Operando Surface X-ray Diffraction Studies of Structurally Defined Co3O4 and CoOOH Thin Films during Oxygen Evolution
Operando Surface X-ray Diffraction Studies of Structurally Defined Co3O4 and CoOOH Thin Films during Oxygen Evolution
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DOI：
10.1021/acscatal.8b04823
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发表时间：
2019-05-01
期刊：
影响因子：
12.9
通讯作者：
Magnussen, Olaf M.
中科院分区：
文献类型：
--
作者：
Reikowski, Finn;Maroun, Fouad;Magnussen, Olaf M.
在寻找用于电化学水分解的无贵金属电极材料的过程中，过渡金属氧化物作为阳极析氧反应（OER）的活性和稳定的电催化剂最近受到了极大的关注。本文介绍了两种结构明确的外延钴氧化物薄膜：Co_3O_4（111）和CoOOH（001）在Au（111）上电沉积的表面X射线衍射研究。在循环伏安过程中的电位依赖性的结构变化进行了监测与高时间分辨率高达OER电流密度高达150 mA cm（-2）。发现CoOOH（001）膜是光滑的并且在宽电位范围内是完全稳定的。在Co 3 O 4（111）的情况下，观察到快速和完全可逆的结构变化。具体地说，Co 3 O 4（111）的表面区域在OER开始的负300 mV电位处开始重构，表明该过程与化学预测的Co 3 O 4/CoOOH（001）转变有关，而不是与催化反应有关。所形成的皮层具有限定的厚度，其随所施加的电势线性变化，并且是OER活性相。令人惊讶的是，如果考虑真实的微观表面积，则表层覆盖的Co 3 O 4膜的催化活性和光滑的CoOOH（001）的催化活性几乎相同。这表明，OER活性位点的两种氧化物的数量是相似的，尽管有非常不同的缺陷密度，是在与以前的建议，二-KT-氧代桥接的钴阳离子是专门负责的钴氧化物的OER活性的差异。对于光滑的CoOOH（001），在400 mV的过电位下测定的每个表面原子的转换频率为4.2 s（-1）。此外，我们的研究表明，赝电容充电电流在预OER电位范围内必须分配给一个散装过程，伴随着潜在的依赖性变化的Co 3 O 4散装的晶胞体积。
In the search for precious-metal-free electrode materials for electrochemical water splitting, transition-metal oxides have been receiving much recent interest as active and stable electrocatalysts for the anodic oxygen evolution reaction (OER). We present operando surface X-ray diffraction studies of two structurally well-defined epitaxial cobalt oxide thin films: Co3O4(111) and CoOOH(001) electrodeposited on Au(111). The potential-dependent structural changes during cyclic voltammograms were monitored with high time resolution up to OER current densities as high as 150 mA cm(-2). The CoOOH(001) film is found to be smooth and perfectly stable over a wide potential range. In the case of Co3O4(111), fast and fully reversible structural changes are observed. Specifically, the surface region of Co3O4(111) starts restructuring at potentials 300 mV negative of the onset of the OER, indicating that the process is related to the thermodynamically predicted Co3O4/CoOOH(001) transition rather than to the catalytic reaction. The formed skin layer is of defined thickness, which changes linearly with applied potential, and is the OER active phase. Surprisingly, the catalytic activity of the skin layer covered Co3O4 film and that of the smooth CoOOH(001) are almost identical, if the true microscopic surface area is taken into account. This indicates that the number of OER active sites on the two oxides is similar, despite the very different defect density, and is at variance with previous suggestions that di-kt-oxo bridged Co cations are exclusively responsible for the OER activity of Co oxides. For the smooth CoOOH(001), a turnover frequency of 4.2 s(-1) per surface atom is determined at an overpotential of 400 mV. Furthermore, our studies demonstrate that the pseudocapacitive charging current in the pre-OER potential range must be assigned to a bulk process that is accompanied by potential-dependent changes of the unit cell volume in the Co3O4 bulk.