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Study on the Structure-Activity Relationship Between Single-Atom, Cluster and Nanoparticle Catalysts in a Hierarchical Structure for the Oxygen Reduction Reaction.

Literature reports have shown that in primary structures, single-atom catalysts exhibit better performance than cluster and nanoparticles due to their maximum atom utilization and the fine-tuning of the electronic structure of the active sites. Hierarchical structures have recently been extensively studied because of increased active sites and orderliness of channels significantly improves the catalytic performance compare to primary structures especially in nanoparticles, however, the different sized effect of catalysts research has not been reported. Herein, a unique hollow double-shell structure (a distinct cavity-containing) is used as a hierarchical model to study the possible difference between single atom, cluster, and nanoparticle and to establish the corresponding structure-activity relationship. Three Co catalysts are prepared: single atoms (Co-Catalyst-1), clusters (Co-Catalyst-2, 0.5-1 nm), and nanoparticles (Co-Catalyst-3, ≈5 nm) and their oxygen-reduction capacity is evaluated. The unique electronic interactions, the strong electron-withdrawing ability of N in Co-N4 (Co-Catalyst-1), attract electrons from the electrode to Co, specifically by expediting the generation and transformation of the rate-determining step intermediates *OOH. The variant spatial structure which is caused by Co atom aggregation, and led to surface area, pore size, and carbon disorder, is a distinct, therefore significant variation in mass and charge transport efficiency, and activities.