Characterization of microstructure,hydrogen storage kinetics and thermodynamics of ball-milled Mg90Y1.5Ce1.5Ni7 alloy

In this work, the Mg₉₀Y_1.5Ce_1.5Ni₇ sample is successfully prepared by combining the vacuum induction melting and the mechanical milling. The phase composition and microstructure characteristics are studied by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The hydrogenated sample is composed of MgH₂, Mg₂NiH₄, CeH_2.73 phases, whereas only the MgH₂ and Mg₂NiH₄ phases are decomposed during dehydrogenation. The hydrogen storage properties of Mg₉₀Y_1.5Ce_1.5Ni₇ samples are measured by semi-automatic Sievert type apparatus. It is found that the samples could be fully activated within three cycles of absorption and dehydrogenation, with a reversible hydrogen storage capacity of about 5.6 wt%. Also, the “optimal hydrogenation temperature” is reduced to 200 °C, and the dehydrogenation activation energy is calculated to be 68.2 kJ/mol and 65.8 kJ/mol by using the Arrhenius and Kissinger equations, respectively. This work provides a scientific approach to promote the practical application of Mg-based alloy.     

Dual-tuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation

Mg₂In_0.1Ni solid solution with an Mg₂Ni-type structure has been synthesized and its hydrogen storage properties have been investigated. The results showed that the introduction of In into Mg₂Ni not only significantly improved the dehydrogenation kinetics but also greatly lowered the thermodynamic stability. The dehydrogenation activation energy (E_a) and enthalpy change (ΔH) decreased from 80 kJ/mol and 64.5 kJ/mol H₂ to 28.9 kJ/mol and 38.4 kJ/mol H₂, respectively. The obtained results point to a method for improving not only the thermodynamic but also the kinetic properties of hydrogen storage materials.     

Preparation and hydrogen storage property of Mg-based hydrogen storage composite embedded by polymethyl methacrylate

A novel embedded Mg-based hydrogen storage nanocomposite was prepared by mechanical milling of hydriding combustion synthesized (HCS) Mg-based hydride and hydrogen permissive/oxygen prohibitive polymer. The Mg-based hydride was mechanically milled with tetrahydrofuran solution of polymethyl methacrylate (PMMA) under argon atmosphere. It is determined by X-ray diffraction (XRD) analysis that the average grain size of all the milled nanocomposites become smaller and the nanocomposites exhibit a good air-stable property. The microstructures of the nanocomposites obtained by Field emission scanning electron microscopy (FESEM) and High-resolution transmission electron microscopy (HRTEM) analyses show that Mg₉₅Ni₅ particles embedded by PMMA have a diameter of smaller than 100 nm, approximately. The nanocomposites show the optimal hydriding/dehydriding properties, requiring 60 min to absorb 3.37 wt.% hydrogen at low temperature of 473 K, and desorbing as high as 1.02 wt.% hydrogen within 120 min at the same temperature. The onset dehydriding temperature of the composites is about 373 K, which is 150 K lower than that of HCS products Mg₉₅Ni₅.     

Effects of annealing temperature on the structure and properties of the LaY2Ni10Mn0.5 hydrogen storage alloy

The effects of annealing at 1123, 1148, 1173 and 1198 K for 16 h on the structure and properties of the LaY₂Ni₁₀Mn_0.5 hydrogen storage alloy as the active material of the negative electrode in nickel–metal hydride (Ni–MH) batteries were systematically investigated by X-ray diffraction (XRD), scanning electron microscopy linked with an energy dispersive X-ray spectrometer (SEM–EDS), pressure-composition isotherms (PCI) and electrochemical measurements. The quenched and annealed LaY₂Ni₁₀Mn_0.5 alloys primarily consist of Ce₂Ni₇- (2H) and Gd₂Co₇-type (3R) phases. The homogeneity of the composition and plateau characteristics of the annealed alloys are significantly improved, and the lattice strain is effectively reduced. The alloys annealed at 1148 K and 1173 K have distinctly greater hydrogen storage amounts, 1.49 wt% (corresponding to 399 mAh g^?1 in equivalent electrochemical units) and 1.48 wt%, respectively, than the quenched alloy (1.25 wt%, corresponding to 335 mAh g^?1 in equivalent electrochemical units). The alloys annealed at 1148 K and 1173 K have relatively good activation capabilities. The annealing treatment slightly decreases the discharge potentials of the alloy electrodes but markedly increases their discharge capacity. The maximum discharge capacities of the annealed alloy electrodes (372–391 mAh g^?1) are greater than the extreme capacity of the LaNi₅-type alloy (370 mAh g^?1). The cycling stability of the annealed alloy electrodes was improved.     

The role of spark plasma sintering on the improvement of hydrogen storage properties of Mg-based composites

Spark plasma sintering (SPS) is a newly developed material preparation technology and is very suitable for the multi-component and/or dissimilar materials preparation. In this paper, Mg–V_77.8Zr_7.4Ti_7.4Ni_7.4, Mg–V₃₈Zr₂₅Ti₁₅Ni₂₂ and Mg–ZrMn₂ composites were synthesized by SPS method and their hydrogen storage properties were evaluated. The results showed that with the addition of the second alloys, the hydrogen desorption temperature of pure Mg decreased apparently, with the reversible hydrogen storage capacity increased from nearly 0 of pure Mg to near 95% of its total absorption at 573 K. The hydrogen ab/desorption kinetics were also greatly improved, with the hydrogen absorption mechanism changed from surface reaction of pure Mg to three-dimension diffusion of the composite. TEM observation indicated that a thin transition zone of nanocrystalline Mg was produced at the sintering interface during SPS, which may be responsible for the improvement of hydrogen storage properties of these Mg-based composites.     

Electrochemical properties of titanium-based hydrogen storage alloy prepared by solid phase sintering

The structure and electrochemical properties of titanium-based hydrogen storage alloy prepared by solid phase sintering at 1123 K were investigated. The result of X-ray diffraction (XRD) showed that the sintered alloy mainly consists of Ti₂Ni phase coexisting with TiNi, TiNi₃ and Ni phases. The alloy had a maximum discharge capacity of 205 mAh/g at a discharge current density of 60 mA/g and showed a discharge capacity of 146 mAh/g at 150 mA/g. The results of linear polarization (LP) and potential-step measurement presented that the exchange current density and hydrogen diffusion efficient of the alloy were 100 mA/g and 4.2 × 10⁻⁹ cm²/s, respectively. The electrochemical performance of the alloy could be effectively improved by using solid phase sintering.     

Study on the eutectic formation and its correlation with the hydrogen storage properties of Mg98Ni2-xLax alloys

Transition metals and rare-earth elements have excellent catalytic effects on improving the de-/hydrogenation properties of Mg-based alloys. In this study, a small amount of La is used to substitute the Ni in Mg₉₈Ni₂ alloy, and some Mg₉₈Ni_2-xLa_x (x = 0, 0.33, 0.67, and 1) alloys show the better overall hydrogen storage properties. The effects of La on the solidification and de-/hydrogenation behaviors of the alloys are revealed. The results indicate that different factors dominate the processes of hydrogen absorption and desorption. The Mg₉₈Ni_1·67La_0.33 alloy absorb 7.04 wt % hydrogen at 300 °C, with the highest isothermal absorption rate, the Mg₉₈Ni_1·33La_0.67 hydride show the highest isothermal desorption rates and the lowest peak desorption temperature of 327 °C. The La addition can increase the driving force of hydrogenation, thus the hydrogenation rates and capacities of the Mg₉₈Ni_1·67La_0.33 and Mg₉₈Ni_1·33La_0.67 alloys are improved. The formation of refined eutectic structures is a key factor that facilitates the desorption processes of the Mg₉₈Ni_2-xLa_x hydrides with x = 0.67 and 1. High-density LaH₃ nanophses are in-situ formed from the LaMg_x (8.5 < x < 12) phase, which results in the improved de-/hydrogenation properties. The further La addition deteriorates the hydrogen storage properties of Mg₉₈Ni_2-xLa_x alloy.     

Scaled-up production of a promising Mg-based hydride for hydrogen storage

The feasibility of scaling up the production of a Mg-based hydride as material for solid state hydrogen storage is demonstrated in the present work. Magnesium hydride, added with a Zr–Ni alloy as catalyst, was treated in an attritor-type ball mill, suitable to process a quantity of 0.5–1 kg of material. SEM–EDS examination showed that after milling the catalyst was well distributed among the magnesium hydride crystallites. Thermodynamic and kinetic properties determined by a Sievert's type apparatus showed that the semi-industrial product kept the main properties of the material prepared at the laboratory scale. The maximum amount of stored hydrogen reached values between 5.3 and 5.6 wt% and the hydriding and dehydriding times were of the order of few minutes at about 300 °C.     

Effects of surface coating with Cu-Pd on electrochemical properties of A2B7- type hydrogen storage alloy

La_0.75Mg_0.25Ni_3.2Co_0.2Al_0.1 hydrogen storage alloy, the nickel-metal hydride (MH/Ni) secondary battery negative electrode, was modified by CuSO₄ solution (3 wt% in Cu in contrast with alloy weight) and PdCl₂ solution varied from 1 wt% to 4 wt% in Pd in contrast with alloy weight with a simplified pollution-free replacement plating method, aiming at improving its comprehensive electrochemical properties. The XRD analysis and SEM images combined with EDS results reveal that Cu and Pd nanoparticles are uniformly plated on the pristine alloy surface. The relative amount of Pd on the Cu-Pd coated alloy surface increases notably as the PdCl₂ concentration increases in the plating solution. Electrochemical tests indicate that alloy electrodes modified by Cu-Pd composite coating show perfect activation performance, which achieve the maximum discharge capacity at the first charge-discharge cycle. Moreover, alloy electrodes coated with Cu-Pd perform dramatically enhanced high rate dischargeability (HRD). The enhancement increases firstly and then decreases as the content of Pd increases in the Cu-Pd coating. Meanwhile, the cycle life of modified alloys is also improved significantly. Among all the samples, the Cu-Pd coated alloy with 3 wt% Pd content in the PdCl₂ solution reinforces the comprehensive electrochemical properties most sufficiently, with dischargeability of 86.4% under 1500 mA/g and remaining capacity of 82.7% after 100 cycles.     

Improved hydrogen storage properties of Mg-based nanocomposite by addition of LaNi5 nanoparticles

Mg (200 nm) and LaNi₅ (25 nm) nanoparticles were produced by the hydrogen plasma-metal reaction (HPMR) method, respectively. Mg–5 wt.% LaNi₅ nanocomposite was prepared by mixing these nanoparticles ultrasonically. During the hydrogenation/dehydrogenation cycle, Mg–LaNi₅ transformed into Mg–Mg₂Ni–LaH₃ nanocomposite. Mg particles broke into smaller particles of about 80 nm due to the formation of Mg₂Ni. The nanocomposite showed superior hydrogen sorption kinetics. It could absorb 3.5 wt.% H₂ in less than 5 min at 473 K, and the storage capacity was as high as 6.7 wt.% at 673 K. The nanocomposite could release 5.8 wt.% H₂ in less than 10 min at 623 K and 3.0 wt.% H₂ in 16 min at 573 K. The apparent activation energy for hydrogenation was calculated to be 26.3 kJ mol⁻¹. The high sorption kinetics was explained by the nanostructure, catalysis of Mg₂Ni and LaH₃ nanoparticles, and the size reduction effect of Mg₂Ni formation.     

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.     

Improved hydrogen storage properties of MgH2 using transition metal sulfides as catalyst

In this study, some transition metal sulfides (TiS₂, NbS₂, MoS₂, MnS, CoS₂ and CuS) are used as catalyst to enhance the hydrogen storage behaviors of MgH₂. The MgH₂-sulfide composites with different sulfides addition are prepared by ball-milling. The phase composition and hydrogen storage properties are studied in detail. The results confirm that all these sulfides can significantly increase the hydrogen desorption and absorption kinetics of MgH₂. The MgH₂–TiS₂ has the best hydrogenation and dehydrogenation kinetics, followed by the MgH₂–NbS₂, MgH₂–MnS, MgH₂–MoS₂, MgH₂–CoS₂, MgH₂–CuS and MgH₂. Also, the onset dehydrogenation temperature of the MgH₂–TiS₂ is about 204 °C, which is lower about 126 °C than that of the MgH₂. The dehydrogenation activation energy can be reduced to 50.8 kJ mol^?1 when doping TiS₂ in MgH₂. The beneficial catalytic effects of the sulfides can be ascribed to the in-situ formation of MgS, TiH₂, NbH, Mo, Mn, Mg₂CoH₅ and MgCu₂ phases.     

NiB nanoparticles: A new nickel-based catalyst for hydrogen storage properties of MgH2

In this paper, amorphous NiB nanoparticles were fabricated by chemical reduction method and the effect of NiB nanoparticles on hydrogen desorption properties of MgH₂ was investigated. Measurements using temperature-programmed desorption system (TPD) and volumetric pressure–composition isotherm (PCI) revealed that both the desorption temperature and desorption kinetics have been improved by adding 10 wt% amorphous NiB. For example, the MgH₂–10 wt%NiB mixture started to release hydrogen at 180 °C, whereas it had to heat up to 300 °C to release hydrogen for the pure MgH₂. In addition, a hydrogen desorption capacity of 6.0wt% was reached within 10 min at 300 °C for the MgH₂–10 wt%NiB mixture, in contrast, even after 120 min only 2.0 wt% hydrogen was desorbed for pure MgH₂ under the same conditions. An activation energy of 59.7 kJ/mol for the MgH₂/NiB composite has been obtained from the desorption data, which exhibits an enhanced kinetics possibly due to the additives reduced the barrier and lowered the driving forces for nucleation. Further cyclic kinetics investigation using high-pressure differential scanning calorimetry technique (HP-DSC) indicated that the composite had good cycle stability.     

Kinetics of dehydrogenation of the Mg-Ti-H hydrogen storage system

Among the proposed hydrogen storage systems, magnesium alloys have proved to be promising since they are rechargeable with high hydrogen capacities (theoretically up to 7.6 wt.%), reversibility and low costs. Small particle size, which can be achieved by milling, and small amounts of transition-metal compounds as catalysts result in increased hydrogen release/uptake kinetics. In this work, we developed the rate expression for the dehydrogenation of milled 7MgH₂/TiH₂, 10MgH₂/TiH₂, and MgH₂ samples. The complete rate expressions, together with the values of activation energy and other rate parameters, were determined for the three milled samples by analyzing data obtained from non-isothermal thermogravimetric analysis (TGA). The MgH₂ doped with TiH₂ by high-energy milling displayed substantially reduced apparent activation energy of 107-118 kJ/mol and significantly faster kinetics, compared with 226 kJ/mol for similarly milled MgH₂ without TiH₂ doping.     

Improvement on hydrogen storage thermodynamics and kinetics of the as-milled SmMg11Ni alloy by adding MoS2

In this experiment, the Mg-based hydrogen storage alloys SmMg₁₁Ni and SmMg₁₁Ni + 5 wt.% MoS₂ (named SmMg₁₁Ni-5MoS₂) were prepared by mechanical milling. By comparing the structures and hydrogen storage properties of the two alloys, it could be found that the addition of MoS₂ has brought on a slight change in hydrogen storage thermodynamics, an obvious decrease in hydrogen absorption capacity, an obvious catalytic action on hydrogen desorption reaction, and a lowered onset desorption temperature from 557 to 545 K. Additionally, the addition of MoS₂ could dramatically improve the alloy in its hydrogen absorption and desorption kinetics. To be specific, the hydrogen desorption times of 3 wt.% H₂ at 593, 613, 633 and 653 K were measured to be 1488, 683, 390 and 192 s respectively for the SmMg₁₁Ni alloy, which were reduced to 938, 586, 296 and 140 s for the MoS₂ catalyzed SmMg₁₁Ni alloy at identical conditions. The activation energies of the alloys with and without MoS₂ for hydrogen desorption are 87.89 and 100.31 kJ/mol, respectively. The 12.42 kJ/mol decrease is responsible for the ameliorated hydrogen desorption kinetics by adding catalyst MoS₂.     

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.     

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.     

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.     

Characteristics of A2B7-type LaYNi-based hydrogen storage alloys modified by partially substituting Ni with Mn

LaY₂Ni_10.5?xMn_x (x = 0.0, 0.5, 1.0, 2.0) alloys are prepared by a vacuum induction-quenching process followed by annealing. The structure, as well as the hydriding/dehydriding and charging/discharging characteristics, of the alloys are investigated via X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), pressure-composition isotherms (PCI), and electrochemical measurement. The alloys have multiphase structures mainly composed of Gd₂Co₇-type (3R) and Ce₂Ni₇-type (2H) phases. Partial substitution of Ni by Mn clearly increases the hydrogen storage capacity of the alloys. The x = 0.5 alloy exhibits a maximum hydrogen storage capacity of 1.40 wt % and a discharge capacity of 392.9 mAh g^?1, which are approximately 1.5 and 1.9 times greater than those of the x = 0.0 alloy, respectively. The high-rate dischargeability (HRD) of the x = 0.5 alloy is higher than that of the other alloys because of its large hydrogen diffusion coefficient D, which is a controlling factor in the electrochemical kinetic performance of alloy electrodes at high discharge current densities. Although the cyclic stability of the x = 0.5 alloy is not as high as that of the other alloys, its capacity retention ratio is as high as 56.3% after the 400th cycle. The thermodynamic characteristics of the x = 0.5 alloy satisfy the requirements of the hydride electrode of metal hydride–nickel (MH–Ni) batteries.