Supercapacitors An Overview of the Applications of Graphene-Based Materials in Supercapacitors Yi Huang , Jiajie Liang , and Yongsheng Chen * From the Contents 1. Introduction . 2 D ue to their unique 2D structure and outstanding intrinsic physical properties, such as extraordinarily high 2. Preparation of Graphene-Based Materials used for Supercapacitor Electrodes . 3 electrical conductivity and large surface area, graphene- based materials exhibit great potential for application 3. Graphene-Based EDLCs: . 3 in supercapacitors. In this review, the progress made so far for their applications in supercapacitors is reviewed, 4. Graphene-Conducting Polymer-Composite- Based Pseudo-capacitors . .. 16 including electrochemical double-layer capacitors, pseudo-capacitors, and asymmetric supercapacitors. 5. Pseudo-capacitors Based on Graphene– Compared with traditional electrode materials, graphene- (Metal Oxide) Composites. 20 based materials show some novel characteristics and 6. Graphene-Based Asymmetric Supercapacitors mechanisms in the process of energy storage and . .. 25 release. Several key issues for improving the structure of graphene-based materials and for achieving better 7. Conclusion and Outlook . 27 capacitor performance, along with the current outlook for the fi eld, are also discussed. small 2012, © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com 1 DOI: 10.1002/smll.201102635 reviews Y. Huang et al. 1. Introduction carbide-derived carbons, [ 49 ] have been investigated for use as electrodes in EDLCs. Extensive explorations have shown 2 Graphene, a one-atom-thick 2D single layer of sp -bonded that to achieve EDLCs with high performance, several factors carbon, has been considered as the basic construction mate- of the carbon-based materials are crucial: the specifi c surface [1–3] rial for carbon materials of all other dimensionalities. As a area (SSA), electrical conductivity, and pore size and distri- result of this unique structural property, graphene is provided bution. In most cases, although porous-carbon-based mate- with a series of prominent intrinsic chemical and physical fea- rials can obtain high SSA, the low conductivity of porous ∼ [4,5] tures, such as strong mechanical strength ( 1 TPa), extraor- carbon materials continues to restrict its application in high- [6–8] dinarily high electrical and thermal conductivity, and large power-density supercapacitors. [ 50 ] As for CNTs, although 2 [9] surface area (2675 m /g), which may rival or even surpass they possess high electrical conductivity and large SSA, CNT- both single- and multi-walled carbon nanotubes. These out- based supercapacitors still cannot meet acceptable perform- standing and intriguing features make this extremely versatile ance, [ 44 , 51–53 ] which is probably due to the observed contact carbon material promising for various practical applications, resistance between the electrode and current collector. [ 48 , 50 ] [10–12] including high-performance nanocomposites, trans- Moreover, the intrinsic impurities of CNTs from catalysts [6,7,13–16] [17,18] [19–22] parent conducting fi lms, sensors, actuators, and amorphous carbon and their high cost have hampered [23,24] [25–28] nanoelectronics, and energy storage devices. Sig- their practical application in supercapacitors to date. Hence, nifi cantly, utilizing graphene as a supercapacitor electrode great efforts are still spent on developing novel carbon-based material has become the focus of a considerable amount of supercapacitor electrode materials with an overall high per- research in the fi eld of clean energy devices due to the ben- formance. Fortunately, the emergence of graphene provides efi cial combination of the excellent mechanical and electrical an excellent alternative to past EDLC electrode materials. [25–28] properties and large surface area. Compared with traditional porous carbon materials, graphene The investigation of novel, low-cost, environmentally has very high electrical conductivity, large surface area, and friendly, and high-performance energy storage systems has profuse interlayer structure. Hence graphene-based materials been under an ever increasing demand as a result of the are hugely favorable for their application to EDLCs. [ 27 ] [29] needs of modern society and emerging ecological concerns. In contrast to EDLCs, pseudo-capacitors store energy Supercapacitors, also named electrochemical capacitors and through a Faradic process, involving fast and reversible redox [30,31] ultracapacitors, are supposed to be a promising candi- reactions between electrolyte and electro-active materials on date for alternative energy storage devices as a result of their the electrode surface. [ 40 ] The most widely explored electro- high rate capability, pulse power supply, long cycle life, simple active materials include three types: a) transition metal oxides principles, high dynamic sof charge propagation, and low or hydroxides, [ 54 , 55 ] such as ruthenium oxide, manganese [31–34] maintenance cost. Supercapacitors store signifi cantly oxide, and nickel hydroxide; b) conducting polymers, [ 39 , 41 , 56 ] higher amounts of energy density than the conventional die- such as polyaniline, polypyrrole, and polythiophene; and lectric capacitor, but they have a similar cell construction as c) materials possessing oxygen- and nitrogen-containing sur- traditional capacitors except for the fact that the metal elec- face functional groups. [ 40 ] Compared with EDLCs, pseudo- [35] trodes are replaced by highly porous electrodes. Further- capacitors can achieve much higher pseudo-capacitance than more, the shortage of other power sources, such as batteries the EDL capacitance. Nevertheless, further practical applica- and fuel cells, could be complemented by supercapacitors, tions of these electro-active materials to pseudo-capacitors because of their long cycle life and rapid charging and dis- are still limited by the low power density that arises from the [34] charging rate at high power densities. Consequently, the poor electrical conductivity restricting fast electron transport, supercapacitor has continued to attract considerable atten- and by the lack of a pure cycling stability owing to the easily tion from both scientists and engineers since it was demon- damaged structure of the materials during the redox process. [36] strated and patented by General Electric in 1957, and it Hence, to resolve these problems, carbon-based materials has generated great interest for a wide and growing range of with high electrical conductivity and large SSA are usu- applications that require high power density such as energy ally used as the backbone materials to combine with these back-up systems, consumer portable devices, and electrical/ active materials for pseudo-capacitor electrodes. Given the [33,34,36–38] hybrid automobiles. many excellent properties of graphene, such as high elec- Actually, based on the different energy storage mecha- trical conductivity and high mechanical strength, graphene is nisms, supercapacitors can be divided into two classes: 1) elec- trochemical double-layer capacitors (EDLCs) which store energy using the adsorption of both anions and cations and Prof. Y. Huang , [+] Dr. J. J. Liang , [+] Prof. Y. S. Chen 2) pseudo-capacitors that store energy through fast surface Key Laboratory of Functional Polymer Materials redox reactions. EDLCs, which are non-Faradaic ultracapaci- and Centre for Nanoscale Science and Technology tors, derive their performance from a so-called double-layer Institute of Polymer Chemistry capacitance. [ 39 , 40 ] The capacitance in these EDLC devices is College of Chemistry stored as a build-up of charge in the layers of the electrical Nankai University double-layer formed at the interface between a high-surface- 300071, Tianjin, China area electrode and an electrolyte. [ 39 , 40 ] Generally, porous E-mail: [email protected] [+] Y. L. and J.J.L. have contributed equally to this review. carbon materials such as activated carbon, [ 41 , 20 ] xerogels, [ 43 ] carbon nanotubes (CNTs), [ 44–47 ] mesoporous carbon, [ 48 ] and DOI: 10.1002/smll.201102635 2 www.small-journal.com © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2012, DOI: 10.1002/smll.201102635 Applications of Graphene-Based Materials in Supercapacitors considered as one of the most suitable substrate materials for preparing pseudo-capacitor electrodes. Yongsheng Chen graduated from the Universi- In this review, we will summarize the progress made so ty of Victoria with a Ph.D. degree in chemistry far in graphene-based supercapacitors by considering to the in 1997 and then joined the University of Ken- tucky and the University of California at Los two basic kinds of supercapacitors (EDLCs and pseudo- Angeles for postdoctoral studies from 1997 to capacitors) and their hybrid supercapacitors (which function 1999. From 2003, he has been a Chair Profes- with simultaneous EDLC and pseudo-capacitor mechanisms); sor at Nankai University. His main research we will discuss their mechanisms and explore their effective interests include: 1) carbon-based nanomateri- ways to achieve high supercapacitor performance. als, including carbon nanotubes and graphene; 2) organic and polymeric functional materials, and 3) energy devices including organic photo- voltaics and supercapacitor. He has published 2. Preparation of Graphene-Based Materials over 130 peer-reviewed papers with over 6000 used for Supercapacitor Electrodes citations and has over 20 patents. Several effective and