The energy cost of water oxidation is mitigated by storing the oxidizing equivalents at different Mn sites, while simultaneously releasing protons to prevent charge accumulation, before performing the four-electron chemistry to circumvent stepwise water oxidation 4. The OEC undergoes a sequence of four light-induced oxidations whereby the Mn centres store four oxidizing equivalents before the formation of the O–O bond, the rate-limiting step, takes place 3. Remarkable advances in the experimental characterization of the structure of the OEC have permitted an atomic-level understanding of the sequence of steps leading to the formation of oxygen. In plants, the OER is catalysed by the oxygen-evolving complex (OEC) in photosystem II, a Mn 4CaO x cluster with a distorted cubane structure. The mechanistic similarity of these two routes is an open question 1, 2. This four-electron half reaction (2H 2O → O 2 + 4H + + 4e −, E° = 1.23 V versus the reversible hydrogen electrode (RHE)) is a key step in both natural photosynthesis and its synthetic counterpart. The oxygen evolution reaction (OER) is the bottleneck in the (photo)electrochemical splitting of water into molecular hydrogen and oxygen, an enabling process for the production of fuels from sustainable energy sources. At variance with the case of metallic oxides, the activation energy of this step is weakly dependent on the surface hole coverage, leading to the observed power law. The key O–O bond formation step occurs by the dissociative chemisorption of a hydroxide ion involving three oxyl sites.
We propose a mechanism wherein the reaction proceeds by accumulating oxidizing equivalents through a sequence of one-electron oxidations of surface hydroxy groups. We show here that the OER rate has a third-order dependence on the surface hole density. In this study, using transient photocurrent measurements, density functional theory simulations and microkinetic modelling, we have uncovered the origin of this behaviour in haematite. Recent measurements on semiconducting oxides have found a power law dependence of the OER rate on surface hole density, suggesting a multihole mechanism.
The oxygen evolution reaction (OER) plays a crucial role in (photo)electrochemical devices that use renewable energy to produce synthetic fuels.
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