Construction of Fe Nanoclusters/Nanoparticles to Engineer FeN₄ Sites on Multichannel Porous Carbon Fibers for Boosting Oxygen Reduction Reaction

Fe–N–C catalysts are emerging as promising alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR), while they still suffer from sluggish reaction kinetics due to the discontented binding affinity between the Fe-N₄ sites and oxygen-containing intermediates, and unsatisfactory stability. Herein, a flexible multichannel carbon fiber membrane immobilized with atomically dispersed Fe-N₄ sites and neighboring Fe nanoclusters/nanoparticles (FeN₄-Fe_NCP@MCF) is synthesized. The optimized geometric and electronic structures of the Fe atomic sites brought by adjacent Fe nanoclusters/nanoparticles and hierarchically porous structure of the carbon matrix endow FeN₄-Fe_NCP@MCF with outstanding ORR activity and stability, considerably outperforming its counterpart with FeN₄ sites only and the commercial Pt/C catalyst. Liquid and solid-state flexible zinc–air batteries employing FeN₄-Fe_NCP@MCF both exhibit outstanding durability. Theoretical calculation reveals that the Fe nanoclusters can trigger remarkable electron redistribution of the FeN₄ sites and modulate the hybridization of central Fe 3d and O 2p orbitals, facilitating the activation of O₂ molecules and optimizing the adsorption capacity of oxygen-containing intermediates on FeN₄ sites, and thus accelerating the ORR kinetic. This work offers an effective approach to constructing coupling catalysts that have single atoms coexisting with nanoclusters/nanoparticles for efficient ORR catalysis.