Deuterium MAS NMR Studies of Mg-Ti Deuteride Formed During High Pressure Ball Milling

摘要

Magnesium hydride has long been considered a candidate onboard hydrogen storage material for hydrogen fuel cell powered vehicles due to its low cost, abundance (of magnesium metal), and high hydrogen content (7.6 wt%). However, its use has been limited by the poor kinetics and thermodynamics of the reversible hydriding/dehydriding reaction. Improvements to hydrogen uptake and release have been previously demonstrated when Mg is alloyed with Sc. The resulting MgSc-hydride adopts a cubic, fluorite crystal structure (as opposed to the rutile, tetragonal structure for bulk MgH2) that exhibits a significantly lower activation energy for thermally driven hydrogen motion. However, Sc is too expensive for widespread implementation. Here we investigate the formation of metastable Mg-Ti-D compounds. Mg and Ti are not mutually soluble at equilibrium, so a molybdenum-lined high pressure vessel (to avoid ferrous and other contamination) has been developed to allow for the ball milling of Mg-Ti powders under high deuterium gas pressures. The local atomic structural properties of the resulting Mg-Ti-D composites have been studied by deuterium Magic Angle Spinning (MAS) nuclear magnetic resonance (NMR), which is sensitive to the deuterium atom siting within the Mg-Ti lattice. The change from rutile to cubic structure is known to play an important role in promoting hydrogen motion within the lattice and thus improving the hydrogen storage properties of the compound. Thus the formation of a fluorite-structured Mg-Ti-D is desired.