Understanding Surface Redox Reactions in Photocatalytic Overall Water Splitting with Pt/SrTiO₃ in the Presence of Oxygen: A Modeling Approach

Understanding nanoscale ion transport and interfacial redox chemistry in photocatalysis is essential for improving solar-to-hydrogen (STH) efficiencies. Herein, a continuum electrochemical model is developed that couples the Nernst–Planck and Poisson equations to resolve proton, hydroxide, buffer-ion, and oxygen transport on the nanoscale at (unprotected) Pt-decorated semiconductor surfaces during photocatalytic overall water splitting (POWS) in oxygen-saturated electrolytes under steady-state conditions. For unbuffered near-neutral electrolytes, asymmetric pH gradients develop rapidly and impose strong Nernstian penalties at both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) sites for reaction rates >∼20 μmol cm⁻² h⁻¹ (∼1% STH). Buffering of the electrolyte mitigates polarization losses, and proton availability for efficient HER is maintained. Importantly, dissolved oxygen accumulates near the Pt HER site resulting in local concentration differences, of both protons and oxygen, increasing the local HER overpotential. Pt particle diameter and HER-OER site spacing are the relevant factors to influence performance. Finally, the simulations indicate that a cocatalyst diameter of 27 nm and a redox-site separation of 10–20 nm, where the effects of pH polarization and oxygen back-diffusion remain minimal, and transport and selectivity are well balanced, result in the optimal POWS. Thus, by correlating electrochemical transport and reactivity at the nanoscale to semiconductor photophysics, design criteria for the efficient design and arrangement of cocatalyst nanoparticles in POWS utilizing unprotected Pt cocatalysts are established.