Phase Transformation and Performance of Mg-Based Hydrogen Storage Material by Adding ZnO Nanoparticles

ZnO nanoparticles in a spherical-like structure were synthesized via filtration and calcination methods, and different amounts of ZnO nanoparticles were added to MgH2 via ball milling. The SEM images revealed that the size of the composites was about 2 μm. The composites of different states were composed of large particles with small particles covering them. After the absorption and desorption cycle, the phase of composites changed. The MgH2-2.5 wt% ZnO composite reveals excellent performance among the three samples. The results show that the MgH2-2.5 wt% ZnO sample can swiftly absorb 3.77 wt% H2 in 20 min at 523 K and even at 473 K for 1 h can absorb 1.91 wt% H2. Meanwhile, the sample of MgH2-2.5 wt% ZnO can release 5.05 wt% H2 at 573 K within 30 min. Furthermore, the activation energies (Ea) of hydrogen absorption and desorption of the MgH2-2.5 wt% ZnO composite are 72.00 and 107.58 KJ/mol H2, respectively. This work reveals that the phase changes and the catalytic action of MgH2 in the cycle after the addition of ZnO, and the facile synthesis of the ZnO can provide direction for the better synthesis of catalyst materials.


Introduction
In recent decades, the overuse of fossil resources and the waste gas produced by fossil resource combustion have led to the problems of energy dilemma and environmental pollution. In order to solve these problems, people urgently need to find clean energy to replace fossil resources. Among the many clean energy sources that have been discovered, hydrogen energy is favored by researchers, owing to its advantages of being clean, pollutionfree, and so on [1,2]. For the applications of the source of hydrogen, hydrogen storage plays a connecting role. Compared with traditional high-pressure gas hydrogen storage and low-temperature liquid hydrogen storage, solid hydrogen storage materials have been widely studied because of their advantages of high safety, small occupation area, low energy consumption, and high hydrogen storage density. For example, rare earth hydrogen storage materials [3][4][5], titanium hydrogen storage materials [6][7][8], vanadium hydrogen storage materials [9,10], magnesium hydrogen storage materials [11][12][13][14][15], etc. MgH 2 is considered one of the best hydrogen storage materials owing to its abundant reserves, high calorific value (1.43×10 8 J/kg), high theoretical hydrogen capacity (7.6 wt% H 2 ), no pollution, and excellent reversibility. However, the shortcomings of high temperature and slow kinetics in the cycle severely hinder the practical applications [16][17][18].
The research shows that domestic and foreign scholars mainly solve the above problems by adding catalysts [19][20][21][22], nano-limiting [11,23,24], and alloying [25][26][27]. Among them, catalysts are considered to be one of the most effective ways to ameliorate these shortcomings in the cycle.  15 alloy, the activation energies for hydrogen absorption and desorption were reduced to 61.6 and 91.3 KJ/mol, respectively [30]. Song et al. prepared Cr 2 O 3 using spray conversion and added 10 wt% Cr 2 O 3 to Mg with ball milling. The Mg-10 wt% Cr 2 O 3 can quickly absorb 4.57 wt% H 2 in 5 min and 5.93 wt% H 2 in 1 h at 593 K and 1.2 MPa hydrogen pressure [30]. Liu et al. synthesized TiO 2 @rGO nanoparticles using the solvothermal method. The TiO 2 nanoparticles are very uniform due to the presence of ethylene glycol and graphene. The MgH 2 -70TiO 2 @rGO-EG composite starts to release hydrogen at 240 • C and can desorb 6.0 wt% H 2 within 6 min at 300 • C. At the same time, it can absorb 5.9 wt% H 2 within 2 min at 200 • C [33]. Chen et al. investigated the addition of ZrO 2 nano-powder to MgH 2 . The results show that the MgH 2 -ZrO 2 composite can still absorb 4.0 wt% H 2 at room temperature [34]. Chuang et al. added Nb 2 O 5 and MWCNT to MgH 2 with ball milling. After 1 h of ball milling, the activation energies of hydrogen absorption and desorption of Mg-Nb 2 O 5 and MWCNT composites were reduced to 12.84 KJ/mol and 102.69 KJ/mol, respectively [35]. The above studies indicate that the addition of oxides can promote the performance of MgH 2 to some extent. As far as we are aware, the influence of ZnO addition on the performance of absorption and desorption of MgH 2 was rarely discussed. At the same time, little research has been performed on Mg-Zn. Zhong et al. synthesized Mg(Sn, Zn) solid solution, and hydrogenated the solid solution to produce MgZn 2 in situ [36]. Deledda et al. added Zn to Mg using the ball milling method and finally generated amorphous Mg 45 Zn 55 , and the influence of amorphous Mg 45 Zn 55 on hydrogen storage material is not obvious [37]. In addition, Liu et al. prepared Mg-Zn alloy using the method of hydrogen plasma-metal reaction. After hydrogen absorption, the Mg in the Mg-Zn alloy is converted to MgH 2 , and a small portion of the Mg reacted with the Zn to form MgZn 2 . The fine particle size and nano-sized grains of the Mg-Zn particles accelerated the nucleation and hydrogen diffusion in the cycle [38]. The studies show that the beneficial effect on Mg with the increase in the amount of ZnO decreases in the cycle. The phase change during the cycle is also closely related to the pressure of hydrogen. When the pressure of hydrogen absorption is higher than 1 MPa, MgZn 2 will be generated, and in the process of desorption, the multi-phase Mg-Zn intermetallics are formed [37][38][39]. In this work, we added the different amount of ZnO into MgH 2 by the most common method of ball milling and investigated the optimal doping amount of ZnO and the phase change in the cycle.

Preparation of the ZnO
The preparation process of ZnO nanoparticles is shown in Figure 1. The preparation method is as follows: Firstly, add 150 mL DMF and 3.75 mmol Zn(CH 3 COO) 2 ·2H 2 O into the beaker and stir for 30 min, named solution A. Next, add 100 mL DMF, 9 mmol PTA and 3.125 mL TEA and stir for 30 min, named solution B. Then, pour solution B into solution A and continue stirring for 1 h. After stirring, the white powder is obtained by vacuum filtration of the transparent solution. Finally, put the white powder into the tube furnace and heat the sample to 700 • C under air at 5 • C/min, and the ZnO is obtained after being left for 10 h.

Preparation of the MgH2-xZnO Nanocomposites (x = 2.5, 5, 7.5 wt%)
The amounts of ZnO particles of 2.5, 5, and 7.5 wt% were added to MgH2 powder by the method of ball milling, respectively. Firstly, the powder of ZnO and MgH2 was mixed in a grinding tank in the glove boxes filled with Ar. Then, the grinding tank was removed from the glove boxes, after vacuuming, and high purity Ar of 0.2 MPa was added. The process was repeated 1~2 times before the ball milling. The technological parameters in the ball milling process were as follows: the total time of the process of ball milling was 720 min (the ratio of run to stop was 60 min 3:1), the rotation speed was 450 r/min and the ratio of ball to powder was 30:1. Finally, the powder was collected in a glove box filled with Ar to prevent avoid oxidation

Characterization
The XRD data of different states of Mg-xZnO nanocomposite were measured in the 2θ range from 20 to 90°and a scanning rate of 6°/min using monochromatic Cu-Kα radiation. The images of morphology and microstructures of the prepared Mg-xZnO nanocomposites were measured with scanning electronic microscopes (SEM, JEOL JSM-7610F, Tokyo, Japan). In order to avoid oxidation, the samples of different states of Mg-xZnO nanocomposite were placed in a mikrouna glove box (C(H2O)<1 mg/L, C(O2)<1 mg/L, China).
The properties of kinetics and thermodynamics of Mg-xZnO composites at each temperature were performed in Sieverts-type equipment. The powders of 300~500 mg after ball milling were packed into a specially designed steel tube for testing and the samples were activated at least 2 times under 673 K and 4 MPa and under vacuum (≤0.1 Pa). In the isothermal de/hydrogenation experiments, the pressure of hydrogen absorption and desorption was 4 MPa and under vacuum (≤0.1 Pa), respectively, and the test time of hydrogen absorption and desorption was 1 h. Figure 2a shows the phase composition of as-prepared ZnO nanoparticles. It shows that the strong diffraction peak at 2θ = 31.77°, 34 Figure 2a, the absence of other phases indicates the high purity of the prepared ZnO nanoparticles. In the SEM image of nanoparticles shown in Figure 2b, it can be seen that the prepared ZnO nanoparticles have a spheroidal structure. The size of ZnO nanoparticles is about 300~500 nm, while the shape is regular and  The amounts of ZnO particles of 2.5, 5, and 7.5 wt% were added to MgH 2 powder by the method of ball milling, respectively. Firstly, the powder of ZnO and MgH 2 was mixed in a grinding tank in the glove boxes filled with Ar. Then, the grinding tank was removed from the glove boxes, after vacuuming, and high purity Ar of 0.2 MPa was added. The process was repeated 1~2 times before the ball milling. The technological parameters in the ball milling process were as follows: the total time of the process of ball milling was 720 min (the ratio of run to stop was 60 min 3:1), the rotation speed was 450 r/min and the ratio of ball to powder was 30:1. Finally, the powder was collected in a glove box filled with Ar to prevent avoid oxidation

Characterization
The XRD data of different states of Mg-xZnO nanocomposite were measured in the 2θ range from 20 to 90 • and a scanning rate of 6 • /min using monochromatic Cu-K α radiation. The images of morphology and microstructures of the prepared Mg-xZnO nanocomposites were measured with scanning electronic microscopes (SEM, JEOL JSM-7610F, Tokyo, Japan). In order to avoid oxidation, the samples of different states of Mg-xZnO nanocomposite were placed in a mikrouna glove box (C(H 2 O) < 1 mg/L, C(O 2 ) < 1 mg/L, China).
The properties of kinetics and thermodynamics of Mg-xZnO composites at each temperature were performed in Sieverts-type equipment. The powders of 300~500 mg after ball milling were packed into a specially designed steel tube for testing and the samples were activated at least 2 times under 673 K and 4 MPa and under vacuum (≤0.1 Pa). In the isothermal de/hydrogenation experiments, the pressure of hydrogen absorption and desorption was 4 MPa and under vacuum (≤0.1 Pa), respectively, and the test time of hydrogen absorption and desorption was 1 h.   Figure 2a, the absence of other phases indicates the high purity of the prepared ZnO nanoparticles. In the SEM image of nanoparticles shown in Figure 2b, it can be seen that the prepared ZnO nanoparticles have a spheroidal structure. The size of ZnO nanoparticles is about 300~500 nm, while the shape is regular and the particle size is homogeneous. To show the composition of the prepared ZnO nanoparticles, the element mapping results show that the O and Zn elements are uniform. Moreover, the O and Zn seem to coincide with each other, suggesting that the nanoparticles are ZnO. The above results all prove the successful preparation of ZnO nanoparticles. the particle size is homogeneous. To show the composition of the prepared ZnO nanoparticles, the element mapping results show that the O and Zn elements are uniform. Moreover, the O and Zn seem to coincide with each other, suggesting that the nanoparticles are ZnO. The above results all prove the successful preparation of ZnO nanoparticles.

Characterization of MgH2-xZnO Composite(x = 2.5, 5, 7.5 wt%)
So as to further determine the effect of ZnO, different contents of ZnO were added to MgH2 to explore the phase changes. Figure 3 shows the XRD patterns of MgH2-xZnO with different states. The XRD patterns show that the most diffraction peaks can be determined with MgH2 (PDF#12-0697), and the diffraction peaks at 31.7°, 34.4°, 36.3°, 47.5°, 56.6°, 62.8, 69.1°, and 76.9° can be ascribed by the ZnO (PDF#36-1451) in Figure 3a. Meanwhile, the diffraction peak is found at 2θ=42.9°, which is due to the slow oxidation of the composite during the test [40]. Figure 3a also shows that with the increase in the content of ZnO, the diffraction peaks of ZnO becomes higher, and the phenomenon shows that ZnO does not decompose after ball milling. All the above results indicate the successful addition of ZnO. As exhibited in Figure 3b, the diffraction peaks of ZnO and MgH2 disappear, and the diffraction peaks of Mg (PDF#12-0697) appear. It is worth noting that some diffraction peaks of uncertainty are detected at 2θ = 38°−50°, which is due to the low hydrogen pressure results in Mg reacting with ZnO to form the multi-phase Mg-Zn intermetallics (MgZn, Mg2Zn3, and Mg2Zn11, etc, named MgxZny) during the process of hydrogen release [38]. In order to have a clearer observation, the above area is enlarged, as shown in Figure 3e. However, the diffraction peak of MgO still exists in Figure 3b, it was caused by oxidation during the test. Figure 3c shows that the XRD patterns of the sample of re-hydrogenation, the conversion of Mg and H2 into MgH2, and the diffraction peaks at 2θ = 38°−50° disappear. At the same time, the diffraction peaks at 20.8°, 22 Figure  3c. In Figure 3d, the diffraction peak of MgZn2 can be seen more clearly, which further proves that the intensity of the diffraction peak increases with the increase in the content of the catalyst. Liu et al. prepared Mg-Zn alloy and the XRD results showed that a small amount of MgZn2 was generated in the cycling process, and the performance test showed that the Mg-Zn composite had better performance than Mg [38]. Zhong et al. synthesized Mg (Sn, Zn) solid solution matrix. After hydrogen absorption, MgZn2 was generated in situ [36]. The superior presence of MgZn2 promoted the hydrogen absorption and release

Characterization of MgH 2 -xZnO Composite (x = 2.5, 5, 7.5 wt%)
So as to further determine the effect of ZnO, different contents of ZnO were added to MgH 2 to explore the phase changes. Figure Figure 3a. Meanwhile, the diffraction peak is found at 2θ = 42.9 • , which is due to the slow oxidation of the composite during the test [40]. Figure 3a also shows that with the increase in the content of ZnO, the diffraction peaks of ZnO becomes higher, and the phenomenon shows that ZnO does not decompose after ball milling. All the above results indicate the successful addition of ZnO. As exhibited in Figure 3b, the diffraction peaks of ZnO and MgH 2 disappear, and the diffraction peaks of Mg (PDF#12-0697) appear. It is worth noting that some diffraction peaks of uncertainty are detected at 2θ = 38−50 • , which is due to the low hydrogen pressure results in Mg reacting with ZnO to form the multi-phase Mg-Zn intermetallics (MgZn, Mg 2 Zn 3, and Mg 2 Zn 11 , etc, named Mg x Zn y ) during the process of hydrogen release [38]. In order to have a clearer observation, the above area is enlarged, as shown in Figure 3e. However, the diffraction peak of MgO still exists in Figure 3b, it was caused by oxidation during the test. Figure 3c shows that the XRD patterns of the sample of re-hydrogenation, the conversion of Mg and H 2 into MgH 2, and the diffraction peaks at 2θ = 38−50 • disappear. At the same time, the diffraction peaks at 20.  Figure 3d, the diffraction peak of MgZn 2 can be seen more clearly, which further proves that the intensity of the diffraction peak increases with the increase in the content of the catalyst. Liu et al. prepared Mg-Zn alloy and the XRD results showed that a small amount of MgZn 2 was generated in the cycling process, and the performance test showed that the Mg-Zn composite had better performance than Mg [38]. Zhong et al. synthesized Mg (Sn, Zn) solid solution matrix. After hydrogen absorption, MgZn 2 was generated in situ [36]. The superior presence of MgZn 2 promoted the hydrogen absorption and release process. These results indicate that MgH 2 -ZnO composites can be converted into MgZn 2 phase and Mg x Zn y phase during the cycle process, which has a positive catalytic effect on the subsequent hydrogenation reaction, thus enhancing the kinetics.
process. These results indicate that MgH2-ZnO composites can be converted into MgZn2 phase and MgxZny phase during the cycle process, which has a positive catalytic effect on the subsequent hydrogenation reaction, thus enhancing the kinetics. In order to study the catalytic effects of ZnO nanoparticles on MgH2, Figure 4 displays the kinetics curves of hydrogen absorption and desorption of MgH2-xZnO. It can be seen that the composite with a different amount of ZnO shows a similar absorption kinetic above 623 K in Figure 4a-c. The hydrogen storage capacity of MgH2-2.5 wt% ZnO, MgH2-5 wt% ZnO, and MgH2-7.5 wt% ZnO were 6.077, 5.994, and 5.957 wt% H2 at 673 K, respectively. At the same time, this phenomenon also indicates that the maximum hydrogen content decreases with the increase in ZnO content. However, the decrease is not particularly obvious, which is well demonstrated by TPD test. This is caused by the inability of ZnO to store hydrogen, thus reducing the hydrogen capacity of MgH2-xZnO composites. When the sample absorbs hydrogen at high temperatures, the sample will also undergo the hydrogen desorption process. However, at this temperature, the rate of hydrogen absorption is much higher than the rate of hydrogen desorption, and the presence of a catalyst also accelerates the process of hydrogen absorption. The hydrogen absorption process belongs to the gas-solid reaction, which will eventually reach the equilibrium state. Due to the existence of the catalyst, the hydrogen absorption rate is too fast, leading to the increase in hydrogen absorption, but it is not in the equilibrium state at this time. After a period of time, the equilibrium state is reached, resulting in the decrease in the maximum hydrogen absorption. It is worth noting that the MgH2-2. 5   In order to study the catalytic effects of ZnO nanoparticles on MgH 2 , Figure 4 displays the kinetics curves of hydrogen absorption and desorption of MgH 2 -xZnO. It can be seen that the composite with a different amount of ZnO shows a similar absorption kinetic above 623 K in Figure 4a-c. The hydrogen storage capacity of MgH 2 -2.5 wt% ZnO, MgH 2 -5 wt% ZnO, and MgH 2 -7.5 wt% ZnO were 6.077, 5.994, and 5.957 wt% H 2 at 673 K, respectively. At the same time, this phenomenon also indicates that the maximum hydrogen content decreases with the increase in ZnO content. However, the decrease is not particularly obvious, which is well demonstrated by TPD test. This is caused by the inability of ZnO to store hydrogen, thus reducing the hydrogen capacity of MgH 2 -xZnO composites. When the sample absorbs hydrogen at high temperatures, the sample will also undergo the hydrogen desorption process. However, at this temperature, the rate of hydrogen absorption is much higher than the rate of hydrogen desorption, and the presence of a catalyst also accelerates the process of hydrogen absorption. The hydrogen absorption process belongs to the gas-solid reaction, which will eventually reach the equilibrium state. Due to the existence of the catalyst, the hydrogen absorption rate is too fast, leading to the increase in hydrogen absorption, but it is not in the equilibrium state at this time. After a period of time, the equilibrium state is reached, resulting in the decrease in the maximum hydrogen absorption. It is worth noting that the MgH 2 -2. 5 [38]. The results show that the MgH 2 -2.5 wt% ZnO composite is only 0.4 wt% lower. In this work, the MgH 2 -2.5 wt% ZnO composite still exhibits better kinetic performance in the three samples as the temperature drops lower. In terms of hydrogen desorption, dehydrogenation kinetics remain similar at high temperatures. In Figure 4e, where α is the percent conversion at t, k is a parameter, and η represents the Avrami exponent. Additionally, η and ηlnk can be gained by the slope and intercept of the fitting lines of ln [−ln(1−α)] vs ln t at different temperatures, and the curves of ln [−ln(1−α)] vs lnt are shown in Figure 5. At the same time, Table 1 details the results of the fit of the kinetic curves with appropriate standard deviation. Table 1 shows that the standard deviation of where α is the percent conversion at t, k is a parameter, and η represents the Avrami exponent. Additionally, η and ηlnk can be gained by the slope and intercept of the fitting lines of ln [−ln(1−α)] vs. ln t at different temperatures, and the curves of ln [−ln(1−α)] vs. lnt are shown in Figure 5. At the same time, Table 1 details the results of the fit of the kinetic curves with appropriate standard deviation. Table 1 shows that the standard deviation of the fitted curves of ln [−ln(1−α)] vs. ln t for the measured temperatures of all samples is lower than 0.043, which also indicates that the data are very stable and the accuracy of the results of the fitting curve of ln [−ln(1−α)] vs. lnt at different temperatures were further verified. Next, the E a can be calculated via the Arrhenius equation in Equation (2) lnk = E a /RT + lnA (2) where A represents a coefficient, T is the temperature, R is the gas constant (8.314 KJ/mol·K) and the lnk is obtained by the JMAK equation. where α is the percent conversion at t, k is a parameter, and η represents the Avrami exponent. Additionally, η and ηlnk can be gained by the slope and intercept of the fitting lines of ln [−ln(1−α)] vs ln t at different temperatures, and the curves of ln [−ln(1−α)] vs lnt are shown in Figure 5. At the same time, Table 1 details the results of the fit of the kinetic curves with appropriate standard deviation. Table 1 shows that the standard deviation of the fitted curves of ln [−ln(1−α)] vs ln t for the measured temperatures of all samples is lower than 0.043, which also indicates that the data are very stable and the accuracy of the results of the fitting curve of ln [−ln(1−α)] vs lnt at different temperatures were further verified. Next, the Ea can be calculated via the Arrhenius equation in Equation (2) lnk = Ea/RT + lnA (2) where A represents a coefficient, T is the temperature, R is the gas constant (8.314 KJ/mol·K) and the lnk is obtained by the JMAK equation.

MgH2-2.5 wt% ZnO
The  The plots of the lnk vs. 1000/T of hydrogen absorption and desorption are shown in Figure 6a,b. In Figure 6a and Table 1, the value of E ab (the activation energy of hydrogen absorption) of MgH 2 -2.5 wt% ZnO, MgH 2 -5 wt% ZnO, and MgH 2 -7.5 wt% ZnO were calculated to be 72.00, 82.66, and 88.43 KJ/mol, respectively. Singh et al. synthesized different sizes of CeO 2 nanoparticles with ball milling, and the kinetics effect of MgH 2 -CeO 2 was studied. The E ab was 84 KJ/mol [28]. The E a for the hydrogenation of the sample of MgH 2 -2.5 wt% ZnO is lower than that of the Mg-TiO 2 @C sample. So as to further study the sample of MgH 2 -2.5 wt% ZnO, the value of E de (the activation energy of hydrogen release) of MgH 2 -2.5 wt% ZnO, MgH 2 -5 wt% ZnO and MgH 2 -7.5 wt% ZnO are calculated to be 107.58, 116.35 and 128.41 KJ/mol, respectively, in Figure 6b and Table 1. In the three samples, the E ab and E de of MgH 2 -2.5 wt% ZnO are the lowest, which also demonstrates the MgH 2 -2.5 wt% ZnO with the best kinetic performance. Research shows that TiO 2 is an excellent catalyst for Mg-based hydrogen storage material. Zhang et al. synthesized the Mg-10 wt% TiO 2 @C composite, the E de for the composite was 106 ± 4 KJ/mol [44]. All the above results show that MgH 2 -2.5 wt% ZnO composites have the best performance among the three samples, and the activation energy of hydrogen absorption and desorption is superior to other materials.  Figure 6c reveals non-isothermal dehydrogenation curves of MgH2 and MgH2-xZnO composites. The test results show that the initial dehydrogenation temperatures of MgH2-2.5 wt% ZnO, MgH2-5 wt% ZnO, and MgH2-7.5 wt% ZnO decreased by 72.5, 66.5, and 54.0 °C, respectively, compared to pure MgH2. Moreover, the hydrogen capacity of MgH2-2.5 wt% ZnO, MgH2-5 wt% ZnO and MgH2-7.5 wt% ZnO are 6.61, 6.33, and 6.22 wt% H2. The capacity of the above three samples is higher than 5.79 wt% H2 of pure MgH2 and Mg85Al15-V2O3@rGO composite [19]. The result of TPD also indicated the lowest dehydrogenation temperature of MgH2-2.5 wt% ZnO in Figure 6c. In a word, the above data indicate that MgH2-2.5 wt% ZnO has the best performance among the three samples. 5,7.5

wt%)
To study the effect of MgH2 by adding ZnO with different amounts, the micromorphology of the MgH2-xZnO composites of ball milling after dehydrogenated and re-hydrogenated are characterized by SEM and shown in Figure 7. The SEM images show that the size of the composites is about 2 μm. Figure 7a shows that ZnO nanoparticles were uniformly attached to the surface of MgH2. This phenomenon shows that the catalyst can be evenly dispersed on the surface of the MgH2 by ball milling.   [19]. The result of TPD also indicated the lowest dehydrogenation temperature of MgH 2 -2.5 wt% ZnO in Figure 6c. In a word, the above data indicate that MgH 2 -2.5 wt% ZnO has the best performance among the three samples. To study the effect of MgH 2 by adding ZnO with different amounts, the micromorphology of the MgH 2 -xZnO composites of ball milling after dehydrogenated and re-hydrogenated are characterized by SEM and shown in Figure 7. The SEM images show that the size of the composites is about 2 µm. Figure 7a shows that ZnO nanoparticles were uniformly attached to the surface of MgH 2 . This phenomenon shows that the catalyst can be evenly dispersed on the surface of the MgH 2 by ball milling. After dehydrogenation, the SEM exhibit the size of the particle to also be about 2 μm and the similar morphology with the state of ball milling. According to the phase change of XRD, MgH2 is transformed into Mg, while small particles on the surface transform from ZnO into MgxZny, see Figure 7b. Moreover, Figure 7c shows the SEM image of MgH2-2.5 wt% ZnO after re-hydrogenation, the results display no significant morphology change from the other two states. According to Figure 3c, the small particles on the surface transform from MgxZny into MgZn2 in Figure 7c. The morphological evolution of the sample of MgH2-5 wt% ZnO and MgH2-7.5 wt% ZnO are similar to MgH2-2.5 wt% ZnO.
In order to further ensure the element composition of the different states of MgH2-ZnO, the element mapping of the composites with different states of ball milling, dehydrogenation, and re-hydrogenation. Figure 8 shows that the elements of Mg, Zn, and O are uniformly distributed, and Mg is the dominant element. The XRD pattern shown in Figure 3a also confirms that ZnO exists in the composite after ball milling, implying that the ZnO remains stable during ball milling. As for MgO, it is inevitable for the formation of MgO for the high activity of the powder after ball milling; therefore, the O element in Figure 8 is also contributed from MgO to some extent. After dehydrogenation, the SEM exhibit the size of the particle to also be about 2 µm and the similar morphology with the state of ball milling. According to the phase change of XRD, MgH 2 is transformed into Mg, while small particles on the surface transform from ZnO into Mg x Zn y , see Figure 7b. Moreover, Figure 7c shows the SEM image of MgH 2 -2.5 wt% ZnO after re-hydrogenation, the results display no significant morphology change from the other two states. According to Figure 3c, the small particles on the surface transform from Mg x Zn y into MgZn 2 in Figure 7c. The morphological evolution of the sample of MgH 2 -5 wt% ZnO and MgH 2 -7.5 wt% ZnO are similar to MgH 2 -2.5 wt% ZnO.
In order to further ensure the element composition of the different states of MgH 2 -ZnO, the element mapping of the composites with different states of ball milling, dehydrogenation, and re-hydrogenation. Figure 8 shows that the elements of Mg, Zn, and O are uniformly distributed, and Mg is the dominant element. The XRD pattern shown in Figure 3a also confirms that ZnO exists in the composite after ball milling, implying that the ZnO remains stable during ball milling. As for MgO, it is inevitable for the formation of MgO for the high activity of the powder after ball milling; therefore, the O element in Figure 8 is also contributed from MgO to some extent. Nanomaterials 2023, 13, x FOR PEER REVIEW 11 of 14 The element mapping of dehydrogenated MgH2-5 wt% ZnO displays the elements of Mg and Zn uniformly distributed which prove the phase change, see Figure 9. The elements mapping of the sample of re-hydrogenated is shown in Figure 10. The elements of Mg and Zn also uniformly distributed, and it is proven that the MgxZny turns into MgZn2. It can be seen that after the addition of ZnO, the catalyst does not decompose. The phase of the catalyst will be transformed when the sample changes to the hydrogen absorption or hydrogen desorption state.

Conclusions
In this work, we synthesize a spheroidal structure ZnO nanoparticle via filtration and calcination methods, the nanoparticle size varied from 300 to 500 nm. Different amounts The element mapping of dehydrogenated MgH 2 -5 wt% ZnO displays the elements of Mg and Zn uniformly distributed which prove the phase change, see Figure 9. The elements mapping of the sample of re-hydrogenated is shown in Figure 10. The element mapping of dehydrogenated MgH2-5 wt% ZnO displays the elements of Mg and Zn uniformly distributed which prove the phase change, see Figure 9. The elements mapping of the sample of re-hydrogenated is shown in Figure 10. The elements of Mg and Zn also uniformly distributed, and it is proven that the MgxZny turns into MgZn2. It can be seen that after the addition of ZnO, the catalyst does not decompose. The phase of the catalyst will be transformed when the sample changes to the hydrogen absorption or hydrogen desorption state.

Conclusions
In this work, we synthesize a spheroidal structure ZnO nanoparticle via filtration and calcination methods, the nanoparticle size varied from 300 to 500 nm. Different amounts  The element mapping of dehydrogenated MgH2-5 wt% ZnO displays the elements o Mg and Zn uniformly distributed which prove the phase change, see Figure 9. The ele ments mapping of the sample of re-hydrogenated is shown in Figure 10. The elements of Mg and Zn also uniformly distributed, and it is proven that th MgxZny turns into MgZn2. It can be seen that after the addition of ZnO, the catalyst doe not decompose. The phase of the catalyst will be transformed when the sample change to the hydrogen absorption or hydrogen desorption state.

Conclusions
In this work, we synthesize a spheroidal structure ZnO nanoparticle via filtration and calcination methods, the nanoparticle size varied from 300 to 500 nm. Different amount The elements of Mg and Zn also uniformly distributed, and it is proven that the Mg x Zn y turns into MgZn 2 . It can be seen that after the addition of ZnO, the catalyst does not decompose. The phase of the catalyst will be transformed when the sample changes to the hydrogen absorption or hydrogen desorption state.

Conclusions
In this work, we synthesize a spheroidal structure ZnO nanoparticle via filtration and calcination methods, the nanoparticle size varied from 300 to 500 nm. Different amounts of ZnO were introduced to the MgH 2 through the ball milling method. After ball milling, the ZnO is not decomposed and is uniformly dispersed on the surface of MgH 2 . After the desorption process, the ZnO reacts with Mg to form Mg x Zn y due to the low pressure of hydrogen. After the absorption, the Mg in the Mg-Zn alloy is converted to MgH 2 , and a small portion of the Mg reacted with the Zn to form MgZn 2 . After the performance test, the MgH 2 -2.5 wt% ZnO composite has the best kinetic performance. Compared with pure MgH 2 , the initial dehydrogenation temperature of MgH 2 -2.5 wt% ZnO is 40 • C lower than it. As for isothermal tests, MgH 2 -2.5 wt% ZnO composite can release about 5.47 wt% at 623 K and can absorb 4.70 wt% H 2 in 1 h at 523 K. However, MgH 2 -5 wt% ZnO and MgH 2 -7.5 wt% ZnO can releases/absorb 4.90 wt%/4.26 wt% H 2 and 4.58 wt%/2.92 wt% H 2 with the same condition, respectively. According to the calculation of activation energy, the E ab and E de of MgH 2 -2.5 wt% ZnO are the lowest, which are 72.00 KJ/mol and 107.58 KJ/mol, respectively. Although the addition of ZnO plays a certain catalytic role, the catalytic effect is not excellent compared with other kinds of catalysts.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

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