Cooperative Effect of Co and Fe Promotes the OER Activity of CoFe ₂ O ₄ (100) and (001) surfaces

Abstract The inverse spinel is a promising anode material for electrochemical water splitting. Understanding how the surface orientation and composition affects the oxygen evolution reaction (OER) activity is of paramount importance for tailoring its functionality. To this end, we employ density functional theory (DFT) calculations with a Hubbard term to assess the OER performance at the iron‐ or cobalt‐rich (001) and the mixed (100) surfaces. While the overpotentials at Fe reaction sites are above 0.44 V and octahedral Co at the Co‐rich (001) surface exhibits an overpotential of 0.27 V, the lowest overpotential of 0.20 V is found for octahedral Co at the A‐layer of the mixed Co–Fe (100) surface. Taking into account implicit solvation shifts the most active site to octahedral iron at the B‐layer of the Fe‐rich (001) surface (=0.37 V). Our analysis reveals that a bulk‐like oxidation state during OER correlates with higher OER activity. Moreover, the reduced overpotential originates from the stabilization of the *OOH intermediate through an unusual bidentate bonding geometry to octahedral Co and neighboring Fe, highlighting the cooperative effect. This is in contrast to the monodentate binding typically assumed in the scaling relationship and demonstrates that the involvement of multiple surface sites can significantly improve the OER performance.