Axial orbital hybridization enables single-atom Fe-N-C hollow microplates for efficient oxygen reduction

Metal single-atoms with optimized coordination structure on highly accessible substrate can maximize the metal utilization efficiency along with enhancing catalytic activities. Herein, axial nitrogen-coordinated Fe-N5 sites on N-doped carbon (denoted as FeN5@N-C) hollow microplates are fabricated via a unique Fe3+-chelated polydopamine assisted hollowing strategy using ZIF-L microplates as multifunctional templates. Due to the powerful chelating and adhesive ability of polydopamine, this hollow-carbon strategy can be extended to fabricate single-atom Fe-N-C hollow structures with different shapes and encapsulate other transition-metal single atoms (Ni, Co, Mn, and Cu) into the N-doped carbon hollow microplates. The FeN5@N-C hollow microplates exhibit outstanding oxygen reduction reaction (ORR) capability with an impressive half-wave potential of 0.93 V vs. reversible hydrogen electrode and high stability, which can serve as air-cathode catalysts for high-performance Zn-air batteries with high peak power density of 225.3 mW cm−2 and stable cyclability of up to 400 h. Comprehensive analysis and theoretical calculations elucidate that axial nitrogen coordination in Fe-N5 catalytic sites, unlike the planar Fe-N4 configuration, can compete well with the bonding of OH* through additional 3d-2p orbital hybridization, thereby giving moderate bonding strength to enhance the ORR activity.