| With the rapid development of human society,the energy crisis and environmental pollution are becoming serious.Zn-air batteries（ZABs）are considered as a promising next-generation energy storage technology due to the high theoretical energy density,safety and environmental friendliness.However,the successful commercialization of ZABs is hindered by the sluggish oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）at the cathode.Single-atom catalysts（SACs）with isolated metal atoms dispersed on solid supports have recently emerged as a new frontier in catalysis science and have attracted extensive attention.Metal single atoms show outstanding electrocatalytic activity owing to the abundant atomic reactive sites and superior stability.However,the preparation of single atoms suffers from inexorable metal aggregation which is harmful to electrocatalytic activity.This work has developed high-efficiency SACs with small size of aggregated metallic particles in the catalyst that can be used in ZABs.Here,we employ ionic covalent organic frameworks（iCOFs）as the sacrificed precursor to mitigate the metal aggregation and subsequent formation of bulky particles because the anionic groups of iCOFs can trap and confine metal ions to eschew the aggregation after pyrolysis.We prepared efficient oxygen electrocatalysis consisting of Co single atoms and homogeneous distribution of Co nanoparticles.Molecular dynamics（MD）simulation shows that iCOFs can trap and confine more Co ions as compared to neutral COFs,resulting in the formation of a catalyst composed of Co single atoms and uniformly distributed Co nanoparticles(Co SA&Co NP-10).However,the neutral COFs derive a catalyst composed of Co atomic clusters and large Co nanoparticles(Co AC&Co NP-25).HAADF-STEM and XANES can tell the coordination environment around the Co atom.The Co SA&Co NP-10 catalyst exhibits higher oxygen bifunctional electrocatalytic activities than Co AC&Co NP-25,coinciding with the density functional theory（DFT）results.Taking the Co SA&Co NP-10 as the air cathode in ZABs,the aqueous ZAB presents a high power density of 181 mW cm-2,a specific capacity of 811 mAh g-1 as well as a long cycle life of 407 h at a current density of 10 mA cm-2,while the quasi-solid state ZAB displays a power density of 179 mW cm-2 and the cycle life of 30 h.This work promote the development of single-atom catalysts and ZABs. |
