Operando spectroelectrochemical analysis is used to determine the water oxidation reaction kinetics for hematite photoanodes prepared using four different synthetic procedures. Whilst these photoanodes exhibit very different current / voltage performance, their underlying water oxidation kinetics are found to be almost invariant. Lower photoanode performance was found to correlate with the observation of optical signals indicative of charge accumulation in mid-gap oxygen vacancy states, indicating these states do not contribute directly to water oxidation.

1Molecular Sciences Research Hub, Imperial College London, Imperial College London, London, UK. 2Present address: Institute of Advanced Materials (INAM), Universitat Jaume I, Castelló, Spain. 3Present address: Departamento de Ingeniería Electrónica, Universidad Central, Bogotá, Colombia. 4Present address: Departament de Química, Universitat Autònoma de Barcelona, Barcelona, Spain. ✉e-mail: j.durrant@imperial.ac.uk 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 density4–6. 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. In this work, we used a ~400-nm-thick Si-doped haematite produced by atmospheric pressure chemical vapour deposition (APCVD)7 and a haematite film prepared by the hydrothermal growth of FeOOH on FTO with a further annealing procedure at 800 °C (‘solution-based regrown’, SBR) to produce a slightly rougher structure of 300 nm thickness8. The data included here are published by Mesa and co-workers3,9,10. Here we discuss the independency of the observed rate constants (krxn) with an applied bias and light intensity for two different oxidation reactions, water (kWO) and methanol (kMO) oxidation, and consequently justify the use of the kinetic model we propose3. We use equation (1) to describe the kinetics:

Impact Of Synthesis Route on the Water Oxidation Kinetics of Hematite Photoanodes

Citations

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

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