Reserach On The Preparation And The Electrochemical Properties Of Manganesedioxide-based Electrode With High Specific Capacitance

Supercapacitor is a new type of energy storage device. It has lots of advantages, such as fast charge and discharge rate, higher capacity, long cycle life, excellent reversibility, high energy density and high power density. Therefore, it has promising applications in the field of energy. An important way to enhance the performance of supercapacitor is to develop high-performance electrode. MnO2 is a good supercapacitor electrode materials that have been widely studied. However, poor conductivity and mechanical stability limits its electrochemical performance and the real applications as higher perforamce electrode materials. The main purpose of this thesis is to prepare high-performance MnO2-based electrode materials, through the optimization of electrodeposition conditions and testing conditions to form the composite film electrode by combining the MnO2 with the conductive polymer composite materials, thus improving the specific capacitance value and the capacitive perforamnce of the MnO2-based electrode material.(1) MnO2 coating is anodically electrodeposited from a Mn(CH3COO)2 solution. We investigate the influence of preparing process and testing conditions on the electrochemical capacitive behaviour of the MnO2 electrode for optimized electrochemcial performance. It has been found that, during the electrodeposition, the specific capacitance of the MnO2 electrode increased to 100F/g when the concentration of Mn(CH3COO)2 solution was raised to 0.01 M. However, further increasing of concentration to 0.02 M resulted in unstable mechanical performance of obtained electrode films. At the fixed concentration of Mn(CH3COO)2 solution, when the temperature of the Mn(CH3COO)2 solution increased from 20 centigrade to 40 centigrade, the specific capacitance of the MnO2 electrode increased from 92F/g to 127 F/g accordingly. However, at a higher temperature of 60 centigrade, no distinct change of specific capacitance was observed but with deteriorated reversibility of electrode. Furthermore, the electrochemical capacitive behaviour of the MnO2 electrode in varied Na2SO4 electrolyte with different concentration and temperature is also investigated. The results indicated that, with the increase of Na2SO4 concentration, the electrode showed faster current response and the capacitance characteristics was improved. It also has been found that, with the Na2SO4 solution temperature increasing from 20 centigrade to 40 centigrade, the reversibility of the MnO2 electrode deteriorated significantly.(2) A Fe-doped MnO2 is anodically electrodeposited from the solution containing Mn(CH3COO)2 and FeCl3. The results indicate that the specific capacitance of the Fe-doped MnO2 electrode increased significantly comparing with the pure MnO2 electrode. A high 184F/g specific capacitance of Fe-doped MnO2 electrode was achieved at the 0.4A/g current density, which also showed reduced resistance compared with pure MnO2 electrode.(3) A two-step method was demonstrated to prepare the MnO2/PEDOT composite film. The effects of different types and the concentration of deposition solution on the electrochemical capacitive behaviour of the composite electrode were investiaged. The results indicated that a Mn(CH3COO)2 solution mixed with KCl and EDOT resulted in formation of Mn O2/PEDOT composite film with good capcitive reversibility, which is more suitable for discharging at larger current. The results indicates that, with the KCl conentration increasing from 0.05 M to 0.2M, the specific capacitance of the composite film electrode increased first and then decreased. With the(CH3COO)2 concentration increasing from 0.006 M to 0.02 M, the specific capacitance of MnO2/PEDOT composite film electrode increased from 100F/g to 166F/g. Futhermore, by using a mixed solution containning Mn(CH3COO)2, KCl or LiClO4 and EDOT, the MnO2/PEDOT composite electrode can be anodically deposited onto the substrates successfully with a one–step electrodeposition process. The results indicated that, compared with electrolytic solution without LiClO4, the specific capacitance of the MnO2/PEDOT composite film deposited from mixed solution containing LiClO4 is higher, and a high 157F/g specific capacitance was achieved. This composite electrode films also showed lower resistance and diffusion resistance for better electrochimcal perforamance.