Creating Asymmetric Fe-N₃C-N Sites in Single-Atom Catalysts Boosts Catalytic Performance for Oxygen Reduction Reaction

Fine tuning of the metal site coordination environment of a single-atom catalyst (SAC) to boost its catalytic activity for oxygen reduction reaction (ORR) is of significance but challenging. Herein, we report a new SAC bearing Fe-N₃C-N sites with asymmetric in-plane coordinated Fe-N₃C and axial coordinated N atom for ORR, which was obtained by pyrolysis of an iron isoporphyrin on polyvinylimidazole (PVI) coated carbon black. The C@PVI-(NCTPP)Fe-800 catalyst exhibited significantly improved ORR activity (E_1/2 = 0.89 V vs RHE) than the counterpart SAC with Fe-N₄-N sites in 0.1 M KOH. Significantly, the Zn-air batteries equipped with the C@PVI-(NCTPP)Fe-800 catalyst demonstrated an open-circuit voltage (OCV) of 1.45 V and a peak power density (P_max) of 130 mW/cm², outperforming the commercial Pt/C catalyst (OCV = 1.42 V; P_max = 119 mW/cm²). The density functional theory (DFT) calculations revealed that the d-band center of the asymmetric Fe-N₃C-N structure shifted upward, which enhances its electron-donating ability, favors O₂ adsorption, and supports O-O bond activation, thus leading to significantly promoted catalytic activity. This research presents an intriguing strategy for the designing of the active site architecture in metal SACs with a structure-function controlled approach, significantly enhancing their catalytic efficiency for the ORR and offering promising prospects in energy-conversion technologies.