Effect of gas back pressure on hydrogen storage properties and crystal structures of Li2Mg(NH)2

The ternary imide Li₂Mg(NH)₂ is considered to be one of the most promising on-board hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%, favorable thermodynamic properties and good cycling stability. In this work, Li₂Mg(NH)₂ was synthesized by dynamically dehydrogenating a mixture of Mg(NH₂)₂–2LiH up to 280 °C under different gas (Ar and H₂) and pressures (0–9.0 bar). The crystal structure of Li₂Mg(NH)₂ was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li₂Mg(NH)₂ and the dehydrogenation/rehydrogenation properties of the Mg(NH₂)₂–2LiH system strongly depend on the gas back pressure in the dehydrogenation process due to the effect of the pressure on the dehydrogenation kinetics. This study provides a new approach for improving the hydrogen storage properties of the amide–hydride systems.     

The mixed lithium-magnesium imide Li2Mg(NH)2 a promising and reliable hydrogen storage material

A statistical theory of the structural phase transformations of the imide Li₂Mg(NH)₂ has been developed, the formation of which is accompanied by the release of free hydrogen. The standard Gibbs free energy formation of the imide's three phases: α, β, γ are calculated. Their dependencies on composition, temperature, pressure, and energy are found. Plots of concentration dependences of the free energy values of these phases are constructed for different temperatures, according to which the imide state diagram is established. Isotherms, isobars and isoplets are investigated. The possibility of hysteresis effect for α and β phases is shown. The results of the calculations are compared with the experimental data.     

Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

A novel dual-cation/anion complex hydride (Li₂Mg(BH₄)₂(NH₂)₂), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH₄NH₂ and Li₂BH₄NH₂. The prepared Li₂Mg(BH₄)₂(NH₂)₂ crystallizes in a triclinic structure, and the [NH₂] and [BH₄] groups remain intact within the structure. Upon heating, the prepared Li₂Mg(BH₄)₂(NH₂)₂ decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li₂Mg(BH₄)₂(NH₂)₂ first decomposes to LiMgBN₂, LiBH₄, BN, LiH and MgBNH₈ at 100–250 °C, and then, the newly formed MgBNH₈ reacts with LiH to form Mg, LiBH₄ and BN at 250–340 °C. Finally, the decomposition of LiBH₄ releases hydrogen and generates LiH and B at 340–450 °C.     

Synthesis,characterization and hydrogen storage behaviour of AB2 (ZrFe2, Zr(Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 type materials

In this paper, we describe and discuss the synthesis, structural-microstructural and hydrogen storage behaviour of three AB₂ type storage materials namely (a) ZrFe₂, (b) Zr(Fe_0.75V_0.25)₂ and (c) Zr(Fe_0.5V_0.5)₂. These alloys were synthesied by radio frequency induction melting in argon atmosphere. X-ray diffraction and transmission electron microscope have been employed for structural and microstructural characterizations. The XRD study reveals that the lattice constants and the unit cell volume of ZrFe₂, Zr (Fe_0.75V_0.25)₂, Zr(Fe_0.5V_0.5)₂ alloys, which has C14 type hexagonal Laves phase. The Surface morphology and elemental composition of these alloys were investigated by scanning electron microscope and energy dispersive X-ray analysis. The pressure composition isotherms of these alloys were investigated at room temperature and pressure ranges of 0–100 atm respectively, measured through a fully computerized PCI apparatus. As we increase the concentration of V (substituted for Fe), the total hydrogen storage capacities increased up to 1.45 wt%. This capacity is achieved in Zr(Fe_0.5V_0.5)₂ alloy, while the reversible hydrogen storage capacity decreases due to the formation of a stable hydride phase. It has been found that the lattice constants increase with higher vanadium concentration. This is indicating that the majority of vanadium atoms reside in the B-site. The broader X-ray diffraction peaks observed in Zr(Fe_0.5V_0.5)₂ alloy indicates a higher degree of disorder for alloys with the higher V-content. The yet another interesting feature observed in our present study is that the plateau pressure remains well below 1 atm for all the compositions.     

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.     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Structural change and hydrogenation behavior of (Sr,Ca)2Al alloys

The phase structure and hydrogenation behavior of the (Sr_1−xCa_x)₂Al (x = 0.5, 0.6, 0.7, 0.8 and 0.9) alloys were investigated. It was found that the (Sr_1−xCa_x)₂Al alloys have different phase components with the change of Ca content. When x = 0.6 and 0.7, the single (Sr, Ca)₂Al phase can be obtained. The lattice parameters of (Sr, Ca)₂Al compound in the (Sr_1−xCa_x)₂Al alloys decrease gradually with the increase of Ca content. The (Sr, Ca)₂Al phase can absorb hydrogen even at 303 K. During hydrogenation, the (Sr, Ca)₂Al compound transforms into distinct phases at different temperatures.     

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.     

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.     

Mechano-chemical synthesis of manganese borohydride (Mn(BH4)2) and inverse cubic spinel (Li2MnCl4) in the (nLiBH4 + MnCl2) (n = 1, 2, 3, 5, 9 and 23) mixtures and their dehydrogenation behavior

Manganese borohydride (Mn(BH₄)₂) was successfully synthesized by a mechano-chemical activation synthesis (MCAS) from lithium borohydride (LiBH₄) and manganese chloride (MnCl₂) by applying high energy ball milling for 30 min. For the first time a wide range of molar ratios n = 1, 2, 3, 5, 9 and 23 in the (nLiBH₄ + MnCl₂) mixture was investigated. During ball milling for 30 min the mixtures release only a very small quantity of H₂ that increases with the molar ratio n but does not exceed ∼0.2 wt.% for n = 23. However, longer milling duration leads to more H₂ released. For the equimolar ratio n = 1 the principal phases synthesized are Li₂MnCl₄, an inverse cubic spinel phase, and the Mn(BH₄)₂ borohydride. For n = 2 a LiCl salt is formed which coexists with Mn(BH₄)₂. With the n increasing from 3 to 23 LiBH₄ is not completely reacted and its increasing amount is retained in the microstructure coexisting with LiCl and Mn(BH₄)₂. Gas mass spectrometry during Temperature Programmed Desorption (TPD) up to 450 °C shows the release of hydrogen as a principal gas with a maximum intensity around 130–150 °C accompanied by a miniscule quantity of borane B₂H₆. The intensity of the B₂H₆ peak is 200–600 times smaller than the intensity of the corresponding H₂ peak. In situ heating experiments using a continuous monitoring during heating show no evidence of melting of Mn(BH₄)₂ up to about 270–280 °C. At 100 °C under 1 bar H₂ pressure the ball milled n = 2 and 3 mixtures are capable of desorbing quite rapidly ∼4 wt.% H₂ which is a very large amount of H₂ considering that the mixture also contains 2 mol of LiCl salt. The H₂ quantities experimentally desorbed at 100 and 200 °C do not exceed the maximum theoretical quantities of H₂ expected to be desorbed from Mn(BH₄)₂ for various molar ratios n. It clearly confirms that the contribution from B₂H₆ evolved is negligibly small (if any) when desorption occurs isothermally in the practical temperature range 100–200 °C. It is found that the ball milled mixture with the molar ratio n = 3 exhibits the highest rate constant k and the lowest apparent activation energy for dehydrogenation, E_A ∼ 102 kJ/mol. Decreasing or increasing the molar ratio n below or above 3 increases the apparent activation energy. Ball milled mixtures with the molar ratio n = 2 and 3 discharge slowly H₂ during storage at room temperature and 40 °C. The addition of 5 wt.% nano-Ni with a specific surface area of 60.5 m²/g substantially enhances the rate of discharge at 40 °C.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Modeling and analyzing the hydriding kinetics of Mg-LaNi5 composites by Chou model

Chou model was used to analyze the influences of LaNi₅ content, preparation method, temperature and initial hydrogen pressure on the hydriding kinetics of Mg-LaNi₅ composites. Higher LaNi₅ content could improve hydriding kinetics of Mg but not change hydrogen diffusion as the rate-controlling step, which was validated by characteristic reaction time t_c. The rate-controlling step was hydrogen diffusion in the hydriding reaction of Mg-30 wt.% LaNi₅ prepared by microwave sintering (MS) and hydriding combustion synthesis (HCS), and surface penetration was the rate-controlling step of sample prepared by mechanical milling (MM). Rising temperature and initial hydrogen pressure could accelerate the absorption rate. The rate-controlling step of Mg-30 wt.% LaNi₅ remained hydrogen diffusion at temperatures ranging from 302 to 573 K, while that of Mg-50 wt.% LaNi₅ changed from surface penetration to hydrogen diffusion with increasing initial hydrogen pressure ranging from 0.2 to 1.5 MPa. Apparent activation energies of absorption for Mg-30 wt.% LaNi₅ prepared by MS and MM were respectively 25.2 and 28.0 kJ/mol H₂ calculated by Chou model. Kinetic curves fitted and predicted by Chou model using temperature and hydrogen pressure were well exhibited.     

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.     

Synthesis and electrochemical properties of Na-doped Li3V2(PO4)3 cathode materials for Li-ion batteries

Na-doped Li_3−xNa_xV₂(PO₄)₃/C (x = 0.00, 0.01, 0.03, and 0.05) compounds have been prepared by using sol-gel method. The Rietveld refinement results indicate that single-phase Li_3−xNa_xV₂(PO₄)₃/C with monoclinic structure can be obtained. Among three Na-doped samples and the undoped one, Li_2.97Na_0.03V₂(PO₄)₃/C sample has the highest electronic conductivity of 6.74 × 10⁻³ S cm⁻¹. Although the initial specific capacities for all Na-doped samples have no much enhancement at the current rate of 0.2 C, both cycle performance and rate capability have been improved. At the 2.0 C rate, Li_2.97Na_0.03V₂(PO₄)₃/C presents the highest initial capacity of 118.9 mAh g⁻¹ and 12% capacity loss after 80 cycles. The partial substitution of Li with Na (x = 0.03) is favorable for electrochemical rate and cyclic ability due to the enlargement of Li₃V₂(PO₄)₃ unit cells, optimizing the particle size and morphology, as well as resulting in a higher electronic conductivity.     

Hydrogen storage properties of the novel crosslinked UiO-66-(OH)2

Metal organic frameworks (MOF) are a type of nanoporous materials with large specific surface area, which are especially suitable for gas separation and storage. In this work, we report a new approach of crosslinking UiO-66-(OH)₂ to enhance its hydrogen storage capacity. UiO-66-(OH)₂ was synthesized using hafnium tetrachloride (HfCl₄) and 2, 5-dihydroxyterephthalic acid (DTPA) through a canonical modulated hydrothermal method (MHT), followed by a post-synthesis modification, which is to form a crosslinking structure inside the porous structure of UiO-66-(OH)₂. During the modification process, the phenolic hydroxyl groups on the UiO-66-(OH)₂ reacted with methanal, and HCl aqueous solution and triethylamine served as catalyst (the products denoted as UiO-66-H and UiO-66-T, respectively). Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) proved that the crosslinking was formed. The BET specific surface area and the average adsorption pore size of UiO-66-H and UiO-66-T significantly increased after modification. The hydrogen storage capacity of UiO-66-H reached a maximum of 3.37 wt% (16.87 mmol/g) at 77 K, 2 MPa. Hydrogen adsorption enthalpy of UiO-66-T was 0.986 kJ/mol, which was higher than that of UiO-66-(OH)₂ (0.695 kJ/mol). This work shows that UiO-66-(OH)₂ is a promising candidate for potential application in high-performance hydrogen storage.     

Synthesis and crystal structure analysis of complex hydride Mg(BH4)(NH2)

Complex hydride Mg(BH₄)(NH₂), which consists of double anion BH₄⁻ and NH₂⁻, was synthesized and the crystal structure was analyzed by synchrotron X-ray diffraction. The mixture sample of Mg(BH₄)₂ + Mg(NH₂)₂ prepared by ball milling was reacted and crystallized to Mg(BH₄)(NH₂) by heating at about 453 K. This crystal phase transforms into amorphous phase above 473 K and subsequently the dehydrogenation begins. The crystal structure of Mg(BH₄)(NH₂) was determined from measurement data at 453 K (chemical formula: Mg_0.94(BH₄)₁(NH₂)_0.88, crystal system: tetragonal, space group: I4₁ (No.80), Z = 8, lattice constants: a = 5.814(1), c = 20.450(4) Å at 453 K). Mg(BH₄)(NH₂) is ionic crystal which the cation (Mg²⁺) and the anions (BH₄⁻ and NH₂⁻) are stacking alternately along the c-axis direction. Two BH₄⁻ and two NH₂⁻ tetrahedrally coordinate around Mg²⁺ ion.     

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.     

Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4

A two-step ball-milling method has been provided to synthesize Mg(BH₄)₂ using NaBH₄ and MgCl₂ as starting materials. The method offers high yield and high purity (96%) of the compound. The as-synthesized Mg(BH₄)₂ is then combined with LiAlH₄ by ball-milling in order to form new multi-hydride systems with high hydrogen storage properties. The structure, the dehydrogenation and the reversibility of the combined systems are studied. Analyses show that a metathesis reaction takes place between Mg(BH₄)₂ and LiAlH₄ during milling, forming Mg(AlH₄)₂ and LiBH₄. Mg(BH₄)₂ is excessive and remains in the ball-milled product when the molar ratio of Mg(BH₄)₂ to LiAlH₄ is over 0.5. The onset dehydrogenation temperature of the combined systems is lowered to ca. 120 °C, which is much lower than that of either Mg(BH₄)₂ or LiAlH₄. The dehydrogenation capacities of the combined systems below 300 °C are all higher than that of both Mg(BH₄)₂ and LiAlH₄. The combined systems are reversible for hydrogen storage at moderate hydrogenation condition, and rapid hydrogenation occurred within the initial 30 min. Moreover, the remained Mg(BH₄)₂ in the combined systems is found also partially reversible. The mechanism of the enhancement of the hydrogen storage properties and the dehydrogenation/hydrogenation process of the combined systems were discussed.     

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.