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.