Hengyang purpose is to describe and evaluate the potential of
aluminum-water reactions for the production of hydrogen for
on-board hydrogen-powered vehicle applications. Although the
concept of reacting aluminum metal with water to produce hydrogen
is not new, there have been a number of recent claims that such
aluminum-water reactions might be employed to power fuel cell
devices for portable applications such as emergency generators and
laptop computers, and might even be considered for possible use as
the hydrogen source for fuel cell-powered vehicles.
In the vicinity of room temperature, the reaction between aluminum
metal and water to form aluminum hydroxide and hydrogen is the
following: 2Al + 6H2O = 2Al(OH)3 + 3H2. The gravimetric hydrogen
capacity from this reaction is 3.7 wt.% and the volumetric hydrogen
capacity is 46 g H2/L.
Although this reaction is thermodynamically favorable, it does not
proceed due to the presence of a coherent and adherent layer of
aluminum oxide that forms on the surface of aluminum particles
which prevents water from coming into direct contact with the
aluminum metal. The key to inducing and maintaining the reaction of
aluminum with water near room temperature is the continual removal
and/or disruption of this coherent/adherent aluminum oxide layer.
A number of reaction-promoting approaches have been investigated
for the aluminum-water reaction. These include additions of
hydroxide promoters such as NaOH, oxide promoters such as Al2O3,
and salt promoters such as NaCl. These additions act to disrupt the
aluminum oxide layer on the aluminum metal. In addition, the
reaction of water with molten aluminum alloys such as
aluminum-lithium and aluminum-gallium has been studied. In this
case, the molten nature of the alloy prevents the development of a
coherent and adherent aluminum oxide layer. However, none of these
approaches have proven commercially viable to date. The concept of
using the aluminum-water reaction to provide onboard hydrogen for
hydrogenpowered vehicles presents a number of difficulties. First,
storage systems using this approach will not be able to meet the
2015 DOE system targets of 5.5 wt.% hydrogen and 40 grams hydrogen
per liter. Second, based on published aluminum-water reaction rate
kinetics, it appears 3 difficult for this approach to meet the DOE
minimum hydrogen flow rate target for fuel-cell powered vehicles.
Finally, the cost of producing hydrogen by this approach is
dictated by the cost of aluminum metal.
While aluminum-water reaction systems cannot meet the targets for
on-board vehicular hydrogen storage, the use of aluminum as a water
splitting agent for generating hydrogen might have utility for
non-vehicular applications.
