Tracking high-valent surface iron species in the oxygen evolution reaction on cobalt iron (oxy)hydroxides

The oxygen evolution reaction (OER) is the bottleneck reaction of water splitting, which can be used to generate green hydrogen from renewable electricity. Cobalt iron oxyhydroxides (CoFeO x H y ) are among the most active OER catalysts in alkaline medium. However, the active sites of these catalysts remain unclear. Here we use operando ultraviolet-visible (UV-Vis), X-ray absorption, and Raman spectroscopy to reveal oxidations of both Fe and Co ions in CoFeO x H y during the OER. By analyzing samples with different Fe contents and thickness, we find that the concentration of Fe 4+ species at the surface, but not the concentration of Co 4+ in the bulk, scales with the catalytic activity. These results indicate an Fe 4+ -containing active site in CoFeO x H y . Multifold operando spectroscopy reveals an Fe 4+ -containing active site at the surface of cobalt iron oxyhydroxides for the oxygen evolution reaction.