Composition dependent microstructure evolution,activation and de-/hydrogenation properties of Mg–Ni–La alloys

In this work, in order to elucidate the effect of different alloying elements on the microstructure, activation and the de-/hydrogenation kinetics performance, the Mg–20La, Mg–20Ni and Mg–10Ni–10La (wt.%) alloys have been prepared by near equilibrium solidification combined with high-energy ball milling treatment to realize the internal optimization as well as particle refinement. The results show that the microstructures of the prepared alloys are significantly refined by forming different types and sizes of intermetallic compounds. Meanwhile, the effects of LaH₃ and Mg₂Ni within the activated samples on de-/hydrogenation kinetics are also discussed. It is observed that the alloy containing LaH₃ preserves stable hydrogenation behavior between 573 and 623 K, while the hydrogenation properties of the alloy containing Mg₂Ni is susceptible to temperature. A preferable hydrogenation performance is observed in Mg–10Ni–10La alloy, which can absorb as high as 5.86 wt% hydrogen within 15 min at 623 K and 3.0 MPa hydrogen pressure. Moreover, the desorption kinetics and the desorption activation energies are evaluated to illustrate the mechanism based on improved dehydrogenation performance. The addition of proper alloying elements Ni and La in combination with reasonable processing is an effective strategy to improve the de-/hydrogenation performance of Mg-based alloys.     

Tunable microstructure,de-/hydrogenation kinetics and thermodynamics performance of Mg–Ni–La–Ti–H systems

In the light of positive effects of rare earth and transition metals on the hydrogen absorption/desorption properties of magnesium, the Mg20La–5TiH₂, Mg20Ni–5TiH₂ and Mg10Ni10La–5TiH₂ composites have been prepared in this work to ameliorate the de-/hydrogenation kinetics and thermodynamic performance. The results indicate that the as-prepared composites are mainly composed of Mg, Mg₂Ni/LaH₃ and TiH₂ phases after activation, and LaH₃ and TiH₂ are stable during de-/hydrogenation cycles. The morphology observations give evidences that LaH₃ with size about ~20 nm and Mg₂Ni with size about ~1 μm are uniformly distributed in the composites. It is noted that the de-/hydriding kinetics of the as-prepared composites are significantly improved after internal and surface modification, of which the Mg10Ni10La–5TiH₂ composite can desorb as high as 5.66 wt% hydrogen within 3 min at 623 K. Moreover, the thermodynamic properties of the experimental composites have also been investigated and discussed according to the pressure-composition isothermal curves and corresponding calculation by Van't Hoff equation. The improved hydrogen storage properties of the as-prepared composites are mainly attributed to the uniformly distributed LaH₃, Mg₂Ni and TiH₂ phases, which provide a large amount of phase boundaries, diffusion paths and nucleation sites for de-/hydrogenation reactions.     

Mechanisms of hydrides’ nucleation and the effect of hydrogen pressure induced driving force on de-/hydrogenation kinetics of Mg-based nanocrystalline alloys

Aiming to gain insight on the hydrogen storage properties of Mg-based alloys, partial hydrogenation and hydrogen pressure related de-/hydrogenation kinetics of Mg–Ni–La alloys have been investigated. The results indicate that the phase boundaries, such as Mg/Mg₂Ni and Mg/Mg₁₇La₂, distributed within the eutectics can act as preferential nucleation sites for β-MgH₂ and apparently promote the hydrogenation process. For bulk alloy, it is observed that the hydrogenation region gradually grows from the fine Mg–Ni–La eutectic to primary Mg region with the extension of reaction time. After high-energy ball milling, the nanocrystalline powders with crystallite size of 12～20 nm exhibit ameliorated hydrogen absorption/desorption performance, which can absorb 2.58 wt% H₂ at 368 K within 50 min and begin to desorb hydrogen from ～508 K. On the other side, variation of hydrogen pressure induced driving force significantly affects the reaction kinetics. As the hydrogenation/dehydrogenation driving forces increase, the hydrogen absorption/desorption kinetics is markedly accelerated. The dehydrogenation mechanisms have also been revealed by fitting different theoretical kinetics models, which demonstrate that the rate-limiting steps change obviously with the variation of driving forces.     

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.     

A comparative study on the microstructure and cycling stability of the amorphous and nanocrystallization Mg60Ni20La10 alloys

Amorphous and nanocrystalline Mg₆₀Ni₂₀La₁₀ alloys were prepared by melt-spun and crystallization of the amorphous alloy respectively. Microstructural evolution of the amorphous and crystallized (CA) alloys during hydrogenation/dehydrogenation cycles was studied and compared in the present work. The CA alloy exhibits homogeneous and fine (<50 nm) multiphase microstructure composed of LaMg₂Ni, Mg₂Ni and LaMgNi₄. The CA alloy has slightly lower hydrogenation ability but far excellent cycling stability compared with the amorphous alloy. The amorphous and CA alloys have identical phase constitution including Mg₂Ni and LaH₃ after cycling. While, microstructures of the two cycled alloys show dramatically distinct characters. Grain size of the cycled CA alloy is almost unchanged compared with the original alloy, which contributes to the better cycling stability. However, grain growth especially coarsening of Mg₂Ni is severe in the cycled amorphous alloy, leading to difficulty to dehydrogenation. The better coarsening resistance of the CA alloy is attributed to the crisscrossed distribution of Mg₂Ni and LaH₃ and well-matched interfacial configuration between Mg₂Ni and LaH₃ that (113)Mg₂Ni ??? ? (111)LaH₃. However, hydrogenation of the amorphous alloy leads to large and inhomogeneous microstructure which is ascribed to the preferential recrystallization and growth of Mg₂Ni, contributing to rapid degradation of the amorphous alloy. The present work illumines a potential way to prepare stable nanocrystalline by introducing secondary phase with interlaced microstructure and well-matched interfacial configuration using nanocrystallization method.     

Hydrogen storage by Mg-based nanocomposites

Hydrogen storage materials research is entered to a new and exciting period with the advance of the nanocrystalline alloys, which show substantially enhanced absorption/desorption kinetics, even at room temperatures. In this work, hydrogen storage capacities and the electrochemical discharge capacities of the Mg₂(Ni, Cu)-, LaNi₅-, ZrV₂-type nanocrystalline alloys and Mg₂Ni/LaNi₅-, Mg₂Ni/ZrV₂-type nanocomposites have been measured. The electronic properties of the Mg₂Ni_1-xCu_x, LaNi₅ and ZrV₂ alloys were calculated. The nanocomposite structure reduced hydriding temperature and enhanced hydrogen storage capacity of Mg-based materials. The nanocomposites (Mg,Mn)₂Ni (50 wt%)-La(Ni,Mn,Al,Co)₅ (50 wt%) and (Mg,Mn)₂Ni (75 wt%)-(Zr,Ti)(V,Cr,Ni)_2.4 (25 wt%) materials releases 1.65 wt% and 1.38 wt% hydrogen at 25 °C, respectively. The strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence hydrogenation properties of Mg-based nanocomposities.     

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.     

Hydrogenation and dehydrogenation behaviours of nanocrystalline Mg20Ni10−xCux (x = 0−4) alloys prepared by melt spinning

In order to improve the hydriding and dehydriding kinetics of the Mg₂Ni-type alloys, Ni in the alloy was partially substituted by element Cu, and the nanocrystalline Mg₂Ni-type Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) alloys were synthesized by melt-spinning technique. The structures of the as-cast and spun alloys were studied by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured using an automatically controlled Sieverts apparatus. The results show that the substitution of Cu for Ni does not change the major phase Mg₂Ni. The hydrogen absorption capacity of the alloys first increases and then decreases with rising Cu content, but the hydrogen desorption capacity of the alloys grows with increasing Cu content. The melt spinning significantly improves the hydrogenation and dehydrogenation capacity and kinetics of the alloys.     

Achieving superior hydrogen storage properties via in-situ formed nanostructures: A high-capacity Mg–Ni alloy with La microalloying

Mg-based hydride is a promising hydrogen storage material, but its capacity is hindered by the kinetic properties. In this study, Mg–Mg₂Ni–LaH_x nanocomposite is formed from the H-induced decomposition of Mg₉₈Ni_1·67La_0.33 alloy. The hydrogen capacity of 7.19 wt % is reached at 325 °C under 3 MPa H₂, attributed to the ultrahigh hydrogenation capacity in Stage I. The hydrogen capacity of 5.59 wt % is achieved at 175 °C under 1 MPa H₂. The apparent activation energies for hydrogen absorption and desorption are calculated as 57.99 and 107.26 kJ/mol, which are owing to the modified microstructure with LaH_x and Mg₂Ni nanophases embedding in eutectic, and tubular nanostructure adjacent to eutectic. The LaH_2.49 nanophase can catalyze H₂ molecules to dissociate and H atoms to permeate due to its stronger affinity with H atoms. The interfaces of these nanophases provide preferential nucleation sites and alleviate the “blocking effect” together with tubular nanostructure by providing H atoms diffusion paths after the impingement of MgH₂ colonies. Therefore, the superior hydrogenation properties are achieved because of the rapid absorption process of Stage I. The efficient synthesis of nano-catalysts and corresponding mechanisms for improving hydrogen storage properties have important reference to related researches.     

Microstructural evolution and hydrogen storage proprieties of melt-spun eutectic Mg76.87Ni12.78Y10.35 alloy with low hydrides formation/decomposition enthalpy

Ternary eutectic Mg_76.87Ni_12.78Y_10.35 (at. %) ribbons with mixed amorphous and nanocrystalline phases were prepared by melt spinning. The microstructures of the melt-spun, hydrogenated and dehydrogenated samples were examined and compared by X-ray diffraction and transmission electron microscopy. The amorphous structure transforms into a thermally stable nanocrystalline structure with a grain size of about 5 nm during hydrogen ab/desorption cycles. The Mg, Mg₂Ni and phases with Y in the melt-spun state transform into MgH₂, Mg₂NiH₄, Mg₂NiH_0.3, YH₂ and YH₃ after hydrogenation, and transform back to Mg, Mg₂Ni and YH₂ upon subsequent dehydrogenation. The reaction enthalpy (ΔH) and entropy (ΔS) of the higher plateau pressure corresponding to Mg₂Ni hydride formation are −53.25 kJ mol⁻¹ and −107.74 J K⁻¹ mol⁻¹, respectively. The amorphous/nanocrystalline structure effectively reduces the enthalpy and entropy of Mg₂Ni hydride formation, but has little effect on Mg. The activation energy for dehydrogenation of the hydrogenated ribbons is 69 kJ mol⁻¹. This suggests that Mg–Ni–Y with ternary eutectic composition can form an amorphous/nanocrystalline structure by melt spinning, and this nanostructure efficiently improves the thermodynamics and kinetics for hydrogen storage.     

Microstructural evolution and hydrogen storage properties of a Ni-modified Mg15Al alloy

On the basis of modification of transition metals on Mg-Al hydrogen storage alloys, Mg15Al5Ni alloy with Ni content of 5 wt% has been prepared by high energy ball mill. The results show that Ni particles uniformly distribute on the surface of particles, while several Ni particles are embedded inside alloy particles. These Ni particles tend to redistribute after hydrogenation. The phase composition analysis reveals the formation of stable Al₃Ni₂ phase in Ni-modified alloy after hydrogenation. The hydrogen absorption performance of Mg15Al5Ni alloy has been improved by introducing Ni, which can absorb 4.36 wt% hydrogen within 5 min at 350 °C. Meanwhile, the activation properties of Mg15Al5Ni alloy can be obviously deteriorated due to the addition of Ni. However, uniformly distributed Al₃Ni₂ nanocrystals with grain sizes around 10 nm hinder grain growth of hydrides, ameliorating hydrogenation kinetics of Mg15Al5Ni alloy. Besides, the modified effect of Ni on hydrogenation kinetics of Mg15Al5Ni alloy has been also discussed in this work.     

Formation of AlNi phase and its influence on hydrogen absorption kinetics of Mg77Ni23-xAlx alloys at intermediate temperatures

In order to reduce the obstacle influence of coarse Mg₂Ni phase on hydrogen absorption kinetics in Mg–Ni alloys, aluminum was doped and Mg₇₇Ni_23-xAl_x (x = 0, 3, 6, 9) alloys were prepared. The results show that AlNi phase was formed when Al was added, the size of primary Mg₂Ni phase decreases with increasing Al content till 6 at.%, while primary Mg₂Ni phase was diminished and primary Mg phase was formed when Al content increased to 9 at.%. The initial hydrogenation rates of Mg₇₇Ni_23-xAl_x alloys were increased, which is resulted from the refined primary Mg₂Ni and the catalytic AlNi phase. More importantly, the hydrogenation rates and capacities were significantly improved at 150 °C, especially for the Mg₇₇Ni₁₇Al₆ alloy. The apparent activation energy of the Mg₇₇Ni₁₇Al₆ alloy for hydrogenation was reduced to 73.68 kJ/mol from 102.27 kJ/mol of the Mg₇₇Ni₂₃ alloy. Its enthalpy changes for hydrogenation at low and high platforms are 72.3 kJ/mol and 53.9 kJ/mol, respectively. The multiple channels and short distance for hydrogen atoms diffusion provided by refined primary Mg₂Ni phase, the solid dissolution of Al in Mg₂Ni lattice, and catalytic effect of AlNi on hydrogenation, leading to the improvement of the hydrogen storage properties.     

Hydrogen sorption behaviour of Mg-5wt.%La alloys after the initial hydrogen absorption process

In our earlier study, it has been shown that trace Na additions can improve the reaction kinetics of Mg–5%La (wt.%) alloys during the first absorption. However, the subsequent hydrogen desorption/absorption process of the Mg–5%La after the first absorption has not been investigated. In this study, we have investigated the hydrogen sorption behaviour of the Mg–5%La alloy after the first absorption in terms of phase evolution, and lattice expansion properties during desorption as function of temperature using in-situ synchrotron Powder X-ray Diffraction (PXRD) and in-situ High Voltage Transmission Electron Microscopy (HVTEM). Two distinct phase evolutions, a continuous phase transformation of LaH₃ → LaH₂ + ½ H₂ (from 250 °C) and decomposition of MgH₂ → Mg + H₂ (between 440 and 460 °C) were identified during the desorption. It is determined that this alloy is cyclable in the absence of Mg₁₂La intermetallic during the subsequent absorption/desorption cycling after the first hydrogen absorption.     

Composition dependent hydrogen storage performance and desorption factors of Mg–Ce based alloys

The widespread application of Mg as a hydrogen storage material has been limited by its slow absorption and desorption kinetics at moderate temperatures. Aiming at improving the de-/absorption kinetics of Mg-based alloys by in situ formed catalysts and understanding the desorption factors, Mg–Ce and Mg–Ce–Ni alloys with different Ce contents are prepared. The phase components, microstructure and hydrogen storage properties have been carefully investigated. It is shown that an 18R-type long-period stacking ordered (LPSO) phase is formed in as-melt Mg–Ce–Ni ternary alloy together with random stacking faults. Abundant in situ formed CeH_2.73 particles with particle size less than 100 nm are observed on the matrix after hydrogenation. It is found in isothermal hydrogenation and dehydrogenation kinetic curves that Ni significantly favors desorption process, while Ce is more conducive to absorption. After partial dehydrogenation of Mg–Ce binary alloy, the initial desorption temperature decreases significantly when desorbing again. The primary-formed Mg phase on the surface of MgH₂ accounts for the improved desorption performance.     

Effects of graphite addition and air exposure on ball-milled Mg–Al alloys for hydrogen storage

Magnesium hydride (MgH₂) is a promising candidate as a hydrogen storage material. However, its hydrogenation kinetics and thermodynamic stability still have room for improvement. Alloying Mg with Al has been shown to reduce the heat of hydrogenation and improve air resistance, whereas graphite helps accelerating hydrogenation kinetics in pure Mg. In this study, the effects of simultaneous Al alloying and graphite addition on the kinetics and air-exposure resistance were investigated on the Mg₆₀Al₄₀ system. The alloys were pulverized through high-energy ball milling (hereinafter HEBM). We tested different conditions of milling energy, added graphite contents, and air exposure times. Structural characterization was conducted via X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). H₂ absorption and desorption properties were obtained through volumetry in a Sieverts-type apparatus and Differential Scanning Calorimetry (DSC). The desorption activation energies were calculated using DSC curves through Kissinger analysis. Mg₆₀Al₄₀ with 10 wt% graphite addition showed fast activation kinetics, even after 2 years of air exposure. Graphite addition provided a catalytic effect on ball-milled Mg–Al alloys by improving both absorption and desorption kinetics and lowering the activation energy for desorption from 189 kJ/mol to 134 kJ/mol. The fast kinetics, reduced heat of reaction, and improved air resistance of these materials make them interesting candidates for potential application in hydride-based hydrogen storage tanks.     

Improved hydrogen storage behaviours of nanocrystalline and amorphous Mg2Ni-type alloy by Mn substitution for Ni

The nanocrystalline and amorphous Mg₂Ni-type alloys with nominal compositions of Mg₂Ni_1−xMn_x (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by melt spinning technique. The structures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The hydrogen absorption and desorption kinetics of the alloys were measured by an automatically controlled Sieverts apparatus. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that the as-spun (x = 0) alloy holds a typical nanocrystalline structure, whereas the as-spun (x = 0.4) alloy displays a nanocrystalline and amorphous structure, confirming that the substitution of Mn for Ni facilitates the glass formation in the Mg₂Ni-type alloy. The hydrogen absorption capacity of the alloys first increases then decreases with rising Mn content, but the hydrogen desorption capacity of the alloys grows with increasing Mn content. Furthermore, the substitution of Mn for Ni significantly improves the electrochemical hydrogen storage performances of the alloys, involving both the discharge capacity and the electrochemical cycle stability. With an increase in the amount of Mn from 0 to 0.4, the discharge capacity of as-spun (30 m/s) alloy grows from 116.7 to 311.5 mAh/g, and its capacity retaining rate at 20th charging and discharging cycle rises from 36.7 to 78.7%.     

Nanocrystalline structure and electrochemical hydrogen storage properties of the as-milled Mg–V–Ni–Fe–Zn-based materials

Among the electrode materials for Ni-MH batteries, the Mg alloy electrodes such as MgNi, Mg₂Ni, REMg₁₂, La₂Mg₁₇ are considered the most suitable anode materials due to their high discharge capacity and low cost. However, the poor electrochemical cycling stability prevents its practical application. In this paper, Mg_50-xV_xNi₄₅Fe₃Zn₂ (x = 0, 1, 2, 3, 4) + 50 wt% Ni alloys were prepared by partially replacing Mg with V and using mechanical ball milling techniques with amorphous and nanocrystalline structures. Electrochemical tests showed that the ball-milled alloy had good electrochemical uptake and release performance. The maximum release performance is achieved in the first cycle. After that, the discharge level and cycle stability increased significantly with increasing ball grinding time and V content.     

Electronic structure of nanocrystalline and polycrystalline hydrogen storage materials

To optimise the choice of the compounds for a selected application, a better understanding of the role of each alloy constituent on the electronic properties of the material is crucial. In this work, we study experimentally the electronic properties of nanocrystalline and polycrystalline (Mg_1−xM_x)₂Ni, (Mg_1−xM_x)₂Cu, La(Ni_1−xM_x)₅, and Ti(Ni_1−xM′_x) (MMn,Al; M′Fe,Mg,Zr) alloys. The nanocrystalline and polycrystalline samples were prepared by mechanical alloying (MA) followed by annealing and arc melting method, respectively. All X-ray photoelectron spectroscopy (XPS) spectra were measured immediately after cleaning of the sample surface in a vacuum of 8×10⁻¹¹ mbar. Furthermore, we have measured XPS spectra of in situ prepared nanocrystalline and polycrystalline LaNi₅, TiNi, and Mg₂Ni thin films and compared with those obtained for ex situ prepared bulk materials. The substitution of Mg in Mg₂Ni and Mg₂Cu, Ni in LaNi₅ and TiNi by transition metals leads to significant modifications of the shape and width of the valence band of the nanocrystalline as well as polycrystalline samples. Especially, the valence bands of the MA nanocrystalline alloys are considerably broader compared to those measured for the polycrystalline samples. Results also showed that the strong modifications of the electronic structure of the nanocrystalline alloys could significantly influence on their hydrogenation properties.     

Microstructure characteristics,hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg_95-X-Ni_x-Y₅ (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH₂, Mg₂NiH₄, and YH₃ phases. And, the in-situ formed YH₃ phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg₂Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni₅, Ni₁₀, and Ni₁₅ samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H₂. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg₈₀Ni₁₅Y₅ alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H₂. Besides, the dehydrogenated activation energy of the Mg₈₀Ni₁₅Y₅ alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni₅ alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.     

Effect of rapid solidification on hydrogen solubility in Mg-rich Mg-Ni alloys

The hydrogenation properties of Mg_100−xNi_x alloys (x = 0.5, 1, 2, 5) produced by melt spinning and subsequent high-energy ball milling were studied. The alloys were crystalline and, in addition to Mg matrix, contained finely dispersed particles of Mg₂Ni and metastable Mg₆Ni intermetallic phases. The alloys exhibited excellent hydrogenation kinetics at 300 °C and reversibly absorbed about 6.5 mass fraction (%) of hydrogen. At the same temperature, the as prepared Mg_99.5Ni_0.5 and Mg₉₅Ni₅ powders dissolved about 0.6 mass fraction (%) of hydrogen at the pressures lower than the hydrogen pressure corresponding to the bulk Mg-MgH₂ two-phase equilibrium, exhibiting an extended apparent solubility of hydrogen in Mg-based matrix. The hydrogen solubility returned to its equilibrium value after prolonged hydrogenation testing at 300 °C. We discuss this unusually high solubility of hydrogen in Mg-based matrix in terms of ultrafine dispersion of nanometric MgH₂ precipitates of different size and morphology formed on vacancy clusters and dislocation loops quenched-in during rapid solidification.