The HR-TEM image of Co-Fe-N-C revealed a morphology similar to Co-N-C and with no obvious aggregated nanoparticles (Fig. ). Many atomically dispersed metal atoms were observed in spherical aberration corrected HAADF-STEM images (Fig. , Fig. ). Elemental EDXS mapping spectra (Fig. , 3d, S15) showed that both Co and Fe were homogeneously distributed throughout the material. Some possible double-atom pairs were visible in HAADF-STEM imaging (Fig. ). While it is tempting to assign them to the Co-Fe dimer, unambiguous confirmation is not possible because, as such images correspond to a 2D projection of a 3D structure, they might also originate from Co atoms that are separated by height in the amorphous carbon but which nearly overlap in the imaging projection. Unequivocal imaging of first-row transition metal double-atom pairs supported in a 3D substrate remains a fundamental challenge to be addressed in the future. Only a very small amount of Co-containing nanoparticles were observed in Co-Fe-N-C (Fig. ). However, nearly no Fe species were observed within these nanoparticles. Their tiny quantity and large size suggested that they are unlikely to be the active species for OER. To better evaluate the activity of these Co-containing nanoparticles, they were purposely prepared at a high pyrolysis temperature of 900 o C (see Supporting Information for preparation and Fig. -S20 for characterization). The resulting material, termed Co-N-C-900, exhibited much lower activity than Co-N-C after the same activation process, despite similar Co loadings compared to Co-N-C (Fig. ). This result further supports the hypothesis that the active sites of Co-Fe-N-C are atomically dispersed reaction centers.