Crystal structure and hydrogen storage properties of Ti-V-Mn alloys

The compositions of TiMn _{(100-x, Ti/Mn=5/8)}V_x (x = 25, 30, 35, 40, 45 and 50) alloys have been investigated comprehensively for their microstructure and hydrogen absorption/desorption properties. The proportion of BCC and C14 Laves phases changes with the V content, and BCC phase increases with increasing V content. With increasing BCC phase, more number of cycles are needed to reach to the saturated hydrogen absorption, and the hydrogen storage capacity first increases and then decreases after 40 at.% of the V content. It is indicated that the brittle C14 Laves phase plays as the “path” for hydrogen atom diffusion into the BCC phase. For the samples of V₄₅Ti₂₁Mn₃₄ and V₅₀Ti₁₉Mn₃₁ with less content of C14 Laves phase, it is difficult for hydrogen to diffuse into the BCC phase leading to low absorption capacity. The results of XRD and DSC analyses show that hydrides are less stable in V-poor samples. V₄₀Ti₂₃Mn₃₇ has the best hydrogen storage properties in this study: Its maximum hydrogen absorption capacity is 3.5 wt% at 293 K, dissociation enthalpy is 34.88 kJ/mol H₂, and desorption plateau platform is 0.05 Mpa at 303 K.     

Influence of partial substitution of Mo for Cr on structure and hydrogen storage characteristics of non-stoichiometric Laves phase TiCrB0.9 alloy

The influence of the partial substitution of Mo for Cr on phase composition and hydrogen storage characteristics of non-stoichiometric Laves phase TiCrB_0.9-based alloys is investigated by X-ray diffraction (XRD), pressure composition isotherm (PCT), and scanning electron microscopy (SEM) characterizations. XRD tests reveal that the phase composition of the alloys gradually changes from single TiB_1.07CrB_1.93 Laves phase to the co-existence of Laves phase and Mo-based BCC phase with increasing substitution of Mo for Cr. The phase composition eventually transforms into a single Mo-based BCC phase when the amount of the substitution surpasses a certain level. PCT tests reveal that the maximum hydrogen storage capacity increases with increasing Mo content. The hydrogenation-induced phase changes are also greatly influenced by the substitution of Mo for Cr. SEM tests of the hydrided alloys show that the increasing Mo content enhances hydrogenation-induced pulverization. Finally, hydrogenation-induced phase changes during the course of activation are also investigated.     

Improvement of hydrogen storage properties of TiCrV alloy by Zr substitution for Ti

The effects of Zr substitution for Ti on the hydrogen absorption–desorption characteristics of Ti_1−xZr_xCrV alloys (x = 0, 0.05, 0.1 and 1.0) have been investigated. The crystal structure, maximum hydrogen absorption capacity, kinetics and hydrogen desorption properties have been studied in detail. While TiCrV crystallizes in body centered cubic (BCC) structure, ZrCrV is a C15 cubic Laves phase compound and the intermediate compositions with 5 and 10 at% Zr substitutions for Ti (x = 0.05 and 0.1) show the presence of a small amount of ZrCr₂ Laves phase along with the main BCC phase. The pressure–composition isotherms have been studied at room temperature. TiCrV shows separation of TiH₂ phase on cycling. A small amount of Zr substitution for Ti is found to have advantageous effects on the hydrogen absorption properties of TiCrV as it suppresses TiH₂ phase separation and decreases hysteresis. It is found that the hydrogen absorption capacity of Ti_1−xZr_xCrV decreases as the Zr content increases due to the increased fraction of Laves phase. Temperature-programmed desorption studies have been carried out on the saturated hydrides in order to find the relative desorption temperatures.     

Hydrogen storage properties of new A3B2-type TiZrNbCrFe high-entropy alloy

Crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFe high-entropy alloy (HEA) were studied. The selected alloy, which had a A₃B₂-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen) was designed to produce a hydride with a hydrogen-to-metal atomic ratio (H/M) higher than those for the AB₂- and AB-type alloys. The phase stability of alloy was investigated through thermodynamic calculations by the CALPHAD method. The alloy after arc melting showed the dominant presence of a solid solution C14 Laves phase (98.4%) with a minor proportion of a disordered BCC phase (1.6%). Hydrogen storage properties investigated at different temperatures revealed that the alloy was able to reversibly absorb and fully desorb 1.9 wt% of hydrogen at 473 K. During the hydrogenation, the initial C14 and BCC crystal structures were fully converted into the C14 and FCC hydrides, respectively. The H/M value was 1.32 which is higher than the value of 1 reported for the AB₂- and AB-type HEAs. The present results show that good hydrogen storage capacity and reversibility at moderate temperatures can be attained in HEAs with new configurations such as A₃B₂/A₃B₂H₇.     

Improving hydrogen storage properties of Laves phase related BCC solid solution alloy by SPS preparation method

Spark plasma sintering (SPS) is a newly developed technique for multiple-phase material preparation. In this article, a Laves phase related BCC solid solution V–Ti–Cr alloy was prepared by SPS method, and its microstructure and hydrogen storage properties were investigated and compared with those of the alloy prepared by arc melting method. The results indicated that the alloy prepared by SPS method possessed not only excellent activation and kinetics properties, but also a much larger hydrogen storage capacity. The maximum hydrogen storage capacity was determined to be 2.89 mass%, which is about 24% higher than that of the arc melting alloy, and the hydrogen desorption ratio was improved from 35% for the arc melting alloy to 60% for the SPS alloy. This was due to the absence of Laves formation elements in the BCC solid solution and the formation of the interface phase between the Laves phase and the BCC solid solution during the SPS process, which fully optimized the hydrogen storage property of the Laves phase related BCC solid solution alloy.     

Role of defect structure on hydrogenation properties of Zr0.9Ti0.1V2 alloy

The pseudobinary Zr_0.9Ti_0.1V₂ compound was prepared by induction melting method. The microstructure and phase compositions were examined by the scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). Hydrogen absorption pressure composition isotherms (P-C isotherms) were investigated by pressure reduction method using Sievert apparatus at temperature ranging from 673 to 823 K. The compositional homogeneity of the alloy was achieved under the homogenizing treatment at 1373 K for 100 h. Twin defects with {111}<022> orientation relationship were observed in the Zr_0.9Ti_0.1V₂ annealed at 1373 K for 100 h. The phase compositions of the annealed Zr_0.9Ti_0.1V₂ alloy could be attributed to ZrV₂ Laves phase and V BCC solid solution. After hydrogenation, the alloy hydrides were consisted of ZrV₂H_3.6 and V₄H_2.88. Titanium substitution in the Zr_0.9Ti_0.1V₂ alloy induced the formation of twin defect structures and the multiphase consisting of Laves (C15 type) related with BCC solid solution phases. The especial phase compositions and structures in the alloy were favorable to decrease the equilibrium pressure and improve the hydrogen absorption kinetics due to the twin defects in Zr_0.9Ti_0.1V₂ comparing with the primary ZrV₂ alloy. The desorption hysteresis could be decreased to a certain extent in the Ti-doped alloy under the experimental condition. V-based BCC phase in the Zr_0.9Ti_0.1V₂ alloy could improve hydrogen absorption and desorption properties by an autocatalytic mechanism.     

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 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.     

Study on hydrogen storage property of (ZrTiVFe)xAly high-entropy alloys by modifying Al content

(ZrTiVFe)_xAl_y high-entropy alloys are potential hydrogen storage materials because of their intermediate properties of high hydrogen uptake capacity and fast kinetics. In this study, equimolar and non-equimolar (ZrTiVFe)₈₀Al₂₀ and (ZrTiVFe)₉₀Al₁₀ alloys were prepared, and the effect of Al content on the microstructure, element distribution, and hydrogen storage properties of (ZrTiVFe)_xAl_y alloys were investigated. The results show that both alloys are composed of C14 Laves phase, a small amount of tetragonal and HCP phases. With the increase of Al content, the content and the size of C14 Laves phase decrease, the V element content in C14 Laves phase also decreases, which is resulted from the contents of Zr, V, and Fe element constituting the C14 Laves phase decrease. The Ti element can combine with the excessive Al to form a tetragonal phase around the C14 phase, and the growth of the tetragonal phase causes the refinement of the C14 phase. The (ZrTiVFe)₉₀Al₁₀ alloy absorbed 1.3 wt. % H at room temperature, which indicates the better hydrogenation capacity and kinetics. The improvement of hydrogen storage property is resulted from the increased C14 Laves phase, more V element in C14 Laves phase and the severe lattice distortion of (ZrTiVFe)₉₀Al₁₀ alloy. It is found that the (ZrTiVFe)₈₀Al₂₀ alloy can be activated easily with only three cycles, which is caused by the refined Laves phase. After hydrogenated, H atoms are dissolved into the lattice space of C14 Laves phase for both alloys, and crystalline structure is not changed.     

Effect of addition of Zr,Ni, and Zr-Ni alloy on the hydrogen absorption of Body Centred Cubic 52Ti-12V-36Cr alloy

The effects of addition of Zr, Ni and Zr₇Ni₁₀ on the crystal structure, microstructure and hydrogen absorption of Body Centred Cubic (BCC) 52Ti-12V-36Cr were investigated. We found that addition of Zr and/or Ni led to the formation of a Ni/Zr rich secondary phase. This secondary phase is responsible for the much faster first hydrogenation of the alloys with additives compared to the bare BCC alloy. Zirconium addition had positive influence on incubation time and intrinsic hydrogenation kinetics while nickel addition improved the hydrogen capacity. Among the additives tested, Zr₇Ni₁₀ is the best for optimized hydrogenation kinetics and capacity.     

Effects of Si addition on the microstructure and the hydrogen storage properties of Ti26.5V45Fe8.5Cr20Ce0.5 BCC solid solution alloys

The microstructure and the hydrogen storage properties of Ti_26.5(V_0.45Fe_0.085)_100−xCr₂₀Ce_0.5Si_x (x = 0 and 1) have been investigated by EPMA, XRD, in situ temperature XRD, neutron diffraction and P-C isotherm. Si addition results in the precipitation of a TiFe₂-type Laves phase and produces chemical heterogeneity in the BCC phase. As a consequence, Si-added alloy exhibits a lower hydrogen capacity and both a higher plateau pressure and slope factor as compared to Si-free alloy. Si enters in both Laves and BCC phases with a higher preference for the former phase. For both alloys, all metal atoms (Ti, V, Fe and Cr) are supposed to be randomly distributed in the 2a sites of the BCC phase and deuterium atoms occupy the 8c sites on fully charged deuterides. Si has no significant influence on the hydrogen occupation. Two hydrides are observed during the desorption process for Ti_26.5(V_0.45Fe_0.085)₁₀₀Cr₂₀Ce_0.5 alloy, a hydrogen rich one with distorted FCC structure (space group: P4/mmm) and a hydrogen poor one with BCT structure (space group: I4/mmm).     

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.     

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 high entropy alloy. It was found that the V_0.3Ti_0.3Cr_0.25Mn_0.1Nb_0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.     

Microstructure evaluations and hydrogenation properties on Ti–xNb–10Cr alloys

Ti–Cr alloys consist of BCC solid solution, C36, C14, C15 Laves phase at high temperature. Among others, the BCC solid solution phase was reported to have a high hydrogen storage capacity. However, activation, wide range of hysteresis at hydrogenation/dehydrogenation, and degradation of hydrogen capacity due to hydriding/dehydriding cycles must be improved for its application. In this study, to improve such problems, we added a Nb. For attaining target material, Ti–1Nb–10Cr, Ti–3Nb–10Cr and Ti–5Nb–10Cr specimens were prepared by a planetary ball mill. The milling process was carried out under a high pressure nitrogen atmosphere. Specimens synthesized were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and thermogravimetric analysis/differential scanning calorimetry (TG/DSC). In order to examine hydrogenation behavior, a pressure-composition-isotherm (PCI) was performed at 323, 373, 423, 473 and 523 K.     

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.     

Formation of BCC TiFe hydride under high hydrogen pressure

We investigated the hydrogenation of a binary TiFe alloy at 5 GPa and 600 °C by in situ synchrotron radiation X-ray diffraction measurements. After formation of a solid solution of hydrogen in TiFe, an order–disorder phase transition in the metal lattice of TiFe occurred, which yielded a BCC TiFe hydride. The unit cell volume of the BCC hydride increased by 21.0% after the hydrogenation reaction. The volume expansion was larger than that of a γ-hydride TiFeH_1.9 prepared by hydrogenation near ambient conditions.     

Hydrogen flux of BCC and FCC PdCuAg membranes

First-principles calculations have been used to study the effects of Ag addition on adsorption and dissociation of H₂ on BCC and FCC PdCu surfaces as well as hydrogen diffusion and recombinative hydrogen desorption through the PdCu membranes. It is found that the Ag addition makes it energetically difficult for the adsorption of H₂ on PdCu surfaces and hydrogen diffusion through PdCu, while could help the recombinative desorption of H atoms from both BCC (110) and FCC (111) surfaces of PdCu. Moreover, substitution of Ag for Pd or Cu would impede or improve the dissociation of H₂ on PdCu surface. Calculations also reveal that the overall hydrogen flux of BCC Pd₈Cu₈ (Pd₈Cu₇Ag) membranes is determined by the recombinative desorption and diffusion when the membrane thickness is smaller and bigger than 10.31 (4.73) μm, respectively. In addition, hydrogen diffusion is the dominant step of hydrogen permeation of FCC PdCu and PdCuAg as well as BCC Pd₇Cu₈Ag membranes. The present results not only agree well with experimental observations in the literature, but also deepen the understanding of the effect of Ag alloying on hydrogen permeation through PdCu membranes.     

Structural characterization and hydrogen storage properties of the Ti31V26Nb26Zr12M5 (M = Fe,Co, or Ni) multi-phase multicomponent alloys

Structure and hydrogen storage properties of three Ti₃₁V₂₆Nb₂₆Zr₁₂M₅ multicomponent alloys with M = Fe, Co and Ni are investigated. The alloys synthesized by arc melting are characterized via X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The as-cast ingots present multi-phase dendritic structures composed mainly of BCC phases and small amounts of C14 Laves phases. Upon hydrogenation, each alloy absorbs around 1.9 H/M (number of hydrogen atoms per metal atoms) at room temperature. XRD of fully hydrogenated samples shows the formation of multi-phase structures composed of FCC and C14 hydrides. Thermo Desorption Spectroscopy (TDS) shows that the hydrogenated alloys present multi-step desorption processes with wide temperature ranges and low onset temperatures. XRD of partially hydrogenated samples indicate the presence of intermediate BCC hydrides. XRD of desorbed samples suggest reversible reactions of absorption/desorption: BCC + C14 alloy ? intermediate BCC hydride + C14 hydride ? FCC + C14 hydrides.     

Optimizing Ti–Zr–Cr–Mn–Ni–V alloys for hybrid hydrogen storage tank of fuel cell bicycle

AB₂-type Ti-based alloys with Laves phase have advantages over other kinds of hydrogen storage intermetallics in terms of hydrogen sorption kinetics, capacity, and reversibility. In this work, Ti–Zr–Cr-based alloys with progressive Mn, Ni, and V substitutions are developed for reversible hydrogen storage under ambient conditions (1–40 atm, 273–333 K). The optimized alloy (Ti_0.8Zr_0.2)_1.1Mn_1.2Cr_0.55Ni_0.2V_0.05 delivers a hydrogen storage capacity of 1.82 wt%, the hydrogenation pressure of 10.88 atm, and hydrogen dissociation pressure of 4.31 atm at 298 K. In addition, fast hydrogen sorption kinetics and low hydriding-dehydriding plateau slope render this alloy suitable for use in hybrid hydrogen tank of fuel cell bicycles.     

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.