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Effect of Al and Ce substitutions of the electrochemical properties of amorphous MgNi-based alloy electrodes
International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 www.elsevier.com/locate/ijhydene
Effect of Al and Ce substitutions of the electrochemical properties of amorphous MgNi-based alloy electrodes Yan Feng, Lifang Jiao, Huatang Yuan ∗ , Ming Zhao Institute of New Energy Material Chemistry, Nankai University, Tianjin, PR China Received 30 March 2006; received in revised form 24 November 2006 Available online 16 January 2007
Abstract Mg-based hydrogen storage alloys Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) were successfully prepared by means of mechanical alloying (MA). The structure and the electrochemical characteristics of these Mg-based electrodes were also studied. The result of X-ray diffraction (XRD) shows that the main phases of the alloys exhibit amorphous structures. The charge–discharge cycle tests indicate these alloys have good electrochemical active characteristics. Among these alloys, the Mg0.9 Al0.08 Ce0.02 Ni has the best cycle stability. After 50 cycles charge–discharge, the discharge capacity of Mg0.9 Al0.08 Ce0.02 Ni alloy is 66.67% higher than MgNi alloy. The cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS) and anticorruption test (potentiodynamic polarization) were also studied, and the results show that the electrochemical cycle stability of these alloys was improved by Al and Ce substitutions. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Mg-based hydrogen storage; Electrochemical property; Cycle stability; Al and Ce substitutions
1. Introduction Mg-based hydrogen storage alloys are attractive materials for energy conversion and hydrogen storage because of their lighter weight, higher hydrogen storage capacity and lower cost compared with other series of hydrogen storage materials [1]. Despite of its merits, practical applications of these materials as secondary battery have been hampered by the following major factor: easy to be corrupted in alkaline solutions, which cause their poor charge–discharge cycle stability. Many efforts have been made on their electrochemical characteristics. Element substitution has been found to be the most effective way to retard the corrosion and to improve the cycle capacity degradation [2–4]. Liu et al. reported that Ti and Al substitute in MgNi-type alloy electrode is an effective way to improve the cycle stability in alkaline solution [5]. Ce is also a good substitute in AB5 -type and AB3 -type alloy electrode to improve the cycle life [6,7].
∗ Corresponding author. Tel.: +86 22 23504527; fax: +86 22 23502604.
E-mail address: [email protected] (H. Yuan).
In this work, Al and Ce substituted Mg in MgNi-type alloys. Amorphous quaternary Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) alloy was prepared by mechanical alloying (MA). The structural and electrochemical properties were characterized. 2. Experimental MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) alloys were each prepared from mixed powder of pure Mg, Al, Ce and Ni by MA. The mixture was then MA for several hours at a speed of 450 rpm in an argon filled stainless steel vessel with a ball to powder weight ratio of 25:1. The purity of all the initial reagents was 99%. The structures and surface patterns of the alloys were characterized by powder X-ray diffraction (XRD) (Rigaku D/Max2500, CuK radiation, graphite monochromator) and scanning electron microscopy (Hitachi X-650), respectively. Electrodes for tests were prepared as follow: 0.8 g of the mixture of as-prepared powder and nickel powder (mass ration 1:3) were pressed into pellet (10 mm in diameter) at 30 MPa.
0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.11.034
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Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706
700 600 500 400 300 200 100 0
Ni
Intensity [cps]
0 700 600 500 400 300 200 100 0
10
700 600 500 400 300 200 100 0
Ni
MgNi
20
Ni
30
40
50
60
70
80
Ni
700 600 500 400 300 200 100 0
20
40
60
40
60
80
20
40
60
80
40
60
80
60
80
Mg0.9Al0.08Ce0.02Ni
Ni
0 700 600 500 400 300 200 100 0
Ni
20
0
90
80
Mg0.9Al0.075Ce0.025Ni
0
Ni
700 600 500 400 300 200 100 0
Mg0.9Al0.09Ce0.01Ni
0
Mg0.9Al0.1Ni
20
Mg0.9Al0.025Ce0.075Ni
Ni
0
20
40
2 Theta [deg.] Fig. 1. XRD patterns of MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) MA 100 h.
A sandwich of the pellet between two foam nickel disks (20 mm in diameter) was pressed at 20 MPa, on which a nickel strip was soldered [6]. Electrochemical tests employed a three-electrode system, asprepared electrode as working electrode, NiOOH/Ni(OH)2 as counter electrode, Hg/HgO as reference electrode and 5mol · dm−3 KOH solution as electrolyte. Charge–discharge cycle tests were performed using an automatic battery-testing instrument controlled by a computer. Test sequence was that charge at 100 mA g−1 for 6 h, discharge at 25 mA g−1 to –0.6 V vs. HgO/Hg, rest for 10 min between charge and discharge. CHI 660b electrochemical workstation was used for Tafel polarization (scan rate: 1 mV s−1 , −1.2 to −0.2 V vs. HgO/Hg), cyclic voltammograms (CV) (scan rate: 1, 5, 10 mV s−1 , −1.2.0.2 V vs. HgO/Hg) and electrochemical impedance spectroscopy (EIS) (open circuit potential, amplitude 5 mV, 104 .10−1 Hz) measurements. All the experiments were conducted at room temperature. 3. Result and discussion 3.1. Phase structure To investigate the microstructure changes of the ball milled Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) and MgNi alloy, X-ray diffractometry of these alloys was carried out. As seen in Fig. 1, only broad peaks appear in the region of 40.45◦ , which suggests the main phase of an
amorphous structure. There are merely some weak Ni peaks co-existed. Fig. 2 shows XRD patterns of Mg0.9 Al0.08 Ce0.02 Ni alloy ball milled for different hours (40, 60 and 100 h). As seen, with the increase of ball-milling time, Ni peaks become weaker, and the main peak becomes broader and lower. After MA 100 h, no Ni peak can be found, which indicates that Ni atoms almost penetrate into the MgNi host lattice. MA 100 h Mg0.9 Al0.08 Ce0.02 Ni alloy exhibits a diffused peak at around 40.45 ◦ , which suggests that the main phase of an amorphous structure. 3.2. Discharge capacity and cycle stability Fig. 3 shows the discharge capacity at varying with cycle number for MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025) alloy electrodes. The discharge capacity of MgNi electrode decreased rapidly with the increase in cycle number. Information on the cycle stability of the MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025) alloy electrodes is given in Table 1. From Table 1, it shows that alloy Mg0.9 Al0.08 Ce0.02 Ni electrode exhibits the best cycle stability which dictates Al and Ce doping has good effect on the cycle life of Mg-based alloy electrodes. The cycle stability of Mg0.9 Al0.075 Ce0.025 was lower than that of Mg0.9 Al0.08 Ce0.02 , which indicates the mass ratio of Ce substitution in MgNi-type alloy electrode should not over 0.025. This probably because that the more content Ce is easy to be oxidized by air in the process of electrode execution.
Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 500 400 300 200 100 0
100h
0 Intensity [cps]
1703
10
20
30
40
50
70
60
80
90
70
60
80
90
80
90
Ni
600 400 80h
200
Ni
0 0
10
20
30
40
1500
50
Ni
1000 500
Ni
40h
Ni
0 0
10
20
30
40
50
70
60
2 Theta [deg.]
Fig. 2. The XRD pattern of Mg0.9 Al0.08 Ce0.02 Ni alloy MA 40, 80, 100 h.
450 400 Discharge capacity/mAhg-1
Fig. 4 shows the discharge capacity of Mg0.9 Al0.08 Ce0.02 Ni alloy electrode at 40, 60, 80 and 100 h ball-milling time. As seen from Fig. 4, with the ball-milling time from 40 to 80 h, the discharge capacity was increased. This is mainly because with the ball-milling time increase, the alloy particle size became smaller and smaller. This small size particle shows the good effect in electrochemical absord–desorded hydrogen in alkaline solution. At ball-milling 100 h, the first cycle discharge capacity was decrease but it has the best cycle stability. When ball-milling time reached 120 h, both the discharge capacity and cycle stability were decreased. Therefore, MA 80 h is the perfect time for the alloy exhibiting good discharge capacity and cycle stability.
Charge:100mAg-1,6h MgNi Mg0.9Al0.1Ni
Discharge:25mAg-1
350
Mg0.9Al0.09Ce0.01Ni
300
Mg0.9Al0.08Ce0.02Ni Mg0.9Al0.075Ce0.025Ni
250
Mg0.9Al0.025Ce0.075Ni
200 150 100 50 0 -5
0
5
10
15
20
25
30
35
40
45
50
55
3.3. Cyclic voltammograms test
Cycle Number
Fig. 3. The discharge capacity of MgNi and Mg0.9 Al1−x Cex (x =0,0.01,0.02, 0.025,0.075) alloys at MA 100 h.
To investigate the phenomena describe above in detail, the electrochemical behavior of each sample was measured.
Table 1 Cycle stability of the MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) alloy electrodes Alloy
1 cycle (mA h g−1 )
15 cycle (mA h g−1 )
30 cycle (mA h g−1 )
50 cycle (mA h g−1 )
C15th /C1st (%)
C30th /C1st (%)
C50th /C1st
MgNi Mg0.9 A10.1 Ni Mg0.9 Al0.09 Ce0.01 Ni Mg0.9 Al0.08 Ce0.02 Ni Mg0.9 Al0.075 Ce0.025 Ni Mg0.9 Al0.025 Ce0.075 Ni
432 401.9 435.9 435.1 398.9 322
155.2 271 224.1 234.6 169.2 119.1
96 110 148.7 151.4 121.3 93.7
74.4 — 120.8 124 — —
35.93 67.93 51.41 53.92 42.42 36.99
22.22 27.37 34.11 34.80 30.41 29.10
17.22 — 27.71% 28.50% — —
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Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 0.2
450 Mg0.9Al0.08Ce0.02Ni
Mg0.9Al0.08Ce0.02Ni
40h 60h 80h 100h 120h
350 300
0.1 scan: 2 cycle 0.0 Current/A
Discharge Capacity/mAhg-1
400
250 200
-0.1 1mv/s 5mv/s 10mv/s
-0.2
150 100
-0.3
50 0
5
10
15
20 25 30 35 Cycle Number
40
45
50
55
-0.4 -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
Potential/V
Fig. 4. The discharge capacity of Mg0.9 Al0.08 Ce0.02 Ni alloy at different milling time (40 h,60 h,80 h and 100 h).
Fig. 6. CV graphs of the Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes at scan speed 1, 5, 10 mV s−1 .
0.3 0.2
Current / A
0.1 0.0 -0.1
MgNi Mg0.9Al0.1Ni
-0.2
Mg0.9Al0.08Ce0.02Ni scan:1mv/s,2 cycles
-0.3 -0.4 -1.2
-1.0
-0.8 -0.6 Potential / V
-0.4
-0.2
Fig. 5. CV graphs of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes at scan speed 1 mV s−1 , two cycles.
Fig. 5 shows the CV of the 100 h MA MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes at 1, 5, 10 mV s−1 scan speed in 5 M KOH solution. The potential interval was −1.2 to −0.2 V vs. HgO/Hg. The anodic peak current at around −600 mv vs. Hg/HgO to −700 mv vs. Hg/HgO was attributed to the oxidation of hydrogen absorbed in the alloy according to the equation [7]: MHn + nOH- → M + nH2 O + ne- . This anodic peak current might be used for evaluating the electrocatalytic activity of hydride electrode for hydrogen oxidation [8]. Fig. 5 indicates the Al and Ce substitutions in MgNitype alloy give a remarkable rise of anodic peak current of the hydrogen oxidation. This suggests that the electrocatalytic activity of MgNi-based alloy electrode for hydrogen oxidation enhanced by some amount of Al and Ce substitutions.
The potential at the rising of the cathodic current of Mg0.9 Al0.08 Ce0.02 Ni attributed to hydrogen absorption shifted at 10.30 mV to the anodic direction compared to MgNi alloy electrode. The respective coulombic charges estimated from each peak area is Mg0.9 Al0.08 Ce0.02 Ni > MgNi > Mg0.9 Al0.1 Ni, agreeing fairly well with the difference between discharge capacities of the samples obtained at the first cycle. These results imply that the amorphous Mg0.9 Al0.08 Ce0.02 Ni has a good electron transfer rate of the hydrogen absorption/desorption reaction, resulting in the increase in the discharge capacity at the first cycle. Fig. 6 is CV graphs of the Mg0.9 Al0.08 Ce0.02 Ni alloy electrode at scan speed 1, 5 and 10 mV s−1 . It can be seen that in Mg0.9 Al0.08 Ce0.02 Ni alloy electrode, the CV shape of each scan is not repeatable, which indicated the alloy has irreversible property in electrochemical behavior test. With the scan speed increased, anodic peak potential moved a little to the positive side which is the property of an irreversible electrode. This is because the effect of contacted resistance and concentrate polarization, the alloy over-potential elevated that lead to anodic peak potential moved to positive side. This reason also can explain why the discharge capacity was decreased after several cycles. 3.4. Corrosion of the alloys There are many investigations about the corrosion behavior of Mg-based alloys, because the corrosion of Mg-based alloys is a barrier to its practical use. In order to find a new method for improvement of the anti-corrosion behavior of the alloys, potentiodynamic polarization has been employed to investigate the corrosion behavior of Mg-based alloys. The potentiodynamic polarization of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni is shown in Fig. 7. The result obtained by Tafel fitting [9] is summarized in Table 2. The result reveals that in the Mg alloy, its values of Ecorr are higher than for
Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 1
-5
0.1
-4
1705
MgNi Mg0.9Al0.1Ni Mg0.9Al0.08Ce0.02Ni
-3
1E-3
Z''/Ω
Current/A
0.01
1E-4
MgNi Mg0.9Al0.1Ni
1E-5
Mg0.9Al0.08Ce0.02Ni
-2
-1
0
1E-6 -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
1 0.5
Potential/V
1.0
1.5
2.0
Z'/Ω
Fig. 7. Potentiodynamic polarization curves of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes.
Fig. 8. EIS plot for composite all MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes.
Rs Table 2 Tafel fitting data of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes Alloy
Ecor (V)
Icorr (A)
MgNi Mg0.9 Al0.01 Ni Mg0.9 Al0.08 Ce0.02 Ni
−0.9328 −0.9089 −0.8280
4.6943E-4 1.2983E-4 3.636E-6
R1
W1
CPE1
Fig. 9. The EIS equivalent circuit of MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes Rs : solution resistance; R1 : electrochemical reaction; CPE1: constant phase element in electric circuit; W1: Warburg resistance.
Icorr : corrosion current density; Ecorr : corrosion potential.
Table 3 The EIS simulated electrochemical parameters of MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes
MgNi, and the values of corrosion rate (Icorr ) are lower than for MgNi. These results suggest that the amount of substitutions of the Al and Ce improves the anti-corrosion behavior of MgNi alloy.
Alloy
Rs ( cm2 )
R1 ( cm2 )
MgNi Mg0.9 Al0.1 Ni Mg0.9 Al0.08 Ce0.02 Ni
0.422 0.573 0.821
1.374 0.701 0.336
Rs : solution resistance; R1 : electrochemical reaction.
3.5. Electrochemical impedance spectroscopy (EIS) analysis 4. Conclusion Fig. 8 shows the EIS Nyquist diagrams for the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni electrode alloy at 50% DOD at the first cycle. Fig. 9 is the responding equivalent circuit. It is clear that each curve consists of one semicircle and a linear Warburg part [10]. The transfer of the electric charge on the interface of alloy/electrolyte and the diffusion of hydrogen atoms in the host lattice control the electrochemical reaction of alloy electrode. Based on the circuit, the solution resistance and electrochemical reaction values were obtained by means of the non-linear least square (NLLS) fitting program in Zplot software set. The results are shown in Table 3. The result shows that the values of electrochemical reaction resistances R1 of the Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy were lower than MgNi. That suggests that amount of Al and Ce substitutions improves the electrochemical catalytic activity on the alloy surface significantly.
The Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) was prepared successfully by MA. XRD analysis refer these ball-milling 100 h alloys have amorphous structures. Mg0.9 Al0.08 Ce0.02 Ni alloy has the best cycle stability and the Ce added content should not over 0.025 (alloy element mass ratio) in order to obtain good cycle stability. The potentiodynamic polarization result shows that Ecorr value of the Mg0.9 Al0.08 Ce0.02 Ni alloy was higher than that of MgNi, and the corrosion rate (Icorr ) was lower than MgNi. These results suggest that amount of Al and Ce substitutions improves the anti-corrosion behavior of MgNi. The EIS test result shows that electrochemical reaction (R1 ) of the Mg0.9 Al0.08 Ce0.02 Ni alloy was lower than MgNi. That suggests that amount of Al and Ce substitutions improves significantly the electrochemical catalytic activity on the alloy surface.
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Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706
Acknowledgments The Project supported by the Natural-science Fund (Nos. 50571046, 50631020 and 20573058) of China. References [1] Yuan HT, Li QD, Song HN, Wang YJ. et al. J Alloys Compd 2003;353:322. [2] Han SC, Lee PS, Lee JY, Züttel A. et al. J Alloys Compd 2000;306:219.
[3] Zhang Y, Zhang SK, Chen LX. et al. Int J Hydrogen Energy 2001;26:801. [4] Lee HY, Goo NH, Jeong WT. et al. J Alloys Compd 2000;313:258. [5] Liu JW, Yuan HT, Cao JS. et al. J Alloys Compd 2005;392:300. [6] Pan HG, Jin QW, Gao MX. et al. J Alloys Compd 2004;376:196. [7] Yang HB, Yang ED, Gao F. et al. J Electrochem Soc 2005;149:A543. [8] Luo JL, Cui N. J Alloys Compd 1998;264:299. [9] Liu W, Lei YQ, Sun D. et al. J Power Sources 1996;58:243. [10] Liu W, Wu H, Lei YQ. et al. J Alloys Compd 2002;346:244.
Effect of Al and Ce substitutions of the electrochemical properties of amorphous MgNi-based alloy electrodes Yan Feng, Lifang Jiao, Huatang Yuan ∗ , Ming Zhao Institute of New Energy Material Chemistry, Nankai University, Tianjin, PR China Received 30 March 2006; received in revised form 24 November 2006 Available online 16 January 2007
Abstract Mg-based hydrogen storage alloys Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) were successfully prepared by means of mechanical alloying (MA). The structure and the electrochemical characteristics of these Mg-based electrodes were also studied. The result of X-ray diffraction (XRD) shows that the main phases of the alloys exhibit amorphous structures. The charge–discharge cycle tests indicate these alloys have good electrochemical active characteristics. Among these alloys, the Mg0.9 Al0.08 Ce0.02 Ni has the best cycle stability. After 50 cycles charge–discharge, the discharge capacity of Mg0.9 Al0.08 Ce0.02 Ni alloy is 66.67% higher than MgNi alloy. The cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS) and anticorruption test (potentiodynamic polarization) were also studied, and the results show that the electrochemical cycle stability of these alloys was improved by Al and Ce substitutions. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Mg-based hydrogen storage; Electrochemical property; Cycle stability; Al and Ce substitutions
1. Introduction Mg-based hydrogen storage alloys are attractive materials for energy conversion and hydrogen storage because of their lighter weight, higher hydrogen storage capacity and lower cost compared with other series of hydrogen storage materials [1]. Despite of its merits, practical applications of these materials as secondary battery have been hampered by the following major factor: easy to be corrupted in alkaline solutions, which cause their poor charge–discharge cycle stability. Many efforts have been made on their electrochemical characteristics. Element substitution has been found to be the most effective way to retard the corrosion and to improve the cycle capacity degradation [2–4]. Liu et al. reported that Ti and Al substitute in MgNi-type alloy electrode is an effective way to improve the cycle stability in alkaline solution [5]. Ce is also a good substitute in AB5 -type and AB3 -type alloy electrode to improve the cycle life [6,7].
∗ Corresponding author. Tel.: +86 22 23504527; fax: +86 22 23502604.
E-mail address: [email protected] (H. Yuan).
In this work, Al and Ce substituted Mg in MgNi-type alloys. Amorphous quaternary Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) alloy was prepared by mechanical alloying (MA). The structural and electrochemical properties were characterized. 2. Experimental MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) alloys were each prepared from mixed powder of pure Mg, Al, Ce and Ni by MA. The mixture was then MA for several hours at a speed of 450 rpm in an argon filled stainless steel vessel with a ball to powder weight ratio of 25:1. The purity of all the initial reagents was 99%. The structures and surface patterns of the alloys were characterized by powder X-ray diffraction (XRD) (Rigaku D/Max2500, CuK radiation, graphite monochromator) and scanning electron microscopy (Hitachi X-650), respectively. Electrodes for tests were prepared as follow: 0.8 g of the mixture of as-prepared powder and nickel powder (mass ration 1:3) were pressed into pellet (10 mm in diameter) at 30 MPa.
0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.11.034
1702
Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706
700 600 500 400 300 200 100 0
Ni
Intensity [cps]
0 700 600 500 400 300 200 100 0
10
700 600 500 400 300 200 100 0
Ni
MgNi
20
Ni
30
40
50
60
70
80
Ni
700 600 500 400 300 200 100 0
20
40
60
40
60
80
20
40
60
80
40
60
80
60
80
Mg0.9Al0.08Ce0.02Ni
Ni
0 700 600 500 400 300 200 100 0
Ni
20
0
90
80
Mg0.9Al0.075Ce0.025Ni
0
Ni
700 600 500 400 300 200 100 0
Mg0.9Al0.09Ce0.01Ni
0
Mg0.9Al0.1Ni
20
Mg0.9Al0.025Ce0.075Ni
Ni
0
20
40
2 Theta [deg.] Fig. 1. XRD patterns of MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) MA 100 h.
A sandwich of the pellet between two foam nickel disks (20 mm in diameter) was pressed at 20 MPa, on which a nickel strip was soldered [6]. Electrochemical tests employed a three-electrode system, asprepared electrode as working electrode, NiOOH/Ni(OH)2 as counter electrode, Hg/HgO as reference electrode and 5mol · dm−3 KOH solution as electrolyte. Charge–discharge cycle tests were performed using an automatic battery-testing instrument controlled by a computer. Test sequence was that charge at 100 mA g−1 for 6 h, discharge at 25 mA g−1 to –0.6 V vs. HgO/Hg, rest for 10 min between charge and discharge. CHI 660b electrochemical workstation was used for Tafel polarization (scan rate: 1 mV s−1 , −1.2 to −0.2 V vs. HgO/Hg), cyclic voltammograms (CV) (scan rate: 1, 5, 10 mV s−1 , −1.2.0.2 V vs. HgO/Hg) and electrochemical impedance spectroscopy (EIS) (open circuit potential, amplitude 5 mV, 104 .10−1 Hz) measurements. All the experiments were conducted at room temperature. 3. Result and discussion 3.1. Phase structure To investigate the microstructure changes of the ball milled Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) and MgNi alloy, X-ray diffractometry of these alloys was carried out. As seen in Fig. 1, only broad peaks appear in the region of 40.45◦ , which suggests the main phase of an
amorphous structure. There are merely some weak Ni peaks co-existed. Fig. 2 shows XRD patterns of Mg0.9 Al0.08 Ce0.02 Ni alloy ball milled for different hours (40, 60 and 100 h). As seen, with the increase of ball-milling time, Ni peaks become weaker, and the main peak becomes broader and lower. After MA 100 h, no Ni peak can be found, which indicates that Ni atoms almost penetrate into the MgNi host lattice. MA 100 h Mg0.9 Al0.08 Ce0.02 Ni alloy exhibits a diffused peak at around 40.45 ◦ , which suggests that the main phase of an amorphous structure. 3.2. Discharge capacity and cycle stability Fig. 3 shows the discharge capacity at varying with cycle number for MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025) alloy electrodes. The discharge capacity of MgNi electrode decreased rapidly with the increase in cycle number. Information on the cycle stability of the MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025) alloy electrodes is given in Table 1. From Table 1, it shows that alloy Mg0.9 Al0.08 Ce0.02 Ni electrode exhibits the best cycle stability which dictates Al and Ce doping has good effect on the cycle life of Mg-based alloy electrodes. The cycle stability of Mg0.9 Al0.075 Ce0.025 was lower than that of Mg0.9 Al0.08 Ce0.02 , which indicates the mass ratio of Ce substitution in MgNi-type alloy electrode should not over 0.025. This probably because that the more content Ce is easy to be oxidized by air in the process of electrode execution.
Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 500 400 300 200 100 0
100h
0 Intensity [cps]
1703
10
20
30
40
50
70
60
80
90
70
60
80
90
80
90
Ni
600 400 80h
200
Ni
0 0
10
20
30
40
1500
50
Ni
1000 500
Ni
40h
Ni
0 0
10
20
30
40
50
70
60
2 Theta [deg.]
Fig. 2. The XRD pattern of Mg0.9 Al0.08 Ce0.02 Ni alloy MA 40, 80, 100 h.
450 400 Discharge capacity/mAhg-1
Fig. 4 shows the discharge capacity of Mg0.9 Al0.08 Ce0.02 Ni alloy electrode at 40, 60, 80 and 100 h ball-milling time. As seen from Fig. 4, with the ball-milling time from 40 to 80 h, the discharge capacity was increased. This is mainly because with the ball-milling time increase, the alloy particle size became smaller and smaller. This small size particle shows the good effect in electrochemical absord–desorded hydrogen in alkaline solution. At ball-milling 100 h, the first cycle discharge capacity was decrease but it has the best cycle stability. When ball-milling time reached 120 h, both the discharge capacity and cycle stability were decreased. Therefore, MA 80 h is the perfect time for the alloy exhibiting good discharge capacity and cycle stability.
Charge:100mAg-1,6h MgNi Mg0.9Al0.1Ni
Discharge:25mAg-1
350
Mg0.9Al0.09Ce0.01Ni
300
Mg0.9Al0.08Ce0.02Ni Mg0.9Al0.075Ce0.025Ni
250
Mg0.9Al0.025Ce0.075Ni
200 150 100 50 0 -5
0
5
10
15
20
25
30
35
40
45
50
55
3.3. Cyclic voltammograms test
Cycle Number
Fig. 3. The discharge capacity of MgNi and Mg0.9 Al1−x Cex (x =0,0.01,0.02, 0.025,0.075) alloys at MA 100 h.
To investigate the phenomena describe above in detail, the electrochemical behavior of each sample was measured.
Table 1 Cycle stability of the MgNi and Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) alloy electrodes Alloy
1 cycle (mA h g−1 )
15 cycle (mA h g−1 )
30 cycle (mA h g−1 )
50 cycle (mA h g−1 )
C15th /C1st (%)
C30th /C1st (%)
C50th /C1st
MgNi Mg0.9 A10.1 Ni Mg0.9 Al0.09 Ce0.01 Ni Mg0.9 Al0.08 Ce0.02 Ni Mg0.9 Al0.075 Ce0.025 Ni Mg0.9 Al0.025 Ce0.075 Ni
432 401.9 435.9 435.1 398.9 322
155.2 271 224.1 234.6 169.2 119.1
96 110 148.7 151.4 121.3 93.7
74.4 — 120.8 124 — —
35.93 67.93 51.41 53.92 42.42 36.99
22.22 27.37 34.11 34.80 30.41 29.10
17.22 — 27.71% 28.50% — —
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Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 0.2
450 Mg0.9Al0.08Ce0.02Ni
Mg0.9Al0.08Ce0.02Ni
40h 60h 80h 100h 120h
350 300
0.1 scan: 2 cycle 0.0 Current/A
Discharge Capacity/mAhg-1
400
250 200
-0.1 1mv/s 5mv/s 10mv/s
-0.2
150 100
-0.3
50 0
5
10
15
20 25 30 35 Cycle Number
40
45
50
55
-0.4 -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
Potential/V
Fig. 4. The discharge capacity of Mg0.9 Al0.08 Ce0.02 Ni alloy at different milling time (40 h,60 h,80 h and 100 h).
Fig. 6. CV graphs of the Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes at scan speed 1, 5, 10 mV s−1 .
0.3 0.2
Current / A
0.1 0.0 -0.1
MgNi Mg0.9Al0.1Ni
-0.2
Mg0.9Al0.08Ce0.02Ni scan:1mv/s,2 cycles
-0.3 -0.4 -1.2
-1.0
-0.8 -0.6 Potential / V
-0.4
-0.2
Fig. 5. CV graphs of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes at scan speed 1 mV s−1 , two cycles.
Fig. 5 shows the CV of the 100 h MA MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes at 1, 5, 10 mV s−1 scan speed in 5 M KOH solution. The potential interval was −1.2 to −0.2 V vs. HgO/Hg. The anodic peak current at around −600 mv vs. Hg/HgO to −700 mv vs. Hg/HgO was attributed to the oxidation of hydrogen absorbed in the alloy according to the equation [7]: MHn + nOH- → M + nH2 O + ne- . This anodic peak current might be used for evaluating the electrocatalytic activity of hydride electrode for hydrogen oxidation [8]. Fig. 5 indicates the Al and Ce substitutions in MgNitype alloy give a remarkable rise of anodic peak current of the hydrogen oxidation. This suggests that the electrocatalytic activity of MgNi-based alloy electrode for hydrogen oxidation enhanced by some amount of Al and Ce substitutions.
The potential at the rising of the cathodic current of Mg0.9 Al0.08 Ce0.02 Ni attributed to hydrogen absorption shifted at 10.30 mV to the anodic direction compared to MgNi alloy electrode. The respective coulombic charges estimated from each peak area is Mg0.9 Al0.08 Ce0.02 Ni > MgNi > Mg0.9 Al0.1 Ni, agreeing fairly well with the difference between discharge capacities of the samples obtained at the first cycle. These results imply that the amorphous Mg0.9 Al0.08 Ce0.02 Ni has a good electron transfer rate of the hydrogen absorption/desorption reaction, resulting in the increase in the discharge capacity at the first cycle. Fig. 6 is CV graphs of the Mg0.9 Al0.08 Ce0.02 Ni alloy electrode at scan speed 1, 5 and 10 mV s−1 . It can be seen that in Mg0.9 Al0.08 Ce0.02 Ni alloy electrode, the CV shape of each scan is not repeatable, which indicated the alloy has irreversible property in electrochemical behavior test. With the scan speed increased, anodic peak potential moved a little to the positive side which is the property of an irreversible electrode. This is because the effect of contacted resistance and concentrate polarization, the alloy over-potential elevated that lead to anodic peak potential moved to positive side. This reason also can explain why the discharge capacity was decreased after several cycles. 3.4. Corrosion of the alloys There are many investigations about the corrosion behavior of Mg-based alloys, because the corrosion of Mg-based alloys is a barrier to its practical use. In order to find a new method for improvement of the anti-corrosion behavior of the alloys, potentiodynamic polarization has been employed to investigate the corrosion behavior of Mg-based alloys. The potentiodynamic polarization of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni is shown in Fig. 7. The result obtained by Tafel fitting [9] is summarized in Table 2. The result reveals that in the Mg alloy, its values of Ecorr are higher than for
Y. Feng et al. / International Journal of Hydrogen Energy 32 (2007) 1701 – 1706 1
-5
0.1
-4
1705
MgNi Mg0.9Al0.1Ni Mg0.9Al0.08Ce0.02Ni
-3
1E-3
Z''/Ω
Current/A
0.01
1E-4
MgNi Mg0.9Al0.1Ni
1E-5
Mg0.9Al0.08Ce0.02Ni
-2
-1
0
1E-6 -1.2
-1.0
-0.8
-0.6
-0.4
-0.2
1 0.5
Potential/V
1.0
1.5
2.0
Z'/Ω
Fig. 7. Potentiodynamic polarization curves of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes.
Fig. 8. EIS plot for composite all MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes.
Rs Table 2 Tafel fitting data of the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes Alloy
Ecor (V)
Icorr (A)
MgNi Mg0.9 Al0.01 Ni Mg0.9 Al0.08 Ce0.02 Ni
−0.9328 −0.9089 −0.8280
4.6943E-4 1.2983E-4 3.636E-6
R1
W1
CPE1
Fig. 9. The EIS equivalent circuit of MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes Rs : solution resistance; R1 : electrochemical reaction; CPE1: constant phase element in electric circuit; W1: Warburg resistance.
Icorr : corrosion current density; Ecorr : corrosion potential.
Table 3 The EIS simulated electrochemical parameters of MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy electrodes
MgNi, and the values of corrosion rate (Icorr ) are lower than for MgNi. These results suggest that the amount of substitutions of the Al and Ce improves the anti-corrosion behavior of MgNi alloy.
Alloy
Rs ( cm2 )
R1 ( cm2 )
MgNi Mg0.9 Al0.1 Ni Mg0.9 Al0.08 Ce0.02 Ni
0.422 0.573 0.821
1.374 0.701 0.336
Rs : solution resistance; R1 : electrochemical reaction.
3.5. Electrochemical impedance spectroscopy (EIS) analysis 4. Conclusion Fig. 8 shows the EIS Nyquist diagrams for the MgNi, Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni electrode alloy at 50% DOD at the first cycle. Fig. 9 is the responding equivalent circuit. It is clear that each curve consists of one semicircle and a linear Warburg part [10]. The transfer of the electric charge on the interface of alloy/electrolyte and the diffusion of hydrogen atoms in the host lattice control the electrochemical reaction of alloy electrode. Based on the circuit, the solution resistance and electrochemical reaction values were obtained by means of the non-linear least square (NLLS) fitting program in Zplot software set. The results are shown in Table 3. The result shows that the values of electrochemical reaction resistances R1 of the Mg0.9 Al0.1 Ni and Mg0.9 Al0.08 Ce0.02 Ni alloy were lower than MgNi. That suggests that amount of Al and Ce substitutions improves the electrochemical catalytic activity on the alloy surface significantly.
The Mg0.9 Al0.1−x Cex Ni (x = 0.00, 0.01, 0.02, 0.025, 0.075) was prepared successfully by MA. XRD analysis refer these ball-milling 100 h alloys have amorphous structures. Mg0.9 Al0.08 Ce0.02 Ni alloy has the best cycle stability and the Ce added content should not over 0.025 (alloy element mass ratio) in order to obtain good cycle stability. The potentiodynamic polarization result shows that Ecorr value of the Mg0.9 Al0.08 Ce0.02 Ni alloy was higher than that of MgNi, and the corrosion rate (Icorr ) was lower than MgNi. These results suggest that amount of Al and Ce substitutions improves the anti-corrosion behavior of MgNi. The EIS test result shows that electrochemical reaction (R1 ) of the Mg0.9 Al0.08 Ce0.02 Ni alloy was lower than MgNi. That suggests that amount of Al and Ce substitutions improves significantly the electrochemical catalytic activity on the alloy surface.
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Acknowledgments The Project supported by the Natural-science Fund (Nos. 50571046, 50631020 and 20573058) of China. References [1] Yuan HT, Li QD, Song HN, Wang YJ. et al. J Alloys Compd 2003;353:322. [2] Han SC, Lee PS, Lee JY, Züttel A. et al. J Alloys Compd 2000;306:219.
[3] Zhang Y, Zhang SK, Chen LX. et al. Int J Hydrogen Energy 2001;26:801. [4] Lee HY, Goo NH, Jeong WT. et al. J Alloys Compd 2000;313:258. [5] Liu JW, Yuan HT, Cao JS. et al. J Alloys Compd 2005;392:300. [6] Pan HG, Jin QW, Gao MX. et al. J Alloys Compd 2004;376:196. [7] Yang HB, Yang ED, Gao F. et al. J Electrochem Soc 2005;149:A543. [8] Luo JL, Cui N. J Alloys Compd 1998;264:299. [9] Liu W, Lei YQ, Sun D. et al. J Power Sources 1996;58:243. [10] Liu W, Wu H, Lei YQ. et al. J Alloys Compd 2002;346:244.