Depth-Profiling Cation Location and Oxidation States of Ni Foam Electrodes in Fe/Co-Enriched Electrolytes under Alkaline Water Oxidation Conditions

This study investigates the surface transformations of nickel foam (NF) electrodes induced by Fe and Co cation incorporation from the electrolyte following activation in the oxygen evolution reaction (OER), using both ex situ time-of-flight secondary ion mass spectrometry (TOF-SIMS) and in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). Electrochemical measurements demonstrate that Fe enhances OER kinetics as expected, while the copresence of Co improves electrode conductivity and electrochemically active surface area. Our surface analyses with depth resolution on cation location and corresponding oxidation states interestingly reveal the formation of a chemically distinct Ni-based layered structures following OER conditions, with a preferential Ni2+M3+O(OH) topmost composition (M = Fe or Co) and a Ni3+M2+O(OH)/Ni metal subsurface layer. These layers evolve through a proposed mechanism involving two-proton, one-electron transfer, which specifically generates Fe3+and Co3+sites in the Ni2+topmost layer that are more active than single Ni3+sites. Upon air exposure, the layered phases become chemically homogeneous, though their signatures remain detectable in TOF-SIMS. This study shows that Fe and Co incorporation from the electrolyte leads to the formation of chemically distinct surface layers on NF electrodes, which are associated with improved OER performance. These findings clarify how secondary metals modify the surface chemistry of Ni electrodes in alkaline electrolysis conditions.