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+.

The rate-determining step of the oxygen evolution reaction on hematite electrodes was switched from the sequential electron/proton transfer process to the concerted proton-coupled electron transfer (CPET) process by adding pyridine derivatives to the electrolyte. By inducing the CPET process, the overpotential for oxygen evolution at neutral pH decreased by approximately 250 mV.

The utilization of a support material has been an effective method for the management of proton transfer on catalytic active centers. Previous studies showed that the large overpotential of α-Fe2O3 for the oxygen evolution reaction (OER) resulted from the sequential transfer of electrons and protons. Here, by combining α-Fe2O3 with La2O3, which functions as a solid proton acceptor, concerted proton–electron transfer (CPET) was induced. The induction of CPET facilitated the formation of intermediate species, leading to the OER activity enhancement at neutral pH. This work may provide useful insight for the design of promising OER catalysts, which is pivotal for energy conversion.