Adjustment of the decomposition path for Na2LiAlH6 by TiF3 addition

The decomposition of Na₂LiAlH₆ is studied by in-situ synchrotron diffraction. By addition of TiF₃ and dehydrogenation-rehydrogenation cycling of the samples new decomposition paths are found. Na₃AlH₆ is formed on decomposition in the presence of TiF₃. The additive brings the system closer to equilibrium, and decomposition through Na₃AlH₆ is demonstrated for the first time. The results are in agreement with previously published computational data. For a cycled sample with 10 mol% TiF₃ Na₂LiAlH₆ decomposes fully into Na₃AlH₆ before further decomposition to NaH and Al. This shows clear changes in the kinetics of the system, and may open possibilities of tailoring the decomposition path by the use of additives.     

Effect of admixing different carbon structural variants on the decomposition and hydrogen sorption kinetics of magnesium hydride

The effect of admixing catalysts comprised of carbon nanostructures, specifically planar, helical and twisted carbon nanofibers, spherical carbon particles and multi-walled carbon nanotubes, on the hydrogen storage properties of magnesium hydride has been investigated. Optimum results were achieved with the mixture containing twisted carbon nanofibers (TCNF) synthesized by Ni catalyst derived by oxidative dissociation of catalyst precursor LaNi₅. The desorption temperature of 2 wt.% TCNF admixed MgH₂ is ∼65 K lower than that of pristine MgH₂ milled for the same duration. The enhancement in hydrogen absorption capacity of MgH₂ admixed with 2 wt.% TCNF has been found to be two-fold in the first 10 minutes at 573 K and under a hydrogen pressure of 2 MPa, i.e. 4.8wt% as compared to 2.5 wt% for MgH₂ alone. The increase in capacity by a factor of about two within the first 10 minutes as a result of the catalytic activity of TCNF is one of the exciting results obtained for hydrogen absorption in catalyzed MgH₂.     

Effect of different sized CeO2 nano particles on decomposition and hydrogen absorption kinetics of magnesium hydride

In present paper, different sizes of CeO₂ nanoparticles were synthesized by ball milling and their effect on the absorption kinetics and decomposition temperature of MgH₂ was studied. It was found that a small amount of admixing of the above said catalysts with MgH₂ exhibits improved hydrogen storage properties. Among these different sizes of CeO₂ nanoparticles, 2 weight % admixed CeO₂ with a particle size of ∼10–15 nm led to decrease in desorption temperature by ∼50 K. Moreover, it also shows 1.5 times better absorption kinetics with respect to pure MgH₂. The samples were characterized using SEM, TEM and XRD techniques. The hydrogenation/dehydrogenation properties were measured by gas reaction controller.     

Interaction of zirconia with magnesium hydride and its influence on the hydrogen storage behavior of magnesium hydride

This study demonstrates how zirconia additive transforms to zirconium hydride and substantially lowers the dehydrogenation temperature of magnesium hydride. We prepared MgH₂+xZrO₂ (x = 0.125 and 0.5) powder samples reacted for 15 min, 1 h, 5 h, 10 h, 15 h, 20 h and 25 h, and monitored the phase changes at each stage of the reaction. Differential scanning calorimetry (DSC) study provides the first crucial evidence regarding the chemical transformation of zirconia. Subsequently, detailed additional sample testing by X-ray diffraction (XRD), energy dispersive x-ray spectroscopy and confocal Raman microscopy provide strong supports that low temperature dehydrogenation of magnesium hydride is a result of formation of an active in situ product (zirconium hydride). This observation is validated by the negative Gibbs free energy values obtained for the formation of zirconium hydride over a broad working temperature range of 0–600 °C. Scanning electron microscopy (SEM) results prove the high dispersion of tiny nanoparticles all across the surface after the chemical interaction between MgH₂ and ZrO₂ and atomic force microscopy (AFM) study further proves that objects with grain sizes of ～10 nm are abundant throughout the scanned surfaces. These observations reiterate that better metal oxide additives interact with MgH₂ and results to the evolution of highly active insitu nanocatalysts.     

Study on hydrogen storage properties of LaNi3.8Al1.2−xMnx alloys

Effects of the Mn substitution on microstructures and hydrogen absorption/desorption properties of LaNi_3.8Al_1.2−xMn_x (x = 0.2, 0.4, 0.6) hydrogen storage alloys were investigated. The pressure-composition (PC) isotherms and absorption kinetics were measured in a temperature range of 433 K ≤ T ≤ 473 K by the volumetric method. XRD analyses showed that with the increase of the Mn content in the LaNi_3.8Al_1.2−xMn_x alloys, the lattice parameter a was decreased, c increased and the unit cell volume V reduced. It was found that the absorption/desorption plateau pressure was increased and the hydrogen storage capacity was enhanced with the increase of Mn content. The absorption/desorption plateau pressure of the alloys was linearly changed with the Mn content x and the lattice parameter a, while the hydrogen storage capacity was linearly increased with the increase of c/a ratio. It was also found that the slope factor S_f was closely correlated with the lattice strain of the alloys.     

Investigations on kinetics properties of hydrogen adsorbing/desorbing reactions for metal hydride electrodes

As we know, the kinetics properties of hydrogen adsorbing/desorbing reactions for metal hydride electrodes are determined by the rates of the charge transfer, hydrogen transfer and hydrogen diffusion reactions. In previous studies, not only the hydrogen transfer process was always ignored, but also the values of the kinetics parameters for the charge transfer and hydrogen diffusion processes were quite different. Therefore, the purpose of this work is to investigate the key issues of the kinetics properties for hydrogen adsorbing/desorbing reactions. Firstly, the hydrogen transfer process was thoroughly studied by electrochemical impedance spectrum (EIS) method, in which it emphasized the corresponding relationship between the electrode process of hydrogen adsorbing/desorbing reactions and the time constants presented in different frequency regions of impedance spectrum. The values of the hydrogen transfer resistance were calculated as a function of depth of discharge (DOD). Meanwhile, almost all the electrochemical techniques including linear polarization curves (LP), constant potential step method (CPSM), galvanostatic intermittent titration technique (GITT), cyclic voltammetry method (CV) and EIS were used to measure the kinetics parameters for the charge transfer and hydrogen diffusion processes. Moreover, the factors causing the discrepancy of the kinetics parameters were analyzed in detail.     

Substitution of nickel in Mg2Ni and its hydride with elements from groups XIII and XIV: An ab initio study

We have performed ab initio calculations with equilibrium supercells of the Mg₂Ni compound and its hydride Mg₂NiH₄ doped with elements X = Al, Ga, In, Si, Ge and Sn. Two concentrations of X in both structures have been set: (1) every 16th, and (2) every fourth Ni atom has been substituted by X. Total energy calculations yielded the Mg₂NiH₄ hydrogen absorption enthalpy ΔH_abs according to the chemical reaction Mg₂Ni + 2H₂ → Mg₂NiH₄. Reduction of the hydrogen absorption enthalpy was reported for both concentrations of X. When doping the Mg₂NiH₄ hydride with X = In in a low concentration (1), the value of hydrogen desorption enthalpy decreases from 68.22 to 55.96 kJ(mol H₂)^?1. Doping with X = In in a high concentration (2) further decreases the hydrogen desorption enthalpy to 5.50 kJ(mol H₂)^?1. Further, the electronic structure of Mg₂(Ni–In)H₄ hydride with a low In concentration indicates weaker Ni–H bonds in comparison with the pristine Mg₂NiH₄. Attraction between H and In atoms induced enhanced bonding between Mg and H atoms compared to the pristine Mg₂NiH₄.     

Influence of intrinsic hydrogenation/dehydrogenation kinetics on the dynamic behaviour of metal hydrides: A semi-empirical model and its verification

Metal hydrides can store hydrogen at low pressures and with high volumetric capacity. For the possible application as storage medium in hydrogen stand-alone power systems, large metal hydride hydrogen storage units are usually required. A reliable and verified kinetic correlation is an important tool in the designing process of a larger storage unit. This paper describes kinetic investigation of a AB₅-type alloy and its corresponding hydride, with the purpose of finding a semi-empirical correlation suitable for use in heat and mass transfer modelling and engineering design of metal hydride storage units.     

Ab initio and thermodynamic investigation on the Ca–H system

The thermodynamic properties of the Ca–H system are evaluated by combining the CALPHAD approach with ab initio predictions. The Gibbs free energies of the individual phases are thermodynamically modeled, where the model parameters are obtained from best-fit optimizations to combined experimental data and ab initio thermodynamic predictions. The ab initio thermodynamic predictions are based upon density functional theory ground state minimizations and finite displacement lattice dynamics. The predictions are proved effective in the assessments whenever experimental measurements are lacking or not feasible. It is demonstrated that the obtained phase equilibria and thermodynamic properties have shown satisfactory agreement with the experimental data in the literature as well as the ab initio calculations from the present work.     

The effects of halide modifiers on the sorption kinetics of the Li-Mg-N-H System

The effects of different transition metal halides (TiCl₃, VCl₃, ScCl₃ and NiCl₂) on the sorption properties of the 1:1 molar ratio of LiNH₂ to MgH₂ are investigated. The modified mixtures were found to contain LiNH₂, MgH₂ and LiCl. TGA results showed that the hydrogen desorption temperature was reduced with the modifier addition in this order: TiCl₃ > ScCl₃ > VCl₃ > NiCl₂. Ammonia release was not significantly reduced resulting in a weight loss greater than the theoretical hydrogen storage capacity of the material. The isothermal sorption kinetics of the modified systems showed little improvement after the first dehydrogenation cycle over the unmodified system but showed drastic improvement in rehydrogenation cycles. X-ray diffraction and Raman spectroscopy identified the cycled material to be composed of LiH, MgH₂, Mg(NH₂)₂ and Mg₃N₂.     

Effect of non-stoichiometry on hydrogen storage properties of La(Ni3.8Al1.0Mn0.2)x alloys

The La(Ni_3.8Al_1.0Mn_0.2)_x (x = 0.94, 0.96, 0.98, 1.0) hydrogen storage alloys have been investigated to examine the effect of non-stoichiometry on the crystal structure, activation performance, hydrogen absorption/desorption properties and cycle life. It was found that for the stoichiometric compound, only single phase with CaCu₅ type structure exists. However, for B-poor compounds of AB₅ alloys, there is a principal CaCu₅ type phase with a small amount of second phase and the amount of second phase increased with decreasing x when x ≥ 0.96 and reached a maximum when x = 0.96. The activation becomes harder with decreasing x until x = 0.96 and easier when x decreased to 0.94. The plateau pressure increased and the hydrogen uptake capacity decreased with decreasing x when x ≥ 0.96, and then decreased and increased, respectively, when x further decreased to 0.94. Both the change in the lattice strain which could be estimated by FWHM (full width at half maximum) and the degree of slope factor S_f in the alloys show the same trend with the change of x, exhibiting a maximum at x = 0.96. The ΔH decreased with decreasing x when x ≥ 0.96 and then increased when x = 0.94 and it was found that the larger the cell volume, the larger the absolute value of the enthalpy. The pulverization resistance of the alloys was greatly improved by the non-stoichiometric. The kinetics of the alloys was very fast and almost not influenced by the change of non-stoichiometric x. After 300 absorption/desorption cycles, the hydrogen uptake capacity of the stoichiometric and non-stoichiometric alloys almost kept the same, but the particle size decreased greatly.     

Decomposition of lithium magnesium aluminum hydride

The quaternary aluminum hydride LiMg(AlH₄)₃ contains 9.7 wt% hydrogen, of which 7.2 wt% can be released in a two-step decomposition reaction via first formation of LiMgAlH₆ and then the binary hydrides MgH₂ and LiH. In-situ synchrotron radiation powder X-ray diffraction and thermal desorption spectroscopy measurements were performed to analyze the product distributions formed during the thermal decomposition of LiMg(AlD₄)₃. The first decomposition step occurs at about 120 °C and the second at about 160 °C for the as-milled sample, while for a purified sample of LiMg(AlD₄)₃, the decomposition temperatures involving release of hydrogen increase to 140 and 190 °C, respectively, suggesting that pure samples of LiMg(AlD₄)₃ are kinetically stabilized. Studies of the purified LiMg(AlD₄)₃ also showed that the second decomposition step can be divided into two reactions: 3LiMgAlD₆ → Li₃AlD₆ + 3MgD₂ + 2Al + 3D₂ and Li₃AlD₆ → 3LiD + Al + 3/2D₂. Addition of TiCl₃ to LiMg(AlD₄)₃ under a variety of ball milling conditions consistently led to decomposition of LiMg(AlD₄)₃ during milling. Correspondingly, all attempts to rehydrogenate the (completely or partially) decomposed samples at up to 200 bar hydrogen pressure failed. Decomposition of MgD₂ was observed at relatively low temperatures. This is ascribed to thermodynamic destabilization due to the formation of different Al_xMg_y phases, and to kinetic destabilization by addition of TiCl₃. A thermodynamic assessment was established for the calculation of phase stability and decomposition reaction relationships within the Li-Mg-Al−H system. The calculations confirmed the metastability of the LiMg(AlH₄)₃ phase and the irreversibility of the Li-Mg alanate phase decomposition reactions. The Li-Mg alanate decomposition pathways followed experimentally could be explained by the endothermicity of the calculated decomposition enthalpies, in that an impure or catalyzed LiMgAlH₆ intermediate phase could more directly access an endothermic decomposition reaction at lower temperatures, while a kinetically-hindered, purified LiMgAlH₆ would require higher temperatures to initiate the two-step decomposition through an exothermic reaction.     

Improvement in the electrochemical properties of gas atomized AB2 metal hydride alloys by hydrogen annealing

The effects of high temperature hydrogen annealing were studied on powders made by gas atomization of both conventional vanadium-containing AB₂ metal hydride alloys and new vanadium-free AB₂ alloys designed for high power and low self-discharge applications. In both alloy systems, annealing in 950 °C hydrogen for 30 min was proven to be effective in improving the capacity, formation, high power, and low temperature performance in the nickel metal hydride battery compared to previous gas atomization trials where each property was reduced. The advantage in improving the cycle life by gas atomization was further extended by the hydrogen annealing process. Reduction in the surface oxide was confirmed by the use of Auger electron spectroscopy depth profiling and magnetic susceptibility. Metallic elements were reduced from the oxide state by hydrogen to react with the metallic nickel particulates originally embedded in the surface oxide in a high temperature environment and created a new surface free of oxygen.     

Effect of molybdenum content on structural, gaseous storage, and electrochemical properties of C14-predominant AB2 metal hydride alloys

The structure, gaseous storage, and electrochemical properties of Mo-modified C14-predominant AB₂ metal hydride alloys were studied. The addition of Mo expands the unit cell volume and stabilizes the metal hydride. This increased metal-to-hydrogen bond strength reduces the equilibrium plateau pressure, reversible hydrogen storage, and the high-rate dischargeability in the flooded cell configuration, but not the high-rate dischargeability in the sealed cell configuration. The low-temperature performance was improved by the addition of Mo through increases in bulk diffusion rate, surface area, and surface catalytic ability. The increase in bulk diffusion is the result of smaller crystallites and larger AB₂-AB₂ grain boundary densities. The increase in surface area is due to the high solubility of Mo in alkaline solution. Even with a higher leaching rate, the Mo-containing alloys still have strong corrosion resistance which contributes positively to both the charge retention and the cycle life performances. As the Mo-content in the alloy increases, the low temperature performance improves at the expense of a lower capacity.     

Studies of off-stoichiometric AB2 metal hydride alloy: Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two-part series of papers, gaseous hydrogen storage and electrochemical properties of three series of alloys with different combinations of Cr/Mn/Co ratios are studied and compared to the structural properties reported in Part 1. As the B/A stoichiometry in each series of alloys increases from 1.8 to 2.2, systematic trends in certain storage properties are found: the hydrogen dissociation pressure and heat of hydride formation increases; the alloy with a AB_2.0 stoichiometry has the highest electrochemical full capacity; and slightly higher and lower B-contents increase the electrochemical high-rate-dischargeability and gaseous phase maximum storage capacity, respectively. Stoichiometric or slightly hyper-stoichiometric AB₂ alloys have lower PCT hysteresis which are expected to reduce pulverization during cycling. The full and high-rate discharge electrochemical capacities correlate well with the maximum and reversible gaseous hydrogen storages, respectively. Slight hyper-stoichiometry increases the high-rate dischargeability. Open circuit voltage, an important parameter in high-power application, is also found to be more relevant to the surface reaction than to the bulk hydride stability.     

A DFT study on how vanadium affects hydrogen storage kinetics in magnesium nickel hydride

Magnesium nickel hydride (MNH), Mg₂NiH₄ is a promising material that was shown enhance the hydrogen storage performance. Also, further modification of such a material included the incorporation of V into the host under hydrogen pressure to form the VH/Mg₂NiH₄ catalyst. However, investigations on its catalytic performance are still in need. Thus, this work studied via density functional theory, the improved hydrogen storage kinetics on VH/Mg₂NiH₄(101). The molecular hydrogen desorption was found to improve on such a system. The hydrogen vacancy sites (H_V) formed at the interfacial sites. Consequently, all H and H_V diffusion pathway and TOF derived from the most favorable diffusion pathway was investigated to understand the desorption and diffusion processes at the active site. Hence, this DFT investigation can be used to guide the design of high-performance Mg-based hydrogen storage materials.     

The structure, hydrogen storage, and electrochemical properties of Fe-doped C14-predominating AB2 metal hydride alloys

A series of Fe-substituting cobalt C14-predoninating AB₂ alloys with the general formula Ti₁₂Zr_21.5V₁₀Cr_7.5Mn_8.1Fe_xCo_8−xNi_32.2Sn_0.3Al_0.4 (x = 0-5) were studied for the impacts of Fe to structure, gaseous, and electrochemical hydrogen storage properties. All alloys exhibit hyper-stoichiometric C14 main phase due to the formation of A-rich non-Laves secondary phases and the loss of Zr and Ti in the melt. Lattice parameters together with the unit cell volume increases and then decreases with increasing Fe-content which indicates the existence of anti-site defects. The amount of TiNi secondary phase increases with the increase of Fe-content up to 4% and shows a detrimental effect to the high-rate dischargeability of the alloys. Most of the gaseous storage characteristics remain unchanged with the addition of Fe. In the electrochemical properties, Fe-addition in the AB₂ alloys facilitates activation, increases the total electrochemical capacity and effective surface reaction area, decreases the half-cell high-rate dischargeability and bulk hydrogen diffusion, and deteriorates both −10 and −40 °C low-temperature performance. Fe-substituting Co in AB₂ alloys as negative electrode of nickel metal hydride battery can reduce the raw material cost with the trade-off being mainly in the low-temperature performance.     

Structures and properties of Mg–La–Ni ternary hydrogen storage alloys by microwave-assisted activation synthesis

It is a challenge to prepare a material meeting two conflicting criteria – absorbing hydrogen strongly enough to reach a stable thermodynamic state and desorbing hydrogen at moderate temperature with a fast reaction rate. With the guide of the Mg–La–Ni phase diagram, microwave sintering (MS) was successfully applied to preparing Mg–La–Ni ternary hydrogen storage alloys from the powder mixture of Mg, La and Ni. Their phase structures, morphologies and hydrogen absorption and desorption (A/D) properties have been studied by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), pressure-composition-isotherm (PCI) and differential scanning calorimetry (DSC). The metal hydride of 70 Mg–9.72 La–20.28 Ni (wt pct) has the best comprehensive hydriding and dehydriding (H/D) properties, which can absorb 4.1 wt.% H₂ in 600 s and desorb 3.9 wt.% H₂ in 1500 s at 573 K. The DSC results reveal its onset temperatures of hydrogen A/D are the lowest among all the samples, which are 671.4 and 600.9 K. Its activation energy of dehydriding reaction is 113.5 kJ/mol H₂, which is the smallest among all the samples. Also, Chou model was used to analyze the reaction kinetic mechanism.     

Design,development, construction and operation of a novel metal hydride compressor

Metal Hydride Compressors (MHC) is a promising technology for thermal compression of hydrogen. Besides the absence of a necessity for significant mechanical or electrical energy input, this type of compressor has the advantage that no moving parts are involved. A brief review on the reported experimental set ups of metal hydride compressors is carried out and compared to the metal hydride compressor developed and constructed by HYSTORE Technologies Ltd in Cyprus. The compressor built by HYSTORE consists of 6 stages using AB₂ and AB₅ – type metal hydride alloys. The MHC is operated between 10 C and 80 °C, which is a temperature range that can be supplied by solar thermal collectors. Furthermore, the experimental results showed, that even lower temperatures of 17 C are sufficient thus reducing the demand for cooling capacity. During the operation, the compressor achieved stable compression of hydrogen from 7 bar more than 220 bar. The specific productivity of the compressor achieved values up to 67.2 l_H2 kg^?1 h^?1.     

Thermodynamic simulation of hydrogen based thermochemical energy storage system

The increasing energy demand needs the attention for energy conservation as well as requires the utilisation of renewable sources. In this perspective, hydrogen provides an eco-friendly and regenerative solution toward this matter of concern. Thermochemical energy storage system working on gas-solid interaction is a useful technology for energy storage during the availability of renewable energy sources. It provides the same during unavailability of energy sources. This work presents a performance analysis of metal hydride based thermal energy storage system (MH-TES), which can transform the waste heat into useful high-grade heat output. This system opens new doors to look at renewable energy through better waste heat recovery systems. Experimentally measured PCIs of chosen metal hydride pairs, i.e. LaNi_4.6Al_0.4/La_0.9Ce_0.1Ni₅ (A-1/A-3; pair 1) and LaNi_4.7Al_0.3/La_0.9Ce_0.1Ni₅ (A-2/A-3; pair 2) are employed to estimate the thermodynamic performance of MH-TES at operating temperatures of 298 K, 373 K, 403 K and 423 K as atmospheric temperature (T_atm), waste heat input temperature (T_m), storage temperature (T_s) and upgraded/enhanced heat output temperature (T_h) respectively. It is observed that the system with alloy pair A-1/A-3 shows higher energy storage density of 121.83 kJ/kg with a higher COP of 0.48 as compared to A-2/A-3 pair. This is due to the favourable thermodynamic properties, and the pressure differential between coupled MH beds, which results in higher transferrable hydrogen. Besides, the effect of operating temperatures on COP is studied, which can help to select an optimum temperature range for a particular application.