Chemical Storage Based on Metal Hydrides and Hydrocarbons - Hydrogen Generation, Storage, and Utilization - page 102

FIGURE 6.8 Targeted range of bond strengths that allow hydrogen release around room temperature.
A given material can exhibit both chemisorption and physisorption. Source : Reproduced with permission
from Berube et al. [22].
application: high adsorption/desorption temperature and slow kinetics. The
strong chemical bonds (>50 kJ mol −1 ) formed between hydrogen and metals
in metal hydrides result in the high storage capacity and stability at room
temperature, and also lead to an enormous energy release during the hydro-
genation process. However, to release hydrogen, the hydride must be heated
to high temperature in order to supply sufficient energy to break the strong
chemical bonds. Thus, to obtain a destabilized hydride with small binding
energy that is in thermodynamic equilibrium with the hydrogen gas closer
to room temperature is the key to reducing the heat of formation which would
in turn reduce the release temperature [22]. Figure 6.8 shows the range of
binding energies E b targeted by DOE. For metal hydrides, the binding ener-
gies are too high for chemisorption, while for carbon structures, metal organic
frameworks, and so on, the binding energies are too low due to physisorption
(<10 kJ·mol −1 ). Therefore it is necessary to raise E b for physisorbed materi-
als, or lower E b for metal hydrides through thermodynamic destabilization.
Since storage capacity remains a priority, it is important to achieve destabi-
lization while minimizing the storage capacity reduction. Many different
methods or processes have been developed to destabilize metal hydrides.
Among them, to make metal hydride as nanometer-size crystals and to add
nano-sized catalysts into metal hydrides have attracted the main attentions
and also led to significant improvement for metal hydride performance.
Previous studies on nanocrystalline metal hydrides have demonstrated the
following potential advantages for hydrogen storage:
1. Nanostructures Will Improve the Hydrogen Adsorption and Desorption
Kinetics. Numerous experiments indicated that for all metal hydrides
the kinetics of both adsorption and desorption can be improved by an