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Influence of adding nano-graphite powders on the microstructure and gas hydrogen storage properties of ball-milled Mg90Al10 alloys
Accepted Manuscript Influences of adding nano-graphite powders on microstructure and gaseous hydrogen storage properties of ball milled Mg90Al10 alloy Hongwei Shang, Yaqin Li, Yanghuan Zhang, Xin Wei, Yan Qi, Dongliang Zhao PII:
S0008-6223(19)30356-2
DOI:
https://doi.org/10.1016/j.carbon.2019.04.028
Reference:
CARBON 14112
To appear in:
Carbon
Received Date: 22 December 2018 Revised Date:
2 April 2019
Accepted Date: 7 April 2019
Please cite this article as: H. Shang, Y. Li, Y. Zhang, X. Wei, Y. Qi, D. Zhao, Influences of adding nanographite powders on microstructure and gaseous hydrogen storage properties of ball milled Mg90Al10 alloy, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.04.028. 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.
Ball milling 20 h
Mg
Al
Absorb hydrogen
Mg Main phase: Mg Minor phase: Al
Absorb hydrogen
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Main phase: MgH2 Minor phase: Al
Mg2Al3
Mg17Al12+H→
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Mg2Al3+MgH2
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Main phase: Mg Minor phase: Mg17Al12 Al
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Mg17Al12 C Al Ball milling 20 h Mg
MgH2
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Main phase: MgH2 Minor phase: Mg2Al3 Al
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Influences of adding nano-graphite powders on microstructure and
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gaseous hydrogen storage properties of ball milled Mg90Al10 alloy
3 4 5 6
Hongwei Shanga,∗∗∗, Yaqin Lia,∗∗, Yanghuan Zhangb, Xin Weib, Yan Qib, Dongliang Zhaob b. Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, China
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a. School of Physics & Electrical Engineering, Anyang Normal University, Anyang 455000, China
Abstract
A small quantity of nano-graphite powders were introduced into the Mg90Al10 alloy
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to obtain the ball milled Mg90Al10+x wt% graphite (x=0, 1, 3, 5 and 8) composites.
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The influences of adding nano-graphite powders on the microstructure and hydrogen
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storage performances were studied. The microstructure analysis showed that adding
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nano-graphite powders not only promotes the formation of Mg17Al12 phase, but also
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increases the number of Al atoms that enter into the crystal lattice of Mg. The
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performance test indicated that adding nano-graphite powders can improve the
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activation performance and enhance the hydrogenation/dehydrogenation kinetics and
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capacity. It also is benificial to lower the thermodynamic stability of the hydride. The
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dehydrogenation enthalpy (∆Hde) and activation energy (Ede(a)) decrease at different
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degrees after adding nano-graphite powders. Dehydrogenation enthalpy of the
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composites is 75.43, 74.63, 73.20, 72.68 and 71.52 kJ mol-1 H2 when the value of x is
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0, 1, 3, 5 and 8, respectively. Dehydrogenation activation energy of the composites is
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162.06, 161.26, 159.07, 156.15 and 158.26 kJ mol-1 H2 when the value of x is 0, 1, 3,
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5 and 8, respectively.
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∗∗ ∗
Corresponding author. E-mail: [email protected] (Hongwei Shang) Corresponding author. E-mail: [email protected] (Yaqin Li) 1
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Keywords: Mg90Al10 alloy; Nano-graphite powder; Ball milling; Kinetics;
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Thermodynamics
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1. Introduction The lack of hydrogen storage system with high safety and high energy conversion
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efficiency is the main obstacle to the commercial application of hydrogen
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fuel-powered vehicle. Numerous studies have been conducted to seek appropriate
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hydrogen storage materials with wider operating temperature, low cost, superior
8
kinetics and high mass hydrogen storage density [1]. Magnesium hydride gains a wide
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attention for the advantages of low cost and high mass hydrogen storage density (7.6
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wt%) [2]. Nevertheless, the decomposition temperature of MgH2 is too high and the
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dehydrogenation kinetics is too poor when the temperature is below 623 K [3,4].
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These defects greatly obstruct the application of Mg-based alloys in the on-board
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hydrogen storage system.
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Alloying has been proved to have ameliorating effect on the hydrogen storage
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properties of Mg and Mg-based alloys. The dehydrogenation enthalpy of MgH2 is
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77.9 kJ mol-1 H2 [5]. Zhong et al. [6] prepared Mg(In) solid solution alloy, and found
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that the thermodynamic stability of MgH2 was reduced to some extent by building
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new reaction path of hydrogenation and dehydrogenation. Especially for the
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Mg0.95In0.05 solid solution, the dehydrogenation enthalpy is only 68.1 kJ mol-1 H2.
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However, indium is a rare element and has trace levels of radioactivity. Vajo et al. [7]
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synthesized the MgH2/Si composite using the method of ball milling. Comparing with
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pure Mg, the composite has a lower dehydrogenation enthalpy, which is only 36.4 kJ
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satisfying. Beyond that, metal aluminium is a nice choice for alloying with Mg due to
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the advantages of substantial reserves, low cost and easy preparation. Mg-Al
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intermetallic compounds mainly include Mg17Al12 and Mg2Al3, which have lower
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decomposition conditions for the hydrides compared to pure Mg. Besides, the solid
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solubility of Al in Mg lattice reaches up to 12.7 wt%. Considering the smaller atomic
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radius of Al, the lattice volume of Mg will reduce when Al atoms enter into Mg lattice,
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which conduces to the reduction of the decomposition conditions for the hydrides [8].
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Unfortunately, the theoretical hydrogen storage capacity of Mg17Al12 phase and
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Mg2Al3 phase is only 4.44 and 3.02 wt% respectively, which makes them unable to
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meet the requirement of DOE (5.50 wt%) [9]. Therefore, the content of Al in the alloy
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needs to be tightly controlled to reduce the thermodynamic stability of MgH2 and
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retain high hydrogen storage capacity as much as possible.
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It has been verified that mechanical milling is suitable to prepare the Mg-based
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alloys with super hydrogen storage performance [8,10,11]. The reduction of the grain
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size caused by mechanical milling is conducive to the improvement of the
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hydrogenation kinetics [12]. In our previous study, it has been reported that the ball
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milled Mg90Al10 solid solution alloy has lower dehydrogenation enthalpy and
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activation energy than Mg, while the improvement is very limited [8]. The work in
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this paper aims to further reduce the decomposition conditions of MgH2 and enhance
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the hydrogenation and dehydrogenation kinetics. Several studies have demonstrated
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that ball milling Mg-based alloys with proper catalysts can greatly facilitate the
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ACCEPTED MANUSCRIPT dissociation of hydrogen molecules on the surface of the alloy particle and the
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diffusion of hydrogen atoms in the inner of the alloy, and reduce the dehydrogenation
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temperature of the hydride [13-22]. Among many catalysts, carbon additives, such as
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carbon nano-tube [23], graphene [24] and graphite [25] have attracted great attention
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of scientists due to the high specific area, good stability and abundant reserves. Lang
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et al. [26-28] prepared the few-layer graphene (FLG) and Mg@FLG composite via
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plasma-assisted ball milling, and found that the dehydrogenation activation energy of
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the Mg@FLG composite is only 155 kJ mol-1 H2. In addition, they also reported that
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plasma milling La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 alloy with graphene can enhance the
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discharge capacity from 172.2 to 263.8 mAh g-1 [29]. Ma et al. [30] improves the
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hydrolysis properties of Mg by plasma milling it with expanded graphite. Shang et al.
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[31] prepared the ball milled MgH2+x mol% graphite (x=1, 10 and 30) composites
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and found that the addition of graphite is beneficial to accelerate the hydrogenation.
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However, the differential scanning calorimetry (DSC) results showed that it does little
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to reduce the decomposition temperature of MgH2. Imamura et al. [32] studied the
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dehydrogenation performances of the Mg/graphite composite using DSC and found
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that the initial dehydrogenation temperature decreases from 707 K for pure Mg to 668
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K for the containing-graphite composite. Lototskyy et al. [4] considered that adding
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graphite does not obviously improve the hydrogenation kinetics. Huot et al. [33]
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reported that adding graphite has little impact on the hydrogenation and
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dehydrogenation kinetics while significantly enhance the activation performance of
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MgH2. Over all, there is no common opinion about the effect of adding graphite on
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ACCEPTED MANUSCRIPT the hydrogen storage performance of Mg or Mg-based alloys. The obvious difference
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of the results may be due to the different test methods. More study is definitely
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needed to determine the role of graphite on the hydrogen storage performance.
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Therefore, in this work, a small quantity of nano-graphite powders were introduced
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into the Mg90Al10 alloy during ball milling for further reducing the decomposition
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conditions of MgH2. And the actions of adding nano-graphite powders on the gaseous
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hydrogen storage performances of the alloy were studied systematically.
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2. Experimental
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2.1 Sample preparation
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The used raw materials were Mg powder (purity: 99.9%, average diameter: 178
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um), Al powder (purity: 99.9%, average diameter: 48 um) and nano-graphite powder
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(purity: 99.9%, average particle thickness: 40 nm, piece of diameter distribution:
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400-2000 nm). The preparation technology was as follows: 10 g mixture of Mg
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powder and Al powder with the atom ratio of 9:1 was ball milled with x wt%
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nano-graphite powders to synthetize the Mg90Al10+x wt% graphite (x=0, 1, 3, 5 and 8)
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composites (abbreviated as Mg-Al-xGR (x=0, 1, 3, 5 and 8)) using a planetary ball
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mill (QM-3SP2) with an operating speed of 350 rpm. The mixed powders together
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with grinding balls were put into the milling jar with the weight ratio of
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ball-to-powder of 20:1. Argon was adopted as protective atmosphere of the milling jar.
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The total milling time was set as 20 h with each 30 min resting after 30 min running
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to prevent overheating. And the particles size of the samples used for performance test
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was strictly limited between 48 and 72 µm.
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2.2 Structural characterization Phase composition and crystal lattice parameters analysis were performed by
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Rigaku D/MAX-RB12 X-ray diffractometer (XRD) using CuKα1 radiation filtered by
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graphite monochromator. The XRD patterns of the samples were collected from 20º to
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90º at a scanning rate of 10º min-1. Surface morphology and size dimension
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observation of the sample particles were carried out using Philips QUANTA 400
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scanning electron microscope (SEM). The crystal structure was observed using
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JEM-2010 Field emission transmission electron microscope (FETEM).
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2.3 Performance test
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The gaseous hydrogen storage properties of the samples were tested using a
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Sieverts type apparatus (General Research Institute for Nonferrous Metals). 0.5 g of
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sample powders was loaded into the stainless steel cylindrical reaction chamber firstly,
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and then heated up to 653 K while pumping vacuum. Then the sample powders were
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activated for obtaining the steady hydrogenation ability. After that, the
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Pressure-composition-temperature (P-C-T) curves were measured at 613, 633 and 653
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K. Finally, the hydrogenation/dehydrogenation kinetic curves were tested at different
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temperature. The initial hydrogenation and dehydrogenation pressure was set to 3 and
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1×10-4 MPa, respectively. And the hydrogenation time was limited to 2 h.
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Dehydrogenation performance of the hydrogenated composites was also analyzed
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using NETZSCH STA 449F3 type DSC analyzer with alumina crucible. 30 mg of the
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hydrogenated composites was measured under purified argon flow (50 mL min-1) in
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the temperature range from 523 to 773 K at the heating rate of 5 K min-1.
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3. Results and discussion
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3.1 Microstructure Figure 1 shows the XRD patterns and the Rietveld refinement results of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The phase composition and
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microstructure of the samples are analyzed by MDI Jade 6.0 software based on the
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International Centre for Diffraction Date (ICDD). It can be seen that the ball milled
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Mg-Al alloy contains the major phase of Mg phase (P63/mmc) and the minor phase of
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Al phase (Fm(-)3m). Adding nano-graphite powders makes the intermetallic
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phase-Mg17Al12 phase (I(-)43m) generate in the ball milled Mg-Al-xGR (x=1, 3, 5 and
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8) composites. The Rietveld refinement of XRD data is analyzed using Maud
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software. The obtained lattice constants and phase abundances of Mg, Al and
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Figure 1 XRD patterns and Rietveld refinement results of ball milled Mg-Al-xGR
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(x=0, 1, 3, 5 and 8) composites. 7
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be seen that the content of Mg17Al12 phase increases gradually with increasing
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nano-graphite powders (see Figure S1 in the supporting information). Additionally,
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the diffraction angle (2θ) of Mg phase diffraction peaks gradually increases with the
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increase of nano-graphite content. Taking the diffraction angle (2θ) of Mg (101) as an
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example, it is 36.58º, 36.62º, 36.66º, 36.67º and 36.70º when the addition of
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nano-graphite powders is 0, 1, 3, 5 and 8 wt%, respectively. Right-shift of the peak
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position means the decrease of the unit cell volume of Mg phase. The Rietveld
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refinement results in Table S1 is consistent with the above analysis. It is the result that
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Al atoms with a smaller atom radius enter into the crystal lattice of Mg. In other
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words, the solid solution of Al in Mg is increased after adding nano-graphite powders.
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It is also important to note that there are no diffraction peaks of graphite in the XRD
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patterns of the containing-graphite composites, which is similar to the result of Du et
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al [34]. This is due to the following two facts: On the one hand, Mg has quite high
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crystallinity and its diffraction peak intensity is much larger than that of graphite. On
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the other hand, the structure of nano-graphite powder will be disrupted during the
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long time ball milling as reported by Bouaricha et al [35].
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Figure 2 displays the XRD patterns and the Rietveld refinement results of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites after hydrogenation for 2 h at 653
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K and 3 MPa H2. It shows that the ball milled Mg-Al alloy contains the major phase
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of MgH2 phase (P42/mnm), the minor phase of Al phase and a small quantity of
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un-hydrogenated Mg phase. It is notable that the diffraction peak intensity of
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Figure 2 XRD patterns and Rietveld refinement results of ball milled Mg-Al-xGR
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(x=0, 1, 3, 5 and 8) composites after hydrogenation for 2 h.
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un-hydrogenated Mg phase has a remarkable decline for Mg-Al-1GR composite. And
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it disappears completely in the ball milled Mg-Al-xGR (x=3, 5 and 8) composites.
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This indicates that adding nano-graphite powders can effectively facilitate the
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hydrogenation. Besides, Mg17Al12 phase, in the ball milled Mg-Al-xGR (x=1, 3, 5 and
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8) composites, decomposes into Mg2Al3 phase during hydrogenation through the
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disproportionation reaction (Mg17Al12+H→Mg2Al3+MgH2) [8]. The obtained lattice
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constants and phase abundances of Mg, Al, Mg2Al3 and MgH2 phases are summarized
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in Table S2 (see the supporting information).
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Figure 3 shows the XRD patterns and the Rietveld refinement results of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites after dehydrogenation at 653 K
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and 1×10-4 MPa. It can be seen that Mg phase and Mg17Al12 phase appear again with 9
ACCEPTED MANUSCRIPT the disappearance of MgH2 phase and Mg2Al3 phase. This illustrates that the
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hydrogenation of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites is
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reversible. In addition, the diffraction angle (2θ) of Mg (101) is 36.62º, 36.70º, 36.82º,
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36.86º and 36.98º when the addition of nano-graphite powders is 0, 1, 3, 5 and 8 wt%,
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which is larger than that of the composites before hydrogenation. It means that the
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hydrogenation/dehydrogenation cycles can further facilitate the process that Al atoms
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enter into the crystal lattice of Mg. The lattice constants and phase abundances of Mg,
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Al and Mg17Al12 phases are summarized in Table S3 (see the supporting information).
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Figure 4 shows the SEM images of the Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The Mg-Al alloy particles form the flaky morphology during the ball milling process
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due to the high ductility of aluminum. And the particles size is about 100 um (see
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Figure 3 XRD patterns and Rietveld refinement results of ball milled Mg-Al-xGR
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(x=0, 1, 3, 5 and 8) composites after dehydrogenation. 10
ACCEPTED MANUSCRIPT Figure 4 (a)). In contrast, the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites are
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nearly spherical particles and the particle size is mostly in the range of 50-100 um
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(see Figure 4 (b-e)). It illustrates that adding nano-graphite powders can reduce the
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particle size to some extent [36,37].
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Figure 5 shows the SAED and HRTEM images of the ball milled Mg-Al alloy
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before and after 2 h of hydrogenation. It can be seen that the Mg-Al alloy is composed
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of Mg grains and Al grains, which is in conformity with the analysis of XRD. After
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hydrogenation for 2 h, there are still residual Mg grains except Al grains and MgH2
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grains, indicating that the alloy cannot be hydrogenated completely within 2 h under
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Figure 4 SEM images of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
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Figure 5 HRTEM and SAED images of ball milled Mg-Al alloy: (a) Before
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hydrogenation; (b) After 2 h of hydrogenation.
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the current experimental conditions.
Figure 6 shows the SAED and HRTEM images of the ball milled Mg-Al-xGR (x=1,
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3, 5 and 8) composites. Besides the grains of Mg and Al, there also are some Mg17Al12
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grains, which is consistent with the analysis of XRD. In addition, it also can be seen
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that there is a small quantity of graphite in the marginal area of the ball milled
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Figure 6 HRTEM and SAED images of ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites: (a) x=1; (b) x=3; (c) x=5; (d) x=8. 12
ACCEPTED MANUSCRIPT Mg-Al-1GR composite. With an increase in the mass fraction of nano-graphite
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powders, more graphite is observed in the interior of the ball milled Mg-Al-xGR (x=3,
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5 and 8) composites. And the disordered structure is increased with increasing
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nano-graphite content. It is because adding nano-graphite powders can enhance the
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ball milling efficiency, which makes the structure of nano-graphite powders change
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from ordered state to disordered state during ball milling [38]. Figure 7 shows the
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SAED and HRTEM images of the ball milled Mg-Al-xGR (x=1, 3, 5 and 8)
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composites after 2 h of hydrogenation. Different from the ball milled Mg-Al alloy,
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there also are some Mg2Al3 grains besides the grains of Mg, Al and MgH2 for the
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Mg-Al-1GR composite after 2 h of hydrogenation, as shown in Figure 7 (a). For the
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ball milled Mg-Al-xGR (x=3, 5 and 8) composites, all of Mg grains have transformed
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Figure 7 HRTEM and SAED images of ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites after 2 h of hydrogenation: (a) x=1; (b) x=3; (c) x=5; (d) x=8.
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ACCEPTED MANUSCRIPT into MgH2 grains. It suggests that adding nano-graphite powders can effectively
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facilitate the hydrogenation, which is consistent with the analysis results of XRD (see
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Figure 2).
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3.2 Gaseous hydrogen storage performance
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3.2.1 Activation performance
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Due to the existence of surface oxides layer, hydrogen storage materials have to
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be activated under high enough hydrogen pressure and temperature before absorbing
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hydrogen [8,39]. Figure 8 shows the first four hydrogenation kinetic curves of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at 653 K and 3 MPa H2. The
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hydrogenation capacity of the ball milled Mg-Al alloy is only 1.465 wt% within
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10000 s in the first hydrogenation cycle, while it increases to 5.044, 6.120, 6.220, and
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5.802 wt% when the addition of nano-graphite powders increases to 1, 3, 5 and 8 wt%,
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respectively (see Figure S2 in the supporting information). The improvement of the
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activation performance is ascribed to the destruction of the surface oxide film during
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ball milling. And graphite coating on alloy particles can effectively inhibit the
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generation of new oxide layer [35]. Therefore, the activation performance is improved
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after adding nano-graphite powders.
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3.2.2 Thermodynamics
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Figure 9 displays the P-C-T curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and
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8) composites at different temperatures. Based on the analysis of the XRD patterns, it
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can be determined that the long and flat plateau reflects the phase transition process of
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hydrogenation or dehydrogenation (Mg+H2↔MgH2). The plateau pressure can be
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Figure 8 First four hydrogenation kinetic curves of ball milled Mg-Al-xGR (x=0, 1, 3,
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5, 8) composites at 653 K and 3 MPa H2 in the activation process: (a) x=0; (b) x=1; (c)
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x=3; (d) x=5; (e) x=8.
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used to assess the thermodynamic stability of the hydride. In general, the increase of
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the dehydrogenation plateau pressure means the decline of the thermodynamic
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stability of the hydride [40]. The values of dehydrogenation plateau pressure ((P(H2))
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of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites are listed in Table S4 15
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Figure 9 P-C-T curves of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at
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613, 633 and 653 K: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
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(see the supporting information). Here, hydrogen pressure in the midpoint of
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dehydrogenation plateau is chosen as P(H2). The value of P(H2) gradually increases
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with increasing the mass fraction of nano-graphite powder. For example, the value of
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P(H2) is 0.5234, 0.5503, 0.5850, 0.5910 and 0.6502 MPa when the addition of
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nano-graphite powders is 0, 1, 3, 5 and 8 wt% at 653 K, respectively (see Figure S3 in
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increase the dehydrogenation plateau pressure and promote the decomposition of the
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hydride. XRD analysis has proved that the unit cell volume of Mg decreases with
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increasing the content of nano-graphite powders. It is the fundamental cause of the
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rising of the dehydrogenation plateau pressure [8].
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To quantify the effect of adding nano-graphite powders on the thermodynamics, the
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dehydrogenation enthalpy (∆Hde) of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
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composites are calculated based on the Van’t Hoff equation [41]:
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P(H 2 ) ∆H de ∆Sde ln − = R P0 RT
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where P0 is atmosphere pressure, P0=0.1 MPa; R is the ideal gas constant, R=8.314 J
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mol-1 K-1; T is the test temperature, and P(H2) is the corresponding dehydrogenation
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plateau pressure which are listed in Table S4 (see the supporting information). The
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dehydrogenation enthalpy (∆Hde) represents the needed minimum energy for the
14
decomposition of the hydride. The smaller the absolute value of ∆Hde is, the less
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P(H 2 ) stable the hydride will be. Figure 10 displays the relationship curves of ln P0
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vs. 1000/RT of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The slope of
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the curve represents ∆Hde. The intercept of the curve on the vertical coordinate
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represents ∆Sde/R. It can be seen that the absolute value of ∆Hde decreases from 75.43
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kJ mol-1 H2 for the ball milled Mg-Al alloy to 74.63, 73.20, 72.68 and 71.52 kJ mol-1
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H2 for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites, suggesting that
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adding nano-graphite powders in the process of ball milling is beneficial to reduce the
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P(H 2 ) Figure 10 Relationship curves of ln vs. 1000/RT of ball milled P0
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Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e)
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x=8.
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thermodynamic stability of MgH2 (see Figure S4 in the supporting information). For
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the ball milled Mg-Al-xGR (x=3, 5 and 8) composites, the value of ∆Hde is smaller
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than that of Mg3La, Mg3Mm, Mg3Pr and Mg17Al12 alloys, but still larger than that of
8
Mg2Ni alloy (as shown in Table S4 in the supporting information) [42-46]. 18
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3.2.3 Kinetics Figure 11 displays the isothermal hydrogenation kinetic curves of the ball milled
3
Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at 533 and 653 K under 3 MPa H2.
4
Obviously, the hydrogenation kinetics is enhanced after adding nano-graphite
5
powders, which is consistent with the results of Shang et al [31]. The improvement of
6
hydrogenation kinetics is ascribed to the following two factors. First, adding
7
nano-graphite powders makes the alloy particle size reduce obviously (see Figure 4).
8
The increase of the specific surface area is beneficial to the diffusion of hydrogen
9
atoms and the formation of hydrides [31]. Second, adding nano-graphite powders
10
leads to the formation of Mg17Al12 grain through enhancing the ball milling efficiency.
11
Mg17Al12 reacts with hydrogen to form Mg2Al3 (Mg17Al12+H→Mg2Al3+MgH2), and
12
Mg2Al3 further absorbs hydrogen to form MgH2 and Al (Mg2Al3+H→MgH2+Al) in
13
the hydrogenation process. The boundaries among the above grains (including Mg,
14
Mg17Al12, Mg2Al3, Al, MgH2 and graphite) can be used as the diffusion channels for
15
hydrogen atoms in the inner of the materials [47,48]. However, the hydrogenation
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Figure 11 Isothermal hydrogenation kinetic curves of ball milled Mg-Al-xGR (x=0, 1,
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3, 5 and 8) composites at 533 and 653 K.
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ACCEPTED MANUSCRIPT kinetics of the composite decreases again when the nano-graphite powder content
2
exceeds 5 wt%. It is because the degree of disorder of the graphite increases with
3
increasing nano-graphite powder content. As shown in Figure 6 (d), there is a lot of
4
disordered structure, which is not conducive to the diffusion of hydrogen atoms. The
5
hydrogenation capacity has a similar trend. Taking T=653 K as an example, the
6
hydrogenation capacity within 7200 s is 4.570, 6.099, 6.396, 6.650 and 5.916 wt%
7
when the addition of nano-graphite powders is 0, 1, 3, 5 and 8 wt%, respectively (see
8
Figure S5 in the supporting information). In general, the hydrogenation capacity is
9
influenced by the phase composition and microstructure of the materials. It has been
10
proved by XRD analysis that the content of Mg17Al12 phase increases with increasing
11
nano-graphite powder content (see Figure S1 in the supporting information). The
12
theoretical capacity of Mg17Al12 phase is only 4.44 wt%, which is lower than that of
13
pure Mg. And the added nano-graphite cannot store hydrogen under the current
14
experimental conditions. Therefore, the increase of the hydrogenation capacity for the
15
ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites is not caused by the phase
16
composition, but is the result of the increase of the grain boundary due to the fact that
17
the concentration of hydrogen is highest in the grain boundary [49]. However, adding
18
excess graphite, for example the Mg-Al-8GR composite, leads to a decrease in
19
hydrogenation capacity due to the fact that graphite cannot store hydrogen atom under
20
the current experimental condition and there is a lot of disordered structure formation.
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Figure 12 shows the isothermal dehydrogenation kinetic curves of the ball milled
22
Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at 533-653 K with an interval of 20 K.
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2
the test temperature drops to 573 K. After adding nano-graphite powders, the
3
dehydrogenation kinetics can be improved to some extent, which is different with the
4
result of Shang et al. [31, 33]). Taking T=573 K as an example, the dehydrogenation
5
capacity is only 0.899 wt% for the Mg-Al alloy at 30000 s, while it increases to 1.631,
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Figure 12 Isothermal dehydrogenation kinetic curves of ball milled Mg-Al-xGR (x=0,
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1, 3, 5 and 8) composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8. 21
ACCEPTED MANUSCRIPT 2.209, 6.316 and 3.759 wt% when the addition of nano-graphite powders increases to
2
1, 3, 5 and 8 wt%, respectively. Similar to the hydrogenation kinetics, the
3
enhancement of the dehydrogenation kinetics is also ascribed to the decrease of the
4
particle size and the generation of the multiphase structure. As for the Mg-Al-8GR
5
composites, the poorer dehydrogenation kinetics and the lower dehydrogenation
6
capacity are the results of the formation of a lot of disordered structure.
The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is introduced to analyze
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the dehydrogenation performance. The JMAK equation is described as follows [50]:
9
ln[− ln(1 − α )] = η ln k + ηln t
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where α represents the fraction of phase transformation (α=Ct/Cmax, Ct represents the
11
dehydrogenation capacity at time t, Cmax represents the maximum of dehydrogenation
12
capacity); η is the Avrami exponent; k is the rate constant. Figure 13 shows the JMAK
13
fitting curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The
14
Equation of the fitting curves of ln[-ln(1-α)] vs. lnt are listed in Table S5 (see the
15
supporting information). The intercept and slope corresponds to ηlnk and η,
16
respectively. The Arrhenius equation is adopted to calculate the dehydrogenation
17
activation energy (Ede(a)), as described below [51]:
18
ln k = ln k 0 −
19
In this equation, k0 is a temperature independent coefficient. The value of Ede(a) can
20
be calculated by fitting the curve of lnk vs. 1000/RT, as shown in Figure 14. The
21
value of Ede(a) is 162.06, 161.26, 159.07, 156.15 and 158.26 kJ mol-1 H2 for the ball
22
milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites, respectively (see Figure S6 in the
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Figure 13 JMAK fitting curves of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
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composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
4
supporting information). The decrease of Ede(a) means the reduction of the needed
5
minimum energy for dehydrogenation [52]. Therefore, the ball milled Mg-Al-5GR
6
composite has optimum dehydrogenation performance.
7
Figure 15 depicts the DSC curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
8
composites. For the Mg-Al alloy, the endothermic desorption peak is located in the
9
temperature range of 662-718 K and the peak temperature is 691 K. Adding 23
ACCEPTED MANUSCRIPT nano-graphite powders makes the peak temperature decrease to 688, 685, 678 and 680
2
K for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites, respectively. It is
3
consistent with the change trend of the dehydrogenation activation energy (Ede(a)),
4
suggesting that adding nano-graphite powders is beneficial to improve the
5
dehydrogenation performance. It is noteworthy that the initial decomposition
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Figure 14 Arrhenius fitting curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
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composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
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Figure 15 DSC curves of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites:
3
(a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8 at heating rate of 5 K min-1.
4
temperature is 662, 657, 645, 633 and 641 K for the ball milled Mg-Al-xGR (x=1, 3, 5
5
and 8) composites (see Figure S7 in the supporting information), which is obviously
6
higher than the lowest dehydrogenation temperature as shown in Figure 12. It is
7
resulted by the different measurement methods. On the one hand, the dehydrogenation
8
kinetic curve is measured at the constant temperature, while DSC is a temperature
9
rising test. Due to the poor dehydrogenation kinetics of the samples, the DSC device
10
cannot detect the dehydrogenation signal in time. On the other hand, the samples are
11
measured under purified argon flow of 50 mL min-1 for the DSC test, while the initial
12
dehydrogenation pressure is 1×10-4 MPa in a vacuum for the dehydrogenation kinetic
13
curve test.
14
4. Conclusions
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This work mainly studied the actions of adding nano-graphite powders on the
2
microstructure and hydrogen storage properties of the ball milled Mg90Al10 alloy. The
3
main conclusions can be drawn as follows: (1) The ball milled Mg-Al alloy is composed of Mg phase and Al phase. Adding
5
nano-graphite powders causes the generation of Mg17Al12 phase and increases the
6
number of Al atoms that enter into the crystal lattice of Mg. Besides, the particle size
7
of the containing-graphite composites has an obvious decrease compared to the
8
Mg-Al alloy.
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(2) Adding nano-graphite powders improves the activation performance of the ball
10
milled Mg-Al alloy through preventing the surface oxide film from forming again.
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The hydrogenation capacity of the Mg-Al alloy is only 1.465 wt% within 10000 s at
12
the first hydrogenation process. And it increases to 5.044, 6.120, 6.220 and 5.802 wt%
13
for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8), respectively.
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(3) Adding nano-graphite powders promotes the reduction of the thermodynamic
15
stability of the hydrides. The dehydrogenation enthalpy (∆Hde) decreases from 75.43
16
kJ mol-1 H2 for the ball milled Mg-Al alloy to 74.63, 73.20, 72.68 and 71.52 kJ mol-1
17
H2 for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites, respectively.
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(4) Adding nano-graphite powders contributes to the enhancement of the
19
hydrogenation and dehydrogenation kinetics. The dehydrogenation activation energy
20
Ede(a) decreases from 162.06 kJ mol-1 H2 for the ball milled Mg-Al alloy to 161.26,
21
159.07, 156.15 and 158.26 kJ mol-1 H2 for the ball milled Mg-Al-xGR (x=1, 3, 5 and
22
8) composites, respectively.
26
ACCEPTED MANUSCRIPT Acknowledgements
2
This work is financially supported by the National Natural Science Foundations of
3
China (51471054).
4
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S0008-6223(19)30356-2
DOI:
https://doi.org/10.1016/j.carbon.2019.04.028
Reference:
CARBON 14112
To appear in:
Carbon
Received Date: 22 December 2018 Revised Date:
2 April 2019
Accepted Date: 7 April 2019
Please cite this article as: H. Shang, Y. Li, Y. Zhang, X. Wei, Y. Qi, D. Zhao, Influences of adding nanographite powders on microstructure and gaseous hydrogen storage properties of ball milled Mg90Al10 alloy, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.04.028. 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.
Ball milling 20 h
Mg
Al
Absorb hydrogen
Mg Main phase: Mg Minor phase: Al
Absorb hydrogen
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Main phase: MgH2 Minor phase: Al
Mg2Al3
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Main phase: Mg Minor phase: Mg17Al12 Al
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Mg17Al12 C Al Ball milling 20 h Mg
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Main phase: MgH2 Minor phase: Mg2Al3 Al
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Influences of adding nano-graphite powders on microstructure and
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gaseous hydrogen storage properties of ball milled Mg90Al10 alloy
3 4 5 6
Hongwei Shanga,∗∗∗, Yaqin Lia,∗∗, Yanghuan Zhangb, Xin Weib, Yan Qib, Dongliang Zhaob b. Department of Functional Material Research, Central Iron and Steel Research Institute, Beijing 100081, China
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a. School of Physics & Electrical Engineering, Anyang Normal University, Anyang 455000, China
Abstract
A small quantity of nano-graphite powders were introduced into the Mg90Al10 alloy
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to obtain the ball milled Mg90Al10+x wt% graphite (x=0, 1, 3, 5 and 8) composites.
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The influences of adding nano-graphite powders on the microstructure and hydrogen
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storage performances were studied. The microstructure analysis showed that adding
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nano-graphite powders not only promotes the formation of Mg17Al12 phase, but also
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increases the number of Al atoms that enter into the crystal lattice of Mg. The
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performance test indicated that adding nano-graphite powders can improve the
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activation performance and enhance the hydrogenation/dehydrogenation kinetics and
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capacity. It also is benificial to lower the thermodynamic stability of the hydride. The
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dehydrogenation enthalpy (∆Hde) and activation energy (Ede(a)) decrease at different
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degrees after adding nano-graphite powders. Dehydrogenation enthalpy of the
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composites is 75.43, 74.63, 73.20, 72.68 and 71.52 kJ mol-1 H2 when the value of x is
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0, 1, 3, 5 and 8, respectively. Dehydrogenation activation energy of the composites is
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162.06, 161.26, 159.07, 156.15 and 158.26 kJ mol-1 H2 when the value of x is 0, 1, 3,
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5 and 8, respectively.
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Corresponding author. E-mail: [email protected] (Hongwei Shang) Corresponding author. E-mail: [email protected] (Yaqin Li) 1
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Keywords: Mg90Al10 alloy; Nano-graphite powder; Ball milling; Kinetics;
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Thermodynamics
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1. Introduction The lack of hydrogen storage system with high safety and high energy conversion
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efficiency is the main obstacle to the commercial application of hydrogen
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fuel-powered vehicle. Numerous studies have been conducted to seek appropriate
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hydrogen storage materials with wider operating temperature, low cost, superior
8
kinetics and high mass hydrogen storage density [1]. Magnesium hydride gains a wide
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attention for the advantages of low cost and high mass hydrogen storage density (7.6
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wt%) [2]. Nevertheless, the decomposition temperature of MgH2 is too high and the
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dehydrogenation kinetics is too poor when the temperature is below 623 K [3,4].
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These defects greatly obstruct the application of Mg-based alloys in the on-board
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hydrogen storage system.
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Alloying has been proved to have ameliorating effect on the hydrogen storage
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properties of Mg and Mg-based alloys. The dehydrogenation enthalpy of MgH2 is
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77.9 kJ mol-1 H2 [5]. Zhong et al. [6] prepared Mg(In) solid solution alloy, and found
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that the thermodynamic stability of MgH2 was reduced to some extent by building
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new reaction path of hydrogenation and dehydrogenation. Especially for the
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Mg0.95In0.05 solid solution, the dehydrogenation enthalpy is only 68.1 kJ mol-1 H2.
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However, indium is a rare element and has trace levels of radioactivity. Vajo et al. [7]
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synthesized the MgH2/Si composite using the method of ball milling. Comparing with
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pure Mg, the composite has a lower dehydrogenation enthalpy, which is only 36.4 kJ
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ACCEPTED MANUSCRIPT mol-1 H2. Nevertheless, the dehydrogenation is irreversible and the kinetics also is not
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satisfying. Beyond that, metal aluminium is a nice choice for alloying with Mg due to
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the advantages of substantial reserves, low cost and easy preparation. Mg-Al
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intermetallic compounds mainly include Mg17Al12 and Mg2Al3, which have lower
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decomposition conditions for the hydrides compared to pure Mg. Besides, the solid
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solubility of Al in Mg lattice reaches up to 12.7 wt%. Considering the smaller atomic
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radius of Al, the lattice volume of Mg will reduce when Al atoms enter into Mg lattice,
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which conduces to the reduction of the decomposition conditions for the hydrides [8].
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Unfortunately, the theoretical hydrogen storage capacity of Mg17Al12 phase and
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Mg2Al3 phase is only 4.44 and 3.02 wt% respectively, which makes them unable to
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meet the requirement of DOE (5.50 wt%) [9]. Therefore, the content of Al in the alloy
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needs to be tightly controlled to reduce the thermodynamic stability of MgH2 and
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retain high hydrogen storage capacity as much as possible.
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It has been verified that mechanical milling is suitable to prepare the Mg-based
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alloys with super hydrogen storage performance [8,10,11]. The reduction of the grain
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size caused by mechanical milling is conducive to the improvement of the
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hydrogenation kinetics [12]. In our previous study, it has been reported that the ball
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milled Mg90Al10 solid solution alloy has lower dehydrogenation enthalpy and
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activation energy than Mg, while the improvement is very limited [8]. The work in
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this paper aims to further reduce the decomposition conditions of MgH2 and enhance
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the hydrogenation and dehydrogenation kinetics. Several studies have demonstrated
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that ball milling Mg-based alloys with proper catalysts can greatly facilitate the
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ACCEPTED MANUSCRIPT dissociation of hydrogen molecules on the surface of the alloy particle and the
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diffusion of hydrogen atoms in the inner of the alloy, and reduce the dehydrogenation
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temperature of the hydride [13-22]. Among many catalysts, carbon additives, such as
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carbon nano-tube [23], graphene [24] and graphite [25] have attracted great attention
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of scientists due to the high specific area, good stability and abundant reserves. Lang
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et al. [26-28] prepared the few-layer graphene (FLG) and Mg@FLG composite via
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plasma-assisted ball milling, and found that the dehydrogenation activation energy of
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the Mg@FLG composite is only 155 kJ mol-1 H2. In addition, they also reported that
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plasma milling La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 alloy with graphene can enhance the
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discharge capacity from 172.2 to 263.8 mAh g-1 [29]. Ma et al. [30] improves the
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hydrolysis properties of Mg by plasma milling it with expanded graphite. Shang et al.
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[31] prepared the ball milled MgH2+x mol% graphite (x=1, 10 and 30) composites
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and found that the addition of graphite is beneficial to accelerate the hydrogenation.
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However, the differential scanning calorimetry (DSC) results showed that it does little
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to reduce the decomposition temperature of MgH2. Imamura et al. [32] studied the
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dehydrogenation performances of the Mg/graphite composite using DSC and found
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that the initial dehydrogenation temperature decreases from 707 K for pure Mg to 668
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K for the containing-graphite composite. Lototskyy et al. [4] considered that adding
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graphite does not obviously improve the hydrogenation kinetics. Huot et al. [33]
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reported that adding graphite has little impact on the hydrogenation and
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dehydrogenation kinetics while significantly enhance the activation performance of
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MgH2. Over all, there is no common opinion about the effect of adding graphite on
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ACCEPTED MANUSCRIPT the hydrogen storage performance of Mg or Mg-based alloys. The obvious difference
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of the results may be due to the different test methods. More study is definitely
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needed to determine the role of graphite on the hydrogen storage performance.
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Therefore, in this work, a small quantity of nano-graphite powders were introduced
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into the Mg90Al10 alloy during ball milling for further reducing the decomposition
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conditions of MgH2. And the actions of adding nano-graphite powders on the gaseous
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hydrogen storage performances of the alloy were studied systematically.
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2. Experimental
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2.1 Sample preparation
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The used raw materials were Mg powder (purity: 99.9%, average diameter: 178
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um), Al powder (purity: 99.9%, average diameter: 48 um) and nano-graphite powder
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(purity: 99.9%, average particle thickness: 40 nm, piece of diameter distribution:
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400-2000 nm). The preparation technology was as follows: 10 g mixture of Mg
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powder and Al powder with the atom ratio of 9:1 was ball milled with x wt%
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nano-graphite powders to synthetize the Mg90Al10+x wt% graphite (x=0, 1, 3, 5 and 8)
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composites (abbreviated as Mg-Al-xGR (x=0, 1, 3, 5 and 8)) using a planetary ball
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mill (QM-3SP2) with an operating speed of 350 rpm. The mixed powders together
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with grinding balls were put into the milling jar with the weight ratio of
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ball-to-powder of 20:1. Argon was adopted as protective atmosphere of the milling jar.
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The total milling time was set as 20 h with each 30 min resting after 30 min running
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to prevent overheating. And the particles size of the samples used for performance test
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was strictly limited between 48 and 72 µm.
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2.2 Structural characterization Phase composition and crystal lattice parameters analysis were performed by
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Rigaku D/MAX-RB12 X-ray diffractometer (XRD) using CuKα1 radiation filtered by
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graphite monochromator. The XRD patterns of the samples were collected from 20º to
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90º at a scanning rate of 10º min-1. Surface morphology and size dimension
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observation of the sample particles were carried out using Philips QUANTA 400
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scanning electron microscope (SEM). The crystal structure was observed using
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JEM-2010 Field emission transmission electron microscope (FETEM).
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2.3 Performance test
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The gaseous hydrogen storage properties of the samples were tested using a
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Sieverts type apparatus (General Research Institute for Nonferrous Metals). 0.5 g of
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sample powders was loaded into the stainless steel cylindrical reaction chamber firstly,
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and then heated up to 653 K while pumping vacuum. Then the sample powders were
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activated for obtaining the steady hydrogenation ability. After that, the
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Pressure-composition-temperature (P-C-T) curves were measured at 613, 633 and 653
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K. Finally, the hydrogenation/dehydrogenation kinetic curves were tested at different
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temperature. The initial hydrogenation and dehydrogenation pressure was set to 3 and
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1×10-4 MPa, respectively. And the hydrogenation time was limited to 2 h.
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Dehydrogenation performance of the hydrogenated composites was also analyzed
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using NETZSCH STA 449F3 type DSC analyzer with alumina crucible. 30 mg of the
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hydrogenated composites was measured under purified argon flow (50 mL min-1) in
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the temperature range from 523 to 773 K at the heating rate of 5 K min-1.
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3. Results and discussion
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3.1 Microstructure Figure 1 shows the XRD patterns and the Rietveld refinement results of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The phase composition and
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microstructure of the samples are analyzed by MDI Jade 6.0 software based on the
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International Centre for Diffraction Date (ICDD). It can be seen that the ball milled
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Mg-Al alloy contains the major phase of Mg phase (P63/mmc) and the minor phase of
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Al phase (Fm(-)3m). Adding nano-graphite powders makes the intermetallic
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phase-Mg17Al12 phase (I(-)43m) generate in the ball milled Mg-Al-xGR (x=1, 3, 5 and
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8) composites. The Rietveld refinement of XRD data is analyzed using Maud
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software. The obtained lattice constants and phase abundances of Mg, Al and
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Figure 1 XRD patterns and Rietveld refinement results of ball milled Mg-Al-xGR
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(x=0, 1, 3, 5 and 8) composites. 7
ACCEPTED MANUSCRIPT Mg17Al12 phases are summarized in Table S1 (see the supporting information). It can
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be seen that the content of Mg17Al12 phase increases gradually with increasing
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nano-graphite powders (see Figure S1 in the supporting information). Additionally,
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the diffraction angle (2θ) of Mg phase diffraction peaks gradually increases with the
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increase of nano-graphite content. Taking the diffraction angle (2θ) of Mg (101) as an
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example, it is 36.58º, 36.62º, 36.66º, 36.67º and 36.70º when the addition of
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nano-graphite powders is 0, 1, 3, 5 and 8 wt%, respectively. Right-shift of the peak
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position means the decrease of the unit cell volume of Mg phase. The Rietveld
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refinement results in Table S1 is consistent with the above analysis. It is the result that
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Al atoms with a smaller atom radius enter into the crystal lattice of Mg. In other
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words, the solid solution of Al in Mg is increased after adding nano-graphite powders.
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It is also important to note that there are no diffraction peaks of graphite in the XRD
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patterns of the containing-graphite composites, which is similar to the result of Du et
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al [34]. This is due to the following two facts: On the one hand, Mg has quite high
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crystallinity and its diffraction peak intensity is much larger than that of graphite. On
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the other hand, the structure of nano-graphite powder will be disrupted during the
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long time ball milling as reported by Bouaricha et al [35].
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Figure 2 displays the XRD patterns and the Rietveld refinement results of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites after hydrogenation for 2 h at 653
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K and 3 MPa H2. It shows that the ball milled Mg-Al alloy contains the major phase
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of MgH2 phase (P42/mnm), the minor phase of Al phase and a small quantity of
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un-hydrogenated Mg phase. It is notable that the diffraction peak intensity of
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Figure 2 XRD patterns and Rietveld refinement results of ball milled Mg-Al-xGR
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(x=0, 1, 3, 5 and 8) composites after hydrogenation for 2 h.
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un-hydrogenated Mg phase has a remarkable decline for Mg-Al-1GR composite. And
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it disappears completely in the ball milled Mg-Al-xGR (x=3, 5 and 8) composites.
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This indicates that adding nano-graphite powders can effectively facilitate the
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hydrogenation. Besides, Mg17Al12 phase, in the ball milled Mg-Al-xGR (x=1, 3, 5 and
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8) composites, decomposes into Mg2Al3 phase during hydrogenation through the
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disproportionation reaction (Mg17Al12+H→Mg2Al3+MgH2) [8]. The obtained lattice
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constants and phase abundances of Mg, Al, Mg2Al3 and MgH2 phases are summarized
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in Table S2 (see the supporting information).
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Figure 3 shows the XRD patterns and the Rietveld refinement results of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites after dehydrogenation at 653 K
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and 1×10-4 MPa. It can be seen that Mg phase and Mg17Al12 phase appear again with 9
ACCEPTED MANUSCRIPT the disappearance of MgH2 phase and Mg2Al3 phase. This illustrates that the
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hydrogenation of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites is
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reversible. In addition, the diffraction angle (2θ) of Mg (101) is 36.62º, 36.70º, 36.82º,
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36.86º and 36.98º when the addition of nano-graphite powders is 0, 1, 3, 5 and 8 wt%,
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which is larger than that of the composites before hydrogenation. It means that the
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hydrogenation/dehydrogenation cycles can further facilitate the process that Al atoms
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enter into the crystal lattice of Mg. The lattice constants and phase abundances of Mg,
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Al and Mg17Al12 phases are summarized in Table S3 (see the supporting information).
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Figure 4 shows the SEM images of the Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The Mg-Al alloy particles form the flaky morphology during the ball milling process
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due to the high ductility of aluminum. And the particles size is about 100 um (see
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Figure 3 XRD patterns and Rietveld refinement results of ball milled Mg-Al-xGR
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(x=0, 1, 3, 5 and 8) composites after dehydrogenation. 10
ACCEPTED MANUSCRIPT Figure 4 (a)). In contrast, the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites are
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nearly spherical particles and the particle size is mostly in the range of 50-100 um
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(see Figure 4 (b-e)). It illustrates that adding nano-graphite powders can reduce the
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particle size to some extent [36,37].
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Figure 5 shows the SAED and HRTEM images of the ball milled Mg-Al alloy
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before and after 2 h of hydrogenation. It can be seen that the Mg-Al alloy is composed
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of Mg grains and Al grains, which is in conformity with the analysis of XRD. After
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hydrogenation for 2 h, there are still residual Mg grains except Al grains and MgH2
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grains, indicating that the alloy cannot be hydrogenated completely within 2 h under
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Figure 4 SEM images of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
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Figure 5 HRTEM and SAED images of ball milled Mg-Al alloy: (a) Before
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hydrogenation; (b) After 2 h of hydrogenation.
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the current experimental conditions.
Figure 6 shows the SAED and HRTEM images of the ball milled Mg-Al-xGR (x=1,
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3, 5 and 8) composites. Besides the grains of Mg and Al, there also are some Mg17Al12
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grains, which is consistent with the analysis of XRD. In addition, it also can be seen
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that there is a small quantity of graphite in the marginal area of the ball milled
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Figure 6 HRTEM and SAED images of ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites: (a) x=1; (b) x=3; (c) x=5; (d) x=8. 12
ACCEPTED MANUSCRIPT Mg-Al-1GR composite. With an increase in the mass fraction of nano-graphite
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powders, more graphite is observed in the interior of the ball milled Mg-Al-xGR (x=3,
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5 and 8) composites. And the disordered structure is increased with increasing
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nano-graphite content. It is because adding nano-graphite powders can enhance the
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ball milling efficiency, which makes the structure of nano-graphite powders change
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from ordered state to disordered state during ball milling [38]. Figure 7 shows the
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SAED and HRTEM images of the ball milled Mg-Al-xGR (x=1, 3, 5 and 8)
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composites after 2 h of hydrogenation. Different from the ball milled Mg-Al alloy,
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there also are some Mg2Al3 grains besides the grains of Mg, Al and MgH2 for the
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Mg-Al-1GR composite after 2 h of hydrogenation, as shown in Figure 7 (a). For the
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ball milled Mg-Al-xGR (x=3, 5 and 8) composites, all of Mg grains have transformed
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Figure 7 HRTEM and SAED images of ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites after 2 h of hydrogenation: (a) x=1; (b) x=3; (c) x=5; (d) x=8.
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ACCEPTED MANUSCRIPT into MgH2 grains. It suggests that adding nano-graphite powders can effectively
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facilitate the hydrogenation, which is consistent with the analysis results of XRD (see
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Figure 2).
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3.2 Gaseous hydrogen storage performance
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3.2.1 Activation performance
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Due to the existence of surface oxides layer, hydrogen storage materials have to
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be activated under high enough hydrogen pressure and temperature before absorbing
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hydrogen [8,39]. Figure 8 shows the first four hydrogenation kinetic curves of the ball
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milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at 653 K and 3 MPa H2. The
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hydrogenation capacity of the ball milled Mg-Al alloy is only 1.465 wt% within
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10000 s in the first hydrogenation cycle, while it increases to 5.044, 6.120, 6.220, and
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5.802 wt% when the addition of nano-graphite powders increases to 1, 3, 5 and 8 wt%,
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respectively (see Figure S2 in the supporting information). The improvement of the
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activation performance is ascribed to the destruction of the surface oxide film during
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ball milling. And graphite coating on alloy particles can effectively inhibit the
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generation of new oxide layer [35]. Therefore, the activation performance is improved
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after adding nano-graphite powders.
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3.2.2 Thermodynamics
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Figure 9 displays the P-C-T curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and
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8) composites at different temperatures. Based on the analysis of the XRD patterns, it
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can be determined that the long and flat plateau reflects the phase transition process of
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hydrogenation or dehydrogenation (Mg+H2↔MgH2). The plateau pressure can be
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Figure 8 First four hydrogenation kinetic curves of ball milled Mg-Al-xGR (x=0, 1, 3,
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5, 8) composites at 653 K and 3 MPa H2 in the activation process: (a) x=0; (b) x=1; (c)
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x=3; (d) x=5; (e) x=8.
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used to assess the thermodynamic stability of the hydride. In general, the increase of
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the dehydrogenation plateau pressure means the decline of the thermodynamic
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stability of the hydride [40]. The values of dehydrogenation plateau pressure ((P(H2))
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of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites are listed in Table S4 15
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Figure 9 P-C-T curves of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at
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613, 633 and 653 K: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
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(see the supporting information). Here, hydrogen pressure in the midpoint of
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dehydrogenation plateau is chosen as P(H2). The value of P(H2) gradually increases
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with increasing the mass fraction of nano-graphite powder. For example, the value of
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P(H2) is 0.5234, 0.5503, 0.5850, 0.5910 and 0.6502 MPa when the addition of
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nano-graphite powders is 0, 1, 3, 5 and 8 wt% at 653 K, respectively (see Figure S3 in
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ACCEPTED MANUSCRIPT the supporting information). It suggests that adding nano-graphite powders can
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increase the dehydrogenation plateau pressure and promote the decomposition of the
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hydride. XRD analysis has proved that the unit cell volume of Mg decreases with
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increasing the content of nano-graphite powders. It is the fundamental cause of the
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rising of the dehydrogenation plateau pressure [8].
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To quantify the effect of adding nano-graphite powders on the thermodynamics, the
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dehydrogenation enthalpy (∆Hde) of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
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composites are calculated based on the Van’t Hoff equation [41]:
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P(H 2 ) ∆H de ∆Sde ln − = R P0 RT
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where P0 is atmosphere pressure, P0=0.1 MPa; R is the ideal gas constant, R=8.314 J
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mol-1 K-1; T is the test temperature, and P(H2) is the corresponding dehydrogenation
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plateau pressure which are listed in Table S4 (see the supporting information). The
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dehydrogenation enthalpy (∆Hde) represents the needed minimum energy for the
14
decomposition of the hydride. The smaller the absolute value of ∆Hde is, the less
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P(H 2 ) stable the hydride will be. Figure 10 displays the relationship curves of ln P0
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vs. 1000/RT of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The slope of
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the curve represents ∆Hde. The intercept of the curve on the vertical coordinate
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represents ∆Sde/R. It can be seen that the absolute value of ∆Hde decreases from 75.43
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kJ mol-1 H2 for the ball milled Mg-Al alloy to 74.63, 73.20, 72.68 and 71.52 kJ mol-1
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H2 for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites, suggesting that
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adding nano-graphite powders in the process of ball milling is beneficial to reduce the
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P(H 2 ) Figure 10 Relationship curves of ln vs. 1000/RT of ball milled P0
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Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e)
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x=8.
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thermodynamic stability of MgH2 (see Figure S4 in the supporting information). For
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the ball milled Mg-Al-xGR (x=3, 5 and 8) composites, the value of ∆Hde is smaller
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than that of Mg3La, Mg3Mm, Mg3Pr and Mg17Al12 alloys, but still larger than that of
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Mg2Ni alloy (as shown in Table S4 in the supporting information) [42-46]. 18
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3.2.3 Kinetics Figure 11 displays the isothermal hydrogenation kinetic curves of the ball milled
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Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at 533 and 653 K under 3 MPa H2.
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Obviously, the hydrogenation kinetics is enhanced after adding nano-graphite
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powders, which is consistent with the results of Shang et al [31]. The improvement of
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hydrogenation kinetics is ascribed to the following two factors. First, adding
7
nano-graphite powders makes the alloy particle size reduce obviously (see Figure 4).
8
The increase of the specific surface area is beneficial to the diffusion of hydrogen
9
atoms and the formation of hydrides [31]. Second, adding nano-graphite powders
10
leads to the formation of Mg17Al12 grain through enhancing the ball milling efficiency.
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Mg17Al12 reacts with hydrogen to form Mg2Al3 (Mg17Al12+H→Mg2Al3+MgH2), and
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Mg2Al3 further absorbs hydrogen to form MgH2 and Al (Mg2Al3+H→MgH2+Al) in
13
the hydrogenation process. The boundaries among the above grains (including Mg,
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Mg17Al12, Mg2Al3, Al, MgH2 and graphite) can be used as the diffusion channels for
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hydrogen atoms in the inner of the materials [47,48]. However, the hydrogenation
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Figure 11 Isothermal hydrogenation kinetic curves of ball milled Mg-Al-xGR (x=0, 1,
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3, 5 and 8) composites at 533 and 653 K.
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ACCEPTED MANUSCRIPT kinetics of the composite decreases again when the nano-graphite powder content
2
exceeds 5 wt%. It is because the degree of disorder of the graphite increases with
3
increasing nano-graphite powder content. As shown in Figure 6 (d), there is a lot of
4
disordered structure, which is not conducive to the diffusion of hydrogen atoms. The
5
hydrogenation capacity has a similar trend. Taking T=653 K as an example, the
6
hydrogenation capacity within 7200 s is 4.570, 6.099, 6.396, 6.650 and 5.916 wt%
7
when the addition of nano-graphite powders is 0, 1, 3, 5 and 8 wt%, respectively (see
8
Figure S5 in the supporting information). In general, the hydrogenation capacity is
9
influenced by the phase composition and microstructure of the materials. It has been
10
proved by XRD analysis that the content of Mg17Al12 phase increases with increasing
11
nano-graphite powder content (see Figure S1 in the supporting information). The
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theoretical capacity of Mg17Al12 phase is only 4.44 wt%, which is lower than that of
13
pure Mg. And the added nano-graphite cannot store hydrogen under the current
14
experimental conditions. Therefore, the increase of the hydrogenation capacity for the
15
ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites is not caused by the phase
16
composition, but is the result of the increase of the grain boundary due to the fact that
17
the concentration of hydrogen is highest in the grain boundary [49]. However, adding
18
excess graphite, for example the Mg-Al-8GR composite, leads to a decrease in
19
hydrogenation capacity due to the fact that graphite cannot store hydrogen atom under
20
the current experimental condition and there is a lot of disordered structure formation.
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Figure 12 shows the isothermal dehydrogenation kinetic curves of the ball milled
22
Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites at 533-653 K with an interval of 20 K.
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ACCEPTED MANUSCRIPT The dehydrogenation kinetics of the ball milled Mg-Al alloy becomes very poor when
2
the test temperature drops to 573 K. After adding nano-graphite powders, the
3
dehydrogenation kinetics can be improved to some extent, which is different with the
4
result of Shang et al. [31, 33]). Taking T=573 K as an example, the dehydrogenation
5
capacity is only 0.899 wt% for the Mg-Al alloy at 30000 s, while it increases to 1.631,
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Figure 12 Isothermal dehydrogenation kinetic curves of ball milled Mg-Al-xGR (x=0,
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1, 3, 5 and 8) composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8. 21
ACCEPTED MANUSCRIPT 2.209, 6.316 and 3.759 wt% when the addition of nano-graphite powders increases to
2
1, 3, 5 and 8 wt%, respectively. Similar to the hydrogenation kinetics, the
3
enhancement of the dehydrogenation kinetics is also ascribed to the decrease of the
4
particle size and the generation of the multiphase structure. As for the Mg-Al-8GR
5
composites, the poorer dehydrogenation kinetics and the lower dehydrogenation
6
capacity are the results of the formation of a lot of disordered structure.
The Johnson-Mehl-Avrami-Kolmogorov (JMAK) model is introduced to analyze
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the dehydrogenation performance. The JMAK equation is described as follows [50]:
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ln[− ln(1 − α )] = η ln k + ηln t
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where α represents the fraction of phase transformation (α=Ct/Cmax, Ct represents the
11
dehydrogenation capacity at time t, Cmax represents the maximum of dehydrogenation
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capacity); η is the Avrami exponent; k is the rate constant. Figure 13 shows the JMAK
13
fitting curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites. The
14
Equation of the fitting curves of ln[-ln(1-α)] vs. lnt are listed in Table S5 (see the
15
supporting information). The intercept and slope corresponds to ηlnk and η,
16
respectively. The Arrhenius equation is adopted to calculate the dehydrogenation
17
activation energy (Ede(a)), as described below [51]:
18
ln k = ln k 0 −
19
In this equation, k0 is a temperature independent coefficient. The value of Ede(a) can
20
be calculated by fitting the curve of lnk vs. 1000/RT, as shown in Figure 14. The
21
value of Ede(a) is 162.06, 161.26, 159.07, 156.15 and 158.26 kJ mol-1 H2 for the ball
22
milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites, respectively (see Figure S6 in the
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Figure 13 JMAK fitting curves of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
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composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
4
supporting information). The decrease of Ede(a) means the reduction of the needed
5
minimum energy for dehydrogenation [52]. Therefore, the ball milled Mg-Al-5GR
6
composite has optimum dehydrogenation performance.
7
Figure 15 depicts the DSC curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
8
composites. For the Mg-Al alloy, the endothermic desorption peak is located in the
9
temperature range of 662-718 K and the peak temperature is 691 K. Adding 23
ACCEPTED MANUSCRIPT nano-graphite powders makes the peak temperature decrease to 688, 685, 678 and 680
2
K for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites, respectively. It is
3
consistent with the change trend of the dehydrogenation activation energy (Ede(a)),
4
suggesting that adding nano-graphite powders is beneficial to improve the
5
dehydrogenation performance. It is noteworthy that the initial decomposition
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Figure 14 Arrhenius fitting curves of the ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8)
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composites: (a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8.
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Figure 15 DSC curves of ball milled Mg-Al-xGR (x=0, 1, 3, 5 and 8) composites:
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(a) x=0; (b) x=1; (c) x=3; (d) x=5; (e) x=8 at heating rate of 5 K min-1.
4
temperature is 662, 657, 645, 633 and 641 K for the ball milled Mg-Al-xGR (x=1, 3, 5
5
and 8) composites (see Figure S7 in the supporting information), which is obviously
6
higher than the lowest dehydrogenation temperature as shown in Figure 12. It is
7
resulted by the different measurement methods. On the one hand, the dehydrogenation
8
kinetic curve is measured at the constant temperature, while DSC is a temperature
9
rising test. Due to the poor dehydrogenation kinetics of the samples, the DSC device
10
cannot detect the dehydrogenation signal in time. On the other hand, the samples are
11
measured under purified argon flow of 50 mL min-1 for the DSC test, while the initial
12
dehydrogenation pressure is 1×10-4 MPa in a vacuum for the dehydrogenation kinetic
13
curve test.
14
4. Conclusions
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This work mainly studied the actions of adding nano-graphite powders on the
2
microstructure and hydrogen storage properties of the ball milled Mg90Al10 alloy. The
3
main conclusions can be drawn as follows: (1) The ball milled Mg-Al alloy is composed of Mg phase and Al phase. Adding
5
nano-graphite powders causes the generation of Mg17Al12 phase and increases the
6
number of Al atoms that enter into the crystal lattice of Mg. Besides, the particle size
7
of the containing-graphite composites has an obvious decrease compared to the
8
Mg-Al alloy.
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(2) Adding nano-graphite powders improves the activation performance of the ball
10
milled Mg-Al alloy through preventing the surface oxide film from forming again.
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The hydrogenation capacity of the Mg-Al alloy is only 1.465 wt% within 10000 s at
12
the first hydrogenation process. And it increases to 5.044, 6.120, 6.220 and 5.802 wt%
13
for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8), respectively.
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(3) Adding nano-graphite powders promotes the reduction of the thermodynamic
15
stability of the hydrides. The dehydrogenation enthalpy (∆Hde) decreases from 75.43
16
kJ mol-1 H2 for the ball milled Mg-Al alloy to 74.63, 73.20, 72.68 and 71.52 kJ mol-1
17
H2 for the ball milled Mg-Al-xGR (x=1, 3, 5 and 8) composites, respectively.
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(4) Adding nano-graphite powders contributes to the enhancement of the
19
hydrogenation and dehydrogenation kinetics. The dehydrogenation activation energy
20
Ede(a) decreases from 162.06 kJ mol-1 H2 for the ball milled Mg-Al alloy to 161.26,
21
159.07, 156.15 and 158.26 kJ mol-1 H2 for the ball milled Mg-Al-xGR (x=1, 3, 5 and
22
8) composites, respectively.
26
ACCEPTED MANUSCRIPT Acknowledgements
2
This work is financially supported by the National Natural Science Foundations of
3
China (51471054).
4
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