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The electrochemical behavior of Ni(OH)2 in KOH solution containing aluminum hydroxide
Materials Chemistry and Physics 86 (2004) 293–297
The electrochemical behavior of Ni(OH)2 in KOH solution containing aluminum hydroxide Qian Zhang a , Yanhui Xu b , Xiaolin Wang a,∗ a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China b Fuel Science Laboratory, Faculty of Mechanic Engineering and Production, Hamburg University of Applied Sciences, 20099 Hamburg, Germany
Received 10 January 2004; received in revised form 4 March 2004; accepted 12 March 2004
Abstract In this paper Co-doped Al-substituted ␣-Ni(OH)2 was prepared by a homogeneous precipitation method and its electrochemical behavior, in two kinds of electrolyte (7 M KOH solution and 7 M KOH saturated with Al(OH)3 ), was investigated by galvanostatic charge-discharge method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical properties of the commercial -Ni(OH)2 in both electrolyte was also studied in order to compare with that of ␣-Ni(OH)2 . The results show that Co-doped Al-substituted ␣-Ni(OH)2 has the largest capacity in 7 M KOH solution (300 mAh g−1 ). The addition of Al to KOH solution is harmful to the reversible capacity and cycle life of both ␣-Ni(OH)2 and -Ni(OH)2 . X-ray diffraction (XRD) patterns show that, for Co-doped Al-substituted ␣-Ni(OH)2 , its turbostratic structure maintains after cycles in both alkaline electrolyte, despite the interlayer distance becoming smaller. For -Ni(OH)2 , however, ␥-NiOOH species appeared after cycling in both electrolyte and its content is higher after cycling in 7 M KOH saturated with Al(OH)3 . And adding Al into KOH solution caused the decrease of solution conductivity and increased the apparent diffusion coefficient which is evidenced by electrochemical impedance spectroscopy. © 2004 Published by Elsevier B.V. Keywords: ␣-Ni(OH)2 ; Impedance spectroscopy; Aluminum hydroxide; Electrochemistry
1. Introduction Ni-MH battery and Ni–Cd battery are important power sources and are extensively used in industry and daily life. Their positive electrode active material is -Ni(OH)2 , compared with ␣-Ni(OH)2 has many advantages—faster proton diffusion rate, smaller expansion during cycling and more numbers of exchange electrons [1]. Pure ␣-Ni(OH)2 , however, is unstable in strong alkaline solution and transforms into -Ni(OH)2 . Since Bode found that replacing Ni atom by other element, such as Co, Al, Fe, and Mn, is effective in stabling its turbostratic structure it has attracted much attention [2–12]. On the other hand as well known, now Ni-MH battery is replacing the Ni–Cd battery because of its superior performance. And the main reason is that metal-hydride electrode has better electrochemical properties compared with Cd electrode. Metal-hydride material includes many types: AB5 type rare-earth/Ni based, AB2 Ti-based and ∗ Corresponding author. Tel.: +86 10 62794741; fax: +86 10 62794742. E-mail address: [email protected] (X. Wang).
0254-0584/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2004.03.029
Zr-based, AB type Ti–Ni based and V-based solid solution phase and Mg–Ni amorphous alloys [13–18]. In order to furthermore improve their cycle life and reversible capacity, the addition of some elements, such as Co, Al, Mn, Nb, Si, Cr, etc., was used frequently. Some researchers found that, however, the dissolution of the added elements into alkaline solution also causes the decrease of discharge capacity of metal-hydride electrode. One possible method is to add the same element into alkaline solution in order to prevent dissolving. But before adding this element into the solution, its influence on the electrochemical behavior of positive electrode needs to be well studied. In this study, because we want to dissolve Al element into 7 M KOH solution in order to prevent the dissolution of Al in metal-hydride alloy and improve properties of metal-hydride electrode, we first investigated the influence of Al additive on Ni(OH)2 . This is just the background of this article, i.e. in this paper we reported the electrochemical behavior of Co-doped Al-substituted ␣-Ni(OH)2 and commercial -Ni(OH)2 in Al-containing 7 M KOH solution by electrochemical techniques.
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2. Experimental 2.1. Synthesis of samples The ␣-Ni(OH)2 material was prepared by a homogeneous precipitation method which was described in our previous paper [12]. In brief, the mixed solution of Ni(NO3 )2 , Al(NO3 )3 , Co(NO3 )2 and urea with certain ratio (the molar ratios of Al/Ni and Co/Ni were set to be 3/16 and 1/16, respectively) was stirred for some time under controlled reaction temperature. Obtained precipitate was washed several times with deionized water to neutralization and filtered. Then the cake was dried in air. The -Ni(OH)2 material is a commercial sample. 2.2. Physical characterization of samples The crystal structure of the samples were determined by X-ray diffraction (XRD) analysis using a BRUKER D8 Advance X-ray diffractometer with Cu K␣ radiation, at a scanning rate of 3◦ (2θ) per minute and a scanning range of 5–80◦ (2θ). And the ␣-Ni(OH)2 and -Ni(OH)2 powders were characterized directly before cycling and the electrodes prepared with both materials were characterized so that nickel peaks resulted from nickel substrate in XRD patterns after cycling. 2.3. Preparation of nickel electrodes and their electrochemical test The pasted nickel electrodes were prepared as follows: 70 wt.% nickel hydroxide and 15 wt.% acetylene black powder or carbon powder were thoroughly mixed with 15 wt.% ploy(tetrafluoroethylene) emulsion. The paste obtained was incorporated into nickel foam with a spatula. The pasted nickel electrodes were dried and then pressed under a pressure of 200 kg cm−2 for 2 min. Thereafter, the electrodes were soaked in 7 M KOH for 24 h before being coupled with porous nickel electrodes on either side as counter electrodes and Hg/HgO electrodes as reference electrodes. Galvanostatic charge-discharge studies were conducted on LAND series battery system instrument (made in China). The working electrode was galvanostatically charged at 120 mA g−1 rate, set for 5 min, and then discharged at 60 mA g−1 rate to 0.2 V versus Hg/HgO/7 M KOH electrode. Electrochemical impedance spectroscopy (EIS) was performed using EG&G PARC Model 283 Potentioatat/Galvanostat and Model 1025 Frequency Analyzer. 3. Results and discussion The morphology of Ni(OH)2 powders was determined by scanning electron microscopy, as shown in Fig. 1, from which it can be seen that the -Ni(OH)2 has sphere or ellipsoid shape and the ␣-Ni(OH)2 has no regular shape. For
Fig. 1. The SEM of Ni(OH)2 powders: (A) -Ni(OH)2 ; (B) ␣-Ni(OH)2 .
industrialized-scale production, the sphere or/and ellipsoid shape is needed for higher tap density. Fig. 2 shows the gavalnostatic charge-discharge results of ␣-Ni(OH)2 and -Ni(OH)2 in electrolyte 1 (7 M KOH) and 2 (7 M KOH saturated with Al(OH)3 ), respectively. In electrolyte 1(7 M KOH), specific discharge capacities are about 300 and 260 mAh g−1 for ␣-Ni(OH)2 and -Ni(OH)2 , respectively. And almost no change after 32 charge-discharge cycles, which show that the cycle stability is good for both ␣-Ni(OH)2 and -Ni(OH)2 . On the other hand, in electrolyte 2(7 M KOH saturated with Al(OH)3 ), specific discharge capacity for ␣-Ni(OH)2 is about 290 mAh g−1 at the first cycle, but decreases quickly in subsequent charge-discharge cycles and declines to about 250 mAh g−1 at the 32nd cycle. And
Q. Zhang et al. / Materials Chemistry and Physics 86 (2004) 293–297
Fig. 2. The cycle performance of ␣-Ni(OH)2 and -Ni(OH)2 in both alkaline electrolytes. The current density: 120 mA g−1 for charge and 60 mA g−1 for discharge.
for -Ni(OH)2 in this electrolyte, specific discharge capacity is greatest at the second cycle and then decreases more quickly than ␣-Ni(OH)2 and drops to about 160 mAh g−1 at the 32nd cycle. Galvanostatic charge-discharge curves of both ␣-Ni(OH)2 and -Ni(OH)2 in electrolyte 1(7 M KOH) and 2(7 M KOH saturated with Al(OH)3 ) are shown in Fig. 3. It can be observed that at the same charge-discharge current density, the polarization of ␣-Ni(OH)2 in electrolyte 1(7 M KOH) is the smallest. In electrolyte 1(7 M KOH), the charge potential of ␣-Ni(OH)2 is 50 mV lower than in electrolyte 2 (7 M KOH saturated with Al(OH)3 ), whereas the discharge potential is almost unchanged. It indicates that charging
Fig. 3. The charge-discharge curves of ␣-Ni(OH)2 and -Ni(OH)2 in both alkaline electrolytes. The current density: 120 mA g−1 for charge and 60 mA g−1 for discharge.
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is more difficult due to AlO2 − anions. For -Ni(OH)2 , in electrolyte 1 and 2, not only charge-discharge potential but also charge-discharge curves shape alter strongly, especially the charge curve. When specific charge capacity is less than 40 mAh g−1 , the charge potential in electrolyte 2 is slightly higher than that in electrolyte 1. And in the range of 40–100 mAh g−1 , two charge curves are almost the same. Subsequently, the charge potential of -Ni(OH)2 in electrolyte 2 is much higher than that in electrolyte 1. Moreover, this difference increases with the charging and reaches to 70 mV at the end of charging. The charge curve of -Ni(OH)2 in electrolyte 2 is very steep and charge potential increases quickly with the charging. And the oxygen evolution reaction occurs earlier for -Ni(OH)2 in electrolyte 2 than in electrolyte 1, which suggests that adding Al in electrolyte is harmful for charge efficiency. For discharge curves, there is a little difference between two curves of -Ni(OH)2 in both alkaline electrolytes. In the initial discharging period, i.e. in the range of 0–20 mAh g−1 discharge capacity, the discharge potential is equal in electrolyte 1 and 2. And in the range of 20–60 mAh g−1 discharge capacity, the discharge potential in electrolyte 2 is a little higher than that in electrolyte 1. With the increase of the discharge capacity, however, the discharge potential in electrolyte 2 becomes lower slightly than that in electrolyte 1. The polarization of -Ni(OH)2 is also enhanced with the addition of Al in electrolyte, just as ␣-Ni(OH)2 . But compared with -Ni(OH)2 , both in electrolyte 1 and 2, there are obvious advantages for Co-doped ␣-Ni(OH)2 —higher discharge potential and lower charge potential. The structural changes of ␣-Ni(OH)2 and -Ni(OH)2 before and after cycling in electrolyte 1 and 2 are shown in Figs. 4 and 5, respectively. The XRD peaks of ␣-Ni(OH)2 before cycling (Fig. 4a) are very narrow and sharp with a low angle reflection at 2θ = 11.3◦ , followed by another at 2θ = 22.6◦ corresponding to (0 0 3) and (0 0 6) planes, respectively. And on the higher 2θ side there are 3 consecutive reflection peaks at 2θ = 34.5–46.0◦ . Both of the XRD patterns of ␣-Ni(OH)2 (discharge state) after cycled 32 times in electrolyte 1 and 2, show the characteristic of ␣-Ni(OH)2 . The reflection peak at 2θ = 18.4◦ is the characteristic of poly(tetrafluoroethylene) used as the binder. And the reflection peaks at 2θ = 44.7◦ and 52.1◦ are the characteristic of nickel used as current collector. Although the structure of ␣-Ni(OH)2 still maintains after cycling in both alkaline electrolyte, there are very many small mussy peaks and the base line shifts in the XRD patterns of ␣-Ni(OH)2 after cycled in electrolyte 2 that indicates that the structure of ␣-Ni(OH)2 is destroyed to some extent. In contrast, ␣-Ni(OH)2 structure is almost unchanged after cycled in electrolyte 1. It indicates that AlO2 − anions accelerate the deterioration of ␣-Ni(OH)2 structure. It also observes that all the reflection peaks of ␣-Ni(OH)2 shift to higher angle of diffraction after cycling and the first peak shifts from 2θ = 11.3◦ to 2θ = 11.8◦ that suggests that interlayer distance decreases
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Fig. 6. The Nyquist plot for Ni(OH)2 electrodes with state of charge of 60%: (䊏) ␣-Ni(OH)2 in KOH, 0.0242 g; (䊉) ␣-Ni(OH)2 in KOH containing Al, 0.03358 g; (䉱) -Ni(OH)2 in KOH, 0.02065; (䉲) -Ni(OH)2 in KOH containing Al, 0.0433 g.
Fig. 4. X-ray diffraction patterns of ␣-Ni(OH)2 before and after cycle in both alkaline solution electrolytes: (a) before cycle; (b) after 32 cycles in electrolyte 1; (c) after 32 cycles in electrolyte 2.
Fig. 5. X-ray diffraction patterns of -Ni(OH)2 before and after cycle in both alkaline electrolytes: (*) ␥-NiOOH and ␣-Ni(OH)2 ; (a) before cycle; (b) after 500 cycles in electrolyte 1; (c) after 32 cycles in electrolyte 2.
slightly. And the width of diffraction peaks increases, which indicates that crystallinity is reduced or the size of crystalline decreases. There are three main XRD peaks of -Ni(OH)2 at 2θ = 19.0, 33.0 and 38.3◦ corresponding to the (0 0 1), (1 0 0) and (1 0 1), respectively. After cycling in two kinds of electrolyte, diffraction peaks of ␥-NiOOH (2θ = 12.8◦ , 25.9◦ and 37.7◦ ) appeared to some extent. It shows that charging process leads to the forming of ␥-NiOOH and it cannot be reduced during discharging, because of which the utility ratio of -Ni(OH)2 declined. To compare the XRD patterns of -Ni(OH)2 after cycling in electrolyte 1 and 2, small ␥-NiOOH diffraction peaks appeared after -Ni(OH)2 cycling 500 times in electrolyte 1, but large ␥-NiOOH diffraction peaks appeared after -Ni(OH)2 cycling only 32 times in electrolyte 2. It suggests that AlO2 − anions accelerate the forming of ␥-NiOOH and the deteriorate the discharge capacity during cycling process. In addition, for the latter, the peak at about 12.8◦ is broad and it is believed that not only ␥-phase but also ␣-phase formed during cycling in electrolyte 2. The possible reason is that aluminum, added in electrolyte 2, play a role in the structure change of -Ni(OH)2 , i.e. aluminum insert into the lattice of -Ni(OH)2 during cycling and turbostratic ␣-Ni(OH)2 formed. Specific discharge capacity, however, decreases with cycling. It may be caused by the large difference of density between ␣-Ni(OH)2 and -Ni(OH)2 , and the ␣-Ni(OH)2 formed cannot take part in charging-discharging process. Fig. 6 is the Nyquist Plots of Ni(OH)2 electrode with state of charge of 60%. The measuring frequency ranged from 11.31 to 0.005 Hz, from which it can be seen that its characterization is the solid-phase H+ proton finite-diffusion reaction. For solid-phase finite-diffusion process, its impedance can be expressed as [19]
Q. Zhang et al. / Materials Chemistry and Physics 86 (2004) 293–297
ZT =
1 (jω)−1/2 tan h(B(jω)1/2 ), Y0
B=
l D1/2
where B can be simulated by using Equivcrt Software; D is the apparent diffusion coefficient; l is the diffusion distance; the average size of particles is about 5 × 10−6 m, as shown in Fig. 1. In the higher frequency region, the intercept at x-axis for ␣-Ni(OH)2 is smaller than that for -Ni(OH)2 , which may be due to the higher conductivity of the former. Adding Al into solution caused the increase of intercept at x-axis, which may be due to the decrease of solution conductivity caused by adding Al. Based on Eq. (1), the apparent diffusion coefficient was calculated and it was about 2.2, 3.0, 3.5 and 4.0 cm2 s−1 for -Ni(OH)2 in electrolyte 1, -Ni(OH)2 in electrolyte 2, ␣-Ni(OH)2 in electrolyte 1 and ␣-Ni(OH)2 in electrolyte 2, respectively. It can be found that the apparent diffusion rate increased with adding Al into solution. For -Ni(OH)2 , the increase of diffusion rate may be due to the formation of turbostratic structure caused by the addition of Al into solution—for ␣-Ni(OH)2 , the Al in solution is inserted into the bulk of active materials, which increases the amount of Al in bulk and the diffusion of proton in bulk materials becomes easier.
cles although the interlayer distance becomes smaller; for -Ni(OH)2 , the ␥-NiOOH is found in the XRD patterns after cycling it in both alkaline electrolyte. The ␥-NiOOH content, moreover, is higher and ␣-type turbostratic structure also can be found after cycling in electrolyte containing Al. The addition of Al into solution caused the poorer conductivity of solution but increased the apparent diffusion rate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusion In this paper the influence of adding Al into KOH solution on the electrochemical behavior of Ni(OH)2 electrode was investigated. The results show that Co-doped Al-substituted ␣-Ni(OH)2 has the largest capacity in KOH solution (300 mAh g−1 ). The Al addition to KOH solution is harmful to the electrochemical reversible capacity and cycle life of both active masses. For Co-doped Al-substituted ␣-Ni(OH)2 , XRD patterns show that its turbostratic structure is still maintained after some electrochemical cy-
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[11] [12] [13] [14] [15] [16] [17] [18] [19]
C. Faure, J. Power Sources 35 (1991) 279. C. Delmas, C. Faure, Y. Borthomieu, Mater. Sci. B13 (1992) 89. C. Faure, C. Delmas, J. Power Sources 35 (1991) 263. V. Ganesh Kumar, N. Munichandraiah, P. Vishnu Kamath, A.K. Shukla, J. Power Sources 56 (1995) 111. P. Vishnu Kamath, J. Electrochem. Soc. 141 (1994) 2956. J. Dai, S.F.Y. Li, T.D. Xiao, D.M. Wang, D.E. Reisner, J. Power Sources 89 (2000) 40. R.S. Jayashree, P. Vishnu Kamath, J. Power Sources 107 (2002) 120. L. Demourgues-Guerlou, C. Denage, C. Delmas, J. Power Sources 52 (1994) 269. Y. Xu, M. Chen, X. Lin, C. Chen, China J. Nonferrous Met. 10 (2000) 656. Q. Zhang, Y. XU, X. Wang, China Rare Met. Mater. Eng. 32 (2003) 825. Q. Zhang, Y. XU, X. Wang, Chin. Rare Met. Mater. Eng. 32 (2003) 919. Q. Zhang, Y. XU, X. Wang, Chin. Rare Met. Mater. Eng., in press. Y. Xu, C. Chen, Q. Wang, Mater. Chem. Phys. 71 (2001) 190. J.M. Nan, Y. Yang, J.K. You, J. Alloys Compd. 293–295 (1999) 747. N. Cui, J.L. Luo, J. Alloys Compd. 265 (1998) 305. J.-H. Jung, H.-H. Lee, D.-M. Kim, J. Alloys Compd. 266 (1998) 266. D.M. Kim, K.J. Jang, J.Y. Lee, J. Alloys Compd. 293–295 (1999) 583. Y. Xu, C. Chen, X. Wang, Q. Wang, J. Alloys Comp. 335 (2002) 262. C. Cao, J. Zhang, An Introduction to Electrochemical Impedance Spectroscopy, Science Press, Beijing, 2002.
The electrochemical behavior of Ni(OH)2 in KOH solution containing aluminum hydroxide Qian Zhang a , Yanhui Xu b , Xiaolin Wang a,∗ a
Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China b Fuel Science Laboratory, Faculty of Mechanic Engineering and Production, Hamburg University of Applied Sciences, 20099 Hamburg, Germany
Received 10 January 2004; received in revised form 4 March 2004; accepted 12 March 2004
Abstract In this paper Co-doped Al-substituted ␣-Ni(OH)2 was prepared by a homogeneous precipitation method and its electrochemical behavior, in two kinds of electrolyte (7 M KOH solution and 7 M KOH saturated with Al(OH)3 ), was investigated by galvanostatic charge-discharge method, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemical properties of the commercial -Ni(OH)2 in both electrolyte was also studied in order to compare with that of ␣-Ni(OH)2 . The results show that Co-doped Al-substituted ␣-Ni(OH)2 has the largest capacity in 7 M KOH solution (300 mAh g−1 ). The addition of Al to KOH solution is harmful to the reversible capacity and cycle life of both ␣-Ni(OH)2 and -Ni(OH)2 . X-ray diffraction (XRD) patterns show that, for Co-doped Al-substituted ␣-Ni(OH)2 , its turbostratic structure maintains after cycles in both alkaline electrolyte, despite the interlayer distance becoming smaller. For -Ni(OH)2 , however, ␥-NiOOH species appeared after cycling in both electrolyte and its content is higher after cycling in 7 M KOH saturated with Al(OH)3 . And adding Al into KOH solution caused the decrease of solution conductivity and increased the apparent diffusion coefficient which is evidenced by electrochemical impedance spectroscopy. © 2004 Published by Elsevier B.V. Keywords: ␣-Ni(OH)2 ; Impedance spectroscopy; Aluminum hydroxide; Electrochemistry
1. Introduction Ni-MH battery and Ni–Cd battery are important power sources and are extensively used in industry and daily life. Their positive electrode active material is -Ni(OH)2 , compared with ␣-Ni(OH)2 has many advantages—faster proton diffusion rate, smaller expansion during cycling and more numbers of exchange electrons [1]. Pure ␣-Ni(OH)2 , however, is unstable in strong alkaline solution and transforms into -Ni(OH)2 . Since Bode found that replacing Ni atom by other element, such as Co, Al, Fe, and Mn, is effective in stabling its turbostratic structure it has attracted much attention [2–12]. On the other hand as well known, now Ni-MH battery is replacing the Ni–Cd battery because of its superior performance. And the main reason is that metal-hydride electrode has better electrochemical properties compared with Cd electrode. Metal-hydride material includes many types: AB5 type rare-earth/Ni based, AB2 Ti-based and ∗ Corresponding author. Tel.: +86 10 62794741; fax: +86 10 62794742. E-mail address: [email protected] (X. Wang).
0254-0584/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2004.03.029
Zr-based, AB type Ti–Ni based and V-based solid solution phase and Mg–Ni amorphous alloys [13–18]. In order to furthermore improve their cycle life and reversible capacity, the addition of some elements, such as Co, Al, Mn, Nb, Si, Cr, etc., was used frequently. Some researchers found that, however, the dissolution of the added elements into alkaline solution also causes the decrease of discharge capacity of metal-hydride electrode. One possible method is to add the same element into alkaline solution in order to prevent dissolving. But before adding this element into the solution, its influence on the electrochemical behavior of positive electrode needs to be well studied. In this study, because we want to dissolve Al element into 7 M KOH solution in order to prevent the dissolution of Al in metal-hydride alloy and improve properties of metal-hydride electrode, we first investigated the influence of Al additive on Ni(OH)2 . This is just the background of this article, i.e. in this paper we reported the electrochemical behavior of Co-doped Al-substituted ␣-Ni(OH)2 and commercial -Ni(OH)2 in Al-containing 7 M KOH solution by electrochemical techniques.
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2. Experimental 2.1. Synthesis of samples The ␣-Ni(OH)2 material was prepared by a homogeneous precipitation method which was described in our previous paper [12]. In brief, the mixed solution of Ni(NO3 )2 , Al(NO3 )3 , Co(NO3 )2 and urea with certain ratio (the molar ratios of Al/Ni and Co/Ni were set to be 3/16 and 1/16, respectively) was stirred for some time under controlled reaction temperature. Obtained precipitate was washed several times with deionized water to neutralization and filtered. Then the cake was dried in air. The -Ni(OH)2 material is a commercial sample. 2.2. Physical characterization of samples The crystal structure of the samples were determined by X-ray diffraction (XRD) analysis using a BRUKER D8 Advance X-ray diffractometer with Cu K␣ radiation, at a scanning rate of 3◦ (2θ) per minute and a scanning range of 5–80◦ (2θ). And the ␣-Ni(OH)2 and -Ni(OH)2 powders were characterized directly before cycling and the electrodes prepared with both materials were characterized so that nickel peaks resulted from nickel substrate in XRD patterns after cycling. 2.3. Preparation of nickel electrodes and their electrochemical test The pasted nickel electrodes were prepared as follows: 70 wt.% nickel hydroxide and 15 wt.% acetylene black powder or carbon powder were thoroughly mixed with 15 wt.% ploy(tetrafluoroethylene) emulsion. The paste obtained was incorporated into nickel foam with a spatula. The pasted nickel electrodes were dried and then pressed under a pressure of 200 kg cm−2 for 2 min. Thereafter, the electrodes were soaked in 7 M KOH for 24 h before being coupled with porous nickel electrodes on either side as counter electrodes and Hg/HgO electrodes as reference electrodes. Galvanostatic charge-discharge studies were conducted on LAND series battery system instrument (made in China). The working electrode was galvanostatically charged at 120 mA g−1 rate, set for 5 min, and then discharged at 60 mA g−1 rate to 0.2 V versus Hg/HgO/7 M KOH electrode. Electrochemical impedance spectroscopy (EIS) was performed using EG&G PARC Model 283 Potentioatat/Galvanostat and Model 1025 Frequency Analyzer. 3. Results and discussion The morphology of Ni(OH)2 powders was determined by scanning electron microscopy, as shown in Fig. 1, from which it can be seen that the -Ni(OH)2 has sphere or ellipsoid shape and the ␣-Ni(OH)2 has no regular shape. For
Fig. 1. The SEM of Ni(OH)2 powders: (A) -Ni(OH)2 ; (B) ␣-Ni(OH)2 .
industrialized-scale production, the sphere or/and ellipsoid shape is needed for higher tap density. Fig. 2 shows the gavalnostatic charge-discharge results of ␣-Ni(OH)2 and -Ni(OH)2 in electrolyte 1 (7 M KOH) and 2 (7 M KOH saturated with Al(OH)3 ), respectively. In electrolyte 1(7 M KOH), specific discharge capacities are about 300 and 260 mAh g−1 for ␣-Ni(OH)2 and -Ni(OH)2 , respectively. And almost no change after 32 charge-discharge cycles, which show that the cycle stability is good for both ␣-Ni(OH)2 and -Ni(OH)2 . On the other hand, in electrolyte 2(7 M KOH saturated with Al(OH)3 ), specific discharge capacity for ␣-Ni(OH)2 is about 290 mAh g−1 at the first cycle, but decreases quickly in subsequent charge-discharge cycles and declines to about 250 mAh g−1 at the 32nd cycle. And
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Fig. 2. The cycle performance of ␣-Ni(OH)2 and -Ni(OH)2 in both alkaline electrolytes. The current density: 120 mA g−1 for charge and 60 mA g−1 for discharge.
for -Ni(OH)2 in this electrolyte, specific discharge capacity is greatest at the second cycle and then decreases more quickly than ␣-Ni(OH)2 and drops to about 160 mAh g−1 at the 32nd cycle. Galvanostatic charge-discharge curves of both ␣-Ni(OH)2 and -Ni(OH)2 in electrolyte 1(7 M KOH) and 2(7 M KOH saturated with Al(OH)3 ) are shown in Fig. 3. It can be observed that at the same charge-discharge current density, the polarization of ␣-Ni(OH)2 in electrolyte 1(7 M KOH) is the smallest. In electrolyte 1(7 M KOH), the charge potential of ␣-Ni(OH)2 is 50 mV lower than in electrolyte 2 (7 M KOH saturated with Al(OH)3 ), whereas the discharge potential is almost unchanged. It indicates that charging
Fig. 3. The charge-discharge curves of ␣-Ni(OH)2 and -Ni(OH)2 in both alkaline electrolytes. The current density: 120 mA g−1 for charge and 60 mA g−1 for discharge.
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is more difficult due to AlO2 − anions. For -Ni(OH)2 , in electrolyte 1 and 2, not only charge-discharge potential but also charge-discharge curves shape alter strongly, especially the charge curve. When specific charge capacity is less than 40 mAh g−1 , the charge potential in electrolyte 2 is slightly higher than that in electrolyte 1. And in the range of 40–100 mAh g−1 , two charge curves are almost the same. Subsequently, the charge potential of -Ni(OH)2 in electrolyte 2 is much higher than that in electrolyte 1. Moreover, this difference increases with the charging and reaches to 70 mV at the end of charging. The charge curve of -Ni(OH)2 in electrolyte 2 is very steep and charge potential increases quickly with the charging. And the oxygen evolution reaction occurs earlier for -Ni(OH)2 in electrolyte 2 than in electrolyte 1, which suggests that adding Al in electrolyte is harmful for charge efficiency. For discharge curves, there is a little difference between two curves of -Ni(OH)2 in both alkaline electrolytes. In the initial discharging period, i.e. in the range of 0–20 mAh g−1 discharge capacity, the discharge potential is equal in electrolyte 1 and 2. And in the range of 20–60 mAh g−1 discharge capacity, the discharge potential in electrolyte 2 is a little higher than that in electrolyte 1. With the increase of the discharge capacity, however, the discharge potential in electrolyte 2 becomes lower slightly than that in electrolyte 1. The polarization of -Ni(OH)2 is also enhanced with the addition of Al in electrolyte, just as ␣-Ni(OH)2 . But compared with -Ni(OH)2 , both in electrolyte 1 and 2, there are obvious advantages for Co-doped ␣-Ni(OH)2 —higher discharge potential and lower charge potential. The structural changes of ␣-Ni(OH)2 and -Ni(OH)2 before and after cycling in electrolyte 1 and 2 are shown in Figs. 4 and 5, respectively. The XRD peaks of ␣-Ni(OH)2 before cycling (Fig. 4a) are very narrow and sharp with a low angle reflection at 2θ = 11.3◦ , followed by another at 2θ = 22.6◦ corresponding to (0 0 3) and (0 0 6) planes, respectively. And on the higher 2θ side there are 3 consecutive reflection peaks at 2θ = 34.5–46.0◦ . Both of the XRD patterns of ␣-Ni(OH)2 (discharge state) after cycled 32 times in electrolyte 1 and 2, show the characteristic of ␣-Ni(OH)2 . The reflection peak at 2θ = 18.4◦ is the characteristic of poly(tetrafluoroethylene) used as the binder. And the reflection peaks at 2θ = 44.7◦ and 52.1◦ are the characteristic of nickel used as current collector. Although the structure of ␣-Ni(OH)2 still maintains after cycling in both alkaline electrolyte, there are very many small mussy peaks and the base line shifts in the XRD patterns of ␣-Ni(OH)2 after cycled in electrolyte 2 that indicates that the structure of ␣-Ni(OH)2 is destroyed to some extent. In contrast, ␣-Ni(OH)2 structure is almost unchanged after cycled in electrolyte 1. It indicates that AlO2 − anions accelerate the deterioration of ␣-Ni(OH)2 structure. It also observes that all the reflection peaks of ␣-Ni(OH)2 shift to higher angle of diffraction after cycling and the first peak shifts from 2θ = 11.3◦ to 2θ = 11.8◦ that suggests that interlayer distance decreases
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Fig. 6. The Nyquist plot for Ni(OH)2 electrodes with state of charge of 60%: (䊏) ␣-Ni(OH)2 in KOH, 0.0242 g; (䊉) ␣-Ni(OH)2 in KOH containing Al, 0.03358 g; (䉱) -Ni(OH)2 in KOH, 0.02065; (䉲) -Ni(OH)2 in KOH containing Al, 0.0433 g.
Fig. 4. X-ray diffraction patterns of ␣-Ni(OH)2 before and after cycle in both alkaline solution electrolytes: (a) before cycle; (b) after 32 cycles in electrolyte 1; (c) after 32 cycles in electrolyte 2.
Fig. 5. X-ray diffraction patterns of -Ni(OH)2 before and after cycle in both alkaline electrolytes: (*) ␥-NiOOH and ␣-Ni(OH)2 ; (a) before cycle; (b) after 500 cycles in electrolyte 1; (c) after 32 cycles in electrolyte 2.
slightly. And the width of diffraction peaks increases, which indicates that crystallinity is reduced or the size of crystalline decreases. There are three main XRD peaks of -Ni(OH)2 at 2θ = 19.0, 33.0 and 38.3◦ corresponding to the (0 0 1), (1 0 0) and (1 0 1), respectively. After cycling in two kinds of electrolyte, diffraction peaks of ␥-NiOOH (2θ = 12.8◦ , 25.9◦ and 37.7◦ ) appeared to some extent. It shows that charging process leads to the forming of ␥-NiOOH and it cannot be reduced during discharging, because of which the utility ratio of -Ni(OH)2 declined. To compare the XRD patterns of -Ni(OH)2 after cycling in electrolyte 1 and 2, small ␥-NiOOH diffraction peaks appeared after -Ni(OH)2 cycling 500 times in electrolyte 1, but large ␥-NiOOH diffraction peaks appeared after -Ni(OH)2 cycling only 32 times in electrolyte 2. It suggests that AlO2 − anions accelerate the forming of ␥-NiOOH and the deteriorate the discharge capacity during cycling process. In addition, for the latter, the peak at about 12.8◦ is broad and it is believed that not only ␥-phase but also ␣-phase formed during cycling in electrolyte 2. The possible reason is that aluminum, added in electrolyte 2, play a role in the structure change of -Ni(OH)2 , i.e. aluminum insert into the lattice of -Ni(OH)2 during cycling and turbostratic ␣-Ni(OH)2 formed. Specific discharge capacity, however, decreases with cycling. It may be caused by the large difference of density between ␣-Ni(OH)2 and -Ni(OH)2 , and the ␣-Ni(OH)2 formed cannot take part in charging-discharging process. Fig. 6 is the Nyquist Plots of Ni(OH)2 electrode with state of charge of 60%. The measuring frequency ranged from 11.31 to 0.005 Hz, from which it can be seen that its characterization is the solid-phase H+ proton finite-diffusion reaction. For solid-phase finite-diffusion process, its impedance can be expressed as [19]
Q. Zhang et al. / Materials Chemistry and Physics 86 (2004) 293–297
ZT =
1 (jω)−1/2 tan h(B(jω)1/2 ), Y0
B=
l D1/2
where B can be simulated by using Equivcrt Software; D is the apparent diffusion coefficient; l is the diffusion distance; the average size of particles is about 5 × 10−6 m, as shown in Fig. 1. In the higher frequency region, the intercept at x-axis for ␣-Ni(OH)2 is smaller than that for -Ni(OH)2 , which may be due to the higher conductivity of the former. Adding Al into solution caused the increase of intercept at x-axis, which may be due to the decrease of solution conductivity caused by adding Al. Based on Eq. (1), the apparent diffusion coefficient was calculated and it was about 2.2, 3.0, 3.5 and 4.0 cm2 s−1 for -Ni(OH)2 in electrolyte 1, -Ni(OH)2 in electrolyte 2, ␣-Ni(OH)2 in electrolyte 1 and ␣-Ni(OH)2 in electrolyte 2, respectively. It can be found that the apparent diffusion rate increased with adding Al into solution. For -Ni(OH)2 , the increase of diffusion rate may be due to the formation of turbostratic structure caused by the addition of Al into solution—for ␣-Ni(OH)2 , the Al in solution is inserted into the bulk of active materials, which increases the amount of Al in bulk and the diffusion of proton in bulk materials becomes easier.
cles although the interlayer distance becomes smaller; for -Ni(OH)2 , the ␥-NiOOH is found in the XRD patterns after cycling it in both alkaline electrolyte. The ␥-NiOOH content, moreover, is higher and ␣-type turbostratic structure also can be found after cycling in electrolyte containing Al. The addition of Al into solution caused the poorer conductivity of solution but increased the apparent diffusion rate. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
4. Conclusion In this paper the influence of adding Al into KOH solution on the electrochemical behavior of Ni(OH)2 electrode was investigated. The results show that Co-doped Al-substituted ␣-Ni(OH)2 has the largest capacity in KOH solution (300 mAh g−1 ). The Al addition to KOH solution is harmful to the electrochemical reversible capacity and cycle life of both active masses. For Co-doped Al-substituted ␣-Ni(OH)2 , XRD patterns show that its turbostratic structure is still maintained after some electrochemical cy-
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[11] [12] [13] [14] [15] [16] [17] [18] [19]
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