Dihydride Phase Research Articles

Evaluation of hydrogen storage performance of Ti0.25Zr0.25V0.15Nb0.15Ta0.2 high-entropy alloy using calorimetric technique

The features of phase transformations during interaction of BCC-type Ti0.25Zr0.25V0.15Nb0.15Ta0.2 high-entropy alloy (HEA) with hydrogen and the accompanying thermal effects have been examined using the Tian-Calvet calorimetric technique. The alloy was produced by the pendent drop melt extraction method in the form of microfibers and then coated with a thin layer of palladium to eliminate the preliminary high-temperature activation and enable first hydrogenation at room temperature. Two reaction stages were distinguished on the measured heat emission curves. Using XRD data, these stages were attributed to the formation of a BCC-type hydrogen solid solution and a phase transition to FCC-type dihydride phase. Recorded values of the reaction enthalpy decrease in absolute terms from 145 kJ/mol H2 (dilute solid solution) to 73–77 kJ/mol H2 (BCC→FCC) and to 20 kJ/mol H2 (H solution in FCC dihydride). The overall heat emission of the entire process as a whole is 110 kJ/ mol H2. The course of the dehydrogenation process depends strongly on the applied temperature and at 493 K results in the formation of a BCT-type monohydride, which was not detected during hydrogenation. The detailed calorimetric data obtained here significantly change the previously existing ideas about thermal effects during this type HEAs hydrogenation, based on calculations using the Van't-Hoff equation. The reported findings are of special importance for various hydrogen related applications of HEAs

Thermochemical Analysis of Hydrogenation of Pd-Containing Composite Based on TiZrVNbTa High-Entropy Alloy

The microcalorimetric hydrogen titration technique combined with conventional volumetric measurements has been used to reveal peculiarities of the hydrogenation of the single-phase TiZrVNbTa equiatomic high-entropy alloy. The alloy has been produced in the form of microfibers by the pendent drop melt extraction technique. Palladium coating of the fibers has been applied to enable first hydrogenation at room temperature without additional activation. An analysis of the obtained data allows us to evaluate the dependence of hydrogenation enthalpy on the hydrogen concentration in the alloy. Three concentration ranges, presumably related to the formation of the hydrogen solid solution, monohydride and dihydride phases, have been identified, and the corresponding ΔH values of about −100, −80 and −60 kJ/mol H2, respectively, have been determined.

Exploring the Hydrogen Sorption Capabilities of a Novel Ti-V-Mn-Zr-Nb High-Entropy Alloy

Hydrogen is considered as a clean energy carrier able to achieve the decarbonization of the economy, but its compact, safe, and efficient storage represents an important challenge. Among many materials forming hydrides, this work reports the study of hydrogen sorption properties of a novel bcc high-entropy alloy, Ti0.30V0.25Mn0.10Zr0.10Nb0.25, synthesized by arc melting. In less than 60 s, the alloy fully absorbs hydrogen at room temperature, reaching a capacity of 2.0 H/M (2.98 wt.%). A two-step reaction with hydrogen is confirmed by pressure-composition isotherms, synchrotron X-ray and neutron diffraction: bcc solid solution ↔ bcc monohydride ↔ fcc dihydride. For the second step transformation, the calculated thermodynamic values indicate the formation of a very stable dihydride, with ΔHabs = −97 kJ/molH2. Moreover, the pair distribution function analysis of high-energy synchrotron X-ray scattering data validates a completely random distribution of metal atoms in the fcc dihydride phase, without noticeable lattice strain nor elemental segregation. In situ synchrotron X-ray and neutron diffraction, performed during hydrogen desorption by heating under vacuum, demonstrated full reversibility of the reaction with hydrogen. On the basis of these results, tuning the chemical composition of high-entropy alloys may have great implications in terms of hydrogen sorption properties.

Compositional effects on the hydrogen storage properties in a series of refractory high entropy alloys

The possible combinations in the multidimensional space of high entropy alloys are extremely broad, which makes the incremental experimental research limited. As a result, establishing trends with well-known empirical parameters (lattice distortion, valence electron concentration etc.) and predicting effects of the chemical composition change are vital to guide future research in the field of materials science. In this context, we propose a strategy to rationalize the effect of chemical composition change on the hydrogen sorption properties in a series of high entropy alloys: Ti0.30V0.25Zr0.10Nb0.25M0.10 with M = Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ta and ∅ (corresponding quaternary alloy). All materials are bcc alloys and absorb hydrogen at room temperature forming fcc or pseudo-fcc dihydride phases. The maximum hydrogen storage capacity at room temperature strongly depends on the valence electron concentration (VEC) of the alloys: the capacity is high (1.5–2.0 H/M) for low values of VEC (<4.9) whereas, a drastic fading is observed for VEC ≥4.9 which is the case for alloys with M being a late 3d transition metal. The structural analysis suggests that steric effects might not be responsible for this trend and electronic reasons may be invoked. Increasing the VEC by alloying with late 3d transition metals will fill the unoccupied valence states and the electrons from interstitial hydrogens can no longer be accommodated, which is unfavorable for hydrogen storage. Moreover, the onset temperature of desorption increases almost linearly with VEC for this composition series. These findings suggest that alloys with low VEC are more likely to become promising candidates for hydrogen storage.

Displacement of hydrogen position in di-hydride of V-Ti-Cr solid solution alloys

Local structure in the di-hydride (di-deuteride) phases of V-Ti-Cr solid solution alloys (V10Ti35Cr55, V50Ti20Cr30 and V80Ti8Cr12) were investigated using synchrotron X-ray and neutron total scattering experiments. Both Rietveld refinement and pair distribution function (PDF) refinement of the X-ray scattering data indicated that the crystal structure of the metal lattice was a face centered cubic (FCC) structure and no difference between their local structure and average structure was observed. Rietveld refinement of neutron scattering data showed that the crystal structure was a CaF2 structure. However, the CaF2 structure model did not reproduce the first peak corresponding to the metal-hydrogen correlation in neutron PDF patterns. When special quasi-random structure (SQS) models based on the CaF2 structure are applied for the refinements, the SQS structure model, relaxed via first-principles calculations, reproduced the whole neutron PDF pattern. Distribution of interatomic distances between hydrogen and metal atoms in the relaxed SQS models showed that interatomic distance of hydrogen with Cr was shorter than that with V and that with Ti was longer than that with V, independently of the chemical compositions. The combination of neutron PDF and first-principles calculations with the SQS models revealed that hydrogen atoms did not occupy the center of tetrahedron but moved toward Cr atoms and away from Ti.

Face-centered-cubic yttrium trihydride high-pressure phase stabilized at ambient pressures by mechanical milling

The crystal structure of yttrium trihydride (YH3) is usually characterized by hexagonal close-packed (HCP) metal ordering at ambient conditions and transforms into an FCC structure (FCC-YH3−δ) at pressures greater than 8 GPa. FCC-YH3−δ was previously considered to only be stable under high pressures, but we found that it can be stabilized even at ambient pressures by the mechanical milling of hexagonal YH3(HCP-YH3) in this study. The lattice constant of the FCC-“YH3−δ” sample was evaluated to be a = 0.52801(2) nm, which corresponds to the expected value calculated in other studies. This structural change from HCP to FCC enlarged band gap of the YH3 from 2.2 eV to 2.45 eV measured by UV–vis spectrometry. FCC-YH3 was dehydrogenated to an FCC “YH2” dihydride phase at 520 K, which is known to be conventionally present at ambient pressures. However, the YH2 generated from FCC-YH3−δ maintained its FCC structure upon re-hydrogenation to trihydride, unlike how YH2 usually changes its structure from FCC to HCP. The volume change after the dehydrogenation of FCC-YH3−δ was approximately 5%, which is smaller than that of HCP-YH3 (approximately 10%).

Hydrogen storage properties of Nb-based solid solution alloys with a BCC structure

Hydrogen storage properties of Nbx(Ti0.7Cr0.3)1-x (x = 0.3, 0.4, 0.5, 0.6) alloys with a body centered cubic (BCC) structure and the crystal structure of their hydrides were investigated. These alloys absorbed hydrogen up to around 1.8 H/M by forming a di-hydride phase in the absorption process, and desorbed to 0.8 H/M to transform to a mono-hydride phase during the desorption process. These behaviors are similar to those of Ti–Cr–V and Ti–Cr–Mo alloys with the BCC structure. XRD patterns indicated that the alloys transformed to the mono-hydride with a slightly distorted BCC structure (orthorhombic structure), and then to the di-hydride with a CaF2 structure. With increasing Nb content, both the lattice constants for all phases, and the equilibrium pressures for the absorption and desorption processes increased. This indicated that Nbx(Ti0.7Cr0.3)1-x does not follow the well-known relationship of equilibrium pressures decreasing with increasing lattice constants. However, the logarithmic equilibrium pressures had a linear relationship with their estimated bulk modulus.

TiVZrNb Multi-Principal-Element Alloy: Synthesis Optimization, Structural, and Hydrogen Sorption Properties

While the overwhelming number of papers on multi-principal-element alloys (MPEAs) focus on the mechanical and microstructural properties, there has been growing interest in these alloys as solid-state hydrogen stores. We report here the synthesis optimization, the physicochemical and the hydrogen sorption properties of Ti0.325V0.275Zr0.125Nb0.275. This alloy was prepared by two methods, high temperature arc melting and ball milling under Ar, and crystallizes into a single-phase bcc structure. This MPEA shows a single transition from the initial bcc phase to a final bct dihydride and a maximum uptake of 1.7 H/M (2.5 wt%). Interestingly, the bct dihydride phase can be directly obtained by reactive ball milling under hydrogen pressure. The hydrogen desorption properties of the hydrides obtained by hydrogenation of the alloy prepared by arc melting or ball milling and by reactive ball milling have been compared. The best hydrogen sorption properties are shown by the material prepared by reactive ball milling. Despite a fading of the capacity for the first cycles, the reversible capacity of the latter material stabilizes around 2 wt%. To complement the experimental approach, a theoretical investigation combining a random distribution technique and first principle calculation was done to estimate the stability of the hydride.

Hydrogen sorption in TiZrNbHfTa high entropy alloy

High Entropy Alloys (HEA), where five or more elements are mixed together in near equiatomic ratios offer promising properties as hydrogen storage materials due to their ability to crystallize in simple cubic structures in the presence of large lattice strain originating from the different sizes of the atoms. In this work, the hydrogen absorption and desorption as well as the cycling properties of the TiZrNbHfTa HEA have been studied by in situ Synchrotron X-Ray diffraction, Pressure-Composition-Isotherm, Thermal Desorption Spectroscopy and Differential Scanning Calorimetry. The alloy crystallizes in a cubic bcc phase and undergoes a two-stage hydrogen absorption reaction to a fcc dihydride phase with an intermediate tetragonal monohydride, very similar to the V-H system. The hydrogen absorption/desorption in TiZrNbHfTa is completely reversible and the activation energy of desorption could be calculated. Furthermore, we have observed an interesting macrostructure following parallel planes after the formation of the dihydride phase, which is retained after desorption.

Стабильность и электронное строение фаз дигидрида магния

Стабильность и электронное строение фаз дигидрида магния

Magnetocaloric properties of hydrogenated Gd, Tb and Dy

The influence of hydrogen on magnetocaloric properties of three high purity heavy rare earths (R), gadolinium, terbium and dysprosium is studied. Hydrogenation procedure is performed using high purity hydrogen in a Sievert-type apparatus. In addition to the main α-RHx phase (where x is the hydrogen concentration), appearance of the second phase RH2 or its traces is detected in nearly all hydrogenated samples. In the samples where the main α-RHx phase prevails and only traces of the dihydride phase are found, the magnetocaloric effect (MCE) remains of the same magnitude as in the hydrogen-free samples. With the increase of hydrogen concentration (and with a corresponding increase of the RH2 phase content), the MCE values decrease. Only Gd samples demonstrate the increased Curie temperatures (TC = 291 and 296 K for Gd and GdH0.2, respectively) related to the increase of exchange interactions in the Gd-Gd pairs. At the same time, the Néel temperatures of Tb and Dy retain their values after hydrogenation.

High-pressure synthesis of tantalum dihydride

The reaction of tantalum with molecular hydrogen was studied by x-ray diffraction in a diamond-anvil cell at room temperature and pressures from 1 to 41 GPa. At pressures up to 5.5 GPa, a substoichiometric tantalum monohydride with a distorted bcc structure was shown to be stable. Its hydrogen content gradually increased with the pressure increase, reaching $\mathrm{H}/\mathrm{Ta}=0.92(5)$ at 5 GPa. At higher pressures, a new dihydride phase of tantalum was formed. This phase had an hcp metal lattice, and its hydrogen content was virtually independent of pressure. When the pressure was decreased, the tantalum dihydride thus obtained transformed back to the monohydride at $P=2.2\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Single-phase samples of tantalum dihydride also were synthesized at a hydrogen pressure of 9 GPa in a toroid-type high-pressure apparatus, quenched to the liquid-${\mathrm{N}}_{2}$ temperature, and studied at ambient pressure. X-ray diffraction showed them to have an hcp metal lattice with $a=3.224(3)$ and $c=5.140(5)\phantom{\rule{0.16em}{0ex}}\AA{}$ at $T=85\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The hydrogen content determined by thermal desorption was $\mathrm{H}/\mathrm{Ta}=2.2(1)$.

Fabrication of YH3 thin film using Pd/Ni co-capping layer: Ni thickness effect

With an aim of decreasing the temperature at which metallic Y reacts with H2 to form the semiconductor phase YH3 (γ phase), we employed Pd and Ni co-capping layers as catalysts. For this purpose, various types of Pd/Ni co-capping layers having different Ni thicknesses varied from 5 to 80nm while having a Pd thickness of 80nm were prepared as a catalytic capping layer on metal Y. We achieved an onset temperature (50°C) of the γ phase formation, which is approximately independent of Ni thickness except for 40nm. A directing principle for decreasing the onset temperature of the γ phase formation was discussed in terms of crystal texture and crystallite size of dihydride phase (β phase).

Ti-based BCC Alloy: Dehydrogenation Characterization Using Synchrotron and Neutron Diffraction

Dehydrogenation of a BCC alloy of composition 52Ti-12V-36Cr doped with 4 wt.% Zr7Ni10 was studied by diffraction using synchrotron and neutron radiations. In the case of synchrotron radiation dehydrogenation was imposed by raising the temperature while keeping the sample under vacuum. We found that prior to final dehydrogenation the FCC dihydride phase is becoming metastable. Neutron in-situ experiment was done by keeping the deuterided sample at 200°C and slowly reducing the pressure. This lead to a reduction of the occupation factor of deuterium in the FCC phase and an increase of the isotropic atomic temperature factor of deuterium.

The Development of High Capacity V-Based BCC Electrodes for Aqueous Metal Hydride Air Systems

The energy density of traditional Ni-MH batteries could be dramatically increased by developing a higher capacity metal hydride alloy. An even larger increase in energy density for MH-type electrochemical cells would be seen by pairing the high capacity metal hydride with an air cathode to create an MH-air battery. Vanadium based BCC alloys are a promising choice for high capacity anode materials, and discharge capacities of greater than 450 mAh/g have been reported in the literature for TiV2.1-xCrxNi0.3 alloys.1 The kinetics and power density of these alloys are further improved by reducing the microstructure via rapid solidification.2 Pure vanadium metal forms a dihydride with a total hydrogen content of 4 wt.% or a theoretical electrochemical capacity of 1068 mAh/g. Practical cycling capacity is limited by the fact that the VH and VH2 phase transitions differ by several orders of magnitude in pressure.3 The plateau pressures of these phase transitions are strongly affected by the composition and lattice parameter of host metal alloy, and the kinetics depend on microstructure and electrode design. We initially identified a ternary TiVNi alloy4 that delivers close to the theoretical capacity of about 1100 mAh/g by accessing both the monohydride and dihydride phase transitions. The transitions manifest as two voltage plateaus in the charge/discharge profile of the metal hydride electrode, as seen in the accompanying figure. The difference in voltage plateaus is directly related to the difference in gas phase plateau pressures via the Nernst equation. From the figure we see a difference of ~120mV, corresponding to a 4 – 5 order of magnitude difference in plateau pressure. This alloy has poor cycle life, so Cr is added to prevent corrosion resistance. We will present results on the effect of composition on the capacity and cycle life of the alloy, and the role that microstructural reduction through rapid solidification and ball milling play on improving the kinetics. 1. H. Inoue, S. Koyama, and E. Higuchi, Electrochimica Acta, 59, 23–31 (2012). 2. H. Tan, N. Weadock, B. Fultz, and R. V. Bugga, Meet. Abstr., MA2015-01, 19–19 (2015). 3. M. Tsukahara et al., J. Alloys Compd., 224, 162–167 (1995). 4. C. Iwakura, W.-K. Choi, R. Miyauchi, and H. Inoue, J. Electrochem. Soc., 147, 2503–2506 (2000). Figure 1

Formation of hydride phase and diffusion of hydrogen in the V–H system varied by substitutional Fe

We elucidate the effect of Fe addition on the phase transformation and the diffusion of hydrogen in the V–H system. V forms monohydride (β) and dihydride (γ) phases by hydrogenation. The γ phase of VH2 has a face centered cubic (FCC) structure but V1−xFex (0.03 ≤ x ≤ 0.10) forms the FCC hydride phase though the hydrogen content is less than half of VH2, resulting in decreasing the hydrogen storage capacity. On the other hand, for the β phase proton nuclear magnetic resonance (1H NMR) demonstrates that addition of Fe increases the density of states at the Fermi energy N(EF) and the activation energy for hydrogen diffusion EH. This indicates that Fe has the effects of stabilizing the hydride phase and of suppressing the diffusion of hydrogen in the V–H system.

Effect of dissolved oxygen on hydrogenation of vanadium and hydrogen diffusion in the monohydride phase

We elucidate the effect of dissolved oxygen on the phase transformation and the diffusion of hydrogen in the V–H system. V forms monohydride (β) and dihydride (γ) phases by hydrogenation but a small amount of dissolved oxygen (O/V ≈ 0.01) leads to formation of another β phase expanded along the c-axis in a body centered tetragonal structure (BCT) rather than the γ phase. This change in the phase transformation is due to trapping hydrogen atoms by oxygen atoms, resulting in the shift of site occupation of hydrogen from octahedral (O) sites to tetrahedral (T) sites. Proton nuclear magnetic resonance (1H NMR) demonstrates that the activation energy for hydrogen diffusion is increased by the trapping effect. Oxygen dissolved in V suppresses not only the formation of the γ phase but also the diffusion of hydrogen.

Synthesis and hydrogen sorption properties of TiV(2−x)Mnx BCC alloys

We report here the effect of activation process of TiV2−xMnx samples (x=0.2,0.4,0.6,0.8,0.9). Crystal structures are investigated for the as-cast, hydrogenated and desorbed states. The as-cast samples crystallise with a Body Centered Cubic (BCC) structure, and the lattice parameter decreases as x increases in the TiV2−xMnx series. Upon hydrogenation, the BCC structure expands and turns into a Body Centered Tetragonal (BCT) structure for the monohydride phase and into a Face Centered Cubic (FCC) for the dihydride phase. Pressure–Composition-Isotherms (P–C–I) measurements were performed to study the thermodynamic of TiV2−xMnx alloys and their hydrogen capacities. The maximum hydrogen uptake is observed for TiV1.2Mn0.8 composition, which store 3.4wt.% of hydrogen at 100°C. The effect of a new activation process on the hydrogen storage capacities of these alloys is discussed.

Phase Transformation and Lattice-Strain Formation in Ti<SUB>1.0</SUB>V<SUB>1.1</SUB>Mn<SUB>0.9</SUB> during First Absorption and Desorption

We used powder X-ray diffraction (XRD) and Rietveld refinement to study phase transformation and the lattice strain introduced into each hydride phase of Ti1.0V1.1Mn0.9 during first absorption and desorption. Hydrogenation proceeded from a solid-solution phase to a dihydride phase via a monohydride phase. Each single-phase region was observed beside two clear plateau regions on the pressure–composition (P–C) isotherm. In contrast, the desorption P–C isotherm showed only one clear plateau, corresponding to a two-phase region of the dihydride and the monohydride. The plateau was connected to a two-phase region of the monohydride and a solid-solution phase, and to another region of solid-solution phases. The monohydride single-phase region was not clearly observed during desorption. Isotropic lattice strain was introduced, and increased with phase transformation during the first absorption. The strain increased further in the subsequent phase transformation during desorption, particularly upon formation of a solid-solution phase.

Appearance of a correlation between the Hall coefficient and electrical resistivity upon dihydrogenation of yttrium

To shed light on the correlation between the Hall coefficient (RH) and electrical resistivity (ρ), we performed simultaneous measurements of these two transport coefficients in fcc dihydride phase of yttrium (YHx), having H/Y values ranging from 1.73 to 2.04. Unlike the typical behavior of metals, an approximately linear relationship was observed between RH and ρ at room temperature after dihydrogenation of yttrium. Interpretation of this relationship, based on the Boltzmann–Bloch scheme, reveals that the transverse (cyclotron) relaxation rate (1/τc) of the carriers is relatively insensitive to the generation of hydrogen defects in the dihydride phase of yttrium, unlike the longitudinal relaxation rate (1/τ), which is affected by the presence of hydrogen defect. Low-temperature (77 K) measurements of RH and ρ on the same samples show that the approximately linear relationship observed at room temperature disappears but a certain nonlinear relationship may exist at 77 K.