Metal oxides of the spinel family have shown great potential towards the oxygen evolution reaction (OER), but the fundamental OER mechanism of spinel oxides is still far from being completely understood, especially for the role of the metal ions. Owing to various coordinated sites of divalent/trivalent metals ions and surface conditions (morphology and defects), it is a great challenge to have a fair assessment of the electrocatalytic performance of spinel systems. Herein, we demonstrated a series of MFe₂O₄ (M = Fe, Co, Ni, Zn) with a well-controlled morphology to achieve a comprehensive study of electrocatalytic activity toward OER. By utilizing several in situ analyses, we could conclude a universal rule that the activities for OER in the metal oxide systems were determined by the occurrence of a phase transformation, and this structural transformation could work well in both crystallographic sites (T_d and O_h sites). Additionally, the divalent metal ion significantly dominated the formation of oxyhydroxide through an epitaxial relationship, which depended on the atomic arrangement at the interface of spinel and metal oxyhydroxide, while trivalent metal ions remained unchanged as a host lattice. The metal oxyhydroxide was formed during a redox reaction rather than being formed during OER. The occurrence of the redox reaction seems to accompany a remarkable increase in resistance and capacitance might result from the structural transformation from spinel to metal oxyhydroxide. We believe that the approaching strategies and information obtained in the present study can offer a guide to designing a promising electrocatalytic system towards the oxygen evolution reaction and other fields.

中文翻译：

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与价和元素有关的水氧化行为：原位X射线衍射，吸收和电化学阻抗谱†

尖晶石家族的金属氧化物在氧释放反应（OER）方面显示出巨大潜力，但仍未完全了解尖晶石氧化物的基本OER机理，特别是对于金属离子的作用。由于二价/三价金属离子的各种配位位置和表面条件（形貌和缺陷），对尖晶石体系的电催化性能进行公正的评估是一个巨大的挑战。在这里，我们展示了一系列具有良好控制形态的MFe ₂ O ₄（M = Fe，Co，Ni，Zn），以实现对OER的电催化活性的全面研究。通过利用几个原位分析中，我们可以得出一个普遍规则，即金属氧化物体系中OER的活性取决于相变的发生，并且这种结构转变在两个晶体学位点（T _d和O _h网站）。另外，二价金属离子通过外延关系显着支配了羟基氧化物的形成，这取决于尖晶石和羟基氧化物的界面处的原子排列，而三价金属离子作为主体晶格保持不变。羟基氧化金属是在氧化还原反应中形成的，而不是在OER中形成的。氧化还原反应的发生似乎伴随着电阻的显着增加，并且电容可能由尖晶石向金属羟基氧化物的结构转变所导致。我们相信，本研究中获得的处理策略和信息可以为设计有前途的电催化系统（用于制氧反应和其他领域）提供指导。