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 (NF) 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 1000 illumination cycles. Density functional theory with the on-site Hubbard U correction (DFT + U) calculations further confirmed that nickel substitution in the Co₃O₄ matrix significantly reduces the bandgap and enhances the 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.