Metal Oxides in Supercapacitors

Metal Oxides in Supercapacitors addresses the fundamentals of metal oxide-based supercapacitors and provides an overview of recent advancements in this area. Metal oxides attract most of the materials scientists use due to their excellent physico-chemical properties and stability in electrochemical systems. This justification for the usage of metal oxides as electrode materials in supercapacitors is their potential to attain high capacitance at low cost.

After providing the principles, the heart of the book discusses recent advances, including: binary metal oxides-based supercapacitors, nanotechnology, ternary metal oxides, polyoxometalates and hybrids. Moreover, the factors affecting the charge storage mechanism of metal oxides are explored in detail.

The electrolytes, which are the soul of supercapacitors and a mostly ignored character of investigations, are also exposed in depth, as is the fabrication and design of supercapacitors and their merits and demerits.

Lastly, the market status of supercapacitors and a discussion pointing out the future scope and directions of next generation metal oxides based supercapacitors is explored, making this a comprehensive book on the latest, cutting-edge research in the field.

• Explores the most recent advances made in metal oxides in supercapacitors
• Discusses cutting-edge nanotechnology for supercapacitors
• Includes fundamental properties of metal oxides in supercapacitors that can be used to guide and promote technology development
• Contains contributions from leading international scientists active in supercapacitor research and manufacturing

• Language: English
• Publisher: Elsevier Science
• Release date: Jul 10, 2017
• ISBN: 9780128104651

Book preview

Metal Oxides in Supercapacitors

Series Editor

Ghenadii Korotcenkov

Editors

Deepak P. Dubal

Pedro Gomez-Romero

Table of Contents

Cover image

Title page

Copyright

List of Contributors

Editors' Biography

Series Editor's Biography

Preface

Preface to the Series

1. Capacitive and Pseudocapacitive Electrodes for Electrochemical Capacitors and Hybrid Devices

1.1. Introduction

1.2. Devices

1.3. Electrodes for Electrochemical Capacitors and for Hybrid Capacitors

1.4. Conclusions

2. Features of Design and Fabrication of Metal Oxide–Based Supercapacitors

2.1. Introduction

2.2. Fundamentals of Symmetric and Asymmetric Supercapacitors

2.3. Configuration Design of Metal Oxide–Based Supercapacitors

2.4. Conclusions and Outlook

3. Electrolytes in Metal Oxide Supercapacitors

3.1. Introduction

3.2. Supercapacitors and Interaction With Electrolytes

3.3. Electrolytes for Metal Oxide Supercapacitors

3.4. Conclusions and Outlooks

4. Fundamentals of Binary Metal Oxide–Based Supercapacitors

4.1. Introduction

4.2. Binary Metal Oxides in Supercapacitors

5. Structure and Basic Properties of Ternary Metal Oxides and Their Prospects for Application in Supercapacitors

5.1. Introduction

5.2. Several Types of Ternary Metal Oxides

5.3. Synthesis Routes

5.4. Nanostructures

5.5. Concluding Remarks

6. Polyoxometalates: Molecular Metal Oxide Clusters for Supercapacitors

6.1. Introduction

6.2. Polyoxometalate Structure and Electrochemistry

6.3. Fabricating Polyoxometalate Composites for Supercapacitor Electrodes

6.4. Application of Polyoxometalate Electrodes in Supercapacitor Devices

6.5. Conclusions and Future Perspectives

7. Metal–Organic Framework (MOF)–Derived Metal Oxides for Supercapacitors

7.1. Introduction

7.2. Metal–Organic Framework–Derived Metal Oxides for Supercapacitors

7.3. Conclusion and Future Perspectives

8. Metal Oxide–Carbon Hybrid Materials for Application in Supercapacitors

8.1. Introduction

8.2. Porous Carbon–Metal Oxide Hybrids

8.3. Carbon Nanofiber–Metal Oxide Nanocomposites

8.4. Graphene–Metal Oxide Nanocomposites

8.5. Conclusion and Future Directions

9. Metal Oxide/Conducting Polymer Hybrids for Application in Supercapacitors

9.1. Introduction

9.2. Conclusions

10. Enhanced Hybrid Supercapacitors Utilizing Nanostructured Metal Oxides

10.1. Introduction

10.2. Li4Ti5O12: Dimension-Controlled Nanosheet/Nanobook, Highly Dispersed on the Carbon Nanotube Surface

10.3. TiO2(B): Dimension Control and Hyperdispersion of Nano Metal Oxides Within a Nanocarbon Matrix

10.4. Li3VO4: Electrochemical Activation; Control of Crystal Structure of Nano Metal Oxides for Li+ Diffusion Enhancement via the Electrochemical Method

10.5. LiFePO4: Defective (Crystalline/Amorphous) Control of Nano Metal Oxides Within the Peculiar Core–Shell LiFePO4/Graphitic Carbon Structure

10.6. Li3V2(PO4)3: Nano Entanglement of Metal Oxides in Carbon Nanotube Matrix

10.7. Conclusions and Remarks

Index

Copyright

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Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary.

Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility.

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ISBN: 978-0-12-810465-1 (online)

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List of Contributors

Nayarassery N. Adarsh,     Catalan Institute of Nanoscience and Nanotechnology (ICN2-CSIC), Bellaterra, Spain

Daniel Bélanger,     Université du Québec à Montréal, Montréal, QC, Canada

Thierry Brousse

Université de Nantes, CNRS, Nantes, France

Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS, Amiens, France

Maria J. Carmezim

ESTSetubal, Instituto Politécnico de Setúbal, Setúbal, Portugal

CQE-IST, Universidade de Lisboa, Lisboa, Portugal

Nilesh R. Chodankar,     Chonnam National University, Gwangju, South Korea

Olivier Crosnier

Université de Nantes, CNRS, Nantes, France

Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS, Amiens, France

Deepak P. Dubal

School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia

Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, The Barcelona Institute of Science and Technology, Barcelona, Spain

Matthew Genovese,     University of Toronto, Toronto, ON, Canada

Pedro Gomez-Romero,     Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC, The Barcelona Institute of Science and Technology, Barcelona, Spain

Rudolf Holze,     Technische Universität Chemnitz, Chemnitz, Germany

Mihnea I. Ionescu,     National Research Council Canada, London, ON, Canada

Etsuro Iwama

Tokyo University of Agriculture and Technology, Tokyo, Japan

Institute of Global Innovation Research, Tokyo, Japan

Do-Heyoung Kim,     Chonnam National University, Gwangju, South Korea

Kazuaki Kisu

Tokyo University of Agriculture and Technology, Tokyo, Japan

Institute of Global Innovation Research, Tokyo, Japan

Keryn Lian,     University of Toronto, Toronto, ON, Canada

Y. Liu

Nanjing Tech University, Nanjing, China

Technische Universität Chemnitz, Chemnitz, Germany

Jeffrey W. Long,     U.S. Naval Research Laboratory, Washington, DC, United States

Katsuhiko Naoi

Tokyo University of Agriculture and Technology, Tokyo, Japan

K & W Inc, Tokyo, Japan

Institute of Global Innovation Research, Tokyo, Japan

Wako Naoi,     K & W Inc, Tokyo, Japan

Catarina F. Santos

ESTSetubal, Instituto Politécnico de Setúbal, Setúbal, Portugal

CQE-IST, Universidade de Lisboa, Lisboa, Portugal

Patrice Simon

Tokyo University of Agriculture and Technology, Tokyo, Japan

Université Paul Sabatier, Toulouse, France

Institute of Global Innovation Research, Tokyo, Japan

Rongming Wang,     Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, People's Republic of China

Jian Wu,     Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing, People's Republic of China

X.W. Wu,     Hunan Agricultural University, Changsha, China

Y.P. Wu

Nanjing Tech University, Nanjing, China

Hunan Agricultural University, Changsha, China

Dongfang Yang,     National Research Council Canada, London, ON, Canada

Y. Zhang,     Nanjing Tech University, Nanjing, China

Y.S. Zhu,     Nanjing Tech University, Nanjing, China

Editors' Biography

Deepak P. Dubal, PhD, is currently working as senior researcher at the University of Adelaide, Australia. He worked as a Marie-Curie Fellow (BP-DGR) at the Catalan Institute of Nanoscience and Nanotechnology, Spain (2014), and a Alexander von Humboldt Fellow (2012) at the Chemnitz University of Technology, Germany. Dr. Dubal received his PhD in Physics from the Shivaji University, Kolhapur, India, in 2011, and since then, he has been awarded at international and national (India) conferences for his research excellence. He is a member of the Editorial Board for Scientific Reports, Nature Publishing Group, and Electrochemical energy technology (De Gruyder publications). Dr. Dubal is the author of over 120 research articles and book chapters and has filed seven patents. His research interest is focused on the chemical synthesis of nanostructured materials and hybrid nanomaterials and their applications in energy storage devices, with special emphasis on Li-ion batteries, supercapacitors, electrochemical flow cells, and Li-ion capacitor.

Pedro Gomez-Romero, PhD, is a full professor at the NEO-Energy Group Leader at the Catalan Institute of Nanoscience and Nanotechnology, Spain. Prof. Gomez-Romero received his PhD in Chemistry from the Georgetown University, United States, in 1987 with distinction; was a CSIC Researcher at ICMAB from 1990 to 2007; and was a sabbatical at the National Renewable Energy Laboratory, United States (1998–99). Dr. Gomez-Romero is the leading scientist of five main national (Spanish) research projects and several international research projects. His research has focused on energy storage and conversion, advanced functional materials and nanocomposites, new oxides, polyoxometalates, and polymers. He is the editor of Functional Hybrid Materials (Wiley-VCH) and two award-winning books in the area of popular science. Dr. Gomez-Romero has published over 200 articles, book chapters, and conference proceedings, as well as filed six patents.

Series Editor's Biography

Ghenadii Korotcenkov received his PhD in Material Sciences from the Technical University of Moldova in 1976 and his Doctor of Science degree in Physics from the Academy of Science of Moldova in 1990 (Highest Qualification Committee of the USSR, Moscow). He has more than 40  years of experience as a scientific researcher. For a long time, he was the leader of the gas sensor group and manager of various national and international projects carried out in the Laboratory of Micro- and Optoelectronics, Technical University of Moldova. His research had financial support from international foundations and programs such as CRDF, MRDA, ICTP, INTAS, INCO-COPERNICUS, COST, and NATO. From 2007 to 2008, he was an invited scientist in the Korea Institute of Energy Research (Daejeon). After which, and until now, Dr. G. Korotcenkov is a research professor at the School of Materials Science and Engineering at the Gwangju Institute of Science and Technology in Korea.

Scientific interests of G. Korotcenkov, starting from 1995, include material sciences, focusing on metal oxide film deposition and characterization, surface science, and the design of thin film gas sensors and thermoelectric convertors. G. Korotcenkov is the author or editor of 35 books, including the 11-volume Chemical Sensors series published by Momentum Press (United States), 15-volume Chemical Sensors series published by Harbin Institute of Technology Press (China), 3-volume Porous Silicon: From Formation to Application published by CRC Press (United States), and 2-volume Handbook of Gas Sensor Materials published by Springer (United States). G. Korotcenkov is author and coauthor of more than 550 scientific publications, including 20 review papers, 35 book chapters, more than 250 articles published in peer-reviewed scientific journals [h-factor  =  37 (Scopus) and h  =  44 (Google Scholar citation)]. He is a holder of 17 patents. He has presented more than 200 reports at national and international conferences, including 15 invited talks. G. Korotcenkov was coorganizer of several international conferences. His research activities are honored by an award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004); prize of the Presidents of the Ukrainian, Belarus, and Moldovan Academies of Sciences (2003); the National Youth Prize of the Republic of Moldova in the field of science and technology (1980); among others. G. Korotcenkov also received a fellowship from the International Research Exchange Board (IREX, United States, 1998), Brain Korea 21 Program (2008–12), and Brainpool Program (Korea, 2015–17).

Preface

As long as silicates remain a mineral category in themselves, oxides will not be the most abundant minerals on the Earth crust. They are however the richest source of chemical and physical variety available to humans on the mesmerizing interphase between our small rocky-planet geosphere and our oxygen-rich atmosphere. Indeed, the multiple combinations between a few dozen metallic elements and oxygen lead to an unexhausted list of fascinating and useful properties from natural magnetism to human-made superconductivity, from a colorful palette of natural pigments to an arsenal of energy-storing materials.

Speaking of energy storage, batteries were the first electrochemical energy storage devices making use of the versatility of oxides by harnessing their rich redox chemistry. Reversible lithium-ion cells reached the highest performances among batteries by refining the concept of Li+ insertion/deinsertion to compensate for stored charges, thus leading to relatively high energy densities at the expense of the low power imposed by slow ion diffusion. Supercapacitors, on the other hand, were low-energy but high-power devices conceptually derived from the electrophysical world of capacitors and originated from the concept of double-layer capacitance accumulated at the interface between an electrolyte and a high-surface-area electrode, which originally was always a carbon electrode. Not anymore. Oxides are catching up as supercapacitor active materials, providing enhanced energy densities and blurring the boundaries between supercapacitors and batteries.

Supercapacitors used to be secondary actors in the energy storage scene, providing support to batteries or occupying small niche applications when high power was momentarily needed. This is quickly changing, and supercapacitors are increasingly contributing to the recent energy storage device booming, reaching the world of portable electronics, sustainable mobility (from tramways to cars), and grid applications and renewable energy sources management. The current success of supercapacitors is in large part due to the use of transition metal oxides in one or both electrodes.

Our book is concerned with supercapacitor electrode materials based on transition metal oxides and their composites. In these materials, capacitance arises through reversible redox reactions taking place at or near the surface of an electrode material that is in contact with an electrolyte, or when these reactions are not limited by solid-state ion diffusion. The most significant practical difference between battery and supercapacitor materials (in particular, metal oxides) is that the charging and discharging processes of supercapacitors occur in the order of seconds and minutes. Thus, a strong motivation for investigating and developing metal oxide–based supercapacitors is that it can lead to both high energy and high power densities in the same material.

To provide the directions for further research and development, we believe a book discussing both the fundamentals and applications of Metal Oxide–Based Supercapacitors is highly needed. Indeed, the famous book in the field by B.E. Conway, Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications, published in 1999 provided the first broad coverage of the development of supercapacitors in the past century. However, in the last 17  years, the field has made significant advancements with new ideas such as better explanation of pseudocapacitance, introduction of new metal oxides, and new device configurations such as hybrid and asymmetric capacitors that require a further comprehensive review.

In this respect, we have taken the efforts to write a book in collaboration with leading scientists in the field of energy storage materials and devices with special focus on supercapacitors. Our book is structured in a total of 10 chapters, which are well organized and provide detailed explanation about the respective titles with adequate examples. Moreover, the content of our book is multidisciplinary, covering different science and engineering disciplines. The first two introductory chapters give a deep knowledge about the pseudocapacitance involved in the metal oxides and the device configurations. In the next chapter, the electrolytes used for metal oxide–based supercapacitors are discussed. Further chapters are focused on the structural properties of different metal oxides, molecular clusters of metal oxides, and metal organic framework (MOF)-derived metal oxides and their contribution to the electrochemical properties. We believe that the content of this book will provide a clear understanding of the fundamental as well as practical aspects and will facilitate the final application of this technology in the industry, while also providing a prospective view of the new advanced materials developed by experts in the field, which may surface in the future.

We are sure that both industrial and academic scientists and engineers, along with undergraduate and graduate students, in the field will benefit by the knowledge gathered in this book and also will help foster ideas for new devices that will help further the technology. Moreover, the readers will find this logical evolution highly appealing, as it introduces a didactic element to the reading of the book apart from the joy of grasping the essentials of an important subject.

Finally, we sincerely acknowledge and thank all our contributors who devoted their valuable time for preparing and developing this wonderful book. We also would like to thank the series editor Prof. Korotcenkov Ghenadii and Elsevier Publishers for inviting us to lead this book project, especially Anna Valutkevich, for their patience and support in smoothing out the book preparation process. Finally, I (DPD) personally would like to dedicate this book to all my supervisors with whom I worked so far, including Prof. Pedro Gomez-Romero (coeditor), for empowering me to fly high in this fascinating field.

Deepak P. Dubal, and Pedro Gomez-Romero

Preface to the Series

The field of synthesis, study, and application of metal oxides is one of the most rapidly progressing areas of science and technology. Metal oxides are one of the most ubiquitous compound groups on Earth, which has a large variety of chemical compositions, atomic structures, and crystalline shapes. In addition, metal oxides are known to possess unique functionalities that are absent or inferior in other solid materials. In particular, metal oxides represent an assorted and appealing class of materials, properties of which exhibit a full spectrum of electronic properties, i.e., from insulating to semiconducting, metallic, and superconducting. Moreover, almost all the known effects, including superconductivity, thermoelectric effects, photoelectrical effects, luminescence, and magnetism, can be observed in metal oxides. Therefore, metal oxides have emerged as an important class of multifunctional materials with a rich collection of properties, which have great potential for numerous device applications. Specific properties of the metal oxides, such as the wide variety of materials with different electrophysical, optical, and chemical characteristics, their high thermal and temporal stability, and their ability to function in harsh environments, make metal oxides very suitable materials for designing transparent electrodes, high-mobility transistors, gas sensors, actuators, acoustical transducers, photovoltaic and photonic devices, photo- and heterogeneous catalysts, solid-state coolers, high-frequency and micromechanical devices, energy harvesting and storage devices, nonvolatile memories, and many others in the electronics, energy, and health sectors. In these devices, metal oxides can be successfully used as sensing or active layers, substrates, electrodes, promoters, structure modifiers, membranes, and fibers; that is, they can be used as active and passive components.

Among the other advantages of metal oxides are the low fabrication cost and robustness in practical applications. Metal oxides can be prepared in various forms such as ceramics, thick films, and thin films. Moreover, for thin-film deposition, techniques that are compatible with standard microelectronic technology can be used. The last factor is very important for large-scale production because the microelectronic approach promotes low cost for mass production, offers the possibility of manufacturing devices on a chip, and guarantees good reproducibility. Various metal oxide nanostructures, including nanowires, nanotubes, nanofibers, core–shell structures, and hollow nanostructures, can also be synthesized. As it is known, the field of metal-oxide nanostructured morphologies (e.g., nanowires, nanorods, nanotubes) has become one of the most active research areas within the nanoscience community.

The ability to create a variety of metal oxide–based composites and to synthesize various multicomponent compounds significantly expand the range of properties that metal oxide–based materials can have, making metal oxides a truly versatile multifunctional material for widespread use. As it is known, small changes in their chemical composition and atomic structure can be accompanied by the spectacular variation in properties and behavior of metal oxides. Even now, advances in synthesizing and characterizing techniques are revealing numerous new functions of metal oxides.

Taking into account the importance of metal oxides for progress in microelectronics, optoelectronics, photonics, energy conversion, sensor, and catalysis, a large number of various books devoted to this class of materials have been published. However, one should note that some books from this list are too general, some books are collections of various original works without any generalizations, and other ones were published many years ago. But during the past decade great progress has been made on the synthesis as well as on the structural, physical, and chemical characterization and the application of metal oxides in various devices, and a large number of papers have been published on metal oxides. In addition, till now many important topics related to metal oxide study and application have not been discussed. To remedy this situation, we decided to generalize and systematize the results of research in this direction and to publish a series of books devoted to metal oxides.

The proposed book series Metal Oxides is the first one devoted to the consideration of metal oxides only. We believe that combining books on metal oxides in a series could help readers in searching required information on the subject. In particular, we plan that the books from our series, which have a clear specialization by its content, will provide interdisciplinary discussion for various oxide materials with a wide range of topics, from material synthesis and deposition to characterizations, processing, and then to device fabrications and applications. This book series is prepared by a team of highly qualified experts, which guarantees it a high quality.

I hope that our books will be useful and comfortable. I would also like to hope that readers will consider this Metal Oxides book series like an encyclopedia of metal oxides that enables to understand the present status of metal oxides, to estimate the role of multifunctional metal oxides in the design of advanced devices, and then based on the observed knowledge, to formulate new goals for further research.

The intended audience of this book series is scientists and researchers who are working or planning to work in the field of materials related to metal oxides, i.e., scientists and researchers whose activities are related to electronics, optoelectronics, energy, catalysis, sensors, electrical engineering, ceramics, biomedical designs, etc. I believe that this Metal Oxides book series will also be interesting for practicing engineers or project managers in industries and national laboratories, which would like to design metal oxide–based devices, but do not know how to do it and how to select optimal metal oxides for specific applications. With many references to the vast resource of the recently published literature on the subject, this book series will be serving as a significant and insightful source of valuable information, providing scientists and engineers with new insights for understanding and improving existing metal oxide–based devices and for designing new metal oxide–based materials with new and unexpected properties.

I believe that this Metal Oxides book series would be very helpful for university students, postdocs, and professors. The structure of these books offers a basis for courses in the field of material sciences, chemical engineering, electronics, electrical engineering, optoelectronics, energy technologies, environmental control, and many others. Graduate students could also find the book series to be very useful in their research and in understanding features of metal oxide synthesis, study, and application of this multifunctional material in various devices. We are sure that all of them will find the information useful for their activity.

Finally, I thank all the contributing authors and book editors who have been involved in the creation of these books. I am thankful that they agreed to participate in this project and for their efforts in the preparation of these books. Without their participation, this project would have not been possible. I also express my gratitude to Elsevier for giving us the opportunity to publish this series. I especially thank the team of editorial office at Elsevier for their patience during the development of this project and for encouraging us during the various stages of preparation.

Ghenadii Korotcenkov

1

Capacitive and Pseudocapacitive Electrodes for Electrochemical Capacitors and Hybrid Devices

Thierry Brousse¹,², Olivier Crosnier¹,², Daniel Bélanger³, and Jeffrey W. Long⁴     ¹Université de Nantes, CNRS, Nantes, France     ²Réseau sur le Stockage Electrochimique de l'Energie (RS2E), FR CNRS, Amiens, France     ³Université du Québec à Montréal, Montréal, QC, Canada     ⁴U.S. Naval Research Laboratory, Washington, DC, United States

Abstract

Electrochemical capacitors are energy storage devices that have intermediate energy and power densities between those of batteries (high energy) and dielectric capacitors (high power). In this chapter, the distinctions between these different devices, as well as emerging devices such as lithium-ion capacitors, are presented in terms of electric and electrochemical properties. The materials for electrochemical capacitors are classified with respect to their charge storage behavior and the electrochemical techniques that are used to characterize them. The different types of carbon that have been investigated as electrode materials are depicted with a special focus on the modern theory on electrochemical double-layer capacitance. The concept of pseudocapacitance is explained and the differences between battery-type electrodes and pseudocapacitive electrodes are detailed, distinguishing intrinsic from extrinsic pseudocapacitance. The role of battery-type electrodes in hybrid capacitors is also explained.

Keywords

Carbon; Dielectric capacitor; Electrochemical double-layer capacitance; High-power battery; Hybrid capacitor; MnO2; Pseudocapacitance; RuO2

1.1. Introduction

Electrochemical capacitors (ECs, also sometimes denoted as supercapacitors or ultracapacitors) are energy storage devices that bridge the performance gap between the high energy density provided by batteries and the high power density (but very limited energy density) derived from dielectric capacitors. Commercially available ECs exhibit gravimetric energy density up to 8.5  Wh  kg−¹ and usable power density up to 9.0  kW  kg−¹ [1].

In the field of ECs, there is often confusion between the electric parameters of a full device and the electrochemical properties of the individual electrodes that comprise the cell. The aim of this chapter is to describe the distinctions between these various devices and their constituents, starting with a comparison of dielectric capacitors and ECs, followed by discussion of other electrochemical energy storage devices with regard to their electric properties.

The electrochemical behavior of common electrode materials used in ECs and related devices will be discussed in terms of capacitive, pseudocapacitive, and faradaic charge storage mechanisms, as well as recommended methods with which such electrodes should be characterized. We highlight the distinctions between carbon-based capacitive electrodes that are commonly found in commercial ECs and pseudocapacitive electrodes [2,3], such as RuO2 [4,5], or MnO2 [6,7], that have the electrochemical signature of a capacitive electrode but express different charge storage mechanisms. In the last part of the chapter, we describe the important distinctions between high-power battery-type electrodes and pseudocapacitive electrodes.

1.2. Devices

1.2.1. Dielectric Capacitors

A dielectric is an electronically insulating material that can be polarized by an applied electric field where electric charges do not flow through the material as they do in an electronic conductor, such as metals, but are only slightly shifted from their equilibrium positions. Positive charges are displaced in the direction of the field and negative charges shift in the opposite direction. This separation of charge creates an internal electric field that reduces the overall field within the dielectric itself [8]. In a dielectric capacitor (e.g., conventional polymer film or ceramic capacitor), the dielectric material is thin and sandwiched between two current collectors that are usually metals. When a voltage is applied to the dielectric capacitor, an electric field is created within the dielectric, and charges (electrons and holes) accumulate in the metallic current collectors at the interface with the dielectric material. Thus, dielectric capacitors store charges through electrostatic interactions but at levels that are much lower than for standard batteries, and they are usually not designed or used where high energy density is required.

Capacitance is the ability of a body to store an electric charge. This capacitance is constant over a given voltage window and can be used to calculate the charge stored using Eq. (1.1),

(1.1)

where ΔQ is the charge stored (expressed in coulombs, C) and ΔU is the width of the voltage window (V). In this case the capacitance, C, is the amount of charge stored when a 1-V window is used. For a given voltage window, there is a direct and simple access to the charge stored. The SI unit of capacitance is farad (symbol: F), named after the English physicist, Michael Faraday.

For conventional dielectric capacitors, charges can be stored over a wide voltage window, sometimes reaching several hundred volts. Thus, capacitance was introduced to compare the performance of dielectric materials: the higher the capacitance, the more charge stored within a given voltage. In a conventional dielectric capacitor where a thin dielectric layer separates the two metallic current collectors, the capacitance is proportional to the surface S of the metallic plates and inversely proportional to the thickness of the dielectric, e; the thinner it is, the larger the capacitance (Eq. 1.2).

(1.2)

where ε0 is the electric permittivity of vacuum and εr is the relative permittivity of the given dielectric material. However, thinner dielectric are more susceptible to breakdown. Indeed, the concept of capacitance is accompanied in dielectric materials by that of breakdown voltage. This is characteristic of an insulator that defines the maximum voltage difference that can be applied across a given dielectric material before it collapses and conducts charges. In solid dielectric materials, the breakdown is usually due to the creation of a weakened pathway within the material that enables charge transfer from one electrode to the other.

Thus, from relation (1.1), high capacitance values and high breakdown voltage lead to high charge storage. Furthermore, the energy stored in the dielectric capacitor is related to capacitance and cell voltage by relation (1.3):

(1.3)

Figure 1.1  Constant-current charge galvanostatic cycling of a