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 on low-vanadium Ti–Zr–Mn–Cr–V based alloys for high-density hydrogen storage

To reduce the cost and modulate hydrogen storage performances of Ti-based Laves phase alloys for the application of inputting 3.2 MPa feed hydrogen and outputting 8 MPa hydrogen with water bath, three series of less-vanadium Ti–Zr–Mn–Cr–V based alloys were prepared by induction levitation melting, and their microstructure and hydrogen storage properties were systematically investigated. All alloys consist of a single C14-type Laves phase with well-distributed elements. With vanadium decreasing in Ti_0.95Zr_0.05Mn_0.9+xCr_0.9+xV_0.2-2x (x = 0–0.02) and Ti_0.93Zr_0.07Mn_1.1+yCr_0.7+zV_0.2-y-z (y = 0, 0.05, z = 0–0.05) stoichiometric alloys, the hydrogen equilibrium pressure increases and hydrogenation kinetics is slightly deteriorated. After introducing Ti hyper-stoichiometry, Ti_0.93+wZr_0.07Mn_1.15Cr_0.7V_0.15 (w = 0–0.04) alloys show decreased hydrogen equilibrium pressure, high hydrogen capacity and enhanced kinetics. Among alloys mentioned, Ti_0.95Zr_0.07Mn_1.15Cr_0.7V_0.15 has optimum performances including useable capacity of 1.07 wt% at working conditions, together with satisfactory cycling durability. This study guides for compositional design of high-density hydrogen storage multi-component alloys.     

Structural and electrochemical properties of Ti1.5Zr5.5VxNi10−x

Quaternary alloys with the formula Ti_1.5Zr_5.5V_xNi_10−x (x between 0 and 3.0) were studied as a potential replacement for Laves phase alloys used as the negative electrode active material in nickel metal hydride batteries. The V-containing alloys all show multi-phase structures. The major phase shifts from a Zr₇Ni₁₀ structure to a Zr₉Ni₁₁ structure and finally to a C14 structure as the vanadium content increases. Other minor phases with C15 and ZrNi crystal structures are also present. The solubility of vanadium is high in AB2 phases (both C14 and C15), moderate for the ZrNi phase and very low for Zr₇Ni₁₀ and Zr₉Ni₁₁ phases. The bulk hydrogen transport property of the alloys is dominated by synergetic effects between major and minor phases. Electrochemical testing shows that the highest discharge capacity, 357 mAh/g, was obtained from an alloy with a chemical composition of Ti_1.5Zr_5.5V_2.5Ni_7.5 and mainly C14 structure. Testing also shows the high rate dischargeability is controlled by the surface reaction and Ti_1.5Zr_5.5V_0.5Ni_9.5 has the best high rate dischargeability.     

Study of the different ZrxNiy phases of Zr-based AB2 materials

Hydride-forming alloys are used as components of the negative electrode of nickel-metal hydride (NiMH) batteries. In previous works, the study of Zr-based AB₂-type alloys indicated that the material without heat treatment (annealing) had better electrochemical characteristics than the annealed one. The effect was attributed to the presence of secondary phases Zr_xNi_y formed during the solidification of the alloy button obtained by arc melting, and to the fact that these phases diminished their concentration or disappeared upon annealing. The main secondary phases formed by microsegregation are Zr₇Ni₁₀, Zr₉Ni₁₁ and Zr₈Ni₂₁.     

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.     

Microstructural features and improved reversible hydrogen storage properties of ZrTiVFe high-entropy alloy via Cu alloying

ZrTiVFe high-entropy alloy has shown desirable hydrogen absorption and desorption properties due to its lattice distortion effect and high content of C14 phase that can store hydrogen. In this study, element Cu was used to improve the reversible hydrogen storage properties of equimolar ZrTiVFe alloy by increasing valence-electron concentration (VEC), and (ZrTiVFe)_1-xCu_x (x = 0.05, 0.1, 0.2) alloys were prepared. After studying their microstructural features and hydrogen storage properties, the results indicate that (ZrTiVFe)_0.95Cu_0.05 and (ZrTiVFe)_0.90Cu_0.10 alloys are mainly consisted of C14 Laves phase and a small amount of α-Ti and α-Zr phases. When the Cu content increases to 20 at. %, the microstructure transforms to reticular ZrTiCu₂ phase around C14 Laves phase, and the Cu₈Zr₃ phase is formed in final solidification stage. The fastest hydrogen absorption rate of (ZrTiVFe)_0.80Cu_0.20 alloy at room temperature suggests the ZrTiCu₂ and Cu₈Zr₃ phases can provide preferential paths for hydrogen atoms diffusion. The amount of hydrogen in (ZrTiVFe)_0.90Cu_0.10 hydride that cannot be desorbed at 400 °C in vacuum is greatly reduced from 0.370 wt% to 0.084 wt% comparing with ZrTiVFe hydride. The addition of element Cu reduces the stability of ZrTiVFe hydride significantly, which favors the hydrogen desorption of the (ZrTiVFe)_1-xCu_x alloys.     

Microstructure and electrochemical properties of Zr–Ti–V–Ni–Mn–LM alloys for application in Ni–MH secondary battery

A series of multi-component Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4; y=0.0,0.02,0.05,0.1,0.2,0.3, LM; lantanum-rich-mischmetal) alloys are prepared and their crystal structure and P–C–T curves are analyzed. The alloys have been modified by adding LM and their gaseous and electrochemical hydrogenation properties are studied to find out the effect of LM elements. Also, the second phase and initial activation performance are investigated. The Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3,0.4; y=0.0,0.02,0.05,0.1,0.2,0.3) alloys have C14 Laves phase hexagonal structure, so the volume expansion ratio of lattice parameters with LM has increased. As the amount of LM in alloy has increased, correspondingly the second phase is also increased. The second phase is LM, Ti and V-rich. The second phase improve the activation of La-rich misch-metal, and also the concentration of elements Ti, V〉LM〉 matrix in alloys.The addition of LM in Zr_1−xTi_xV_0.4Ni_1.2Mn_0.4LM_y (x=0.3, 0.4) alloys have increased the activation rate and hydrogen storage capacity significantly, but the plateau pressure and the discharge capacity have been decreased due to the formation of second phase. For more Zr in electrode alloys, the activation of rate becomes slow.     

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.     

Phase structures and electrochemical properties of the Laves phase hydrogen storage alloys Zr1−xTix(Ni0.6Mn0.3V0.1Cr0.05)2

The effects of the partial substitution of Ti for Zr in Zr_1−xTi_x(Ni_0.6Mn_0.3V_0.1Cr_0.05)₂ alloys are reported in this paper. The main phases C15 and C14 Laves phases, and secondary phase Zr₇Ni₁₀ were found. With increasing Ti content, abundance of C15 phase decreases and that of C14 phase increases. Increase of Ti content leads to decrease of the lattice parameters of both C15 and C14 phases. Pressure-composition isotherms show a decrease of the stability of the alloy hydride. Ti substitution for Zr in the alloys is effective to improve the activation and high-rate dischargeability. A critical substitution content of Ti is found at x=0.2. The cycling stability and the high rate dischargeability are deteriorated for the alloy electrodes with high Ti content (x>0.3).     

Microstructural characterization and hydrogenation properties of non-stoichiometric Zr0.9TixV2 alloys

The non-stoichiometric Zr_0.9Ti_xV₂ (x = 0, 0.2, 0.3, 0.4) alloys are designed to explore the effect of non-stoichiometry on phase constituent, microstructure and hydrogenation properties of Zr-based AB₂ Laves alloys. The alloys are prepared by non-consumable arc melting and annealed at 1273 K for 168 h in argon atmosphere to ensure the homogeneity. Phase structure investigation shows the α-Zr/β-Zr phase and V-BCC phase originating from the non-equilibrium solidification can be reduced after annealing, C15-type ZrV₂ becomes the dominant phase. Meanwhile, a small amount of Zr₃V₃O phase generates when x ≤ 0.2 and the β-Zr transforms to α-Zr when x > 0.2. High density annealing twins are observed in ZrV₂ matrix by TEM. Activation behavior, hydrogenation kinetics and PCT characteristics of annealed Zr_0.9Ti_xV₂ are investigated in the temperature range 673–823 K. With the decrease in B/A ratio or increase in Ti content, the initial hydrogen absorption speed decreases obviously, the plateaus of PCT curves become wide and flat, meanwhile the hydrogen absorption capacity and the stability of metal hydrides increases. Twin defects observed in these alloys play an important role in accelerating the hydrogenation kinetics. In addition, phase constituent after hydrogenation is analyzed.     

Modification of nano-eutectic structure and the relation on hydrogen storage properties: A novel Ti–V–Zr medium entropy alloy

TiV-based alloys present desirable hydrogen storage properties owing to the formation of Body-centered cubic (BCC) solid solutions. However, the nanostructure that helps hydrogen absorption and desorption is hard to be designed and prepared in these alloys. In this study, Ti₄₀Zr_60-xV_x (x = 20, 25, 30) alloys with hyperfine nano-eutectic structures of 50–500 nm in lamellar space are prepared, and the nano-eutectic structures can be refined by increasing Zr content. Ti₄₀Zr_60-xV_x alloy powder exhibits excellent activation and hydrogenation properties. The phase separation and nano-eutectic structure are formed due to the differences of atomic size in Ti₄₀Zr_60-xV_x alloys. The highest total hydrogenation capacity of 2.4 wt% is obtained within 10 min at 200 °C under 1 MPa H₂ by Ti₄₀V₃₅Zr₂₅ alloy, surpassing that of Ti₄₀Zr₄₀V₂₀ and Ti₄₀Zr₃₀V₃₀ alloys of 2.2 wt% in 20min. Based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, lower energy is required for the hydrogenation of Ti₄₀V₃₅Zr₂₅ alloy. Due to the formation of some stable hydrides, the Ti₄₀Zr_60-xV_x alloys show lower reversible hydrogenation capacities. The spinodal decomposition in Ti₄₀V₃₅Zr₂₅ alloy facilitates the formation of reticular eutectics, which provide high-density phase interfaces and produce “synergistic effect”. As a result, the hydrogenation kinetic and capacity are enhanced significantly.     

Hydrogen storage properties and microstructures of Ti0.7Zr0.3(Mn1−xVx)2 (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys

Hydrogen storage properties, activation performance and thermodynamics of Ti_0.7Zr_0.3(Mn_1−xV_x)₂ (x = 0.1, 0.2, 0.3, 0.4, 0.5) alloys and associated microstructures and surface chemical states were investigated by hydrogenation measurements and relevant structure and surface characterization methods. The results showed that the phase composition of the alloy changed from single C14 Laves phase (x ≤ 0.2) to coexistent Laves phase and V-based BCC solid solution phase with increasing V content (x ≥ 0.3). The V in the alloys catalyzed hydrogen dissociation and improved resistivity to oxygen poisoning, so that the alloys could be easily and quickly activated at 293 K even after being exposed in air for a long time. The hydrogen storage capacity of the alloy increased and the plateau pressure decreased with increasing V content. The x   = 0.2 and 0.3 alloys exhibited the best reversible hydrogen storage capacities of above 1.8 wt% at 1 kPa–4 MPa and 293 K. The relative partial molar enthalpy |ΔH|$\left|\Delta H\right|$ increased but the relative partial molar entropy |ΔS|$\left|\Delta S\right|$ decreased with increasing V content, and deviated from the linear relationship for x = 0.4 and 0.5 alloys due to coexisted BCC phase in the alloys.     

Hydrogen solid solution thermodynamics of V1−xAlx (x: 0, 0.18, 0.37, 0.52) alloys

Thermodynamic parameters such as enthalpy and entropy of the vanadium–hydrogen solid solution are investigated as a function of aluminum content using hydrogen solubility data and the Sievert's constant. The enthalpy decreases with increase in the aluminum content. Entropy shows anomalous behavior as it first increases with the aluminum content for V_1−xAl_x (x: 0, 0.18, 0.37) but then substantially decreases for V_0.48Al_0.52. The lattice parameters and the electrical resistivity of the alloys are calculated to explain the mechanical and electronic effects on the thermodynamic parameters. It is found that the electrical resistivity of vanadium systematically decreases and the lattice constant increases with increase in aluminum content. The hardness of the alloys increases with aluminum which indicates that aluminum hardens the vanadium by simple solid solution effect. The variation of enthalpy and entropy is explained on the basis of change in Fermi energy level of the host matrix vanadium, the strong bonding nature of V–Al in the alloy and increased activity of hydrogen due to aluminum in the alloy.     

Microstructure and hydrogen storage properties of Ti10+xV80-xFe6Zr4 (x=0~15) alloys

The microstructure and hydrogen storage properties of Ti_10+xV_80-xFe₆Zr₄ (x = 0, 5, 10, 15) alloys have been studied. XRD and SEM analyses show that all alloys consist of a BCC main phase and a small fraction of C14 Laves secondary phase, in which the latter precipitates along the grain boundary of the former becoming network structure. With increasing Ti content in the alloy, the lattice parameter and cell volume of the BCC main phase of the alloy increase. The chemical composition of each phase is analysed by EDS, from which the lattice parameters of BCC phase have a good linear relationship with their average atomic radii. All bulk alloys have good activation behaviors and hydriding kinetics. With the increase of Ti content, the incubation time for activation decreases first and then increases under an initial hydrogen pressure of 4 MPa at 298 K. The incubation time of Ti₁₅V₇₅Fe₆Zr₄ alloy is only 12 s. It is one of the shortest incubation time in V-based solid solution alloys as far as we know, which may be related to the existence of C14 Laves phase. All the alloys have relatively high hydrogen absorption capacities of above 3 wt%, which increase first and then decrease as the Ti content increases, achieving the maximum capacity of 3.61 wt% at x = 10 at 298 K. With increasing x, the equilibrium plateau pressure of dehydrogenation of the samples at 353 K decreases owing to the expansion of unit cell of main phase, which is far below 0.1 MPa for x = 10 and 15. The maximum desorption capacity of 1.94 wt% (desorbed to 0.001 MPa) is obtained at x = 5, compared to that of 1.6 wt% (desorbed to 0.1 MPa) achieved at x = 0.     

Hydrogen absorption properties of Zr(V1−xFex)2 intermetallic compounds

The Zr(V_1−xFe_x)₂ (x = 0.02, 0.05, 0.10, 0.15, 0.25) alloys were prepared by the arc-melt method and annealed at 1273 K for 168 h in an argon atmosphere. Phase structure investigations of the as-cast and annealed Zr(V_1−xFe_x)₂ alloys indicate the annealing treatment can eliminate the minority phases originating from the non-equilibrium solidification of as-cast alloys. The ZrV₂-type phase becomes the dominant one in each annealed alloy. The substitution of Fe in V sites leads to the contraction of their lattice. For annealed Zr(V_1−xFe_x)₂ alloys, the P–t and PCT curves obtained between 673 K and 823 K give the evidence that the absorption process is controlled by a rate-controlling hydrogen diffusion. With the increase of iron, the equilibrium pressure and the plateau slope increase while the hydrogenation capacity and the absolute value of enthalpy and entropy decrease accordingly. The stability of metal hydride reduces gradually as the Fe content varies from x = 0.02 to 0.25 which promotes the hydrogen release and favors the practical applications of the Zr(V_1−xFe_x)₂ alloys.     

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.     

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.     

Microstructure and hydrogen storage properties of Ti10V84−xFe6Zrx (x = 1–8) alloys

Ti₁₀V_84−xFe₆Zr_x (x = 1, 2, 4, 6, 8) hydrogen storage alloys were prepared by induction melting with magnetic levitation, and the effects of Zr content on the microstructures and hydrogen storage properties have been investigated systematically. The results show that the alloy with x = 1 has a single V-based solid solution phase with BCC structure, while other alloys with x = 2–8 consist of a BCC main phase and a C14 type Laves secondary phase, and the abundance ratio of the secondary phase increases with increasing Zr content. As the Zr content in the alloy increases, the activation behavior is improved, but the hydrogen absorption and desorption capacities decrease gradually. For the alloy with the Zr content of x = 1, the best overall hydrogen storage properties are obtained.     

Microstructure and hydrogen storage properties of non-stoichiometric Zr–Ti–V Laves phase alloys

The non-stoichiometric C15 Laves phase alloys namely Zr_0.9Ti_0.1V_x (x = 1.7, 1.8, 1.9, 2.1, 2.2, 2.3) are designed and expected to investigate the role of defect and microstructure on hydrogenation kinetics of AB₂ type Zr-based alloys. The alloys are prepared by non-consumable arc melting in argon atmosphere and annealed at 1273 K for 168 h to ensure the homogeneity. The microstructure and phase constitute of these alloys are examined by SEM, TEM and XRD. The results indicate the homogenizing can reduce the minor phases α-Zr and abundant V solid solution originating from the non-equilibrium solidification of as-cast alloys. Twin defects with {111}<011 > orientation relationship are observed, and the role of defects on hydrogenation kinetics is discussed. Hydrogen absorption PCT characteristics and hydrogenation kinetics of Zr_0.9Ti_0.1V_x at 673–823 K are investigated by the pressure reduction method using a Sievert apparatus. The results show the hypo-stoichiometric alloys preserve faster hydrogenation kinetics than the hyper-stoichiometric ones due to the decrease of dendritic V. The excess content of Zr₃V₃O phase decreases the hydrogenation kinetics and the stability of hydrides. In addition, the different rate controlled mechanisms during hydrogen absorption are analyzed. The effects of non-stoichiometry on the crystal structure and hydrogen storage properties of Zr_0.9Ti_0.1V_x Laves alloys are discussed.     

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.