Investigation on structure and hydrogen storage performance of as-milled and cast Mg90Al10 alloys

The as-milled (20 h) and cast Mg₉₀Al₁₀ alloys were prepared by mechanical milling and vacuum induction melting, respectively. The differences in the phase composition, apparent morphology and microstructure of the alloys were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) and high resolution transmission microscope (HRTEM). The activation performance, hydrogen absorption/desorption rate and pressure-composition-isotherm (P-C-T) curves of the pure Mg, as-milled (20 h) and cast Mg₉₀Al₁₀ alloys were tested using a Sieverts apparatus. The results show that the alloys both are nanocrystalline structure and consisted of the main phase of Mg phase and the second phase of Al phase or Mg₁₇Al₁₂ phase. Compared to pure Mg, the thermodynamics and kinetics of the as-milled (20 h) and cast Mg₉₀Al₁₀ alloys are improved in different degree. The hydrogen desorption enthalpy (ΔH_de) of the as-milled (20 h) and cast Mg₉₀Al₁₀ alloys are 75.43 and 72.76 kJ mol^?1 H₂, which are smaller than 100.67 kJ mol^?1 H₂ of pure Mg. And the dehydrogenation activation energy (E^de(a)) decreases from 172.61 kJ mol^?1 H₂ of pure Mg to 163.59 and 157.65 kJ mol^?1 H₂ of the as-milled (20 h) and cast Mg₉₀Al₁₀ alloys, respectively. However, the activation performance and the hydrogen absorption capacity have the varying degree to drop.     

Recoverable Ni2Al3 nanoparticles and their catalytic effects on Mg-based nanocomposite during hydrogen absorption and desorption cycling

The Mg-3.9 wt% Ni₂Al₃ nanocomposite is produced by hydrogen plasma-metal reaction method. The particle size of Mg is in range of 40–160 nm with an average size of 90 nm. The Ni₂Al₃ nanoparticles (NPs) of about 9 nm uniformly disperse on the surface of Mg NPs and in situ transform to Mg₂NiH_0.3 and Al after hydrogen absorption process. Surprisingly, the Mg₂NiH_0.3 and Al can recover to the initial state of Ni₂Al₃ after hydrogenation/dehydrogenation cycle. The Mg-Ni₂Al₃ nanocomposite shows enhanced hydrogen sorption rate and storage capacity. It can quickly uptake 6.4 wt% H₂ within only 10 min at 573 K, and release 6.1 wt% H₂ within 10 min at 623 K. The apparent activation energies for hydrogenation and dehydrogenation are calculated to be 55.4 and 115.7 kJ mol^?1 H₂. The enhanced hydrogen storage performances of the Mg-Ni₂Al₃ nanocomposite are attributed to both the nanostructure of Mg and the catalytic effects of Ni₂Al₃ NPs.     

Synergistic effects played by CMK-3 and NbF5 co-additives on de/re-hydrogenation performances of NaAlH4

In the present work, a strategy for simultaneously reducing the thermal stability of NaAlH₄ and enhancing its dehydrogenation kinetics was suggested by means of synergistic effects from co-additives of mesoporous carbon material CMK-3 and NbF₅. The ball milled NaAlH₄ + 10 wt% (NbF₅ + CMK-3) (NbF₅: CMK-3 = 1:1 in weight ratio) composite can liberate hydrogen at an onset temperature of 358 K, which was drastically decreased by 93 K from that of pristine NaAlH₄. By means of Kissinger's method, the activation energy of NaAlH₄ + 10 wt% (NbF₅ + CMK-3) can be identified as 99.2 kJ mol^?1, which was greatly reduced from that of pristine NaAlH₄ (121 kJ mol^?1). Investigations on the dehydrogenation process revealed that CMK-3 was beneficial to reducing the particle size of NaAlH₄ during ball milling, while NbF₅ was actively involved in the decomposition of NaAlH₄ and yielded some Nb-relevant intermediate phases NbH_0.89 during the heating process. The modified dehydrogenation pathway of NaAlH₄ also results in the destabilization of dehydrogenation by 2.13 kJ mol^?1 H₂ from that of pristine NaAlH₄. During the hydrogenation process, the NbH_0.89 and the mesoporous carbon material CMK-3 played synergistic roles in improving the dehydrogenation performance of NaAlH₄.     

Temperature-hydrogen pressure phase boundaries and corresponding thermodynamics for ZrCoH system

The thermodynamically and kinetically stable regions of the temperature–H₂ pressure phase boundaries for the ZrCoH system were established using the Temperature-Concentration-Isobar (TCI) method. Based on this, the enthalpy change and entropy change values of dehydrogenation and disproportionation reactions were successfully obtained. The average enthalpy change (ΔH) and entropy change (ΔS) estimated from the phase boundaries for dehydrogenation of ZrCoH₃ to ZrCo are respectively 103.07 kJ mol^?1H₂ and 148.85 J mol^?1 H₂ K^?1, which are well agreement with the data reported in literature. The average ΔH and ΔS were estimated to be ?120.91 kJ mol^?1H₂ and -149.32 J mol^?1 H₂ K^?1 for the disproportionation of ZrCoH₃, whereas the ΔH and ΔS were calculated to be ?84.6 kJ mol^?1H₂ and -92.29 J mol^?1 H₂ K^?1 for disproportionation of ZrCo. In addition, it was found from the established phase boundaries that the anti-disproportionation property of ZrCo alloy can be enhanced if the phase boundaries of hydrogenation/dehydrogenation are far away from the phase boundaries of disproportionation by adjusting the thermodynamics. Meanwhile, it is possible to keep ZrCo away from disproportionation even at high temperature of 650 °C under hydrogen atmosphere, if the temperature-H₂ pressure trajectory is carefully controlled without crossing the phase boundaries of disproportionation. Therefore, the established phase boundaries can be used as a guide to the eye avoiding disproportionation and improving the anti-disproportionation property of ZrCo alloy.     

Salicylaldimine-Ni complex supported on Al2O3: Highly efficient catalyst for hydrogen production from hydrolysis of sodium borohydride

In this study, the Ni-based complex catalyst containing nickel of 1% supported on Al₂O₃ is used as for the hydrogen production from NaBH₄ hydrolysis. The maximum hydrogen production rate from hydrolysis of NaBH₄ with Ni-based complex catalyst supported on Al₂O₃ containing nickel of 1% is 62535 ml min^?1 g^?1 (complex catalyst containing 1 wt% Ni). The resulting complex catalyst is characterised by XRD, XPS, SEM, FT-IR, UV, and BET surface area analyses. The Arrhenius activation energy is found to be 27.29 kJ mol^?1 for the nickel-based complex catalyst supported on Al₂O₃. The reusability of the catalyst used in this study has also been investigated. The Ni-based complex catalyst supported on Al₂O₃ containing nickel of 1% is maintained the activity of 100% after the fifth use, compared to the first catalytic use. The n value for the reaction rate order of NaBH₄ is found to be about 0.33.     

Monodisperse PtCu alloy nanoparticles as highly efficient catalysts for the hydrolytic dehydrogenation of ammonia borane

Pt-M alloy nanoparticles (NPs) with well-defined size and compositions exhibit dramatically catalytic performance in chemical reactions. In this work, monodisperse PtCu NPs with controlled size and compositions were synthesized by the co-reduction method in the presence of oleylamine. These NPs have excellent catalytic activities in the hydrolytic dehydrogenation of ammonia borane (AB) and their activities were composition dependent. Among the different-composition PtCu NPs, the Cu₅₀Pt₅₀ NPs exhibit the highest catalytic activity with an initial turnover frequency of 102.5 mol_(hydrogen)·mol_(catalyst)^?1·min^?1 and an apparent activation energy of 36 kJ·mol^?1, which demonstrate the validity of partly replacing Pt by a first-row transition metal on constructing high performance heterogeneous nanocatalysts for the hydrolytic dehydrogenation of AB.     

Facile synthesis of nickel-vanadium bimetallic oxide and its catalytic effects on the hydrogen storage properties of magnesium hydride

To improve the dehydrogenation/hydrogenation performance of magnesium hydride (MgH₂), a nickel-vanadium bimetallic oxide (NiV₂O₆) was prepared by a simple hydrothermal method using ammonium metavanadate and nickel nitrate as raw materials. This oxide was used to improve the hydrogen storage performance of MgH₂. NiV₂O₆ reacted with Mg to form Mg₂Ni and V₂O₅; Mg₂Ni and V₂O₅ played an important role in improving the hydrogen storage properties of MgH₂. The NiV₂O₆-doped MgH₂ had an excellent hydrogen absorption and desorption kinetics performance, and it could absorb 5.59 wt% of hydrogen within 50 min at 150 °C and release about 5.3 wt% of hydrogen within 12 min. The apparent activation energies for the dehydrogenation and hydrogenation of MgH₂-NiV₂O₆ were 92.9 kJ mol^?1 and 24.9 kJ mol^?1, respectively. These were 21.7% and 66.3% lower than those of MgH_2, respectively. The mechanism analysis demonstrated that the improved kinetic properties of MgH₂ resulted from the heterogeneous catalysis of vanadium and nickel.     

Hydrogen storage properties of nano-structured 0.65MgH2/0.35ScH2

The intermetallic compound Mg_0.65Sc_0.35 was found to form a nano-structured metal hydride composite system after a (de)hydrogenation cycle at temperatures up to 350 °C. Upon dehydrogenation phase separation occurred forming Mg-rich and Sc-rich hydride phases that were clearly observed by SEM and TEM with the Sc-rich hydride phase distributed within Mg/MgH₂-rich phase as nano-clusters ranging in size from 40 to 100 nm. The intermetallic compound Mg_0.65Sc_0.35 showed good hydrogen uptake, ca. 6.4 wt.%, in the first charging cycle at 150 °C and in the following (de)hydrogenation cycles, a reversible hydrogen capacity (up to 4.3 wt.%) was achieved. Compared to the as-received MgH₂, the composite had faster cycling kinetics with a significant reduction in activation energy E_a from 159 ± 1 kJ mol⁻¹ to 82 ± 1 kJ mol⁻¹ (as determined from a Kissinger plot). Two-dehydrogenation events were observed by DSC and pressure–composition-isotherm (PCI) measurements, with the main dehydrogenation event being attributed to the Mg-rich hydride phase. Furthermore, after the initial two cycles the hydrogen storage capacity remained unchanged over the next 55 (de)hydrogenation cycles.     

Investigation on gaseous and electrochemical hydrogen storage performances of as-cast and milled Ti1.1Fe0.9Ni0.1 and Ti1.09Mg0.01Fe0.9Ni0.1 alloys

The AB-type Ti_1.1Fe_0.9Ni_0.1 (Mg₀ for short) and Ti_1.09Mg_0.01Fe_0.9Ni_0.1 (Mg_0.01 for short) alloys were fabricated by vacuum induction melting and mechanical milling. The effects of partly substituting Ti with Mg and/or mechanical milling on the structure, morphology, gaseous thermodynamics and kinetics, and electrochemical performances were studied. The results reveal that the as-cast Mg₀ alloy contains the main phase TiFe and a small number of TiNi₃ and Ti₂Ni phases. Substituting Ti with Mg and/or mechanical milling results in the disappearance of the secondary phases. The discharge capacities of the as-cast Mg₀ and Mg_0.01 alloys are 12.6 and 8.8 mAh g^?1, which increase to 52.6 and 80.4 mAh g^?1 after 5 h of mechanical milling. By milling the as-cast alloy powders with carbonyl nickel powders, they are greatly enhanced to 191.6 mAh g^?1 for the Mg₀+7.5 wt% Ni alloy and 205.9 mAh g^?1 for the Mg_0.01+5 wt% Ni alloy at the current density of 60 mA g^?1, respectively. The values of dehydrogenation enthalpy (ΔH_des) and dehydrogenation activation energy (E^des_(a)) are very small, meaning that the thermal stability and the desorption kinetics of the hydrides are not the key influence factors for the discharge capacity. The reduction of the particle size and the generation of the new surfaces without oxide layers have slight improvements on the discharge capacity, while the enhancement of the charge transfer ability of the surfaces of the alloy particles can significantly promote the electrochemical reaction of the alloy electrodes.     

Hydrogen absorption and its influence on the thermal stability of Zr65Al10Ni10Cu15 amorphous alloy

The hydrogen absorption properties of Zr₆₅Al₁₀Ni₁₀Cu₁₅ amorphous alloy with a wide supercooled liquid region were evaluated using a Sieverts-type apparatus. The amorphous alloy absorbs 0.34, 0.80 and 0.85 wt.% hydrogen within 10, 6 and 5 min at 373, 473 and 523 K, respectively. According to Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory, the hydrogen absorption activation energy of the amorphous alloy was 1.27 kJ mol⁻¹. The pressure–composition (P–C) isotherms of the amorphous Zr₆₅Al₁₀Ni₁₀Cu₁₅ alloy at 573, 623 and 673 K did not show a plateau, and the hydrogen absorption capacities were 0.8, 1.3 and 1.7 wt.%, respectively. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) analysis demonstrated that the thermal stability of the amorphous alloy was improved with an enlarged supercooled liquid region after the hydrogen uptake below 473 K, but was decreased after the hydrogenation above 523 K. The alloy still kept the amorphous structure after hydrogenation at 573 K, and transformed into the crystalline phases of ZrH₂, ZrNi and AlCu after the hydrogenation at 673 K.     

Fabrication of Mg–Ni–Sn alloys for fast hydrogen generation in seawater

Mg-2.7Ni-x wt.% Sn(x = 0–2) alloys were fabricated to promote hydrogen generation kinetics of Mg-2.7Ni alloy. The Sn in Mg-2.7Ni-Sn alloys exists as Mg₂Sn phase at the grain boundary and solid solution at the Mg matrix. The Mg₂Sn at the grain boundary acts as the initiation site for pitting corrosion and the dissolved Sn in the alloy causes pitting corrosion by locally breaking the surface oxide film in the Mg matrix in seawater. The Mg-2.7Ni-1Sn alloy showed an excellent hydrogen generation rate of 28.71 ml min^?1 g^?1, which is 1700 times faster than that of pure Mg due to the combined action of galvanic and intergranular corrosion as well as pitting corrosion in seawater. As the solution temperature was increased from 30 to 70 °C, the hydrogen generation rate from the hydrolysis of the Mg-2.7Ni-1Sn alloy was dramatically increased from 34 to 257.3 ml min^?1 g^?1. The activation energy for the hydrolysis of Mg was calculated to be 43.13 kJ mol^?1.     

Synergistic effect of TiF3@ graphene on the hydrogen storage properties of Mg–Al alloy

Mg–Al alloy was prepared by sintering and mechanical alloying, and the effects of graphene (Gp), TiF₃ and Gp/titanium (III) fluoride (TiF₃) on the hydrogen storage properties of the Mg–Al alloy were studied. The results show that Gp and TiF₃ could improve the hydrogen storage properties of Mg–Al alloy. In particular, Gp and TiF₃ showed good synergistic effect for enhancing the hydrogen storage properties of Mg–Al alloy. For example, when 1.0 wt% of H₂ was absorbed/desorbed, the hydrogen adsorption/desorption temperature of the Mg–Al alloy and Mg–Al-M (M = Gp, TiF₃, and TiF₃@Gp) composites were 241/343 °C, 185/310 °C, 229/292 °C and 159/280 °C, respectively. For the Mg–Al alloy, the apparent activation energy was 176.5 kJ mol^?1, and it decreased to 139.8 kJ mol^?1, 171.6 kJ mol^?1, and 94.3 kJ mol^?1, with the addition of Gp, TiF₃ and TiF₃@Gp composites, respectively. Evidently, the comprehensive hydrogen storage properties of Mg–Al alloy were improved remarkably under the synergistic effect of Gp and TiF₃.     

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.     

Structure, morphology and hydrogen storage properties of a Ti0.97Zr0.019V0.439Fe0.097Cr0.045Al0.026Mn1.5 alloy

The Ti_0.97Zr_0.019V_0.439Fe_0.097Cr_0.045Al_0.026Mn_1.5 alloy is a hexagonal C14 Laves phase material that reversibly stores hydrogen under ambient temperatures. Structural changes are studied by XRD and SEM with regard to hydrogenation and dehydrogenation cycling at 25, 40 and 60 °C. The average particle size is reduced after hydrogenation and dehydrogenation cycling through decrepitation. The maximum hydrogen capacity at 25 °C is 1.71 ± 0.01 wt. % under 78 bar H₂, however the hydrogen sorption capacity decreases and the plateau pressure increases at higher temperatures. The enthalpy (ΔH) and entropy (ΔS) of hydrogen absorption and desorption have been calculated from a van’t Hoff plot as −21.7 ± 0.1 kJ/mol H₂ and −99.8 ± 0.2 J/mol H₂/K for absorption and 25.4 ± 0.1 kJ/mol H₂ and 108.5 ± 0.2 J/mol H₂/K for desorption, indicating the presence of a significant hysteresis effect.     

Solvent-free synthesis of Rh/meso-Al2O3 via mechanochemistry for hydrolytic dehydrogenation of ammonia borane

An effective strategy synthesis of Rh/meso-Al₂O₃ catalysts was demonstrated by mechanochemistry for hydrolytic dehydrogenation of ammonia borane (AB). These catalysts are characterized systematically by N₂ adsorption-desorption isotherms, X-ray diffraction (XRD), X-ray photoelectron spectrometry (XPS), scanning electron microscope (SEM), and transmission electron microscope (TEM). The results show that the turnover frequency (TOF) and activation energy (E_a) are 246.8 mol_H2·mol_Rh^?1·min^?1 and 47.9 kJ mol^?1 for hydrolytic dehydrogenation of at 298 K catalyzed by Rh/Al₂O₃-CTAB-400, obviously higher than those previously reported catalysts. Furthermore, catalyst Rh/Al₂O₃-CTAB-400 can be recycled by simple centrifugal separation and the catalytic activity is still well maintained after five cycles. In addition, a plausible mechanism for hydrolytic dehydrogenation of AB has also been proposed. This mechanochemical synthesis method exhibits great application prospects for the preparation of heterogeneous catalysts.     

Enhanced hydrogen storage properties of LiBH4-MgH2 composite by the catalytic effect of MoCl3

Though LiBH₄-MgH₂ system exhibits an excellent hydrogen storage property, it still presents high decomposition temperature over 350 °C and sluggish hydrogen absorption/desorption kinetics. In order to improve the hydrogen storage properties, the influence of MoCl₃ as an additive on the hydrogenation and dehydrogenation properties of LiBH₄-MgH₂ system is investigated. The reversible hydrogen storage performance is significantly improved, which leads to a capacity of about 7 wt.% hydrogen at 300 °C. XRD analysis reveals that the metallic Mo is formed by the reaction between LiBH₄ and MoCl₃, which is highly dispersed in the sample and results in improved dehydrogenation and hydrogenation performance of LiBH₄-MgH₂ system. From Kissinger plot, the activation energy for hydrogen desorption of LiBH₄-MgH₂ system with additive MoCl₃ is estimated to be ∼43 kJ mol⁻¹ H₂, 10 kJ mol⁻¹ lower than that for the pure LiBH₄-MgH₂ system indicating that the kinetics of LiBH₄-MgH₂ composite is significantly improved by the introduction of Mo.     

Microstructure and hydrogen absorption/desorption properties of Mg24Y3M (M = Ni,Co, Cu,Al) alloys

Ternary alloys with the nominal composition of Mg₂₄Y₃M (M = Ni, Co, Cu, Al) have been fabricated by using vacuum induction melting method. Their microstructure and phase composition are characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The isothermal hydrogen absorption and desorption kinetics are measured by a Sievert's-type apparatus. The dehydrogenation behaviors of the full hydrogenated alloys are also analyzed by differential scanning calorimetry (DSC) method. Results show that each and every alloy has a distinct multiphase structure containing the main phase Mg₂₄Y₅ and some amount of Mg. Intermetallic compounds of YCo₂ and Al₂Y are detected in the M = Co and M = Al alloy, while long-period stacking ordered (LPSO) phase can be also observed in M = Ni and M = Cu alloy. The hydrogen absorption and desorption kinetics shows a decreased trend in the following order: (M = Ni) > (M = Al) > (M = Co) > (M = Cu). The M = Ni alloy has the best hydrogen storage performance among the investigated alloys. The dehydrogenation activation energy (E_a) of the M = Ni alloy decreases to 66 kJ/mol, and its decomposition peak temperature is also reduced to 313 °C. Moreover, the p–c–T (pressure-composition isotherms) curves of the studied alloys are also discussed.     

Alumina nanofiber-stabilized ruthenium nanoparticles: Highly efficient catalytic materials for hydrogen evolution from ammonia borane hydrolysis

Effective catalysts for hydrogen generation from ammonia borane (AB) hydrolysis should be developed for the versatile applications of hydrogen. In this study, ruthenium nanoparticles (NPs) supported on alumina nanofibers (Ru/Al₂O₃-NFs) were synthesized by reducing the Ru(Ш) ions impregnated on Al₂O₃-NFs during AB hydrolysis. Results showed that the Ru NPs with an average size of 2.9 nm were uniformly dispersed on the Al₂O₃-NFs support. The as-synthesized Ru/Al₂O₃-NFs exhibited a high turnover frequency of 327 mol H₂ (mol Ru min)^?1 and an activation energy of 36.1 kJ mol^?1 for AB hydrolysis at 25 °C. Kinetic studies showed that the AB hydrolysis catalyzed by Ru/Al₂O₃-NFs was a first-order reaction with regard to the Ru concentration and a zero-order reaction with respect to the AB concentration. The present work reveals that Ru/Al₂O₃-NFs show promise as a catalyst in developing a highly efficient hydrogen storage system for fuel cell applications.     

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.     

Dual-tuning of de/hydrogenation kinetic properties of Mg-based hydrogen storage alloy by building a Ni-/Co-multi-platform collaborative system

The Mg-based hydrogen storage alloy with multiple platforms is successfully prepared by ball milling Co powder and Mg-RE-Ni precursor alloy, and its hydrogen storage behavior was investigated in detail by XRD, EDS, TEM, PCI, and DSC methods. The ball-milled alloy consists of the main phase Mg, the catalytic phases Mg₂Ni, Mg₂Co as well as a small amount of Mg₁₂Ce, and convert into the MgH₂–CeH_2.73-Mg₂NiH₄–Mg₂CoH₅ composite after hydrogenation. The composite has three PCI platforms corresponding to the reversible de/hydrogenation reaction of Mg/MgH₂, Mg₂Ni/Mg₂NiH₄ and Mg₆Co₂H₁₁/Mg₂CoH₅. Among them, the transformation between Mg₂Ni and Mg₂NiH₄ triggers the “spill-over” effect which promote the decomposition of MgH₂ phases and enhances the hydrogen desorption kinetics. Meanwhile, the conversion of the Mg₆Co₂H₁₁ to Mg₂CoH₅ phase induces the “chain reaction” effect, which leads to preferential nucleation of Mg phase and improves the hydrogen absorption kinetics. Therefore, the Mg-RE-Ni-Co alloy has a double improvement on hydrogen absorption and desorption kinetics. Concretely, the alloy has an optimal hydrogen absorption temperature of 200 °C, at which it can absorb 5.5 wt. % H₂ within 40 s. Under the conditions, the capacity of absorption almost reaches the maximum reversible value (about 5.6 wt. %). Besides, the alloy has a dehydrogenation activation energy of 67.9 kJ/mol and can desorb 5.0 wt. % H₂ within 60 min at the temperature of 260 °C.