Journal
ENERGY & ENVIRONMENTAL SCIENCE
Volume 11, Issue 9, Pages 2476-2485Publisher
ROYAL SOC CHEMISTRY
DOI: 10.1039/c8ee01063c
Keywords
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Funding
- Bundesministerium fur Bildung und Forschung (BMBF) [CO2EKAT (03SF0523B), 05K16KE2]
- Deutsche Forschungsgemeinschaft through Berlin cluster of excellence
- UniCat [EXC 314-2]
- Alexander von Humboldt Foundation
- [SPP 1613 (DA 402/7-2)]
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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-mu-hydroxo bridged Ni-II-Ni-II motifs that is preserved following a symmetric contraction of Ni-O bond lengths when oxidized to di-mu-oxo Ni-IV-Ni-IV. Incorporation of Fe ions into the structure generates di-mu-hydroxo Ni-II-Fe-III 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--hydroxo Ni-II-Fe-III motifs and neighboring di-mu-hydroxo Ni-II-Ni-II 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-mu-hydroxo Ni-II-Fe-III motifs, which we propose is the process relevant to catalytic oxygen evolution.
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