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Dehydriding properties of g-AlH3

Haizhen Liu a, Xinhua Wang a,*, Zhaohui Dong a, Guozhou Cao a,Yongan Liu a, Lixin Chen a, Mi Yan b,**aKey Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province and Department of

Materials Science and Engineering, Zhejiang University, Hangzhou 310027, Chinab State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

Article history:

Received 7 November 2012

Received in revised form

13 February 2013

Accepted 14 February 2013

Available online 19 March 2013

Keywords:

Hydrogen storage materials

Hydrogen storage properties

Aluminum hydride

Ball milling

Dehydriding

* Corresponding author. Tel./fax: þ86 571 87** Corresponding author.

E-mail addresses: xinhwang@zju.edu.cn0360-3199/$ e see front matter Copyright ªhttp://dx.doi.org/10.1016/j.ijhydene.2013.02.0

a b s t r a c t

AlH3 is a promising hydrogen storage material due to its high hydrogen capacity (10 wt%)

and relatively low dehydriding temperature. In this work, g-AlH3 was prepared by organ-

ometallic synthesis method and the effects of ball milling on dehydriding properties of

g-AlH3 were investigated systematically. Experimental results shows that as-prepared

g-AlH3 releases about 8.3 wt% of hydrogen in the temperature range of 130e160 �C at a

heating rate of 2 �C/min. Ball milling significantly improves the dehydriding behavior of

g-AlH3. DSC-MS analysis reveals that the dehydriding temperature of g-AlH3 ball-milled for

10 h decreases by around 30 �C. In addition, the dehydriding activation energy of g-AlH3

ball-milled for 2 h decreased from 87 to 68 kJ/mol. Isothermal dehydriding measurements

demonstrate that duration needed to release 90% hydrogen for as-prepared g-AlH3 is

280 min, but it takes only 82 min after ball milled for 10 h to release the same amount

of hydrogen. Moreover, the dehydriding path of g-AlH3 is changed by ball milling.

As-prepared g-AlH3 transforms to a-AlH3 before dehydriding, while ball-milled g-AlH3

prefers to dehydride directly without firstly transforming to a-AlH3.

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction dehydrides in the temperature range of 150e200 �C. This

The major challenge for the application of hydrogen-fueled

vehicles is on-board hydrogen storage [1,2]. Aluminum hy-

dride (AlH3) is a promising hydrogen storage material and

receives more and more attention due to its high hydrogen

storage capacity (10 wt%) and relatively low dehydriding

enthalpy [3e16]. AlH3 has been found to have at least seven

polymorphs depending on the synthesis route and a-AlH3 is

the most stable, while b-, g-AlH3 are less stable and would

undergo a transition to a-AlH3 when it is heated [17].

Sandrock et al. [3] demonstrated that a-AlH3 with large

particles (50e100 mm) prepared by DOW Chemical Company

95 2716.

(X. Wang), mse_yanmi@z2013, Hydrogen Energy P95

temperature is still high for AlH3 to be used as an on-board

hydrogen storage material. They also reported that ball mill-

ing can effectively reduce the dehydriding temperature of

a-AlH3. Orimo et al. [4] have also studied the effect of ball

milling on the dehydriding behavior of a-, b- and g-AlH3.

However, their investigation is limited to the dehydriding

thermodynamics of ball-milled AlH3, and more work need to

be done to thoroughly understand the dehydriding thermo-

dynamics, kinetics, mechanisms of ball-milled AlH3.

In this work, we firstly prepared g-AlH3 by wet chemical

synthesis method. Then studies was focused on the effect of

ball milling on the dehydriding thermodynamics, kinetics and

ju.edu.cn (M. Yan).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 8 5 1e1 0 8 5 610852

mechanisms of g-AlH3 by using X-ray diffraction (XRD), tem-

perature programmed desorption (TPD), differential scanning

calorimeter (DSC), mass spectroscopy (MS), and scanning

electron microscope (SEM).

Fig. 1 e XRD patterns of g-AlH3: (a) as-prepared, (bed) ball-

milled for 0.1 h, 2 h and 10 h respectively. The inset shows

an expanded view of the XRD patterns at the 2q range

between 35� and 45�.

2. Experimental

g-AlH3 was prepared using the modified organometallic

methods described by Brower et al. [17]. A 1 mol/L ether

(Sinopharm Group) solution of LiAlH4 (TCI, 98%) (6.84 g of

LiAlH4 and 180 mL of ether) was mixed with a 1 mol/L ether

solution of AlCl3 (Aldrich, 99.99%) (5.334 g of AlCl3 and 40mL of

ether) to produce a mixed ether solution of LiAlH4 and AlCl3with a molar ratio of 4.5:1. The mixed solution was stirred for

2e3min to ensure the reaction between LiAlH4 and AlCl3 went

to completion (reaction (1)):

4.5LiAlH4 þ AlCl3 þ Et2O(l) / 4AlH3$nEt2O þ 3LiClY þ1.5LiAlH4 þ Et2O(l) (1)

Then, the solution was filtered to remove the LiCl precipi-

tate and the ether was removed under vacuum at room tem-

perature. The dry residue was ground with a mortar and

pestle, and heated under vacuum at 60 �C for 5 h. The final

productwaswashedwith ether to remove the excess LiAlH4. It

is worthy to note that all the above procedures were operated

under inert gas.

Freshly prepared g-AlH3 was mechanically milled using a

QM-3SP4 planetary ball mill. Generally, 1 g sample together

with 50 g balls (ten F10 balls and ten F6 balls) were loaded in a

100-mL stainless vial. The rotation speed is 150 rpm and the

milling atmosphere was 0.1 MPa of argon. To prevent the

temperature rising resulting from long-time milling, milling

was paused every 0.1 h for cooling. The amount of hydrogen

released during milling was measured by drainage method.

Non-isothermal and isothermal dehydridings were carried

out on a Sieverts-type apparatus. About 200 mg sample was

loaded into a stainless holder connected to a thermocouple to

detect the sample temperature. The temperature and pres-

sure of the sample were monitored and recorded by a com-

puter. Temperature programmed desorption (TPD) (that is

non-isothermal dehydriding) was performed from room

temperature to about 300 �C with a heating rate of 2 �C/min.

Concerning the isothermal dehydriding kinetics, the samples

were heated quickly to about 97 �C and kept at 97 �C till the

completion of dehydriding.

Differential scanning calorimetry (DSC) and mass spec-

troscopy (MS) analysis of the dehydriding processwere carried

out on a Netzsch STA449F3 synchronous thermal analysis

system equippedwith a Netzsch QMS403Cmass spectrometer

in a flow of high purity argon (50 mL/min). Powder X-ray

diffraction (XRD) measurements were carried out by using a

PANalytical X-ray diffractometer (X’Pert Pro, Cu-Ka, 40 kV,

40 mA). The samples for XRD measurements were mounted

onto a 1-mm-thick glass board in the Ar-filled glove box and

sealed with an amorphous membrane to avoid oxidation

during XRD measurements.

3. Results and discussion

3.1. Structure and dehydriding properties of as-preparedg-AlH3

Fig. 1(a) shows the XRD pattern of the freshly prepared g-AlH3,

which is well identified to be g-AlH3. The diffraction peak at

around 15� is attributed to the amorphous membrane. TPD

curve of the as-prepared g-AlH3 is presented in Fig. 2, from

which it can be seen that g-AlH3 starts to dehydride at 130 �Cand releases about 8.3 wt% of hydrogen when it is heated to

around 160 �C. The hydrogen desorption capacity is somehow

lower than the theoretical hydrogen storage capacity of AlH3

(10 wt%), which is ascribed to the oxidation and the slightly

decomposition during preparation process. The former is due

to that the ether used contained some trace of water

unavoidably, and the latter is attributed to that g-AlH3 is

metastable at ambient temperature and pressure conditions

and will decompose slowly both during preparation process

and under storage. So, to ensure the accuracy of measure-

ment, the g-AlH3 samples used are all freshly prepared.

The activation energy of g-AlH3 dehydrogenation can be

determined by Kissinger’s method [18]. The Kissinger’s

equation (Equation (2)) below demonstrates the relationship

among the activation energy (Ea), the heating rate (c), and the

peak temperature of dehydriding (TP) in the DSC curve:

lnc

T2P

¼ � Ea

RTPþ A (2)

where c is the heating rate in the DSC measurement, TP is the

peak temperature of the related reaction in the DSC curve, Ea is

the activation energy of the reaction. In addition, R is the uni-

versal gas constant and A is a constant. By experiment, the DSC

Fig. 2 e TPD curve of as-prepared g-AlH3. The heating rate

is 2 �C/min.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 8 5 1e1 0 8 5 6 10853

curves of g-AlH3 at various heating rates (in this study, c ¼ 2, 4,

8, 16 �C/min) are measured to obtain the peak temperatures.

Then lnðc=T2PÞ is plotted vs 1/TP and linear fitting is done to

calculate the activation energy from the slop of the fitting line.

In this way, the activation energy of dehydriding reaction for

the as-prepared g-AlH3 is estimated to be 87.9 kJ/mol (Fig. 4(a)

and (d)). The activation energy of dehydriding of g-AlH3 in this

work is close to that reported by Maehlen et al. [7].

3.2. Structure and dehydriding properties of ball-milledg-AlH3

The g-AlH3 was ball-milled for 0.1e10 h and their XRD pat-

terns are shown in Fig. 1(bed). From Fig. 1, one can see that,

before milling, g-AlH3 sample exists in the form of a single

phase while after milling, there are some traces of Al in the

g-AlH3 samples. From the inset in Fig. 1, it is clearly shown

that as themilling time increases, more Al exists in the g-AlH3

sample. This indicates that a longer duration ofmilling results

Fig. 3 e DSC-MS curves of g-AlH3 ball-milled for 0, 0.1, 2,

and 10 h, respectively. The heating rate is 6 �C/min.

in more decomposition of g-AlH3. This agrees with the

amount of hydrogen released during milling. According to the

volumetric method, about0.03, 0.24 and 0.60 wt% of hydrogen

is released when g-AlH3 is milled for 0.1, 2 and 10 h,

respectively.

g-AlH3 was reported by Orimo et al. [4] to transform to

a-AlH3 after ball milled for 1 h at a rotation speed of 400 rpm.

But in this work, no polymorph transition was detected even

though the milling duration was extended to 10 h (Fig. 1(d)).

This may be ascribed to the relatively low rotation speed of

150 rpm used in our experiments. It is believed that a low

rotation speed is insufficient to supplying enough mechanical

energy needed for that polymorph transition.

In order to clarify the dehydriding thermodynamics of ball-

milled g-AlH3, non-isothermal dehydridingmeasurementwas

done using thermal analysis. Fig. 3 shows the DSC-MS curves

of g-AlH3 ball-milled for various durations at a heating rate of

6 �C/min. The DSC curve of as-prepared g-AlH3 contains one

exothermic peak and one endothermic peak. It has been

verified that the exothermic reaction corresponds the poly-

morph transformation from g-AlH3 to a-AlH3, and the endo-

thermic reaction is attributed to the dehydriding of the formed

a-AlH3 [5]. From the DSC peaks and mass spectroscopy of

dehydriding in Fig. 3, it can be found that the dehydriding

temperature of ball-milled g-AlH3 is remarkably reduced. For

instance, the peak dehydriding temperature of g-AlH3 ball-

milled for 10 h reduced from 163.4 �C for as-prepared g-AlH3

to 135.3 �C, and the starting dehydriding temperature

decreased from around 130 �C for as-prepared g-AlH3 to

around 90 �C for g-AlH3 ball milled for 10 h.

It is interesting to find that the endothermic peak in the

DSC curve of the ball-milled g-AlH3 is composed of two

overlapping peaks, and the same phenomena are observed in

the mass spectroscopy related to the hydrogen releasing

reaction of ball-milled g-AlH3. It is reported that as prepared

g-AlH3 is less stable than a-AlH3 and will transform to a-AlH3

before dehydriding [17]. But in this work, we found that the

case changes after g-AlH3 is ball-milled, namely, parts of

g-AlH3 would directly dehydride without firstly transforming

to a-AlH3. So, the low-temperature and high-temperature

peaks may be ascribed respectively to the direct dehydriding

of g-AlH3 and the dehydriding of a-AlH3 which is transformed

from g-AlH3. Besides, Fig. 3 also shows that the area of the

low-temperature endothermic dehydriding peak increases

while the high-temperature one decreases as the milling

duration extends, which indicates thatmore g-AlH3 dehydride

directly without polymorph transition when ball-milled for

longer duration. As can be seen from the DSC curve of g-AlH3

ball-milled for 10 h in Fig. 3, the exothermic peak corre-

sponding to the polymorph transition of g-AlH3 to a-AlH3

almost disappears, which suggests that most of g-AlH3 will

dehydride directly without polymorph transition when ball

milled for 10 h. More kinetics and structure information to

support this finding will be revealed below. All in all, ball

milling plays a vital role in the dehydriding thermodynamic of

g-AlH3.

Fig. 4(d) shows the activation energy of the dehydriding

reaction of g-AlH3 using the Kissinger’s method (Equation (2))

with the parameters obtained from the DSC measurements

(Fig. 3(aec)). As can be seen that the activation energy of

Fig. 4 e DSC curves of ball-milled g-AlH3 at various heating rate ( c [ 2, 4, 8, 16 �C/min): (a) 0 h, (b) 0.1 h, (c) 2 h. (d) Shows the

activation energy Ea using Kissinger’s method with the parameters obtained from the DSC measurements.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 8 5 1e1 0 8 5 610854

dehydriding reaction of g-AlH3 ball-milled for 2 h is

68.1 kJ/mol, which is significantly lower than that of

as-prepared g-AlH3 (87.9 kJ/mol).

The dehydriding kinetics of g-AlH3 was measured using

isothermal dehydriding at 97 �C. Fig. 5(a) shows the dehy-

driding curves of g-AlH3 ball-milled for different durations. All

isothermal dehydriding curves behave sigmoidal features

with induction, acceleration, and decay periods. Moreover,

ball-milled g-AlH3 exhibits much faster dehydriding kinetics

Fig. 5 e Isothermal decomposition curves of ball milled

g-AlH3 at 97 (±1) �C plotted as (a) fractional decomposition,

a, vs time, t and (b) [Lln(1 L a)]1/3 vs t.

than as-prepared g-AlH3. Time needed for g-AlH3 samples to

release 90% hydrogen during isothermal decomposition at

97 �C (t0.9) is shortened as the milling process is lengthened.

For example, about 280 min is needed for as-prepared g-AlH3

to release 90% hydrogen, but it takes only 80 min for g-AlH3

ball-milled for 10 h to release 90% hydrogen. So, ball milling

obviously improves the dehydriding kinetics of g-AlH3.

Many solid state reactions can be represented by the

following equation:

FðaÞ ¼ kt (3)

where a is the fraction of material reacted in one interval,

t. The designation of F(a) depends on the reactionmechanism.

In studying the kinetics of dehydriding, certain equations

need to be considered. Some of these equations have been

summarized and tabulated in Ref. [19].

Graetz et al. [6] have investigated the dehydriding kinetic

models of as-prepared a-AlH3, b-AlH3, g-AlH3, and found that

their dehydriding reactions are controlled by nucleation and

growthofaluminumphase in twoand threedimensions.As for

g-AlH3, the fit of the experimental data of isothermal dehy-

driding to the kinetic equations for nucleation and growth

models did not come to a good linearity over the whole range.

Graetz et al. supposed that the odd shape of the isothermal

dehydriding curve for g-AlH3 was ascribed to that the dehy-

driding of g-AlH3 may be accelerated by the excess energy

produced by the exothermic transition from g-AlH3 to a-AlH3.

In this work, the fraction reacted for g-AlH3 is plotted as

[�ln(1 � a)]1/3 for the three-dimensional nucleation and

growthmodel and fine linearity is obtained within two stages,

which indicates that the dehydriding kinetics of g-AlH3 is

divided into two stages (Fig. 5(b)). In order to clarify the

dehydriding kinetic mechanisms of the two stages for g-AlH3,

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 8 ( 2 0 1 3 ) 1 0 8 5 1e1 0 8 5 6 10855

XRDmeasurements were carried out for g-AlH3 ball-milled for

0.1 h before dehydriding, after isothermal dehydriding at 97 �Cto the turning point marked in Fig. 5(b), and after complete

isothermal dehydriding. The XRD results are presented in

Fig. 6. Before dehydriding, the sample contains mainly the

g-AlH3 phase, and after heated to the turning point, the

sample consistsmainly of Al and a-AlH3, as well as some trace

of non-reacted g-AlH3. This indicates that at the first stage,

two major reactions occur:

(I) polymorph transition:

g-AlH3 / a-AlH3, (4)

(II) direct dehydriding of g-AlH3 to Al and H2:

g-AlH3 / Al þ H2. (5)

And at the second stage, there is onemajor reaction related

to the dehydriding of a-AlH3 transformed from g-AlH3 through

reaction (4):

a-AlH3 / Al þ H2. (6)

Thedifferencebetweentheslopesof the twostages inFig. 6(b)

is attributed to the different energies needed for Al to nucleate

and grow from g-AlH3 crystal and from a-AlH3 crystal. Since the

crystal structure of g-AlH3 is more open and looser than a-AlH3

[20,21], it is believed that the nucleation and growth of Al from

g-AlH3 crystal is easier due to its lower relaxation energy

required for structure transition. This is supported by the fact

that g-AlH3 is metastable at ambient conditions and would lose

hydrogen to form Al under storage, while a-AlH3 is stable and

could be stored for years without decomposition [22], thus Al

would not nucleate that easy from a-AlH3 crystal structure.

Fig. 6(b) also shows that the longer themillingprocess, the longer

is the first stage over the second stage, which suggests that

longer milling duration will lead to more g-AlH3 to dehydride

Fig. 6 e XRD patterns of g-AlH3 ball milled for 0.1 h: (a)

before isothermal dehydriding, (b) after isothermal

dehydriding at 97 �C for 120 min (namely the turning point

marked in Fig. 5(b) by the arrow), (c) after complete

isothermal dehydriding.

directly without firstly transforming to a-AlH3. This is in agree-

mentwith the resultsof thermodynamicmeasurementsbyDSC-

MS analysis (Fig. 3). As can be seen from the DSC-MS results, the

area of the low-temperature dehydriding peak that is related to

the direct dehydriding of g-AlH3 is increasing with the milling

duration, andsimultaneously the exothermicpeak related to the

polymorph transition from g-AlH3 to a-AlH3 disappears gradu-

ally. So, combining the thermodynamic and kinetic results, we

find that as-prepared g-AlH3 is likely to transform to a-AlH3

before dehydridingwhile ball-milled g-AlH3 prefers to dehydride

directly without firstly transforming to a-AlH3. The change in

dehydridingmechanismofball-milledg-AlH3 is favorable for the

dehydriding of g-AlH3 for that ball-milled g-AlH3 would dehy-

dride directly without firstly transforming to the more stable

a-AlH3. To sum up, ball milling has a great influence on the

dehydriding kinetic andmechanism of g-AlH3.

4. Conclusion

Single phase g-AlH3was prepared by organometallicmethods.

As-prepared g-AlH3 releases about 8.3 wt% of hydrogen in the

temperature of 130e160 �C. The activation energy of dehy-

driding of as-prepared g-AlH3 was estimated to be 87.9 kJ/mol

using Kissinger’s method. Ball milling remarkably effects the

dehydriding properties of g-AlH3. The dehydriding tempera-

ture and activation energy of ball-milled g-AlH3 are both

reduced remarkably. Besides, ball-milled g-AlH3 exhibits

faster kinetic than as-prepared g-AlH3. Combining the dehy-

driding thermodynamic and kinetics results, a new phenom-

ena was found for the dehydriding mechanism of g-AlH3, that

is, as-prepared g-AlH3 mostly transform to a-AlH3 before

dehydriding, but ball milled g-AlH3 prefers to dehydride

directly without firstly transforming to a-AlH3.

Acknowledgment

This work was supported by the National Basic Research

Program of China (973 Program) (NO.2010CB631304), Key Sci-

ence and Technology Innovation Team of Zhejiang Province

(NO. 2010R50013), National Natural Science Foundation of

China (NO.51171168), Zhejiang Provincial Natural Science

Foundation of China (NO. Y4110147).

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