2-Methylimidazole assisted binder-free synthesis approach for hexagonal nickel-cobalt mixed oxide nanoplate as high-rate capability electrode for supercapacitor

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A binder-free synthetic approach employing a hydrothermal route followed by heat treatment has been utilized to synthesize nickel-cobalt mixed oxide. The as-prepared mixed oxide exhibits a hexagonal nanoplate morphology. It was deduced that the precursor ratios and the annealing medium significantly affected the electrochemical behavior of the respective mixed oxide sample. The binary oxide with equimolar nickel and cobalt annealed at 300 °C in an inert atmosphere exhibits an optimum specific capacitance of 1058 F g−1 at 1 A g−1 with capacity retention of 91 % at 10 A g−1. The sample shows excellent cycling stability of ∼100 % after 10,000 cycles, maintaining 95 % coulombic efficiency. The assembled asymmetric supercapacitor device with nickel-cobalt mixed oxide as the cathode and reduced graphene oxide as the anode achieved a specific capacitance of 197.5 F g−1 at 1 A g−1, retaining 63.2 % at 10 A g−1. The asymmetric supercapacitor device exhibits ∼100 % capacitance retention after 10,000 cycles, with a maximum specific energy of 17.5 Wh·kg−1 at a specific power of 800 W·kg−1.