Pure MgH2 was milled under a hydrogen atmosphere (reactive mechanical grinding, RMG). The hydrogen storage properties of the prepared samples were studied at a relatively low temperature of 423 K and were compared with those of pure Mg. The hydriding rate of the Mg was extremely low (0.0008 wt% H/min at n = 4), and the MgH2 after RMG had higher hydriding rates than that of Mg at 423 K under 12 bar H2. The initial hydriding rate of MgH2 after RMG at 423 K under 12 bar H2 was the highest (0.08 wt% H/min) at n = 2. At n = 2, the MgH2 after RMG absorbed 0.39 wt% H for 5 min, and 1.21 wt% H for 60 min at 423K under 12 bar H2. At 573 K under 12 bar H2, the MgH2 after RMG absorbed 4.86 wt% H for 5 min, and 5.52 wt% H for 60 min at n = 2. At 573 K and 423 K under 1.0 bar H2, the MgH2 after RMG and the Mg did not release hydrogen. The decrease in particle size and creation of defects by reactive mechanical grinding are believed to have led to the increase in the hydriding rate of the MgH2 after RMG at a relatively low temperature of 423 K.

A 90 wt% Mg-10 wt% NbF5 sample was prepared by mechanical milling under H2 (reactive mechanical grinding). Its hydriding and dehydriding properties were then examined. Activation of the 90 wt% Mg-10 wt% NbF5 sample was not required. At n=1, the sample absorbed 3.11 wt% H for 2.5 min, 3.55 wt% H for 5 min, 3.86 wt% H for 10 min, and 4.23 wt% H for 30 min at 593K under 12 bar H2. At n=1, the sample desorbed 0.17 wt% H for 5 min, 0.74 wt% H for 10 min, 2.03 wt% H for 30 min, and 2.81 wt% H for 60 min at 593K under 1.0 bar H2. The XRD pattern of the 90 wt% Mg-10 wt% NbF5 after reactive mechanical grinding showed Mg, β-MgH2 and small amounts of γ-MgH2, NbH2, MgF2 and NbF3. The XRD pattern of the 90 wt% Mg-10 wt% NbF5 dehydrided at n=3 revealed Mg, β-MgH2, a small amount of MgO and very small amounts of MgH2 and NbH2. The 90 wt% Mg-10 wt% NbF5 had a higher initial hydriding rate and a larger quantity of hydrogen absorbed for 60 min than the 90 wt% Mg-10 wt% MnO and the 90 wt% Mg-10 wt% Fe2O3, which were reported to have quite high hydriding rates and/or dehydriding rates. The 90 wt% Mg-10 wt% NbF5 had a higher initial dehydriding rate (after an incubation period) and a larger quantity of hydrogen desorbed for 60 min than the 90 wt% Mg-10 wt% MnO and the 90 wt% Mg-10 wt% Fe2O3.

The hydrogen storage properties of pure MgH2 were studied and compared with those of pure Mg. At the first cycle,pure MgH2 absorbed hydrogen very slowly at 573 K under 12 bar H2. The activation of pure MgH2 was completed after threehydriding-dehydriding cycles. At the 4th cycle, the pure MgH2 absorbed 1.55wt% H for 5 min, 2.04wt% H for 10 min, and3.59wt% H for 60 min, showing that the activated MgH2 had a much higher initial hydriding rate and much larger Ha (60min), quantity of hydrogen absorbed for 60 min, than did activated pure Mg. The activated pure Mg, whose activation wascompleted after four hydriding-dehydriding cycles, absorbed 0.80wt% H for 5 min, 1.25wt% H for 10 min, and 2.34wt%H for 60 min. The particle sizes of the MgH2 were much smaller than those of the pure Mg before and after hydriding-dehydriding cycling. The pure Mg had larger hydrogen quantities absorbed at 573K under 12 bar H2 for 60 min, Ha (60 min),than did the pure MgH2 from the number of cycles n=1 to n=3; however, the pure MgH2 had larger Ha (60 min) than didthe pure Mg from n=4 to n=6.