This study investigates the origin of the multihole oxygen evolution reaction (OER) mechanism in haematite photoanodes using transient photocurrent measurements, density functional theory (DFT) simulations, and microkinetic modelling. The OER rate shows a third-order dependence on surface hole density, explained by a mechanism involving sequential one-electron oxidations of surface hydroxy groups, with O-O bond formation through dissociative hydroxide ion chemisorption involving three oxyl sites. Unlike metallic oxides, the activation energy of this step is weakly dependent on surface hole coverage, resulting in the observed power law.

Giulia Righi, Julius Plescher, Franz-Philipp Schmidt, R. Kramer Campen, Stefano Fabris, Axel Knop-Gericke, Robert Schlögl, Travis E. Jones, Detre Teschner, Simone Piccinin