Electrolyte‐Dependent Charge Storage in ZnFe 2 O 4 : Insights Into Fe Mixed‐Valence Electronic Structure via Near‐Edge X‐Ray Absorption Fine Structure ( NEXAFS ) and Symmetric Supercapacitor Performance

ABSTRACT This study establishes a direct correlation between Fe mixed‐valence electronic structure and electrolyte‐dependent charge‐storage behavior in ZnFe 2 O 4 . ZnFe 2 O 4 were synthesized via a co‐precipitation route and systematically investigated for their structural, electronic, and electrochemical properties. Near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy at the Fe L 3,2 ‐ and O K‐edges revealed a partially inverse spinel structure with mixed Fe 2+ /Fe 3+ valence states, where linear combination fitting indicated ~74.4% Fe 3+ and ~25.6% Fe 2+ . The enhanced Fe‐O hybridization arising from this mixed‐valence configuration is beneficial for charge transport. Electrochemical performance of the ZnFe 2 O 4 electrode was evaluated in a three‐electrode configuration using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy in 1.0 M KOH and 1.0 M Na 2 SO 4 electrolytes. The ZnFe 2 O 4 electrode exhibited significantly superior capacitive behavior in Na 2 SO 4 , delivering a high specific capacitance of 692.2 F/g at 1 A/g and an energy density of 96.13 Wh/kg. Kinetic analysis revealed a mixed charge‐storage mechanism dominated by surface‐controlled capacitive processes. Furthermore, the electrode demonstrated excellent cyclic stability with ~91% capacitance retention after 2000 charge–discharge cycles. The enhanced electrochemical performance in Na 2 SO 4 is attributed to improved ion transport, reduced interfacial resistance, and greater electrochemical stability. When employed as an electrode in a symmetric supercapacitor device, ZnFe 2 O 4 delivered a maximum specific capacitance of 201.28 F/g at a current density of 0.25 A/g, along with a high energy density of 71.57 Wh/kg at a power density of 400 W/kg. Furthermore, the SSCD exhibited notable cycling stability, retaining 46.93% of its initial capacitance after 10 000 galvanostatic charge–discharge cycles at a current density of 2.5 A/g. These results highlight ZnFe 2 O 4 as a promising electrode material for high‐performance supercapacitor applications, particularly in neutral electrolytes.