d-Orbital Electron Delocalization Realized by Axial Fe C Atomic Clusters Delivers High-Performance Fe-N-C Catalysts for Oxygen Reduction Reaction.

Fe-N-C catalyst for oxygen reduction reaction (ORR) has been considered as the most promising nonprecious metal catalyst due to its comparable catalytic performance to Pt in proton exchange membrane fuel cells (PEMFCs). The active centers of Fe-pyrrolic N have been proven to be extremely active for ORR. However, forming a stable Fe-pyrrolic N structure is a huge challenge. Here, a Cyan-Fe-N-C catalyst with Fe-pyrrolic N as the intrinsic active center is constructed with the help of axial Fe C atomic clusters, which shows a half-wave potential of up to 0.836 V (vs. RHE) in the acid environment. More remarkably, it delivers a high power density of 870 and 478 mW cm at 1.0 bar in H -O and H -Air fuel cells, respectively. According to theoretical calculation and in situ spectroscopy, the axial Fe C can provide strong electronic perturbation to Fe-N active centers, leading to the d-orbital electron delocalization of Fe and forming the Fe-pyrrolic N bond with high charge distribution, which stabilizes the Fe-pyrrolic N structure and optimizes the OH* adsorption during the catalytic process. This work proposes a new strategy to adjust the electronic structure of single-atom catalysts based on the strong interaction between single atoms and atomic clusters.