Enhanced Hydrogen Generation Properties of MgH₂-Based Hydrides by Breaking the Magnesium Hydroxide Passivation Layer

Abstract

Due to its relatively low cost, high hydrogen yield, and environmentally friendly hydrolysis byproducts, magnesium hydride (MgH₂) appears to be an attractive candidate for hydrogen generation. However, the hydrolysis reaction of MgH₂ is rapidly inhibited by the formation of a magnesium hydroxide passivation layer. To improve the hydrolysis properties of MgH₂-based hydrides we investigated three different approaches: ball milling, synthesis of MgH₂-based composites, and tuning of the solution composition. We demonstrate that the formation of a composite system, such as the MgH₂/LaH₃ composite, through ball milling and in situ synthesis, can improve the hydrolysis properties of MgH₂ in pure water. Furthermore, the addition of Ni to the MgH₂/LaH₃ composite resulted in the synthesis of LaH₃/MgH₂/Ni composites. The LaH₃/MgH₂/Ni composites exhibited a higher hydrolysis rate—120 mL/(g·min) of H₂ in the first 5 min—than the MgH₂/LaH₃ composite—95 mL/(g·min)—without the formation of the magnesium hydroxide passivation layer. Moreover, the yield rate was controlled by manipulation of the particle size via ball milling. The hydrolysis of MgH₂ was also improved by optimizing the solution. The MgH₂ produced 1711.2 mL/g of H₂ in 10 min at 298 K in the 27.1% ammonium chloride solution, and the hydrolytic conversion rate reached the value of 99.5%.

1. Introduction

The global energy crisis and our current ecological problems have stimulated the development of new clean energies [1,2,3,4]. Hydrogen has a high energy density of 142 MJ/kg, three times higher than that of petroleum, with 47 MJ/kg [5,6]. Upon combustion, the byproduct is water, which can subsequently be used to regenerate hydrogen [7,8,9,10]. Therefore, hydrogen is regarded as a potential candidate for clean and sustainable fuel to replace fossil fuels [11,12,13]. Currently, hydrogen is primarily produced through the processing of fossil fuels, biomass and water. The main methods include fossil fuel reforming [14], biological hydrogen production [15], generating hydrogen from water decomposition [16] and hydrogen production by metal or hydride hydrolysis [17]. Fuel processing of methane is the most common hydrogen production method in commercial use today [18,19]. Biohydrogen technologies include direct biophotolysis [20,21,22], indirect biophotolysis [23], photo-fermentation [24,25], and dark-fermentation [26,27,28]. Electrolysis technology is an effective method for transforming electrical energy into chemical energy and can be traced back to 1890 [29,30,31]. Photocatalytic decomposition of water was first used by Fujishima and Honda [32] and the catalytic efficiency reached 93% under the UV irradiation [33,34].

In recent years, more and more attention has been paid to hydrogen generation by hydrolysis of metals or metal hydrides for their high theoretical hydrogen yield. The main hydrogen generation metals include Al and Mg alloys, due to their light weight, abundance, and low cost and the byproduct being benign. The self-corrosion of hydrogen evolution reactions of aluminum has led to many studies focusing on hydrogen generation with aluminum and its alloys. However, aluminum has a strong affinity for oxygen, in the form of a dense oxide layer formed on the surface of aluminum and its alloys, making the corrosion potential shift nearly 1 V in the positive direction, thereby interrupting the corrosion reaction of aluminum [35]. This becomes the major challenge to realizing continuous hydrogen generation through aluminum corrosion. How to remove or prevent the formation of a dense aluminum oxide layer is the key issue in the hydrogen generation process by hydrolysis of aluminum alloys. Many methods have been reported to improve the hydrolysis properties of aluminum as, for example, changing the reaction solution (alkali solution, NaAlO₂ solution, Na₂SnO₃ solution, etc.) [36,37,38] and improving the reaction activity of aluminum alloys by ball milling and/or some special metal doping [39,40,41]. However, the corrosion of equipment in alkali or salt solution and the risk of corrosion of materials restrict its commercial utilization. Additional research is still needed to further improve the hydrogen generation performance with Al alloys. The hydrolysis reaction of Mg is rapidly interrupted because of the formation of a passive magnesium hydroxide layer.

Compared with pure metals or alloys, the metal hydrides can generate double the amount of hydrogen by hydrolysis, and the generated high purity hydrogen can be passed directly into the cells as fuel [42]. In addition, the fuel cell-generated water could be recycled and used for hydrolysis, thus achieving reduced weight of the system. Due to the hydrogen generation by hydrolysis having the excellent properties of widely raw material sources, high hydrogen generation yield, high hydrogen purity, and environmental friendliness, a large amount of metal hydrides have been studied as the mobile hydrogen source of fuel cell, such as LiH [43], CaH₂ [44], MgH₂ [45,46,47,48], NaBH₄ [49,50,51], LiBH₄ [52,53], LiAlH₄ [42,52]. Among them, LiH, CaH₂, and LiAlH₄ can react with water violently, so the uncontrollable reaction leads to poor practicability. Among these methods, production of hydrogen using NaBH₄ is safety and controllability, but the regeneration of NaBH₄ needs a great deal of energy leading to a higher cost. On the other hand, the efficiency and service life of the catalyst and the price of the whole system restrict the promotion and application of NaBH₄ hydrogen production technology. Therefore, looking for a cheap and practical technology to generate hydrogen always is a fervent focus of research all over the world. Comparing the hydrolysis of MgH₂ with that of NaBH₄, it is not necessary for a highly basic solution to stabilize the NaBH_4, whereas MgH₂ can generate hydrogen in the absence of a catalyst at room temperature. Furthermore, as the existence of Mg(OH)₂, the hydrolysis product could be easily recycled. In comparison to ammonia borane (AB), there is no requirement for a catalyst to improve the hydrolysis rate, only an adjustment to the particle size or solution, which influence hydrolysis rate and hydrogen yield.

MgH₂-based materials have higher theoretical hydrogen content (15.2%) and Mg element is abundant in the earth’s crust (2.4%). They are low cost, have a high hydrogen yield, and are gentle on the environment, making them a potential candidate for high-quality hydrogen generation material. The progress of reaction can be concluding as in Equation (1):

$${\text{MgH}}_2+2{\text{H}}_2\text{O}=\text{Mg}{\left(\text{OH}\right)}_2+2{\text{H}}_2\mathrm{\Delta }{\text{H}}^{\text{θ}}\left(298\text{K}\right)=-134.325\text{ KJ}/\text{mol}\cdot {\text{H}}_2$$

(1)

The hydrolysis production of MgH₂-based hydride (Mg(OH)₂) is harmless to the environment and easily reused and recycled [54]. Fortunately, the hydrolysis reaction of MgH₂ occurred immediately when it had contact with water at room temperature. However, the hydrolysis reaction of MgH₂ is rapidly interrupted because of the formation of a passive magnesium hydroxide layer, which prevents the diffusion of the water to the particle inside, so that the hydrolysis reaction after the early high-speed reaction stage is quickly stagnated.

In order to improve the hydrolysis efficiency and the reaction rate, the main methods such as ball milling, alloying, changing hydrolysis solution composition, catalyst introduction, etc. were adopted. Research has shown that although adding acid to form Mg²⁺ ion is an effective method, it may pollute the environment, introduce an additional hazard to the equipment, and lower overall safety [55,56]. The addition of CaH₂ appears to be very promising, but this reagent is too reactive and needs an extended milling time of 10 h that is not propitious for industrial production in terms of feasibility and safety [57]. In addition, hydrolyses of high-energy ball-milled MgH₂ composites in aqueous NaCl solution [58,59,60], aqueous KCl solution [61], or in different alcoholic solutions [62] have been used to produce hydrogen, but the state-of-the-art hydrolysis performance is still far from commercial utilization. Even when Ni was added to MgH₂ in KCl solution, the hydrolysis reaction did not show any significant reactivity improvement [61]. Recently, ultrasonication has been shown to enhance hydrogen generation during the hydrolysis process of magnesium hydride, but it has the disadvantages of requiring extra ultrasonic equipment and the associated extra energy consumption [63]. Therefore, the key issue relating to MgH₂ hydrolysis remains the circumvention of the barrier of oxide/hydroxide layer formation during the hydrolysis process so as to realize complete hydrolysis [47,64]. In this paper, we review our recent research results about hydrolysis of MgH₂-based hydrides and confirm the effect of the ball-milling and catalyst-introducing methods to break the magnesium hydroxide passivation layer to enhance the hydrolysis rate and hydrogen generation yield. The hydrolysis rate and yield in the pure water of in situ synthetic MgH₂-based composites could be further adjusted by controlling the particle size via ball milling. The ammonium chloride solution can destroy the Mg(OH)₂ passivation layer and accelerate the hydrolysis process of MgH₂, which can produce 1711.2 mL/g hydrogen in 10 min at 298 K with a hydrolytic conversion rate of 99.5% in the 27.1% ammonium chloride solution. Moreover, the optimization of the MgCl₂ aqueous solution was also investigated. It not only eliminates the introduction of impure gas and byproduct, but also simplifies the regeneration process. Such a result opens a promising route for improving hydrolysis properties for commercial hydrogen production.

2. Experimental

2.1. Sample Preparation

The reagent of MgH₂ was purchased from Sigma-Aldrich Inc. The rare earth ingot was broken into small particles and filtered through a 400-mesh sieve, then hydrogenated at 573 K for 4 h with the hydrogen pressure of 4 MPa by using an AMC gas reaction controller (Advance Materials Corporation, Pittsburgh, PA, USA) to prepare REH₃ hydride. The MgH₂ was ball milled under hydrogen atmosphere for 3 h using QM-3SP4 planetary ball mill (Nanjing NanDa Instrument Plant, Nanjing, China) with a ball-to-powder mass ratio of 20:1 at rotational speed of 500 rpm. The MgH₂/REH₃ composite was obtained by ball milling LaH₃ and MgH₂ with different atomic ratios of 3:1, 5:1, 8.5:1 under hydrogen atmosphere for 4 h. The ball milling was performed on a QM-3SP4 planetary ball mill rotating at 500 rpm with the ball-to-powder mass ratio of 20:1. To prevent samples and raw materials from oxidation and/or hydroxide formation, all samples were stored and handled in an Ar filled glove box. The solution of MgCl₂ and NH₄Cl was prepared to be used as hydrolysis solution: 2.38 g MgCl₂ was dissolved into 500 mL deionized water to form a solution of 0.5 mol/L MgCl₂. 4.7 g, 9.3 g, 18.6 g and 37.2 g NH₄Cl were dissolved into 100 mL of deionized water to form solutions with the mass ratio of 4.5%, 8.5%, 15.7%, and 27.1%, respectively.

2.2. Hydrolysis Experiment

The hydrolysis reactions of the MgH₂ or MgH₂/LaH₃ composite were carried out in a 250 mL Pyrex glass reactor with three openings, one for water addition, one for hydrogen exhausting and one for inserting the thermometer. All the experiments were carried out at room temperature (298 K), without external heating. Figure 1 shows the schematic hydrolysis reaction equipment which was used to quantify the hydrogen production rate and yield. Samples of 0.1–0.2 g were added into the Pyrex glass, before 20 mL water or MgCl₂ solution were added to react with the alloys. Hydrogen production reaction started when the alloys contacted with pure water or solution. The generated hydrogen was exhausted through a Tygon tube, then passed through a Monteggia washing bottle filled with water at room temperature in order to condense the water vapor, and collected hydrogen by extracting water in a beaker which was put on an electronic scales to recorded the weight changes over time, in order to measure the quantity of hydrogen. The hydrogen generation rate and yield can be calculated from the reaction time and hydrogen volume, which was measured and analyzed by the computer. Each experiment test was repeated at least two times in order to confirm its reproducibility.

Figure 1. The schematic of equipment used to quantify hydrogen production rates and yields: (1) iron support; (2) Pyrex glass reactor; (3) thermometer; (4) injector; (5) double gum plug; (6) piston joint; (7) Monteggia washing bottle filled with water at room temperature; (8) beaker to collect the extracted water; (9) electronic scales; (10) computer.

Figure 1. The schematic of equipment used to quantify hydrogen production rates and yields: (1) iron support; (2) Pyrex glass reactor; (3) thermometer; (4) injector; (5) double gum plug; (6) piston joint; (7) Monteggia washing bottle filled with water at room temperature; (8) beaker to collect the extracted water; (9) electronic scales; (10) computer.

2.3. Sample Characterization

The morphological characteristics of fabricated structures were studied by field emission scanning electron microscopy (FE-SEM, JEOL XL-30) (JEOL, Tokyo, Japan) using secondary electrons with the acceleration voltage of 10 kV, and their structural and chemical characteristics were investigated using transmission electron microscopy (TEM, JEOL-2100 (JEOL, Tokyo, Japan) equipped with an energy dispersive spectroscopy (EDS) system with an operating voltage of 200 kV).

3. Results and Discussion

3.1. Enhancement of the Hydrolysis Properties of MgH₂-Based Hydrides by Formation of a Composite Structure

The hydrolysis reaction of MgH₂ is interrupted by the formation of a magnesium hydroxide passivation layer. To improve the hydrolysis performance of MgH₂-based materials, a composite structure of MgH₂ with a different hydride was synthesized through ball milling. By adjusting the atomic ratio, MgH₂/LaH₃ composites were designed to adjust the hydrolysis performance of the MgH₂-based composite. Figure 2 shows the hydrogen production curves by hydrolysis of the MgH₂/LaH₃ composites with the different atomic ratios of 3:1, 5:1, and 8.5:1 obtained after milling for 4 h. As shown in Figure 2, the overall hydrolysis rate of the composites improved; the hydrolysis rate was the same for all the composites during the first 5 min, and then increased with the LaH₃ content. The hydrolysis of LaH₃ can produce conductive ions on the surface of the composite, which is beneficial for the work of the micro galvanic cells and results in a synergetic effect leading to a higher hydrolysis rate [65]. The MgH₂/LaH₃ composites with the MgH₂/LaH₃ atomic ratio of 3:1 and 8.5:1 can generate 706.7 mL/g of H₂ in 40 min and 473.0 mL/g of H₂ in 60 min, respectively. The highest observed hydrolysis yield is equal to 1195 mL/g of H₂ in 80 min and belongs to the MgH₂/LaH₃ composite with the MgH₂/LaH₃ atomic ratio of 5:1. Notably, by decreasing the LaH₃ content, the hydrolysis synergetic effect was enhanced and consequently the hydrogen yield and rate increased. However, with the increase of the LaH₃ content, the theoretical hydrogen generation yield of the composites decreased because of the relatively low hydrogen yield of LaH₃.

Figure 2. The hydrogen generation curves of ball-milled MgH₂ and LaH₃ with different atomic ratio.

Figure 2. The hydrogen generation curves of ball-milled MgH₂ and LaH₃ with different atomic ratio.

The ball-milling method used to synthesize the composite structure of MgH₂ with other hydrides is relatively expensive, as LaH₃ and MgH₂ are obtained upon hydrogenation of Mg and La at relative high temperature. To decrease the costs of the MgH₂-based composites, an Mg-based alloy hydrogenated at room temperature with low pressure was designed. The Mg₃La and Mg₃LaNi_0.1 alloys prepared by induction melting with pure Mg and La could be hydrogenated in situ at room temperature and 3.8 MPa of hydrogen pressure to form the LaH₃/MgH₂ (hereafter referred to as H-Mg₃La) and LaH₃/MgH₂/Ni (hereafter referred to as H-Mg₃LaNi_0.1) composites [66]. To further study the influence of the light rare earth (RE) elements on the hydrolysis performance of the composite formed in situ, hydrogenated Mg₃Mm (H-Mg₃Mm, Mm denotes the mischmetal), Mg₃La (H-Mg₃La), Mg₃Ce (H-Mg₃Ce), Mg₃Pr (H-Mg₃Pr), and Mg₃Nd (H-Mg₃Nd) composites were also prepared by hydrogenating the Mg₃Mm, Mg₃La, Mg₃Ce, Mg₃Pr, and Mg₃Nd (H-Mg₃Nd) alloys at 298 K [67]. The hydrolysis performances of the H-Mg₃Mm, H-Mg₃La, H-Mg₃Ce, H-Mg₃Pr, and H-Mg₃Nd composites were investigated at 298 K, as shown in Figure 3. The H-Mg₃Mm, H-Mg₃La, H-Mg₃Ce, H-Mg₃Pr, and H-Mg₃Nd composites generated 1097 mL/g, 949 mL/g, 1025 mL/g, 905 mL/g, and 657 mL/g of hydrogen by hydrolysis, with total hydrogen yields of 9.79 wt.%, 8.47 wt.%, 9.15 wt.%, 8.08 wt.%, and 5.86 wt.%, respectively.

Notably, H-Mg₃Mm exhibited the best hydrolysis performance (Figure 3, magenta line), showing the fastest hydrolysis rate and producing 695 mL/g of H₂ in 5 min, 784 mL/g of H₂ in 10 min, 828 mL/g of H₂ in 15 min, and 1097 mL/g of H₂ in 36 h. H-Mg₃La generated 463 mL/g of H₂ in 5 min and 653 mL/g of H₂ in 10 min (Figure 3, black line). H-Mg₃Ce produced 542 mL/g of H₂ in 5 min and 705 mL/g of H₂ in 10 min (Figure 3, red line). The blue line in Figure 3 indicates the hydrolysis of the H-Mg₃Pr sample; the conversion yields were equal to 545 mL/g of H₂ in 5 min and 699 mL/g of H₂ in 10 min. For the hydrolysis of the H-Mg₃Nd sample (Figure 3, green line), the hydrolysis yield was relatively low and the sample only generated 434 mL/g of H₂ in 5 min and 581 mL/g of H₂ in 10 min. All the H-Mg₃RE samples exhibited a faster hydrolysis rate than Mg or MgH₂, indicating that the rare-earth hydrides may significantly improve the hydrolysis performance of MgH₂. For commercial use, pure rare-earth metal La was replaced by the mischmetal to reduce the costs. An inexpensive and efficient hydrolysis material, namely, as-hydrogenated Mg₃Mm (abbreviated as H-Mg₃Mm, where Mm denotes the La-rich and Ce-rich mischmetal), was identified as an excellent hydrolysis material. Pure REH₃ can fully react with water. Conversely, the hydrolysis of MgH₂ can be rapidly interrupted by the formation of a magnesium hydroxide passivation layer onto the reactive materials. During the H-Mg₃RE hydrolysis, REH₃ could continually hydrolyze and produce a reaction tunnel for the further hydrolysis of MgH₂, leading to the break of the magnesium hydroxide passivation layer. Thus, REH₃ can accelerate the hydrolysis rate of MgH₂ and lead to the completion of the hydrolysis process.

Figure 3. The hydrogen evolution curves for the hydrolysis of (a) H-Mg₃Mm; (b) H-Mg₃La; (c) H-Mg₃Ce; (d) H-Mg₃Pr and (e) H-Mg₃Nd alloys at 298 K [67]. Reproduced with permission from [67], copyright 2013 Elsevier.

Figure 3. The hydrogen evolution curves for the hydrolysis of (a) H-Mg₃Mm; (b) H-Mg₃La; (c) H-Mg₃Ce; (d) H-Mg₃Pr and (e) H-Mg₃Nd alloys at 298 K [67]. Reproduced with permission from [67], copyright 2013 Elsevier.

Comparing the hydrogen evolution curves for the hydrolysis of the LaH₃/MgH₂ and LaH₃/MgH₂/Ni composites at room temperature (298 K), the hydrogen generation rate of LaH₃/MgH₂/Ni is higher than that of LaH₃/MgH₂ during the first 3 min [66]. The addition of Ni plays an active role in enhancing the reaction rate. However, to study the hydrolysis performance of in situ formed MgH₂/LaH₃ composites affected by the different atomic ratio of MgH₂ to LaH₃, MgH₂/LaH₃ composites hydrogenated from Mg₃La (MgH₂:LaH₃ = 3:1, hereafter referred to as H-Mg₃La) and La₂Mg₁₇ (MgH₂:LaH₃ = 17:2, hereafter referred to as H-La₂Mg₁₇) were synthesized [47]. The results show that the hydrolysis rate of H-Mg₃La was higher than that of H-La₂Mg₁₇ (918.4 mL/g and 851.7 mL/g for H-Mg₃La and H-La₂Mg₁₇, respectively, in 21 min), while the hydrogen yield decreased from 1224.5 mL/g to 983.7 mL/g in 4.4 h for H-La₂Mg₁₇ and H-Mg₃La, respectively. By increasing the content of LaH₃ from H-La₂Mg₁₇ to H-Mg₃La, the interface density between the LaH₃ and MgH₂ phases significantly increased, leading a change in the rate controlling steps from a one-dimensional diffusion process to a three-dimensional interface reaction process. As the hydrolytic rate depends on the content of LaH₃, the hydrolytic rate of MgH₂ is lower than that of the hydrides in the hydrogenated La-Mg system. As a result, the kinetic properties of the LaH₃/MgH₂ composites hydrogenated from Mg₃La and La₂Mg₁₇ alloys are significantly superior to those of pure MgH₂.

To further examine the composite structure, we measured the microstructure by analyzing the TEM images. As shown in Figure 4a, the bright field image of H-Mg₃Ce demonstrates the presence of MgH₂ and CeH_2.73. Figure 4b shows the selected area electron diffraction (SAED) patterns, which was obtained by selecting the diffraction spots of a circular area. The MgH₂(PDF card 012-0697) phase with a tetragonal structure and CeH_2.73 (PDF card 089-3694) phase with an FCC structure are shown in the SAED image. A very fine plate-like or lamellar mixture of CeH_2.73 (dark) and MgH₂ (white) is observed, as identified by the SAED patterns. In the bright field image in Figure 4a, the CeH_2.73 platelet has an average width of 20 nm and a planar length of about 150 nm, whereas the MgH₂ matrix is nanocrystalline with an average grain size of approximately 20 nm. The CeH_2.73 plate is embedded in the MgH₂ phases, uniformly. The hydrolysis of CeH_2.73 can produce conductive ions on the surface of the composite results in a synergetic effect leading to a higher hydrolysis rate.

Figure 4. The bright field image of H-Mg₃Ce composite structure (a) and selected area diffraction patterns (b).

Figure 4. The bright field image of H-Mg₃Ce composite structure (a) and selected area diffraction patterns (b).

3.2. Acceleration of the Hydrolysis Reaction of MgH₂ by Controlling the Particle Size via Ball Milling

The composite structure formed via ball milling could significantly improve the hydrolysis properties. Consequently, the ball milling effect should also be investigated. Figure 5 shows the hydrogen evolution curves by hydrolysis of the as-received (untreated) and 3 h ball-milled MgH₂ in pure water. As shown in Figure 5a, the hydrolysis rate of untreated MgH₂ in pure water is very low, and only 27.4 mL/g of H₂ could be collected in the Monteggia washing bottle after 20 min. In contrast, after 3 h ball milling, the hydrolysis rate of MgH₂ in pure water increased significantly, as shown in Figure 5b, and produced 394.6 mL/g of H₂ in 5 min. Unfortunately, the hydrolysis reaction is interrupted and there is no hydrogen produced after 5 min because of the formation of the magnesium hydroxide passivation layer. According to the hydrogen evolution curves of untreated MgH₂ and ball-milled MgH₂ in pure water by hydrolysis shown in Figure 5, the ball-milling method can effectively improve the reaction rate and yield of hydrogen generation by hydrolysis. This phenomenon could be attributed to the fact that the ball milling process refines the particle and grain size, and increases the specific surface area. In addition, the introduction of structural defects, phase change, and nanocrystalline structures is also beneficial for the MgH₂ hydrolysis. Although ball milling could improve the hydrolysis properties of MgH₂-based materials, the hydrolysis reaction of MgH₂ was still interrupted because of the formation of a magnesium hydroxide passivation layer.

Figure 5. The hydrogen evolution curves by hydrolysis of as-received MgH₂ (a) and 3 h ball-milled MgH₂ (b) in pure water.

Figure 5. The hydrogen evolution curves by hydrolysis of as-received MgH₂ (a) and 3 h ball-milled MgH₂ (b) in pure water.

As a consequence, the ball-milling method has only a limited effect on the hydrolysis properties for pure MgH₂ and the MgH₂ particle size is difficult to control. To understand the effects of the particle size and ball-milling effect on the hydrolysis properties of the MgH₂-based hydrides, H-Mg₃La with different particle sizes but fixed grain sizes of LaH₃ (~17 nm) and MgH₂ (~33 nm) was prepared in situ by hydrogenating Mg₃La and controlling the hydrogenation conditions [68].

Figure 6 shows the hydrogen evolution curves of H-Mg₃La with different particle sizes. Clearly, the hydrolysis rate and hydrogen yield were significantly affected by the particle size. The hydrogenated Mg₃La with the smallest particle size (<12 µm) exhibited a higher hydrolysis yield of 863 mL/g (7.70 wt.%) of H₂. The final hydrogen hydrolysis yield for the samples with particle sizes of >38 µm, 38–23 µm, and 23–18 µm were 1.67 wt.%, 5.06 wt.%, and 6.26 wt.%, respectively. The smaller the particle size, the higher the observed hydrolysis rate. The above results reveal that ball milling is an effective and simple method to improve the hydrolysis properties of MgH₂-based materials.

Figure 6. The hydrogen evolution curves of H-Mg₃La with different particle sizes [68]. Reproduced with permission from [68], copyright 2014 Elsevier.

Figure 6. The hydrogen evolution curves of H-Mg₃La with different particle sizes [68]. Reproduced with permission from [68], copyright 2014 Elsevier.

Analyzing the hydrolysis curves and the hydrolysis products, we found that, as the particle size decreased, the hydrolysis rate accelerated and the hydrogen yield increased, promoting the hydrolysis of H-Mg₃La. Figure 7a,b shows the SEM images of H-Mg₃La with particle sizes >38 μm and <12 μm. The H-Mg₃La sample with particle sizes of <12 μm exhibited larger surface areas and more defects than the sample with particle size >38 μm. As the particle size decreased, the surface area and defects of the sample increased, providing larger surfaces and available sites for the water. As more contact area with water was available, the reaction proceeded more violently, preventing the magnesium hydroxide layer from covering the sample surface. The hydrolysis reaction can spontaneously continue until fully completed. Therefore, reducing the particle size is an effective and simple method to improve the hydrolysis properties of the MgH₂-based materials.

Figure 7. SEM images of H-Mg₃La with particle sizes of (a) [>38] μm and (b) [<12] μm [68]. Reproduced with permission from [68], copyright 2014 Elsevier.

Figure 7. SEM images of H-Mg₃La with particle sizes of (a) [>38] μm and (b) [<12] μm [68]. Reproduced with permission from [68], copyright 2014 Elsevier.

3.3. Enhancement of the Hydrolysis Properties by Optimization of the Solution Composition

The hydrolysis reaction of MgH₂ can be accelerated both by controlling the particles size via ball milling and by forming composite structures. It would be significant to develop a simple method to get the high hydrolysis reaction rate of MgH₂. To break the Mg(OH)₂ passivation layer and accelerate the hydrolysis process, acids were used as effective hydrolytic media to enhance the hydrolysis properties. However, acids may cause corrosion damage to the reaction equipment and increase the cost of the process. The hydrolysis of MgH₂ at different concentrations of NH₄Cl aqueous solution was investigated in this work. The hydrolysis kinetics of MgH₂ at different concentrations of ammonium chloride solution is shown in Figure 8. The hydrolytic rate and hydrogen yield increased with the increase of the NH₄Cl concentration. The hydrogen evolution curves also show that the hydrogen yield kinetics of MgH₂ improved with the increase of the ammonium chloride concentration. This result confirms that the hydrolysis speed can be controlled by adjusting the solution concentration. The hydrolysis of MgH₂ produced 1711.2 mL/g of H₂ in 10 min at 298 K in the 27.1% ammonium chloride solution, and the hydrolytic conversion rate reached the value of 99.5%.

Figure 8. The hydrolysis kinetics of MgH₂ in different concentrations of ammonium chloride solution.

Figure 8. The hydrolysis kinetics of MgH₂ in different concentrations of ammonium chloride solution.

The introduction of a high concentration NH₄Cl solution leads to a high hydrolysis rate, but it also causes some issues, such as resource waste and equipment corrosion damage. Besides, the hydrolysis reaction in NH₄Cl solution introduces impurities and byproducts, which make the regeneration process and hydrolysis gas more complex. Thus, adding a stable catalyst to improve the hydrolysis rate may be a simple way to solve the key issue of hydrogen generation. The 0.05 mol/L MgCl₂ aqueous solution has been optimized for the hydrolysis of MgH₂. The hydrogen evolution curves by hydrolysis of the ball-milled MgH₂ in pure water and in a 0.05 mol/L MgCl₂ solution are shown in Figure 9. The hydrolysis of ball-milled MgH₂ produced 394.6 mL/g of H₂ in 5 min at 298 K, either in the MgCl₂ solution or in pure water, indicating that the hydrolysis rate and yield are similar in the initial stage. The hydrolysis of ball-milled MgH₂ produced 1137.4 mL/g in 90 min at 298 K in the 0.05 mol/L MgCl₂ solution, whereas the hydrolysis in pure water was interrupted. This result suggests that the presence of MgCl₂ played a key role for the hydrolysis of MgH₂. The Mg²⁺ and Cl⁻ ions can actively affect the formation of a relatively loose magnesium hydroxide layer to further accelerate the hydrolysis of MgH₂ in the MgCl₂ solution. Furthermore, the byproduct is the MgO and MgCl₂ composite and no separation process is necessary for the regeneration. A simple method involves the reaction of the MgO and MgCl₂ composite with chlorine at 900 °C to produce MgCl₂; Mg can then be industrially produced by electrolysis of fused MgCl₂. This strategy opens a novel and effective route to modulate the hydrogen yield and hydrogen generation rate in the hydrolysis process.

Figure 9. The hydrogen evolution curves by hydrolysis of 3 h ball-milled MgH₂ in pure water (a) and in 0.05M MgCl₂ solution (b).

Figure 9. The hydrogen evolution curves by hydrolysis of 3 h ball-milled MgH₂ in pure water (a) and in 0.05M MgCl₂ solution (b).

4. Conclusions

MgH₂/LaH₃ composites were prepared by ball milling of MgH₂ and LaH₃ and showed a synergetic effect during the hydrolysis process. A low-cost method for the synthesis of MgH₂/REH₃-based composites was developed by in situ hydrogenation of Mg₃RE-based alloys at room temperature, and showed an enhanced hydrolysis rate. The H-Mg₃Mm, H-Mg₃La, H-Mg₃Ce, H-Mg₃Pr, and H-Mg₃Nd composites generated 1097 mL/g, 949 mL/g, 1025 mL/g, 905 mL/g, and 657 mL/g of hydrogen by hydrolysis, with the total hydrogen yields of 9.79 wt.%, 8.47 wt.%, 9.15 wt.%, 8.08 wt.%, and 5.86 wt.%, respectively. The hydrolysis properties of the MgH₂/REH₃ composites could be further adjusted by controlling the particle size via ball milling. The ammonium chloride solution, which can destroy the Mg(OH)₂ passivation layer and accelerate the hydrolysis process, with the MgH₂ produced 1,711.2 mL/g of H₂ in 10 min at 298 K with a hydrolytic conversion rate of 99.5%. To eliminate the impure gas and simplify the regeneration process, the 0.05 mol/L MgCl₂ aqueous solution was optimized for the hydrolysis of MgH₂ and the hydrogen yield was 1137.4 mL/g in 90 min at 298 K. In conclusion, MgH₂-based hydride composites, ball milling, and tuning of the solution composition are all effective methods to enhance the hydrolysis reaction rate and hydrogen generation yield of MgH₂.