Enhanced Hydrogen Generation Properties of MgH 2-Based Hydrides by Breaking the Magnesium Hydroxide Passivation Layer

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. OPEN ACCESS Energies 2015, 8 4238 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%.


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 NaBH4, 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): 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 Mg 2+ 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 1711.2mL/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.

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.

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 0.1-0.2g 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.

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).

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.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 Mg3LaNi0.1 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-Mg3LaNi0.1)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.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 CeH2.73. 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 CeH2.73 (PDF card 089-3694) phase with an FCC structure are shown in the SAED image.A very fine plate-like or lamellar mixture of CeH2.73 (dark) and MgH2 (white) is observed, as identified by the SAED patterns.In the bright field image in Figure 4a, the CeH2.73 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 CeH2.73 plate is embedded in the MgH2 phases, uniformly.The hydrolysis of CeH2.73 can produce conductive ions on the surface of the composite results in a synergetic effect leading to a higher hydrolysis rate.

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.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.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.

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%.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 Mg 2+ 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.

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 1,711.2mL/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.

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

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

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

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

Figure 9 .
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).