Morphology-dependent solvothermal synthesis of spinel NiCo2O4 nanostructures for enhanced energy storage device application

Spinel NiCo2O4 is a remarkable electrode material for enhanced energy storage devices due to its unique tunable structure and shape. Herein, we have demonstrated a facile solvothermal approach to synthesize various nanostructures such as nanoparticles, nanoflowers self-assembled with nanoflakes, nanorods, and nanosheets of spinel NiCo2O4. The influence of solvents (such as water, ethanol, water-ethanol, and ethylene glycol etc.), urea, the molar concentration of reactant precursors Ni(NO3)2·6H2O and Co(NO3)2·6H2O, and calcination temperatures are varied for designing the various nanostructures of NiCo2O4. Further all electrode materials are tested for supercapacitor application. Among all as-synthesized nickel cobaltites, NiCo2O4 nanoflowers (NC-2) is found to be the best electrode materials for supercapacitor application with a high specific capacitance (415 F/g at 2 mV/s scan rate and 144.3 F/g at 1 A/g current density) under 1 M Na2SO4 electrolyte using three-electrode system. The higher specific capacitance of NiCo2O4 nanoflowers is attributed to its larger surface area, the higher number of active sites, and lower charge transfer resistance. Moreover, the NiCo2O4 nanoflowers electrodes were further fabricated for a two-electrode symmetric supercapacitor device and showed a specific capacitance of 80.2 F/g at 1 A/g current density and exhibit higher energy density of 17.4 Wh/kg with a power density of 329.7 W/kg. The fabricated symmetric device of NiCo2O4 nanoflowers displays outstanding durability upto 10,000 continuous charge-discharge cycles at 5 A/g.