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Weikai Xiang, Nating Yang, Xiaopeng Li, Julia Linnemann, Ulrich Johannes Hagemann, Olaf Rüdiger, Markus Heidelmann, Tobias Falk, Matteo Aramini, Serena DeBeer, Martin Muhler, Kristina Tschulik, Tong Li

• The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized \(Co_{2}FeO_{4}\) and \(CoFe_{2}O_{4}\) nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine \(Co_{2}FeO_{4}\). The interfaces of Co-rich and Fe-rich nanodomains of \(Co_{2}FeO_{4}\) become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of \(CoFe_{2}O_{4}\). However, the activity of \(Co_{2}FeO_{4}\) drops considerably due to concurrent irreversible transformation towards \(Co^{IV}O_{2}\) and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for \(CoFe_{2}O_{4}\) after OER, except for surface structural transformation towards (\(Fe^{III}, Co^{III})_{2}O_{3}\). Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER.