Atomic-scale insights into the oxygen evolution reaction on hematite: a detailed mechanistic investigation of different surface coverages

Abstract

The oxygen evolution reaction (OER)—2H₂O → O₂ + 4H⁺ + 4e⁻—is the limiting half-cell reaction in the electrochemical water splitting process for the production of green hydrogen. In photoelectrochemical cells for solar water splitting, hematite (α-Fe₂O₃) is among the state-of-the-art anode materials. Despite extensive research, the mechanistic understanding of the OER on hematite is still incomplete, and previous computational work suggested that the less stable Fe–Fe–O–, rather than the most stable Fe–O–Fe– surface termination of α-Fe₂O₃(0001), is responsible for its high OER activity. Using a comprehensive density functional theory framework, we investigate seven different OER mechanisms for five different surface coverages of the most stable Fe–O–Fe– surface termination and find that the OER preferentially proceeds via Walden-type mechanisms that have been overlooked in previous work. The present contribution advances the understanding of OER mechanisms on hematite, emphasizing the importance of considering multiple pathways in the analysis of the elementary steps, and proposes to exploit the most stable Fe–O–Fe– termination for the development of advanced hematite-based photoanodes.