The hydrogen storage performance of a 4MgH2-LiAlH4-TiH2 composite system

Article abstract of DOI:10.1016/j.jallcom.2016.03.194

The hydrogen storage performance of a 4MgH₂-LiAlH₄ composite system was greatly improved by adding TiH₂. The temperature-programmed release curve of the 4MgH₂-LiAlH₄-TiH₂ composite reflecte

Full text of DOI:10.1016/j.jallcom.2016.03.194

Journal of Alloys and Compounds 676 (2016) 557e564
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Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
The hydrogen storage performance of a 4MgH2eLiAlH4eTiH2
composite system
,
,
, ***
Wenpei Sun a, Yao Zhang a *, Yunfeng Zhu b **, Xiangyang Zhuang a, Jun Dong c
,
Yi Qu d, Xinli Guo a, Jian Chen a, Zengmei Wang a, Liquan Li b
a School of Materials Science and Engineering, Jiangsu Key Laboratory of Advanced Metallic Materials, Southeast University, Nanjing 211189, China
b College of Materials Science and Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, China
c School of Information Science and Engineering, Southeast University, Nanjing 210096, China
d Dalian Environmental Monitoring Centre, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The hydrogen storage performance of a 4MgH2eLiAlH4 composite system was greatly improved by
adding TiH2. The temperature-programmed release curve of the 4MgH2eLiAlH4eTiH2 composite re-
flected that the onset temperature of dehydrogenation decreased remarkably to 54 ꢀC from that of
4MgH2eLiAlH4 (100 ꢀC). X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR)
analyses indicated that TiH2 was not involved in the decomposition of either LiAlH4 or MgH2 or their
interactions. In the ternary composite, TiH2 can be regarded as an effective catalyst for LiAlH4, Li3AlH6
and MgH2, of which the activation energies of dehydrogenation were reduced by 23.3 kJ/mol, 29.1 kJ/mol
and 45.5 kJ/mol from those of 4MgH2eLiAlH4, respectively. The rehydrogenation of those products at
350 ꢀC cannot be fully integrated into their original phases, and the reversible capacity was ascribed only
to the formation of MgH2. The activation energy of rehydrogenation of MgH2 was greatly decreased to
108.9 kJ/mol from 158.8 kJ/mol of 4MgH2eLiAlH4. Isothermal hydrogenation curves and fitted lines from
the Arrhenius equation demonstrated the enhancement of hydrogenation kinetics.
Received 26 December 2015
Received in revised form
23 March 2016
Accepted 23 March 2016
Available online 25 March 2016
Keywords:
MgH2eLiAlH4TiH2 system
De/re-hydrogenation
Kinetics
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
elemental reserves on earth, MgH2 has already received consider-
able attention. However, many severe thermodynamic and kinetic
Hydrogen is an environmentally friendly energy carrier. Hy-
drogen's production, storage/transportation and utilization
constitute a perfect hydrogen cycle, which can possibly accomplish
a so-called hydrogen economy society [1]. However, hydrogen
storage has seemed to be a bottleneck for the cycle especially in
recent years. Although available materials for hydrogen storage are
rare at present, some solid-state hydrogen storage materials are
still potential candidates for possible applications, for example the
metal hydride MgH2 [2,3], the complex hydride LiAlH4 [4e6], LiBH4
[7e10], composite systems such as LiNH2eLiH [11e13] and
Mg(NH2)2e2LiH [14e16], etc.
issues remain in this material, which hamper its further applica-
tions [17]. LiAlH4 is also competitive among solid-state hydrogen
storage materials owing to its large theoretical capacity (10.5 wt%)
and low onset temperature of dehydrogenation (below 150 ꢀC).
Unfortunately, it has sluggish dehydrogenation kinetics and poor
reversibility.
The integration of MgH2 with LiAlH4 was individually proposed
by Zhang et al. [18] and Chen et al. [19]. Both groups found that the
thermal stability of MgH2 in the composite system was significantly
lowered while the kinetics was greatly improved. Ismail et al.
[20,21] observed that introducing a catalyst, such as Fe2O3, TiF3,
etc., into the 4MgH2eLiAlH4 system likely improved the hydrogen
de/re-sorption properties of the composite.
Because of its large capacity of 7.6 wt% H2 and its abundant
Simultaneously, other researchers found that TiH2 played the
role of a catalyst on MgH2 or LiAlH4, respectively. Shao et al. [22]
synthesized a MgH2e0.1TiH2 composite and reduced the dehy-
drogenation temperature by more than 100 ꢀC from as-received
MgH2. However, the initial dehydrogenation temperature of the
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: zhangyao@seu.edu.cn (Y. Zhang), yfzhu@njtech.edu.cn
(Y. Zhu), djhf2003@hotmail.com (J. Dong).
http://dx.doi.org/10.1016/j.jallcom.2016.03.194
0925-8388/© 2016 Elsevier B.V. All rights reserved.

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W. Sun et al. / Journal of Alloys and Compounds 676 (2016) 557e564
Fig. 1. TPD (temperature-programmed desorption) curves of 4MgH2eTiH2 (x)eLiAlH4
(1 h) samples milled for different time (x ¼ 1 h, 5 h, 10 h and 20 h).
Fig. 3. XRD patterns of the 4MgH2eTiH2 (x)eLiAlH4 (1 h) samples after ball milling for
various time. (a) x ¼ 1 h; (b) x ¼ 5 h; (c) x ¼ 10 h; (d) x ¼ 20 h
reduced thermal stability. Therefore, we prepared a 4MgH2e-
LiAlH4eTiH2 composite system by ball milling. Using volumetric
release measurements, we found that the onset temperature for the
ternary composite could be reduced by 90 ꢀC and 54 ꢀC from
4MgH2eTiH2 and 4MgH2eLiAlH4, respectively. We also investi-
gated the product of the 4MgH2eTiH2eLiAlH4 system during de/re-
hydrogenation processes using X-ray diffraction (XRD), Fourier
transform infrared spectroscopy (FTIR) and differential scanning
calorimetry (DSC). Insights into the reaction pathway of the ternary
system will also be discussed.
2. Experimental
The raw materials in the present work, such as commercial
MgH2 (98%, Alfa Aesar), TiH2 (97%, Alfa Aesar) and LiAlH4 (99.99%,
Alfa Aesar) powders, were used as received without any treatment.
All materials were stored and handled in a glove box filled with
high-purity argon (99.999%, O2 < 0.1 ppm, H2O < 0.1 ppm) to isolate
them from oxygen and humidity. The 4MgH2eLiAlH4 and
4MgH2eTiH2eLiAlH4 mixtures were prepared in a QM-1SP plane-
tary ball mill for various periods using a ball-to-powder weight
ratio of 40:1 and a rotation rate of 450 rpm.
The non-isothermal dehydrogenation kinetics of the samples
were characterized by temperature programmed desorption (TPD)
measurements at a ramping rate of 2 ꢀC minꢁ1 on a Sieverts-type
apparatus (Advanced Materials Corporation). The activation
Fig. 2. XRD patterns of the 4MgH2eTiH2 (x) samples after ball milling for different
time. (a) x ¼ 1 h; (b) x ¼ 5 h; (c) x ¼ 10 h; (d) x ¼ 20 h
TiH2-doped LiAlH4 decreased to 75 ꢀC from that of as-received
LiAlH4 (155 ꢀC) according to the investigation by Liu et al. [23].
Based on these works, we imagined that TiH2-doped MgH2
reacting with LiAlH4 might achieve both enhanced kinetics and
Table 1
The grain sizes of the samples calculated by Scherrer's equation based on their XRD patterns in Fig. 2.
Samples
4MgH2eTiH2 (1 h)
4MgH2eTiH2 (5 h)
4MgH2eTiH2 (10 h)
4MgH2eTiH2 (20 h)
The grain size of MgH2 (nm)
The grain size of TiH2 (nm)
28 0.1
12 0.1
17 0.3
10 0.1
15 0.4
15 0.2
9
0.4
9
0.3
Table 2
The grain sizes of the samples calculated by Scherrer's equation based on their XRD patterns in Fig. 3.
Samples 4MgH2eTiH2 (1 h)eLiAlH4 (1 h) 4MgH2eTiH2 (5 h)eLiAlH4 (1 h) 4MgH2eTiH2 (10 h)eLiAlH4 (1 h) 4MgH2eTiH2 (20 h)eLiAlH4 (1 h)
The grain size of MgH2 (nm) 18 0.1
12 0.2
10 0.4
38 0.3
11.0 0.1
0.2
34 0.1
10 0.3
0.4
31 0.2
The grain size of TiH2 (nm) 12 0.2
The grain size of LiAlH4 (nm) 48 0.3
9
8

W. Sun et al. / Journal of Alloys and Compounds 676 (2016) 557e564
559
Fig. 4. XRD patterns of 4MgH2eTiH2 (5 h)eLiAlH4 (1 h) samples collected at different temperatures after TPD measurements, respectively. (a) after ball milling; (b) 171 ꢀC; (c)
198 ꢀC; (d) 220 ꢀC; (e) 237 ꢀC; (f) 257 ꢀC; (g) 275 ꢀC; (h) 312 ꢀC.
tested upon the same Sieverts-type apparatus as the TPD mea-
surements. A hydrogen pressure of 4 MPa was exerted on the
samples while the temperature was maintained at a certain value.
The phases in our prepared and produced samples were iden-
tified by X-ray diffraction (XRD, Rigaku SmartLab, Cu Ka, 40 kV and
30 mA). The grains sizes of the samples were calculated by Scher-
rer's equation using Jade software. Simultaneously, the Fourier
transformation infrared spectrometer (FTIR-7600) was used to
confirm the alanate phases. All samples were handled in a glove
box. An amorphous polymer film covered the sample holders
tightly enough to shield them from the air.
3. Results and discussion
3.1. Influence of ball milling time
LiAlH4 is well known to be able to decompose itself and liberate
hydrogen during ball milling [24]. In order to retain the high ca-
pacity of the 4MgH2eLiAlH4eTiH2 system after ball milling, we first
prepared 4MgH2eTiH2 composite for different milling times (1 h,
5 h, 10 h and 20 h), and then we milled the samples with LiAlH4 for
another 1 h. According to some previous studies [25], varying the
ball milling time likely affected the dehydrogenation performance
of the MgH2eTiH2 composite. We therefore needed to investigate
the influence of milling time on the 4MgH2eLiAlH4eTiH2 sample to
optimize it.
In this case, four samples were synthesized using the above-
mentioned method and denoted as 4MgH2eTiH2 (xh)-LiAlH4
(1 h) (x ¼ 1, 5, 10, 20). Their non-isothermal dehydrogenation be-
haviours (TPD curves) are presented by Fig. 1. All these samples
exhibited two significant dehydrogenation steps, similar to the
behaviour of binary 4MgH2eLiAlH4 [18]. Furthermore, with the ball
Fig. 5. FTIR spectra of (a) as-received Li3AlH6, (b) 4MgH2eTiH2 (5 h)eLiAlH4(1 h) after
ball milling and (c) 4MgH2eTiH2 (5 h)eLiAlH4 (1 h) after TPD measurements at 171 ꢀC.
energies of these samples were determined from Kissinger's plots
and tested by differential scanning calorimetry (DSC, TA Q2000) at
various ramping rates of 2 ꢀC minꢁ1, 4 ꢀC minꢁ1, 6 ꢀC minꢁ1 and
8
ꢀC minꢁ1. The temperature ranged from room temperature to
450 ꢀC. Before the tests, 7 mg samples were loaded into a platinum
crucible in a glove box and then laid in the instrument's chamber.
High-purity argon (99.999%) was employed as the carrier gas in the
DSC tests with a flux of 50 ml minꢁ1
.
The isothermal hydrogenation performance of the samples was

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W. Sun et al. / Journal of Alloys and Compounds 676 (2016) 557e564
Fig. 6. The dehydrogenation process of the ternary 4MgH2eTiH2 (5 h)eLiAlH4 (1 h) system.
Table 3
Reactions in the 4MgH2eTiH2eLiAlH4 composite in different temperature ranges.
Temperature range
Reaction equation
Identified products by XRD analysis
54 ꢀCe171 ꢀC
171 ꢀCe198 ꢀC
198 ꢀCe220 ꢀC
198 ꢀCe257 ꢀC
237 ꢀCe275 ꢀC
3 LiAlH4 / Li3AlH6 þ 3 Al þ 3 H2
2 Li3AlH6 / 6LiH þ 2 Al þ 3 H2
6 LiH þ 14 MgH2 / 2 Li3Mg7 þ 17 H2
12 Al þ 17 MgH2 / Al12Mg17 þ 17 H2
MgH2 / Mg þ H2
Li3AlH6, Al
LiH, Al
Li3Mg7
Al12Mg17
Mg
(5 h)eLiAlH4 (1 h) were reduced to the lowest of these samples.
Furthermore, this condition possesses the largest amount of
dehydrogenation capacity (up to 5.2 wt%) among these ternary
phase samples.
Fig. 2 presents the XRD patterns of the 4MgH2eTiH2 (x h) (x ¼ 1,
5, 10, 20) samples after different ball milling times. Scherrer's
equation was used to calculate the average grain size of TiH2 and
MgH2 after different ball milling times as follows.
D ¼ K
g=B cos
q
(1)
where D is the average grain size (nm), K is the Scherrer constant,
g
is the wavelength of the X-rays, which is 0.154056 nm, B is the
FWHM of diffraction peaks of the samples, and is the Bragg angle.
q
The calculated grain sizes of MgH2 and TiH2 in binary and
ternary samples are summarized in Tables 1 and 2, respectively. The
values suggested that the TiH2 grain size gradually decreased with
longer milling times. Therefore, with the increase in the milling
time, more TiH2 grain boundaries formed, which would continu-
ously provide more catalytic sites for LiAlH4 and MgH2 decompo-
sition. As shown in Fig. 3, Li3AlH6 was generated even after milling
the ternary phase mixture for 1 h. If the balance between the onset
temperature and the dehydrogenation capacity is taken into ac-
count, the MgH2eTiH2 (5 h)eLiAlH4 (1 h) can be regarded as the
most optimized sample among the prepared composites.
Fig. 7. TPD curves of 4MgH2eTiH2 (5 h)eLiAlH4 (1 h) and 4MgH2 (5 h)-LiAlH4 (1 h).
milling time increasing from 1 h to 20 h, the initial dehydrogena-
tion temperature gradually decreased. The onset temperatures of
the initial step (54 ꢀC) and the second step (165 ꢀC) of 4MgH2eTiH2

W. Sun et al. / Journal of Alloys and Compounds 676 (2016) 557e564
561
Fig. 8. DSC curves at various heating rates (
b
¼ 2, 4, 6, 8 ꢀC minꢁ1) (a) 4MgH2 (5 h)eLiAlH4 (1 h); (b) 4MgH2eTiH2 (5 h)eLiAlH4 (1 h). The inset plot was achieved by Kissinger's
method. (A) The apparent activation energy of the decomposition of LiAlH4; (B) The apparent activation energy of the decomposition of Li3AlH6; (C) The apparent activation energy
of MgH2-relevanted dehydrogenation.
3.2. Dehydrogenation behaviour
confirming the results of the XRD analyses.
3 LiAlH4 / Li3AlH6 þ 3 Al þ 3 H2
Fig. 4 shows the XRD patterns of the 4MgH2eTiH2eLiAlH4
samples after reaching different temperatures of 171 ꢀC, 198 ꢀC,
220 ꢀC, 237 ꢀC, 257 ꢀC, 275 ꢀC and 312 ꢀC.
LiAlH4 fully decomposed into Li3AlH6 and Al at the temperature
of 171 ꢀC as shown in equation (2), which can be observed in the
XRD patterns in Fig. 4. The FTIR spectra in Fig. 5 shows the presence
of the AleH bands of Li3AlH6 in the range of 1600e800 cmꢁ1 [26],
(2)
According to Fig. 4, LiAlH4 completely disappeared in the sample
at the temperature of 171 ꢀC, and Li3AlH6 remained in the first step
of the dehydrogenation in accordance with reaction (2). For the
samples that reached 171 ꢀCe198 ꢀC, their XRD patterns demon-
strate the presence of LiH and Al phases and the absence of Li3AlH6,

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W. Sun et al. / Journal of Alloys and Compounds 676 (2016) 557e564
Table 4
The activation energies for de/re-hydrogenation of the 4MgH2eLiAlH4 and 4MgH2eTiH2eLiAlH4 samples.
4MgH2eLiAlH4
4MgH2eTiH2eLiAlH4
Activation energy of dehydrogenation
(kJ molꢁ1
Decomposition of LiAlH4
Decomposition of Li3AlH6
MgH2-relavanted decomposition
111.4 5.1
114.7 6.6
174.3 8.6
158.8 8.3
88.1 10.8
85.6 6.2
128.8 4.0
108.9 6.9
)
Activation energy of rehydrogenation
(kJ molꢁ1
)
Fig. 9. XRD pattern for the 4MgH2eLiAlH4eTiH2 composite rehydrogenated at 350 ꢀC.
which implies that Li3AlH6 was decomposed gradually within the
temperature range. Therefore, the following reaction (3) should
have taken place in that temperature range.
2 Li3AlH6 / 6LiH þ 2Al þ 3 H2
(3)
The Li3Mg7 phase can be found in the XRD pattern at a tem-
perature of 220 ꢀC, implying the occurrence of an interaction be-
tween MgH2 with LiH as described by reaction (4). LiH disappeared
from the XRD pattern when the temperature reached 220 ꢀC, which
means that LiH was thoroughly consumed at that temperature.
6 LiH þ 14 MgH2 / 2 Li3Mg7 þ 17 H2
(4)
Al12Mg17 can be identified in the sample collected at 220 ꢀC,
while Al completely disappeared from the XRD pattern of the
sample at 257 ꢀC. These phenomena indicate that the produced Al
should react with the remaining MgH2 and facilitate the decom-
position of MgH2 in this case. The interaction occurring within the
temperature range of 198 ꢀCe257 ꢀC can be described as follows.
Fig. 10. Isothermal hydrogenation curves for the dehydrogenated samples. (a)
4MgH2eLiAlH4 under 4 MPa hydrogen pressure at 254 ꢀC, 275 ꢀC and 315 ꢀC (527, 548,
and 588 K). (b) 4MgH2eLiAlH4eTiH2 under 4 MPa hydrogen pressure at 248 ꢀC, 285 ꢀ
C
and 312 ꢀC (521, 558, and 585 K). The inset plots were achieved by Arrhenius equation.
12 Al þ 17 MgH2 / Al12Mg17 þ 17 H2
(5)
The reflection peaks of metallic Mg in the XRD pattern of the
sample collected at 237 ꢀC indicates that MgH2 likely started self-
decomposing from this point. The absence of the MgH2 phase in
the XRD patterns demonstrated that the self-decomposition would
be terminated at 275 ꢀC. Furthermore, possibly due to the presence
of TiH2, the barely decomposed Mg2Al3 phase in previous work [18]
was not identified in the dehydrogenation process. One possible
reason is that the dehydrogenated product Mg17Al12 tends to
transform into Mg2Al3 at high temperature (over 400 ꢀC) in the
presence of hydrogen [18]. In this case, however, the major dehy-
drogenation was observed below 350 ꢀC due to the introduction of
TiH2. Thus, all reactions avoided forming Mg2Al3.
The whole dehydrogenation of the system can be summarized
in both Fig. 6 and Table 3. The first step of dehydrogenation can
surely be attributed to the decomposition of LiAlH4, and the second
step was composed of Li3AlH6 and MgH2-relevant reactions.

W. Sun et al. / Journal of Alloys and Compounds 676 (2016) 557e564
563
Table 5
Typical kinetic equations used for fitting the experimental hydrogen sorption data.
Model equation
Equation description
R2 (T ¼ 315 ꢀC)
Chemical adsorption
John-Mehl-Avrami-Kolmogorov equation
0.84
0.99
a
¼ kt
½ꢁlnð1 ꢁ
a2 ¼ kt
a
Þ1=nꢂ ¼ kt
One-dimensional diffusion
0.88
0.90
0.78
Two-dimensional diffusion
Shrinking core model three-dimensional growth controlled by diffusion
a
þ ð1 ꢁ
a
Þlnð1 ꢁ Þ ¼ kt
a
ꢀ
ꢁ
1 ꢁ
ꢁ ð1 ꢁ
a
Þ1=n ¼ kt
2
3
a
½1 ꢁ ð1 ꢁ
a
Þ1=3ꢂ2 ¼ kt
Jander equation for three-dimensional diffusion
0.85
3.3. Dehydrogenation kinetics
2 Li3Mg7 þ 17 H2 / 6 LiH þ 14 MgH2
(7)
(8)
Fig. 7 exhibits the temperature-programmed desorption (TPD)
curves of the 4MgH2eTiH2eLiAlH4 and 4MgH2eLiAlH4 samples for
comparison. Each sample shows two significant dehydrogenation
steps, as mentioned above. The initial dehydrogenation tempera-
ture of the ternary phase sample was found to be greatly reduced
by 46 ꢀC from that of the binary phase sample, 100 ꢀC. For the
second step, the onset temperature of the ternary phase sample
was greatly reduced by 60 ꢀC from that of the binary one (225 ꢀC) as
well. Furthermore, the final temperature of the end of the dehy-
drogenation reaction of 4MgH2eTiH2eLiAlH4 was also reduced by
12 ꢀC from 287 ꢀC of the 4MgH2eLiAlH4 sample. This result dem-
onstrates that the hydrogen release kinetics of the 4MgH2eLiAlH4
system was greatly improved due to the introduction of TiH2. The
dehydrogenation capacity of the ternary system was approximately
1.0 wt% in the first step, which nearly attained the theoretical ca-
pacity of the reaction (2) (up to 1.04 wt%). However, the total
release capacity was inevitably reduced from that of binary system
because of the presence of TiH2, which does not contribute any
hydrogen storage capacity during the whole reaction.
Al12Mg17 þ 17 H2 / 12 Al þ 17 MgH2
The isothermal absorption curves of 4MgH2eTiH2eLiAlH4 at
different temperatures are shown in Fig. 10 (b), which suggest that
the reaction rate was greatly improved with the increase of tem-
perature from 254 ꢀC (527 K) to 315 ꢀC (588 K). The activation
energies reflecting the hydrogen absorption kinetics are well
known to be described by the Arrhenius equation as follows:
Ea ¼ ꢁRTln(K/K0)
(9)
where Ea is the activation energy, R is the gas constant, and T is the
absolute temperature for hydrogen absorption. K is a temperature-
dependent reaction rate constant, which varied with different
temperatures. If plotting the dependence of ln(k) on 1/T, one may
calculate the reaction activation energy Ea. Therefore, K is funda-
mentally important to make such a plot.
Equation (10) was proposed to determine the values of K [28].
d
a
/dt ¼ Kf(
a
)
(10)
The thermal decomposition behaviours of 4MgH2eLiAlH4 and
4MgH2eTiH2eLiAlH4 were investigated by DSC measurements, and
their patterns are shown in Fig. 8. As in previous studies, the
exothermic peak was attributed to the decomposition of LiAlH4 into
Li3AlH6 and Al. The endothermic events occurring at approximately
200 ꢀC and 325 ꢀC should belong to the decomposition of Li3AlH6
and MgH2-relavanted dehydrogenation, respectively [20]. Both
processes merged together in the second step.
where is the reacted fraction and f(
a
a
) is the differential form of
the reaction mechanism function. We plotted the results as shown
in Supplementary Figs. 1 and 2 on the basis of Fig. 10 and tried
several reaction models as listed in Table 5 [29] to fit those points.
The Johnson-Mehl-Avrami (JMA) equation (11) was found to be the
best most matched model [30e33]. Therefore, we may identify the
values of K corresponding to different temperatures.
To calculate the apparent activation energy of the two dehy-
drogenation steps, different endothermic peak temperatures at
)]1/n ¼ Kt
(11)
various heating rates (
b
¼ 2, 4, 6, 8 ꢀC minꢁ1) were fitted on the
[ꢁln(1ꢁ
a
basis of Kissinger's equation [27] to obtain the slopes and describe
the dehydrogenation kinetics of the samples. As shown in Table 4,
the apparent activation energy of the decomposition of LiAlH4 was
reduced from 111.4 kJ/mol for the binary sample to 88.1 kJ/mol for
the ternary one. Similarly, the activation energies belonging to the
Li3AlH6 and MgH2 decompositions in the ternary sample were
reduced by 29.1 kJ/mol and 45.5 kJ/mol, respectively. This result
indicates that the added TiH2 was beneficial to enhancing the
dehydrogenation kinetics of the 4MgH2eLiAlH4 system.
Using the method mentioned above, the activation energy (Ea)
of 4MgH2eTiH2eLiAlH4 can be determined to be 108.9 kJ/mol H2,
which was greatly decreased from that of binary 4MgH2eLiAlH4,
158.8 kJ/mol H2 (as shown in Fig. 10 and Table 4). This result
demonstrates again that the hydrogen absorption kinetics was
greatly improved by adding TiH2.
4. Conclusions
3.4. Hydrogen-absorbing performance
In this paper, the hydrogen storage kinetics of 4MgH2eLiAlH4
was greatly improved after doping with TiH2. The initial dehydro-
genation temperature of 4MgH2eLiAlH4eTiH2 significantly
decreased by nearly 50 ꢀC from that of 4MgH2eLiAlH4. Its second
release step also decreased by 60 ꢀC from that of the latter.
TiH2 was not involved in the dehydrogenation process of
4MgH2eLiAlH4eTiH2. However, it serves as an effective catalyst for
both MgH2 and LiAlH4. The apparent activation energy of the
decomposition of LiAlH4 was reduced from 111.4 kJ/mol to 88.1 kJ/
mol, and the activation energy of MgH2-relavanted dehydrogena-
tion in 4MgH2eLiAlH4eTiH2 was 128.8 kJ/mol, which was lowered
The XRD pattern of the product of the hydrogenated sample is
shown in Fig. 9. MgH2, TiH2, Al and LiH can be observed after being
isothermally hydrogenated at 350 ꢀC. Combined with Fig. 4 (h), we
can clearly identify the pathways of hydrogen absorption and
describe them by equations (6)e(8). LiAlH4/Li3AlH6 phases were
barely achieved, and the reversible capacity of the ternary com-
posite was only relevant to the de/re-sorption of MgH2.
Mg þ H2 / MgH2
(6)

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storage: a review, Int. J. Hydrogen Energy 36 (2011) 14512e14526.
[11] Y. Song, Z.X. Guo, Electronic structure, stability and bonding of the Li-N-H
hydrogen storage system, Phys. Rev. B 74 (2006) 19.
[12] S.D. Beattie, H.W. Langmi, G.S. McGrady, In situ thermal desorption of H2 from
LiNH2-2LiH monitored by environmental SEM, Int. J. Hydrogen Energy 34
(2009) 376e379.
[13] K. Luo, Y.F. Liu, F.H. Wang, et al., Hydrogen storage in a Li-Al-N ternary system,
Int. J. Hydrogen Energy 34 (2009) 8101e8107.
[14] Z.T. Xiong, J.J. Hu, G.T. Wu, et al., Thermodynamic and kinetic investigations of
the hydrogen storage in the Li-Mg-N-H system, J. Alloy Compd. 398 (2005)
235e239.
[15] Y. Zhang, Z.T. Xiong, Cao Hujun, et al., The enhanced hydrogen storage per-
formance of (Mg-B-N-H)-doped Mg(NH2)-2LiH system, Int. J. Hydrogen En-
ergy 39 (2014) 1710e1718.
[16] M.Y. Yan, F. Sun, X.P. Liu, J.H. Ye, Effects of compaction pressure and graphite
content on hydrogen storage properties of Mg(NH2)(2)-2LiH hydride, Int. J.
Hydrogen Energy 39 (2014) 19656e19661.
[17] I.P. Jain, C. Lal, A. Jain, Hydrogen storage in Mg: a most promising material, Int.
J. Hydrogen Energy 35 (2010) 5133e5144.
[18] Y. Zhang, Q.-F. Tian, S.-S. Liu, L.-X. Sun, The destabilization mechanism and de/
re-hydrogenation kinetics of MgH2-LiAlH4 hydrogen storage system, J. Power
Sources 185 (2008) 1514e1518.
by 45.5 kJ/mol from that of 4MgH2eLiAlH4. The activation energy
for rehydrogenation was also reduced by 49.9 kJ/mol due to the
addition of TiH2.
The first step of dehydrogenation for 4MgH2eLiAlH4eTiH2
composite can be attributed to the decomposition of LiAlH4, while
the second step was constituted of the decomposition of Li3AlH6,
the interactions between MgH2 with LiH or Al, and the self-
decomposition of MgH2 as well. The produced LiH and Al modi-
fied the dehydrogenation pathway of MgH2.
Acknowledgments
Foundation item: These projects (51571112, 51471087,
11204031 and 11472080) were supported by the National Natural
Science Foundation of China; the Natural Science Foundation of
Jiangsu Province of China (BK20151405); Jiangsu Key Laboratory for
Advanced Metallic Materials (BM2007204); The Six Talent Peaks
Project in Jiangsu Province (2015-XNY-002); The Fundamental
Research Funds for the Central Universities.
[19] R. Chen, X. Wang, L. Xu, L. Chen, S. Li, C. Chen, An investigation on the reaction
mechanism of LiAlH4-MgH2 hydrogen storage system, Mater Chem. Phys. 124
(2010) 83e87.
[20] N.S. Mustafa, M. Ismail, Enhanced hydrogen storage properties of 4MgH2-
LiAlH4 composite system by doping with Fe2O3 nanopowder, Int. J. Hydrogen
Energy 39 (2014) 7834e7841.
Appendix A. Supplementary data
[21] J.F. Mao, Z.P. Guo, X.B. Yu, M. Ismail, H. Liu, Enhanced hydrogen storage
performance of LiAlH4-MgH2-TiF3 composite, Int. J. Hydrogen Energy 36
(2011) 5369e5374.
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2016.03.194.
[22] H. Shao, M. Felderhoff, F. Schüth, Hydrogen storage properties of nano-
structured MgH2/TiH2 composite prepared by ball milling under high
hydrogen pressure, Int. J. Hydrogen Energy 36 (2011) 10828e10833.
[23] S.S. Liu, Z.B. Li, C.L. Jiao, et al., Improved reversible hydrogen storage of LiAlH4
by nano-sized TiH2, Int. J. Hydrogen Energy 38 (2013) 2770e2777.
[24] J.R. Ares, K.F. Aguey-Zinsou, M. Porcu, et al., Thermal and mechanically acti-
vated decomposition of LiAlH4, Mater. Res. Bull. 43 (2008) 1263e1275.
[25] Andrew W. Vittetoe, Michael U. Niemann, Sesha S. Srinivasan, et al., Desta-
bilization of LiAlH4 by nanocrystalline MgH2, Int. J. Hydrogen Energy 34
(2009) 2333e2339.
[26] S.S. Liu, L.X. Sun, Y. Zhang, et al., Effect of ball milling time on the hydrogen
storage properties of TiF3-doped LiAlH4, Int. J. Hydrogen Energy 34 (2009)
8079e8085.
[27] H.E. Kissinger, Anal. Chem. 29 (1957) 1702e1706.
[28] E.Y. Davydov, Kinetics Peculiarities of Solid Phase Reactions, John Wiley &
Sons, Ltd, New York, 1998.
References
[1] F.S. Feng, Progress metal hydrogen storage materials, Chem. Propellants
Polym. Mater. 8 (2010) 15e18.
[2] N. Hanada, T. Ichikawa, H. Fujii, Catalytic effect of nanoparticle 3d-transition
metals on hydrogen storage properties in magnesium hydride MgH2 prepared
by mechanical milling, J. Phys. Chem. B 109 (2005) 7188e7194.
[3] B. Bogdanovic, K. Bohmhammel, B. Christ, et al., Thermodynamic investigation
of the magnesium-hydrogen system, J. Alloy Compd. 282 (1999) 84e92.
[4] M. Resan, M.D. Hampton, J.K. Lomness, D.K. Slattery, D.K. Slattery, Effects of
various catalysts on hydrogen release and uptake characteristics of LiAlH4, Int.
J. Hydrogen Energy 30 (2005) 1413e1416.
[5] R.A. Varin, L. Zbroniec, T. Czujko, Z.S. Wronski, The effects of nanonickel ad-
ditive on the decomposition of complex metal hydride LiAlH4 (lithium ala-
nate), Int. J. Hydrogen Energy 36 (2011) 1167e1176.
[6] M. Ismail, Y. Zhao, X.B. Yu, I.P. Nevirkovets, S.X. Dou, Significantly improved
dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder, Int. J. Hydrogen
Energy 36 (2011) 8327e8334.
[7] A. Züttel, P. Wenger, S. Rentsch, P. Sudan, et al., LiBH4 a new hydrogen storage
material, J. Power Sources 118 (2003) 1e7.
[8] A. Züttel, S. Rentsch, P. Fischer, Wenger, et al., Hydrogen storage properties of
LiBH4, J. Alloy Compd. 356 (2003) 515e520.
[9] Y. Kojima, Y. Kawai, M. Kimbara, H. Nakanishi, S. Matsumoto, Hydrogen
generation by hydrolysis reaction of lithium borohydride, Int. J. Hydrogen
Energy 29 (2004) 1213e1217.
[29] Y. Zhang, Q.F. Tian, J. Zhang, S.S. Liu, L.X. Sun, The dehydrogenation reactions
and kinetics of 2LiBH4-Al composite, J. Phys. Chem.
18424e18430.
C 113 (2009)
[30] M.J. Avrami, Chem. Phys. 7 (1939) 1103e1112.
[31] M.H. Mintz, Y. Zeiri, Hydriding kinetics of powders, J. Power Sources 216
(1994) 159e175.
[32] J. Huot, G. Liang, S. Boily, et al., Structural study and hydrogen sorption ki-
netics of ball-milled magnesium hydride, J. Power Sources 216 (1994)
159e175, 293e295 (1999) 495e500.
[33] N. Bazzanella, R. Checchetto, A. Miotello, Catalytic effect on hydrogen
desorption in Nb-doped microcrystalline MgH2, Appl. Phys. Lett. 85 (2004)
5212e5214.
[10] C. Li, P. Peng, D.W. Zhou, L. Wan, Research progress in LiBH4 for hydrogen