Potential-dependent polaron formation activates TiO2 for the hydrogen evolution reaction

Polarons and defects are recognized to be crucial in semiconductor (photo)electrocatalysis, yet their precise impact on charge transfer and electrocatalytic activity has remained elusive. In this study, we investigated the influence of potential-dependent polaron and defect formation on a prototypical TiO2 semiconductor for a hydrogen evolution reaction (HER), combining grand canonical ensemble density functional theory (GCE-DFT) calculations with multiple (spectro)electrochemical experiments. Our joint experimental and theoretical exploration unveiled notable changes in the TiO2 electronic structure with the application of an electrode potential resulting in the reduction of Ti4+ to Ti3+ surface polarons at reducing electrode potentials. We demonstrate that potential-dependent polaron formation creates highly active sites for HER, enhances conductivity, and breaks the Butler-Volmer-like kinetics highlighting the qualitatively different behavior between semiconducting TiO2 and metallic electrodes. Overall, our findings furnish compelling evidence and atomistic understanding of the pivotal role that potential-dependent polaron formation plays in semiconductor (photo)electrocatalysis.