Why Do Weak-Binding M–N–C Single-Atom Catalysts Possess Anomalously High Oxygen Reduction Activity?

Single-atom catalysts (SACs) with metal–nitrogen–carbon (M–N–C) structures are widely recognized as promising candidates in oxygen reduction reactions (ORR). According to the classical Sabatier principle, optimal 3d metal catalysts, such as Fe/Co–N–C, achieve superior catalytic performance due to the moderate binding strength. However, the substantial ORR activity demonstrated by weakly binding M–N–C catalysts such as Ni/Cu–N–C challenges current understandings, emphasizing the need to explore new underlying mechanisms. In this work, we integrated a pH-field coupled microkinetic model with detailed experimental electron state analyses to verify a novel key step in the ORR reaction pathway of weak-binding SACs─the oxygen adsorption at the metal–nitrogen bridge site. This step significantly altered the adsorption scaling relations, electric field responses, and solvation effects, further impacting the key kinetic reaction barrier from HOO^* to O^* and pH-dependent performance. Synchrotron spectra analysis further provides evidence for the new weak-binding M–N–C model, showing an increase in electron density on the antibonding π orbitals of N atoms in weak-binding M–N–C catalysts and confirming the presence of N–O bonds. These findings redefine the understanding of weak-binding M–N–C catalyst behavior, opening up new perspectives for their application in clean energy.