Cu-substituted NiCo2O4 binder-free electrodes for high-performance bifunctional energy storage and oxygen evolution reaction

Nickel cobaltite (NiCo₂O₄) has emerged as a promising pseudocapacitive electrode material owing to its mixed-metal spinel structure and rich redox chemistry; however, its practical electrochemical performance is often limited by moderate conductivity and insufficient utilization of active sites. In this work, binder-free Cu-substituted NiCo 2 O 4 electrodes were successfully fabricated via a simple and scalable reflux condensation method, and the influence of Ni/Cu molar ratios on structural, morphological, and electrochemical properties was systematically investigated. Structural and microscopic studies revealed the formation of a cubic spinel structure with phase purity and an average particle size of 20 nm, providing enhanced electroactive surface area and shortened ion diffusion pathways. Electrochemical studies demonstrated that the optimized Cu-substituted NiCo 2 O 4 electrode delivered a high specific capacitance of 1406 F g −1 at a current density of 1 A g −1 , along with excellent cycling stability. Electrochemical kinetic analysis indicated that the charge storage process is predominantly diffusion-controlled, with a b-value of 0.46 and 0.51, and 97.8% diffusion contribution at 5 mV s −1 , highlighting the role of faradaic redox reactions. The optimized electrode showed better electrocatalytic activity towards OER with a minimal overpotential of 360 mV to achieve a current density of 25 mA cm −2 . The enhanced performance is attributed to synergistic cation substitution, improved electronic conductivity, and the binder-free electrode architecture. This study provides a viable approach for designing high-performance spinel oxide electrodes for advanced supercapacitor and OER applications.