The Effect of Graphite and Fe2O3 Addition on Hydrolysis Kinetics of Mg-Based Hydrogen Storage Materials

In this paper, graphite and Fe2O3 are introduced into MgH2 powder by the method of hydrogenation after magnetic grinding. Hydrogen storage materials which composite of MgH2–5 wt.% C and MgH2–5 wt.% C–5 wt.% Fe2O3 are successfully prepared. The physical structure of these materials was analyzed and characterized by XRD, SEM, etc. Furthermore, the influence of graphite and Fe2O3 on the hydrolysis of MgH2 was systematically investigated. The results show that MgH2–C–Fe2O3 composite powder has the fastest hydrogen release rate in municipal drinking water and the highest conversion rate. Graphite and Fe2O3 can effectively reduce the activation energy of the hydrolysis reaction of MgH2 and improve the hydrolysis kinetics of MgH2. The synergistic effect of the coaddition of graphite and Fe2O3 can significantly increase the hydrolysis conversion rate of MgH2 and improve the hydrolysis kinetics.


Introduction
Due to the increasing environmental pollution caused by the development and use of fossil fuels such as petroleum, the research and use of green and clean energy has become an increasing focus of the society [1][2][3]. Hydrogen is an ideal clean energy, whose energy density (142 MJ kg -1 ) is three times that of gasoline, and the combustion product is only water [4]. In the past few decades, most of the commercial hydrogen was obtained through partial oxidation of natural gas and coal gasification [5], which consumes petrochemical energy to cause certain environmental pollution. Recently, hydrogen production by hydrolysis has become a new focus due to its simple process, mild reaction conditions, safety, clean, and efficiency [6][7][8]. Researchers have reported several materials for hydrogen production by hydrolysis, such as metals [9], metal hydrides [10,11], and borohydrides [12][13][14]. Among all the hydrogen storage materials, MgH 2 has the advantages of large hydrogen storage capacity (7.6 wt.%) and high theoretical hydrolysis hydrogen production (15.2 wt.%). In recent years, it has attracted much atten-tion from scientific researchers. Hydrogen can be obtained by the reaction of MgH 2 with water, and the reaction equation is as follows: The hydrolysis reaction of MgH 2 is not harsh and can proceed spontaneously in contact with water at room temperature. However, a passivation layer can be formed on the surface of the unreacted MgH 2 during the hydrolysis process, which prevents the water from diffusing into the interior, thus, making the hydrolysis stop rapidly from the high-speed reaction stage. Kojima et al. [15] reported that the hydrogen conversion rate of MgH 2 hydrolysis is less than 30% in municipal drinking water for 1 hour, which hinders the practical application of MgH 2 . For improving the low conversion rate of MgH 2 hydrolysis, many studies have been reported, such as reducing the particle size of MgH 2 to nanometer [16,17] and using other aqueous solutions (such as acid solution [18] and salt solution [19][20][21]) to prevent the generation of Mg(OH) 2 passivation layer. In addition, adding chloride [22], metal hydride [23,24], carbon material [25,26], and metal oxide [27,28] to MgH 2 by ball grinding and other methods can also significantly improve the hydrolysis kinetic performance of MgH 2 . For example, Yang et al. [28] studied the effects of TiO 2 , MgAl 2 O 4 , and Fe on the hydrolysis properties of MgH 2 . As an amphoteric oxide, TiO 2 can lower the pH value around MgH 2 , which can therefore enhance the hydrolysis properties of MgH 2 ; MgAl 2 O 4 has a catalytic effect on the hydrolysis of MgH 2 ; Fe as the cathode of galvanic cell and thereby reduce the activation energy for hydrolysis reaction of MgH 2 . Awad et al. [26] studied the effect of adding carbon materials, metals (Ni, Fe, and Al), and metal oxides (Nb 2 O 5 and V 2 O 5 ) by using the ball grinding method on the hydrolysis of magnesium-based materials; it was found that the mixture of Mg-5 wt.% C-5 wt.% Ni had the best hydrolysis rate (95% of the theoretical hydrogen production within 2 minutes) and the lowest hydrolysis activation energy (14.34 kJ mol -1 ). It was proven that the carbon material and the transition metal Ni could promote the hydrolysis of magnesium-based materials. However, the synergistic effect of carbon and metal oxides on magnesium has not been systematically studied, while the mechanism has not been analyzed [25,26,28].
Considering the above, in this paper, several MgH 2 -C and MgH 2 -C-Fe 2 O 3 materials were prepared by hydrogenation after magnetic grinding. The synergistic effect of graphite and Fe 2 O 3 on the hydrolytic kinetics of magnesium-based hydrogen storage materials was investigated systematically. The hydrolytic conversion rate, hydrolytic reaction rate, and hydrolytic activation energy Ea of pure MgH 2 and composite powders with different additives were obtained.
Then, the furnace was cooling under room temperature and stood for 12 h. The samples were taken out after the reaction kettle which naturally cooled to room temperature. The powder composite of MgH 2 -5 wt.% C and MgH 2 -5 wt.% Fe 2 O 3 -5 wt.% C was prepared. The pure MgH 2 powder is also annealed in hydrogen gas.

Hydrolysis Experiment.
The hydrolysis device is shown in Figure 1, which mainly consists of a reaction device and a collection device. The reaction device includes a thermostatic water bath pot, three-necked flask, and condensing tube. The flask's three mouths are, respectively, used for inserting a thermometer, connecting the condensing tube, and adding powder samples. The collecting device consists of a Monteggia washing bottle, beaker, and electronic balance. Adding 100 mL of municipal drinking water into a three-necked flask and using a water bath to control temperature. Then, adding 0.05 g powder into the three-necked flask and recording the time. Through the condenser tube, the condensed hydrogen enters the Monteggia washing bottle. The hydrogen drains the water from the Monteggia washing bottle to the beaker. The hydrogen released is estimated through calculating the weight of the discharged water.
The hydrogen conversion rate is the ratio of the amount of hydrogen produced at time T to the total amount of hydrogen produced by adding excess of 0.1 mol L -1 HCl solution [29] (see Table 1).

Sample Characterization.
Components and crystal structures of the samples were examined by powder Xray diffraction apparatus (XRD, Ultima IV, Rigaku Corporation) equipped with a Cu Kα radiation source. XRD analyses were performed over a range from 10°to 80°at a scanning rate of 10°min -1 . Using a scanning electron microscope (SEM, Quanta 250, FEI, equipped with energy dispersive spectroscopy (EDS) system, working voltage 20 kV) to observe the structure and element composition of the composite powder. Another reason for appearing MgO is the part of Mg that has not been completely hydrogenated quickly producing a dense MgO layer on the surface when it comes in contact with air. In addition, we also found that the diffraction peak strength of C and Fe 2 O 3 is relatively low, which may be caused by the low content of these two phases, the excessively small particle size, and the relatively high dispersion.

Results and Discussion
The SEM measurements were taken out to characterize the structure of the MgH 2 -C ( Figure 3) and MgH 2 -C-Fe 2 O 3 (Figure 4). In Figures 3(c) and 3(d) and 4(c) and 4(d), it can be seen that the particle size of MgH 2 is relatively uniform, and the particle size of the two samples is about 300-700 nm. This is because the fine graphite particles are distributed on the surface and gap of the Mg during the magnetic grinding process to play a role of lubrication and dispersion, which reduces the cold welding phenomenon of Mg particles. This effectively prevents the agglomeration of Mg particles, so that the size of the hydrogenated MgH 2 particles is smaller and uniform, which is similar to the research conclusion of the ball milling reaction of C and Mg reported by Awad et al. [26]. Compared with the MgH 2 -C powder (Figure 3(c)), the MgH 2 -C-Fe 2 O 3 powder (Figure 4(c)) has fewer large particles and a more even particle size, which indicates that the addition of Fe 2 O 3 can further reduce the size of MgH 2 particles.
The SEM image and EDS spectrum of the MgH 2 -C-Fe 2 O 3 powder (Figure 4(b)) show that there is a certain amount of Fe on the surface of the particle. Combined with the XRD pattern of the sample (Figure 2(b)), it shows that the Fe element comes from Fe 2 O 3 , which further proves the presence of Fe 2 O 3 . Figure 5 shows the hydrogen release curves of pure MgH 2 (a), MgH 2 -C powder (b), and MgH 2 -C-Fe 2 O 3 powder (c) in municipal drinking water at 353 K. It can be observed that the hydrolysis rate of pure MgH 2 and the hydrogen conversion rate are relatively low. At 353 K, pure MgH 2 produces only 113 mL g -1 hydrogen in 2 minutes, 201.9 mL g -1 hydrogen in 5 minutes, and the hydrolysis conversion rate in 60 minutes is only 19.3%. It is possible that after the first few minutes of rapid reaction, a dense Mg(OH) 2 layer was formed on the surface of MgH 2 , which prevented MgH 2 from further reacting with water. In contrast, when the C added sample is at 353 K, the hydrogen production is 268.5 mL g -1 in 2 minutes, 416 mL g -1 hydrogen is produced in 5 minutes, and the hydrolysis conversion rate is 52.5% in 60 minutes. The sample with C and Fe 2 O 3 showed the fastest hydrolysis rate at 353 K, with 280 mL g -1 hydrogen produced in 2 minutes, 468 mL g -1 hydrogen produced in 5 minutes, and the hydrolysis conversion rate increased to 62.8% in 60 minutes. Compared with pure MgH 2 , the two have better hydrolysis kinetics and higher hydrolysis conversion rate, which proves that C and Fe 2 O 3 can promote the hydrolysis of MgH 2 . This is because, in the magnetic grinding process, C can effectively reduce the agglomeration of magnesium particles, make the particle size of the hydrogenated powder smaller, increase the area in contact with water during hydrolysis, and thus effectively improve the hydrolysis reaction rate. Tayeh et al. [16] also showed a similar effect after ball milling of MgH 2 with C added. Fe 2 O 3 can further reduce the particle size of MgH 2 and may have a catalytic effect on the hydrolysis reaction of MgH 2 . Furthermore, it can improve the hydrolysis kinetic performance of MgH 2 .

International Journal of Photoenergy
In order to investigate the synergistic effect of the addition of C and Fe 2 O 3 on the hydrolytic hydrogen production performance of MgH 2 , we tested the hydrogen liberation performance of pure MgH 2 , MgH 2 -C powder, and MgH 2 -C-Fe 2 O 3 powder in municipal drinking water under different temperature conditions. As shown in Figure S1, compared with pure MgH 2 and MgH 2 -C powders, the hydrogen production rate and hydrogen conversion rate of MgH 2 -C-Fe 2 O 3 powder are greatly improved at different temperatures. Figure S1(c) shows that MgH 2 -C-Fe 2 O 3 powder has the highest hydrogen production rate and hydrogen conversion rate. Compared with MgH 2 -C powder, the hydrogen conversion rate increases from 28.6% to 36.4% at 333 K (see Table S1) and from 50.9% to 60.7% in the first 30 min at 353 K (see Table S2), respectively. The results show that the synergistic effect of C and Fe 2 O 3 together can significantly improve the hydrolysis kinetics of MgH 2 and increase the hydrogen conversion rate.
Two main models, the diffusion-controlled and the phase-boundary controlled, describe the experimental kinetic curves of MgH 2 hydrolysis [30]. The hydrolysis process of MgH 2 and MgH 2 -additives in municipal drinking water can be described by the Avramie-Erofeev equation (Equation (2)) [20]: αðtÞ is the reaction rate (the ratio of the amount of reacted material to the total amount of material, it can be regarded as hydrogen conversion rate), t is the reaction time, and B and m are constants. Values of B and m obtained by fitting and R 2 (correlation coefficient) are shown in Figure 6. The R 2 values indicate that the fitted results are in good agreement with the experimental data. Different m values represent different nucleation growth rate control steps, and m values for one-dimensional diffu-sion and for three-dimensional interfacial reaction are 0.62 and 1.07, respectively [20]. According to Figure 6, the m values of MgH 2 -C-Fe 2 O 3 , MgH 2 -C, and pure MgH 2 samples are 0.56, 0.54, and 0.52, respectively, which are closer to 0.62. This indicates that the hydrolysis of three samples at 353 K follows a one-dimensional diffusion mechanism.
3.3. Activation Energy of Hydrolysis. The apparent activation energy for hydrolysis of MgH 2 can be determined by the Arrhenius equation (Equation (3)): where k is the reaction rate constant, Ea is the apparent activation energy (J mol -1 ), R is the molar gas constant (8.314 J mol -1 K -1 ), and T is the reaction temperature (K). By fitting the lnk-1000/T line, the slope of the line was multiplied by the R value, and the apparent activation energy Ea of different samples was finally obtained. Figure 7 shows the apparent activation energy of three different samples calculated.

Data Availability
All data used to support the this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare no conflict of interest.