Perspective pubs.acs.org/JPCL
Graphene Materials for Electrochemical Capacitors Ji Chen, Chun Li, and Gaoquan Shi* Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: Electrochemical capacitors (ECs) have been widely applied in electronics, electric vehicles, aircrafts, energy storage devices, uninterrupted or emergency power supplies, and so on. An ideal EC should have high energy and/or powder density, good rate capability, and long cycling life. Recently, graphene, graphene derivatives, and their composites have been explored as the electrode materials of ECs to satisfy these requirements. In this Perspective, we review the recent development in synthesizing graphene materials for ECs and discuss the strategies of fabricating graphene-based macroscopic electrodes. Particularly, we highlight the importance of the specific surface area, conductivity, and heteroatom-doping of graphene sheets and the micro/nanostructures of their electrodes for controlling the performances of graphene-based ECs.
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performance ECs, including the methodology of synthesis, selfassembly of graphene materials, and the main factors that affect the performances of graphene-based ECs. Moreover, we will also discuss the challenges in developing graphene-based ECs for practical applications. Graphene is a unique and attractive electrode material because of its atom-thick two-dimensional (2D) structure and excellent properties.10 First, graphene has a theoretical specific surface areas as high as ∼2600 m2 g−1, being twice that of single-walled carbon nanotubes (CNTs) and much higher than those of most carbon black and activated carbons. This structure characteristic makes graphene materials highly desirable for the formation of electrochemical double layers (EDLs).11 The interfacial double-layer capacitance on one side of a graphene sheet was measured to be about 21 μF cm−2;12 thus, the theoretical maximum gravimetric specific double-layer capacitance of graphene electrode was calculated to be approximately 550 F g−1. Second, graphene materials, especially chemically modified graphene (CMG), can be obtained at relatively low costs in a large scale by using graphite, graphite oxide, and its derivatives as the precursors.13,14 Third, the superior electron mobility of graphene facilitates the electron propagation during the charge/discharge processes, improving the performances of ECs. Fourth, graphene sheets are unique 2D building blocks for self-assembling into three-dimensional (3D) macroscopic materials with controlled microstructures that can be used for fabricating EC electrodes without the requirements of blending conductive additives or binders.15 Finally, graphene also has high mechanical, chemical, thermal, and electrochemical stabilities, which can guarantee the lifetime of ECs. However, most graphene-based ECs have low volumetric energy densities caused by the low densities of
lectrochemical capacitors (ECs) are devices that can store and release energies rapidly and reversibly.1 According to the storage mechanisms, ECs can be classified as electrochemical double-layer capacitors (EDLCs) and pseudocapacitors (PsCs).1,2 EDLCs store energies physically by reversible ion adsorption at the electrode/electrolyte interfaces, thus they can be charged/discharged within seconds and over 100 000 cycles. On the other hand, PsCs store energies chemically by fast surface redox reactions of the electrodes. Because redox reactions are involved in the charge-storage processes, the gravimetric specific capacitance (SC) of a PsC usually exceeds that of an EDLC, while to some extent at the sacrifice of its rate performance and cycling life. Actually, these two storage mechanisms can occur simultaneously.3 For the convenience of studies, however, electrode materials are divided into both groups according to their dominant storage mechanism. An ideal EC should have high energy density (1−10 Wh kg−1), power density (103∼106 W kg−1), and ultralong cycling life (>100 000 cycles).4 Furthermore, in comparison with batteries, ECs usually have much higher rate capabilities suitable for the special requirements in electronics, ac-line filtering, electric vehicles, aircrafts, energy storage devices, uninterrupted or emergency power supplies, and so on.4,5 For these purposes, a variety of carbon materials such as activated carbons, carbon nanotubes, graphene, graphene derivatives, and their composites have been applied as electrode materials in commercialized or developing ECs.2,6 Among them, activated carbons are the most widely used electrode materials in commercial EDLCs mainly due to their reasonable costs and scalable manufacture,1 whereas graphene materials have been attracting the most intensive attention recently. Several reviews have covered preliminary results of this research area,7−10 but the tremendous activity in this field allows us to further provide a comprehensive overview to elucidate the basics, challenges, and opportunities of these unique materials. In this Perspective, we will outline the graphene materials used for fabricating high© XXXX American Chemical Society
Received: January 23, 2013 Accepted: March 28, 2013 1244
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Perspective
ques including self-assembly, chemical activation, and light treatment have been developed. In the initial stages, much effort has been paid to construct 3D graphene architectures with opened porous structure, minimizing the restacking of graphene sheets and maximizing their SSAs exposed to electrolytes.15 From the standpoint of polymer and colloid science, CMG sheets are 2D amphiphilic polyelectrolytes with hydrophobic conjugated basal planes and hydrophilic oxygenated groups. The hydrophilic−hydrophobic balance between interplanar van der Waals force and electrostatic repulsion of CMG sheets dominates their solution behavior, and regulates their self-assembly properties in aqueous media. In 2010, we reported a convenient one-step method to prepare self-assembled reduced graphene oxide (rGO) hydrogel via a hydrothermal process.22 The obtained rGO hydrogel has an interconnected 3D porous structure with pore sizes in the range of submicrometer to several micrometers and a large SSA of 964 m2 g−1 (Figure 1a),23 which facilitates the accessibility of graphene sheets to the electrolyte for forming EDL charges. The EDLC based on this rGO hydrogel exhibited an SC of 152 F g−1,22 being about 50% higher than that of EDLC with rGO agglomerate particles as electrodes tested under identical conditions (100 F g−1 at a scan rate of 20 mV s−1).11 The SC of rGO hydrogel has been further optimized to 222 F g−1 (at the discharge rate of 1 A g−1) by further chemical reduction of the rGO hydrogel with hydrazine.23 Simultaneously, this EDLC exhibited good rate performance, maintaining a high SC of 165 F g−1 at a fast discharge rate of 100 A g−1, being 74% of that (222 F g−1) measured at the discharge current density of 1 A g−1. Chemical reduction of GO sheets in their aqueous dispersions can also produce rGO hydrogels, providing an alternative route to prepare 3D rGO architectures.24 Very
their electrodes. Furthermore, the energy densities of graphenebased ECs are also limited by their much less mass loadings of graphene materials compared with those of activated carbons in commercial ECs. Nevertheless, graphene is a promising electrode material for fabricating high-rate, high-power, flexible, or small ECs.1 Graphene materials have been explored to fabricate EDLCs since 2008; Ruoff and co-workers are the pioneers in this field.11,16−21 In their earliest work, a CMG material was synthesized by the reduction of graphene oxide (GO) in its aqueous dispersion using hydrazine hydrate.11 During the process of chemical reduction, the CMG sheets aggregated into irregular particles with diameters in the range of 15 to 25 μm. The specific surface area (SSA) of this CMG agglomerate was determined to be 705 m2 g−1 by a N2 absorption Brunauer− Emmett−Teller (BET) test. The powdery CMG was processed into active electrodes of ECs using polytetrafluoroethylene (PTFE) as a polymeric binder. The SCs of these CMG-based EDLCs were measured to be about 135 and 99 F g−1 in aqueous and organic electrolytes, respectively. These values are much lower than the theoretical maximum of 550 F g−1 calculated for single-layer graphene. The relatively small SSAs and the uncontrolled microstructures of these CMG electrodes greatly decreased the SCs of CMG-based ECs.
Graphene materials with large SSAs and high conductivities are required for fabricating high-performance EDLCs. To improve the SSAs of graphene materials without decreasing their electrical properties, several effective techni-
Figure 1. SEM images of different microstructures of graphene electrodes. (a) The interior microstructures of an rGO hydrogel. Reprinted with permission from ref 22. Copyright 2010 American Chemical Society. (b,c) Cross-section of a freeze-dried (b) rGO-Gel/NF electrode and (c) SSG film. Reprinted with permission from refs 25 and 26. Copyright 2012, 2011 Wiley-VCH. (d) The porous a-MEGO and (e) cross-section of LSG. Reprinted with permission from refs 17 and 27. Copyright 2011, 2012 AAAS. (f) Top-view of oriented microstructures of rGO electrode prepared by electrochemical deposition. Reprinted with permission from ref 29. Copyright 2012 Nature Publishing Group. Insets show the photographs or schematic illustrations of microstructures. 1245
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Figure 2. Electrochemical characterization of rGO-Gel/NF EC. (a) CV curves of rGO-Gel/NF EC in 5 M KOH at different scan rates. (b) Discharge curves of rGO-Gel/NF EC at different current densities. (c) Nyquist plot of rGO-Gel/NF EC. Inset: Plot on an enlarged scale. (d) Cycling stability of rGO-Gel/NF EC upon charging/discharging at a current density of 10 mA cm−2. Inset: CV curves recorded before and after charging/discharging for 2000 or 10 000 cycles; scan rate = 500 mV s−1. Reprinted with permission from ref 25. Copyright 2012 Wiley-VCH.
cycles of charging/discharging test at a high operation current density of 10 mA cm−2 (Figure 2d). The excellent performances of rGO-Gel/NF ECs, including high SCs in terms of area, long durability, and high rate capability, were attributed to the 3D interpenetrating microstructure of rGO-Gel/NF electrodes and the short distances of ion and electron transportation in the electrodes. In another approach, Li et al. developed a flow-directed selfassembly method to prepare self-stacked solvated graphene (SSG) films (Figure 1c).26 It was found that the flexible SSG film prepared by vacuum filtration of CMG dispersions contains 92 wt % of water and can be directly peeled off from the filter membrane. The water trapped in the SSG film is functionalized as a “spacer” to prevent the restacking of CMG sheets, endowing the SSG films with highly opened pore structure. The SSG film-based supercapacitor displays superior performance with a high SC up to 215.0 F g−1 in an aqueous electrolyte. More significantly, the CV profile still remains a rectangular shape at a high scan rate of 2.0 V s−1, and an SC of 156.5 F g−1 can be obtained even at an ultrafast charge/ discharge rate of 1080 A g−1, signifying its excellent rate performance. Additionally, the SSG film exhibited an excellent electrochemical stability as confirmed by retaining over 97% of its initial capacitance after a lifetime test of 10 000 charging/ discharging cycles at a high operation current of 100 A g−1. Chemical activation has also been adopted for fabricating porous activated CMG to improve SSA.17,19 In this approach, graphite oxide powder prepared by a modified Hummers’ method was further treated through a microwave-assisted exfoliation process, which was denominated as microwave exfoliated graphite oxide (MEGO).17 First, fluffy MEGO powders were dispersed and soaked in an aqueous KOH
recently, we applied this facile method to deposit rGO in the pores of nickel foam (NF) by chemical reduction of the GO suspension soaked in the interior voids of NF, forming rGO hydrogel/NF (rGO-Gel/NF) composite electrode.25 The cross-sectional scanning electron microscopy (SEM) image indicates that the rGO-Gel was coated on the framework of NF, and it has a 3D interpenetrating porous structure with SSAs in the range of 1260 to 1725 m2 g−1 (Figure 1b). A symmetric EDLC was constructed to evaluate the performance of rGOGel/NF composite electrode. It is evident that the cyclic voltammogram (CV) remains a near-rectangular shape at a scan rate as high as 2.0 V s−1 (Figure 2a), suggesting that the rGOGel/NF EC has a high rate capability and a low internal resistance. The low internal resistance was also confirmed by the observation of a small voltage drop (0.15 V) even at a high discharge density of 100.0 mA cm−2 (281.7 A g−1). The SC in geometric area unit of rGO-Gel/NF EDLC was calculated to be 44.6, 41.1, and 36.2 mF cm−2 at discharge current densities of 2.7, 50.0, and 183.3 mA cm−2, respectively (Figure 2b). It can be seen that even at a discharge current density as high as 183.3 mA cm−2 (516.3 A g−1), an SC of 36.2 mF cm−2 was still achieved, which is about 80% of that measured at 0.67 mA cm−2 (1.9 A g−1), implying the excellent rate performance. The performance of the rGO-Gel/NF EC was further studied by electrochemical impedance spectroscopy (EIS), and it can be seen that the Nyquist plot of the rGO-Gel/NF EC starts with a short 45° region (Figure 2c), being characteristic of the porous structure of the rGO-Gel/NF electrode. At low frequencies, the straight line is nearly perpendicular to the real axis, indicating the ideal capacitive behavior of the rGO-Gel/NF EC. The rGOGel/NF EC also has an excellent cycling stability as confirmed by retaining ∼90% of its initial capacitance even after 10 000 1246
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Perspective
Figure 3. Supercapacitor performances of a-MEGO (SSA ∼ 2400 m2 g−1) in the BMIM BF4/AN electrolyte. (a) CV curves for different scan rates. (b) Galvanostatic charge/discharge curves of a-MEGO-based supercapacitor under different constant currents. (c) Nyquist plot, showing the imaginary part versus the real part of impedance. (Inset) The data at high-frequency ranges, with frequency values corresponding to the transition of the curves marked. (d) Testing of the a-MEGO (with surface area of ∼3100 m2 g−1) based supercapacitor in neat BMIM BF4 over 10 000 cycles. Reprinted with permisison from ref 17. Copyright 2011 AAAS.
LightScribe is a standard optical disc recording technique, and it has been applied to laser reduction of GO films.27 The laser-scribed graphene (LSG) films have 3D open networks with a large SSA of 1520 m2 g−1 (Figure 1e). Furthermore, they are mechanically robust and electrically conductive (1738 S m−1), and thus can be directly processed into EDLC electrodes without the requirements of polymer binder and current collectors. A superthin EDLC device with a total thickness of
