Simultaneously Engineering the First and Second Coordination Shells of Single Iron Catalysts for Enhanced Oxygen Reduction.

The atomically dispersed Fe-N 4 active site presents enormous potential for various renewable energy conversions. Despite its already remarkable catalytic performance, the local atomic microenvironment of each Fe atom can be regulated to further enhance its efficiency. Herein, a novel conceptual strategy that utilizes a simple salt-template polymerization method to simultaneously adjust the first coordination shell (Fe-N 3 S 1 ) and second coordination shell (C-S-C, a structure similar to thiophene) of Fe-N 4 isolated atoms is proposed. Theoretical studies suggest that this approach can redistribute charge density in the MN 4 moiety, lowering the d-band center of the metal site. This weakens the binding of oxygenated intermediates, enhancing oxygen reduction reaction (ORR) activity when compared to only implementing coordination shell regulation. Based on the above discovery, a single Fe atom electrocatalyst with the optimal Fe-N 3 S 1 -S active moiety incorporated in nitrogen, sulfur co-doped graphene (Fe-SAc/NSG) is designed and synthesized. The Fe-SAc/NSG catalyst exhibits excellent alkaline ORR activity, exceeding benchmark Pt/C and most Fe-SAc ORR electrocatalysts, as well as superior stability in Zn-air battery. This work aims to pave the way for creating highly active single metal atom catalysts through the localized regulation of their atomic structure.