The topmost atomic layers of electrocatalysts determine the mechanism and kinetics of reactions in many important industrial processes, such as water splitting, chlor-electrolysis or fuel cells. Optimizing the performance of electrocatalysts requires a detailed understanding of surface-state changes during the catalytic process, ideally at the atomic scale. Here, we use atom probe tomography to reveal the three-dimensional structure of the first few atomic layers of electrochemically grown iridium oxide, an efficient electrocatalyst for the oxygen evolution reaction. We unveil the formation of confined, non-stoichiometric Ir–O species during oxygen evolution. These species gradually transform to IrO₂, providing improved stability but also a decrease in activity. Additionally, electrochemical growth of oxide in deuterated solutions allowed us to trace hydroxy-groups and water molecules present in the regions of the oxide layer that are favourable for the oxygen evolution and iridium dissolution reactions. Overall, we demonstrate how tomography with near-atomic resolution advances the understanding of complex relationships between surface structure, surface state and function in electrocatalysis.

电催化剂的最顶层原子层决定了许多重要工业过程中的反应机理和动力学，例如水分解，氯电解或燃料电池。要优化电催化剂的性能，需要详细了解催化过程中的表面状态变化，最好是在原子尺度上。在这里，我们使用原子探针层析成像技术来揭示电化学生长的氧化铱的前几个原子层的三维结构，该氧化铱是一种有效的氧释放反应电催化剂。我们揭示了氧气逸出过程中非化学计量的受限Ir-O物种的形成。这些物质逐渐转变为IrO ₂，不仅提高了稳定性，而且降低了活性。另外，在氘代溶液中氧化物的电化学生长使我们能够追踪存在于氧化物层区域的羟基和水分子，这些羟基和氧分子有利于氧的释放和铱的溶解反应。总的来说，我们展示了具有近原子分辨率的层析成像技术如何增进对表面结构，表面状态和电催化功能之间复杂关系的理解。