Study On The Characteristics Of Supercapacitors Based On MnO₂ And Graphene Electrodes

Supercapacitor is a new type of energy storage devices between the traditional capacitor and the secondary cell. It has many advantages, such as short charging time and high power density, but its energy density needs to be improved. The electrode material is a key factor affecting the performance of the supercapacitor. There are two ways to improve the energy density. The first one is increasing the specific capacitance of the electrode material. The second one is widening the working voltage window by the assembly of different electrodes with different potential windows into asymmetric supercapacitor. Based on these two methods, 8-MnO2, reduction of graphene oxide and MnO2/ graphene composite electrode materials were prepared. The morphology of these materials were characterized by X- ray diffraction, scanning electron microscopy and Raman test. Cyclic voltammetry, galvanostatic charge discharge and A.C. impedance measurements were carried out on three kinds of electrode materials and devices in 1 M Na2SO4 solution.The main work and innovation of this paper are given as follows:1. Manganese oxide materials were prepared by hydrothermal method. In the process of preparation, the effects of hydrothermal time, temperature, material ratio,dispersant and pH on the structure and electrochemical properties of MnO2 were studied.The gradient temperature methods were used innovatively to optimize MnO2 materials.Finally, it is found that the amorphous and micro crystalline coexistence of ?-MnO2 has the best electrochemical performance, which is obtained on the gradient temperature of 140-120-100?. The maximum specific capacitance is up to 180.25 F/g. Even when the current density rises to 13 A/g, the specific capacitance could still reach 117 F/g. After 1000 cycles of a scan rate of 50 mV/s, the specific capacitance remains as high as 94%.2. Graphene oxide was prepared by the improved Hummers method, and three kinds of reduction modes of hydrazine hydrate chemical reduction, thermal reduction and hydrothermal reduction were studied. It is found that the performance of hydrothermal reduction graphene oxide is the best. The maximum specific capacitance is up to 128.81 F/g. Even when the current density rises to 13 A/g, the specific capacitance could still reach 94.25 F/g. After 1000 cycles of a scan rate of 50 mV/s, the specific capacitance remains as high as 99%.3. MnO2 and graphene were synthesized by one step hydrothermal method. The excessive KMnO4 was used innovatively in the process of compounding, and therefore there were two compounding modes in the compound process, which were intercalation compound and in situ compound, and which resulted in the formation of MnO2/graphene /MnO2 sandwich structure. The maximum specific capacitance is up to 193.38 F/g. After 1000 cycles of a scan rate of 50 mV/s, the specific capacitance remains as high as 98%.4. The composite with the best performance was selected as the anode and the hydrothermally reduced graphene oxide as the negative electrode, which then were assembled into a button type asymmetric supercapacitor for electrochemical test. It was found that under the current density of 0.5 A/g, the specific capacitance of the device is about 46.68 F/g, the corresponding energy density is 17.31 Wh/Kg,and the power density is still up to 171.65 W/Kg at the same time.