Pressure-composition Isotherms Research Articles

Design of Nb-based multi-phase alloy membranes for high hydrogen permeability and suppressed hydrogen embrittlement

Nb-Hf-Co alloys, consisting of a multi-phase microstructure with primary BCC-(Nb, Hf) and eutectic {BCC-(Nb, Hf) + HfCo} phases, are a potential alloys for hydrogen separation and purification. However, the compositional window, comprising of the above phases, has not yet been established because of lack of important information concerning the phase diagrams. To address this need, this study first calculated the phase diagram and related phase equilibria information of this class of alloys, by using the CALPHAD approach, and established a new compositional window suitable for hydrogen permeation. The alloys in this window exhibit very high hydrogen permeability. In particular, Nb45Hf27.5Co27.5 exhibits the highest permeability of 5.32 × 10−8 mol H2 m−1s−1Pa−0.5 at 673 K, which is the best among all the Nb-based multi-phase alloys. Secondly, the boundary for ductile to brittle transition-hydrogen concentration (DBTC) of the Nb-HfCo multi-phase alloy was determined for the first time and found to occur at around 0.72–0.82 H/M, which reveals the fundamental reasons for brittle failure occurring during hydrogen permeation. From the point of DBTC, a new design concept for Nb-based hydrogen permeable membranes is proposed and the factors affecting their brittle failure are analysed. Thirdly, to verify the feasibility of the above concept, Nb45Hf27.5-xTixCo27.5 (x = 0 … 10) alloys were designed and comprehensively studied. It is demonstrated that Nb45Hf20Ti7.5Co27.5 alloy exhibits excellent hydrogen permeability with enhanced hydrogen embrittlement under appropriate permeation conditions. The significance of the study is that the best composition range for hydrogen-permeable Nb-HfCo alloys is explored, and some novel hydrogen-permeable alloys can be easily developed by analysing their absorption pressure-composition isotherms in combination with the newly explored DBTC.

A new strategy for remarkably improving anti-disproportionation performance and cycling stabilities of ZrCo-based hydrogen isotope storage alloys by Cu substitution and controlling cutoff desorption pressure

ZrCo1-xCux (x = 0–0.3) alloys for hydrogen isotope storage were prepared via induction levitation melting. The effects of partial substitution of Cu for Co on the microstructure, hydrogen storage properties including hydriding-dehydriding kinetics, thermodynamic characteristics, cycling stability and related mechanism have been systematically investigated. It is found that all synthesized alloy ingots consist of ZrCo main phase, the grain size is further refined but the segregation of Cu element at grain boundary is more serious with the increase of Cu content. The pressure-composition isotherms for dehydrogenation show that both ZrCoH3 and ZrCo0.9Cu0.1H3 hydrides undergo one-step desorption, while ZrCo0.8Cu0.2H3 and ZrCo0.7Cu0.3H3 hydrides experience two-step desorption since a new middle hydride phase of ZrCoH0.8 with CrB-type orthorhombic structure is discovered during their dehydrogenation. This result is proposed to be linked with the changed electronic structure and lattice distortion induced by Cu substitution. To compare their dehydriding kinetics, the apparent activation energies for hydrogen desorption of different hydride phases in the alloys are also calculated systematically, and the value of Ea for hydrogen desorption of ZrCoH3 phase is decreased from 118.02 kJ/mol to 95.90 kJ/mol, 77.98 kJ/mol and 78.65 kJ/mol, respectively. Prominent cycling stabilities of the Cu-substituted alloys are obtained and follow the trend: ZrCo0.8Cu0.2 > ZrCo0.7Cu0.3 > ZrCo0.9Cu0.1 > ZrCo. Specifically, the optimum cycling stable capacity of ZrCo-based alloy is increased from 0.4 wt % to 1.21 wt %. Furthermore, the ZrCo0.8Cu0.2 alloy exhibits further enhanced cycling stability during the cycle by controlling cutoff desorption pressure at 0.253 bar, where only the first-step dehydrogenation happens. Therefore, a new strategy for improving anti-disproportionation performance of ZrCo-based alloys by controlling cutoff desorption pressure is also proposed, which is in favor of the enhanced cycling stability and further application of Cu-substituted ZrCo-based alloys for hydrogen isotope storage and delivery in the International Thermonuclear Experimental Reactor (ITER).

Effect of Hafnium Addition on the Hydrogenation Process of TiFe Alloy

The alloy TiFe has interesting hydrogen storage properties for practical applications: low cost, operation at room temperature, and good hydrogen capacity. However, the first hydrogenation is difficult and increases the cost of the alloy. In this work, we studied the effect of adding hafnium to TiFe in order to enhance the first hydrogenation process. TiFe + x Hf alloys, with x = 0, 4, 8, 12, and 16 wt.%, were synthesized by arc melting. The microstructure of the as-cast alloys was investigated by scanning electron microscopy and electron microprobe analysis. These alloys consisted of B2-TiFe, C14-Laves, and BCC (Body Centered Cubic) phases. A minimum of 8 wt.% of hafnium is required to obtain an enhancement of the first hydrogenation. In the first hydrogenation, the material reaches its maximal hydrogen capacity in less than two hours at room temperature and under 20 bars of hydrogen. Hafnium addition also had the effect of lowering the plateau pressure in the pressure-composition isotherm. It could be concluded that hafnium has a positive effect on the activation properties of TiFe.

Measurements of Pressure-Stress Coupling in Hydrated Palladium

Hydrogen is considered to be a promising element which has specific energy over 120 MJ/kg highest energy per mass of any fuel. However, its low ambient temperature density leads to low volumetric density, therefore requiring the development of energy storage systems that have potential for higher volumetric density. In this context, we are storing hydrogen within metals by absorption. Here, we report on the stress evolution of metal during hydrogen absorption which is due to the lattice expansion of the hydrogen absorbing phase during hydrogen uptake. The stress induced by material based hydrogen absorption and desorption in very thin palladium layers was recorded in situ using a specially designed cell. Influences of gas pressure, temperature and metal geometry on the stress evolution were considered. In this regard, so called pressure-stress isotherms are proposed in thermomechanical analogy to pressure composition isotherms. The importance of the coupling is discussed in interpreting the hysteresis observed in the pressure versus hydrogen content plots and the role of stress in modifying the maximum hydrogen capacity of a metal hydride under stress.

Evaluation of hydrogen storage performance of ZrTiVNiCrFe in electrochemical and gas-solid reactions

In the present study, the hydrogen storage performance of multi-principal-component ZrTiVNiCrFe alloy produced through rapid solidification has been examined by electrochemical methods and gas-solid reactions. XRD and EBSD analyses reveal the hexagonal Laves phase structure (type C14) with average grain size of 300 nm and root-mean-square microstrain of 0.19%. Cyclic voltammetry and electrochemical impedance spectroscopy analyses in the hydrogen sorption/desorption region give insight to the sorption/desorption kinetics and the change in the desorption charge in terms of the applied potential. The pressure-composition isotherms measured in course of gas-solid reaction confirm the hydrogen storage capacity reaching 1.6 wt% at the first hydrogenation at room temperature, then reducing to 1.3–1.4% during subsequent cycling. According to the calorimetric titration study, there is a significant hysteresis primarily caused by the non-equilibrium character of the hydrogenation process.

Effect of annealing temperature on the microstructure of Pr2Co7 alloy and its hydrogen absorption-desorption kinetics

In this work, we studied the effect of annealing temperature on the kinetics of hydrogen absorption/desorption in nanocrystalline Pr2Co7 alloy. The pressure-composition (P–C) isotherm measurements revealed that the plateau pressure of Pr2Co7 alloy after annealing becomes comparatively flat. The maximum hydrogen absorption content of as-milled alloy and the annealed one reached 1.01 and 1.2 H/M (H/M: hydrogen per metal), respectively. The derived formation enthalpy ΔH and Gibbs free energy ΔG of the hydride in the annealed alloy are higher than those of the as-milled compound, indicating that the hydrides stability increases after annealing. The mechanism of hydrogen absorption-desorption process in the as-milled and annealed alloys was further investigated according to the Johnson-Mehl-Avrami formula. The hydrogenation of the as-milled and annealed Pr2Co7 alloys was found to be controlled by the three-dimensional-interface process. The activation energies Ea were determined and indicate that annealing is a very beneficial way to improve hydrogen absorption-desorption kinetics of the nanocrystalline Pr2Co7 alloy. The latter has a very fast absorption kinetic at 298 K. Throughout the study, it has been shown that Pr2Co7 alloy exhibits excellent hydrogenation properties which open a new perspective to be a new candidate for important uses in the field of hydrogen storage such as negative electrodes for nickel metal hydride (Ni-MH) batteries, mobile and short range vehicular applications.

Hydrogen isotope effect in a Ni32Nb28Zr30Cu10 amorphous membrane

The pressure-composition isotherms and the absorption kinetics of both hydrogen and deuterium were investigated for an amorphous membrane, potentially useful for hydrogen purification and separation in membrane reactors, with atomic composition Ni32Nb28Zr30Cu10. In the temperature range between 294 and 394 °C, the equilibrium pressure between the gas and the solid was higher for deuterium than for hydrogen, with a typical ratio P(D2)/P(H2)≈1.5. Moreover, the absorption of H2 is faster than that of D2. Both behaviors of the presently investigated amorphous membrane closely resemble those of the Pd–Ag alloys, that are currently used in membrane reactors and separators.

Deuterium absorption properties of V85Ni15 and evidence of isotope effect

The pressure-composition absorption isotherms of deuterium in an alloy with nominal composition V85Ni15 were measured between 87 and 400 °C for pressures up to ≈17 bar. For T ≤ 144 °C one observes the occurrence of a solid solution α phase for D/M < 0.15, the coexistence of α and β phase up to D/M ≈ 0.5 and a subsequent change to a different phase for higher deuterium concentrations. For temperatures higher than 200 °C only a solid solution with a deuteration enthalpy ΔHdeutr = 47 ± 2 kJ/mol is detected. The critical temperature of this V-Ni alloy (144 °C ≤ T*≤200 °C) is much higher than that of pure V (100 °C). Moreover, well above the critical temperature, one can observe a “normal” isotope effect, i.e. the equilibrium pressure of the V85Ni15-H system is higher than that of V85Ni15-D.

Preparation and characterization of (CeO2)x–(Fe2O3)1−x nanocomposites: reduction kinetics and hydrogen storage

A series of nanosized CeO2–Fe2O3 mixed-oxide nanocomposites with different Ce4+/Fe3+ molar ratios were successfully prepared by a co-precipitation technique. The surface area increased with Fe2O3 content increasing up to 60 wt% in the composite. However, with further increase in Fe2O3 content, the surface area began to decrease. The reduction processes of the CeO2–Fe2O3 nanocomposites were studied in a hydrogen atmosphere at 300–600 °C. The reduction rates increased by increasing both the temperature and Fe2O3 content in the nanocomposites. The microstructure of the reduced composites at 500 °C illustrated the presence of a considerable number of macro- and micro-pores. The activation energy values were calculated which were in the range of 3.56–5.37 kJ·mol−1 at the initial stages (up to 35% reduction) and 5.21–10.2 kJ·mol−1 at the final stages (up to 80% reduction) of reduction. The rate-controlling mechanisms in both the initial and final reduction stages were determined, and the initial reaction stage was controlled by combined gaseous diffusion and interfacial chemical reaction mechanisms for all the composites except for pure CeO2, which was controlled by a chemical reaction mechanism. The final reaction stage was controlled by a gaseous diffusion mechanism for some composites, while for the others it was controlled by combined gaseous diffusion and interfacial chemical reaction mechanisms. The hydrogen sorption properties of the nanocomposites were studied by pressure composition isotherms using a volumetric method. Hydrogen storage in the nanocomposites increased by increasing the temperature because of the formation of oxygen vacancies which enhance atomic H adsorption and function as strong adsorption sites forming more metal hydride covalent bonds.

Mechanochemical synthesis and hydrogenation behavior of (TiFe)100-xNix alloys

Single phase ternary (TiFe)100-xNix alloys were synthesized through high-energy ball milling. Ni solubility in equiatomic TiFe reaches 10 at. %, exceeding the values known from the equilibrium diagram of state and those obtainable by conventional arc melting. The reactivity of mechanically alloyed (TiFe)95Ni5 with hydrogen was investigated in detail by a combination of volumetric and calorimetric methods. The influence of Ni addition on the pressure-composition isotherms and the reaction enthalpies was evaluated. Finally, the hydrogenation behavior was compared with Ni-free mechanically alloyed TiFe and with literature data on different TiFe-Ni alloys.

Accurate measurement of pressure-composition isotherms and determination of thermodynamic and kinetic parameters of metal hydrides

Abstract This work presents an in-depth investigation of different techniques used to accurately determine pressure-composition isotherms (pcI) as well as thermodynamic and kinetic parameters of metal hydrides. A case study is presented for an alloy of the type LaNi5. The results combine experimental measurements with theoretical modeling. Firstly, the numerical work presented discusses a numerical method for the evaluation of pcI raw data in order to minimize the experimental error. A simple, efficient iterative algorithm is presented to solve for the van der Waals equation of state in order to take into account the compressibility of hydrogen. Secondly, the measurements of dynamic (mass-flow) and quasi-static (Sievert's type) isotherms at different temperatures and different flow rates are presented. The dead volume of the apparatus as well as its experimental error, which is key parameter for the accurate calculation of hydrogen capacity, are also measured. Finally, important thermodynamic and kinetic parameters are deduced from the pcI curves. The kinetics of the absorption and desorption reactions are modelled and the reaction rate constants are found using both dynamic and quasi-static methods. The presented work goes beyond the calculation of the entropy and enthalpy of reaction, as the lattice gas model is also used to determine the critical temperature as well as the interaction energy between hydrogen-metal atoms and hydrogen-hydrogen atoms.

Evaluation of a BCC alloy as metal hydride compressor via 100 MPa-class high-pressure hydrogen apparatus

In the study presented here, an apparatus that correctly measured pressure–composition isotherms (PCi) at high pressures (up to 100 MPa) and temperatures (up to 200 °C) was developed. The PCi characteristics of a vanadium–titanium–chromium alloy at high pressure and temperature were examined with the apparatus to develop a metal hydride (MH) compressor. It was revealed that it is possible to use a 40 V20Ti40Cr (at%) alloy with a heat source below 200 °C for hydrogen compression in the approximate range of 2.1–30.0 MPa. The compressed content reached approximately 1.8 wt%, which is almost the same as the reversible hydrogen capacity at 20 °C.

Boosting low-temperature de/re-hydrogenation performances of MgH2 with Pd-Ni bimetallic nanoparticles supported by mesoporous carbon

Bimetallic Pd-Ni nano-particles supported by a mesoporous carbon material CMK-3 (denoted as Pd30Ni70/CMK-3) were synthesized through solution impregnation and hydrogen reduction methods. Among those hierarchical Ni-Pd nano-particles, majorly large ones (>10 nm) are dispersed over the surface of CMK-3, while a litter small ones (<10 nm) are embedded into the pores. It significantly improves the de/re-hydrogenation performances of MgH2 at low temperature. The onset desorption temperature of MgH2-Pd30Ni70/CMK-3 is lowered by 150 K from that of pristine MgH2 (above 593 K). About 6 wt% hydrogen could be released during its decomposition below 561 K. Noticeably, MgH2-Pd30Ni70/CMK-3 is capable of releasing 1.3 wt% H2 even at 373 K. 4 wt% hydrogen can be absorbed at 343 K under a hydrogen pressure of 3 MPa within 18000 s. Activation energy values of both hydrogen decomposition (65.9 kJ mol−1) and absorption (78.9 kJ mol−1) for MgH2-Pd30Ni70/CMK-3 are greatly improved from those of as-milled MgH2. Thermal stability of the composite system is remarkably destabilized by 4.3 kJ mol H2−1 from pristine MgH2 according to pressure-composition isotherm curves and van't Hoff plots. The enhanced performances can be ascribed to the synergistic effects of both destabilization and catalysis from nano-dispersed Pd and Ni particles, respectively.

Microstructure and hydrogen storage properties of V48Fe12Ti15-xCr25Alx (x=0, 1) alloys

The microstructure and hydrogen storage characteristics of V48Fe12Ti15-xCr25Alx (x = 0, 1) alloys prepared by vacuum arc melting were studied by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and pressure–composition isotherm measurements. It was confirmed that all of the alloys comprise a BCC phase, a Ti-rich phase, and a TiFe phase. Al as a substitute for part of the Ti content caused an increase of lattice parameters of the BCC phase and of the equilibrium pressures of hydrogen desorption, but decrease of the hydrogen storage capacities. The kinetic mechanism of the hydrogenation and dehydrogenation of the alloys was investigated by the classical Johnson–Mehl–Avrami equation. The reaction enthalpies (ΔH) for the dehydrogenation of alloys without and with Al were calculated by the Van't Hoff equation based on the PCI measurement data, which are 30.12 ± 0.14 kJ/mol and 28.02 ± 0.46 kJ/mol, respectively. The thermal stability of the metal hydride was measured by differential scanning calorimetry. The hydrogen desorption activation energies were calculated using the Kissinger method as 79.41 kJ/mol and 83.56 kJ/mol for x = 0 and 1, respectively. The results suggest that the substitution of titanium with aluminum improves the thermodynamic properties of hydrogen storage and reduces the kinetic performance of hydrogen desorption.

Pressure artifacts at isothermal operation of a metal hydride tank

The paper presents the results of experimental investigations on hydrogen absorption and desorption in the metal hydride reactor RS-1 containing 81 kg of La0.5Nd0.5Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 °C isothermal conditions. During the reactor-scale measurements of pressure-composition isotherms we observe pressure artifacts; pressures for absorption are higher and for desorption are lower than equilibrium pressures obtained after cooling down and reheating of the reactor. This thermal relaxation procedure removes the pressure artifacts. The observed effect is similar to the large aliquot effect, and mostly affects the desorption isotherm. The highest measured pressure difference is 0.7 bar (11%).

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.

Tuning the de/hydriding thermodynamics and kinetics of Mg by mechanical alloying with Sn and Zn

Mg95Sn3Zn2 alloy was prepared by mechanical alloying. The phase constituents and phase transition were analyzed by X-ray diffraction (XRD) method. The microstructure was characterized by scanning electron microscope (SEM). The hydrogen storage properties were evaluated in detail by the measurements of isothermal hydrogen absorption and desorption, and pressure-composition isotherms (PCI) using the Sieverts method. The addition of Zn benefits to extend the solubility of Sn in the Mg lattice, as a result supersaturated Mg(Sn, Zn) ternary solid solution was synthesized by mechanical alloying, which decomposed to MgH2, Sn and MgZn2 in the hydrogenating process. The in situ formed nanostructure Mg2Sn and MgZn2 have positive effects on the hydrogen absorption and desorption of Mg. Mg95Sn3Zn2 alloy showed significantly improved kinetics with lowered hydrogen absorption and desorption activation energies of 38.1 kJ/mol and 86.6 kJ/mol respectively, and exhibited a reduced dehydriding enthalpy of 67.0 ± 1.9 kJ/(mol·H2).

Structure and hydrogen storage properties of AB3-type Re2Mg(Ni0.7\u2009\u2212\u2009xCo0.2Mn0.1Alx)9 (x\u2009=\u20090\u20120.04) alloys

Re2Mg(Ni0.7 − xCo0.2Mn0.1Alx)9 (x = 0‒0.04) alloys are prepared by induction melting, and the influence of the partial substitution of Ni by Al on the structure, hydrogen storage, and electrochemical properties of the alloys are investigated systematically. These alloys mainly consist of two main phases with LaNi5 phase and (La,Mg)2Ni7 phase, and minor LaNi2 phase. The pressure-composition isotherms shows that, with Al content increasing in the alloys, the maximum hydrogen storage capacity decreased from 1.16 wt% (x = 0) to 0.99 wt% (x = 0.04). The changes of enthalpy and entropy reveal that the thermodynamic stability and the disordered degree of the hydride alloys increase with the Al addition. Results of electrochemical studies indicate that the substitution of Al for Ni can noticeably improve the cycle stability of the alloy electrode. The capacity retention after 80 cycles is enhanced from 63.6% (x = 0) to 76.5% (x = 0.04). However, the maximum discharge capacity of the alloys decreases. The Re2Mg(Ni0.7 − xCo0.2Mn0.1Alx)9 (x = 0‒0.04) alloys exhibit excellent dischargeability.

Experimental and computational modelling study of Ni substitution for Fe in Zr3Fe and its hydride

Zr3Fe and Ni-substituted Zr3Fe alloys with 30 and 50 at.% Ni were synthesized and their hydrogen absorption/desorption characteristics were compared experimentally (pressure–composition isotherms, thermal desorption spectroscopy, in-situ neutron diffraction) and by computational methods (ab-initio molecular dynamics (AMD), nudged elastic band theory (NEB)). All the alloys absorbed hydrogen to a hydrogen-to-metal atomic ratio of about 1.7, but the hydrides formed were stable at room temperature. The Zr3Fe0.5Ni0.5 alloy and its hydrided form were multi-phase. The Zr3Fe0.7Ni0.3 alloy was single-phase and retained the Cmcm structure of the parent intermetallic. In-situ neutron diffraction with D2 in place of H2 showed that the hydride formed in the isotherm measurements, Zr3Fe0.7Ni0.3H6.88, had the same structure (Cmcm) as Zr3FeH7, while disproportionation was observed in the hydrogenation of Zr3Fe. The kinetics of hydride formation was slower in both the Ni-substituted alloys. Thermal desorption spectroscopy showed that substitution of 0.3Ni significantly destabilized the hydride, lowering the temperature of the principal desorption peak by about 300 K relative to Zr3Fe–H2, without loss of hydrogen capacity, and avoiding disproportionation. Based on the structures determined by neutron diffraction, AMD and NEB calculations were conducted to compare Zr3Fe and Zr3Fe0.7Ni0.3 and their hydrides. The AMD calculations predicted that H diffusion was slower in Ni-substituted Zr3Fe, in agreement with the experimental observation of slower kinetics, implying a higher activation energy for H migration. The NEB calculations also predicted a higher energy barrier for H migration in Ni-substituted Zr3Fe.

Hydrogen absorption and desorption behavior of Ni catalyzed Mg–Y–C–Ni nanocomposites

In order to improve the hydrogen storage properties of Mg-based materials, ternary Mg24Y3–3 wt.% graphite (C)–x wt.% Ni (x = 0–20) nanocomposites were synthesized by mechanical ball-milling. Micro-area elemental analysis shows that the Ni and C are evenly distributed in the samples. Effect of Ni content on hydrogen absorption and desorption behavior at various temperatures was performed. Hydrogenation leads to the in situ formation of YH2/YH3 and Mg2Ni/Mg2NiH4, which has a significant catalytic effect on the hydrogen absorption and desorption kinetics. Composites with more than 3 wt% of Ni can almost reach their maximum hydrogen storage capacity within 1 min at 100 °C. When the Ni content increases from 0 to 20 wt%, the dehydrogenation activation energy (Ea) is reduced to 58 kJ/mol, while the desorption peak temperature is also lowered down to 282 °C. Two equilibrium plateaus are clearly observed in pressure–composition isotherms (p–c–T) curves, which can be ascribed to the hydrogen absorption/desorption reactions of Mg/MgH2 and Mg2Ni/Mg2NiH4. In order to take account to the reversible hydrogen storage capacity and absorption/desorption kinetics, Mg24Y3–3 wt.% C–5 wt.% Ni composite is considered having the optimized hydrogen storage performance.