A new ternary phase, called LaCuMg8, for solid hydrogen storage. Influence of ball milling and cold rolling.

R. Boidin1,a, S. Couillaud2,b, E. Gaudin2,c, J. Huot1,d, JL. Bobet2,e

1 Physics Department and Institut de Recherche sur l’Hydrogène, Université du Québec à

Trois-Rivières, 3351 Boul. Des Forges, Trois-Rivières, Québec G9A 5H7, Canada 2 CNRS, Université de Bordeaux, ICMCB, 87 avenue du Docteur Albert Schweitzer, 33608 Pessac

Cedex, France

aboidinremi@aol.com, bcouillaud@icmcb-bordeaux.cnrs.fr, cgaudin@icmcb-bordeaux.cnrs.fr, djacques.huot@uqtr.ca, ejean-louis.bobet@u-bordeaux1.fr

Abstract

Recently, our group found a new phase with a composition close to LaCuMg8. This compound

crystallizes in the La2Mg17 structure type. A similar hydrogen sorption behavior was observed for

both La2Mg17 and LaCuMg8 compounds with a two steps mechanism. During the first step,

LaCuMg8 absorbs around 3wt% with a low kinetic; the decomposition into LaH3, MgH2 and Cu2Mg

is also observed. This first step is considered as the activation process. The second step consists in

the reversible desorption/absorption of hydrogen by magnesium and also the reversible

transformation of Mg2Cu into MgCu2. In order to improve the activation process, the influence of

ball milling and cold rolling was tested. LaCuMg8 can not be synthesized by ball milling starting

from elemental powders (LaCu + 8Mg). However, cold rolling on as-cast LaCuMg8 compound

improves the activation. Indeed, more than 3%wt is absorbed after 1 hour which is 20 times faster

than LaCuMg8 as cast. An effect of CuMg2, induced by cold rolling was proposed.

1 - Introduction

In order for hydrogen to be used as a fuel for mobile applications, there must be a method

for its storage that results in a high volumetric and gravimetric density while being safe and

economical. Although there is currently no ideal storage system, metal hydrides have long been

considered to be excellent for hydrogen storage. Some of the advantages of metal hydrides are the

high volumetric density possible (higher than that of liquid hydrogen), the inherent safety of

hydrides, and the ability to deliver high purity hydrogen at a constant pressure. Just as there is no

idea1 storage system, there is no ideal metal hydride. Up to now, magnesium appears as a serious

candidate as it offers the highest gravimetric capacity (i.e. 7.6%). Nevertheless, the poor hydrogen

sorption kinetics and the high operating temperatures limit its application. Therefore, some

researches are devoted to some new binary or ternary system based on magnesium.

Recently, a new phase with a composition close to LaCuMg8 (exactly La10.5Cu9Mg80.5 but

named LaCuMg8 here for simplicity) has been highlighted in our group [1]. The structure has been

determined and it derived from that of the well known La2Mg17. In the early 80’s, La2Mg17 has been

studied for its hydrogen sorption properties [2-5] but receive a supplement of interest after the work

of Dutta et al [6] in 1990. As the previous authors were observing absorption at high temperature,

Dutta et al claimed that the compound can reversibly absorbs and desorbs close to 4wt%H2 at room

temperature. Unfortunately, this exceptional result was not confirmed and the results of

Khrussanova et al [3-4] were then confirmed by other authors [7-9]. Then, interest on La2Mg17 was

focussed on the addition with AB5 compounds by ball milling [10-11] with an improvement of the

kinetics. More recently, this compound receives a lot of attention because of the interesting

electrochemical properties [12-14]. With the addition of 200wt% of Ni (added to improve the

electric conductivity), the measured capacity was more that 1000 mAh/g (but taking into account

only the active materials and not all the mixtures) with is 3 times higher than that of currently used

materials (i.e. MmNi5-xMx). Recently, La2Mg17 was also used to produce hydrogen by electrolysis

with promising results [15].

Solid State Phenomena Vol. 170 (2011) pp 102-108Online available since 2011/Apr/19 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.170.102

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LaCuMg8 have some similarities from both the structural and hydrogen sorption behaviour

with La2Mg17 [1]. The kinetics of the reaction was supposed to be a two step reactions (like for

La2Mg17) but the activation of the samples was not easy. Therefore, in this study, we present the

preliminary results on (i) the synthesis of the material by mechanical alloying in order to obtain a

compound with a high quantity of defects and grains boundaries which could have a positive role on

the hydrogen sorption and (ii) the activation of LaCuMg8 by cold rolling.

2 - Experimental details

Starting materials for the preparation of LaCuMg8 were lanthanum pieces (Stream Chemicals, >

99.9 %), Copper rod (Good fellow, > 99.9 %) and magnesium rod (alpha Aesar, > 99.8 %). The

elemental pieces were weighted in the appropriate amounts and sealed in tantalum crucibles under

argon. The ampoules were placed in a high frequency furnace under argon and heated at about

1100°C and kept at that temperature for 2 min. The ampoules were then annealed under vacuum at

400°C for two weeks.

Cold rolling on the as-cast LaCuMg8 was done using a Durston apparatus with 65mm diameter

rolls. The as-cast sample was inserted between two Stainless Steel (316) plates and rolled in air..

After each roll, the foil was folded in two and rolled again thus giving a 50% thickness reduction on

each roll. The final thickness of the foil was 0.3 mm. The prepared samples were stored in air

before hydrogen activation and sorption measurements.

For the synthesis by ball milling, the starting powder were LaCu synthesized by arc melting and

crushed into powder and Magnesium powder (325 mesh). Stoichiometric proportion of each

element were weighted for a total weight of 3 g and put in a 55 cc crucible, with three stainless steel

balls for a powder to balls weight ratio of 30. Milling was performed on a high-energy shaker mill

(Spex 8000)

The samples were investigated by electron probe microanalyses (EPMA) with La (L, Cu

(K, and Mg (K, as standards. The cold rolled sample was used as synthesized and the ball

milled sample was pelleted and cold pressed. Samples were embedded in a methylmetacrylate

matrix and the surface was polished with different silica and diamond pastes. The surface remained

unetched for the EPMA measurements.

All polycrystalline samples were characterized with X-ray powder diffraction using a Philips

PW 1050 diffractometer with CuKα radiation (λ = 0.15405 nm). These patterns were scanned by

steps of 0.02° (2θ) from 5° to 80° with a constant counting time of 30 s. In order to determine the

structural properties of the compound, the structure has been refined using the Rietveld method. The

diffraction patterns being analyzed by a whole pattern fitting procedure using the FULLPROF

program [16].

Hydrogen sorptions kinetics were investigated with an automatic Sievert-type volumetric

apparatus (HERA, Hydrogen Storage System) in the temperature range from room temperature to

623 K and with a H2 pressure of 3 MPa for absorption and 50 kPa for desorption..

3 – Results and discussion

3 - 1 – Ball milling and cold rolling of “LaCu + 8Mg” and LaCuMg8

Ball milling has been widely used to synthesis many intermetallics. As the compound

prepared by melting was difficult to obtain, it provides the impetus to try mechanical alloying to

produce more rapidly and more simply a homogeneous compound. The X-ray diffraction patterns

obtained after various duration of milling are presented in figure 1. It shows that after the first hour

of milling, the elemental powder are still detected and only a decrease of the crystallites size could

be observed. Nevertheless, after 2 hours of milling, some new peaks appear. They could be indexed

to the La2Mg17 phase (with the lattice parameters : a = 10.34 A and c = 10.23A). The elemental

components can also be detected but the amount and the crystallite size decrease. The same

observation can be done after 5 hours of milling but a few more peaks can also be detected. They

could be attributed to Mg2Cu compound. After 10 hours of milling, only two phases can be detected

Solid State Phenomena Vol. 170 103

: La2Mg17 and Mg2Cu. After 20 hours of milling, the intensity of the peaks related to La2Mg17

decreases as the ones related to Mg2Cu are increasing.

BM 1h

BM 2h

BM 5h

BM 10h

BM 20h

La2Mg17

CuMg2

10 90403020 50 807060

2 (°)

Inte

nsi

ty(u

.a.)

BM 1h

BM 2h

BM 5h

BM 10h

BM 20h

La2Mg17

CuMg2

10 90403020 50 807060

2 (°)

Inte

nsi

ty(u

.a.)

Fig 1 : XRD pattern for the (LaCu + 8Mg) mixture ball milled for 1, 2, 5, 10 and 20 hours.

The calculated amounts (derived from the Rietveld refinement) of La2Mg17 and Mg2Cu in

weight % are respectively 54% and 46% after 10h of milling compare to respectively 39% and 61%

after 20h of milling. Such result is rather unexpected as it would indicate that the chemical

composition of the mixture is changing along milling. It is not the case but as the crystallite size are

rather small (8 and 20 nm for La2Mg17 and Mg2Cu respectively), it should be concluded that a

certain amount of amorphous phase should exist and is not taken into account from the XRD

calculations. It is worth pointing out that milling for longer time does not induce significant change

on the La2Mg17/Mg2Cu ratio.

Both as melted material (mainly LaCuMg8 phase) and ball milled mixture (i.e. La2Mg17 +

Mg2Cu) have been subjected to cold rolling. The chemical analyses (EPMA) of cold rolled samples

are presented in figure 2a and 2b. Figure 2a, shows a small amount of Mg2Cu that was produced by

cold rolling. Moreover, some traces of oxides are also observed as the cold rolling was performed in

air. The morphology of the ball milled and cold rolled sample is slightly different. Two main phases

can be highlighted but because of the very intimate mixture, it was not possible to determine

accurately the chemical composition of each ones. Some traces of oxides can also be observed but

mainly in or close to the porosities.

LaCu0.75Mg8

La-Cu-Mg-O CuMg2

Oxides

Dark grey

phase(« La2Mg17 »)

Light grey phase

(« Mg2Cu) LaCu0.75Mg8

La-Cu-Mg-O CuMg2

Oxides

Dark grey

phase(« La2Mg17 »)

Light grey phase

(« Mg2Cu)

Figure 2 : (a) as melted LaCuMg8 and (b) ball milled « La+Cu+8Mg » cold rolled 20 times

104 Solid Compounds of Transition Elements

Therefore, from a chemical point of view, cold rolling does not induce some drastic change on both

samples but Mg2Cu and some oxides appear (as the cold rolling is done under air) in the case of the

melted sample. We are testing the influence of the formation of Mg2Cu and oxides on the reaction

path.

3 – b –Hydrogen sorption properties

It has been shown that the as melted LaCuMg8 compound absorbs more than 2wt% of hydrogen at

573K under 30 bars in 19 hours [1]. The absorption results in the decomposition into LaH3, MgH2

and MgCu2. The first absorption cycle is very long and it should be considered as an activation

process as the following absorption (after desorption at 330°C under 0.2 bar) are much more rapid

as seen in figure 3. At 300°C, the second absorption is completed in less than 30 minutes. As

expected, the kinetic decreases with temperature. Nevertheless, at 200°C, 3wt% is absorbed after

about 1 hour and the maximum capacity is reached after few hours. When the temperature decreases

to 100°C, the kinetic still decreases but the maximum amount of hydrogen absorbed is lower. This

observation is in good agreement with previous works of Reilly [17] about the conversion reaction

Mg2Cu/MgCu2.

At T > 200°C, the reaction can be written as follow:

LaH3-x + Mg2Cu + 6Mg + 15/2 H2 LaH3 + ½ MgCu2 + 15/2 MgH2

With a theoretical hydrogen capacity of 3.61 wt%

At T< 100°C, the reaction is slightly different as the conversion Mg2Cu/MgCu2 does not

exist any longer and it should be written as follow:

LaH3-x + Mg2Cu + 6Mg + 6 H2 LaH3 + Mg2Cu + 6 MgH2

The theoretical hydrogen capacity is then only 2.90 wt%.

Finally, it is worth pointing out that the absorption is possible even at room temperature and

almost 1.5 wt% is absorbed after 2hours.

0 2000 4000 6000 8000 10000

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

300°C / 1st cycle

(which is considered as an activation)

Room Temperature / 6th cycle

100°C / 5th cycle

200°C / 4th cycle

300°C / 2nd

cycle

We

igh

t %

hyd

rog

en

ab

so

rbe

d

Time (s)

Figure 3 : Absorption kinetics of as-cast LaCuMg8 at various temperatures under 10 bars of

hydrogen

Solid State Phenomena Vol. 170 105

As the sorption properties of as-cast and ball milled LaCuMg8 analogous mixtures are

discussed elsewhere [2-4, 11-12], here we presents only the results for cold-rolled LaCuMg8. As

shown from figure 4, it clearly appears that the absorption for cold-rolled LaCuMg8 is already very

quick for the first cycle. More than 3wt% is absorbed after 1hour and the full absorption is reached

after 3 hours as it takes almost 20 hours for the as melted sample. The following cycles are even

quicker and a full absorption is reached in 15 minutes for the third cycle. This result clearly shows

that cold rolling acts as an activation enhancer and facilitates the absorption of hydrogen as already

showed by Dufour et al [18]. The maximum hydrogen absorbed is slightly lower than 3wt% which

is 20% lower than for the second cycle of the as melted sample. Such decrease could be directly

linked to the formation of oxides occurring during the cold rolling process. As preliminary

conclusion, the formation of Mg2Cu during the cold rolling process could be considered to be the

agent responsible for fast activation as the initial reaction is a decomposition reaction. Nevertheless,

more experiments are underway to verify this hypothesis. The preliminary study of the desorption

highlight a complete desorption within the first 20 minutes. The comparison between the curve of

the as melted sample and the cold rolled sample clearly shows that cold rolling is very efficient for

activation process. For the cold rolled sample, the second cycle and subsequent ones give the same

results meaning that the activation is complete. A complete study of the desorption process is

underway.

0

1

2

3

4

0

1

2

3

4

0

1000 2000 3000 4000

Cycle 1

Cycle 3

Wei

gh

t%

H a

bso

rbed

As melted sample

(3rdcycle)

Cycle 1

Cycle 2

As melted (cycle 1)-1

-2

-3

-4

-1

-2

-3

-4

Wei

gh

t%

H d

esso

rbed

Time (s)

Figure 4 : Absorption at 300°C-10 bars and desorption at 330°C -0.2 bar kinetics under 10 bars

for LaCuMg8 cold rolled sample. The first and 3rd cycle at 300°C are represented and for a

matter of comparison the previous results for as melted sample after 3rd cycle for absorption and

1st cycle for desorption are also presented.

106 Solid Compounds of Transition Elements

4 – Conclusion

This preliminary study allow us to draw the following conclusion:

(i) the synthesis of LaCuMg8 can not be achieved by ball milling process and only an intimate

mixture of nanocrystalline La2Mg17 and Mg2Cu can be obtained

(ii) cold rolling applied to ―La+Cu+8Mg‖ ball milled mixture or to LaCuMg8 melted compound

induced the formation of oxides which in the case of the intermetallic probably induces the

formation of Mg2Cu particles.

(iii)LaCuMg8 melted sample absorbs reversibly almost 3.5 wt% of hydrogen at T>200°C.. At

lower temperature, the capacity drop to less than 3wt% because the conversion reaction 2

Mg2Cu MgCu2 + 3Mg is no longer observed. The first absorption cycle is considered as

an activation step and takes approximately 20 hours.

(iv) cold rolling process allows a full absorption for the melted LaCuMg8 compound in less than

3 hours (6 times faster than the melted one) which confirms the efficiency of cold rolling for

the activation of hydrides. The decrease of the maximum hydrogen absorbed is due to some

oxidation occurring during the cold rolling process.

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108 Solid Compounds of Transition Elements

Solid Compounds of Transition Elements 10.4028/www.scientific.net/SSP.170 A New Ternary Phase, Called LaCuMg8, for Solid Hydrogen Storage. Influence of Ball Milling and

Cold Rolling 10.4028/www.scientific.net/SSP.170.102

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http://dx.doi.org/10.1016/j.jallcom.2005.07.039