Substrate strain tunes operando geometric distortion and oxygen reduction activity of CuN₂C₂ single-atom sites

Single-atom catalysts are becoming increasingly significant to numerous energy conversion reactions. However, their rational design and construction remain quite challenging due to the poorly understood structure-function relationship. Here we demonstrate the dynamic behavior of CuN₂C₂ site during operando oxygen reduction reaction, revealing a substrate-strain tuned geometry distortion of active sites and its correlation with the activity. Our best CuN₂C₂ site, on carbon nanotube with 8 nm diameter, delivers a sixfold activity promotion relative to graphene. Density functional theory and X-ray absorption spectroscopy reveal that reasonable substrate strain allows the optimized distortion, where Cu bonds strongly with the oxygen species while maintaining intimate coordination with C/N atoms. The optimized distortion facilitates the electron transfer from Cu to the adsorbed O, greatly boosting the oxygen reduction activity. This work uncovers the structure-function relationship of single-atom catalysts in terms of carbon substrate, and provides guidance to their future design and activity promotion.