Supersonically sprayed self-aligned rGO nanosheets and ZnO/ZnMn2O4 nanowires for high-energy and high-power-density supercapacitors

Abstract

• ZnO nanospheres and ZnMn 2 O 4 nanowires were produced by a hydrothermal method for supercapacitor applications • The MnSO 4 concentration was varied from 0.05 to 0.15 mol/L to identify the electrode with the highest capacitance. • The optimal electrode exhibited a specific capacitance of 276.3 mF cm −2 at a current density of 0.5 mA cm −2 . • The highest energy density was 98.2 μWh cm −2 at a power density of 1600 μW cm −2 . Core-shell-type bimetallic oxide and carbon composites comprising zinc oxide (ZnO) nanospheres and zinc manganese oxide (ZnMn 2 O 4 ) nanowires were produced by a hydrothermal method, and supersonically sprayed together with reduced graphene oxide (rGO) nanosheets onto Ni foil to fabricate flexible supercapacitors. The supersonic impact facilitated the exfoliation of the rGO nanosheets, thereby increasing the surface area and adhesion of the composite particles to the substrate. The rGO nanosheets were vertically aligned during the supersonic impact and formed localized zones, enabling optimal accommodation of the ZnO/ZnMn 2 O 4 particles. This localization, with the addition of rGO, reduced the agglomeration of ZnO/ZnMn 2 O 4 particles. The molar concentration of MnSO 4 used in the synthesis of ZnO/ZnMn 2 O 4 was varied from 0.05 to 0.15 mol/L to determine the optimal MnSO 4 concentration that would result in the highest energy storage capacitance. The unique nanostructure of ZnO/ZnMn 2 O 4 and the self-alignment of rGO sheets facilitated a favorable environment for high energy storage capability with a specific capacitance of 276.3 mF cm −2 at a current density of 0.5 mA cm −2 and an energy density of 98.2 μWh cm −2 at a power density of 1600 μW cm −2 . The width of the potential window was increased to 1.2 V, implying a significant increase in the energy storage capability of the supercapacitor. Capacitance retention of 88% was achieved after 10,000 charge/discharge cycles for the supercapacitor fabricated using an optimal MnSO 4 concentration (0.10 mol/L) during the composite synthesis.