Unveiling Surface Species Formed on Ni-Fe Spinel Oxides During the Oxygen Evolution Reaction at the Atomic Scale

Optimizing electrocatalyst performance requires an atomic-scale understanding of surface state changes and how those changes affect activity and stability during the reaction. This is particularly important for the oxygen evolution reaction (OER) since the electrocatalytically active surfaces undergo substantial reconstruction and transformation. Herein, a multimodal method is employed that combines X-ray photoemission spectroscopy, transmission electron microscopy, atom probe tomography, operando surface-enhanced Raman spectroscopy with electrochemical measurements to examine the surface species formed on NiFe₂O₄, P-doped NiFe₂O₄ and Ni_1.5Fe_1.5O₄ upon OER cycling. The activated NiFe₂O₄ and P-doped NiFe₂O₄ exhibit a significantly lower Tafel slope (≈40 mV dec^-1) than Ni_1.5Fe_1.5O₄ (≈90 mV dec^-1), although oxyhydroxides are grown on all three Ni-Fe spinels during OER. This is likely attributed to the formation of a ≈1 nm highly defective layer with a higher oxygen concentration on the activated NiFe₂O₄ and P-doped NiFe₂O₄ nanoparticle surfaces (than that in bulk), which improves the charge transfer kinetics toward OER. Such surface species are not formed on Ni_1.5Fe_1.5O₄. Overall, this study provides a mechanistic understanding of the role of Fe, P, and Ni in forming active oxygen species in the Ni-based spinels toward OER.