Therefore, we addressed the following Heyrovsky step (H* + H3O + + e -= H2 + H2O), which was found to have very high reaction barriers on pristine TiO2 (101) even under highly reducing conditions where HER is experimentally seen to take place. Consequently, the Heyrovsky step is kinetically unfeasible on the ideal surface and limits HER activity due to the strongly bound surface proton. In the presence of single oxygen vacancy (VO-TiO2 forming two Ti 3+ polarons), we studied the Volmer step to the VO-site and found that this step has very high activation energies at all consider potentials (Table ). Hence, the Volmer step limits the HER activity of VO-TiO2.