Water Oxidation on Oxygen-Deficient Barium Titanate: A First-Principles Study

We present a study of the effects of oxygen vacancies (O_vac) on the oxygen evolution reaction (OER) on the TiO₂-terminated (001) surface of cubic BaTiO₃ (cBTO-TiO₂) using spin-polarized DFT+U calculations and the standard (cation-based) four proton-coupled-electron-transfer methodology. We find that the excess electrons associated with O_vac's are involved in charge transfer (CT) to the intermediate adsorbate species HO∗, O∗, and HOO∗ and/or new surface oxygen hole states that we identified. The CT is responsible for an increase in these species' binding energies to the oxygen-deficient surface (cBTO-TiO_2-x) to an extent consistent with their electronegativity. The much stronger stabilization of HO∗ and O∗ compared to HOO∗ results in an increased overpotential η^OER on the reduced oxide. This result is at odds with experiment that shows a significantly increased efficiency for oxygen-deficient BTO, suggesting that a different mechanism and/or surface must be involved under the experimental conditions. We also identify heretofore unreported HO∗ and O∗ intermediate adsorbate structures whereby these species oxidize the surface and a surface oxygen hole is formed adjacent to the adsorption site. We assign the facile surface oxidation to the 2-fold coordination of the surface oxygen atoms in Ti-O-Ti surface moieties and a resulting low work function.