Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/absorption kinetics of MgH2, yet this catalysis could be enhanced further by supporting NPs on carbon-based matrix materials. In this work, Co NPs with a uniform size of 10 nm loaded on carbon nanotubes (Co@CNTs) were synthesized in situ by carbonizing zeolitic imidazolate framework-67 (ZIF-67). The novel Co@CNTs nanocatalyst was subsequently doped into MgH2 to remarkably improve its hydrogen storage properties. The MgH2-Co@CNTs starts to obviously release hydrogen at 267.8 degrees C, displaying complete release of hydrogen at the capacity of 6.89 wt% at 300 degrees C within 15 min. For absorption, the MgH2-Co@CNTs uptakes 6.15 wt% H-2 at 250 degrees C within 2 min. Moreover, both improved hydrogen capacity and enhanced reaction kinetics of MgH2-Co@CNTs can be well preserved during the 10 cycles, which confirms the excellent cycling hydrogen storage performances. Based on XRD, TEM and EDS results, the catalytic mechanism of MgH2-Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg2Co to Mg2CoH5, and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg2Co/Mg2CoH5 can act as "hydrogen gateway" to catalytically accelerate the de/rehydrogenation kinetics of MgH2. Meanwhile, the carbon pieces coated on the surfaces of MgH2 particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH2 NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH2-Co@CNTs system. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.