Stability Of Hydrides Research Articles

Structure, electronic characteristic and thermodynamic properties of K2ZnH4 hydride crystal: A first-principles study

Density functional theory calculations have been used to investigate the structural stability, electronic characteristics and thermodynamic properties of K2ZnH4 hydride. We find that the orthorhombic structure of K2ZnH4 is more stable than the tetragonal structure base on the total energy and phonon density of states. The calculated lattice parameters of the orthorhombic structure of K2ZnH4 are in good agreement with the experimental data. Analysis of the electronic characteristic suggests that K2ZnH4 crystal is an insulating material with a band gap of 4.010eV. The calculated average H site energy is 4.540eV/H, demonstrating that strong covalent bond exist between Zn and H atom. The formation enthalpy of K2ZnH4 is found to be −1.318eV/f.u at 298K. Hydrogen desorption from K2ZnH4 takes place via a two-step process, firstly forming KH, Zn and H2, and then K and H2. The first decomposition reaction is predicted at 524K, and the second decomposition reaction is at 505K. The reaction enthalpy is estimated to be 49.164kJ/mol at 524K for the first decomposition reaction, and 62.523kJ/mol at 505K for the second decomposition reaction.

Influence of Cr on the hydrogen storage properties of Ti-rich Ti–V–Cr alloys

In the present work, we studied the effects of Cr on the crystal structures and hydrogen storage properties of ternary alloys, Ti0.7V0.3−xCrx and Ti0.8V0.2−xCrx. Metal–hydrogen interactions were characterised by Thermal Desorption Spectroscopy (TDS) and in situ Synchrotron X-ray diffraction (SR-XRD). All initial alloys crystallise with body-centred cubic (BCC) crystal structures formed as solid solutions of V and Cr in Ti. Upon hydrogenation, the dihydrides (Ti,V,Cr)H2 with face-centred cubic (FCC) structures are formed. An increase in the Cr content leads to systematic changes in the structure and hydrogenation behaviours. The changes include (a) contraction of the unit cells for the initial alloys and for the corresponding dihydrides; (b) slower hydrogen absorption kinetics and an increase in the incubation period for hydrogenation; (c) a decrease in the thermal stability of the saturated hydrides; and (d) a reduction in the apparent activation energy of hydrogen desorption. In situ SR-XRD and TDS studies of the FCC Ti–V–Cr hydrides indicated that their decomposition consists of five individual desorption events.

Π-Bond maximization of graphene in hydrogen addition reactions

Thermodynamic stability of graphene hydrides increases in an approximately linear way with the numbers of π-bonds they contain. Thus, π-bond maximization is the primary driving force for hydrogen addition reactions of graphene. The previously reported thermal preference of sp(2)/sp(3)-phase separation of graphene hydrides is a straightforward effect of π-bond maximization. Although not well applicable to hydroxylation and epoxidation, the π-bond maximization principle also holds approximately for the fluorination reactions of graphene. The findings can be used to help locate the lowest-energy structures for graphene hydrides and to estimate the hydrogenation energy without first-principles calculations.

High Pressure Synthesis of Novel Li-TM Hydrides (TM=Nb, Ta)

High-pressure synthesis has been widely used for exploration of novel materials. In this study, new Li-based hydrides in Li-Nb and Li-Ta systems have been successfully synthesized under GPa-order by using a cubic-anvil-type apparatus. And crystal structures, thermal stabilities of novel hydrides were investigated.For Li-Nb-H system, novel hydride with primitive hexagonal structure was synthesized at 973K under 5GPa. Its lattice parameter was a = 0.56250(5) nm, c = 0.57416(4) nm. A structural model of the hydride was CaCu5-type structure (P6/mmm, No. 191). For Li-Ta system, novel hydride with C-face centered monoclinic structure was obtained at 973K under 5GPa. Its lattice parameter was a = 0.7918(5) nm, b = 0.8012(4) nm, c = 1.030(7) nm, β =111.27 (6)˚. These novel hydrides in Li-Nb and Li-Ta systems were decomposed at 510K and 573K, respectively, and these values were about 400K lower than that of LiH (>953K).

Stability of erbium hydrides studied by DFT calculations

The reaction between hydrogen and erbium has been studied by DFT calculations. The enthalpies of formation of the compounds Er, ErH, ErH2 and ErH3 generated by the ordered insertion of hydrogen atoms in the tetrahedral and octahedral (or triangular) sites of the fcc and hcp structures have been calculated. In addition, ErH3 has been calculated with its reported trigonal superstructure. The results of the calculations are consistent with the experimental observation of the sequence Er (hcp) → ErH2 (fcc) → ErH3 (trigonal), qualitatively, and with the measured enthalpies of formation, quantitatively. Additionally, the densities of states of all the compounds are studied in detail.

P102 Couscous à base de riz et de féverole pour malades cœliaques

The cathodic discharge of hydrogen on the surface of massive specimens of NiZr intermetallic compound readily forms the trihydride NiZrH3 which exhibits the same crystallographic parameters as the trihydride obtained by gas phase charging. Moreover microstructural studies of the first stage of the hydriding process during cathodic charging of massive samples allow to follow the influence of the microstructure of NiZr on the germination of cracks and on the first stage of the decrepitation associated with H absorption. These results show that the NiZr microstructure could strongly affect both the hydriding kinetic, the hydride stability and further the amount of hydrogen stored in the intermetallic compound.The XPS analysis of the surface of NiZr after cathodic charging shows the coexistence of zirconium core levels characteristic of Zr–H and Zr–O bonds.The hydrogen absorption affects the electronic properties of Zr atoms in the intermetallic compound in a similar way of what is observed in pure Zr: an H-induced band is observed about 6 eV below the Fermi level together with a chemical shift of the Zr core levels characteristic of the hydride formation. The presence of oxygen on the surface decreases the intensity of the Ni2p core levels whereas the intensity drop of the satellite located at 5 eV (below Ni 2p) is associated with an H-induced filling of the 3d band of Ni atoms in NiZr.The cosegregation of oxygen and zirconium on the surface is clearly evidenced by XPS in the 20–380∘C temperature range. A preferential Zr enrichment of the surface is observed at higher temperature; the occupancy fraction of the surface sites by Zr atoms is as large as 98% at 600∘C. This is presumably a consequence of both the oxygen dissolution in NiZr and a preferential segregation of Zr atoms on the surface.

Hydrogen absorption properties of Zr(V 1− xFe x) 2 intermetallic compounds

The Zr(V 1− x Fe x ) 2 ( x = 0.02, 0.05, 0.10, 0.15, 0.25) alloys were prepared by the arc-melt method and annealed at 1273 K for 168 h in an argon atmosphere. Phase structure investigations of the as-cast and annealed Zr(V 1− x Fe x ) 2 alloys indicate the annealing treatment can eliminate the minority phases originating from the non-equilibrium solidification of as-cast alloys. The ZrV 2-type phase becomes the dominant one in each annealed alloy. The substitution of Fe in V sites leads to the contraction of their lattice. For annealed Zr(V 1− x Fe x ) 2 alloys, the P–t and PCT curves obtained between 673 K and 823 K give the evidence that the absorption process is controlled by a rate-controlling hydrogen diffusion. With the increase of iron, the equilibrium pressure and the plateau slope increase while the hydrogenation capacity and the absolute value of enthalpy and entropy decrease accordingly. The stability of metal hydride reduces gradually as the Fe content varies from x = 0.02 to 0.25 which promotes the hydrogen release and favors the practical applications of the Zr(V 1− x Fe x ) 2 alloys.

Theoretical studies on hydrogen adsorption properties of lithium decorated diborene (B 2H 4Li 2) and diboryne (B 2H 2Li 2)

Using ab initio based quantum chemical calculations, we have studied the structure, stability and hydrogen adsorption properties of different boron hydrides decorated with lithium, examples of the corresponding anions being dihydrodiborate dianion, B 2H 2 2− and tetrahydrodiborate dianion, B 2H 4 2− which can be considered to be analogues and isoelectronic to acetylene (C 2H 2) and ethelene (C 2H 4) respectively. It is shown that there exists a B–B double bond in B 2H 4Li 2 and a B–B triple bond in B 2H 2Li 2. In both the complexes, the lithium sites are found to be cationic in nature and the calculated lithium ion binding energies are found to be very high. The cationic sites in these complexes are found to interact with molecular hydrogen through ion-quadrupole and ion-induced dipole interactions. In both the complexes, each lithium site is found to bind a maximum of three hydrogen molecules which corresponds to a gravimetric density of ∼23 wt% in B 2H 4Li 2 and ∼24 wt% in B 2H 2Li 2. We have also studied the hydrogen adsorption in a model one-dimensional nanowire with C 6H 4B 2Li 2 as the repeating unit and found that it can adsorb hydrogen to the extent 9.68 wt% and the adsorption energy is found to be −2.34 kcal/mol per molecular hydrogen.

Structure and hydrogen absorbing properties of ScCrMn alloy

The crystal structures of ScCrMn alloy and its hydride are determined by XRD Rietveld analysis and TEM examination. The ScCrMn alloy exhibits a rarely ideal close-packed C14 type hexagonal Laves phase structure with lattice parameters a = 5.064(1), c = 8.263(2) Å, deviating from the ANOE dependent rule on Laves phase structure. The hydrogenation of the alloy to ScCrMnH 3.9 results in significant lattice expansion of 27%, but does not alter the matrix lattice structure. The XRD pattern of the hydride also shows close-packed structure with little line broadening, comparable to that of the alloy, indicating a high degree of crystallinity, where the hydrogen atoms are homogeneously distributed in evenly distributed interstitials with minimal defects. The alloy demonstrates extremely easy activation property. The alloy can be activated at a low pressure of 0.46 kPa and exhibit very fast absorption rate at sub-atmosphere at room temperature. The hydrogenation thermodynamics of the alloy are evaluated by P– C– T measurements at different temperatures. At hydrogen concentration H/M = 0.66 corresponding to a defined room temperature plateau pressure, the relative partial molar enthalpy Δ H and entropy Δ S derived by Van’t Hoff equation are −63 kJ/mol and −111 J/mol K, respectively. These results manifest that the hydride stability of the ScCrMn alloy is comparable to those of the hydrogen isotope storage alloys of ZrCo and ZrTi 0.2V 1.8, and would be superior on relevant applications. The DSC–TG measurements of the hydride reveal that the total release of hydrogen can basically be achieved at 653 K, accompanied by an oxidation reaction with residual oxygen to form water vapor. The results indicate that the alloy is an effective catalyst for the dissociation of hydrogen and combination of oxygen.

Propyne and 1,3-butadiene adsorption and co-adsorption over palladium catalysts

The adsorption of propyne, 1,3-butadiene and the co-adsorption of propyne and 1,3-butadiene has been studied over a series of palladium catalysts. The adsorption of propyne shows a “secondary” adsorption region typical of hydrocarbon adsorption over supported metal catalysts. Hydrogenation occurs via the hydrogen associated with the β-palladium hydride formed during reduction. A support effect is seen with the titania support reducing the stability of the palladium hydride resulting in a lower hydrogen concentration in the titania supported hydrides. The behaviour of 1,3-butadiene is similar to that found with propyne. The catalyst with the lowest dispersion, as measured by carbon monoxide adsorption, has the highest butadiene adsorption and vice versa, whereas propyne adsorption followed the same trend as carbon monoxide. This behaviour is linked to the mode of adsorption and site requirements. Sequential adsorption revealed that the catalysts had sites that were specific to each adsorbate. Co-adsorption revealed a reduction in adsorption for both gases but with a larger reduction for 1,3-butadiene due to site requirements.

Hydrogen in layered iron arsenides: Indirect electron doping to induce superconductivity

Utilizing the high stability of calcium and rare-earth hydrides, CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--1.0) and SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ ($x$ $=$ 0.0--0.47) have been synthesized using high pressure to form hydrogen-substituted 1111-type iron-arsenide superconductors. Neutron diffraction and density functional calculations have demonstrated that the hydrogens are incorporated as H${}^{\ensuremath{-}}$ ions occupying F${}^{\ensuremath{-}}$ sites in the blocking layer of CaFeAsF. The resulting CaFeAsF${}_{1\ensuremath{-}x}$H${}_{x}$ is nonsuperconducting, whereas, SmFeAsO${}_{1\ensuremath{-}x}$H${}_{x}$ is a superconductor, with an optimal ${T}_{\mathrm{c}}$ $=$ 55 K at $x$ \ensuremath{\sim} 0.2. It was found that up to 40% of the O${}^{2\ensuremath{-}}$ ions can be replaced by H${}^{\ensuremath{-}}$ ions, with electrons being supplied into the FeAs layer to maintain neutrality (O${}^{2\ensuremath{-}}$ $=$ H${}^{\ensuremath{-}}$ $+$ ${e}^{\ensuremath{-}}$). When $x$ exceeded 0.2, ${T}_{\mathrm{c}}$ was reduced corresponding to an electron overdoped region.

Studies of off-stoichiometric AB 2 metal hydride alloy: Part 2. Hydrogen storage and electrochemical properties

In Part 2 of this two-part series of papers, gaseous hydrogen storage and electrochemical properties of three series of alloys with different combinations of Cr/Mn/Co ratios are studied and compared to the structural properties reported in Part 1. As the B/A stoichiometry in each series of alloys increases from 1.8 to 2.2, systematic trends in certain storage properties are found: the hydrogen dissociation pressure and heat of hydride formation increases; the alloy with a AB 2.0 stoichiometry has the highest electrochemical full capacity; and slightly higher and lower B-contents increase the electrochemical high-rate-dischargeability and gaseous phase maximum storage capacity, respectively. Stoichiometric or slightly hyper-stoichiometric AB 2 alloys have lower PCT hysteresis which are expected to reduce pulverization during cycling. The full and high-rate discharge electrochemical capacities correlate well with the maximum and reversible gaseous hydrogen storages, respectively. Slight hyper-stoichiometry increases the high-rate dischargeability. Open circuit voltage, an important parameter in high-power application, is also found to be more relevant to the surface reaction than to the bulk hydride stability.

Gas Phase Reactions of 1,3,5-Triazine: Proton Transfer, Hydride Transfer, and Anionic σ-Adduct Formation

The gas phase reactivity of 1,3,5-triazine with several oxyanions and carbanions, as well as amide, was evaluated using a flowing afterglow-selected ion flow tube mass spectrometer. Isotopic labeling, H/D exchange, and collision induced dissociation experiments were conducted to facilitate the interpretation of structures and fragmentation processes. A multi-step (→ HCN + HC(2)N (2) (-) → CN(-) + 2 HCN) and/or single-step (→ CN(-) + 2 HCN) ring-opening collision-induced fragmentation process appears to exist for 1,3,5-triazinide. In addition to proton and hydride transfer reactions, the data indicate a competitive nucleophilic aromatic addition pathway (S(N)Ar) over a wide range of relative gas phase acidities to form strong anionic σ-adducts (Meisenheimer complexes). The significant hydride acceptor properties and stability of the anionic σ-adducts are rationalized by extremely electrophilic carbon centers and symmetric charge delocalization at the electron-withdrawing nitrogen positions. The types of anion-arene binding motifs and their influence on reaction pathways are discussed.

Role of Transition Elements in Stability of Magnesium Hydride: A Review of Theoretical Studies

During the last decades hydrogen has attracted worldwide attention as an energy carrier. MgH2 is one of the most promising materials for hydrogen storage due to its high hydrogen uptake, large reserves and low cost. However, the potential for practical use of MgH2 is severely limited because of its high temperature of hydrogen discharge, slow desorption kinetics and a high reactivity toward air and oxygen. Nevertheless, the transition metals doping of Mg greatly enhances the kinetics of hydrogen uptake and release and in particular cases decreases its stability. Despite a huge number of experimental studies fundamental aspects of these phenomena remain unclear. Theoretical researches could provide an insight in metal-hydrogen bonding that governs both the thermodynamic stability and the hydrogen sorption kinetics. In this paper a brief review of the recent theoretical works concerning the influence of transition metals on the electronic structure and stability of magnesium hydride MgH2 is given.

Electronic and bonding properties of MgH 2–Nb containing vacancies

The magnesium hydride stability and bonding have been studied using density functional theory (DFT). To this aim, calculations on the electronic structure were performed. We also modeled the bulk hydride with a Nb atom as a substitutional impurity. Furthermore, both systems were modeled containing different types of vacancies (Mg, H or H–Mg complex). The crystal orbital overlap population for both the metal–metal and metal–hydrogen bonds was also computed. The influence of vacancy-like defects was studied through the calculation of the positron lifetimes in defected MgH 2 and defected MgH 2–Nb. For the pure hydride, the results show an increment in the atom bonds in correlation with an increase of the positron localization reflected in a rise of the positron lifetimes. On the other hand, in all considered cases for Mg or/and H vacancies, the presence of Nb reduces the hydride bond about 36%. This decrease in the hydride stability was associated with a decrease in the probability of the positron localization and a consequently reduction of the positron lifetimes.

Pressure detector based on the itinerant-electron metamagnetic transition of La(Fe0.88Si0.12)13Hy

The ferromagnetic (FM)–paramagnetic (PM) first-order transition is induced by pressure around room temperature for La(Fe0.88Si0.12)13H1.25 with the Curie temperature TC = 293 K. The spontaneous magnetization at room temperature is significantly decreased due to the phase transition induced by pressure. In addition, the low-field magnetic susceptibility exhibits a response against hydrostatic pressures. From these results, the pressure detection using FM–PM transition has been demonstrated. By dispersing the coarse-grained La(Fe0.88Si0.12)13H1.25 into a column-shaped epoxy resin, the susceptibility change against the unidirectional stress is detected by the inductance of a coil. The inductance response is scarcely changed after 400 cycles of loading. Relating to endurance, the thermal phase stability of hydrides annealed at various temperatures has been investigated. The hydrogen desorption rate obtained from the Arrhenius plots is negligibly small around room temperature, proving that La(Fe0.88Si0.12)13H1.25 has high thermal phase stability over a long period of time.

A study of stability of MgH 2 in Mg–8at%Al alloy powder

To investigate the effect of Al addition on the stability of magnesium hydride, the hydrogenation characteristics of a Mg–8at%Al alloy powder synthesized using the electrodeposition technique were evaluated. The characterization of the hydrogenation behavior within the 180 °C–280 °C temperature range and the subsequent microstructural analysis elucidated that the amount of Al present in the hydride decreased with increasing temperature. This observation suggests that Al has very low solubility in magnesium hydride but Al can be accommodated in MgH 2 by processing under non-equilibrium conditions. Pressure–composition isotherms were developed at different temperatures for the Mg–Al powder as well as pure Mg powder. The results indicate that the enthalpy of formation was slightly lower for the Mg–8at%Al powder while the enthalpy of dissociation did not change. The absence of noticeable influence of Al addition on the stability of magnesium hydride is attributed to its lack of solubility.

Isomerization of the hydride complexes [HFe2(SR)2(PR3)(x)(CO)(6-x)]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases.

The stepwise formation of bridging (mu-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(pdt)(CO)(4)(dppv) (2) followed an isomerization pathway similar to that for [1H](+), except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)(3) or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.

Effects of aluminum substitution in C14-rich multi-component alloys for NiMH battery application

The effects of aluminum substitution to the structural, electrochemical, and gas phase hydrogen storage properties of C14-rich alloys are reported. Minor phases, including C15 and TiNi, were identified by X-ray diffraction analysis. Entropy and enthalpy were estimated from equilibrium pressure at a fixed hydrogen concentration due to the large slope factor in pressure–concentration–temperature isotherms. The stability of hydrides from these materials, determined from the pressure–concentration isotherm equilibrium pressure and maximum storage capacities has a better correlation with the change in entropy than that in enthalpy. Alloys having smaller unit cell volume, relatively low hydride heat of formation, and relatively higher degree of disorder exhibit lower plateau pressure, higher storage capacity, and smaller hydrogen diffusion coefficient. Comparing to the Co substitution in the same base alloy, Al substitution makes better contribution to both bulk hydrogen transport and surface reaction. Substituting 0.4% Al and 1.5% Co to AB2 alloy is found to be the best combination in terms of general nickel metal hydride battery performance.

Hydrogen storage performance of Mg-based composites prepared by spark plasma sintering

The hydrogen storage capacities, hydrogen absorption mechanism and hydride stability of Mg-based composites prepared by spark plasma sintering (SPS) were investigated in this paper. The results showed that the composites had a double-phase microstructure of Mg phase and V-based solid solution, with nanocrystalline magnesium existing at their sintering interface. With the addition of the V-based solid solution in 20% volume fraction, the composite exhibited a maximum reversible hydrogen storage capacity of 4.2 wt.% at 573 K, compared with that of pure Mg of almost zero. DSC results indicated that the hydride decomposition temperature of MgH 2 decreased sharply from 708 K in pure Mg to 636 K and to 591 K as the volume of V-based solid solution increased from 20 vol.% to 50 vol.%. With the addition of V-based solid solution, the hydrogen absorption kinetics of pure Mg was greatly improved at 573 K, and its hydrogen absorption mechanism changed from surface reaction control to diffusion control in the composite. Based on these experimental results, a model was put forward to describe the hydrogen absorption/desorption mechanism in these composites.