Hydrogen desorption mechanism in a Li-N-H system by means of the isotopic exchange technique

The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH2 --> Li2NH + NH3 and LiH + NH3 --> LiNH2 + H2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.     

Mechanism of hydrogenation reaction in the Li-Mg-N-H system

The Li-Mg-N-H system composed of 3 Mg(NH2)2 and 8 LiH reversibly desorbs/absorbs approximately 7 wt % of H2 at 120-200 degrees C and transforms into 4 Li2NH and Mg3N2 after dehydrogenation. In this work, the mechanism of the hydrogenation reaction from 4 Li2NH and Mg3N2 to 8 LiH and 3 Mg(NH2)2 was investigated in detail. Experimental results indicate that 4 Li2NH is first hydrogenated into 4 LiH and 4 LiNH2. At the next step, 4 LiNH2 decomposes into 2 Li2NH and 2 NH3, and the emitted 2 NH3 reacts with (1/2) Mg3N2 and produces the (3/2) Mg(NH2)2 phase, while the produced 2 Li2NH is hydrogenated into 2 LiH and 2 LiNH2 again. Such successive steps continue until all 4 Li2NH and Mg3N2 completely transform into 8 LiH and 3 Mg(NH2)2 by hydrogenation.     

Destabilisation of the Li-N-H hydrogen storage system with elemental Si

A significant improvement in the dehydrogenation kinetics of the (LiNH(2) + LiH) system was obtained upon doping with elemental Si. Whilst, complete dehydrogenation of the (LiNH(2) + LiH) system requires more than 2 h, the time required for full dehydrogenation was reduced to less than 30 min by doping with elemental Si. It is observed that Si thermodynamically destabilises the system through the formation of novel intermediate phases resulting from the reaction of Si with both LiNH(2) and LiH. Such intermediate phases are also believed to enhance reaction kinetics by providing a path for accelerated dehydrogenation and the rapid release of hydrogen at the early stages of the reaction. It is believed that the dehydrogenation kinetics of the (LiNH(2) + LiH) system, which is controlled by the diffusion of H(-) from LiH and H(+) from LiNH(2), becomes independent of diffusion upon Si addition due to an enhanced concentration gradient in reactive ionic species.     

Hydrogen storage properties of Li-Mg-N-H systems with different ratios of LiH/Mg(NH2)2

In this work, the hydrogen desorption and structural properties of the Li-Mg-N-H systems with different LiH/Mg(NH2)2 ratios are systemically investigated. The results indicate that the system with the LiH/Mg(NH2)2 ratio of 6/3 transforms into Li2NH and MgNH, and then, the mixture forms an unknown phase by a solid-solid reaction, which presumably is the ternary imide Li2Mg(NH)2; the system with the LiH/Mg(NH2)2 ratio of 8/3 transforms into 4Li2NH and Mg3N2 after releasing H2 at T < 400 degrees C; the system with the LiH/Mg(NH2)2 ratio of 12/3 transforms into 4Li3N and Mg3N2 after releasing H2 at T > 400 degrees C, where the LiMgN phase is formed by the reaction between Li3N and Mg3N2. The characteristics of the phase transformations and the thermal gas desorption behaviors in these Li-Mg-N-H systems could be reasonably explained by the ammonia mediated reaction model, irrespective of the difference in the LiH/Mg(NH2)2 ratios.     

Intrinsic defects and dopants in LiNH2: a first-principles study

The lithium amide (LiNH(2)) + lithium hydride (LiH) system is one of the most attractive light-weight materials options for hydrogen storage. Its dehydrogenation involves mass transport in the bulk (amide) crystal through lattice defects. We present a first-principles study of native point defects and dopants in LiNH(2) using density functional theory. We find that both Li-related defects (the positive interstitial Li(i)(+) and the negative vacancy V(Li)(-)) and H-related defects (H(i)(+) and V(H)(-)) are charged. Li-related defects are most abundant. Having diffusion barriers of 0.3-0.5 eV, they diffuse rapidly at moderate temperatures. V(H)(-) corresponds to the [NH](2-) ion. It is the dominant species available for proton transport with a diffusion barrier of ～0.7 eV. The equilibrium concentration of H(i)(+), which corresponds to the NH(3) molecule, is negligible in bulk LiNH(2). Dopants such as Ti and Sc do not affect the concentration of intrinsic defects, whereas Mg and Ca can alter it by a moderate amount. Ti and Mg are easily incorporated into the LiNH(2) lattice, which may affect the crystal morphology on the nano-scale.     

Quantitative estimation of NH3 partial pressure in H2 desorbed from the Li-N-H system by Raman spectroscopy

The partial pressure of NH3 gas estimated by Raman spectroscopy indicates that approximately 0.1% NH3 inevitably contaminates the H2 desorbed from a hydrogen storage material composed of LiH and LiNH2 at any temperature up to 400 degrees C in a closed system.     

Ab initio study on the hydrogen desorption from MH-NH3 (M = Li, Na, K) hydrogen storage systems

The hydrogen storage system LiH + NH(3) ? LiNH(2) + H(2) is one of the most promising hydrogen storage systems, where the reaction yield can be increased by replacing Li in LiH with other alkali metals (Na or K) in order of Li < Na < K. In this paper, we have studied the alkali metal M (M = Li, Na, K) dependence of the reactivity of MH with NH(3) by calculating the potential barrier of the H(2) desorption process from the reaction of an M(2)H(2) cluster with an NH(3) molecule based on the ab initio structure optimization method. We have shown that the height of the potential barrier becomes lower in order of Li, Na, and K, where the difference of the potential barrier in Li and Na is relatively smaller than that in Na and K, and this tendency is consistent with the recent experimental results. We have also shown that the H-H distance of the H(2) dimer at the transition state takes larger distance and the change of the potential energy around the transition state becomes softer in order of Li, Na, and K. There are almost no M dependence in the charge of the H atom in NH(3) before the reaction, while that of the H atom in M(2)H(2) takes larger negative value in order of Li, Na, and K. We have also performed molecular dynamics simulations on the M(2)H(2)-NH(3) system and succeeded to reproduce the H(2) desorption from the reaction of Na(2)H(2) with NH(3).     

Enhancement of lithium amide to lithium imide transition via mechanical activation

The decomposition of lithium amide (LiNH2) to lithium imide (Li2NH) and ammonia (NH3) with and without high-energy ball milling is investigated to lay a foundation for identifying methods to enhance the hydrogen uptake/release of the lithium amide and lithium hydride mixture. A wide range of analytical instruments are utilized to provide unambiguous evidence of the effect of mechanical activation. It is shown that ball milling reduces the onset temperature for the decomposition of LiNH2 from 120 degrees C to room temperature. The enhanced decomposition via ball milling is attributed to mechanical activation related to the formation of nanocrystallites, the reduced particle size, the increased surface area, and the decreased activation energy. The more mechanical activation there is, then the more improvement there is in enhancing the decomposition of LiNH2. It also is found that the activation energy for the decomposition of LiNH2 without ball milling is 243.98 kJ/mol, which is reduced to 222.20 kJ/mol after ball milling at room temperature for 45 min and is further reduced to 138.05 kJ/mol after ball milling for 180 min. The rate of the isothermal decomposition at the later phase of the LiNH2 decomposition is controlled by diffusion of NH3 through the Li2NH layer.     

LiAlH4与LiNH2的相互作用机理及储氢特性

将LiAlH4和LiNH2按摩尔比1：2进行球磨复合,随后将复合物进行加热放氢特性研究,然后对其完全放氢后的产物进行再吸氢特性研究。通过X射线衍射分析(XRD)、热分析(DSC)和红外 (FTIR)分析等测试手段对其反应过程进行了系统分析研究。研究结果表明,LiAlH4/2LiNH2加热放氢分为3个反应阶段,放氢后生成Li3AlN2,总放氢量达到8.65wt%。放氢生成的Li3AlN2在10MPaH2压力和400℃条件下,可以可逆吸氢5.0wt%,吸氢后的产物为 LiNH2 、AlN和LiH,而不能再生成LiAlH4。本文对LiAlH4/2LiNH2复合物放氢/再氢化过程机理进行了分析。     

Identification of destabilized metal hydrides for hydrogen storage using first principles calculations

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through alloying with other elements. A very large number of possible destabilized metal hydride reaction schemes exist. The thermodynamic data required to assess the enthalpies of these reactions, however, are not available in many cases. We have used first principles density functional theory calculations to predict the reaction enthalpies for more than 100 destabilization reactions that have not previously been reported. Many of these reactions are predicted not be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low. More importantly, our calculations identify five promising reaction schemes that merit experimental study: 3LiNH(2) + 2LiH + Si --> Li(5)N(3)Si + 4H(2), 4LiBH(4) + MgH(2) --> 4LiH + MgB(4) + 7H(2), 7LiBH(4) + MgH(2) --> 7LiH + MgB(7) + 11.5H(2), CaH(2) + 6LiBH(4) --> CaB(6) + 6LiH + 10H(2), and LiNH(2) + MgH(2) --> LiMgN + 2H(2).     

Dehydrogenation of a combined LiAlH4/LiNH2 system

Although there have been numerous materials systems studied as potential candidates for hydrogen storage applications, none of the materials known to date has demonstrated enough hydrogen capacity or efficiency at required operating temperature ranges. There are still considerable opportunities for discovery of new materials or material systems that could lead to advances in science as well as commercial technologies in this area. LiAlH(4) is one of the most promising materials owing to its high hydrogen content. In the present work, we investigated dehydrogenation properties of the combined system of LiAlH(4) and LiNH(2) under atmospheric argon. Thermogravimetric analysis (TGA) of 2LiAlH(4)/LiNH(2) mixtures without any catalysts indicated that a large amount of hydrogen (approximately 8.1 wt %) can be released between 85 and 320 degrees C under a heating rate of 2 degrees C/min in three dehydrogenation reaction steps. It is found that LiNH(2) effectively destabilizes LiAlH(4) by reacting with LiH during the dehydrogenation process of LiAlH(4).     

Synthesis and crystal structure of Li4BH4(NH2)3

The solid solution, (LiNH2)x(LiBH4)(1-x), formed through the reaction of the two potential hydrogen storage materials, LiNH2 and LiBH4, is dominated by a compound that has an ideal stoichiometry of Li4BN3H10 and forms a body-centred cubic structure with a lattice constant of ca. 10.66 A.     

On the composition and crystal structure of the new quaternary hydride phase Li4BN3H10

X-ray data on single crystals of the quaternary metal hydride near the composition LiB(0.33)N(0.67)H(2.67), previously identified as "Li3BN2H8", reveal that its true composition is Li4BN3H10. The structure has body-centered-cubic symmetry [space group I2(1)3, cell parameter a = 10.679(1)-10.672(1) Angstroms] and contains an ordered arrangement of BH4- and NH2- anions in the molar ratio 1:3. The borohydride anion has an almost ideal tetrahedral geometry (angleH-B-H approximately 108-114 degrees), while the amide anion has a nearly tetrahedral bond angle (angleH-N-H approximately 106 degrees). Three symmetry-independent Li atom sites are surrounded by BH4- and NH2- anions in various distorted tetrahedral configurations, one by two B and two N atoms, another by four N atoms, and the third by one B and three N atoms. The Li configuration around B is nearly tetrahedral, while that around N resembles a distorted saddlelike configuration, similar to those in LiBH4 and LiNH2, respectively.     

Li+ ionic conductivities and diffusion mechanisms in Li-based imides and lithium amide

In this study, both experimental ionic conductivity measurements and the first-principles simulations are employed to investigate the Li(+) ionic diffusion properties in lithium-based imides (Li(2)NH, Li(2)Mg(NH)(2) and Li(2)Ca(NH)(2)) and lithium amide (LiNH(2)). The experimental results show that Li(+) ions present superionic conductivity in Li(2)NH (2.54 × 10(-4) S cm(-1)) and moderate ionic conductivity in Li(2)Ca(NH)(2) (6.40 × 10(-6) S cm(-1)) at room temperature; while conduction of Li(+) ions is hardly detectable in Li(2)Mg(NH)(2) and LiNH(2) at room temperature. The simulation results indicate that Li(+) ion diffusion in Li(2)NH may be mediated by Frenkel pair defects or charged vacancies, and the diffusion pathway is more likely via a series of intermediate jumps between octahedral and tetrahedral sites along the [001] direction. The calculated activation energy and pre-exponential factor for Li(+) ion conduction in Li(2)NH are well comparable with the experimentally determined values, showing the consistency of experimental and theoretical investigations. The calculation of the defect formation energy in LiNH(2) reveals that Li defects are difficult to create to mediate the Li(+) ion diffusion, resulting in the poor Li(+) ion conduction in LiNH(2) at room temperature.     

Investigation of hydrogen absorption in Li7VN4 and Li7MnN4

The hydrogen storage properties of Li(7)VN(4) and Li(7)MnN(4) were investigated both by experiment and by density functional theory calculations. Li(7)VN(4) did not sorb hydrogen under our experimental conditions. Li(7)MnN(4) was observed to sorb 7 hydrogen atoms through the formation of LiH, Mn(4)N, and ammonia gas. An applied pressurized mixture of H(2)/Ar and H(2)/N(2) gases was helpful to mitigate the release of NH(3) but could not prevent its formation. The introduction of N(2) also caused weight gain of the sample by re-nitriding the absorbed products LiH and Mn(4)N, which correlated with the presence of Li(2)NH, LiNH(2), and Mn(2)N detected by X-ray diffraction. While our observed results for Li(7)VN(4) and Li(7)MnN(4) differ in detail, they are in overall qualitative agreement with our theoretical work, which strongly suggests that both compounds are unlikely to form quaternary hydrides.     

A new Li-Al-N-H system for reversible hydrogen storage

Complex metal hydrides are considered as a class of candidate materials for hydrogen storage. Lithium-based complex hydrides including lithium alanates (LiAlH(4) and Li(3)AlH(6)) are among the most promising materials owing to its high hydrogen content. In the present work, we investigated dehydrogenation/rehydrogenation reactions of a combined system of Li(3)AlH(6) and LiNH(2). Thermogravimetric analysis (TGA) of Li(3)AlH(6)/3LiNH(2)/4 wt % TiCl(3)-(1)/(3)AlCl(3) mixtures indicated that a large amount of hydrogen (approximately 7.1 wt %) can be released between 150 degrees C and 300 degrees C under a heating rate of 5 degrees C/min in two dehydrogenation reaction steps. The results also show that the dehydrogenation reaction of the new material system is nearly 100% reversible under 2000 psi pressure hydrogen at 300 degrees C. Further, a short-cycle experiment has demonstrated that the new combined material system of alanates and amides can maintain its hydrogen storage capacity upon cycling of the dehydrogenation/rehydrogenation reactions.     

Hydrogen desorption exceeding ten weight percent from the new quaternary hydride Li3BN2H8

Mobile applications of hydrogen power have long demanded new solid hydride materials with large hydrogen storage capacities. We report synthesis of a new quaternary hydride having the approximate composition Li(3)BN(2)H(8) with 11.9 wt % theoretical hydrogen capacity. It forms by reacting LiNH(2) and LiBH(4) powders in a 2:1 molar ratio either by ball milling or by heating the mixed powders above 95 degrees C. This new quaternary hydride melts at approximately 190 degrees C and releases > or =10 wt % hydrogen above approximately 250 degrees C. A small amount of ammonia (2-3 mol % of the generated gas) is released simultaneously. Preliminary calorimetric measurements suggest that hydrogen release is exothermic and, hence, not easily reversible.     

In situ hybridization of LiNH2-LiH-Mg(BH4)2 nano-composites: intermediate and optimized hydrogenation properties

Nano-composites of LiNH(2)-LiH-xMg(BH(4))(2) (0 ≤ x ≤ 2) were prepared by plasma metal reaction followed by a nucleation growth method. Highly reactive LiNH(2)-LiH hollow nanoparticles offered a favorable nucleus during a precipitation process of liquid Mg(BH(4))(2)·OEt(2). The electron microscopy results suggested that more than 90% of the obtained nano-composites were in the range 200-400 nm. Because of the short diffusion distance and ternary mixture self-catalyzing effect, this material possesses enhanced hydrogen (de)sorption attributes, including facile low-temperature kinetics, impure gases attenuation and partial reversibility. The optimal hydrogen storage properties were found at the composition of LiNH(2)-LiH-0.5Mg(BH(4))(2), which was tentatively attributed to a Li(4)(NH(2))(2)(BH(4))(2) intermediate. 5.3 wt% hydrogen desorption could be recorded at 150 °C, with the first 2.2 wt% release being reversible. This work suggests that controlled in situ hybridization combined with formula optimization can improve hydrogen storage properties.     

Correlation between composition and hydrogen storage behaviors of the Li2NH-MgNH combination system

Hydrogen storage performances of a Li(2)NH-xMgNH combination system (x = 0, 0.5, 1 and 2) are investigated for the first time. It is found that the hydrogenated samples with MgNH exhibit a significant reduction in the dehydrogenation temperatures. Mechanistic investigations reveal that there is a strong dependence of the hydrogen storage reaction process on the molar ratio between MgNH and Li(2)NH. As a consequence, tuning of thermodynamics is achieved for hydrogen storage in the Li(2)NH-xMgNH system by changing the reaction routes, which is ascertained to be the primary reason for the reduction in the operating temperature for hydrogen desorption. Specifically, it is found that under 105 atm hydrogen (140-280 °C) 5.6 wt% hydrogen is reversibly stored in the Li(2)NH-0.5MgNH combination system, which is greater than in the well-investigated Mg(NH(2))(2)-2LiH system.     

Hydrogen storage of metal nitride by a mechanochemical reaction

Metal imides (Li(2)NH, CaNH), a metal amide (LiNH(2)) and metal hydrides (LiH, CaH(2)) were synthesized by ball milling of their respective metal nitrides (Li(3)N, Ca(3)N(2)) in a H(2) atmosphere at 1 MPa and at room temperature.