Ionic liquids as an efficient medium for the mechanochemical synthesis of α-AlH3 nano-composites

Article abstract of DOI:10.1039/c8ta00533h

Aluminum hydride (AlH₃) is one of the most promising hydrogen storage materials that has a high theoretical hydrogen storage capacity (10.08 wt%) and relatively low dehydriding temperature (100-200 °C). In this work, we present a cost-effective route to synthesize the α-AlH₃ nano-composite by using cheap metal hydrides and aluminum chloride as starting reagents and to achieve liquid state reactive milling. The LiH/AlCl₃ and MgH₂/AlCl₃ reaction systems were systemically explored. The phase identification of the obtained products was carried out by XRD and the morphology observed by TEM characterization. It was found that the α-AlH₃ nano-composite can be successfully synthesized by reactive milling of commercial AlCl₃ and LiH in a neutral ionic liquid ([2-Eim] OAc). Based on XRD analysis and TEM observation, an average grain size of 56 nm can be obtained by the proposed mechanochemical process. By setting the isothermal dehydrogenation temperature between 80 and 160 °C, the as-synthesized α-AlH₃ nano-composite exhibits an advantage in hydrogen desorption capacity and has fast dehydriding kinetics. The hydrogen desorption content of 9.93 wt% was achieved at 160 °C, which indicates the potential utilization of the prepared nanocomposite in hydrogen storage applications.

Full text of DOI:10.1039/c8ta00533h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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J oP ul er na as el od f oM na ot et rai ad lj su Cs th me ma ri gs ti nr ys A
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ARTICLE
Ionic liquids as an efficient media assisted mechanochemical synthesis of αꢀ
AlH nanoꢀcomposite
3
a
b
c
Received 00th January 20xx,
Accepted 00th January 20xx
C. W. Duan *, L. X. Hu and J. L. Ma
Aluminum hydride (AlH
storage capacity (10.08 wt. %) and relatively low dehydriding temperature (100-200 °C). In this work, we present a cost-
effective route to synthesize α-AlH nano-composite by using cheaper metal hydrides and aluminum chloride as starting
reagents and to achieve liquid state reactive milling. The LiH/AlCl and MgH /AlCl reaction systems were systemically
explored. The phase identification of the obtained products was carried out by XRD and the morphology observed by TEM
characterization. It was found that the α-AlH nano-composite can be successfully synthesized by reactive milling of
commercial AlCl and LiH in a neutral ionic liquid ([2-Eim] OAc). Based on XRD analysis and TEM observation, an average
grain size of 56 nm can be obtained by the proposed mechanochemical process. By setting the isothermal dehydrogenation
temperature between 80 and 160°C, the as-synthesized α-AlH nano-composite exhibits an advantage in hydrogen
desorption capacity and has fast dehydriding kinetics. The hydrogen desorption content of 9.93 wt. % has been achieved at
3
) is one of the most promising hydrogen storage materials that has a high theoretical hydrogen
DOI: 10.1039/x0xx00000x
3
www.rsc.org/
3
2
3
3
3
3
1
60 °C, which indicates the potential utilization of the prepared nanocomposite in hydrogen storage applications.
distance that is between that expected for ionic and covalent
bonding and therefore has become one of the most researched
Introduction
1
2-14
binary metal hydrides.
and temperature of α-AlH
Additionally, the dehydriding capacity
are well above the threshold, set by US
An extensive amount of research has been focused on developing a
viable, safe and efficient technique for hydrogen storage. Most
importantly, various hydrogen storage materials, such as the light
metal hydride and their complexes, have been extensively explored
3
Department of Energy (DOE) 2015. The target hydrogen storage
requirement of 9 wt. % can also be achieved by controlling the
particle size or hybridizing with other metal hydrides, such as LiH
1
, 2
over the recent years. Among these materials, aluminium hydride
AlH ) is one of the most promising candidates due to its high
4
, 5
and MgH
Developing cost-effective and efficient synthesis approaches
for α-AlH remain key issues and limit the practical utilization of this
material. Apart from the structural investigations, Brower et al.
have synthesized the non-solvated AlH through an organometallic
synthesis route in 1967. By adding LiAlH and AlCl into the ether
O at 65 °С for 6.5 h under
2
.
(
3
theoretical hydrogen storage capacity (10.08 wt. %) and low
3
-8
3
decomposition temperature (100-200 °С). Compared to other
metal hydride or hydrogen storage materials, AlH3 is an ideal
candidate for a wide array of applications, such as solid rocket
3
1
7
9
-11
4
3
propellant and hydrogen-fuel cells.
Hence, several research
.
3 2
and heating the resulting AlH nEt
groups have investigated the synthesis methods and hydrogen-
4
, 5, 12-16
vacuum, the α-AlH
3
can be preferentially formed according to the
storage performance of AlH polymorphs.
3
Eqn. (1).
In 1969, Turley and Rinn, firstly, confirmed the crystal structure
.
3
LiAlH4(sln)+AlCl3 (sln)+4nEt
2
O→4AlH
3
nEt
2
O
(sln)+3LiCl↓
(1)
of α-AlH phase, which crystallizes in a trigonal space group with a
3
1
2
It was later discovered by Bulychev et al. that the α-AlH
3
could
hexagonal unit cell. Afterwards, other structural variations of AlH3
be directly synthesized by use of Lewis and a Bronsted acid as
reagents and crystallizing from diethyl ether-benzene solutions, the
polymorphs (α′, β, γ, δ, ε, ζ, orthorhombic Cmcm and cubic F 3 dm)
were reported by various research groups.
1
3-16
Among these
18, 19
reactions are expressed in Eqns. (2) and (3).
However, the
polymorphs, the α-AlH3 is recognized as the most stable and,
consequently, the most investigated phase. The α-AlH is stable at
room temperature, whereas thermodynamically at
abovementioned method for α-AlH synthesis is highly sensitive to
3
3
the temperature and time and not suitable for large-scale AlH
production.
3
a higher
temperature (150 °C) with an equilibrium decomposition pressure
1
4
6MAlH4(s)+7Al
2
Br6 (sln)→8AlH
3
↓+6MAl
2
Br
7
(2)
(3)
of 10 kbar. Recently, the α-polymorph, has an interatomic Al-H
2
LiAlH +H
4
2
SO
4
→2AlH
3
+Li SO +H
2
4
2
It has been reported that the mechanochemical synthesis
process is under the umbrella of green chemistry and is a
sustainable method to synthesize metal hydrides due to favorable
solid-state reactions during ball milling. For instance, Kim et al. have
4 2
reported that the nanocrystalline Mg(AlH ) (~ 18 nm) can be
synthesized by the mechanochemically activated reaction between
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2
0
NaAlH
reported by Møller et al. that the NaBH
formed by mechanochemical treatment of NaAlH
4
and MgCl
2
without the use of any solvents. It has been suitable for synthesis of α-AlH
and Ca(AlH
and Ca(BH
mixtures. Birke et al. have suggested that the mechanochemical heterogeneity of the solid state reaction and stabilize the
method offers distinct economic benefits, such as a low energy of synthesized α-AlH during the mechanochemical process, the [2-
3
because the protonic acid is
4
4
)
2
can be eliminated and the mechanochemical reaction can be performed at
a lower temperature under neutral conditions. To overcome the
4
4 2
)
2
1
3
2
2
consumption and utilization of cost-effective reagents. Moreover, Eim] OAc ionic liquid has been induced as the efficient neutral
it has been demonstrated by Tigineh et al. that the media.
mechanochemical process is solvent-free and consumes less
3
Enormous interest in α-AlH has urged us to explore green and
energy, which makes them greener and sustainable than the straightforward mechanochemical route. Herein, we propose a
23
3
conventional solution-based processes. Furthermore, Park et al. sustainable and green synthesis process for α-AlH by employing
have demonstrated that the dry-nature of the mechanochemical liquid state reaction milling process and using cheap metal hydrides
method makes it compatible with the current push to green- and aluminum chloride as reagents. In the present investigation, the
chemistry alternatives by avoiding the use of expensive and LiH/AlCl
3
and MgH
hazardous organic solvents during the synthesis process. Thus, separately. The [2-Eim] OAc (2-ethyl imidazolium acetate) is used as
this method is considered as a powerful tool to employ solid-state an efficient neutral media ionic liquid for α-AlH synthesis.
reactions for metal hydrides synthesis. Based on the drawbacks of The proposed synthesis approach is more sustainable,
above-mentioned wet chemical techniques, Brinks et al. have convenient and eco-friendly than the previously reported methods.
2 3
/AlCl are considered as the reaction system,
24
3
1
4
employed a mechanochemical method to synthesize AlH
been demonstrated that the AlH /LiCl composite can be synthesized materials, such as LiH and AlCl
by a mechanically assisted reaction, between LiAlH and AlCl at energy consumption indicate the distinct advantages of the applied
room temperature and 77 K. The reaction can be described by synthesis process. For instance, the LiH utilized in this study is
Eqn. (4). cheaper than LiAlH . As a conventional raw materials, LiH was
(4) always be used for producing LiAlH . In contrast, the conventional
can method for synthesizing AlH is conducted in ether solvent using
, as a reagent, and FeF LiAlH and AlCl as reagents and the whole process is not
as a catalyst. It has been demonstrated by Paskevicius et al. that sustainable and green. In addition, the MgH was prepared by
the AlH can be formed via a mechanochemical reaction of LiAlH milling cheap Mg powder under lower hydrogen pressure, which is
and AlCl at lower or room temperature, which results in several a green process with high energy efficiency and does not require
polymorphs (α-, α’-, and γ -AlH
nm. Nevertheless, this work was solely focused on the carried out by milling AlCl
mechanochemical synthesis of AlH composite rather than aiming any catalyst. Moreover, the utilized [2-Eim] OAc ionic liquid is eco-
for a specific crystal structure. The as-milled AlH , with some friendly and facilitates the stabilization of AlH . The adopted lower
3
.
It has Furthermore, the effective utilization of cost-efficient raw
3
3
, the catalyst-free synthesis and low
4
3
1
4
4
3
LiAlH4(s)+AlCl3(s)→4AlH
Lately, Sartori et al. have discovered that the yield of AlH
be increased to 49.9 mol % by using NaAlH
3
+3LiCl
4
3
3
4
3
,
4
3
2
5
26
2
3
4
3
3
6
3
) with the crystalline size of 15 – 17 hazardous solvents. The synthesis of α-AlH
3
nano-composite was
3
and LiH powders in hydrogen, without
3
3
3
variations in its crystal structures, ultimately leads to the complexity ball-to-powder ratio results in less energy consumption, which
of the extraction. This solid state mechanochemical method has makes this process more sustainable. Furthermore, Paskevicius et
also been adopted for use by other researchers to produce nano- al. have demonstrated the synthesis of several polymorphs of AlH
3
2
7-31
sized α-AlH
evident that mechanochemical method is a valid and well- AlCl
established method for the synthesis of nano-sized α-AlH AlH
3
composite.
Because of its widespread use, it is (α-, α’-, β-, and γ -AlH
3
) via a chemical reaction between LiAlH
4
and
2
6
3
at 77 K and RT. However, in our proposed method, the α-
phase can be directly obtained during ball milling, which
3
3
composite. However, it has been reported that the metallic Al facilitates of the following extraction.
forms during the room temperature ball milling, which can be
As outlined by thermodynamic processes, the Gibbs free
3 2
partially attributed to the heterogeneity of solid-state reaction and energy change for the reactions between LiH and AlCl , MgH and
leads to the decomposition of AlH
has the potential to prevent the AlH
lower temperature also raises technical challenges on large-scale kJ/mol, as shown in Fig. S1, which indicates that these reactions can
production of α-AlH take place at room temperature. However, according to the
3
3
. Though the cryo-milling process AlCl are calculated from the Eqns (5) and (6). The Gibbs free energy
3
from dehydriding, the much changes for the reactions from 273K to 373K are both less than 0
3
.
Ionic liquids (ILs) are considered as promising environment- reaction kinetics, the feasibility of the mechanochemical reaction
friendly solvents in organic synthesis and offer distinct advantages, has to be experimentally demonstrated by ball milling.
such as non-flammability, low volatility, low toxicity and high
3LiH+AlCl
3
→3LiCl+AlH
→2AlH +3MgCl
2
3
(5)
(6)
3
2, 33
thermal and chemical stability.
ILs are also extremely important
3MgH +2AlCl
2
3
3
because they can be used as a solvent in catalyst-free reactions. To
date, series of catalyst-free reactions are employed and completed
in ILs due to the ease of the experimental procedure, low cost and
Experimental
3
4, 35
eco-friendly process.
traditional approaches, we aimed to employ ILs solvent to
synthesize α-AlH . In order to stabilize the obtained α-AlH during
Due to the advantages offered by ILs over
The nanocrystalline MgH
7%), AlCl (Sigma Aldrich, 99%) and [2-Eim] OAc were employed as
starting reagents and solvent to synthesize α-AlH composite. The
nanocrystalline MgH powder was prepared by mechanochemical
reaction between Mg powder and H
2
(self-made, 99%), LiH (Sigma Aldrich,
9
3
3
3
3
the mechanochemical process, the selection of ionic liquid is
critical. Compared with acidic ionic liquids, the neutral ILs are more
2
3
6
2
, as reported previously. The
2
| J. Name., 2012, 00, 1-3
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LiH, AlCl
3
and [2-Eim] OAc were used without any further
kλ
cos θ
D =
(7)
purification (refer to ESI, Fig. S2 and S3). Due to the high sensitivity
β
of the reagents and as-milled products to moisture content, all Where D represents the average grain size, k is a constant with a
manipulations including charging and discharging of vials value of 0.89, λ corresponds to the applied X-ray wavelength (λCu-Kα
throughout experiments were conducted in an Ar-filled gloves box, = 1.54 Å), β is defined as the half-width after subtracting for the
with oxygen and water content of less than 5 and 2 ppm, instrumental broadening and θ corresponds to the diffraction angle
respectively.
For
LiH/AlCl
of the diffraction peak.
a
typical mechanochemical reaction, the reagents,
The solid-state NMR experiments were carried out at 14.1 T
and MgH /AlCl as well as [2-Eim] OAc were weighed and magnet on a Bruker Avance NMR spectrometer, equipped with a
3
2
3
mixed in the various molar ratios(3:1, 4:1, 6:1, 9:1 and 3:2) and magic angle spinning probe head for rotors of 2.5 mm diameter.
transferred to a high-pressure planetary type ball milling machine Due to the air and moisture sensitivity of the materials used in
3
(
QM-SP4), with 500 cm ball milling chamber. The reaction mixture experiments, all the solid samples were packed into a 2.5 mm ZrO2
was sealed into the container under Ar atmosphere in gloves box. rotor in a gloves box under argon atmosphere. To maximize
The argon atmosphere was purged twice, with zero grade H
quantitative accuracy, the direct polarization spectra were spun
before the container was finally pressurized to 5 MPa H pressure. under 20 KHz to identify the resonances and spinning sidebands.
2
,
2
During the mechanochemical reaction, the ball to powder mass The chemical shift scale was referenced externally with potassium
ratio were 20, 40 and 60:1 and high-energy milling was carried out alum to -0.033 ppm as a secondary reference, and recycle delay for
2
7
for various time intervals at 200 rpm. The abovementioned ball to
Al was set to 3 sec. The transmission electron microscopy (TEM)
powder ratio is lower than the study carried out by Sartori et al., analysis was carried out on the JEOL JEM2011 apparatus. For TEM
25,30
where the ball to powder ratio was 200:1.
The lower ball to observations, the powders were loaded onto the carbon support
powder ratio results in less energy consumption, which makes it films, with 200 mesh copper grids. It should be noted that the as-
more sustainable. To keep the average temperature in the vial as milled products, mixed with [2-Eim] OAc, were filtrated and dried in
close to room temperature as possible, the milling sequence was a vacuum oven before dehydriding, TEM and NMR measurements.
altered every twenty minutes, with an intermittent pause of ten
minutes. After completion of the mechanochemical reaction, the
as-milled product was filtered and distilled in a vacuum. The crude Results and discussion
ionic liquid [2-Eim] OAc was washed with diethyl ether and dried in
Selection of mechanochemical reaction reagents
The solid-state synthesis of alane via mechanochemical reaction of
a vacuum oven at 80 °C for further experiments.
To study the dehydriding properties of as-milled samples,
LiH or MgH precursors with AlCl has been demonstrated as an
2
3
dehydriding measurements were carried out on a home-made
special vacuum apparatus. The as-milled powder was loaded into a
stainless steel holder. The evacuated volume inside the apparatus
efficient and green method of producing adduct-free AlH in large
3
5, 26, 31, 37-40
31
quantities.
Hlova et al. have found that the amorphous
AlH can be formed by milling LiH and AlCl at room temperature.
−
2
3
3
can reach up to 1×10 Pa, at the initial stages of isothermal
desorption measurements. In order to investigate the dehydriding
kinetics of composite systemically, the as-milled samples were
powdered at different temperatures of 80, 120, 140 and 160 °C.
Moreover, the successful synthesis of γ-AlH /MgCl nano-composite
3
2
has been demonstrated by solid-state milling of nanocrystalline
MgH and AlCl , which is also a cost-effective method to obtain γ-
2
3
37-40
AlH nano-composite.
However, the formation of α phase has
3
The time required for the full dehydriding reaction was fixed at 6.5
not been demonstrated by above reports mentioned. Based on the
previous investigation, the MgH /AlCl and LiH/AlCl as the reaction
3
×
10 s. The amount of hydrogen decomposed from the composite
2
3
3
was measured and calculated in terms of the vial vacuum change.
Based on the H pressure change, the ideal gas equation, as well as
the stoichiometric weight of AlH calculated by the chemical
reaction, the content of AlH in nanocomposite, could be obtained.
The dehydriding curves of the AlH nano-composite were obtained.
systems are employed and introduced into ILs to synthesize α-AlH3.
2
Fig. 1 shows the XRD patterns of the MgH and AlCl , with a ratio of
2
3
3
3
:2, milled in [2-Eim] OAc for different time intervals. As suggested
3
by the XRD patterns, an amorphous-like background between 10°
to 20° can ascribe to the [2-Eim] OAc, which was used as a solvent
3
The differential thermal analysis measurements were performed by
using a TG-DSC METTLER TOLEDO instruments. The samples were
handled and transferred to the instrument in T-zero pans and
in the mechanochemical synthesis of AlH . Only the diffraction
3
peaks, corresponding to Mg(AlH ) and MgCl were found at milling
4
2
2
time = 1 hrs, which result suggests that the following reaction has
occurred initially (Eqn.8). This is mainly due to the higher chemical
◦
heated from 50 to 280 C, by using different heating rates. In order
to prevent the air exposure, the composite was sealed in an
aluminum crucible.
reactivity of nanocrystalline MgH (Fig. S2). It also indicates that the
2
formation of Mg(AlH4)2 requires lower activation energy, as
The X-ray powder diffraction (XRD) analysis of the as-milled
products was performed by using a diffractometer, equipped with a
graphite monochromator and rotating anode tube, operating with
Cu Kα radiation source at 45 kV and 40 mA. The XRD patterns were
collected by using step scanning mode, at a rate of 0.5°/min, in the
compared to the AlH , synthesized in [2-Eim] OAc. Even with
3
prolonged reaction times, the XRD patterns did not indicate the
presence of AlH traces (Fig. 1 b and c). However, the Mg(AlH ) was
3
4 2
still found to be a major phase in the as-milled product. This
phenomenon is in good agreement with the previously published
2
θ range of 10 - 90°. The average grain size of the α-AlH
3
phase (G)
41
report, where Mg(AlH ) was obtained by ball milling MgH and
4
2
2
was obtained by using Scherrer formula according to the following
equation:
AlCl in a molar ratio of 3:2 at low temperature. Moreover, the grain
3
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size of Mg(AlH
4
)
2
3
, obtained after 2.5h milling, reduced to 32 nm. It reaction mixture. In order to obtain AlH by mechanochemical
can be concluded that Mg(AlH
nano-crystallite form. Based on above analysis, it can be concluded present investigation. During the second stage of the reaction
that it is difficult to obtain AlH by mechanochemical reaction of process, 1/3 molar equivalents of AlCl were added to the products
in [2-Eim] OAc. Hence, LiH was used to replace obtained from the 4:1 reaction of LiH and AlCl (LiAlH , LiCl at τBM =
60 min) to reach the desired ratio of 3:1. Fig. 4 demonstrates the
(8) XRD patterns of the LiAlH /LiCl composite with a mixture of AlCl
Fig.2 shows the XRD patterns of fresh samples, prepared by ball milled for 20, 40, 60 and 80 min, and with the same ball to powder
milling a mixture of LiH and AlCl (3:1) in [2-Eim] OAc for different mass ratio as previously mentioned. It can be seen from that the
time intervals. After milling for h, the diffraction peaks diffraction peaks corresponding to the unreacted AlCl , along with
corresponding to the starting reagents were still observed, which the previously present LiCl and LiAlH , were observed after 20
indicates that the AlH cannot be synthesized under this condition. minutes of milling. This could be ascribed to the incomplete
When milling time was increased to 2 h, the observed diffraction reaction of LiAlH and AlCl . It is noted that a new set of diffraction
peaks correspond to the starting reagents (LiH and AlCl ) and LiAlH peaks appeared at 28-29° and 48°, which could be unambiguously
Even at a milling time of 2.5 h, the formation of AlH was not assigned to LiAlCl . This result is in good agreement with the
observed (Fig. 2c). However, an alternate crystalline phase, LiAlH previously published literature. Upon further milling for 60 min,
has formed during the ball milling. It can be concluded that the LiH the peaks of LiAlH , LiAlCl and AlCl phase disappeared from the
and AlCl have reacted to form LiAlH , instead of AlH . It is worth XRD patterns and the complete mechanochemical reaction has
noting that molar ratio in these experiments was fixed at 3:1. It has been achieved. Meanwhile, there was a change in the mixtures for
been reported that AlH can be formed by mechanochemical the as-milled samples, reacted between 20 and 60 min, as
reaction of Li precursors (LiAlH and LiH) if the mole ratio of reagent compared to the starting precursors. The intensities of the peaks
Therefore, we aimed to optimize the between 10° and 20° gradually increased, indicating that an
synthesis process by varying the molar ratios, ball to powder ratio, amorphous intermediate has formed. According to the latest
4 2 3
) phase has transformed into a reaction of LiH with AlCl , the same pathway was adopted in our
3
3
MgH
MgH
2
2
with AlCl
3
3
4
.
4
MgH
2
3
+2AlCl →2AlH
3
+MgH
2
+3MgCl
2
→Mg(AlH
4
)
2
+3MgCl
2
4
3
3
1
3
4
3
4
3
3
4
.
3
4
4
3
4
,
4
4
3
3
4
3
3
4
5, 38
is chosen properly.
3
1
the mill shaft rotation speed and milling time to obtain α-AlH
mechanochemical process.
3
by report, it has been demonstrated that LiH/AlCl
undergo an amorphization stage before the final AlH
formation. Thus, with the increased milling time, the crystallization
3
reagents may
3
phase
Determination of mechanochemical reaction conditions
Based on above analysis, when a stoichiometric 3:1 molar mixture and stabilization of the intermediate phase have been achieved
was used to start the reaction, the LiAlH is formed during the ball (Fig. 4). From XRD pattern of the as-milled samples, milled for 60
4
3
milling. The effect of molar ratio (4:1, 6:1 and 9:1) was studied by min, it is obvious that the crystallization of α, γ-AlH was achieved.
changing the relative amount of LiH and AlCl . Fig. 3 displays the This phenomenon also demonstrates that the formation of α and γ
3
XRD patterns of LiH with various ratio of AlCl milled for 1h, with the phases is more favorable than other less stable phases. With
3
3
ball to powder mass ratio of 40:1. When the ratio was fixed at 4:1, prolonged reaction time, it can be concluded that only α-AlH phase
the results were similar to those prepared by milling the LiH/AlCl3 was eventually formed and transformed into a nano-crystallite form.
mixture with a ratio of 3:1, but it appeared that the reaction process The crystallite size, calculated from XRD patterns, demonstrated
3
had been accelerated. Complete reaction of LiH and AlCl with the that the average crystallite size of the α-AlH phase obtained by 80
3
formation of LiAlH4 was achieved on milling the mixture for 1h. min milling could reach 128 nm. From the XRD patterns, it can be
Therefore, the ball-to-powder ratio is regarded as an important observed that no impurity phase was present in the final as-milled
parameter during the mechanochemical process, which can products. This may be attributed to the stabilization of the [2-Eim]
effectively enhance the chemical reactivity between the reagents. OAc, which can not react with a product, such as AlH
The complexity and uniqueness of the mechanochemical reaction, the milling process.
3
or LiCl during
with respect to the precursors, indicates that the above result may
In order to obtain α-AlH
3
composite as the final product in a
/LiCl mixture was milled with
to carry out an in-depth investigation on the ratio of reagents. a ratio of 60:1 for various time intervals. When milled for 20 min,
When initial reactions between LiH and AlCl were performed for the intensity of the peaks, assigned to LiAlH and AlCl3, decreased
not be fully applicable to the LiH-AlCl system, which motivated us short milling time, the as-milled LiAlH
4
3
3
4
different molar ratios (4:1 to 6:1), it can be readily observed that significantly, suggesting that the decrement of reagents and the
the LiAlH , Li AlH and LiCl were major products (Fig. 3). The increment of the mechanochemical reaction extent (Fig. 5). In
4
3
6
presence of LiAlH4 and Li AlH , after 1 h of ball milling, can be addition, it can be seen that no diffraction peaks of LiAlH
4
and AlCl
explained by the following reaction (Eqn 9). As shown in Fig. 3, for were detected in the sample milled for 40 min. It appeared from
the starting mixture of 9:1, only Li AlH , LiCl and unreacted LiH Fig. 5 that the complete reaction of LiAlH and AlCl , with the
, has been achieved in final milling stage. It can
excess of Li hydride source leads to the formation of Li AlH . This be concluded that the ball-to-powder ratio is an important
3
3
6
4
2
3
6
4
3
diffraction peaks were observed. Thus, it can be concluded that the formation of α-AlH
3
3
6
observation is in good agreement with our previous investigation, parameter, which effectively increases the reaction kinetics and
38
where Li AlH was observed as the final product.
decreases the reaction time to form α-AlH
(9) the milling time was increased to 60 min, the final crystallite size of
have reported a transformation pathway for α-AlH reduced to 56 nm. In short, the adopted mechanochemical
synthesizing AlH , where the solid-state mechanochemical reaction pathway can be regarded as a green and simple way to synthesize
3
nano-composite. When
3
6
2
LiH + LiAlH → Li AlH
4 3 6
3
1
Hlova et al.
3
3
can be controlled by step-wise addition of AlCl3 to the initial α-AlH
3
nano-composite by step-wise addition of AlCl
3
into LiH
4
| J. Name., 2012, 00, 1-3
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Journal Name ARTICLE
reaction system. Furthermore, the elimination of hazardous crystalline nature of nanocomposite with (111), (200), (220) and
solvents from the synthesis process is the distinct advantage of this (400) planes of the LiCl. The morphology of α-AlH nanoparticles is
similar to the results of TEM investigations carried out by
3
process.
2
6
Identification of as-milled powders
Paskevicius et al.,
where the product consists of α-AlH
3
nanoparticles embedded in a LiCl by-product.
To confirm the XRD results and the existence of an amorphous
phase in the as-milled product, the sequence of events during the
3
De-hydriding property of α-AlH composite
mechanochemical process (stage II) was also monitored by NMR Fig. 8 shows the dehydriding kinetics of the as-milled α-AlH
3
spectroscopy. The as-milled samples during the mechanochemical composite at various temperatures and time intervals. It is
27
process were characterized by Al DPMAS spectroscopy, which is noteworthy that the dehydriding content is calculated by dividing
shown in Fig. 6. It can be seen that a peak centered around -2 ppm actual hydrogen content by the stoichiometric AlH weight. As seen
was detected in Fig. 6a, which can be ascribed to the presence of from Fig. 8, all the isothermal dehydriding curves present sigmoidal
AlCl . This indicates that AlCl was not entirely consumed within the features with introduction, acceleration and decay periods. The
first 20 min of milling. The dominant spectral band around 100 ppm results indicate that the rate of dehydrogenation accelerated as the
3
3
3
IV
(
Fig. 6a) represents the resulting four-coordinated (Al ) species. The temperature rose to 160 °C. Additionally, from the
VI
Al signal consists of two superimposed central transition powder dehydrogenation curves, it can be conjectured that dehydriding
patterns ascribed to LiAlH and LiAlCl . However, the relative temperature has a remarkable effect on the dehydrogenation
contributions of LiAlH and LiAlCl are difficult to quantify and be reaction of α-AlH /LiCl composite, which can be expressed by Eqn.
4
4
4
4
3
resolved in a DPMAS spectrum due to severe spectral overlap. This (10).
assignment has been elucidated in Hlova’s study, based on the
AlH
3
→2Al+3H
2
↑
(10)
31
3
DPMAS spectra of LiAlH
milling time was increased to 40 min, the NMR spectrum suggested s, it can be seen from the Fig. 8a that the hydrogen desorption
that AlCl , LiAlH and LiAlCl got consumed in the as-milled content of as-milled product reached merely 2.0 wt. %, indicating
4 4
and LiAlCl in neat form. When the When the dehydriding temperature was fixed at 80 °C for 2.84*10
3
4
4
composite. On the contrary, it consists of a prominent peak, the partial dehydriding reactions. Although the dehydriding curves
centered around 4-8 ppm, indicating the formation of six- exhibited an undesirable property, the as-milled product still has an
VI
coordinated Al species in the composite. It is difficult to find any advantage in dehydriding properties, compared to the much lower
crystal structure report or NMR data for these hydrogen- hydrogen content of 1.9 wt. % derived from the as-milled AlH
3
5
,
5
substituted aluminum halides. According to Gupta’s report, it has which entirely decomposed from room temperature to 200 °C.
been suggested that this intermediate might still contain more than Furthermore, the value described above, is higher than the 0.48 wt.
6
, 8, 47
one Al species. Even after milling for 60 min, the DPMAS spectrum % hydrogen content of crude α-AlH
was similar to the one observed for 40 min. This result confirms By increasing the reaction temperature to 120 °C, the hydrogen
that no peaks indicative of LiAlCl or LiAH were identified in the content of α-AlH composite increased rapidly to 7.29 wt. %,
spectrum, which suggests the termination of the mechanochemical suggesting that a large part of AlH decomposed at a higher
3
, measured by Graetz. et al.
4
4
3
3
reaction. However, it can be seen from Fig. 6 that there is a temperature. Furthermore, 9.93 wt. % of hydrogen desorption
VI
substantial change in the peak of six-coordinated Al species. The could be obtained by heating the reaction mixture at 140 °C for the
VI
evolution of the Al peak may be attributed to the ensuing phase same time, which implies that the dehydrogenation rate has a direct
3
conversion of intermediates such as Li-Al-H or Al-H into AlH during relationship with the temperature. Afterwards, the as-milled
extended milling. It has previously been demonstrated by composite was annealed at 160 °C for 860 s and the hydrogen
Humphries et al. that the resonance bands around 5.9 ppm are content reached a maxima up to 9.93 wt. %, which is quite close to
4
4, 45
indicative of a α-AlH
formation of α-AlH , as the final product, after 60 min milling.
The transmission electron microscopy (TEM) is uniquely suited organometallic method, can only release 8.0 wt. % of H
3
phase.
Therefore, this result confirmed the the theoretical hydrogen storage capacity (10.08 wt. %) of pure
AlH . It is worth mentioning that the AlH , prepared by the
in the
to study the phase transition at nano-scale. It has been temperature range of 20-160 °C. Although the freshly synthesized
demonstrated by Nakagawa et al. that the a-AlH can be clearly nanoscale α-AlH has an enhanced dehydrogenation property, it has
identified from the selected area electron diffraction (SAED) been reported by Graetz et al. that the complete decomposition at
3
3
3
4
8
2
4
6
3
3
7
49
pattern. Moreover, it has been reported that AlH
3
is stable under 138°C can be achieved even within 1800 s. Therefore, it can be
Thus, TEM observation and associated SAED concluded that nano-structured α-AlH composite, characterized by
patterns were monitored and these results indicate the presence of much finer grain size, exhibits excellent dehydriding performance.
α-AlH and LiCl phases in as-milled products. It can be seen from Fig. Though, the dehydriding temperature of the α-AlH
that the crystallization of α-AlH was complete at following the nanocomposite is higher than usual and lower dehydriding
end of the milling stage. Furthermore, Fig. 7a shows that the temperature is favorable in hydrogen storage materials. However, it
average grain size of α-AlH was 56 nm after 60 min milling. This has been demonstrated by Graetz et al. that the AlH with a
result is in good agreement with our previous work, where only α- dehydriding temperature of 100 to 200 °C can be considered as an
37, 38
the electron beam.
3
3
3
7
3
3
3
6
, 8
AlH
lattice spacings of 0.323, 0.234 and 0.222 nm correspond to the dehydriding temperature, the AlH
012), (104) and (110) planes of α-AlH , respectively (Fig. 7b). hydrogen storage due to its high theoretical hydrogen storage
Moreover, the corresponding SAED patterns (Fig. 7b) indicate the capacity. The dehydriding temperature can be reduced by a number
3
and LiCl phases were observed in XRD patterns. The typical ideal candidate for hydrogen storage.
Despite the high
3
is an excellent candidate for
(
3
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Journal of Materials Chemistry A
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ARTICLE
Journal Name
of approaches. For instance, the effective doping enhances the composite had lower activation energy and exhibited faster
dehydriding kinetics of AlH and fundamental understanding of the dehydriding kinetics than as-prepared α-AlH with larger grain size.
additives’ effect is required to optimize the AlH performance. It has
been reported that the desorption properties can be improved by
the reactive hydride composites between AlH and other
3
3
3
3
3
9
50, 51
52
Conclusions
compounds, such as MgCl
considered as an effective catalyst for enhancing the desorption
properties of AlH . The significant enhancement in dehydriding
kinetics was observed by adding TiCl in the solution, during the
synthesis. Moreover, it has been reported by Sandrock et al.
that the onset decomposition temperature of AlH can be reduced
to below 100 °C by LiH addition. According to the Sandrock’s
report, 2 mol % of LiH was added to the α-AlH /LiCl nanocomposite.
2
,
MgH
2
and LiBH
4
.
Ti is also
In this paper, a benign and cost-effective route was developed to
synthesize α-AlH (alane) at room temperature by employing liquid
state reactive milling. The proposed mechanochemical method was
proved to be green and convenient synthesis route for the α-AlH
composite. By selecting the appropriate reagents and optimizing
the relative amount and milling parameters, the α-AlH nano-
composite could be synthesized by reactive milling of commercial
AlCl and LiH in [2-Eim] OAc. Most importantly, the reaction
pathway was controlled by step-wise addition of AlCl into the
reaction system and determined as the most critical parameter in
the mechanochemical synthesis of α-AlH /LiCl nano-composite. In
the second stage, when 1/3 molar equivalent of AlCl was added to
the milling product (the product obtained from ball milling of LiH
and AlCl for 60 min), the reactional system undergoes an
amorphization stage before the final product, with the desired
product α-AlH /LiCl being formed. The average crystallite size of α-
AlH , embedded with LiCl obtained by mechanochemical reaction in
2-Eim] OAc for 2 h, reached to 56 nm. In particular, the crude ionic
liquid [2-Eim] OAc can be recycled for the secondary use. In
comparison with the pure α-AlH , the as-milled α-AlH nano-
3
3
3
5
3
AlH
3
3
3
5
4
3
3
It can be observed, from Fig. S4, that when the dehydriding
temperature was fixed at 80 °C, the time to attain complete
3
3
decomposition decreased rapidly. Compared with the α-AlH
3
/LiCl
nano-composite without adding LiH, the AlH in the presence of LiH
3
3
has shown enhanced dehydriding performance, with 9.86 wt. % of
hydrogen content at 80 °C for 3400 s. This observation is consistent
3
5
4
with the Sandrock’s report,
decomposition kinetics of AlH
We suggest that if LiCl can be fully separated from this composite,
the obtained pure AlH doped with LiH will have an excellent
which has shown that the
3
3
can be accelerated by LiH doping.
3
3
3
dehydriding property and can be used as an onboard hydrogen
storage material.
[
In order to get a deep insight into the dehydriding process of
3
3
the α-AlH
the DSC curves. Fig. 9 presents the DSC curves of the as-milled α-
AlH composite at several heating rates. It can be clearly observed
that as-milled α-AlH curves contain only one endothermic peak, at
an elevated temperature of 50 – 280 °C, corresponding to the α-
3
/LiCl nano-composite, further evidence was obtained by
composite exhibited an advantage in hydrogen desorption capacity,
particularly in the de-hydriding kinetics, with desirable hydrogen
desorption amount of 9.93 wt. % within 860 s at 160 °C. Thus, the
proposed mechanochemical method offers a green and sustainable
room-temperature approach for the large-scale production of α-
3
3
51, 55, 56
AlH
Liu et al. have found that the endothermic peak between 80 and
90 °C can be assigned to the dehydriding reaction of the α phase.
3
decomposition and consistent with published literature.
AlH
3
nano-composite. The unsolvated alane can be successfully
applied as a promising material for hydrogen storage material.
1
5
5, 56
For instance, for as-milled α-AlH
ranging from 15 to
temperature reduced from 157.4 °C to 121.6 °C. To further explore
the dehydrogenation property of α-AlH nano-composite, the
dehydriding kinetics of α-AlH /LiCl were investigated by Kissinger’s
method (Eq. 11), particularly in terms of apparent activation energy
).
3
composite with a heating rate
3
°C/min, the corresponding absolute Conflicts of interest
The authors declare that they have no conflict of interest.
3
3
Acknowledgements
(E
a
The present work was supported financially by the Natural
Science Foundation of Hebei Province (Grant E2018502054).
2
p
ln(c/T ) = -(E /RT )+A
(11)
a
p
Where c corresponds to the heating rate, T
p
represents the This work was also supported by the Fundamental Research
absolute temperature, corresponding to the maximum desorption Funds for the Central Universities (Grant 2017MS141).
rate, R refers to the universal gas constant and A is a constant. Fig.
9
b shows the activation energy of the dehydriding reaction based
Notes and references
on parameters obtained from the DSC measurements (Fig. 9a). The
a
activation energy of dehydriding reaction of as-milled α-AlH
KJ/mol, a value much lower than those calculated by Herley et al.
who have reported the activation energy of 157 KJ/mol for α-AlH
3
is 56.8
Department of Environmental Science and Engineering, North China
Electric Power University, Baoding, 071003, China. Tel: +86 0312 7525228,
Eꢀmail: duancw@ncepu.edu.cn.
57
3
b
with an average particle size of 50 μm. This value is also lower than
School of Materials Science and Engineering, Harbin Institute of
5
8
Technology, Harbin, 150001, China. Tel: +86 0451 86418613.
result reported by Cao et al.,
4 3
activation energy of the first dehydrogenation step of Y(AlH )
who found that the apparent
is
2.1 KJ/mol. Gabis et al. have demonstrated that by grain size
refinement, the activation energy for dehydriding reaction of α-
AlH , can be reduced to 104 KJ/mol. Therefore, the as-milled α-AlH
c
School of Information Science and Engineering, Hebei University of
Science and Technology, Shijiazhuang, 050018, China.
5
9
9
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| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C8TA00533H
Journal Name ARTICLE
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DOI: 10.1039/C8TA00533H
Mg(AlH )
4
2
MgCl
2
c)
2.5h
b)
2h
a)
1h
10
20
30
40
50
60
70
80
90
2theta(deg.)
2 3
Fig. 1 XRD patterns of asꢀmilled MgH /AlCl powders milled with BPR of 20:1 at 200 rpm for
different times: a) 1h, b)2h, c)2.5h
AlCl
LiH
LiCl
3
LiAlH
4
c)
b)
a)
2
h
1.5h
1h
10
20
30
40
50
60
70
80
90
2theta(deg.)
Fig. 2 XRD patterns of asꢀmilled LiH/AlCl
3
powders milled with BPR of 20:1 at 200 rpm for
different times: a) 1h, b)2h, c)2.5h
Li AlH LiH
3
6
LiAlH
LiCl
4
c)
b)
a)
10
20
30
40
50
60
70
80
90
2theta(deg.)
3
Fig. 3 The XRD patterns of the asꢀmilled samples with LiH:AlCl starting ratios of a) 4:1, b) 6:1
and c) 9:1 after ballꢀmilling for 1h with a milling speed of 200 rpm and a BPR of 40.
1

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DOI: 10.1039/C8TA00533H
LiAlCl
LiAlH
AlCl
γ−AlH
3
4
3
LiCl
α−AlH
3
4
d)
80min
c)
6
0min
0min
b)
4
a)
20min
10
20
30
40
50
60
70
80
90
2theta(deg.)
3 4
Fig. 4 XRD patterns of samples obtained from the 4:1 reaction of LiH and AlCl (LiAlH , LiCl at
τBM = 60 min) after ballꢀmilling for the various times: a) 20min, b)40min, c)60min and d)80min,
with a milling speed of 200 rpm and BPR of 40.
LiAlCl
AlCl
LiAlH
4
4
3
LiCl
a AlH
3
c)
60min
40min
b)
a)
20min
10
20
30
40
2
50
60
70
80
90
theta(deg.)
3 4
Fig. 5 XRD patterns of samples obtained from the 4:1 reaction of LiH and AlCl (LiAlH , LiCl at
τBM = 60 min) after ballꢀmilling for the various times: a) 20min, b)40min and c)60min, with a
milling speed of 200 rpm and BPR of 60.
α
AlH3
c) 60min
b) 40min
LiAlH4
LiAlCl4
AlCl3
a) 20min
140 120 100
80
δ
60
Al(ppm)
40
20
0
-20
27
2
7
Fig. 6 Solidꢀstate Al NMR spectra of obtained samples after ballꢀmilling for the 20, 40,
0min with a milling speed of 200 rpm and a b:p of 60. Noted that the obtained samples were
6
3
synthesized from the 4:1 reaction of LiH and AlCl milling for 60 min.
2

Journal of Materials Chemistry A
Page 10 of 11
View Article Online
DOI: 10.1039/C8TA00533H
a)
b)
Fig. 7 TEM images as well as electron diffraction patterns of αꢀAlH
0:1 ratios for 60min.
3
composite milled with
6
10
8
6
4
2
80℃
120℃
1
40
℃
1
60
℃
3
3
3
0.0
2.0x10
4.0x10
6.0x10
Desorptiontime(s)
3
Fig. 8 The dehydriding curves of αꢀAlH /LiCl composite at different temperatures.
Hydrogen content is calculated by percentage of the calculated nonꢀsalt portion of the sample.
0.4
a)
b)
157.4°C
3
5
°C/min
°C/min
10°C/min
°C/min
-9.5
0.3
149.2°C
15
K=-6830
-10.0
0.2
Ea=56.8KJ/mol
135.5°C
0.1
121.6°C
-10.5
0.0
-11.0
80
120
160
Temperature(
200
C)
240
280
0.0023
0.0024
1/Tp
0.0025
°
3
Fig. 9 (a) DSC curves of αꢀAlH /LiCl composite in temperature ranges from 50 to
280 °C; (b) The apparent energy for the decomposition obtained from DSC measurements.
3

Page 11 of 11
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA00533H