Regulating the Coordination Geometry and Oxidation State of Single-Atom Fe Sites for Enhanced Oxygen Reduction Electrocatalysis.

FeNC catalysts demonstrate remarkable activity and stability for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells and Zn-air batteries (ZABs). The local coordination of Fe single atoms in FeNC catalysts strongly impacts ORR activity. Herein, FeNC catalysts containing Fe single atoms sites with FeN 3 , FeN 4 , and FeN 5 coordinations are synthesized by carbonization of Fe-rich polypyrrole precursors. The FeN 5 sites possess a higher Fe oxidation state (+2.62) than the FeN 3 (+2.23) and FeN 4 (+2.47) sites, and higher ORR activity. Density functional theory calculations verify that the FeN 5 coordination optimizes the adsorption and desorption of ORR intermediates, dramatically lowering the energy barrier for OH - desorption in the rate-limiting ORR step. A primary ZAB constructed using the FeNC catalyst with FeN 5 sites demonstrates state-of-the-art performance (an open circuit potential of 1.629 V, power density of 159 mW cm -2 ). Results confirm an intimate structure-activity relationship between Fe coordination, Fe oxidation state, and ORR activity in FeNC catalysts.