Structure, thermal analysis and dehydriding kinetic properties of Na1-xLixMgH3 hydrides

Article abstract of DOI:10.1016/j.jallcom.2015.11.127

NaMgH₃ hydride with perovskite structure has been synthesized by high-energy ball milling, the maximum hydrogen-desorbed amount of which is 3.42 wt.% at 638 K. Two decomposition steps have been detected for perovskite-type NaMgH₃ hydride, calculated values of activation energy for the two steps are 180.25 ± 8.25 kJ/mol and 156.23 ± 18.54 kJ/mol by Kissinger method. In comparison with NaMgH₃ hydride, Li_0.5Na_0.5MgH₃ hydride has better dehydriding kinetic properties and higher hydrogen-desorbed amount (4.11 wt.%) due to partial replacement of Na by Li. LiMgH₃ hydride with perovskite structure cannot be synthesized by milling of the mixture of LiH and MgH₂ hydrides. However, the maximum hydrogen-desorbed amount of this milled mixture is 5.54 wt.% at 638 K, this may suggest that LiH is a good catalyst for dehydrogenation of MgH₂, but further research is needed.

Full text of DOI:10.1016/j.jallcom.2015.11.127

Journal of Alloys and Compounds 660 (2016) 402e406
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Structure, thermal analysis and dehydriding kinetic properties of
Na1ꢀxLixMgH3 hydrides
Zhong-min Wang*, Jia-jun Li, Song Tao, Jian-qiu Deng, Huaiying Zhou, Qingrong Yao
School of Material Science and Engineering, Guilin University of Electronic Technology, Guangxi, Guilin, 541004, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
NaMgH3 hydride with perovskite structure has been synthesized by high-energy ball milling, the
maximum hydrogen-desorbed amount of which is 3.42 wt.% at 638 K. Two decomposition steps have
been detected for perovskite-type NaMgH3 hydride, calculated values of activation energy for the two
Received 6 October 2015
Received in revised form
17 November 2015
Accepted 19 November 2015
Available online 23 November 2015
steps are 180.25
8.25 kJ/mol and 156.23
18.54 kJ/mol by Kissinger method. In comparison with
NaMgH3 hydride, Li0.5Na0.5MgH3 hydride has better dehydriding kinetic properties and higher hydrogen-
desorbed amount (4.11 wt.%) due to partial replacement of Na by Li. LiMgH3 hydride with perovskite
structure cannot be synthesized by milling of the mixture of LiH and MgH2 hydrides. However, the
maximum hydrogen-desorbed amount of this milled mixture is 5.54 wt.% at 638 K, this may suggest that
LiH is a good catalyst for dehydrogenation of MgH2, but further research is needed.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
NaMgH3
Perovskite structure
Decomposition
Dehydriding kinetic property
Ball milling
1. Introduction
86.6 1.0 kJ/(mol H2) and 132.2 1.3 kJ/(mol H2 K) respectively,
indicating that NaMgH3 is thermodynamically more stable than
Hydrogen has great potential as an efficient fuel and is a good
energy carrier, especially in the terms of energy sustainability and
environmental conservation. However, it's large-scale use for mo-
bile applications is still limited by storage issues. Hydrogen storage
in the solid state is currently considered as the best option for a safe
and long-term solution [1e10]. Recently, Mg-based perovskite-type
MgH2 [18].
It is shown that the thermodynamic properties of NaMgH3 can
be tuned by incorporating new elements or doping catalytic addi-
tives [19,20]. For example, the hydrogen storage properties of
NaMgH3 in situ embedded of Mg2NiH4 and YH3 nanoparticles was
studied by Li's group and they found that the composite material
exhibited greatly enhanced kinetics for hydrogen absorption and
desorption cycling after doping [10]. Moreover the onset temper-
ature of decomposition was successfully decreased with the addi-
tions of Si element doped into NaMgH3 by mechanical milling as
reported by Anna-Lisa Chaudhary and co-workers [20]. Martínez-
Coronado et al. [21] prepared Na1ꢀxLixMgH3 (X ¼ 0, 0.25, 0.5)
ternary hydrides using a high-pressure technique, and found that
as Li is introduced in the system, the unit-cell parameters decrease,
the tilting angle increases and the MgH6 octahedra become more
distorted due to the smaller ionic size of Liþ vs Naþ. The hydrogen
desorption temperature decreases due to the lower structural sta-
bility of the Li-containing perovskites. Xiao et al. [22] also predicted
the same results based on density functional theory calculations.
The replacement of Na by Li would be advantageous for the
hydrogen storage ability of NaMgH3 hydrides [23,24] due to the
following reasons: (i) Li is lighter than Na, so the amount of
hydrogen per hydride mass should be improved and (ii) the sta-
bility of Li perovskite is found to be lower, therefore the hydrogen
hydrides, AMgH3 (A
considerable attention [11e16].
¼
alkali-earth element), have received
NaMgH3 was studied as a potential candidate for on-board ap-
plications because it has high gravimetric and volumetric densities
(6 wt.% and 88 kg/m3) as well as reversible hydriding and dehy-
driding reactions [14]. Also, it was pointed out as a potential ma-
terial for future generation of electronic devices due to the superior
hydrogen mobility of the perovskite structure [11]. NaMgH3 [17]
has an orthorhombic perovskite structure analogous to the
GdFeO3 type (space group Pnma) which is common in low toler-
ance factor oxide perovskites, where the singly charged Na cation
occupies eight-fold coordinated voids. Sheppard et al. measured the
kinetic and thermodynamic data for the decomposition of NaMgH3
into NaH and Mg. The desorption enthalpy and entropy are
* Corresponding author.
E-mail address: ghab1987@163.com (Z.-m. Wang).
http://dx.doi.org/10.1016/j.jallcom.2015.11.127
0925-8388/© 2015 Elsevier B.V. All rights reserved.

Z.-m. Wang et al. / Journal of Alloys and Compounds 660 (2016) 402e406
403
desorption proceeds at lower temperatures. In this work, high-
phases. This results indicate that the crystallinity of NaMgH3
sample can be improved by heat-treatment at 623 K for 5 h (H2,
3 MPa). There are MgH2 and NaH phases after heat treatment. This
is why we think maybe heat treatment can make MgH2 and NaH
phases which are unreactive and have a broken crystal structure to
form new crystals. The existence of MgO phase can be attributed to
the slight oxidization of samples in the handling process [27].
Fig. 2 shows the isothermal dehydriding properties of as-milled
NaMgH3 samples at 638 K, and the hydrogen-desorption pressure is
10ꢀ2 bar. As illustrated in Fig. 2, dehydriding kinetics of as-milled
NaMgH3 hydride without heat-treatment is superior to that after
heat-treatment, the maximum hydrogen-desorbed amount of the
former is 3.42 wt.%, which will release above 3 wt.% of the amount
within 40 min. However, the dehydriding process of the latter is
quite slow with hydrogen-desorbed amount less than 3 wt.% after
170 min. Possible reasons may be: (1) Partial decomposition of
NaMgH3 perovskite hydride will occur in the heat-treated process,
the products is MgH2 and NaH hydrides, the dehydriding temper-
atures of the two products are higher than that of NaMgH3 hydride
[27e29], so results in the decrease of the total hydrogen-desorbed
amount of as-milled NaMgH3 hydride. (2) As-milled sample pos-
sesses some amorphous structure, more defects and large grain
boundaries which will lead to the improvement of dehydriding
kinetic properties, but such structure feature will greatly be dis-
appeared after heat-treated process. In other word, the heat-
treatment will increase the crystallinity of the sample, but reduce
its dehydriding kinetic properties. The hydrogen-desorbed
amounts of the sample before/after heat-treatment are also listed
in Table 1.
energy ball-milling under H2, is employed to prepared Na1ꢀx
-
LixMgH3 (x ¼ 0, 0.5, 1.0) hydrides, a comprehensive study of the
phase structure before/after heat-treatment, thermal analysis and
hydrogen-desorbed amount and dehydriding kinetic properties
have been investigated.
2. Experimental procedure
Nominal composition Na1ꢀxLixMgH3 (x ¼ 0, 0.5, 1.0) samples,
were prepared from stoichiometric mixtures of MgH2 (>98% purity,
Alfa-Aesar), NaH and LiH (>95% Purity, Sigma-Aldrich). The mixed
powders were milled with stainless steel balls under H2 atmo-
sphere (0.8 MPa) for 45 h at 320 rpm using a YXQM-2l planetary
mill at the ambient temperature. The weight ratio of ball to powder
is 80:1. Partial as-milled samples were selected and heat-treated for
5 h under H2 atmosphere (3 MPa) at 623 K, then cooled to room
temperature for structure analysis and dehydriding test.
X-ray diffraction (XRD) analysis of samples were performed on
Empyrean PIXcel 3D (Cu Ka radiation) diffractometer. The powder
samples were sealed in a special cell, coated with Kapton film to
prevent exposure to air. The evaluation of hydrogen-desorbed
amount and dehydriding kinetic properties of samples were
measured on automatic Sievert-type apparatus (PCTpro2000,
Setaram Co.). Thermostability properties of as-milled samples were
investigated by DSC analysis (DSC, NETZSCH STA 449F3) at different
heating rates (5, 10, 15 K/min) under a continuous argon flow
(20 ml/min) from 298 K to 725 K, and testing samples were sealed
with aluminum metal crucible. All sample handlings were under-
taken in an argon atmosphere glovebox (Mbraun, Labstar, 1 ppm
H2O, 1 ppm O2).
Fig. 3 shows the isothermal dehydriding properties of as-milled
NaMgH3 hydride at different temperature, and the hydrogen-
desorbed amounts are also listed in Table 1. As can been seen, the
hydrogen-desorption rate accelerated with increasing temperature.
The maximum hydrogen-desorbed amount of NaMgH3 sample is
3.42 wt.%, and about 90% of the amount is released within 47 min at
638 K, but the hydrogen-desorbed amount is 2.03 wt.% at 613 K, and
only 1.13 wt.% (after 150 min) at 593 K.
3. Results and discussion
3.1. Synthesis, dehydriding properties and thermal analysis of
NaMgH3 hydrides
The XRD patterns of NaMgH3 samples after dehydriding at
different temperature are shown in Fig. 4. As can been seen, peaks
of NaMgH3 phase are still in existence, and peaks of NaH phase and
Mg phase can also be observed. With the increase of temperature
from 593 K to 638 K, peaks of NaMgH3 phase become weakened,
and peaks of NaH phase and Mg phase become strengthened. The
Fig. 1 displayed the XRD patterns of as-milled NaMgH3 samples
before/after heat-treatment. The peaks of NaMgH3 phase can be
observed in as-milled sample, which is an orthorhombic perovskite
structure, similar to the GdFeO3 type (space group Pnma) [25,26].
More sharp peaks of NaMgH3 phase can be seen in the sample after
heat-treatment, accompanied with the peaks of MgH2 and NaH
Fig. 2. Dehydriding kinetic curves of as-milled NaMgH3sample before/after heat-
Fig. 1. XRD patterns of as-milled NaMgH3 hydrides before/after heat-treatment.
treatment at 638 K.

404
Z.-m. Wang et al. / Journal of Alloys and Compounds 660 (2016) 402e406
Table 1
The hydrogen-desorbed amounts of NaMgH3 sample before/after heat-treatment.
Method
593 K
613 K
638 K
As-milled
Heat-treated
1.13 wt.%
/
2.03 wt.%
/
3.42 wt.%
2.28 wt.%
Fig. 3. Dehydriding kinetic curves of as-milled NaMgH3 hydride at different
temperature.
Fig. 5. DSC curves of as-milled NaMgH3 hydride at different heating rate.
Table 2
Dehydriding temperatures of NaMgH3 hydride at different heating rate.
Rate
Tx1
Tp1
Tx2
Tp2
5 K/min
10 K/min
20 K/min
623.3 K
632.8 K
639.3 K
639.4 K
651.0 K
665.0 K
642.6 K
654.6 K
669.9 K
649.0 K
660.8 K
679.0 K
DE
180.25 8.25 kJ/mol
156.23 18.54 kJ/mol
NaMgH3 / NaH þ Mg þ H2
NaH / Na þ 1/2H2[
(1)
(2)
This explanation agrees well with the previous works [19,20].
Based on the two decomposition steps of NaMgH3 hydride, the
theoretical hydrogen-desorbed amount of the first step is 4 wt.%,
and 2 wt.% for the second step. In this work, the experimental data
is 3.42 wt.% at 638 K, which is about 85.5% of the theoretical
hydrogen-desorbed amount of the first step. The second step was
not carried out due to the limitation of working temperature of PCI
apparatus.
Fig. 4. XRD patterns of as-milled NaMgH3 hydride after dehydriding at different
temperature.
result indicates that the decomposition of NaMgH3 hydride will
occurred in the dehydriding process and the decomposition prod-
ucts are MgH2, NaH and H2. The decomposed H2 will contribute to
the hydrogen-desorbed amount of the sample, but this decompo-
sition is not fully finished at this condition.
Fig. 5 shows the DSC curves of as-milled NaMgH3 hydride at
different heating rates (5, 10, 20 K/min) under a continuous argon
flow from 298 to 750 K. Herein Tx and Tp stand for the start and
peak temperature of dehydrogenation, respectively. We observed
that there are two dehydriding processes, Tx and Tp rise clearly
with increased heating rate and these data are listed in Table 2.
According to results of Figs. 4 and 5, the following dehydrogenation
steps can be deduced:
In order to estimate the activation energy (DE) for dehydroge-
nation of the as-milled NaMgH3 hydride, the Kissinger relation is
frequently used for non-isothermal DSC analysis, and can be
expressed in the form as [30].
ln (Tp2/
y) ¼
D
E/RT þ ln (
D
E/Ru0).
(3)
where R is the gas constant, and
y
and u0 are heating rate and
frequency factor respectively. Fitting curves by Kissinger method
are shown in Fig. 6, which represent a good linear relation between

Z.-m. Wang et al. / Journal of Alloys and Compounds 660 (2016) 402e406
405
diffraction angles as Na atoms are replaced by Li. This is due to the
smaller size of the Li cations.
In addition, tolerance factor (t) can be used to predict structure
stability of ABX3 perovskite-type compound. When H is X, the
equation is expressed as follow:
.
pffiffiffi
t ¼ ðRA þ RHÞ
2ðRB þ RHÞ
(4)
Here RA and RB stand for cationic Ionic radius of A and B respec-
tively, RH is hydrogen anion radius. Electronic distribution of H is
not spherical-scattering type in hydride pattern, RH is not fixed, and
the covalent bond length varied with different cationic ions. The
calculation of tolerance factor (t) is based on the fixed ion radius, so
RH is set as 0.140 nm [26,31]. No accurate tolerance factor range for
perovskite-type hydrides is given at present. However, by analogy
with perovskite-type oxides and halides, Ikeda et al. reported that a
reasonable range of tolerance factor is 0.77 < t < 1.00, and
perovskite-type hydride with stable structure are suggested to be
existent within this range [32]. The calculated tolerance factor (t)
are listed in Table 3. The results illustrate that perovskite-type
NaMgH3 hydride can be stably existing at t ¼ 0.807, wheres t
value of LixNa1ꢀxMgH3(x ¼ 0.5) perovskite structures is 0.767 (near
0.77), indicating that partial replacement of Na with Li will reduce
the tolerance factor, and form distorted perovskite structure.
LiMgH3 perovskite hydride cannot be formed for t ¼ 0.727. In fact,
LiMgH3 ternary hydride has not been successfully synthesized so
far. Vajeeston et al. reported that LiMgH3 is a stable trigonal
structure of LiTaO3 via first-principles prediction [33].
Fig. 6. The relationships between ln (Tp2/v) and (1000/Tp) for NaMgH3 hydride fitted
by Kissinger method.
ln (Tp2/
E1 for the first decomposition step of as-milled NaMgH3 hydride is
180.25 kJ/mol, and 156.23 kJ/mol ( E2) for the second step. Where
y) and (1000/Tp) with a slope of DE/R. It can be found that
D
D
measured NaMgH3 sample was synthesized by a two-step method
(consisting of ball milling and hydriding combustion synthesis).
This work suggests that as-milled NaMgH3 hydride has better
dehydriding property.
The replacement of Na by Li would be advantageous for the
hydrogen storage ability of the parent material [23,24] since (i) Li is
lighter than Na, so the amount of hydrogen per hydride mass
should be improved and (ii) the stability of the Li perovskite is
found to be lower, therefore the hydrogen desorption proceeds at
lower temperatures. Fig. 8 shows dehydriding kinetic curves of
LixNa1ꢀxMgH3 hydrides at 638 K. The maximum hydrogen-
desorbed amount increases with the increase of x, In comparison
with NaMgH3 hydride, Li0.5Na0.5MgH3 hydride has better dehy-
driding kinetic properties, and released about 90% of the maximum
hydrogen-desorbed amount within 28 min, which is possibly due
to the formation of distorted perovskite structure caused by partial
replacement of Na by Li. However, LixNa1ꢀxMgH3(x ¼ 1.0) can be
regard as a ball-milled mixture of LiH and MgH2 hydrides, which
has 5.54 wt. % of the maximum hydrogen-desorbed amount, and
releases about 90% of the maximum hydrogen-desorbed amount
within 23 min at 638 K further indicating that LiH has good cata-
lytic effect for dehydriding properties of MgH2 in this milling
condition.
3.2. Structure and dehydriding properties of LixNa1ꢀxMgH3
hydrides
The XRD patterns of nominal composition LixNa1ꢀxMgH3(x ¼ 0,
0.5, 1.0) samples are shown in Fig. 7. The following points can be
deduced from Fig. 7, (1) In the case of x ¼ 1.0, peaks of unreacted LiH
and MgH2 are detected, no new phase can be formed indicating
that LiMgH3 hydride cannot be synthesized by mechanical ball-
milling of the mixture of LiH and MgH2 hydrides. (2) In the case
of x ¼ 0.5, Peaks of NaMgH3 and MgH2 phases have been observed,
LiH is not observed, probably because the latter is eventually pre-
sent as an amorphous phase after ball-milling [21]. (3) In
Li0.5Na0.5MgH3 sample, peaks of NaMgH3 phase shift to higher
Measured and theoretical hydrogen-desorbed amounts of
nominal composition LixNa1ꢀxMgH3 hydrides are listed in Table 3.
The theoretical value is related to the first decomposition step of
NaMgH3 hydride. The hydrogen-desorbed amount increases ac-
cording to x ¼ 1.0 > x ¼ 0.5 > x ¼ 0, which suggest Li doping is
beneficial for dehydriding properties of LieNaeMgeH system hy-
drides. In fact, based on density functional theory calculation, Xiao's
group [22] found that the dehydrogenation enthalpy of perovskite-
Table 3
Calculated tolerance factor (t) of LixNa1ꢀxMgH3 samples.
Sample
x ¼ 0
x ¼ 0.5
x ¼ 1.0
t
0.807
3.42
4.01
0.767
4.11
4.77
0.727
5.54
5.88
Desorp. H2 (wt. %)
Theoretical H2 (wt. %)
Percentage (%)
Fig. 7. The XRD patterns of nominal composition LixNa1ꢀxMgH3 samples (a) x ¼ 1(b)
x ¼ 0.5(c) x ¼ 0.
85.5
86.2
94.2

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1) NaMgH3 hydride with perovskite structure has been synthe-
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desorbed amount is 3.42 wt. % at 638 K. The crystallinity of as-
milled NaMgH3 will be improved by above-adopted heat-
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obviously.
2) Two decomposition steps have been detected for perovskite-
type NaMgH3 hydride. Based on Kissinger method, calculated
values of activation energy for the two steps are
180.25
8.25 kJ/mol (DE1) and 156.23
18.54 kJ/mol (DE2)
respectively.
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This work was financially supported by the National Natural
Science Foundation of China (No. 51261003, 51471055, 51371061
and 21363005) and Guangxi Experiment Center of Information
Science, Guilin University of Electronic Technology (20130113).
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