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Characteristics of electrodes using amorphousAB2 hydrogen storage alloys
Journal of the Less-Common Metals, 172-174 (1991) 1205-1210
1205
Characteristics of electrodes using amorphous AB2 hydrogen storage alloys Hiroshi M i y a m u r a , N o b u h i r o K u r i y a m a , T e t s u o Sakai, Keisuke Oguro, A k i h i k o Kato and Hiroshi I s h i k a w a Government Industrial Research Institute, Osaka, Midorigaoka 1-8-31, Ikeda, Osaka 563 (Japan)
Toshikatsu Iwasaki Osaka Electro-Communication University, Hatsu-cho 18-8, Neyagawa, Osaka 572 (Japan)
Abstract The hydrogen sorption behaviour of RENi2 (RE = La, Ce, Pr, Mm (misch metal)) Laves phase alloys was investigated by both the coulometric titration technique and the gas volumetric method. These alloys became amorphous by charging and kept the amorphous structure during charge-discharge cycles. The shapes of the pressurecomposition isotherms indicated that the amorphous hydrides were compounds of the solid solution type. The discharge capacities of the amorphous hydride electrodes in the electrochemical cell were less than one-fourth of the capacities evaluated from the gas volumetric measurement, though they increased with increasing temperature. The discharge capacity did not decay during the first 500 cycles.
1. I n t r o d u c t i o n Metal hydrides are promising materials for negative electrodes in nickel-hydrogen batteries. However, rapid decay of the discharge capacity by repetition of charge and discharge is a remaining problem [1]. Disintegration of alloys arising from grain boundaries is thought to be one of the causes of the capacity decay. Therefore the cycle lives of the electrodes are expected to be improved by using amorphous materials because they have no distinct grain boundaries. Oesterreicher et al. found that some L a - N i binary alloys became amorphous/on hydrogenation [2]. Aoki et al. further investigated RENi2 (RE, rare earth) alloys with the C15 Laves phase structure and found that many of these alloys also became amorphous on hydrogenation [3]. Miyamura et al. investigated the hydrogenation behaviour of some R E - N i binary alloys and found that the amorphous LaNi2 hydride was very stable [4] and that the LaNi 2 and CeNi 2 alloys were easily amorphized also by charging in an electrochemical cell, giving considerably long-life electrodes [5]. In the present work the hydrogen sorption behaviour and electrode characteristics of some RENi2 (RE = Ce, La, Pr, Mm (mischmetal)) alloys were extensively studied. 0022-5088/91/$3.50
9 Elsevier Sequoia/Printed in The Netherlands
1206
2. E x p e r i m e n t a l procedure CeNi 2 and PrNi2 alloys with the C15 Laves phase structure were prepared by arc melting under an argon atmosphere followed by annealing in vacuum for 12 h at 770 ~ LaNi 2 and MmNi2 alloys with the C15 Laves phase structure were prepared by the melt-spinning method with a copper roller under an argon atmosphere. The alloys were pulverized mechanically into powder about 100 ~tm in diameter. The surface of the powder was coated with a thin layer of copper by an electrodeless plating process (microencapsulation [6, 7]). The copper layer amounted to 20 wt.% of the coated alloy powder. The copper-plated alloy powder was mixed with F E P powder (Daikin Indust., tetrafluoroethylene-hexafluoroethylene copolymer) and pressed into pellets (diameter 1.3 cm; thickness 0.3 mm; net weight of alloy 200 mg). Each pellet was sandwiched by a pair of nickel nets and connected to a nickel wire for use as an electrode. P r e s s u r e - c o m p o s i t i o n isotherms ( P - C isotherms) in the pressure range between 0.1 and 38 atm were determined by gas volumetric measurement using a Sieverts-type apparatus at 20 ~ P - C isotherms in the pressure range below 0.1 atm were determined by the coulometric titration technique in an electrochemical cell, using Nernst's relation to derive the dissociation pressure [8-10]. The discharge capacity of each electrode was measured at various temperatures and current densities. Cycle lives were evaluated from the decay rate of discharge capacity with increasing repeated cycle numbers. These electrochemical measurements were carried out with an open cell using 6M KOH as electrolyte, H g - H g O as reference electrode and N i O O H Ni(OH)2 as counterelectrode. The cut-off potential for each discharge was set to be - 0 . 6 V vs. H g - H g O . The lattice structures of the hydrides were investigated by X-ray diffraction using Cu Kct radiation.
3. Results and d i s c u s s i o n 3.1. Gas volumetric and electrochemical P - C isotherms Figure 1 shows the P - C isotherms of the alloys by the gas volumetric method at 20 ~ It was found that the alloys absorbed hydrogen in H/M ratios of more than 1.2 at 10 atm and that the hydrides did not desorb hydrogen easily around atmospheric pressure. Figure 2 shows the coulometrically determined P - C isotherm of the CeNi2 hydride at 20 ~ The isotherm was sloped and had no pressure plateau, suggesting that the hydride absorbed hydrogen in solid solution. With decreasing hydrogen concentration the dissociation pressure became so low that more than 75% of absorbed hydrogen still remained in the hydride at a pressure of 10 -s atm. The coulometrically determined P - C isotherms for the other three hydrides (LaNi2, PrNi2 and MmNi2 hydrides) were almost the
1207 Hydrogen concentration, H / M 0.7 -~L,
0.8 ,
0.9
1.0 I
1.1 ,
1.2 ,
50. E 10-
20 *C
I.
O -4
a_e1~ o CeNi2 9 LoNi2 PrNi2 9 MrnNiz
~n0.5 ' - 0.10
. . . .
B10
.'- 16 8
9
,
I
.
.
.
.
I
150
.
.
.
100
.
.
.
.
.
I
5O
,
0
<
i
Discharge capacity,
H/M
C / rnAh-(j I
Fig. 1. P - C isotherms of the RENi2 hydrides at 20 ~ by the gas volumetric method. Fig. 2. P - C isotherm of the CeNi 2 hydride at 20 ~ by the coulometric titration technique.
same as that of the CeNi2 hydride. The capacity data are summarized in Table 1. In order to confirm the continuity of the results between the volumetrically obtained P - C isotherms and the coulometrically obtained ones, the slopes of the isotherms obtained by the two methods were compared. The slope factor S was defined as S = d(log P) (1) d(H/M) where P is the dissociation pressure and H/M is the concentration of hydrogen. The slope factors obtained by the two different methods are shown in Table 1, expressed as Sg and So. These two values were close to each other, indicating that the two isotherms can be smoothly connected. TABLE 1 Characteristic parameters of the RENi2 hydrides Alloy
M...
H/M(1)
Cth ( m A h g -1)
C,~.,, ( m A h g -1)
No.,,
C(500) ( m A h g -1)
C(1000) ( m A h g -1)
Sc
Sg
CeNi2 PrNi.~ LaNi 2 MmNi2
85.83 86.10 85.43 85.79
1.22 1.36 1.31 1.13
381 423 411 353
81 71 70 61
100 150 500 100
74 60 70 48
56 46 -
27 23 32 26
30 31 29 21
May, average atomic weight of alloy; H/M(1), hydrogen concentration at 1.0 atm obtained by the gas volumetric measurement; Cth, calculated capacity by the relation Cth = {[H/M(1)]/Mav} x F/ 3.6 (F is the Faraday); Cm,x, maximum of discharge capacity obtained during cyclic capacity measurement (current 10 mA); N~,ax, number of repeated cycles at which Cm,x is realized; C(500), discharge capacity at the repeated cycle number of 500; C(1000), discharge capacity at the repeated cycle number of 1000; Sr slope factor (eqn. (1)) by the coulometric titration technique; Sg, slope factor (eqn. (1)) by the gas volumetric measurement.
1208 '7(: D
~ IOC
(J 8 g-
,
50
u
o CeNiz 9 LaNi2 Pr Ni2 9 MmNi2
g ._~ O
0
10 20 30 40 Temperature. t / ~
50
Fig. 3. Discharge capacities of the amorphous hydrides at various temperatures; current 50 mA g- '.
3.2. Rate capability Figure 3 shows the discharge capacity of each hydride electrode at various temperatures (current 50 mA g-l). The discharge capacities increased with increasing temperature. Figures 4(a) and 4(b) show the discharge curves of a CeNi 2 hydride electrode at 40 and 0 ~ respectively at various currents. With increasing temperature and decreasing discharge c u r r e n t the discharge capacity and potential became high. The other three hydride electrodes showed almost the same curves as those of the CeNi2 hydride electrode. These results suggest t h a t the diffusivity of hydrogen played a significant role in the discharge behaviour. Richter and Hempelmann studied the diffusivity of hydrogen in crystalline LaNisH6 by quasi-elastic n e u t r o n scattering and found t h a t the diffusion coefficient of hydrogen (DH) was around 1.0 x 10 -7 cm 2 s -1 at room temperature [11]. Kirchheim investigated DH in amorphous P d - C u - S i - H by an electrochemical method and reported t h a t it was about 1.0 x 10 -s cm 2 s -1 at room temperature [12]. However, as we have reported, DH in an amorphous LaNi2 hydride thin film was extremely small and decreased with decreasing hydrogen concentration [13]. It was around 10 -1~ cm 2 s -1 at 40 ~ -1.0 > -0.9 o z
~ -o.~ LJJ -0.7 --2 ._o -0.6 *5 a_ _ 0.50
(a) CeNi2 40 ~
~5 ,
.
.
i
.
.
.
.
-1.0 CeNiz
>-0.9
(b)
0~
~-0.8
-o71 i
I
,
,
.
.
100 Discharge capacity, C / mAh.g-~
~-0.6 "5 a_ _0.50 . . . .
2345 I .... 100~. . . . Discharge capacity, C / mAh.g-1
Fig. 4. Discharge curves at various currents and temperatures: (a) 40 ~ 2, 100mAg-1; 3, 5 0 m A g - 1 ; 4, 2 5 m A g - 1 ; 5, 1 0 m A g -l.
(b) 0 ~
1,200 mA g - l ;
1209 7cn150 <~ E
- - CeNiz -~ LaNiz -.- PrNi2
uIOG >: .
- - .
-
u
~ 5c Y
260
460
,ooo
Cycles, n
Fig. 5. Discharge capacity ature 20 ~
vs.
cycle curves for the RENi 2 electrodes; current 50 mA g - l , temper-
at an electrode potential of - 0 . 8 V vs. Hg-HgO, which corresponded to a hydrogen pressure of 8.0 • 10 -5 atm. The small discharge capacities of the RENi2 hydride electrodes would be ascribed to the extremely low diffusivity of stored hydrogen with the equilibrium pressure below 10 -4 atm. 3.3. Cycle lives Figure 5 shows the discharge capacity vs. cycle number curves at 20 ~ for each hydride electrode. The electrodes kept more than 80% of their maximum discharge capacities after 400 cycles. In particular, the CeNi 2 and LaNi 2 electrodes retained nearly 70% of their maximum discharge capacities even after 1000 cycles. 3.4. Crystal structures after cyclic measurements Figure 6 shows X-ray diffraction profiles of the discharged hydrides after 1000 cycles for the CeNi2 and LaNi2 hydrides and after 500 cycles for the PrNi2 and MmNi2 hydrides. Although there were small reflections corresponding to hydroxides, no outstanding reflection from the alloys was found in the profile, showing that the hydrides were stable and kept their amor-
Tergel:
Cu
e*0 kvI 150 mA
~
o
a:Cu
~
PrNiz
~
o MmNiz ,
10
20
30
40 2 0
50
60
70
/ deg
Fig. 6. X-ray diffraction profiles of the amorphous hydrides after cyclic charge and discharge. The peaks marked with "o" are from the plated copper.
1210
phous structure after many charge-discharge cycles at room temperature. However, at higher temperature, recrystallization of CeNi2 from amorphous CeNi2-H has been reported by Paul-Boncour et al. [14].
4. C o n c l u s i o n s
Electrodes using hydrogenation-induced amorphous RENi2 (RE = Ce, La, Pr, Mm) had very long cycle lives, though the effective discharge capacities were only one-fourth of those expected from the amount of absorbed hydrogen. The apparently low capacities of these electrodes were attributed to the extremely low dissociation pressures and thereby small diffusivity of hydrogen in the hydrides. It seems that a suitable adjustment of the dissociation pressure is needed in order to utilize all the hydrogen stored in amorphous RENi2. Further investigations on multicomponent alloys are in progress. If their discharge capacities could be improved, these alloys would become promising electrode materials in metal hydride batteries.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
H. F. Bittner and C. C. Badcock, J. Electrochem. Soc., 130 (1983) 193C. H. Oesterreicher, J. Clinton and H. Bittner, Mater, Res. Bull., 11 (1976) 1241. K. Aoki, T. Yamamoto and T. Masumoto, Scr. Metall., 21 (1987) 27. H. Miyamura, T. Sakai, K. Oguro, A. Kato and H. Ishikawa, J. Less-Common Met., 146(1989) 197. H. Miyamura, T. Sakai, K. Oguro, A. Kato and H. Ishikawa, M R S Int. Meeting on Advanced Materials, 1988, Vol. 2, 1989, p. 15. H. Ishikawa, K. Oguro, A. Kato, H. Suzuki and E. Ishii, J. Less-Common Met., 107(1985) 105. T. Sakai, H. Ishikawa, K. Oguro, C. Iwakura and H. Yoneyama, J. Electrochem. Soc, 134 (1987) 558. C. Iwakura, T. Asaoka, H. Yoneyama, T. Sakai, K. Oguro and H. Ishikawa, Nippon Kagaku Kaishi, (1988) 1482. T. Sakai, H. Miyamura, N. Kuriyama, A. Kato, K. Oguro and H. Ishikawa, J. Electrochem. Soc., 137 (1990) 795. T. Sakai, H. Miyamura, N. Kuriyama, A. Kato, K. Oguro and H. Ishikawa, J. Less-Common Met., 159 (1990) 127. D. Richter and R. Hempelmann, J. Less-Common Met., 88 (1982) 353. R. Kirchheim, Acta Metall., 30 (1982) 1059. H. Miyamura, T. Sakai, N. Kuriyama, K. Oguro, A. Kato and H. Ishikawa, Extended Abstracts 40th ISE Meeting, 1989, Vol. 2, 1989, p. 1228. V. Paul-Boncour, C. Lartigue, A. Percheron-Guegan and J. C. Achard, J. Less-Common Met., 143 (1988) 301.
1205
Characteristics of electrodes using amorphous AB2 hydrogen storage alloys Hiroshi M i y a m u r a , N o b u h i r o K u r i y a m a , T e t s u o Sakai, Keisuke Oguro, A k i h i k o Kato and Hiroshi I s h i k a w a Government Industrial Research Institute, Osaka, Midorigaoka 1-8-31, Ikeda, Osaka 563 (Japan)
Toshikatsu Iwasaki Osaka Electro-Communication University, Hatsu-cho 18-8, Neyagawa, Osaka 572 (Japan)
Abstract The hydrogen sorption behaviour of RENi2 (RE = La, Ce, Pr, Mm (misch metal)) Laves phase alloys was investigated by both the coulometric titration technique and the gas volumetric method. These alloys became amorphous by charging and kept the amorphous structure during charge-discharge cycles. The shapes of the pressurecomposition isotherms indicated that the amorphous hydrides were compounds of the solid solution type. The discharge capacities of the amorphous hydride electrodes in the electrochemical cell were less than one-fourth of the capacities evaluated from the gas volumetric measurement, though they increased with increasing temperature. The discharge capacity did not decay during the first 500 cycles.
1. I n t r o d u c t i o n Metal hydrides are promising materials for negative electrodes in nickel-hydrogen batteries. However, rapid decay of the discharge capacity by repetition of charge and discharge is a remaining problem [1]. Disintegration of alloys arising from grain boundaries is thought to be one of the causes of the capacity decay. Therefore the cycle lives of the electrodes are expected to be improved by using amorphous materials because they have no distinct grain boundaries. Oesterreicher et al. found that some L a - N i binary alloys became amorphous/on hydrogenation [2]. Aoki et al. further investigated RENi2 (RE, rare earth) alloys with the C15 Laves phase structure and found that many of these alloys also became amorphous on hydrogenation [3]. Miyamura et al. investigated the hydrogenation behaviour of some R E - N i binary alloys and found that the amorphous LaNi2 hydride was very stable [4] and that the LaNi 2 and CeNi 2 alloys were easily amorphized also by charging in an electrochemical cell, giving considerably long-life electrodes [5]. In the present work the hydrogen sorption behaviour and electrode characteristics of some RENi2 (RE = Ce, La, Pr, Mm (mischmetal)) alloys were extensively studied. 0022-5088/91/$3.50
9 Elsevier Sequoia/Printed in The Netherlands
1206
2. E x p e r i m e n t a l procedure CeNi 2 and PrNi2 alloys with the C15 Laves phase structure were prepared by arc melting under an argon atmosphere followed by annealing in vacuum for 12 h at 770 ~ LaNi 2 and MmNi2 alloys with the C15 Laves phase structure were prepared by the melt-spinning method with a copper roller under an argon atmosphere. The alloys were pulverized mechanically into powder about 100 ~tm in diameter. The surface of the powder was coated with a thin layer of copper by an electrodeless plating process (microencapsulation [6, 7]). The copper layer amounted to 20 wt.% of the coated alloy powder. The copper-plated alloy powder was mixed with F E P powder (Daikin Indust., tetrafluoroethylene-hexafluoroethylene copolymer) and pressed into pellets (diameter 1.3 cm; thickness 0.3 mm; net weight of alloy 200 mg). Each pellet was sandwiched by a pair of nickel nets and connected to a nickel wire for use as an electrode. P r e s s u r e - c o m p o s i t i o n isotherms ( P - C isotherms) in the pressure range between 0.1 and 38 atm were determined by gas volumetric measurement using a Sieverts-type apparatus at 20 ~ P - C isotherms in the pressure range below 0.1 atm were determined by the coulometric titration technique in an electrochemical cell, using Nernst's relation to derive the dissociation pressure [8-10]. The discharge capacity of each electrode was measured at various temperatures and current densities. Cycle lives were evaluated from the decay rate of discharge capacity with increasing repeated cycle numbers. These electrochemical measurements were carried out with an open cell using 6M KOH as electrolyte, H g - H g O as reference electrode and N i O O H Ni(OH)2 as counterelectrode. The cut-off potential for each discharge was set to be - 0 . 6 V vs. H g - H g O . The lattice structures of the hydrides were investigated by X-ray diffraction using Cu Kct radiation.
3. Results and d i s c u s s i o n 3.1. Gas volumetric and electrochemical P - C isotherms Figure 1 shows the P - C isotherms of the alloys by the gas volumetric method at 20 ~ It was found that the alloys absorbed hydrogen in H/M ratios of more than 1.2 at 10 atm and that the hydrides did not desorb hydrogen easily around atmospheric pressure. Figure 2 shows the coulometrically determined P - C isotherm of the CeNi2 hydride at 20 ~ The isotherm was sloped and had no pressure plateau, suggesting that the hydride absorbed hydrogen in solid solution. With decreasing hydrogen concentration the dissociation pressure became so low that more than 75% of absorbed hydrogen still remained in the hydride at a pressure of 10 -s atm. The coulometrically determined P - C isotherms for the other three hydrides (LaNi2, PrNi2 and MmNi2 hydrides) were almost the
1207 Hydrogen concentration, H / M 0.7 -~L,
0.8 ,
0.9
1.0 I
1.1 ,
1.2 ,
50. E 10-
20 *C
I.
O -4
a_e1~ o CeNi2 9 LoNi2 PrNi2 9 MrnNiz
~n0.5 ' - 0.10
. . . .
B10
.'- 16 8
9
,
I
.
.
.
.
I
150
.
.
.
100
.
.
.
.
.
I
5O
,
0
<
i
Discharge capacity,
H/M
C / rnAh-(j I
Fig. 1. P - C isotherms of the RENi2 hydrides at 20 ~ by the gas volumetric method. Fig. 2. P - C isotherm of the CeNi 2 hydride at 20 ~ by the coulometric titration technique.
same as that of the CeNi2 hydride. The capacity data are summarized in Table 1. In order to confirm the continuity of the results between the volumetrically obtained P - C isotherms and the coulometrically obtained ones, the slopes of the isotherms obtained by the two methods were compared. The slope factor S was defined as S = d(log P) (1) d(H/M) where P is the dissociation pressure and H/M is the concentration of hydrogen. The slope factors obtained by the two different methods are shown in Table 1, expressed as Sg and So. These two values were close to each other, indicating that the two isotherms can be smoothly connected. TABLE 1 Characteristic parameters of the RENi2 hydrides Alloy
M...
H/M(1)
Cth ( m A h g -1)
C,~.,, ( m A h g -1)
No.,,
C(500) ( m A h g -1)
C(1000) ( m A h g -1)
Sc
Sg
CeNi2 PrNi.~ LaNi 2 MmNi2
85.83 86.10 85.43 85.79
1.22 1.36 1.31 1.13
381 423 411 353
81 71 70 61
100 150 500 100
74 60 70 48
56 46 -
27 23 32 26
30 31 29 21
May, average atomic weight of alloy; H/M(1), hydrogen concentration at 1.0 atm obtained by the gas volumetric measurement; Cth, calculated capacity by the relation Cth = {[H/M(1)]/Mav} x F/ 3.6 (F is the Faraday); Cm,x, maximum of discharge capacity obtained during cyclic capacity measurement (current 10 mA); N~,ax, number of repeated cycles at which Cm,x is realized; C(500), discharge capacity at the repeated cycle number of 500; C(1000), discharge capacity at the repeated cycle number of 1000; Sr slope factor (eqn. (1)) by the coulometric titration technique; Sg, slope factor (eqn. (1)) by the gas volumetric measurement.
1208 '7(: D
~ IOC
(J 8 g-
,
50
u
o CeNiz 9 LaNi2 Pr Ni2 9 MmNi2
g ._~ O
0
10 20 30 40 Temperature. t / ~
50
Fig. 3. Discharge capacities of the amorphous hydrides at various temperatures; current 50 mA g- '.
3.2. Rate capability Figure 3 shows the discharge capacity of each hydride electrode at various temperatures (current 50 mA g-l). The discharge capacities increased with increasing temperature. Figures 4(a) and 4(b) show the discharge curves of a CeNi 2 hydride electrode at 40 and 0 ~ respectively at various currents. With increasing temperature and decreasing discharge c u r r e n t the discharge capacity and potential became high. The other three hydride electrodes showed almost the same curves as those of the CeNi2 hydride electrode. These results suggest t h a t the diffusivity of hydrogen played a significant role in the discharge behaviour. Richter and Hempelmann studied the diffusivity of hydrogen in crystalline LaNisH6 by quasi-elastic n e u t r o n scattering and found t h a t the diffusion coefficient of hydrogen (DH) was around 1.0 x 10 -7 cm 2 s -1 at room temperature [11]. Kirchheim investigated DH in amorphous P d - C u - S i - H by an electrochemical method and reported t h a t it was about 1.0 x 10 -s cm 2 s -1 at room temperature [12]. However, as we have reported, DH in an amorphous LaNi2 hydride thin film was extremely small and decreased with decreasing hydrogen concentration [13]. It was around 10 -1~ cm 2 s -1 at 40 ~ -1.0 > -0.9 o z
~ -o.~ LJJ -0.7 --2 ._o -0.6 *5 a_ _ 0.50
(a) CeNi2 40 ~
~5 ,
.
.
i
.
.
.
.
-1.0 CeNiz
>-0.9
(b)
0~
~-0.8
-o71 i
I
,
,
.
.
100 Discharge capacity, C / mAh.g-~
~-0.6 "5 a_ _0.50 . . . .
2345 I .... 100~. . . . Discharge capacity, C / mAh.g-1
Fig. 4. Discharge curves at various currents and temperatures: (a) 40 ~ 2, 100mAg-1; 3, 5 0 m A g - 1 ; 4, 2 5 m A g - 1 ; 5, 1 0 m A g -l.
(b) 0 ~
1,200 mA g - l ;
1209 7cn150 <~ E
- - CeNiz -~ LaNiz -.- PrNi2
uIOG >: .
- - .
-
u
~ 5c Y
260
460
,ooo
Cycles, n
Fig. 5. Discharge capacity ature 20 ~
vs.
cycle curves for the RENi 2 electrodes; current 50 mA g - l , temper-
at an electrode potential of - 0 . 8 V vs. Hg-HgO, which corresponded to a hydrogen pressure of 8.0 • 10 -5 atm. The small discharge capacities of the RENi2 hydride electrodes would be ascribed to the extremely low diffusivity of stored hydrogen with the equilibrium pressure below 10 -4 atm. 3.3. Cycle lives Figure 5 shows the discharge capacity vs. cycle number curves at 20 ~ for each hydride electrode. The electrodes kept more than 80% of their maximum discharge capacities after 400 cycles. In particular, the CeNi 2 and LaNi 2 electrodes retained nearly 70% of their maximum discharge capacities even after 1000 cycles. 3.4. Crystal structures after cyclic measurements Figure 6 shows X-ray diffraction profiles of the discharged hydrides after 1000 cycles for the CeNi2 and LaNi2 hydrides and after 500 cycles for the PrNi2 and MmNi2 hydrides. Although there were small reflections corresponding to hydroxides, no outstanding reflection from the alloys was found in the profile, showing that the hydrides were stable and kept their amor-
Tergel:
Cu
e*0 kvI 150 mA
~
o
a:Cu
~
PrNiz
~
o MmNiz ,
10
20
30
40 2 0
50
60
70
/ deg
Fig. 6. X-ray diffraction profiles of the amorphous hydrides after cyclic charge and discharge. The peaks marked with "o" are from the plated copper.
1210
phous structure after many charge-discharge cycles at room temperature. However, at higher temperature, recrystallization of CeNi2 from amorphous CeNi2-H has been reported by Paul-Boncour et al. [14].
4. C o n c l u s i o n s
Electrodes using hydrogenation-induced amorphous RENi2 (RE = Ce, La, Pr, Mm) had very long cycle lives, though the effective discharge capacities were only one-fourth of those expected from the amount of absorbed hydrogen. The apparently low capacities of these electrodes were attributed to the extremely low dissociation pressures and thereby small diffusivity of hydrogen in the hydrides. It seems that a suitable adjustment of the dissociation pressure is needed in order to utilize all the hydrogen stored in amorphous RENi2. Further investigations on multicomponent alloys are in progress. If their discharge capacities could be improved, these alloys would become promising electrode materials in metal hydride batteries.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
H. F. Bittner and C. C. Badcock, J. Electrochem. Soc., 130 (1983) 193C. H. Oesterreicher, J. Clinton and H. Bittner, Mater, Res. Bull., 11 (1976) 1241. K. Aoki, T. Yamamoto and T. Masumoto, Scr. Metall., 21 (1987) 27. H. Miyamura, T. Sakai, K. Oguro, A. Kato and H. Ishikawa, J. Less-Common Met., 146(1989) 197. H. Miyamura, T. Sakai, K. Oguro, A. Kato and H. Ishikawa, M R S Int. Meeting on Advanced Materials, 1988, Vol. 2, 1989, p. 15. H. Ishikawa, K. Oguro, A. Kato, H. Suzuki and E. Ishii, J. Less-Common Met., 107(1985) 105. T. Sakai, H. Ishikawa, K. Oguro, C. Iwakura and H. Yoneyama, J. Electrochem. Soc, 134 (1987) 558. C. Iwakura, T. Asaoka, H. Yoneyama, T. Sakai, K. Oguro and H. Ishikawa, Nippon Kagaku Kaishi, (1988) 1482. T. Sakai, H. Miyamura, N. Kuriyama, A. Kato, K. Oguro and H. Ishikawa, J. Electrochem. Soc., 137 (1990) 795. T. Sakai, H. Miyamura, N. Kuriyama, A. Kato, K. Oguro and H. Ishikawa, J. Less-Common Met., 159 (1990) 127. D. Richter and R. Hempelmann, J. Less-Common Met., 88 (1982) 353. R. Kirchheim, Acta Metall., 30 (1982) 1059. H. Miyamura, T. Sakai, N. Kuriyama, K. Oguro, A. Kato and H. Ishikawa, Extended Abstracts 40th ISE Meeting, 1989, Vol. 2, 1989, p. 1228. V. Paul-Boncour, C. Lartigue, A. Percheron-Guegan and J. C. Achard, J. Less-Common Met., 143 (1988) 301.