Smith, R.D.L.R.D.L.SmithPasquini, C.C.PasquiniLoos, S.S.LoosChernev, P.P.ChernevKlingan, K.K.KlinganKubella, P.P.KubellaMohammadi, M.R.M.R.MohammadiGonzález-Flores, D.D.González-FloresDau, H.H.Dau2022-03-052022-03-052018https://publica.fraunhofer.de/handle/publica/25677610.1039/c8ee01063cThe dramatic change in electrochemical behavior of nickel (oxy)hydroxide films upon incorporation of Fe ions provides an opportunity to establish effective electrocatalyst design principles. We characterize a photochemically deposited series of Fe-Ni (oxy)hydroxides by X-ray absorption spectroscopy and track the voltage- and composition-dependence of structural motifs. We observe a trigonal distortion in di-m-hydroxo bridged NiII-NiII motifs that is preserved following a symmetric contraction of Ni-O bond lengths when oxidized to di-m-oxo NiIV-NiIV. Incorporation of Fe ions into the structure generates di-m-hydroxo NiII-FeIII motifs in which Ni-Fe distances are dependent on nickel oxidation state, but Fe-O bond lengths are not. This asymmetry minimizes the trigonal distortion in di-m-hydroxo NiII-FeIII motifs and neighboring di-m-hydroxo NiII-NiII sites in the reduced state, but exacerbates it in the oxidized state. We attribute both the Fe-induced anodic shift in nickel-based redox peaks and the improved ability to catalyze the oxygen evolution reaction to this inversion in geometric distortions. Spectroelectrochemical experiments reveal a previously unreported change in optical absorbance at ca. 1.5 V vs. RHE in Fe-containing samples. We attribute this feature to oxidation of nickel ions in di-m-hydroxo NiII-FeIII motifs, which we propose is the process relevant to catalytic oxygen evolution.en620333journal article