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Preparation of a nickel–metal hydride (Ni–MH) rechargeable battery and its application to a solar vehicle
International Journal of Hydrogen Energy 26 (2001) 873–877
www.elsevier.com/locate/ijhydene
Preparation of a nickel–metal hydride (Ni–MH) rechargeable battery and its application to a solar vehicle H. Hoshinoa , H. Uchidaa; ∗ , H. Kimuraa , K. Takamotoa , K. Hiraokaa , Y. Matsumaeb b Research
a School of Engineering, Tokai University, 1117 Kita-Kaname, Hiratsuka-city, Kanagawa 259-1292, Japan Institute of Science and Technology, Tokai Educational System, 2-28-4 Tomigaya, Shibuya-ku, Tokyo 151-0063, Japan
Abstract This paper reports the preparation of a nickel–metal hydride (Ni–MH) rechargeable battery with a high capacity of 96 V-14 Ah and a high energy density of 1:4 kWh. The negative electrode was prepared with a rare earth based MmNi5 type hydrogen storage alloy and graphite as a conductive material. The prepared Ni–MH battery was installed into a solar vehicle. The data obtained from a three-day long world solar car rally yielded high discharge-to-charge coulomb e7ciency (76%) and solar-to-electric energy conversion e7ciency (60%) in average, in spite of severe rally conditions. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Recently, the nickel–metal hydride (Ni–MH) rechargeable battery using rare earth based hydrogen storage alloys has been commercialized as small size batteries and actually applied to electronic devices such as personal computers and small machine tools or hybridized vehicles in Japan. However, there are few detailed reports on the characteristics of a large scaled Ni–MH battery and the actual performance under practical conditions. In previous papers, we reported the preparations and the characteristics of Ni–MH batteries with capacities, 9:6 V-7 Ah [1] and 4:8 V − 14 Ah [2]. Fundamental requirements for Ni–MH battery are the ready initial activation, high charge-discharge rates and a long cyclic life, where the control of the surface condition of hydrogen storage alloys in the negative electrode and the choice of the conducting material for the electrode are important factors. For the alloy, many studies point out the importance of the surface segregation of nickel atoms on
∗ Corresponding author. Tel.: +81-463-58-1211; fax: +81463-58-9461. E-mail address: [email protected] (H. Uchida).
the alloy surface. However, both in the gas reaction and electrochemical process [3– 6], we found the marked enhancement of the initial activation and hydriding rate by the addition of alkaline atoms such as Li, Na and K in the surface oxide layers of rare earth based hydrogen storage alloys. The alkaline atoms penetrate into the surface oxide layers by heating the alloys in alkaline hydroxide solutions such as LiOH, NaOH and KOH. And, the presence of these alkaline atoms reduce the work function of electrons of the alloy surface over 1 eV, where the reduction rate as well as the hydriding rate can be increased with increasing the concentration of alkaline atoms on the surface [6]. This reduction in work function seems responsible for the ease of the dissociation of covalent molecules like H2 and H2 O because the dissociation of a covalent molecule needs the electron exchange with the surface [7,8]. The resultant acceleration of the rate of dissociation of H2 or H2 O enhances the hydriding rate both in the gas phase and electrochemical process [2– 4]. We applied this result to the preparation of a large scaled Ni–MH battery acting as a solar energy storage device in a solar vehicle. The enhancement of the rate of electrochemical hydriding (electric charge) by the alkaline pretreatment is reported elsewhere [4,6]. In this paper, we report mainly the discharge characteristics of the prepared battery in which the alkaline pretreatment was found very eHective also for the discharge
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 1 3 - 1
874
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
Fig. 1. Geometrical factors of the prepared negative and positive electrodes.
characteristics. In addition, we examined the use of graphite as a conducting material for the negative electrode of the battery in order to reduce the battery weight. Details for the conducting materials such as graphite, copper, cobalt or nickel are reported elsewhere [9].
2. Experimental 2.1. Preparation of a paste type negative electrode A rare earth based MmNi5 type alloy with a particle size 38 m and a spherical graphite powder with a size 5 m were used as hydrogen storage and conducting materials of negative electrode, respectively. The alloy was produced by a new rapid-quenching method developed at SHOWA DENKO K.K., Japan. The alloy powder was produced by the cyclic hydriding and dehydriding reactions [10] and then the produced alloy powder was heated in a 6 M-KOH solution to realize the ready activation and high rates of charge and discharge [3– 6]. In this study, a paste type segmental negative electrode with a rectangular form of 7 × 6 cm (Fig. 1) was formed by mixing the alloy powder (9 g), the graphite powder (0:74 g) and polyvinyl alcohol (PVA) (0:12 g), respectively. As a positive electrode, a sintered nickel hydroxide plate was used (Fig. 1). Nickel ribbon plates as lead collectors were welded to the electrodes. 2.2. Preparation of a Ni–MH battery Fig. 2 shows a battery module composed of four cells, and each of which was composed of 13 positive and 12 negative electrodes. The positive and negative electrodes were placed in alternate layers, where hydrophilic polypropylene cloths were used as separators among the electrodes. The electrodes were immersed in a 6 M-KOH electrolyte. The capacity of one module was 4:8 V-14 Ah. Twenty modules were
Fig. 2. A battery module.
connected as a series circuit to yield the Lnal 96 V-14 Ah battery for a solar vehicle. 2.3. Measurements of electrochemical characteristics The electrochemical characteristics of a prepared negative electrode were measured at 298 K using a reference electrode of Hg=HgO and a positive electrode with a much higher electric capacity than that of the negative electrode by a factor of at least three . The sample electrode was cyclically charged and discharged until the discharge capacity became reproducibly stable. The measurement of charge characteristics was made at a current of 50 mA per 1 g-negative electrode for 7 h, then the current was cut oH for 1 h. Subsequently, the measurement of discharge characteristics was made at a current of 50 mA per 1 g-negative electrode until the potential with respect to the reference electrode became −0:6 V. 3. Results and discussion 3.1. Discharge characteristics of negative electrodes Fig. 3 shows the change in discharge capacity in mAh per 1 g-negative electrode as a function of the mixture ratio of alloy to conducting material (graphite) in weight. The optimal mixture ratio lies at 8–16 in weight ratio, where the discharge capacity is lying around 200 mAh per 1 g-electrode. From this data, the mixture ratio of the alloy to graphite was determined as described in Section 2.1. Fig. 4 shows the changes in the discharge capacity in mAh per one piece of the negative electrode and the voltage in V vs. the Hg=HgO reference electrode as a function of discharge current. With increasing the current from 0.2 to 3:2 A, the discharge capacity becomes lower from 1700 to 1300 mAh. A discharge current of 3:2 A for a negative electrode corresponds to about 40 A discharge current for one
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
875
Fig. 3. The change in the discharge capacity as a function of the mixture ratio of the alloy to graphite in weight. Fig. 5. The changes in the discharge capacity and voltage for a battery module as a function of the number of charge-discharge cycling. The alloy in the negative electrode was not pretreated with KOH.
Fig. 4. The changes in the discharge capacity and voltage for a negative electrode as a function of discharge current.
battery module. With increasing discharge current, the interfacial electric resistance between the electrode surface and electrolyte increases, leading to a positive shift of the negative electrode potential, and to the decrease in discharge capacity. In addition, when the fact that the increase in temperature of hydrogen storage alloys increases discharge potential and decreases hydrogen storage capacity is taken into account, the joule heat eHect in the electrode may also be responsible for the decreased discharge capacity at high currents. 3.2. Charge characteristics The charging rate is essential for a solar vehicle when the e7ciency of solar energy storage is concerned. In a previous
study for the solar energy storage system using a hydrogen storage alloy [11], we found that the most important factor to determine the whole energy conversion e7ciency lies neither in the hydrogen production rate of a solid polymer electrolyte (SPE) nor the hydriding rate of a hydrogen storage alloy but in the solar-to-electric energy conversion rate of photo voltaic solar cells. However, for a solar vehicle, solar energy is stored as electric energy in rechargeable batteries. In our case, the solar energy is stored as hydrogen in a Ni–MH battery. Therefore, the enhancement of hydriding rate by the alkaline pretreatment is of great importance. The marked enhancement of charging (electrochemical hydriding) rates can be induced by the surface modiLcation of a hydrogen storage alloy with alkaline atoms, which are reported elsewhere [4,6]. 3.3. Discharge characteristics of a battery module Fig. 5 shows typical discharging characteristics of a battery module with 12 pieces of negative electrodes and 13 pieces of positive electrodes. Each charge was made at a current of 650 mA for 24 h, and the discharge measurement was made at a current 650 mA and the voltage drop of 0:7 V. The cyclic charge and discharge treatments were made at an interval of 1 h. The data shown in Fig. 5 demonstrates the results for the alloy powder without alkaline pretreatment before the preparation of the electrode. The discharge capacity increases from 8 to 12 Ah per module with increasing the number of the charge–discharge cycle. This indicates that several hydriding and dehydriding reactions are needed until the surface of the whole alloy particles becomes activated. Fig. 6 shows typical discharging characteristics of a similar module in which the alloy surface was pretreated with a
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H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
Fig. 7. The discharge characteristics of a battery module. Fig. 6. The changes in the discharge capacity and voltage for a battery module as a function of the number of charge-discharge cycling. The alloy in the negative electrode was pretreated with a 6M-KOH solution.
6 M-KOH solution before the preparation of the electrodes. In this case, the discharge capacity exhibits a maximum value of 14 Ah per module even at the Lrst discharge. This result suggests that the alkaline treatment is eHective not only for the dissociation of H2 O but for the recombination: H+ + OH− → H2 O on the electrode surface. Two possible explanations for this eHect are considered. The Lrst one is that the initial hydrogen charge (hydriding) accelerated by the alkaline treatment activates the electrode surface by the inductions of micro cracks and fresh surfaces, and the subsequent hydrogen discharge (dehydriding) also becomes activated. The second is that the reduced work function of electrons of the KOH pretreated alloy surface [6] facilitates the recombination of the hydrogen atoms and hydroxyl to form H2 O in discharge. 3.4. Discharge characteristics of a battery module The cyclic charge and discharge test was extended to the 170th cycle, where the charge was made at a current of 650 mA for 24 h and the discharge was made at a current of 650 mA and a voltage cut-oH of 0:7 V. An interval was made between each charge and discharge at the 170th cycle, the capacity was maintained still over 75% of a maximum value 14 Ah. Fig. 7 shows typical discharging characteristics of a battery module. The charge was made at a current of 1:4 A for 10 h. The measurements of the voltage change in V and discharge capacity in Ah were made by changing the discharge current from 0.65 to 14 A. These results indicate that one battery module yields maximum discharge capacities Q0 of about 15 to 13 Ah in the discharge rate from 0.1 to 1:0 C. The energy density per weight of the prepared module is
Fig. 8. A solar vehicle used for this study - Tokai Spirit
higher by a factor 1.6 than those of conventional lead batteries. Because the maximum charge capacity Q1 was 15:6 Ah, the discharge-to-charge coulomb e7ciency Q deLned as the ratio of discharge to charge capacities is about 90%. 3.5. Running test of the prepared Ni–MH battery in a solar vehicle The twenty pieces of the prepared battery module (4:8 V-14 Ah) were connected in a series circuit to yield a Ni–MH battery with an energy capacity, about 1:4 kWh and an energy density per weight, about 50 Wh=kg. Fig. 8 shows a solar vehicle - Tokai Spirit into which the prepared Ni–MH battery was installed. The actual data in running conditions were obtained in the 98-World Solar Car Rally held for three days in Akita, Japan, August 1998. Fig. 9 shows the change in the coulomb capacity stored in the battery during the rally as a function of time in day for three days. The total mileage for three days was about 9; 000 km. All data concerning energy supply and consumption were recorded using a microcomputer in the vehicle. The initial battery capacity is designated as zero in the Lrst day (Fig. 9). From these data, coulomb and energy conversion e7ciencies were calculated in average for three days.
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
877
4. Conclusion A large scaled Ni–MH rechargeable battery, which was prepared for the installation on to a solar vehicle, demonstrated a high discharge-to-charge coulomb e7ciency, 76% and a high solar-to-electric energy conversion e7ciency, 60% even under a severe rally condition. The prepared battery was found very suitable for the practical driving of solar energy assisted vehicles and devices, particularly because of its high responses of charge and discharge characteristics under intermittent and changeable weather conditions. Acknowledgements
Fig. 9. The change in the coulomb capacity recorded for three days long during the 98 World Solar Car Rally in Akita, Japan.
The discharge-to-charge coulomb e7ciency, Q∗ is deLned as the following relation, Q∗ = (Q0 − QC )=Q1 ;
(1)
where Q0 is the total discharge capacity, 56:98 Ah; QC is the initial charge capacity, 14 Ah, and QI is the total energy supplied to the battery, 56:39 Ah, respectively. From this relation, the average discharge-to-charge coulomb e7ciency, Q∗ = 76% was obtained. The energy conversion e7ciency, w∗ is deLned as follows, w∗ = (W0 − WC )=WI ;
(2)
where W0 is the total output power, 4:96 kWh; WC is the initial energy capacity, 1:4 kWh, and WI is the total energy supply from solar cells to the battery, 5:96 kWh, respectively. From this, the average solar-to-electric energy conversion e7ciency, W ∗ = 60% was obtained. During the rally, the battery was used in conditions near the fully discharged states (see Fig. 9). In spite of that, as the coulomb e7ciency exhibits, the battery was very active in the storage of solar energy even in rainy days. These high responses in charge and discharge are characteristic of the Ni–MH battery.
This study was supported by the Energy Project of the General Research Organization, Tokai Educational System and by a special bounty of Tokai University Graduate School for the promotion of the research of solar vehicles in 1998. The authors are grateful to Showa Denko K.K., Japan for the supply of large amounts of hydrogen storage alloys produced by a newly developed method. The authors express their appreciation to Mr. S. Watanabe, the graduate course of Applied Science, Tokai University, for his kind cooperation to this work. References [1] Hoshino H, Kimura H, Takamoto K, Morii K, Uchida H. J Hydrogen Energy System 1998;23:22–8. [2] Hoshino H, Kimura H, Takamoto K, Uchida H. J Hydrogen Energy System 1999;24:2–8. [3] Uchida H, Kawachi M, Goto K, Watanabe Y, Uchida HH, Matsumura Y. Z Phys Chm 1994;183:303. [4] Uchida HH, Watanabe Y, Matsumura Y, Uchida H. J Alloys Compounds 1995;231:684. [5] Uchida HH, Moriai K, Aoyama K, Kondo H, Uchida H. J Alloys Compounds 1997;525:253–4. [6] Uchida H, Yamashita Y, Tabata Y. J Alloys Compounds in press. [7] Uchida H, Ohtani Y, Ozawa M, Kawahata T, Suzuki T. J Less-Common Met 1991;983:172–4. [8] Lang ND, Holloway S, Norskov JK. Surf Sci 1985;150:24. [9] Uchida H, Matsumoto T, Watanabe S, Kobayashi K, Hoshino H. A paste type electrode using Mm-Ni based hydrogen storage alloys for Ni–MH battery. Paper to be presented at IAHE, conference Int J Hydrogen Energy, USA to be published. [10] Uchida H, Uchida HH, Huang YC. J Less-Common Met 1984;101:459. [11] Huang YC, Goto H, Sato A, Hayashi T, Uchida H. Z Phys Chem NF 1989;164:1398.
www.elsevier.com/locate/ijhydene
Preparation of a nickel–metal hydride (Ni–MH) rechargeable battery and its application to a solar vehicle H. Hoshinoa , H. Uchidaa; ∗ , H. Kimuraa , K. Takamotoa , K. Hiraokaa , Y. Matsumaeb b Research
a School of Engineering, Tokai University, 1117 Kita-Kaname, Hiratsuka-city, Kanagawa 259-1292, Japan Institute of Science and Technology, Tokai Educational System, 2-28-4 Tomigaya, Shibuya-ku, Tokyo 151-0063, Japan
Abstract This paper reports the preparation of a nickel–metal hydride (Ni–MH) rechargeable battery with a high capacity of 96 V-14 Ah and a high energy density of 1:4 kWh. The negative electrode was prepared with a rare earth based MmNi5 type hydrogen storage alloy and graphite as a conductive material. The prepared Ni–MH battery was installed into a solar vehicle. The data obtained from a three-day long world solar car rally yielded high discharge-to-charge coulomb e7ciency (76%) and solar-to-electric energy conversion e7ciency (60%) in average, in spite of severe rally conditions. ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.
1. Introduction Recently, the nickel–metal hydride (Ni–MH) rechargeable battery using rare earth based hydrogen storage alloys has been commercialized as small size batteries and actually applied to electronic devices such as personal computers and small machine tools or hybridized vehicles in Japan. However, there are few detailed reports on the characteristics of a large scaled Ni–MH battery and the actual performance under practical conditions. In previous papers, we reported the preparations and the characteristics of Ni–MH batteries with capacities, 9:6 V-7 Ah [1] and 4:8 V − 14 Ah [2]. Fundamental requirements for Ni–MH battery are the ready initial activation, high charge-discharge rates and a long cyclic life, where the control of the surface condition of hydrogen storage alloys in the negative electrode and the choice of the conducting material for the electrode are important factors. For the alloy, many studies point out the importance of the surface segregation of nickel atoms on
∗ Corresponding author. Tel.: +81-463-58-1211; fax: +81463-58-9461. E-mail address: [email protected] (H. Uchida).
the alloy surface. However, both in the gas reaction and electrochemical process [3– 6], we found the marked enhancement of the initial activation and hydriding rate by the addition of alkaline atoms such as Li, Na and K in the surface oxide layers of rare earth based hydrogen storage alloys. The alkaline atoms penetrate into the surface oxide layers by heating the alloys in alkaline hydroxide solutions such as LiOH, NaOH and KOH. And, the presence of these alkaline atoms reduce the work function of electrons of the alloy surface over 1 eV, where the reduction rate as well as the hydriding rate can be increased with increasing the concentration of alkaline atoms on the surface [6]. This reduction in work function seems responsible for the ease of the dissociation of covalent molecules like H2 and H2 O because the dissociation of a covalent molecule needs the electron exchange with the surface [7,8]. The resultant acceleration of the rate of dissociation of H2 or H2 O enhances the hydriding rate both in the gas phase and electrochemical process [2– 4]. We applied this result to the preparation of a large scaled Ni–MH battery acting as a solar energy storage device in a solar vehicle. The enhancement of the rate of electrochemical hydriding (electric charge) by the alkaline pretreatment is reported elsewhere [4,6]. In this paper, we report mainly the discharge characteristics of the prepared battery in which the alkaline pretreatment was found very eHective also for the discharge
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 1 ) 0 0 0 1 3 - 1
874
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
Fig. 1. Geometrical factors of the prepared negative and positive electrodes.
characteristics. In addition, we examined the use of graphite as a conducting material for the negative electrode of the battery in order to reduce the battery weight. Details for the conducting materials such as graphite, copper, cobalt or nickel are reported elsewhere [9].
2. Experimental 2.1. Preparation of a paste type negative electrode A rare earth based MmNi5 type alloy with a particle size 38 m and a spherical graphite powder with a size 5 m were used as hydrogen storage and conducting materials of negative electrode, respectively. The alloy was produced by a new rapid-quenching method developed at SHOWA DENKO K.K., Japan. The alloy powder was produced by the cyclic hydriding and dehydriding reactions [10] and then the produced alloy powder was heated in a 6 M-KOH solution to realize the ready activation and high rates of charge and discharge [3– 6]. In this study, a paste type segmental negative electrode with a rectangular form of 7 × 6 cm (Fig. 1) was formed by mixing the alloy powder (9 g), the graphite powder (0:74 g) and polyvinyl alcohol (PVA) (0:12 g), respectively. As a positive electrode, a sintered nickel hydroxide plate was used (Fig. 1). Nickel ribbon plates as lead collectors were welded to the electrodes. 2.2. Preparation of a Ni–MH battery Fig. 2 shows a battery module composed of four cells, and each of which was composed of 13 positive and 12 negative electrodes. The positive and negative electrodes were placed in alternate layers, where hydrophilic polypropylene cloths were used as separators among the electrodes. The electrodes were immersed in a 6 M-KOH electrolyte. The capacity of one module was 4:8 V-14 Ah. Twenty modules were
Fig. 2. A battery module.
connected as a series circuit to yield the Lnal 96 V-14 Ah battery for a solar vehicle. 2.3. Measurements of electrochemical characteristics The electrochemical characteristics of a prepared negative electrode were measured at 298 K using a reference electrode of Hg=HgO and a positive electrode with a much higher electric capacity than that of the negative electrode by a factor of at least three . The sample electrode was cyclically charged and discharged until the discharge capacity became reproducibly stable. The measurement of charge characteristics was made at a current of 50 mA per 1 g-negative electrode for 7 h, then the current was cut oH for 1 h. Subsequently, the measurement of discharge characteristics was made at a current of 50 mA per 1 g-negative electrode until the potential with respect to the reference electrode became −0:6 V. 3. Results and discussion 3.1. Discharge characteristics of negative electrodes Fig. 3 shows the change in discharge capacity in mAh per 1 g-negative electrode as a function of the mixture ratio of alloy to conducting material (graphite) in weight. The optimal mixture ratio lies at 8–16 in weight ratio, where the discharge capacity is lying around 200 mAh per 1 g-electrode. From this data, the mixture ratio of the alloy to graphite was determined as described in Section 2.1. Fig. 4 shows the changes in the discharge capacity in mAh per one piece of the negative electrode and the voltage in V vs. the Hg=HgO reference electrode as a function of discharge current. With increasing the current from 0.2 to 3:2 A, the discharge capacity becomes lower from 1700 to 1300 mAh. A discharge current of 3:2 A for a negative electrode corresponds to about 40 A discharge current for one
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
875
Fig. 3. The change in the discharge capacity as a function of the mixture ratio of the alloy to graphite in weight. Fig. 5. The changes in the discharge capacity and voltage for a battery module as a function of the number of charge-discharge cycling. The alloy in the negative electrode was not pretreated with KOH.
Fig. 4. The changes in the discharge capacity and voltage for a negative electrode as a function of discharge current.
battery module. With increasing discharge current, the interfacial electric resistance between the electrode surface and electrolyte increases, leading to a positive shift of the negative electrode potential, and to the decrease in discharge capacity. In addition, when the fact that the increase in temperature of hydrogen storage alloys increases discharge potential and decreases hydrogen storage capacity is taken into account, the joule heat eHect in the electrode may also be responsible for the decreased discharge capacity at high currents. 3.2. Charge characteristics The charging rate is essential for a solar vehicle when the e7ciency of solar energy storage is concerned. In a previous
study for the solar energy storage system using a hydrogen storage alloy [11], we found that the most important factor to determine the whole energy conversion e7ciency lies neither in the hydrogen production rate of a solid polymer electrolyte (SPE) nor the hydriding rate of a hydrogen storage alloy but in the solar-to-electric energy conversion rate of photo voltaic solar cells. However, for a solar vehicle, solar energy is stored as electric energy in rechargeable batteries. In our case, the solar energy is stored as hydrogen in a Ni–MH battery. Therefore, the enhancement of hydriding rate by the alkaline pretreatment is of great importance. The marked enhancement of charging (electrochemical hydriding) rates can be induced by the surface modiLcation of a hydrogen storage alloy with alkaline atoms, which are reported elsewhere [4,6]. 3.3. Discharge characteristics of a battery module Fig. 5 shows typical discharging characteristics of a battery module with 12 pieces of negative electrodes and 13 pieces of positive electrodes. Each charge was made at a current of 650 mA for 24 h, and the discharge measurement was made at a current 650 mA and the voltage drop of 0:7 V. The cyclic charge and discharge treatments were made at an interval of 1 h. The data shown in Fig. 5 demonstrates the results for the alloy powder without alkaline pretreatment before the preparation of the electrode. The discharge capacity increases from 8 to 12 Ah per module with increasing the number of the charge–discharge cycle. This indicates that several hydriding and dehydriding reactions are needed until the surface of the whole alloy particles becomes activated. Fig. 6 shows typical discharging characteristics of a similar module in which the alloy surface was pretreated with a
876
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
Fig. 7. The discharge characteristics of a battery module. Fig. 6. The changes in the discharge capacity and voltage for a battery module as a function of the number of charge-discharge cycling. The alloy in the negative electrode was pretreated with a 6M-KOH solution.
6 M-KOH solution before the preparation of the electrodes. In this case, the discharge capacity exhibits a maximum value of 14 Ah per module even at the Lrst discharge. This result suggests that the alkaline treatment is eHective not only for the dissociation of H2 O but for the recombination: H+ + OH− → H2 O on the electrode surface. Two possible explanations for this eHect are considered. The Lrst one is that the initial hydrogen charge (hydriding) accelerated by the alkaline treatment activates the electrode surface by the inductions of micro cracks and fresh surfaces, and the subsequent hydrogen discharge (dehydriding) also becomes activated. The second is that the reduced work function of electrons of the KOH pretreated alloy surface [6] facilitates the recombination of the hydrogen atoms and hydroxyl to form H2 O in discharge. 3.4. Discharge characteristics of a battery module The cyclic charge and discharge test was extended to the 170th cycle, where the charge was made at a current of 650 mA for 24 h and the discharge was made at a current of 650 mA and a voltage cut-oH of 0:7 V. An interval was made between each charge and discharge at the 170th cycle, the capacity was maintained still over 75% of a maximum value 14 Ah. Fig. 7 shows typical discharging characteristics of a battery module. The charge was made at a current of 1:4 A for 10 h. The measurements of the voltage change in V and discharge capacity in Ah were made by changing the discharge current from 0.65 to 14 A. These results indicate that one battery module yields maximum discharge capacities Q0 of about 15 to 13 Ah in the discharge rate from 0.1 to 1:0 C. The energy density per weight of the prepared module is
Fig. 8. A solar vehicle used for this study - Tokai Spirit
higher by a factor 1.6 than those of conventional lead batteries. Because the maximum charge capacity Q1 was 15:6 Ah, the discharge-to-charge coulomb e7ciency Q deLned as the ratio of discharge to charge capacities is about 90%. 3.5. Running test of the prepared Ni–MH battery in a solar vehicle The twenty pieces of the prepared battery module (4:8 V-14 Ah) were connected in a series circuit to yield a Ni–MH battery with an energy capacity, about 1:4 kWh and an energy density per weight, about 50 Wh=kg. Fig. 8 shows a solar vehicle - Tokai Spirit into which the prepared Ni–MH battery was installed. The actual data in running conditions were obtained in the 98-World Solar Car Rally held for three days in Akita, Japan, August 1998. Fig. 9 shows the change in the coulomb capacity stored in the battery during the rally as a function of time in day for three days. The total mileage for three days was about 9; 000 km. All data concerning energy supply and consumption were recorded using a microcomputer in the vehicle. The initial battery capacity is designated as zero in the Lrst day (Fig. 9). From these data, coulomb and energy conversion e7ciencies were calculated in average for three days.
H. Hoshino et al. / International Journal of Hydrogen Energy 26 (2001) 873–877
877
4. Conclusion A large scaled Ni–MH rechargeable battery, which was prepared for the installation on to a solar vehicle, demonstrated a high discharge-to-charge coulomb e7ciency, 76% and a high solar-to-electric energy conversion e7ciency, 60% even under a severe rally condition. The prepared battery was found very suitable for the practical driving of solar energy assisted vehicles and devices, particularly because of its high responses of charge and discharge characteristics under intermittent and changeable weather conditions. Acknowledgements
Fig. 9. The change in the coulomb capacity recorded for three days long during the 98 World Solar Car Rally in Akita, Japan.
The discharge-to-charge coulomb e7ciency, Q∗ is deLned as the following relation, Q∗ = (Q0 − QC )=Q1 ;
(1)
where Q0 is the total discharge capacity, 56:98 Ah; QC is the initial charge capacity, 14 Ah, and QI is the total energy supplied to the battery, 56:39 Ah, respectively. From this relation, the average discharge-to-charge coulomb e7ciency, Q∗ = 76% was obtained. The energy conversion e7ciency, w∗ is deLned as follows, w∗ = (W0 − WC )=WI ;
(2)
where W0 is the total output power, 4:96 kWh; WC is the initial energy capacity, 1:4 kWh, and WI is the total energy supply from solar cells to the battery, 5:96 kWh, respectively. From this, the average solar-to-electric energy conversion e7ciency, W ∗ = 60% was obtained. During the rally, the battery was used in conditions near the fully discharged states (see Fig. 9). In spite of that, as the coulomb e7ciency exhibits, the battery was very active in the storage of solar energy even in rainy days. These high responses in charge and discharge are characteristic of the Ni–MH battery.
This study was supported by the Energy Project of the General Research Organization, Tokai Educational System and by a special bounty of Tokai University Graduate School for the promotion of the research of solar vehicles in 1998. The authors are grateful to Showa Denko K.K., Japan for the supply of large amounts of hydrogen storage alloys produced by a newly developed method. The authors express their appreciation to Mr. S. Watanabe, the graduate course of Applied Science, Tokai University, for his kind cooperation to this work. References [1] Hoshino H, Kimura H, Takamoto K, Morii K, Uchida H. J Hydrogen Energy System 1998;23:22–8. [2] Hoshino H, Kimura H, Takamoto K, Uchida H. J Hydrogen Energy System 1999;24:2–8. [3] Uchida H, Kawachi M, Goto K, Watanabe Y, Uchida HH, Matsumura Y. Z Phys Chm 1994;183:303. [4] Uchida HH, Watanabe Y, Matsumura Y, Uchida H. J Alloys Compounds 1995;231:684. [5] Uchida HH, Moriai K, Aoyama K, Kondo H, Uchida H. J Alloys Compounds 1997;525:253–4. [6] Uchida H, Yamashita Y, Tabata Y. J Alloys Compounds in press. [7] Uchida H, Ohtani Y, Ozawa M, Kawahata T, Suzuki T. J Less-Common Met 1991;983:172–4. [8] Lang ND, Holloway S, Norskov JK. Surf Sci 1985;150:24. [9] Uchida H, Matsumoto T, Watanabe S, Kobayashi K, Hoshino H. A paste type electrode using Mm-Ni based hydrogen storage alloys for Ni–MH battery. Paper to be presented at IAHE, conference Int J Hydrogen Energy, USA to be published. [10] Uchida H, Uchida HH, Huang YC. J Less-Common Met 1984;101:459. [11] Huang YC, Goto H, Sato A, Hayashi T, Uchida H. Z Phys Chem NF 1989;164:1398.