Environmental Engineering Reference
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FIGURE 6.9 (Left) SEM image of the Mg nanoblades, and (right) the sorption curves of (a) hydrogen
absorption under a hydrogen pressure of 10 bar and (b) hydrogen desorption under vacuum at varying
temperatures for V decorated Mg nanoblade array. Source : Reproduced with permission from He
et al. [31].
nanostructures with metal ions such as Ti 3+/4+ is another useful method
to improve the hydrogen storage properties of the nanostructures [25].
As an example, Figure 6.9 shows a 4.6 at% vanadium decorated Mg
nanoblade structure made by oblique angle deposition [31]. The blade thick-
ness is about 65 nm, and the height is about 50 μm. The hydrogen sorption
kinetic curves at various temperatures ( T   ≤  570  K) are also plotted. The
reversible hydrogen amount is less than 6 wt%, which is slightly lower than
the theoretical value 7.6 wt% of MgH 2 . The V-decorated Mg nanoblades can
absorb hydrogen to saturation within 7 minutes at 570 K with a rate constant
of k  ≈ 131 h −1 . The hydrogenated nanoblades can desorb hydrogen almost
completely within 15 minutes at 570  K with k  ≈  26  h −1 . By plotting the
obtained rate constant k versus reciprocal temperature 1/ T in Figure 6.10,
the activation energy is estimated to be E a
35 0
1 2
kJ mol H
absorption process and E a d
. . for desorption process. These
activation energies are much lower than the desorption energy of
141  kJ·(mol·H 2 ) −1 for MgH 2 film [32] and 156  kJ·(mol·H 2 ) −1 for MgH 2
powder [33], indicating the catalytic effect of the V coating on the MgH 2
formation and decomposition.
However, the fundamental understanding of how those nanostructures
improve the thermodynamics and kinetics of metal hydrides is still under
debate, although some consensuses have been reached. In the past 5 years,
a number of research groups have conducted extensive experimental and
theoretical research toward such an understanding. We summarize the main
results and challenges from those studies following the arguments by the
MIT groups [22, 34]:
65 0
0 3
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