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Supercapacitors based on self-assembled graphene organogelw
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Yiqing Sun, Qiong Wu and Gaoquan Shi* Received 26th July 2011, Accepted 15th August 2011 DOI: 10.1039/c1cp22409c Self-assembled graphene organogel (SGO) with 3-dimensional (3D) macrostructure was prepared by solvothermal reduction of a graphene oxide (GO) dispersion in propylene carbonate (PC). This SGO was used as an electrode material for fabricating supercapacitors with a PC electrolyte. The supercapacitor can be operated in a wide voltage range of 0–3 V and exhibits a high specific capacitance of 140 F g 1 at a discharge current density of 1 A g 1. Furthermore, it can still keep a specific capacitance of 90 F g 1 at a high current density of 30 A g 1. The maximum energy density of the SGO based supercapacitor was tested to be 43.5 Wh kg 1, and this value is higher than those of the graphene based supercapacitors with aqueous or PC electrolytes reported previously. Furthermore, at a high discharge current density of 30 A g 1, the energy and power densities of the supercapacitor were measured to be 15.4 Wh kg 1 and 16 300 W kg 1, respectively. These results indicate that the supercapacitor has a high specific capacitance and power density, and excellent rate capability.
Introduction The supercapacitor is one of the most attractive energy storage devices because of its high power density, long cycle life, and broad application prospects, especially in electro-vehicles.1–4 However, in comparison with batteries, the low energy density of supercapacitors limits their practical applications.5 Increasing the operating voltage of a supercapacitor is an efficient approach to improve its energy density. For this purpose, organic electrolytes are widely used to replace aqueous media because of their high electrochemical stabilities,6 whereas the relatively high viscosities and low conductivities of organic electrolytes usually limit the specific capacitances and rate capabilities of supercapacitors.7 Therefore, it is still a challenge for developing high-performance supercapacitors with organic electrolytes. Supercapacitors are divided into electrical double-layer capacitors (EDLCs) and redox capacitors by their different energy storage mechanisms.2,8 The capacitances of EDLCs mainly result from the accumulations of electrostatic charges at electrode/electrolyte interfaces. Therefore, they strongly depend on the specific surface areas of electrode materials.9 As a result, porous carbon materials, such as activated carbon,10,11 carbon nanotubes,12–14 carbon fibers 15,16 are the most attractive electrode materials.6 However, the reported specific capacitances of activated carbon are usually only Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China. E-mail: [email protected] w Electronic supplementary information (ESI) available: Galvanostatic charge/discharge curves of SGO and SGH based supercapacitors at a current density of 1 or 30 A g 1. Life cycling stability of the SGO and SGH. See DOI: 10.1039/c1cp22409c
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about 10–20% of their theoretical values. This is mainly due to the fact that most of the micropores of activated carbon cannot be accessed or wetted by electrolytes.5 Although vertically grown carbon nanotubes possess a better rate performance,17 the high costs and low specific capacitances restrain their practical applications. On the other hand, graphene, an atom thick graphite sheet,18 is a promising candidate for supercapacitor electrodes, due to its high theoretical surface area (2630 m2 g 1),19 excellent electronic conductivity,18 and great mechanical strength.20 Actually, a great deal effort has been devoted to exploring graphene based supercapacitors.1,5,21–26 Ruoff and co-workers firstly reported a graphene material with specific capacitances of 135 and 99 F g 1 in aqueous and organic electrolytes, respectively.25 To date, the highest specific capacitances of graphene in aqueous or organic electrolytes were reported to be 264 or 122 F g 1.23 However, in these cases, polymer binders and conducting additives have to be mixed with graphene powders for fabricating mechanically stable conductive electrodes and improve their conductivities. These additives increased the total weights of the electrodes and decreased their energy densities.24 Although free-standing graphene films have been explored for fabricating supercapacitors, their specific capacitances are unsatisfactory because of low specific surface areas.27 In order to develop high-performance supercapacitors, high operating voltages, low internal resistances, appropriate pore sizes of electrodes, and good wetting properties between electrodes and electrolytes should be considered. Recently, self-assembled graphene hydrogels (SGH) with 3D networks were prepared by a convenient one-step hydrothermal method or chemical reduction.28,29 These hydrogels have strong mechanical strengths, high electrical conductivities, excellent Phys. Chem. Chem. Phys., 2011, 13, 17249–17254
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thermal stabilities and large pore sizes. Thus, they are promising electrode materials for fabricating supercapacitors without adding polymer binders or conducting additives.28–31 Unfortunately, these materials were prepared in aqueous media. They cannot be directly used for fabricating supercapacitors with organic electrolytes. In this paper, we prepared a SGO by solvothermal reduction of a graphene oxide (GO) dispersion in propylene carbonate (PC). PC was chosen as the solvent of the gel, mainly due to it being the main component of widely used organic electrolytes in supercapacitors. The supercapacitor based on this SGO with PC electrolyte exhibited a high specific capacitance of 140 F g 1 at a discharge current density of 1 A g 1. Moreover, the maximum energy density of the supercapacitor was measured to be as high as 43.5 Wh kg 1, and this value is higher than those of graphene based supercapacitors with aqueous or PC electrolytes reported previously. The supercapacitor also has a high rate capability. Its specific capacitance, energy and power densities were tested to be 90 F g 1, 15.4 Wh kg 1 and 16 300 W kg 1, correspondingly, at a high discharge current density of 30 A g 1.
Experimental procedure
advance (Bruker) X-ray diffractometer with Cu-Ka radiation (l = 1.5418 A˚). Conductivities were measured by using a conventional four-probe technique. An atomic force microscope (AFM, SPM-9600, Shimadzu) was used to study the morphology of GO sheets. All of the electrochemical measurements were carried out in a conventional two electrode system. 1.0 mol L 1 PC solution of TEABF4 was used as the electrolyte. The electrolyte was well dehydrated and deoxygenated. The graphene gels were obtained as small cylinders. The size of each cylinder was p 0.42 0.2 cm3 and it was cut into to two slices with the same volumes and used as the both electrodes of a supercapacitor. SGH was freeze dried and then immersed in PC electrolyte for over 4 h. The masses of the electrodes were measured by weighing their freeze dried samples. Each electrode has a weight of over 1.0 mg. Cyclic voltammetry (CV), galvanostatic charge/discharge tests and electrochemical impedance spectroscopy (EIS) were carried out on a CHI 760D electrochemical analyzer. The energy (E) and power (P) densities of the electrodes were calculated by using following equations: E = 1/8CsDV2 and P = E/Dt, respectively, where Cs is specific capacitance, Dt is the discharge time, and DV is the operating voltage with the IR voltage drop subtracted (I and R are the current density and resistance of the supercapacitor, respectively).
Chemicals Natural graphite powder (325 meshes) was purchased from Qingdao HuaTai Lubricant Sealing S&T Co. Ltd (Qingdao, China). PC (98%) and tetraethylammonium tetrafluoroborate (TEABF4, 98%) were bought from Alfa Aesar. Synthesis of GO GO was prepared by the oxidation of natural graphite powder according to a modified Hummers’ method,32 and the details are described in the literature.33 The GO dispersion was purified by dialysis and successively freeze dried for use. Preparation of SGO GO was mixed with PC by sonication for 1 h to form a homogeneous dispersion (2 mg mL 1). Then, 2 mL of the GO dispersion was put into a 16-mL Teflon-lined autoclave and maintained at 180 1C for 12 h. Preparation of SGH
Results and discussion Preparation and characterizations of SGO GO can be well dispersed in PC, as reported by Ruoff et al.34 A 2 mg mL 1 GO dispersion is stable and no precipitation was observed after aging for one month (Fig. 1a, left). Atomic force Microscope (AFM) study indicated that the thickness of GO nanosheets dispersed in PC is about 1.1 nm (Fig. 1b). This value is in agreement with that of GO monolayer.24 After 12 h of solvothermal reduction at 180 1C, a columniform SGO was obtained (Fig. 1a, right). According to our previous report, the formation of the macroscopic gel is attributed to the regional overlapping and coalescing of flexible reduced GO nanosheets through p–p interactions.28 The microstructure of the SGO was studied using a scanning electron microscope (SEM, Fig. 2). The SEM images show an interconnected 3-dimensional (3D) porous network with pore sizes in the range of submicrometer to several micrometres. The interconnected graphene nanosheets
SGH was prepared via hydrothermal reduction of the aqueous dispersion of graphene oxide.28 Details are described as follows. GO was dispersed in water by ultrasonication for 1 h to make a homogeneous dispersion (2 mg mL 1). Then, 2 mL of the dispersion was put into a 16-mL Teflon-lined autoclave and maintained at 180 1C for 12 h. Finally, the SGH was freeze dried for use. Characterizations Scanning electron microscope (SEM) images were recorded using a HITACHI S-4500 scanning electron microscope. X-Ray photoelectron spectra (XPS) were obtained using a PHI 550 EACA/SAM photoelectron spectrometer (Perkin-Elmer PHI) with Al Ka (1486.6 eV) radiation. X-Ray diffraction (XRD) patterns were recorded on a D8 17250
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Fig. 1 (a) Photographs of a 2 mg L 1 GO dispersion in PC before and after solvothermal reduction at 180 1C for 12 h. (b) An AFM image of graphene oxide nanosheets and the corresponding height profile.
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Fig. 2 SEM images of freeze dried SGO with different magnifications.
provide the SGO with a high electrical conductivity of 2 S m 1. This value is about 4 times that of SGH (0.5 S m 1)28 and comparable to that of the SGH prepared by chemical reduction.29 This result implies that the solvothermal reduction of GO in PC is more effective than in an aqueous medium. A similar phenomenon was also observed by Ruoff and co-workers.34 They reported that the electrical conductivity of a graphene paper prepared via the thermal reduction of graphene oxide in PC is comparable to that of graphene paper prepared by chemical reduction.35–37 The structure of SGO was also studied by X-ray diffraction (XRD). According to the XRD patterns shown in Fig. 3, the interlayer spacing of SGO was calculated to be 0.38 nm (2y = 23.71), and this value is between those of graphene oxide (0.74 nm, 2y = 11.91) of SGH and natural graphite (0.336 nm, 2y = 26.51), and comparable to that of SGH. This result confirms that the formation mechanism of SGO is similar to that of SGH, mainly due to the regional stacking of reduced GO sheets. The emerging of the broad peak centered at 2y = 23.71 indicates the poor ordering of graphene sheets along their stacking direction and the SGO framework is composed of few layer stacked graphene sheets.28 C 1s X-ray photoelectron spectra (XPS) of graphene oxide and freeze dried SGO are illustrated in Fig. 4. The peaks at 284.5, 285.7, 286.8, 288.4 eV are assigned to C–C, C–OH, CQO, and OQC–OH groups.38–40 After solvothermal reduction, the peaks with binding energies higher than 284.5 eV were highly suppressed. This result reflects that the oxygencontaining groups of GO have been effectively removed. Actually, the atomic ratio of C/O was increased from 2.7 for GO to 10.8 for freeze dried SGO. Thermal gravimetric analysis
Fig. 3
XRD patterns of graphite, freeze dried GO and SGO.
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Fig. 4
C 1s XPS spectra of freeze dried GO (a) and SGO (b).
(TGA) also confirms the XPS results described above (Fig. 5). The TGA curve of GO exhibits a weight loss before 100 1C due to the release of water trapped between GO sheets.41 A significant weight loss is shown in the range of 170–230 1C. This is associated with the removing of oxygen-containing groups.41–43 In comparison, there is only a small weight loss in the temperature range of 170–230 1C in the curve of freeze dried SGO. This result implies that most of the oxygen-containing groups have been removed by thermal reduction of GO in PC. Electrochemical performances As described above, SGO is mechanically stable and has high conductivity and a porous structure. More importantly, it has good affinity with PC solvent. Therefore, it is an attractive electrode material for supercapacitors with PC electrolyte. SGO can be directly used for fabricating the electrodes, no polymer binder or conducting additive is needed. The supercapacitor
Fig. 5 TGA curves of freeze dried GO and SGO.
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Fig. 6 (a) A schematic of the configuration of the supercapacitor device. (b) CV curves of the supercapacitor in different potential ranges and at a scan rate of 10 mV s 1.
devices were assembled with a configuration shown in Fig. 6a. The performance of the supercapacitor was studied by cyclic voltammetry (CV). The CV curves of the supercapacitor tested in a two-electrode system keep a quasi-rectangular shape in the potential range of 0 to 3 V (Fig. 6b). This is mainly due to the superior electrochemical stability of the PC electrolyte and SGO electrodes. The quasi-rectangular shaped CV curves also indicate that the capacitance of the electrode is predominately determined by its electrical double layers. The specific capacitance of the electrode materials were measured by galvanostatic discharge (Figs S1 and S2, ESIw). The specific capacitance of SGO was tested to be about 140 F g 1 at a discharge current density of 1 A g 1. The solid content density of the electrode in a squeezed state was measured to be about 0. 05 g cm 3. Accordingly, the volume-specific capacitance of the electrode was calculated to be 7 F cm 3. For comparison, freeze dried SGH was also used as an electrode material for the supercapacitor with PC electrolyte and its specific capacitance was measured to be only 115 F g 1 under the same conditions (Fig. 7a). Accordingly, although the microstructures of SGO and SGH are similar, their electrochemical performances in PC electrolyte are quite different. Furthermore, with the increase of the discharge current density, the capacitance of SGH decreases much more dramatically than that of SGO. As the discharge current density increased from 1 to 5 A g 1, the specific capacitance of SGH dropped about 50%, while the capacitance of SGO decreased only approximately 14%. At the discharge current density of 30 A g 1, the capacitance of SGH decreased to close to zero and the capacitance of the SGO still kept at a high value of 90 F g 1. Considering that the specific area of the freeze dried SGH (166 m2 g 1) is larger than that of the SGO (146 m2 g 1), the high performance of SGO is mainly due to its better wetting property with PC electrolyte. It was also confirmed by the XPS results that the atomic ratio of C/O of SGO (10.8) is much higher than that of SGH (5.3). Therefore, SGO is more hydrophobic than SGH, which provided it with better wetting properties with the organic PC electrolyte. The improved affinity of SGO greatly increased its effective surface area accessible by electrolyte ions.44 The poor rate performance of activated carbon based supercapacitor electrodes is one of their main drawbacks. Aligned carbon nanotubes possess better rate performances than activated carbon due to their regular and short ion transportation paths.17,45 17252
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However, the cost of aligned carbon nanotubes is much higher than that of thermally reduced GO. Furthermore, aligned carbon nanotubes were reported to have a specific capacitance of only 14 F g 1, which dropped to 6 F g 1 at a discharge current density at 12 A g 1.45 On the basis of these facts, it is reasonable to conclude that the supercapacitor based on SGO has much better performance than that of the supercapacitors based on other carbon materials such as activated carbon, carbon nanotubes and SGH. The CV curves carried out at different potential scan rates for the supercapacitor based on SGO also confirm its outstanding rate performance (Fig. 7b). With the increase of the potential scan rate, no obvious distortion in the curves was observed. Even at a high scan rate of 500 mV s 1, the CV curve still keeps its quasirectangular shape. These results reflect that the SGO has a low equivalent series resistance and a fast diffusion rate of electrolyte ions.21,46 The electrochemical stability during long term cycling is one of the major advantages of supercapacitors. The SGO electrode maintained about 83% of its original specific capacitance after 1000 charge/discharge cycles, which is slightly higher than that (80%) of the SGH electrode (Fig. S3, ESIw). The IR drops at different current densities for the supercapacitor based on SGO or SGH are shown in Fig. 7c. Accordingly, at a given current density, the IR drop of SGO based supercapacitor is lower than that of the SGH counterpart. This result reflects that SGO electrode has a lower internal resistance than that of SGH electrode. Lower internal resistance will waste less energy to produce unwanted heat during charging/discharging processes.24 Thus, SGO is more suitable for fabricating supercapacitors with organic electrolytes, especially for the requirement of rapid charging/ discharging. It should be noted here that the voltage drop of the supercapacitor based on SGH was increased to about 3 V at a current density of 30 A g 1, indicating that nearly all of the energy was wasted to produce unwanted heat. Therefore, its rate performance is much worse than that of SGO. The energy density of the SGO electrode was also tested to be much higher than that of SGH at the same power density (Fig. 7d). SGO has a maximum energy density of 43.5 Wh kg 1, which is much higher than the values of 4.7–9.2 or 21.5–39 Wh kg 1 for chemically modified graphene based supercapacitors with aqueous5,23,25,26 or organic electrolytes.5,25,47 Furthermore, SGO can still keep an energy This journal is
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Fig. 7 (a) Plots of the specific capacitance of SGO or SGH based supercapacitors measured by galvanostatic charge/discharge as a function of current density. (b) CV curves of the SGO based supercapacitor at different potential scan rates. (c) IR drops of the supercapacitors based on SGO or SGH versus current density. (d) Plots of energy density versus power density at an operating voltage of 3 V for SGO or SGH based supercapacitors.
density of 15.4 Wh kg 1 at a high current density of 30 A g 1. This result implies that this supercapacitor can be rapidly charged or discharged within 10 s, keeping an acceptable energy density (15.4 Wh kg 1) and a high powder density (16 000 W kg 1). Two main criteria for evaluating the performance of supercapacitors are as follows: (1) high energy densities (>10 Wh kg 1) with a power density substantially higher than those of batteries, (2) rapid charge/discharge processes within several seconds. Considering these two requirements, the supercapacitor based on SGO should have wide application prospects. Due to the regular and short ion transfer paths, aligned carbon nanotubes were considered to keep a large density at a high charge/discharge rate. However, the energy density of our SGO based supercapacitor is much higher than the maximum energy density of aligned carbon nanotube based supercapacitors (4.3 Wh kg 1) reported previously.45 We further investigated the frequency responses of the supercapacitors by electrical impedance spectroscopy. The Nyquist plots of SGO and SGH in the frequency range of 0.01 Hz to 1 MHz are shown in Fig. 8. Each plot has a small semicircle region, a 451 Warburg region, and a pure capacitor region. The semicircle region of SGO is more obvious and integrated than that of SGH. This region is related to the faradic resistance, which is modelled by a parallel combination of an interfacial charge transfer resistance.44 The observation that this region is nearly missing in SGH is mainly due to the interlayers of stacked graphene sheets in freeze dried SGH are hard to be reached by PC electrolyte, especially at high frequencies.48 It is also clear from this figure that the 451 Warburg region of SGO (176–1.5 Hz) is much shorter than that of SGH (561–0.3 Hz). This region corresponds to This journal is
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Fig. 8 Nyquist plots of SGO or SGH based supercapacitors. The insert shows the magnified high frequency regions.
Warburg impedance caused by ion diffusion and transport in the electrolyte.44 A shorter Warburg region can be explained by the enhanced ion diffusion in SGO because of its improved compatibility with PC electrolyte. This is why the SGO based supercapacitor exhibited much better performance than that of the SGH counterpart. The vertical lines at low frequencies indicate pure capacitor behaviour. The more vertical curve of SGO compared to that of SGH reflects that the SGO based supercapacitor is more like an ideal capacitor.33
Conclusions SGO with a 3D macrostructure has been successfully prepared by thermal reduction of GO dispersed in PC. Because the Phys. Chem. Chem. Phys., 2011, 13, 17249–17254
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reaction occurred in PC and the effective reduction, the resulting SGO has a good affinity with this solvent. Furthermore, SGO has a highly conductive and a porous 3D network. Therefore, the ion diffusion property of SGO based supercapacitors in TEABF/PC electrolyte was greatly improved. The supercapacitor based on SGO exhibited a high specific capacity of 140 F g 1 and a high maximum energy density of 43.5 Wh kg 1. They also showed excellent rate capabilities. Their performances are the best among the supercapacitors based on carbon materials including activated carbon, carbon nanotubes, chemically modified graphene or SGH with PC electrolytes. The high operating voltage of the SGO based supercapacitor also makes its power and energy densities much higher than those of the chemically modified graphene based supercapacitors with aqueous electrolytes.
Acknowledgements This work was supported by natural science foundation of China (91027028, 50873092) and open research fund of state key laboratory of bioelectronics (Southeast University).
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Supercapacitors based on self-assembled graphene organogelw
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Yiqing Sun, Qiong Wu and Gaoquan Shi* Received 26th July 2011, Accepted 15th August 2011 DOI: 10.1039/c1cp22409c Self-assembled graphene organogel (SGO) with 3-dimensional (3D) macrostructure was prepared by solvothermal reduction of a graphene oxide (GO) dispersion in propylene carbonate (PC). This SGO was used as an electrode material for fabricating supercapacitors with a PC electrolyte. The supercapacitor can be operated in a wide voltage range of 0–3 V and exhibits a high specific capacitance of 140 F g 1 at a discharge current density of 1 A g 1. Furthermore, it can still keep a specific capacitance of 90 F g 1 at a high current density of 30 A g 1. The maximum energy density of the SGO based supercapacitor was tested to be 43.5 Wh kg 1, and this value is higher than those of the graphene based supercapacitors with aqueous or PC electrolytes reported previously. Furthermore, at a high discharge current density of 30 A g 1, the energy and power densities of the supercapacitor were measured to be 15.4 Wh kg 1 and 16 300 W kg 1, respectively. These results indicate that the supercapacitor has a high specific capacitance and power density, and excellent rate capability.
Introduction The supercapacitor is one of the most attractive energy storage devices because of its high power density, long cycle life, and broad application prospects, especially in electro-vehicles.1–4 However, in comparison with batteries, the low energy density of supercapacitors limits their practical applications.5 Increasing the operating voltage of a supercapacitor is an efficient approach to improve its energy density. For this purpose, organic electrolytes are widely used to replace aqueous media because of their high electrochemical stabilities,6 whereas the relatively high viscosities and low conductivities of organic electrolytes usually limit the specific capacitances and rate capabilities of supercapacitors.7 Therefore, it is still a challenge for developing high-performance supercapacitors with organic electrolytes. Supercapacitors are divided into electrical double-layer capacitors (EDLCs) and redox capacitors by their different energy storage mechanisms.2,8 The capacitances of EDLCs mainly result from the accumulations of electrostatic charges at electrode/electrolyte interfaces. Therefore, they strongly depend on the specific surface areas of electrode materials.9 As a result, porous carbon materials, such as activated carbon,10,11 carbon nanotubes,12–14 carbon fibers 15,16 are the most attractive electrode materials.6 However, the reported specific capacitances of activated carbon are usually only Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China. E-mail: [email protected] w Electronic supplementary information (ESI) available: Galvanostatic charge/discharge curves of SGO and SGH based supercapacitors at a current density of 1 or 30 A g 1. Life cycling stability of the SGO and SGH. See DOI: 10.1039/c1cp22409c
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about 10–20% of their theoretical values. This is mainly due to the fact that most of the micropores of activated carbon cannot be accessed or wetted by electrolytes.5 Although vertically grown carbon nanotubes possess a better rate performance,17 the high costs and low specific capacitances restrain their practical applications. On the other hand, graphene, an atom thick graphite sheet,18 is a promising candidate for supercapacitor electrodes, due to its high theoretical surface area (2630 m2 g 1),19 excellent electronic conductivity,18 and great mechanical strength.20 Actually, a great deal effort has been devoted to exploring graphene based supercapacitors.1,5,21–26 Ruoff and co-workers firstly reported a graphene material with specific capacitances of 135 and 99 F g 1 in aqueous and organic electrolytes, respectively.25 To date, the highest specific capacitances of graphene in aqueous or organic electrolytes were reported to be 264 or 122 F g 1.23 However, in these cases, polymer binders and conducting additives have to be mixed with graphene powders for fabricating mechanically stable conductive electrodes and improve their conductivities. These additives increased the total weights of the electrodes and decreased their energy densities.24 Although free-standing graphene films have been explored for fabricating supercapacitors, their specific capacitances are unsatisfactory because of low specific surface areas.27 In order to develop high-performance supercapacitors, high operating voltages, low internal resistances, appropriate pore sizes of electrodes, and good wetting properties between electrodes and electrolytes should be considered. Recently, self-assembled graphene hydrogels (SGH) with 3D networks were prepared by a convenient one-step hydrothermal method or chemical reduction.28,29 These hydrogels have strong mechanical strengths, high electrical conductivities, excellent Phys. Chem. Chem. Phys., 2011, 13, 17249–17254
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thermal stabilities and large pore sizes. Thus, they are promising electrode materials for fabricating supercapacitors without adding polymer binders or conducting additives.28–31 Unfortunately, these materials were prepared in aqueous media. They cannot be directly used for fabricating supercapacitors with organic electrolytes. In this paper, we prepared a SGO by solvothermal reduction of a graphene oxide (GO) dispersion in propylene carbonate (PC). PC was chosen as the solvent of the gel, mainly due to it being the main component of widely used organic electrolytes in supercapacitors. The supercapacitor based on this SGO with PC electrolyte exhibited a high specific capacitance of 140 F g 1 at a discharge current density of 1 A g 1. Moreover, the maximum energy density of the supercapacitor was measured to be as high as 43.5 Wh kg 1, and this value is higher than those of graphene based supercapacitors with aqueous or PC electrolytes reported previously. The supercapacitor also has a high rate capability. Its specific capacitance, energy and power densities were tested to be 90 F g 1, 15.4 Wh kg 1 and 16 300 W kg 1, correspondingly, at a high discharge current density of 30 A g 1.
Experimental procedure
advance (Bruker) X-ray diffractometer with Cu-Ka radiation (l = 1.5418 A˚). Conductivities were measured by using a conventional four-probe technique. An atomic force microscope (AFM, SPM-9600, Shimadzu) was used to study the morphology of GO sheets. All of the electrochemical measurements were carried out in a conventional two electrode system. 1.0 mol L 1 PC solution of TEABF4 was used as the electrolyte. The electrolyte was well dehydrated and deoxygenated. The graphene gels were obtained as small cylinders. The size of each cylinder was p 0.42 0.2 cm3 and it was cut into to two slices with the same volumes and used as the both electrodes of a supercapacitor. SGH was freeze dried and then immersed in PC electrolyte for over 4 h. The masses of the electrodes were measured by weighing their freeze dried samples. Each electrode has a weight of over 1.0 mg. Cyclic voltammetry (CV), galvanostatic charge/discharge tests and electrochemical impedance spectroscopy (EIS) were carried out on a CHI 760D electrochemical analyzer. The energy (E) and power (P) densities of the electrodes were calculated by using following equations: E = 1/8CsDV2 and P = E/Dt, respectively, where Cs is specific capacitance, Dt is the discharge time, and DV is the operating voltage with the IR voltage drop subtracted (I and R are the current density and resistance of the supercapacitor, respectively).
Chemicals Natural graphite powder (325 meshes) was purchased from Qingdao HuaTai Lubricant Sealing S&T Co. Ltd (Qingdao, China). PC (98%) and tetraethylammonium tetrafluoroborate (TEABF4, 98%) were bought from Alfa Aesar. Synthesis of GO GO was prepared by the oxidation of natural graphite powder according to a modified Hummers’ method,32 and the details are described in the literature.33 The GO dispersion was purified by dialysis and successively freeze dried for use. Preparation of SGO GO was mixed with PC by sonication for 1 h to form a homogeneous dispersion (2 mg mL 1). Then, 2 mL of the GO dispersion was put into a 16-mL Teflon-lined autoclave and maintained at 180 1C for 12 h. Preparation of SGH
Results and discussion Preparation and characterizations of SGO GO can be well dispersed in PC, as reported by Ruoff et al.34 A 2 mg mL 1 GO dispersion is stable and no precipitation was observed after aging for one month (Fig. 1a, left). Atomic force Microscope (AFM) study indicated that the thickness of GO nanosheets dispersed in PC is about 1.1 nm (Fig. 1b). This value is in agreement with that of GO monolayer.24 After 12 h of solvothermal reduction at 180 1C, a columniform SGO was obtained (Fig. 1a, right). According to our previous report, the formation of the macroscopic gel is attributed to the regional overlapping and coalescing of flexible reduced GO nanosheets through p–p interactions.28 The microstructure of the SGO was studied using a scanning electron microscope (SEM, Fig. 2). The SEM images show an interconnected 3-dimensional (3D) porous network with pore sizes in the range of submicrometer to several micrometres. The interconnected graphene nanosheets
SGH was prepared via hydrothermal reduction of the aqueous dispersion of graphene oxide.28 Details are described as follows. GO was dispersed in water by ultrasonication for 1 h to make a homogeneous dispersion (2 mg mL 1). Then, 2 mL of the dispersion was put into a 16-mL Teflon-lined autoclave and maintained at 180 1C for 12 h. Finally, the SGH was freeze dried for use. Characterizations Scanning electron microscope (SEM) images were recorded using a HITACHI S-4500 scanning electron microscope. X-Ray photoelectron spectra (XPS) were obtained using a PHI 550 EACA/SAM photoelectron spectrometer (Perkin-Elmer PHI) with Al Ka (1486.6 eV) radiation. X-Ray diffraction (XRD) patterns were recorded on a D8 17250
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Fig. 1 (a) Photographs of a 2 mg L 1 GO dispersion in PC before and after solvothermal reduction at 180 1C for 12 h. (b) An AFM image of graphene oxide nanosheets and the corresponding height profile.
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Fig. 2 SEM images of freeze dried SGO with different magnifications.
provide the SGO with a high electrical conductivity of 2 S m 1. This value is about 4 times that of SGH (0.5 S m 1)28 and comparable to that of the SGH prepared by chemical reduction.29 This result implies that the solvothermal reduction of GO in PC is more effective than in an aqueous medium. A similar phenomenon was also observed by Ruoff and co-workers.34 They reported that the electrical conductivity of a graphene paper prepared via the thermal reduction of graphene oxide in PC is comparable to that of graphene paper prepared by chemical reduction.35–37 The structure of SGO was also studied by X-ray diffraction (XRD). According to the XRD patterns shown in Fig. 3, the interlayer spacing of SGO was calculated to be 0.38 nm (2y = 23.71), and this value is between those of graphene oxide (0.74 nm, 2y = 11.91) of SGH and natural graphite (0.336 nm, 2y = 26.51), and comparable to that of SGH. This result confirms that the formation mechanism of SGO is similar to that of SGH, mainly due to the regional stacking of reduced GO sheets. The emerging of the broad peak centered at 2y = 23.71 indicates the poor ordering of graphene sheets along their stacking direction and the SGO framework is composed of few layer stacked graphene sheets.28 C 1s X-ray photoelectron spectra (XPS) of graphene oxide and freeze dried SGO are illustrated in Fig. 4. The peaks at 284.5, 285.7, 286.8, 288.4 eV are assigned to C–C, C–OH, CQO, and OQC–OH groups.38–40 After solvothermal reduction, the peaks with binding energies higher than 284.5 eV were highly suppressed. This result reflects that the oxygencontaining groups of GO have been effectively removed. Actually, the atomic ratio of C/O was increased from 2.7 for GO to 10.8 for freeze dried SGO. Thermal gravimetric analysis
Fig. 3
XRD patterns of graphite, freeze dried GO and SGO.
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Fig. 4
C 1s XPS spectra of freeze dried GO (a) and SGO (b).
(TGA) also confirms the XPS results described above (Fig. 5). The TGA curve of GO exhibits a weight loss before 100 1C due to the release of water trapped between GO sheets.41 A significant weight loss is shown in the range of 170–230 1C. This is associated with the removing of oxygen-containing groups.41–43 In comparison, there is only a small weight loss in the temperature range of 170–230 1C in the curve of freeze dried SGO. This result implies that most of the oxygen-containing groups have been removed by thermal reduction of GO in PC. Electrochemical performances As described above, SGO is mechanically stable and has high conductivity and a porous structure. More importantly, it has good affinity with PC solvent. Therefore, it is an attractive electrode material for supercapacitors with PC electrolyte. SGO can be directly used for fabricating the electrodes, no polymer binder or conducting additive is needed. The supercapacitor
Fig. 5 TGA curves of freeze dried GO and SGO.
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Fig. 6 (a) A schematic of the configuration of the supercapacitor device. (b) CV curves of the supercapacitor in different potential ranges and at a scan rate of 10 mV s 1.
devices were assembled with a configuration shown in Fig. 6a. The performance of the supercapacitor was studied by cyclic voltammetry (CV). The CV curves of the supercapacitor tested in a two-electrode system keep a quasi-rectangular shape in the potential range of 0 to 3 V (Fig. 6b). This is mainly due to the superior electrochemical stability of the PC electrolyte and SGO electrodes. The quasi-rectangular shaped CV curves also indicate that the capacitance of the electrode is predominately determined by its electrical double layers. The specific capacitance of the electrode materials were measured by galvanostatic discharge (Figs S1 and S2, ESIw). The specific capacitance of SGO was tested to be about 140 F g 1 at a discharge current density of 1 A g 1. The solid content density of the electrode in a squeezed state was measured to be about 0. 05 g cm 3. Accordingly, the volume-specific capacitance of the electrode was calculated to be 7 F cm 3. For comparison, freeze dried SGH was also used as an electrode material for the supercapacitor with PC electrolyte and its specific capacitance was measured to be only 115 F g 1 under the same conditions (Fig. 7a). Accordingly, although the microstructures of SGO and SGH are similar, their electrochemical performances in PC electrolyte are quite different. Furthermore, with the increase of the discharge current density, the capacitance of SGH decreases much more dramatically than that of SGO. As the discharge current density increased from 1 to 5 A g 1, the specific capacitance of SGH dropped about 50%, while the capacitance of SGO decreased only approximately 14%. At the discharge current density of 30 A g 1, the capacitance of SGH decreased to close to zero and the capacitance of the SGO still kept at a high value of 90 F g 1. Considering that the specific area of the freeze dried SGH (166 m2 g 1) is larger than that of the SGO (146 m2 g 1), the high performance of SGO is mainly due to its better wetting property with PC electrolyte. It was also confirmed by the XPS results that the atomic ratio of C/O of SGO (10.8) is much higher than that of SGH (5.3). Therefore, SGO is more hydrophobic than SGH, which provided it with better wetting properties with the organic PC electrolyte. The improved affinity of SGO greatly increased its effective surface area accessible by electrolyte ions.44 The poor rate performance of activated carbon based supercapacitor electrodes is one of their main drawbacks. Aligned carbon nanotubes possess better rate performances than activated carbon due to their regular and short ion transportation paths.17,45 17252
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However, the cost of aligned carbon nanotubes is much higher than that of thermally reduced GO. Furthermore, aligned carbon nanotubes were reported to have a specific capacitance of only 14 F g 1, which dropped to 6 F g 1 at a discharge current density at 12 A g 1.45 On the basis of these facts, it is reasonable to conclude that the supercapacitor based on SGO has much better performance than that of the supercapacitors based on other carbon materials such as activated carbon, carbon nanotubes and SGH. The CV curves carried out at different potential scan rates for the supercapacitor based on SGO also confirm its outstanding rate performance (Fig. 7b). With the increase of the potential scan rate, no obvious distortion in the curves was observed. Even at a high scan rate of 500 mV s 1, the CV curve still keeps its quasirectangular shape. These results reflect that the SGO has a low equivalent series resistance and a fast diffusion rate of electrolyte ions.21,46 The electrochemical stability during long term cycling is one of the major advantages of supercapacitors. The SGO electrode maintained about 83% of its original specific capacitance after 1000 charge/discharge cycles, which is slightly higher than that (80%) of the SGH electrode (Fig. S3, ESIw). The IR drops at different current densities for the supercapacitor based on SGO or SGH are shown in Fig. 7c. Accordingly, at a given current density, the IR drop of SGO based supercapacitor is lower than that of the SGH counterpart. This result reflects that SGO electrode has a lower internal resistance than that of SGH electrode. Lower internal resistance will waste less energy to produce unwanted heat during charging/discharging processes.24 Thus, SGO is more suitable for fabricating supercapacitors with organic electrolytes, especially for the requirement of rapid charging/ discharging. It should be noted here that the voltage drop of the supercapacitor based on SGH was increased to about 3 V at a current density of 30 A g 1, indicating that nearly all of the energy was wasted to produce unwanted heat. Therefore, its rate performance is much worse than that of SGO. The energy density of the SGO electrode was also tested to be much higher than that of SGH at the same power density (Fig. 7d). SGO has a maximum energy density of 43.5 Wh kg 1, which is much higher than the values of 4.7–9.2 or 21.5–39 Wh kg 1 for chemically modified graphene based supercapacitors with aqueous5,23,25,26 or organic electrolytes.5,25,47 Furthermore, SGO can still keep an energy This journal is
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Fig. 7 (a) Plots of the specific capacitance of SGO or SGH based supercapacitors measured by galvanostatic charge/discharge as a function of current density. (b) CV curves of the SGO based supercapacitor at different potential scan rates. (c) IR drops of the supercapacitors based on SGO or SGH versus current density. (d) Plots of energy density versus power density at an operating voltage of 3 V for SGO or SGH based supercapacitors.
density of 15.4 Wh kg 1 at a high current density of 30 A g 1. This result implies that this supercapacitor can be rapidly charged or discharged within 10 s, keeping an acceptable energy density (15.4 Wh kg 1) and a high powder density (16 000 W kg 1). Two main criteria for evaluating the performance of supercapacitors are as follows: (1) high energy densities (>10 Wh kg 1) with a power density substantially higher than those of batteries, (2) rapid charge/discharge processes within several seconds. Considering these two requirements, the supercapacitor based on SGO should have wide application prospects. Due to the regular and short ion transfer paths, aligned carbon nanotubes were considered to keep a large density at a high charge/discharge rate. However, the energy density of our SGO based supercapacitor is much higher than the maximum energy density of aligned carbon nanotube based supercapacitors (4.3 Wh kg 1) reported previously.45 We further investigated the frequency responses of the supercapacitors by electrical impedance spectroscopy. The Nyquist plots of SGO and SGH in the frequency range of 0.01 Hz to 1 MHz are shown in Fig. 8. Each plot has a small semicircle region, a 451 Warburg region, and a pure capacitor region. The semicircle region of SGO is more obvious and integrated than that of SGH. This region is related to the faradic resistance, which is modelled by a parallel combination of an interfacial charge transfer resistance.44 The observation that this region is nearly missing in SGH is mainly due to the interlayers of stacked graphene sheets in freeze dried SGH are hard to be reached by PC electrolyte, especially at high frequencies.48 It is also clear from this figure that the 451 Warburg region of SGO (176–1.5 Hz) is much shorter than that of SGH (561–0.3 Hz). This region corresponds to This journal is
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Fig. 8 Nyquist plots of SGO or SGH based supercapacitors. The insert shows the magnified high frequency regions.
Warburg impedance caused by ion diffusion and transport in the electrolyte.44 A shorter Warburg region can be explained by the enhanced ion diffusion in SGO because of its improved compatibility with PC electrolyte. This is why the SGO based supercapacitor exhibited much better performance than that of the SGH counterpart. The vertical lines at low frequencies indicate pure capacitor behaviour. The more vertical curve of SGO compared to that of SGH reflects that the SGO based supercapacitor is more like an ideal capacitor.33
Conclusions SGO with a 3D macrostructure has been successfully prepared by thermal reduction of GO dispersed in PC. Because the Phys. Chem. Chem. Phys., 2011, 13, 17249–17254
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reaction occurred in PC and the effective reduction, the resulting SGO has a good affinity with this solvent. Furthermore, SGO has a highly conductive and a porous 3D network. Therefore, the ion diffusion property of SGO based supercapacitors in TEABF/PC electrolyte was greatly improved. The supercapacitor based on SGO exhibited a high specific capacity of 140 F g 1 and a high maximum energy density of 43.5 Wh kg 1. They also showed excellent rate capabilities. Their performances are the best among the supercapacitors based on carbon materials including activated carbon, carbon nanotubes, chemically modified graphene or SGH with PC electrolytes. The high operating voltage of the SGO based supercapacitor also makes its power and energy densities much higher than those of the chemically modified graphene based supercapacitors with aqueous electrolytes.
Acknowledgements This work was supported by natural science foundation of China (91027028, 50873092) and open research fund of state key laboratory of bioelectronics (Southeast University).
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