Exploring Properties of Spinel-Structured Co-Doped ZnFe2O4 for Hydrogen and Oxygen Evolution and Supercapacitor Applications

Abstract

The eco-friendly, low-cost catalysts with high activity and stability are key to advancing efficient water splitting and energy storage for sustainable green energy. To address this, cobalt-doped zinc ferrite (Co_xZn_(1−x)Fe₂O₄, x = 0.0–0.4) nanoparticles were synthesized using hibiscus leaves extract via microwave-assisted solution combustion method. Structural, morphological, and optical analysis (HR-XRD, field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), UV-DRS, EDX, FTIR, and brunauer emmett teller (BET)) confirmed the successful incorporation of Co²⁺ ions into the spinel lattice. Rietveld refinement further verified the cubic spinel structure with the Fd-3m space group. Co-doping greatly enhanced the electrochemical performance of ZnFe₂O₄ with Co_0.3Zn_0.7Fe₂O₄, achieving the highest specific capacitance of 161.72 and 453.63 F/g at 1 A/g under different electrolyte concentrations, along with excellent cycling stability (87.18% after 1000 cycles). Solid-state symmetric supercapacitor based on this composition delivered 111.25 F/g with 86.93% retention after 3000 galvanostatic charge discharge (GCD) cycles, demonstrating favorable energy and power densities. In parallel, the optimized Co_0.2Zn_0.8Fe₂O₄ catalyst exhibited superior HER and OER performance, with an overpotential of 115 and 288 mV, respectively, at 10 mA/cm². These improvements are attributed to enhanced electronic conductivity from partial Zn²⁺ substitution by Co²⁺ and synergistic effects within the spinel matrix. Overall, Co-doped ZnFe₂O₄ offers a promising and sustainable pathway for bifunctional applications in both high-performance supercapacitors and water electrolysis.