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A secondary battery composed of multilayer Prussian Blue and its reaction characteristics
45
J. Electroanal. Chem., 255 (1988) 45-52 Elsevier Sequoia S.A., Lausamte - Printed in The Netherlands
A secondary battery composed of multilayer Prussian Blue and its reaction characteristics Masao Kaneko and Tokuo Okada The Institute of Physical and Chemical Research, Wako, Saitama, 351-01 (Japan) (Received 4 May 1988; in revised form 4 July 1988)
ABSTRACT A secondary battery composed of multilayer Prussian Blue (PB) was fabricated. A trilayer system composed of PB/Nafion/PB showed reversible charge/discharge characteristics, and worked as a secondary battery. Two types of cells were fabricated; one contained sandwiched PB layers and the other parallel PB layers, the latter of which has been used to study spectral changes of the cathode and the anode. Redox reactions accompanying the charging and discharging have been studied electrochemically as well as spectrophotometrically.
INTRODUCTION
Polynuclear complexes such as Prussian Blue (PB) whose composition is expressed as Fe~+[Fe”(CN)& are attracting attention as electrode-coating materials for electrochromic displays [l-4a], electrocatalysis [4b], photosensing devices [5], etc. Since PB is a Fe3+/Fe2+ mixed-valent complex and can be both reduced and oxidized reversibly, it can store both negative and positive charges, which allows its use as active material for both the anode and the cathode of a secondary battery. Secondary batteries have been fabricated from a pair of PB-coated electrodes [6,7] or from a Nafion film on which PB was deposited chemically on both the surfaces [8]. In these works the redox reactions of PB have been studied only electrochemically, no spectrophotometrical studies having been made. The present authors have adopted a simple fabrication method for a semi-dry multilayer type battery by sandwiching a Nafion film between two membranes of PB. A cell was fabricated by putting two PB membranes in parallel on a Nafion film, enabling us to follow the spectral changes of each PB membrane accompanying charge/discharge cycles. The redox reactions of this secondary battery following charge/discharge cycles were studied electrochemically and spectrophotometrically, and are reported here. 0022-0728/88/$03.50
6 1988 Elsevier Sequoia S.A.
46 EXPERIMENTAL
PB membrane
PB was coated on an IT0 glass by electrodeposition [l-3] from an aqueous mixture of 10 m M K,Fe(CN), + 10 m M FeCl, and 10 mM HCl under galvanostatic conditions of 0.2 mA cme2 cathodic current, with charges of 10 or 20 mC cmp2 in total being passed. A potentio-galvanostat (model HA-301) and a function generator (HB-104) from Hokuto Denko Co. Ltd. were used for the electrodeposition and electrochemical measurements. PB secondary batteries
Two types of PB secondary batteries were fabricated. Type A is composed of two serial PB membranes which are sandwiched between IT0 glasses with a Nafion@ film separator between them (Fig. 1). The area of one PB membrane was 5 X 4 = 20 mm2. The Nafion film was purchased from Aldrich Chemical Co., Inc. (Nafion 117); its thickness was 180 pm. The Nafion film was used after adsorbing an aqueous solution of 1 M KC1 in order to maintain electrolytes required for the battery reactions. The type B battery was fabricated in order to measure its visible spectrum with charge/discharge cycles. It is composed of two parallel PB membranes placed on a Nafion film (Fig. 2). The area of one PB membrane was 5x7=35mm2. RESULTS
AND DISCUSSION
The composition of PB has been studied electrochemically [l] and, from the composition of PB crystals [9], reported as Fe~+[Fe”(CN)& [1,9] where the ratio
Naf ion nsulat
ing
epoxy
Fig. 1. Sandwich type PB secondary battery (type A).
ating
epoxy
Fig. 2. PB secondary battery (type B) for spectral studies.
l’,,‘I”‘,“‘,,‘,“‘1
-0.5
0
0.5 EAT (vs.
1
1.5
Ag-AgCl)
Fig. 3. Cyclic voltammogram of a PB membrane coated on a IT0 glass dipped in an aqueous mixture of 0.1 M KNO, and 0.01 M HNO,. Scan rate, 20 mV s-‘.
a)
^. o.8 c,;arge
_. . ;iscna;pe
~
~
c
3.0
2.0
1.0 Charge/&
Charqe
.
:4J
I 0
I
I
I
1.0
I
2.0
I
I
3.0
Charge/W Fig. 4. Charge/discharge behavior.
characteristics of a PB secondary battery. (a) Normal behavior, (b) degraded
48
L
I
-1.4
I
-1.0
1
-0.5
,
0
I
I
0.5
1.0
Potential/V
4
1.4
Fig. 5. Cyclic scanning of potential difference between the electrodes of a PB battery.
of Fe/CN is 7/18. A composition containing iron and cyanide ions in the ratio of 1 to 3 (Fe/CN) has also been reported [3]. The cyclic voltammogram of a PB membrane coated on an IT0 as used in the present work is shown in Fig. 3. It shows a pattern typical of PB [l-3], with two reversible redox couples at 0.20 V and 0.89 V vs. Ag/AgCl. The reduced state (Fe*+‘*+ complex) is called Prussian White (PW), and the oxidized form (Fe 3+‘3+ state) Berlin Green (BG) or Berlin Brown. The charge/discharge characteristics of the type A battery are shown in Fig. 4a. Although this new type of battery still has a problem with its long-term stability, it showed stable charge/discharge characteristics for at least 100 cycles. The charge/discharge characteristics are sometimes not as good as those shown in Fig.
o 400
500 Wavelength
600
700
800
/nm
Fig. 6. Visible absorption spectra of PB coated on an IT0 1.1 V (BG,- - -), and -0.1 V (PW; . . . . .). (PB, -),
glass at applied potentials of 0.5 V,
49
b O.D.
400
0.1
600
800
400
600
800
Wavelengthlnm Fig. 7. Spectral change of PB accompanying charging (battery (B) at negative (a) and positive (b) electrodes.
-
-)
and discharging
( -)
of the PB
4b, where tails of the charge/discharge curves are observed. In order to study chemical reactions occurring on the charge/discharge processes, the following experiments were done. When the potential difference between the two electrodes of the battery (either A or B) is scanned between - 1.4 V and 1.4 V as a cyclic voltammogram, the potential-current curves show a pattern as shown in Fig. 5. The coupled redox peaks centered at the potential differences of 0.7 V and -0.7 V correspond to the reactions of BG/PB and PB/PW. During repeated scanning, new redox peaks appear centered at 0 V which increase with the number of scans, while the redox peaks at 0.7 V decrease. It would be rational in discussing battery reactions to consider that these redox peaks at around 0 V are the reason for the tails in the bad charge/discharge characteristics shown in Fig. 4b. Redox reactions occurring on the charge/discharge cycles have been studied as follows. Visible spectra of PB, BG, and PW films coated on an IT0 electrode were measured at applied potentials of 0.5 V, 1.1 V, and -0.1 V vs. Ag/AgCl, respectively, and are shown in Fig. 6. PB has an absorption maximum at about 690 nm, BG shows an absorption at around 420 rnn, and PW has almost no absorption in the visible region. Battery B showed almost the same charge/discharge characteristics and cyclic voltammetric behavior as battery A (see Figs. 4 and 5, respectively). The spectral changes of battery B accompanying charge/discharge are shown in Fig. 7. At the positive electrode, the film becomes BG after charging, and turns back to PB after discharging. At the negative electrode, the film becomes PW after charging, returning to PB after discharging.
50
Electrode a
b
-1.4
0
0
1.4
0
Potential/V Fig. 8. Absorbance change (690 nm) of the PB battery (B) induced by cyclic scanning of the potential difference between the electrodes with electrode a polarized first positively and then negatively. ( -) First scan; (- - -) second scan; (. . . . . .) third scan. Amount of PB coated, a/b = 2/l.
I
Electrode a
b
I
0
I
0.5
Potential/V
1.0
I 1.4
1
I
I
0
1.4
0
4 Potential/V
Fig. 9. Recovery of cell characteristics by repeated scanning between 0 and 1.4 V with electrode a (excess PB) polarized only positively.
Fig. 10. Recovery of cell characteristics, represented by the absorbance change at 690 nm, upon repeated scanning of the potential difference as described in Fig. 9. () First scan; (. . . . -) last scan.
c I
0
51
I
I
4
0.5
1.0
1.4
Potential/V Fig. 11. Stable redox behavior upon repeated scanning of the potential difference with electrode a (excess PB) polarized positively.
The changes in the absorbance at A,, (690 nm) accompanying cyclic scarming of the potential difference of battery B are shown in Fig. 8. In this battery the amount of PB coated at electrodes a and b is 2/l for a/b. The absorbance decrease represents BG formation at the positive electrode, and PW formation at the negative one. In the first cycle of 0 --, 1.4 --, 0 + - 1.4 + 0 V, formation of BG and PW occurs at electrodes a and b, respectively, as expected. In the second and third cycles, however, BG formation at electrode a at positive potential is suppressed. When the potential difference is swept from 1.4 to 0 V and further negative in the second and the third cycles, reduction of PB to PW and oxidation of PW to PB takes place near 0 V. It is evident that this redox couple, PB + PW and PW + PB, is responsible for the redox waves observed at 0 V in Fig. 5. The increase of the redox peaks near 0 V (Fig. 5) parallels the increase of the tails in Fig. 4b. The redox waves that appear at 0 V disappear when, in scanning the potential difference of the battery between 0 and 1.4 V, the electrode with excess PB (here electrode a) is kept in the positive potential region. The disappearance of the redox waves at 0 V and the simultaneous recovery of the redox couples (PB/BG and PB/PW) at 0.7 V are shown in Fig. 9. The absorbance changes at 690 nm accompanying these cyclic scans are shown in Fig. 10. In the first cycle, the PB/PW coupled reaction is predominant. Along with the repeated scans between 0 and 1.4 V, the couple PB/BG and PB/PW recovers and becomes predominant. Concomitantly with this recovery of the redox characteristics, the charge/discharge characteristics of Fig. 4b are restored to those of Fig. 4a. When the electrode coated with excess PB is always taken as the positive one, the battery shows stable cyclic characteristics, as shown in Fig. 11, and its charge/discharge characteristics are those of Fig. 4a. The behavior described above could be ascribed to the relatively unfavorable reduction of PB to PW as well as to the oxidation of PB to BG, represented by eqns. (1) and (2), respectively, when the composition of Fe/CN = 7/18 is assumed. Fe~+[Fe”(CN),]~-(PB)
+ 4 IS++ 4 e- + 4 K+Fe~+[Fe”(CN),]~-(PW)
Fe~+(Fe”(CN),]~-(PB)
+ 3 X-+
Fe~+[Fe”‘(CN),]~-
.3 X-(BG)
(1) + 3 e-
(2)
52
These are unfavorable reactions because cations or anions must enter the PB lattice from the outer solution for the reduction or oxidation to occur. The reactions must then be slower for a thicker PB film than for a thinner one. When the potential difference is scanned cyclically both to positive (1.4 V) and negative (- 1.4 V) values on a thicker PB film, the rather unfavorable reaction series of PB + PW + PB --, BG hinders the formation of BG at positive potentials (see Fig. 8, electrode a) in comparison with the smooth redox reactions on a thinner PB film (electrode b) in repeated cyclic scanning, thus causing the PB/PW redox couple to appear near 0 V. However, when electrode a (excess PB) is kept in the positive potential region, the side reaction of PW formation at 0 V is suppressed, and the PB/BG couple increases again at 1.4 V on repeated scanning (Figs. 9 and 10). The short circuit current of the present battery was of the order of 0.1 mA cme2. In conclusion, PB batteries have been manufactured, and the redox reactions of PB in these batteries have been elucidated electrochemically and spectrophotometritally. REFERENCES 1 2 3 4 5 6 7 8 9
K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res., 19 (1986) 162. V.D. Neff, J. Electrochem. Sot., 125 (1978) 886. R.J. Mortimer and D.R. Ross&sky, J. Electroanal. Chem., 151 (1983) 133. (a) K. Itaya and A.J. Bard, Anal. Chem., 50 (1978) 1487; (b) K. Itaya, N. Shoji and I. Uchida, J. Am. Chem. Sot., 106 (1984) 3423. M. Kaneko, S. Hara and A. Yamada, J. Electroanal. Chem., 194 (1985) 165. V.D. Neff, J. Ekxtrochem. Sot., 132 (1985) 1382. M. Kaneko, J. Polym. Sci., Polym. Lett. Ed., 24 (1986) 435. K. Honda, J. Dchiai and H. Hayasbi, J. Chem. Sot., Chem. Commun., 1986 (1986) 168. H.J. Busser, D. Schwatzenbach, W. Petter and A. Ludi, Inorg. Chem., 16 (1977) 2704.
J. Electroanal. Chem., 255 (1988) 45-52 Elsevier Sequoia S.A., Lausamte - Printed in The Netherlands
A secondary battery composed of multilayer Prussian Blue and its reaction characteristics Masao Kaneko and Tokuo Okada The Institute of Physical and Chemical Research, Wako, Saitama, 351-01 (Japan) (Received 4 May 1988; in revised form 4 July 1988)
ABSTRACT A secondary battery composed of multilayer Prussian Blue (PB) was fabricated. A trilayer system composed of PB/Nafion/PB showed reversible charge/discharge characteristics, and worked as a secondary battery. Two types of cells were fabricated; one contained sandwiched PB layers and the other parallel PB layers, the latter of which has been used to study spectral changes of the cathode and the anode. Redox reactions accompanying the charging and discharging have been studied electrochemically as well as spectrophotometrically.
INTRODUCTION
Polynuclear complexes such as Prussian Blue (PB) whose composition is expressed as Fe~+[Fe”(CN)& are attracting attention as electrode-coating materials for electrochromic displays [l-4a], electrocatalysis [4b], photosensing devices [5], etc. Since PB is a Fe3+/Fe2+ mixed-valent complex and can be both reduced and oxidized reversibly, it can store both negative and positive charges, which allows its use as active material for both the anode and the cathode of a secondary battery. Secondary batteries have been fabricated from a pair of PB-coated electrodes [6,7] or from a Nafion film on which PB was deposited chemically on both the surfaces [8]. In these works the redox reactions of PB have been studied only electrochemically, no spectrophotometrical studies having been made. The present authors have adopted a simple fabrication method for a semi-dry multilayer type battery by sandwiching a Nafion film between two membranes of PB. A cell was fabricated by putting two PB membranes in parallel on a Nafion film, enabling us to follow the spectral changes of each PB membrane accompanying charge/discharge cycles. The redox reactions of this secondary battery following charge/discharge cycles were studied electrochemically and spectrophotometrically, and are reported here. 0022-0728/88/$03.50
6 1988 Elsevier Sequoia S.A.
46 EXPERIMENTAL
PB membrane
PB was coated on an IT0 glass by electrodeposition [l-3] from an aqueous mixture of 10 m M K,Fe(CN), + 10 m M FeCl, and 10 mM HCl under galvanostatic conditions of 0.2 mA cme2 cathodic current, with charges of 10 or 20 mC cmp2 in total being passed. A potentio-galvanostat (model HA-301) and a function generator (HB-104) from Hokuto Denko Co. Ltd. were used for the electrodeposition and electrochemical measurements. PB secondary batteries
Two types of PB secondary batteries were fabricated. Type A is composed of two serial PB membranes which are sandwiched between IT0 glasses with a Nafion@ film separator between them (Fig. 1). The area of one PB membrane was 5 X 4 = 20 mm2. The Nafion film was purchased from Aldrich Chemical Co., Inc. (Nafion 117); its thickness was 180 pm. The Nafion film was used after adsorbing an aqueous solution of 1 M KC1 in order to maintain electrolytes required for the battery reactions. The type B battery was fabricated in order to measure its visible spectrum with charge/discharge cycles. It is composed of two parallel PB membranes placed on a Nafion film (Fig. 2). The area of one PB membrane was 5x7=35mm2. RESULTS
AND DISCUSSION
The composition of PB has been studied electrochemically [l] and, from the composition of PB crystals [9], reported as Fe~+[Fe”(CN)& [1,9] where the ratio
Naf ion nsulat
ing
epoxy
Fig. 1. Sandwich type PB secondary battery (type A).
ating
epoxy
Fig. 2. PB secondary battery (type B) for spectral studies.
l’,,‘I”‘,“‘,,‘,“‘1
-0.5
0
0.5 EAT (vs.
1
1.5
Ag-AgCl)
Fig. 3. Cyclic voltammogram of a PB membrane coated on a IT0 glass dipped in an aqueous mixture of 0.1 M KNO, and 0.01 M HNO,. Scan rate, 20 mV s-‘.
a)
^. o.8 c,;arge
_. . ;iscna;pe
~
~
c
3.0
2.0
1.0 Charge/&
Charqe
.
:4J
I 0
I
I
I
1.0
I
2.0
I
I
3.0
Charge/W Fig. 4. Charge/discharge behavior.
characteristics of a PB secondary battery. (a) Normal behavior, (b) degraded
48
L
I
-1.4
I
-1.0
1
-0.5
,
0
I
I
0.5
1.0
Potential/V
4
1.4
Fig. 5. Cyclic scanning of potential difference between the electrodes of a PB battery.
of Fe/CN is 7/18. A composition containing iron and cyanide ions in the ratio of 1 to 3 (Fe/CN) has also been reported [3]. The cyclic voltammogram of a PB membrane coated on an IT0 as used in the present work is shown in Fig. 3. It shows a pattern typical of PB [l-3], with two reversible redox couples at 0.20 V and 0.89 V vs. Ag/AgCl. The reduced state (Fe*+‘*+ complex) is called Prussian White (PW), and the oxidized form (Fe 3+‘3+ state) Berlin Green (BG) or Berlin Brown. The charge/discharge characteristics of the type A battery are shown in Fig. 4a. Although this new type of battery still has a problem with its long-term stability, it showed stable charge/discharge characteristics for at least 100 cycles. The charge/discharge characteristics are sometimes not as good as those shown in Fig.
o 400
500 Wavelength
600
700
800
/nm
Fig. 6. Visible absorption spectra of PB coated on an IT0 1.1 V (BG,- - -), and -0.1 V (PW; . . . . .). (PB, -),
glass at applied potentials of 0.5 V,
49
b O.D.
400
0.1
600
800
400
600
800
Wavelengthlnm Fig. 7. Spectral change of PB accompanying charging (battery (B) at negative (a) and positive (b) electrodes.
-
-)
and discharging
( -)
of the PB
4b, where tails of the charge/discharge curves are observed. In order to study chemical reactions occurring on the charge/discharge processes, the following experiments were done. When the potential difference between the two electrodes of the battery (either A or B) is scanned between - 1.4 V and 1.4 V as a cyclic voltammogram, the potential-current curves show a pattern as shown in Fig. 5. The coupled redox peaks centered at the potential differences of 0.7 V and -0.7 V correspond to the reactions of BG/PB and PB/PW. During repeated scanning, new redox peaks appear centered at 0 V which increase with the number of scans, while the redox peaks at 0.7 V decrease. It would be rational in discussing battery reactions to consider that these redox peaks at around 0 V are the reason for the tails in the bad charge/discharge characteristics shown in Fig. 4b. Redox reactions occurring on the charge/discharge cycles have been studied as follows. Visible spectra of PB, BG, and PW films coated on an IT0 electrode were measured at applied potentials of 0.5 V, 1.1 V, and -0.1 V vs. Ag/AgCl, respectively, and are shown in Fig. 6. PB has an absorption maximum at about 690 nm, BG shows an absorption at around 420 rnn, and PW has almost no absorption in the visible region. Battery B showed almost the same charge/discharge characteristics and cyclic voltammetric behavior as battery A (see Figs. 4 and 5, respectively). The spectral changes of battery B accompanying charge/discharge are shown in Fig. 7. At the positive electrode, the film becomes BG after charging, and turns back to PB after discharging. At the negative electrode, the film becomes PW after charging, returning to PB after discharging.
50
Electrode a
b
-1.4
0
0
1.4
0
Potential/V Fig. 8. Absorbance change (690 nm) of the PB battery (B) induced by cyclic scanning of the potential difference between the electrodes with electrode a polarized first positively and then negatively. ( -) First scan; (- - -) second scan; (. . . . . .) third scan. Amount of PB coated, a/b = 2/l.
I
Electrode a
b
I
0
I
0.5
Potential/V
1.0
I 1.4
1
I
I
0
1.4
0
4 Potential/V
Fig. 9. Recovery of cell characteristics by repeated scanning between 0 and 1.4 V with electrode a (excess PB) polarized only positively.
Fig. 10. Recovery of cell characteristics, represented by the absorbance change at 690 nm, upon repeated scanning of the potential difference as described in Fig. 9. () First scan; (. . . . -) last scan.
c I
0
51
I
I
4
0.5
1.0
1.4
Potential/V Fig. 11. Stable redox behavior upon repeated scanning of the potential difference with electrode a (excess PB) polarized positively.
The changes in the absorbance at A,, (690 nm) accompanying cyclic scarming of the potential difference of battery B are shown in Fig. 8. In this battery the amount of PB coated at electrodes a and b is 2/l for a/b. The absorbance decrease represents BG formation at the positive electrode, and PW formation at the negative one. In the first cycle of 0 --, 1.4 --, 0 + - 1.4 + 0 V, formation of BG and PW occurs at electrodes a and b, respectively, as expected. In the second and third cycles, however, BG formation at electrode a at positive potential is suppressed. When the potential difference is swept from 1.4 to 0 V and further negative in the second and the third cycles, reduction of PB to PW and oxidation of PW to PB takes place near 0 V. It is evident that this redox couple, PB + PW and PW + PB, is responsible for the redox waves observed at 0 V in Fig. 5. The increase of the redox peaks near 0 V (Fig. 5) parallels the increase of the tails in Fig. 4b. The redox waves that appear at 0 V disappear when, in scanning the potential difference of the battery between 0 and 1.4 V, the electrode with excess PB (here electrode a) is kept in the positive potential region. The disappearance of the redox waves at 0 V and the simultaneous recovery of the redox couples (PB/BG and PB/PW) at 0.7 V are shown in Fig. 9. The absorbance changes at 690 nm accompanying these cyclic scans are shown in Fig. 10. In the first cycle, the PB/PW coupled reaction is predominant. Along with the repeated scans between 0 and 1.4 V, the couple PB/BG and PB/PW recovers and becomes predominant. Concomitantly with this recovery of the redox characteristics, the charge/discharge characteristics of Fig. 4b are restored to those of Fig. 4a. When the electrode coated with excess PB is always taken as the positive one, the battery shows stable cyclic characteristics, as shown in Fig. 11, and its charge/discharge characteristics are those of Fig. 4a. The behavior described above could be ascribed to the relatively unfavorable reduction of PB to PW as well as to the oxidation of PB to BG, represented by eqns. (1) and (2), respectively, when the composition of Fe/CN = 7/18 is assumed. Fe~+[Fe”(CN),]~-(PB)
+ 4 IS++ 4 e- + 4 K+Fe~+[Fe”(CN),]~-(PW)
Fe~+(Fe”(CN),]~-(PB)
+ 3 X-+
Fe~+[Fe”‘(CN),]~-
.3 X-(BG)
(1) + 3 e-
(2)
52
These are unfavorable reactions because cations or anions must enter the PB lattice from the outer solution for the reduction or oxidation to occur. The reactions must then be slower for a thicker PB film than for a thinner one. When the potential difference is scanned cyclically both to positive (1.4 V) and negative (- 1.4 V) values on a thicker PB film, the rather unfavorable reaction series of PB + PW + PB --, BG hinders the formation of BG at positive potentials (see Fig. 8, electrode a) in comparison with the smooth redox reactions on a thinner PB film (electrode b) in repeated cyclic scanning, thus causing the PB/PW redox couple to appear near 0 V. However, when electrode a (excess PB) is kept in the positive potential region, the side reaction of PW formation at 0 V is suppressed, and the PB/BG couple increases again at 1.4 V on repeated scanning (Figs. 9 and 10). The short circuit current of the present battery was of the order of 0.1 mA cme2. In conclusion, PB batteries have been manufactured, and the redox reactions of PB in these batteries have been elucidated electrochemically and spectrophotometritally. REFERENCES 1 2 3 4 5 6 7 8 9
K. Itaya, I. Uchida and V.D. Neff, Act. Chem. Res., 19 (1986) 162. V.D. Neff, J. Electrochem. Sot., 125 (1978) 886. R.J. Mortimer and D.R. Ross&sky, J. Electroanal. Chem., 151 (1983) 133. (a) K. Itaya and A.J. Bard, Anal. Chem., 50 (1978) 1487; (b) K. Itaya, N. Shoji and I. Uchida, J. Am. Chem. Sot., 106 (1984) 3423. M. Kaneko, S. Hara and A. Yamada, J. Electroanal. Chem., 194 (1985) 165. V.D. Neff, J. Ekxtrochem. Sot., 132 (1985) 1382. M. Kaneko, J. Polym. Sci., Polym. Lett. Ed., 24 (1986) 435. K. Honda, J. Dchiai and H. Hayasbi, J. Chem. Sot., Chem. Commun., 1986 (1986) 168. H.J. Busser, D. Schwatzenbach, W. Petter and A. Ludi, Inorg. Chem., 16 (1977) 2704.