Energy crises is one of the big problem in nowadays and supercapacitor is one of the solution for it which shows hope for future of storing energy due to their high power density, long cycle life, wide operation temperature range, and green for environment 1-3. In general, supercapacitors are divided into two categories based upon their working procedure involved during the charge storage processes (i) electric double layer capacitor (EDLC) and (ii) Pseudo capacitor. The EDLC (like active carbon as a electrode) stores the charges via electrostatic adsorption/desorption at the electrode surface whereas, in pseudo-capacitors 4, it is due to fast faradaic redox reaction. The pseudocapacitors afford a higher specifc capacitance than the EDLCs due to their fast and reversible redox reaction 5-6. There is therefore a growing interest in using pseudocapacitive materials for supercapacitor. In recent years, a tremendous research efforts have been triggered in order to develop efficient electrodes based on different transition metal sulfide and oxide for high-performance supercapacitor electrodes due to their enhanced electrochemical properties associated with multiple oxidation states. Till now, transition metal oxides such as Co3O47-8 , NiO9, NiCo2O410-11, CuCo2O4 12, MnCo2O413-14 and metal sulfides such as CoS15, Co3S4 16-17, Ni3S218, NiS19, NiCo2S4 20 have been investigated as high performance electrode materials for super-capacitor.
In between different compositions cobalt oxide Co3O4 is particularly attractive because of its high theoretical capacity. When falling in the nano sized regime, it is expected to lead to even more attractive properties. Xiangmei Liu, Qing Long et al.21 prepared mesoporous Co3O4 nanocubes by a hydrothermal method, and shows a capacitance of 220 Fg-1 at current density of 1Ag-1 in the three-electrode measurement. Qiu Yang, Zhiyi Lu et al.16 prepared Ultrathin Co3O4 nanosheet arrays by a hydrothermal method on a NI foam , and the supercapacitor based on these Co3O4 showed a capacitance of 1782 Fg-1 under a current density of 1.8Ag-1 in the three-electrode measurement. Huatao Wang, Li Zhang et al.17 synthesize Co3O4 nanostructures by hydrothermal method and with a heavy mass loading of material their Co3O4 shows a super capacitance of 1090Fg-1 under the scan rate of 10mV s_1 in the three electrode test. Although there are more work based on Co3O4 and its composite with different material.22-23.
Cobalt sulfides have a series of from, such as Co1?xS, CoS, CoS2, Co9S8, Co3S4, etc24-26. When nano structure of cobalt sulfide serves as electrode, the Faraday oxide-reduction reaction can occur between the cobalt ions with different valences similar to that of the cobalt oxides 27. Q. Chen, H.X. Li, C.Y. Cai et al.28 prepared Co3S4 through simple shape and phase transformation route, which exhibits high specific capacitance 1081 F g-1 at 1.61 A-1. Q.H. Wang, L.F. Jiao et al.29 synthesized hollow Co3S4-graphene composite nanospheres shows 675.9 Fg-1 at 0.5 Ag-1. Bo Xu, Lu Pan et al. prepared mesoporous Co3S4 nanosheets by a hydrothermal method, and shows a capacitance of 1037Fg-1 current density of 1Ag-1 in the three-electrode measurement.