Structure and hydrogen storage properties of mechanically alloyed Ti-V alloys

Ti-V alloys are potential candidates for hydrogen storage materials. In this study, mechanical alloying under an argon atmosphere was used to produce Ti_2?xV_x nanocrystalline alloys (x = 0.5, 0.75, 1, 1.25, 1.5). Shaker type ball mill was used. An objective of the present study was to investigate an influence of chemical composition and method of production on hydrogenation and dehydrogenation properties of Ti-V alloys. X-ray diffraction analyses revealed formation of BCC solid solution after 14 h of milling. It is the first time of obtaining this phase directly from mechanical alloying method. HRTEM images confirmed formation of nanocrystalline materials. Synthesized materials were studied by a conventional Sievert's type apparatus at 303 K. It was observed that the maximum hydrogen storage capacity is increased with increased Ti content in the alloy. Ti_1.5V_0.5 alloy showed high hydrogen storage capacity at room temperature, which reached about 3.67 wt.%. Simultaneously, it was noticed that Ti-rich alloys form more stable hydride phases than V-rich alloys. Observed properties resulted mainly from structure of studied materials.     

An open-source code to calculate pressure-composition-temperature diagrams of multicomponent alloys for hydrogen storage

Recently, multicomponent alloys have been studied for hydrogen storage because of their vast compositional field, which opened an exciting path for designing alloys with optimized properties for any specific application, in a properties-on-demand approach. Since the experimental measurements of hydrogen storage properties are very time-consuming, computational tools to assist the exploration of the endless compositional field of multicomponent alloys are needed. In a previous work reported by Zepon et al. (2021), a thermodynamic model to calculate pressure-composition-temperature (PCT) diagrams for body-centered-cubic (BCC) multicomponent alloys was proposed. In the present work, we implemented this model in an open-source code with an user-friendly interface to calculate PCT diagrams for BCC multicomponent alloys having any of the following elements: Mg, Al, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, Mo, Pd, Hf, and Ta. The open-source code aims to allow the use of the thermodynamic model for alloy design as well as to encourage other researchers to improve the inputs and the initial thermodynamic model. As an example of application of the model for alloy design, the code was employed to investigate the effect of different metals (M) on the PCT diagrams of Ti_0.3V_0.3Nb_0.3M_0.1 alloys.     

Hydrogenation properties of nanocrystalline TiVMn body-centered-cubic alloys

An effect of Mn content in nanocrystalline TiVMn alloy on hydrogenation properties has been investigated systematically in this work. Ti_0.5V_1.5-xMn_x (x = 0, 0.1, 0.2, 0.3) alloys are synthesized by mechanical alloying method. It has been shown that all alloys need to be activated in order to present the best hydrogenation properties. With increasing Mn content in TiVMn alloys, the maximum hydrogen storage capacity at room temperature firstly increases to reach 3.07 wt% for Ti_0.5V_1.4Mn_0.1 alloy and then gradually decreases. Reversibility of hydriding-dehydriding process improves after chemical modification of TiV alloy. Hydrogen storage properties result from phase composition and structure of alloys. All samples are composed of a major body-centered-cubic (BCC) phase. In Mn-containing materials also a second BCC phase has been detected. Its abundance increases with higher content of Mn in the alloy. Moreover, with increasing Mn content, the lattice parameters of both phases decrease.     

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.     

Hydrogen storage in TiZrNbFeNi high entropy alloys,designed by thermodynamic calculations

The hydrogen storage properties of the novel equiatomic TiZrNbFeNi and non-equiatomic Ti₂₀Zr₂₀Nb₅Fe₄₀Ni₁₅ high entropy alloys (HEAs) were studied. These alloys were designed with the aid of thermodynamic calculations using the CALPHAD method due to their tendency to form single C14 Laves phase, a phase desirable for room-temperature hydrogen storage. The alloys, which were synthesized by arc melting, showed a dominant presence of C14 Laves phases with the (Zr, Ti)₁(Fe, Ni, Nb, Ti)₂ constitution and small amounts of cubic phases (<1.4 wt%), in good agreement with the thermodynamic predictions. Hydrogen storage properties, examined at room temperature without any activation procedure, revealed that a maximum hydrogen storage capacity was reached for the equiatomic alloy in comparison to the non-equiatomic alloy (1.64 wt% vs 1.38 wt%) in the first cycle; however, the non-equiatomic alloy presented superior reversibility of 1.14 wt% of hydrogen. Such differences on reversibility and capacity among the two alloys were discussed based on the chemical fluctuations of hydride-forming and non-hydride-forming elements, the volume per unit cell of the C14 Laves phases and the distribution of valence electrons.     

Tuning the hydrogenation properties of Ti1+yCr2-xMnx laves phase compounds for high pressure metal-hydride compressors

Ti_1+yCr_2-xMn_x compounds with a hexagonal MgZn₂-type structure were investigated for their potential use in metal-hydride (MH) compressor applications. Here, the targeted MH compressor should absorb hydrogen below 30 MPa at 30 °C and desorb hydrogen at 80 MPa when heated at 80 °C. The pressure (P)-composition (C) isotherms were measured at pressures up to 40 MPa. Because of the very high pressures, the fugacity was taken into account. Details are given on how to determine the absorption and desorption enthalpies and entropies from the fugacity. The Van't Hoff plots were used to estimate the critical temperatures (T_C). The P–C isotherms of all the compounds exhibit relatively small hysteresis and flat plateaus but the absorption and desorption pressures need to be increased. Superstoichiometry of y = 0.05 tends to decrease the plateau pressure and increase its slope, which is not desired for our target. On the other hand, the increase of Mn content led to higher T_C as well as higher and flatter plateaus. Cr is a key element to get small hysteresis.     

Development of large MH tank system for renewable energy storage

Metal Hydrides (MH) can absorb large quantities of hydrogen at room temperature and ordinary pressure. Because MH can store hydrogen at a pressure less than 0.1 MPa safely and compactly, it is looked to as a method of storing hydrogen produced by electricity derived from renewable energy sources. To study this method of storing renewable energy, we made a MH tank system which could store hydrogen in the range of 1000 Nm³. A Mm-NiMnCo alloy was used for this MH tank system. MH becomes pulverized with absorbing and desorbing hydrogen, and this causes the problem of MH tank transformation owing to the partial distribution of the pulverized MH powders. Our MH material, named “Hydrage?,” was made using a technique to compose the MH powders with polymer materials without decreasing the hydrogen absorption and desorption rate. With this technique, the MH powders were immobilized, and strain on the MH tank was reduced. Furthermore, this technique enabled uniform dispersion of the MH powders, and high-density filling in MH tank was achieved relative to that attainable in a conventional MH tank. An MH tank system with a capacity of 1000 Nm³ is 1,800 mm in width, 3,150 mm in length, and 2,145 mm in height. The system for renewable energy storage consists of 9 tanks. About 7.2 tons of MH were used in this system. This system could work at temperatures from 25 to 35° C, and its maximum hydrogen absorption and desorption rate is 70 Nm³/h with a medium flow rate of 30 NL/min. This type of MH tank system, which can store a large amount of hydrogen safely and compactly, has the potential to become popular with various applications in the future.     

Hydrogenation properties of Mg–Al alloys

In this paper the properties of Mg–Al alloys in relation to hydrogen storage are reviewed. The main topics of this paper are materials preparation, hydrogen capacity, thermodynamics of hydride formation, and the kinetics of hydride formation and decomposition.     

Thermodynamics and reaction pathways of hydrogen sorption in Mg6(Pd,TM) (TM = Ag,Cu and Ni) pseudo-binary compounds

Mg₆(Pd,TM) (TM = Ag, Cu and Ni) pseudo-binary compounds have been synthesized at the TM solubility limit to determine the influence of TM on the thermodynamics and reaction pathways of the Mg₆Pd–H system. All compounds exhibit a two-plateau pressure behaviour, being the value of the high plateau pressure well above that of the Mg/MgH₂ system. Such destabilization is explained by the formation of different Mg–(Pd,TM) intermetallics and/or Mg₂NiH₄ hydride phases during the hydrogenation reaction. The formation of these phases not only increases the enthalpy of hydrogenation but also enhances disorder leading to a limited destabilization of the hydrogenated state. This compensation effect is characterized by a linear correlation between enthalpy and entropy terms. In addition, this work also provides the assessment at 623 K of the ternary Mg–Pd–Cu phase diagram in the Mg-rich corner.     

Compositional effects on the hydrogen cycling stability of multicomponent Ti-Mn based alloys

Intermetallic alloys such as AB, AB₂, and AB₅ type have been studied due to their capability to reversibly store hydrogen. These alloys exhibit varying hydrogen storage properties depending on the crystal structure and composition. Compositional modification is commonly known as an effective method to modify the alloys thermodynamic and kinetics for various applications such as metal hydride batteries, metal hydrides hydrogen storage and compression. However, the effects of the compositional modification on the cyclic stability of these alloys are not usually well studied.Here, the hydrogen cycling stabilities of Ti-Mn based alloys with C14 type structure are studied. Hyper-stoichiometry, stoichiometry and hypo-stoichiometry alloys were prepared accordingly: Ti_30.6V_16.4Mn_48.7 (Zr_0.7Cr_0.8Fe_2.8) (B/A = 2.19), Ti_32.8V_15.1Mn_47.1 (Zr_0.9Cr_1.2Fe_2.9) (B/A = 1.97) and Ti_34.5V_15.4Mn_44.7 (Zr_0.9Cr_1.3Fe_3.2) (B/A = 1.87). Whilst the hyper-stoichiometry alloy showed almost a stable (about 9% capacity reduction) hydrogen capacity after 1000 cycles of hydrogenation and dehydrogenation, the stoichiometry and hypo-stoichiometry alloys failed to hydrogenate after about 950 and 500 cycles respectively. A limited reduction in the calculated crystalline size of the alloys was observed before and after the hydrogen cycling, denoting that pulverisation plays a less significant role on the observed hydrogen capacity loss. In addition, a reduction in the B/A ratio from 2.19 to 1.82 (hyper to hypo-stoichiometry) encouraged the formation of more stable hydride and a higher level of heterogeneous lattice strain. Whilst a small loss of hydrogen capacity (9%) in the hyper-stoichiometry alloy was attributed to the trapped hydrogen, the complete loss of hydrogen capacity in the stoichiometry and hypo-stoichiometry alloys seemed to originate from the formation of stable hydride and the lattice distortion.     

Metal hydride hydrogen storage tank for fuel cell utility vehicles

The “low-temperature” intermetallic hydrides with hydrogen storage capacities below 2 wt% can provide compact H₂ storage simultaneously serving as a ballast. Thus, their low weight capacity, which is usually considered as a major disadvantage to their use in vehicular H₂ storage applications, is an advantage for the heavy duty utility vehicles. Here, we present new engineering solutions of a MH hydrogen storage tank for fuel cell utility vehicles which combines compactness, adjustable high weight, as well as good dynamics of hydrogen charge/discharge. The tank is an assembly of several MH cassettes each comprising several MH containers made of stainless steel tube with embedded (pressed-in) perforated copper fins and filled with a powder of a composite MH material which contains AB₂- and AB₅-type hydride forming alloys and expanded natural graphite. The assembly of the MH containers staggered together with heating/cooling tubes in the cassette is encased in molten lead followed by the solidification of the latter. The tank can provide >2 h long H₂ supply to the fuel cell stack operated at 11 kWe (H₂ flow rate of 120 NL/min). The refuelling time of the MH tank (T = 15–20 °C, P(H₂) = 100–150 bar) is about 15–20 min.     

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.     

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.     

Ternary LaNi4.75M0.25 hydrogen storage alloys: Surface segregation,hydrogen sorption and thermodynamic stability

Modification of compounds like LaNi₅ toward ternary compositions change alloy hydrogen storage properties and influence resistance to hydrogen contamination. Below thermodynamic properties of ternary alloys LaNi_4.75M_0.25 are investigated with ab initio methods and synthesized in order to select the composition with hydrogen sorption properties not worse than LaNi₅. The specific volume change, surface segregation energy and change of the hydride formation enthalpy are calculated for 34 elements (M: Ag, Al, Au, B, Bi, Ca, Cd, Cu, Cr, Fe, Ga, Ge, In, Ir, K, Mg, Mn, Mo, Nb, Pb, Pd, Pt, Rh, Ru, Sb, Sn, Ti, V, W, Y, Zn, Zr) substituting Ni. Five ternary compounds are synthesized and analyzed with respect to crystal structure and hydrogen sorption properties. Compounds like LaNi_4.75Ag_0.25. and LaNi_4.75Pb_0.25 show favorable stability and H₂ sorption thermodynamics. The substituting elements segregating toward the surface are expected to be catalytically active for hydrogen contamination gasses.     

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.     

Metal hydride hydrogen storage tank for light fuel cell vehicle

We describe a metal hydride (MH) hydrogen storage tank for light fuel cell vehicle application developed at HySA Systems. A multi-component AB₂-type hydrogen storage alloy was produced by vacuum induction melting (10 kg per a load) at our industrial-scale facility. The MH alloy has acceptable H sorption performance, including reversible H storage capacity up to ∼170 NL/kg (1.5 wt% H). The cassette-type MH tank was made up of 2 cylindrical aluminium canisters with transversal internal copper fins and external aluminium fins for improving the heat exchange between the heating medium and the MH tank. Heat supply and removal was provided from the outside using air at T = 15–25 °C. The MH tank was tested at the conditions of natural or forced (velocity ∼2 m/s) air convection. The tests included H₂ charge of the tank at P = 15–40 bar and its discharge at P = 1 bar. The tank in the H₂ discharge mode was also tested together with open cathode low-temperature proton exchange membrane fuel cell (LT PEMFC).     

Hydrolysis of Mg-based alloys and their hydrides for efficient hydrogen generation

The hydrolysis of Mg-based alloys and their hydrides with high abundance on the earth and low cost could produce hydrogen with high theoretical capacity and the formation of by-products that have no pollution to the environment. Hence, it has been regarded as one of the most promising way for hydrogen generation. Particularly, a gravimetric capacity of 6.4 wt% and 3.4 wt% H₂ could be produced from the hydrolysis of pure Mg and MgH₂, respectively, even when stoichiometric water is included for calculation. The formation of passive magnesium hydroxides with dense structure, however, could immediately interrupt the hydrolysis reaction of Mg/MgH₂, which leads to ultralow yield and sluggish hydrogen generation rate. Recent studies have demonstrated that the hydrolysis reaction of Mg/MgH₂ could be effectively enhanced in terms of both yield and kinetics by the formation of Mg-based alloys and their hydrides. This review aims to summarize the recent progress in the hydrolysis of Mg-based alloys and their hydrides and the involved hydrolysis mechanisms.     

A BCC-C14 alloy suitable for EV application of Ni/MH battery

Hydrogen energy in the electrochemical form was studied in nickel/metal hydride (Ni/MH) battery. We have developed a Laves phase related body-centered-cubic metal hydride alloy showing a discharge capacity over 400 mAh g⁻¹ at a 100 mA g⁻¹ rate (C/4) suitable for electric vehicle (EV) application. The improvements in both capacity and high-rate capability were originated from the synergetic effect with a fourth phase (Ti₂Ni) besides the main constituent phases of BCC, C14, and TiNi through the composition and process refinements. Together with the pouch design and advanced high-capacity core-shell β-αNi(OH)₂, a prismatic cell with an energy density of 145 Wh kg⁻¹ is designed. The simplicity in both battery and thermal management systems for Ni/MH battery EV pack provides extra advantages in both design and energy density.     

Multi-criteria analysis for screening of reversible metal hydrides in hydrogen gas storage and high pressure delivery applications

Metals and alloys forming reversible hydrides with hydrogen gas are potential building blocks for compact, solid state hydrogen storage systems. Based on the materials’ thermodynamic characteristics, their use as temperature-swing gas compression and delivery systems in the hydrogen economy is also possible. Given the wide variety of materials developed and tested at laboratory and pilot scales, a harmonized method of selecting the feasible material(s) for a particular real-life application is required. This study proposes a system selection framework based on a normalized, multi-criteria metric. Using calculated values of multi-criteria metric, multi-criteria screening and ranking of potential materials has been demonstrated for a particular use case. It is found that the alloy TiMn_1.52 having value of additive metric between 0.25 and 0.35 represents the best material for a single stage system. The alloy pair CaNi₅–Ti_1.5CrMn represents the best alternative for a two-stage system with additive metric values between 0.63 and 0.82. Energy and economic characteristics of the metal hydride gas compression and delivery systems are evaluated and compared with an equivalent mechanical compression system producing the same final effect (i.e., delivery of a given quantity of gas at a defined pressure).