The Preparation Of Graphene/Metal Oxide Composites And Its Application In Supercapacitors

Due to the high surface area, excellent thermal conductivity, remarkable electronic conductivity and mechanical properties, graphene exhibits lots of advantages as the supercapacitor electrode material. However, graphene materials are very easy to aggregate which decreases the surface area, resulting in reduced capacitance. This greatly destroys the prospect for graphene in pratical applications. Combination of metal oxides with graphene offers a pathway for material selection and optimization. The porous structures of graphene are favorable for fast ion/electron transport, and facilitate sufficient contact between electrolyte and graphene materials. Faradaic materials are well-known for their high capacitance and highly reversible redox reactions, while graphene is an ideal matrix for composites due to the chemical stability and sophisticated porosity. The synergetic effects can provide full scope to the potentiality of each component. As the electrode material, graphene-based composites have great application prospects and research value.In this paper, we study the electrochemical properties of graphene, and synthesis the composite materials of graphene and MnO₂, NiO by solvent thermal method in order to improve the specific capacitance. We characterize the structure and morphology of composite materials, as well as the electrochemical performance. The main contents and results are as follows:（1） Modify the traditional Hummers method to preparation of graphene by using the natural flake graphite instead of Expandable graphite. The graphene oxide obtained is with higher degree of oxidation, and more homogeneous in thickness. With the different temperature from 200℃ to 800℃ to reduce graphene oxide, a series of graphene are obtained with different levels of oxidation. We discussed the impact of annealing temperature on the oxygen-containing functional groups and electrochemical properties of graphene. Electrochemical tests show that the specific capacitance of graphene reduced at 200℃ is 286 F/g at a current density of 1A/g. And as the annealing temperature increases, the specific capacitance of abtained graphene is decreasing.（2） Use the solvothermal method and the following simple heat treatment to synthesize graphene/MnO₂ composite materials. And study the effects of solvothermal conditions on composite materials, such as feed ratio, reaction temperature and time. The results show that when the ratio of graphene and manganese source is 1:8, the solvothermal reaction temperature is 160 ℃, and the reaction time is 8h, the graphene/MnO₂ composite materials synthsized perform the best electrochemical properties. At a current density of 1A/g, the specific capacitance can reach 445F/g. And the composite materials still maintain 93.6% of the capacitance after 5000 cycles, showing excellent cycle stability.（3） As an active Faradaic material, NiO has aroused great academic interests because of the extensive sources and the low prices. However, its relatively poor electronic conductivity limits the practical application in a certain extent. We use solvothermal method to prepare the precursor, and then reduce the precursor by high-temperature annealing to the final product—G/NiO composite material. The composite material’s special capacitance is 456 F/g at the current density of 1 A/g, and after 5000 cycles, the capacity retentivity is 87%, showing a good cycle performance. It is one of the ideal supercapacitor electrode materials.