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electron mixed conductors based on polymer electrolytes
Ekmchimico km. Vol. 37. No. 9. pp. 1521~1523, 1992 tinted in Great Britain.
0013-4686/92 S5.M)+ 0.00
Q 1992. Pergmon Press Ltd.
ION/ELECTRON MIXED CONDUCTORS POLYMER ELECTROLYTES
BASED ON
MASAY~SHI WATANABE,* HIDEAKI NAGASAKA,* KOHEI SANUI,* NAOYA OGATA* ROYCE W. MunuYt
and
*Department of Chemistry, Sophia University, Chiyoda-ku, Tokyo 102, Japan and TDepartment of Chemistry, The University of North Carolina, Chapel Hill, NC 27599-3290, U.S.A. (Received 20 December 1991) Ahatrae-This paper demonstrates that ion/electron mixed conductors (redox conductors) based on polymer electrolytes can be obtained by incorporating redox molecules into ion-conducting polymer phases. The incorporation has been attained by simple dissolution of mdox molecules into polymer electrolytes and by copolymerization of a redox monomer with an ion-conducting monomer. The former examples are the poly(ethylene oxide) (PEO) networks in which LiTCNQ and LiClO, are dissolved, and the latter examples are the copolymers of vinylferrocene and methoxy-nona(ethylene oxide)methacrylate in which LiClO, is dissolved. Oxidation of TCNQ- in the PEO networks occurs via both of physical diffusion of TCNQ- toward the electrode and the electron self exchange between TCNQ- and TCNQO in the diffusion layer. The electron diffusivity at 42°C is 10-9-IO-scm* s-l, linearly increases with
increasing LiTCNQ concentration, and surpasses the physical diffusivity at [LiTCNQ] = 0.05 M. Reversible redox reaction of ferrocene sites occurs in the copolymers where the ferrocene sites am covalently fixed to the polymer backbone and can not largely diffuse. The redox reaction is caused by the electron hopping (redox conduction). The electron diffusivity at 40°C is estimated at ca 10-10cm2 s-r. Key words: polymer electrolyte, ion/electron mixed conductor, redox conduction, electron self exchange, electron hopping, solid state voltammetry, TCNQ, ferrocene.
ping in the copolymers where the redox active sites are fixed to the polymer backbone by covalent bond.
INTRODUCTION
Combined use of polymer electrolytes and ultramicroelectrodes has made it possible to conduct quantitative electrochemical measurements (solid state voltammetry) of redox active molecules dissolved in or attached to polymer electrolytes[l-51. Electrochemical reactions of the redox molecules in the bulk polymeric phase occur via the diffusion of the redox molecules to electrodes, but electron self exchange reaction or electron hopping between redox centers sometimes plays an important role for the charge transport[6]. Electroactive polymers having the latter charge transport process is called “ion/electron mixed conductor” or “redox conductor”. In this study, it is shown that solid state redox conduction in polymer electrolytes is observed in two systems: one is polymer electrolyte in which redox molecule, having high electron self exchange rate constant, is dissolved at a high concentration. Lithium salt of 7,7,8,8-tetracyanoquinodimethane radical anion (LiTCNQ) is selected as a molecular solute and dissolved in network poly(ethylene oxide) (PEO) electrolytes containing LiClO.,, and the electron hopping rate between TCNQ-lo couple has been studied as a function of concentrations of LiTCNQ and LiClO,, and temperature[q. The other system is copolymer p(VFc/MEOg)] consisting of redox active monomer, vinylferrocene (VFc), and ion conductive monomer, methoxy-nona(ethylene oxide)-methacrylate (MEO,), in which LiClO, is dissolved. It is demonstrated that reversible and diffusion-controlled electrochemical reaction occurs via the electron hop-
VFc-MEO, Copolymer
EXPERIMENTAL
Electrochemical measurements (solid state voltammetry) were made by using a three electrode microcell and a highly sensitive potentiostat in a Faraday cage. The three electrode microcell consists of tips of three electrodes; Pt microelectrode (10 or 25 pm diameter), and Pt counter and Ag reference electrodes, exposing in an insulating plane[3]. The surface of the microcell is polished by using diamond and alumina pastes. Network polymer electrolytes were prepared directly onto the microcell surface by cross-linking reaction of PEO trio1 (mol. wt = 3000) with toluene2Jdiisocyanate in the presence of LiClO, and LiTCNQ, for the solid state voltammetry[3,8,9]. P(VFC/MEO~)S, having several compositions, were prepared by radical copolymerization of VFc and ME4 (unpublished observations). The solutions of the copolymers obtained, containing LiClO,, were cast on the surface of the microcells, followed by evaporation of the solvent, for the measurements.
1521
M. WATANABE et al.
1522 CLiTCNQl:
.m 50mM
I/
I
OO
0.05
I
I
I
I
0.15 0.10 CLi TCNQI I M
I
I
0.20
Fig. 3. Diffusion coefficients as a function of [LiTCNQJ in network PEO/LiCIO, electrolyte- (Li/O = l/50) at 42°C: 0, D aPP(TCNQ-'"couple); 0, Dpg (TCNQ-flcouple); 0, D,, (electron dt usivity). I
I
-1.0
-0.5
0 E I V vs Ag
I
0.5
,
1.0
Fig. 1. Solid state cyclic voltammograms (20 mV s-r) at 25pm dia. Pt disk electrode for LiTCNQ dissolved in
network PEO/LiC104 electrolyte (Li/O = l/50) at 42°C. Diffusion coefficient of redox molecules in polymer electrolytes and that for electron between mixedvalent redox centers are determined by using potential step chronoamperometry. The methodological detail is described elsewhere[3,4].
RESULTS
AND DISCUSSION
In the solid state cyclic voltammetry of LiTCNQ in network PEO electrolyte, two reversible waves, corresponding to TCNQ-lo and TCNQ-‘2- reactions, are observed, as shown in Figure 1. Although the molecule diffusing in the polymeric phase, toward oxidizing and reducing microdisk electrodes, is the same TCNQ- species, the oxidation wave, the TCNQ-/’ couple, gives a larger current than does the reduction wave, the TCNQ-12- couple, and the difference increases with increasing the concentraton of LiTCNQ. Since the electron self exchange rate constant for the TCNQ-‘O couple is very large (k,, > lo9 M-l s-l in acetonitrile[lO]) and that for the TCNQ-/2- couple is much smaller, the difference in the voltammetric current has been explained by the contribution of the electron self exchange reaction to the TCNQ-lo reaction[7J. In other words, transport of TCNQ- to the electrode for the TCNQ-‘O reaction is brought about (a)
by both diffusion (Fig. 2a) and electron hopping between mixed-valent redox centers in the diffusion layer (Fig. 2b), while that for the TCNQ-12- is brought about by only diffusion. This coupling of physical diffusion and electron self exchange reaction in the diffusion layer has been interpreted by Ruff [ll, 121: D npp = Dphys + Dcke= Dphp+ (1/6)k,d2c,
where Dsppis experimental diffusivity, D,, is physical diffusivity, D,, is electron diffusivity, d is inter-site distance at electron transfer, k,, is the electron self exchange rate constant, and C is the solute concentraton. When the diffusion rate for the TCNQ-@ wave is taken as D,, and that for the TCNQ-12- wave is taken as Dphyl,their difference (electron diffusivity) is 10-9-10-8 cm2 s-r at 42”C, and linearly increases up to [LiTCNQ] = 0.1 M (solubility limit of LiTCNQ in n
(b)
t 0
I
0.5
1.0
EfVvsAg
Fig. 2. Charge transport mechanisms for redox reaction: (a) physical diffusion process; (b) electron self exchange (electron hopping) process.
Fig. 4. Solid state cyclic voltammograms (IOmVs-‘) at 10 pm dia. Pt disk electrode for P(VFc/MEO,) containing LiClO, (Li/O = l/30) at 40°C. VFc mol/%: (a) 79 mol%; (b) 48 mol%; (c) 32 mol%.
Ion/electron mixed conductors
the network PEO), as shown in Fig. 3. This is consistent with Ruffs equation. It should be noted that the electron diffusivity surpasses the physical diffusivity at [LiTCNQ] = 0.05 M. It is shown here that solid state redox conductors based on polymer electrolytes can be obtained by dissolving redox molecules having high electron exchange rate constants at high concentrations. Reversible redox reaction in the bulk polymeric phase is also observed for ferrocene sites in P(VFc/MEO,), as shown in Fig. 4. In order to illustrate the redox reaction in the copolymers, the range of reacted ferrocene sites during the redox process is calculated here. From the area under oxidation wave (O-O.8 V vs. Ag), oxidized ferrocene sites during the oxidation process are estimated as 8.7 x lo-” mol for P(VFc/MEO,) (VFc = 48 mol%, Fig. 4b). On the contrary, the maximum amount of ferrocene covered on the electrode surface (10 pm diameter disk) in monolayer level is 1.0 x lo-l5 mol, assuming that ferrocene is a sphere of 4 8, diameter. Consequently, it is revealed that more than 800 layers of monolayer ferrocene are oxidized during the oxidation process. Ferrocene sites are covalently fixed to the polymer backbone in the copolymers and their large displacement is unlikely. Thus, the above fact that the amount of electrochemically reacted ferrocene sites during the reaction is far larger than the possible amount of ferrocene sites on the electrode surface, strongly suggests that the redox reaction in P(VFc/MEO,) is brought about by the
1523
electron hopping mechanism (Fig. lb). Electron diffusivity at 40°C in the copolymer is estimated at ca lo-‘Ocm* s-1.
REFERENCES 1. L. Geng, M. L. Longmire, R. A. Reed, M.-H. Kim, T. T. Wooster, B. N. Oliver, J. Egekeze, R. T. Kennedy, J. W. Jorgenson, J. F. Parcher and R. W. Murray, J. Am. Chem. Sot. 111. 1619 (1989). 2. M. Watanabe, H. Shibuya, K.‘Sanui and N. Ogata, in Second International Sykposium on Polymer ,%ectrolvtes (Edited bv B. Scrosati).I, .DD. . 165-174. Elsevier. London (1990).3. M. Watanabe, M. L. Longmire and R. W. Murray, J. Phys. Chem. 94, 2614 (1990). 4. M. L. Longmire, M. Watanabe, H. Zhang, T. T. Wooster and R. W. Murray, Anal. Chem. 62, 747 (1990). 5. M. J. Pinkerton, Y. Le Mest, H. Zhang, M. Watanabe and R. W. Murray, J. Am. Chem. Sot. 112,373O (1990). 6. N. A. Surridge, J. C. Jernigan, E. F. Dalton, B. P. Buck, M. Watanabe, H. Zhang, M. Pinkerton, T. T. Wooster, M. L. Longmire, J. S. Facci and R. W. Murray, Faraday Discuss. Chem. Sot. 88, 1 (1989). 7. M. Watanabe, T. T. Wooster and R. W. Murray, J. Phys. Gem. 95, 4573 (1991). 8. M. Watanabe, S. Nagano, K. Sanui and N. Ogata, Polym. J. 18, 809 (1986). 9. M. Watanabe, M. Itoh, K. Sanui and N. Ogata, Macromolecules 20, 569 (1987). 10. W. Harrer. G. Gramoo and W. Jaenicke. Chem. Phvs. . Lett. 112, 263 (1984): 11. I. Ruff and V. J. Friedrich, J. Chem. Phys. 75, 3297 (1971). 12. L. Botar and I. Ruff, Chem. Phys. L&t. 149,99 (1988).
0013-4686/92 S5.M)+ 0.00
Q 1992. Pergmon Press Ltd.
ION/ELECTRON MIXED CONDUCTORS POLYMER ELECTROLYTES
BASED ON
MASAY~SHI WATANABE,* HIDEAKI NAGASAKA,* KOHEI SANUI,* NAOYA OGATA* ROYCE W. MunuYt
and
*Department of Chemistry, Sophia University, Chiyoda-ku, Tokyo 102, Japan and TDepartment of Chemistry, The University of North Carolina, Chapel Hill, NC 27599-3290, U.S.A. (Received 20 December 1991) Ahatrae-This paper demonstrates that ion/electron mixed conductors (redox conductors) based on polymer electrolytes can be obtained by incorporating redox molecules into ion-conducting polymer phases. The incorporation has been attained by simple dissolution of mdox molecules into polymer electrolytes and by copolymerization of a redox monomer with an ion-conducting monomer. The former examples are the poly(ethylene oxide) (PEO) networks in which LiTCNQ and LiClO, are dissolved, and the latter examples are the copolymers of vinylferrocene and methoxy-nona(ethylene oxide)methacrylate in which LiClO, is dissolved. Oxidation of TCNQ- in the PEO networks occurs via both of physical diffusion of TCNQ- toward the electrode and the electron self exchange between TCNQ- and TCNQO in the diffusion layer. The electron diffusivity at 42°C is 10-9-IO-scm* s-l, linearly increases with
increasing LiTCNQ concentration, and surpasses the physical diffusivity at [LiTCNQ] = 0.05 M. Reversible redox reaction of ferrocene sites occurs in the copolymers where the ferrocene sites am covalently fixed to the polymer backbone and can not largely diffuse. The redox reaction is caused by the electron hopping (redox conduction). The electron diffusivity at 40°C is estimated at ca 10-10cm2 s-r. Key words: polymer electrolyte, ion/electron mixed conductor, redox conduction, electron self exchange, electron hopping, solid state voltammetry, TCNQ, ferrocene.
ping in the copolymers where the redox active sites are fixed to the polymer backbone by covalent bond.
INTRODUCTION
Combined use of polymer electrolytes and ultramicroelectrodes has made it possible to conduct quantitative electrochemical measurements (solid state voltammetry) of redox active molecules dissolved in or attached to polymer electrolytes[l-51. Electrochemical reactions of the redox molecules in the bulk polymeric phase occur via the diffusion of the redox molecules to electrodes, but electron self exchange reaction or electron hopping between redox centers sometimes plays an important role for the charge transport[6]. Electroactive polymers having the latter charge transport process is called “ion/electron mixed conductor” or “redox conductor”. In this study, it is shown that solid state redox conduction in polymer electrolytes is observed in two systems: one is polymer electrolyte in which redox molecule, having high electron self exchange rate constant, is dissolved at a high concentration. Lithium salt of 7,7,8,8-tetracyanoquinodimethane radical anion (LiTCNQ) is selected as a molecular solute and dissolved in network poly(ethylene oxide) (PEO) electrolytes containing LiClO.,, and the electron hopping rate between TCNQ-lo couple has been studied as a function of concentrations of LiTCNQ and LiClO,, and temperature[q. The other system is copolymer p(VFc/MEOg)] consisting of redox active monomer, vinylferrocene (VFc), and ion conductive monomer, methoxy-nona(ethylene oxide)-methacrylate (MEO,), in which LiClO, is dissolved. It is demonstrated that reversible and diffusion-controlled electrochemical reaction occurs via the electron hop-
VFc-MEO, Copolymer
EXPERIMENTAL
Electrochemical measurements (solid state voltammetry) were made by using a three electrode microcell and a highly sensitive potentiostat in a Faraday cage. The three electrode microcell consists of tips of three electrodes; Pt microelectrode (10 or 25 pm diameter), and Pt counter and Ag reference electrodes, exposing in an insulating plane[3]. The surface of the microcell is polished by using diamond and alumina pastes. Network polymer electrolytes were prepared directly onto the microcell surface by cross-linking reaction of PEO trio1 (mol. wt = 3000) with toluene2Jdiisocyanate in the presence of LiClO, and LiTCNQ, for the solid state voltammetry[3,8,9]. P(VFC/MEO~)S, having several compositions, were prepared by radical copolymerization of VFc and ME4 (unpublished observations). The solutions of the copolymers obtained, containing LiClO,, were cast on the surface of the microcells, followed by evaporation of the solvent, for the measurements.
1521
M. WATANABE et al.
1522 CLiTCNQl:
.m 50mM
I/
I
OO
0.05
I
I
I
I
0.15 0.10 CLi TCNQI I M
I
I
0.20
Fig. 3. Diffusion coefficients as a function of [LiTCNQJ in network PEO/LiCIO, electrolyte- (Li/O = l/50) at 42°C: 0, D aPP(TCNQ-'"couple); 0, Dpg (TCNQ-flcouple); 0, D,, (electron dt usivity). I
I
-1.0
-0.5
0 E I V vs Ag
I
0.5
,
1.0
Fig. 1. Solid state cyclic voltammograms (20 mV s-r) at 25pm dia. Pt disk electrode for LiTCNQ dissolved in
network PEO/LiC104 electrolyte (Li/O = l/50) at 42°C. Diffusion coefficient of redox molecules in polymer electrolytes and that for electron between mixedvalent redox centers are determined by using potential step chronoamperometry. The methodological detail is described elsewhere[3,4].
RESULTS
AND DISCUSSION
In the solid state cyclic voltammetry of LiTCNQ in network PEO electrolyte, two reversible waves, corresponding to TCNQ-lo and TCNQ-‘2- reactions, are observed, as shown in Figure 1. Although the molecule diffusing in the polymeric phase, toward oxidizing and reducing microdisk electrodes, is the same TCNQ- species, the oxidation wave, the TCNQ-/’ couple, gives a larger current than does the reduction wave, the TCNQ-12- couple, and the difference increases with increasing the concentraton of LiTCNQ. Since the electron self exchange rate constant for the TCNQ-‘O couple is very large (k,, > lo9 M-l s-l in acetonitrile[lO]) and that for the TCNQ-/2- couple is much smaller, the difference in the voltammetric current has been explained by the contribution of the electron self exchange reaction to the TCNQ-lo reaction[7J. In other words, transport of TCNQ- to the electrode for the TCNQ-‘O reaction is brought about (a)
by both diffusion (Fig. 2a) and electron hopping between mixed-valent redox centers in the diffusion layer (Fig. 2b), while that for the TCNQ-12- is brought about by only diffusion. This coupling of physical diffusion and electron self exchange reaction in the diffusion layer has been interpreted by Ruff [ll, 121: D npp = Dphys + Dcke= Dphp+ (1/6)k,d2c,
where Dsppis experimental diffusivity, D,, is physical diffusivity, D,, is electron diffusivity, d is inter-site distance at electron transfer, k,, is the electron self exchange rate constant, and C is the solute concentraton. When the diffusion rate for the TCNQ-@ wave is taken as D,, and that for the TCNQ-12- wave is taken as Dphyl,their difference (electron diffusivity) is 10-9-10-8 cm2 s-r at 42”C, and linearly increases up to [LiTCNQ] = 0.1 M (solubility limit of LiTCNQ in n
(b)
t 0
I
0.5
1.0
EfVvsAg
Fig. 2. Charge transport mechanisms for redox reaction: (a) physical diffusion process; (b) electron self exchange (electron hopping) process.
Fig. 4. Solid state cyclic voltammograms (IOmVs-‘) at 10 pm dia. Pt disk electrode for P(VFc/MEO,) containing LiClO, (Li/O = l/30) at 40°C. VFc mol/%: (a) 79 mol%; (b) 48 mol%; (c) 32 mol%.
Ion/electron mixed conductors
the network PEO), as shown in Fig. 3. This is consistent with Ruffs equation. It should be noted that the electron diffusivity surpasses the physical diffusivity at [LiTCNQ] = 0.05 M. It is shown here that solid state redox conductors based on polymer electrolytes can be obtained by dissolving redox molecules having high electron exchange rate constants at high concentrations. Reversible redox reaction in the bulk polymeric phase is also observed for ferrocene sites in P(VFc/MEO,), as shown in Fig. 4. In order to illustrate the redox reaction in the copolymers, the range of reacted ferrocene sites during the redox process is calculated here. From the area under oxidation wave (O-O.8 V vs. Ag), oxidized ferrocene sites during the oxidation process are estimated as 8.7 x lo-” mol for P(VFc/MEO,) (VFc = 48 mol%, Fig. 4b). On the contrary, the maximum amount of ferrocene covered on the electrode surface (10 pm diameter disk) in monolayer level is 1.0 x lo-l5 mol, assuming that ferrocene is a sphere of 4 8, diameter. Consequently, it is revealed that more than 800 layers of monolayer ferrocene are oxidized during the oxidation process. Ferrocene sites are covalently fixed to the polymer backbone in the copolymers and their large displacement is unlikely. Thus, the above fact that the amount of electrochemically reacted ferrocene sites during the reaction is far larger than the possible amount of ferrocene sites on the electrode surface, strongly suggests that the redox reaction in P(VFc/MEO,) is brought about by the
1523
electron hopping mechanism (Fig. lb). Electron diffusivity at 40°C in the copolymer is estimated at ca lo-‘Ocm* s-1.
REFERENCES 1. L. Geng, M. L. Longmire, R. A. Reed, M.-H. Kim, T. T. Wooster, B. N. Oliver, J. Egekeze, R. T. Kennedy, J. W. Jorgenson, J. F. Parcher and R. W. Murray, J. Am. Chem. Sot. 111. 1619 (1989). 2. M. Watanabe, H. Shibuya, K.‘Sanui and N. Ogata, in Second International Sykposium on Polymer ,%ectrolvtes (Edited bv B. Scrosati).I, .DD. . 165-174. Elsevier. London (1990).3. M. Watanabe, M. L. Longmire and R. W. Murray, J. Phys. Chem. 94, 2614 (1990). 4. M. L. Longmire, M. Watanabe, H. Zhang, T. T. Wooster and R. W. Murray, Anal. Chem. 62, 747 (1990). 5. M. J. Pinkerton, Y. Le Mest, H. Zhang, M. Watanabe and R. W. Murray, J. Am. Chem. Sot. 112,373O (1990). 6. N. A. Surridge, J. C. Jernigan, E. F. Dalton, B. P. Buck, M. Watanabe, H. Zhang, M. Pinkerton, T. T. Wooster, M. L. Longmire, J. S. Facci and R. W. Murray, Faraday Discuss. Chem. Sot. 88, 1 (1989). 7. M. Watanabe, T. T. Wooster and R. W. Murray, J. Phys. Gem. 95, 4573 (1991). 8. M. Watanabe, S. Nagano, K. Sanui and N. Ogata, Polym. J. 18, 809 (1986). 9. M. Watanabe, M. Itoh, K. Sanui and N. Ogata, Macromolecules 20, 569 (1987). 10. W. Harrer. G. Gramoo and W. Jaenicke. Chem. Phvs. . Lett. 112, 263 (1984): 11. I. Ruff and V. J. Friedrich, J. Chem. Phys. 75, 3297 (1971). 12. L. Botar and I. Ruff, Chem. Phys. L&t. 149,99 (1988).