Destabilization of LiAlH4 by nanocrystalline MgH2

In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH₄ + nanoMgH₂). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH₂ is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH₂. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH₄ + nanoMgH₂ possess ∼5.0 wt.% H₂ reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH₄ + nanoMgH₂) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H₂) to their pristine counterparts, LiAlH₄ and MgH₂. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.     

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

The present paper reports the effect of graphitic nanofibres (GNFs) for improving the desorption kinetics of LiMg(AlH₄)₃ and LiAlH₄. LiMg(AlH₄)₃ has been synthesized by mechano-chemical metathesis reaction involving LiAlH₄ and MgCl₂. The enhancement in dehydrogenation characteristics of LiMg(AlH₄)₃ has been shown to be higher when graphitic nanofibres (GNFs) were used as catalyst. Out of two different types of nanofibres namely planar graphitic nanofibre (PGNF) and helical graphitic nanofibre (HGNF), the latter has been found to act as better catalyst. We observed that helical morphology of fibres improves the desorption kinetics and decreases the desorption temperature of both LiMg(AlH₄)₃ and LiAlH₄. The desorption temperature for 8 mol% HGNF admixed LiAlH₄ gets lowered from 159 °C to 128 °C with significantly faster kinetics. In 8 mol% HGNF admixed LiMg(AlH₄)₃ sample, the desorption temperature gets lowered from 105 °C to ∼70 °C. The activation energy calculated for the first step decomposition of LiAlH₄ admixed with 8 mol% HGNF is ∼68 kJ/mol, where as that for pristine LiAlH₄ it is 107 kJ/mol. The activation energy calculated for as synthesized LiMg(AlH₄)₃ is ∼66 kJ/mol. Since the first step decomposition of LiMg(AlH₄)₃ occurs during GNF admixing, the activation energy for initial step decomposition of GNF admixed LiMg(AlH₄)₃ could not be estimated.     

Reversible hydrogen storage in a 3NaBH4/YF3 composite

A 3NaBH₄/YF₃ hydrogen storage composite was prepared through ball milling and its hydrogen sorption properties were investigated. It is shown that NaBH₄ does not react with YF₃ during ball milling. The dehydrogenation of the composite starts at 423 °C, which is about 100 °C lower than the dehydrogenation temperature of pure NaBH₄, with a mass loss of 4.12 wt%. Pressure–Composition–Temperature tests reveal that the composite has reversible hydrogen sorption performance in the temperature range from 350 °C to 413 °C and under quite low hydrogenation plateau pressures (<1 MPa). Its maximum hydrogen storage capacity can reach up to 3.52 wt%. The dehydrogenated composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. Based on the Pressure–Composition–Temperature analyses, the hydrogenation enthalpy of the composite is determined to be −46.05 kJ/mol H₂, while the dehydrogenation enthalpy is 176.76 kJ/mol H₂. The mechanism of reversible hydrogen sorption in the composite involves the decomposition and regeneration of NaBH₄ through the reaction with YF₃. Therefore, the addition of the YF₃ to NaBH₄ as a reagent forms a reversible hydrogen storage composite.     

Destabilization effects of Mg(AlH4)2 on MgH2: Improved desorption performances and its reaction mechanism

Both kinetics and thermodynamics properties of MgH₂ are significantly improved by the addition of Mg(AlH₄)₂. The as-prepared MgH₂–Mg(AlH₄)₂ composite displays superior hydrogen desorption performances, which starts to desorb hydrogen at 90 °C, and a total amount of 7.76 wt% hydrogen is released during its decomposition. The enthalpy of MgH₂-relevant desorption is 32.3 kJ mol⁻¹ H₂ in the MgH₂–Mg(AlH₄)₂ composite, obviously decreased than that of pure MgH₂. The dehydriding mechanism of MgH₂–Mg(AlH₄)₂ composite is systematically investigated by using x-ray diffraction and differential scanning calorimetry. Firstly, Mg(AlH₄)₂ decomposes and produces active Al^∗. Subsequently, the in-situ formed Al^∗ reacts with MgH₂ and forms Mg–Al alloys. For its reversibility, the products are fully re-hydrogenated into MgH₂ and Al^∗, under 3 MPa H₂ pressure at 300 °C for 5 h.     

Hydrogen sorption performance of MgH2 doped with mesoporous nickel- and cobalt-based oxides

The effect of mesoporous Co₃O₄, NiCo₂O₄ and NiO on the hydrogen sorption performance of MgH₂ was investigated. These oxides were synthesized by multi-step nanocasting and introduced during the high-energy ball milling of MgH₂ powder to act as catalysts. Hydrogen desorption on the as-milled powders was assessed upon heating the samples from room temperature to 400 °C. In all cases, the onset temperature for desorption was lowered by taking advantage of the introduced additives. The NiO-doped sample displayed the best response, the desorption rate being 7 times faster than in pure MgH₂. Complementary kinetic studies on this particular sample revealed that the sorption activation energies were much lower (50 kJ/mol for absorption and 335 kJ/mol for desorption) than the corresponding ones for undoped MgH₂ (57 kJ/mol for absorption and 345 kJ/mol for desorption), thus proving the catalytic activity of the mesoporous NiO oxide. Significantly, the X-ray powder diffraction (XRPD) patterns taken on the NiO-doped sample after discharging/charging cycles revealed that Mg could fully hydrogenate at the end of the charging process, while Mg metal was still detected in the undoped (pure) sample. Favored conditions for dissociative chemisorption of hydrogen could be ascribed to the formation of metallic Ni arising from complete or partial reduction of NiO, as observed in the XRPD patterns.     

Intensive investigation on hydrogen storage properties and reaction mechanism of the NaBH4-Li3AlH6 destabilized system

Notable effects of Li₃AlH₆ on the hydrogen storage properties of the NaBH₄ are studied intensively. Li₃AlH₆ is synthesized by milling 2LiH-LiAlH₄ mixture for 12 h. The best molar ratio of the NaBH₄- Li₃AlH₆ destabilized system is 1:1 which has decomposed at two stages; Li₃AlH₆ decomposition stage at 170 °C and NaBH₄decomposition stage at 400 °C. As no significant effect on the decomposition temperature between 1 h and 24 h of milling time can be observed, the 1-hour milling preparation method is selected for the characterization. Isothermal absorption has shown that the system is able to absorb 4.2 wt% and 6.1 wt% of hydrogen in 60 min at 330 °C and 420 °C under 30 atm of hydrogen pressure. In contrast, only about 3.4 wt% and 3.7 wt% of hydrogen can be absorbed by the milled NaBH₄ under a similar condition. Meanwhile, the system is able to desorb 2.0 wt% and 4.1 wt% of hydrogen in 60 min at 330 °C and 420 °C in isothermal desorption while only 0.3 wt% and 2.1 wt% can be released by the milled NaBH₄ under the similar condition. The decomposition activation energy and enthalpy of the NaBH₄ stage are calculated to be 162.1 kJ/mol and 68.1 kJ/mol H₂. Based on the X-ray diffraction analysis, Na, Al and AlB₂ are formed during the dehydrogenation process. The formation of Al and AlB2 are the keys to the improvement of hydrogenation properties. It is concluded that Li₃AlH₆ is a good destabilizing agent for the NaBH₄ system.     

Synthesis and hydrogen storage thermodynamics and kinetics of Mg(AlH4)2 submicron rods

Mg(AlH₄)₂ submicron rods with 96.1% purity have been successfully synthesized in a modified mechanochemical reaction process followed by Soxhlet extraction. ∼9.0 wt% of hydrogen is released from the as-prepared Mg(AlH₄)₂ at 125–440 °C through a stepwise reaction. Upon dehydriding, Mg(AlH₄)₂ decomposes first to generate MgH₂ and Al. Subsequently, the newly produced MgH₂ reacts with Al to form a Al_0.9Mg_0.1 solid solution. Finally, further reaction between the Al_0.9Mg_0.1 solid solution and the remaining MgH₂ gives rise to the formation of Al₃Mg₂. The first step dehydrogenation is a diffusion-controlled reaction with an apparent activation energy of ∼123.0 kJ/mol. Therefore, increasing the mobility of the species involved in Mg(AlH₄)₂ will be very helpful for improving its dehydrogenation kinetics.     

TiF4-doped Mg(AlH4)2 with significantly improved dehydrogenation properties

A significant decrease in the dehydrogenation temperature of Mg(AlH₄)₂ was achieved by low-energy ball milling with TiF₄. Approximately 8.0 wt% of hydrogen was released from the Mg(AlH₄)₂-0.025TiF₄ sample with an on-set temperature of 40 °C, which represents a decrease of 75 °C relative to pristine Mg(AlH₄)₂. In contrast to the three-step reaction for pristine Mg(AlH₄)₂, hydrogen desorption from the TiF₄-doped sample involves a two-step process because the Ti-based species participates in the dehydrogenation reaction. The presence of TiF₄ alters the nucleation and growth of the dehydrogenation product, significantly decreasing the activation energy barrier of the first step in the dehydrogenation of Mg(AlH₄)₂. Further hydrogenation measurements revealed that the presence of the Ti-based species was also advantageous for hydrogen uptake, as the on-set hydrogenation temperature was only 100 °C for the dehydrogenated TiF₄-doped sample, compared with 130 °C for the additive-free sample.     

Influence of metal oxide on LiBH4/2LiNH2/MgH2 system for hydrogen storage properties

In this study, various nanoscale metal oxide catalysts, such as CeO₂, TiO₂, Fe₂O₃, Co₃O₄, and SiO₂, were added to the LiBH₄/2LiNH₂/MgH₂ system by using high-energy ball milling. Temperature programmed desorption and MS results showed that the Li–Mg–B–N–H/oxide mixtures were able to dehydrogenate at much lower temperatures. The order of the catalytic effect of the studied oxides was Fe₂O₃ > Co₃O₄ > CeO₂ > TiO₂ > SiO₂. The onset dehydrogenation temperature was below 70 °C for the samples doped with Fe₂O₃ and Co₃O₄ with 10 wt.%. More than 5.4 wt.% hydrogen was released at 140 °C. X-ray diffraction indicated that the addition of metal oxides inhibited the formation of Mg(NH₂)₂ during ball milling processes. It is thought that the changing of the ball milling products results from the interaction of oxide ions in metal oxide catalysts with hydrogen atoms in MgH₂. The catalytic effect depends on the activation capability of oxygen species in metal oxides on hydrogen atoms in hydrides.     

Effects of NbF5 addition on the hydrogen storage properties of LiAlH4

We investigated the effects of NbF₅ addition by ball milling on the hydrogen storage properties of LiAlH₄. Pressure-composition-temperature (PCT) experiments showed that addition of 0.5 and 1 mol% NbF₅ in LiAlH₄ improves the onset desorption temperature and results in little decrease in hydrogen capacity, with approximately 7.0 wt% released by 188 °C. Isothermal dehydriding kinetics measurements indicated that the NbF₅-doped sample shows an average dehydrogenation rate 5–6 times faster than that of the as-received LiAlH₄ sample. In the x-ray diffraction results, there are distinct peaks of Al and LiH that appear after desorption. There is no peak of NbF₅ before or after desorption. Desorption kinetics measurements indicated that the activation energy, E_A, for LiAlH₄ + 1 mol% NbF₅ is about 67 kJ/mol for first reaction stage and about 77 kJ/mol for second reaction stage. The desorption process was further characterised by differential scanning calorimetry, and the possible mechanism of the effects of NbF₅ addition is discussed.     

Role of particle size,grain size,microstrain and lattice distortion in improved dehydrogenation properties of the ball-milled Mg(AlH4)2

The decreased dehydrogenation temperature and improved dehydrogenation kinetics were achieved by high-energy ball milling Mg(AlH₄)₂. The particle size, grain size, microstrain and lattice distortion of the post-milled samples, i.e., from macro- to micro-scale, were systematically characterized by means of SEM and XRD measurements. The results indicated that the high-energy ball milling process led to not only a decrease in the particle size and grain size but also an increase in the microstrain and lattice distortion, which provides a synergetic effect of the thermodynamics and kinetics on lowering the dehydrogenation temperatures of the post-milled Mg(AlH₄)₂ samples. From the kinetic point of view, the refinement of the particles and grains shortens the diffusion distance, and the increase of the microstrain and lattice distortion enhances the diffusivity, which work together to decrease the apparent activation energy for hydrogen desorption. Besides, the presence of microstrain and lattice distortion increased the free energy of the post-milled samples, which was released by recovery and recrystallization processes upon heating. This offers more heat release during the first-step dehydrogenation, consequently leading to thermodynamically decline in dehydrogenation temperatures of the post-milled samples. Such a finding provides insights into the mechanistic understanding on decreased dehydrogenation temperature and improved dehydrogenation kinetics of the post-milled metal hydrides as hydrogen storage materials.     

Complex quaternary hydrides Mg2(Fe,Co)Hy for hydrogen storage

Complex ternary hydrides based in Mg and transition metals are very attractive materials for hydrogen and energy storage due to their large volumetric capacity, up to 150 kgH₂/m³ in Mg₂FeH₆ and their high dissociation enthalpies. These compounds may be produced at room temperature by mechanical milling of the constituents in H₂ atmosphere. This technique has also served to explore the synthesis of quaternary hydrides Mg₂T_1−zT’_zH_y, combining two transition metals to optimize the properties of the resulting hydride. In the present work we analyze the mechanical synthesis of the compounds Mg₂Fe_1−zCo_zH_y (z = 0, ¼, ½, ¾, 1) by mechanical alloying at room temperature Mg-Fe-Co powder mixtures in adequate proportion, at 0.3 MPa H₂. We follow the mechanosynthesis process trough the analysis of the hydrogen absorption kinetic curves. Samples obtained after a steady state was reached were characterized by X ray diffraction and Mössbauer spectroscopy. The different stages in the mechanosynthesis of these complex hydrides are discussed in terms of the composition and initial state of the powder mixture.     

Hydrogen production from solid reactions between MAlH4 and NH4Cl

Solid reactions between alkali aluminum hydrides (MAlH₄, M = Li or Na) and NH₄Cl (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 H₂ from per mole of MAlH₄ was measured, corresponding to 5.6 wt.% H₂ capacity for the NaAlH₄/NH₄Cl system and 6.6 wt.% for LiAlH₄/NH₄Cl, respectively. By ball milling of the precursor compounds prior to the mixing, the reaction proceeded fast and NH₃ production as the by-product could be avoided. The quick solid reactions may be attributed to the low melting temperatures of MAlH₄ and the exothermic nature of the reactions. The reaction mechanism was also discussed.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Dual-tuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation

Mg₂In_0.1Ni solid solution with an Mg₂Ni-type structure has been synthesized and its hydrogen storage properties have been investigated. The results showed that the introduction of In into Mg₂Ni not only significantly improved the dehydrogenation kinetics but also greatly lowered the thermodynamic stability. The dehydrogenation activation energy (E_a) and enthalpy change (ΔH) decreased from 80 kJ/mol and 64.5 kJ/mol H₂ to 28.9 kJ/mol and 38.4 kJ/mol H₂, respectively. The obtained results point to a method for improving not only the thermodynamic but also the kinetic properties of hydrogen storage materials.     

Thermal stability of Vale Inco nanonometric nickel as a catalytic additive for magnesium hydride (MgH2)

The structure stability of nanometric-Ni (n-Ni) produced by Vale Inco Ltd. Canada as a catalytic additive for MgH₂ has been investigated. Each n-Ni filament is composed of nearly spherical interconnected particles having a mean diameter of 42 ± 16 nm. After ball milling of the MgH₂ + 5 wt.%n-Ni mixture for 15 min the n-Ni particles are found to be uniformly embedded within the particles of MgH₂ and at their surfaces. Neither during ball milling of the MgH₂ + 5 wt.%n-Ni mixture nor its first decomposition at temperatures of 300, 325, 350 and 375 ^°C the elemental n-Ni reacts with the elemental Mg to form the Mg₂Ni intermetallic phase (and eventually the Mg₂NiH₄ hydride). The n-Ni additive acts as a strong catalyst accelerating the kinetics of desorption. From the Arrhenius and Johnson–Mehl–Avrami–Kolmogorov theory the activation energy for the first desorption is determined to be ∼94 kJ/mol. After cycling at 300 ^°C the activation energy for desorption is determined to be ∼99 kJ/mol. This is much lower than ∼160 kJ/mol observed for the undoped and ball milled MgH₂. During cycling at 275 and 300 ^°C the n-Ni additive is converted into Mg₂Ni (Mg₂NiH₄). The newly formed Mg₂NiH₄ has a nanosized grain on the order of 20 nm. Its catalytic potency seems to be similar to its n-Ni precursor. The formation of Mg₂Ni (Mg₂NiH₄) may be one of the factors responsible for the systematic decrease of hydrogen capacity observed upon cycling at 275 and 300 ^°C.     

Improved dehydrogenation of MgH2-Li3AlH6 mixture with TiF3 addition

MgH₂-Li₃AlH₆ mixture shows a mutual activation effect between the components. But the dehydrogenation kinetics is still slow, especially at temperature as low as 250 °C. Hereby, an additive (TiF₃) was introduced into the mixture in the present study. The reaction mechanisms were studied by the combined analyses of X-ray diffraction (XRD), thermogravimetric analysis (TGA), as well as thermodynamic calculations. A two-step ball milling method could reduce the mechanical decomposition of Li₃AlH₆ effectively and was adopted. During milling, Li₃AlH₆ reacts with TiF₃ and produces Al₃Ti while MgH₂ remains stable. All the species are well mixed after milling and the grain size is as small as 100 nm. During TGA test, all the reactions occur at lower temperatures compared with undoped mixture, especially the dehydrogenation of MgH₂, which shows a decrease of 60 °C. Its activation energy is reduced by 32.0 kJ mol⁻¹. The first three isothermal (250 °C) cycles indicate that the kinetics of dehydrogenation has been greatly enhanced, showing a reversible capacity of 4.5 wt.% H₂. The time needed for the 1st dehydrogenation has been shortened to 3600 s from 8000 s for the undoped mixture. These improvements are mainly attributed to the catalytic effect of the in-situ formed Al₃Ti. But there is no influence on the rehydrogenation kinetics and the enthalpy of the dehydrogenation of MgH₂ is unchanged.     

Engineering the hydrogen storage properties of the perovskite hydride ZrNiH3 by uniaxial/biaxial strain

In the present work, the bonding length, electronic structure, stability, and dehydrogenation properties of the Perovskite-type ZrNiH₃ hydride, under different uniaxial/biaxial strains are investigated through ab-initio calculations based on the plane-wave pseudo-potential (PW-PP) approach. The findings reveal that the uniaxial/biaxial compressive and tensile strains are responsible for the structural deformation of the ZrNiH₃ crystal structure, and its lattice deformation becomes more significant with decreasing or increasing the strain magnitude. Due to the strain energy contribution, the uniaxial/biaxial strain not only lowers the stability of ZrNiH₃ but also decreases considerably the dehydrogenation enthalpy and decomposition temperature. Precisely, the formation enthalpy and decomposition temperature are reduced from ?67.73 kJ/mol.H₂ and 521 K for non-strained ZrNiH₃ up to ?33.73 kJ/mol.H₂ and 259.5 K under maximal biaxial compression strain of ε = ?6%, and to ?50.99 kJ/mol.H₂ and 392.23 K for the maximal biaxial tensile strain of ε = +6%. The same phenomenon has been also observed for the uniaxial strain, where the formation enthalpy and decomposition temperature are both decreased to ?39.36 kJ/mol.H₂ and 302.78 K for a maximal uniaxial compressive strain of ε = - 12%, and to ?51.86 kJ/mol.H₂ and 399 K under the maximal uniaxial tensile strain of ε = +12%. Moreover, the densities of states analysis suggests that the strain-induced variation in the dehydrogenation and structural properties of ZrNiH₃ are strongly related to the Fermi level value of total densities of states. These ab-initio calculations demonstrate insightful novel approach into the development of Zr-based intermetallic hydrides for hydrogen storage practical applications.     

The role of differently distributed vanadium nanocatalyst in the hydrogen storage of magnesium nanostructures

Using a glancing angle (co)deposition technique, ∼4.6 at.% V has been coated on the surface of individual Mg nanoblades and doped into Mg nanostructures fabricated at different deposition angles. The hydrogen storage properties of the formed V-decorated and V-doped Mg nanostructures depend strongly on how the nanocatalyst V is surrounded by the host Mg. The V-doped Mg sample has lower activation energies for hydrogen absorption (E_a^a = 35.3 ± 0.9 kJ/mol H₂) and hydrogen desorption (E_a^d = 38.9 ± 0.3 kJ/mol H₂) than the V-decorated Mg sample when deposited at the same deposition angle of θ = 70°. The activation energies of the doped samples increase gradually with the decrease of the θ angle. We also find that the porosity of the Mg nanostructures plays a secondary role. A phenomenological model based on a heterogeneous reaction is proposed to explain the different hydrogen desorption activation energies obtained for different V–Mg nanostructured samples.