ZIF-67 derived Co@CNTs nanoparticles: Remarkably improved hydrogen storage properties of MgH 2 and synergetic catalysis mechanism

© 2018 Hydrogen Energy Publications LLC Transition-metal nanoparticles (NPs) can catalytically improve the hydrogen desorption/absorption kinetics of MgH 2 , 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 MgH 2 to remarkably improve its hydrogen storage properties. The MgH 2 -Co@CNTs starts to obviously release hydrogen at 267.8 °C, displaying complete release of hydrogen at the capacity of 6.89 wt% at 300 °C within 15 min. For absorption, the MgH 2 -Co@CNTs uptakes 6.15 wt% H 2 at 250 °C within 2 min. Moreover, both improved hydrogen capacity and enhanced reaction kinetics of MgH 2 -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 MgH 2 -Co@CNTs can be ascribed to the synergetic effects of reversible phase transformation of Mg 2 Co to Mg 2 CoH 5 , and physical transformation of CNTs to carbon pieces. It is demonstrated that phase transformation of Mg 2 Co/Mg 2 CoH 5 can act as “hydrogen gateway” to catalytically accelerate the de/rehydrogenation kinetics of MgH 2 . Meanwhile, the carbon pieces coated on the surfaces of MgH 2 particles not only offer diffusion channels for hydrogen atoms but also prevent aggregation of MgH 2 NPs, resulting in the fast reaction rate and excellent cycling hydrogen storage properties of MgH 2 -Co@CNTs system.