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Facile synthesis of small MgH2 nanoparticles confined in different carbon materials for hydrogen storage
Journal Pre-proof Facile synthesis of small MgH2 nanoparticles confined in different carbon materials for hydrogen storage Qiuyu Zhang, Yike Huang, Tiancai Ma, Ke Li, Fei Ye, Xuechao Wang, Lifang Jiao, Huatang Yuan, Yijing Wang PII:
S0925-8388(20)30316-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.153953
Reference:
JALCOM 153953
To appear in:
Journal of Alloys and Compounds
Received Date: 1 October 2019 Revised Date:
14 January 2020
Accepted Date: 20 January 2020
Please cite this article as: Q. Zhang, Y. Huang, T. Ma, K. Li, F. Ye, X. Wang, L. Jiao, H. Yuan, Y. Wang, Facile synthesis of small MgH2 nanoparticles confined in different carbon materials for hydrogen storage, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153953. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit author statement Qiuyu Zhang: Conceptualization, Writing - Original Draft, Visualization Yike Huang: Validation, Software, Investigation Tiancai Ma: Methodology Ke Li: Formal analysis, Data Curation Fei Ye: Validation Xuechao Wang: Writing - Review & Editing Lifang Jiao: Project administration Huatang Yuan: Supervision Yijing Wang: Validation, Resources
Facile synthesis of small MgH2 nanoparticles confined in different carbon materials for hydrogen storage Qiuyu Zhanga, Yike Huangb, Tiancai Mac,*, Ke Lia, Fei Yea, Xuechao Wanga, Lifang Jiaob, Huatang Yuanb, Yijing Wangb,* a
Water A airs Research Institute, North China University of Water Resources and Electric Power,
Zhengzhou 450000, China b
Key Laboratory of Advanced Energy Materials Chemistry (MOE), Renewable Energy
Conversion and Storage Center (ReCast), College of Chemistry, Nankai University, Tianjin 300071, China c
College of Automotive Engineering, Tongji University, Shanghai 200092, China
Abstract: We introduce a facile chemical solid state method to in situ grow MgH2 nanoparticles in various carbon materials. Commercial carbon materials, containing coconut shell charcoal (CSC), multi-walled carbon nanotube (CNT), graphite (G) and activated carbon (AC) are employed as the templates. The MgH2@X (X=CSC, CNT, G and AC) composites were successfully obtained by the simple solid state method. The hydrogen storage properties of MgH2@X (X=CSC, CNT, G and AC) composites are systematically studied by temperature-programmed desorption system, isothermal de/hydrogenation apparatus and differential scanning calorimetry measurements. Experimental results reveal that the MgH2@CSC composites have the most fascinating hydrogen absorption and desorption performance, followed by MgH2@CNT, MgH2@G and MgH2@AC composites. The dehydrogenation of MgH2@CSC composites begins at 245 °C. Moreover, the MgH2@CSC composites exhibit superior de/hydrogenation kinetic performance. The composites could desorb 5.4 wt% hydrogen within 10 min at 325 °C, and the dehydrogenated composites take up 5.0 wt% hydrogen within 5 min at 250 °C under 2 MPa H2 pressure. Among the carbon
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materials, CSC with layered structure composed of interconnected wrinkles is most beneficial to maintain the high dispersity and nano size of MgH2 nanoparticles, resulting in the superior de/hydrogenation performance.
1. Introduction Hydrogen, as clear and renewable energy, has been suggested as one of the most prospective energy carriers [1, 2]. However, safe, efficient and inexpensive hydrogen storage material is still the main challenge for hydrogen economy. As one of the most promising hydrogen storage materials, MgH2/Mg system has received great attention owing to its relatively cheap cost (< 3 € kg-1), high hydrogen capacity (gravimetric capacity of 7.6 wt%, volumetric capacity of 111 kg m-3 H2) and abundance of Mg (7th element on Earth) [3-5]. But, the unfavorable thermodynamic and kinetic performance of de/hydrogenation seriously restrict its practical applications. In the past several years, various strategies including alloying [6-11], catalyzing [12-25] and nanostructuring [26-38] have been proposed in order to overcome these drawbacks. Nanostructuring MgH2/Mg system is proved to be a significantly efficient way, because the strategy can both improve the thermodynamic and kinetic performance. Indeed, as the MgH2/Mg particles are sufficiently small, Mg and H atoms would expose to the surface, resulting in the destabilization of Mg-H bonds. Moreover, nanosizing Mg-based materials would also generate active surface/interface and shorten diffusion distance of hydrogen atoms, which is obviously beneficial to decrease the kinetics barrier of the Mg-based system. Theoretically, Wagemans et al. [39] calculated the hydrogen desorption enthalpy of MgH2 nanoparticles by density functional theories. The results show that the dehydrogenation enthalpy of MgH2 nanoparticles (~ 0.9 nm)
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reduces to 63 kJ mol-1. The dehydrogenation temperature of MgH2 nanoparticles can decrease to 200 oC. Li et al. [40] studied the hydrogen storage behavior of nanosized Mg-based materials by experimentally. Mg nanowires with different diameters (30~170 nm) were synthesized by physical vapor deposition. The Mg nanowire with the smallest diameter (30 nm) absorbs 7.6 wt% and desorbs 6.8 wt% hydrogen in 30 min at 300 oC. Moreover, hydrogenation and dehydrogenation activation energies of 30 nm Mg nanowires reduce to 34 and 39 kJ mol-1 H2, respectively. Those investigation results clearly observe that nanostructuring is an efficient way to enhance hydrogen storage property of MgH2/Mg system. In sight of the great potential for nanostructuring, the available synthesis for nanosized MgH2/Mg still remains a challenge. The common nanostructuring techniques contain mechanical milling [17-19], chemical reduction [26-29], hydrogenation method [30-37], melting [41] and vapor deposition [10]. Mechanical milling is widely used to acquire nanoscaled hydrides. Unfortunately, this method does not desirably control the nano size and homogenous dispersity of the Mg-based materials. The chemical reduction and hydrogenation methods also cause much attention, basing that the two methods can control the Mg-based nanomaterials with uniform and small size. The chemical reduction method for the synthesis of nanosized Mg grains is achieved by the reduction of magnesium salts, as shown in reaction (1) and (2). Obviously, this method is unfavorable due to high cost of the reagents and undesirable complexity of the synthesis process. The synthesis of nanosized MgH2 via hydrogenation methods involves the reaction between organometallic precursor and hydrogen, as displayed in reaction (3). However, this method needs high hydrogen pressure (3-8 MPa) and reaction temperature (170-200 °C), which leads to limitation of large-scale production of nanosized MgH2. Thus, it still remains the urgent need for 3
searching facile and low-cost method to obtain the nanoscaled Mg-based materials.
(1) (2) (3) Recently, there has been great interest in a facile chemical solid state synthesis for nanoscale metal hydrides, based on the low cost and portable operating condition of the method. Indeed, the nanosized AlH3 and Mg(AlH4)2 are efficiently synthesized by the solid state method [42, 43]. For instance, Duan et al. [43] employed LiH/AlCl3, MgH2/AlCl3 and CaH2/AlCl3 reaction systems to synthesize AlH3 nanoparticles by solid state method. The as-prepared AlH3 nanoparticles possess small size (8.5 nm) and high hydrogen storage capacity (9.71 wt%). MgH2 nanoparticles can also be obtained by the facile solid state method. The corresponding synthesis reaction is displayed in reaction (4). The synthesis for MgH2 nanoparticles can be carried out in a mild condition, based on the low standard Gibbs free energy change (-75.06 kJ mol-1). Furthermore, MgH2 can be purified using tetrahydrofuran (THF) as solvent to remove LiCl [44]. However, MgH2 nanoparticles tend to agglomerate in large particles upon purifying progress due to the small size and high surface energy. Clearly, it would lead to the unfavorable degradation of hydrogen storage properties. Therefore, it is significantly necessary for searching available nano-confinement system to reject the agglomerating and growing. 2LiH
MgCl → MgH
2LiCl
(4)
Carbon materials is one of the most potential materials for confining the MgH2/Mg nanoparticles, due to its light weight, high chemical stability and superior mechanical strength 4
[26-31]. Many investigations have employed various carbon materials to confine MgH2/Mg particles. It is observed that the carbon materials can confine MgH2/Mg particles and maintain the nano size. Thus, carbon materials may be employed as the attractive confinement templates for in situ growing MgH2 nanoparticles using the solid state method. More importantly, it is still a challenge, but of great importance, to determine the key factors of carbon materials on the structure and hydrogen storage property of Mg-based materials. In this paper, the MgH2 nanoparticles in situ grow in various carbon materials by the facile chemical solid state method. The commercial carbon materials, including coconut shell charcoal (CSC), multi-walled carbon nanotube (CNT), graphite (G) and activated carbon (AC), are chosen as confinement templates. These carbon materials are well studied and can be easily obtained. The corresponding MgH2@X (X=CSC, CNT, G and AC) composites are obtained. The effect of various carbon materials on the phase and structure of Mg-based composites is systematically investigated by the XRD and SEM analyses. Meanwhile, the hydrogen storage properties and mechanisms of MgH2@X (X=CSC, CNT, G and AC) composites are also discussed.
2. Experimental 2.1 Sample preparation All chemicals were commercial and used as received. The commercial carbon materials, containing coconut shell charcoal (CSC), multi-walled carbon nanotube (CNT), graphite (G) and activated carbon (AC), are used as the templates. Typically, the MgH2@10wt% carbon material were prepared by the facile solid state method, employing mechanochemical milling technique to carry out the synthesis. LiH/MgCl2 with the molar ratio of 2:1 and the corresponding carbon
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material were mixed in a steel milling chamber loaded with steel balls. The mixtures were milled at 500 rpm for 30 h under 0.5 MPa H2 pressure. The ball-to-powder mass ratio was 70:1. After milling, the as-obtained mixtures were further purified by washing with THF. The final MgH2@X (X=CSC, CNT, G and AC) composites were obtained after drying. All of the handling and transferring was performed in purified argon filled glove box. 2.2 Characterization The phase structure of carbon materials and the Mg-based composites was detected by a Mini FlexII X-ray diffraction (XRD, Rigaku). The morphology of the samples was observed by employing scanning electron microscopy (SEM, JEOL, JSM7500). The surface area of carbon materials was carried out by NOVA-2200e surface area analyzer (Quantachrome Instruments). The dehydrogenation performance was tested from room temperature to 500 °C with a temperature-programmed desorption system (TPD, PX200). The isothermal de/hydrogenation kinetic performance was examined using a Sievert's type apparatus. Differential scanning calorimetry (DSC) was conducted on a TA Q20P analyzer under high purified Ar atmosphere.
3. Results and discussion 3.1. Characterization of Mg-based composites The phase of various carbon materials and as-prepared MgH2@X (X=CSC, CNT, G and AC) composites was collected and analyzed by XRD measurements. As exhibited in Fig.1a, the main phase of all the carbon materials is hexagonal C. For CNT, G and AC, there are strong diffraction peaks at 26o, meaning that carbon materials are with good crystallinity. However, there is only a weak peak at 26° in the XRD pattern of CSC. It reveals that CSC exists as amorphous state. The
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amorphous CSC possesses high active sites, which may be beneficial for the synthesis of MgH2. The XRD patterns of MgH2@X (X=CSC, CNT, G and AC) composites are displayed in Fig. 1b. The diffraction peaks of MgH2@X (X=CSC, CNT, G and AC) composites could be assigned to tetragonal MgH2 according to PDF# 72-1687. It is calculated that the lattice parameters of the as-prepared MgH2 are a=b=4.481 Å and c=3.000 Å, which agrees well with the standard values. The average crystal sizes of MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC composites are respectively 10.9, 10.4, 12.0 and 11.7 nm, calculated from the Scherrer equation. Besides, there exists a weak peak at 45° ascribed to MgO, which may be caused by oxidation of Mg-based materials upon preparation process. The microstructure of the various carbon materials and as-prepared MgH2@X (X=CSC, CNT, G and AC) composites was observed by SEM examination. As displayed in Fig. 2a-d, carbon materials display the different microstructures. The CSC has a layered structure with interconnected wrinkles. The wrinkle structure could provide large surface for the growth of MgH2 nanoparticles. Meanwhile, it is beneficial to maintain uniform dispersity of the as-prepared MgH2 nanoparticles. The CNT displays the network structure with the interconnected nanotubes. Furthermore, it is observed that the G has a sheet structure with the thickness of about 10 nm, and the AC has an irregular block structure. The morphology of MgH2@X (X= CSC, CNT, G and AC) composites is also observed, as displayed in Fig. 2e-h. It can be observed that the MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC composites are all composed of nanoparticles with the size of ~ 10 nm. However, notably, the MgH2 nanoparticles of MgH2@X (X= CSC, CNT, G and AC) composites exhibit different dispersity degree. It is noteworthy to find that the MgH2 nanoparticles of MgH2@CSC composites homogeneously distribute in CSC. There is no obvious 7
agglomeration among the nanoparticles. Compared with MgH2@CSC compositions, the MgH2@CNT, MgH2@G and MgH2@AC composites show obvious agglomeration and obscure boundary among the MgH2 nanoparticles. The agglomeration degree exhibits an increasing tend from MgH2@CNT composites to MgH2@AC composites. Especially, the MgH2 nanoparticles in MgH2@AC composites heavily agglomerate together to consequently form a sheet structure (Fig. 2h). The SEM observation results clearly reveal that carbon materials significantly affect the dispersity and size of the nanoparticles. CSC template is most favorable to maintain the small size and homogeneous dispersity of MgH2 nanoparticles. 3.2. Hydrogen storage properties of Mg-based composites The hydrogen decomposition properties of MgH2@X (X=CSC, CNT, G and AC) composites were characterized by TPD. The onset dehydrogenation temperatures of MgH2@X (X=CSC, CNT, G and AC) composites are compared. The MgH2@CSC composites exhibit the lowest onset hydrogen desorption temperature among all the Mg-based composites. The onset dehydrogenation temperature of MgH2@CSC composites reduces to 245 °C. By comparison, dehydrogenation for MgH2@CNT, MgH2@G and MgH2@AC composites begins at 257 °C, 265 °C and 267 °C, respectively. Although the average crystal size of MgH2@CNT (10.4 nm) is smaller than that of MgH2@CSC (10.9 nm), the MgH2@CSC composites show better kinetic performance. This may attribute to the obvious agglomeration among crystals in MgH2@CNT confirmed by SEM, which hinders hydrogen transfer. Further, the peak dehydrogenation temperatures of MgH2@X (X=CSC, CNT, G and AC) composites were also investigated. As displayed in Fig. 3a, there is one peak at 311 °C in the hydrogen desorption curve of MgH2@CSC composites, which indicates that MgH2@CSC composites undertake one-step reaction for the dehydrogenation. However, there are 8
two peaks (at ~ 310 °C and 350 °C) in the hydrogen desorption curves of MgH2@CNT, MgH2@G and MgH2@AC composites, which reveals that MgH2 particles of these composites exist as two states. For the peak at 310 °C, the MgH2 particles are confined in the carbon materials, thus possessing small size and resulting in the low dehydrogenation peak. The other peak at 350 °C is ascribed as the MgH2 particles undertaking agglomeration during the purifying process. The MgH2 particles are with large size, thus having the high dehydrogenation temperature. The comparison of dehydrogenation peak reveals that the CSC as template can protect the MgH2 particles from agglomerating/growing into large particles, thus resulting in the fascinating hydrogen storage performance. Additionally, the hydrogen capacity of MgH2@X (X=CSC, CNT, G and AC) composites extracted from TPD is presented in Fig. 3b. The MgH2@CSC composites show the highest dehydrogenation capacity among the composites. Approximately 6.3 wt% of hydrogen can be released form MgH2@CSC composites. The dehydrogenation capacities of the MgH2@CNT, MgH2@G and MgH2@AC composites decrease to 5.5, 5.3 and 5.1 wt%, respectively. The high dehydrogenation capacity of MgH2@CSC composites may be attributed to the amorphous state of CSC. Amorphous CSC has high activity sites during the ball-milling, which may be favorable for destabilization of Mg-H bonds and rapid diffusion of H atoms. As the result, the dehydrogenation capacity enhances. Although AC also has amorphous nature, the irregular block structure of AC is not so helpful to maintain the nanosize of the as-prepared MgH2 nanoparticles (Fig. 2d). The MgH2 nanoparticles of MgH2@AC composites heavily agglomerate together, and even form a sheet structure (Fig. 2h), leading to the two dehydrogenation peaks in the TPD curves. Thus, less of MgH2 in MgH2@AC could release hydrogen in comparison with other Mg-based materials. The TPD results reveal that MgH2@CSC composites have the most 9
remarkable dehydrogenation property among all the materials. The isothermal hydrogen absorption and desorption performance were detected to further study the kinetic performance of MgH2@X (X= CSC, CNT, G and AC) composites. Fig. 4a presents the isothermal hydrogen desorption curves of the Mg-based composites. It is clear that MgH2@CSC composites have the most superior hydrogen desorption kinetic performance among all the Mg-based composites. The MgH2@CSC composites could release 5.4 wt% hydrogen within 10 min. Totally, 5.8 wt% hydrogen can be desorbed within 120 min. Compared with the MgH2@CSC composites, the MgH2@X (X= CNT, G and AC) composites display considerably decreased desorption rates and capacities. The MgH2@CNT, MgH2@G and MgH2@AC composites respectively release 2.7, 2.0 and 1.5 wt% H2 within 10 min at 325 °C. Moreover, the hydrogen capacities of the MgH2@CNT, MgH2@G and MgH2@AC composites within 120 min are 4.8, 4.5 and 4.0 wt%, respectively. Fig. 4b displays the isothermal hydrogen absorption curves of MgH2@X (X= CSC, CNT, G and AC) composites. The MgH2@CSC composites show a more superior hydrogen adsorption performance in compared with other Mg-based composites. The MgH2@CSC composites absorb 5.0 wt% hydrogen within 5 min at 250 °C. By comparison, the MgH2@AC composites only take up 4.0 wt% hydrogen within 30 min. The isothermal de/hydrogenation results clearly reveal that MgH2@CSC composites have the best kinetic property, following with MgH2@CNT, MgH2@G and MgH2@AC composites. In addition, the rate constant of the MgH2@CSC composites is estimated using the tangent line of the dehydrogenation and rehydrogenation curves. The dehydrogenation rate is 1.09 wt% min-1 at 325 °C, and the hydrogenation rate is 3.01 wt% min-1 at 250 °C. The hydrogen desorption and absorption rate of Mg-based materials with nano size synthesized by different methods is contrasted, as displayed in 10
Table 1. The results obviously reveal that MgH2@CSC composites prepared by the facile solid state method possess the fascinating hydrogenation and dehydrogenation kinetic performance. To further understand the remarkable promotion of the kinetic performance, the activation energy (Ea) for dehydrogenation of MgH2@X (X=CSC, CNT, G and AC) composites was determined using DSC curves, according to Kissinger method: [ln (
)]
( )=−
(5)
where β, Te and R represent heating speed, the temperature for endothermic peak and the gas constant, respectively. The specific DSC curves and Kissinger plots are displayed in Fig. 5 and Fig. 6, respectively. It is found that the Ea for MgH2@CSC composites is much lower compared with other Mg-based materials. The Ea of MgH2@CSC composites is 120.19 kJ mol-1. Moreover, the Ea of MgH2@CNT, MgH2@G and MgH2@AC composites is 133.77 kJ mol-1、142.17 kJ mol-1 and145.05 kJ mol-1, respectively. The reduction of activation energy for MgH2@CSC composites reveals that the confinement of CSC is contributed to the enhancement of the hydrogen desorption performance. In sight of studying the hydrogen storage mechanism of MgH2@X (X=CSC, CNT, G and AC) composites, the phase change of the composites after dehydrogenation and rehydrogenation was detected by XRD measurements. The XRD patterns of MgH2@X (X=CSC, CNT, G and AC) composites after dehydrogenation and rehydrogenation are exhibited in Fig. 7. It is found that the diffraction peaks are ascribed to Mg in dehydrogenated composites, which reveals that MgH2 was fully transformed to Mg upon dehydrogenation process. After rehydrogenation, the main diffraction peaks are identified as MgH2. From the result, it can be determined that the Mg-based materials are fully reversible. Furthermore, the crystal size change of the Mg-based materials after 11
cycling are also studied. It is found that the average crystal size of the MgH2@CSC composites is 23 nm. However, the average crystal sizes of other Mg-based composites are larger than 100 nm. The results reveal that CSC as template could efficiently reject the growth and agglomeration of MgH2/Mg nanoparticles upon de/hydrogenation process, which results in the best hydrogen storage behavior of MgH2@CSC composites. Among MgH2@X (X=CSC, CNT, G and AC) composites, the MgH2@CSC composites have the most fascinating de/hydrogenation performance, followed by MgH2@CNT, MgH2@G and MgH2@AC. CSC is composed of interconnected wrinkles has a high surface area with 1090 m2 g-1, as shown in Fig. 8. The wrinkles with high surface area are helpful to hold the nanosize and high dispersity of MgH2 particles. The porous structure of CSC with average diameter of 3.8 nm is also beneficial for the confinement of MgH2 nanoparticles. Moreover, CSC exists as amorphous state. The amorphous CSC can introduce more defect sites and active sites compared with other carbon materials during the ball-milling, leading to destabilization of Mg-H bonds and diffusion of hydrogen atoms. In addition, the morphology of MgH2@CSC composites in different states is further detected by TEM examination, as displayed in Fig. 9. The as-prepared MgH2 nanoparticles of MgH2@CSC composites show the dense arrangement and obvious grain boundary, which is consistent with the SEM images (Fig. 2). Meanwhile, after dehydrogenation and rehydrogenation, the nanoparticles still show homogenous distribution. Thus, CSC as template has the most favorable effect on the dehydrogenation and rehydrogenation of MgH2. The introduce of CNT as template is also attributed for enhancement of hydrogen storage performance of MgH2@CNT. The in situ formed MgH2 nanoparticles significantly adhere the CNT. The highly curved surface of CNT can alter the charge distribution of MgH2, which can weaken the interaction between Mg and 12
H atoms [45]. The MgH2@G composites also show improved hydrogen storage properties. It is attributed that G could undertake exfoliating or crushing during the ball-milling [46]. Thus, the surface area and the active sites of G can enhance, resulting in improvement of the de/hydrogenation behavior of MgH2@G composites. The limited improvement of the dehydrogenation and rehydrogenation properties for MgH2@AC is resigned to the block structure of AC. The block structure can not efficiently reject the growth and agglomeration of MgH2 nanoparticles, thus the hydrogen storage performance of MgH2@AC composites is not efficiently enhanced. The analysis results reveal that the structure of carbon material significantly affect the hydrogen storage performance of the Mg-based materials. By use of CSC with layered wrinkle structure as the template, the MgH2 nanoparticles can possess the homogeneous distribution and small size, thus exhibiting the most superior hydrogen storage performance.
4. Conclusions The MgH2@X (X=CSC, CNT, G and AC) composites are successfully synthesized by the facile chemical solid state method. The hydrogen storage performance of the Mg-based composites ranks as MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC. The hydrogen desorption of MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC composites begin at 245 °C, 257 °C, 265 °C and 267 °C, respectively. Moreover, the composites respectively desorb 5.4, 2.7, 2.0 and 1.5 wt% H2 within 10 min at 325 °C. The hydrogen storage property of MgH2@X (X=CSC, CNT, G and AC) composites is significantly influenced by the structure of the carbon materials. It is found the layered structure of carbon material relates to the superior de/hydrogenation, which is contributed to the high surface area for maintaining of nano size and high dispersity of MgH2
13
nanoparticles.
Acknowledgement We greatly appreciate the financial support by National Key R&D Program of China (2018YFB1502102), NSFC (51571124, 51571125, 51871123, 51501072), 111 Project (B12015), MOE (IRT13R30).
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Mater. Chem. A 4 (2016) 14761-14772. [23] L. Zhang, X. Xiao, C. Xu, J. Zheng, X. Fan, J. Shao, S. Li, H. Ge, Q. Wang, L. Chen, Remarkably improved hydrogen storage performance of MgH2 catalyzed by multivalence NbHx nanoparticles, J. Phys. Chem. C 119 (2015) 8554-8562. [24] Y. Jia, C. Sun, Y. Peng, W. Fang, X. Yan, D. Yang, J. Zou, S.S. Mao, X. Yao, Metallic Ni nanocatalyst in situ formed from a metal-organic-framework by mechanochemical reaction for hydrogen storage in magnesium, J. Mater. Chem. A 3, (2015) 8294-8299. [25] F.A. Halim Yap, N.S. Mustafa, M. Ismail, A study on the effects of K2ZrF6 as an additive on the microstructure and hydrogen storage properties of MgH2, RSC Adv. 5 (2015) 9255-9260. [26] K.J. Jeon, H.R. Moon, A.M. Ruminski, B. Jiang, C. Kisielowski, R. Bardhan, J.J. Urban, Air-stable magnesium nanocomposites provide rapid and high-capacity hydrogen storage without using heavy-metal catalysts, Nat. Mater. 10 (2011) 286-290. [27] W. Liu, K.F. Aguey-Zinsou, Size effects and hydrogen storage properties of Mg nanoparticles synthesised by an electroless reduction method, J. Mater. Chem. A 2 (2014) 9718-9726. [28] N.S. Norberg, T.S. Arthur, S.J. Fredrick, A.L. Prieto, Size-dependent hydrogen storage properties of Mg nanocrystals prepared from solution, J. Am. Chem. Soc. 133 (2011) 10679-10681. [29] Y. Liu, J. Zou, X. Zeng, X. Wu, D. Li, W. Ding. Hydrogen storage properties of a Mg–Ni nanocomposite coprecipitated from solution, J. Phys. Chem. C 118 (2014) 18401-18411. [30] M. Konarova, A. Tanksale, J.N. Beltramini, G.Q. Lu, Effects of nano-confinement on the hydrogen desorption properties of MgH2, Nano Energy 2 (2013) 98-104. [31] R. Bardhan, A.M. Ruminski, A. Brand, J.J. Urban, Magnesium nanocrystal-polymer composites: A new platform for designer hydrogen storage materials, Energy Environ. Sci. 4 (2011) 4882-4895. 17
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[40] W.Y. Li, C.S. Li, H. Ma, J. Chen, Magnesium nanowires: Enhanced kinetics for hydrogen absorption and desorption, J. Am. Chem. Soc. 129 (2007) 6710-6711. [41] M. Ponthieu, Y.S. Au, K. Provost, C. Zlotea, E. Leroy, J.F. Fernandez, M. Latroche, P.E. de Jongh, F. Cuevas, Nanoconfinement of Mg6Pd particles in porous carbon: size effects on structural and hydrogenation properties, J. Mater. Chem. A 2 (2014) 18444-18453. [42] M. Fichtner, O. Fuhr, Synthesis and structures of magnesium alanate and two solvent adducts, J. Alloys Comp. 345 (2002) 286-296. [43] C.W. Duan, L.X. Hu, D. Xue, Solid state synthesis of nano-sized AlH3 and its dehydriding behaviour, Green Chem. 17 (2015) 3466-3474. [44] X. Xiao, Z. Liu, S. Saremi-Yarahmadi, D.H. Gregory, Facile preparation of β-/γ-MgH2 nanocomposites under mild conditions and pathways to rapid dehydrogenation, Phys. Chem. Chem. Phys. 18 (2016) 10492-10498. [45] W. Cai, X. Zhou, L. Xia, K. Jiang, S. Peng, X. Long, J. Liang, Effects of carbon nanotubes on the dehydrogenation behavior of magnesium hydride at relatively low temperatures, J. Mater. Chem. A 2 (2014) 16369-16372. [46] M. Lototskyy, J.M. Sibanyoni, R.V. Denys, M. Williams, B.G. Pollet, V.A. Yartys, Magnesium-carbon hydrogen storage hybrid materials produced by reactive ball milling in hydrogen, Carbon 57 (2013) 146-160.
19
Table 1 The initial hydrogen absorption and desorption rate of nanoscale Mg-based materials prepared by different methods. Absorption
Desorption
kinetics
kinetics
Ball-milling
2.24-250°C
0.45-320°C
[19]
Chemical reduction
0.80-200°C
--
[26]
Mg
Chemical reduction
0.26-300°C
0.03-300°C
[27]
Mg
Chemical reduction
2.19-260°C
0.60-335°C
[28]
MgH2/CMK-3
Hydrogenation
0.33-250°C
--
[30]
MgH2/graphene
Hydrogenation
0.17-200°C
0.03-200°C
[32]
Hydrogenation
2.63-250°C
0.72-300°C
[35]
Solid phase method
3.01-250°C
1.09-325°C
Samples Mg-graphene Mg/Poly(methyl methacrylate)
Synthesis method
Ref.
MgH2/bamboo-s haped carbon nanotube
MgH2@CSC
This work
Note: x−y°C represents the initial hydrogen absorption and desorption rate x wt % min-1 at y °C.
Figure captions Fig. 1. The XRD patterns of (a) various carbon materials and (b) the MgH2@X (X=CSC, CNT, G and AC) composites. Fig. 2. SEM images of various carbon materials and the MgH2@X (X=CSC, CNT, G and AC)
composites, (a) CSC, (b) CNT, (c) G, (d) AC, (e) MgH2@CSC, (f) MgH2@CNT, (g) MgH2@G and (h) MgH2@AC.
Fig. 3. (a) Thermally programmed H2 desorption curves heating at 2 °C min-1, (b) corresponding thermally programmed H2 desorption capacity curves. Fig. 4. (a) Hydrogen desorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC) composites at 325 °C; (b) hydrogen adsorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC) composites at 250 °C under 2MPa H2 pressure. Fig. 5. DSC curves of the MgH2@X (X=CSC, CNT, G and AC) composites, (a) MgH2@CSC, (b) MgH2@CNT, (c) MgH2@G and (d) MgH2@AC. Fig. 6. The Kissinger plots of the MgH2@X (X=CSC, CNT, G and AC) composites. Fig. 7. The XRD patterns of the MgH2@X (X=CSC, CNT, G and AC) composites: (a) after dehydrogenation and (b) after rehydrogenation. Fig. 8. (a) Nitrogen absorption-desorption isotherms and (b) the corresponding pore-size distribution (calculated by the BJH method) of CSC. Fig. 9 TEM images of MgH2@CSC composites after (a-b) ball-milling, (c-d) dehydrogenation and (e-f) rehydrogenation with different magnification.
Fig. 1. The XRD patterns of (a) various carbon materials and (b) the MgH2@X (X=CSC, CNT, G and AC) composites.
Fig. 2. SEM images of various carbon materials and the MgH2@X (X=CSC, CNT, G and AC)
composites, (a) CSC, (b) CNT, (c) G, (d) AC, (e) MgH2@CSC, (f) MgH2@CNT, (g) MgH2@G and (h) MgH2@AC.
Fig. 3. (a) Thermally programmed H2 desorption curves heating at 2 °C min-1, (b) corresponding thermally programmed H2 desorption capacity curves.
Fig. 4. (a) Hydrogen desorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC)
composites at 325 °C; (b) hydrogen adsorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC) composites at 250 °C under 2MPa H2 pressure.
Fig. 5. DSC curves of the MgH2@X (X=CSC, CNT, G and AC) composites, (a) MgH2@CSC, (b) MgH2@CNT, (c) MgH2@G and (d) MgH2@AC.
-10.0
MgH2@CNT
MgH2@CSC Ea = 120.19 kJ/mol
-10.5
2
ln(ß /Te )
Ea = 133.77 kJ/mol
MgH2@AC Ea = 145.05 kJ/mol
-11.0 MgH2@G Ea = 142.17 kJ/mol
-11.5 1.50
1.55
1.60
1.65
-1
1000/T (K ) Fig. 6. The Kissinger plots of the MgH2@X (X=CSC, CNT, G and AC) composites.
Fig. 7. The XRD patterns of the MgH2@X (X=CSC, CNT, G and AC) composites: (a) after dehydrogenation and (b) after rehydrogenation.
Fig. 8. (a) Nitrogen absorption-desorption isotherms and (b) the corresponding pore-size distribution (calculated by the BJH method) of CSC.
Fig. 9. TEM images of MgH2@CSC composites after (a-b) ball-milling, (c-d) dehydrogenation and (e-f) rehydrogenation with different magnification.
Highlights The MgH2 nanoparticles loaded in different carbon materials were in situ synthesized by a facile chemical solid state method. The MgH2 nanoparticles confined in coconut shell charcoal exhibit the most superior hydrogen storage performance among the Mg-based materials. The MgH2 nanoparticles confined in coconut shell charcoal could desorb 5.4 wt% hydrogen in 10 min at 325 °C, and absorb 5.0 wt% hydrogen in 5 min at 250 °C.
Declaration of interests √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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0925-8388(20)30316-9
DOI:
https://doi.org/10.1016/j.jallcom.2020.153953
Reference:
JALCOM 153953
To appear in:
Journal of Alloys and Compounds
Received Date: 1 October 2019 Revised Date:
14 January 2020
Accepted Date: 20 January 2020
Please cite this article as: Q. Zhang, Y. Huang, T. Ma, K. Li, F. Ye, X. Wang, L. Jiao, H. Yuan, Y. Wang, Facile synthesis of small MgH2 nanoparticles confined in different carbon materials for hydrogen storage, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153953. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Credit author statement Qiuyu Zhang: Conceptualization, Writing - Original Draft, Visualization Yike Huang: Validation, Software, Investigation Tiancai Ma: Methodology Ke Li: Formal analysis, Data Curation Fei Ye: Validation Xuechao Wang: Writing - Review & Editing Lifang Jiao: Project administration Huatang Yuan: Supervision Yijing Wang: Validation, Resources
Facile synthesis of small MgH2 nanoparticles confined in different carbon materials for hydrogen storage Qiuyu Zhanga, Yike Huangb, Tiancai Mac,*, Ke Lia, Fei Yea, Xuechao Wanga, Lifang Jiaob, Huatang Yuanb, Yijing Wangb,* a
Water A airs Research Institute, North China University of Water Resources and Electric Power,
Zhengzhou 450000, China b
Key Laboratory of Advanced Energy Materials Chemistry (MOE), Renewable Energy
Conversion and Storage Center (ReCast), College of Chemistry, Nankai University, Tianjin 300071, China c
College of Automotive Engineering, Tongji University, Shanghai 200092, China
Abstract: We introduce a facile chemical solid state method to in situ grow MgH2 nanoparticles in various carbon materials. Commercial carbon materials, containing coconut shell charcoal (CSC), multi-walled carbon nanotube (CNT), graphite (G) and activated carbon (AC) are employed as the templates. The MgH2@X (X=CSC, CNT, G and AC) composites were successfully obtained by the simple solid state method. The hydrogen storage properties of MgH2@X (X=CSC, CNT, G and AC) composites are systematically studied by temperature-programmed desorption system, isothermal de/hydrogenation apparatus and differential scanning calorimetry measurements. Experimental results reveal that the MgH2@CSC composites have the most fascinating hydrogen absorption and desorption performance, followed by MgH2@CNT, MgH2@G and MgH2@AC composites. The dehydrogenation of MgH2@CSC composites begins at 245 °C. Moreover, the MgH2@CSC composites exhibit superior de/hydrogenation kinetic performance. The composites could desorb 5.4 wt% hydrogen within 10 min at 325 °C, and the dehydrogenated composites take up 5.0 wt% hydrogen within 5 min at 250 °C under 2 MPa H2 pressure. Among the carbon
1
materials, CSC with layered structure composed of interconnected wrinkles is most beneficial to maintain the high dispersity and nano size of MgH2 nanoparticles, resulting in the superior de/hydrogenation performance.
1. Introduction Hydrogen, as clear and renewable energy, has been suggested as one of the most prospective energy carriers [1, 2]. However, safe, efficient and inexpensive hydrogen storage material is still the main challenge for hydrogen economy. As one of the most promising hydrogen storage materials, MgH2/Mg system has received great attention owing to its relatively cheap cost (< 3 € kg-1), high hydrogen capacity (gravimetric capacity of 7.6 wt%, volumetric capacity of 111 kg m-3 H2) and abundance of Mg (7th element on Earth) [3-5]. But, the unfavorable thermodynamic and kinetic performance of de/hydrogenation seriously restrict its practical applications. In the past several years, various strategies including alloying [6-11], catalyzing [12-25] and nanostructuring [26-38] have been proposed in order to overcome these drawbacks. Nanostructuring MgH2/Mg system is proved to be a significantly efficient way, because the strategy can both improve the thermodynamic and kinetic performance. Indeed, as the MgH2/Mg particles are sufficiently small, Mg and H atoms would expose to the surface, resulting in the destabilization of Mg-H bonds. Moreover, nanosizing Mg-based materials would also generate active surface/interface and shorten diffusion distance of hydrogen atoms, which is obviously beneficial to decrease the kinetics barrier of the Mg-based system. Theoretically, Wagemans et al. [39] calculated the hydrogen desorption enthalpy of MgH2 nanoparticles by density functional theories. The results show that the dehydrogenation enthalpy of MgH2 nanoparticles (~ 0.9 nm)
2
reduces to 63 kJ mol-1. The dehydrogenation temperature of MgH2 nanoparticles can decrease to 200 oC. Li et al. [40] studied the hydrogen storage behavior of nanosized Mg-based materials by experimentally. Mg nanowires with different diameters (30~170 nm) were synthesized by physical vapor deposition. The Mg nanowire with the smallest diameter (30 nm) absorbs 7.6 wt% and desorbs 6.8 wt% hydrogen in 30 min at 300 oC. Moreover, hydrogenation and dehydrogenation activation energies of 30 nm Mg nanowires reduce to 34 and 39 kJ mol-1 H2, respectively. Those investigation results clearly observe that nanostructuring is an efficient way to enhance hydrogen storage property of MgH2/Mg system. In sight of the great potential for nanostructuring, the available synthesis for nanosized MgH2/Mg still remains a challenge. The common nanostructuring techniques contain mechanical milling [17-19], chemical reduction [26-29], hydrogenation method [30-37], melting [41] and vapor deposition [10]. Mechanical milling is widely used to acquire nanoscaled hydrides. Unfortunately, this method does not desirably control the nano size and homogenous dispersity of the Mg-based materials. The chemical reduction and hydrogenation methods also cause much attention, basing that the two methods can control the Mg-based nanomaterials with uniform and small size. The chemical reduction method for the synthesis of nanosized Mg grains is achieved by the reduction of magnesium salts, as shown in reaction (1) and (2). Obviously, this method is unfavorable due to high cost of the reagents and undesirable complexity of the synthesis process. The synthesis of nanosized MgH2 via hydrogenation methods involves the reaction between organometallic precursor and hydrogen, as displayed in reaction (3). However, this method needs high hydrogen pressure (3-8 MPa) and reaction temperature (170-200 °C), which leads to limitation of large-scale production of nanosized MgH2. Thus, it still remains the urgent need for 3
searching facile and low-cost method to obtain the nanoscaled Mg-based materials.
(1) (2) (3) Recently, there has been great interest in a facile chemical solid state synthesis for nanoscale metal hydrides, based on the low cost and portable operating condition of the method. Indeed, the nanosized AlH3 and Mg(AlH4)2 are efficiently synthesized by the solid state method [42, 43]. For instance, Duan et al. [43] employed LiH/AlCl3, MgH2/AlCl3 and CaH2/AlCl3 reaction systems to synthesize AlH3 nanoparticles by solid state method. The as-prepared AlH3 nanoparticles possess small size (8.5 nm) and high hydrogen storage capacity (9.71 wt%). MgH2 nanoparticles can also be obtained by the facile solid state method. The corresponding synthesis reaction is displayed in reaction (4). The synthesis for MgH2 nanoparticles can be carried out in a mild condition, based on the low standard Gibbs free energy change (-75.06 kJ mol-1). Furthermore, MgH2 can be purified using tetrahydrofuran (THF) as solvent to remove LiCl [44]. However, MgH2 nanoparticles tend to agglomerate in large particles upon purifying progress due to the small size and high surface energy. Clearly, it would lead to the unfavorable degradation of hydrogen storage properties. Therefore, it is significantly necessary for searching available nano-confinement system to reject the agglomerating and growing. 2LiH
MgCl → MgH
2LiCl
(4)
Carbon materials is one of the most potential materials for confining the MgH2/Mg nanoparticles, due to its light weight, high chemical stability and superior mechanical strength 4
[26-31]. Many investigations have employed various carbon materials to confine MgH2/Mg particles. It is observed that the carbon materials can confine MgH2/Mg particles and maintain the nano size. Thus, carbon materials may be employed as the attractive confinement templates for in situ growing MgH2 nanoparticles using the solid state method. More importantly, it is still a challenge, but of great importance, to determine the key factors of carbon materials on the structure and hydrogen storage property of Mg-based materials. In this paper, the MgH2 nanoparticles in situ grow in various carbon materials by the facile chemical solid state method. The commercial carbon materials, including coconut shell charcoal (CSC), multi-walled carbon nanotube (CNT), graphite (G) and activated carbon (AC), are chosen as confinement templates. These carbon materials are well studied and can be easily obtained. The corresponding MgH2@X (X=CSC, CNT, G and AC) composites are obtained. The effect of various carbon materials on the phase and structure of Mg-based composites is systematically investigated by the XRD and SEM analyses. Meanwhile, the hydrogen storage properties and mechanisms of MgH2@X (X=CSC, CNT, G and AC) composites are also discussed.
2. Experimental 2.1 Sample preparation All chemicals were commercial and used as received. The commercial carbon materials, containing coconut shell charcoal (CSC), multi-walled carbon nanotube (CNT), graphite (G) and activated carbon (AC), are used as the templates. Typically, the MgH2@10wt% carbon material were prepared by the facile solid state method, employing mechanochemical milling technique to carry out the synthesis. LiH/MgCl2 with the molar ratio of 2:1 and the corresponding carbon
5
material were mixed in a steel milling chamber loaded with steel balls. The mixtures were milled at 500 rpm for 30 h under 0.5 MPa H2 pressure. The ball-to-powder mass ratio was 70:1. After milling, the as-obtained mixtures were further purified by washing with THF. The final MgH2@X (X=CSC, CNT, G and AC) composites were obtained after drying. All of the handling and transferring was performed in purified argon filled glove box. 2.2 Characterization The phase structure of carbon materials and the Mg-based composites was detected by a Mini FlexII X-ray diffraction (XRD, Rigaku). The morphology of the samples was observed by employing scanning electron microscopy (SEM, JEOL, JSM7500). The surface area of carbon materials was carried out by NOVA-2200e surface area analyzer (Quantachrome Instruments). The dehydrogenation performance was tested from room temperature to 500 °C with a temperature-programmed desorption system (TPD, PX200). The isothermal de/hydrogenation kinetic performance was examined using a Sievert's type apparatus. Differential scanning calorimetry (DSC) was conducted on a TA Q20P analyzer under high purified Ar atmosphere.
3. Results and discussion 3.1. Characterization of Mg-based composites The phase of various carbon materials and as-prepared MgH2@X (X=CSC, CNT, G and AC) composites was collected and analyzed by XRD measurements. As exhibited in Fig.1a, the main phase of all the carbon materials is hexagonal C. For CNT, G and AC, there are strong diffraction peaks at 26o, meaning that carbon materials are with good crystallinity. However, there is only a weak peak at 26° in the XRD pattern of CSC. It reveals that CSC exists as amorphous state. The
6
amorphous CSC possesses high active sites, which may be beneficial for the synthesis of MgH2. The XRD patterns of MgH2@X (X=CSC, CNT, G and AC) composites are displayed in Fig. 1b. The diffraction peaks of MgH2@X (X=CSC, CNT, G and AC) composites could be assigned to tetragonal MgH2 according to PDF# 72-1687. It is calculated that the lattice parameters of the as-prepared MgH2 are a=b=4.481 Å and c=3.000 Å, which agrees well with the standard values. The average crystal sizes of MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC composites are respectively 10.9, 10.4, 12.0 and 11.7 nm, calculated from the Scherrer equation. Besides, there exists a weak peak at 45° ascribed to MgO, which may be caused by oxidation of Mg-based materials upon preparation process. The microstructure of the various carbon materials and as-prepared MgH2@X (X=CSC, CNT, G and AC) composites was observed by SEM examination. As displayed in Fig. 2a-d, carbon materials display the different microstructures. The CSC has a layered structure with interconnected wrinkles. The wrinkle structure could provide large surface for the growth of MgH2 nanoparticles. Meanwhile, it is beneficial to maintain uniform dispersity of the as-prepared MgH2 nanoparticles. The CNT displays the network structure with the interconnected nanotubes. Furthermore, it is observed that the G has a sheet structure with the thickness of about 10 nm, and the AC has an irregular block structure. The morphology of MgH2@X (X= CSC, CNT, G and AC) composites is also observed, as displayed in Fig. 2e-h. It can be observed that the MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC composites are all composed of nanoparticles with the size of ~ 10 nm. However, notably, the MgH2 nanoparticles of MgH2@X (X= CSC, CNT, G and AC) composites exhibit different dispersity degree. It is noteworthy to find that the MgH2 nanoparticles of MgH2@CSC composites homogeneously distribute in CSC. There is no obvious 7
agglomeration among the nanoparticles. Compared with MgH2@CSC compositions, the MgH2@CNT, MgH2@G and MgH2@AC composites show obvious agglomeration and obscure boundary among the MgH2 nanoparticles. The agglomeration degree exhibits an increasing tend from MgH2@CNT composites to MgH2@AC composites. Especially, the MgH2 nanoparticles in MgH2@AC composites heavily agglomerate together to consequently form a sheet structure (Fig. 2h). The SEM observation results clearly reveal that carbon materials significantly affect the dispersity and size of the nanoparticles. CSC template is most favorable to maintain the small size and homogeneous dispersity of MgH2 nanoparticles. 3.2. Hydrogen storage properties of Mg-based composites The hydrogen decomposition properties of MgH2@X (X=CSC, CNT, G and AC) composites were characterized by TPD. The onset dehydrogenation temperatures of MgH2@X (X=CSC, CNT, G and AC) composites are compared. The MgH2@CSC composites exhibit the lowest onset hydrogen desorption temperature among all the Mg-based composites. The onset dehydrogenation temperature of MgH2@CSC composites reduces to 245 °C. By comparison, dehydrogenation for MgH2@CNT, MgH2@G and MgH2@AC composites begins at 257 °C, 265 °C and 267 °C, respectively. Although the average crystal size of MgH2@CNT (10.4 nm) is smaller than that of MgH2@CSC (10.9 nm), the MgH2@CSC composites show better kinetic performance. This may attribute to the obvious agglomeration among crystals in MgH2@CNT confirmed by SEM, which hinders hydrogen transfer. Further, the peak dehydrogenation temperatures of MgH2@X (X=CSC, CNT, G and AC) composites were also investigated. As displayed in Fig. 3a, there is one peak at 311 °C in the hydrogen desorption curve of MgH2@CSC composites, which indicates that MgH2@CSC composites undertake one-step reaction for the dehydrogenation. However, there are 8
two peaks (at ~ 310 °C and 350 °C) in the hydrogen desorption curves of MgH2@CNT, MgH2@G and MgH2@AC composites, which reveals that MgH2 particles of these composites exist as two states. For the peak at 310 °C, the MgH2 particles are confined in the carbon materials, thus possessing small size and resulting in the low dehydrogenation peak. The other peak at 350 °C is ascribed as the MgH2 particles undertaking agglomeration during the purifying process. The MgH2 particles are with large size, thus having the high dehydrogenation temperature. The comparison of dehydrogenation peak reveals that the CSC as template can protect the MgH2 particles from agglomerating/growing into large particles, thus resulting in the fascinating hydrogen storage performance. Additionally, the hydrogen capacity of MgH2@X (X=CSC, CNT, G and AC) composites extracted from TPD is presented in Fig. 3b. The MgH2@CSC composites show the highest dehydrogenation capacity among the composites. Approximately 6.3 wt% of hydrogen can be released form MgH2@CSC composites. The dehydrogenation capacities of the MgH2@CNT, MgH2@G and MgH2@AC composites decrease to 5.5, 5.3 and 5.1 wt%, respectively. The high dehydrogenation capacity of MgH2@CSC composites may be attributed to the amorphous state of CSC. Amorphous CSC has high activity sites during the ball-milling, which may be favorable for destabilization of Mg-H bonds and rapid diffusion of H atoms. As the result, the dehydrogenation capacity enhances. Although AC also has amorphous nature, the irregular block structure of AC is not so helpful to maintain the nanosize of the as-prepared MgH2 nanoparticles (Fig. 2d). The MgH2 nanoparticles of MgH2@AC composites heavily agglomerate together, and even form a sheet structure (Fig. 2h), leading to the two dehydrogenation peaks in the TPD curves. Thus, less of MgH2 in MgH2@AC could release hydrogen in comparison with other Mg-based materials. The TPD results reveal that MgH2@CSC composites have the most 9
remarkable dehydrogenation property among all the materials. The isothermal hydrogen absorption and desorption performance were detected to further study the kinetic performance of MgH2@X (X= CSC, CNT, G and AC) composites. Fig. 4a presents the isothermal hydrogen desorption curves of the Mg-based composites. It is clear that MgH2@CSC composites have the most superior hydrogen desorption kinetic performance among all the Mg-based composites. The MgH2@CSC composites could release 5.4 wt% hydrogen within 10 min. Totally, 5.8 wt% hydrogen can be desorbed within 120 min. Compared with the MgH2@CSC composites, the MgH2@X (X= CNT, G and AC) composites display considerably decreased desorption rates and capacities. The MgH2@CNT, MgH2@G and MgH2@AC composites respectively release 2.7, 2.0 and 1.5 wt% H2 within 10 min at 325 °C. Moreover, the hydrogen capacities of the MgH2@CNT, MgH2@G and MgH2@AC composites within 120 min are 4.8, 4.5 and 4.0 wt%, respectively. Fig. 4b displays the isothermal hydrogen absorption curves of MgH2@X (X= CSC, CNT, G and AC) composites. The MgH2@CSC composites show a more superior hydrogen adsorption performance in compared with other Mg-based composites. The MgH2@CSC composites absorb 5.0 wt% hydrogen within 5 min at 250 °C. By comparison, the MgH2@AC composites only take up 4.0 wt% hydrogen within 30 min. The isothermal de/hydrogenation results clearly reveal that MgH2@CSC composites have the best kinetic property, following with MgH2@CNT, MgH2@G and MgH2@AC composites. In addition, the rate constant of the MgH2@CSC composites is estimated using the tangent line of the dehydrogenation and rehydrogenation curves. The dehydrogenation rate is 1.09 wt% min-1 at 325 °C, and the hydrogenation rate is 3.01 wt% min-1 at 250 °C. The hydrogen desorption and absorption rate of Mg-based materials with nano size synthesized by different methods is contrasted, as displayed in 10
Table 1. The results obviously reveal that MgH2@CSC composites prepared by the facile solid state method possess the fascinating hydrogenation and dehydrogenation kinetic performance. To further understand the remarkable promotion of the kinetic performance, the activation energy (Ea) for dehydrogenation of MgH2@X (X=CSC, CNT, G and AC) composites was determined using DSC curves, according to Kissinger method: [ln (
)]
( )=−
(5)
where β, Te and R represent heating speed, the temperature for endothermic peak and the gas constant, respectively. The specific DSC curves and Kissinger plots are displayed in Fig. 5 and Fig. 6, respectively. It is found that the Ea for MgH2@CSC composites is much lower compared with other Mg-based materials. The Ea of MgH2@CSC composites is 120.19 kJ mol-1. Moreover, the Ea of MgH2@CNT, MgH2@G and MgH2@AC composites is 133.77 kJ mol-1、142.17 kJ mol-1 and145.05 kJ mol-1, respectively. The reduction of activation energy for MgH2@CSC composites reveals that the confinement of CSC is contributed to the enhancement of the hydrogen desorption performance. In sight of studying the hydrogen storage mechanism of MgH2@X (X=CSC, CNT, G and AC) composites, the phase change of the composites after dehydrogenation and rehydrogenation was detected by XRD measurements. The XRD patterns of MgH2@X (X=CSC, CNT, G and AC) composites after dehydrogenation and rehydrogenation are exhibited in Fig. 7. It is found that the diffraction peaks are ascribed to Mg in dehydrogenated composites, which reveals that MgH2 was fully transformed to Mg upon dehydrogenation process. After rehydrogenation, the main diffraction peaks are identified as MgH2. From the result, it can be determined that the Mg-based materials are fully reversible. Furthermore, the crystal size change of the Mg-based materials after 11
cycling are also studied. It is found that the average crystal size of the MgH2@CSC composites is 23 nm. However, the average crystal sizes of other Mg-based composites are larger than 100 nm. The results reveal that CSC as template could efficiently reject the growth and agglomeration of MgH2/Mg nanoparticles upon de/hydrogenation process, which results in the best hydrogen storage behavior of MgH2@CSC composites. Among MgH2@X (X=CSC, CNT, G and AC) composites, the MgH2@CSC composites have the most fascinating de/hydrogenation performance, followed by MgH2@CNT, MgH2@G and MgH2@AC. CSC is composed of interconnected wrinkles has a high surface area with 1090 m2 g-1, as shown in Fig. 8. The wrinkles with high surface area are helpful to hold the nanosize and high dispersity of MgH2 particles. The porous structure of CSC with average diameter of 3.8 nm is also beneficial for the confinement of MgH2 nanoparticles. Moreover, CSC exists as amorphous state. The amorphous CSC can introduce more defect sites and active sites compared with other carbon materials during the ball-milling, leading to destabilization of Mg-H bonds and diffusion of hydrogen atoms. In addition, the morphology of MgH2@CSC composites in different states is further detected by TEM examination, as displayed in Fig. 9. The as-prepared MgH2 nanoparticles of MgH2@CSC composites show the dense arrangement and obvious grain boundary, which is consistent with the SEM images (Fig. 2). Meanwhile, after dehydrogenation and rehydrogenation, the nanoparticles still show homogenous distribution. Thus, CSC as template has the most favorable effect on the dehydrogenation and rehydrogenation of MgH2. The introduce of CNT as template is also attributed for enhancement of hydrogen storage performance of MgH2@CNT. The in situ formed MgH2 nanoparticles significantly adhere the CNT. The highly curved surface of CNT can alter the charge distribution of MgH2, which can weaken the interaction between Mg and 12
H atoms [45]. The MgH2@G composites also show improved hydrogen storage properties. It is attributed that G could undertake exfoliating or crushing during the ball-milling [46]. Thus, the surface area and the active sites of G can enhance, resulting in improvement of the de/hydrogenation behavior of MgH2@G composites. The limited improvement of the dehydrogenation and rehydrogenation properties for MgH2@AC is resigned to the block structure of AC. The block structure can not efficiently reject the growth and agglomeration of MgH2 nanoparticles, thus the hydrogen storage performance of MgH2@AC composites is not efficiently enhanced. The analysis results reveal that the structure of carbon material significantly affect the hydrogen storage performance of the Mg-based materials. By use of CSC with layered wrinkle structure as the template, the MgH2 nanoparticles can possess the homogeneous distribution and small size, thus exhibiting the most superior hydrogen storage performance.
4. Conclusions The MgH2@X (X=CSC, CNT, G and AC) composites are successfully synthesized by the facile chemical solid state method. The hydrogen storage performance of the Mg-based composites ranks as MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC. The hydrogen desorption of MgH2@CSC, MgH2@CNT, MgH2@G and MgH2@AC composites begin at 245 °C, 257 °C, 265 °C and 267 °C, respectively. Moreover, the composites respectively desorb 5.4, 2.7, 2.0 and 1.5 wt% H2 within 10 min at 325 °C. The hydrogen storage property of MgH2@X (X=CSC, CNT, G and AC) composites is significantly influenced by the structure of the carbon materials. It is found the layered structure of carbon material relates to the superior de/hydrogenation, which is contributed to the high surface area for maintaining of nano size and high dispersity of MgH2
13
nanoparticles.
Acknowledgement We greatly appreciate the financial support by National Key R&D Program of China (2018YFB1502102), NSFC (51571124, 51571125, 51871123, 51501072), 111 Project (B12015), MOE (IRT13R30).
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19
Table 1 The initial hydrogen absorption and desorption rate of nanoscale Mg-based materials prepared by different methods. Absorption
Desorption
kinetics
kinetics
Ball-milling
2.24-250°C
0.45-320°C
[19]
Chemical reduction
0.80-200°C
--
[26]
Mg
Chemical reduction
0.26-300°C
0.03-300°C
[27]
Mg
Chemical reduction
2.19-260°C
0.60-335°C
[28]
MgH2/CMK-3
Hydrogenation
0.33-250°C
--
[30]
MgH2/graphene
Hydrogenation
0.17-200°C
0.03-200°C
[32]
Hydrogenation
2.63-250°C
0.72-300°C
[35]
Solid phase method
3.01-250°C
1.09-325°C
Samples Mg-graphene Mg/Poly(methyl methacrylate)
Synthesis method
Ref.
MgH2/bamboo-s haped carbon nanotube
MgH2@CSC
This work
Note: x−y°C represents the initial hydrogen absorption and desorption rate x wt % min-1 at y °C.
Figure captions Fig. 1. The XRD patterns of (a) various carbon materials and (b) the MgH2@X (X=CSC, CNT, G and AC) composites. Fig. 2. SEM images of various carbon materials and the MgH2@X (X=CSC, CNT, G and AC)
composites, (a) CSC, (b) CNT, (c) G, (d) AC, (e) MgH2@CSC, (f) MgH2@CNT, (g) MgH2@G and (h) MgH2@AC.
Fig. 3. (a) Thermally programmed H2 desorption curves heating at 2 °C min-1, (b) corresponding thermally programmed H2 desorption capacity curves. Fig. 4. (a) Hydrogen desorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC) composites at 325 °C; (b) hydrogen adsorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC) composites at 250 °C under 2MPa H2 pressure. Fig. 5. DSC curves of the MgH2@X (X=CSC, CNT, G and AC) composites, (a) MgH2@CSC, (b) MgH2@CNT, (c) MgH2@G and (d) MgH2@AC. Fig. 6. The Kissinger plots of the MgH2@X (X=CSC, CNT, G and AC) composites. Fig. 7. The XRD patterns of the MgH2@X (X=CSC, CNT, G and AC) composites: (a) after dehydrogenation and (b) after rehydrogenation. Fig. 8. (a) Nitrogen absorption-desorption isotherms and (b) the corresponding pore-size distribution (calculated by the BJH method) of CSC. Fig. 9 TEM images of MgH2@CSC composites after (a-b) ball-milling, (c-d) dehydrogenation and (e-f) rehydrogenation with different magnification.
Fig. 1. The XRD patterns of (a) various carbon materials and (b) the MgH2@X (X=CSC, CNT, G and AC) composites.
Fig. 2. SEM images of various carbon materials and the MgH2@X (X=CSC, CNT, G and AC)
composites, (a) CSC, (b) CNT, (c) G, (d) AC, (e) MgH2@CSC, (f) MgH2@CNT, (g) MgH2@G and (h) MgH2@AC.
Fig. 3. (a) Thermally programmed H2 desorption curves heating at 2 °C min-1, (b) corresponding thermally programmed H2 desorption capacity curves.
Fig. 4. (a) Hydrogen desorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC)
composites at 325 °C; (b) hydrogen adsorption kinetic curves of the MgH2@X (X=CSC, CNT, G and AC) composites at 250 °C under 2MPa H2 pressure.
Fig. 5. DSC curves of the MgH2@X (X=CSC, CNT, G and AC) composites, (a) MgH2@CSC, (b) MgH2@CNT, (c) MgH2@G and (d) MgH2@AC.
-10.0
MgH2@CNT
MgH2@CSC Ea = 120.19 kJ/mol
-10.5
2
ln(ß /Te )
Ea = 133.77 kJ/mol
MgH2@AC Ea = 145.05 kJ/mol
-11.0 MgH2@G Ea = 142.17 kJ/mol
-11.5 1.50
1.55
1.60
1.65
-1
1000/T (K ) Fig. 6. The Kissinger plots of the MgH2@X (X=CSC, CNT, G and AC) composites.
Fig. 7. The XRD patterns of the MgH2@X (X=CSC, CNT, G and AC) composites: (a) after dehydrogenation and (b) after rehydrogenation.
Fig. 8. (a) Nitrogen absorption-desorption isotherms and (b) the corresponding pore-size distribution (calculated by the BJH method) of CSC.
Fig. 9. TEM images of MgH2@CSC composites after (a-b) ball-milling, (c-d) dehydrogenation and (e-f) rehydrogenation with different magnification.
Highlights The MgH2 nanoparticles loaded in different carbon materials were in situ synthesized by a facile chemical solid state method. The MgH2 nanoparticles confined in coconut shell charcoal exhibit the most superior hydrogen storage performance among the Mg-based materials. The MgH2 nanoparticles confined in coconut shell charcoal could desorb 5.4 wt% hydrogen in 10 min at 325 °C, and absorb 5.0 wt% hydrogen in 5 min at 250 °C.
Declaration of interests √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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: