In situ confinement pyrolysis of ZIF-67 nanocrystals on hollow carbon spheres towards efficient electrocatalysts for oxygen reduction

The design and preparation of metal-organic frameworks (MOFs) as self-sacrificed precursors/templates has been considered as a promising strategy in recent years for fabricating metal/carbon electrocatalysts with intriguing architectures and outstanding properties. However, the serious aggregation during the calcination and the poor electron conductivity are still obstacles for these electrocatalysts which need to be urgently solved. Herein, an in situ confinement pyrolysis protocol is reported to transform ZIF-67 nanocrystals on hollow carbon spheres (HCS) to cobalt and nitrogen-enriched carbon shell, resulting in the formation of hierarchical HCS@Co/NC. This is the first study of electrochemistry for HCS decorated with MOFs or MOFs derivatives. In the structure, metallic Co nanoparticles (NPs) and N species are strongly anchored and dispersed in the network of nanocarbon shell, which not only affords a boosting conductivity but also greatly alleviates the aggregation of active sites. Meanwhile, the unique structure with hollow feature provides an effective pathway for mass transport and shortens the transmission path of electrons. Thanks to the advantages of structure and composition, the HCS@Co/NC catalyst exhibits a superb performance of oxygen reduction reaction, which outperforms the commercial Pt/C benchmark.