TiO2 Protective Layers Shape Photocathode Performance in Photoelectrochemical CO2 Reduction

Photoelectrochemical carbon dioxide reduction (PEC CO2R) enables direct solar-to-chemical conversion but requires protective layers to stabilize photocathodes under corrosive conditions. 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.