Monohydride Phase Research Articles

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.

Diffusion of hydrogen in the body centered tetragonal hydride phase of the V–H system controlled by multiple substitutional metal elements

The diffusion and site occupation of hydrogen in the monohydride phase (β phase) of the V1−x−yCrxMoy–H (x + y ≤ 0.15) system have been studied by means of proton nuclear magnetic resonance. The β phase has a body centered tetragonal (BCT) structure and hydrogen atoms occupy the octahedral (Oz) sites along the c-axis. Cr has a smaller atomic radius than V but Mo has a larger one, resulting in no obvious change in the unit cell volume of the BCT lattice by substitution of both Cr and Mo for a part of V, although the multiple metal elements of V, Cr, and Mo induce lattice strain and distortion. The temperature dependence of 1H spin-lattice relaxation time has demonstrated that the substitution of both Cr and Mo offsets the change in the activation energy for hydrogen diffusion EH for the Oz sites, while our previous study has indicated that Cr increases EH and Mo decreases that. The diffusion behavior of hydrogen is explained by the dimension of the interstitial sites attributing to the weighted mean atomic radius of the substitutional metal elements, which is independent of their affinities for hydrogen and the lattice strain.

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.

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.

Enhancement of hydrogen diffusion in the body-centered tetragonal monohydride phase of the V–H system by substitutional Al studied by proton nuclear magnetic resonance

The diffusion and site occupation of hydrogen in the monohydride phase (β phase) of the V1−xAlx–H (x⩽0.1) system was studied by means of proton nuclear magnetic resonance (1H NMR). Hydrogen atoms which occupied the octahedral (Oz) sites along the c-axis in a body-centered tetragonal structure shifted to the tetrahedral (T) sites with the Al content. V0.90Al0.10H0.68 had a body-centered cubic structure and the component observed in the temperature dependence of 1H spin-lattice relaxation time was due to the contribution of hydrogen on the T sites. The addition of Al reduced the site radius of the Oz sites and resulted in reducing the activation energy for hydrogen diffusion, EH, for the Oz sites. Moreover, the diffusion of hydrogen was enhanced by a shift of site occupation from the Oz sites to the T sites, which had lower EH than the Oz sites. The addition of larger substitutional Al atoms than V enhanced the diffusion of hydrogen in the β phase of V–H.

Effect of substitutional Cr on hydrogen diffusion and thermal stability for the BCT monohydride phase of the V–H system studied by 1H NMR

The diffusion of hydrogen in the monohydride phase (β phase) of the V1−xCrx–H (x≤0.1) system was studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in the hydrides occupied the octahedral (O) sites in a body centered tetragonal (BCT) structure. The BCT lattice was contracted by addition of Cr because Cr has a smaller atomic radius than V. The activation energy for hydrogen diffusion, EH, and the Korringa constant, T1eT, were estimated from the temperature and frequency dependence of 1H spin–lattice relaxation times, T1. The value of EH increased with increase in the Cr content. The contraction of the BCT lattice by addition of Cr suppressed the diffusion of hydrogen which occupied the interstitial sites. Addition of Cr also increased the T1eT, which was accompanied by reducing in the stability of the monohydride phase of the V–H system.

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.

Effect of substitutional Mo on diffusion and site occupation of hydrogen in the BCT monohydride phase of V–H system studied by 1H NMR

The diffusion and site occupation of hydrogen in the monohydride phase of V 1− x Mo x –H (0 ≤ x ≤ 0.1) were studied by means of 1H nuclear magnetic resonance (NMR). Hydrogen atoms in VH 0.68 occupied the octahedral ( O) sites in a body centered tetragonal (BCT) structure. The addition of Mo to V reduced the activation energy for hydrogen diffusion, E H, for the O sites. In V 0.9Mo 0.1H 0.68 hydrogen atoms occupied both the O and tetrahedral ( T) sites, which was demonstrated by two components observed in the temperature dependence of the 1H spin-lattice relaxation time. The value of E H for the T sites was lower than that for the O sites. Hydrogen atoms in V 0.9Mo 0.1H 0.68 diffused faster than those in VH 0.68.

H ordering in hcp M–H systems (M=Sc, Y; Ti, Zr)

The H-sublattice ordering and the effect of local atomic relaxations due to H interstitial atoms on the structural stability of hexagonal hydrogen-M solid solutions (M = Sc, Ti, Y, Zr) is studied by means of the density functional theory. A first-principle based model is presented which, by considering the relaxation of the host metal cell, yields for the first time a suitable explanation for the observed chain-like short range ordering through a coherent stress field along the chain as well as its limitation to only six hcp metals, namely Sc, Y, Ho, Er, Tm, and Lu, and the absence of a mono-hydride phase in the same elements.

The effect of Cr content on the structural and hydrogen storage characteristics of Ti 10V 80− xFe 6Zr 4Cr x ( x = 0–14) alloys

Ti 10V 80− x Fe 6Zr 4Cr x ( x = 0, 6, 10 and 14) alloys were prepared by levitation induction melting, and the effects of Cr content on the structural and hydrogen storage characteristics have been investigated systematically. The results show that alloys investigated consist of a V-based solid solution main phase with BCC structure and a small amount of C14 Laves secondary phase. As the Cr content increases, the unit cell volumes of both phases decrease. It is found that all these alloys have good activation behavior and hydriding/dehydriding kinetics. As the Cr content in the alloys increases, the maximum hydrogen absorption capacity at room temperature decreases gradually, and the effective hydrogen desorption capacity increases first and then decreases. The XRD and DSC analyses show that the stability of mono-hydride phase (β) in the sample is heightened with increasing Cr content. Among the alloys studied, Ti 10V 74Fe 6Zr 4Cr 6 alloy has the best overall property.

Evolution of Two-Dimensional Structure Phase Transitions (3 ×1) →(2 ×1) and (1 ×1)→(2 ×1) on Hydrogen-Terminated Si(100) Surface

On the Si(100) surface, monohydride dimers (H–Si–Si–H or M–M) and dihydride (H–Si–H or D) species can form an ordered mixture with (3 ×1), (1 ×1) and (2 ×1) phases. Thermal annealing at elevated temperatures causes both the (3 ×1) and (1 ×1) domains to transform to the (2 ×1) monohydride phase. We utilize scanning tunneling microscopy to observe these two-dimensional structure phase transitions on the atomic scale. The results show that the coverage of the (3 ×1) and (1 ×1) domains decays linearly with a common half-life time of ∼9.8 h at 570 K. In good agreement with a previous report, this finding suggests that the two different transitions are governed by the same reaction mechanism, i.e., the dihydride-pair recombination mechanism.

Simulation of SiH 4 adsorption on H/Si(100) surfaces

Using quantum-chemical calculations, we have studied the adsorption mechanism, geometry and energy states of silane on H/Si(100) surfaces (with a monohydride phase). Adsorption can take place both with and without dimer bond rupture. As a result of the calculations, it has been established that adsorption of silane on the H/Si(100) surface can result in the formation of structures of different types. The SiH 2-fragment can be adsorbed onto atoms of one surface dimer (models 1 and 3) and onto atoms of two neighboring dimers of one dimer row (models 2 and 4).

Degrading Mechanism on Hydrogen Absorbing-Desorbing Cycle Durability of V- and Ti-Cr-Based BCC-Type Solid Solutions

V- and Ti-Cr-based solid solutions with body-centered cubic (BCC) type structure were investigated on hydrogen absorbing-desorbing cycle durability by using H2 without and with 10ppm CO (Hereafter they are expressed as H2 and CO/H2, respectively.). The solid solutions showed excellent cycle durability even after 1,000 cycles of hydrogen absorption and desorption under H2 atmosphere. On the other hand, the capacities decreased rapidly during hydrogen absorbing-desorbing cycles in the beginning of the tests under CO/H2 one. The solutions were not disproportionate though the stable monohydride phase increased, that is, stored hydrogen increased gradually. It was observed that not only microscopic pulverization but also nanoscopically fine-grained crystallization occurred in degraded particles. It is considered that the impurity CO influences bulk structure as an intrinsic factor as well as surface area as an extrinsic factor as regarded as ever. This causes the degradation of the cycle capacities.

Diffusion of hydrogen isotopes in the monohydride phase of Ti 1− zV zH xD y studied by 1H and 2H NMR spin–lattice relaxation times

The 1H and 2H spin–lattice relaxation times in the monohydride phase of the V–H–D and Ti 0.1V 0.9–H–D systems have been re-analyzed by Bloembergen–Purcell–Pound (BPP) equations with a log-normal distribution of correlation times, and the obtained parameters have been compared with those of the Ti 0.33V 0.67–H–D system. The mean activation energy and the magnitude of the distribution of H and D diffusions are discussed in relation to the metal compositions, the hydrogen site, the crystal structure and the [H]/[D] ratios.

Hydrogen elimination reactions in the thermal decomposition of alcohols on Si(100) surfaces

The thermal decomposition of 1-pentanol and 2-methyl-1-butanol on clean Si(100)-2×1 in an ultrahigh vacuum has been examined using temperature programmed desorption, intergrated desorption mass spectrometry, and low-energy electron diffraction. The results show that the alkoxy species formed on Si(100) are stable up to temperatures of about 480 K. Above 500 K, the alkoxy species decompose on Si(100) via the γ-hydrogen elimination mechanism to yield alkene in the gas phase, together with adsorbed hydrogen. The H2 gas is evolved by the recombinative desorption of hydrogen atoms that are generated by the adsorption of alcohols and/or the decomposition of alkoxy species. The H2 thermal desorption exhibits a peak near 800 K, which indicates that the H2 gas is evolved from the monohydride phase on Si(100).

Thermal desorption spectra of hydrogen isotopes in the monohydride phase of V–H–D and Ti–V–H–D systems

Thermal desorption of hydrogen isotopes in VH x D y ( x+ y≈0.6), Ti 0.1V 0.9H x D y ( x+ y≈0.6) and Ti 0.33V 0.67H x D y ( x+ y≈0.9) alloys has been studied by means of simultaneous measurements of thermogravimetric and differential thermal analyses and mass spectrometry. H desorbs more easily than D in all the samples studied and thus H is enriched in the gas phase during desorption. The H enrichment factor in the gas phase is in the following order, V≥Ti 0.1V 0.9>Ti 0.33V 0.67. The H enrichment factor also depends on the [H]/[D] ratio, which suggests that H and D in the solid phase influence the desorption kinetics of the other isotope probably through an indirect interaction mediated by metal atoms. The equilibrium pressure is in the order of Ti 0.1V 0.9>V>>Ti 0.33V 0.67. Deuterides have higher equilibrium pressures than protides, and this isotope effect is Ti 0.33V 0.67>Ti 0.1V 0.9≈V. There are no correlations in the isotope effects between the thermal desorption and the equilibrium pressure. Hydrogen diffusion in solids might play an important role in the thermal desorption.

Kinetics of H2 (D2) desorption from a Ge(100)-2×1:H (D) surface studied using scanning tunneling microscopy and temperature programmed desorption

The kinetics of H2 (D2) desorption from a Ge(100)-2×1:H (D) surface was studied using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). Inspection of STM images of surfaces at the saturation coverage of H (D) (θH(D)≃1.0 ML) revealed a 2×1 monohydride (monodeuteride) phase in which most H (D) atoms were paired on Ge-dimers. By counting the sites of H2 (D2) desorption in STM images taken after desorption of H2 (D2) at temperatures in the range Ts=500−550 K, the desorption of H2 (D2) was found to follow first order kinetics with an activation energy of Ed=1.65±0.1 eV (1.65±0.1 eV) and a pre-exponential factor of νd=(2.7±0.5)×1013 s−1 [(1.2±0.5)×1013 s−1]. These values of Ed and νd were used to simulate TPD spectra for the desorption of H2 (D2) from a Ge(100)-2×1:H (D) surface. The simulated spectra were in good agreement with the experimental TPD spectra. In contrast to the surfaces with saturated H coverage, which are characterized by pairs of H atoms on Ge-dimers, at the low H coverage of θH≃0.05 ML unpaired H atoms as well as paired H atoms were observed on the Ge-dimers on the surface, causing the desorption process to follow second order kinetics. At Ts∼300 K, the singly occupied dimers (SODs) appear to be favored over doubly occupied dimers (DODs). However, upon increasing the temperature (Ts) from 300 to 500 K, most SODs were rapidly converted into the thermodynamically favored DODs by the migration of H atoms. On the other hand, it is observed that even above Ts∼500 K, the onset temperature for H2 desorption from DODs, a non-negligible number of SODs remain on the surface due to the large entropic barrier to pairing. These results suggest that H adsorption in the low coverage is strongly influenced by the energetics of the pairing of H atoms.