Mesopore-Confined Synthesis of Cobalt Nanoparticles with Single Atoms in Close Proximity for Efficient Oxygen Reduction

Although the synergy between single atom sites (SAs)and nanoparticles(NPs) can enhance the oxygen reduction performance, current syntheticstrategies are not able to precisely control the distribution of SAs/NPsor the size of NPs under high metal loading, undesirably limitingthe overall performance. Herein, we report that the pyrolysis of nanosizedzeolitic imidazolate frameworks (ZIF-67) that are preconfined in thepores of mesoporous carbon (MC) can produce Co SAs and NPs (mean sizeof 4.3 nm at 15 wt % loadings) in close proximity to each other. Asthe movement of finely dispersed ZIF-67 is restricted by the pore-confinementconfiguration, the pyrolysis of spatially isolated ZIF-67 units canproduce abundant Co-N-4 sites distributed in thefirst or second graphitic shell encapsulating the NPs. Benefittingfrom the high density of active sites, the maximized synergetic interactionof SAs/NPs, and the rich mesoporosity of MC, Co-SAs/NPs@MC exhibitsa half-wave potential (E (1/2)) of 0.91 Vand excellent durability under alkaline conditions, outperformingcommercial Pt/C and most reported counterparts. Control experimentscombined with density functional theory calculations evidence thatthe presence of Co NPs is essential to optimize the adsorption ofintermediates on the intimately coupled, catalytically active Co-N-4 sites toward ORR. Moreover, it is revealed that the distancebetween Co SAs and NPs is critical for superior activity. As controlledfabrication of SA/NP hybrid catalysts is desired, the pore-confinementmethodology reported herein and the advantages demonstrated by ORRare expected to guide the design of other functional catalysts.