Enhanced dehydriding thermodynamics and kinetics in Mg(In)-MgF2 composite directly synthesized by plasma milling

Article abstract of DOI:10.1016/j.jallcom.2013.10.029

The reversible formation of Mg(In) solid solution provides a new way to tune the dehydriding thermodynamics of MgH₂. However, the preparation of this solid solution is quite difficult and its dehydriding kinetics is rather sluggish. This work offers a novel technique, plasma milling (P-milling), to solve the two problems simultaneously. The efficiency of the synthesis of Mg(In) solid solution, with a hydrogen capacity of up to 5.16 wt.%, is improved significantly. Meanwhile, the kinetics is also modified by the catalyzing effect of in situ synthesized MgF₂.

Full text of DOI:10.1016/j.jallcom.2013.10.029

Journal of Alloys and Compounds 586 (2014) 113–117
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Enhanced dehydriding thermodynamics and kinetics in Mg(In)–MgF2
composite directly synthesized by plasma milling
L.Z. Ouyang a,d, Z.J. Cao a,d, H. Wang a,d, J.W. Liu a,d, D.L. Sun b,d, Q.A. Zhang , M. Zhu
c
a,d,
⇑
a
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, People’s Republic of China
b
Department of Materials Science, Fudan University, Shanghai 200433, People’s Republic of China
c
School of Materials Science and Engineering, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China
Key Laboratory of Advanced Energy Storage Materials of Guangdong Province, South China University of Technology, Guangzhou 510641, People’s Republic of China
d
a r t i c l e i n f o
a b s t r a c t
Article history:
The reversible formation of Mg(In) solid solution provides a new way to tune the dehydriding
thermodynamics of MgH . However, the preparation of this solid solution is quite difficult and its
2
dehydriding kinetics is rather sluggish. This work offers a novel technique, plasma milling (P-milling),
to solve the two problems simultaneously. The efficiency of the synthesis of Mg(In) solid solution, with
a hydrogen capacity of up to 5.16 wt.%, is improved significantly. Meanwhile, the kinetics is also modified
Received 23 August 2013
Received in revised form 4 October 2013
Accepted 4 October 2013
Available online 16 October 2013
2
by the catalyzing effect of in situ synthesized MgF .
Keywords:
Ó 2013 Elsevier B.V. All rights reserved.
Hydrogen storage
Magnesium alloys
Thermodynamics
Kinetics
1
. Introduction
for even as long as 24 h because the addition of NbF
protective layers on the surfaces of MgH particles, which contrib-
uted to the mitigation of degradation in the dehydrogenation
behavior of air-exposed MgH . Meanwhile, surface treatment with
fluoride resulted in the formation of magnesium fluoride (MgF ) on
the particle surface of Mg-based alloys, which also effectively im-
proved their surface activity and promoted the hydriding kinetics
under conditions of low temperature and pressure [16–18].
Although the hydrogenation and dehydrogenation kinetics of these
F-containing Mg-based alloys have been dramatically improved,
their high thermodynamic stabilities remain almost unchanged,
and a low dehydrogenation pressure at ambient temperature is re-
quired for their hydrogen desorption.
5
could form
2
Hydrogen storage systems have been considered as potential
solutions for the dwindling fossil fuel resources and growing envi-
ronmental problems [1,2]. Among them, MgH has shown promise
as an energy carrier medium due to its high hydrogen content
7.6 wt.%) and low cost [3,4]. However, its on-board applications
are obstructed by sluggish desorption kinetics and high thermody-
namic stability ( H = 75 kJ/mol H ). The dehydrogenation of MgH
2
2
2
(
D
2
2
requires a high temperature (>350 °C), and the hydrogenation of
Mg without catalysts is very difficult which generally needs a tem-
perature higher than 300 °C and pressure more than 20 bar [5].
During the past decades, considerable efforts have been devoted
to destabilizing Mg-based systems, aiming either at improving the
kinetic properties or decreasing the reaction enthalpy change.
Different approaches have been explored, which have mainly
involved alloying [6,7], nanostructuring [8,9], doping with catalytic
additives [10–15] or surface modification [16–18]. Of these strate-
gies, doping with F-containing species led to superior catalysis
through improving both the hydrogen absorption and desorption
In recent works, it has been found that the desorption enthalpy
change of MgH could be substantially altered by reversibly form-
2
ing an Mg(In) or Mg(In, Y) solid solution during dehydrogenation
[23,24], but the kinetic properties of these solid solution systems
were rather sluggish. The dehydrogenation activation energy of
Mg(In) solid solution was even as high as 161 kJ/mol. Thus,
improving its kinetic properties would be significantly beneficial
for making use of this thermodynamic tuning effect. Zhou et al.
[5] found that adding the Ti intermetallic alloys as catalysts could
significantly improve both dehydrogenation and hydrogenation
kinetics of MgH
alyzed with NbF
2
[19–22]. Park et al. [22] reported that MgH
could release 5 wt.% H after being exposed to air
2
cat-
5
2
2 2
kinetics of MgH , especially for TiMn -doped Mg system, which
⇑
Corresponding author at: School of Materials Science and Engineering, South
China University of Technology, Guangzhou 510641, People’s Republic of China.
Tel.: +86 20 87114253; fax: +86 20 87112762.
demonstrated extraordinary hydrogen storage properties at room
temperature and 1 bar hydrogen pressure. Subsequently, they used
this catalyst to catalyze the dehydriding kinetics of Mg(In) solid
E-mail address: memzhu@scut.edu.cn (M. Zhu).
0
925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.10.029

1
14
L.Z. Ouyang et al. / Journal of Alloys and Compounds 586 (2014) 113–117
solution, and found that this thermodynamic destabilized Mg-0.1In
alloy showed excellent dehydrogenation kinetics: this catalyzed
alloy began to dehydrogenate at approximately 100 °C and was
fully dehydrogenated at 150 °C within 3 h [25], which realized
2
the thermodynamic and kinetic destabilization of MgH simulta-
neously. Additionally, as Zhong’s report [23], the synthesis of
Mg(In) solid solution requires the combination of prolonged sinter-
ing and ball milling due to the quite different atomic radii between
the soft metal In (1.93 Å) and Mg (1.73 Å) [23]. Through the above
analysis, it would be very valuable to find a simple way to synthe-
size Mg(In) solid solution and to combine it with the effective
catalyst doping.
In previous works [26,27], dielectric barrier discharge plasma-
assisted milling (P-milling), a new material processing method,
has been reported. It showed much higher mechanical alloying
efficiency, and may thus provide a new means of synthesizing
Mg-based alloys. In this work, P-milling has been used to synthe-
size Mg(In) solid solution. On the other hand, polytetrafluoroethyl-
ene has been deployed in the milling process in a certain way to
2
realize the in situ formation of MgF catalyst in Mg(In) solid solu-
tion. By this new method, the effective synthesis of Mg(In) solid
solution and catalyst doping have been achieved simultaneously.
2
The Mg(In)–MgF composite exhibited greatly improved hydrogen
desorption thermodynamics and kinetics. The present work
provides a new strategy for simultaneous tuning of the thermody-
namics and kinetics of Mg-based hydrogen storage alloys.
2
. Experimental details
Fig. 1. XRD patterns for Mg–In–F system at different stages: (a) the mixture of the
Mg and In, (b) P-milling for 2 h, (c) the first hydrogenated sample, (d) the first
dehydrogenated sample, (e) hydrogenated product after 10 hydrogenation/dehy-
drogenation cycles, and (f) dehydrogenated product after 10 cycles.
Mg and In powders (both with a purity of 99.9%, 200 mesh) were mixed in a
molar ratio of 95:5. This powder mixture was loaded together with steel balls into
a cylindrical stainless steel vial filled with high purity argon (0.1 MPa), and the ball-
to-powder weight ratio was chosen as 30:1. The vial was vibrated for 2 h at a double
amplitude of 10 mm and a frequency of 25 Hz. The dielectric barrier discharge plas-
ma was generated by high-voltage (24 kV) alternating current at a frequency of
5 2 2
Mg(In) solid solution, besides the formation of Mg In and Mg In.
1
2 kHz. Polytetrafluoroethylene was introduced to induce the in situ formation of
MgF catalyst. Before milling, all sample handlings were performed in a glovebox
with a moisture content of less than 3 ppm and an oxygen content of less than
ppm.
The microstructures of all of the samples were characterized by X-ray diffrac-
tion analysis (XRD, Philips X’pert-MPD) using Cu K radiation (k = 0.15406 nm).
This outcome was consistent with the results of Zhong et al. [23]
and Busk [28], who also noted that the incorporation of In led to
a decrease in the lattice constants of the host Mg. More impor-
2
5
tantly, as initially designed, MgF
between Mg and polytetrafluoroethylene under the influence of
the plasma-assisted ball milling process. The amount of MgF
2
was formed due to the reaction
a
Lattice constants were calculated from the XRD data by the Rietveld model employ-
ing highscore software. As the lattice parameter was the only variable during the
transformation between Mg and Mg(In) solid solution, here we only considered
the refinement information of Mg in the XRD profile. Before the measurements, a
high purity (99.9999%) silicon wafer was used to calibrate the instrumental zero-
shift. Scanning electron microscopy (SEM) was carried out on a Philips XL-30 FEG
equipped with an EDX accessory. Differential scanning calorimetric (DSC) analysis
was performed using a NETZSCH STA409PC at a heating rate of 2 K/min. The hydro-
gen desorption properties were evaluated using a Sieverts-type automatic gas reac-
tion controller (Pct Pro2000). Samples of about 0.3 g were used for these
measurements, and the reaction cell was inductively heated with an accuracy of
2
was approximately 12.9 wt.%, as determined from an XRD profile
refinement. Fig. 1c shows the XRD pattern of P-milled Mg–In–F
powder after the first hydrogenation at 623 K. Compared with
the pattern of the P-milled product, the diffraction peaks of Mg(In)
solid solution, Mg
5
In
2
and Mg
and MgIn (b ) phase with L1
be concluded that Mg(In) solid solution, Mg
2
In disappeared, while peaks due to
-structure appeared. It could
In , and Mg In phases
0
0
MgH
2
0
5
2
2
0
0
were transformed into MgH2 and b , while the MgF phase re-
2
±1 K.
mained unchanged. Fig. 1d shows the XRD pattern of the dehydro-
0
0
genated sample. After dehydrogenation, the
b
phase had
3
. Results and discussion
disappeared and no In-containing phases had emerged. Mean-
while, calculation showed that the lattice constants of the Mg
phase further decreased to a = 0.32071(7) nm, c = 0.52099(4) nm,
which suggested that an Mg(In) solid solution with an increased
3.1. Phase transition of the Mg–In–F system during hydrogenation and
dehydrogenation
0
0
solubility of In had been recovered through reaction of the b phase
and MgH . MgF was stable even when the operation temperature
was above 1573 K [29,30]. Fig. 2 shows SEM images of the Mg(In)–
MgF composite. The morphology of this composite was largely
composed of flocculent structures. Very fine MgF particles
(ꢀ300 nm) were separated out along the bulk Mg(In). Fig. 3 shows
the element mapping of this Mg(In)–MgF particle. The mapping
results showed that MgF was uniformly dispersed in the
Mg(In)–MgF composite, and its grain size was 38.6 nm, as
Fig. 1 shows the XRD patterns of the Mg–In–F system at differ-
ent stages. The diffraction peaks of the In phase could be clearly
identified in the initial powder mixture (Fig. 1a), and it should be
noted that no F was present in the sample before milling. Fig. 1b
shows the XRD pattern of Mg-In powder after P-milling for 2 h. It
can be observed that the In peaks disappeared and peaks attribut-
2
2
2
2
2
able to Mg
In
5 2
and Mg
2
In developed. Lattice constant calculations
2
showed that those of Mg decreased slightly from
a = 0.32098(6) nm, c = 0.52116(0) nm in the initial powder mixture
to a = 0.32091(1) nm, c = 0.52099(5) nm in the P-milled product,
which indicated that part of the In had dissolved in the Mg to form
2
calculated from the XRD data by the Rietveld method. These results
showed that the P-milling method yielded an Mg(In) solid solution
with an in situ synthesized MgF phase homogeneously distributed
2

L.Z. Ouyang et al. / Journal of Alloys and Compounds 586 (2014) 113–117
115
2
Fig. 2. SEM images of (a) Mg(In)–MgF composite and (b) its partial enlargement.
2
Fig. 3. (a) The mapping results of the Mg(In)–MgF composite, and the element distribution of (b) Mg, (c) F, (d) In, respectively.
within it. Fig. 1e and
f
show the XRD patterns for the
To further reveal the reaction temperature and mechanism, DSC
curves of the hydrogenated product and pure MgH were mea-
sured, and the results are shown in Fig. 4. The endothermic peak
of the hydrogenated product of Mg(In)–MgF composite was obvi-
hydrogenation and dehydrogenation products of the Mg–In–F sys-
tem after ten hydrogenation/dehydrogenation cycles at 623 K,
respectively. These patterns confirmed that the products after ten
cycles were stable and identical to those from the first hydrogena-
tion/dehydrogenation.
2
2
ously broader than that for pure Mg, suggesting an overlap of mul-
ti-step reactions upon dehydrogenation, i.e., decomposition of
0
0
Based on the above structural analysis, an alloy containing
2
MgH and dissolution of the b phase in Mg to form Mg(In) solid
Mg(In) solid solution, Mg
P-milling, then transformed into MgH
hydrogenation, and finally reverted to Mg(In)–MgF
lowing dehydrogenation. The reaction mechanism can be ex-
pressed as follows:P-milling:
5
In
2
, Mg
2
In and MgF
2
was obtained after
, b , and MgF phases after
composite fol-
solution. This result is consistent with the results of Zhong et al.
[23] and Luo et al. [24], who found the endothermic peaks of both
Mg(In) binary and Mg(In, Y) ternary solid solutions to be broader
than that of pure Mg. Furthermore, the hydrogenated Mg(In)–
0
0
2
2
2
MgF
temperature compared with pure MgH
2
dehydriding thermodynamics and kinetics of MgH .
2
composite showed
a
decrease in the decomposition
2
, indicating tuning of the
Mg þ In þ ðC
2
F
4
Þ ! MgðInÞ þ Mg In
2
þ Mg In þ MgF
n
5
2
2
1
st hydrogenation:
3.2. Dehydriding thermodynamic properties of Mg(In)–MgF
2
0
0
composite
MgðInÞ þ Mg In
2
þ Mg In þ H
2
! MgH þ b
5
2
2
Following hyd./dehyd. cycles:
Fig. 5a shows the pressure-composition isotherm (PCI) curves
for hydrogen desorption from the Mg(In)–MgF composite and
pure MgH . It is impressive that the decomposition equilibrium
2
0
0
MgðInÞ þ H
2
$ MgH þ b
2
2

1
16
L.Z. Ouyang et al. / Journal of Alloys and Compounds 586 (2014) 113–117
2
der, the Mg(In)–MgF composite had a much lower DH than that
of pure Mg. According to the above results, the improved thermo-
dynamics and related mechanism of the present Mg-In system
incorporating MgF
lid solution reported by Zhong et al. [23]. The incorporation of
MgF had no influence on the dehydriding thermodynamics. In
2
should be the same as those for the Mg(In) so-
2
addition, we would like to point out that the system still had a high
hydrogen storage capacity of 5.16 wt.% at 609 K.
3
.3. Dehydriding kinetic properties of Mg(In)–MgF
2
composite
In addition to the above thermodynamic tuning, the kinetics
was also greatly improved in the present work. Fig. 6a displays
the hydrogen desorption curves for the hydrogenated Mg(In)–
MgF
the composite could release the stored hydrogen in less than
0 min, whereas for MgH the dehydrogenation process was
incomplete even after 120 min. The activation energy for the dehy-
drogenation process of Mg(In)–MgF was obtained by fitting the
2 2
composite compared with that for pure MgH . At 623 K,
3
2
2
isothermal dehydriding curves at different temperatures using
the Johanson–Mehl–AvramiKolmogorov (JMAK) equation [31–33]:
In½ꢁlnð1 ꢁ
aÞꢂ ¼
g
lnt þ
g
lnk
ð1Þ
where is the reaction fraction, k is the rate constant, and
a
g
is the
Avrami exponent. The hydrogen desorption fraction ranging from
0
g
.2–0.5 was adopted to fit the kinetic curves. The Avrami exponent
was in the range 1.10 to 2.41, implying that the dehydrogenation
2 2
Fig. 4. DSC curves for (a) Pure MgH , (b) hydrogenated Mg(In)–MgF composite, (c)
hydrogenated Mg(In) solid solution [23] and (d) hydrogenated Mg(In, Y) solid
solution [24], with a heating rate of 2 K/min.
reaction for the sample followed a diffusion-controlled mechanism
31,32,34]. The values of k were applied to calculate the dehydroge-
nation activation energy according to the Arrhenius equation. As
shown in Fig. 6b (1), E for the dehydrogenation was calculated to
be 127.7 kJ/mol, much lower than the values for pure MgH
ꢀ160 kJ/mol) [35–38], Mg(In) binary (161 kJ/mol) [23], and Mg(In,
[
a
2
(
Y) ternary solid solution (147.41 kJ/mol) [24]. To further verify the
reaction mechanism and the obtained dehydrogenation activation
energy, the observed dehydrogenation rate curves were also fitted
using the rate equations proposed by Li et al. [39]. As shown in
Fig. 6b (2), the activation energy fitted by the Jander diffusion model
was 125.7 kJ/mol, in good agreement with the result from the JMAK
model. This also provides strong evidence that the rate-limiting
step of the desorption process follows the diffusion-controlled
mechanism as proposed by the JMAK model.
Fig. 5. (a) PCI curves for the hydrogen desorption of the Mg(In)–MgF
and pure MgH , (b) Van’t Hoff plots of the Mg(In)–MgF composite, pure MgH
Mg(In) [23] and Mg(In, Y) [24] solid solutions.
2
composite
2
2
2
,
pressures of this system were much higher than that of pure MgH
2
at the same temperature, which confirmed the thermodynamic
destabilization indicated by the DSC analysis. Based on the PCI
data, Van’t Hoff plots for this sample and MgH
and the results are shown in Fig. 5b. Van’t Hoff plots from refes
23,24] are also presented for comparison. The H for the dehydro-
genation of the hydrogenated Mg(In)–MgF composite was calcu-
lated as 69.2 kJ/mol H , which is comparable to the experimental
68.1 kJ/mol H ) and calculated (67.8 kJ/mol H ) values obtained
by Zhong et al. [23]. Although the experimental value for MgH
2
were obtained,
[
D
2
2
(
2
2
2
2
Fig. 6. (a) Dehydriding kinetic curves for the hydrogenated Mg(In)–MgF compos-
ite, (b): (1) and (2) are the Arrhenius plots fitted by the JMAK model and the Jander
diffusion model, respectively.
(
2
79.1 kJ/mol H ) slightly deviated from the standard value (75 kJ/
2
mol H ) due to experimental error or a different type of Mg pow-

L.Z. Ouyang et al. / Journal of Alloys and Compounds 586 (2014) 113–117
117
As is well known, the poor dehydriding kinetics arises from the
poor permeability of the MgH layer to H atoms. By P-milling, MgF
particulates were formed in situ and dispersed homogeneously in
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9
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2
[
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[
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were produced along the interfaces of MgH
viding channels for hydrogen diffusion in the dehydriding process
16–18]. More importantly, these MgF particulates, which exhibit
2 2
and MgF , thus pro-
[
[
[
[
[
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(
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2
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The dehydrogenation activation energy (E
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) of Mg(In) solid solu-
[
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[
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