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High-Performance Supercapacitors Based on Poly(Ionic Liquid)-Modified Graphene Electrodes

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High-Performance Supercapacitors Based on Poly(Ionic Liquid)-Modified Graphene Electrodes ARTICLE High-Performance Supercapacitors Based on Poly(ionic liquid)-Modified Graphene Electrodes Tae Young Kim,† Hyun Wook Lee,† Meryl Stoller,‡ Daniel R. Dreyer,§ Christopher W. Bielawski,§ Rodney S. Ruoff,‡,* and Kwang S. Suh†,* †Department of Materials Science and Engineering, Korea University, 5-1 Anam-dong Seongbuk-gu, Seoul 136-713, South Korea, ‡Department of Mechanical Engineering and the Texas Materials Institute, The University of Texas at Austin, One University Station C2200, Austin, Texas 78712, United States, and §Department of Chemistry and Biochemistry, The University of Texas at Austin, One University Station A5300, Austin, Texas 78712, United States upercapacitors (also called electric S double-layer capacitors or ultracapac- ABSTRACT We report a high-performance supercapacitor incorporating a poly(ionic liquid)-modified reduced itors) are electrochemical capacitors graphene oxide (PIL:RG-O) electrode and an ionic liquid (IL) electrolyte (specifically, 1-ethyl-3-methylimidazolium that store energy through reversible ion ad- bis(trifluoromethylsulfonyl)amide or EMIM-NTf2). PIL:RG-O provides enhanced compatibility with the IL electrolyte, sorption onto active materials that have thereby increasing the effective electrode surface area accessible to electrolyte ions. The supercapacitor assembled high specific surface area.1 Because of their many advantageous properties, such as with PIL:RG-O electrode and EMIM-NTf2 electrolyte showed a stable electrochemical response up to 3.5 V operating high power density, high capacitance, and voltage and was capable of yielding a maximum energy density of 6.5 W · h/kg with a power density of 2.4 kW/ long cycle life (Ͼ100 000 cycles), these sys- kg. These results demonstrate the potential of the PIL:RG-O material as an electrode in high-performance tems play an important role in electrical en- supercapacitors. ergy storage.1Ϫ3 To generate a high spe- cific capacitance, the specific surface area KEYWORDS: supercapacitors · electrical double layers · graphene · ionic liquids · of the electrode materials needs to be as thermal reduction · ultracapacitors high as possible to accommodate a large number of electrolyte ions at the electrode/ Hence, recent efforts have been focused electrolyte interface thereby promoting on the development of supercapacitors 1Ϫ3 the electrical double-layer capacitance. with high energy densities, which may be Graphene, an atom-thick 2D achieved both by enhancing the operating 4,5 nanostructure, is a promising material for voltage of the devices and by improving the supercapacitor electrodes owing to its low accessibility of the ions from the electro- mass density, excellent electronic conduc- lyte to the active regions of electrode ϳ 2 tivity, and high surface area ( 2630 m /g, materials. 6Ϫ8 theoretical). Reduced graphene oxide Toward this goal, graphene-based elec- (we use the generic term “RG-O”), a compo- trodes combined with ionic liquid (IL) elec- sition closely related to graphene, is a prom- trolytes can provide an attractive alternative ising material for supercapacitor applica- for supercapacitors since such combina- tions, as specific capacitance values of 135 tions result in an optimal pairing of high and 99 F/g based on RG-O-based electrodes specific surface area electrodes and wider in aqueous and organic electrolytes, respec- operating potentials that may be afforded tively, have been obtained.6 However, dis- by some IL electrolytes. Generally, ILs fea- persed graphene oxide (G-O) platelets can ture moderately high ion conductivity, non- agglomerate during reduction by, for ex- ample, hydrazine in a solvent system such volatility, high decomposition tempera- as water, resulting in the possible decrease tures, and wide electrochemical stability *Address correspondence to of effective surface area, resulting in a lower windows, and many ILs are being consid- [email protected], [email protected]. specific capacitance than might be ex- ered as electrolytes to increase supercapac- 10Ϫ13 pected for an ideal graphene-based super- itor operating voltages. Despite the po- Received for review August 10, 2010 capacitor.6 Moreover, current supercapaci- tential of ILs as electrolytes, further work is and accepted December 01, 2010. tors have energy densities well below the needed to explore their potential for super- values required to provide power assists in capacitors assembled with graphene-based 10.1021/nn101968p various applications including hybrid elec- electrodes. One of the challenges is achiev- 2,9 tric vehicles or other high energy uses. ing graphene-based © XXXX American Chemical Society www.acsnano.org VOL. XXX ▪ NO. XX ▪ 000–000 ▪ XXXX A ARTICLE Figure 1. (a) Optical images of a suspension of a graphene oxide (G-O) in propylene carbonate (PC) and a poly(ionic liquid)- modified reduced graphene oxide (PIL:RG-O) in PC. (b) Scanning electron microscopy (SEM) and (c) transmission electron micro- scopy (TEM) image of PIL:RG-O platelets. (d) Schematic diagram of the supercapacitor based on the PIL:RG-O electrodes and ionic liquid electrolyte (EMIM-NTf2). electrode materials capable of being well wetted by should provide enhanced compatibility with certain IL the chosen ILs,2,10 which may be attainable by the sur- electrolytes and improved accessibility of IL electrolyte face modification of graphene. ions into the graphene electrodes. Therefore, an addi- Herein, we report our progress toward high- tional aim of this paper was to illustrate the potential for performance supercapacitors based on poly(ionic using PIL:RG-O as an electrode material in supercapaci- liquid)-modified RG-O electrodes and an IL electrolyte, tors incorporating IL electrolytes such as EMIM-NTf2. Ul- 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfo- timately, a supercapacitor assembled with such a PIL: nyl)amide or EMIM-NTf2. Poly(ionic liquid) (PIL) poly- RG-O electrode and with EMIM-NTf2 as the electrolyte mers formed from IL monomers can be prepared by exhibited (i) a specific capacitance of 187 F/g, (ii) an en- the polymerization of unsaturated salts. Specifically, the ergy density of 6.5 W · h/kg, and (iii) a power density of use of poly(1-vinyl-3-ethylimidazolium) salts bearing 2.4 kW/kg. Ϫ the bis(trifluoromethylsulfonyl)amide anion (NTf2 or The production of PIL:RG-O electrode materials was CF3SO2ϪNϪSO2CF3) has been reported by our group achieved through a thermal reduction of G-O suspen- to effectively stabilize hydrazine-reduced graphene ox- sion in propylene carbonate (PC)15 containing PIL; the ide (RG-O) platelets via electrostatic and cationϪ␲ in- synthesis procedure is given in the Methods section. teractions, resulting in the formation of PIL-modified Briefly, 75 mg of PIL was dissolved in PC (20 mL) and RG-O materials.14 The PIL-modified RG-O material (PIL: 20 mg of graphite oxide (GO) powder was exfoliated RG-O) can be produced simply by reduction in propy- and suspended by mild sonication in the PC/PIL mix- lene carbonate (PC) at elevated temperature as recently ture. Then, the suspension was heated at 150 °C for 1 h, reported,15 where graphene oxide (G-O) platelets were which yielded a homogeneous black-colored suspen- suspended with the PIL in propylene carbonate and the sion without any visible precipitation. The reduction of suspension was thermally treated at ϳ150 °C15 to yield G-O platelets in PC in the presence of PIL at 150 °C oc- PIL:RG-O (in PIL-modified RG-O material, the PIL is likely curred more readily than in a control experiment where physisorbed to surface of RG-O platelet and not co- no PIL was present,15 as indicated by a lower surface re- valently linked). These PIL-modified reduced G-O materi- sistivity value for a thin PIL:RG-O film (3 k⍀/sq) as com- als (referred to here as PIL:RG-O) are expected to offer pared to a thin film composed of overlapped and advantages for supercapacitor applications in that they stacked reduced G-O platelets (505 k⍀/sq). The rapid re- B VOL. XXX ▪ NO. XX ▪ KIM ET AL. www.acsnano.org ARTICLE duction of suspended G-O platelets in PC with PIL by thermal treatment described here may provide a facile and economical processing route for such applications as electrode materials in supercapacitors, which had been previously suggested for suspensions of G-O platelets when heat treated in PC alone.15 The PIL: RG-O electrode material was then recovered by vacuum filtration, and excess PIL was washed away with addi- tional PC. The PIL:RG-O was composed of overlapped and stacked RG-O platelets that we believe were coated with PIL. The value of the zeta potential from the GO suspension in PC was Ϫ9.3 mV due to the negatively charged surface of the GO platelets. In contrast, the in- troduction of the PIL into the GO suspension in PC re- sulted in the reversal of zeta potential with a value of 4.1 mV, which suggested to us that the negatively charged functional groups on the GO platelets were covered and stabilized by the positively charged polycations in PIL. From the scanning electron microscopy (SEM) and high-resolution tunneling electron microscopy (TEM) images shown in Figure 1b,c, most of the PIL:RG-O platelets were observed to form separated thin sheets with single- or few-layered structures, which may con- tribute to the higher effective surface area accessible by the electrolyte ions. X-ray photoelectron spectroscopy (XPS) spectra of the PIL:RG-O material (Supporting Infor- mation, Figure S1) showed two N 1s peaks for the imi- dazolium cation and NTf2 (CF3SO2ϪN-SO2CF3) anion, which reflected the coating of PIL on the RG-O plate- Figure 2. (a) Cyclic voltammogram (CV) at various scan rates for lets, presumably via an attractive cationϪ␲ interaction. the supercapacitor cell. The test cell was cycled to 3.0 V. (b) Gal- We believe that the PIL intercalated between the over- vanostatic charge/discharge curve at different constant currents.
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