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Activation behaviour of mechanically Ni-coated Zr-based laves phase hydride electrode
Journal o[
ALLOYS AND CON~OUND$ ELSEVIER
Journal of Alloys and Compounds 257 (1997) 302-305
Activation behaviour of mechanically Ni-coated Zr-based Laves phase hydride electrode b* Dalin S u n a, M. Latroche b, A. Percheron-Guegan '
"Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China bLaboratoire de Chimie M&allurgique des Terms Rares, CNRS, l, Place A. Briand, 92195 Meudon Cedex, France Received 6 January I997
Abstract A type of composite particle with pure nickel on the surface of Zr-based Laves phase compound was successfully prepared by means of mechanical ball-milling. X-Ray diffraction shows that after milling with 10 wt% nickel from 30 to 60 min, Zr(Cro.,Nio,o) 2 maintains its crystalline state. Further scanning electron microscopy and energy dispersive X-ray analysis reveal that some nickel particles were decorated on the surface of Zr(Cro.4Nio.~)v and in the meantime some fresh surfaces were simultaneously generated. It is found that these fresh surfaces increase the activation behaviour and the discharge capacity of Zr(Cro.4Nio.6)2. While when nickel particles are coated on its surfaces, not only the discharge capacity increase further, but the activation cycles for the electrode are apparently shortened. According to these results, it is believed that the existence of pure nickel on the surface would make Zr(Cro.4Nio.~)2 to be easily activated in alkaline electrolyte. © 1997 Elsevier Science S.A. Keywords: Zr-based Laves phase; Nickel; Ball-miLling; Activation
1. Introduction Contrary to LaNis-type of compounds, the electrochemical capacity of Zr-based Laves phase compounds is often lower than that determined from solid-H a reaction. This phenomenon is related to either surface oxidation during the preparation of electrode in air or surface passivation in the alkaline electrolyte. Both of them lead to difficult activation, poor kinetics and even jamming of electrochemical reactions. In order to overcome this problem, a method involving the precipitation of second phases in the matrix which have good electrocatalytic activity was developed [1]. In the meantime, we also notice that LaNi 5related alloys show a good activation behaviour even after being air-exposed, which is mainly associated with the formation of metallic nickel in sub-surface owing to the surface segregation [2]. Based on this idea, in the present study, a type of composite particle with pure nickel on the surfaces of the parent compound is proposed and assumed also to show the good activation behaviour like LaNi 5 related alloys. Over the past decades, mechanical ball-milling has been extensively used in the synthesis of amorphous or/and
*Corresponding author, 0925-8388/97/$17.00 © 1997 Elsevier Science S.A, All rights reserved. PII S 0 9 2 5 - 8 3 8 8 ( 9 7 ) 0 0 0 2 5 - X
nanocrystalline materials [3,4]. In the present study, this technique was successfully employed to coat nickel particles on the surfaces of Zr(Cr0.4Nio.~,)v In accordance with our expectation, the resulting particles show an improved activation behaviour in alkaline electrolyte. In order to understand the mechanism of this improvement in relation to the process of ball-milling, X-ray diffraction (XRD) and scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) were used to examine the structure and morphology of the powders with the change of milling time.
2. Experimental details 2.1. Preparation of ran' materials Zr(Cro~4Nio,6) 2 was chosen as a parent compound for its high expected electrochemical capacity according to solid gas experiments (3.5 H/f.u. which correspond to 400 mA h g - i in electrochemical units). It was synthesized from pure elements by induction-melting in a water-cooled copper crucible under argon atmosphere and was annealed 1 month at 1000 °C. XRD and microprobe analysis show that Zr(Cro.4Nio.6) 2 consists of cubic C15 Laves phase with a few of Zr7Ni10 precipitation, The lattice parameter
303
D, 3un et a L / Journal of Alloys and Cotnpounds 257 (1997) 3 0 2 - 3 0 5
of the Laves phase is 7.059(1)A. Then the ingot was mechanically crushed into less than 36 o.m in the air for the following ball-milling. Nickel powder (99.999 wt%100 mesh) from Aldrich Chemical Company, USA.
5000
2.2. Procedures for mechanical ball-milling
3000
I
i
4000,.,,,.~2~
t=60 rain
1
!
I
2
As it has already been recognised that the end products of mechanical ball-milling depend strongly on the experimental parameters, such as milling time, milling speed and weight ratio between balls and sample, and that generally a phase transformation from starting materials to other ones will take place. Unlike these works, the aim of the present study is just to coat nickel particles on the surfaces of the parent compound without damage to the structure of the parent compound. In order to realise this purpose, the following adequate procedures were determined through some preliminary experiments: (i) mixing 1 g. of Zr(Cro.4Ni0.6) 2 with 10 wt% of nickel powder; (ii) sealing the mixed powders in a stainless steel vial (12 rot) under argon atmosphere, and the weight ratio between bails and powders is fixed at 4:i; and (iii) to increase the probability of collision between the two "kinds of particles in a relatively less time, a Spex model 8000 Mixer/Mill was chosen to perform ball-milling because its vial can rotate in the three dimensions in the course of milling. The apparatus was run for 30, 45 and 60 min at the speed of 400 rounds per minute.
2.3. XRD, SEM and EDXA experiments After being ball-milled for the given time, the mixed particles were characterised by XRD, SEM and EDXA. XRD patterns were registered on a Philips PW 11710 diffractometer which was operated at 40 kV and 20 mA. The morphology of particles was observed on a JSM-840 SEM, and the distribution of nickel was analysed on a Philips 501B SEM equipped with an energy-dispersive X-ray analysis.
4
2000~)q~ 1000 ?
t=3o mJn
)
.,
Ni
I
(200)
I
0
t=O
,
i
r
30
40
50
60
70
80
gO
100
110
Fig. I. XRD patterns of milling Zr(Cro4Ni~~)_,with I0 wt% nickel.
comparison. As the milling time increases from 30 to 60 rain, some diffraction peaks are slightly broadened and their maxima are gradually decreased (see the peak (200) of pure nickel), which is likely to be caused by the increase of mechanical strains and the reduction of particle sizes as the ball-milling proceeds. However, most of Bragg peaks still remain sharp. Therefore, it can be deduced that the parent compoundl Zr(Cro.4Ni0.~) 2, maintains its crystalline state after milling for 60 min, which complies with the aim of the present study. The typical SEM micrograph of the ball-milled powders is shown in Fig. 2. It demonstrates that the surfaces of the milled particles become rough, which means some fresh surfaces were generated during the process of ball-milling. Fig. 3 shows the morphology of a selected Zr(Cro4Nio 6) 2 particle after 45 min of milling and the EDXA image of nickel (bright areas in figure) on its surface, which clearly indicates that nickel particles were deposited on the surfaces of Zr(Cr0.4Nio.6) 2 particles.
2.4. Activation behaviour in alkaline eIecn'olyte The dependence of discharge capacities on chargingdischarging cycles was used to compare the activation behaviour before and after ball-milling. The preparation of electrodes and the electrochemical cell were the same as described in previous study [5].
3. Results
3.1. Characteristics before and after ball-milling XRD patterns of milling Zr(Cro.4Nio.6) 2 with 10 wt% nickel as a function of milling time are presented in Fig. 1. The initial pattern prior to milling is also shown for
120
20 (o)
Fig. 2. The typical SEM micr0graph after ball-milling.
304
D. Sun et al. / Journal of Alloys and Compounds 257 (1997) 302-305
Fig. 3, The SEM micrograph and EDXA image of a selected Zr(Cro4Nio6)2 particle. as observed above, the reduction of initial discharge capacities after being bali-milled is modified in this case.
3.2. Activation behaviour in alkaline electrolyte 3.2.1. Effect o f ball-milling without nickel The discharge capacity vs. cycle before and after ballmilling for 30 and 45 rain is shown in Fig. 4. For Zr(Cr0 4Ni0.6) 2 without milling, it takes eight cycles before -I reaching the maximum discharge capacity, 95 mA h g While after millimg for 30 and 45 rain, the cycles needed to reach the maximum were shortened from eight to five, -I and the maximum value increases to 140 m A h g although the initial discharge capacity is reduced. 3.2.2. Effect o f ball-milling with nickel When coated with nickel, both the activation behaviour and the discharge capacity of Zr(Cro.4Ni0.6) 2 were further increased. Fig. 5 shows the discharge capacity vs. cycle after milling with nickel for a different amount of time. It indicates that all three samples take only two cycles to reach the maximum capacity, 195 mA h g - 1 higher than that of without nickel. Further increasing milling time from 30 to 60 min does not change the maximum value. Unlike 160 140
4. Discussion
Milling Zr(Cro.4Nio.6) 2 with I0 wt% nickel from 30 to 60 min, XRD reveals that the parent compound maintains its crystalline state. Further SEM observation and EDXA show that along with the formation of fresh surfaces, some nickel particles are coated on the surfaces of Zr(Cro.4Nio.6),- during the process of milling. The process for the above observation may be depicted as: when nickel particles collide with Zr(Cro.4Nio.6) = particles by the action of steel balls at high speed, the high kinetic energy causes cold-welding or surface alloying at the points of impact. Hence it is expected that the chemical bonding between nickel and the parent compound may occur to some extent. Bratanich et al. [6] showed that ball-milling of LaNi 5 or Mg2Ni for a shorter time made their activation process easier, and they attributed this to the formation of fresh
I
I
I
|
2
4
6
8
I
-
'7
,.-
120 I00 -
0
80-
t'J
0
ID
tO
/,1 a
60
-'
4020 -
Cycles Fig. 4. The discharge capactty vs. cycle before and after ball-milling•
10
D, Sun et al. I Journal o f Alloys and Compounds 257 (1997) 3 0 2 - 3 0 5
305
200 -
"7
180 -
'4 / / ' 7
I --e--t=45min
¢-
r
<
E o ffz,
0
.--;~J"
/
Zr(Cro,4Nio,,) ' * 10
wt%
[
I Ni
140 120 100 -
o ¢o to
8060.
--
40 .
~ 1
2
3
4
5
6
7
8
Cycles Fig. 5. Discharge capacity vs. cycle after coating with nickel for different times.
surfaces. Although all of ball-milled powders of the present study were exposed to the air when preparing the electrodes, this effect was still investigated firstly in order to clarify the role of ball-milling with nickel. Ball-milling to create fresh surfaces is found to increase the activation behaviour and the discharge capacity of Zr(Cr0.gNi0.6) 2. However when nickel particles are coated on its surfaces, not only does the discharge capacity increase further, but the activation cycles for the electrodes are apparently shortened. According to these results, it is believed that existence of pure nickel on the surface would make Zr(Cr04Nio.6) 2 more easily activated. The role of nickel particles is to catalyse the dissociation of molecular hydrogen on the surfaces, just as observed in LaNi 5 and Mg2Ni [7,8]. Concerning the reduction of the initial discharge capacity as observed in Fig. 4, it can be explained by the unrelaxed mechanical strain accumulated in the samples, which inhibits the hydrogen dissociation on the surfaces of the parent compound. In fact, this phenomenon was also found in the hydrogen absorption of mechanically alloyed Mg2Ni and FeTi [9].
5. Conclusion In order to impr0ve the activation behaviour of Zr-based Laves phase hydride electrode, a type of composite particle with pure nickel on the surface of parent comp6find is proposed. In the present study, these particles were successfully prepared through adequate procedures by means of mechanical ball-milling. XRD shows that after milling with 10 wt% nickel from 30 to 60 min, Zr(Cr0.4Nio.6) 2 maintains its crystalline state. SEM and EDXA reveal that
some nickel particles were coated on the surface of Zr(Cr0.4Ni0.6)2. It is found that for this alloy, when nickel particles are coated on its surfaces, the activation process for the electrode is apparently Shortened. Furthermore, the discharge capacity increases due to the formation of new fresh surfaces. According to these results, it is believed that the existence of pure nickel on the surface would make Zr(Cro.4Nio.6): more easily activated.
Acknowledgments Dalin Sun wants to give his great thanks for the financial support from the CNRS during his stay in France.
References [1] J. M. Joubert, Dalin Sun, M. Latroche and A. Percheron-Gu6gan, Int. Symp. Metal-Hydrogen Systems, Fundamentals and Applications, Les Diablerets, Switzerland, August 1996, in press in J. Alloys Comp. [2] L. Schlapbach, Hydrogen in Intermetallic Compounds, II, SpringerVerlag, Berlin, 1992. [3] E. Gaffet and M. Harmelin, J. Less-Common Met., 157 (1990) 201. [4] C. Suryanarayana and F. H. Froes, Nanostruct. Mater., I (1992) 191. [5] D. Sun, J. M. Joubert, M. Latroche and A. Percheron-Gu6gan, J. Alloys Comp., 239 (1996) 193. [6] T.I. Bratanich, S.M. Solonin and MV. Skorokhod, Int. J. Hydrogen Energy, 20 (1995) 353. [7] X.L. Wang and S. Suda, J. Alloys Comp., 194 (I993) 73. [8] H.Y. Zhu, C.R Chen, Y.Q. Lei, J. Wu and Q.D. Wang, J . LessCommon Met., 173 (1991) 873. [9] L. Zaluski, A. Zaluska, R Tessier, J. O. Str6m-Olsen and R. Schulz, J. Alloys Comp., 217 (1995) 295.
ALLOYS AND CON~OUND$ ELSEVIER
Journal of Alloys and Compounds 257 (1997) 302-305
Activation behaviour of mechanically Ni-coated Zr-based Laves phase hydride electrode b* Dalin S u n a, M. Latroche b, A. Percheron-Guegan '
"Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China bLaboratoire de Chimie M&allurgique des Terms Rares, CNRS, l, Place A. Briand, 92195 Meudon Cedex, France Received 6 January I997
Abstract A type of composite particle with pure nickel on the surface of Zr-based Laves phase compound was successfully prepared by means of mechanical ball-milling. X-Ray diffraction shows that after milling with 10 wt% nickel from 30 to 60 min, Zr(Cro.,Nio,o) 2 maintains its crystalline state. Further scanning electron microscopy and energy dispersive X-ray analysis reveal that some nickel particles were decorated on the surface of Zr(Cro.4Nio.~)v and in the meantime some fresh surfaces were simultaneously generated. It is found that these fresh surfaces increase the activation behaviour and the discharge capacity of Zr(Cro.4Nio.6)2. While when nickel particles are coated on its surfaces, not only the discharge capacity increase further, but the activation cycles for the electrode are apparently shortened. According to these results, it is believed that the existence of pure nickel on the surface would make Zr(Cro.4Nio.~)2 to be easily activated in alkaline electrolyte. © 1997 Elsevier Science S.A. Keywords: Zr-based Laves phase; Nickel; Ball-miLling; Activation
1. Introduction Contrary to LaNis-type of compounds, the electrochemical capacity of Zr-based Laves phase compounds is often lower than that determined from solid-H a reaction. This phenomenon is related to either surface oxidation during the preparation of electrode in air or surface passivation in the alkaline electrolyte. Both of them lead to difficult activation, poor kinetics and even jamming of electrochemical reactions. In order to overcome this problem, a method involving the precipitation of second phases in the matrix which have good electrocatalytic activity was developed [1]. In the meantime, we also notice that LaNi 5related alloys show a good activation behaviour even after being air-exposed, which is mainly associated with the formation of metallic nickel in sub-surface owing to the surface segregation [2]. Based on this idea, in the present study, a type of composite particle with pure nickel on the surfaces of the parent compound is proposed and assumed also to show the good activation behaviour like LaNi 5 related alloys. Over the past decades, mechanical ball-milling has been extensively used in the synthesis of amorphous or/and
*Corresponding author, 0925-8388/97/$17.00 © 1997 Elsevier Science S.A, All rights reserved. PII S 0 9 2 5 - 8 3 8 8 ( 9 7 ) 0 0 0 2 5 - X
nanocrystalline materials [3,4]. In the present study, this technique was successfully employed to coat nickel particles on the surfaces of Zr(Cr0.4Nio.~,)v In accordance with our expectation, the resulting particles show an improved activation behaviour in alkaline electrolyte. In order to understand the mechanism of this improvement in relation to the process of ball-milling, X-ray diffraction (XRD) and scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDXA) were used to examine the structure and morphology of the powders with the change of milling time.
2. Experimental details 2.1. Preparation of ran' materials Zr(Cro~4Nio,6) 2 was chosen as a parent compound for its high expected electrochemical capacity according to solid gas experiments (3.5 H/f.u. which correspond to 400 mA h g - i in electrochemical units). It was synthesized from pure elements by induction-melting in a water-cooled copper crucible under argon atmosphere and was annealed 1 month at 1000 °C. XRD and microprobe analysis show that Zr(Cro.4Nio.6) 2 consists of cubic C15 Laves phase with a few of Zr7Ni10 precipitation, The lattice parameter
303
D, 3un et a L / Journal of Alloys and Cotnpounds 257 (1997) 3 0 2 - 3 0 5
of the Laves phase is 7.059(1)A. Then the ingot was mechanically crushed into less than 36 o.m in the air for the following ball-milling. Nickel powder (99.999 wt%100 mesh) from Aldrich Chemical Company, USA.
5000
2.2. Procedures for mechanical ball-milling
3000
I
i
4000,.,,,.~2~
t=60 rain
1
!
I
2
As it has already been recognised that the end products of mechanical ball-milling depend strongly on the experimental parameters, such as milling time, milling speed and weight ratio between balls and sample, and that generally a phase transformation from starting materials to other ones will take place. Unlike these works, the aim of the present study is just to coat nickel particles on the surfaces of the parent compound without damage to the structure of the parent compound. In order to realise this purpose, the following adequate procedures were determined through some preliminary experiments: (i) mixing 1 g. of Zr(Cro.4Ni0.6) 2 with 10 wt% of nickel powder; (ii) sealing the mixed powders in a stainless steel vial (12 rot) under argon atmosphere, and the weight ratio between bails and powders is fixed at 4:i; and (iii) to increase the probability of collision between the two "kinds of particles in a relatively less time, a Spex model 8000 Mixer/Mill was chosen to perform ball-milling because its vial can rotate in the three dimensions in the course of milling. The apparatus was run for 30, 45 and 60 min at the speed of 400 rounds per minute.
2.3. XRD, SEM and EDXA experiments After being ball-milled for the given time, the mixed particles were characterised by XRD, SEM and EDXA. XRD patterns were registered on a Philips PW 11710 diffractometer which was operated at 40 kV and 20 mA. The morphology of particles was observed on a JSM-840 SEM, and the distribution of nickel was analysed on a Philips 501B SEM equipped with an energy-dispersive X-ray analysis.
4
2000~)q~ 1000 ?
t=3o mJn
)
.,
Ni
I
(200)
I
0
t=O
,
i
r
30
40
50
60
70
80
gO
100
110
Fig. I. XRD patterns of milling Zr(Cro4Ni~~)_,with I0 wt% nickel.
comparison. As the milling time increases from 30 to 60 rain, some diffraction peaks are slightly broadened and their maxima are gradually decreased (see the peak (200) of pure nickel), which is likely to be caused by the increase of mechanical strains and the reduction of particle sizes as the ball-milling proceeds. However, most of Bragg peaks still remain sharp. Therefore, it can be deduced that the parent compoundl Zr(Cro.4Ni0.~) 2, maintains its crystalline state after milling for 60 min, which complies with the aim of the present study. The typical SEM micrograph of the ball-milled powders is shown in Fig. 2. It demonstrates that the surfaces of the milled particles become rough, which means some fresh surfaces were generated during the process of ball-milling. Fig. 3 shows the morphology of a selected Zr(Cro4Nio 6) 2 particle after 45 min of milling and the EDXA image of nickel (bright areas in figure) on its surface, which clearly indicates that nickel particles were deposited on the surfaces of Zr(Cr0.4Nio.6) 2 particles.
2.4. Activation behaviour in alkaline eIecn'olyte The dependence of discharge capacities on chargingdischarging cycles was used to compare the activation behaviour before and after ball-milling. The preparation of electrodes and the electrochemical cell were the same as described in previous study [5].
3. Results
3.1. Characteristics before and after ball-milling XRD patterns of milling Zr(Cro.4Nio.6) 2 with 10 wt% nickel as a function of milling time are presented in Fig. 1. The initial pattern prior to milling is also shown for
120
20 (o)
Fig. 2. The typical SEM micr0graph after ball-milling.
304
D. Sun et al. / Journal of Alloys and Compounds 257 (1997) 302-305
Fig. 3, The SEM micrograph and EDXA image of a selected Zr(Cro4Nio6)2 particle. as observed above, the reduction of initial discharge capacities after being bali-milled is modified in this case.
3.2. Activation behaviour in alkaline electrolyte 3.2.1. Effect o f ball-milling without nickel The discharge capacity vs. cycle before and after ballmilling for 30 and 45 rain is shown in Fig. 4. For Zr(Cr0 4Ni0.6) 2 without milling, it takes eight cycles before -I reaching the maximum discharge capacity, 95 mA h g While after millimg for 30 and 45 rain, the cycles needed to reach the maximum were shortened from eight to five, -I and the maximum value increases to 140 m A h g although the initial discharge capacity is reduced. 3.2.2. Effect o f ball-milling with nickel When coated with nickel, both the activation behaviour and the discharge capacity of Zr(Cro.4Ni0.6) 2 were further increased. Fig. 5 shows the discharge capacity vs. cycle after milling with nickel for a different amount of time. It indicates that all three samples take only two cycles to reach the maximum capacity, 195 mA h g - 1 higher than that of without nickel. Further increasing milling time from 30 to 60 min does not change the maximum value. Unlike 160 140
4. Discussion
Milling Zr(Cro.4Nio.6) 2 with I0 wt% nickel from 30 to 60 min, XRD reveals that the parent compound maintains its crystalline state. Further SEM observation and EDXA show that along with the formation of fresh surfaces, some nickel particles are coated on the surfaces of Zr(Cro.4Nio.6),- during the process of milling. The process for the above observation may be depicted as: when nickel particles collide with Zr(Cro.4Nio.6) = particles by the action of steel balls at high speed, the high kinetic energy causes cold-welding or surface alloying at the points of impact. Hence it is expected that the chemical bonding between nickel and the parent compound may occur to some extent. Bratanich et al. [6] showed that ball-milling of LaNi 5 or Mg2Ni for a shorter time made their activation process easier, and they attributed this to the formation of fresh
I
I
I
|
2
4
6
8
I
-
'7
,.-
120 I00 -
0
80-
t'J
0
ID
tO
/,1 a
60
-'
4020 -
Cycles Fig. 4. The discharge capactty vs. cycle before and after ball-milling•
10
D, Sun et al. I Journal o f Alloys and Compounds 257 (1997) 3 0 2 - 3 0 5
305
200 -
"7
180 -
'4 / / ' 7
I --e--t=45min
¢-
r
<
E o ffz,
0
.--;~J"
/
Zr(Cro,4Nio,,) ' * 10
wt%
[
I Ni
140 120 100 -
o ¢o to
8060.
--
40 .
~ 1
2
3
4
5
6
7
8
Cycles Fig. 5. Discharge capacity vs. cycle after coating with nickel for different times.
surfaces. Although all of ball-milled powders of the present study were exposed to the air when preparing the electrodes, this effect was still investigated firstly in order to clarify the role of ball-milling with nickel. Ball-milling to create fresh surfaces is found to increase the activation behaviour and the discharge capacity of Zr(Cr0.gNi0.6) 2. However when nickel particles are coated on its surfaces, not only does the discharge capacity increase further, but the activation cycles for the electrodes are apparently shortened. According to these results, it is believed that existence of pure nickel on the surface would make Zr(Cr04Nio.6) 2 more easily activated. The role of nickel particles is to catalyse the dissociation of molecular hydrogen on the surfaces, just as observed in LaNi 5 and Mg2Ni [7,8]. Concerning the reduction of the initial discharge capacity as observed in Fig. 4, it can be explained by the unrelaxed mechanical strain accumulated in the samples, which inhibits the hydrogen dissociation on the surfaces of the parent compound. In fact, this phenomenon was also found in the hydrogen absorption of mechanically alloyed Mg2Ni and FeTi [9].
5. Conclusion In order to impr0ve the activation behaviour of Zr-based Laves phase hydride electrode, a type of composite particle with pure nickel on the surface of parent comp6find is proposed. In the present study, these particles were successfully prepared through adequate procedures by means of mechanical ball-milling. XRD shows that after milling with 10 wt% nickel from 30 to 60 min, Zr(Cr0.4Nio.6) 2 maintains its crystalline state. SEM and EDXA reveal that
some nickel particles were coated on the surface of Zr(Cr0.4Ni0.6)2. It is found that for this alloy, when nickel particles are coated on its surfaces, the activation process for the electrode is apparently Shortened. Furthermore, the discharge capacity increases due to the formation of new fresh surfaces. According to these results, it is believed that the existence of pure nickel on the surface would make Zr(Cro.4Nio.6): more easily activated.
Acknowledgments Dalin Sun wants to give his great thanks for the financial support from the CNRS during his stay in France.
References [1] J. M. Joubert, Dalin Sun, M. Latroche and A. Percheron-Gu6gan, Int. Symp. Metal-Hydrogen Systems, Fundamentals and Applications, Les Diablerets, Switzerland, August 1996, in press in J. Alloys Comp. [2] L. Schlapbach, Hydrogen in Intermetallic Compounds, II, SpringerVerlag, Berlin, 1992. [3] E. Gaffet and M. Harmelin, J. Less-Common Met., 157 (1990) 201. [4] C. Suryanarayana and F. H. Froes, Nanostruct. Mater., I (1992) 191. [5] D. Sun, J. M. Joubert, M. Latroche and A. Percheron-Gu6gan, J. Alloys Comp., 239 (1996) 193. [6] T.I. Bratanich, S.M. Solonin and MV. Skorokhod, Int. J. Hydrogen Energy, 20 (1995) 353. [7] X.L. Wang and S. Suda, J. Alloys Comp., 194 (I993) 73. [8] H.Y. Zhu, C.R Chen, Y.Q. Lei, J. Wu and Q.D. Wang, J . LessCommon Met., 173 (1991) 873. [9] L. Zaluski, A. Zaluska, R Tessier, J. O. Str6m-Olsen and R. Schulz, J. Alloys Comp., 217 (1995) 295.