Environmental Engineering Reference
In-Depth Information
change with temperature, typically the concentration difference decreases when
temperature increases, and becomes zero when the critical point is reached. Until
the two solid phases coexist the temperature remains practically constant, and the
hydrogen concentration increases inside the intra-metallic mixture, evidencing that
the hydrogen amount is reversibly stored at the fixed temperature with slight
pressure variations. Finally, the intermediate solution disappears and all hydrogen
atoms are entangled in the hydride solid solution. Further hydrogen absorption
could occur when the hydrogen concentration increases up to the maximum value
compatible with the atom insertion inside the voids of the metal framework.
The strong interest towards metal hydrides is strictly related to the potential
of significantly increasing the volumetric density of hydrogen packing (up to
0.2 kg/m
3
) that largely exceeds the density of liquid hydrogen (about 0.07 kg/m
3
).
The operative temperature and pressure of the absorption process depend on the
type of hydride, and are the key parameters affecting the efficiency of a hydrogen
storage device for automotive application.
The different metal hydrides can be classified in two main categories:
1. Low desorption temperature.
2. High desorption temperature.
The first class is characterized by those metals and metal alloys (iron, titanium
and nickel) which require only a small amount of heat to release hydrogen. This
heat might be easily withdrawn by the process fluids of a fuel cell powertrain. On
the other hand, some hydrogen can be released also at room temperature, but this
problem could be overcome by preliminary tank pressurizing, followed by a
gradual pressure diminution when hydrogen content within the hydride decreases.
Beside the basic advantages of high volumetric density and low energy inputs,
this class of hydrogen storage materials is attractive also for the operational safety.
Unfortunately the gravimetric energy density does not reach the targets useful for
road vehicle application, essentially because of the high molar mass of metal
framework.
The second class includes other hydrides based on light elements, such as alkali
and alkaline-earth metals, first of all the metal alloys of magnesium [
107
,
117
-
121
].
These materials are very interesting because of their acceptable gravimetric
hydrogen storage capability, but their poor kinetic properties at moderate temper-
ature strongly limit the practical application. In particular MgH could contain
theoretically hydrogen up to about 8% in MgH form, but absorption and desorption
processes occur only at elevated temperatures ([200C), and then require large
amount of heat to release hydrogen. The catalytic role of different additional
materials such as metal oxides or transition metals has been extensively investi-
gated in magnesium alloys to reduce release temperature and improve kinetics
[
122
,
123
], but the results do not evidence significant improvements of their
performance.
Recently, metal-doped aluminium hydrides have been proposed as further
potential hydrogen storage materials [
107
,
123
]. Among lightweight metal
hydrides the lithium and sodium alanates (NaAlH
4
, LiAlH
4
) have been widely
