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The electrode material is the key factor for developing asymmetric supercapacitors. A simple water-boiling treatment was adopted to fabricate a Mn3O4/Ni(OH)2 nanocomposite on a large scale, which exhibited a splendid electrochemical performance with a boiling time of 3 h, achieving a capacitance of 742 F g–1 at 1 A g–1. When assembling the asymmetric capacitor with the Mn3O4/Ni(OH)2 composite and activated carbon separately as positive and negative electrodes, a capacitance of 43 F g–1 was attained at 0.2 A g–1 with an energy density of 15.3 Wh kg–1 at a power density of 168.8 W kg–1. This simple, reproducible, ecofriendly, and large-scale fabrication method is practical for preparing other transition metal oxides for the purpose of use in asymmetric supercapacitors with superior properties.

Pure and Ni-doped manganese dioxides were prepared by liquid-phase reaction method. The morphology of the samples was characterized by SEM and the electrochemical capacitance behaviors of pure MnO2 and Ni-doped MnO2 electrode were investigated by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy tests. The results show that nickel doping can enhance the specific capacitance of MnO2. The 1 % Ni-doped MnO2 electrode shows a relative high specific capacitance of 451 F g-1at a scan rate of 5 mV s-1 and 267.3 F g-1 at a current density of 0.3 A g-1 in 1 M Na2SO4 electrolyte.