Highly Dispersed MgH2 NanoparticleâGraphene Nanosheet

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Cite This: ACS Appl. Nano Mater. 2019, 2, 3828−3835

Highly Dispersed MgH2 Nanoparticle−Graphene Nanosheet Composites for Hydrogen Storage Qiuyu Zhang,†,‡ Yike Huang,‡ Li Xu,*,§ Lei Zang,‡ Huinan Guo,‡ Lifang Jiao,‡ Huatang Yuan,‡ and Yijing Wang*,‡ †

Water Affairs Research Institute, North China University of Water Resource and Electric Power, Zhengzhou 450000, China Key Laboratory of Advanced Energy Materials Chemistry (MOE), Renewable Energy Conversion and Storage Center (ReCast), College of Chemistry, Nankai University, Tianjin 300071, China § Material Laboratory of State Grid Corporation of China, Global Energy Interconnection Research Institute, Beijing 102211, China Downloaded via BUFFALO STATE on July 18, 2019 at 01:09:31 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: We report a facile solid state method to in situ synthesize highly dispersed MgH2 nanoparticle−graphene nanosheet composites and their improved dehydrogenation/hydrogenation properties. The graphene is used as the support to in situ prepare the MgH2 nanoparticles. The MgH2 nanoparticle−10 wt % graphene nanosheet composites possess the best hydrogen storage properties among Mg-based materials. The onset dehydrogenation temperature of the MgH2 nanoparticle−10 wt % graphene nanosheet composites decreases to 255 °C. More importantly, the MgH2 nanoparticle−10 wt % graphene nanosheet composites can release 5.1 wt % hydrogen in 20 min at 325 °C. Moreover, the dehydrogenated composites could absorb 5.2 wt % hydrogen in 10 min at 250 °C under the hydrogen pressure of 2 MPa. The well-dispersed MgH2 nanoparticles (∼3 nm) and the confinement effect of graphene result in the improved hydrogen storage properties. The novel solid state method of in situ synthesizing MgH2 nanoparticles provides a new horizon for designing high performance Mg-based materials. KEYWORDS: MgH2, graphene nanosheet, confinement, hydrogen storage, solid state method it is significantly important to find simple and efficient methods to prepare the nanosized Mg-based materials. Recently, the available nanostructuring operations contain melting,47 vapor deposition,48,49 mechanical milling,50−53 chemical reduction,54−59 and hydrogenation method.60−65 Among these methods, chemical reduction and hydrogenation method, which make the Mg-based nanomaterials have the controllable and uniform size, cause much attention. For instance, Norberg et al.58 reported the study regarding the hydrogen storage behaviors of Mg particles prepared by chemical reduction. The Mg particles with size of 25 nm can absorb 5.1 wt % hydrogen in 10 min at 260 °C and desorb 5.1 wt % hydrogen within 20 min at 335 °C. However, chemical reduction can only decrease the size of Mg particles to about 30 nm. Thus, though the kinetics properties of Mg particles are obviously enhanced, the thermal properties are not efficiently improved. The hydrogenation method could in situ prepare nanostructured Mg-based materials by confining the MgH2 particles within the porous carbon materials. Konarova et al.62 used the hydrogenation method to confine MgH2 nano-

1. INTRODUCTION Hydrogen, with advantages of high abundance and energy density, is a prospective energy carrier to substitute the traditional fuels.1−4 However, safe and efficient hydrogen storage becomes the technology barrier for hydrogen application. MgH2/Mg system is one of the most promising candidates for hydrogen storage due to the low cost and high hydrogen storage density (7.6 wt % in weight and 110 kg cm−3 in volume).5−7 However, the unfavorable thermodynamics of MgH 2 (ΔH = 75 kJ mol −1 H 2 ) leads to the high dehydrogenation temperature (>300 °C). Moreover, hydrogen sorption/desorption kinetics in the Mg/MgH2 system is slow below 300 °C. Many strategies are employed to improve hydrogen storage performance of MgH2/Mg system.8−40 Particularly, nanostructuring, which can simultaneously alter the thermodynamics and kinetics of MgH2/Mg system, is one of the most efficient approaches.7,31−44 Theoretically, nanostructuring Mgbased materials would lead to high active interface/surface and short H diffusion distance, which can decrease the kinetics barrier. More importantly, as the particles have a sufficiently small size, most atoms would be exposed to the surface, which is beneficial to the destabilization of Mg−H bonds.45,46 Thus, © 2019 American Chemical Society

Received: April 15, 2019 Accepted: May 17, 2019 Published: May 22, 2019 3828

DOI: 10.1021/acsanm.9b00694 ACS Appl. Nano Mater. 2019, 2, 3828−3835

Article

ACS Applied Nano Materials

composites was carried out at 325 °C under static vacuum. The hydrogenation performance for the composites was evaluated at 250 °C under 2 MPa H2 pressure. Further, the dehydrogenation performance of MgH2 nanoparticle−10 wt % graphene nanosheet composites was detected at different temperatures and hydrogen pressures. The thermal properties and cycling performance were detected by TA Instruments (DSC, Q20P).

particles into CMK3. The dehydrogenation enthalpy even reduces to 52.38 kJ mol−1 H2. Xia et al.64 employed the hydrogenation method to synthesize graphene-supported MgH2 nanoparticles. The as-prepared MgH2 nanoparticles possess small size (about 5.7 nm) and high dispersity, which results in superior thermodynamics and kinetics properties. However, the hydrogenation method should be operated under a high temperature (>150 °C) and hydrogen pressure (>3.5 MPa), leading to high cost and limited application. Obviously, it is still necessary to search for simple and efficient methods to synthesize the nanostructured Mg-based materials with controllable size and high dispersity. Herein, by use of graphene as supporting material, a simple chemical solid state method is used for in situ synthesis of MgH2 nanoparticles with controllable size and high dispersity. On the basis of high surface area and active sites, graphene is chosen as support material to in situ grow MgH2 nanoparticles.64,66 The graphene shows a confinement effect, which efficiently holds the small size and high dispersity of MgH2 nanoparticles. As a result, the MgH2 nanoparticles could maintain the high activity and superior hydrogen storage properties. More significantly, the solid state method is a bottom-up approach, leading the MgH2 nanoparticles to be in situ confined in graphene nanosheets. Meanwhile, the solid state method could be operated under mild conditions. The MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites are prepared, and the corresponding hydrogen storage performance is systematically studied. The MgH2 nanoparticle−10 wt % graphene nanosheet composites can release 5.1 wt % hydrogen in 20 min at 325 °C and absorb 5.2 wt % hydrogen in 10 min at 250 °C under 2 MPa hydrogen pressure. These results show that in situ synthesized MgH2 nanoparticle−graphene nanosheet composites exhibit superior hydrogen storage properties.

3. RESULTS AND DISCUSSION The preparation route of in situ formed MgH2 nanoparticle− graphene nanosheet composites by solid state method is shown in Scheme 1. The replacement reaction is used for the Scheme 1. Illustration of Synthesis of MgH2 Nanoparticle− Graphene Nanosheet Composites by the Solid State Method

synthesis of MgH2, as displayed in eq 1. According to thermodynamic calculation, the standard Gibbs free energy change for this reaction is −75.06 kJ mol−1. Thermodynamically, the reaction takes place at room temperature. The simple high-energy ball milling technology is employed to undertake the synthesis reaction. After ball milling, MgH2 nanoparticles are in situ synthesized in the surfaces and wrinkle of graphene. However, LiCl is also formed with the replacement reaction. MgH2 is further purified by employing THF to remove LiCl, based on the high solubility of LiCl in THF (1.14 mmol g−1).67 Finally, the MgH2 nanoparticle−graphene nanosheet composites are obtained.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All chemicals with analytical grade were used without further purification. Graphene oxide (GO) was prepared by the Hummer’s method. Graphene nanosheet is obtained by reducing GO at 800 °C under H2−Ar (5:95 v/v) flow. Typically, the MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites were synthesized by the solid state method via the technology of high energy mechanical milling. Amounts of 0.08 g of LiH, 0.48 g of MgCl2, and different weights of graphene nanosheet were mechanically milled in a steel vessel with steel balls. The mass ratio of ball to powder was 70:1. The mixtures were then ball-milled for 30 h at room temperature with 0.5 MPa H2 pressure. The rotational speed for the ball-milling was 500 rpm. After milling, the asobtained mixtures were washed with tetrahydrofuran (THF). The final MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites were obtained after drying by vacuum pumping to remove the residual THF. All of the preparation was undertaken in glovebox filled with purified argon (99.999%) to protect samples from oxidation or hydroxide reactions. 2.2. Characterization. The structural and morphological features of as-synthesized samples were characterized by powder X-ray diffraction (XRD, Rigaku Mini FlexII, Cu Kα radiation), transmission electron microscopy (TEM, JEOL JEM-2010FEF), scanning electron microscopy (SEM, JEOL JSM7500), and Fourier transform infrared spectrometry (FT-IR, FTIR-650). The samples were coated by a film to reject the oxidation or hydroxide for XRD measurement. The decomposition performance was measured by a temperatureprogrammed desorption system (TPD, PX200). The isothermal hydrogen absorption and desorption kinetics was determined by Sievert’s measurement. The dehydrogenation test of MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet

2LiH + MgCl2 → MgH2 + 2LiCl

(1)

According to the preparation process, it is obvious that the milling condition and the graphene addition affect significantly the hydrogen storage properties of MgH2 nanoparticle− graphene nanosheet composites. First, the effect of milling condition is studied. The XRD patterns of the as-milled samples at different stages are shown in Figure 1. By

Figure 1. XRD patterns of the as-milled samples for different milling times. 3829


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ACS Applied Nano Materials comparison, the standard XRD patterns of reactants including LiH and MgCl2 are also provided. As displayed in Figure 1, the main phase is MgCl2 for the 1 h milled sample. The diffraction peaks of LiH are even not detectable, which may be caused by the amorphization in the process of the high energy mechanical milling. Furthermore, it is observed that the intensity of diffraction peaks of MgCl2 decreases with the increase of milling time. As the milling time increases to 20 h, it is seen that there exist relatively weak peaks ascribing to MgH2 (Figure S1), and the diffraction peaks of MgCl2 disappear. It implies that the synthesis reaction for MgH2 is finally completed for ball-milling 20 h. In addition, the intensities of LiCl diffraction peaks are enhanced for 30 h milled sample. The enhancement may be contributed by the increase of the crystalline degree as the replacement reaction proceeds. However, with further ball-milling, the crystal of LiCl was destructed by the continuous milling and impact, which led to the reduction of diffraction intensity, as displayed for the 40 h milled sample (Figure S1). In order to further explore the effect of milling time on the hydrogen storage performance, the DSC measures of the samples milled for 20, 30, and 40 h were performed. The resultant DSC curves are displayed in Figure S2. The peak temperature of hydrogen decomposition displays the trend of the lower value with increase of milling time, from 335.5 °C for the 20 h milled sample to 327.5 °C for the 40 h milled sample. However, there is only a slight peak temperature decline for the samples milled for 30 and 40 h. Thus, 30 h is proved to be the optimal milling time for the replacement reaction. The structure and hydrogen storage performance of MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites were systematically investigated. The MgH2 nanoparticle−graphene nanosheet composites were synthesized by milling 30 h. The phases of the composites were observed by XRD, as displayed in Figure S3. The diffraction peaks in MgH2 nanoparticle−graphene nanosheet composites could be indexed with the tetragonal MgH2 by contrast with the data of JCPDS card file no. 72-1687. The calculated crystal parameters of MgH2 are a = b = 4.481 Å and c = 3.000 Å, matching well with the standard values. In addition, there is a weak diffraction peak at 45°, which indicates the formation of MgO. The existence of MgO is ascribed to the oxidation of Mg-based materials during the synthesis process. The morphology of MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites is characterized by SEM. As shown in Figure 2, MgH2 exhibits a plate-like structure with thickness of 10 nm. By comparison, with the addition of graphene, the MgH2 particles show nanoscale size and high dispersity. It is obviously found that the MgH2 nanoparticles in situ grow on the surface of graphene nanosheets. The addition of graphene is beneficial for holding the nanostructure of MgH2. It is observed that the dispersity of the MgH2 nanoparticles become better with the increase of graphene. Meanwhile, the size of MgH2 nanoparticles shows a decreasing shift (Figure 2). Those observations indicate that the MgH2 nanoparticles with small size and high dispersity are synthesized using graphene as support material. The hydrogen storage performance of MgH2 nanoparticle− graphene nanosheet composites was systematically investigated. The dehydrogenation properties of MgH2 nanoparticle− x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites were explored by TPD. As displayed in Figure 3, with the

Figure 2. SEM images of MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites: (a) x = 0, (b) x = 5, (c) x = 10, and (d) x = 15.

Figure 3. Thermally programmed dehydrogenation curves of MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites.

increasing addition of graphene, both the onset and peak hydrogen desorption temperatures obviously decrease. The MgH2 nanoparticle−15 wt % graphene nanosheet composites start to release hydrogen at 240 °C, 62 °C lower than that of the MgH2. Meanwhile, the peak temperature of MgH2 nanoparticle−15 wt % graphene nanosheet composites is 304 °C, which is reduced by 76 °C in comparison with the MgH2. The peak dehydrogenation temperatures of MgH2 nanoparticle−10 wt % graphene nanosheet composites and MgH2 nanoparticle−5 wt % graphene nanosheet composites respectively decrease by 62 and 23 °C, compared with 380 °C of the MgH2. The comparison results of onset and peak hydrogen desorption temperatures reveal that the addition of graphene appreciably enhances the hydrogen storage performance of Mg-based materials. The isothermal hydrogen desorption and absorption measurements were performed to further detect the kinetics 3830


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dehydrogenation/rehydrogenation properties among the MgH2 nanoparticle−graphene nanosheet composites. Thus, the dehydrogenation properties of MgH2 nanoparticle−10 wt % graphene nanosheet composites were further investigated. Figure S5a demonstrates the isothermal dehydrogenation behavior of MgH2 nanoparticle−10 wt % graphene nanosheet composites at different temperatures. As displayed in Figure S5a, the MgH2 nanoparticle−10 wt % graphene nanosheet composites have higher dehydrogenation amount and faster kinetics with higher operating temperature. Moreover, the MgH2 nanoparticle−10 wt % graphene nanosheet composites can still desorb 4.1 wt % hydrogen within 2 h at 275 °C. Figure S5b demonstrates the isothermal dehydrogenation behavior of MgH2 nanoparticle−10 wt % graphene nanosheet composites under different initial hydrogen pressure at 325 °C. The hydrogen desorption amount and kinetics decrease with increasing initial hydrogen pressure. However, 4.0 wt % hydrogen can still be released with 1 bar hydrogen pressure. The isothermal dehydrogenation results imply that the MgH2 nanoparticle−10 wt % graphene nanosheet composites possess fascinating dehydrogenation kinetics. In addition, the hydrogen storage performance of in situ formed nanoscaled Mg-based materials prepared by different methods is compared, and the results are listed in Table 1. It is observed that the MgH2 nanoparticle−10 wt % graphene nanosheet composites synthesized by solid state method have relatively comparable hydrogen storage properties. Additionally, the reversibility of the MgH2 nanoparticle−10 wt % graphene nanosheet composites was investigated using DSC with an initial 0.4 MPa H2 pressure, as shown in Figure 5. The endothermic and exothermic peaks are respectively ascribed as the hydrogen desorption and absorption reactions of the MgH2 nanoparticle−10 wt % graphene nanosheet composites. The DSC curves display that hydrogen absorption process begins around 310 °C (peak at 350 °C) and hydrogen desorption starts near 394 °C (peak around 410 °C). It is found that both the dehydrogenation and hydrogenation areas do not obviously diminish even after five cycles. Thus, the MgH2 nanoparticle−10 wt % graphene nanosheet composites possess good cycle stability and reversibility. In order to explore the hydrogen storage mechanism of MgH2 nanoparticle−10 wt % graphene nanosheet composites, TEM examination was used to characterize the microstructure of graphene nanosheet, MgH2 nanoparticle, and MgH2

properties of MgH2 nanoparticle−graphene nanosheet composites. The resultant curves and corresponding dehydrogenation and rehydrogenation amounts with different time are shown in Figure 4. As displayed in Figure 4, the MgH2 nanoparticle−

Figure 4. (a) Hydrogen desorption kinetic curves and (b) hydrogen absorption kinetic curves of MgH2 nanoparticle−x wt % (x = 0, 5, 10, and 15) graphene nanosheet composites.

graphene nanosheet composites possess fast hydrogen desorption and sorption rates with increasing addition of graphene nanosheet. However, the hydrogen capacity of MgH2 nanoparticle−graphene nanosheet composites exhibits a decreasing shift as the addition of graphene nanosheet increases. Considering the kinetics property and hydrogen capacity, MgH2 nanoparticle−10 wt % graphene nanosheet composites have the best hydrogen storage performance. The MgH2 nanoparticle−10 wt % graphene nanosheet composites can release 5.1 wt % hydrogen within 20 min at 325 °C. Moreover, a hydrogen desorption of 5.6 wt % is obtained after 2 h. The dehydrogenated MgH2 nanoparticle−10 wt % graphene nanosheet composites could take up 5.2 wt % hydrogen in 10 min at 250 °C under 2 MPa H2. It is observed that the MgH2 nanoparticle−10 wt % graphene nanosheet composites have the most fascinating

Table 1. Hydrogen Storage Performance of Nanoscaled Mg-Based Materials Prepared by Different Methods sample

synthesis method

capacity (wt %)

Mg6Pd/graphite Mg/Ti Mg/graphene Mg/poly(methyl methacrylate) Mg Mg Mg/carbon aerogels MgH2/carbon aerogel MgH2/CMK-3 MgH2/activated carbon fiber MgH2/graphene MgH2 nanoparticle−10 wt % graphene nanosheet composites

melting vapor deposition ball-milling chemical reduction chemical reduction chemical reduction chemical reduction hydrogenation hydrogenation hydrogenation hydrogenation solid phase method

1.7 7.2 ∼4 5.2 6.5 1.7 1.4 5.2 1.67 5.4 5.6

absorption kineticsa 1.4−10−300 2.3−10−300 5.8−10−250 2.7−10−200 4.3−10−300 5.3−10−260 0.9−10−350

desorption kineticsa

4.1−20−300 0.2−20−300 5.1−20−335

2.1−10−250 1.7−10−200 5.2−10−250

1.1−20−290 0.4−20−200 5.1−20−325

ref 47 49 51 54 55 56 60 61 62 63 64 this work

Note: x−y−z represents x wt % hydrogen released/absorbed within y min at z °C.

a

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may be caused by the growth and agglomeration in the synthesis process of using THF to purify MgH2. Due to high surface to volume ratio and short diffusion, the adjacent MgH2 nanoparticles tend to agglomerate and finally grow into the larger particles.32 After the addition of graphene nanosheet, on the basis of high surface area and active sites of graphene nanosheet, the MgH2 nanoparticles can in situ generate in the surfaces and pores of graphene nanosheet. The wrinkle structure of graphene nanosheet also protects MgH2 particles from agglomerating and growing in the synthesis process and dehydrogenation/hydrogenation cycling.64 Thus, the MgH2 nanoparticle−10 wt % graphene nanosheet composites can maintain the nanoscaled MgH2 nanoparticles. The nanoscaled MgH2 and confinement effect of graphene result in improved hydrogen storage properties. In order to further analyze the hydrogen storage mechanism of MgH 2 nanoparticle−10 wt % graphene nanosheet composites, the FT-IR spectra of commercial MgH2, MgH2, and MgH2 nanoparticle−10 wt % graphene nanosheet composites prepared by solid state method were collected. As shown in Figure 7, all of the FT-IR spectra exhibit two

Figure 5. DSC curves of the MgH2 nanoparticle−10 wt % graphene nanosheet composites with 0.4 MPa H2 pressure (heating ramp, 10 °C min−1; cooling ramp, 10 °C min−1).

nanoparticle−10 wt % graphene nanosheet composites. As shown in Figure 6a, graphene nanosheet exhibits a 2D sheet-

Figure 7. FT-IR spectra of commercial MgH2, as-prepared MgH2, and MgH2 nanoparticle−10 wt % graphene nanosheet composites.

broad peaks at 500−800 cm−1 and 800−1400 cm−1, which respectively correspond to Mg−H bending and stretching bands. It proves the existence of MgH2 in the three samples. Moreover, the bending bands of commercial MgH2, MgH2, and MgH2 nanoparticle−10 wt % graphene nanosheet composites tend to shift to low wavenumber, indicating the weakness of Mg−H bond. Owing to the weakened stability of MgH2, MgH2 nanoparticle−10 wt % graphene nanosheet composites have the improved hydrogen storage properties. Furthermore, the activation energy of hydrogen desorption for commercial MgH2, MgH2, and MgH2 nanoparticle−10 wt % graphene nanosheet composites prepared by solid state method was determined using DSC curves. Kissinger method is employed to calculate the activation energy. The DSC curves and resultant Kissinger plot are shown in Figure 8. The value of activation energy for MgH2 nanoparticle−10 wt % graphene nanosheet composites is 118.9 kJ mol−1, which is even 90 kJ mol−1 lower than that of commercial MgH2. The reduction in the desorption activation energy is contributed by the improvement in the hydrogen desorption kinetics of MgH2 nanoparticle−graphene nanosheet composites. In addition, the enthalpy change of commercial MgH2, MgH2, and MgH2 nanoparticle−10 wt % graphene nanosheet composites prepared by solid state method for dehydrogenation was extracted from the DSC curves, as displayed in Figure S6. The enthalpy change of MgH2 nanoparticle−10 wt % graphene

Figure 6. TEM images of (a, b) graphene nanosheet, (c) MgH2, (d− f) MgH2 nanoparticle−10 wt % graphene nanosheet composites.

like structure with many wrinkles. Meanwhile, about five individual graphene layers are visible in the edge of a typical graphene nanosheet (Figure 6b). It can be seen that the MgH2 prepared by the solid state method exhibits a plate-like structure formed by coalescence of nanoparticles (Figure 6c). However, after the introduction of graphene nanosheet, MgH2 nanoparticle−10 wt % graphene nanosheet composites exhibit a micronanostructure such that tetragonal MgH2 nanoparticles with the size of about 3 nm homogeneously are distributed in the graphene nanosheet (Figure 6e,d). It is found that the high resolution TEM exhibits regular stripes (Figure 6e). The lattice spacing is 0.286 nm, which corresponds to the (111) plane of MgH2. Moreover, it can be observed from Figure 6f that MgH2 nanoparticles are confined in the wrinkles or pores of graphene nanosheet. The morphology changes between and MgH2 and MgH2 nanoparticle−10 wt % graphene nanosheet composites can be discussed as follows. The plate-like structure of MgH2 3832


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Figure 8. DSC curves of (a) MgH2 nanoparticle−10 wt % graphene nanosheet composites, (b) MgH2, and (c) commercial MgH2 at different heating rates and (d) corresponding Kissinger plots.

hydrogen desorption kinetics of MgH2 nanoparticle−graphene nanosheet composites is also significantly improved. The MgH2 nanoparticle−10 wt % graphene nanosheet composites can release 5.1 wt % hydrogen in 20 min at 325 °C. Moreover, a total of 5.6 wt % hydrogen can be desorbed within 2 h. Thus, it is reasonable to conclude that our work provides a facile strategy to in situ prepare nanostructured Mg-based materials with superior hydrogen storage properties.

nanosheet composites is 1153 J/g H2, even lower 1155 J/g H2 compared with the bulk MgH2. Thus, MgH2 nanoparticle−10 wt % graphene nanosheet composites exhibit an enhanced thermodynamics property. Additionally, the phase and morphology changes of MgH2 nanoparticle−10 wt % graphene nanosheet composites after dehydrogenation, rehydrogenation, and cycling were investigated. The XRD patterns of MgH2 nanoparticle−10 wt % graphene nanosheet composites in different states are shown in Figure S7. It is observed that the diffraction peaks of Mg dominate in XRD pattern of dehydrogenated composites. Even after five cycles, MgH2 can completely desorb hydrogen to completely transform into Mg, which indicates the good reversibility of MgH2 nanoparticle−10 wt % graphene nanosheet composites. The SEM images of MgH2 nanoparticle−10 wt % graphene nanosheet composites in different states are presented in Figure S8. It is observed that the MgH2 particles could maintain uniform structure and nanoscaled size even after five cycles. The observations reveal that the addition of graphene nanosheet can efficiently restrict the growing and agglomerating of the MgH 2 /Mg particles during the dehydrogenation and hydrogenation processes. Therefore, the MgH2 nanoparticle−10 wt % graphene nanosheet composites can hold its superior hydrogen storage properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00694. XRD and DSC results of the samples milled for 20, 30, and 40 h; XRD patterns of MgH2 nanoparticle− graphene nanosheet composites; XRD pattern and SEM image of graphene nanosheet; dehydrogenation performance, XRD patterns, and SEM images of the MgH2 nanoparticle−10 wt % graphene nanosheet composites (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*L.X.: e-mail, [email protected]. *Y.W.: e-mail, [email protected].

4. CONCLUSIONS The highly dispersed MgH2 nanoparticle−graphene nanosheet composites are prepared using the facile solid state method. On the basis of the high surface area of the graphene nanosheet, the MgH2 nanoparticles are in situ formed and confined in the wrinkles and pores of the graphene nanosheet. The MgH2 nanoparticles are with small size (∼3 nm) and high dispersity. The MgH2 nanoparticle−graphene nanosheet composites exhibit improved hydrogen storage properties. The peak hydrogen desorption temperature of MgH2 nanoparticle−10 wt % graphene nanosheet composites is 318 °C, which is 62 °C lower than the pure MgH2. Meanwhile, the

ORCID

Lifang Jiao: 0000-0002-4676-997X Yijing Wang: 0000-0002-9584-5406 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the financial support by National Key R&D Program of China (Grant 2018YFB1502102), NSFC (Grants 51571124, 51571125, 51871123, 51501072), 111 Project (Grant B12015), and MOE (Grant IRT13R30). 3833


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