Detection of Intermediate Species in Oxygen Evolution on Hematite Electrodes Using Spectroelectrochemical Measurements

The development of active oxygen evolution catalysts based on understanding of the underlying reaction mechanisms is the key to achieving efficient solar-to-chemical energy conversion. In this study, we synthesized hematite nanoparticle film electrodes and applied them to spectro-electrochemical measurements to detect the intermediate species in oxygen evolution reaction. In situ UV–vis absorption spectra showed that the formation of the species exhibiting absorption at 580 nm was the rate-determining step of oxygen evolution reaction on hematite over a wide range of pH from 4 to 13. In addition, the pH dependences of the onset potentials for oxygen evolution and formation of the intermediate species revealed that there were two reaction mechanisms, which switched at approximately pH 10. On the basis of careful inspection of the observed spectra and the possible active species with reference to previous reports on anomalously oxidized iron compounds, the detected intermediate species was assigned to iron in the oxidation state of 4+.