Tuning axial ligand coordination for enhanced activity and selectivity in metal–nitrogen–carbon single-atom catalysts for the oxygen reduction reaction

Axial ligand coordination plays a critical role in modulating the catalytic activity and selectivity of metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) for the oxygen reduction reaction (ORR). In this study, we systematically investigate FeN4, CoN4, and NiN4 active sites coordinated with a broad range of axial ligands to elucidate how ligand environments influence reaction energetics across different metal centers. Free energy analyses reveal that axial ligands can substantially modify key steps in the ORR pathway, including *OOH formation and *OH desorption, with distinct effects observed for each metal. FeN4 exhibits tunable activity and selectivity depending on the ligand, balancing between 4e- and 2e- pathways. CoN4 shows a pronounced preference toward the 2e- pathway under specific ligand environments, highlighting its potential for selective H2O2 production. In contrast, NiN4 displays generally high energy barriers and limited ORR activity across all axial ligands. These results establish a comparative framework for understanding how axial ligand coordination governs ORR performance and provide guidance for the rational design of M-N-C SACs with tailored activity and selectivity.