Detection of high-valent iron species in alloyed oxidic cobaltates for catalysing the oxygen evolution reaction

Iron alloying of oxidic cobaltate catalysts results in catalytic activity for oxygen evolution on par with Ni-Fe oxides in base but at much higher alloying compositions. Zero-field 57 Fe Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) are able to clearly identify Fe 4+ in mixed-metal Co-Fe oxides. The highest Fe 4+ population is obtained in the 40–60% Fe alloying range, and XAS identifies the ion residing in an octahedral oxide ligand field. The oxygen evolution reaction (OER) activity, as reflected in Tafel analysis of CoFeO x films in 1 M KOH, tracks the absolute concentration of Fe 4+ . The results reported herein suggest an important role for the formation of the Fe 4+ redox state in activating cobaltate OER catalysts at high iron loadings.