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