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

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.