Stability Of Hydrides Research Articles

Effect of Ce substitution on hydrogen absorption/ desorption of Laves phase-related BCC solid solution Ti33V37Mn30 alloy

In order to understand the effects of rare element substitution on activation performance of Laves phase-related BCC solid solution alloys, cerium (Ce) is used to partial substitute for Mn in Ti33V37Mn30 alloy. Results show that Ce substitution leads to the formation of Ce/CeO2 while decreases C14 Laves phase. Ce substituting alloys can absorb hydrogen without being activated at a high temperature, which means the activation performance of Ti33V37Mn30 alloys is improved greatly by the Ce/CeO2 rather than C14 Laves phase. The maximum hydrogen absorption capacity also increases when Ce content increases. The hydrogen absorption capacity and effective hydrogen storage capacity reach the maximum when the composition is Ti33V37Mn29.4Ce0.6, with the values of 3.35 wt% at 293 K and 2.25 wt% at 423 K. The dehydriding enthalpy (ΔH) of 0.6 at.%-Ce-substituting alloy is higher than that of Ce-free alloy, which indicates that Ce substitution can increase the stability of hydrides.

Impact of Hydrogen Content on the Thermal Stability of Hydride Phases in Zirconium Alloys

The fast and spontaneous hydrogen diffusion in HCP structures leads to the hydride precipitation. It is often pointed as causing embrittlement and rupture in zirconium alloys for applications in the nuclear industry. In our previous works TEM, DSC, SEM-EBSD and XRD were used to study the hydride stability after many precipitation-dissolution thermal cycles as well as the crystallographic hydride phase nature and the hydride-substrate crystallographic orientation relationships as a function of the hydrogen content. Results showed that the evolution of the dissolution and precipitation energies is correlated to the concentration of hydrogen atoms available to reprecipitate, which is submitted to a diffusion controlled by the misfit dislocation migration. In the present work in-situ TEM thermal cycling was performed in order to locally investigate the crystallographic stability of zirconium hydrides of different structures after many dissolution-reprecipitation cycles.

Temperature-Dependent Lattice Vibration of Magnesium Hydride

The thermodynamic stability of magnesium hydride hinders its application as a hydrogen storage medium. Efforts to destabilize MgH2 demonstrate the undesired reduction of entropy concomitant with the decrement of enthalpy. Lattice vibration contributes to entropy as well as thermal properties, whereas its temperature dependence is rarely reported for MgH2. Here, vibrational spectra of MgH2 are detected by means of Raman scattering between room temperature and the critical temperature of significant dehydrogenation. The laser power is changed, and the localized sample temperature in the irradiated region is evaluated according to the bandshift of the incorporated carbon with MgH2. With increasing temperature, the frequency for the B1g mode increases, while phonon softening is exhibited for both Eg and A1g modes. It is found that quasiharmonicity decides the thermal frequency shifts for Raman-active vibrations. Phonon–phonon interactions result in significant phonon broadenings for B1g and Eg modes. By contr...

Facile Synthesis of Magnetically Recoverable Pd and Ru Catalysts for 4-Nitrophenol Reduction: Identifying Key Factors.

This paper reports the development of robust Pd- and Ru-containing magnetically recoverable catalysts in a one-pot procedure using commercially available, branched polyethyleneimine (PEI) as capping and reducing agent. For both catalytic metals, ∼3 nm nanoparticles (NPs) are stabilized in the PEI shell of magnetite NPs, whose aggregation allows for prompt magnetic separation. The catalyst properties were studied in a model reaction of 4-nitrophenol hydrogenation to 4-aminophenol with NaBH4. A similar catalytic NP size allowed us to decouple the NP size impact on the catalytic performance from other parameters and to follow the influence of the catalytic metal type and amount as well as the PEI amount on the catalytic activity. The best catalytic performances, the 1.2 min–1 rate constant and the 433.2 min–1 turnover frequency, are obtained for the Ru-containing catalyst. This is discussed in terms of stability of Ru hydride facilitating the surface-hydrogen transfer and the presence of Ru4+ species on the Ru NP surface facilitating the nitro group adsorption, both leading to an increased catalyst efficiency. High catalytic activity as well as the high stability of the catalyst performance in five consecutive catalytic cycles after magnetic separation makes this catalyst promising for nitroarene hydrogenation reactions.

Microstructure and Hydrogen Absorption Properties of a BCC Phase Accompanied Laves Alloy

A non-stoichiometry Zr0.7Ti0.4V1.5Cr0.4 alloy has been synthesized by arc melting following annealing treatment or melt-spinning to obtain the bulk and ribbon samples, respectively. XRD investigation reveals the multiphase structure consisting of C15-Laves, V-BCC and a small amount of α-Zr or Zr3V3O. The alloy shows easy activation and fast hydrogenation kinetics. The annealed alloy absorbs 2.51 wt% H at room temperature, higher than the melt-spun ribbons. Refined grains by melt-spinning accelerates the hydrogenation of bulk alloy. The absorption behavior in presence of 1 mol% air has been tested to evaluate the anti-poisoning ability. Pressure–composition–temperature characteristics and thermodynamics parameters indicate the low equilibrium pressure and high hydrides stability. Hydrides investigation reveals that the Laves phase dominant multiphase structure contributes to the enhanced hydrogen capacity and multi-stage hydrogen release in DSC curve.

Toward the design of interstitial nonmetals co-doping for Mg-based hydrides as hydrogen storage material

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Revealing the role of nitrogen on hydride nucleation and stability in pure niobium using first-principles calculations

Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are not well known. Using first-principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties.

Effect of Gd and Co content on electrochemical and electronic properties of La1.5Mg0.5Ni7 alloys: A combined experimental and first-principles study

In this work we investigate the effect of Gd and Co substitutions on the electrochemical and electronic properties of La1.5Mg0.5Ni7 alloy. Two series of La1.5−xGdxMg0.5Ni7 (x = 0.0, 0.25, 1.0) and La1.5Mg0.5Ni7−yCoy (y = 0.0, 0.5, 1.5) alloys are produced using mechanical alloying technique. The X-ray diffraction indicates multi-phase character of the samples with majority of hexagonal Ce2Ni7-type and rhombohedral Gd2Co7-type structures of (La,Mg)2Ni7 phase. Partial substitutions of La by Gd or Ni by Co in La1.5Mg0.5Ni7 phase result in increase of cycle stability of the metal hydride (MHx) electrodes. All considered alloys reach the maximum discharge capacity after three charging-discharging cycles. Two optimal compositions, in respect of electrochemical properties, are subsequently investigated by the X-ray photoelectron spectroscopy (XPS). The experimental analysis of the valence band is further extended by the density functional theory (DFT) calculations. We also discuss the effects of alloying and site preference of dopants on the position of the van Hove-type singularity as observed in the electronic densities of states (DOS) in proximity of the Fermi level.

Air-stable metal hydride-polymer composites of Mg(NH2)2–LiH and TPX™

Light metal hydrides are prone to react with oxygen and/or water to produce oxides and/or hydroxides leading to reduction of hydrogen capacities, and deterioration of the hydrogen storage properties. It is therefore critical to address these issues when the materials are to be exposed to air or moisture. In this work, the combination of light metal hydrides, Mg(NH2)2–nLiH with polymethylpentene (TPX™), an air/moisture protective barrier is presented. It was found that the fabricated composites exhibit significant improvement of the metal hydrides stability in air. No oxidation reactions in air can be proven even after air exposure for 90 min. Extending the air-exposure time to 12 h, the reversible hydrogen capacities of these composites are much higher and more stable than they are in the case of the pure metal hydrides. In comparison to the pure metal hydrides, the composites retain the same hydrogen loading capacities and kinetic properties, with respect to the metal hydrides contents. Further, in situ synchrotron radiation powder X-ray radiation diffraction (SR-PXRD) experiments reveal that the thermal decomposition reaction pathways of the 90 min air-exposed composites are the same under air or H2 atmosphere. Moreover, morphology analysis confirms that the metal hydrides remain stable in the polymeric matrix and the three-dimensional integrity is retained, even after performing tens of de/re-hydrogenation cycles. The present study shows a promising way to fabricate air-stable metal hydride-polymer composite hydrogen storage materials that can be handled in ambient conditions.

Why do higher VDEs of superhalogen not ensure improved stabilities of the noble gas hydrides promoted by them? A high-level ab initio case study.

A series of 20 composite structures, consisting of superhalogen and noble gas (Ng) hydrides, was explored via high-level coupled-cluster single, double and perturbative triple excitations calculations in this work. The existence of these composites, as local minima on the potential energy surface, arises from the charge transfer from the Ng hydride part to the superhalogen moiety. Clearly, this transfer could lead to stabilizing the interaction of the ionic type between the two components. The driving force of the charge transfer should be the high vertical electron detachment energy (VDE) of the superhalogen part leading to its enough capability of extracting the electron from the Ng hydride moiety. However, except triggering the ionic attractive interaction, there is nomonotonic correlation between the VDE value and the thermodynamic stability of the whole composite. This counter-intuitive result actually originates from the fact that, irrespective of various superhalogens, only two of their F ligands interact with the Ng atoms directly. Thus, although leading to higher VDE values, the increase in the number of electronegative ligands of the superhalogen moiety does not affect the stabilizing interaction of the composites here directly. In other words, with the necessary charge transfer generated, further increase of the VDE does not ensure the improvement of the thermodynamic stabilities of the whole composite. Moreover, in the transition state of the exothermic dissociation channel, more F atoms will give rise to higher probability of additional attractions between the F and H atoms which should lower the energy barrier. That is to say, increasing VDE, i.e., having more F atoms in many cases, will probably reduce the kinetic stability. Knowing the inevitable existence of the exothermic channel, kinetic stability is crucial to the ultimate goal of experimental observation of these Ng hydrides. Thus, in some cases, only the superhalogen itself may not provide enough information for the correct prediction on the properties of the whole composites. The understanding of the superhalogen-based composites will provide valuable information on the functional properties as well as the application potential of superhalogen clusters. Thus, the corresponding researches should focus on not only the superhalogen itself but also other related aspects, especially the details of the interaction between different parts.

Bulk nanocrystalline gamma magnesium hydride with low dehydrogenation temperature stabilized by plastic straining via high-pressure torsion

MgH2 with the α tetragonal structure was plastically strained using the high-pressure torsion (HPT) method and fully transformed to a nanonocrystalline γ orthorhombic phase with increasing the strain. The formation of nanocrystalline high-pressure γ phase resulted in decreasing the dehydrogenation temperature by 80 K. First-principles phonon calculations showed that both α and γ phases are dynamically stable, but the γ phase with the ionic binding has weaker hydrogen binding energy and accordingly lower dehydrogenation temperature. This study confirms the significance of crystal structure on thermal stability of hydrides for hydrogen storage applications.

Microstructure and Electrochemical Performance of CeMg12/Ni/TiF3 Composites for Hydrogen Storage

Ball milling was used to prepare CeMg12/Ni/TiF3 composite alloys in this study. Microstructures of experimental alloys were analyzed by scanning electron microcopy (SEM), x-ray diffraction (XRD), and high-resolution transmission electron microscopy (HRTEM) with electron diffraction (ED). The electrochemical and kinetic characteristics of the ball-milled composite alloys were further evaluated based on their galvanostatic charge–discharge measurement, high-rate dischargeability (HRD), hydrogen diffusion behavior, as well as electrochemical impedance spectrum (EIS). Results indicated that the ball-milled alloy with 3 wt.% TiF3 exhibited the best electrochemical discharge capacity, which was not only related to the formation of amorphous and/or nanocrystalline phase but to the formation of MgF2 because it can effectively reduce the thermodynamic stability of hydride. With increasing TiF3, the cycle degradation rates of the milled alloys were ameliorated remarkably. This improvement was attributed to the formation of an amorphous phase (which possesses strong anticorrosive and antioxidation abilities) as well as the formation of MgF2 and TiNi secondary phases. The ball-milled alloy with 3 wt.% TiF3 additive exhibited the strongest electrochemical kinetic properties, related to the highest hydrogen diffusion rate in the alloy (which was associated with multiple defects and grain boundary of amorphous and/or nanocrystalline phases) and the electrochemical reaction on the surface of alloy (which was related to the lowest apparent activation energy).

Vibration analysis of hydrogen, deuterium and tritium in metals: consequences on the isotope effect

The present study focuses on the impact of the vibrational frequencies on the thermodynamic behavior of hydrides, deuterides and tritides, using high scale harmonic phonon calculations based on first-principle calculations. 115 MHy hydrides were considered, for with M among 30 metallic elements. The results were found to be in good agreement with the available experimental data and pointed out trends on the evolution of the hydride zero point energy as a function of the crystal structure and the host metal nature. Based on this information, the vibration contribution to the formation enthalpy was deduced. This contribution is responsible for the differences between the enthalpies and therefore pressures of formation of the hydride, deuteride and tritide compounds. This so-called ‘isotope effect’ is experimentally observed but has never been studied by large scale calculations. A straightforward method has been developed allowing to quantify the isotope effect at non zero temperature. It explains the experimentally observed relative stability of hydride, deuteride and tritide compounds. As a major achievement, a new phenomenon was highlighted, which has never been anticipated, consisting in an inversion of the isotope effect when the temperature increases.

Achieving the dehydriding reversibility and elevating the equilibrium pressure of YFe2 alloy by partial Y substitution with Zr

To overcome the hydrogen-induced amorphization and phase disproportionation in the fast de-/hydrogenation of YFe2, the alloying of partial substituting Y with Zr was carried out to obtain Y1−xZrxFe2 (x = 0.1, 0.2, 0.3, 0.5) alloys. All YZrFe alloys remained single C15 Laves phase structure at states of as-annealed, hydrogenated and dehydrogenated. With the increasing of Zr content, the YZrFe alloys showed the decrease in the lattice constants and hydrogenation capacity, but the increase in the dehydrogenation capacity and dehydriding equilibrium pressure. The alloy Y0.9Zr0.1Fe2 showed maximum initial hydrogenation capacity of 1.87 wt% H, while the alloy Y0.5Zr0.5Fe2 showed highest desorption capacity of 1.26 wt% with obvious dehydriding plateau. Based on experiment analysis and first principle calculation of binding energy, the great improvement in the dehydriding thermodynamics for YZrFe alloys is attributed to the change in the unit cell volume, electron concentration and stability of hydrides due to the Zr substitution.

Degradation behaviors of La–Mg–Ni-based metal hydride alloys: structural stability and influence on hydrogen storage performances

The present work focuses on the structural stability upon hydrogenation of three typical La–Mg–Ni-based alloys: La2MgNi9, La3MgNi14 and La4MgNi19. Structural changes during gaseous and electrochemical cycles were characterized, and the influence of the structure distortion on the hydrogen storage properties was concerned. Hydrogen-induced amorphization (HIA) and disproportionation of the three alloys have occurred during both the gaseous and electrochemical cycles. Structural stability of the phase structures in the La–Mg–Ni system is found to follow the order: LaNi5 > (La,Mg)5Ni19 > (La,Mg)2Ni7 > (La,Mg)Ni3 > (La,Mg)Ni2. HIA increases thermal stability of the metal hydrides and difficulty to dehydrogenation and leads to degradation of both the gaseous and electrochemical capacities. Interestingly, La2MgNi9 with poor stability presents elevated discharge capability even at 60 °C which can be attributed to increase in the hydrogen desorption capability and inhibition of the self-discharge induced by severe HIA at higher temperatures. In addition, HIA in the electrochemical reactions is obviously weaker than the extent during the gaseous cycles, which is mainly due to the slower hydrogenation speed. The development of HIA in the gaseous and electrochemical process is considered to follow the direct and gradual modes, respectively.

Increase in Thermal Stability of Hydride of the Titanium

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High pressure synthesis and stability of cobalt hydrides.

In situ high-pressure high-temperature X-ray powder diffraction studies of the cobalt-hydrogen system reveal the direct synthesis of both the binary cobalt hydride (CoH) and a novel cobalt dihydride (CoH2). We observe the formation of fcc CoH at pressures of 4 GPa, which persists to pressures of 45 GPa. At this pressure, we see the emergence with time of a further expanded fcc lattice, which we identify as CoH2, where the hydrogen atoms occupy the tetrahedral vacancies. We have explored alternative synthesis routes of CoH2 and can lower the synthesis pressure to 35 GPa by the application of high temperature. CoH2 is stable to at least 55 GPa and decomposes into CoH below 10 GPa, releasing molecular hydrogen before further decomposing completely into its constituent elements below 3 GPa. As a first-row transition metal, cobalt has a relatively lower mass than other hydride-forming transition metals, and as a result, CoH2 has a high hydrogen content of 3.3 wt. % and a volumetric hydrogen density of 214 g/l.

Actinium Hydrides AcH10, AcH12, and AcH16 as High-Temperature Conventional Superconductors.

The stability of numerous unexpected actinium hydrides was predicted via the evolutionary algorithm USPEX. The electron-phonon interaction was investigated for the hydrogen-richest and most symmetric phases: R3̅ m-AcH10, I4/ mmm-AcH12, and P6̅ m2-AcH16. Predicted structures of actinium hydrides are consistent with all previously studied Ac-H phases and demonstrate phonon-mediated high-temperature superconductivity with TC in the range of 204-251 K for R3̅ m-AcH10 at 200 GPa and 199-241 K for P6̅ m2-AcH16 at 150 GPa, which was estimated by directly solving the Eliashberg equation. Actinium belongs to the series of d1 elements (Sc-Y-La-Ac) that form high- TC superconducting (HTSC) hydrides. Combining this observation with previous predictions of p0-HTSC hydrides (MgH6 and CaH6), we propose that p0 and d1 metals with low-lying empty orbitals tend to form phonon-mediated HTSC metal polyhydrides.

Solid-gas and electrochemical hydrogenation properties of the La1-xNdxMgNi4-yCoy alloys

New La1-xNdxMgNi4-yCoy (x = 0, 0.5, 1; y = 0, 0.5, 1) alloys have been synthesized by powder sintering method, and their crystal structure and hydrogen storage properties have been systematically studied. X-ray diffraction analysis showed that La1-xNdxMgNi4-yCoy alloys belong to the MgCu4Sn-type structure. Hydrogen storage capacity of the alloys is in the 1.1–1.7 wt % H range. The alloy hydrides of are stable in air at room temperature. XRD studies have shown that hydrides retain the crystalline structure of the starting alloy while increasing the cellular volume by 15–23%. Pressure–composition–temperature (PСT) measurements demonstrated that thermodynamic stability of hydrides decreases with an increasing Nd content whereas increasing Co content in the alloys leads to the increase their stability which is demonstrated by the lowering desorption plateau pressure with Co content. Electrochemical studies showed that La-rich electrode materials exhibit the maximum discharge capacity (300–330 mAh/g), but Nd-doped ones show better cyclic stability.

Molybdenum incorporation on AB2 alloys-Part II. On the synergetic effects of Laves and non-Laves phases

Molybdenum substitution by chromium in ZrCr1-xNiMox alloys produces the segregation of ZrxNiy phases, ZrCr2 and ZrMo2 Laves phases as it was stated in Part I of this paper. In order to study the contribution of these phases in ZrCr1-xNiMox alloys performance, secondary Zr9Ni11 and Zr7Ni10, together with ZrCr2 and ZrMo2 Laves phases were synthesized and their crystalline structures determined with XRD patterns. Zr7Ni10 exhibits an orthorhombic structure, whereas Zr9Ni11 depicts a tetragonal structure. ZrCr2 presents a hexagonal structure (C14), while ZrMo2 depicts a cubic structure (C15).ZrCr2 shows zero discharge capacities, while ZrMo2 exhibits a first capacity value of 245 mAh g−1. Zr9Ni11 and Zr7Ni10 phases exhibit maximum discharge capacities of 119 and 157 mAh g−1, respectively.Theoretical results show that the hydride stability for ZrCr2 is larger than that for ZrMo2. However, the great hydrogen storage capacity yields a slower desorption kinetics since it requires a considerable de-hydrogenation energy. The opposite effect is observed for the proposed hexagonal structure ZrMo2.The 100% charge of the alloys shows different results for the hydrogen evolution reaction. Zr7Ni10 phase depicts a better performance since the required potentials are lower than −1.05 V. There is a change in the rate determining step (rds) from a chemical recombination of surface hydrogen (Tafel pathway) in Zr7Ni10 to a fast first electron transfer (Volmer step) for Zr9Ni11. Thus, the rds is strongly affected by the type of secondary phase dominant in each sample, being Zr7Ni10 the most favorable.