Efficient photoelectrochemical (PEC) systems rely on cocatalysts integrated with light-absorbing semiconductors, where performance depends on effective charge transfer across the semiconductor/cocatalyst/solution interfaces. However, directly probing these interfacial processes under operando conditions, and elucidating how cocatalysts promote interfacial charge transfer, remains challenging. Here, we directly monitor electrochemical potentials at SrTiO3/CoOOH/water interfaces during PEC operation, providing mechanistic insights into cocatalyst function. We show that hole transfer into CoOOH dynamically increases its electrochemical potential, establishing an adaptive junction at the SrTiO3/CoOOH interface and enhancing the system photovoltage. Further tracking charge transfer to the solution, we find that CoOOH increases the water oxidation rate constant by a factor of 1.8 at surface potentials below the thermodynamic water-oxidation potential, corresponding to a population-controlled regime. More importantly, at surface potentials above the water-oxidation potential, CoOOH induces a transition to a Butler-Volmer-controlled regime, in which water oxidation is catalyzed with an exponentially increasing rate constant. These findings provide fundamental understanding and mechanistic insights into the role of cocatalysts in facilitating charge transfer across multiple interfaces during PEC operation.
Zhang et al. (2026) studied this question.