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Effect of Ti–Al substitution on the electrochemical properties of amorphous MgNi-based secondary hydride electrodes
Journal of Alloys and Compounds 392 (2005) 300–305
Effect of Ti–Al substitution on the electrochemical properties of amorphous MgNi-based secondary hydride electrodes Jing-Wang Liua,b,∗ , Hua-Tang Yuanb , Jian-Sheng Caob , Yi-Jing Wangb b
a Department of Chemistry, Tianjin Normal University, Tianjin 300074, PR China Institute of New Energy Material Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, PR China
Received 30 July 2004; received in revised form 30 August 2004; accepted 31 August 2004 Available online 28 October 2004
Abstract A new amorphous Mg0.7 Ti0.225 Al0.075 Ni alloy was prepared by means of mechanical alloying (MA), its discharge capacity reached 218.32 mAh g−1 after 50 cycles, which retains 63.57% of the initial capacity of 343.44 mAh g−1 . On the surface of the alloy during cycling in the electrolyte of 5 M KOH, the X-ray photoelectron spectroscopy (XPS) shows that the majority of magnesium and aluminum in the alloy exist in metallic state; Ti exists in the +3 oxidation state. Electrochemical impedance spectroscopy (EIS) shows that the aluminum lowered the charge transfer resistance. The structure of the alloy was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). And the improvement of cycle life of the electrode can be related to the formation of MgTi2 O4 . A study on the anti-corrosion properties in alkaline solution of the alloy electrode has been done, and it is shown that Ti and Al substitution of Mg improves the anti-corrosion properties significantly. It is found that Ti and Al substitution of Mg could improve the electrochemical behavior of MgNi-based electrodes. © 2004 Elsevier B.V. All rights reserved. Keywords: Nickel-metal hydride battery; Hydrogen storage alloy; Amorphous electrochemical properties; Substitution
1. Introduction Immense efforts were undertaken worldwide to investigate synthesis, structure, and potential applications of hydrogen storage alloys [1]. The alloys are capable of reversibly storing large amounts of hydrogen via absorbing gaseous hydrogen at certain pressure and temperatures or by electrochemistry to form metal hydride (MH). Rechargeable alkaline Ni-MH batteries using metal hydride as negative materials have become the dominant advanced battery technology for portable electronic devices, electric tools, electric vehicle (EV), and hybrid electric vehicle (HEV) applications due to their high reversible energy storage capacity, excellent long-term cycling stability, and good electrochemical reaction kinetics. The various alloy families including AB5 , AB2 , AB, AB3 , Mg-based alloy, and their composites have been largely investigated as negative materials of Ni/MH batteries. ∗
Corresponding author. Tel.: +86 22 2350 4527; fax: +86 22 2350 2604. E-mail address: [email protected] (J.-W. Liu).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.060
Mg-based hydrogen storage alloys are considered as one of the most promising candidates of third generation alloys, because of their high discharge capacity, lower gravity and rich natural resources [2]. However, the practical application of Mg-based alloys is prevented by their poor hydriding/dihydriding kinetics at room temperature and their poor charge–discharge cycling stability. The capacity degradation is associated with the irreversible oxidation of the alloy by the electrolyte (KOH) leading to the formation of a Mg(OH)2 layer on the surface of the alloy particles [3,4]. In order to improve the cycle life, many efforts have been made on their electrochemical characteristics. Multi-component alloying has been found to be the most effective way to retard the corrosion and to improve the cycling capacity degradation [5–9]. Han et al. [5] and Zhang et al. [6] demonstrated that Ti is very effective as a partial substitute for Mg in MgNi alloy by improving the cycle life, and the Mg0.7 Ti0.3 Ni alloy shows the best cycle life. The analysis of Ruggeri et al. [9] and the studies of other research groups [5,6] have shown that a few elements can be
J.-W. Liu et al. / Journal of Alloys and Compounds 392 (2005) 300–305
used as candidates for partial substitution of Mg and Ni, and the elements should have properties such as: (i) a higher vacancy formation energy than Mg [10]; (ii) they can form a passive film or oxide layer on the Mgbased alloy; (iii) the passive film is compact; (iv) the passive film is resistant to the penetration of KOH solution; (v) the passive film has some ability of hydrogen diffusion; (vi) improved ability to avoid cracking on cycling. Among the substitution elements, Ti and Al seem to be the better choice, and Ti is the key element for improving the cycle life of MgNi-based electrodes. In this study, an amorphous quaternary Mg0.7 Ti0.225 Al0.075 Ni alloy was prepared by mechanical alloying (MA). The phase structure, electrochemical capacity and cycling stability of the alloy were tested. The effect of Ti and Al on the surface of the alloy was studied by means of X-ray photoelectron spectroscopy (XPS).
2. Experimental details MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys were prepared from mixed powers of pure Mg, Ti, Al and Ni by MA. MA was carried out using a planetary ball mill in a stainless steel vessel at a speed of 450 rpm under argon atmosphere for 100 h. The crystal structure and surface configuration of the alloy were characterized by X-ray diffraction (Rigaku D/Max-2500, Cu ␣ radiation), scanning electron microscopy (Hitachi X-650); the surface element analysis was performed on a PHI-5300 ESCA spectrometer (PHI Company, USA) using Mg K␣ radiation. Negative electrodes were constructed through mixing asprepared composites with carbonyl nickel powders in a weight ratio of 1:3. The powder mixture was pressed under 30 MPa pressure into a small pellet of 10 mm in diameter and 1.5 mm thickness. Electrochemical measurements were conducted in a threecompartment cell using a Land battery test instrument. NiOOH/Ni(OH)2 and Hg/HgO were used as the counter electrode and the reference electrode. In each charge–discharge cycle tests, the negative electrode was charged for 6 h at 100 mAg−1 and discharged at 25 mAg−1 up to the cut-off voltage set at −0.5 V (versus Hg/HgO). The electrolyte solution is a 5 M KOH aqueous solution. The testing time between charge and discharge was 10 min. After 50 charge–discharge cycles, the pellet was crushed to powder, and washed with distilled water and dried in a vacuum drier before testing. A Solartron 1287 electrochemical interface (EI) coupled with a Solartron 1250 frequency response analyzer was used for polarization and EIS measurements. The AC amplitude was 5 mV and the frequency range employed between 104 and 10−2 Hz. The potentiodynamic polarization experiment
301
was performed using the S1287 EI. The potential range employed was between −1.2 V and −0.2 V (versus HgO/Hg). The sweep rate was 0.2 mV/s. All the measurements were conducted at room temperature.
3. Results and discussion 3.1. Phase structure In order to investigate the microstructures changes of the ternary Mg0.7 Ti0.225 Al0.075 Ni alloy during charge–discharge cycling, X-ray diffractometry for MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloy was carried out. As can be seen in Fig. 1(A). In the XRD patterns of the three alloys after 100 h milling, only broad peaks appear in the region of 40–45◦ , suggesting that the main phase of the alloy has an amorphous structure, and some weak Ni peaks co-exists with the main phase of Mg0.7 Ti0.225 Al0.075 Ni alloy. The ternary phase MgTi2 O4 was identified in the Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys. The XRD patterns of the electrodes after 50 charge–discharge cycles (recovered in the dehydrided state) are shown in Fig. 1(B). They have been zoomed to see the peaks ascribed to the active material (the larger peaks are attributed to nickel and nickel foam). Some
Fig. 1. XRD patterns of alloys and electrode: (A) as milled alloys of MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni; (B) after 50 cycles of Mg0.7 Ti0.225 Al0.075 Ni electrode.
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Fig. 2. SEM micrographs of Mg0.7 Ti0.225 Al0.075 Ni alloy and electrode: (A) secondary electron image of the as milled alloy; (B) backscattered electron image of the as milled alloy; (C) secondary electron image of the electrode after 50 cycles; (D) backscattered electron image of the electrode after 50 cycles.
oxide films may have formed on the alloy surface after cycling. However, the amount of oxide turns out to be too small to be observed by XRD. So oxide peaks were not detected [7], even no diffraction peaks of Mg(OH)2 can be found (not thick enough to be detected by XRD) [8]. However, as regards a surface passivation effect as reported for MgNi [9], these facts indicate that the spinel MgTi2 O4 may be the main reason to prevent the alloy particles from degradation or oxidation. The particles of Mg0.7 Ti0.225 Al0.075 Ni alloy milled 100 h are cubical grains with large interfaces and defects, as shown in Fig. 2(A). The grain size varies between 2 and 3 m in diameter. These observations do not all agree with results of the MgNi alloy reported by other authors [12–14]. The backscattered electron image in Fig. 2(B) indicates that all elements in the alloy are uniformly distributed. The SEM micrographs of particles of the electrode after 50 charge–discharge cycles are shown in Fig. 2, It can be seen in Fig. 2(C) that a loose surface layer is formed on the gray amorphous particle matrix, and that the particle surface becomes smoother on cycling, the grain size varying between 2 and 3 m in diameter. The backscattered electron image shows a homogeneous distribution of the phases in Fig. 2(D), the bright phase corresponding to crystalline of Ni. The gray phase corresponding to the main phase suggests that the samples are still in the amorphous state. All those results agree with the XRD analysis shown in Fig. 1. Fig. 2 also shows that there is no filamentous structure on the particles [15] and there is no clearer evidence that the size of
the particles has become smaller. These observations do not agree with results reported for the MgNi electrode by other authors [11–15], but they show that Al might improve the ability to avoid cracking. 3.2. Discharge capacity and cycle life Fig. 3 shows the discharge capacity at 25 ◦ C varying with cycle number for MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloy electrodes. In the case of the MgNi electrode, the discharge capacity decreased considerably with the increase in cycle number. Furthermore, the substitution of Mg by Ti and Al in the quaternary alloy improves
Fig. 3. The cycling discharge-ability of the MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni electrode alloys at 25 ◦ C.
J.-W. Liu et al. / Journal of Alloys and Compounds 392 (2005) 300–305 Table 1 Cycle stability of the three alloys Alloy (mAh g−1 )
C1st C25th (mAh g−1 ) C50th (mAh g−1 ) C25th/C1st (%) C50th/C1st (%)
MgNi
Mg0.7 Ti0.3 Ni
Mg0.7 Ti0.225 Al0.075 Ni
495.62 107.86
401.2 205 121.8 51.10 30.36
343.44 254.8 218.32 74.19 63.57
21.76
evidently the cycling stability of the alloys. The discharge capacity at 50 cycles for the Mg0.7 Ti0.225 Al0.075 Ni alloy is still higher than 218.32 mAh g−1 . Information on the cycling stability of the three alloys is given in Table 1, where C1st refers to the discharge capacity at the first cycle, C25th refers to 25 cycles, and C50th refers to 50 cycles. From Table 1, it is ascertained that Ti substitution for Mg improves the cycle life of MgNi, and Al substitution for Ti improves the cycle life of Mg0.7 Ti0.3 Ni significantly. 3.3. The effect on the properties of passivation film The only way to inhibit the corrosion of an Mg-based alloy is to build up a good passivation film on its external surface [16]. In order to find the reason for the improvement on cycling stability brought forth by Ti and Al, the corrosion products on the surface of the alloys before charging and after 50
303
charge–discharge cycles were investigated by means of XPS surface analysis. Fig. 4A–D shows XPS data for the surface of Mg0.7 Ti0.225 Al0.075 Ni alloy balled milled for 50 cycles. Both O 1s and Ti 2p peaks shift to higher binding energy during charging, and after 50 cycles, the Ti 2p peak exhibited a maximum at about 458.27 eV, lower than 458.70 eV of TiO2 and higher than 455.10 eV of TiO, indicating that the majority of Ti exists in the +3 oxidation state [17], so that, this peak is most likely attributed to MgTi2 O4 . On the other hand, the peak binding energy of Mg and Al are lower, the majority of magnesium and aluminum in the alloy existing in metallic state even after 50 cycles [5], and that agrees with the longer cycle life of the electrodes. 3.4. Corrosion of the alloys There are many investigations about the corrosion behavior of Mg-based alloys, because the corrosion of Mgbased alloys is a barrier to its practical use [18]. 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 Mgbased alloys. The potentiodynamic polarization of MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys are shown in Fig. 5 with the results obtained by Tafel fitting [19] and are summarized in Table 2. The results reveal that in the
Fig. 4. XPS of Mg0.7 Ti0.225 Al0.075 Ni alloy as ball milled and after 50 cycles: (A) Mg 2p; (B) Ti 2p; (C) Al 2p; (D) O 1s.
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J.-W. Liu et al. / Journal of Alloys and Compounds 392 (2005) 300–305 Table 3 Simulated electrochemical parameters of the MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys electrodes Alloy
Rct ()
MgNi Mg0.7 Ti0.3 Ni Mg0.7 Ti0.225 Al0.075 Ni
0.48 0.35 0.07
Rct : charge transfer resistance.
Fig. 5. Anodic polarization curves of MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys. Table 2 Tafel fitting data of the MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloy electrodes Alloy
icorr (mA/cm2 )
Ecorr (V)
MgNi Mg0.7 Ti0.3 Ni Mg0.7 Ti0.225 Al0.075 Ni
5.32 6.49 4.38
−0.939 −0.938 −0.902
Calculated based on Tafel equation [15]. icorr : corrosion current density; Ecorr : corrosion potential.
Mg0.7 Ti0.225 Al0.075 Ni alloy, its values of Ecorr is higher than for MgNi and Mg0.7 Ti0.3 Ni, and the corrosion rate (icorr ) is lower than for MgNi and Mg0.7 Ti0.3 Ni. These results suggest that the substitution of Mg by Ti and Al improves the anti-corrosion behavior of MgNi-based alloy.
charge transfer resistance values were obtained. The results are shown in Table 3. The results reveal that in the Mg0.7 Ti0.225 Al0.075 Ni alloy, the values of the charge transfer resistance Rct is much lower than in MgNi and Mg0.7 Ti0.3 Ni. These results suggest that in the Mg0.7 Ti0.225 Al0.075 Ni alloy, Ti helps to improve the charge transfer on the surface [21], and Al helps to improve the charge transfer on the surface significantly.
4. Conclusions The amorphous Mg0.7 Ti0.225 Al0.075 Ni alloy was synthesized successfully by mechanical alloying, it possesses higher cycling capacity than other MgNi-based alloys. The discharge capacity of the 100 h milled Mg0.7 Ti0.225 Al0.075 Ni alloy at 25 mAg−1 discharge current density reached 218.32 mAh g−1 after 50th cycles. XPS showed that the majority of magnesium and aluminum in this alloy exist in the metallic state. It is worth to point out that from XRD and XPS analysis, a spinel MgTi2 O4 can be found, and no direct evidence of Mg(OH)2 exists. Further detailed investigations on the effects of Ti and Al are now in progress.
3.5. The effect on polarization resistance and the surface exchange current Acknowledgements Fig. 6 shows the EIS Nyquist diagrams for the two electrode alloys at 50% DOD at the first cycle and their proposed equivalent circuit. It can be seen clearly that the spectra consist of three overlapping semicircles and a linear Warburg part [20]. On the basis of the circuit and by means of the nonlinear least squares (NLLS) fitting program EQUIVCT, the
The work is subsidized by the National High Tech. ‘863’ (2001AA515022), the Special Founds for Major State Basic Project (G200026405) and the Natural Science Foundation of Tianjin, China (01360211) and Tian-Nan Union Found.
References
Fig. 6. Nyquist plot for MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni electrode.
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[16] Q.-D. Wang, Y. Zhang, Y.-Q. Lei, J. Alloys Compd. 356–357 (2003) 784. [17] M. Schmidt, W. Ratcliff II, P.G. Radaelli, K. Refson, N.M. Harrison, S.W. Cheong, Phys. Rev. Lett. 92 (5) (2004) 056402-1-4. [18] H. Yang, H.S.E. Yang, F. Gao, H. Yuan, Z. Zhou, J. Electrochem. Soc. 149 (5) (2002) A543–A545. [19] D. Johnson, CVIEW for Windows, Ver. 2.3 [d]1990-2000, Scribner Associates Inc. [20] W. Liu, H. Wu, Y. Lei, Q. Wang, J. Alloys Compd. 346 (2002) 244–249. [21] S. Barnartt, Electrochim. Acta 15 (1970) 1313.
Effect of Ti–Al substitution on the electrochemical properties of amorphous MgNi-based secondary hydride electrodes Jing-Wang Liua,b,∗ , Hua-Tang Yuanb , Jian-Sheng Caob , Yi-Jing Wangb b
a Department of Chemistry, Tianjin Normal University, Tianjin 300074, PR China Institute of New Energy Material Chemistry, Nankai University, 94 Weijin Road, Tianjin 300071, PR China
Received 30 July 2004; received in revised form 30 August 2004; accepted 31 August 2004 Available online 28 October 2004
Abstract A new amorphous Mg0.7 Ti0.225 Al0.075 Ni alloy was prepared by means of mechanical alloying (MA), its discharge capacity reached 218.32 mAh g−1 after 50 cycles, which retains 63.57% of the initial capacity of 343.44 mAh g−1 . On the surface of the alloy during cycling in the electrolyte of 5 M KOH, the X-ray photoelectron spectroscopy (XPS) shows that the majority of magnesium and aluminum in the alloy exist in metallic state; Ti exists in the +3 oxidation state. Electrochemical impedance spectroscopy (EIS) shows that the aluminum lowered the charge transfer resistance. The structure of the alloy was characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). And the improvement of cycle life of the electrode can be related to the formation of MgTi2 O4 . A study on the anti-corrosion properties in alkaline solution of the alloy electrode has been done, and it is shown that Ti and Al substitution of Mg improves the anti-corrosion properties significantly. It is found that Ti and Al substitution of Mg could improve the electrochemical behavior of MgNi-based electrodes. © 2004 Elsevier B.V. All rights reserved. Keywords: Nickel-metal hydride battery; Hydrogen storage alloy; Amorphous electrochemical properties; Substitution
1. Introduction Immense efforts were undertaken worldwide to investigate synthesis, structure, and potential applications of hydrogen storage alloys [1]. The alloys are capable of reversibly storing large amounts of hydrogen via absorbing gaseous hydrogen at certain pressure and temperatures or by electrochemistry to form metal hydride (MH). Rechargeable alkaline Ni-MH batteries using metal hydride as negative materials have become the dominant advanced battery technology for portable electronic devices, electric tools, electric vehicle (EV), and hybrid electric vehicle (HEV) applications due to their high reversible energy storage capacity, excellent long-term cycling stability, and good electrochemical reaction kinetics. The various alloy families including AB5 , AB2 , AB, AB3 , Mg-based alloy, and their composites have been largely investigated as negative materials of Ni/MH batteries. ∗
Corresponding author. Tel.: +86 22 2350 4527; fax: +86 22 2350 2604. E-mail address: [email protected] (J.-W. Liu).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.08.060
Mg-based hydrogen storage alloys are considered as one of the most promising candidates of third generation alloys, because of their high discharge capacity, lower gravity and rich natural resources [2]. However, the practical application of Mg-based alloys is prevented by their poor hydriding/dihydriding kinetics at room temperature and their poor charge–discharge cycling stability. The capacity degradation is associated with the irreversible oxidation of the alloy by the electrolyte (KOH) leading to the formation of a Mg(OH)2 layer on the surface of the alloy particles [3,4]. In order to improve the cycle life, many efforts have been made on their electrochemical characteristics. Multi-component alloying has been found to be the most effective way to retard the corrosion and to improve the cycling capacity degradation [5–9]. Han et al. [5] and Zhang et al. [6] demonstrated that Ti is very effective as a partial substitute for Mg in MgNi alloy by improving the cycle life, and the Mg0.7 Ti0.3 Ni alloy shows the best cycle life. The analysis of Ruggeri et al. [9] and the studies of other research groups [5,6] have shown that a few elements can be
J.-W. Liu et al. / Journal of Alloys and Compounds 392 (2005) 300–305
used as candidates for partial substitution of Mg and Ni, and the elements should have properties such as: (i) a higher vacancy formation energy than Mg [10]; (ii) they can form a passive film or oxide layer on the Mgbased alloy; (iii) the passive film is compact; (iv) the passive film is resistant to the penetration of KOH solution; (v) the passive film has some ability of hydrogen diffusion; (vi) improved ability to avoid cracking on cycling. Among the substitution elements, Ti and Al seem to be the better choice, and Ti is the key element for improving the cycle life of MgNi-based electrodes. In this study, an amorphous quaternary Mg0.7 Ti0.225 Al0.075 Ni alloy was prepared by mechanical alloying (MA). The phase structure, electrochemical capacity and cycling stability of the alloy were tested. The effect of Ti and Al on the surface of the alloy was studied by means of X-ray photoelectron spectroscopy (XPS).
2. Experimental details MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys were prepared from mixed powers of pure Mg, Ti, Al and Ni by MA. MA was carried out using a planetary ball mill in a stainless steel vessel at a speed of 450 rpm under argon atmosphere for 100 h. The crystal structure and surface configuration of the alloy were characterized by X-ray diffraction (Rigaku D/Max-2500, Cu ␣ radiation), scanning electron microscopy (Hitachi X-650); the surface element analysis was performed on a PHI-5300 ESCA spectrometer (PHI Company, USA) using Mg K␣ radiation. Negative electrodes were constructed through mixing asprepared composites with carbonyl nickel powders in a weight ratio of 1:3. The powder mixture was pressed under 30 MPa pressure into a small pellet of 10 mm in diameter and 1.5 mm thickness. Electrochemical measurements were conducted in a threecompartment cell using a Land battery test instrument. NiOOH/Ni(OH)2 and Hg/HgO were used as the counter electrode and the reference electrode. In each charge–discharge cycle tests, the negative electrode was charged for 6 h at 100 mAg−1 and discharged at 25 mAg−1 up to the cut-off voltage set at −0.5 V (versus Hg/HgO). The electrolyte solution is a 5 M KOH aqueous solution. The testing time between charge and discharge was 10 min. After 50 charge–discharge cycles, the pellet was crushed to powder, and washed with distilled water and dried in a vacuum drier before testing. A Solartron 1287 electrochemical interface (EI) coupled with a Solartron 1250 frequency response analyzer was used for polarization and EIS measurements. The AC amplitude was 5 mV and the frequency range employed between 104 and 10−2 Hz. The potentiodynamic polarization experiment
301
was performed using the S1287 EI. The potential range employed was between −1.2 V and −0.2 V (versus HgO/Hg). The sweep rate was 0.2 mV/s. All the measurements were conducted at room temperature.
3. Results and discussion 3.1. Phase structure In order to investigate the microstructures changes of the ternary Mg0.7 Ti0.225 Al0.075 Ni alloy during charge–discharge cycling, X-ray diffractometry for MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloy was carried out. As can be seen in Fig. 1(A). In the XRD patterns of the three alloys after 100 h milling, only broad peaks appear in the region of 40–45◦ , suggesting that the main phase of the alloy has an amorphous structure, and some weak Ni peaks co-exists with the main phase of Mg0.7 Ti0.225 Al0.075 Ni alloy. The ternary phase MgTi2 O4 was identified in the Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys. The XRD patterns of the electrodes after 50 charge–discharge cycles (recovered in the dehydrided state) are shown in Fig. 1(B). They have been zoomed to see the peaks ascribed to the active material (the larger peaks are attributed to nickel and nickel foam). Some
Fig. 1. XRD patterns of alloys and electrode: (A) as milled alloys of MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni; (B) after 50 cycles of Mg0.7 Ti0.225 Al0.075 Ni electrode.
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Fig. 2. SEM micrographs of Mg0.7 Ti0.225 Al0.075 Ni alloy and electrode: (A) secondary electron image of the as milled alloy; (B) backscattered electron image of the as milled alloy; (C) secondary electron image of the electrode after 50 cycles; (D) backscattered electron image of the electrode after 50 cycles.
oxide films may have formed on the alloy surface after cycling. However, the amount of oxide turns out to be too small to be observed by XRD. So oxide peaks were not detected [7], even no diffraction peaks of Mg(OH)2 can be found (not thick enough to be detected by XRD) [8]. However, as regards a surface passivation effect as reported for MgNi [9], these facts indicate that the spinel MgTi2 O4 may be the main reason to prevent the alloy particles from degradation or oxidation. The particles of Mg0.7 Ti0.225 Al0.075 Ni alloy milled 100 h are cubical grains with large interfaces and defects, as shown in Fig. 2(A). The grain size varies between 2 and 3 m in diameter. These observations do not all agree with results of the MgNi alloy reported by other authors [12–14]. The backscattered electron image in Fig. 2(B) indicates that all elements in the alloy are uniformly distributed. The SEM micrographs of particles of the electrode after 50 charge–discharge cycles are shown in Fig. 2, It can be seen in Fig. 2(C) that a loose surface layer is formed on the gray amorphous particle matrix, and that the particle surface becomes smoother on cycling, the grain size varying between 2 and 3 m in diameter. The backscattered electron image shows a homogeneous distribution of the phases in Fig. 2(D), the bright phase corresponding to crystalline of Ni. The gray phase corresponding to the main phase suggests that the samples are still in the amorphous state. All those results agree with the XRD analysis shown in Fig. 1. Fig. 2 also shows that there is no filamentous structure on the particles [15] and there is no clearer evidence that the size of
the particles has become smaller. These observations do not agree with results reported for the MgNi electrode by other authors [11–15], but they show that Al might improve the ability to avoid cracking. 3.2. Discharge capacity and cycle life Fig. 3 shows the discharge capacity at 25 ◦ C varying with cycle number for MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloy electrodes. In the case of the MgNi electrode, the discharge capacity decreased considerably with the increase in cycle number. Furthermore, the substitution of Mg by Ti and Al in the quaternary alloy improves
Fig. 3. The cycling discharge-ability of the MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni electrode alloys at 25 ◦ C.
J.-W. Liu et al. / Journal of Alloys and Compounds 392 (2005) 300–305 Table 1 Cycle stability of the three alloys Alloy (mAh g−1 )
C1st C25th (mAh g−1 ) C50th (mAh g−1 ) C25th/C1st (%) C50th/C1st (%)
MgNi
Mg0.7 Ti0.3 Ni
Mg0.7 Ti0.225 Al0.075 Ni
495.62 107.86
401.2 205 121.8 51.10 30.36
343.44 254.8 218.32 74.19 63.57
21.76
evidently the cycling stability of the alloys. The discharge capacity at 50 cycles for the Mg0.7 Ti0.225 Al0.075 Ni alloy is still higher than 218.32 mAh g−1 . Information on the cycling stability of the three alloys is given in Table 1, where C1st refers to the discharge capacity at the first cycle, C25th refers to 25 cycles, and C50th refers to 50 cycles. From Table 1, it is ascertained that Ti substitution for Mg improves the cycle life of MgNi, and Al substitution for Ti improves the cycle life of Mg0.7 Ti0.3 Ni significantly. 3.3. The effect on the properties of passivation film The only way to inhibit the corrosion of an Mg-based alloy is to build up a good passivation film on its external surface [16]. In order to find the reason for the improvement on cycling stability brought forth by Ti and Al, the corrosion products on the surface of the alloys before charging and after 50
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charge–discharge cycles were investigated by means of XPS surface analysis. Fig. 4A–D shows XPS data for the surface of Mg0.7 Ti0.225 Al0.075 Ni alloy balled milled for 50 cycles. Both O 1s and Ti 2p peaks shift to higher binding energy during charging, and after 50 cycles, the Ti 2p peak exhibited a maximum at about 458.27 eV, lower than 458.70 eV of TiO2 and higher than 455.10 eV of TiO, indicating that the majority of Ti exists in the +3 oxidation state [17], so that, this peak is most likely attributed to MgTi2 O4 . On the other hand, the peak binding energy of Mg and Al are lower, the majority of magnesium and aluminum in the alloy existing in metallic state even after 50 cycles [5], and that agrees with the longer cycle life of the electrodes. 3.4. Corrosion of the alloys There are many investigations about the corrosion behavior of Mg-based alloys, because the corrosion of Mgbased alloys is a barrier to its practical use [18]. 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 Mgbased alloys. The potentiodynamic polarization of MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys are shown in Fig. 5 with the results obtained by Tafel fitting [19] and are summarized in Table 2. The results reveal that in the
Fig. 4. XPS of Mg0.7 Ti0.225 Al0.075 Ni alloy as ball milled and after 50 cycles: (A) Mg 2p; (B) Ti 2p; (C) Al 2p; (D) O 1s.
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J.-W. Liu et al. / Journal of Alloys and Compounds 392 (2005) 300–305 Table 3 Simulated electrochemical parameters of the MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys electrodes Alloy
Rct ()
MgNi Mg0.7 Ti0.3 Ni Mg0.7 Ti0.225 Al0.075 Ni
0.48 0.35 0.07
Rct : charge transfer resistance.
Fig. 5. Anodic polarization curves of MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloys. Table 2 Tafel fitting data of the MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni alloy electrodes Alloy
icorr (mA/cm2 )
Ecorr (V)
MgNi Mg0.7 Ti0.3 Ni Mg0.7 Ti0.225 Al0.075 Ni
5.32 6.49 4.38
−0.939 −0.938 −0.902
Calculated based on Tafel equation [15]. icorr : corrosion current density; Ecorr : corrosion potential.
Mg0.7 Ti0.225 Al0.075 Ni alloy, its values of Ecorr is higher than for MgNi and Mg0.7 Ti0.3 Ni, and the corrosion rate (icorr ) is lower than for MgNi and Mg0.7 Ti0.3 Ni. These results suggest that the substitution of Mg by Ti and Al improves the anti-corrosion behavior of MgNi-based alloy.
charge transfer resistance values were obtained. The results are shown in Table 3. The results reveal that in the Mg0.7 Ti0.225 Al0.075 Ni alloy, the values of the charge transfer resistance Rct is much lower than in MgNi and Mg0.7 Ti0.3 Ni. These results suggest that in the Mg0.7 Ti0.225 Al0.075 Ni alloy, Ti helps to improve the charge transfer on the surface [21], and Al helps to improve the charge transfer on the surface significantly.
4. Conclusions The amorphous Mg0.7 Ti0.225 Al0.075 Ni alloy was synthesized successfully by mechanical alloying, it possesses higher cycling capacity than other MgNi-based alloys. The discharge capacity of the 100 h milled Mg0.7 Ti0.225 Al0.075 Ni alloy at 25 mAg−1 discharge current density reached 218.32 mAh g−1 after 50th cycles. XPS showed that the majority of magnesium and aluminum in this alloy exist in the metallic state. It is worth to point out that from XRD and XPS analysis, a spinel MgTi2 O4 can be found, and no direct evidence of Mg(OH)2 exists. Further detailed investigations on the effects of Ti and Al are now in progress.
3.5. The effect on polarization resistance and the surface exchange current Acknowledgements Fig. 6 shows the EIS Nyquist diagrams for the two electrode alloys at 50% DOD at the first cycle and their proposed equivalent circuit. It can be seen clearly that the spectra consist of three overlapping semicircles and a linear Warburg part [20]. On the basis of the circuit and by means of the nonlinear least squares (NLLS) fitting program EQUIVCT, the
The work is subsidized by the National High Tech. ‘863’ (2001AA515022), the Special Founds for Major State Basic Project (G200026405) and the Natural Science Foundation of Tianjin, China (01360211) and Tian-Nan Union Found.
References
Fig. 6. Nyquist plot for MgNi, Mg0.7 Ti0.3 Ni and Mg0.7 Ti0.225 Al0.075 Ni electrode.
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