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Effect of air exposure on hydrogen storage properties of catalyzed magnesium hydride
Journal of Power Sources 454 (2020) 227936
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Effect of air exposure on hydrogen storage properties of catalyzed magnesium hydride Huang Liu a, b, Pei Sun b, Robert C. Bowman Jr. c, Zhigang Zak Fang b, Yong Liu a, **, Chengshang Zhou a, * a b c
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, PR China Department of Metallurgical Engineering, The University of Utah, Salt Lake City, UT, 84112-0114, United States RCB Hydrides, LLC, 117 Miami Ave., Franklin, OH, 45005, United States
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Desorption temperature of MgH2–5% TiMn2 increased with air exposure prolonged. � Short time air exposure has limited impact on performance of MgH2–5% VTiCr. � Long time air exposure deteriorates hydrogen storage capacity of MgH2–5% VTiCr. � Solvent protection during air exposure is effective for MgH2–5%TiMn2.
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium hydride Air exposure Hydrogen storage properties Kinetics Solvent protection
Air exposure of the magnesium hydride materials can lead to severe degradation on the hydrogen storage properties, and therefore is one of the key remaining challenges for its application. In this work, the effects of direct air exposure and solvent-protected air exposure for catalyzed MgH2 on the hydrogen storage properties are systematically investigated. Results show that the direct air exposure of catalyzed MgH2 leads to reduction of the hydrogen storage capacity, and moderate deterioration of the hydrogen absorption/desorption kinetics, but has limited impact on reversibility and cycle stability. A short-time air exposure (15 min) causes the decrease of hydrogen storage capacity of MgH2–5%VTiCr by 0.2 wt%, and long-time air exposure (2400 min) significantly deteriorates the hydrogen storage capacity by 1.95 wt%. During the direct exposure in ambient air, MgH2 reacts with oxygen and moisture, and the surface forms a layer consisting of Mg(OH)2 and MgO. Various solvents, hexane, acetone, and ethanol, are used to protect MgH2–5%TiMn2 material. The hydrogen storage properties of the solvent-protected materials after air exposure for 1500 min are evaluated, both the hydrogen storage capacity and kinetics are degraded after exposure. The acetone-protected MgH2–5%TiMn2 shows better kinetics and capacities compare to those protected by hexane and ethanol.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Liu), [email protected] (C. Zhou). https://doi.org/10.1016/j.jpowsour.2020.227936 Received 16 October 2019; Received in revised form 18 February 2020; Accepted 20 February 2020 Available online 27 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
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Journal of Power Sources 454 (2020) 227936
1. Introduction
Table 1 Solvent information.
Magnesium based hydride has attracted continuing attention as a promising material for hydrogen storage and thermal energy storage owing to its high hydrogen storage capacity (7.76 wt%), low cost and good reversibility [1–6]. However, pure MgH2 is not able to meet the requirements for practical application, due to its sluggish reaction ki netics and high decomposition temperature [7–9]. Researches of past decades have shown that the hydrogen storage properties of MgH2 can be effectively improved by nano-structuring and catalytic doping using transition metals [10–12], metal oxides [13–18], and intermetallic compounds [19–21]. A variety of physical or chemical methods including ball-milling [22–24], thin-film deposition [25,26], combus tion synthesis [27,28] have been employed to synthesize Mg-based nanocomposites. Air exposure is an important and practical problem to metal hydride materials because it commonly deteriorates the hydrogen storage properties [29–32]. Oxidation of Mg/MgH2 is highly exothermic, which can cause heat accumulation. There is also a chance of self-ignition when material is inappropriately handled. Oxidation and hydration often raise serious concerns, especially for nanostructured Mg-based hydride with high surface areas. Given the importance mentioned above, air exposure and its effects of nanosized Mg-based hydride desires comprehensive investigation. It has been widely recognized that highly reactive surfaces facilitate hydrogen reaction kinetics, but these surfaces also increase detrimental reactions when exposed to oxygen or H2O. An oxidized layer on the surface of Mg/MgH2 hinders the penetration of hydrogen atom [33]. Friedrichs et al. found that air exposure of nanocrystalline MgH2 resulted in formation of a surface layer composing of oxide and amor phous hydroxide [34]. Dobrovolsky [35] reported that different surface states were obtained when MgH2 films hydrogenated under different hydrogen pressures were exposed to air. For example, the MgH2 film hydrogenated under a high hydrogen pressure (11.5 MPa) tended to contain higher concentration of Mg(OH)2 compared to those obtained under low hydrogen pressures. Bouaricha et al. [36] discovered that oxygen exposure permanently reduced the hydrogen storage capacity of MgH2-5%V, but did not affect its cycling kinetics. Dehouche [37] found that H2O contamination during prolonged cycling tests facilitated hydrogen absorption kinetics but deteriorated the hydrogen desorption kinetics of nanostructured MgH2–V–Ti composite. In addition, previous efforts have been made to prevent air exposure by using various approaches, such as using polymer protection [38], carbon nanoconfinement [39] and wet milling method [40–45]. Ball milling Mg-based hydrides in organic solvent, such as tetrahydrofuran [40,41],ethanol [42,43], toluene [44] and benzene [42,45] has been proposed. The solvent addition can prevent the material from severe oxidation by ambient air. Therefore, the use of solvents as milling media has been demonstrated as an effective method. However, the effect of solvent protection on air-resistance of catalyzed MgH2 has rarely been studied in detail. The present research is dedicated to understanding the effect of air exposure on hydrogen storage properties of catalyzed MgH2. Our pre vious investigations have demonstrated that VTiCr and TiMn2 catalyzed MgH2 systems own good properties in terms of hydrogen storage ca pacity, kinetics and cyclic stability [20,46,47]. Therefore, the MgH2–5% VTiCr and MgH2–5%TiMn2 were selected and synthesized for this work. The composition, surface state and hydrogen storage properties of catalyzed MgH2 after exposed to air for different time were investigated. In order to study the effect of solvent protection on air resistance of catalyzed MgH2, three common organic solvents, hexane, acetone and ethanol, were employed to investigate their relative effectiveness to provide protection for catalyzed MgH2. The kinetics of hydrogen ab sorption and desorption of solvent-protected MgH2–5%TiMn2 after air exposure were characterized. The desorption and absorption reactions of these systems were analyzed by using the Johnson Mehl Avrami (JMA) model [20,24,48].
Solvent
Acetone
Hexane
Ethanol
Purity Water Supplier Catalog No.
�99.5% �0.5% Fisher Scientific A18-500
�98.5% �0.5% Fisher Scientific H292-500
�99.8% �0.2% Fisher Scientific A4094
2. Experimental section 2.1. Materials and reagents MgH2 (>98%, Sigma-Aldrich, 683043), TiMn2 (Sigma-Aldrich, 685941) were used as received without further purification. The 75V–5Ti–20Cr alloy was prepared via the same procedure as our pre vious reports [46,47]. The information of solvents (hexane, acetone and ethanol) were given in Table 1. Two catalyzed MgH2 samples were prepared by ball milling MgH2 with 5 M percent TiMn2 or VTiCr using a custom-made ultrahigh-energy-high-pressure (UHEHP) planetary ball-milling machine under a 100-bar hydrogen pressure. The ball-to-powder ratio was 20:1 by volume and the milling was carried out for 4 h at room temperature. All materials handling was carried out in an argon-filled glove box except the air exposure experiments. The glove box is filled with purified argon (99.999%), with water vapor and ox ygen concentrations both at less than 1 ppm. The air exposure of MgH2–5%VTiCr was carried out by placing the ~0.3g powder sample on a plate on a laboratory bench that was spread out to ensure maximum exposure. The subsequent weight gain of the sample was recorded by using a standard analytical balance (OHAUS CP214). MgH2–5%TiMn2 with a mass of ~0.2g, and solvents (hexane, acetone or ethanol) with volumes of ~ 5 ml, were mixed in the glove box, and then exposed to air for 1500 min. After that, the powdersolvent mixtures were dried by active vacuum pumping for 10 h to remove the solvents. The solvent-free samples were then stored in the glove box for characterization. 2.2. Characterization The isothermal hydrogenation, dehydrogenation, and cycle stability tests were performed by using a commercial Sieverts-type apparatus (Hy-Energy LLC., PCTPro-2000) that have been used in our previous studies [8,20]. About 0.1 g of powder sample was loaded into a stainless-steel vessel and sealed in the PCT sample holder. Before tests, each sample was vacuum tested to ensure that there is neither leak nor residual solvent existed in the powders. During the experiments, the sample temperature was held constant by a PID controller (Watlow, PID controller SD). The amount of hydrogen release or uptake was calculated based on the changes of pressures in calibrated volumes. The dehydro genation experiments were conducted by the using a thermogravimetric analyzer (TGA, Shimadzu TGA50) in which ~10 mg samples were heated under 100 mL/min flowing argon up to 450 � C at heating rates of 5, 10, 15 and 20 � C/min. The crystallographic properties of MgH2–5% VTiCr and MgH2–5%TiMn2 with different air exposure time were characterized by powder X-ray diffraction (XRD, D500, Siemens). Morphological and crystallographic properties of MgH2 after expose to ambient air for 15 min were investigated with transmission electron microscopy (TEM, JEOL JEM-2100F). The surface of samples was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-ALPHA). The XPS data was analyzed by the peak fitting software XPS Peak 4.1. The calculation of Gibbs free energy was carried out by using thermodynamic databases and the software HSC Chemistry 6.0.
2
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Fig. 1. (a) The weight gain for a MgH2-5% VTiCr sample with the air exposure time. (b) XRD patterns of MgH2–5%VTiCr after air exposure for 5, 300, 1500 and 3000 min, respectively. (c) XRD patterns of MgH2–5%TiMn2 after air exposure for 5, 300, 1500 and 3000 min, respectively.
Fig. 2. (a) TGA curves of MgH2–5%TiMn2 after air exposure for 0 min, 10 min, 200 min, 500 min and 1500 min, respectively. (b) Corresponding DSC curves of MgH2–5%TiMn2 after air exposure for 10 min, 200 min, 500 min and 1500 min, respectively. (c) TGA curves of MgH2–5%TiMn2 and MgH2–5%VTiCr after air exposure for 10 min and 500 min, respectively. (d) TGA curves for the MgH2–5%TiMn2 after air exposure for 10 min under various heating rates. (e) The activation energy (Ea) for the dehydrogenation of MgH2–5%TiMn2 after air exposure for 0 min, 10 min, 200 min, 500 min and 1500 min, respectively. (f) The activation energies (Ea) of MgH2-5% TiMn2 with the air exposure time.
3. Results
However, there is no obvious peaks of Mg(OH)2 were observed via XRD, this probably due to the poor crystallinity or amorphous structure of the surface contaminants [34]. The presence of Mg(OH)2 phase will be discussed later.
3.1. Behavior of direct air exposure The 5 mol% VTiCr doped MgH2 after high-energy ball milling was removed from the glove box and exposed to ambient air. During the air exposure, the black color of MgH2–5%VTiCr powder showed no obvious change. As summarized in Fig. 1 (a), the weight of the MgH2–5%VTiCr increased during the air exposure, and the weight gain presented approximate linear relation with the air exposure time. After air expo sure for 2880 and 4320 min, the weight gain of the sample reached 10.67% and 15.64%, respectively. The XRD patterns of MgH2–5%VTiCr and MgH2–5%TiMn2 that were recorded after air exposure for 5, 300, 1500 and 3000 min, respectively. As shown in Fig. 1(b) and (c), only MgH2 and small amount of MgO phases were detected by XRD. It is found that even after 3000 min air exposure, the XRD signal of MgH2 was almost unchanged. It also can be seen that the peak of MgO slightly increased after air exposure for longer time. The MgH2–5%VTiCr and MgH2–5%TiMn2 samples with 5 min air exposure also exhibited the existence of MgO phase, suggesting the oxidation may occur during a short period of time. It is also noted that synthesis and/or storage process may still lead to contamination.
3.2. TGA/DSC analysis Fig. 2 (a) compared the TGA curves of different MgH2–5%TiMn2 samples after air exposure for 0 min, 10 min, 200 min, 500 min and 1500 min, respectively. As expected, the MgH2–5%TiMn2 sample without air exposure exhibited the best dehydrogenation behavior, showing an on-set dehydrogenation temperature of about 170 � C. It is noted that even a short-time air exposure (10 min) caused a significant increase of decomposition temperature of MgH2–5%TiMn2. Comparing the MgH2–5%TiMn2 sample without air exposure and the sample exposed to ambient air for 10 min, the on-set decomposition tempera ture of increased from 170 � C to 250 � C. As seen in Fig. 2 (b), the endothermic peaks of dehydrogenation of air-exposed MgH2–5%TiMn2 samples shifted to higher temperature, showing that the on-set decom position temperature increased progressively as the air exposure pro longed. Therefore, the air exposure of MgH2–5%TiMn2 led to a deterioration of dehydrogenation behavior. A similar trend was found 3
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Fig. 3. (a) Hydrogen absorption and desorption cycle tests conducted at 270 � C of MgH2–5%VTiCr with prior air exposure for 1000 min. (b) Hydrogen absorption and desorption cycle tests at 300 � C of MgH2–5%VTiCr with prior air exposures for 15 min and 2400 min.
with the MgH2–5%VTiCr sample, as shown in Fig. 2 (c). Further, we observed that the TGA curves of the air-exposed MgH2–5%TiMn2 and MgH2–5%VTiCr almost coincided. This behavior suggests that the air exposure impacts the two catalyzed system to the same extent, pre sumably due to similar surface structures that are independent of the catalyst. In addition, it can be seen that the weight losses of the samples did not decrease, but instead showed a slight increase as the air exposure prolonged. For instance, 0 min, 10 min, 200 min, 500 min and 1500 min air-exposed samples exhibited the 5.42%, 5.60%, 5.72%, 5.72% and 5.81% of weight losses, respectively. This is because of the formation of Mg(OH)2, which would decompose into MgO and release the heavier H2O in addition to H2 during heating, as the following equation. Mg(OH)2 ¼ MgO þ H2O
activation energies results for the dehydrogenation of MgH2–5%TiMn2 after air exposure for 10, 200, 500 and 1500 min. The results showed that the activation energy (Ea) of the 10 min air-exposed MgH2–5% TiMn2 is 72.06 kJ/mol, which is nearly the same as MgH2–5%TiMn2 without air exposure (74.22 kJ/mol), according to our previous study [20]. This result indicated that short time (10 min) air exposure has little impact on the dehydrogenation kinetics of MgH2–5%TiMn2. It is also noted that the activation energy (Ea) for the 10 min air-exposed MgH2–5%TiMn2 (72.06 kJ/mol) is much lower than the 200 min (142.80 kJ/mol), 500 min (147.62 kJ/mol) and 1500 min (276.12 kJ/mol) air exposed MgH2–5%TiMn2. As summarized in Fig. 2 (f), the activation energies of MgH2–5%TiMn2 increase with the increase of air exposure time. It suggests that long time air exposure result in the degradation of the catalytic effects of MgH2–5%TiMn2.
(1)
Consequently, the weight losses of the air-exposed samples corre spond to the total amount of both H2 and H2O being released from the samples concurrently. Activation energies of dehydrogenation were also calculated to evaluate the effect of direct air exposure on the kinetics of MgH2–5% TiMn2. The TGA profiles of 10 min air-exposed MgH2–5%TiMn2 under various heating rates were shown in Fig. 2 (d). It can be seen the TGA profiles of 10 min air exposed MgH2–5%TiMn2 move to a higher tem perature with increasing heating rate from 5 to 20 � C/min, as expected. By using the Ozawa–Flynn–Wall method [49–51], the activation en ergies of desorption of air-exposed MgH2–5%TiMn2 can be calculated from the data of non-isothermal TGA results. The method is based on the following rate equation: dα ¼ Af ðαÞexpð dt
Ea Þ RT
3.3. Effect of direct air exposure on cycling stability In order to examine the cycle stability of air-exposed MgH2–5% VTiCr, cycle tests of hydrogen absorption and desorption were per formed, as shown in Fig. 3. Fig. 3(a) provides the cycling curves (20 cycles) at 270 � C of the MgH2–5%VTiCr sample air-exposed for 1000 min. This result showed that the MgH2–5%VTiCr maintained a good cycle stability even after 1000 min air exposure. In the first dehydro genation process, the air-exposed MgH2–5%VTiCr showed 2.34 wt% gas releasing, which should be attributed to decomposition of both MgH2 and Mg(OH)2. The curves of 1st-5th cycles of hydrogen absorption/ desorption exhibited unstable behavior. In contrast, in the 6th-20th cycles, the kinetics and capacities showed more reproducible stability at a reversible hydrogen capacity approximately 1.5 wt%. It is noted that the absorption and desorption capacities of MgH2–5%VTiCr exhibit slightly recovery, which is known as the “activation process” [52,53]. Furthermore, Fig. 3(b) compared the 6th-10th cyclic curves at 300 � C for the MgH2–5%VTiCr samples air-exposed for 15 min and 2400 min. It is seen that the hydrogen storage capacities of the samples air-exposed for 15 min or 2400 min are approximately 5 wt% and 2 wt%, respectively. This result indicates that long-time air exposure significantly deterio rated hydrogen storage capacity of the material. However, the air-exposed MgH2–5%VTiCr can still maintain a good stability. This result implies that ambient air exposure had only limited impact on reversibility and cycle stability of MgH2–5%VTiCr.
(2)
Where α is the fractional conversion, t is the reaction time, A is the preexponential factor of the Arrhenius equation, f(α) is a kinetic function that is related to the reaction mechanism and R is the gas constant. Integration of Eq. (2) and the subsequent approximation under the condition with constant heating rate (T ¼ T0 þ βt; β ¼ the heating rate; T0 ¼ the starting temperature) result in the following equation: Z a 0:457Ea R dα logβ ¼ 2:315 logð Þ (3) AEa 0 f ðαÞ RT Based on Eq. (3), the activation energy (Ea) can be calculated from the slope of a plot of logβ vs. 1/T at a given value of α. The activation energy is then evaluated using the heating rates (β ¼ 5, 10, 15, and 20 � C/min) and a fractional conversion (α ¼ 0.4) from the TGA profiles by plotting logβ vs. 1000/T. Fig. 2 (e) presents the
3.4. Effect of direct air exposure on kinetics Hydrogenation and dehydrogenation kinetic tests of MgH2–5%VTiCr with and without air exposure were performed at 25 � C under 1 bar 4
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Fig. 4. (a) Isothermal hydrogenation and (b) dehy drogenation curves of as-milled MgH2–5%VTiCr, and MgH2–5%VTiCr with air exposure for 15 and 2400 min, respectively. (c)The first 25 min of hydrogena tion and (d) the first 1.5 min of dehydrogenation curves of as-milled MgH2–5%VTiCr, and MgH2–5% VTiCr with air exposure for 15 and 2400 min, respectively. (e) Isothermal hydrogenation and (f) dehydrogenation curves of as-milled MgH2 and MgH2 with air exposure for 15 and 2400 min, respectively.
pressure, and 300 � C under 0.1 bar pressure, respectively. The kinetic tests were conducted after the samples were hydrogen cycled at 300 � C for 3 times. Fig. 4(a) and (b) compared the isothermal hydrogen ab sorption and desorption curves of MgH2–5%VTiCr before and after air exposure for 15 min and 2400 min. Similar to the cycling results, the decreases of hydrogen storage capacity were observed in the air-exposed samples. For example, at 25 � C, the as-milled sample absorbed about 3.17 wt% of hydrogen in 200 min, but the 2400 min air-exposed samples only absorbed 1.22 wt% of hydrogen during the same time. However, a short time air exposure (15 min) only resulted in slight degradation regarding the hydrogen absorption and desorption kinetics. Fig. 4(c) and (d) depict the initial stages of kinetic curves of MgH2–5%VTiCr. We can
see that the initial stages of hydrogen absorption and desorption curves of the three samples almost coincide. These observations indicate that air exposure resulted in a decrease in hydrogen storage capacity of MgH2–5%VTiCr, but it only slightly impacted the kinetics of hydrogen absorption and desorption. Furthermore, the hydrogenation and dehydrogenation kinetic tests of pure MgH2 with and without air exposure were first performed at 350 � C under 20 bar and 0 bar pressure after ball milling, respectively. The results showed that the air exposure lead to the reduction of hydrogen storage capacity of pure MgH2, which is similar to MgH2–5%VTiCr. However, it is noted that air exposure for pure MgH2 can deteriorate its hydrogen absorption and desorption kinetics. For example, at 350 � C,
Fig. 5. (a) 25 � C, (b) 60 � C and (c) 100 � C isothermal hydrogenation curves of as-milled MgH2–5%TiMn2, and as-milled MgH2–5%TiMn2 after expose to air for 1500 min with hexane, acetone and ethanol protection, respectively. 5
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Fig. 6. (a) 300 � C, (b) 270 � C and (c) 240 � C isothermal dehydrogenation curves of as-milled MgH2–5%TiMn2, and as-milled MgH2–5%TiMn2 after expose to air for 1500 min with hexane, acetone and ethanol protection, respectively.
the as-milled MgH2 released 5.2 wt% of hydrogen within 3 min, but 15 min and 2400 min air-exposed MgH2 only desorbed 4.13 wt% and 2.72 wt% of hydrogen during the same time, respectively. It should be noted that the actual hydrogen desorption capacity of air-exposed samples mentioned above might be lower due to the decomposition of Mg(OH)2 in the first dehydrogenation process. Comparing the samples of MgH2–5%VTiCr and pure MgH2 after same duration of air exposure, it is obvious that the VTiCr catalyzed MgH2 exhibited better hydrogenation and dehydrogenation kinetics.
protected samples. The acetone-protected sample also exhibited the fastest dehydrogenation kinetics, even better than that of as-milled MgH2–5%TiMn2. This indicates that surface contaminants generated by air exposure can somehow lead to kinetic improvement [37]. It must be pointed out that the solvent protection by using hexane, acetone and ethanol result in significant difference in kinetic behaviors. Since the dose of solvents is very small (~5 ml), the kinetic degradation would not merely due to the presence of a trace amount of impurities including H2O. Nevertheless, oxygen dissolved in the solvent should be the main reason leading to the oxidation. The mechanisms in solvent-protected air exposure and direct air exposure should be different, which will be discussed in the discussion section.
3.5. Solvent-protected air exposure Three solvents, hexane, acetone and ethanol, were added to the asmilled MgH2–5%TiMn2, and the hydride-solvent mixtures were then air-exposed for 1500 min. After the solvent-protected air exposure and dried by the vacuum, hydrogenation kinetics of the samples were characterized under hydrogen of 1 bar at 25 � C, 60 � C and 100 � C, respectively, as shown in Fig. 5. As expected, the as-milled MgH2–5% TiMn2 exhibited better hydrogenation properties than all solventprotected samples after air exposure. For example, as-milled MgH2–5%TiMn2 absorbed 3.54 wt% of hydrogen at 25 � C within 600 min, while the hexane-, acetone- and ethanol-protected samples only absorbed 1.50 wt%, 2.58 wt% and 1.72 wt% of hydrogen within 600 min, respectively. Fig. 6 compared the dehydrogenation kinetics of as-milled and solvent-protected MgH2–5%TiMn2 samples. Fig. 6 (a), (b) and (c) pro vide the hydrogen desorption curves at 300 � C, 270 � C and 240 � C, respectively. Similar with hydrogenation kinetics, the solvent-protected samples exhibited reduced hydrogen storage capacities compared to asmilled sample. Comparing the samples prepared by solvent-protected and direct air exposure, it is concluded that using solvent protection can preserve hydrogen storage capacity in some extent indicating the oxidation of MgH2 was retarded in the presence of the organic solvents. The acetone-protected samples showed a higher hydrogen storage capacity and better kinetics than those of the hexane- and ethanol-
3.6. Surface analysis TEM analysis of MgH2–5%TiMn2 after air exposure for 15 min was conducted and the surface microstructure is shown in Fig. 7 (a). It should be pointed out that hexane was used to disperse the MgH2–5%TiMn2 powder for the purpose of depositing the hydride particles onto the copper (Cu) grid. After dispersion, hexane was removed by air drying and then the sample was inevitably exposed to ambient air for a short time (few minutes) before TEM analysis. Undoubtfully, this process does lead to finite contamination on the surface of the particle. From Fig. 7 (a), a surface layer with a thickness of few nanometers can be found. The surface consists of nanocrystalline of MgO and Mg(OH)2 with sizes around 3–5 nm. XPS analysis of MgH2–5%TiMn2 after air exposure for 15 min and 1500 min was carried out, and the XPS spectra are shown in Fig. 7(b) and (c). The spectra show peaks of OH and O2 , indicating the for mation of Mg(OH)2 and MgO during the air exposure of MgH2–5% TiMn2. Comparing the relative peak intensities of the air-exposed sam ples, it can be seen that the OH peak intensity increased and the O2 peak weakened as air exposure extended. This suggests that the nano crystalline MgO gradually transformed into Mg(OH)2 during air exposure.
Fig. 7. (a) The TEM image of MgH2–5%TiMn2 after expose to ambient air for 15 min. The XPS spectra of MgH2–5%TiMn2 expose to ambient air for 15 min (b) and 1500 min (c), respectively. 6
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Table 2 Thermodynamic calculations of MgH2 for environmental air exposure. No.
Reactions
(4) (5) (6)
MgH2 þ O2(g) ¼ Mg(OH)2 MgH2 þ O2 (g) ¼ MgO þ H2O(g) MgH2þ2H2O(g) ¼ Mg(OH)2þ2H2(g)
Table 3 Corresponding reaction rate constant k and Avrami exponent n from the best-fit of kinetic data of the MgH2–5%VTiCr samples with different air exposure time.
ΔG (kJ/mole) at 298 K
Samples (MgH2–5%VTiCr)
797.32 761.59 340.16
25 C (hydrogenation) 300 � C (dehydrogenation) �
As-milled
Air exposure for 15 min
Air exposure for 2400 min
n
n
n
0.67 1.74
ln(k) 5.10 7.08
0.85 1.56
ln(k) 6.72 6.30
0.80 1.31
ln(k) 6.26 5.77
significant reduction of hydrogen storage capacity. It is noted that both the Mg(OH)2 and MgH2 would decompose when the air-exposed sample was heated (equation (1) and (8)). The decom position of MgH2 is given below: MgH2 ¼ Mg þ H2
(8)
It has been reported the decomposition of Mg(OH)2 took place around 350 � C [54]. As a result, the surface of the dehydrogenated sample should be mainly MgO, as shown in Fig. 8. Because of the exis tence of Mg(OH)2, the first-cycle weight losses of samples exposed to air for different time were similar to that of fresh sample (Fig. 2). This is also corresponded with PCT cycling result (Fig. 3), where the air-exposed catalyzed MgH2 samples showed a larger gas release in the first desorption reaction comparing to those of the rest cycles. 4.2. Kinetic analysis In order to further understand the kinetic behavior of the catalyzed MgH2 with different air exposure time, Johnson-Mehl-Avrami (JMA) model was applied to fit the experimental hydrogenation and dehy drogenation data. The kinetic equation of JMA model is expressed as: lnð Fig. 8. Schematic illustrations of reactions in surface during air exposure.
ξÞÞ ¼ ln k þ n ln t
(9)
ξ is the reaction fraction, t is the reaction time, k stands for reaction rate constant, and n is Avrami exponent. It was found that the hydro genation and dehydrogenation kinetics of the as-milled and aix exposed MgH2–5%VTiCr can be fitted using JMA model. The slope and intercept of the fitted straight lines stands for n and lnk, which is marked in Fig. S1. Besides the fitting using JMA model, Jander diffusion model, GinslingBraunshteinn model, 3D contracting model were employed, but unsat isfactory fitting results were obtained. These results suggest that the kinetics behavior of oxidized materials was controlled by nucleationgrowth mechanism [24]. Furthermore, we observed some of the JMA fits (Figs. S1 and S2) slightly deviate from a linear relation. This could be attributed to non-isothermal effect [20,24] and/or inhomogeneous passivation of the powder samples. The values of the reaction rate constant k from the best-fits of the kinetic data are listed in Table 3. The data shows that the as-milled sample has the fastest hydrogenation rate, the air exposure results in decrease of hydrogenation rate. The dehydrogenation k of the long-time exposed sample exhibits a slight increase. These results suggest that the addition of catalyst could effectively mitigate the kinetic degradation during the air exposure. Similar observations can be found in several other investigations [30,31,36,55–57]. However, it should not be ignored that the catalysts may also be oxidized during air exposure, which could lead to the kinetic degrada tion. As mentioned before, the kinetics of MgH2 without catalyst decreased more significantly than those of catalyzed MgH2. The calcu lation of reaction rate constant k also show that air exposure has limited impact on kinetics of catalyzed MgH2. As the air exposure prolonged, some of the catalysts might be oxidized, meanwhile the remaining cat alysts still plays the role. This means that the kinetic degradation can be mitigated as long as the catalyst are homogeneously dispersed. It has been demonstrated in our previous studies [8,58] that most of the size of the catalysts are few nanometers and homogeneously dispersed in MgH2
4. Discussion 4.1. Reactions during air exposure According to the calculation by using thermodynamic databases and the software HSC Chemistry, the Gibbs free energies (ΔG) of possible reactions during air exposure are listed in Table 2. As the mentioned equation (4) in Table 2, the oxidation of MgH2 has the lowest Gibbs free energy ( 797.32 kJ/mol at 298 K). This means that MgH2 is likely to react with O2 to produce Mg(OH)2 when exposed to air. It is also noted that the ΔG of MgH2 with O2 produce MgO and H2O 761.59 is kJ/mol, which is very close to the ΔG of oxidation of MgH2 (equation (4)). It suggests that the reaction (5) may also occur with thermodynamic feasibility, which corresponds the existence of MgO form XRD and XPS result. Moreover, the weight gain of the catalyzed MgH2 cannot exclude the reactions of MgH2 with H2O in air and thus formation of Mg(OH)2, as given by equation (6) in Table 2. Fig. 8 illustrates the air exposure induced surface microstructure evolution of catalyzed MgH2. In the initial stage of air exposure, the catalyzed MgH2 reacts with the O2 and H2O, which produced the contaminated surface that composed of Mg(OH)2 and a small amount of MgO, as demonstrated by the TEM and XPS results (Fig. 7). As the air exposure prolonged, the MgO of the surface continuously absorbed H2O and transformed to Mg(OH)2, as the following equation: MgO þ H2O → Mg(OH)2
lnð1
(7)
In the meantime, the surface layer will continue to grow. After long time air exposure, the composition of surface layer become a large fraction of Mg(OH)2 and a small fraction of MgO. The progressive air exposure caused continuously consumption of MgH2, resulting in a 7
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Journal of Power Sources 454 (2020) 227936
Table 4 Corresponding reaction rate constant k and Avrami exponent n of the MgH2–5%TiMn2 samples with different solvent protection after air exposure for 1500 min. Samples (MgH2–5%TiMn2)
As-milled
25 � C hydrogenation 60 � C hydrogenation 100 � C hydrogenation 300 � C dehydrogenation 270 � C dehydrogenation 240 � C dehydrogenation
0.36 0.41 0.29 1.33 1.17 1.35
n
Hexane protected ln(k) 3.25 2.89 1.63 6.11 6.18 8.77
n
Acetone protected
ln(k)
0.91 0.67 0.67 1.23 1.31 1.09
9.71 6.25 5.56 6.42 7.76 8.18
particles. In order to compare absorption and desorption kinetics of the 1500 min air-exposed MgH2–5%TiMn2 protected by different solvents, the JMA model was applied to fit the experimental hydrogenation and dehydrogenation data. As shown in Fig. S2, the experimental data can be fit by straight lines. According to Table 4, the value of the hydrogenation k of as-milled sample is higher than those of the solvent-protected samples with air exposure for 1500 min. It suggests that solventprotected air exposure result in decrease of the hydrogenation rate. Interestingly, the dehydrogenation kinetics after solvent-protected air exposure show different kinetic behavior. The value of the dehy drogenation k of acetone-protected sample was higher than hexane and ethanol protected samples, and even slightly higher than that for the asmilled MgH2–5%TiMn2. In addition, an attempt to derive reaction activation energies was conducted using lnk versus 1/T. Unfortuantely, relations of lnk versus 1/ T obtained from most of the solvent-protected samples (Table 4) are found to be non-linear, making the calculation of activation energy impractical. It should be noted that the passivated surface, in contrary to the catalytic additive, imposes a negative effect by adding an extra ki netic barrier to the reaction [59]. We suspect that the kinetic barrier may be temperature-dependent due to changes of the surface coverage and thickness of passivation surface as temperature increasing. Therefore, it is not possible to derive the activation energy using Arrhenius equation under these conditions.
n 0.68 0.55 0.31 0.96 1.01 1.26
Ethanol protected
ln(k) 6.28 4.49 2.35 4.25 5.19 7.90
n 0.57 0.63 0.70 1.21 1.52 1.02
ln(k) 6.76 6.04 5.72 5.96 9.15 7.89
5. Conclusions In summary, the effect of air exposure and solvent-protected air exposure on hydrogen storage behaviors of TiMn2 and VTiCr catalyzed MgH2 were studied, and conclusions are made as follows: (1) Exposure the catalyzed MgH2 in air resulted in gradual weight gain. During the exposure, layer consisting of Mg (OH)2 and MgO formed on the particles and grew on and into the surface of the catalyzed MgH2. (2) The decomposition temperature of the MgH2–5%TiMn2 increased as the air exposure prolonged. The prolonged air exposure also resulted in the increased of activation energy of MgH2–5%TiMn2. Both MgH2 and Mg(OH)2 decomposed more or less concurrently when the air exposed samples were heated. (3) Short time air exposure has limited impact on the kinetics and capacity of MgH2–5%VTiCr. As the air exposure prolonged, the hydrogen storage capacity decreased significantly. However, the reversibility and cycle stability were not significantly affected by the air exposure. (4) Solvent protection using hexane, acetone and ethanol are effec tive to preserve the hydrogen storage properties of MgH2–5% TiMn2 during air exposure, although moderate decreases of hydrogen storage capacities and kinetics were observed. In particular, the acetone-protected MgH2–5%TiMn2 shows better hydrogen storage properties compared to those protected by hexane and ethanol.
4.3. Effects of MgO and Mg(OH)2
Declaration of competing interest
From the present kinetics analyses, it was inferred that the MgO contamination has only moderate or limited negative effect on the ki netics of these catalyzed MgH2 samples. In some cases, the presence of surface MgO could promote the dehydrogenation kinetics. This is not surprising, given the fact that Ares-Fern� andez et al. [60] and Aguey-Zinsou et al. [61] has been reported that the addition of MgO can improve the kinetic of MgH2. The co-milling of MgO and MgH2 resulted in decrease of the hydrogen desorption temperature from 336 � C to 262 � C [61]. According to our TGA observation, the formation of Mg(OH)2 acts as passivation surface that hinders the penetration of H atom. Nevertheless, the kinetics of following cycles recovered, which may attribute to two aspects. First, the decomposition of Mg(OH)2 might cause surface cracking (as illustrated in Fig. 8), which allows hydrogen to readily penetrate the surface. Second, the catalyst particles existed in the MgO layer may still provide contribution to the catalysis. The formation of MgO and Mg(OH)2 caused decrease of hydrogen capacity, which observed in both direct air-exposed and solventprotected air-exposed samples. However, difference between the direct air exposure and solvent-protected air exposure is obvious. The former condition leaves the samples surface regions directly expose to gaseous oxygen, the latter limits reaction to attack by dissolved oxygen in the solvent. Moreover, effect of solvents themselves may change the surface state of catalyzed MgH2. However, such mechanisms were not assessed during these experiments and will need further investigations.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Huang Liu: Conceptualization, Formal analysis, Investigation, Writing - original draft. Pei Sun: Writing - review & editing, Resources. Robert C. Bowman: Writing - review & editing. Zhigang Zak Fang: Writing - review & editing. Yong Liu: Writing - review & editing, Su pervision. Chengshang Zhou: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Acknowledgements Chengshang Zhou acknowledges the financial support from the Na tional Natural Science Funds for Young Scientists of China (Grant No. 51704336) and Hunan Provincial Natural Science Foundation of China (Grant No. 2018JJ3653). The authors acknowledge the financial and facility support of the Powder Metallurgy Laboratory in the Department of Metallurgical Engineering of the University of Utah.
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Appendix A. Supplementary data
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Effect of air exposure on hydrogen storage properties of catalyzed magnesium hydride Huang Liu a, b, Pei Sun b, Robert C. Bowman Jr. c, Zhigang Zak Fang b, Yong Liu a, **, Chengshang Zhou a, * a b c
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan, 410083, PR China Department of Metallurgical Engineering, The University of Utah, Salt Lake City, UT, 84112-0114, United States RCB Hydrides, LLC, 117 Miami Ave., Franklin, OH, 45005, United States
H I G H L I G H T S
G R A P H I C A L A B S T R A C T
� Desorption temperature of MgH2–5% TiMn2 increased with air exposure prolonged. � Short time air exposure has limited impact on performance of MgH2–5% VTiCr. � Long time air exposure deteriorates hydrogen storage capacity of MgH2–5% VTiCr. � Solvent protection during air exposure is effective for MgH2–5%TiMn2.
A R T I C L E I N F O
A B S T R A C T
Keywords: Magnesium hydride Air exposure Hydrogen storage properties Kinetics Solvent protection
Air exposure of the magnesium hydride materials can lead to severe degradation on the hydrogen storage properties, and therefore is one of the key remaining challenges for its application. In this work, the effects of direct air exposure and solvent-protected air exposure for catalyzed MgH2 on the hydrogen storage properties are systematically investigated. Results show that the direct air exposure of catalyzed MgH2 leads to reduction of the hydrogen storage capacity, and moderate deterioration of the hydrogen absorption/desorption kinetics, but has limited impact on reversibility and cycle stability. A short-time air exposure (15 min) causes the decrease of hydrogen storage capacity of MgH2–5%VTiCr by 0.2 wt%, and long-time air exposure (2400 min) significantly deteriorates the hydrogen storage capacity by 1.95 wt%. During the direct exposure in ambient air, MgH2 reacts with oxygen and moisture, and the surface forms a layer consisting of Mg(OH)2 and MgO. Various solvents, hexane, acetone, and ethanol, are used to protect MgH2–5%TiMn2 material. The hydrogen storage properties of the solvent-protected materials after air exposure for 1500 min are evaluated, both the hydrogen storage capacity and kinetics are degraded after exposure. The acetone-protected MgH2–5%TiMn2 shows better kinetics and capacities compare to those protected by hexane and ethanol.
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Liu), [email protected] (C. Zhou). https://doi.org/10.1016/j.jpowsour.2020.227936 Received 16 October 2019; Received in revised form 18 February 2020; Accepted 20 February 2020 Available online 27 February 2020 0378-7753/© 2020 Elsevier B.V. All rights reserved.
H. Liu et al.
Journal of Power Sources 454 (2020) 227936
1. Introduction
Table 1 Solvent information.
Magnesium based hydride has attracted continuing attention as a promising material for hydrogen storage and thermal energy storage owing to its high hydrogen storage capacity (7.76 wt%), low cost and good reversibility [1–6]. However, pure MgH2 is not able to meet the requirements for practical application, due to its sluggish reaction ki netics and high decomposition temperature [7–9]. Researches of past decades have shown that the hydrogen storage properties of MgH2 can be effectively improved by nano-structuring and catalytic doping using transition metals [10–12], metal oxides [13–18], and intermetallic compounds [19–21]. A variety of physical or chemical methods including ball-milling [22–24], thin-film deposition [25,26], combus tion synthesis [27,28] have been employed to synthesize Mg-based nanocomposites. Air exposure is an important and practical problem to metal hydride materials because it commonly deteriorates the hydrogen storage properties [29–32]. Oxidation of Mg/MgH2 is highly exothermic, which can cause heat accumulation. There is also a chance of self-ignition when material is inappropriately handled. Oxidation and hydration often raise serious concerns, especially for nanostructured Mg-based hydride with high surface areas. Given the importance mentioned above, air exposure and its effects of nanosized Mg-based hydride desires comprehensive investigation. It has been widely recognized that highly reactive surfaces facilitate hydrogen reaction kinetics, but these surfaces also increase detrimental reactions when exposed to oxygen or H2O. An oxidized layer on the surface of Mg/MgH2 hinders the penetration of hydrogen atom [33]. Friedrichs et al. found that air exposure of nanocrystalline MgH2 resulted in formation of a surface layer composing of oxide and amor phous hydroxide [34]. Dobrovolsky [35] reported that different surface states were obtained when MgH2 films hydrogenated under different hydrogen pressures were exposed to air. For example, the MgH2 film hydrogenated under a high hydrogen pressure (11.5 MPa) tended to contain higher concentration of Mg(OH)2 compared to those obtained under low hydrogen pressures. Bouaricha et al. [36] discovered that oxygen exposure permanently reduced the hydrogen storage capacity of MgH2-5%V, but did not affect its cycling kinetics. Dehouche [37] found that H2O contamination during prolonged cycling tests facilitated hydrogen absorption kinetics but deteriorated the hydrogen desorption kinetics of nanostructured MgH2–V–Ti composite. In addition, previous efforts have been made to prevent air exposure by using various approaches, such as using polymer protection [38], carbon nanoconfinement [39] and wet milling method [40–45]. Ball milling Mg-based hydrides in organic solvent, such as tetrahydrofuran [40,41],ethanol [42,43], toluene [44] and benzene [42,45] has been proposed. The solvent addition can prevent the material from severe oxidation by ambient air. Therefore, the use of solvents as milling media has been demonstrated as an effective method. However, the effect of solvent protection on air-resistance of catalyzed MgH2 has rarely been studied in detail. The present research is dedicated to understanding the effect of air exposure on hydrogen storage properties of catalyzed MgH2. Our pre vious investigations have demonstrated that VTiCr and TiMn2 catalyzed MgH2 systems own good properties in terms of hydrogen storage ca pacity, kinetics and cyclic stability [20,46,47]. Therefore, the MgH2–5% VTiCr and MgH2–5%TiMn2 were selected and synthesized for this work. The composition, surface state and hydrogen storage properties of catalyzed MgH2 after exposed to air for different time were investigated. In order to study the effect of solvent protection on air resistance of catalyzed MgH2, three common organic solvents, hexane, acetone and ethanol, were employed to investigate their relative effectiveness to provide protection for catalyzed MgH2. The kinetics of hydrogen ab sorption and desorption of solvent-protected MgH2–5%TiMn2 after air exposure were characterized. The desorption and absorption reactions of these systems were analyzed by using the Johnson Mehl Avrami (JMA) model [20,24,48].
Solvent
Acetone
Hexane
Ethanol
Purity Water Supplier Catalog No.
�99.5% �0.5% Fisher Scientific A18-500
�98.5% �0.5% Fisher Scientific H292-500
�99.8% �0.2% Fisher Scientific A4094
2. Experimental section 2.1. Materials and reagents MgH2 (>98%, Sigma-Aldrich, 683043), TiMn2 (Sigma-Aldrich, 685941) were used as received without further purification. The 75V–5Ti–20Cr alloy was prepared via the same procedure as our pre vious reports [46,47]. The information of solvents (hexane, acetone and ethanol) were given in Table 1. Two catalyzed MgH2 samples were prepared by ball milling MgH2 with 5 M percent TiMn2 or VTiCr using a custom-made ultrahigh-energy-high-pressure (UHEHP) planetary ball-milling machine under a 100-bar hydrogen pressure. The ball-to-powder ratio was 20:1 by volume and the milling was carried out for 4 h at room temperature. All materials handling was carried out in an argon-filled glove box except the air exposure experiments. The glove box is filled with purified argon (99.999%), with water vapor and ox ygen concentrations both at less than 1 ppm. The air exposure of MgH2–5%VTiCr was carried out by placing the ~0.3g powder sample on a plate on a laboratory bench that was spread out to ensure maximum exposure. The subsequent weight gain of the sample was recorded by using a standard analytical balance (OHAUS CP214). MgH2–5%TiMn2 with a mass of ~0.2g, and solvents (hexane, acetone or ethanol) with volumes of ~ 5 ml, were mixed in the glove box, and then exposed to air for 1500 min. After that, the powdersolvent mixtures were dried by active vacuum pumping for 10 h to remove the solvents. The solvent-free samples were then stored in the glove box for characterization. 2.2. Characterization The isothermal hydrogenation, dehydrogenation, and cycle stability tests were performed by using a commercial Sieverts-type apparatus (Hy-Energy LLC., PCTPro-2000) that have been used in our previous studies [8,20]. About 0.1 g of powder sample was loaded into a stainless-steel vessel and sealed in the PCT sample holder. Before tests, each sample was vacuum tested to ensure that there is neither leak nor residual solvent existed in the powders. During the experiments, the sample temperature was held constant by a PID controller (Watlow, PID controller SD). The amount of hydrogen release or uptake was calculated based on the changes of pressures in calibrated volumes. The dehydro genation experiments were conducted by the using a thermogravimetric analyzer (TGA, Shimadzu TGA50) in which ~10 mg samples were heated under 100 mL/min flowing argon up to 450 � C at heating rates of 5, 10, 15 and 20 � C/min. The crystallographic properties of MgH2–5% VTiCr and MgH2–5%TiMn2 with different air exposure time were characterized by powder X-ray diffraction (XRD, D500, Siemens). Morphological and crystallographic properties of MgH2 after expose to ambient air for 15 min were investigated with transmission electron microscopy (TEM, JEOL JEM-2100F). The surface of samples was analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-ALPHA). The XPS data was analyzed by the peak fitting software XPS Peak 4.1. The calculation of Gibbs free energy was carried out by using thermodynamic databases and the software HSC Chemistry 6.0.
2
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Fig. 1. (a) The weight gain for a MgH2-5% VTiCr sample with the air exposure time. (b) XRD patterns of MgH2–5%VTiCr after air exposure for 5, 300, 1500 and 3000 min, respectively. (c) XRD patterns of MgH2–5%TiMn2 after air exposure for 5, 300, 1500 and 3000 min, respectively.
Fig. 2. (a) TGA curves of MgH2–5%TiMn2 after air exposure for 0 min, 10 min, 200 min, 500 min and 1500 min, respectively. (b) Corresponding DSC curves of MgH2–5%TiMn2 after air exposure for 10 min, 200 min, 500 min and 1500 min, respectively. (c) TGA curves of MgH2–5%TiMn2 and MgH2–5%VTiCr after air exposure for 10 min and 500 min, respectively. (d) TGA curves for the MgH2–5%TiMn2 after air exposure for 10 min under various heating rates. (e) The activation energy (Ea) for the dehydrogenation of MgH2–5%TiMn2 after air exposure for 0 min, 10 min, 200 min, 500 min and 1500 min, respectively. (f) The activation energies (Ea) of MgH2-5% TiMn2 with the air exposure time.
3. Results
However, there is no obvious peaks of Mg(OH)2 were observed via XRD, this probably due to the poor crystallinity or amorphous structure of the surface contaminants [34]. The presence of Mg(OH)2 phase will be discussed later.
3.1. Behavior of direct air exposure The 5 mol% VTiCr doped MgH2 after high-energy ball milling was removed from the glove box and exposed to ambient air. During the air exposure, the black color of MgH2–5%VTiCr powder showed no obvious change. As summarized in Fig. 1 (a), the weight of the MgH2–5%VTiCr increased during the air exposure, and the weight gain presented approximate linear relation with the air exposure time. After air expo sure for 2880 and 4320 min, the weight gain of the sample reached 10.67% and 15.64%, respectively. The XRD patterns of MgH2–5%VTiCr and MgH2–5%TiMn2 that were recorded after air exposure for 5, 300, 1500 and 3000 min, respectively. As shown in Fig. 1(b) and (c), only MgH2 and small amount of MgO phases were detected by XRD. It is found that even after 3000 min air exposure, the XRD signal of MgH2 was almost unchanged. It also can be seen that the peak of MgO slightly increased after air exposure for longer time. The MgH2–5%VTiCr and MgH2–5%TiMn2 samples with 5 min air exposure also exhibited the existence of MgO phase, suggesting the oxidation may occur during a short period of time. It is also noted that synthesis and/or storage process may still lead to contamination.
3.2. TGA/DSC analysis Fig. 2 (a) compared the TGA curves of different MgH2–5%TiMn2 samples after air exposure for 0 min, 10 min, 200 min, 500 min and 1500 min, respectively. As expected, the MgH2–5%TiMn2 sample without air exposure exhibited the best dehydrogenation behavior, showing an on-set dehydrogenation temperature of about 170 � C. It is noted that even a short-time air exposure (10 min) caused a significant increase of decomposition temperature of MgH2–5%TiMn2. Comparing the MgH2–5%TiMn2 sample without air exposure and the sample exposed to ambient air for 10 min, the on-set decomposition tempera ture of increased from 170 � C to 250 � C. As seen in Fig. 2 (b), the endothermic peaks of dehydrogenation of air-exposed MgH2–5%TiMn2 samples shifted to higher temperature, showing that the on-set decom position temperature increased progressively as the air exposure pro longed. Therefore, the air exposure of MgH2–5%TiMn2 led to a deterioration of dehydrogenation behavior. A similar trend was found 3
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Fig. 3. (a) Hydrogen absorption and desorption cycle tests conducted at 270 � C of MgH2–5%VTiCr with prior air exposure for 1000 min. (b) Hydrogen absorption and desorption cycle tests at 300 � C of MgH2–5%VTiCr with prior air exposures for 15 min and 2400 min.
with the MgH2–5%VTiCr sample, as shown in Fig. 2 (c). Further, we observed that the TGA curves of the air-exposed MgH2–5%TiMn2 and MgH2–5%VTiCr almost coincided. This behavior suggests that the air exposure impacts the two catalyzed system to the same extent, pre sumably due to similar surface structures that are independent of the catalyst. In addition, it can be seen that the weight losses of the samples did not decrease, but instead showed a slight increase as the air exposure prolonged. For instance, 0 min, 10 min, 200 min, 500 min and 1500 min air-exposed samples exhibited the 5.42%, 5.60%, 5.72%, 5.72% and 5.81% of weight losses, respectively. This is because of the formation of Mg(OH)2, which would decompose into MgO and release the heavier H2O in addition to H2 during heating, as the following equation. Mg(OH)2 ¼ MgO þ H2O
activation energies results for the dehydrogenation of MgH2–5%TiMn2 after air exposure for 10, 200, 500 and 1500 min. The results showed that the activation energy (Ea) of the 10 min air-exposed MgH2–5% TiMn2 is 72.06 kJ/mol, which is nearly the same as MgH2–5%TiMn2 without air exposure (74.22 kJ/mol), according to our previous study [20]. This result indicated that short time (10 min) air exposure has little impact on the dehydrogenation kinetics of MgH2–5%TiMn2. It is also noted that the activation energy (Ea) for the 10 min air-exposed MgH2–5%TiMn2 (72.06 kJ/mol) is much lower than the 200 min (142.80 kJ/mol), 500 min (147.62 kJ/mol) and 1500 min (276.12 kJ/mol) air exposed MgH2–5%TiMn2. As summarized in Fig. 2 (f), the activation energies of MgH2–5%TiMn2 increase with the increase of air exposure time. It suggests that long time air exposure result in the degradation of the catalytic effects of MgH2–5%TiMn2.
(1)
Consequently, the weight losses of the air-exposed samples corre spond to the total amount of both H2 and H2O being released from the samples concurrently. Activation energies of dehydrogenation were also calculated to evaluate the effect of direct air exposure on the kinetics of MgH2–5% TiMn2. The TGA profiles of 10 min air-exposed MgH2–5%TiMn2 under various heating rates were shown in Fig. 2 (d). It can be seen the TGA profiles of 10 min air exposed MgH2–5%TiMn2 move to a higher tem perature with increasing heating rate from 5 to 20 � C/min, as expected. By using the Ozawa–Flynn–Wall method [49–51], the activation en ergies of desorption of air-exposed MgH2–5%TiMn2 can be calculated from the data of non-isothermal TGA results. The method is based on the following rate equation: dα ¼ Af ðαÞexpð dt
Ea Þ RT
3.3. Effect of direct air exposure on cycling stability In order to examine the cycle stability of air-exposed MgH2–5% VTiCr, cycle tests of hydrogen absorption and desorption were per formed, as shown in Fig. 3. Fig. 3(a) provides the cycling curves (20 cycles) at 270 � C of the MgH2–5%VTiCr sample air-exposed for 1000 min. This result showed that the MgH2–5%VTiCr maintained a good cycle stability even after 1000 min air exposure. In the first dehydro genation process, the air-exposed MgH2–5%VTiCr showed 2.34 wt% gas releasing, which should be attributed to decomposition of both MgH2 and Mg(OH)2. The curves of 1st-5th cycles of hydrogen absorption/ desorption exhibited unstable behavior. In contrast, in the 6th-20th cycles, the kinetics and capacities showed more reproducible stability at a reversible hydrogen capacity approximately 1.5 wt%. It is noted that the absorption and desorption capacities of MgH2–5%VTiCr exhibit slightly recovery, which is known as the “activation process” [52,53]. Furthermore, Fig. 3(b) compared the 6th-10th cyclic curves at 300 � C for the MgH2–5%VTiCr samples air-exposed for 15 min and 2400 min. It is seen that the hydrogen storage capacities of the samples air-exposed for 15 min or 2400 min are approximately 5 wt% and 2 wt%, respectively. This result indicates that long-time air exposure significantly deterio rated hydrogen storage capacity of the material. However, the air-exposed MgH2–5%VTiCr can still maintain a good stability. This result implies that ambient air exposure had only limited impact on reversibility and cycle stability of MgH2–5%VTiCr.
(2)
Where α is the fractional conversion, t is the reaction time, A is the preexponential factor of the Arrhenius equation, f(α) is a kinetic function that is related to the reaction mechanism and R is the gas constant. Integration of Eq. (2) and the subsequent approximation under the condition with constant heating rate (T ¼ T0 þ βt; β ¼ the heating rate; T0 ¼ the starting temperature) result in the following equation: Z a 0:457Ea R dα logβ ¼ 2:315 logð Þ (3) AEa 0 f ðαÞ RT Based on Eq. (3), the activation energy (Ea) can be calculated from the slope of a plot of logβ vs. 1/T at a given value of α. The activation energy is then evaluated using the heating rates (β ¼ 5, 10, 15, and 20 � C/min) and a fractional conversion (α ¼ 0.4) from the TGA profiles by plotting logβ vs. 1000/T. Fig. 2 (e) presents the
3.4. Effect of direct air exposure on kinetics Hydrogenation and dehydrogenation kinetic tests of MgH2–5%VTiCr with and without air exposure were performed at 25 � C under 1 bar 4
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Fig. 4. (a) Isothermal hydrogenation and (b) dehy drogenation curves of as-milled MgH2–5%VTiCr, and MgH2–5%VTiCr with air exposure for 15 and 2400 min, respectively. (c)The first 25 min of hydrogena tion and (d) the first 1.5 min of dehydrogenation curves of as-milled MgH2–5%VTiCr, and MgH2–5% VTiCr with air exposure for 15 and 2400 min, respectively. (e) Isothermal hydrogenation and (f) dehydrogenation curves of as-milled MgH2 and MgH2 with air exposure for 15 and 2400 min, respectively.
pressure, and 300 � C under 0.1 bar pressure, respectively. The kinetic tests were conducted after the samples were hydrogen cycled at 300 � C for 3 times. Fig. 4(a) and (b) compared the isothermal hydrogen ab sorption and desorption curves of MgH2–5%VTiCr before and after air exposure for 15 min and 2400 min. Similar to the cycling results, the decreases of hydrogen storage capacity were observed in the air-exposed samples. For example, at 25 � C, the as-milled sample absorbed about 3.17 wt% of hydrogen in 200 min, but the 2400 min air-exposed samples only absorbed 1.22 wt% of hydrogen during the same time. However, a short time air exposure (15 min) only resulted in slight degradation regarding the hydrogen absorption and desorption kinetics. Fig. 4(c) and (d) depict the initial stages of kinetic curves of MgH2–5%VTiCr. We can
see that the initial stages of hydrogen absorption and desorption curves of the three samples almost coincide. These observations indicate that air exposure resulted in a decrease in hydrogen storage capacity of MgH2–5%VTiCr, but it only slightly impacted the kinetics of hydrogen absorption and desorption. Furthermore, the hydrogenation and dehydrogenation kinetic tests of pure MgH2 with and without air exposure were first performed at 350 � C under 20 bar and 0 bar pressure after ball milling, respectively. The results showed that the air exposure lead to the reduction of hydrogen storage capacity of pure MgH2, which is similar to MgH2–5%VTiCr. However, it is noted that air exposure for pure MgH2 can deteriorate its hydrogen absorption and desorption kinetics. For example, at 350 � C,
Fig. 5. (a) 25 � C, (b) 60 � C and (c) 100 � C isothermal hydrogenation curves of as-milled MgH2–5%TiMn2, and as-milled MgH2–5%TiMn2 after expose to air for 1500 min with hexane, acetone and ethanol protection, respectively. 5
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Fig. 6. (a) 300 � C, (b) 270 � C and (c) 240 � C isothermal dehydrogenation curves of as-milled MgH2–5%TiMn2, and as-milled MgH2–5%TiMn2 after expose to air for 1500 min with hexane, acetone and ethanol protection, respectively.
the as-milled MgH2 released 5.2 wt% of hydrogen within 3 min, but 15 min and 2400 min air-exposed MgH2 only desorbed 4.13 wt% and 2.72 wt% of hydrogen during the same time, respectively. It should be noted that the actual hydrogen desorption capacity of air-exposed samples mentioned above might be lower due to the decomposition of Mg(OH)2 in the first dehydrogenation process. Comparing the samples of MgH2–5%VTiCr and pure MgH2 after same duration of air exposure, it is obvious that the VTiCr catalyzed MgH2 exhibited better hydrogenation and dehydrogenation kinetics.
protected samples. The acetone-protected sample also exhibited the fastest dehydrogenation kinetics, even better than that of as-milled MgH2–5%TiMn2. This indicates that surface contaminants generated by air exposure can somehow lead to kinetic improvement [37]. It must be pointed out that the solvent protection by using hexane, acetone and ethanol result in significant difference in kinetic behaviors. Since the dose of solvents is very small (~5 ml), the kinetic degradation would not merely due to the presence of a trace amount of impurities including H2O. Nevertheless, oxygen dissolved in the solvent should be the main reason leading to the oxidation. The mechanisms in solvent-protected air exposure and direct air exposure should be different, which will be discussed in the discussion section.
3.5. Solvent-protected air exposure Three solvents, hexane, acetone and ethanol, were added to the asmilled MgH2–5%TiMn2, and the hydride-solvent mixtures were then air-exposed for 1500 min. After the solvent-protected air exposure and dried by the vacuum, hydrogenation kinetics of the samples were characterized under hydrogen of 1 bar at 25 � C, 60 � C and 100 � C, respectively, as shown in Fig. 5. As expected, the as-milled MgH2–5% TiMn2 exhibited better hydrogenation properties than all solventprotected samples after air exposure. For example, as-milled MgH2–5%TiMn2 absorbed 3.54 wt% of hydrogen at 25 � C within 600 min, while the hexane-, acetone- and ethanol-protected samples only absorbed 1.50 wt%, 2.58 wt% and 1.72 wt% of hydrogen within 600 min, respectively. Fig. 6 compared the dehydrogenation kinetics of as-milled and solvent-protected MgH2–5%TiMn2 samples. Fig. 6 (a), (b) and (c) pro vide the hydrogen desorption curves at 300 � C, 270 � C and 240 � C, respectively. Similar with hydrogenation kinetics, the solvent-protected samples exhibited reduced hydrogen storage capacities compared to asmilled sample. Comparing the samples prepared by solvent-protected and direct air exposure, it is concluded that using solvent protection can preserve hydrogen storage capacity in some extent indicating the oxidation of MgH2 was retarded in the presence of the organic solvents. The acetone-protected samples showed a higher hydrogen storage capacity and better kinetics than those of the hexane- and ethanol-
3.6. Surface analysis TEM analysis of MgH2–5%TiMn2 after air exposure for 15 min was conducted and the surface microstructure is shown in Fig. 7 (a). It should be pointed out that hexane was used to disperse the MgH2–5%TiMn2 powder for the purpose of depositing the hydride particles onto the copper (Cu) grid. After dispersion, hexane was removed by air drying and then the sample was inevitably exposed to ambient air for a short time (few minutes) before TEM analysis. Undoubtfully, this process does lead to finite contamination on the surface of the particle. From Fig. 7 (a), a surface layer with a thickness of few nanometers can be found. The surface consists of nanocrystalline of MgO and Mg(OH)2 with sizes around 3–5 nm. XPS analysis of MgH2–5%TiMn2 after air exposure for 15 min and 1500 min was carried out, and the XPS spectra are shown in Fig. 7(b) and (c). The spectra show peaks of OH and O2 , indicating the for mation of Mg(OH)2 and MgO during the air exposure of MgH2–5% TiMn2. Comparing the relative peak intensities of the air-exposed sam ples, it can be seen that the OH peak intensity increased and the O2 peak weakened as air exposure extended. This suggests that the nano crystalline MgO gradually transformed into Mg(OH)2 during air exposure.
Fig. 7. (a) The TEM image of MgH2–5%TiMn2 after expose to ambient air for 15 min. The XPS spectra of MgH2–5%TiMn2 expose to ambient air for 15 min (b) and 1500 min (c), respectively. 6
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Table 2 Thermodynamic calculations of MgH2 for environmental air exposure. No.
Reactions
(4) (5) (6)
MgH2 þ O2(g) ¼ Mg(OH)2 MgH2 þ O2 (g) ¼ MgO þ H2O(g) MgH2þ2H2O(g) ¼ Mg(OH)2þ2H2(g)
Table 3 Corresponding reaction rate constant k and Avrami exponent n from the best-fit of kinetic data of the MgH2–5%VTiCr samples with different air exposure time.
ΔG (kJ/mole) at 298 K
Samples (MgH2–5%VTiCr)
797.32 761.59 340.16
25 C (hydrogenation) 300 � C (dehydrogenation) �
As-milled
Air exposure for 15 min
Air exposure for 2400 min
n
n
n
0.67 1.74
ln(k) 5.10 7.08
0.85 1.56
ln(k) 6.72 6.30
0.80 1.31
ln(k) 6.26 5.77
significant reduction of hydrogen storage capacity. It is noted that both the Mg(OH)2 and MgH2 would decompose when the air-exposed sample was heated (equation (1) and (8)). The decom position of MgH2 is given below: MgH2 ¼ Mg þ H2
(8)
It has been reported the decomposition of Mg(OH)2 took place around 350 � C [54]. As a result, the surface of the dehydrogenated sample should be mainly MgO, as shown in Fig. 8. Because of the exis tence of Mg(OH)2, the first-cycle weight losses of samples exposed to air for different time were similar to that of fresh sample (Fig. 2). This is also corresponded with PCT cycling result (Fig. 3), where the air-exposed catalyzed MgH2 samples showed a larger gas release in the first desorption reaction comparing to those of the rest cycles. 4.2. Kinetic analysis In order to further understand the kinetic behavior of the catalyzed MgH2 with different air exposure time, Johnson-Mehl-Avrami (JMA) model was applied to fit the experimental hydrogenation and dehy drogenation data. The kinetic equation of JMA model is expressed as: lnð Fig. 8. Schematic illustrations of reactions in surface during air exposure.
ξÞÞ ¼ ln k þ n ln t
(9)
ξ is the reaction fraction, t is the reaction time, k stands for reaction rate constant, and n is Avrami exponent. It was found that the hydro genation and dehydrogenation kinetics of the as-milled and aix exposed MgH2–5%VTiCr can be fitted using JMA model. The slope and intercept of the fitted straight lines stands for n and lnk, which is marked in Fig. S1. Besides the fitting using JMA model, Jander diffusion model, GinslingBraunshteinn model, 3D contracting model were employed, but unsat isfactory fitting results were obtained. These results suggest that the kinetics behavior of oxidized materials was controlled by nucleationgrowth mechanism [24]. Furthermore, we observed some of the JMA fits (Figs. S1 and S2) slightly deviate from a linear relation. This could be attributed to non-isothermal effect [20,24] and/or inhomogeneous passivation of the powder samples. The values of the reaction rate constant k from the best-fits of the kinetic data are listed in Table 3. The data shows that the as-milled sample has the fastest hydrogenation rate, the air exposure results in decrease of hydrogenation rate. The dehydrogenation k of the long-time exposed sample exhibits a slight increase. These results suggest that the addition of catalyst could effectively mitigate the kinetic degradation during the air exposure. Similar observations can be found in several other investigations [30,31,36,55–57]. However, it should not be ignored that the catalysts may also be oxidized during air exposure, which could lead to the kinetic degrada tion. As mentioned before, the kinetics of MgH2 without catalyst decreased more significantly than those of catalyzed MgH2. The calcu lation of reaction rate constant k also show that air exposure has limited impact on kinetics of catalyzed MgH2. As the air exposure prolonged, some of the catalysts might be oxidized, meanwhile the remaining cat alysts still plays the role. This means that the kinetic degradation can be mitigated as long as the catalyst are homogeneously dispersed. It has been demonstrated in our previous studies [8,58] that most of the size of the catalysts are few nanometers and homogeneously dispersed in MgH2
4. Discussion 4.1. Reactions during air exposure According to the calculation by using thermodynamic databases and the software HSC Chemistry, the Gibbs free energies (ΔG) of possible reactions during air exposure are listed in Table 2. As the mentioned equation (4) in Table 2, the oxidation of MgH2 has the lowest Gibbs free energy ( 797.32 kJ/mol at 298 K). This means that MgH2 is likely to react with O2 to produce Mg(OH)2 when exposed to air. It is also noted that the ΔG of MgH2 with O2 produce MgO and H2O 761.59 is kJ/mol, which is very close to the ΔG of oxidation of MgH2 (equation (4)). It suggests that the reaction (5) may also occur with thermodynamic feasibility, which corresponds the existence of MgO form XRD and XPS result. Moreover, the weight gain of the catalyzed MgH2 cannot exclude the reactions of MgH2 with H2O in air and thus formation of Mg(OH)2, as given by equation (6) in Table 2. Fig. 8 illustrates the air exposure induced surface microstructure evolution of catalyzed MgH2. In the initial stage of air exposure, the catalyzed MgH2 reacts with the O2 and H2O, which produced the contaminated surface that composed of Mg(OH)2 and a small amount of MgO, as demonstrated by the TEM and XPS results (Fig. 7). As the air exposure prolonged, the MgO of the surface continuously absorbed H2O and transformed to Mg(OH)2, as the following equation: MgO þ H2O → Mg(OH)2
lnð1
(7)
In the meantime, the surface layer will continue to grow. After long time air exposure, the composition of surface layer become a large fraction of Mg(OH)2 and a small fraction of MgO. The progressive air exposure caused continuously consumption of MgH2, resulting in a 7
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Table 4 Corresponding reaction rate constant k and Avrami exponent n of the MgH2–5%TiMn2 samples with different solvent protection after air exposure for 1500 min. Samples (MgH2–5%TiMn2)
As-milled
25 � C hydrogenation 60 � C hydrogenation 100 � C hydrogenation 300 � C dehydrogenation 270 � C dehydrogenation 240 � C dehydrogenation
0.36 0.41 0.29 1.33 1.17 1.35
n
Hexane protected ln(k) 3.25 2.89 1.63 6.11 6.18 8.77
n
Acetone protected
ln(k)
0.91 0.67 0.67 1.23 1.31 1.09
9.71 6.25 5.56 6.42 7.76 8.18
particles. In order to compare absorption and desorption kinetics of the 1500 min air-exposed MgH2–5%TiMn2 protected by different solvents, the JMA model was applied to fit the experimental hydrogenation and dehydrogenation data. As shown in Fig. S2, the experimental data can be fit by straight lines. According to Table 4, the value of the hydrogenation k of as-milled sample is higher than those of the solvent-protected samples with air exposure for 1500 min. It suggests that solventprotected air exposure result in decrease of the hydrogenation rate. Interestingly, the dehydrogenation kinetics after solvent-protected air exposure show different kinetic behavior. The value of the dehy drogenation k of acetone-protected sample was higher than hexane and ethanol protected samples, and even slightly higher than that for the asmilled MgH2–5%TiMn2. In addition, an attempt to derive reaction activation energies was conducted using lnk versus 1/T. Unfortuantely, relations of lnk versus 1/ T obtained from most of the solvent-protected samples (Table 4) are found to be non-linear, making the calculation of activation energy impractical. It should be noted that the passivated surface, in contrary to the catalytic additive, imposes a negative effect by adding an extra ki netic barrier to the reaction [59]. We suspect that the kinetic barrier may be temperature-dependent due to changes of the surface coverage and thickness of passivation surface as temperature increasing. Therefore, it is not possible to derive the activation energy using Arrhenius equation under these conditions.
n 0.68 0.55 0.31 0.96 1.01 1.26
Ethanol protected
ln(k) 6.28 4.49 2.35 4.25 5.19 7.90
n 0.57 0.63 0.70 1.21 1.52 1.02
ln(k) 6.76 6.04 5.72 5.96 9.15 7.89
5. Conclusions In summary, the effect of air exposure and solvent-protected air exposure on hydrogen storage behaviors of TiMn2 and VTiCr catalyzed MgH2 were studied, and conclusions are made as follows: (1) Exposure the catalyzed MgH2 in air resulted in gradual weight gain. During the exposure, layer consisting of Mg (OH)2 and MgO formed on the particles and grew on and into the surface of the catalyzed MgH2. (2) The decomposition temperature of the MgH2–5%TiMn2 increased as the air exposure prolonged. The prolonged air exposure also resulted in the increased of activation energy of MgH2–5%TiMn2. Both MgH2 and Mg(OH)2 decomposed more or less concurrently when the air exposed samples were heated. (3) Short time air exposure has limited impact on the kinetics and capacity of MgH2–5%VTiCr. As the air exposure prolonged, the hydrogen storage capacity decreased significantly. However, the reversibility and cycle stability were not significantly affected by the air exposure. (4) Solvent protection using hexane, acetone and ethanol are effec tive to preserve the hydrogen storage properties of MgH2–5% TiMn2 during air exposure, although moderate decreases of hydrogen storage capacities and kinetics were observed. In particular, the acetone-protected MgH2–5%TiMn2 shows better hydrogen storage properties compared to those protected by hexane and ethanol.
4.3. Effects of MgO and Mg(OH)2
Declaration of competing interest
From the present kinetics analyses, it was inferred that the MgO contamination has only moderate or limited negative effect on the ki netics of these catalyzed MgH2 samples. In some cases, the presence of surface MgO could promote the dehydrogenation kinetics. This is not surprising, given the fact that Ares-Fern� andez et al. [60] and Aguey-Zinsou et al. [61] has been reported that the addition of MgO can improve the kinetic of MgH2. The co-milling of MgO and MgH2 resulted in decrease of the hydrogen desorption temperature from 336 � C to 262 � C [61]. According to our TGA observation, the formation of Mg(OH)2 acts as passivation surface that hinders the penetration of H atom. Nevertheless, the kinetics of following cycles recovered, which may attribute to two aspects. First, the decomposition of Mg(OH)2 might cause surface cracking (as illustrated in Fig. 8), which allows hydrogen to readily penetrate the surface. Second, the catalyst particles existed in the MgO layer may still provide contribution to the catalysis. The formation of MgO and Mg(OH)2 caused decrease of hydrogen capacity, which observed in both direct air-exposed and solventprotected air-exposed samples. However, difference between the direct air exposure and solvent-protected air exposure is obvious. The former condition leaves the samples surface regions directly expose to gaseous oxygen, the latter limits reaction to attack by dissolved oxygen in the solvent. Moreover, effect of solvents themselves may change the surface state of catalyzed MgH2. However, such mechanisms were not assessed during these experiments and will need further investigations.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Huang Liu: Conceptualization, Formal analysis, Investigation, Writing - original draft. Pei Sun: Writing - review & editing, Resources. Robert C. Bowman: Writing - review & editing. Zhigang Zak Fang: Writing - review & editing. Yong Liu: Writing - review & editing, Su pervision. Chengshang Zhou: Conceptualization, Writing - review & editing, Supervision, Funding acquisition. Acknowledgements Chengshang Zhou acknowledges the financial support from the Na tional Natural Science Funds for Young Scientists of China (Grant No. 51704336) and Hunan Provincial Natural Science Foundation of China (Grant No. 2018JJ3653). The authors acknowledge the financial and facility support of the Powder Metallurgy Laboratory in the Department of Metallurgical Engineering of the University of Utah.
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Appendix A. Supplementary data
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