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Theoretical Studies of Aluminum Based Nano Scale Materials for Hydrogen Storage


Hydrogen is, for many reasons, an appealing energy carrier. The main problem for
using hydrogen as fuel in mobile application is the onboard storage. Many studies
have been done on so called metal hydrides for the purpose of storing hydrogen.
For a metal hydride system to fulfill its task of being a good hydrogen storage
it needs to release the hydrogen close to 100◦ C and the gravimetric portion of
hydrogen in the system should be high, at least 6 wt% Former studies done in the
H. Jónsson group showed that hydrogen binds more strongly to Mg, compared
to large crystalline MgH2 , if the Mg is in nano scale clusters. Unfortunately,
for Mg the hydrogen binding energy is already to large for the MgH2 crystal.
These results have motivated us to look at the hydrogen binding in nano scale Al
clusters, since the alane crystal (AlH3 ) is thermodynamically unstable at room
temperatures but has a very high gravimetric portion of hydrogen, 10.1 wt%
We have studied the stability and structure of nano scaled aluminum and alu-
minum hydride clusters with plane wave based density functional theory (DFT).
The structures of Aln H3n clusters are surprisingly open and all aluminum atoms
are connected through a hydrogen bridging bonds. These bridging bonds make
the clusters very open and floppy. The binding energy of hydrogen in the alu-
minum hydride clusters has been calculated for clusters with 2 to 30 Al atoms.
As for MgH2 , the binding energy is larger for small clusters of AlH3 than for the
crystalline material. A model has been fitted to our data, to predict the bind-
ing energies of hydrogen for even larger clusters. By comparing the stabilities
of the pure aluminum clusters to the stability of aluminum hydride clusters, one
sees that the reason for the increase in binding energy is due to the fact that
the pure aluminum clusters destabilize more than the aluminum hydride clusters
with decreasing size.


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