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

Polarons are known to play a crucial role in determining the (photo)electrocatalytic activity of semiconductors. Traditionally polarons are introduced ex situ and irreversibly through doping or defect engineering during catalyst synthesis. In this work, we present a fundamentally different approach by introducing polarons in situ in a reversible manner, through the application of an external electrode potential. This study investigates the potential-dependent polaron formation and its impact on electrocatalysis, specifically focusing on a prototypical TiO2 semiconductor electrode for the acidic hydrogen evolution reaction (HER). By combining grand canonical ensemble density functional theory (GCE-DFT) calculations with multiple (in situ spectro)electrochemical experiments, we demonstrate notable changes in TiO2´s electronic structure driven by the reduction of Ti4+ to Ti3+ surface polarons at reducing potentials. Our results show that potential-dependent polaron formation creates highly active sites for HER, breaks down the linear relationship between adsorption energy and electrode potential, and leads to complex electrochemical reaction kinetics. We also discuss how the in situ polaron generation can be leveraged to design more active semiconductor (photo)electrodes. Overall, our findings provide compelling evidence and atomistic understanding on the pivotal role of potential-dependent polaron formation in semiconductor (photo)electrocatalysis.