Comparative study of CoO(111), CoFeO(111), and FeO(111) thin film electrocatalysts for the oxygen evolution reaction.

Water electrolysis to produce 'green H' with renewable energy is a promising option for the upcoming green economy. However, the slow and complex oxygen evolution reaction at the anode limits the efficiency. CoO with added iron is a capable catalyst for this reaction, but the role of iron is presently unclear. To investigate this topic, we compare epitaxial CoO(111), CoFeO(111), and FeO(111) thin film model electrocatalysts, combining quasi in-situ preparation and characterization in ultra-high vacuum with electrochemistry experiments. The well-defined composition and structure of the thin epitaxial films permits the obtention of quantitatively comparable results. CoFeO(111) is found to be up to about four times more active than CoO(111) and about nine times more than FeO(111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFeO(111), the oxyhydroxide's Fe/Co concentration ratio is stabilized by partial iron dissolution.