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 Ni1.5Fe1.5O4 upon OER cycling. The activated NiFe2O4 and P-doped NiFe2O4 exhibit a significantly lower Tafel slope (approximate to 40 mV dec-1) than Ni1.5Fe1.5O4 (approximate to 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 approximate to 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 Ni1.5Fe1.5O4. 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.