Preparation and electrochemical properties of nanostructured porous spherical NiCo₂O₄ materials

Porous spherical NiCo₂O₄ powders with a micro-nano structure were prepared by the spray drying method using citric acid as the chelating agent and soluble salts as cobalt and nickel sources. By calcination at 300 °C, spherical and single-phase spinel-type NiCo₂O₄ powders were obtained. The powders with particle sizes of 0.5-3 μm were aggregations of nano-sized grains (about 15-30 nm). The electrical property tests demonstrated that the synthesized NiCo₂O₄ has a specific capacitance of 430.67 F g^-1 at a current density of 1 A g^-1 and a capacitance retention rate of 100% after 3000 cycles at a current density of 4 A g^-1, indicating excellent cycling stability. On assembly into an asymmetric supercapacitor device, a specific capacitance of 37.06 F g^-1 and energy density of 13.18 W h kg^-1 at a power density of 800 W kg^-1 and a current density of 1 A g^-1 were demonstrated. The micro-nano structured porous NiCo₂O₄ powders have a larger specific surface area, which can allow the sample to come in full contact with the electrolyte. The nanopore channels are favourable for releasing the lattice distortion stress during the charge-discharge process, maintaining the structural stability of the crystal and improving the cycle life.