Oxygen Evolution at Hematite Surfaces: The Impact of Structure and Oxygen Vacancies on Lowering the Overpotential

Abstract

Simulations of the oxygen evolution reaction (OER) are essential for understanding the limitations of water splitting. Most research has focused so far on the OER at flat metal oxide surfaces. The structure sensitivity of the OER has, however, recently been highlighted as a promising research direction. To probe the structure sensitivity, we investigate the OER at 11 hematite (Fe 2 O 3 ) surfaces with density functional theory + Hubbard U (DFT+U) calculations. The results show that the O–O coupling (O–O bond formation via two adjacent terminal Os at dual site) OER mechanism at the (110) surface is competing with the mechanism of OOH formation at single site. We study the effects of surface orientation (110 vs 104), active surface sites (bridge vs terminal site), presence of surface steps and oxygen vacancy concentration on the OER and explore strategies to reduce the OER overpotential. It is found that the oxygen vacancy concentration is the most effective parameter in reducing the overpotential. In particular, an overpotential of as low as 0.47 V is obtained for the (110) surface with an oxygen vacancy concentration of 1.26 vacancies/nm 2 .