Mn-based oxides are widely investigated as electrode materials for electrochemical supercapacitors, because of their high specific capacitance in addition to the high abundance, low cost, and environmental friendliness of Mn. The pre-insertion of alkali metal ions is found to improve the capacitance properties of MnO2. While the capacitance properties of MnO2, Mn2O3, P2-Na0.5MnO2, and O3-NaMnO2etc. are reported, there is no report yet on the capacitive performance of P2-Na2/3MnO2, which has already been studied as a potential positive electrode material for Na-ion batteries. In this work, we have synthesized sodiated manganese oxide, P2-Na2/3MnO2 by a hydrothermal method followed by annealing at a high temperature of about 900 °C for 12 h. For comparison, manganese oxide Mn2O3 (without pre-sodiation) is synthesized by following the same method, but annealing at 400 °C. While P2-Na2/3MnO2 exhibits a high specific capacitance of 234 F g-1, Mn2O3 can deliver only 115 F g-1 when cycled at 0.4 A g-1 in an aqueous electrolyte of 1.0 M Na2SO4 in a three-electrode cell. An asymmetric supercapacitor Na2/3MnO2‖AC is assembled, which can exhibit a SC of 37.7 F g-1 at 0.1 A g-1 with an energy density of 20.9 W h kg-1, based on the total weight of Na2/3MnO2 and AC with an operational voltage of 2.0 V and possesses excellent cycling stability. This asymmetric Na2/3MnO2‖AC supercapacitor can be cost-effective considering the high abundance, low-cost and environmental friendliness of Mn-based oxides and aqueous Na2SO4 electrolyte.
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