  Single-atom catalysts (SACs) with metal-nitrogen-carbon (M-N-C) structures
are promising candidates for oxygen reduction reactions (ORR). Based on the
adsorption strength, theory predicts that an optimal catalyst should possess
moderate binding strength (e.g., M=Fe/Co). However, the considerable ORR
activity observed from weak-binding M-N-C catalysts (M=Ni/Cu/Zn) in alkaline
electrolytes contradicts theoretical prediction, challenging the
well-established Sabatier principle and urging for identifying new underlying
mechanisms. This study reveals a new bridge-adsorbed oxygen mechanism in
weak-binding SACs by incorporating a pH-field coupled microkinetic model and
experiments. We found that the O* favours bridge adsorption in weak-binding
SACs that differs significantly from the typical atop-O* in terms of scaling
relations, electric field responses, and solvation effects. These variations
further impact the pH dependence and the kinetic barriers of HOO* activation,
for which a unified scaling relation model with kinetics incorporated has been
developed. This new model shows improved alignment with experimental data,
offering a design principle for high-performance SACs by leveraging
weak-binding atoms to modify reaction pathways, potentially advancing SAC
development for ORR and beyond.
