The Preparation Of MnO₂/Graphene Composites And Its Application On Supercapacitor

Supercapacitors as new energy storage device with the characteristics of high power density, fast charge and discharge and long cycle life, have been widely used. The electrode material is the core of the supercapacitors. Transition metal oxide with high theoretical specific capacity have poor electrical conductivity, add the graphene can effectively improve this shortcoming. In this paper, we prepared GO based on the modified Hummers' method, followed by treating it under microwave irradiation to form microwave exfoliated GO (MEGO). Then we used KMnO4, NH4Cl, MEGO as raw materials to synthetise a series of MnO2-graphene composites with different amount of MnO2 (by changing the reaction time) and pure MnO2. The XRD, Raman and SEM were applied to study the morphology and structure of the composites' and electrochemical performance of composite was studied.(1) With the increase of reaction time, the weight percentage of MnO2 in the MnO2-graphene composites are increased. Through SEM tests: the nano sheet MnO2 superimpose to form honeycomb-like three-dimensional structure, and disperse uniformly on the surface of graphene layers.(2) Composites' electrochemical performance was studied through the three electrode system. The results show that the electrochemical properties of composites are better than pure MnO2 and MEGO; when the mass fraction of MnO2 is 35.4%, composite has the best electrochemical performance; when the current density is 1 A/g, the specific capacitance is 339 F/g and the capacitance retention is 85.71% after 1000 cycles at a current density of 10 A/g. It shows that the material has high specific capacitance and cyclic stability.(3) Asymmetric supercapacitor was assembled with composites, and the influence of MEGO's mass on the performance of supercapacitor was studied. With composites as positive electrode, graphene as negative electrode, 1 M Na2SO4 as electrolyte, the results show that working voltage is 1.8 V; the optimal mass ratio of composites/graphene is 0.55:1; at the current density of 0.1 A/g, asymmetric supercapacitor capacitance is 76.56 F/g; the capacitance retention is 87.72% after 1000 cycles at the current density of 1 A/g;a maxium energy density of asymmetric supercapacitor is 37.45 Wh/Kg (for a power density of 100 W/Kg). Under the same test conditions, compared with graphene's and composites' symmetry supercapacitors, the asymmetric supercapacitor has higher energy density and power density.