Unveiling Charge Dynamics in High-Performance Binder-Free Photo-Rechargeable Supercapacitors

Abstract

In this study, binder-free nickel cobalt oxide (NiCo2O4) nanowire arrays with a cubic spinel structure were directly grown on nickel foam via an in-situ hydrothermal process. The resulting one-dimensional nanowires exhibited a uniform morphology and a favourable bandgap of approximately 1.67 eV, making them ideal candidates as electrode materials for photo-assisted supercapacitors. Electronic structure analysis revealed the coexistence of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ redox pairs, significantly enhancing electrochemical kinetics and facilitating efficient photo-assisted charge storage. Under illumination, the NiCo2O4@NF nanowires demonstrated a remarkable 54% increase in areal capacitance, from 570 to 880 mF cm⁻² at 15 mA cm⁻², attributed to the efficient separation and storage of photo-generated charges driven by surface polarization effects. An asymmetric supercapacitor device was fabricated with activated carbon (AC) as the anode and NiCo2O4@NF nanowires as the photoactive cathode, maintaining 88% capacitance retention after 1,000 illumination cycles. Density Functional Theory (DFT+U) calculations further confirmed that nickel substitution in the Co3O4 matrix significantly reduces the bandgap and enhances magnetic moment, supported by asymmetric spin-resolved density of states and band structure analyses. This research provides valuable insights for developing next-generation photo-assisted energy storage solutions.