The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels. OER is a 4-hole process leading to the oxidation of two water molecules and the release of molecular oxygen and is considered the bottleneck of water splitting. While the mechanism of this reaction is still debated, recent measurements on semiconducting oxides have shown that the dependence of the rate of OER on the surface hole density is a power law, suggesting a multihole mechanism via surface hole accumulation. This is reminiscent of the mechanism for OER promoted by the oxygen evolving complex in Photosystem II and in stark contrast with metallic oxides like IrO₂, where the dependence is exponential. In this talk I will discuss our recent findings where, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we try to shed light on the origin of this behavior in hematite, -Fe₂O₃. We find an approximate third order behavior of the OER rate on the surface hole density. We propose a model where the reaction proceeds through a sequence of 1-electron oxidations of surface hydroxy groups leading to surface hole accumulation at oxygen sites. The key O-O bond formation proceeds through a chemical step, the dissociative chemisorption of a hydroxide ion at an oxyl active site in the presence of two nearby surface holes, leading to the direct formation of the superoxo intermediate via a three-hole step. We find that, at variance with the IrO₂ case, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law.

Contact : Mariachiara Pastore