Feasibility study on the controlled hydriding combustion synthesis of Mg–La–Ni ternary hydrogen storage composite

International Journal of Hydrogen Energy 32 (2007) 1875 – 1884 www.elsevier.com/locate/ijhydene

Feasibility study on the controlled hydriding combustion synthesis of Mg–La–Ni ternary hydrogen storage composite Qian Li a,∗ , Xiong-Gang Lu a , Kuo-Chih Chou a,b , Kuang-Di Xu a , Jie-Yu Zhang a , Shuang-Lin Chen a,c a School of Materials Science and Engineering, Shanghai University, No. 275 mailbox, 149 Yanchang Road, Shanghai 200072, PR China b Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, PR China c CompuTherm, LLC, 437 S. Yellowstone Dr., Suite 217, Madison, WI 53719, USA

Received 13 August 2006; accepted 16 August 2006 Available online 10 October 2006

Abstract The feasibility of using the controlled hydriding combustion synthesis (CHCS) under a high magnetic field to prepare the Mg–La–Ni ternary hydrogen storage alloys was studied. Comparison was made between the conventional hydriding combustion synthesis (HCS) and the CHCS. The influence of a high magnetic field on the physicochemical properties (thermodynamic and kinetic characteristics, hydrogen absorption/desorption (A/D) properties, thermal behavior, phase composition and morphology) of the Mg–La–Ni composite was analyzed and the results suggested that a high magnetic field can change the microstructure and phase compositions, decrease the hydriding/dehydriding (H/D) temperature and the particle size of the composite, and increase the H/D rates. Based on this study, the lower-temperature metal hydrides prepared by CHCS seem to be technically feasible for lab-scale and it could represent a possible and attractive alternative to prepare the hydrogen storage materials. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Mg-based hydrogen storage materials; Controlled hydriding combustion synthesis; High magnetic field; Thermodynamic and kinetic properties; Structure and phase transformation

1. Introduction Hydrogen as an energy carrier plays an important role as an alternative to conventional fuels provided that the technical problems of production, storage and transportation of hydrogen can be resolved satisfactorily and the overall cost could be brought down to acceptable limit. Hydrogen storage is a key issue in the practical application of hydrogen energy. Efficient, high performance and low cost technologies for storing hydrogen are urgently needed to encourage hydrogen utilization in the future. Hydrogen can be stored by various kinds of hydrogen storage materials. It is well known that Mg based alloys are promising materials for hydrogen storage carriers due to their high hydrogen capacity and low cost. These alloys, however, reveal relatively high H-desorption temperatures and poor kinetics of H-sorption [1], which make them difficult for practical applications. In order to ameliorate the properties, ∗ Corresponding author. Tel.: +86 21 56338065; fax: +86 21 56338065.

E-mail address: [email protected] (Q. Li).

much effort has been made on several aspects: element substitution [2,3], surface modification [4,5], the addition of catalytic components [6], formation of amorphous or nanocrystalline structure [7,8], synthesis of composite by ball-milling magnesium with other hydrogen storage materials [9,10] and new method to synthesize Mg-based hydrogen storage alloys with superior hydrogenation properties [11,12]. Among these approaches, the preparation process of Mg-based composite is one of the key issues since it has strong influence on the microstructure, morphology and macro properties of the materials. The Mg-based composites have been received much attention recently, such as Mg–Mg2 Ni [13], Mg(La2 Mg17 )–LaNi5 [14] and Mg–FeTi [15]. Although enhanced activity of magnesium with respect to hydrogen has been improved to a certain extent, there are still some problems existing in these systems. Mg2 Ni and La2 Mg17 possess high hydrogen storage capacity, but both hydrogen absorption/desorption (A/D) take place at high temperature (> 573 K) [13]; FeTi will bring its activation problem into the composite system [15].

0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.08.035

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Q. Li et al. / International Journal of Hydrogen Energy 32 (2007) 1875 – 1884

So far worldwide researchers still face the predicament that Mg-based alloys cannot keep their excellent hydrogenation properties at a lower temperature. Recently, Varin et al. [16,17] have applied a novel magneto-ball milling technique in a UniBall-Mill5 to process Mg-based hydrogen storage materials by highly efficient modes based on carefully controlled ball movement by varying both the intensity and the direction of the magnetic field and found that the mode of ball milling can also have a profound effect on the hydrogenation properties of Mgbased hydrogen storage materials. Moreover, the advantage of the novel magneto-ball milling technique is exactly what the phrase means: one can control to a large extent the movement of balls and as such induce desired mode of milling, i.e., shearing or impact with controlled energy throughout the entire milling duration. In all other mills, the movement of balls is uncontrollable and at every instant of milling, it is impossible to get a ratio of shearing to impact and the ratio changes continually and chaotically during the entire milling duration. The high magnetic field technology has been successfully applied to materials science, such as solidification, orientation of structure, phase transformation, heat treatment, convection in material processing and so on [18]. A lot of exploring experimental and theoretical work had been done on the study of magnetics and many important experimental phenomena with great theoretical value was achieved. Its application on hydrogen storage materials had been considered to be promising. However, up to date, only a few high magnetic field processing experiments have been done on hydrogen storage materials, and the mechanisms through which the high magnetic field plays a role on microstructures, phase transformation and hydrogenation properties are still unclear [12]. This paper describes a feasibility study of the preparation process of the Mg–4 mol%LaNi2.5 composite by controlled hydriding combustion synthesis (CHCS) in the magnetic field of several Tesla. Some fundamental knowledge of this attractive

technological approach and some understanding of the effect of a strong magnetic field on the thermodynamic and kinetic properties of this composite will be presented. 2. Experimental details A stoichiometric mixture of La (purity > 99.9%) and Ni (purity > 99.9%) was melted in a Cu hearth of an arc furnace with non-consumable electrode under a high-purity argon atmosphere. To achieve homogeneity, the initially prepared alloy was inverted upside down and remelted several times. They were then well-mixed with Mg powder (purity > 99.9%) in stoichiometric proportion of the Mg–4 mol%LaNi2.5 by an ultrasonic homogenizer in ethanol. After completely dried in air, they were compressed to form wafers with 4 mm in height and 15 mm in diameter by a uniaxial single-acting press at 33.4 MPa. Then, the wafers were crushed to small fragments. Some characteristics of the furnace used in this study are maximum 9.0 MPa of pressure, maximum 1400 K of temperature and 300 mm long uniform temperature region. An autoclave of 32 mm in diameter and 1000 mm in length is used to contain the samples and is set to the middle of the furnace tube made of iron. Before the experiment, the autoclave is evacuated to 3 × 10−4 Pa by an oil diffusion pump. The wafer fragments were put in the heart of the furnace and were synthesized without magnetic field by hydriding combustion synthesis (HCS) for batch 1 and with magnetic field by CHCS for batch 2, respectively. These samples were first heated up to 913 K in 80 min and kept at 913 K for 600 min for batch 1 and 300 min for batch 2. They were then cooled down to 303 K in 90 min. Before heating, the autoclave was filled with 3 MPaH2 . The composite of batch 1 was prepared without magnetic field. The composite of batch 2 was prepared under magnetic field, which was carried out in the experimental apparatus for magnet shown in Fig. 1 schematically.

1 7

11

8 2 3 5

4 9 ~ 10 5

6 (a)

6 (b)

Fig. 1. A schematic diagram for preparing hydrogen storage alloy under the strong magnetic field from the superconductor magnetizer (1-sample frame; 2-water-cool copper set; 3-heat furnace; 4-superconductor magnet; 5-thermocouple; 6-sample; 7-resistance; 8-commutating circuit; 9-temperature automation controller; 10-power supply; 11-valve) (a) Sample reactor (b) Experimental system.

Q. Li et al. / International Journal of Hydrogen Energy 32 (2007) 1875 – 1884

3. Results and discussion 3.1. Structural and morphological characterization of the Mg–4 mol%LaNi2.5 composites prepared by HCS and CHCS Fig. 2 shows the XRD spectra of the raw materials and the Mg–4 mol%LaNi2.5 composites prepared by HCS and CHCS under different conditions. Since there was no standard powder diffraction file corresponding for LaNi2.5 and its hydride, the XRD spectra of LaNi2.5 after H/D were also presented in

Mg

MgH2

LaNi2.5

LaNi2.5Hx

La

(f)

Intensity(a.u.)

(e)

(d) (c)

(b) (a)

10

20

30

40

(I) Mg

MgH2

LaNi2.5

50 2θ

60

LaNi2.5Hx

70

La

80

90

80

90

Ni

(f)

Intensity(a.u.)

The apparatus consists of superconductor static magnet, furnace, stainless steel crucible, and temperature controller. Magnitude of the static magnetic field on the vertical direction could be as high as 14 T. The field has the homogeneity of 0.1% over 10 mm D.S.V. The temperature in the furnace could reach 1273 K and was controlled with the precision of ±1 K. The microstructures of the samples were examined by X-ray diffraction (XRD, model D\MAX-2550) and scanning electron microscopy (SEM, model S250MK3) with an energy dispersive X-ray spectrometer (EDX), respectively. Size measurements were performed using a Malvern Mastersizer hydro 2000S in liquid (ethanol) media. The sample (20 mg) was dispersed in ethanol using an ultrasonic device (80 kHz, 150 W, 20 min). The thermal stability and the structural transformation of the samples were studied by differential scanning calorimetry (DSC) (SDT Q600 V5.0 Build 63). The powder sample (∼ 40 mg) was heated at a rate of 5 K/min from 303 to 673 K and kept at the maximum temperature for 60–80 min with a pure argon flowing at the rate of 100 ml/min. A gas chromatogram (GC9800) was used to measure the dehydriding properties of samples. The amounts of desorbed hydrogen in the samples can be monitored using a hydrogen analyzer in the gas chromatogram. The powder samples weighting 72 mg was put in a quartz tube that was thoroughly purged with high pure argon. When the recorded curve reached the steady state, the samples were heated up from room temperature T0 (298 K) to given temperature T1 (298 K for LaNi2.5 Hx , 673 K for (Mg–4 mol%LaNi2.5 )Hx and 773 K for MgH2 ) at a rate of 5 K/min and maintained at T1 in high pure argon flowing at a rate of 30 ml/min. Meanwhile, the thermal desorption spectrum was recorded. The H/D characteristics of the composite were determined by an isovolumetric method without activation. Measurements of kinetic properties were performed in the temperature range from 523 to 573 K. The overall hydriding kinetics measurement was performed by applying a single allotment of hydrogen (9.5 ml at 4 MPaH2 ) to fully hydride the sample in one step. To study the overall dehydriding kinetics, a fully hydrogenated sample was put into a large container with a much low pressure (15 000 ml at 0.1 PaH2 ) than the plateau pressure. The temperature of the reactor was controlled within ±1 K. The mass of the sample was kept at 1500 mg in order to get good heat and mass transfer. The system volumes were all known. The magnitude of pressure change was monitored at regular time intervals. Then the hydrogen content in the sample versus time could be calculated by recording the change of hydrogen pressure.

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(d)

(b)

(a)

10

(II)

20

30

40

50 2θ

60

70

Fig. 2. (I) XRD spectra of the Mg–4 mol%LaNi2.5 prepared without magnetic field and (II) XRD spectra of the Mg–4 mol%LaNi2.5 prepared with 10 T magnetic field: (a) raw LaNi2.5 ; (b) raw Mg; (c) LaNi2.5 after hydriding; (d) Mg–4 mol%LaNi2.5 after hydriding; (e) LaNi2.5 after dehydriding; (f) Mg–4 mol%LaNi2.5 after dehydriding.

Fig. 2 (see I(c) and I(e)) for comparison. Without the high magnetic field, the fourth pattern (see I(d)) shows that the hydriding composite is mainly composed of MgH2 (JCPDS 74-0934) and LaNi2.5 Hx with trace of Mg (JCPDS 35-0821) and minor La (JCPDS 1-0718). After subsequent dehydriding reaction (see I(f)), the structure changed to that involving Mg + LaNi2.5 Hx + minor La, which indicates that all MgH2 decomposes to Mg completely, but LaNi2.5 Hx only partially changes to LaNi2.5 during the H/D processes and no characteristic peak of Ni is observed in this composition. For the batch 2, its XRD spectra indicate that it includes the MgH2 +Mg+LaNi2.5 Hx +minor La after hydriding (see II(d)). From Fig. 2 (II(f)), it can be seen that the after dehydrogenation batch 2 is made of Mg + La + LaNi2.5 Hx + LaNi2.5 + trace of Ni (JCPDS 88-2326). For the sake of clarity, Table 1 shows the phase composition of these batches after H/D. Comparison between the phase

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Table 1 Phase composition of hydrided and dehydrided Mg–4 mol%LaNi2.5 Samples

Phase composition of hydrided

Phase composition of dehydrided

Batch 1 Batch 2

MgH2 , LaNi2.5 Hx , traces of Mg and La MgH2 , LaNi2.5 Hx , Mg, trace of La

Mg, traces of La and LaNi2.5 Hx Mg, traces of LaNi2.5 , La, Ni and LaNi2.5 Hx

Fig. 3. SEM and EDX observation of the Mg–4 mol%LaNi2.5 prepared under the different conditions: (I) without magnetic field and (II) with 10 T magnetic field.

compositions of the two batch composites shows that the MgH2 phase is the largest contributor to the hydrogen storage capacity of the Mg–4 mol%LaNi2.5 . It can be found that the amount of Mg in hydrided batch 2 is larger than that in hydrided batch 1 and it can lead to the decrease in the hydrogen content of batch 2. La exists in both batches and traces of LaNi2.5 and Ni appear only in dehydrided batch 2. The results above mentioned attributes to the profound effect of the high magnetic field on the ferromagnetic constitutes such as LaNi2.5 and Ni. It is well known that both La and Ni have good catalysis for the H/D processes. Srivastava et al. [19] found that La–Ni secondary phase was effective in improving the hydrogenation characteristics, e.g., higher storage capacity and faster kinetics. The existence of LaNi2.5 in batch 2 is favorable to improve the hydrogenation of the Mg–4 mol%LaNi2.5 composite. Fig. 3 shows the results of SEM and EDX analysis of the surface of the Mg–4 mol%LaNi2.5 . It is seen that the batch 1

appears to consist of a mixture of irregular shaped particles with slits and the batch 2 has the irregular morphology and roughness. In terms of particle size of powder, the batch 1 has the particle size of more than 20 m estimated by SEM micrographs, which is larger than that in the batch 2 (less than 20 m). The results suggest the particle size decreases with the external magnetic field. A quantitative comparison of the surface compositions of the two batches shows that Mg is the predominant composite in both batches, but the fractions of La and Ni on the surface of the batch 1 are smaller than those in the batch 2. Both La and Ni have good catalysis on the H/D reactions. The presence of clusters in batch 2 as seen in Fig. 3 indicates that the agglomeration may cause the inhomogeneous composition in these composites. The particle size distribution of the Mg–4 mol%LaNi2.5 composite as a function of strength of the static magnetic field was

Q. Li et al. / International Journal of Hydrogen Energy 32 (2007) 1875 – 1884

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Fig. 4. Size distributions of the Mg–4 mol%LaNi2.5 composites for different magnetic intensities: (a) 10 T magnetic field and (b) 0 T magnetic field.

3.2. Thermal behavior of the Mg–4 mol%LaNi2.5 composites prepared by HCS and CHCS Hydrogen desorption was investigated by two methods: DSC and GC (a gas chromatogram). Fig. 5 gives the curves of the heat flow vs. time from DSC measurements under pure argon for the Mg–4 mol%LaNi2.5 samples prepared by HCS and CHCS, respectively. Decomposition of the hydrogenated composite started at 306 K with 2 min for both batches and finished at 673 K with 140 min for batch 1 and 121 min for batch 2. The results are illustrated in Fig. 5, which indicates that the desorption kinetics property of batch 2 is better than that of the batch 1. A peak shape of DSC curves was observed and the dehydriding kinetics of the batch 1 is slower than in the case of batch 2. Five endothermic peaks are clearly observed in DSC curve of the batch 1 but only four endothermic peaks exist in desorp-

700 610 K, 61 min

121 min 81 min

Exothermic Heat Flow (a.u.)

600 140 min

74 min 470 K, 34 min

500

T(K)

obtained by means of granulometric measurements and shown in Fig. 4. The median of the distribution changes from 60 m for batch 1 to 30 m for batch 2. The tendency of the size change is consistent with the information provided by SEM techniques. However, their numerical values are inconsistent. This discrepancy might be caused by the different frequencies of the ultrasonic device and the different times used in the process of pretreatment before the SEM experiment and size measurements. The comparison of the median size under the different conditions suggested that the effect of the static magnetic field on the size distribution of the particles was obvious. In other words, the observed results showed that the particle size deceased with the external magnetic field. This is consistent with the experience that the smaller the particle size, the larger the surface area, and the faster the reaction rate. From the discussion above, it can be concluded that the high magnetic field has influence on many characterizations of the hydrogen storage alloys, such as the structure, morphological characterization, phase composition, particle size, and element distribution. The hydrogenation properties of hydrogen storage alloys prepared under the high magnetic field would be changed accordingly and will be discussed in the next section.

400 10T 0T

300 96 min

-20

0

20

40

60 80 t (min)

105 min

100

120

140

160

Fig. 5. Comparison of DSC traces of dehydrogenation of the Mg–4 mol%LaNi2.5 composites prepared by HCS (0 T magnetic intensity, batch 1) and CHCS (10 T magnetic intensity, batch 2).

tion reaction of batch 2. However, despite the presence of the two dissimilar hydrides, the DSC curves show only one prominent endothermic peak at 96 min for batch 2 and at 105 min for batch 1. The results described above can be reasonably explained from the change of phase composition and the particle size reduction due to the influence of the high magnetic field on the Mg–4 mol%LaNi2.5 composites. In order to further investigate the effect of the high magnetic field on the dehydrogenation properties of the Mg–4 mol%LaNi2.5 composites, the corresponding GC curves as a function of time and temperature for MgH2 , LaNi2.5 Hx and Mg–4 mol%LaNi2.5 hydrides are shown in Fig. 6 to compare their dehydriding ability. Fig. 6(I) indicates that LaNi2.5 Hx thoroughly desorbs most hydrogen within 5 min at 298 K. However, as shown in Fig. 6(II) MgH2 desorbs hydrogen at a temperature range from 589 to 773 K and takes 42 min for completing the dehydriding reaction and the maximum dehydriding peak appears at 686 K. In the case of the dehydriding curve of the (Mg–4 mol%LaNi2.5 )Hx composite (Fig. 6(III)),

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600000 LaNi2.5Hx

T=298K

500000

Hydrogen signal

400000 300000 200000 100000 0 -2

0

2

4

6

8

10

12

14

t (min)

(I)

16000 MgH2

686K

800

14000

12000 600 10000

500

8000

T (K)

Hydrogen signal

700

400 773K

6000

300 589K

4000

200 0

20

40

60

80

100

120

140

180

t (min)

(II)

(Mg-4mol%LaNi2.5)Hx

82min, 673K

119min, 673K

13min, 369K

700

600

(a) 10T 500

103min, 673K

400

92min, 673K 16min, 373K 0

20

300

(b) 0T 40

60

80

T (K)

Hydrogen signal (a.u.)

6min, 321K

(III)

160

100

120

140

160

180

t (min)

Fig. 6. Hydrogen desorption signal as function of time and temperature: (a) with 10 T magnetic field and (b) without magnetic field, (I) LaNi2.5 Hx , (II) MgH2 , (III) (Mg–4 mol%LaNi2.5 )Hx .

Q. Li et al. / International Journal of Hydrogen Energy 32 (2007) 1875 – 1884

lg(Peq/0.1MPa)

1

523K 553K 573K

1E-3 0

1

2

(a)

3

4

5

mass% H

1

0.1

523K

3.3. Thermodynamic properties of the Mg–4 mol%LaNi2.5 composites prepared by HCS and CHCS In order to evaluate the effect of a high magnetic field on the thermodynamics properties of the Mg–4 mol%LaNi2.5 composite, the pressure-composition isotherms (PCT) of the two batches were measured. Fig. 7 shows that the PCT curves of hydrogen A/D for the composites at the temperatures from 523 to 573 K without activation. The isotherms in Fig. 7(a) indicate that there is only one long and flat plateau region (.) present. A high performance hydrogen storage alloy should have not only a larger capacity for hydrogen absorption but also a sufficient capacity to desorb the absorbed hydrogen. In other words, the alloy should have a good characterization for reversible hydrogen storage. It should be emphasized that these values were obtained directly from the products by HCS without any activation process. The pressure discrepancy between hydrogen A/D is called hysteresis, which is usually expressed by a hysteresis coefficient defined as Hf = ln(Pab. /Pde. ), Pab. and Pde. are hydrogen A/D plateau pressures. Table 2 summarizes the PCT parameters, which indicate that the plateau pressures of hydrogen A/D of batch 1 are lower than those of batch 2, whereas the latter’s hysteresis coefficient is smaller than that of the formers’. Unfortunately, the reversible hydrogen storage capacity of batch 2 is slightly smaller than that of batch 1 because the amount of the Mg phase in batch 2 is greater than that in batch 1 after hydriding (see Fig. 2). The difference of the composites leads to the difference in the ability of hydrogen absorption for the two batches, which is agreement with the results from GC measurement.

0.1

0.01

lg(Peq/0.1MPa)

its onset temperature of decomposition is lower than that of MgH2 due to the presence of LaNi2.5 , which has better hydrogen A/D properties under the moderate ambient condition [19] and is a good catalyst for H/D reaction in Mg–4 mol%LaNi2.5 system. Comparing the hydrogen desorption peak temperatures of both batches indicates that the onset dehydriding temperature of batch 1 is higher than that of batch 2. The discrepancy of dehydriding temperatures in both batches is caused by the difference of the phase compositions and morphological characterization between batches 1 and 2. From Fig. 6(III), the hydrogen desorption capacities of the composites can be compared by calculating the peak areas in the desorption curves. The total of the hydrogen desorption peak area of batch 1 (S = 15.3) is slightly larger than that of batch 2 (S = 13.6). As mentioned above, if a Mg–4 mol%LaNi2.5 composites is prepared under an external high magnetic field, the decomposition temperature of the composite decreases and the dehydriding reaction rate becomes faster, which may be explained in terms of the change of the structure and crystalline resulting from the action of the high magnetic field. The experimental results of DSC and GC measurements both indicated the similar conclusion described above, although a small difference appears in the measurements, which may be caused by the sensitivity and ability of these experimental apparatus or the nonidentical experimental conditions.

1881

553K

0.01

573K

1E-3 0

(b)

1

2

3

4

5

mass% H

Fig. 7. Pressure-composition isotherms of hydrogen absorption and desorption for the Mg–4 mol%LaNi2.5 at the different temperatures: (a) batch 1; (b) batch 2.

Taking into account the information presented above, it could be seen that a high magnetic field has an effect on the thermodynamics properties. The composite prepared under an external high magnetic field has higher plateau pressures of hydrogen A/D and smaller hysteresis coefficient at the cost of lower hydrogen storage capacity. According to the Van’t Hoff equation and the experimental data in Table 2, Van’t Hoff diagrams of the H/D process of the Mg–4 mol%LaNi2.5 composite are given in Fig. 8. The relationships between the temperature and the equilibrium plateau pressure, and the value of hydrogen A/D for hydride formation of the composite is estimated and summarized in Table 3. Table 3 shows that the absolute values of the entropy and enthalpy of H/D reactions for batch 2 are larger than those for batch 1 and the influence of a high magnetic field on the thermodynamics properties is visible.

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Table 2 The comparison of the hydrogenation properties of the Mg–4 mol%LaNi2.5 prepared by HCS and CHCS without magnetic field and with a 10 T magnetic field T /K

Pab. /MPa

Pde. /MPa

Hf

Cab. /mass%

Cde. /mass%

DRate /%

0T

573 553 523

0.298 0.179 0.086

0.227 0.128 0.061

0.27 0.34 0.34

5.06 4.34 3.69

5.00 4.28 3.60

98.81 98.62 97.56

10 T

573 553 523

0.388 0.237 0.109

0.329 0.191 0.085

0.16 0.22 0.25

4.76 4.00 3.42

4.70 3.88 3.31

98.74 97.00 96.78

Pab. and Pde. are hydrogen A/D plateau pressure, Hf is hysteresis coefficient, Cab. and Cde. are capacities of hydrogen A/D, DRate is the percent of desorbed hydrogen.

Hydriding Dehydriding

lg(Peq/0.1MPa)

0.6

0.4

0.2

0.0

0.00174

0.00180

0.00186

0.00192

-1

(a)

1/T (K )

0.6

lg(Peq/0.1MPa)

0.4

0.2

Hydriding Dehydriding

0.0

-0.2 0.00174

(b)

0.00180

0.00186

reaction rates become faster with the increasing temperature. Considering the overall H/D rates of both batches, it is worthy to point out that the strength of the static magnetic field has an effect on the hydriding rate, as shown in Fig. 9. In terms of the shape of the reacted fraction curves in given time, it can be seen that within 300 s, the ratio of the weight absorbed hydrogen to the batch weight (H a) were 79% and 82% of the maximum hydrogen absorption capacity for batch 1 and 92% and 98% of the maximum values for batch 2 at 523 and 553 K, respectively. The ratio of the weight desorbed hydrogen to the batch weight (H d) within 600 s were 84% and 85% of the maximum hydrogen desorption capacity for batch 1 and 90% and 95% for batch 2 at 523 and 553 K, respectively. From the quantitative comparison of the H/D rates described above, it can be seen that the kinetic properties of the materials prepared by CHCS was improved to a certain extent. However, the origin of the effect induced by the high magnetic field on the kinetic property of hydrogen storage alloys is still unclear. In this particular case, a possible explanation for the improvement of kinetics could be the formation of new structure and the decrease of reaction activation energy due to the transmission of energy into materials from the high magnetic field. On the other hand, the important improvement in the H/D rates for the batch 2 attributes to the existence of multiple phases including Mg, traces of LaNi2.5 , La, Ni and LaNi2.5 Hx , which have better catalysis on the H/D reactions, and small particle size domino effect. Determination of the hydrogenation properties of hydrogen storage materials prepared by different magnetic intensities and the application of the CHCS technique in other intermetallic systems are ongoing research program.

0.00192

1/T (K-1)

Fig. 8. Relationship of plateau pressure with temperature for the Mg–4 mol%LaNi2.5 : (a) batch 1; (b) batch 2.

3.4. Kinetic properties of the Mg–4 mol%LaNi2.5 composites prepared by HCS and CHCS Fig. 9 shows the typical H/D kinetic curves in the range of temperature form 523 to 573 K for the Mg–4 mol%LaNi2.5 composite made by HCS and CHCS, respectively. The shapes of the curves indicate that the hydrogen capacity increases and the

4. Conclusion A technique saving time and energy has been developed for preparing the hydrogen storage alloys and the feasibility study of the controlled hydriding combustion synthesis (CHCS) applying in the Mg–La–Ni system was investigated. The influence of the high magnetic field on the physicochemical properties of the Mg–4 mol%LaNi2.5 composite is analyzed and major conclusions from this research are listed below. (a) It is confirmed that the Mg–4 mol%LaNi2.5 composition can be obtained successfully with magnetic field by CHCS (batch 2) and it can absorb/desorb 4.76/4.70 mass%H

Q. Li et al. / International Journal of Hydrogen Energy 32 (2007) 1875 – 1884

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Table 3 Van’t Hoff equations for the Mg–4 mol%LaNi2.5 and entropy and enthalpy of hydriding (H) and dehydriding (D) at different temperatures Batch 1

Batch 2

H (523 K  T  573 K) D (523 K  T  573 K) Entropy (H) Entropy (D) Enthalpy (H) Enthalpy (D)

lg(P /0.1 MPa) = −3299/T + 6.345 lg(P /0.1 MPa) = −3510/T + 6.637 −121 ± 2 J/(mol K) 127 ± 2 J/(mol K) −63 ± 1 kJ/mol 67 ± 2 kJ/mol

lg(P /0.1 MPa) = −3519/T + 6.746 lg(P /0.1 MPa) = −3632/T + 6.859 −129 ± 2 J/(mol K) 131 ± 2 J/(mol K) −67 ± 1 kJ/mol 69 ± 2 kJ/mol

5

0

4

-1

3 573K with magnetic field 10T 573K without magnetic field 553K with magnetic field 10T 553K without magnetic field 523K with magnetic field 10T 523K without magnetic field

2 1

Hydrogen content(wt.%H)

Hydrogen content(wt.%H)

Item

573K with magnetic field 10T 573K without magnetic field 553K with magnetic field 10T 553K without magnetic field 523K with magnetic field 10T 523K without magnetic field

-2 -3 -4 -5

0 0

500

0

1.0

0.8

0.8

0.6

553K with magnetic field 10T 553K without magnetic field 523K with magnetic field 10T 523K without magnetic field

0.2

1500

0.6 0.4

553K with magnetic field 10T 553K without magnetic field 523K with magnetic field 10T 523K without magnetic field

0.2 0.0

0.0 0

(a)

1000 t (s)

1.0

0.4

500

(b)

Reacted fraction

Reacted fraction

1000 t (s)

(a)

500

0

1000 t (s)

(b)

500

1000

1500

t (s)

Fig. 9. The hydriding and dehydriding kinetic curves of the Mg–4 mol%LaNi2.5 : (a) absorption; (b) desorption.

at 573 K, which are slightly lower than that (5.06/5.00 mass%H for hydrogen absorption/desorption) of batch 1 prepared without magnetic field by HCS. (b) Comparing the kinetic properties of these batches, it can be concluded that the batch 2 has faster reaction rate due to the influence of a high magnetic field. Within 300 s, the ratio of the weight absorbed hydrogen to the batch weight was more than 92% of the maximum hydrogen absorption capacity and within 600 s the ratio of the weight desorbed hydrogen to the batch weight was larger than 90% of the maximum values above 523 K.

(c) The relationships between the temperature and the equilibrium plateau pressure in the Mg–4 mol%LaNi2.5 were obtained and the influence of the high magnetic field on the thermodynamic properties of the composite was studied. (d) From the DSC and GC measurements, the onset decomposition temperature of the batch 2 is obviously lower than that of the batch 1 and the more phase transformations appear in the batch 2 at the dehydriding reaction. (e) The phenomena from (a) to (d) mentioned above maybe ascribe to the change of phase composition and decrease of particle size due to the effect of high magnetic field

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and more catalytic phase(traces of LaNi2.5 , La, Ni and LaNi2.5 Hx ) existing in the batch 2. Acknowledgments The authors thank Prof. Z.M Ren for useful advice. Financial support by Science and Technology Committee of Shanghai (No. 0452NM002 and 05JC14064), and Shanghai Rising-Star Program (No. 06QA14021), are gratefully acknowledged. References [1] Huot J, Akiba E, Takada T. J Alloys Compd 1995;231:815. [2] Kuji T, Nakayama S, Hanzawa N, Tabira Y. J Alloys Compd 2003;356–357:456. [3] Li Q, Chou KC, Xu KD, Jiang LJ, Lin Q, Lin GW, Lu XG, Zhang JY. Int J Hydrogen Energy 2006;31:497. [4] Wang CY, Yao P, Bradhurst DH. J Alloys Compd 1999;285:267. [5] Hamptona MD, Lomnessa JK, Giannuzzi LA. Int J Hydrogen Energy 2002;27:79.

[6] Oelerich W, Klassen T, Bormann R. J Alloys Compd 2001;315:237. [7] Solsona P, Doppiu S, Spassov T, Suriñach S, Baró MD. J Alloys Compd 2004;381:66. [8] Tessier J-P, Palau P, Huot J, Schulz R, Guay D. J Alloys Compd 2004;376:180. [9] Huot J, Liang G, Schulz R. Appl Phys A 2001;72:187. [10] Zhu M, Wang H, Ouyang LZ, Zeng MQ. Int J Hydrogen Energy 2006;31:251. [11] Varin RA, Chiu Ch. J Alloys Compd 2005;397:276. [12] Li Q, Xu KD, Chou KC, Lu XG, Zhang JY, Lin GW. Intermetallics, in press. [13] Zaluska A, Zaluski L, Ström-Olsen JO. J Alloys Compd 1999;289:197. [14] Liang G, Huot J, Boily S, Van Neste A, Schulz R. J Alloys Compd 2000;297:261. [15] Mandal P, Srivastava ON. J Alloys Compd 1994;205:111. [16] Varin RA, Li S, Wronski Z, Morozova O, Khomenko T. J Alloys Compd 2005;390:282. [17] Bystrzycki J, Czujko T, Varin RA. J Alloys Compd 2005;404–406:507. [18] Tsurekawa S, Kawahara K, Okamoto K, Watanabe T, Faulkner R. Mater Sci Eng A 2004;387–389:442. [19] Srivastava S, Srivastava ON. J Alloys Compd 1999;282:197.