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

Authors: Kun Yang; Hongyun Qin; Junnan Lv; Rujun Yu; Xia Chen; Zengdian Zhao; Yongjie Li; Fang Zhang; Xianchang Xia; Qiang Fu; Ming Wang
Journal: International Journal of Photoenergy
 (2021)

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## Abstract

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.


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## Body

## 1. 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 [1–3]. 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 [6–8]. Researchers have reported several materials for hydrogen production by hydrolysis, such as metals [9], metal hydrides [10, 11], and borohydrides [12–14]. Among all the hydrogen storage materials, MgH2 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 attention from scientific researchers. Hydrogen can be obtained by the reaction of MgH2 with water, and the reaction equation is as follows:
(1)MgH2+2H2O→MgOH2+2H2↑ΔH=–277kJmol−1.The hydrolysis reaction of MgH2 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 MgH2 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 MgH2 hydrolysis is less than 30% in municipal drinking water for 1 hour, which hinders the practical application of MgH2. For improving the low conversion rate of MgH2 hydrolysis, many studies have been reported, such as reducing the particle size of MgH2 to nanometer [16, 17] and using other aqueous solutions (such as acid solution [18] and salt solution [19–21]) 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 MgH2 by ball grinding and other methods can also significantly improve the hydrolysis kinetic performance of MgH2. For example, Yang et al. [28] studied the effects of TiO2, MgAl2O4, and Fe on the hydrolysis properties of MgH2. As an amphoteric oxide, TiO2 can lower the pH value around MgH2, which can therefore enhance the hydrolysis properties of MgH2; MgAl2O4 has a catalytic effect on the hydrolysis of MgH2; Fe as the cathode of galvanic cell and thereby reduce the activation energy for hydrolysis reaction of MgH2. Awad et al. [26] studied the effect of adding carbon materials, metals (Ni, Fe, and Al), and metal oxides (Nb2O5 and V2O5) 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 MgH2–C and MgH2–C–Fe2O3 materials were prepared by hydrogenation after magnetic grinding. The synergistic effect of graphite and Fe2O3 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 MgH2 and composite powders with different additives were obtained.


## 2. Experimental

### 2.1. Sample Preparation

Magnesium-based hydride composite materials were prepared by hydrogenation after magnetic grinding. The starting materials are Mg powder (purity≥99%, 200 mesh, Sinopharm Chemical Reagent Co., Ltd), MgH2, (purity≥99.5%, MG Power Technology Co., Ltd), Fe2O3 (average particle size 20 nm, purity≥99.5%, Shanghai Yaoyi), and graphite (average particle size 80 nm, purity≥99.5%, Shanghai Yaoyi). Place the required raw materials (total weight 3 g, MgH2–C (Mg : C massratio=0.95:0.05), MgH2–C–Fe2O3 (Mg :C : Fe2O3massratio=0.9:0.05:0.05) into the stainless steel reaction kettle (volume 100 mL, 60 g of steel needle for grinding is installed in the reaction kettle, 0.3×5mm:1×5mmmassratio=1:1). In order to prevent Mg oxidation during grinding, high purity Ar gas (99.999%) was used to purge the reactor chamber and pipeline for 20 min before grinding, and a small amount of Ar gas was filled in the chamber and pipeline for protection. Then, the grinding experiment was performed (rotationalspeed=2400rad/min), and the grinding time was 2 h (grinding 30min+cooling30min, repeated four times). Then, the reactor was placed on a constant temperature heating furnace (~ 300°C) and filled with hydrogen gas (~ 2.3 MPa) for 2 h hydrogenation. 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 MgH2–5 wt.% C and MgH2–5 wt.% Fe2O3–5 wt.% C was prepared. The pure MgH2 powder is also annealed in hydrogen gas.


### 2.2. Hydrolysis Experiment

The hydrolysis device is shown in Figure1, 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.Figure 1
The experimental apparatus used for hydrolyzing hydrogen production.The hydrogen conversion rate is the ratio of the amount of hydrogen produced at timeT to the total amount of hydrogen produced by adding excess of 0.1 mol L-1 HCl solution [29] (see Table 1).Table 1
The amount of hydrogen released from each sample after complete hydrolysis.
SamplesTotal hydrogen released [mL/g]Pure MgH21730MgH2–C1602MgH2–C–Fe2O31503


### 2.3. Sample Characterization

Components and crystal structures of the samples were examined by powder X–ray 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.


## 2.1. Sample Preparation

Magnesium-based hydride composite materials were prepared by hydrogenation after magnetic grinding. The starting materials are Mg powder (purity≥99%, 200 mesh, Sinopharm Chemical Reagent Co., Ltd), MgH2, (purity≥99.5%, MG Power Technology Co., Ltd), Fe2O3 (average particle size 20 nm, purity≥99.5%, Shanghai Yaoyi), and graphite (average particle size 80 nm, purity≥99.5%, Shanghai Yaoyi). Place the required raw materials (total weight 3 g, MgH2–C (Mg : C massratio=0.95:0.05), MgH2–C–Fe2O3 (Mg :C : Fe2O3massratio=0.9:0.05:0.05) into the stainless steel reaction kettle (volume 100 mL, 60 g of steel needle for grinding is installed in the reaction kettle, 0.3×5mm:1×5mmmassratio=1:1). In order to prevent Mg oxidation during grinding, high purity Ar gas (99.999%) was used to purge the reactor chamber and pipeline for 20 min before grinding, and a small amount of Ar gas was filled in the chamber and pipeline for protection. Then, the grinding experiment was performed (rotationalspeed=2400rad/min), and the grinding time was 2 h (grinding 30min+cooling30min, repeated four times). Then, the reactor was placed on a constant temperature heating furnace (~ 300°C) and filled with hydrogen gas (~ 2.3 MPa) for 2 h hydrogenation. 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 MgH2–5 wt.% C and MgH2–5 wt.% Fe2O3–5 wt.% C was prepared. The pure MgH2 powder is also annealed in hydrogen gas.


## 2.2. Hydrolysis Experiment

The hydrolysis device is shown in Figure1, 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.Figure 1
The experimental apparatus used for hydrolyzing hydrogen production.The hydrogen conversion rate is the ratio of the amount of hydrogen produced at timeT to the total amount of hydrogen produced by adding excess of 0.1 mol L-1 HCl solution [29] (see Table 1).Table 1
The amount of hydrogen released from each sample after complete hydrolysis.
SamplesTotal hydrogen released [mL/g]Pure MgH21730MgH2–C1602MgH2–C–Fe2O31503


## 2.3. Sample Characterization

Components and crystal structures of the samples were examined by powder X–ray 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.


## 3. Results and Discussion

### 3.1. Composition and Structure of Mg Hydrogen Storage Materials Prepared

XRD patterns of the MgH2–C and MgH2–C–Fe2O3 powders are shown in Figures 2(a) and 2(b). It can be seen from Figure 2 that the MgH2–C–Fe2O3 powder has diffraction peaks of MgH2, Mg, MgO, and C, as well as a small diffraction peak of Fe2O3. And the MgH2–C powder only showed diffraction peaks of MgH2, Mg, MgO, and C. With the exception of MgH2, neither of the two powders produced any new substances, which proved that no other side reactions during the magnetic grinding and hydrogenation. The appearance of the Mg diffraction peak is due to the fact that part of the Mg particles not sufficiently ground or agglomerated during the magnetic grinding process, resulting in the large particles of Mg not being completely hydrogenated. And the one reason for appearing MgO is that there is an oxide layer on the surface of the raw material Mg particles. The steel needle in the grinding chamber uses mechanical force to cut the Mg particles while peeling off the surface MgO layer. MgO is mixed in the hydrogenated MgH2 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 Fe2O3 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.Figure 2
XRD patterns of MgH2–C (a) and MgH2–C–Fe2O3 (b) powders prepared by magnetic grinding and hydrogenation.The SEM measurements were taken out to characterize the structure of the MgH2–C (Figure 3) and MgH2–C–Fe2O3 (Figure 4). In Figures 3(c) and 3(d) and 4(c) and 4(d), it can be seen that the particle size of MgH2 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 MgH2 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 MgH2–C powder (Figure 3(c)), the MgH2–C–Fe2O3 powder (Figure 4(c)) has fewer large particles and a more even particle size, which indicates that the addition of Fe2O3 can further reduce the size of MgH2 particles.Figure 3
SEM micrograph of the MgH2–C (a, c, d) and EDS analysis of (a).
(a)(b)(c)(d)Figure 4
SEM micrograph of the MgH2–C–Fe2O3 (a, c, d) and EDS analysis of (a).
(a)(b)(c)(d)The SEM image and EDS spectrum of the MgH2–C–Fe2O3 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 Fe2O3, which further proves the presence of Fe2O3.


### 3.2. Kinetics of Hydrolysis

Figure5 shows the hydrogen release curves of pure MgH2 (a), MgH2–C powder (b), and MgH2–C–Fe2O3 powder (c) in municipal drinking water at 353 K. It can be observed that the hydrolysis rate of pure MgH2 and the hydrogen conversion rate are relatively low. At 353 K, pure MgH2 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 MgH2, which prevented MgH2 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 Fe2O3 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 MgH2, the two have better hydrolysis kinetics and higher hydrolysis conversion rate, which proves that C and Fe2O3 can promote the hydrolysis of MgH2. 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 MgH2 with C added. Fe2O3 can further reduce the particle size of MgH2 and may have a catalytic effect on the hydrolysis reaction of MgH2. Furthermore, it can improve the hydrolysis kinetic performance of MgH2.Figure 5
The hydrogen evolution curves for the hydrolysis of pure MgH2 (a), MgH2–C (b), and MgH2–C–Fe2O3 (c) at 353 K.In order to investigate the synergistic effect of the addition of C and Fe2O3 on the hydrolytic hydrogen production performance of MgH2, we tested the hydrogen liberation performance of pure MgH2, MgH2–C powder, and MgH2–C–Fe2O3 powder in municipal drinking water under different temperature conditions. As shown in Figure S1, compared with pure MgH2 and MgH2–C powders, the hydrogen production rate and hydrogen conversion rate of MgH2–C–Fe2O3 powder are greatly improved at different temperatures. Figure S1(c) shows that MgH2–C–Fe2O3 powder has the highest hydrogen production rate and hydrogen conversion rate. Compared with MgH2–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 Fe2O3 together can significantly improve the hydrolysis kinetics of MgH2 and increase the hydrogen conversion rate.Two main models, the diffusion-controlled and the phase-boundary controlled, describe the experimental kinetic curves of MgH2 hydrolysis [30]. The hydrolysis process of MgH2 and MgH2–additives in municipal drinking water can be described by the Avramie–Erofeev equation (Equation (2)) [20]:
(2)αt=1–exp–Btm.αt 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 R2 (correlation coefficient) are shown in Figure 6. The R2 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 diffusion and for three-dimensional interfacial reaction are 0.62 and 1.07, respectively [20]. According to Figure 6, the m values of MgH2–C–Fe2O3, MgH2–C, and pure MgH2 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.Figure 6
The kinetic curves of hydrogen generation for the hydrolysis of (a) pure MgH2, (b) MgH2–C, and (c) MgH2–C–Fe2O3 at 353 K in municipal drinking water.


### 3.3. Activation Energy of Hydrolysis

The apparent activation energy for hydrolysis of MgH2 can be determined by the Arrhenius equation (Equation (3)):
(3)k=A·exp–Ea/RT,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. R2 values in all cases are >0.98, indicating that the Arrhenius equation is appropriate for describing the hydrolysis of MgH2. From the slope of the fitted line, Ea of pure MgH2 was found to be 55.57 kJ mol-1. Similarly, Ea values of MgH2–C and MgH2–C–Fe2O3 samples were 43.40 and 36.92 kJ mol-1, respectively, which were lower than the value for pure MgH2. It can be seen that adding C can reduce the hydrolysis activation energy of MgH2; adding C and Fe2O3 at the same time can further reduce the hydrolysis activation energy of MgH2. A lower Ea value generally indicates a higher reaction activity. Therefore, the addition of C and Fe2O3 simultaneously has a synergistic effect on improving the hydrolysis properties of MgH2.Figure 7
Arrhenius plot for the activation energy calculation hydrolysis of MgH2–C–Fe2O3 (a), MgH2–C (b), and pure MgH2 (c) in municipal drinking water.


## 3.1. Composition and Structure of Mg Hydrogen Storage Materials Prepared

XRD patterns of the MgH2–C and MgH2–C–Fe2O3 powders are shown in Figures 2(a) and 2(b). It can be seen from Figure 2 that the MgH2–C–Fe2O3 powder has diffraction peaks of MgH2, Mg, MgO, and C, as well as a small diffraction peak of Fe2O3. And the MgH2–C powder only showed diffraction peaks of MgH2, Mg, MgO, and C. With the exception of MgH2, neither of the two powders produced any new substances, which proved that no other side reactions during the magnetic grinding and hydrogenation. The appearance of the Mg diffraction peak is due to the fact that part of the Mg particles not sufficiently ground or agglomerated during the magnetic grinding process, resulting in the large particles of Mg not being completely hydrogenated. And the one reason for appearing MgO is that there is an oxide layer on the surface of the raw material Mg particles. The steel needle in the grinding chamber uses mechanical force to cut the Mg particles while peeling off the surface MgO layer. MgO is mixed in the hydrogenated MgH2 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 Fe2O3 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.Figure 2
XRD patterns of MgH2–C (a) and MgH2–C–Fe2O3 (b) powders prepared by magnetic grinding and hydrogenation.The SEM measurements were taken out to characterize the structure of the MgH2–C (Figure 3) and MgH2–C–Fe2O3 (Figure 4). In Figures 3(c) and 3(d) and 4(c) and 4(d), it can be seen that the particle size of MgH2 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 MgH2 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 MgH2–C powder (Figure 3(c)), the MgH2–C–Fe2O3 powder (Figure 4(c)) has fewer large particles and a more even particle size, which indicates that the addition of Fe2O3 can further reduce the size of MgH2 particles.Figure 3
SEM micrograph of the MgH2–C (a, c, d) and EDS analysis of (a).
(a)(b)(c)(d)Figure 4
SEM micrograph of the MgH2–C–Fe2O3 (a, c, d) and EDS analysis of (a).
(a)(b)(c)(d)The SEM image and EDS spectrum of the MgH2–C–Fe2O3 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 Fe2O3, which further proves the presence of Fe2O3.


## 3.2. Kinetics of Hydrolysis

Figure5 shows the hydrogen release curves of pure MgH2 (a), MgH2–C powder (b), and MgH2–C–Fe2O3 powder (c) in municipal drinking water at 353 K. It can be observed that the hydrolysis rate of pure MgH2 and the hydrogen conversion rate are relatively low. At 353 K, pure MgH2 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 MgH2, which prevented MgH2 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 Fe2O3 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 MgH2, the two have better hydrolysis kinetics and higher hydrolysis conversion rate, which proves that C and Fe2O3 can promote the hydrolysis of MgH2. 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 MgH2 with C added. Fe2O3 can further reduce the particle size of MgH2 and may have a catalytic effect on the hydrolysis reaction of MgH2. Furthermore, it can improve the hydrolysis kinetic performance of MgH2.Figure 5
The hydrogen evolution curves for the hydrolysis of pure MgH2 (a), MgH2–C (b), and MgH2–C–Fe2O3 (c) at 353 K.In order to investigate the synergistic effect of the addition of C and Fe2O3 on the hydrolytic hydrogen production performance of MgH2, we tested the hydrogen liberation performance of pure MgH2, MgH2–C powder, and MgH2–C–Fe2O3 powder in municipal drinking water under different temperature conditions. As shown in Figure S1, compared with pure MgH2 and MgH2–C powders, the hydrogen production rate and hydrogen conversion rate of MgH2–C–Fe2O3 powder are greatly improved at different temperatures. Figure S1(c) shows that MgH2–C–Fe2O3 powder has the highest hydrogen production rate and hydrogen conversion rate. Compared with MgH2–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 Fe2O3 together can significantly improve the hydrolysis kinetics of MgH2 and increase the hydrogen conversion rate.Two main models, the diffusion-controlled and the phase-boundary controlled, describe the experimental kinetic curves of MgH2 hydrolysis [30]. The hydrolysis process of MgH2 and MgH2–additives in municipal drinking water can be described by the Avramie–Erofeev equation (Equation (2)) [20]:
(2)αt=1–exp–Btm.αt 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 R2 (correlation coefficient) are shown in Figure 6. The R2 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 diffusion and for three-dimensional interfacial reaction are 0.62 and 1.07, respectively [20]. According to Figure 6, the m values of MgH2–C–Fe2O3, MgH2–C, and pure MgH2 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.Figure 6
The kinetic curves of hydrogen generation for the hydrolysis of (a) pure MgH2, (b) MgH2–C, and (c) MgH2–C–Fe2O3 at 353 K in municipal drinking water.


## 3.3. Activation Energy of Hydrolysis

The apparent activation energy for hydrolysis of MgH2 can be determined by the Arrhenius equation (Equation (3)):
(3)k=A·exp–Ea/RT,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. R2 values in all cases are >0.98, indicating that the Arrhenius equation is appropriate for describing the hydrolysis of MgH2. From the slope of the fitted line, Ea of pure MgH2 was found to be 55.57 kJ mol-1. Similarly, Ea values of MgH2–C and MgH2–C–Fe2O3 samples were 43.40 and 36.92 kJ mol-1, respectively, which were lower than the value for pure MgH2. It can be seen that adding C can reduce the hydrolysis activation energy of MgH2; adding C and Fe2O3 at the same time can further reduce the hydrolysis activation energy of MgH2. A lower Ea value generally indicates a higher reaction activity. Therefore, the addition of C and Fe2O3 simultaneously has a synergistic effect on improving the hydrolysis properties of MgH2.Figure 7
Arrhenius plot for the activation energy calculation hydrolysis of MgH2–C–Fe2O3 (a), MgH2–C (b), and pure MgH2 (c) in municipal drinking water.


## 4. Conclusion

In this work, MgH2–C and MgH2–C–Fe2O3 composite powders were prepared by the hydrogenation after magnetic grinding. The effects of graphite and Fe2O3 on the hydrolysis kinetics of MgH2 were studied. It was found that both MgH2–C and MgH2–C–Fe2O3 samples have better performance of hydrolysis reaction rate and hydrogen conversion rate compared with pure MgH2. Among them, MgH2–C–Fe2O3 sample has the fastest hydrolytic yield rate and the highest conversion rate. The conversion rate of 30 minutes at 353 K is 9.8% higher than that of the MgH2–C sample, while that is 42.6% higher than that of pure MgH2. It was also found that the Ea value of hydrolysis activation energy of pure MgH2 was 55.57 kJ mol-1. The addition of C and Fe2O3 could effectively reduce the Ea value of MgH2–C and MgH2–C–Fe2O3 samples to 43.40 and 36.92 kJ mol-1, respectively. The results show that the addition of graphite and Fe2O3 has an obvious synergistic effect on improving the hydrolysis kinetics and hydrogen conversion rate of MgH2.



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