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 synthetic strategies are not able to precisely control the distribution of SAs/NPs or the size of NPs under high metal loading, undesirably limiting the overall performance. Herein, we report that the pyrolysis of nanosized zeolitic imidazolate frameworks (ZIF-67) that are preconfined in the pores of mesoporous carbon (MC) can produce Co SAs and NPs (mean size of 4.3 nm at 15 wt % loadings) in close proximity to each other. As the movement of finely dispersed ZIF-67 is restricted by the pore-confinement configuration, the pyrolysis of spatially isolated ZIF-67 units can produce abundant Co–N4 sites distributed in the first or second graphitic shell encapsulating the NPs. Benefitting from the high density of active sites, the maximized synergetic interaction of SAs/NPs, and the rich mesoporosity of MC, Co-SAs/NPs@MC exhibits a half-wave potential (E1/2) of 0.91 V and excellent durability under alkaline conditions, outperforming commercial Pt/C and most reported counterparts. Control experiments combined with density functional theory calculations evidence that the presence of Co NPs is essential to optimize the adsorption of intermediates on the intimately coupled, catalytically active Co–N4 sites toward ORR. Moreover, it is revealed that the distance between Co SAs and NPs is critical for superior activity. As controlled fabrication of SA/NP hybrid catalysts is desired, the pore-confinement methodology reported herein and the advantages demonstrated by ORR are expected to guide the design of other functional catalysts.