HYDROGEN STORAGE IN COMPLEX HYDRIDES

D. Lupu, A.R. Biriş, I. Mişan

National Institute for R&D of Isotopic and Molecular Technologies, 400293 Cluj Napoca, P.O.Box 700, Romania

	Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in mobile and stationary applications. A relevant role of the fuel cell powered vehicles on the market of the transportation systems will be achieved only if the research and development of on-board vehicular hydrogen storage are able to allow a driving range of 500 km. The on-board hydrogen storage systems are more challenging due to the space, weight and cost limitations. This range of autonomy between refueling requires materials able to store at least 6.5% weight hydrogen, available at moderate pressures, at the working temperature of the fuel cells and with acceptable cycling stability.
	The intensive research on the hydrogen storage in alloys and intermetallic of the LaNi5, FeTi or  Laves phase type compounds, started more than three decades ago did not result in storage materials of more than about 3% H storage capacities. The 7.5%H content of the Mg hydride is still attracting interest  but though the absorption has been achieved at lower temperatures by ball milling magnesium with various amounts of nickel, the desorption can not be attained at 1 bar H2 below 280o C and the kinetics of the process is too slow. In the last decade, the attention is focused on another class of compounds, the complex hydrides of aluminum with alkali metals (alanates), due to their high hydrogen content. It was discovered that doping with Ti-based catalysts improve the hydrogenation/dehydrogenation conditions [1] of NaAlH4. Later, it was shown [2] that ball milling with solid state catalysts greatly improve the hydrogen desorption kinetics of NaAlH4, and this also helps to the rehydriding process. The hydrogen desorption from NaAlH4  occurs in three steps, it shows a reversible storage capacity of 5.5%H and this led to further research work for a better knowledge of its application related properties[3].
	In this work, ball milling experiments on NaAlH4 with some hydrogen absorbing alloys and carbon nanofibers are reported, using a specially designed vial for the planetary mill, which allows the in situ study of the ball milling effects on the desorption characteristics, avoiding  some additional contamination of the sample surface which could arise during a handling step in a glove box. The results are comparable with the literature data, showing reversible storage capacities of 4.1%H.The results on LiAlH4  are also reported and discussed comparatively. Reversible absorption/desorption cycles were performed on NaAlH4 while LiAlH4 could not be rehydrogenated.
	The results are compared with the literature data on alanates and suggest some new directions of research to find new materials, with improved hydrogen storage capacity.

REFERENCES
[1] B. Bogdanovic, M. Schwickardi, J. Alloys Comp. 253(1997)1
[2] A. Zaluska, L. Zaluski, J.O. Ström-Olsen, J. Alloys Comp. 298(2000)125
[3] G. Sandrock, K. Gross, G. Thomas, C. Jensen, D. Meeker, S. Takara, J. Alloys Comp. 330-332  (2002)696