Coral-Shaped Bifunctional NiCo2O4 Nanostructure: A Material for Highly Efficient Electrochemical Charge Storage and Electrocatalytic Oxygen Evolution Reaction

Multifunctional materials are quite fascinating and conveniently serve the purpose where two or more efficient materials are required. Herein, we report such a bifunctional material which is new by its morphology and enables the provision of a reliable power output as a supercapacitor electrode as well as oxygen evolution in water splitting as an efficient electrocatalyst material. A coral-shaped NiCo2O4 nanostructure was developed by the oriented attachment pathway of nanocrystal building blocks, which can provide efficient energy storage and energy conversion bifunctional properties which are not realized earlier. Here, the less stable and highly reactive (111) planes of NiCo2O4 small single crystals grow at the expense of the (100) planes in the ⟨111⟩ direction to decrease the total interfacial free energy and get attached with each other to form the coral-shaped nanostructure. The outstanding battery-like capacitive features (e.g., maximum specific capacitance of 1297 F·g–1 or specific capacity of 180 mA·h·g–1, energy density of 45 W·h·kg–1 at high charge–discharge rates, and a minimum stability of 10,000 cycles) with a high Coulombic efficiency (>96%) are attributed to faster ion intercalation between an electrode and an electrolyte and redox pseudocapacitance, high conductivity, and highly porous coral morphology that decreases diffusion distances and exposure of mixed metal valence at the surface. In addition to these features, a higher Ni2+ content enhances the adsorption of OH– species on the material surface and a high electrochemically active surface area of the material attributed to a lower overpotential (0.29 V) and longer stability at a higher current density during precious metal-free electrocatalytic oxygen evolution reaction.