Destabilization of the LiNH2–LiH hydrogen storage system by aluminum incorporation

The lithium amide–lithium hydride system (LiNH₂–LiH) is one of the most attractive light-weight materials for hydrogen storage. In an effort to improve its hydrogen sorption kinetics, the effect of 1 mol% AlCl₃ addition to LiNH₂–LiH system was systematically investigated by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis and hydrogen volumetric measurements. It is shown that Al³⁺ is incorporated into the LiNH₂ structure by partial substitution of Li⁺ forming a new amide in the Li–Al–N–H system, which is reversible under hydriding/dehydriding cycles. This new substituted amide displays improved hydrogen storage properties with respect to LiNH₂–LiH. In fact, a stable hydrogen storage capacity of about 4.5–5.0 wt% is observed under cycling and is completely desorbed in 30 min at 275 °C for the Li–Al–N–H system. Moreover, the concurrent incorporation of Al³⁺ and the presence of LiH are effective for mitigating the ammonia release. The results reveal a common reaction pathway for LiNH₂–LiH and LiNH₂–LiH plus 1 mol% AlCl₃ systems, but the thermodynamic properties are changed by the inclusion of Al³⁺ in the LiNH₂ structure. These findings have important implications for tailoring the properties of the Li–N–H system.     

New insights into the thermal desorption of the 2LiNH2 + KBH4 + LiH mixture

In situ two-dimensional synchrotron X-ray powder diffraction investigation combined with Rietveld method data analysis were performed in order to yield a complete and quantitative phases structure evolution of the polycrystalline mixture 2LiNH₂ + KBH₄ + LiH during H₂ desorption. While a first-principles, purely thermodynamics approach of the system predicted a single dehydrogenation step reaction at relatively low temperatures, it is assessed experimentally that the reaction occurs in two steps with first the formation of Li₂NH at ca. 230 °C due to the reaction between LiNH₂ and LiH plus hydrogen and ammonia evolution, followed by an additional reaction of the resulting phases with KBH₄ at 360 °C, which releases hydrogen and leads to the formation of the monoclinic and tetragonal Li₃BN₂ polymorphs. Besides pointing out possible limits of a purely thermodynamics approach inevitably relying exact knowledge of experimental quantities, it is concluded that before assuming it viable for on-board vehicle use, additional stoichiometries may be worth of investigation in order to assess any existence of lower hydrogen desorption temperature of such system.     

Kinetic rate-limiting steps in dehydrogenation of Li-N-H and Li-Mg-N-H systems - Effects of elemental Si and Al

The study is focused on the (LiNH₂ + LiH) and the (2LiNH₂ + MgH₂) hydrogen storage systems. Efforts were made to achieve an in-depth understanding of the dehydrogenation mechanisms following the introduction of elemental Si and Al to both systems. A variety of analytical instruments were employed, such as Temperature-Programmed Desorption, Mass Spectrometry, X-ray diffraction (XRD) and InfraRed spectroscopy. Unlike the effect of elemental Al in the (LiNH₂ + LiH) system, which showed no promising improvement in the dehydrogenation, a significant kinetic improvement in the (LiNH₂ + LiH) system was achieved upon addition of elemental Si. Kinetic improvement by elemental Si was described as a result of the LiH destabilisation through the formation of a Li₂Si phase and an increase in H⁻ anion concentration. On the other hand, once elemental Al is added to the (2LiNH₂ + MgH₂) system, the overall dehydrogenation kinetics of the system is delayed through the formation of a LiAl Phase. The results suggest that dehydrogenation mechanism in both the (LiNH₂ + LiH) and the (2LiNH₂ + MgH₂) systems are identical. Dehydrogenation reaction starts with the electrostatic interaction of the oppositely charged hydrogen atoms in amide and hydride and proceeds by mass transfer of reactant species across the product layer at the later stage of the dehydrogenation. However, it was particularly identified that each system has a unique kinetic rate limiting step. Dehydrogenation kinetics seems to be controlled by the diffusion of the H⁻ anion in the (LiNH₂ + LiH) system but by the diffusion of the Li⁺ cation in the (2LiNH₂ + MgH₂) system.     

Enhanced hydrogen storage performance of LiAlH4-MgH2-TiF3 composite

The mutual destabilization of LiAlH₄ and MgH₂ in the reactive hydride composite LiAlH₄-MgH₂ is attributed to the formation of intermediate compounds, including Li-Mg and Mg-Al alloys, upon dehydrogenation. TiF₃ was doped into the composite for promoting this interaction and thus enhancing the hydrogen sorption properties. Experimental analysis on the LiAlH₄-MgH₂-TiF₃ composite was performed via temperature-programmed desorption (TPD), differential scanning calorimetry (DSC), isothermal sorption, pressure-composition isotherms (PCI), and powder X-ray diffraction (XRD). For LiAlH₄-MgH₂-TiF₃ composite (mole ratio 1:1:0.05), the dehydrogenation temperature range starts from about 60 °C, which is 100 °C lower than for LiAlH₄-MgH₂. At 300 °C, the LiAlH₄-MgH₂-TiF₃ composite can desorb 2.48 wt% hydrogen in 10 min during its second stage dehydrogenation, corresponding to the decomposition of MgH₂. In contrast, 20 min was required for the LiAlH₄-MgH₂ sample to release so much hydrogen capacity under the same conditions. The hydrogen absorption properties of the LiAlH₄-MgH₂-TiF₃ composite were also improved significantly as compared to the LiAlH₄-MgH₂ composite. A hydrogen absorption capacity of 2.68 wt% under 300 °C and 20 atm H₂ pressure was reached after 5 min in the LiAlH₄-MgH₂-TiF₃ composite, which is larger than that of LiAlH₄-MgH₂ (1.75 wt%). XRD results show that the MgH₂ and LiH were reformed after rehydrogenation.     

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.     

Improvement of reaction kinetics by metal chloride on ammonia and lithium hydride system

Ammonia NH₃ and lithium hydride LiH system releases hydrogen even at room temperature to form lithium amide LiNH₂. LiNH₂ is recycled back to NH₃ and LiH below 300 °C under hydrogen H₂ flow condition. However, the reaction rate of the system is slow for a practical application. In this work, various kinds of transition metal chlorides were examined as a potential catalyst to improve the kinetics. For hydrogen desorption reaction, the reaction kinetics of titanium chloride TiCl₃ dispersing LiH was about 8 times faster than the raw LiH, suggesting that TiCl₃ possessed an excellent catalytic effect. In the case of the regeneration reaction, the reaction kinetics was also improved by the addition of TiCl₃. It was mainly caused by physical effects in contrast to the hydrogen desorption process, in other words, the small crystallite and/or particle were formed by the milling with the additive.     

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.     

Significantly improved dehydrogenation of LiAlH4 destabilized by K2TiF6

The effects of K₂TiF₆ on the dehydrogenation properties of LiAlH₄ were investigated by solid-state ball milling. The onset decomposition temperature of 0.8 mol% K₂TiF₆ doped LiAlH₄ is as low as 65 °C that 85 °C lower than that of pristine LiAlH₄. Isothermal dehydrogenation properties of the doped LiAlH₄ were studied by PCT (pressure–composition–temperature). The results show that, for the 0.8 mol% K₂TiF₆ doped LiAlH₄ that dehydrogenated at 90 °C, 4.4 wt% and 6.0 wt% of hydrogen can be released in 60 min and 300 min, respectively. When temperature was increased to 120 °C, the doped LiAlH₄ can finish its first two dehydrogenation steps in 170 min. DSC results show that the apparent activation energy (E_a) for the first two dehydrogenation steps of LiAlH₄ are both reduced, and XRD results suggest that TiH₂, Al₃Ti, LiF and KH are in situ formed, which are responsible for the improved dehydrogenation properties of LiAlH₄.     

Enhancement of the H2 desorption properties of LiAlH4 doping with NiCo2O4 nanorods

Lithium aluminum hydride (LiAlH₄) is considered as an attractive candidate for hydrogen storage owing to its favorable thermodynamics and high hydrogen storage capacity. However, its reaction kinetics and thermodynamics have to be improved for the practical application. In our present work, we have systematically investigated the effect of NiCo₂O₄ (NCO) additive on the dehydrogenation properties and microstructure refinement in LiAlH₄. The dehydrogenation kinetics of LiAlH₄ can be significantly increased with the increase of NiCo₂O₄ content and dehydrogenation temperature. The 2 mol% NiCo₂O₄-doped LiAlH₄ (2% NCO–LiAlH₄) exhibits the superior dehydrogenation performances, which releases 4.95 wt% H₂ at 130 °C and 6.47 wt% H₂ at 150 °C within 150 min. In contrast, the undoped LiAlH₄ sample just releases <1 wt% H₂ after 150 min. About 3.7 wt.% of hydrogen can be released from 2% NCO–LiAlH₄ at 90 °C, where total 7.10 wt% of hydrogen is released at 150 °C. Moreover, 2% NCO–LiAlH₄ displayed remarkably reduced activation energy for the dehydrogenation of LiAlH₄.     

Ti-doped LiAlH4 for hydrogen storage: Rehydrogenation process,reaction conditions and microstructure evolution during cycling

One-step rehydrogenation of Ti-doped LiAlH₄ from its dehydrogenated precursor (Ti-doped LiH/Al), proceeds in Me₂O solution under mild conditions (25 °C; 50–100 bar H₂). The formation process, the effects of reaction conditions (temperature and H₂ pressure) on the yield of LiAlH₄, and the microstructure evolution during hydrogenation–dehydrogenation cycles were investigated comprehensively. No evidence was obtained for the intermediate Li₃AlH₆ in the product. Higher temperatures adversely affect the yield and stability of the Ti-doped LiAlH₄ product. High hydrogen pressure favors the formation of LiAlH₄ but the influence of pressure in the range 50–100 bar is slight if other variables are maintained constant. The cycling performance depends on the microstructure of the material: larger particle size; lower surface area; and the formation of less active Ti clusters and Ti–Al alloys upon cycling correspond to a decrease in cycling performance. Incorporation of an extra ball milling stage before each rehydrogenation significantly improves the cycling performance.     

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.     

Hydrogen storage in a Li–Al–N ternary system

Hydrogen-storage properties and mechanisms of a novel Li–Al–N ternary system were systematically investigated by a series of performance evaluation and structural examinations. It is found that ca. 5.2 wt% of hydrogen is reversibly stored in a Li₃N–AlN (1:1) system, and the hydrogenated product is composed of LiNH₂, LiH, and AlN. A stepwise reaction is ascertained for the dehydrogenation of the hydrogenated Li₃N–AlN sample, and AlN is found reacting only in the second step to form the final product Li₃AlN₂. The calculation of the reaction enthalpy change indicates that the two-step dehydrogenation reaction is more thermodynamically favorable than any one-step reaction. Further investigations exhibit that the presence of AlN in the LiNH₂–2LiH system enhances the kinetics of its first-step dehydrogenation with a 10% reduction in the activation energy due likely to the higher diffusivity of lithium and hydrogen within AlN.     

Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

The effects of TiO₂ nanopowder addition on the dehydrogenation behaviour of LiAlH₄ have been studied. The 5 wt.% TiO₂-added LiAlH₄ sample showed a significant improvement in dehydrogenation rate compared to that of undoped LiAlH₄, with the dehydrogenation temperature reduced from 150 °C to 60 °C. Kinetic desorption results show that the added LiAlH₄ released about 5.2 wt% hydrogen within 30 min at 100 °C, while the as-received LiAlH₄ just released below 0.2 wt.% hydrogen within same time at 120 °C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 114 kJ/mol for pure LiAlH₄ and 49 kJ/mol for the 5 wt.% TiO₂ added LiAlH₄, indicating that TiO₂ nanopowder adding significantly decreased the activation energy for hydrogen desorption of LiAlH₄. X-ray diffraction and Fourier transform infrared analysis show that there is no phase change in the cell volume or on the Al-H bonds of the LiAlH₄ due to admixture of TiO₂ after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO₂ after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH₄ in the presence of TiO₂ is believed to be due to the high defect density introduced at the surfaces of the TiO₂ particles during the milling process.     

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.     

Synthesis and dehydrogenation of LiCa(NH2)3(BH3)2

We report the synthesis of a new hydrogen storage material with a composition of LiCa(NH₂)₃(BH₃)₂. The theoretical hydrogen capacity of LiCa(NH₂)₃(BH₃)₂ is 9.85 wt.%. It can be prepared by ball milling the mixture of calcium amidoborane (Ca(NH₂BH₃)₂) and lithium amide (LiNH₂) in a molar ratio of 1:1. The experimental results show that this material starts to release hydrogen at a temperature as low as ca. 50 °C, which is ca. 70 °C lower than that of pure Ca(NH₂BH₃)₂ possibly resulting from the active interaction of NH₂⁻ in LiNH₂ with BH₃ in Ca(NH₂BH₃)₂. ca. 4.1 equiv. or 6.8 wt.% hydrogen can be released at 300 °C. The dehydrogenation is a mildly exothermic process forming stable nitride products.     

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.     

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.     

Thermal dehydrogenation behaviors and mechanisms of Mg(BH4)2∙6NH3-xLiH combination systems

A reactive composite of Mg(BH₄)₂⋅6NH₃-xLiH is prepared, and the effects of the LiH content on the dehydrogenation/hydrogenation properties of the material are investigated. The results show that the presence of LiH with x = 3 reduces the onset dehydrogenation temperature of Mg(BH₄)₂⋅6NH₃ from 130 °C to 80 °C in TPD mode. Approximately 14.3 wt% hydrogen is released from the Mg(BH₄)₂⋅6NH₃-6LiH composite with distinctly reduced ammonia evolution while heating to 340 °C. Upon heating, Mg(BH₄)₂⋅6NH₃ first reacts with LiH to form Mg(NH₂)₂, Li₃BN₂H₈ and LiBH₄ with the release of H₂ and the evolution of a minor amount of NH₃. The newly formed Mg(NH₂)₂ then reacts with LiH to produce H₂ and Li₂Mg(NH)₂. Further elevating the operating temperature induces chemical reactions between Li₂Mg(NH)₂, LiBH₄ and Li₃BN₂H₈, causing the release of additional H₂ and production of Li₃BN₂, LiMgBN₂ and LiH. The dehydrogenated sample at 210 °C absorbs 2.2 wt% of hydrogen, exhibiting partial reversibility for hydrogen storage.     

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.     

The reaction pathway and rate-limiting step of dehydrogenation of the LiHN2 + LiH mixture

The reaction pathway and rate-limiting step of dehydrogenation of the LiNH₂ + LiH mixture have been investigated. The study reveals that dehydrogenation of the LiNH₂ + LiH mixture is diffusion-controlled and the rate-limiting step is NH₃ diffusion through the Li₂NH product layer outside the LiNH₂ shrinking core. This phenomenon is explained based on a model describing the major steps of the dehydriding reaction of the mixture, and related to the evidence obtained from X-ray diffraction and specific surface area measurements of the mixture before and after isothermal hydrogen uptake/release cycles at high homologous temperatures.