Decomposition of lithium magnesium aluminum hydride (original) (raw)

First-principles determination of crystal structures, phase stability, and reaction thermodynamics in the Li-Mg-Al-H hydrogen storage system

Physical Review B, 2009

First-principles density-functional theory ͑DFT͒ calculations have been used to investigate the crystal structures, thermodynamic stability, and decomposition pathways of Li-Mg-Al-H hydrogen storage compounds. We find that the recently discovered LiMg͑AlH 4 ͒ 3 compound is stable with respect to solid-state decomposition into LiAlH 4 and Mg͑AlH 4 ͒ 2 ; however, we also find that LiMg͑AlH 4 ͒ 3 is unstable with respect to hydrogen release and decomposes exothermically into LiMgAlH 6 , Al, and H 2 with a calculated T = 300 K enthalpy of −7.3 kJ/ ͑mol H 2 ͒, in excellent agreement with the weakly exothermic value of −5 kJ/ ͑mol H 2 ͒ obtained from differential scanning calorimetry measurements ͓M. Mamatha et al., J. Alloys Compd. 407, 78 ͑2006͔͒. LiMgAlH 6 is a stable intermediate, which has two competing endothermic decomposition pathways for H 2 release: one going directly into the binary hydrides of Li and Mg and the other proceeding via the formation of an intermediate Li 3 AlH 6 phase, with room-temperature enthalpies of +18.6 and +16.6 kJ/ ͑mol H 2 ͒, respectively. Using database searching based on known crystal structures from the inorganic crystal structure database, we predict that the hypothetical MgAlH 5 compound should assume the orthorhombic BaGaF 5 prototype structure, in contrast to a previous DFT study of MgAlH 5 , ͓A. Klaveness et al., Phys. Rev. B 73, 094122 ͑2006͔͒. However, the decomposition enthalpy of MgAlH 5 is only weakly endothermic, +1.1 kJ/ ͑mol H 2 ͒, and therefore this compound is not expected to occur in the high-temperature decomposition sequence of Mg alanate. We also present a comprehensive investigation of the phonon spectra and vibrational thermodynamics of Li-Mg-Al-H compounds, finding that vibrations typically decrease reaction enthalpies by up to 10 kJ/ mol H 2 at ambient temperatures and significantly lower reaction entropies.

An investigation on the hydrogen storage properties and reaction mechanism of the destabilized MgH2-Na3AlH6 (4:1) system

Hydrogen storage Destabilization mechanism a b s t r a c t A novel hydrogen storage composite system, MgH 2 eNa 3 AlH 6 (4:1), was prepared by mechanochemical milling, and its hydrogen storage properties and reaction mechanism were studied. Temperature-programmed desorption results showed that a mutual destabilization effect exists between the components. First, Na 3 AlH 6 reacts with MgH 2 to form a perovskite-type hydride, NaMgH 3 , Al, and H 2 at a temperature of about 170 C, which is about 55 C lower than the decomposition temperature of as-milled Na 3 AlH 6 . Then, at a temperature of about 275 C, the as-formed Al can destabilize MgH 2 to form the intermetallic compound Mg 17 Al 12 , which is accompanied by the self-decomposition of the residual MgH 2 . This temperature is about 55 C lower than the decomposition temperature for as-milled MgH 2 . Furthermore, when heated up to 345 C, NaMgH 3 starts to decompose into NaH, Mg, and H 2 , which is followed by the decomposition of NaH at a temperature of about 370 C. Rehydrogenation processes show that Mg 17 Al 12 and NaMgH 3 are fully reversible. It is believed that the Mg 17 Al 12 and NaMgH 3 formed in situ provide synergetic thermodynamic and kinetic destabilization, leading to the dehydrogenation of MgH 2 , which is responsible for the distinct reduction in the operating temperatures of the as-prepared MgH 2 eNa 3 AlH 6 (4:1) composite system.

Effects of helical GNF on improving the dehydrogenation behavior of LiMg(AlH4)3 and LiAlH4

The present paper reports the effect of graphitic nanofibres (GNFs) for improving the desorption kinetics of LiMg(AlH4)3 and LiAlH4. LiMg(AlH4)3 has been synthesized by mechano-chemical metathesis reaction involving LiAlH4 and MgCl2. The enhancement in dehydrogenation characteristics of LiMg(AlH4)3 has been shown to be higher when graphitic nanofibres (GNFs) were used as catalyst. Out of two different types of nanofibres namely planar graphitic nanofibre (PGNF) and helical graphitic nanofibre (HGNF), the latter has been found to act as better catalyst. We observed that helical morphology of fibres improves the desorption kinetics and decreases the desorption temperature of both LiMg(AlH4)3 and LiAlH4. The desorption temperature for 8 mol% HGNF admixed LiAlH4 gets lowered from 159 C to 128 C with significantly faster kinetics. In 8 mol% HGNF admixed LiMg(AlH4)3 sample, the desorption temperature gets lowered from 105 C to ~70 C. The activation energy calculated for the first step decomposition of LiAlH4 admixed with 8 mol% HGNF is ~68 kJ/mol, where as that for pristine LiAlH4 it is 107 kJ/mol. The activation energy calculated for as synthesized LiMg(AlH4)3 is w66 kJ/mol. Since the first step decomposition of LiMg(AlH4)3 occurs during GNF admixing, the activation energy for initial step decomposition of GNF admixed LiMg(AlH4)3 could not be estimated.

Kinetics and hydrogen storage performance of Li-Mg-N-H systems doped with Al and AlCl3

Journal of Alloys and Compounds

Recent investigations showed the formation of new amide-chloride phases between LiNH 2 and AlCl 3 after milling and/or heating under hydrogen pressure. These phases exhibited a key role in the improvement of the hydrogen storage properties of the LiNH 2-LiH composite. In the present work, we studied the effects of Al and AlCl 3 additives on the hydrogen storage behavior of the Li-Mg-N-H system. The dehydrogenation kinetics and the reaction pathway of Al and AlCl 3 modified LiNH 2-MgH 2 composite were investigated through a combination of kinetic measurements and structural analyses. During the first cycle, the addition of Al catalytically accelerates the hydrogen release at 200°C. In the subsequent cycles, the formation of a new phase of unknown nature is probably responsible for both increased equilibrium hydrogen pressure and decreased dehydrogenation rate. In contrast, AlCl 3 additive reacts with LiNH 2-MgH 2 through the milling and continues during heating under hydrogen pressure. Addition of AlCl 3 leads to the formation of two cubic structures identified in the Li-Al-N-H-Cl system, which improves dehydrogenation rate by modifying the thermodynamic stability of the material. This study evidences positive effect of cation and/or anion substitution on hydrogen storage properties of the Li-Mg-N-H system.

Regeneration of Lithium Aluminum Hydride

Journal of the American Chemical Society, 2008

Lithium aluminum hydride (LiAlH 4) is a promising compound for hydrogen storage, with a high gravimetric and volumetric hydrogen density and a low decomposition temperature. Similar to other metastable hydrides, LiAlH 4 does not form by direct hydrogenation at reasonable hydrogen pressures; therefore, there is considerable interest in developing new routes to regenerate the material from the dehydrogenated products LiH and Al. Here we demonstrate a low-energy route to regenerate LiAlH 4 from LiH and Ti-catalyzed Al. The initial hydrogenation occurs in a tetrahydrofuran slurry and forms the adduct LiAlH 4 • 4THF.Thethermodynamicsofthisreversiblereactionwereinvestigatedbymeasuringpressure-composition isotherms, and the free energy was found to be small and slightly negative (∆G)-1.1 kJ/mol H 2), suggesting an equilibrium hydrogen pressure of just under 1 bar at 300 K. We also demonstrate that the adduct LiAlH 4 • 4THF can be desolvated at low temperature to yield crystalline LiAlH 4 .