Impact of halogen axial coordination on the electronic and magnetic properties of MN4 single-atom catalysts for oxygen reduction

Abstract

The rational design of single-atom catalysts (SACs) through coordination engineering is a pivotal strategy for enhancing the oxygen reduction reaction (ORR). While axial ligand coordination has been identified as a promising approach, the specific role of halogen atoms and their influence on both electronic and magnetic properties remain insufficiently explored. Herein, we present a systematic first-principles study on the impact of axial halogen ligands (X = –F, –Cl, –Br, –I) on the ORR performance of MN₄/C SACs across the 3d transition metal series. Our computations reveal that halogen coordination universally enhances the structural stability of the single-atom sites and, in most cases, significantly improves the ORR activity. Among the studied metals, iron-based catalysts exhibit the most pronounced promotion, which we attribute to the competitive coordination between the halogen and reaction intermediates for the Fe dz² orbital, leading to an optimized adsorption strength of oxygenated species. Crucially, we identify a robust linear correlation between the metal-centered magnetic moment and the adsorption free energy of key intermediates. This relationship establishes the magnetic moment as a novel and effective descriptor for predicting ORR activity, providing a fresh perspective for catalyst design. These findings underscore the role of axial halogen ligands in modulating electronic and magnetic properties, offering a viable pathway for developing high-performance M–N–C electrocatalysts.