»HYDROGEN STORAGE IN MAGNESIUM BASED ALLOYS
Introduction
~ hydrogen storage remains a critical problem
~ while high pressure tanks are being used in several prototype cars today, the capacity is small and the volume large
~ and it is likely that safety concerns will also call for alternative solutions
~ light metal hydrides are a possible solution to this problem
~ one example is magnesium hydride, MgH2, which contains 7.6% hydrogen by weight
~ there are two serious problems though:
~ (1) the hydrogen binds so strongly in magnesiumhydride that temperature in excess of 700 K is needed to release the hydrogen,
~ while a release temperature around 400 K would be preferable, and
~ (2) the diffusion of hydrogen through the hydride is so slow that loading and unloading of hydrogen takes very long time
~ the question is whether the binding energy can be reduced and the rate of diffusion increased
~ by adding other elements to the magnesium while not reducing the mass ratio of hydrogen in the hydride too much
Results
~ the addition of aluminum was found to reduce the binding energy of hydrogen in the hydride
~ with a, Al molefraction of 0.15, the binding energy has been reduced from 0.38 eV to 0.24 eV,
~ corresponding to hydrogen release at 1 bar and 350 K
~ the addition of a rather small amount of a more electronegative element to the magnesium can significantly reduce the binding energy of the hydrogen in the hydride
~ experimental measurements have indicated that the addition of small amount of transition metal, for example titanium,
~ can speed up the unloading of hydrogen from magnesium hydride
~ a cubic crystal structure with a unit cell of Mg7TiHx was found from X-ray crystallography
~ the compound was synthesized under high pressure
~ calculations shown that the hydrogen ions sit in tetrahdedral holes in the lattice
~ (unlike the rutile structure of pure magnesium hydride) and that x=16
~ a large increase in diffusivity of hydrogen in the magnesium-titanium hydride is predicted from the theoretical calculations
~ as compared with pure magnesium hydride, largely because of greatly reduced formation energy of hydrogen interstitials in the hydride
~ the lowering of binding energy that can be accomplished by adding a less electronegative element such as aluminum and
~ the acceleration of diffusion that can be accomplished by adding a transition metal
~ do NOT exclude each other and that a ternary alloy could both have reduced binding energy and fast diffusion
~ the task remains,however, to find a stable alloy where both the pure metal alloy and the hydride are stable