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

Core-shell-type bimetallic oxide and carbon composites comprising zinc oxide (ZnO) nanospheres and zinc manganese oxide (ZnMn₂O₄) 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/ZnMn₂O₄ particles. This localization, with the addition of rGO, reduced the agglomeration of ZnO/ZnMn₂O₄ particles. The molar concentration of MnSO₄ used in the synthesis of ZnO/ZnMn₂O₄ was varied from 0.05 to 0.15 mol/L to determine the optimal MnSO₄ concentration that would result in the highest energy storage capacitance. The unique nanostructure of ZnO/ZnMn₂O₄ 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 MnSO₄ concentration (0.10 mol/L) during the composite synthesis.