Enhanced hydrogen storage properties of MgH₂ co-catalyzed with K₂NiF₆ and CNTs

The composite of MgH₂/K₂NiF₆/carbon nanotubes (CNTs) is prepared by ball milling, and its hydrogenation properties are studied for the first time. MgH₂ co-catalyzed with K₂NiF₆ and CNTs exhibited an improvement in the onset dehydrogenation temperature and isothermal de/rehydrogenation kinetics compared with the MgH₂-K₂NiF₆ composite. The onset dehydrogenation temperature of MgH₂ doped with 10 wt% K₂NiF₆ and 5 wt% CNTs is 245 °C, which demonstrated a reduction of 25 °C compared with the MgH₂ + 10 wt% K₂NiF₆ composite. In terms of rehydrogenation kinetics, MgH₂ doped with 10 wt% K₂NiF₆ and 5 wt% CNTs samples absorbed 3.4 wt% of hydrogen in 1 min at 320 °C, whereas the MgH₂ + 10 wt% K₂NiF₆ sample absorbed 2.6 wt% of hydrogen under the same conditions. For dehydrogenation kinetics at 320 °C, the MgH₂ + 10 wt% K₂NiF₆ + 5 wt% CNTs sample released 3.3 wt% hydrogen after 5 min of dehydrogenation. By contrast, MgH₂ doped with 10 wt% K₂NiF₆ released 3.0 wt% hydrogen in the same time period. The apparent activation energy, E_a, for the dehydrogenation of MgH₂ doped with 10 wt% K₂NiF₆ reduced from 100.0 kJ mol^-1 to 70.0 kJ mol^-1 after MgH₂ was co-doped with 10 wt% K₂NiF₆ and 5 wt% CNTs. Based on the experimental results, the hydrogen storage properties of the MgH₂/K₂NiF₆/CNTs composite is enhanced because of the catalytic effects of the active species of KF, KH and Mg₂Ni that are formed in situ during dehydrogenation, as well as the unique structure of CNTs.