Hydrogen storage properties of destabilized MgH_2-Li_3AlH_6 system

摘要

To improve the dehydrogenation properties of MgH_2, a novel hydrogen storage system, MgH_2-Li_3AlH_6, is prepared by mechanochemical milling. Three physical mixtures containing different mole ratios (1:4, 1:1 and 4:1) of MgH_2 and Li_3AlH_6 are studied and there exists a mutual destabilization effect between the components. The last mixture shows a capacity of 6.5 wt% H_2 with the lowest starting temperature of dehydrogenation (170 ℃). First, Li_3AlH_6 decomposes into Al, LiH and H_2, and then the as-formed Al can easily destabilize MgH_2 to form the intermetallic compound Mg_(17)Al_(12) at a temperature of 235 ℃, which is about 180 ℃ lower than the decomposition temperature of pristine MgH_2. Finally, the residual MgH_2 undergoes a self-decomposition whose apparent activation energy has been reduced by about 22 kJ mol~(-1) compared with pristine MgH_2. At a constant temperature of 250 ℃, the mixture can dehydrogenate completely under an initial vacuum and rehydrogenate to form MgH_2 under 2 MPa H_2, showing good cycle stability after the first cycle with a capacity of 4.5 wt% H_2. The comparison between 4 MgH_2 + Li_3AlH_6 and 4 MgH_2 + LiAlH_4 mixtures is also investigated.