Synthesis and hydriding properties of Li2Mg(NH)2

The phase pure Li₂Mg(NH)₂ has been synthesized via a dehydriding treatment of a ball milled 2LiNH₂ + MgH₂ mixture. This phase pure Li₂Mg(NH)₂ has been utilized to investigate its hydriding kinetics at the temperature range 180-220 °C. It is found that the hydriding process of Li₂Mg(NH)₂ is very sluggish even though it has favorable thermodynamic properties for near the ambient temperature operation. Holding at 200 °C for 10 h only results in 3.75 wt.% H₂ uptake. The detailed kinetic analysis reveals that the hydriding process of Li₂Mg(NH)₂ is diffusion-controlled. Thus, this study unambiguously indicates that the future direction to enhance the hydriding kinetics of this promising hydrogen storage material system should be to minimize the diffusion distance and increase the diffusion rate.     

The mixed lithium-magnesium imide Li2Mg(NH)2 a promising and reliable hydrogen storage material

A statistical theory of the structural phase transformations of the imide Li₂Mg(NH)₂ has been developed, the formation of which is accompanied by the release of free hydrogen. The standard Gibbs free energy formation of the imide's three phases: α, β, γ are calculated. Their dependencies on composition, temperature, pressure, and energy are found. Plots of concentration dependences of the free energy values of these phases are constructed for different temperatures, according to which the imide state diagram is established. Isotherms, isobars and isoplets are investigated. The possibility of hysteresis effect for α and β phases is shown. The results of the calculations are compared with the experimental data.     

Hydrogen storage properties of nanostructured MgH2/TiH2 composite prepared by ball milling under high hydrogen pressure

Nanostructured MgH₂/0.1TiH₂ composite was synthesized directly from Mg and Ti metal by ball milling under an initial hydrogen pressure of 30 MPa. The synthesized composite shows interesting hydrogen storage properties. The desorption temperature is more than 100 °C lower compared to commercial MgH₂ from TG-DSC measurements. After desorption, the composite sample absorbs hydrogen at 100 °C to a capacity of 4 mass% in 4 h and may even absorb hydrogen at 40 °C. The improved properties are due to the catalyst and nanostructure introduced during high pressure ball milling. From the PCI results at 269, 280, 289 and 301 °C, the enthalpy change and entropy change during the desorption can be determined according to the van’t Hoff equation. The values for the MgH₂/0.1TiH₂ nano-composite system are 77.4 kJ mol⁻¹ H₂ and 137.5 J K⁻¹ mol⁻¹ H₂, respectively. These values are in agreement with those obtained for a commercial MgH₂ system measured under the same conditions. Nanostructure and catalyst may greatly improve the kinetics, but do not change the thermodynamics of the materials.     

Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

A novel dual-cation/anion complex hydride (Li₂Mg(BH₄)₂(NH₂)₂), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH₄NH₂ and Li₂BH₄NH₂. The prepared Li₂Mg(BH₄)₂(NH₂)₂ crystallizes in a triclinic structure, and the [NH₂] and [BH₄] groups remain intact within the structure. Upon heating, the prepared Li₂Mg(BH₄)₂(NH₂)₂ decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li₂Mg(BH₄)₂(NH₂)₂ first decomposes to LiMgBN₂, LiBH₄, BN, LiH and MgBNH₈ at 100–250 °C, and then, the newly formed MgBNH₈ reacts with LiH to form Mg, LiBH₄ and BN at 250–340 °C. Finally, the decomposition of LiBH₄ releases hydrogen and generates LiH and B at 340–450 °C.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Synthesis and hydrogen storage thermodynamics and kinetics of Mg(AlH4)2 submicron rods

Mg(AlH₄)₂ submicron rods with 96.1% purity have been successfully synthesized in a modified mechanochemical reaction process followed by Soxhlet extraction. ∼9.0 wt% of hydrogen is released from the as-prepared Mg(AlH₄)₂ at 125–440 °C through a stepwise reaction. Upon dehydriding, Mg(AlH₄)₂ decomposes first to generate MgH₂ and Al. Subsequently, the newly produced MgH₂ reacts with Al to form a Al_0.9Mg_0.1 solid solution. Finally, further reaction between the Al_0.9Mg_0.1 solid solution and the remaining MgH₂ gives rise to the formation of Al₃Mg₂. The first step dehydrogenation is a diffusion-controlled reaction with an apparent activation energy of ∼123.0 kJ/mol. Therefore, increasing the mobility of the species involved in Mg(AlH₄)₂ will be very helpful for improving its dehydrogenation kinetics.     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Role of particle size,grain size,microstrain and lattice distortion in improved dehydrogenation properties of the ball-milled Mg(AlH4)2

The decreased dehydrogenation temperature and improved dehydrogenation kinetics were achieved by high-energy ball milling Mg(AlH₄)₂. The particle size, grain size, microstrain and lattice distortion of the post-milled samples, i.e., from macro- to micro-scale, were systematically characterized by means of SEM and XRD measurements. The results indicated that the high-energy ball milling process led to not only a decrease in the particle size and grain size but also an increase in the microstrain and lattice distortion, which provides a synergetic effect of the thermodynamics and kinetics on lowering the dehydrogenation temperatures of the post-milled Mg(AlH₄)₂ samples. From the kinetic point of view, the refinement of the particles and grains shortens the diffusion distance, and the increase of the microstrain and lattice distortion enhances the diffusivity, which work together to decrease the apparent activation energy for hydrogen desorption. Besides, the presence of microstrain and lattice distortion increased the free energy of the post-milled samples, which was released by recovery and recrystallization processes upon heating. This offers more heat release during the first-step dehydrogenation, consequently leading to thermodynamically decline in dehydrogenation temperatures of the post-milled samples. Such a finding provides insights into the mechanistic understanding on decreased dehydrogenation temperature and improved dehydrogenation kinetics of the post-milled metal hydrides as hydrogen storage materials.     

Reversible hydrogen storage in LiBH4/Ca(AlH4)2 systems

To improve the hydrogen storage property of LiBH₄, the LiBH₄/Ca(AlH₄)₂ reactive systems with various ratios were constructed, and their de-/hydrogenation properties as well as the reaction mechanisms were investigated experimentally. It was found that the sample with the LiBH₄ to Ca(AlH₄)₂ molar ratio of 6:1 exhibits the best comprehensive hydrogen storage properties, desorbing hydrogen completely (8.2 wt.%) within 35 min at 450 °C and reversibly absorbing 4.5 wt.% of hydrogen at 450 °C under a hydrogen pressure as low as 4.0 MPa. During the first dehydrogenation process of the LiBH₄/Ca(AlH₄)₂ systems, the CaH₂ and Al particles were in situ precipitated via the self-decomposition of Ca(AlH₄)₂, and then reacted with LiBH₄ to form CaB₆, AlB₂ and LiH. Whereafter, the sample can cycle between LiBH₄ + Ca(BH₄)₂ + Al in the hydrogenated state and CaB₆ + AlB₂ + LiH in the dehydrogenated state.     

Multi-hydride systems with enhanced hydrogen storage properties derived from Mg(BH4)2 and LiAlH4

A two-step ball-milling method has been provided to synthesize Mg(BH₄)₂ using NaBH₄ and MgCl₂ as starting materials. The method offers high yield and high purity (96%) of the compound. The as-synthesized Mg(BH₄)₂ is then combined with LiAlH₄ by ball-milling in order to form new multi-hydride systems with high hydrogen storage properties. The structure, the dehydrogenation and the reversibility of the combined systems are studied. Analyses show that a metathesis reaction takes place between Mg(BH₄)₂ and LiAlH₄ during milling, forming Mg(AlH₄)₂ and LiBH₄. Mg(BH₄)₂ is excessive and remains in the ball-milled product when the molar ratio of Mg(BH₄)₂ to LiAlH₄ is over 0.5. The onset dehydrogenation temperature of the combined systems is lowered to ca. 120 °C, which is much lower than that of either Mg(BH₄)₂ or LiAlH₄. The dehydrogenation capacities of the combined systems below 300 °C are all higher than that of both Mg(BH₄)₂ and LiAlH₄. The combined systems are reversible for hydrogen storage at moderate hydrogenation condition, and rapid hydrogenation occurred within the initial 30 min. Moreover, the remained Mg(BH₄)₂ in the combined systems is found also partially reversible. The mechanism of the enhancement of the hydrogen storage properties and the dehydrogenation/hydrogenation process of the combined systems were discussed.     

Mixed-metal Li3N-based systems for hydrogen storage: Li3AlN2 and Li3FeN2

The hydrogen storage systems Li₃AlN₂ and Li₃FeN₂ were synthesized mechanochemically by two different routes. In each case an intermediate material formed after milling, which transformed into Li₃MN₂ (M = Al or Fe) upon annealing. The synthesis route had a measurable effect on the hydrogen storage properties of the material: Li₃AlN₂ prepared from hydrogenous starting materials (LiNH₂ and LiAlH₄) performed better than that synthesized from non-hydrogenous materials (Li₃N and AlN). For both Li₃AlN₂ materials, the hydrogen storage capacity and the absorption kinetics improved significantly upon cycling. Ti-doped Li₃AlN₂ synthesized from LiNH₂ and LiAlH₄ showed the best hydrogen storage characteristics of all, with the best kinetics for hydrogen uptake and release, and the highest hydrogen storage capacity of 3.2 wt.%, of which about half was reversible. Meanwhile, Li₃FeN₂ synthesized from Li₃N and Fe displayed similar kinetics to that synthesized from Li₃N and Fe_xN (2 ≤ x ≤ 4), but demonstrated lower gravimetric hydrogen storage capacities. Li₃FeN₂ displayed a hydrogen uptake capacity of 2.7 wt.%, of which about 1.5 wt.% was reversible. For both Li₃AlN₂ and Li₃FeN₂, doping with TiCl₃ resulted in enhancement of hydrogen absorption kinetics. This represents the first study of a ternary lithium-transition metal nitride system for hydrogen storage.     

Improved hydrogen storage properties of Mg/MgH2 thanks to the addition of nickel hydride complex precursors

In order to improve the hydrogenation/dehydrogenation properties of the Mg/MgH₂ system, the nickel hydride complex NiHCl(P(C₆H₁₁)₃)₂ has been added in different amounts to MgH₂ by planetary ball milling. The hydrogen storage properties of the formed composites were studied by different thermal analyses methods (temperature programmed desorption, calorimetric and pressure-composition-temperature analyses). The optimal amount of the nickel complex precursor was found to be of 20 wt%. It allows to homogeneously disperse 1.8 wt% of nickel active species at the surface of the Mg/MgH₂ particles. After the decomposition of the complex during MgH₂ dehydrogenation, the formed composite is stable upon cycling at low temperature. It can release hydrogen at 200 °C and absorb 6.3 wt% of H₂ at 100 °C in less than 1 h. The significantly enhanced H₂ storage properties are due to the impact of the highly dispersed nickel on both the kinetics and thermodynamics of the Mg/MgH₂ system. The hydrogenation and dehydrogenation enthalpies were found to be of −65 and 63 kJ/mol H₂ respectively (±75 kJ/mol H₂ for pure Mg/MgH₂) and the calculated apparent activation energies of the hydrogen uptake and release processes are of 22 and 127 kJ/mol H₂ respectively (88 and 176 kJ/mol H₂ for pure Mg/MgH₂). The change in the thermodynamics observed in the formed composite is likely to be due to the formation of a Mg_0.992Ni_0.008 phase during dehydrogenation/hydrogenation cycling. The impact of another hydride nickel precursor in which chloride has been replaced by a borohydride ligand, namely NiH(BH₄)(P(C₆H₁₁)₃)₂, is also reported.     

Hydrogen storage properties of the novel crosslinked UiO-66-(OH)2

Metal organic frameworks (MOF) are a type of nanoporous materials with large specific surface area, which are especially suitable for gas separation and storage. In this work, we report a new approach of crosslinking UiO-66-(OH)₂ to enhance its hydrogen storage capacity. UiO-66-(OH)₂ was synthesized using hafnium tetrachloride (HfCl₄) and 2, 5-dihydroxyterephthalic acid (DTPA) through a canonical modulated hydrothermal method (MHT), followed by a post-synthesis modification, which is to form a crosslinking structure inside the porous structure of UiO-66-(OH)₂. During the modification process, the phenolic hydroxyl groups on the UiO-66-(OH)₂ reacted with methanal, and HCl aqueous solution and triethylamine served as catalyst (the products denoted as UiO-66-H and UiO-66-T, respectively). Powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), ¹³C nuclear magnetic resonance spectroscopy (¹³C NMR) proved that the crosslinking was formed. The BET specific surface area and the average adsorption pore size of UiO-66-H and UiO-66-T significantly increased after modification. The hydrogen storage capacity of UiO-66-H reached a maximum of 3.37 wt% (16.87 mmol/g) at 77 K, 2 MPa. Hydrogen adsorption enthalpy of UiO-66-T was 0.986 kJ/mol, which was higher than that of UiO-66-(OH)₂ (0.695 kJ/mol). This work shows that UiO-66-(OH)₂ is a promising candidate for potential application in high-performance hydrogen storage.     

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH₂ + MgH₂) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO₂, O₂, N₂ and CH₄ are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH₂ + MgH₂) system. The results indicate that the hydrogen capacity of the (Mg(NH₂)₂ + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li₂CN₂ and MgO appear after (2LiNH₂ + MgH₂) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH²⁻, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.     

Synthesis,characterization and hydrogen storage behaviour of AB2 (ZrFe2, Zr(Fe0.75V0.25)2, Zr(Fe0.5V0.5)2 type materials

In this paper, we describe and discuss the synthesis, structural-microstructural and hydrogen storage behaviour of three AB₂ type storage materials namely (a) ZrFe₂, (b) Zr(Fe_0.75V_0.25)₂ and (c) Zr(Fe_0.5V_0.5)₂. These alloys were synthesied by radio frequency induction melting in argon atmosphere. X-ray diffraction and transmission electron microscope have been employed for structural and microstructural characterizations. The XRD study reveals that the lattice constants and the unit cell volume of ZrFe₂, Zr (Fe_0.75V_0.25)₂, Zr(Fe_0.5V_0.5)₂ alloys, which has C14 type hexagonal Laves phase. The Surface morphology and elemental composition of these alloys were investigated by scanning electron microscope and energy dispersive X-ray analysis. The pressure composition isotherms of these alloys were investigated at room temperature and pressure ranges of 0–100 atm respectively, measured through a fully computerized PCI apparatus. As we increase the concentration of V (substituted for Fe), the total hydrogen storage capacities increased up to 1.45 wt%. This capacity is achieved in Zr(Fe_0.5V_0.5)₂ alloy, while the reversible hydrogen storage capacity decreases due to the formation of a stable hydride phase. It has been found that the lattice constants increase with higher vanadium concentration. This is indicating that the majority of vanadium atoms reside in the B-site. The broader X-ray diffraction peaks observed in Zr(Fe_0.5V_0.5)₂ alloy indicates a higher degree of disorder for alloys with the higher V-content. The yet another interesting feature observed in our present study is that the plateau pressure remains well below 1 atm for all the compositions.     

Effect of NaH/MgB2 ratio on the hydrogen absorption kinetics of the system NaH + MgB2

In this work the effect of the ratio of starting reactants on the hydrogen absorption reaction of the system xNaH + MgB₂ is investigated. At a constant hydrogen pressure of 50 bar, depending on the amount of NaH present in the system NaH + MgB₂, different hydrogen absorption behaviors are observed. For two system compositions: NaH + MgB₂ and 0.5NaH + MgB₂, the formation of NaBH₄ and MgH₂ as only crystalline hydrogenation products is achieved. The relation between the ratio of the starting reactants and the obtained hydrogenation products is discussed in detail.     

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

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

Improved hydrogen storage performance of Mg(NH2)2/LiH mixture by addition of carbon nanostructured materials

The effect of different carbon nanostructures specifically carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the improvement of the de/re-hydrogenation characteristics of a Mg(NH₂)₂/LiH mixture have been studied. Amongst CNTs and CNFs, the improvement in the hydrogenation properties for the Mg(NH₂)₂/LiH mixture is higher when CNFs are used as a catalyst. Investigations are also focused on the deployment of two different types of CNF (a) CNF1 (synthesized using a ZrFe₂ catalyst) and (b) CNF2 (synthesized using a LaNi₅ catalyst). The results show that CNF2 is better. The maximum decomposition temperature for the pristine Mg(NH₂)₂/LiH mixture is found to be ∼250 °C, which is reduced to ∼180 and ∼150 °C for the sample mixed with 4 wt% of multi-walled carbon nanotubes (MWCNTs) and CNF2 respectively. The activation energy for the dehydrogenation reaction is found to be 74 and 68 kJ mol⁻¹ for the samples mixed with MWCNT and CNF2 respectively, whereas the activation energy for the dehydrogenation reaction of the pristine Mg(NH₂)₂/LiH mixture is 97 kJ mol⁻¹. The catalytic activity and the de/re-hydrogenation characteristics of the Mg(NH₂)₂/LiH mixture mixed with different carbon nanostructures are described and discussed.     

Effect of chromium,manganese and yttrium on microstructure and hydrogen storage properties of TiFe-based alloy

In this paper, we report the microstructure and hydrogen storage properties of TiFe-based alloys containing chromium (Cr), manganese (Mn) and yttrium (Y). Four alloy samples with chemical composition of TiFe_0.9Cr_0.1, TiFe_0.9Cr_0.1Y_0.05, TiFe_0.9Mn_0.1 and TiFe_0.9Mn_0.1Y_0.05 were prepared by arc melting, and the effects of alloying elements Cr, Mn and Y on microstructure and hydrogenation kinetics and thermodynamics were investigated in detail. It was found that all the four alloys have the main phase of TiFe intermetallic compound. A small amount of secondary phase can be also detected in the alloy samples. Cr substituted alloys have larger lattice parameters than that of Mn substituted alloys. Y in the alloys is mainly existed in the form of α-Y phase, and it transform into YH₃ during hydrogenation process. Very sloped equilibrium plateau regions are observed in Cr substituted alloys, while the Mn substituted alloys have flat equilibrium plateaus. Y addition has almost no influence on pressure–composition–isotherm (p–c–T) curves of Cr substituted alloy, but slightly decrease the equilibrium plateaus of Mn substituted alloys. Hydrogen absorption and desorption kinetics strongly depend on the equilibrium plateau pressures. As a result, the Cr substituted alloys with lower equilibrium plateau pressure have faster hydrogen absorption and slower desorption kinetics compared with Mn substituted alloys. The Cr substituted alloys have poor powdering resistance compared with Mn substituted alloys during hydrogenation cycles, which can be ascribed to the higher hardness of alloy matrix caused by Cr substitution.     

Effects of ultra-high pressure on phase compositions,phase configurations and hydrogen storage properties of LaMg4Ni alloys

The influences of ultrahigh pressure (UHP, under 5 GPa) on phase compositions, phase morphologies and hydrogen storage properties of LaMg₄Ni alloys were studied. The X-ray diffraction patterns show that the as-cast alloy consists of La₂Mg₁₇, LaMg₂Ni and Mg₂Ni phases, whereas a new LaMg₃ phase is observed in the UHP samples in addition to LaMg₂Ni and Mg₂Ni phases. The scanning electron microscopy graphs indicate that the phase distribution is more homogenous in the UHP alloys than in the as-cast one. Additionally, the microstructure of the UHP alloy heat-treated at 973 K is finer than that at 823 K. Both the reversible hydrogen storage capacity and the plateau of hydrogen pressure of the UHP alloys are close to those of the as-cast one. Of particular interest is that both UHP alloys exhibit better activation properties compared with the as-cast alloy. Moreover, the dehydriding onset temperature of the UHP alloys (5 GPa at 973 K) is about 490 K, which is obviously lower than that of the as-cast alloy. The amount of hydrogen desorption in the UHP alloy (5 GPa at 973 K) is 2.69 wt.% at 573 K, which corresponds to 89.6% of the saturated capacity. However, the corresponding values change to 1.75 wt.% and 58.3% in the as-cast alloy, respectively. It is confirmed the UHP treatment is one of effective approaches to tune the hydrogen storage performances of those rare earth–magnesium–nickel alloys.