Engineering d–p orbital hybridization of single-atom Fe sites <i>via</i> axial B-mediation for the oxygen reduction reaction

Single-atom Fe-N-C catalysts have demonstrated promising potential in the oxygen reduction reaction (ORR), yet their intrinsic activity remains less than ideal. Orbital hybridization provides a versatile means to modulate the thermodynamic and kinetic properties during electrochemical processes. In this study, we adopt an "axial ligand boron-modulation" strategy to regulate the electronic structure of single-atom Fe sites through d<i>-</i>p orbital hybridization. The synthesized FeN₄-B/NC demonstrates exceptional ORR activity with a half-wave potential of 0.915 V, surpassing planar FeN₄/NC and commercial Pt/C. <i>In situ</i> XAS results reveal the dynamic stretching of Fe-N/O and Fe-B bonds during the ORR process, providing an intuitive confirmation that the single-atom sites undergo reversible structural changes to optimize the adsorption of reaction intermediates. Theoretical investigations combined with zero-field cooling temperature dependence analyses demonstrate that in the intermediate spin state, hybridization occurs between the central Fe's 3d orbitals and B's 2p orbitals, which results in increased e_g orbital occupancy and positions the d-band center closer to the Fermi level, which enhances charge transfer efficiency and O₂ adsorption capabilities. Furthermore, the newly developed FeN₄-B/NC catalyst shows remarkable performance in liquid and quasi-solid-state zinc-air batteries.