Study on Hydrolysis of Magnesium Hydride by Interface Control

Magnesium hydride (MgH2) is one of the competitive hydrogen storage materials on account of abundant reserves and high hydrogen content. The hydrolysis of MgH2 is an ideal and controllable chemical hydrogen generation process. However, the hydrolyzed product of MgH2 is a passivation layer on the surface of the magnesium hydride, which will make the reaction continuity worse and reduce the rate of hydrogen release. In this work, hydrogen generation is controllably achieved by regulating the change of the surface tension value in the hydrolysis, a variety of surfactants were systematically investigated for the effect of the hydrolysis of MgH2 In the meantime, the passivation layer of MgH2 was observed by scanning electron microscope (SEM), and the surface tension value of the solution with different surfactants were monitored, investing the mechanism of hydrolysis adding different surfactants. Results show that different surfactants have different effects on hydrogen generation. The hydrogen generation capacity from high to low is as follows: tetrapropylammonium bromide (TPABr), sodium dodecyl benzene sulfonate (SDBS), Ecosol 507, octadecyl trimethyl ammonium chloride (OTAC), sodium alcohol ether sulfate (AES), and fatty methyl ester sulfonate (FMES-70). When the ratio of MgH2 to TPABr was 5 : 1, the hydrogen generation was increased by 52% and 28.3%, respectively, at the time of 100 s and 300 s. When hydrolysis time exceeds 80 s, the hydrogen generation with AES and FMES-70 began to decrease; it was reduced by more than 20% at the time of 300 s. SEM reveals that surfactants can affect the crystalline arrangement of Mg(OH)2 and make the passivation layer three-dimensionally layered providing channels for H2O molecules to react with MgH2.


Introduction
With the rapid development of industrial technology, the global demand for energy is growing exponentially. Fossil fuels, as the most widely used energy materials, are not renewable, environmental pollution, and other defects; therefore, it is an urgent problem to find new clean and efficient energy sources at the present stage. Hydrogen has the characteristics of environmental protection, renewable, and high heat energy. Since the 1970s, it has been widely concerned by researchers. Hydrogen storage and release technology limits the development and application of hydrogen energy [1][2][3][4]. Among many hydrogen storage materials, the hydrogen content of MgH 2 reaches 7.69% (wt. %), and its theoretical hydrolysis hydrogen yield is 15.3% (wt. %) [5,6]. Magnesium hydride is considered one of the best choices for portable hydrogen fuel cells due to stable storage and mild hydrolysis [7][8][9][10][11][12][13][14][15][16][17][18][19]. The chemical equation of the reaction between MgH 2 and H 2 O is as following: The magnesium hydroxide is difficult to dissolve in water whose solubility product is 5:6 × 10 −12 mol 3 · L −3 . It easily forms the passivation layer during the hydrolysis process, prevents the diffusion of water molecules toward the surface of magnesium hydride, reduces the rate, and shortens the duration of hydrolysis reaction [20][21][22]. Eliminating the cladding effect of magnesium hydroxide passivation layer on MgH 2 has become an urgent problem to be solved in the hydrogen yield of hydrolysis [23].
As we all know, surfactants can change the surface tension of the solution by means of forming an adsorbed layer with a certain orientation at the solid-liquid interface [24][25][26][27]. Based on the above principles, this study selected several typical surfactants to study the influence on the quantity of hydrogen generation and the rate of hydrogen generation. These data were combined with the surface tension value of the aqueous solution and the scanning electron micrograph of the product to analyze the corresponding mechanism. The study of interface control in this work provides the theoretical basis for the future researches of H 2 generation by hydrolysis of MgH 2 .

Experimental
Device and Process. The experimental device ( Figure 1) is composed of the reaction system and the metering system. The reaction system is composed of a 250 mL three-necked flask, a condenser, a thermometer, and a water bath; the metering system is composed of a gas washing bottle, a beaker, and an analytical balance. The two systems are connected with silicone tubes.
Firstly, add 200 mL of water to a 250 mL three-necked flask. After the temperature of the water bath reaches 70°C, surfactant and stir were added. Then, the magnesium hydride was put to the three-necked flask. The hydrogen produced by hydrolysis is condensed by the condenser. An equal volume of water of gas washing bottle was discharge. Finally, the volume of hydrogen generated was calculated as follows:

Results and Discussion
Surfactants with different groups, structures, and dosages have different impacts on the surface energy and the wetting effect between solid and liquid.
3.1. Effect of Anionic Surfactant. According to the structure of hydrophilic groups, anionic surfactants mainly include sulfonate and sulfate ester salts. Typical anionic surfactants were used in this study: SDBS, AES, and FMES-70.  (7) analytical balance).

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Under the same condition, the volume of the water, the temperature of the water bath, and the mass of MgH 2 , surfactants with different dosages were added, time (seconds) is the x-coordinate, and hydrogen yield (mL/g) is the y-coordinate.
It is shown in Figure 2 that the hydrogen generation rate are approximately the same at 110 s when MgH 2 : SDBS = 300 : 1, MgH 2 : SDBS = 50 : 1, and MgH 2 : SDBS = 10 : 1. While the hydrogen generation rate is slightly lower when MgH 2 : SDBS = 100 : 1 at 110 s. When the hydrolysis time is 120 s, the hydrogen generation rate of MgH 2 : SDBS = 300 : 1 is greater than the other. When the hydrolysis time is 155 s, the hydrogen generation rate of MgH 2 : SDBS = 100 : 1 is higher than MgH 2 : SDBS = 10 : 1 and The hydrogen generation rate will improve with the increase of SDBS dosages (time ≤100 s). It is possible that the surface tension of water becomes lower as surfactants increase (it is presented in Table 1). However, the lower the surface tension, the easier bubbles will foam. These bubbles gather into a foam layer, which binds H 2 . It is difficult for H 2 escape from the reaction system (e.g., the curve of MgH 2 : SDBS = 10:1 in the Figure 2). Besides, the SDBS concentration is bigger than the critical micelle concentration (CMC = 1.47 × 10 −3 − 1.60 × 10 −3 mol/L) when MgH 2 : SDBS = 10 : 1. Self-polymerization process of SDBS molecules possibly employed, which inhibits the detachment of Mg(OH) 2 from the surface of MgH 2 and the dispersion of Mg(OH) 2 in the water.
It is observed from Table 2 that surface tension of solutions becomes lower as the ratio of MgH 2 to surfactants increases. On one hand, the system foams more easily [28,29], and the foam layer weakens the diffusion of H 2 . On the other hand, the hydrolysis reaction is essentially a     It is shown in Figure 4 that the hydrogen generation of MgH 2 : OTAC = 5 : 1 and MgH 2 : OTAC = 100 : 1 are 70% and 29% higher than that without surfactants at time of 100 s. When the hydrolysis time is over 100 s, the hydrogen generation curves gradually become smooth. At the time of 300 s, the hydrogen generation of MgH 2 : OTAC = 40 : 1 reaches the maximum which is 11.0% higher than that without surfactants.
Within 120 s of the hydrolysis, the hydrogen generation and the hydrogen generation rate improve as the ratio of MgH 2 to OTAC. After 120 s, the foam layer hinders the continuation of the hydrolysis. It can be seen from Table 3 that the surface tension of MgH 2 : OTAC = 100 : 1 decrease to 36.87 mN/m. When MgH 2 : OTAC = 5 : 1, the surface tension decreases to 32.99 mN/m slightly. Therefore, OTAC reduces the surface energy of solution preventing the agglomeration of Mg(OH) 2 between crystals.
It is also possible that the positively charged groups of the cationic surfactants may combine with OHinhibiting the combination of OHand Mg 2+ and delay the formation of the passivation layer in the surface of MgH 2 . Zheng et al. [30] studied the hydrolysis of Mg in different solutions (MgCl 2 , MnCl 2 , NiCl 2 , AlCl 3 , NH 4 Cl, and HCl). They found that there was a significant improvement in NH 4 Cl and HCl solution, the conversion efficiency increased by more than 60%. Researchers believe that the higher the affinity between the cations and OHin the solution, the more effective it is to inhibit the formation of the passivation layer.
In addition, the groups of the cationic surfactants can absorb on Mg(OH) 2 along the direction of growth and prevent its growth [31]. Figure 5 reveals the effect of TPABr on hydrogen generation. When the time is 100 s, the hydrogen generation of MgH 2 : TPABr = 5 : 1 and MgH 2 : TPABr = 100 : 1 increases by 52% and 29% compared with that without surfactants, respectively. At the time of 300 s, the hydrogen generation of MgH 2 :TPABr = 5 : 1 and MgH 2 : TPABr = 100 : 1 increases by 28.3% and 3%.

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According to the previous results in this work, the surface tension of solution decreases with the increase of the ratio of MgH 2 to surfactants. At the initial stage of the reaction, the lower the surface tension, the more hydrogen generation. And at the middle and late stages, the foam layer caused by the low surface tension impedes the hydrogen generation. However, there are some differences when TPABr added to the solution. As shown in Table 4, the surface tension decreases from 71.57 mN/m to 61.54 mN/m and then increases to 69.23 mN/m with increase of the ratio of MgH 2 to TPABr. And the hydrogen generation has the same trend, the final hydrogen yield are ranked as follows: MgH 2 : TPABr = 5 : 1 > MgH 2 : TPABr = 1 : 1 > MgH 2 : TPABr = 10 : 1 > MgH 2 : TPABr = 100 : 1 > MgH 2 . Figure 6(d) illustrates that Mg(OH) 2 grows evenly around the surface of MgH 2 , like a blooming flower. The "petals"-Mg(OH) 2 form uniform channels instead of coating MgH 2 in the form of flakes. Because of the better passability, H 2 O molecules can contact MgH 2 through these channels. Contrarily, the high concentration of TPABr may cause Mg(OH) 2 to grow as many points as possible on the surface of MgH 2 forming narrow and long channels. As a result, it is difficult for H 2 O molecules to reach the surface of MgH 2 through the channels.
When the surface tension decreases to 61.54 mN/m, there is no obvious foam layer. Thus, H 2 can easily escape from the solution. In addition, TPABr inhibits the formation of passivation layer and prolongs the time for Mg(OH) 2 to reach the critical volume. It may also because of the adsorptive effect of TPABr. When TPABr absorbs on the surface of Mg(OH) 2 newly formed and the adsorption layer cannot only inhibit the growth of the Mg(OH) 2 [31,32], it also reduces the surface energy. Therefore, the interaction among the particles is weakened, and it prevents the agglomeration among the particles [33][34][35] 3.3. Effect of Nonionic Surfactant. Nonionic surfactants are different in structure from other ionic surfactants. The main hydrophilic group is an ether group that does not dissociate in aqueous solution. It has an excellent wetting effect and superior antideposition ability for particles.
As shown in Figure 7, when the time is 100 s, the hydrogen generation of MgH 2 : Ecosol 507 = 100 : 1 is 50% higher 5 International Journal of Photoenergy than that without surfactants. When the time is 300 s, the hydrogen generation of MgH 2 : Ecosol 507 = 10 : 1 is 40% lower than that without surfactants, but the hydrogen generation of MgH 2 : Ecosol 507 = 100 : 1 is 11.7% higher than it.
It can be seen from Table 5 that Ecosol 507 significantly reduces the surface tension. In this experiment, we observed the volume of the foam was far much larger than the other surfactants. When the reaction time is over 100 s, the foam of MgH 2 : Ecosol 507 = 10 : 1 fills the whole three-necked flask and seriously weakens diffusion of H 2 . 3.5. Analysis of SEM. In order to further explore the influence of surfactants on the hydrogen generation and the hydrogen generation rate, samples are observed and analyzed by SEM. Figure 6 shows the scanning electron microscope pictures of the products of four illustrative experiments. The surface of MgH 2 particles used in the experiments can be observed in Figure 6(a) with a smooth surface and localized layered deposition. Figure 6(b) shows the morphology of MgH 2 after the hydrolysis reaction without surfactants. The smooth surface is pitted by H 2 O molecules. There are two forms of Mg(OH) 2 on the surface of MgH 2 . One forms a dense passiv-ation layer that prevents H 2 O molecules from contacting with MgH 2 , and the other forms a discontinuous layered structure in the form of three-dimensional stacking that provides channels for H 2 O molecules.
Surfactants can improve the morphology of the hydrolysate through interface control. After adding SDBS (MgH 2 : SDBS = 300 : 1), it is observed that the area of the coating layer in Figure 6(c) is significantly smaller than that in Figure 6(b), the relative stratigraphic structure becomes more and more evenly distributed on the hydrogenated, and the number and diameter of the channels have also increased. Therefore, the addition of SDBS increases the hydrogen generation and the hydrogen generation rate of the reaction system compared without surfactants. On one hand, adding TPABr increases the growth sites of Mg(OH) 2 to make the distribution of Mg(OH) 2 more uniform; on the other hand, it makes the directional growth of Mg(OH) 2 , which further changes the morphology. Compared with Figure 6(b), the hydrolyzed product in Figure 6(d), there is no cladding layer; instead, a layered structure is formed by three-dimensional accumulation. Compared with Figure 6(c), the layered structure in Figure 6(d) is more uniform and stable (i.e., the distribution and the diameter of channels is more uniform and larger), and it allows more H 2 O molecules to reach the surface of MgH 2 improving the hydrogen generation rate. At the same time, Mg(OH) 2 crystals formed by the reaction grow in the form of threedimensional accumulation instead of encapsulating MgH 2 , so that H 2 O can react with MgH 2 constantly.  International Journal of Photoenergy generation and the hydrogen generation rate at the initial stage of the hydrolysis (2) With the addition of surfactants, the reaction system is more prone to foam. When these foams gather to form a foam layer, it hinders the escape of hydrogen and reduces the hydrogen generation and the hydrogen generation rate

Conclusion
(3) Surfactants can change the morphology of Mg(OH) 2 . Mg(OH) 2 stake in three-dimensions to form a discontinuous layered structure improving the continuity of the hydrolysis reaction (4) When the surfactant concentration reaches CMC, the self-polymerization of surfactants inhibits the separation of Mg(OH) 2 and affects the dispersion effect (5) On one hand, the negatively charged groups of surfactants may combine with H + in the water, which reduces the concentration of H + affecting the hydrogen generation; on the other hand, the positively charged groups may combine with OHinhibiting the nucleated growth of Mg(OH) 2

Data Availability
The experimental data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare no conflict of interest.