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Geometric distortions in nickel (oxy)hydroxide electrocatalysts by redox inactive iron ions

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Abstract

The 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-μ-hydroxo bridged NiII–NiII motifs that is preserved following a symmetric contraction of Ni–O bond lengths when oxidized to di-μ-oxo NiIV–NiIV. Incorporation of Fe ions into the structure generates di-μ-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-μ-hydroxo NiII–FeIII motifs and neighboring di-μ-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-μ-hydroxo NiII–FeIII motifs, which we propose is the process relevant to catalytic oxygen evolution.

Graphical abstract: Geometric distortions in nickel (oxy)hydroxide electrocatalysts by redox inactive iron ions

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Publication details

The article was received on 10 Apr 2018, accepted on 15 Jun 2018 and first published on 25 Jun 2018


Article type: Paper
DOI: 10.1039/C8EE01063C
Citation: Energy Environ. Sci., 2018, Advance Article
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    Geometric distortions in nickel (oxy)hydroxide electrocatalysts by redox inactive iron ions

    R. D. L. Smith, C. Pasquini, S. Loos, P. Chernev, K. Klingan, P. Kubella, M. R. Mohammadi, D. González-Flores and H. Dau, Energy Environ. Sci., 2018, Advance Article , DOI: 10.1039/C8EE01063C

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