- Email: [email protected]
High power density of graphene-based supercapacitors in ionic liquid electrolytes
Materials Letters 68 (2012) 475–477
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High power density of graphene-based supercapacitors in ionic liquid electrolytes Yao Chen a, b, Xiong Zhang a, Dacheng Zhang a, b, Yanwei Ma a,⁎ a b
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Graduate University of Chinese Academy Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2011 Accepted 4 November 2011 Available online 12 November 2011 Keywords: Carbon materials Energy storage and conversion
a b s t r a c t Reduced graphene oxide (RGO) has been prepared with HBr as a reducing reagent. RGO-based supercapacitors in two-electrode systems have been fabricated in ionic liquid electrolytes of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4), respectively. RGO in BMIBF4 shows higher capacitance of 74 F g−1 at 10 mV s−1, while RGO in BMIPF6 merely exhibits 45 F g−1. However, due to wider potential window of 4 V for BMIPF6, RGO in BMIPF6 has higher energy and power densities. The highest power density of 27.8 kW kg−1 is obtained at 14 A g−1 and the maximum energy density of 18.9 Wh kg−1 at 1 A g−1 for BMIPF6. The exciting results infer potential application for RGO in BMIPF6. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Material and methods
Supercapacitors, one of the most important energy storage devices, can store and deliver energy at high power density. Carbon materials as electrical double-layer capacitance electrodes are ideal for supercapacitors [1]. Graphene is a 2D flat material consisting of monolayer carbon atoms [2]. Since 2008, graphene has been employed on supercapacitors [3]. However, the limited energy density is still a major drawback of graphene-based supercapacitors due to the narrow potential window of aqueous electrolytes. The larruping properties of ionic liquids include wide electrochemical window, long lifetime and high safety which make them more promising electrolytes than organic electrolytes [4]. So far, a few studies on graphene-based supercapacitors with ionic liquids have been reported. N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide was employed in graphene supercapacitors by Vivekchand et al. [5]. Ultrahigh energy density of 85.6 Wh kg −1 at the current density of 1 A g −1 was acquired using 1-ethyl-3-methylimidazolium tetrafluoroborate with a potential window of 4 V by Liu et al. [6]. Yet 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) has not been reported to be used on graphene-based supercapacitors. Here we have prepared reduced graphene oxide (RGO). Supercapacitors based on RGO are fabricated using BMIPF6 and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) as electrolytes. The highest power density of RGO in BMIPF6 is 27.8 kW kg−1 at 14 A g−1 and the maximum energy density 18.9 Wh kg−1 at 1 A g−1.
Graphite oxide (GO) was synthesized by the modified Hummers' method [7,8]. 0.1 g of GO in 100 mL of water was sonicated and then loaded in a round-bottom flask. Next, hydrobromic acid (3 mL, 40 wt.%) was added into the GO colloids. The mixture was refluxed in an oil bath at 110 °C for 24 h. Finally RGO was obtained after filtrating, washing with water and desiccating [9]. X-ray diffraction (XRD) analyses were performed using an X'Pert Pro system with Cu Kα radiation. The transmission electron microscopy (TEM) images were observed with JEOL JSM 2010. The electrode materials are composed of RGO, acetylene black and polyvinylidene difluoride with a weight ratio of 7:2:1. Then the electrode materials were pasted on titanium foils as current collectors. Coin-size capacitor cells were assembled in an Ar glove box using a polypropylene separator. The electrolyte is BMIPF6. Finally, two-electrode systems are measured using CHI 660 C workstation at room temperature. For comparison purpose, BMIBF4 is used instead of BMIPF6.
⁎ Corresponding author. Tel.: +86 10 82547129; fax: +86 10 82547137. E-mail address: [email protected] (Y. Ma). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.008
3. Results and discussion The XRD patterns of GO and RGO are shown in Fig. 1. The (002) peaks at 10° of GO appears and a broad amorphous peak between 18 and 22° emerges in RGO, indicative of partial reduction of GO. Fig. 2 shows the low and high magnification TEM images of RGO. As the dried powders were dispersed in water, corrugation and scrolling are viewed in thin and transparent RGO sheets (Fig. 2a). However, from Fig. 2b, it is confirmed that RGO consists of 2–3 graphene layers. The cyclic voltammetry (CV) profiles of RGO in BMIPF6 are exhibited in Fig. 3a. These similar rectangular shapes in CV curves illustrate that RGO is ideal to be used as a capacitor electrode. The capacitance of 45 F g −1 for a single electrode is obtained at 10 mV s −1. Compared with BMIPF6, RGO in BMIBF4 shows higher capacitance (Fig. 3b). For
476
Y. Chen et al. / Materials Letters 68 (2012) 475–477
Fig. 1. XRD patterns of GO and RGO.
instance, the capacitance at 10 mV s −1 corresponds to 74 F g −1. And even if at 200 mV s −1, it reaches 43 F g −1, almost the same as that in BMIPF6 at 10 mV s −1. Considering that smaller ions are accessible to electrode more easily, the increased capacitance of RGO in BMIBF4 is due to enhanced ion transport for smaller anion BF4−[10]. Fig. 4a shows the charge/discharge measurements of RGO in BMIPF6 at 1, 7 and 14 A g −1. Energy (E) and power densities (P) of supercapacitors are estimated based on total mass of activated electrode materials. The Ragone plots of RGO in BMIPF6 and BMIBF4 are shown in Fig. 4b. The maximum energy density in BMIPF6 is 18.9 Wh kg −1 at 1 A g −1, while the corresponding power density is 2.8 kW kg −1. The highest power density of 27.8 kW kg −1 is obtained
Fig. 3. CV plots of RGO electrodes in ionic liquid electrolytes of (a) BMIPF6 and (b) BMIBF4.
at 14 A g −1 with the energy density of 8.1 Wh kg −1. Interestingly, the ultrahigh power density is much higher than in the other reports [5,6]. The excellent results are certainly attributed to the wide potential window of BMIPF6. For the control experiment using BMIBF4, although the maximum energy density of 16.5 Wh kg−1 is obtained at 1 A g −1, the corresponding power density is only 1.6 kW kg−1, smaller than 2.8 kW kg−1 in BMIPF6. More importantly, when retaining high power density, only BMIPF6 can show high energy density. Typically, at 5.5 A g−1, its energy density reaches 12.1 Wh kg−1 when the power density is as high as 11.1 kW kg−1. While at the same condition for BMIBF4, the energy and power densities are only 7.9 Wh kg −1 and 8 kW kg−1. Further, when the energy density is 15.1 Wh kg−1, the power density values are 5.6 and 2 kW kg−1 in BMIPF6 and BMIBF4, respectively. Obviously, the electrochemical performance of RGO in BMIPF6 is much better than that in BMIBF4. The cycling durability of RGO in BMIPF6 is revealed at 7 A g −1 in Fig. 5. The capacitance increases till the 236th cycle and the max value is nearly four times the initial one. The increased trend is similar to functionalized graphene [9]. The peculiar phenomenon may arise whether from the reduction of partial unstable oxygen functional groups in RGO or exertion of pseudocapacitance in a slow kinetics course. After 520 cycles, the capacitance begins to decay due to decreased pseudocapacitance resulting from stable oxygen functional groups in the electrochemical system. The final retention reaches 141% after 3000 cycles. 4. Conclusions
Fig. 2. (a) Low magnification and (b) high resolution TEM images of RGO.
The electrochemical properties of supercapacitors based on RGO in BMIPF6 and BMIBF4 have been investigated. Although RGO in BMIBF4
Y. Chen et al. / Materials Letters 68 (2012) 475–477
477
Fig. 5. The life performance of RGO in BMIPF6.
Acknowledgments This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (no. KJCX2-YW-W26), Beijing Municipal Science and Technology Commission (no. Z111100056011007), and the National Natural Science Foundation of China (no. 21001103 and 51025726). References
Fig. 4. (a) Charge/discharge curves of RGO electrodes in BMIPF6 electrolyte at the current densities of 1, 7 and 14 A g−1. (b) Ragone plots of RGO electrodes in BMIPF6 and BMIBF4 electrolytes.
shows higher capacitance of 74 F g−1 at 10 mV s−1, ultrahigh power density of 27.8 kW kg −1 and high energy density of 18.9 Wh kg−1 are obtained by RGO in BMIPF6. RGO in BMIPF6 has higher energy and power densities simultaneously. The life retention is 141% of the initial value after 3000 cycles. The outstanding results depend on the wider electrochemical window (4 V) of BMIPF6.
[1] Chen Y, Zhang X, Yu P, Ma Y. Electrophoretic deposition of graphene nanosheets on nickel foams for electrochemical capacitors. J Power Sources 2010;195:3031–5. [2] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666–9. [3] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8:3498–502. [4] Lu W, Qu LT, Henry K, Dai LM. High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes. J Power Sources 2009;189:1270–7. [5] Vivekchand SRC, Rout CS, Subrahmanyam KS, Govindaraj A, Rao CNR. Graphene-based electrochemical supercapacitors. J Chem Sci 2008;120:9–13. [6] Liu C, Yu Z, Neff D, Zhamu A, Jang BZ. Graphene-based supercapacitor with an ultrahigh energy density. Nano Lett 2010;10:4863–8. [7] Chen Y, Zhang X, Yu P, Ma Y. Stable dispersions of graphene and highly conducting graphene films: a new approach to creating colloids of graphene monolayers. Chem Commun 2009:4527–9. [8] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [9] Chen Y, Zhang X, Zhang D, Yu P, Ma Y. High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes. Carbon 2011;49: 573–80. [10] Makino S, Takasu Y, Sugimoto W. Electrochemical capacitor properties of NiO in ionic liquids. Chem Lett 2010;39:544–5.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
High power density of graphene-based supercapacitors in ionic liquid electrolytes Yao Chen a, b, Xiong Zhang a, Dacheng Zhang a, b, Yanwei Ma a,⁎ a b
Institute of Electrical Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Graduate University of Chinese Academy Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 31 May 2011 Accepted 4 November 2011 Available online 12 November 2011 Keywords: Carbon materials Energy storage and conversion
a b s t r a c t Reduced graphene oxide (RGO) has been prepared with HBr as a reducing reagent. RGO-based supercapacitors in two-electrode systems have been fabricated in ionic liquid electrolytes of 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4), respectively. RGO in BMIBF4 shows higher capacitance of 74 F g−1 at 10 mV s−1, while RGO in BMIPF6 merely exhibits 45 F g−1. However, due to wider potential window of 4 V for BMIPF6, RGO in BMIPF6 has higher energy and power densities. The highest power density of 27.8 kW kg−1 is obtained at 14 A g−1 and the maximum energy density of 18.9 Wh kg−1 at 1 A g−1 for BMIPF6. The exciting results infer potential application for RGO in BMIPF6. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Material and methods
Supercapacitors, one of the most important energy storage devices, can store and deliver energy at high power density. Carbon materials as electrical double-layer capacitance electrodes are ideal for supercapacitors [1]. Graphene is a 2D flat material consisting of monolayer carbon atoms [2]. Since 2008, graphene has been employed on supercapacitors [3]. However, the limited energy density is still a major drawback of graphene-based supercapacitors due to the narrow potential window of aqueous electrolytes. The larruping properties of ionic liquids include wide electrochemical window, long lifetime and high safety which make them more promising electrolytes than organic electrolytes [4]. So far, a few studies on graphene-based supercapacitors with ionic liquids have been reported. N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide was employed in graphene supercapacitors by Vivekchand et al. [5]. Ultrahigh energy density of 85.6 Wh kg −1 at the current density of 1 A g −1 was acquired using 1-ethyl-3-methylimidazolium tetrafluoroborate with a potential window of 4 V by Liu et al. [6]. Yet 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF6) has not been reported to be used on graphene-based supercapacitors. Here we have prepared reduced graphene oxide (RGO). Supercapacitors based on RGO are fabricated using BMIPF6 and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4) as electrolytes. The highest power density of RGO in BMIPF6 is 27.8 kW kg−1 at 14 A g−1 and the maximum energy density 18.9 Wh kg−1 at 1 A g−1.
Graphite oxide (GO) was synthesized by the modified Hummers' method [7,8]. 0.1 g of GO in 100 mL of water was sonicated and then loaded in a round-bottom flask. Next, hydrobromic acid (3 mL, 40 wt.%) was added into the GO colloids. The mixture was refluxed in an oil bath at 110 °C for 24 h. Finally RGO was obtained after filtrating, washing with water and desiccating [9]. X-ray diffraction (XRD) analyses were performed using an X'Pert Pro system with Cu Kα radiation. The transmission electron microscopy (TEM) images were observed with JEOL JSM 2010. The electrode materials are composed of RGO, acetylene black and polyvinylidene difluoride with a weight ratio of 7:2:1. Then the electrode materials were pasted on titanium foils as current collectors. Coin-size capacitor cells were assembled in an Ar glove box using a polypropylene separator. The electrolyte is BMIPF6. Finally, two-electrode systems are measured using CHI 660 C workstation at room temperature. For comparison purpose, BMIBF4 is used instead of BMIPF6.
⁎ Corresponding author. Tel.: +86 10 82547129; fax: +86 10 82547137. E-mail address: [email protected] (Y. Ma). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.008
3. Results and discussion The XRD patterns of GO and RGO are shown in Fig. 1. The (002) peaks at 10° of GO appears and a broad amorphous peak between 18 and 22° emerges in RGO, indicative of partial reduction of GO. Fig. 2 shows the low and high magnification TEM images of RGO. As the dried powders were dispersed in water, corrugation and scrolling are viewed in thin and transparent RGO sheets (Fig. 2a). However, from Fig. 2b, it is confirmed that RGO consists of 2–3 graphene layers. The cyclic voltammetry (CV) profiles of RGO in BMIPF6 are exhibited in Fig. 3a. These similar rectangular shapes in CV curves illustrate that RGO is ideal to be used as a capacitor electrode. The capacitance of 45 F g −1 for a single electrode is obtained at 10 mV s −1. Compared with BMIPF6, RGO in BMIBF4 shows higher capacitance (Fig. 3b). For
476
Y. Chen et al. / Materials Letters 68 (2012) 475–477
Fig. 1. XRD patterns of GO and RGO.
instance, the capacitance at 10 mV s −1 corresponds to 74 F g −1. And even if at 200 mV s −1, it reaches 43 F g −1, almost the same as that in BMIPF6 at 10 mV s −1. Considering that smaller ions are accessible to electrode more easily, the increased capacitance of RGO in BMIBF4 is due to enhanced ion transport for smaller anion BF4−[10]. Fig. 4a shows the charge/discharge measurements of RGO in BMIPF6 at 1, 7 and 14 A g −1. Energy (E) and power densities (P) of supercapacitors are estimated based on total mass of activated electrode materials. The Ragone plots of RGO in BMIPF6 and BMIBF4 are shown in Fig. 4b. The maximum energy density in BMIPF6 is 18.9 Wh kg −1 at 1 A g −1, while the corresponding power density is 2.8 kW kg −1. The highest power density of 27.8 kW kg −1 is obtained
Fig. 3. CV plots of RGO electrodes in ionic liquid electrolytes of (a) BMIPF6 and (b) BMIBF4.
at 14 A g −1 with the energy density of 8.1 Wh kg −1. Interestingly, the ultrahigh power density is much higher than in the other reports [5,6]. The excellent results are certainly attributed to the wide potential window of BMIPF6. For the control experiment using BMIBF4, although the maximum energy density of 16.5 Wh kg−1 is obtained at 1 A g −1, the corresponding power density is only 1.6 kW kg−1, smaller than 2.8 kW kg−1 in BMIPF6. More importantly, when retaining high power density, only BMIPF6 can show high energy density. Typically, at 5.5 A g−1, its energy density reaches 12.1 Wh kg−1 when the power density is as high as 11.1 kW kg−1. While at the same condition for BMIBF4, the energy and power densities are only 7.9 Wh kg −1 and 8 kW kg−1. Further, when the energy density is 15.1 Wh kg−1, the power density values are 5.6 and 2 kW kg−1 in BMIPF6 and BMIBF4, respectively. Obviously, the electrochemical performance of RGO in BMIPF6 is much better than that in BMIBF4. The cycling durability of RGO in BMIPF6 is revealed at 7 A g −1 in Fig. 5. The capacitance increases till the 236th cycle and the max value is nearly four times the initial one. The increased trend is similar to functionalized graphene [9]. The peculiar phenomenon may arise whether from the reduction of partial unstable oxygen functional groups in RGO or exertion of pseudocapacitance in a slow kinetics course. After 520 cycles, the capacitance begins to decay due to decreased pseudocapacitance resulting from stable oxygen functional groups in the electrochemical system. The final retention reaches 141% after 3000 cycles. 4. Conclusions
Fig. 2. (a) Low magnification and (b) high resolution TEM images of RGO.
The electrochemical properties of supercapacitors based on RGO in BMIPF6 and BMIBF4 have been investigated. Although RGO in BMIBF4
Y. Chen et al. / Materials Letters 68 (2012) 475–477
477
Fig. 5. The life performance of RGO in BMIPF6.
Acknowledgments This work was supported by the Knowledge Innovation Program of the Chinese Academy of Sciences (no. KJCX2-YW-W26), Beijing Municipal Science and Technology Commission (no. Z111100056011007), and the National Natural Science Foundation of China (no. 21001103 and 51025726). References
Fig. 4. (a) Charge/discharge curves of RGO electrodes in BMIPF6 electrolyte at the current densities of 1, 7 and 14 A g−1. (b) Ragone plots of RGO electrodes in BMIPF6 and BMIBF4 electrolytes.
shows higher capacitance of 74 F g−1 at 10 mV s−1, ultrahigh power density of 27.8 kW kg −1 and high energy density of 18.9 Wh kg−1 are obtained by RGO in BMIPF6. RGO in BMIPF6 has higher energy and power densities simultaneously. The life retention is 141% of the initial value after 3000 cycles. The outstanding results depend on the wider electrochemical window (4 V) of BMIPF6.
[1] Chen Y, Zhang X, Yu P, Ma Y. Electrophoretic deposition of graphene nanosheets on nickel foams for electrochemical capacitors. J Power Sources 2010;195:3031–5. [2] Novoselov KS, Geim AK, Morozov SV, Jiang D, Zhang Y, Dubonos SV, et al. Electric field effect in atomically thin carbon films. Science 2004;306:666–9. [3] Stoller MD, Park S, Zhu Y, An J, Ruoff RS. Graphene-based ultracapacitors. Nano Lett 2008;8:3498–502. [4] Lu W, Qu LT, Henry K, Dai LM. High performance electrochemical capacitors from aligned carbon nanotube electrodes and ionic liquid electrolytes. J Power Sources 2009;189:1270–7. [5] Vivekchand SRC, Rout CS, Subrahmanyam KS, Govindaraj A, Rao CNR. Graphene-based electrochemical supercapacitors. J Chem Sci 2008;120:9–13. [6] Liu C, Yu Z, Neff D, Zhamu A, Jang BZ. Graphene-based supercapacitor with an ultrahigh energy density. Nano Lett 2010;10:4863–8. [7] Chen Y, Zhang X, Yu P, Ma Y. Stable dispersions of graphene and highly conducting graphene films: a new approach to creating colloids of graphene monolayers. Chem Commun 2009:4527–9. [8] Hummers WS, Offeman RE. Preparation of graphitic oxide. J Am Chem Soc 1958;80:1339. [9] Chen Y, Zhang X, Zhang D, Yu P, Ma Y. High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes. Carbon 2011;49: 573–80. [10] Makino S, Takasu Y, Sugimoto W. Electrochemical capacitor properties of NiO in ionic liquids. Chem Lett 2010;39:544–5.