Low temperature milling of the LiNH2 + LiH hydrogen storage system

Ball milling of the LiNH₂ + LiH storage system was performed at 20 °C, −40 °C, and −196 °C, and the resulting powders were analyzed using X-ray diffraction, scanning electron microscopy, nuclear magnetic resonance (NMR), specific surface area analysis, and kinetics cycling measurements. Ball milling at −40 °C showed no appreciable deviations from the 20 °C sample, but the −196 °C powder exhibited a significant increase in the hydrogen desorption kinetics. NMR analysis indicates that a possible explanation for the kinetics increase is the retention of internal defects generated during the milling process that are annealed at the collision site at higher milling temperatures.     

X-ray diffraction study on LixCoO2 below ambient temperature

In order to elucidate the structural change of Li_xCoO₂ with temperature (T), powder X-ray diffraction measurements have been carried out using a synchrotron radiation source in the T range between 300 and 90 K for the samples with x=1.02, 0.60, 0.56, and 0.53. The samples with x<1.02 were prepared by an electrochemical reaction in a non-aqueous lithium cell. The x=1.02 and 0.60 samples are in a rhombohedral phase () in the whole T range measured. On the other hand, the x=0.56 and 0.53 samples exhibit a structural transition around 140 K, although the both samples are in a monoclinic phase (C2/m) down to 90 K. That is, the angle between a_M- and c_M-axis (β_M) increases monotonically down to 150 K, then increases more rapidly with further lowering T. The values of and a_M/b_M, which are parameters to characterize a monoclinic distortion from the hexagonal symmetry, are and a_M/b_M<1.732 above 140 K, while and a_M/b_M≈1.732 below 140 K. This suggests that the monoclinic distortion below 140 K is mainly caused by a gliding along the basal plane.     

Reversible hydrogen storage in LiBH4/Ca(AlH4)2 systems

To improve the hydrogen storage property of LiBH₄, the LiBH₄/Ca(AlH₄)₂ reactive systems with various ratios were constructed, and their de-/hydrogenation properties as well as the reaction mechanisms were investigated experimentally. It was found that the sample with the LiBH₄ to Ca(AlH₄)₂ molar ratio of 6:1 exhibits the best comprehensive hydrogen storage properties, desorbing hydrogen completely (8.2 wt.%) within 35 min at 450 °C and reversibly absorbing 4.5 wt.% of hydrogen at 450 °C under a hydrogen pressure as low as 4.0 MPa. During the first dehydrogenation process of the LiBH₄/Ca(AlH₄)₂ systems, the CaH₂ and Al particles were in situ precipitated via the self-decomposition of Ca(AlH₄)₂, and then reacted with LiBH₄ to form CaB₆, AlB₂ and LiH. Whereafter, the sample can cycle between LiBH₄ + Ca(BH₄)₂ + Al in the hydrogenated state and CaB₆ + AlB₂ + LiH in the dehydrogenated state.     

Comparisons between MgH2- and LiH-containing systems for hydrogen storage applications

The present study compares the dehydrogenation kinetics of (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ systems and their vulnerabilities to the NH₃ emission problem. The (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixtures with different degrees of mechanical activation are investigated in order to evaluate the effect of mechanical activation on the dehydrogenation kinetics and NH₃ emission rate. The activation energy for dehydrogenation, the phase changes at different stages of dehydrogenation, and the level of NH₃ emission during the dehydrogenation process are studied. It is found that the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ mixture has a higher rate for hydrogen release, slower rate for approaching a certain percentage of its equilibrium pressure, higher activation energy, and more NH₃ emission than the (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixture. On the basis of the phenomena observed, the reaction mechanism for the dehydrogenation of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system has been proposed for the first time. Approaches for further improving the hydrogen storage behavior of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system are discussed in light of the newly proposed reaction mechanism.     

Structural and electrochemical behavior of LiMn0.4Ni0.4Co0.2O2

Layered LiMn_0.4Ni_0.4Co_0.2O₂ with the α-NaFeO₂ structure was synthesized by the “mixed hydroxide” method, followed by a high temperature calcination at 800 °C giving a single phase material of surface area 5 m² g⁻¹. A combined X-ray/neutron diffraction Rietveld refinement showed that the transition metals in the 3b layer are randomly distributed at room temperature, and that only nickel migrates to the lithium layer and in this case 4.4%. Addition of excess lithium reduces the amount of nickel on the lithium sites. The magnetic susceptibilities of the compounds LiMn_yNi_yCo_1−2yO₂ (y = 0.5, 0.4, 0.333) follow the Curie–Weiss law above 100 K and are consistent with the presence of Ni²⁺, Mn⁴⁺ and Co³⁺ cations; their magnetization curves, measured at 5 K and showing a pronounced hysteresis, are also consistent with the nickel content on the lithium sites increasing with decreasing cobalt content. This material shows a stable capacity of 140–170 mA h g⁻¹ for more than 90 cycles within the voltage window of 2.5–4.4 V. The layered rhombohedral structure is maintained as lithium is removed down to at least a lithium content of 0.05; the total volume change on cycling is under 2%. The nickel ions pin the lattice so that MO₂ slab sliding to form the 1T structure cannot readily occur. The capability of aqueous acids to leach lithium from the lattice decreases with increasing nickel content in the lithium layer; however, the thermal stability of the delithiated compounds increases with cobalt content.     

Enhanced hydrogen storage properties of LiBH4-MgH2 composite by the catalytic effect of MoCl3

Though LiBH₄-MgH₂ system exhibits an excellent hydrogen storage property, it still presents high decomposition temperature over 350 °C and sluggish hydrogen absorption/desorption kinetics. In order to improve the hydrogen storage properties, the influence of MoCl₃ as an additive on the hydrogenation and dehydrogenation properties of LiBH₄-MgH₂ system is investigated. The reversible hydrogen storage performance is significantly improved, which leads to a capacity of about 7 wt.% hydrogen at 300 °C. XRD analysis reveals that the metallic Mo is formed by the reaction between LiBH₄ and MoCl₃, which is highly dispersed in the sample and results in improved dehydrogenation and hydrogenation performance of LiBH₄-MgH₂ system. From Kissinger plot, the activation energy for hydrogen desorption of LiBH₄-MgH₂ system with additive MoCl₃ is estimated to be ∼43 kJ mol⁻¹ H₂, 10 kJ mol⁻¹ lower than that for the pure LiBH₄-MgH₂ system indicating that the kinetics of LiBH₄-MgH₂ composite is significantly improved by the introduction of Mo.     

Li2(NH2BH3)(BH4)/LiNH2BH3: The first metal amidoborane borohydride complex with inseparable amidoborane precursor for hydrogen storage

A novel lithium amidoborane borohydride complex, Li₂(NH₂BH₃)(BH₄), was synthesized using mechanochemical method and its crystal structure was successfully determined by a combination of X-ray diffraction (XRD) analysis and first-principles calculations. Interestingly, this compound does not exist as a pure phase, but requires almost equivalent amount of amorphous LiAB as a stabilizing agent. In this paper, we report a careful study of the structure, property, and dehydrogenation mechanism of the 1:1 Li₂(NH₂BH₃)(BH₄)/LiAB composite. This composite can release ∼8 wt% H₂ at 100 °C via a two-step dehydrogenation process, with dehydrogenation kinetics better than the parenting phases. The composite and its dehydrogenation products were characterized by the combined XRD, Fourier transformation infrared (FTIR) spectroscopy, and solid-state ¹¹B MAS NMR techniques. Selective deuterium labeling was performed to elucidate a reaction sequence for the hydrogen release by analyzing the released gases.     

Hydrogen absorption enhancement of nanocrystalline Li3N/Li2C2 composite

A nanocrystalline composite of lithium nitride and lithium carbide was synthesized through melt infiltration of lithium metal into the mesopores of carbon aerogels followed by nitrogenation with nitrogen gas. The structure, surface properties, and morphology of the prepared samples were examined by XRD, N₂ adsorption at 77 K, FE-SEM, FE-TEM, and TPD/MS. It was found that some of the lithium metal reacted with the carbon to form lithium carbide, and some of the lithium metal was transformed into lithium nitride by nitrogenation, yielding a composite of lithium nitride and lithium carbide. Relative to the bulk lithium nitride, the lithium nitride in the composite showed a significantly enhanced sequential hydrogen absorption capacity and a lowered temperature of hydrogenation/dehydrogenation.     

Hydrogenation of LiH/Al catalyzed with TiN,TiMn2 and LaNi5

In order to investigate the catalytic effect of TiN, TiMn₂ and LaNi₅ on the hydrogen storage capacity of LiAlH₄, 2 mol% of the catalyst was milled with LiH/Al and then hydrogenated in Me₂O. Doping with TiN, TiMn₂ or LaNi₅ led to substantial hydrogenation of LiH/Al in accordance with the formation of LiAlH₄. In each case the amount of hydrogen absorbed was dependent on the catalyst and the ball-to-powder ratio used during milling. A high ball-to-powder ratio results in an improvement in the hydrogen storage capacity of LiAlH₄. For each ball-to-powder ratio the highest hydrogen storage capacity was recorded for the TiN-catalyzed sample; hydrogen storage capacity increased from 3.2 to 4.8 to 6.0 wt.% H as the ball to-powder ratio increased from 10:1 to 20:1 to 40:1. The high levels of hydrogenation of LiH/Al catalyzed with TiN, TiMn₂ and LaNi₅ are remarkable because for the LiAlH₄ system only a TiCl₃ catalyst has previously been shown to result in rehydrogenation of the dehydrogenated products to LiAlH₄.     

Destabilization of LiBH4 by SrF2 for reversible hydrogen storage

The de-/rehydrogenation features of the 6LiBH₄/SrF₂ reactive hydride system have been systematically investigated. It was found that the thermal stability of LiBH₄ can be reduced markedly by combining it with SrF₂. Dehydrogenation of the 6LiBH₄/SrF₂ system proceeds via the 6LiBH₄ + SrF₂ → SrB₆ + 2LiF + 4LiH + 10H₂ reaction, which involves SrH₂ as the intermediate product. The dehydrogenation enthalpy change was experimentally determined to be 52 kJ/mol H₂ based on the P–C isotherm analysis. For rehydrogenation, LiBH₄ and SrF₂ were regenerated along with LiSrH₃ at 450 °C under ～8 MPa hydrogen pressure; thus, approximately 5.2 wt% of hydrogen can be released during the second dehydrogenation process.     

Environmental exposure of 2LiBH4+MgH2 using empirical and theoretical thermodynamics

It has been shown that the consequence of environmental exposure can be qualitatively predicted by modeling the heat generated as a result of environmental exposure of reactive hydrides along with heat loss associated with conduction and convection with the ambient surroundings. To this end, an idealized finite volume model was developed to represent the behavior of dispersed hydride from a breached system. Semi-empirical thermodynamic calculations and substantiating calorimetric experiments were performed in order to quantify the energy released, energy release rates and to quantify the reaction products resulting from water and air exposure of a lithium borohydride and magnesium hydride combination. The hydrides, LiBH₄ and MgH₂, were studied in a 2:1 “destabilized” mixture which has been demonstrated to be reversible. Liquid water hydrolysis reactions were performed in a Calvet calorimeter equipped with a mixing cell using pH-neutral water. Water vapor and gaseous oxygen reactivity measurements were performed at varying relative humidities and temperatures by modifying the calorimeter and utilizing a gas circulating flow cell apparatus. The results of these calorimetric measurements were used to develop quantitative kinetic expressions for hydrolysis and air oxidation in these systems. Thermodynamic parameters obtained from these tests were then incorporated into a computational fluid dynamics model to predict both the hydrogen generation rates and concentrations along with localized temperature distributions. The results of these numerical simulations can be used to predict ignition events and the resultant conclusions will be discussed.     

Mixed-metal Li3N-based systems for hydrogen storage: Li3AlN2 and Li3FeN2

The hydrogen storage systems Li₃AlN₂ and Li₃FeN₂ were synthesized mechanochemically by two different routes. In each case an intermediate material formed after milling, which transformed into Li₃MN₂ (M = Al or Fe) upon annealing. The synthesis route had a measurable effect on the hydrogen storage properties of the material: Li₃AlN₂ prepared from hydrogenous starting materials (LiNH₂ and LiAlH₄) performed better than that synthesized from non-hydrogenous materials (Li₃N and AlN). For both Li₃AlN₂ materials, the hydrogen storage capacity and the absorption kinetics improved significantly upon cycling. Ti-doped Li₃AlN₂ synthesized from LiNH₂ and LiAlH₄ showed the best hydrogen storage characteristics of all, with the best kinetics for hydrogen uptake and release, and the highest hydrogen storage capacity of 3.2 wt.%, of which about half was reversible. Meanwhile, Li₃FeN₂ synthesized from Li₃N and Fe displayed similar kinetics to that synthesized from Li₃N and Fe_xN (2 ≤ x ≤ 4), but demonstrated lower gravimetric hydrogen storage capacities. Li₃FeN₂ displayed a hydrogen uptake capacity of 2.7 wt.%, of which about 1.5 wt.% was reversible. For both Li₃AlN₂ and Li₃FeN₂, doping with TiCl₃ resulted in enhancement of hydrogen absorption kinetics. This represents the first study of a ternary lithium-transition metal nitride system for hydrogen storage.     

In situ X-ray diffraction study of dehydrogenation of MgH2 with Ti-based additives

Synchrotron based in situ x-ray diffraction measurements and an analysis of the dehydrogenation of MgH₂ and MgH₂ with Ti-based additives, including TiH₂ and TiMn₂, are presented. γ-MgH₂ to β-MgH₂ phase transformation in ball milled MgH₂ samples was observed prior to the dehydrogenation reaction. During the dehydrogenation of MgH₂ there were no significant phase transformations observed of the additives. These Ti-based additives functioned as catalysts during the dehydrogenation process resulting in lower temperature of dehydrogenation.     

In situ X-ray diffraction under H2 of the pseudo-AB2 compounds: YNi3.5Al0.5Mg

The hydrogenation and dehydrogenation behaviours of the YNi_3.5Al_0.5Mg compound were studied by in situ X-ray diffraction under hydrogen pressure and at room temperature. The changes of (i) the lattice parameters, (ii) the crystallite size and (iii) the lattice strain during the sorption process (i.e. along the PC isotherms) were studied. These results indicate that the crystallite size decreases by a factor of 2. The micro deformations increase at first and then tend to almost zero at the end of the sorption cycle. This behaviour is explained in terms of co-existence of the metal (i.e. α$\alpha$ phase) and metal hydride (i.e. β$\beta$ phase) phases. The change in crystallinity is consistent with the hydrogen induced amorphisation process existing in a lot of AB₂ compounds. No anisotropic effects can be highlighted on this pseudo-AB₂ compounds in contrary with what could be observed in AB₅ compounds.     

Improving solid-state hydriding and dehydriding properties of the LiBH4 plus MgH2 system with the addition of Mn and V dopants

The hydriding process of the 2LiH + MgB₂ mixture is controlled by outward diffusion of Mg and inward diffusion of Li and H within MgB₂ crystals to form LiBH₄. This study explores the feasibility of using transition metal dopants, such as Mn and V, to enhance the diffusion rate and thus the hydriding kinetics. It is found that Mn can indeed enhance the hydriding kinetics of the 2LiH + MgB₂ mixture, while V does not. The major factor in enhancing the diffusion rate and thus the hydriding kinetics is related to the dopant's ability to induce the lattice distortion of MgB₂ crystals. This study demonstrates that the kinetics of the diffusion controlled solid-state hydriding process can be improved by doping if the dopant is properly selected.     

Hydrogen storage behaviour of Li3N doped with Li2O and Na2O

Mixtures of Li₂O/Li₃N and Na₂O/Li₃N have been investigated for hydrogen storage. When Li₃N is doped with ca. 5 mol% Li₂O and annealed, both binary compounds exist as separate phases as evident from powder X-ray diffraction. Li₂O acts as a spectator in the hydrogen storage reactions and there is no evidence of enhanced Li⁺ or H⁺ mobility. Na₂O (5 mol%) interacts more strongly with Li₃N, leading to the generation of an unidentified phase, which also appears to play no part in the hydrogen storage reactions of the composite system. We conclude that addition of these levels of Li₂O or Na₂O to Li₃N followed by annealing does not improve the hydrogen storage properties of Li₃N.     

Study of the multipeak deuterium thermodesorption in YFe2Dx (1.3 ≤ x ≤ 4.2) by DSC,TD and in situ neutron diffraction

The deuterium thermal desorption of various YFe₂D_x (x = 1.3, 2.5, 3.5, 4.2) compounds has been studied using differential scanning calorimetry (DSC) and thermal desorption (TD) experiments. These studies show that the number of desorption peaks increases with the deuterium content. In order to understand the origin of this multipeak behaviour, in situ neutron diffraction experiments during thermal desorption have been performed from 290 K to 680 K on YFe₂D_4.2. Upon heating, a multipeak TD spectrum is observed. It relates to the existence of several YFe₂D_x phases with different stabilities. The rate limiting step of this thermal desorption has been therefore attributed to several successive phase transformations rather than to different types of interstitial sites as proposed in previous TD models reported for C15-Laves phase compounds.     

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH₂ + MgH₂) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO₂, O₂, N₂ and CH₄ are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH₂ + MgH₂) system. The results indicate that the hydrogen capacity of the (Mg(NH₂)₂ + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li₂CN₂ and MgO appear after (2LiNH₂ + MgH₂) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH²⁻, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.     

Dust cloud combustion characterization of a mixture of LiBH4 destabilized with MgH2 for reversible H2 storage in mobile applications

This study discusses results of an experimental program to determine dust cloud combustion characteristics of 2LiBH₄ + MgH₂ binary system in air. The determined parameters of hydrided and partially-dehydrided states of this system include: maximum deflagration pressure rise (P_MAX), maximum rate of pressure rise (dP/dt)_MAX, minimum ignition temperature (T_C), minimum explosible concentration (MEC), minimum ignition energy (MIE), and explosion severity index (K_St). Impact of dust particle size on the measured parameters is evaluated for the partially-dehydrided state. For dust of same mean particle size, results show the hydrided state to be more explosible in air compared to its partially-dehydrided state. Moreover, MIE of the partially-dehydrided mixture is identified in the test with lowest ignition delay time (IDT) and highest dust cloud concentration (DC). Taguchi's mixed-levels design of experiments (DoE) methodology is employed to calculate dust's MIE response surface as a function of DC and IDT. The one-at-a-time effect and interaction effect between DC and IDT on dust MIE are determined. The core insights of this contribution are useful for quantifying risks in mobile and stationary H₂ storage applications, informing H₂ safety standards, and augmenting property databases of H₂ storage materials.     

Enhanced hydrogen sorption kinetics of Mg50Ni-LiBH4 composite by CeCl3 addition

Mg₅₀Ni-LiBH₄ and Mg₅₀Ni-LiBH₄-CeCl₃ composites have been prepared by short times of ball milling under argon atmosphere. Combination of HP-DSC and volumetric techniques show that Mg₅₀Ni-LiBH₄-CeCl₃ composite not only uptakes hydrogen faster than Mg₅₀Ni-LiBH₄, but also releases hydrogen at a lower temperature (225 °C). The presence of CeCl₃ has a catalytic role, but it does not modify the thermodynamic properties of the composite which corresponds to MgH₂. Experimental studies on the hydriding/dehydriding mechanisms demonstrate that LiBH₄ and Ni lead to the formation of MgNi₃B₂ in both composites. In addition, XRD/DSC analysis and thermodynamic calculations demonstrate that the addition of CeCl₃ accounts for the enhancement of the hydrogen absorption/desorption kinetics through the interaction with LiBH₄. The in situ formation and subsequent decomposition of Ce(BH₄)₃ provides a uniform distribution of nanosize CeB₄ compound, which plays an important role in improving the kinetic properties of MgH₂.