Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1−xCexNi2.8Co0.5 (x = 0–0.20) electrode alloys

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 6 ( 2 0 1 1 ) 3 0 1 6 e3 0 2 1

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Microstructure and electrochemical hydrogen storage characteristics of (La0.7Mg0.3)1LxCexNi2.8Co0.5 (x [ 0e0.20) electrode alloys Zhenwei Dong a,b, Liqun Ma a,*, Yaoming Wu b, Limin Wang b, Xiaodong Shen a,* a b

College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, China State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China

article info

abstract

Article history:

The

Received 30 August 2010

(La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0, 0.05, 0.10, 0.15 and 0.20) alloys have been investigated.

microstructure

and

electrochemical

hydrogen

storage

characteristics

of

Received in revised form

The results show that all alloys consist of (La, Mg)Ni3 and LaNi5 phases. The cyclic

9 October 2010

stability (S100) of the alloy electrodes increases from 58.7% (x ¼ 0) to 69.8% (x ¼ 0.20) after

Accepted 9 November 2010

100 charge/discharge cycles. The high rate dischargeability (HRD) increases from 66.8%

Available online 15 January 2011

(x ¼ 0) to 69.6% (x ¼ 0.10), then decreases to 65.1% (x ¼ 0.20) at the discharge current

Keywords:

electrodes are also improved by increasing Ce content.

density of 1200 mA/g. Moreover, the electrochemical kinetic characteristics of the alloy ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

LaeMgeNi system alloy Hydrogen storage Metal hydride electrode Metal

hydride/nickel

secondary

battery

1.

Introduction

For metal hydride/nickel (MH/Ni) secondary batteries, it is very important to improve the overall properties by using novel alloy electrode materials with large discharge capacity and long cycle life. In recent years, LaeMgeNi system hydrogen storage alloys have been investigated extensively as one of new type negative electrode candidates for traditional AB5-type alloys due to their larger discharge capacity [1e4]. For example, Kohno et al. [5] have reported that La0.7Mg0.3Ni2.8Co0.5 alloy electrode exhibits large discharge capacity (410 mA h/g) and good cyclic stability during 30 charge/ discharge cycles. The main problem for LaeMgeNi system alloys to be used for industrialization is the poor cycle life. It is well known that Ce is a necessary element in commercial AB5-type alloys, and

a certain amount of Ce content can effectively improve the cycle life of MH/Ni secondary batteries. Consequently, some studies have been devoted to improve the cycle life of LaeMgeNi system alloys through substituting La with Ce [6e10]. Unfortunately, separate substitution of Ce for La cannot solve the poor cycle life thoroughly, which is caused by ignoring the critical effect of Mg in LaeMgeNi system alloys, since Mg is an important factor that influences the cyclic stability of alloy electrodes in KOH aqueous alkali according to previous studies [11,12]. Therefore, it is significant to study the cooperative effect of partially substituting La and Mg with Ce simultaneously on the overall electrochemical properties of LaeMgeNi system hydrogen storage alloys. In this work, in order to illustrate the effect of Ce on electrochemical properties of LaeMgeNi system hydrogen storage alloys distinctly, we select La0.7Mg0.3Ni2.8Co0.5 as a basic alloy,

* Corresponding authors. Tel.: þ86 25 83587243/7234; fax: þ86 25 83240205. E-mail addresses: [email protected] (L. Ma), [email protected] (X. Shen). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.11.028

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Table 1 e Composition of (La0.7Mg0.3)1LxCexNi2.8Co0.5 (x [ 0e0.20) alloys detected by ICP-AES analysis. x x¼0 x ¼ 0.05 x ¼ 0.10 x ¼ 0.15 x ¼ 0.20

La (mg/g)

Mg (mg/g)

Ce (mg/g)

Ni (mg/g)

Co (mg/g)

Composition formula

321.02 302.77 290.56 268.04 255.32

25.73 22.51 20.71 20.01 18.03

0 23.10 45.79 68.82 90.44

548.33 550.52 539.08 543.24 535.57

105.11 101.22 104.03 100.19 100.70

La0.7Mg0.32Ni2.83Co0.54 La0.66Mg0.28Ce0.05Ni2.84Co0.52 La0.64Mg0.26Ce0.10Ni2.81Co0.54 La0.59Mg0.25Ce0.15Ni2.83Co0.52 La0.57Mg0.23Ce0.20Ni2.83Co0.53

and investigate the microstructure and electrochemical properties of (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) electrode alloys.

2.

Experimental details

The LaeCeeNieCo alloys were prepared by arc-melting the constituent metals (La, Ce, Ni and Co; >99.9% purity) on a water-cooled copper hearth under an argon atmosphere. The alloys were turned over and melted for four times to make them homogeneous. Then parts of the alloys were crushed and grinded to powder for composites and powder sintering. Before sintering, LaeCeeNieCo alloys, MgNi2 alloy and Ni powders (200e300 mesh) at a certain molar ratio were premixed using a planetary ball mill QM-1 SP for 10 min under an argon atmosphere. The ball to powder mass ratio was 5: 1 and the rotation speed was 200 rpm. After the milling, the mixtures were cold pressed into green compacts under a pressure of 20 MPa and subsequently powder sintered under an argon atmosphere for 12 h at 1273 K. The (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) hydrogen storage alloys were crushed mechanically into powders of 200e300 mesh in a glove box under a dry argon atmosphere. XRD measurements were carried out using a Rigaku D/Max-3B diffractometer with Cu Ka radiation at 40 kV and 30 mA. The surface morphology of the alloys was observed using a scanning electron microscopy (SEM) linked with energy dispersive X-ray spectrometer (EDS). The testing electrodes were constructed by mixing the composite powders with carbonyl nickel powders at a weight ratio of 1:5. The powder mixtures were pressed into a small

pellet under a pressure of 20 MPa. Electrochemical charge/ discharge testing was carried out at 298 K by using a DC-5 battery testing system. The electrolyte was 6 M KOH aqueous solution. The negative electrodes were charged for 7 h at 60 mA/g and discharged at 60 mA/g to the cut-off potential of 0.6 V. High rate dischargeability (HRD) was measured by a ratio of the discharge capacity at a given discharge current density (300e1200 mA/g) to the discharge capacity at 60 mA/g. Linear polarization curves were performed on a EG&G PARC’s Model 273 Potentiostat/Galvanostat station by scanning the electrode potential at the rate of 0.1 mV/s from
5 mV to 5 mV (versus open circuit potential) at 50% depth of discharge (DOD). Potential step experiments were performed on the same instrument (using the M352 CorrWare electrochemical/ corrosion software) at 100% charge state. Electrochemical impedance spectra (EIS) were obtained in the frequency range from 10 kHz to 1 mHz at 50% DOD.

3.

Results and discussion

3.1.

Microstructures

The chemical composition of (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) alloys measured by ICP-AES analysis is shown in Table 1. It can be seen that the results of chemical analysis agree with those of target compositions. Fig. 1 shows the SEM image and EDS analysis pattern for (La0.7Mg0.3)0.90Ce0.10Ni2.8Co0.5 hydrogen storage alloy as a representative example of (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) alloys. The contents of all elements in the alloy are listed in Table 2. The abundances of all elements detected by the EDS analysis agree with

Fig. 1 e SEM image and EDS pattern of the (La0.7Mg0.3)0.90Ce0.10Ni2.8Co0.5 alloy particles. (a) SEM image; (b) EDS pattern.

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x ¼ 0.10 Theoretical Experimental

Element abundances (wt.%) La

Mg

Ce

Ni

Co

29.68 28.67

2.60 2.54

4.99 5.04

52.25 53.04

10.48 10.61

0.05

23.9 5.08

550

5.06

545

540

5.02 0.00

0.05

(La, Mg)Ni3 LaNi 5

Intensity (arb.units)

0.10

0.15

0.20

xin (La 0.7 Mg0.3 )1-xCex Ni2.8Co0.5

Lattice constant (Å)

b

5.1

0.00

0.05

0.10

0.15

0.20

89 a c v

5.0

88 4.9 4.02

87

4.00 3.98

86

3.96 0.05

0.10

0.15

0.20

xin (La 0.7 Mg0.3 )1-xCex Ni2.8Co0.5

The relationship between the discharge capacity and cycle number for (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) alloy electrodes is shown in Fig. 4. It is obvious to see that the maximum discharge capacities of the alloy electrodes decrease enormously by increasing Ce content. The maximum discharge capacities (Cmax) of the alloy electrodes are summarized in Table 4. The Cmax decreases from 387.0 mA h/g (x ¼ 0) to 322.5 mA h/g (x ¼ 0.20), which is ascribed to that Ce can exist randomly in AB5 or AB2 structural units [13,14], whereas Mg can only exist in AB2 structural unit [15], therefore, the abundance of (La, Mg)Ni3 phase decreases and the abundance

Fig. 3 e Lattice parameters and unit cell volumes as a function of x in the alloys. (a) (La, Mg)Ni3 phase; (b) LaNi5 phase.

of LaNi5 phase increases with the increase of Ce content. It is believed that the discharge capacity of (La, Mg)Ni3 phase is larger than LaNi5 phase. Accordingly, the maximum discharge capacities of the alloy electrodes decrease by increasing x. Moreover, the unit cell volumes of the phases decrease by

Table 3 e Characteristics of the phases in the (La0.7Mg0.3)1LxCexNi2.8Co0.5 (x [ 0e0.20) alloys.

x= 0.20

x

Phase

Phase type

x = 0.15 x = 0.10

x¼0

x = 0.05

x ¼ 0.05

x=0

x ¼ 0.10

40

555

24.0

5.04

Electrochemical characteristics

30

0.20 560 a c v

0.00

20

0.15

24.1

as-designed alloy, which illustrates that the alloy has been synthesized successfully. The XRD patterns of (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) alloys are shown in Fig. 2. The results indicate that all alloys consist of (La, Mg)Ni3 phase with PuNi3-type rhombohedral structure and LaNi5 phase with CaCu5-type hexagonal structure. Moreover, the patterns exhibit similar diffraction peaks, which illustrates that the microstructures of all alloys are almost unchanged by increasing Ce content. Fig. 3 represents the function of lattice parameters and unit cell volumes with Ce content. It can been seen that the a, c parameters and unit cell volumes v of the phases decrease linearly with the increase of Ce content, which is mainly ascribed to that the atomic radius of Ce (1.82  A) is smaller than that of La (1.88  A). In addition, the c/a of (La, Mg)Ni3 phase almost remains constant (4.75e4.76) when the cell volume A3 (x ¼ 0.20), which shrinks from 557.31  A3 (x ¼ 0) to 545.17  indicates that the crystal structures of all alloys are kept isotropic when the unit cell shrinks (Table 3).

3.2.

0.10

Unit cell volume(Å3)

Calculated source

Lattice constant (Å)

x

0.00 24.2

Unit cell volume(Å3)

a

Table 2 e The element abundances of (La0.7Mg0.3)0.90Ce0.10Ni2.8Co0.5 alloy from EDS pattern.

50

60

2θ (Degree)

70

80

Fig. 2 e XRD patterns of the (La0.7Mg0.3)1LxCexNi2.8Co0.5 (x [ 0e0.20) hydrogen storage alloys.

x ¼ 0.15 x ¼ 0.20

(La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5 (La, Mg)Ni3 LaNi5

PuNi3 CaCu5 PuNi3 CaCu5 PuNi3 CaCu5 PuNi3 CaCu5 PuNi3 CaCu5

Lattice Cell parameter ( A) volume ( A3) a

c

c/a

5.080 5.019 5.071 5.011 5.057 5.002 5.046 4.991 5.035 4.988

24.179 3.982 24.138 3.985 24.072 3.987 24.018 3.991 23.919 3.993

4.76 4.76 4.76 4.76 4.75

557.31 88.81 554.63 88.35 551.22 87.75 547.91 87.14 545.17 86.73

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High rate dischargeability (%)

Discharge capacity (mAh/g)

400

300

200

x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20

100

0 0

20

40

60

80

100

100 90 80

x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20

70 60 50

300

Cycle number (n) Fig. 4 e Cyclic stability of the alloy electrodes after 100 charge/discharge cycles.

Cd  100 Cd þ C60

900

(1)

where Cd is the discharge capacity of the alloy electrode at the current density of Id to cut-off potential of 0.6 V, C60 is the residual discharge capacity at the discharge current density of 60 mA/g to the same cut-off potential. The HRDs of the alloy electrodes at the discharge current of 1200 mA/g are listed in Table 4. It is found that the HRD1200 increases from 66.8%

(x ¼ 0) to 69.6% (x ¼ 0.10), then decreases to 65.1% (x ¼ 0.20), which is due to the acceleration of dehydriding process of the alloys after partially substituting La and Mg with Ce. However, when Ce content exceeds a critical content, the effect of lattice parameters and unit cell volumes will become dominant and give rise to a decrease of the HRD.

3.3.

Kinetic characteristics

In order to examine the effect of partially substituting La and Mg with Ce on discharge kinetic characteristics, the potential step and linear polarization curves are preformed on the alloy electrodes. The potential step curves are measured and shown in Fig. 6. According to the model proposed by Zheng et al. [16], the hydrogen diffusion coefficient D in bulk electrode, which is used to characterize the diffusion rate of hydrogen, can be estimated through the slope of linear region of the corresponding plots by following formula: log i ¼ log

  6FDðc0
cs Þ p2 D
t da2 2:303a2

(2)

where D is the hydrogen diffusion coefficient (cm2/s); a is the radius of the spherical particle (cm); i is the diffusion current

1 x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20

Table 4 e Summary of the electrochemical performances of the alloy electrodes. Cmax (mA/g)

Na

C100 (mA/g)

S100 (%)

HRD1200 (%)

387.0 382.9 370.8 344.2 322.5

4 4 4 5 5

201.2 213.1 222.5 209.5 205.1

52.0 55.7 60.0 60.9 63.6

66.8 68.3 69.6 66.4 65.1

Log i (i (A/g))

0

x

1200

Fig. 5 e High rate dischargeability of the alloy electrodes at 298 K.

increasing x, which can also lead to decreasing hydrogen storage capacities of the alloy electrodes. The reason for the decay of discharge capacity is generally due to the pulverization and oxidation of alloy electrode in KOH alkaline electrolyte. In this work, the discharge capacity retention (S100) increases from 52.0% (x ¼ 0) to 63.6% (x ¼ 0.20) after 100 charge/discharge cycles, which is mainly ascribed to that the Ce3þ ions can be oxidized to Ce4þ ions in KOH alkaline electrolyte and form a compact CeO2 oxide film on the alloy surface, which rather effectively inhibits further oxidation and corrosion of the electrode alloys. In addition, the abundance of LaNi5 phase that possesses better cyclic stability than (La, Mg)Ni3 phase increases with the increase of Ce content. Therefore, the cyclic stability of the alloy electrodes is improved by increasing x. The high rate dischargeability (HRD) of the alloy electrodes is shown in Fig. 5. The HRD is defined and calculated according to the following formulation: HRDð%Þ ¼

600

Discharge current density (mA/g)

-1

-2

-3 x¼0 x ¼ 0.05 x ¼ 0.10 x ¼ 0.15 x ¼ 0.20

a The cycle numbers needed to activate the alloy electrodes.

-4

0

600

1200

1800

2400

3000

3600

Time (s) Fig. 6 e Potential step curves of the alloy electrodes at 298 K.

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x

Rp (mU)

I0 (mA/g)

D (cm2/s)

Rct (mU)

113.0 107.8 103.3 118.9 128.8

227.3 238.7 248.5 216.0 199.4

11.7  10
11 12.7  10
11 12.9  10
11 12.6  10
11 12.2  10
11

103.8 97.1 94.4 108.5 117.5

x¼0 x ¼ 0.05 x ¼ 0.10 x ¼ 0.15 x ¼ 0.20

density (A/g); c0 is the initial hydrogen concentration in bulk electrode (mol/cm3); cs is the hydrogen concentration on alloy surface (mol/cm3); d is the density of hydrogen storage materials (g/cm3); and t is the discharge time. Assuming that the average particle radius is 50 mm, the hydrogen diffusion coefficient D in the bulk electrodes can be estimated by Eq. (2) and the results are listed in Table 5. Fig. 7 represents linear polarization patterns of (La0.7Mg0.3) 1
xCexNi2.8Co0.5 (x ¼ 0e0.20) alloy electrodes at 50% DOD. The polarization current density (I0), as an important kinetic parameter to evaluate reversibility of electrode reactions, can be obtained from the slope of linear polarization curves and calculated by the following formula [17]: I0 ¼

RT FRp

(3)

where R is the gas constant, T is the absolute temperature, F is the Faraday constant and Rp is the polarization resistance. Based on the measured linear polarization curves, the values of polarization current density I0 and polarization resistance Rp are calculated and also summarized in Table 5. It can be found that I0 first increases from 227.2 mA/g (x ¼ 0) to 242.5 mA/g (x ¼ 0.10), then decreases to 199.4 mA/g (x ¼ 0.20), which indicates that the reaction rate of hydrogen on the (La0.7Mg0.3)0.9Ce0.1Ni2.8Co0.5 alloy surface is better than that of the others. The HRD1200 as a function of I0 of the alloy electrodes are shown in Fig. 8. The HRD1200 shows a linear relationship with the polarization current density I0. It can be found that the hydrogen diffusion coefficient D maintains almost unchanged in Table 5 (11.7e12.9  10
11 cm2/s). Therefore, the HRDs are essentially controlled by the charge-transfer reaction of hydrogen on alloy electrode surface at the discharge current density of 1200 mA h/g.

70

High rate dischargeability (%)

Table 5 e Electrochemical kinetic parameters of the alloy electrodes at 298 K.

69 68 67 66 65 200

210

220

230

240

250

Polarization current density, I0 (mA/g) Fig. 8 e HRD1200 as a function of I0 for the alloy electrodes at 298 K.

The charge-transfer resistance of hydrogen storage alloy electrode can be determined by electrochemical impedance spectra (EIS). The EIS of the alloy electrodes at 50% depth of discharge (DOD) are shown in Fig. 9. It can be seen that each spectroscopy consists of a small semicircle in the highfrequency region, a large semicircle in the low frequency region and a straight line. According to the model developed by Kuriyama et al. [18], the small semicircle in EIS curve indicates the contact resistance between the alloy powder and conductive material, and the large semicircle represents the charge-transfer resistance on alloy electrode surface. The EIS data is analyzed using an equivalent circuit represented by the inset in Fig. 9. The Re1 is the electrolyte resistance between the metal hydride electrode and reference electrode, the Rcp and Ccp are the contact resistance between the alloy particles and current collector in the high-frequency region, the Rpp and Cpp stand for the contact resistance and capacitance between the alloy particles, respectively, the Rct and Cct contribute to the charge-transfer reaction resistance and double layer capacitance in the low frequency region, the Rw is the Warburg impedance. As shown in Fig. 9, the contact impedance remains almost unchanged, whereas the radius of large semicircle in the low frequency region first decreases and then increases during charge/discharge cycles. The result

1.0 0.8

x=0 x = 0.05 x = 0.10 x = 0.15 x = 0.20

25

-Z" ( /g)

Current density (mA/g)

50

0

0.6

Re1 Ccp x=0 x = 0.05 x = 0.10 Rcp x = 0.15 x = 0.20

Cpp

Rct R w

Rpp

Cct

0.4 0.2

-25

-50 -6

-4

-2

0

2

4

6

Overpotential (mV) Fig. 7 e Linear polarization curves of the alloy electrodes at 50% DOD.

0.0 0.0

0.4

0.8

1.2

1.6

2.0

Z' ( /g ) Fig. 9 e Electrochemical impedance spectra of the alloy electrodes at 50% DOD.

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 6 ( 2 0 1 1 ) 3 0 1 6 e3 0 2 1

illustrates that the charge-transfer resistance on the alloy surface first decreases from x ¼ 0 to x ¼ 0.10, then increases from x ¼ 0.10 to x ¼ 0.20, which is ascribed to the cooperative effect of charge-transfer resistance of MgO and CeO2 film on the alloy surface due to the corrosion during charge/discharge cycles. The result is consistent with the change of HRDs completely, which verifies that the effect of charge-transfer reaction of hydrogen on alloy electrode surface is the essentially controlled step once again.

4.

Conclusions

The (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) hydrogen storage alloys studied in this work are composed of (La, Mg)Ni3 and LaNi5 phases. The maximum discharge capacity of the alloy electrodes decreases from 387.0 mA h/g (x ¼ 0) to 322.5 mA h/g (x ¼ 0.20), the cyclic stability (S100) increases from 58.7% (x ¼ 0) to 69.8% (x ¼ 0.20) after 100 charge/discharge cycles, and the HRD1200 first increases from 66.8% (x ¼ 0) to 69.6% (x ¼ 0.10), then decreases to 65.1% (x ¼ 0.20). The entire results indicate that the substitution of La and Mg with a suitable Ce content is effective to improve the overall electrochemical properties of the alloy electrodes, which is mainly ascribed to the effect of Ce on the phase abundances and anti-corrosion of (La0.7Mg0.3)1
xCexNi2.8Co0.5 (x ¼ 0e0.20) alloys.

Acknowledgements This work was financially supported by the National High Technology Research and Development Program of China (2007AA03Z228), Project supported by the Major Program for the Fundamental Research of the Chinese Academy of Sciences (KJCX2-Y1W-H21), the Science and Technology Cooperation Project of Chinese Academy of Sciences and Jilin province (2008SYHZ0008), the NSFC Fund for Creative Research Group (20921002) and the Open Subject of State Key Laboratory of Rare Earth Resource Utilization.

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