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

Abstract

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+.

Electrochemical Formation of Fe(IV)=O Derived from H2O2 on a Hematite Electrode as an Active Catalytic Site for Selective Hydrocarbon Oxidation Reactions

“…This increase in anodic current by the addition of H 2 O 2 is assignable to the H 2 O 2 oxidative decomposition to oxygen molecules. The lower overpotential for the HPOR on α‐Fe 2 O 3 than that for the WOR is consistent with the results of previous studies ,. Note that the product of both the HPOR and WOR is molecular oxygen under this condition.…”

“…Figures (b) and (c) show diffuse transmission UV‐vis absorption ( ΔT ) spectra of the α‐Fe 2 O 3 electrode obtained at various U with and without H 2 O 2 , respectively (also see Figure S2). In the absence of H 2 O 2 , a new broad absorption peak at 580 nm appears above +1.81 V vs. RHE, which is consistent with the previous report . However, after the addition of H 2 O 2 , the same peak appears from +1.71 V, which is 100 mV more negative than that in the absence of H 2 O 2 .…”

“…Takashima et al . previously demonstrated that the Fe(IV)=O species, which serves as the reactive intermediate of the WOR on α‐Fe 2 O 3 , exhibits a broad absorption peak centered at 580 nm . Here, we compared in situ UV‐vis spectra acquired with and without the presence of H 2 O 2 .…”

“…In contrast, Fe(IV)=O has recently been also identified as a reactive intermediate during the electrochemical water oxidation reaction (WOR) on hematite (α‐Fe 2 O 3 ) electrodes, as shown in Scheme . The Fe(IV)=O on α‐Fe 2 O 3 electrodes is generated via the following reactions:, …”

“…In the WOR, the oxidation of Fe(III) to Fe(IV) (reaction (2)) requires the high overpotential (>0.4 V vs. RHE) ,,. Considering that H 2 O 2 serves as an oxidant to form Fe(IV)=O in the peroxide shunt of P450, we can propose a working hypothesis that the Fe(IV)=O is formed derived from H 2 O 2 lower overpotential than that from H 2 O.…”