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Since the initial assessment four decades ago of zirconium diberyllide, ZrBe2, as a potential hydride-forming intermetallic for hydrogen-storage applications, structural and dynamical studies to date have been chiefly limited to the hydride composition, ZrBe2H1.5, which exists as a single-phase disordered hydride with hexagonal P6/mmm symmetry that undergoes hydrogen sublattice ordering below ~200 K (~235–250 K for ZrBe2D1.5). It is desirable from both fundamental and technological viewpoints to have a more complete understanding of the ZrBe2Hx phase diagram. In the present study, both neutron powder diffraction and neutron vibrational spectroscopy measurements of ZrBe2Hx at lower hydrogen contents (x < 1.5) indicate that at least two other ordered phases exist at low temperatures, coinciding with respective nominal x values of 1 and 0.67. Compared to ZrBe2H1.5, these more-hydrogen-dilute phases possess different structural symmetries (orthorhombic) with different H-sublattice orderings and undergo much-higher-temperature order-disorder transitions at ≈ 460 K (x = 1) and ≈ 490 K (x = 0.67) to the characteristic H-disordered hexagonal P6/mmm structure associated with ZrBe2H1.5.

The use of the conventional pressure-composition-temperature (PCT) method to determine the thermodynamics of metal hydrides is a time-consuming process. This work presents an efficient method based on thermogravimetric analysis (TGA), to characterize the thermodynamic parameters. Through cycling catalyzed magnesium hydride in a TGA apparatus under a flowing gas with a constant hydrogen partial pressure, TGA curves can be used to determine absorption/desorption equilibrium temperatures. Based on the van't Hoff analysis, the reaction enthalpies and entropies can be derived from the equilibrium temperatures as a function of hydrogen pressure. Using the results of this work we calculated the hydrogenation and dehydrogenation enthalpies, which are 79.8 kJ per mol per H2 and 76.5 kJ per mol per H2, respectively. These values are in good agreement with those reported values using the PCT method. These results demonstrate that the TGA can be an efficient and effective method for measuring thermodynamic parameters of metal hydrides.

Magnesium-based hydrides are considered as promising candidates for solid-state hydrogen storage and thermal energy storage, due to their high hydrogen capacity, reversibility, and elemental abundance of Mg. To improve the sluggish kinetics of MgH2, catalytic doping using Ti-based catalysts is regarded as an effective approach to enhance Mg-based materials. In the past decades, Ti-based additives, as one of the important groups of catalysts, have received intensive endeavors towards the understanding of the fundamental principle of catalysis for the Mg-H2 reaction. In this review, we start with the introduction of fundamental features of magnesium hydride and then summarize the recent advances of Ti-based additive doped MgH2 materials. The roles of Ti-based catalysts in various categories of elemental metals, hydrides, oxides, halides, and intermetallic compounds were overviewed. Particularly, the kinetic mechanisms are discussed in detail. Moreover, the remaining challenges and future perspectives of Mg-based hydrides are discussed.

Magnesium hydride has long been regarded as a promising candidate material for hydrogen and heat storage due to its high hydrogen capacity, reversibility, and low cost. Catalytic doping has been demonstrated as one of the most effective methods to improve hydrogen storage properties of MgH

Hydrogen absorption and desorption isotherms have been measured for several metal hydride alloys identified as possible candidates in the high-pressure (i.e. >80 MPa) stage of a two-stage hydrogen compressor. The isotherms were obtained using two independent Sieverts volumetric test systems built specifically for measuring hydrogen absorption and desorption parameters from 0.10 to 100 MPa. The results obtained enabled us to identify the alloy Ti0.8Zr0.2Fe1.6V0.4 as the most viable of the candidates investigated for use in the high-pressure stage of a prototype two-stage 80+ MPa compressor, as it produced the highest desorption pressures at moderate temperatures. Issues and challenges in determining reliable isotherms at pressures >50 MPa are also described.

Air exposure of the magnesium hydride materials can lead to severe degradation on the hydrogen storage properties, and therefore is one of the key remaining challenges for its application. In this work, the effects of direct air exposure and solvent-protected air exposure for catalyzed MgH2 on the hydrogen storage properties are systematically investigated. Results show that the direct air exposure of catalyzed MgH2 leads to reduction of the hydrogen storage capacity, and moderate deterioration of the hydrogen absorption/desorption kinetics, but has limited impact on reversibility and cycle stability. A short-time air exposure (15 min) causes the decrease of hydrogen storage capacity of MgH2–5%VTiCr by 0.2 wt%, and long-time air exposure (2400 min) significantly deteriorates the hydrogen storage capacity by 1.95 wt%. During the direct exposure in ambient air, MgH2 reacts with oxygen and moisture, and the surface forms a layer consisting of Mg(OH)2 and MgO. Various solvents, hexane, acetone, and ethanol, are used to protect MgH2–5%TiMn2 material. The hydrogen storage properties of the solvent-protected materials after air exposure for 1500 min are evaluated, both the hydrogen storage capacity and kinetics are degraded after exposure. The acetone-protected MgH2–5%TiMn2 shows better kinetics and capacities compare to those protected by hexane and ethanol.

A concept of thermal battery based on advanced metal hydrides was studied for heating and cooling of cabins in electric vehicles. The system utilized a pair of thermodynamically matched metal hydrides as energy storage media. The pair of hydrides that was identified and developed was: (1) catalyzed MgH 2 as the high temperature hydride material, due to its high energy density and enhanced kinetics; and (2) TiV 0.62 Mn 1.5 alloy as the matching low temperature hydride. Further, a proof-of-concept prototype was built and tested, demonstrating the potential of the system as HVAC for transportation vehicles.

Variable temperature (VT) ^2H magic angle spinning (MAS) spectroscopy was employed to measure deuterium diffusion behavior in the Zr_2NiD_(4.8) phase. ^2H MAS NMR spectrum at ∼190 K provides with well-resolved 4 different site occupancies which can be assigned based on the crystal structure (16k (Zr_2Ni_2), 32m (Zr_3Ni), Zr_4 (16/ and 4b)). As the temperature rises, the ^2H peaks sensitively reflect the exchange behavior among the sites with evident change at around 230 K and reaching a uniform distribution of site occupancies, indistinguishable in NMR timescale, above 245 K. This behavior is reflected by the collapse of the ^2H MAS spectrum into a single peak. From analyses of VT MAS NMR spectra, we were able to extract multiple hopping rates and activation energies among face sharing interstices: for example, 32m ↔ 16/ hopping shows _(τc)=2.8×10^(-4)s at 245 K and E_a = 62.2 kJ/mol.

Magnesium hydride is widely recognized as a promising material for hydrogen storage and thermal energy storage. Cyclic stability of the kinetics of catalyzed MgH2 was systematically investigated in this study. Three systems including MgH2–TiH2, MgH2–TiMn2, and MgH2–VTiCr were prepared by high energy ball milling. Volumetric hydrogen desorption and absorption cycles were performed using the pressure–composition–isothermal (PCI) method. Results show that the kinetics of high-temperature (300 °C) hydrogenation and dehydrogenation maintained a good stability during 100 hydrogen cycles. However, when testing the hydrogenation kinetics at the low-temperature range from 25 to 150 °C, a severe degradation was observed after hydrogen cycles. Among the three materials, the MgH2–VTiCr system shows better cyclic performance. The degradation of low-temperature kinetics can be mainly related to the hydrogenation–dehydrogenation cycling reactions.

In Part I, the cyclic stabilities of the kinetics of catalyzed MgH2 systems including MgH2–TiH2, MgH2–TiMn2, and MgH2–VTiCr were investigated, showing stable kinetics at 300 °C but deteriorations of the hydrogenation kinetics at temperatures below 150 °C. The present Part II describes the characterization of uncycled and cycled catalyzed MgH2 by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. XRD analysis shows the crystallite sizes of the Mg and MgH2 significantly increased after the cycling. The mean crystallite sizes of the catalysts (TiH2 and VTiCr) increased moderately after the cycling. SEM and TEM imaging were used to compare the microstructures of uncycled (as-milled) and cycled materials, revealing a drastic change of the microstructure after 100 cycles. In particular, results from energy-dispersive spectroscopy (EDS) mapping show that a change of distribution of the catalyst particles in the...

We present first-principles solubility calculations of H–/[OH]− mixing in binary alkali metal hydrides MH and their corresponding hydroxides MOH, for M ={Li, Na, K, Rb, Cs}. Solid solubility in the...

Deuterium magic-angle spinning NMR spectra of MgD2/TiD2 composites are presented. After high-energy, high-pressure ball-milling of an 80:20 Mg:Ti hydride, the material was H → D exchanged using high-pressure D2 gas. This material displayed the deuterium NMR spinning sideband pattern of the MgD2 component that is typical of rutile-structured MgD2. After heat treatment at 450 °C under high D2 pressure, the sidebands became much weaker, indicating that most D atoms were now in sites of local cubic symmetry. This is in agreement with a previous proposal of a nearly fully Mg–Ti segregated, lattice coherent fluorite (fcc) structure of the composite. Because H(D) diffusion is expected to be more rapid in the fluorite structure, compared to the rutile form, this may be important to the kinetics of hydrogen storage and release from MgH2/TiH2 composites.

Metal hydride (MH) thermal sorption compression is an efficient and reliable method allowing a conversion of energy from heat into a compressed hydrogen gas. The most important component of such a thermal engine – the metal hydride material itself – should possess several material features in order to achieve an efficient performance in the hydrogen compression. Apart from the hydrogen storage characteristics important for every solid H storage material (e.g. gravimetric and volumetric efficiency of H storage, hydrogen sorption kinetics and effective thermal conductivity), the thermodynamics of the metal–hydrogen systems is of primary importance resulting in a temperature dependence of the absorption/desorption pressures). Several specific features should be optimised to govern the performance of the MH-compressors including synchronisation of the pressure plateaus for multi-stage compressors, reduction of slope of the isotherms and hysteresis, increase of cycling stability and life time, together with challenges in system design associated with volume expansion of the metal matrix during the hydrogenation. The present review summarises numerous papers and patent literature dealing with MH hydrogen compression technology. The review considers (a) fundamental aspects of materials development with a focus on structure and phase equilibria in the metal–hydrogen systems suitable for the hydrogen compression; and (b) applied aspects, including their consideration from the applied thermodynamic viewpoint, system design features and performances of the metal hydride compressors and major applications.

1H and 27Al NMR is used to reveal the motions of AlH4 anions in KAlH4. Line-narrowing from rotations and from translational diffusion is observed in the NMR of both nuclei. Unlike the anions in NaAlH4 and LiAlH4 that are not rotating on the NMR time scale at room temperature, the KAlH4 anions are already rotating rapidly at 23 °C. Based on the onset of rotation-induced line narrowing, the 1H T1 minimum, and the low- temperature hydrogen T1ρ minimum associated with reorientations, the rotational activation energy Erot,act = 0.28 eV is determined. Similarly, we use the onset temperature of translational motion-induced line narrowing and the high-temperature T1ρ minimum to determine the diffusion activation energy Ediff,act = 0.70 eV. Lack of sharp structure in the first-order quadrupole pattern and the absence of second-order quadrupole structure in the 27Al NMR data suggest asymmetry (η ≠ 0) and/or variations in the anion electric field gradients from structural disorder.

Hydrogenation mechanism of magnesium-based hydrides has attracted considerable interest but many details still remained unclear or controversial. This work aimed to investigate the isothermal hydrogenation kinetics for a number of un-catalyzed and catalyzed MgH2 samples prepared by high-energy ball milling. The effects of processing parameters on the hydrogenation kinetics of magnesium hydrides were studied. X-ray diffraction and transmission electron microscopy were carried out in order to relate MgH2 crystalline size and catalytic effect to the kinetics of hydrogenation. Different kinetic models were employed to further understand the mechanism of hydrogenation. The best kinetic model to describe the hydrogenation behavior of the ball-milled Mg/MgH2 in this work was found to be the JMA (Johnson-Mehl-Arrami) model. In addition, hydrogen diffusion can be a controlling step when the kinetic rates are slow. Adding catalysts contributes to lower hydrogenation activation energy in two ways: reducing crystalline size of the as-milled MgH2 powder, as well as facilitating the hydrogen diffusion in Mg/MgH2.

The presence of approximately 10 mol % NaOH mixed with NaH powder is shown to result in much more rapid hydrogen motion in the NaH above 150 °C, as indicated by the onset of hydrogen NMR line narrowing at this temperature. A similar result appears for air-exposed NaH due to the formation of hydroxide from atmospheric H2O. The NMR line narrowing is too rapid, as a function of temperature, to be described by thermal activation; rather, it is suggestive of a phase transition. Indeed, differential scanning calorimetry finds, after an initial thermal cycle, a reversible thermal anomaly indicating a phase transition near 150 °C. Powder X-ray diffraction with an excess of NaOH displays a remarkable lattice expansion of the NaH in the temperature range of 100–240 °C where the (200), (220), and (311) reflections indicate a volume lattice expansion of up to 11%; the expansion is reversible upon cycling. The data thus point to a reversible phase transition in which NaOH enters the NaH structure above 150 °C and exit...

Hydrogen absorption isotherms (273–333 K) have been measured for closely stoichiometric, annealed, un-activated LaNi5, i.e., a virgin un-activated LaNi5 sample was employed at each temperature. This is the first time that such isotherms have been measured for annealed, un-activated LaNi5 at such low temperatures ⩽298 K. After measurement of the initial absorption isotherms, the second absorption and desorption isotherms have been measured at each temperature. Van’t Hoff plots have been made from these absorption and desorption plateau pressure data and, from these, Δ H plat values have been obtained for the initial and subsequent absorption and desorption reactions. The ∣ΔHplat∣ determined from the van’t Hoff plot is smaller for the initial isotherm for the annealed, un-activated form of LaNi5 than it is for activated LaNi5.

LaNi4.78Sn0.22Hx hydride samples were held at a hydrogen content of x > 5.0 (x is H/La atomic ratio) and temperatures above 465 K to accelerate the intrinsic degradation processes. Although Sn-substituted alloys are much more resistant to disproportionation than nearly all other LaNi5 alloys, the present test conditions did produce substantial degradation. Effects observed included reduction in hydrogen storage capacity, decreases in the plateau pressures, increased slopes of the plateaus, and smaller hysteresis ratios. A regeneration process nearly completely restored the behavior of the degraded LaNi4.78Sn0.22 hydride to its initial value. First-principles chemical reaction kinetics and statistical thermodynamics simulations have replicated experimental pressure–composition hydrogen gas absorption isotherms for both initial and degraded LaNi4.78Sn0.22 hydride. Neutron diffraction characterization of phase compositions, crystal structures, and hydrogen content have been performed on undamaged, degraded, and regenerated LaNi4.78Sn0.22 deuterides.

Metal hydride (MH) thermal sorption compression is one of the more important applications of the MHs. The present paper reviews recent advances in the field based on the analysis of the fundamental principles of this technology. The performances when boosting hydrogen pressure, along with two- and three-step compression units, are analyzed. The paper includes also a theoretical modelling of a two-stage compressor aimed at describing the performance of the experimentally studied systems, their optimization and design of more advanced MH compressors. Business developments in the field are reviewed for the Norwegian company HYSTORSYS AS and the South African Institute for Advanced Materials Chemistry. Finally, future prospects are outlined presenting the role of the MH compression in the overall development of the hydrogen-driven energy systems. The work is based on the analysis of the development of the technology in Europe, USA and South Africa.

Hydrogen and 23Na NMR were used to probe diffusive motions of the ions in several NaH powders. In three NaH samples, the H resonance is a superposition of broad and narrow components, reflecting th...

Solid state NMR is widely applied in studies of solid state chemistries for hydrogen storage reactions. Use of ^(11)B MAS NMR in studies of metal borohydrides (BH_4) is mainly focused, revisiting the issue of dodecaborane formation and observation of ^(11)B{^1H} Nuclear Overhauser Effect.

In Part 1, the promising hydrogen storage properties of the combined systems of Li3AlH6/LiBH4 and Al/LiBH4, exhibiting the favorable formation of AlB2 during dehydrogenation, were presented based on TGA and XRD analyses. The present Part 2 describes the characterization of the intermediate and final products of the dehydrogenation and rehydrogenation of the above systems by multinuclear solid-state NMR characterization. This work has also verified that the presence of Al resulted in the re-formation of LiBH4 occurring at a much lower temperature and H2 pressure, under which conditions the dehydrogenation product from LiBH4 alone does not show any degree of rehydrogenation. NMR studies mainly identified various reaction intermediates for LiBH4 dehydrogenation/rehydrogenation reactions. Unlike the XRD studies, the AlB2 formation, in particular, could not be unambiguously confirmed by NMR. 27Al NMR showed that aluminum was mainly involved in various Li—Al alloy formations. The catalytic role of Al in the LiB...

Neutron radiography and tomography were used to study the transient and steady-state distributions, respectively, of hydrogen within a prototypical, LaNi 4.78 Sn 0.22 -based hydrogen-storage bed during and after various absorption and desorption steps. It was shown that using deuterium instead of hydrogen enabled the imaging of thicker beds. These measurements serve to demonstrate the unique utility of neutron imaging as an important diagnostic tool for in situ , real-time characterization of hydrogen-concentration profiles in practical hydrogen-storage systems.

LiBH4 doped with 1.6 mol % well-dispersed C60 is studied with solid-state nuclear magnetic resonance (NMR). Variable-temperature hydrogen NMR shows large changes between the data upon first heating and after exposure to 300 °C. After heating, a large fraction on the order of 50% of the hydrogen signal appears in a motionally narrowed peak, similar to a previous report of LiBH4 in a porous carbon aerogel nanoscaffold. Magic-angle spinning (MAS) NMR of 13C in a 13C-enriched sample finds the C60 has reacted already in the as-mixed (unheated) material. Dehydriding and rehydriding result in further 13C spectral changes, with nearly all intensity being found in a broad peak corresponding to aromatic carbons. It thus appears that the previously reported improved dehydriding and rehydriding kinetics of this material at least partially result from in situ formation of a carbon framework. The method may offer a new route to dispersal of hydrides in carbon support structures.

Objectives This study examined the effect of increased physician numbers overall on the geographic distribution of the physicians in Japan and the US. Methods Equity of physician distribution with reference to community population and community income was evaluated in all municipalities in Japan, and all counties in the US. Results Between 1980 and 2005, Japan and the US experienced a 55% and 47% increase in the number of physicians per unit population, respectively. The Gini coefficients against population were at similar values between Japan and the US, and have been almost unchanged in the past 25 years in both countries. The Gini coefficient against income in the US was lower than the coefficient in Japan, and the US value has decreased since 1980. Correlation between physician-to-population ratio and per capita income among the communities was stronger in the US than in Japan and has increasingly been strengthened during the period examined. Conclusions In spite of constant growth of physician numbers, physicians do not diffuse according to population distribution in both countries. Rather, US physicians seem to diffuse according to income distribution. In order to reverse the continuing maldistribution of physicians, political intervention is required in both countries.

The NaAlH4 system remains the archetype of hydrogen storage solids. However, the detailed mechanisms of the hydrogen reactions of NaAlH4 remain unknown. We report 27Al in situ nuclear magnetic resonance (NMR) spectroscopic data revealing an Al-bearing mobile species that could provide the large scale metal-atom transport needed for rehydriding. This new species forms under reaction conditions but can be captured at ambient temperature using excess H2 pressure. The NMR measurements demonstrate that the species is highly mobile (at 20 °C) and carries both Al and H atoms. On the basis of the 27Al shift (close to NaAlH4) and the disorder evident in X-ray diffraction, the species is identified as highly defective NaAlH4, likely having a large AlH3 vacancy concentration.

Hydrogen NMR of LiBH4 in the pores of carbon aerogel nanoscaffolds shows the coexistence of motionally narrowed and broad components. The fraction of mobile, diffusing hydrogen, already evident at room temperature, increases continuously with temperature. Thus, a broad distribution of environments is present, as in some ball-milled hydrides. With decreasing pore size from 25 to 13 nm, the narrowed fraction increases, suggesting that the narrow resonance is from the most defective regions, the grain boundaries. The broad component eventually exhibits narrowing in the same temperature window as for bulk material, confirming the bulk-like structure of those regions. Hole-burning measurements reveal magnetization exchange between the broad and narrow resonance lines, confirming the close spatial proximity of the atoms in each line. The solid−solid transition is clearly evident in 7Li line shapes, with a 10−15 °C depression from the bulk. More rapid decay of the quadrupolar satellite signals in spin echoes, co...

A lithium salt of anionic scandium tetraborohydride complex, LiSc(BH_4)_4, was studied both experimentally and theoretically as a potential hydrogen storage medium. Ball milling mixtures of LiBH_4 and ScCl_3 produced LiCl and a unique crystalline hydride, which has been unequivocally identified via multinuclear solid-state nuclear magnetic resonance (NMR) to be LiSc(BH_4)_4. Under the present reaction conditions, there was no evidence for the formation of binary Sc(BH_4)_3. These observations are in agreement with our first-principles calculations of the relative stabilities of these phases. A tetragonal structure in space group I (#82) is predicted to be the lowest energy state for LiSc(BH_4)_4, which does not correspond to structures obtained to date on the crystalline ternary borohydride phases made by ball milling. Perhaps reaction conditions are resulting in formation of other polymorphs, which should be investigated in future studies via neutron scattering on deuterides. Hydrogen desorption while heating these Li−Sc−B−H materials up to 400 °C yielded only amorphous phases (besides the virtually unchanged LiCl) that were determined by NMR to be primarily ScB_2 and [B_(12)H_(12)]^(−2) anion containing (e.g., Li_2B_(12)H_(12)) along with residual LiBH_4. Reaction of a desorbed LiSc(BH_4)_4 + 4LiCl mixture (from 4LiBH_4/ScCl_3 sample) with hydrogen gas at 70 bar resulted only in an increase in the contents of Li_2B_(12)H_(12) and LiBH_4. Full reversibility to reform the LiSc(BH_4)_4 was not found. Overall, the Li−Sc−B−H system is not a favorable candidate for hydrogen storage applications.

To clarify the effect of substitution of Sn for Ni of LaNi 5 on cyclic durability of hydrogen storage, the behavior of lattice defects during the hydrogenation cycles at 288 K in LaNi 4.93 Sn 0.27 was investigated by in situ positron lifetime measurements. Mean positron lifetime increased to 175 ps by vacancy formation during initial hydrogenation and then decreased to 135 ps by vacancy recovery during dehydrogenation. Vacancies were reversibly introduced and removed at 288 K, although quenched-in vacancy cannot migrate below 448 K in LaNi 4.93 Sn 0.27 . Reversible change of the positron lifetime was observed in subsequent cycles, and dislocation density and vacancy concentration remained almost constant at around 6 × 10 9 cm −2 and 10 ppm, respectively, which are two orders of magnitude lower than those in binary LaNi 5 . These results indicate that the degradation of hydrogen capacity during a cycle closely relates to the concentration of lattice defects accumulated with hydrogenation.

We report here boron-11 and hydrogen NMR measurements of the lifetime of BH4 units in molten LiBH4, with respect to hydrogen exchange. A well-resolved 11B pattern of B−H J-couplings yields a lower ...

The decomposition of crystalline magnesium borohydride upon heating was studied using thermal desorption, calorimetry, in situ X-ray diffraction, and solid state NMR. Hydrogen release from Mg(BH4)2 occurs in at least four steps via formation of several polyborane intermediate species and includes an exothermic reaction yielding crystalline MgH2 as an intermediate. The decomposition products may be only partially recharged after the very first step and also via hydrogenation of Mg metal. The intermediate formation of amorphous MgB12H12, was confirmed by 11B NMR. A four-stage pathway for the thermal decomposition of Mg(BH4)2 is proposed.

Aluminoborane compounds AlB_4H_(11), AlB_5H_(12), and AlB_6H_(13) were reported by Himpsl and Bond in 1981, but they have eluded the attention of the worldwide hydrogen storage research community for more than a quarter of a century. These aluminoborane compounds have very attractive properties for hydrogen storage: high hydrogen capacity (i.e., 13.5, 12.9, and 12.4 wt % H, respectively) and attractive hydrogen desorption temperature (i.e., AlB_4H_(11) decomposes at ~125 °C). We have synthesized AlB_4H_(11) and studied its thermal desorption behavior using temperature-programmed desorption with mass spectrometry, gas volumetric (Sieverts) measurement, infrared (IR) spectroscopy, and solid state nuclear magnetic resonance (NMR). Rehydrogenation of hydrogen-desorbed products was performed and encouraging evidence of at least partial reversibility for hydrogenation at relatively mild conditions is observed. Our chemical analysis indicates that the formula for the compound is closer to AlB_4H_(12) than AlB_4H_(11).

Reversible dehydrogenation and hydrogenation reactions have been reported for a number of reactions based on lithium alanate and lithium amide materials. The dehydrogenation and hydrogenation reactions involving these materials are, however, usually very complex. Significant discrepancies exist among different studies published in literature. Understanding the reaction mechanism and the dependence of the reaction pathway on material preparation processes and processing parameters is critical. In this paper, the hydrogenation reactions of the mixture of 3Li2NH/Al/4 wt%TiCl3 were investigated as a function of the heating rate. The hydrogenated products were characterized by means of TGA, XRD and solid-state NMR. These new results showed that the re-formation of Li3AlH6 depends strongly on the heating rate during the hydrogenation process. The dehydrogenation and rehydrogenation reaction pathways and possible mechanisms of the combined system are, however, still under investigation.

In coarse-grained MgH2, the diffusive motion of hydrogen remains too slow (

1H, 7Li, and 11B NMR measurements were used to understand atomic translational motions in both the low- and high-temperature phases (LT, HT) of LiBH4. In the HT phase 7Li spectra, spin−echo T2, and T1 all indicate very rapid lithium ion diffusion. Just above the phase transition, the hydrogen resonance is broad, about 22 kHz fwhm (full width at half of maximum), showing that H translations remain slow. From 120 to 170 °C, a rapidly decreasing T1D (relaxation time of dipolar spin-order) shows that the hydrogens diffuse increasingly rapidly. This motion eventually results in marked hydrogen line-narrowing centered near 190 °C; the hydrogen diffusion is likely relevant to the kinetics of dehydriding. The extent of 11B line-narrowing demonstrates that the boron atoms also diffuse rapidly at temperatures above 200 °C. In the LT phase, the hydrogen T1D decreases rapidly with increasing temperature, here due to 7Li diffusion which is too slow for line-narrowing.

In an effort to develop reversible metal borohydrides with high hydrogen storage capacities and low dehydriding temperature, doping LiBH4 with various metal halides and hydrides has been conducted. Several metal halides such as TiCl3, TiF3, and ZnF2 effectively reduced the dehydriding temperature through a cation exchange interaction. Some of the halide doped LiBH4 are partially reversible. The LiBH4 + 0.1TiF3 desorbed 3.5 wt % and 8.5 wt % hydrogen at 150 and 450 °C, respectively, with subsequent reabsorption of 6 wt % hydrogen at 500 °C and 70 bar observed. XRD and NMR analysis of the rehydrided samples confirmed the reformation of LiBH4. The existence of the (B12H12)−2 species in dehydrided and rehydrided samples gives insight into the resultant partial reversibility. A number of other halides, MgF2, MgCl2, CaCl2, SrCl2, and FeCl3, did not reduce the dehydriding temperature of LiBH4 significantly. XRD and TGA-RGA analyses indicated that an increasing proportion of halides such as TiCl3, TiF3, and ZnCl2 from 0.1 to 0.5 mol makes lithium borohydrides less stable and volatile. Although the less stable borohydrides such as LiBH4 + 0.5TiCl3, LiBH4 + 0.5TiF3, and LiBH4 + 0.5ZnCl2 release hydrogen at room temperature, they are not reversible due to unrecoverable boron loss caused by diborane emission. In most cases, doping that produced less stable borohydrides also reduced the reversible hydrogen uptake. It was also observed that halide doping changed the melting points and reduced air sensitivity of lithium borohydrides.

Solid aluminum hydride, AlH 3 , has been proposed and studied for applications in hydrogen storage. In some samples, a comparatively narrow feature in the proton NMR spectrum is observed; we demonstrate here that this peak is due to molecular hydrogen (H 2 ) trapped within the solid, presumably from earlier processing procedures or partial decomposition. Static and magic-angle spinning NMR show that the responsible species is highly mobile, even at 11 K. Neutron-energy-gain spectra obtained at 3.5 K yield a feature at or near the free-rotor H 2 energy difference between the J = 1 and J = 0 states. Both NMR and neutron scattering demonstrate ortho – para conversion at low temperatures. Similar NMR signatures in other hydrogen-storage solids such as NaAlH 4 may also be due to trapped H 2 .

The hydrogen storage reaction ScH_2 + 2LiBH_4 → ScB_2 + 2LiH + 4H_2 (8.91 wt %), based on the thermodynamic destabilization of LiBH_4, is predicted to have a reaction enthalpy of ΔH_(300K) = 34.1 kJ/mol H_2. The isothermal kinetic desorption behavior in this system was measured. At temperatures up to 450 °C, less than 5 wt % H_2 is released, which is only half of the theoretical capacity. Powder X-ray diffraction data indicate that LiBH_4 has decomposed into LiH in the final desorption product, but the data provide no evidence that ScH_2 has participated in the reaction. Magic angle spinning NMR (MAS NMR) results do not show that the expected ScB_2 equilibrium product phase has formed during desorption. While the addition of 2 mol % TiCl_3 catalyst does improve desorption kinetics at 280 °C, it does not otherwise assist the destabilization reaction. The calculated reaction enthalpy suggests that this system should be of interest at moderate temperatures, but the large heats of formation of the reactant phases in this system appear to play a critical role in determining overall kinetics. Furthermore, the formation of a Li_2B_(12)H_(12) intermediate phase was determined by MAS NMR, which is an undesirable stable product if reaction reversibility is to be accomplished.

High pressure hydrides of V 0.995 C 0.005 were thermally cycled between β 2 - and γ-phases hydrides for potential use in cryocoolers/heat pumps for space applications. The effect of addition of carbon to vanadium, on the plateau enthalpies of the high pressure β 2 + γ region is minimal. This is in contrast to the calculated plateau enthalpies for low pressure (α + β 1 ) mixed phases which showed a noticeable lowering of the values. Thermal cycling between β 2 -and γ-phase hydrides increased the absorption pressures but desorption pressure did not change significantly and the free energy loss due to hysteresis also increased. Hydriding of the alloy with prior cold-work increased the pressure hysteresis significantly and lowered the hydrogen capacity. In contrast to the alloy without any prior straining (as-cast), desorption pressure of the alloy with prior cold-work also decreased significantly. Microstrains, 〈 ɛ 2 〉 1/2 , in the β 2 -phase lattice of the thermally cycled hydrides decreased after 778 cycles and the domain sizes increased. However, in the γ-phase, both the microstrains and the domain sizes decreased after thermal cycling indicating no particle size effect. The dehydrogenated α-phase after 778 thermal cycles also showed residual microstrains in the lattice, similar to those observed in intermetallic hydrides. The effect of thermal cycling (up to 4000 cycles between β 2 - and γ-phases) and cold working on absorption/desorption pressures, hydrogen storage capacity, microstrains, long-range strains, and domain sizes of β 2 - and γ-phase hydrides of V 0.995 C 0.005 alloys are presented.

^(11)B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH_4, Mg(BH_4)_2, LiSc(BH_4)_4, and the mixed Ca(AlH_4)_(2-)LiBH_4 system. The ^(11)B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K2B_(12)H_(12) that contains the [B_(12)H_(12)]^(2-) anion. The presence of [B_(12)H_(12)]^(2-) in the partially decomposed borohydrides was further confirmed by high-resolution solution ^(11)B and ^1H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.