3D atomic-scale imaging of mixed Co-Fe spinel oxide nanoparticles during oxygen evolution reaction

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 Co₂FeO₄ and CoFe₂O₄ nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co₂FeO₄. The interfaces of Co-rich and Fe-rich nanodomains of Co₂FeO₄ become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe₂O₄. However, the activity of Co₂FeO₄ drops considerably due to concurrent irreversible transformation towards Co^IVO₂ and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe₂O₄ after OER, except for surface structural transformation towards (Fe^III, Co^III)₂O₃. 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.