MgH2 Hydride Research Articles

Superior effect of Ni-substitution on the hydrogenation kinetics of Mg6Pd1−xTMx (TM = Ag, Cu, Ni) pseudo-binary compounds

In this investigation, the H-sorption kinetics of Mg6Pd and Mg6Pd1−xTMx (TM=Ag, Cu, Ni) pseudo-binary compounds at the TM solubility limit have been studied by isothermal hydrogen absorption, thermal desorption spectroscopy and in situ neutron diffraction. Among all studied compounds, the fastest absorption kinetics takes place for the Ni-substituted one. The fit of the absorption curves to established model equations for solid–gas reaction shows that hydrogenation is controlled by diffusion. As for desorption, the peak temperature for the Ni-substituted compound is 90K below that of MgH2/Mg system and is characterised by a low activation energy of 68kJ/molH2. To better understand these results, neutron diffraction experiments during in␣situ thermal desorption of deuterated Mg6Pd and Mg6Pd0.25Ni0.75 compounds were carried out. These experiments demonstrate a synergetic effect between MgH2 and Mg2NiH4 hydrides as responsible for the remarkable kinetics of the Ni-containing compound.

The new ternary intermetallic NdNiMg5: Hydrogen sorption properties and more

The hydrogen sorption properties of the new compound NdNiMg5 have been investigated. The compound starts to absorb hydrogen at 573K under a pressure of 1MPa. The absorption leads to the decomposition into two binary hydrides MgH2 and NdH2.6, and a ternary hydride Mg2NiH4. The maximum hydrogen sorption capacity reached is 3.2wt.% and the cycling properties of the mixture decreases quickly due to grain growth. The activation energies for absorption and desorption are estimated to be equal to 14(3)kJ/molH2 and 110(9) kJ/molH2, respectively. Preliminary studies of the mechanical properties were also performed. Despite the high content in magnesium, NdNiMg5 exhibits an average microhardness of 210Hv and a reduced Young Modulus of 9(1)GPa.

Structural Characterization of Mg2CoH5-based Nanocomposites for Hydrogen Storage

Hydrogen is considered the ideal energy carrier, mainly due to its heating power, the highest among all chemical fuels, and to possibility of using it in fuel cells, therefore with efficiency and producing only water as by-product. However, the development of safe and effective hydrogen storage solutions remains as a challenge of applied research. MgH2, Mg2FeH6 and Mg2CoH5complex hydrides are promising materials for hydrogen storage, avoiding the inconvenient of gaseous or liquid storage alternatives. The main attractives of these phases are their volumetric and gravimetric hydrogen capacities, their reversibility during absorption/desorption cycles and the relatively low cost. Recently, we have achieved an important control of the synthesis of Mg-based complex hydrides with nanocrystalline structure, using reactive milling (RM) under hydrogen atmosphere as processing route [1, 2]. In this study, we present new results concerning the synthesis, hydrogen storage properties and structural characterization of MgH2–Mg2CoH5nanocomposites prepared by RM. The nanocomposites were produced by milling different Mg-Co starting compositions (2:1, 3:1, 5:1, 7:1, 1:0) for 12 h in a planetary mill under 3 MPa of H2. All samples were fully hydrogenated during milling, generating different MgH2–Mg2CoH5hydride mixtures. Mg presents the tendency of agglomerate during milling, so the sample that presents more MgH2shows larger agglomerates. This behavior causes a slight increase in the temperature of hydrogen desorption and the presence of two peaks, showed by DSC analysis for those samples which presents MgH2and Mg2CoH5. Using in-situ XRD and XANES during hydrogen desorption revealed that Mg and Co tend to remain coupled forming intermetallics after the complex hydride decomposition, differently from that was observed for Mg2FeH6. This effect is correlated to the high-reversibility exhibited by the Mg2CoH5phase. Furthermore, the nanocomposites of MgH2+Mg2CoH5showed better H-absorption/desorption kinetics than the Mg2CoH5or MgH2alone, as shown by volumetric measurements. The combination of MgH2and Mg2CoH5is therefore a promising strategy to produce hydrogen storage materials, matching the good reversibility and high capacity of magnesium hydride with the lower thermal stability.

Aluminum integral foams with tailored density profile by adapted blowing agents

The goal of the present work is the variation of the structure of aluminum integral foams regarding the thickness of the integral solid skin as well as the density profile. A modified die casting process, namely integral foam molding, is used in which an aluminum melt and blowing agent particles (magnesium hydride MgH2) are injected in a permanent steel mold. The high solidification rates at the cooled walls of the mold lead to the formation of a solid skin. In the inner region, hydrogen is released by thermal decomposition of MgH2 particles. Thus, the pore formation takes place parallel to the continuing solidification of the melt. The thickness of the solid skin and the density profile of the core strongly depend on the interplay between solidification velocity and kinetics of hydrogen release. By varying the melt and blowing agent properties, the structure of integral foams can be systematically changed to meet the requirements of the desired field of application of the produced component.

Electrochemical reactivity of magnesium hydride toward lithium: New synthesis route of nano-particles suitable for hydrogen storage

Electrochemical conversion reaction of MgH2 with Li ion enables the production of in-situ nanometric Mg and MgH2 particles so-called (nano-Mg) INSITU and [(nano-MgH2)INSITU] showing interesting hydrogen sorption properties with hydrogen absorption at 100 °C under 10 bars of hydrogen pressure (PH2) and desorption at 200 °C under primary vacuum, respectively. Differential Scanning Calorimetry (DSC) measurements of MgH2 electrochemically prepared confirmed a decrease in the desorption temperature from 416 °C to 295 °C and in the heat of formation from −74 kJ mole mol−1 (H2)−1 to −56 kJ mol−1 (H2)−1 for commercial (particle size diameter: 10 μm–100 μm) and as prepared MgH2 hydride (particle size: 10 nm–40 nm), respectively.

Influence of Exposure to Air and Mechanical Dispersion upon Thermal Stability and Decomposition Temperature of β-MgH<sub>2</sub> Phase, a Component of Composites Derived by Different Methods

Influence of mechanical dispersion and exposure to air on hydrogen-sorption properties and thermal stability of β-MgH2 synthesized by two different methods, namely through direct hydriding in gaseous medium and by reactive mechanical alloying, is studied employing thermodesorption spectroscopy and X-ray photoelectron spectroscopy. It is established that the temperature of the beginning of desorption of the β-MgH2 phase of the composite synthesized through direct hydriding in gaseous medium depends upon exposure to air after its synthesis. Exposure of the sample to air for 20 min (for more than 5 h) increases the temperature of decomposition of the β-MgH2 phase from 360 to 375 oC (from 360 to 440 oC). Following dispersion of the composite in the ball mill decreases the decomposition temperature of the hydride phase from 440 to 340 oC. Exposure of the dispersed composite to air for 5 h and even for several days does not increase the decomposition temperature of the MgH2 phase. Significant role of the surface state in increase of the decomposition temperature (and thermal stability) due to exposure to air of MgH2 hydride derived by hydriding of magnesium powder in gaseous medium is revealed. It is found that dispersion of the hydride in the ball mill reduces the temperature of decomposition of the MgH2 phase.

Short communication: High-pressure synthesis and crystal structure of a novel Mg3CuHx ternary hydride

A novel ternary hydride in the Mg–Cu–H system has been identified by a new synthesis method based on the direct reaction of simple hydrides under high-pressure and moderate-temperature conditions. A well-crystallized sample was obtained in a piston-cylinder hydrostatic press at moderate pressures of 3.5 GPa and temperatures around 850 °C starting from mixtures of MgH2 and Cu enclosed in platinum capsules. X-ray and neutron powder diffraction analysis were used to identify the purity of the samples and provide an accurate description of the crystal structure features. The crystal structure of Mg3CuHx was determined by ab-initio procedures from neutron powder diffraction (NPD) data in the cubic space group F-43m (No. 216), with lattice parameter a = 6.285 Å. The hydrogen positions were determined by Fourier synthesis revealing the introduction of hydrogen atoms in the system. A final Rietveld refinement of the structure gives up a Mg3CuH0.6 composition, which is consistent with the Fourier synthesis. Thermogravimetric analysis (TGA) has been carried out to determine the hydrogen desorption temperature, and to confirm the hydrogen contents. A destabilization with respect to MgH2 hydride, with a reduction of the desorption temperature of approximately 240 °C, suggests that Cu-doping of MgH2 may find applications in the hydrogen storage field.

Sorption behavior of the MgH2–Mg2FeH6 hydride storage system synthesized by mechanical milling followed by sintering

The hydrogen sorption behavior of the Mg2FeH6–MgH2 hydride system is investigated via in-situ synchrotron and laboratory powder X-ray diffraction (SR-PXD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), particle size distribution (PSD) and volumetric techniques. The Mg2FeH6–MgH2 hydride system is obtained by mechanical milling in argon atmosphere followed by sintering at high temperature and hydrogen pressure. In-situ SR-PXD results show that upon hydriding MgH2 is a precursor for Mg2FeH6 formation and remained as hydrided phase in the obtained material. Diffusion constraints preclude the further formation of Mg2FeH6. Upon dehydriding, our results suggest that MgH2 and Mg2FeH6 decompose independently in a narrow temperature range between 275 and 300 °C. Moreover, the decomposition behavior of both hydrides in the Mg2FeH6–MgH2 hydride mixture is influenced by each other via dual synergetic-destabilizing effects. The final hydriding/dehydriding products and therefore the kinetic behavior of the Mg2FeH6–MgH2 hydride system exhibits a strong dependence on the temperature and pressure conditions.

MgH2 and LiH metal hydrides crystals as novel hydrogen storage material: Electronic structure and optical properties

We have performed a comprehensive theoretical investigation of the electronic band structure, density of states, electronic charge density and optical properties of the novel hydrogen storage material MgH2 and LiH compounds. The all electron full potential linear augmented plane wave method was employed. The local density approximation (LDA), the generalized gradient approximation (GGA) and the Engle Vosko generalize gradient approximation (EVGGA) were used to treat the exchange-correlation potential. The calculations show that the MgH2 compound is indirect gap semiconductor as the conduction band minimum (CBM) situated at R point of the Brillouin zone (BZ), while the valence band maximum (VBM) located between Λ and Γ points of the BZ, whereas LiH is a direct gap material as the CBM and the VBM located at X point of BZ. The values of the calculated energy band gap of MgH2 (LiH) compounds are 3.372 (2.769), 3.735 (3.067) and 5.104 (4.488) eV for LDA, GGA, and EVGGA, respectively. From the partial density of states and the electronic charge density in (0 0 1) and (1 0 1) crystallographic planes we conclude that there exists strong ionic bonds. The bond lengths were calculated and compared with the available experimental and theoretical results, our results show better agreement with the experimental values than the other theoretical results. The frequency dependent dielectric function's dispersions were calculated and analyzed so as to obtain further insight into the electronic structure. The calculated dielectric function's dispersions confirm the semiconducting nature of MgH2 and LiH compounds.

Electrochemical hydriding performance of Mg–TM–Mm (TM=transition metals, Mm=mischmetal) alloys for hydrogen storage

Eighteen as-cast binary Mg–Ni, Mg–Mm and ternary Mg–Ni–Mm and Mg–Ni–TM (TM=transition metals (Cu, Zn, Mn and Co); Mm = mischmetal containing Ce, La, Nd and Pr) alloys were hydrided by an electrochemical process to determine the alloys with the most potential for electrochemical hydrogen storage. The alloys were hydrided in a 6 mol/L KOH solution at 80 °C for 480 min and at 100 A/m2. To assess the electrochemical hydriding performance of alloys, maximum hydrogen concentrations, hydrogen penetration depths and total mass of absorbed hydrogen in the alloys were measured by glow discharge spectrometry. In addition, the structures and phase compositions of the alloys both before and after hydriding were studied by optical and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. It was determined that the highest total amount of hydrogen was absorbed by the Mg–25Ni–12Mm and Mg–26Ni (mass fraction, %) alloys. The maximum hydrogen concentrations in the Mg–25Ni–12Mm and Mg–26Ni alloys were 1.0% and 1.6%, respectively. The main hydriding product was the binary MgH2 hydride, and the ternary Mg2NiH4 hydride was also detected in the Mg–25Ni–12Mm alloy. The electrochemical hydriding parameters achieved are discussed in relation to the structures of alloys, alloying elements and hydriding mechanisms.

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.

Electrochemical hydriding of Mg–Ni–Mm (Mm = mischmetal) alloys as an effective method for hydrogen storage

In this work, the electrochemical hydriding method was used for storing hydrogen in four binary Mg–Ni (Ni content from 15 to 34 wt.%) alloys and one ternary Mg–26Ni–12Mm alloy. Both the as-cast and powdered alloys were hydrided in a 6 M KOH solution at 80 °C for 120–480 min. The structures and phase compositions of the alloys, both before and after hydriding, were studied using optical and scanning electron microscopy, energy dispersive spectrometry and X-ray diffraction. Differential scanning calorimetry and mass spectrometry were used to study the dehydriding process. In the case of as-cast alloys, the best combination of hydriding parameters (maximum hydrogen concentration on surface; depth of hydrogen penetration) was achieved in the Mg–26Ni alloy. In the case of powdered alloys, the Mg–34Ni alloy absorbed the highest amount of hydrogen, nearly 4.5 wt.%. The only hydride formed during hydriding was the MgH2 hydride. The results of the mass spectrometry analysis reveal a significant thermodynamic destabilization of magnesium hydride due to Ni and Mm. The decomposition temperature of MgH2 was reduced by more than 200 °C. The results are discussed in relation to the electronic structure and atomic size of the alloying elements and the structural variations in the alloys.

Effective synthesis of Mg2CoH5 by reactive mechanical milling and its hydrogen sorption behavior after cycling

Mg2CoH5 was synthesized by reactive mechanical milling (RMM) under hydrogen atmosphere (0.5 MPa) from 2MgH2–Co and 3MgH2–Co mixtures, with a yield >80%. The microstructure, structure and thermal behavior of the phases formed during the processing were investigated by transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. Kinetic properties of the reaction with hydrogen of the 2MgH2–Co and 3MgH2–Co mixtures after RMM were evaluated using modified Sieverts-type equipment. The 3MgH2–Co mixture showed better properties for storage applications, with its highest rate of hydrogen absorption and desorption at 300 °C, its storage capacity of about 3.7 wt% in less than 100 s, and good stability after cycling. Although the starting material presents Mg2CoH5 as majority phase, the cycling leads to disproportion between Mg and Co. We obtained a mixture of Mg2CoH5, Mg6Co2H11 and MgH2 hydrides, as well as other phases such as Co and/or Mg, depending on experimental conditions.

Mechanochemistry and H-sorption properties of Mg2FeH6-based nanocomposites

Abstract Mg2FeH6 is a promising material for hydrogen storage applications, since it presents the highest known volumetric capacity of 150 kg m−3 of H2 and its metallic constituents are inexpensive. The major drawbacks for its application are the difficulties associated with its synthesis and also its high thermal stability. In this paper, Mg2FeH6-based nanocomposites were prepared from the elements via reactive milling. A high-yield of the complex hydride synthesis was obtained after a systematic processing study, and the best conditions were successfully extended to the mechanochemical synthesis of Mg2CoH5. The influence of different additives such as transition metals, transition metal fluorides and graphite on the H-sorption behavior of the Mg2FeH6-based nanocomposites was evaluated. Mixtures rich in both MgH2 and Mg2FeH6 hydrides present lower temperature ranges of hydrogen release than those which are rich in only one of these hydrides. The MgH2–Mg2FeH6-based nanocomposite presents ultra-fast H-sorption kinetics at 300°C with partial reversible formation of Mg2FeH6.

Simplified mechano-synthesis procedure of Mg2NiH4

We describe a simplified synthesis method for the metal hydride Mg2NiH4, based on the direct reaction of MgH2 hydride and Ni metal (2MgH2 + Ni → Mg2NiH4) by ball-milling under nitrogen atmosphere at room temperature. Different milling times and different MgH2:Ni atomic stoichiometries have been evaluated, in order to optimize the synthesis conditions. X-ray diffraction (XRD) analysis was used to identify the purity of the samples and quantify the present phases, through Rietveld analyses of the crystal structure, which can be defined in the cubic space group Fm3¯m. Thermogravimetric analysis has been carried out to determine not only the hydrogen desorption temperature but also the hydrogen contents. Scanning electron microscopy images show an average particle size of 1–2 μm for Mg2NiH4; the microstructure presents a sponge-like aspect adequate to favor the absorption and desorption processes.

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.

Structural and H2 sorption properties of MgH2–10 wt%ZrCrM (M = Cu, Ni) nano-composites

Magnesium and its hydride MgH2 are widely regarded as promising candidates for hydrogen storage materials due to its benefits of high gravimetric and volumetric capacity, excellent reversibility, abundance in the earth and a low cost. Much attention has been paid to improve its absorption/desorption kinetics, trying to make it useful for practical applications. To make composite of MgH2 with other hydrogen storage compounds is an effective method to improve the hydrogen storage properties. In this study nano-composite of MgH2 with ZrCrCu alloy was prepared using high energy ball-milling for 5 h under Ar atmosphere. Microstructure and morphology of the composites were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM). XRD patterns show that no alloy formation between MgH2 and elements of the alloys takes place during milling. Different morphology of the powders as-milled and after cycling was observed by SEM. Pressure–composition isotherms of these composites have been obtained in the pressure range 0.1–10 bar at 275 and 300 °C. The absorption/desorption kinetics data have been analyzed using pressure 0.1–5.0 bar at 275 and 300 °C to understand the mechanism of the hydriding/dehydriding reaction processes. A comparison of these results has been attempted with our previous published results of MgH2–10 wt%ZrCrNi in order to find the better composite for storage applications. It is observed by DSC curves that the onset temperature of hydrogen desorption is decreased for MgH2–10%ZrCrNi in comparison to MgH2 which further decreased for MgH2–10%ZrCrCu. However, little loss in hydrogen absorption/desorption capacity is also observed for ZrCrCu composite in comparison to that of ZrCrNi composite.

Metallographic and Numerical Studies of the Role of Catalyst Particles of MgH<sub>2</sub>-Mg System

Magnesium is one of the most promising materials for hydrogen storage due to its high capacity and low cost. Unfortunately, practical applications are for the moment limited by the slow kinetics and the high operating temperature. Nanostructuring magnesium hydride MgH2, generally by ball milling, introduces plastic deformations and catalysts that highly enhances the H2 absorption and desorption. However a fundamental understanding of the role played by catalysts and interfaces in MgH2 is still lacking. Microscopic characterization of MgH2-Mg system with and without heavy metal catalysts, is achieved by combining accurate SEM observations of samples after partial desorption process and atomic level ab-initio molecular dynamics simulations of MgH2-Mg interfaces. The experimental method is based on low voltage SEM observations of cross sectional powder samples, prepared by a new specific metallographic process. Identification of nucleation sites of the sorption reaction and their correlation with the presence of catalyst particles is achieved by suitable experimental conditions. Moreover ab-initio molecular dynamics clarifies the interplay of interfaces and the deformations induced during desorption by the presence of catalysts that are able to lower binding energies and free hydrogen atoms toward interfaces. Both approaches confirm and characterize the nucleation step in the catalysts driven phase transformation.

ステンレス鋼/純鉄シース MgB2 細径線材の加工性と超伝導特性

MgB2 superconducting thin wires sheathed with stainless steel (SS) and pure iron (Fe) have been prepared by in-situ powder-in-tube process. Using Magnesium hydride MgH2 and amorphous B powders with and without SiC nano-sized powder addition, SS/Fe/MgB2 composites have been fabricated through grooved rolling and drawing to form the round wires of 0.53~0.10 mm in diameter. Both SS and Fe sheaths were hardened to be a Vickers hardness of around Hv 650 and Hv 510 through cold-working at room temperature. The transport critical current (Ic) at 4.2 K for the MgB2 wire increases with decreasing the amount of SiC addition in lower magnetic field than 1 T. The Ic at 4.2 K and self-field for the MgB2 wire of 0.10 mm in diameter without SiC addition is 14 A, which corresponds to the critical current density (Jc) of around 1×104 A/mm2. The present MgB2 thin wires are promising as current leads and level sensor for liquid hydrogen.

The ethyl cation as proton donor for dihydrogen bonds in the (m=1 or 2 and n=1 or 2) complex: A theoretical study

BHandHLYP/6-31G(d,p) calculations and molecular integrations of the quantum theory of atoms in molecules (QTAIM) were applied to study a new class of dihydrogen-bonded complexes (mC2H5+⋯nMgH2 with m = 1 or 2 and n = 1 or 2) based on the assumption that earth alkaline hydride MgH2 can bind with the non-localized hydrogen H+δ of the ethyl cation C2H5+.