Exploring Properties of Spinel‐Structured Co‐Doped ZnFe ₂ O ₄ for Hydrogen and Oxygen Evolution and Supercapacitor Applications

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 x Zn (1 − x ) Fe 2 O 4 , 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 Co 2+ 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 ZnFe 2 O 4 with Co 0.3 Zn 0.7 Fe 2 O 4 , 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 0.2 Zn 0.8 Fe 2 O 4 catalyst exhibited superior HER and OER performance, with an overpotential of 115 and 288 mV, respectively, at 10 mA/cm 2 . These improvements are attributed to enhanced electronic conductivity from partial Zn 2+ substitution by Co 2+ and synergistic effects within the spinel matrix. Overall, Co‐doped ZnFe 2 O 4 offers a promising and sustainable pathway for bifunctional applications in both high‐performance supercapacitors and water electrolysis.