  The influence of surface oxygen vacancies on the oxygen evolution reaction on
bismuth vanadate is studied using hybrid density functional theory. Our
findings reveal the thermodynamic instability of the neutral unionized defect
(${\rm V}_{\rm O}^0$), leading to spontaneous ionization into ${\rm V}_{\rm
O}^{2+}$. By investigating the oxygen evolution reaction mechanism on both
stoichiometric and oxygen-deficient surfaces, we find that surface oxygen
vacancies reduce the reaction's thermodynamic overpotential, but only when the
defects are ionized. Moreover, the reaction pathway involves the formation of
surface-bound peroxide and superoxide ions as intermediates. Our work provides
insight into the nature of surface oxygen vacancies and shines new light on how
they may enhance the photoelectrochemical properties of semiconductors.
