Stability Of Hydrides Research Articles

Microstructure and hydrogenation properties of a melt-spun non-stoichiometric Zr-based Laves phase alloy

Alloy with composition of Zr0.9Ti0.1V1.7 off normal stoichiometric proportion is selected to investigate the effect of defects introduced by non-stoichiometry on hydrogenation kinetics of Zr–Ti–V Laves phase alloys. Microstructure and phase constituent of melt-spun ribbons have been investigated in this work. The activation process, hydrogenation kinetics, thermodynamics characteristics and hydride phase constituent of as-cast alloy and melt-spun ribbons are also compared. Comparing with the as-cast alloy, the dominant Laves phase ZrV2 is preserved, V-BCC phase is reduced and α-Zr phase is replaced by a small amount of Zr3V3O phase in melt-spun ribbons. Melt-spun ribbons exhibit easy activation and fast initial hydrogen absorption on account of the increased specific surface area. However, the decrease in unit cell volume of the dominant phase leads to the decrease in hydrogen absorption capacity. Melt-spinning technique raises the equilibrium pressure and decreases the stability of hydride due to the decrease of unit cell volume and the elimination of α-Zr phase, respectively. Melt-spun ribbons with fine grains show improved hydrogen absorption kinetics comparing with that of the as-cast alloy. Meanwhile, the prevalent micro twins observed within melt-spun ribbons are believed to account for the improved hydrogen absorption kinetics.

Liquid Carbon Dioxide as Anti‐Solvent Coating Aluminum Hydride

AbstractThis paper presents a novel surface coating to resolve the serviceability, stability, compatibility, and dehydrogenation of aluminum hydride (alane) using liquid carbon dioxide as an anti‐solvent, dispersant, drying medium, and inert atmosphere. The coated alane sample was characterized by SEM, XPS, XRD, and FT‐IR spectroscopy. The coating agent was uniformly applied on the surface of alane and the crystalline phase remained unchanged. The alane molecules became more stable after coating. The thermal stability of alane before and after coating was analyzed by DSC. The enthalpy of the coated alane increased under different heating rates, indicating increased thermal stability. The lower electric spark sensitivity of the coated alane showed improved serviceability and stability. The method developed in this study may be suitable for coating other hydrogen storage and energetic materials.

Hydrogen permeance studies in ordered ternary Cu–Pd alloys

Considerable attention has recently been given to bimetallic systems such as Cu–Pd alloys as hydrogen storage material. These alloys are attractive alternatives to pure Pd membranes because of their sulphur tolerance and high permeability to hydrogen. The density functional theory based full potential linearized augmented plane wave and projector augmented wave methods have been implemented to understand the phenomenon of hydrogen diffusion in Cu–Pd alloys. The AuCu3 structure of CuPd3 and Cu3Pd allows intercalation of hydrogen atoms in its octahedral sites. The lattice parameters and volume of the unit cell expand on hydrogenation, without phase change.We have studied the formation and stability of hydrides of the ternary alloys. The heats of formation support the hydrogenation of CuPd3 alloy whereas the endothermic reactions indicate that hydrogen will be absorbed in the Cu3Pd alloy at the expense of energy. The phonon dispersion relations of the hydrides of CuPd3 exhibits instability due to the Pd–H bonds which leads to distortion of Pd-octahedra in the unit cell. Softening of acoustic modes is not observed in the Cu3Pd alloy; instead the optical modes due to H–H bonds have very high vibrational frequencies.The permeance of H atoms in the alloy can be quantified from the extent of hybridization between H s-states, Cu and Pd d-states. From the density of states, the movement of the H s-band and centre of d-bands towards the Fermi energy level EF was observed. Both the factors indicated that the monohydride of CuPd3 was most stable. The temperature dependent Hall and Seebeck coefficients reflect the modifications in the electronic structure due to formation of hydrides. At ambient temperatures, the free charges carriers (electrons) are found to increase in the monohydride of Cu3Pd, whereas it decreases in the monohydride of CuPd3. The electronic component of specific heats γ obtained from low temperature specific heats is found to decrease in the hydrides of both the alloys, which implies an increase in the lattice contribution through the electron-phonon coupling. From the optical properties, it is possible to monitor the hydrogen uptake in the alloys by studying the variations in reflectance, absorption coefficients in the visible regions.The electronic and dynamical studies suggest that the alloy with excess Pd has potential applications as hydrogen storage materials. The monohydride of CuPd3 appears to be appropriately stable for applications such as electrode material in rechargeable batteries.

Formation of metastable phases in magnesium–titanium system by high-pressure torsion and their hydrogen storage performance

No binary phases exist in the Mg–Ti binary equilibrium phase diagram and the two elements are totally immiscible even in liquid form. This study shows that four metastable phases (two with the bcc and fcc structures and two with the hcp structures) are formed in the Mg–Ti system by severe plastic deformation (SPD) through the process of high-pressure torsion (HPT). Investigation of hydrogenation properties reveals that these metastable phases are decomposed to pure Mg and Ti during heating before they can absorb the hydrogen in the form of ternary Mg–Ti hydrides. First-principles calculations show that the hydrogenation reaction should occur thermodynamically, and ternary Mg–Ti hydrides with the cubic structure should form at low temperature. However, the slow kinetics for this reaction appears to be the limiting step. Calculations show that the binding energy of hydrogen increases and the thermodynamic stability of hydrides undesirably increases by addition of Ti to Mg.

Effects of Al content on the electrochemical properties of La0.78Ce0.22Ni3.95-xCo0.65Mn0.3Si0.1Alx alloys at 20–80 °C

The various properties of La0.78Ce0.22Ni3.95-xCo0.65Mn0.3Si0.1Alx (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys at 20–80 °C have been investigated systematically. The equilibrium pressure and hydrogen storage capacity decrease with the increment of Al content, especially at higher temperature. Thus the high stability of the counterpart hydride means the high-Al alloy is more suitable for working at higher temperature. With the increase of Al content, both the discharge capacity and high-rate dischargeability deteriorate at room temperature, but increase gradually at higher temperature (40–80 °C). Electrochemical kinetics and scanning electron microscope analyses were conducted to investigate the mechanism of Al effect on high-temperature properties. The results demonstrate that the dissolution of Al leads to the outcrop of high electro-catalytic Ni atoms on the alloy surface, thus charge-transfer resistance decreases and exchange current density increases, making the high-temperature dischargeability and high-rate dischargeability better for high-Al alloys at 60 °C. Furthermore, the existence of Al improves the stability of the oxide film, enhancing the anti-corrosion property of high-Al alloy at elevated temperature, and consequently results in excellent high-temperature recoverability.

Phase-Structural and Electrochemical Properties of La2MgNi9 Alloys

Multiphase La 2 MgNi 9 alloys are synthesized by induction melting. The crystalline structures of all phases in the alloys are determined with X-ray diffraction. The La 2 MgNi 9 electrodes with nickel as a binder and electrical conductor are prepared and their charge–discharge characteristics are studied. The influence of homogenizing annealing and mechanical grinding on the discharge capacity and cyclic stability of the metal hydride (MH) electrodes is shown. The best electrochemical parameters are observed for powdered as-cast alloys.

Structure and stability of high pressure synthesized MgTM2H6 (TM = Zr, Nb) hydrides

The structure of two isomorphous high pressure synthesized metal hydrides MgZr2H6 and MgNb2H6 is described in the R-3m space group with the hexagonal unit cells a=3.3592(2)Å, c=25.131(3)Å and a=3.2901(9)Å, c=23.09(1)Å, respectively (Z=3).The metal atom structures were refined from powder XRD data, with Mg in the 3a site (0,0,0,) and Zr and Nb in the 6c site (0,0,z) with the only refineable atomic parameter z=0.225 and z=0.220, respectively.As only a minute sample could be produced in the anvil cell, a conventional structural determination with neutron diffraction on a deuterated sample could not be made. Instead formation energies of the all proposed structural models were calculated using density functional theory (DFT) for accessing the most stable structure of the lowest energy level. The measured lattice parameters and the refined atomic positional parameters from the powder X-ray diffraction pattern were reproduced well by the DFT calculation. The hydrogen atoms were found to fully occupy an octahedral site coordinated by 3Mg and 3Zr atoms, a tetrahedral site coordinated by 4Zr atoms and another tetrahedral site coordinated by 3Mg and 1Zr atoms. Chemical, mechanical and dynamical stabilities of the MgZr2H6 structure were further discussed based on a chemical potential equilibrium diagram constructed from the calculated formation energies, calculated elastic constants and phonon spectra respectively. It is also shown that a high pressure increases the stability of the new hydrides.

Achieving reversibility of ultra-high mechanical stress by hydrogen loading of thin films

Nano-materials are commonly stabilized by supports to maintain their desired shape and size. When these nano-materials take up interstitial atoms, this attachment to the support induces mechanical stresses. These stresses can be high when the support is rigid. High stress in the nano-material is typically released by delamination from the support or by the generation of defects, e.g., dislocations. As high mechanical stress can be beneficial for tuning the nano-materials properties, it is of general interest to deduce how real high mechanical stress can be gained. Here, we show that below a threshold nano-material size, dislocation formation can be completely suppressed and, when delamination is inhibited, even the ultrahigh stress values of the linear elastic limit can be reached. Specifically, for hydrogen solved in epitaxial niobium films on sapphire substrate supports a threshold film thickness of 6 nm was found and mechanical stress of up to (−10 ± 1) GPa was reached. This finding is of basic interest for hydrogen energy applications, as the hydride stability in metals itself is affected by mechanical stress. Thus, tuning of the mechanical stress-state in nano-materials may lead to improved storage properties of nano-sized materials.

Microstructures and hydrogenation properties of (ZrTi)(V1−xAlx)2 Laves phase intermetallic compounds

In this work, the (ZrTi)(V1−xAlx)2 (x=0.02, 0.05, 0.10, 0.15, 0.25) Laves phase intermetallic compounds were prepared by the arc-melting method. The microstructure and phase compositions were examined by SEM and XRD. Hydrogen absorption pressure composition isotherms (P–C isotherms) were obtained by the pressure reduction method using a Sievert type apparatus at different temperatures. The thermodynamic and kinetic properties of the alloys were investigated in this work. The results show that the (ZrTi)(V1−xAlx)2 alloys consist of a dominant C15 Laves phase with cubic structure and a V-based solid solution phase with BCC structure. With further increasing Al content, C15 cubic type Laves phase and C14 hexagonal type Laves phase coexist in the range x⩾0.15 in this (ZrTi)(V1−xAlx)2 alloys. The crystal lattice parameter of the C15 phase increases with the increase of Al content. The PCT curves give the evidence that the maximum hydrogen absorption capacity decreases with the increase of Al content, which results from the existence of ZrAl2 which hardly absorb hydrogen. There is no obvious hysteresis between absorption and desorption in the (ZrTi)(V1−xAlx)2 alloys at 823K. The (ZrTi)(V1−xAlx)2 alloys with x=0.25 preserves higher temperature of phase transformation (β→α). The existence of C14 phase (including ZrV2 and ZrAl2) decreases the stability of hydrides.

Selective reversible hydrogenation of Mg(B3H8)2/MgH2 to Mg(BH4)2: pathway to reversible borane-based hydrogen storage?

Mg(B3H8)2·2THF (THF = tetrahydrofuran) was prepared by the addition of BH3·THF to Mg/Hg amalgam. Heating a 1:2 molar mixture of Mg(B3H8)2·2THF and MgH2 to 200 °C under 5 MPa H2 for 2 h leads to nearly quantitative conversion to Mg(BH4)2. The differential scanning calorimetry profile of the reaction measured under 5 MPa H2 shows an initial endothermic feature at ∼65 °C for a phase change of the compound followed by a broad exothermic feature that reaches a maximum at 130 °C corresponding to the hydrogenation of Mg(B3H8)2 to Mg(BH4)2. Heating Mg(B3H8)2·2THF to 200 °C under 5 MPa H2 pressure in the absence of MgH2 gives predominantly MgB12H12 as well as significant amounts of MgB10H10 and Mg(BH4)2. Hydrogenation of a mixture of Mg(B3H8)2·2THF and LiH in a 1:4 molar ratio at 130 °C under 5 MPa H2 yields [B12H12](2-) in addition to [BH4](-), while a 1:4 molar ratio of Mg(B3H8)2·2THF and NaH yields [BH4](-) and a new borane, likely [B2H7](-). Hydrogenation of the NaH-containing mixture at 130 °C gives primarily the alternative borane, indicating it is an intermediate in the two-step conversion of the triborane to [BH4](-). The solvent-free triborane Mg(B3H8)2, derived from the low-temperature dehydrogenation of Mg(BH4)2, also produces Mg(BH4)2, but higher temperature and pressure is required to effect the complete transformation of the Mg(B3H8)2. These results show that the reversible transformation of the triborane depends on the stability of the metal hydride. The more stable the metal hydride, that is, LiH > NaH > MgH2, the lower is the "regeneration" efficiency.

Preparation of hydroentangled CMC composite nonwoven fabrics as high performance separator for nickel metal hydride battery

Hydroentangling nonwoven process has been attempted to fabricate carding fiber web/meltblown/carding fiber web (CMC) composite nonwoven fabrics with the view to improve the chemical stability and long-term performance of nickel metal hydride (Ni-MH) battery. Hydrophilic polypropylene (PP) meltblown nonwoven fabric was prepared by adding hydrophilic additives in PP melt. Then it was sandwiched into two layers of carding PP fiber web and bonded by hydroentangling process. The mechanical strength, pore size, electrolyte absorption and retention capacity, chemical stability, and electrolytic resistance were investigated in comparison with a commercial sulfonated post-treated separator. The cell performance of hydroentangled CMC composite nonwoven fabrics before and after electrolyte storage was tested by using a stacked Ni-MH battery. These results indicate that hydroentangled CMC composite nonwoven fabric is a kind of promising materials to replace the traditional wet-laid nonwoven fabric as Ni-MH battery separator with better long-term cell performance.

First-principles study of the relative stability of various zirconium hydrides using the special quasirandom structures approach

We present a detailed study on the bulk properties and relative stability of the ordered and disordered phases of zirconium hydrides by using first-principles calculations combined with the special quasirandom structures (SQS) method. The formation energy, density of states (DOS) and elastic constants are calculated to judge the relative stability of these phases. For the ordered phases, the relative stability of different configurations of the γ and δ phase is especially concerned. It is found that the ζ-Zr2H, “diamond-like” γ-ZrH, [111] configurations of the δ-ZrH1.5 and fcc-ZrH2 phase are not stable. For the disordered phases, the SQS method, which is a well-proven method to mimic the random alloys, is used to construct supercells of zirconium hydrides with four structures. It is found that the random alloy gradually transforms from an hcp (solid solution phase) into an fcc (δ phase) and then an fct (c/a < 1, ε phase) phase with the increasing H concentration. The equilibrium states are also determined. The γ phase is energetically not favorable at all H concentrations.

Mechanical properties of zirconium alloys and zirconium hydrides predicted from density functional perturbation theory.

The elastic properties and mechanical stability of zirconium alloys and zirconium hydrides have been investigated within the framework of density functional perturbation theory. Results show that the lowest-energy cubic Pn3[combining macron]m polymorph of δ-ZrH1.5 does not satisfy all the Born requirements for mechanical stability, unlike its nearly degenerate tetragonal P42/mcm polymorph. Elastic moduli predicted with the Voigt-Reuss-Hill approximations suggest that mechanical stability of α-Zr, Zr-alloy and Zr-hydride polycrystalline aggregates is limited by the shear modulus. According to both Pugh's and Poisson's ratios, α-Zr, Zr-alloy and Zr-hydride polycrystalline aggregates can be considered ductile. The Debye temperatures predicted for γ-ZrH, δ-ZrH1.5 and ε-ZrH2 are θD = 299.7, 415.6 and 356.9 K, respectively, while θD = 273.6, 284.2, 264.1 and 257.1 K for the α-Zr, Zry-4, ZIRLO and M5 matrices, i.e. suggesting that Zry-4 possesses the highest micro-hardness among Zr matrices.

Effect of stoichiometry and Cu-substitution on the phase structure and hydrogen storage properties of Ml-Mg-Ni-based alloys

To improve the electrochemical properties of rare-earth-Mg-Ni-based hydrogen storage alloys, the effects of stoichiometry and Cu-substitution on the phase structure and thermodynamic properties of the alloys were studied. Nonsubstituted Ml0.80Mg0.20(Ni2.90Co0.50-Mn0.30Al0.30)x (x = 0.68, 0.70, 0.72, 0.74, 0.76) alloys and Cu-substituted Ml0.80Mg0.20(Ni2.90Co0.50−yCuyMn0.30Al0.30)0.70 (y = 0, 0.10, 0.30, 0.50) alloys were prepared by induction melting. Phase structure analysis shows that the nonsubstituted alloys consist of a LaNi5 phase, a LaNi3 phase, and a minor La2Ni7 phase; in addition, in the case of Cu-substitution, the Nd2Ni7 phase appears and the LaNi3 phase vanishes. Thermodynamic tests show that the enthalpy change in the dehydriding process decreases, indicating that hydride stability decreases with increasing stoichiometry and increasing Cu content. The maximum discharge capacity, kinetic properties, and cycling stability of the alloy electrodes all increase and then decrease with increasing stoichiometry or increasing Cu content. Furthermore, Cu substitution for Co ameliorates the discharge capacity, kinetics, and cycling stability of the alloy electrodes.

Electronic structure and crystal phase stability of palladium hydrides

The results of electronic structure calculations for a variety of palladium hydrides are presented. The calculations are based on density functional theory and used different local and semilocal approximations. The thermodynamic stability of all structures as well as the electronic and chemical bonding properties are addressed. For the monohydride, taking into account the zero-point energy is important to identify the octahedral Pd-H arrangement with its larger voids and, hence, softer hydrogen vibrational modes as favorable over the tetrahedral arrangement as found in the zincblende and wurtzite structures. Stabilization of the rocksalt structure is due to strong bonding of the 4d and 1s orbitals, which form a characteristic split-off band separated from the main d-band group. Increased filling of the formerly pure d states of the metal causes strong reduction of the density of states at the Fermi energy, which undermines possible long-range ferromagnetic order otherwise favored by strong magnetovolume effects. For the dihydride, octahedral Pd-H arrangement as realized, e.g., in the pyrite structure turns out to be unstable against tetrahedral arrangement as found in the fluorite structure. Yet, from both heat of formation and chemical bonding considerations, the dihydride turns out to be less favorable than the monohydride. Finally, the vacancy ordered defect phase Pd3H4 follows the general trend of favoring the octahedral arrangement of the rocksalt structure for Pd:H ratios less or equal to one.

Microstructural characterization and hydrogenation properties of non-stoichiometric Zr0.9TixV2 alloys

The non-stoichiometric Zr0.9TixV2 (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 Zr0.9TixV2 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.

Research progress in Mg-based hydrogen storage alloys

Magnesium and magnesium-based alloy hydrides remain attractive hydrogen storage materials owing to high hydrogen capacity and rich reserves in the earth’s crust. A high stability of hydride and sluggish hydriding/dehydriding kinetics at practical temperatures for the materials drove researchers into alloying with other elements, using different preparation techniques, using catalyst and thin film hydride to improve the hydrogen absorption/desorption properties. In this review, the development of these approaches and their effects on the thermodynamic and kinetics properties of magnesium and magnesium-based alloy hydrides were descript in details.

Investigation into the Palladium‐Europium Acetate Reductive Decomposition with Synchrotron Radiation‐Based X‐ray Diffraction and X‐ray Absorption Spectroscopy

AbstractThe reductive decomposition of the complex Pd2Eu2(μ,η2‐OOCMe)2(μ‐OOCMe)8(H2O)2·(HOOCMe)2upon heating from room temperature to 500 °C in a hydrogen atmosphere was investigated by in situ synchrotron radiation‐based X‐ray absorption fine structure spectroscopy (XAFS) including observations of X‐ray near‐edge structure (XANES) and extended absorption fine structure (EXAFS) at Pd K‐ and Eu L3‐edges and synchrotron X‐ray diffraction (XRD). The Pd‐Eu2O3nanocomposite was found to be the final thermolysis product. The decomposition proceeds via series of intermediate crystalline phases, including europium(II) and (III) anhydrous acetates and EuO(CH3COO) oxidoacetate. Surprisingly, the major fraction of europium occurs in the reduced form Eu2+within a limited temperature range of 200–300 °C. Presumably, the reversible Eu3+↔Eu2+redox‐process is associated with the stability of nanocrystalline palladium hydride, which acts as a stronger reducing agent as compared to molecular hydrogen.

Atomic-Layer Electroless Deposition on Noble Metal Powders

Palladium has important applications in an array of fields including hydrogen storage and sensing, chemical catalysis and fuel cells. It offers high volumetric power density in metal hydride batteries and pseudocapacitors. However, the rates of hydride formation and hydrogen release in such applications are often limited by the high stability of the surface hydride formed on Pd. Theoretical and experimental reports suggest that adlayers of metals that form weaker surface Pd-H interactions can destabilize the surface hydride, which should facilitate hydrogen transport.We have developed a room-temperature electroless atomic layer deposition (ALD) technique in which we grow such layers on palladium powder. This is in contrast to electrochemical ALD, which requires applying a current to a substrate, and to gas-phase ALD, which requires elevated temperatures that can damage some substrates. Exposing a suspension of powder to dilute hydrogen/nitrogen gas mixtures causes a surface hydride to form. Subsequent addition of a solution containing noble metal salts displaces the surface hydride with the reduced metal in a surface-limited reaction. This process can be repeated, and the thickness of the surface coverage increases, as shown by X-ray photoelectron spectroscopy.This technique requires no specialized equipment, and makes use of benign reagents. It is surface-limited in nature, but is not limited to an electrode. This confers important advantages of high scalability and compatibility with complex surface topologies; conductive substrates are not required. We believe that this versatile technique could be widely applied by synthetic chemists to take advantage of enhanced catalytic properties of near-surface alloys.In addition to details on the preparation and characterization of Pd powder with adlayers of other noble metals, we will show evidence of enhanced hydrogen transport in these samples. A kinetic model for this process that includes gas flow, solid-state hydrogen diffusion and surface hydride formation will be presented.Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. SAND2013-8925A

Electronic structure and phase stability of plutonium hydrides: Role of Coulomb repulsion and spin-orbital coupling

The electronic and magnetic states, chemical bonding and reactions, and phonon spectrum of the plutonium hydrides PuHx (x = 2, 3) are investigated by employing first-principles calculations by means of the density functional theory (DFT) + U approach. The strong correlation and the spin-orbit coupling (SOC) effects on these 5f electrons systems are systematically studied. Results show that both the strong correlation and the SOC play critical roles in correctly describing their ground-state properties. The antiferromagnetic configuration of PuH2 is found energetically most stable while for PuH3 the ferromagnetic state is the most stable state. Our calculated phonon spectra clearly indicate the dynamical stability of these magnetic configurations. For PuH3, more electrons from the Pu atoms are released to bond with H than that in PuH2. As a result, the lattice constant is contracted by increasing the H concentration. Reacting from metal Pu and molecule H2, more PuH3 should be produced than PuH2 in low temperature condition.