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Improved dehydriding property of polyvinylpyrrolidone coated Mg-Ni hydrogen storage nano-composite prepared by hydriding combustion synthesis and wet mechanical milling
Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx
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Original Research
Improved dehydriding property of polyvinylpyrrolidone coated Mg-Ni hydrogen storage nano-composite prepared by hydriding combustion synthesis and wet mechanical milling☆ ⁎
Linglong Yaoa,b, Huihui Hana,b, Yana Liua,b, Yunfeng Zhua,b, , Yao Zhangb,c, Liquan Lia,b,
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a
College of Materials Science and Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, China c School of Materials Science and Engineering, Southeast University, Nanjing 211189, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mg-Ni-PVP Composite Mg-based alloy Wet mechanical milling Dehydriding temperature
In this work, polyvinylpyrrolidone (PVP) coated Mg95Ni5 nano-composites were prepared by hydriding combustion synthesis (HCS) plus wet mechanical milling (WM) with tetrahydrofuran (THF) and donated as WM-x wt % PVP (x = 1, 3, 5 and 7) respectively. The phase compositions, microstructures and dehydriding property, as well as the co-effect of PVP and THF were investigated in detail. XRD results showed that the average crystal size of MgH2 in the milled Mg95Ni5 decreased from 23 nm without PVP to 18 nm with 1 wt% PVP. The peak temperature of dehydrogenation of MgH2 in the milled Mg95Ni5 decreased from 293.0 °C without THF to 250.4 °C with THF. The apparent activation energy for decomposition of MgH2 in WM-7 wt% PVP was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of milled Mg95Ni5 without THF and PVP. PVP and THF can facilitate the refinement of particle size during mechanical milling process. Attributed to small particle sizes and synergistic effect of PVP and THF, the composites exhibit markedly improved dehydriding properties.
1. Introduction Nowadays, the increasing demand for energy results in excessive exploitation and use of fossil fuels and cause adverse effects on the environment. As a new energy carrier, hydrogen is regarded as an ideal substitution to fossil fuel for its high energy density, abundant resources and environmental harmless. Storage of hydrogen in an efficient and safe way is the key to large-scale utilization of it [1,2]. In numerous kinds of hydrogen storage materials, magnesium is considered to be one of the most promising solid hydrogen storage media with high hydrogen storage capacity (7.6 wt% for MgH2) and abundant in natural resources. However, the practical application of Mg-based hydrogen storage materials is restricted by high thermodynamic stability of MgH2 and harsh terms required for hydrogenation and dehydrogenation. Up to now, variety of strategies, such as nano-structuring [3], alloying [4–6], catalyzing by various additives [7,8] and multiphase compositing [9], have been taken to improve hydrogenation and dehydrogenation properties of Mg-based hydrogen storage materials. Among them, nanosizing strategy has considerable effects on improving thermodynamics and kinetic properties [10]. Calculation results showed that the hydrogen desorption energy is decreased significantly
when the crystal size is smaller than 1.3 nm [11]. Mg-based hydride with nanostructure can be synthesized by methods of solution reduction [12], vapor deposition [13], hydrogenolysis of precursor [14], mechanical milling [15], etc. Among them, mechanical milling is a convenient and effective method in preparing nanostructured Mg based hydrogen storage materials with improved hydrogen absorption and desorption properties [10,16]. However, cold welding and aggregation of nanoparticles occurred during mechanical milling process always impede the further refining of crystal and particle size. Therefore, various kinds of milling additives, including different carbon additives [17,18], surfactants [19], metal oxides and halides [20,21], are used during the mechanical milling process. Imamura et al. [22] prepared nanosized magnesium by mechanical milling of MgH2 with solutions of Al(C2H5)3 in benzene and it can reach a maximum absorption capacity of 7.3 wt% at a 0.1 MPa H2 atmosphere. MgH2 nanoparticles embedded in a LiCl salt matrix prepared by mechanochemical method shows decreasing in the decomposition reaction enthalpy [23]. However, metals or hydrides nanoparticles with high surface energy coalesce easily, which may hinder the diffusion of H atom. Besides, it may be contaminated by oxygen easily due to its high surface activity [24]. Therefore the concept of nano-confinement has been developed
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Peer review under responsibility of Chinese Materials Research Society. Corresponding authors. E-mail addresses: [email protected] (Y. Zhu), [email protected] (L. Li).
https://doi.org/10.1016/j.pnsc.2018.01.011 Received 22 August 2017; Received in revised form 10 January 2018; Accepted 10 January 2018 1002-0071/ © 2018 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Yao, L., Progress in Natural Science: Materials International (2018), https://doi.org/10.1016/j.pnsc.2018.01.011
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2.2. Characterization
and numerous studies have been carried out to synthesis metal hydride nanoparticles with different nano-confined matrixes [25–27]. Polymers or organics, with excellent modulation ability in structures and characteristics [28,29], are widely used as nano-confined matrixes [30]. Significant improvements in hydrogenation kinetics and air stability have been obtained in systems like Mg-PMMA [31], NH3BH3-PVP [32], TiFe-PTFE [33], LiBH4-PPP [34], etc. In our previous works, hydriding combustion synthesis (HCS), firstly proposed by Akiyama et al. [35,36], was used to synthesize Mgbased hydrogen storage materials. The HCS has advantages such as energy conservation, high activity of the product and short processing time. By combining HCS with mechanical milling (MM), Mg95Ni5 composites with improved hydriding properties were synthesized [37]. However, the dehydriding properties of these products still cannot satisfied the requirements of actual application. Besides, Mg95Ni5 with high activity is sensitive to oxygen and humidity and may be contaminated by air easily. Polyvinylpyrrolidone (PVP) was selected as a gas selective barrier to improve the air stability of Mg95Ni5 for its inferior oxygen permeability. In this study, PVP coated Mg95Ni5 nanocomposites were synthesized by HCS and wet mechanical milling with tetrahydrofuran (THF) solvent. PVP and THF also act as milling assistant agents helped to improve dispersibility of nanoparticles and to prevent cold welding and aggregation of particles during mechanical milling process, thus help to further decrease the crystal and particle size of Mg95Ni5. Therefore, the effects of PVP and THF were mainly investigated in this study. The dehydriding property of composites with different amount of PVP was also investigated systematically.
X-ray diffraction (XRD; Smart Lab TM, Rigaku, Tokyo, Japan) with Cu-Kα radiation (40 kV and 30 mA) was used to characterize the phase compositions and crystal structures of samples. All XRD patterns were obtained with 0.02° step and scan speed of 10°/min. The average crystallite sizes of MgH2 in different samples were estimated according to Eq. (1):
FW (S )* cos(θ ) = k *λ /Size + 4*Strain* sin(θ)
(1)
where FW(S) is the specimen broadening. θ is the peak position. k is the shape factor, typically 0.89. λ is the X-ray wavelength of 0.154 nm, and strain is the lattice strain. Scanning electron microscopy (SEM; JSM-6360LV, JEOL, Tokyo, Japan) and high-resolution transmission electron microscope (HRTEM, JEM-2100 UHR, JEOL, Tokyo, Japan) were used to investigate the microstructures and surface morphologies of the composite powders. Fourier transform infrared spectrometry (FTIR, NEXUS670) analysis was carried out to analyze the structure characteristics of composites. Differential scanning calorimetry (DSC) was performed on a highpressure apparatus from TA Instruments (DSC Q2000). For dehydrogenation kinetics tests, a flow of purified Ar (50 ml/min) was used with various heating rates of 3 °C/min, 5 °C/min, 7 °C/min and 10 °C/ min from room temperature (RT) to 400 °C. 3. Results and discussion 3.1. Phase compositions and structural characterization
2. Experimental
Fig. 1 shows the XRD patterns of Mg95Ni5 milled with and without additives. For MM-Mg95Ni5, MM-1% PVP, WM-1% PVP and WM-0% PVP, they all consist of MgH2, Mg2NiH4, Mg and Mg2NiH0.3, indicating that the phase compositions of the samples have no change after the additions of PVP and THF. The XRD patterns of Mg95Ni5 wet milled with different amount of PVP are showed in Fig. 2. The appearance of MgO and Mg(OH)2 phases may be attributed to the air exposure of starting materials and the reaction between Mg and residual moisture of THF. After the addition of PVP or co-addition of PVP and THF, the crystalline diffraction peaks of MgH2 phase are broadened compared with MM-Mg95Ni5, which can be ascribed to the grain refinement after the addition of additives during mechanical milling process. The average crystal sizes of MgH2 in different composites were estimated from the diffraction peaks of MgH2 (2θ = 27.9°, 35.7° and 54.5°) in Figs. 1 and 2 according to Eq. (1), and the results are presented in Table 1. This indicates that all of the composites have MgH2 nanocrystalline due to the brittleness of HCS products. The average crystallite size of MgH2 in MM-Mg95Ni5 decreased from 23 to 18 nm in MM-
2.1. Sample preparation Analytical reagent (AR) grade polyvinylpyrrolidone (PVP) and tetrahydrofuran (THF) used in this study were obtained commercially. The THF was further purified by distillation under the reaction of sodium piece. The PVP was dried in a vacuum drying oven overnight at 368 K. Commercial Mg (99 wt% in purity and < 74 µm in diameter; TangShan WeiHao Magnesium Powder Co., Ltd, Hebei, China) and Ni powder (99 wt% in purity and 2–3 µm in diameter; JiangYou HeBao Nanomaterial Co., Ltd, Beijing, China) powders with an atomic ratio of 95:5 designated as Mg95Ni5 were homogenized by ultrasonic vibration in acetone for 60 min. After drying in air, the mixtures were used directly for hydriding combustion synthesis (HCS). During the HCS process, the sample was firstly heated to 853 K at 10 K/min under 2 MPa hydrogen pressure and held for 1 h. It was then cooled to 613 K and held for 4 h. Finally, the sample was cooled to room temperature. More details about HCS were described in Ref [37]. The HCS product of Mg95Ni5 (1 g) was pre-ball milled in a 50 ml stainless steel vial with stainless steel balls at a speed of 400 rpm for 8 h under 0.1 MPa argon atmosphere via a planetary milling apparatus (QM-3SP2, Nanjing University Instrument Plant). The ball to powder ratio was 40:1. Then, the pre-ball milled Mg95Ni5 was wet-milled (WM) with 3 ml THF and different amount of PVP (0 wt%, 1 wt%, 3 wt%, 5 wt % and 7 wt%, vs. the Mg95Ni5) under the same mechanical milling conditions for another 2 h and donated as WM-x % PVP (x = 0, 1, 3, 5 and 7) respectively. Upon completion of ball milling, the stainless steel vial was opened to take out the products under inert atmosphere in an argon-filled glovebox (< 1 ppm O2 and H2O) from Mikrouna, and 2 ml THF was added into the product and dispersed by ultrasonic vibration for 20 min. Finally, the resultant products were dried in a vacuum drying oven overnight at 358 K before test. For comparison, the Mg95Ni5 pre-ball milled for 8 h, was milled for another 2 h without any additives and donated as MM-Mg95Ni5. Mg95Ni5 pre-ball milled for 8 h, was milled for another 2 h with 1 wt% PVP and donated as MM-1% PVP.
Fig. 1. XRD patterns of (a) MM-Mg95Ni5, (b) MM-1% PVP, (c) WM-1% PVP and (d) WM0% PVP.
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Mg95Ni5, characteristic absorption band for Mg-H and Ni-H can be observed at 560 cm−1 and 1640 cm−1, respectively [38]. In PVPMg95Ni5 composites, the peak at 3692 cm−1 is assigned to O-H stretching vibration of Mg(OH)2. And bands from Mg-H and Ni-H did not change after the additions of PVP and THF. C-N bending vibration at 1290 cm−1 could still be observed in WM-7% PVP, suggesting that PVP successfully incorporated with Mg95Ni5 and remained stable after wet milling process. Indistinctive characteristic absorption bands of PVP in WM-1% PVP is mainly due to the traces amount of PVP in WM1% PVP composite. 3.2. Microstructures and morphologies TEM images of WM-7% PVP are shown in Fig. 4. In the bright field image as shown in Fig. 4a, different contrast is easy to be distinguished. It is more clearly recognized from the dark field image in Fig. 4b that the white points are Ni, which are corresponding to Mg–Ni hydrides [39]. Mg–Ni hydrides embedded in the composites may have catalytic effect on the dehydrogenation of MgH2 [40]. The structural information of PVP coated Mg95Ni5 nano-composite was further explored by HRTEM. The interface between the crystalline region and amorphous region is distinguishable in Fig. 4c, implying the good physical compatibility between PVP and Mg-Ni composite. The lattice fringes with a separation of 0.2281 nm agree well with the (200) interplanar spacing of MgH2. Amorphous of polymerized PVP with the thickness about 7 nm was coated on the particle surfaces. During wet mechanical milling process, well contacting between PVP and MgH2 plays an important role in improving milling efficiency thus improving the dehydriding properties of MgH2. Mechanical milling is usually used to refine the particle size and crystallite size and to create various defects on particle surfaces, which shorten the diffusion distance of hydrogen and increase the number of active sites for hydrogenation, and hence to improve the hydriding/ dehydriding properties of hydrogen storage materials. In order to investigate the influence of additives on particle size and morphology of MM-Mg95Ni5, MM-1% PVP, WM-0% PVP, WM-1% PVP and WM-7% PVP, SEM was used, and the images with different magnifications are shown in Fig. 5. Fig. 5a and b are the SEM images of MMMg95Ni5. It is clear that the particle has coarse surface and agglomerates seriously, which hinders the diffusion of hydrogen, and thus decreases the hydriding/dehydriding kinetics of the materials. Compared with MM-Mg95Ni5, MM-1% PVP possesses smaller and uniform particle size with an average particle size of 440 nm (Fig. 5d). PVP acted as a dispersing agent during mechanical milling process, which was coated on the surface of MgH2 particle and had space hindrance effect to prevent the aggregation of nanoparticles. Compared Figs. 5e with 5a and c, it is clear that WM-0%PVP shows higher dispersibility of particles with an average size of 390 nm. Ball milling efficiency of Mg95Ni5 was improved after the addition of THF solvent. The existence of THF made particles suspend and disperse in solvent, which alleviated the aggregation and cold welding of Mg and MgH2 particles. As for WM-1% PVP and WM-7% PVP, the results are similar to that of WM-0% PVP. For PVP-Mg95Ni5 composites prepared by wet milling process, the distributions of PVP were more uniform in Mg95Ni5 matrix, and PVP contacts with Mg and MgH2 particles more sufficient after dissolving in THF solvent. But the average particle sizes of WM-1% PVP and WM-7% PVP increased slightly (~ 500 nm) owing to crosslinking effect between polymer chains.
Fig. 2. XRD patterns of (a) WM-3% PVP, (b) WM-5% PVP and (c) WM-7% PVP.
Table 1 Average crystallite size of MgH2 mechanical milled with different milling assistant agents. Sample
MM-Mg95Ni5 MM-1% PVP WM-0% PVP WM-1% PVP WM-5% PVP
Assistant agents of milling THF (ml)
PVP amount (wt%)
0 0 3 3 3
0 1 0 1 5
Average crystallite size (nm)
23 18 24 20 23
1% PVP. But the crystallite size of MgH2 increased to 24 nm in WM-0% PVP, implying that ball milling in a liquid environment is not conducive to the crystallite refinement. After the co-addition of 1 wt% PVP and THF, the crystallite size of MgH2 was about 20 nm. As a kind of dispersion agent, PVP together with THF solvent, helped to prevent the cold welding and particle agglomeration during mechanical milling process, which is helpful for the refinement of crystallite size. Nevertheless, the crystallite size of MgH2 was similar to that of MM-Mg95Ni5 when the PVP content increased to 5 wt%, which demonstrates that excessive amount of PVP added during mechanical milling process has no obvious effect on crystallite refining. The FTIR spectra of pure PVP, MM-Mg95Ni5, WM-1% PVP and WM7% PVP are showed in Fig. 3. For pure PVP, the characteristic absorption band for C=O stretching vibration and C-N stretching vibration were found at 1651 cm−1 and 1290 cm−1 respectively. The peak at 1450 cm−1 is attributed to C-H bending vibration. The peak at 2958 cm−1 is attributed to C-H stretching vibration. As for MM-
3.3. Dehydriding property of the composites Dehydrogenation performance of different composites was studied by DSC under Ar atmosphere. Fig. 6(A) shows the DSC curve of MMMg95Ni5 with heating rate of 10 °C/min, and the corresponding XRD patterns obtained when DSC ran and stopped at specified temperatures (RT, 260, 323, 368 and 450 °C) are shown in Fig. 6(B) in order to
Fig. 3. FT-IR spectra for (a) pure PVP, (b) MM-Mg95Ni5 (c) WM-1% PVP and (d) WM-7% PVP.
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Fig. 4. TEM images of WM-7% PVP. (a) Bright field image, (b) dark field image, (c) HRTEM image.
obtained when DSC ran and stopped at specified temperatures (RT, 230, 300 and 360 °C) are presented in Fig. 9. It is clear that the first endothermic peak at 230.7 °C results from the dehydrogenation of Mg2NiH4 and Mg2NiH0.3, together with the decomposition of a small amount of MgH2. The endothermic peak at 252.2 °C mainly comes from the dehydrogenation of MgH2. Different from the DSC results of WM0% PVP, WM-1% PVP, WM-3% PVP and WM-5% PVP, there was a weak endothermic peak at 328.5 °C for WM-7% PVP, which is mainly attributed to slightly excessive amount of PVP coated on particle surfaces, making MgH2 decompose incompletely under 250 °C with faster heating rate of 10 °C/min, and the residual MgH2 decompose completely with the increasing of temperature. Xiao et al. [42] showed that ball milled MgH2 with LiCl and then treated mixture with THF can promote the formation of γ-MgH2. Volume contraction effect during the decomposition process of γ-MgH2 generates stress on the adjacent β-MgH2 and accelerates the dehydrogenation of β-MgH2 [17]. Therefore, there is a synergetic effect in the γ-MgH2 and β-MgH2 mixture, which helps to reduce the dehydrogenation temperature of β-MgH2 during dehydrogenation process [43]. However, the XRD results clearly show that γ-MgH2 phase didn’t form during wet milling process following by THF treatment, indicating that the decreasing in dehydriding temperature is not related to the formation of γ-MgH2 phase. With the additions of THF and PVP in mechanical milling process, the dehydrogenation temperature of MgH2 in WM-0% PVP and WM-7% PVP are 42.6 °C and 49.7 °C lower than 293.0 °C in MM-Mg95Ni5, respectively. Imamura et al. [22] showed that after 1 h ball milling of MgH2 with benzene, the dehydrogenation
determine the corresponding reaction occurred during heating process. The endothermic peak at 249.2 °C comes from the decomposition of Mg2NiH4 and Mg2NiH0.3 to Mg2Ni and H2. Due to the small amount and the incomplete dehydrogenation of Mg2NiH4 during the mechanical milling process, the corresponding dehydrogenation peak was weak. The endothermic peak at 323 °C and 368 °C are assigned to the decomposition of MgH2. These two peaks may be due to bimodal particle size distribution of MgH2 formed during ball milling process [41], which is also in accordance with SEM result showed in Fig. 5. DSC curves of MM-Mg95Ni5, MM-1% PVP, WM-0% PVP and MM-1% PVP with heating rate of 5 °C/min are shown in Fig. 7. MM-Mg95Ni5 displayed two endothermic peaks at 293.0 °C and 352.9 °C. After the single addition of PVP, there was only one endothermic peak at 290.5 °C, and the peak temperature was similar to the first endothermic peak of MMMg95Ni5 (293.0 °C). It may be mainly attributed to the uniform distribution of particle size obtained by the dispersion effect of PVP. As showed in Fig. 7c, there were also two endothermic peaks for WM-0% PVP, but the peak temperature was nearly 40 °C lower than that of MMMg95Ni5. These two endothermic peaks of MM-Mg95Ni5 at 293.0 and 352.9 °C, respectively, decreased to 227.3 and 250.4 °C when THF was added, indicating that the THF is significantly conductive to the decrease of dehydriding temperature. Fig. 8 shows the DSC curves of WM-x % PVP (x = 1, 3, 5 and 7). With the increasing amount of PVP used in wet milling process, the endothermic peak temperature decreased from 251.4 °C in WM-1% PVP to 243.3 °C in WM-7% PVP. DSC curve for WM-7% PVP with heating rate of 10 °C/min and the corresponding XRD patterns of WM-7% PVP 4
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Fig. 5. SEM images of (a), (b) MM-Mg95Ni5; (c), (d) MM-1% PVP; (e), (f) WM-0% PVP; (g) WM-1% PVP and (h) WM-7% PVP.
The oxygen atoms on the inner amide polar group show prominent electron-donor ability so as to provide polymer ring with higher electron density. Therefore, PVP shows excellent surface affinity and complexation ability to active H atoms of MgH2. As a result, synergetic effects between THF and PVP help to significantly weaken the Mg–H bond. Besides, Mg2NiH4 has less thermal stability and lower dehydriding temperature. It serves as diffusion channels for hydrogen, and the lattice strain caused by Mg2NiH4 and Mg2Ni phase transformation provides the driving force to trigger the dehydrogenation of MgH2, and therefore reducing the dehydriding temperature of the composite [40].
temperature was around 327 °C. By mechanical grinding Mg and graphite with benzene or THF, the endothermic DSC peaks corresponding to decomposition of the magnesium hydride obviously shifted to lower temperatures. Charge-transfer sites formed during mechanical grinding play an important role in the improvement of hydrogen uptake and release properties, which consistent with catalysis by electron donor–acceptor complexes with charge-transfer as well [44]. Careful analysis shows that the dehydrogenation peak temperature of MgH2 gets further decreased from 251.4 °C to 243.3 °C with the increment of PVP amount from 1 wt% to 7 wt%. PVP is a polymer with flexible chain structure. 5
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Fig. 8. DSC curves of (a) WM-1% PVP, (b) WM-3% PVP, (c) WM-5% PVP and (d) WM-7% PVP. (The heating rate was 5 °C/min from RT to 400 °C under Ar atmosphere.).
Fig. 6. (A) DSC curve of MM-Mg95Ni5 with heating rate of 10 °C/min from RT to 500 °C under Ar atmosphere and (B) the corresponding XRD patterns of MM-Mg95Ni5 obtained when DSC ran and stopped at specified temperatures (RT, 260, 323, 350, 368 and 450 °C, respectively).
Fig. 9. (A) DSC curve of WM-7% PVP with heating rate of 10 °C/min from RT to 400 °C under Ar atmosphere and (B) the corresponding XRD patterns of WM-7% PVP obtained when DSC ran and stopped at specified temperatures (RT, 230, 300 and 360 °C, respectively).
Fig. 7. DSC curves of (a) MM-Mg95Ni5, (b) MM-1% PVP and (c) WM-0% PVP. (The heating rate was 5 °C/min from RT to 400 °C under Ar atmosphere.).
energy for decomposition of MgH2 in MM-Mg95Ni5 was estimated to be 104.64 kJ/mol, which was greatly reduced compared with the reported value of commercial MgH2 (213 ± 6 kJ/mol) [42]. Further improvement was obtained in WM-7% PVP and the apparent activation energy for decomposition of MgH2 was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that in MM-Mg95Ni5. The above results indicate that the dehydrogenation barrier of the whole system has been greatly reduced. The improved performance for dehydrogenation of PVP-Mg95Ni5 composites prepared by wet mechanical milling is mainly attributed to the refining of particle size and the formation of crystal defects and phase interfaces [46]. It can be deduced that nano-crystallites and nanoparticles of Mg95Ni5 and PVP-
The dehydrogenation apparent activation energy of MgH2 in MMMg95Ni5 and PVP-Mg95Ni5 composites can be estimated via Kissinger's method as indicated in Eq. (2) [45]:
ln(β / TP 2) = −(Ea/ RTP ) + α
(2)
where Tp is peak temperature, β is heating rate, Ea is activation energy, R is gas constant for the dehydrogenation reaction and α is the linear constant. DSC curves measured with the heating rates of 3, 5 and 7 °C/ min for MM-Mg95Ni5 and WM-7% PVP are presented in Fig. 10. The peak temperature moves to higher temperature with the increasing of heating rate. Fig. 10(C) shows Kissinger's plots of the dehydrogenation of MgH2 in MM-Mg95Ni5 and WM-7% PVP. The apparent activation 6
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and the lattice strain caused by Mg2NiH4 and Mg2Ni phase transformation provides the driving force to trigger the dehydrogenation of MgH2. Therefore, the synergistic dehydrogenation effect of Mg2NiH4 and MgH2 in PVP-Mg95Ni5 composite prepared by HCS and wet milling might be enhanced by THF solvent, which needs further investigations. 4. Conclusions The WM-x % PVP (x = 1, 3, 5 and 7) composites have been prepared by HCS and wet mechanical milling with THF solvent, and the effects of PVP and THF investigated in detail in this study. PVP and THF have positive effect on alleviating the aggregation and cold welding of Mg and MgH2 particles during mechanical milling process, which facilitates the refinement of particle size. The dehydriding peak temperature of MgH2 decreases from 293.0 °C milled without THF to 250.4 °C milled with THF, and it gets further decreased from 251.4 °C to 243.3 °C with the increment of PVP amount from 1 wt% to 7 wt% owing to the co-effect between THF and PVP. The apparent activation energy for decomposition of MgH2 in WM-7% PVP is estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of MM-Mg95Ni5. Synergistic dehydrogenation effect between Mg2NiH4 and MgH2 in PVP-Mg95Ni5 composite prepared by HCS and wet milling might be enhanced by THF solvent, which needs further investigations. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, Grant nos. 51571112, 51771092, 51471087), the Natural Science Foundation of Jiangsu Province (BK20151405, BK20161004), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. References [1] X.B. Yu, Z.W. Tang, D.L. Sun, L.Z. Ouyang, M. Zhu, Prog. Mater. Sci. 88 (2017) 1–48. [2] Y.P. Pang, Y.F. Liu, M.X. Gao, L.Z. Ouyang, J.W. Liu, H. Wang, M. Zhu, H.G. Pan, Nat. Commun. 5 (2014) 3519. [3] E.J. Setijadi, C. Boyer, K. Aguey-Zinsou, RSC Adv. 4 (2014) 39934–39940. [4] W. Lv, Y.F. Shi, W.P. Deng, J.G. Yuan, Y.H. Yan, Y. Wu, Prog. Nat. Sci.: Mater. Int. 26 (2016) 177–181. [5] L.Z. Ouyang, X.S. Yang, H.W. Dong, M. Zhu, Scr. Mater. 61 (2009) 339–342. [6] D.L. Zhu, J.G. Zhang, Y.F. Zhu, Y. Zhang, Y.N. Liu, H.J. Lin, W.F. Zhang, L.Q. Li, Prog. Nat. Sci.: Mater. Int. 27 (2017) 144–148. [7] J.G. Yuan, Y.F. Zhu, L.Q. Li, Chem. Commun. 50 (2014) 6641–6644. [8] Y.F. Liu, H.F. Du, X. Zhang, Y.X. Yang, M.X. Gao, H.G. Pan, Chem. Commun. 52 (2016) 705–708. [9] I.H. Cho, S. Gang, H. Lee, J.H. Shim, M. Park, Y.N. Choi, Int. J. Hydrog. Energy 41 (2016) 22090–22096. [10] V. Berube, G. Chen, M.S. Dresselhaus, Int. J. Hydrog. Energy 33 (2008) 4122–4131. [11] R. Wagemans, J.H. van Lenthe, P.E. de Jongh, A.J. van Dillen, K.P. de Jongh, J. Am. Chem. Soc. 127 (2005) 16675–16680. [12] K. Jeon, H.R. Moon, A.M. Ruminski, B. Jiang, C. Kisielowski, R. Bardhan, J.J. Urban, Nat. Mater. 10 (2011) 286–290. [13] J.G. Zhang, Y.F. Zhu, H.J. Lin, Y.N. Liu, Y. Zhang, S.Y. Li, Z.L. Ma, L.Q. Li, Adv. Mater. 29 (2017) 1700760. [14] Y.Q. Huang, G.L. Xia, J. Chen, B.P. Zhang, Q. Li, X.B. Yu, Prog. Nat. Sci.: Mater. Int. 27 (2017) 81–87. [15] J. Huot, G. Liang, S. Boily, A. Van Neste, R. Schulz, J. Alloy. Compd. 293 (1999) 495–500. [16] L.E.A. Berlouis, E. Cabrera, E. Hall-Barientos, P.J. Hall, S. Dodd, S. Morris, M.A. Imam, J. Alloy. Compd. 305 (2000) 82–89. [17] Z.G. Huang, Z.P. Guo, A. Calka, D. Wexler, H.K. Liu, J. Alloy. Compd. 427 (2007) 94–100. [18] L.J. Wei, Y.F. Zhu, L.Q. Li, J. Alloy. Compd. 539 (2012) 215–220. [19] S. Hong, M.Y. Song, Met. Mater. Int. 22 (2016) 544–549. [20] M. Ismail, N. Juahir, N.S. Mustafa, J. Phys. Chem. C 118 (2014) 18878–18883. [21] M. Ismail, Energy 79 (2015) 177–182. [22] H. Imamura, K. Masanari, M. Kusuhara, H. Katsumoto, T. Sumi, Y. Sakata, J. Alloy. Compd. 386 (2005) 211–216. [23] M. Paskevicius, D.A. Sheppard, C.E. Buckley, J. Am. Chem. Soc. 132 (2010) 5077–5083. [24] S.S. Shinde, D.H. Kim, J.Y. Yu, J.H. Lee, Nanoscale 9 (2017) 7094–7103. [25] E.S. Cho, A.M. Ruminski, S. Aloni, Y. Liu, J.H. Guo, J.J. Urban, Nat. Commun. 7 (2016) 10804.
Fig. 10. DSC curves of (A) MM-Mg95Ni5 and (B) WM-7% PVP measured at different heating rates (β = 3, 5, 7 °C/min, respectively), and (C) the ln (β/Tp2) − 1/Tp linear diagram of MM-Mg95Ni5 and WM-7% PVP for the decomposition of MgH2.
Mg95Ni5 composites prepared by wet milling process lower the driving force for Mg nucleation and promote the dehydrogenation of MgH2, which results in a reduced dehydrogenation temperature. Therefore, the improved dehydrogenation properties of Mg95Ni5 composites wet milled with THF and PVP might be due to the following reasons. Firstly, the addition of PVP and THF has positive effects on alleviating the aggregation and cold welding of Mg and MgH2 particles during mechanical milling process, which facilitates the refinement of particle size and crystal size and so the uniform distribution of the particle size, resulting in more active sites and the reducing of the hydrogen diffusion distance. Secondly, as a polar solvent, THF fully contact with MgH2 particles during ball milling process. And the synergetic effect between THF and PVP help to significantly weaken the Mg-H bond, which improves the dehydrogenation property of the composite. Finally, Mg2NiH4 has catalytic effect on the dehydrogenation of MgH2 7
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Original Research
Improved dehydriding property of polyvinylpyrrolidone coated Mg-Ni hydrogen storage nano-composite prepared by hydriding combustion synthesis and wet mechanical milling☆ ⁎
Linglong Yaoa,b, Huihui Hana,b, Yana Liua,b, Yunfeng Zhua,b, , Yao Zhangb,c, Liquan Lia,b,
⁎
a
College of Materials Science and Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, China c School of Materials Science and Engineering, Southeast University, Nanjing 211189, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Mg-Ni-PVP Composite Mg-based alloy Wet mechanical milling Dehydriding temperature
In this work, polyvinylpyrrolidone (PVP) coated Mg95Ni5 nano-composites were prepared by hydriding combustion synthesis (HCS) plus wet mechanical milling (WM) with tetrahydrofuran (THF) and donated as WM-x wt % PVP (x = 1, 3, 5 and 7) respectively. The phase compositions, microstructures and dehydriding property, as well as the co-effect of PVP and THF were investigated in detail. XRD results showed that the average crystal size of MgH2 in the milled Mg95Ni5 decreased from 23 nm without PVP to 18 nm with 1 wt% PVP. The peak temperature of dehydrogenation of MgH2 in the milled Mg95Ni5 decreased from 293.0 °C without THF to 250.4 °C with THF. The apparent activation energy for decomposition of MgH2 in WM-7 wt% PVP was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of milled Mg95Ni5 without THF and PVP. PVP and THF can facilitate the refinement of particle size during mechanical milling process. Attributed to small particle sizes and synergistic effect of PVP and THF, the composites exhibit markedly improved dehydriding properties.
1. Introduction Nowadays, the increasing demand for energy results in excessive exploitation and use of fossil fuels and cause adverse effects on the environment. As a new energy carrier, hydrogen is regarded as an ideal substitution to fossil fuel for its high energy density, abundant resources and environmental harmless. Storage of hydrogen in an efficient and safe way is the key to large-scale utilization of it [1,2]. In numerous kinds of hydrogen storage materials, magnesium is considered to be one of the most promising solid hydrogen storage media with high hydrogen storage capacity (7.6 wt% for MgH2) and abundant in natural resources. However, the practical application of Mg-based hydrogen storage materials is restricted by high thermodynamic stability of MgH2 and harsh terms required for hydrogenation and dehydrogenation. Up to now, variety of strategies, such as nano-structuring [3], alloying [4–6], catalyzing by various additives [7,8] and multiphase compositing [9], have been taken to improve hydrogenation and dehydrogenation properties of Mg-based hydrogen storage materials. Among them, nanosizing strategy has considerable effects on improving thermodynamics and kinetic properties [10]. Calculation results showed that the hydrogen desorption energy is decreased significantly
when the crystal size is smaller than 1.3 nm [11]. Mg-based hydride with nanostructure can be synthesized by methods of solution reduction [12], vapor deposition [13], hydrogenolysis of precursor [14], mechanical milling [15], etc. Among them, mechanical milling is a convenient and effective method in preparing nanostructured Mg based hydrogen storage materials with improved hydrogen absorption and desorption properties [10,16]. However, cold welding and aggregation of nanoparticles occurred during mechanical milling process always impede the further refining of crystal and particle size. Therefore, various kinds of milling additives, including different carbon additives [17,18], surfactants [19], metal oxides and halides [20,21], are used during the mechanical milling process. Imamura et al. [22] prepared nanosized magnesium by mechanical milling of MgH2 with solutions of Al(C2H5)3 in benzene and it can reach a maximum absorption capacity of 7.3 wt% at a 0.1 MPa H2 atmosphere. MgH2 nanoparticles embedded in a LiCl salt matrix prepared by mechanochemical method shows decreasing in the decomposition reaction enthalpy [23]. However, metals or hydrides nanoparticles with high surface energy coalesce easily, which may hinder the diffusion of H atom. Besides, it may be contaminated by oxygen easily due to its high surface activity [24]. Therefore the concept of nano-confinement has been developed
☆ ⁎
Peer review under responsibility of Chinese Materials Research Society. Corresponding authors. E-mail addresses: [email protected] (Y. Zhu), [email protected] (L. Li).
https://doi.org/10.1016/j.pnsc.2018.01.011 Received 22 August 2017; Received in revised form 10 January 2018; Accepted 10 January 2018 1002-0071/ © 2018 Chinese Materials Research Society. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Please cite this article as: Yao, L., Progress in Natural Science: Materials International (2018), https://doi.org/10.1016/j.pnsc.2018.01.011
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2.2. Characterization
and numerous studies have been carried out to synthesis metal hydride nanoparticles with different nano-confined matrixes [25–27]. Polymers or organics, with excellent modulation ability in structures and characteristics [28,29], are widely used as nano-confined matrixes [30]. Significant improvements in hydrogenation kinetics and air stability have been obtained in systems like Mg-PMMA [31], NH3BH3-PVP [32], TiFe-PTFE [33], LiBH4-PPP [34], etc. In our previous works, hydriding combustion synthesis (HCS), firstly proposed by Akiyama et al. [35,36], was used to synthesize Mgbased hydrogen storage materials. The HCS has advantages such as energy conservation, high activity of the product and short processing time. By combining HCS with mechanical milling (MM), Mg95Ni5 composites with improved hydriding properties were synthesized [37]. However, the dehydriding properties of these products still cannot satisfied the requirements of actual application. Besides, Mg95Ni5 with high activity is sensitive to oxygen and humidity and may be contaminated by air easily. Polyvinylpyrrolidone (PVP) was selected as a gas selective barrier to improve the air stability of Mg95Ni5 for its inferior oxygen permeability. In this study, PVP coated Mg95Ni5 nanocomposites were synthesized by HCS and wet mechanical milling with tetrahydrofuran (THF) solvent. PVP and THF also act as milling assistant agents helped to improve dispersibility of nanoparticles and to prevent cold welding and aggregation of particles during mechanical milling process, thus help to further decrease the crystal and particle size of Mg95Ni5. Therefore, the effects of PVP and THF were mainly investigated in this study. The dehydriding property of composites with different amount of PVP was also investigated systematically.
X-ray diffraction (XRD; Smart Lab TM, Rigaku, Tokyo, Japan) with Cu-Kα radiation (40 kV and 30 mA) was used to characterize the phase compositions and crystal structures of samples. All XRD patterns were obtained with 0.02° step and scan speed of 10°/min. The average crystallite sizes of MgH2 in different samples were estimated according to Eq. (1):
FW (S )* cos(θ ) = k *λ /Size + 4*Strain* sin(θ)
(1)
where FW(S) is the specimen broadening. θ is the peak position. k is the shape factor, typically 0.89. λ is the X-ray wavelength of 0.154 nm, and strain is the lattice strain. Scanning electron microscopy (SEM; JSM-6360LV, JEOL, Tokyo, Japan) and high-resolution transmission electron microscope (HRTEM, JEM-2100 UHR, JEOL, Tokyo, Japan) were used to investigate the microstructures and surface morphologies of the composite powders. Fourier transform infrared spectrometry (FTIR, NEXUS670) analysis was carried out to analyze the structure characteristics of composites. Differential scanning calorimetry (DSC) was performed on a highpressure apparatus from TA Instruments (DSC Q2000). For dehydrogenation kinetics tests, a flow of purified Ar (50 ml/min) was used with various heating rates of 3 °C/min, 5 °C/min, 7 °C/min and 10 °C/ min from room temperature (RT) to 400 °C. 3. Results and discussion 3.1. Phase compositions and structural characterization
2. Experimental
Fig. 1 shows the XRD patterns of Mg95Ni5 milled with and without additives. For MM-Mg95Ni5, MM-1% PVP, WM-1% PVP and WM-0% PVP, they all consist of MgH2, Mg2NiH4, Mg and Mg2NiH0.3, indicating that the phase compositions of the samples have no change after the additions of PVP and THF. The XRD patterns of Mg95Ni5 wet milled with different amount of PVP are showed in Fig. 2. The appearance of MgO and Mg(OH)2 phases may be attributed to the air exposure of starting materials and the reaction between Mg and residual moisture of THF. After the addition of PVP or co-addition of PVP and THF, the crystalline diffraction peaks of MgH2 phase are broadened compared with MM-Mg95Ni5, which can be ascribed to the grain refinement after the addition of additives during mechanical milling process. The average crystal sizes of MgH2 in different composites were estimated from the diffraction peaks of MgH2 (2θ = 27.9°, 35.7° and 54.5°) in Figs. 1 and 2 according to Eq. (1), and the results are presented in Table 1. This indicates that all of the composites have MgH2 nanocrystalline due to the brittleness of HCS products. The average crystallite size of MgH2 in MM-Mg95Ni5 decreased from 23 to 18 nm in MM-
2.1. Sample preparation Analytical reagent (AR) grade polyvinylpyrrolidone (PVP) and tetrahydrofuran (THF) used in this study were obtained commercially. The THF was further purified by distillation under the reaction of sodium piece. The PVP was dried in a vacuum drying oven overnight at 368 K. Commercial Mg (99 wt% in purity and < 74 µm in diameter; TangShan WeiHao Magnesium Powder Co., Ltd, Hebei, China) and Ni powder (99 wt% in purity and 2–3 µm in diameter; JiangYou HeBao Nanomaterial Co., Ltd, Beijing, China) powders with an atomic ratio of 95:5 designated as Mg95Ni5 were homogenized by ultrasonic vibration in acetone for 60 min. After drying in air, the mixtures were used directly for hydriding combustion synthesis (HCS). During the HCS process, the sample was firstly heated to 853 K at 10 K/min under 2 MPa hydrogen pressure and held for 1 h. It was then cooled to 613 K and held for 4 h. Finally, the sample was cooled to room temperature. More details about HCS were described in Ref [37]. The HCS product of Mg95Ni5 (1 g) was pre-ball milled in a 50 ml stainless steel vial with stainless steel balls at a speed of 400 rpm for 8 h under 0.1 MPa argon atmosphere via a planetary milling apparatus (QM-3SP2, Nanjing University Instrument Plant). The ball to powder ratio was 40:1. Then, the pre-ball milled Mg95Ni5 was wet-milled (WM) with 3 ml THF and different amount of PVP (0 wt%, 1 wt%, 3 wt%, 5 wt % and 7 wt%, vs. the Mg95Ni5) under the same mechanical milling conditions for another 2 h and donated as WM-x % PVP (x = 0, 1, 3, 5 and 7) respectively. Upon completion of ball milling, the stainless steel vial was opened to take out the products under inert atmosphere in an argon-filled glovebox (< 1 ppm O2 and H2O) from Mikrouna, and 2 ml THF was added into the product and dispersed by ultrasonic vibration for 20 min. Finally, the resultant products were dried in a vacuum drying oven overnight at 358 K before test. For comparison, the Mg95Ni5 pre-ball milled for 8 h, was milled for another 2 h without any additives and donated as MM-Mg95Ni5. Mg95Ni5 pre-ball milled for 8 h, was milled for another 2 h with 1 wt% PVP and donated as MM-1% PVP.
Fig. 1. XRD patterns of (a) MM-Mg95Ni5, (b) MM-1% PVP, (c) WM-1% PVP and (d) WM0% PVP.
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Mg95Ni5, characteristic absorption band for Mg-H and Ni-H can be observed at 560 cm−1 and 1640 cm−1, respectively [38]. In PVPMg95Ni5 composites, the peak at 3692 cm−1 is assigned to O-H stretching vibration of Mg(OH)2. And bands from Mg-H and Ni-H did not change after the additions of PVP and THF. C-N bending vibration at 1290 cm−1 could still be observed in WM-7% PVP, suggesting that PVP successfully incorporated with Mg95Ni5 and remained stable after wet milling process. Indistinctive characteristic absorption bands of PVP in WM-1% PVP is mainly due to the traces amount of PVP in WM1% PVP composite. 3.2. Microstructures and morphologies TEM images of WM-7% PVP are shown in Fig. 4. In the bright field image as shown in Fig. 4a, different contrast is easy to be distinguished. It is more clearly recognized from the dark field image in Fig. 4b that the white points are Ni, which are corresponding to Mg–Ni hydrides [39]. Mg–Ni hydrides embedded in the composites may have catalytic effect on the dehydrogenation of MgH2 [40]. The structural information of PVP coated Mg95Ni5 nano-composite was further explored by HRTEM. The interface between the crystalline region and amorphous region is distinguishable in Fig. 4c, implying the good physical compatibility between PVP and Mg-Ni composite. The lattice fringes with a separation of 0.2281 nm agree well with the (200) interplanar spacing of MgH2. Amorphous of polymerized PVP with the thickness about 7 nm was coated on the particle surfaces. During wet mechanical milling process, well contacting between PVP and MgH2 plays an important role in improving milling efficiency thus improving the dehydriding properties of MgH2. Mechanical milling is usually used to refine the particle size and crystallite size and to create various defects on particle surfaces, which shorten the diffusion distance of hydrogen and increase the number of active sites for hydrogenation, and hence to improve the hydriding/ dehydriding properties of hydrogen storage materials. In order to investigate the influence of additives on particle size and morphology of MM-Mg95Ni5, MM-1% PVP, WM-0% PVP, WM-1% PVP and WM-7% PVP, SEM was used, and the images with different magnifications are shown in Fig. 5. Fig. 5a and b are the SEM images of MMMg95Ni5. It is clear that the particle has coarse surface and agglomerates seriously, which hinders the diffusion of hydrogen, and thus decreases the hydriding/dehydriding kinetics of the materials. Compared with MM-Mg95Ni5, MM-1% PVP possesses smaller and uniform particle size with an average particle size of 440 nm (Fig. 5d). PVP acted as a dispersing agent during mechanical milling process, which was coated on the surface of MgH2 particle and had space hindrance effect to prevent the aggregation of nanoparticles. Compared Figs. 5e with 5a and c, it is clear that WM-0%PVP shows higher dispersibility of particles with an average size of 390 nm. Ball milling efficiency of Mg95Ni5 was improved after the addition of THF solvent. The existence of THF made particles suspend and disperse in solvent, which alleviated the aggregation and cold welding of Mg and MgH2 particles. As for WM-1% PVP and WM-7% PVP, the results are similar to that of WM-0% PVP. For PVP-Mg95Ni5 composites prepared by wet milling process, the distributions of PVP were more uniform in Mg95Ni5 matrix, and PVP contacts with Mg and MgH2 particles more sufficient after dissolving in THF solvent. But the average particle sizes of WM-1% PVP and WM-7% PVP increased slightly (~ 500 nm) owing to crosslinking effect between polymer chains.
Fig. 2. XRD patterns of (a) WM-3% PVP, (b) WM-5% PVP and (c) WM-7% PVP.
Table 1 Average crystallite size of MgH2 mechanical milled with different milling assistant agents. Sample
MM-Mg95Ni5 MM-1% PVP WM-0% PVP WM-1% PVP WM-5% PVP
Assistant agents of milling THF (ml)
PVP amount (wt%)
0 0 3 3 3
0 1 0 1 5
Average crystallite size (nm)
23 18 24 20 23
1% PVP. But the crystallite size of MgH2 increased to 24 nm in WM-0% PVP, implying that ball milling in a liquid environment is not conducive to the crystallite refinement. After the co-addition of 1 wt% PVP and THF, the crystallite size of MgH2 was about 20 nm. As a kind of dispersion agent, PVP together with THF solvent, helped to prevent the cold welding and particle agglomeration during mechanical milling process, which is helpful for the refinement of crystallite size. Nevertheless, the crystallite size of MgH2 was similar to that of MM-Mg95Ni5 when the PVP content increased to 5 wt%, which demonstrates that excessive amount of PVP added during mechanical milling process has no obvious effect on crystallite refining. The FTIR spectra of pure PVP, MM-Mg95Ni5, WM-1% PVP and WM7% PVP are showed in Fig. 3. For pure PVP, the characteristic absorption band for C=O stretching vibration and C-N stretching vibration were found at 1651 cm−1 and 1290 cm−1 respectively. The peak at 1450 cm−1 is attributed to C-H bending vibration. The peak at 2958 cm−1 is attributed to C-H stretching vibration. As for MM-
3.3. Dehydriding property of the composites Dehydrogenation performance of different composites was studied by DSC under Ar atmosphere. Fig. 6(A) shows the DSC curve of MMMg95Ni5 with heating rate of 10 °C/min, and the corresponding XRD patterns obtained when DSC ran and stopped at specified temperatures (RT, 260, 323, 368 and 450 °C) are shown in Fig. 6(B) in order to
Fig. 3. FT-IR spectra for (a) pure PVP, (b) MM-Mg95Ni5 (c) WM-1% PVP and (d) WM-7% PVP.
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Fig. 4. TEM images of WM-7% PVP. (a) Bright field image, (b) dark field image, (c) HRTEM image.
obtained when DSC ran and stopped at specified temperatures (RT, 230, 300 and 360 °C) are presented in Fig. 9. It is clear that the first endothermic peak at 230.7 °C results from the dehydrogenation of Mg2NiH4 and Mg2NiH0.3, together with the decomposition of a small amount of MgH2. The endothermic peak at 252.2 °C mainly comes from the dehydrogenation of MgH2. Different from the DSC results of WM0% PVP, WM-1% PVP, WM-3% PVP and WM-5% PVP, there was a weak endothermic peak at 328.5 °C for WM-7% PVP, which is mainly attributed to slightly excessive amount of PVP coated on particle surfaces, making MgH2 decompose incompletely under 250 °C with faster heating rate of 10 °C/min, and the residual MgH2 decompose completely with the increasing of temperature. Xiao et al. [42] showed that ball milled MgH2 with LiCl and then treated mixture with THF can promote the formation of γ-MgH2. Volume contraction effect during the decomposition process of γ-MgH2 generates stress on the adjacent β-MgH2 and accelerates the dehydrogenation of β-MgH2 [17]. Therefore, there is a synergetic effect in the γ-MgH2 and β-MgH2 mixture, which helps to reduce the dehydrogenation temperature of β-MgH2 during dehydrogenation process [43]. However, the XRD results clearly show that γ-MgH2 phase didn’t form during wet milling process following by THF treatment, indicating that the decreasing in dehydriding temperature is not related to the formation of γ-MgH2 phase. With the additions of THF and PVP in mechanical milling process, the dehydrogenation temperature of MgH2 in WM-0% PVP and WM-7% PVP are 42.6 °C and 49.7 °C lower than 293.0 °C in MM-Mg95Ni5, respectively. Imamura et al. [22] showed that after 1 h ball milling of MgH2 with benzene, the dehydrogenation
determine the corresponding reaction occurred during heating process. The endothermic peak at 249.2 °C comes from the decomposition of Mg2NiH4 and Mg2NiH0.3 to Mg2Ni and H2. Due to the small amount and the incomplete dehydrogenation of Mg2NiH4 during the mechanical milling process, the corresponding dehydrogenation peak was weak. The endothermic peak at 323 °C and 368 °C are assigned to the decomposition of MgH2. These two peaks may be due to bimodal particle size distribution of MgH2 formed during ball milling process [41], which is also in accordance with SEM result showed in Fig. 5. DSC curves of MM-Mg95Ni5, MM-1% PVP, WM-0% PVP and MM-1% PVP with heating rate of 5 °C/min are shown in Fig. 7. MM-Mg95Ni5 displayed two endothermic peaks at 293.0 °C and 352.9 °C. After the single addition of PVP, there was only one endothermic peak at 290.5 °C, and the peak temperature was similar to the first endothermic peak of MMMg95Ni5 (293.0 °C). It may be mainly attributed to the uniform distribution of particle size obtained by the dispersion effect of PVP. As showed in Fig. 7c, there were also two endothermic peaks for WM-0% PVP, but the peak temperature was nearly 40 °C lower than that of MMMg95Ni5. These two endothermic peaks of MM-Mg95Ni5 at 293.0 and 352.9 °C, respectively, decreased to 227.3 and 250.4 °C when THF was added, indicating that the THF is significantly conductive to the decrease of dehydriding temperature. Fig. 8 shows the DSC curves of WM-x % PVP (x = 1, 3, 5 and 7). With the increasing amount of PVP used in wet milling process, the endothermic peak temperature decreased from 251.4 °C in WM-1% PVP to 243.3 °C in WM-7% PVP. DSC curve for WM-7% PVP with heating rate of 10 °C/min and the corresponding XRD patterns of WM-7% PVP 4
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Fig. 5. SEM images of (a), (b) MM-Mg95Ni5; (c), (d) MM-1% PVP; (e), (f) WM-0% PVP; (g) WM-1% PVP and (h) WM-7% PVP.
The oxygen atoms on the inner amide polar group show prominent electron-donor ability so as to provide polymer ring with higher electron density. Therefore, PVP shows excellent surface affinity and complexation ability to active H atoms of MgH2. As a result, synergetic effects between THF and PVP help to significantly weaken the Mg–H bond. Besides, Mg2NiH4 has less thermal stability and lower dehydriding temperature. It serves as diffusion channels for hydrogen, and the lattice strain caused by Mg2NiH4 and Mg2Ni phase transformation provides the driving force to trigger the dehydrogenation of MgH2, and therefore reducing the dehydriding temperature of the composite [40].
temperature was around 327 °C. By mechanical grinding Mg and graphite with benzene or THF, the endothermic DSC peaks corresponding to decomposition of the magnesium hydride obviously shifted to lower temperatures. Charge-transfer sites formed during mechanical grinding play an important role in the improvement of hydrogen uptake and release properties, which consistent with catalysis by electron donor–acceptor complexes with charge-transfer as well [44]. Careful analysis shows that the dehydrogenation peak temperature of MgH2 gets further decreased from 251.4 °C to 243.3 °C with the increment of PVP amount from 1 wt% to 7 wt%. PVP is a polymer with flexible chain structure. 5
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Fig. 8. DSC curves of (a) WM-1% PVP, (b) WM-3% PVP, (c) WM-5% PVP and (d) WM-7% PVP. (The heating rate was 5 °C/min from RT to 400 °C under Ar atmosphere.).
Fig. 6. (A) DSC curve of MM-Mg95Ni5 with heating rate of 10 °C/min from RT to 500 °C under Ar atmosphere and (B) the corresponding XRD patterns of MM-Mg95Ni5 obtained when DSC ran and stopped at specified temperatures (RT, 260, 323, 350, 368 and 450 °C, respectively).
Fig. 9. (A) DSC curve of WM-7% PVP with heating rate of 10 °C/min from RT to 400 °C under Ar atmosphere and (B) the corresponding XRD patterns of WM-7% PVP obtained when DSC ran and stopped at specified temperatures (RT, 230, 300 and 360 °C, respectively).
Fig. 7. DSC curves of (a) MM-Mg95Ni5, (b) MM-1% PVP and (c) WM-0% PVP. (The heating rate was 5 °C/min from RT to 400 °C under Ar atmosphere.).
energy for decomposition of MgH2 in MM-Mg95Ni5 was estimated to be 104.64 kJ/mol, which was greatly reduced compared with the reported value of commercial MgH2 (213 ± 6 kJ/mol) [42]. Further improvement was obtained in WM-7% PVP and the apparent activation energy for decomposition of MgH2 was estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that in MM-Mg95Ni5. The above results indicate that the dehydrogenation barrier of the whole system has been greatly reduced. The improved performance for dehydrogenation of PVP-Mg95Ni5 composites prepared by wet mechanical milling is mainly attributed to the refining of particle size and the formation of crystal defects and phase interfaces [46]. It can be deduced that nano-crystallites and nanoparticles of Mg95Ni5 and PVP-
The dehydrogenation apparent activation energy of MgH2 in MMMg95Ni5 and PVP-Mg95Ni5 composites can be estimated via Kissinger's method as indicated in Eq. (2) [45]:
ln(β / TP 2) = −(Ea/ RTP ) + α
(2)
where Tp is peak temperature, β is heating rate, Ea is activation energy, R is gas constant for the dehydrogenation reaction and α is the linear constant. DSC curves measured with the heating rates of 3, 5 and 7 °C/ min for MM-Mg95Ni5 and WM-7% PVP are presented in Fig. 10. The peak temperature moves to higher temperature with the increasing of heating rate. Fig. 10(C) shows Kissinger's plots of the dehydrogenation of MgH2 in MM-Mg95Ni5 and WM-7% PVP. The apparent activation 6
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and the lattice strain caused by Mg2NiH4 and Mg2Ni phase transformation provides the driving force to trigger the dehydrogenation of MgH2. Therefore, the synergistic dehydrogenation effect of Mg2NiH4 and MgH2 in PVP-Mg95Ni5 composite prepared by HCS and wet milling might be enhanced by THF solvent, which needs further investigations. 4. Conclusions The WM-x % PVP (x = 1, 3, 5 and 7) composites have been prepared by HCS and wet mechanical milling with THF solvent, and the effects of PVP and THF investigated in detail in this study. PVP and THF have positive effect on alleviating the aggregation and cold welding of Mg and MgH2 particles during mechanical milling process, which facilitates the refinement of particle size. The dehydriding peak temperature of MgH2 decreases from 293.0 °C milled without THF to 250.4 °C milled with THF, and it gets further decreased from 251.4 °C to 243.3 °C with the increment of PVP amount from 1 wt% to 7 wt% owing to the co-effect between THF and PVP. The apparent activation energy for decomposition of MgH2 in WM-7% PVP is estimated to be 66.94 kJ/mol, which is 37.70 kJ/mol lower than that of MM-Mg95Ni5. Synergistic dehydrogenation effect between Mg2NiH4 and MgH2 in PVP-Mg95Ni5 composite prepared by HCS and wet milling might be enhanced by THF solvent, which needs further investigations. Acknowledgements This work was supported by the National Natural Science Foundation of China (NSFC, Grant nos. 51571112, 51771092, 51471087), the Natural Science Foundation of Jiangsu Province (BK20151405, BK20161004), and the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions. References [1] X.B. Yu, Z.W. Tang, D.L. Sun, L.Z. Ouyang, M. Zhu, Prog. Mater. Sci. 88 (2017) 1–48. [2] Y.P. Pang, Y.F. Liu, M.X. Gao, L.Z. Ouyang, J.W. Liu, H. Wang, M. Zhu, H.G. Pan, Nat. Commun. 5 (2014) 3519. [3] E.J. Setijadi, C. Boyer, K. Aguey-Zinsou, RSC Adv. 4 (2014) 39934–39940. [4] W. Lv, Y.F. Shi, W.P. Deng, J.G. Yuan, Y.H. Yan, Y. Wu, Prog. Nat. Sci.: Mater. Int. 26 (2016) 177–181. [5] L.Z. Ouyang, X.S. Yang, H.W. Dong, M. Zhu, Scr. Mater. 61 (2009) 339–342. [6] D.L. Zhu, J.G. Zhang, Y.F. Zhu, Y. Zhang, Y.N. Liu, H.J. Lin, W.F. Zhang, L.Q. Li, Prog. Nat. Sci.: Mater. Int. 27 (2017) 144–148. [7] J.G. Yuan, Y.F. Zhu, L.Q. Li, Chem. Commun. 50 (2014) 6641–6644. [8] Y.F. Liu, H.F. Du, X. Zhang, Y.X. Yang, M.X. Gao, H.G. Pan, Chem. Commun. 52 (2016) 705–708. [9] I.H. Cho, S. Gang, H. Lee, J.H. Shim, M. Park, Y.N. Choi, Int. J. Hydrog. Energy 41 (2016) 22090–22096. [10] V. Berube, G. Chen, M.S. Dresselhaus, Int. J. Hydrog. Energy 33 (2008) 4122–4131. [11] R. Wagemans, J.H. van Lenthe, P.E. de Jongh, A.J. van Dillen, K.P. de Jongh, J. Am. Chem. Soc. 127 (2005) 16675–16680. [12] K. Jeon, H.R. Moon, A.M. Ruminski, B. Jiang, C. Kisielowski, R. Bardhan, J.J. Urban, Nat. Mater. 10 (2011) 286–290. [13] J.G. Zhang, Y.F. Zhu, H.J. Lin, Y.N. Liu, Y. Zhang, S.Y. Li, Z.L. Ma, L.Q. Li, Adv. Mater. 29 (2017) 1700760. [14] Y.Q. Huang, G.L. Xia, J. Chen, B.P. Zhang, Q. Li, X.B. Yu, Prog. Nat. Sci.: Mater. Int. 27 (2017) 81–87. [15] J. Huot, G. Liang, S. Boily, A. Van Neste, R. Schulz, J. Alloy. Compd. 293 (1999) 495–500. [16] L.E.A. Berlouis, E. Cabrera, E. Hall-Barientos, P.J. Hall, S. Dodd, S. Morris, M.A. Imam, J. Alloy. Compd. 305 (2000) 82–89. [17] Z.G. Huang, Z.P. Guo, A. Calka, D. Wexler, H.K. Liu, J. Alloy. Compd. 427 (2007) 94–100. [18] L.J. Wei, Y.F. Zhu, L.Q. Li, J. Alloy. Compd. 539 (2012) 215–220. [19] S. Hong, M.Y. Song, Met. Mater. Int. 22 (2016) 544–549. [20] M. Ismail, N. Juahir, N.S. Mustafa, J. Phys. Chem. C 118 (2014) 18878–18883. [21] M. Ismail, Energy 79 (2015) 177–182. [22] H. Imamura, K. Masanari, M. Kusuhara, H. Katsumoto, T. Sumi, Y. Sakata, J. Alloy. Compd. 386 (2005) 211–216. [23] M. Paskevicius, D.A. Sheppard, C.E. Buckley, J. Am. Chem. Soc. 132 (2010) 5077–5083. [24] S.S. Shinde, D.H. Kim, J.Y. Yu, J.H. Lee, Nanoscale 9 (2017) 7094–7103. [25] E.S. Cho, A.M. Ruminski, S. Aloni, Y. Liu, J.H. Guo, J.J. Urban, Nat. Commun. 7 (2016) 10804.
Fig. 10. DSC curves of (A) MM-Mg95Ni5 and (B) WM-7% PVP measured at different heating rates (β = 3, 5, 7 °C/min, respectively), and (C) the ln (β/Tp2) − 1/Tp linear diagram of MM-Mg95Ni5 and WM-7% PVP for the decomposition of MgH2.
Mg95Ni5 composites prepared by wet milling process lower the driving force for Mg nucleation and promote the dehydrogenation of MgH2, which results in a reduced dehydrogenation temperature. Therefore, the improved dehydrogenation properties of Mg95Ni5 composites wet milled with THF and PVP might be due to the following reasons. Firstly, the addition of PVP and THF has positive effects on alleviating the aggregation and cold welding of Mg and MgH2 particles during mechanical milling process, which facilitates the refinement of particle size and crystal size and so the uniform distribution of the particle size, resulting in more active sites and the reducing of the hydrogen diffusion distance. Secondly, as a polar solvent, THF fully contact with MgH2 particles during ball milling process. And the synergetic effect between THF and PVP help to significantly weaken the Mg-H bond, which improves the dehydrogenation property of the composite. Finally, Mg2NiH4 has catalytic effect on the dehydrogenation of MgH2 7
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