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Effects of in-situ formed Mg2Si phase on the hydrogen storage properties of MgLi solid solution alloys
Effects of in-Situ formed Mg 2 Si phase on the hydrogen storage properties of Mg-Li solid solution alloys Yongqing Wang, Zhou Zhiyan, Wenzheng Zhou, Liqin Xu, Jin Guo, Zhiqiang Lan PII: DOI: Reference:
S0264-1275(16)31143-1 doi: 10.1016/j.matdes.2016.08.080 JMADE 2233
To appear in: Received date: Revised date: Accepted date:
12 May 2016 15 August 2016 29 August 2016
Please cite this article as: Yongqing Wang, Zhou Zhiyan, Wenzheng Zhou, Liqin Xu, Jin Guo, Zhiqiang Lan, Effects of in-Situ formed Mg2 Si phase on the hydrogen storage properties of Mg-Li solid solution alloys, (2016), doi: 10.1016/j.matdes.2016.08.080
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ACCEPTED MANUSCRIPT Effects of in-Situ formed Mg2Si phase on the hydrogen storage properties of Mg-Li solid solution alloys
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Yongqing Wang a) Zhou Zhiyan a) Wenzheng Zhou a),b) Liqin Xu a) Jin Guo a), b) Zhiqiang Lan a), b)
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a) Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related
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Technology, College of Physics Science and Technology, Guangxi University, Nanning 530004, P. R. China
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b) Guangxi Collaborative Innovation Center of Structure and Performance for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology,
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Guilin, 541004, P. R. China
Corresponding author: zhiqiang Lan; E-mail: [email protected]
ACCEPTED MANUSCRIPT Abstract Mg-Li solid solution alloys, Mg90Li10-xSix (x=0,2,4,6), were synthesized via sintering and
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ball-milling. Analysis of the samples, via X-ray diffraction (XRD), revealed that the Mg2Si phase
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formed with the addition of Si. Moreover, measurements of the pressure composition isotherms
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(PCT) revealed reversible hydrogen storage capacities of 6.47 wt.%, 5.69 wt.%, 5.56 wt.%, and 5.28 wt.%, respectively, at 623 K, for the Mg90Li10-xSix (x=0,2,4,6) alloys. The Mg2Si acted as a
reaction,
and
improved
the
kinetic
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catalyst that reduced the apparent activation energy (Ea), enhanced the reversible hydrogenation and
thermodynamic
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hydrogenation/dehydrogenation process of the Mg-Li solid solution.
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Keywords: Hydrogen storage property; Mg-Li solid solution; Mg2Si phase
performance
of
the
ACCEPTED MANUSCRIPT 1. Introduction Mg has a reversible hydrogen storage capacity of 7.76 wt.% and constitutes, therefore, one of
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the most promising candidates for use as on-board hydrogen storage materials. However, the
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relatively high dehydrogenation temperature (>623 K) and poor kinetic properties have prevented
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the use of MgH2 in practical applications [1,2]. Some methods such as mechanical alloying [3–5], the use of transition metal additives [6–8], nanostructuring [9–11], and intermetallic compounds
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[12,13] have proven effective in overcoming these drawbacks. For example, a MgH2+10wt.%LaCl3 composite prepared via mechanical alloying, absorbed 5.1wt.% of H within 2 min at 573 K, and the
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activation energy of dehydrogenation was 143.0 kJ/mol [5].A Mg hydride decomposed at 498 K,
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with the addition of a transition metal [6]. In addition, a Mg-5wt.%LaNi5 nanocomposite absorbed
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3.5 wt.% of H2 in <5 min at 473 K, and the hydrogen storage capacity increased to 6.7 wt.% at 673 K; the apparent activation energy for hydrogenation was only 26.3 kJ mol.−1 [13]. Although the
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aforementioned methods are effective in improving the hydrogen storage properties of Mg or Mg-based alloys, implementing Mg or Mg-based alloys in practical applications remains difficult.
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In order to improve the integrated hydrogen storage properties of Mg or Mg-based alloys to meet the practical applications demands, further researches have been made. Recently, hydrogen storage materials have been prepared by using modern nanotechnology, the method was introduced to perform microstructure, composition and surface modification to Mg or Mg-based alloys[14-18]. For example, 100 h of milling of a Mg50Co50 composite yielded a body-centered cubic (BCC)-structured Mg-Co solid solution, which absorbed hydrogen at 100 °C [16]. The BCC structure remained unchanged during hydrogenation at temperature lower than 160 °C [17]. Furthermore, the activation energy for Mg2Co crystallization prepared by mechanical alloying decreased from 206 to 184 kJ/mol. when the milling time was increased from 12 to 36 h [18]. These
ACCEPTED MANUSCRIPT results indicated that the hydrogen storage properties of Mg-based composites can be improved by using modern nanotechnology. Li also can be used as a hydrogen storage material when combined
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with other elements to form binary or ternary compounds such as LiAlH4 and LiBH4. For Lithium
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hydride, though a high hydrogen storage capacity of 11.5wt.%, the hydride is very stable and do not
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release hydrogen until the temperature increases to ~ 910℃ under an equilibrium pressure of 1 bar. The practical application of Li as a direct hydrogen carrier faces severe challenges. It is known that
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the Mg-Li binary alloy has a broad solid solution region, and solid solution typically has the same crystal structure as the matrix. Hence, the Mg-Li solid solution may have a high hydrogen storage Meanwhile, it was reported that the presence of Si facilitates the dehydrogenation of
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capacity.
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MgH2 via formation of Mg2Si alloy[19]. Therefore, it is reasonable to assume that Mg2Si would act
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as a catalyst promoting Mg-Li solid solution system hydrogen storage properties. As such, in this work, the Mg-Li solid solution was prepared and the effect of the Mg2Si phase on hydrogen storage
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properties of the Mg-Li solid solution was investigated.
ACCEPTED MANUSCRIPT 2. Experimental detail Powders of Mg (purity >99.5%), LiH (purity >99.5%), and Si (>99.9 wt.%), purchased from
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Alfa Aesar, were used as the raw materials. These powders were mixed in a molar ratio of
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90:(10-x):x(x=0,2,4,6) and compressed into round tablets, under a pressure of 25 MPa. The tablets
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were sintered at 773 K for 2 h in a high vacuum resistance furnace under an argon atmosphere, and then cooled with furnace temperature. Afterwards, the as-sintered samples were crushed into a
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powder and milled at 300 rpm for 50 h in a planetary miller (QM-WX04), at a ball-to-powder mass ratio of 40:1. The pressure composition isotherms (PCT) of the powders were measured under a
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hydrogen pressure of 3.5MPa at 598K and 623K by using an automatic Sievert-type apparatus. The
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temperature-programmed hydrogenation was performed under a hydrogen pressure of 6.5MPa with
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a heating rate of 2K/min from room temperature to 660K by using a homemade Sievert-type apparatus. Similarly, for the dehydrogenation process, the sample was heated from room
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temperature to 660K with a heating rate of 2K/min after the reactor was evacuated to 0.06atm. All the samples were fully activated (absorption under 35atm H2 for 4h at 623K, and desorption under
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0.06atm H2 for 4h at 673K) before PCT and hydrogenation/dehydrogenation kinetics testing. The phase composition in each sample was identified via X-ray diffraction (XRD; Rinku Miniflex X-ray diffractometer), using CuKα radiation at 40 kV and 200 mA, over a 2 range of 20°–80°. The samples were also examined via differential scanning calorimetry (DSC), by using a simultaneous thermal analyzer (Linseis STA PT-1000). During these measurements, the samples were heated at rates of 5 K/min, 10 K/min, 15 K/min, and 20 K/min, under an argon atmosphere.
ACCEPTED MANUSCRIPT 3. Results and discussions The XRD patterns of the Mg90Li10-xSix (x=0,2,4,6) alloys are shown in Fig.1. As the Fig.1(a)
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shows, the Mg90Li10 (x=0) alloy consists of a single Mg phase. Second phase Mg2Si was formed
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with the addition of Si. Moreover, diffraction peaks corresponding to Li were absent from the
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patterns, indicating that Li atoms diffused into the Mg matrix, thereby forming a Mg-Li solid solution (i.e., Mg(Li)). Table 1 lists the cell parameters of Mg and Mg2Si phase in the Mg90Li10-xSix
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(x=0,2,4,6) alloys. The cell volume of Mg in the Mg90Li10-xSix (x=0,2,4,6) alloys is 0.04587nm3 (x=0), 0.04599 nm3(x=2), 0.04628 nm3(x=4) and 0.04621 nm3(x=6), respectively, and the phase
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abundance of Mg2Si phase also increases as increasing Si content. The reaction process and the
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mechanism governing this process were investigated via XRD analyses (Fig.1(b) and Fig.1(c)) of
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the hydrogenated and dehydrogenated Mg90Li10-xSix (x=0,2,4,6) alloys. During hydrogenation, the Mg-Li solid solution phase was transformed to Mg(Li)H2 (Fig.1(b)), whereas the Mg-Li solid
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solution was the only phase formed after dehydrogenation (Fig.1(c)). As hydrogenation production, the cell volume of MgH2 is almost the same in the Mg90Li10-xSix (x=0,2,4,6) hydrides as listed in
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Table 1. The experimental cell parameters for α-MgH2 are matched very well with data reported in the literature[20]. A comparison of Fig.1(a) and Fig.1(c) reveals that the two XRD patterns are identical, indicating that the structure of the non-hydrogenated Mg-Li solid solution is restored after the hydrogenation/dehydrogenation cycle. In addition, the Mg2Si phase remained unchanged during the hydrogenation/dehydrogenation process. These results show that the hydrogenation reaction is reversible. This reaction is given as:
Mg(Li) H 2 Mg(Li)H 2
(1)
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Fig.1. XRD patterns of the Mg90Li10-xSix (x=0,2,4,6) alloys (a) as-milled (b) hydrogenation and (c) dehydrogenation
ACCEPTED MANUSCRIPT Table1 The phase characteristics of the Mg90Li10-xSix (x=0,2,4,6) alloys
c
0.3197
0.3197
0.5182
0.04587
α-MgH2 0.4505
0.4505
0.3014
0.06116
0.3200
0.3200
0.5186
0.04599
Mg2Si
0.6355
0.6355
0.6355
α-MgH2 0.4512
0.4512
Note
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As-milled Hydrogenation
98.0
As-milled
0.2567
2.0
As-milled
0.3018
0.06143
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Hydrogenation
0.04628
94.4
As-milled
0.3204
0.3204
0.5204
Mg2Si
0.6346
0.6346
0.6346
0.2556
5.6
As-milled
α-MgH2 0.4510
0.4510
0.3017
0.06135
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Hydrogenation
0.5194
0.04621
91.1
As-milled
0.6346
0.2556
8.9
As-milled
0.3016
0.06132
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Hydrogenation
0.3205
0.3205
Mg2Si
0.6346
0.6346
α-MgH2 0.4509
0.4509
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Mg
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Mg
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Mg
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abundance/%
a
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Phase
volume/nm3
Mg x=0
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Lattice parameter/nm Sample
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A plateau in the PCT curve corresponds to a hydrogenation reaction process. The PCT measurement
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results coincide with the XRD analysis (Fig.1(b) and Fig.1(c)). The curves in Fig.2(a) indicate that
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no dehydrogenation process was found in Mg90Li10-xSix(x=0,2) alloy, whereas it was observed in Mg90Li10-xSix(x=4,6) alloy at 598K; this indicates that the Mg90Li10-xSix (x=0,2) alloy hydrides are
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very stable. The stability of the Mg-Li solid solution hydride decreases with the addition of Si. As previously mentioned, the Mg2Si phase, which formed with the addition of Si, remained unchanged
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during the hydrogenation/dehydrogenation process. The Si element acted a very important role to
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lower the stability of MgH2 by reducing the band gaps of MgH2, which made the Mg–H bond more
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susceptible to dissociation[21]. In fact, the Mg2Si phase acted as a catalyst that lowered the reaction barrier for dehydrogenation of the Mg-Li solid solution. This catalytic action gives rise to the
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reversible hydrogenation reaction that occurs in the case of the Mg90Li10-xSix (x=4,6) alloys at 598 K. Dehydrogenation at 598 K did not occur in the case of the Mg90Li10-xSix (x=2) alloy, which is due to
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only a small amount of the Mg2Si catalyst contained in the alloy. The Mg90Li10-xSix (x=0,2) alloys also showed a reversible hydrogenation/dehydrogenation process (Fig.2(b)), however, when the temperature was increased to 623 K. The Mg90Li10-xSix (x=0,2,4,6) alloy exhibited a reversible hydrogen storage capacity of 6.47 wt.%, 5.69 wt.%, 5.56 wt.%, and 5.28 wt.%, respectively, at 623K.
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Fig.2. PCT curves of the Mg90Li10-xSix (x=0,2,4,6) alloys measured at (a) 598 K and (b) 623 K
ACCEPTED MANUSCRIPT The effect of the Mg2Si phase on the hydrogen storage properties of the Mg-Li solid solution was investigated in further detail, by comparing the hydrogenation/dehydrogenation kinetic curves
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of the Mg90Li10-xSix (x=0,2,4,6) alloys. During the measurements, the temperature was increased
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from room temperature to 660 K (as shown in Fig. 3(a)) at a rate of 2 K/min and under a hydrogen
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pressure of 6.5 MPa. The Mg90Li10-xSix (x=0,2,4,6) alloys had the same onset temperature of hydrogenation, as shown in Fig. 3(a). Nevertheless, the steeper slope of the curve corresponding to
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the Mg90Li10-xSix (x=2,4,6) alloys, indicating that the Mg90Li10-xSix (x=2,4,6) alloy is hydrogenated at a faster rate than the Mg90Li10 alloys. It appears that the Mg90Li10-xSix (x=2,4,6) alloy had better
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hydrogenation properties than the Mg90Li10(x=0) alloy. These results can be explained by assuming
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that the Mg2Si phase acts as a catalyst that reduces the hydrogenation barrier and improves the
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hydrogenation properties of the Mg-Li solid solution. As shown in Fig. 3(b), in the case of the dehydrogenation reaction, the Mg90Li4Si6(x=6) alloy had a significantly lower onset temperature of
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dehydrogenation than that of the Mg90Li10-xSix (x=0,2,4) alloys. The onset temperature of dehydrogenation for the Mg90Li10(x=0) alloy was 643K, and it decreased to 625K when x value
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increases to 2 and 4. In the case of Mg90Li4Si6(x=6) alloy, the onset temperature of dehydrogenation was 610K, and it was completely dehydrogenated when the temperature was increased to 650 K. And the Mg90Li8Si2(x=2) and Mg90Li6Si4(x=4) alloy could release 82.4% and 98.9% of the hydrogen absorbed, respectively. Whereas only 28.6% of the hydrogen absorbed in the Mg90Li10(x=0) alloy was released in the same condition. As shown in Fig.1, the Mg2Si phase remained unchanged during hydrogenation/dehydrogenation process, indicating that the Mg2Si phase is stable in hydrogenation reaction. The stability of the Mg2Si phase could reduce the standard enthalpy of dehydrogenation of MgH2[19]. Therefore, the Mg2Si phase acted as a catalyst during both hydrogenation and dehydrogenation. This catalytic action enhances the hydrogenation
ACCEPTED MANUSCRIPT reaction of the Mg-Li solid solution and improves the kinetic performances of the hydrogenation/dehydrogenation. These results are fairly consistent with the report that the hydrogen
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desorption temperature of MgH2 was reduced with the addition of Si [22].
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Fig.3. Kinetic curves obtained during (a) hydrogenation and (b) dehydrogenation of the
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Mg90Li10-xSix (x=0,2,4,6) alloys
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Fig.4. DSC curves of the Mg90Li10-xSix (x=0, 2, 4, 6) hydrides
ACCEPTED MANUSCRIPT Fig. 4 shows DSC curves of the Mg90Li10-xSix (x=0, 2, 4, 6) hydrides. Each of the DSC curves consists of an endothermic peak that occur at temperatures of 688 K, 670K, 666K and 653 K in the
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case of the Mg90Li10-xSix (x=0, 2, 4, 6) alloy hydrides, respectively. The result has again shown that
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the Mg2Si phase acts as an effective catalyst in reducing the dehydrogenation temperature of the
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Mg-Li solid solution. It is known that the melting temperature of Li is 453K. The endothermic peak temperatures in Fig.4 were much higher than 453K, indicating that Li was completely dissolved in Mg
matrix,
and
did
not
be
separated
from
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the
the
solid
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hydrogenation/dehydrogenation process, as previously indicated by Fig.1.
solution
during
the
ACCEPTED MANUSCRIPT The activation energy barrier of dehydrogenation of the Mg-Li solid solution was estimated in order to further understand the role of the Mg2Si phase in improving the hydrogen storage
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thermodynamic performance of the Mg-Li solid solution. The apparent activation energy (Ea) for
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2 p
) A
Ea RT p
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ln(
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the dehydrogenation was determined by using the Kissinger equation, which is given as [23]: (2)
Where β, Tp, R, and A are the heating rate, peak temperature in the DSC curve, gas constant, and
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linear constant, respectively. The DSC measurements were performed at heating rates of 5
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K/min,10 K/min,15 K/min, and 20 K/min, under a fixed argon flow of 30 mL/min. According to the Kissinger equation, Ea is determined from the slope of the plot of ln 2 vs. 1000/Tp, as Tp
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shown in Fig.5. The Ea of the Mg-Li solid solution is 179.4kJ·mol-1, and decreases to163.6 kJ·mol-1, 155.2kJ·mol-1 and 128.3 kJ·mol-1 for the Mg90Li10-xSix (x=0, 2, 4, 6) alloys, respectively. The results
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show that the activation energy decreases with the increase of the Mg2Si content. This confirms that the activation energy of the Mg-Li solid solution is reduced under the catalytic action of the Mg2Si
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phase during the hydrogenation/dehydrogenation process.
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Fig.5. Kissinger curves corresponding to the dehydrogenation of Mg90Li10-xSix (x=0,2,4,6)
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ACCEPTED MANUSCRIPT 4. Conclusions In summary, the Mg-Li solid solution was prepared via sintering and ball-milling and its
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hydrogen storage performance was investigated. Analysis of the XRD data indicated that the
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hydrogenation reaction, Mg(Li) H2 Mg(Li)H 2 , of the Mg-Li solid solution is irreversible. In
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addition, a Mg2Si phase formed when Si was added to the alloy. The Mg2Si phase acted as a catalyst that improved the hydrogenation and dehydrogenation properties of the Mg-Li solid solution. For
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example, the Mg90Li4Si6(x=6) alloy was completely dehydrogenated when heated at a rate of 2 K/min from room temperature to 650 K; only 28.6% of the hydrogen absorbed in the Mg90Li10(x=0)
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alloy was released, under the same conditions. The temperature for the onset of dehydrogenation of
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the Mg90Li4Si6(x=6) alloy was significantly lower than that of the Mg90Li10(x=0) alloy. Furthermore,
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the apparent activation energy (Ea) of the Mg90Li4Si6(x=6) alloy is 128.3 kJ/mol, which is 51.2 kJ/mol lower than that of the Mg90Li10 alloy. The Mg2Si phase acted as a catalyst that reduced the
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reaction barrier of the hydrogenation/dehydrogenation process of the Mg-Li solid solution.
ACCEPTED MANUSCRIPT Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 51571065,
51401055),
the Natural Science Foundation of
Guangxi Province
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51271061,
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(Grant no.2013GXNSFGA019007), Foundation of Guangxi Educational Committee (Grant
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No.2013YB006) and the Key Laboratory of Guangxi for Nonferrous Metals and Materials
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Processing Technology (Grant No. 12-A-01-07).
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Graphical abstract
ACCEPTED MANUSCRIPT Highlihghts:
Effect of Mg2Si on hydrogen storage properties of Mg-Li alloys was investigated.
The Mg2Si phase could improve the hydrogenation and dehydrogenation properties of the Mg-Li
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The Mg90Li6Si4 (x=6) alloy showed the best hydrogen storage property.
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alloys.
S0264-1275(16)31143-1 doi: 10.1016/j.matdes.2016.08.080 JMADE 2233
To appear in: Received date: Revised date: Accepted date:
12 May 2016 15 August 2016 29 August 2016
Please cite this article as: Yongqing Wang, Zhou Zhiyan, Wenzheng Zhou, Liqin Xu, Jin Guo, Zhiqiang Lan, Effects of in-Situ formed Mg2 Si phase on the hydrogen storage properties of Mg-Li solid solution alloys, (2016), doi: 10.1016/j.matdes.2016.08.080
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT Effects of in-Situ formed Mg2Si phase on the hydrogen storage properties of Mg-Li solid solution alloys
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Yongqing Wang a) Zhou Zhiyan a) Wenzheng Zhou a),b) Liqin Xu a) Jin Guo a), b) Zhiqiang Lan a), b)
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a) Guangxi Colleges and Universities Key Laboratory of Novel Energy Materials and Related
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Technology, College of Physics Science and Technology, Guangxi University, Nanning 530004, P. R. China
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b) Guangxi Collaborative Innovation Center of Structure and Performance for New Energy and Materials, School of Material Science and Engineering, Guilin University of Electronic Technology,
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Guilin, 541004, P. R. China
Corresponding author: zhiqiang Lan; E-mail: [email protected]
ACCEPTED MANUSCRIPT Abstract Mg-Li solid solution alloys, Mg90Li10-xSix (x=0,2,4,6), were synthesized via sintering and
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ball-milling. Analysis of the samples, via X-ray diffraction (XRD), revealed that the Mg2Si phase
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formed with the addition of Si. Moreover, measurements of the pressure composition isotherms
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(PCT) revealed reversible hydrogen storage capacities of 6.47 wt.%, 5.69 wt.%, 5.56 wt.%, and 5.28 wt.%, respectively, at 623 K, for the Mg90Li10-xSix (x=0,2,4,6) alloys. The Mg2Si acted as a
reaction,
and
improved
the
kinetic
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catalyst that reduced the apparent activation energy (Ea), enhanced the reversible hydrogenation and
thermodynamic
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hydrogenation/dehydrogenation process of the Mg-Li solid solution.
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Keywords: Hydrogen storage property; Mg-Li solid solution; Mg2Si phase
performance
of
the
ACCEPTED MANUSCRIPT 1. Introduction Mg has a reversible hydrogen storage capacity of 7.76 wt.% and constitutes, therefore, one of
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the most promising candidates for use as on-board hydrogen storage materials. However, the
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relatively high dehydrogenation temperature (>623 K) and poor kinetic properties have prevented
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the use of MgH2 in practical applications [1,2]. Some methods such as mechanical alloying [3–5], the use of transition metal additives [6–8], nanostructuring [9–11], and intermetallic compounds
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[12,13] have proven effective in overcoming these drawbacks. For example, a MgH2+10wt.%LaCl3 composite prepared via mechanical alloying, absorbed 5.1wt.% of H within 2 min at 573 K, and the
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activation energy of dehydrogenation was 143.0 kJ/mol [5].A Mg hydride decomposed at 498 K,
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with the addition of a transition metal [6]. In addition, a Mg-5wt.%LaNi5 nanocomposite absorbed
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3.5 wt.% of H2 in <5 min at 473 K, and the hydrogen storage capacity increased to 6.7 wt.% at 673 K; the apparent activation energy for hydrogenation was only 26.3 kJ mol.−1 [13]. Although the
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aforementioned methods are effective in improving the hydrogen storage properties of Mg or Mg-based alloys, implementing Mg or Mg-based alloys in practical applications remains difficult.
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In order to improve the integrated hydrogen storage properties of Mg or Mg-based alloys to meet the practical applications demands, further researches have been made. Recently, hydrogen storage materials have been prepared by using modern nanotechnology, the method was introduced to perform microstructure, composition and surface modification to Mg or Mg-based alloys[14-18]. For example, 100 h of milling of a Mg50Co50 composite yielded a body-centered cubic (BCC)-structured Mg-Co solid solution, which absorbed hydrogen at 100 °C [16]. The BCC structure remained unchanged during hydrogenation at temperature lower than 160 °C [17]. Furthermore, the activation energy for Mg2Co crystallization prepared by mechanical alloying decreased from 206 to 184 kJ/mol. when the milling time was increased from 12 to 36 h [18]. These
ACCEPTED MANUSCRIPT results indicated that the hydrogen storage properties of Mg-based composites can be improved by using modern nanotechnology. Li also can be used as a hydrogen storage material when combined
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with other elements to form binary or ternary compounds such as LiAlH4 and LiBH4. For Lithium
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hydride, though a high hydrogen storage capacity of 11.5wt.%, the hydride is very stable and do not
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release hydrogen until the temperature increases to ~ 910℃ under an equilibrium pressure of 1 bar. The practical application of Li as a direct hydrogen carrier faces severe challenges. It is known that
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the Mg-Li binary alloy has a broad solid solution region, and solid solution typically has the same crystal structure as the matrix. Hence, the Mg-Li solid solution may have a high hydrogen storage Meanwhile, it was reported that the presence of Si facilitates the dehydrogenation of
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capacity.
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MgH2 via formation of Mg2Si alloy[19]. Therefore, it is reasonable to assume that Mg2Si would act
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as a catalyst promoting Mg-Li solid solution system hydrogen storage properties. As such, in this work, the Mg-Li solid solution was prepared and the effect of the Mg2Si phase on hydrogen storage
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properties of the Mg-Li solid solution was investigated.
ACCEPTED MANUSCRIPT 2. Experimental detail Powders of Mg (purity >99.5%), LiH (purity >99.5%), and Si (>99.9 wt.%), purchased from
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Alfa Aesar, were used as the raw materials. These powders were mixed in a molar ratio of
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90:(10-x):x(x=0,2,4,6) and compressed into round tablets, under a pressure of 25 MPa. The tablets
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were sintered at 773 K for 2 h in a high vacuum resistance furnace under an argon atmosphere, and then cooled with furnace temperature. Afterwards, the as-sintered samples were crushed into a
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powder and milled at 300 rpm for 50 h in a planetary miller (QM-WX04), at a ball-to-powder mass ratio of 40:1. The pressure composition isotherms (PCT) of the powders were measured under a
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hydrogen pressure of 3.5MPa at 598K and 623K by using an automatic Sievert-type apparatus. The
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temperature-programmed hydrogenation was performed under a hydrogen pressure of 6.5MPa with
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a heating rate of 2K/min from room temperature to 660K by using a homemade Sievert-type apparatus. Similarly, for the dehydrogenation process, the sample was heated from room
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temperature to 660K with a heating rate of 2K/min after the reactor was evacuated to 0.06atm. All the samples were fully activated (absorption under 35atm H2 for 4h at 623K, and desorption under
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0.06atm H2 for 4h at 673K) before PCT and hydrogenation/dehydrogenation kinetics testing. The phase composition in each sample was identified via X-ray diffraction (XRD; Rinku Miniflex X-ray diffractometer), using CuKα radiation at 40 kV and 200 mA, over a 2 range of 20°–80°. The samples were also examined via differential scanning calorimetry (DSC), by using a simultaneous thermal analyzer (Linseis STA PT-1000). During these measurements, the samples were heated at rates of 5 K/min, 10 K/min, 15 K/min, and 20 K/min, under an argon atmosphere.
ACCEPTED MANUSCRIPT 3. Results and discussions The XRD patterns of the Mg90Li10-xSix (x=0,2,4,6) alloys are shown in Fig.1. As the Fig.1(a)
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shows, the Mg90Li10 (x=0) alloy consists of a single Mg phase. Second phase Mg2Si was formed
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with the addition of Si. Moreover, diffraction peaks corresponding to Li were absent from the
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patterns, indicating that Li atoms diffused into the Mg matrix, thereby forming a Mg-Li solid solution (i.e., Mg(Li)). Table 1 lists the cell parameters of Mg and Mg2Si phase in the Mg90Li10-xSix
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(x=0,2,4,6) alloys. The cell volume of Mg in the Mg90Li10-xSix (x=0,2,4,6) alloys is 0.04587nm3 (x=0), 0.04599 nm3(x=2), 0.04628 nm3(x=4) and 0.04621 nm3(x=6), respectively, and the phase
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abundance of Mg2Si phase also increases as increasing Si content. The reaction process and the
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mechanism governing this process were investigated via XRD analyses (Fig.1(b) and Fig.1(c)) of
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the hydrogenated and dehydrogenated Mg90Li10-xSix (x=0,2,4,6) alloys. During hydrogenation, the Mg-Li solid solution phase was transformed to Mg(Li)H2 (Fig.1(b)), whereas the Mg-Li solid
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solution was the only phase formed after dehydrogenation (Fig.1(c)). As hydrogenation production, the cell volume of MgH2 is almost the same in the Mg90Li10-xSix (x=0,2,4,6) hydrides as listed in
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Table 1. The experimental cell parameters for α-MgH2 are matched very well with data reported in the literature[20]. A comparison of Fig.1(a) and Fig.1(c) reveals that the two XRD patterns are identical, indicating that the structure of the non-hydrogenated Mg-Li solid solution is restored after the hydrogenation/dehydrogenation cycle. In addition, the Mg2Si phase remained unchanged during the hydrogenation/dehydrogenation process. These results show that the hydrogenation reaction is reversible. This reaction is given as:
Mg(Li) H 2 Mg(Li)H 2
(1)
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Fig.1. XRD patterns of the Mg90Li10-xSix (x=0,2,4,6) alloys (a) as-milled (b) hydrogenation and (c) dehydrogenation
ACCEPTED MANUSCRIPT Table1 The phase characteristics of the Mg90Li10-xSix (x=0,2,4,6) alloys
c
0.3197
0.3197
0.5182
0.04587
α-MgH2 0.4505
0.4505
0.3014
0.06116
0.3200
0.3200
0.5186
0.04599
Mg2Si
0.6355
0.6355
0.6355
α-MgH2 0.4512
0.4512
Note
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As-milled Hydrogenation
98.0
As-milled
0.2567
2.0
As-milled
0.3018
0.06143
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Hydrogenation
0.04628
94.4
As-milled
0.3204
0.3204
0.5204
Mg2Si
0.6346
0.6346
0.6346
0.2556
5.6
As-milled
α-MgH2 0.4510
0.4510
0.3017
0.06135
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Hydrogenation
0.5194
0.04621
91.1
As-milled
0.6346
0.2556
8.9
As-milled
0.3016
0.06132
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Hydrogenation
0.3205
0.3205
Mg2Si
0.6346
0.6346
α-MgH2 0.4509
0.4509
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Mg
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Mg
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Mg
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x=6
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b
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x=4
abundance/%
a
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x=2
Phase
volume/nm3
Mg x=0
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ACCEPTED MANUSCRIPT Fig.2 shows the PCT curves of the Mg90Li10-xSix(x=0,2,4,6) alloys measured at 598K and 623K. Only a plateau appeared in the PCT curves of the Mg90Li10-xSix(x=0,2,4,6) alloys as shown in Fig.2.
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A plateau in the PCT curve corresponds to a hydrogenation reaction process. The PCT measurement
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results coincide with the XRD analysis (Fig.1(b) and Fig.1(c)). The curves in Fig.2(a) indicate that
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no dehydrogenation process was found in Mg90Li10-xSix(x=0,2) alloy, whereas it was observed in Mg90Li10-xSix(x=4,6) alloy at 598K; this indicates that the Mg90Li10-xSix (x=0,2) alloy hydrides are
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very stable. The stability of the Mg-Li solid solution hydride decreases with the addition of Si. As previously mentioned, the Mg2Si phase, which formed with the addition of Si, remained unchanged
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during the hydrogenation/dehydrogenation process. The Si element acted a very important role to
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lower the stability of MgH2 by reducing the band gaps of MgH2, which made the Mg–H bond more
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susceptible to dissociation[21]. In fact, the Mg2Si phase acted as a catalyst that lowered the reaction barrier for dehydrogenation of the Mg-Li solid solution. This catalytic action gives rise to the
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reversible hydrogenation reaction that occurs in the case of the Mg90Li10-xSix (x=4,6) alloys at 598 K. Dehydrogenation at 598 K did not occur in the case of the Mg90Li10-xSix (x=2) alloy, which is due to
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only a small amount of the Mg2Si catalyst contained in the alloy. The Mg90Li10-xSix (x=0,2) alloys also showed a reversible hydrogenation/dehydrogenation process (Fig.2(b)), however, when the temperature was increased to 623 K. The Mg90Li10-xSix (x=0,2,4,6) alloy exhibited a reversible hydrogen storage capacity of 6.47 wt.%, 5.69 wt.%, 5.56 wt.%, and 5.28 wt.%, respectively, at 623K.
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Fig.2. PCT curves of the Mg90Li10-xSix (x=0,2,4,6) alloys measured at (a) 598 K and (b) 623 K
ACCEPTED MANUSCRIPT The effect of the Mg2Si phase on the hydrogen storage properties of the Mg-Li solid solution was investigated in further detail, by comparing the hydrogenation/dehydrogenation kinetic curves
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of the Mg90Li10-xSix (x=0,2,4,6) alloys. During the measurements, the temperature was increased
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from room temperature to 660 K (as shown in Fig. 3(a)) at a rate of 2 K/min and under a hydrogen
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pressure of 6.5 MPa. The Mg90Li10-xSix (x=0,2,4,6) alloys had the same onset temperature of hydrogenation, as shown in Fig. 3(a). Nevertheless, the steeper slope of the curve corresponding to
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the Mg90Li10-xSix (x=2,4,6) alloys, indicating that the Mg90Li10-xSix (x=2,4,6) alloy is hydrogenated at a faster rate than the Mg90Li10 alloys. It appears that the Mg90Li10-xSix (x=2,4,6) alloy had better
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hydrogenation properties than the Mg90Li10(x=0) alloy. These results can be explained by assuming
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that the Mg2Si phase acts as a catalyst that reduces the hydrogenation barrier and improves the
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hydrogenation properties of the Mg-Li solid solution. As shown in Fig. 3(b), in the case of the dehydrogenation reaction, the Mg90Li4Si6(x=6) alloy had a significantly lower onset temperature of
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dehydrogenation than that of the Mg90Li10-xSix (x=0,2,4) alloys. The onset temperature of dehydrogenation for the Mg90Li10(x=0) alloy was 643K, and it decreased to 625K when x value
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increases to 2 and 4. In the case of Mg90Li4Si6(x=6) alloy, the onset temperature of dehydrogenation was 610K, and it was completely dehydrogenated when the temperature was increased to 650 K. And the Mg90Li8Si2(x=2) and Mg90Li6Si4(x=4) alloy could release 82.4% and 98.9% of the hydrogen absorbed, respectively. Whereas only 28.6% of the hydrogen absorbed in the Mg90Li10(x=0) alloy was released in the same condition. As shown in Fig.1, the Mg2Si phase remained unchanged during hydrogenation/dehydrogenation process, indicating that the Mg2Si phase is stable in hydrogenation reaction. The stability of the Mg2Si phase could reduce the standard enthalpy of dehydrogenation of MgH2[19]. Therefore, the Mg2Si phase acted as a catalyst during both hydrogenation and dehydrogenation. This catalytic action enhances the hydrogenation
ACCEPTED MANUSCRIPT reaction of the Mg-Li solid solution and improves the kinetic performances of the hydrogenation/dehydrogenation. These results are fairly consistent with the report that the hydrogen
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desorption temperature of MgH2 was reduced with the addition of Si [22].
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Fig.3. Kinetic curves obtained during (a) hydrogenation and (b) dehydrogenation of the
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Mg90Li10-xSix (x=0,2,4,6) alloys
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Fig.4. DSC curves of the Mg90Li10-xSix (x=0, 2, 4, 6) hydrides
ACCEPTED MANUSCRIPT Fig. 4 shows DSC curves of the Mg90Li10-xSix (x=0, 2, 4, 6) hydrides. Each of the DSC curves consists of an endothermic peak that occur at temperatures of 688 K, 670K, 666K and 653 K in the
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case of the Mg90Li10-xSix (x=0, 2, 4, 6) alloy hydrides, respectively. The result has again shown that
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the Mg2Si phase acts as an effective catalyst in reducing the dehydrogenation temperature of the
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Mg-Li solid solution. It is known that the melting temperature of Li is 453K. The endothermic peak temperatures in Fig.4 were much higher than 453K, indicating that Li was completely dissolved in Mg
matrix,
and
did
not
be
separated
from
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the
the
solid
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hydrogenation/dehydrogenation process, as previously indicated by Fig.1.
solution
during
the
ACCEPTED MANUSCRIPT The activation energy barrier of dehydrogenation of the Mg-Li solid solution was estimated in order to further understand the role of the Mg2Si phase in improving the hydrogen storage
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thermodynamic performance of the Mg-Li solid solution. The apparent activation energy (Ea) for
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2 p
) A
Ea RT p
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ln(
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the dehydrogenation was determined by using the Kissinger equation, which is given as [23]: (2)
Where β, Tp, R, and A are the heating rate, peak temperature in the DSC curve, gas constant, and
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linear constant, respectively. The DSC measurements were performed at heating rates of 5
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K/min,10 K/min,15 K/min, and 20 K/min, under a fixed argon flow of 30 mL/min. According to the Kissinger equation, Ea is determined from the slope of the plot of ln 2 vs. 1000/Tp, as Tp
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shown in Fig.5. The Ea of the Mg-Li solid solution is 179.4kJ·mol-1, and decreases to163.6 kJ·mol-1, 155.2kJ·mol-1 and 128.3 kJ·mol-1 for the Mg90Li10-xSix (x=0, 2, 4, 6) alloys, respectively. The results
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show that the activation energy decreases with the increase of the Mg2Si content. This confirms that the activation energy of the Mg-Li solid solution is reduced under the catalytic action of the Mg2Si
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phase during the hydrogenation/dehydrogenation process.
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Fig.5. Kissinger curves corresponding to the dehydrogenation of Mg90Li10-xSix (x=0,2,4,6)
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ACCEPTED MANUSCRIPT 4. Conclusions In summary, the Mg-Li solid solution was prepared via sintering and ball-milling and its
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hydrogen storage performance was investigated. Analysis of the XRD data indicated that the
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hydrogenation reaction, Mg(Li) H2 Mg(Li)H 2 , of the Mg-Li solid solution is irreversible. In
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addition, a Mg2Si phase formed when Si was added to the alloy. The Mg2Si phase acted as a catalyst that improved the hydrogenation and dehydrogenation properties of the Mg-Li solid solution. For
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example, the Mg90Li4Si6(x=6) alloy was completely dehydrogenated when heated at a rate of 2 K/min from room temperature to 650 K; only 28.6% of the hydrogen absorbed in the Mg90Li10(x=0)
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alloy was released, under the same conditions. The temperature for the onset of dehydrogenation of
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the Mg90Li4Si6(x=6) alloy was significantly lower than that of the Mg90Li10(x=0) alloy. Furthermore,
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the apparent activation energy (Ea) of the Mg90Li4Si6(x=6) alloy is 128.3 kJ/mol, which is 51.2 kJ/mol lower than that of the Mg90Li10 alloy. The Mg2Si phase acted as a catalyst that reduced the
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reaction barrier of the hydrogenation/dehydrogenation process of the Mg-Li solid solution.
ACCEPTED MANUSCRIPT Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 51571065,
51401055),
the Natural Science Foundation of
Guangxi Province
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51271061,
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(Grant no.2013GXNSFGA019007), Foundation of Guangxi Educational Committee (Grant
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No.2013YB006) and the Key Laboratory of Guangxi for Nonferrous Metals and Materials
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Processing Technology (Grant No. 12-A-01-07).
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Graphical abstract
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Effect of Mg2Si on hydrogen storage properties of Mg-Li alloys was investigated.
The Mg2Si phase could improve the hydrogenation and dehydrogenation properties of the Mg-Li
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The Mg90Li6Si4 (x=6) alloy showed the best hydrogen storage property.
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alloys.