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Promoting the catalyst performance for oxygen reduction reaction (ORR) in energy conversion devices through controlled manipulation of 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–N5active sites and found that the catalytic activity of the catalyst with shrinkable Fe–N5–C11sites for ORR was significantly improved compared with the catalyst bearing normal Fe–N5–C12sites. The catalyst C@PVI-(TPC)Fe-800, prepared by pyrolyzing an axial-imidazole-coordinated iron corrole precursor, exhibited positive shifted half-wave potential (E1/2= 0.89 V vs RHE) and higher peak power density (Pmax= 129 mW/cm2) than the iron porphyrin-derived counterpart C@PVI-(TPP)Fe-800 (E1/2= 0.81 V, Pmax= 110 mW/cm2) in 0.1 M KOH electrolyte and Zn–air batteries, respectively. X-ray absorption spectroscopy (XAS) analysis of C@PVI-(TPC)Fe-800 revealed a contracted Fe–N5–C11structure with iron in a higher oxidation state than the porphyrin-derived Fe–N5–C12counterpart. Density functional theory (DFT) calculations demonstrated that C@PVI-(TPC)Fe-800 possesses a higher HOMO energy level than C@PVI-(TPP)Fe-800, which can increase its electron-donating ability and thus help achieve enhanced O2adsorption as well as O–O bond activation. This work provides a new approach to tune the active site structure of SACs with unique contracted Fe–N5–C11sites that remarkably promote the catalyst performance, suggesting significant implications for catalyst design in energy conversion devices.

There is an urgent need for developing nonprecious metal catalysts to replace Pt-based electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Atomically dispersed M–Nx/C catalysts have shown promising ORR activity; however, enhancing their performance through modulating their active site structure is still a challenge. In this study, a simple approach was proposed for preparing atomically dispersed iron catalysts embedded in nitrogen- and fluorine-doped porous carbon materials with five-coordinated Fe–N5sites. The C@PVI-(DFTPP)Fe-800 catalyst, obtained through pyrolysis of a bio-inspired iron porphyrin precursor coordinated with an axial imidazole from the surface of polyvinylimidazole-grafted carbon black at 800 °C under an Ar atmosphere, exhibited a high electrocatalytic activity with a half-wave potential of 0.88 V versus the reversible hydrogen electrode for ORR through a four-electron reduction pathway in alkaline media. In addition, an anion-exchange membrane electrode assembly (MEA) with C@PVI-(DFTPP)Fe-800 as the cathode electrocatalyst generated a maximum power density of 0.104 W cm–2and a current density of 0.317 mA cm–2. X-ray absorption spectroscopy demonstrated that a single-atom catalyst (Fe–Nx/C) with an Fe–N5active site can selectively be obtained; furthermore, the catalyst ORR activity can be tuned using fluorine atom doping through appropriate pre-assembling of the molecular catalyst on a carbon support followed by pyrolysis. This provides an effective strategy to prepare structure-performance-correlated electrocatalysts at the molecular level with a large number of M–Nxactive sites for ORR. This method can also be utilized for designing other catalysts.

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 (E1/2= 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 (Pmax) of 130 mW/cm2, outperforming the commercial Pt/C catalyst (OCV = 1.42 V; Pmax= 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 O2adsorption, 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.