Abstract: Clew-like MnO2(CMnO2) with a specific capacitance of 154 F/g within a potential window of 0~1.0 V vs saturated calomel electrode has been synthesized by hydrothermally treating the precursor of as synthesized MnO2 in a solution of 1 mol/L NaOH at 150 ℃ for 12 h. The morphology and structure of the obtained material were examined by XRD, SEM and N2 adsorption-desorption. The capacitive properties were characterized by cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopies. The results indicate that the CMnO2 exhibits good capacitive performance in a three-electrode cell with a mass specific capacitance of 154 F/g. An asymmetrical electrochemical capacitor(EC) was assembled using CMnO2 as the positive electrode and graphene(GR) as the negative electrode, respectively. The asymmetrical EC can cycle reversibly in a cell potential of 0~1.8 V and gives a high energy density of 21.6 W·h/kg, which is much higher than those of symmetrical ECs based on CMnO2(4.86 W·h/kg) and GR(4.3 W·h/kg). Moreover, the asymmetrical EC presents a high power density(9 kW/kg at 8.1 W·h/kg) and excellent cycling performance of 83% retention after 1000 cycles.

Key words: asymmetrical electrochemical capacitor, manganese dioxide, graphene, energy density, cycle stability