The consequence of silicon additive in isothermal decomposition of hydrides LiH,NaH, CaH2 and TiH2

The study focuses on hydrogen desorption characteristics of lithium hydride (LiH), sodium hydride (NaH), calcium hydride (CaH₂), and titanium hydride (TiH₂) which permits a possible path regarding challenging goals of US DOE standards. This research reported the consequences of enrichment in hydrogen uptake 7.02 wt% in lithium hydride, 7.71 wt% in sodium hydride, 5.91 wt% in calcium hydride, and 5.00 wt% in titanium hydride using silicon as an additive when evaluated with the help of volumetric technique. The ball milling process with silicon additive for LiH, NaH, CaH₂, and TiH₂ shows depletion in dehydrogenation temperatures 523 K, 453 K, 488 K, and 463 K respectively. Similarly, the reduction in the activation energies reported due to ball milling process with silicon additive are 37 kJ/mol for lithium hydride, 42 kJ/mol for sodium hydride, 56 kJ/mol for calcium hydride and 45 kJ/mol for titanium hydride compared with crystalline powder samples of the respective materials. The outcome of Fourier-transform infrared spectroscopy of milled hydride samples after decomposition intimate rapid decrease in transmittance intensities due to hydrogen release because of the destabilization effect caused by silicon additive. The porosity and sponginess in high-resolution Transmission electron microscopy images after dehydrogenation reveals the hydrogen desorption from the sample materials.     

Heat transfer characteristics of the metal hydride vessel based on the plate-fin type heat exchanger

Heat transfer characteristics of the metal hydride vessel based on the plate-fin type heat exchanger were investigated. Metal hydride beds were filled with AB₂ type hydrogen-storage alloy’s particles, Ti_0.42Zr_0.58Cr_0.78Fe_0.57Ni_0.2Mn_0.39Cu_0.03, with a storage capacity of 0.92 wt.%. Heat transfer model in the metal hydride bed based on the heat transfer mechanism for packed bed proposed by Kunii and co-workers is presented. The time-dependent hydrogen absorption/desorption rate and pressure in the metal hydride vessel calculated by the model were compared with the experimental results. During the hydriding, calculated hydrogen absorption rates agreed with measured ones. Calculated thermal equilibrium hydrogen pressures were slightly lower than the measured hydrogen pressures at the inlet of metal hydride vessel. Taking account of the pressure gradient between the inlet of metal hydride vessel and the metal hydride bed, it is considered that this discrepancy is reasonable. During the dehydriding, there were big differences between the calculated hydrogen desorption rates and measured ones. As calculated hydrogen desorption rates were lower than measured ones, there were big differences between the calculated thermal equilibrium hydrogen pressures and the measured hydrogen pressures at the inlet of metal hydride vessel. It is considered that those differences are due to the differences of the heat transfer characteristics such as thermal conductivity of metal hydride particles and porosity between the assumed and actual ones. It is important to obtain the heat transfer characteristics such as thermal conductivity of metal hydride particles and porosity both during the hydriding and dehydriding to design a metal hydride vessel.     

Performance of high power metal hydride reactors

Metal hydride reactors were built with porous powder metal hydride (PMH) compacts. An improved reactor built with copper coated PMH compacts of LaNi₅ with a 1.27 cm diameter produced a nominal specific cooling power of 1.5 kW/kg hydride. A similar reactor, built with copper coated PMH compacts of Ca_0.4Mm_0.6Ni₅, showed 2.2 kW/kg hydride. Results with copper coated PMH compacts showed improved thermal conductivity. The compacts are structurally strong and prevent migration of fine metal hydride particles. Life-cycle tests were performed on the reactor with LaNi₅ for over 3000 cycles and the cooling power of the reactor gradually decreased by approximately 55%.     

Thermodynamics,kinetics and microstructural evolution of Ti0.43Zr0.07Cr0.25V0.25 alloy upon hydrogenation

Zr substituted Ti₂CrV alloy with Ti_0.43Zr_0.07Cr_0.25V_0.25 composition was synthesized by arc melting method and its crystal structure, microstructure and hydrogen storage performance were investigated. XRD and microstructural analyses confirmed that the alloy forms Laves phase related BCC solid solution. The enthalpy of hydride formation as derived from pressure composition absorption isotherms is ?56.33 kJ/mol H₂. The desorption temperature of the hydride is significantly lower (by ～50 K) than that of Ti₂CrV hydride indicating lower thermal stability of the hydride compared to its unsubstituted analogue. The alloy shows better cyclic stability over the unsubstituted one. This work also offers mechanistic insight into hydrogen absorption reaction of Ti_0.43Zr_0.07Cr_0.25V_0.25 alloy by analyzing the hydriding kinetics data with standard kinetic models. The rate-determining steps of hydrogen absorption reaction were identified as random nucleation and growth of hydride followed by 1D and 3D diffusion of hydrogen atoms through the hydride layer. The present study is expected to provide valuable information for the better development of Ti–Cr–V based hydrogen storage alloys.     

Mg2(Fe,Cr, Ni)HX complex hydride synthesis from austenitic stainless steel and magnesium hydride

Mg₂(Fe, Cr, Ni)H_X synthesis was successfully performed using magnesium hydride and 316L austenitic stainless steel. It was proven that despite being widely used and considered inert, under some conditions, this steel could react with magnesium hydride and be used as a reaction substrate for complex hydride synthesis. The properties of the product are different from those of pure Mg₂FeH_6, which suggests the partial substitution of iron atoms with chromium and nickel. The influence of milling time on hydrogen content and phase composition was studied and compared to the reference material. It was found to be likely that mechanically induced martensite formed in austenitic stainless steel during ball milling may enable the reaction between steel and magnesium hydride.     

Rapid activation of MmNi5−xMx based MH alloy through Pd nanoparticle impregnation

Faster activation of a multi-component AB₅ based alloy metal hydride electrode through Pd nanoparticle (NP) impregnation is demonstrated. Pd nanoparticle impregnated MmNi_5−xM_x based alloy was prepared and characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and elemental mapping techniques. Electro-catalytic activity of laminar metal hydride electrodes containing Pd nanoparticles and micrometer size Ni particles was studied. Hydrogen absorption efficiency of the nanocomposite electrodes was compared with the metal hydride electrodes without Pd nanoparticles. The incorporation of nanostructured materials in the metal hydride alloy increased its hydrogen absorption capacity at the initial stage and activated much faster, indicating its good prospect for energy storage applications.     

Performance analysis of a compressor driven metal hydride cooling system

A compressor-driven metal hydride cooling system is analyzed in terms of its energetic and exergetic efficiencies. Applying the first and second laws of thermodynamics, the system COP, contribution of individual irreversibilities and the second law efficiency of the system are obtained. Effects of important design and operating parameters on system performance are presented. Performance comparison is made between systems working with established metal hydrides (MmNi_4.5Al_0.5 and Fe_0.85Mn_0.15Ti) and an assumed metal hydride of high hydrogen storage capacity. Studies reveal that the system performance depends mainly on the temperature drops at the high- and low-temperature reactors and compressor efficiency. It is observed that when the temperature drops are high, there is no significant difference between the well-established hydrides and the assumed hydride. It is observed that in order to compete with commercially available vapour compression refrigeration systems in terms of performance, the metal hydride reactors need to be designed for minimum temperature difference between the external medium and the hydride beds.     

Theoretical prediction of structure,electronic and optical properties of VH2 hydrogen storage material

The vanadium hydrides have better hydrogen storage capacity in comparison to the other metal hydrides. Although the structure of VH₂ hydride has been reported, the structural stability, electronic and optical properties of VH₂ hydride are unclear. To solve these problems, we apply the first-principles method to study the structural stability, electronic and optical properties of VH₂ hydrides. Similar to the metal dihydrides, four possible VH₂ hydrides such as the cubic (Fm-3m), tetragonal (I4/mmm), tetragonal (P4₂/mnm) and orthorhombic (Pnma) are designed. The result shows that the cubic VH₂ hydride is a thermodynamic and dynamical stability. In particular, the tetragonal (I4/mmm) and the orthorhombic (Pnma) VH₂ hydrides are firstly predicted. It is found that these VH₂ hydrides show metallic behavior. The electronic interaction of V (d-state)-H (s-state) is beneficial to improve the hydrogen storage in VH₂ hydride. In addition, the formation of V–H bond can improve the structural stability of VH₂ hydride. Based on the analysis of optical properties, it is found that all VH₂ hydrides show the ultraviolet response. Compared to the tetragonal and orthorhombic VH₂ hydrides, the cubic VH₂ hydride has better storage optical properties. Therefore, we believe that the VH₂ hydride is a promising hydrogen storage material.     

Imaging the hydrogenation of Mg thin films

Among the metal hydride materials, magnesium (Mg) and its alloys show excellent performance for hydrogen storage. The main drawback is the slow hydrogen absorption and desorption kinetics, the sole barrier to commercial adoption. In this work we use Mg thin films as model materials in order to study these kinetics, and observe the growth process of the hydride. Palladium (Pd) is used as a catalyst coating for improving the conditions of hydrogenation. The hydride formation is followed by in-situ X-ray diffraction. Microscopic imaging of the co-existence of Mg and MgH₂ is presented. The microstructure change is clearly visible in the micrographs, despite the fact that sample preparation damages the hydride phase. The transformation from columnar grains of the as-deposited Mg thin film, to a grainy equi-axed structure film indicate that the hydride is observed. The hydride is immediately formed at the interface between the Pd and the Mg thin film and grows in a layer-like reaction towards the substrate (SiO₂). These combined techniques provide an efficient methodology to follow the kinetics of hydride formation within the layer, and study further the diffusion coefficients and mechanism of hydrogenation.     

High temperature metal hydrides for energy systems Part B: Comparison between high and low temperature metal hydride reservoirs

A dynamic analysis model aimed at describing the hydrogen absorption and desorption phases of a metal hydride has been calibrated for magnesium hydride in Part A of the present work. We can make use of the estimate of the main kinetic parameters associated to this kind of hydride in order to study the behaviour of a metal hydride-based energy system.A metal hydride becomes the basis of an energy system when the enthalpy related to its hydriding/dehydriding reactions is used by an applicator. Therefore, magnesium hydride, which is a high-temperature hydride, can be virtually placed in an energy system thanks to the model and its main energy-related characteristics can be calculated. This allows us to get a glimpse of the performances of magnesium hydride in the field of energy production and to compare them to those of well-established low-temperature hydrides, such as LaNi₅ hydride.     

An investigation on the thermodynamics and kinetics of magnesium hydride decomposition based on isotope effects

This work investigates the thermodynamics and kinetics of magnesium hydride decomposition by analyzing isotope effects in hydride and deuteride samples. Complete pressure composition desorption isotherm measurements of MgD₂ are reported for the first time. Deuterium desorption enthalpy and entropy obtained from the van’t Hoff plot of the middle plateau fugacities are 73.8 ± 0.4 kJ/mol and 135.5 ± 0.6 J/mol K, respectively, which are in good accordance with the values obtained more than fifty years ago from plateau pressure measurements. This result reveals that the enthalpy of desorption of MgD₂ is slightly lower than that of MgH₂, whereas the entropy change is higher for the deuteride than for the hydride. Although the differences in the enthalpy and entropy of both isotopes are weak, the synergy of both effects is capable of explaining the higher equilibrium pressures for the deuteride than for the hydride.On the other hand, kinetics of magnesium hydride decomposition has been investigated by simultaneous H and D desorption experiments from mixed hydride-deuteride samples. The obtained results reveal that that decomposition is controlled by the nucleation and growth of the Mg phase. Because this reaction step is not affected by the isotopic replacement of H for D no isotope effect is observed in the kinetics of magnesium hydride decomposition. On the contrary, a marked isotope effect is observed in the kinetics of H₂(D₂) absorption by magnesium. In this case, the lighter isotope shows faster kinetics than the heavier one, what has been related to the fact that absorption is rate limited by H(D) diffusion through the hydride(deuteride) phase.     

Simulation of double-stage double-effect metal hydride heat pump

This paper presents the operational characteristics of a double-stage double-effect metal hydride heat pump (DSDE-MHHP) working with LaNi_4.1Al_0.52Mn_0.38/LmNi_4.91Sn_0.15/Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 as high/medium/low temperature alloys. The performances of the DSDE-MHHP are predicted by solving the transient, two-dimensional, conjugate heat and mass transfer characteristics between the paired metal hydride reactors of cylindrical configuration using the finite volume method (FVM). The designed operating temperatures chosen for the present analysis are 568, 361, 296, and 289 K as heat driven (T_D), heat rejection (T_H), heat sink (T_M) and refrigeration (T_C) temperatures, respectively. The variations in hydrogen concentrations, hydride equilibrium pressures, and temperatures within the hydride beds, and the heat exchange between the hydride beds with the heat transfer fluids are presented for a complete cycle. The operating cycle of a DSDE-MHHP is explained on dynamic pressure–concentration–temperature (PCT) plot. The variation of temperatures in the reactors during hydriding and dehydriding processes is compared with experimental data and a good agreement was observed between them. For given operating temperatures of 568/361/296/289 K, the average coefficient of performance (COP) and the specific cooling power (SCP) of the system are found to be 0.471 and 28.4 W/kg of total hydride mass, respectively.     

Experimental investigations and a simple balance model of a metal hydride reactor

The metal hydride reactor filled with 5 kg of the AB₅-type (LaFe_0.5Mn_0.3Ni_4.8) alloy was investigated with respect to the hydrogen discharge rates classified using C-rate value, which is discharge of the maximum hydrogen capacity 750 st L within 1 h. The reactor cannot be fully discharged with a constant flow rate, for each temperature of hot water and flow rate there exists a moment of crisis at which the hydrogen flow drops under the constant value. The nominal capacity of the reactor reaches 80% of maximum capacity if sufficient heat transfer is provided. The simple balance model of a metal hydride reactor is developed based on the assumption of uniform temperature and pressure inside a metal hydride bed. The model permits to predict behavior of the metal hydride reactor in different operation regimes, quantitative agreement is obtained for low C-rates (less than 4) and sub-critical modes.     

The structures of hydride phases in the Ti3Al/H system

The large strength-to-weight ratio of titanium makes this element animportant structural component in the aerospace industry. The strength and other mechanicalproperties may be further improved by alloying with aluminum to form aluminides, e.g. Ti₃Al and TiAl, but at a large loss in ductility. Exposure of the aluminides to hydrogen cancause profound reduction in mechanical stabilities through hydride formation. This paper focuseson the structures of hydride phases in the Ti₃Al/H system as determined by neutrondiffraction. Another reason for our interest in the aluminides is their possible role as dopants orcatalysts in certain complex hydride reactions. A recent report suggests that Ti-doped alkali metalaluminum hydrides may be potentially novel hydrogen storage materials and while the nature ofthe dopant or catalyst is not known, a titanium aluminide or hydride may be involved.     

Metal hydride hydrogen storage and compression systems for energy storage technologies

Along with a brief overview of literature data on energy storage technologies utilising hydrogen and metal hydrides, this article presents results of the related R&D activities carried out by the authors. The focus is put on proper selection of metal hydride materials on the basis of AB₅- and AB₂-type intermetallic compounds for hydrogen storage and compression applications, based on the analysis of PCT properties of the materials in systems with H₂ gas. The article also presents features of integrated energy storage systems utilising metal hydride hydrogen storage and compression, as well as their metal hydride based components developed at IPCP and HySA Systems.     

Thermal Modeling of Mg2Ni-Based Solid-State Hydrogen Storage Reactor

In this paper, a numerical study of coupled heat and hydrogen transfer characteristics in an annular cylindrical hydrogen storage reactor filled with Mg₂Ni is presented. An unsteady, two-dimensional (2-D) mathematical model of a metal hydride reaction bed of cylindrical configuration is developed for predicting the hydrogen storage capacity. The effect of volumetric radiation is accounted in the thermal model. Effects of hydride bed thickness, initial absorption temperature, hydride bed thermal conductivity, and hydrogen supply pressure on the hydrogen storage capacity are studied. A thinner hydride bed is found to enhance the hydriding rate, accomplishing a rapid reaction. At an operating condition of 20 bar supply pressure and 573 K initial absorption temperature, Mg₂Ni stores about 36.7 g hydrogen per kg alloy. For a given bed thickness and an overall heat transfer coefficient, there exists an optimum value of hydride bed thermal conductivity. The present numerical results are compared with the experimental data reported in the literature, and good agreement was observed.     

Interaction in the Ce3Co8Si–H2 system

Interaction of hydrogen with Ce₃Co₈Si intermetallic compound (IMC) has been studied. IMC Ce₃Co₈Si absorbs hydrogen and forms a hydride phase at 11 atm and 50 °C. X-ray analysis of Ce₃Co₈Si H_10.2 saturated hydride phase lattice showed that it has the symmetry of the initial compound and is expanded with strong anisotropy due to increased c parameter. Analysis of hydrogen desorption isotherms in Ce₃Co₈Si–H₂ system has revealed that the decomposition of hydride phase occurred in one stage. The heat of hydride phase formation was calculated on the base of obtained equilibrium pressures data at 50, 60 and 70 °C. The results obtained demonstrate that Ce₃Co₈Si intermetallic compound may be used as reversible accumulator of hydrogen in medium temperatures interval.     

Study on absorption/desorption characteristics of a metal hydride tank for boil-off gas from liquid hydrogen

We have been performing research on the Totalized Hydrogen Energy Utilization System (THEUS) which has applications to commercial buildings and a planned added function of supplying energy to stations for hydrogen and electric vehicles. In that case we will utilize liquid hydrogen transported from a hydrogen station and all Boil-Off Gas (BOG) will be recovered in THEUS’s metal hydride tanks. It is known that BOG is chiefly composed of para-hydrogen, which has different thermo-physical properties from normal hydrogen. It has been reported that some metal hydride alloys work as a catalyst to accelerate the para-ortho conversion and the conversion proceeds relatively fast in the case of La–Ni₅. The conversion is considered to be an endothermic reaction. A misch metal (Mm)-Ni₅ metal hydride alloy, which contained La and Ni, was used in our THEUS metal hydride tank. To examine the effect of the para-ortho conversion on the THEUS operation, we investigated the absorption/desorption characteristics of the metal hydride tank with BOG. We confirmed that the effect of the heat of conversion was very small and BOG could be treated as normal hydrogen for practical application.     

Electronic and bonding properties of MgH2–Nb containing vacancies

The magnesium hydride stability and bonding have been studied using density functional theory (DFT). To this aim, calculations on the electronic structure were performed. We also modeled the bulk hydride with a Nb atom as a substitutional impurity. Furthermore, both systems were modeled containing different types of vacancies (Mg, H or H–Mg complex). The crystal orbital overlap population for both the metal–metal and metal–hydrogen bonds was also computed. The influence of vacancy-like defects was studied through the calculation of the positron lifetimes in defected MgH₂ and defected MgH₂–Nb. For the pure hydride, the results show an increment in the atom bonds in correlation with an increase of the positron localization reflected in a rise of the positron lifetimes. On the other hand, in all considered cases for Mg or/and H vacancies, the presence of Nb reduces the hydride bond about 36%. This decrease in the hydride stability was associated with a decrease in the probability of the positron localization and a consequently reduction of the positron lifetimes.     

A thermally coupled metal hydride hydrogen storage and fuel cell system

This paper examines the ability of metal hydride storage systems to supply hydrogen to a fuel cell with a time varying demand, when the metal hydride tanks are thermally coupled to the fuel cell. A two-dimensional mathematical model is utilized to compare different heat transfer enhancements and storage tank configurations. The scenario investigated involves two metal hydride tanks containing the alloy Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5, located in the air exhaust stream of a fuel cell. Three cases are simulated: a base case with no heat transfer enhancements, a case with external fins attached to the outside of the tank, and a case where an annular tank design is used. For the imposed duty cycle, the base case is insufficient to provide the hydrogen demands of the system, while both the finned and annular cases are able to meet the demands. The finned case yields higher pressures and occupies more space, while the annular case yields acceptable pressures and requires less space. Furthermore, the annular metal hydride tank meets the requirements of the fuel cell while providing a more robust and compact hydrogen storage system.