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Metal hydride electrodes based on solid solution type alloy TiV3Nix (0 ≦ x ≦ 0.75)
Journal of
ALLOY5
AHD CO, POUND5 ELSEVIER
Journal of Alloysand Compounds226 (1995) 203-207
Metal hydride electrodes based on solid solution type alloy TiV3Nix
(0
Received 24 December1994; in final form28 January 1995
Abstract Microslmctures, pressure-composition isotherms, and charge-discharge characteristics of TiV2Nix (0 0.25 composed of a vanadium rich (/3Ti, V) main phase and a TiNi based b.c.c, secondary phase. The secondary phase formed a ihree dimensional network, enhancing electrode kinetics of the alloys. The maximum discharge capacity was 420 A h kg- l for x=0.56. Keywords: Metal hydride electrodes; Secondary phases; Three dimensional network
1. Introduction Group V metals such as vanadium, niobium, and tantalum are known to form monohydride and dihydride. A lot of investigations have been reported as reviewed by Schober and WenzA [ 1 ]. Reaction between V and VH = 1occurs below 10° Pa at 373 K [2] which is too stable to use for practical applications. However, reaction between VH = t and VH = z occurs reversibly at moderate pressure and temperature [3] with more hydrogen capacity per mass than that for the reaction between LaNi5 and LaNisH6. The diffusion rate of hydrogen in vanadium is extremely high. The diffusion coefficient of hydrogen atoms in VH
of TiV3Nix, the phase structure changes with increasing the nickel content x from a single/3-phase to (/3 + 8)-phases and finally to (/3 + 6 + r/)-phases, where the/3-phase, the 6-phase, and the r/-phase are defined as the (/3ri, v ) solid solutiontype b.c.c, phase, the TiNi-type b.c.c, phase, and the Ti2Ni-type phase respectively. It was found from an electrochemical pressure-composition isotherm (PCT curve) that the TiV3Nix electrode (x = 0.56) showed very high storage capacity (approximately 800 A h k g - ~). In the present paper, the microstructures, the PCT curves, and the charge--discharge characteristics of TiV3Ni~ (0 < x < 0.75) are reported.
2. Experimental details The alloy samples of TiV3Nix ( x = 0, 0.25, 0.50, 0.56, 0.75) were prepared by arc melting of metal components on a water-cooled copper hearth in argon gas. Each ingot was turned and remelt several times to be homogenized. The ascast samples were pulverized by hydriding at high temperature (less than 673 K) under high pressure hydrogen gas (less than 3.3 MPa). The metallurgical microstructures were examined by scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA) on polished crosssections. The samples were crushed mechanically to a powder under 100/zm in diameter and degassed at 673 K under vacuum.
204
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207 i
1
o 0
A
(a)
0
==
(b)
e 0
0
40
/~
(d)
,j~
(e)
60
50
20 / deg Fig. 1. X-ray diffraction patterns of TiVsNi~ (a) x=O, (b) x=0.25, (c) x=0.50, (d) x=0.56 and (e) x=0.75. F o r x ~ 0 the alloys have the main phase O and the secondary phase V.
Crystal structures were analyzed by X-ray powder diffraction (XRD) using Cu K a radiation. The PCT curves were measured with a Sieverts-type apparatus. Each sample was mechanically crushed, put in an SUS 316 reactor tube, and activated as follows. The reactor was evacuated and heated up to 673 K, followed by admitting
hydrogen (purity>99.9999%) up to 3.3 MPa and then cooled down to room temperature. Just before measuring the PCT curve, the reactor was evacuated at 673 K for 4 h in order to get a hydrogen zero point. The discharge capacities Cd of the alloys were measured in a half cell at discharge rates of 25-400 A k g - 1 to - 0.7 V vs. Hg/HgO at 293 K using an Hg/HgO reference electrode, a 6 M KOH electrolyte, and an Ni(OH) 2 counter electrode as reported previously [8]. Alloy electrodes were prepared by hot pressing 20 mass% copper coated powder sample with 10 mass% FEP binder (Daikin Co. tetrafluoroethylenehexafluoropropylene copolymer) onto a nickel mesh at 573 K. The charge-discharge cycle test of the electrode was conducted by charging at 100 A kg- ~ and discharging at 50 A k g - 1 to the cut off voltage of - 0.7 V vs. Hg/HgO at 293 K. The electrode was rinsed with pure water and dried at 333 K for the SEM-EPMA observations and the PCT measurement.
3. Results and discussion
The XRD patterns of TiV3Nix (x = 0, 0.25, 0.50, 0.56, 0.75) are shown in Fig. 1. All alloys had a main phase with a b.c.c, type structure. The peaks for the main phase shifted to higher angle with increasing the nickel content x, which indicated that the unit cell contracted. A secondary phase was
(a)
(b)
(c)
(d
50 lun
Fig. 2. The scanning electron micrographs of TiV3Nix (a) x=0, (b) x=0.25, (c) x=0.50, and (d) x=0.75.
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207 100 -..
75 50 25
o 0.25
X
(o)
0.5
0.75
0.5
0.75
IO0
-
~"
50
i_._v i L-,_Ni
25
0 0.25 X
(b)
Fig. 3. The composition of (a) the main phase and (b) the secondary phase of TiV3Ni~ (x = 0-0.75).
5 ,ff
,-
0.5
0.1 ,- 0.05
=a
i
3.010
i
Hydrogen
(M/H)
content
Fig. 4. Pressure-composition isotherms for TiV3Nix (x = 0-0.75) at 353 K: O, x = 0; E], x = 0.25; ~ , x = 0.50; O, x = 0.56, and V, x = 0,75. 500
--
I
"7i&e e-.
400
300 o
/ 1O0
/'
/
--
0.0
'
0.2
/
,/
7",~
400
[]
// /
/ '
co 0
c~
//
t-.
SEM images of the cross-sections of the alloys are shown in Fig. 2. The alloy ofTiV 3 looked a single phase of the b,c.c. solid solution. By adding the nickel to the alloy up tox = 0.25, in TiV3Nio.25, the secondary phase (light gray) was formed like islands in the main phase (dark gray). With increasing nickel content, in the alloys of TiV3Nio.5 and TiV3Nio.75, the secondary phases were dispersed along the grain boundary of the main phase, forming a three dimensional network. In TiV3Nio.75, the tertiary phase (black) with composition of Ti~V=oNi=o.5 appeared in addition to the secondary phase. The compositional change of vanadium, titanium, and nickel with increasing nickel content x in the main phase and in the secondary phase are shown in Figs. 3(a) and 3(b) respectively. It is considered that the main phase is the (/3Ti, V) based alloy containing nickel while the secondary phase is the TiNi based alloy containing vanadium. The titanium content in the main phase decreased with increasing total nickel content x because the volume ratio of the TiNi based secondary phase increased as observed in Fig. 2. The unit cell contraction of the main phase would be ascribed to the reducing titanium content because the atomic radius of the titanium is the largest among the component elements. The PCT curves for TiV3Nix (0 0.56 the discharge capacity at 25 A kg-~ was in good agreement with the half value (dashed line 500
// //
230
205
/
/~A 0.4
"\
~.~ 300 0.6
0.8
\ \\
1.0
3C
Fig. 5. Discharge capacities of TiV3Nix ( x = 0 - 0 . 7 5 ) at 293 K for various current densities: Q, 25 A k g - l; II, 50 A k g - 1; A, 100 A k g - ~; O, 200 A kg-~; D, 300 A kg-~ and A, 400 A kg-~. The dashed line shows the discharge capacities calculated from the half of hydrogen content at 293 K at 1.0 MPa obtained from PCT curves.
observed in the TiV3Nix ( x > 0.25) alloys, having the same b.c.c, type structure with a smaller unit cell dimension. The peak intensity due to the secondary phase increased and the unit cell of the main phase contracted with increasing the nickel content x.
,\ "\\\
2o0 g
"~ loo
0
i
i
i
10
20
30
,
i
i
L
i
40
5C
GO
7C
8@
Charge / dischargecyc/e nurri~er Fig. 6. Discharge capacity vs. cycle number of TiV:~Nio56 electrode at 293 K.
206
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207
Cu shell
INi0.56
30
l,m ........
(o)
c
.6I,,= V
03 e"
.^.~,.Ti
,',
,..,
.,~
e" m
' \/
grain edge
Ni
grain center
/",
grain edge
Capacity decay of the TiV3Nio.s6 electrode with chargedischarge cycle is shown in Fig. 6. This alloy showed very quick activation, reaching a maximum value of 420 A h kg- 1 at the third cycle. The discharge capacity was rapidly decreased and vanished at the 77th cycle. Fig. 7(a) shows a cross-section of a copper coated powder of the degraded electrode. The secondary phase almost disappeared by the corrosion of both titanium and vanadium. The main phase grain remained though the size of it diminished. Fig. 7(b) shows the compositional change across one of the main phase grains and the secondary phase surrounding it measured by EPMA. The titanium content of the secondary phase was significantly low while the nickel content remained unchanged. In the main phase grain the titanium and vanadium contents were selectively decreased on the grain edges compared with the grain center. Titanium and vanadium would be dissolved from the grain surface into the KOH solution, causing enrichment of nickel in the main phase. Fig. 8 compares PCT curves at 313 K for the TiV3Nio.s6 alloy between the as-prepared and degraded samples in the electrode after the cycle test, though the net mass of the alloy in the electrode was not accurate. After the cycle test the plateau pressure slightly increased together with the sloping plateau. This result would be explained by the enrichment of nickel in the surface layer of the main phase because the plateau pressure is increased by increasing the nickel content in the alloy [7]; this would be ascribed to the unit cell contraction. Evidently the alloy in the electrode after the cycle test absorbed and desorbed hydrogen in a gas process although it was inactive in the electrochemical process. After the secondary phase disappeared, the alloy lost discharge ability. This result suggests that the TiNi based secondary phase enhances the electrochemical reaction. It is considered that the main matrix phase is responsible for hydrogen storage and the TiNi based secondary phase for the electrochemical reaction as a catalyst and/or a current collector. This model would explain the fact that the discharge
(b)
Fig. 7. (a) Backscatteredscanningelectronmicrographof onepowdergrain of TiV3Ni0.56electrodeafter 77 cycles;and (b) line scans ofTi Ka, V Ka and Ni Ka across one grain of the main phase and the secondaryphase surroundingit. in Fig. 5) of the total hydrogen storage capacity obtained from PCT curves at 293 K at 1.0 MPa. The total storage capacity would be obtained from whole reaction (VH~ --+V) while the dischargeable capacity would be obtained from the half reaction (VH2 ~ VH) because the vanadium monohydride would be too stable to discharge hydrogen at 293 K by this discharge rate [7]. The discharge capacity at higher rate increased with increasing total nickel content in the alloy though the nickel content in the main phase kept constant as shown in Fig. 3. This result suggests that the TiNi based secondary phase would have a very important role for enhancing the reaction rate of the (/3Ti, V) based main phase.
5 Z
e-
~.~='-0 . 5 "
0.1
o 0.05
"0
0"010
'
'
'
i
. . . .
2
'
Hydrogen oontent (M/H)
Fig. 8. Pressure-compositionisothermsfor TiV3Nio.s6(©) and the alloy taken from the electrode after the charge-discharge cycle test (VI) at 313K.
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207
capacity at high rate increases with increasing volume ratio of the secondary phase as shown in Fig. 5.
4. Conclusions The TiV3Nix (x < 0.25) alloys are composed of the (flTi, V) based main phase and the TiNi based secondary phase with a b.c.c, structure. With increasing nickel content the hydrogen storage capacity decreases but the utilization efficiency c f the metal hydride electrode significantly increases. The maximum discharge capacity (420 A h kg -1) was obtained for x = 0.56. After 77 cycles the discharge capacity was completely lost and the TiNi based secondary phase disappeared. It has been concluded that the TiNi based secondary phase would help A h kg--1 the electrochemical reaction on the grain of the
207
(ffri, v ) based main phase as a catalyst and/or a current collector.
References [ 1] T. Schoberand H. Wenzl, in G. Alefeldand J. V61kl (eds,), Hydrogen in Metals II, Topics in Applied Physics, Vol. 29, Springer-Verlag,Berlin, 1978, p. 11. [2] K. Fujita, Y.C. Huang and M. Tada, Nippon Kinzoku Gakkaishi, 43 (1979) 601. [3] J.J. Reilly and R.H. Wiswall, lnorg. Chem., 9 (1970) 1678. [4] J.E. Kleiner, E.H. Sevillaand R.M. Cotts,Phys. Rev. B, 33 (1986) 6662. [5] A.J. Maeland, G.G. Libowitz,J.F. Lynch and G. Rak, .L Less-Common Met., 104 (1984) 133. [6] G.G. Libowitz and A.J. Maeland, Mater. Sci. Forum, 31 (1988) 177. [7] M. Tsukahara, K. Takahashi, T. Mishima, T. Sakai, H. Miyamura, N. Kuriyamaand I. Uehara, J. Alloys Comp., submitted for publication. [8] T. Sakal, H. Yoshinaga,H. Miyamura,N. Kuriyama,A. Kato, K. Oguro and H. Ishikawa,J. Alloys Comp., 180 (1992) 37.
ALLOY5
AHD CO, POUND5 ELSEVIER
Journal of Alloysand Compounds226 (1995) 203-207
Metal hydride electrodes based on solid solution type alloy TiV3Nix
(0
Received 24 December1994; in final form28 January 1995
Abstract Microslmctures, pressure-composition isotherms, and charge-discharge characteristics of TiV2Nix (0
1. Introduction Group V metals such as vanadium, niobium, and tantalum are known to form monohydride and dihydride. A lot of investigations have been reported as reviewed by Schober and WenzA [ 1 ]. Reaction between V and VH = 1occurs below 10° Pa at 373 K [2] which is too stable to use for practical applications. However, reaction between VH = t and VH = z occurs reversibly at moderate pressure and temperature [3] with more hydrogen capacity per mass than that for the reaction between LaNi5 and LaNisH6. The diffusion rate of hydrogen in vanadium is extremely high. The diffusion coefficient of hydrogen atoms in VH
of TiV3Nix, the phase structure changes with increasing the nickel content x from a single/3-phase to (/3 + 8)-phases and finally to (/3 + 6 + r/)-phases, where the/3-phase, the 6-phase, and the r/-phase are defined as the (/3ri, v ) solid solutiontype b.c.c, phase, the TiNi-type b.c.c, phase, and the Ti2Ni-type phase respectively. It was found from an electrochemical pressure-composition isotherm (PCT curve) that the TiV3Nix electrode (x = 0.56) showed very high storage capacity (approximately 800 A h k g - ~). In the present paper, the microstructures, the PCT curves, and the charge--discharge characteristics of TiV3Ni~ (0 < x < 0.75) are reported.
2. Experimental details The alloy samples of TiV3Nix ( x = 0, 0.25, 0.50, 0.56, 0.75) were prepared by arc melting of metal components on a water-cooled copper hearth in argon gas. Each ingot was turned and remelt several times to be homogenized. The ascast samples were pulverized by hydriding at high temperature (less than 673 K) under high pressure hydrogen gas (less than 3.3 MPa). The metallurgical microstructures were examined by scanning electron microscopy (SEM) and electron probe X-ray microanalysis (EPMA) on polished crosssections. The samples were crushed mechanically to a powder under 100/zm in diameter and degassed at 673 K under vacuum.
204
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207 i
1
o 0
A
(a)
0
==
(b)
e 0
0
40
/~
(d)
,j~
(e)
60
50
20 / deg Fig. 1. X-ray diffraction patterns of TiVsNi~ (a) x=O, (b) x=0.25, (c) x=0.50, (d) x=0.56 and (e) x=0.75. F o r x ~ 0 the alloys have the main phase O and the secondary phase V.
Crystal structures were analyzed by X-ray powder diffraction (XRD) using Cu K a radiation. The PCT curves were measured with a Sieverts-type apparatus. Each sample was mechanically crushed, put in an SUS 316 reactor tube, and activated as follows. The reactor was evacuated and heated up to 673 K, followed by admitting
hydrogen (purity>99.9999%) up to 3.3 MPa and then cooled down to room temperature. Just before measuring the PCT curve, the reactor was evacuated at 673 K for 4 h in order to get a hydrogen zero point. The discharge capacities Cd of the alloys were measured in a half cell at discharge rates of 25-400 A k g - 1 to - 0.7 V vs. Hg/HgO at 293 K using an Hg/HgO reference electrode, a 6 M KOH electrolyte, and an Ni(OH) 2 counter electrode as reported previously [8]. Alloy electrodes were prepared by hot pressing 20 mass% copper coated powder sample with 10 mass% FEP binder (Daikin Co. tetrafluoroethylenehexafluoropropylene copolymer) onto a nickel mesh at 573 K. The charge-discharge cycle test of the electrode was conducted by charging at 100 A kg- ~ and discharging at 50 A k g - 1 to the cut off voltage of - 0.7 V vs. Hg/HgO at 293 K. The electrode was rinsed with pure water and dried at 333 K for the SEM-EPMA observations and the PCT measurement.
3. Results and discussion
The XRD patterns of TiV3Nix (x = 0, 0.25, 0.50, 0.56, 0.75) are shown in Fig. 1. All alloys had a main phase with a b.c.c, type structure. The peaks for the main phase shifted to higher angle with increasing the nickel content x, which indicated that the unit cell contracted. A secondary phase was
(a)
(b)
(c)
(d
50 lun
Fig. 2. The scanning electron micrographs of TiV3Nix (a) x=0, (b) x=0.25, (c) x=0.50, and (d) x=0.75.
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207 100 -..
75 50 25
o 0.25
X
(o)
0.5
0.75
0.5
0.75
IO0
-
~"
50
i_._v i L-,_Ni
25
0 0.25 X
(b)
Fig. 3. The composition of (a) the main phase and (b) the secondary phase of TiV3Ni~ (x = 0-0.75).
5 ,ff
,-
0.5
0.1 ,- 0.05
=a
i
3.010
i
Hydrogen
(M/H)
content
Fig. 4. Pressure-composition isotherms for TiV3Nix (x = 0-0.75) at 353 K: O, x = 0; E], x = 0.25; ~ , x = 0.50; O, x = 0.56, and V, x = 0,75. 500
--
I
"7i&e e-.
400
300 o
/ 1O0
/'
/
--
0.0
'
0.2
/
,/
7",~
400
[]
// /
/ '
co 0
c~
//
t-.
SEM images of the cross-sections of the alloys are shown in Fig. 2. The alloy ofTiV 3 looked a single phase of the b,c.c. solid solution. By adding the nickel to the alloy up tox = 0.25, in TiV3Nio.25, the secondary phase (light gray) was formed like islands in the main phase (dark gray). With increasing nickel content, in the alloys of TiV3Nio.5 and TiV3Nio.75, the secondary phases were dispersed along the grain boundary of the main phase, forming a three dimensional network. In TiV3Nio.75, the tertiary phase (black) with composition of Ti~V=oNi=o.5 appeared in addition to the secondary phase. The compositional change of vanadium, titanium, and nickel with increasing nickel content x in the main phase and in the secondary phase are shown in Figs. 3(a) and 3(b) respectively. It is considered that the main phase is the (/3Ti, V) based alloy containing nickel while the secondary phase is the TiNi based alloy containing vanadium. The titanium content in the main phase decreased with increasing total nickel content x because the volume ratio of the TiNi based secondary phase increased as observed in Fig. 2. The unit cell contraction of the main phase would be ascribed to the reducing titanium content because the atomic radius of the titanium is the largest among the component elements. The PCT curves for TiV3Nix (0
// //
230
205
/
/~A 0.4
"\
~.~ 300 0.6
0.8
\ \\
1.0
3C
Fig. 5. Discharge capacities of TiV3Nix ( x = 0 - 0 . 7 5 ) at 293 K for various current densities: Q, 25 A k g - l; II, 50 A k g - 1; A, 100 A k g - ~; O, 200 A kg-~; D, 300 A kg-~ and A, 400 A kg-~. The dashed line shows the discharge capacities calculated from the half of hydrogen content at 293 K at 1.0 MPa obtained from PCT curves.
observed in the TiV3Nix ( x > 0.25) alloys, having the same b.c.c, type structure with a smaller unit cell dimension. The peak intensity due to the secondary phase increased and the unit cell of the main phase contracted with increasing the nickel content x.
,\ "\\\
2o0 g
"~ loo
0
i
i
i
10
20
30
,
i
i
L
i
40
5C
GO
7C
8@
Charge / dischargecyc/e nurri~er Fig. 6. Discharge capacity vs. cycle number of TiV:~Nio56 electrode at 293 K.
206
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207
Cu shell
INi0.56
30
l,m ........
(o)
c
.6I,,= V
03 e"
.^.~,.Ti
,',
,..,
.,~
e" m
' \/
grain edge
Ni
grain center
/",
grain edge
Capacity decay of the TiV3Nio.s6 electrode with chargedischarge cycle is shown in Fig. 6. This alloy showed very quick activation, reaching a maximum value of 420 A h kg- 1 at the third cycle. The discharge capacity was rapidly decreased and vanished at the 77th cycle. Fig. 7(a) shows a cross-section of a copper coated powder of the degraded electrode. The secondary phase almost disappeared by the corrosion of both titanium and vanadium. The main phase grain remained though the size of it diminished. Fig. 7(b) shows the compositional change across one of the main phase grains and the secondary phase surrounding it measured by EPMA. The titanium content of the secondary phase was significantly low while the nickel content remained unchanged. In the main phase grain the titanium and vanadium contents were selectively decreased on the grain edges compared with the grain center. Titanium and vanadium would be dissolved from the grain surface into the KOH solution, causing enrichment of nickel in the main phase. Fig. 8 compares PCT curves at 313 K for the TiV3Nio.s6 alloy between the as-prepared and degraded samples in the electrode after the cycle test, though the net mass of the alloy in the electrode was not accurate. After the cycle test the plateau pressure slightly increased together with the sloping plateau. This result would be explained by the enrichment of nickel in the surface layer of the main phase because the plateau pressure is increased by increasing the nickel content in the alloy [7]; this would be ascribed to the unit cell contraction. Evidently the alloy in the electrode after the cycle test absorbed and desorbed hydrogen in a gas process although it was inactive in the electrochemical process. After the secondary phase disappeared, the alloy lost discharge ability. This result suggests that the TiNi based secondary phase enhances the electrochemical reaction. It is considered that the main matrix phase is responsible for hydrogen storage and the TiNi based secondary phase for the electrochemical reaction as a catalyst and/or a current collector. This model would explain the fact that the discharge
(b)
Fig. 7. (a) Backscatteredscanningelectronmicrographof onepowdergrain of TiV3Ni0.56electrodeafter 77 cycles;and (b) line scans ofTi Ka, V Ka and Ni Ka across one grain of the main phase and the secondaryphase surroundingit. in Fig. 5) of the total hydrogen storage capacity obtained from PCT curves at 293 K at 1.0 MPa. The total storage capacity would be obtained from whole reaction (VH~ --+V) while the dischargeable capacity would be obtained from the half reaction (VH2 ~ VH) because the vanadium monohydride would be too stable to discharge hydrogen at 293 K by this discharge rate [7]. The discharge capacity at higher rate increased with increasing total nickel content in the alloy though the nickel content in the main phase kept constant as shown in Fig. 3. This result suggests that the TiNi based secondary phase would have a very important role for enhancing the reaction rate of the (/3Ti, V) based main phase.
5 Z
e-
~.~='-0 . 5 "
0.1
o 0.05
"0
0"010
'
'
'
i
. . . .
2
'
Hydrogen oontent (M/H)
Fig. 8. Pressure-compositionisothermsfor TiV3Nio.s6(©) and the alloy taken from the electrode after the charge-discharge cycle test (VI) at 313K.
M. Tsukahara et al. / Journal of Alloys and Compounds 226 (1995) 203-207
capacity at high rate increases with increasing volume ratio of the secondary phase as shown in Fig. 5.
4. Conclusions The TiV3Nix (x < 0.25) alloys are composed of the (flTi, V) based main phase and the TiNi based secondary phase with a b.c.c, structure. With increasing nickel content the hydrogen storage capacity decreases but the utilization efficiency c f the metal hydride electrode significantly increases. The maximum discharge capacity (420 A h kg -1) was obtained for x = 0.56. After 77 cycles the discharge capacity was completely lost and the TiNi based secondary phase disappeared. It has been concluded that the TiNi based secondary phase would help A h kg--1 the electrochemical reaction on the grain of the
207
(ffri, v ) based main phase as a catalyst and/or a current collector.
References [ 1] T. Schoberand H. Wenzl, in G. Alefeldand J. V61kl (eds,), Hydrogen in Metals II, Topics in Applied Physics, Vol. 29, Springer-Verlag,Berlin, 1978, p. 11. [2] K. Fujita, Y.C. Huang and M. Tada, Nippon Kinzoku Gakkaishi, 43 (1979) 601. [3] J.J. Reilly and R.H. Wiswall, lnorg. Chem., 9 (1970) 1678. [4] J.E. Kleiner, E.H. Sevillaand R.M. Cotts,Phys. Rev. B, 33 (1986) 6662. [5] A.J. Maeland, G.G. Libowitz,J.F. Lynch and G. Rak, .L Less-Common Met., 104 (1984) 133. [6] G.G. Libowitz and A.J. Maeland, Mater. Sci. Forum, 31 (1988) 177. [7] M. Tsukahara, K. Takahashi, T. Mishima, T. Sakai, H. Miyamura, N. Kuriyamaand I. Uehara, J. Alloys Comp., submitted for publication. [8] T. Sakal, H. Yoshinaga,H. Miyamura,N. Kuriyama,A. Kato, K. Oguro and H. Ishikawa,J. Alloys Comp., 180 (1992) 37.