Environmental Engineering Reference
FIGURE 6.10 Arrhenius plots of hydrogen absorption and desorption rate constant k versus reciprocal
temperature 1/ T for V-decorated Mg nanoblades. Source : Reproduced with permission from He et al .
1. The Effect of Nanostructure on the Thermodynamics of Hydriding/
Dehydriding of Metal Hydrides. Four potential factors could be account
to affect the thermodynamics of hydrogen storage process: the presence
of a metastable phase, nanostructure size (surface area), stress/strain at
the grain boundary, and excess volume in deform regions.
(a) New chemical species can be introduced to react with metal and
form an intermediate state. This can alter the formation path of
metal hydride, and potentially reduce the heat of formation [22,
35]. Strictly speaking, this effect is not limited to nanostructures.
(b) The size of the metal nanostructure could also have an effect on
lowering the heat of formation . Smaller size means more
surface energy. If the surface energy of the hydride layer is larger
than that of the metal, due to the extra energy deposited on the
surface, the heat of formation will be reduced [22, 36]. Calculations
show that in order to achieve a significant reduction of the heat of
formation, for Mg, the nanoparticle's diameter should be smaller
than 2 nm [22, 36].
(c) The grain boundaries caused by mismatched crystal plane orienta-
tion could also supply extra energy for lowering the heat of forma-
tion . According to a conservative estimate, for grain size
on the order of 7-9 nm, the maximum reduction in the heat of