J. Phys. Chem. 1993,97, 9736-9740
9736
Faradaic Charge Transfer with Double-Layer Charging and/or Adsorption-Related Charging at Polymer-Modified Electrodes As Observed by Color Impedance Spectroscopy T. Amemiya,? K. Hashimoto, and A. Fujishima' Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Bunkyo- ku, Tokyo 113, Japan Received: March 8, 1993; In Final Form: July 8, 1993"
The interfacial processes at polymer-modified electrodes are investigated using electrochemical impedance and color impedance (electromodulated optical transmittance) spectroscopies. Direct in situ evidence for faradaic charge transfer with double-layer charging is obtained for polypyrrole/ polystyrenesulfonate (PPy/PSS-) composite films in 1 M aqueous LiC1, NaC1, KC1, CsC1, and Me4NC1. On the other hand, the high-frequency charging is found to be coupled with adsorption-related (faradaic) charging for the film in 1 M aqueous Bu4NCl. These results are modeled with an equivalent circuit taking into account both the double-layer and adsorption-related capacitances at the film/solution interface. W.E.
Introduction The redox kinetics of conducting polymer films such as polypyrrole, polythiophene, and polyaniline on electrodes is composed of two kinds of faradaic processes. One is electron transfer at the electrode/fidm and ion transfer at the film/solution interfaces, and the other is charge transport within the films. The charge-transport process is observed at low frequencies (- 10 Hz to "Hz) by ac impedance measurement and is so far studied intensively.' We have also discussed* in detail the chargetransport processes within polypyrrole films on the basis of both electrochemical impedance and color impedance (electromodulated optical re~ponse)~ of the films. In particular, the usefulness of the color impedancespectroscopy (CIS) to investigatefaradaic processes within conducting polymer films is demonstrated in a series of papers.2 Our present investigations aim at accounting for the interfacial process at thin-film (polypyrro1e)-modified electrodes. This process is expected to comprise a semicircle at high frequencies (-kHz to 10 Hz) in complex impedance plots on the basis of the Randles equivalent ~ i r c u i t .This ~ circuit is also applicable for thin-film-modifiedelectrodes if the conventionalsemiinfinite Warburg impedances is replaced with a finite Warburg term6 or a finite length transmission line.7 The high-frequencysemicircle is probably due to a charge- and/or ion-transfer resistance in parallel with the double-layers or an adsorption-related(faradaic) capacitance7*at the film/solution i n t e r f a ~ e . ~ . ' ~ We address the following two points concerning the highfrequency response of polypyrrole (PPy)-modifiedelectrodes. One is that the PPy film changes its color at high frequencies (- kHz to 10 Hz)where the reaction is controlled by the charge- and/ or ion-transfer process at the interfaces. The other is that the high-frequency charging can be assigned either to the doublelayer or an adsorption-related (faradaic) capacitance at the film/ solution interface on the basis of the color impedance behavior. First, we present electrical and optical ac responses of a modei circuit and show that the two responses differ from one another at high frequencies where faradaic charge transfer and nonfaradaic double-layer charging occur simultaneously. Second, experimental results of the electrochemicalimpedance and color impedance spectroscopies are shown for polypyrrole/polystyrenesulfonate(PPy/PSS-) composite films2b.11 in 1 M aqueous CsCl or Bu4NCl. The interfacial capacitance for the film in the CsCl
-
-
t Present address: Department of Chemical Systems, National Institute of Materials and Chemical Research (NIMC), Tsukuba, Ibaraki 305,Japan. *Abstract published in Advance ACS Abstracts. September 1, 1993.
0022-3654/93/2091-9136$04.00/0
AE
r
R.E. -1
Figure 1. Equivalent circuit for a thin-film-coatedelectrode. This circuit is used as a model to calculate the electrical and optical ac responses as
shown in Figure 2. c d is thedouble-layercapacitanceatthe film/solution interface,& is a charge and/or ion-transfer resistance, ZDis a chargetransport impedance within the film, and Ro is an ohmic resistance of a solution plus underlying electrode. AE is the applied ac potential; A Q is a total charge divided into the double-layer charging (A&) and the faradaic process (AQr). solution is found to be due to the double-layer charging, while that in the Bu4NC1 soiution is found to be due to adsorptionrelated (faradaic) charging at the film/solution interface.
Electrical and Optical Ac Response Based on a Model Circuit Derivation of Electrical and Optical Ac Response. Figure 1 shows the faradaic (AQf)and nonfaradaic(AQd)processesthrough the equivalent circuit for thin-film-coated electrodes6v7aunder ac electromodulation (AE).We employ this circuit as a model system to investigate the relationship between electrical and optical ac response. The ac optical transmittance ( A n or reflectance (AR) of an electrochromic material on electrodes is expected to be proportional to the modulation of the faradaic charge (AQf). Thus, the optical ac response (ATIAE or ARIAE) can be derived from the modulated faradaic charge (AQf/AE= CAW)).The ac voltage (AE)applied to the circuit is given by where Ra is the charge- and/or ion-transfers resistance, Z Dis the chargetransport impedance within the film as represented by the finite length transmission line,'J2 Rn is the ohmic resistance due to an electrolyte and underlying electrode, and AIf (=AQf/ 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9737
Charge Transfer at Polymer-Modified Electrodes At) and AI ( = A Q / A t ) are the faradaic and total current, respectively, passed through the circuit as shown in Figure 1. Arrangement of eq 1 yields the faradaic complex capacitance (Cr(w))
C f b ) = J ( N f / W dt = (l/h)[{1
0.01
u.
E . E h
TO
+
-&(N/m))/(&t zD)l
C(Re)/mF
= (l/.b)[il - R Q Y ( w ) ) / ( R , + zD)l (2) where Y(w)is the total admittance of the circuit and is given by
Y(w) = 1/Z(o)
(3)
150
with z(w) = R Q + l/il/(&,+ zD) + . b c d l (4) Here Z(o) is the total impedance of the circuit, and c d is the double-layer capacitance. The frequency response of the Cf(w) can numericallybe obtained from eqs 2-4 with appropriatevalues of the parameters. Alternatively, the total capacitance (C(o)) of the circuit is given by
c x
ZO
v
C(w) = (1/.b)(l/Z(w)) (5) In addition, we define the faradaic complex impedance ( Z d w ) ) as Z&) = (1/jm)(1/Cb)) (6) The functional form of this impedance is similar to the total impedance (Z(o))of the circuit. We show below the difference between the total (Z(w)and C(w))and the faradaic (Zr(o)and Cr(w))response at high frequencies in the complex plane plots. We now regard the faradaic response (Zr(o)and Cr(w))as the optical response (Zopt(w)and Copt(o))3hfor convenience. In practice, the optical impedance (Zop,(w))and the capacitance (Copt(@))are proportional to the complex faradaic impedance (Zdw)) and the capacitance (Cdo)), respectively. ComplexPlanePlots. Figure 2 shows the electrical and optical ac response obtained from numerical calculation of eqs 2-6. The optical response is revealed to differ from the electrical response at high frequencies,as shown in the shaded areas.I3 In particular, complex plane plots clearly show that optical impedance yields a straight line parallel to the imaginary axis instead of the semicircle at high frequencies. The shapesof electrical and optical responses are similar at low frequencies where the double-layer charging is complete and has no effect on the faradaic process. The double-layer charging is revealed to affect the optical ac response only at high frequencies. In addition, it should be noted that the electrical and optical responses are similar in shape at all frequencies,in the case where the charging of the double layer is coupled with a faradaic process, e.g., faradaic adsorption at the film/solution interface. From the above considerations, we can distinguishthe double-layercharging from an adsorption-related (faradaic) charging at high frequencies. Experimental results concerning the two kinds of high-frequency capacitances are presented below.
Experimental Section Chemicals. Pyrrole, sodium polystyrenesulfonate (NaPSS), LiCl, NaC1, KCI, CsCl, tetramethylammonium chloride (Me4NCl), and tetra-n-butylammonium chloride (Bu4NCl) were purchased from Tokyo Kasei Co. and used as received. Solutions were prepared with distilled water. Polypyrrole Film Deposition. An electrochemical cell and electrodes used in this study were the same as those reported previously.2 Pyrrole and PSS-composite (PPy/PSS-) films were grown, at constant current of 1.3 mA/cm2 for 60 s on indiumdoped tin oxide-coated (ITO) glasses (area 0.78 cm2), from an aqueous solution containing 0.1 M (M = mol/dm3) pyrrole and
0
150
0
1000
Z( R e p F i p e 2. Complex plane plots of electrical (0)and optical (+) ac responses in the form of capacitance(C)and impedancc (2).These plots are calculated from eqs 2-6 on the basis of the equivalent circuit as shown in Figure 1 with the following parameters: cd = 10 mF, lpft = 10 Q,Ro = 10 a, T = 0.1 s, and CT= 3 mF, where 7 and CTare the characteristic time constant and the total distributed capacity of the transmission line (ZD), respectively (see ref 12). Discrepancies are found between the electrical and optical responses at high frequencies (shaded areas) where faradaic charge and/or ion transfer occurs in parallel with the doublelayer charging. 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 ca. 0.26 pm measured with a talystep apparatus (Taylar-Hobson). All potentials are reported relative to the SCE. Electrochemistry and Electromodulation Spectroscopies. After the polymerization,the cell and the PPy/PSS- films were washed with distilled water. Cyclic voltammetry and electromodulation spectroscopiesfor the films were performed in 1 M aqueous LiCl, NaC1, KCl, CsC1, Me4NC1,and BqNCl electrolyte: a constant anion but different cations. Electrochemicalimpedanceand color impedance spectroscopies for the films were carried out after cyclic voltammetry for the first two cycles. These electromodulation measurementswere carried out at an amplitude of 5 mV, in the frequency range from 1 kHz to 10mHz using a potentiostat (Model 2020, Toho TechnicalResearch) and a frequencyresponse analyzer (Model S5720B, NF Circuit, Ltd.). The transmittance of the films was monitored with 700-nm light from a tungsten lamp through a monochromator. Apparatus and the experimental methods for the electromodulation spectroscopies have been described in detail previously.2 Results Cyclic Voltammetry. Figure 3 shows typical cyclic voltammograms of PPy/PSS- films on the second cycle in 1 M aqueous CsCl (solid curve) or BudNCl (dash-dotted curve). Multicycle voltammograms were qualitatively identical to the second voltammograms. The voltammograms in 1 M aqueous LiC1, NaCl, KCl, and Me4NCl (not shown) were nearly the same as that in 1 M aqueous CsC1. Large differencesin the voltammogramwere seen only in 1 M Bu4NCl. The current density for the film in the Bu4NCl solution decreased approximately by one-fifth, and
9738 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 T
Amemiya et al. (a) 0.2 V
10.2 mA L
0.054
f\
Y
A\
E
-
-gE 0 2
1.995
N
0 Copt(Re)/a.u.
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
E I V vs. SCE Figure 3. Cyclic voltammograms of a PPy/PSS- film in 1 M aqueous
CsCl (-) or BbNCl(- - -). The film was electrochemicallypolymerized (Q= 77 mC/cm*) on an IT0 glass. Electrode area was 0.78 cm2, and scan speed was 50 mV/s. the redox peak shifted ca. 0.4 V to positive potentials compared with the film in the CsCl solution. This result combined with those of the modulation spectroscopies as shown later indicates that mostly cations are inserted and removed tocompensate charge in the f i l m ~ . ~ BThe J ~ modulation measurements for the films were carried out at 0.2 and -0.3 V in the CsCl solution and 0.3 and 0 V in the Bu4NCl solution, as shown in Figure 3. Impedance and Color Impedance Spectroclcopies for a PPy/psSFilm in 1 M Aqueous CsCl. Figure 4 shows complex plane plots of the electrical response (capacitance, C, and impedance, Z) and optical response (capacitance, Copt,and impedance, Zopt)for the PPy/PSS- film in 1 M aqueous CsCl a t 0.2 V (a) and -0.3 V (b).14 The open circles are the experimental data, and the closed squares are the simulated plots as shown later. The optical capacitance, Copt,is the frequency response of the modulated transmittance (ATIAE)a t 700 nm obtained from the experiment for the film. The optical impedance, Zoptis calculated from the optical capacitance using eq 6. When the PPy/PSS- film was oxidized (at 0.2 V), the electrical (C, Z)and optical (Copt,Zopt)responses were similar in shape at all frequencies from 1 kHz to 10 mHz. The high-frequency semicircle was not observed in the impedance plots. These results indicate that both the transfer resistance and the double-layer capacitance are small for oxidized PPy f i l m ~ . ~Further, ~ J ~ the coincidence between the electrical and the optical responses shows that the electrochemical reaction in the system is coupled to the faradaic process due to the PPy film.* On the other hand, when the film was slightly oxidized (at -0.3 V), apparent discrepancies are found between the electrical and the optical responses at high frequencies. In particular, the highfrequency semicircle was observed in the impedance ( Z ) plots, while the straight line parallel to the imaginary axis was observed in the optical impedance (Zopt)as shown in the shaded areas. These observations clearly indicate that the PPy film changes its color at high frequencies where the reaction is kinetically controlled and that faradaic charge and/or ion transfer occurs in parallel with the double-layer charging at the filmlsolution interface as mentioned previously. In addition, a slow faradaic process was observed a t low frequencies (- 10 mHz) at -0.3 V as shown in both the capacitance (C and Copt)plots. The previous paper& has discussed the fast and slow faradaic processes in PPy films in detail. Impedance and Color Impedance Spectmpcopiesfor a PPy/psS Film in 1M Aqueous BmCI. Figure 5 shows the same plots as shown in Figure 4 for the PPy/PSS- film in 1 M aqueous BudNC1. A lot of noise was observed in the optical responses. This
0.08
Zopt(Re)/a.u.
~
(b)-0.3V 0.024
0.5012
Y
E =i E 8 0
E
0
0.024 0 CIReWmF
5
0.012 0 Copt(Re)/a.u.
2
100
0 Z(Re)/R
I
0.012
--4 ,"
E
go
Y 0
1
Zopt(Re)/a.u.
Figure 4. Complex plane plots of the electrical response (capacitance, C, and impedance, Z)and the optical response (C, and Z ,) at 700 nm for a PPy/PSS- film in 1 M aqueous CsCl at 0.2 V (a) and -0.3 V (b), respectively. The open circles ( 0 )are the experimental data, and the closed squares (B) are the simulated plots using an equivalent circuit as shown in Figure 6.
(a) 0.3 V 0.1
.
LL
E E
-
E
50.12
oTo 0.1 0 CIReVmF
0
0
1.5
200 Z(Re)/R
0.01
i
4
1
3
-=E
3 E
G
O
go
Y
$
0.01
0
0
0
0.2
1
Zopt(Re)/a.u.
Copt(Re)/a.u.
(b) 0 V 0.1
-. y.
E E oT0
39.81
0 0.02 0.02 I
*
I
?
.
.
200 Z(Re)/R
C(Re)/mF
n ~ r
%! E O
0
Y
0
0.02 0 Copt(Re)/a.u.
0.4
0
1
Zopt(Re)/a.u.
Figure 5. Same plots as shown in Figure 4 for a PPy/PSS- film in 1 M aqueous BudNCl at 0.3 V (a) and 0 V (b), respectively.
The Journal of Physical Chemistry, Vol. 97, No. 38, I993 9739
Charge Transfer at Polymer-Modified Electrodes
TABLE I: Fitting Parameters bulk process
electrolyte 1 M CsCl/HzO 1 M TBACl/H20
E, V
nus
0.2 -0.3 0.3 0
av3 0.1 11 13
TZ/S
CTI,O~F
40 60 45 70
3.3 4.4 1.5 1.4
interfacial proccss C T ~ , " ~ F &, CHF,cF 0.8 0 20 7.0 1.3
25 150 1400
1.1
R Q , ~
20 200 200
1
K,bau
17 17 46 46
350 400 150 300
and CT,(i = 1, 2) are the characteristic time constant and total capacity of the transmission line (&) (see ref 7) which represents a chargetransport process within the films. Proportional coefficient used to fit the optical impedance and optical capacitance plots. Ti
e-
f
electrode
x
film
.o
a TBA+ d
adsorption s k TBA+
soh.
Figure 7. Schematic representation of the charge- and/or ion-transfer
processes at the PPy/PSS- film/solution interface.
r------
capacitance (Cdo)) for the film in 1 M aqueous CsC1,
Copt= KCf(0) Figure6. Equivalent circuit used to obtain the simulated plots in Figures 4 and 5 . CHFiSa high-frequency capacitanceraponsiblefor the interfacial processes and is composed of a parallel combination of the double-layer
capacitance(cd)with an adsorption-related(faradaic) capacitance(CA) at the film/solution interface. ZDl and ZDZrepresent a fast and a slow charge-transportprocess within the film and are expressed by the finite length transmission line as shown in Figure 1. Other symbols are the same as those in Figure 1. noise probably arises from less optical changes in the film in 1 M aqueous Bu4NC1. The film in this solution was electrochemically less active as shown in the cyclic voltammograms (Figure 3). The electrical and optical ac responses were qualitatively similar in shape at all frequencies for both the oxidized (0.3 V) and slightly oxidized (0 V) films as shown in Figure 5 . The two responses were also similar in shape at other dc potentials such as 0.2,0.1, and 0 V (not shown). These results indicate that the electrochemical reaction in the system is coupled to the faradaic process due to the PPy film at all frequencies from 1 kI-Iz to 10 mHz. It must be mentioned here that the impedance at high frequencies (1 kHz-3.981 Hz)asshowninFigure 5a,biscomposed of a large charge- and/or ion-transfer resistance (150 or 1400 Q) in parallel with an interfacial capacitance (200 pF) as described below.
-
Equivalent Circuit and Fitting of the Data. Figure 6 shows an equivalent circuit employed to reproducethe electrical and optical ac responses for the PPy/PSS- film in both 1 M aqueous CsCl and Bu4NC1. This circuit is a modification of that as shown in Figure 1 and takes into account both a fast ( Z m ) and a slow (ZD2)charge-transport process within PPy films as previously reported.2 Further, this circuit has a high-frequencycapacitance (CHF)in place of the double-layercapacitance. This capacitance (CHF) is composed of a parallel combination of the double-layer capacitance (Cd) with an adsorption-related (faradaic) capacitance (CA)7a at the film/solution interface. The simulated plots on the basis of the equivalent circuit are also depicted in Figures 4 and 5 . The simulated optical capacitance (Copt)was obtained from the complex faradaic
= K(AQ,/W
(7) and from the total capacitance (C(w))for the film in 1 M aqueous Bu4NCl. Copt= W o )
=K(AQ/W (8) where K is a constant. Table I summarizes the values of the fitting parameters. Both the charge- and/or ion-transfer resistance (&) and the high-frequency capacitance (CHF) for the film in 1 M aqueous BudNCl have been found to be much larger than those in 1 M aqueous CsC1. In addition, the characteristic time constants (71) due to the fast charge transport process2within the film were over 2 orders of magnitude larger in 1 M aqueous Bu4NCl than those in 1 M aqueous CsCl. The sum of the charging-discharging capacities (cT1 and C T ~within ) the films was small in 1 M aqueous Bu4NCl compared with that in 1 M aqueous CsC1. These differences in the charge-transfer and charge-transport processes for the filmin the different electrolytes can mostly be attributed to the mobility of the charge-compensating cations.'gJ6 Discussion Figure 7 shows the schematic representation of the chargeand/or ion-transfer process at the PPy/PSS- film/solution interface. When CsCl was used as electrolyte, faradaic charge and/or ion transfer was found to occur in parallel with nonfaradaic charging at -0.3 V at high frequencies. Thus, the high-frequency capacitance (CHF)as shown in Figure 6 is composed of only the double-layer capacitance (Cd) at the film/solution interface.9.10 When Bu4NCl was used as electrolyte, the high-frequency capacitance (CHF)has been found to be faradaic. Namely, this capacitance (CHF)is considered to be a parallel combination of an adsorption-related(faradaic) capacitance (CA)with the doublelayer capacitance (Cd); CHF= CA+ Cd." The faradaic adsorption of Bu4N+ at the film/solution interface probabl results from its larger cationic size, which makes it diffult to enter into the film, than that of Cs+ or the other cations (Li+, Na+, K+, and Me4N+), as schematicallyshown in Figure 7. The radius of theunhydrated
9740 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
ions is in the order Li+ (0.78)< Na+ (0.95) < K+ (1 -33) < Cs+ (1.69)< Me4N+ (3.47)< Bu4N+ (4.52)in A. These values are said to be preferable to those of hydrated ions for the description of the ionic sizes in aqueous solutions.l6a.l* However, it seems unlikely that there would be such a larger difference between Me4N+ and Bu4N+ merely on the basis of the small difference in the ionic size. Thus, other possibilities such as ion pairing, solvation, and anion movement must be consideredto account for the results.
Conclusions The electrochemical impedance and color impedance spectroscopies have been employed to investigate the charge- and/or ion-transfer processes at PPy/PSS- film/solution interfaces in 1 M aqueous LiC1, NaC1, KCl, CsC1, Me4NC1, and Bu4NCl. Apparent discrepancies have been found between the electrical and the optical responses at high frequencies (1 kHz-10 Hz) for the film at -0.3 V in aqueous LiC1, NaCl, KCl, CsCl, and Me4NCl. In particular, the high-frequency semicircle was observed in the impedance (2)plots, while a straight line parallel to the imaginary axis was observed in the optical impedance (Zopt)plots which is analogous to 2. This result demonstrates that faradaic charge and/or ion transfer occurs in parallel with the doublelayer chargingat the film/solutioninterfaceas previously expected using a model circuit. On the other hand, the electrical and the optical responses were similar in shape for the film in the aqueous Bu4NCl at all frequencies and at any dc potentials investigated here. Thus, the high-frequencycapacitance (CHF) is found to be composed of an adsorption-related (faradaic) capacitance (CA) at the film/solutioninterface in this case. The faradaic adsorption of Bu4N+ at the film/solution interface probably results from its larger cationic size, which makes it difficult to enter into the films. In conclusion, color impedancespectroscopyhas been found to be a very powerful technique to investigate faradaic processes taking place at electrochemic film/solution interfaces as well as within the films.
Acknowledgment. Discussions with Professor K. Itoh of Yokohama National University has been stimulating and aided this work. We thankDr. K. Ohkawa both for technical assistance in the computer program and for fruitful discussions. This work was supported by a grant From Ministry of Education, Science and Culture of Japan. References and Notes (1) (a) Burgmayer, P.; Murray, R. W. J. Phys. Chem. 1984,88,2515. (b) Rubinstein, I.; Sabatani, E.; Rishpon, J. J. Electrochem. Soc. 1987,134, 3078. (c) Glarum, S. H.; Marshall, J. H. J. Electrochem. Soc. 1987, 134, 142. (d) Osaka, T.; Naoi, K.; Ogano, S.;Nakamura, S.J. Electrochem. Soc. 1987, 134, 2096. (e) Tsai, E. W.; Rajeshwar, K.; Reynolds, J. R. J. Phys. Chem. 1988,92,3560. (f) Penner, R. M.; Dyke, L. S.V.; Martin, C. R. J. Phys. Chem. 1988,92,5274. (g) Paulse, C. D.; Pickup, P. G. J. Phys. Chem.
Amemiya et al. 1988,92,7002. (h) Penner, R. M.; Martin, C. R. J. Phys. Chem. 1989.93, 984. (2) (a) Amemiya,T.; Hashimoto,K.; Fujishima, A. J. Phys. Chem. 1993, 97,4187. (b) Amemiya, T.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1993, 97, 4192. (3) (a) Adzic, R.; Cahan, B.; Yeager, E. J. Chem. Phys. 1973,58,1780. (b) Horio, K.; Adachi, Y.; Ikoma, T. Denki Kagaku 1984,52,69. (c) Rao, A. V.;Chazalviel, J.-N.; Ozanam, F. J. Appl. Phys. 1986,60,696. (d) Grcef, R.; Kalaji, M.; Peter, L. M. Faraday Discuss. Chem. Soc. 1989,88,277. (e) Hutton, R. S.;Kalaji, M.; Peter, L. M. J . Electroanal. Chem. 1989,270,429. (f) Chazalviel, J.-N. Electrochim. Acta 1990, 35, 1545. (g) Gabrielli, C.; Keddam, M.; Takenouti, H. Electrochim. Acta 1990, 35, 1553. (h) Kalaji, M.; Peter, L. M. J. Chem. Soc., Faraday Trans. 1991,87,853. (i) Sagara, T.; Igarashi, S.;Sato, H.; Nib, K. hngmuir 1991, 7, 1005. (4) Randles, J. E. B. Discuss. Faraday Soc. 1947, I , 11. (5) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiely & Sons: New York, 1980; Chapter 9. (6) Ho, C.; Raistrick, I. D.; Huggins, R. A. J . Electrochem. Soc. 1980, 127, 343. (7) (a) Franceschetti, D. R.;Macdonald, J. R. J. Electrochem.Soc. 1982, 129, 1754. (b) Rishpon, J.; Gottesfeld, S.J . Electrochem. Soc. 1984, 131, 1960. (c) Rubinstein, I.; Rishpon, J.; Gottesfeld, S.J . Electrochem. Soc. 1986,133,729. (d) Albery, W. J.; Chen, Z.; Horrocks, B. R.; Mount, A. R.; Wilson, P. J.; Bloor, D.; Monkman, A. T.; Elliot, C. M. Faraday Discuss. Chem. Soc. 1989, 88, 247. (e) Albery, W. J.; Elliot, C. M.; Mount, A. R. J. Electroanal. Chem. 1990, 288, 15. (f) Albery, W. J.; Mount, A. R. J. Electroonal. Chem. 1991,305,3. (g) Elliot, C. M.; Kopelove, A. B.; Albery, W. J.; Chen, Z . J . Phys. Chem. 1991,95, 1743. (8) (a) Dalahay, P. J. Phys. Chem. 1966, 70, 2067. (b) Delahay, P. J . Phys. Chem. 1966, 70, 2373. (c) Delahay, P.; Subbielles, G. G. J. Phys. Chem. 1966, 70, 3150. (9) (a) Hunter, T. B.; Tyler, P. S.;Smyrl, W. H.; White, H. S.J. Electrochem. Soc. 1987, 134, 2198. (b) Jow, T. R.; Shacklette, L. W. J . Electrochem. Sot. 1988, 135, 541. (c) Tanguy, J.; Mermilliod, N.; Hoclet, J. J. Electrochem. Soc. 1987,134,795. (d) Albery, W. J.; Mount, A. R. The Electrochemical Society Extended Abstracts, Abstract 671; American Chemical Society: Washington, DC, May 5-10, 1991; Vol. 91-1, p 965. (10) The doublelayer Capacitance (&) at the electrode/fiIm interface can be negligible for a conducting polymer unless it is reduced state since the effect of c d l is swamped by the film (bulk) capacitance. Thus, the double layer capacitance (cd) appeared in the previous papers2 should be ascribed to the film/solution interface. (1 1) (a) Schimidzu, 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. (12) The mathematical form of the transmission line' is given by ZD= (T/CT)coth[Ciw7)1/z]/(iw~)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. (13) The complex plane analysis of optical ac response (reflectance) has first been reportedby Adzicandco-workersin 1973.k Thedbcrepancybetwcen the electrical and the optical response at high frequencies is mentioned in ref 3a. However, their analysis concerning the origin of the discrepancy is somewhat different from our present analysis. (14) The electrical and optical responses for the PPy/PPS- in 1 M aqueous LiC1, NaCI, KCl, and Me4NCI are qualitatively identical with those in 1 M csc1. (15) Ren, X.;Pickup, P. G. J . Electrochem. Sot. 1992, 139, 2097. (16) (a) Kuwabata, S.;Nakamura, J.; Yoneyama, H. J. Electrochem. Soc. 1990, 137,2147. (b) Baker, C. K.; Qiu, Y.-J.; Reynolds, J. R. J. Phys. Chem. 1991, 95, 4446. (17) The charging process of the C, as shown in Figure 6 cannot be separately observed from that of the cd by means of the color impedance spectroscopy or any other electrical measurements. (18) Horvath, A. L. Handbook of Aqueous Electrolyte Solutions;Ellis Horwood: England, 1985; Parts 1 and 2.14.
