IntroductionElectrochemical capacitor has emerged with wide application potential because it can provide higher energy density than conventional capacitors, and greater power density and longer cycling life than batteries through the use of high-surface-area electrodes and fast surface-charge-storage processes, which bridges the gap between conventional capacitors and batteries1.Graphene (RGO) is emerging as one of the most appealing materials because of its unique superior electrical conductivity, excellent mechanical flexibility, and high thermal and chemical stabilities2. It has been found that the capacitance of grapheme can be improved by combining with some pseudocapacitive electrode materials, such as conducting polymers and transition-metal oxides3,4. In this presentation, a series of grapheme-metal oxide hybrid electrode materials (RGO-RuO2, RGO-MnO2, RGO-Co3O4, and RGO-VO2) are prepared by a nanosheet assembly technology between metal oxide nanosheets or nano-particles and graphene. The high dispersion of metal oxide nanosheets and RGO nanosheets maintains a high structural stability for the hybrid materials, and causes an obvious synergistic effect between the metal oxide nanosheets and RGO nanosheets. The RGO-metal oxide hybrid electrodes show excellent capacitance and cycle performances compared with both pure RGO and as-pristine metal oxides5. On the basis of the grapheme-metal oxide hybrid electrode materials, an asymmetrical electrochemical capacitor is assembled, with 1 M Na2SO4solution as the electrolyte. The asymmetrical electrochemical capacitor can cycle reversibly in a voltage range of 0�2 V and gives a high energy density and good cycling performance.Experimental Graphite oxide (GO) was fabricated from crude flake graphite by a modified Hummers method. The as-prepared GO was then treated by ultrasonication treatment using a KQ-600kDE Digital Ultrasonic cleaning device (600 W, 80% amplitude) in water bath for an hour, and GO nanosheet homogeneous dispersion (0.3 mg mL-1) was obtained. The GO nanosheet homogeneous dispersion (100 mL) was mixed with hydrazine solution (35 mL) and ammonia solution (400 mL), followed by stirring for 10 min. The suspension was then heated to 95 �C and stirred for an hour, GO was reduced to RGO and RGO nanosheet suspension was obtained.Grapheme-metal oxide hybrid electrode materials (RGO-RuO2, RGO-MnO2, RGO-Co3O4, and RGO-VO2) were prepared by a nanosheet assembly technology between metal oxide nanosheets or nano-particles and graphene. RGO-RuO2 hybrid material with RuO2 amount of 40% was prepared by a controllable nanosheet reassemble technology between RuO2 nanosheets and RGO nanosheets at room temperature. The colloidal suspension of RuO2 nanosheets (2.2 mg mL-1, 18 mL) and RGO nanosheet dispersion (0.25 mg mL-1, 240 mL) were respectively taken in two separate constant pressure funnels and simultaneously dropped into a beaker with constant stirring. Then the suspension was stirred for 2 days at room temperature. The obtained precipitate was filtrated with a nylon membrane (0.22 �m) and then freeze-dried for 24 h, the RGO-RuO2 hybrid material was finally obtained. The RGO-MnO2 hybrid material was prepared by adjusting the pH value of the obtained layered manganese oxide slurry (15 mL) to that of the graphene nanosheet dispersion (300 mL), and then two dispersions were mixed and stirred for 7 days at room temperature. The obtained precipitate was filtrated with a nylon membrane (0.22 �m) and freeze-dried for 24 h to obtain the RGO�MnO2hybrid material.The RGO-Co3O4hybrid material was prepared by two step method. GO suspension with a concentration of 0.1 mg mL-1 was slowly added into the exfoliated Co(OH)2 nanosheet suspension (0.5 mg mL-1) with different mass ratios, and the obtained suspension was stirred for 24 h with N2 purging. After centrifugation, washed with degassed water, ethanol, and dried by N2 purging, GO/Co(OH)2 layered nanocomposite was firstly obtained. Then the GO/Co(OH)2 layered nanocomposite was calcined in a muffle furnace at 300-450 �C for 2 h and followed by thermal annealing in tubular furnace at 500-750 �C for 90 min with Ar atmosphere, the RGO-Co3O4hybrid material was prepared. 0.3 g NH4VO3 powder was added into 40 mL GO dispersion (1.0 mg mL-1), followed by stirring for 10 min. Then, 5 mL formic acid was added to the suspension. After being vigorously stirring for another 10 min, the orange liquid dispersion was transferred into an autoclave and was heated at 180 �C for 12 h. The resulted precipitate was washed with water for several times, dried in room temperature, RGO-VO2hybrid material was obtained.Results and Discussion The high dispersion of RuO2 and RGO nanosheets maintains a high structural stability for the hybrid material, and causes an obvious synergistic effect between the RuO2 and RGO nanosheets. A specific capacitance of 479 F g-1 has been obtained for the hybrid material with RuO2 mass content of 40%, and a high specific capacitance of 998 F g-1 obtained for RuO2 in the electrode (Figure 1a). An asymmetric electrochemical capacitor with high energy and power densities based on the RGO-MnO2 hybrid material as positive electrode and graphene as negative electrode in a neutral aqueous Na2SO4 solution as electrolyte is assembled. The asymmetrical electrochemical capacitor could cycle reversibly in a voltage of 0-1.7 V and give an energy density of 10.03 Wh kg-1 even at an average power density of 2.53 kW kg-1 (Figure 1b). The RGO-Co3O4 hybrid composite with 94.3 wt% Co3O4 exhibits a high specific capacitance of 331 F g-1 at the current density of 5 A g-1, and shows the excellent electrochemical cycle stability with capacitance retention over 122% after 5000 continuous charge-discharge cycles (Figure 1c). The hydrothermal treatment makes the reduction of GO into RGO and the formation of VO2 particles with starfruit morphology. The starfruit-like VO2 particles are uniformly embedded in the hole constructed by RGO nanosheets, which makes the electrode-electrolyte contact better. A high specific capacitance of 225 F g-1 has been achieved for RGO-VO2 electrode with RGO content of 26 wt% in 0.5 mol L-1 K2SO4electrolyte. Conclusion A series of grapheme-metal oxide hybrid electrode materials (RGO-RuO2, RGO-MnO2, RGO-Co3O4, and RGO-VO2) are prepared by a nanosheet assembly technology between metal oxide nanosheets or nano-particles and graphene. These hybrid electrode materials show excellent capacitive performance and can be used as promising electrodes for the electrochemical energy storage.Acknowledgement . This work is supported by the National Natural Science Foundation of China (51172137), Changjiang Scholars and Innovative Research Team in University (IRT1070) and the Fundamental Research Funds for the Central Universities (GK201301002).References (1) Liu C., Li F., Ma L., Cheng H. Adv. Mater. 2010 , 22, E28-E62.(2) Geim A. Science 2009 , 324, 1530-1534.(3) Wang G., Zhang L., Zhang J. Chem. Soc. Rev. 2012 , 41, 797-828.(4) Wang Y., Xia Y. Adv. Mater. 2013 , 25, 5336-5342.(5) Deng L., Wang J., Zhu G., Kang L., Hao Z., Lei Z., Yang Z., Liu Z.-H. J. Power Sources 2014  , 248, 407-415.

Graphene-metal oxide hybrid materials with high capacitance

248th National Meeting (2014)

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