The oxygen evolution reaction (OER, 2H2 O → O2 + 4H+ + 4e- ) has received considerable attention in recent years as a promising method for utilizing water as a sustainable electron source. By coupling the OER with a reduction half-reaction such as hydrogen evolution (2H+ + 2e- → H2 ) or carbon dioxide reduction (ex. CO2 + 8H+ + 8e- → CH4 + 2H2 O), the intermittent energy from renewable sources, including solar energy or wind power, can be captured in the form of chemical fuels.
However, driving the four-electron process to oxidize water to oxygen has proven difficult due to the large overpotential. Among the various OER catalysts examined to date, RuO2 and IrO2 have been reported to show good catalytic performance. However, the scarcity and high costs associated with precious metals limit the large-scale applicability of these materials, and therefore it is highly desirable to substitute them with catalysts derived from earth-abundant 3d metals such as Mn, Co, and Fe.
This article reports the investigation of the OER mechanism of hematite ( -Fe2 O3 ) to gain insight into the critical factors determining its electrocatalytic activity. Using spectroelectrochemimcal measurements, an intermediate species assignable to Fe4+ was identified as the precursor of OER across the entire pH range investigated (pH 4−13). The pH dependences of the onset potentials for OER and Fe4+ formation revealed that -Fe2 O3 showed higher catalytic activity under alkaline conditions and that there were two reaction mechanisms which switched at approximately pH 10. From the results of kinetic isotope effect experiments, it was found that the manner of proton and electron transfer during Fe4+ formation (Fe3+–OH → Fe4+=O + H+ + e- ) was different between the two mechanisms. Whereas proton transfer and electron transfer proceed as discrete reaction steps at neutral pH, they proceed simultaneously at alkaline pH. Because the simultaneous transfer of proton and electron enables to avoid formation of high-energy protonated or deprotonated intermediates, the simultaneous transfer is energetically more favorable than the stepwise one, resulting in the higher activity under alkaline pH conditions. Notably, by adding appropriate pyridine derivatives as proton accepting reagents, the switching of the rate-determining step from the stepwise process to the simultaneous process was demonstrated. As a result, a decrease in overpotential of 250 mV was achieved at pH 7 and the OER activity of -Fe2 O3 under neutral pH conditions became comparable to that under alkaline pH conditions. Furthermore, when the pyridine derivatives were replaced with a solid proton accepting material that is robust against anodic decomposition, the enhanced activity was maintained in the long-term operation. These findings will be helpful to design active Fe-based OER catalysts and offer the opportunities to achieve high OER activity under neutral pH using 3d-metal catalysts which often show remarkable activities under alkaline pH conditions.
Audience Take Away:
- Reaction mechanism of oxygen evolution reaction on iron oxide.
- Strategy for design of active oxygen evolution catalyst.
- Spectroelectrochemical method for identification of intermediate species.