Effects of metal oxides addition on the performance of La1.3CaMg0.7Ni9 hydrogen storage alloy

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33 (2008) 1304– 1309

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Effects of metal oxides addition on the performance of La1.3CaMg0.7Ni9 hydrogen storage alloy Peng Zhang, Xuedong Wei, Yongning Liu, Jiewu Zhu, Guang Yu State Key Laboratory for Mechanical Behavior of Materials, Xi’ an Jiaotong University, Xi’ an 710049, PR China

ar t ic l e i n f o

abs tra ct

Article history:

The La1:3 CaMg0:7 Ni9 alloy was modified with various metal oxides (Fe2 O3 , TiO2 , Cr2 O3 , ZnO),

Received 13 June 2007

and the effects of metal oxides addition on the electrochemical properties of the

Received in revised form

La1:3 CaMg0:7 Ni9 hydrogen storage alloy were investigated. The catalytic effects of metal

11 December 2007

oxides are found. Not only the discharge capacity but also the high-rate dischargeability

Accepted 11 December 2007

(HRD) is improved by addition of 5 wt% TiO2 , Cr2 O3 , and ZnO, while the cyclic stability does

Available online 24 January 2008

not change except for addition of Fe2 O3 . The low-temperature property is enhanced more

Keywords: Hydrogen storage alloys Metal oxides Ni–MH battery

obviously by addition of TiO2 , Cr2 O3 , and ZnO. The electrochemical kinetics is also measured by the linear polarization and electrochemical impedance spectroscopy (EIS). In addition, the hydrogen absorption kinetic behavior is measured by gas–solid reaction. & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Electrochemical properties

1.

Introduction

Hydrogen storage alloys have been widely used as negative electrode materials of Ni–MH batteries due to their higher energy density and better environmental compatibility compared to the Ni–Cd battery [1–3]. A new series of R–Mg–Ni (R ¼ rare earth, Ca, Y) hydrogen storage alloys with PuNi3 type structure were studied extensively because of their higher discharge capacity (380–410 mAh/g) than conventional AB5 type commercial alloys [4–8]. Khrussanova et al. [9,10] reported that some 3d transition metal oxides (TiO2 , Fe2 O3 , and MnO2 Þ could be used as catalysts to improve the hydriding and dehydriding kinetics of metal Mg. Oelerich et al. [11] also discussed the catalytic effects of different metal oxides for nanocrystalline Mg-base materials. They found that only the oxides of the transition metals Ti, V, Cr, Mn, Fe, and Cu, which have different valences, have a catalytic effect for the solid–gas reaction. As for the electrochemical performance, Iwakura et al. [12,13] found that the discharge capacity of MmNi3:6 Mn0:4 Al0:3 Co0:7 was greatly increased by mixing it with RuO2 and Co3 O4 . The

electrochemical performances of mechanically alloyed Mg1:9 Y0:1 Ni0:9 Al0:1 25% MO (MO ¼ RuO2 , V2 O5 ) composites [14] and nanocrystalline LaMg12 2Ni modified with a small amount of TiO2 , Fe3 O4 [15] were improved significantly. The AB5 type alloys modified with Bi2 O3 [16] and nanometer CuO [17] also have good electrochemical performance because Bi2 O3 and CuO were reduced to metal Bi and Cu after the first charging–discharging cycle. In this paper, in order to improve overall electrochemical properties of the La1:3 CaMg0:7 Ni9 hydrogen storage alloy, the La1:3 CaMg0:7 Ni9 alloy was modified with different metal oxides (Fe2 O3 , TiO2 , Cr2 O3 , ZnO), and the catalytic effects of different metal oxides on the hydrogen absorption kinetic behavior of the La1:3 CaMg0:7 Ni9 alloy were also investigated.

2.

Experimental

The La1:3 CaMg0:7 Ni9 hydrogen storage alloy was prepared by induction melting under argon atmosphere two times for good homogeneity. The metals La, Ca, Mg, and Ni have

Corresponding author. Tel.: +86 29 8266 4602; fax: +86 29 8266 3453.

E-mail address: [email protected] (P. Zhang). 0360-3199/$ - see front matter & 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2007.12.030

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purities of more than 99.9 wt%. In order to avoid evaporation of Ca and Mg, a slight excess of Ca and Mg was necessary in melting. The alloy ingot was mechanically pulverized for electrochemical measurements and X-ray diffraction (XRD) analysis. The crystal structures of the alloy powders were determined by XRD using Cu Ka radiation. For electrochemical measurement, 1 g of alloy powders was mixed with 5 wt% metal oxides as additives. The mixture was pasted into two porous nickel foams (size 4  3 cm2 Þ with adding 5 wt% polyvinyl alcohol (PVA) as a binder, and then dried in vacuum condition at 395 K. The metal hydride working electrode was produced by compressing two porous nickel foams under 20 MPa pressure. The NiðOHÞ2 electrode was used as the counter electrode, the capacity of which is four times higher than that of the working electrode. The electrochemical measurements were performed in 6 M KOH electrolyte at 303 K. Before measurement, the working electrode was placed in 6 M KOH electrolyte at 333 K for 2 h to activate. The charge and discharge capacity test was as follows: For the electrode activation and measurements of the maximum discharge capacity, the electrodes were charged at 100 mA/g for 5 h, then rested for 10 min and discharged at 100 mA/g to a cut-off potential of 1.0 V. The measurements for cyclic stability were carried out at a charge/discharge current of 300 mA/g. To investigate the high-rate dischargeability (HRD) of the alloy electrodes, discharge capacities at different

1.5

Voltage (V)

1.4 1.3 1.2

1 2 3 4 5

1.1 1.0

1 2 345

0

50

100 150 200 250 300 350 Discharge capacity (mAh/g)

400

450

Fig. 1 – Discharge curves of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides.

1305

discharge current densities (100, 300, 600, 900, 1200 mA/g) were measured. In order to measure electrochemical impedance spectroscopy (EIS) and linear polarization, an Hg/HgO reference electrode was used. EIS of the electrodes were measured in a frequency from 10 kHz to 5 mHz with AC amplitude of 5 mV on an electrochemical workstation (Chi 650c) at 50% depth of discharge (DOD). The linear polarization curves of the electrodes were also measured at the scan rate of 0.1 mV/s range from 5 to 5 mV (vs. open circuit potential) at 50% DOD. The hydrogen absorption kinetic behavior of the La1:3 CaMg0:7 Ni9 alloy was measured using a Sievert’s type volumetric gas reaction controller at 303 K. The reaction container (including 1.5 g alloy powder) was heated to 353 K and air was evacuated for 3 h prior to the measurement.

3.

Results and discussion

Fig. 1 shows the discharge curves of the La1:3 CaMg0:7 Ni9 alloy modified with various metal oxides (Fe2 O3 , TiO2 , Cr2 O3 , and ZnO). The electrochemical properties of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides are also presented in Table 1. As it can be seen, different metal oxide additions impose various effects on the discharge behavior. Modification of the La1:3 CaMg0:7 Ni9 alloy with 5% TiO2 , 5% Cr2 O3 , and 5% ZnO result in an increase on the discharge capacity from 380 to 390.4, 396.8, and 399 mAh/g, respectively. In contrast, addition of 5% Fe2 O3 has a negative effect on the discharge capacity which decreases to 370 mAh/g. The activation property of the alloy electrodes is also improved by addition of metal oxides. Fig. 2 shows the HRD of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides. The HRD value is defined and calculated according to the following formulation: HRD ¼

La1.3CaMg0.7Ni9 + 5% Fe2O3 La1.3CaMg0.7Ni9 La1.3CaMg0.7Ni9 + 5% TiO2 La1.3CaMg0.7Ni9 + 5% Cr2O3 La1.3CaMg0.7Ni9 + 5% ZnO

33 (2008) 1304 – 1309

Cd  100%, Cd þ C100

(1)

where Cd is the discharge capacity of the alloy electrode at the current density Id to the cut-off potential of 1 V, C100 is the residual discharge capacity at the discharge current density I ¼ 100 mA=g to the same cut-off potential after the alloy electrodes have been fully discharged at the discharged current Id and rested for 5 min. The HRD value at the discharge current density 1200 mA/g is increased in the order of Fe2 O3 ð41:93%Þounmodified ð50%ÞoTiO2 ð53:16%ÞoCr2 O3

Table 1 – The electrochemical properties of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides Samples x ¼ 0:0 5%Fe2 O3 5%TiO2 5%Cr2 O3 5%ZnO a

Cmax (mAh/g)

Na

C150 /Cmax (%)

HRD1200 (%)

I0 (mA/g)

380 370 390 396.8 399

6 4 4 4 4

53 63.73 51.35 53.64 53.24

42.2 41.93 53.16 58.88 65.3

122.1 118.85 143.77 164.1 184.58

The cycle numbers needed to activate the alloy electrode.

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50

100 90 Current density (mA /g)

High rate dischargeability (%)

40

80 70 60

La1.3CaMg0.7 Ni9 + 5% Fe2O3 La1.3CaMg0.7 Ni9

50

La1.3CaMg0.7 Ni9 + 5% TiO2 La1.3CaMg0.7 Ni9 + 5% Cr2O3

0

20 10 0 -10 -20 -30

La1.3CaMg0.7 Ni9 + 5% ZnO

40

30

La1.3CaMg0.7Ni9 + 5% Fe2O3 La1.3CaMg0.7Ni9 La1.3CaMg0.7Ni9 + 5% TiO2 La1.3CaMg0.7Ni9 + 5% Cr2O3 La1.3CaMg0.7Ni9 + 5% ZnO

-40

300 600 900 Discharge current density (mA/g)

-50 -6

1200

Fig. 2 – The high rate dischargeability (HRD) of the La1:3 CaMg0:7 Ni9 alloy modified with various metal oxides.

-4

-2 0 2 Overpotential (mV)

4

6

Fig. 4 – Linear polarization curves of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides at 50% DOD and 303 K.

Fe2O3 TiO2

intensity (arbitrary units)

(e) + 5% ZnO

Cr2 O3 cannot be reduced or oxidized during the electrochemical process. Disappearance of ZnO is attributed to dissolving in KOH electrolyte. Fig. 4 shows linear polarization curves of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides at 50% DOD and 303 K. Exchange current density reflects the speed of charge transfer on the surface of the alloy electrodes, which can affect the HRD property. Exchange current density I0 can be calculated from the slope of polarization curves according to the following equation [18]:

Cr2O3

(d) + 5% Cr2O3

(c) + 5% TiO2

(b) + 5% Fe2O3

(a) La1.3CaMg0.7Ni9

I0 ¼ 20

30

40

50

60

70

80

2 theta (deg.)

Fig. 3 – XRD patterns of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides after the first electrochemical cycle.

ð58:88%ÞoZnO ð65:3%Þ. Based on the above results, it seems that addition of metal oxides (TiO2 , Cr2 O3 , and ZnO) can improve the discharge capacity, especially at high-discharge currents. From above experimental details, we can conclude that metal oxides have surface catalytic effects. In order to investigate what the effect of metal oxides is taken, for electrochemical action, linear polarization and electrochemical impedance spectra of the alloy electrodes are measured. For solid–gas reaction, hydrogen absorption kinetic of the alloy powders is also investigated. First, XRD patterns of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides after the first electrochemical charging and discharging cycle are shown in Fig. 3. Comparing with XRD pattern of the La1:3 CaMg0:7 Ni9 alloy, it can be seen that diffraction peaks of Fe2 O3 , TiO2 , and Cr2 O3 appear, while ZnO cannot be detected. It means that Fe2 O3 , TiO2 , and

RTId , FZ

(2)

where R is the gas constant, T is the absolute temperature, Id is the applied current density, F is the Faraday’s constant, and Z is the total overpotential. The exchange current density I0 is obtained and listed in Table 1. In the order of Fe2 O3 o unmodifiedoTiO2 oCr2 O3 oZnO, the I0 value increases. Electrochemical impedance spectra of the La1:3 CaMg0:7 Ni9 alloy modified with various metal oxides are shown in Fig. 5. According to the EIS model for hydrogen storage alloys [19], the large semicircle in the low-frequency region represents the electrochemical reaction resistance on the surface of the alloy electrodes. It can be seen that the large semicircles in the low-frequency region for electrochemical reaction resistance are different. This result indicates that the electrochemical reaction resistance changes reversely with the value of I0 . Variation of I0 and electrochemical reaction resistance are consistent with that of the maximum discharge capacity and the HRD value. It can be concluded that the charge transfer reaction is the dominant step which controls the electrochemical reaction speed. The speed of charge transfer on the surface of the alloy electrode is increased by modifying with TiO2 , Cr2 O3 , and ZnO. By addition of metal oxide, the surface of the alloy powder is covered with metal oxide layers. For one aspect, RuO2 and Co3 O4 layers as electrochemical catalysts

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0.20

2.0

0.18

La1.3CaMg0.7Ni9 + 5% Fe2O3 La1.3CaMg0.7Ni9 La1.3CaMg0.7Ni9 + 5% TiO2 La1.3CaMg0.7Ni9 + 5% Cr2O3 La1.3CaMg0.7Ni9 + 5% ZnO

0.14 0.12

1.8 Hydrogen concentration (wt.%)

0.16 -Z imag. (Ω)

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0.10 0.08 0.06 0.04 0.02

1.6 1.4 1.2 1.0

La1.3CaMg0.7Ni9 + 5% Fe2O3 La1.3CaMg0.7Ni9 La1.3CaMg0.7Ni9 + 5% TiO2 La1.3CaMg0.7Ni9 + 5% Cr2O3

0.8 0.6 0.4

La1.3CaMg0.7Ni9 + 5% ZnO

0.2

0.00

0.0

0.1

0.2

0.3

0.4

Zreal (Ω)

0

100

200

300 Time (s)

400

500

600

Fig. 5 – Electrochemical impedance spectra of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides at 50% DOD and 303 K.

Fig. 6 – Hydrogen absorption kinetic curves of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides at 303 K under 3.5 MPa.

can accelerate the Volmer reaction [13].

phases. In this stage, over 90% of the maximum storage capacity is reached. After 250 s, the alloy is almost saturated with hydrogen. And since then absorbing and desorbing of hydrogen atom achieves equilibrium. It is obviously observed that hydrogen absorption kinetics of the alloy powder is accelerated by modifying with TiO2 , Cr2 O3 , and ZnO, while addition of Fe2 O3 causes a decreasing kinetics between 100 and 250 s. The results of hydrogen absorption kinetics of the solid–gas reaction are in accordance with that of electrochemical reaction kinetics. The one difference is that addition of TiO2 , Cr2 O3 , and ZnO almost shows the same hydrogen absorption kinetics. This may be due to the fact that the hydrogen absorption kinetics of the alloy powder modified with TiO2 , Cr2 O3 , and ZnO reach a limiting point because that of the alloy without modification is also fast. Addition of metal oxide by ball milling can introduce high-density defects, which is believed to be an important reason for the improved kinetics of the solid–gas reaction [11,21]. In our experiment, metal oxides, which are added to the alloy by mixture, still have catalytic effects. As reported in Ref. [22], metal V shows no significant catalytic effect; however, the absorption kinetics is obviously improved when the MgH2 =V0:01 powder is oxidized during 11 months of storage in an Ar atmosphere and during exposure to air, respectively. This means that metal oxides have excellent catalytic properties due to the presence of oxygen. In gas reaction, the catalytic role of metal oxides is decomposition of hydrogen into hydrogen atom, the same as electrochemical reaction. The cyclic stability of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides is shown in Fig. 7. The cyclic stability is improved by addition of Fe2 O3 , while not changed by addition of TiO2 , Cr2 O3 , and ZnO. The degradation is mainly due to the common effects of pulverization of the alloy particles and corrosion of the alloy compounds. Addition of Fe2 O3 causes a low-discharge capacity, leading to a small lattice expansion and contraction. It can explain the improvement of the cyclic stability. For addition of other metal oxides, it can be seen that catalytic effect is obvious for

3H2 O þ e ! HðaÞ þ OH

Volmer reaction.

(3)

This results in the abundance of adsorbed hydrogen atoms on the surface of the electrode. The charging efficiency is eventually improved. For another aspect, Zander D et al. [20] reported that addition of Nb2 O5 reduces the hydrogen evolution at the electrode surface by decreasing the hydrogen overpotential, which also improved the charging efficiency. At the large charge/discharge current density, the alloy electrode has the high speed of charge transfer. So the amount of hydrogen atom on the electrode surface determines the electrochemical reaction speed. For unmodified alloys, Ni particles on the surface take a catalytic role. Shown from the results, the electrocatalytic activity of TiO2 and Cr2 O3 is better than Ni particles, while that of Fe2 O3 is weaker. It can be indicated that addition of TiO2 and Cr2 O3 as catalysts improves the charging efficiency. This explains why HRD value at the discharge current density of 1200 mA/g is obviously enhanced by modifying with TiO2 and Cr2 O3 . It is still unclear why the electrode addition of ZnO shows the best electrochemical performance. Known from above experimental results, ZnO has dissolved into KOH electrolyte from the electrode. Maybe another unknown reaction mechanism is included and it will be summarized in our future research. The hydrogen absorption kinetic curves of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides at 303 K are indicated in Fig. 6. As shown in Fig. 6, the hydrogen absorption kinetic curve contains three parts. It is well known that hydrogen atom is first absorbed, forming a solid solution phase, then hydrogen atom react with saturated a solid solution phase, generating b hydride phases, which existed with a solid solution phase together, at last a solid solution phase is changed into b hydride phases completely. When time interval is shorter than 100 s, there is mainly a solid solution phase. In the range of 100–250 s, saturated a solid solution hydrogen coexists with b hydride

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400

where CT is the discharge capacity of the alloy electrode at T K, C303 is the discharge capacity of the alloy electrode at 303 K. The LTD value of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides increases from 44.59 % (Fe2 O3 Þ, to 45.47 % (unmodified), 54.4 % ðTiO2 Þ, 59.43 % ðCr2 O3 Þ, and 64.01% (ZnO). It can be found that the low-temperature property is improved more obviously by addition of TiO2 , Cr2 O3 , and ZnO.

Discharge capacity (mAh/g)

350 300 250 200 La1.3CaMg0.7Ni9 + 5% Fe2O3 La1.3CaMg0.7Ni9 La1.3CaMg0.7Ni9 + 5% TiO2 La1.3CaMg0.7Ni9 + 5% Cr2O3

150 100

4.

Conclusions

La1.3CaMg0.7Ni9 + 5% ZnO

50 0 0

20

40

60 80 100 Cycle number (N)

140

120

160

Fig. 7 – Discharge capacity vs. cycle number for the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides.

1.6 1 La1.3CaMg0.7Ni9 + 5% Fe2O3 2 La1.3CaMg0.7Ni9

1.4

3 La1.3CaMg0.7Ni9 + 5% TiO2 4 La1.3CaMg0.7Ni9 + 5% Cr2O3

Voltage (V)

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5 La CaMg Ni + 5% ZnO 1.3 0.7 9

1.2

Acknowledgment

1.0

The authors are grateful for the financial support of the Education Ministry Key Project of China (104266).

0.8 0

Effects of metal oxides modification on the performance of La1:3 CaMg0:7 Ni9 hydrogen storage alloy were investigated in this paper. The catalytic effects of metal oxides are discovered. The discharge capacity increases with the oxides addition in the order Fe2 O3 ounmodifiedoTiO2 oCr2 O3 oZnO. The HRD value at the discharge current density 1200 mA/g is also improved in the same order. The results of the linear polarization and EIS show the speed of charge transfer on the surface of the alloy electrode is enhanced by modifying with TiO2 , Cr2 O3 , and ZnO. Hydrogen absorption kinetics of the alloy powder is also accelerated by modifying with TiO2 , Cr2 O3 , and ZnO, while addition of Fe2 O3 causes a bad kinetics. The well low-temperature property is also obtained by addition of TiO2 , Cr2 O3 , and ZnO at 255 K.

1 2

50

100

150

3

200

4

5

250

300

R E F E R E N C E S

Discharge capacity (mAh/g) Fig. 8 – Discharge curves of the La1:3 CaMg0:7 Ni9 alloy modified with different metal oxides at 255 K.

the low-temperature dischargeability (LTD) and (HRD), especially in discharge current density 1200 mA/g. In cycle measurement, though addition of TiO2 , Cr2 O3 , and ZnO could improve the discharge capacity, the increasing extent is small. As a result, the pulverization and corrosion behavior of the alloy electrode do not change. The same phenomena also can be found in Ref. [15]. Because the La1:3 CaMg0:7 Ni9 alloy modified with many metal oxides show excellent electrochemical kinetic property, the lowtemperature property of which are investigated and shown in Fig. 8. At 255 K, the discharge capacity of the alloy electrodes decreases because of the low-electrochemical reaction rate and slow hydrogen diffusion rate at low temperature [23]. Shown in Fig. 8, the discharge capacity is still increased in the order of Fe2 O3 ounmodifiedoTiO2 oCr2 O3 oZnO. The LTD value is defined and calculated according to the following formulation: LTD ¼

CT  100%, C303

(4)

[1] Willems JJG. Philips J Res 1984; 39 (Suppl. 1): 1. [2] Sakai T, Miyamara H, Kauriyama N, Kato A, Oguro K, Ishikawa H. J Electrochem Soc 1990;137:795. [3] Schlapbach L, Zuttel A. Nature. 2001;414:353. [4] Kadir K, Sakai T, Uehare I. J Alloys Compd 2000;302:112. [5] Kadir K, Kuriyama N, Sakai T, Uehare I, Eriksson L. J Alloys Compd 1999;284:145. [6] Kadir K, Sakai T, Uehare I. J Alloys Compd 1999;287:264. [7] Chen J, Kuriyama N, Takeshita HT, Tanaka H, Sakai T, Haruta M. Electrochem Solid-State Lett 2000;3(6):249. [8] Kohno T, Yoshida H, Kawashima F, Inaba T, Sakai I, Yamamoto M, et al. J Alloys Compd 2000;311:L5. [9] Khrussanova M, Peshev P, Ivanov EY, Terzieva M. Mater Res Bull 1987;22:405. [10] Terzieva M, Khrussanova M, Peshev P. Int J Hydrogen Energy 1991;16:265. [11] Oelerich W, Klassen T, Bormann R. J Alloys Compd 2001;315:237. [12] Iwakura C, Fukumoto Y, Matsuoka M, Kohno T, Shinmou K. J Alloys Compd 1993;192:152. [13] Iwakura C, Matsuoka M, Kohno T. J Electrochem Soc 1994;141:2306. [14] Cui N, Luo JL. Electrochim Acta 1998;44:711. [15] Wang Y, Gao XP, Lu ZW, Hu WK, Zhou Z, Qu JQ, et al. Electrochim Acta 2005;50:2187. [16] Cheng SA, Lei YQ, Leng YJ, Wang QD. J Alloys Compd 1998;264:104. [17] Zhang S, Shi PF, Deng C. Solid State Ionics 2006;177:1193.

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[18] Notten PHL, Hokkeling P. J Electrochem Soc 1991;138:1877. [19] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H, Iwasaki T. J Alloys Compd 1993;202:183. [20] Zander D, Lyubenova L, Ko¨ster U, Dornheim M, Aguey-Zinsou F, Klassen T. J Alloys Compd 2006;413:298.

33 (2008) 1304 – 1309

1309

[21] Barkhordarian G, Klassen T, Bormann R. Scr Mater 2003;49:213. [22] Oelerich W, Klassen T, Bormann R. J Alloys Compd 2001;322:L5. [23] Liu YF, Pan HG, Jin QW, Li R, Li SQ, Ge HW, et al. Chin J Nonferrous Metals 2004;14:802.