Synergistic zinc-iron oxide composites for enhanced hydrogen evolution in alkaline electrolysis

Developing affordable and efficient electrocatalysts for the hydrogen evolution reaction (HER) is essential for sustainable hydrogen production. In this work, zinc-iron oxide composites (Zn x Fe 3-x O) with varying Zn:Fe ratios (0:1, 1:3, 1:1) were synthesized via a facile hydrothermal method and evaluated as HER catalysts in alkaline medium (1 M NaOH). Rietveld refinement of XRD data reveals that the optimal composite (ZnFe 0.75 ) consists of 50.5 wt% ZnFe₂O₄ spinel and 49.5 wt% α-Fe₂O₃ hematite, forming a heterojunction structure. HR-TEM directly visualizes lattice fringes of 0.254 nm (ZnFe₂O₄ (311)) and 0.270 nm (α-Fe₂O₃ (104)) within the same nanoparticle agglomerate, confirming intimate contact between phases. XPS analysis demonstrates exclusive Zn²⁺ and Fe³ ⁺ oxidation states with a substantial oxygen vacancy concentration of 38%, induced by Zn incorporation. Electrochemical measurements show that ZnFe 0.75 achieves an overpotential of 443 mV at 10 mA cm⁻², outperforming pure Fe (468 mV) and ZnFe 0.5 (558 mV). The enhanced performance arises from quantified synergy: the observed overpotential is 70 mV lower than the weighted average of the pure phases. Tafel analysis (72 mV dec⁻¹) suggests a Volmer-Heyrovsky mechanism modulated by oxygen vacancy-induced site heterogeneity. The catalyst exhibits excellent stability with only 28 mV degradation after 1000 cycles, and post-stability characterization confirms preserved morphology and composition. This work provides a comprehensive mechanistic understanding of Zn-induced activation of iron oxides, establishing design principles for earth-abundant HER electrocatalysts.