Unlocking superior energy storage in ZnFe₂O₄ nanospheres via advanced electrode design: An integrated experimental and computational approach

Zinc ferrite (ZnFe₂O₄) is a compelling electrode material for supercapacitors due to its high theoretical capacitance, magnetic behaviour, and environmental abundance. However, its practical application has been limited by low intrinsic conductivity and poor structural stability, challenges often exacerbated using polymer binders. This study presents a binder and carbon-free electrode fabrication approach by directly depositing ZnFe₂O₄ Nanospheres (NS) onto Ni foam, resulting in superior electrochemical performance. Particle size was effectively controlled by tuning the surfactant concentration during hydrothermal synthesis. The optimized sample, synthesized with 20 % surfactant, exhibited a remarkable specific capacitance of 1819.6 F/g at 10 A/g and outstanding cycling performance, retaining 121.9 % of its initial capacitance after 2500 cycles. To understand the underlying charge storage mechanisms, Density Functional Theory (DFT) simulations were performed on bulk ZnFe₂O₄ as well as its (111) and (311) surface terminations, chosen to reflect the material's nanoscale morphology, where surface interactions dominate. The results showed that potassium (K⁺) adsorption from the KOH electrolyte introduces new energy states near the Fermi level, facilitating improved charge transfer. Additionally, the presence of oxygen vacancies, detected via Electron Paramagnetic Resonance (EPR), significantly enhanced redox activity. Magnetic characterization at both macro and nanoscale levels further revealed that variations in magnetic properties correlated with vacancy concentrations and electrochemical performance. This work provides comprehensive insight into how surface engineering, defect chemistry, and magnetic interactions collectively enhance the energy storage capabilities of ZnFe₂O₄ nanostructures.