Effects of adding nano-CeO2 powder on microstructure and hydrogen storage performances of mechanical alloyed Mg90Al10 alloy

For pushing Mg-based alloys developing to the practical applications, nano-CeO₂ powders were added into the mechanical alloyed (MA) Mg₉₀Al₁₀ alloy. The aim of it is to improve the thermodynamics and kinetics through generating new intermetallic compound, reducing the grain size and increasing the solid solubility of Al in Mg. XRD analysis showed that adding nano-CeO₂ powder causes the generation of Mg₁₇Al₁₂ phase and grain refinement of the MA Mg₉₀Al₁₀ + x wt% CeO₂ (x = 1, 3, 5 and 8) composites. It also increases the solid solubility of Al in Mg, while results in the reduction of Mg lattice volume. The dehydrogenation enthalpy (ΔH_de), calculated from Van't Hoff equation, is reduced from 75.43 kJ mol⁻¹ H₂ for the MA Mg₉₀Al₁₀ alloy to 74.22, 72.70, 70.28 and 73.71 kJ mol⁻¹ H₂ for the MA Mg₉₀Al₁₀ + x wt% CeO₂ (x = 1, 3, 5 and 8) composites, respectively. The increased grain boundaries, caused by the grain refinement and formation of the mutilphase structure, are beneficial to reduce the dehydrogenation activation energy (E^de(a)). It is obtained through Johnson-Mehl-Avrami-Kolmogorov model, which is 162.06, 121.86, 103.73, 101.83 and 109.08 kJ mol⁻¹ H₂ for the MA Mg₉₀Al₁₀ + x wt% CeO₂ (x = 0, 1, 3, 5 and 8) composites, respectively.     

Comparative investigation on the hydrogenation/dehydrogenation characteristics and hydrogen storage properties of Mg3Ag and Mg3Y

The hydrogenation/dehydrogenation characteristics and hydrogen storage properties of nominal Mg₃Ag and Mg₃Y alloys prepared by induction melting were investigated. The as-melted Mg₃Ag alloy was composed of Mg₅₄Ag₁₇ phase, while Mg₃Y consisted of Mg₂₄Y₅ and Mg₂Y phases. Mg₅₄Ag₁₇ transformed into MgAg and MgH₂ during the first hydrogenation, and the phase transition of the following hy/dehydrogenation cycles was Mg₃Ag + 2H₂ ↔ MgAg + 2MgH₂. Both Mg₂₄Y₅ and Mg₂Y undertook disproportion reactions and decomposed into MgH₂ and YH₃. Experimental and calculated results demonstrated that there was no necessary relation between the thermodynamic stabilities and the size interstices in these alloys. The dehydrogenation enthalpy change (ΔH) and entropy change (ΔS) of Mg₃Ag were calculated and compared with that of pure Mg, which indicated that the increase of ΔS could counteract the stabilization effect of ΔH, which offered a method for tuning the thermodynamic properties of Mg-based alloys.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.     

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.     

Microstructures and electrochemical characteristics of Mm0.3Ml0.7Ni3.55Co0.75Mn0.4Al0.3 hydrogen storage alloys prepared by mechanical alloying

The as-cast alloy with the composition of Mm_0.3Ml_0.7Ni_3.55Co_0.75Mn_0.4Al_0.3 prepared by induction melting was milled for 6, 15, 40 and 50 h in this work. The microstructures of alloys were analyzed by XRD and DSC. Two broadening effects of XRD peak caused by crystallite and microstrain were separated by approximate function method and least square method. Crystallite sizes and microstrains of the alloys were calculated. The results show that alloys milled for 6 h or 15 h consist of nanocrystalline and polycrystalline phase. Lattice parameters (a, c) and volumes of alloy increase with the increasing milling time, whereas the ratio of c/a keep constant. Moreover, the crystallite sizes decrease and the microstrains in the alloys increase first and then decrease with the increase of milling time. The alloys milled for 40 h or 50 h transform partly into amorphous structures. The maximum discharge capacities of alloys decrease with the increase of milling time. The cycle stabilities of the milled alloys are better than those of the as-cast. And they increase with the increasing milling time.     

Kinetics and electrochemical characteristics of Mg2NiH4-x wt.% MmNi3.8Co0.75Mn0.4Al0.2 (x = 5, 10, 20, 40) composites for Ni-MH battery

The structure, kinetics and electrochemical characteristics of Mg₂NiH₄-x wt.% MmNi_3.8Co_0.75Mn_0.4Al_0.2 (x = 5, 10, 20, 40) composites prepared by mechanical milling have been investigated in this paper. XRD results indicate that the as-milled Mg₂NiH₄ shows nanocrystalline or amorphous-like structure, and it does not react with MmNi_3.8Co_0.75Mn_0.4Al_0.2 during mechanical milling. As the amount of MmNi_3.8Co_0.75Mn_0.4Al_0.2 increases, the maximum discharge capacity decreases initially from 508 mAh/g (x = 5) to 440 mAh/g (x = 10) and then increases to 509 mAh/g (x = 40). Meanwhile, the capacity retention (R₁₀) increases from 12.8% (x = 5) to 23.4% (x = 40), and the corrosion potential of electrode (E_corr) increases from −0.930 V to −0.884 V (vs. Hg/HgO). Especially, the more MmNi_3.8Co_0.75Mn_0.4Al_0.2 content the composite contains, the higher high rate dischargeability (HRD) the electrode exhibits, which could be attributed to the catalytic reaction and reduction of the Mg₂NiH₄ grain size brought by MmNi_3.8Co_0.75Mn_0.4Al_0.2. The improvement in electrode kinetics has been depicted from the bulk hydrogen diffusion coefficient (D), the exchange current density (I₀) and the charge transfer resistance (R_ct) on the alloy surface.     

Enhanced hydrogen sorption kinetics of Mg50Ni-LiBH4 composite by CeCl3 addition

Mg₅₀Ni-LiBH₄ and Mg₅₀Ni-LiBH₄-CeCl₃ composites have been prepared by short times of ball milling under argon atmosphere. Combination of HP-DSC and volumetric techniques show that Mg₅₀Ni-LiBH₄-CeCl₃ composite not only uptakes hydrogen faster than Mg₅₀Ni-LiBH₄, but also releases hydrogen at a lower temperature (225 °C). The presence of CeCl₃ has a catalytic role, but it does not modify the thermodynamic properties of the composite which corresponds to MgH₂. Experimental studies on the hydriding/dehydriding mechanisms demonstrate that LiBH₄ and Ni lead to the formation of MgNi₃B₂ in both composites. In addition, XRD/DSC analysis and thermodynamic calculations demonstrate that the addition of CeCl₃ accounts for the enhancement of the hydrogen absorption/desorption kinetics through the interaction with LiBH₄. The in situ formation and subsequent decomposition of Ce(BH₄)₃ provides a uniform distribution of nanosize CeB₄ compound, which plays an important role in improving the kinetic properties of MgH₂.     

Study on the gaseous and electrochemical hydrogen storage properties of as-milled CeMgNi-based alloys

For gaining further insight into the involvement of the gaseous and electrochemical hydrogen storage properties of CeMg₁₂-type alloys, partial substitution and ball milling were both used to synthesize the nanocrystalline and amorphous CeMg₁₁Ni + x wt.% Ni (x = 100, 200) samples. This research aims at elucidating the functional roles of Ni content and milling time on samples' structure and hydrogen storage performance. X-Ray diffraction and high-resolution transmission electron microscope were used to reveal the micro constructions of alloys. To determine the gaseous hydrogen storage property, Sievert's apparatus and a thermal gravity analysis bonded with a H₂ probe were adopted. The dehydrogenation activation energy was computed in the Kissinger method. The electrochemical performances of the as-milled samples were measured through a constant current system. Further researches showed that the electrochemical performance of as-milled samples had been dramatically improved by increasing Ni content. With milling duration lengthens, the gaseous hydrogen absorption capacity, gaseous hydriding rate and high rate discharge capability of samples reached the maximal values, but electrochemical discharge capacity and gaseous dehydriding rate always increased. The dehydrogenation activation energy decrease resulted by improving Ni percent and milling duration was deemed as the cause of the excellent gaseous kinetics of samples.     

Investigation on gaseous and electrochemical hydrogen storage performances of as-cast and milled Ti1.1Fe0.9Ni0.1 and Ti1.09Mg0.01Fe0.9Ni0.1 alloys

The AB-type Ti_1.1Fe_0.9Ni_0.1 (Mg₀ for short) and Ti_1.09Mg_0.01Fe_0.9Ni_0.1 (Mg_0.01 for short) alloys were fabricated by vacuum induction melting and mechanical milling. The effects of partly substituting Ti with Mg and/or mechanical milling on the structure, morphology, gaseous thermodynamics and kinetics, and electrochemical performances were studied. The results reveal that the as-cast Mg₀ alloy contains the main phase TiFe and a small number of TiNi₃ and Ti₂Ni phases. Substituting Ti with Mg and/or mechanical milling results in the disappearance of the secondary phases. The discharge capacities of the as-cast Mg₀ and Mg_0.01 alloys are 12.6 and 8.8 mAh g^?1, which increase to 52.6 and 80.4 mAh g^?1 after 5 h of mechanical milling. By milling the as-cast alloy powders with carbonyl nickel powders, they are greatly enhanced to 191.6 mAh g^?1 for the Mg₀+7.5 wt% Ni alloy and 205.9 mAh g^?1 for the Mg_0.01+5 wt% Ni alloy at the current density of 60 mA g^?1, respectively. The values of dehydrogenation enthalpy (ΔH_des) and dehydrogenation activation energy (E^des_(a)) are very small, meaning that the thermal stability and the desorption kinetics of the hydrides are not the key influence factors for the discharge capacity. The reduction of the particle size and the generation of the new surfaces without oxide layers have slight improvements on the discharge capacity, while the enhancement of the charge transfer ability of the surfaces of the alloy particles can significantly promote the electrochemical reaction of the alloy electrodes.     

Effect of hydrogenation on the mechanical property of amorphous Ni90Al10 membranes

To investigate the mechanism of hydrogen embrittlement in amorphous membranes, changes in the stress-strain curves of an amorphous Ni₉₀Al₁₀ model membrane as a function of hydrogen concentration were examined using Molecular Dynamics (MD) simulations. In addition, the fractional change of short-range ordered (SRO) structures during uniaxial tensile deformation was scrutinized using the Voronoi tessellation method. By correlating these structural evolutions related to volume expansion with changes in the mechanical property, the hydrogen embrittlement phenomenon occurring in the amorphous membrane was discussed with respect to the abrupt increase in the degree of strain localization during deformation. The possibility, not only of hydrogen embrittlement, but also of hydrogen-induced ductility enhancement, was demonstrated by the decrease in the degree of strain localization. Finally, we proposed a fundamental mechanism that can explain these overall phenomena occurring in the amorphous membrane by the introduction of hydrogen.     

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.     

Influence of multiple oxide (Cr2O3/Nb2O5) addition on the sorption kinetics of MgH2

The influence of multiple additions of two oxides, Cr₂O₃ and Nb₂O₅, as additives on the hydrogen sorption kinetics of MgH₂ after milling was investigated. We found that the desorption kinetics of MgH₂ were improved more by multiple oxide addition than by single addition. Even for the milled MgH₂ micrometric size powders, the high hydrogen capacity with fast kinetics were achieved for the powders after addition of 0.2 mol% Cr₂O₃ + 1 mol% Nb₂O₅. For this composition, the hydride desorbed about 5 wt.% hydrogen within 20 min and absorbed about 6 wt.% in 5 min at 300 °C. Furthermore, the desorption temperature was decreased by 100 °C, compared to MgH₂ without any oxide addition, and the activation energy for the hydrogen desorption was estimated to be about 185 kJ mol⁻¹, while that for MgH₂ without oxide was about 206 kJ mol⁻¹.     

Effect of gas back pressure on hydrogen storage properties and crystal structures of Li2Mg(NH)2

The ternary imide Li₂Mg(NH)₂ is considered to be one of the most promising on-board hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%, favorable thermodynamic properties and good cycling stability. In this work, Li₂Mg(NH)₂ was synthesized by dynamically dehydrogenating a mixture of Mg(NH₂)₂–2LiH up to 280 °C under different gas (Ar and H₂) and pressures (0–9.0 bar). The crystal structure of Li₂Mg(NH)₂ was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li₂Mg(NH)₂ and the dehydrogenation/rehydrogenation properties of the Mg(NH₂)₂–2LiH system strongly depend on the gas back pressure in the dehydrogenation process due to the effect of the pressure on the dehydrogenation kinetics. This study provides a new approach for improving the hydrogen storage properties of the amide–hydride systems.     

Catalytic effects of V and V2O5 on hydrogen storage property of Mg17Al12 alloy

A Mg₁₇Al₁₂ alloy was synthesized via sintering, and the catalytic effects of V and V₂O₅ on the hydrogen (H₂)-storage properties of this alloy were investigated. The results revealed that the hydrogenation/dehydrogenation temperature of Mg₁₇Al₁₂ decreased markedly and the reversible hydrogen storage properties improved with the addition of V or V₂O₅. For example, at 250 °C, the Mg₁₇Al₁₂ alloy underwent hydrogenation only and a hydrogen absorption capacity of 2.22 wt.% was realized. However, with the addition of V and V₂O₅, (i) reversible hydrogen absorption/desorption occurred, (ii) the hydrogen absorption capacity increased to 2.95 wt.% and 3.35 wt.%, and (iii) the hydrogenation/dehydrogenation enthalpy of the Mg₁₇Al₁₂alloy decreased from 65.7/83.1 kJ·mol^?1 to 62.6/69.3 kJ·mol^?1 and 59.9/68.1 kJ·mol^?1, respectively.     

Low temperature milling of the LiNH2 + LiH hydrogen storage system

Ball milling of the LiNH₂ + LiH storage system was performed at 20 °C, −40 °C, and −196 °C, and the resulting powders were analyzed using X-ray diffraction, scanning electron microscopy, nuclear magnetic resonance (NMR), specific surface area analysis, and kinetics cycling measurements. Ball milling at −40 °C showed no appreciable deviations from the 20 °C sample, but the −196 °C powder exhibited a significant increase in the hydrogen desorption kinetics. NMR analysis indicates that a possible explanation for the kinetics increase is the retention of internal defects generated during the milling process that are annealed at the collision site at higher milling temperatures.     

Hydrogen storage properties of Mg2Ni affected by Cr catalyst

The effect of Cr as a catalyst in different proportions was investigated to monitor the hydrogen storage properties of Mg₂Ni including their thermodynamic aspects. The P–C–T isotherms for absorption/desorption were measured at 225 °C, 250 °C, 275 °C and 300 °C temperatures. A significant increment in the plateau pressures at different temperature was observed, which shows the positive impact of Cr content in the formation of less stable hydrides. The active sites produced by the ball milling may be the reason for the formation of less stable hydrides. Decrements in the storage capacity with the Cr content were attributed to the formation of MgNi₂ phase which does not absorb hydrogen at the employed temperature-pressure conditions. XRD and SEM technique were used to identify the structural and morphological changes induced by the hydrogenation cycles.     

Catalytic mechanism of Nb2O5 and NbF5 on the dehydriding property of Mg95Ni5 prepared by hydriding combustion synthesis and mechanical milling

The catalytic mechanism of Nb₂O₅ and NbF₅ on the dehydriding property of Mg₉₅Ni₅ prepared by hydriding combustion synthesis and mechanical milling (HCS + MM) was studied. It was shown that NbF₅ was more efficient than Nb₂O₅ in improving the dehydriding property. In particular, the dehydriding temperature onset decreases from 460 K for Mg₉₅Ni₅ to 450 K for Mg₉₅Ni₅with 2.0 at.% Nb₂O₅, whereas it decreases to 410 K for that with 2.0 at.% NbF₅. By means of X-ray diffraction and X-ray photoelectron spectroscopy, it was confirmed that the interaction between the Nb ions and the H atoms and that between the anions (O²⁻ or F⁻) and Mg²⁺ existed in Mg₉₅Ni₅ doped with Nb₂O₅ or NbF_5. Further, the pressure–concentration-isotherms analysis clarified that these interactions destabilized the Mg–H bonding, and that NbF₅ had a better effect on the destabilization of the Mg–H bonding than Nb₂O₅ contributing to the better dehydriding property of (Mg₉₅Ni₅)2.0−NbF₅.     

Dynamic synthesis of ternary Mg2FeH6

This work presents new results on the dynamic synthesis and decomposition of ternary Mg₂FeH₆. A novel synthesis method was applied for the rapid and effective synthesis of a ternary Mg–Fe hydride. This method consists of two processing routes. The first route involves high-energy ball milling of the initial MgH₂–Fe powder mixture, while the second is composed of a unique pressurizing and heating cycle route to obtain a full phase transformation within half an hour. The structural investigations carried out by X-ray diffraction revealed that almost all of the initial powder mixture transforms into the ternary hydride. Furthermore, the sample, which was synthesized, was also decomposed and reloaded with hydrogen. The formation of Mg₂FeH₆ consists of two steps that involve MgH₂ as an intermediate compound. In contrast, the decomposition of Mg₂FeH₆ consists of only one step and does not follow the inverse route. Some traces of iron were found in the reaction products. TDP results show that a desorption peak occurs at 315 °C, and this is in good agreement with DSC measurements showing only a single endothermic peak around 340 °C. Microstructural examinations revealed that the synthesized Mg₂FeH₆ powder generally exhibits a duplex structure that consists of plate-like particles larger than 1 μm in diameter and spherical particles smaller than 50 nm that show a tendency to agglomerate and form larger particles exhibiting a sponge-like structure. The formation of Mg₂FeH₆ takes place at the phase boundary between Fe seeds and the growing hydride phase. In contrast, the decomposition of the Mg₂FeH₆ phase takes place with the formation of the separate nanosized Mg and Fe phases. The dehydrogenated powder sample shows oval Fe precipitates of 10–100 nm in size that are embedded in the Mg-based matrix.     

Structure, morphology and hydrogen storage properties of a Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy

The Ti_0.97Zr_0.019V_0.439Fe_0.097Cr_0.045Al_0.026Mn_1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H₂, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H₂ and −99.8 ± 0.2 J/mol H₂/K for absorption and 25.4 ± 0.1 kJ/mol H₂ and 108.5 ± 0.2 J/mol H₂/K for desorption, indicating the presence of a significant hysteresis effect.