Zn _1−x Ni _x Fe ₂ O ₄ Electrodes for Symmetric Supercapacitors: Sustained Electrochemical Stability With Multiple Charge Storage Pathways

ABSTRACT The electrochemical performance of ZnFe 2 O 4 is enriched at 50% Ni substitution in Zn sites with a specific capacitance of 209 F/g. Lindstrom's power law and Dunn's model studies in cyclic voltammograms of Zn 1−x Ni x Fe 2 O 4 electrodes reveal that charge storage in the electrodes occurs through redox reactions involving the intercalation of electrolyte ions deep into the electrodes (intercalation pseudocapacitance) and the adsorption of electrolyte ions on the surface or near the surface of the electrodes (redox pseudocapacitance), simultaneously. This enhancement in specific capacitance and contribution of intercalation and redox pseudocapacitance is the cumulative involvement of various facets such as: (i) crystallite/grain size, (ii) electronic transitions, (iii) availability of active sites, and (iv) the ion diffusion pathways in the electrodes. The variation in crystallite size with respect to Ni substitution has been evaluated through quantitative analysis of XRD patterns. Quantification of XPS spectra reveals the existence of multivalent states of metal ions and the oxygen vacancies, which influence the active sites and ion diffusion pathways in the electrode. Qualitative analysis of the optical properties in the UV–Vis–NIR region divulges that Zn 1−x Ni x Fe 2 O 4 electrodes exhibit several electronic transitions, such as ligand to metal transitions (LMCT) between oxygen and metal ions, intervalence charge transitions (IVCT) between (Fe 2+ /Ni 2+ ) and Fe 3+ ions, and crystal field (CF) transitions occurring in Fe 3+ , Fe 2+ , and Ni 2+ ions. The Ni 2+ incorporation affects the gap between the electronic states involved in each transition by creating defect states between them. This, in turn, impacts the faradaic charge transfer between the electrolyte ions and electrodes during intercalation and adsorption of electrolyte ions. To evaluate practical performance, a symmetric 2‐electrode cell was assembled using Zn 0.5 Ni 0.5 Fe 2 O 4 electrodes, which had shown superior specific capacitance in the three‐electrode assembly. The symmetric cell exhibits 116% cyclic stability after 10 000 cycles, and delivers an energy density of 5 Wh/kg and a power density of 2050 W/Kg. This highly capacitive retention nature of Ni‐substituted ZnFe 2 O 4 electrodes makes them potential candidates for energy storage devices, and the exhibition of multiple pseudocapacitive charge storage mechanisms delivers high energy and power density.