Partially substituted magnetic ferrites as Pt-free catalysts for hydrogen evolution from water photo-electrolysis

The global energy landscape, still heavily dependent on fossil fuels, has intensified the search for sustainable alternatives. Specifically, the interest is focused on a clean energy source such as hydrogen, through the development of cost-effective and efficient photo-electrocatalysts based on earth-abundant elements. In this context, magnetic spinel ferrites have attracted attention as low-cost, Pt-free catalysts for the photo-electrocatalytic hydrogen evolution reaction (HER). In this work, a series of spinel ferrites, CuFe₂O₄, CoFe₂O₄, ZnFe₂O₄, and partially substituted CuFe₂O₄ with Co²⁺ or Zn²⁺ (CuXCoYFe or CuXZnYFe, where X is the % of Cu, and Y is the % of substituting cation) were synthesized via a soft-chemistry co-precipitation route and systematically evaluated as photo-electrocatalysts for HER in neutral media. Structural and morphological analyses confirmed the formation of single-phase spinel with tunable lattice parameters and crystallite sizes. SEM revealed a progressive morphology transition from anisotropic structures to compact particles in Co- and Zn-substituted systems, while magnetic characterization evidenced superparamagnetic behavior in all samples, with Co substitution enhancing the saturation magnetization and Zn substitution modulating the saturation magnetization depending on the concentration. Electrochemical and photo-electrochemical testing demonstrated that controlled Co²⁺ and Zn²⁺ doping significantly improved charge transfer, HER activity, and selectivity. Notably, in photo-electrocatalytic experiments, Cu95Zn5Fe achieved the highest hydrogen production (304.3 mmol g^-1) with a Faradaic Efficiency of 94%, while Cu75Co25Fe reached nearly 100% efficiency in the same conditions. Electrochemical impedance spectroscopy further confirmed reduced interfacial resistance and improved charge transport in the substituted ferrites. These findings highlight the potential of tailored magnetic spinel ferrites for efficient, sustainable hydrogen generation and provide design guidelines for optimizing photo-electrocatalytic performance by controlling Co²⁺ and Zn²⁺ content into CuFe₂O₄.