O4/reduced graphene oxide hybrid cathode. This device exhibited capacity discharge of 390 mAh g−1 and 900 mAh g−1 after 100 cycles, owing to the exceptional structural and morphological properties of LiMn2O4.186 Sivakkumar and Kim fabricated PANI/CNT by employing in situ chemical polymerization of aniline in well-dispersed CNT solution. In LIBs, this acts as

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an active cathode material. The discharge capacity of the cell was maximum of 86 mAh g−1 at the 80th cycle along with 98% efficiency retention.187 Special nano/micro hierarchical structure using three-dimensional network of LiNi1/3Mn1/3Co1/3O2 cathode and reduced graphene oxide (RGO) was fabricated by Jiang et al. The exhibited capacity of the device was 115 mAh g−1 at 6°C. RGO network greatly decreases the resistance of the device.188 Wang and Kumta reported the synthesis of 1D heterostructures comprising vertically aligned multiwall CNTs (VACNTs) containing nanoscale amorphous/ nanocrystalline Si droplets deposited using a simple two-step liquid injection CVD process. The device exhibited high reversible capacity of 2050 mAh g−1 and capacity reduced about 20% after 20 cycles.189 V2O5/SWNT mesoporous structure is prepared using a simple floating CVD method and employed as cathodes for lithium-ion batteries exhibits a high-rate capacity of 548 mAh g−1 at the discharge rate of 300 mA g−1 (~1 C). Chemical interaction between the components and excellent Li diffusivity are main reasons for excellent electrochemical performance (Cao and Wei).190 Vazri et al. fabricated a fuel cell of lithium ion having excellent electrochemical performance using ZnFe2O4 nanoparticle-based anode coated with carbon and a LiFePO-MWCN-based cathode. The device using this innovative structure exhibited remarkable gravimetric energy (202 Wh/kg) and power density (3.72 W/kg) values.191 Micro-arc oxidation technique was used to fabricate anode layer of TiO–CNTs on titanium foil for lithium-ion batteries (Lo et al.). Coin cell assembled using Ti/TiO–CNTs composite as the anode shows capacity of discharge of 201 mAh g−1 measured after the 20th cycle at a discharge rate of 0.2°C and the performance is better than that of device without the addition of CNTs.192 A novel nanocomposite (Si-CNT/ PANi) was developed by Xiao et al. for Li-ion batteries as material for anode. This nanocomposite incorporates high capacity of Si nanoparticles and excellent electrical conductivity. Mechanically flexible network of CNTs and PANi coating guards the silicon structure and improves the electrical conductivity further. The structure demonstrates 2430 mAh g−1 reversible capacity with good capacity retention over 500 cycles.193 Outstanding cycling performance was exhibited by electrode with structure CoFe2O4/G. At 0.2 Ag−1 electrode exhibits 1109 mAh g−1 after 100 cycles and 835 mAh g−1 after 200 cycles at 1 Ag−1 and excellent rate capability (420 mAh g−1 at 5 Ag−1).194 Hydrothermal route could be selected to fabricate ultrathin, undersized MoS2/graphene composites for lithium-ion battery anodes. Method makes use of acetic acid and post-annealing. For the initial cycle 1229 mAh g−1 reversible capacity was shown by the device. And even after 50 cycles, it

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maintains 942.6 mAh g−1 at a current density of 100 mA g−1.195 Electrospinning and subsequent thermal treatment can be utilized for fabricating flexible CoO–graphene–carbon nanofiber mats as binder-free anodes for Li-ion batteries. The device fabricated using this anode material, at 500 mA g−1 current density showed discharge capacities of 760 mAh g−1 at 252 cycles and a discharge capacity of 690 mAh g−1 at 352 cycles, respectively. Better device performance might be due to fast diffusion of Li+ owing to the framework, conductivity enhancement due to graphene, and the presence of defective sites in the structure.196 Nanoparticles of tin (~5 nm) grown between graphene interlayer were developed as anode material for Li-ion batteries with structure Sn nanoparticle/graphene nanosheets (G/Sn). Better material was obtained by undergoing optimization procedure and hence fabricated device exhibited an enhanced capacity of 838.4 mAh g−1 at 0.1 Ag−1 even after 100 cycles. The cycling stability is also excellent (684.5, 639.7, 552.3, and 359.7 mAh g−1 at 0.5, 1, 2, and 5 Ag−1, respectively after 100 cycles).197 KEYWORDS • • • • • •

nanomaterials carbon nanotubes green energy environmental friendly nanofibers storage systems

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172. Mehdinia, A.; Ziaei, E.; Jabbari, A. Multi-walled Carbon Nanotube/SnO2 Nanocomposite: A Novel Anode Material for Microbial Fuel Cells. Electrochim. Acta 2014, 130, 512–518. 173. Johansson, A.-C.; Yang, R. B.; Haugshoj, K. B.; Larsen, J. V.; Christensen, L. H.; Thmsen, E. V. Ru-decorated Pt Nanoparticles on N-doped Multi-walled Carbon Nanotubes by Atomic Layer Deposition for Direct Methanol Fuel Cells. Int. J. Hydrogen Energy 2013, 38 (26), 11406–11414. 174. Xie, X.; Ye, M.; Hu, L.; Liu, N.; McDonough, J. R.; Chen, W.; Alshareef, H. N.; Criddle, C. S.; Cui, Y. Carbon Nanotube-coated Macroporous Sponge for Microbial Fuel Cell Electrodes. Energy Environ. Sci. 2012, 5, 5265–5270. 175. Jafri, R. I.; Rajalakshmi, N.; Ramaprabhu, S. Nitrogen Doped Graphene Nanoplatelets as Catalyst Support for Oxygen Reduction Reaction in Proton Exchange Membrane Fuel Cell. J. Mater. Chem. 2010, 20, 7114–7117. 176. Xin, S.; Shen, J.; Liu, G.; Chen, Q.; Xiao, Z.; Zhang, G.; Xin, Y. Electricity Generation and Microbial Community of Single-Chamber Microbial Fuel Cells in Response to Cu2O Nanoparticles/reduced Graphene Oxide as Cathode Catalyst. Chem. Eng. J. 2020, 380, 122446. 177. Lee, H.; Han, J.; Kim, K.; Kim, J.; Kim, E.; Shin, H.; Lee, J. Highly Sulfonated Polymergrafted Graphene Oxide Composite Membranes for Proton Exchange Membrane Fuel Cells. Ind. Eng. Chem. Res. 2019, 74, 223–232. 178. Dong, H.; Liu, X.; Xu, T. Hydrogen Peroxide Generation in Microbial Fuel Cells Using Graphenebased Air-cathodes. Bioresour. Technol. 2018, 247, 684–689. 179. Song, X.; Liua, J.; Jiang, Q. Enhanced Electricity Generation and Effective Water Filtration Using Graphene-based Membrane Air-cathodes in Microbial Fuel Cells. J. Power Sources 2018, 395, 221–227. 180. Wu, J.; Pan, Z.; Zhang, Y.; Wang, B.; Peng, H. The Recent Progress of Nitrogen-doped Carbon Nanomaterials for Electrochemical Batteries. J. Mater. Chem. A 2018. 181. Etacheri, V. K.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 3243–3262. 182. Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652–657. 183. Scrosati, B.; Garche, J. Lithium Batteries: Status, Prospects and Future. J. Power Sources 2010, 195, 2419–2430. 184. Hordé, T. 28th International Congress of the Aeronautical Sciences, 2012, Brisbane, Australia 185. Tao, L.; Zai, J.; Wang, K.; Zhang, H.; Xu, M.; Shen, J.; Su, Y.; Qian, X. Co3O4 Nanorods/ graphene Nanosheets Nanocomposites for Lithium Ion Batteries with Improved Reversible Capacity and Cycle Stability. J. Power Sources 2012, 202, 230–235. 186. Bak, S.-M.; Nam, K.-W.; Lee, C.-W.; Kim, K.-H.; Jung, H.-C.; Yang, X.-Q., Kim, K.-B. Spinel LiMn2O4/reduced Graphene Oxide Hybrid for High Rate Lithium Ion Batteries. J. Mater. Chem. 2011, 21, 17309–17315. 187. Sivakkumar, S. R.; Kim, D.-W. Polyaniline/Carbon Nanotube Composite Cathode for Rechargeable Lithium Polymer Batteries Assembled with Gel Polymer Electrolyte. J. Electrochem. Soc. 2007, 154, A134–A139. 188. Jiang, K.-C., Xin, S.; Lee, J.-S., Kim, J.; Xiao, X.-L., Guo, Y.-G. Improved Kinetics of LiNi1/3Mn1/3Co1/3O2 Cathode Material Through Reduced Graphene Oxide Networks. Phys. Chem. Chem. Phys. 2012, 14, 2934–2939. 189. Wang, W.; Kumta, P. N. Nanostructured Hybrid Silicon/Carbon Nanotube Heterostructures: Reversible High-capacity Lithium-ion Anodes. ACS Nano 2010, 4, 2233–2241.

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190. Cao, Z.; Wei, B. V2O5/single-walled carbon nanotube hybrid mesoporous films as cathodes with high-rate capacities for rechargeable lithium ion batteries. Nano Energy 2013, 2, 481–490. 191. Varzi, A.; Bresser, D.; von Zamory, J.; Müller, F.; Passerini, S. ZnFe2O4-C/LiFePO4-CNT: A Novel High-power Lithium-ion Battery with Excellent Cycling Performance. Adv. Energy Mater. 2014, 4, 1–9; 1400054. 192. Lo, W.-C., Su, S.-H., Chu, H.-J., He, J.-L. TiO2-CNTs Grown on Titanium as An Anode Layer for Lithium-ion Batteries. Surf. Coat. Technol. 2018, 337, 544–551. 193. Xiao, L.; Sehlleier, Y. H.; Dobrowolny, S.; Mahlendorf, F.; Heinzel, A.; Schulz, C.; Wiggers, H. Novel Si-CNT/polyaniline Nanocomposites as Lithium-ion Battery Anodes for Improved Cycling Performance. Mater. Today: Proc. 2017, 4, S263–S268. 194. Yang, Z.; Huang, Y.; Hu, J.; Xiong, L.; Luo, H.; Wan, Y. Nanocubic CoFe2O4/Graphene Composite for Superior Lithium-ion Battery Anodes. Synth. Met. 2018, 242, 92–98. 195. Chen, L.; Yang, Y.; Gao, Y.; Tronganh, N.; Chen, F.; Lu, M.; Jiang, Y.; Jiao, Z.; Zhao, B. Facile Synthesis of Ultrathin, Undersized MoS2/Graphene for Lithium-ion Battery Anodes. RSC Adv. 2016, 6, 99833–99851. 196. Zhang, M.; Yan, F.; Tang, X.; Li, Q.; Wang, T.; Cao, G. Flexible CoO-graphene-carbon Nanofiber Mats as Binder-free Anodes for Lithium-ion Batteries with Superior Rate Capacity and Cyclic Stability. J. Mater. Chem. A 2014b, 2, 5890–5897. 197. Zhou, X.; Zou, Y.; Yang, J. Periodic Structures of Sn Self-inserted Between Graphene Interlayers as Anodes for Li-ion Battery. J. Power Sources 2014, 253, 287–293.

CHAPTER 5

Carbon Nanotubes: Application in Energy Harvesting and Storage SNEHA MATHEW, BINILA K. KORAH, ANU ROSE CHACKO, and BEENA MATHEW School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala, India

ABSTRACT Over the last few years, carbon-based nanomaterials have captivated consideration in energy storage due to their high storage capacity, better durability, and fast rechargeability. Carbon nanotubes (CNTs) are one type of nanostructured material with beneficial electrical and mechanical properties due to one-dimensional containment, combined with surface properties that contribute to improved overall behavior. Their unique structural, electronic, mechanical, and electrical properties made them an attractive system standing for a wide range of applications, including energy storage. CNTs have been used as active material and supporters in the electrodes of batteries, fuel cells, and supercapacitors. This chapter focuses on hydrogen storage, lithium-ion batteries, metal-air batteries, solar cells, supercapacitors, fuel cells, and the current state and projected development trends of carbon nanotubes for energy storage. 5.1 INTRODUCTION Energy became humanity’s most important issue. Nanoscience and nanotechnology promise scientific breakthroughs and innovative technologies to Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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tackle the global energy problem. The combination of concerns about the contribution of fossil fuels to the climate change induced by greenhouse gases and the long-term sustainability of their supply has required an increasingly urgent development of alternative energy generation and storage approaches. Nanotechnology plays a critical role in transforming energy conversion device applications and addressing these issues, storage, monitoring the environment, and green engineering of materials friendly to the environment.1 Due to their unique electronic, optical, magnetic, mechanical, and chemical properties, nanoparticles with sizes between 1 nm and 100 nm have attracted considerable interest. The discovery of carbon nanotubes has brought about unparalleled largescale nanotechnology research activities.2 Carbon nanotubes (CNTs), due to their exceptional properties, find applications in various multidisciplinary fields like physics, chemistry, biology, electronic, and material science.3 CNT exhibits thermal conductivity twice that of diamond and has received significant recognition as a candidate electrode substrate for batteries, owing to their favorable electrochemical properties. CNTs also play a critical role in supercapacitors or electrochemical double-layer condensers (EDLCs), energy-saving devices,4,5 Their one-dimensional (1D) morphology and exceptional electrical conductivity make them the most suitable candidate for manufacturing coming generation energy harvesting electrochemical devices. With the introduction of CNTs, research has demonstrated significant performance improvements for various energy storage devices. In this chapter, efforts are made to emphasize the energy applications of carbon nanotubes. This chapter highlights the applications of carbon nanotubes in hydrogen storage, lithium-ion batteries, metal-air batteries, solar cells, supercapacitors, and fuel cells. Figure 5.1 depicts the energy applications of carbon nanotubes. 5.2 CARBON NANOTUBES Carbon nanotubes (CNTs) are large cylindrical molecules composed of hybridized carbon atoms hexagonal arrangement. Iijima discovered these in 1991. They can be classified into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Single-walled carbon nanotubes are rolled up graphene sheets with sp2 hybridization. If additional coaxial graphene tubes are around single-walled carbon nanotubes, they are called multi-walled carbon nanotubes.6–9 Since their discovery, these one-dimensional carbon allotropes have attracted significant research interest

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because of their outstanding material properties. Carbon nanotubes (CNTs) and their composites have unique chemical, physical, and electrical properties, which open up new prospects for electronic applications on a nanoscale scale.

FIGURE 5.1

Schematic illustration of applications of carbon nanotubes.

The diameter of SWCNTs is about 0.4–3 nm and has a length of several micrometers. MWCNT is a stack of graphene sheets rolling in concentrated cylinders between 2 and 25 nm in diameter, and the distance between sheets or interlayer spacing is approximately 0.34 nm. The atoms are arranged helically along the axis of the tube along each tube. The outer diameter of these MWNTs is about tens of nanometers, and they are 10–100 nm long. CNTs can be further classified into armchairs, zigzags, and chiral depending on chirality. Only armchair nanotubes are genuinely metallic, while other tubes zigzag together with chiral 1/3 are narrow gap and 2/3 wide gap semiconductor.10 Different varieties of CNTs are depicted in Figure 5.2.11 Iijima was the first to notice that nanotubes were rolled concentratively graphene sheets with multiple possibilities of helicities and chiralities. CNTs possess some unique physical characteristics, such as 100 times steel’s tensile strength, thermal conductivity superior to other materials,

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except purest diamond and copper-like electrical conductivity but carry significantly higher currents. CNTs have a high transfer rate of the electron over the sidewall, which brings in high CNT electrical conductivity. The chemical reactivity of CNTs is primarily derived from the sidewall curvature and structural defects. Van der Waals interactions between sidewalls in SWCNTs lead to hexagonally close-packed bundles. The existence of structural defects such as vacancies increases chemical reactivity and surface area, whereas the porous nature of the material impacts adsorption properties.1

FIGURE 5.2 Structural varieties of carbon nanotubes. Reproduced with permission from Ref. [11]. Copyright (2011), ACS.

Their properties and structure are in a direct relationship. Usually, the structure of tubulenes is defined in terms of the sp2-network. The hexagonal network’s mutual orientation and a nanotube’s longitudinal axis determine its chirality. Two integers (n and m) define the chirality of a nanotube that locates the hexagon of the network that matches after nanotube rolling with the hexagon that is at the coordinates center. The nanotube’s chirality can also be determined uniquely by the angle Θ (either the orientation angle or the chiral angle) created by the rolling direction of the nanotube and the direction of the distinctive edge. Among the different rolling possibilities, those at Θ=0 and 30 arc deg corresponding to chiralities (n,0) and (n, n) are

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preferred. The angle of orientation (or rolling) establishes CNT’s electrical properties. Chiral and achiral CNTs constitute other subclassifications of CNTs. They are the strongest and toughest materials known in nature.12 CNTs’ unique electric and mechanical properties attracted them to energy storage and conversion applications. Beyond energy storage and conversion, they laid their footprints in nano transistors,13 electron field emission,14 and biological sensing devices.15–17 As they are chemically stable, they prevent degradation at the electrode surface and are thus used as electrode materials.18 5.2.1 SYNTHESIS OF CARBON NANOTUBES The three primary methods for synthesizing CNTs are chemical vapor deposition (CVD), electric arc, and pulsed laser. • Chemical vapor deposition: This involves the decomposition of carbon atom-containing gas or vapor by heat treatment in the presence of a metal catalyst. The nanotubes are formed by the disintegration of the volatile precursors and deposited on the substrate. The essential parameters in this method are the substrate temperature (between 500°C and 700°C), the type of catalyst, and the type of substrate, which determines the growth region of the CNTs. • Electric arc: Arc-current is applied between two cylindrical graphite electrodes immersed in inert gas. To generate a plasma between the electrodes, the cathode and the anode are held at a distance less than 1 nm. The anode graphite undergoes sublimation in the plasma region (temperature is about 4000 K) and is deposited on the cathode or chamber’s inner walls (soot) where the CNTs are contained. MWNTs will be obtained using catalysts for shaping beams of nanotubes of this kind. This is one of the earliest and most widely used CNT synthesis techniques. • Pulsed laser: It is identical to the electric arc method. In the presence of inert gas, the carbon is vaporized by irradiation by a laser. The graphite is placed within a quartz tube and put in a tubular oven. The temperature is then increased to 1200°C after emptying the tube, and then the tube is filled with an inert gas (He or Ar). The laser vaporizes the graphite surface and forms CNTs. Both SWCNT and MWCNT can be prepared by this method.3

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5.3 APPLICATIONS 5.3.1 HYDROGEN STORAGE Hydrogen is a clean and potentially renewable energy source with no environmental implications. Hydrogen energy systems are in the future progressively expected replacement of current fossil fuels. Although hydrogen has significant advantages, its utilization also has significant drawbacks. Manufacturing costs and storage facilities are the most important. CNTs and their modified forms have proved promising as prominent future candidates for hydrogen storage in recent years. Carbon nanotube’s high surface-to-volume ratio, chemical stability, and light mass make them potential candidates for hydrogen storage.19 Dillon et al. in 1997 discovered that as produced, CNTs can store 5 to 10 wt% of hydrogen at RT. Hydrogen stores on the outer walls, inside the mesoporous tube-like channels, and in the space between individual nanotubes for bundles of nanotubes. In addition, hydrogen can also occur in the interstitial spaces inside 2-D triangular lattices formed by combining large diameters of individual CNTs. While the exact mechanism of hydrogen adsorption by CNT is still unclear, it has been reported that adsorption happens primarily by two means, namely chemisorption and physisorption. Chemisorption in CNT happens as the hydrogen adsorbs exothermically to the top locations of carbon atoms on the tube walls. In contrast, physisorption occurs by trapping hydrogen molecules within the nanotube’s cylindrical structure. This facilitates sp3 like tube hybridization, which results in diameter expansion. A larger tube diameter yields a greater capacity to store hydrogen.20 Hydrogen adsorption by CNT is influenced by specific surface area, purity, temperature, pressure, etc. For a given temperature and tube diameter, the hydrogen adsorption capacity of SWCNTs increases with the rise in pressure, whereas this capacity decreases with an increase in temperature.21–23 Several research using carbon nanotubes to increase hydrogen conservation have been conducted in recent decades. It was found that, at temperatures and pressures similar to room conditions, SWCNTs could have high energy storage. SWCNTs with a diameter between 1.6 and 2 nm could achieve 6.5 wt percent of the target H2 adsorption density.24 The presence of structural defects in CNTs raises adsorption binding energy by 50%, which has a significant impact on hydrogen storage. SWCNTs with small diameters exhibit higher adsorption rates.

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Modification of CNTs is needed to boost further the hydrogen storage ability of CNTs at moderate temperature and pressure. Complexes of CNTs with metal hydrides, chemical hydrides, and complex hydrides result in higher hydrogen adsorption. Activation of CNTs via heating and treating with NaOH or KOH increases the surface area and thus hydrogen uptake. Doping or decorating with metal or metal oxide is another strategy. Transition metal-doped CNTs showed approximately 40% more hydrogen uptake capacity than pristine CNT. This enhanced hydrogen absorption is due to initial hydrogen adsorption by metal nanoparticles, which eventually dissociate hydrogen molecules and spill them over to carbon nanomaterials.25–28 Coating SWCNTs directly with hydrides, which have a passivated metal atom, increases the hydrogen storage capacity by reducing the activation energy on the CNT surface. Besides the hydrogen in them, the metal hydrides can absorb some amount of additional hydrogen.25 Ion irradiation is another activation method. It was reported that γ irradiation increases structural defects in MWCNTs and hence hydrogen adsorption capacity.29 Besides these, acid treatment, primary treatment, heat treatment, and treating with fluorine and bromine, loading hetero atoms like N, P, B, Si, and hetero atom-metal mixed doping has been reported to improve the hydrogen storage capacity of CNTs.28 Green synthesized carbon nanotubes are also reported to have a high hydrogen storage efficiency. There are reports of CNTs synthesized from camphor, oilseeds, and plant-based fibers.30 Carbon nanotube technology constitutes a new facet for solid hydrogen storage, particularly if these materials can be modified to store vast quantities of hydrogen. 5.3.2 LITHIUM-ION BATTERY A technological breakthrough happened due to the introduction of batteries. Recent advances in nanoscience and nanotechnology provide promising opportunities to produce new, nanostructured electrode materials for betterperforming batteries of the next decade. The praising electrochemical performance of LIBs in terms of energy and power densities and developments in cell design and manufacturing have made LIBs highly competitive for mobile electronics. CNTs with outstanding electrical and thermal conductivity, mechanical stability, and considerably large surface area are examined as suitable additives for enriching their electrodes. By their high strength and energy efficiency, Li-ion

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batteries (LIBs) have been receiving considerable interest. The properties of anode material determine the performance of LIBs. Carbon-containing materials are usually used as electrode materials.31 Due to its high electric conductivity, graphite has been the prominent anode material for LIBs. But the sp2 hybridization causes the force between any two carbon atoms to be stronger than those between adjacent sheets; this limits the specific capacity. As lithium ions can only be mixed in the graphite sheets for any second carbon hexagon, the ability of LIBs based on graphite as an anode medium is technically reduced. So then, another member of the carbonaceous material family, CNT, and composites of CNT have attracted extensive attention as anode material for batteries.32,33 Many research groups have reported lithium storage capacities of various CNTs.34,35 The inclusion of CNT as a conductive add-on presents an efficient approach to electrical networks. CNT can also assemble to form free-standing electrodes (free of any binder or current collector). Alternatively, CNTs can act as an effective lithium-ion storage medium or physical reinforcement for high-performance anode materials such as silicon or germanium. The lithium-ion capacity of CNT is due to their diffusion to the stable sites on the surface or in the inside walls of CNTs. As an additive in LIBs, CNT increases the reverse capacity, improves cyclability and rate capability. For purified materials, the theoretical electrical conductivity at room temperature measurements is greater than 5 × 105 S m−1.36 Li-ion capacities of CNT-based anodes are about three times the graphite electrodes. Because of their unique structure (one-dimensional cylindrical tubule of graphite sheet), high conductivity (106 Sm−1 at 300 K for SWCNTs and >105 Sm−1 for MWCNTs), high tensile strength (up to 60 GPa), low density, carbon nanotubes (CNTs), an allotrope of graphite, have been accepted as a promising anode material for lithium batteries. CNTs significantly increase the capacity of LIBs. Surface-modified CNTs are an excellent approach to improve electrochemical performance. Metal nanoparticles such as tin and tin antimonide deposited CNTs showed high lithium storage capacity.37 LIBs with vertically aligned carbon nanotubes (VA-CNTs) also exhibit higher capacity. VA-CNTs with or without heteroatom doping have similar effects. VA-CNT can also be used as a conductive substrate to store electroactive materials, thus producing high rate and high capacity electrodes.28 To improve the performance of batteries, modified CNT is used. Zhai et al. observed a better lithium storage performance when a nanocomposite of MWCNTs anchored with SnS2 nanosheets (NS) (SnS2 NS@MWCNTs coaxial nano cables) has been used as anode material for LIBs.38 V2O5 encapsulated

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carbon nanotubes battery cell reported to have high flexibility and Li storage performance.39 Morphology of CNTs (diameter, length of individual tube) has significant effects on Li absorption.40,41 Small-diameter CNTs are preferred as intercalated Li-ions tend to locate near CNT sidewalls also, large diameter ones have a lot of empty space. MWCNTs grant a short reversible path for Li-ions. Yan et al. reported short carbon nanotubes (CNTs-1, length of 150–400 nm, outer diameter of 35–65 nm, and inner diameter 15–40 nm) and long carbon nanotubes (CNTs-2, length more than several micrometers, outer diameter of 15–30 nm, and inner diameter of 5–10 nm) and a better rate performance is observed for CNTs-1.42 Heteroatom-doped CNT anodes significantly increase the Li-ion absorption in LIBs.43 5.3.3 METAL-AIR BATTERY Metal–air batteries constitute one class of attractive power sources for applications in electronics of the next decade, electrified transport, and smart grid storage due to the surprisingly high potential energy efficiency.44 Metal−air batteries have a potential energy capacity that is far greater than that of lithium-ion batteries and is also proposed as a solution to electrochemical energy storage of the next decade for uses like hybrid vehicles or grid energy storage. They consist of a metal anode and an air-breathing cathode in a suitable electrolyte. Alkali metals (e.g., Li, Na, and K), alkaline earth metals (e.g., Mg), or first-row transition metals (e.g., Fe and Zn) with good electrochemical equivalence can be the metal anode; external cathode of ambient air and based on the nature of anode the electrolyte may be aqueous or nonaqueous. Redox reaction between metal and oxygen in air generates electricity.45–47 A schematic representation of a metal-air battery is shown in Figure 5.3.44 5.3.3.1 ELECTRODE REACTIONS The general electrode reactions for metal-air batteries with aqueous electrolytes are as follows: Metal electrode: M ↔ Mn+ + ne−

(5.1)

Air electrode: O2 + 4e- ↔ 2H2O + 4OH−

(5.2)

where M is metal ion and n is metal ion charge number.45

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FIGURE 5.3 Schematic representation of metal-air battery. Reproduced with permission from Ref. [44]. Copyright (2012), RSC.

When the cell is discharged, the metal anode becomes oxidized, and electrons are released to the external circuit. Simultaneously, the diffused oxygen at the cathode is reduced to oxygen-containing species by the electrons received from the anode. The dissociated metal ions and the molecules reduced in oxygen migrate over the electrolyte fuse together, forming metal oxides. The cycle is reversed with anode metal plating and cathode oxygen for rechargeable metal-air batteries. Battery performance is influenced by the oxygen transport property of electrolytes.44 Reports on metal-air batteries such as lithium-air,48–50 sodium-air,51,52 magnesium-air,53,54 zinc-air,55,56 aluminum-air,57,58 etc., are available in the literature. All the above batteries have about 2–10 folds higher theoretical energy density than lithium-ion batteries. The lack of effective and reliable bifunctional air-electrode catalysts that dramatically restrict battery efficiency, both in terms of rate power and long-term stability, is a challenge to these batteries.59 Carbon materials, especially carbon nanotubes, have been used as supports and catalyst elements for enhancing catalytic operation and stability. Song et al. synthesized cobalt and nitrogen-embedded CNT for rechargeable

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Zn-air batteries with stable charge–discharge durability over 12 h and a high open-circuit potential of 1.45 V.60 Introduction of CNT on catalyst surface good electronic conductivity and fast oxygen reduction reaction kinetics (ORR).61 Doped CNT such as nitrogen-doped CNT (N-CNT) produced a better discharge capacity for the electrode.62 5.3.4 SOLAR CELLS The global demand for clean, low cost, and renewable energy sources has never been higher. Solar energy is a key to clean future energy. We get much more energy from sun, and unlike fossil fuels, it will not run out soon at any time. As a source of renewable energy, the only limitation of solar power is our ability to convert it into electricity efficiently and cost-effectively. Solar cells harness energy from sunlight and convert it into usable electricity. They have great potential as a sustainable and alternate source of electricity production since the Sun is an inexhaustible source. Conventional silicon and photovoltaic semiconductor systems have many disadvantages, including poor illumination efficiency and high cost. CNTs are ideal replacements in photovoltaic devices due to their unique optoelectronic properties.63,64 CNT has been used as light absorbers, transparent electrodes, additives, and carrier transporters in solar cells, and they also studied as a means to enhancing electron transfer, replacing conventional platinum-based counter electrodes, particularly in dye-sensitized solar cells (DSSCs).65,66 The 1D nanoscale structure, outstanding electrical and mechanical properties, high mobility, environmental stability, etc., make CNTs ideal for the synthesizing of high-performance solar cells. The key technique for introducing CNTs into solar cells is to combine CNTs with an electrondonating conjugated polymer solution. This mixture is spin-coated on a transparent conductive electrode. The high electron affinity of polymersCNTs mixture facilitates exciton dissociation, producing free electrons in a high yield. This helps better flexibility of energy conversion.68,67 CNT/ Si heterojunction-based solar cells are reported to have high efficiency. A schematic illustration of SWCNT-assembled silicon solar cell is depicted in Figure 5.4. The SWCNT has been introduced to capture incident photons and assist in electronic transport at the silicon solar cell interface.69 Singlewall carbon nanotubes (SWCNT) have gained much research interest due to their outstanding electron-accepting and transportation ability.70 Dye-sensitized solar cells (DSSC) constitute another class of solar cells. They are introduced to overcome the drawbacks of traditional solar cells

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by combining low fabrication costs and greater conversion efficiencies. They generate electricity by the absorption photon by dye molecule. Carbon nanotubes (CNT) can be regarded as attractive alternatives to costly platinum (Pt) in the counter electrode of DSSC due to their exceptional properties, such as strong electrochemical behavior, high conductivity and low cost. CNT-based counter electrodes also exhibit high stability.71–73 Surfacemodified MWCNTs reported having high performance as counter electrodes of DSSC.74 TiO2-MWCNT as a photoanode in DSSC improved the physicochemical properties. MWCNTs cause electron–hole recombination and larger quantities of adsorbed dye, leading to enhanced performance.75

FIGURE 5.4

Schematic diagram of SWCNT-assembled silicon solar cell.

Perovskite solar cells (PSCs) are other classes of solar cells, with general formula ABX3 (A—monovalent cation, B—Sn or Pb, X—halide) gaining much research interest due to their easy manufacturing process and high efficiency. Even though it has these merits, PSCs are unstable in the presence of humidity and high temperature. This instability issue can be addressed by introducing CNT into PSCs.76 Mechanical strength, charge transfer capacity, chemical inertness of CNTs improve the stability of PSCs. The mechanical resilience of CNTs can be particularly helpful in developing flexible PSCs.77

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5.3.5 SUPERCAPACITOR Supercapacitors can be considered an alternative to conventional electrical energy storage devices because of their fascinating properties like high energy density, excellent reversibility, long-term operational stability, faster mode of operation, low heating, and safety. Supercapacitors find applications in varieties of domains such as mobile telecommunications, personal electronics, electric vehicles, pulse power systems, industrial power, and energy management.78–81 Based on their energy storage mechanism, supercapacitors are categorized into two: electric double-layer capacitors (EDLC) and pseudocapacitors. In EDLC energy is stored by adsorbing charges electrostatically, whereas in pseudocapacitor energy is stored by the reverse Faradaic redox reaction.82 Niu et al. were the first to propose (1997) that CNTs could be used in supercapacitors. Supercapacitors or EDLCs have fairly large power and energy densities and longer life cycles than the traditional ones. They can store large amounts of energy in their double layer and thus have higher power than batteries. Electrodes of supercapacitors can be made of polymers, oxides, or carbon and their composites. CNTs are desirable electrode materials for producing high-performance supercapacitors due to their appreciable properties of high electrical conductivity, high specific surface area, high charging power, and high electrolyte accessibility.83 Both MWCNTs and SWCNTs are studied for electrodes of supercapacitors due to their desirable properties.84–87 Because of their potentially higher surface area and larger conductivity, SWCNTs have been mainly used as electrodes for EDLC. The typical value of 40 Fg−1 specific capacitance is observed for 500 m2 g−1 surface area purified SWCNT. But the cost is a problem here.88 Research has been done to produce various types of CNT electrode materials and combine them with different electrolytes to further improve supercapacitor performance. Composite of CNT with transition metal oxides, conducting polymers, and heteroatoms are efficient supercapacitor materials.89 Mazurenko et al. fabricated CNT-carbon composite with uniform pore size and found an improvement in specific capacitance from 6 mF cm2 to 200 mF cm2.90,91 Cheng et al. fabricated graphene and single-walled carbon nanotube (SWCNT) composite film as the electrode for supercapacitor with a specific capacitance of 290.4 F g−1.92 CNTs of crystalline cylindrical shapes can be assembled into the 1D fiber, 2D film, 3D aerogels, and have superior mechanical properties and lightweight is highly useful in the production of flexible supercapacitor electrodes.93 Better performance is achieved with CNFs@CNTs, electrochemically inert functional groups, oxygenated groups,

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nitrogen-containing surface functional groups, composites with polymers, etc.88 The capacitance of CNTs can be enhanced by forming nanocomposites of CNTs. MnO2/CNT nanocomposite as electrode material for SC exhibited enhanced capacitance.94 The electrochemical performance of some MnO2/ CNT material is given in Table 5.1. TABLE 5.1

Electrochemical Performance of Different MnO2/CNT Materials.

Material

Electrolyte

Specific capacitance (Fg-1)

Stability

Reference

MnO2/MWCNT

Na2SO4 (0.2 M)

276 (3Ag−1)

MnO2/MWCNT

Na2SO4 (0.5 M)

793 (5 mV s−1)

91.6%

[97]

97%

[98]

Na2SO4 (0.2 M)

−1

MnO2/CNT MnO2/MWCNT

642 (10 mVs )

70%

[95]

KCl (1 M)

251 (0.5 Ag−1)

97%

[96]

5.3.6 FUEL CELLS A fuel cell is any class of devices that convert the chemical energy directly into electricity by electrochemical reactions. It has high energy conversion efficiency and low emissions. Fuel cells are regarded as favorable methods to solve the future energy crisis and environmental issues.99–102 Carbon nanotube’s magnificent conductive properties, powerful physical properties, and small mass density make them optimal and long-lasting material for fuel cells.103 A schematic representation of the CNT composite fuel cell is given in Figure 5.5. Usually, a catalyst like noble metal Pt is used to improve the efficiency of fuel cells. Even though Pt has good catalytic efficiency, it is expensive, which is a great hurdle for the fuel cell to be commercialized. A supported catalyst like CNT can help improve the capability of catalyst.104,105 CNT improves the performance of the fuel cell by improving the performance of the catalyst.106,107 Fuel cell using CNT as a support catalyst has larger current density, high catalytic activity, and the quality of transmission. It also helps to improve the corrosion resistance108 and stability of the catalyst and enhances the electrocatalytic properties. Carbon nanotubes used in fuel cells can reduce the need for noble metals used for catalysts, increase fuel cell efficiency, decrease fuel cell cost, and increase transmission capacity. In microbial fuel cells, MWCNTs improve anode electron transmission potential.109 CNT as a cathode catalyst display enhanced power and current density.110 Ramesh et al. reported that a combination of MWCNTs and

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SWNTs can improve the cathode quality activities and mass transfer when used as a Pt support.111

FIGURE 5.5

Representation of CNT composite hydrogen fuel cell.

CNTs desirable properties made them find use in polymer electrolyte membrane fuel cells. Usually, carbon black is the electrode material for fuel cells and contains a significant amount of organosulfur impurities. Replacing carbon black by CNTs reduces impurity levels. The presence of surface defects on CNTs improves the performance. CNT/CC (carbon cloth) composite provides better performance fuel cells by optimizing mass/ electron transport.88 Despite the above-mentioned merits, MWCNTs possess some demerits like weak interaction with the catalyst, which may result in low stability and faster corrosion, so modified MWCNTs are used. Modified MWCNT as electrode support in fuel cells showed enhanced performance.112–114 5.4 CONCLUSION Since their discovery by Iijima, carbon nanotubes (CNTs) have attracted much attention in various domains of life like electronics, energy, physics,

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chemistry, and material science. With their outstanding structural, electronic, and mechanical properties, CNTs are surprisingly ubiquitous in energy harvesting and storage. Since its introduction, there have been bountiful publications and patents aimed at scrutinizing the synthesis and applications of this material, and academic researchers and industry have made a promising breakthrough. CNTs, the most common 1D nanomaterial, may serve as a conductive component and electroactive substance, making CNTs a suitable energy storage system nanomaterial for supercapacitors, fuel cells, Li-ion batteries, metal-air batteries, etc. CNTs offer their outstanding benefits, such as lightweight, high electrical conductivity, superior electrochemical efficiency, and good physical robustness relative to other carbon materials used in conventional lithium-ion batteries and supercapacitors. They find beneficial application as electrode material for energy storage and harvesting systems, CNTs as electrode contribute an alternate storage mechanism that is not conceivable with bulk materials, the extensive electrode–electrolyte surface of contact for LIBs, etc. Carbon nanotubes technology constitutes a new facet for solid hydrogen storage, particularly if these materials can be modified to store huge quantities of hydrogen. The design of compact and self-powered high-performance electronic devices built on a single chip is a hot research spot. In addition, using CNTs may make the energy storage systems lightweight and stretchable, which is an essential aspect for wearable applications. While various energy applications of CNT have already been published, extensive opportunities remain for producing novel CNT materials for highly efficient energy conversion and storage systems. KEYWORDS • • • • • •

carbon nanotube fuel cells hydrogen storage Li-ion battery metal air battery supercapacitor

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CHAPTER 6

Piezoelectric Materials and Their Configurations for Energy-Harvesting Applications KAVYA RAVINDRAN1, V. T. JOHNSON2, C. J. ROSEMARY2, B. JAYASREE2, and PIUS AUGUSTINE1,2,3 Material Research Laboratory, Sacred Heart College (Autonomous), Thevara, Kochi, India 1

Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, India

2

3

Material Research Centre, Indian Institute of Science, Bangalore, India

ABSTRACT With the advancement in technologies, researches are widely been carried out to develop devices that meet the global energy crisis and environmental pollution. For the last two decades, the demand for wireless sensor devices, wearable and implantable electronic devices, high power density, and longer lifespan power sources1 are increasing day by day. Energy harvesters (EHs) possess the high potential to be considered as a promising candidate for independent power sources utilized in low-power electronic devices. Energy harvesting can be considered as an alternative energy solution for batteries and thereby developing pollution-free systems. There are several technologies adopted for energy harvesting which includes harvesting from light, temperature, mechanical motion, and temperature gradients.1 The mechanical EHs utilize vibration, kinetic energy, or deformations to be converted to electrical energy, and piezoelectric effect is a process in Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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which mechanical energy is converted to electrical energy and vice versa.1,2 Piezoelectric devices are rapidly increasing their demand due to its high efficiency for energy conversion, miniaturization, easy implementation, and high power output.2,3 Thus, piezoelectric energy harvesters (PEHs) with favorable features like enhanced piezoelectric coefficient, figure of merit, electromechanical coupling factor, and flexibility have received the keen interest amongst the scientific community.2,3 This chapter discusses the piezoelectric energy conversion principles, different piezoelectric materials both lead and lead-free systems, and piezoelectric EHs both in nanoscale and MEMS scale. This chapter also deals with the limitations and benefits; applications and possible improvements in piezoelectric energy-harvesting technologies. 6.1 INTRODUCTION Researchers paid a great deal of attention to scavenge the ambient-wasted energy mainly due to the advancement in wireless sensors and communication node networks so as to replace the batteries.1,8 The conventional energy production faces many challenges due to the environmental pollution, climatic changes, and global energy crisis. So, it is a need of the hour to develop renewable energy sources that are environmental friendly and costeffective.4 The piezoelectric materials are useful in converting mechanical strain and vibrational energy into electrical energy, and this property can be manipulated for developing renewable and sustainable energy which is achieved through power harvesting and self-sustained sensing devices.5 The development in the field of Internet of Things (IoT), wearable electronic technology, large-scale integration design, and device size downscaling laid a pathway for powering wireless sensors.1,6 Different technologies of energy harvesting like harvesting from ambient light, temperature, temperature gradient, mechanical motion, solar energy, and vibrations are utilized in this field.2 The main aim of piezoelectrics materials to be incorporated for EHs is to develop dense, highly efficient mechanical harvesters that can be integrated with sensors, electronics, transceiver, and other electronic devices in the form of a single chip.2 PEH can replace the electromechanical batteries so that an environment-friendly device can be generated.2 If lead-free piezoelectric materials are used as EHs, a complete pack of hazard-free devices can be fabricated. Not only commercial batteries, PEHs can also replace the complex wiring for powering microsystems and thereby benefits like overcoming toxicity, periodic replacing or recharging of batteries, battery disposal issues,

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and installation.2 The smaller size and unique and enhanced mechanical and electrical properties of the piezoelectric nanomaterials make them ideal for various applications like actuators, sensors, transducers, and even for energyharvesting applications. The challenges as well as opportunities in the field of small-scale power generation and harvesting of renewable energy sources emerged with the advent and progress of microscale and nanoscale devices.7 6.1.1 THE NEED FOR PIEZOELECTRIC ENERGY HARVESTING Presently, three kinds of energy-harvesting devices such as electromagnetic, electrostatic, and PEHs are used to harvest mechanical energy from the vibrations. The electromagnetic induction is the basic principle used in the electromagnetic vibration EHs.8 In these devices, when an external vibration is given, a magnetic flux is generated due to the relative movement between the induction coil and magnets and results in the formation of an induced voltage. Even though larger output is produced, limitation in device miniaturization with higher output voltage is a major disadvantage of such devices. The electrostatic vibration EH or capacitor vibration EH works on the principle that under the influence of an external power, a potential difference is generated above the plates of the capacitor to produce a stable output voltage and further changes are occurred due to the changes in the capacitance between the electrode plates.8 These devices can be incorporated to the MEMS technology and the output power generated is higher than the electromagnetic EHs but achieving the requirement of additional power is a major drawback. Thus, considering the drawbacks of both electromagnetic and electrostatic EHs, to achieve additional electric power, piezoelectric energy harvesting (PEH) became meaningful in further development and applications of devices. PEHs are highly compatible with MEMS technology and expected to produce 3–5 times larger power density than electromagnetic and electrostatic devices. Environmental adaptability, high voltage, compact structure, and working without an additional power supply are the major advantages of PEHs.8 However, PEHs also has its own limitations, and researches are still progressing to exploit the actual efficiency of PEHs and to expand their applications to self-powering, wireless sensor networks, and low-power electronic devices. Recent studies focus mainly to enhance the power density (output power per unit volume) of energy harvesting and to improve the efficiency of energy conversion.8 Figure 6.1 represents the large-scale and small-scale sources for energy harvesting.

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FIGURE 6.1

Classification of energy-harvesting sources.

6.2 PIEZOELECTRIC EFFECT Piezoelectric effect was first discovered by Pierre Curie and Jacques Curie in the year 1880.8 Piezoelectric materials possess the ability of electromechanical coupling either with developing electric charges on exposure to the applied mechanical stress which is known as the direct piezoelectric effect or the induction of mechanical stress or strain due to the applied electric field known as the converse piezoelectric effect.8 The direct piezoelectric effect finds its application even from the flashing lights in shoes to the motion or vibration-powered wireless sensors.10 The converse piezoelectric effect causes shape deformations and are utilized in actuator applications, piezoelectric transducers to create high-frequency acoustic waves for ultrasound medical imaging, to enable nanoscale positioning in scanning probe microscope.9 A pictorial representation of direct and converse piezoelectric effect is given in Figure 6.2, in which (a) represents the direct piezoelectric effect in which charge is generated due to the applied force and in (b) converse piezoelectric effect with developed strain due to the applied field is shown. In other words, piezoelectricity is defined as the coupling between the polarization generated and the applied stress or strain. The direct piezoelectric effect is represented in Einstein’s notation as9

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Pi = dijkσjk Pi = eijkxjk The converse piezoelectric effect is expressed as xij = dijkEk σij = eijkEk where dijk and eijk are the piezoelectric coefficients, i, j, and k are the integers varies as 1, 2, and 3. xij is the strain and σij is the stress.9

FIGURE 6.2 Schematic representation of piezoelectric effects.

The piezoelectric effects originate in particular materials due to the distribution of positive and negative ions in their crystal structure.10 In the absence of an applied external force, the ions will be in a state of equilibrium and materials, therefore remain neutral. In most of the piezoelectrics, the molecular dipoles are randomly arranged, and in order to obtain enhanced piezoelectric responses, poling is done.10 Poling is a process in which a high electric field is applied along with a high temperature to the material so as to align the molecular dipoles in the required direction and then subsequent cooling of the material after poling to sustain the orientation.10 6.2.1 UTILIZATION OF PIEZOELECTRIC EFFECT IN ENERGY HARVESTING Direct piezoelectric effect is utilized in energy harvesting. The two primary operating modes of a piezoelectric energy-harvesting device are 33-mode, where the applied stress would be in the direction of the polarization and the 31-mode in which the applied stress is perpendicular to the direction of polarization.4,8 Once the electric field is applied to the EH, the dipoles align in the same direction as the applied field.4,8 On applying the compressive force to the material in vertical direction, the material is polarized due to

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compressive strain, which induces a piezoelectric potential between the electrodes. A tensile force is developed to induce a reverse piezoelectric potential on releasing the applied force.4,8 The charge, Q, generated across the opposite faces of a piezoelectric material of area, A, is4,8 Q = d33 A∆σ where d33 is the longitudinal piezoelectric coefficient; voltage can be calculated as = V

d33

ε 33T

h∆σ

The energy, E, due to applied stress is = E

1 d332 C T At ∆σ 2 2 ε 33

Thus, energy obtained can be maximized for a certain thickness and area by choosing large

d332

which is known as harvesting figure of merit.4,8 ε 33T There is no effective power at open and at closed circuit conditions because the current is zero at the open circuit and there is no potential difference at the closed circuit. Thus, instantaneous power density is calculated as4,8 p=

V2 RA

where A is the effective area of electrodes and voltage across load resistance R is denoted by V. The performance of the piezoelectric EH depends on the mechanical to electrical conversion efficiency E% of the transducer and expressed as E% =

Pout 100

Pin

where Pout is the electrical output power and Pin is the mechanical input power defined as Pout = vp ip Pin = Fv where vp is the total voltage between the transducer’s electrodes, ip is the current passing through the piezoelectric transducer when the circuit is closed, F is the external mechanical force, and v is the speed of the moving object.4,8

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The working with piezoelectric material is also influenced by the change in polarization under the given mechanical stress. Three important factors that influence the strength and direction of the polarization under mechanical stress are as follows: orientation of dipoles or the polarization direction within the crystal, the crystal symmetry, and the amount of stress applied to develop mechanical deformation to the system. Any amount of change in polarization can be measured as change in surface charge density at the crystal faces and it is measured in Cm–2 or μC/cm2.4,8 6.3 PIEZOELECTRIC ENERGY HARVESTING Basically, an energy-harvesting structure has three parts which include the sources of energy, harvesting mechanism, and the load. The energy sources can be either ambient like sunlight, heat, wind, or external like lightening, vibrations, and heat that can be utilized to generate electrical power.4 The structure that converts ambient or natural energy into electrical energy is the harvesting system and the sink which stores or consumes the energy output is the load. In general, a piezoelectric EH has two basic parts and they are • mechanical module that generates electrical energy. • external module comprising an electrical circuit which converts and rectifies the generated voltage. The three general phases of a piezoelectric EH are depicted in Figure 6.3. • Mechanical energy conversion that accounts to the mechanical strength of the PEH under a large extend of stress/strain and the mechanical impedance matching. • Mechanical energy to electrical energy conversion which connects the electromechanical coupling and the piezoelectric coefficients of the PEH structure. • Electrical to electrical energy conversion accounts to the electrical impedance matching. • A suitable DC converter is used for the accumulation of electrical energy from a piezoelectric device having large impedance into a low impedance rechargeable cell or battery.8 The mechanical loss can be classified as mechanical strength, damping factor, or mechanical impedance. The coupling factor and piezoelectric coefficients can be attributed to the mechanical–electrical transduction loss.

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Electrical impedance and circuit loss lead to electric loss.4 As shown in Figure 6.3, the flow of energy between the different domains in phase 1 and phase 2 can be considered as the basic principle of piezoelectric cantileverbased EHs. The output power of the PEHs is influenced by the factors such as mechanical strength, piezoelectric coefficient, coupling and damping factors, electrical and mechanical impedance, circuit loss, and so on.8

FIGURE 6.3

Schematic representation of the three phases of piezoelectric energy harvesting.

Advantages • • • • • • • • • •

Miniaturized simple structure with ease of application High power density and high energy No external voltage sources required Scalability is good Versatile shapes are possible in piezoelectric transducers For a broad range of voltage production, piezoelectric materials could be meshed into hybrid materials Transducers can be easily incorporated to energy-harvesting structures High curie temperature (Tc) of piezoelectric materials Ease of installation and application Both macro- and microscale fabrications are possible

Disadvantages • Harvested power is low in comparison with other harvesting techniques (e.g., thermo-electric generator devices generate up to 125 W)4 • The EHs need rectification, extraction of maximum power, and regulation of output voltage • PEHs are not suitable for certain applications such as low-voltage CMOS processes because of their high voltage generation and low current output

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6.3.1 FREQUENCY RESPONSE IN PEHS An important feature of PEH is the frequency response because better performance of a PEH is obtained when resonance frequency and their input frequency resemble. In other words, high efficiency for a resonance-based EH is achieved when harvester’s resonant frequency matches the source frequency and even a small mismatch would influence the output voltage or output power.11 Thus, piezoelectric materials are selected accordingly to match the application frequencies, and also, the size and shape of the piezoelectric layers can be optimized according to the natural frequency of the system.11 Examples: Piezoelectric ceramics have superior piezoelectric properties and high stiffness, so they are used for applications that require high vibration frequencies (>100 Hz), while the piezoelectric polymers and composites are used in low frequency (≤30 Hz) applications. The frequency response functions (FRFs) can be classified into two groups; they are:11 • frequency response function: FRF of linear field variables is a function that relates the excitation force and output response velocity and also called mobility function.11 • Frequency response function for power variables: It is a function that relates the output power/squared voltage and the input power.11 6.4 PIEZOELECTRIC MATERIALS FOR PEHS Out of the 32 crystal classes present, 20 exhibit the direct or positive piezoelectricity and the other 10 classes are polar crystals which exhibit a spontaneous polarization (Ps) even in the absence of mechanical stress. Piezoelectrics are that class of crystals which lacks a center of inversion, that is, they are noncentrosymmetric materials. The relation between piezoelectrics, ferroelectrics, and pyroelectric is given below. Figure 6.4 represents the classification of the 32 different crystal classes. Those materials exhibit spontaneous polarization even in the absence of an applied field and this spontaneous polarization can be reversed with the application of a large electric field. In other words, dipole moment is reversible in ferroelectric materials on application of sufficiently large electric field.4 Almost around 200 piezoelectric materials are expected to be used in energy-harvesting applications and some of the major classifications of piezoelectric materials are discussed below:

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• Single crystals like Rochelle salt, lithium niobite, and quartz crystals • Lead and lead-free piezoelectric ceramics like lead–zirconium–titanate (PZT), barium titanate (BT), and so on • Polymers like polylactic acid, polyvinylidene fluoride (PVDF), copolymers, cellulose, and so on

FIGURE 6.4

Classification of the crystal classes.

The piezoelectric materials can also be classified as naturally occurring like Quartz, Rochelle salt, and Topaz and synthetic such as PZT, BT, and PMN-PT.4,9 The piezoelectric responses, design flexibility, and frequency of application are considered to select a piezoelectric material for energyharvesting applications. For the past several decades, PZT-based systems are commonly used for energy harvesting and concerning the toxicity of these lead-based systems; lead-free BaTiO3 (BT) piezoceramics are widely explored in this field. Even though the piezoceramics are highly efficient in energy-harvesting purposes, certain parameters like rigidity, brittleness, high density, lower voltage coefficient, and limitation in reduction of size inhibit their performances.4 Piezoelectric polymers are also a promising material for energy harvesting due to their mechanical flexibility to withstand high strain, ability to generate suitable voltage and output power, low fabrication

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cost, resist higher driving fields, and processing is faster than ceramics composites.4 A summary of the various piezoelectric materials used for energyharvesting applications is described in Table 6.1:4,9 TABLE 6.1

Different Piezoelectric Materials for PEH Applications are Tabulated.4,9

Piezoelectric materials

Properties

Lead-based piezoelectric • Polycrystalline ceramics (PZT, PMN-PZT, • Perovskite crystal structure PMN-PT, PZT-ZnO, • Easy fabrication process (e.g., solid-state reaction route) PZT-5A) • Reduced dielectric loss • High electrical properties • Toxicity due to the presence of lead is its demerit Lead-free piezoceramics (barium titanate (BT), sodium bismuth titanate (NBT), etc.)

• Environment friendly

Single crystal piezoelectric materials (ZnO, lead magnesium niobate (PMN-PT))

• Monocrystalline in nature and various methods like Bridgeman, flux methods are used for synthesis

• Perovskite structure • Lower transduction efficiency • Could replace toxic lead systems

• High piezoelectric properties • Mostly used in actuators and sensors • Nanostructure formation varies according to the synthesis methods adopted

Piezoelectric polymers (polyvinylidene fluoridederived polymers)

• Flexible • Not toxic • Light weighted • Electromagnetic coupling is small • Rapid processing and low manufacturing cost • Biocompatible, biodegradable • Low power consumption

6.5 PEH DEVICE CONFIGURATIONS The piezoelectric bulk ceramics exhibits higher Young’s modulus which suppresses the piezoelectric effect by stretching the piezoelectric materials.8 Various designs and fabrication methods like cantilever type, cymbal, stacks etc., are adopted to generate larger strains. Some of them are discussed in this section.8

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Cantilever PEH structure is one of the simplest and easier method to generate strain under the base excitation in a vibrating system.8 The lower energy conversion efficiency is one of the limitations of this configuration, which can further be improved by the adoption of different sizes and structure modifications.8 6.5.1.1 BIOMORPH OR UNIMORPH CANTILEVER-TYPE PEH STRUCTURE The biomorph or unimorph straight cantilever-type structure is mostly used in PEH usually for harvesting mechanical energy from the vibrations.13 Larger mechanical strains will be produced by the vibrations in the piezoelectric materials. The lower frequency of cantilever’s fundamental flexural modes is an important factor than any other structures or configurations. Figure 6.6 shows the cantilever PEH in different modes and the most common 31-mode bimorph cantilever contains two separate piezoelectric sheets which is bonded together by a center rim.

FIGURE 6.5

Two different modes in PEH applications.

Figure 6.5 represents the two different modes (i.e., 31-mode and 33-mode) of the piezoelectric energy-harvesting applications. In order to operate in bending mode, the cantilever-type structure is designed in such a way that the top layer of the elements has tension and bottom layers face compression or vice versa; then electric charges are developed due to the piezoelectric effect.13 The parallel or series poling of the top and bottom layers can be achieved by poling in the same direction or in the opposite direction and this would induce accumulated current/voltage by each layer. The piezoelectric elements are made of multiple layers with proper wiring and electrodes between each layer in bending-type cantilever.13

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In all these structures, the potential for power conversion is the same and the voltage or current rate is influenced by the poling direction and number of piezoelectric layers.13

FIGURE 6.6

Biomorph and unimorph cantilever structures in 33-mode and 31-mode.

For microelectromechanical (MEMS) implementation, unimorph cantilever structures are mostly used. As in Figure 6.6, in the 31-mode structure, the piezoelectric layers lie between the top and bottom electrodes, while in 33-mode, the electrode is place on the top of the piezoelectric layer with interdigital electrode pattern. In both 33-mode and 31-mode, the thin film is coated to the elastic substrate. Some of the conclusions from these cantilevertype PEH structures are as follows: • The piezoelectric mode (33-mode or 31-mode) determines the figure of merits and performances of the unimorph cantilevers.8,13 • In 33-mode devices, the voltage generation depends on the distance between the electrode fingers.13 • The electrode distance in 31-mode is shorter than the 33-mode due to the thin PZT layer. • The 33-mode EH is superior in high voltage output while 31-mode produces larger current output. Thus, we can conclude that the 33-mode

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cantilevers produce high voltage and power output by optimizing the IDE designs.13 6.5.2 PIEZOELECTRIC FILM CONFIGURATION FOR PEH Earlier researchers focused only on the rectangular type cantilever beams as its ease of implementation and high strain distribution for the given force input made them so popular. But in such kind of structures, the stress is concentrated only near the clamp ends during bending and the parts of nonstressed piezoelectric layers do not take part in the power generation. On the basis of theoretical and experimental evidences, tapered or triangular-shaped cantilever structures are proposed so that they are expected to replace the rectangular structures and also to produce more energy output. This structure could also help to reduce the weight of cantilever.13 To reduce the stiffness of cantilever structure, certain studies proposed zigzag or meandering beam shapes and also to lower the resonance frequency. Circular diaphragm is another mechanical structure adopted for PEH, but it is stiffer than cantilever of same size which results in higher resonance frequencies in the vibration mode operation.8,13 6.5.3 STACK CONFIGURATION OF PEH In the 33-mode PEH structure, the piezoelectric layers are completely stacked together and the piezoelectric coefficient d33 is found to be higher than the 31-mode. A 300-layered PZT stack used in PEH was reported by Xu et al.10 Since there are multiple stacked layers, the poling directions can be altered for the subsequent layers which enable the direction of the electric field toward the same electrode. However, due to the high stiffness of the stacks, high compressive force or coupling to mechanical force amplifiers is required.13 A cymbal-type force amplifying structure is proposed in utilizing these stack properties. A cymbal type comprises piezoelectric layers or stack and a metallic end cap on each side and on applying an axial stress to radial stress;13 a higher d33 is achieved due to charge generation in PZT stacks. Tufekcioglu and Dogan proposed a PEH prototype in which a cantilever beam is sandwiched between two cymbal transducers at the clamped ends.13,14 A compound double-staged force amplification frame with the piezoelectric layered stacks was form to improve the force even in small structures. This structure can obtain a force amplification 21 times enhanced

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with an energy transmission ratio of 18% so as to develop electric energy up to 79 times more than the PZT piezoelectric stack structure.13,15 Figure 6.7 gives the schematic summary of the various commonly used structures of piezoelectric EHs. Figure 6.7(a) represents the cantilever (zigzag) structure and (b) represents the unimorph or biomorph-type cantilever structure and both structures are of resonance type. Then, Figure(c) represents the cymbal structure in which a dynamic excitation is shown and (d) stacks structure for PEHs. Figure 6.7(c) and (d) belongs to the nonresonance category.

FIGURE 6.7

(a–d) The commonly used PEH structures are depicted.

6.6 CHALLENGES FACED AND SOLUTIONS The major challenges faced in the PEH include the bandwidth, CMOS compatibility, and biocompatibility. The detailed descriptions along with the solutions are given below. 6.6.1 BANDWIDTH The vibrational EHs operate mainly in resonance mode but their half-power bandwidth is found to be smaller, which is a major challenge in PEH. Since the frequency of ambient vibrations varies from 1 Hz to 240 Hz or several other wider frequency ranges depending on the applications, enhanced bandwidth properties are necessary. The abovementioned issue is mostly seen in the random ambient vibration sources because it is not practical to compute the vibration continuously and to physically tune the resonance frequency of

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the EH. Several researches to solve this issue proposed two major solutions, that is,16 • •

Dynamic resonance frequency tuning Wide bandwidth harvesters

The possibility of actively tuning the resonance frequency by using actuators was studied by Roundly and Zhang,16,17 and they also considered the requirements for increasing the net power. Sadegi et al.18 suggested to add a deformable support to the cantilever-type EH as a method to expand the bandwidth frequency of it. 6.6.2 CMOS COMPATIBILITY In monolithic circuits, the mechanical unit and the electrical unit are fabricated on the same structure. CMOS-compatible MEMS technologies allow the fabrication of these kinds of monolithic transducers and sensors. The advantages of monolithic fabrication include reduction of overall device size, cost and parasitic effects, and increased reliability. By developing a CMOS-compatible process, mechanical parts for a monolithic device can be fabricated without harming the electrical circuits.16 Thin-film PZT is the most commonly used material in MEMS-scale PEHs that is usually deposited with sol–gel method. In order to obtain piezoelectric characteristics, the coated films must be annealed at temperatures around 600–700°C at which crystallization occurs.16 Other methods for thin-film deposition like aerosol deposition or epitaxial growth require high temperatures, but they are not CMOS compatible. It was proposed that integration of bulk PZT on silicon substrate by a low-temperature bonding method is a solution to this issue. The bulk PZT crystal which is about 200 µm is mechanically thinned after the bonding since it is too thick for a MEMS-scale device. However, the proposed models are expected to ensure CMOS compatibility even at the increased fabrication complexity. ZnO and AlN can be used to form CMOScompatible piezoelectric layers as they need only low temperature and subsequent poling is not required for deposition. AlN makes a better candidate for CMOS-compatible energy-harvesting applications. AlN is found to be more compatible than ZnO with silicon semiconductor technology and also possess higher resistivity. Also, the reported output power density of AlN devices is relatively promising. Piezoelectric polymers are another class of piezoelectric material that can be used for CMOS-compatible EHs.16 PVDF and its copolymer, polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE)

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are the most commonly explored piezoelectric polymers.16 PVDF cannot be used on nonflexible substrates for piezoelectric applications as it requires mechanical stretching to switch to its piezoelectric phase. On the other hand, PVDF-TrFE films can crystallize into their piezoelectric β-phase without mechanical stretching if they are annealed at a temperature between its Curie temperature and melting temperature which is typically lower than 150°C, thus making PVDF-TrFE a suitable candidate for CMOS-compatible piezoelectric applications.16 6.6.3 BIOCOMPATIBILITY FOR THE ENERGY HARVESTERS The most promising applications of PEHs are found in implantable medical devices (IMDs). There is a significant progress in efforts to develop selfsustained IMDs using PEHs. One of the most important considerations in IMDs is the biocompatibility of the used materials.16 PZT is one of the mostly used piezoelectric materials but is not favorable for implants due to its lead content, which is a toxic material. As an alternative to PZT, the lead-free piezoelectric material, potassium sodium niobate (KNN) was proposed as they exhibit piezoelectric properties comparable to PZT. It was found that the performance of KNN-based PEHs was comparable to PZT-based devices. Biocompatibility of another widely used piezoelectric material, AlN, has been verified by in-vivo and in-vitro tests. A corrugated membrane-type AlN-based EH designed for aperiodic vibrations at low acceleration levels was proposed, which is the expected input type for an implanted device.16 It was also possible to demonstrate the AlN thin film deposition on polyimide to fabricate flexible devices that are excellent for implanted devices. Other alternative technique is using PVDF-based polymers to create a flexible, implantable EH. Biocompatibility of PVDF-TrFE and PVDF has also been verified. Biocompatibility and flexibility of these polymers make them a worthy candidate for implantable EHs.16 6.7 PEH IN MEMS SCALE The possibility of high-level incorporation of both modern CMOS technology and MEMS fabrication techniques influenced the manufacturing of monolithic sensor devices, especially in sub-cm-sized chips. During the last decade, many remarkable studies were reported on piezoelectric MEMS EHs. The functionality of vibrational EHs depends strongly on the dimensions, and as

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the mass gets smaller, the harvestable energy is also reduced because these devices harvest the mechanical energy from the moving mass.16 One of the major challenges in MEMS devices is to maximize the output power developed per unit volume for a certain input acceleration and it is a serious issue in thin films due to their lower electromechanical conversion efficiency. The unimorph configuration is mostly explored in MEMS PEH due to its lower electromechanical conversion than biomorphs and also the microfabrication techniques are not suitable for biomorphs. The resonance frequency is also a major issue in smaller dimensions. Most widely explored piezoelectric material in MEMS PEH is the lead-based highly efficient lead-zirconium titanate, PZT. The first proposed model by Jeon et al. (2013)16,19 was a rectangular cantilever which consists of a structural oxide layer and a sol–gel-coated thin film layer of PZT. This model also consists of an interdigital electrode layer that enables the d33-mode operation in cantilever structure and also a SU-8 proof mass layer. The electrodes also allow the output voltage to be determined irrespective of the material thickness by the finger spacing. The other material commonly used in MEMS-type EHs is the aluminum nitride.16 The MEMS-scale PEHs are mostly available in the cantilever-type structure to harvest the ambient vibrations. But at the same time, pressure mode EHs also developed in MEMS scale in which pressure differences in the surroundings are converted to energy. Diaphragm PEHs in MEMS scale are also developed and tested their efficiencies.16 Recent studies in this MEMS PEHs focus mainly to improve the operation bandwidth and to optimize the mechanical design to achieve maximum electromechanical conversion efficiency.16 6.8 PEHS IN NANOSCALE The PEH using the nanowires synthesized using piezoelectric materials falls to this category of PEHs in nanoscale.16 The piezoelectric nanomaterials are very smart and functional materials that found applications in automobiles, consumer products, medical diagnosis, and sophisticated scientific instruments. The characteristics like smaller size, lower power consumption, superior mechanical robustness, and enhanced performance of piezoelectric materials attracted many researchers. They are expected to develop as a potential candidate for renewable energy and sensor applications.7 This idea of piezoelectric nanowire generators using ZnO was first proposed by Dr. Wang’s group at Georgia Institute of Technology in 2006.16,20,21 In their experiment, a single nanowire was twisted by a conductive AFM

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probe which is in contact mode and a Schottky contact was created between the conductive tip of that AFM probe and ZnO nanowire due to the semiconducting characteristics of ZnO. Thus, a rectifying structure was formed which developed an output voltage in the external load resistance. The nanowires grown on flexible plastic substrates suitable for implantable EHs are also proposed by this same group. From these studies, the efficiency of ZnO nanowires in PEHs is exposed but thousands of such wires are required in practical applications. Various structures were developed to enhance their efficiencies; a zigzag-shaped top electrode on a vertically aligned ZnO nanowire array was proposed by Wang et al.16,22 In such devices, the power output is influenced by the density and height uniformity of nanowires and also on the distance between the nanowire array and top electrode. The conical-shaped nanowires are also adopted to expand the contribution of number of nanowires to the electrical output and a flexible multilayer generator of conical nanowire was used in LCD screen power supply. Self-powered wireless sensors to be used in pressure and speed sensors are also developed by using these nanowires.16,23,24 ZnO nanostructures are widely explored and a well-established synthesis method can exploit the ZnO nanowire properties in a better way and it will be a novel approach for renewable energy generation. Further studies and development and successful integration of piezoelectric nanogenerators with the sensors, telemetry, and control units can develop self-powered autonomous systems that could revolutionize the society in future.7 Energy harvesting using nanosystems for next-generation IoT devices is widely being studied recently. IoT devices used in various areas such as in medical and health, smart homes, transportation, and infrastructures helped to enhance the lifestyle. Wirelessly interconnected devices are an inevitable part of today’s world. These self-powered autonomous systems both in microscale and nanoscale are widely incorporated in sensors, actuators, and communication devices.25 One of the major challenges faced in this new generation powering IoT devices is their scaling to minimized sizes and to provide power to these tiny devices. Various sources like ambient or externally supplied sources including radio-frequency, optical, mechanical, chemical, thermal, nuclear, and biological methods can be adopted to develop EHs and to supply electric power for such micro and nanosystems.25 On reducing the dimension to nanoscale, nanowire technology can be used in which the piezoelectric nanogenerators will be used in powering microelectronics.25,26 Moorthy et al.27 discussed about the potential of dimensional scaling to power nanosystems using a single nanowire instead of the array of

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nanowires.25,27 Generally, the piezoelectric nanogenerators efficiently operate at a frequency of 100 Hz. Triboelectric effect and electrostatic induction can also be explored to generate electricity in mechanical systems.25,28 6.9 STRATEGIES TO ENHANCE PERFORMANCE OF PEH Nonlinearity, double pendulum system, frequency upconversion, and circuit management are some of the techniques to increase the EH efficiency.11 Nonlinearity has been proposed to be an effective method to improve the bandwidth for the PEHs. But certain properties inhibit the complete performance of nonlinear PEHs such as the nonlinear EHs have multiple vibration orbits. It also has a region of hysteresis under more excitations. Moreover, enhanced performance could be achieved only during the vibration of the system at its highest orbit. Under weak excitations, the performance of nonlinear harvesters is very poor. If the initial conditions are zero, small amplitude voltage responses are exhibited as these EHs follow the lowenergy orbits.1 Some methods are adopted to overcome these demerits of nonlinear PEHs. Zhou et al. proposed an initial impact method to overcome the potential barriers as discussed above and also to obtain high-energy interwell oscillations even under low-level excitations.1,30 Other research by Mallicket et al. presented an electronic control method. In this model, switching between low- and high-energy orbit of the nonlinear electromagnetic EH is achieved by utilizing the strong interaction between their electrical and mechanical degrees of freedom.1,31 A recent study by Lan et al. represented a voltage impulse perturbation approach using negative resistance which is helpful in triggering high-energy orbit responses of nonlinear kind of piezoelectric EHs.1,32 A robust and adaptive sliding mode controller was proposed by Hosseinloo et al. This model helps to shift the nonlinear EH to the desired attractor by short variations or excitations on that particular attractor.1,33 There are five mainstreams for enhancing the performance mechanism for frequency bandwidth broadening and power amplification:13 • • • • •

Multi-degree-of-freedom (multi-DOF) harvesting mechanism Monostable nonlinear mechanism Bi-stable nonlinear mechanism Frequency upconversion mechanism Hybrid-harvesting mechanism

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6.10 SUMMARY

FIGURE 6.8 A general outline of the piezoelectric energy harvesting is depicted. This represents the various groups of piezoelectric materials used in EHs and also the different fabrication techniques of PEHs. The configurations and mechanisms exploited for PEHs are also pictured here. Finally, the various applications of PEHs are represented.

6.11 CONCLUSION The different natural sources of mechanical energy in the form of vibration, human movements, wind, ocean waves, and so on can be exploited properly to harvest energy using the piezoelectric effect. Thus, the PEH can bring out novel research in producing renewable energy and self-powered autonomous systems. This chapter discusses the different piezoelectric materials and configurations of piezoelectric EHs. The strategies to enhance PEH performances are also discussed. Even though energy-harvesting technologies have been widely explored for decades, a considerable gap lies between the achieved and expected results. So far, EHs are developed for general purposes and then evaluated their performances, that is, they are tested under simplified harmonic excitations. But this approach does not provide a potential candidate for practical applications. However, certain purpose-oriented EHs are tested for their reliability, stiffness, stability, and compatibility but lack their establishments in the market. Thus, to cope up with these challenges, more and more research in this field is expected. Recent researches on the system-level investigations in which the PEHs are integrated with sensors and control units, energy storage elements, power

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conditioning circuits, energy storage elements, and so on are expected to bring a huge boom in this area and could change the decades of research efforts into a tangible benefit in the day-to-day life. However, a significant progress in piezoelectric energy-harvesting technology has been achieved during the last decade and a huge progress in various applications like selfpowered and autonomous working of wearable electronic devices, medical devices, wireless sensors, and sensor monitoring units is witnessed. Moreover, the recent advances in the IoT, energy harvesting, offers opportunities to establish smart cities, homes, smart health, agriculture, intelligent transportation, industry, security, and so on along with enhanced progress in the self-powered systems that can replace the batteries in the future. ACKNOWLEDGMENTS Pius acknowledges the Science and Engineering Research Board (SERB), Department of Science and Technology (DST), Government of India for the SERB_TARE grant (TAR/2020/000241) to associate with MRC, IISc, Bangalore, India. The financial support received under the Major Research Project from Sacred Heart College (SHMRP/2021/001) is also acknowledged. KEYWORDS • • • • • • •

piezoelectric energy harvesting piezoelectric effect piezoelectric materials frequency response cantilever type MEMS scale nanoscale

REFERENCES 1. Yang, Z. et al. High-performance Piezoelectric Energy Harvesters and Their Applications. Joule 2018, 2 (4), 642–697.

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2. Yeo, H. G.; Trolier-McKinstry, S. Piezoelectric Energy Generation. Ferroelectr. Mat. Energy App. 2018, 33–59. 3. Sezer, N.; Koç, M. A Comprehensive Review on the State-of-the-art of Piezoelectric Energy Harvesting. Nano Energy 2021, 80, 105567. 4. Covaci, C.; Gontean, A. Piezoelectric Energy Harvesting Solutions: A Review. Sensors 2020, 20 (12), 3512. 5. Chen, J. et al. Piezoelectric Materials for Sustainable Building Structures: Fundamentals and Applications. Renew. Sustain. Energy Rev. 2019, 101, 14–25. 6. Todaro, M. T. et al. Piezoelectric MEMS Vibrational Energy Harvesters: Advances and Outlook. Microelectron. Eng. 2017, 183, 23–36. 7. Jenkins, K.; Yang, R. Piezoelectric Nanomaterials for Energy Harvesting. In: Nanomaterials for Sustainable Energy. Springer, Cham, 2016; pp 193–213. 8. Li, L. et al. Recent Progress on Piezoelectric Energy Harvesting: Structures and Materials. Adv. Comp. Hybrid Mat. 2018, 1 (3), 478–505. 9. Mahapatra, S. D. et al. Piezoelectric Materials for Energy Harvesting and Sensing Applications: Roadmap for Future Smart Materials. Adv. Sci. 2021, 8 (17), 2100864. 10. Eom, C.-B.; Trolier-McKinstry, S. Thin-film Piezoelectric MEMS. Mrs Bulletin 2012, 37 (11), 1007–1017. 11. Gareh, S. et al. Optimization of the Compression-based Piezoelectric Traffic Model (CPTM) for Road Energy Harvesting Application. Int. J. Renew. Energy Res. 2019, 9 (3), 1272–1282. 12. Maamer, B. et al. A Review on Design Improvements and Techniques for Mechanical Energy Harvesting Using Piezoelectric and Electromagnetic Schemes. Energy Convers. Manag. 2019, 199, 111973. 13. Liu, H. et al. A Comprehensive Review on Piezoelectric Energy Harvesting Technology: Materials, Mechanisms, and Applications. Appl. Phys. Rev. 2018, 5 (4), 041306. 14. Tufekcioglu, E.; Dogan, A. A Flextensional Piezo-composite Structure for Energy Harvesting Applications. Sens. Actuator A Phys. 2014, 216, 355–363. 15. Wang, L. et al. Piezoelectric Vibration Energy Harvester with Two-stage Force Amplification. J. Intell. Mater. Syst. Struct. 2017, 28 (9), 1175–1187. 16. Toprak, A.; Tigli, O. Piezoelectric Energy Harvesting: State-of-the-art and Challenges. Appl. Phys. Rev. 2014, 1 (3), 031104. 17. Roundy, S.; Zhang, Y. Toward Self-tuning Adaptive Vibration-based Microgenerators. In Smart Structures, Devices, and Systems II. Vol. 5649. International Society for Optics and Photonics, 2005. 18. Sadeqi, S.; Arzanpour, S.; Hajikolaei, K. H. Broadening the Frequency Bandwidth of a Tire-embedded Piezoelectric-based Energy Harvesting System Using Coupled Linear Resonating Structure. IEEE ASME Trans Mechatron 2014, 20 (5), 2085–2094. 19. Jeon, Y. et al. Energy Harvesting MEMS Devices based on d33 Mode Piezoelectric Pb (Zr, Ti) O3 Thin Film Cantilever. CIRP Seminar on Micro and Nano Technology 2003. 20. Wang, Z. L.; Song, J. Piezoelectric Nanogenerators Based on Zinc Oxide Nanowire Arrays. Science 2006, 312 (5771), 242–246. 21. Song, J.; Zhou, J.; Wang, Z. L. Piezoelectric and Semiconducting Coupled Power Generating Process of a Single ZnO Belt/wire. A Technology for Harvesting Electricity from the Environment. Nano Letters 2006, 6 (8), 1656–1662. 22. Wang, X. et al. Direct-current Nanogenerator Driven by Ultrasonic Waves. Science 2007, 316 (5821), 102–105.

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23. Hu, Y. et al. High-output Nanogenerator by Rational Unipolar Assembly of Conical Nanowires and Its Application for Driving a Small Liquid Crystal Display. Nano Letters 2010, 10 (12), 5025–5031. 24. Hu, Y. et al. A Nanogenerator for Energy Harvesting from a Rotating Tire and Its Application as a Self-Powered Pressure/Speed Sensor. Adv. Mater. 2011, 23 (35), 4068–4071. 25. Phillips, J. D. Energy Harvesting in Nanosystems: Powering the Next Generation of the Internet of Things. Front. Nanosci. 2021, 3, 5. 26. Xu, S.; Hansen, B. J.; Wang, Z. L. Piezoelectric-nanowire-enabled Power Source for Driving Wireless Microelectronics. Nat. Commun. 2010, 1 (1), 1–5. 27. Moorthy, B. et al. Piezoelectric Energy Harvesting from a PMN–PT Single Nanowire. RSC Adv. 2017, 7 (1), 260–265. 28. Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology for Self-powered Systems and as Active Mechanical and Chemical Sensors. ACS Nano 2013, 7 (11), 9533–9557. 29. Wang, Z. L. On the First Principle Theory of Nanogenerators from Maxwell’s Equations. Nano Energy 2020, 68, 104272. 30. Zhou, S. et al. Impact-induced High-energy Orbits of Nonlinear Energy Harvesters. Appl. Phys. Lett. 2015, 106 (9), 093901. 31. Mallick, D.; Amann, A.; Roy, S. Surfing the High Energy Output Branch of Nonlinear Energy Harvesters. Phys. Rev. Lett. 2016, 117 (19), 197701. 32. Lan, C.; Tang, L.; Qin, W. Obtaining High-energy Responses of Nonlinear Piezoelectric Energy Harvester by Voltage Impulse Perturbations. EPJ Appl. Phys. 2017, 79 (2), 20902. 33. Haji Hosseinloo, A.; Slotine, J.-J.; Turitsyn, K. Robust and Adaptive Control of Coexisting Attractors in Nonlinear Vibratory Energy Harvesters. J. Vib. Control. 2018, 24 (12), 2532–2541.

CHAPTER 7

Light-Energy Harvesting Using Two-Dimensional Transition Metal Dichalcogenide MoS2 VIDHYA SIVAN1, E. P. JIJO2, TANIYA TOMY3, and PIUS AUGUSTINE1,3,4 Material Research Laboratory, Sacred Heart College (Autonomous), Thevara, Kochi, India 1

Department of Physics, St. Berchmans College, Changanassery, Kottayam, Kerala, India

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Department of Physics, Sacred Heart College (Autonomous), Thevara, Kochi, India

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Material Research Centre, Indian Institute of Science, Bangalore, India

ABSTRACT The extreme demand for energy is skyrocketing in order to keep up with rapid technological advancement. This growing demand for energy has paved the way for exploring advanced materials for the development of new green energy-harvesting systems as a part of sustainable energy management. Two-dimensional materials are the latest focus materials in the scientific world. They are eco-friendly and have high potential. It opens up wide applications beyond research limits.1 Among 2D, transition metal dichalcogenides (TMDs) represent the biggest class of layered materials. This chapter enumerates the unique structural and optical properties of the most promising TMD, molybdenum disulfide (MoS2), and its various applications in light-energy-harvesting systems. Advances in Energy Materials: New Composites and Techniques for Future Energy Applications. Iuliana Stoica, Ann Rose Abraham, & A. K. Haghi (Eds.) © 2024 Apple Academic Press, Inc. Co-published with CRC Press (Taylor & Francis)

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Molybdenum disulfide is a classic two-dimensional material that exhibits band gap tunability from indirect (bulk) to direct (mono-few layer) form.2 The unique structural, electrical, and optical properties of this layered material make it ideal for application in photodetectors and solar cells.3,4 This chapter begins with a brief overview of two-dimensional materials, followed by the TMD MoS2 structure and properties. The main content of this chapter will discuss the important light-harvesting applications of 2D MoS2 in the next-generation ultrathin solar cells, photocatalytic, and photovoltaic applications. 7.1 INTRODUCTION 7.1.1 STRUCTURE AND PROPERTIES OF MoS2 The urge to understand more about the properties of layered materials has led to the development of new 2D materials such as transition metal dichalcogenides (TMDs), topological insulators, and other novel materials. TMDs were discovered for the first time in 1923 by Linus Pauling. The structural phases of TMDs are triagonal prismatic 2H and 3R, and octahedral 1T metal atom coordination. ABA stacking is present in the 2H phase, with chalcogen atoms in the same place throughout all layers. The 1T phase is characterized by ABC stacking.17

FIGURE 7.1

Crystal structure of MoS2.

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TMDs may be generalized using the formula MX2 (where M is a transition metal and X is a chalcogen). It is possible to employ semiconductors, semimetals, metals, and even superconductors.5 Their intralayer covalent connections are strong, while their interlayer van der Waals forces are weak. Due to its abundance on the planet and its one-of-a-kind properties, by recent years, MoS2 has become the material of choice. The bandgap of MoS2 may be adjusted from indirect (1.2 eV in bulk form) to direct (1.8 eV in monolayer form).6 Its unique features make it ideal for optoelectronic applications. Individual layers are piled by the weak van der Waals force in the crystal structure of MoS2, which is similar to graphene. Each MoS2-stacked layer has a thickness of 0.65 nm. Hexahedral (H), tetrahedral (T), and rhombohedral (R) crystal forms exist in MoS2. The octahedral structure of the 1T phase contrasts with the triagonal prismatic structures of the 2H and 3R phases. The 2H and 3R phases are semiconducting, but the 1T phase is metallic. The 2H phase of MoS2 is the most stable configuration with lattice parameters: a = 3.15 Å and c = 12.30 Å. 3R, which is found in less than 3% of molybdenite ore, is another stable form of MoS2. The lattice parameters of these phases are a = 3.17 Å and c =18.38 Å.18 The 1T phase is metastable with lattice parameters a = 5.60 Å and c = 5.99 Å. These 1T phases can coexist in other phases like 1T′, 1T″, and 1T‴. Of these coexisting phases, 1T′ is superconducting, and 1T‴ can be either superconducting or insulating. Metallic MoS2 does not exist naturally and can be obtained by the transition of the 2H phase under specific conditions like laser irradiation, electron beam, and ion intercalation. The conductivity of the metallic phase of MoS2 is higher than that of the semiconducting phase and it also possesses high catalytic activity.7 The covalently bonded S–Mo–S crystal structure in two dimensions consists of a plane with hexagonally ordered Mo atoms sandwiched between two planes with hexagonally arranged sulfur (S) atoms. Each layer of the arrangement is stacked by the weak van der Waals forces. MoS2 also has a high mechanical strength and a high Young’s modulus, making it a good candidate for flexible electronics. It may also be utilized in photodetectors due to its configurable band gap and ability to detect a spectrum extending from UV–vis to NIR. There are two types of energy applications for MoS2: energy storage and energy generating.8 7.1.2 METHODS OF PREPARATION TMD synthesis can be done in two ways: top-down or bottom-up. Topdown methods include mechanical exfoliation, liquid-phase exfoliation, and

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sputtering. Physical vapor deposition (PVD), chemical vapor deposition, atomic layer deposition (ALD), and chemical solution processes are examples of bottom-up techniques. 7.1.2.1 TOP-DOWN APPROACH 7.1.2.1.1 Mechanical Exfoliation Mechanical exfoliation is a simple way to obtain MoS2 flakes. Sticky tapes can be used to peel off MoS2 flakes from bulk MoS2 crystal. Repeating the technique, a sufficient number of times, flakes of various sizes, shapes, and layers are obtained. Flakes thus obtained are deposited on a suitable substrate. Most TMDs have a lower weak van der Waals adhesion force between the layers than graphene; however by tweaking, MoS2 with a large area can be produced. Despite the fact that the process is relatively easy and direct, it does not produce flakes of the necessary quality.19

FIGURE 7.2

Synthesis methods of MoS2.

7.1.2.1.2 Liquid-Phase Exfoliation Exfoliation in the liquid phase begins with a bulk form of MoS2 that has a random shape, size, and layer number. Mechanical means and atomic

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intercalation are the two methods of synthesis. Sonication, shearing, stirring, grinding, and bubbling are examples of the first mechanical pathway. Surfactants can be added to solutions to prevent flakes from recombination. Electrolysis can also be used to exfoliate by squeezing the bubbles created during the process. Despite its low efficiency, tape-assisted exfoliation can improve the yield and efficiency of this process. Atomic intercalation is the second path. Intercalation of atoms such as Li into MoS2 can enhance layer spacing and make exfoliation easier. Liquid-phase exfoliation is a low-cost preparation process for producing nanosheets of the desired grade.18,19

FIGURE 7.3

Mechanical exfoliation of MoS2.

7.1.2.2 BOTTOM-UP APPROACH 7.1.2.2.1 Physical Vapor Deposition PVD is a bottom-up process for thin film and nanostructure deposition. There are three steps to the PVD process. 1. Solid precursor evaporation. 2. Transport vapor phase onto the substrate. 3. Condensation of vapor on substrate.

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This approach is suitable for the deposition of various materials in the form of thin films and nanostructures and it does not require any chemical processes. PVD technique includes (1) thermal evaporation, (2) sputtering, and (3) arch discharge. 7.1.2.2.2 Molecular Beam Epitaxy (MBE) Molecular beam epitaxy (MBE) is a sophisticated thermal evaporation technology that uses multiple vapor sources and extremely high vacuum to epitaxially produce thin layer crystals on crystalline substrates. Due to the existence of effusion cells and a high vacuum, MBE enables for regulated deposition thickness. One of the most significant advantages of this deposition approach is the ability to adjust the rate of deposition at both the atomic and molecular levels. The presence of effusion cells in the system aids in the generation of molecular beams with reduced interparticle collision before transit to the substrate surface. This means that evaporated atoms escape from the cell’s orifice by effusion, have a long mean-free route, and do not interact with other atoms or gases in the vacuum chamber.18–21 7.1.2.2.3 Electron Beam Evaporation (E-Beam Evaporation) Electron beam evaporation, in comparison to MBE, allows for large-area deposition at high rates. EBPVD entails evaporating anode material with an electron beam in a high-vacuum environment. A multigun and multisource EBPVD system is used. Either thermionic emission or filament field emission creates electron beams. High kinetic energy accelerates the beam, which focuses on the ingot. As the beam hits the ingot’s surface, the energy of the electrons is transformed into thermal energy. This method offers a high rate of deposition without the use of corrosive materials. The filament deterioration of the electron gun that probably causes non-uniform evaporation is a significant disadvantage of this approach.21 7.1.2.2.4 Pulsed Laser Deposition Pulsed laser deposition (PLD) is a PVD technique in which high-power laser sources create vapor from the target, which then condenses on the substrate.

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Through the chamber viewport, laser pulses will be focused on the target surface during film deposition, resulting in a laser spot with high irradiance and a local electric field. This local field can break the bonds between atoms, leading dielectric breakdown of target and the target to evaporate. The leftover laser pulses are subsequently absorbed by the vapor, which causes electrons and ions to heat up, resulting in laser plasma. A laser plume arises as a result of the continuing absorption, and it flows toward the substrate, which is directly opposite the target. PLD can be performed in either a vacuum or a low-pressure gas environment. It is known as reactive PLD when done in a reactive gas environment. In plasma-enhanced PLD, plasma is utilized to improve the interaction between the vapor and the gas environment, and in ion-beam aided PLD, ion beams are irradiated on the substrate surface during film deposition. The wavelength and energy of the pulsed laser have an influence on the quality of PLD-deposited samples.19,21

FIGURE 7.4

Schematic diagram of PLD.

7.1.2.2.5 Sputtering Sputtering is a popular PVD process with benefits over PLD and MBE. When a target collides with energetic particles like ions or atoms, atoms are released from the target. These atoms are then deposited onto the substrate

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and is called PVD by sputtering. The technique is based on the conservation of energy and momentum. Sputter can be produced in a variety of ways, including DC, AC, RF plasmas, and external ion beams. Sputtering has several benefits over other processes, including a broad deposition area, ability to deposit alloys/composite materials and materials with high melting points, and prevent from X-ray damage caused by e-beam evaporation. This approach also has certain disadvantages, such as the possibility of sample contamination owing to low vacuum levels, film morphology being rough/ even destroyed by collision of growth species, and growth species aggregation with regard to deposition pressure.20,21

FIGURE 7.5

Schematic diagram of standard sputtering technique.

7.1.2.2.6 Chemical Vapor Deposition Chemical vapor deposition is a compatible deposition method to semiconducting technology. Growth of films on the substrate involves a suitable chemical reaction. Optimizing the temperature and use of suitable metal catalyst favor to produce desired quality of films. Plasma-enhanced chemical vapor deposition and metal-organic chemical vapor deposition are other chemical deposition techniques used to produce single crystal epitaxial films.19

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FIGURE 7.6

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Schematic illustration of CVD synthesis of clean and pure MoS2.

7.1.2.2.7 Atomic Layer Deposition ALD is a technique for producing thin and thick films with low impurities. The approach entails spraying gas precursors over the substrate surface, which causes the substrate to react with molecules in a resistive way, allowing the formation of thick layers. The thickness of the film may be successfully controlled using this method. ALD is a suitable technique for factor applications such as catalysts and sensors.18,21 7.1.2.2.8 Chemical Solution Process The solvothermal and hydrothermal are two solution chemical processes of preparation of MoS2 nanosheets. The reaction between sulfide and molybdnate takes place in an autoclave where a number of physicochemical reactions occur. The temperature and pressure are controlled to obtain the desired product. The resultant MoS2 powder possesses varied shapes. The difference between solvothermal and hydrothermal methods is that former one is not aqueous.19

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7.1.3 GENERAL APPLICATIONS Because of its unique electrical and optical properties, 2D TMD MoS2 is used in electronics such as photodetectors and storage devices. The key reason for this material’s emphasis is its unusual structure and the capability of band gap engineering. Because of the lack of a band gap, graphene, an amazing 2D material with high electrical performance, fails to switch control. In contrast, MoS2 has a configurable band gap with modification in the number of layers, but its mobility is not comparable to graphene, preventing it from being used as a transparent electrode. Thus, new research emphasis areas include the manufacturing of hybrid structures based on 2D materials and their use in energy storage and harvesting. Because it is less hazardous, it is also used for biological applications. 7.2 ENERGY STORAGE APPLICATIONS 7.2.1 LITHIUM-ION BATTERY Growing demand for energy has increased tremendously over the past few decades. Advances in electronic devices and automobiles have brought great concern for the development of high-energy density batteries with high storage efficiency, a long-life span, and environmental friendliness.9 Because MoS2 has a layered structure and a theoretical capacity-based storing mechanism, as well as a wide interlayer distance, lithium ions may diffuse quickly without expanding in volume. TMDs have sparked interest in their application for energy storage and conversion due to their structural, optical, and mechanical properties. Thus, MoS2 can play a vital role in Li-ion batteries as both anode and cathode because of its high capacitance.10 It has high theoretical capacity of about 670 mA hg−1. The conductivity of the 1T phase is 107 times greater than that of the 2H phase between the two crystal phases (1T and 2H). Hence, the 1T phase MoS2 is the most preferred phase in LIBs. From the previous reports by Song et al., excellent performance of lithium-ion battery with interlayer metallic MoS2 aligned vertically on graphene has shown interlayer distance of 0.98 nm much larger than actual 0.6 nm and high capacity of ~1700 mA h g−1. At a current density of 5 A g−1, the metallic nanotubes are said to have a reversible specific capacity of ~935 mA h g−1 up to 200 cycles.11,12

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7.2.2 SODIUM-ION BATTERY Sodium, being an earth-abundant, low-cost, less toxic, and readily accessible material, can be a promising option for storage batteries. Sodium-ion batteries (SIBs) can thus be a good alternative to Li-ion storage batteries. The radius of the sodium atom (99 pm) is an influential parameter for the overall performance of the batteries. Sodium being an attractive element located in same group of lithium in periodic table can easily replace lithium. The normal potential of Na+/Na (−2.71 V) is comparatively higher than Li+/Li (−3.05 V). These are few important parameters that point out the potential application of SIBs for renewable energy and smart grids.16 Similar to LIBs, interlayer spacing between MoS2 layers enables Na+ ion insertion/extraction to/from the layered structure and enables reversible storage capability of MoS2. It may be used to store and convert renewable energy sources like wind and solar energy on a huge scale. Furthermore, the large surface area of MoS2 allows sodium ions to be absorbed, which can also improve the performance of SIBs. Ultrathin nanosheets of the TMD MoS2 used in SIB have provided high storage, good cycling stability, and excellent performance rate.13 7.2.3 LITHIUM SULFUR BATTERY Due to its high theoretical capacity (1672 mA g−1), high energy density (2600 W h kg−1), and low cost, batteries with sulfur cathode have attracted attention as an energy storage technology for the next generation. These batteries have a sulfur-based cathode, binder, separator, organic liquid electrolyte, lithium anode, and collector and are rechargeable. When MoS2, a graphene-like two-dimensional layered material, combines with Li+, it has a potential storage capacity of 670 mA hg−1. The high theoretical lithium storage capacity of LS batteries is because of the special structural characteristics of MoS2. In MoS2, S–Mo–S are covalent bonds in a hexagonal sandwich-like structure. Since interaction between layers is limited, it is easy to embed materials into it. Using MoS2 as the cathode material, the electrochemical interaction between MoS2 and Li+ enables for adsorption of MoS2 and inhibits polysulfide groups from dissolving Li–S batteries. Over thousands of charge/discharge cycles, it provides less volume growth and good stability. In applications such as Li–S batteries, 2D MoS2 offers unique properties that are utilized. The enormous surface area of these 2D materials

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creates an active location for electrode materials and electrolyte to interact, allowing for rapid electrochemical reaction kinetics. It can also prevent the shuttling effect of polysulfides. Thus, MoS2 as a cathode material allows strong adsorption of polysulfides, fast electron transfer, and catalysis. As a cathode material, it reduces the shuttling effect. Strong adsorption of MoS2 with Sulphur containing groups yields good cycling stability over thousands of charge/discharge cycles.14 7.2.4 SUPERCAPACITORS Supercapacitors are the most widely used energy storage devices that store energy by means of electrical double layer or by redox reaction. Supercapacitors feature a large power density, a quick charge time, a long cycle life, and a low cost when compared to regular capacitors. Depending on storage mechanism, these are categorized as 1. Electrical double-layer capacitor 2. Pseudocapacitors In electrical double-layer capacitors, their storage mechanism is nonFaradic and no transfer of carrier takes place during charge and discharge cycle of electrode–electrolyte interface and thus storage is found to be electrostatic in nature. Carbon-based materials such as activated carbon, carbon nanotubes, and graphene are examples of electrical double-layer capacitors.14,15 In pseudocapacitors, reversible reaction occurs between electrode and electrolyte and here the mechanism is Faradic. Metal oxides and conducting polymers are examples of pseudocapacitors. Although many carbon-based and porous materials are used in supercapacitors, graphene analog TMD MoS2 with electrochemical properties are found promising. Supercapacitor applications take advantage of MoS2’s unique chemical and physical characteristics, which are comparable to those of graphene. Ions are introduced by effective intercalation because MoS2 is made up of layers that provide a wide surface area for charge storage. Electrochemical features can be obtained by proper exfoliation and restacking to form electrodes. Even though carbon-based supercapacitors have gained much attention, MoS2-based supercapacitors have the capability to achieve capacitance of 400–700 F cm.−3 In ionic liquids, 1T MoS2 with an interlayer spacing of 0.615 to 1.615 nm may achieve large volumetric and gravimetric capacitances of around 118 F cm−3 and 42 F g−1. Various MoS2-based materials such as MoS2 nanosheets; nanospheres,

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nanoporous, and mesoporous MoS2; porous tubular C/MoS2; MoS2 graphene; and MoS2/PANI have shown good supercapacitor performance.15 7.3 ENERGY GENERATION APPLICATIONS 7.3.1 HYDROGEN EVOLUTION REACTION The energy crisis and effective energy generation mechanisms are always topics under discussion among the research communities. Availability and effective use of suitable, cost-effective, and pollution-free sources and catalysts pose a challenge in energy production. Among many innovative approaches in this regard, sustainable hydrogen production has attracted growing attention. Hydrogen as an effective source of energy marks its relevance primarily because of its unlimited and nontoxic natural resource, that is, water and also its high fuel value (~143 MJ kg−1) which is almost three times that of gasoline.7,22 Hydrogen is a secondary form of energy since it must be produced from natural resources. One of few properties that make hydrogen a unique fuel is that it burns rapidly and has a high-octane number and zero toxicity. Among all fuels, it produces the least pollution when ignited in the air.23 There are various sources of hydrogen, like natural gas and other light hydrocarbons, oil, coal, fossil fuels, and so on. Fossil fuel and carbon sources as sources of hydrogen generate carbon dioxide as a coproduct, which in fact adds to the problem of greenhouse effect.24,25 Hydrogen may also be created from sustainable energy sources such as biomass, including both solid biomass like pellets from specific energy crops and waste biomass, and liquid manure fermentation. However, the yield in the above methods is found to be low.23 Production of hydrogen from water proposes a better environmental solution due to its abundant availability and absence of hazardous by-products of any kind. Hydrogen evolution reaction (HER) in simple terms is the cathode reaction in the electrochemical water splitting. 2H+ + 2e– → H2 HER is thus an example of a two-electron transfer reaction with one catalytic intermediate.25 Platinum is the most well-known HER catalyst. However, its scarcity and high cost restrict its use in large-scale hydrogen production. One of the most cost-effective alternatives for platinum for hydrogen production is MoS2.26 It is a low-cost and earth-abundant catalyst

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both in electrochemical HER and water-splitting reactions for the production of hydrogen.27,28 Water splitting is a catalyst-assisted chemical reaction in which water is separated into hydrogen and oxygen.26 High activity, excellent stability, and precious metal-free composition make MoS2 and its composites a promising class of catalysts for HER.29 MoS2 has a large number of electrostatic active edges and structural defects on its basal planes which make it a good catalyst.7 Unsaturated Mo and S atoms are generated at the edge of MoS2 due to the S–Mo–S coordination in the crystal lattices, which results in the so-called “edge activity” of MoS2.22 Most of the studies show that catalytic activity of 2H-MoS2 in the HER correlates with its active edge sites rather than basal planes, that is, structural defects. Improving the active sites is also, therefore, vital in improving its catalytic properties.30 Defect on the basal plane would imply a molybdenum or sulfur vacancy which enhances the catalytic activity of MoS2 for the dissociation of H2O and hence HER. MoS2 with Mo vacancies are found to have better catalytic activities for the HER when compared with the well-known Pt catalyst.27 One of the studies shows that activation and optimization of the basal plane of 2H-MoS2 by creating sulfur (S) vacancies and straining the S-vacancies improve hydrogen adsorption resulting in improved HER activity.31 There have been many other important studies on improvising the catalytic activity of 1T-MoS2 in the recent years. When compared to the 2H phase of MoS2, monolayers of MoS2 produced by chemical exfoliation exhibit greater catalytic activity in HER. In the 1T phase, the hydrogen binding energy is brought near to zero (∆G ~ 0), and ∆G ~ 0 for a catalyst would suggest that the catalyst’s binding and release of hydrogen in the HER is simple. In the case of 1T-MoS2, it is found that the basal planes are more active than the edges in the hydrogen evolution.15 One of the best current MoS2 catalysts available for hydrogen evolution was obtained by a structural alteration done on it. In this method, which is based on a microwave-assisted strategy, MoS2 nanosheets with edgeterminated structure and increased interlayer spacing were produced. Edge termination creates more vacancies and thereby active sites enhancing catalysis. MoS2 prepared by this method shows an interlayer spacing which is 1.5 times the interlayer spacing of the standard 2H-MoS2. The expansion of interlayers modifies the electronic structures and electrical conductivity of MoS2 resulting in enhanced catalytic activity. This method can be conceived as one of the cheapest methods to produce catalysts for HER, with very high efficiency.28

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Formation of composites of MoS2 with various compounds is also reported to bring out excellent result in HER activity and is often found to be superior to that of the pristine MoS2 nanosheets. For instance, an n-type graphene with p-type MoS2 is one such composite with enhanced photocatalytic and electrocatalytic activity for hydrogen production. Graphene acts as electron collector, allowing better charge separation leading to more H+–e− union to form H2 molecules.15 7.3.2 OXYGEN EVOLUTION REACTION In the water-splitting process, the oxygen evolution reaction (OER) is a complimentary reaction. HER occurs at the cathode while OER occurs at the anode in a water-splitting cell. OER is given below as H2O →

1 O 2 + 2H + + 2e − 2

Water splitting involves the proper catalytic activities both in HER and OER, the two half reactions resulting in the production of H2 and O2, and the latter being more complex of the two.29 Like in the case of HER, the benchmark catalysts for OER like precious metals and their oxides such as IrO2, RuO2, and Pt are costly and rare. This has led the research community to devote enormous effort to looking for cost-effective and earth-abundant catalysts. In addition, the slow pace of OER had to be overcome too. MoS2 has been identified as a suitable alternative here as well, due to its exposed edges and vacancy defects.32 Improvisations done on MoS2 in terms of structural alterations and development of different composites are found to better its catalytic activity. MoS2 quantum dots,33 direct growth of MoS2 microspheres on Ni foam,32 and Co3O4–MoS2 heterostructure,34 are a few to mention. 7.3.3 CO2 REDUCTION REACTION (CO2RR) Electrochemical reduction of CO2 has become one of the major sources of fuel production. It has two simultaneous effects in total; one being a solution to the environmental problem of global warming, the second is the actual production of fuel, which is called e-fuel (electro fuel). This involves the production of fuels from hydrogen and CO2 through photocatalytic or electrocatalytic activity.35

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Few electrochemical reductions of CO2 to various chemicals and fuels are given below: CO 2 + 2H + + 2e − → HCOOH CO 2 + 2H + + 2e − → CO + H 2 O CO 2 + 8H + + 8e − → CH 4 + 2H 2 O 2CO 2 +12H + +12e − → C2 H 4 + 4H 2 O 2CO 2 +12H + +12e − → C2 H 5 OH + 3H 2 O36

In recent decades, many catalysts have been tried out for CO2RR. Metallic electrodes like Ag, Cu, Zn, and Pd were reported to demonstrate acceptable performance on electrocatalytic CO2RR. However, requirement of relatively large overpotential, less possibility of large-scale production of Ag and Pd make them less opted choice as suitable catalysts. Furthermore, metallic electro-catalysts create a mixture of H2, C1, C2, and C3 hydrocarbon species, limiting their practical applicability. Recently, nanostructured MoS2 has attracted attention as one of the most suitable catalysts for CO2RR due to its atomic arrangement and layered crystal structure. Its good electrical transport, excellent light absorption and possibility of large-scale production are its added advantages to be used as catalyst for CO2RR.37 The inclined MoS2 edges can absorb CO2 and reduce it to CO.38 Heterostructures of MoS2 are found to be more effective as catalysts in the CO2RR. Bi2S3–MoS2 composite is one such example. The p–n junction Bi2S3–MoS2 has better light absorption and CO2 adsorption when compared to the pristine MoS2 or any other single compound.39 7.3.4 SOLAR CELL The overuse and use of conventional fuel have resulted in a shortage of these sources. As a result, ecologically friendly sustainable energy technologies such as solar, wind, and water are being used. Si-based solar cells are extremely costly and need complicated production procedures. As a result, low-cost, highly efficient perovskite-type solar cells, organic solar cells, and silicon-based solar cells were developed. Graphene is one of the most widely researched and utilized two-dimensional materials for energy-harvesting applications. However, the lack of an inherent band gap limits its applicability. As a result, the graphene equivalent MoS2 with a large band gap emerged as a promising option to replace graphene. MoS2’s

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outstanding light-matter interaction and optical qualities enable it to be used in energy-harvesting applications.40,41 2D-layered materials having band gaps along the visible region of the electromagnetic spectrum may absorb 5–10% of incoming light within 1 nm thickness and reach at least one order of magnitude greater absorption than GaAs and Si4. Thus, MoS2 plays vital role in the newest solar cells, where MoS2 flakes can be used as electron transport layer, hole transport layer, or buffer layer to enhance efficiency of the solar cells. From the previous reports of Capasso et al., and Abd Malek et al., the power conversion efficiency of MoS2 as ETL and HTL were 3.9% and 3.36%, respectively.42,43

FIGURE 7.7

Schematic illustration of typical n-MoS2/p-silicon solar cell.

7.4 CHALLENGES MoS2’s unique features have allowed it to be employed in a variety of energy storage and generation applications; nevertheless, it faces several obstacles and requires more development before it can be used on a commercial scale. Synthesis approaches face obstacles in producing MoS2 on a big scale and at a reasonable cost. The conductivity of MoS2’s 2H phase has to be improved, while the stability of the 1T phase needs to be addressed.45

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LIB is really competitive, but they still need to improve cycling stability and manufacturing costs. The use of MoS2 as an interlayer in these batteries can operate as a protective layer; however, oxidation and direct contact with air can have an impact on the device’s storage and quality. As a result, MoS2 composites should be targeted for future energy-generating applications. The crystallinity, purity, and particle size must all be engineered.44,45 7.5 CONCLUSION This chapter describes the structure, characteristics, and synthesis techniques of 2D TMD, molybdenum disulfide, as well as its energy storage and generation applications. As a graphene analog with a distinct structure, it finds use in energy harvesting and generation. MoS2’s extraordinarily high conductivity and weak van der Waals forces between layers make it an excellent choice for use as an anode material in LIBs. MoS2 as a cathode permits MoS2 adsorption and prevents polysulfide dissolution in Li–S batteries, ensuring stability over thousands of charge/discharge cycles. MoS2 is also used in supercapacitors, which have a high capacitance. The HER, OER, and CO2 reduction process are all examples of MoS2’s energygenerating applications. MoS2 also plays an important role in the latest solar cells as a buffer layer, ETL, and HTL to boost efficiency.7 Thus, the unique optical and electrical features of MoS2 with adjustable band gaps in various energy-harvesting and storage applications are described in detail.

FIGURE 7.8

Illustrative diagram of challenges faced by MoS2.

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ACKNOWLEDGMENTS Pius acknowledges the research grant, Teacher Associate for Research Excellence (TARE-TAR/2020/000241) by the Science and Engineering Research Board (SERB), DST, Govt. of India, which facilitated the establishment of Material Research Laboratory in SH College and research association with Prof. Karuna Kar Nanda, Material Research Centre, Indian Institute of Science Bangalore and Institute of Physics (IoP), Bhubaneswar. The financial support received under the Major Research Project from Sacred Heart College (SHMRP/2021/001) is also acknowledged. KEYWORDS • • • • • •

TMD PLD hydrogen evolution reaction Li-ion battery supercapacitor solar cell

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30. Wang, H. et al. Structural and Electronic Optimization of MoS2 Edges for Hydrogen Evolution. J. Am. Chem. Soc. 2019, 141 (46), 18578–18584. 31. Li, H. et al. Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat. Mater. 2016, 15 (1), 48–53. 32. Yan, K.; Lu, Y. Direct Growth of MoS2 Microspheres on Ni Foam as a Hybrid Nanocomposite Efficient for Oxygen Evolution Reaction. Small 2016, 12 (22), 2975–2981. 33. Mohanty, B. et al. MoS2 Quantum Dots as Efficient Catalyst Materials for the Oxygen Evolution Reaction. ACS Catal. 2018, 8 (3), 1683–1689. 34. Muthurasu, A.; Maruthapandian, V.; Kim, H. Y. Metal-organic Framework Derived Co3O4/MoS2 Heterostructure for Efficient Bifunctional Electrocatalysts for Oxygen Evolution Reaction and Hydrogen Evolution Reaction. Appl. Catal. B: Environ. 2019, 248, 202–210. 35. He, J.; Janaky, C. Recent Advances in Solar-driven Carbon Dioxide Conversion: Expectations versus Reality. ACS Energy Lett. 2020, 5 (6), 1996–2014. 36. Sun, Z. et al. Fundamentals and Challenges of Electrochemical CO2 Reduction Using Two-Dimensional Materials. Chem 2017, 3 (4), 560–587. 37. Xie, Y. et al. Reaction Mechanisms for Reduction of CO2 to CO on Monolayer MoS2. Appl. Surf. Sci. 2020, 499, 143964. 38. Asadi, M. et al. Robust Carbon Dioxide Reduction on Molybdenum Disulphide Edges. Nat. Commun. 2014, 5 (1), 1–8. 39. Kim, R. et al. Carbon Dioxide Photoreduction on the Bi2S3/MoS2 Catalyst. Catalysts 2019, 9 (12), 998. 40. Ma, J. et al. High Efficiency Graphene/MoS2/Si Schottky Barrier Solar Cells Using Layer-controlled MoS2 Films. Sol. Energy 2018, 160, 76–84. 41. Tsai, M.-L. et al. Monolayer MoS2 Heterojunction Solar Cells. ACS Nano 2014, 8 (8), 8317–8322. 42. Capasso, A. et al. In Spray Deposition of Exfoliated MoS2 Flakes as Hole Transport Layer in Perovskite-based Photovoltaics, 2015 IEEE 15th International Conference on Nanotechnology (IEEE-NANO), IEEE, 2015. 43. Malek, A.; Ain, N., et al. Ultra-thin MoS2 Nanosheet for Electron Transport Layer of Perovskite Solar Cells. Opt. Mater. 2020, 104, 109933. 44. Cha, E., Kim, D. K.; Choi, W. Advances of 2D MoS2 for High-Energy Lithium Metal Batteries. Front. Energy Res. 2021, 9, 44. 45. Shi, S.; Sun, Z.; Hu, Y. H. Synthesis, Stabilization and Applications of 2-dimensional 1T Metallic MoS2. J. Mater. Chem. A 2018, 6 (47), 23932–23977.

CHAPTER 8

The Smart Chromogenic Hydrated WO3 for Energy-Storage Applications M. MANUJA1, PIUS AUGUSTINE2,3, and GIJO JOSE1 Research and Postgraduate Department of Physics,

St. Berchmans College (Autonomous), Changanasserry, Kottayam, India

1

Materials Research Laboratory, Sacred Heart College (Autonomous) Thevara, Kochi, India

2

3

Materials Research Centre, Indian Institute of Science, Bangalore, India

ABSTRACT Layered transition metal oxides find various applications in electrochromic devices, supercapacitors, photocatalytic activity, electrochemical properties, and so on to name a few. The smart chromogenic and layered hydrated WO3 is a promising candidate as a good electrode substitute for lithium ion, in solid-state battery fabrication for future energy markets. This can overcome the hurdles of shortage of Li-ion in the earth’s crust and the higher initial cost of battery fabrication. Being a smart chromogenic material, it can respond to external stimuli such as temperature, pressure, and voltage with very fast switching time ( J. Mater. Chem. 2008, 18, 153–157. 77. Zhang, M.; Zhao, N.; Li, W.; He, C.; Li, J.; Shi, C.; Liu, E. A Novel Synthesis of CNTs/ TiO2 Nanocomposites with Enhanced Performance as Photoanode of Solar Cell. Mater. Lett. 2013, 109, 240–242. 78. Akhtar, M. S.; Park, J. G.; Lee, H. C.; Lee, S. K.; Yang, O. B. Carbon NanotubesPolyethylene Oxide Composite Electrolyte for Solid-State Dye-Sensitized Solar Cells. Electrochim. Acta 2010, 55, 2418–2423. 79. Nath, B. C.; Gogoi, B.; Boruah, M.; Sharma, S.; Khannam, M.; Ahmed, G. A.; Dolui, S. K. High Performance Polyvinyl Alcohol/Multi Walled Carbon Nanotube/Polyaniline Hydrogel (PVA/MWCNT/PAni) Based Dye Sensitized Solar Cells. Electrochim. Acta 2014, 146, 106–111. 80. Notarianni, M.; Liu, J.; Vernon, K.; Motta, N. Synthesis and Applications of Carbon Nanomaterials for Energy Generation and Storage. Beilstein J. Nanotechnol. 2016, 7, 149–196. 81. Wang, X.; Zhi, L.; Tsao, N.; Tomovic, Z.; Li, J.; Mullen, K. Transparent Carbon Films as Electrodes in Organic Solar Cells. Angew. Chem. Int. Ed. 2008, 47, 2990–2992. 82. Liu, Z.; Liu, Q.; Huang, Y.; Ma, Y.; Yin, S.; Zhang, X.; Sun, W.; Chen, Y. Organic Photovoltaic Devices Based on a Novel Acceptor Material: Graphene. Adv. Mater. 2008, 20, 3924–3930. 83. Liu, Q.; Liu, Z.; Zhang, X.; Zhang, N.; Yang, L.; Yin, S.; Chen, Y. Organic Photovoltaic Cells Based on an Acceptor of Soluble Graphene. Appl. Phys. Lett. 2008, 92, 1–3. 84. Ma, J.; Li, C.; Yu, F.; Chen, J. “Brick-Like” N-Doped Graphene/Carbon Nanotube Structure Forming Three-Dimensional Films as High Performance Metal-Free Counter Electrodes in Dye-Sensitized Solar Cells. J. Power Sources 2015, 273, 1048–1055. 85. Xu, D.; Yu, X.; Gao, D.; Mu, X.; Zhong, M.; Yuan, S.; Xie, J.; Ye, W.; Huang, J.; Yang, D. Room Temperature Processed, Air-Stable and Highly Efficient Graphene/Silicon Solar Cells With an Organic Interlayer. J. Mater. Chem. A 2016, 4, 11284–11291. 86. Liscio, A.; Veronese, G. P.; Treossi, E.; Suriano, F.; Rossella, F.; Bellani, V.; Rizzoli, R.; Samori, P.; Palermo V. Charge Transport in Graphene–Polythiophene Blends as Studied by Kelvin Probe Force Microscopy and Transistor Characterization. J. Mater. Chem. 2011, 21, 2924–2931. 87. Stylianakis, M. M.; Stratakis, E.; Koudoumas, E.; Kymakis, E.; Anastasiadis, S. H. Organic Bulk Heterojunction Photovoltaic Devices Based on Polythiophene−Graphene Composites. ACS Appl. Mater. Interfaces 2012, 4, 4864–4870. 88. Wang, J.; Wang, Y.; He, D.; Liu, Z.; Wu, H.; Wang, H.; Zhao, Y.; Zhang, H.; Yang, B. Composition and Annealing Effects in Solution-Processable Functionalized Graphene Oxide/P3HT Based Solar Cells. Synth. Met. 2010, 160, 2494–2500. 89. Fan, B.; Mei, X.; Sun, K.; Ouyang, J. Conducting Polymer/Carbon Nanotube Composite as Counter Electrode of Dye-Sensitized Solar Cells. Appl Phys. Lett. 2008, 93, 1–3.

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Index

A Applications of carbon nanotubes (CNTs) electrode reactions, 123–125 fuel cells, 128–129 hydrogen storage, 120–121 lithium-ion battery (LIBs), 121–123 metal-air battery, 123–125 solar cells, 125–126 supercapacitor, 127–128 Atomic layer deposition (ALD), 171

B Biomorph, 150 Bottom-up approach atomic layer deposition (ALD), 171 chemical solution process, 171 chemical vapor deposition, 170–171 electron beam evaporation (e-beam evaporation), 168 general applications, 172 molecular beam epitaxy (MBE), 168 physical vapor deposition, 167–168 pulsed laser deposition (PLD), 168–169 sputtering, 169–170

C Cantilever type, 150 Carbon nanomaterials for energy applications, 77 DSSC-based, 81–82 carbon nanodots, 83

carbon nanofibers, 82–83

carbon nanotubes, 82

graphene, 83–84

lithium ion batteries (LIBS), 99–102 nano-carbon materials for fuel cells, 96–99 organic solar cell (OSC), 84

fullerene, 84–85

graphene, 85–86

perovskite solar cells (PSCs), 86

CNT in, 86–87

fullerene in, 88–89

graphene, 87–88

solar cell, 78–79

SI-based, 79–80

thin-film chalcogenide, 81

supercapacitor, 89–96 Carbon nanotubes, 115–119 applications electrode reactions, 123–125 fuel cells, 128–129 hydrogen storage, 120–121 lithium-ion battery (LIBs), 121–123 metal-air battery, 123–125 solar cells, 125–126 supercapacitor, 127–128 synthesis, 119 Carbon quantum dots, 209 electronic energy applications, 216–217 batteries, 217–219 light emitting diodes (LED), 224–226 solar cells, 221–224 supercapacitors, 219–221 properties, 215–216 synthesis, 211

bottom-up, 214–215

top-down, 211–214

D Dye-sensitized solar cells carbon nanomaterials, 251–252 fullerene, 252–253 graphene, 253–254 metal-oxide nanomaterials, 247–248 nanomaterials, 254 plasmonic nanoparticles, 250–251 SNO2, 249–250 TiO2-based, 248–249 ZnO, 249

Index

342 E Electron transport layer (ETL), 256 Electronic energy applications, 216–217 batteries, 217–219 light emitting diodes (LED), 224–226 solar cells, 221–224 supercapacitors, 219–221 Electronics energy applications, 209 Energy generation applications challenges, 179–180 CO2 reduction reaction (CO2RR), 177–178

hydrogen evolution reaction (HER),

175–177 lithium sulfur (LS) battery, 173–174 lithium-ion battery, 172 oxygen evolution reaction (OER), 177 sodium-ion batteries (SIBs), 173 solar cell, 178–179 supercapacitors, 174–175 Energy-harvesting, piezoelectric materials, 139, 159 challenges faced and solutions, 153 bandwidth, 153–154 biocompatibility, 155 CMOS compatibility, 154–155 effect, 142–143 utilization, 143–145

nanoscale, 156–158

need, 141–142

PEH device configurations, 149

biomorph, 150 cantilever type, 150 piezoelectric film configuration, 152 stack configuration, 152–153 unimorph, 150–152 PEH in MEMS scale, 155–156 piezoelectric energy harvesting, 145–146 frequency response, 147 piezoelectric materials, 147–149 strategies, enhance performance, 158 Energy-storage mechanisms, 195–196 faradaic, 197–198 faradaic capacitive charge storage, 198 faradaic noncapacitive energy storage, 198–199

non-faradaic, 196–197

F Faradaic, 197–198 capacitive charge storage, 198 noncapacitive energy storage, 198–199 Fuel cells, 128–129

H Hole transport layer (HTL), 255 Hydrogen evolution reaction (HER), 175–177 Hydrogen storage, 120–121

L Light emitting diodes (LED), 224–226 Lithium ion batteries (LIBS), 99–102

M Magnetorheological and electrorheological, 25 smart polymer systems, rheological techniques, 28–31

electrorheology, 35–38

magnetorheology, 31–35

SPCS energy-related applications of, 38–40 Metal-air battery, 123–125 Metal-organic frameworks (MOFS), 47 for energy applications, 58–61

CO2 reduction, 62

hydrogen storage and production,

61–62 supercapacitors and batteries, 62–64 perspectives, 64–65 strategies, 55 design, 55–56

flexibility, 57–58

porosity, 56–57

structural and chemical versatility, 50–54 Molecular beam epitaxy (MBE), 168 Molybdenum disulfide, 164

N Nano- and smart materials, 239 CNT and graphene, 245

charge transport layer, 246

incorporation, 245–246

photoactive layer, 245–246

Index

343

dye-sensitized solar cells

carbon nanomaterials, 251–252

fullerene, 252–253

graphene, 253–254

metal-oxide nanomaterials, 247–248

nanomaterials, 254

plasmonic nanoparticles, 250–251

SNO2, 249–250

TiO2-based, 248–249

ZnO, 249

organic solar cells, 242

carbon-based, effect, 245

fullerenes, 246–247

gold, effect, 243–244

nanomaterial, 247

nanomaterials, 242–243

plasmonic nanoparticles, 243

silver nanoparticles, effect, 244–245

perovskite solar cells

carbon nanotube (CNT), 260

carbon-based compounds, 259

CNT as HTL, 260–261

electron transport layer (ETL), 256

fullerene, 261

gold (Au) ETL, 256–257

gold (Au) HTL, 256

gold (Au) nanoparticle, 259

graphene, 259–260

hole transport layer (HTL), 255

MNP in active layer, 258

silver (Ag) ETL, 257–258

silver (Ag) HTL, 255

silver (Ag) nanoparticle, 258

O Organic solar cell (OSC), 84, 242

carbon-based, effect, 245

fullerene, 84–85

fullerenes, 246–247

gold, effect, 243–244

graphene, 85–86

nanomaterial, 247

nanomaterials, 242–243

plasmonic nanoparticles, 243

silver nanoparticles, effect, 244–245

Oxygen evolution reaction (OER), 177

P Perovskite solar cells (PSCs), 86

carbon nanotube (CNT), 260

carbon-based compounds, 259

CNT as HTL, 260–261

CNT in, 86–87

electron transport layer (ETL), 256

fullerene, 261

fullerene in, 88–89

gold (Au) ETL, 256–257

gold (Au) HTL, 256

gold (Au) nanoparticle, 259

graphene, 87–88, 259–260

hole transport layer (HTL), 255

MNP in active layer, 258

silver (Ag) ETL, 257–258

silver (Ag) HTL, 255

silver (Ag) nanoparticle, 258

Piezoelectric energy harvesters (PEH),

145–146

device, configurations, 149

biomorph, 150

cantilever type, 150

piezoelectric film configuration, 152

stack configuration, 152–153

unimorph, 150–152

frequency response, 147

piezoelectric materials, 147–149

Piezoelectric film configuration, 152

Polymer nanocomposites, 1

energy applications, 7–8

filled polymers, 15–18

self-cleaning properties, 8–11

self-healing, 12–15

molecular modeling some smart, 3–7 Pulsed laser deposition (PLD), 168–169

S Smart chromogenic hydrated WO3, 185

emergence of layered nature, 190–191

energy saving technologies, 194–195

energy-storage mechanisms, 195–196

faradaic, 197–198

faradaic capacitive charge storage, 198

faradaic noncapacitive energy storage,

198–199

non-faradaic, 196–197

Index

344 forms and crystal structure, 188–190 WO3 0.5H2O, 189 WO3 0.33H2O, 190 WO3.H2O, 188–189 WO3.2H2O, 188 material, 191–193 electrochromism, 194 mechanism of photochromism, 193 solid-state batteries

energy-storage technology and

importance, 199–202 substitute for LI electrode, 202–203 transition metal oxides energy-storage devices, 187–188 Smart polymer systems rheological techniques, 28–31

electrorheology, 35–38

magnetorheology, 31–35

Sodium-ion batteries (SIBs), 173 Solar cells, 78–79, 125–126 SI-based, 79–80 thin-film chalcogenide, 81 Solar energy conversion and storage, 239 Stack configuration, 152–153 Supercapacitor, 127–128

T Transition metal dichalcogenides (TMDs) bottom-up approach atomic layer deposition (ALD), 171 chemical solution process, 171 chemical vapor deposition, 170–171

electron beam evaporation (e-beam evaporation), 168 general applications, 172 molecular beam epitaxy (MBE), 168 physical vapor deposition, 167–168 pulsed laser deposition (PLD), 168–169

sputtering, 169–170

energy generation applications

challenges, 179–180

CO2 reduction reaction (CO2RR),

177–178 hydrogen evolution reaction (HER), 175–177 oxygen evolution reaction (OER), 177 solar cell, 178–179 energy storage applications lithium sulfur (LS) battery, 173–174 lithium-ion battery, 172 sodium-ion batteries (SIBs), 173 supercapacitors, 174–175 MoS2 structure and properties

crystal structure, 164

intralayer covalent, 165

preparation, methods, 165–166

top-down approach

liquid-phase exfoliation, 166–167

mechanical exfoliation, 166

Transition metal oxides energy-storage devices, 187–188

U Unimorph, 150–152