Hydrogen Generation by Hydrolysis of MgH₂-LiH Composite

Abstract

As a most promising material for hydrogen generation by hydrolysis, magnesium hydride (MgH₂) is also trapped by its yielded byproduct Mg(OH)₂ whose dense passivated layers prevent the further contact of intimal MgH₂ with water. In this work, LiH, as a destroyer, has been added to promote the hydrogen properties of MgH₂. The results demonstrate that even 3 wt % LiH was added into MgH₂-G, the hydrogen generation yield can increase about 72% compared to the hydrogen generation yield of MgH₂-G. The possible mechanism is that Mg²⁺ from the hydrolysis of MgH₂ preferentially bound with OH⁻ ions from the hydrolysis of LiH to form Mg(OH)₂ precipitation, which is dispersed in water rather than coated on the surface of MgH₂. Moreover, adding MgCl₂ into hydrolysis solution, using ball milling technology, and increasing the hydrolysis temperature can make the hydrolysis rate higher and reaction process more complete. It is noted that a too high weight ratio of LiH with too high of a hydrolysis temperature will make the reaction too violent to be safe in the experiment. We determinate the best experimental condition is that the LiH ratio added into MgH₂ is 3 wt %, the hydrolysis temperature is 60 °C, and the concentration of MgCl₂ hydrating solution is 1 M. MgH₂-LiH composite hydrogen generation technology can meet the needs of various types of hydrogen supply and has broad application prospects.

1. Introduction

Currently, the contradiction between the decreasing of global fossil energy and the infinite demand for energy is the main factor that restricts the sustainable development of society [1,2]. Hydrogen energy is considered to be the most potential energy carrier for replacing traditional fossil fuels in the future because of its advantages such as abundant reserves, high energy density (142 MJ/kg), environmental protection, and renewability [3,4,5,6]. Hydrogen fuel cell is the most attractive new energy, and its “fuel” is hydrogen, so efficient and safe hydrogen generation technology is particularly important [7]. Hydrogen generation by hydrolysis is a kind of on-site hydrogen generation method, which can be easily applied to various mobile devices [8,9,10].

At present, scientists have paid their attention into the hydrogen generation by the hydrolysis of metal or metal hydrides and chemical hydrides, such as MgH₂, NaBH₄, CaH₂, and LiH, and their theoretical hydrogen yield of are 15.32 wt %, 21.32 wt %, 9.58 wt %, and 25.36 wt % (without H₂O), respectively. NaBH₄ is considered as a promising hydrogen storage material; however, NaBH₄ needs noble catalysts to improve the kinetic performance of hydrolysis, and high cost of recovery of byproducts [11,12,13], while CaH₂ and LiH all react violently with water, making it difficult to control the course of experiments [14,15].

By comparison, MgH₂ has not only a high theoretical hydrogen generation but also environmentally friendly hydrolysis byproduct Mg(OH)₂, what is more, the hydrolysis of Mg-based materials is considered as a clean hydrogen generation technique and Mg element is abundant on Earth [16]. MgH₂ has a lower cost than other hydrolyzed materials, which is regarded the most promising hydrolysis of material. The reaction equations are as follows:

$$\mathrm{Mg}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Mg}{\left(\mathrm{OH}\right)}_2+{\mathrm{H}}_2$$

(1)

$${\mathrm{MgH}}_2+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{Mg}{\left(\mathrm{OH}\right)}_2+2{\mathrm{H}}_2$$

(2)

However, the dense passivation layer of Mg(OH)₂ will prevent the further contact of water with intimal MgH₂, which results in sluggish hydrolysis kinetics [17,18]. In the process of hydrolysis, in order to remove the passivation layer and promote the hydrolysis reaction of MgH₂, researchers have made great efforts, such as the introduction of various cations/anions in the solution [19,20,21], the addition of additives [21,22,23], alloying [16,24,25,26,27], reduce Mg particle size [28], and so on. These methods play an important role in improving the hydrolysis kinetics and hydrogen generation capacity to a certain extent.

In this paper, to improve the hydrolysis properties of MgH₂, we choose the LiH with high hydrogen storage capacity and light weight to add into MgH₂. The effects of milling technology, solution, water solution temperature, and the amount of adding LiH on the hydrolysis performance of MgH₂ were studied.

2. Experimental Details

2.1. Sample Preparation

The Mg powder was prepared by the hydrogen plasma metal reaction (HPMR), the detailed description is given in the published article [29]. Then, the Mg powder was placed in a steel reactor which vacuum to 10⁻³ Pa, and heated to 400 °C for 5 h, followed by hydrogenation under 4 MPa hydrogen atmosphere at 673 K for 10 h. Finally, the reactor was vacuumed, and the sample was taken out to obtain the MgH₂. The MgH₂ and LiH powders were mixed in different mass ratios by obtained by planetary ball milling (BM) and by grinding (G) in agate mortar, respectively, which operated in the argon atmosphere glove box. The LiH was purchased from Alfa Aesar (Lancashire, UK), there is no further purification.

The phase composition of the samples was determined by a powder X-ray diffraction (XRD) method. The XRD patterns were obtained on an X-Pert3 powder diffractometer (PANalytical, Almelo, The Netherlands) in the 2θ range from 5° to 80° using the CuKα radiation. The morphologies of the samples were observed using a JSM-IT300 (JEOL, Tokyo, Japan) scanning electron microscopy (SEM).

2.2. Hydrolysis Experiment

The hydrolysis reactions were tested on a self-assembled system in room temperature. The 50 mg of sample was added into the flask with three opening in the glove box and take out the glove box and quickly connect it to the test system. Then 50 mL of 1 M MgCl₂ aqueous solution was injected into the conical flask with a syringe. After opening the peristaltic pump, MgCl₂ aqueous solution dropped in the three-mouth flask, and the hydrogen was collected by draining water gathering of gas law and recorded the volume of discharged water.

3. Results and Discussion

Figure 1a shows the hydrogen generation curves of MgH₂-G and MgH₂-3 wt % LiH-G in deionized water. As shown in Figure 1a, the hydrogen yield of the MgH₂-G and MgH₂-3 wt % LiH-G are about 130 mL/g, and 230 mL/g, respectively. Although the hydrogen generation yield of MgH₂-3 wt % LiH-G and MgH₂-G are only about 10% of their theoretical value, respectively, the hydrogen generation yield of MgH₂-3 wt % LiH-G increase about 72% compared to the hydrogen generation yield of MgH₂-G. At the same time, from Figure 1b, the hydrolysis byproducts mainly contain a major phase unreacted MgH₂ and a secondary phase Mg(OH)₂. The XRD results indicate that the Mg(OH)₂ layer formed on the surface samples hindered the hydrolysis reaction. However, it also shows that the LiH addition increased the hydrogen yield properties.

In order to improve the hydrogen yield, adding a small amount of MgCl₂ is a common way to promote the hydrolysis of MgH₂. Figure 2a,b are the hydrogen yield and the hydrogen generation rate of the MgH₂-3 wt % LiH-G in the different concentrations (0, 0.1, 0.5, 1) of MgCl₂ aqueous solution, respectively. As shown in Figure 2a, the hydrogen yield curves in 0.1 M, 0.5 M, and 1 M MgCl₂ solutions are compared. The hydrogen production rate diagram from Figure 2b is converted from the data in Figure 2a. The results indicate that MgCl₂ aqueous solution is beneficial for improvement of the MgH₂-3 wt % LiH hydrolysis; their hydrolysis reaction is relatively complete, almost 100%. In addition, in the 1M MgCl₂ aqueous solution, the sample has the fastest hydrolysis rate. The result is consistent with XRD results from Figure 3. The hydrolysis byproduct mainly contains Mg(OH)₂; unreacted MgH₂ is not found.

In order to obtain the effect of LiH on MgH₂-LiH system, we tested the hydrogen generation yield and hydrogen generation rate of MgH₂ with the different amounts of LiH, as shown in Figure 4a,b, respectively. From Figure 4a, the MgH₂-G, MgH₂-1.5 wt % LiH-G, and MgH₂-3 wt % LiH-G generated 1753 mL/g, 1840 mL/g, and 1870 mL/g hydrogen, respectively. Obviously, the addition of LiH improves the hydrogen yield. Furthermore, we can see that the hydrogen generation rate is also improved in Figure 4b. The results indicate the addition of LiH not only enhance hydrogen generation yield but also increase hydrogen generation rate. The mechanism of the reaction will be described later in this paper.

Figure 5 shows the XRD pattern of MgH₂-3 wt % LiH-BM and MgH₂-3 wt % LiH-G. We compared the hydrolytic properties of MgH₂-3 wt % LiH-BM and MgH₂-3 wt % LiH-G. The two samples were composed of MgH₂ and LiH, in addition, small peaks of MgO and LiOH could also be observed. This MgO is introduced during preparation and LiOH is introduced XRD characterization process.

Figure 6a,b show the hydrogen generation curves and hydrogen generation rate of MgH₂-3 wt % LiH-BM and MgH₂-3 wt % LiH-G, respectively. It can be seen that the MgH₂-3 wt % LiH-BM can generate about 1830 mL/g in 800 s, continue to generate hydrogen until 1890 mL/g in 1500 s, which can close to the maximum hydrogen yield for the studied samples. The MgH₂-3 wt % LiH-G can generate 1820 mL/g in 3000 s. Finally, the maximum value is reached more than 3500 s. Obviously, the MgH₂-3 wt % LiH-BM exhibits a higher hydrogen generation rate than the MgH₂-3 wt % LiH-G. From the above experimental results, the ball milling in a short time plays an important role in increasing the rate of hydrogen generation. The results should be closely related to the particle size of the sample. In order to provide more direct experimental evidence, the SEM images of the two samples are given in Figure 7. From the SEM images of the two samples, we can see that the MgH₂-3 wt % LiH-BM are obviously smaller than the MgH₂-3 wt % LiH-G, proving the above hydrogen generation properties analysis results of two samples.

Furthermore, in order to study the hydrolysis properties of the sample in deionized water, the MgH₂-3 wt % LiH-BM in deionized water at the different water bath temperatures were investigated as shown in Figure 8. The hydrogen generation yield and hydrogen generation rate are enhanced with the increase of water bath temperatures. One possible reasons is that the higher temperature is conducive to the dissolution of Mg(OH)₂. Figure 9 shows XRD patterns of the byproducts after hydrolysis in deionized water at water bath temperatures with 50 °C and 60 °C. From Figure 9, the byproducts are composed of MgH₂, Mg(OH)₂, and Li₂CO₃. In the byproducts, Li₂CO₃ may come from CO₂ absorbed by the LiOH byproduct during the drying and testing process. LiOH is dissolved in water, so there is no associated diffraction peak in XRD [26]. According to the XRD results, the reaction of Mg, MgH₂ and LiH could be described as follow:

$$\mathrm{LiH}+2{\mathrm{H}}_2\mathrm{O}\to \mathrm{LiOH}+{\mathrm{H}}_2$$

(3)

$$\mathrm{LiOH}+{\mathrm{CO}}_2\to {\mathrm{LiCO}}_3+{\mathrm{H}}_2\mathrm{O}$$

(4)

As the water bath temperature increases from 50 °C to 60 °C, the diffraction peaks intensity of Mg(OH)₂ increases while the intensity of MgH₂ decreases. The hydrolysis properties of the sample is obviously dependent on temperature [30]. When the water bath temperature is 60 °C, the final hydrolysis yield reaches about 1510 mL/g until 250 min, with the conversion rate up to about 80%. These results also prove that the suitable water temperature is beneficial to the hydrolysis reaction.

We carefully compared hydrogen generation of the MgH₂-BM and MgH₂-3 wt % LiH-BM at 60 °C in Figure 10a. As can be seen, the hydrogen generation rate of MgH₂-3 wt % LiH-BM is improved. The presence of LiH can increase the hydrolysis rate compared to pure MgH₂. The result can also be seen visually from the hydrogen generation rate diagram, as shown in Figure 10b.

The possible hydrolysis mechanism of the MgH₂-LiH system in MgCl₂ aqueous solution was detailed described in previous work of our research group [21]. In pure water, the MgH₂ hydrolysis reaction leads to an increase −OH concentration on the particles surface. The precipitation of −OH and Mg²⁺ mainly occurs on the surface of particles, so the byproduct Mg(OH)₂ rapidly deposits on the surface of particles, forming a dense passivation layer. The passivation layer prevents further hydrolysis of MgH₂. In MgCl₂ aqueous solution, due to the presence of a large amount of Mg²⁺ in the whole solution system, the Mg²⁺ in solution competes with the MgH₂ on the surface during the formation of Mg(OH)₂ precipitation, that is to say, Mg²⁺ in solution combine with OH⁻ on the surface of MgH₂. In this case, the resulting precipitate is dispersed in the solution rather than forming a passivated layer on the surface.

For hydrolysis properties of the MgH₂-3 wt % LiH-BM and MgH₂-3 wt % LiH-G, the MgH₂-3 wt % LiH-BM has better hydrolysis kinetics than MgH₂-3 wt % LiH-G. This result is that the reduction of particle size of the samples after planetary ball milling cause the larger specific surface area of the samples, which is more conducive to the rapid hydrolysis reaction [23,30].

For the role of LiH, by XRD, SEM, and reaction byproduct analysis, LiH is uniformly attached to the surface of MgH₂. LiH hydrolyzes rapidly in water, and the reaction equation is shown as (3): LiH + H₂O → LiOH + H₂. Then, Mg²⁺ preferentially binds with OH⁻ ions to form Mg(OH)₂ precipitation, which is dispersed in water rather than coated on the surface of MgH₂ nanoparticles. The result is in agreement with reported results for MgLi alloy and MgH₂-LiNH₂ composites [8,26].

In addition, in the process of planetary ball milling and grinding in agate mortar, LiH will also enter between MgH₂ nanoparticles to disperse MgH₂ and conduce to MgH₂ fully contact with aqueous solution.

4. Conclusions

In this work, the hydrolysis properties of the MgH₂-LiH system have been studied. The 3 wt % LiH is added into MgH₂, the hydrogen generation yield can increase about 72% compared to the hydrogen generation yield of MgH₂-G. The MgH₂-LiH system hydrolysis is relatively complete, almost 100%, also has the fastest hydrolysis rate in the 1M MgCl₂ aqueous solution. In a short time, the ball milling reduces the particle size of the samples and cause the larger specific surface area of the samples, which is more conducive to the rapid hydrolysis reaction. The higher water solution temperature is helpful to improve the hydrolysis properties. When the water bath temperature is 60 °C, the final hydrolysis yield reaches to about 1510 mL/g until 250 min.