(457f) Dynamics of Ti-Al-H Compounds In Ti-Doped NaAlH4: A DFT-MD Stability and Transport Study

It is believed that the diffusion
of atomic/molecular hydrogen and ionic species in metal hydrides is improved by
the presence of dopants [1].
However, the role of transition metal dopants in improved thermodynamics and
kinetics of hydrogen ab/desorption is still debated in solid state hydrogen
storage research until date. Sodium aluminum Hydride (NaAlH₄) also
known as sodium alanate, decomposes through a two step reaction to yield 5.6
wt% of hydrogen, which results in Na₃AlH₆, Aluminum and
NaH phases [2].
Information regarding the formation of these phases provides insight into the
rate limiting step for the hydrogen desorption process from metal hydrides [3].
Moreover, when Ti is present in the sodium alanate lattice, the stability and
transport properties of the Ti-Al-H complexes
that are formed would shed light into the role of Ti (dopant or catalyst or
both) when added to sodium alanates.

In this work, Ti-dopant
effects in the sodium alanate bulk, (001) surface, and on-top (001) surface are
studied considering Na and interstitial lattice sites.
The
stability of the formed Ti-Al-H complexes and their
dynamics
over time at different temperatures are
investigated using periodic Density Functional Molecular Dynamics (DFT-MD)
simulations [4]. From
calculations of the mean squared displacement (MSD) for different species,
transport properties such as diffusion coefficients are calculated from the
slope of the MSD curve. From these results, the role of Ti and its effect on the
diffusion of atomic/ionic species and groups within the system can be elucidated.

[1] D. L. Anton, Hydrogen
Desorption Kinetics in Transition Metal Modified NaAlH₄, J.
Alloys Compd. 356-357 (2003), pp. 400-404.

[2] B.
Bogdanović and M.Schwickardi, Ti-doped alkali metal aluminium hydrides
as potential novel reversible hydrogen storage materials, J. Alloys Compd.
253 (1997), pp. 1-9.

[3] H.
Gunaydin, K. N. Houk and V. Ozolins, Vacancy-mediated dehydrogenation of
sodium alanate, Proc. Natl. Acad. Sci. U. S. A. 105 (2008), pp. 3673-3677.

[4]
G.K.P. Dathara and D.S. Mainardi, Structure and Dynamics of Ti-Al-H
Compounds in Ti-Doped NaAlH₄, Mol. Sim. 34 (2008), pp. 201-210.