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

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.


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 MgH 2 and enhance the hydrogen reaction kinetics of MgH 2 [31,32]. Mg-Si alloys formed by alloying Mg and Si elements [33] can significantly reduce the thermal stability of the 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.

Hydrogen Storage Measurements
Hydrogen sorption kinetic tests were performed in a Sievert's apparatus under highpurity 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).

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.   Figure 3a Mg 77 Ni 20 La 3 is determined to be the Mg 2 Ni 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 La 2 Mg 17 and LaMg 12 phases. In Figure 3b, Mg 82 Ni 15 La 3 , the long band-like Mg 2 Ni 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 La 2 Mg 17 and LaMg 12 phases in the bright region of Mg 87 Ni 10 La 3 is more regular. Figure 3d The microstructure and structure of the alloy changes considerably with increasing Mg content in Mg 92 Ni 5 La 3 . The grey and bright areas are uniformly distributed in the Mg matrix and show a typical eutectic structure consisting of Mg-Mg 2 Ni, 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 LaH 3.05 and MgH 2 after reacting with H 2 , which are formed from La 2 Mg 17 and LaMg 12 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 MgH 2 , LaH 3.05 and Mg 2 NiH 4 . Thus, the first hydrogenation exothermic process of Mg 77+x Ni 20−x La 3 (x = 0, 5, 10, 15) alloy can be expressed by the following chemical equations: Mg  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: 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: 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 Moreover, after the dehydrogenation endothermic process, the MgH 2 phase is completely transformed into Mg phase, but the rare earth hydride phase (LaH 3.05 ) 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 Mg 77+x Ni 20−x La 3 (x = 0, 5, 10, 15) alloy after activation can be represented by the following two chemical equations: The stable LaH 3.05 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/MgH 2 and Mg 2 Ni/Mg 2 NiH 4 , but also enhances the diffusion rate of hydrogen atoms in the alloy. Meanwhile, the existence of Mg-MgH 2 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.  Figure 4a, the long-term hydrogen absorption curve of Mg 77 Ni 20 La 3 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 Mg 82 Ni 15 La 3 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 Mg 77 Ni 20 La 3 alloy at 653 K requires 3 hydrogenation cycles, while the Mg 82 N 15 La 3 alloy requires 2 hydrogenation cycles. In Figure 5a,b, the short-term hydrogen absorption curves of Mg 77 Ni 20 La 3 and Mg 82 Ni 15 La 3 alloys demonstrate that Mg 77 Ni 20 La 3 alloy takes much less time than Mg 82 Ni 15 La 3 alloy to reach the saturated hydrogen absorption (90%) in the second and third activations. This implies that Mg 77 Ni 20 La 3 alloy has a better activation performance than Mg 82 Ni 15 La 3 alloy. Moreover, it can also be seen from the short-term activation curves that the initial activation of the alloy is very slow.

Activation Behaviors and Hydrogen Absorption Kinetics
rectly improve the kinetic properties of hydrogen absorption and desorption in the

Activation Behaviors and Hydrogen Absorption Kinetics
Figures 4 and 5 shows the activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloy at 653 shown in Figure 4a, the long-term hydrogen absorption curve of Mg77Ni20La3 alloy ind that the alloy has an initial activation hydrogen absorption of 4.67 wt.%, which is much h than the second and third activation hydrogen absorptions. In Figure 4b, the long-term h gen absorption curve of Mg82Ni15La3 alloy reveals that the alloy has an initial activation h gen absorption of 5.37 wt.%, while the second and third activation hydrogen absorption 5.16 wt.%, and the two curves almost overlap. This suggests that complete activation Mg77Ni20La3 alloy at 653 K requires 3 hydrogenation cycles, while the Mg82N15La3 alloy req 2 hydrogenation cycles. In Figure 5a,b, the short-term hydrogen absorption curv Mg77Ni20La3 and Mg82Ni15La3 alloys demonstrate that Mg77Ni20La3 alloy takes much less than Mg82Ni15La3 alloy to reach the saturated hydrogen absorption (90%) in the second third activations. This implies that Mg77Ni20La3 alloy has a better activation performance Mg82Ni15La3 alloy. Moreover, it can also be seen from the short-term activation curves th initial activation of the alloy is very slow.    rectly improve the kinetic properties of hydrogen absorption and desorption in the a

Activation Behaviors and Hydrogen Absorption Kinetics
Figures 4 and 5 shows the activation curves of Mg77+xNi20−xLa3 (x = 0, 5) alloy at 653 K shown in Figure 4a, the long-term hydrogen absorption curve of Mg77Ni20La3 alloy indi that the alloy has an initial activation hydrogen absorption of 4.67 wt.%, which is much h than the second and third activation hydrogen absorptions. In Figure 4b, the long-term hy gen absorption curve of Mg82Ni15La3 alloy reveals that the alloy has an initial activation hy gen absorption of 5.37 wt.%, while the second and third activation hydrogen absorption 5.16 wt.%, and the two curves almost overlap. This suggests that complete activation o Mg77Ni20La3 alloy at 653 K requires 3 hydrogenation cycles, while the Mg82N15La3 alloy req 2 hydrogenation cycles. In Figure 5a,b, the short-term hydrogen absorption curv Mg77Ni20La3 and Mg82Ni15La3 alloys demonstrate that Mg77Ni20La3 alloy takes much less than Mg82Ni15La3 alloy to reach the saturated hydrogen absorption (90%) in the second third activations. This implies that Mg77Ni20La3 alloy has a better activation performance Mg82Ni15La3 alloy. Moreover, it can also be seen from the short-term activation curves tha initial activation of the alloy is very slow.     Figure 6b reveals that the hydrogen absorption of Mg 82 Ni 15 La 3 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 Mg 77 Ni 20 La 3 and Mg 82 Ni 15 La 3 alloys is significantly higher than that of the alloy at each temperature, which is due to the formation of LaH 3.05 phase with high thermal stability and difficult to decompose after the initial hydrogenation of the alloy. In summary, Mg 82 Ni 15 La 3 alloy has better hydrogen absorption kinetics performance than Mg 77 Ni 20 La 3 alloy.
Mg content, the stronger the hydrogen absorption ability of the alloy. The hydrogen absorp tion 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.   Figure 7a tha 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 tha Mg82Ni15La3 alloy has a hydrogen desorption amount of up to 4.69 wt.% at 683 K. This is be cause the higher the Mg content, the stronger the hydrogen desorption ability of the alloy. I 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 amoun 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.  that Mg 77 Ni 20 La 3 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 Mg 82 Ni 15 La 3 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 Mg 77 Ni 20 La 3 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 Mg 82 Ni 15 La 3 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, Mg 82 Ni 15 La 3 alloy reaches a hydrogen desorption amount of 3.8 wt.% within 47 s. with high thermal stability and difficult to decompose after the initial hydrogenation of th alloy. In summary, Mg82Ni15La3 alloy has better hydrogen absorption kinetics performanc than Mg77Ni20La3 alloy.  Figures 7 and 8 show the hydrogen desorption endothermic reaction kinetics curves o Mg77+xNi20−xLa3 (x = 0, 5) alloys at different temperatures. It can be seen from Figure 7a tha Mg77Ni20La3 alloy has the best hydrogen desorption working temperature above 623 K an reaches a hydrogen desorption amount of 4.1 wt.% at 683 K. It can be seen from Figure 7b tha Mg82Ni15La3 alloy has a hydrogen desorption amount of up to 4.69 wt.% at 683 K. This is be cause the higher the Mg content, the stronger the hydrogen desorption ability of the alloy. can be seen from Figure 8a that the short-term hydrogen desorption endothermic kinetic curve of Mg77Ni20La3 alloy shows that the maximum hydrogen desorption amount of the allo is 3.8 wt.% at 653 K and reaches the saturated hydrogen desorption amount within 125 s. A the temperature increases, it only takes 71 s to reach a maximum hydrogen desorption amoun of 3.8 wt.% at 683 K. This is mainly due to the increase of temperature, which increases th energy of reactants in the alloy, accelerates the hydrogen desorption rate of the alloy, and thu increases the hydrogen desorption amount. At the same time, Figure 8b shows that the shor term hydrogen desorption exothermic kinetics curve of Mg82Ni15La3 alloy shows that the allo reaches a saturated hydrogen desorption amount of 4.39 wt.% within 140 s at 653 K. And whe the temperature rises to 683 K, Mg82Ni15La3 alloy reaches a hydrogen desorption amount of 3. wt.% within 47 s. The phase change heat of the alloy was calculated by using the test method o DSC curve of the alloy hydride warming and exothermic reaction. In order to calc the phase transition heat of the alloy, we adopted the method of testing the DSC cur the hydrogen desorption endothermic of the alloy hydride. Figure 9 shows the DSC cu of Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy hydrides after saturated hydrogen absorpti different heating rates. The test temperature range was 400~700 K. Firstly, the initial perature of the hydrogen desorption endothermic reaction of the alloy can be obta from the DSC curve, and the activation energy of the hydrogen desorption endothe reaction can be calculated according to the Kissinger method, so as to judge the diffi of the reaction. It can be seen from Figure 9 that there is a clear exothermic main peak a smaller peak on each DSC curve at each heating rate. The smaller exothermic pe related to the phase transition of Mg2NiH4, by the coupled hydrogen desorption e thermic of MgH2 and Mg2NiH4 phases. The temperature range of the smaller exothe peak is between 513-522 K. In Figure 9a, the endothermic main peak consists of tw vious peaks, which confirms that the exothermic main peak is caused by two hydr desorption phases of MgH2 and Mg2NiH4, while in Figure 9b-d, the exothermic main becomes an obvious peak, because two kinds of hydrogen desorption phases have a ergistic hydrogen desorption endothermic phenomenon. In Figure 9e, the exothe peak temperature at 20 K/min heating rate shows that the phase transition peak temp ture of Mg2NiH4 is little affected by Mg content and remains around 520K. While the temperature of MgH2 is more affected by Mg content and shows a trend of first rising then falling. The increase of hydrogen desorption peak temperature causes the increa activation energy of hydrogen desorption endothermic reaction of alloy hydride, w will be calculated in detail below. By comparing the hydrogen desorption endothe temperatures corresponding to the exothermic main peaks of four alloys with diff Mg contents at the same rate, it is found that with the increase of Mg content, the hydr desorption peak temperature of the alloy gradually increases and then decre Mg87Ni10La3 alloy has the highest hydrogen desorption peak temperature at 20 K heating rate, which is 629.8 K; while Mg92Ni5La3 alloy is 624.1 K, which is 5.7 K lowe 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 Mg 77+x Ni 20−x La 3 (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 Mg 2 NiH 4 , by the coupled hydrogen desorption endothermic of MgH 2 and Mg 2 NiH 4 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 MgH 2 and Mg 2 NiH 4 , 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 Mg 2 NiH 4 is little affected by Mg content and remains around 520 K. While the peak temperature of MgH 2 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. Mg 87 Ni 10 La 3 alloy has the highest hydrogen desorption peak temperature at 20 K/min heating rate, which is 629.8 K; while Mg 92 Ni 5 La 3 alloy is 624.1 K, which is 5.7 K lower. Table 1 lists the peak temperatures of DSC endothermic dehydrogenation reaction of Mg 77+x Ni 20−x La 3 (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.  Table 1 lists the peak temperatures of DSC endothermic dehydrogenation react Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy hydrides at different heating rates. Using diff heating rates and corresponding peak temperatures, the activation energy (Ea) of d drogenation endothermic reaction of the alloy can be calculated by Kissinger metho According to the Kissinger method, the activation energy can be calculated by the following Equation (6):  According to the Kissinger method, the activation energy can be calculated by using the following Equation (6):

Hydrogen Desorption Kinetics and Activation Energy
where R is the gas constant, k 0 is a constant, T p is the peak temperature of the exothermic peak, and β is the heating rate. This method was used to calculate the activation energy of Mg 77+x Ni 20−x La 3 (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 Mg 82 Ni 15 La 3 alloy is (72.7 ± 4.1) kJ/mol, the dehydrogenation activation energy of Mg 87 Ni 10 La 3 alloy is larger, which is (75.2 ± 1.5) kJ/mol, and the dehydrogenation activation energy of Mg 92 Ni 5 La 3 alloy is smaller, which is (68.5 ± 3.8) kJ/mol. Compared with Mg 87 Ni 10 La 3 alloy, the dehydrogenation endothermic activation energy of Mg 92 Ni 5 La 3 alloy is reduced by about 9%. Figure 10 shows that the activation energy values of Mg 77 Ni 20 La 3 , Mg 82 Ni 15 La 3 , Mg 87 Ni 10 La 3 and Mg 92 Ni 5 La 3 are between 68.5~75.2 kJ/mol. After comparison, these values are obviously lower than (150~160 kJ/mol) of MgH 2 . 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 LaH 3.05 phase, and LaH 3.05 phase has "hydrogen pumping", which greatly reduces the hydrogen activation energy value of the alloy.
where R is the gas constant, k0 is a constant, Tp is the peak temperature of the exoth peak, and β is the heating rate. This method was used to calculate the activation ene 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 equ The fitting results show that the dehydrogenation activation energy of Mg82Ni15La3 is (72.7 ± 4.1) kJ/mol, the dehydrogenation activation energy of Mg87Ni10La3 alloy is l which is (75.2 ± 1.5) kJ/mol, and the dehydrogenation activation energy of Mg92Ni5L loy is smaller, which is (68.5 ± 3.8) kJ/mol. Compared with Mg87Ni10La3 alloy, the drogenation endothermic activation energy of Mg92Ni5La3 alloy is reduced by abou Figure 10 shows that the activation energy values of Mg77Ni20La3, Mg82Ni15La3, Mg87N and Mg92Ni5La3 are between 68.5~75.2 kJ/mol. After comparison, these values are ously lower than (150~160 kJ/mol) of MgH2. The reason for the low activation energy of the alloy is the effect of alloying and rare earth hydride formed by adding rare elements. In addition, due to the addition of rare earth element La to form rare ear dride LaH3.05 phase, and LaH3.05 phase has "hydrogen pumping", which greatly re the hydrogen activation energy value of the alloy. The increase of dehydrogenation temperature and dehydrogenation activatio ergy of alloy hydride is not conducive to the improvement of dehydrogenation ki performance of alloy. In Mg77+xNi20−xLa3 (x = 0, 5, 10, 15) alloy, Mg content increases Mg77 to Mg92, and Ni content decreases from Ni20 to Ni5. It can be seen from the hyd absorption and dehydrogenation kinetics curves of the alloy that the hydrogen absor amount of the alloy shows an increasing trend. This is because after the increase o content, the hydrogen absorption and desorption ability of Mg phase is higher tha  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 Mg 77+x Ni 20−x La 3 (x = 0, 5, 10, 15) alloy, Mg content increases from Mg 77 to Mg 92 , and Ni content decreases from Ni 20 to Ni 5 . 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 Mg 2 Ni 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 Mg 77+x Ni 20−x La 3 (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 Mg 77 Ni 20 La 3 alloy is 1177 J/g at a heating and desorption hydrogen rate of 5 K/min; the phase change heat of Mg 87 Ni 10 La 3 alloy is 1529 J/g; the phase change heat of Mg 92 Ni 5 La 3 alloy is 1428 J/g, which is 251 J/g higher than that of Mg 77 Ni 20 La 3 alloy. The phase change heat of Mg 92 Ni 5 La 3 alloy is second only to Mg 87 Ni 10 La 3 alloy, while the peak hydrogen desorption temperature is significantly lower than that of Mg 87 Ni 10 La 3 , 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 Mg 87 Ni 10 La 3 alloy is significantly higher than that of the first two alloys and slightly higher than that of Mg 92 Ni 5 La 3 alloy. At the same time, the higher the Mg content in the alloy, the more likely the alloy will not desorption hydrogen. In summary, Mg 87 Ni 10 La 3 alloy is more suitable as an energy storage material.

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Conflicts of Interest:
The authors declare no conflict of interest.