Introduction

Solid-state hydrogen storage was considered as the safest and most effective way to use the H2 as prominent energy carrier in the future1. Magnesium-based alloys have been extensively investigated as potential materials for solid-state hydrogen storage due to its reasonably high hydrogen capacity (7.6wt.%) and could be interesting if it is considered as a heat storage material in heat power plants2. However, the relatively slow kinetics of H/D reaction makes them far from practical application. Extensive efforts have been made to improve the H/D properties of magnesium-based alloys by adopting novel preparation techniques3,4 to reduce the particles sizes or/and adding catalytic components such as transition metals (Ni3, Nb5, Ti6) and rare earth elements (RE = La7, Ce8, Pr9, Nd10). The addition of RE into Mg-based hydrogen storage alloys facilitates hydrogen absorption through the formation of rare earth hydrides (REHx). The REHx act as active nucleation sites for magnesium hydride by chemisorbing hydrogen atoms and transferring them to the Mg-metal interfaces7,11. Among the catalytic elements, Nd shows good catalytic effect on the H/D kinetics of Mg-Ni alloys, especially for the samples of ultrafine crystalline and small particle size10,12,13,14,15,16,17,18,19. Tanaka et al.12 found that the nanocrystallized Nd-Mg-Ni alloys prepared through melting-spinning followed by crystallizing exhibited excellent hydrogen absorbing kinetics and Pressure-Composition-Temperature (PCT) characteristics in comparison with those of the corresponding as-cast alloys with coarse eutectic structures. However, the growth up of the grain size would impair the hydrogen storage performance. Denys et al.20 found that the kinetic properties and cyclic stabilities of nanostructured Mg-based and Mg-8wt.%Mm-20wt.%Ni based hydrides degraded due to the grain growth of Mg nanocrystalline during thermal desorption above 300 °C. Therefore, it is important to keep the grain sizes of NdHx, Mg and Mg2Ni/Ni small as long as possible.

Zhu et al.21,22,23 reported the method of in-situ formation of CeH2.73-MgH2-Ni and YH3-MgH2-Mg2NiH4 nanocomposites through directly hydriding the Mg80Ce18Ni2 and Mg12YNi alloys. They found that the composites of CeH2.73-MgH2-Ni remained its excellent performance after 500 H/D cycles because the in-situ formed nanocomposite structure suppressed the grain growth of Mg and MgH221. However, the as-melted Mg80Ce18Ni2 alloy was a multiphase mixture which was composed of 57 wt.% CeMg3, 29 wt.% Ce2Mg17, 7 wt.% CeMg and 5wt.% CeMgNi421. That is to say, the catalytic elements Ce and Ni didn’t distribute uniformly in the alloy, which would lead to the non-uniformity of in-situ formed CeH2.73-MgH2-Ni composites. It is reasonable to assume that if the catalytic elements distribute uniformly in the whole alloy rather than concentrate in a few phases, the in-situ formed nanocomposites will become more homogenous. One of homogenous alloys is amorphous. Nanocrystallisation of the amorphous LaMg11Ni alloy led to the formation of Mg2Ni/Mg1.9La0.1Ni and La1.8Mg17Ni1 nanocrystalline, which greatly improved the hydrogenation rate and lowered the temperature of hydrogen desorption24,25. Another easily available and stable alloy with homogenous microstructure is the single phase or intermetallic compound. Therefore, searching for the Mg-based multicomponent compound and synthesizing it with catalyst elements Nd and Ni are significant for in-situ formation of ultrafine NdHx-MgH2-Ni/Mg2NiH4 composites with excellent hydrogen storage properties. However, seeking for the Mg-based multicomponent compounds with good hydrogen storage properties is time-consuming through traditional trial-and-error method. A precise Nd-Mg-Ni phase diagram could help us to find a suitable target alloy for hydrogen storage and to select the reasonable process parameters for preparing it. The method combining the computational materials with the experimental verification would save a lot of time to explore new material.

Therefore, the purpose of this study is that how to explore the Nd-Mg-Ni hydrogen storage alloy with excellent properties for meeting the practical application by material design method in order to avoid the drawback of trial-and-error method, then the target alloy is prepared to verify validity. Moreover, the relationships between the thermodynamic property, kinetic performance and microstructure are systematically investigated in order to clarify the H/D mechanism. Specifically, through the first hydrogen-induced decomposition of the new ternary compound Nd4Mg80Ni8, the nanocomposites of NdH2.61-MgH2-Mg2NiH0.3 are in-situ formed. This alloy shows fast H/D rates and good cycling behavior. The mechanism of hydrogen-induced decomposition and phase evolution during hydriding reaction are systematically analyzed by HR-TEM and in-situ SR-PXRD. Moreover, the kinetics mechanism is theoretically investigated by RPP model to clarify our experimental results.

Results and Discussions

The phase diagram of Nd-Mg-Ni system in Mg-rich corner at 400 °C

The phase equilibria at 400 °C were determined by analyzing the phase composition of the four quenched samples. The XRD and EDS analysis of the alloys are listed in Table S1. In the Samples #1 and #2 annealed at 400 °C, an unknown phase (named as Nd4Mg80Ni8) is observed with an average composition of at.% 4.86Nd-87.46Mg-7.68Ni, which is different from Nd5Mg41, Mg, Mg2Ni and any other known ternary compounds. The morphology and XRD patterns of Sample #1 shown in Fig. 1(a,b) confirm the existence of the new ternary compound. The SEM images of Samples #2~4 in Fig. 1(c~e) show that the Nd4Mg80Ni8 equilibrates with Mg, Mg2Ni, NdMg8Ni and Nd5Mg41 phases. According to the experimental results, the determined phase diagram in the Mg-rich corner at 400 °C is plotted in Fig. 1(f).

Figure 1
figure 1

(a–e) The SEM images and XRD pattern of samples and (f) the isothermal section of Nd-Mg-Ni system in the Mg-rich corner at 400 °C.

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The Nd-Mg-Ni ternary compounds are expected to be the hydrogen storage alloys with excellent properties because the uniform composition would leads to in-situ formation of the ultrafine MgH2-NdHx-Ni/Mg2NiH4 composites11,21,26. From the reported literature and our experimental work, it can be found that there are seven ternary compounds in the Nd-Mg-Ni system: Nd4Mg80Ni8, NdMg8Ni, NdMg5Ni27, NdMg2Ni9, Nd2MgNi228, NdMgNi429 and NdMg2Ni930. The new compound Nd4Mg80Ni8 is selected as the target alloy because of its highest content of Mg among those ternary compounds which indicates the maximum hydrogen capacity.

The crystal structure of Nd4Mg80Ni8

In order to determine the crystal structure of Nd4Mg80Ni8, the single phase was synthesized and examined by SR-PXRD. The actual composition of Sample #5 and #6 are Nd4.08Mg89.98Ni5.94 and Nd4.45Mg84.64Ni10.91, respectively. The SR-PXRD pattern of Sample #5 was indexed with a tetragonal unit cell using DICVOL0631. The structure solution started using the charge-flipping algorithm implemented in the program TOPAS v4.232. The Ni and Nd atoms were easily located in the electron density maps. The structure was subsequently solved in the space group of I41/amd (No. 141) by global optimization in direct space with 5 Mg atoms with no constraint using the program FOX33. Rietveld refinement was performed using TOPAS v4.2 and the refined lattice parameters were a = b = 11.2743(1) Å, c = 15.9170(2) Å, V = 2023.19(4) Å3. The diffraction profile fitted by Rietveld refinement using these parameters is shown in Fig. 2(a), with the agreement factors of Rwp = 8.1%, RB = 6.4% and GoF = 1.44. The fitting result suggests that there were 87.8 wt.% Nd4Mg80Ni8 and 12.2 wt.% Mg in the Sample #5. The details of the structure determination and crystallographic data are presented in Tables 1 and 2. The crystal structure of Nd4Mg80Ni8 is shown in Fig. 2(b).

Table 1 Crystallographic data and structure refinement of Nd4Mg80Ni8.
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Table 2 The atomic parameters of Nd4Mg80Ni8.
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Figure 2
figure 2

(a) The experimental (blue), fitted (red) and difference (grey line below observed and calculated patterns) SR-PXRD profiles for Sample #5 at a wavelength of 0.8262 Å and (b) the schematic crystal structure of Nd4Mg80Ni8.

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Nd4Mg80Ni8 has a distinguished structure from other reported M-Mg-Ni (M = metal) ternary alloys. There is one symmetry independent Ni atom in the unit cell coordinates with six Mg atoms forming [NiMg6] in trigonal antiprism shape. Two different Ni-Nd bond distances are observed at 2.8186 and 2.8218 Å, which are close to the sum of the metallic radii rNi + rMg = 2.85 Å and larger than the sum of the covalent radii rNi + rMg = 2.75 Å27. While Nd atom in the unit cell coordinates with sixteen Mg atoms forming [NdMg16], where Nd atom sits in the middle of tetrahexahedron with the Nd-Mg bond distances equaling to 3.4501, 3.5464 and 3.5660 Å. The shortest bond distance is slightly larger than the sum of the metallic radii 3.42 Å. The trigonal antiprisms of [NiMg6] are linked together through vertices and the complex polyhedra of [NdMg16] also linked together via shared vertices. Figure 2(b) shows both [NiMg6] and [NdMg16] complexes form two independent three-dimensional network in the structure.

The phase transformation of Nd4Mg80Ni8 during as-cast → anneal → hydrogenation process

The XRD patterns of as-cast Sample #5 and #6 shown in Fig. 3 show that both of the samples are multiphase alloys containing NdMg12, Mg and Mg2Ni. NdMg12 is a metastable phase in Nd-Mg system which forms if a nucleation barrier for Nd5Mg41 exists34. Therefore, the as-cast sample is not suitable for hydrogenation to in-situ form uniform nanocomposites of hydrides. According the established phase diagram, the equilibrated phase should be 90.7 wt.% Nd4Mg80Ni8 with 9.3 wt.% Mg in Sample #5 and 88.3 wt.% Nd4Mg80Ni8 with 11.7 wt.% Mg2Ni in Sample #6 at 400 °C. In order to obtain a homogeneous alloy, both alloys were annealed at 400 °C for 2 days. It is found that NdMg12 disappeared in the annealed samples and supplanted by Nd4Mg80Ni8. The predicted phase fraction in Sample #5 is consistent with the result determined by SR-PXRD. Figure 3 shows the compared XRD patterns of the as-cast and annealed samples. A small amount of Mg2Ni and Mg are still observed in the annealed samples due to the composition of prepared samples deviating from the designed value. On the other hand, it suggests that the solid solution range of Nd4Mg80Ni8 is limited.

Figure 3
figure 3

The XRD patterns of Nd4Mg80Ni8 with different heat treatment conditions.

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After Sample #6 powder is hydrogenated at 350 °C under 2.0 MPa H2 for 1 h, the diffraction peaks of Nd4Mg80Ni8 disappear and the pattern could be well indexed by NdH2.61, MgH2, Mg2NiH0.3, Mg2NiH4 and little Mg. Compared the XRD pattern of hydrogenated sample with that of the annealed, it can be seen that the overall amount of Mg2Ni and Mg2NiH4 in hydrogenated sample is obviously larger than that in the annealed one. The evidence suggests that Nd4Mg80Ni8 is decomposed to NdH2.61, MgH2, Mg2NiH0.3, Mg2NiH4 and Mg, because the elements Nd and Mg could only come from Nd4Mg80Ni8. In order to make sure whether NdH2.61, MgH2 and Mg2NiH4 are formed in-situ or not and understand the hydrogen-induced decomposition mechanism of Nd4Mg80Ni8, the microstructure and phase composition of the incompletely hydrogenated bulk Sample #5 was investigated by HR-TEM.

The mechanism of hydrogen-induced decomposition and the formation of NdH2.61-MgH2-Mg2NiH0.3 nanocomposites

The comparison of the microstructure of Nd4Mg80Ni8 and incompletely hydrogenated sample is shown in Fig. 4. The starting material shows plate-like Mg2Ni in the Nd4Mg80Ni8 matrix. After hydrogenation the surface of Nd4Mg80Ni8 becomes rough. Many ultrafine white particles are observed on the Mg2Ni. The original Nd4Mg80Ni8/Mg2Ni phase boundaries in the starting alloy are still visible. By excavating holes on the hydrogenated sample, the inner microstructure is shown in Fig. 4(c). Many second phases precipitate from the Nd4Mg80Ni8 matrix after hydrogenation.

Figure 4
figure 4

The microstructure of Nd4Mg80Ni8: (a) before hydrogenation, (b) after hydriding at 350 °C for 1 h and (c) the vertical section of hydrogenated sample.

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HR-TEM was used to determine the phase composition and analyze the microstructure evolution during hydrogenation. The bright-field image, selected area electron diffraction (SAED) pattern and HR-TEM image of the Nd4Mg80Ni8 are shown in Fig. 5 (a~c). The composition of the selected area agrees well with the nominal composition of Nd4Mg80Ni8. The indexing of SAED pattern is consistent with the result of SR-PXRD. In addition, a weak polycrystalline diffraction ring is observed with interplanar spacing d = 2.105 Å which corresponds to the crystal plane of (110) of Nd2O3 (d = 1.9735 Å). It indicates that a little Nd2O3 formed on the surface of the thin slice. From the aspect of thermodynamics, the oxide of Nd should form earlier than the oxide of Mg and Ni because of the lower Gibbs free energy of formation of Nd oxide. The interplanar spacing of d1 and d2 in Fig. 5(c) correspond to the crystal planes of and of Nd4Mg80Ni8. Both values agree with 5.6323 Å and 4.8066 Å determined by SR-PXRD with relative error of 1.36% and 1.80%, respectively. The HR-TEM image shows that the Nd4Mg80Ni8 compound is homogeneous and highly crystallized.

Figure 5
figure 5

(a) The TEM bright-field image of Nd4Mg80Ni8, (b) the SAED pattern of Nd4Mg80Ni8, (c) HR-TEM image of Nd4Mg80Ni8, (d) the bright field image of incompletely hydrogenated Sample #6, (e) SAED pattern of Mg2NiH0.3, (f) the magnified image of the incompletely hydrogenated Nd4Mg80Ni8, (g) the magnified image of MgH2 and Mg2NiH0.3 and the SAED pattern of dark phase, (h) the HR-TEM image of MgH2/Mg2NiH0.3 interface and (i) the composition map of Nd, Mg and Ni.

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Figure 5(d) shows the bright field image of the incompletely hydrogenated sample. The plate-like Mg2Ni in starting alloy dissolves little hydrogen to become the solid solution of Mg2NiH0.3, which is identified by the SAED pattern given in Fig. 5(e). The composition of the Mg2NiH0.3 at the position marked by yellow point is at.% 0.46Nd-66.05Mg-33.49Ni. The hydrogen cannot be detected from EDS. From Fig. 5(d), an original grain boundary in the starting alloy of Nd4Mg80Ni8 is observed. Many fine equiaxed dark particles with size in the range of 58 ~ 250 nm distributed randomly in the bright matrix.

Figure 5(f) shows the magnified TEM image of the region which was single phase Nd4Mg80Ni8 in starting material. From Fig. 5(g), it can be seen that numerous ultrafine particles distribute in both bright and dark phases. The composition of dark phase is at.% 4.47Nd-71.78Mg-23.76Ni. It is Mg2NiH0.3 which is identified by SAED pattern shown in Fig. 5(g). The index of the diffraction rings indicates that the high density particles in Mg2NiH0.3 are NdH2.61 nanoparticles. The high content of 4.47 at.% Nd on the Mg2NiH0.3 is contributed by the NdH2.61. Figure 5(h) shows the high resolution TEM image of the region marked as blue frame in Fig. 5(g). The interplanar spacing of the bright matrix is d = 2.250 Å which corresponds to the crystal plane (110) of MgH2 (d = 2.2570 Å). Numerous equiaxed grains of NdH2.61 are distributed in the matrix MgH2. The average composition of bright region is at.% 5.70Nd-93.23Mg-1.07Ni. The difference of Nd content in Mg2NiH0.3 and MgH2 phases is small, which also indicates that the NdH2.61 particles distribute uniformly in both MgH2 and Mg2NiH0.3 phases. The NdH2.61 is nanocrystalline with grain size of about 4 ~ 40 nm. Figure 5(i) gives the composition map of Nd, Mg and Ni. It shows that the Nd element concentrates in the nanoparticles and Ni element concentrates in Mg2NiH0.3 phases.

Based on the TEM results, the mechanism of hydrogen-induced microstructure evolution can be revealed. When the alloy reacts with hydrogen, the Nd atoms firstly disassociate from Nd4Mg80Ni8 to generate NdH2.61 because the enthalpy of formation of NdH2.61 (−207.2 ~ −187.6 kJ/mol35,36) is relatively lower than that of MgH2 and Mg2NiH4. After the ultrafine nanoparticles of NdH2.61 in-situ generating from the original Nd4Mg80Ni8 compound, the structure of Nd4Mg80Ni8 becomes unstable because the absence of Nd atoms leads to the polyhedra of [NdMg16] crumbling. The released Mg atoms make the structure highly disorder. According to the equilibrated phase diagram, the rest composition will shift to Mg + Mg2Ni two-phase region. The Mg and Ni atoms diffuse fast owing to the disordered structure. Then large particles of Mg and Mg2Ni (58 ~2 50 nm) form to reduce the Gibbs free energy of system. After the Nd4Mg80Ni8 transforming to NdH2.61-Mg-Mg2Ni nanocomposites, the Mg reacts with hydrogen to generate MgH2 and the hydrogen atoms dissolve in Mg2Ni to generate Mg2NiH0.3. The high density NdH2.61 nanoparticles, numerous interfaces between MgH2 and Mg2NiH0.3 and a large number of grain boundaries in the nanocomposites of NdH2.61-Mg-Mg2Ni may provide a great deal of hydrogen atoms diffusion channels and nucleation sites of hydrides. Thus, the NdH2.61-MgH2-Mg2NiH0.3 nanocomposites should exhibit excellent H/D kinetics.

The thermodynamic and kinetic properties of H/D reactions in Nd4Mg80Ni8

The Sample #6 shows a good activation behavior at 350 °C. At the second H/D cycle, it reaches a maximum hydrogen capacity of 5.15 wt.% which is near to the theoretical value 5.18 wt.% H2. All the PCT curves at different temperatures shown in Fig. 6(a) manifests two flat plateaus, indicating that there are two phases reacting with hydrogen during the H/D processes. One of phases exhibits larger storage capacity and wider plateau of H/D reactions marked as the first plateau in Fig. 6(a). The second phase shows higher equilibrium pressure of hydrogen and narrower plateau marked as the second plateau. Table 3 gives plateau pressures, maximum hydrogen capacities at different temperatures and thermodynamic data for the different phases. The hysteresis factor defined as Hf = ln(Pab/Pde) are 0.144, 0.206 and 0.134 at 350, 300 and 250 °C respectively for the first plateau and 0.474, 0.693 and 1.366 for the second plateau. The enthalpies and entropies are derived from the Van’t Hoff equation. The enthalpies of H/D reactions for the first phase are consistent with the reported values of MgH2 in the range of 71.9 ~ 78.0 kJ/mol8,17,21,37. The enthalpies of H/D reactions for the second phase are consistent with the reported values of Mg2NiH4 in the range of 53.23 ~ 72.9 kJ/mol38,39,40,41,42,43. The comparison indicates that both thermodynamic properties of MgH2 and Mg2NiH4 are basically unchanged by adding 4.5 at.% NdH2.61.

Table 3 The H/D properties and thermodynamic data of Nd4Mg80Ni8.
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Figure 6
figure 6

(a) The PCT curves of Nd4Mg80Ni8 at 250, 300 and 350 °C, (b) the absorption kinetic curves in the temperature range of 100 ~ 350 °C, (c) the desorption kinetic curves at 291, 315 and 347 °C and (d) the comparison of hydrogen storage capacity and hydriding rate for Nd-Mg-Ni alloys at 300 °C.

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Figure 6(b) shows the hydriding behavior of Nd4Mg80Ni8 in the temperature range from 100 to 350 °C. It absorbs 85% of the maximum hydrogen content above 250 °C within 5.8 min. After that the hydrogen absorption content increases slowly with prolonging time. At 1 h, the alloy absorbs 4.82 wt.% hydrogen at 350 °C, which is 93% of the theoretical hydrogen storage content. The sample exhibits good desorption kinetics as shown in Fig. 6(c). It releases the absorbed hydrogen thoroughly within 8.3 min when the sample is heated up to 291 °C.

Lots of scholars developed kinetic models for the gas-solid reaction, such as Jander model44, Ginstling-Brounshtein equation45, etc. Evard et al.46 developed a mathematical model to describe the non-isothermal decomposition process of MgH2, which took into account relative rates of hydrogen desorption, chemical transformation on the MgH2-Mg interface and size distribution of the powder particles. In our previous work, Chou et al.47,48,49 proposed a series of formulae concerning the isothermal kinetics of gas-solid reaction based on a real physical picture. All parameters in RPP model have clear physical meanings and the effects of temperature, pressure, particle size, sample shape, density change of resultant on the reaction fraction can be analyzed quantitatively. The treatment of this model avoids the multistep calculation error at multi-temperatures and multi-pressures50. up to now, the RPP model has been successfully used in analyzing the H/D kinetics of Mg-Ni alloy50,51,52, LaNi5-based alloy9, Mg-LaNi553, La2Mg17-based composites54, etc. Therefore, the isothermal H/D kinetics of the Nd4Mg80Ni8 are analyzed by fitting the observed curves using the RPP model. It is found that the rate controlling step is the diffusion of hydrogen in the hydride during hydrogenation by fitting the experimental data with Eq. (5).

where

ξ the reacted fraction equaling to the ratio of hydrogen absorption weight Δm at time t to the maximum hydrogen absorption weight Δmmax, tc(d) the characteristic reaction time representing the required time of a completely hydriding or dehydriding of the sample, PH2 the partial pressure of hydrogen in gas phase, Peq the hydrogen partial pressure in equilibrium with hydride and ΔE the activation energy. The characteristic time tc is regarded as a criterion for reaction rate: the larger the characteristic time, the slower the reaction rate. The corresponding squared correlation coefficient, r2, reflects the level of agreement between fitting curve and experimental data. Using Eq. (1) to fit the hydrogenation data, the calculated tc(d) decreases from 153.5 to 1.6 min when temperature increases from 100 to 300 °C, indicating that the hydriding reaction rate increases with the temperature rising. The apparent activation energy for hydrogenation is determined to be 82.3 kJ/mol by fitting the experimental data using Eq. (2).

There is an interesting phenomenon that the fastest hydriding rate is observed at 300 °C for Nd4Mg80Ni8 (tc(d) = 1.6 min) alloy within the investigated temperature range from 100 to 350 °C. It is known that both the forward reaction rate (hydriding reaction) and reverse reaction rate (dehydriding reaction) are accelerated with the increasing temperature. In addition, the hydrogenation of Mg and Mg2Ni is exothermic, while the dehydrogenation reaction is endothermic. The increase of temperature is propitious for the reverse reaction. If the reverse reaction rate increases more rapidly than the forward reaction rate, an apparent fastest hydriding rate would be found in the temperature range.

In order to compare the hydriding rate of Nd4Mg80Ni8 with that of other Nd-Mg-Ni alloys in literatures12,13,16,18,26, the calculated results of characteristic reaction time are listed in Table S2. Figure 6(d) shows the comparison of hydrogen storage capacity and hydriding rate of those alloys at 300 °C. Except the samples prepared by melt-spinning, the fastest two hydrogenation are observed from Nd14Mg72Ni14 (tc(d) = 1.5 min) and Nd4Mg80Ni8 (tc(d) = 1.6 min) alloys at 300 °C. However, the maximum hydrogen absorption content of Nd14Mg72Ni14 is only 2.91 wt.%, relatively smaller than that of Nd4Mg80Ni8 (4.77 wt.%) at the same temperature. The composition of as-cast Nd4Mg86Ni10 alloy13 is very close to Nd4Mg80Ni8 developed in present work, but the as-cast Nd4Mg86Ni10 is a multiphase alloy consisted of NdMg12, Mg and Mg2Ni phases. This indicates that the composition distribution in the annealed Nd4Mg80Ni8 is more uniform than that in as-cast Nd4Mg86Ni10, which results in that the in-situ formed NdH2.61-Mg-Mg2Ni nanocomposites more homogenous than that in as-cast Nd4Mg86Ni10. As calculated from RPP model, the characteristic hydriding time at 300 °C for Nd5Mg80Ni15 (12, 3.5 MPa), Nd4Mg80Ni8 (this work, 3.4 MPa) and Nd4Mg86Ni10 (13, 3.0 MPa) are 0.5, 1.6 and 4.7 min, respectively. It indicated that the hydriding rate of Nd5Mg80Ni15 alloy is the fastest, while the hydriding rate of Nd4Mg86Ni10 is much slower than that of the other two alloys. The hydriding rate is related to many factors, such as temperature, hydrogen pressure, microstructure, powder size, composition, etc. Both of Nd5Mg80Ni15 and Nd4Mg80Ni8 using for hydriding are nanocrystalline structure, but the particle size of Nd5Mg80Ni15 is much smaller than the micro-particles of Nd4Mg80Ni8 (−100 mesh) and Nd4Mg86Ni10 (~70 mesh). The smaller the sample size, the faster the hydriding rate. In addition, the Ni content in Nd5Mg80Ni15 is higher than that in Nd4Mg80Ni8 and Nd4Mg86Ni10, which means the quantity of catalytic element is more than Nd4Mg80Ni8 and Nd4Mg86Ni10. Therefore, the hydriding rate of melt-spinning and crystallized Nd5Mg80Ni15 showed better hydriding kinetics than that of Nd4Mg80Ni8 and Nd4Mg86Ni10. But the increase of Ni content reduced the hydrogen storage capacity (Nd5Mg80Ni15: 4.10wt.%, Nd4Mg80Ni8: 4.77wt.% and Nd4Mg86Ni10: 4.70wt.%). The comparison of all alloys in Fig. 6(d) shows that the optimal Nd-Mg-Ni alloy is the Nd4Mg80Ni8 designed in present work by considering the hydriding kinetics and hydrogen storage capacity.

The rate controlling steps are surface penetration (sp) of hydrogen atoms for dehydriding reaction at 291 ~ 347 °C through fitting the experimental data with Eq. (7).

where

The calculated tc(sp) are 8.8, 3.0 and 1.5 min with temperature increasing from 291 to 347 °C. This means that the dehydriding reaction rate increases with the temperature rising. The activation energy for dehydrogenation is calculated to be 97.5 kJ/mol, which is much smaller than 160 kJ/mol for ball milled pure MgH255, 124.6 kJ/mol for induction melted Mg90Ce5Ni5 alloy8 and comparable to 104 kJ/mol for the as-cast CeMg321. Combining Eqs (3 and 4), the dehydriding kinetic curves at any other temperatures can be predicted by RPP model as follows:

where R the gas constant, T temperature in Kelvin and 5.0 wt.% the largest desorption hydrogen content from experimental. The calculated and predicted curves are shown in Fig. 6(c), which suggests that the theoretical calculation agree well with experimental data.

The cycle life kinetics was examined at 300 °C under 3.0 MPa H2. The hydriding behaviors of the 1st ~ 5th, 10th, 39th and 58th cycles are showed in Fig. 7(a). It can be seen that the hydriding rate increases with the increase of cycle times from 1st to 5th. After the 5th cycle, the hydriding rate becomes very fast. Figure 7(b) shows the hydrogen storage capacity versus cycle times. The hydrogen capacity increases sharply from 2.36 to 4.54 wt.% in the first 3 cycles and then increases gradually to the maximum value of 4.77 wt.%. Until the 58th cycle, the hydrogen storage capacity still remains stable, which suggests that the Nd4Mg80Ni8 has a good cycle ability. In order to investigate the relationship between grain size and cycle times, the XRD pattern was collected after the 1st ~ 5th and 10th cycles, shown in Fig. 7(c). The samples were vacuumed at 300 °C for 2 h. A part of powders after the 5th cycle were further vacuumed at 350 °C for 2 h.

Figure 7
figure 7

(a) The hydriding behavior of Nd4Mg80Ni8 at 300 °C under 3.0 MPa H2 at different cycles, (b) the hydrogen storage capacity versus cycle times, (c) the XRD patterns of completely dehydrogenated samples after different cycles and (d) the relationship between grain size and cycle number.

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The calculated grain size versus cycle numbers is showed in Fig. 7(d). It can be seen that the grain size of NdH2.61 increase slowly with the cycle number, but the grain sizes of Mg and Mg2Ni decrease in the first 3 cycles and then increase with the increase of cycle number. The Nd4Mg80Ni8 disappears until the 4th cycle and the phase fractions of Nd4Mg80Ni8 in the first 3 cycles are 7.0 ± 0.6, 4.1 ± 0.2 and 2.5 ± 0.5 wt.%, respectively. Although most of Nd4Mg80Ni8 decomposed in the 1st cycle, the hydrogen absorption content is only 2.36wt.%. It suggests the generated Mg and Mg2Ni didn’t absorb hydrogen fully. Therefore, in the 2nd and 3rd cycle, the uncompleted phase transformation of Mg ↔ MgH2 and Mg2Ni ↔ Mg2NiH4 reduced the grain size of Mg and Mg2Ni. After the sample is completely activated, the phase transformation can be finished at the initial stage of H/D process. The long holding time at this temperature leads to the growth of grain size. Therefore, after the 4th cycle the grain sizes of Mg and Mg2Ni growth obviously.

The grain size of the sample further vacuumed at 350 °C is larger than that dehydriding at 300 °C. It suggests that the grain size grows with the raise of temperature and the extension of time. The growth of NdH2.61 is slowly with cycle times, but the growth of Mg is obviously from 60 ± 2 nm after the 3rd cycle to 87 ± 3 nm after the 10th cycle. The grain size of Mg vacuumed at 350 °C is about 83 ± 3 nm which is smaller than the value of Mg (150 nm) reported by Denys et al.20 at the same temperature. This is because it was pure Mg sample used in their study, while the well-distributed NdH2.61 and Mg2Ni in present work can restrain the growth of Mg21.

The phase transformation of NdH2.61-Mg-Mg2Ni nanocomposites during hydrogenation

The phase evolution mechanism of RE-Mg-Ni alloys during hydrogenation/dehydrogenation process was well clarified by Denys et al.20,24,25 combining in-situ SR-PXRD. The effect of solidification rate on the microstructure of alloy, phase structural and microstructural state of constituents during reversible process of synthesis and decomposition of hydrides and kinetic mechanism during hydriding and dehydriding process were studied in detail. Inspiring by their work, the in-situ SR-PXRD assisted with Rietveld refinement was also applied to study the mechanism of phase transformation under different hydrogen pressures and at different time. The SR-PXRD patterns under different hydrogen pressures at 350 °C are shown in Fig. 8(a). The indexation of the pattern of the activated powders indicates the existence of NdH2.61, MgH2 and Mg2Ni. Combined with the TEM results, the reaction taking place during the first hydrogenation is assumed to be:

Figure 8
figure 8

(a) The SR-PXRD patterns of NdH2.61-Mg-Mg2Ni composites under different hydrogen pressures at 350 °C, (b) the weight fraction of each phase versus pressure, (c) the SR-PXRD patterns of NdH2.61-Mg-Mg2Ni composites during hydriding process at different time at 300 °C and (d) the weight fraction of each phase versus time.

Full size image

The calculated fraction of each phase versus pressure is plotted in Fig. 8(b). The sample of complete dehydrogenation contained 18.8 wt.% NdH2.61, 39.5 wt.% Mg and remainder Mg2Ni. As temperature increased to 350 °C under 0.10 MPa H2, the phase fraction of Mg and Mg2Ni didn’t change. However, 10.0 wt.% Nd2O3 emerged because of the oxidation of NdH2.61 during heating process. Then the content of Nd2O3 remained stable at this level under hydrogen atmosphere. 0.47 MPa H2 is close to the pressure of the lower flat plateau at 350 °C. It is found that 7.8 wt.% MgH2 appeared and the fraction of Mg reduced to 33.3 wt.%. The result indicates that the flat plateau at 0.47 MPa H2 corresponds to the phase equilibrium of Mg + MgH2 + NdH2.61 + Mg2NiH0.3. The hydrogen absorption content of 0.15 wt.% H2 before the lower flat plateau is contributed by the solid solution of hydrogen in Mg and Mg2NiH0.3. When the Mg transforms to MgH2 entirely, the hydrogen absorption content reaches to 3.74 wt.% (As shown in Fig.6 (a)). The Mg2NiH4 doesn’t emerge until hydrogen pressure increases to 0.90 MPa. The second flat plateau corresponds to the equilibrium of Mg2NiH0.3 + Mg2NiH4 + MgH2 + NdH2.61. At 2.00 MPa H2, the Mg is depleted, but 28.4 wt.% Mg2NiH0.3 (the solid solution of hydrogen) is remained because of the alloy doesn’t reach the equilibrium state during the measurement. Based on those results, the sequence of the phase transformations with equilibrium hydrogen pressure during hydrogenation at 350 °C is presented as follows:

Seen from the isothermal hydriding kinetic curves as shown in Fig. 6(b), the hydriding process of the alloy above 200 °C can be separated as two stages. The first was the rapid hydriding stage, while the second stage exhibited relatively slow hydriding rate. The phase composition during the hydriding process at 300 °C under 2.00 MPa H2 was analyzed by in-situ SR-PXRD, shown in Fig. 8(c). The Mg2Ni phase peaks shifts left toward lower 2θ values as time increases to 8 min (marked as the red arrows) which suggests the lattice expansion of the Mg2Ni phase causing by the solid solution of hydrogen atoms. The intensity of Mg and Mg2Ni decreases with time prolonging, indicating the Mg and Mg2Ni transforms to MgH2 and Mg2NiH4, respectively. The change of phase fractions refined by Rietveld method is plotted in Fig. 8(d). About 11.3 wt.% Nd2O3 emerges and doesn’t change significantly later. There is no hydride appearing until 14 min. The delay of the hydrogenation may be due to the slight oxidation of the powders during heating process. The MgH2 appeared earlier than Mg2NiH4 and its fraction increases with the prolongation of time. At 40 min, the Mg transforms into MgH2 almost completely, but 26.5 wt.% Mg2NiH0.3 is left. The hydrogen absorption process of Mg8Mm20Ni alloy identified by Denys et al.20 was as following sequence: (1) Mg2Ni → Mg2NiH0.3, (2) MmH2 → MmH3 and Mg → MgH2 and (3) Mg2NiH0.3 → Mg2NiH4. They thought that the transformation of Mg2Ni → Mg2NiH0.3 occurred earlier than other transformations is because the instantly formed α-solid solution Mg2NiH0.3 catalyzed the hydrogenation of Mg. Therefore, one can believe that the fast hydriding rate of the first stage results from the fast hydrogenation of Mg.

In summary, the isothermal section of Nd-Mg-Ni system in the Mg-rich corner at 400 °C was established based on the phase relationships determined from equilibrated alloys. A new ternary compound Nd4Mg80Ni8 was found and it exhibited excellent H/D kinetic properties as a novel hydrogen storage alloy. It has structure of space group I41/amd (No.142), Z = 4, a = b = 11.2743(1) Å, c = 15.9170(2) Å. HR-TEM results revealed that the hydrogen-induced decomposition of Nd4Mg80Ni8 lead to in-situ formation of NdH2.61-MgH2-Mg2NiH0.3 nanocomposites. The high density grain boundaries in the nanocomposites of NdH2.61-Mg-Mg2Ni provided a great deal of hydrogen atoms diffusion channels and nucleation sites of hydrides, which greatly enhances the H/D kinetics and improved the cycle ability. The grain size of NdH2.61 grows slowly with cycle number, but the grain sizes of Mg and Mg2Ni decrease in the first 3 cycles and then increase with the increase of cycle times. The growth of grain size is related with temperature and vacuum time. The kinetics mechanism is analyzed by RPP model, which suggests that the rate controlling step was diffusion for hydrogenation and surface penetration for dehydrogenation. The in-situ SR-PXRD results revealed that the sequence of phase transformation during hydrogenation at 350 °C was NdH2.61 + Mg + Mg2Ni → NdH2.61 + Mg(H) + Mg2NiH0.3 → NdH2.61 + MgH2 + Mg2NiH0.3 → NdH2.61 + MgH2 + Mg2NiH4 with the increase of equilibrium hydrogen pressure from 0.0 to 2.0 MPa. Mg absorbed hydrogen earlier and faster than Mg2Ni during isothermal hydrogenation at 300 °C under 2.0 MPa H2.

Experimental Methods

The preparation and examination of the equilibrated alloys

The Nd-Mg-Ni samples were prepared by a medium frequency induction furnace using blocks of Nd (≥99.99 wt.%), Mg (≥99.99 wt.%) and Ni (≥99.99 wt.%) as the starting materials. The as-cast samples were enclosed by tantalum foils for subsequently sealing in evacuated quartz tubes. The samples were annealed at 400 °C for 30 days and then quenched in ice-water. The sample compositions and heat treatment conditions were listed in Table S1.

The actual composition of each alloy was determined by inductively coupled plasma atomic emission spectrometry (ICP). The microstructure and composition of phases in the bulk samples were investigated by HITACHI SU-1500 scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectrometer (EDS). The phase composition of annealed samples were characterized by X-ray diffraction (XRD) using 18KW D/MAX2500V + /PC diffractometer with Cu Kα radiation.

The solution of crystal structure

According to the average composition of at.% 4.86Nd-87.46Mg-7.68Ni detected from EDS, the Nd4Mg80Ni8 compound (Sample #5) was prepared by annealing an induction melted ingot at 400 °C for 30 days followed by ice-water quenching. The actual composition determined by ICP located in the region of Nd4Mg80Ni8 + Mg two-phase equilibrium. SR-PXRD data for Sample #5 were collected at a wavelength of 0.8262 Å by a Mythen-II detector on powder diffraction beamline, Australian synchrotron. The powdered samples were loaded into pre-dried 0.7 mm quartz capillaries fitted with a flow cell under an atmosphere of argon. The Rietveld refinement was performed using TOPAS v4.232.

The microstructure of Nd4Mg80Ni8 and its hydrogen-induced microstructure evolution

A bulk sample of Nd4Mg80Ni8 compound (Sample #6, actual composition is Nd4.5Mg84.6Ni10.9) with size of 3 × 3 × 2 mm was polished to obtain a smooth surface. The sample was consist of major phase Nd4Mg80Ni8 and minor Mg2Ni. Then the bulk sample was incompletely hydrogenated at 350 °C under 2.0 MPa H2 for 1 h. The microstructure of the sample was examined by FEI Helios Nanolab 600i dual beam focused ion beam FIB. In order to compare the microstructure and phase composition of Nd4Mg80Ni8 and hydrogenated sample, two thin slices with thickness less than 100 nm were cut from the annealed Nd4Mg80Ni8 and hydrogenated sample respectively by FEI Helios Nanolab 600i dual beam FIB. SAED patterns and HR-TEM images were collected by Tecnai G2 F20 S-Twin TEM.

The measurement of H/D properties

The H/D properties of annealed Nd4Mg80Ni8 (Sample #6) was tested using automatic PCT characteristics measurement system from SUZUKI HOKAN. CO., LTD. in Japan. The Nd4Mg80Ni8 was mechanically crushed into micro-particles (−100 mesh, <150 μm) and activated at 350 °C under 4.0 MPa H2 for hydrogen absorption and at the same temperature in vacuum for hydrogen desorption. The PCT curves were measured at 250 ~ 350 °C with the maximum equilibrated time of 40 min. The hydrogen absorption kinetics were examined at 100 ~ 350 °C under initial hydrogen pressure of 3.4 MPa. Before hydrogenation the sample was kept in vacuum at 350 °C for 2 h to ensure its complete dehydrogenation. The isothermal dehydriding kinetics was examined at 291 ~ 347 °C in vacuum after the sample completely hydriding at 350 °C for 2 h under initial hydrogen pressure of 3.4 MPa. The cycling behavior of the Nd4Mg80Ni8 was determined at 300 °C. The time for hydriding under initial pressure of 3.0 MPa H2 was 2 h and for dehydriding in vacuum was 2.8 h. In order to study the relationship between grain size and cycle number, the XRD pattern was collected after the 1st ~ 5th and 10th cycles. The samples were vacuumed at 300 °C for 2 h and then air cooled to room temperature. A part of powders after the 5th cycle were further vacuumed at 350 °C for 2 h to observe the growth of grain size. The XRD patterns were collected by Bruker AXS D8 diffractometer with Cu Kα radiation. The sizes of the crystallites in the samples were calculated from the refinements of XRD patterns using Scherrer equation.

The evolution of phase composition during hydrogenation

The in-situ SR-PXRD data of Nd4.5Mg84.6Ni10.9 powders were collected at by a wavelength of 0.8262 Å by a Mythen-II detector on powder diffraction beamline, Australian synchrotron. The completely dehydrogenated Nd4.5Mg84.6Ni10.9 powders were loaded into pre-dried 0.7 mm quartz capillaries fitted with a flow cell under an atmosphere of argon. The sample was heating to the set temperatures by a Cybostar hot air blower with heating rate of 20 °C/min under vacuum, then hydrogen was imported in and the detector started to collect the XRD data. Before heating, the quartz capillaries didn’t scrubbing with argon, which led to some oxidation of the sample during heating process. Therefore, about 10.0 wt.% Nd2O3 emerged at the initial stage of examination. Then the quantity of Nd2O3 didn’t change any more. The phase transformation of Mg and Mg2Ni during the hydriding process still can be observed. The diffraction data under different hydrogen pressures from 0.0 to 2.0 MPa were collected at 350 °C and proceeded for 4 min at one pressure. Before every collection, the sample was kept under this pressure for 30 min. The diffraction data at different time was measured every 2 min at 300 °C under a constant pressure of 2.0 MPa H2.

Additional Information

How to cite this article: Luo, Q. et al. Phase Equilibria, Crystal Structure and Hydriding/Dehydriding Mechanism of Nd4Mg80Ni8 Compound. Sci. Rep. 5, 15385; doi: 10.1038/srep15385 (2015).