Oxidation of interfacial cobalt controls the pH-dependence of the oxygen evolution reaction

Transition metal oxides (e.g., cobalt oxides) often undergo a dynamic surface reconstruction under oxygen evolution reaction (OER) conditions to form the active state, which differs in response to the electrolyte pH. The resulting pH-dependency of OER activity is commonly observed but poorly understood. Herein, we demonstrate that operando X-ray absorption spectroscopy (XAS) characterization enables tracking of the Co oxidation transformation at different pH-directed (hydr)oxide/electrolyte interfaces. Combined with in situ electrochemical analyses, correlations between Co redox dynamics, flat band potential and Co oxidation transformations are established to explain the pH-dependency of OER activity. In alkaline environments, the low flat band potential allows a low-potential Co redox transformation, which in turns favors surface reconstruction. In neutral and acidic environments, an anodic shift of the Co redox transformation increases the OER overpotential, particularly in an acidic environment. The largest OER overpotential, in a neutral environment, is further attributable to the poor polarizability of Co atoms and the slowest Co oxidation transformation with respect to the change in applied potential (or OER current). These findings reveal that the Co oxidation transformation at the interface is the factor directly determining the pH-dependency of OER activity, therefore providing insight into designing efficient OER catalysts in different pH environments.

过渡金属氧化物（如钴氧化物）在析氧反应（Oxygen Evolution Reaction, OER）工况下常会发生动态表面重构，形成活性态，且该过程对电解液pH存在响应差异。目前OER活性的pH依赖性现象已被广泛观测，但相关机理仍未得到充分阐明。本文中，我们证实operando X射线吸收光谱（X-ray absorption spectroscopy, XAS）可实现在不同pH调控的（氢氧化物/氧化物）-电解液界面处追踪钴的氧化态转变。结合原位电化学分析手段，我们建立了钴氧化还原动力学、平带电位与钴氧化态转变之间的关联，以此阐释OER活性的pH依赖性。在碱性环境中，较低的平带电位可促使钴氧化还原转变在更低电位下发生，进而助力表面重构过程。在中性与酸性环境中，钴氧化还原转变的阳极偏移会提升OER过电位，其中酸性环境下的影响尤为显著。而中性环境中出现的最大OER过电位，还可进一步归因于钴原子的极化能力较弱，以及相较于外加电位（或OER电流）的变化而言，钴氧化态转变的速率最为缓慢。本研究揭示，界面处的钴氧化态转变是直接决定OER活性pH依赖性的核心因素，由此为不同pH环境下高效OER催化剂的设计提供了理论参考与设计思路。