To grasp the atomic-scale mechanisms of HER on the TiO2 (101) surface, we studied the acidic HER thermodynamics and kinetics across a wide electrode potential range from 0 to -1.2 VSHE, see Tables and-S6. To elucidate the influence of polaronic state on HER activity, we initially focused on the first elementary step of HER, the Volmer reaction (H3O + + e -→ H* + H2O), on both pristine and VO -TiO2 (101) surfaces. As shown in Figures and and as discussed in the Supplementary section S3, the Volmer step is very facile and cannot therefore be the rate-limiting step.