Understanding the inter-site distance effect in single-atom catalysts for oxygen electroreduction

Regulating the site density of single-atom catalysts (SACs) promotes the potential to remarkably improve the performance of electrocatalysis, such as the oxygen reduction reaction (ORR). However, the catalytic behaviour governed by individual and interacting sites is particularly elusive and has yet to be understood. Here we demonstrate the origin of the enhancement of the ORR activity of isolated Fe-N-4 SACs over inter-site distances down to the subnanometre level. Strong interactions between adjacent Fe-N-4 moieties alter the electronic structure when the inter-site distance is less than about 1.2 nm, resulting in increased intrinsic ORR activity. The marked improvement in site performance continues until neighbouring Fe atoms approach as close as about 0.7 nm, below which the intrinsic activity is slightly diminished. The present study highlights the significance of identifying the fundamental mechanism of the inter-site distance effect in Fe-N-4 catalysts for the ORR, which may promote the full potential of densely populated SACs.