Although aqueous asymmetric supercapacitors are promising technologies because of their high-energy density and enhanced safety, their voltage window is still limited by the narrow stability window of water. Redox reactions at suitable electrodes near the water splitting potential can increase the working potential. Here, we demonstrate a kinetic approach for expanding the voltage window of aqueous asymmetric supercapacitors using in situ activated Mn3O4 and VO2 electrodes. The underlying mechanism indicates a specific potential of ∼1 V vs Ag/AgCl for the oxidation of Mn4+-to-Mn7+ at the positive electrode and ∼ -0.8 V vs Ag/AgCl for the reduction of V3+-to-V2+ at the negative electrode, which limits oxygen and hydrogen evolution reactions, respectively. The as-fabricated aqueous asymmetric supercapacitor exhibited a working voltage of 2.2 V with a high-energy density of 42.7 Wh/kg and a power density of ∼1.1 kW/kg. This mechanism improves the voltage window and energy and power densities.
Keywords: aqueous asymmetric supercapacitor; energy density; power density; voltage window; water splitting potential.