Electronic Structure and Hydrogen Desorption in NaAlH 4
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Electronic Structure and Hydrogen Desorption in NaAlH4 S. Li and P. Jena Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284-2000 USA C. M. Araujo and R. Ahuja Condensed Matter Theory Group, Department of Physics, Uppsala University, Box530, SE-751 21 Uppsala, Sweden ABSTRACT First principles calculations based on gradient corrected density functional theory are carried out to understand the electronic structure and mechanisms responsible for desorption of hydrogen from Ti doped and vacancy containing sodium-alanate (NaAlH4). The energy necessary to remove a hydrogen atom from Ti doped NaAlH4 is significantly smaller than that from pristine NaAlH4 irrespective of whether Ti substitutes the Na or the Al site. However, the presence of Na and Al vacancies is shown to play an even more important role: The removal of hydrogen associated with both Na and Al vacancies is found to be exothermic. It is suggested that this role of vacancies can be exploited in the design and synthesis of complex light metal hydrides suitable for hydrogen storage. INTRODUCTION The success of a new hydrogen economy1-5 relies on our ability in finding storage materials with high gravimetric and volumetric density as well as fast kinetics6 and favorable thermodynamics.7 Unfortunately there are currently no materials that can satisfy all these requirements. In some inter-metallic alloys, the volumetric density of H is larger than that of liquid hydrogen, however, the gravimetric density of hydrogen seldom exceeds 3 wt %. To store hydrogen with a 10 wt % capacity, one has to consider host materials that are composed of light elements such as Li, Na, B, C, N, and Al. Unfortunately, hydrogen in these materials are held by strong covalent and/or ionic bonds and desorption of hydrogen requires rather high temperatures. Thus, ways of changing the chemistry of these light complex metal hydrides has to be found so that they can be considered as viable hydrogen storage materials. A class of materials that has attracted considerable attention in recent years is called alanates. These materials have the formula unit [M(AlH4), M=Li, Na, K; Mg(AlH4)2] where the [AlH4]- unit is ionically bonded to the metal atoms. Although these materials can store hydrogen up to 18 wt %, desorption of hydrogen requires rather high temperatures and the materials are not ideal for applications. Recently NaAlH4, which has a theoretical hydrogen content of 7.5 wt %, has attracted the most attention. This is due to the discovery by Bogdanovic and coworkers8 that addition of TiCl3 not only makes the hydrogen storage capacity of sodium alanates reversible, but also significantly reduces the hydrogen desorption temperature. Several experiments have been recently carried out to understand the mechanisms for hydrogen desorption and the role Ti precursors play in the process. However, the results are not conclusive as an atomic understanding of where Ti atoms reside and how they influence the bonding
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between hydrogen and metal atoms are lac
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