Electronic Structure and Stability of the Active Surface Phase of NixCo3–xO₄ Spinel Alkaline O₂ Evolution Electrocatalysts: From an Epitaxial Model Catalyst Perspective

In this work, we investigate the relationship between the surface stability, electronic structure of Ni3+ hole states, and oxygen evolution reaction (OER) activity in epitaxially grown NixCo3–xO4 (x = 0, 0.3, 1.0) model electrocatalysts through analysis of their electronic structure before and after electrochemical treatments. The use of flat, structurally well-defined models allows us to apply advanced characterization methods, not applicable to powder catalysts. The OER activity of all NixCo3–xO4 samples increases consistently upon cyclic voltammetry (CV) between +1.22 V and +1.92 V vs RHE with proceeding cycles. Quasi in situ synchrotron X-ray photoemission spectroscopy (SXPS) results show a gradual upshift of the Fermi level (EF) for all NixCo3–xO4 after proceeding with the OER electrochemical treatment, while near edge X-ray absorption fine structure spectroscopy (NEXAFS) of the O K-edge as well as the Co and Ni L-edges show no significant changes in the hole state structure near the conduction band minimum as well as the oxidation state of Ni and Co upon the OER treatment. A combination of atomic force microscopy, spectroscopic ellipsometry, X-ray diffraction, and X-ray reflectivity measurements reveals that the surface of NixCo3–xO4 reconstructs and builds up oxygen deficiency upon OER treatment. While the upshift of the Fermi level is disadvantageous for the OER, the emerging oxygen deficiency together with morphology changes lead to an overall increase of OER activity.