Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3

MgH₂ nanoparticles were obtained by hydriding ultrafine magnesium particles which were prepared by hydrogen plasma–metal reaction. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the obtained sample is almost pure MgH₂ phase, without residual magnesium and with an average particle size of ∼300 nm. Milled with 5 wt.% TiF₃ as a doping precursor in a hydrogen atmosphere, the sample desorbed 4.5 wt.% hydrogen in 6 min under an initial hydrogen pressure of ∼0.001 bar at 573 K and absorbed 4.2 wt.% hydrogen in 1 min under ∼20 bar hydrogen at room temperature. Compared with MgH₂ micrometer particles doped with 5 wt.% TiF₃ under the same conditions as the MgH₂ nanoparticles, it is suggested that decrease of particle size is beneficial for enhancing absorption capacity at low temperatures, but has no effect on desorption. In addition, the catalyst was mainly responsible for improving the sorption kinetics and its catalytic mechanism is discussed.     

XRD studies of electrochemically hydrided–dehydrided thin films of the alloy AZ31 and AZ31 with Ti

A magnetron sputtered thin films of the AZ31 alloy and AZ31 alloy with Ti capped with Pd were electrochemically hydrogenated and dehydrogenated in a 3 M KOH solution. A phase composition and structure of the films were studied by XRD. It has been determined that the behaviour of magnetron sputtered alloy AZ31 during electrochemical charging with hydrogen was alike that of pure Mg. The shift of the XRD peak Mg (0 0 0 2) to lower angles indicates that a hydrogen solid solution in the AZ31 alloy was formed along with MgH₂. When the AZ31 alloy with 18 at.% of Ti was electrochemically hydrogenated the whole film was transformed into hydride. The minor part of the hydride was in the nanocrystalline state with a structure of rutile and a major part of the hydride was in the amorphous state. After dehydrogenation only a part of the alloy recovered and the rest remained in the state of amorphous hydride. A positive shift of peak Pd (1 1 1) was observed in all of the XRD patterns for hydrogenated films. At least partially the shift should be associated with the compressive stresses in the top-cap layer of Pd, which arose due to the hydrogenation of the AZ31 alloy.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Improvement in hydrogen sorption kinetics of MgH2 with Nb hydride catalyst

Nanocrystalline MgH₂ with fine and evenly dispersed Nb hydride was prepared by ball milling a mixture of MgH₂ and 1 mol.% NbF₅. This NbH-catalyzed MgH₂ desorbed 6.3 wt.% H₂ in 15 min and absorbed more than 90% of its initial hydrogen capacity within 5 min at 573 K. Moreover, this fast sorption kinetics was maintained after 10 cycles. Based on X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy analyses, it is suggested that NbF₅ melts during high-energy ball milling and this promotes the formation of extremely fine, film-like Nb hydride preferentially along the grain boundaries of nanocrystalline MgH₂ by a liquid/solid reaction. This unique nanostructured Nb hydride is believed to suppress the grain growth of MgH₂ quite effectively and thus maintain its initial catalytic effect throughout repeated hydrogenation–dehydrogenation cycles.     

Thermodynamic and structural characterization of the Mg–Li–N–H hydrogen storage system

The Mg–Li–N–H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. Some of thermodynamic and structural properties for this system are characterized here. Pressure-composition isotherms are measured and presented in this paper for absorption–desorption at 220, 200 and 180 °C. Powder X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) analysis were carried out for samples at various degrees of hydrogenation. These results provide information about the structural changes during absorption/desorption. The mixture of (2LiNH₂ + MgH₂) partially converts to (Mg(NH₂)₂ + 2LiH) when heated at 220 °C and 100 bar of hydrogen without undergoing desorption. Based on two distinct parts which appear in all of the pressure-composition isotherms (180–220 °C), two reactions taking place isothermally in hydrogen absorption/desorption are proposed for the material starting with (2LiNH₂ + MgH₂) or (Mg(NH₂)₂ + 2LiH). These reactions include a single solid-phase reaction, corresponding to the sloping region for hydrogen weight percent (Hwt%) smaller than 1.5%, and a multiple-phase reaction, corresponding to a plateau region for Hwt.% > 1.5 in the isotherms. During hydrogen absorption/desorption, the single-solid-phase reaction corresponds to the forming/consuming of NH₂ which is bonded to Li and the multiple-solid-phase reaction corresponds to forming/consuming Mg(NH₂)₂ and LiH. A mechanism for the sorption reactions has been proposed.     

Dehydriding and rehydriding processes of well-crystallized Mg(BH4)2 accompanying with formation of intermediate compounds

Dehydriding and rehydriding properties of well-crystallized Mg(BH₄)₂ were systematically investigated by thermogravimetry (TG) and pressure–composition–temperature (PCT) measurements. The dehydriding reaction of Mg(BH₄)₂ starts at approximately 500 K, and about 14.4 mass% of hydrogen is desorbed according to the following multi-step reaction:Mg(BH₄)₂ → some intermediate compounds → MgH₂ + 2B + 3H₂ → Mg + 2B + 4H₂The apparent enthalpy change in the dehydriding reaction from Mg(BH₄)₂ to MgH₂ is estimated to be 57 ± 5 kJ mol⁻¹ H₂ based on the result of the PCT measurement. It is proved that approximately 6.1 mass% of hydrogen can be reversibly stored for the sample of Mg(BH₄)₂ after the dehydriding reaction, through the formation of a possible intermediate compound such as MgB₁₂H₁₂.     

Cubic MgH2 stabilized by alloying with transition metals: A density functional theory study

The stability of two crystallographic modifications of Mg-transition metal (Sc, Ti, Zr, Hf) dihydrides was studied using density functional theory. Beyond a certain transition metal content, the rutile structure characteristic of pure MgH₂ is no longer stable, and the hydride transforms into a fluorite-type structure, similar to that of the transition metal dihydride. The transition point for both Mg–Sc and Mg–Ti hydrides is estimated to be at 20 at.% Sc/Ti, which is in very good agreement with previous experimental studies. For Mg–Zr and Mg–Hf, no experimental data are available for comparison, so the calculations have a predictive value for these systems. For Zr and Hf, the transition point is predicted to be at lower transition metal content than for Sc and Ti, at 13 at.%. This means that the Mg–Zr hydride is also of practical importance, because a fluorite structured hydride is predicted with a hydrogen content in excess of 6 wt.%.     

Hydrogen sorption properties of MgH2–LiBH4 composites

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH₂ + 2LiBH₄ ↔ MgB₂ + 2LiH + 4H₂ was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH₂ + 2LiBH₄ ↔ Mg + 2LiBH₄ + H₂ ↔ MgB₂ + 2LiH + 4H₂. However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH₄ and MgH₂ are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250–300°C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.     

Corrosion of ultra-high-purity Mg in 3.5% NaCl solution saturated with Mg(OH)2

Corrosion was evaluated for ultra-high-purity magnesium (Mg) immersed in 3.5% NaCl solution saturated with Mg(OH)₂. The intrinsic corrosion rate measured with weight loss, P_W = 0.25 ± 0.07 mm y⁻¹, was slightly smaller than that for high-purity Mg. Some specimens had somewhat higher corrosion rates attributed to localised corrosion. The average corrosion rate measured from hydrogen evolution, P_AH, was lower than that measured with weight loss, P_W, attributed to dissolution of some hydrogen in the Mg specimen. The amount of dissolution under electrochemical control was a small amount of the total dissolution. A new hydride dissolution mechanism is suggested.     

Analyses of hydrogen sorption kinetics and thermodynamics of magnesium–misch metal alloys

The multi-component Mg–x wt.% Mm alloys are synthesized using the mechanical ball-milling technique and their hydrogen storage capacities, absorption/desorption kinetics, and thermodynamic parameters are quantified. The analysis of kinetic properties presented here is based on the method of exponential peeling. It is seen that the presence of misch metal (Mm) in the alloy samples dramatically decreases their rate of decrepitation and increases their cyclic stability. However, the increased concentration of misch metal in the samples has an adverse effect on their hydrogen storage capacity and their reaction rate. The hydrogen storage properties also vary with reaction temperature. The best hydrogen absorption kinetics are observed at temperatures around 300 °C and the desorption kinetics are quite fast at temperatures of 400 °C and above. The hydrogen desorption activation energy of Mg–x wt.% Mm hydride is much lower than that of MgH₂. The pressure–composition–isotherm (P–C–T) plots of the samples at 300–420 °C indicate that the alloys possess good cyclic stability but very poor reversibility, making the study of their thermodynamic properties difficult. The P–C–T plots also indicate that the increase of the concentration of the misch metal's rare earths leads to an increase of the hydrogen equilibrium pressure and decrease of hydrogen storage capacity.     

Mechanochemical preparation and investigation of properties of magnesium,calcium and lithium–magnesium alanates

Ball milling of MgCl₂ and CaCl₂ with NaAlH₄ or LiAlH₄ can be used for the preparation of magnesium, calcium and lithium–magnesium alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it was possible to obtain these alanates in nearly pure state. The alanates were characterized by X-ray diffractometry, solid-state ²⁷Al NMR and IR spectroscopy and thermovolumetric (TV) and differential scanning calorimetry (DSC) measurements. Mg(AlH₄)₂ dissociates thermally in one step to MgH₂, Al and hydrogen; at a higher temperature, MgH₂ and Al transform to Mg–Al alloy and hydrogen. Thermal dissociations of Ca(AlH₄)₂ and of LiMg(AlH₄)₃ (in mixture with NaCl or LiCl) proceeds in several steps, of which the first two can be assigned to the formation of CaH₂ and of a MgH₂/LiH mixture, respectively, in addition to Al and H₂. Possible intermediates of these two steps are CaAlH₅ and LiMgAlH₆. Higher temperature dissociations include formation of MgH₂ (LiH) and Ca–Al alloys from CaH₂, CaH₂ and Al, respectively. Upon ball milling of MgCl₂ or CaCl₂ with NaAlH₄ or LiAlH₄ in the presence of Ti catalysts, only the thermal dissociation products of the expected alanates are obtained. This indicates that dehydrogenation discharge of earth alkali metal alanates can be catalyzed by Ti. According to DSC measurements, the thermodynamic stability of Mg(AlH₄)₂ (ΔH = 1.7 kJ/mol) is too low for the purpose of reversible hydrogen storage. Determination of ΔH values for the second, endothermal step of calcium and lithium–magnesium alanate dissociations gave values of around 31.6 and 13.1 kJ/mol, respectively.     

Structure of a new ternary compound with high magnesium content,so-called Gd13Ni9Mg78

The magnesium-rich composition Gd₁₃Ni₉Mg₇₈ was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd_10–15Ni_8–12Mg_72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd₁₃Ni₉Mg₇₈. In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h^?1) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h^?1. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 Å could be assumed. A modulation along both a¹ and b¹ axis with vectors of modulation q1 = 0.42a¹ and q2 = 0.42b¹ was observed. This compound, so-called Gd₁₃Ni₉Mg₇₈, absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH₂, GdH₂ and NiMg₂H₄ has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.     

Hydrogen diffusion in Mg–H and Mg–Ni–H alloys

Coefficients of hydrogen diffusion in Mg₂NiH₄ (D_I), MgH₂ (D_M), and in (Mg + Mg₂Ni)−H eutectic (D_E) were measured in the temperature interval 449–723 K. Experimental material was prepared by two techniques: by melting and casting and by ball-milling and compacting into pellets. Hydrogen charging was carried out from the hydrogen gas phase at high temperature and pressure. The Mg₂NiH₄ pellets were prepared in different regimes resulting either in a structure with a high fraction or with a low fraction of twinned low-temperature phase LT2. It was found that the LT2 slows down the hydrogen desorption rate considerably – values of D_I in low-temperature untwinned phase LT1 are by a multiplication factor of about 20 higher than those in the twinned phase LT2. Obtained values of D_M are significantly lower than the literature values reported for the pure Mg. Hydrogen diffusion coefficients in interphase boundary were estimated from D_E.     

Formation of Na3AlH6 from a NaH/Al mixture and Ti-containing catalyst

To investigate the effects of Ti-containing catalyst on the hydride formation in the Na–Al–H system, Ti(OBuⁿ)₄- and TiF₃-doped NaH/Al powders were hydrogenated into Na₃AlH₆ by controlling the hydrogen pressure at 5 MPa and the temperature at 100–140 °C. X-ray diffraction and differential scanning calorimetry showed that TiF₃ was more catalytically favorable than Ti(OBuⁿ)₄ to enhance the formation of Na₃AlH₆. X-ray absorption spectroscopy revealed that the initial Ti⁴⁺ or Ti³⁺ ions were reduced to Ti¹⁺ or Ti⁰ and form Ti–Al pairs with two coordination shells during the doping and hydrogenation processes. The difference in catalytic activity between the two Ti-containing catalysts can be attributed to their ability to form Ti–Al pairs with the NaH/Al mixture.     

Reaction chemistry in the Mg–B2O3–MoO3 system reactive mixtures

Fundamental aspects of reaction path in the MoO₃ + B₂O₃ + Mg system to synthesize molybdenum boride have been investigated. The phase transformation and structural evaluation were studied by means of differential thermal analysis (DTA) techniques, X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Thermodynamic evaluations indicated that the reaction was highly exothermic and should be a mechanically induced self-sustaining reaction (MSR). According to DTA results, for unmilled sample, the reaction's sequence includes the following reactions: MoO₃ reduction → B₂O₃ reduction → molybdenum boride formation. For 3 h-milled sample, the temperature of exothermic reaction decreased significantly and all the reactions occurred, simultaneously. Based on XRD results, the mechanochemical products including MgO and mix of molybdenum boride phases were achieved after 4 h of high energy ball milling. SEM and TEM observations confirmed that the range of particle size was within 100 nm.     

Phase transformation in an yttrium–hydrogen system studied by TEM

Phase transformations in Pd-capped epitaxial yttrium films grown on (0 0 0 1) sapphire substrates covered with a Ti buffer layer and hydrogenated for different times were studied using transmission electron microscopy (TEM). For short hydrogen charging times, the phase transformation from α-Y to β-YH₂ is associated with the nucleation and growth of two orientational variants, which after coalescence form twin-related lamellae of the β-YH₂ phase with twin interfaces parallel to the substrate plane. Shockley partial dislocations are present at the twin boundaries; their glides during phase transformation are responsible for the formation of the twin lamellae. Superlattice reflections were observed for β-YH₂, and the existence of a new long-range ordered superstoichiometric YH_2+x phase was suggested. A structural model of the ordering based on the occupation of octahedral interstitial sites by H in a doubled cell of Y-face-centered cubic was offered. For samples hydrogenated for longer times, β-YH₂-to-γ-YH₃ phase transformation was accompanied by cracking along the twin boundaries, which eventually developed into a network of pores and caused significant swelling of the films. No γ-YH₃ phase was observed directly in TEM because of its unstable nature under the high vacuum of a microscope column. The fully transformed YH₃ films have over a 60% increase in its thickness, which is mostly accounted for by the high volume fraction of pores.     

High-pressure synthesized nanostructural magnesium diboride-based materials for superconductive electromotors,generators and pumps

Additions of Zr can increase critical current density (j_c) of high-pressure synthesized MgB₂ (HPS-MgB₂) in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen (to form ZrH₂). The formation in HPS-MgB₂ of ZrB₂ phase at higher synthesis temperatures (about 950 °C) does not result in the j_c increase. Some increase in j_c of HPS-MgB₂ at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH₂ impurity phase from appearing and may prevent hydrogen from being introduced into the material structure and besides, their presence in HPS-MgB₂ promotes the formation of a higher amount of Mg–B (most likely MgB₂) inclusions in the Mg–B–O material “matrix” that in turn leads to the increase of j_c in magnetic fields. The high level of superconductive (SC) and mechanical characteristics attained for HPS-MgB₂ and the possibility to manufacture large samples make its application in the superconductive electromotors, generators, pumps, etc., very promising.     

The phase diagram and superabundant vacancy formation in Co–H alloys

The phase diagram of the Co–H system was determined by in situ XRD up to 1300 °C and hydrogen pressure of p_H = 7.4 GPa, encompassing ϵ(hcp), γ(fcc) and liquid phases. A steep drop of the ϵ–γ phase boundary at p_H ≥ 6 GPa is shown to be concomitant with an abrupt increase of the solubility in the γ phase at p_H ∼ 5 GPa, one of the characteristic features of supercritical anomaly. Prolonged heat treatments in the γ phase caused gradual lattice contraction, indicating that superabundant vacancies (SAVs) are formed. From the systematic variation of observed lattice parameters at various p_H, T conditions, it is inferred that, irrespective of initial hydride compositions, the same defect hydride of composition Co₃VacH₄ was formed after the heat treatments. Vacancy-hydrogen binding energies are deduced from thermal desorption data measured on heat-treated samples, and on this basis the energetics of SAV formation is discussed. The migration volume of a vacancy-H cluster was estimated from the pressure dependence of the rate of lattice contraction.