Articoli Scientifici

Simulazione di reazioni e processi in materiali per l'hydrogen storage


Abstract


The adoption of hydrogen as fuel for automotive applications requires the development
of effective technologies for its storage on board. The most promising materials for this
application are those in which hydrogen is stored in solid phase, being part of the structure
of the material. This technology allows an high hydrogen density together with
an high weight percentage and a low risk in case of accident.
The development of the next generation of this class of materials requires a proper
modelization of the chemical reactions and physical processes occurring during the release
and uptake of hydrogen, and in particular the role played by catalysts. In fact, together
with an higher capacity, the next generation of solid state hydrogen storage
materials, the first expected to be used in real applications, must be able to achieve a
higher life cycle (higher number of charge-discharge cycles).
Our research focused on the study of the de-hydrogenation path in one of the most
known material: sodium alanates (NaAlH4, Na3AlH6). Combining modern methods for
the reconstruction of the free energy surface and an ab-initio calculation for the modelization
of interatomic forces (density function theory in generalized gradient approximation),
we were able to demonstrate that the limiting step of the de-hydrogenation
process in this material is the diffusion of hydrogen atoms from one defective AlH6
unit to another in its nearest neighbor shell. We also shown that this process is not affected
by the catalyst. We therefore concluded that a further study of this process, focusing
on the chemical reaction rather than on the diffusion process is needed to
understand the role played by the catalyst.

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