Spin polarization regulation of Fe-N₄ by Fe₃ atomic clusters for highly active oxygen reduction reaction

The Fe-N₄ motif is regarded as a leading non-precious metal catalyst for the oxygen reduction reaction (ORR) with the potential to replace platinum (Pt), yet achieving or surpassing the performance of Pt-based catalysts remains a significant challenge. In this study, we introduce a modification strategy employing homogeneous few-atom Fe₃ cluster to regulate the spin polarization of Fe-N₄. Experimental research and theoretical calculations show that the incorporation of the Fe₃ cluster significantly enhances the adsorption of Fe-N₄ motif toward OH ligands, leading to a structural transformation from a square-planar field (Fe-N₄) to a square-pyramid field structure (Fe(OH) -N₄). This structural transformation reduces the spin polarization of 3d_xz, 3d_yz, and 3d_z² orbitals of Fe-N₄, resulting in a decrease in unpaired electrons within 3d orbitals. As a result, this modulation leads to moderate adsorption/desorption energies of reaction intermediates, thereby facilitating the ORR process. Moreover, the in-situ spectroscopy confirms that the desorption of OH* on Fe₃/Fe(OH) -NC motif is more favorable compared to atomic Fe-NC, indicating a lower energy barrier for ORR. Consequently, the Fe₃/Fe-NC catalyst demonstrates outstanding ORR performance with a half-wave potential of 0.836 V vs. reversible hydrogen electrode (RHE) in 0.1 mol L^-1 HClO₄ solution and 0.936 V vs. RHE in 0.1 mol L^-1 KOH solution, even surpassing commercial Pt/C catalyst. It also exhibits excellent Zn-air battery efficiency. Our study introduces a novel approach to modulating the electronic structure of single atoms catalysts by leveraging the robust interaction between single atoms and atomic clusters.