Despite the current prominence of cobalt oxides as electrocatalysts for the alkaline oxygen evolution reaction (OER), there is a lack of unambiguous demonstration for the presence and the role of Co⁴⁺ prior to/during the OER. Here, we combine electrochemistry with in situ UV–Vis absorption spectroscopy to investigate and discuss the previously unaddressed effect of different OH⁻ concentrations in the range from 1 M to 0.016 M on the population of Co⁴⁺ in thin films of CoO_x and its concomitant impact on their OER performance. Evidence for Co⁴⁺ is provided by in-situ X-ray absorption spectroscopy. Our UV–Vis absorption spectroscopic findings indicate that, not only can the overall redox conversion of Co be qualitatively monitored as a function of potential and OH⁻ concentration, but also the formation of oxidized Co (i.e.,Co³⁺ and Co⁴⁺) assigned to a peak at 800 nm can be more quantitatively tracked in situ via stepped potential spectroelectrochemistry; with their optical signals becoming stronger at higher OH⁻concentrations above 1.2 V vs. RHE, which is consistent with voltammetric redox couples, indicating an enhancement in Co oxidation state and the consequent predominance of Co⁴⁺ under conditions of elevated OH⁻ concentrations. Furthermore, the evolved oxygen due to OER does not depend on Co⁴⁺ or OH⁻ activity at 1.54 V vs. RHE, while a correlation with both Co⁴⁺ and OH⁻ is identified at 1.59 V vs. RHE. This study not only provides spectral insight into the redox chemistry of Co at OER-relevant potentials but also highlights the importance of Co⁴⁺ in facilitating the alkaline OER at high OH⁻ concentrations and current densities.

中文翻译：

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原位紫外-可见吸收光谱对氧化钴催化剂进行水电氧化的研究


尽管目前钴氧化物作为碱性析氧反应 （OER） 的电催化剂很突出，但在 OER ⁴⁺ 之前/期间缺乏明确的证明 Co 的存在和作用。在这里，我们将电化学与原位紫外-可见吸收光谱相结合，研究和讨论以前未解决的不同 OH ⁻ 浓度在 1 M 至 0.016 M 范围内对 CoO 薄膜 ⁴⁺ 中 Co 数量的影响 _x 及其对其 OER 性能的随之影响。原位 X 射线吸收光谱提供了 Co ⁴⁺ 的证据。我们的紫外-可见吸收光谱结果表明，不仅可以定性监测 Co 的整体氧化还原转化率作为电位和 OH ⁻ 浓度的函数，而且可以通过阶跃电位光谱电化学更定量地原位跟踪分配给 800 nm 峰的氧化 Co（即 Co ³⁺ 和 Co ⁴⁺ ）的形成;与 RHE 相比，在 1.2 V 以上的较高 OH ⁻ 浓度下，它们的光信号变得更强，这与伏安法氧化还原对一致，表明 ⁴⁺ 在 OH ⁻ 浓度升高的条件下，Co 氧化态增强，因此 Co 占主导地位。此外，OER 产生的释放氧不取决于 1.54 V 与 RHE 时的 Co ⁴⁺ 或 OH ⁻ 活性，而在 1.59 V 与 RHE 时确定与 Co ⁴⁺ 和 OH ⁻ 的相关性。 这项研究不仅提供了对 Co 在 OER 相关电位下的氧化还原化学的光谱见解，还强调了 Co ⁴⁺ 在促进高 OH ⁻ 浓度和电流密度下的碱性 OER 方面的重要性。