# Study on Hydrolysis of Magnesium Hydride by Interface Control

Authors: Yanyan Chen; Ming Wang; Fenggang Guan; Rujun Yu; Yuying Zhang; Hongyun Qin; Xia Chen; Qiang Fu; Zeyao Wang
Journal: International Journal of Photoenergy
 (2020)

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## Abstract

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.


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## Body

## 1. 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 [1–4]. Among many hydrogen storage materials, the hydrogen content of MgH2 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 [7–19]. The chemical equation of the reaction between MgH2 and H2O is as following:
(1)MgH2+2H2O⟶MgOH2↓+2H2↑⋯ΔH=−277KJmol−1.The magnesium hydroxide is difficult to dissolve in water whose solubility product is5.6×10−12mol3·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 [20–22]. Eliminating the cladding effect of magnesium hydroxide passivation layer on MgH2 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 [24–27]. 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 H2 generation by hydrolysis of MgH2.


## 2. Experimental

### 2.1. Experimental Materials

H2 is generated from the reaction of MgH2 (purity ≥99.5%, MG Power Technology Co., Ltd) and H2O (deionized water). To investigate the effects of surfactants on the generation of H2, different series of surfactants were added into deionized water: sodium dodecyl benzene sulfonate (SDBS, purity ≥90.0%, Shandong Yousuo Chemical Technology Co., Ltd), fatty methyl ester sulfonate (FMES-70, purity ≥70.0%, Shandong Yousuo Chemical Technology Co., Ltd), sodium alcohol ether sulfate (AES, purity ≥70%, Shandong Yousuo Chemical Technology Co., Ltd), tetrapropylammonium bromide (TPABr, purity ≥99.0%, Sinopharm Chemical Reagent Co., Ltd), octadecyl trimethyl ammonium chloride (OTAC, purity ≥99.5%, Shandong Yousuo Chemical Technology Co., Ltd), and Ecosol 507 (purity ≥90.0%, Shandong Yousuo Chemical Technology Co., Ltd).


### 2.2. Experimental Device and Process

The experimental device (Figure1) 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.Figure 1
Hydrolysis unit. ((1) water bath, (2) three-necked flask, (3) thermometer, (4) condenser, (5) gas washing bottle, (6) beaker, and (7) analytical balance).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:(2)VH2=mH2OρH2O


### 2.3. Analytical Method

The surface morphology and dispersion state of magnesium hydride and hydrolyzed products are tested by SEM (Quanta 250, FEI); the surface tension (ST) of aqueous solutions with different surfactants is tested by surface tension meter K100C-MK2.


## 2.1. Experimental Materials

H2 is generated from the reaction of MgH2 (purity ≥99.5%, MG Power Technology Co., Ltd) and H2O (deionized water). To investigate the effects of surfactants on the generation of H2, different series of surfactants were added into deionized water: sodium dodecyl benzene sulfonate (SDBS, purity ≥90.0%, Shandong Yousuo Chemical Technology Co., Ltd), fatty methyl ester sulfonate (FMES-70, purity ≥70.0%, Shandong Yousuo Chemical Technology Co., Ltd), sodium alcohol ether sulfate (AES, purity ≥70%, Shandong Yousuo Chemical Technology Co., Ltd), tetrapropylammonium bromide (TPABr, purity ≥99.0%, Sinopharm Chemical Reagent Co., Ltd), octadecyl trimethyl ammonium chloride (OTAC, purity ≥99.5%, Shandong Yousuo Chemical Technology Co., Ltd), and Ecosol 507 (purity ≥90.0%, Shandong Yousuo Chemical Technology Co., Ltd).


## 2.2. Experimental Device and Process

The experimental device (Figure1) 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.Figure 1
Hydrolysis unit. ((1) water bath, (2) three-necked flask, (3) thermometer, (4) condenser, (5) gas washing bottle, (6) beaker, and (7) analytical balance).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:(2)VH2=mH2OρH2O


## 2.3. Analytical Method

The surface morphology and dispersion state of magnesium hydride and hydrolyzed products are tested by SEM (Quanta 250, FEI); the surface tension (ST) of aqueous solutions with different surfactants is tested by surface tension meter K100C-MK2.


## 3. 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.Under the same condition, the volume of the water, the temperature of the water bath, and the mass of MgH2, 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 Figure2 that the hydrogen generation rate are approximately the same at 110 s when MgH2 : SDBS = 300 : 1, MgH2 : SDBS = 50 : 1, and MgH2 : SDBS = 10 : 1. While the hydrogen generation rate is slightly lower when MgH2 : SDBS = 100 : 1 at 110 s. When the hydrolysis time is 120 s, the hydrogen generation rate of MgH2 : SDBS = 300 : 1 is greater than the other. When the hydrolysis time is 155 s, the hydrogen generation rate of MgH2 : SDBS = 100 : 1 is higher than MgH2 : SDBS = 10 : 1 and MgH2 : SDBS = 50 : 1. All the hydrogen generation rate reduces gradually over 200 s. The final hydrogen yield are ranked as follows: MgH2:SDBS=300:1>MgH2:SDBS=100:1>MgH2:SDBS=50:1>MgH2:SDBS=10:1=MgH2.Figure 2
The effect of different SDBS dosages on hydrogen generation.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 Table1). However, the lower the surface tension, the easier bubbles will foam. These bubbles gather into a foam layer, which binds H2. It is difficult for H2 escape from the reaction system (e.g., the curve of MgH2 : 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 MgH2 : SDBS = 10 : 1. Self-polymerization process of SDBS molecules possibly employed, which inhibits the detachment of Mg(OH)2 from the surface of MgH2 and the dispersion of Mg(OH)2 in the water.Table 1
Surface tension values of aqueous solutions with different SDBS dosages.
Ratio∗ST (mN/m)Water71.57MgH2 : SDBS = 300 : 170.51MgH2 : SDBS = 100 : 159.00MgH2 : SDBS = 50 : 147.80MgH2 : SDBS = 10 : 138.48∗: the ratio represents dosages of surfactants in experiments without MgH2.From Figure3, the hydrogen generation of two experiments (MgH2 : FMES = 10 : 1, MgH2 : AES = 10 : 1) are 100% and 153.8% at the hydrolysis time of 50 s, which is higher than that without surfactant. When the hydrolysis time is over 75 s, the generation rate of four experiments (MgH2 : FMES-70 = 300 : 1, MgH2 : FMES-70 = 10 : 1, MgH2 : AES = 300 : 1, and MgH2 : AES = 10 : 1) are lower than that without surfactants.Figure 3
The effect of different FMES-70, AES dosages on hydrogen generation.It is observed from Table2 that surface tension of solutions becomes lower as the ratio of MgH2 to surfactants increases. On one hand, the system foams more easily [28, 29], and the foam layer weakens the diffusion of H2. On the other hand, the hydrolysis reaction is essentially a reaction with H+, while the negatively charged groups of FMES-70 and AES may combine with H+ decreasing the hydrogen generation.Table 2
Surface tension values of aqueous solutions with different FMES-70, AES dosages.
RatioST (mN/m)Water71.57MgH2 : FMES-70 = 300 : 145.25MgH2 : FMES-70 = 10 : 139.74MgH2 : AES = 300 : 130.05MgH2 : AES = 10 : 137.86


### 3.2. Effect of Cationic Surfactant

Cationic surfactants are mainly nitrogen-containing organic amine derivatives, composed of a long-chain hydrophobic group and a positive charged hydrophilic group. With good emulsification, wetting effect, and other properties, it is easier to absorb on the solid surface and improves the solid-liquid interface effect.It is shown in Figure4 that the hydrogen generation of MgH2 : OTAC = 5 : 1 and MgH2 : 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 MgH2 : OTAC = 40 : 1 reaches the maximum which is 11.0% higher than that without surfactants.Figure 4
The effect of different OTAC dosages on hydrogen generation.Within 120 s of the hydrolysis, the hydrogen generation and the hydrogen generation rate improve as the ratio of MgH2 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 MgH2 : OTAC = 100 : 1 decrease to 36.87 mN/m. When MgH2 : 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.Table 3
Surface tension values of aqueous solutions with different OTAC dosages.
RatioST (mN/m)Water71.57MgH2 : OTAC = 100 : 136.87MgH2 : OTAC = 40 : 135.79MgH2 : OTAC = 10 : 134.24MgH2 : OTAC = 5 : 132.99It is also possible that the positively charged groups of the cationic surfactants may combine with OH- inhibiting the combination of OH- and Mg2+ and delay the formation of the passivation layer in the surface of MgH2. Zheng et al. [30] studied the hydrolysis of Mg in different solutions (MgCl2, MnCl2, NiCl2, AlCl3, NH4Cl, and HCl). They found that there was a significant improvement in NH4Cl and HCl solution, the conversion efficiency increased by more than 60%. Researchers believe that the higher the affinity between the cations and OH- in 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].Figure5 reveals the effect of TPABr on hydrogen generation. When the time is 100 s, the hydrogen generation of MgH2 : TPABr = 5 : 1 and MgH2 : TPABr = 100 : 1 increases by 52% and 29% compared with that without surfactants, respectively. At the time of 300 s, the hydrogen generation of MgH2 :TPABr = 5 : 1 and MgH2 : TPABr = 100 : 1 increases by 28.3% and 3%.Figure 5
The effect of different TPABr dosages on hydrogen generation.According to the previous results in this work, the surface tension of solution decreases with the increase of the ratio of MgH2 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 MgH2 to TPABr. And the hydrogen generation has the same trend, the final hydrogen yield are ranked as follows: MgH2:TPABr=5:1>MgH2:TPABr=1:1>MgH2:TPABr=10:1>MgH2:TPABr=100:1>MgH2.Table 4
Surface tension values of aqueous solutions with different TPABr dosages.
RatioST (mN/m)Water71.57MgH2 : TPABr = 100 : 161.54MgH2 : TPABr = 10 : 165.39MgH2 : TPABr = 5 : 168.67MgH2 : TPABr = 1 : 169.23Figure6(d) illustrates that Mg(OH)2 grows evenly around the surface of MgH2, like a blooming flower. The “petals”-Mg(OH)2 form uniform channels instead of coating MgH2 in the form of flakes. Because of the better passability, H2O molecules can contact MgH2 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 MgH2 forming narrow and long channels. As a result, it is difficult for H2O molecules to reach the surface of MgH2 through the channels.When the surface tension decreases to 61.54 mN/m, there is no obvious foam layer. Thus, H2 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 [33–35].Figure 6
(a) Magnesium hydride. (b) Hydrolyzed product without surfactants. (c) MgH2 : SDBS = 300 : 1, hydrolyzed product. (d) MgH2 : TPABr = 1 : 1, hydrolyzed product (×20000).
(a)(b)(c)(d)


### 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 Figure7, when the time is 100 s, the hydrogen generation of MgH2 : Ecosol 507 = 100 : 1 is 50% higher than that without surfactants. When the time is 300 s, the hydrogen generation of MgH2 : Ecosol 507 = 10 : 1 is 40% lower than that without surfactants, but the hydrogen generation of MgH2 : Ecosol 507 = 100 : 1 is 11.7% higher than it.Figure 7
The effect of different Ecosol 507 dosages on hydrogen generation.It can be seen from Table5 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 MgH2 : Ecosol 507 = 10 : 1 fills the whole three-necked flask and seriously weakens diffusion of H2.Table 5
Surface tension values of aqueous solutions with different Ecosol 507 dosages.
RatioST (mN/m)Water71.57MgH2 : Ecosol 507 = 100 : 128.38MgH2 : Ecosol 507 = 10 : 120.49


### 3.4. Comparison of Maximum Hydrogen Generation

Sort by maximum hydrogen generation (Figure8): TPABr>SDBS>Ecosol507>OTAC>nosurfactants>AES≈FMES−70.Figure 8
Comparison of hydrogen generation curves of MgH2.


### 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.Figure6 shows the scanning electron microscope pictures of the products of four illustrative experiments. The surface of MgH2 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 MgH2 after the hydrolysis reaction without surfactants. The smooth surface is pitted by H2O molecules. There are two forms of Mg(OH)2 on the surface of MgH2. One forms a dense passivation layer that prevents H2O molecules from contacting with MgH2, and the other forms a discontinuous layered structure in the form of three-dimensional stacking that provides channels for H2O molecules.Surfactants can improve the morphology of the hydrolysate through interface control. After adding SDBS (MgH2 : 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 H2O molecules to reach the surface of MgH2 improving the hydrogen generation rate. At the same time, Mg(OH)2 crystals formed by the reaction grow in the form of three-dimensional accumulation instead of encapsulating MgH2, so that H2O can react with MgH2 constantly.


## 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.Under the same condition, the volume of the water, the temperature of the water bath, and the mass of MgH2, 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 Figure2 that the hydrogen generation rate are approximately the same at 110 s when MgH2 : SDBS = 300 : 1, MgH2 : SDBS = 50 : 1, and MgH2 : SDBS = 10 : 1. While the hydrogen generation rate is slightly lower when MgH2 : SDBS = 100 : 1 at 110 s. When the hydrolysis time is 120 s, the hydrogen generation rate of MgH2 : SDBS = 300 : 1 is greater than the other. When the hydrolysis time is 155 s, the hydrogen generation rate of MgH2 : SDBS = 100 : 1 is higher than MgH2 : SDBS = 10 : 1 and MgH2 : SDBS = 50 : 1. All the hydrogen generation rate reduces gradually over 200 s. The final hydrogen yield are ranked as follows: MgH2:SDBS=300:1>MgH2:SDBS=100:1>MgH2:SDBS=50:1>MgH2:SDBS=10:1=MgH2.Figure 2
The effect of different SDBS dosages on hydrogen generation.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 Table1). However, the lower the surface tension, the easier bubbles will foam. These bubbles gather into a foam layer, which binds H2. It is difficult for H2 escape from the reaction system (e.g., the curve of MgH2 : 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 MgH2 : SDBS = 10 : 1. Self-polymerization process of SDBS molecules possibly employed, which inhibits the detachment of Mg(OH)2 from the surface of MgH2 and the dispersion of Mg(OH)2 in the water.Table 1
Surface tension values of aqueous solutions with different SDBS dosages.
Ratio∗ST (mN/m)Water71.57MgH2 : SDBS = 300 : 170.51MgH2 : SDBS = 100 : 159.00MgH2 : SDBS = 50 : 147.80MgH2 : SDBS = 10 : 138.48∗: the ratio represents dosages of surfactants in experiments without MgH2.From Figure3, the hydrogen generation of two experiments (MgH2 : FMES = 10 : 1, MgH2 : AES = 10 : 1) are 100% and 153.8% at the hydrolysis time of 50 s, which is higher than that without surfactant. When the hydrolysis time is over 75 s, the generation rate of four experiments (MgH2 : FMES-70 = 300 : 1, MgH2 : FMES-70 = 10 : 1, MgH2 : AES = 300 : 1, and MgH2 : AES = 10 : 1) are lower than that without surfactants.Figure 3
The effect of different FMES-70, AES dosages on hydrogen generation.It is observed from Table2 that surface tension of solutions becomes lower as the ratio of MgH2 to surfactants increases. On one hand, the system foams more easily [28, 29], and the foam layer weakens the diffusion of H2. On the other hand, the hydrolysis reaction is essentially a reaction with H+, while the negatively charged groups of FMES-70 and AES may combine with H+ decreasing the hydrogen generation.Table 2
Surface tension values of aqueous solutions with different FMES-70, AES dosages.
RatioST (mN/m)Water71.57MgH2 : FMES-70 = 300 : 145.25MgH2 : FMES-70 = 10 : 139.74MgH2 : AES = 300 : 130.05MgH2 : AES = 10 : 137.86


## 3.2. Effect of Cationic Surfactant

Cationic surfactants are mainly nitrogen-containing organic amine derivatives, composed of a long-chain hydrophobic group and a positive charged hydrophilic group. With good emulsification, wetting effect, and other properties, it is easier to absorb on the solid surface and improves the solid-liquid interface effect.It is shown in Figure4 that the hydrogen generation of MgH2 : OTAC = 5 : 1 and MgH2 : 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 MgH2 : OTAC = 40 : 1 reaches the maximum which is 11.0% higher than that without surfactants.Figure 4
The effect of different OTAC dosages on hydrogen generation.Within 120 s of the hydrolysis, the hydrogen generation and the hydrogen generation rate improve as the ratio of MgH2 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 MgH2 : OTAC = 100 : 1 decrease to 36.87 mN/m. When MgH2 : 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.Table 3
Surface tension values of aqueous solutions with different OTAC dosages.
RatioST (mN/m)Water71.57MgH2 : OTAC = 100 : 136.87MgH2 : OTAC = 40 : 135.79MgH2 : OTAC = 10 : 134.24MgH2 : OTAC = 5 : 132.99It is also possible that the positively charged groups of the cationic surfactants may combine with OH- inhibiting the combination of OH- and Mg2+ and delay the formation of the passivation layer in the surface of MgH2. Zheng et al. [30] studied the hydrolysis of Mg in different solutions (MgCl2, MnCl2, NiCl2, AlCl3, NH4Cl, and HCl). They found that there was a significant improvement in NH4Cl and HCl solution, the conversion efficiency increased by more than 60%. Researchers believe that the higher the affinity between the cations and OH- in 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].Figure5 reveals the effect of TPABr on hydrogen generation. When the time is 100 s, the hydrogen generation of MgH2 : TPABr = 5 : 1 and MgH2 : TPABr = 100 : 1 increases by 52% and 29% compared with that without surfactants, respectively. At the time of 300 s, the hydrogen generation of MgH2 :TPABr = 5 : 1 and MgH2 : TPABr = 100 : 1 increases by 28.3% and 3%.Figure 5
The effect of different TPABr dosages on hydrogen generation.According to the previous results in this work, the surface tension of solution decreases with the increase of the ratio of MgH2 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 MgH2 to TPABr. And the hydrogen generation has the same trend, the final hydrogen yield are ranked as follows: MgH2:TPABr=5:1>MgH2:TPABr=1:1>MgH2:TPABr=10:1>MgH2:TPABr=100:1>MgH2.Table 4
Surface tension values of aqueous solutions with different TPABr dosages.
RatioST (mN/m)Water71.57MgH2 : TPABr = 100 : 161.54MgH2 : TPABr = 10 : 165.39MgH2 : TPABr = 5 : 168.67MgH2 : TPABr = 1 : 169.23Figure6(d) illustrates that Mg(OH)2 grows evenly around the surface of MgH2, like a blooming flower. The “petals”-Mg(OH)2 form uniform channels instead of coating MgH2 in the form of flakes. Because of the better passability, H2O molecules can contact MgH2 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 MgH2 forming narrow and long channels. As a result, it is difficult for H2O molecules to reach the surface of MgH2 through the channels.When the surface tension decreases to 61.54 mN/m, there is no obvious foam layer. Thus, H2 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 [33–35].Figure 6
(a) Magnesium hydride. (b) Hydrolyzed product without surfactants. (c) MgH2 : SDBS = 300 : 1, hydrolyzed product. (d) MgH2 : TPABr = 1 : 1, hydrolyzed product (×20000).
(a)(b)(c)(d)


## 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 Figure7, when the time is 100 s, the hydrogen generation of MgH2 : Ecosol 507 = 100 : 1 is 50% higher than that without surfactants. When the time is 300 s, the hydrogen generation of MgH2 : Ecosol 507 = 10 : 1 is 40% lower than that without surfactants, but the hydrogen generation of MgH2 : Ecosol 507 = 100 : 1 is 11.7% higher than it.Figure 7
The effect of different Ecosol 507 dosages on hydrogen generation.It can be seen from Table5 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 MgH2 : Ecosol 507 = 10 : 1 fills the whole three-necked flask and seriously weakens diffusion of H2.Table 5
Surface tension values of aqueous solutions with different Ecosol 507 dosages.
RatioST (mN/m)Water71.57MgH2 : Ecosol 507 = 100 : 128.38MgH2 : Ecosol 507 = 10 : 120.49


## 3.4. Comparison of Maximum Hydrogen Generation

Sort by maximum hydrogen generation (Figure8): TPABr>SDBS>Ecosol507>OTAC>nosurfactants>AES≈FMES−70.Figure 8
Comparison of hydrogen generation curves of MgH2.


## 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.Figure6 shows the scanning electron microscope pictures of the products of four illustrative experiments. The surface of MgH2 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 MgH2 after the hydrolysis reaction without surfactants. The smooth surface is pitted by H2O molecules. There are two forms of Mg(OH)2 on the surface of MgH2. One forms a dense passivation layer that prevents H2O molecules from contacting with MgH2, and the other forms a discontinuous layered structure in the form of three-dimensional stacking that provides channels for H2O molecules.Surfactants can improve the morphology of the hydrolysate through interface control. After adding SDBS (MgH2 : 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 H2O molecules to reach the surface of MgH2 improving the hydrogen generation rate. At the same time, Mg(OH)2 crystals formed by the reaction grow in the form of three-dimensional accumulation instead of encapsulating MgH2, so that H2O can react with MgH2 constantly.


## 4. Conclusion

(1)
The addition of surfactants reduces the surface tension of the liquid, improves the wetting effect of the liquid on the MgH2, and increases the hydrogen 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(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 OH- inhibiting the nucleated growth of Mg(OH)2



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*Source: 8859770-2020-12-17.xml*