As both the pristine and VO-TiO2 exhibit high barriers for HER at experimentally relevant potentials, we considered the influence of a double oxygen vacancy (V2O-TiO2 forming four Ti 3+ polarons, Figure ) on the Volmer step and focused on the potential range between U = 0 VSHE and U = -1.3 VSHE. In this scenario, the H3O + molecule in the inner solvation layer donates a proton to a V2O site leading to the formation of V2O-H bridging two Ti atoms (Figure ). V2O accumulates more positive charge than that single oxygen vacancy, and thus hinders proton trapping as stable OHads species, as detailed in Figure . The Volmer reaction approaches thermoneutrality at U = 0 VSHE with the activation energy of 1.35 eV. By increasing the potential to the experimental onset potential of -0.3 VSHE the activation energy decreased to 1.0 eV, see Table . Notably the V2O-TiO2 (101) exhibits more favorable reaction kinetics than the VO-TiO2 (101) surface. We attribute this behavior to a stronger orbital hybridization role between H-1s and Ti4c-3d as a function of electrode potential below the Fermi energy on V2O-site than the VO-site, therefore facilitating the Volmer reaction, see detailed discussion in Figure .