We have used in situ and operando X-ray absorption spectroscopy at the cobalt K-edge to study the formation of cobalt nanoparticles from a molecular precursor, as well as their structural evolution under hydrogen-evolving electrocatalytic conditions. We show that these particles, which are about 100–150 nm in diameter overall, are made of an uncommon form of amorphous metallic cobalt, the smallest ordered unit being 1 nm clusters of ∼50 cobalt atoms. In aqueous solution, these porous particles are partly oxidized into cobalt(II), a fraction of which remains present as an outer shell during hydrogen evolution electrocatalysis, even at very high cathodic potentials. Our operando measurements show that the activity of the particles is correlated to the oxidized layer thickness, a thinner layer exposing a larger fraction of the active metallic cobalt and leading to a higher activity. These findings expand our current understanding of the solid–liquid interface in hydrogen evolution catalytic species in neutral pH and suggest new directions for the improvement of hydrogen-evolving catalytic systems.

中文翻译：

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析氢无定形钴电催化剂的形成与结构的原位观察

我们已经在钴K边缘上使用了原位和操作X射线吸收光谱技术来研究由分子前体形成的钴纳米颗粒，以及它们在氢电催化条件下的结构演变。我们表明，这些粒子的总直径约为100-150 nm，是由稀有形式的非晶态金属钴制成的，最小的有序单位是〜50个钴原子的1 nm簇。在水溶液中，这些多孔颗粒被部分氧化成钴（II），即使在非常高的阴极电势下，在氢气析出电催化过程中，一部分钴仍作为外壳存在。我们的操作测量结果表明，颗粒的活性与氧化层厚度有关，较薄的层暴露出较大比例的活性金属钴并导致较高的活性。这些发现扩展了我们目前对中性pH下析氢催化物种中固液界面的理解，并为改善析氢催化体系提出了新的方向。