Effect of partial niobium and iron substitution on short-term cycle durability of hydrogen storage Ti–Cr–V alloys

The effect of partial niobium and iron substitution on the short-term (up to 10 cycles) cycle durability of hydrogen absorption and desorption was evaluated for Ti–Cr–V alloys. Partial iron substitution improved the durability of Ti₁₆Cr₃₄V₅₀ alloy, but reduced its hydrogen storage capacity. In contrast, partial niobium substitution improves its durability while not affecting its hydrogen storage capacity. Similar experimental results were obtained for Ti₂₅Cr₅₀V₂₅ alloy. The effective hydrogen storage capacity decreased to 84.9 and 94.2% of its initial value after 10 cycles of hydrogen absorption and desorption for Ti₂₅Cr₅₀V₂₅ and Ti₂₅Cr₄₅V₂₅Nb₅ alloys, respectively. This reduction in the effective hydrogen storage capacity of Ti₂₅Cr₅₀V₂₅ alloy during hydrogen absorption and desorption is attributed to reduced hydrogen storage capacity during absorption and a greater residual hydrogen during desorption.     

Effect of V content on hydrogen storage properties and cyclic durability of V–Ti–Cr–Fe alloys

Vanadium-based body-centered-cubic (BCC) alloys are ideal hydrogen storage media because of their high reversible hydrogen capacities at moderate conditions. However, the rapid capacity decay in hydrogen ab-/desorption cycles prevents their practical application. In this work, V-based BCC alloys with three different V contents (V₂₀Ti₃₈Cr_41.4Fe_0.6, V₄₀Ti_28.5Cr_30.1Fe_1.4, V₆₀Ti₁₉Cr₁₉Fe₂, named as V20, V40, V60) were prepared by arc melting, and their microstructures and hydrogen ab-/desorption properties were investigated systematically. XRD results show that there is a number of C15-Laves phase presence in V20, which does not appear in V40 and V60. Meanwhile, the lattice constant of the BCC phase clearly decreases as the V content rises. These differences result in a hydrogen storage capacity of only 1.82 wt% for V20 alloy, but 2.13 wt% for V40 and 2.14 wt% for V60, and an increment in hydrogen ab-/desorption plateau pressure. The V40 and V60 alloys are chosen in de-/hydrogenation cycle test owing to higher effective storage capacities, and the results show that the V60 alloy has better cycle durability. According to the microstructural analysis of the two alloys during the cycles, the micro-strain accumulates, the cell volume expands, the particles pulverizes and the defects increase during the cycles, which eventually lead to the attenuation of the hydrogen storage capacity. The increment of the V content obviously improves the elastic properties of the alloy, which further diminishes the micro-strain accumulation, cell volume expansion, particle pulverization and defect increase, eventually resulting in a higher capacity retention and better cyclic durability.     

Effect of LaH3 additive on microstructures and hydrogen storage properties of V40Ti26Cr26Fe8 alloys prepared by hydride powder sintering method

Hydride-Powder-Sintering(HPS) as a new preparation approach was applied in the low cost V₄₀Ti₂₆Cr₂₆Fe₈ hydrogen storage alloys successfully, which provides an alternative preparation route for other types of hydrogen storage alloys. V₄₀Ti₂₆Cr₂₆Fe₈ alloy exhibits ideal hydrogen absorption-desorption performance after adding moderate content of LaH₃. Optimizing sintering process by increasing the sintering temperature and time has improved hydrogen desorption plateau evidently. X-ray diffraction analysis results show that the Ti-Oxide phase basically no longer exists when the LaH₃ content is greater than or equal to 3 wt% as compared to that of LaH₃ content below 3 wt%. Pressure-Compose-Temperature(PCT) curves show that the hydrogen absorption and desorption capacities increase gradually with the increase of LaH₃ content. The lattice parameters calculated by Rietveld refinement has the same variety rule as the hydrogen absorption-desorption capacities with the increase of LaH₃ content. It can be inferred that the addition of LaH₃ has reduced the content of Ti-Oxide and then the lattice parameters increase, which leads to the improvement of hydrogen performance of the alloy. The most appropriate preparation process is 3 wt% of LaH₃ additive with a sintering process 1673 K-6h, correspondingly, the hydrogen absorption and desorption capacities are 3.13 wt% and 1.97 wt%, respectively.     

Hydrogen storage and cyclic properties of V60Ti(21.4+x)Cr(6.6−x)Fe12 (0 ≤ x ≤ 3) alloys

Hydrogen storage and cyclic properties of V₆₀Ti_(21.4+x)Cr_(6.6−x)Fe₁₂ (0 ≤ x ≤ 3) alloys were investigated systematically. All alloys were composed of single BCC phase and exhibited good activation performance. V₆₀Ti_22.4Cr_5.6Fe₁₂ showed the highest desorption capacity of 2.12 wt% with the plateau pressure of 0.061 MPa. In the absorption–desorption cycle tests, both the hydrogen desorption capacity and the micro-strain of V₆₀Ti_22.4Cr_5.6Fe₁₂ alloy showed exponential relationship with the increase of cycle numbers, which indicated that the micro-strain induced and thereafter accumulated during the absorption–desorption cycles might lead to the decrease of the desorption capacity.     

The effect of Sc addition on the hydrogen storage capacity of Ti0.32Cr0.43V0.25 alloy

The effect of the addition of 4th element on the hydrogen storage capacity of Ti_0.32Cr_0.43V_0.25 alloy was evaluated by simulation and confirmed experimentally. The crystal lattice volume, phase formation energy, and hydrogen absorption energy of the alloys were calculated by ab initio calculation for the alloys containing the third-period transition metals as Sc, Cr, Mn, Fe, Co, Ni, Cu, and Zn. It was postulated that the hydrogen absorption would be favored by large crystal volume and low hydrogen absorption energy. The calculation suggested Sc as the most suitable element and the hydrogen capacities of a series of Ti_0.32Cr_0.43−xV_0.25Sc_x alloys (x = 0.02–0.1) were determined accordingly. Among the alloys, the capacities of Ti_0.32Cr_0.41V_0.25Sc_0.02 and Ti_0.32Cr_0.39V_0.25Sc_0.04 alloys were higher than that of the Ti_0.32Cr_0.43V_0.25 alloy. The capacity of both alloys could be enhanced further by the heat treatment at 1250 °C due to the elimination of the second-phase TiCr₂.     

Influence of Fe content on the microstructure and hydrogen storage properties of Ti16Zr5Cr22V57−xFex (x = 2–8) alloys

The influence of Fe content on the microstructure and hydrogen storage properties of Ti₁₆Zr₅Cr₂₂V_57−xFe_x (x = 2–8) alloys was investigated systematically. The results show that all alloys consist of a BCC main phase and a small amount of C14 Laves secondary phase. The crystal lattice parameters of the BCC main phase in the alloys decrease with the increase of the Fe content. Under moderate conditions, all the alloys have good activation behaviors and hydriding/dehydriding kinetics. As the x increases, the hydrogen desorption plateau pressure of the alloys increases consequently. Among the studied alloys, Ti₁₆Zr₅Cr₂₂V₅₅Fe₂ alloy has suitable hydrogen desorption plateau pressures indicated by the middle value of pressure range. (0.1–1 MPa) at 298 K and the best overall hydrogen storage properties.     

Microstructure and hydrogen storage properties of Ti–V–Cr based BCC-type high entropy alloys

In this work, the crystal structure and hydrogen storage properties of V₃₅Ti₃₀Cr₂₅Fe₁₀, V₃₅Ti₃₀Cr₂₅Mn₁₀, V₃₀Ti₃₀Cr₂₅Fe₁₀Nb₅ and V₃₅Ti₃₀Cr₂₅Fe₅Mn₅ BCC-type high entropy alloys have been investigated. It was found that high entropy promotes the formation of BCC phase while large atomic difference (δ) has the opposite effect. Among the four alloys, the V₃₅Ti₃₀Cr₂₅Mn₁₀ alloy shows the highest hydrogen absorption capacity while the V₃₅Ti₃₀Cr₂₆Fe₅Mn₅ alloy exhibits the highest reversible capacity. The cause of the loss of desorption capacity is mainly due to the high stability of the hydrides. The higher room-temperature desorption capacity of the V₃₅Ti₃₀Cr₂₅Fe₅Mn₅ alloy is due to higher hydrogen desorption pressure. After pumping at 400 °C, the hydrides can return to the original BCC structure with only a small expansion in the cell volume.     

High‐pressure hydrogen storage properties of TixCr1 − yFeyMn1.0 alloys

Ti_xCr_{1 ? y}Fe_yMn_1.0 (x = 1.02, 1.05, 1.1, 0.05 ≤ y ≤ 0.25) alloys were prepared by plasma arc melting and annealing at 1273 K for 2 hours. The XRD results show that the main phase of all alloys is the C14 type Laves phase, and a little secondary phase exists in a mixture of the binary alloy phase. The lattice parameters increase with Ti super‐stoichiometry ratio increasing, whereas smaller lattice parameters emerge with increasing Fe stoichiometry content. Additionally, as the Ti super‐stoichiometry ratio decreases, the pressure‐composition‐temperature measurements indicated that hydrogen absorption and desorption plateau pressures of Ti_xCr_0.9Fe_0.1Mn_1.0 (x = 1.1, 1.05, 1.02) alloys increase from 3.15, 0.67, to 5.94, 1.13 MPa at 233 K, respectively. On the other hand, with the Fe content increasing in Ti_1.05Cr_{1 ? y}Fe_yMn_1.0 (0.1 ≤ y ≤ 0.25) alloys from 0.1 to 0.25, the hydrogen desorption plateau pressures increased from 1.41 to 2.46 MPa at 243 K. The hydrogen desorption plateau slopes reduce to 0.2 with Ti super‐stoichiometry ratio decreasing to 1.02, but the alloys are very difficult to activate for hydrogen absorption and cannot activate when the Fe substituting for Cr exceeds 0.2. The maximum hydrogen storage capacities were more than 1.85 wt% at 201 K. The reversible hydrogen storage capacities can remain more than 1.55 wt% at 271 K. The enthalpy and entropy for all hydride dehydrogenation are in the range of 21.0 to 25.5 kJ/mol H₂ and 116 to 122 J mol^?1 K^?1, respectively. Our results suggest that Ti_1.05Cr_0.75Fe_0.25Mn_1.0 alloy with low enthalpy holds great promise for a high hydrogen pressure hybrid tank in a hydrogen refueling station (45 MPa at 333 K), and the other alloys of low cost may be used for a potable hybrid tank due to high dissociation pressure at 243 K and high volumetric density exceeding 40 kg/m³.     

Development of Ti1.02Cr2-x-yFexMny (0.6≤x≤0.75, y=0.25, 0.3) alloys for high hydrogen pressure metal hydride system

To save compressor investment and promote operation efficiency of hydrogen refueling station, the hydrogen storage alloys for high-pressure hydrogen metal hydride tank is developed. Ti_1.02Cr_2-x-yFe_xMn_y (0.6 ≤ x ≤ 0.75, y = 0.25, 0.3) alloys with main structure of C14 type Laves phase and low dehydrogenation enthalpy were prepared by plasma arc melting and heat treatment. Pressure-composition-temperature measurements show that hydrogen desorption plateau pressures increase with Cr substituted by Fe increasing. The maximum and reversible hydrogen storage capacities are more than 1.85 and 1.65 wt% at 201 K respectively. The hydrogen desorption plateau slopes are all less than 0.5. The symmetry weakening of 2a sites may deteriorate the plateau slop characteristic. Ti_1.02Cr_0.95Fe_0.75Mn_0.3 and Ti_1.02Cr_1.0Fe_0.75Mn_0.25 alloys are suitable for high pressure hybrid tank which can supply the effective hydrogen (more than 70 MPa) about 40.0, 44.2, 46.9 kg/m³ with 45, 70, 90 MPa compressor, respectively.     

Development of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption pressures for hybrid hydrogen storage vessel application

Three series of Ti–Cr–Mn–Fe based alloys with high hydrogen desorption plateau pressures for hybrid hydrogen storage vessel application were prepared by induction levitation melting, as well as their crystallographic characteristics and hydrogen storage properties were investigated. The results show that all of the alloys were determined as a single phase of C14-type Laves structure. As the Fe content in the TiCr_1.9−xMn_0.1Fe_x (x = 0.4–0.6) alloys increases, the hydrogen absorption and desorption plateau pressures increase, and the hydrogen storage capacity and plateau slope factor decrease respectively. The same trends are observed when increasing the Mn content in the TiCr_1.4−yMn_yFe_0.6 (y = 0.1–0.3) alloys, except for the plateau slope factor. Compared with the stoichiometric TiCr_1.1Mn_0.3Fe_0.6 alloy, the titanium super-stoichiometric Ti_1+zCr_1.1Mn_0.3Fe_0.6 (z = 0.02, 0.04) alloys have larger hydrogen storage capacities and lower hydrogen desorption plateau pressures. Among the studied alloys, Ti_1.02Cr_1.1Mn_0.3Fe_0.6 has the best overall properties for hybrid hydrogen storage application. Its hydrogen desorption pressure at 318 K is 41.28 MPa, its hydrogen storage capacity is 1.78 wt.% and its dissociation enthalpy (ΔH_d) is 16.24 kJ/mol H₂.     

Hydrogen storage properties of Ti2−xCrVMx (M = Fe,Co, Ni) alloys

In this work, quaternary alloys having compositions Ti_1.9CrVM_0.1 and Ti_1.8CrVM_0.2 (M = Fe, Co and Ni) have been studied in detail for their structural aspects and hydrogen absorption–desorption properties. All the alloys form bcc phase solid solutions and after hydrogen absorption the structures change to fcc. The pressure composition isotherms, hydrogen storage capacities and hydrogen absorption kinetics were studied using Sievert's type of volumetric setup. The Ti_1.9CrVFe_0.1, Ti_1.9CrVCo_0.1 and Ti_1.9CrVNi_0.1 alloys are found to absorb maximum 3.80, 3.68 and 3.91 wt.% of hydrogen respectively; whereas, Ti_1.8CrVCo_0.2 and Ti_1.8CrVNi_0.2 alloys show 3.52 and 3.67 wt.% of hydrogen at room temperature. All the alloys show fast hydrogen absorption kinetics at the room temperature. From differential scanning calorimetric measurements, it has been found that Fe, Ni and Co substitution in place of Ti decreased the hydrogen desorption temperature drastically compared to the parent alloy.     

Investigation on Ti–Zr–Cr–Fe–V based alloys for metal hydride hydrogen compressor at moderate working temperatures

Ti_0.85Zr_0.17Cr_1.2-xFe_0.8V_x (x = 0–0.2), Ti_0.85Zr_0.17Cr_1.2-yFe_0.7+yV_0.1 (y = 0–0.25) and Ti_0.87-zZr_0.15+zCr_0.95Fe_0.95V_0.1 (z = 0–0.04) alloys for metal hydride hydrogen compressor at moderate working temperatures were prepared by induction levitation melting. Their microstructures and hydrogen storage properties were systematically investigated. The results show that all Ti–Zr–Cr–Fe–V based alloys have a single C14 Laves phase structure. As the V content in the Ti_0.85Zr_0.17Cr_1.2-xFe_0.8V_x (x = 0–0.2) alloys increases, better activation kinetics and larger hydrogen storage capacity are achieved, while the plateau pressure decreases and the plateau slope factor increases. Similarly, the hydrogen storage capacity, the plateau pressure and the plateau slope factor of the Ti_0.87-zZr_0.15+zCr_0.95Fe_0.95V_0.1 (z = 0–0.04) alloys vary identically with Zr content increasing. Conversely, these three properties vary oppositely with increasing Fe content in the Ti_0.85Zr_0.17Cr_1.2-yFe_0.7+yV_0.1 (y = 0–0.25) alloys. Among the studied alloys, Ti_0.85Zr_0.17Cr_0.95Fe_0.95V_0.1 possesses the best overall properties for the designed moderate hydrogen compression application.     

Study of the structural,thermodynamic and cyclic effects of vanadium and titanium substitution in laves-phase AB2 hydrogen storage alloys

The substitution of the high-purity and expensive raw materials vanadium (V) and titanium (Ti) by their low-cost, low-purity alternatives ferrovanadium (FeV) and Ti sponge in Ti_0.98Zr_0.02V_0.43Fe_0.09Cr_0.05Mn_1.5 was investigated and the microstructural, thermodynamic and cyclic properties were tested of these compounds. Four different samples were prepared and studied: one material prepared with high-purity V and Ti, a second one prepared with FeV in substitution for V and Fe, a third prepared with Ti sponge in substitution for Ti, and a fourth prepared with FeV and Ti sponge in substitution for V, Fe and Ti. The substitution of Ti with Ti sponge and of V and Fe by FeV had negligible effects on the microstructural properties. 2.0 mass% H were absorbed in both the pristine (high-purity V and Ti) material and after replacing Ti by Ti sponge. Substitution of V by FeV reduced the initial hydrogen absorption capacity to approximately 1.7 mass%. All materials exhibited equilibrium hydrogen absorption and dissociation pressures of ca. 1.5 MPa. They reversibly stored 1.8 mass% H for both the pristine and Ti-substituted samples and 1.6 mass% H after substitution of V and Fe by FeV or after substitution of V, Fe and Ti by FeV and Ti sponge, respectively.Long-term cyclic experiments over 1000 de-/hydrogenation pressure swing cycles were performed for the pristine material and after substitution of V, Fe and Ti by FeV and Ti sponge, respectively. Both materials exhibited similar activation and degradation behavior upon cycling. A reversible capacity of 1.5 mass% H was recorded for the pristine material after 1000 cycles, and 1.4 mass% H were reversibly stored in the material prepared with FeV and Ti sponge subjected to 1000 cycles. The raw material cost to store an equal amount of hydrogen can be reduced by 83% when V and Ti are substituted by FeV and Ti sponge.     

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V₄₈Fe₁₂Ti_15-xCr₂₅Al_x (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.     

Study of cyclic performance of V-Ti-Cr alloys employed for hydrogen compressor

The development of a suitable hydrogen compressor plays one of the key roles to realize the fuel cell vehicle as well as for many other stationary and mobile applications of hydrogen. V-Ti-Cr BCC alloys are considered as promising candidates for effective hydrogen storage. The cyclic durability of hydrogen absorption and desorption is very important for these alloys to be realized as practical options. In connection to this, two alloys of V-Ti-Cr, (1) V₄₀Ti_21.5Cr_38.5 and (2) V₂₀Ti₃₂Cr₄₈, were selected and their cyclic hydrogen absorption-desorption performance was evaluated up to 100 cycles for temperature and pressure ranges of 20–300 °C and 5–20 MPa, respectively. It has been found that the cyclic hydrogen storage capacity continuously decreased for one composition while it was stable after 10 cycles for another composition. This performance difference of the alloys was studied in terms of their structural and microscopic properties and the results are presented in this paper.     

A low-cost BCC alloy prepared from a FeV80 alloy with a high hydrogen storage capacity

A V₃₀Ti₃₂Cr₃₂Fe₆ alloy prepared from a FeV80 master alloy is reported. It has a high hydrogen absorption/desorption capacity, good activation performance and kinetics. Heat-treatment at 1673 K is an effective way to increase the capacity and flatten the plateau due to the homogenization of the compositions in the alloy and the disappearance of Laves phase after heat-treatment. The heat-treated alloy can absorb 3.76 wt.%H at 298 K. It desorbs 2.35 wt.%H at 298 K and 2.56 wt.%H at 373 K. The development of this alloy could be of great significance to the application of V-based BCC hydrogen storage alloys.     

A 70 MPa hydrogen-compression system using metal hydrides

The objective of this work was to develop a 70 MPa hydride-based hydrogen compression system. Two-stage compression was adopted with AB₂ type alloys as the compression alloys. Ti_0.95Zr_0.05Cr_0.8Mn_0.8V_0.2Ni_0.2 and Ti_0.8Zr_0.2Cr_0.95Fe_0.95V_0.1 alloys were developed for the compression system. With these two alloys, a 70 MPa two-stage hydride-based hydrogen compression system was designed and built with hot oil as the heat source, and composite materials formed by mixing hydrogen storage alloys with Al fiber were used to prevent hydride bed compaction and to prevent strain accumulation. The experimental results showed that Ti_0.95Zr_0.05Cr_0.8Mn_0.8V_0.2Ni_0.2 and Ti_0.8Zr_0.2Cr_0.95Fe_0.95V_0.1 alloys could well meet the requirements of compression system. Composite materials formed by mixing hydrogen storage alloys with Al fiber were an effective way to prevent strain accumulation for hydride compression. With cold oil (298 K) and hot oil (423 K) as the cooling and heating sources, the built compression system could convert hydrogen pressure from around 4.0 MPa to over 70 MPa.     

Activation and de/ hydriding behavior in Ti23V40Mn37 alloy by Hf and Hf/Cr substitutions

To improve absorption/desoprtion rate and hydrogen desorption capacity of Ti–V–Mn alloy, Ti₂₃V₄₀Mn₃₇ alloys by Hf and Hf/Cr substitutions were prepared, the activation and hydriding/dehydriding behaviors of the alloys are investigated. Results show that the lattice parameter of BCC phase increases and the ratio of C14 Laves phase also increases by the substitutions. Ti₁₉Hf₄V₄₀Mn₃₅Cr₂ alloy exhibits the rapid absorption/desoprtion rate and the highest hydrogen desorption capacity of 1.58 wt% H₂ at 293 K. The Hydrogenation kinetic mechanism of the alloys is transformed from nucleation-growth to diffusion, and the dehydrogenation kinetic mechanism is only nucleation-growth. The activation energy of Hf/Cr substituted alloy is lower than that of Hf-free alloy, with the values of 53.79 kJ mol⁻¹ H₂ and 90.13 kJ mol⁻¹ H₂ respectively, which is accounted for the easily absorption of hydrogen molecules on the particle surface and the rapid H diffusion of the interior of alloy, thus the substituted alloys have rapid absorption/desoprtion rate.     

Measurement of thermodynamic properties of some hydrogen absorbing alloys

In this paper, the effect of hydrogen concentration on the reaction enthalpies of some metal hydride alloys during hydriding and dehydring is presented. Pressure–concentration–temperature characteristics of the metal hydride alloys are measured under nearly isothermal condition during both absorption and desorption. Reaction enthalpies and entropies of LaNi₅, LaNi_4.7Al_0.3, LmNi_4.91Sn_0.15, Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 and MmCo_0.72Al_0.87Fe_0.04Ni_3.91 are estimated by constructing van't Hoff plots at different hydrogen concentrations. It is observed that the effect of hydrogen concentration on reaction enthalpies is more significant for the alloys having larger plateau slopes. At the initial stage of hydrogenation, metal hydrides are found to have larger reaction enthalpies which decrease gradually by about 5–15% at the end of the hydrogen absorption. At any given temperature, desorption enthalpies of LaNi₅, LmNi_4.91Sn_0.15, MmCo_0.72Al_0.87Fe_0.04Ni_3.91, LaNi_4.7Al_0.3 and Ti_0.99Zr_0.01V_0.43Fe_0.09Cr_0.05Mn_1.5 are found to be higher by about 5, 8, 10, 28 and 32% than their respective absorption enthalpies. Reaction enthalpies of the selected metal hydride alloys are expressed as a function of hydrogen concentration by a fourth order polynomial equation obtained from fitting with the experimental data.     

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.