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.