Hydrogen storage properties of 2Mg-Fe mixtures processed by hot extrusion at different temperatures

2Mg-Fe mixtures produced by high-energy ball milling were consolidated into bulk form by hot extrusion at different processing temperatures (573 K (300 °C), 623 K (350 °C) and 673 K (400 °C)), aiming to evaluate their influence on the structure and microstructure of bulk materials and their consequent influence on the hydrogen sorption properties. In spite being in the nanosize range, the highest the processing temperature, the larger the grain sizes. However, the nanometric grain size remained after any hot extrusion condition, as estimated by Rietveld refinement. The pinning effect of Fe on Mg grain boundaries explained this effect. In the first absorption (activation), powders showed a hydrogen storage capacity of ～4.53 wt%, while the extruded samples (bulk materials) reached almost the same capacity during the period of hydrogenation (～94% of the maximum hydrogen storage capacity for Mg₂FeH₆ - 5.5 wt%). The smallest crystallite sizes and highest surface area for hydrogenation explain the good performance of powders. However, when comparing only extruded samples, it was observed that the highest capacity and the lowest incubation times were mainly related to grain sizes and to the favorable texture along (002) plane of αMg. The desorption temperature of bulk materials was very similar to that of powders, which is good considering the lower surface area of bulk materials.     

High pressure DSC study of hydrogen sorption in MgH2/graphite mixtures: Effects of sintering and oxidation

Hydrogen absorption and desorption properties of ball milled Mg and Mg/graphite materials were analyzed by high pressure differential scanning calorimetry. The influence on hydrogen sorption kinetics of different graphite distribution, oxygen poisoning and magnesium sintering was studied. The Mg/graphite mixture with graphite distributed in the bulk showed better kinetics than the material with graphite located on the surface and Mg without additive. The effect of sintering and oxygen poisoning was a progressive storage capacity loss, due to a kinetic limitation in the case of sintering, and due to irreversible magnesium oxidation in the case of poisoning. The mixtures with graphite exhibited more resistance toward oxygen contamination, particularly in the case where graphite was primarily located on the surface compared to the material with graphite well dispersed in the bulk.     

Fast synthesis of TiNi by mechanical alloying and its hydrogenation properties

Mechanical alloying is widely used for the synthesis of hydrogen storage materials. However, amorphization and contamination triggered by long-time milling are serious drawbacks for obtaining efficient hydrogen storage. In this work, short-time ball milling synthesis is explored for a representative hydride forming compound: TiNi. Through structural, morphological and chemical characterizations, we evidence that formation of TiNi is complete in only 20 min with minor Fe contamination (0.2 wt%). Cross-sectional analysis of powder stuck on milling balls reveals that alloy formation occurs through the interdiffusion between thin layers of co-laminated pure elements. Hydrogenation thermodynamics and kinetics of short-time mechanically alloyed TiNi are similar to those of coarse-grained compounds obtained by classical high-temperature melting. Mechanical alloying is a suitable method for fast and energy-efficient synthesis of intermetallic compounds such as TiNi.     

Effect of Al and Mo substitution on the structural and hydrogen storage properties of CaNi5

A simple mechanical milling and annealing process has been used to synthesize CaNi₅-based hydrogen storage alloys. Heat treatment at 800 °C under vacuum results in the formation of a crystalline CaNi₅ phase. Secondary phases, including Ca₂Ni₇ and Mo–Ni, are formed when substituting Mo for Ni. Replacement of Ni by Al or Mo leads to an increase in the unit cell volume of the CaNi₅ phase. The hydrogen storage capacity of all substituted alloys is reduced and the plateau pressures are lower than those of pure CaNi₅. Fairly flat plateau regions are retained for all compositions except the CaNi_4.8Mo_0.2 composition where a Ca₂Ni₇ phase is dominant. The incorporation of Mo also causes slow sorption kinetics for the CaNi_4.9Mo_0.1 alloy. CaNi_4.9Al_0.1 maintains its initial hydrogen absorption capacity for 20 cycles performed at 85 °C but the other substituted alloys lose their capacity rapidly, especially the CaNi_4.8Mo_0.2 composition.     

Catalytic effect of monoclinic WO3, hexagonal WO3 and H0.23WO3 on the hydrogen sorption properties of Mg

The H sorption properties of mixtures Mg + WO₃ (having various structures) and Mg + H_0.23WO₃ are reported. First, the higher conversion of Mg into MgH₂ during reactive mechanical grinding (under 1.1 MPa of H₂) for higher WO₃ content is due to the improvement of the milling efficiency. Then, it is shown that the hydrogen absorption properties are almost independent of the crystal structure of the catalyst and that only the particles' size and the specific surface play a major role. Finally, for the desorption process, it appears that the chemical composition and structure of the catalyst, together with the particle size and specific surface have an effect.     

Hydrogen storage nanocrystalline TiFe intermetallic compound: Synthesis by mechanical alloying and compacting

Equiatomic nanocrystalline TiFe intermetallic compound was synthesized from elementary metals by mechanical alloying in a planetary ball mill. Then the powder produced was compacted by cold pressing into bulk samples keeping the nanocrystalline structure. The hydrogen capacity of TiFe bulk samples was found to be of 1.4 wt.%. The absorption isotherm had a long plateau corresponding to pressures of 0.3–0.4 MPa at room temperature. The bulk samples demonstrated high durability after 20 absorption–desorption cycles.     

Improved ball milling method for the synthesis of nanocrystalline TiFe compound ready to absorb hydrogen

In this study, we propose a method to produce nanocrystalline TiFe powder by high-energy ball milling, in order to avoid the common sticking problem of the material to the milling tools, assuring a material prompt to absorb hydrogen as well. The method consists of making a preliminary milling operation with the elemental powders (50:50 stoichiometric ratio) to form a strong adhered layer of the milled material on the surfaces of the vial and balls. The main milling operation is then performed with a new powder charge (same composition as before), but now adding a process control agent (stearic acid). Various processing times - 2, 6, 10 and 20 h - were used in the milling experiments. Nanocrystalline TiFe was synthesized in this way with low oxygen contamination, full yields for milling times of 6 h or over, requiring no heat treatments for the first hydrogen absorption. Hydrogen storage capacity of 1.0 wt% at room temperature under 20 bar was attained by the sample milled for 6 h. Kinetic data from samples milled for 2 h and 6 h agreed with Jander model for the rate limiting step of the hydriding reaction, which is based on diffusion with constant interface area.     

Hydrogen storage properties of TiFe-based ternary mechanical alloys with cobalt and niobium. A thermochemical approach

Ternary alloys of general composition (TiFe)_100-xM_x (M = Co, Nb) have been synthesized from pure metals through high-energy ball milling. The maximum concentration of alloying components allowing formation of single phase TiFe-type compounds has been defined as 2 at.%. The hydrogenation behavior of the mechanical alloys in comparison with the arc-melted ones of the same composition has been studied by a combination of volumetric and calorimetric techniques. Influence of the alloy composition and the synthesis mode on the crystal structure of TiFe and its hydrides has been evaluated. It has been shown that the thermochemical method based on calorimetric titration provides more accurate information about phase transformations in the nanocrystalline metal hydride systems. The obtained results show that the third components slightly affect the hydrogen storage performance of non-equilibrium mechanical alloys in contrast with alloys produced by conventional melting.     

Effect of the heating rate on crystallization behavior of mechanically alloyed amorphous alloy

The mechanical alloying is the most convenient method to produce Mg–Ni alloys. In this study, the effect of ball-to-powder weight ratios and the mechanical alloying time on amorphization of Mg₅₀Ni₅₀ alloy and its thermal stabilities were investigated. Mg₅₀Ni₅₀ alloy has been produced by using Spex 8000 D mixer/mill with different ball-to-powder weight ratios (5:1, 10:1, 20:1). Amorphization times by XRD analysis are found to be 60 h for 5:1 ball-to-powder weight ratio, 10 h for 10:1 ball-to-powder weight ratio and 5 h for 20:1 ball-to-powder weight ratio. The thermal stabilities of amorphous Mg₅₀Ni₅₀ alloys, obtained by different ball-to-powder weight ratios, have been determined and the effect of heating rates on the crystallization temperatures have also been investigated by DSC. The heating rates employed were 5, 10, 15, 20 °C/min. During the first crystallization reaction, the amorphous and Mg₂Ni intermetallic phases occurred. DSC studies show that increase in heating rates increased the crystallization temperatures for all samples. The apparent activation energies were determined by means of the Kissinger method.     

Hydrogen sorption properties of M0.2Ca0.8MgH4 (M = Li,Na)

The synthesis, thermodynamic destabilisation and hydrogen sorption properties of the M_0.2Ca_0.8MgH₄ hydride system, where (M = Na or Li) have been investigated. Samples were mechanically milled under argon for 5, 10 and 15 h; then characterised by X-ray diffraction (in/ex-situ), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) coupled with a mass spectrometer (MS). On Li and Na substitution into Ca–Mg–H ternary hydride, diffraction peaks shifted to higher angles. 2 and 3 endothermic reactions were observed for the Li_0.2Ca_0.8MgH₄ and Na_0.2Ca_0.8MgH₄. The maximum amount of hydrogen release was achieved for the 5 h milled Na_0.2Ca_0.8MgH₄ hydride from 298 °C to 386 °C accounting for 3.5 wt % of H₂.     

Hydrogen storage properties of Mg-10 wt% Ni alloy co-catalysed with niobium and multi-walled carbon nanotubes

Mg-10 wt% Ni alloys containing up to 1 wt% Nb were fabricated by a casting technique, followed by ball-milling with 5 wt% multi-walled carbon nanotubes. Further mechanical alloying with 1.5, 3, and 5 at % Nb was applied to a cast Mg-10 wt% Ni-370 ppm Nb alloy to investigate the catalytic role of Nb in hydrogen dissociation. The microstructure and distribution of Nb and Mg₂Ni in the alloys were characterised by SEM. The absorption and desorption kinetics of the samples were measured by Sieverts’ apparatus at various temperatures. The results show that addition of Nb during casting accelerates the hydrogen diffusion compared to the cast binary Mg-10 wt% Ni alloy. Moreover, ball-milling of the alloy with metallic niobium leads to the formation of BCC phase of Mg-Nb solid solution, which significantly improves the hydrogenation properties of the alloy. DSC results show that mechanical alloying of Mg-10 wt%Ni-370 ppm Nb with Nb in excess of 1.5 wt% decreases the desorption temperature by approximately 100 °C compared to the ball-milled cast alloy.     

Hydrogen storage and electrochemical properties of mechanically alloyed La1.5-xGdxMg0.5Ni7 (0 ≤ x ≤ 1.5)

RE-Mg-Ni-based alloys with their unique superlattice structures have been considered to be possible candidates for hydrogen storage containers as well as electrodes in Ni-MH_x batteries. In this study, mechanical alloying with subsequent annealing in the argon atmosphere at 1123 K for 0.5 h, was applied to produce the La_1.5-xGd_xMg_0.5Ni₇ (0 ≤ x ≤ 1.5) alloys. Hydrogen storage and electrochemical properties of the synthesized material have been investigated. For example, at the 50^th cycle the La_1.5Mg_0.5Ni₇ material shows a much lower reversible electrochemical capacity than La_1.25Gd_0.25Mg_0.5Ni₇ alloy. Additionally, the partial substitution of La by Gd improves the kinetics of hydrogen absorption. On the other hand, the stability of the electrochemical discharge capacity increases with the increasing value Gd up to x = 1.0. However, a significant reduction in the discharge capacity was observed for the Gd content above x = 0.25. From the application point of view, only La_1.25Gd_0.25Mg_0.5Ni₇ alloy show great potential in the future application as electrode material in Ni-MH_x batteries.     

Effect of Ti-based nanosized additives on the hydrogen storage properties of MgH2

MgH₂-based nanocomposites were synthesized by high-energy reactive ball milling (RBM) of Mg powder with 0.5–5 mol% of various catalytic additives (nano-Ti, nano-TiO₂, and Ti₄Fe₂O_x suboxide powders) in hydrogen. The additives were shown to facilitate hydrogenation of magnesium during RBM and substantially improve its hydrogen absorption-desorption kinetics. X-ray diffraction analysis showed the formation of nanocrystalline MgH₂ and hydrogenation of nano-Ti and Ti₄Fe₂O_x. The possible reduction of TiO₂ during RBM in hydrogen was not observed, which is in agreement with lower hydrogenation capacity of the corresponding composite, 5.7 wt% for Mg + 5 mol% nano-TiO₂ compared to 6.5 wt% for Mg + 5 mol% nano-Ti. Hydrogen desorption from the as-prepared composites was studied by Thermal Desorption Spectroscopy (TDS) in vacuum. A significant lowering of the hydrogen desorption temperature of MgH₂ by 30–90 °C in the presence of the additives is associated with lowering activation energy from 146 kJ/mol for nanosized MgH₂ down to 74 and 67 kJ/mol for MgH₂ modified with nano-TiO₂ and Ti₄Fe₂O_0.3 additives, respectively. After hydrogen desorption at 300–350 °C, these materials are able to absorb hydrogen even at room temperature. It is shown that nano-structuring and addition of Ti-based catalysts do not decrease thermodynamic stability of MgH₂. The thermodynamic parameters, obtained from hydrogen desorption isotherms for the Mg–Ti₄Fe₂O_0.3 nanocomposite, ΔH_des = 76 kJ/mol H₂ and ΔS_des = 138 J/K·mol H₂, correspond to the reported literature values for pure polycrystalline MgH₂. Hydrogen absorption-desorption characteristics of the composites with nano-Ti remain stable during at least 25 cycles, while a gradual decay of the reversible hydrogen capacity occurred in the case of TiO₂ and Ti₄Fe₂O_x additives. Cycling stability of Mg/Ti₄Fe₂O_x was substantially improved by introduction of 3 wt% graphite into the composite.     

Effect of Cr substitution by Ni on the cycling stability of Mg2Ni alloy using EXAFS

This paper describes the hydrogen storage properties of Mg₂Ni_0.9Cr_0.1 alloy and aims to elucidate the effect of doping Cr on the hydrogen sorption/desorption kinetics upon cycling. Mg₂Ni_0.9Cr_0.1 alloy shows stable absorption capacity, and its absorption/desorption rates further improve after cycling. The calculated activation energy for dehydrogenation was 53 kJ/mol at the 3rd cycle, and decreased to 36 kJ/mol at the 20th cycle. XRD combined with SEM exhibits that Cr dopant substitutes for Mg or Ni after ball milling and the lattice structure remains stable over 20 cycles. EXAFS was used to investigate the local coordination of Ni and Cr atoms in the ball-milled and cycled samples. For the ball-milled sample, the strong Cr–Ni bonds weaken the Cr–Mg bonds, thereby destabilizing all Cr-doped phases. After 20 cycles, the stable Ni₁–Mg₁ bonds may be dominant and control the structural stability of Mg₂Ni phases.     

Dehydriding and re-hydriding properties of high-energy ball milled LiBH4 + MgH2 mixtures

Here we report the first investigation of the dehydriding and re-hydriding properties of 2LiBH₄ + MgH₂ mixtures in the solid state. Such a study is made possible by high-energy ball milling of 2LiBH₄ + MgH₂ mixtures at liquid nitrogen temperature with the addition of graphite. The 2LiBH₄ + MgH₂ mixture ball milled under this condition exhibits a 5-fold increase in the released hydrogen at 265 °C when compared with ineffectively ball milled counterparts. Furthermore, both LiBH₄ and MgH₂ contribute to hydrogen release in the solid state. The isothermal dehydriding/re-hydriding cycles at 265 °C reveal that re-hydriding is dominated by re-hydriding of Mg. These unusual phenomena are explained based on the formation of nanocrystalline and amorphous phases, the increased defect concentration in crystalline compounds, and possible catalytic effects of Mg, MgH₂ and LiBH₄ on their dehydriding and re-hydriding properties.     

Effect of Ni on electrochemical and hydrogen storage properties of V-rich body-centered-cubic solid solution alloys

V-rich solid solution alloys are potential candidates for Ni-MH_x negative electrodes and hydrogen sorbing materials. Mechanical alloying (MA) is used in this paper to produce Ti_0.5V_1.5?xNi_x nanocrystalline alloys (x = 0, 0.1, 0.2, 0.3). A SPEX 8000 M mill is used. The aim of this work is to study the effect of chemical modification by Ni on hydrogen sorption/desorption and electrochemical properties of V-rich body-centered-cubic (BCC) alloys. Presented measurements results show formation of BCC phase after 14 h of MA. The nanocrystallinety of obtained materials is confirmed by high resolution transmission electron microscopy images. MA alloys are tested by a Sievert's device at near room temperature. Partial substitution of V by Ni causes improved hydrogenation kinetics, reduced hysteresis and increased hydrogenation/dehydrogenation reversibility. Observed properties are mainly due to differences in structures of studied materials. Electrochemical studies on chemically modified V-rich alloys show that capacity retaining rate and discharge capacity increase with higher Ni content in the material.     

Hydrogen desorption properties of MgH2-TiCr1.2Fe0.6 nanocomposite prepared by high-energy mechanical alloying

In the present work, high-energy mechanical alloying (MA) was employed to synthesize a nanostructured magnesium-based composite for hydrogen storage. The preparation of the composite material with composition of MgH₂-5 at% (TiCr_1.2Fe_0.6) was performed by co-milling of commercial available MgH₂ powder with the body-centered cubic (bcc) alloy either in the form of Ti-Cr-Fe powder mixture with the proper mass fraction (sample A) or prealloyed TiCr_1.2Fe_0.6 powder (sample B). The prealloyed powder with an average crystallite size of 14 nm and particle size of 384 nm was prepared by the mechanical alloying process. It is shown that the addition of the Ti-based bcc alloy to magnesium hydride yields a finer particle size and grain structure after mechanical alloying. As a result, the desorption temperature of mechanically activated MgH₂ for 4 h decreased from 327 °C to 262 °C for sample A and 241 °C for sample B. A high dehydrogenation capacity (∼5 wt%) at 300 °C is also obtained. The effect of the Ti-based alloy on improvement of the dehydrogenation is discussed.     

Improvement of as-milled properties of mechanically alloyed LaNi5 and application to hydrogen thermal compression

LaNi₅ was obtained from raw materials by low energy mechanical alloying. Hydriding properties of as-milled intermetallic were improved by annealing. Pressure-composition isotherms showed flat plateaus when annealing temperature was 600 °C, this value is at least 300 °C lower than the synthesis and annealing temperature of standard equilibrium methods. This low energy mechanical alloying - low temperature annealing procedure reduces the number of intermediate stages needed to scale up the fabrication of the intermetallic. After cycling this material in hydrogen, its hydriding properties were studied in the 25-90 °C range. From these results, we propose a one-stage hydrogen thermal compression scheme working between 25 °C (absorption) and 90 °C (desorption) with a compression ratio of 2.5 and a useful capacity of 1.0 mass %.     

The effects of Co and Ni addition on the hydrogen storage properties of Mg3Mm

The effects of Ni and Co addition on the hydrogen storage properties of Mg₃Mm alloy was studied by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and pressure-composition isotherm (PCI) measurement. The hydrogen absorption kinetics and the thermodynamic parameters (apparent ΔH, ΔS) for Mg₃Mm dehydrogenation reactions in Mg₃Mm, Mg₃MmNi_0.1 and Mg₃MmNi_0.1Co_0.1 alloys have been also investigated. The maximum hydrogen storage content of Mg₃Mm, Mg₃MmNi_0.1 and Mg₃MmNi_0.1Co_0.1 alloys was improved due to that the addition of Ni and/or Co further spurred the MmH₃ phase transforming to MmH₂ phase. On the other side, the kinetics curves show the addition of Co could enhance hydrogen absorption rate while the addition of Ni change the hydrogenation reaction mechanism.