Uncovering the Nonmonotonic Relationship between Total Activity and Single-Atom Density for Oxygen Reduction Catalysis

In common sense, the total activity of single-atom catalysts (SACs) increases monotonically with the densification of single-atom sites, encouraging a general effort in developing high-density SACs for a variety of reactions, such as the oxygen reduction reaction (ORR). However, the intrinsic activity of each single-atom site may not remain constant with increasing density, since their growing interactions at the subnanometer scale can no longer be ignored. Here we report the nonmonotonic relationship between ORR activity and single-atom density, as revealed by theoretical calculations and experimental validation. Taking cobalt-embedded carbon as the model SAC for ORR, when the distance between neighboring Co sites is reduced below about 0.5 nm, proximity effects including hydrogen bonding and steric hindrance between adjacent intermediates dominate the ORR energetics, leading to a unexpected drop in the intrinsic activity. Our experiments unambiguously verified that both the total and mass activities of Co-SACs show turning points with increasing single-atom density. This counterintuitive nonmonotonic relationship between total activity and single-atom density may guide the rational design of high-performance SACs with optimal site densities.