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.     

Microstructure and hydrogenation thermokinetics of ZrTi0.2V1.8 alloy

The microstructures and phase composition of the pseudobinary ZrTi_0.2V_1.8 alloy were examined by scan electron microscope (SEM) and X-ray diffraction (XRD). Before hydrogenation, the hypoeutectic structure accompanied with ZrV₂ + (ZrV₂ + Zr) spherical-like texture has been observed in ZrTi_0.2V_1.8 and the dominant phase could be ascribed to the C15 Laves phase. Hydrogen absorption pressure–composition isotherms (P–C isotherms) and hydriding kinetics of ZrTi_0.2V_1.8 were investigated by pressure reduction method using Sievert apparatus from 673 to 823 K. At hydrogen concentration 0.65 (H/A), the relative partial molar enthalpy and entropy calculated by Van’t Hoff equation are −60 ± 1 kJ mol⁻¹ and −119 ± 1 J mol⁻¹ K⁻¹, respectively. In addition, two stages in the hydrogen absorption reaction between 673 and 823 K could be attributed to the different hydrogen absorption mechanisms including redistribution of the hydrogen atoms in the hydride phase and the diffusion of hydrogen in the β-phase. The activation energy E_a of the alloy is ∼3.6 kJ mol⁻¹ for the first absorption stage and ∼61.9 kJ mol⁻¹ for the second one.     

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.     

Effects of substituting Al for Cr in the Ti0.32Cr0.43V0.25 alloy on its microstructure and hydrogen storage properties

Substituting Al for part of Cr in Ti_0.32Cr_0.43V_0.25 alloy caused BCC lattice parameter and crystallite size to increase and lattice strain to decrease. These microstructual changes caused the decrease in the hydrogen storage capacity and the increase in both the plateau pressure and the hysteresis. The results were contradictory to the general observation that the plateau pressure decreases with the increase in the lattice parameter. The fact that the bond structure between H and Al and that between H and the transition metals differ can account for this discrepancy. This difference also resulted in the decrease in the hydrogen storage capacity. The increased hysteresis resulting from the increase in the Al content can be ascribed to the increased crystallite size and the decreased lattice strain.     

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.     

Effect of in-situ boron doping on hydrogen adsorption properties of carbon nanotubes

Floating catalyst chemical vapor deposition method was used for the synthesis of boron doped carbon nanotubes (BCNTs) using ethanol, triethyl borate and ferrocene as carbon source, boron source and catalyst precursor, respectively. The synthesized BCNTs were characterized by transmission electron microscopy, Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy (XPS). The hydrogen adsorption activity was studied for BCNTs along with undoped single walled and multi walled carbon nanotubes. Significant enhancement in the hydrogen storage value was found in doped CNTs as compared to the other undoped CNTs. Hydrogen storage for BCNTs was found to be 2.5 wt% at 10 bar and 77 K. In-situ doped BCNTs gives higher hydrogen adsorption as compared to ex-situ doped BCNTs. The Langmuir adsorption isotherm was found to be suitable for describing the adsorption isotherm as compared with Freundlich isotherm. Maximum adsorption capacity was about 9.8 wt% at 77 K. Pseudo second order kinetics was followed by BCNTs for hydrogen adsorption.     

Hydrogen storage properties and thermal stability of V35Ti20Cr45 alloy by heat treatment

The crystal structure, microstructure, hydrogen storage properties and thermal stability of the as-cast and annealed V₃₅Ti₂₀Cr₄₅ alloys prepared by arc-melting were studied in this work. It was confirmed that the as-cast alloy is a body-centered cubic (bcc) single phase, while it consists of bcc main phase and C14-typed Laves secondary phase after annealed at 973 K for 72 h. As a result of the microstructure change, the activation performance and kinetic properties of the annealed alloy are improved greatly due to the catalysis of C14-typed Laves secondary phase in the annealed alloy. The kinetic mechanism of hydrogen absorption/desorption processes in the as-cast and annealed alloys was discussed using the Johnson-Mehl-Avrami (JMA) equation. Based on the plateau pressure data from pressure-composition-temperature (PCT) measurements with the Van't Hoff equation, the calculated formation enthalpies of the hydride formed in the as-cast and annealed alloys indicate that heat treatment results in lower thermal stability of the hydride in the as-cast alloy. Furthermore, using the Kissinger method with the peak temperatures from differential scanning calorimeter (DSC) measurements at different heating rates, the calculated activation energies of the dehydrogenation in the as-cast and annealed alloys suggest that heat treatment is very beneficial to improve hydrogen absorption/desorption capacities in the alloy.     

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).     

Thermodynamic and electrochemical hydrogenation properties of LaNi5 − xInx alloys

Hydrogenation properties of LaNi_5 − xIn_x alloys (x = 0.1, 0.2 and 0.5) were examined by their direct reaction with gaseous hydrogen and by cathodic charging in 6 M KOH solution. The gas phase measurements were carried out using Sievert's type apparatus in 300–400 K temperature range and at hydrogen pressures up to 40 bars. Indium substitution for Ni in LaNi₅ significantly modifies the hydrogenation behavior, decreasing the equilibrium pressure of hydrogen and limiting the hydrogen capacity as compared to LaNi₅. The LaNi_4.9In_0.1 revealed a distinct presence of two pressure plateaus on the high temperature isotherms. Apart from the α-phase (hydrogen solid solution) and β-phase (LaNi₅H₆ hydride), formation of a new σ*-hydride phase was postulated at the hydrogen content extended over the region of H/f.u. = 1.3–1.8. Thermodynamic functions: enthalpy and entropy of the hydrogen absorption process were calculated from the H₂-pressure/composition (p–c) isotherms at several temperatures, applying the Van't Hoff's (lnp − 1/T) dependence. Electrochemical galvanostatic hydrogenation experiments at 185 mA/g charge/discharge rate revealed the greatest discharge current capacity of 319 mAh/g for LaNi_4.9In_0.1 alloy after 4–5 cycles. The hydrogen discharge capacities decrease with further increase of indium content in the alloy.     

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.     

The effect of rapid solidification on the microstructure and hydrogen storage properties of V35Ti25Cr40 hydrogen storage alloy

The microstructures and hydrogen storage properties of as-cast and rapidly solidified V₃₅Ti₂₅Cr₄₀ alloys have been investigated in this paper. The results showed that the rapid solidification refined the dendritic microstructure and altered the element distribution of the alloy. And through the positron annihilation measurements of the vacancy trapping rate (K_d1) and vacancy-trapped positron annihilation lifetime (τ₂), it was found that the rapid solidification increased the vacancy concentration and at the same time decreased the vacancy size in the alloy. The XRD results showed that the rapid solidification also significantly enlarged the alloy's lattice parameter. As a result of the microstructure change, the hydrogen absorption capacity and hydrogen absorption rate were increased; and the kinetic mechanism of hydrogen absorption was changed from 3-D diffusion control in the as-cast alloy to chemical reaction control in the rapidly solidified alloy; but the activation property was to some extent weakened after the rapid solidification.     

Deuterium storage of Ti40Zr40Ni20 icosahedral quasicrystal

Ti₄₀Zr₄₀Ni₂₀ icosashedral quasicrystal was observed to load hydrogen in a much lower capacity than similar Ti–Zr–Ni alloys. To verify the result, the alloy is further studied by using deuterium instead of hydrogen in this work. With a home-made gas–solid reaction system, XRD and XPS techniques, the investigation was conducted on deuterium absorption and desorption properties of Ti₄₀Zr₄₀Ni₂₀ alloy and its phase stability during the deuteration course. It is shown that the quasicrystal can load deuterium rapidly in an elevated volume of 11.5 mmol·D₂/g·M (D₂ denotes deuterium molecular and M the metal). After the full storage of deuterium, the quasicrystal phase remained, however the quasilattice expanded at a rate of 6.28%, revealing the occurrence of severe quasilattice stress. The solution of deuterium in the alloy caused the increase of binding energy of the metal elements, as much as 0.4 eV for Ti, 0.6 eV for Zr and 0.1 eV for Ni, which reflects the location of deuterium near Ti and Zr. The deuterium release was very slow at low temperature and could be complete at least above 610 °C. Based on the gained results, the quasilattice shrink would be more reasonable to explain the big difficulty of the desorption.     

Hydrogenation study of suction-cast Ti40Zr40Ni20 quasicrystal

Suction casting was predicted to be an usable method for improving the hydriding kinetics of Ti/Zr-based icosahedral quasicrystals (IQCs) in our previous work. To further determine it, a suction-cast Ti₄₀Zr₄₀Ni₂₀ IQC alloy was used for hydrogenation studies by Pressure Composition Isotherm (PCI) and Temperature Programmed Desorption (TPD) techniques. The results showed that, this alloy absorbed hydrogen rapidly with obvious hydrogen pressure plateau and some reversibility, however, displayed very limited hydrogen capacity (about 0.7 wt.%) and low equilibrium pressure. After several hydrogenation/dehydrogenation cycles, the IQC structure transformed into two hydride phases, ZrH_2−x and one unknown, both of which decomposed at above 600 °C, suggesting high thermo-stability for them. On the whole, indeed the suction-casting method can increase the hydrogen absorption rate of Ti/Zr-based IQCs, however, the hydrogenation properties of the Ti₄₀Zr₄₀Ni₂₀ IQC alloy still need a mighty advancement.     

Hydrogenation of C14 Laves phase alloy: CaLi2

The CaLi₂ alloy which was prepared by the induction melting method has been successfully hydrogenated. The CaLi₂ alloy synthesized had a hexagonal C14-type Laves phase structure and absorbed 6.8–7.1 mass% hydrogen under the temperature range from 273 K to 393 K. The hydrogenated CaLi₂ which consisted of CaH₂ and LiH hydride phases did not desorb hydrogen under 10 kPa-H₂ at the same temperatures. The hydrogen absorption kinetics measured under 3.1 MPa-H₂ at room temperature showed that the hydrogen content reached to 6 mass% in 10 s. No obvious hydrogen desorption from the hydrogenated CaLi₂ was observed even after evacuation for 20 h at 623 K.     

Structure, morphology and hydrogen storage properties of a Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy

The Ti_0.97Zr_0.019V_0.439Fe_0.097Cr_0.045Al_0.026Mn_1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H₂, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H₂ and −99.8 ± 0.2 J/mol H₂/K for absorption and 25.4 ± 0.1 kJ/mol H₂ and 108.5 ± 0.2 J/mol H₂/K for desorption, indicating the presence of a significant hysteresis effect.     

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 permeation properties of Pd-coated V89.8Cr10Y0.2 alloy membrane using WGS reaction gases

The influence of co-existing gases on the hydrogen permeation was studied through a Pd-coated V_89.8Cr₁₀Y_0.2 alloy membrane. Preliminary hydrogen permeation experiments have been confirmed that hydrogen flux was 6.26 ml/min/cm² for a Pd-coated V_89.8Cr₁₀Y_0.2 alloy membrane (thick: 0.5 mm) using pure hydrogen as feed gas. Also, the hydrogen permeation flux decreased with decrease of hydrogen partial pressure at constant pressure when H₂/CO₂ and H₂/CO₂/H₂S mixture applied as feed gas respectively and permeation fluxes were satisfied with Sievert's law in different feed conditions. It was found from XRD and SEM results after permeation test that the Pd-coated V_89.8Cr₁₀Y_0.2 alloy membrane had good stability and durability for various mixture feeding conditions.     

Distinct synergistic effect in Ti0.10Zr0.15V0.35Cr0.10Ni0.30 + 1.0 wt.% LaNi5 hydrogen storage composite electrode

Distinct synergistic effect of Ti_0.10Zr_0.15V_0.35Cr_0.10Ni_0.30 + 1.0 wt.% LaNi₅ hydrogen storage composite prepared by two-step arc-melting has been analyzed and discussed systematically. X-ray diffractometry (XRD) and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS) show that the main phase of composite alloy is composed of V-based solid solution phase with a BCC structure and C14 Laves phase with hexagonal structure, while secondary phase also exists in the composite alloy. Electrochemical properties of the composite alloy electrode have significantly been improved. It is suggested that distinct synergistic effect appears in the activation process, in the composite process, in the cyclic process, in the discharge process at low/high temperature and at different current density, and in the charge-transfer resistance and the exchange current density for the hydrogen storage composite electrode, which is probably ascribed to the formation of the secondary phase in the composite alloy.