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Reply to: Questioning the rate law in the analysis of water oxidation catalysis on haematite photoanodes

Nature Chemistry volume 12, pages 1099–1101 (2020)Cite this article

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The Original Article was published on 09 November 2020

replying to S. Zhang and W. Leng Nature Chemistry https://doi.org/10.1038/s41557-020-00569-y (2020)

Water oxidation is widely accepted to be a kinetically demanding reaction that hinders solar-to-fuel transformations, such as photoelectrochemical water splitting, especially when using metal oxide photoanodes. Broadly, two different kinetic models are used, according to the literature, to model electron transfer reactions at metal oxide–electrolyte interfaces. The first is the Butler–Volmer model, which in its standard form has been derived for charge transfer at metal-electrolyte interfaces1. It assumes a continuum of accessible energy levels within the electrode such that an applied potential is manifested solely as a potential drop across the Helmholtz layer, thereby changing the energetics of the barrier associated with electron transfer. This model is used in the Matters Arising2 to describe our data3. However, this model is arguably less suited to describe charge transfer at semiconducting interfaces, as an increasing light intensity results in the photoinduced generation of an increased population of localized holes, which are approximately degenerate owing to their relaxation to the valence band edge. In addition, in the case of band-edge pinning, the potential drop between the valence band and the solution redox (Helmholtz potential) is constant and an increase in photovoltage increases the potential across the space charge layer. For these interfaces, a population model is more suitable and assumes that the kinetics correlate with the hole concentration. Such a model has been previously used to describe the kinetics of reactions at semiconductor interfaces, with a first-order dependence on the hole density4,5,6. In our work, we demonstrate that in multielectron redox processes, such as water oxidation, the order of the reaction can deviate from one under one sun conditions. In the following sections, we describe the suitability of the population model to describe water oxidation kinetics under the experimental conditions described previously3.

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Fig. 1: Mechanistic analyses of water and methanol oxidation on α-Fe2O3.
Fig. 2: Arrhenius slope analysis on α-Fe2O3.

Data availability

The dataset used here can be found in the respective references or can be shared upon request.

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Acknowledgements

J.R.D. acknowledges financial support from the European Research Council (project Intersolar 291482). C.A.M. thanks COLCIENCIAS (now Ministry of Science, Technology and Innovation, call 568) for funding. J.R.D., C.A.M., R.R. and S.C. acknowledge H2020 project A-LEAF (732840) and L.F. thanks the European Union for a Marie Curie fellowship (658270). We also thank S. Zhang and W. Leng for their interest in our research and for encouraging insightful scientific debate.

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Author notes

  1. Camilo A. Mesa

    Present address: Institute of Advanced Materials (INAM), Universitat Jaume I, Castelló, Spain

  2. Camilo A. Mesa

    Present address: Departamento de Ingeniería Electrónica, Universidad Central, Bogotá, Colombia

  3. Laia Francàs

    Present address: Departament de Química, Universitat Autònoma de Barcelona, Barcelona, Spain

Authors and Affiliations

  1. Molecular Sciences Research Hub, Imperial College London, Imperial College London, London, UK

    Camilo A. Mesa, Reshma R. Rao, Laia Francàs, Sacha Corby & James R. Durrant

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Contributions

C.A.M., L.F. and J.R.D. conceived and designed the experiments. C.A.M. performed the data collection, treatment and analysis. C.A.M., R.R. and J.R.D. co-wrote the manuscript. All the authors discussed the results, commented on and revised the manuscript.

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Correspondence to James R. Durrant.

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Mesa, C.A., Rao, R.R., Francàs, L. et al. Reply to: Questioning the rate law in the analysis of water oxidation catalysis on haematite photoanodes. Nat. Chem. 12, 1099–1101 (2020). https://doi.org/10.1038/s41557-020-00570-5

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