Magnesium and Doped Magnesium Nanostructured Materials for Hydrogen Storage - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 292

Fig. 1 A comparison of a range of materials as a function of stored hydrogen per volume and per
mass. Reproduced with permission from Schlapbach 2001
due to their high theoretical hydrogen storage capacities (e.g., 7.6 wt.% for MgH 2 )
[ 3 , 4 ]. Pure Mg particles can store 1 kg of H 2 in 6.9 L of solid metal hydride.
However, bulk Mg is less than ideal as a hydrogen storage material due to the slow
kinetics and high temperatures required for hydrogen absorption/desorption.
One common approach to mitigating the sluggish kinetics for bulk magnesium
is to increase the surface area to volume ratio of the material. The reaction of bulk
Mg and molecular hydrogen first involves the adsorption of molecular hydrogen
onto the surface of the magnesium, the breaking of the hydrogen-hydrogen bond,
and then the diffusion of the atomic hydrogen into the magnesium. Once a critical
concentration is reached, MgH 2 nucleates and grows (shown in Fig 2 ). One can
imagine that by reducing the physical dimensions of the material to the nanoscale,
the diffusion length required for hydrogen to diffuse by solid-state diffusion
from the surface of the material to the center is dramatically reduced, which can
reduce the time necessary for hydrogenation (and the same effect should be true for
the reverse reaction, removing hydrogen from the hydride).
Recent studies have shown that hydrogen storage properties can be greatly
improved by preparing these materials with nanocrystalline grain sizes, often
achieved by mechanical ball-milling, and/or by alloying Mg with other metals [ 3 - 5 ].
Previous studies have shown that reducing the particle size of metal hydrides such as
Mg 2 Ni significantly increases the kinetics of hydrogen absorption [ 4 ]. The increase
in the kinetics for hydrogen absorption may be a result of a decrease in the diffusion
length for hydrogen due to the small sizes of the particles (as mentioned previously),
but the lack of control over the size and homogeneity of the samples has made it
difficult to determine this conclusively. The synthetic methods used (such as ball
milling [ 4 ] and hydrogen plasma-metal reactions [ 6 ]) can produce samples that are