An exploratory kinetic analysis of photoelectrochemical oxygen evolution on hematite

The kinetics of the photoelectrochemical oxygen evolution reaction (POER) on hematite photoanodes are explored using a simple reaction scheme involving an initial pre-equilibrium hole/proton transfer step to form an Fe(IV) surface species, Fe = O, followed by a rate-determining hole/proton transfer step to form the Fe(II) peroxo species, FeOOH, which then reacts rapidly with two more holes to form oxygen. The modelling considers how the kinetics of these two reaction steps are affected by changes in \({V}_{\text{H}}\), the potential drop across the Helmholtz layer that arises from the build-up of positive charge at the interface under illumination. The model, which also considers electron–hole recombination and back reaction of Fe = O with conduction band electrons, is used to calculate steady-state photocurrent/voltage characteristics, pseudocapacitance-voltage plots, and transient absorbance (TAS) responses that can be compared with published results. The model is also used to show that the slopes of double logarithmic reaction order plots of photocurrent vs. hole or reaction intermediate concentrations are influenced by light-induced changes in \({V}_{\text{H}}\). The insights from this analysis should be relevant to the ongoing discussion of multi-hole mechanisms for the POER on hematite photoanodes.