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.     

Dual-tuning effect of In on the thermodynamic and kinetic properties of Mg2Ni dehydrogenation

Mg₂In_0.1Ni solid solution with an Mg₂Ni-type structure has been synthesized and its hydrogen storage properties have been investigated. The results showed that the introduction of In into Mg₂Ni not only significantly improved the dehydrogenation kinetics but also greatly lowered the thermodynamic stability. The dehydrogenation activation energy (E_a) and enthalpy change (ΔH) decreased from 80 kJ/mol and 64.5 kJ/mol H₂ to 28.9 kJ/mol and 38.4 kJ/mol H₂, respectively. The obtained results point to a method for improving not only the thermodynamic but also the kinetic properties of hydrogen storage materials.     

The effects of Co and Ni addition on the hydrogen storage properties of Mg3Mm

The effects of Ni and Co addition on the hydrogen storage properties of Mg₃Mm alloy was studied by X-ray diffraction (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDX) and pressure-composition isotherm (PCI) measurement. The hydrogen absorption kinetics and the thermodynamic parameters (apparent ΔH, ΔS) for Mg₃Mm dehydrogenation reactions in Mg₃Mm, Mg₃MmNi_0.1 and Mg₃MmNi_0.1Co_0.1 alloys have been also investigated. The maximum hydrogen storage content of Mg₃Mm, Mg₃MmNi_0.1 and Mg₃MmNi_0.1Co_0.1 alloys was improved due to that the addition of Ni and/or Co further spurred the MmH₃ phase transforming to MmH₂ phase. On the other side, the kinetics curves show the addition of Co could enhance hydrogen absorption rate while the addition of Ni change the hydrogenation reaction mechanism.     

Ab initio calculations on energetics and electronic structures of cubic Mg3MNi2 (M = Al, Ti, Mn) hydrogen storage alloys

Mg₃MNi₂ (M = Al, Ti, Mn) ternary intermetallic compounds with cubic structure are a new type of potential hydrogen storage alloys. Using ab initio density functional theory (DFT) calculations, the energetics and electronic structures of Mg₃MNi₂ (M = Al, Ti, Mn) compounds are systematically investigated. The optimized structural parameters including lattice constants and internal atomic positions are close to experimental data determined from X-ray powder diffraction. The calculated results of formation enthalpy ΔH_form show that the stabilities of cubic Mg₃MNi₂ (M = Al, Ti, Mn) compound, compared with hexagonal Mg₂Ni, increase in the order of Mg₃MnNi₂, Mg₂Ni, Mg₃TiNi₂ and Mg₃AlNi₂, whereas the stabilities of their saturated Mg₃MNi₂H₃ (M = Al, Ti, Mn) hydrides, compared with monoclinic Mg₂NiH₄, decrease in the order of Mg₂NiH₄, Mg₃AlNi₂H₃, Mg₃TiNi₂H₃ and Mg₃MnNi₂H₃. Further calculations of hydrogen desorption enthalpy ΔH_des indicate that these cubic Mg₃MNi₂ (M = Al, Ti, Mn) compounds possess promising dehydrogenation properties for their relatively lower ΔH_des values. Among of them, the dehydrogenation ability of Mg₃TiNi₂ is the most pronounced. Analysis of electronic structures suggests that the strong covalent bonding interactions between Ni and M within cubic Mg₃MNi₂ (M = Al, Ti, Mn) are dominant and directly control the structural stabilities of these compounds.     

Interactions of Y and Cu on Mg2Ni type hydrogen storage alloys: A study based on experiments and density functional theory calculation

Evolution of microstructure and hydrogen storage performances were studied in a Y substituted Mg₂₄Ni₁₀Cu₂ hydrogen storage alloy. Interactions of Y and Cu on the phase structure and hydrogen storage properties were explore. Substitution by Y refined the microstructure and yield existence of YMgNi₄. Furthermore, Y addition promoted the replacement of Cu for Ni in the Mg₂Ni.The study of the alloy's dehydrogenation performance and mechanism showed that the addition of Y did not alter the mechanism of random nucleation and subsequent growth, but reduced the activation energy of the dehydrogenation of the alloy from 77.4 kJ/mol to 67.6 kJ/mol. The thermodynamic energy of the dehydrogenation was also improved, and the enthalpy change (ΔH) and entropy change (ΔS) of the Mg₂NiH₄ phase decreased from 67.1 J/K/mol H₂ and 123.1 J/K/mol H₂ to 61.1 J/K/mol H₂ and 115.4 J/K/mol H₂, respectively. Furthermore, the density functional theory calculation showed that the addition of Y promoted the substitution of Cu for Ni, further reduced the stability of the main hydride Mg₂NiH₄, facilitated the release of hydrogen, and reduced the ΔH and ΔS of the hydride dehydrogenation.     

Enhanced reversible hydrogen storage properties of a Mg–In–Y ternary solid solution

A Mg(In, Y) ternary solid solution was successfully synthesized by two-step method, namely sintering the elemental powders and subsequent milling. The formation of Mg(In, Y) indicates that the solubility of Y in the Mg lattice is expanded due to the existence of In. The as-synthesized Mg₉₀In₅Y₅ solid solution transformed to MgH₂, YH₃, In₃Y and MgIn compound upon hydrogenation, the hydrogenated products except for the YH₃ recovered to Mg(In, Y) solid solution after dehydrogenation. The Mg₉₀In₅Y₅ solid solution exhibited a decreased reaction enthalpy of 62.9 kJ/(mol H₂), reduction by ca. 5 kJ/(mol H₂) or 12 kJ/(mol H₂) than the Mg₉₅In₅ binary solid solution and pure Mg, respectively. The working temperature as well as the activation energies for the hydriding and dehydriding were also decreased in comparison with those of Mg(In) binary solid solution, which are attributed to the reduced reaction enthalpy and the catalytic role of YH₃. Our work indicates that the thermodynamic and kinetic tuning of MgH₂ are realized in the Mg(In, Y) ternary solid solution.     

Effect of in-situ formed Mg2Ni/Mg2NiH4 compounds on hydrogen storage performance of MgH2

Magnesium-based hydrogen storage materials (MgH₂) are promising hydrogen carrier due to the high gravimetric hydrogen density; however, the undesirable thermodynamic stability and slow kinetics restrict its utilization. In this work, we assist the de/hydrogenation of MgH₂ via in situ formed additives from the conversion of an MgNi₂ alloy upon de/hydrogenation. The MgH₂–16.7 wt%MgNi₂ composite was synthesized by ball milling of Mg powder and MgNi₂ alloy followed by a hydrogen combustion synthesis method, where most of the Mg converted to MgH₂, and the others reacted with the MgNi₂ generating Mg₂NiH₄, which produced in situ Mg₂Ni during dehydrogenation. Results showed that the Mg₂Ni and Mg₂NiH₄ could induce hydrogen absorption and desorption of the MgH₂, that it absorbed 2.5 wt% H₂ at 473 K, much higher than that of pure Mg, and the dehydrogenation capacity increased by 2.6 wt% at 573 K. Besides, the initial dehydrogenation temperature of the composite under the promotion of Mg₂NiH₄ decreased greatly by 100 K, whereas it is 623 K for MgH₂. Furthermore, benefiting from the catalyst effect of Mg₂NiH₄ during dehydrogenation, the apparent activation energy of the composite reduced to 73.2 kJ mol⁻¹ H₂ from 129.5 kJ mol⁻¹ H₂.     

Hydrogen storage properties of Mg98.5Gd1Zn0.5 and Mg98.5Gd0.5Y0.5Zn0.5 alloys containing LPSO phases

In this work, the as-cast Mg-rich Mg_98.5Gd₁Zn_0.5 and Mg_98.5Gd_0.5Y_0.5Zn_0.5 alloys are prepared by the semi-continuous casting method, and their hydrogen storage performance and catalytic mechanisms are investigated by experimental and first-principles calculations approaches. The results show that the LPSO phases decompose and in-situ form the RE(Gd/Y)H_x(x = 2,3) nano-hydrides upon hydrogenation. These nano-hydrides not only serve as the in-situ catalysts to promote the hydrogen ab/desorption of Mg matrix, but also present the pinning effect to inhibit the growth of Mg/MgH₂ grains during hydrogenation and dehydrogenation. Comparatively, the two alloys exhibit the similar hydrogen absorption kinetics, while the hydrogen desorption kinetics of Mg_98.5Gd₁Zn_0.5 is superior to that of Mg_98.5Gd_0.5Y_0.5Zn_0.5. The first-principles calculations reveal that the GdH₂ and YH₂ hydrides exhibit different catalytic effects on weakening the bond strength of H–H within H₂ and Mg–H within MgH₂, which interprets well the differences in the hydrogen ab/desorption kinetics between Mg_98.5Gd₁Zn_0.5 and Mg_98.5Gd_0.5Y_0.5Zn_0.5 alloys.     

Enhanced Hydrogen Ab/De-sorption of Mg(Zn) solid solution alloy catalyzed by YH2/Y2O3 nanocomposite

YH₂/Y₂O₃ nanocomposite was prepared and introduced to Mg_0.97Zn_0.03 solid solution alloy forming a nanocomposite of Mg_0.97Zn_0.03-10 wt%YH₂/Y₂O₃ by mechanical milling. The phase components and microstructure were systematically investigated by XRD, SEM and STEM. Hydrogenation of Mg_0.97Zn_0.03 solid solution resulted in phase segregation into MgH₂ and intermetallic compound MgZn₂. The in-situ formed ultra-fine MgZn₂ homogeneously dispersed in MgH₂ matrix, and returned to Mg(Zn) solid solution through dehydrogenation. This reversible phase transition of Mg(Zn) solid solution benefited to thermodynamic destabilization of MgH₂. The co-dopant of YH₂ and Y₂O₃ exhibited synergistic catalytic effects on the hydrogen absorption and desorption of Mg_0.97Zn_0.03 solid solution alloy. As a result, Mg_0.97Zn_0.03-10 wt%YH₂/Y₂O₃ nanocomposite showed significantly improved kinetics with obviously lowered hydriding and dehydriding activation energy of 45.8 kJ⋅mol⁻¹⋅H₂ and 74.7 kJ⋅mol⁻¹⋅H₂, respectively, and the enthalpy of hydrogen desorption was reduced to 72.2 kJ⋅mol⁻¹⋅H₂.     

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.     

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.     

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.     

The hydrogen storage properties and reaction mechanism of the MgH2-NaAlH4 composite system

In this study, we report the hydrogen absorption/desorption properties and reaction mechanism of the MgH₂-NaAlH₄ (4:1) composite system. This composite system showed improved dehydrogenation performance compared with that of as-milled NaAlH₄ and MgH₂ alone. The dehydrogenation process in the MgH₂-NaAlH₄ composite can be divided into four stages: NaAlH₄ is first reacted with MgH₂ to form a perovskite-type hydride, NaMgH₃ and Al. In the second dehydrogenation stage, the Al phase reacts with MgH₂ to form Mg₁₇Al₁₂ phase accompanied with the self-decomposition of the excessive MgH₂. NaMgH₃ goes on to decompose to NaH during the third dehydrogenation stage, and the last stage is the decomposition of NaH. Kissinger analysis indicated that the apparent activation energy, E_A, for the MgH₂-relevent decomposition in MgH₂-NaAlH₄ composite was 148 kJ/mol, which is 20 kJ/mol less than for as-milled MgH₂ (168 kJ/mol). X-ray diffraction patterns indicate that the second, third, and fourth stages are fully reversible. It is believed that the formation of Al₁₂Mg₁₇ phase during the dehydrogenation process alters the reaction pathway of the MgH₂-NaAlH₄ (4:1) composite system and improves its thermodynamic properties.     

Hydrogen desorption performance of high-energy ball milled Mg2NiH4 catalyzed by multi-walled carbon nanotubes coupling with TiF3

The Mg₂NiH₄ complex hydrides were synthesized by high-energy ball milling (HEBM) MgH₂ + Ni mixtures. Multi-walled carbon nanotubes (MWCNTs) or TiF₃ as catalysts were added and the catalytic-dehydrogenation behaviors were investigated. All prepared samples are characterized by X-ray diffraction (XRD) spectroscopy, scanning electron microscope (SEM) and differential scanning calorimetry (DSC) to acquire information of microstructure, phase compositions, surface and dehydrogenation properties. The results indicate that the method of adding catalysts by HEBM is reasonable and the hydrogen desorption property of Mg₂NiH₄ is improved by catalysts. It is worth noting that the dehydrogenation temperature (T_D) and the activation energy (E_a) of Mg₂NiH₄ catalyzed by MWCNTs coupling with TiF₃ are reduced to 230 °C (243.6 °C of Mg₂NiH₄) and 53.24 kJ/mol (90.13 kJ/mol of Mg₂NiH₄), respectively. The addition of proper catalysts is proved to be an effective strategy to decrease T_D and E_a of Mg₂NiH₄ hydrides.     

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.     

Improved dehydrogenation of MgH2-Li3AlH6 mixture with TiF3 addition

MgH₂-Li₃AlH₆ mixture shows a mutual activation effect between the components. But the dehydrogenation kinetics is still slow, especially at temperature as low as 250 °C. Hereby, an additive (TiF₃) was introduced into the mixture in the present study. The reaction mechanisms were studied by the combined analyses of X-ray diffraction (XRD), thermogravimetric analysis (TGA), as well as thermodynamic calculations. A two-step ball milling method could reduce the mechanical decomposition of Li₃AlH₆ effectively and was adopted. During milling, Li₃AlH₆ reacts with TiF₃ and produces Al₃Ti while MgH₂ remains stable. All the species are well mixed after milling and the grain size is as small as 100 nm. During TGA test, all the reactions occur at lower temperatures compared with undoped mixture, especially the dehydrogenation of MgH₂, which shows a decrease of 60 °C. Its activation energy is reduced by 32.0 kJ mol⁻¹. The first three isothermal (250 °C) cycles indicate that the kinetics of dehydrogenation has been greatly enhanced, showing a reversible capacity of 4.5 wt.% H₂. The time needed for the 1st dehydrogenation has been shortened to 3600 s from 8000 s for the undoped mixture. These improvements are mainly attributed to the catalytic effect of the in-situ formed Al₃Ti. But there is no influence on the rehydrogenation kinetics and the enthalpy of the dehydrogenation of MgH₂ is unchanged.     

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.     

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.     

Hydrogenation and dehydrogenation of Mg2Co nanoparticles and carbon nanotube composites

This study investigated the hydrogenation and dehydrogenation behavior of Mg₂Co nanoparticles and carbon nanotube (CNT) composites using temperature-programmed deposition, Raman spectroscopy, and X-ray diffraction (XRD). We used the mechanical alloying method to prepare nanosized Mg₂Co particles on CNTs with three loadings of alloys. The introduction of CNTs showed dehydrogenation, hydrogen desorption starting at 370 °C, with the majority of hydrogen being below 500 °C. This can be explained by the fact that Mg₂Co alloy deposited on CNT surface induced the dissociation of hydrogen into two atoms, which were spilt over and then intercalated into the interlayer of CNT. Accordingly, the atomic intercalation enabled the reduction of the hydrogen desorption activation barrier. The spillover mechanism of hydrogen storage can be confirmed by XRD and Raman spectroscopy because of larger interspacing (d_0 0 2) and weaker graphite degree (I_D/I_G) of CNTs after hydrogenation.