Effect of Ni Substitution on the Structural and Electrochemical Properties of ZnFe2O4 for Supercapacitor Applications

Abstract

Ni_xZn_1-xFe₂O₄ (x = 0, 0.3, 0.5) nanoparticles were synthesised via a sol-gel auto-combustion method to examine the effect of Ni substitution on ZnFe₂O₄, with emphasis on their structural, optical, morphological, compositional, oxidation state and electrochemical properties. PXRD analysis confirmed the formation of a single-phase cubic spinel structure with crystallite size from 39.74 to 36.06 nm, while UV-Vis analysis showed that the bandgap energy gradually decreases with increasing nickel concentration. FTIR and Raman spectra validated the characteristic M–O vibrational modes of the spinel structure. FESEM studies revealed particle-like morphologies with moderate clusters, and a decrease in average NP size from 48 nm to 41 nm as the Ni content increased. EDX and XPS analyses verified the elemental composition and oxidation states without the presence of impurity phases. Electrochemical studies demonstrated pronounced pseudocapacitive behaviour, with the Ni_0.5Zn_0.5Fe₂O₄ electrode delivering a maximum specific capacitance of 250 F g^–1 at 1 A g^–1. Furthermore, it exhibited an energy density of 22.31 Wh kg^–1 and a power density of 400 W kg^–1, along with reasonable cycling stability, retaining 91.54% of its initial capacitance after 200 cycles. Electrochemical impedance spectroscopy indicated reduced charge-transfer resistance and improved ion diffusion kinetics upon Ni substitution. Among all compositions, the x = 0.5 sample demonstrated the most balanced electrochemical performance, highlighting its potential as an efficient electrode material for high-performance supercapacitor applications.