Elastic properties of perovskite-type hydride NaMgH3 for hydrogen storage

Mechanical properties of NaMgH₃ were investigated using the norm-conserving pseudopotentials and plane waves (PP–PW) within the general gradient approximation (GGA) in the frame of density functional theory (DFT). The elastic constants of NaMgH₃ were calculated for the first time. The NaMgH₃ compound is found to be mechanically stable at ambient pressure. The linear bulk modulus and the bulk modulus along crystallographic axes of single crystals have been derived using elastic constants. The calculated linear bulk moduli are found to be in good agreement with the theoretical value reported in the literature. Shear and Young's moduli as well as Poisson's ratio for ideal polycrystalline NaMgH₃ are also calculated. According to the obtained results, NaMgH₃ can be classified as brittle material. The shear anisotropic factors and the elastic anisotropy are also discussed. A Debye temperature of 648 K was also determined using theoretical elastic constants.     

Properties of BaYO3 perovskite and hydrogen storage properties of BaYO3Hx

In this study, Density Functional Theory (DFT) calculations have been performed for BaYO₃ perovskite with the generalized gradient approximation (GGA) as implemented in Vienna Ab-initio Simulation Package (VASP). The structural optimization of BaYO₃ perovskite have been studied for the five possible phases: cubic, tetragonal, hexagonal, orthorhombic and rhombohedral to determine the most stable phase of BaYO₃ perovskite. It has been found that the cubic phase is the most stable one and electronic and mechanical properties of this phase have been investigated. Moreover, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's and Shear moduli for cubic phase. Then, hydrogen bonding to BaYO₃ perovskite has been conducted and hydrogen storage properties of BaYO₃H_x (x = 3 and 9) such as: formation energy, cohesive energy and gravimetric hydrogen storage capacity have been analyzed. Having no study about BaYO₃ perovskite and hydrogen bonding in the literature makes this study the first considerations of BaYO₃ perovskite. Hence, this work could enlighten the possible future studies.     

Role of vacancy defects on the dehydrogenation properties of the ternary hydride ZrNiH3: Ab-initio insights

ZrNi is considered a promising candidate for hydrogen storage and nickel-metal hydride rechargeable batteries (Ni-MH). The effect of creating zirconium and nickel vacancy defects on the dehydrogenation properties of ZrNiH₃ is investigated by means of first-principles calculations. The results indicate that nickel vacancy is energetically more favorable to form in ZrNiH₃ than zirconium vacancy, because of the lesser formation energy of Ni-vacancy. For both Zr and Ni vacancy defects, the formation enthalpy decreases with increasing the concentration of vacancy and, vice versa. In particular, it is found that with ~2.4% of zirconium vacancy defects or with ~4.5% of nickel vacancy defects in ZrNiH₃, the formation enthalpy is around - 40 kJ/mol.H₂, which is recommended by the U.S. Department of Energy (DOE). It is worth noting also that with slightly higher vacancy defects ~2.8 of Zr-vacancy or ~5.3% of Ni-vacancy in ZrNiH₃, it becomes harder to store hydrogen in these systems without cooling. Moreover, the density of states (DOS) analysis indicates that the stability of ZrNiH₃ decreases with increasing Zr-vacancy and Ni-vacancy concentrations, through the shrinkage in the size of the total DOS and shifting in the valence bands near to Fermi level.     

Formation of aluminium hydride (AlH3) via the decomposition of organoaluminium and hydrogen storage properties

Aluminium hydride (AlH₃) is a promising hydrogen storage material due to its competitive hydrogen storage density and moderate decomposition temperature. However, there is no convenient way to prepare/regenerate AlH₃ from (spent) Al by direct hydrogenation. Herein, we report on a novel approach to generate AlH₃ from the decomposition of triethylaluminium (Et₃Al) under mild hydrogen pressures (10 MPa) with the use of surfactants. With tetraoctylammonium bromide (TOAB), the synthesis led to the formation of nanosized AlH₃ with the known α phase, and these nanoparticles released hydrogen from 40 °C instead of the 125 °C observed with bulk α-AlH₃. However, when tetrabutylammonium bromide (TBAB) was used instead of TOAB, larger nanoparticles believed to be related to the formation of β-AlH₃ were obtained, and these decomposed through a single exothermic process. Despite the possibility to form α-AlH₃ under low conditions of temperature (180 °C) and pressure (10 MPa), TOAB stabilised AlH₃ was found to be irreversible when subjected to hydrogen cycling at 150 °C and 7 MPa hydrogen pressure.     

Enhanced hydrogen desorption/absorption properties of magnesium hydride with CeF3@Gn

In order to improve the hydrogen storage performance of MgH₂, graphene and CeF₃ co-catalyzed MgH₂ (hereafter denoted as MgH₂+CeF₃@Gn) were prepared by wet method ball milling and hydriding, which is a simple and time-saving method. The effect of CeF₃@Gn on the hydrogen storage behavior of MgH₂ was investigated. The experimental results showed that co-addition of CeF₃@Gn greatly decreased the hydrogen desorption/absorption temperature of MgH₂, and remarkably improved the dehydriding/hydriding kinetics of MgH₂. The onset hydrogen desorption temperature of Mg + CeF₃@Gn is 232 °C,which is 86 °C lower than that of as-milled undoped MgH₂, and its hydrogen desorption capacity reaches 6.77 wt%, which is 99% of its theoretical capacity (6.84 wt%). At 300 °C and 200 °C the maximum hydrogen desorption rates are 79.5 and 118 times faster than that of the as-milled undoped MgH₂. Even at low temperature of 150 °C, the dedydrided sample (Mg + CeF₃@Gn) also showed excellent hydrogen absorption kinetics, it can absorb 5.71 wt% hydrogen within 50 s, and its maximum hydrogen absorption rate reached 15.0 wt% H₂/min, which is 1765 times faster than that of the undoped Mg. Moreover, no eminent degradation of hydrogen storage capacity occurred after 15 hydrogen desorption/absorption cycles. Mg + CeF₃@Gn showed excellent hydrogen de/absorption kinetics because of the MgF₂ and CeH_2-3 that are formed in situ, and the synergic catalytic effect of these by-products and unique structure of Gn.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

Enhanced hydrogen storage properties and mechanisms of magnesium hydride modified by transition metal dissolved magnesium oxides

Magnesium hydride (MgH₂) is a promising on-board hydrogen storage material due to its high capacity, low cost and abundant Mg resources. Nevertheless, the practical application of MgH₂ is hindered by its poor dehydrogenation ability and cycling stability. Herein, the influences and mechanisms of thin pristine magnesium oxide (MgO) and transition metals (TM) dissolved Mg(TM)O layers (TM = Ti, V, Nb, Fe, Co, Ni) on hydrogen desorption and reversible cycling properties of MgH₂ were investigated using first-principles calculations method. The results demonstrate that either thin pristine MgO or Mg(TM)O layer weakens the MgH bond strength, leading to the decreased structural stability and hydrogen desorption energy of MgH₂. Among them, the Mg(Nb)O layer exhibits the most pronounced destabilization effect on MgH₂. Moreover, the Mg(Nb)O layer presents a long-acting confinement effect on MgH₂ due to the stronger interfacial bonding strength of Mg(Nb)O/MgH₂ and the lower brittleness of Mg(Nb)O itself. Further analyses of electronic structures indicate that these thin oxide layers coating on MgH₂ surface reduce the bonding electron number of MgH₂, which essentially accounts for the weakened MgH bond strength and enhanced hydrogen desorption properties of modified MgH₂ systems. These findings provide a new avenue for enhancing the hydrogen desorption and reversible cycling properties of MgH₂ by designing and adding suitable MgO based oxides with high catalytic activity and low brittleness.     

MgTiO3Hx and CaTiO3Hx perovskite compounds for hydrogen storage applications

The first principle calculations are used to investigate hydrogen storage properties of MgTiO₃H_x and CaTiO₃H_x (x = 0, 3, 6, and 8) perovskite compounds in cubic phase (Pm3 m). In order to examine the stability of these compounds, formation enthalpies are calculated and all compounds (except MgTiO₃H₆ and MgTiO₃H₈) are found to be stable. The second order elastic constants and related polycrystalline elastic moduli (e.g., shear modulus, Young's modulus, Poisson's ratio, Debye temperature, sound velocities) are determined and the results are discussed in detail. The mechanical stability determination indicates that MgTiO₃, CaTiO₃, and CaTiO₃H₆ compounds are only stable compounds and also MgTiO₃ and CaTiO₃H₆ are ductile while CaTiO₃ is a brittle material. Also, the mechanical anisotropy is discussed via two-dimensional (2D) and three-dimensional (3D) surfaces for mechanical stable compounds and they are found to have anisotropic behaviour (except linear compressibility for MgTiO₃, CaTiO₃). Electronic band structure and corresponding partial density of states (PDOS) and charge density have been plotted. Bader charge analysis have been done. MgTiO₃ has metallic behaviour whereas CaTiO₃ and CaTiO₃H₆ have semiconductor behaviour. Among all compounds, CaTiO₃H₆ is found to be only one that could be used in the hydrogen storage applications. For this compound, the gravimetric hydrogen storage capacity is calculated as 4.27 wt% and the hydrogen desorption temperature is obtained as 827.1 K.     

Three-dimensional modeling of hydrogen sorption in metal hydride hydrogen storage beds

This paper conducts a three-dimensional (3D) modeling study to investigate the hydrogen absorption process and associated mass and heat transport in a metal hydride (LaNi₅) hydrogen storage tank. The 3D model is further implemented numerically for validation purpose and the detailed investigation on absorption process. Results indicate that at the very initial absorption stage the bed temperature evolves almost uniformly, while it varies greatly spatially at the latter stage. At the initial seconds, most hydrogen is absorbed in the region near the cooling wall due to the better heat removal. The absorption in the core is slow at the beginning, but becomes important at the very end stage. It also shows that the initial hydrogen flow in the bed is several-fold larger than the latter stage and the flow may provide extra cooling to the hydriding process. By analyzing the Peclet number, we find that the heat convection by the hydrogen flow may play an important role in local heat transfer. This work provides an important platform beneficial to the fundamental understanding of multi-physics coupling phenomena during hydrogen absorption and the development of on-board hydrogen storage technology.     

On the optimization of hydrogen storage in metal hydride beds

This work presents a novel systematic approach for the optimal design and control of metal hydride beds used for hydrogen storage. A detailed 2-D mathematical model is developed and validated against experimental and theoretical literature results. Based on recent advances in dynamic optimization, the objective is then to find the optimal process design (e.g. cooling systems design) and operating strategy (e.g. cooling fluid profile over time, hydrogen charging profile, etc.) so as to minimize the storing time, while satisfying, a number of operating constraints. Such constraints account for pressure drop limitations, cooling fluid availability and maximum tank temperature. Optimization results indicate that almost 60% improvement of the storage time can be achieved, over the case where the system is not optimized, for a minimum storage capacity of 99% of the total bed capacity. Trade-offs between various objectives, alternative design options and optimal cooling control policies are systematically revealed illustrating the potential offered by modern optimization techniques.     

Iron and niobium based additives in magnesium hydride: Microstructure and hydrogen storage properties

In this study, powder mixtures of MgH₂ + 2 mol.% X, with X = Nb, Nb₂O₅, NbF₅, Fe, Fe₂O₃, FeF₃, were processed by mechanical milling at liquid nitrogen temperature (cryomilling). The effect of additives on crystalline structure, thermal properties and hydrogen storage properties of the mixtures were investigated. Morphological investigations indicated a heterogeneous particle size distribution of the powder mixtures and a fine dispersion of additive particles (FeF₃) in the MgH₂ matrix. High resolution synchrotron radiation X-ray diffraction (SR-XRD) data followed by Rietveld refinements showed a significant reduction on crystallite size for the samples containing fluorides (11 nm) in comparison with the pure MgH₂ sample (29 nm). This was related to the mechanical behavior of fluorides during milling with MgH₂, which act as a lubricant, dispersing and/or cracking agent during milling, and thus helping to further reduce MgH₂ particle size. DSC analysis revealed that fluorides (NbF₅, FeF₃) are much more effective than oxides (Nb₂O₅, Fe₂O₃) and the transition metals (Nb and Fe), respectively, in reduction the desorption temperature. Furthermore, Nb₂O₅ is more efficient than Fe₂O₃. Finally, the best results for desorption kinetics were observed for the fluorides: NbF₅ and FeF₃ (equivalent effect and consistent to the DSC analysis) followed by the oxides: Nb₂O₅, Fe₂O₃ and Nb. The addition of Fe was not efficient in comparison with the pure cryomilled sample.     

Active disturbance rejection control of metal hydride hydrogen storage

Metal hydride (MH) hydrogen storage is used in both mobile and stationary applications. MH tanks can connect directly to high-pressure electrolyzers for on-demand charging, saving compression costs. To prevent high hydrogen pressure during charging, hydrogen generation needs to be controlled with consideration for unknown disturbances and time-varying dynamics. This work presents a robust control system to determine the appropriate mass flow rate of hydrogen, which the water electrolyzer should produce, to maintain the gaseous hydrogen pressure in the tank for the hydriding reaction. A control-oriented model is developed for MH hydrogen storage for control system design purposes. A proportional-integral (PI) and an active disturbance rejection control (ADRC) feedback controllers are investigated, and their performance is compared. Simulation results show that both the PI and ADRC controllers can reject both noises from the output measurements and unknown disturbances associated with the heat exchanger. ADRC excels in eliminating disturbances produced by the input mass flow rate, maintaining the pressure of the tank at the charging pressure with little oscillations. Additionally, the parameters estimated by the ADRC's extended state observer was used to predict the state-of-charge (SOC) of the MH.     

First-principles studies of lithium hydride series for hydrogen storage

The application of hydrogen as a clean energy source is based on storage of hydrogen. In metal hydrides is possible, since many metals react readily with hydrogen forming a stable metal hydride. Thus, saline hydrides such as lithium hydride have appeared as new alternatives to this, because of their high reactivity and reversibility. The first principles calculations based on density functional theory (DFT) have been used to study the physical properties of several Li–H compounds. The crystal structure, electronic properties and internal optimization parameters are treated by the LAPW method implemented in the WIEN2k code. In the present study we show the comparison of three different phases of lithium hydride compounds, in six different crystal structures, with the purpose of comparing the formation energies in all cases, and determine which is the structure, with the best structural properties for applications as hydrogen reservoir. The comparisons between the results obtained in the structures of lithium–hydride are discussed in this work.     

Analysis of a metal hydride reactor for hydrogen storage

In this paper a two-dimensional model of an annular cylindrical reactor filled with metal hydride suitable for hydrogen storage is presented. Comparison of the computed bed temperatures with published experimental data shows a reasonably good agreement except for the initial period. Effects of hydrogen pressure and external fluid temperatures on heat transfer and entropy generation are obtained. Results show that the time required for hydrogen charging and discharging is higher when the thermal capacity of the reactor wall is considered. The time required for absorption and desorption can be reduced significantly by varying the hydrogen gas pressure and external fluid temperatures. However, along with reduction in time the entropy generated during hydrogen storage and discharge increases significantly. Results also show that for the given input conditions, heat transfer between the external fluid and hydride bed is the main source of entropy generation.     

Effect of multi-wall carbon nanotubes supported palladium addition on hydrogen storage properties of magnesium hydride

To improve the hydrogen storage performance of magnesium hydride, multi-wall carbon nanotubes supported palladium (Pd/MWCNTs) was introduced to the magnesium-based materials. Pd/MWCNTs catalysts with different amounts of Pd (20 wt.%, 40 wt.%, 60 wt.%, 80 wt.%) were synthesized by a solution chemical reduction method. Afterwards, Mg₉₅–Pd_m/MWCNTs_5−m (m = 0, 1, 2, 3, 4, 5) were prepared for the first time by hydriding combustion synthesis (HCS) and mechanical milling (MM). It is determined by X-ray diffraction (XRD) analysis that Pd/MWCNTs can significantly increase the hydrogenation degree of magnesium during the HCS process. The microstructures of the composites obtained by transmission electron microscope (TEM) and field emission scanning electronic microscopy (FESEM) analyses show that Pd nanoparticles are well supported on the surface of carbon nanotubes and the Pd/MWCNTs are dispersed uniformly on the surface of MgH₂ particles. Moreover, it is revealed that there is a synergistic effect of MWCNTs and Pd on the hydrogen storage properties of the composites. The Mg₉₅–Pd₃/MWCNTs₂ shows the optimal hydriding/dehydriding properties, requiring only 100 s to reach its saturated hydrogen absorption capacity of 6.67 wt.% at 473 K, and desorbing 6.66 wt.% hydrogen within 1200 s at 573 K. Additionally, the dehydrogenation activation energy of MgH₂ in this system is decreased to 78.6 kJ/mol H₂, much lower than that of as-received MgH₂.     

Accelerating hydrogen absorption in a metal hydride storage tank by physical mixing

A novel method to improve the hydrogen absorption rate in a metal hydride tank is proposed by introducing physical mixing of the metal hydride powder to promote heat removal and accelerate the kinetics of the hydriding process. Experiments were conducted with and without mixing to demonstrate that the hydrogen absorption rate can be improved significantly by mixing. Mixing was achieved by tilting the cylindrical metal hydride tank back and forth by 90° during charging. A mathematical model was also developed to simulate the effects of physical mixing. The model results indicate that physical mixing enhances heat transfer by redistributing the hydride powder from the hot core to the boundary and facilitates heat removal by convection at the tank walls. After validating the model against experimental results, the effect of physical mixing on accelerating hydrogen storage was explored by changing the mixing rate and the convection coefficient at the tank wall, and by increasing the thermal conductivity of the hydride bed by adding aluminum foam. It was found that while higher mixing rates generally improve the absorption rate, the benefits of mixing are reduced for higher convection coefficients, and for higher weight fractions of Al foam. Simulations were also conducted with and without mixing as a function of tank size. The results show that the benefit of physical mixing increases with tank size.     

Recent progress in hydrogen storage alloys for nickel/metal hydride secondary batteries

This paper reviews the development of hydrogen storage alloys prepared by an effective method of mechanical alloying and milling. It emphasizes alloys based on Mg or that contain Mg due to their low cost, low weight and high hydrogen storage capacity. Hydrogen absorption/desorption and electrochemical measurements are briefly discussed. The electrochemical properties of the alloys that contain Mg are covered in detail, emphasizing the effects of changes in alloy composition. The system of Ti–Ni-based alloys is also introduced. At present, composite hydrogen storage alloys may be the most effective materials for practical application in new nickel/metal hydride secondary batteries. The steps of hydrogen absorption/desorption such as charge-transfer and hydrogen diffusion for evaluating the electrochemical properties of hydrogen storage alloys are discussed. The relationship between alloy composition and electrochemical properties is noted and evaluated.     

Development of a hydrogen catalytic heater for heating metal hydride hydrogen storage systems

This paper describes the design, fabrication and performance evaluation of a high efficiency, compact heater that uses the catalytic oxidation of hydrogen to provide heat to a hydrogen storage system. The heater was designed to transfer up to 30 kW of heat from the catalytic reaction to the hydrogen storage system via a recirculating heat transfer fluid.