Hydrogen storage properties of Mg–Ni–Cu prepared by hydriding combustion synthesis and mechanical milling (HCS + MM)

international journal of hydrogen energy 34 (2009) 2654–2660

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Hydrogen storage properties of Mg–Ni–Cu prepared by hydriding combustion synthesis and mechanical milling (HCS D MM) Hao Gu1, Yunfeng Zhu, Liquan Li* College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, People’s Republic of China

article info

abstract

Article history:

The effect of Cu-doping on the hydrogen storage properties of Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2)

Received 21 September 2008

prepared by hydriding combustion synthesis and mechanical milling (HCS þ MM) was

Received in revised form

studied. For dehydriding properties, the dehydriding temperature onset decreases from

7 December 2008

450 K for Mg95Ni5 to 420 K for Mg95Ni5Cu2. Additionally, the activation energy for dehy-

Accepted 26 January 2009

driding decreases from 116 kJ/mol for Mg95Ni5 to 98 kJ/mol for Mg95Ni5Cu2, indicating that

Available online 20 February 2009

the dehydriding reaction is activated by the catalytic effect of Cu. Moreover, the hydrogen

Keywords:

pretreatment before HCS. The factors for the improvement of the hydrogen storage

Mg-based materials

properties are discussed in this paper.

Hydrogen storage

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

absorption capacity of Mg95Ni5Cu2 at 373 K in 100 s increases from 0.95 to 4.16 wt.% by MM

Hydriding combustion synthesis

reserved.

Mechanical milling

1.

Introduction

Mg is a very promising candidate for hydrogen storage due to its high hydrogen storage capacity (7.6 wt.%), abundant resources and low cost. The major obstacle for the application of MgH2 is the thermodynamic stability (the enthalpy of hydride formation is 74.5 kJ/mol), and thus, it requires about 553 K for dehydriding at 1 bar of hydrogen pressure [1]. It has been found that alloying Mg with other elements is an effective way to destabilize MgH2. For example, compared with pure Mg, Mg2Ni and Mg2Cu alloys have lower hydride formation enthalpy [2]. Additionally, the destabilization effect on MgH2 increases in the following order: Ti, Nb, Al, Fe, Ni and Cu on the basis of first-principles electronic simulations [3]. Another problem for MgH2 is its sluggish hydriding and dehydriding kinetics. In recent years, mechanical milling, i.e.,

MM, has been widely used to prepare nanostructured Mgbased materials with large amounts of micro defects and surface areas for hydrogen diffusion [4,5]. Moreover, during MM, different kinds of additives, including non-metals [6–8], transition metals [9,10], metal oxides and fluorides [11,12] and other hydrogen storage materials [13–15], are added into Mg to improve the hydriding and dehydriding kinetics. Besides MM, hydriding combustion synthesis (HCS), proposed in 1997 by Akiyama et al. [16–18], has been regarded as an innovative method to produce Mg-based hydrogen storage alloys since HCS has the advantages of a short processing time, a low energy requirement and high activity of the products. In our previous work, we suggested that the combined processing of HCS with MM (i.e., HCS þ MM) has the potential to synthesize Mg-based hydrogen storage materials for vehicular applications [19]. The HCS þ MM product of

* Corresponding author. Tel.: þ86 25 83587255. E-mail address: [email protected] (L. Li). 1 PhD candidate. 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.068

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2.

Experimental

Original powders of Mg (<100 mm in size and 99 wt.% in purity), Ni (2z3 mm in size and 99 wt.% in purity) and Cu (<100 mm in size and 99 wt.% in purity) were commercially gotten. The powder mixtures with the molar ratio of Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) were homogenized by ultrasonic vibration in acetone for 30 min. After being completely dried in air, the mixtures were used directly for HCS, and more details about HCS are described in our previous study [27]. The composites of Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) prepared by HCS were denoted as HCS-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2). Besides, HCS-Mg95Ni5Cu2 that was milled for 5 h before HCS, i.e., MM pretreatment, was denoted as MM-HCS-Mg95Ni5Cu2. Then, the HCS products were milled in 50 ml stainless steel vials with stainless steel balls (a ball to powder ratio of 30:1) at 200 rpm for 10 h under 0.1 MPa argon by a planetary milling apparatus. After MM, HCS-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) and MM-HCS-Mg95Ni5Cu2 were denoted as HCS þ MMMg95Ni5Cux (x ¼ 0, 0.5, 1, 2) and MM-HCS þ MM-Mg95Ni5Cu2, respectively. The microstructures of the HCS and HCS þ MM products were examined by means of X-ray diffraction (XRD) with Cu Ka radiation and scanning electron microscopy (SEM). The average crystal sizes from the XRD patterns were estimated based on Eq. (1) [28]:

FWðSÞ  cosðqÞ ¼ kl=size þ 4  strain  sinðqÞ

(1)

where FW(S ) is the specimen broadening, q is the peak position, k is the shape factor, l is the wavelength of the X-ray, and strain is the lattice strain. The hydrogen storage properties of the HCS þ MM products were measured using Sieverts’ method. The transfer of samples to the sample chamber was performed in a glove box under an argon atmosphere. Since the HCS þ MM products were in their hydride states, they were dehydrogenated by heating to 603 K in the evacuated sample chamber prior to the hydriding kinetics measurement, and the amount of hydrogen desorbed as a function of temperature was derived from the recorded data of temperature and pressure. A similar method used for the examination of the dehydriding property is reported in Refs. [29,30]. Additionally, the hydriding and dehydriding kinetics were measured. Afterwards, the pressure–concentration-isotherms (PCIs) of HCS þ MM-Mg95Ni5Cux (x ¼ 0, 2) at 523 K were measured.

3.

Results and discussion

3.1. XRD analysis of HCS-Mg95Ni5Cux and HCS þ MMMg95Ni5Cux (x ¼ 0, 2) Fig. 1 shows the XRD patterns of HCS-Mg95Ni5Cux (x ¼ 0, 2). HCS-Mg95Ni5 consisted of MgH2, Mg2NiH4 and Mg2NiH0.3. In addition, part of the un-reacted Mg remained in the HCS product due to the slow hydriding kinetics of Mg. During HCS of the Mg–Ni–Cu system, Mg2Ni and Mg2Cu phases are the intermediate products [26]. In the XRD pattern of HCS-Mg95Ni5Cu2, on the one hand, we cannot detect Ni and Cu peaks. On the other hand, the peaks of Mg2NiH0.3, Mg2NiH4 and MgCu2, rather than the Mg2Ni and Mg2Cu peaks appeared. The above results show that the intermediate products of Mg2Cu and Mg2Ni were synthesized and hydrogenated completely during the process of HCS. The XRD patterns of HCS þ MM-Mg95Ni5 and HCS þ MMMg95Ni5Cu2 are given in Fig. 2. It was found that the phase

MgH2

Mg

Mg2NiH4

MgCu2

Mg2NiH0.3

HCS-Mg95Ni5Cu2

Intensity (a.u.)

Mg2Ni alloy shows the maximum hydrogen capacity of 2.76 wt.% at 313 K in 100 s [19–21]. A higher hydrogen absorption capacity of 5.60 wt.% at 373 K in 100 s is obtained for the Mg–Ni alloys prepared by HCS þ MM when the Mg content increases from 66.7 at.% (Mg2Ni) to 98 at.% (Mg98Ni2) [22]. Such hydriding performance is comparable with the excellent literature result that MgH2 þ 5 wt.% NbF5 þ 5 wt.% SWNTs composite absorbs 4.8 wt.% hydrogen at 373 K in 300 s [23]. For the Mg–Ni–Cu system, the substitution of Ni by Cu in as-melted Mg2Ni decreases the dissociation temperature of the hydride from 523 to 500 K under 0.1 MPa hydrogen [24]. Besides, Mg–23.5 wt.% Ni and Mg–23.5 wt.% Ni–5 wt.% Cu alloys prepared by melt spinning followed by crystallization heat treatment absorb 4.08 and 4.76 wt.% hydrogen at 573 K in 20 min, respectively, and they desorb 3.58 and 4.59 wt.% hydrogen at the same temperature and duration condition, respectively, indicating that the addition of Cu is favorable for the improvement of hydriding and dehydriding kinetics of Mg-based materials [25]. Moreover, Li et al. [26] also suggested that Ni or Cu has a catalytic effect on the hydriding reaction during HCS of Mg2Ni0.75Cu0.25 and Mg2Ni0.5Cu0.5. In the present paper, we emphasize the effect of Cudoping on the hydrogen storage properties of the HCS þ MM products of Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2). We hope to combine the advantage of high activity of the HCS þ MM product and the catalytic effect of Cu in Mg-based materials. As a result, the addition of Cu decreases the dehydriding temperature onset by 30 K, and the activation energy for dehydriding decreases from 116 kJ/mol for Mg95Ni5 to 98 kJ/mol for Mg95Ni5Cu2.

HCS-Mg95Ni5

20

30

40

50

60

2 Theta (degree) Fig. 1 – XRD patterns of HCS-Mg95Ni5Cux (x [ 0, 2).

70

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international journal of hydrogen energy 34 (2009) 2654–2660

100 Absorption, HCS+MM-Mg95Ni5 Desorption, HCS+MM-Mg95Ni5

Equilibrium Pressure, 0.1 MPa

compositions did not change after MM. Besides, a small amount of MgO was detected in each XRD pattern, which is coincident with previous results [7,20]. After MM, the diffraction peaks broadened due to crystal grain refinement and lattice stress. The average crystal sizes of the milled composites were calculated by means of Eq. (1), and MgH2 peaks in the patterns were used for this calculation. As a result, the average crystal sizes of HCS þ MM-Mg95Ni5 and HCS þ MM-Mg95Ni5Cu2 were roughly estimated to be 56 and 38 nm, respectively, and their lattice strains were 0.10 and 0.14%, respectively. This is in agreement with the result that MM is an effective way to provide nanostructured Mg and Ti hydrides [31,32]. Moreover, it is clear that the presence of Cu (in the form of MgCu2) promoted the decrease of crystal size during MM. However, the reduced crystal size of HCS þ MM-Mg95Ni5Cu2 did not improve the hydriding kinetics at 373 K, which can be observed below.

B

Absorption, HCS+MM-Mg95Ni5Cu2 10

Desorption, HCS+MM-Mg95Ni5Cu2

D

A

C

1

F

E

G

H

0.1

0

1

2

3

4

5

6

Hydrogen content, wt.% Fig. 3 – Pressure–concentration-isotherms (PCIs) of HCS D MM-Mg95Ni5 and HCS D MM-Mg95Ni5Cu2 measured at the temperature of 523 K.

3.2. Thermodynamic properties of HCS þ MMMg95Ni5Cux(x ¼ 0, 2) The pressure–concentration-isotherms (PCIs) of HCS þ MMMg95Ni5 and HCS þ MM-Mg95Ni5Cu2 measured at 523 K are shown in Fig. 3. The isotherm of HCS þ MM-Mg95Ni5 had two hydriding/dehydriding plateaus, and in these plateaus, the hydrogen concentration was independent of pressure, indicating a two-phase region [33]. Moreover, the lower plateau can be assigned to Mg and the higher plateau to Mg2Ni [34]. As for HCS þ MM-Mg95Ni5Cu2, both of the hydriding and dehydriding PCIs showed two plateaus. The hydrogen pressures in the lower plateaus are the same as those of HCS þ MM-Mg95Ni5, indicating that Cu-doping did not alter the thermodynamic properties of MgH2 in HCS þ MMMg95Ni5Cu2. It should be noted that Mg2Cu was generated and transformed to MgCu2 during HCS, as demonstrated in Fig. 2, and that the plateau pressure corresponding to the hydriding of Mg2Cu is higher than that of Mg2Ni [2].

2Mg2Cu þ 3H2 / MgCu2 þ 3MgH2

(2)

Hence, Mg2Cu could react with hydrogen reversibly according to reaction (2) since the hydrogen pressures in the higher plateaus (the interval between A and B for hydriding and that between E and F for dehydriding) of the PCIs of HCS þ MM-Mg95Ni5Cu2 were higher compared with those in the higher plateaus (the interval between C and D for hydriding and that between G and H for dehydriding) of HCS þ MM-Mg95Ni5. As shown in Fig. 3, the reversible hydrogen storage capacity of HCS þ MM-Mg95Ni5 was larger than that of HCS þ MM-Mg95Ni5Cu2 by about 0.34 wt.% because the theoretical value was 6.80 wt.% for the former and 6.40 wt.% for the latter.

MgH2

Mg

Desorbed hydrogen capacity, wt.%

HCS+MM-Mg95Ni5 Mg2NiH0.3

Mg2NiH4 MgCu2

MgO

Intensity (a.u.)

HCS+MM-Mg95Ni5Cu2

HCS+MM-Mg95Ni5

HCS+MM-Mg95Ni5Cu0.5 HCS+MM-Mg95Ni5Cu1

4

HCS+MM-Mg95Ni5Cu2 0.4

0.2

2

0.0 400

0 400

420

420

440

440

460

460

480

480

500

520

540

560

580

600

620

Temperature, K 20

30

40

50

60

70

2 Theta (degree) Fig. 2 – XRD patterns of HCS D MM-Mg95Ni5Cux (x [ 0, 2).

Fig. 4 – The amount of hydrogen desorbed as a function of the temperature of HCS D MM-Mg95Ni5Cux (x [ 0, 0.5, 1, 2). The average heating rate was 20 K/min.

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international journal of hydrogen energy 34 (2009) 2654–2660

-6

a

0.2

HCS+MM-Mg95Ni5Cu0.5

-8

HCS+MM-Mg95Ni5Cu1

lnk

Desorbed transformed fraction

HCS+MM-Mg95Ni5

HCS+MM-Mg95Ni5Cu2 0.1

HCS+MM-Mg95Ni5

-10

HCS+MM-Mg95Ni5Cu2

1.90

1.95

2.00

2.05

2.10

2.15

1000/T, 1/K 0.0

0

200

400

600

800

1000

1200

1400

1600

1800

Time, s

b

Fig. 6 – Arrhenius plots of the dehydriding reaction rate for HCS D MM-Mg95Ni5Cux (x [ 0, 2).

0.5

Desorbed transformed fraction

HCS+MM-Mg95Ni5 HCS+MM-Mg95Ni5Cu0.5

0.4

3.3. Effect of Cu-doping on the dehydriding properties of HCS þ MM-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2)

HCS+MM-Mg95Ni5Cu1 HCS+MM-Mg95Ni5Cu2

0.3

0.2

0.1

0.0

0

200

400

600

800

1000

1200

1400

1600

1800

Time, s 1.0

0.8 5 0.6

4

0.4

Hydrogen content, wt.%

Desorbed transformed fraction

c

Fig. 4 presents the amounts of hydrogen desorbed as a function of temperature for HCS þ MM-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2). As x increased from 0 to 1, the dehydriding temperature onset decreased slightly. With the increase of x to 2, the dehydriding temperature onset was about 420 K, which was 30 K lower than that of HCS þ MM-Mg95Ni5. This implies that Cu (in the form of MgCu2) worked as the catalyst for hydrogen desorption during heating, and more amount of Cu was favorable for the decrease of the dehydriding temperature onset. The dehydriding kinetics of HCS þ MM-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) at 473, 493 and 523 K are shown in Fig. 5, in which the vertical coordinate is expressed as the transformed fraction

HCS+MM-Mg95Ni5 HCS+MM-Mg95Ni5Cu0.5

0.2

HCS+MM-Mg95Ni5Cu1 HCS+MM-Mg95Ni5Cu2

0.0

0

200

400

600

800

1000

1200

1400

1600

1800

3 2 HCS+MM-Mg95Ni5 1

HCS+MM-Mg95Ni5Cu0.5 HCS+MM-Mg95Ni5Cu1

0

HCS+MM-Mg95Ni5Cu2

Time, s -1

Fig. 5 – The dehydriding kinetics of HCS D MM-Mg95Ni5Cux (x [ 0, 0.5, 1, 2) measured at different temperatures: (a) 473 K under vacuum; (b) 493 K under vacuum; (c) 523 K under the initial pressure of 0.005 MPa.

MM-HCS+MM-Mg95Ni5Cu2 0

100

200

300

400

500

600

Time, s Fig. 7 – Hydriding kinetics of HCS D MM-Mg95Ni5Cux (x [ 0, 0.5, 1, 2) and MM-HCS D MM-Mg95Ni5Cu2 measured at 373 K under 3.0 MPa hydrogen.

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Fig. 8 – SEM graphs of the HCS and HCS D MM products: (a) HCS-Mg95Ni5; (b) HCS-Mg95Ni5Cu2; (c) MM-HCS-Mg95Ni5Cu2; (d) HCS D MM-Mg95Ni5; (e) HCS D MM-Mg95Ni5Cu2; (f) MM-HCS D MM-Mg95Ni5Cu2.

(the ratio of the hydrogen desorption capacity to the saturated hydrogen absorption capacity). Clearly, the addition of Cu promoted the dehydriding rate at these temperatures. For example, the desorbed transformed fraction of HCS þ MMMg95Ni5Cu2 at 523 K in 1800 s was 80%, which was 7% higher than that of HCS þ MM-Mg95Ni5. Additionally, the desorption curves at 473, 493 and 523 K in Fig. 5 can be fitted well by the rate expression ln(1  a) ¼ kt (the relative coefficients were larger than 0.99), where a is the dehydriding transformed fraction at time t, and k is the rate constant. This kind of rate expression suggests that the rate limiting step was nucleation and growth. In order to understand the catalytic effect of Cu on the dehydriding kinetics, the activation energies of HCS þ MM-Mg95Ni5Cux (x ¼ 0, 2) for the hydrogen desorption reaction were calculated by Arrhenius plots in Fig. 6. The activation energy of 116 kJ/mol for HCS þ MM-Mg95Ni5 was close to the value for MgH2 obtained by Luo et al. [35]. With the addition of Cu, the activation energy decreased to 98 kJ/mol. This indicates that Cu (in the form of MgCu2) decreased the mass transfer barrier for dehydriding, i.e., Cu had an obvious

catalytic effect on the dehydriding reaction of Mg–Ni composite. For the catalytic mechanism of MgCu2, it was reported that the addition of MgCu2 in Mg provides an oxide-reduced surface, promoting hydrogen recombination [36–38]. In our study, Mg2Cu in the Mg–Ni–Cu system absorbed and desorbed hydrogen reversibly according to reaction (2), as demonstrated by the PCIs. On the other hand, the decomposition of MgH2 with the existence of MgCu2 is easier since the absolute value of the enthalpy for reaction (2) is lower than that for the hydriding reaction of Mg [38]. Hence, the existence of MgCu2 promoted the cleavage of the Mg–H bond by reaction (2), improving the dehydriding properties of HCS þ MM-Mg95Ni5Cux (x ¼ 0.5, 1, 2).

3.4. Effect of Cu-doping on the hydriding kinetics of HCS þ MM-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) Fig. 7 shows the hydriding kinetics of HCS þ MM-Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) at 373 K. For HCS þ MM-Mg95Ni5, it reached the

international journal of hydrogen energy 34 (2009) 2654–2660

saturated hydrogen absorption capacity of 4.90 wt.% in 100 s. However, the addition of Cu led to the drastic decline of the hydriding kinetics, e.g., HCS þ MM-Mg95Ni5Cu2 absorbed 0.95 wt.% hydrogen in 100 s. A similar phenomenon was reported in Ref. [2], where the disproportionation reaction (2) slows down the hydriding rate of the Mg–Ni–Cu system. In Fig. 8(a) and (b), it is clear that more micro-cracks were present in HCS-Mg95Ni5 than those in HCS-Mg95Ni5Cu2. This was because two eutectic reactions of Mg–Ni and Mg–Cu occur during HCS of the Mg–Ni–Cu system, but only one eutectic reaction of Mg–Ni occurs during HCS of the Mg–Ni system [26]. After milling of the HCS products, the powders were ground into fine particles in Fig. 8(d) and (e). However, comparing the particle feature of HCS þ MM-Mg95Ni5 with HCS þ MMMg95Ni5Cu2, we found that the average particle size and the particle agglomeration degree of the latter were larger. It is guessed that such particle characteristics of HCS þ MMMg95Ni5Cu2 contributed to the much slower hydriding rate at 373 K. In order to clarify this, MM-HCS-Mg95Ni5Cu2 with loose and porous particles in Fig. 8(c) was prepared, and it had the same phase compositions with those of HCS-Mg95Ni5Cu2. (In our preliminary experiments, we found that MM pretreatment for 5 h before HCS of Mg95Ni5Cu2 reduced particle size and agglomeration degree effectively.) Besides, compared with HCS þ MM-Mg95Ni5Cu2, the powders of MM-HCS þ MMMg95Ni5Cu2 were fine and dispersive in Fig. 8(f). Hence, it is revealed that the HCS product with loose and porous particles was beneficial to particle refinement and dispersion during MM. The hydriding kinetics of MM-HCS þ MM-Mg95Ni5Cu2 at 373 K is shown in Fig. 7. Only 100 s was required for MMHCS þ MM-Mg95Ni5Cu2 to reach the saturated hydrogen capacity of 4.16 wt.%, while HCS þ MM-Mg95Ni5Cu2 absorbed 0.95 wt.% hydrogen in 100 s. Therefore, it is certain that the optimized structural features played a key role in improving the hydriding kinetics. In detail, the reduced particle size and agglomeration degree as a result of MM pretreatment increased the surface areas for the hydriding reaction, in favor of the improvement of the hydriding kinetics. Apart from this, it is believed that the loose and porous particle of the HCS product is responsible for the improvement of hydriding kinetics [18]. Comparing the hydriding kinetics at 373 K of HCS þ MM-Mg95Ni5 and MM-HCS þ MM-Mg95Ni5Cu2, we found that both of them required only 100 s to reach their saturated hydrogen capacities, and that the addition of Cu decreased the saturated hydrogen capacity by 0.74 wt.% in Fig. 7. The decrease was related to their different theoretical hydrogen storage capacities, as mentioned in Fig. 3. Recently, it was reported that the melt-spun and heattreated alloy of Mg–23.5 wt.% Ni–5 wt.% Cu after milling absorbs 3.90 wt.% hydrogen in 300 s and 4.50 wt.% in 600 s at 573 K after two hydriding/dehydriding cycles [25]. Compared with the reported results, the hydrogen absorption capacity of the Mg–Ni–Cu system in this study was 4.16 wt.% in 100 s even at 373 K without any activation, showing the advantage of the HCS þ MM method in preparing Mg-based materials with good hydriding kinetics at moderate temperature. Additionally, it should be noted that MM-HCS þ MMMg95Ni5Cu2 had the same dehydriding properties as that of

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HCS þ MM-Mg95Ni5Cu2, indicating that MM pretreatment before HCS had little effect on the dehydriding properties of the Mg–Ni–Cu system.

4.

Conclusion

Mg95Ni5Cux (x ¼ 0, 0.5, 1, 2) composites were prepared by HCS þ MM, and the effect of Cu-doping on the hydrogen storage properties of the products was investigated. (1) As for the dehydriding properties, the dehydriding temperature onset decreased from 450 K for Mg95Ni5 to 420 K for Mg95Ni5Cu2. The activation energy for hydrogen desorption decreased from 116 kJ/mol for Mg95Ni5 to 98 kJ/ mol for Mg95Ni5Cu2, indicating that Cu had an obvious catalytic effect on the dehydriding properties. (2) Structural characteristic rather than catalysis of Cu played a key role in the hydriding kinetics. In detail, MM pretreatment before HCS which reduced particle size and agglomeration degree increased the hydrogen absorption capacity of Mg95Ni5Cu2 at 373 K in 100 s from 0.95 to 4.16 wt.%. (3) We suggested that the HCS þ MM method had the advantage in the preparation of the Mg–Ni–Cu system with excellent hydrogen storage properties. Specially, the addition of Cu in the HCS process resulted in the improved hydrogen desorption properties of Mg-based materials.

Acknowledgements This research is supported by the National Natural Science Foundation of China (grant no. 50601014, 50871052), the National Hi-Tech Research and Development Program of China (863 Program) (grant no. 2007AA05Z110), the Hi-Tech Research Program of Jiangsu Science and Technology Department of China (grant no. BG2007052) and the Doctor Discipline Fund of Nanjing University of Technology (grant no. BSCX200703).

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