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The reduction of cycling capacity degradation of Mg–Ni-based electrode alloys by Fe substitution
International Journal of Hydrogen Energy 27 (2002) 501 – 506
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The reduction of cycling capacity degradation of Mg–Ni-based electrode alloys by Fe substitution Yao Zhang ∗ , Li-Xin Chen, Yong-Quan Lei, Qi-Dong Wang Department of Materials Science and Engineering, Zhejiang University, Jade Spring, Hangzhou 310027, People’s Republic of China
Abstract A new approach to reduce the cycling capacity degradation of Mg–Ni-based alloy through lowering the polarization resistance by means of element substitution is suggested in this study. A new Mg-based amorphous alloy Mg45 Fe5 Ni50 was prepared by means of mechanical alloying. The cycling stability of this alloy which is found superior to that of Mg50 Ni50 and several other A side-substituted Mg-based ternary alloys. A comparative study on the surface layer formed on Mg50 Ni50 and Mg45 Fe5 Ni50 during cycles was made by the X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). XPS analysis demonstrates that, the degree of oxidation of the two alloys are almost the same. Mg on the surface of both alloys transforms into Mg(OH)2 passive ;lm and the Mg (2P) binding energy peaks of both alloys before and after di=erent cycles are also similar. The only cause for the di=erence in cycling stability of the two alloys is the di=erence in polarization resistance RP , which leads to di=erent potential drops in the charge=discharge processes. On the basis of the EIS Nyquist diagrams and corresponding equivalent, the RP values of di=erent cycles of both alloys were calculated through non-linear least-squares ;tting. For Mg45 Fe5 Ni50 , the polarization resistance is lower. The metallic Fe was believed to be bene;cial for suppressing the polarization resistance RP and elevating the cycling discharge plateau, thus improving the cycling discharge-ability. This phenomenon may be useful in preparing more durable Mg–Ni-based electrode alloys. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Cycling capacity degradation; Mg–Ni-based electrode alloy; Polarization resistance; Fe substitution
1. Introduction Mechanically alloyed (MA) Mg–Ni-based amorphous hydrogen storage electrode alloys have been extensively studied recently because of their high discharge capacity (discharge capacity of Mg50 Ni50 reaching 500 mA h=g), low cost and good initial activation property in electrochemical processes [1–3]. However, its poor cycling stability discourages its further development and hampers its immediate application. Until now, the method of partial substitution of A side element (Mg) and=or B side element (Ni) in Mg– Ni-based alloys has been widely adopted for improving their electrochemical performances in the 6M alkaline electrolyte ∗ Corresponding author. Tel.: +86-571-8795-2615; fax: +86-571-8795-1152. E-mail address: [email protected] (Y. Zhang).
[4 –11]. The main cause for cycling capacity degradation is believed to be due to the loss of Mg in KOH solution and the formation of a permeable Mg(OH)2 passive ;lm. Yet, the present authors believe that there exist other factors such as the polarization resistance and the surface activation which also a=ect the cycling capacity degradation. A comparative study between Mg50 Ni50 and Mg45 Fe5 Ni50 is presently studied. According to Liu’s investigation [4], Mg50 Ni45 Fe5 possesses the lowest cycling degradation among the fourteen B side-substituted Mg50 Ni45 M5 ternary alloys (M = Fe, W, Cu, C, Mn, Cr, Zr, Ti, Zn, Se, Sb, Co, Si and Al). Fe was used by us to substitute Mg in Mg50 Ni50 partially to form a new alloy Mg45 Fe5 Ni50 , considering that A side substitution might possess higher cycling stability than B side substitution. Through the comparisons of the electrochemical performance with Mg50 Ni50 and Mg50 Ni45 Fe5 alloys and the
0360-3199/02/$ 20.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 1 8 2 - 3
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Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
surface status with Mg50 Ni50 , the e=ect of surface polarization resistance on cycling discharge capacity degradation was noticed and studied. 2. Experimental details MA technique was used to prepare the amorphous Mg50 Ni50 and Mg45 Fe5 Ni50 alloys. 99.9% pure metal powders were sealed each time in a 100 ml planetary mill vial according to the stoichiometric ratio of the sample required. The vial was ;rst evacuated then ;lled with high purity argon and set to mill for 120 h. In the vial, the weight ratio of ball to powder was 30:1. The test electrode was fabricated according to the following procedure: 100 mg of alloy powders prepared were ;rst thoroughly mixed with 200 mg ;ne copper powder (below 200 mesh). The mixtures were then pressed at 20 MPa into pellets of 10 mm diameter. Electrochemical measurements were performed in a three-electrode system, in which the alloy electrode pellet was separated from the counter electrode by a porous polypropylene ;lm. Ni(OH)2 =NiOOH and Hg=HgO were used as the counter electrode and the reference electrode, respectively. The electrolyte was a 6M KOH aqueous solution. Measurements were executed by an automatic galvanostatic system (DC-5). For each cycle, charging was conducted at the current density of 300 mA=g for 3 h and discharging at 100 mA=g, with a cut-o= voltage set at −0:6 V (vs. Hg=HgO). After the ;rst and 10th cycle of charge and discharge, one pellet was taken out separately, crunched into ;ne powder, washed with distilled water for 3 times and then dried in vacuum for X-ray photoelectron spectroscopy (XPS) analysis. XPS measurements were carried out on a VG ESCALAB MKII spectrometer using Al K radiation (1486:6 eV). The operating pressure was below 5 × 10−5 mbar. The electrochemical impedance spectroscopy (EIS) analysis was executed on EG& G potentiostat=galvanostat Model 273A frequency response analyzer. The frequency range was from 120 kHz to 5 mHz. The Nyquist diagrams were plotted at the steady-state of the 50% depth of discharge (DOD) at the 1st and 10th cycles.
Fig. 1. The XRD patterns of Mg45 Fe5 Ni50 and Mg50 Ni50 alloys after 120 h ball milling.
Fig. 2. The discharge potential curves of amorphous Mg45 Fe5 Ni50 and Mg50 Ni50 after the ;rst cycle.
3. Results and discussion 3.1. Discharge performance The ball-milled Mg45 Fe5 Ni50 alloy powder was in amorphous state as evidenced by its XRD pattern (Fig. 1). Fig. 2 shows that the discharge potential curve of the amorphous Mg45 Fe5 Ni50 possesses a higher discharge potential plateau than the amorphous Mg50 Ni50 and a lower initial discharge capacity (dropping from 463 to 313 mA h=g). From Fig. 3, it can be seen that its capacity retention rate after 20 cycles, denoted by (C20 =C1 ) × 100% (C20 stands for the
Fig. 3. The cycle discharge ability of the ternary Mg–Fe–Ni and Mg50 Ni50 electrodes.
discharge capacity after the 20th cycle, and C1 for that after the 1st cycle), increases from 32.3% to 62.3%. This means that the cycle stability is improved signi;cantly. Compared to Liu’s results for Mg50 Ni45 Fe5 , Mg45 Fe5 Ni50 still possesses
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a higher initial capacity and a higher charge–discharge cycle stability as shown in Table 1. From Fig. 3 and Table 1, it is ascertained that Mg-side iron substitution is superior to that of Ni-side substitution in discharge performance. The Fe substitution leads to the best cycle stability in the four Mg-side substitution elements studied here as indicated in Table 2. The same conclusion was also reached in Liu’s study [4]. In our present test, the sequence of the cycling stability of the Mg45 M5 Ni50 (M = Al, Zn, Cu, Fe) ternary electrode alloys and the binary Mg50 Ni50 alloy, from low to high is as follows: Mg45 Al5 Ni50 ¡ Mg45 Zn5 Ni50 ¡ Mg50 Ni50 ¡ Mg45 Cu5 Ni50 ¡ Mg45 Fe5 Ni50 :
(1)
For this reason, we chose Mg50 Ni50 and Mg45 Fe5 Ni50 as the alloys for investigating the other factors for cycling degradation of Mg-based electrode alloys besides the oxidation of Mg. 3.2. Surface analysis Fig. 4 is the XPS wide scan spectra. Fig. 4(a) exhibits the spectra of Mg45 Fe5 Ni50 alloy after di=erent cycles. However, the Fe peaks cannot be found in these spectra, suggesting that iron might have been dissolved in the 6M KOH solution, which agrees well with the pH–E plots [12]. For both alloys, the atomic percentages of Mg, O and Ni content on the surface after di=erent cycles are listed in Tables 3 and 4. From Fig. 4(a), we can see that as the charge–discharge cycling proceeds, all peaks shift from lower binding energies to higher ones. The same tendency is also noticed in Fig. 4(b) for the XPS wide scan spectra of Mg50 Ni50 . Moreover, the surface oxide contribution of Mg50 Ni50 is very close to that of Mg45 Fe5 Ni50 on comparing data in Table 3 with those in Table 4. In Fig. 5(a), the Mg (2p) spectra of Mg45 Fe5 Ni50 before and after di=erent cycles are shown. Before Table 1 Comparison of cycle stability between Mg45 Fe5 Ni50 Mg50 Ni45 Fe5 [4]
Fig. 4. (a) The XPS wide scan spectra of Mg45 Fe5 Ni50 electrode before and after cycles; and (b) the XPS wide scan spectra of Mg50 Ni50 electrode before and after 1 cycle.
Table 3 Atomic percent of Mg, O and Ni of di=erent cycles of Mg45 Fe5 Ni50 electrode
Before cycle After 1 cycle After 10 cycles
Mg (at%)
O (at%)
Ni (at%)
35.3 33.6 28.4
34.6 37.2 43.1
30.1 29.2 28.5
and
Alloys
C1 (mAh=g)
C9 (mAh=g)
C9 =C1 (%)
Mg50 Ni45 Fe5 Mg45 Fe5 Ni50 Mg50 Ni50
273.3 313 463
108 215 150
39.5 68.7 32.2
charge–discharge cycle, the 2p peak is located at 49:6 eV, rather close to the standard binding energy of metallic magnesium (49:75 eV), which means that Mg in alloy is in the metallic form and has not been oxidized. After 1 cycle and 10 cycles, the 2p peak shifts to 49.8 and 50:2 eV, respectively. They move much closer to the binding energy of Mg(OH)2
Table 2 Cycle stability of mechanical alloying Mg45 M5 Ni50 (M = Al, Zn, Cu, Fe)
C1 (mAh=g) C20 (mAh=g) C20 =C1 (%)
Mg45 Al5 Ni50
Mg45 Zn5 Ni50
Mg45 Cu5 Ni50
Mg45 Fe5 Ni50
Mg50 Ni50
420 117 27.9
350 111 31.7
365 180 51
313 195 62.3
463 150 32.2
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Table 4 Atomic percent of Mg, O and Ni of di=erent cycles of Mg50 Ni50 electrode
Before cycle After 1 cycle After 10 cycles
Mg (at%)
O (at%)
Ni (at%)
37.8 35.4 30.6
35.4 38.3 43.7
26.8 26.3 25.7
Fig. 6. (a) The impedance spectra of the Mg45 Fe5 Ni50 electrode at the 50% DOD of the ;rst cycle and the 10th cycle; and (b) the impedance spectra of the Mg50 Ni50 electrode at the 50% DOD of the ;rst cycle and the 10th cycle.
Fig. 5. (a) The Mg(2p) spectra of Mg45 Fe5 Ni50 electrode before and after cycles; and (b) the Mg(2p) spectra of Mg50 Ni50 electrode before and after 1 cycle.
(50:4 eV). This change makes us believe that the Mg(OH)2 passive ;lm grows thicker and thicker over the surface of the Mg-based alloy as the cycle number increases. Table 3 shows that the content of Mg drops from 33:6 at% after 1 cycle to 28:4 at% after 10 cycles. This also con;rms that more Mg(OH)2 are formed on the surface with the augmentation of charge and discharge cycles. From Fig. 5(b), it can be seen that the Mg (2p) peak displacement of Mg50 Ni50 is very close to that of Mg45 Fe5 Ni50 . It demonstrates that the substituting element Fe does not change the degree of oxidation of Mg on the surface of the alloy, and thus does not help to inhibit the corrosion of Mg. From Fig. 5, Tables 3 and 4, it is clear that iron does not protect Mg from oxidation, but leads to a drop in polarization resistance as indicated below.
Fig. 7. The equivalent circuit of Mg45 Fe5 Ni50 electrode at the 50% DOD of the ;rst cycle and the 10th cycle.
3.3. Cycle impedance analysis Fig. 6 shows the impedance spectra of the Mg45 Fe5 Ni50 and Mg50 Ni50 electrodes at the 50% DOD of the ;rst cycle and the 10th cycle, respectively. Each spectrum consists of several semicircles, which overlap one another and are not very distinguishable. By means of the nonlinear least-squares (NLLS) ;tting program EQUIVCT, the equivalent circuit is shown in Fig. 7. For Mg45 Fe5 Ni50 , four components exist in the circuit. They are resistance R1 and three impedance components, (R2 ; Q2 ), (R3 ; Q3 ) and (R4 ; Q4 ) corresponding to semicircles A, B and C in Fig. 6(a), respectively. For Mg50 Ni50 , there are three components that exist in the circuit, namely R1 and two impedance components (R2 ; Q2 ) and (R3 ; Q3 ) corresponding to semicircles 1 and 2, respectively in Fig. 6(b).
Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
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Table 5 R and Q values of Mg45 Fe5 Ni50 electrode’s equivalent circuit
Cycle 1 Cycle 10
R1 (S)
R2 (S)
Q2 (F)
R3 (S)
Q3 (F)
R4 (RP =S)
Q4 (F)
0.826 0.830
0.187 0.215
0.0045 0.0077
2.261 3.137
0.08 0.09
2.25 2.76
0.87 1.23
Table 6 R and Q values of Mg50 Ni50 electrode’s equivalent circuit
Cycle 1 Cycle 10
R1 (S)
R2 (S)
Q2 (F)
R3 (RP =S)
Q3 (F)
1.129 1.096
0.1996 0.1625
0.0082 0.0077
3.68 4.902
0.1434 0.1573
According to Ref. [13], R1 represents the electrolyte resistance between the working electrode and reference electrode for both alloys. Tables 5 and 6 con;rm that it remains constant during cycling. The circuit of the parallelly connected R2 and Q2 , which corresponds to the small semicircle A in Fig. 6(a) and semicircle 1 in Fig. 6(b), is induced by high-frequency signal. The semicircle B in Fig. 6(a) stands for the contact resistance between the current collector and alloy pellets, and the double-layer capacitance on the surface of pellets and the capacitance on the current collectors, respectively. In Table 5, R3 and Q3 are used to represent such resistance and capacitance of Mg45 Fe5 Ni50 , respectively. Also, according to Ref. [13], the components in low-frequency zone of the parallel circuit R4 and Q4 for Mg45 Fe5 Ni50 and R3 and Q3 for Mg50 Ni50 are considered as the polarization resistance for the charge–discharge process and the double-layer capacitance over the external surface of the electrode, respectively. This impedance is a=ected sensitively by the alloy surface conditions, and varies as the charge–discharge process proceeds. From Tables 5 and 6, it can be seen that as the cycling number increases, the reaction resistance and double-layer capacitance for both alloys increase considerably. This could be ascribed to the formation of a passive Mg(OH)2 ;lm which leads to the deterioration of the electrocatalytic activity and increase of polarization resistance. R4 in Table 5 and R3 in Table 6 increase all the time as Mg corrodes due to corrosion current. Comparing the R3 values of Mg50 Ni50 with R4 of Mg45 Fe5 Ni50 electrodes at the same DOD of the same cycle, we can see that the Mg45 Fe5 Ni50 electrode possesses a lower polarization resistance. This means that the Fe substitution for Mg signi;cantly drops the polarization resistance and increases the cycle stability of the Mg50 Ni50 hydrogen storage electrode. The reasons for considering the polarization resistance of an electrode RP as a factor for cycling capacity degradation are: (1) The cycling capacity of any electrode is evaluated at a certain cut-o= voltage, below which the electrochemical capacity of the electrode is not counted;
Fig. 8. The discharge curves of Mg50 Ni50 electrode of cycle 1 and cycle 10.
(2) The actual voltage (VA ) of the electrode is VA =V0 −IRP ; (3) RP increases with the number of cycling. As with the same V0 , the actual voltage of the electrode is lower when the polarization resistance is large. The potential drop due to this resistance RP lowers the discharge plateau when a current Tows through, and causes the discharge process end in advance. This relation was usually disregarded in capacity degradation studies. In Fig. 2, the potential di=erence (between Mg50 Ni50 and Mg45 Fe5 Ni50 at 50% DOD of initial cycle) is 0:015 V. The result agrees very well with the value 0:0143 V obtained by multiplying the URP between the electrodes (1:43 S) with the discharge current density 100 mA=g for an electrode of 100 mg alloy powder at 50% DOD. This makes us believe that the increase of RP reduces the discharge plateau and thus inTuences the potential-capacity characteristic of the Mg– Ni-based alloys as shown in Figs. 8 and 9. For Mg50 Ni50 , URP is larger than that of Mg45 Fe5 Ni50 , this makes the drop of discharge plateaus from cycle 1 to cycle 10 become larger than Mg45 Fe5 Ni50 . Therefore, the discharge deterioration of
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Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
polarization resistance RP and to elevate the discharge plateau of the Mg-based alloy electrodes and thus leads to a slower deterioration of cycling stability. The role of substituted Fe in improving the surface activation and decreasing the di=erence of discharge plateau and RP between di=erent cycles and thus improving the cycling stability is a new approach to combat the poor cycling capacity degradation of Mg–Ni-based alloys and should be studied more thoroughly and systematically later.
Acknowledgements Fig. 9. The discharge curves of Mg45 Fe5 Ni50 electrode of cycle 1 and cycle 10.
the former is much more serious than that of the latter. The above analysis also indicates that the Fe substitution not only elevates the discharge plateau of Mg-based alloys, but also decreases the di=erence of discharge plateau and RP between di=erent cycles. It demonstrates again that the mechanism of inhibiting cycling degradation by additional Fe in Mg-based alloy is to decrease both the value of RP and the di=erence of discharge plateau between di=erent cycles. 4. Conclusions In the present work, we prepared by means of mechanical alloying (MA) the Mg45 Fe5 Ni50 amorphous alloy, whose cycling discharge ability is superior to that of the B-side Fe-substituting ternary alloy Mg50 Ni45 Fe5 and the parent alloy Mg50 Ni50 . The X-ray photoelectron spectrum study (XPS) demonstrates that Mg on the surface of the alloy forms an Mg(OH)2 passive ;lm, and the degree of oxidation of the two alloys are almost the same. This indicates that Fe does not form a strong protective ;lm to inhibit the corrosion of Mg. But from the Nyquist diagrams of Mg45 Fe5 Ni50 and Mg50 Ni50 electrodes at two di=erent cycles by means of the electrochemical impedance spectroscopy (EIS), we found that the substituted Fe does help to decrease the
The authors wish to express their gratitude and appreciation for the support from the National Natural Science Foundation of China (No. 59971047).
References [1] Lei YQ, Wu YM, Yang QM, Wu J, Wang QD. Z Phys Chem 1994;183:379. [2] Nohara S, Inoue H, Fukumoto Y, Iwakura C. J Alloys Compounds 1997;259:183. [3] Iwakura C, Inoue H, Zhang SG, Nohara S. J Alloys Compounds 1998;270:142. [4] Liu WH, Wu HQ, Lei YQ, Wang QD, Wu J. J Alloys Compounds 1997;261:289. [5] Goo NH, Jeong WT, Lee KS. J Power Sources 2000;87:118. [6] Han SC, Lee PS, Lee JY, ZVuttel A, Schlapbach L. J Alloys Compounds 2000;306:219. [7] Cui N, He P, Luo JL. Electrochim Acta 1999;44:3549. [8] Ye H, Lei YQ, Chen LS, Zhang H. J Alloys Compounds 2000;311:194. [9] Zhang SG, Yorimitsu K, Nohara S, Morikawa T, Inoue H, Iwakura C. J Alloys Compounds 1998;270:123. [10] Jiang JJ, Gasik M. J Power Sources 2000;89:117–24. [11] Chen J, Yao P, Bradhurst DH, Dou SX, Liu HK. J Alloys Compounds 2000;293–295:675. [12] Pourbaix M. Atlas of electrochemical equilibrium in aqueous solutions. Oxford: Pergamon Press, 1974. [13] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H. J Alloys Compounds 1993;202:183.
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The reduction of cycling capacity degradation of Mg–Ni-based electrode alloys by Fe substitution Yao Zhang ∗ , Li-Xin Chen, Yong-Quan Lei, Qi-Dong Wang Department of Materials Science and Engineering, Zhejiang University, Jade Spring, Hangzhou 310027, People’s Republic of China
Abstract A new approach to reduce the cycling capacity degradation of Mg–Ni-based alloy through lowering the polarization resistance by means of element substitution is suggested in this study. A new Mg-based amorphous alloy Mg45 Fe5 Ni50 was prepared by means of mechanical alloying. The cycling stability of this alloy which is found superior to that of Mg50 Ni50 and several other A side-substituted Mg-based ternary alloys. A comparative study on the surface layer formed on Mg50 Ni50 and Mg45 Fe5 Ni50 during cycles was made by the X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). XPS analysis demonstrates that, the degree of oxidation of the two alloys are almost the same. Mg on the surface of both alloys transforms into Mg(OH)2 passive ;lm and the Mg (2P) binding energy peaks of both alloys before and after di=erent cycles are also similar. The only cause for the di=erence in cycling stability of the two alloys is the di=erence in polarization resistance RP , which leads to di=erent potential drops in the charge=discharge processes. On the basis of the EIS Nyquist diagrams and corresponding equivalent, the RP values of di=erent cycles of both alloys were calculated through non-linear least-squares ;tting. For Mg45 Fe5 Ni50 , the polarization resistance is lower. The metallic Fe was believed to be bene;cial for suppressing the polarization resistance RP and elevating the cycling discharge plateau, thus improving the cycling discharge-ability. This phenomenon may be useful in preparing more durable Mg–Ni-based electrode alloys. ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. Keywords: Cycling capacity degradation; Mg–Ni-based electrode alloy; Polarization resistance; Fe substitution
1. Introduction Mechanically alloyed (MA) Mg–Ni-based amorphous hydrogen storage electrode alloys have been extensively studied recently because of their high discharge capacity (discharge capacity of Mg50 Ni50 reaching 500 mA h=g), low cost and good initial activation property in electrochemical processes [1–3]. However, its poor cycling stability discourages its further development and hampers its immediate application. Until now, the method of partial substitution of A side element (Mg) and=or B side element (Ni) in Mg– Ni-based alloys has been widely adopted for improving their electrochemical performances in the 6M alkaline electrolyte ∗ Corresponding author. Tel.: +86-571-8795-2615; fax: +86-571-8795-1152. E-mail address: [email protected] (Y. Zhang).
[4 –11]. The main cause for cycling capacity degradation is believed to be due to the loss of Mg in KOH solution and the formation of a permeable Mg(OH)2 passive ;lm. Yet, the present authors believe that there exist other factors such as the polarization resistance and the surface activation which also a=ect the cycling capacity degradation. A comparative study between Mg50 Ni50 and Mg45 Fe5 Ni50 is presently studied. According to Liu’s investigation [4], Mg50 Ni45 Fe5 possesses the lowest cycling degradation among the fourteen B side-substituted Mg50 Ni45 M5 ternary alloys (M = Fe, W, Cu, C, Mn, Cr, Zr, Ti, Zn, Se, Sb, Co, Si and Al). Fe was used by us to substitute Mg in Mg50 Ni50 partially to form a new alloy Mg45 Fe5 Ni50 , considering that A side substitution might possess higher cycling stability than B side substitution. Through the comparisons of the electrochemical performance with Mg50 Ni50 and Mg50 Ni45 Fe5 alloys and the
0360-3199/02/$ 20.00 ? 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 1 8 2 - 3
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Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
surface status with Mg50 Ni50 , the e=ect of surface polarization resistance on cycling discharge capacity degradation was noticed and studied. 2. Experimental details MA technique was used to prepare the amorphous Mg50 Ni50 and Mg45 Fe5 Ni50 alloys. 99.9% pure metal powders were sealed each time in a 100 ml planetary mill vial according to the stoichiometric ratio of the sample required. The vial was ;rst evacuated then ;lled with high purity argon and set to mill for 120 h. In the vial, the weight ratio of ball to powder was 30:1. The test electrode was fabricated according to the following procedure: 100 mg of alloy powders prepared were ;rst thoroughly mixed with 200 mg ;ne copper powder (below 200 mesh). The mixtures were then pressed at 20 MPa into pellets of 10 mm diameter. Electrochemical measurements were performed in a three-electrode system, in which the alloy electrode pellet was separated from the counter electrode by a porous polypropylene ;lm. Ni(OH)2 =NiOOH and Hg=HgO were used as the counter electrode and the reference electrode, respectively. The electrolyte was a 6M KOH aqueous solution. Measurements were executed by an automatic galvanostatic system (DC-5). For each cycle, charging was conducted at the current density of 300 mA=g for 3 h and discharging at 100 mA=g, with a cut-o= voltage set at −0:6 V (vs. Hg=HgO). After the ;rst and 10th cycle of charge and discharge, one pellet was taken out separately, crunched into ;ne powder, washed with distilled water for 3 times and then dried in vacuum for X-ray photoelectron spectroscopy (XPS) analysis. XPS measurements were carried out on a VG ESCALAB MKII spectrometer using Al K radiation (1486:6 eV). The operating pressure was below 5 × 10−5 mbar. The electrochemical impedance spectroscopy (EIS) analysis was executed on EG& G potentiostat=galvanostat Model 273A frequency response analyzer. The frequency range was from 120 kHz to 5 mHz. The Nyquist diagrams were plotted at the steady-state of the 50% depth of discharge (DOD) at the 1st and 10th cycles.
Fig. 1. The XRD patterns of Mg45 Fe5 Ni50 and Mg50 Ni50 alloys after 120 h ball milling.
Fig. 2. The discharge potential curves of amorphous Mg45 Fe5 Ni50 and Mg50 Ni50 after the ;rst cycle.
3. Results and discussion 3.1. Discharge performance The ball-milled Mg45 Fe5 Ni50 alloy powder was in amorphous state as evidenced by its XRD pattern (Fig. 1). Fig. 2 shows that the discharge potential curve of the amorphous Mg45 Fe5 Ni50 possesses a higher discharge potential plateau than the amorphous Mg50 Ni50 and a lower initial discharge capacity (dropping from 463 to 313 mA h=g). From Fig. 3, it can be seen that its capacity retention rate after 20 cycles, denoted by (C20 =C1 ) × 100% (C20 stands for the
Fig. 3. The cycle discharge ability of the ternary Mg–Fe–Ni and Mg50 Ni50 electrodes.
discharge capacity after the 20th cycle, and C1 for that after the 1st cycle), increases from 32.3% to 62.3%. This means that the cycle stability is improved signi;cantly. Compared to Liu’s results for Mg50 Ni45 Fe5 , Mg45 Fe5 Ni50 still possesses
Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
503
a higher initial capacity and a higher charge–discharge cycle stability as shown in Table 1. From Fig. 3 and Table 1, it is ascertained that Mg-side iron substitution is superior to that of Ni-side substitution in discharge performance. The Fe substitution leads to the best cycle stability in the four Mg-side substitution elements studied here as indicated in Table 2. The same conclusion was also reached in Liu’s study [4]. In our present test, the sequence of the cycling stability of the Mg45 M5 Ni50 (M = Al, Zn, Cu, Fe) ternary electrode alloys and the binary Mg50 Ni50 alloy, from low to high is as follows: Mg45 Al5 Ni50 ¡ Mg45 Zn5 Ni50 ¡ Mg50 Ni50 ¡ Mg45 Cu5 Ni50 ¡ Mg45 Fe5 Ni50 :
(1)
For this reason, we chose Mg50 Ni50 and Mg45 Fe5 Ni50 as the alloys for investigating the other factors for cycling degradation of Mg-based electrode alloys besides the oxidation of Mg. 3.2. Surface analysis Fig. 4 is the XPS wide scan spectra. Fig. 4(a) exhibits the spectra of Mg45 Fe5 Ni50 alloy after di=erent cycles. However, the Fe peaks cannot be found in these spectra, suggesting that iron might have been dissolved in the 6M KOH solution, which agrees well with the pH–E plots [12]. For both alloys, the atomic percentages of Mg, O and Ni content on the surface after di=erent cycles are listed in Tables 3 and 4. From Fig. 4(a), we can see that as the charge–discharge cycling proceeds, all peaks shift from lower binding energies to higher ones. The same tendency is also noticed in Fig. 4(b) for the XPS wide scan spectra of Mg50 Ni50 . Moreover, the surface oxide contribution of Mg50 Ni50 is very close to that of Mg45 Fe5 Ni50 on comparing data in Table 3 with those in Table 4. In Fig. 5(a), the Mg (2p) spectra of Mg45 Fe5 Ni50 before and after di=erent cycles are shown. Before Table 1 Comparison of cycle stability between Mg45 Fe5 Ni50 Mg50 Ni45 Fe5 [4]
Fig. 4. (a) The XPS wide scan spectra of Mg45 Fe5 Ni50 electrode before and after cycles; and (b) the XPS wide scan spectra of Mg50 Ni50 electrode before and after 1 cycle.
Table 3 Atomic percent of Mg, O and Ni of di=erent cycles of Mg45 Fe5 Ni50 electrode
Before cycle After 1 cycle After 10 cycles
Mg (at%)
O (at%)
Ni (at%)
35.3 33.6 28.4
34.6 37.2 43.1
30.1 29.2 28.5
and
Alloys
C1 (mAh=g)
C9 (mAh=g)
C9 =C1 (%)
Mg50 Ni45 Fe5 Mg45 Fe5 Ni50 Mg50 Ni50
273.3 313 463
108 215 150
39.5 68.7 32.2
charge–discharge cycle, the 2p peak is located at 49:6 eV, rather close to the standard binding energy of metallic magnesium (49:75 eV), which means that Mg in alloy is in the metallic form and has not been oxidized. After 1 cycle and 10 cycles, the 2p peak shifts to 49.8 and 50:2 eV, respectively. They move much closer to the binding energy of Mg(OH)2
Table 2 Cycle stability of mechanical alloying Mg45 M5 Ni50 (M = Al, Zn, Cu, Fe)
C1 (mAh=g) C20 (mAh=g) C20 =C1 (%)
Mg45 Al5 Ni50
Mg45 Zn5 Ni50
Mg45 Cu5 Ni50
Mg45 Fe5 Ni50
Mg50 Ni50
420 117 27.9
350 111 31.7
365 180 51
313 195 62.3
463 150 32.2
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Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
Table 4 Atomic percent of Mg, O and Ni of di=erent cycles of Mg50 Ni50 electrode
Before cycle After 1 cycle After 10 cycles
Mg (at%)
O (at%)
Ni (at%)
37.8 35.4 30.6
35.4 38.3 43.7
26.8 26.3 25.7
Fig. 6. (a) The impedance spectra of the Mg45 Fe5 Ni50 electrode at the 50% DOD of the ;rst cycle and the 10th cycle; and (b) the impedance spectra of the Mg50 Ni50 electrode at the 50% DOD of the ;rst cycle and the 10th cycle.
Fig. 5. (a) The Mg(2p) spectra of Mg45 Fe5 Ni50 electrode before and after cycles; and (b) the Mg(2p) spectra of Mg50 Ni50 electrode before and after 1 cycle.
(50:4 eV). This change makes us believe that the Mg(OH)2 passive ;lm grows thicker and thicker over the surface of the Mg-based alloy as the cycle number increases. Table 3 shows that the content of Mg drops from 33:6 at% after 1 cycle to 28:4 at% after 10 cycles. This also con;rms that more Mg(OH)2 are formed on the surface with the augmentation of charge and discharge cycles. From Fig. 5(b), it can be seen that the Mg (2p) peak displacement of Mg50 Ni50 is very close to that of Mg45 Fe5 Ni50 . It demonstrates that the substituting element Fe does not change the degree of oxidation of Mg on the surface of the alloy, and thus does not help to inhibit the corrosion of Mg. From Fig. 5, Tables 3 and 4, it is clear that iron does not protect Mg from oxidation, but leads to a drop in polarization resistance as indicated below.
Fig. 7. The equivalent circuit of Mg45 Fe5 Ni50 electrode at the 50% DOD of the ;rst cycle and the 10th cycle.
3.3. Cycle impedance analysis Fig. 6 shows the impedance spectra of the Mg45 Fe5 Ni50 and Mg50 Ni50 electrodes at the 50% DOD of the ;rst cycle and the 10th cycle, respectively. Each spectrum consists of several semicircles, which overlap one another and are not very distinguishable. By means of the nonlinear least-squares (NLLS) ;tting program EQUIVCT, the equivalent circuit is shown in Fig. 7. For Mg45 Fe5 Ni50 , four components exist in the circuit. They are resistance R1 and three impedance components, (R2 ; Q2 ), (R3 ; Q3 ) and (R4 ; Q4 ) corresponding to semicircles A, B and C in Fig. 6(a), respectively. For Mg50 Ni50 , there are three components that exist in the circuit, namely R1 and two impedance components (R2 ; Q2 ) and (R3 ; Q3 ) corresponding to semicircles 1 and 2, respectively in Fig. 6(b).
Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
505
Table 5 R and Q values of Mg45 Fe5 Ni50 electrode’s equivalent circuit
Cycle 1 Cycle 10
R1 (S)
R2 (S)
Q2 (F)
R3 (S)
Q3 (F)
R4 (RP =S)
Q4 (F)
0.826 0.830
0.187 0.215
0.0045 0.0077
2.261 3.137
0.08 0.09
2.25 2.76
0.87 1.23
Table 6 R and Q values of Mg50 Ni50 electrode’s equivalent circuit
Cycle 1 Cycle 10
R1 (S)
R2 (S)
Q2 (F)
R3 (RP =S)
Q3 (F)
1.129 1.096
0.1996 0.1625
0.0082 0.0077
3.68 4.902
0.1434 0.1573
According to Ref. [13], R1 represents the electrolyte resistance between the working electrode and reference electrode for both alloys. Tables 5 and 6 con;rm that it remains constant during cycling. The circuit of the parallelly connected R2 and Q2 , which corresponds to the small semicircle A in Fig. 6(a) and semicircle 1 in Fig. 6(b), is induced by high-frequency signal. The semicircle B in Fig. 6(a) stands for the contact resistance between the current collector and alloy pellets, and the double-layer capacitance on the surface of pellets and the capacitance on the current collectors, respectively. In Table 5, R3 and Q3 are used to represent such resistance and capacitance of Mg45 Fe5 Ni50 , respectively. Also, according to Ref. [13], the components in low-frequency zone of the parallel circuit R4 and Q4 for Mg45 Fe5 Ni50 and R3 and Q3 for Mg50 Ni50 are considered as the polarization resistance for the charge–discharge process and the double-layer capacitance over the external surface of the electrode, respectively. This impedance is a=ected sensitively by the alloy surface conditions, and varies as the charge–discharge process proceeds. From Tables 5 and 6, it can be seen that as the cycling number increases, the reaction resistance and double-layer capacitance for both alloys increase considerably. This could be ascribed to the formation of a passive Mg(OH)2 ;lm which leads to the deterioration of the electrocatalytic activity and increase of polarization resistance. R4 in Table 5 and R3 in Table 6 increase all the time as Mg corrodes due to corrosion current. Comparing the R3 values of Mg50 Ni50 with R4 of Mg45 Fe5 Ni50 electrodes at the same DOD of the same cycle, we can see that the Mg45 Fe5 Ni50 electrode possesses a lower polarization resistance. This means that the Fe substitution for Mg signi;cantly drops the polarization resistance and increases the cycle stability of the Mg50 Ni50 hydrogen storage electrode. The reasons for considering the polarization resistance of an electrode RP as a factor for cycling capacity degradation are: (1) The cycling capacity of any electrode is evaluated at a certain cut-o= voltage, below which the electrochemical capacity of the electrode is not counted;
Fig. 8. The discharge curves of Mg50 Ni50 electrode of cycle 1 and cycle 10.
(2) The actual voltage (VA ) of the electrode is VA =V0 −IRP ; (3) RP increases with the number of cycling. As with the same V0 , the actual voltage of the electrode is lower when the polarization resistance is large. The potential drop due to this resistance RP lowers the discharge plateau when a current Tows through, and causes the discharge process end in advance. This relation was usually disregarded in capacity degradation studies. In Fig. 2, the potential di=erence (between Mg50 Ni50 and Mg45 Fe5 Ni50 at 50% DOD of initial cycle) is 0:015 V. The result agrees very well with the value 0:0143 V obtained by multiplying the URP between the electrodes (1:43 S) with the discharge current density 100 mA=g for an electrode of 100 mg alloy powder at 50% DOD. This makes us believe that the increase of RP reduces the discharge plateau and thus inTuences the potential-capacity characteristic of the Mg– Ni-based alloys as shown in Figs. 8 and 9. For Mg50 Ni50 , URP is larger than that of Mg45 Fe5 Ni50 , this makes the drop of discharge plateaus from cycle 1 to cycle 10 become larger than Mg45 Fe5 Ni50 . Therefore, the discharge deterioration of
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Y. Zhang et al. / International Journal of Hydrogen Energy 27 (2002) 501 – 506
polarization resistance RP and to elevate the discharge plateau of the Mg-based alloy electrodes and thus leads to a slower deterioration of cycling stability. The role of substituted Fe in improving the surface activation and decreasing the di=erence of discharge plateau and RP between di=erent cycles and thus improving the cycling stability is a new approach to combat the poor cycling capacity degradation of Mg–Ni-based alloys and should be studied more thoroughly and systematically later.
Acknowledgements Fig. 9. The discharge curves of Mg45 Fe5 Ni50 electrode of cycle 1 and cycle 10.
the former is much more serious than that of the latter. The above analysis also indicates that the Fe substitution not only elevates the discharge plateau of Mg-based alloys, but also decreases the di=erence of discharge plateau and RP between di=erent cycles. It demonstrates again that the mechanism of inhibiting cycling degradation by additional Fe in Mg-based alloy is to decrease both the value of RP and the di=erence of discharge plateau between di=erent cycles. 4. Conclusions In the present work, we prepared by means of mechanical alloying (MA) the Mg45 Fe5 Ni50 amorphous alloy, whose cycling discharge ability is superior to that of the B-side Fe-substituting ternary alloy Mg50 Ni45 Fe5 and the parent alloy Mg50 Ni50 . The X-ray photoelectron spectrum study (XPS) demonstrates that Mg on the surface of the alloy forms an Mg(OH)2 passive ;lm, and the degree of oxidation of the two alloys are almost the same. This indicates that Fe does not form a strong protective ;lm to inhibit the corrosion of Mg. But from the Nyquist diagrams of Mg45 Fe5 Ni50 and Mg50 Ni50 electrodes at two di=erent cycles by means of the electrochemical impedance spectroscopy (EIS), we found that the substituted Fe does help to decrease the
The authors wish to express their gratitude and appreciation for the support from the National Natural Science Foundation of China (No. 59971047).
References [1] Lei YQ, Wu YM, Yang QM, Wu J, Wang QD. Z Phys Chem 1994;183:379. [2] Nohara S, Inoue H, Fukumoto Y, Iwakura C. J Alloys Compounds 1997;259:183. [3] Iwakura C, Inoue H, Zhang SG, Nohara S. J Alloys Compounds 1998;270:142. [4] Liu WH, Wu HQ, Lei YQ, Wang QD, Wu J. J Alloys Compounds 1997;261:289. [5] Goo NH, Jeong WT, Lee KS. J Power Sources 2000;87:118. [6] Han SC, Lee PS, Lee JY, ZVuttel A, Schlapbach L. J Alloys Compounds 2000;306:219. [7] Cui N, He P, Luo JL. Electrochim Acta 1999;44:3549. [8] Ye H, Lei YQ, Chen LS, Zhang H. J Alloys Compounds 2000;311:194. [9] Zhang SG, Yorimitsu K, Nohara S, Morikawa T, Inoue H, Iwakura C. J Alloys Compounds 1998;270:123. [10] Jiang JJ, Gasik M. J Power Sources 2000;89:117–24. [11] Chen J, Yao P, Bradhurst DH, Dou SX, Liu HK. J Alloys Compounds 2000;293–295:675. [12] Pourbaix M. Atlas of electrochemical equilibrium in aqueous solutions. Oxford: Pergamon Press, 1974. [13] Kuriyama N, Sakai T, Miyamura H, Uehara I, Ishikawa H. J Alloys Compounds 1993;202:183.