Identifying the bottleneck of water oxidation by ab initio analysis of in situ optical absorbance spectrum

Hematite's (α-Fe₂O₃) major limitation to efficiently splitting water using sunlight is the low rate of the oxygen evolution reaction (OER). Thus, identifying the OER rate limiting step is a cornerstone to enhancing the current under low applied potential. Different measurement techniques showed similar absorption difference spectra during a change in applied potential on the hematite anode below and above the onset of the OER in the dark and under light. This absorption change was shown to result from surface modification during the OER, but the specific surface species could not be resolved. On the basis of ab initio calculations, we analyze the calculated absorption spectra in relation to previous measurements. We provide for the first time solid evidence to specify H₂O + *O → *OOH + H⁺ + e^- as the rate limiting step and *O as the bottleneck intermediate of the hematite OER.