Hydrogen Storage Properties of a Combined Li3AlH6-LiBH4 System
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Hydrogen Storage Properties of a Combined Li3AlH6-LiBH4 System Young Joon Choi, Jun Lu, Hong Yong Sohn, and Zak Fang Metallurgical Engineering, University of Utah, 135 S 1460 E Room 412, Salt Lake City, UT, 84112-0114 1. INTRODUCTION In recent years, the growing demand for efficient and clean alternative fuels has increased attention to the development of hydrogen storage materials, including research on reversible solid storage materials. However, one of the key obstacles to the use of hydrogen as a fuel, especially for vehicles, is the lack of practical methods to store it. Although substantial progress has been made in the past few years in the discovery of new materials, none of them have demonstrated a sufficient reversible storage capacity in terms of the combined gravimetric and volumetric content of hydrogen required for practical application. Among them, lithium based complex hydrides of light metals such as alanates1-10 and borohydrides11-14 have attracted much interest for on-board application due to the potential of releasing a large amount of hydrogen at relatively low temperatures (< 300°C). However, the dehydrogenation reaction of LiAlH4 is not easily reversible, which is a critical requirement for on-board applications.3,4,15-17 In addition, one of the reaction steps during the complete dehydrogenation of LiAlH4 is the decomposition of LiH that requires temperatures higher than 700°C, which is also unacceptable for practical vehicular applications.18 Recently, however, Jang et al.19 have shown that the reaction 3LiH + Al + 1.5H2 = Li3AlH6 is thermodynamically feasible, whereas the reaction from Li3AlH6 to LiAlH4 is difficult. Lithium borohydride has recently received attention as a superior hydrogen storage material because of its high theoretical storage capacity (18.5 wt. %). Unfortunately, LiBH4 is thermodynamically stable and its dehydrogenation temperature under 1 bar of hydrogen pressure is above 405oC. Also, the rehydrogenation conditions, which has 600oC at 350 bar, are still impractical for on-board application.12,14,20 In an effort to lower the dehydrogenation temperature, Vajo et al.13 studied the destabilization of LiBH4 for reversible hydrogen storage using MgH2 as a destabilizing additive. On the basis of the above information, we thought that the combined system of Li3AlH6 and LiBH4 in a proper ratio would be reversible. The overall reaction equation of combined system would be given by Li3AlH6 + 2LiBH4 = 5LiH + AlB2 + 4.5H2 In the present work, we investigated that the reversible hydrogen storage properties of the Li3AlH6-LiBH4 combined system. This new system has a high theoretical hydrogen capacity of 9.2 wt. %. The experimental results on the dehydrogenation and rehydrogenation behaviors of this new material system will be discussed below. Possible mechanisms and reaction paths of the combined system will also be discussed. 2. EXPERIMENT The starting materials, lithium borohydride (LiBH4, 90%), titanium chloride (TiCl3, 99.999%) from Sigma-Aldrich, and lithium aluminu
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