Contracted Fe-N₅-C₁₁ Sites in Single-Atom Catalysts Boosting Catalytic Performance for Oxygen Reduction Reaction

Promoting the catalyst performance for oxygen reductionreaction(ORR) in energy conversion devices through controlled manipulationof the structure of catalytic active sites has been a major challenge.In this work, we prepared Fe-N-C single-atom catalysts(SACs) with Fe-N-5 active sites and found that thecatalytic activity of the catalyst with shrinkable Fe-N-5-C-11 sites for ORR was significantly improvedcompared with the catalyst bearing normal Fe-N-5-C-12 sites. The catalyst C@PVI-(TPC)Fe-800, prepared by pyrolyzingan axial-imidazole-coordinated iron corrole precursor, exhibited positiveshifted half-wave potential (E (1/2) = 0.89V vs RHE) and higher peak power density (P (max) = 129 mW/cm(2)) than the iron porphyrin-derived counterpartC@PVI-(TPP)Fe-800 (E (1/2) = 0.81 V, P (max) = 110 mW/cm(2)) in 0.1 M KOH electrolyteand Zn-air batteries, respectively. X-ray absorption spectroscopy(XAS) analysis of C@PVI-(TPC)Fe-800 revealed a contracted Fe-N-5-C-11 structure with iron in a higher oxidationstate than the porphyrin-derived Fe-N-5-C-12 counterpart. Density functional theory (DFT) calculationsdemonstrated that C@PVI-(TPC)Fe-800 possesses a higher HOMO energylevel than C@PVI-(TPP)Fe-800, which can increase its electron-donatingability and thus help achieve enhanced O-2 adsorption aswell as O-O bond activation. This work provides a new approachto tune the active site structure of SACs with unique contracted Fe-N-5-C-11 sites that remarkably promote the catalystperformance, suggesting significant implications for catalyst designin energy conversion devices.