Comparative study of Co 3 O 4 (111), CoFe 2 O 4 (111), and Fe 3 O 4 (111) thin film electrocatalysts for the oxygen evolution reaction.

Water electrolysis to produce 'green H 2 ' 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. Co 3 O 4 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 Co 3 O 4 (111), CoFe 2 O 4 (111), and Fe 3 O 4 (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. CoFe 2 O 4 (111) is found to be up to about four times more active than Co 3 O 4 (111) and about nine times more than Fe 3 O 4 (111), with the activity depending acutely on the Co/Fe concentration ratio. Under reaction conditions, all three oxides are covered by oxyhydroxide. For CoFe 2 O 4 (111), the oxyhydroxide's Fe/Co concentration ratio is stabilized by partial iron dissolution.