Syntheses And Electrochemcal Capacitive Performance Of Reduced Graphene/NiCo₂O₄ Composites

As a new type of electrochemical energy storage device,supercapacitors have several advantages that include high power density, long cycle life, wide range of operating temperatures, and fast charge–discharge processes. Many researches have focused on researching and developing electrode materials, which are the key factor to affect supercapacitors performance. Graphene as a new two-dimensional carbon material exhibits many advantages, such as high surface area, extraordinarily high electrical conductivity, and chemical stability, making it a potential candidate for applications in supercapacitors field. In addition, transition metal oxides have been considered as promising electrodes to improve the performance of supercapercitor due to several advantages, including their low cost,environmentally friendly and high specific/volumetric capacity. Among them, ternary nickel cobaltite(Ni Co2O4) has recently been investigated as electrode material for supercapercitor. Therefore, the development of graphene-based transition metal oxide composites formed a new research direction.(1) Graphite oxide was prepared by flake graphite via modified Hummers method, and chemical reduction method had been used to synthesize graphene, using hydrazine and urea as reducing agents. The results showed that the reduced graphene had multilayer restacked.The BET specific surface area of two graphene is difference, r GN-2 to be340 m2 g-1 and r GN-1 to be 180m2 g-1. At a current density of 0.5A/g,the specific capacitance of the r GN-2 and r GN-1 is calculated to be174 F g-1 and 161 F g-1, respectively. At a high current density of 5 A g-1,they still remain larger capacitance, 152 F g-1 and 136 F g-1 respectively.The capacitance retention rate is 87.4% and 84.4%, respectively, which shouwed that r GN-1 had better electrochemical performance than r GN-2.(2) We prepared r GO/Ni Co2O4-NW composite for supercapacitor material has been fabricated by a co-precipitation method coupled witha simple post-annealing treatment when r GO as substrate and urea as hyderolyzing agent. Ni Co2O4 nanowires with a diameter of 200-400 nm and a diameter around 20 nm are densely and uniformly grown on the surface of r GO. After 1000 cycles at 2 A g–1, r GO/Ni Co2O4-NW composite has remained 89% of their initial specific capacitance.(3) The morphology of Ni Co2O4 can be controlled by changed the hyderolyzing agent in the reaction solution. Ultrathin Ni Co2O4 nanosheets can assemble into a film that fully anchored on the thin graphene sheets, which can improve specific surface area and the electric conductivity. With the special two-dimension structural and compositional advantages, the as-synthesized r GO/Ni Co2O4-NS nanocomposite is expected to manifest enhanced supercapacitors storage properties of specific capacitance and cycling stability, because r GO in the nanocomposites could not only increase the electric conductivity, but also faciliate the charge transport in charge/discharge progresss. As a result, the r GO/Ni Co2O4-NS nanocomposite manifests a very stable high reversible capacitance of around 1217.4 F g- 1at a current density of 1 A g- 1, and the capacitance still remain 983 F g-1at a current density of 2 A g-1after 1000 cycles, only lost 6% of the initial specific capacitance.