Encapsulating metal organic framework into hollow mesoporous carbon sphere as efficient oxygen bifunctional electrocatalyst

Applying metal organic frameworks (MOFs) in electrochemical systems is a currently emerging field owing to the rich metal nodes and highly specific surface area of MOFs. However, the problems for MOFs that need to be solved urgently are poor electrical conductivity and low ion transport. Here we present a facile in situ growth method for the rational synthesis of MOFs@hollow mesoporous carbon spheres (HMCS) yolk-shell-structured hybrid material for the first time. The size of the encapsulated Zeolitic Imidazolate Framework-67 (ZIF-67) is well controlled to 100 nm due to the spatial confinement effect of HMCS, and the electrical conductivity of ZIF-67 is also increased significantly. The ZIF@HMCS-25% hybrid material obtained exhibits a highly efficient oxygen reduction reaction activity with 0.823 V (vs. reversible hydrogen electrode) half-wave potential and an even higher kinetic current density (J _K = 13.8 mA cm^-2) than commercial Pt/C. ZIF@HMCS-25% also displays excellent oxygen evolution reaction performance and the overpotential of ZIF@HMCS-25% at 10 mA cm^-2 is 407 mV. In addition, ZIF@HMCS-25% is further employed as an air electrode for a rechargeable Zn-air battery, exhibiting a high power density (120.2 mW cm^-2 at 171.4 mA cm^-2) and long-term charge/discharge stability (80 h at 5 mA cm^-2). This MOFs@HMCS yolk-shell design provides a versatile method for the application of MOFs as electrocatalysts directly.