Water oxidation is the key kinetic bottleneck of photoelectrochemical devices for fuel synthesis. Despite advances in the identification of intermediates, elucidating the catalytic mechanism of this multi-redox reaction on metal–oxide photoanodes remains a significant experimental and theoretical challenge. Here, we report an experimental analysis of water oxidation kinetics on four widely studied metal oxides, focusing particularly on haematite. We observe that haematite is able to access a reaction mechanism that is third order in surface-hole density, which is assigned to equilibration between three surface holes and M(OH)–O–M(OH) sites. This reaction exhibits low activation energy (Ea ≈ 60 meV). Density functional theory is used to determine the energetics of charge accumulation and O–O bond formation on a model haematite (110) surface. The proposed mechanism shows parallels with the function of the oxygen evolving complex of photosystem II, and provides new insights into the mechanism of heterogeneous water oxidation on a metal oxide surface.
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The complete optical and electrochemical dataset is available at http://zenodo.org with the identifier 10.5281/zenodo.851635.
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J.R.D. acknowledges financial support from the European Research Council (project Intersolar 291482) and H2020 project A-LEAF (732840). C.A.M. thanks COLCIENCIAS (call 568) for funding. L.F. thanks the EU for a Marie Curie fellowship (658270) and E.P. thanks the EPRSC for a DTP scholarship. V.S.B. acknowledges support from the Air Force Office of Scientific Research (AFSOR) grant no. FA9550-17-0198 and high performance computer time from the National Energy Research Scientific Computing Center (NERSC). P.G.B. acknowledges “la Caixa” foundation for the PhD grant. A.K. thanks Imperial College for a Junior Research Fellowship. M.G. acknowledges support from the Swiss National Science Foundation (project 140709) and Swiss Federal Office for Energy (project PECHouse 3; contract no. SI/500090–03). T.E.R. thanks the EPSRC for a DTC studentship and E.R. the Christian Doppler Research Association (Austrian Federal Ministry of Science, Research and Economy, and the National Foundation for Research, Technology and Development) and the OMV Group for financial support. M.T.M. acknowledges the Helmholtz Association’s Initiative and Networking Fund.
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Supplementary Figs. 1–16, Tables 1–3, experimental methods, density functional theory calculations, optical and photoelectrochemical characterization of the materials used, supporting data for the activation energy calculation as well as the kinetic isotope effect and the pH dependence studies and discussion of the first-order mechanism and optimized geometries.
Optimized geometries for the density functional theory calculations.
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Mesa, C.A., Francàs, L., Yang, K.R. et al. Multihole water oxidation catalysis on haematite photoanodes revealed by operando spectroelectrochemistry and DFT. Nat. Chem. 12, 82–89 (2020). https://doi.org/10.1038/s41557-019-0347-1
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