Understanding surface charging limitations of hematite photoanodes through combining cathodic discharge measurements and computational modeling

We investigated the oxygen evolution reaction and recombination on Zn-doped, Sn-doped, and undoped hematite ( α -Fe 2 O 3 ) (0 0 0 1) surface, as a model system for earth abundant, chemically stable photoanode material for hydrogen production by light driven water splitting. Charge transfer at the photoanode/water interface was previously found to play a significant role in determining the water oxidation efficiency. However, the fine details of the oxygen evolution reaction, recombination, and surface charging mechanisms remain elusive. In this work we combine preconditioned linear sweep voltammetry (LSV) measurements on well-defined heteroepitaxial hematite films with Density Functional Theory (DFT) + U ab-initio calculations to get atomic level understanding of the oxygen evolution reaction and recombination mechanism and kinetic limitations. From preconditioned LSV measurements, we get the unstable charge which contributes to surface recombination and the stable charge which contributes to the reaction. In agreement with the unstable and stable charges measured in the preconditioned LSV, the density of states for the Zn-doped sample shows larger amount of oxygen mid gap states than in the Sn-doped sample, which contribute to more stable charge for the reaction and to less unstable charge for recombination. The number of calculated reaction steps involving discharge under cathodic bias for doped and pure hematite is also in agreement with the number of peaks observed in the preconditioned LSV measurement. We find that the LSV rate dependency is correctly described only after polaronic effects are accounted in the charge transfer (CT) rate equation using the Marcus CT model. By fitting the revised rate equation we extracted the surface recombination rate for differently doped hematite films, finding the following trend: Zn-doped < undoped < Sn-doped. This insightful association between conventional LSV experiment and DFT provides a means to identify the limiting steps of surface charging in electrochemistry.