Hydrogen storage properties and microstructure of melt-spun Mg90Ni8RE2 (RE = Y, Nd, Gd)

For hydrogen storage applications a nanocrystalline Mg₉₀Ni₈RE₂ alloy (RE = Y, Nd, Gd) was produced by melt spinning. The microstructure in the as-cast, melt-spun and hydrogenated state was characterized by X-ray diffraction and electron microscopy. Its activation, hydrogenation/dehydrogenation properties and cycle stability were examined by thermogravimetry in the temperature range from 50 °C to 385 °C and pressures up to 30 bar H₂. It was found that the activated alloy can reach a reversible gravimetric hydrogen storage density of up to 5.6 wt.%-H. Furthermore, the reversible gravimetric hydrogen storage density increases with the number of hydrogenation/dehydrogenation cycles, while the dehydrogenation rate remained unchanged. This observation was attributed to the increase of the specific surface area of the ribbon due to cracking during repeated cycling. However, the microstructure of the hydrogenated alloy remained nanocrystalline throughout cycling.     

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.     

Microstructure and hydrogen storage properties of melt-spun Mg-Cu-Ni-Y alloys

Microstructural and hydrogen storage properties of three nanocrystalline melt-spun Mg-base alloys (Mg₉₀Cu_2.5Ni_2.5Y₅, Mg₈₅Cu₅Ni₅Y₅ and Mg₈₀Cu₅Ni₅Y₁₀) have been investigated in view of their application as reversible hydrogen storage materials. The activation procedure and the hydrogen sorption kinetics of these alloys were studied by thermogravimetry at different temperatures in the range from 100 °C to 380 °C. It has been found that these alloys can reach reversible gravimetric hydrogen storage densities of up to 4.8 wt.%-H₂. Even at a low temperature of 100 °C, the hydrogenation kinetics of the investigated alloys is rather high in the range of 1.5 wt.%-H₂ per hour. In the hydrogenated state, these alloys consist of MgH₂, high temperature Mg₂NiH₄, Mg₂NiH_0.3, YH₂, YH₃ as well as MgCu₂. The presence of MgCu₂ indicates the reaction of Mg₂Cu with hydrogen. After repeated hydrogenation/dehydrogenation the preservation of a nanocrystalline grain structure has been confirmed by scanning electron microscopy, energy-filtered and conventional transmission electron microscopy. Additionally, the distribution of hydrogen in the hydrogenated sample was mapped by means of electron energy loss spectroscopy.     

Structural and hydrogen storage properties of melt-spun Mg–Ni–Y alloys

Nanocrystalline magnesium-rich Mg–Ni–Y alloys were produced by melt-spinning. They were characterized regarding their microstructure, crystallization behaviour, and cyclic hydrogenation/dehydrogenation properties in view of their application as reversible hydrogen storage materials. Transmission electron microscopy reveals that these alloys consist in the as-spun state of mixtures of nanocrystalline Mg(Ni;Y) grains that are embedded in an amorphous matrix. Differential scanning calorimetry and X-ray diffraction analysis show that these alloys undergo several crystallization steps in the temperature range between 180 and 370 °C. It was found that only a few thermal activation cycles of the as-quenched ribbons are required in order to reach excellent hydrogenation/dehydrogenation properties of these alloys. In thermogravimetric analyses using a magnetic suspension balance it could be shown that these alloys can reach reversible gravimetric hydrogen storage densities of up to 5.3 wt.%-H with hydrogenation and dehydrogenation rates of up to 1 wt.%-H/min even at temperatures of 250 °C. The structure of the alloys remains nanocrystalline even after several hydrogenation/dehydrogenation cycles.     

Improved hydrogen storage kinetics of Mg-based alloys by substituting La with Sm

Element substitution is an effective strategy for improving Mg-based alloys in their hydrogenation/dehydrogenation property. Thereby, in this paper, Sm was selected to partially replace La in a La–Mg-based alloy for improving its hydriding and dehydriding performance. The alloys with the compositions of Mg₈₀Ni₁₀La_10-xSm_x (x = 0–4) were manufactured through vacuum induction melting. Their microstructures and phase compositions were measured by XRD, SEM and HRTEM. The isothermal hydrogen storage property was tested through an automatic Sieverts apparatus. Non-isothermal hydrogen desorption performance was measured through TGA and DSC. Arrhenius and Kissinger methods were adopted to calculate the dehydrogenation activation energy of alloys. The results reveal that all of the experimental alloys can reversibly absorb and release a large amount of H₂ at appropriate temperatures. The substitution of Sm for La ameliorates the hydriding and dehydriding kinetics, but it results in an undesired reduction of hydrogen absorption and desorption capacities. Substituting La by Sm decreases the initial hydrogen release temperature of the hydride visibly. Furthermore, substituting Sm for La engenders the dehydrogenation activation energy decline clearly, which is considered as the main reason for the improved hydrogen desorption kinetics resulted from Sm replacing La.     

Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1−xCexNi2.8Co0.5 (x = 0-0.20) electrode alloys

The microstructure and electrochemical hydrogen storage characteristics of (La_0.7Mg_0.3)_1−xCe_xNi_2.8Co_0.5 (x = 0, 0.05, 0.10, 0.15 and 0.20) alloys have been investigated. The results show that all alloys consist of (La, Mg)Ni₃ and LaNi₅ phases. The cyclic stability (S₁₀₀) of the alloy electrodes increases from 58.7% (x = 0) to 69.8% (x = 0.20) after 100 charge/discharge cycles. The high rate dischargeability (HRD) increases from 66.8% (x = 0) to 69.6% (x = 0.10), then decreases to 65.1% (x = 0.20) at the discharge current density of 1200 mA/g. Moreover, the electrochemical kinetic characteristics of the alloy electrodes are also improved by increasing Ce content.     

Structural characteristics and hydrogen storage properties of Ca3.0−xMgxNi9 (x = 0.5, 1.0, 1.5 and 2.0) alloys

The effect of Mg content on the structural characteristics and hydrogen storage properties of the Ca_3.0−xMg_xNi₉ (x = 0.5, 1.0, 1.5 and 2.0) alloys was investigated. The lattice parameters and unit cell volume of the PuNi₃-type (Ca, Mg)Ni₃ main phase decreased with increasing Mg content. The 6c site of PuNi₃-type structure was occupied by both Ca and Mg atoms. Moreover, the occupation factor of Ca on the 6c site decreased with the increase of Mg content. The hydrogen absorption capacity of the alloys decreased due to higher Mg content. However, the thermodynamic properties of hydrogen absorption and desorption were improved and the plateau pressures were increased. When x = 1.5–2.0, the Ca_3.0−xMg_xNi₉ alloys had favorable enthalpy (ΔH) and entropy (ΔS) of hydride formation.     

Thermodynamical properties of La–Ni–T (T = Mg, Bi and Sb) hydrogen storage systems

The hydrogen absorption properties of LaNi_4.8T_0.2 (T = Mg, Bi and Sb) alloys are reported. The effects of the substitution of Ni in the LaNi₅ compound with Mg, Bi and Sb are investigated. The ability of alloys to absorb hydrogen is characterized by the pressure–composition (p–c) isotherms. The p–c isotherms allow the determining thermodynamic parameters enthalpy (ΔH^des) and entropy (ΔS^des) of the dehydrogenation processes. The calculated ΔH^des and ΔS^des data helps to explain the decrease of hydrogen equilibrium pressure in alloys doped with Al, Mg and Bi and its increase in the Sb-doped LaNi₅ compound. Generally, partial substitution of Ni in LaNi₅ compound with Mg, Bi and Sb cause insignificant changes of hydrogen storage capacity compared to the hydrogen content in the initial LaNi₅H₆ hydride phase. However, it is worth to stress that, in the case of LaNi_4.8Bi_0.2, a small increase of H/f.u. up to 6.8 is observed. The obtained results in these investigations indicate that the LaNi_4.8T_0.2 (T = Al, Mg and Bi) alloys can be very attractive materials dedicated for negative electrodes in Ni/MH batteries.     

Influence of annealing treatment on the hydrogen storage properties of La2−xTixMgNi9 (x = 0.2, 0.3) alloys

La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La_2−xTi_xMgNi₉ (x = 0.2, 0.3) alloys, LaNi₅, LaMg₂Ni₉ and LaNi₃ are the main phases and a Ti₂Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La_1.8Ti_0.2MgNi₉ alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.     

Effects of REF3 (RE = Y,La, Ce) additives on dehydrogenation properties of LiAlH4

The catalytic effects of rare earth fluoride REF₃ (RE = Y, La, Ce) additives on the dehydrogenation properties of LiAlH₄ were carefully investigated in the present work. The results showed that the dehydrogenation behaviors of LiAlH₄ were significantly altered by the addition of 5 mol% REF₃ through ball milling. The destabilization ability of these catalysts on LiAlH₄ has the order: CeF₃>LaF₃>YF₃. For instance, the temperature programmed desorption (TPD) analyses showed that the onset dehydrogenation temperature of CeF₃ doped LiAlH₄ was sharply reduced by 90 °C compared to that of pristine LiAlH₄. Based on differential scanning calorimetry (DSC) analyses, the dehydriding activation energies of the CeF₃ doped LiAlH₄ sample were 40.9 kJ/mol H₂ and 77.2 kJ/mol H₂ for the first and second dehydrogenation stages, respectively, which decreased about 40.0 kJ/mol H₂ and 60.3 kJ/mol H₂ compared with those of pure LiAlH₄. In addition, the sample doped with CeF₃ showed the fastest dehydrogenation rate among the REF₃ doped LiAlH₄ samples at both 125 °C and 150 °C during the isothermal desorption. The phase changes in REF₃ doped LiAlH₄ samples during ball milling and dehydrogenation were examined using X-ray diffraction and the mechanisms related to the catalytic effects of REF₃ were proposed.     

Enhanced hydrogen storage properties of 4MgH2 + LiAlH4 composite system by doping with Fe2O3 nanopowder

In this paper, the hydrogen storage properties and reaction mechanism of the 4MgH₂ + LiAlH₄ composite system with the addition of Fe₂O₃ nanopowder were investigated. Temperature-programmed-desorption results show that the addition of 5 wt.% Fe₂O₃ to the 4MgH₂ + LiAlH₄ composite system improves the onset desorption temperature to 95 °C and 270 °C for the first two dehydrogenation stage, which is lower 40 °C and 10 °C than the undoped composite. The dehydrogenation and rehydrogenation kinetics of 5 wt.% Fe₂O₃-doped 4MgH₂ + LiAlH₄ composite were also improved significantly as compared to the undoped composite. Differential scanning calorimetry measurements indicate that the enthalpy change in the 4MgH₂–LiAlH₄ composite system was unaffected by the addition of Fe₂O₃ nanopowder. The Kissinger analysis demonstrated that the apparent activation energy of the 4MgH₂ + LiAlH₄ composite (125.6 kJ/mol) was reduced to 117.1 kJ/mol after doping with 5 wt.% Fe₂O₃. Meanwhile, the X-ray diffraction analysis shows the formation of a new phase of Li₂Fe₃O₄ in the doped composite after the dehydrogenation and rehydrogenation process. It is believed that Li₂Fe₃O₄ acts as an actual catalyst in the 4MgH₂ + LiAlH₄ + 5 wt.% Fe₂O₃ composite which may promote the interaction of MgH₂ and LiAlH₄ and thus accelerate the hydrogen sorption performance of the MgH₂ + LiAlH₄ composite system.     

Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg₃MNi₂ (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg₃MNi₂ (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔH_form show that the stabilities of cubic Mg₃MNi₂ (M = Al, Ti, Mn) compound, compared with hexagonal Mg₂Ni, increase in the order of Mg₃MnNi₂, Mg₂Ni, Mg₃TiNi₂ and Mg₃AlNi₂, whereas the stabilities of their saturated Mg₃MNi₂H₃ (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg₂NiH₄, decrease in the order of Mg₂NiH₄, Mg₃AlNi₂H₃, Mg₃TiNi₂H₃ and Mg₃MnNi₂H₃. Further calculations of hydrogen desorption enthalpy ΔH_des indicate that these cubic Mg₃MNi₂ (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔH_des values. Among of them, the dehydrogenation ability of Mg₃TiNi₂ is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg₃MNi₂ (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.     

Comparisons between MgH2- and LiH-containing systems for hydrogen storage applications

The present study compares the dehydrogenation kinetics of (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ systems and their vulnerabilities to the NH₃ emission problem. The (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ and (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixtures with different degrees of mechanical activation are investigated in order to evaluate the effect of mechanical activation on the dehydrogenation kinetics and NH₃ emission rate. The activation energy for dehydrogenation, the phase changes at different stages of dehydrogenation, and the level of NH₃ emission during the dehydrogenation process are studied. It is found that the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ mixture has a higher rate for hydrogen release, slower rate for approaching a certain percentage of its equilibrium pressure, higher activation energy, and more NH₃ emission than the (_LiNH2+LiH)$({\mathrm{LiNH}}_2+\mathrm{LiH})$ mixture. On the basis of the phenomena observed, the reaction mechanism for the dehydrogenation of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system has been proposed for the first time. Approaches for further improving the hydrogen storage behavior of the (2_LiNH2+_MgH2)$(2{\mathrm{LiNH}}_2+{\mathrm{MgH}}_2)$ system are discussed in light of the newly proposed reaction mechanism.     

Effects of TiF3 addition on the hydrogen storage properties of 4MgH2 + Cd composite

In this paper, the best performance of the MgH₂ destabilized system with different ratios of Cd (1:1, 2:1, 3:1 and 4:1) have been studied for the first time. Remarkable enhancements on the onset dehydrogenation temperature, as well as the isothermal de/rehydrogenation kinetics were shown by the 4MgH₂ + Cd composite. In order to improve the hydrogen storage properties of the 4MgH₂ + Cd, TiF₃ was added and its catalytic effects were investigated. Temperature programmed dehydrogenation result had revealed that the onset dehydrogenation temperature was improved once the 10 wt% TiF₃ was incorporated into the 4MgH₂ + Cd system. The absorption and desorption kinetics were also improved compared to the un-doped 4MgH₂ + Cd composite system. The scanning electron microscope result had displayed that the 4MgH₂ + Cd + 10 wt% TiF₃ had the smallest particle size compared to the pure and the ball-milled MgH₂, as well as the 4MgH₂ + Cd composite system. The X-ray diffraction results had demonstrated the formation of an intermediate compound, Mg₃Cd, which was formed during the heating process. For the TiF₃-doped sample, it is reasonable to conclude that the in-situ formed TiH₂ and F-containing species play a synergetic role to encourage interactions between the MgH₂ and the Cd and thus further ameliorate the performances of the hydrogen storage of 4MgH₂ + Cd composite system.     

Investigation on hydrogen storage properties of as-cast,extruded and swaged Mg–Y–Zn alloys

In this work, three different states of Mg-9.1Y-1.8Zn alloys including as-cast, extruded and swaged were prepared by semi-continuous casting, extrusion and swaging processes, respectively. Their compositions, microstructures and hydrogen storage properties were investigated. The results show that Mg-9.1Y-1.8Zn alloys in three different states are all composed of Mg and long-period stacking ordered (LPSO) phases. The LPSO phases occurs to break and decompose after hydrogenation and in-situ forms the YH_{χ(χ = 2,3)} nano-hydrides. The nano-hydrides can be used as in-situ catalysts to improve the hydrogen storage properties of alloys. Meanwhile, many nanocrystalline grains appear in the core of alloy after swaging, and the average grain size ranges from 80 to 200 nm. The presence of nanocrystals may increase the specific surface area of alloy, facilitating the diffusion and absorption of hydrogen. Comparatively, the swaged alloy exhibits the largest hydrogen storage capacity and excellent hydrogen sorption kinetics relative to other states of alloys.     

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.     

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.     

Dehydrogenation and hydrogenation characteristics of MgH2 with transition metal fluorides

The effect of transition metal fluorides on the dehydrogenation and hydrogenation of MgH₂ has been investigated. Many of the fluorides show a considerable catalytic effect on both the dehydrogenation temperature and hydrogenation kinetics of MgH₂. Among them, NbF₅ and TiF₃ most significantly enhance the hydrogenation kinetics of MgH₂. It is suggested that hydride phases formed by the reaction between MgH₂ and these transition metal fluorides during milling and/or hydrogenation play a key role in improving the hydrogenation kinetics of MgH₂.     

Effect of NaH/MgB2 ratio on the hydrogen absorption kinetics of the system NaH + MgB2

In this work the effect of the ratio of starting reactants on the hydrogen absorption reaction of the system xNaH + MgB₂ is investigated. At a constant hydrogen pressure of 50 bar, depending on the amount of NaH present in the system NaH + MgB₂, different hydrogen absorption behaviors are observed. For two system compositions: NaH + MgB₂ and 0.5NaH + MgB₂, the formation of NaBH₄ and MgH₂ as only crystalline hydrogenation products is achieved. The relation between the ratio of the starting reactants and the obtained hydrogenation products is discussed in detail.     

Electrochemical hydrogen storage properties of Mg2−xAlxNi (x = 0, 0.3, 0.5, 0.7) prepared by hydriding combustion synthesis and mechanical milling

Mg_2−xAl_xNi (x = 0, 0.3, 0.5, 0.7) hydrogen storage alloys used as the negative electrode in a nickel–metal hydride (Ni–MH) battery were successfully prepared by means of hydriding combustion synthesis (HCS) and the selected alloy Mg_1.5Al_0.5Ni was further modified by mechanical milling (MM). The structural and electrochemical hydrogen storage properties of Mg_2−xAl_xNi alloys have been investigated in detail. XRD results show that a new phase Mg₃AlNi₂ that possesses an excellent cycling stability is observed with the substitution of Al for Mg. A short-time mechanical milling has a significant effect on improving the discharge capacity of the HCS product of Mg_1.5Al_0.5Ni. The maximum discharge capacity of Mg_1.5Al_0.5Ni ascends with increasing mechanical milling time and reaches the maximum 245.5 mAh/g when milled for 10 h. The alloy milled for 5 h shows the best electrochemical kinetics, which is due to its smaller mean particle size and uniform distribution of the particles. Further increasing in mechanical milling time could not bring about better electrochemical kinetics, which might be attributed to the agglomeration of the alloy particles and thus the charge-transfer reaction and hydrogen diffusion are restrained. It is suggested that the novel method of HCS + MM is promising to prepare ternary Mg-based intermetallic compound for electrochemical hydrogen storage.