Crystal structure and hydrogen storage properties of AB-type TiZrNbCrFeNi high-entropy alloy

The crystal structure and hydrogen storage properties of a novel equiatomic TiZrNbCrFeNi high-entropy alloy (HEA) were studied. The alloy, which had an AB-type configuration (A: elements forming hydride, B: elements with low chemical affinity with hydrogen), was selected with the aid of thermodynamic calculations employed by the CALPHAD method. The arc-melted AB-type TiZrNbCrFeNi alloy showed the presence of two C14 Laves phases in different fractions but with slight differences in unit cell parameters. Hydrogen storage properties investigated through pressure-composition-temperature absorption and desorption isotherms at different temperatures revealed that the alloy could absorb 1.5 wt% of hydrogen at room temperature without applying any activation procedure, but full desorption was not obtained. At 473 K, the alloy was able to reversibly absorb and fully desorb 1.1 wt% of hydrogen. After full hydrogenation at 473 K, the initial metallic C14 Laves phases were converted into their respective Laves phase hydrides. Under cycling, the fractions of two C14 Laves phases changed while one of the phases was more active to accommodate the hydrogen atoms. After dehydrogenation at 473 K, the alloy presented a single C14 Laves phase. The microstructural analysis, before and after cycling, showed a very well homogeneous microstructure and good distribution of elements.     

Machine learning analysis of alloying element effects on hydrogen storage properties of AB2 metal hydrides

Zirconium-titanium-based AB₂ is a potential candidate for hydrogen storage alloys and NiMH battery electrodes. Machine learning (ML) has been used to discover and optimize the properties of energy-related materials, including hydrogen storage alloys. This study used ML approaches to analyze the AB₂ metal hydrides dataset. The AB₂ alloy is considered promising owing to its slightly high hydrogen density and commerciality. This study investigates the effect of the alloying elements on the hydrogen storage properties of the AB₂ alloys, i.e., the heat of formation (ΔH), phase abundance, and hydrogen capacity. ML analysis was performed on the 314 pairs collected and data curated from the literature published during 1998–2019, comprising the chemical compositions of alloys and their hydrogen storage properties. The random forest model excellently predicts all hydrogen storage properties for the dataset. Ni provided the most contribution to the change in the enthalpy of the hydride formation but reduced the hydrogen content. Other elements, such as Cr, contribute strongly to the formation of the C14-type Laves phase. Mn significantly affects the hydrogen storage capacity. This study is expected to guide further experimental work to optimize the phase structure of AB₂ and its hydrogen sorption properties.     

Effects of H2O2 addition to the cell balance and self-discharge of Ni/MH batteries with AB5 and A2B7 alloys

In this paper we have compared nickel/metal hydride batteries made from AB₅ and Nd-only A₂B₇ alloys with or without addition of hydrogen peroxide (H₂O₂). The biggest advantages Nd-only A₂B₇ alloys have over AB₅ alloys are: a higher positive electrode utilization rate, lower initial internal resistance and less resistance increase after a 60 °C storage, and higher capacity and resistance degradation during cycling. The hydrogen peroxide was used as an oxidation agent and was added into the electrolyte before closing the cells. The H₂O₂ can oxidize both Co(OH)₂ in the positive electrode and MH alloy in the negative electrode. From the test results, H₂O₂ oxides the MH alloy preferentially over the Co(OH)₂ in the case of AB₅ alloy. This preferential oxidation is reversed in the case of the A₂B₇ alloy in which Co(OH)₂ is oxidized first. In cells made from both alloys, the addition of H₂O₂ prevented the venting of cells during formation, increased the utilization of positive electrode, improved the 60 °C charge retention, and increased the mid-point voltage after 300 cycles. Additionally the H₂O₂ also improved the cell balance for A₂B₇ alloy by decreasing the over-discharge reservoir in the negative electrode and reducing the capacity degradation in A₂B₇ alloy. However, the addition of H₂O₂ in cells made with AB₅ alloy deteriorated the cell balance by increasing the over-discharge reservoir in the negative electrode. The different cell balance and failure mechanisms for the two alloy compositions and H₂O₂ additive were compared and discussed.     

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.     

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.     

Synthesis,characterization and first hydrogen absorption/desorption of the Mg35Al15Ti25V10Zn15 high entropy alloy

In this work, a novel Mg-containing high entropy alloy (HEA), namely Mg₃₅Al₁₅Ti₂₅V₁₀Zn₁₅, was synthesized, characterized, and the phases formed after synthesis and after first hydrogen absorption/desorption were analyzed. The alloy composition was conceptualized aiming at increasing the atomic fraction of lightweight elements (35 at.% Mg and 15 at.% Al) to increase its gravimetric hydrogen storage capacity. Ti and V were selected to increase the hydrogen affinity of the alloy. Finally, Zn was selected as an alloying element because of its negative enthalpy of mixing with Mg, which could favor the formation of solid solution and avoid Mg segregation. The Mg₃₅Al₁₅Ti₂₅V₁₀Zn₁₅ HEA was synthesized by high-energy ball milling (HEBM) in two different routes: under argon atmosphere (mechanical alloying – MA) and under hydrogen pressure (reactive milling – RM). The sample produced by MA was composed of a body-centered cubic (BCC) phase with an amount of unmixed Mg. When hydrogenated at 375 °C and under 4.0 MPa of H₂, the MA sample absorbed about 2.5 wt% of H₂ by forming a mixture of MgH₂, a face-centered cubic (FCC) hydride, a body-centered tetragonal (BCT) phase and MgZn₂. The desorption sequence upon heating was investigated and it was shown that the MgH₂ is the first to desorb. The sequence of desorption is completed by the decomposition of the FCC hydride and BCT phase leading to the formation of the BCC phase. On the other hand, the alloy produced by RM was composed of a mixture of an FCC hydride and MgH₂. During desorption upon heating, the MgH₂ is also the first to desorb. At lower temperatures (below 350 °C), the FCC hydride decomposes partially forming the BCT phase. At higher temperatures, both FCC hydride and BCT phase decomposes forming the BCC phase. The desorption capacity of the RM sample was 2.75 wt% H₂.     

On structural model of AB5-type multi-element hydrogen storage alloy

The present study is focused on the development of a structural model for multi−element AB₅−type alloy which can correlate the hydrogenation characteristics with structural parameters. At present no such model is available, which can predict the trend of hydrogenation properties like hydrogen storage capacity, heat of formation and plateau pressure of multi−element hydrogen storage material. It is done by trial and error method. In present investigation efforts are made to correlate atomic radius of substituted elements with heat of formation of hydride, plateau pressure and hydrogen storage capacity by calculating equivalent radius of B (r_B*) in multi−element composition, contraction in A−B bond, radius of voids and ratio of r_A with r_B*. The heat of formation decreases and hydride stability increases with increasing value of r_B* and contraction in A−B bond. While hydrogen storage capacity decreases with increasing value of r_B* and contraction in A−B bond. Based on this, correlation between thermodynamic and structural properties has been established and structural model has been developed.     

Substitutional effect of Ti-based AB2 hydrogen storage alloys: A density functional theory study

Stability of AB₂ alloy in Laves phases C14 and C15 were studied by first-principle density functional theory simulations. A range of different combinations of B and C elements in the Ti_1−xC_xB₂ alloys were considered. The formation energies of these alloys generally increase with the unit cell volumes of alloys. The volume also affects the stability of the corresponding metal hydride. We find that the formation energies and the hydrogenation enthalpies of AB₂ alloys are likely to be determined by at least three factors: electronegativity, atomic radius and covalent radius. The enthalpies of AB₂ hydrides increase with increasing compositionally-averaged electronegativity and volume change upon hydrogenation. However, the enthalpies of AB₂ hydrides decrease with increasing compositionally-averaged atomic and covalent radii. This study provides useful insights for future exploration of AB₂-type alloys for hydrogen storage applications.     

Hydrogen-induced phase transition of MgZrTiFe0.5Co0.5Ni0.5 high entropy alloy

In this study, the MgZrTiFe_0.5Co_0.5Ni_0.5 high entropy alloy was processed by high-energy ball milling under both argon and hydrogen atmospheres. The hydrogen storage behavior of the samples was evaluated by combination of thermal analyses and in-situ synchrotron powder X-ray diffraction. It is shown that this alloy forms a body-centered cubic (BCC) structure when milled under argon pressure. The BCC phase is capable to absorb up to 1.2%wt. of hydrogen and during absorption it undergoes a phase transition forming a face-centered cubic (FCC) high entropy hydride. This FCC hydride can be directly synthesized by high-energy ball milling under hydrogen pressure.     

Structural,hydrogen storage,and electrochemical properties of Laves phase-related body-centered-cubic solid solution metal hydride alloys

Structure, gaseous phase hydrogen storage, and electrochemical properties of a series of Laves phase-related BCC solid solution metal hydride alloys with BCC/C14 ratios ranging from 0.09 to 8.52 were studied. Some properties are correlated to the phase abundance and V-content in the alloy with monotonic evolutions, for example, lattice constant, phase abundance, and hydrogen storage pressure. Other properties such as gaseous phase capacities, PCT hysteresis, high-rate dischargeability, and bulk hydrogen diffusion correlate better with the C14 phase crystallite size, which are considered to be more related to the synergetic effect between main and secondary phases. In contrast with conventional metal hydride alloys used in NiMH batteries, the electrochemical discharge capacities of these alloys are not between the maximum and the reversible hydrogen storage measured in the gaseous phase. The current study's alloys have electrochemical capacities that are insensitive to composition but have much room for improvement, with high-rate dischargeabilities that are superior compared to other commercially available alloys. With further research, these alloys show potential for high-rate battery applications.     

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.     

Superior electrochemical performances of LaMgNi alloys with A2B7/A5B19 double phase

Here, phase transformation and electrochemical characteristics of non-stoichiometry La₄MgNi_x (x = 16, 17 and 18) hydrogen storage alloys were studied. It is found that after annealed at 1223 K for 24 h, the minor AB₃ and AB₅ phases in La₄MgNi₁₆ alloy transform into A₂B₇ phase by a peritectic reaction and the La₄MgNi₁₆ alloy shows a A₂B₇ single phase structure. Double phase structures of A₂B₇/A₅B₁₉ are obtained in La₄MgNi₁₇ and La₄MgNi₁₈ alloys after annealed at the same condition. The abundance of A₅B₁₉ phase increases as x increases, indicating the increasing x value contributes to the formation of A₅B₁₉ phases. Electrochemical studies show that the maximum discharge capacity and capacity retention at the 100^th charge/discharge cycles (S₁₀₀) of A₂B₇ single phase La₄MgNi₁₆ alloy is 373 mAh g⁻¹ and 78.4%, respectively. The appearance of A₅B₁₉ (minor) phase in the La₄MgNi₁₇ alloy makes a remarkable improvement in the discharge capacity from 373 mAh g⁻¹ to 388.8 mAh g⁻¹, as well as the S₁₀₀ from 78.4% to 90.1%. It is believed that the LaMgNi-based alloy with the A₂B₇(main)/A₅B₁₉(minor) phase structure possesses the good overall electrochemical properties and is applicable to the high-power and long-cycle life negative electrode material for nickel metal hydride batteries.     

Function of aluminum in crystal structure of rare earth–Mg–Ni hydrogen-absorbing alloy and deterioration mechanism of Nd0.9Mg0.1Ni3.5 and Nd0.9Mg0.1Ni3.3Al0.2 alloys

We investigated in detail the effect of incorporating Al in the crystal structure of rare earth–Mg–Ni hydrogen-absorbing alloys, which were developed as candidate materials for the metal hydride (MH) negative electrode in commercial Ni–MH batteries, using synchrotron powder X-ray diffraction. Partially substituting the Ni part with Al changes the lattice parameter of the major A₂B₇ phase, eliminating a mismatch between the AB₂ units and the AB₅ units. The change of the lattice parameter in the alloy leads to good hydrogen reversibility and good durability. Furthermore, we observed the alloy after 30 hydrogen absorption–desorption cycles using a scanning transmission electron microscope image. Consequently, we found that the surface layer of the Nd_0.9Mg_0.1Ni_3.5 alloy had an amorphous state, while the surface layer of the Nd_0.9Mg_0.1Ni_3.3Al_0.2 alloy did not. We confirmed that the deterioration mechanism of the Nd_0.9Mg_0.1Ni_3.3Al_0.2 alloy was due to partial expansion of the AB₂ unit along the c-axis; the local area of deterioration expanded during the cycling period.     

Insights into the structure–performance relationship in La–Y–Ni-based hydrogen storage alloys

La–Y–Ni-based alloys with different phase structures possess various physicochemical properties and thereby different hydrogen storage performances. In this work, the hydrogen storage and electrochemical properties of LaY_1·9Ni₁₀Mn_0·5Al_0.2 alloys with different phase structures (2H-A₂B₇, 3R-A₂B₇ and 2H-A₅B₁₉) are investigated. All the investigated phases present two plateaus in pressure-composition-temperature (PCT) curves, which is induced by the different hydrogen location (A₂B₄ or AB₅) during the hydrogen absorption process. All of the LaY_1·9Ni₁₀Mn_0·5Al_0.2 series alloys possess good hydrogen storage capacities and electrochemical properties. The cyclic stability of the alloys is determined by the anti-corrosive properties of the alloys to electrolyte, neither the phase transition nor the previously believed pulverization. This work, by systematically investigating phase transitions during the annealed process and elucidating the key factors of influencing the cyclic stability, is useful for the design of La–Y–Ni-based alloy for the application of hydrogen storage and beyond.     

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.     

Theoretical model on the electronic properties of multi-element AB5-type metal hydride

Nowadays, multi-element alloys are preferred over binary alloys for application point of view. The hydrogenation properties strongly depend on the thermodynamic, structural and electronic properties of the alloys. At present, no model is available which can predict the hydrogen storage properties of the multi-element alloy, before actual synthesis of the alloy. In the present investigation, efforts are made to develop a theoretical mathematical model to predict the hydrogenation properties of multi-element AB₅-type metal hydride. The present investigation deals with the various electronic parameters which may affect the hydrogenation characteristics of the metal hydride. Based on all such parameters, an electronic factor has been proposed for AB₅-type alloys. Electronic factor has been combined with the structural and thermodynamical factor to propose a new combined factor, which was further correlated with the hydrogen storage capacity of the alloy. Atomic radius and electronic configuration of substituted elements in the multi-element AB₅-type hydrogen storage alloy have been found as key players to predict the hydrogenation properties of the alloys before synthesis. It has been shown that in the case of alloy series with multiple substitutions, the combined factor is more relevant in deciding the hydrogen storage capacity in comparison to electronic factor alone. Combined factor is directly proportional to the hydrogen storage capacity. All the three factors thermodynamic, structural and electronic together may lead to the prediction of pressure-composition isotherm of the multi-element AB₅-type hydrogen storage alloy.     

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.     

Effects of lithium addition in AB2 metal hydride alloy by solid-state diffusion

In this study, microstructures and electrochemical properties of a Zr-based AB₂-type metal hydride alloy (Ti_12.1Zr_21.3V_10.5Cr_7.5Mn_8.1Co_8.0Ni_31.7Al_0.8) doped with 6 wt% LiH by solid-state diffusion at different temperatures were investigated and compared with those of the base alloy sintered at the same conditions. Structural study by x-ray diffraction analysis exhibited a lattice expansion in the main C14 phase of the Li-doped alloys with a relatively low sintering temperature, indicating that Li incorporated into the C14 Laves phase but was evaporated at higher sintering temperatures. Addition of Li deteriorated both the electrochemical capacity and activation easiness; however, it improved the high-rate dischargeability (HRD). The benefit of Li addition on HRD can be associated with the increase in amount of metallic nickel clusters embedded in the surface oxide (estimated by the saturated magnetic susceptibility) and was a strong function of sintering temperature. As the sintering temperature increased, HRD increased due to the increase in amount of surface catalytic metallic nickel.     

Hydrogen storage in magnesium based-composite hydride through hydriding combustion synthesis

Mg and Zr-based AB₂ hydride composite was prepared by hydriding combustion synthesis (HCS) and the hydriding–dehydriding properties of HCS Mg–(20, 40 wt%)AB₂ products were extensively examined. The dehydriding onset temperatures of the HCS Mg–20AB₂ and Mg–40AB₂ composites were 533 K and 493 K, respectively, which were lower than that of the MgH₂. It is suggested that the well-dispersed Zr-based AB₂ phase in a Mg composite prepared by HCS plays a crucial role in significantly improving its kinetic properties. Especially, the HCS Mg–20AB₂ composite showed fully activated hydrogenation within the 8th cycle and reached a saturated H₂ absorption capacity of 5.7 wt.% at 573 K in 10 min. In addition, the hydrogen capacity did not show any significant decrease even after 86 cycles. These results display a potential excellence of HCS processing in preparing Mg-based hydrogen storage materials.     

Effect of heat treatment on microstructure and hydrogen storage properties of mass-produced Ti0.85Zr0.13(Fex–V)0.56Mn1.47Ni0.05 alloy

Hydrogen storage as a metal hydride is the most promising alternative because of its relatively large hydrogen storage capacities near room temperature. TiMn₂-based C14 Laves phases alloys are one of the promising hydrogen storage materials with easy activation, good hydriding-dehydriding kinetics, high hydrogen storage capacity and relatively low cost. In this work, multi-component, hyper-stoichiometric TiMn₂-based C14 Laves phase alloys were prepared by a vacuum induction melting method for a hydrogen storage tank and electrochemical applications. Since TiMn₂ alloy shows high equilibrium plateau pressure, Ti and Mn were partially replaced by other metallic elements such as Zr, V, and Ni. Since pure vanadium (V) is quite expensive, the substitution of the V element in these alloys has been tried and some interesting results were achieved by replacing V by commercial ferrovanadium, FeV, raw material. XRD pattern and SEM analysis of the as-cast VIM Ti_0.85Zr_0.13(Fe_x–V)_0.56Mn_1.47Ni_0.05 alloy revealed that the main phase of the C14 Laves phase and the secondary phase of FeO formed along the grain boundary. Also, the as-cast VIM Ti_0.85Zr_0.13(Fe_x–V)_0.56Mn_1.47Ni_0.05 alloy showed a high plateau pressure slope from measurement of P–C-isotherms, which led to a decrease in reversible hydrogen storage capacity. It was found that a suitable heat treatment was very effective in the formation of the C14 single phase and improving the sloping properties. The improvement of sloping properties was mainly attributed to the strain energy effect and the homogeneity of chemical composition. In this work, hydrogen storage capacity was evaluated by a volumetric method using P–C-isotherms and a gravimetric method using magnetic suspension balance.