Maneuver of the d-orbital spin polarization of Fe single atomic sites via a near-range axial ligand effect for boosting oxygen reduction reaction

As one of the most competitive alternatives to precious Pt-based electrocatalysts towards the oxygen reduction reaction (ORR), Fe-N-C single atom catalysts (SACs) are still eagerly anticipated to undergo further performance enhancement to advance their practical applications in diverse renewable energy systems. The regulation of energy level of Fe site through the manipulation of d-orbital splitting manner provides a novel route to improve the intrinsic activity, yet still in its infancy. We herein present a Fe d-orbital splitting modulation strategy via crafting atomically dispersed Fe-N 5 moieties with axial N ligand anchored on “lotus root”-like N-doped multichannel carbon nanofibers (abbreviated as FeN 5 @NMCNFs hereafter). Symmetrical experimental and theoretical findings reveal that near-range coordinated effect of axial N ligand results in the deformation of local symmetry, deep valence band spin-polarization of d orbitals and downshifted d-band center of Fe center site, endowing the optimized adsorption energetics of ORR intermediates and reduced reaction energy barrier. Accordingly, the as-developed FeN 5 @NMCNFs deliver Pt-competitive ORR activity and extraordinary long-term durability in KOH electrolyte with a halfwave potential of 0.87 V, exceeding a majority of recently reported non-precious SACs. Furthermore, when incorporated into liquid and flexible all-solid-state Zn air batteries (ZABs), the FeN 5 @NMCNFs||RuO 2 -assembled ZABs demonstrate superior electrochemical performance to the state-of-the-art Pt/C||RuO 2 counterpart. This work offers a new perspective for the performance optimization of SACs via the manipulation of the local coordination manner and the precise spin-polarization regulation strategy.