Summing up, the XAS data reveal the following structural evolution of the single-atom Co precatalyst during activation and OER (Fig. ): the as-prepared sample comprised of atomically dispersed Co atoms stabilized by three N and one C atoms from the N-doped carbon support and one oxygen from OH -/H2O; upon immersion into an alkaline electrolyte, the ligand environment of Co ions changes significantly, and the Co ions are now coordinated to 1 C and 1 N from the support and 4 OH -/OH2 groups coming from the electrolyte. After electrochemical activation, Fe is incorporated and a dimeric Co-Fe moiety is formed, probably through one or two bridging OHs. This structure remains stable during OER. Taking into consideration of the essential role of Fe in catalysis and the activity scales with the amount of Fe, we attribute this dimeric, Co-Fe doubleatom site as the active site of Co-Fe-N-C. As the XAS results above indicated the active site of Co-Fe-N-C is the Co-Fe dimeric unit, the turnover frequencies (TOFs) of the catalyst could be calculated using the total amount of such units (Fig. ). With TOFs of higher than 1 s -1 at 300 mV and about 12 s -1 at = 350 mV, the Co-Fe-N-C has activity comparable to the most active nanomaterials based on transition metals (Table ). It is noteworthy that the Co-Fe-N-C also exhibited the best OER performance compared to previously reported OER catalysts consisted of single atoms or subnano clusters (Table ).