TiO₂ protective layers shape photocathode performance in photoelectrochemical CO₂ reduction

Photoelectrochemical carbon dioxide reduction (PEC CO₂R) enables direct solar-to-chemical conversion. However, few photocathodes are intrinsically stable light absorbers. The application of protective layers remains a critical approach for stabilizing photocathodes in corrosive environments. TiO₂ is the most widely adopted stabilizer, yet its influence on catalytic performance remains poorly understood. Here, we examine TiO₂-protected Cu₂O photocathodes integrated with Au, Cu and Bi cocatalysts, combining experiments and simulations to unravel the role of the TiO₂ overlayer. We find that TiO₂ profoundly alters catalytic selectivity, suppressing CO and multicarbon (C₂ ⁺) pathways, while its impact on formate production with Bi, In and Sn cocatalysts is comparatively negligible. These results demonstrate that TiO₂ is not an inert stabilizer, but an active component that reshapes interfacial reaction pathways. This work establishes critical design principles for integrating protective layers with photocathodes to achieve selective and efficient PEC CO₂R.