Tunning cation occupancy in MgFe2O4 spinel and effect of interfacial voltage drop on catalytic efficiency in photoelectrochemical water splitting

Applying an external magnetic field during chemical vapor deposition of thin films can induce texturing effects that are beneficial in photoelectrochemical (PEC) water splitting. In this work, thin films of MgFe2O4 spinel were grown in a chemical vapor deposition (CVD) reactor operating under external magnetic field (0 – 1 T). The X-ray diffraction analyses suggested that the degree of inversion (structural disorder due to movement of cations from tetrahedral into octahedral holes) in MgFe2O4 can be controlled by varying the magnetic field intensity during CVD. The MgFe2O4 films with a normal spinel configuration show a broader absorption range in the UV-visible spectra compared to the inverse counterpart, indicating the potential tunability of degree of inversion for enhanced PEC water splitting performance. Despite extensive efforts to develop efficient photocatalysts by structural and compositional modulations, evaluation frameworks for PEC water splitting primarily rely on current-based parameters, such as the onset potential and current density at 1.23 V versus the reversible hydrogen electrode. Functional performance in PEC water splitting is typically evaluated using current density. However, such measurements often underestimate activity due to voltage losses arising from resistances (iR) within the PEC cell, compounded by the absence of standardized iR compensation protocols. In this work, the voltage drops associated with charge transfer (Vct) and pH gradient (VpH) voltage drops at the solid/liquid interface are quantitatively analyzed as direct indicators of PEC oxygen evolution performance. While open circuit potential (OCP) decay transients have previously been employed to monitor interfacial pH changes during hydrogen evolution reaction (HER) on a platinum electrode involving two electrons, this work extends the approach to the more complex oxygen evolution reaction (OER) on an illuminated TiO2 semiconductor photoanode involving four electrons. The individual roles of Vct and VpH, examined across various pH values and illumination intensities, reveal that Vct reflects intrinsic interfacial voltage loss under bias, whereas VpH serves as a complementary descriptor to photocurrent density for evaluating catalytic activity in PEC systems. This work demonstrates that the degree of inversion in MgFe2O4 can be systematically tuned by applying an external magnetic field, thereby linking cation disorder to improved spectral response. In addition, the quantitative evaluation Vct and VpH provides a diagnostic framework for assessing catalytic efficiencies beyond conventional current-based parameters, thus offering practical guidelines for the rational design and optimization of high performance PEC water splitting systems.