Oxygen vacancies and tailored redox activity encountered with NiCo₂O₄ nanostructures for promising applications in supercapacitor and water oxidation

It is well known that transition metal oxides play a significant role in the development of efficient electrochemical energy storage and conversion systems. However, their application is limited by low electrical conductivity, a limited number of electroactive sites, and limited stability. In this study, nickel‑cobalt bimetallic oxide (NiCo₂O₄) nanostructures were prepared using orange peel extract and its fruit juice during hydrothermal method. The use of green chemical components from orange peel extract and its fruit juice was to demonstrate their ability to manipulate NiCo₂O₄'s structural and functional properties. For this reason, the optimized 1 mL of orange peel extract and its fruit juice was found highly active for improving the electrochemical properties of NiCo₂O₄ nanostructures. The electrochemical supercapacitor performance of optimized NiCo₂O₄ nanostructures showed a specific capacitance of 997.50 Fg⁻¹ with a cycling stability of more than thirty thousand cycles, resulting in a capacitance retention rate of almost 100 % at 1.5 Ag⁻¹. In addition, during the oxygen evolution reaction (OER), the overpotential was 120 mV at 20 mA/cm² and the Tafel slope was 110 mVdec⁻¹, with a prolonged stability exceeding 45 h were observed for optimized NiCo₂O₄ nanostructures. With an electrode material comprising NiCo₂O₄ nanostructures and an orange peel extract, outstanding electrochemical performance was obtained. This could be explained by several factors, including high electroactive site exposure, rapid charge transfer kinetics, high hydroxide adsorption, oxygen vacancies on the surface, and abundance of Co(III). The approach described here can be used for the facile synthesis of a wide range of nanostructured functional materials that exhibit impressive electrochemical properties, particularly for energy storage and conversion applications.