Environmental Engineering Reference
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formation is about 9-10  kJ·mol −1 [35]. This reduction cannot
explain large enthalpy changes observed in experiments.
(d) The most probable enthalpy reduction cause may be the excess
volume effect. In heavily milled metal hydride samples, it is likely
that noncrystalline regions arise and the material is deformed.
These deformations could be gathered around grains or particles
surfaces. The resulting lattice distortions will change the energy
level of the metal and hydride and therefore could explain the for-
mation enthalpy change [35].
2. The Effect of Nanostructure on the Kinetics of Metal Hydriding/
Dehydriding. The kinetic process of hydrogen storage comprises
several steps: the dissociation and penetration of hydrogen at the inter-
face, the formation of metal hydride with moving boundary, the diffu-
sion of hydrogen, along with the heat dissipation and stress/strain
change. As the hydrogenation reaction progresses, the rate limiting
process changes from the dissociation and penetration of hydrogen at
the interface to the nucleation of the β-phase, and finally to the diffu-
sion of hydrogen through the β-phase layer formed around the particle
(see Figure 6.5). For the desorption process, the main rate limiting
processes are the slow diffusion through the β-phase layer and the high
hydrogen dissociation energy barrier. The absorption kinetics is accel-
erated by the high reaction rate, the large diffusion coefficient, and the
small diffusion length, that is, small particle size. By engineering metal
hydrides into nanostructures, the increased surface area and porosity
of nanostructures can offer a larger number of dissociation sites and
allow fast gaseous diffusion to the center of the material [22]. If the
nanostructures accompany with increased volume of grain boundaries,
those grain boundaries will weaken the binding between metal and
hydrogen atoms, which helps the site-to-site hopping for hydrogen and
enhances diffusion in the α -phase. Thus, as nanostructures favor the
thermodynamics of hydrogen absorption, they also improve the kinet-
ics. Several experiments have already demonstrated that the hydrogen
diffusivity increases with decreasing particle size [37, 38]. The grain
boundaries and internal strains in nanostructures also promote a fast
kinetics [22]. In addition, the kinetics can be dramatically improved by
a process known as spillover through the use of a proper surface cata-
lyst on the metal surface as shown in Figure 6.11 [22]. In the spillover
process, the hydrogen molecule is dissociated on the metal catalyst,
and the resulting hydrogen atom diffuses to the surrounding storage
media. Such a process can also make the diffusion through the surface
insensitive to the oxide layer, which prevents the need for an activation
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