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The catalytic effect of rare-earth hydrogen storage alloy is investigated for dehydrogenation of alane, which shows a significantly reduced onset dehydrogenation temperature (86 °C) with a high-purity hydrogen storage capacity of 8.6 wt% and an improved dehydrogenation kinetics property (6.3 wt% of dehydrogenation at 100 °C within 60 min). The related mechanism is that the catalytic sites on the surface of the hydrogen storage alloy and the hydrogen storage sites of the entire bulk phase of the hydrogen storage reduce the dehydrogenation temperature of AlH3 and improve the dehydrogenation kinetic performance of AlH3. This facile and effective method significantly improves the dehydrogenation of AlH3 and provides a promising strategy for metal hydride modification.

AlH 3 (alane) is considered as an ideal one-off compact hydrogen storage material thanks to its high storage capacity and moderate decomposition temperature. However, application has been hindered by the lack of direct hydrogen reversibility, i.e. regeneration of AlH 3 directly from the reaction of spent Al with hydrogen under mild conditions. Herein, we investigated the effect of nanoconfinement on the properties of AlH 3 and the potential of this approach toward the direct release and uptake of hydrogen by nanosized Al. AlH 3 was confined within the mesopores of a high surface area graphite (HSAG). The decomposition of the nanoconfined AlH 3 was detected 50 °C lower than the bulk, showing some effect of nanoconfinement on the overall temperature for hydrogen release. Some hydrogen uptake was also observed at 150 °C and 60 bar hydrogen pressure, and this indicates that reversible release of hydrogen with alane under moderate conditions of pressure and temperature may be possible.

Aluminium hydride (AlH3) is a promising hydrogen storage material due to its competitive hydrogen storage density and moderate decomposition temperature. However, there is no convenient way to prepare/regenerate AlH3 from (spent) Al by direct hydrogenation. Herein, we report on a novel approach to generate AlH3 from the decomposition of triethylaluminium (Et3Al) under mild hydrogen pressures (10 MPa) with the use of surfactants. With tetraoctylammonium bromide (TOAB), the synthesis led to the formation of nanosized AlH3 with the known α phase, and these nanoparticles released hydrogen from 40 °C instead of the 125 °C observed with bulk α-AlH3. However, when tetrabutylammonium bromide (TBAB) was used instead of TOAB, larger nanoparticles believed to be related to the formation of β-AlH3 were obtained, and these decomposed through a single exothermic process. Despite the possibility to form α-AlH3 under low conditions of temperature (180 °C) and pressure (10 MPa), TOAB stabilised AlH3 was found to be irreversible when subjected to hydrogen cycling at 150 °C and 7 MPa hydrogen pressure.

Aluminium hydride (AlH3) is a promising hydrogen storage material because it possesses a high theoretical hydrogen capacity of 10.01 wt%. However, the stability and decomposition mechanism of some AlH3 polymorphs (e.g., γ-AlH3) still remain unclear. In this work, the hydrogen desorption behaviours of γ-AlH3 with or without TiF3 addition were investigated by hydrogen desorption measurement and thermal analysis. It was revealed that the decompositions of the outer layer and the inner part of γ-AlH3 particle follow diverse decomposition mechanisms. The outer layer of γ-AlH3 particle tends to decompose directly, while the inner part of γ-AlH3 particle prefers to first transform to more stable α-AlH3 and then decompose. TiF3 addition significantly lowers the temperature for the direct decomposition of outer layer γ-AlH3 by about 30 °C but scarcely impacts the decomposition of inner part γ-AlH3, which further confirms the decomposition mechanism of γ-AlH3. It was suggested that the particle size plays an important role in the thermal stability of γ-AlH3. The results of this work will help understanding the decomposition mechanisms of other AlH3 polymorphs for hydrogen storage.

Ternary hydride of LiBH 4 –MgH 2 –NaAlH 4 confined into carbo n aerogel scaffold (CAS) via melt infiltration for reversible hydrogen storage is proposed. Nanoconfinement of hydrides into CAS is obtained together with surface occupation of some phases, such as Al and/or LiH. Regarding nanoconfinement, not only multiple-step decomposition of LiBH 4 –MgH 2 –NaAlH 4 hydride reduces to about single step, but also reduction of dehydrogenation temperature is significantly observed, for example, ∆ T up to 70 °C regarding last dehydrogenation step. Moreover, decomposition of NaBH 4 in nanoconfined sample can be done at 360 °C (dehydrogenation temperature in this study), which is 115 and 180 °C lower than that of NaBH 4 in milled LiBH 4 –MgH 2 –NaAlH 4 and bulk NaBH 4 , respectively. The reaction of LiBH 4 +NaAlH 4 →LiAlH 4 +NaBH 4 takes place during nanoconfinement and the decomposition of LiAlH 4 is observed, resulting deficient hydrogen content liberated. However, hydrogen content released (1st cycle) and reproduced (2nd–4th cycles) from this ternary hydride enhances up to 11% and 22% of full hydrogen storage capacity due to nanoconfinement. After rehydrogenation ( T =360 °C and P (H 2 )=50 bar H 2 for 12 h), NaBH 4 , MgH 2 , and Li 3 AlH 6 are reversible, whereas Li 3 AlH 6 and NaBH 4 in milled sample cannot be recovered due to deficient hydrogen pressure ( T =360 °C and P (H 2 )=80 bar) and probably evaporation of molten sodium during dehydrogenation, respectively. The latter results in inferior hydrogen content reproduced from milled sample to nanoconfined sample.

The effect of additives and mechanochemical processing at low temperature on β-AlD 3 has been studied. β-AlD 3 synthesized by wet chemistry was mixed with selected fluorides and cryomilled. Investigations of the fractions of α-, α′- and β-AlD 3 in the samples show that the additive and cryomilling affect the ratio of the polymorphs. Addition of AlF 3 leads to the highest ratio of α-AlD 3 , while addition of TiF 3 leads to full decomposition to Al. Formation of α′-AlD 3 during milling with NaF or MgF 2 is observed. The presence of fluorides in the synthesis of alane by cryomilling showed no notable impact on the ratios of the alane polymorphs compared to cryomilling without additives.

Hydrogen desorption energies are considered an important factor in the selection of hydrogen storage materials. Among metal hydrides, aluminum hydride seems to be a promising material for hydrogen storage. We report the theoretical calculations of the hydrogen desorption energies of AlnH3n (n=1, 2, 3…) clusters based on Density Functional Theory (DFT). Except for very small clusters, desorption energy is seen to steadily decrease with cluster size n and reach a value of 0.19 eV per H2 for n=20, showing that for large cluster sizes approximating bulk behavior, aluminum hydride tends to be unstable. But AlnH3n clusters of sizes n=8–16 have desorption energies in the range 0.6–0.4 eV per H2, which is suitable for hydrogen storage application.

Solid reactions between alkali aluminum hydrides (MAlH 4 , M = Li or Na) and NH 4 Cl (at mole ratio 1:1) at 170 °C were investigated quantitatively using temperature programmed reaction (TPR), thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC) and x-ray diffraction (XRD). The release of 3 mol of H 2 from per mole of MAlH 4 was measured, corresponding to 5.6 wt.% H 2 capacity for the NaAlH 4 /NH 4 Cl system and 6.6 wt.% for LiAlH 4 /NH 4 Cl, respectively. By ball milling of the precursor compounds prior to the mixing, the reaction proceeded fast and NH 3 production as the by-product could be avoided. The quick solid reactions may be attributed to the low melting temperatures of MAlH 4 and the exothermic nature of the reactions. The reaction mechanism was also discussed.

Light metal hydrides represent a promising class of materials for hydrogen storage. However, there are several technical challenges to overcome before their potential can be realized. Key among these is the often adverse absorption and desorption kinetics, which are a function of intrinsic reaction rates and practical operating temperatures. Modifying and controlling these kinetics require a thorough understanding of the hydrogen absorption and desorption processes. In this study, we have investigated the thermal decomposition of aluminum hydride, AlH3, with Magnetic Resonance Imaging in order to visualize spatially the progress and extent of the reaction.

In an effort to thermodynamically stabilize the alane (i.e., to increase the desorption enthalpy), partial substitution of Mg for Al was investigated by ball milling the mixtures (1 − x )AlH 3 / x MgH 2 ( x = 0.1, 0.2 and 0.3). Rietveld analysis of the XRD profiles showed that the cell volume of α-AlH 3 increased with the Mg substitution rate, and thereby formation of solid solutions was assumed ( x ≤ 0.05). In agreement with the experimental results, theoretical calculations indicated that a hypothetical supercell structure (MgAl 15 )H 47 ( x = 0.0625), which contained a hydrogen vacancy, was at least metastable. However, the effect of alane stabilization by Mg substitution for Al was not observed, either by experiment or by simulation, and only an increase in the activation energy was measured.

Transition metal dopants added to complex metal hydrides, specifically to sodium aluminum hydride (NaAlH 4 ), by high energy ball milling enhances dehydrogenation kinetics and induces dehydrogenation reaction reversibility. This study uses the power-law scattering parameter, p , gained from ultrasmall-angle X-ray scattering (USAXS) data to elucidate differences in NaAlH 4 particle morphology as dopant type and mill time is varied. Four dopant types were used. Two dopant types were used to represent the best kinetic enhancements having high desorption rates (e.g. TiCl 2 , TiCl 3 ) and were compared with two dopant types which do not perform as well (e.g. ZrCl 3 and VCl 3 ). USAXS data for the doped hydrides were compared with undoped and milled NaAlH 4 powders. Mill times used were 0 min (blended), 1, 5, and 25 min. As indicated by the USAXS power-law scattering data, the undoped NaAlH 4 powders are comprised of primary particles each having a high surface area. The high particle surface area in the undoped NaAlH 4 particle system persists as mill time increases—with only the 25 min sample undergoing a marked a decrease in primary particle surface area. Alternatively, the doped powders milled for 1, 5, and 25 min show uniformly decreasing hydride particle surface area. These decreases in particle surface area may be explained by either the colloidal particles increasing in surface smoothness or decreasing internal void space. TiCl 3 -doped NaAlH 4 powders show the trend of maintaining particles having a morphology comprised of higher particle surface area during the high energy milling stage of powder processing compared with other dopants.

Multinuclear and multidimensional solid state NMR techniques including magic-angle-spinning (MAS) and multiple-quantum (MQ) MAS experiments have been used to characterize various AlH_3 samples. At least three distinct polymorphic AlH_3 phases have been prepared by desolvating the alane etherate product from its organometallic synthesis. MAS-NMR spectra for the ^(1)H and ^(27)Al nuclei have been obtained on a variety of AlH_3 samples that include the β- and γ-phases as well as the α-phase. ^(27)Al MAS NMR was found to respond with high sensitivity for showing differences in spatial arrangements of AlH_6 octahedra in the three polymorphs studied. Based on the characteristic NMR signatures determined, phase transition of the γ-AlH_3 to the α-AlH_3 was studied at room and high temperatures. Direct decomposition of the γ-AlH_3 to aluminum metal at room temperature was also unambiguously confirmed by NMR studies.

When Li,Al is coupled-substituted into the MgH2 structure to assume the composition of LiAlMg10H24, the resulted structure was found to retain that of the parent material but assumes substantial differences in bonding and associated properties. Density functional theory (DFT) method was used in deriving the stable structure, computing the thermochemical parameters and evaluating the reaction/activation energies of dehydriding. The stability of the optimized LiAl-substituted structure was verified through the lattice dynamics calculations of the density functional perturbation theory (DFPT) method. The coupled substitution weakens the H-metal bonds that results in reductions in the reaction energy and potentially the activation barriers of dehydriding of the materials.

Thermal properties of AlH3-etherate and its desolvation reaction into AlH3 were experimentally investigated under heating and isothermal processes. AlH3-ehterate was desolvated to γ-AlH3 which immediately transformed into α-AlH3 in the heating process. The results under the isothermal process provide appropriate conditions to effectively obtain α- and γ-AlH3 by restraining the occurrence of the dehydriding reaction into metallic Al.

The thermal decomposition of alane was investigated by application of synchrotron X-ray diffraction (SR-XRD) and thermal desorption spectroscopy (TDS). Two polymorphs were studied, α- and γ-AlH3. Activation energies, anisotropic volume expansions, and phase transformation paths were found. In addition, the crystal structure data, including structure of hydrogen sublattice, and small charge transfer from the aluminium towards the hydrogen sites were observed during a high-resolution SR-XRD study of α-AlH3.

A 1-D model has been developed to evaluate various designs of metal hydride reactors with planar, cylindrical or spherical geometry. It simulates a cycling loop (absorption–desorption) focusing attention on the heat transfer inside the hydride bed, which is considered the rate-limiting factor. We have validated this model with experimental data collected on two reactors, respectively, containing 1 and 25 g of LaNi 5 , the second being equipped with aluminium foam. The simulation program reproduces accurately our experimental results. The impact of the foam cell size has been studied. According to our model, the use of aluminium foam allows the reactor diameter to be increased by 7.5 times, without losing its performance.

This paper reports mischmetal (Mm) as an effective catalyst for fast desorption kinetics (3.7 wt% in 60 min in which ∼3.3 wt% observed in 30 min and ∼5 wt% in 180 min at the desorption temperature of 150 °C for the 2 mol% Mm-doped material) and rehydrogenation (up to 35 cycles) of the light weight hydrogen storage material NaAlH 4 . In fact this catalyst Mm has been shown to be better than the presently known catalyst Ti. For Mm, its presence when admixed in NaAlH 4 is discernible. Also the higher reversible hydrogen storage capacity (4.77 wt% which is 86% of the total reversible storage capacity) of the total is achievable in the 12th cycle. The possible modes of catalyst Mm for fast desorption kinetics has been outlined and the most feasible mechanism in terms of weakening of Na and AlH 4 bonding has been put forward.

In the present paper, we investigate the thermal and mechanical decomposition of lithium alanate ( LiAlH 4 ) milled with different metal-halides ( VCl 3 , VBr 3 and AlCl 3 ) . We observed that the thermal decomposition temperature of LiAlH 4 does not depend on the metal–halide and it is decreased by 25 ∘ C as compared to LiAlH 4 without additives. Moreover, metal halides enhance the decomposition of LiAlH 4 and Li 3 AlH 6 . The ability of the metal halides to decompose LiAlH 4 and Li 3 AlH 6 during milling follows the order: VCl 3 > VBr 3 > AlCl 3 . X-ray diffraction and IR spectroscopy did not allow detecting any change on the cell volume or on Al–H bond of the doped alanate suggesting that the additives seem to do not act as substitutes into the lattice of the alanate.

Thermodynamic properties of hydrides, NaH, Na 3 AlH 6 and NaAlH 4 which can be a promising candidate for a hydrogen storage material system, were critically assessed by analyzing available piecewise literature information on thermodynamic properties using the CALPHAD method. Through the assessment, a critical evaluation on the reliability of individual literature data on thermodynamic properties of the NaH–Na 3 AlH 6 –NaAlH 4 hydride system could be made, and a self-consistent thermodynamic description could be obtained. The finally assessed thermodynamic description can reproduce various thermodynamic properties of individual hydrides and phase equilibria among hydrides in reasonable agreement with relevant experimental and high-level calculation data, and provide a reference data that can be used for comparison with high-level calculations and for further thermodynamic assessments of higher order systems.

Al(3)Ti was directly utilized as catalyst precursor to examine its catalytic activity in the reversible dehydrogenation of NaAlH(4). It was observed that Al(3)Ti possessed considerable catalytic effect on the de-/hydriding reactions of NaAlH(4). The catalytic enhancement by Al(3)Ti doping significantly increased with increasing ball-milling time. Moreover, it was noted that dehydriding kinetics and cycling performance of the Al(3)Ti-doped NaAlH(4) were highly dependent on the starting material, 1:1 NaH/Al mixture or NaAlH(4). The combined property and phase examinations suggest that Al(3)Ti may act as active species to catalyze the reversible de-/hydrogenation of NaAlH(4). (c) 2006 Published by Elsevier B.V.

A systematic investigation was performed on the hydrogen-storage performance of TiF(3) doped NaAlH(4) that was prepared under varied conditions. It was found that the starting material, milling atmosphere (inert or reactive) and milling time all exerted substantial influence on the hydrogen-storage performance of the materials. In particular, the variation of milling time or milling atmosphere was observed to influence only the NaAlH(4)/Na(3)AlH(6) + Al step, but not the following decomposition/reconstruction of Na(3)AlH(6). Moreover, it was observed that the specific hydrogen-storage performance of the doped hydrides arising from the variation of the preparation conditions persisted through the following dehydriding/rehydriding cycles. These results elucidate the optimized preparation conditions of the NaAlH(4)-TiF(3) system. More significantly, they provide valuable hint in exploring the nature of active Ti-species in the doped NaAlH(4). (c) 2005 Elsevier B.V. All rights reserved.

The effect of Ti-doping on the dehydrogenation kinetics of lithium aluminum hydride has been investigated in this paper. For the decomposition of lithium aluminum hydride into trilithium hexahydridoaluminate apparent activation energies of 81 and 89 kJ/mol are found for the un-doped and Ti-doped sample, respectively. For the decomposition of trilithium hexahydridoaluminate into lithium hydride apparent activation energies of 108 and 103 kJ/mol are found. The differences in apparent activation energies between the un-doped and the Ti-doped sample are within experimental uncertainty, suggesting that the effect of Ti-doping on the kinetic parameters of dehydrogenation of lithium aluminum hydride, is mainly a prefactor effect.

The decomposition of various alkali and alkaline earth complex alanates and the formation of intermediate compounds was studied by in situ X-ray high-temperature diffraction and differential scanning calorimetry (DSC) experiments. Differences of the reaction pathways during thermolysis for the alkali and the alkaline earth aluminum hydrides were determined. During the thermolysis of Mg(AlH4)2 and Ca(AlH4)2, the appearance of metal hydrides in combination with alloys was observed, whereas for the alkali alanates LiAlH4 and KAlH4, intermediate aluminum hydrides but no alloys are formed. For the alkali salt-containing KAlH4 systems a strong influence due to the presence of salts on the decomposition temperatures are observed. In addition, the decomposition temperatures are also significantly influenced by the type of salt present. For the first time, the decomposition of the LiMg(AlH4)3 and Na2LiAlH6 systems was studied by in situ methods.

The complex hydride NaAlH4 is attracting much attention right now because of high hydrogen storage capacity and a practical desorption temperature that can be brought down below 150 ° C. However, this is still far from the working temperature of conventional perfluorosulfonic acid based polymer fuel cells (e.g. Nafion cells). A hydrogen desorption temperature below 80 ° C is mandatory if the excess heat of such a fuel cell shall be used for hydrogen desorption from a NaAlH4 storage system. Our approach is to increase the working temperature of the polymer fuel cell. The present paper reports our recent work on polymer fuel cells operating at temperatures up to 200 ° C. The key component to make this possible is an electrolyte membrane of polybenzimidazole (PBI) doped with phosphoric acid. Cells are operated at temperatures up to 200 ° C without any humidification. Continuous service for more than 6 months at 150 ° C is demonstrated. The temperature and amount of the excess heat is more than enough to run a metal hydride tank based on NaAlH4.

In situ studies of the transition from NaAlH 4 to Na 3 AlH 6 are performed by proton NMR for samples doped with TiCl 3 and with Ti 13 -nanoclusters. The local hydrogen dynamics in the different compounds is studied by the nuclear spin-lattice relaxation. For the Ti-doped NaAlH 4 samples a double-exponential recovery of the nuclear magnetization is observed, indicating two fractions of hydrogen with different mobilities. In Na 3 AlH 6 a transition from hindered rotation of the AlH 6 groups to full isotropic reorientation of these groups is observed in the temperature range 200–260 K.

Mg–Al thin films with a compositional gradient are co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. We study their hydride formation by monitoring their optical transmission during hydrogenation under defined pressure and temperature conditions. We find that Mg(AlH 4 ) 2 is already formed from the elements at p ( H 2 ) = 1 bar and T = 100 ° C. A thin layer of Ti acts as a catalyst. Doping of Mg–Al with Ti has a negative influence on hydrogen absorption.

In this paper, we have carried out investigations on admixing carbon nanotubes (CNT) to increase the desorption rate of hydrogen in NaAlH 4 . So far only transition metal Ti has been attempted as the suitable dopant for making NaAlH 4 (purified version) a viable hydrogen storage material by increasing the hydrogen desorption rate. Out of the various materials corresponding to NaAlH 4 – x mol% CNT ( x = 2, 4, 6, 8 and 12), we have found that the material with x = 8 mol% is the optimum material. It shows highest desorption rate leading to 3.3 wt.% of H 2 at ∼160 °C within 2 h for the first dissociation reaction. The CNT admixed NaAlH 4 has also been found to exhibit good rehydrogenation characteristics (reversibility on hydrogenation up to ∼4.2 wt.%). A feasible mechanism for the improvement of hydrogen desorption characteristics has been put forward.

The potential for using aluminum hydride, AlH3, for vehicular hydrogen storage is explored. It is shown that particle-size control and doping of AlH3 with small levels of alkali-metal hydrides (e.g. LiH) results in accelerated desorption rates. For AlH3–20 mol % LiH, 100 °C desorption kinetics are nearly high enough to supply vehicles. It is highly likely that 2010 gravimetric and volumetric vehicular system targets (6 wt % H2 and 0.045 kg/L) can be met with onboard AlH3. However, a new, low-cost method of off-board regeneration of spent Al back to AlH3 is needed.

CeMn1.8Al0.2 absorbs more than 4.4 hydrogen atoms near ambient conditions, thereby undergoing a volume expansion (ΔV/Ṽ43%) that is the largest known among metal hydrides. While the Mn/Al distribution in the C14-type alloy [P63/mmc, a = 5.37393(6), c = 8.76321(12) A] is partially ordered (preference of Al for site 2a), it tends to become disordered in the hydride (equal Mn/Al occupancies for sites 2a and 6h). Only very slow hydrogenation while cooling the sample to -70°C is capable of maintaining a partial Mn/Al order. Deuterium in CeMn1.8Al0.2D4.36(7) [a = 5.9788(6), c = 9.7600(13) A] occupies exclusively tetrahedral Ce2(Mn,Al)2 type interstices. Magnetic susceptibility data suggest that the volume expansion during hydrogenation is enhanced by a valence change of cerium (CeIV → CeIII). © 2003 Elsevier B.V. All rights reserved.

We investigated the dehydrogenation of NaAlH 4 and the reversible low-pressure rehydrogenation from NaH to Na 3 AlH 6 . Highly purified NaAlH 4 requires relatively high temperatures to decompose to NaH and long durations to rehydride to the Na 3 AlH 6 phase in hydrogen gas. However, any degradation of the purity of this material, whether through ball milling with diamond powder or ball milling with diamond plus Al powders, mixing the purified material with Pt powder, or doping with a Ti organometallic compound, lowers the decomposition temperature and facilitates rehydriding the product NaH+Al to Na 3 AlH 6 . Diamond ball milling of NaAlH 4 seems to be the best of these procedures; it substantially decreases the decomposition temperatures, with significant dehydrogenation starting at 180 °C rather than 250 °C for the purified material, and with formation of NaH substantially complete at 235 °C rather than 290 °C. Rather surprisingly, it also facilitates rehydrogenation from NaH+Al to Na 3 AlH 6 . Similarly, NaAlH 4 doped with Ti according to the recipe of Bogdanovic lowers the decomposition temperatures and improves the hydrogenation kinetics for the low pressure transition from NaH+Al to Na 3 AlH 6 . Pressure–composition isotherms show that the rehydrogenation of the resulting NaH+Al decomposition phases into the Na 3 AlH 6 intermediate phase at pressures below 3.6 MPa is similar for the diamond ball milled and Ti-doped material. Diamond ball milling NaAlH 4 with excess Al did not improve the rehydrogenation kinetics.

The hydrogen storage properties of catalyzed NaAlH 4 (and associated Na 3 AlH 6 ) were studied in relation to various practical engineering considerations. Properties measured were cyclic capacity, charging and discharging rates, thermal effects, gaseous impurities, volume changes, low temperature plateau pressures and detailed isothermal desorption kinetics over the temperature range 23–180°C. Two materials were evaluated, one mechanically milled with the liquid alkoxides Ti(OBu n ) 4 and Zr(OPr i ) 4 and one milled with dry TiCl 3 as catalyst precursors. The alkoxide-catalyzed materials had low reversible capacities and released significant levels of hydrocarbon impurities during H 2 discharge. These problems were virtually eliminated with the inorganic TiCl 3 catalyst precursor. The NaAlH 4 and Na 3 AlH 6 decomposition kinetics of TiCl 3 -catalyzed Na-alanate conform to Arrhenius behavior with activation energies of 79.5 and 97 kJ/mol H 2 , respectively. Measured absorption and desorption kinetics were surprisingly good and it is shown that 3–4.5 wt.% H 2 can be stored and recovered in reasonable times at 100–125°C. It may even be ultimately possible to use the NaAlH 4 decomposition reaction to provide 3 wt.% H 2 at room temperature for low-rate applications.

The discovery that hydrogen can be reversible absorbed and desorbed from NaAlH{sub 4} by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. NaAlH{sub 4} releases hydrogen through the following set of decomposition reactions: NaAlH{sub 4} {r_arrow} 1/3({alpha}-Na{sub 3}AlH{sub 6}) + 2/3Al + H{sub 2} {r_arrow} NaH + Al + 3/2H{sub 2}. These decomposition reactions as well as the reverse recombination reactions were directly observed using time-resolved in-situ x-ray powder diffraction. These measurements were performed under conditions similar to those found in PEM fuel cell operations (hydrogen absorption: 50--70 C, 10--15 bar Hz, hydrogen resorption: 80--110 C, 5--100 mbar H{sub 2}). Catalyst doping was found to dramatically improve kinetics under these conditions. In this study, the alanate was doped with a catalyst by dry ball-milling NaAlH{sub 4} with 2 mol.% solid TiCl{sub 3}. X-ray diffraction clearly showed that TiCl{sub 3} reacts with NaAlH{sub 4} to form NaCl during the doping process. Partial desorption of NaAlH{sub 4} was even observed to occur during the catalyst doping process.

Mechanochemical processing of polycrystalline LiAlH4 revealed good stability of this complex aluminohydride during high-energy ball-milling in a helium atmosphere for up to 35 h. The decomposition of lithium aluminohydride into Li3AlH6, Al and H2 is observed during prolonged mechanochemical treatment for up to 110 h and is most likely associated with the catalytic effect of a vial material, iron, which is introduced into the hydride as a contaminant during mechanical treatment.

New reversible hydrogen storage systems are proposed, based on catalyzed reactions (Eqs. 4–6). The catalytic acceleration of the reactions in both directions is achieved by doping alkali metal aluminium hydrides with a few mol% of selected Ti compounds. The PCI diagrams of the Ti catalyzed systems show an absence of hysteresis and nearly horizontal pressure plateaus. The PCI of the NaAlH4 system reveals two temperature-dependent pressure plateaus, corresponding to the two-step reversible dissociation of NaAlH4. The PCI of the Na3AlH6 system shows only one pressure plateau; the latter can be lowered by partial substitution of Na by Li. In cyclic tests, reversible H2 capacities of 4.2–3.1 and 2.7–2.1 wt% H have been achieved.

Reaction of DIPPnacnacAlH(2) with DIPPNH2BH3 did not give the anticipated deprotonation but nucleophilic substitution at B was observed instead. The product DIPPnacnacAl(BH4)(2) was isolated and structurally characterized. Nucleophilic displacement at B might play a role in mechanistic pathways related to metal amidoborane complexes.

Aluminium hydride has been synthesized electrochemically, providing a synthetic route which closes a reversible cycle for regeneration of the material and bypasses expensive thermodynamic costs which have precluded AlH(3) from being considered as a H(2) storage material.

An in situ thermal desorption study of solvated aluminum hydride (alane) by transmission electron microscopy and selected area diffraction has permitted characterisation of the structural and morphological changes during desorption of solvent and hydrogen in real-time; this powerful technique for studying hydrogen storage materials complements several others already employed.

The effects of various catalysts on the H 2 release characteristics of LiAlH 4 were studied. It was established that the catalytic activity defined by the H 2 desorption capacity by a mechanochemical reaction decreased in the series TiCl 3 > ZrCl 4 > VCl 3 > NiCl 2 > ZnCl 2 . Transition metal trialuminides Al 3 X (X = Ti, Zr, V, Ni) were prepared by the mechanochemical reaction of LiAlH 4 and the metal chlorides, while the reaction of LiAlH 4 and ZnCl 2 yields Zn. The high catalytic activity was attributed to the lower percentage d-character and the electronegativity of those transition metals. The catalytic activity of Ni was also improved with decreasing the Ni particle size. Those catalysts were classified to two groups (1: TiCl 3 , ZrCl 4 ; 2: VCl 3 , NiCl 2 , Ni, nano-Ni) from the viewpoint of the H 2 desorption mechanism.

The hydrogen storage properties of doped and undoped NaAlH4 samples are studied after mixing them with different percentages of high surface carbon. Manually mixed samples are compared with ball milled ones; it was found that manual mixing was a simple and effective way to dope NaAlH4. A morphological and micro-structural analysis has been carried out in order to understand the effect of carbon. Carbon added samples show a marked enhancement of hydrogen desorption rate. The desorption temperature and the total hydrogen content remain almost unchanged for undoped sample. The desorption temperatures of Ti-doped samples increase with carbon content. (C) 2006 Elsevier B.V. All rights reserved.

Aluminium hydride is a compound that is well known for its high gravimetric and volumetric hydrogen densities and favorable hydrogen storage properties. Tertiary amine–aluminium hydride complexes have gained interest due to their application as chemical reducing agents and in aluminium thin-film deposition. Various complexes of these amine alane compounds have been created and studied previously, but these compounds were not formed directly using pressurized hydrogen. Here, we demonstrate the direct reaction of catalyzed aluminium, a tertiary amine, and hydrogen in a common solvent proceeds to form an amine alane adduct at moderate pressures and temperatures. A complex of aluminium hydride has been formed with dimethylethylamine by this technique. A vibrational analysis of the product of these reactions by Raman and infrared spectroscopy is presented, including experimental and theoretical data. The results clarify the molecular and vibrational structure of amine alane complexes formed by direct hydrogenation and are compared with previously determined experimental information. In addition, we demonstrate a new method for the formation of triethylamine alane using the direct hydrogenation of dimethylethylamine and catalyzed aluminium followed by transamination with triethylamine. Finally, we propose a new low energy method to regenerate AlH3 from catalyzed aluminium and hydrogen gas.

Photorechargeable air batteries which discharge by reducing oxygen in the air and which can he charged by light irradiation, are new types of secondary batteries We have investigated recharging this type of battery via the photoelectrochemical reaction that occurs at its metal hydride-semiconductor/electrolyte interface. This paper discusses the photorecharging capability of an air hattery using a metal hydride anode, namely, a SrTiO 3 -LaNi 3.76 Al 1.24 H n |KOH|O 2 cell. We have confirmed that our new negative electrode not only prevents self-discharge hut is also capable of semiconductor hand bending which is the most important requirement for photorecharging We have demonstrated that this battery can be photorecharged and discharged by using a metal hydride electrode and oxygen from air.