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Catalytic effect of ZrCrNi alloy on hydriding properties of MgH2

Shivani Agarwal a,b,*, Annalisa Aurora a, Ankur Jain a,b, I.P. Jain b, Amelia Montone a

a ENEA, C.R. Casaccia, FIM Department, Via Anguillarese 301, 00123 Rome, Italyb Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur, India

a r t i c l e i n f o

Article history:

Received 29 July 2009

Received in revised form

12 September 2009

Accepted 12 September 2009

Available online 4 October 2009

Keywords:

Hydrogen storage

Magnesium hydride

Ball milling

X-ray diffraction

Microstructures

Kinetics

Cycling

Thermodynamics

* Corresponding author. Tel.: þ91 141 270160E-mail address: shivaniphy@gmail.com (

0360-3199/$ – see front matter ª 2009 Profesdoi:10.1016/j.ijhydene.2009.09.034

a b s t r a c t

MgH2 nanocomposites with ZrCrNi alloy obtained by high energy ball-milling were studied

as-milled and after several hydriding-deydriding cycles. The microstructure and

morphology of the samples was characterized by means of X-ray diffraction (XRD) and

scanning electron microscopy (SEM). XRD patterns show that no phase formation between

MgH2 and elements of the alloys takes place during milling and after cycling. Different

morphology of the powders as-milled and after cycling was observed by SEM. Pressure-

composition isotherms of these composites were obtained in the pressure and temperature

range of 0.1–15 bar and 200–300 �C respectively. The maximum reversible storage capacity

was found to be 6.2 wt% at 300 �C. Absorption/desorption kinetics data at pressures of

0.1–5.0 bar and temperatures of 275 �C and 300 �C show that an activation process of about

20 cycles at 300 �C is necessary for stabilization of the kinetics and for achievement of the

full hydrogen capacity. The thermodynamic parameters, i.e. enthalpy of formation and

dissociation calculated using Van’t Hoff plots, were found to be 73.53 kJ mol�1 and

87.63 kJ mol�1 respectively, in agreement with MgH2 data reported in literature.

ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction MmNi5 [8], FeTi1.2 [9], ZrFe1.4Cr0.6 [10], TiO2 [11], Cr2O3 [12],

Among the metals and alloys known for their potential use in

hydrogen storage, Mg-based alloys have attracted much

interest for their high hydrogen capacity and low cost [1].

However, slow reaction kinetics, high dissociation tempera-

ture and a hard activation process are the main drawback of

the Mg-based system. Mg-based materials treated by high

energy ball milling in inert and active media possess better

sorption kinetics and higher hydrogen capacity [2–4].

Hydrogen absorption properties of Mg-based systems can be

improved by different approaches, i.e. by i) adding an alloying

element like Mg17Al12 [5], Mg2Ni [6], ii) formation of composite

materials with different catalysts such as metals, alloy,

intermetallic, oxides and carbon materials like LaNi5 [7],

2; fax: þ91 141 2711049.S. Agarwal).sor T. Nejat Veziroglu. Pu

Nb2O5 [13], carbon nano-tubes [14] and, iii) surface modifica-

tion of Mg [15,16]. Liang et al. [17] first reported the superiority

of MgH2–V composite over mechanically milled and unmilled

MgH2 due to the catalytic behavior of V. Among all the alloys

used to prepare composites with Mg, Zr based AB2 type alloys,

in particular the Zr–Cr–Ni ternary alloy offer great improve-

ment in the sorption behavior due to their high charging/

discharging rate [18–21]. Recently Dehouche et al [22] reported

the hydrogenation properties of MgH2 nanocomposites with

different families of alloys. In the present work, we chose Zr–

Cr–Ni ternary alloy to prepare the composite MgH2-

10wt%ZrCrNi and to explore its hydrogenation properties in

more detail viz. in terms of its structural, morphological,

kinetics and thermodynamic properties.

blished by Elsevier Ltd. All rights reserved.

Fig. 1 – XRD pattern of (a) as-milled (b) Cycled & Desorbed at

300 8C MgH2-10wt%ZrCrNi composite.

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 e n e r g y 3 4 ( 2 0 0 9 ) 9 1 5 7 – 9 1 6 29158

2. Experimental

MgH2 powder (60 mm particle size) was purchased from Gold-

schmidt AG with a purity of 95%, the remaining 5% being

metallic magnesium. ZrCrNi alloy was prepared by melting

the pure elements in an arc furnace under an argon atmo-

sphere. The purity of the initial elements was: Zr (99.8%), Ni

(99.98%), Cr (99.9%). The sample was remelted several times

until good homogeneity was ensured. The ingot was pulver-

ized into granules in air until 20–100 mm of particle size was

reached. 90 wt% MgH2 powder with 10 wt% ZrCrNi was milled

for 5 h in a SPEX 8000 mixer-miller. In order to avoid too high

working temperature due to high energy ball milling, a 5 min

break was set after every 30 min of milling. Ball milling was

carried out in a hardened steel vial equipped with a needle

valve made by Cantil Srl in order to perform the milling under

a pressurized Ar atmosphere (6 bar). Hardened steel balls were

used with a ball-to-powder ratio of 10:1. After ball milling, the

powder was handled in an inert atmosphere using a glove box

filled with Ar.

XRD spectra have been obtained using Cu Ka radiation in

a Philips powder diffractometer with Bragg–Brentano geom-

etry equipped with a graphite monochromator positioned in

the diffracted beam. The microstructure and morphology

were analyzed by a Scanning Electron Microscope (SEM)

(ZEISS EVO MA15) equipped with EDS microanalysis and

backscattered electron detector.

Hydrogen absorption/desorption behavior of the

composite was quantitatively characterized by pressure-

composition-temperature (P–C–T) curves in the pressure and

temperature range of 0.1–15 bar and 200–300 �C with 0.15 g

sample using a volumetric system (Gas Reaction Controller by

Advanced Materials Corporation). Kinetic measurements for

both absorption and desorption have also been performed by

the same system at the pressure of 5 bar and 0.1 bar respec-

tively and in the temperature range from 250 �C to 300 �C.

0.16 g samples were used for hydrogenation measurements.

3. Results and discussion

Fig. 1 shows the XRD pattern of as-milled and 20 times-cycled

MgH2-10wt%ZrCrNi composite. There is no phase formation

between MgH2 and elements of the alloy. It can be seen that

MgH2 and Mg peaks are dominant in comparison with ZrCrNi

in both diffractograms which is due to the fraction ratio of

MgH2 and alloy. The broadening of the X-ray peaks reflects the

reduction in crystallite size. From the pattern of the as-milled

sample [Fig. 1(a)] different phases i.e. g-MgH2, b-MgH2 and

ZrCrNi with a small amount of MgO phase are indicated. A

possible explanation of the abundance of the b and g MgH2

phases could be given on the basis of hydrogenation processes

occurring during milling. Milling-induced formation of g-

MgH2 has been reported by several authors [23–27]; this phase

can be considered a distortion of b-MgH2 since the two

hydrides exhibit the same packing type and coordination

number [28].The appearance of small peaks of MgO is due to

the oxidation of metallic Mg present as an impurity in the

starting material. Fig. 1(b) shows the XRD pattern of desorbed

composite material at 300 �C after 20 hydrogen absorption/

desorption cycles. It consists of clear separate peaks corre-

sponding to Mg, b-MgH2 and ZrCrNi alloy. No other peaks

could be recognized indicating that there is no intermetallic

phase formed, even after 20 hydrogenation cycles. MgH2 was

also found to be present in very small amount (7.5% as

calculated by Rietveld analysis) in the powder due to the fact

that MgH2 could not be converted completely into Mg at

300 �C.

Fig. 2 shows the microstructures of as-milled and cycled

samples using the SEM technique in backscattered mode. EDX

analysis confirmed that bright spots represent ZrCrNi phase

while the dark areas represent MgH2/Mg phase. It is clearly

evident from the figure that the alloy is homogeneously

dispersed in the MgH2/Mg matrix. The morphology of the

composites before and after cycling is illustrated in SEM

images using secondary electron (SE) signal (Fig. 3). The

reduction in particle size is obvious in a comparison of both

micrographs. Moreover cycling increases the agglomeration of

the particles.

Fig. 4 shows the PC isotherm of the composite without any

activation at 3 different temperatures. It can be seen that the

sample desorbed 5.0 wt% of hydrogen at 300 �C, while

absorbing 5.7 wt%, which is much less than expected. At

200 �C and 250 �C neither desorption nor absorption took

Fig. 2 – Microstructures of (a) as-milled (b) Cycled &

Desorbed at 300 8C MgH2-10wt%ZrCrNi composite.Fig. 3 – Morphology of (a) as-milled (b) Cycled & Desorbed at

300 8C MgH2-10wt%ZrCrNi composite.

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 e n e r g y 3 4 ( 2 0 0 9 ) 9 1 5 7 – 9 1 6 2 9159

place. We also performed kinetics measurements at this

condition, but found a very slow rate of absorption and

desorption even at 300 �C. This may be due to the sample not

having been activated for hydrogen uptake. Thus, we per-

formed 5 absorption/desorption cycles to activate the

composite material and the results after 5 cycles are shown in

Fig. 5. The absorption and desorption capacities were found to

be increased i.e. 6.0 wt% at 300 �C. Moreover, the sample

successfully desorbed hydrogen at lower temperature; about

4.0 wt% and 5.0 wt% hydrogen desorbed at 250 �C and 275 �C,

respectively. Another few hydrogenation cycles were per-

formed to get stabilized isotherms and kinetics measure-

ments. After about 20 cycles we found no further change in

the PC isotherms. The fully activated isotherms are shown in

Fig. 6. We observed that 6.2 wt% of hydrogen could be absor-

bed and desorbed at 300 �C, which is in agreement with the

expected value for this composite. If we compare this value

with the capacity of pure MgH2, we find that it is nearly equal

to 90% of total desorbed hydrogen from MgH2 [26]. This means

that the ZrCrNi alloy acts as a catalyst in our case, it didn’t

work as a hydriding phase. Generally, the hydrogen sorption

temperatures for MgH2 is above 300 �C, and it takes many

hours to complete an absorption desorption cycle. However,

in this work these shortcomings have been greatly overcome.

Addition of ZrCrNi alloy can reduce the temperature of

absorption and desorption. This reduction in the desorption

temperature strongly indicates that even such a low quantity

of alloy along with the milling process greatly improves the

hydrogen desorption properties of MgH2. A mechanical ball

milling produces a nanocomposite with particles of various

sizes and a metastable g-MgH2 phase. According to a recent

study by Varin et al [30], the observed desorption peak doublet

is to a great extent associated with the distribution of reduced

particulate sizes and the presence of the g-MgH2 phase, which

might occupy the finest particle fraction. In total 6.0 wt% and

5.8 wt % of hydrogen could reversibly be stored at 275 �C and

250 �C respectively. These results are very good in comparison

to existing literature data [17,18,22,29]. In particular, on

comparison with the recent report of Dehouche et al [22] on

the same system prepared by 20 h milling, we could achieve

0.5 wt% and 1.0 wt% more hydrogen desorption at 250 �C and

300 �C respectively. Therefore, milling for such a long time is

not required to prepare these composites; only 5 h milling is

sufficient for getting much better results.

The change in absorption/desorption kinetics at 300 �C

with the number of cycles is shown in Fig. 7. Although it is

believed that alloy does not require any activation after

Fig. 4 – P–C isotherms of MgH2-10wt%ZrCrNi composite

without any activation.Fig. 6 – Stabilized P–C isotherms of MgH2-10wt%ZrCrNi

composite after 20 hydrogenation cycles.

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 e n e r g y 3 4 ( 2 0 0 9 ) 9 1 5 7 – 9 1 6 29160

prolonged milling in an argon atmosphere, the alloy present in

our composite is highly reactive to the atmosphere condition.

Therefore, we needed an activation process before stabilized

results could be achieved, as can be seen in Fig. 7. In the first

cycle, hydrogen is absorbed and desorbed at a very slow rate

and it took about 45 min to reach its maximum capacity, yet

the capacity was very low compared to the expected value.

After 5 activation cycles, complete absorption process was

achieved in only 30 min with improvement in capacity also.

However, desorption still took the same amount of time as the

first cycle, i.e. 45 min to reach the maximum level. Although

the kinetics were improved compared to the first cycle, the

material took 16 min to desorb 80% of its maximum hydrogen

uptake in the first cycle, while it took only 12 min after 5

cycles. Further improvement continued with more cycles, and

by the 20th cycle we finally achieved consistent results. At this

stage the sample absorbed and desorbed about 90% of its

maximum hydrogen uptake, i.e. 5.5 wt%, in about same time,

Fig. 5 – P–C isotherms of MgH2-10wt%ZrCrNi composite

after 5 hydrogenation cycles.

Fig. 7 – (a) Desorption (P [ 0.1 bar) (b) Absorption (P [ 5 bar)

kinetics for the composite at 300 8C with no. of cycles; To

show clearly the kinetics for 20th & 22nd cycles are shown

for shorter time (Inset).

Fig. 8 – Absorption (P [ 5 bar) & Desorption (P [ 0.1 bar)

curves at different temperatures after 20th cycle.

Fig. 9 – Van’t Hoff plots for Hydrogen Absorption &

Desorption.

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 e n e r g y 3 4 ( 2 0 0 9 ) 9 1 5 7 – 9 1 6 2 9161

i.e. 7 min. While the absorption rate at 275 �C and 250 �C was

almost comparable to that at 300 �C, there was a big difference

in the desorption rate. This is due to the fact that in the case of

absorption, the working pressure was kept at 5 bar, which is

higher than that of the equilibrium pressure of the material,

i.e. less than 3 bar as measured by PCT experiments, so at this

pressure the thermodynamic instead of a kinetic effect is

dominant. On contrary, in desorption, the working pressure

was very close to the equilibrium pressure, so that the kinetic

curves reflect quasi – equilibrium conditions. As a conse-

quence, the driving forces are low. The desorption rate was

slower at 275 �C, and slower still at 250 �C. This can be clearly

seen in Fig. 8. Only half of the hydrogen could be desorbed in

50 min at 250 �C. In addition to the slow kinetics, the desorp-

tion plateau pressure was below 1 bar at 250 �C, therefore

study of PCT as well as kinetics was not considered suitable

for temperatures below 250 �C.

For the analysis of the kinetics at 300 �C before and after

the cycling process, we have taken into consideration both

the Johnson–Mehl–Avrami (JMA) [31] and Contracting Volume

[32] models. The best fits for samples after cycling were

obtained with a JMA function and the reaction order was

determined to be n¼ 1 for absorption and n¼ 2 for desorp-

tion. The former value, in particular, clearly indicates that

precipitation of the hydride occurs with instant nucleation,

with a rate limiting step attributable either to a diffusion-

controlled bi-dimensional growth or interface-controlled

mono-dimensional growth [31]. Concerning the kinetics

before cycling, the complexity of the curves has not allowed

straightforward determination of the model that best fits the

curves. However, among the different possibilities, a diffu-

sion-controlled mechanism of reaction emerges, both for

absorption and desorption even if it was impossible to

determine whether nucleation occurred from the particle

surface or bulk.

The enthalpy (DH) and entropy (DS) of hydride formation/

decomposition have been derived utilizing a Van’t Hoff plot of

ln Peq versus 1/T as shown in Fig. 9, according to the equation:

RTlnPH2¼ DH� TDS

where R is the Universal gas constant and T is the absolute

temperature.

The values for the enthalpy of formation and decomposi-

tion are found to be 73.53 kJ mol�1 and 87.63 kJ mol�1,

respectively, which are in close agreement with the reported

data [33] for MgH2. The higher difference between decompo-

sition and formation enthalpies could be due to the inaccur-

acy of enthalpy calculation based on three points only. The

absorption and desorption plateau pressures for this

composite showed no considerable difference compared to

those of pure MgH2, so the thermodynamic properties of the

Mg–H bond were not changed.

4. Conclusions

The MgH2–10 wt% ZrCrNi nano-composite prepared by

mechanical milling for 5 h under 6 bar Argon pressure exhibits

good hydrogen sorption properties. Hydrogenation properties

in terms of their structural, morphological, kinetics and

thermodynamics aspects were analyzed for the as-milled and

cycled sample. There is no phase formation between MgH2

and elements of ZrCrNi alloys in the as-milled and after

cycling composite. A 20-cycle activation process at 300 �C was

found necessary for stabilization of the kinetics and for

achievement of the full hydrogen capacity. The sorption

kinetics of these nanocomposites is found to be much better

compared to 40 h milled pure MgH2. The sample absorbed and

desorbed about 90% of its maximum hydrogen uptake in only

7 min at 300 �C. Reduction in desorption temperature could

also be achieved; even at lower temperatures, e.g. 250 �C, the

sample was able to absorb and desorb 5.8 wt%. The achieve-

ment of this superior hydrogenation performance was

attributed to the combined effects of the full realization of the

catalytic function of ZrCrNi alloy and the nanostructure of

MgH2.

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 e n e r g y 3 4 ( 2 0 0 9 ) 9 1 5 7 – 9 1 6 29162

Acknowledgement

The authors gratefully acknowledge ICTP, Italy for financial

support under the TRIL programme. One of us (Ankur Jain) is

also thankful to the Department of Science and Technology,

New Delhi, India for financial assistance in the form of

a Young Scientist project under the FAST TRACK scheme.

Financial support from the Italian Ministry for University and

Research under Project No. FISR-TEPSI is gratefully acknowl-

edged. The authors are grateful to Daniele Mirabile Gattia for

Rietveld analysis of the samples.

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