Effects of Heat Treatment on the Physical and Electrochemical Properties of Clew-like Manganese Dioxide

The clew-like manganese dioxide nanostructures composed of interleaving nanoplates were successfully prepared by adding 8 L dilute potassium permanganate solution (0.015 mol.L-1) to 8 L dilute manganese chloride solution (0.01 mol.L-1) with the dropping rate of 20 mL.min(-1) under a stirring condition. The effects of heat treatments on the physical and electrochemical properties of the obtained manganese dioxide were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area measurement, cyclic voltametry (CV), electrochemical impedance spectroscopy (EIS) and constant current charge-discharge method. The pore size of the obtained clew-like manganese dioxide was ca. 6.5 nm and its distribution was uniform. The results of physical characterization showed that heat treatment could adjust the microstructure of manganese dioxide. The increase of the treating temperature induced the decrease of the specific surface area and the amount of the micropores while the mean pore size increased with the increase of the treating temperature. The MnO2 particles remained amorphous below 300 degrees C. The cyclic voltametry measurements indicated that all the amorphous MnO2 samples have characteristic of the ideal pesudocapacitor. The impedance spectroscopy tests showed that a suitable treating temperature could promote the ability of faradic charger-transfer and the diffusion of ion on the interface between the active material particles and electrolyte. Though the MnO2, sample treated at 200 degrees C did not have the highest specific surface area, it had the lowest faradic charger-transfer resistance and the highest specific capacitance, which was 210.6 F.g(-1) at the scan rate of 2 mV.s(-1) in 1 mol.L-1Na2SO4 solution. The constant charge-discharge curves of this sample are approximately symmetrical and linear for both charge and discharge portions, which indicated this sample had an ideal capacitive characterization. The cycling life test was conducted by the CV method. The first CV capacitance at 5 mV.s(-1) was 178.1 F.g(-1). 93.5% of its initial capacitance was remained after 1000 cycles, showing an excellent cycling performance. The effects of heat treatment on physical and electrochemical of manganese dioxide indicated that a suitable heat treatment can promote the electrochemical performance of manganese dioxide through adjusting its microstructure.