Unusual formation of NiCo2O4@MnO2/nickel foam/MnO2 sandwich as advanced electrodes for hybrid supercapacitors

Abstract

A facile three-step method is designed for large-scale preparation of a NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwich architecture with robust adhesion as an advanced electrode for high-performance supercapacitors. The synthesis contains the hydrothermal reaction of a cobalt–nickel hydroxide precursor on a nickel foam (NF) support and subsequent thermal conversion into spinel mesoporous NiCo₂O₄ nanowire arrays, followed by a hydrothermal oxidation reaction to synthesize NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches. Moreover, the tactics reported in this study enable easy control of the growth of NiCo₂O₄ on one side of the NF and MnO₂ nanosheets on both sides of the NF to obtain novel NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches. Because of the unusual structural and compositional features, the obtained NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches manifest excellent performance with high specific capacitance (1.70 C cm⁻² at 2 mA cm⁻²), exceptional rate capability (78.5% retention at 20 mA cm⁻²) and ultralong cycling stability (91% retention over 30 000 cycles at 20 mA cm⁻²) as a battery-type electrode material for supercapacitors. When further assembled into an aqueous hybrid supercapacitor, it can deliver an energy density of 53.5 W h kg⁻¹ at a power density of 80 W kg⁻¹ and 20.7 W h kg⁻¹ at 8 kW kg⁻¹. This novel sandwich electrode provides a new idea for improving the electrochemical performance of hybrid supercapacitors.