Hydrogen storage in LiAlH4: predictions of the crystal structures and reaction mechanisms of intermediate phases from quantum mechanics

We use the density functional theory and x-ray and neutron diffraction to investigate the crystal structures and reaction mechanisms of intermediate phases likely to be involved in decomposition of the potential hydrogen storage material LiAlH(4). First, we explore the decomposition mechanism of monoclinic LiAlH(4) into monoclinic Li(3)AlH(6) plus face-centered cubic (fcc) Al and hydrogen. We find that this reaction proceeds through a five-step mechanism with an overall activation barrier of 36.9 kcal/mol. The simulated x ray and neutron diffraction patterns from LiAlH(4) and Li(3)AlH(6) agree well with experimental data. On the other hand, the alternative decomposition of LiAlH(4) into LiAlH(2) plus H(2) is predicted to be unstable with respect to that through Li(3)AlH(6). Next, we investigate thermal decomposition of Li(3)AlH(6) into fcc LiH plus Al and hydrogen, occurring through a four-step mechanism with an activation barrier of 17.4 kcal/mol for the rate-limiting step. In the first and second steps, two Li atoms accept two H atoms from AlH(6) to form the stable Li-H-Li-H complex. Then, two sequential H(2) desorption steps are followed, which eventually result in fcc LiH plus fcc Al and hydrogen: Li(3)AlH(6)(monoclinic)-->3 LiH(fcc)+Al(fcc)+3/2 H(2) is endothermic by 15.8 kcal/mol. The dissociation energy of 15.8 kcal/mol per formula unit compares to experimental enthalpies in the range of 9.8-23.9 kcal/mol. Finally, we explore thermal decomposition of LiH, LiH(s)+Al(s)-->LiAl(s)+12H(2)(g) is endothermic by 4.6 kcal/mol. The B32 phase, which we predict as the lowest energy structure for LiAl, shows covalent bond characters in the Al-Al direction. Additionally, we determine that transformation of LiH plus Al into LiAlH is unstable with respect to transformation of LiH through LiAl.     

Crystal structure, Raman spectroscopy, and ab initio calculations of a new bialkali alanate K2LiAlH6

A new bialkali alanate K2LiAlH6 was synthesized at 320-330 degrees C and 100-700 bar. It was structurally characterized by powder X-ray diffraction. It crystallizes in space group R3m (No. 166) with unit cell parameters a = 5.62068(8) and c = 27.3986(6) A. The Li and K cation sites are mutually exclusive, and Rietveld refinement finds no cation mixing. First-principles total energy calculations were performed for nine competing database structures of the stoichiometry A2BCX6, taken from fluoride and oxide compounds in the Inorganic Crystal Structure Database (ICSD). The relaxed structures were compared via their total energies and their agreement with experimental diffraction spectra. Two database structures K2LiAlF6 (R3m) and Cs2NaAlF6 (C2/m) were found to have the lowest total energies, but with the Rietveld method the K2LiAlF6 structure type was shown to be the most favorable. Ab initio total energy calculations support the validity of the structure determination. First-principles calculations also indicate that cation mixing is energetically unfavorable. Hydride properties such as plateau pressure are therefore more difficult to manipulate through alloying in this class of compounds.     

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).     

Investigation of hydrate formation in the system H2-CH4-H2O at a pressure up to 250 MPa

Phase equilibria in the system H2-CH4-H2O are investigated by means of differential thermal analysis within hydrogen concentration range 0-70 mol % and at a pressure up to 250 MPa. All the experiments were carried out under the conditions of gas excess. With an increase in hydrogen concentration in the initial gas mixture, decomposition temperature of the formed hydrates decreased. X-ray diffraction patterns and Raman spectra of the quenched hydrate samples obtained at a pressure of 20 MPA from a gas mixture containing 40 mol % hydrogen were recorded. It turned out that the hydrate has cubic structure I under these conditions. The Raman spectra showed that hydrogen molecules are not detected in the hydrate within the sensitivity of the method, that is, almost pure methane hydrate is formed. The general view of the phase diagram of the investigated system is proposed. A thermodynamic model was proposed to explain a decrease in hydrate decomposition temperature in the system with an increase in the concentration of hydrogen in the initial mixture.     

机械球磨固相化学反应制备AlH3及其放氢性能

以LiAlH4和AlCl3为原料, 采用机械球磨固相化学反应方法制备了铝氢化合物, 通过X射线衍射(XRD)、热分析(TG-DSC)和质谱(MS)分析等方法对反应产物进行分析和表征, 研究了不同球磨时间(4、8、15和20 h)对LiAlH4+AlCl体系的固相反应转变规律﹑合成产物和放氢性能的影响. 研究结果表明, 随球磨时间的增加, 球磨固相反应按3LiAlH4+AlCl3→4AlH3+3LiCl方向进行, 形成了非晶态铝氢化合物AlH3, 球磨20 h时反应基本完全. 球磨产物的放氢动力学特性随球磨时间增加而改善, 其放氢起始温度均低于100 ℃, 最大放氢量达到2.6%-3.6%(H2)(w), 接近反应体系的理论储氢量4.85%(H2)(w). 球磨过程中反应产物形成LiCl·H2O以及少量AlH3发生分解是影响球磨产物最大放氢量的主要因素.     

Ionic motion in crystalline cryolite

The character of the ion dynamics in crystalline cryolite, Na(3)AlF(6), a model double perovskite-structured mineral, has been examined in computer simulations using a polarizable ionic potential obtained by force-fitting to ab initio electronic structure calculations. NMR studies, and conductivity measurements, have indicated a high degree of mobility, in both Na(+) ion diffusion and reorientation of the AlF(6) octahedral units. The simulations reproduce the low-temperature (tilted) crystal structure and the existence of a transition to a cubic structure at elevated temperatures, in agreement with diffraction measurements, though the calculated transition temperature is too low. The reorientational dynamics of the AlF(6) octahedra is shown to consist of a hopping motion between the various tilted positions of the low-temperature form, even above the transition temperature. The rate of reorientation estimated by extrapolation to the temperature régime of the NMR measurements is consistent with the experimental data. In addition, we report a novel cooperative "tilt-swapping" motion of the differently tilted sublattices, just below the transition temperature. The perfect crystals show no Na(+) diffusion, in apparent disagreement with observation. We argue, following previous analyses of the cryolite phase diagram, that the diffusion observed in the experimental studies is a consequence of defects that are intrinsic to the thermodynamically stable form of cryolite. By introducing defects into the simulation cell, we obtain diffusion rates that are consistent with the NMR and conductivity measurements. Finally, we demonstrate a link between diffusion of the Na(+) ions and the reorientation of AlF(6) units, though the correlation between the two is not very strong.     

Coordination-cluster model for analyzing interactions in Na-O-H melts

A procedure for calculating the thermodynamic activity coefficients of oxygen and hydrogen in dilute Na-O-H system melts over the temperature range 300–600°C was suggested. The thermodynamic activity coefficients of hydrogen in liquid sodium calculated by the coordination-cluster model equations were used to determine the equilibrium hydrogen pressure over melts. The calculation results were compared with the experimental partial hydrogen pressures over the Na(excess)-Na₂O-NaH system at x _O = x _H. The calculated values were in qualitative agreement with the experimental data.     

Physiochemical pathway for cyclic dehydrogenation and rehydrogenation of LiAlH4

A five-step physiochemical pathway for the cyclic dehydrogenation and rehydrogenation of LiAlH4 from Li3AlH6, LiH, and Al was developed. The LiAlH4 produced by this physiochemical route exhibited excellent dehydrogenation kinetics in the 80-100 degrees C range, providing about 4 wt % hydrogen. The decomposed LiAlH4 was also fully rehydrogenated through the physiochemical pathway using tetrahydrofuran (THF). The enthalpy change associated with the formation of a LiAlH4.4THF adduct in THF played the essential role in fostering this rehydrogenation from the Li3AlH6, LiH, and Al dehydrogenation products. The kinetics of rehydrogenation was also significantly improved by adding Ti as a catalyst and by mechanochemical treatment, with the decomposition products readily converting into LiAlH4 at ambient temperature and pressures of 4.5-97.5 bar.     

气态配合物Na2HfCl6的热力学性质

Thermodynamic studies were carried out for the vapor complex of sodium chloride with hafnium tetrachloride at 712-778 K and 0.5-3.1 kPa by using high temperature phase equilibrium-quenching experiments, with closed Pyrex glass ampoules as the reaction containers. The results show that the sole predominant vapor complex is Na2HfCl6 for the HfCl4-NaCl system under the experimental conditions. The thermodynamic equilibrium constants and other thermodynamic functions of the reaction 2NaCl(s)+HfCl4(g)=Na2HfCl6(g) have been derived from the measurements. The results for the changes in enthalpy and entropy are -65.5§1.5 kJ/mol and -99.6§2.0 J/(mol K) in the temperature range.     

Multiple crossed isopolymorphism: two-component systems CCl4 +CBr2Cl2 and CBrCl3 + CBr2Cl2; inference of a metastable rhombohedral phase of CBr2Cl2

The phases diagrams of the two-component systems CCl4 +CBr2Cl2 and CBrCl3 + CBr2Cl2 have been determined by means of X-ray powder diffraction and thermal analysis techniques from the low-temperature ordered phase to the liquid state. The isomorphism relationship between the stable orientationally disordered (OD) face-centered cubic (FCC) phases of CBrCl3 and CBr2Cl2 and the metastable OD FCC phase (monotropic behavior with respect to the OD rhombohedral stable phase) of CCl4 has been put into evidence throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] for the whole range of composition in both two-component systems. This equilibrium interferes, for the CCl4 +CBr2Cl2 system, with a rhombohedral (R) plus liquid ([R + L]) equilibrium giving rise to a peritectic invariant. In addition, whatever the system, [R + FCC] equilibrium also interferes with the low-temperature equilibria between the low-temperature monoclinic (C2/c) phase and the OD R and FCC phases. In regards to the low-temperature monoclinic phases, isomorphism is evidenced, and by means of Rietveld profile refinement, any ordering of the molecules by varying the fractional occupancy of the halogen sites has been detected. The thermodynamic assessment, conducted by means of the concept of crossed isopolymorphism, coherently reproduces all the involved equilibria and provides a coherent set of data for the thermodynamic properties of nonexperimentally available phase transitions of pure compound CBr2Cl2 which enables us to obtain the topological properties of its pressure-temperature phase diagram and to infer the existence of a high-pressure R phase for such a compound.     

Ring-expansion reaction of oximes with aluminum reductants

The ring-expansion reactions of heterocyclic ketoximes and carbocyclic ketoximes with several reductants such as AlHCl2, AlH3 (alane), LiAlH4, LiAlH(OtBu)3, and (MeOCH2CH2O)2AlH2Na (Red-Al) were examined. Among reductants, AlHCl2 (LiAlH4:AlCl3 = 1:3) in cyclopentyl methyl ether (CPME) has been found to be a suitable reagent for the reaction, and the rearranged cyclic secondary amines were obtained in good to excellent yields.     

Hydrogen encapsulation in a silicon clathrate type I structure: Na5.5(H2)2.15Si46: synthesis and characterization

A hydrogen-encapsulated inorganic clathrate, which is stable at ambient temperature and pressure, has been prepared in high yield. Na5.5(H2)2.15Si46 is a sodium-deficient, hydrogen-encapsulated, type I silicon clathrate. It was prepared by the reaction between NaSi and NH4Br under dynamic vacuum at 300 degrees C. The Rietveld refinement of the powder X-ray diffraction data is consistent with the clathrate type I structure. The type I clathrate structure has two types of cages where the guest species, in this case Na and H2, can reside: a large cage composed of 24 Si, in which the guest resides in the 6d crystallographic position, and a smaller one composed of 20 Si, in which the guest occupies the 2a position. Solid-state 23Na, 1H, and 29Si MAS NMR confirmed the presence of both sodium and hydrogen in the clathrate cages. 23Na NMR shows that sodium completely fills the small cage and is deficient in the larger cage. The 1H NMR spectrum shows a pattern consistent with mobile hydrogen in the large cage. 29Si NMR spectrum is consistent with phase pure type I clathrate framework. Elemental analysis is consistent with the stoichiometry Na5.5(H2.15)2Si46. The sodium occupancy was also examined using spherical aberration (Cs) corrected scanning transmission electron microscopy (STEM). The high-angle annular dark-field (HAADF) STEM experimental and simulated images indicated that the Na occupancy of the large cage, 6d sites, is less than 2/3, consistent with the NMR and elemental analysis.     

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。     

TiCl3-enhanced NaAlH4: impact of excess al and development of the Al1-yTiy phase during cycling

NaAlH(4) with TiCl(3) and Al were mixed by ball-milling and cycled three times. The hydrogen storage properties were monitored during cycling, and the products were characterized by synchrotron X-ray diffraction. Because of the previously described formation of Al(1)(-)(y)Ti(y) with y approximately 0.15 during cycling that traps Al beyond the amount associated with the formation of NaCl, some Na(3)AlH(6) has no free Al to react with to form NaAlH(4). This was counteracted in the present work by adding a stoichiometric amount of Al that increases the theoretical storage capacity. Due to limitations in metal diffusion small amounts of Na(3)AlH(6) were still detected. When approximately 7 mol % more Al than the stoichiometric amount was added, the observed storage capacity increased significantly, and the Na(3)AlH(6) content was negligible after prolonged rehydrogenation. Cycled NaAlH(4) + 10 mol % TiCl(3) were desorbed to two different levels, and the diffraction patterns were compared. There is no change in unit-cell dimensions during desorption, and there is no sign of changes in the bulk composition of the Al(1)(-)(y)Ti(y) phase during a cycle. Adding pure Ti to a NaH + Al mixture by ball-milling in argon or hydrogen results in formation of TiH(2) that is stable during at least one cycle.     

A first-principles study of the electronic structure and stability of a lithium aluminum hydride for hydrogen storage

LiAlH4 holds great promise for reversible hydrogen storage, where a fundamental understanding of hydrogen interaction with the metal elements is essential to further improve its properties. The present paper reports a first-principles study of its stability and electronic structure, using a full potential linearized augmented plane wave (FLAPW) method within the generalized gradient approximation (GGA) for high accuracy. The theoretically calculated heat of formation agrees well with experiment. The electronic structures show that the H atoms bond nonequivalently with the Al in the [AlH4]- ligand, which leads to complex dehydrogenation characteristics of LiAlH4.     

烟酸钠Na(C6H4NO2)(s)的低温热容和热化学

选择分析纯烟酸和无水醋酸钠作为反应物, 用室温固相合成方法合成了无水烟酸钠. 利用FTIR和X射线粉末衍射等方法进行了表征, 利用化学分析和元素分析确定其组成为Na(C6H4NO2). 用精密自动绝热热量计测量其在78~400 K温度区间的低温热容. 研究结果表明, 该化合物在此温度区间无热异常现象发生. 用最小二乘法将实验摩尔热容对温度进行拟合, 得到热容随温度变化的多项式方程. 用此方程进行数值积分, 得到在此温度区间每隔5 K的舒平热容值和相对于298.15 K时的热力学函数值. 在此基础上, 通过设计合理的热化学循环, 选用1 mol/L NaOH溶液作为量热溶剂, 利用等温环境溶解-反应热量计分别测得固相反应的反应物和产物在所选溶剂中的溶解焓, 得到固相反应的反应焓. 最后, 计算出无水烟酸钠的标准摩尔生成焓为: ΔfHm0[Na(C6H4NO2), s]=-(548.96±1.11) kJ/mol.     

Hydrogen dynamics in Na3AlH6: a combined density functional theory and quasielastic neutron scattering study

Understanding the elusive catalytic role of titanium-based additives on the reversible hydrogenation of complex hydrides is an essential step toward developing hydrogen storage materials for the transport sector. Improved bulk diffusion of hydrogen is one of the proposed effects of doping sodium alanate with TiCl3, and here we study hydrogen dynamics in doped and undoped Na3AlH6 using a combination of density functional theory calculations and quasielastic neutron scattering. The hydrogen dynamics is found to be vacancy mediated and dominated by localized jump events, whereas long-range bulk diffusion requires significant activation. The fraction of mobile hydrogen is found to be small for both undoped and doped Na3AlH6, even at 350 K, and improved hydrogen diffusion as a result of bulk-substituted titanium is found to be unlikely. We also propose that previously detected low-temperature point defect motion in sodium alanate could result from vacancy-mediated sodium diffusion.     

Decomposition mechanism and the effects of metal additives on the kinetics of lithium alanate

First-principles density functional theory studies have been carried out for native defects and transition-metal (Ti and Ni) impurities in lithium alanate (LiAlH(4)), a potential material for hydrogen storage. On the basis of our detailed analysis of the structure, energetics, and migration of lithium-, aluminium-, and hydrogen-related defects, we propose a specific atomistic mechanism for the decomposition and dehydrogenation of LiAlH(4) that involves mass transport mediated by native point defects. We also discuss how Ti and Ni impurities alter the Fermi-level position with respect to that in the undoped material, thus changing the concentration of charged defects that are responsible for mass transport. This mechanism provides an explanation for the experimentally observed lowering of the temperature for the onset of decomposition and of the activation energy for hydrogen desorption from LiAlH(4).     

Experimental study and thermodynamic modeling of the MgO–NiO–SiO2 system

The MgO–NiO–SiO₂ system has been studied by a combination of thermodynamic modeling and experimental measurements of phase equilibria. A complete literature review, critical evaluation and thermodynamic modeling of phase diagrams and thermodynamic properties of all oxide phases in the MgO–NiO–SiO₂ system at 1 atm total pressure are presented. To resolve the contradictions in the literature data, a new experimental investigation has been carried out over the temperature range from (1400 to 1650) °C using an equilibration and quenching technique followed by electron probe X-ray microanalysis (EPMA). Tie-lines between olivine and monoxide, olivine and proto-pyroxene, liquid and olivine and liquid and cristobalite have been measured. The whole set of experimental data, including the new experimental results and previously published data, has been taken into consideration in thermodynamic modeling of oxide phases in the MgO–NiO–SiO₂ system. The Modified Quasichemical Model has been used for the liquid phase. A simple random mixing model with a polynomial expansion of the excess Gibbs energy has been used for the monoxide solid solution. The models for olivine and proto-pyroxene were developed within the framework of the Compound Energy Formalism. The optimized model parameters reproduce all available thermodynamic and phase diagram data within experimental error limits.     

Mg2NiQ6(Q=H,D, T)体系的热力学氢同位素效应

为研究Mg2NiQ4(Q=H, D, T)体系的热力学氢同位素效应, 基于量子力学第一原理, 采用密度泛函与赝势平面波相结合的方法, 计算了Mg2NiQ6(Q=H, D, T)体系的声子色散谱, 利用声子色散谱得到了热力学函数. 利用文献报道的氢同位素气体分子的热力学数据, 采用热力学方法分析了Mg2Ni吸氢形成Mg2NiQ6的同位素效应. 研究结果表明, Mg2Ni吸氢的同位素效应主要是原子相对振动的频率不同导致的. CaF2结构的Mg2NiQ6的氢同位素效应随温度升高由负同位素效应转变为正同位素效应.