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

中文翻译：

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氧化还原惰性铁离子在氢氧化镍（氢氧根）催化剂中的几何变形†

掺入Fe离子后，羟基氧化镍薄膜的电化学行为发生了巨大变化，这为建立有效的电催化剂设计原理提供了机会。我们通过X射线吸收光谱法表征了一系列光化学沉积的Fe–Ni（羟基）氢氧化物，并跟踪结构图案的电压和成分依赖性。我们观察到二μ-羟基桥接的Ni ^II -Ni ^II基序中的三角畸变，当Ni-O键长被对称氧化为二μ-羟基Ni ^IV -Ni ^IV后，该畸变得以保留。在结构中掺入铁离子会生成二μ-羟基Ni ^II -Fe ^IIINi-Fe的距离取决于镍的氧化态，但Fe-O的键长不依赖于这些基序。这种不对称性最大程度地减少了还原状态下的di- hydroxo Ni ^II -Fe ^III基序和邻近的di- hydroxo Ni ^II -Ni ^II位的三角畸变，但加剧了其在氧化态下的变形。我们将归因于镍基氧化还原峰的铁诱导的阳极位移和归因于这种几何畸变反转的提高的催化氧气析出反应的能力归因于此。光谱电化学实验表明，以前在光吸收方面未曾报道过的变化。1.5 V与含铁样品中的RHE。我们将此特征归因于二μ-羟基Ni ^II -Fe ^III图案中镍离子的氧化，我们提出这是与催化氧释放有关的过程。