Nanocarbons for Advanced Energy Storage E-Book
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- Herausgeber: Wiley-VCH
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This first volume in the series on nanocarbons for advanced applications presents the latest achievements in the design, synthesis, characterization, and applications of these materials for electrochemical energy storage. The highly renowned series and volume editor, Xinliang Feng, has put together an internationally acclaimed expert team who covers nanocarbons such as carbon nanotubes, fullerenes, graphenes, and porous carbons. The first two parts focus on nanocarbon-based anode and cathode materials for lithium ion batteries, while the third part deals with carbon material-based supercapacitors with various applications in power electronics, automotive engineering and as energy storage elements in portable electric devices. This book will be indispensable for materials scientists, electrochemists, physical chemists, solid state physicists, and those working in the electrotechnical industry.
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Veröffentlichungsjahr: 2015
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Table of Contents
Cover
Related Titles
Title Page
Copyright
Preface
List of Contributors
Chapter 1: Nanostructured Activated Carbons for Supercapacitors
1.1 Supercapacitors
1.2 Activated Carbon as Electrode for Supercapacitors
1.3 Synthesis of ACs
1.4 Various Forms of ACs as Supercapacitor Electrodes
1.5 Key Factors Determining the Electrochemical Performance of AC-Based Supercapacitors
1.6 Self-discharge of ACs-Based Supercapacitors
1.7 Summary
References
Chapter 2: Nanocarbon Hybrids with Silicon, Sulfur, or Paper/Textile for High-Energy Lithium Ion Batteries
2.1 Introduction
2.2 Nanocarbon/Silicon Hybrid Anodes
2.3 Nanocarbon/Sulfur Hybrid Cathodes
2.4 Nanocarbon/Paper/Textile Hybrids as Conductive Substrates
2.5 Conclusion and Perspective
References
Chapter 3: Precursor-Controlled Synthesis of Nanocarbons for Lithium Ion Batteries
3.1 Introduction
3.2 Precursor-Controlled Synthesis of Nanocarbons
3.3 Nanocarbons in LIBs
3.4 Summary and Outlook
References
Chapter 4: Nanocarbon/Metal Oxide Hybrids for Lithium Ion Batteries
4.1 Metal Oxides (MOs) for Lithium Ion Batteries
4.2 Carbon Nanocoating/MO Hybrids for LIBs
4.3 CNFs/MO Hybrids and CNTs/MO Hybrids
4.4 Graphene/MO Hybrids
4.5 Hierarchical Nanocarbon/MO Hybrids
4.6 Summary and Perspectives
Acknowledgments
References
Chapter 5: Graphene for Flexible Lithium-Ion Batteries: Development and Prospects
5.1 Introduction
5.2 Types of Flexible LIBs
5.3 Current Status of Graphene-Based Electrodes for Bendable LIBs
5.4 Characterization of Graphene-Based Bendable Electrodes
5.5 Prospects of Flexible LIBs
5.6 Summary and Perspective
Acknowledgment
References
Chapter 6: Supercapatteries with Hybrids of Redox Active Polymers and Nanostructured Carbons
6.1 Introduction
6.2 Electrochemical Capacitance
6.3 Supercapattery
6.4 Carbon Nanotubes and Redox Active Polymers
6.5 Carbon Nanotube-Polymer Hybrids
6.6 Electrode and Cell Fabrication
6.7 Electrolytes and Separator
6.8 Recycling of Materials
6.9 Conclusion
Abbreviations
References
Chapter 7: Carbon-Based Supercapacitors Produced by the Activation of Graphene
7.1 Introduction
7.2 Supercapacitors Produced from activated graphene
7.3 Conclusion and Remarks
Acknowledgments
References
Chapter 8: Supercapacitors Based on Graphene and Related Materials
8.1 Introduction
8.2 Characteristics of Supercapacitors
8.3 Activated Carbons
8.4 Carbon Nanotubes
8.5 Graphene-Based Supercapacitors
8.6 Graphene Micro-Supercapacitors
8.7 Nitrogen-Doped Graphene
8.8 Boron-Doped Graphene
8.9 Graphene Pseudocapacitors
8.10 Graphene-Conducting Polymer Composites
8.11 Graphene-Transition Metal Oxide Composites
References
Chapter 9: Self-Assembly of Graphene for Electrochemical Capacitors
9.1 Introduction
9.2 The Chemistry of Chemically Modified Graphene
9.3 The Self-Assembly of CMGs into 2D Films
9.4 Self-Assembling CMG Sheets into 3D Architectures
9.5 Self-Assembled Graphene Materials for ECs
9.6 Conclusions and Perspectives
References
Chapter 10: Carbon Nanotube-Based Thin Films for Flexible Supercapacitors
10.1 Introduction
10.2 Solution-Processed CNT Films
10.3 Solution-Processed Composite CNT Films
10.4 Directly Synthesized SWCNT Films
10.5 The Composite Films Based on Directly Synthesized SWCNT Films
10.6 Conclusions and Outlook
References
Chapter 11: Graphene and Porous Nanocarbon Materials for Supercapacitor Applications
11.1 Introduction
11.2 Construction and Classification of Supercapacitors
11.3 A Performance Study of EDLCs Based on Nanocarbon Materials
11.4 Porous Nanocarbon Materials for Supercapacitors
11.5 Summary
Acknowledgments
References
Chapter 12: Aligned Carbon Nanotubes and Their Hybrids for Supercapacitors
12.1 Introduction
12.2 Synthesis of Aligned CNT Materials
12.3 Properties of Aligned CNT Materials
12.4 Planar Supercapacitors
12.5 Fiber-Shaped Supercapacitors
12.6 Summary and Outlook
References
Chapter 13: Theoretic Insights into Porous Carbon-Based Supercapacitors
13.1 Introduction
13.2 Classical Density Functional Theory
13.3 Ionic Liquid-Based Electric Double-Layer Capacitors
13.4 Organic Electrolyte Based Electric Double-Layer Capacitors
13.5 Summary and Outlook
Acknowledgments
References
Chapter 14: Nanocarbon-Based Materials for Asymmetric Supercapacitors
14.1 Introduction
14.2 Activated Carbons for ASCs
14.3 Graphene for ASCs
14.4 Nanocarbon Composites for ASCs
14.5 Other Carbon Materials and Their Composites for ACSs
14.6 All Solid State ASCs Based on Nanocarbon Materials
14.7 Summary and Prospects
Acknowledgments
References
Chapter 15: Nanoporous Carbide-Derived Carbons as Electrode Materials in Electrochemical Double-Layer Capacitors
15.1 Introduction
15.2 Synthesis and Materials
15.3 Application of CDCs in EDLCs
15.4 Electrosorption Mechanisms in CDC-Based EDLCs
15.5 Conclusions and Outlook
Acknowledgments
References
Index
EULA
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Guide
Cover
Table of Contents
Preface
Begin Reading
List of Illustrations
Figure 1.1
Figure 1.2
Figure 1.3
Figure 1.4
Figure 1.5
Figure 1.6
Figure 1.7
Figure 2.1
Figure 2.2
Figure 2.3
Figure 2.4
Figure 2.5
Figure 2.6
Figure 2.7
Figure 2.8
Figure 2.9
Figure 2.10
Figure 2.11
Figure 3.1
Figure 3.2
Figure 3.3
Figure 3.4
Figure 3.5
Figure 3.6
Figure 3.7
Figure 3.8
Figure 3.9
Figure 3.10
Figure 3.11
Figure 3.12
Figure 3.13
Figure 3.14
Figure 3.15
Figure 4.1
Figure 4.2
Figure 4.3
Figure 4.4
Figure 4.5
Figure 4.6
Figure 4.7
Figure 4.8
Figure 4.9
Figure 4.10
Figure 5.1
Figure 5.2
Figure 5.3
Figure 5.4
Figure 5.5
Figure 5.6
Figure 5.7
Figure 5.8
Figure 5.9
Figure 5.10
Figure 5.11
Figure 5.12
Figure 5.13
Figure 5.14
Figure 5.15
Figure 5.16
Figure 5.17
Figure 5.18
Figure 5.19
Figure 5.20
Figure 5.21
Figure 5.22
Figure 5.23
Figure 5.24
Figure 5.25
Figure 5.26
Figure 5.27
Figure 5.28
Figure 5.29
Figure 5.30
Figure 5.31
Figure 5.32
Figure 6.1
Figure 6.2
Figure 6.3
Figure 6.4
Figure 6.5
Figure 6.6
Figure 6.7
Figure 6.8
Figure 6.9
Figure 6.10
Figure 6.11
Figure 6.12
Figure 6.13
Figure 6.14
Figure 7.1
Figure 7.2
Figure 7.3
Figure 7.4
Figure 7.5
Figure 7.6
Figure 8.1
Figure 8.2
Figure 8.3
Figure 8.4
Figure 8.5
Figure 8.6
Figure 8.7
Figure 8.8
Figure 8.9
Figure 9.1
Figure 9.2
Figure 9.3
Figure 9.4
Figure 9.5
Figure 9.6
Figure 9.7
Figure 9.8
Figure 9.9
Figure 9.10
Figure 9.11
Figure 9.12
Figure 9.13
Figure 9.14
Figure 10.1
Figure 10.2
Figure 10.3
Figure 10.4
Figure 10.5
Figure 10.6
Figure 11.1
Figure 11.2
Figure 11.3
Figure 11.4
Figure 11.5
Figure 11.6
Figure 11.7
Figure 11.8
Figure 11.9
Figure 11.10
Figure 11.11
Figure 11.12
Figure 11.13
Figure 11.14
Figure 11.15
Figure 11.16
Figure 11.17
Figure 11.18
Figure 11.19
Figure 11.20
Figure 12.1
Figure 12.2
Figure 12.3
Figure 12.4
Figure 12.5
Figure 12.6
Figure 12.7
Figure 12.8
Figure 12.9
Figure 12.10
Figure 12.11
Figure 12.12
Figure 12.13
Figure 12.14
Figure 13.1
Figure 13.2
Figure 13.3
Figure 13.4
Figure 13.5
Figure 13.6
Figure 13.7
Figure 13.8
Figure 13.9
Figure 13.10
Figure 13.11
Figure 13.12
Figure 13.13
Figure 14.1
Figure 14.2
Figure 14.3
Figure 14.4
Figure 14.5
Figure 14.6
Figure 14.7
Figure 14.8
Figure 14.9
Figure 14.10
Figure 14.11
Figure 15.1
Figure 15.2
Figure 15.3
Figure 15.4
Figure 15.5
Figure 15.6
Figure 15.7
Figure 15.8
Figure 15.9
Figure 15.10
Figure 15.11
Figure 15.12
Figure 15.13
Figure 15.14
Figure 15.15
List of Tables
Table 1.1
Table 1.3
Table 2.1
Table 4.1
Table 5.1
Table 5.2
Table 5.3
Table 5.4
Table 5.5
Table 5.6
Table 6.1
Table 8.1
Table 8.2
Table 8.3
Table 14.1
Table 14.2
Table 14.3
Table 14.4
Table 14.5
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Edited by
Xinliang Feng
Nanocarbons for Advanced Energy Storage
Volume 1
The Editors
Prof. Xinliang Feng
Technische Universität Dresden
01062 Dresden
Germany
Cover
Background Photo. Source: iStock © BCFC
All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.
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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at <http://dnb.d-nb.de>.
© 2015 Wiley-VCH Verlag GmbH & Co.
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Print ISBN: 978-3-527-33665-4
ePDF ISBN: 978-3-527-68008-5
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Mobi ISBN: 978-3-527-68006-1
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Preface
With the rapid change and increasing concerns about the climatic warming and oil consumption, development of new clean energy storage systems (e.g., electricity, hydrogen) with high energy efficiency has become more and more important and urgent in our modern society. One of the major directions to overcome these challenging issues is the production of electricity, for instance, an electric vehicle could be powered by a rechargeable battery or/and supercapacitor instead of oil or coal. Taking this into account, high‐performance electrochemical energy storage systems must be developed to meet the growing industrial and societal demands. In this respect, searching for novel materials with exceptional electrochemical properties for energy storage is essential.
Among of all newly developed functional materials, nanocarbons ranging from pristine nanocarbons to carbon‐based nanohybrids are playing a key role in high‐performance electrochemical energy storage devices. With the rapid growth of nanotechnology, nanocarbon materials such as activated carbon, porous carbons, carbon nanotubes, and graphene have been dramatically developed in the past two decades. Their unique electrical properties and tailored porous structures facilitate fast ion and electron transportation. In order to further improve the power and energy densities of the lithium‐ion batteries and electrochemical capacitors, carbon‐based hybrids that combine the synergistic properties of carbon and hybrid components (such as metal, metal oxide, polymer) have been extensively explored. These nanocarbon‐based materials exhibit not only enhanced specific capacitance, rate capability, but also improved cyclability and energy/power densities. Undoubtedly, advanced nanocarbon materials show great potential in improving current or even further developing high‐performance electrochemical energy storage devices. Therefore, the goal of this book is to present the latest advancements associated with the design and synthesis, characterizations, and applications of nanocarbon materials for advanced electrochemical energy storage, in particular, involving nanostructured carbon materials as cathodes and anodes for lithium‐ion batteries, and as electrodes for supercapacitors.
In this book, world‐leading scientists working in the field of nanocarbons and energy storage applications are joining together to write a book for students (graduate and undergraduate level), researchers, and possible investors interested in supporting materials research. This book consists of 15 chapters: 11 chapters are devoted to electrochemical capacitors (electrochemical double‐layer capacitors, supercapacitors), in which 8 chapters address the general nanocarbon materials, including activated carbons, porous carbon, carbide‐derived carbons, aligned carbon nanotubes, carbon nanotube thin films, graphene, and activated graphene, for supercapacitors; 1 chapter describes the theoretical insights into carbon‐based supercapacitors; 2 chapters present nanocarbon‐based materials and their hybrids for asymmetric supercapacitors and hybrid supercapacitors. And the remaining four chapters discuss the applications of nanocarbons in lithium‐ion batteries, of which two are nanocarbon hybrids with metal oxide, silicon, sulfur, or paper/textile, one is related to the precursor‐controlled synthesis of nanocarbons and one is graphene for flexible battery devices. Each chapter aims at presenting the most detailed information using familiar terms from the point of view of both research and industrial applications.
Finally, we would like to thank all scientists who have been helpful in the preparation of this book and all colleagues who kindly devoted their time and efforts to contribute chapters.
Xinliang FengDresden, Germany
List of Contributors
Lars Borchardt
Dresden University of Technology
Department of Inorganic Chemistry
Bergstraße 66
01069, Dresden
Germany
Junghoon Chae
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division, Faculty of Engineering, University Park
Nottingham, NG72RD
UK
Zheng Chang
School of Energy
Nanjing Tech University
Nanjing 211816
Jiangsu Province
China
George Z. Chen
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division, Faculty of Engineering, University Park
Nottingham, NG72RD
UK
Guanxiong Chen
Department of Materials Science and Engineering and CAS Key Laboratory of Materials for Energy Conversion
University of Science and Technology of China
Jinzhai Road 96, Hefei
Anhui 230026
China
Xiaodong Chen
Nanyang Technological University
School of Materials Science and Engineering
50 Nanyang Avenue
Singapore, 639798
Singapore
Yongsheng Chen
Nankai University
Key Laboratory of Functional Polymer Materials
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)
Center for Nanoscale Science and Technology
Institute of Polymer Chemistry
College of Chemistry
Wenjin Road 94
Tianjin, 300071
China
Hui-Ming Cheng
Chinese Academy of Sciences
Shenyang National Laboratory for Materials Science
Institute of Metal Research
72 Wenhua Road
Shenyang, 110016
China
Yi Cui
Stanford University
Department of Materials Science and Engineering
476 Lomita Mall, McCullough Building Rm. 343
Stanford, CA 94305
USA
and
SLAC National Accelerator Laboratory
Stanford Institute for Materials and Energy Sciences
2575 Sand Hill Road
Menlo Park, CA 94025
USA
Sheng Dai
The University of Tennessee
Department of Chemistry
1420 Circle Dr.
Knoxville, TN 37996
USA
and
Chemical Sciences Division
Oak Ridge National Laboratory
1 Bethel Valley Rd.
Oak Ridge, TN 37831
USA
Shoushan Fan
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
Tsinghua University
Beijing, 100084
China
Kothandam Gopalakrishnan
Jawaharlal Nehru Centre for Advanced Scientific Research
Chemistry and Physics of Materials Unit
International Centre for Materials Science
CSIR Centre of Excellence in Chemistry and Sheik Saqr Laboratory
Jakkur Campus
Bangalore, 560064
India
Achutharao Govindaraj
Jawaharlal Nehru Centre for Advanced Scientific Research
Chemistry and Physics of Materials Unit
International Centre for Materials Science
CSIR Centre of Excellence in Chemistry and Sheik Saqr Laboratory
Jakkur Campus
Bangalore, 560064
India
and
Indian Institute of Science
Solid State and Structural Chemistry Unit
Bangalore, 566012
India
Denys G. Gromadskyi
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division
Faculty of Engineering
University Park
Nottingham, NG72RD
UK
Wentian Gu
Department of Materials Science and Engineering
Georgia Institute of Technology
771 Ferst Drive, N.W., Love Building, Room 372
Atlanta, GA 30332-0245
USA
Li Guan
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division
Faculty of Engineering
University Park
Nottingham, NG72RD
UK
Guang-Ping Hao
Dresden University of Technology
Department of Inorganic Chemistry
Bergstraße 66
01069, Dresden
Germany
Di Hu
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division
Faculty of Engineering
University Park
Nottingham, NG72RD
UK
De-en Jiang
University of California
Department of Chemistry
501 Big Springs Rd.
Riverside, CA 92521
USA
Stefan Kaskel
Dresden University of Technology
Department of Inorganic Chemistry
Bergstraße 66
01069, Dresden
Germany
Feng Li
Chinese Academy of Sciences
Shenyang National Laboratory for Materials Science
Institute of Metal Research
72 Wenhua Road
Shenyang, 110016
China
Minxia Li
School of Energy
Nanjing Tech University
Nanjing 211816
Jiangsu Province
China
Hong-Ze Luo
Council for Scientific and Industrial Research
Naude Road
Brummeria Pretoria, 0001
South Africa
Kaili Jiang
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
Tsinghua University
Beijing, 100084
China
Mengya Li
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
Tsinghua University
Beijing, 100084
China
Xianglong Li
National Center for Nanoscience and Technology
Beiyitiao 11, Zhongguancun
Beijing, 100190
China
Lili Liu
Nanyang Technological University
School of Materials Science and Engineering
50 Nanyang Avenue
Singapore, 639798
Singapore
Nian Liu
Stanford University
Department of Materials Science and Engineering
476 Lomita Mall, McCullough Building Rm. 217
Stanford, CA 94305
USA
Yanhong Lu
Nankai University
Key Laboratory of Functional Polymer Materials
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)
Center for Nanoscale Science and Technology
Institute of Polymer Chemistry
College of Chemistry
Wenjin Road 94
Tianjin, 300071
China
Nada Mehio
The University of Tennessee
Department of Chemistry
1420 Circle Dr.
Knoxville, TN 37996
USA
Zhiqiang Niu
Nanyang Technological University
School of Materials Science and Engineering
50 Nanyang Avenue
Singapore, 639798
Singapore
and
Nankai University
Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education)
College of Chemistry
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)
Tianjin, 300071
China
Martin Oschatz
Dresden University of Technology
Department of Inorganic Chemistry
Bergstraße 66
01069, Dresden
Germany
Huisheng Peng
Fudan University
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science, and Laboratory of Advanced Materials
2205 Songhu Road
Shanghai, 200438
China
C. N. R. Rao
Jawaharlal Nehru Centre for Advanced Scientific Research
Chemistry and Physics of Materials Unit
International Centre for Materials Science
CSIR Centre of Excellence in Chemistry and Sheik Saqr Laboratory
Jakkur Campus
Bangalore, 560064
India
and
Indian Institute of Science
Solid State and Structural Chemistry Unit
Bangalore, 566012
India
Shuling Shen
University of Shanghai for Science and Technology
School of Materials Science and Engineering
Jungong Road 516
Shanghai, 200093
China
Gaoquan Shi
Tsinghua University
Department of Chemistry
Beijing, 100084
China
Anthony J. Stevenson
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division
Faculty of Engineering
University Park
Nottingham, NG72RD
UK
Hao Sun
Fudan University
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science, and Laboratory of Advanced Materials
2205 Songhu Road
Shanghai, 200438
China
Li Sun
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
Tsinghua University
Beijing, 100084
China
Xuemei Sun
Fudan University
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science, and Laboratory of Advanced Materials
2205 Songhu Road
Shanghai, 200438
China
Yiqing Sun
Tsinghua University
Department of Chemistry
Beijing, 100084
China
Ziqi Tan
Department of Materials Science and Engineering and CAS Key Laboratory of Materials for Energy Conversion
University of Science and Technology of China
Jinzhai Road 96, Hefei
Anhui 230026
China
Faxing Wang
School of Energy
Nanjing Tech University
Nanjing 211816
Jiangsu Province
China
Jiaping Wang
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
Tsinghua University
Beijing, 100084
China
Xinran Wang
Department of Materials Science and Engineering
Georgia Institute of Technology
771 Ferst Drive, N.W., Love Building, Room 372
Atlanta, GA 30332-0245
USA
and
Chinese Academy of Sciences
Institute of Process Engineering
China
Lei Wen
Chinese Academy of Sciences
Shenyang National Laboratory for Materials Science
Institute of Metal Research
72 Wenhua Road
Shenyang, 110016
China
Jianzhong Wu
University of California
Department of Chemical and Environmental Engineering
900 University Ave.
Riverside, CA 92521
USA
Yang Wu
Department of Physics and Tsinghua-Foxconn Nanotechnology Research Center
Tsinghua University
Beijing, 100084
China
Yuping Wu
School of Energy
Nanjing Tech University
Nanjing 211816
Jiangsu Province
China
Sishen Xie
Chinese Academy of Sciences
Beijing National Laboratory for Condensed Matter Physics
Institute of Physics
Beijing, 100190
China
Zhibin Yang
Fudan University
State Key Laboratory of Molecular Engineering of Polymers
Department of Macromolecular Science, and Laboratory of Advanced Materials
2205 Songhu Road
Shanghai, 200438
China
Linpo Yu
University of Nottingham
Department of Chemical and Environmental Engineering and Energy and Sustainability Research Division, Faculty of Engineering, University Park
Nottingham, NG72RD
UK
Gleb Yushin
Department of Materials Science and Engineering
Georgia Institute of Technology
771 Ferst Drive, N.W., Love Building, Room 372
Atlanta, GA 30332-0245
USA
Guangyuan Zheng
Stanford University
Department of Materials Science and Engineering
476 Lomita Mall, McCullough Building Rm. 228
Stanford, CA 94305
USA
Linjie Zhi
University of Shanghai for Science and Technology
School of Materials Science and Engineering
Shanghai, 200093
China
and
National Center for Nanoscience and Technology
Beiyitiao 11, Zhongguancun
Beijing, 100190
China
Weiya Zhou
Chinese Academy of Sciences
Beijing National Laboratory for Condensed Matter Physics
Institute of Physics
Beijing, 100190
China
Yanwu Zhu
Department of Materials Science and Engineering and CAS Key Laboratory of Materials for Energy Conversion
University of Science and Technology of China
Jinzhai Road 96, Hefei
Anhui 230026
China
1Nanostructured Activated Carbons for Supercapacitors
Wentian Gu, Xinran Wang and Gleb Yushin
1.1 Supercapacitors
Production and storage of clean and renewable energy has become one of the most exciting yet challenging topics in recent decades. The pressing need for green energy production and efficient energy storage has been further emphasized by the shortage of conventional energy sources and the continuous environment deterioration. While many forms of natural energy, such as solar, wind, and water power, have been considered candidates for the next-generation energy sources, electrochemical energy storage devices, such as rechargeable batteries and supercapacitors, dominate the solutions for the transmittance and storage of renewable energy. By now, these devices have been commercialized and applied in a wide range of industries, ranging from portable electronics to transportation to military and aerospace.
A significant performance gap exists between the energy and power performance characteristics of batteries and electrolytic capacitors, as shown in Figure 1.1. Batteries offer very high specific energy and energy density (energy stored per unit mass or volume of a device), but suffer from relatively low specific power and power density. Conversely, electrolytic capacitors offer excellent power density characteristic at the expense of lower energy density. Electrochemical capacitors (which are often called supercapacitors) nearly bridge the existing gap in performance, by offering moderate energy and power characteristics. In contrast to batteries, supercapacitors additionally offer significantly longer cycle stability and broader temperature window of efficient applications.
Figure 1.1 Schematic illustration of the specific power versus specific energy for various electrical energy storage devices.
On the basis of the differences in energy storage mechanisms, supercapacitors can be classified into two broad categories. One is the electrical double-layer capacitor (EDLC), in which the capacitance comes from the pure electrostatic charge accumulated across the so-called double layer at the electrode/electrolyte interface. The large surface area of the EDLC electrodes combined with a small thickness of the double layer results in a specific and volumetric capacitance two orders of magnitude larger than that of the electrolytic capacitors (Figure 1.1). The second category is a pseudocapacitor, in which fast and reversible Faradic (charge transfer) processes take place across the electrode/electrolyte interface. Quite often, these two mechanisms may function simultaneously in many supercapacitors.
The energy density of supercapacitors is dependent on the capacitance of their electrodes and the maximum operating voltage. The latter is determined by the window of electrochemical stability of the electrolyte. Such stability windows, however, may be influenced by the surface chemistry and other properties of the supercapacitor electrodes as well as electrolyte purity.
The energy of an EDLC could be estimated according to following equation:
where E is the energy, Vmax is the maximum voltage difference between two electrodes, C+ and C− are the capacitances of the positive and negative electrodes, respectively. The energy of an EDLC is maximized when C+ and C− are identical:
In a symmetric EDLC, the specific capacitance of each electrode (capacitance per unit mass of the electrode material) could be identified by a galvanostatic (constant current) charge–discharge test, where the specific capacitance is calculated using the following equation:
where C is the specific (normalized by an electrode mass) capacitance, I is the specific current, and dv/dt is the changing rate of the voltage. In an ideal EDLC, the voltage slope, dv/dt, is constant for a fixed current.
The calculation of a pseudocapacitance (or a total capacitance, which includes both pseudocapacitors and a double-layer capacitance) could be similar to that of a pure double-layer capacitance, in case when dv/dt stays constant, in spite of the additional Faradic reactions. If dv/dt varies with time, one may approximate the capacitance by using an average value of the voltage slope. We shall note that because of lower cost, faster rate, longer cycle life, and lower self-discharge, EDLC-type of supercapacitors currently dominate the market. The current market fraction for pseudocapacitors is tiny.
1.2 Activated Carbon as Electrode for Supercapacitors
In addition to high capacitance, other desirable properties of electrode materials for EDLCs include (i) Free of uncontrolled side reactions with utilized electrolyte to achieve a low self-discharge and long cycle life; (ii) low cost; (iii) abundance; (iv) low toxicity and health hazard; (v) scalability of the synthesis; (vi) mechanical, chemical, and electrochemical stability during the device assembling and operation; (vii) high packing density; and (viii) reliable and reproducible properties.
By now, high-surface-area carbon materials are utilized in EDLCs, with activated carbons (ACs) taking nearly all of the current market. The large specific surface area (SSA) of ACs, their relatively high chemical stability, somewhat reasonable cost, abundance, and diversity of AC precursors, biocompatibility, scalable synthesis, and other useful properties make ACs the choice of the device manufacturers. ACs could be produced in various shapes and forms, such as powders and fibers of various size and pore size distributions, mats, monoliths, films, foils.
Many raw materials, natural and artificial, have been utilized as precursors for AC synthesis. The pore size of ACs can be partially controllable by selecting particular precursor chemistry, activation method, and conditions. Still, commercial ACs for use in EDLCs suffer from some limitations, such as the presence of bottle neck pores, high resistance to ion diffusion and limited volumetric and gravimetric capacitance, to name a few. With the goal of efficient ion diffusion and reduction in equivalent series resistance (ESR), several routes for a more delicate control on the pore size distribution and microstructure of ACs have recently been explored.
In this chapter, we review the development of nanostructured ACs as electrode materials for EDLCs. In Sections 1.3 and 1.4, we review the precursors and processes for AC synthesis in various shapes and forms. In Section 1.5, we review the key factors determining the performance of AC-based EDLCs, including the porous texture of the electrode, the electrical and ionic conductivity within the electrode, and the electrolyte selection. In Section 1.6, we discuss some of the AC properties, which may induce self-discharge within EDLCs.
1.3 Synthesis of ACs
1.3.1 Precursors
ACs are prepared by thermal treatment and partial oxidation of organic compounds, including a very wide selection of natural and synthetic precursors. Most of the pores in ACs are in the 0.4–4 nm range, and the pore size distribution is generally relatively broad. Some of the most common natural precursors for AC synthesis include nutshells (mostly coconut shells [1–8]), waste wood products, coal, petroleum coke, pitch, peat, lignite, while other precursors, such as starch, sucrose, corn grain, leaves, seaweed, alginate, straw, coffee grounds are also occasionally used (Tables 1.1–1.3) [9–28]. More advanced (and unfortunately more expensive) ACs with reproducible properties, more uniform microstructure and pores, and often better developed porosity (higher SSA) can be produced from synthetic polymers, such as polyacrylonitrile (PAN), polyvinylidene chloride (PVDC), polyfurfuryl alcohol (PFA), polyvinyl chloride (PVC), polypyrrole (PPy), polyaniline (PANI), polydivinylbenzene (PDVB) [9, 29–35], to mention a few. Most organic materials rich in carbon that do not fuse upon thermal decomposition can be used as precursors. Some physical properties of the selected precursors for ACs synthesis are listed the following sections:
Table 1.1 Physical properties of the ACs from various precursors
Precursors
Density of carbon (g cm
−3
)
Carbon yield (wt%)
Conductivity
References
Natural precursor
Coconut shell
1.834–2.131
25–40
—
[1, 2, 4–8, 36, 37]
Pitch
0.54–0.75
33.6
22 Ω
[16, 17, 38–41]
Starch
—
—
0.1 Ω
[13, 42, 43]
Seaweed
0.47–0.80
16
–
[44, 45]
Coal
—
40
—
[16, 19]
Apricot shell
0.504
23.2
—
[20]
Ramie
—
38.1
0.08 Ω
[46]
Sugarcane bagasse
—
34.2
—
[18]
Wheat straw
—
37
0.62–1.63 Ω
[22]
Petroleum residue (ethylene-tar)
—
—
0.6 Ω
[47]
Egg shell
—
—
0.018 Ω m
[28]
Artificial precursor
Polyacrylonitrile
—
30
4.91 (S·cm
−1
)/0.5 Ω
[48]
Poly(vinylidene chloride)
—
18–22
0.1 Ω
[49–51]
Poly(amide imide)
—
55
—
[52]
Phenol formaldehyde resin
—
40
—
[53]
Polybenzimidazole
1.2
49
9 S·cm
−1
[54]
Sulfonated poly(divinylbenzene)
0.66
—
—
[33]
Polystyrene
—
48
—
[55]
Table 1.2 Activated method on the pore characterization of different ACs precursors
Table 1.3 Capacitive performance of AC-based supercapacitors
1.3.2 Activation Method
Current methods for the preparation of ACs are often classified into two categories: physical (or thermal) activation and chemical activation. On the basis of some of the representative works, the porous structures of ACs activated from different activation methods are summarized in Table 1.2.
1.3.2.1 Physical Activation
Production of ACs by physical activation commonly involves two steps: carbonization of a precursor (removal of noncarbon species by thermal decomposition in inert atmosphere) and gasification (development of porosity by partial etching of carbon during annealing with an oxidizing agent, such as CO2, H2O, or a mixture of both) [78, 79]. In some cases, low-temperature oxidation in air (at temperatures of 250–350 °C) is occasionally performed on polymer precursors to increase the carbon yield. The reactions occurring during the physical activation could be simplified to the following:
It has to be pointed out that all these reactions are endothermic [80, 81]. This provides better control over the temperature uniformity and activation rate within the powder, but requires sufficient thermal energy (commonly heating to above 800 °C) and a relatively long (hours) period of activation for generating high SSA and pore volume. Such porosity development during activation commonly results in 20–30 wt% yield (oxidation of 60–80 wt% of the initial carbon to CO), which may be considered a critical drawback of physical activation.
1.3.2.2 Chemical Activation
Chemical activation is one-pot preparation method for ACs, utilizing the microexplosion behavior of the activating agent. Production of ACs by chemical activation generally involves the reaction of a precursor with a chemical reagent (such as KOH [40, 41, 49, 61, 68], H3PO4 [74], ZnCl2 [65], H2SO4 [73], among a few) at elevated temperatures. Compared to physical activation, chemical activation generally results in smaller pores, higher carbon yield, and more uniform pore size distribution [82–84].
KOH and NaOH are among the most effective chemical agents for porosity development. The pores are believed to be created via both exfoliation and partial oxidation of carbon [85]. This commonly leads to a larger volume of micropores formed (Table 1.2) and often high carbon yield. The mechanism of carbon etching during metal hydroxide activation can be qualitatively expressed as follows:
Chemical activation methods commonly result in higher specific capacitance in both aqueous and organic electrolytes [86–88]. For example, Kierzek et al. [16] chemically activated highly volatile coal by utilizing KOH. The produced AC exhibited SSA of 3150 m2 g−1 with the pore volume of 1.61 cm3 g−1. Its application in EDLC showed a specific capacitance of 300 F g−1 and 9.9 μF cm−2 in 1 M H2SO4 (aq. solution) electrolyte. This capacitance is very high and superior to the reported performance of one of the most promising commercial ACs, PX 21 (240 F g−1 and 8 μF cm−2) when measured under the same conditions.
For some precursors, however, well-developed porosity is difficult to achieve even by using chemical activation. For example, Hwang et al. [89] systematically investigated variations in the activation process of sewage sludge and coal tar pitch as carbon precursors, by varying the activation temperature, operation time, and activating agent concentration. The total surface area of AC from KOH and NaOH were found to be only 450 and 381 m2 g−1 with the pore volume of 0.394 and 0.37 cm3 g−1, respectively. The more open porous texture from KOH activation is attributed to the larger ionic radius of K+, which is 0.27 nm, compared to 0.19 nm of Na+.
Alkali metal carbonates, such as K2CO3, Na2CO3, and Li2CO3, can be alternatively used as the activating agent. The reaction involved is listed as follows:
The remaining alkali metal and redundant carbonate salts are then removed by using HCl and subsequent distilled water wash.
According to Addoun's research, the radii of cations plays an important role in the development of porosity via alkali metal carbonates [90]. With the increase in cation radii, the pore volume increases as well. Besides, carbonate agents with larger alkali metal cations are usually thermally unstable, which promotes CO2 bubbling.
ZnCl2 is another promising activating agent for ACs synthesis. Different from alkali hydroxides and alkali metal carbonates, ZnCl2 can be impregnated into the precursor and remove hydrogen and oxygen from the precursor with the formation of H2O, resulting in the development of porosity. For example, Du et al. [46] prepared activated carbon hollow fibers (ACHFs) from renewable ramie fibers through ZnCl2 activation for the EDLC electrode. ACHFs calcined at 400 °C for 2 h exhibited an SSA of 2087 m2 g−1 with 38.1% carbon yield. In 6 M KOH electrolyte (aq. solution), the optimal ACHFs showed an impressive capacitance of 287 F g−1 under a specific current of 50 mA g−1.
1.3.2.3 Electrochemical Activation
Sullivan et al. [91, 92] reported the surface activation of glassy carbon via electrochemical process. The activation consisted of applying a large positive potential in an aqueous electrolytic solution (e.g., 1 M H2SO4 aq. solution), during which a reduced thin layer of “activated” (partially oxidized to increase ion accessible surface area) glassy carbon was formed on the surface of the electrode. Besides the development of porous structure, in this process, surface functionalities were also obtained, which contributed to pseudocapacitance. The thickness of the active layer could be controlled in between <1 µm and around 100 µm. They found a strong correlation between the capacitance provided by the active layer and its thickness. An outstanding areal capacitance as high as 460 F cm−2 was achieved in this work. This activation method was followed by several works, with various results [93–97].
1.4 Various Forms of ACs as Supercapacitor Electrodes
Various forms of ACs have been synthesized, with different morphologies, porosities, and conductivities (Figure 1.2). In this section, we provide a brief review on the basic properties and pros/cons of each form of ACs as electrode material for supercapacitors.
Figure 1.2 SEM images of (a) activated carbon powder [98] (Copyright © 2014 ACS Publications); (b) activated carbon fibers [69] (Copyright © 2012 Elsevier); (c) activated carbon fabric [69] (Copyright © 2012 Elsevier); and (d) macroporous activated carbon monoliths [99] (Copyright © 2007 Wiley).
(All figures reproduced with permission.)
1.4.1 Activated Carbon Powders
Commercial AC powders (Figure 1.2a) commonly offer SSA in the range of 700–2200 m2 g−1 and moderately high specific capacitance in the range of 70–200 F g−1 in aqueous and 50–120 F g−1 in organic electrolytes [70, 71, 100–102]. Furthermore, the recent developments in the synthesis of ACs having greatly enhanced specific capacitance (up to 250–300 F g−1 in aqueous, organic, and IL-based electrolytes) demonstrate that for a significant portion of EDLC applications, ACs may remain the material of choice [14, 29, 36, 44, 45, 55, 103–107].
In order to minimize the ion diffusion distance within individual carbon particles, the particle size can be reduced to submicrons and even 10–30 nm range [108]. However, the use of porous nanoparticles reduces both the electrode density (and thus the energy density of the fabricated device) and the size of pores between the individual particles, which may ultimately lead to high ionic resistance in thick electrodes and reduced power density. In a systematic study performed on microporous carbon particles having different particle size but similar electrode mass per unit current collector area, small 20 nm size particles (also having small interparticle pore size), in fact, demonstrated power performance inferior to that of 600 nm particles [108]. In addition, with the exception of AC aerogels having interconnected nanoparticles, the major particle size reduction leads to a lower electrical conductivity of the electrodes (due to point contacts between individual particles), which may become significant enough to impact the EDLC's power characteristics. In addition, handling nanoparticles is difficult and they are difficult to pack densely, which reduces the volumetric device performance. Therefore, commercial EDLC electrodes continue to adopt microsized AC powder with large mesopores between the individual particles in the assembled and compressed electrodes.
1.4.2 Activated Carbon Films and Monoliths
Formation of EDLC electrodes from films and porous monoliths of ACs (Figure 1.2d) allows for a significant increase in their electrical conductivity (due to the elimination of both the nonconductive binder and the high resistance particle-to-particle point contacts) and, in cases when the pore volume in monoliths is relatively small, their volumetric capacitance increases (due to the elimination of the large macropores between the particles) [33, 47, 57, 75, 76, 109]. For example, meso/microporous AC monoliths (1 mm thickness) produced by chemical activation (KOH) of mesophase pitch precursors and exhibiting SSA of up to 2650 m2 g−1 and surface area of micropores of up to 1830 m2 g−1 showed an outstanding initial capacitance of up to 334 F g−1 in 1 M H2SO4 [47], which is one of the highest capacitance values reported for carbon materials. Nitrogen-doped macro/meso/microporous AC monoliths (cylindrical shape with up to 17 mm diameter) having a very moderate SSA of 772 m2 g−1 were recently shown to exhibit high specific capacitance of up to ∼200 F g−1 in 6 M KOH electrolyte [76]. In another recent study, S-containing macro/meso/microporous AC monoliths produced by physical (CO2) activation of carbonized PDVB also demonstrated a specific capacitance of up to ∼200 F g−1 in 2 M H2SO4 electrolyte [33] (volumetric capacitance was not provided in the last two studies, but it is not expected to exceed ∼40 F cm−3 due to the presence of high content of macropores). A recent publication on patterned thin (1–3 µm) AC films reported a specific capacitance greater than 325 F g−1 (>250 F cm−3) in 1 M H2SO4 electrolyte [110].
1.4.3 Activated Carbon Fibers
AC fibers/fabrics (Figure 1.2c) commonly exhibit high electrical conductivity [39, 48, 51, 52, 54, 56, 59, 60, 63, 64, 111–116]. In contrast to monolithic electrodes, AC fabric electrodes could offer very high mechanical flexibility. Their higher power characteristics often originate from the smaller electrode thickness, high volume of macro/mesopores between the individual fibers, and higher electrical conductivity. Depending on the fiber diameter and the activation process utilized, the ion transport and the overall specific power of AC fiber-based EDLCs may vary in a broad range.
Apart from catalyst-grown carbon fibers, the smallest diameter AC fibers are produced by carbonization and activation of electrospun polymer solutions [48, 53, 54, 69, 77, 117–119]. The produced AC nanofiber electrodes exhibit an outstanding rate capability, but suffer from low density [48, 54]. In fact, the density of high-power AC fiber electrodes is often noticeably lower than that of AC powder electrodes, which leads to their lower volumetric capacitance. However, several studies have demonstrated very promising performance of dense AC fiber–based EDLCs. For example, pitch-derived carbon fiber electrodes (individual fiber diameter in the range of 2–30 µm) physically activated in an H2O stream to moderately high SSA of 880 m2 g−1 while retaining high density (up to 0.8 g cm−3) exhibited a specific capacitance of up to 112 F g−1 (90 F cm−3) in 1 M KCl electrolyte [59]. Chemical activation can similarly be used to control the density and porosity of carbon fibers. For example, chemically (KOH) activated mesophase-pitch–based carbon fibers showed an SSA increase from 510 to 2436 m2 g−1 upon increase in the KOH-to-C ratio from 1.5 to 4 [114]. The highest gravimetric capacitance in both ILs and tetraethyl ammonium tetrafluoroborate (TEATFB)-based organic electrolytes (up to ∼180 F g−1) was achieved in the sample with the highest SSA, while the highest volumetric capacitance (up to ∼88 F cm−3) was achieved in moderately activated fibers with an SSA of 1143 m2 g−1 [114].
Oxygen-containing plasma treatment of AC fibers was found to increase their SSA and specific capacitance in aqueous (0.5 M H2SO4) electrolytes [115, 116]. Interestingly, treatment in a pure O2 atmosphere at moderate temperatures (∼250 °C) did not significantly change the SSA, but introduced a higher content of C=O functional groups, which resulted in an increase in specific capacitance from 120 to 150 F g−1 in 1 M H2SO4 electrolyte, presumably owing to improved wetting and a higher contribution from pseudocapacitance produced by the introduced functional groups [66].
As summary to this section, the capacitive performance of AC-based electrodes reported by representative previous works is listed in Table 1.3.
1.5 Key Factors Determining the Electrochemical Performance of AC-Based Supercapacitors
The capacitive performance of AC-based supercapacitors is dependent on several key factors. Independent contributions of each factor could be challenging to separate, because most of these factors are strongly correlated. In this section, we summarize some of the most critical properties of ACs that affect their performance in cells.
1.5.1 Pore Size and Pore Size Distribution
According to the simplified equation for the capacitance calculation,
The specific capacitance provided by carbons should be proportional to their SSA, often approximated as Brunauer–Emmett–Teller (BET) SSA. This linear dependence was indeed suggested in early studies for small SBET values [19, 120], but the capacitance was found to be almost constant for SBET at 1200–3000 m2 g−1. To explain this nonlinear behavior, the complicated pore structures of ACs need to be carefully characterized for better understanding of their electrochemical performance. ACs are highly porous materials with different types of pores, which are classified on the basis of their diameters: micropores(<2 nm), mesopores(2–50 nm), and macropores(>50 nm) [121]. Micropores play an essential role in the formation of electrical double layers. Shi [122] studied the relation between SSAs of microbeads and carbon fibers and their specific capacitances. It was suggested that the micropore surface is the most efficient in capacitance contribution (15–20 μF cm−2), while capacitance from external (meso- and macropore) surface is dependent on the morphology of pores and surface functionalization. Since then, many works have confirmed the outstanding capacitive performance of micropores, especially ones with diameters <1 nm [123, 124]. Because electrolyte ions are generally shielded with solvent molecules, one may hypothesize that electrode pores that contribute capacitance should be at least larger than solvated ions. This theory was soon challenged by experimental observations [123, 125]. It was reported that pores that are smaller than solvated ions could still contribute capacitance [123]. In this case, the ion adsorption is realized by distortion of the solvation shell. This process provides a closer approach of the ion center to the electrode surface, which may result in anomalous increase in capacitance (Figure 1.3). The follow-up report applied solvent-free ionic liquid (IL) and revealed that the pore size leading to the maximum double-layer capacitance is very close to the ion size.
Figure 1.3 Carbon capacitance normalized by a BET specific surface area SBET [123]. Anomalous capacitance increase happens when the pore size is reduced to less than 1 nm.
(Reproduced with permission. Copyright © 2006 AAAS.)
However, determination of the true SSA of AC with irregular shape of pores, reduction of the SSA during electrode processing, and the difference between the electrolyte ion accessible SSA and the SSA determined using gas sorption studies (N2, Ar, CO2) may induce significant discrepancies. Some of the smallest micropores accessible by gas molecules may not be accessible by ions and thus may not contribute to double-layer capacitance. Similar, ion intercalation might open some surface areas, which were previously inaccessible by gas molecules.
The use of simplistic BET model may induce significant inaccuracies in SSA determination; and more advanced models for SSA and pore size distribution measurements are constantly being developed. For example, Ravikovitch and Neimark [126] developed a method for pore size distribution calculation on the basis of nonlocal density functional theory (NLDFT) of capillary condensation hysteresis in cylindrical pores. Chmiola et al. [124] compared the correlation of the specific capacitance of microporous carbide-derived carbons (CDCs) with their SSAs calculated on the basis of both BET and NLDFT, respectively. The researchers found a better correlation between the specific capacitance and DFT (density functional theory) SSA (SSADFT) than between the specific capacitance and BET SSA (SSABET) (Figure 1.4). Compared to BET method, DFT method allows a further comparison of double-layer capacitance with ultra-small micropores (<1 nm). On the basis of the analysis of pore size distribution, they found that the specific double-layer capacitance of CDC is mostly dependent on the surface area of the pores <2 nm (Figure 1.5).
Figure 1.4 The evolution of (a) SSABET; (b) SSADFT; (c) gravimetric capacitance; and (d) volumetric capacitance of ZrC-CDC and TiC-CDC with chlorination temperature [124].
(Figure reproduced with permission. Copyright © 2006 Elsevier.)
Figure 1.5 Micropore (<2 nm) and mesopore (>2 nm) surface area in comparison with gravimetric specific capacitance for (a) TiC-CDC and (b) ZrC-CDC [124]. A direct correlation between the micropore surface area and specific capacitance is evident.
(Figure reproduced with permission. Copyright © 2006 Elsevier.)
While micropores lead to a higher volumetric and gravimetric capacitance than mesopores, electrodes with only micropores may suffer from the slow ion transport and the resulting high ionic resistance even under moderate current densities. Conversely, mesopores allow smooth entry for electrolyte ions and therefore enhance the capacitance and the power density of devices [87, 127–129]. Frackowiak et al. studied the coal-based ACs as electrode material for EDLC and found the optimized range of mesopore content within 20–50% [128]. This result, although still qualitative, shows that one needs to consider the balance in the pore size distribution of ACs for optimal EDLC performance in a given application. Chen et al. synthesized a series of micro- and mesoporous carbon materials with different SSA and pore size distribution by using different carbon sources and preparation methods. On the basis of the capacitive performance of these materials, they developed a general model for the estimation of capacitance, which is linearly proportional to “effective SSA,” as determined by both the measured SSA and pore size distribution [130]. Further detailed models have been developed by taking pore curvature into consideration. Meunier et al. developed separate models for the calculation of micropore, mesopore, and macropore capacitances, respectively [131].
1.5.2 Pore Alignment
A recent study revealed the significant effect of pore alignment on the transport of ions within carbon nanopores [132]. In that work, zeolite-templated carbons were synthesized using low-pressure chemical vapor deposition (CVD). Identical micropore (<2 nm) size distributions with varying levels of pore alignment (as well as pore tortuosity) were achieved by manipulating the annealing conditions. Electrochemical impedance spectroscopy measurements and cyclic voltammetry studies showed up to three orders of magnitude enhancements in the ion transport and frequency response accompanying the micropore alignment and a decrease in the concentration of obstacles for ion diffusion. This finding proves that other than introduction of mesopores as conducting paths, designing ACs with straight pores and low concentration of defects might be an alternative strategy for achieving rapid ion diffusion within individual electrode particles.
In response to the discovery of the essential role of pore alignment and optimization of pore size distribution for fast electrolyte diffusion and high power output of carbon EDLCs, researchers proposed various strategies to control the pore size and alignment of AC-based electrode materials. In order to achieve a higher SSA and eliminate bottle-neck pores, while uniformly enlarging the smallest micropores produced in the course of carbonization of organic precursors, several promising routes were proposed. According to one method, an equilibrium content of oxygen-containing functional groups is uniformly formed on the porous carbon surface during room temperature treatment in acids [32]. These groups together with the carbon atoms are later removed via heat treatment at 900 °C. Repetition of the process of uniform formation of chemisorbed oxygen functional groups, and subsequently removing them, allows for the uniform pore broadening needed to achieve the optimum pore size distribution [32]. In another study, an environmentally friendly low-temperature hydrothermal carbonization was utilized in order to introduce a network of uniformly distributed oxygen within the carbon structure in one step [14]. This material, produced from natural precursors (such as wood dust and potato starch), was then transformed into microporous carbons with high SSA of 2100–2450 m2 g−1 via simultaneous heat treatment (and thus uniform removal of the oxygen-containing functional groups from the internal material surface) and opening of closed and bottle-neck pores by activation. A very high specific capacitance of 140–210 F g−1 was demonstrated in a TEATFB-based organic electrolyte [14]. Another efficient method for synthesis of carbons with controlled pore size distribution and alignment is the template method. In this method, highly ordered porous oxides, such as mesoporous silica are used as sacrificial template materials. Carbon precursors (sucrose solution, propylene, pitch, resin, etc.) are deposited inside the pores of the template and then carbonized. Then, the template is removed by dissolution in the hydrofluoric acid. The detailed discussion on highly ordered porous carbon is beyond the scope of this review, but readers are encouraged to refer to the representative works [133–137] on this important category of carbons.
1.5.3 Surface Functionalization
A variable amount of heteroatoms (oxygen, nitrogen, sulfur, etc.) can be added into the structure of carbon materials and functionalize their surfaces. Functionalized carbons can be synthesized by applying precursors containing heteroatoms [72, 138, 139] or posttreatment of carbons in heteroatom-enriched atmosphere [66, 140]. The resulted functional groups can considerably enhance the gravimetric capacitance of electrodes by contributing additional pseudocapacitance, which involves rapid charge transfer reactions between the electrolyte and the functional moieties. The usually denser functionalized carbons, although with smaller SSA, provide a higher volumetric capacitance than highly porous intrinsic ACs. However, pseudocapacitance based on surface functionalization also suffers from intrinsic shortcomings, including relatively sluggish charge transfer process, serious self-discharge, and high leakage currents. These limitations are further discussed in Section 1.6.
1.5.4 Electrical Conductivity of the Electrode
To achieve high power output, charge carriers must move quickly and smoothly through electrodes, which requires high electrical conductivity. The electrical conductivity of AC-based electrodes strongly depends on the thermal treatment in the synthesis process, porous texture of the electrodes, and the content of heteroatoms [101].
Most carbon precursors are good insulators with a high content of σ- or sp3- bonded carbon structures. During the thermal treatment at elevated temperatures, conductivity of the material is rapidly increased with the increasing content of sp2-bonded conjugated carbon, because electrons associated with π-bonds are delocalized and become available as charge carriers [141]. The structural disorders and defects are healed to various extents, depending on the temperature applied for this process. Conductivity of carbon begins to increase at 600–700 °C, which corresponds to the range where loss of surface functionalities happens, until the formation of perfect crystalline graphite structure at over 2500 °C.
Generally, higher porosity leads to poorer electrical conductivity because of higher content of insulating voids. To enhance the electrical conductivity of highly porous ACs, several strategies could be applied, such as adding conductive agents, high-pressure packing of AC particles, and additional bonding network connecting particles.
In the previous section, we have discussed the enhancement of the capacitive performance of AC-based electrodes by surface functionalization. However, the effect of surface functionalization on the electric conductivity of the AC-based electrodes is largely negative. However, exceptions exist. For example, some functionalized carbons have higher density and, therefore, better electrical conductivity than porous intrinsic ACs [45]. Considering different categories of functionalities, oxygen functionalities, which preferentially form at the edge sites of graphite-like microcrystallites, increase the local barrier for electrons to transfer in between neighboring crystallite elements [142, 143]. Conversely, nitrogen functionalities are often grafted on basal planes of graphite crystallites, with an increase in local free electrons. As a result, proper amount of nitrogen doping increases the electrical conductivity of carbons. However, previous works also reported aggravation of the capacitive performance of carbon materials from excess amount of nitrogen doping [144]. In summary, an optimized content of functionality is necessary for enhancement in capacitive performance of AC-based supercapacitors.
1.5.5 Electrolyte Selection
Electrolytes used in EDLCs may be divided into three classes: (i) aqueous (solutions of acids, bases, and salts), (ii) organic, and (iii) ILs. Each electrolyte has been intensively studied and widely acknowledged for its pros and cons [145]. ILs are nonflammable, are nontoxic, and offer a higher operating voltage than their counterparts. Serious shortcomings of ILs are their very high (often prohibitively high) current cost and relatively low ionic mobility at room temperature and below, which limits the charge/discharge rate of IL-based EDLCs. The advantages of using aqueous electrolytes include their very low cost, safety, and high ionic conductivity. Their disadvantages, however, include their low narrow electrochemical window and corrosion of EDLC electrodes observed at higher temperatures and voltages (particularly for acid-based electrolytes, such as H2SO4 solutions), which limits the cycle life of the EDLCs and contributes to self-discharge. Organic electrolytes are somewhat in between aqueous ones and ILs in terms of the price, voltage, and charge–discharge time. Organic electrolyte-based EDLCs offer cycle life in excess of 500 000 and are used in the majority of commercial EDLCs. In addition, EDLCs with organic electrolytes are much less flammable than Li-ion batteries.
Besides the selection of the electrolyte solvent, it is important to properly match the size of the electrolyte ions and the electrode pores of ACs. Aurbach et al. demonstrated that it was possible to selectively electroadsorb ions based on size [32, 146, 147]. They applied CVD of carbon on active porous carbon fibers to reduce the average pore size to 0.5–0.6 nm, which is in between the size of solvated monovalent (Na+, ∼0.4 nm) and bivalent cations (Ca2+; Mg2+
