Effect of cycling on hydrogen storage properties of Ti2CrV alloy

In the present work, we have studied the hydrogen absorption–desorption properties of the Ti₂CrV alloy, and effect of cycling on the hydrogen storage capacity. The material has been characterized for the structure, morphology, pressure composition isotherms, hydrogen storage capacity, hydrogen absorption kinetics and the desorption profile at different temperatures in detail. The Ti₂CrV crystallizes in body centered cubic (bcc) structure like TiCrV. The pressure composition isotherm of the alloy has been measured at room temperature and at 373K. The Ti₂CrV alloy shows maximum hydrogen storage capacity of 4.37 wt.% at room temperature. The cyclic hydrogen absorption capacity of Ti₂CrV alloy has been investigated at room temperature upto 10th cycle. The hydrogen storage capacity decreased progressively with cycling initially, but the alloy can maintain steady cyclic hydrogen absorption capacity 3.5 wt.% after 5th cycle. To get insight about the desorption behavior of the hydride in-situ desorption has been done at different temperatures and the amount of hydrogen desorbed has been calculated. The TG (Thermo gravimetric) and DTA analysis has been done on uncycled hydride shows that the surface poisoned sample gives a desorption onset temperature of 675K. The DSC measurement of uncycle and multi-cycled saturated hydrides shows that the hydrogen desorption temperature decreasing with cycling.     

Understanding crystal structure roles towards developing high-performance V-free BCC hydrogen storage alloys

Current bottlenecks in the supply and high cost of V have negatively impacted their application. There is great interest in developing V-based hydrogen storage alloys that use less or free V. Here, we investigate the role of V in deliberately designed V-based alloys. Our results affirm that V plays an undeniable role in enhancing hydrogen storage properties. It is found that V maintains the stable single BCC structure but leads to more residual hydrogen (1.4 wt%) because of the high stability of the dihydride and smaller hydriding rate because of the small lattice parameter, which offers unexpected but encouraging perspectives towards reducing the need of V in such alloys. Mo substitution for V effectively alleviates the higher residual hydrogen to achieve a high dehydriding capacity of 2.5 wt%. Moreover, the suction-cast (Ti_0.46Cr_0.54)_97.5Mo_2.5 alloy, which keeps BCC structure after suction-cast process and contains a low-Mo content, also exhibits dehydriding capacity of 2.3 wt%. The enthalpy change as well as dehydriding capacity of V-Free alloys obtained were similar to those reported V-based alloys. These findings are attractive for developing new V-free BCC hydrogen storage alloys and higher hydrogen capacity.     

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.     

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.     

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.     

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.     

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.     

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.     

Influence of Mn or Mn plus Fe on the hydrogen storage properties of the Ti-Cr-V alloy

To increase the hydrogen storage capacity and the plateau pressure of the Ti_0.32Cr_0.43V_0.25 alloy, a fraction of the Cr was replaced with Mn or a combination of Mn and Fe. When Mn was used alone, the effective hydrogen storage capacity increased to about 2.5 wt% though the plateau pressure showed no significant change. When Fe was added with Mn, however, both the effective hydrogen storage capacity and the plateau pressure increased. The BCC (body centered cubic) lattice parameter of the alloy decreased linearly with the Fe content, but it was not affected by Mn alone. The effective hydrogen storage capacity of the Ti_0.32Cr_0.32V_0.25Fe_0.03Mn_0.08 alloy was about 2.5 wt%, higher than 2.35 wt% in the original alloy. The estimated usable hydrogen stored in the Ti_0.32Cr_0.32V_0.25Fe_0.03Mn_0.08 alloy was 2.71 wt% in the temperature and pressure range of 293–353 K and 5–0.002 MPa, respectively.     

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.     

First-principles predictions of potential hydrogen storage materials: Novel sandwich-type ethylene dimetallocene complexes

The hydrogen storage capacities of a sandwich-type ethylene dimetallocene complex (Cp₂Ti₂C₂H₄) are studied using first-principles calculations. It is found that the TiC₂H₄Ti molecule can intercalate into the two cyclopentadienyl (Cp) rings and form a stable sandwich-type complex. Each Ti atom can adsorb a maximum of three H₂ molecules, which corresponds to a gravimetric storage capacity of 4.73 wt%. This hydrogen storage capacity is close to the 2015 target of 5.5% set by the US Department of Energy (DOE) in 2009. Furthermore, the Cp₂Ti₂C₂H₄ molecule proposed in this paper is favorable for both adsorption and desorption of hydrogen molecules at room temperature and ambient pressure because its average binding energy of 0.34 eV/H₂.     

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.     

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.     

Hydrogen storage properties of Ti0.72Zr0.28Mn1.6V0.4 alloy prepared by mechanical alloying and copper boat induction melting

In the present study, two process techniques, mechanical alloying and innovative vacuum copper boat induction melting, were used to produce Ti_0.72Zr_0.28Mn_1.6V_0.4 alloy for hydrogen storage applications. The hydrogen absorption and desorption properties of the alloy were studied. The material structure and phases were characterized by XRD and SEM. The hydrogen absorption and desorption properties of the alloy were measured by an automatically controlled Sieverts apparatus. The results showed that the samples consisted of two main phases, C14 Lave phase and V-base solid solution phase. The maximum capacity of abs/desorption was achieved at mediate temperature (150 °C). The hydrogen capacity of the induction melted samples in various temperatures was higher than that for the samples produced by mechanical alloying method. The maximum absorption capacity of the induction melted and mechanically alloyed samples were 2 and 1.2 wt%, respectively. The maximum desorption capacity of the induction melted and mechanically alloyed samples were 0.45 and 0.1 wt%, respectively.     

Microstructure and hydrogen storage properties of (Ti0.32Cr0.43V0.25) + x wt% La (x = 0–10) alloys

The composite alloy of Ti_0.32Cr_0.43V_0.25 with x wt% La (where x = 0–10) was prepared by arc melting technique. The effect on hydrogen storage capacity, flatness of the plateau pressure, and residual hydrogen was investigated in La added Ti_0.32Cr_0.43V_0.25. Crystalline phase and microstructure of the prepared composite alloy were investigated and characterized by XRD, SEM and TEM. The crystal structure was refinement using Rietveld analysis. The effective hydrogen storage capacity of the composite alloy was found comparable to the parent alloy, when 5 wt% La was added. The effective hydrogen capacity (∼2.31 wt%) was close to that of the parent alloy (2.35 wt%) and the plateau slope was significantly improved from 30.5 of the parent alloy to 14.6. Appropriate mechanisms associated with the improved flatness by the La addition has been discussed in terms of the refined crystalline structure. Using TG/DTA method we have shown the differences in the interaction of residual hydrogen with the BCC phase of both parent alloy and 5 wt % La mixed alloy.     

Enhanced hydrogen storage properties of Ti–Cr–Nb alloys by melt-spin and Mo-doping

Ti–Cr–Nb hydrogen storage alloys with a body centered cubic (BCC) structure have been successfully prepared by melt-spin and Mo-doping. The crystalline structure, solidification microstructural evolution, and hydrogen storage properties of the corresponding alloys were characterized in details. The results showed that the hydrogen storage capacity of Ti–Cr–Nb ingot alloys increased from 2.2 wt% up to around 3.5 wt% under the treatment of melt-spin and Mo-doping. It is ascribed that the single BCC phase of Ti–Cr–Nb alloys was stabilized after melt-spin and Mo-doping, which has a higher theoretical hydrogen storage site than the Laves phase. Furthermore, the melt-spin alloy after Mo doping can further effectively increase the de-/absorption plateau pressure. The hydrogen desorption enthalpy change ΔH of the melt-spin alloy decreased from 48.94 kJ/mol to 43.93 kJ/mol after Mo-doping. The short terms cycling test also manifests that Mo-doping was effective in improving the cycle durability of the Ti–Cr–Nb alloys. And the BCC phase of the Ti–Cr–Nb alloys could form body centered tetragonal (BCT) or face center cubic (FCC) hydride phase after hydrogen absorption and transform to the original BCC phase after desorption process. This study might provide reference for developing reversible metal hydrides with favorable cost and acceptable hydrogen storage characteristics.     

Characterization of microstructure and surface oxide of Ti1.2Fe hydrogen storage alloy

In this study, we investigated the microstructures, hydrogen absorption kinetics, and oxide layers of TiFe and Ti_1.2Fe hydrogen storage alloys. Whereas the TiFe alloy has a single phase, the Ti_1.2Fe alloy is composed of three phases: TiFe, Ti₂Fe, and Ti₄Fe. Under no thermal activation process, the TiFe alloy does not absorb hydrogen, though the Ti_1.2Fe alloy starts to absorb hydrogen after 4 min of incubation time. From the XPS results, it is revealed that the Ti concentration in the oxide layer on the Ti₄Fe phase is higher than that on the TiFe phase, indicating that the Ti concentration in the oxide layer would be important in improving hydrogen absorption kinetics. Based on these results, the hydrogen absorption kinetics could be improved by adjusting composition, enabling the formation of a Ti-rich oxide layer.     

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.     

Effects of Zr doping on activation capability and hydrogen storage performances of TiFe-based alloy

Activation difficulty is the key problem limiting the application of TiFe-based hydrogen storage alloys. The addition of transition group elements helps to improve the activation properties of TiFe-based hydrogen storage alloy. In our previous work, the Ti_1.08Y_0.02Fe_0.8Mn_0.2 alloy exhibits extremely high hydrogen storage capacity (1.84 wt%) at room temperature with excellent kinetic properties, but it still needs an incubation period of about 1500s. In this study, the composition of Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_x (x = 0, 0.02, 0.04, 0.06, 0.08) alloys was prepared by electromagnetic induction melting. The quantitative analysis of elements by energy dispersive spectrometer shows that in the second phase region containing Zr, the content of Ti element is significantly higher than that of Fe. Meanwhile, the first-principle calculation on Zr-doped TiFe system indicates that Zr is more attractive to substitute Ti than Fe. Therefore, the doping of Zr partially replaces the Ti. The solubility of Zr in TiFe is limited, when x ≤ 0.04, the alloy consists of pure TiFe phase. When x > 0.4, the excess Zr forms precipitates, which reduces the reversible hydrogen absorption and desorption capacity of the TiFe alloy. The addition of Zr significantly shortens the activation time and reduces the plateau pressure of TiFe alloys. The Ti_1.08Y_0.02Fe_0.8Mn_0.2Zr_0.04 alloy can be directly activated without the incubation period and its absolute values of enthalpy change (ΔH) and entropy change (ΔS) are minima (ΔH for 23.2 kJ/mol and ΔS for 83.1 J/mol/K).     

High-performance La–Mg–Ni-based alloys prepared with low cost raw materials

La–Mg–Ni-based hydrogen storage alloys showed good application prospects owing to their high hydrogen storage capacity. However, the poor cycling stability was a key problem. In order to improve the cycling stability, low cost YFe_0.85 master alloy was used as raw material to prepare La–Mg–Ni-based La_0.8-xY_xMg_0.2Ni_3-0.85xFe_0.85x (x = 0.50, 0.55, 0.60) hydrogen storage alloys by powder sintering method. The alloys were mainly composed of PuNi₃ phase and MgCu₄Sn phase. With the increase of Y and Fe, the cell parameters of PuNi₃ phase decreased. Lower mismatch coefficient promoted the cycling stability. As the case of x = 0.60, the capacity retention rate rose up to 95.45%. Aside from the cycling stability, appropriate substitution content contributed to higher capacity and satisfactory kinetics. As the case of x = 0.55, the hydrogen storage capacity reached 1.529 wt%, and hydriding time for the x = 0.60 alloy shrank to 76.7% of that for alloys without Y and Fe at 303 K.