Supramolecular confinement of single Cu atoms in hydrogel frameworks for oxygen reduction electrocatalysis with high atom utilization

Single-atom catalysis has emerged as a cutting-edge field in heterogeneous catalysis. Considerable efforts have been devoted to developing versatile methodologies for synthesizing single-atom catalysts, however, the main thrust in this field is to fundamentally understand the intricacies of a catalyst and their performance on a specific reaction by complimenting different techniques. Here we demonstrate a supramolecular hydrogel strategy to effectively isolate copper atoms on interconnected carbon fibers as efficient electrocatalysts for the alkaline oxygen reduction reaction (ORR). The Cu–N2 coordination state and the atomic dispersion were confirmed by X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy. Additionally, the atom utilization (η_atom), which is the ratio between Cu(I) sites participating in the catalysis and the total Cu(I) sites available, has been investigated via surface-interrogation scanning electrochemical microscopy (SI-SECM) technique. The over 90% atom utilization of the synthesized Cu SACs is very close to the theoretical value (100%) for SACs, implying the proposed supramolecular approach can enable the ultrahigh exposure of Cu sites. The in situ time-resolved titration of SI-SECM and first-principal calculations further support the remarkable ORR activity on isolated Cu–N2 sites.