Environmental Engineering Reference
In-Depth Information
Table 1 Targets set by the Department of Energy (released in 2009)
Storage parameters
Units
2010
2015
Ultimate
kWh kg
-1
System gravimetric
capacity
1.5
1.8
2.5
kg H
2
kg
-1
system
0.045
0.055
0.075
kWh L
-1
System volumetric capacity
0.9
1.3
2.3
kg H
2
L
-1
system
0.028
0.040
0.070
Min/max delivery
temperature
C
-40/85
-40/85
-40/95-105
Cycle life (1/4 tank to full)
Cycles
1,000
1,500
1,500
Min/max delivery pressure
for fuel cell
Bar (abs)
5/12
5/12
3/12
Min/max delivery pressure
for internal combustion
engine
Bar (abs)
35/100
35/100
35/100
Onboard reversible system
efficiency
%
90
90
90
kg H
2
min
-1
System fill rate
1.2
1.5
2.0
Used with permission by Chen 2012
Current research in the storage of hydrogen is focused on several areas,
including the synthesis of new materials with high porosity and controllable
functionality, [
1
] the adsorption of H
2
in zeolites and carbonaceous materials
including carbon nanotubes, [
2
] and the modification of existing metal hydride
systems [
3
]. A comparison of a range of materials as a function of their volumetric
and gravimetric hydrogen storage is shown in Fig.
1
. Among these research areas,
metal hydrides containing light elements such as MgH
2
offer high density storage
at low cost.
The significant advantage to light metal hydrides such as MgH
2
is the poten-
tially high hydrogen capacity in small volumes at ambient pressures and tem-
peratures. However, essentially all of the target compounds are plagued by slow
kinetics for the hydrogenation and dehydrogenation reactions, which means that
they are currently cycled at elevated temperatures (typically near 300 C) and low/
high pressure for the dehydrogenation/hydrogenation reactions. The compounds
that can cycle at or near room temperature are hindered by low mass density for
hydrogen (such as Pd, which converts to PdH
0.6
)[
3
]. A fundamental understanding
of the structure and properties of complex hydrides would be extremely helpful in
decoupling the challenges in terms of thermodynamic stability of the relevant
phases as well as the kinetics. Without an understanding of the interplay among
structure, bonding, and diffusion in these materials, optimization to improve the
kinetics at lower temperatures and ambient pressures relies on trial and error. For
that reason a portion of this chapter is dedicated to theoretical predictions that are
guiding current research.
A reasonable argument can be made that only light hydrogen storage materials
with high gravimetric capacities will be able to achieve the DOE targets. For that
reason, Mg, doped Mg, and Mg alloys are promising materials for hydrogen storage
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