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Article

Microstructure Characteristics and Hydrogen Storage Kinetics of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) Alloys

by
Hongxiao Tian
1,
Qichang Wang
1,
Xia Li
1,
Long Luo
1 and
Yongzhi Li
1,2,*
1
School of Science, Inner Mongolia University of Science and Technology, Baotou 014010, China
2
Baotou Materials Research Institute of Shanghai Jiao Tong University, Baotou 014010, China
*
Author to whom correspondence should be addressed.
Materials 2023, 16(13), 4576; https://doi.org/10.3390/ma16134576
Submission received: 22 May 2023 / Revised: 14 June 2023 / Accepted: 19 June 2023 / Published: 25 June 2023
(This article belongs to the Topic Metal Hydrides: Fundamentals and Applications)
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Abstract

:
Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys were successfully prepared by the vacuum induction melting method. The structural characterizations of the alloys were performed by using X-ray diffraction and scanning electron microscope. The effects of nickel content on the microstructure and hydrogen storage kinetic of the as-cast alloys were investigated. The results showed that the alloys are composed of a primary phase of Mg2Ni, lamella eutectic composites of Mg + Mg2Ni, and some amount of LaMg12 and La2Mg17. Nickel addition significantly improved the hydrogen-absorption kinetic performance of the alloy. At 683 K, Mg77Ni20La3 alloy and Mg82Ni15La3 alloy underwent hydrogen absorption and desorption reactions for 2 h, respectively, and their hydrogen absorption and desorption capacities were 4.16 wt.% and 4.1 wt.%, and 4.92 wt.% and 4.69 wt.%, respectively. Using the Kissinger equation, it was calculated that the activation energy values of Mg77Ni20La3, Mg82Ni15La3, Mg87Ni10La3 and Mg92Ni5La3 alloys were in the range of 68.5~75.2 kJ/mol, much lower than 150~160 kJ/mol of MgH2.

1. Introduction

Fossil fuels, the main energy source for the past two centuries, have caused energy crisis and environmental pollution problems due to excessive consumption [1,2,3]. Therefore, finding new energy sources is urgent [4]. Hydrogen is a clean energy source with many advantages such as versatility, high utilization efficiency, environmental compatibility, and inexhaustible [5]. Hydrogen energy economy involves hydrogen production, storage, transportation and application. Among them, hydrogen storage and transportation are the core and bottleneck technologies [6]. To achieve large-scale applications, the hydrogen storage system needs to be safe, efficient, and cost-effective [7]. Traditional gas storage, which can be pressurized up to 70 MPa with technical progress, pose considerable safety risk for mobile use [8]. Cryogenic liquid hydrogen storage, which consume more than 30% calorific value of the liquid hydrogen, are not suitable for popularization in the civil field [9]. Solid-state hydrogen storage materials are the first choice for large-scale hydrogen storage systems because of their safety, high capacity, and low pressure [10,11,12,13]. Magnesium-based alloys are the most viable solid-state hydrogen storage materials, owning to their low cost, high hydrogen storage capacity, high cycling stability and high safety [14,15,16,17]. However, the slow kinetics and high thermodynamics hinder their development [18].
A great deal of experimental researchers have been conducted in the last few decades to address the drawback of Mg-based alloys, such as alloying [19,20,21], nano-crystallization [22,23,24], doping catalyst [25,26,27] and exploring new treatment methods [28,29,30], etc. By alloying various transition metals (TM) with Mg, TM was found to reduce the thermal stability of MgH2 and enhance the hydrogen reaction kinetics of MgH2 [31,32]. Mg-Si alloys formed by alloying Mg and Si elements [33] can significantly reduce the thermal stability of the alloy. When Mg is alloyed with Ni elements, the hydrogen absorption/desorption phases transform from Mg/MgH2 to Mg2Ni/Mg2NiH4, and the dehydrogenation enthalpy decreases from 76 kJ/mol H2 to 64 kJ/mol H2 [34,35,36]. Ding et al. reported that the Mg-Mg2Ni eutectic with a large amount of phase/grain boundaries is beneficial for the dissociation of H2 and permeation of hydrogen atoms [34]. In addition, the enthalpy of hydrogen desorption of the Mg-In alloy [37] is reduced to 67.8 kJ/mol, which is very close to the Mg2Ni system. The TM elements can be used as a special catalyst to lower the energy barrier for the recombination of hydrogen atoms and the decomposition of hydrogen molecules [38]. However, the Mg-TM alloys possess better hydrogen absorption/desorption kinetics with the expense of reversible hydrogen storage capacity [39,40].
Beside of TM, rare earth (RE) metals were also used widely for producing Mg-based alloys to enhance the hydrogenation kinetics. The in situ formed nano-scale rare earth hydrides (REHx) bring interfacial channels and therefore facilitates the diffusion of hydrogen in the alloy [41,42,43]. Moreover, REHx has a unique “hydrogen pumping” function, which can effectively reduce the apparent activation energy [44,45,46]. According to the Mg-La phase diagram [47], it is known that Mg-LaMgx eutectic can be formed even at very high Mg content. Lass et al. [48] found that after alloying with La, the reaction enthalpy of MgH2 and Mg2NiH4 in the Mg-Ni alloy were reduced by 8 kJ/mol and 5 kJ/mol, respectively. REHx in Mg-RE-TM alloys, which can be uniformly dispersed at the grain boundaries of the Mg matrix, effectively reduces the catalytic dead zone and improves the catalytic efficiency [49]. In addition, the stability of the nano phase structure of Mg-based alloys can be improved by pinning of REHx to the Mg nano grain boundaries, which inhibits the growth of Mg/MgH2 grains [50].
The balance between improving of reaction kinetics and decreasing of hydrogen storage capacity should be investigated carefully when increasing the proportion of Ni or RE in Mg-Ni-RE alloys. In the present study, Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys were prepared by induction melting method. The microstructure, phase transformation and hydrogen storage kinetic of the alloys with different Mg and Ni ratios are studied. The synergistic catalytic effect of LaH3.05 phase and Mg2Ni phase on the hydrogen absorption and desorption process of the alloy was analyzed.

2. Materials and Methods

2.1. Material Preparation Methods

Alloys with the nominal composition of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) were made from the appropriate amounts of pure magnesium, nickel, and lanthanum by using vacuum induction melting. The purity of the starting materials was not less than 99.5%, and the overall weight of the alloy was ~5 kg. Pure argon was used to fill in the furnace to prevent oxidation. The metallic solution was poured into an iron mold and cooled to indoor temperature. Afterward, the alloy ingots were mechanically pulverized into powder samples with a particle size of <200 mesh.

2.2. Structural Characterizations

The physical composition of the alloy was analysed by X-ray powder diffraction (XRD). The test instrument was a PANalytical X’pert Powder X-ray diffractometer from Panacor, Eindhoven, The Netherlands, using a Cu Kα target, a tube voltage of 40 kV, a current of 150 mA, sampling in a continuous scan mode, a scan speed of 2°/min, a step size of 0.02° and a scan range of 10–80°. To determine the phase composition of the alloy, XRD patterns were analysed using X’Pert HighScore Plus software, V3.8. After the ingot sections had been ground and polished in alcohol, the morphological structure of the ingot sections was observed using a TESCAN GAIA-3 (Tescan, Brno, Czech Republic) field emission scanning electron microscope (SEM) and the phase composition of the different morphological micro-zones was analysed using the energy spectrum analysis (EDS) supplied with the equipment. The block and powdered samples are glued to the metal base with conductive adhesive and the SEM is performed on the block before hydrogen absorption.

2.3. Hydrogen Storage Measurements

Hydrogen sorption kinetic tests were performed in a Sievert’s apparatus under high-purity hydrogen (99.999% purity). Each sample weighed ~200 mg. The reactor was heated by a resistance furnace with an accuracy of ±0.5 K. Before measuring the hydrogenation kinetics, the samples were activated to three hydrogen absorption and desorption cycles at 653 K. Isothermal hydrogen absorption kinetic measurements were made in the temperature range from 473 to 683 K and with 3 MPa of hydrogen pressure. Calorimetric measurements of the full hydrogenated samples were also made using a NETZSCH (Selb, Germany) DSC 204 HP differential scanning calorimeter (DSC). The samples were heated from indoor temperature to 750 K with a heating rate of 5, 10, 20 and 40 K/min. The tests were performed under a pure argon flow (50 mL/min).

3. Results

3.1. Microstructure Characteristics

The X-ray diffraction patterns (XRD) of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys with four different Mg and Ni contents (x = 0, 5, 10, 15) are shown in Figure 1. The alloys before hydrogenation mainly consist of four phases: LaMg12, Mg2Ni, La2Mg17 and Mg. As the Mg content increases, the intensities of LaMg12, Mg2Ni and La2Mg17 phases gradually decrease for Mg82Ni15La3 alloy. It is speculated that this is because the Mg content in the Mg phase increases, while the Mg content in the LaMg12, Mg2Ni and La2Mg17 phases decreases accordingly. Therefore, the intensities of the diffraction peaks of LaMg12, Mg2Ni and La2Mg17 phases decrease gradually. The grain boundaries formed between the multi-phase structures can provide channels for the diffusion of hydrogen sites and hydrogen atoms in the alloy. The defects and dislocations on the grain boundaries can lower the diffusion energy barrier of hydrogen atoms and enhance the diffusion coefficient of hydrogen atoms, thus accelerating the migration of hydrogen atoms in the alloy, which is conducive to improving the kinetic properties of the hydrogen absorption and desorption processes in the alloy. The C peak in the alloy is caused by the graphite crucible used to prepare the alloy in our experiment. The graphite crucible reacts with some metals at high temperatures to produce carbides, which are transferred to the alloy, resulting in the C-peak.
Figure 2 shows the XRD patterns of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys after saturated hydrogenation. It can be seen from the figure that the main phases of Mg77Ni20La3 alloy after saturated hydrogenation are Mg2NiH4, MgH2 and LaH3.05, which are manifested as peaks with different positions and intensities on the XRD pattern. As the Mg content increases and the Ni content decreases, the intensity of the Mg2NiH4 phase diffraction peak gradually decreases, while the intensities of the MgH2 and LaH3.05 phase diffraction peaks increase significantly. This indicates that the La2Mg17 and La2Mg17 phases before hydrogenation of the alloy are completely transformed into LaH3.05 and MgH2 phases, and no unmelted Mg and Ni elements are generated.
Figure 3 shows the scanning electron microscope (SEM) images of the cross-sections of the Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy ingots, showing the microstructure and phase composition of the alloys. As can be seen from the figure, the alloys are typical multiphase structures, represented by regions of different brightness. Combined with the EDS spectrum analysis of the corresponding regions in Figure 3a and the XRD test results, it can be known that the Mg77Ni20La3 alloy mainly consists of four phases: Mg2Ni, Mg, La2Mg17 and LaMg12, corresponding to the gray band-like regions, dark regions and bright continuous regions in the figure, respectively. The grey band in Figure 3a Mg77Ni20La3 is determined to be the Mg2Ni phase in combination with the phases identified by EDS spectroscopy and XRD in the corresponding region [51]. Similarly, the EDS energy spectrum analysis shows that the bright region is a coexistence of La2Mg17 and LaMg12 phases. In Figure 3b, Mg82Ni15La3, the long band-like Mg2Ni phase can be seen split into small pieces. The Mg phase gradually increases in the darker areas. The continuous bright areas begin to split into irregular polygons. Figure 3c The polygonal structure of the coexisting La2Mg17 and LaMg12 phases in the bright region of Mg87Ni10La3 is more regular. Figure 3d The microstructure and structure of the alloy changes considerably with increasing Mg content in Mg92Ni5La3. The grey and bright areas are uniformly distributed in the Mg matrix and show a typical eutectic structure consisting of Mg-Mg2Ni, a layered structure of several hundred nanometers [52]. This layered eutectic structure provides an efficient channel for the diffusion of hydrogen atoms during the hydrogen absorption and release process of the alloy [53]. These changes are consistent with the above XRD analysis results.
The XRD and SEM results show that the alloy decomposes into LaH3.05 and MgH2 after reacting with H2, which are formed from La2Mg17 and LaMg12 in the alloy. The XRD patterns of the hydrogenated alloy samples reveal that the diffraction peaks of the as-cast alloy phases disappear completely, and new peaks of metal hydrides emerge, including MgH2, LaH3.05 and Mg2NiH4. Thus, the first hydrogenation exothermic process of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy can be expressed by the following chemical equations:
Mg + H2 → MgH2
Mg2Ni + H2 → Mg2NiH4
LaMg12 + La2Mg17 + H2 → LaH3.05 + MgH2
Moreover, after the dehydrogenation endothermic process, the MgH2 phase is completely transformed into Mg phase, but the rare earth hydride phase (LaH3.05) remains. This is because this kind of rare earth hydride has a high thermal stability [47], which cannot decompose under the current experimental conditions. Therefore, the reversible hydrogen absorption and desorption process of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy after activation can be represented by the following two chemical equations:
MgH2 ↔ Mg + H2
Mg2NiH4 ↔ Mg2Ni + H2
The stable LaH3.05 nanocrystals uniformly distributed on the surface and bulk of the alloy during hydrogen absorption and desorption create a large number of boundaries and active sites for hydrogen atom diffusion. This not only facilitates the nucleation of Mg/MgH2 and Mg2Ni/Mg2NiH4, but also enhances the diffusion rate of hydrogen atoms in the alloy. Meanwhile, the existence of Mg-MgH2 eutectic layered structure significantly increases the diffusion pathways of hydrogen atoms in the alloy. These two factors directly improve the kinetic properties of hydrogen absorption and desorption in the alloy.

3.2. Activation Behaviors and Hydrogen Absorption Kinetics

Figure 4 and Figure 5 shows the activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloy at 653 K. As shown in Figure 4a, the long-term hydrogen absorption curve of Mg77Ni20La3 alloy indicates that the alloy has an initial activation hydrogen absorption of 4.67 wt.%, which is much higher than the second and third activation hydrogen absorptions. In Figure 4b, the long-term hydrogen absorption curve of Mg82Ni15La3 alloy reveals that the alloy has an initial activation hydrogen absorption of 5.37 wt.%, while the second and third activation hydrogen absorptions are 5.16 wt.%, and the two curves almost overlap. This suggests that complete activation of the Mg77Ni20La3 alloy at 653 K requires 3 hydrogenation cycles, while the Mg82N15La3 alloy requires 2 hydrogenation cycles. In Figure 5a,b, the short-term hydrogen absorption curves of Mg77Ni20La3 and Mg82Ni15La3 alloys demonstrate that Mg77Ni20La3 alloy takes much less time than Mg82Ni15La3 alloy to reach the saturated hydrogen absorption (90%) in the second and third activations. This implies that Mg77Ni20La3 alloy has a better activation performance than Mg82Ni15La3 alloy. Moreover, it can also be seen from the short-term activation curves that the initial activation of the alloy is very slow.
Figure 6 shows the hydrogen absorption kinetics curves of Mg77+xNi20−xLa3 (x = 0, 5) alloy at different temperatures. As shown in Figure 6a, the hydrogen absorption of Mg77Ni20La3 alloy reaches 4.02 wt.% at 533 K for 2 h, and the hydrogen absorption kinetics curves of Mg77Ni20La3 alloy are almost identical when the temperature is above 533 K, indicating excellent hydrogen absorption kinetics properties. The hydrogen absorption at 623 K and 683 K are 4.3 wt.% and 4.16 wt.%, respectively. Figure 6b reveals that the hydrogen absorption of Mg82Ni15La3 alloy attains as high as 4.92 wt.% at 683 K for 2 h. This is because the higher the Mg content, the stronger the hydrogen absorption ability of the alloy. The hydrogen absorption amount of the initial activation curve in Mg77Ni20La3 and Mg82Ni15La3 alloys is significantly higher than that of the alloy at each temperature, which is due to the formation of LaH3.05 phase with high thermal stability and difficult to decompose after the initial hydrogenation of the alloy. In summary, Mg82Ni15La3 alloy has better hydrogen absorption kinetics performance than Mg77Ni20La3 alloy.

3.3. Hydrogen Desorption Kinetics and Activation Energy

Figure 7 and Figure 8 show the hydrogen desorption endothermic reaction kinetics curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at different temperatures. It can be seen from Figure 7a that Mg77Ni20La3 alloy has the best hydrogen desorption working temperature above 623 K and reaches a hydrogen desorption amount of 4.1 wt.% at 683 K. It can be seen from Figure 7b that Mg82Ni15La3 alloy has a hydrogen desorption amount of up to 4.69 wt.% at 683 K. This is because the higher the Mg content, the stronger the hydrogen desorption ability of the alloy. It can be seen from Figure 8a that the short-term hydrogen desorption endothermic kinetics curve of Mg77Ni20La3 alloy shows that the maximum hydrogen desorption amount of the alloy is 3.8 wt.% at 653 K and reaches the saturated hydrogen desorption amount within 125 s. As the temperature increases, it only takes 71 s to reach a maximum hydrogen desorption amount of 3.8 wt.% at 683 K. This is mainly due to the increase of temperature, which increases the energy of reactants in the alloy, accelerates the hydrogen desorption rate of the alloy, and thus increases the hydrogen desorption amount. At the same time, Figure 8b shows that the short-term hydrogen desorption exothermic kinetics curve of Mg82Ni15La3 alloy shows that the alloy reaches a saturated hydrogen desorption amount of 4.39 wt.% within 140 s at 653 K. And when the temperature rises to 683 K, Mg82Ni15La3 alloy reaches a hydrogen desorption amount of 3.8 wt.% within 47 s.
The phase change heat of the alloy was calculated by using the test method of the DSC curve of the alloy hydride warming and exothermic reaction. In order to calculate the phase transition heat of the alloy, we adopted the method of testing the DSC curve of the hydrogen desorption endothermic of the alloy hydride. Figure 9 shows the DSC curves of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy hydrides after saturated hydrogen absorption at different heating rates. The test temperature range was 400~700 K. Firstly, the initial temperature of the hydrogen desorption endothermic reaction of the alloy can be obtained from the DSC curve, and the activation energy of the hydrogen desorption endothermic reaction can be calculated according to the Kissinger method, so as to judge the difficulty of the reaction. It can be seen from Figure 9 that there is a clear exothermic main peak and a smaller peak on each DSC curve at each heating rate. The smaller exothermic peak is related to the phase transition of Mg2NiH4, by the coupled hydrogen desorption endothermic of MgH2 and Mg2NiH4 phases. The temperature range of the smaller exothermic peak is between 513–522 K. In Figure 9a, the endothermic main peak consists of two obvious peaks, which confirms that the exothermic main peak is caused by two hydrogen desorption phases of MgH2 and Mg2NiH4, while in Figure 9b–d, the exothermic main peak becomes an obvious peak, because two kinds of hydrogen desorption phases have a synergistic hydrogen desorption endothermic phenomenon. In Figure 9e, the exothermic peak temperature at 20 K/min heating rate shows that the phase transition peak temperature of Mg2NiH4 is little affected by Mg content and remains around 520 K. While the peak temperature of MgH2 is more affected by Mg content and shows a trend of first rising and then falling. The increase of hydrogen desorption peak temperature causes the increase of activation energy of hydrogen desorption endothermic reaction of alloy hydride, which will be calculated in detail below. By comparing the hydrogen desorption endothermic temperatures corresponding to the exothermic main peaks of four alloys with different Mg contents at the same rate, it is found that with the increase of Mg content, the hydrogen desorption peak temperature of the alloy gradually increases and then decreases. Mg87Ni10La3 alloy has the highest hydrogen desorption peak temperature at 20 K/min heating rate, which is 629.8 K; while Mg92Ni5La3 alloy is 624.1 K, which is 5.7 K lower.
Table 1 lists the peak temperatures of DSC endothermic dehydrogenation reaction of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy hydrides at different heating rates. Using different heating rates and corresponding peak temperatures, the activation energy (Ea) of dehydrogenation endothermic reaction of the alloy can be calculated by Kissinger method.
According to the Kissinger method, the activation energy can be calculated by using the following Equation (6):
ln ( β T p 2 ) = − E a R T P +   ln ( k 0 )
where R is the gas constant, k0 is a constant, Tp is the peak temperature of the exothermic peak, and β is the heating rate. This method was used to calculate the activation energy of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys at different heating rates.
Figure 10 shows the curve diagram after fitting according to the Kissinger equation. The fitting results show that the dehydrogenation activation energy of Mg82Ni15La3 alloy is (72.7 ± 4.1) kJ/mol, the dehydrogenation activation energy of Mg87Ni10La3 alloy is larger, which is (75.2 ± 1.5) kJ/mol, and the dehydrogenation activation energy of Mg92Ni5La3 alloy is smaller, which is (68.5 ± 3.8) kJ/mol. Compared with Mg87Ni10La3 alloy, the dehydrogenation endothermic activation energy of Mg92Ni5La3 alloy is reduced by about 9%. Figure 10 shows that the activation energy values of Mg77Ni20La3, Mg82Ni15La3, Mg87Ni10La3 and Mg92Ni5La3 are between 68.5~75.2 kJ/mol. After comparison, these values are obviously lower than (150~160 kJ/mol) of MgH2. The reason for the low activation energy value of the alloy is the effect of alloying and rare earth hydride formed by adding rare earth elements. In addition, due to the addition of rare earth element La to form rare earth hydride LaH3.05 phase, and LaH3.05 phase has “hydrogen pumping”, which greatly reduces the hydrogen activation energy value of the alloy.
The increase of dehydrogenation temperature and dehydrogenation activation energy of alloy hydride is not conducive to the improvement of dehydrogenation kinetics performance of alloy. In Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy, Mg content increases from Mg77 to Mg92, and Ni content decreases from Ni20 to Ni5. It can be seen from the hydrogen absorption and dehydrogenation kinetics curves of the alloy that the hydrogen absorption amount of the alloy shows an increasing trend. This is because after the increase of Mg content, the hydrogen absorption and desorption ability of Mg phase is higher than that of Mg2Ni phase. And with the increase of hydrogen absorption and desorption cycle times, the hydrogen absorption and desorption ability of the alloy does not weaken obviously, and the alloy can achieve complete hydrogen absorption and desorption.
The peak temperature of hydrogen desorption endothermic reaction and phase transition heat for Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy is presented in Table 2. The phase change heats are based on the DSC curves of the exothermic hydrides of the alloy after saturated hydrogen absorption at 5, 10, 20 and 40 K/min heating rates, calculated by the phase change heat (area of the peak) calculation function that comes with the DSC software. From the table, it can be seen that the phase change heat of Mg77Ni20La3 alloy is 1177 J/g at a heating and desorption hydrogen rate of 5 K/min; the phase change heat of Mg87Ni10La3 alloy is 1529 J/g; the phase change heat of Mg92Ni5La3 alloy is 1428 J/g, which is 251 J/g higher than that of Mg77Ni20La3 alloy. The phase change heat of Mg92Ni5La3 alloy is second only to Mg87Ni10La3 alloy, while the peak hydrogen desorption temperature is significantly lower than that of Mg87Ni10La3, which is more favorable to the hydrogen desorption and heat absorption reaction of the alloy, but due to the high Mg content, it will lead to the alloy not releasing hydrogen. It can be seen from the analysis that with the increase of Mg content, the peak endothermic temperature of the alloy gradually increases, and the corresponding phase transition heat also gradually increases. It can be seen that the phase transition heat of Mg87Ni10La3 alloy is significantly higher than that of the first two alloys and slightly higher than that of Mg92Ni5La3 alloy. At the same time, the higher the Mg content in the alloy, the more likely the alloy will not desorption hydrogen. In summary, Mg87Ni10La3 alloy is more suitable as an energy storage material.

4. Conclusions

The Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy prepared by induction melting method mainly consists of three main phases: Mg2Ni, LaMg12, La2Mg12 and residual Mg phase. After saturated hydrogen absorption, Mg2Ni phase transforms into MgH2 and LaH3.05 phase. With the increase of Mg content, the long strip-like Mg2Ni phase gradually disappears and transforms into Mg-Mg2Ni eutectic lamellar structure. Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys can be fully activated after 3 hydrogenation cycles at 653 K. The hydrogen absorption capacity of Mg77Ni20La3 and Mg82Ni15La3 alloys at 683 K for 2h are 4.92 wt.% and 4.69 wt.%, respectively. The hydrogen desorption activation energy of Mg77Ni20La3 alloy is (70.9 ± 4.2) kJ/mol, Mg82Ni15La3 alloy is (72.7 ± 4.1) kJ/mol, Mg87Ni10La3 alloy is (75.2 ± 1.5) kJ/mol, and Mg92Ni5La3 alloy is (68.5 ± 3.8) kJ/mol. The DSC test results show that the phase transition heat of Mg82Ni15La3 alloy reaches 1529 J/g, which is 352 J/g higher than that of Mg77Ni20La3 alloy.

Author Contributions

Conceptualization, Y.L.; Investigation, H.T., X.L. and Y.L.; Resources, L.L.; Writing—original draft, Q.W.; Writing—review & editing, Y.L.; Funding acquisition, Y.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundations of China (52261041) and the Natural Science Foundation of Inner Mongolia, China (2020LH01006, 2020LH05024 and 2022MS05011).

Institutional Review Board Statement

The study did not require ethical approval. The study did not involve the use of humans or animals. Therefore, ethical review and approval were not required for this study. As per the guidelines, we confirm that the study was conducted in accordance with the Declaration of Helsinki.

Informed Consent Statement

Not applicable.

Data Availability Statement

Due to privacy and ethical restrictions, the data supporting the reported results of this study are unavailable for sharing. We acknowledge the importance of data sharing and encourage researchers to comply with data availability policies.

Conflicts of Interest

The authors declare no conflict of interest.

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Materials 16 04576 g001 550
Figure 1. XRD patterns of the as-cast Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys.
Figure 1. XRD patterns of the as-cast Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys.
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Materials 16 04576 g002 550
Figure 2. XRD patterns of the full hydrogenated Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys.
Figure 2. XRD patterns of the full hydrogenated Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys.
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Materials 16 04576 g003 550
Figure 3. SEM images of the as-cast alloy ingots: (a) Mg77Ni20La3; (b) Mg82Ni15La3; (c) Mg87Ni10La3, (d) Mg92Ni5La3.
Figure 3. SEM images of the as-cast alloy ingots: (a) Mg77Ni20La3; (b) Mg82Ni15La3; (c) Mg87Ni10La3, (d) Mg92Ni5La3.
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Materials 16 04576 g004 550
Figure 4. Long-time activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) long-time hydrogen absorption by Mg77Ni20La3; (b) long-time hydrogen absorption by Mg82Ni15La3.
Figure 4. Long-time activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) long-time hydrogen absorption by Mg77Ni20La3; (b) long-time hydrogen absorption by Mg82Ni15La3.
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Materials 16 04576 g005 550
Figure 5. Short-time activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) short-time hydrogen absorption by Mg77Ni20La3; (b) short-time hydrogen absorption by Mg82Ni15La3.
Figure 5. Short-time activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) short-time hydrogen absorption by Mg77Ni20La3; (b) short-time hydrogen absorption by Mg82Ni15La3.
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Materials 16 04576 g006 550
Figure 6. Hydrogen absorption kinetic curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at various temperatures: (a) Mg77Ni20La3; (b) Mg82Ni15La3.
Figure 6. Hydrogen absorption kinetic curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at various temperatures: (a) Mg77Ni20La3; (b) Mg82Ni15La3.
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Materials 16 04576 g007 550
Figure 7. Long-time hydrogen desorption kinetic curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) long-time hydrogen desorption by Mg77Ni20La3; (b) long-time hydrogen desorption by Mg82Ni15La3.
Figure 7. Long-time hydrogen desorption kinetic curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) long-time hydrogen desorption by Mg77Ni20La3; (b) long-time hydrogen desorption by Mg82Ni15La3.
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Materials 16 04576 g008 550
Figure 8. Short-time hydrogen desorption kinetic curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) short-time hydrogen desorption by Mg77Ni20La3; (b) short-time hydrogen desorption by Mg82Ni15La3.
Figure 8. Short-time hydrogen desorption kinetic curves of Mg77+xNi20−xLa3 (x = 0, 5) alloys at 653 K: (a) short-time hydrogen desorption by Mg77Ni20La3; (b) short-time hydrogen desorption by Mg82Ni15La3.
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Materials 16 04576 g009 550
Figure 9. DSC curves of saturated hydrides of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys: (a) Mg77Ni20La3; (b) Mg82Ni15La3; (c) Mg87Ni10La3; (d) Mg92Ni5La3; (e) two peak heat absorption temperatures with increase of Mg content at 20 K/min heating rate contention.
Figure 9. DSC curves of saturated hydrides of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys: (a) Mg77Ni20La3; (b) Mg82Ni15La3; (c) Mg87Ni10La3; (d) Mg92Ni5La3; (e) two peak heat absorption temperatures with increase of Mg content at 20 K/min heating rate contention.
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Materials 16 04576 g010 550
Figure 10. Kissinger plot corresponding peak temperature of DSC at different heating rates of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys: (a) Mg77Ni20La3; (b) Mg82Ni15La3; (c) Mg87Ni10La3; (d) Mg92Ni5La3.
Figure 10. Kissinger plot corresponding peak temperature of DSC at different heating rates of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloys: (a) Mg77Ni20La3; (b) Mg82Ni15La3; (c) Mg87Ni10La3; (d) Mg92Ni5La3.
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Table
Table 1. The peak temperature of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy hydrides during DSC dehydrogenation reaction at different heating rates.
Table 1. The peak temperature of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy hydrides during DSC dehydrogenation reaction at different heating rates.
Alloy5 K/min10 K/min20 K/min40 K/min
Mg2NiH4MgH2Mg2NiH4MgH2Mg2NiH4MgH2Mg2NiH4MgH2
Mg77Ni20La3515.7551.9516.6569.8519.2598.9522.3623.7
Mg82Ni15La3514.4572.3514.9591.4517.5621.9520.5647.6
Mg87Ni10La3513.2579.3514.8604.5515.8629.8518.4656.3
Mg92Ni5La3513.6575.9513.6598.4514.9624.1517.5659.4
Table
Table 2. The peak temperature of hydrogen desorption endothermic reaction and phase transition heat for Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy.
Table 2. The peak temperature of hydrogen desorption endothermic reaction and phase transition heat for Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy.
AlloyPeak Temperature/°CMelting Enthalpy/J·g−1
5
K/min		10 K/min20 K/min40 K/min5 K/min10 K/min20 K/min40 K/min
Mg77Ni20La3545.4561.1589.7613.7−1177−1176−1067−1112
Mg82Ni15La3572.3591.4621.9647.6−1218−1107−1062−1142
Mg87Ni10La3579.3604.5629.8656.3−1529−1322−1328−1406
Mg92Ni5La3575.9598.4624.1659.4−1428−1271−1314−1180
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Tian, H.; Wang, Q.; Li, X.; Luo, L.; Li, Y. Microstructure Characteristics and Hydrogen Storage Kinetics of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) Alloys. Materials 2023, 16, 4576. https://doi.org/10.3390/ma16134576

AMA Style

Tian H, Wang Q, Li X, Luo L, Li Y. Microstructure Characteristics and Hydrogen Storage Kinetics of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) Alloys. Materials. 2023; 16(13):4576. https://doi.org/10.3390/ma16134576

Chicago/Turabian Style

Tian, Hongxiao, Qichang Wang, Xia Li, Long Luo, and Yongzhi Li. 2023. "Microstructure Characteristics and Hydrogen Storage Kinetics of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) Alloys" Materials 16, no. 13: 4576. https://doi.org/10.3390/ma16134576

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