Stability Of Hydrides Research Articles

Stability of vacancy-hydrogen clusters in nickel from first-principles calculations

The interactions of hydrogen (H) atoms with vacancies are investigated by means of ab initio calculations. The lowest segregation energies are −0.27 and −0.41eV at single vacancies and divacancies, respectively. These values are in excellent agreement with those corresponding to the two characteristic peaks of the thermal desorption spectra. The microscopic interpretation of the experimental data is therefore clarified. An energetic model is built from the ab initio data and used to study the influence of H bulk concentration and temperature on the concentration of vacancy-H clusters. Analytical expressions, validated by Monte Carlo simulations, are given. The mean vacancy occupation and the H-induced vacancy enrichment are calculated at two temperatures representative of H embrittlement experiments and stress corrosion cracking at high temperatures. The stability domain of VH6 clusters is found to significantly overlap with the experimental conditions for embrittlement. Therefore, vacancy clustering at high concentrations can be qualitatively discussed based on VH6–VH6 interactions that are found weakly repulsive. Consequences on H damage in Ni are discussed. The effect of metal vibrations on segregation and local hydride stability is qualitatively evaluated by off-lattice Monte Carlo simulations using a semi-empirical Ni–H potential. They are shown to shift local hydride stability towards higher H concentrations.

Hydrogen storage properties of as-cast and annealed low-Co La1.8Ti0.2MgNi8.9Co0.1 alloys

Low-Co La1.8Ti0.2MgNi8.9Co0.1 alloys were prepared by magnetic levitation melting followed by annealing treatment. The effect of annealing on the hydrogen storage properties of the alloys was investigated systematically by X-ray diffraction (XRD), pressure-composition isotherm (PCI), and electrochemical measurements. The results show that all samples contain LaNi5 and LaMg2Ni9 phases. LaCo5 phase appears at 1,000 °C. The enthalpy change of all hydrides is close to −30.6 kJ·mol−1 H2 of LaNi5 compound. Annealing not only increases hydrogen capacity and improves cycling stability but also decreases plateau pressure at 800 and 900 °C. After annealing, the contraction of cell volume and the increase of hydride stability cause the high rate dischargeability to reduce slightly. The optimum alloy is found to be one annealed at 900 °C, with its hydrogen capacity reaching up to 1.53 wt%, and discharge capacity remaining 225.1 mAh·g−1 after 140 charge–discharge cycles.

First-principles study of structural stability and elastic properties of MgPd3 and its hydride

Abstract Theoretical study of structural stability and elastic properties of α - and β -MgPd 3 intermetallic compounds as well as their hydrides have been carried out based on density functional theory. The results indicate α -MgPd 3 is more stable than β phase with increased stability in their hydrides. The calculated elastic constants of α -MgPd 3 are overall larger than β phase. After hydrogenation, the elastic constants are enlarged. And the elastic moduli exhibit similar tendency. The anisotropy of α -MgPd 3 is larger than β phase, and the hydrides demonstrate larger anisotropy. Their ductility follows the order of α -MgPd 3 H 0.5 α -MgPd 3 β -MgPd 3 H β -MgPd 3 . Compared with β phase, higher Debye temperature of α -MgPd 3 implies stronger covalent interaction, and the Debye temperature of hydrides increases slightly. The electronic structures demonstrate that the Pd–Pd interaction is stronger than Pd–Mg, and Pd–H bonds play a significant role in the phase stability and elastic properties of hydrides.

Enhanced tunability of thermodynamic stability of complex hydrides by the incorporation of H– anions

First-principles calculations were employed to investigate hypothetical complex hydrides (M,M′)4FeH8 (M = Na, Li; M′=Mg, Zn, Y, Al). Besides complex anion [FeH6]4–, these materials contain two H– anions, which raise the total anionic charge state from tetravalent to hexavalent, and thereby significantly increasing the number of combinations of countercations. We have determined that similar to complex hydrides (M,M′)2FeH6 containing only [FeH6]4–, the thermodynamic stability is tuned by the average cation electronegativity. Thus, the chemical flexibility provided by incorporating H– enhances the tunability of thermodynamic stability, which will be beneficial in obtaining optimal stability for hydrogen storage materials.

Ti(Ni,Cu) pseudobinary compounds as efficient negative electrodes for Ni–MH batteries

The effect of Ni by Cu substitution on the structural, solid–gas and electrochemical hydrogenation properties of TiNi has been investigated. Pseudo-binary TiNi1−xCux (x ≤ 0.5) compounds have been synthesized by induction melting. They crystallize in B2 structure above 350 K and either in B19′ (x < 0.1) or B19 (0.2 ≤ x ≤ 0.5) at room temperature (RT). For all compounds, Pressure-Composition Isotherms at 423 K exhibit a single slopping plateau pressure within the range 10−3–1 MPa of hydrogen pressure revealing a metal to hydride transformation. Both the hydrogenation capacity and the hydride stability decrease with Cu-content. The hydrided pseudobinary compounds crystallize in the tetragonal S.G. I4/mmm structure as for TiNi hydride. The electrochemical discharge capacity increases with Cu content from 150 mAh g−1 for TiNi up to 300 mAh g−1 for TiNi0.8Cu0.2 and then decreases again for larger Cu amounts. Electrochemical isotherms and in-situ neutron diffraction measurements at RT demonstrate that such a capacity increase results from a metal to hydride phase transformation in which the hydride phase is destabilized by Cu substitution. The TiNi0.8Cu0.2 compound exhibits interesting cycling stability for 30 cycles and good high-rate capability at D/2 rate. This compound has promising electrochemical properties as compared to commercial LaNi5-type alloys with the advantage of being rare-earth metal free.

Hydrogenation properties and crystal structure of YMgT4 (Т = Co, Ni, Cu) compounds

New two ternary YMgCo4 and YMgCu4 and one quaternary YMgCo2Ni2 compounds have been synthesized by mechanical alloying with further annealing. The hydrogenation capacity of YMgCo4 reaches 6.8at.H/f.u. The Pressure-Composition-Temperature studies of YMgCo4–H2 and YMgNi4–H2 systems revealed that introduction of magnesium, accompanied by shrinking of the unit cell, decreases the stability of hydrides comparing to binary YCo2 and YNi2 compounds. The values of heat and entropy of the YMgCo4H6.8 hydride formation were calculated: ΔH=−27.9±0.8kJmol–1H2 and ΔS=−93.4±2.6Jmol−1H2K−1. The YMgCo2Ni2–H2 system shows intermediate thermodynamic properties compared to the ternary hydrides (ΔH=−28.8±0.2kJmol–1H2 and ΔS=−117.6±2.4Jmol–1H2K−1). The YMgCo4H6.8 and YMgCo2Ni2H4.9 hydrides keep the cubic structure of the parent compounds with a cell volume expansion of 23 and 14.4% respectively. It is shown that the YMgCu4 compound does not interact with hydrogen under normal conditions.

Selective oxidation using supported gold bimetallic and trimetallic nanoparticles

Supported gold nanoparticles are highly effective for a range of redox reactions. In these reactions the activity is often enhanced by the addition of a second or indeed a third metallic component. A model reaction that is often investigated is the selective oxidation of benzyl alcohol using molecular oxygen as terminal oxidant. In the presence of a solvent the complexity of this reaction can often be missed. However, in the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold palladium nanoparticles as catalysts, there are two pathways to the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O2, while the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. The formation of toluene is an unwanted side reaction. In this paper the research on this reaction will be reviewed and two strategies described that can be used to switch off the formation of the non desired toluene. The first involves the use of basic supports for the gold palladium nanoparticles, which is highly effective in suppressing the formation of toluene and this may be related to the morphology of the gold palladium nanoparticles and their interaction with the support. The second involves the introduction of platinum to the gold palladium nanoparticles which also switches off toluene formation on supports that permit toluene formation in the absence of platinum. This effect may be related to the relative stability of platinum hydride.

New high-pressure phase of MgH2: An ab initio constant-pressure study

The stability of magnesium hydride (MgH2) at high pressure is studied using a constant-pressure ab initio technique. Two phase transformations are successfully observed through the simulations. The rutile structure undergoes a phase transformation into a CaCl2-type phase. Further increase in pressure results into a first-order phase transition into an orthorhombic state within Pbcm symmetry. This phase can be considered as a distorted CaF2-type crystal and does not correspond to the previously proposed MgH2 phases. The transformation mechanism of the CaCl2-Pbcm phase change at the atomistic level is successfully characterized and it is found that the CaCl2-to-Pbcm phase change proceeds via an ideal CaF2-type intermediate phase. These phase transformations are also analyzed using total energy-volume calculations.

The effect of storage time on the thermal behavior of nanocrystalline magnesium hydride with metal halide additives

This work presents results concerning the thermal stability of nanocrystalline magnesium hydride milled with 7 wt% of metal halide additives such as ZrF4, NbF5, TaF5, FeF2, FeF3, TiCl3 and VCl3 after one year of storage in a glovebox under a continuously purified argon atmosphere. The amounts of oxygen and water in the glovebox were below 0.1 ppm. The gases released from the materials during the thermal decomposition of all samples were analyzed using a quadrupole mass spectrometer. The oxidation of MgH2 at low oxygen and water vapor pressures leads to an increase in the hydride decomposition temperatures of the samples compared to their initial (as-prepared) counterparts, which are shown in Ref. [1]. The differences in the decomposition temperatures between the powders stored for one year in a glovebox and the initial as-prepared powders were greater than 60 °C for MgH2 with NbF5, ZrF4, TaF5, TiCl3 and VCl3. The release of water from all long-term-stored materials and the emission of hydrogen fluoride (HF) from MgH2 with metal fluorides were observed during heating. The observed results demonstrate that even a protective argon atmosphere did not prevent the degradation of the nanocrystalline MgH2 powders with various metal halide additives during their long-term storage in a professional glovebox, which is an important practical concern.

The application of Pettifor structure maps to binary metal hydrides

The aim of this paper is the application of Pettifor stability structure maps to MHx binary metal (M) hydrides in accordance with their hydrogenation properties. The formation enthalpy (ΔHf) of selected hydrides, their stoichiometry (MHx) and hydrogen capacity (Hcap) were extracted from the literature, where these values were either measured or calculated using various models. A comparison of the enthalpy values obtained from experiments or calculations based on the Miedema, Born–Haber and Energetic models is also presented. An assessment of the effect of the ΔHf values obtained by various methods on the structure stability maps was performed by plotting the enthalpy vs. the Mendeleev number M and the value of the chemical scale, χ. Moreover, based on two- and three-dimensional Pettifor plots, the enthalpy of hydride formation and hydrogen capacity of selected hydrides are presented with respect to the hydride stoichiometry, Hx, and the Mendeleev number M. Higher values of M indicate less hydride stability and, thus, larger stability variations. The metal hydrides with only one hydrogen atom (x = 1) and M from 0 to 21 exhibit stability that is adequate for practical applications. For M from 21 to 53, all hydrides exhibit high stability regardless of Hx. For M from 53 to 102, all hydrides exhibit large variations in stability. Thus, the hydrides with x = 1–3 exhibit either high stability or stability that is adequate for practical applications, and those with x = 4 are highly unstable hydrides.

Microstructure and hydrogen storage properties of non-stoichiometric Zr–Ti–V Laves phase alloys

The non-stoichiometric C15 Laves phase alloys namely Zr0.9Ti0.1Vx (x = 1.7, 1.8, 1.9, 2.1, 2.2, 2.3) are designed and expected to investigate the role of defect and microstructure on hydrogenation kinetics of AB2 type Zr-based alloys. The alloys are prepared by non-consumable arc melting in argon atmosphere and annealed at 1273 K for 168 h to ensure the homogeneity. The microstructure and phase constitute of these alloys are examined by SEM, TEM and XRD. The results indicate the homogenizing can reduce the minor phases α-Zr and abundant V solid solution originating from the non-equilibrium solidification of as-cast alloys. Twin defects with {111}<011 > orientation relationship are observed, and the role of defects on hydrogenation kinetics is discussed. Hydrogen absorption PCT characteristics and hydrogenation kinetics of Zr0.9Ti0.1Vx at 673–823 K are investigated by the pressure reduction method using a Sievert apparatus. The results show the hypo-stoichiometric alloys preserve faster hydrogenation kinetics than the hyper-stoichiometric ones due to the decrease of dendritic V. The excess content of Zr3V3O phase decreases the hydrogenation kinetics and the stability of hydrides. In addition, the different rate controlled mechanisms during hydrogen absorption are analyzed. The effects of non-stoichiometry on the crystal structure and hydrogen storage properties of Zr0.9Ti0.1Vx Laves alloys are discussed.

Photoelectrochemistry of silicon in HF solution

Photoelectrochemical, photoelectrocatalytic, and electrochemical processes of silicon anodic oxidation and hydrogen evolution in aqueous HF solution are discussed in terms of thermodynamic stability of Si, oxides SiO, SiO2, and Si surface hydrides. It is shown that photoelectrochemical oxidation of n-type low-resistivity silicon to SiO2 is catalyzed by Si\(^{+}\) photo-hole formation, whereas in the case of p-type Si, the feasibility of this reaction is predetermined by p-type conductivity. It is suggested that anodic oxidation of Si goes through the stage of SiO oxide formation and its subsequent oxidation to SiO2. Such mechanism accounts for chemical inertness of Si phase in HF solutions as well as for selective, anisotropic, and isotropic etching of Si within E ranges from \(-0.5\) to 0.35 V, \(0.35-0.8~V,\) and \(E > 0.8\) V, respectively. Hydrogen evolution reaction on Si surface proceeds at very large overpotential (\(\geq 0.5\) V) through the stage of surface Si hydride formation: \(\mathrm {Si + H_{2}O + e^{-} \rightarrow (SiH)_{surf} + OH^{-}}\) (the rate determining step) and \(\mathrm {(SiH)_{surf} + H_{2}O + e^{-} \rightarrow Si + H_{2} + OH^{-}}\). Illumination-related effects of surface reactions relevant to selective and anisotropic etching and nano/micro-structuring of Si surface are discussed.

Influence of Pd addition on the electrochemical performance of Mg–Ni–Ti–Al-based metal hydride for Ni–MH batteries

Amorphous Mg0.9Ti0.1NiAl0.05 and Mg0.9Ti0.1NiAl0.05Pd0.1 alloys were prepared by high energy ball milling and evaluated as metal hydride electrodes for Ni–MH batteries. The Mg0.9Ti0.1NiAl0.05Pd0.1 alloy showed a much higher cycle life with a capacity retention of 72% after 100 cycles (C100th = 288 mAh g−1) compared to 26% for the Pd-free alloy (C100th = 117 mAh g−1). This was mainly attributed to the improvement of the alloy oxidation resistance in KOH electrolyte with Pd addition, as confirmed by cyclic voltammetry experiments and X-ray diffraction analyses on cycled electrodes. In addition, in situ acoustic emission (AE) measurements revealed that the energy of the AE signals related to the particle cracking is lower for the Mg0.9Ti0.1NiAl0.05Pd0.1 electrode, suggesting that the cracks are smaller in size than with the Pd-free alloy. The Mg0.9Ti0.1NiAl0.05Pd0.1 electrode also displayed a higher discharge rate capability than the Mg0.9Ti0.1NiAl0.05 electrode. On the basis of their respective electrochemical pressure–composition isotherm, it was shown that the presence of Pd in the alloy decreases the thermodynamic stability of the metal hydride. Through a comparative analysis of discharge polarization curves, it was also shown that Pd addition decreases substantially the H-diffusion resistance in the alloy whereas its positive effect on the charge-transfer resistance is limited.

Improvement of hydrogen sorption properties of compounds based on Vanadium “bcc” alloys by mean of intergranular phase development

Body centered cubic structure (bcc) type alloys based on Vanadium [1] reveal promising characteristics for mobile applications. These disordered solid solutions have particular metal/hydride equilibrium and some regulation aspects have leaded us to pay special attention to this type of material [2]. Compounds based on Vanadium-rich solid solution have been elaborated in order to destabilize γ hydride phase (corresponding to the face centered cubic (fcc) structure of VH2). Addition of Ni and Zr-rich Laves phase as a secondary phase results in the development of a particular microstructure composed of a principal bcc matrix rounded by intergranular activating phase. This results in a facilitated and faster activation of these compounds. The present study shows that some constituting species of the secondary phase have diffused in the main matrix and therefore have modified the thermodynamic of hydride. In fact, chromium diffusion into the bcc matrix destabilizes hydride. It is correlated to the lower stability of chromium hydride compared to Vanadium hydride. The enthalpic terms of each sample have been measured (assuming standard entropy of 130 J mol−1 K−1). The equilibrium metal/hydride can be easily switched in order to adapt it to a mobile hydride tank and obtain low working temperature in regard to the potential use.

Correlation between electrochemical behavior and hydrogen storage properties of Li–Sn system

Abstract The high stability of Lithium hydride is a bottleneck to use it as hydrogen storage media. The present paper is an effort to destabilize it using Sn as a dopant. Li 4.4 Sn is chosen due to its highest lithium content among all the possible stoichiometries. It is found that the real composition is Li 17 Sn 4 rather than Li 4.4 Sn. Hydrogenation measurements suggest the existence of two step reaction. Li 17 Sn 4 first react with hydrogen to form a mixture of LiH and Li 13 Sn 5 , which further transformed into a mixed phase of LiSn and LiH as final product under studied hydrogenation condition. The enthalpy values for the above mentioned reactions are found to be −93 ± 11 kJ/mol H 2 and −37 ± 2 kJ/mol H 2 for 1st and 2nd step respectively. These values are much lower than the values observed for pure LiH. Finally these hydrogenation properties were used to calculate the electrochemical potential values required for lithiation/delithiation. The value for potential for 1st step and 2nd step reaction is found to be 0.456 V and 0.746 V for Li–Sn system, which is in good agreement with the values calculated through direct electrochemical methods.

Synthesis and structure of a dinuclear NiIIIrIII hydride complex supported by a N3S2 bridging ligand: Comparison of property and reactivity of dinuclear NiII(μ-H)MIII units (M = Rh, Ir) in aqueous media

A water-soluble dinuclear NiIIIrIII complex with a bridging hydride, [Cp*Ir(μ-H)Ni(TsTACN(C2H4S)2)]NO3 (TsTACN(C2H4SH)2 = 1,4-bis(2-mercaptoethyl)-7-tosyl-1,4,7-triazacyclononane, Cp* = η5-penta-methylcyclopentadienyl, [2b]NO3), was synthesized by reacting a dinuclear NiIIIrIII complex [Cp*Ir(NO3)Ni(TsTACN(C2H4S)2)]NO3 ([1b]NO3) with HCOONa in water. The structure of [2b]NO3 was unequivocally determined by an X-ray analysis to allow comparing its structure, property, and reactivity in aqueous media to those of the corresponding rhodium counterpart, [Cp*Rh(μ-H)Ni(TsTACN(C2H4S)2)]NO3 ([2a]NO3). The Ni⋯Ir distance (av. 2.721 Å) is slightly longer than the Ni⋯Rh separation (av. 2.691 Å). The hydrido ligand of NiII(μ-H)IrIII complex ([2b]NO3) bears protic character and is stable in water above pH 2, which is contrasted to the fact that the hydrido ligand of NiII(μ-H)RhIII ([2a]NO3) has no protic character and is stable in water only above pH 6. The difference of the hydride stability in water may lead to a difference of their reductivity, in which [2b]NO3 barely reduced benzaldehyde to benzyl alcohol only in acidic media (pH 3) although the rhodium analog [2a]NO3 did promote the reaction in neutral water. Theoretical calculations with DFT methods on model compounds [Cp*M(μ-H)Ni(HTACN(CH2CH2S)2)]+ (M = Rh ([3a]+), Ir ([3b]+)) revealed that the hydrido ligand of the Ni(μ-H)Ir has less bridging character with a stronger interaction with the IrIII center than that with the RhIII center in the Ni(μ-H)Rh unit.

Strain effect on structural and dehydrogenation properties of MgH2 hydride from first-principles calculations

The structure, stability, dehydrogenation thermodynamic and kinetic properties of MgH2 hydride under different biaxial strain conditions were investigated by using first-principles calculations based on the density functional theory (DFT). The results show that either biaxial tensile or compressive strain is likely to cause the structural deformation of MgH2 crystal, and its lattice distortion becomes severe with increasing magnitude of strain. Due to the contribution of strain energy, the biaxial strain not only weakens the structural stability of MgH2, but also lowers its hydrogen desorption enthalpy and dehydrogenation temperature. Furthermore, the diffusion activation energy of hydrogen atom in MgH2 host is also decreased, which results in a remarkable improvement of dehydrogenation properties. Noticeably, the effect of tensile strain in improving dehydrogenation thermodynamics is relatively superior to that of compressive one, while the role of the latter in enhancing dehydrogenation kinetics is relatively stronger than that of the former. Further analysis of electronic structures suggests the strain-induced changes in structural and dehydrogenation properties of MgH2 are closely associated with the value of total densities of states at the Fermi level as well as the bonding electrons number below Fermi level. These results provide an insight for developing better MgH2-based nanocomposite hydrogen storage materials by introducing suitable interface misfit strain.

Theoretical study on the structure and stability of aluminum hydride (Al n H3n ) clusters

Density functional theory (DFT) calculations have been carried out to study the linear polymeric Al n H3n (n = 1–12) clusters and cage Al n H3n (n = 6, 8, 10, and 12) clusters. In particular, a stable cage-chain (Al12H36) m (m = 2, 3) structure has been predicted for the first time, with the basic unit, cage Al12H36 structure connected by double hydrogen-bridged bonds (i.e., Al–2H–Al). The stability of these clusters has been confirmed by the large binding energies and substantial energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, and the cage-chain structure is more stable than the corresponding chain and cage forms. Furthermore, with the cluster size increasing, the aluminum hydride clusters tend to exist with the cage structure compared with the chain structure, even though the formation of the chain structure is much easier than others. Also, the heats of formation for some clusters have been calculated by the atomization reaction method based on the results from DFT calculations, and it is revealed that the cage and cage-chain clusters can store more energy and, especially the latter, is more promising as a kind of energetic material.

Structural, electronic and thermodynamic properties of R3ZnH5 (R=K, Rb, Cs): A first-principle calculation

R3ZnH5 (R=K, Rb, Cs) series have been investigated with respect to the crystal structure, electronic and thermodynamic properties using first-principle methods based on density functional theory with generalized gradient approximation. The optimized structures and atomic coordinates are in good agreement with the experimental data. The strong covalent interactions are obtained between Zn and H atoms in the 18-electron [ZnH4]2− complex, while an ionic interaction is found between [ZnH4]2− and R atom. The formation enthalpies show that the formations of R3ZnH5 hydrides are all exothermic at 298K. The vibration free energies of R3ZnH5 show that the thermodynamic stabilities of R3ZnH5 hydrides decrease with the increasing diameter of R atom. Two possible decomposition reactions of R3ZnH5 series have been suggested in our work. One (reaction one) is that R3ZnH5 hydrides decomposes to elements directly, and the other (reaction two) is that R3ZnH5 hydrides decomposes to RH hydride. The results show that the first decomposition reaction is more favorable one. The spontaneous decomposition reaction of K3ZnH5 hydrides occur upon 465K via reaction one, and 564K via reaction two, respectively.

Computationally efficient determination of hydrogen isotope effects on the thermodynamic stability of metal hydrides

Although the thermodynamics of metal hydrides at low to moderate temperatures has been successfully described with density functional theory (DFT) calculations using 0 K total energies and simple harmonic models, it is unclear if this approach is valid for hydrides that are stable at high temperatures. To aid development of computationally efficient methods, this paper uses DFT to explore the predicted stabilities of ZrH${}_{2}$, HfH${}_{2}$, TiH${}_{2}$, LiH${}_{,}$ and NaH with four levels of theory. We also investigate isotope effects to understand if these should be accounted for in screening of deuterated or tritiated materials. We show that calculations that account for vibrational corrections to the crystal lattice are not necessary to get an accurate description of relative stabilities of metal hydrides. The shifts in dissociation temperatures due to isotope substitutions are 50 K for all materials, with larger shifts for lighter materials, as expected. We show that accounting for vibrational effects due to isotope substitution in metal hydrides is unnecessary to accurately predict the relative stabilities of metal hydrides at high temperatures.