Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 103

order of magnitude simply by reducing the grain size of the metal
[23-26]. For example, while it is very hard to activate and hydrogenate
polycrystalline Mg films at 300°C, when the grain size is reduced to
30-50 nm, the Mg films absorb hydrogen more quickly [23-26].
Research performed on thin films of pure magnesium demonstrated
that the thinner the magnesium sheet is, the faster it achieves complete
formation of MgH 2 [27-29]. Similar results have been observed for
other materials, such as Mg 2 Ni, FeTi, and LaNi 5 , as well [23-26].
The crystalline structure of the resulting materials has been found to
play a significant role. For instance, crystalline, amorphous, nanocrys-
talline, or the mixture of amorphous and nanocrystalline phases
affect the thermodynamics of hydrogen adsorption for FeTi. In some
cases, a partially amorphized alloy may result in a further increment
of the hydrogen solubility [23-26]. It has been suggested that the
grain boundaries play a critical role in improving the hydrogenation
properties.
2. Nanostructures Can Lower the Desorption Activation Energy. For
example, theoretical calculations based on quantum chemistry have
shown that small MgH 2 clusters have much lower desorption energy
than bulk MgH 2 [30]. The hydrogen desorption energy decreases sig-
nificantly when the crystal grain size becomes smaller than 1.3 nm.
3. Nanoscaled Catalyst Can Greatly Improve Hydrogen Adsorption.
Effective catalysts, even added in small amounts, enhance the forma-
tion of a hydride to a reasonable extent. Small nanoscale catalysts
distributed over the surface of nanocrystalline hydrides can result in a
spectacular improvement of sorption properties, such as elimination of
the need for activation and overall improvement of sorption kinetics
[23-26]. Previous experiments have shown that with proper catalysts,
the activation time for LaNi 5 nanocrystalline film to absorb hydrogen
can be eliminated [25]. Therefore, by eliminating the need for activa-
tion, the combination of nanostructure and nanocatalysis yields a tre-
mendous gain in sorption kinetics.
4. Nanocomposites Can Further Improve the Quality of Hydrogen Adsorp-
tion. A specifically designed nanocomposite, where both nanostruc-
ture and nanocatalyst can be incorporated into a more complex system,
can have properties surpass that of the individual components alone.
For example, a composite of a nano-mixture of a high temperature
hydride (Mg) with a low temperature hydride (FeTi, or LaNi 5 ) can be
used for “cold-starting.” [25]. Or the “cascade” of components can be
operated at various temperatures and hydrogen pressures. Doping of