Super-exchange effect induced by early 3d metal doping on NiFe2O4(0 0 1) surface for oxygen evolution reaction

Understanding the intrinsic activity of oxygen evolution reaction (OER) is crucial for catalyst design. To date, different metal-doping strategies have been developed to achieve this, but the involving mechanisms remain unclear. Here, the electronic structure of the transition metal-doped NiFe2O4(0 0 1) surface is scrutinized for OER intrinsic activity using density functional theory calculations. Five 3d-orbital filling metals (Ti, V, Cr, Mn, and Co) are introduced as dopants onto A- and B-layers of the NiFe2O4(0 0 1) surface, and variation of oxidation states over Fe sites is observed on B-layer. Analyzing the magnetic moment and charge transfer of surface cation sites reveals that the variation of Fe oxidation states originates from the super-exchange effect and is influenced by the t2g-electron configuration of 3d metal dopants. This trend governs the generation of highly-active Fe3+ sites on the B-layer, the adsorption strength of OER intermediates, i.e., *O and *OH, and therefore the intrinsic activity. The finding of super-exchange mechanism induced by 3d early metal doping offers insights into electronic structure tailoring strategies for improving the intrinsic activity of OER electrocatalysts.