Operando x-ray absorption spectroscopy unveils light-driven redox dynamics at the semiconductor/cocatalyst interface.

Cobalt-based mixed oxides are widely studied as oxygen evolution reaction (OER) catalysts, yet their role as photoelectrochemical cocatalysts remains debated due to scarce operando studies probing irradiation-induced structural changes. Here, we unveil redox dynamics of cobalt-iron oxide (CoFeO) cocatalysts in semiconductor photoanodes for solar water splitting. By combining operando x-ray absorption spectroscopy (XAS) with fixed-energy x-ray absorption voltammetry (FEXRAV) at semiconductor/cocatalyst interfaces, we provide an element-selective probe of Co oxidation states under dark and illuminated conditions. Our results reveal a previously unrecognized interfacial Co state, highlighting interface structure's role in tuning catalytic activity. We observe light-induced reduction in oxidation state and cathodic shift in Co redox potentials, offering insights into hole transfer and catalytic behavior. Identification of a specific photocatalytic cycle, distinct from dark-state electrocatalysis, advances understanding of how light modulates rate-determining steps in OER. These findings underscore the power of operando x-ray techniques in elucidating interfacial charge transfer and guiding design of more efficient photoelectrochemical systems.