The uniformity in electrochemical behavior of Fe_xNi_1−x(OH)₂ is remarkable given the diversity in published fabrication protocols, composition-dependent structural changes, and multitude of electrochemical reaction mechanisms proposed. We use complementary techniques to show that multiple unique Fe(III) coordination environments exist in Fe_xNi_1−x(OH)₂ synthesized by alkaline precipitation. Samples synthesized with formamide added to the reaction vessel yield Mössbauer and Raman spectra consistent with Fe(III) successfully incorporated into the Ni(OH)₂ lattice. Additional spectroscopic features emerge for samples synthesized in the absence of formamide, revealing a second Fe(III) coordination environment. Correlations in structural information derived from the numerous characterization techniques suggest that the secondary environment sits atop the 2-dimensional Ni(OH)₂ sheets. X-ray diffraction and X-ray absorption spectroscopy fail to resolve this second site, but show that Fe(III) induces a non-uniform structural contraction that distorts the Ni(OH)₂ lattice for all samples. Electrocatalytic oxygen evolution is qualitatively similar for both sample series, but samples synthesized in the presence of formamide consistently outperform those from conventional pH precipitation. Iron ions within the Ni(OH)₂ lattice are therefore deemed to be the catalytically relevant structural feature. The identification of a secondary iron site demonstrates that qualitative similarities in electrochemical behavior mask quantitative differences in structure and electrocatalytic competence.

中文翻译：

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氢氧化镍晶格中铁离子非传统配位环境的鉴定

_{Fe x} Ni _{1− x} (OH) ₂的电化学行为的一致性是显着的，因为已发表的制造方案的多样性、与成分相关的结构变化以及提出的多种电化学反应机制。我们使用互补技术来证明碱沉淀法合成的Fe _x Ni _{1− x} (OH) ₂中存在多种独特的 Fe( III ) 配位环境。将甲酰胺添加到反应容器中合成的样品产生的穆斯堡尔和拉曼光谱与成功结合到 Ni(OH) ₂中的 Fe( III ) 一致格子。在没有甲酰胺的情况下合成的样品出现了额外的光谱特征，揭示了第二个 Fe( III ) 配位环境。来自众多表征技术的结构信息的相关性表明，次生环境位于二维 Ni(OH) ₂片层之上。X 射线衍射和 X 射线吸收光谱无法分辨第二个位点，但表明 Fe( III ) 会引起不均匀的结构收缩，从而扭曲 Ni(OH) ₂所有样品的晶格。两个样品系列的电催化析氧在性质上相似，但在甲酰胺存在下合成的样品始终优于传统 pH 沉淀法。_{Ni(OH) 2}晶格内的铁离子因此被认为是催化相关的结构特征。二次铁位点的鉴定表明，电化学行为的定性相似性掩盖了结构和电催化能力的定量差异。