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Discharge behavior of MmNi3.66(CoAlMn)1.34 hydrogen storage alloys
Journal of Alloys and Compounds 438 (2007) 298–302
Discharge behavior of MmNi3.66(CoAlMn)1.34 hydrogen storage alloys Z.M. Wang a,∗ , C.Y.V. Li b , S.L.I. Chan b a
Center of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China b School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia Received 6 July 2006; accepted 16 August 2006 Available online 21 December 2006
Abstract Discharge behaviors of MmNi3.66 (CoAlMn)1.34 alloy electrodes at different discharge current (DC) have been investigated by simulated battery tests. The results indicate that discharge capacity is dependant on its DC adopted in the tests, in which discharge capacities decrease with increasing discharge current. More than 80% of capacity retention has been observed where DC is less than 1C, and 21.91% of capacity retention for DC = 3C. A second (additional) “activation process” is required in order to fully activate the MmNi3.66 (CoAlMn)1.34 alloy electrodes to reach its active charge/discharge states again, when discharged at a higher DC above 0.2C. In addition to normal discharge step, an extra low-current discharge process indeed can effectively discharge the retained capacity; however such act damages the quality of MmNi3.66 (CoAlMn)1.34 alloy electrodes, thereby resulting in an apparent decline in cycle life. © 2006 Elsevier B.V. All rights reserved. Keywords: MmNi3.66 (CoAlMn)1.34 ; Hydrogen storage; Discharge current (DC); Discharge capacity
1. Introduction AB5 -type hydrogen storage alloys have been extensively studied and applied as negative electrode materials in NiMH batteries; such alloys display capacities of 300–330 mAh/g, where 70–80% of capacity can be retained after 300 charging/ discharging cycles [1–3]. In order to employ this technology in the field of electric tools and hybrid electric vehicles (HEV), etc. [4,5], further improvement must be made on the ability of high-rate discharge, self-discharge, cycle life for AB5 -type hydrogen storage alloys so as to meet various requirements for the extended applications. Elemental substitution is proved as an effective method to enhance the overall properties of the hydrogen storage alloys [6–9]. Yuan et al. [10] reported that the discharge capacity, highrate discharge ability of LaNi5 -based alloys could be greatly improved by partially substituting La for Ce. Zhang et al. [11] reported that La0.7 Mg0.3 Ni2.975−x Co0.525 Mnx (x = 0.3) showed an impressive high-rate discharge (HRD) ability of 81.5% and discharge capacity of 356 mAh/g. Extensive studies also indicated that the characteristics of negative electrode are affected by
∗
Corresponding author. Tel.: +86 773 5605903. E-mail address: [email protected] (Z.M. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.072
correlated conditions. Our group [12,13] investigated the effect of electrolyte and MmNi5 -based alloy electrodes on high-rate discharge capacity by cross-examining different combinations of metal hydride electrode and electrolyte systems; and found that HRD capacity of Ni-MH battery is mostly affected by the type of electrolyte, then the type of alloy. Alloy electrode performed good durability in electrolyte, where an appropriate amount of Al2 (SO4 )3 or MnSO4 was added to the original electrolyte (6 M KOH + 1 wt% LiOH), respectively. In this paper, we focus on the discharge behavior of MmNi5 -based hydrogen storage alloys affected by the discharge current (DC) adopted in the tests, and discuss the relationships between the discharge capacity and different DC. 2. Experimental details MmNi3.66 (CoAlMn)1.34 hydrogen storage alloys were prepared in an arcmelting furnace under argon atmosphere, and turned over and remelted several times to assure homogeneity, then mechanically crushed and milled to powders below 250 mesh under a protective argon atmosphere. X-ray diffraction (XRD) analysis was carried out with a D8 Advance diffractometer using Cu K␣ radiation. The P–C isotherms were measured using a gas reaction controller. The electrode properties were measured by simulated battery tests with these alloys as negative electrodes. The negative electrodes were fabricated by the following procedures: Firstly, the alloy powder (0.6 g) was mixed with nickel powder (0.6 g) and a small amount of polytetrafluoroethylene (PTFE). Secondly, the mixture was pasted onto both sides of the nickel foam sheet (2.5 cm × 2.5 cm).
Z.M. Wang et al. / Journal of Alloys and Compounds 438 (2007) 298–302
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Table 2 Fundamental properties of hydrogen storage alloy powder used in this experiment Sample Composition Structure ˚ & c (A) ˚ Lattice constant a (A) ˚ 3) Unit cell volume (A Hydrogen uptake (wt%) Plateau pressure (atm)
Fig. 1. XRD patterns of MmNi3.66 (CoAlMn)1.34 hydrogen storage alloy. Finally, the pasted nickel sheets were cold pressed to a pellet at a compacting pressure of 155–175 MPa. Commercial nickel hydroxide Ni(OH)2 was used as the counter electrode (positive). The negative electrode was separated from the counter electrode (Ni(OH)2 ) by a porous frit in aqueous electrolyte. Charge and discharge tests were carried out on a computer-controlled battery testing instrument (CT2001A). The charge/discharge tests were focused mainly on the electrochemical capacity and stability of the negative alloy electrode, thus the capacity of the positive electrode plate was designed to be much higher than that of the negative electrode. Test battery (after being activated) will be overcharged to 120% state of charge (S.O.C.) at 0.2C, and discharged at different current to cut-off voltage of 0.9 V. 0.2C, 1.0C, 1.5C, 2.0C, 2.5C, 3.0C were selected as the discharge current, respectively. In some cases, an additional low-current discharge process (0.2C) is followed after the normal discharge step to discharge the remaining capacity within the test battery. Capacity retention can be calculated by this equation: capacity retention (%) = (Q2 /Qmax ) × 100%, where Qmax denotes the max value of discharge capacity of the test battery, and Q2 denotes the actual value of discharge capacity.
3. Results and discussion 3.1. Characterization and PCI curves of alloy MmNi3.66 (CoAlMn)1.34
MmNi3.66 Co0.74 Mn0.41 Al0.18 Mm = (La0.58 Ce0.25 Pr0.06 Nd0.11 ) AB5 , CaCu5 a = 4.988; c = 4.08 87.911 1.7 (under 10 atm) 0.3
where the lattice constants (a and c) of AB5 phase belonging to the CaCu5 crystal structure were obtained by exact calculation, the unit cell volume was √ calculated by the following equation: V = a2 c sin 120◦ = a2 c 3/2. The calculated unit cell volume ˚ 3 to the alloy. In general, the size of the crysis about 87.911 A tal lattice in AB5 -based hydrogen storage alloys significantly affects the hydrogen storage capacity and plateau pressure, larger crystal lattice size leads to higher hydrogen storage capacity and lower plateau pressure. Good cycle life is observed due to less distortion caused by hydrogen absorption/desorption in large lattice. The value of H/M and plateau pressure in hydrogen absorbing process are 1.7 wt% and 0.3 atm for the alloy (illustrated in Fig. 2), performing typical absorption/desorption characteristics of AB5 type hydrogen storage alloys. 3.2. Discharge capacity of MmNi3.66 (CoAlMn)1.34 alloys at different DC Discharge behaviors of Ni-MH battery are important, especially in the field of electric tools and electric vehicles. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloy electrode at different discharge currents (0.2C, 1.0C, 1.5C, 2.0C, 2.5C, 3.0C) are shown in Fig. 3, respectively. Firstly, MmNi5 -based hydrogen storage alloys require five to six charge/discharge cycles to be fully activated. After activation, different discharge current is selected to characterize the discharge behavior of
The XRD patterns of MmNi5 -based alloy MmNi3.66 (CoAlMn)1.34 are shown in Fig. 1, which demonstrated that it is a single-phase alloy belonging to CaCu5 hexagonal structure. The results of ICP analysis for alloy MmNi3.66 Co0.74 Mn0.41 Al0.18 are shown in Table 1, indicating the alloy with abundant La. Fundamental properties of these alloys are listed in Table 2, Table 1 ICP analysis of the alloy powders Wt%
Sample
La Ce Pr Nd Ni Co Mn Al
20.53 8.96 2.09 4.10 43.22 8.73 4.57 1.00
Fig. 2. PCI curve for MmNi3.66 (CoAlMn)1.34 alloy powders at room temperature (298 K).
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Fig. 3. Discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current.
Fig. 5. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current (DC). (a) DC = 1C of DC = 1C + 0.2C; (b) additional low-current discharge step (DC = 0.2C); (c) discharge by 1C only.
From both Figs. 3 and 4, we observed that discharge capacity of MmNi3.66 (CoAlMn)1.34 alloy electrode decreases with increasing discharge current. Compared to the results obtained by discharging at 0.2C, there are obvious capacity losses in
these batteries being discharged at a higher discharge current. In order to determine whether such loss in capacity is reversible or irreversible, an additional low-current discharge (0.2C) step is followed after the normal discharge step to discharge the retained capacity in the testing batteries for each cycle. The result of the testing battery discharged by DC = 1C only and DC = 1C + 0.2C (Fig. 5). DC = xC + 0.2C means the nominal discharge current is xC follows with an additional 0.2C low-current discharge for each cycle; equivalent representation applies to x = 1, 2, 3. Discharge capacity at DC = 1C where an extra 0.2C discharge is applied, was around 270 mAh/g (Fig. 5—line (a)) and declines after 30 cycles. The capacity obtained by the subsequent low-current discharge was about 40 mAh/g (Fig. 5—line (b)), indicating this extra step at each cycle can discharge some retained capacity of the alloy electrode, thus the capacity loss is partly reversible. For the testing electrode discharged solely by DC = 1C (i.e. without additional 0.2C step), the alloy electrode maintained stable charge/discharge state at 260 mAh/g throughout its cycle life (Fig. 5—line (c)). Fig. 6 showed the results of the alloy electrode discharged at DC = 2C only and DC = 2C + 0.2C, respectively. When discharges occur at 2C only (Fig. 6—line (c)), the
Fig. 4. Capacity retentions of MmNi3.66 (CoAlMn)1.34 alloy electrodes at different discharge current after 60 charge/discharge cycles.
Fig. 6. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current (DC). (a) DC = 2C of DC = 2C + 0.2C; (b) additional low-current discharge step (DC = 0.2C); (c) discharge by 2C only.
these samples. It can be seen clearly from Fig. 3 that the discharge capacity decreases with increasing discharge current adopted in tests. Another interesting phenomenon is that a second “activation process” is required for these testing batteries (previously been activated at DC = 0.2C) in order to reach the stable charge/discharge state when the discharge current is higher than the nominal 0.2C. Capacity retention after 60 cycles, were shown in Fig. 4, about 85.41% of capacity retention for battery discharged at DC = 0.2C; 80.56% for the one discharged at DC = 1C. An apparent decrease in capacity retention (41.66%) was observed when discharged at DC = 2.5C, only 21.91% at DC = 3.0C. 3.3. Effect of additional low-current discharge process on the discharge behavior of MmNi3.66 (CoAlMn)1.34 alloy electrode
Z.M. Wang et al. / Journal of Alloys and Compounds 438 (2007) 298–302
Fig. 7. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current (DC). (a) DC = 3C of DC = 3C + 0.2C; (b) additional low-current discharge step (DC = 0.2C); (c) discharge by 3C only.
charge/discharge behaviour is fairly stable at 200 mAh/g. Similar to DC = 1C + 0.2C, when DC = 2C + 0.2C, there is a decline in capacity (<200 mAh/g); while the discharge capacity increases to 100–140 mAh/g resulting from the additional step of 0.2C discharge. Fig. 7 illustrates the discharge behaviour when discharged at DC = 3C and DC = 3C + 0.2C. When discharged at DC = 3C (line c) and DC = 3C + 0.2C (line a), the discharge capacity is merely 75 and 60 mAh/g, respectively. In contrast, the capacity discharged by the extra 0.2C step ranges from 230 to 260 mAh/g, which is a lot higher than that obtained by the nominal 3C discharge step. Such behaviour can be explained in terms of high-rate discharge. In general, the discharge process of the alloy electrode mainly consists of two steps, one is the diffusion of Hatoms through the alloy electrode, the other is electron transfer rate on the electrode surface. As the rate of discharge increased, some H-atoms were unable to release from the electrode before reaching the cut-off voltage as the rate of diffusion was slower than electron transfer rate on the electrode surface. Thus, at lower discharge current, there was sufficient time for hydrogen diffusion allowing the electrode to be fully desorbed. Therefore, the capacity obtained at low DC is higher than the one obtained at high DC, in which the retained capacity of the alloy electrode can be further discharged by a subsequent low-current discharge. Discharge curve at different discharge currents is shown on Fig. 8, in which the behaviour when discharge solely at 0.2C (DC = 0.2C) is compared to the combined effect of the extra low-current step (i.e. DC = 1C + 0.2C, DC = 2C + 0.2C, DC = 3C + 0.2C). When DC = 1C + 0.2C, the total capacity increases slightly before 30 cycles and declines significantly after 30 cycles. For DC = 2C + 0.2C and DC = 3C + 0.2C, the total capacities were apparently higher than the one obtained by DC = 0.2C. According to the results in Figs. 5–8, it can be concluded that the discharge capacity of MmNi3.66 (CoAlMn)1.34 alloy electrodes were dependant on the discharge current, the alloy electrode discharges more capacity at lower DC, and discharges
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Fig. 8. The overall effect of the extra 0.2C discharge step at different discharge currents (i.e. DC = 1C + 0.2C, DC = 2C + 0.2C, DC = 3C + 0.2C) by comparing to the electrode discharged at 0.2C only (i.e. DC = 0.2C).
less capacity at higher DC. From Figs. 5–7, we observed that the additional low-current discharge (0.2C) process at each cycle decreases the electrode capacity obtained by nominal discharge, though it subsequently discharges the retained capacity within the testing electrodes. As shown in Fig. 8, the total discharge capacities obtained by DC = 1C + 0.2C, DC = 2C + 0.2C, DC = 3C + 0.2C is higher than the one obtained by DC = 0.2C to a limited extent, but this extra discharge process lowers the quality of MmNi3.66 (CoAlMn)1.34 alloy electrodes, resulting in an apparent decline in cycle life (illustrated in Fig. 5(a)). 4. Conclusions A second “activation process” is required for MmNi3.66 (CoAlMn)1.34 alloy electrodes (initially activated at DC = 0.2C), when the discharge current is above 0.2C. Discharge capacity of the alloys depends greatly on the adopted discharge current. The discharge efficiency decreases with the increase in discharge current. More than 80% capacity retention has been observed in cases where DC is less than 1C, and 21.91% of capacity retention when DC = 3C. Excluding the normal charge/discharge steps of the alloy electrodes, an additional low-current discharge process is followed after each discharge step. It effectively discharges the retained capacity within testing electrodes, but slightly lowers the quality of MmNi3.66 (CoAlMn)1.34 alloy electrodes and causes the cycle life to decline. References [1] C. Iwakura, K. Fukuda, H. Senoh, H. Inoue, Electrochim. Acta 43 (1998) 2041. [2] J. Han, F. Feng, M. Geng, R. Buxbaun, D.O. Northwood, J. Power Sources 80 (1999) 39. [3] A. Zuttel, D. Chartouni, K. Gross, P. Spatz, M. Bachler, F. Lichtenberg, A. Folzer, N.J.E. Adkins, J. Alloys Compds. 253–254 (1997) 626–631. [4] P. Gifford, J. Adams, D. Corrigan, S. Venkatesan, J. Power Sources 80 (1999) 157. [5] F. Haschka, W. Warthmann, G. Benczru-Urmossy, J. Power Sources 72 (1998) 32.
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[6] X. Zhang, Y. Chai, W. Yin, M. Zhao, J. Solid State Chem. 177 (2004) 2373–2377. [7] X. Zhang, D. Sun, W. Yin, Y. Chai, M. Zhao, Scripta Mater. (in press). [8] D. Lu, W. Li, S. Hu, F. Xiao, R. Tang, Int. J. Hydrogen Energy (in press). [9] H. Pan, N. Chen, M. Gao, R. Li, Y. Lei, Q. Wang, J. Alloys Compd. 397 (2005) 306–312.
[10] X.X. Yuan, H.-S. Liu, Z.-F. Ma, N.X. Xu, J. Alloys Compd. 359 (2003) 300–306. [11] X. Zhang, D. Sun, W. Yin, Y. Chai, M. Zhao, Electrochim. Acta 50 (2005) 2911–2918. [12] W. Weng, Thesis (master degree), Taiwan, 2003. [13] Z. Jian, Thesis (master degree), Taiwan, 2002.
Discharge behavior of MmNi3.66(CoAlMn)1.34 hydrogen storage alloys Z.M. Wang a,∗ , C.Y.V. Li b , S.L.I. Chan b a
Center of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China b School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia Received 6 July 2006; accepted 16 August 2006 Available online 21 December 2006
Abstract Discharge behaviors of MmNi3.66 (CoAlMn)1.34 alloy electrodes at different discharge current (DC) have been investigated by simulated battery tests. The results indicate that discharge capacity is dependant on its DC adopted in the tests, in which discharge capacities decrease with increasing discharge current. More than 80% of capacity retention has been observed where DC is less than 1C, and 21.91% of capacity retention for DC = 3C. A second (additional) “activation process” is required in order to fully activate the MmNi3.66 (CoAlMn)1.34 alloy electrodes to reach its active charge/discharge states again, when discharged at a higher DC above 0.2C. In addition to normal discharge step, an extra low-current discharge process indeed can effectively discharge the retained capacity; however such act damages the quality of MmNi3.66 (CoAlMn)1.34 alloy electrodes, thereby resulting in an apparent decline in cycle life. © 2006 Elsevier B.V. All rights reserved. Keywords: MmNi3.66 (CoAlMn)1.34 ; Hydrogen storage; Discharge current (DC); Discharge capacity
1. Introduction AB5 -type hydrogen storage alloys have been extensively studied and applied as negative electrode materials in NiMH batteries; such alloys display capacities of 300–330 mAh/g, where 70–80% of capacity can be retained after 300 charging/ discharging cycles [1–3]. In order to employ this technology in the field of electric tools and hybrid electric vehicles (HEV), etc. [4,5], further improvement must be made on the ability of high-rate discharge, self-discharge, cycle life for AB5 -type hydrogen storage alloys so as to meet various requirements for the extended applications. Elemental substitution is proved as an effective method to enhance the overall properties of the hydrogen storage alloys [6–9]. Yuan et al. [10] reported that the discharge capacity, highrate discharge ability of LaNi5 -based alloys could be greatly improved by partially substituting La for Ce. Zhang et al. [11] reported that La0.7 Mg0.3 Ni2.975−x Co0.525 Mnx (x = 0.3) showed an impressive high-rate discharge (HRD) ability of 81.5% and discharge capacity of 356 mAh/g. Extensive studies also indicated that the characteristics of negative electrode are affected by
∗
Corresponding author. Tel.: +86 773 5605903. E-mail address: [email protected] (Z.M. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.072
correlated conditions. Our group [12,13] investigated the effect of electrolyte and MmNi5 -based alloy electrodes on high-rate discharge capacity by cross-examining different combinations of metal hydride electrode and electrolyte systems; and found that HRD capacity of Ni-MH battery is mostly affected by the type of electrolyte, then the type of alloy. Alloy electrode performed good durability in electrolyte, where an appropriate amount of Al2 (SO4 )3 or MnSO4 was added to the original electrolyte (6 M KOH + 1 wt% LiOH), respectively. In this paper, we focus on the discharge behavior of MmNi5 -based hydrogen storage alloys affected by the discharge current (DC) adopted in the tests, and discuss the relationships between the discharge capacity and different DC. 2. Experimental details MmNi3.66 (CoAlMn)1.34 hydrogen storage alloys were prepared in an arcmelting furnace under argon atmosphere, and turned over and remelted several times to assure homogeneity, then mechanically crushed and milled to powders below 250 mesh under a protective argon atmosphere. X-ray diffraction (XRD) analysis was carried out with a D8 Advance diffractometer using Cu K␣ radiation. The P–C isotherms were measured using a gas reaction controller. The electrode properties were measured by simulated battery tests with these alloys as negative electrodes. The negative electrodes were fabricated by the following procedures: Firstly, the alloy powder (0.6 g) was mixed with nickel powder (0.6 g) and a small amount of polytetrafluoroethylene (PTFE). Secondly, the mixture was pasted onto both sides of the nickel foam sheet (2.5 cm × 2.5 cm).
Z.M. Wang et al. / Journal of Alloys and Compounds 438 (2007) 298–302
299
Table 2 Fundamental properties of hydrogen storage alloy powder used in this experiment Sample Composition Structure ˚ & c (A) ˚ Lattice constant a (A) ˚ 3) Unit cell volume (A Hydrogen uptake (wt%) Plateau pressure (atm)
Fig. 1. XRD patterns of MmNi3.66 (CoAlMn)1.34 hydrogen storage alloy. Finally, the pasted nickel sheets were cold pressed to a pellet at a compacting pressure of 155–175 MPa. Commercial nickel hydroxide Ni(OH)2 was used as the counter electrode (positive). The negative electrode was separated from the counter electrode (Ni(OH)2 ) by a porous frit in aqueous electrolyte. Charge and discharge tests were carried out on a computer-controlled battery testing instrument (CT2001A). The charge/discharge tests were focused mainly on the electrochemical capacity and stability of the negative alloy electrode, thus the capacity of the positive electrode plate was designed to be much higher than that of the negative electrode. Test battery (after being activated) will be overcharged to 120% state of charge (S.O.C.) at 0.2C, and discharged at different current to cut-off voltage of 0.9 V. 0.2C, 1.0C, 1.5C, 2.0C, 2.5C, 3.0C were selected as the discharge current, respectively. In some cases, an additional low-current discharge process (0.2C) is followed after the normal discharge step to discharge the remaining capacity within the test battery. Capacity retention can be calculated by this equation: capacity retention (%) = (Q2 /Qmax ) × 100%, where Qmax denotes the max value of discharge capacity of the test battery, and Q2 denotes the actual value of discharge capacity.
3. Results and discussion 3.1. Characterization and PCI curves of alloy MmNi3.66 (CoAlMn)1.34
MmNi3.66 Co0.74 Mn0.41 Al0.18 Mm = (La0.58 Ce0.25 Pr0.06 Nd0.11 ) AB5 , CaCu5 a = 4.988; c = 4.08 87.911 1.7 (under 10 atm) 0.3
where the lattice constants (a and c) of AB5 phase belonging to the CaCu5 crystal structure were obtained by exact calculation, the unit cell volume was √ calculated by the following equation: V = a2 c sin 120◦ = a2 c 3/2. The calculated unit cell volume ˚ 3 to the alloy. In general, the size of the crysis about 87.911 A tal lattice in AB5 -based hydrogen storage alloys significantly affects the hydrogen storage capacity and plateau pressure, larger crystal lattice size leads to higher hydrogen storage capacity and lower plateau pressure. Good cycle life is observed due to less distortion caused by hydrogen absorption/desorption in large lattice. The value of H/M and plateau pressure in hydrogen absorbing process are 1.7 wt% and 0.3 atm for the alloy (illustrated in Fig. 2), performing typical absorption/desorption characteristics of AB5 type hydrogen storage alloys. 3.2. Discharge capacity of MmNi3.66 (CoAlMn)1.34 alloys at different DC Discharge behaviors of Ni-MH battery are important, especially in the field of electric tools and electric vehicles. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloy electrode at different discharge currents (0.2C, 1.0C, 1.5C, 2.0C, 2.5C, 3.0C) are shown in Fig. 3, respectively. Firstly, MmNi5 -based hydrogen storage alloys require five to six charge/discharge cycles to be fully activated. After activation, different discharge current is selected to characterize the discharge behavior of
The XRD patterns of MmNi5 -based alloy MmNi3.66 (CoAlMn)1.34 are shown in Fig. 1, which demonstrated that it is a single-phase alloy belonging to CaCu5 hexagonal structure. The results of ICP analysis for alloy MmNi3.66 Co0.74 Mn0.41 Al0.18 are shown in Table 1, indicating the alloy with abundant La. Fundamental properties of these alloys are listed in Table 2, Table 1 ICP analysis of the alloy powders Wt%
Sample
La Ce Pr Nd Ni Co Mn Al
20.53 8.96 2.09 4.10 43.22 8.73 4.57 1.00
Fig. 2. PCI curve for MmNi3.66 (CoAlMn)1.34 alloy powders at room temperature (298 K).
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Fig. 3. Discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current.
Fig. 5. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current (DC). (a) DC = 1C of DC = 1C + 0.2C; (b) additional low-current discharge step (DC = 0.2C); (c) discharge by 1C only.
From both Figs. 3 and 4, we observed that discharge capacity of MmNi3.66 (CoAlMn)1.34 alloy electrode decreases with increasing discharge current. Compared to the results obtained by discharging at 0.2C, there are obvious capacity losses in
these batteries being discharged at a higher discharge current. In order to determine whether such loss in capacity is reversible or irreversible, an additional low-current discharge (0.2C) step is followed after the normal discharge step to discharge the retained capacity in the testing batteries for each cycle. The result of the testing battery discharged by DC = 1C only and DC = 1C + 0.2C (Fig. 5). DC = xC + 0.2C means the nominal discharge current is xC follows with an additional 0.2C low-current discharge for each cycle; equivalent representation applies to x = 1, 2, 3. Discharge capacity at DC = 1C where an extra 0.2C discharge is applied, was around 270 mAh/g (Fig. 5—line (a)) and declines after 30 cycles. The capacity obtained by the subsequent low-current discharge was about 40 mAh/g (Fig. 5—line (b)), indicating this extra step at each cycle can discharge some retained capacity of the alloy electrode, thus the capacity loss is partly reversible. For the testing electrode discharged solely by DC = 1C (i.e. without additional 0.2C step), the alloy electrode maintained stable charge/discharge state at 260 mAh/g throughout its cycle life (Fig. 5—line (c)). Fig. 6 showed the results of the alloy electrode discharged at DC = 2C only and DC = 2C + 0.2C, respectively. When discharges occur at 2C only (Fig. 6—line (c)), the
Fig. 4. Capacity retentions of MmNi3.66 (CoAlMn)1.34 alloy electrodes at different discharge current after 60 charge/discharge cycles.
Fig. 6. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current (DC). (a) DC = 2C of DC = 2C + 0.2C; (b) additional low-current discharge step (DC = 0.2C); (c) discharge by 2C only.
these samples. It can be seen clearly from Fig. 3 that the discharge capacity decreases with increasing discharge current adopted in tests. Another interesting phenomenon is that a second “activation process” is required for these testing batteries (previously been activated at DC = 0.2C) in order to reach the stable charge/discharge state when the discharge current is higher than the nominal 0.2C. Capacity retention after 60 cycles, were shown in Fig. 4, about 85.41% of capacity retention for battery discharged at DC = 0.2C; 80.56% for the one discharged at DC = 1C. An apparent decrease in capacity retention (41.66%) was observed when discharged at DC = 2.5C, only 21.91% at DC = 3.0C. 3.3. Effect of additional low-current discharge process on the discharge behavior of MmNi3.66 (CoAlMn)1.34 alloy electrode
Z.M. Wang et al. / Journal of Alloys and Compounds 438 (2007) 298–302
Fig. 7. The discharge curves of MmNi3.66 (CoAlMn)1.34 alloys at different discharge current (DC). (a) DC = 3C of DC = 3C + 0.2C; (b) additional low-current discharge step (DC = 0.2C); (c) discharge by 3C only.
charge/discharge behaviour is fairly stable at 200 mAh/g. Similar to DC = 1C + 0.2C, when DC = 2C + 0.2C, there is a decline in capacity (<200 mAh/g); while the discharge capacity increases to 100–140 mAh/g resulting from the additional step of 0.2C discharge. Fig. 7 illustrates the discharge behaviour when discharged at DC = 3C and DC = 3C + 0.2C. When discharged at DC = 3C (line c) and DC = 3C + 0.2C (line a), the discharge capacity is merely 75 and 60 mAh/g, respectively. In contrast, the capacity discharged by the extra 0.2C step ranges from 230 to 260 mAh/g, which is a lot higher than that obtained by the nominal 3C discharge step. Such behaviour can be explained in terms of high-rate discharge. In general, the discharge process of the alloy electrode mainly consists of two steps, one is the diffusion of Hatoms through the alloy electrode, the other is electron transfer rate on the electrode surface. As the rate of discharge increased, some H-atoms were unable to release from the electrode before reaching the cut-off voltage as the rate of diffusion was slower than electron transfer rate on the electrode surface. Thus, at lower discharge current, there was sufficient time for hydrogen diffusion allowing the electrode to be fully desorbed. Therefore, the capacity obtained at low DC is higher than the one obtained at high DC, in which the retained capacity of the alloy electrode can be further discharged by a subsequent low-current discharge. Discharge curve at different discharge currents is shown on Fig. 8, in which the behaviour when discharge solely at 0.2C (DC = 0.2C) is compared to the combined effect of the extra low-current step (i.e. DC = 1C + 0.2C, DC = 2C + 0.2C, DC = 3C + 0.2C). When DC = 1C + 0.2C, the total capacity increases slightly before 30 cycles and declines significantly after 30 cycles. For DC = 2C + 0.2C and DC = 3C + 0.2C, the total capacities were apparently higher than the one obtained by DC = 0.2C. According to the results in Figs. 5–8, it can be concluded that the discharge capacity of MmNi3.66 (CoAlMn)1.34 alloy electrodes were dependant on the discharge current, the alloy electrode discharges more capacity at lower DC, and discharges
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Fig. 8. The overall effect of the extra 0.2C discharge step at different discharge currents (i.e. DC = 1C + 0.2C, DC = 2C + 0.2C, DC = 3C + 0.2C) by comparing to the electrode discharged at 0.2C only (i.e. DC = 0.2C).
less capacity at higher DC. From Figs. 5–7, we observed that the additional low-current discharge (0.2C) process at each cycle decreases the electrode capacity obtained by nominal discharge, though it subsequently discharges the retained capacity within the testing electrodes. As shown in Fig. 8, the total discharge capacities obtained by DC = 1C + 0.2C, DC = 2C + 0.2C, DC = 3C + 0.2C is higher than the one obtained by DC = 0.2C to a limited extent, but this extra discharge process lowers the quality of MmNi3.66 (CoAlMn)1.34 alloy electrodes, resulting in an apparent decline in cycle life (illustrated in Fig. 5(a)). 4. Conclusions A second “activation process” is required for MmNi3.66 (CoAlMn)1.34 alloy electrodes (initially activated at DC = 0.2C), when the discharge current is above 0.2C. Discharge capacity of the alloys depends greatly on the adopted discharge current. The discharge efficiency decreases with the increase in discharge current. More than 80% capacity retention has been observed in cases where DC is less than 1C, and 21.91% of capacity retention when DC = 3C. Excluding the normal charge/discharge steps of the alloy electrodes, an additional low-current discharge process is followed after each discharge step. It effectively discharges the retained capacity within testing electrodes, but slightly lowers the quality of MmNi3.66 (CoAlMn)1.34 alloy electrodes and causes the cycle life to decline. References [1] C. Iwakura, K. Fukuda, H. Senoh, H. Inoue, Electrochim. Acta 43 (1998) 2041. [2] J. Han, F. Feng, M. Geng, R. Buxbaun, D.O. Northwood, J. Power Sources 80 (1999) 39. [3] A. Zuttel, D. Chartouni, K. Gross, P. Spatz, M. Bachler, F. Lichtenberg, A. Folzer, N.J.E. Adkins, J. Alloys Compds. 253–254 (1997) 626–631. [4] P. Gifford, J. Adams, D. Corrigan, S. Venkatesan, J. Power Sources 80 (1999) 157. [5] F. Haschka, W. Warthmann, G. Benczru-Urmossy, J. Power Sources 72 (1998) 32.
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