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Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrodes at 233 K
JOURNAL OF RARE EARTHS, Vol. 26, No. 3, Jun. 2008, p. 402
Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrodes at 233 K ZHANG Xiaoyan(张晓燕), CHEN Yungui (陈云贵), TAO Mingda (陶明大), WU Chaoling (吴朝玲) (School of Materials Science and Engineering, Sichuan University, Chengdu, 610065, China) Received 2 August 2007; revised 25 December 2007
Abstract: The effect of KOH electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrodes at 233 K was studied. The results indicated that the electrolyte concentration had great influence on discharge capacity and discharge voltage plateau of LaNi5 alloy electrode at 233 K, and the highest discharge capacity and discharge voltage plateau were both obtained at 6 mol/L KOH. When the KOH electrolyte concentration changed from 5 to 9 mol/L at 233 K, the high rate discharge ability (HRD) had the same change tendency as the diffusion coefficient, but the exchange current density did not change significantly, which implied that hydrogen diffusion was the control step at low temperature 233 K for discharge process of LaNi5 alloy electrode. Keywords: hydrogen storage alloy; low-temperature electrochemical characteristic; LaNi5; rare earths
Normal Ni/MH batteries cannot meet the requirements of military battery because of their poor low-temperature performances[1]. In military devices and gelid areas, Ni/MH battery has to satisfy stable performance at low temperatures, especially at 233 K, so the low-temperature electrochemical performance of hydrogen storage alloys has been paid great attention to[2–4]. At present, studies of Ni/MH batteries under low temperature include alloy composition[4–6], structure[7], low-temperature kinetic property[8], etc. The main factor that determines the low-temperature performances of Ni/MH battery is the hydrogen storage alloy. To develop hydrogen storage alloy with excellent low-temperature performances, the discharge process, affecting factors and the mechanism must be studied. So the simplest system should be researched first. In this paper, the effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrode at 233 K was studied.
1 Experimental The purities of the raw materials, i.e., lanthanum and nickel in this experiment were 99.5 wt.%. The samples were arc-melted into button-shaped ingots under argon atmosphere. To ensure composition homogeneity, samples were turned over and remelted for four times. They were then crushed into 200-mesh powders under an air atmosphere for electrode tests. Alloy electrodes were prepared by pressing the mixture of
alloy power and carbonyl nickel power with a mass ratio of 1:3 under 65 MPa pressure to form pellets 10 mm in diameter. Electrochemical measurements were performed at 233 K in a standard open tri-electrode electrolysis cell consisting of the alloy electrode, sintered Ni (OH)2/NiOOH counter electrode and Hg/HgO reference electrode immersed in KOH electrolyte. The concentrations of KOH electrolyte are 5, 6, 7, 8 and 9 mol/L, respectively. The discharge capacities of hydride electrodes were determined by galvanostatic method. Each electrode was discharged to cut-off potential −0.6 V versus Hg/HgO reference electrode. Electrodes were charged/discharged at 55 mA/g 10 times first and then 275 mA/g 10 times, respectively at room temperature. After that they were charged at 275 mA/g for 1.2 h at room temperature and discharged at several assigned current densities at 233 K. Liner polarization curves of the electrodes were measured with the LK98C electrochemical measurement system by scanning the electrode potential at a rate of 0.1 mV/s from 0 to 10 mV (vs. open-circuit potential) at 0% depth of discharge (DOD) at 233 K. Potentiostatic test was carried out after all the electrodes were fully charged, with a potential step of 0.1 V for 1000 s using LK98C electrochemical measurement system. DWB semi-conductor cooler was used as the cooling system. When alloy electrodes were fully charged, standard open tri-electrode electrolysis cells were put into the semi-conductor cooler for 4 h at 233 K to ensure that the discharge temperature of alloy electrodes (the temperature
Foundation item: Project supported by the National Natural Science Foundation of China (NSFC 50571072) Corresponding author: CHEN Yungui (E-mail: [email protected]; Tel.: +86-28-85407335)
ZHANG X Y et al., Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5…
of KOH electrolyte) was identical, and then electrochemical test was carried out. KOH electrolyte was in a liquid form at 233 K.
2 Results and discussion Fig.1 shows the discharge capacity curves of the LaNi5 alloy electrode at different discharge current densities at 298 and 233 K. It can be seen that with the increase of the concentration of KOH electrolytes, the discharge capacity and discharge voltage plateau of the LaNi5 alloy electrode change little at 298 K. However, with the discharge rate increased at 233 K, the difference of discharge capacity and discharge voltage amongst various electrolyte concentrations becomes larger. When the concentration of the electrolyte is 6 mol/L, the discharge capacity and discharge voltage plateau reach maximum. The high rate dischargeability (HRD) at low temperature is defined and calculated according to the following formula: HRD=(Cd/C13.75)×100% (1) where, Cd and C13.75 are the discharge capacities at the current density Id and 13.75 mA/g to the cut-off potential of −0.6 V versus Hg/HgO reference electrode at 233 K, respec-
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tively. Fig.2 indicates the relationship between HRD and the electrolyte concentration at 233 K. When Id is 27.5 mA/g, HRD of LaNi5 electrodes has no significant difference. When Id is 55 mA/g, HRD shows a descending trend with the increase of electrolyte concentration. When Id is 1 mA/g, HRD increases first, then decreases, and reaches maximum when the electrolyte concentration is 6 mol/L. HRD reflects the behavior of electrochemical kinetics of the alloy electrodes. And the kinetic property of the hydrogen storage alloy electrodes is related to electrochemical reaction on the surface of alloy particles and diffusion of hydrogen within the alloy, which are characterized by exchange current density and diffusion coefficient, respectively. Thereby exchange current density and diffusion coefficient were examined. The linear polarization curves of LaNi5 alloy electrodes are illustrated in Fig.3. From the slopes of the curves, it is shown that the reaction resistance of the electrodes does not change significantly. The exchange current density I0 was calculated according to the slope of the following formula (2)[10] and the results are shown in Fig.4: I0=(RT/F)(I/η)η→0 (2) where, R is the gas constant, T the absolute temperature, F the Faraday constant and I/η the slope of the linear polarization curve. It can be seen that electrolyte concentration has
Fig.1 Discharge capacity versus discharge voltage of LaNi5 at different discharge current densities at 298 and 233 K (a) 298 K, Id=275 mA/g ; (b) 233 K, Id=27.5 mA/g; (c) 233 K, Id=55 mA/g; (d) 233 K, Id=110 mA/g
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as shown in Fig.2. So it can be confirmed that hydrogen diffusion may be the control factor in the electrode reaction process. Fig.6 shows the hydrogen diffusion model of the negative electrode. H1, H2, H3 indicate the location of H in the alloy particle. H1 is in the alloy, H2 is near the surface of the alloy, and H3 is on the surface of the electrode. H1 and H2 are in the absorption state, while H3 is the adsorbed state. The diffusion coefficient from H1 to H2 is a constant, but the diffusion
Fig.2 HRD of LaNi5 alloy electrodes at 233 K
Fig.4 Relationship between electrolyte concentration and I0, D of LaNi5 alloy
Fig.3 Linear polarization curves of the LaNi5 alloy electrodes at 0% depth of discharge
little influence on the exchange current density. However, HRD of LaNi5 alloy electrodes increases first, and then decreases when the discharge current density is 110 mA/g. The tendency of the change of I0 and HRD is inconsistent; therefore it can be indicated that electrode reaction was not controlled by the charge-transfer reaction. The diffusion coefficient D of hydrogen atoms can be obtained by chronoamperometry. It can be calculated with Eq.(3)[9]. lgi=lg[±(6FD)/(δd2)(C0–Cs)]–(π2D/2.303d2)t (3) where, i is the diffusion current density, C0 the initial hydrogen concentration in the bulk of the alloy, Cs the hydrogen concentration on the surface of the alloy particles, d the alloy particle radius, t the discharge time, δ the thickness of diffusion coating, F the Faraday constant and D the diffusion coefficient. Fig.5 shows the semi-logarithmic plot of anodic current versus time response of LaNi5 alloy electrodes. It can be seen from Fig.4 that the diffusion coefficient increases first, and then decreases when electrolyte concentration increases from 5 to 9 mol/L. When the electrolyte concentration is 6 mol/L, D reaches the maximum. These experimental results are consistent with that of HRD
Fig.5 Current-time responses of the LaNi5 alloy electrodes after application of a potential step of 100 mV at 233 K
Fig.6 Hydrogen diffusion model of negative electrode
ZHANG X Y et al., Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5…
from H2 to H3 changes with different surface states of electrodes. In this article, the low-temperature diffusion coefficient D is an apparent diffusion coefficient which includes diffusion processes from H1 to H2 and H2 to H3. So the diffusion coefficient D changes with electrolyte concentration. The reasons about the control of hydrogen diffusion are speculated as follows. As the hydrogen storage alloy electrodes are all the same, different discharge capacities and HRD can be attributed to electrolyte concentration. Different electrolyte concentrations affect the surface state of the electrodes which affects the diffusion coefficient. With the increase of electrolyte concentration, OH– concentration of the electrode surface increases, but viscosity of the electrolyte also increases, slowing the diffusion velocity of OH–; so the OH– concentration at the electrode surface decreases. When the electrolyte concentration is low, electrolyte concentration plays a major role on OH– concentration of the electrode surface, so the value of D is low. On the other hand, the viscosity of electrolyte plays a dominating role on OH– concentration of the electrode surface when the electrolyte concentration is high, and so the value of D is low, too.
3 Conclusions 1. With the increase of discharge current, the electrolyte concentration had great influence on the discharge capacity and discharge voltage plateau of LaNi5 alloy electrode at 233 K, and the highest discharge capacity and discharge voltage plateau were obtained at 6 mol/L KOH electrolyte concentration. 2. When the KOH electrolyte concentration changed from 5 to 9 mol/L at 233 K, the high rate discharge ability changed with the same tendency as the diffusion coefficient, but the exchange current density did not change significantly, which implied that hydrogen diffusion may be the control factor at 233 K for discharge process of LaNi5 alloy electrode.
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References: [1] Wu Zhu, Wang Ke, Xia Baojia. Low-temperature characteristics of Ni-MH battery and their modification. Chinese Journal of Power Sources, 2001, 25(5): 350. [2] Hiroshi Senoh, Yasutaka Hara, Hiroshi Inoue, Chiaki Iwakura. Charge efficiency of misch metal-based hydrogen storage alloy electrodes at relatively low temperatures. Electrochimica Acta, 2001, 46: 967. [3] Iwakura C, Senoh H, Morimoto K. Influence of temperature on discharge process of misch metal-based hydrogen storage alloy electrode. Electrochemistry, 2002, 70: 2. [4] Zhang Wenkuan, Shi Jingxian, Lou Yuwan. Studies on low temperature performance of rechargeable MH-Ni batteries. Electrochemistry, 1995, 1(2): 198. [5] Tetsuo Sakai, Hiroshi Miyamura, Nobuhiro Kuriyama, Akihiko Kato, Keisuke Oguro, Hiroshi Ishikawa, Chiaki Iwakura. The influence of small amounts of added elements on various anode performance characteristics for LaNi2.5Co2.5-based alloys. J. Less-Common Met., 1990, 159:127. [6] Ye Hui, Zhang Hong. Improvement of the low temperature performances of MmNi3.55Co0.75Mn0.4Al0.6 hydrogen storage alloy. Journal of the Electrochemical Society, 2002, 149(2): A122. [7] Guo Jinghong, Pan Chengjiu, Mao Yongzhi. Study on lowtemperature discharge characteristics of Ni-MH batteries. Chinese Journal of Power Sources, 2000, 24(4): 189. [8] Yuan Xianxia, Xu Naixin. Effects of temperature on the kinetic performance of hydrogen storage alloy MmNi3.75Co0.65Mn0.4Al0.2. Acta Chimica Sinica, 2002, 60(1): 13. [9] Zhang G, Popov B N, White R E. Electrochemical determination of the diffusion coefficient of hydrogen through an LaNi4.25Al0.75 electrode in alkaline aqueous solution. J. Electrochem. Soc., 1995, 142: 2695. [10] Chen W X, Qi J Q, Chen Y, Chen C P, Wang Q D, Zhou J M. Effects of pretreatments on the electrochemical properties of MlNi4.0Co0.6Al0.4 alloy electrode and the performance of a Ni/metal hydride battery. J. Alloys Compd., 1999, 728: 293.
Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrodes at 233 K ZHANG Xiaoyan(张晓燕), CHEN Yungui (陈云贵), TAO Mingda (陶明大), WU Chaoling (吴朝玲) (School of Materials Science and Engineering, Sichuan University, Chengdu, 610065, China) Received 2 August 2007; revised 25 December 2007
Abstract: The effect of KOH electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrodes at 233 K was studied. The results indicated that the electrolyte concentration had great influence on discharge capacity and discharge voltage plateau of LaNi5 alloy electrode at 233 K, and the highest discharge capacity and discharge voltage plateau were both obtained at 6 mol/L KOH. When the KOH electrolyte concentration changed from 5 to 9 mol/L at 233 K, the high rate discharge ability (HRD) had the same change tendency as the diffusion coefficient, but the exchange current density did not change significantly, which implied that hydrogen diffusion was the control step at low temperature 233 K for discharge process of LaNi5 alloy electrode. Keywords: hydrogen storage alloy; low-temperature electrochemical characteristic; LaNi5; rare earths
Normal Ni/MH batteries cannot meet the requirements of military battery because of their poor low-temperature performances[1]. In military devices and gelid areas, Ni/MH battery has to satisfy stable performance at low temperatures, especially at 233 K, so the low-temperature electrochemical performance of hydrogen storage alloys has been paid great attention to[2–4]. At present, studies of Ni/MH batteries under low temperature include alloy composition[4–6], structure[7], low-temperature kinetic property[8], etc. The main factor that determines the low-temperature performances of Ni/MH battery is the hydrogen storage alloy. To develop hydrogen storage alloy with excellent low-temperature performances, the discharge process, affecting factors and the mechanism must be studied. So the simplest system should be researched first. In this paper, the effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5 alloy electrode at 233 K was studied.
1 Experimental The purities of the raw materials, i.e., lanthanum and nickel in this experiment were 99.5 wt.%. The samples were arc-melted into button-shaped ingots under argon atmosphere. To ensure composition homogeneity, samples were turned over and remelted for four times. They were then crushed into 200-mesh powders under an air atmosphere for electrode tests. Alloy electrodes were prepared by pressing the mixture of
alloy power and carbonyl nickel power with a mass ratio of 1:3 under 65 MPa pressure to form pellets 10 mm in diameter. Electrochemical measurements were performed at 233 K in a standard open tri-electrode electrolysis cell consisting of the alloy electrode, sintered Ni (OH)2/NiOOH counter electrode and Hg/HgO reference electrode immersed in KOH electrolyte. The concentrations of KOH electrolyte are 5, 6, 7, 8 and 9 mol/L, respectively. The discharge capacities of hydride electrodes were determined by galvanostatic method. Each electrode was discharged to cut-off potential −0.6 V versus Hg/HgO reference electrode. Electrodes were charged/discharged at 55 mA/g 10 times first and then 275 mA/g 10 times, respectively at room temperature. After that they were charged at 275 mA/g for 1.2 h at room temperature and discharged at several assigned current densities at 233 K. Liner polarization curves of the electrodes were measured with the LK98C electrochemical measurement system by scanning the electrode potential at a rate of 0.1 mV/s from 0 to 10 mV (vs. open-circuit potential) at 0% depth of discharge (DOD) at 233 K. Potentiostatic test was carried out after all the electrodes were fully charged, with a potential step of 0.1 V for 1000 s using LK98C electrochemical measurement system. DWB semi-conductor cooler was used as the cooling system. When alloy electrodes were fully charged, standard open tri-electrode electrolysis cells were put into the semi-conductor cooler for 4 h at 233 K to ensure that the discharge temperature of alloy electrodes (the temperature
Foundation item: Project supported by the National Natural Science Foundation of China (NSFC 50571072) Corresponding author: CHEN Yungui (E-mail: [email protected]; Tel.: +86-28-85407335)
ZHANG X Y et al., Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5…
of KOH electrolyte) was identical, and then electrochemical test was carried out. KOH electrolyte was in a liquid form at 233 K.
2 Results and discussion Fig.1 shows the discharge capacity curves of the LaNi5 alloy electrode at different discharge current densities at 298 and 233 K. It can be seen that with the increase of the concentration of KOH electrolytes, the discharge capacity and discharge voltage plateau of the LaNi5 alloy electrode change little at 298 K. However, with the discharge rate increased at 233 K, the difference of discharge capacity and discharge voltage amongst various electrolyte concentrations becomes larger. When the concentration of the electrolyte is 6 mol/L, the discharge capacity and discharge voltage plateau reach maximum. The high rate dischargeability (HRD) at low temperature is defined and calculated according to the following formula: HRD=(Cd/C13.75)×100% (1) where, Cd and C13.75 are the discharge capacities at the current density Id and 13.75 mA/g to the cut-off potential of −0.6 V versus Hg/HgO reference electrode at 233 K, respec-
403
tively. Fig.2 indicates the relationship between HRD and the electrolyte concentration at 233 K. When Id is 27.5 mA/g, HRD of LaNi5 electrodes has no significant difference. When Id is 55 mA/g, HRD shows a descending trend with the increase of electrolyte concentration. When Id is 1 mA/g, HRD increases first, then decreases, and reaches maximum when the electrolyte concentration is 6 mol/L. HRD reflects the behavior of electrochemical kinetics of the alloy electrodes. And the kinetic property of the hydrogen storage alloy electrodes is related to electrochemical reaction on the surface of alloy particles and diffusion of hydrogen within the alloy, which are characterized by exchange current density and diffusion coefficient, respectively. Thereby exchange current density and diffusion coefficient were examined. The linear polarization curves of LaNi5 alloy electrodes are illustrated in Fig.3. From the slopes of the curves, it is shown that the reaction resistance of the electrodes does not change significantly. The exchange current density I0 was calculated according to the slope of the following formula (2)[10] and the results are shown in Fig.4: I0=(RT/F)(I/η)η→0 (2) where, R is the gas constant, T the absolute temperature, F the Faraday constant and I/η the slope of the linear polarization curve. It can be seen that electrolyte concentration has
Fig.1 Discharge capacity versus discharge voltage of LaNi5 at different discharge current densities at 298 and 233 K (a) 298 K, Id=275 mA/g ; (b) 233 K, Id=27.5 mA/g; (c) 233 K, Id=55 mA/g; (d) 233 K, Id=110 mA/g
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as shown in Fig.2. So it can be confirmed that hydrogen diffusion may be the control factor in the electrode reaction process. Fig.6 shows the hydrogen diffusion model of the negative electrode. H1, H2, H3 indicate the location of H in the alloy particle. H1 is in the alloy, H2 is near the surface of the alloy, and H3 is on the surface of the electrode. H1 and H2 are in the absorption state, while H3 is the adsorbed state. The diffusion coefficient from H1 to H2 is a constant, but the diffusion
Fig.2 HRD of LaNi5 alloy electrodes at 233 K
Fig.4 Relationship between electrolyte concentration and I0, D of LaNi5 alloy
Fig.3 Linear polarization curves of the LaNi5 alloy electrodes at 0% depth of discharge
little influence on the exchange current density. However, HRD of LaNi5 alloy electrodes increases first, and then decreases when the discharge current density is 110 mA/g. The tendency of the change of I0 and HRD is inconsistent; therefore it can be indicated that electrode reaction was not controlled by the charge-transfer reaction. The diffusion coefficient D of hydrogen atoms can be obtained by chronoamperometry. It can be calculated with Eq.(3)[9]. lgi=lg[±(6FD)/(δd2)(C0–Cs)]–(π2D/2.303d2)t (3) where, i is the diffusion current density, C0 the initial hydrogen concentration in the bulk of the alloy, Cs the hydrogen concentration on the surface of the alloy particles, d the alloy particle radius, t the discharge time, δ the thickness of diffusion coating, F the Faraday constant and D the diffusion coefficient. Fig.5 shows the semi-logarithmic plot of anodic current versus time response of LaNi5 alloy electrodes. It can be seen from Fig.4 that the diffusion coefficient increases first, and then decreases when electrolyte concentration increases from 5 to 9 mol/L. When the electrolyte concentration is 6 mol/L, D reaches the maximum. These experimental results are consistent with that of HRD
Fig.5 Current-time responses of the LaNi5 alloy electrodes after application of a potential step of 100 mV at 233 K
Fig.6 Hydrogen diffusion model of negative electrode
ZHANG X Y et al., Effect of electrolyte concentration on low-temperature electrochemical properties of LaNi5…
from H2 to H3 changes with different surface states of electrodes. In this article, the low-temperature diffusion coefficient D is an apparent diffusion coefficient which includes diffusion processes from H1 to H2 and H2 to H3. So the diffusion coefficient D changes with electrolyte concentration. The reasons about the control of hydrogen diffusion are speculated as follows. As the hydrogen storage alloy electrodes are all the same, different discharge capacities and HRD can be attributed to electrolyte concentration. Different electrolyte concentrations affect the surface state of the electrodes which affects the diffusion coefficient. With the increase of electrolyte concentration, OH– concentration of the electrode surface increases, but viscosity of the electrolyte also increases, slowing the diffusion velocity of OH–; so the OH– concentration at the electrode surface decreases. When the electrolyte concentration is low, electrolyte concentration plays a major role on OH– concentration of the electrode surface, so the value of D is low. On the other hand, the viscosity of electrolyte plays a dominating role on OH– concentration of the electrode surface when the electrolyte concentration is high, and so the value of D is low, too.
3 Conclusions 1. With the increase of discharge current, the electrolyte concentration had great influence on the discharge capacity and discharge voltage plateau of LaNi5 alloy electrode at 233 K, and the highest discharge capacity and discharge voltage plateau were obtained at 6 mol/L KOH electrolyte concentration. 2. When the KOH electrolyte concentration changed from 5 to 9 mol/L at 233 K, the high rate discharge ability changed with the same tendency as the diffusion coefficient, but the exchange current density did not change significantly, which implied that hydrogen diffusion may be the control factor at 233 K for discharge process of LaNi5 alloy electrode.
405
References: [1] Wu Zhu, Wang Ke, Xia Baojia. Low-temperature characteristics of Ni-MH battery and their modification. Chinese Journal of Power Sources, 2001, 25(5): 350. [2] Hiroshi Senoh, Yasutaka Hara, Hiroshi Inoue, Chiaki Iwakura. Charge efficiency of misch metal-based hydrogen storage alloy electrodes at relatively low temperatures. Electrochimica Acta, 2001, 46: 967. [3] Iwakura C, Senoh H, Morimoto K. Influence of temperature on discharge process of misch metal-based hydrogen storage alloy electrode. Electrochemistry, 2002, 70: 2. [4] Zhang Wenkuan, Shi Jingxian, Lou Yuwan. Studies on low temperature performance of rechargeable MH-Ni batteries. Electrochemistry, 1995, 1(2): 198. [5] Tetsuo Sakai, Hiroshi Miyamura, Nobuhiro Kuriyama, Akihiko Kato, Keisuke Oguro, Hiroshi Ishikawa, Chiaki Iwakura. The influence of small amounts of added elements on various anode performance characteristics for LaNi2.5Co2.5-based alloys. J. Less-Common Met., 1990, 159:127. [6] Ye Hui, Zhang Hong. Improvement of the low temperature performances of MmNi3.55Co0.75Mn0.4Al0.6 hydrogen storage alloy. Journal of the Electrochemical Society, 2002, 149(2): A122. [7] Guo Jinghong, Pan Chengjiu, Mao Yongzhi. Study on lowtemperature discharge characteristics of Ni-MH batteries. Chinese Journal of Power Sources, 2000, 24(4): 189. [8] Yuan Xianxia, Xu Naixin. Effects of temperature on the kinetic performance of hydrogen storage alloy MmNi3.75Co0.65Mn0.4Al0.2. Acta Chimica Sinica, 2002, 60(1): 13. [9] Zhang G, Popov B N, White R E. Electrochemical determination of the diffusion coefficient of hydrogen through an LaNi4.25Al0.75 electrode in alkaline aqueous solution. J. Electrochem. Soc., 1995, 142: 2695. [10] Chen W X, Qi J Q, Chen Y, Chen C P, Wang Q D, Zhou J M. Effects of pretreatments on the electrochemical properties of MlNi4.0Co0.6Al0.4 alloy electrode and the performance of a Ni/metal hydride battery. J. Alloys Compd., 1999, 728: 293.