4192
J. Phys. Chem. 1993,97, 4192-4195
Dynamics of Faradaic Processes in Polypyrrole/PolystyrenesulfonateComposite Films in the Presence and Absence of a Redox Species in Aqueous Solutions T. Amemiya, K. Hashimoto, and A. Fujishima' Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo- ku, Tokyo 1 1 3, Japan Received: January 12, 1993
Electrochemical impedance and color impedance (electromodulated optical transmittance) methods were used to separate two processes: a faradaic process in polypyrrole films and a redox process of [Fe(CN)613-/" dissolved in a solution. The modulated transmittance ( T = AT/AE) a t 700 nm followed the modulation of faradaic charge only in the films. Analysis of the capacitance (C = AQ/AE) and the modulated transmittance data using an equivalent circuit has revealed that the faradaic process in the films and the redox reaction of the redox species occur in parallel. The latter reaction is found to occur a t the film/solution interface.
Introduction The redox kinetics of conducting polymers such as polypyrrole,' polythiophene,2 and polyaniline3 or redox polymers4 have been investigated by an ac impedance technique. This technique has also been applied to the kinetic study of a redox species such as [Fe(CN)6]3-/4-or Fe2+/3+in a solution at modified electrode^.^ These impedance data are mixtures of nonfaradaic charging of the electrical double layer and faradaic reactions due to both the films themselves and the redox species in the solution. Each process cannot be monitored separately by the conventional impedance method. An optical measurement has recently been combined with the ac impedance method6 in order to better understand the faradaic contributions to the kinetics of conducting polymers or inorganic electrochromic films. The above optical method is called color impedance s p e c t r o ~ c o p yhere ~ ~ ~from the analogy to ordinary electrochemical impedance methods. Using the color impedance method, the previous paper8 has described in detail the faradaic processes in PPy/Cl- films as a function of dc potentials in the absence of a redox species in a solution. In this paper, color impedance and electrochemical impedance measurements have been carried out on two model systems; one consists of a PPy/PSS- (PSS- = polystyrenesulfonate) composite film in an aqueous solution of 1 M KC1, the other consists of the film in the same solution but also containing 10 mM K4[Fe(CN)6] as a redox species. The electrical response will follow the overall electrochemical process in the systems, while the modulated transmittance (ATIAE) at 700 nm is expected to follow only the faradaic process in the films. Taking into account both the faradaic process in the films and the redox reaction of the redox species, we propose a charge transport model for the system which is an extension of the previous one.8
Experimental Section Chemicals. Pyrrole, sodium polystyrenesulfonate (NaPSS), KCl, and K4[Fe(CN),] were purchased from Tokyo Kasei Co. andusedasreceived. Solutions were prepared with distilled water. Electrochemical Cells and Polypyrrole Film Deposition. The electrochemical cell and electrodes used in this study were the same as those reported previously.8 Polypyrrole and PSScomposite (PPy/PSS-) films were grown, at a constant current of 1.3 mA/cm2 for 60 s on indium-doped tin oxide coated (ITO) glasses (area, 0.78 cm2), from an aqueous solution containing 0.1 M ( M = mol/dm3) pyrrole and 0.2 M NaPSS. The potential of the working electrode was ca. 0.63 V vs saturated calomel electrode (SCE) during the polymerization. The thickness of the films
was nearly the same as that (0.26 pm) of the PPy/Cl- composite films.8 All potentials are reported relative to the SCE. Electrochemistryand ElectromodulationSpectroscopies. After the polymerization, the cell and the PPy/PSS- films were washed with distilled water. Cyclic voltammetry and electromodulation spectroscopies for the films were performed in either 1 M KCl or 1 M KCl 10 mM &[Fe(CN)6] aqueous solutions. The apparatus for these measurements has been described previously.8 Electrochemical impedance and color impedance spectra for the films were taken after the first two cyclic voltammetry measurements between 0.2 and -0.7 V. The redox couple, [Fe(CN)6]4-/3-,in the solution, does not absorb the monochromatic light of 700 nm used to monitor the transmittance of the PPy/PSSfilms.
+
Results Cyclic Voltammetry. Figure 1 shows typical cyclic voltammograms of PPy/PSS- films on the second cycle in 1 M KCl and 1 M KC1 + 10 mM &[Fe(CN),] aqueous solutions. Multicycle voltammograms were qualitatively identical to the second voltammograms. In the presence of &[Fe(CN),], one pair of welldefined current peaks was observed around 0.25 V. These peaks correspond to the redox reaction of the [Fe(CN)6]3-/". The large cathodic current over the potential range from 0 to -0.4 V was observed only when the potential was swept up to 0.5 V. This cathodic current is probably due to excessive oxidation of the PPy films. When the potential sweep was limited to between 0.2 and -0.7 V, the voltammograms were nearly the same between those in the presence (- - -) and the absence (- - -) of the redox species. The electromodulation measurements were carried out a t 0.2, 0, and -0.15 V as shown in Figure 1. ElectromodulationSpectroscopiesfor PPy/PSS- Films without the Redox Speciesin Solution. Figure 2 shows the complex plane plots of capacitance (C) and modulated transmittance ( T ) for the PPy/PSS- film at 0.2 V in 1 M KCl aqueous solution. The complex impedance ( Z ) plots for the film are also shown in the inset. Good coincidence between the Cand the Tplots indicates that the charging4ischarging process is coupled to the faradaic process in the PPy/PSS- films. The C, Z , and T plots were simulated using an equivalent circuit as shown in Figure 3. This circuit takes into account both a fast and a slow faradaic process in the film and is similar to that for the PPy/CI- film reported previously.8 The difference between the equivalent circuits for the PPy/Cl- film and for the PPy/PSS- film is that the fast faradaic process is represented by a finite length transmission linegfor thePPy/Cl-film but bya purecapacitorforthePPy/PSS-
0022-3654/93/2097-4 192%04.00/0 0 1993 American Chemical Society
Polypyrrole/PolystyrenesulfonateComposite Films
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4193
1
1 + + 1
I
I
I
-0.9
-0.6
-0.3
1
Cd40,RCt=O
I +
0 E / V vs. SCE
I
I
0.3
0.6
Figure 1. Cyclic voltammograms of PPy/PSS- films, polymerized (Q = 77 mC/cm2) on I T 0 coated glasses, in aqueous solutions containing 1 M KCI (- - -), 1 M + 10 mM &[Fe(CN)6] cycled between 0.2 and -0.7 V (- - -), and 1 M KCI + 10 mM K4[Fe(CN)6] cycled between 0.5 V and -0.7 V (-). Electrode area was 0.78 cm2, and scan speed was 50 mV/s. 100
0.5012 HZ
-
Figure 3. Equivalent circuit for the PPy/PSS- film in the absence of a d and R,, are the double layer capacitance redox species in solution. c and the charge-transfer resistance at the ITO/ film interface, respectively. These two values are negligible in the present case. Rn is the uncompensated resistance of solution plus ITO. Z,,, is the faradaic impedance of the film and consists of a fast and a slow process.
30 1.995 Hz
0
0
50
1.995Hz
100
Z(Re)/n
\
P
1
10mH
U
E
0
2 0
15
Y
-
0
0
3 C(Re)/mF
6 0
. -
h
E
0.5
15 C(Re)/mF
30
7 1
F
0
0
0.5 T(Re)/a.u.
1
Figure 2. Complex plane plots of capacitance (C) and modulated transmittance ( r ) at 700 nm for the PPy/PSS- film at 0.2 V in 1 M KCI aqueous solution. Inset shows the impedance plots corresponding to the Cplots. Experimental plots (0)and simulated curves (B) obtained from an equivalent circuit as shown in Figure 3 are presented together. The simulation was carried out with the following parameters: c d = 0, R,, = 0, Cf,,, = 3.4 mF, Rn = 21 Q, CT = 0.3 mF, T = 1 S, and K = 155, where K was used for the simulation of Tplots by assuming T = KCand is arbitrary.
film. 10 The slow faradaic process represented by the finite length transmission line is due to a charging-discharging process of deeply trapped ionslaJ near the polymer chains. The mathematical form for the finite length transmission line is9.l I
2, = ( 7 / C T )coth[ ( i w ) 1’2] /(.i07)”~ (1) where T is the characteristic time constant of the line, w is an angular frequency, and CT.is the total distributed charge capacity of the line. The simulated plots constructed from the equivalent circuit are also shown in Figure 2.
0
0.5 T(Re)/a.u.
1
Figure 4. The same plots as shown in Figure 2 for the PPy/PSS- film at 0.2 V in 1 M + 10 mM Kd[Fe(CN)6] aqueous solution.
Electromodulation Spectroscopies for PPy/PSS- Films with the Redox Species in Solution. Figure 4 shows the C and the T plots for the PPy/PSS- film at 0.2 V in 1 M KCl 10 mM K4[Fe(CN)6] aqueous solution. The coincidence between the C and the T plots was not maintained. A straight line with a slope of 4 5 O was observed at low frequencies in the C plots, while a deformed semicircular shape was seen in the T plots. When the dc potential was fixed at 0 or -0.15 V so that the redox reaction of [Fe(CN)6]3-/4-did not occur, both the Cand the T plots were nearly identical to one another regardless of the existence of the redox species (Figure 5 ) . From theseresults, the faradaic process in the PPy/PSS- film is found to be affected by the redox reaction of [Fe(CN)6]3-/4-in the solution.
+
4194 The Journal of Physical Chemistry, Vol. 97, No. 16, 1993
Amemiya et al.
(a)
. E
i o mHz 1 kHz
0
3 C(Re)/m F
6
0
0.5
1
T(Re)/a.u.
0.5 0
3 C( Re)/mF
6
0
0.5 T(Re)/e.u.
=!
1
1.995 Hz
Figure 5. The same plots as shown in Figure 2 for the PPy/PSS- films at 0 V (a) and at -0.15 V (b) in 1 M KCI solution in the presence ( 0 ) and absence (0)of 10 mM Kd[Fe(CN)6].
0
0.5
1
T( Re)/a.u. Figure 7. Simulated complex plane plots of (a) capacitance (C = AQI/ AE)and (b) modulated transmittance ( T ) in the PPy/PSS- film at 0.2 V in the presence of the redox species. The C plots were produced from eq 4 and the equivalent circuit as shown in Figure 6 and the Tplots from eq 5 (T= KC), where K is 207 (arbitrary). (0)The same experimental plots as shown in Figure 4, (m) simulated curves.
Figure 6. Equivalent circuit for the PPy/PSS- film in the presence of a redox species in solution. A series combination of the charge-transfer resistance (R:,) and the Warburg impedance ( Z Wis) added to thecircuit asshown in Figure 3. Thisseriescombination represents theredoxreaction of [Fe(CN)6]3-/4-.
I
H
E 150
Model for the Faradaic Process in the PPy/PSS- Films with the Redox Couple, [Fe(cN),]%/', in Solution. Figure 6 shows an equivalent circuit employed to simulate the modulated transmittance (T) plots for the PPy/PSS- film in the presence of the redox species. The faradaic impedance, Zppy, for the film is the same as that described in Figure 3 and connected in parallel with a series combination of the charge-transfer resistance (RLJ and the Warburg impedance (Zw).I2 This series combination is assumed to represent the redox reaction of [Fe(CN)6l3+-, At the moment, we can proceed with the simulation without considering whether the redox reaction will occur at the film/ solution interface or within the film. This point will be discussed later. The total charge ( Q ) passed through the circuit is equal to the sum of the two charges (Ql and Q2) through the different branches as shown in Figure 6. The T plots are expected to follow the modulation of the charge only in the film (AQl/AE). The ac electromodulation (A,!?) applied to the circuit is represented as follows:
m(w)= AZ(w)& + AQ,(w)/Cppy(a)
(2)
with
CPPY(4 = 1/(iwZpp,(4) The value of A Q I / A Eis given by solving eq 2
(3)
AQ,((J)/mb) = Cppy(4[1- RfI(ww)/m(o))l (4)
I
10mHz '
I
I I
I
Discussion
E
I
I
8
I
"0
150
300
Z(Re)/n Figure 8. Impedance plots obtained from the C plots as shown in Figure 4 after subtracting the uncompensated resistance (Rn) and the faradaic as shown in Figure 6. impedance (Zppy)
where Y(w)is the electrical admittance for the system. The value of AQl/AE can numerically be calculated from eq 4, because both the Cppy(w)and the Y(w)can be acquired from the Cor Z plots as shown in Figure 2. The T (=ATILT) plots for the film are simulated by assuming Faraday's law
A T / S = K(AQ,/AE) (5) where K is a constant. The calculated C (=AQl/AE) plots and the T ( = A T / A E ) plots are shown in Figure 7. These Tplots fit well to the experimental plots. This result indicates that the equivalent circuit is valid for the present system. Thus we conclude that the faradaic process in the film and the electrochemical reaction of the redox species occur in parallel. The Redox Processes of [Fe(CN)6P/4-at the Film. Figure 8 shows the impedance plots obtained from the C plots as shown in Figure 4 after subtracting the uncompensated resistance (&) of the solution plus I T 0 and the faradaic impedance (Zppy) of the film. The straight line from the origin with a slope of 4 5 O indicates that the charge-transfer resistance (RLJ is negligible and that the impedance for the redox reaction of [Fe(CN)6]3-/4-
Polypyrrole/PolystyrenesulfonateComposite Films
The Journal of Physical Chemistry, Vol. 97, No. 16, 1993 4195
TABLE I: Parameters for the Diffusion Process of the Redox Species, [Fe(CN)6>I4-, at Different Electrodes in 1 M KCI 10 mM K@e(CN\6] Aqueous Solution
+
+ L
electrodes
Q s-1/2
A/Aptb
PPy/PPSIT0 Pt
67.8 72.5 79.4
1.1
1.2
1
From eq 6. Ratio of the apparent surface area ( A ) of the electrodes to that (Apl) of the Pt plate. Calculated from eq 7 by assuming that the diffusion coefficients and bulk concentrations of ferri- and ferrocyanide are identical regardless of the electrodes. *ply
/
Ion rh llowly trapped Ion
9
reaction of [Fe(CN)6]3-/” occur in parallel. The latter reaction occurs not within the film but at the film/solution interface. A possible reason why the redox species do not enter into the film is that PPy/PSS- films are known as cation exchange films.I4
Conclusions Electrochemical impedance and cobr impedance spectroscopies were applied to a PPy/PSS- film immersed in aqueous solutions in the presence and absence of a redox species, [Fe(CN)s]3-/4-. A comparison of the complex capacitance with the modulated optical transmittance clearly shows that optical response follows the faradaic process only in the film. Color impedance spectroscopy is thus found to be a very powerful technique to separate the faradaic process in the films from the other electrochemical reactions. Analysis of both the capacitance and the modulated transmittance data using a new equivalent circuit has revealed that the faradaic process in the film and the redox reaction of [Fe(CN)6]3-/”occur in parallel and that thelatter reaction occurs at the film/solution interface.
Acknowledgment. Discussions with Professor K. Itoh of Yokohama National University have been stimulating and aided a series of these works. We thank Miss K. Ohkawa for her skillful assistance in the computer program and Dr. L. A. Nagahara for critical reading of the article. This work was supported by a grant from Ministry of Education, Science and Culture of Japan. References and Notes (1) (a) Tanguy, J.; Mermilliod, N.; Hoclet, M. J . Elecfrochem. SOC.
film
soh.
Figure 9. Schematic representation of the faradaic process in the PPy/PSS- film and the redox reaction of [Fe(CN)6I3-le at the film. The dotted lines indicate the double layers near the polymer chains and other spaces are bulk in the film.
is represented by the Warburg impedanceI2 as supposed above
where A is the surface area of the electrode, n is the number of electrons involved in the redox reaction, C*Oand C*Rare the bulk concentrations of oxidized and reduced species, DO and DR are the diffusion coefficients of oxidized (ferricyanide) and reduced (ferrocyanide) species, and R, T,and F have their usual significance. The value of u (67.852 s-I/*)was obtained from eq 6 and Figure 8 using the least-squares method. This value is compared with other values obtained with different electrodes such as an I T 0 coated glass and a Pt plate under the same conditions, as listed in Table I. Small differences in the u values between the three electrodes suggests that the diffusion process at the PPy/PSSfilm-coated electrodeoccurs not within the film but in the solution. This difference in the uvalues probably arises from the difference in the apparent surface areas of these electrodes as predicted from eq 7. Because both the diffusion coefficients of ferri- and ferrocyanide in 1 M KCl aqueous solution (DO= 7.63 X 10-6, D R= 6.32 X 10-6 cm2/s)13and the bulkconcentrations of the two species must be identical regardless of the electrodes. Thus the ratios of surface areas of the two electrodes to that of the Pt plate (A/Ap,)were obtained from eq 7. The relatively small value of A/Apl (=1.2) for the PPy/PSS- film indicates that the surface of the PPy/PSS- film is as smooth as that of the Pt plate. On the basis of the above results, Figure 9 shows a schematic representation of the electrochemical reactions in the system investigated here. The faradaic process in the film and the redox
1987, 134,795. (b) Penner, R. M.; Martin, C. R. J . Phys. Chem. 1989,93, 984. (c) Elliott, C. M.; Kopelove, A. B.; Albery, W. J.; Chen, Z. J . Phys. Chem. 1991, 95, 1743. (2) Otero, T. F.; de Larreta, E. J . Electroanal. Chem. 1988, 244, 311. (3) (a) Mermilliod, N.; Tanguy, J.; Hoclet, M.; Syed, A. A. Synth. Mer. 1987,18,359. (b) Glarum, S. H.; Marshall, J. H. J . Electrochem. Soc. 1987, 134, 142. (c) Rubinstein, 1.; Sabatani, E.; Rishpon, J. J. Electrochem. SOC. 1987, 134, 3078. (4) (a) Hunter, T. B.; Tyler, P. S.; Smyrl, W. H.; White, H. S. J . Electrochem. SOC.1987, 134, 2198. (b) Gabrielli, C.; Takenouti, H.; Hass, 0.;Tsukada, A. J. Elertroanab Chem. 1991, 302, 59. (5) (a) van der Sluijs, M. J.; Underhill, A. E.; Zaba, B. N. J . Phys. D: Appl. Phys. 1987, 20, 1411. (b) Deslouis, C.; Musiani, M. M.; Tribollet, B. J . Electroanal. Chem. 1989, 264, 37. (c) Deslouis, C.; Musiani, M. M.; Tribollet, B. J . Electroanal. Chem. 1989,264, 57. (d) Deslouis, C.; Musiani, M. M.; Pagura, C.; Tribollet, B. J . Electrochem. SOC.1991, 138, 2606. (6) (a) Hutton, R. S.; Kalaji, M.; Peter, L. M. J . Elecfroanal. Chem. 1989, 270, 429. (b) Greef, R.; Kalaji, M.; Peter, L. M. Faraday Discuss. Chem. SOC.1989, No. 88, 277. (c) Gabrielli, C.; Keddam, M.; Takenouti, H. Electrochim. Acta 1990,35, 1553. (d) Kalaji, M.; Peter, L. M. J . Chem. SOC.,Faraday Trans. 1991, 87, 853. (7) (a) Chazalviel, J.-N. Electrochim. Acta 1990,35,1545. (b) Rao, A. V.; Chazalviel, J.-N.; Ozanam, F. J . Appl. Phys. 1986, 60, 696. (8) Amemiya, T.; Hashimoto, K.; Fujishima, A. J.Phys. Chem.,preceding
paper in this issue. (9) (a) Albery, W. J.; Chen, Z.; Horrocks, B. R.; Mount, A. R.; Wilson, P. J.; Bloor, D.; Monkman, A. T.; Elliott, C. M. Faraday Discuss. Chem. SOC. 1989, No. 88, 247. (b) Albery, W. J.; Ellitto, C. M.; Mount, A. R. J . Electroanal. Chem. 1990, 288, 15. (c) Albery, W. J.; Mount, A. R. J. Electroanal. Chem. 1991, 305, 3. (10) This difference in the circuit elements for the fast faradaic processes comes from the difference in the impedance responses of the two films at high frequencies. A straight line with a slope of 45” was observed in the 2 plots for the PPy/CI- film,nhowever, it was never observed for the PPy/PSS film over the dc potential range from 0.2 to 4 . 5 V. (1 1) ,(a) Rishpon, J.; Gottesfeld, S. J. Electrochem. SOC.1984.131, 1960. (b) Rubinstein, I.; Rishpon, J.; Gottesfeld, S. J . Electrochem. Soc. 1986,133, 729. (c) Glarum, S. H.; Marshall, J. H. J . Electrochem. SOC.1980, 127, 1467. (d) Raistrick, I. D. Electrochim. Acta 1990, 35, 1579. (12) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980; Chapter 9. (1 3) Adams, R. N. Electrochemistry at solid electrodes; Marcel Dekker: New York, 1969; p 223. (14) (a) Shimidzu, T.; Ohtani, A,; Iyoda, T.; Honda, K. J . Electroanal. Chem. 1987, 224, 123. (b) Shimidzu, T.; Ohtani, A.; Iyoda, T.; Honda, K. J. Electroanal. Chem. 1988,251,323. (c) Baker, C. K.; Qiu, Y.-J.; Reynolds, F. R. J. Phys. Chem. 1991, 95,4446.
