Efficient catalysts for the anodic oxygen evolution reaction (OER) are critical for electrochemical H₂ production. Their design requires structural knowledge of their catalytically active sites and state. Here, we track the atomic-scale structural evolution of well-defined CoO_x(OH)_y compounds into their catalytically active state during electrocatalytic operation through operando and surface-sensitive X-ray spectroscopy and surface voltammetry, supported by theoretical calculations. We find clear voltammetric evidence that electrochemically reducible near-surface Co³⁺–O sites play an organizing role for high OER activity. These sites invariably emerge independent of initial metal valency and coordination under catalytic OER conditions. Combining experiments and theory reveals the unified chemical structure motif as µ₂-OH-bridged Co^2+/3+ ion clusters formed on all three-dimensional cross-linked and layered CoO_x(OH)_y precursors and present in an oxidized form during the OER, as shown by operando X-ray spectroscopy. Together, the spectroscopic and electrochemical fingerprints offer a unified picture of our molecular understanding of the structure of catalytically active metal oxide OER sites.

用于阳极氧放出反应（OER）的高效催化剂对于电化学H _2的生产至关重要。他们的设计需要了解其催化活性位点和状态的结构知识。在这里，我们通过理论计算支持的操作，表面敏感的X射线光谱和表面伏安法，在电催化操作过程中，追踪了定义明确的CoO _x（OH）_y化合物进入其催化活性状态的原子尺度结构演变。我们找到了清晰的伏安法证据，即电化学可还原的近表面Co ³⁺–O站点在OER高活动中起组织作用。在催化OER条件下，这些位点始终独立于初始金属化合价和配位而出现。结合实验和理论揭示了统一的化学结构基序，即µ ₂ -OH桥联的Co ^{2 + / 3 +}离子簇形成在所有三维交联和分层的CoO _x（OH）_y前体上，并在氧化过程中以氧化形式存在如操作X射线光谱法所示。光谱指纹图谱和电化学指纹图谱共同提供了我们对催化活性金属氧化物OER位点结构的分子理解的统一图片。