- Email: [email protected]
Charge-transport rates in polyelectrolyte films on electrodes
339
J. Electroanal. Chew., 241(1988) 339-343 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note CHARGETRANSPORT ON ELECTRODES
RATES IN POLYELECTROLYTE
DISPARATE DIFFUSION COEFFICIENTS WITH TWO SEPARATE ELECTROACTIVE
TAKE0
FILMS
FOR ALIZARIN RED S CENTERS
OHSAKA and NOBORU OYAMA *
Department of Applied Chemistry for Resources, Tokyo University of Agnculture Tokyo 184 (Japan) YOUKO TAKAHIRA Department
and Technology, Koganei,
and SADAKO NAKAMURA
of Chemistq
Japan Women’s University, Bunkyo-ku,
Tokyo I12 (Japan)
(Received 6th April 1988)
INTRODUCTION
Recently, the charge-transport processes through electroactive, thin polymer films on electrodes have drawn considerable attention from the viewpoint of fundamental studies on the mechanism and kinetics of charge transport as well as possible applications based on their electron-transfer catalysis (and mediation), electrochromic properties, photovoltaic properties, etc. [l]. In the case of the electroactive reactants incorporated in solvent-swollen polymer films, which are not attached to the polymer chains permanently, charge (electron or ion) transport through the films is generally believed to occur via electron hopping (electron exchange) between the reactants and/or physical diffusion (molecular motion) of the reactants themselves (which are temporarily confined in polymer domains) [2-41. Both processes are followed by a charge-compensating motion of the counter-ion. Motion of the solvent and segmental motion of the polymer chain are also involved in the overall charge-transport process. However, it is not easy to elucidate which process controls the charge-transport rate in a given system [2-41. During the course of our continuing study concerning the above-mentioned subjects [4], we found that for an electroactive alizarin red S dye (1,2-dihydroxyanthracene-9,10-dione-3-sulfonate, 1) with two separate redox centers (i.e., l,Zdihydroxy and 9,10-dione groups) incorporated electrostatically into a cationic perfluo* To whom correspondence should be addressed. OC22-0728/88/$03.50
0 1988 EIsevier Sequoia S.A.
340 0
OH
0 alizarin red
S
I
tCF2-$Ffg-------fCF2-CF2jii? CF2 F3C-
dF-0-CF2-CH2-N-
n/m= 6.5
CH3 ‘t
20 pA
CH3 CI-
dH3
CPFP
-0.5
I 0.5
0 E/V
vs.
I 1.0
SSCE
Fig. 1. Structures of alizarin red S (1) and cationic perfluoro polymer (CPFP). Fig. 2. A typical cyclic voltammogram for 1 confined in the CPFP fii in 0.2 M CF,COONa solution (pH 1.0) containing 0.1 mM of 1. Electrode substrate: IT0 (0.25 cm2 s-l). CPFP film thickness: 1.0 pm. Concentration of 1 in the CPFP fiim: 0.46 M. Scan rate: 50 mV s-l.
ropolymer (CPFP) film on electrodes (Fig. l), the charge transport based on molecular motion and electron exchange can be differentiated depending on which redox center is employed in the electrochemical measurement of charge-transport rates. A similar idea was applied recently by Tsou and Anson to the heterobinuclear metal complexes within Nafion@ coatings on electrodes [2]. In this paper, we report on the preliminary results concerning the charge transport through l-incorporating CPFP films undergoing oxidation or reduction of two separate redox centers of 1. EXPERIMENTAL
The preparation of CPFP film-coated In-Sn oxide Matsuzaki Shinku Co., 10 Q/O) electrodes, the electrostatic the CPFP film-coated electrodes, and the electrochemical ried out as described previously [5]. The experiments were atmosphere at 25 o C.
conducting glass (ITO, incorporation of 1 into measurements were carconducted in a nitrogen
RESULTS AND DISCUSSION
Figure 2 shows a typical cyclic voltammogram for 1 confined in the CPFP film on an IT0 electrode in 0.2 M CF,COONa solution (pH 1.0) containing 0.1 mM of 1. In this case, the concentration (cfilm) of 1 incorporated in the CPFP film was 0.46 M, and it was about three orders of magnitude larger than the concentration (0.1 mM) of 1 in the bathing solution in which the CPFP film-coated electrode was soaked, indicating that 1 can be concentrated into the CPFP film by an electrostatic
341
interaction between the sulfonyl group of 1 and the quatemized ammonium site of the CPFP film. The reversible oxidation-reduction response at ca. -0.25 V vs. a sodium chloride saturated calomel electrode (SSCE) corresponds to the two-electron reduction of 1 to 1,2-dihydroxyanthracene-9,10-diol-3-sulfonate, 2, and the oxidation of 2 to 1 (process I), while the irreversible response (anodic peak), at ca. 0.8 V vs. SSCE, corresponds to the oxidation of 1 to 1,2-dioxoanthracene-9,10-dione-3sulfonate, 3 (process II) [6]. These reactions are 2 H+, 2 e- reactions. The mechanism of these electrode reactions can be represented by the following scheme:
2
3
I
From the potential scan rate (u) dependence of the peak currents (i,), processes I and II were found to be diffusion-controlled. Here, potential-step chronoamperometry and chronocoulometry were employed to estimate the charge-transport rates for these processes as apparent diffusion coefficients (Dapps). Potential-step chronoamperometric and chronocoulometric experiments were carried out in 0.2 it4 CF,COONa solution (PH 1.0) in the presence of 0.1 mM of 1 (in order to hold crilm constant) within the experimental time scale (typically 2-40 ms) satisfying the conditions that the diffusion layer thickness +z the film thickness (1.0 pm). In Fig. 3 a typical example of potential-step chronocoulometric Cottrell plots is shown for the oxidations of 2 to 1 and 1 to 3 and the reduction of 1 to 2. These plots are linear, in agreement with the result of the dependence of the i,s on U. The point to be noted in Fig. 3 is that the slopes for the 1 to 2 and 2 to 1 processes are almost the same, but they are significantly larger than that for the 1 to 3 process. From these slopes, __
25 20uJ_ '5>
lo50 0
! t III 12
,.p _.*;.* .*$' . .:* . .NC .. . /" .* :. ..** I .a* 1. . . III 3 4 5 t"2/ (d"2
6
Fig. 3. Typical Cottrell plots for the oxidations of (A) 2 to 1 and (C) 1 to 3 and (B) the reduction of 1 to 2. Electrode potentials were stepped (A) from - 0.6 to 0.2, (B) from 0.2 to - 0.6, and (C) from 0.2 to 1.3 V vs. SSCE. Other experimental conditions are the same as those in Fig. 2.
342
1 to 3 processes (abbreviated as Dapp(I+ 2), Dapp(2 + 1) and D&l --) 3), respectively) were estimated to be (1.5 + 0.2) x lo-“, (1.6 f 0.1) x lo-” and (6.1 f 0.1) X 10-l’ cm2 s-l, respectively. The same results were also obtained chronoamperometrically. It is necessary for an understanding of the charge-transport mechanism to compare the Dapp values with the diffusion coefficients (0,‘) corresponding to the motion of supporting electrolyte ions (i.e., CF$OO- and Na+) which is necessarily (for charge neutrality) coupled to electron transfer. The 0,’ values can be estimated approximately from the diffusion coefficient (KD,; K is the distribution coefficient of an ion between solution and film) for film permeation of a dissolved redox species (e.g., Co(tpy)‘,+, tpy: 2,2’,2”-terpyridine) which is not confined in the polymer matrix. Co(tpy);+ is electroactive in the potential region where substantially no redox response of 1 is observed. The formal redox potential (E O’) is 0.035 V vs. SSCE in 0.2 M CF,COONa solution (pH 1.0). Therefore, using the l-incorporating CPFP film-coated electrode KD, could be estimated to be (2.6 + 0.2) X lo-* cm2 s-l under the same experimental conditions as those used in the estimation of the Dap,s. The KD, value was determined from the Koutecky-Levich plot of the steady-state current-potential curves obtained for the oxidation of Co(tpy)“,’ at a rotating-disk basal-plane pyrolytic graphite electrode coated with the CPFP film incorporating 0.46 M of 1 in 0.2 M CFsCOONa solution (pH 1.0) containing 0.1 mM of 1 and 0.5 mM Co(tpy)‘,‘. On the basis of the charge and dimensions of Co(tpy)‘,’ and Na+ (and CF,COO-), the 0,’ values seem larger than the KD, value for Co(tpy)‘,+. Thus, the 0,’ values are considered to be more than two orders of magnitude larger than the Dapp values, suggesting that the rate of the charge transport in the film is not controlled by motion of the counterion in the polymer matrix, which is coupled to electron transfer and molecular motion of the reactants undergoing electrolysis. The above-mentioned results lead to the following order of the diffusion coefficients for each charge-transport process: 0,’ B Dapp(l --3r2) - Dapp(2 --, 1) > Dapp(l --, 3). The Dapp value may include contributions from both molecular motion (physical diffusion of reactants in the film) and electron exchange between the reactants which will be represented as Do and D,, in units of cm2 s-l, respectively. In process I (specificlly the reduction process) and process II, the contribution (Do) from molecular motion should be the same within experimental error, because the redox species taking part in both processes is the same one (i.e., 1). Thus the difference in Dapps may originate from that in D,:s. The degree of the contribution from electron exchange to the overall charge-transport rate depends upon the electron self-exchange rate between electroactive reactants, i.e., the homogeneous electron self-exchange rate constant (k,,): The larger k,, is, the larger is D,,, and thus the larger is the contribution of electron exchange [2,4]. Alternatively, k,, can be correlated to the heterogeneous electron-transfer rate constant (k o ) [7], i.e., in a qualitative sense to the reversibility of the electrode reaction: For a redox system with large k,,, the k o value is large. As can readily be seen from Fig. 2, process I is reversible, while process II is irreversible. Thus, the k o value for process I should be
the Dappvalues for the 1 to 2, 2 to 1 and
343
larger than that for process II. In conclusion, the difference in Dap,s for processes I and II may be considered to be due to different contributions of the electron exchange between electroactive centers to the overall charge-transport rates. Further study concerning the charge-transport kinetics of the present dye-containing polymer film system is in progress and the details will be reported in the near future [S]. ACKNOWLEDGEMENTS
The present work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of “Dynamic Interactions and Electronic Processes of Macromolecular Complexes” No. 62612504, for N. Oyama, from the Ministry of Education, Science and Culture, Japan. REFERENCES 1 See, for example, R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 191; A.R. Hillman in R.G. Linford (Ed.), Electrochemical Science and Technology, Vol. 1, Elsevier Applied Science, Barking, 1987, p. 103. 2 Y.M. Tsou and F.C. Anson, J. Phys. Chem., 89 (1985) 3818 and references cited therein. 3 K.M. O’Connell, E. Waldner, L. Roullier and E. Laviron, J. Electroanal. Chem., 162 (1984) 77. 4 N. Oyama, T. Ohsaka, Y. Yamamoto and M. Kaneko, J. Phys. Chem., 90 (1986) 3850; K. Chiba, T. Ohsaka and N. Oyama, J. Electroanal. Chem., 217 (1987) 239; T. Oh&a, H. Yamamoto and N. Oyama, J. Phys. Chem., 91(1987) 3775; T. Ohsaka, T. Okajima and N. Oyama, J. Electroanal. Chem., 215 (1986) 191; N. Oyama, T. Ohsaka, M. Kaneko, K. Sato and H. Matsuda, J. Am. Chem. Sot., 105 (1983) 6003. 5 N. Oyama, T. Ohsaka and T. Okajima, Anal. Chem., 58 (1986) 979. 6 H.E. Zittel and T.M. Florence, Anal. Chem., 39 (1967) 320. 7 R.A. Marcus, J&ctrochim. Acta, 13 (1968) 995. 8 T. Oh&a, Y. Takahira, 0. Hatozaki and N. Oyama, J. Phys. Chem., to be submitted.
J. Electroanal. Chew., 241(1988) 339-343 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note CHARGETRANSPORT ON ELECTRODES
RATES IN POLYELECTROLYTE
DISPARATE DIFFUSION COEFFICIENTS WITH TWO SEPARATE ELECTROACTIVE
TAKE0
FILMS
FOR ALIZARIN RED S CENTERS
OHSAKA and NOBORU OYAMA *
Department of Applied Chemistry for Resources, Tokyo University of Agnculture Tokyo 184 (Japan) YOUKO TAKAHIRA Department
and Technology, Koganei,
and SADAKO NAKAMURA
of Chemistq
Japan Women’s University, Bunkyo-ku,
Tokyo I12 (Japan)
(Received 6th April 1988)
INTRODUCTION
Recently, the charge-transport processes through electroactive, thin polymer films on electrodes have drawn considerable attention from the viewpoint of fundamental studies on the mechanism and kinetics of charge transport as well as possible applications based on their electron-transfer catalysis (and mediation), electrochromic properties, photovoltaic properties, etc. [l]. In the case of the electroactive reactants incorporated in solvent-swollen polymer films, which are not attached to the polymer chains permanently, charge (electron or ion) transport through the films is generally believed to occur via electron hopping (electron exchange) between the reactants and/or physical diffusion (molecular motion) of the reactants themselves (which are temporarily confined in polymer domains) [2-41. Both processes are followed by a charge-compensating motion of the counter-ion. Motion of the solvent and segmental motion of the polymer chain are also involved in the overall charge-transport process. However, it is not easy to elucidate which process controls the charge-transport rate in a given system [2-41. During the course of our continuing study concerning the above-mentioned subjects [4], we found that for an electroactive alizarin red S dye (1,2-dihydroxyanthracene-9,10-dione-3-sulfonate, 1) with two separate redox centers (i.e., l,Zdihydroxy and 9,10-dione groups) incorporated electrostatically into a cationic perfluo* To whom correspondence should be addressed. OC22-0728/88/$03.50
0 1988 EIsevier Sequoia S.A.
340 0
OH
0 alizarin red
S
I
tCF2-$Ffg-------fCF2-CF2jii? CF2 F3C-
dF-0-CF2-CH2-N-
n/m= 6.5
CH3 ‘t
20 pA
CH3 CI-
dH3
CPFP
-0.5
I 0.5
0 E/V
vs.
I 1.0
SSCE
Fig. 1. Structures of alizarin red S (1) and cationic perfluoro polymer (CPFP). Fig. 2. A typical cyclic voltammogram for 1 confined in the CPFP fii in 0.2 M CF,COONa solution (pH 1.0) containing 0.1 mM of 1. Electrode substrate: IT0 (0.25 cm2 s-l). CPFP film thickness: 1.0 pm. Concentration of 1 in the CPFP fiim: 0.46 M. Scan rate: 50 mV s-l.
ropolymer (CPFP) film on electrodes (Fig. l), the charge transport based on molecular motion and electron exchange can be differentiated depending on which redox center is employed in the electrochemical measurement of charge-transport rates. A similar idea was applied recently by Tsou and Anson to the heterobinuclear metal complexes within Nafion@ coatings on electrodes [2]. In this paper, we report on the preliminary results concerning the charge transport through l-incorporating CPFP films undergoing oxidation or reduction of two separate redox centers of 1. EXPERIMENTAL
The preparation of CPFP film-coated In-Sn oxide Matsuzaki Shinku Co., 10 Q/O) electrodes, the electrostatic the CPFP film-coated electrodes, and the electrochemical ried out as described previously [5]. The experiments were atmosphere at 25 o C.
conducting glass (ITO, incorporation of 1 into measurements were carconducted in a nitrogen
RESULTS AND DISCUSSION
Figure 2 shows a typical cyclic voltammogram for 1 confined in the CPFP film on an IT0 electrode in 0.2 M CF,COONa solution (pH 1.0) containing 0.1 mM of 1. In this case, the concentration (cfilm) of 1 incorporated in the CPFP film was 0.46 M, and it was about three orders of magnitude larger than the concentration (0.1 mM) of 1 in the bathing solution in which the CPFP film-coated electrode was soaked, indicating that 1 can be concentrated into the CPFP film by an electrostatic
341
interaction between the sulfonyl group of 1 and the quatemized ammonium site of the CPFP film. The reversible oxidation-reduction response at ca. -0.25 V vs. a sodium chloride saturated calomel electrode (SSCE) corresponds to the two-electron reduction of 1 to 1,2-dihydroxyanthracene-9,10-diol-3-sulfonate, 2, and the oxidation of 2 to 1 (process I), while the irreversible response (anodic peak), at ca. 0.8 V vs. SSCE, corresponds to the oxidation of 1 to 1,2-dioxoanthracene-9,10-dione-3sulfonate, 3 (process II) [6]. These reactions are 2 H+, 2 e- reactions. The mechanism of these electrode reactions can be represented by the following scheme:
2
3
I
From the potential scan rate (u) dependence of the peak currents (i,), processes I and II were found to be diffusion-controlled. Here, potential-step chronoamperometry and chronocoulometry were employed to estimate the charge-transport rates for these processes as apparent diffusion coefficients (Dapps). Potential-step chronoamperometric and chronocoulometric experiments were carried out in 0.2 it4 CF,COONa solution (PH 1.0) in the presence of 0.1 mM of 1 (in order to hold crilm constant) within the experimental time scale (typically 2-40 ms) satisfying the conditions that the diffusion layer thickness +z the film thickness (1.0 pm). In Fig. 3 a typical example of potential-step chronocoulometric Cottrell plots is shown for the oxidations of 2 to 1 and 1 to 3 and the reduction of 1 to 2. These plots are linear, in agreement with the result of the dependence of the i,s on U. The point to be noted in Fig. 3 is that the slopes for the 1 to 2 and 2 to 1 processes are almost the same, but they are significantly larger than that for the 1 to 3 process. From these slopes, __
25 20uJ_ '5>
lo50 0
! t III 12
,.p _.*;.* .*$' . .:* . .NC .. . /" .* :. ..** I .a* 1. . . III 3 4 5 t"2/ (d"2
6
Fig. 3. Typical Cottrell plots for the oxidations of (A) 2 to 1 and (C) 1 to 3 and (B) the reduction of 1 to 2. Electrode potentials were stepped (A) from - 0.6 to 0.2, (B) from 0.2 to - 0.6, and (C) from 0.2 to 1.3 V vs. SSCE. Other experimental conditions are the same as those in Fig. 2.
342
1 to 3 processes (abbreviated as Dapp(I+ 2), Dapp(2 + 1) and D&l --) 3), respectively) were estimated to be (1.5 + 0.2) x lo-“, (1.6 f 0.1) x lo-” and (6.1 f 0.1) X 10-l’ cm2 s-l, respectively. The same results were also obtained chronoamperometrically. It is necessary for an understanding of the charge-transport mechanism to compare the Dapp values with the diffusion coefficients (0,‘) corresponding to the motion of supporting electrolyte ions (i.e., CF$OO- and Na+) which is necessarily (for charge neutrality) coupled to electron transfer. The 0,’ values can be estimated approximately from the diffusion coefficient (KD,; K is the distribution coefficient of an ion between solution and film) for film permeation of a dissolved redox species (e.g., Co(tpy)‘,+, tpy: 2,2’,2”-terpyridine) which is not confined in the polymer matrix. Co(tpy);+ is electroactive in the potential region where substantially no redox response of 1 is observed. The formal redox potential (E O’) is 0.035 V vs. SSCE in 0.2 M CF,COONa solution (pH 1.0). Therefore, using the l-incorporating CPFP film-coated electrode KD, could be estimated to be (2.6 + 0.2) X lo-* cm2 s-l under the same experimental conditions as those used in the estimation of the Dap,s. The KD, value was determined from the Koutecky-Levich plot of the steady-state current-potential curves obtained for the oxidation of Co(tpy)“,’ at a rotating-disk basal-plane pyrolytic graphite electrode coated with the CPFP film incorporating 0.46 M of 1 in 0.2 M CFsCOONa solution (pH 1.0) containing 0.1 mM of 1 and 0.5 mM Co(tpy)‘,‘. On the basis of the charge and dimensions of Co(tpy)‘,’ and Na+ (and CF,COO-), the 0,’ values seem larger than the KD, value for Co(tpy)‘,+. Thus, the 0,’ values are considered to be more than two orders of magnitude larger than the Dapp values, suggesting that the rate of the charge transport in the film is not controlled by motion of the counterion in the polymer matrix, which is coupled to electron transfer and molecular motion of the reactants undergoing electrolysis. The above-mentioned results lead to the following order of the diffusion coefficients for each charge-transport process: 0,’ B Dapp(l --3r2) - Dapp(2 --, 1) > Dapp(l --, 3). The Dapp value may include contributions from both molecular motion (physical diffusion of reactants in the film) and electron exchange between the reactants which will be represented as Do and D,, in units of cm2 s-l, respectively. In process I (specificlly the reduction process) and process II, the contribution (Do) from molecular motion should be the same within experimental error, because the redox species taking part in both processes is the same one (i.e., 1). Thus the difference in Dapps may originate from that in D,:s. The degree of the contribution from electron exchange to the overall charge-transport rate depends upon the electron self-exchange rate between electroactive reactants, i.e., the homogeneous electron self-exchange rate constant (k,,): The larger k,, is, the larger is D,,, and thus the larger is the contribution of electron exchange [2,4]. Alternatively, k,, can be correlated to the heterogeneous electron-transfer rate constant (k o ) [7], i.e., in a qualitative sense to the reversibility of the electrode reaction: For a redox system with large k,,, the k o value is large. As can readily be seen from Fig. 2, process I is reversible, while process II is irreversible. Thus, the k o value for process I should be
the Dappvalues for the 1 to 2, 2 to 1 and
343
larger than that for process II. In conclusion, the difference in Dap,s for processes I and II may be considered to be due to different contributions of the electron exchange between electroactive centers to the overall charge-transport rates. Further study concerning the charge-transport kinetics of the present dye-containing polymer film system is in progress and the details will be reported in the near future [S]. ACKNOWLEDGEMENTS
The present work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of “Dynamic Interactions and Electronic Processes of Macromolecular Complexes” No. 62612504, for N. Oyama, from the Ministry of Education, Science and Culture, Japan. REFERENCES 1 See, for example, R.W. Murray in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 13, Marcel Dekker, New York, 1984, p. 191; A.R. Hillman in R.G. Linford (Ed.), Electrochemical Science and Technology, Vol. 1, Elsevier Applied Science, Barking, 1987, p. 103. 2 Y.M. Tsou and F.C. Anson, J. Phys. Chem., 89 (1985) 3818 and references cited therein. 3 K.M. O’Connell, E. Waldner, L. Roullier and E. Laviron, J. Electroanal. Chem., 162 (1984) 77. 4 N. Oyama, T. Ohsaka, Y. Yamamoto and M. Kaneko, J. Phys. Chem., 90 (1986) 3850; K. Chiba, T. Ohsaka and N. Oyama, J. Electroanal. Chem., 217 (1987) 239; T. Oh&a, H. Yamamoto and N. Oyama, J. Phys. Chem., 91(1987) 3775; T. Ohsaka, T. Okajima and N. Oyama, J. Electroanal. Chem., 215 (1986) 191; N. Oyama, T. Ohsaka, M. Kaneko, K. Sato and H. Matsuda, J. Am. Chem. Sot., 105 (1983) 6003. 5 N. Oyama, T. Ohsaka and T. Okajima, Anal. Chem., 58 (1986) 979. 6 H.E. Zittel and T.M. Florence, Anal. Chem., 39 (1967) 320. 7 R.A. Marcus, J&ctrochim. Acta, 13 (1968) 995. 8 T. Oh&a, Y. Takahira, 0. Hatozaki and N. Oyama, J. Phys. Chem., to be submitted.