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.     

Characterization of microstructure,hydrogen storage kinetics and thermodynamics of a melt-spun Mg86Y10Ni4 alloy

Ternary Mg₈₆Y₁₀Ni₄ alloy was successfully prepared by vacuum induction melting and subsequent melt-spinning technique. The phase composition and microstructure of the melt-spun and hydrogenated samples were characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy measurements. The melt-spun alloy had an amorphous structure, and it transformed into nanocrystalline during the first hydrogenation process. The hydrogenated sample was composed of MgH₂, Mg₂NiH₄, YH₂, and a small amount of YH₃. The hydrogen absorption/desorption kinetics and thermodynamics were measured by Sievert's apparatus at various temperatures. It was found that the melt-spun Mg₈₆Y₁₀Ni₄ alloy could be fully activated after five hydrogenation and dehydrogenation cycles at 380 °C, and it exhibited a reversible gravimetric hydrogen storage capacity of about 5.3 wt%. The enhanced hydrogen sorption kinetics during the first few cycles can be attributed to the increased specific surface caused by the pulverization and cracking of the alloy particles. The activation energy for dehydrogenation reaction was determined to be 67 kJ/mol and 71 kJ/mol by using Arrhenius equation and Kissinger equation respectively. The thermodynamics of the sample was also evaluated by pressure–composition–isotherms, and the results shown that the enthalpy and entropy changes of Mg/MgH₂ transformation in the Mg₈₆Y₁₀Ni₄ alloy were slightly higher than that of pure Mg/MgH₂.     

Facilitating de/hydrogenation by long-period stacking ordered structure in Mg based alloys

We propose a simple strategy to effectively improve the hydrogenation and dehydrogenation kinetics of Mg based hydrogen storage alloys. We designed and prepared an Mg_91.9Ni_4.3Y_3.8 alloy consisting of a large quantity of long-period stacking ordered (LPSO) phases. A type of highly dispersed multiphase nanostructure, which can markedly promote the de/hydrogenation kinetics, has been obtained utilizing the decomposition of LPSO phases at first a few of hydrogenation reactions. The fine structures of LPSO phases and the microstructural evolutions of the alloy during hydrogenation and dehydrogenation reactions were in detail characterized by means of transmission electron microscopy (TEM). The LPSO phases transformed to MgH₂, Mg₂NiH₄, and YH₃ after the first hydrogenation. The highly dispersed nanostructure at macro and micro (nano) scale range remains even after several de/hydrogenation cycles. The alloy shows excellent hydrogen storage properties and its reversible hydrogen absorption/desorption capacities are about 5.8 wt% at 300 °C. Particularly, the alloy exhibits very fast dehydrogenation kinetics. The dehydrogenated sample can release approximately 5 wt% hydrogen at 300 °C within 200 s and 5.5 wt% within 600 s. We elucidate the structural mechanism of the alloy with outstanding hydrogen storage performance.     

Microstructure characteristics,hydrogen storage kinetic and thermodynamic properties of Mg80–xNi20Yx (x = 0–7) alloys

Mg_80–xNi₂₀Y_x (x = 0–7) alloys were successfully prepared by using the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy measurements. The effects of yttrium content on the microstructure and hydrogen storage properties of the as-cast alloys were investigated. The alloys are composed of a primary phase of Mg₂Ni, lamella eutectic composites of Mg + Mg₂Ni, and some amount of YNi₃. The YNi₃ has completely transformed into in situ formed nanoscale YH₂ and YH₃, with the formation of Mg₂NiH₄. Pulverization of the alloy particles was observed during the hydrogen absorption and desorption cycles. However, the phase composition remains unchanged even after 20 hydrogenation cycles. Yttrium addition significantly improved the hydrogen-absorption kinetic performance of the alloy. In addition, pressure-composition-temperature measurements indicated that the entropy and enthalpy changes for the formation and decomposition of MgH₂ and Mg₂NiH₄ gradually decreased with the increase of yttrium content.     

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.     

Gas hydrogen absorption and electrochemical properties of Mg24Ni10Cu2 alloys improved by Y substitution,ball milling and Ni addition

Mg₂₄Ni₁₀Cu₂ and Mg₂₂Y₂Ni₁₀Cu₂ alloys were prepared via vacuum induction melting, and the nanocrystalline/amorphous Mg₂₄Ni₁₀Cu₂ and Mg₂₄Ni₁₀Cu₂ + 100 wt% Ni alloys were synthesized through ball milling method. Microstructure and hydrogen storage properties of the alloys were investigated and compared as well. The results suggest that adding Ni in the milling process significantly promotes formation of amorphous and nanocrystalline structure. For these four alloys, the as-milled Mg₂₄Ni₁₀Cu₂ with 100 wt% Ni shows the best hydrogen storage performances that 2.03 wt% hydrogen content can be absorbed just in 1 min, and the electrochemical capacity reaches to 899.2 mAh/g. Furthermore, ball milling with Ni promotes the charge transfer reaction and hydrogen diffusion ability which is advantageous to the high rate discharge ability.     

In-situ hydrogen-induced evolution and de-/hydrogenation behaviors of the Mg93Cu7-xYx alloys with equalized LPSO and eutectic structure

Nanosizing is efficient as the dual-tuning of thermodynamics and kinetics for Mg-based hydrogen storage materials. The in-situ synthesis of nanocomposites through hydrogen-induced decomposition from long-period stacking ordered phase is proved effective to achieve active nano-sized catalysts with uniform dispersion. In this study, the Mg₉₃Cu_7-xY_x (x = 0.67, 1.33, and 2) alloys with equalized Mg–Mg₂Cu eutectic and 14H long-period stacking ordered phase of Mg₉₂Cu_3.5Y_4.5 are prepared. Its solidification path is determined as α-Mg, 14H–Mg₂Cu pair and Mg–Mg₂Cu eutectic. The increased Y/Cu atomic ratio lowers the first-cycle hydrogenation rate of the alloys due to the increased 14H–Mg₂Cu structure and reduced Mg–Mg₂Cu eutectic interfaces. After the hydrogen-induced decomposition of the long-period stacking ordered phase, MgCu₂ and YH₃ nanoparticles are in-situ formed, and the following activation process significantly accelerates. The YH₃ nanoparticles partly decompose to YH₂ at 300 °C in vacuum and Mg–Mg₂Cu-YH_x nanocomposites are in-situ formed. The nano-sized YH₂ helps catalyze H₂ dissociation and the YH_x/Mg interfaces stimulate H diffusion and the nucleation of MgH₂. Therefore, the Mg₉₃Cu₅Y₂ composite shows the fastest absorption rates. However, due to the positive effect of YH_x/Mg interfaces on H diffusion and the negative effect of YH₃ nanophases on the hydride decomposition, the minimum activation energy of 115.43 kJ mol⁻¹ is obtained for the desorption of the Mg₉₃Cu_5.67Y_1.33 hydride.     

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.     

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.     

Structure and hydrogen storage properties of Mg2Cu1−xNix (x = 0–1) alloys

The effect of Ni-substitution on the structure and hydrogen storage properties of Mg₂Cu_1−xNi_x (x = 0, 0.2, 0.4, 0.6, 0.8, 1) alloys prepared by a method combining electric resistance melting with isothermal evaporation casting process (IECP) has been studied. The X-ray single-crystal diffraction analysis results showed that the cell volume decreases with increasing Ni concentration, and crystal structure transforms Mg₂Cu with face-centered orthorhombic into Ni-containing alloys with hexagonal structure. The Ni-substitution effects on the hydriding reaction indicated that absorption kinetics and hydrogen storage capacity increase in proportion to the concentration of the substitutional Ni. The activated Mg₂Cu and Mg₂Ni alloys absorbed 2.54 and 3.58 wt% H, respectively, at 573 K under 50 bar H₂. After a combined high temperature and pressure activation cycle, the charged samples were composed of MgH₂, MgCu₂ and Mg₂NiH₄ while the discharged samples contained ternary alloys of Mg–Cu–Ni system with the helpful effect of rising the desorption plateau pressures compared with binary Mg–Cu and Mg–Ni alloys. With increasing nickel content, the effect of Ni is actually effective in MgH₂ and Mg₂NiH₄ destabilization, leading to a decrease of the desorption temperature of these two phases.     

Comparative investigations on the hydrogenation characteristics and hydrogen storage kinetics of melt-spun Mg10NiR (R = La,Nd and Sm) alloys

The hydrogenation characteristics and hydrogen storage kinetics of the melt-spun Mg₁₀NiR (R = La, Nd and Sm) alloys have been studied comparatively. It is found that the Mg₁₀NiNd and Mg₁₀NiSm alloys are in amorphous state but the Mg₁₀NiLa alloy is composed of an amorphous phase and minor crystalline La₂Mg₁₇ after melt-spinning. The alloys can be hydrogenated into MgH₂, Mg₂NiH₄ and a rare earth metal hydride RH_x. The rare earth metal hydride and Mg₂NiH₄ synergistically provide a catalytic effect on the hydrogen absorption–desorption reactions in the Mg−H₂ system. The hydrogen storage kinetics is not influenced by different rare earth metal hydrides but by the particle size of the rare earth metal hydrides.     

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.     

Remarkable synergistic effects of Mg2NiH4 and transition metal carbides (TiC,ZrC, WC) on enhancing the hydrogen storage properties of MgH2

Nanostructuring and catalyzing are effective methods for improving the hydrogen storage properties of MgH₂. In this work, transition-metal-carbides (TiC, ZrC and WC) are introduced into Mg–Ni alloy to enhance its hydrogen storage performance. 5 wt% transition-metal-carbide containing Mg₉₅Ni₅ (atomic ratio) nanocomposites are prepared by mechanical milling pretreatment followed by hydriding combustion synthesis and mechanical milling process, and the synergetic enhancement effects of Mg₂NiH₄ and transition-metal-carbides are investigated systematically. Due to the inductive effect of Mg₂NiH₄ and charge transfer effect between Mg/MgH₂ and transition-metal-carbides, Mg₉₅Ni₅-5 wt.% transition-metal-carbide samples all exhibit excellent hydrogen storage kinetic at moderate temperature and start to release hydrogen around 216 °C. Among them, 2.5 wt% H₂ (220 °C) and 4.7 wt% H₂ (250 °C) can be released from the Mg₉₅Ni₅-5 wt.% TiC sample within 1800 s. The unique mosaic structure endows the Mg₉₅Ni₅-5 wt.% TiC with excellent structural stability, thus can reach 95% of saturated hydrogen capacity within 120 s even after 10 cycles of de-/hydrogenation at 275 °C. And the probable synergistic enhancement mechanism for hydrogenation and dehydrogenation is proposed.     

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.     

An electrochemical investigation of melt-spun nanocrystalline Mg20Ni10−xCux (x = 0–4) electrode alloys

The nanocrystalline Mg₂Ni-type electrode alloys with nominal compositions of Mg₂₀Ni_10−xCu_x (x = 0, 1, 2, 3, 4) were synthesized by the melt spinning technique. The microstructures of the as-cast and spun alloys were characterized by XRD, SEM and HRTEM. The electrochemical hydrogen storage performances were tested by an automatic galvanostatic system. The results show that all the as-spun alloys hold typical nanocrystalline structure instead of an amorphous phase. The melt spinning does not modify the major phase Mg₂Ni, but it leads to the formation of crystal defects such as stacking faults, dislocations, sub-grain boundary and twin-grain boundary. The melt spinning significantly improves the electrochemical hydrogen storage capacity of the alloys, whereas it slightly impairs the electrochemical cycle stability of the alloys. The substitution of Cu for Ni significantly ameliorates the electrochemical hydrogen storage performances of the alloys, involving both the electrochemical hydrogen storage capacity and the electrochemical charging and discharging stability.     

Hydrogen storage characteristics of the nanocrystalline and amorphous Mg–Nd–Ni–Cu-based alloys prepared by melt spinning

Nanocrystalline and amorphous Mg–Nd–Ni–Cu-based (Mg₂₄Ni₁₀Cu₂)_100−xNd_x (x = 0–20) alloys were prepared by melt spinning and their structures as well as hydrogen storage characteristics were investigated. The analysis of XRD, TEM and SEM linked with EDS reveal that all the as-cast alloys hold a multiphase structure, containing Mg₂Ni-type major phase as well as some secondary phases Mg₆Ni, Nd₅Mg₄₁ and NdNi, whose amounts clearly grow with Nd content rising. Furthermore, the as-spun Nd-free alloy displays an entire nanocrystalline structure whereas the as-spun Nd-added alloys have a mixed structure of nanocrystalline and amorphous, moreover, the amorphization degree of the alloys visibly increases with Nd content rising, implying that the addition of Nd facilitates the glass forming in the Mg₂Ni-type alloy. The addition of Nd results in a slight decrease in the hydrogen absorption capacity of the as-cast and spun alloys, but it significantly enhances their hydrogen storage kinetics and hydriding/dehydriding cycle stability of the alloy. In order to reveal the capacity degradation mechanism of the as-spun alloy, the structure evolution of the nanocrystalline and amorphous alloys during the hydriding–dehydriding cycles was investigated. It is found that the root causes of leading to the capacity degradation of the nanocrystalline and amorphous alloys are nanocrystalline coarsening, crystal defect decreasing and amorphous phase crystallizing.     

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.     

First-principles studies of the structures and properties of Al- and Ag-substituted Mg2Ni alloys and their hydrides

The structures and properties of hydrogen storage alloy Mg₂Ni, of aluminum and silver substituted alloys Mg_2−xM_xNi (M = Al and Ag, x = 0.16667), and of their hydrides Mg₂NiH₄, Mg_2−xM_xNiH₄ (M = Al and Ag, x = 0.125) have been calculated from first-principles. Results show that the primitive cell sizes of the intermetallic alloys and hydrides were reduced by substitution of Mg with Al or Ag. Also, the interaction of Ni–Ni was weakened by the substitution. A strong covalent interaction between H and Ni atoms forms tetrahedral NiH₄ units in Mg₂NiH₄. The NiH₄ unit near the Al/Ag atom became tripod-like NiH₃ in Mg_2−xM_xNiH₄ (M = Al, Ag), indicating that the hydrogen storage capacity was decreased by the substitution. The calculated enthalpies of hydrogenation for Mg₂Ni, Mg_2−xAl_xNi and Mg_2−xAg_xNi are −65.14, −51.56 and −53.63 kJ/mol H₂, respectively, implying that the substitution destabilizes the hydrides. Therefore, the substitution is an effective technique for improving the thermodynamic behavior of hydrogenation/dehydrogenation in magnesium-based hydrogen storage materials.     

First-principles study on the dehydrogenation properties and mechanism of Al-doped Mg2NiH4

Mg₂NiH₄, with fast sorption kinetics, is considered to be a promising hydrogen storage material. However, its hydrogen desorption enthalpy is too high for practical applications. In this paper, first-principles calculations based on density functional theory (DFT) were performed to systematically study the effects of Al doping on dehydrogenation properties of Mg₂NiH₄, and the underlying dehydrogenation mechanism was investigated. The energetic calculations reveal that partial component substitution of Mg by Al results in a stabilization of the alloy Mg₂Ni and a destabilization of the hydride Mg₂NiH₄, which significantly alters the hydrogen desorption enthalpy ΔH_des for the reaction Mg₂NiH₄ → Mg₂Ni + 2H₂. A desirable enthalpy value of ∼0.4 eV/H₂ for application can be obtained for a doping level of x ≥ 0.35 in Mg_2−xAl_xNi alloy. The stability calculations by considering possible decompositions indicate that the Al-doped Mg₂Ni and Mg₂NiH₄ exhibit thermodynamically unstable with respect to phase segregation, which explains well the experimental results that these doped materials are multiphase systems. The dehydrogenation reaction of Al-doped Mg₂NiH₄ is energetically favorable to perform from a metastable hydrogenated state to a multiphase dehydrogenated state composed of Mg₂Ni and Mg₃AlNi₂ as well as NiAl intermetallics. Further analysis of density of states (DOS) suggests the improving of dehydrogenation properties of Al-doped Mg₂NiH₄ can be attributed to the weakened Mg-Ni and Ni-H interactions and the decreasing bonding electrons number below Fermi level. The mechanistic understanding gained from this study can be applied to the selection and optimization of dopants for designing better hydrogen storage materials.     

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.