A stepwise thermal migration for inducing copper nanoparticles to boost oxygen reduction activity of single-atomic copper sites

Although single-atom M-N-C electrocatalysts have demonstrated their potential in metal-air batteries and fuel cells, their activity for oxygen reduction reaction (ORR) needs to be further improved. Regulating the electronic structure of M-Nx single sites to accelerate the activity and simultaneously directly identify the enhancement mechanisms is highly desirable but remains a challenge. Here we propose a stepwise thermal migration strategy to in situ introducing Cu nanoparticles (Cu-NPs) adjacent to single atoms Cu (Cu-SAs) confined into hierarchically porous carbon. The optimization of the electronic structure of the Cu-Nx site by adjacent Cu nanoparticles is confirmed by both enhanced ORR activity and theoretical calculations. The optimized CuSA-NP@NC exhibits a more positive half-wave potential (0.870 V) than CuSA@NC (0.842 V) and superior stability with an 86.6% ORR current retention after 24 hours of chronoamperometric test in alkaline electrolyte. When utilized as an air cathode in rechargeable Zn-air batteries, CuSA-NP@NC displays a high-power density of 224.1 mWcm−2 and a specific capacity of 802.0 mAh g−1. Theoretical calculations demonstrate that the introduction of Cu-NPs endows the Cu-N4 site with an enriched electron density, which optimizes the adsorption/desorption of ORR intermediates on the Cu-N4 sites. This work reports an effective way and provides insights into the ORR activity improvement of single-atom electrocatalysts.